Name: Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication (Video) | JoVE Journal
Uploaded: 2017-11-28T13:00:00.000+00:00
Duration: 8 min 50 s
Description: Learn to fabricate large-area vertical 2D crystal heterostructures via sulfurization of transition metal films for enhanced device performance. This method enables precise layer control in MoS₂ and WS₂ films.

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5 minutes

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/pubmed?term=((High-Yield+Production+and+Transfer+of+Graphene+Flakes+Obtained+by+Anodic+Bonding%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">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://onlinelibrary.wiley.com/doi/10.1002/adma.201201909/abstract">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/pubmed?term=((Channel+Length+Scaling+of+MoS2+MOSFETs%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">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/pubmed?term=((Electronics+and+optoelectronics+of+two-dimensional+transition+metal+dichalcogenides%5BTitle%5D)+AND+%22Nat.+Nanotechnol%22%5BJournal%5D)">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/pubmed?term=((Single-layer+MoS2+transistors%5BTitle%5D)+AND+%22Nat.+Nanotechnol%22%5BJournal%5D)">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/pubmed?term=((Synthesis+of+Large-Area+MoS2+Atomic+Layers+with+Chemical+Vapor+Deposition%5BTitle%5D)+AND+%22Adv.+Mater%22%5BJournal%5D)">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/pubmed?term=((Controlled+Scalable+Synthesis+of+Uniform%2C+High-Quality+Monolayer+and+Few-layer+MoS2+Films%5BTitle%5D)+AND+%22Sci.+Rep%22%5BJournal%5D)">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/pubmed?term=((Role+of+the+Seeding+Promoter+in+MoS2+Growth+by+Chemical+Vapor+Deposition%5BTitle%5D)+AND+%22Nano+Lett%22%5BJournal%5D)">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/pubmed?term=((Synthesis+of+wafer-scale+uniform+molybdenum+disulfide+films+with+control+over+the+layer+number+using+a+gas+phase+sulfur+precursor%5BTitle%5D)+AND+%22Nanoscale%22%5BJournal%5D)">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://aip.scitation.org/doi/abs/10.1063/1.4883359">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://iopscience.iop.org/article/10.1088/0022-3727/48/43/435101/meta">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/pubmed?term=((Epitaxial+growth+of+a+monolayer+WSe2-MoS2+lateral+p-n+junction+with+an+atomically+sharp+interface%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">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://onlinelibrary.wiley.com/doi/10.1002/smll.201102654/abstract">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://pubs.acs.org/doi/abs/10.1021/acsnano.5b06126">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="https://www.ncbi.nlm.nih.gov/pubmed/28176836">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://iopscience.iop.org/article/10.1088/1361-6463/aa52b6/pdf">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/pubmed?term=((Establishment+of+2D+Crystal+Heterostructures+by+Sulfurization+of+Sequential+Transition+Metal+Depositions%3A+Preparation%2C+Characterization%2C+and+Selective+Growth%5BTitle%5D)+AND+%22Nano+Lett%22%5BJournal%5D)">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://aip.scitation.org/doi/abs/10.1063/1.4893448">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/pubmed?term=((Anomalous+Lattice+Vibrations+of+Single+and+Few-Layer+MoS2%5BTitle%5D)+AND+%22ACS+Nano%22%5BJournal%5D)">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://iopscience.iop.org/article/10.1088/2053-1583/aa75a7">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, &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

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="Chemistry symbol E1/2g diagram for molecular orbital theory; educational use....

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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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.

Fabricating van der Waals Heterostructures with Precise Rotational Alignment

This work presents a detailed protocol for the microfabrication of nanostructured α-quartz cantilever on a Silicon-On-Insulator(SOI) technology substrate starting from the epitaxial growth of quartz film with the dip coating method and then nanostructuration of the thin film via nanoimprint lithography.

Epitaxial Nanostructured &alpha;-Quartz Films on Silicon: From the Material to New Devices

This study presents a residue-free fabrication methodology for producing single flakes of two-dimensional materials and assembling them into complex heterostructures using only van der Waals interactions. The technique eliminates the need for external substances and specific experimental conditions, enabling complex heterostructure assemblies through bottom-up, top-down, and modular stacking processes.

Residue-Free Fabrication of van der Waals Heterostructures of Two-Dimensional Materials

The two-thermal annealing method enables the fabrication of thin films of type II semiconductor silicon clathrate. Post-synthesis treatments, including thermal pressing and reactive ion etching, were performed to improve the material properties. This approach provides an accessible pathway for fabricating tunable clathrate films suitable for next-generation electronic and photonic devices.

Fabrication and Optimization of Type II Silicon Clathrate Films

Learn to fabricate large-area vertical 2D crystal heterostructures via sulfurization of transition metal films for enhanced device performance. This method enables precise layer control in MoS₂ and WS₂ films.

Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication (Video) | JoVE Journal

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

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