CHEMICAL VAPOR DEPOSITION METHOD FOR FABRICATING TWO-DIMENSIONAL MATERIALS

Abstract

A method of synthesis of two-dimensional metal chalcogenide monolayers, such as WSe.sub.2 and MoSe.sub.2, is based on a chemical vapor deposition approach that uses H.sub.2Se or alkyl or aryl selenide precursors to form a reactive gas. The gaseous selenium precursor may be introduced into a tube furnace containing a metal precursor at a selected temperature, wherein the selenium and metal precursors react to form metal chalcogenide monolayers.

Claims

1. A method of synthesizing a metal chalcogenide nanosheet comprising: reacting a gaseous selenium precursor with a metal precursor.

2. The method of claim 1, wherein the metal chalcogenide nanosheet is selected from the group consisting of: WSe.sub.2; MoSe.sub.2; NbSe.sub.2; PtSe.sub.2; ReSe.sub.2; TaSe.sub.2; TiSe.sub.2; ZrSe.sub.2; ScSe.sub.2; VSe.sub.2; GaSe; Ga.sub.2Se.sub.3; Bi.sub.2Se.sub.3; GeSe; InSe; In.sub.2Se.sub.3; SnSe.sub.2; SnSe; SbSe.sub.3; ZrSe.sub.3; MnIn.sub.2Se.sub.4; MgIn.sub.2Se.sub.4; Pb.sub.2Bi.sub.2Se.sub.5; SnPSe.sub.3; and PdPSe; and alloys and doped derivatives thereof.

3. The method of claim 1, wherein the metal precursor is selected from the group consisting of: a metal; a metal diselenide bulk powder; a metal oxide; an inorganic precursor; an organometallic precursor; a metal alkyl precursor; an ethylhexanoate salt; and bis(ethylbenzene)molybdenum.

4. The method of claim 1, wherein the gaseous selenium precursor is selected from the group consisting of: H.sub.2Se; an alkyl selenide; and an aryl selenide.

5. The method of claim 1, further comprising reacting the gaseous selenium precursor with the metal precursor in the presence of a reducing gas.

6. The method of claim 1, further comprising reacting the gaseous selenium precursor with the metal precursor in the presence of H.sub.2S.

7. The method of claim 1, wherein the gaseous selenium precursor is mixed with a ligand.

8. The method of claim 7, wherein the ligand is selected from the group consisting of: an alkane thiol; an alkane selenol; and a combination of an alkane thiol and an alkane selenol.

9. The method of claim 1, further comprising reacting the gaseous selenium precursor with the metal precursor in a chemical vapor deposition reactor.

10. The method of claim 1, further comprising reacting the gaseous selenium precursor and the metal precursor at a temperature, or a range of temperatures, between 100° C. and 550° C.

11. The method of claim 1, further comprising reacting the gaseous selenium precursor and the metal precursor at a temperature, or a range of temperatures, above 550° C.

12. The method of claim 1, further comprising reacting the gaseous selenium precursor and the metal precursor in the presence of an inert carrier gas.

13. The method of claim 1, wherein the nanosheet has lateral dimensions less than 10 nm.

14. The method of claim 1, wherein the nanosheet has lateral dimensions between 10 nm and 100 μm.

15. The method of claim 1, wherein the nanosheet has lateral dimensions greater than 100 μm.

16. The method of claim 1, wherein reacting the gaseous selenium precursor with the metal precursor is conducted at a pressure below atmospheric pressure.

17. The method of claim 1, wherein reacting the gaseous selenium precursor with the metal precursor is conducted at atmospheric pressure.

18. The method of claim 1, wherein reacting the gaseous selenium precursor with the metal precursor is conducted at a pressure above atmospheric pressure.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0027] FIG. 1 is a schematic diagram illustrating the synthesis of WSe.sub.2 monolayers using H.sub.2Se gas according to an embodiment of the invention.

[0028] FIG. 2 is a schematic diagram illustrating the synthesis of MoSe.sub.2 monolayers using H.sub.2Se gas according to an embodiment of the invention.

[0029] FIG. 3 is a tube furnace temperature profile for the synthesis of MoSe.sub.2 monolayers using H.sub.2Se gas according to an embodiment of the invention.

