We acknowledge the following financial support; Grant-in-Aid for Specially Promoted Research (No. 25000003) from JSPS, Grant-in-Aid for Research Activity Start-up (No.15H06133) and Challenging Research (Exploratory) (No. JP17K18748) from MEXT of Japan.

1. Dispersion of WS 2 Nanotubes (NTs) on substrate
<ol>
	<li>Disperse WS2 NT powders into isopropyl alcohol (IPA, concentration more than 99.8%) with proper diluted ratio (about 0.1 mg/mL) by sonication for 20 min. 
	NOTE: The long-time sonication helps to make WS2 NTs uniformly suspended in IPA liquid and to separate well-formed individual WS2 NTs from amorphous WS2 or other junks, as well as to remove the garbage accumulating on WS2 NTs surface. Figure 2b shows the final suspension of WS2 NTs. Since the suspension might be heated up during the sonication process, it is better to stop sonication at every 5 min and continue sonication after 1 min.</li>
	<li>Spin-coating procedure to disperse WS2 NTs on substrate.
	<ol>
		<li>Start the spin-coater machine and vacuum pump. Put a Si/SiO2 (3000 &#197;) substrate (1 cm x 1 cm) on the center of the chuck and fix it by vacuum with pumping speed of 80 L/min and ultimate pressure of 20 kPa. 
		NOTE: There is a hole located at the center of the chuck connecting to the vacuum pump, thus the substrate is fixed by vacuum pressure (pumping speed is 80 L/min and ultimate pressure is 20 kPa). The vacuum pressure can be different depending on the pump.</li>
		<li>Set relevant parameters of the procedure by control panel of the spin-coater machine. 
		NOTE: There are three steps during the spin-coating procedure: (1) slowly speed up to 500 rpm within the first 3 s, (2) rapidly speed up to 4000 rpm and continue for 50 s, (3) slow down and stop spinning for the last 3 s. Those parameters can be different depending on the use of spin-coater.</li>
		<li>Put one droplet (about 0.01 mL) of the suspension of WS2 NT made in (1.1) by a pipette onto the substrate until the substrate is fully covered by the suspension (if not, put more droplets). Then start spin-coating with relevant parameters in (1.2.2).</li>
	</ol>
	</li>
</ol>
2. Preparation of thin flake on substrate via scotch tape method
<ol>
	<li>Put a small bulk sample of WS2 (grown via chemical vapor transport method) on scotch tape. Fold the scotch tape and unfold it slowly to mechanically exfoliate the thin layer from the bulk. Repeat this procedure for several times, until the exfoliated samples are thin enough. 
	NOTE: Figure 2g and 2h show the initial tape with little bulk sample of WS2 and the final tape after multiple folding procedures, respectively.</li>
	<li>Paste the scotch tape on the top of substrate upside down, slightly press the tape, and carefully remove the tape from the top of substrate. 
	NOTE: After removing the tape, there are many thin flakes left on the substrate.</li>
</ol>
3. Device fabrication by electron beam lithography.
<ol>
	<li>Spin-coating process to cover the resist for electron beam lithography.
	<ol>
		<li>Follow the same spin-coating procedure described in (1.2.1) and (1.2.2).</li>
		<li>Put one droplet (about 0.04 mL) of polymethyl methacrylate (PMMA) by a pipette onto the substrate until the substrate is fully covered by PMMA. Then start spin-coating procedure to uniformly cover PMMA on WS2 sample to prevent it from being exposed in air. 
		Note: PMMA is one of the resists for electron beam lithography.</li>
		<li>After spin-coating, put the substrate on hot-plate at 180 &#176;C and heat it for 1 min. 
		NOTE: Those parameters can be different depending on resist types.</li>
	</ol>
	</li>
	<li>Sample selection by optical microscopy.
	<ol>
		<li>Start the optical microscopy and camera. Put the substrate on the stage.</li>
		<li>Move stage and scan the whole region of the substrate with proper magnification (20X) and in the meantime, select isolated samples with suitable size. 
