WO2016190818A1

WO2016190818A1 - Synthesis and application of tungsten chalcogenide hetero-structured nanomaterials

Info

Publication number: WO2016190818A1
Application number: PCT/SG2016/050249
Authority: WO
Inventors: Hua Zhang; Shikui HAN
Original assignee: Nanyang Technological University
Priority date: 2015-05-26
Filing date: 2016-05-26
Publication date: 2016-12-01

Abstract

The present invention demonstrates a facile method for the synthesis of tungsten chalcogenide hetero-structured nanomaterials in a colloidal solution process, wherein the tungsten precursor is a tungsten salt, a tungsten complex or a combination thereof, and the chalcogen precursor is a chalcogen element, a chalcogen compound, a chalcogen complex, or a metal chalcogenide. By modifying the reaction conditions as described herein, different forms of tungsten chalcogenide hetero-structured nanomaterials can be produced. The thus obtained nanomaterials may be employed in the economical and environmental friendly fabrication of electronic devices, such as organic photovoltaic devices, field emission transistors, gas sensors, and in particular light emitting diodes. As demonstrated for quantum dot light-emitting diodes, outstanding results with respect to thermal stability, luminescence, brightness, and overall performance can be achieved when replacing materials commonly employed for the assembling of hole transport layers or buffer layers in the fabrication of light-emitting diodes, such as PEDOTPSS, with the tungsten chalcogenide hetero- structured nanomaterials according to the present invention.

Description

SYNTHESIS AND APPLICATION OF TUNGSTEN CHALCOGENIDE HETERO-STRUCTURED

NANOMATERIALS

CROSS-REFERENCE TO RELATED APPLICATION

[001 ] The application claims the benefit of priority of Singapore Patent Application No. 10201504144R, filed May 26, 2015, the contents of which being hereby incorporated by reference in its entirety for all purposes.

FIELD OF THE TECHNOLOGY

[002] The present invention relates to a method for the preparation of tungsten chalcogenide hetero- structured nanomaterials in a colloidal solution process. Moreover, the present invention relates to a method of using the tungsten chalcogenide hetero-structured nanomaterial obtained from a method as disclosed herein, as well as to an electronic device, in particular a quantum dot light-emitting diode, wherein said tungsten chalcogenide hetero-structured nanomaterial is used in the fabrication process thereof.

BACKGROUND ART

[003] Recent research has shown that in addition to the well studied zero-dimensional (0D), one- dimensional (1 D), and three-dimensional (3D) crystalline objects, 2D nanomaterials have attracted extensive attention and have been considered as ideal candidates for many future applications owing to their unique optical, electronic, catalytic and mechanical properties (Huang, X. ; Zeng, Z. Y.; Zhang, H. Chem. Soc. Rev. 2013, 42, 1 934).

[004] In the rich family of 2D materials, layered transition metal dichalcogenides (TMDs) and transition metal oxides (TMOs) have been placed under the limelight due to their superior electronic and structural properties and unique technological potential (Chhowalla, M. ; Shin, H. S. ; Eda, G. ; Li, L. J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5, 263). Among all the TMDs and TMOs, WSe₂ and W0₃ are two well-known semiconductors, which have been investigated in a lot of areas, such as photovoltaic devices, field effect transistors, photoelectrochemical hydrogen evolution, light-emitting diodes, photodetectors, sensors, and supercapacitors.

[005] For example, polyethylene dioxythiophene:polystyrene sulfonate (PEDOT:PSS) is traditionally used as a buffer layer material on an indium tin oxide (ITO) electrode for the fabrication of quantum dot light-emitting diodes (QD-LEDs) (Dai, X. L ; Zhang, Z. X. ; Jin, Y. Z. ; Niu, Y. ; Cao, H. J. ; Liang, X. Y. ; Chen, L. W. ; Wang, J. P.; Peng, X. G. Nature 2014, 515, 96). However, there are two disadvantages of PEDOT:PSS as the buffer layer material (Murase, S.; Yang, Y. Adv. Mater. 2012, 24, 2459). Firstly, due to its hygroscopic nature, PEDOT:PSS will corrode the ITO electrode, which will reduce the lifetime of the device. Secondly, the organic interfacial buffer layers have inferior thermal stability compared with the inorganic materials. Lately, it has been demonstrated that TMOs can be a powerful alternative to replace PEDOT SS as the buffer layer material (Yang, X. Y. ; Mutlugun, E. ; Zhao, Y. B.; Gao, Y.; Leek, K. S. ; Ma, Y. Y.; Ke, L; Tan, S. T.; Demir, H. V.; Sun, X. W. Small 2014, 10, 247). In order to be compatible with low-cost solution-processed QD-LEDs for future roll-to-roll scalable manufacturing, a solution-processable inorganic material based buffer layer material is in great demand for high-performance QD-LEDs

[006] As a new class of nano-sized building blocks, hetero-nanostructures containing two or more chemically distinct components have recently attracted great interest because they can combine optical and/or electrical, magnetic, catalytic properties. As another kind of hetero-nanostructures, TMD based hetero-nanostructures have been widely investigated. Duan's group has synthesized M0S2- MoSe2 and WS2-WSe2 lateral hetero-nanostructures and investigated their electrical transport properties (Duan, X. D. ; Wang, C ; Shaw, J. C ; Cheng, R.; Chen, Y.; Li, H. L ; Wu, X. P. ; Tang, Y. ; Zhang, Q. L.; Pan, A. L ; Jiang, J. H. ; Yu, R. Q. ; Huang, Y. ; Duan, X. F. Nat. Nanotechnol. 2014, 9, 1024). Monolayer MoSe2-WSe2 hetero-nanostructures showing enhanced photoluminescence have been fabricated by Xu et al (Huang, C. M. ; Wu, S. F. ; Sanchez, A. M.; Peters, J. J. P. ; Beanland, R. ; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H. ; Xu, X. D. Nat. Mater. 2014, 13, 1 096). WS2-M0S2 (Gong, Y. J. ; Lin, J. H. ; Wang, X. L; Shi, G.; Lei, S. D. ; Lin, Z. ; Zou, X. L; Ye, G. L ; Vajtai, R. ; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T. ; Liu, Z. ; Zhou, W. ; Ajayan, P. M. Nat. Mater. 2014, 13, 1 135), Van der Waals coupled MoS₂-MoSe₂ (Ceballos, F.; Bellus, M. Z. ; Chiu, H.-Y. ; Zhao, H. ACS Nano 2014, 8, 12717), and Van der Waals coupled MoS₂-graphene (Zhou, K. G.; Withers, F.; Cao, Y.; Hu, S. ; Yu, G. L ; Casiraghi, C. ACS Nano 2014, 8, 9914) hetero- nanostructures have also been fabricated and investigated.

