WO2010051598A1 - A process for producing silica nanowires - Google Patents

A process for producing silica nanowires Download PDF

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Publication number
WO2010051598A1
WO2010051598A1 PCT/AU2009/001458 AU2009001458W WO2010051598A1 WO 2010051598 A1 WO2010051598 A1 WO 2010051598A1 AU 2009001458 W AU2009001458 W AU 2009001458W WO 2010051598 A1 WO2010051598 A1 WO 2010051598A1
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Prior art keywords
nanowires
silica
metal catalyst
silicon
silicon substrate
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PCT/AU2009/001458
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French (fr)
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Robert Glen Elliman
Taehyun Kim
Avi Shalav
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The Australian National University
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Priority claimed from AU2008905796A external-priority patent/AU2008905796A0/en
Application filed by The Australian National University filed Critical The Australian National University
Publication of WO2010051598A1 publication Critical patent/WO2010051598A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/0657—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape of the body
    • H01L29/0665—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape of the body the shape of the body defining a nanostructure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y15/00—Nanotechnology for interacting, sensing or actuating, e.g. quantum dots as markers in protein assays or molecular motors
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C01B33/181—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by a dry process
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C01B33/181—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by a dry process
    • C01B33/183—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by a dry process by oxidation or hydrolysis in the vapour phase of silicon compounds such as halides, trichlorosilane, monosilane
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof  ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/0657—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape of the body
    • H01L29/0665—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions characterised by the shape of the body the shape of the body defining a nanostructure
    • H01L29/0669—Nanowires or nanotubes
    • H01L29/068—Nanowires or nanotubes comprising a junction

Definitions

  • the present invention relates to a process for producing silica nanowires.
  • Silica nanowires have enormous potential as functional materials for photonics and chemical sensing, and can be grown on various substrates by metal-catalyzed vapour- liquid-solid or vapour-solid-solid processes.
  • a metal catalyst promotes the growth of the nanowires by dissolving gas-phase reactants and acting as a transport medium for the deposition of the nanowire constituents.
  • a metal typically Au, Pt, Pd, or Fe, acts as a catalyst for the growth of silica nanowires by dissolving gas- phase reactants containing silicon and oxygen, and transporting them through the metal to deposit SiO 2 or Si-rich SiO 2 wires.
  • a metal catalyst is first either deposited as a thin film 102 or as colloidal particles 104 on a substrate 106, or alternatively is ion-implanted 108 into the near-surface of a substrate 106. Irrespective of how the metal is added to the substrate 106, subsequent annealing 112 then causes the metal to form a high density of small diameter (typically ⁇ 200 nm) islands 114 on the surface of the substrate 106, each island 114 acting as a 'seed' for the growth of one or more nanowires 116 from the surface of the substrate 106.
  • small diameter typically ⁇ 200 nm
  • the result is a substrate (typically, as shown, being a silicon wafer) densely populated with nanowires 116, as shown in the plan- view scanning electron microscope image of Figure 2, each nanowire having a metal catalyst particle 118 at its base or at its tip.
  • a substrate typically, as shown, being a silicon wafer
  • nanowires 116 as shown in the plan- view scanning electron microscope image of Figure 2, each nanowire having a metal catalyst particle 118 at its base or at its tip.
  • nano wires 116 An important feature of these nano wires 116 is their large surface-to-volume ratio, a characteristic that provides the basis for many applications, including sensing. Any increase in this ratio would therefore be desirable.
  • the rigid and relatively thick wafers on which the nanowires are grown may not be convenient for many applications or potential applications of nanowires, and may even preclude others.
  • a process for producing silica nanowires including: adding a metal catalyst to first nanowires; and heating the first nanowires and metal catalyst and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that forms second silica nanowires extending from the first nanowires.
  • said first nanowires may be silica nanowires.
  • the process may include forming the first nanowires using a metal catalyst.
  • At least one of the metal catalyst used to form the first nanowires and the metal catalyst used to form the second nanowires may be an optically active metal so that at least one of said first nanowires and said second nanowires is photoluminescent.
  • the optically active metal may be Er.
  • the first nanowires may be attached to a silicon substrate, and said reaction of said oxygen with said silicon substrate forms volatile SiO, thereby etching said silicon substrate and at least partially separating said nanowires from said substrate.
  • the etching of the silicon substrate may completely separate said nanowires from said silicon substrate in the form of a thin film or sheet of said nanowires.
  • the present invention also provides a process for producing nanowires, including: adding a metal catalyst to silica nanowires attached to a silicon substrate; and heating the silica nanowires, silicon substrate and metal catalyst in an atmosphere containing oxygen and at a temperature such that said metal catalyst migrates to said silicon substrate and catalyses a reaction between said oxygen and said silicon substrate to form volatile SiO, thereby etching said silicon substrate and at least partially separating said silica nanowires from said substrate.
  • the etching of the silicon substrate may completely separate said nanowires from said silicon substrate in the form of a thin film or sheet of said nanowires.
  • the atmosphere may include a low partial pressure of oxygen in an inert carrier gas.
  • the low partial pressure may be about 3-5 ppm.
  • the metal catalyst may be Au.
  • the present invention also provides a process for producing nanowires, including: adding a metal catalyst to a non-silicon substrate; and heating the metal catalyst and a source of silicon in proximity to said metal catalyst in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that interacts with said metal catalyst on said substrate to form silica nanowires on said substrate.
  • the source of silicon may be a silicon wafer that is used to cap said substrate and metal catalyst during said heating.
  • the substrate may be a silica substrate.
  • the process may include: depositing a non-silica material over said first nanowires, and; heating the first nanowires and non-silica material and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that forms a cylindrical shell of silica over said non-silica material.
  • the process may include repeating steps (i) and (ii) one or more times to form multi-layer composite silica nanowires.
  • the non-silica material may include a layer of metal.
  • the non-silica material may include nanoparticles of metal.
  • the first nanowires may include pea-pod structures containing nanoparticles of metal.