		Note: In total, 6 to 10 isolated samples can be usually selected for each substrate of 1 cm x 1 cm.</li>
		<li>Take photos of each selected sample with different magnification of 5X, 20X, and 100X. Those photos are used to identify the location of each sample.</li>
	</ol>
	</li>
	<li>Design of the large-scale device pattern.
	<ol>
		<li>Activate the AutoCAD software, and load the format of substrate lattice. Insert photos taken in (3.2) and identify the size and location of each photo depending on the marks on the substrate.</li>
		<li>Insert a large square with length of 1200 &#181;m and a small square with length of 300 &#181;m which should surround each sample.</li>
		<li>Design large-scale patterns including gate, source, drain, and other pads in the large square except for fine structures near the sample. Design small marks close to the sample to precisely identify the location of the samples in the later process of design for the small-scale device pattern.</li>
		<li>Repeat (3.3.2) to (3.3.3) for every sample.</li>
		<li>Record the coordinates of the center of each large and small square, respectively.</li>
		<li>Delete inserted photos, large and small squares, and substrate lattice format, leaving only the designed large patterns and small marks. Export large patterns and small marks as dxf files, respectively.</li>
	</ol>
	</li>
	<li>First electron beam lithography.
	<ol>
		<li>Put the substrate on the stage and fix it, and insert the stage into the main chamber of electron beam lithography machine.</li>
		<li>Activate ECA program (a program for generating a file used in electron beam process). Set field size as 300 for the lithography of small marks. Use tool of dxf converter to transfer dxf file to cell file.</li>
		<li>Load the cell file generated in (3.4.2), enter the file name, identify origin and identify points with coordinates of small squares noted in (3.3.5). Finally, identify the coordinates of large A and B marks and small A and B marks for each point. 
		Note: The large A and B marks are used to correct the direction of stage, while small A and B marks are used to identify the mismatch between designed pattern and printed pattern, during the process of small-scale pattern design.</li>
		<li>Save the file as con file and wait until the pressure inside the main chamber is lower than 5x10-5 Pa. 
		NOTE: Because the electron beam is high energy (50 kV of accelerating voltage), the high quality of vacuum is needed.</li>
		<li>When the pressure of the main chamber becomes low enough for the electron beam lithography process, activate electron beam control program ESL-7500 and subsequently turn on the electron gun.</li>
		<li>Turn on the scanning electron microscope (SEM) and move the stage to the position where the substrate is in the screen. Tune the brightness, contrast, and focus.</li>
		<li>Adjust the angle of the stage by judging from the relative position of large A and B marks originally designed on the substrate until the error of direction is negligible at magnification of 5000X. After correcting the direction of the stage, register the position of the large A mark.</li>
		<li>Set the amplitude of electron beam for lithography; 100 pA is for small marks lithography. Move the stage to the position for amplitude tuning and select the spot mode for the camera, change the amplitude of electron beam until it reaches 100 pA by ampere meter. After setting the amplitude of electron beam, tune the brightness, contrast, and focus.</li>
		<li>Load saved con file in (3.4.4) in the ECA program. Set relevant parameters: 2 s for dose time and 300 for field size. Finally start the exposure process. 
		Note: Dose time can be different depending on the resist.</li>
		<li>Back to electron beam control program ESL-7500, set 5000X of magnification and move the stage to the registered position of the large A mark. Confirm the position of the large A and B marks.</li>
		<li>Set 30000X of magnification and confirm the position of small A and B marks for the lithography of the first small marks designed in (3.3.3). 
		Note: The lithography starts after conforming small marks and takes several second.</li>
		<li>After finishing the lithography, repeat this procedure for all samples. At the last, quit the exposure process and close the ECA program.</li>
		<li>Follow the same process from (3.4.2) to (3.4.10) with different parameters for the lithography of the large pattern designed in (3.3.3). In (3.4.2), set field size as 1200. In (3.4.3), only identify coordinates of large A and B marks except for small A and B marks. In (3.4.8), set the amplitude of electron beam as 1000 pA for the lithography of the large pattern. In (3.4.9), select 1200 of field size. 
