This work was supported in part by projects MOST 105-2221-E-001-011-MY3 and MOST 105-2622-8-002-001 funded by the Ministry of Science and Technology, Taiwan, and in part by the focused project funded by the Research Center for Applied Sciences, Academia Sinica, Taiwan.

1. Growth of Individual 2D material (MoS2 and WS2)
<ol>
	<li>Transition metal deposition using an RF sputtering system
	<ol>
		<li>A clean 2 x 2 cm2 sapphire substrate is placed on the sample holder with the polished side toward the targets of the sputtering system for the transition metal deposition. Sapphire substrates are chosen due to sapphire&#39;s chemical stability at high temperatures and atomic-flat surfaces.</li>
		<li>Pump down the sputtering chamber to 3 x 1...

<ol>
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D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Channel+Length+Scaling+of+MoS2+MOSFETs.">Channel Length Scaling of MoS2 MOSFETs.</a> ACS Nano. 6, 8563-8569 (2012).</li><li>Wang, Q. H., Kalantar-Zadeh, K., Kis, A., Coleman, J. N., Strano, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electronics+and+optoelectronics+of+two-dimensional+transition+metal+dichalcogenides.">Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.</a> Nat. Nanotechnol. 7, 699-712 (2012).</li><li>Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V., Kis, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-layer+MoS2+transistors.">Single-layer MoS2 transistors.</a> Nat. 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Rep. 3, 1866 (2013).</li><li>Ling, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+the+Seeding+Promoter+in+MoS2+Growth+by+Chemical+Vapor+Deposition.">Role of the Seeding Promoter in MoS2 Growth by Chemical Vapor Deposition.</a> Nano Lett. 14, 464-472 (2014).</li><li>Lee, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synthesis+of+wafer-scale+uniform+molybdenum+disulfide+films+with+control+over+the+layer+number+using+a+gas+phase+sulfur+precursor.">Synthesis of wafer-scale uniform molybdenum disulfide films with control over the layer number using a gas phase sulfur precursor.</a> Nanoscale. 6, 2821-2826 (2014).</li><li>Lin, M. Y., Su, C. F., Lee, S. C., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Growth+Mechanisms+of+Graphene+Directly+on+Sapphire+Substrates+using+the+Chemical+Vapor+Deposition.">The Growth Mechanisms of Graphene Directly on Sapphire Substrates using the Chemical Vapor Deposition.</a> J. Appl. Phys. 115, 223510 (2014).</li><li>Wu, C. R., Chang, X. R., Chang, S. W., Chang, C. E., Wu, C. H., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multilayer+MoS2+prepared+by+one-time+and+repeated+chemical+vapor+depositions:+anomalous+Raman+shifts+and+transistors+with+high+ON/OFF+ratio.">Multilayer MoS2 prepared by one-time and repeated chemical vapor depositions: anomalous Raman shifts and transistors with high ON/OFF ratio.</a> J. Phys. D Appl. Phys. 48, 435101 (2015).</li><li>Li, M. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epitaxial+growth+of+a+monolayer+WSe2-MoS2+lateral+p-n+junction+with+an+atomically+sharp+interface.">Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface.</a> Science. 349, 524-528 (2015).</li><li>Zhan, Y., Liu, Z., Najmaei, S., Ajayan, M. P., Lou, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large-area+vapor-phase+growth+and+characterization+of+MoS2+atomic+layers+on+a+SiO2+substrate.">Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate.</a> Small. 8, 966 (2012).</li><li>Woods, J. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=One-Step+Synthesis+of+MoS2/WS2+Layered+Heterostructures+and+Catalytic+Activity+of+Defective+Transition+Metal+Dichalcogenide+Films.">One-Step Synthesis of MoS2/WS2 Layered Heterostructures and Catalytic Activity of Defective Transition Metal Dichalcogenide Films.</a> ACS Nano. 10, 2004-2009 (2016).</li><li>Wu, C. R., Chang, X. R., Wu, C. H., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Growth+Mechanism+of+Transition+Metal+Dichalcogenides+using+Sulfurization+of+Pre-deposited+Transition+Metals+and+the+2D+Crystal+Hetero-structure+Establishment.">The Growth Mechanism of Transition Metal Dichalcogenides using Sulfurization of Pre-deposited Transition Metals and the 2D Crystal Hetero-structure Establishment.</a> Sci. Rep. 7, 42146 (2017).</li><li>Chen, K. C., Chu, T. W., Wu, C. R., Lee, S. C., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Layer+Number+Controllability+of+Transition-metal+Dichalcogenides+and+The+Establishment+of+Hetero-structures+using+Sulfurization+of+Thin+Transition+Metal+Films.">Layer Number Controllability of Transition-metal Dichalcogenides and The Establishment of Hetero-structures using Sulfurization of Thin Transition Metal Films.</a> J. of Phys. D: Appl. Phy. 50, 064001 (2017).</li><li>Wu, C. R., Chang, X. R., Chu, T. W., Chen, H. A., Wu, C. H., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Establishment+of+2D+Crystal+Heterostructures+by+Sulfurization+of+Sequential+Transition+Metal+Depositions:+Preparation,+Characterization,+and+Selective+Growth.">Establishment of 2D Crystal Heterostructures by Sulfurization of Sequential Transition Metal Depositions: Preparation, Characterization, and Selective Growth.</a> Nano Lett. 16, 7093-7097 (2016).</li><li>Lin, M. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+epitaxially+grown+two-dimensional+crystal+hetero-structures:+Single+and+double+MoS2/graphene+hetero-structures+by+chemical+vapor+depositions.">Toward epitaxially grown two-dimensional crystal hetero-structures: Single and double MoS2/graphene hetero-structures by chemical vapor depositions.</a> Appl. Phys. Lett. 105, 073501 (2014).</li><li>Lee, C., Yan, H., Brus, L. E., Heinz, T. F., Hone, J., Ryu, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anomalous+Lattice+Vibrations+of+Single+and+Few-Layer+MoS2.">Anomalous Lattice Vibrations of Single and Few-Layer MoS2.</a> ACS Nano. 4, 2695-2700 (2010).</li><li>Chen, K. C., Chu, T. W., Wu, C. R., Lee, S. C., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+Layer+Etchings+of+Transition+Metal+Dichalcogenides+with+Post+Healing+Procedures:+Equivalent+Selective+Etching+of+2D+Crystal+Hetero-structures.">Atomic Layer Etchings of Transition Metal Dichalcogenides with Post Healing Procedures: Equivalent Selective Etching of 2D Crystal Hetero-structures.</a> 2D Mater. 4, 034001 (2017).</li></ol>