[007] However, all of these hetero-nanostructures are prepared by physical or chemical vapor deposition methods that need high reaction temperature and yield very little products compared with solution processes. Until now, only a few TMD based hetero-nanostructures have been reported to be fabricated by solution process, such as M0O3-M0S2 (Yin, Z.; Zhang, X. ; Cai, Y.; Chen, J.; Wong, J. I. ; Tay, Y.-Y. ; Chai, J.; Wu, J. ; Zeng, Z. ; Zheng, B. ; Yang, H. Y. ; Zhang, H. Angew. Chem. Inter. Ed. 2014, 53, 12560), WS₂-CdS (Chen, J. ; Wu, X.-J. ; Yin, L; Li, B. ; Hong, X.; Fan, Z.; Chen, B.; Xue, C ; Zhang, H. Angew. Chem. Int. Ed. 2015, 54, 1210), TiS₂-CuS (Tan, C ; Zeng, Z. ; Huang, X. ; Rui, X.; Wu, X.-J. ; Li, B. ; Luo, Z. ; Chen, J. ; Chen, B. ; Yan, Q. ; Zhang, H. Angew. Chem. Int. Ed. 2014, 54, 1841 ), and MoS₂-Pt (Huang, X.; Zeng, Z.; Bao, S. ; Wang, M. ; Qi, X.; Fan, Z. ; Zhang, H. Nat. Commun. 2013, 4, 1444). Therefore, there is still a need to find new ways to fabricate TMD based hetero- nanostructures using wet-chemical methods.

SUMMARY OF THE INVENTION [008] In a first aspect, the present invention thus relates to a method for the preparation of tungsten chalcogenide hetero-structured nanomaterial in a colloidal solution process, comprising the steps of:

(i) providing a tungsten precursor solution comprising at least one tungsten precursor and at least one solvent as a first solution;

(ii) providing a chalcogen precursor solution comprising at least one chalcogen precursor and at least one solvent as a second solution; and

(iii) combining the first solution and the second solution to obtain a reaction solution and reacting the first solution with the second solution under conditions sufficient to produce a tungsten chalcogenide hetero-structured nanomaterial,

wherein the tungsten precursor is a tungsten salt, a tungsten complex or a combination thereof, and the chalcogen precursor is a chalcogen element, a chalcogen compound, a chalcogen complex, or a metal chalcogenide.

[009] In a further aspect, the present invention relates to a method of using the tungsten chalcogenide hetero-structured nanomaterial as disclosed herein.

[0010] In another aspect, the present invention further relates to a quantum dot light-emitting diode, wherein the tungsten chalcogenide hetero-structured nanomaterial disclosed herein is used in the fabrication process of said quantum dot light emitting diode.

BRIEF DESCRIPTION OF THE DRAWINGS

[001 1 ] Figure 1 depicts a transmission electron microscopy (TEM) image (Figure 1 a), a high- resolution TEM (HRTEM) image (Figure 1 b), an X-ray diffraction (XRD) pattern (Figure 1 c), and a high-resolution X-ray photoelectron spectroscopy (XPS) W 4f spectrum (Figure 1 d) of the WO3NP- WSe2 hetero-structured nanosheets.

[0012] Figure 2 depicts TEM images (Figures 2a and 2b), an XRD pattern (Figure 2c), and an HRTEM image (Figure 2d), , and a high-resolution XPS W 4f spectrum (Figure 6d) of the flower-like W03-WSe2 hetero-nanostructures using elemental selenium as the Se source.

[0013] Figure 3 depicts a STEM image (Figure 3a) and a TEM image (Figure 3d) of WO3NP-WS2 hetero-structured nanosheets. The corresponding fast-Fourier-transform (FFT)-generated selected area electron diffraction (SAED) patterns of the 2H phase (Figure 3b) and the 1 T phase (Figure 3c) are shown in Figure 3a. Figure 3e depicts a high-resolution XPS W 4f spectrum of WO3-NP-WS2 hetero-structured nanosheets.

[0014] Figure 4 depicts a TEM image (Figure 4a), an XRD pattern (Figure 4b), an HRTEM image (Figure 4c), and a side view HRTEM image (Figure 4d) of the W03NW-WSe2 hetero-structured nanosheets. [0015] Figure 5 depicts a TEM image (Figure 5a) and an HRTEM image (Figure 5b) of the WO3NW- WS2 hetero-structured nanosheets.

[0016] Figure 6 depicts configurational and operational details of a QD-LED fabricated according to the present invention. Figure 6a is a schematic illustration of the configuration of the fabricated QD- LED. Figure 6b depicts an energy level diagram of the fabricated QD-LED. Figure 6c depicts a photograph of a typical QD-LED working at applied voltage of 6 V. Figure 6d depicts a normalized energy level spectrum of the QD-LED at applied voltage of 6 V. Figure 6e depicts the current density and peak luminescence of the QD-LED as a function of bias voltage. Figure 6f depicts the external quantum efficiency (EQE) and current efficiency (CE) of the QD-LED as a function of current density.