		Note: After the process of (3.4.10), the lithography of the large pattern starts and takes several hours.</li>
		<li>After finishing the lithography of the large pattern, move the stage to the original position, turn off the electron beam, and quit the exposure process and the ECA program. Open the main chamber and take out the substrate.</li>
	</ol>
	</li>
	<li>First developing.
	<ol>
		<li>Make a solution of methyl isobutyl ketone (MIBK) and IPA with the ratio of MIBK: IPA = 1: 3. Dip the substrate in the solution for 30 s, and wash it by IPA liquid and dry it by nitrogen gun. 
		NOTE: Developing time may change depending on the environmental conditions such as temperature and humidity.</li>
		<li>Take photos by optical microscopy for each printed pattern with different magnification of 5X, 20X, and 100X.</li>
	</ol>
	</li>
	<li>Design of small-scale device pattern.
	<ol>
		<li>Follow the same process in (3.3). In (3.3.1), load the pattern of substrate lattice including the designed small marks in (3.3.3), and insert photos taken after first developing. 
		NOTE: The size and location of each photo depends on the small marks designed in (3.3.3), instead of marks on the substrate.</li>
		<li>Design the fine structure of device pattern with source, drain and other electrodes in small squares in a Hall bar configuration, which is connected to the printed large pattern. After designing small patterns for all the devices, record the coordinates of small squares.</li>
		<li>Delete inserted photos, small squares, and substrate pattern, leaving only the designed small patterns. Export the small pattern as dxf file.</li>
	</ol>
	</li>
	<li>Second electron beam lithography.
	<ol>
		<li>Follow the same process from (3.4.1) to (3.4.11) with the same parameters for the lithography of the small pattern designed in (3.6); set 300 for field size and select 100 pA for the amplitude of electron beam. 
		NOTE: The lithography process takes several minutes for each small pattern.</li>
		<li>After the lithography of small pattern, move the stage to the original position, turn off the electron beam, quit the exposure process and close the ECA program. Open the main chamber and take out the substrate.</li>
	</ol>
	</li>
	<li>Second developing.
	<ol>
		<li>Follow the same process in (3.5) with the same developing time as 30 s.</li>
		<li>Take photos by optical microscopy for each pattern with different magnification of 5X, 20X, and 100X.</li>
	</ol>
	</li>
</ol>
4. Deposition of electrodes
<ol>
	<li>Deposition of gold electrodes.
	<ol>
		<li>Fix the substrate on substrate holder, put the substrate holder on transfer rod, and insert it into the main chamber of the evaporator. Start rotating the substrate holder.</li>
		<li>First deposit Cr of 5 nm in thickness as the adhesion layer. When the pressure inside the main chamber becomes less than 10-4 Pa, turn on the high voltage source.</li>
		<li>Increase the current of electron gun carefully with the fixed accelerating voltage of 4 kV, until the depositing rate measured by thickness monitor becomes stable about 0.5 &#197;/s (usually pre-evaporate Cr of about 5 nm).</li>
		<li>Open the shutter and deposit Cr until it reaches 5 nm in thickness. Close the shutter, slowly decrease the current of electron gun to zero, and turn off the high voltage source.</li>
		<li>Subsequently deposit Au of appropriate thickness. Turn on the current source and slowly increase the current up to 30 A. Evaporate Au by keeping the current of 30 A, until the depositing rate measured by thickness monitor becomes stable about 1 &#197;/s (usually pre-evaporate Au of about 10 nm ).</li>
		<li>Open the shutter and start depositing Au. After reaching the intended thickness, close the shutter, slowly decrease the current to zero, and turn off the current source. 
		Note: We use 60 nm for thin flake and 90 nm for NT. The appropriate thickness is depending on the sample.</li>
		<li>Since the substrate is heated up during deposition process, remain the substrate inside the chamber for 1 h in order to cool down its temperature to near room temperature. Stop rotating the substrate holder and take it out by transfer rod.</li>
	</ol>
	</li>
	<li>Deposition of SiO2 protection layer.
	<ol>
		<li>With the help of optical microscopy, cover the pads and the gate electrodes by tape. 
