WO2020061617A1

WO2020061617A1 - Optical data storage material and method

Info

Publication number: WO2020061617A1
Application number: PCT/AU2019/050990
Authority: WO
Inventors: Simone LAMON; Qiming Zhang; Min Gu
Original assignee: Royal Melbourne Institute Of Technology
Priority date: 2018-09-26
Filing date: 2019-09-13
Publication date: 2020-04-02

Abstract

An optical data storage material comprising graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage, nanoparticles configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission, and a support material that (i) embeds the GO and the nanoparticles and (ii) comprises a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas.

Description

OPTICAL DATA STORAGE MATERIAL AND METHOD

TECHNICAL FIELD

The present disclosure relates generally to optical data storage, and is particularly directed to an optical data storage material, a method of manufacturing the optical data storage material, and a method of recording optically readable data.

BACKGROUND

Optical data storage involves recording digital data in a bit-by-bit fashion through the use of photons. The recording process produces localised physical or chemical changes of the optical properties of a recording medium to form a recorded medium. Once recorded, the digital data can be optically retrieved (i.e., read) by scanning the recorded medium, e.g., with a tightly-focused laser beam.

In this context, optical storage media allow for digital data to be stored in an optically readable manner, so that they can be read, for example, by means of a laser and a photodetector in a pickup. Current generation commercial optical storage media include single- and dual-layer DVD and Blu-ray discs, in which recording and playback are based on controlling or detecting returned light from reflective layers within the medium.

The storage capacity of conventional optical data storage systems is limited by the diffractive nature of light. To increase the storage capacity in a single optical device, the diffraction limit may be passed using super-resolution optical data storage. Recent breakthroughs in nanophotonics such as the development of super-resolution optical microscopy and nanolithography have provided new pathways for light-matter interaction allowing nanoscale manipulation of materials using optical beams. In particular, stimulated emission depletion (STED) microscopy and super-resolution photo-induction inhibited nanolithography (SPIN) have enabled imaging and writing of features at the nanoscale, respectively.

However, high optical energy may be required to optically record data bits on conventional optical data storage materials beyond the diffraction limit, and these energy levels may be sufficiently high to trigger undesirable thermal effects in the material in areas surrounding the focal point, and such thermal effects may be detrimental to the resolution of the recording process, and thus detrimental to the storage capacity of the medium. High optical energy usage may also be undesirable in large scale optical data storage applications that have limited power available.

It is desired to address or ameliorate one or more disadvantages or limitations of the prior art, or to at least provide a useful alternative.

SUMMARY

There is provided described herein an optical data storage material comprising graphene oxide (GO) configured to be photo-chemically reduced on selected areas (or in selected regions) for optical data storage, nanoparticles configured to photo-chemically reduce the GO on the selected areas (or in the selected regions) by optical upconversion emission, and a support material (or a thermal material) that (i) embeds (or surrounds) the GO and the nanoparticles and (ii) comprises (i.e.,“includes”) a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas (or of the GO in the selected regions).

The local photo-chemical reduction of GO by upconversion emission of the nanoparticles may allow for optical data storage at the nanoscale. In addition, by having the GO and the nanoparticles embedded within a support material comprising a thermal conductor in thermal contact with the GO, the photo-thermal reduction of GO in the irradiated areas can be mitigated, thereby reducing photo-thermal reduction that may cause reduction of a larger area or region than the area or region reduced by the photo-chemical reduction alone. In addition, the photo-chemical reduction of the GO by optical upconversion emission of the nanoparticles may require significantly lower energy consumption relative to conventional super-resolution writing/reading techniques, making the optical data storage material described herein potentially sustainable for implementation of high-capacity optical data storage devices on a large scale.

Upconversion emission may be achieved, for example, by having nanoparticles containing an ion of a rare-earth element. Nanoparticles containing rare-earth ions can provide a pathway for super-resolution optical activation in both writing and reading by exploiting their characteristic electronic transitions and significant thermal stability.

Furthermore, the metastability of the energy levels in nanoparticles containing rare-earth ions guarantees low saturation intensity which results in a decrease of the inhibition beam power during super-resolution photo -induction inhibited nanolithography (SPIN). In some embodiments, the nanoparticles contain one or more ions of a rare-element selected from ytterbium ions (Yb³⁺), thulium ions (Tm³⁺), and a combination thereof.

In some embodiments, the nanoparticles are core-shell nanoparticles. Advantageously, the upconversion luminescence of core-shell nanoparticles is more intense relative to the corresponding nanoparticle absent the shell. As a result, the photo-chemical reduction of the GO by optical upconversion emission is significantly more efficient.

The use of core-shell nanoparticles may advantageously provide enhanced up-conversion emission efficiency and/or fluorescence lifetime, increased functional multiplicity, and/or tunable optical properties.

There is also provided herein a method of manufacturing an optical data storage material, the method comprising the steps of providing graphene oxide (GO) configured to be photo- chemically reduced on selected areas (or in selected regions) for optical data storage, providing nanoparticles on the GO, the nanoparticles being configured to photo-chemically reduce the GO on the selected areas (or in the selected regions) by optical upconversion emission, and providing a support material (or a thermal material) that (i) embeds (or surrounds) the nanoparticles and the GO and (ii) comprises a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas (or of the GO in the selected regions).

The step of embedding the GO and the nanoparticles into a thermal conductor may be performed by any means known to a skilled person, for example, hydrolysis and condensation of metal alkoxides which may be a highly flexible, inexpensive and/or low-energy synthesis route.

There is also provided herein a method of recording optically readable data, the method comprising optically upconverting an input beam using nanoparticles to form an upconverted beam, photo-chemically reducing GO in selected areas (or selected regions) by the upconverted beam, and mitigating photo-thermal reduction of the selected areas by having the nanoparticles and GO embedded in a support material that comprises a thermal conductor in thermal contact with the GO.

In some aspects, the invention also relates to an optical data storage material comprising graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage, and nanoparticles configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission.

In some aspects, there is also provided a method of manufacturing an optical data storage material, the method comprising the steps of providing graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage, and providing nanoparticles on the GO, the nanoparticles being configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission.

In additional aspects, the invention also provides a method of recording optically readable data, the method comprising optically upconverting an input beam using nanoparticles to form an upconverted beam, and photo-chemically reducing GO in selected areas by the upconverted beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be now described with reference to the following non-limiting drawings, in which:

Figure 1 shows a) a transmission electron microscopy (TEM) image of a batch of NaYF₄ nanoparticles containing Yb³⁺ and Tm³⁺ ions, and corresponding b) a size distribution determination plot and c) an emission spectrum,

Figure 2 shows (a) up-conversion fluorescence lifetime data for NaYF₄ nanoparticles containing Yb³⁺ and Tm³⁺ ions conjugated with GO in water solution, and (b) upconversion fluorescence spectra of 4% Tm-doped nanoparticles (10 pmol) mixed with GO at increasing concentration up to 60 pg mL ¹ in water solution under 980-nm CW laser excitation,

Figure 3 shows (a) TEM imaging (Scale bar: 20 nm) and (b) corresponding size distribution of as- synthesized NaGdF₄:Yb/Tm core nanoparticles, before deposition of a NaGdF₄ shell,

Figure 4 shows (a) TEM imaging (Scale bar: 20 nm) and (b) corresponding size distribution of NaGdF₄:Yb/Tm@ NaGdF₄ core-shell nanoparticles,

Figure 5 shows upconversion luminescence spectra of as-synthesized NaGdF₄:Yb/Tm core and NaGdF₄:Yb/Tm@ NaGdF₄ core-shell nanoparticles under excitation using a NIR CW laser at 980 nm,

Figure 6 shows a schematic of a dual-beam optical system setup for implementation of super-resolution optical data storage on the material described herein,

Figure 7 shows a schematic of the principle for recording optically readable data by super- resolution optical data storage using the material described herein,

Figure 8 shows (a) schematic of upconversion luminescence (“ON”) of nanoparticles of the kind described herein when integrated on GO under a 980 nm excitation light, (b) schematic of quenched upconversion luminescence (“OFF”) of the nanoparticles when integrated on rGO under a 980 nm excitation light due to rGO resonance energy transfer (RET) quenching effect, (c) progressive 450-nm upconversion luminescence emission spot quenching of nanoparticles integrated on GO due to increasing GO to rGO conversion upon GO irradiation with 375-nm CW laser at 10, 25, 50, 75 and 100 pW, normalized to before irradiation (exposure time: 100 ms), the inset showing a detailed imaging of the modulation of an individual pixel (Scale bar: 1 pm), and (d) 450-nm emission lifetime before (rightmost line) and after (progressive right to left) irradiation of GO with a 375-nm CW laser at 10, 25, 50, 75 and 100 pW,

Figure 9 shows normalized upconversion luminescence scan of optical patterns in a nanoparticle-GO/rGO nanocomposite displaying representations of (a) a leaf, (b) the Sydney Opera House, and (c) a kangaroo, the bright area being upconversion emission from nanoparticles integrated on GO areas and the dark areas corresponding to quenched emission corresponding to areas where GO was reduced to rGO. The patterns are 50 ^c 50 pixels, with a pixel spacing of 0.97 pm, exposure time was 100 ms, and 20 pm scale bar. Image (d) shows normalized line profile of the upconversion luminescence signal between the two black arrows imaging of the individual pixels (scale bar: 1 pm),

Figure 10 shows optical data storage (writing/reading) using the material described herein, in which optical data bits were written and read out by a) decrease of GO’s 650-nm fluorescence under 880-nm fs laser excitation, and b) quenching of the nanoparticles’ 450- nm upconversion fluorescence under 980-nm CW laser excitation. The insets display the formation of optical data bits (Scale bar: 500 nm),

Figure 11 shows optical data read out using the optical data storage material described herein based on a) decrease of 650-nm fluorescence from GO under 880-nm fs laser excitation and b) quenching of 450-nm upconversion fluorescence from the nanoparticles under 980-nm CW laser excitation,

Figure 12 shows fluorescence spectra of NaYF₄ nanoparticles containing 30 mol% Yb³⁺ and 4 mol% Tm³⁺ ions, showing highly efficient optical depletion of the nanoparticles upconversion fluorescence emission under a 980-nm CW excitation beam and a 808-nm CW depletion beam, together with negligible upconversion fluorescence emission under 808-nm CW excitation laser beam only,

Figure 13 shows confocal imaging of the 4% Tm-doped nanoparticles (a) under CW excitation laser at 980 nm and under dual-laser irradiation with the 808-nm CW depletion laser at (b) 1.5 and (c) 15 mW (scale bar: 500 nm), and (d) shows corresponding upconversion fluorescence intensity profile along the white dashed line for an individual nanoparticle, confirming the expected 50% and -90% upconversion fluorescence depletion,

Figure 14 shows power-dependent depletion efficiency of 450-nm upconversion fluorescence of NaYF₄ nanoparticles containing 30 mol% Yb³⁺ and 4 mol% Tm³⁺ using a CW laser at 980 nm with power of 0.5 mW for excitation and 808-nm CW laser for depletion at increasing power, confirming theoretical prediction for optical depletion efficiency of -90% and a value of saturation intensity I_sat of -375 kW cm^-2 (-1.5 mW),

Figure 15 shows power-dependent depletion efficiency of 450-nm upconversion fluorescence of NaYF₄ nanoparticles containing 30 mol% Yb³⁺ and 4 mol% Tm³⁺ in conjugation with graphene oxide using a CW laser at 980 nm with power of 0.5 mW for excitation and 808-nm CW laser for depletion at increasing power, confirming theoretical prediction for optical depletion efficiency of -95% and a value of saturation intensity Z_{sa t} of -250 kW cm^-2 (-1.0 mW),

Figure 16 shows inhibition of photo-chemical reduction of the GO in the optical data storage material described herein during optical writing process under combined irradiation of a 980- nm CW excitation beam and a 808-nm CW deactivation beam, Figure 17 shows the resolution improvement obtained by increasing the 808-nm doughnut shaped depletion laser intensity in a composite 980nm-808nm beam, for super-resolution imaging of NaYF₄ nanoparticles with 30 mol% Yb³⁺ and 4 mol% Tm³⁺ ions by STED microscopy, with insets showing STED microscopy imaging of individual nanoparticles at different depletion laser intensities (scale bar: 500 nm),

Figure 18 shows imaging of individual nanoparticles by (a) confocal microscopy and (b) STED microscopy and (c) the corresponding line profiles of the image, using a 980-nm CW excitation laser and 808-nm CW depletion laser at intensities of 0.13 and 11.25 MW cm^-2, respectively (Scale bar: 500 nm),

Figure 19 shows the resolution improvement obtained by increasing the 808-nm doughnut shaped depletion laser intensity in a composite 980nm-808nm beam, for super-resolution imaging of NaYF₄ nanoparticles with 30 mol% Yb³⁺ and 4 mol% Tm³⁺ ions deposited on GO by STED microscopy, with insets showing STED microscopy imaging of individual nanoparticles at different depletion laser intensities (scale bar: 500 nm),

Figure 20 shows (a)-(b) confocal and (c)-(d) corresponding STED microscopy imaging of individual nanoparticles integrated on GO before ((a) and (c)) and after ((b) and (d)) nanoscale optical data writing, and intensity profiles for both confocal and STED microscopy along the dotted lines (e) before and (f) after nanoscale optical data writing,

Figure 21 shows the optical depletion of NaYF₄ nanoparticles with 30 mol% Yb³⁺ and 4 mol% Tm³⁺ ions upconversion fluorescence under a composite 980-nm CW excitation beam and a l550-nm CW depletion beam, and

Figure 22 shows a simulation of feature size in the optical data storage material described herein containing NaYF₄ nanoparticles with 30 mol% Yb³⁺ and 4 mol% Tm³⁺ ions under a composite 980-nm CW (for photo-induction, Gaussian shape) and l550-nm CW (for photo inhibition, Laguerre-Gaussian shape) beam. DETAILED DESCRIPTION

The present disclosure relates to an optical data storage material. By the expression "optical data storage" is meant storage by any digital data storage method in which data is written and read with an optical means, such as a laser. The disclosure also relates to a medium for super-resolution optical recording and reading that includes the optical data storage material. The optical data storage material is suitable for effective super-resolution optical recording and reading at the nanoscale, and has the potential for application in optical data storage devices on a large scale.