[0017] Figure 7 depicts characteristics of a QD-LED fabricated according to the present invention using various hole injection layers obtained by different precursors in terms of current density-voltage (Figure 7a), luminescence-voltage (Figure 7b), EQE-current density (Figure 7c), and CE-current density (Figure 7d).

DETAILED DESCRIPTION OF THE INVENTION

[0018] As used in this specification, the singular forms "a," "an" and "the" include plural forms unless the context clearly dictates otherwise. Thus, for example, the term "a material" is intended to mean one or more materials, or a combination thereof.

[0019] "One or more", as used herein, relates to at least one and comprises 1 , 2, 3, 4, 5, 6, 7, 8, 9 or more of the referenced species.

[0020] "About", as used herein, for instance in connection with certain values such as temperatures, is defined to be ranging from ± 5 units, preferably ± 2 units. Thus, for instance, a temperature of about 300 °C is understood to be ranging from 295 °C to 305 °C, preferably 298 °C to 302 °C.

[0021 ] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as those commonly understood to one of ordinary skill in the art to which this invention pertains. Although any known methods, devices, and materials may be used in the practice or testing of the invention, the methods, devices, and materials in this regard are described herein.

[0022] As used herein, the term "nanomaterial" denotes a material in the form of, without limitation, particles, flakes, sheets, rods, and the like, wherein at least one dimension of said material is in the nano-size range. Accordingly, at least one dimension of the nanomaterial, as used herein, has a size between about 1 nm to about 1000 nm, preferably, between about 1 nm to about 100 nm, in particular between about 1 nm and about 50 nm. [0023] As used herein, the term "precursor" denotes an organic or inorganic compound, which is used as a reactant in a (physico)chemical reaction.

[0024] The present invention addresses and solves the problems of conventional preparation methods as in detail described herein.

[0025] The present invention provides a method for the preparation of tungsten chalcogenide hetero- structured nanomaterials in a colloidal solution process. The preparation of such nano-structured materials is achieved through an anion exchange process as the crucial step using a chalcogen precursor material as the chalcogen source. Thus, in a first embodiment of the present invention, a method for the preparation of tungsten chalcogenide hetero-structured nanomaterials in a solution process is disclosed.

[0026] In a first step of the method according to the present invention, a tungsten precursor solution is provided as a first solution. Said first solution comprises at least one tungsten precursor and at least one solvent.

[0027] According to the present invention, the tungsten precursor of the first solution is a tungsten salt, a tungsten complex or a combination thereof.

[0028] As non-limiting examples of tungsten salts, tungsten halides may be mentioned, in particular tungsten chloride.

[0029] As non-limiting examples of tungsten complexes, tungsten carbonyls may be mentioned, in particular tungsten hexacarbonyl.

[0030] According to the present invention, the first solution, that is, the tungsten precursor solution, comprises at least one tungsten precursor. However, a tungsten precursor, as defined herein, may be used alone or in combination with another tungsten precursor, as defined herein. Accordingly, in certain embodiments according to the present invention, two or more tungsten precursors may be comprised in the first solution of the herein disclosed method.

[0031 ] In certain embodiments, the at least one tungsten precursor is selected from the group consisting of tungsten chloride and tungsten hexacarbonyl.

[0032] In certain embodiments of the present invention, the at least one solvent is an organic solvent. According to another embodiment, the at least one solvent to be comprised in the first solution is a high-boiling solvent having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C. In certain embodiments, the at least one solvent of the first solution of the method according to the present invention is an organic solvent having a boiling point of at least about 300 °C. For instance, the at least one solvent may be an alkane, an alkene, or a derivative of the aforementioned. The at least one solvent of the first solution may be a linear alkane or alkene having at least 12, preferably at least 14, and more preferably at least 16 carbon atoms in the alkyl or alkenyl chain, respectively. Suitable solvents are known in the art and the skilled artisan will recognize those best suited for carrying out the method disclosed herein.

[0033] In certain embodiments of the present invention, the tungsten precursor of the first solution is soluble or insoluble in the at least one solvent of the first solution. Accordingly, the tungsten precursor may be dissolved completely in the at least one solvent of the first solution, or it may be suspended therein.

[0034] In certain embodiments of the present invention, the provision of a first solution, as described herein, may encompass degassing and/or heating of the first solution. Furthermore, the first solution may be kept in an inert atmosphere. For instance, the first solution may be provided in a nitrogen atmosphere.

[0035] In certain embodiments, the first solution is degassed by introduction of an inert gas into the solution. In certain embodiments, the first solution is heated to a temperature of about 50 °C to about 150 °C, preferably about 80 °C to about 120 °C while degassing it. In order to thoroughly degas, a vacuum may be applied. As a non-limiting example, the degassing may be performed for a period of about 10 minutes to about 60 minutes.

[0036] In a second step of the method according to the present invention, a chalcogen precursor solution is provided as a second solution. Said second solution comprises at least one chalcogen precursor and at least one solvent.

[0037] As the at least one solvent to be comprised in the second solution of the invention disclosed herein, a solvent as defined above for the first solution may be used. Accordingly, in certain embodiments, the at least one solvent to be comprised in the second solution is a high-boiling solvent having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C. In other embodiments, the at least one solvent of the second solution of the method according to the present invention is an organic solvent having a boiling point of at least about 300 °C.

[0038] According to the present invention, the chalcogen precursor of the second solution is selected from the group consisting of a chalcogen element, a chalcogen compound, a chalcogen complex, and a metal chalcogenide. [0039] A chalcogen element, as used herein, is defined to be an element contained in group 16 of the Periodic Table, or the chalcogen group of the Periodic Table, respectively (CAS Version: group VIA of the Periodic Table) in its elemental form. In the context of the present invention, sulfur, selenium, and tellurium are preferred. For instance, as a chalcogen element, sulfur or selenium may be used.