  • Figure 1 is a schematic diagram illustrating prior art processes used to form silica nanowires on a silicon substrate;
  • Figure 2 is a plan-view electron microscope image of nanowires formed by a prior art process of Figure 1;
  • Figure 3 is a flow diagram of one embodiment of a process for producing nanowires
  • Figure 4 is a scanning electron microscope (SEM) image of nanowires formed by the first step of the process of Figure 3 where silica nanowires are formed on a (100) silicon substrate by depositing a 10 nm film of Au and heating the sample to HOO 0 C for 1 hour;
  • SEM scanning electron microscope
  • Figures 5 and 6 are SEM images of nanowires formed by the process of Figure 3 where Er is ion-implanted into silica nanowires that were previously formed on a silicon substrate using Au as the initial catalyst, showing the secondary growth of additional nanowires on the initially formed nanowires;
  • Figures 7 to 9 are photographs illustrating the separation of a sheet, film, or layer of nanowires from a silicon substrate in accordance with a second embodiment of the present invention
  • Figure 10 is a cross-sectional electron microscope image showing the etching of the silicon substrate under the nanowire layer of Figures 7 to 9;
  • Figures 11 and 12 are plan- view SEM images of nanowires produced in accordance with a third embodiment of the present invention, whereby silica nanowires are formed by annealing a substrate and metal catalyst capped with a silicon wafer;
  • Figures 13 and 14 are plan-view electron microscope images of etch pits produced on the (100) silicon capping wafer used to produce the nanowires of Figures 11 and 12;
  • Figure 15 is a phase diagram for the reaction between Si and O 2 as a function of temperature and oxygen partial pressure
  • Figure 16 is a schematic illustration of the coating of a nanowire with a metal layer
  • Figures 17 and 18 are plan-view and side-view SEM images of a silica nanowire coated with a thin layer of Er metal
  • Figure 19 is an SEM image of silica nanowires decorated with Au nanoparticles
  • Figure 20 is a graph of absorbance as a function of wavelength for both undecorated silica nanowires and the Au-decorated silica nanowires of Figure 19, showing a plasmon resonance at a wavelength of about 500 nm due to the presence of the Au nanoparticles;
  • Figures 21 and 22 are SEM images of Ni and Au nanoparticles, respectively, embedded in silica nanowires to provide 'pea-pod' nanostructures;
  • Figure 23 is a series of SEM images of silica nanowires as a function of annealing time, illustrating the radial growth of the nanowires with continued thermal processing;
  • Figure 24 is a graph showing the linear increase in diameter of the silica nanowires of Figure 23 as a function of annealing time;
  • Figure 25 is a schematic diagram illustrating how the deposition of silica can be combined with metal deposition to form coaxial silica nanowires with buried cylindrical metal shells;
  • Figure 26 is a cross-sectional electron microscope image of a coaxial nanowire of the form shown in Figure 25, incorporating a buried cylindrical shell of Er metal;
  • Figure 27 is a schematic diagram illustrating the result of a more complex combination of different processes to form a composite or hybrid coaxial nanowire incorporating a buried cylindrical metal shell and metal nanoparticles embedded within an outer silica cylindrical shell;
  • Figures 28 and 29 are SEM images of composite coaxial nanowires of the form shown in Figure 27, incorporating a buried cylindrical shell of Er metal and an outer cylindrical shell of silica in which Au nanoparticles are embedded.
  • a process 300 for producing silica nanowires begins at step 302 by forming nanowires using essentially any suitable process, including prior art processes such as those described above.
  • a metal catalyst is first either deposited as a thin film or as colloidal particles on a substrate, or alternatively is ion- implanted into the near-surface of a substrate.
  • the substrate is typically but not necessarily silicon, as described further below.
  • the substrate and metal are then heated in a nominally 'high-purity' Ar or N 2 ambient, but under conditions that produce volatile SiO.
  • silica nanowire growth is mediated by SiO vapour produced by a reaction between residual oxygen (typically present at about 3-5 ppm) and the silicon substrate (or other source of silicon, as described further below), in accordance with the phase diagram of Figure 15.
  • suitable catalysts for silica nanowire growth or deposition include one or more of the metals Pd, Au, Er, Ni, Pt, Co, Ga, and Fe. Other metals may also be suitable.
  • a suitable metal catalyst (which may be the same as or different to the catalyst used to form the initial nano wires at step 302, except that Au is not used for the secondary growth, as discussed below) is either deposited onto the surfaces of the nano wires (e.g., by physical vapour deposition), or is ion implanted into the surfaces of the nanowires.
  • the coated or implanted nanowires are then heated under conditions that are the same as or similar to those used for the initial nanowire growth at step 302 to generate volatile SiO and thereby promote silica nanowire growth.
  • This second heating step 306 causes the deposited or implanted metal catalyst to diffuse and form islands on the surfaces of the nanowires. These islands then act as nuclei and growth media for secondary nanowire growth from the original nanowires.
  • the nanowires can be doped with one or more optically active metallic impurities (such as Er) that are also catalytically active for silica growth by using one or more of those metals as the catalysts for the first stage 302 and/or the second stage 306 growth.
  • silica nanowires were formed on a silicon substrate by depositing a 10 run Au metal catalyst film onto the substrate and heating it to a temperature of about 1100 0 C for about 1 hour in a nominally inert ambient such as N 2 or Ar, but containing trace (e.g., about 3-5 ppm) amounts of O 2 .
  • Figure 4 is a scanning electron microscope (SEM) image illustrating the resulting silica nanowires.
  • the initial nanowires were then themselves implanted with Erbium (Er) using ErO " ions at an energy of about 30 keV and to fluences in the range from about IxIO 15 to 2xlO 16 Er cm "2 .
  • the implanted nanowires were then heated in an Ar or N 2 ambient containing 3-5 ppm of O 2 to a temperature of about 1100 ° C for about 10-80 minutes to generate secondary nanowires from the initial nanowires.
  • Figure 5 is a representative scanning electron microscope (SEM) image of the resulting nanowires, showing the secondary growth of short nanowires on the longer initial nanowires formed during the first heating step, giving them a 'hairy' appearance.
  • Figure 6 is a higher magnification image showing more detail of the secondary nanowires formed on one of the primary or first stage nanowires.
  • nanowires formed by such multistage growth processes have a complex morphology with a considerably increased surface- to-volume ratio and surface area relative to the original nanowires formed by the initial heating step 302, as shown in Figure 4.
  • the nanowires with secondary growth also exhibit photoluminescence from the Er incorporated within them. More generally, it will be apparent to those skilled in the art that the use of an optically active metal as the catalyst for silica nanowire growth is generally applicable to the production of luminescent silica nanowires, and is not limited to secondary nanowire growth.
