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 10-6 torr sequentially using a mechanical pump followed by a diffusion pump.</li>
		<li>Inject the Ar gas into the sputtering system and keep the gas flow at 40 mL/min using a mass flow controller (MFC).</li>
		<li>Keep the chamber pressure at 5 x 10-2 torr using a manual pressure control valve and ignite the Ar plasma. Keep the output power at 40 W.</li>
		<li>Decrease the chamber pressure to 5 x 10-3 torr using a manual wheel Angle poppet valve.</li>
		<li>Manually open the shutter between the sapphire substrate and the 2-inch metal target and start the metal deposition. During the deposition procedure, keep the sputtering power at 40 W for both Mo and W. The background pressure is kept at 5 &#215; 10-3 torr with 40 mL/min Ar gas flow.</li>
		<li>Control the sputtering time to deposit transition metal films with different thicknesses. Due to the thin metal thickness, sputtering times will provide better control over the film thickness than the readings from the quartz crystal resonator. 
		NOTE: The layer numbers of MoS2 and WS2 grown using the method discussed in the current manuscript are proportional to the sputtering times of the pre-deposited Mo and W films. The determination of sputtering times to obtain MoS2 and WS2 with required layer numbers is based on the cross-sectional high-resolution transmission electron microscopy (HRTEM) images for samples with different sputtering times. However, if the pre-deposited Mo and W films are too thick, Mo and W oxide segregation will become the dominant growth mechanism, instead of planar MoS2 and WS2 film growth. Therefore, the proportionality of layer numbers with the sputtering times is limited to few-layer TMDs. With the growth condition of MoS2 in the current manuscript, the layer numbers will be proportional to the sputtering times when the MoS2 film is fewer than 10 layers. The sputtering time is 30 s for the growth of 5-layer MoS2.</li>
	</ol>
	</li>
	<li>The sulfurization of the transition metal film
	<ol>
		<li>Place the sapphire substrates with pre-deposited transition metal films in the center of a hot furnace for sulfurization.</li>
		<li>Place the sulfur (S) powder upstream of the gas flow, 2 cm away from the heating zone of the furnace. In this position, the evaporation temperature for the S powder will be 120 &#176;C when the substrate temperature increases to 800 &#176;C. Precisely control the S powder weight for different transition metals for sulfurization. In the work, the S powder weight is 1.5 g for Mo and 1.0 g for W. 
		NOTE: We determine sulfur powder amounts for the preparation of MoS2 and WS2 films based on the results obtained for each material prepared using different amounts of sulfur powder.</li>
		<li>Keep the furnace pressure at 0.7 torr. During the sulfurization procedure, 130 mL/min Ar gas was used as carrier gas.</li>
		<li>Ramp the temperature of the furnace from room temperature to 800 &#176;C in 40 min with a temperature ramping rate of 20 &#176;C/min. Keep the temperature at 800 &#176;C until the sulfur powder is fully evaporated. After that, the heater power is switched off to lower the furnace temperature. It takes about 30 to 40 min for the furnace to reach room temperature from 800 &#176;C.</li>
	</ol>
	</li>
	<li>Perform Raman spectrum measurements using a 488 nm laser15,16,17. Obtain the cross-sectional HRTEM images to verify the layer numbers of the 2D crystals15,16,17.</li>
</ol>
2. The Growth of the WS2/MoS2 Vertical Single Hetero-structure
NOTE: This section is used to create a single hetero-structure consisting of a sapphire layer with 5 layers of MoS2 and 4 layers of WS2.
<ol>
	<li>Follow the same procedure as step 1.1. Deposit the Mo film on the sapphire substrate using the RF sputtering system with a 30 s sputtering time.</li>
	<li>Sulfurize the Mo film following the same sulfurization procedures as step 1.2 for the growth of MoS2. Five layers of MoS2 will be obtained after the sulfurization procedure.</li>
	<li>Follow the same procedures as step 1.1. Deposit the W film on the MoS2/sapphire substrate using the RF sputtering system with a 30 s sputtering time.</li>
	<li>Sulfurize the W film following the same sulfurization procedure of step 1.2 for the growth of WS2. Four layers of WS2 will be obtained on top of the MoS2 after the sulfurization procedure. 
	NOTE: The metal deposition and sulfurization procedure is the same as the individual material. The sputtering times of the Mo and W films are determined depending on the required layer numbers of MoS2 and WS2 layers. Double- or multi-hetero-structures can be established by repeating the same growth procedure. The sequence of the TMDs in the vertical hetero-structures can also be changed depending on the sample structure.</li>
</ol>
3. The Film Transferring and Device Fabrication Procedures
<ol>
	<li>The film transferring procedure of the 2D crystal films
	<ol>
		<li>Spin coat three drops of poly(methyl methacrylate) (PMMA) on the TMD film to cover the whole film at room temperature. The two-stage rotation speeds of the spinner are 500 rpm for 10 s and 800 rpm for 10 s. After curing at 120 &#176;C for 5 min, the PMMA thickness is around 3 &#181;m.</li>