The authors have nothing to disclose.

Compared with conventional semiconductor materials such as Si and GaAs, the advantage of 2D materials for device applications lies in the possibility of device fabrication with very thin bodies down to several atomic layers. When the Si industry advances into the &#60;10 nm technology node, the high aspect ratio of Si fin FET will make the device architecture unsuitable for practical applications. Thus, 2D materials have emerged due to their potential to replace Si for electronic device applications.
<p class="jove_c...

One of the most common approaches to obtain 2D crystal films is using mechanical exfoliation from bulk materials1,2,3,4,5. Although 2D crystal films with high crystalline quality can be easily obtained using this method, scalable 2D crystal films are not available through this approach, which is disadvantageous for practical applications. It has been demonstrated in previous publications that using chemical vapor deposition (CVD), large-area and uniform 2D crystal films can be prepared6,7,8,9. Direct growth of graphene on sapphire substrates and layer-number-controllable MoS2 films prepared by repeating the same growth cycle are also demonstrated using the CVD growth technique10,11. In one recent publication, in-plane WSe2/MoS2 hetero-structure flakes are also fabricated using the CVD growth technique12. Although the CVD growth technique is promising in providing scalable 2D crystal films, the major disadvantage of this growth technique is that different precursors have to be located for different 2D crystals. The growth conditions also vary between different 2D crystals. In this case, the growth procedures will become more complicated when demand grows for 2D crystal hetero-structures.
Compared with the CVD growth technique, the sulfurization of pre-deposited transition metal films has provided a similar but much simpler growth approach for TMDs13,14. Since the growth procedure involves only metal deposition and the following sulfurization procedure, it is possible to grow different TMDs through the same growth procedures. On the other hand, the layer number controllability of the 2D crystals may also be achieved by changing the pre-deposited transition metal thicknesses. In this case, growth optimization and layer number control down to a single layer are required for different TMDs. Understanding growth mechanisms is also very important for the establishment of complicated TMD hetero-structures using this method.
In this paper, MoS2 and WS2 films are prepared under similar growth procedures of the metal deposition followed by the sulfurization procedure. With the results obtained from the sulfurization of Mo films under sulfur sufficient and deficient conditions, two growth mechanisms are observed during the sulfurization procedure15. Under the sulfur sufficient condition, a uniform and layer-number-controllable MoS2 film can be obtained after the sulfurization procedure. When the sample is sulfurized under the sulfur deficient condition, the background sulfur is not sufficient to form a complete MoS2 film such that the Mo oxide segregation and coalescence will be the dominant mechanism at the early growth stage. A sample with Mo oxide clusters covered by few layers of MoS2 will be obtained after the sulfurization procedure15. Through sequential metal deposition and following sulfurization procedures, WS2/MoS2 vertical hetero-structures with layer number controllability down to a single layer can be prepared15,16. Using this technique, a sample is obtained on a single sapphire substrate with four regions: (I) blank sapphire substrate, (II) standalone MoS2, (III) WS2/MoS2 hetero-structure, and (IV) standalone WS217. The results demonstrate that the growth technique is advantageous for the establishment of vertical 2D crystal hetero-structure and is capable of selective growth. The enhanced device performances of 2D crystal hetero-structures will mark the first step toward practical applications for 2D crystals.