[0030] FIG. 4 is a Raman spectrum of MoSe.sub.2 monolayers grown using H.sub.2Se gas.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Herein, a method of synthesis of metal chalcogenide monolayers, for example, TMDC monolayers such as WSe.sub.2 and MoSe.sub.2, is described. The method is based on a CVD approach, using H.sub.2Se or alkyl or aryl selenide precursors to form the reactive gas. The process is illustrated for the synthesis of WSe.sub.2 monolayers using H.sub.2Se gas in FIG. 1. The gaseous selenium precursor is introduced into a tube furnace containing a metal precursor at a given temperature, where the selenium and metal precursors react to form metal chalcogenide monolayers.

[0032] The method may be used to synthesize TMDC monolayers including, but not restricted to: WSe.sub.2; MoSe.sub.2; NbSe.sub.2; PtSe.sub.2; ReSe.sub.2; TaSe.sub.2; TiSe.sub.2; ZrSe.sub.2; ScSe.sub.2; and VSe.sub.2, and including alloys and doped derivatives thereof. Further, the method may be used to synthesize other metal selenide monolayers including, but not restricted to: GaSe; Ga.sub.2Se.sub.3; Bi.sub.2Se.sub.3; GeSe; InSe; In.sub.2Se.sub.3; SnSe.sub.2; SnSe; SbSe.sub.3; ZrSe.sub.3; MnIn.sub.2Se.sub.4; MgIn.sub.2Se.sub.4; Pb.sub.2Bi.sub.2Se.sub.5; SnPSe.sub.3; and PdPSe, and including alloys and doped derivatives thereof.

[0033] The metal precursor may include, but is not restricted to: a metal, such as W or Mo; a metal diselenide bulk powder, e.g. WSe.sub.2, or MoSe.sub.2; a metal oxide, e.g. WO.sub.3 or MoO.sub.3; inorganic precursors, e.g. WCI.sub.n (n=4-6), Mo.sub.6Cl.sub.12, MoCl.sub.3, [MoCl.sub.5].sub.2, WO.sub.2Cl.sub.2, MoO.sub.2Cl.sub.2, WF.sub.6, MoF.sub.6, (NH.sub.4).sub.6H.sub.2W.sub.12O.sub.40 or (NH.sub.4).sub.6H.sub.2Mo.sub.12O.sub.40; and organometallic precursors such as carbonyl salts, e.g. Mo(CO).sub.6 or W(CO).sub.6 and their alkyl and aryl derivatives; metal alkyl precursors, e.g. W(CH.sub.3).sub.6; ethylhexanoate salts, e.g. Mo[OOCH(C.sub.2H.sub.5)C.sub.4H.sub.9]x; or bis(ethylbenzene)molybdenum [(C.sub.2H.sub.5).sub.yC.sub.6H.sub.6-y].sub.2Mo (y=1-4).

[0034] In one embodiment, the gaseous selenium precursor is H.sub.2Se. H.sub.2Se acts not only as a reactive selenium source but also as a carrier gas. In one embodiment, H.sub.2Se is mixed with other gases, for example H.sub.2, to promote a strongly reducing atmosphere and control the oxidation state of the metal. In the case of WF.sub.6 as a metal precursor, a reduction of the W atom from the +VI oxidation state in WF.sub.6 to the +IV oxidation state in WSe.sub.2 is required. H.sub.2Se itself has strong reducing character; in an alternative embodiment H.sub.2Se facilitates the reduction of the metal precursor from the +VI oxidation state to the +IV oxidation state in WSe.sub.2 or MoSe.sub.2 without the need for an additional reducing agent. The high reactivity of H.sub.2Se compared to elemental selenium may favor better crystallinity and sheet growth.

[0035] In another embodiment, the gaseous selenium precursor is a selenium compound with low volatility, such as an alkyl or aryl selenide. Examples include, but are not restricted to: di-tert-butyl selenide, Se(C(CH.sub.3).sub.3).sub.2; dimethyl selenide, (C.sub.2H.sub.5).sub.2Se; diphenyl selenide, Ph.sub.2Se; and diphenyl diselenide, Ph.sub.2Se.sub.2. The aforementioned precursors are particularly suitable as they have low boiling points, i.e. around or below 100° C. Low-volatility alkyl and aryl diselenides decompose at low temperature and with a clean decomposition pathway that yields only gaseous by-products.