  • the initial nanowires formed at step 302 in the described embodiments are silica nanowires, it will be apparent to those skilled in the art that nanowires of other compositions can be used (providing that they do not melt at the silica deposition/growth temperatures of about 1000 0 C or higher), with the secondary nanowires composed of silica growing from the initial non-silica nanowires via the catalyst and volatile SiO-mediated growth processes described herein.
  • silica nanowire growth is mediated by SiO vapour produced by a reaction between the low partial pressure of oxygen (typically present at about 3-5 ppm) and a source of silicon (typically, but not necessarily, a silicon substrate), in accordance with the phase diagram of Figure 15.
  • the low partial pressure of oxygen typically present at about 3-5 ppm
  • a source of silicon typically, but not necessarily, a silicon substrate
  • the delamination is often found to be localised and hence only partial separation is usually achieved.
  • the second heating step 306 is performed under conditions that cause the additional catalyst to migrate along the nano wires to the surface of the silicon substrate, the presence of the catalyst at the attachment locations between the silicon substrate and the silica nanowires causes extensive etching of the silicon substrate at the bases of the nanowires, resulting in complete delamination of the nanowires from the silicon substrate to provide a self-supporting thin film, sheet, or layer of nanowires.
  • the extent of secondary nano wire growth depends on the specific annealing conditions and materials involved.
  • the second catalyst and the annealing temperature can be selected so that the annealing temperature is above the melting point of the metal catalyst, so that the catalyst will be molten and hence migrate rapidly.
  • the annealing temperature is above the melting point of the metal catalyst, so that the catalyst will be molten and hence migrate rapidly.
  • bulk Au melts at about 1063 0 C, and can form a liquid Au:Si eutectic phase at temperatures as low as 365 0 C, and hence should be molten at an annealing temperature of 1100 0 C. Accordingly, when Au metal is used, the delamination is complete. However, in the case of Au the secondary nanowire growth is suppressed due to the metallic Au reacting with Si to form a Au:Si eutectic phase.
  • nanowires were grown on a silicon wafer by depositing a 10 nm Au layer onto the wafer and heating it to HOO 0 C in a N 2 ambient containing trace amounts of O 2 , as described above. Complete separation of the nanowire layer from the substrate was then achieved by depositing a second 10 nm Au layer onto the resulting nanowire layer and then re-annealing the sample under the same conditions employed for the initial growth.
  • Figures 7 to 9 are photographs illustrating the separation of the layer of nano wires from the silicon substrate to provide a large-area (in this example, ⁇ lcm 2 ), self-supporting layer or film of nanowires.
  • Figure 10 is a cross-sectional scanning electron microscope image showing the etching of the silicon substrate under the nanowire layer prior to separation.
  • Self-supporting nanowire films formed by the processes described herein can be used for a wide variety of applications.
  • the films when removed from the underlying rigid silicon substrate, the films are flexible and can be formed into various shapes and/or attached to non-planar substrates or other objects, which would not be possible otherwise. They can also be integrated into other devices and structures without further high-temperature processing.
  • the films are porous and could, for example, be stacked into a gas or fluid sensor, either to provide an overall increased surface area (possibly in the form of a quasi-three dimensional network of nanowires), or, where films of different nanowire compositions or differently processed nanowires are stacked to provide, for example, different sensitivity ranges and/or sensitivities to different species in the one sensor.
  • silica is advantageous as a sensor material, at least because it is essentially inert, is easily functionalised, and can be made optically active by the addition of optically active impurities.
  • the etching process described above is not used to etch the substrate on which the nanowires are grown, but rather to etch another silicon sample in close proximity to the substrate. This allows silica nanowires to be grown on substrates other than silicon.
  • silica nanowires can be grown on a substrate which may be silica (but equally can be essentially any material that does not react substantially with the metal catalyst or ambient) by depositing a 10 nm thick layer of catalyst on the silica (or other composition) wafer, capping this with a silicon wafer superstrate (which may be of (100) orientation), and then annealing the substrate-superstrate pair as described above.
  • the capping wafer is the source of Si that forms the volatile SiO that is transported in the gas phase to be deposited on the silica (or other) substrate via the catalyst.
  • FIGS. 11 and 12 are plan- view electron microscope images of the Si capping wafer or superstrate after the heating step, showing the etch pits formed in a (100) silicon wafer. Etching is also observed on other orientations of silicon, with the etch pits having a different symmetry that corresponds to the substrate orientation.
  • Silica nanowires formed by any of the processes described above can be modified by a wide variety of processes involving other materials.
  • silica nanowires are coated with a metal film by a standard deposition process such as thermal evaporation or sputter deposition, or with a metal-oxide film by first coating the nanowires with a metal film by a standard deposition process such as thermal evaporation or sputter deposition, and then subjecting them to a high temperature (e.g., 900 to HOO 0 C) anneal in O 2 , as shown schematically in Figure 16.
  • a high temperature e.g., 900 to HOO 0 C
  • Figures 17 and 18 are cross-sectional and side views of a silica nanowire formed as described above, coated with a 30 nm layer of erbium (Er) metal by sputtering, and then annealed for 1 hour at HOO 0 C in O 2 to form Er 2 O 3
  • Selected area x-ray florescence data confirms that the cylindrical shells coating the silica nanowires contain erbium.
  • Erbium oxide exhibits characteristic luminescence at a wavelength of 1.5 micrometres. The intensity of this luminescence is affected by chemical adsorption on the nanowire surface making it useful as a potential chemical sensor.
  • metallic nanoparticles are formed on the surfaces of nanowires by depositing metal on the nanowires (e.g., by thermal evaporation or sputtering), and then heating the metal coated nanowires at high temperatures (e.g., 900 to 1100 0 C) in an O 2 , Ar, or N 2 ambient to form metal islands, thereby providing metal decorated nanowires.
  • Figure 19 is an SEM image of silica nanowires formed as described above, which were then coated with a 10 nm layer of gold (Au) by thermal evaporation and then heated for 1 hour at 900 0 C in a N 2 ambient.
  • the nanowires in this particular example have a diameter ranging from 50 to 350 nanometres, the gold nanoparticles having diameters ranging from about 10 to 150 nm.
  • nanowires are formed with 'pea-pod' structures in which mutually spaced metal nanoparticles are distributed within and along the longitudinal axes of nanowires, as shown in Figures 21 and 22.