<table>
	<tbody>
		<tr>
			<td>RF sputtering system</td>
			<td>Kao Duen Technology</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Furnace for sulfurization</td>
			<td>Creating Nano Technologies</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Polymethyl methacrylate (PMMA)</td>
			<td>Microchem</td>
			<td>8110788</td>
			<td>Flammable</td>
		</tr>
		<tr>
			<td>KOH, &#62; 85%</td>
			<td>Sigma-Aldrich</td>
			<td>30603</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone, 99.5%</td>
			<td>Echo Chemical</td>
			<td>CMOS110</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfur (S), 99.5%</td>
			<td>Sigma-Aldrich</td>
			<td>13803</td>
			<td></td>
		</tr>
		<tr>
			<td>Molybdenum (Mo), 99.95%</td>
			<td>Summit-Tech</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Tungsten (W), 99.95%</td>
			<td>Summit-Tech</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>C-plane Sapphire substrate</td>
			<td>Summit-Tech</td>
			<td>X171999</td>
			<td>(0001) &#177; 0.2 &#176; one side polished</td>
		</tr>
		<tr>
			<td>300 nm SiO2/Si substrate</td>
			<td>Summit-Tech</td>
			<td>2YCDDM</td>
			<td>P-type Si substrate, resistivity: 1-10 &#937; &#183; cm.</td>
		</tr>
		<tr>
			<td>Sample holder (sputtering system)</td>
			<td>Kao Duen Technology</td>
			<td>N/A</td>
			<td>Ceramic material</td>
		</tr>
		<tr>
			<td>Mechanical pump (sputtering system)</td>
			<td>Ulvac</td>
			<td>D-330DK</td>
			<td></td>
		</tr>
		<tr>
			<td>Diffusion pump (sputtering system)</td>
			<td>Ulvac</td>
			<td>ULK-06A</td>
			<td></td>
		</tr>
		<tr>
			<td>Mass flow controller</td>
			<td>Brooks</td>
			<td>5850E</td>
			<td>The maximum Argon flow is 400 mL/min</td>
		</tr>
		<tr>
			<td>Manual wheel Angle poppet valve</td>
			<td>King Lai</td>
			<td>N/A</td>
			<td>Vacuum range from 2500 ~1 &#215; 10-8 torr</td>
		</tr>
		<tr>
			<td>Raman measurement system</td>
			<td>Horiba</td>
			<td>Jobin Yvon LabRAM HR800</td>
			<td></td>
		</tr>
		<tr>
			<td>Transmission electron microscopy</td>
			<td>Fei</td>
			<td>Tecnai G2 F20</td>
			<td></td>
		</tr>
		<tr>
			<td>Petri dish</td>
			<td>Kwo Yi</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Tweezer</td>
			<td>Venus</td>
			<td>2A</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital dry cabinet</td>
			<td>Jwo Ruey Technical</td>
			<td>DRY-60</td>
			<td></td>
		</tr>
		<tr>
			<td>Dual-channel system sourcemeter</td>
			<td>Keithley</td>
			<td>2636B</td>
			<td></td>
		</tr>
	</tbody>
</table>

preparation of large-area vertical 2d crystal hetero-structures through the sulfurization of transition metal films for device fabrication

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.

The Raman spectrum and the cross-sectional HRTEM images of individual MoS2 and WS2 fabricated using the sulfurization of pre-deposited transition metals are shown in Figure 1a-b17, respectively. Two characteristic Raman peaks are observed for both MoS2 and WS2, which correspond to in-plane <img alt="Equation 1" src="/files/ftp_upload/56494/56494eq1.jpg" /> and out-of-plane A<...

Watch this Scientific Journal Video about Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication at JoVE.com

Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication

Through the sulfurization of pre-deposited transition metals, large-area and vertical 2D crystal hetero-structures can be fabricated. The film transferring and device fabrication procedures are also demonstrated in this report.

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition ...