		<li>Place the PMMA/TMD/Sapphire sample in a Petri dish which is filled with deionized (DI) water.</li>
		<li>Peel off one corner of the PMMA/TMD film from the sapphire substrate using tweezers in DI water.</li>
		<li>Heat 250 mL of 1 M KOH aqueous solution (14 g KOH pellets mixed with 250 mL water) in a beaker to 100 &#176;C. Move the sample in the heated KOH aqueous solution and continue peeling the PMMA/TMD film until the film is completely peeled off from the substrate. Peeling requires about 1 min to complete.</li>
		<li>Use a separate sapphire substrate to scoop up the PMMA/TMD film from the KOH solution. Move the film to a 250 mL beaker filled with DI water to wash off residue KOH on the film. At this stage, the adhesion between the PMMA/TMD film and the sapphire substrate to scoop the film is weak. Therefore, the film will de-attach from the sapphire substrate after immersion into the DI water.</li>
		<li>Repeat steps 3.1.4 - 3.1.5 three times using new DI water to make sure that most of the residue KOH is removed from the film. 
		NOTE: The adhesion between each TMD layer is much stronger than TMDs with the sapphire substrate. Therefore, the same transferring procedure can be applied to either individual MoS2/WS2 materials or their hetero-structures. The 2D crystal films will be completely peeled off from the substrate, which is similar to the peeling of the MoS2/graphene hetero-structure discussed in a previous publication18. Depending on the purpose of transferring the film, the substrate mentioned in this step can either be a sapphire substrate or a SiO2/Si substrate with pre-deposited electrodes, as described in step 3.2. Other substrates can also be used for this purpose.</li>
	</ol>
	</li>
	<li>The fabrication of 2D crystal transistors.
	<ol>
		<li>Use standard photolithography to define electrode patterns on SiO2/Si substrates15,16,17. Source and drain electrodes made of 10 nm titanium (Ti) or 100 nm gold (Au) are fabricated on a 300 nm SiO2/p-type Si substrate.</li>
		<li>Immerse the SiO2/Si substrate with pre-patterned source/drain electrodes into the beaker filled with DI water and attach to the TMD side of the PMMA/TMD film as prepared in step 3.1.</li>
		<li>Bake the sample at 100 &#176;C for 3 min, after the film is attached to the SiO2/Si substrate, to remove water residue.</li>
		<li>Drip three drops of PMMA on the sample with the PMMA/TMD film to cover the whole surface and make the film more firmly attached to the substrate.</li>
		<li>Place the sample in an electronic dry cabinet for at least 8 h before moving to the next step.</li>
		<li>Fill two different 250 mL beakers with acetone. Immerse the sample attached with the PMMA/TMD film sequentially into the two different beakers filled with acetone for 50 and 10 min, respectively, to remove the top PMMA layer.</li>
		<li>Define the transistor channel using standard photo-lithography and reactive-ion etching15,16,17. Back-gate MoS2 and WS2/MoS2 hetero-structure transistors are fabricated15,16,17. The channel length and width of the devices are 5 and 150 &#956;m, respectively.</li>
		<li>Use a dual-channel system sourcemeter instrument to measure the current-voltage characteristics of the transistors15,16,17.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Moldt, T., 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=High-Yield+Production+and+Transfer+of+Graphene+Flakes+Obtained+by+Anodic+Bonding.">High-Yield Production and Transfer of Graphene Flakes Obtained by Anodic Bonding.</a> ACS Nano. 5, 7700-7706 (2011).</li><li>Choi, W., 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=High-Detectivity+Multilayer+MoS2+Phototransistors+with+Spectral+Response+from+Ultraviolet+to+Infrared.">High-Detectivity Multilayer MoS2 Phototransistors with Spectral Response from Ultraviolet to Infrared.</a> Adv. Mater. 24, 5832-5836 (2012).</li><li>Liu, H., Neal, A. T., Ye, P. 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. Nanotechnol. 6, 147-150 (2011).</li><li>Lee, Y. H., 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+Large-Area+MoS2+Atomic+Layers+with+Chemical+Vapor+Deposition.">Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition.</a> Adv. Mater. 24, 2320-2325 (2012).</li><li>Yu, Y., Li, C., Liu, Y., Su, L., Zhang, Y., Cao, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Controlled+Scalable+Synthesis+of+Uniform,+High-Quality+Monolayer+and+Few-layer+MoS2+Films.">Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-layer MoS2 Films.</a> Sci. 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.
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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, &gt; 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) &amp;plusmn; 0.2 &amp;deg; one side polished</td></tr><tr><td>300 nm SiO&lt;sub&gt;2&lt;/sub&gt;/Si substrate</td><td>Summit-Tech</td><td>2YCDDM</td><td>P-type Si substrate, resistivity: 1-10 &amp;Omega; &amp;middot; 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 &amp;times; 10&lt;sup&gt;-8&lt;/sup&gt; 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<...