  • Figure 21 is an electron microscope image showing Ni nanoparticles embedded within a silica nanowire formed by depositing a IOnm Ni film onto a fused silica (SiO 2 ) substrate and capping it with a silicon wafer before annealing at HOO 0 C for 1 hour in nitrogen. This capping growth stage promotes a growth regime whereby the Ni nanoparticles remain at the tips of the growing silica nanowires.
  • Figure 22 is an electron microscope image showing Au nanoparticles of about 40.5 nanometre diameter spaced apart by about 92.2 nm in a silica nanowire formed by depositing a IOnm Au film on a silicon substrate and capping it with a silicon wafer before annealing at 1100 0 C for 1 hour in nitrogen.
  • the silicon capping layer was removed and the sample annealed for a further 20 minutes at HOO 0 C in nitrogen. Once formed, continued heating of silica nanowires under the same conditions increases their diameter by way of an absorption reaction involving volatile SiO.
  • Figure 23 is a sequence of SEM images (all at the same magnification) of silica nanowires for annealing times of 20, 40, 60, and 120 minutes, showing the increasing thickness of the nanowires with annealing time.
  • Figure 24 is a graph showing the diameter of the silica nanowires as a function of annealing time, showing that the growth in diameter is linear with time, and in this case indicating that the effective atomic flux being absorbed at the surface of the nanowires is about 2*10 14 cm "2 s "1 .
  • silica nanowires can be combined with the deposition of other materials on the nanowires to form composite or multi-layer nanowire structures that would not otherwise be possible.
  • other materials can be deposited or grown on the surface of nanowires, and resulting structure can be encased in silica by exposing the coated nanowires to SiO vapour, as described above.
  • this radial growth/deposition can be combined with processes such as those described above for coating nanowires with metals, forming nanoparticles on the surface of nanowires, and forming nanowires with embedded metal nanoparticles can be combined in essentially arbitrary combinations with lateral growth processes to form multi-layer nanowires.
  • the initial nanowires need not even be composed of silica.
  • Figure 25 is a schematic diagram illustrating how a nanowire A can be coated with a continuous metal layer to produce a coated nanowire B, and the radial silica growth/deposition processes described above used to deposit a cylindrical shell of silica over the deposited metal layer to provide the composite multi-layer nanowire structure C.
  • Figure 26 is an SEM image of a silica nanowire coated with a 30 nanometre sputter-deposited film of Er, and subsequently coated in silica by annealing the sample for 1 hour at HOO 0 C in N 2 with trace amounts of O 2 , as described above.
  • More complex structures can also be formed, such as the structure illustrated schematically in Figure 27.
  • This Figure shows a silica nanowire core 2702 coated with a continuous layer of a metal 2704 which was subsequently coated with a second metal of lower melting point, and the entire sample then subjected to thermal processing under conditions that provide radial growth of silica to form a composite or hybrid coaxial nanowire structure 2700.
  • Figures 28 and 29 are SEM images of a composite coaxially nanowire formed by coating initial silica nanowires formed as described above with a 30 nm layer of Er, followed by a 1 hour thermal anneal at HOO 0 C (in an O 2 ambient to inhibit radial growth), followed by deposition of a 10 nm layer of Au, followed by a 1 hour anneal at 1100 0 C in N 2 with trace amounts of O 2 to simultaneously cause the Au to form nanoscale islands, and also to induce radial deposition and growth of silica to provide a hybrid coaxial nanowire of the form shown in Figure 27. It will be apparent to those skilled in the art that a wide variety of such modifications and combinations can be used to form a wide variety of composite coaxial nanostructures.
  • the processes described above can be used to form different types of nanowire structures in different regions of a single substrate or self-supporting thin film (e.g., arranged in an array or checkerboard layout).
  • self-supporting thin films these can also be stacked as described above to provide a three-dimensional array of different nanowire compositions, structures, or functions (e.g., by selective functionalisation).
  • the embodiments described above provide substantial advantages over the prior art, including (i) the ability to provide a substantially increased total surface area of nanowires, (ii) the ability to grow nanowires on substrates where the constituents of the nanowires do not come from the substrate, (iii) the ability to produce self-supporting and flexible sheets, films, or layers of nanowires, and (iv) the ability to form composite or multi-layer nanowire structures, in some cases with at least outer silica shells. Additionally, if an optically active metal such as Erbium is used as a metal catalyst for nanowire growth, the resulting nanowires are luminescent, with the luminescence depending on the environment.
  • an optically active metal such as Erbium

Abstract

A process for producing silica nanowires, including adding a metal catalyst to first nanowires; and heating the first nanowires and metal catalyst and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of the oxygen with the silicon forms volatile SiO that forms second silica nanowires extending from the first nanowires.

Description

A PROCESS FOR PRODUCING SILICA NANOWIRES
TECHNICAL FIELD
The present invention relates to a process for producing silica nanowires.
BACKGROUND
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 the 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.
Silica nanowires have enormous potential as functional materials for photonics and chemical sensing, and can be grown on various substrates by metal-catalyzed vapour- liquid-solid or vapour-solid-solid processes. In these processes, a metal catalyst promotes the growth of the nanowires by dissolving gas-phase reactants and acting as a transport medium for the deposition of the nanowire constituents. For example, a metal, typically Au, Pt, Pd, or Fe, acts as a catalyst for the growth of silica nanowires by dissolving gas- phase reactants containing silicon and oxygen, and transporting them through the metal to deposit SiO2 or Si-rich SiO2 wires.
In practice, this is achieved as follows. As shown in Figure I5 a metal catalyst is first either deposited as a thin film 102 or as colloidal particles 104 on a substrate 106, or alternatively is ion-implanted 108 into the near-surface of a substrate 106. Irrespective of how the metal is added to the substrate 106, subsequent annealing 112 then causes the metal to form a high density of small diameter (typically < 200 nm) islands 114 on the surface of the substrate 106, each island 114 acting as a 'seed' for the growth of one or more nanowires 116 from the surface of the substrate 106. The result is a substrate (typically, as shown, being a silicon wafer) densely populated with nanowires 116, as shown in the plan- view scanning electron microscope image of Figure 2, each nanowire having a metal catalyst particle 118 at its base or at its tip.
An important feature of these nano wires 116 is their large surface-to-volume ratio, a characteristic that provides the basis for many applications, including sensing. Any increase in this ratio would therefore be desirable. In addition, the rigid and relatively thick wafers on which the nanowires are grown may not be convenient for many applications or potential applications of nanowires, and may even preclude others.