The optical data storage material described herein comprises graphene oxide (GO). The expression "graphene oxide" used herein refers to a compound of carbon, oxygen and hydrogen obtained by oxidizing graphite. GO is an electrically insulating material composed of one or more graphene carbon sheet(s) with oxygen functional groups bonded perpendicular to the graphene basal-plane.

Thanks to its characteristic electrical and optical properties, the chemical structure of GO can be modified by, for example, chemical-, thermal-, photo-chemical-, and catalytic- reduction of the oxygen-containing groups. GO can be photo-chemically reduced on selected areas for optical data storage, and the energy boundary for triggering photo-chemical reduction of GO is estimated to be in the range from 3.06 eV to 3.4 eV. Accordingly, by irradiating GO with photons having higher energy than the energy boundary it is possible to induce dissociation of chemical bonds between oxygen function groups and the graphene basal-plane, resulting in removal of the oxygen function groups and recovery of the aromatic double-bonded carbon.

The product of the photo-chemical reduction is reduced graphene oxide (rGO), which shows similar properties and morphological characteristics to pristine graphene. Different to GO, however, rGO has significant reduced intrinsic fluorescence owing to the reduction of the concentration of oxygen-containing groups which are responsible for the emission of native GO. The change in fluorescence can therefore be used to generate high-contrast images for data reading.

The GO may be provided in any form, as long as it photo-chemically reduces to rGO in the irradiated regions. In some embodiments, the GO is provided in the form of multi-layer GO. In that form, the graphene basal-planes of the GO are stacked in a multi-sheet arrangement. In other embodiments, the GO is provided in the form of single layer (or monolayer) GO. In that arrangement, the graphene basal-planes of the GO are provided as single discrete sheets. When in single layer form, the GO can provide efficient heat dissipation, and is therefore less prone to photo-thermal reduction when irradiated.

The optical data storage material described herein comprises nanoparticles configured to photo-chemically reduce the GO by optical upconversion emission.

By being capable of optical "upconversion emission", the nanoparticles of the application are nanoparticles that absorb light at a first energy and a first wavelength and emit light at a second energy and a second wavelength wherein the second energy is higher than the first energy and the second wavelength is shorter than the first wavelength according to an Anti- Stokes Emission process.

In addition, by being "configured" to photo-chemically reduce the GO by optical upconversion emission, the nanoparticles of the application are characterised by emitting, through an upconversion emission mechanism, photons having energies of at least 3.06 eV to 3.4 eV, i.e. above the energy boundary for triggering photo -chemical reduction in GO as outlined above. Accordingly, the nanoparticles described herein may be capable of upconversion emission of photons having energies of at least about 3.2 eV, corresponding to fluorescence at wavelengths shorter than 390 nm.

Any nanoparticles that can fluoresce at wavelengths shorter than about 390 nm through an upconversion emission mechanism would be suitable for use in the optical data storage material described herein. In some embodiments, the nanoparticles comprise an inorganic host matrix selected from an oxide of a rare-earth element, an oxysulfide of a rare-earth element, an oxyhalide of a rare- earth element, a phosphate of a rare-earth element, a molybdate of a rare-earth element, a tungstate of a rare-earth element, a gallate of a rare-earth element, a vanadate of a rare-earth element, a fluoride of a rare-earth element, and a combination thereof.

In some embodiments, the nanoparticles comprise an inorganic host matrix selected from Y2O3, LU₂0₃, La₂0₃, Gd₂0₃, Y2O2S, Gd₂0₂S, La₂0₂S, GdOF, YOF, YAG, LaP0₄, LuP0₄, La₂(Mo0₄)₃, NaY(W0₄)₂, Gd₃Ga₅0i₂, YV0₄, LaF₃, YF₃, LuF₃, NaYF₄, LiYF₄, NaGdF₄, KY3F10, KGd2F₇, BaYFs, and a combination thereof. Examples of suitable materials for use as the nanoparticles host matrix are provided in Wang M, Abbineni G, Clevenger A, Mao C, Xu S. Upconversion Nanoparticles: Synthesis, Surface Modification, and Biological Applications . Nanomedicine : nanotechnology, biology, and medicine. 2011, Volume 7, Issue 6, pages 710-729, the content of which is incorporated herein in its entirety.

In some embodiments, the nanoparticles comprise at least an ion of a rare-earth element. Any ion of a rare-earth element may be suitable for use in the nanoparticle, provided the ion can produce upconversion emission with energy of at least about 3.2 eV. Examples of suitable ions include trivalent lanthanides ions such as Yb³⁺, Tm³⁺, Pr³⁺, Nd³⁺, Eu³⁺, Gd³⁺, _Tb ³+, _Dy ³+, _and _Er3+

In some embodiments, the nanoparticles comprise a host lattice which is based on cations with similar ionic radii to those of the rare-earth element ions. This may prevent the formation of defects and stress in the nanoparticles' crystalline structure.

In some embodiments, the nanoparticles have a NaYF₄ host matrix and contain Yb³⁺ and Tm³⁺ ions.

The nanoparticles may contain any suitable amount of the at least one rare-element ion, provided the nanoparticles can fluoresce at wavelengths shorter than 390 nm through an upconversion emission mechanism.

In some embodiments, the nanoparticles contain at least an ion of a rare-earth element in an amount from about 0.1 mol% to about 50 mol%, from about 0.1 mol% to about 40 mol%, from about 0.1 mol% to about 30 mol%, from about 0.5 mol% to about 30 mol%, from about 1 mol% to about 30 mol%, from about 5 mol% to about 30 mol%, or from about 5 mol% to about 15 mol%. For example, the nanoparticles may contain at least an ion of a rare-earth element in an amount of about 0.1 mol%, about 0.5 mol%, about 1 mol%, about 2 mol%, about 3 mol%, about 4 mol%, about 5 mol%, about 10 mol%, about 15 mol%, about 20 mol%, about 30 mol%, about 40 mol%, or about 50 mol%.

In some embodiments, the nanoparticles contain between about 10 mol% and 50 mol% Yb³⁺ and between about 0.2 mol% to about 20 mol% Tm³⁺.

For example, the nanoparticles may have a NaYF₄ host matrix containing about 30 mol% Yb³⁺ and about 4 mol% Tm³⁺. The nanoparticles of these embodiments may allow for high efficient GO reduction at low radiation energy, making optical data storage inexpensive and suitable for large scale use.

The nanoparticles may be of any suitable size, provided the nanoparticles can fluoresce at wavelengths shorter than about 390 nm through an upconversion emission mechanism. The dimension of the nanoparticles is a parameter that can be tuned to control the energy transfer between the nanoparticles and the GO, thus allowing for efficient photo-chemical reduction of GO. In some embodiments, the largest dimension of the nanoparticles is from about 1 nm to about 500 nm, from about 1 nm to about 250 nm, from about 1 nm to about 200 nm, from about 1 nm to about 150 nm, from about 1 nm to about 100 nm, from about 1 nm to about 50 nm, from about 1 nm to about 25 nm, and from about 1 nm to about 10 nm. In some embodiments, the largest dimension of the nanoparticles is from about 5 nm to about 50 nm.

In some embodiments, the nanoparticles are core-shell nanoparticles. By being a“core- shell” nanoparticle, the particle comprises an inner core that is either wholly covered or otherwise surrounded by an outer shell layer. As a result of the core being either wholly covered or otherwise surrounded by the outer shell layer, the upconversion luminescence of the nanoparticles is more intense relative to the nanoparticle absent the shell. In other words, the core-shell nanoparticles are brighter than their corresponding core nanoparticles absent the shell layer. Without wishing to be limited by theory, the increase of luminescence intensity conferred by the shell results from electronic surface passivation of the core provided by the shell layer.

The inner core of the core-shell nanoparticles may be made of any material capable to promote upconversion luminescence. For example, the inner core material may comprise an inorganic host matrix of the kind described herein. The inner core may also comprise an ion of a rare-earth element of the kind described herein.

The outer shell of the core- shell nanoparticles may be made of any material that confers the nanoparticles with more intense upconversion luminescence relative to the particle absent the shell. A skilled person would be capable to select an appropriate shell material based on the electronic characteristics of the inner core material.

Examples of suitable shell materials include Y2O3, Lu₂0₃, La₂0₃, Gd₂0₃, Y₂0₂S, Gd₂0₂S, La₂0₂S, GdOF, YOF, YAG, LaP0₄, LuP0₄, La₂(Mo0₄)₃, NaY(W0₄)₂, Gd₃Ga₅0i₂, YV0₄, LaF₃, YF₃, LUF₃, NaYF₄, LiYF₄, NaGdF₄, KY₃FI_O, KGd₂F₇, BaYFs, and a combination thereof. In some embodiments, the shell may comprise the same inorganic host matrix of the inner core.

In some embodiments, the nanoparticles are NaGdF₄:Yb/Tm@NaGdF₄(“core@shell”) core shell nanoparticles.

In some embodiments, the nanoparticles may be selected from the group consisting of NaLnF₄, (in which Ln is selected from Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Lu, Pr, Yb, Lu, and Gd), Ce0₂, MF₂ (in which M is selected from Mg, Ca, and Sr), LnF₃ (in which Ln is selected from Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Lu, and Er), LnOF (in which Ln is selected from Er, Tm, Yb, Lu, Tb, Dy, Ho, Y, Ce, Pr, Nd, and Sm), NaScF₄, LaOF, FnOCl, EuOF, Fn₂0₃ (in which Fn is selected from Fa, Pr, Nd, Sm, Eu, Y, Tb, Dy, Ho, Er, Tm, Yb, and Fu), FnP0₄ (in which Fn is selected from Fn, Eu, Tb, Y, and Ho), Eu₂0₂S, NaFnS₂ (in which Fn is selected from Fa, Sm, Tb, and Ce), KMnF₃, NaMnF₃, Er₂0₃, SrF₂, CaF₂, GdF₃, CeF₃, NaCeF₄, Na₃MF₇ (in which M is selected from Zr, and Hf), BaY₂F₈, YbP0₄, KYF₄, NaFuF₄, Y₃A1_SO _{I 2,} Gd₃GaOi₂, ZnO, CaS, Zr0₂, BaTi0₃, Ti0₂, NH₄Fu₂F₇, InGaP, InP, M₂0₃ (in which M is selected from Pr, Nd, Sm, Eu, Gd, Tb, and Dy), X0₂ (in which X is selected from Ce, Sn, and Zr), SrTi0₃, Sr₂Ti0₄, Ba₂Ti0₄, XZr0₃ (in which X is selected from Sr, Ba, and Pb), Si0₂, Ga₂0_3, phosphate glass, Y₂Ti₂0₇, fluoride glass, chalcogenide glass ~ P-NaGdF₄, YCl₃, FaCl₃, FaBr₃, and a combination thereof.

It is believed that the GO/nanoparticles composite described herein may be unique in its own right. Accordingly, the present invention also provides, in some aspects, an optical data storage material comprising graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage, and nanoparticles configured to photo- chemically reduce the GO on the selected areas by optical upconversion emission. Accordingly, in some aspects there is also provided a method of manufacturing an optical data storage material, the method comprising the steps of providing graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage, and providing nanoparticles on the GO, the nanoparticles being configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission. In additional aspects, the invention also provides a method of recording optically readable data, the method comprising optically upconverting an input beam using nanoparticles to form an upconverted beam, and photo-chemically reducing GO in selected areas by the upconverted beam.

The optical data storage material described herein also comprises a support material that embeds the GO and the nanoparticles. Stating that the support material "embeds" the GO and the nanoparticles means that the GO and the nanoparticles are surrounded at least partially by the support material. In some embodiments, the support material embeds the GO and the nanoparticles such that the GO and the nanoparticles are located within the volume of the support material to be entirely enclosed within the support material.

The support material comprises a thermal conductor. By the support material "comprising" a thermal conductor is meant that at least a portion of the support material is made of a thermal conductor as described herein. The remainder of the support material, if any, may be made of any material that is (i) compatible with the thermal conductor, the nanoparticles, and the GO, and (ii) suitable for use in optical data storage systems. The portion of support material which is not the thermal conductor may be a polymer that is transparent to the irradiation beam used for data storage. For example, the portion of support material which is not the thermal conductor may be polycarbonate or an acrylic polymer.