[0036] In a further embodiment, the gaseous selenium precursor is used in combination with other gases, such as, but not restricted to, H.sub.2S, to create a gradient composition. This enables modulation of the band gap of the 2-D metal chalcogenide material to form, for example, WS.sub.xSe.sub.2-x, MoS.sub.xSe.sub.2-x, GaS.sub.xSe.sub.1-x, GeS.sub.xSe.sub.1-x, SnS.sub.xSe.sub.2-x, and Zr(S.sub.xSe.sub.1-x).sub.3. Gas mixtures may also be used to form doped metal chalcogenide materials. Doping may alter the electronic properties of the metal chalcogenide material, which may lead to, for example, improved photoluminescence quantum yield.

[0037] In a further embodiment, the gaseous selenium precursor is mixed with a ligand having a low boiling point, such as a thiol or selenol, capable of coordinating the atoms and influencing the growth of the metal chalcogenide monolayer. This may provide both a pathway to doping and also favor a uniform size distribution and tailored sheet growth. Suitable ligands include, but are not restricted to: alkane thiols, e.g. 1-octane thiol or 1-dodecane thiol; alkane selenols, e.g. 1-octane selenol or 1-dodecane selenol; and combinations thereof.

[0038] The gaseous selenium precursors herein described have low decomposition temperatures, below the softening point of glass (600° C.), enabling the use of glass reactors that are significantly less expensive than the quartz reactors required for CVD growth at temperatures above 600° C. In addition, a lower reaction temperature allows monolayer growth on flexible substrates, such as low-cost, thermally sensitive polymer substrates that would otherwise warp, melt or degrade at the high temperatures employed for CVD growth of TMDC monolayers in the prior art.

[0039] In one embodiment, the gaseous selenium precursor is introduced into the tube furnace at room temperature, then the temperature is ramped up systematically from room temperature to a temperature to induce growth of the metal chalcogenide monolayers. In an alternative embodiment, the gaseous selenium precursor is introduced into the tube furnace at elevated temperature. This may prevent any side-reactions when heating the furnace. It will be obvious to one of ordinary skill in the art that the reaction temperature, or range of temperatures, will depend on the choice of precursors. In one embodiment, the reaction proceeds at a temperature, or a range of temperatures, below the softening point of glass. For example, the reaction may proceed at temperatures in the region of 100° C. to 550° C. In another embodiment, the reaction proceeds at a temperature, or range of temperatures, above 550° C.

[0040] In one particular embodiment, the gaseous selenium precursor is used neat. In another embodiment, the gaseous selenium precursor is mixed with an inert carrier gas, such as, but not restricted to, N.sub.2 or Ar. In one embodiment, the supply of the gaseous selenium precursor is controlled during the growth process to create concentration gradients. For example, when using H.sub.2Se, a fast gas-exchange step may be introduced, whereby the flow of H.sub.2Se into the furnace may be rapidly stopped at any point during the process by a combination of an increased inert gas purge and pumping capacity and replaced by an inert gas, such as N.sub.2 or Ar.

[0041] The flow rate of any gaseous precursor(s) and/or carrier gas(es) may be controlled, for example, using a mass flow controller. One of ordinary skill in the art will recognize that the required flow rate or any precursor(s) and/or carrier gas(es) depends on the how far down the reactor the precursor vapors are required to travel. The required flow rates are also related to the diameter of the reaction tube; with increasing diameter, a higher flow rate is required to achieve the same vapor flow down the tube.

[0042] The pressure of the reaction chamber may be used to assist in controlling nucleation, and the thickness of the nanosheets. In one embodiment, the reaction is conducted at a reduced pressure, for example, below atmospheric pressure down to approximately 2 mbar. In another embodiment, the reaction is conducted at atmospheric pressure. In yet another embodiment, the reaction is conducted at a slight overpressure, for example, greater than atmospheric pressure up to approximately 1.2 bar.

[0043] The metal chalcogenide monolayers described herein may be used for a wide range of applications including, but not restricted to: optoelectronic devices, e.g. photodiodes, phototransistors, photodetectors, photovoltaics, light-emitting diodes, laser diodes; memory devices; field-effect transistors; inverters; logic gates; sensors; catalysis; fuel cells; batteries; plasmonic devices; photoluminescence applications, e.g. displays, lighting, optical barcoding, anti-counterfeiting; electroluminescence applications, e.g. displays, lighting; and biological applications, e.g. bioimaging, biosensing, photothermal therapy, photodynamic therapy, antibacterial activity, drug delivery.