It is desired to provide a process for producing silica nanowires that alleviates one or more difficulties of the prior art, or at least provides a useful alternative.
SUMMARY
In accordance with the present invention, there is provided a process for producing silica nanowires, including: adding a metal catalyst to first nanowires; and heating the first nanowires and metal catalyst and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that forms second silica nanowires extending from the first nanowires.
Advantageously, said first nanowires may be silica nanowires.
Advantageously, the process may include forming the first nanowires using a metal catalyst.
At least one of the metal catalyst used to form the first nanowires and the metal catalyst used to form the second nanowires may be an optically active metal so that at least one of said first nanowires and said second nanowires is photoluminescent. The optically active metal may be Er. The first nanowires may be attached to a silicon substrate, and said reaction of said oxygen with said silicon substrate forms volatile SiO, thereby etching said silicon substrate and at least partially separating said nanowires from said substrate.
The etching of the silicon substrate may completely separate said nanowires from said silicon substrate in the form of a thin film or sheet of said nanowires.
The present invention also provides a process for producing nanowires, including: adding a metal catalyst to silica nanowires attached to a silicon substrate; and heating the silica nanowires, silicon substrate and metal catalyst in an atmosphere containing oxygen and at a temperature such that said metal catalyst migrates to said silicon substrate and catalyses a reaction between said oxygen and said silicon substrate to form volatile SiO, thereby etching said silicon substrate and at least partially separating said silica nanowires from said substrate.
The etching of the silicon substrate may completely separate said nanowires from said silicon substrate in the form of a thin film or sheet of said nanowires.
The atmosphere may include a low partial pressure of oxygen in an inert carrier gas. The low partial pressure may be about 3-5 ppm.
The metal catalyst may be Au.
The present invention also provides a process for producing nanowires, including: adding a metal catalyst to a non-silicon substrate; and heating the metal catalyst and a source of silicon in proximity to said metal catalyst in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that interacts with said metal catalyst on said substrate to form silica nanowires on said substrate. The source of silicon may be a silicon wafer that is used to cap said substrate and metal catalyst during said heating. The substrate may be a silica substrate.
The process may include: depositing a non-silica material over said first nanowires, and; heating the first nanowires and non-silica material and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that forms a cylindrical shell of silica over said non-silica material.
The process may include repeating steps (i) and (ii) one or more times to form multi-layer composite silica nanowires.
At least two different non-silica materials may be used in respective instances of steps (i) and (ii). The non-silica material may include a layer of metal. The non-silica material may include nanoparticles of metal.
The first nanowires may include pea-pod structures containing nanoparticles of metal.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention are hereinafter described, by way of example only, with reference to the accompanying drawings, wherein:
Figure 1 is a schematic diagram illustrating prior art processes used to form silica nanowires on a silicon substrate; Figure 2 is a plan-view electron microscope image of nanowires formed by a prior art process of Figure 1;
Figure 3 is a flow diagram of one embodiment of a process for producing nanowires;
Figure 4 is a scanning electron microscope (SEM) image of nanowires formed by the first step of the process of Figure 3 where silica nanowires are formed on a (100) silicon substrate by depositing a 10 nm film of Au and heating the sample to HOO0C for 1 hour;
Figures 5 and 6 are SEM images of nanowires formed by the process of Figure 3 where Er is ion-implanted into silica nanowires that were previously formed on a silicon substrate using Au as the initial catalyst, showing the secondary growth of additional nanowires on the initially formed nanowires;
Figures 7 to 9 are photographs illustrating the separation of a sheet, film, or layer of nanowires from a silicon substrate in accordance with a second embodiment of the present invention; Figure 10 is a cross-sectional electron microscope image showing the etching of the silicon substrate under the nanowire layer of Figures 7 to 9;
Figures 11 and 12 are plan- view SEM images of nanowires produced in accordance with a third embodiment of the present invention, whereby silica nanowires are formed by annealing a substrate and metal catalyst capped with a silicon wafer; Figures 13 and 14 are plan-view electron microscope images of etch pits produced on the (100) silicon capping wafer used to produce the nanowires of Figures 11 and 12;
Figure 15 is a phase diagram for the reaction between Si and O2 as a function of temperature and oxygen partial pressure;
Figure 16 is a schematic illustration of the coating of a nanowire with a metal layer; Figures 17 and 18 are plan-view and side-view SEM images of a silica nanowire coated with a thin layer of Er metal;
Figure 19 is an SEM image of silica nanowires decorated with Au nanoparticles;
Figure 20 is a graph of absorbance as a function of wavelength for both undecorated silica nanowires and the Au-decorated silica nanowires of Figure 19, showing a plasmon resonance at a wavelength of about 500 nm due to the presence of the Au nanoparticles;
Figures 21 and 22 are SEM images of Ni and Au nanoparticles, respectively, embedded in silica nanowires to provide 'pea-pod' nanostructures;
Figure 23 is a series of SEM images of silica nanowires as a function of annealing time, illustrating the radial growth of the nanowires with continued thermal processing; Figure 24 is a graph showing the linear increase in diameter of the silica nanowires of Figure 23 as a function of annealing time;
Figure 25 is a schematic diagram illustrating how the deposition of silica can be combined with metal deposition to form coaxial silica nanowires with buried cylindrical metal shells;
Figure 26 is a cross-sectional electron microscope image of a coaxial nanowire of the form shown in Figure 25, incorporating a buried cylindrical shell of Er metal;
Figure 27 is a schematic diagram illustrating the result of a more complex combination of different processes to form a composite or hybrid coaxial nanowire incorporating a buried cylindrical metal shell and metal nanoparticles embedded within an outer silica cylindrical shell; and
Figures 28 and 29 are SEM images of composite coaxial nanowires of the form shown in Figure 27, incorporating a buried cylindrical shell of Er metal and an outer cylindrical shell of silica in which Au nanoparticles are embedded.