		NOTE: In principle, only the fine structures of electrodes are exposed to deposit SiO2 layer for the protection of electrodes against the chemical reaction during the electrolyte gating.</li>
		<li>Follow the same process from (4.1.1) to (4.1.4) to deposit Cr of 5 nm in thickness as adhesion layer.</li>
		<li>Subsequently follow the same process from (4.1.1) to (4.1.4) to deposit SiO2 of 20 nm in thickness. 
		Note: The depositing rate of SiO2 is about 1 &#197;/s, while pre-evaporating SiO2 of about 10 nm.</li>
		<li>Cool down the substrate inside the chamber for 1 h. Stop rotating the substrate holder and take it out by the transfer rod. Remove the tape under microscopy.</li>
	</ol>
	</li>
</ol>
5. Completion of the device
<ol>
	<li>Substrate scribing.
	<ol>
		<li>Turn on the scribing machine and vacuum pump with pumping speed of 50 L/min and ultimate pressure of 30 kPa. Fix the substrate on the stage by vacuum chuck and adjust the angle and position of the substrate.</li>
		<li>Scribe the substrate into small pieces (usually about 3 mm x 3 mm). 
		Note: The size of each piece is depending on the location of each selected sample and the designed pattern.</li>
	</ol>
	</li>
	<li>Device lift-off.
	<ol>
		<li>Select one device and immerse it into acetone (concentration more than 99.5%) for 1 h at room temperature to remove the redundant PMMA and gold. Only the fabricated electrodes are left on substrate.</li>
		<li>After the lift-off process, wash the substrate by IPA and dry it by nitrogen gun.</li>
	</ol>
	</li>
	<li>Wire-bonding.
	<ol>
		<li>Turn on the wire-bonding machine. Fix the substrate on the chip-carrier via sliver paste. 
		NOTE: For WS2 NT case, we use horizontal rotator shown in Figure 2n.</li>
		<li>With the help of wire-bonding machine, connect each electrode pad and electrode of chip carrier one by one with a gold wire.</li>
	</ol>
	</li>
	<li>Electrolyte droplet putting.
	<ol>
		<li>Put a droplet of electrolyte (less than 0.5 &#181;L) on the top of device by a tweezer after dipping in the electrolyte liquid. 
		NOTE: The amount of electrolyte is very little; it covers the fine structure of device and gate pad but avoids covering the electrode pads. We use the electrolyte of KClO4 (concentration more than 99%) dissolved in polyethylene glycol (PEG; Mw = 600) with a [K]:[O] ratio of 1:20 according to the previous publication38.</li>
	</ol>
	</li>
</ol>
6. Transport measurements
<ol>
	<li>Fix the chip-carrier on the sample holder, and put it inside the chamber of physical properties measurement system by the transfer rod. Pump the chamber by the high vacuum mode.</li>
	<li>Connect measurement system including lock-in amplifiers, nano-voltmeter, source meter, and amplifier. Apply a constant alternating current (AC) with the frequency of 13 Hz to perform the AC-lock-in measurements.</li>
	<li>Run the Keysight VEE program (measurement programs).</li>
	<li>In the measurement of gate response, when the gate voltage is applied to the electrolyte (i.e., between source and gate electrodes), sweep the gate voltage with speed of 50 mV/s at 300 K, under high-vacuum condition to reduce the influence of the air on the gating performance.</li>
	<li>In the measurement of temperature dependence of resistance, first cool down to 200 K with the cooling rate of 1 K/min in high-vacuum condition, and then change to the He-purged condition and keep cooling down to 10 K with the cooling rate of 1 K/min. When the temperature is lower than 10 K, cool down and warm up with the rate of 0.2 K/min. 
	Note: In the He-purged condition, the thermal conductance and resultant temperature stability is better than those in high-vacuum condition.</li>
</ol>

The authors have nothing to disclose.

In both WS2 NTs and flakes, we have successfully controlled the electric properties by electrostatic or electro chemical carrier doping.
In electrostatic doping region, ambipolar transistor operation has been observed. Such ambipolar transfer curve with a high on/off ratio (&#62;102) observed in low bias voltage indicates the effective carrier doping at the interface of electrolyte gating technique for tuning the Fermi level of these systems.