The overall goal of this growth technique is to establish complex vertical 2D crystal heterostructures, with good control over the number of 2D layers, using similar growth procedures for each material. The main advantage of this simple technique is that it uses similar growth procedures for different 2D materials and it has good layer number controllability. This technique can help investigate practical applications of vertical 2D material heterostructures as it can produce larger vertical 2D crystal heterostructures with enhanced transitor properties compared to single material structures.To begin the procedure, mount a clean C-plane two centimeter by two centimeter sapphire substrate on the sample stage of an RF sputtering system, with the polished side facing the molybdenum target. Pump down the sample chamber to three times 10 to the negative six tores. Inject argon gas into the system at 40 milliliters per minute and ensure that the chamber pressure is stable at five times 10 to the negative two tores.Ignite the plasma, and set the power output to 40 watts. Reduce the chamber pressure to five times 10 to the negative three tores. Then, manually open the molybdenum target shutter and deposit the metal for 30 seconds.Ensure that the power output remains constant throughout deposition. When sputtering has finished, place the substrate in a quart sample holder with the metal film facing up. Set the substrate in the center of the heating zone of a calibrated tube furnace.Place 1.5 grams of sulfur powder in a alumina heating boat. Place the boat two centimeters upstream of the heating zone so that the sulfur temperature will be 120 degrees Celsius when the substrate reaches 800 degrees Celsius. Close and pump down the furnace to five times 10 to the negative three tores.Then, flow argon gas through the furnace at 130 milliliters per minute. Ensure that the furnace pressure stabilizes at 0.7 tores. Ramp the furnace from room temperature to 800 degrees Celsius at 20 degrees Celsius per minute.Hold the furnace at 800 degrees Celsius until the sulfur has fully evaporated. Then, turn off the furnace heat and allow the substrate to cool to room temperature under a flow of argon. Next, mount the substrate in the RF sputtering system with the molybdenum disulfide film facing the tungsten target.Sputter tungsten on the substrate for 30 seconds using the same settings as for molybdenum. Place the tungsten-coated substrate in the center of the furnace heating zone and one gram of sulfur powder two centimeters upstream of the heating zone. Sulfurize the tungsten film using the same parameters as for the molybdenum film.To begin the thin film transfer, spin coat three drops of polymethyl methacrylate on a prepared transition metal dichalcogenide film for 10 seconds each at 500 and 800 rotations per minute. Cure the PMMA at 120 degrees Celsius for five minutes. Then, place the PMMA-coated substrate in a Petri dish filled with deionized water.Use tweezers with round, flat tips to carefully peel one corner of the PMMA with the attached TMD film from the substrate. Transfer the substrate to a one molar aqueous potassium hydroxide solution heated to 100 degrees Celsius. Slowly and carefully peel the entire PMMA TMD film from the substrate using tweezers.Scoop up the PMMA TMD film from the potassium hydroxide solution with another clean sapphire substrate with the polished side facing the film. Transfer the film to a beaker of deionized water. Transfer the film to fresh deionized water three more times in the same way, to ensure that residual potassium hydroxide is completely removed.Next, obtain a 300 nanometer thick P type silicon dioxide on silicon substrate patterned with titanium or gold electrodes. Immerse the patterned substrate in the final beaker of deionized water and carefully attach the TMD film to the substrate. Bake the substrate with the attached film at 100 degrees Celsius for three minutes to evaporate the water.Cover the substrate surface with PMMA to ensure that the film is firmly attached to the substrate. Dry the substrate in an electronic drying cabinet for eight hours. Then, remove the PMMA layer and use photolithography and reactive ion etching to fabricate a TMD heterostructure transistor.The molybdenum sulfurization mechanism was examined with HRTEM. Under sulfur rich conditions, the sulfur rapidly replaced the oxygen of molybdenum oxides, eventually forming a uniform film with a controllable number of layers. Molybdenum oxide segregation and coalescence was the dominant early growth mechanism in sulfur deficient conditions.Raman spectroscopy of individual molybdenum disulfide and tungsten disulfide films each showed two characteristic peaks. HRTEM showed five molybdenum disulfide layers and four tungsten disulfide layers. Sequential metal deposition resulted in a tungsten disulfide molybdenum disulfide vertical heterostructure.Both sets of characteristic peaks were apparent in the Ramen spectrum. The formation of a vertical TMD heterostructure was supported by repeated atomic etching of individual tungsten disulfide layers until only molybdenum disulfide remained. A transistor fabricated with a tungsten disulfide molybdenum disulfide single heterostructure showed a substantially greater drain current when compared to a transistor fabricated with only molybdenum disulfide.The single heterostructure transistor had a higher field effect mobility value, which could have been the result of electron injection from tungsten disulfide to molybdenum disulfide and from channels with higher electron concentrations under thermal equilibrium. After watching this video, you should have a good understanding of how to use a equipment and procedures shown to easily establish vertical 2D material heterostructures for different device applications. When using this protocol, you should only need to optimize the sputtering powers, the position times and separation temperatures for your equipment to obtain high quality 2D materials and heterostructures.

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9.1K visualizações.  National Taiwan University. The overall goal of this growth technique is to establish complex vertical 2D crystal heterostructures, with good control over the number of 2D layers, using similar growth procedures for each material. The main advantage of this simple technique is that it uses similar growth procedures for different 2D materials and it has good layer number controllability. This technique can help investigate practical applications of vertical 2D material heterostructures as it can produce larger vertical 2...