The expression "thermal conductor" is used herein to mean a thermally conductive material that provides thermal communication between the GO and the environment outside the material. In the optical data storage material described herein, the thermal conductor is in thermal contact with the GO. By the thermal conductor being in“thermal contact” with the GO means that conductive heat transfer can occur between the thermal conductor and the GO.

By having a thermal conductor in thermal contact with the GO, the material described herein may provide mitigation of photo-thermal reduction effects of irradiated areas of the GO which may be induced by an incident input beam during optical data writing. When an irradiation beam for optical data storage such as a laser beam is incident on the GO, the irradiated area absorbs part of the beam energy and converts it rapidly into local heat. The intense heating raises the temperature of the irradiated area and may result in the chemical degradation of the GO by, inter alia, localised photo-thermal reduction. A main drawback of photo-thermal reduction is that it affects an area that is larger than the irradiated area due to thermal diffusion of heat through the GO, thereby drastically reducing writing resolution. Accordingly, by providing thermal communication between the GO and the external environment, the thermal conductor can effectively mitigate photo-thermal reduction of irradiated areas. Any thermal conductor may be suitable for use in the optical data storage material described herein, provided it (i) effectively mitigates photo-thermal reduction of the GO, and (ii) allows optical transmission at writing and reading wavelengths.

In some embodiments, the thermal conductor is an inorganic thermal conductor selected from silica, alumina, quartz, zirconia, yttrium aluminium garnet (YAG), hafnia, and a combination thereof. The specific combination of an inorganic thermal conductor and the nanoparticles of the kind described herein may be particularly advantageous in that it provides for chemically stable and inert optical data storage materials. This provides the materials of the present application with significantly longer lifetime relative to conventional ones, in terms of both shelf-life and ability to preserve optically written data over long periods of time.

In some embodiments, the thermal conductor is the support material. That is, the support material is entirely made of the thermal conductor such that the GO and the nanoparticles are entirely embedded within the thermal conductor. Accordingly, there is also provided an optical data storage material comprising (a) graphene oxide (GO) configured to be photo- chemically reduced on selected areas for optical data storage, (b) nanoparticles configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission, and (c) a thermal conductor that embeds the GO and the nanoparticles to mitigate photo - thermal reduction of the selected areas. In these embodiments the thermal conductor is in thermal contact with the GO by surrounding the GO in its entirety.

There is also provided a method of manufacture an optical data storage material comprising, inter alia, the provision of graphene oxide (GO) configured to be photo-chemically reduced on selected areas (or on selected regions) for optical data storage. The GO may be provided by any means known to the skilled person. For example, GO may be synthesised by Hummers’ method, or one of its modifications, as described in Hummers Jr, W. S. & Offeman, R. E. "Preparation of graphitic oxide" J. Am. Chem. Soc. 80, 1339-1339 (1958), the content of which is incorporated herein in its entirety. The method is based on the oxidation of graphite by concentrated acids in the presence of strong oxidants, and subsequent exfoliation under sonication in aqueous solution. The GO may be provided, for example, in the form of a suspension in water.

The method of manufacturing also comprises providing nanoparticles of the kind described herein on the GO. This may be achieved by any means known to the skilled person. For example, the nanoparticles described herein may be positively charged on their surface due to the present of cations, while the GO can have negatively charged domains deriving from the oxygen-containing groups. This allows for fast electrostatic attraction between the nanoparticles and the GO.

In some embodiments, the nanoparticles are provided on the GO by a one-step procedure. For example, the nanoparticles and the GO may be dispersed in the same dispersion medium to facilitate nanoparticle adsorption on the GO. The electrostatic affinity between the nanoparticles and the GO ensures spontaneous adsorption of the nanoparticles on the GO. Examples of suitable dispersion media for use in these embodiments include, for example, water, ethanol, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and acetonitrile.

In some embodiments, the nanoparticles are provided on the GO by a two-step procedure. In this case, the GO is first deposited on the surface of a substrate. This may be achieved, for example, by drop-casting a liquid (e.g. water) suspension of GO on a surface of the substrate and removing the excess dispersion medium. The substrate may be any substrate suitable for use in optical data storage. In embodiments, the substrate is a thermal conductor as described herein, and the liquid suspension of GO is therefore drop-casted on the thermal conductor substrate. Subsequently, a dispersion of the nanoparticles may be drop-casted on the previously deposited GO. The electrostatic affinity between the nanoparticles and the GO ensures spontaneous adsorption of the nanoparticles on the GO.

The two-step procedure may ensure a high degree of control over the morphology of the resulting nanoparticle-GO assembly. For example, by controlling the concentration of the nanoparticles in the drop-cast dispersion and the deposition time, the density of the nanoparticles on the GO can be tuned precisely. In general, the density of the nanoparticle layer increases with increasing concentration and deposition time. This may allow for the deposition of single layers of nanoparticles on the GO, which is beneficial for increased resolution and efficiency of data writing.

Any concentration of GO in the drop-casting solution may be suitable for use in the procedure described herein, provided the concentration is effective in achieving deposition of GO on a substrate. In embodiments, the concentration of the GO in the dispersion medium is between about 1 pg/ml and about 500 pg/ml, between about 1 pg/ml and about 250 pg/ml, between about 1 pg/ml and about 100 pg/ml, between about 1 pg/ml and about 75 pg/ml, between about 1 pg/ml and about 50 pg/ml, between about 1 pg/ml and about 25 pg/ml, or between about 1 pg/ml and about 10 pg/ml.

Any amount of nanoparticles may be used in the drop-cast solution for deposition of the nanoparticles on the GO, provided the amount is effective in achieving deposition of the nanoparticles on the GO. In some embodiments, the concentration of the nanoparticles in the dispersion medium is from about 1 nM to about 1M, from about 1 nM to about 0.1 M, from about 1 nM to about 0.01 M, from about 1 nM to about 1 pM, from about 1 nM to about 0.1 pM, or from about 1 nM to about 50 nM.

Any deposition time may be allowed for nanoparticle adsorption on the GO, provided the deposition is effective in achieving adsorption. In some embodiments, the deposition time is between about 1 second to about 1 hour, between about 1 second to about 45 minutes, between about 1 second and 30 minutes, between about 30 seconds and 30 minutes, between about 5 minutes to about 30 minutes, or between about 15 minutes and 30 minutes.

The method of manufacturing further comprises the step of providing a support material that (i) embeds the nanoparticles and the GO and (ii) comprises a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas. The provision of the support material may be achieved by any procedure known to the skilled person, provided (i) and (ii) are satisfied. In the context of the two-step procedure outlined above, for example, the support material may be obtained by performing a further step of coating the GO and the nanoparticles provided on the substrate with a layer of the thermal conductor. The support material would be in this case the combination of the substrate and the thermal conductor.

In some embodiments, the thermal conductor is provided by promoting hydrolysis and condensation of metal alkoxides in the presence of the nanoparticles and the GO, such that the resulting support material embeds the nanoparticles and the GO.

Any metal alkoxides may be used to form the thermal conductor, provided the resulting thermal conductor is in thermal contact with the GO. In some embodiments, the metal alkoxides are selected from alkoxides of silicon, titanium, aluminium, zirconium, hafnium, and a combination thereof.

In some embodiments, functional metal alkoxides are used in combination with or instead of the metal alkoxides. Functional metal alkoxides in the context of the application comprise at least one functional moiety F. By F being a“functional” moiety is meant that F contains at least one element other than carbon and hydrogen. For example, F may contain an element selected from oxygen, nitrogen, sulphur, bromine, chlorine, or fluorine.

F may be any functional moiety. In some embodiments, F is selected from a hydroxyl functionality, an amino functionality, a thiol functionality, a phosphate functionality, an epoxy functionality, an alkyl halide functionality, an isocyanate functionality, a hydrazide functionality, a semicarbazide functionality, an azide functionality, an ester functionality, a carboxylic acid functionality, an aldehyde functionality, a ketone functionality, or a disulfide functionality.

In some embodiments, the metal alkoxides comprise alkoxysilanes and/or functional alkoxysilanes. The use of alkoxysilanes and/or functional alkoxysilanes results in the formation of a silica-based thermal conductor. The term“alkoxysilane” used in isolation means compounds that contain one to four organic groups covalently bonded to a silicon atom through an oxygen atom, as opposed to being covalently bonded directly to the silicon atom. For the optical storage material disclosed herein, the alkoxysilanes may be selected from (1) tetraalkoxysilanes, (2) trialkoxy silanes, (3) dialkoxysilanes, (4) monoalkoxysilanes, (5) trialkoxysilanes, or a combination thereof, respectively represented by the following formulae (1), (2), (3), (4) and (5):

Si(OR¹)₄ (1),

R²R³R⁴Si(OR¹) (4), and

(R¹0)₃Si— R⁵— SiCOR¹^ (5), in which each of R¹, R², R³ and R⁴ independently represents an organic group R of the kind described herein, and R⁵ represents a divalent hydrocarbon group having 1 to 20 carbon atoms. In some embodiments, R¹, R², R³, and R⁴ are the same organic group.

Example of specific alkoxysilanes compounds of this type include methyltriethoxysilane (MTES), phenyltriethoxysilane (PTES), diethyldiethoxysilane, methyltrimethoxysilane (MTMS), dimethyldimethoxysilane, phenyltrimethoxysilane (PTMS), vinyltrimethoxysilane (VTMS), vinylriethoxysilane (VTES), tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS), tetrabutoxysilane (TBOS), and a combination thereof.

By the expression“functional alkoxysilanes” is meant compounds that contain one to three organic groups covalently bonded to a silicon atom through an oxygen atom, and at least one to three groups of formula -R-F covalently bonded directly to the silicon atom, as appropriate such that the silicon atom is tetra-coordinated.

In some embodiments, the functional alkoxysilanes are selected from functional trialkoxysilanes, functional dialkoxysilanes, functional monoalkoxysilanes, and a combination thereof, respectively represented by the following formulae (6)-(l l):

(^OfcSi^-F) (6),

(R¹0)Si(R²)(R³F)(R⁴F’) (10), and

(R¹0)Si(R²F)(R³F’)(R⁴F”) (11). in which each of R¹, R², R³ and R⁴ independently represents an R group of the kind described herein, and each of F, F’, and F” is independently selected from a hydroxyl functionality, an amino functionality, a thiol functionality, a phosphate functionality, an epoxy functionality, an alkyl halide functionality, an isocyanate functionality, a hydrazide functionality, a semicarbazide functionality, an azide functionality, an ester functionality, a carboxylic acid functionality, an aldehyde functionality, a ketone functionality, a vinyl functionality, and a disulfide functionality. In some embodiments, R¹, R², R³ and R⁴ are the same organic group. In some embodiments, F, F’, and F” are the same functional moiety.

Examples of specific functional alkoxysilanes compounds suitable for use in the optical storage material described herein include: 3-aminopropyl triethoxysilane, 3-aminopropyl trimethoxy silane, 3-isocyanatopropyl triethoxysilane, 3-isocyanatopropyl trimethoxy silane, 3-azidopropyl triethoxysilane, 3-azidopropyl trimethoxy silane, 3-thiolpropyl trimethoxy silane (or 3-mercaptopropyl trimethoxy silane or trimethoxy silyl propanethiol), 3- thiolpropyl triethoxysilane (or 3-mercaptopropyl triethoxysilane or triethoxysilyl propanethiol), 3-cyanopropyl trimethoxysilane, 3-cyanopropyl triethoxysilane, N-(2- aminoethyl)-3-aminopropyl trimethoxysilane, N-(2-aminoethyl)-3-aminopropyl triethoxysilane, (aminoethylaminomethyl) phenethyl trimethoxysilane, (3- acetamidopropyl) trimethoxysilane, acetoxyethyl trimethoxysilane, 3-acrylamidopropyl trimethoxysilane, acryloxymethyl trimethoxysilane 3-bromopropyl trimethoxysilane, 3- chloropropyl trimethoxysilane, (heptadecafluoro-l,l,2,2-tetrahydrodecyl) trimethoxysilane, (heptadecafluoro- 1 , 1 ,2,2-tetrahydrodecyl) triethoxysilane, 2-[methoxy(polyethyleneoxy)_2i- ₂₄propyl]trimethoxy silane, and a combination thereof.

The metal alkoxides and/or functional metal alkoxides can be used in solution form at any concentration that is suitable to ensure that their hydrolysis and condensation provide the thermal conductor as intended. In some embodiments, the metal alkoxides and/or functional metal alkoxides are mixed with the nanoparticles and the GO in a concentration between about 50 vol% and about 99 vol%, between about 50 vol% and about 90 vol%, between about 50 vol% and about 75 vol%, or between about 50 vol% and about 60 vol%.

Condensation of hydrolised metal alkoxides and/or functional metal alkoxides can be promoted at any temperature suitable to obtain a thermal conductor that inhibits the thermal reduction of the GO under irradiation. In some embodiments, condensation of the metal alkoxides and/or functional metal alkoxides is obtained by heating at a temperature of between about 300K and about 700K, between about 300K and about 600K, between about 300K and about 500K, or between about 300K and about 400K. For example, condensation of the metal alkoxides and/or functional metal alkoxides is obtained by heating at a temperature of 313K.