Although this method is as advantageous for tuning large amount of carrier number in small gate bias compared with the conventional solid gating method, there are several limitations of this technique. First, because the carrier number control is realized via liquid gate, it is not able to tune the carrier number below the frozen temperature of electrolyte/ionic liquid12,28,29,30. Conventional solid gate, on the other hand, is valid for even low temperature, although it is not as efficient as electrolyte/ionic liquid gate in high temperature (near room temperature). Second, many materials are known to show the chemical reaction with electrolyte/ionic liquid in specific conditions59,60,61,62,63. Such chemical reaction easily breaks the devices and limits the successful ratio of experiments or application to the materials.
However, people have recently recognized that the chemical reaction might help for the future application, such as chemical etching for thinning films59,60 and electrochemical intercalation for heavily electron doping9,11,34,38,50,51,52,53 and phase transformation61,62,63. A similar technique has been also adapted for solid ion conductor51,52,53 and even photoactive EDLT has been developed64.
In the electrochemical doping region, we have observed the electric-field-induced superconductivity. The difference of the superconducting transition temperature between WS2 NTs and flakes, which is possibly due to the lower dimensionality of NTs, should be further pursued in the future.
As clearly demonstrated in the results of this study, carrier number control by the ionic liquid gating provides a powerful method for searching the quantum phase transition in nanomaterials.

Control of the carrier number is the key technique for realizing the quantum phase transition in solids1. In the conventional field effect transistor (FET), it is achieved by use of the solid gate1,2. In such a device, electric potential gradient is uniform throughout the dielectric materials so that induced carrier number at the interface is limited, shown in Figure 1a.
On the other hand, we can achieve the higher carrier density at the interface or bulk by replacing the solid dielectric materials with ionic gels/liquids or polymer electrolytes3,4,5,6,7,8,9,10,11 (Figure 1b). In the electrostatic doping by use of the ionic liquid, electric double layer transistor (EDLT) structure is formed at the interface between ionic liquid and sample, generating strong electric field (&#62;0.5 V/&#197;) even at low bias voltage. Resultant high carrier density (&#62;1014 cm-2) induced at the interface10,12,13 cause the novel electronic properties or quantum phase transition such as electric-field-induced ferromagnetism14, Coulomb blockade15, ambipolar transport16,17,18,19,20,21,22,23,24,25,26,27, formation of p-n junction and resultant electroluminance28,29,30, large modulation of thermoelectric powers31,32, charge density wave and Mott transitions33,34,35, and electric-field-induced insulator-metal transition36,37 including electric-field-induced superconductivity9,10,11,38,39,40,41,42,43,44,45,46,47,48,49.
In the electrolyte gating (Figure 1c), ions are not only accumulated at the interface to form EDLT, but can be also intercalated into layers of two-dimensional materials via thermal diffusion without damaging sample under applying the large gate voltage, leading to the electrochemical doping8,9,11,34,38,50,51,52,53. Thus, we can drastically change the carrier number compared to the conventional field effect transistor using the solid gate. In particular, the electric-field-induced superconductivity9,11,34,38,50 is realized by use of electrolyte gating in region of large carrier number where we cannot access by the conventional solid gating method.