[0040] A chalcogen compound, as used herein, is defined to be denoting a compound comprising at least one element contained in group 16 of the Periodic Table, or the chalcogen group of the Periodic Table, respectively (CAS Version: group VIA of the Periodic Table), which is covalently bound to at least one atom of another element. Accordingly, a chalcogen compound may be an organic chalcogen compound, or a chalcogen oxide. As non-limiting examples of such chalcogen compounds, hydrogen sulfide, sodium sulfide, hydrogen selenide, sodium selenide, diethyl selenide, diphenyl diselenide, thiourea, selenourea and selenium dioxide may be mentioned.

[0041 ] A chalcogen complex, as used herein, is defined to be denoting a complex comprising at least one element contained in group 16 of the Periodic Table, or the chalcogen group of the Periodic Table, respectively (CAS Version: group VIA of the Periodic Table). As non-limiting examples of such chalcogen complexes, oleylamine-sulfur complex, trioctylphosphine-sulfur complex, and trioctylphosphine-selenium complex may be mentioned.

[0042] A metal chalcogenide, as used herein, is defined to be denoting a compound comprising at least one element contained in group 16 of the Periodic Table, or the chalcogen group of the Periodic Table, respectively (CAS Version: group VIA of the Periodic Table) in the anionic form (chalcogenide anion) and at least one more electropositive metal atom. Accordingly, a metal chalcogenide may be a sulfide, a selenide, a telluride, or any binary, tertiary or quaternary compound thereof. As non-limiting examples, copper(l) oxide, zinc oxide, copper (I) sulfide, copper (I) selenide, tin(ll) oxide, zinc (II) sulfide, Cu2SnS3, and Cu4SnS4 may be mentioned.

[0043] According to the present invention, the second solution, that is, the chalcogen precursor solution, comprises at least one chalcogen precursor. However, a chalcogen precursor, as defined herein, may be used alone or in combination with another chalcogen precursor, as defined herein. Accordingly, in certain embodiments according to the present invention, two or more chalcogen precursors may be comprised in the second solution of the herein disclosed method.

[0044] In certain embodiments, the at least one chalcogen precursor is selected from the group consisting of sulfur, selenium, diphenyl selenium, selenourea, and selenium dioxide.

[0045] In certain embodiments of the present invention, the chalcogen precursor of the first solution is soluble or insoluble in the at least one solvent of the first solution. Accordingly, the chalcogen precursor may be dissolved completely in the at least one solvent of the first solution, or it may be suspended. In order to promote solution or suspension of the chalcogen precursor, the second solution may be heated to a temperature of about 150 °C to about 280 °C, preferably about 200 °C to about 250 °C.

[0046] In certain embodiments of the present invention, the provision of a second solution, as described herein, may encompass degassing and/or heating of the second solution. Furthermore, the second solution may be kept in an inert atmosphere. For instance, the first solution may be provided in a nitrogen atmosphere.

[0047] In certain embodiments, the second solution is degassed by introduction of an inert gas into the solution. In certain embodiments, the second solution is heated to a temperature of about 50 °C to about 150 °C, preferably about 80 °C to about 120 °C while degassing it. In order to thoroughly degas, a vacuum may be applied. As a non-limiting example, the degassing may be performed for a period of about 10 minutes to about 60 minutes.

[0048] In a third step of the method according to the present invention, the first solution, as defined above, and the second solution, as defined above, are combined in order to obtain a reaction solution. In certain embodiments, the first solution is preheated to a temperature of at least about 200 °C, preferably at least about 250 °C, in particular at least about 300 °C prior to combining it with the second solution. In certain embodiments, the second solution may be preheated as well, as described above for the first solution.

[0049] The combining of the first solution and the second solution is not limited to a specific kind of order or method, and may, for instance, encompass injecting and/or pouring of either one or both of the solutions. It may involve the use of syringes and other devices commonly used for such purposes, and may vary according to the actual reaction conditions, such as temperatures of the respective solutions, inert atmospheres, etc. For instance, the first and the second solution may be combined by injecting the first solution into the second solution or vice versa, for instance by using syringes or injection tubes.

[0050] In certain embodiments, the molar ratio of the tungsten precursor of the first solution and the chalcogen precursor of the second solution is from 1 :1 to 1 :3.

[0051 ] After combining, the thus obtained reaction solution is reacted under conditions sufficient to produce the tungsten chalcogenide hetero-structured nanomaterial according to the present invention. Accordingly, in certain embodiments, the reaction temperature is controlled to be within the range of about 200 °C to about 400 °C, in particular about 250 °C to about 350 °C, preferably about 280 °C to about 320 °C, more preferably about 290 °C to about 31 0 °C. Most preferably, the reaction temperature is about 300 °C. Furthermore, in certain embodiments, the reaction time is from about 30 minutes to about 8 hours, preferably from about 60 minutes to about 5 hours. The reaction may be performed in an inert gas atmosphere, for instance in a nitrogen atmosphere. [0052] In order to collect the thus produced tungsten chalcogenide hetero-structured nanomaterial, the reaction solution may be cooled down to room temperature, and the nanomaterial may be isolated via centrifugation of filtration or the like.

[0053] According to one embodiment of the present invention, the tungsten chalcogenide hetero- structured nanomaterial that is obtainable from a method as described herein is a WO3-WX2 hetero- structured nanomaterial, wherein X denotes an element of the chalcogen group.

[0054] The reaction conditions may be varied in order to obtain different kinds and forms of tungsten chalcogenide hetero-structured nanomaterials. For instance, the varying of the reaction conditions may encompass different molar ratios of components, different solvents used, different volumes of solvents used, different preheating as well as reaction temperatures, different reaction times, different tungsten and chalcogen precursors, and further additives to be comprised in the first solution and/or the second solution of the method according to the present invention.