DETAILED DESCRIPTION
As shown in Figure 3, a process 300 for producing silica nanowires begins at step 302 by forming nanowires using essentially any suitable process, including prior art processes such as those described above. In the described embodiments, a metal catalyst is first either deposited as a thin film or as colloidal particles on a substrate, or alternatively is ion- implanted into the near-surface of a substrate. The substrate is typically but not necessarily silicon, as described further below. The substrate and metal are then heated in a nominally 'high-purity' Ar or N2 ambient, but under conditions that produce volatile SiO. This can be achieved by including a low partial pressure of oxygen in the otherwise nominally inert ambient, and heating the sample to temperatures above about 1000°C (e.g., in the range of about 1000-HOO0C) for times of about 10-60 minutes. Under these conditions, silica nanowire growth is mediated by SiO vapour produced by a reaction between residual oxygen (typically present at about 3-5 ppm) and the silicon substrate (or other source of silicon, as described further below), in accordance with the phase diagram of Figure 15. In general, suitable catalysts for silica nanowire growth or deposition include one or more of the metals Pd, Au, Er, Ni, Pt, Co, Ga, and Fe. Other metals may also be suitable.
At step 304, a suitable metal catalyst (which may be the same as or different to the catalyst used to form the initial nano wires at step 302, except that Au is not used for the secondary growth, as discussed below) is either deposited onto the surfaces of the nano wires (e.g., by physical vapour deposition), or is ion implanted into the surfaces of the nanowires. At step 306, the coated or implanted nanowires are then heated under conditions that are the same as or similar to those used for the initial nanowire growth at step 302 to generate volatile SiO and thereby promote silica nanowire growth.
This second heating step 306 causes the deposited or implanted metal catalyst to diffuse and form islands on the surfaces of the nanowires. These islands then act as nuclei and growth media for secondary nanowire growth from the original nanowires. The nanowires can be doped with one or more optically active metallic impurities (such as Er) that are also catalytically active for silica growth by using one or more of those metals as the catalysts for the first stage 302 and/or the second stage 306 growth.
In one example, silica nanowires were formed on a silicon substrate by depositing a 10 run Au metal catalyst film onto the substrate and heating it to a temperature of about 11000C for about 1 hour in a nominally inert ambient such as N2 or Ar, but containing trace (e.g., about 3-5 ppm) amounts of O2. Figure 4 is a scanning electron microscope (SEM) image illustrating the resulting silica nanowires.
The initial nanowires were then themselves implanted with Erbium (Er) using ErO" ions at an energy of about 30 keV and to fluences in the range from about IxIO15 to 2xlO16 Er cm"2. The implanted nanowires were then heated in an Ar or N2 ambient containing 3-5 ppm of O2 to a temperature of about 1100°C for about 10-80 minutes to generate secondary nanowires from the initial nanowires. Figure 5 is a representative scanning electron microscope (SEM) image of the resulting nanowires, showing the secondary growth of short nanowires on the longer initial nanowires formed during the first heating step, giving them a 'hairy' appearance. Figure 6 is a higher magnification image showing more detail of the secondary nanowires formed on one of the primary or first stage nanowires. Clearly, nanowires formed by such multistage growth processes have a complex morphology with a considerably increased surface- to-volume ratio and surface area relative to the original nanowires formed by the initial heating step 302, as shown in Figure 4. The nanowires with secondary growth also exhibit photoluminescence from the Er incorporated within them. More generally, it will be apparent to those skilled in the art that the use of an optically active metal as the catalyst for silica nanowire growth is generally applicable to the production of luminescent silica nanowires, and is not limited to secondary nanowire growth.
Although the initial nanowires formed at step 302 in the described embodiments are silica nanowires, it will be apparent to those skilled in the art that nanowires of other compositions can be used (providing that they do not melt at the silica deposition/growth temperatures of about 10000C or higher), with the secondary nanowires composed of silica growing from the initial non-silica nanowires via the catalyst and volatile SiO-mediated growth processes described herein.
As described above, in the described embodiments silica nanowire growth is mediated by SiO vapour produced by a reaction between the low partial pressure of oxygen (typically present at about 3-5 ppm) and a source of silicon (typically, but not necessarily, a silicon substrate), in accordance with the phase diagram of Figure 15. This is a heterogeneous process that causes etching of the silicon via the reaction 2Si+O2 -> 2SiO (rather than the formation of stable SiO2 via the reaction Si + O2 -ϊ SiO2), thereby forming facetted etch- pits in the silicon surface simultaneously with the growth of silica nanowires, and, with sufficient thermal processing and where the source of silicon is a silicon substrate, typically resulting in at least partial delamination of the nanowires from the silicon substrate. However, under the conditions described above, the delamination is often found to be localised and hence only partial separation is usually achieved.
However, if the second heating step 306 is performed under conditions that cause the additional catalyst to migrate along the nano wires to the surface of the silicon substrate, the presence of the catalyst at the attachment locations between the silicon substrate and the silica nanowires causes extensive etching of the silicon substrate at the bases of the nanowires, resulting in complete delamination of the nanowires from the silicon substrate to provide a self-supporting thin film, sheet, or layer of nanowires. As will be appreciated by those skilled in the art, the extent of secondary nano wire growth depends on the specific annealing conditions and materials involved. In order to ensure that the second catalyst migrates to the surface of the silicon substrate, the second catalyst and the annealing temperature can be selected so that the annealing temperature is above the melting point of the metal catalyst, so that the catalyst will be molten and hence migrate rapidly. For example, bulk Au melts at about 10630C, and can form a liquid Au:Si eutectic phase at temperatures as low as 3650C, and hence should be molten at an annealing temperature of 1100 0C. Accordingly, when Au metal is used, the delamination is complete. However, in the case of Au the secondary nanowire growth is suppressed due to the metallic Au reacting with Si to form a Au:Si eutectic phase. Conversely, metallic erbium (Er, bulk melting temperature 1522 0C) is not transported to the silicon surface under these conditions. Consequently, when a metal catalyst is used to promote further silica growth but under conditions such that the metal does not diffuse or otherwise travel to the surface of the silicon substrate, only partial delamination usually occurs.
In one example, nanowires were grown on a silicon wafer by depositing a 10 nm Au layer onto the wafer and heating it to HOO0C in a N2 ambient containing trace amounts of O2, as described above. Complete separation of the nanowire layer from the substrate was then achieved by depositing a second 10 nm Au layer onto the resulting nanowire layer and then re-annealing the sample under the same conditions employed for the initial growth. Figures 7 to 9 are photographs illustrating the separation of the layer of nano wires from the silicon substrate to provide a large-area (in this example, ~lcm2), self-supporting layer or film of nanowires. Under these conditions, the delamination is complete, so that the layer of nanowires slides off the substrate when the sample is tilted, without any need to apply additional force to the layer. Figure 10 is a cross-sectional scanning electron microscope image showing the etching of the silicon substrate under the nanowire layer prior to separation.