In this article, we introduce this unique technique of carrier number control in solids and overview the transistor operation and electric-field-induced superconductivity in semiconducting WS2 samples such as WS2 flakes and WS2 nanotubes54,55,56,57.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Sonication machine</td>
			<td>SND Co., Ltd.</td>
			<td>US-2</td>
			<td>http://www.senjyou.jp/</td>
		</tr>
		<tr>
			<td>Spin-coater machine</td>
			<td>ACTIVE Co.,Ltd.</td>
			<td>ACT-300AII</td>
			<td>http://www.acti-ve.co.jp/spincoater/standard/act300a2.html</td>
		</tr>
		<tr>
			<td>Hot-plate</td>
			<td>TAIYO</td>
			<td>HP131224</td>
			<td>http://www.taiyo-kabu.co.jp/products/detail.php?product_id=431</td>
		</tr>
		<tr>
			<td>Optical Microscopy</td>
			<td>OLYMPUS</td>
			<td>BX51</td>
			<td>https://www.olympus-ims.com/ja/microscope/bx51p/</td>
		</tr>
		<tr>
			<td>Electron Beam Lithography machine</td>
			<td>ELIONIX INC.</td>
			<td>ELS-7500I</td>
			<td>https://www.elionix.co.jp/index.html</td>
		</tr>
		<tr>
			<td>Scribing machine</td>
			<td>TOKYO SEIMITSU CO., LTD.</td>
			<td>A-WS-100A</td>
			<td>http://www.accretech.jp/english/product/semicon/wms/aws100s.html</td>
		</tr>
		<tr>
			<td>Wire-bonding machine</td>
			<td>WEST&#183;BOND&#160;</td>
			<td>7476D-79</td>
			<td>https://www.hisol.jp/products/bonder/wire/mgb/b.html</td>
		</tr>
		<tr>
			<td>Physical Properties Measurement System</td>
			<td>Quantum Design</td>
			<td>PPMS</td>
			<td>http://www.qdusa.com/products/ppms.html</td>
		</tr>
		<tr>
			<td>Lock-in amplifier</td>
			<td>Stanford Research Systems</td>
			<td>SRS830</td>
			<td>http://www.thinksrs.com/products/SR810830.htm</td>
		</tr>
		<tr>
			<td>Source meter</td>
			<td>Textronix</td>
			<td>KEITHLEY 2612A</td>
			<td>http://www.tek.com/keithley-source-measure-units/smu-2600b-series-sourcemeter</td>
		</tr>
		<tr>
			<td>KClO4</td>
			<td>Sigma-Aldrich</td>
			<td>241830</td>
			<td>http://www.sigmaaldrich.com/catalog/product/sigald/241830?lang=ja&#38;region=JP</td>
		</tr>
		<tr>
			<td>PEG</td>
			<td>WAKO</td>
			<td>168-09075</td>
			<td>http://www.siyaku.com/uh/Shs.do?dspCode=W01W0116-0907</td>
		</tr>
		<tr>
			<td>IPA</td>
			<td>WAKO</td>
			<td>169-28121</td>
			<td>http://www.siyaku.com/uh/Shs.do?dspWkfcode=169-28121</td>
		</tr>
		<tr>
			<td>MIBK</td>
			<td>WAKO</td>
			<td>131-05645</td>
			<td>http://www.siyaku.com/uh/Shs.do?dspCode=W01W0113-0564</td>
		</tr>
		<tr>
			<td>PMMA</td>
			<td>MicroChem</td>
			<td>PMMA</td>
			<td>http://microchem.com/Prod-PMMA.htm</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>WAKO</td>
			<td>012-26821</td>
			<td>http://www.siyaku.com/uh/Shs.do?dspWkfcode=012-26821</td>
		</tr>
	</tbody>
</table>

electric-field control of electronic states in ws2 nanodevices by electrolyte gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.

The typical transistor operations of an individual WS2 NT and a WS2 flake devices are shown in Figure 3a and 3b, respectively, where the source drain current (IDS) as a function of the gate voltage (VG) nicely operates in an ambipolar mode, showing a remarkable contrast to the unipolar gate response by the conventional solid gated FET in previous publication58. Considering the ambipolar behavior is reversible and repeatable, these transistor operations are likely due to the electrostatic doping. In the electrostatic doping, ions are accumulated on the surface of the samples, which leads to the large electric potential drop and resultant carrier doping at the interface (Figure 1b).
Electrochemical doping by the intercalation (Figure 1c), on the other hand, is realized in large gate voltage region, causing much higher electron concentration in the bulk of the sample, instead of on the surface of the sample by electrostatic doping. A typical intercalation process is shown in the Figure 3c. When the gate voltage is firstly increased up to 8 V with a constant rate of 50 mV/s, IDS displays a saturation behavior indicating the electrostatic doping, similarly to the 2D WS2 case as shown in Figure 3b. When the gate voltage is kept at 8 V for a couple of minutes, another drastic increase of IDS by more than two orders of magnitude has been observed as shown in Figure 3c. This source drain current increase is presumably attributed to intercalation of K+ ions defusing into WS2 layers without damaging the crystal structure. This process causes much higher carrier concentration in the bulk compared with the electrostatic doping at the surface.