By R being an“organic group” is meant that R includes at least one carbon atom. R may therefore be an alkyl group, an alkenyl group, an aryl group, or a carbocyclyl group.

As used herein, the term“alkyl”, used either alone or in compound words, describes a group composed of at least one carbon and hydrogen atom, and denotes straight chain, branched or cyclic alkyl, for example C1-20 alkyl, e.g. C1-10 or C1-6. Examples of straight chain and branched alkyl include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec -butyl, t-butyl, n- pentyl, l,2-dimethylpropyl, 1,1 -dimethyl -propyl, hexyl, 4-methylpentyl, l-methylpentyl, 2- methylpentyl, 3-methylpentyl, l,l-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,2- dimethylbutyl, l,3-dimethylbutyl, l,2,2-trimethylpropyl, l,l,2-trimethylpropyl, heptyl, 5- methylhexyl, l-methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, l,2-dimethylpentyl, l,3-dimethylpentyl, 1, 4-dimethyl pentyl, l,2,3-trimethylbutyl, 1,1,2- trimethylbutyl, l,l,3-trimethylbutyl, octyl, 6-methylheptyl, l-methylheptyl, 1,1, 3, 3- tetramethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-methyloctyl, 1-, 2-, 3-, 4- or 5-ethylheptyl,

1-, 2- or 3-propylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8-methylnonyl, 1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9- methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4- or 5-propyloctyl, 1-, 2- or 3- butylheptyl, l-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9- or lO-methylundecyl, 1-,

2-, 3-, 4-, 5-, 6-, 7- or 8-ethyldecy 1, 1-, 2-, 3-, 4-, 5- or 6-propylnonyl, 1-, 2-, 3- or 4- butyloctyl, l-2-pentylheptyl and the like. Examples of cyclic alkyl include mono- or polycyclic alkyl groups such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl and the like. Where an alkyl group is referred to generally as‘propyl’, butyl’ etc., it will be understood that this can refer to any of straight, branched and cyclic isomers where appropriate. In some embodiments, R is a linear alkyl group having from 1 to 20 carbon atoms, such as methyl, ethyl, propyl, butyl, hexyl, heptyl, octyl, and the like.

The term“alkenyl” as used herein denotes groups formed from straight chain, branched or cyclic hydrocarbon residues containing at least one carbon to carbon double bond including ethylenically mono-, di- or polyunsaturated alkyl or cycloalkyl groups as previously defined, including C2-20 alkenyl (e.g. C2-10 or C2-6). Examples of alkenyl include vinyl, allyl, 1- methylvinyl, butenyl, iso-butenyl, 3-methyl-2-butenyl, l-pentenyl, cyclopentenyl, l-methyl- cyclopentenyl, l-hexenyl, 3-hexenyl, cyclohexenyl, l-heptenyl, 3-heptenyl, l-octenyl, cyclooctenyl, l-nonenyl, 2-nonenyl, 3-nonenyl, l-decenyl, 3-decenyl, l,3-butadienyl, 1,4- pentadienyl, l,3-cyclopentadienyl, l,3-hexadienyl, l,4-hexadienyl, l,3-cyclohexadienyl, l,4-cyclohexadienyl, l,3-cycloheptadienyl, l,3,5-cycloheptatrienyl and 1, 3,5,7- cyclooctatetraenyl.

The term“aryl” (or‘carboaryl’) denotes any of single, polynuclear, conjugated and fused residues of aromatic hydrocarbon ring systems (e.g. C₆-24 or Ce-is). Examples of aryl include phenyl, biphenyl, terphenyl, quaterphenyl, naphthyl, tetrahydronaphthyl, anthracenyl, dihydroanthracenyl, benzanthracenyl, dibenzanthracenyl, phenanthrenyl, fluorenyl, pyrenyl, idenyl, azulenyl, chrysenyl. The aryl include phenyl and naphthyl. The term“carbocyclyl” includes any of non-aromatic monocyclic, polycyclic, fused or conjugated hydrocarbon residues, inclding C3-20 (e.g. C3-10 or C3-8). The rings may be saturated, e.g. cycloalkyl, or may possess one or more double bonds (cycloalkenyl) and/or one or more triple bonds (cycloalkynyl). Particular carbocyclyl moieties are 5-6-membered or 9-10 membered ring systems. Suitable examples include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, cyclopentenyl, cyclohexenyl, cyclooctenyl, cyclopentadienyl, cyclohexadienyl, cyclooctatetraenyl, indanyl, decalinyl and indenyl. A carbocyclyl group may be optionally substituted by one or more optional substituents as herein defined. The term‘carbocyclylene’ is intended to denote the divalent form of carbocyclyl.

In some embodiments, the thermal conductor is provided to entirely embed the GO and the nanoparticles. That is, in these embodiments the thermal conductor corresponds to the support material.

In the context of those embodiments where the thermal conductor corresponds to the support material, the provision of a thermal conductor may be achieved by any procedure known to the skilled person, provided the resulting thermal conductor entirely embeds the GO and the nanoparticles.

In some embodiments, when the thermal conductor is provided by promoting hydrolysis and condensation reactions of metal alkoxides and/or functional metal alkoxides of the kind described herein, the provision of the thermal conductor to entirely embed the GO and the nanoparticles may be achieved, for example, through a single or multi-step procedure as described below.

Single-step procedure

In this procedure the nanoparticles are first adsorbed on the GO in a liquid suspension according to a procedure described herein. Subsequently, one or more metal alkoxides and/or functional metal alkoxides of the kind described herein are added to the suspension of GO with adsorbed nanoparticles. Optionally, the one or more metal alkoxides and/or functional metal alkoxides may be pre-hydrolised or partially pre-hydrolised prior to being added to the suspension of GO with adsorbed nanoparticles. The resulting solution is then deposited on a suitable substrate by means known to a skilled person (e.g. drop-cast, spin-coating, dip coating). Optionally, before depositing the resulting solution on the substrate, the substrate itself may be pre-coated with metal alkoxides and/or functional metal alkoxides of the kind described herein, which may optionally be pre-hydrolised or at least partially hydrolysed before being deposited on the substrate. The solution of alkoxides, nanoparticles and GO is then deposited on such a pre-coated substrate. Hydrolysis (if needed) and condensation reactions of all metal alkoxides and/or functional metal alkoxides are subsequently promoted, e.g. by heating, to induce formation and consolidation of the thermal conductor embedding the GO with adsorbed nanoparticles.

Multi-step procedure

First, metal alkoxides are deposited on a substrate of the kind described herein. In this step the metal alkoxides may be provided in solution with a suitable solvent. A layer of the solution may be deposited on the substrate by means known to a skilled person, for example by dip-coating or spin-coating. The deposited metal alkoxides may then be hydrolysed and at least partially condensed to promote at least partial formation of the thermal conductor. In an alternative procedure step, the metal alkoxides are first hydrolysed or partially hydrolysed in a separate medium before being deposited on the substrate, in which case the metal alkoxides are deposited on the substrate in a hydrolysed or partially hydrolysed state.

Second, the GO and the nanoparticles are subsequently provided on the at least partially condensed metal alkoxides by procedures of the kind described herein (e.g. one- or two-step drop-cast, spin-coating, dip-coating, etc.).

Third, additional metal alkoxides and/or functional metal alkoxides are deposited on the GO with adsorbed nanoparticles to sandwich them against the initial layer of metal alkoxides and/or functional metal alkoxides. In an alternative procedure step, the additional metal alkoxides and/or functional metal alkoxides may be first hydrolysed or partially hydrolysed in a separate medium before being deposited on the GO with adsorbed nanoparticles.

Fourth, hydrolysis and condensation reactions of all metal alkoxides and/or functional metal alkoxides are subsequently promoted, e.g. by heating, to induce formation and consolidation of the thermal conductor embedding the GO with adsorbed nanoparticles.

In some embodiments, the metal alkoxide and/or functional metal-alkoxide are used together with at least one surfactant additive. This may be selected from, for example, known surfactants such as a glycol surfactant (e.g. poly(ethylene glycol) -block-poly(propylene glycol) -block-poly (ethylene glycol) (Pluronic P123), or Poly(Ethylene Glycol) (PEG)), a pyrrolidone surfactant (e.g. Polyvinylpyrrolidone (PVP)), polyethyleneoxide (PEO), or a combination thereof.

Providing the thermal conductor by hydrolysis and condensation reaction of metal alkoxides and/or functional metal alkoxides advantageously allows the sintering-free incorporation of the nanoparticles conjugated with the GO into the thermal conductor without significantly altering their chemical and physical properties. Also, and without wanting to be bound by theory, the incorporation of the nanoparticles conjugated with the GO into the thermal conductor increases the local Young’s modulus and the thermal conductivity around the nanoparticles conjugated with the GO to either improve their lifespan (e.g. by removing the unwanted degradation susceptible to environmental thermal perturbations) and to prevent thermal-reduction of the GO under irradiation.

Disclosed herein is also a method of recording optically readable data, the method comprising (a) optically upconverting an input beam using nanoparticles to form an upconverted beam, (b) photo-chemically reducing GO in selected areas by the upconverted beam, and (c) mitigating photo-thermal reduction of the selected areas by having the nanoparticles and GO embedded in a support material that comprises a thermal conductor in thermal contact with the GO. In this method, the nanoparticles may be nanoparticles of the kind described herein. Optical data recording (writing) is based on the photo-chemical reduction of GO to rGO induced by the nanoparticles' resonance energy transfer (RET). Optical read out of the recorded data is based either on GO’s fluorescence decrease in correspondence of the rGO, and/or the nanoparticles' up-conversion fluorescence quenching.

For optical data recording, selected areas of the GO and nanoparticles are illuminated by an input beam, which wavelength is sufficient to excite the nanoparticles to produce upconversion to high-energy state triggering the photo-chemical reduction of GO to rGO through an energy transfer process.

The wavelength of the input beam may be any wavelength that achieves upconversion emission of the nanoparticles to produce photo-chemical reduction of GO to rGO. In some embodiments, the input beam is laser beam having a wavelength between about 700 nm and about 1800 nm, between about 700 nm and about 1600 nm, between about 700 nm and about 1400 nm, between about 700 nm and about 1200 nm, between about 700 nm and about 1000 nm, between about 800 nm and about 1000 nm, or between about 900 nm and about 1000 nm. In some embodiments, the input beam is laser beam having a wavelength of between about 930 nm to about 1030 nm, about 750 nm to about 850 nm, about 900 nm to 950 nm, or about 1500 nm to about 1600 nm. Suitable wavelengths include the central values of those ranges indicated above. For example, the input beam may be laser beam having a wavelength of about 808 nm, about 925 nm, about 980 nm, or about 1550 nm.

The power of the input beam may be any power that achieves upconversion emission of the nanoparticles to produce photo-chemical reduction of GO to rGO. In some embodiments, the input beam is laser beam having a power from about 1 pW to about 1 W, from about 10 pW to about 0.5 W, from about 10 pW to about 0.1 W, from about 1 mW to about 50 mW, from about 1 mW to about 25 mW, from about 1 mW to about 10 mW, or from about 1 mW to about 5 mW. In some embodiments, the input beam is a laser beam having power selected from about 1 mW, about 5 mW, about 10 mW, and about 25 mW. The recorded data can be optically read based on, for example, the decrease of fluorescence intensity from rGO relative to GO, and/or the quenching of the upconversion fluorescence from the nanoparticles.

In some embodiments, the input beam results from the combination of an excitation beam and a deactivation beam, in which (i) the excitation beam having a wavelength of X_e induces up-conversion emission of the nanoparticles, thereby promoting, in a central portion of the irradiated region, photo-chemical reduction of the graphene oxide (GO), and (ii) the deactivation beam having a wavelength of Xa inhibits the photo-chemical reduction of the graphene oxide in a region surrounding the central portion of the irradiated region, resulting in recorded optically readable data in the central portion of the irradiated region.

X_e and Xa may be any wavelength, provided the excitation beam induces upconversion emission of the nanoparticles and the deactivation beam inhibits the photo-chemical reduction of the graphene oxide in a region surrounding the central portion of the irradiated region.

In some embodiments, X_e is between about 700 nm and about 1800 nm, between about 700 nm and about 1600 nm, between about 700 nm and about 1400 nm, between about 700 nm and about 1200 nm, between about 700 nm and about 1000 nm, between about 800 nm and about 1000 nm, or between about 900 nm and about 1000 nm. In some embodiments, X_e is from about 930 nm to about 1030 nm, about 750 nm to about 850 nm, about 900 nm to 950 nm, or about 1500 nm to about 1600 nm. For example, X_e may be about 808 nm, about 925 nm, about 980 nm, or about 1550 nm. The skilled person would be capable to identify suitable values of X_e depending on the electronic characteristics of the nanoparticles in order to induce upconversion emission. In some embodiments X_e is about 980 nm.