In the above example, despite Au being both the initial and the second catalyst, complete delamination does not occur under comparable annealing conditions in the absence of the secondary Au deposition. It is believed that this is due to Au nanoparticles remaining on the silicon surface being coated in silica, thereby inhibiting their catalytic function. Hence the silicon etching process is accelerated by providing a fresh source of metal catalyst at the silicon surface during the second heating step.
Self-supporting nanowire films formed by the processes described herein can be used for a wide variety of applications. In particular, when removed from the underlying rigid silicon substrate, the films are flexible and can be formed into various shapes and/or attached to non-planar substrates or other objects, which would not be possible otherwise. They can also be integrated into other devices and structures without further high-temperature processing. Furthermore, the films are porous and could, for example, be stacked into a gas or fluid sensor, either to provide an overall increased surface area (possibly in the form of a quasi-three dimensional network of nanowires), or, where films of different nanowire compositions or differently processed nanowires are stacked to provide, for example, different sensitivity ranges and/or sensitivities to different species in the one sensor. In this respect, silica is advantageous as a sensor material, at least because it is essentially inert, is easily functionalised, and can be made optically active by the addition of optically active impurities. In yet another embodiment, the etching process described above is not used to etch the substrate on which the nanowires are grown, but rather to etch another silicon sample in close proximity to the substrate. This allows silica nanowires to be grown on substrates other than silicon. For example, silica nanowires can be grown on a substrate which may be silica (but equally can be essentially any material that does not react substantially with the metal catalyst or ambient) by depositing a 10 nm thick layer of catalyst on the silica (or other composition) wafer, capping this with a silicon wafer superstrate (which may be of (100) orientation), and then annealing the substrate-superstrate pair as described above. In this embodiment, the capping wafer is the source of Si that forms the volatile SiO that is transported in the gas phase to be deposited on the silica (or other) substrate via the catalyst.
For example, such nanowires are shown in the SEM images of Figures 11 and 12 (the latter showing Au catalyst particles at the ends of the silica nanowires). Figures 13 and 14 are plan- view electron microscope images of the Si capping wafer or superstrate after the heating step, showing the etch pits formed in a (100) silicon wafer. Etching is also observed on other orientations of silicon, with the etch pits having a different symmetry that corresponds to the substrate orientation.
Silica nanowires formed by any of the processes described above can be modified by a wide variety of processes involving other materials.
In one embodiment, silica nanowires are coated with a metal film by a standard deposition process such as thermal evaporation or sputter deposition, or with a metal-oxide film by first coating the nanowires with a metal film by a standard deposition process such as thermal evaporation or sputter deposition, and then subjecting them to a high temperature (e.g., 900 to HOO0C) anneal in O2, as shown schematically in Figure 16. For example, Figures 17 and 18 are cross-sectional and side views of a silica nanowire formed as described above, coated with a 30 nm layer of erbium (Er) metal by sputtering, and then annealed for 1 hour at HOO0C in O2 to form Er2O3 Selected area x-ray florescence data (not shown) confirms that the cylindrical shells coating the silica nanowires contain erbium. Erbium oxide exhibits characteristic luminescence at a wavelength of 1.5 micrometres. The intensity of this luminescence is affected by chemical adsorption on the nanowire surface making it useful as a potential chemical sensor.
In another embodiment, metallic nanoparticles are formed on the surfaces of nanowires by depositing metal on the nanowires (e.g., by thermal evaporation or sputtering), and then heating the metal coated nanowires at high temperatures (e.g., 900 to 11000C) in an O2, Ar, or N2 ambient to form metal islands, thereby providing metal decorated nanowires. For example, Figure 19 is an SEM image of silica nanowires formed as described above, which were then coated with a 10 nm layer of gold (Au) by thermal evaporation and then heated for 1 hour at 9000C in a N2 ambient. The nanowires in this particular example have a diameter ranging from 50 to 350 nanometres, the gold nanoparticles having diameters ranging from about 10 to 150 nm.
In yet another embodiment, nanowires are formed with 'pea-pod' structures in which mutually spaced metal nanoparticles are distributed within and along the longitudinal axes of nanowires, as shown in Figures 21 and 22. For example, Figure 21 is an electron microscope image showing Ni nanoparticles embedded within a silica nanowire formed by depositing a IOnm Ni film onto a fused silica (SiO2) substrate and capping it with a silicon wafer before annealing at HOO0C for 1 hour in nitrogen. This capping growth stage promotes a growth regime whereby the Ni nanoparticles remain at the tips of the growing silica nanowires. The silicon capping layer was then removed and the sample annealed for a further 20 minutes at 11000C in nitrogen, which continues the silica nanowire growth and forms the embedded metal beads. Similarly, Figure 22 is an electron microscope image showing Au nanoparticles of about 40.5 nanometre diameter spaced apart by about 92.2 nm in a silica nanowire formed by depositing a IOnm Au film on a silicon substrate and capping it with a silicon wafer before annealing at 11000C for 1 hour in nitrogen. The silicon capping layer was removed and the sample annealed for a further 20 minutes at HOO0C in nitrogen. Once formed, continued heating of silica nanowires under the same conditions increases their diameter by way of an absorption reaction involving volatile SiO. For example, Figure 23 is a sequence of SEM images (all at the same magnification) of silica nanowires for annealing times of 20, 40, 60, and 120 minutes, showing the increasing thickness of the nanowires with annealing time. Figure 24 is a graph showing the diameter of the silica nanowires as a function of annealing time, showing that the growth in diameter is linear with time, and in this case indicating that the effective atomic flux being absorbed at the surface of the nanowires is about 2*1014 cm"2 s"1.
The radial growth of silica nanowires can be combined with the deposition of other materials on the nanowires to form composite or multi-layer nanowire structures that would not otherwise be possible. In particular, other materials can be deposited or grown on the surface of nanowires, and resulting structure can be encased in silica by exposing the coated nanowires to SiO vapour, as described above.
For example, this radial growth/deposition can be combined with processes such as those described above for coating nanowires with metals, forming nanoparticles on the surface of nanowires, and forming nanowires with embedded metal nanoparticles can be combined in essentially arbitrary combinations with lateral growth processes to form multi-layer nanowires. Moreover, it will be apparent that the initial nanowires need not even be composed of silica.
