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>
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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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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 ...

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9.1K Views.  National Taiwan University. 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.

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

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Fabrication of VB2/Air Cells for Electrochemical Testing

A technique to investigate the properties and performance of new multi-electron metal/air battery systems is proposed and presented. A method for synthesizing nanoscopic VB2 is presented as well as step-by-step procedure for applying a zirconium oxide coating to the VB2 particles for stabilization upon discharge. The process for disassembling existing zinc/air cells is shown, in addition construction of the new working electrode to replace the conventional zinc/air cell anode with a the nanoscopic VB2 anode. Finally, discharge of the completed VB2/air battery is reported. We show that using the zinc/air cell as a test bed is useful to provide a consistent configuration to study the performance of the high-energy high capacity nanoscopic VB2 anode.

Fabrication of VB2/Air Cells for Electrochemical Testing

A protocol is presented to study multi-electron metal/air battery systems by using previous technology developed for the zinc/air cell. Electrochemical testing is then performed on fabricated batteries to evaluate performance.

A technique to investigate the properties and performance of new multi-electron metal/air battery systems is proposed and presented. A method for ...

Fabrication of Low Temperature Carbon Nanotube Vertical Interconnects Compatible with Semiconductor Technology

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive thin-films. Due to the low growth temperature, the process allows integration with modern low-&#954; dielectrics and some flexible substrates. The process is compatible with standard semiconductor fabrication, and a method for the fabrication of electrical 4-point probe test structures for vertical interconnect test structures is presented. Using scanning electron microscopy the morphology of the CNT bundles is investigated, which demonstrates vertical alignment of the CNT and can be used to tune the CNT growth time. With Raman spectroscopy the crystallinity of the CNT is investigated. It was found that the CNT have many defects, due to the low growth temperature. The electrical current-voltage measurements of the test vertical interconnects displays a linear response, indicating good ohmic contact was achieved between the CNT bundle and the top and bottom metal electrodes. The obtained resistivities of the CNT bundle are among the average values in the literature, while a record-low CNT growth temperature was used.

A method for the growth of low temperature vertically-aligned carbon nanotubes, and the subsequent fabrication of vertical interconnect electrical test structures using semiconductor fabrication is presented.

We demonstrate a method for the low temperature growth (350 &#176;C) of vertically-aligned carbon nanotubes (CNT) bundles on electrically conductive ...

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. ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve desired molecular orientation states in LC displays, the "static" surface property of LCs, so-called surface anchoring, has been intensively studied. As a rule of thumb, once the initial orientation of LCs is "locked" by specific surface treatments, such as rubbing or treatment with a specific alignment layer, it hardly changes with temperature. Here, we present a system exhibiting an orientational transition upon temperature variation, which conflicts with the consensus. Right on the transition, the bulk LC molecules experience the orientational rotation, with 90&#176; between the planar (P) orientation at high temperatures and the vertical (V) orientation at low temperatures in the first-order transitional manner. We have tracked thermodynamic surface anchoring behavior by means of polarizing optical microscopy (POM), dielectric spectroscopy (DS), high-resolution differential scanning calorimetry (HR-DSC), and grazing incidence X-ray diffraction (GI-XRD) and reached a plausible physical explanation: that the transition is triggered by a growth of surface wetting sheets, which impose the V orientation locally against the P orientation in the bulk. This landscape would provide a general link explaining how the equilibrium bulk orientation is affected by surface-localized orientation in many LC systems. In our characterization, POM and DS are advantageous by offering information on the spatial distribution of the orientation of LC molecules. HR-DSC gives information about the precise thermodynamic information on transitions, which cannot be addressed by conventional DSC instruments due to limited resolution. GI-XRD provides information on surface-specific molecular orientation and short-range orderings. The goal of this paper is to present a protocol for preparing a sample that exhibits the transition and to demonstrate how the thermodynamic structural variation, both in the bulk and on surfaces, can be analyzed through the abovementioned methods.

Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve ...

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 &#937;/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) &#8211; up to 1.5 x 104 &#8211; which are amongst the best values in the published literature.

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a ...

Preparation of Liquid Crystal Networks for Macroscopic Oscillatory Motion Induced by Light

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous light irradiation. The photo-excitation of dopants that can quickly dissipate light into heat, coupled with anisotropic thermal expansion and self-shadowing of the film, gives rise to the self-sustained deformation. The oscillations observed are influenced by the dimensions and the modulus of the film, and by the directionality and intensity of the light. The system developed offers applications in energy conversion and harvesting for soft-robotics and automated systems.
The general method described here consists of creating free-standing liquid crystalline films and characterizing the mechanical and thermal effects observed. The molecular alignment is achieved using alignment layers (rubbed polyimide), commonly used in the display manufacturing industry. To obtain actuators with large deformation, the mesogens are aligned and polymerized in a splay/bend configuration, i.e., with the director of the liquid crystals (LCs) going gradually from planar to homeotropic through the film thickness. Upon irradiation, the mechanical and thermal oscillations obtained are monitored with a high-speed camera. The results are further quantified by image analysis using an image processing program.

The goal of the protocol is to create liquid crystalline polymer films that can mechanically oscillate under continuous light irradiation. We describe in great detail the conception of free-standing films, from the liquid crystal alignment method to the photo-actuation. The experimental protocol applied to prepare this material is broadly applicable.

A strategy based on doped liquid crystalline networks is described to create mechanical self-sustained oscillations of plastic films under continuous ...

Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present an advanced method for fabricating ultra-thin color films with improved characteristics. The proposed process addresses several fabrication issues, including large area processing. Specifically, the protocol describes a process for fabricating ultra-thin color films using an electron beam evaporator for oblique angle deposition of germanium (Ge) and gold (Au) on silicon (Si) substrates.&#160; Film porosity produced by the oblique angle deposition induces color changes in the ultra-thin film. The degree of color change depends on factors such as deposition angle and film thickness. Fabricated samples of the ultra-thin color films showed improved color tunability and color purity. In addition, the measured reflectance of the fabricated samples was converted into chromatic values and analyzed in terms of color. Our ultra-thin film fabricating method is expected to be used for various ultra-thin film applications such as flexible color electrodes, thin film solar cells, and optical filters. Also, the process developed here for analyzing the color of the fabricated samples is broadly useful for studying various color structures.

We present a detailed method for fabricating ultra-thin color films with improved characteristics for optical coatings. The oblique angle deposition technique using an electron beam evaporator allows improved color tunability and purity. Fabricated films of Ge and Au on Si substrates were analyzed by reflectance measurements and color information conversion.

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present ...

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...