In some embodiments, Xa is between about 700 nm and about 1800 nm, between about 700 nm and about 1600 nm, between about 700 nm and about 1400 nm, between about 700 nm and about 1200 nm, between about 700 nm and about 1000 nm, between about 800 nm and about 1000 nm, or between about 900 nm and about 1000 nm. For example Xa may be between about 700 nm and about 850 nm, or between about 1500 nm and about 1600 nm. The skilled person would be capable to identify suitable values of X_d for inhibiting the photo chemical reduction of the graphene oxide in a region surrounding the central portion of the irradiated region. In some embodiments X_d is about 808 nm or about 1550 nm.

A schematic of an optical layout suitable for performing the method of recording optically readable data of the application is shown in Figure 6, in which M indicates a mirror, FM indicates a flipping mirror, L indicates a lens, DC indicates a dichroic mirror, HWP indicates a half-wave plate, QWP indicates a quarter-wave plate; VPP indicates a vortex phase plate, and SPAD indicates a single -photon avalanche diode.

A single-mode fibre-coupled A nm diode laser can be used as the excitation source. After collimation, the excitation beam is transmitted through a long-pass dichroic mirror, then reflected by a second short-pass dichroic mirror and focused through an oil-immersion objective (e.g. UPlanAPO, Olympus; lOOx, NA = 1.4) onto the sample slide. The first dichroic mirror also allows the X_d nm probing beam to merge with the A nm beam.

The luminescence signal from the sample is collected by the same objective, split from the excitation and probing beams by the second dichroic mirror before being coupled into a multi-mode fibre. The other end of the fibre is connected to a single-photon avalanche diode (SPAD, e.g. SPCM-AQRH-13-FC, PerkinElmer) capable of being time-gated electronically.

To select up-conversion emission bands, different band-pass filters are inserted in the detection path for both transient response measurement and confocal imaging. A flipping mirror is also inserted and the signal coupled with a spectrometer (e.g. by Andor) for measuring up-conversion emission spectra.

For acquiring optical super-resolution images, a quarter-wave plate is used to first transform the X_d nm beam from linear polarization to circular polarization. In practice, a half-wave plate is also used to facilitate the adjustment towards quality circular polarization. Then, a vortex phase plate (e.g. VPP- la, RPC Photonics) is inserted in the X_d nm beam path so that a doughnut- shaped point spread function (PSF) is generated at the focal plane. In both cases of dual-laser confocal and super-resolution imaging, the two X_e and Xa nm beams are carefully aligned to ensure precise overlapping of their PSFs in both X-Y and Z directions.

With a projected maximal single-disc capacity approaching 700 TB, harnessing individual nanoparticles as information storing units in the nanoparticles-GO system using far-field techniques may be a paradigm shift compared with conventional optical data storage and offers a pathway to immediately boost the capacity of standard optical memories from current Gigabytes towards Petabytes. Further, the use of nanoparticles with reduced transition rates in sub-diffraction limit applications allows a 1, 000-fold decrease of the laser intensities from up to hundreds of MW cm ² required by other fluorescent nanomaterials down to hundreds of kW cm ² and therefore drastically reduce the energy per recorded data bit from picojoule to femtojoule level.

EXAMPLES

EXAMPLE 1

Synthesis of Yb, Tm-codoped NaYF4 upconversion nanoparticles

NaYF₄ containing Yb³⁺ and Tm³⁺ ions were synthesised as nanoparticles displaying upconversion emission. The NaYF₄ matrix guarantees high upconversion efficiency of the rare-earth ions and their chemical stability.

NaYF₄ nanoparticles were prepared by co -precipitation of the lanthanide fluorides with long- chain hydrocarbons (e.g. l-octadecene) and unsaturated fatty acids, such as oleic acid. The unsaturated fatty acid is used as the surface ligand to control particle growth and subsequently stabilize the resultant nanoparticles against agglomeration. The reagents of the synthesis of Yb,Tm-doped nanoparticles include ytterbium(III) acetate hydrate (99.9%), yttrium(III) acetate tetrahydrate (99.9%), thulium(III) acetate hydrate (99.9%), sodium hydroxide (NaOH, > 98%), ammonium fluoride (NH4F, > 98%), l-octadecene (90%), oleic acid (OA) (90%). The reagents were purchased from Sigma-Aldrich and used as received.

NaYF₄ nanoparticles were prepared using a procedure adapted from Wang, F., Deng, R. & Liu, X., "Preparation of core-shell NaGdE₄ nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes", Nature Protocols Volume 9, page 1634 (2014), the content of which is incorporated herein in its entirety. The typical inorganic crystalline matrix NaYF₄ was selected to host the Yb³⁺ and Tm³⁺ ions to guarantee intense upconversion fluorescence and chemical stability of the nanoparticles. The OA ligand was removed from the surface of the as-synthesized nanoparticles for water dispersibility.

Synthesis 1

In a typical ssynthesis of NaYF₄:Yb/Tm (30%, X%, X = 1, 2, 4, 6 and 8) nanoparticles, a 4- mL water solution of Ln(CH₃C0₂)₃ (0.2 M, Ln=Y, Yb and Tm) was added to a 50-mL flask containing 6 mL of oleic acid and 14 mL of l-octadecene. The mixture was heated to l60°C for 60 min to form the lanthanide-oleate precursor solution and then cooled down to 50°C naturally. Thereafter, a mixture of NaOH (2 mmol) and NH₄F (3.16 mmol) in methanol was added and stirred for 60 min. The resultant solution was heated at l00°C for 30 min under vacuum to remove the methanol. After purging with argon, the solution was heated to 300°C and kept for 1 h before cooling down to room temperature. The as-prepared nanoparticles were precipitated by additional of ethanol, collected by centrifugation at 6000 rpm for 3 min, and washed with ethanol for several times. The core nanoparticles were stored in cyclohexane (4 mL). The final compositions of the synthesised nanoparticles were:

• NaYF₄:Yb 30%, Tm 1% (sample 1),

• NaYF₄:Yb 30%, Tm 2% (sample 2),

• NaYF₄:Yb 30%, Tm 4% (sample 3),

• NaYF₄:Yb 30%, Tm 6% (sample 4), and

• NaYF₄:Yb 30%, Tm 8% (sample 5). This synthesis gives nanoparticles which are protected by oleic acid ligands. To make the nanoparticles hydrophilic and easy to disperse in water solution the ligand was removed according to the following procedure. The solution was centrifuged and the supernatant was discarded. Oleic acid-capped nanoparticles were then dispersed in a 2-mL HC1 solution (0.1 M) and ultra- sonicated for 15 minutes to remove the surface ligands. After the reaction, the nanoparticles were collected via centrifugation at 16,500 rpm for 20 min. The resulting products were washed with ethanol two times, and re-dispersed in deionized water.

Synthesis 2

NaYF₄:Yb/Tm (30%, X%, X = 1, 2, 4, 6 and 8) nanoparticles were also synthesised according to the following procedure. 2 mL of RE(CH₃C0₂)₃ (0.4 mmol, RE = Y, Yb and Tm) in water were poured into a 50-mL flask containing 3 mL of OA and 7 mL of 1- octadecene. The mixture was heated at l50°C for 1.5 hours to promote the formation of the RE-oleate complexes, and subsequently allowed to cool to 50°C. Then, 6 mL of methanol solution with NELF (1.6 mmol) and NaOH (1 mmol) was poured into the flask, and the resulting solution was stirred for 30 minutes. The temperature was then raised to 100°C to prompt the evaporation of methanol. After degassing for 20 minutes, the resulting mixture was heated to 290°C, maintained under a flow of nitrogen for 2 hours, and cooled to room temperature. The as-synthesized Yb,Tm-codoped NaYF₄ nanoparticles were collected via centrifugation, washed with cyclohexane and ethanol, and finally re-dispersed in cyclohexane (4 mL).

The as- synthesized nanoparticles capped with OA were dispersed in a 2 mL of HC1 solution (0.1 mol) and ultrasonic ated for 15 minutes to allow removal of the surface ligands. After that, the nanoparticles were collected via centrifugation at 16,500 rpm for 20 minutes and purified by addition of acidic ethanol solution (pH 4; prepared by mixing 0.1 mol HC1 aqueous solution with absolute ethanol). Ethanol and deionized water were used to wash the nanoparticles several times, which were then re-dispersed in deionized water.

The final compositions of the synthesised nanoparticles were: • NaYF₄:Yb 30%, Tm 1% (sample 6),

• NaYF₄:Yb 30%, Tm 2% (sample 7),

• NaYF₄:Yb 30%, Tm 4% (sample 8),

• NaYF₄:Yb 30%, Tm 6% (sample 9), and

• NaYF₄:Yb 30%, Tm 8% (sample 10).

TEM imaging and size distribution of the as-synthesized Yb,Tm-codoped NaYF₄ nanoparticles (not shown) confirmed that the nanoparticles were monodispersed with an average diameter of 21.5, 23.1, 24.2, 22.6 and 20.8 nm for the samples from 6 to 10, respectively.

EXAMPLE 2

Morphology and Structure Characterisation of the nanoparticles

The morphology and structure characterisation of the as-prepared core Yb,Tm-doped nanoparticles were characterised using a JEOL-1010 transmission electron microscope (TEM) operating at an acceleration voltage of 100 kV. Figure 1 (a) shows a TEM micrograph showing nanoparticles synthesised according to the procedure described in Example 1. Figure 1(b) shows size distribution analysis of the synthesised particles based on TEM micrographs, showing the particles have an average diameter of 21.5 nm.

Up-conversion Fluorescence of the nanoparticles

The UV and visible absorption spectra of the nanoparticles were determined using an Agilent Technologies Cary 60 ETV-Vis spectrometer. The upconversion fluorescence emission spectra were analyzed using an Andor Shamrock SR-500i imaging spectrometer equipped with iXon EMCCD Camera using a 980-nm continuous wave (CW) laser (Thorlabs, BL976- PAG900) as near-infrared (NIR) light source at a power of 1 mW. The up-conversion emission intensity of the rare-earth doped nanocrystals versus a constant power excitation source with tunable wavelength varies from 700 to 1050 nm. Optimal excitation wavelength is located at around 980 nm for maximised up-conversion emission intensity of the sample. Figure 1(c) shows the upconversion emission spectrum of the sample of NaYF₄ nanoparticles containing Yb 30 mol% and Tm 4 mol%. Visible in the spectrum are emissions corresponding to the ^lle ³¾, ' D₂ ³H₆ and 'D₂ ³F₄ energy level transitions.

The optical characterisation confirms that the rare-earth doped nanocrystals emit intense UV and blue up-conversion fluorescence with peak centred at 349 nm, 364 nm and 453 nm corresponding to the ^lle ³¾, ' D₂ ³ He and 'D₂ ³F₄ energy levels. The power dependence of the rare-earth doped nanocrystals, fluorescence emission at the different wavelength under excitation with the 976-nm CW laser was conducted to confirm the multi photon nature of emission. By fitting the data points and obtaining the slopes of the power dependence of up-conversion fluorescence, we confirmed that the emissions at 349 nm result from five-photon processes while the emissions at 364 and 453 come from four-photon and three-photon processes, respectively.

EXAMPLE 3

The nanoparticles were conjugated with commercially-available GO (2 mg mL ¹ in water solution) through electrostatic bonds due to their opposite charge in water solution. Imaging of the nanoparticles-GO nanocomposite was carried out by detection of 650-nm fluorescence from GO under 880-nm fs laser illumination, and 453-nm up-conversion fluorescence from the nanoparticles under 980-nm CW laser illumination. Highly efficient RET (efficiency > 80%) between the nanoparticles and GO was determined through measurements of up conversion fluorescence lifetime for the nanoparticles conjugated with GO in water solution, in function of increasing GO concentration (Figures 2(a) and (b)).

The reactivity of the nanocrystals and graphene oxide is closely related to their size, shape and surface properties. For instance, after the removal of oleate ligands, rare-earth doped nanocrystals were determined to be positively charged due to surface protonation. In the case of graphene oxide, surface properties such as the surface charge may depend upon the quantity and type of functional groups on the graphene oxide surface. The functional groups on the basal plane and edges of the graphene oxide sheets can weakly develop negative charges in the solution due to deprotonation, yielding a hydrophilic nature. Zeta potential measurements were carried out for the rare-earth doped nanocrystals and graphene oxide sheets in water solution, and data is shown in Table 1.

The values of zeta potential indicated a positive charge for the rare-earth doped nanocrystals and a negative charge for graphene oxide in water solution suggesting the possibility of electrostatic self-assembly when mixed.