[0055] By varying the chalcogen precursor comprised in the second solution, the chalcogen source can be varied, thereby producing different kinds of WO3-WX2 hetero-structured nanomaterials. For instance, a selenium precursor may be used as a chalcogen precursor of the second solution, whereby tungsten chalcogenide hetero-structured nanomaterials of the basic formula W03-WSe2 may be obtained. For instance, elemental selenium or diphenyl diselenide may be used as the chalcogen precursor. Alternatively, a sulfur precursor may be used as a chalcogen precursor of the second solution, whereby tungsten chalcogenide hetero-structured nanomaterials of the basic formula WO3- WS2 may be obtained. For instance, elemental sulfur may be used as the chalcogen precursor. Thus, according to certain embodiments, in the basic formula WO3-WX2 of the tungsten chalcogenide hetero-structured nanomaterials according to the present invention, X denotes S or Se.

[0056] In certain embodiments, the first solution of the first step according to the present invention and/or the second solution of the second step according to the present invention may comprise at least one further additive. Thus, the first solution may comprise at least one solvent, at least one tungsten precursor, and at least one additive. Likewise, the second solution may comprise at least one solvent, at least one chalcogen precursor, and at least one additive.

[0057] As suitable additives to be employed in the first solution and/or second solution of the method according to the present invention, organic bases, inorganic bases, organic acids, and inorganic acids may be mentioned.

[0058] As non-limiting examples of organic bases, nitrogen-containing organic compounds may be mentioned, such as amine-substituted alkyl, alkenyl, alkinyl, aryl, and heteroaryl compounds. [0059] As non-limiting examples of inorganic bases, carbonates, bicarbonates, and hydroxides of alkali metals and alkaline earth metals may be enlisted, but also ammonia and ammonium compounds.

[0060] As non-limiting examples of organic acids, carboxylic acids, sulfonic acids, alcohols, thiols, and derivatives of the aforementioned may be enlisted.

[0061 ] As non-limiting examples of inorganic acids, mineral acids such as hydrochloric acid, hydrofluoric, phosphoric acid, and sulfuric acid may be enlisted.

[0062] In certain embodiments, an organic acid is used as an additive to be employed in the first solution and/or the second solution of the method according to the present invention. For instance, the at least one organic acid to be comprised in the first solution and/or the second solution, respectively, may be a fatty acid with at least 10 carbon atoms, in particular at least 12 carbon atoms, preferably at least 14 carbon atoms, and more preferably at least 1 6 carbon atoms in the carbon chain. The organic acid may be saturated or unsaturated. Suitable examples of the organic acid to be comprised in the first solution and/or the second solution, as defined herein, include, without limitation, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, cerotic acid, myristoleic acid, palmitoleic acid, sapienic acid, oleic acid, elaidic acid, vaccenic acid, linoleic acid, linoelaidic acid, a-linolenic acid, and arachidonic acid.

[0063] In certain embodiments, a suitable organic acid is an organic acid having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

[0064] Thus, in certain embodiment of the present invention, the first solution may further comprise, in addition to the at least one tungsten precursor and the at least one solvent, as defined herein, at least one organic acid. In certain embodiments, the at least one organic acid has a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

[0065] According to another embodiment, the second solution may further comprise, in addition to the at least one chalcogen precursor and the at least one solvent, as defined herein, at least one organic acid. In certain embodiments, the at least one organic acid has a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

[0066] In certain embodiments, both the first and the second solution may comprise at least one organic acid, as defined herein. [0067] Another class of suitable additives to be comprised in the first solution and/or the second solution in a method according to the present invention is the alkyl amine class. The at least one alkyl amine to be comprised in the first solution and/or the second solution, respectively, may be a primary, secondary or tertiary alkyl amine, wherein each alkyl chain, independently, may be linear or branched, and may have at least 10 carbon atoms, in particular at least 12 carbon atoms, preferably at least 14 carbon atoms, and more preferably at least 16 carbon atoms. In certain embodiments, the alkyl amine, as defined herein, may be a fatty amine having, in the carbon chain thereof, at least 12 carbon atoms, in particular at least 14 carbon atoms, preferably at least 16 carbon atoms. Suitable examples of alkyl amines to be comprised in the first solution and/or the second solution, as defined herein, include, without limitation, oleylamine, hexadecylamine, and octadecylamine.

[0068] In certain embodiments, a suitable alkyl amine to be employed in the first solution and/or second solution of the method according to the present invention has a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

[0069] Thus, in some embodiments, the first solution may comprise at least one alkyl amine. In certain embodiments, the at least one alkyl amine to be comprised in the first solution, as defined herein, has a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

[0070] According to another embodiment, the second solution may comprise at least one alkyl amine. In certain embodiments, the at least one alkyl amine to be comprised in the second solution, as defined herein, has a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

[0071 ] In certain embodiments, both the first and the second solution may comprise at least one alkyl amine, as defined herein.

[0072] In certain embodiments, the tungsten chalcogenide hetero-structured nanomaterial may vary in form. For instance, the tungsten chalcogenide hetero-structured nanomaterial may be in the form of nanoparticles, nanowires, nanorods, nanoflakes, nanosheets, or flower-like nanostructures. Furthermore, the tungsten chalcogenide hetero-structured nanomaterial may be an overall nano-sized combination of the aforementioned structures. For instance, the tungsten chalcogenide hetero- structured nanomaterial may comprise one component of the hetero-structure being in the form of nanoparticles, and another component of the hetero-structure being in the form of nanosheets, whereby the components of the hetero-structure are assembled together. As a non-limiting example thereof, the WO3 species of the herein disclosed nanomaterial may be in the form of nanoparticles, and the WX2 species may be in the form of nanosheets, wherein the WO3 species nanoparticles are assembled on the WX2 species nanosheets, and wherein the thus assembled hetero-structured nanomaterial forms the nanomaterial according to the present invention.