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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 ...

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Engineering

9.1K Views.  National Taiwan University. Via de sulfurization van eerder afgegeven overgangsmetalen, kunnen groot-gebied en verticale 2D hetero-kristalstructuren worden vervaardigd. De film overbrengen en apparaat fabricage procedures zijn ook aangetoond in dit verslag.

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

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Atomically Traceable Nanostructure Fabrication

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

We report a protocol for combining the atomic metrology of the Scanning Tunneling Microscope for surface patterning with selective Atomic Layer Deposition and Reactive Ion Etching. Using a robust process involving numerous atmospheric exposures and transport, 3D nanostructures with atomic metrology are fabricated.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film ...

Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. ...

Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with ...

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 &#176;C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 &#176;C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, ...

Chemistry

Synthesis of a Thiol Building Block for the Crystallization of a Semiconducting Gyroidal Metal-sulfur Framework

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and porous metal-dithiolene network in the highly ordered single crystalline state. Unlike the highly reactive free-standing thiols, which tend to decompose and complicate the crystallization of metal-thiolate open frameworks, the thioester reacts in situ to provide the thiol species, serving to mitigate the reaction between the mercaptan units and the metal centers, and to improve crystallization consequently. Specifically, the thioester was synthesized in a one-pot procedure: an aromatic bromide (hexabromotriphenylene) reacted with excess sodium thiomethoxide under vigorous conditions to first form the thioether intermediate product. The thioether was then demethylated by the excess thiomethoxide to provide the thiolate anion that was acylated to form the thioester product. The thioester was conveniently purified by standard column chromatography, and then used directly in the framework synthesis, wherein NaOH and ethylenediamine serve to revert in situ the thioester to the thiol linker for assembling the single-crystalline Pb(II)-dithiolene network. Compared with other methods for thiol synthesis (e.g., by cleaving alkyl thioether using sodium metal and liquid ammonia), the thioester synthesis here uses simple conditions and economical reagents. Moreover, the thioester product is stable and can be conveniently handled and stored. More importantly, in contrast to the generic difficulty in accessing crystalline metal-thiolate open frameworks, we demonstrate that using the thioester for in situ formation of the thiol linker greatly improves the crystallinity of the solid-state product. We intend to encourage broader research efforts on the technologically important metal-sulfur frameworks by disclosing the synthetic protocol for the thioester as well as the crystalline framework solid.

Here, we present a one-pot, transition-metal-free synthesis of thiols and thioesters from aromatic halides and sodium thiomethoxide, followed by the preparation of single crystals of a metal-dithiolene network using thiol species generated in situ from the more stable and tractable thioester.

We present a method for preparing thioester molecules as the masked form of the thiol linkers and their utilization for accessing a semiconducting and ...

A Standard and Reliable Method to Fabricate Two-Dimensional Nanoelectronics

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D materials is still in nascent stages, scientists cannot fully rely on traditional semiconductor techniques for related research. Delicate processes from locating the materials to electrode definition need to be well controlled. In this article, a universal fabrication protocol required in manufacturing nanoscale electronics, such as 2D quasi-heterojunction bipolar transistors (Q-HBT), and 2D back-gated transistors are demonstrated. This protocol includes the determination of material position, electron beam lithography (EBL), metal electrode definition, et al. A step by step narrative of the fabrication procedures for these devices are also presented. Furthermore, results show that each of the fabricated devices has achieved high performance with high repeatability. This work reveals a comprehensive description of process flow for preparing 2D nano-electronics, enables the research groups to access this information, and pave the way toward future electronics.

The article aims to introduce a standard and reliable fabrication procedure for the development of future low dimensional nanoelectronics.

Two-dimensional (2D) materials have attracted huge attention due to their unique properties and potential applications. Since wafer scale synthesis of 2D ...

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We demonstrate not only lateral control but, importantly, also control over the angular alignment of adjacent layers. The core of the technique is represented by a home-built transfer setup which allows the user to control the position of the individual crystals involved in the transfer. This is achieved with sub-micrometer (translational) and sub-degree (angular) precision. Prior to stacking them together, the isolated crystals are individually manipulated by custom-designed moving stages that are controlled by a programmed software interface. Moreover, since the entire transfer setup is computer controlled, the user can remotely create precise heterostructures without coming into direct contact with the transfer setup, labeling this technique as &#8220;hands-free&#8221;. In addition to presenting the transfer set-up, we also describe two techniques for preparing the crystals that are subsequently stacked.

In this work we describe a technique that is used to create new crystals (van der Waals heterostructures) by stacking ultrathin layered 2D materials with precise control over position and relative orientation.

In this work we describe a technique for creating new crystals (van der Waals heterostructures) by stacking distinct ultrathin layered 2D materials. We ...

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

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