Table 1 Zeta potential measurements for rare-earth doped nanocrystals and graphene oxide sheets in water solution

The design of the nanocomposite based on rare-earth doped nanocrystals and graphene oxide followed two steps, namely:

• Deposition of single-layer graphene oxide sheets onto coverslip substrate

• Self-assembly of rare-earth doped nanocrystals onto single-layer graphene oxide

Step 1 - Deposition of single -layer graphene oxide sheets onto coverslip substrate

Single-layer graphene oxide sheets were deposited from water solution onto pre-cleaned and silanised PΌ-coated coverslip glass substrates. First, coverslip substrates were treated with silane solution to covalently link positively charged aminoalkyl groups to the glass. The resultant surface is "sticky", promoting the binding of negatively charged graphene oxide sheets to the glass. Coverslips were soaked in acetone for about two minutes, then air-dried to ensure the absence of oil or water. This assures that the slides are free of any trace of oil or water. A silane solution was then prepared by mixing 200 pL of 3- triethoxysilylpropylamine solution (Sigma-Aldrich, [3-aminopropyl]triethoxysilane 99%) with 5 mL of acetone, obtaining a 2% v/v solution. The clean substrates were dipped in a 2% silane in ethanol solution for 2 minutes. Then, the slides were washed in 2 changes of distilled water and dried. The substrates were silanised by immersing in aqueous silane solution for 30 minutes and then washed thoroughly with Millipore water. GO sheets were deposited onto silanised silicon substrates by immersing a silicon substrate into the GO dispersion (50 pg ml-l) for 5 seconds, then into a second container containing Millipore water for 30 seconds and then air-dried.

Step 2: Self-assembly of nanoparticles onto single -layer GO

Self-assembly was adopted in this preparation by exploiting the electrostatic attraction between nanoparticles and GO nanosheets, which have positive and negative charge, respectively, in an aqueous environment. The nanoparticles were self-assembled electrostatically onto single-layer GO nanosheets from colloidal solutions. Briefly, 100 pL of 0.1 pmol nanoparticles solution was placed onto the PΌ-coated coverslip substrates where single-layer GO was previously deposited, according to Step 1. The solution was kept for 30 minutes to enable self-assembly of the nanoparticles to single-layer GO, and then the substrate was rinsed with deionized water to eliminate excess nanoparticles solution. Typically, the density of the layer of nanoparticles increased with incrementing concentration and deposition time. By tuning the concentration of the nanoparticles in water and the deposition time, the density of the self-assembled nanoparticle layer could be controlled accurately. The nanoparticles deposited prevalently onto the GO nanosheets with a small residue onto the substrate because of the electrostatic repulsion between the nanoparticles and silane groups previously formed onto the ITO-coated coverslip glass.

GO sheets were examined by observation under an optical microscope in reflection mode and AFM analysis (MFP-3D AFM Asylum Research, CA; tapping mode under ambient conditions) to ensure uniform GO sheet deposition was achieved. EXAMPLE 4

The nanoparticles and the GO were embedded within a silica-based thermal conductor by a sol-gel procedure. The regents used were tetramethoxysilane (TMOS), Poly(Ethylene Glycol) (PEG) (Mw = 400), Poly(ethylene glycol)-block-poly(propylene glycol)-block- poly(ethylene glycol) (P123) (Mw = 5800), and hydrochloric acid (HC1) from Sigma- Aldrich. The procedure was adapted from one described in the supplementary information of Zhang Q, Xia Z, Cheng Y-B, Gu M. High-capacity optical long data memory based on enhanced Young’s modulus in nanoplasmonic hybrid glass composites. Nature Communications. 2018, volume 9, page 1183.

PEG was first dissolved in water, followed by addition of a poly(ethylene glycol)-block- poly(propylene glycol) -block-poly (ethylene glycol) (P123) solution (20 wt% P123 in water) and TMOS. A number of solutions of TMOS were made, in which the concentration of TMOS to be mixed with PEG and rare-earth doped nanocrystals adsorbed on GO ranged from 50% to > 99% in volume. After being stirred at room temperature for 10 min, an aqueous HC1 solution (10 wt% hydrochloric acid solution of 32 wt% HC1 in water) was introduced. The resulting solution was stirred unsealed at room temperature.

Rare-earth doped nanocrystals previously conjugated to GO via electrostatic bonds in water solution were added into the hybrid sol, and the resulting mixture drop-casted on cover glass slide substrates. The coated substrates were then placed in an oven at 313K and kept there for a week to bring hydrolysis and condensation reaction of TMOS to completion.

EXAMPLE 5 -Yb,Tm-codoped NaGdF4 core and core-shell nanoparticles

Gadolinium (III) acetate hydrate (99.9%), ytterbium (III) acetate tetrahydrate (99.9%), thulium (III) acetate hydrate, sodium hydroxide (98+%), ammonium fluoride (98+%), 1- octadecene (90%), oleic acid (OA) (90%) and graphene oxide (GO) (2 mg mL ¹) in water solution were purchased from Sigma- Aldrich. All chemicals were used as received without further purification. The Yb,Tm-codoped NaGdF₄ core and core-shell nanoparticles were synthesized by using a modified wet-chemical method based on a procedure described in F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, X. Liu, Nat. Mater. 2011, 10, 968, the content of which is incorporated herein in its entirety. The NaGdF₄:Yb/Tm core nanoparticles were first prepared and then used as seeds for epitaxial growth of NaGdF₄:Yb/Tm@NaGdF₄ core shell nanoparticles. Subsequently, the OA ligand was removed from the surface of the nanoparticles for water dispersibility.

Synthesis of NaGdF . Yb/Tm (49%, 1 %) core

For the preparation of NaGdF₄:Yb/Tm (49%, 1%) core nanoparticles, 2 mL of Ln(CH₃C0₂)₃ (0.2 mol, Ln = Yb, Tm and Gd) water solution was poured into a 50 mL flask that contained 4 mL of OA. The mixture was heated at l50°C for 30 minutes to allow the elimination of water. A solution of l-octadecene (6 mL) was then rapidly poured into the flask and the resulting mixture was heated at l50°C for an additional 30 minutes and then cooled to 50°C. Subsequently, 5 mL of methanol solution that contained NH₄F (1.36 mmol) and NaOH (1 mmol) was poured and the resultant solution was stirred for 30 minutes. After the evaporation of methanol, the solution was heated to 290°C under argon for 1.5 hours and then cooled to room temperature. The as-synthesized nanoparticles with a yield of 80 mg were precipitated by adding ethanol, collected through centrifugation at 6,000 rpm for five minutes, washed with ethanol and re-dispersed in 4 mL of cyclohexane.

Synthesis ofNaGdF4 shell

NaGdF₄:Ln shell precursor was first prepared by mixing 2 mL of Ln(CH₃C0₂)₃ (0.2 mol, Ln = Gd) water solution and 4 mL of OA in a 50-mL flask and subsequently heating at 150°C for 30 minutes. Then l-octadecene (6 mL) was added and the mixed solution was heated at 150°C for an additional 30 minutes before cooling to 50°C. Later, the NaGdF₄:Yb/Tm (49%, 1%) core nanoparticles (40 mg) dispersed in 2 mL of cyclohexane were added, along with 5 mL methanol solution of NH₄F (1.36 mmol) and NaOH (1 mmol). The resulting mixture was stirred at 50°C for 30 minutes and then heated to 290°C under argon for 1.5 hours. Finally, the solution was cooled to room temperature. The resulting NaGdF₄:Yb/Tm (49%, l%)@NaGdF₄ core-shell nanoparticles were precipitated by adding ethanol, collected by centrifugation at 6,000 rpm for five minutes, washed with ethanol several times and re-dispersed in 4 mL of cyclohexane.

Removal of OA ligand

The as- synthesized nanoparticles capped with OA were dispersed in a 2mL of HC1 solution (0.1 mol) and ultrasonic ated for 15 minutes to allow removal of the surface ligands. After that, the nanoparticles were collected via centrifugation at 16,500 rpm for 20 minutes and purified by addition of acidic ethanol solution (pH 4; prepared by mixing 0.1 mol HC1 aqueous solution with absolute ethanol). Ethanol and deionized water were used to wash the nanoparticles several times, which were then re-dispersed in deionized water.

Characterization of NaGdF . Yb/Tm (49%, !%)@NaGdF₄ core-shell nanoparticles

Transmission electron microscopy (TEM) was performed using a JEOL 1010 TEM (2001) at an accelerating voltage of 100 kV equipped with Gatan Orius SC600 CCD Camera (2014). The X-ray powder diffraction (XRD) patterns were obtained on a Bruker Axs D4 Endeavor Wide angle XRD instrument with a Cu Ka radiation source at l = 1.54056 A. Energy- dispersive X-ray spectroscopy (EDX) of the core-shell nanoparticles was conducted using a JEOL 2100F FEGTEM/STEM (2011) at an accelerating voltage of 200 kV equipped with an Oxford X-MaxN 80T EDX Detector (2014) and a Gatan OneView 4k CCD Camera.

TEM images and corresponding particle size determination for the core and core- shell nanoparticles are shown in Figures 3(a)-(b) and 4(a)-(b), respectively. The images show TEM imaging and corresponding size distribution of the as-synthesized NaGdF4:Yb/Tm (49%, 1%) core (left) and NaGdF₄:Yb/Tm (49%, l%)@NaGdF₄ core-shell (right) nanoparticles. The as synthesised core nanoparticles are predominantly spherical with average diameter of 33.4 nm (SD ~ 1 nm). The corresponding core-shell nanoparticles have average diameter of 35.5 nm (SD ~ 0.9 nm). As shown in Figures 3(b) and 4(b), the coefficient of variation (i.e. the ratio of the standard deviation, a_d, to the mean diameter, dmean ) is below 3%, calculated considering about 200 nanoparticles, which is indicative of monodispersed nanoparticles. Scale bar: 20 nm.

Upconversion luminescence spectra were analysed using an Andor Shamrock SR-500i imaging spectrometer equipped with iXon EMCCD Camera using a 980-nm CW laser (Thorlabs, BL976-PAG900) as near-infrared (NIR) light source at a power of 1 mW. Upconversion luminescence spectra of the as-synthesized core and core-shell nanoparticles is shown in Figure 5. The spectra confirms that co-doping of Yb³⁺ as sensitizer and Tm³⁺ and Gd³⁺ as emitters enabled efficient upconversion luminescence through the gadolinium sublattice-mediated Yb³⁺ Tm³⁺ Gd³⁺ energy transfer pathway.

EXAMPLE 6 - Preparation of GO and rGO thin films integrated with NaGdF 4. Yb/Tm (49%, !%)@NaGdF₄ core-shell nanoparticles

A GO thin film integrated with NaGdF₄:Yb/Tm (49%, l%)@NaGdF₄ core-shell nanoparticles prepared in accordance with the procedure described in Example 9 was prepared by vacuum filtration method. 200 pL of GO water solution was diluted to 10 mL with deionized water and then sonicated for 30 minutes. Then, 20 pL of nanoparticle solution (20 mmol, dispersion in water) was diluted up to 10 mF using deionized water and sonicated for 30 minutes. The solutions of GO and nanoparticles were combined and stirred for 10 minutes. The mixture was filtrated through a mixed cellulose ester membrane filter with 0.22 pm pore size, forming a superimposed thin film with controllable thickness onto the membrane filter. The thin film was dried at room temperature and subsequently immersed in acetone to remove the membrane filter. Finally, the free-floating thin film was fished out using a coverslip substrate and let dry at room temperature. Accompanied by the volatilization of acetone, the free-standing GO thin film integrated with nanoparticles was easily separated from the membrane filter and deposited onto coverslip substrates.

Corresponding rGO thin films integrated with NaGdF₄:Yb/Tm (49%, l%)@NaGdF₄ core shell nanoparticles were obtained by irradiating the GO-based samples with a 375-nm continuous wave (CW) laser. The laser induced photo-chemical reduction of GO to rGO. The process of reduction was accompanied by an instantaneous increase in the absorption coefficient of rGO, which was the basis for the ultrafast, high-efficiency modulation of the upconversion luminescence from the nanoparticles.

EXAMPLE 7 Optical laser setup

The optical system setup for implementation of super-resolution optical data storage in the nanoparticles-GO nanocomposite comprised a dual-beam configuration in which a 980-nm CW laser was used for activation and an 808-nm CW laser was used for the deactivation of the nanoparticles. In particular, the deactivation laser beam could be modulated into a Laguerre-Gaussian shape by combined use of a quarter wave place (QWP) and a vortex phase plate (VPP) (Figure 6).

In a typical set-up according to the schematic of Figure 6, a dual-laser super-resolution experimental optical system setup was built on a custom-made confocal microscopy system. The sample consisting of single layer GO loaded with nanoparticles was placed on a computer-controlled high-precision nanopositioning system based on a three-axis piezo stage (Physik Instrumente, P-562.3CD). A single-mode 980-nm CW activation laser (Thorlabs, BL976-PAG900) was used to excite the nanoparticles in the nanocomposite. After being collimated, the activation laser passed through two long-pass dichroic mirrors and was focused onto the sample by an oil-immersion objective lens (Olympus, MPLAPON 100X02).

The first dichroic mirror DC1 (Semrock, FF850-Di0l-tl-25x36) also allowed a collimated 808-nm CW deactivation laser (Lumics, LU0808M250) to combine with the activation laser. The upconversion fluorescence emission was collected by the same objective, separated from the activation and deactivation lasers by the second dichroic mirror DC2 (Semrock, FF705-Di0l-25x36), and coupled into a multimode fiber (Thorlabs, FG050LGA), which was connected to an SPAD (Excelitas Technologies, SPCM-AQRH-14-FC). A band pass filter (Semrock, FFO 1-442/46-25) and a short pass filter (Semrock, FF02- 694/SP-25) were inserted in the detection pathway to select the upconversion fluorescence emission band for imaging and lifetime measurements. A flipping mirror was also inserted to couple the signal with a spectrometer (Andor Shamrock SR-500i imaging spectrometer equipped with iXon EMCCD Camera) to measure upconversion fluorescence emission spectra. For super-resolution imaging, a quarter waveplate (Thorlabs, WPQ10M-808) was employed to convert the 808-nm laser into circular polarized.