[0044] By careful tuning of the reaction conditions, the lateral dimensions of the metal chalcogenide monolayers may be controlled. For example, in the prior art H.sub.2 has been introduced into the CVD reaction chamber to inhibit the lateral growth of MoS.sub.2 nanosheets formed from MoO.sub.3 and sulphur powder. [J. Jeon, J. Lee, G. Yoo, H.-H. Park, G. Y. Yeom, Y. H. Jang and S. Lee, Nanoscale, 2016, 8, 16995] In one embodiment, the gaseous selenium precursor is mixed with a reducing gas such as, but not restricted to, H.sub.2. In a further embodiment, the gaseous selenium precursor is mixed with a reducing gas and an inert carrier gas. The ratio of the reducing gas to the gaseous selenium precursor and/or the inert carrier gas may be varied to tune the lateral dimensions of the metal chalcogenide monolayers. One of ordinary skill in the art will recognize that the lateral dimensions of the metal chalcogenide monolayers may also be manipulated by varying reaction parameters such as, but not restricted to, temperature, pressure, time, gaseous precursor flow rate(s), and choice of precursor(s).

[0045] In some embodiments, the lateral dimensions of the metal chalcogenide monolayers are greater than 100 μm. “Large” (>100 μm) nanosheets may be advantageous for the growth of numerous electronic circuits on a single nanosheet. In further embodiments, the lateral dimensions of the metal chalcogenide monolayers are between 10 μm to 100 μm (“medium-sized” nanosheets). Medium-sized nanosheets are suitable for a range of electronics applications. In yet further embodiments, the lateral dimensions of the metal chalcogenide monolayers are less than 10 μm (“small” nanosheets). More particularly, the lateral dimensions of the metal chalcogenide monolayers may be in the quantum confinement regime, wherein the optical, electronic and chemical properties of the nanosheets may be manipulated by changing their lateral dimensions. For example, metal chalcogenide monolayer nanosheets of materials such as MoSe.sub.2 and WSe.sub.2 with lateral dimensions of approximately 10 nm or less may display properties such as size-tunable emission when excited by an energy source such as electricity or light. These size-tunable emission properties are particularly advantageous for applications such as displays, lighting, optical barcoding, anti-counterfeiting and biological imaging. Further, small nanosheets with a hydrodynamic diameter less than the glomerular filtration threshold of the kidneys are particularly suited to in vivo biological applications as they may be readily excreted via the kidneys.

Example: Synthesis of MoSe.SUB.2 .Nanosheets

[0046] The reaction set-up is illustrated in FIG. 2. MoO.sub.3 powder (10 mg) was placed in an alumina boat. A pre-cleaned SiO.sub.2/Si substrate was placed face-down on top of the boat. The boat was loaded into the center of a quartz reaction tube. The assembled reaction tube was placed within a tube furnace and connected to N.sub.2 and H.sub.2Se reaction gas lines, which were controlled by mass flow controllers, and exhaust lines. Prior to reaction, the tube was purged with vacuum/N.sub.2 cycles, before re-filling the chamber with N.sub.2 gas and maintaining the carrier gas flow at 90 sccm. The tube furnace was turned on, and followed the pre-programmed temperature profile shown in FIG. 3. When the furnace reached 730° C., H.sub.2Se was introduced at a rate of 10 sccm.

[0047] The reaction resulted in the growth of MoSe.sub.2 nanosheets on the SiO.sub.2/Si substrate. The lateral dimensions of the nanosheets ranged from sub-micron to 20 μm. The formation of monolayer MoSe.sub.2 was supported by Raman spectroscopy (FIG. 4), with the position of the A.sub.1g band matching well with that reported for MoSe.sub.2 monolayers in the literature [J. C. Shaw, H. Zhou, Y. Chen, N. O. Weiss, Y. Liu, Y. Huang and X. Duan, Nano Res., 2014, 7, 511] and lack of definition of the B.sup.1.sub.2g band.

[0048] These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it is to be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It is to be understood that this invention is not limited to the particular embodiments described herein and that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.