In perhaps the most straightforward example of such a process, Figure 25 is a schematic diagram illustrating how a nanowire A can be coated with a continuous metal layer to produce a coated nanowire B, and the radial silica growth/deposition processes described above used to deposit a cylindrical shell of silica over the deposited metal layer to provide the composite multi-layer nanowire structure C. Figure 26 is an SEM image of a silica nanowire coated with a 30 nanometre sputter-deposited film of Er, and subsequently coated in silica by annealing the sample for 1 hour at HOO0C in N2 with trace amounts of O2, as described above.
More complex structures can also be formed, such as the structure illustrated schematically in Figure 27. This Figure shows a silica nanowire core 2702 coated with a continuous layer of a metal 2704 which was subsequently coated with a second metal of lower melting point, and the entire sample then subjected to thermal processing under conditions that provide radial growth of silica to form a composite or hybrid coaxial nanowire structure 2700.
For example, Figures 28 and 29 are SEM images of a composite coaxially nanowire formed by coating initial silica nanowires formed as described above with a 30 nm layer of Er, followed by a 1 hour thermal anneal at HOO0C (in an O2 ambient to inhibit radial growth), followed by deposition of a 10 nm layer of Au, followed by a 1 hour anneal at 11000C in N2 with trace amounts of O2 to simultaneously cause the Au to form nanoscale islands, and also to induce radial deposition and growth of silica to provide a hybrid coaxial nanowire of the form shown in Figure 27. It will be apparent to those skilled in the art that a wide variety of such modifications and combinations can be used to form a wide variety of composite coaxial nanostructures.
Moreover, with the addition of selective masking during deposition and/or ion implantation, the processes described above can be used to form different types of nanowire structures in different regions of a single substrate or self-supporting thin film (e.g., arranged in an array or checkerboard layout). In the case of self-supporting thin films, these can also be stacked as described above to provide a three-dimensional array of different nanowire compositions, structures, or functions (e.g., by selective functionalisation).
The embodiments described above provide substantial advantages over the prior art, including (i) the ability to provide a substantially increased total surface area of nanowires, (ii) the ability to grow nanowires on substrates where the constituents of the nanowires do not come from the substrate, (iii) the ability to produce self-supporting and flexible sheets, films, or layers of nanowires, and (iv) the ability to form composite or multi-layer nanowire structures, in some cases with at least outer silica shells. Additionally, if an optically active metal such as Erbium is used as a metal catalyst for nanowire growth, the resulting nanowires are luminescent, with the luminescence depending on the environment.
Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

CLAIMS:
1. A process for producing silica nano wires, including: adding a metal catalyst to first nanowires; and heating the first nanowires and metal catalyst and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that forms second silica nanowires extending from the first nanowires.
2. The process of claim 1, wherein said adding includes depositing said metal catalyst onto said first silica nanowires.
3. The process of claim 1, wherein said adding includes ion implanting said metal catalyst into said first nanowires.
4. The process of any one of claims 1 to 3, wherein said first nanowires are silica nanowires.
5. The process of any one of claims 1 to 4, including forming the first nanowires using a metal catalyst.
6. The process of claim 5, wherein at least one of the metal catalyst used to form the first nanowires and the metal catalyst used to form the second nanowires is an optically active metal so that at least one of said first nanowires and said second nanowires is photoluminescent.
7. The process of claim 6, wherein the optically active metal includes Er.
8. The process of any one of claims 5 to 7, wherein the metal catalyst used to form the first nanowires is a different metal to the metal catalyst used to form the second nano wires.
9. The process of any one of claims 1 to 8, wherein said first nanowires are attached to a silicon substrate, and said reaction of said oxygen with said silicon substrate forms volatile SiO, thereby etching said silicon substrate and at least partially separating said nanowires from said substrate.
10. The process of claim 9, wherein the etching of the silicon substrate completely separates said nanowires from said silicon substrate in the form of a thin film or sheet of said nanowires.
11. A process for producing nanowires, including: adding a metal catalyst to silica nanowires attached to a silicon substrate; and heating the silica nanowires, silicon substrate and metal catalyst in an atmosphere containing oxygen and at a temperature such that said metal catalyst migrates to said silicon substrate and catalyses a reaction between said oxygen and said silicon substrate to form volatile SiO, thereby etching said silicon substrate and at least partially separating said silica nanowires from said substrate.
12. The process of claim 11, wherein the etching of the silicon substrate completely separates said nanowires from said silicon substrate in the form of a thin film or sheet of said nanowires.
13. The process of any one of claims 1 to 12, wherein said atmosphere includes a low partial pressure of oxygen in an inert carrier gas.
14. The process of claim 13, wherein said low partial pressure is about 3-5 ppm.
15. The process of any one of claims 11 to 14, wherein said metal catalyst includes Au.
16. A film or sheet of silica nanowires produced by the process of any one of claims 11 to 15.
17. A process for producing nanowires, including: adding a metal catalyst to a non-silicon substrate; and heating the metal catalyst and a source of silicon in proximity to said metal catalyst in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that interacts with said metal catalyst on said substrate to form silica nanowires on said non-silicon substrate.
18. The process of claim 17, wherein said source of silicon is a silicon wafer that is used to cap said substrate and metal catalyst during said heating.
19. The process of claim 17 or 18, wherein said substrate is a silica substrate.
20. The process of any one of claims 1 to 19, including: (i) depositing a non-silica material over said first nanowires, and;
(ii) heating the first nanowires and non-silica material and a source of silicon in an atmosphere containing oxygen and under conditions such that a reaction of said oxygen with said silicon forms volatile SiO that forms a cylindrical shell of silica over said non-silica material.
21. The process of claim 20, including repeating steps (i) and (ii) one or more times to form multi-layer composite silica nanowires.
22. The process of claim 21, wherein at least two different non-silica materials are used in respective instances of steps (i) and (ii).
23. The process of any one of claims 20 to 22, wherein said non-silica material includes a layer of metal.
24. The process of any one of claims 20 to 23, wherein said non-silica material includes nanoparticles of metal.
25. The process of any one of claims 20 to 24, wherein said first nanowires include pea-pod structures containing nanoparticles of metal.
26. Nanowires produced by the process of any one of claims 1 to 25.
27. A sensor having an array or stack of sheets of nanowires, each sheet being formed by the process of any one of claims 9 to 15.
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