A half waveplate (Newport, 10RP52-2) was also employed to optimize the quality of the circular polarization and control the power of the laser beams in combination with Glan- Thompson prisms (Thorlabs, GTH10-A and GTH10-B). Finally, a vortex phase plate (RPC Photonics, VPP-la) was placed in the path of the deactivation laser to generate a donut shaped PSF in the focal plane.

Determination of the spectral overlap integral J

The spectral overlap integral J of the resonance energy transfer (RET) pair comprising nanoparticles conjugated with GO was of 7xl0¹⁴ nm⁴ m ¹ cm calculated using the software FluorTools a | e. The wavelength range between 200 nm and 600 nm was evaluated. For GO, the maximum absorption coefficient was estimated to be of 180,000 L m ¹ cm⁴.

Upconversion fluorescence lifetime measurements setup

For the measurements of upconversion fluorescence lifetime in the nanocomposite based on single-layer GO and nanoparticles, a 980-nm CW excitation laser (Thorlabs, BL976- PAG900) was modulated using an acousto-optic modulator (AOM) (AA OPTO ELECTRONIC, MT110-A1-VIS/IR/1064) for 50-ps pulses with frequency of 100 Hz for excitation of upconversion fluorescence emission. The emitted photons went through a band pass filter (Semrock, FFO 1-442/46-25) and a short pass filter (Semrock, FF02-694/SP-25) and were detected by a SPAD (Excelitas Technologies, SPCM-AQRH-14-FC). The trigger signal from the AOM was synchronized with the SPAD using a data acquisition (DAQ) card (National Instruments, CDAQ-9171). The effective luminescence decay time was calculated by:

Where I(t) denotes the luminescence intensity as a function of time t and h represents the maximum luminescence intensity.

The schematic of Figure 7 shows how optical data writing in the nanoparticles-GO nanocomposite can be achieved using the optical setup described above. The set-up allows irradiation of individual nanoparticles integrated onto the GO. The particles are irradiated by a composite 980nm-808nm beam, obtained by a 980-nm Gaussian-shaped activation laser spatially overlapped with an 808-nm donut-shaped deactivation laser. This allows generation of a spatially confined upconversion emission from the nanoparticles, which locally reduces the GO to rGO through RET. Once rGO forms, the upconversion emission is quenched, providing a pattern of nanosized recorded bits that can be read optically.

EXAMPLE 8 - Quenching effect of rGO vs GO on the upconversion luminescence of NaGdF - Yb/Tm (49%, !%)@NaGdF₄ core-shell nanoparticles.

Reduction of GO to rGO induces controllable quenching of the upconversion luminescence emission from the nanoparticles. This basic mechanism was tested on the GO thin films integrated with core-shell particles obtained according to the procedure described in Example 6.

The GO thin film integrated with NaGdF₄:Yb/Tm (49%, l%)@NaGdF₄ core-shell nanoparticles is highly luminescent under NIR laser excitation at 980 nm, with the emission arising from the nanoparticles. This is schematically shown in Figure 8(a). However, along with the photochemical reduction of GO, the variation of absorption coefficient between GO and rGO produced quenching of upconversion luminescence emission and therefore decreased emission intensity from the nanoparticles. The quenching effect of rGO on the upconversion luminescence of the nanoparticles can be appreciated from the schematic of Figure 8(b).

Figures 8(c) and 8(d) show 450-nm upconversion luminescence emission quenching in the nanocomposite following the conversion of GO to rGO through exposure to 375-nm CW laser at 10, 25, 50, 75 and 100 pW normalized to before irradiation (Exposure time: 100 ms). The inset of Figure 8(c) shows detailed imaging of the modulation of an individual pixel (Scale bar: 1 pm). Figure 8(d) shows 450-nm emission lifetime before (rightmost line) and after irradiation with a 375-nm CW laser at 10, 25, 50, 75 and 100 pW (from right to left lines) decreasing from 403 to 175 ps. The data allows appreciating the extent of upconversion luminescence quenching achievable by the progressive reduction of GO to rGO.

It can be clearly seen from the data in Figure 8 that the progressive reduction of GO to rGO causes a significant decrease in 450-nm upconversion luminescence emission of the core shell nanoparticles due to quenching by rGO. Further, the extent of quenching of the 450- nm upconversion luminescence is proportional to the extent of reduction of GO to rGO (Figure 8(c)). Values of 450-nm upconversion luminescence emission quenching of up to -90% were achieved, which is a l0⁶-fold decrease of the time for modulation of upconversion luminescence from nanoparticles through our approach compared with the use of photochromic molecules requiring up to tens of minutes. The data proves the effectiveness of an underlying optical writing/reading mechanism afforded by integrating GO with upconversion luminescent nanoparticles.

Micron-scale representations of a leaf (Figure 9(a)), the Sydney Opera House (Figure 9(b)), and a kangaroo (Figure 9(c)) were optically written onto a 2D GO/core-shell nanoparticle composite by spatially reducing GO to rGO along the designated area, resulting in spatially confined quenching of the nanoparticles upconversion luminescence. The patterned areas can be read optically by raster scanning the sample. Areas of pristine GO show unchanged upconversion emission from the nanoparticles (bright areas), while areas where GO was reduced to rGO are dark due to the luminescence quenching effect of rGO. Figure 9 (d) shows normalized line profile of the upconversion luminescence signal between the two black arrows imaging of the individual pixels (scale bar: 1 pm).

EXAMPLE 9 - Minimization of thermal-reduction of GO

The temperature rise in GO under laser irradiation can lead to photo-thermal reduction, causing photo-damage of the sample, and prevent the achievement of writing and reading of information bits beyond the diffraction-limit barrier because of the intrinsic lack of nanoscale control of this process. The single-layer configuration of GO offers efficient heat dissipation to the silica support, acting as the thermal conductor, which avoids unwanted temperature increase in the GO-nanoparticle system.

The theoretical temperature rise in single-layer GO under CW laser irradiation was estimated based on a model that relies on an energy balance argument, as discussed in Wang, D., Carlson, M. T., & Richardson, H. H. (2011). Absorption cross section and interfacial thermal conductance from an individual optically excited single-walled carbon nanotube. Acs Nano, 5(9), 7391-7396.

The temperature profile along the SWCNT can be modeled using Eq. 1, where v is the direction along the wire, k_wllv is the thermal conductivity of the nanowire, q is the heat generation per unit volume, A is the cross sectional area of the wire, h is the thermal transfer coefficient for heat dissipation into the substrate, and P is the perimeter at x under the wire in contact with the substrate. In this model, the heat only dissipates into the substrate and not through air.

When the wire is excited in the middle and the temperature is measured at that point, heat can propagate in both directions to either end of the wire. The magnitude of heat dissipation increases with the temperature gradient between the SWCNT and the substrate. The term of Eq. lthat models heat dissipation into the substrate is /iR(DG) dx. At steady state, the heat generation term, / qA dx' = / (C_abs/(x'))/(V_exc) A dx', and the heat dissipation term, 12.8 / hP(AT) dx, are equal. The variable x ' refers to the distance along the SWCNT that is excited with the laser light. In the heat generation term, the laser intensity is not constant over the SWCNT but varies as a Gaussian profile along the wire. We approximate the laser intensity in the direction of the wire as a constant average intensity using the average intensity of the laser over the length of SWCNT (300 nm). The average laser intensity is 7 x 1010 W/m2 with a peak intensity of 8 x 1010 W/m2. The factor 12.8 in the heat dissipation term relates the local temperature of the nanowire to the measured temperature. (25) This factor considers that our optical temperature measurement is resolution-limited and needs to be convoluted with the true thermal image in the substrate and the collection volume of our microscope.

The steady-state thermal increase DT experienced by single-layer GO nanosheets upon absorbing laser irradiation is:

where I is the incident laser intensity, Ax is the absorbance at the irradiation wavelength and h_air and h_sub are the interfacial thermal conductance between GO and the surrounding air and support, respectively. In the case of this study’s dual -laser super-resolution laser system, the considered laser wavelengths are 980 and 808 nm. The absorbance of a single-layer GO sheet under laser irradiation at 980 and 800 nm was estimated to be A9so = 3xl0⁴ and AHOH = 5xl0⁴, respectively. Values of h_air and h_sub for single-layer GO nanosheets have been estimated using the values for graphene— in particular, h_air ~ lxlO⁵ W m ² K ¹ and h_sub ~ 5xl0⁷ W m ² K ¹ for silica (Si0₂) and coverslip glass. Further, because h_air « h_sub, Equation S2 can be simplified as follows:

Photothermal reduction of GO under laser irradiation has been reported at a temperature of ~230°C; therefore, this study considered that DT = 200 K can produce photothermal reduction of single-layer GO initially at room temperature.

Estimations of the 980-nm CW laser and 808-nm CW laser intensities yielding DT = 200 K from room temperature in single-layer GO deposited onto a coverslip glass substrate were several orders of magnitude higher than those used throughout this research. Therefore, the thermal increase induced by the considered laser intensities was far below this threshold and thus negligible. The reason for this outcome was that thermal increase in the sample strongly depended on the properties of heat dissipation by the thermal conductor onto which the single-layer GO was deposited (silica in this case). In this context, the glass support offers efficient heat dissipation away from GO, preventing thermal increase under the considered laser intensities. Thus, photothermal reduction of single-laser GO deposited onto a coverslip glass substrate could be excluded.

EXAMPLE 10

Optical data bits were written in the nanoparticles-GO nanocomposite and subsequently read out. In Figure 10(a), optical data bits were written using different powers of the 980-nm CW laser and read out through the detection of GO’s fluorescence decrease at 650 nm.

Figure 10(b) shows optical data bits written using different powers of the 980-nm CW laser and read out through the detection of single nanoparticles’ up-conversion fluorescence quenching measured at 453nm.

Spatial optical data read out in the nanoparticles-GO nanocomposite was achieved through the detection of GO’s fluorescence decrease and nanoparticles’ up-conversion fluorescence quenching, which accompanied the reduction from GO to rGO (Figure 11). EXAMPLE 11 - SPIN writing and STED reading

Highly efficient optical depletion (efficiency > 95%) of 453-nm up-conversion fluorescence emission from NaYF₄ nanoparticles containing 30 mol% Yb³⁺ and 4 mol% Tm³⁺ ions was achieved by combined irradiation of the 980-nm CW laser (Gaussian) with the 808-nm CW laser (Laguerre-Gaussian), as shown in Figure 12. The image shows the strong nanoparticle upconversion emission generated under 980nm excitation (sharp peak line), and the complete absence of emission under a 808nm excitation (bottom flat line). This allows to use a 980nm beam as the excitation beam and the 808nm beam as the depletion beam in the setup described in Example 7. The resulting optically combined 980nm-808nm composite beam can be used to induce spatially confined upconversion excitation of the nanoparticles only in the central area of the beam, as shown in the weaker (because spatially confined) 453nm emission peak in the middle line of Figure 12.

Under irradiation by the activation laser (908nm) confined at the center of the composite beam, the nanoparticles undergo upconversion to the high-energy ^2 and ¹1₆ levels in Tm³⁺ and induce the photochemical reduction of GO by RET. By contrast, the nanoparticles located on the outer region of the doughnut- shaped beam are optically switched off by the deactivation laser (808nm), resulting in inhibited upconversion and consequently prevented RET. Therefore, the rGO spots that form are confined to the proximity of the activated nanoparticles only and result at the nanoscale for SPIN-like data writing.

Optical depletion of 450-nm upconversion fluorescence from the nanoparticles under combined 980-nm excitation and 808-nm depletion indicated strong dependence on the Tm³⁺ doping concentration, with the highest value of -90% for 4% Tm³⁺-doped nanoparticles. The depletion efficiency relative to the irradiation power is shown in Figures l3(a)-(d). The Figures relate to confocal imaging of the 4% Tm-doped nanoparticles (a) under CW laser at 980 nm and under dual-laser irradiation with the 808-nm CW laser at (b) 1.5 and (c) 15 mW (scale bar: 500 nm). The plot in Figure 13(d) shows corresponding upconversion fluorescence intensity profile along the white dashed line for an individual nanoparticle, confirming the expected 50% and -90% upconversion fluorescence depletion.

The power-dependent depletion efficiency of 453-nm up-conversion fluorescence indicated a value of saturation intensity Is of -0.4 MW cm ², which results in 2-order of magnitude reduction in energy consumption compared to other available materials for super-resolution optical data storage.

Figure 14 also shows power-dependent depletion efficiency of 450-nm upconversion fluorescence from the 4% Tm-doped nanoparticles using a CW laser at 980 nm with power of 0.5 mW for excitation and 808-nm CW laser for depletion at increasing power. The theoretical prediction for optical depletion efficiency was confirmed, obtaining -90% depletion efficiency and a value of saturation intensity I_sat of -375 kW cm^-2 (-1.5 mW).