[0073] Thus, according to certain embodiments, tungsten chalcogenide hetero-structured nanomaterials of the basic formula W03-WSe2 may be obtained, wherein the WO3 species is in the form of nanoparticles and assembled on the surface of the WSe2 nanosheet-like species. In the context of the present invention, such tungsten chalcogenide hetero-structured nanomaterials shall be denoted WO3 nanoparticle (NP)-WSe2 hetero-structured nanosheets (Figure 1 ). For the production of said W03NP-WSe2 hetero-structured nanosheets, diphenyl diselenide may be used as the chalcogen precursor serving as the selenium source.

[0074] According to other embodiments, tungsten chalcogenide hetero-structured nanomaterials of the basic formula W03-WSe2 may be obtained, wherein the material is assembled in the form of a flower-like nanomaterial. In the context of the present invention, such tungsten chalcogenide hetero- structured nanomaterials shall be denoted flower-like W03-WSe2 hetero-structures (Figure 2). For the production of said flower-like W03-WSe2 hetero-structures, elemental selenium may be used as the chalcogen precursor serving as the selenium source in the method according to the present invention.

[0075] According to other embodiments, tungsten chalcogenide hetero-structured nanomaterials of the basic formula WO3-WS2 may be obtained, wherein the WO3 species is in the form of nanoparticles and assembled on the surface of the WS2 nanosheet-like species. In the context of the present invention, such tungsten chalcogenide hetero-structured nanomaterials shall be denoted WO3 nanoparticle (NP)-WS2 hetero-structured nanosheets (Figure 3). For the production of said WO3NP- WS2 hetero-structured nanosheets, elemental sulfur may be used as the chalcogen precursor serving as the sulfur source in the method according to the present invention.

[0076] As another non-limiting example of alternative nanomaterial forms, the WO3 species of the herein disclosed nanomaterial may be in the form of nanowires, and the WX2 species may be in the form of nanosheets, wherein the WO3 species nanowires are assembled on the WX2 species nanosheets, and wherein the thus assembled hetero-structured nanomaterial forms the nanomaterial according to the present invention.

[0077] Thus, according to other embodiments, tungsten chalcogenide hetero-structured nanomaterials of the basic formula W03-WSe2 may be obtained, wherein the WO3 species is in the form of nanowires and assembled on the surface of the WSe2 nanosheet-like species. In the context of the present invention, such tungsten chalcogenide hetero-structured nanomaterials shall be denoted WO3 nanowire (NW)-WSe2 hetero-structured nanosheets (Figure 4). For the production of said W03NW-WSe2 hetero-structured nanosheets, an alkyl amine may be added to the first solution in the first step of the presently disclosed method. [0078] According to another embodiment, tungsten chalcogenide hetero-structured nanomaterials of the basic formula WO3-WS2 may be obtained, wherein the WO3 species is in the form of nanowires and assembled on the surface of the WS2 nanosheet-like species. In the context of the present invention, such tungsten chalcogenide hetero-structured nanomaterials shall be denoted WO3 nanowire (NW)-WS2 hetero-structured nanosheets (Figure 5). For the production of said WO3NW- WSe2 hetero-structured nanosheets, an alkyl amine may be added to the first solution in the first step of the presently disclosed method.

[0079] The present invention is further directed to a tungsten chalcogenide hetero-structured nanomaterial, which is obtainable from a method as disclosed herein. In certain embodiments, the tungsten chalcogenide hetero-structured nanomaterial is a WO3-WX2 hetero-structured nanomaterial, wherein X denotes an element of the chalcogen group, preferably S or Se.

[0080] The tungsten chalcogenide hetero-structured nanomaterials according to the present invention exhibit long-term stability when kept and stored in the form of suspensions. Suitable solvents for preparing such suspensions include organic solvents such as, without limitation, acetone, toluene, and mixtures thereof. Considering this advantageous property, the forming of uniform thin films on target substrates through solution process is comparably facile, convenient, and economical, and allows for great potential applications, especially in the industrial-scale fabrication of electronic devices

[0081 ] Accordingly, the present invention is further directed to a method of using the tungsten chalcogenide hetero-structured nanomaterial, as disclosed herein.

[0082] In certain embodiments, the tungsten chalcogenide hetero-structured nanomaterial according to the present invention is used in the fabrication of electronic devices, such as organic photovoltaic devices, field emission transistors, gas sensors, and in particular light emitting diodes. The tungsten chalcogenide hetero-structured nanomaterial, as disclosed herein, may be used in the forming of thin films on a substrate, for instance on a flexible or a rigid conductive substrate. In the course of such fabrication processes, additional layers may be disposed on top of the thus formed tungsten chalcogenide hetero-structured nanomaterial layer.

[0083] In certain embodiments, the tungsten chalcogenide hetero-structured nanomaterial is used in the fabrication of a quantum dot light emitting diode (QD-LED). Methods of fabrication of QD-LEDs are known in the art (Dai, X. L ; Zhang, Z. X.; Jin, Y. Z. ; Niu, Y. ; Cao, H. J. ; Liang, X. Y.; Chen, L. W. ; Wang, J. P. ; Peng, X. G. Nature 2014, 515, 96), (Bozyigit, D. ; Wood, V. MRS. Bull. 2013, 38, 731 ), (Qian, L; Zheng, Y. ; Xue, J. G.; Holloway, P. H. Nat. Photon. 2011 , 5, 543), and are thus explicitly incorporated herein by reference. In certain embodiments, a QD-LED according to the present invention is fabricated using the tungsten chalcogenide hetero-structured nanomaterial, as disclosed herein, as the hole transport layer material. The tungsten chalcogenide hetero-structured nanomaterial may replace materials commonly used as the hole transport layer material or buffer layer material, such as polyethylene dioxythiophene:polystyrene sulfonate (PEDOT SS), in particular on an indium tin oxide (ITO) electrode. However, the tungsten chalcogenide hetero-structured nanomaterial disclosed herein may also be used in combination with materials that are already known in the art and commonly used for such purposes.