Figure 15 shows power-dependent depletion efficiency of 450-nm upconversion fluorescence of NaYF₄ nanoparticles containing 30 mol% Yb³⁺ and 4 mol% Tm³⁺ in conjugation with graphene oxide using a CW laser at 980 nm with power of 0.5 mW for excitation and 808-nm CW laser for depletion at increasing power, confirming theoretical prediction for optical depletion efficiency of -95% and a value of saturation intensity 7_{sa t} of -250 kW cm^-2 (-1.0 mW)

Inhibition of GO reduction in the nanoparticles-GO nanocomposite during the writing process was achieved under combined irradiation of the 980-nm CW laser (Gaussian) for activation with the 808-nm CW laser (Laguerre-Gaussian) for inhibition, paving the way for writing of the optical bits at the nanoscale through SPIN (Figure 16).

Super-resolution imaging of the nanoparticles through STED microscopy was achieved under combined irradiation of 980-nm CW laser for excitation with 808-nm CW laser for depletion, paving the way for read out of the optical bits at the nanoscale. Improvement of the optical resolution from -300 nm to < 80 nm has been achieved. Figure 17 allows appreciation of the improvement of resolution obtained by increasing the 808-nm doughnut- shaped depletion laser intensity for super-resolution imaging of the 4% Tm-doped nanoparticles by STED microscopy. The insets relate to STED microscopy imaging of the same nanoparticles obtained at different depletion laser intensities (Scale bar: 500 nm). Individual nanoparticles on GO were imaged by confocal microscopy and STED microscopy, obtaining a significant improvement in spatial resolution by introducing the donut-shaped 808-nm depletion laser (Figure l 8(a)-(c)). The figure shows imaging of individual nanoparticles by (a) confocal microscopy compared to (b) STED microscopy. The corresponding line profiles of the image as shown in Figure 18(c).

EXAMPLE 12 - SPIN writing / STED reading single particle resolution

Figure 20 provides a further outline of the concept of ultralow-power nanoscale optical data storage afforded by the material described herein. The images relate to a GO sample integrated with Yb,Tm-codoped NaYF₄ nanoparticles which is irradiated by a dual-laser configuration comprising a 980-nm Gaussian-shaped activation laser spatially overlapped with an 808-nm donut-shaped deactivation laser. Figure 20 shows confocal microscopy imaging and corresponding STED microscopy imaging of individual nanoparticles integrated on GO before (Figure 20(a) and 20(c)) and after (Figures 20(b) and 20(d)) nanoscale optical data writing. The images relate to the same sample location, and allow appreciating the significant improvement of the writing/reading resolution afforded by material of the invention.

Under irradiation by the activation laser, the nanoparticles undergo upconversion to the high- energy 1H₂ and ¹1₆ levels in Tm³⁺ and induce the photochemical reduction of GO by RET. By contrast, the nanoparticles located on the outer region of the activation laser are optically switched off by the deactivation laser, resulting in inhibited upconversion and consequently prevented RET. Therefore, the rGO spots are confined to the proximity of the activated nanoparticles only and result at the nanoscale for SPIN-like data writing.

When it forms, rGO quenches the proximal nanoparticles for super-resolution data read-out by detection of reduced upconversion fluorescence by STED microscopy. In this test, two neighboring nanoparticles with sub-diffraction limit separation (below 200 nm) were deposited onto GO. The particles, which are initially undistinguishable by conventional confocal microscopy (Figures 20(a)), can be individually discriminated by STED microscopy with a resolution of ~54 nm and depletion intensity of 11.25 MW cm ² (Figure 20(c)). Each of the nanoparticles could thus be considered as a nanoscale digital information bit with initial state‘O’(fluorescent).

Spatially directing the dual-laser configuration so that only a single nanoparticle was selectively irradiated by the activation laser, the localized photochemical reduction of adjacent GO was achieved, converting the first bit to state‘G (quenched). By contrast, the other nanoparticle was not irradiated by the focused excitation beam and thus the local reduction of GO was prevented, maintaining the second bit with unvaried state ‘O’ (fluorescent). Unresolvable by confocal microscopy (Figure 20(b)), the nanoscale bits with different final states were distinguished by STED microscopy (Figure 20(d)). Nm-scale resolution was maintained at all times (Figure 20(e)-(f)).

EXAMPLE 13 - Use of 1550 -nm CW deactivation beam

Ultra-low power and ultra-high capacity super-resolution optical data storage were achieved using the nanoparticles obtained according to the procedures described in Example 1 under 980-nm CW activation and l550-nm CW deactivation beams. Optical depletion of nanoparticles’ up-conversion fluorescence has been simulated under the combined use of 980-nm CW laser for excitation and l550-nm CW laser for depletion (Figure 21). The simulation results indicated ultra-low intensity required for optical depletion of the nanoparticles’ up-conversion fluorescence under combined use of 980-nm CW excitation and l550-nm CW depletion with value of saturation intensity I_sat of ~ 15 W cm ².

Reduction of the feature size in the nanoparticles-GO system under combined use of 980- nm CW for photo -induction (Gaussian shape) and l550-nm CW for photo -inhibition (Laguerre- Gaussian shape) was simulated (Figure 22). The simulation results indicated that 25-nm feature size (towards a petabyte-level capacity) in the nanoparticles-GO system under combined use of 980-nm CW for photo-induction (Gaussian shape) and l550-nm CW for photo-inhibition (Laguerre-Gaussian or "donut" shape) can be achieved with an intensity of ~ 10 kW cm ² of the l550-nm CW laser beam.

The combined use of 980-nm CW laser for activation and l550-nm CW laser for deactivation indicated a reduction of several orders of magnitude of the intensity of the deactivation beam towards ultra-low power, ultra-high capacity super-resolution optical data storage for implementation in sustainable data storage devices.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word‘comprise’, and variations such as‘comprises’ and‘comprising’, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS

1. An optical data storage material comprising:

graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage,

nanoparticles configured to photo-chemically reduce the GO on the selected areas by optical upconversion emission, and

a support material that (i) embeds the GO and the nanoparticles and (ii) comprises a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas.

2. The optical data storage material of claim 1, in which the thermal conductor comprises silica, alumina, quartz, zirconia, yttrium aluminium garnet (YAG), hafnia, or a combination thereof.

3. The optical data storage material of claim 1 or 2, in which the thermal conductor is the support material.

4. The optical data storage material of any one of claims 1-3, in which the nanoparticles comprise an ion of a rare-earth element.

5. The optical data storage material of any one of claims 1-4, in which the nanoparticles comprise one or more ions selected from Yb³⁺, Tm³⁺, Pr³⁺, Nd³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, and Er³⁺.

6. The optical data storage material of any one of claims 1-5, in which the nanoparticles comprise about 10-50 mol% Yb³⁺ and about 0.2-20 mol% Tm³⁺.

7. The optical data storage material of anyone of claims 1-6, in which the nanoparticles comprise NaYF₄ nanoparticles containing about 30 mol% Yb³⁺ and about 4 mol% Tm³⁺.

8. The optical data storage material of any one of claims 1-7, in which the nanoparticles have a largest dimension of about 5 nm to about 50 nm.

9. The optical data storage material of any one of claims 1-8, in which the nanoparticles comprise an inorganic host matrix selected from Y2O3, Lu₂0₃, La₂0₃, Gd₂0₃, Y₂0₂S, Gd₂0₂S, La₂0₂S, GdOF, YOF, YAG, LaP0₄, LuP0₄, La₂(Mo0₄)₃, NaY(W0₄)₂, Gd₃Ga₅0i₂, YV0₄, LaF₃, YF₃, LUF₃, NaYF₄, LiYF₄, NaGdF₄, KY₃FI_O, KGd₂F₇, BaYFs, and a combination thereof.

10. The optical data storage material of any one of claims 1-9, in which the GO is provided in the form of single-layer GO.

11. A method of manufacturing an optical data storage material, the method comprising the steps of:

providing graphene oxide (GO) configured to be photo-chemically reduced on selected areas for optical data storage,

providing nanoparticles on the GO, the nanoparticles being configured to photo- chemically reduce the GO on the selected areas by optical upconversion emission, and providing a support material that (i) embeds the nanoparticles and the GO and (ii) comprises a thermal conductor in thermal contact with the GO to mitigate photo-thermal reduction of the selected areas.

12. The method of claim 11, in which the thermal conductor comprises silica, alumina, quartz, zirconia, yttrium aluminium garnet (YAG), hafnia, or a combination thereof.

13. The method of claim 11 or 12, in which the thermal conductor is the support material.

14. The method of any one of claims 11-13, in which the thermal conductor is obtained from hydrolysis and condensation of metal alkoxides and/or functional metal alkoxides.

15. The method of claim 14, in which the metal alkoxides and/or functional metal alkoxides comprise alkoxysilanes and/or functional alkoxy silanes, in which:

(i) the alkoxysilanes are selected from methyltriethoxysilane (MTES), phenyltriethoxysilane (PTES), diethyldiethoxysilane, methyltrimethoxysilane (MTMS), dimethyldimethoxysilane, phenyltrimethoxysilane (PTMS), vinyltrimethoxysilane (VTMS), vinylriethoxysilane (VTES), tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS), tetrabutoxysilane (TBOS), and a combination thereof, and

(ii) the functional alkoxysilanes are selected from 3-aminopropyl triethoxysilane, 3- aminopropyl trimethoxysilane, 3-isocyanatopropyl triethoxysilane, 3-isocyanatopropyl trimethoxysilane, 3-azidopropyl triethoxysilane, 3-azidopropyl trimethoxysilane, 3- thiolpropyl trimethoxysilane (or 3-mercaptopropyl trimethoxysilane or trimethoxysilyl propanethiol), 3-thiolpropyl triethoxysilane (or 3-mercaptopropyl triethoxysilane or triethoxy silyl propanethiol), 3-cyanopropyl trimethoxysilane, 3-cyanopropyl triethoxysilane, N-(2-aminoethyl)-3-aminopropyl trimethoxysilane, N-(2-aminoethyl)-3- aminopropyl triethoxysilane, (aminoethylaminomethyl) phenethyl trimethoxysilane, (3- acetamidopropyl) trimethoxysilane, acetoxyethyl trimethoxysilane, 3-acrylamidopropyl trimethoxysilane, acryloxymethyl trimethoxysilane 3-bromopropyl trimethoxysilane, 3- chloropropyl trimethoxysilane, (heptadecafluoro-l,l,2,2-tetrahydrodecyl) trimethoxysilane, (heptadecafluoro- 1 , 1 ,2,2-tetrahydrodecyl) triethoxysilane, 2-[methoxy(polyethyleneoxy)_2i- ₂₄propyl] trimethoxysilane, and a combination thereof.

16. The method of any one of clams 11-15, in which the nanoparticles comprise an ion of a rare-earth element.

17. The method of any one of claims 11-16, in which the nanoparticles comprise one or more ions selected from Yb³⁺, Tm³⁺, Pr³⁺, Nd³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, and Er³⁺.

18. The method of any one of claims 11-17, in which the nanoparticles comprise about 10-50 mol% Yb³⁺ and about 0.2-20 mol% Tm³⁺.

19. The method of any one of claims 11-18, in which the nanoparticles comprise an inorganic host matrix selected from Y2O3, Lu₂0₃, La₂0₃, Gd₂0₃, Y₂0₂S, Gd₂0₂S, La₂0₂S, GdOF, YOF, YAG, LaP0₄, LuP0₄, La₂(Mo0₄)₃, NaY(W0₄)₂, Gd₃Ga₅0i₂, YV0₄, LaF₃, YF₃, LUF₃, NaYF₄, LiYF₄, NaGdF₄, KY₃F_IO, KGd₂F₇, BaYFs, and a combination thereof.

20. The method of any one of claims 11-19, in which the nanoparticles comprise NaYF₄ nanoparticles containing about 30 mol% Yb³⁺and about 4 mol% Tm³⁺.

21. The method of any one of claims 11-20, in which the nanoparticles have a largest dimension of about 5 nm to about 50 nm.

22. The method of any one of claims 11-21, in which the GO is provided in the form of single-layer GO.

23. A method of recording optically readable data, the method comprising:

optically upconverting an input beam using nanoparticles to form an upconverted beam,

photo-chemically reducing GO in selected areas by the upconverted beam, and mitigating photo-thermal reduction of the selected areas by having the nanoparticles and GO embedded in a support material that comprises a thermal conductor in thermal contact with the GO.

24. The method of claim 23, in which the nanoparticles are nanoparticles as defined in any one of claims 4-9.

25. The method of claim 23 or 24, in which the input beam results from the combination of an excitation beam and a deactivation beam, in which

(i) the excitation beam induces up-conversion emission of the nanoparticles, thereby promoting, in a central portion of the irradiated region, photo-chemical reduction of the graphene oxide (GO), and

(ii) the deactivation beam inhibits the photo-chemical reduction of the graphene oxide in a region surrounding the central portion of the irradiated region, resulting in recorded optically readable data in the central portion of the irradiated region.

26. The method of claim 25, in which the excitation beam is a laser beam having a wavelength of about 980 nm, and the deactivation beam is a laser beam having a wavelength of about 808 nm or 1550 nm.