[0084] For a non-limiting example of a method of fabrication of a QD-LED using the tungsten chalcogenide hetero-structured nanomaterial according to the present invention, reference is made to Example 6 of the present invention.

[0085] An exemplary QD-LED according to the present invention is depicted in Figure 6. The QD- LED of Figure 6 is a multilayer structure of ITO/ W0₃-WSe₂/ poly(9-vinylcarbazole) (PVK/ CDSe/ZnS core/shell QDs/ 2,2',2"-(1 ,3,5-benzinetriyl)-tris(1 -phenyl-1 -H-benzimidazole) (TPBi)/ LiF/ Al. The tungsten hetero-structured nanomaterial prepared in a method according to the present invention is in the form of W03NP-WSe2 hetero-structured nanosheets and used as the anode interfacial buffer layer material. The current density and luminescence increase steeply once the voltage reaches about 5.2 V, yielding a maximum brightness of over 67,000 cd/m² (Figure 6e, f). A peak external quantum efficiency of 8.53 % is achieved.

[0086] In a further aspect, the present invention is directed to a quantum dot light-emitting diode, which is fabricated using the tungsten chalcogenide hetero-structured nanomaterial according to the present invention. In certain embodiments, such a quantum dot light-emitting diode is fabricated using the tungsten chalcogenide hetero-structured nanomaterial, as described herein, as the hole transport layer.

[0087] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject-matter from the genus, regardless of whether or not the excised material is specifically recited herein. Other embodiments are within the following claims. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

[0088] One skilled in the art would readily appreciate that the present invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. Further, it will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. The compositions, methods, procedures, treatments, molecules and specific compounds described herein are presently representative of preferred embodiments are exemplary and are not intended as limitations on the scope of the invention. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention are defined by the scope of the claims. The listing or discussion of a previously published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

[0089] The invention illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including," containing", etc. shall be read expansively and without limitation. The word "comprise" or variations such as "comprises" or "comprising" will accordingly be understood to imply the inclusion of a stated integer or groups of integers but not the exclusion of any other integer or group of integers. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by exemplary embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0090] The content of all documents and patent documents cited herein is incorporated by reference in their entirety.

EXAMPLES

Example 1 : Synthesis of WO3 nanoparticles (NP)-WSe2 hetero-structured nanosheets

[0091 ] WCIe (0.5 mmol), stearic acid (SA, 1 g) and 9 ml_ octadecene (ODE) were added in a 100 mL three-necked flask. The mixture was degassed under a vacuum at 1 10 °C for 10 min. Then the solution was heated under nitrogen to 300 °C. At the same time, diphenyl diselenide (0.5 mmol) dissolved into 2 ml_ ODE was injected into the above solution when the temperature reached 300 °C, and kept at 300 °C for 60 min and cooled to room temperature. The product was collected by centrifugation (9000 rpm, 5 min) and washed several times with toluene and acetone (technical grade) for further characterization.

Example 2: Synthesis of assembled flower-like W03-WSe2 hetero-nanostructures

[0092] WCIe (0.5 mmol), SA (1 g) and 9 ml_ ODE were added in a 100 ml_ three-necked flask. The mixture was degassed under a vacuum at 1 10 °C for 10 min. Then the solution was heated under nitrogen to 300 °C. At the same time, selenium (1 mmol) was dissolved in 2 ml_ ODE at 250 °C. Then this solution was cooled to 130 °C, which would be injected into the above solution when the temperature reached 300 °C, and kept at 300 °C for 30 min and cooled to room temperature. The product was collected by centrifugation (9000 rpm, 5 min) and washed several times with toluene and acetone (technical grade) for further characterization.

Example 3: Synthesis of WO3 NP-WS2 hetero-structured nanosheets

[0093] WCIe (0.5 mmol), SA (1 g) and 9 ml_ ODE were added in a 100 ml_ three-necked flask. The mixture was degassed under a vacuum at 1 10 °C for 10 min. Then the solution was heated under nitrogen to 300 °C. At the same time, sulfur (1 mmol) was dissolved in 1 ml_ oleylamine (OLA) and 1 ml_ ODE, which would be injected into the above solution when the temperature reached 300 °C, and kept at 300 °C for 3 h and cooled to room temperature. The product was collected by centrifugation (9000 rpm, 5 min) and washed several times with toluene and acetone (technical grade) for further characterization.

Example 4: Synthesis of WO3 nanowire (NW)-WSe2 hetero-structured nanosheets

[0094] WCIe (0.5 mmol), SA (1 g), 1 ml_ OLA and 8 mL ODE were added in a 100 mL three-necked flask. The mixture was degassed under a vacuum at 1 10 for 10 min. Then the solution was heated under nitrogen to 300 °C. At the same time, diphenyl diselenide (0.5 mmol) was dissolved in 2 mL ODE, which would be injected into the above solution when the temperature reached 300 °C, and kept at 300 °C for 1 h and cooled to room temperature. The product was collected by centrifugation (9000 rpm , 5 min) and washed several times with toluene and acetone (technical grade) for further characterization.

Example 5: Synthesis of WO3 NW-WS2 hetero-structured nanosheets

[0095] WCIe (0.5 mmol), 1 ml_ OLA and 8 ml_ ODE were added in a 1 00 ml_ three-necked flask. The mixture was degassed under a vacuum at 1 10 °C for 10 min. Then the solution was heated under nitrogen to 300 °C. At the same time, sulfur (1 mmol) was dissolved in 2 ml_ ODE, which would be injected into the above solution when the temperature reached 300 °C, and kept at 300 °C for 1 h and cooled to room temperature. The product was collected by centrifugation (9000 rpm , 5 min) and washed several times with toluene and acetone (technical grade) for further characterization.

Example 6: Fabrication of QD-LED Devices

[0096] The patterned ITO substrates were cleaned by sonication sequentially in detergent, de-ionized water, acetone, and isopropyl alcohol. The WO3 NP-WSe2 anode buffer layer was spin-coated on the 02-plasma treated ITO substrate from diluted 5 mg/mL of W03-WSe2 toluene/acetone (1 :1 v:v) solution at 5000 rpm for 60 s and treated with 02-plasma. The 2 wt-% of poly-TPD (50 nm) in chlorobenzene was also spin-coated on the WO3 NP-WSe2 layer at 4000 rpm for 60 s, followed by thermal annealing at 150 °C for 30 min in a nitrogen glove box. The QD layer was then deposited on the ITO/WO3 NP-WSe₂/poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)-benzidine] (poly-TPD) layer by spin-coating the QD dispersion (QDs were synthesized according to a modified method reported in the literature Yang, X. Y. ; Mutlugun, E. ; Zhao, Y. B. ; Gao, Y. ; Leek, K. S. ; Ma, Y. Y. ; Ke, L ; Tan, S. T. ; Demir, H. V. ; Sun, X. W. Small 2014, 10, 247) and dispersed in toluene with 15 mg/mL) at a rate of 1000-4000 rpm for 60 s, and cured at 90°C under N₂ atmosphere for 30 min. The TPBi (35 nm), LiF (0.5 nm), and Al (190 nm) layers were thermally deposited under a base pressure of -2x10^-4 Pa.

[0097] Table 1 : Summary of the optical and electrical properties of QD-LEDs fabricated in a method according to the present invention

As apparent from the results enlisted in Table 1 , the QD-LED device featuring the WO3 NP-WSe2 hetero-structured nanosheets according to the present invention showed the best performance with respect to luminescence voltage, EQE-current density, and CE-current density.

Claims

1 . A method for the preparation of tungsten chalcogenide hetero-structured nanomaterial in a colloidal solution process, comprising the steps of:

2. The method according to claim 1 , wherein the tungsten chalcogenide hetero-structured nanomaterial is a WO3-WX2 hetero-structured nanomaterial, wherein X denotes an element of the chalcogen group, preferably S or Se.

3. The method according to claim 1 or 2, wherein

the at least one tungsten precursor is selected from the group consisting of a tungsten halide and a tungsten carbonyl; and/or

wherein the at least one chalcogen precursor is selected from the group consisting of sulfur, selenium, diphenyl selenium, selenourea, and selenium dioxide.

4. The method according to any one of the preceding claims, wherein the reaction conditions of step iii) are such that

the reaction temperature is in the range of about 200 °C to about 400 °C, in particular about 250 °C to about 350 °C, preferably about 280 °C to about 320 °C, more preferably about 290 °C to about 310 °C; and/or

the reaction time is from about 30 minutes to about 8 hours, preferably from about 60 minutes to about 5 hours; and/or

the reaction is performed in an inert gas atmosphere.

5. The method according to any one of the preceding claims, wherein

the at least one solvent of the first solution is a high-boiling solvent having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C; and/or the at least one solvent of the second solution is a high-boiling solvent having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

6. The method according to any one of the preceding claims, wherein the first solution and/or the second solution further comprises at least one organic acid having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

7. The method according to any one of the preceding claims, wherein the first solution and/or the second solution further comprises at least one alkyl amine having a boiling point of at least about 100 °C, preferably at least about 150 °C, more preferably at least about 200 °C, in particular at least about 250 °C, and most preferably at least about 300 °C.

8. The method according to any one of the preceding claims, wherein the molar ratio of the tungsten precursor of the first solution and the chalcogen precursor of the second solution is from 1 :1 to 1 :3.

9. The method according to any one of the preceding claims, wherein the nanomaterial is a WO3 nanoparticle (NP)-WSe2 hetero-structured nanosheet, a WO3 NP-WS2 hetero-structured nanosheet, a WO3 nanowire (NW)-WSe2 hetero-structured nanosheet, a WO3 NW-WS2 hetero-structured nanosheet, or a flower-like W03-WSe2 hetero-nanostructure.

10. A tungsten chalcogenide hetero-structured nanomaterial obtainable from a method according to any one of claims 1 to 9.

1 1 . A WO3 nanoparticle (NP)-WSe2 hetero-structured nanosheet obtainable from a method according to any one of claims 1 to 9.

12. A WO3 nanoparticle (NP)-WS2 hetero-structured nanosheet obtainable from a method according to any one of claims 1 to 9.

13. A WO3 nanowire (NW)-WSe2 hetero-structured nanosheet obtainable from a method according to any one of claims 1 to 9.

14. A WO3 nanowire (NW)-WS2 hetero-structured nanosheet obtainable from a method according to any one of claims 1 to 9.

15. A flower-like W03-WSe2 hetero-nanostructure obtainable from a method according to any one of claims 1 to 9.

16. A tungsten chalcogenide hetero-structured nanomaterial.

17. A WO3 nanoparticle (NP)-WSe2 hetero-structured nanosheet.

18. A WO3 nanoparticle (NP)-WS2 hetero-structured nanosheet.

19. A WO3 nanowire (NW)-WSe2 hetero-structured nanosheet.

20. A WO3 nanowire (NW)-WS2 hetero-structured nanosheet.

21 . A flower-like W03-WSe2 hetero-nanostructure.

22. A method of using the tungsten chalcogenide hetero-structured nanomaterial according to any one of claims 1 0 to 21 .

23. The method according to claim 22, wherein the tungsten chalcogenide hetero-structured nanomaterial is used in the fabrication of an electronic device.

24. The method according to claim 23, wherein the tungsten chalcogenide hetero-structured nanomaterial is used in the fabrication of a quantum dot light emitting diode (QD-LED).

25. The method according to claim 24, wherein the tungsten chalcogenide hetero-structured nanomaterial is used as a hole transport layer material of the QD-LED.

26. A quantum dot light-emitting diode obtainable from a method according to claim 24 or 25.