As shown in Figure 3d, the similar intercalation process is also realized in WS2 flake. When the gate voltage is firstly increased up to 6 V, IDS displays a similar saturation behavior. On the other hand, carrier density estimated by Hall effect does not significantly change, displaying the similar saturation behavior. When the gate voltage becomes higher than 6 V, IDS increases again because of the occurrence of intercalation which is proofed by the clear increase of the carrier density.
Temperature dependences of the resistance of WS2 NTs and flakes after electrochemical doping are shown in the Figure 3e and 3f, respectively. In both cases, the resistance shows the metallic behavior and superconductivity appears in low temperature region.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56862/56862fig1.jpg" /> 
Figure 1: Illustration of electrolyte gating. (a) The schematic figure of conventional field effect transistor by solid gate. (b) The schematic figure of electrostatic doping by electrolyte gating. By take placing the solid dielectric intermedium into electrolyte, the electrostatic doping effect is more efficient since the dielectric constant of liquid is much larger than solid. A large number of carriers are accumulated on the surface of sample. (c) The schematic figure of electrochemical doping by electrolyte-gating-induced intercalation. The positive ions are intercalated into sample, inducing much more carriers in the bulk. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56862/56862fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56862/56862fig2.jpg" /> 
Figure 2: Device fabrication of WS2 nanotube and flake. (a) and (b) WS2 NT, which was first powder configuration, are dispersed into IPA liquid. (c) The photograph of an individual WS2 NT selected by optical microscopy after dispersion of WS2 NT onto the substrate and covered by PMMA. (d) The schematic figure of the designed pattern for WS2 NT by AutoCAD. (e) The photograph of the device pattern of an individual WS2 NT after electron beam lithography process and developing process. (f) The photograph of the device of an individual WS2 NT after deposition of electrodes. (g) and (h) The photograph of WS2 bulk samples on a tape, and the photograph of exfoliated WS2 samples after folding and unfolding tape several times. (i) The photograph of a WS2 flake selected by optical microscopy after transferring it onto the substrate and covered by PMMA. (j) The schematic figure of the designed pattern for a WS2 flake by AutoCAD. (k) The photograph of the device pattern of a WS2 flake after electron beam lithography process and developing process. (l) The photograph of the device of a WS2 flake after deposition of electrodes. (m) The photograph of an isolated device after scribing process and lift-off process. The typical device pattern of the device for electrolyte gating is shown. In addition to electrodes for transport measurements, a side gate was positioned near the sample. (n) The photograph of the device on the horizontal rotator after wire-bonding process. (o) The photograph of the device after wire-bonding process. (p) The photograph of device for electrolyte gating with a droplet of ionic liquid on the top covering both sample and the side gate electrodes. (q) The photograph of typical measurement systems (PC and physical properties measurement system). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56862/56862fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56862/56862fig3.jpg" /> 
Figure 3: Transistor operation, electrochemical intercalation, and electric-field-induced superconductivity in WS2 nanotube and flake device. (a) The ambipolar transfer curve of WS2 NT at 300 K. Source-drain voltage VDS is 0.2 mV and sweeping speed of gate voltage VG is 50 mV/s. (b) The ambipolar transfer curve of WS2 flake at 300 K. VDS is 0.1 V and sweeping speed of gate voltage is 20 mV/s. (c) Source-drain current IDS as a function of VG and waiting time during the electrochemical intercalation in WS2 NT. A saturation behavior of IDS has been observed when increasing VG, and a second dramatically increase of IDS has been observed at fixed VG during waiting couple of minutes. (d) IDS at 300 K (left) and carrier density estimated by Hall effect at 200 K (right) as a function of VG in the WS2 flake. Saturation and second increase of IDS has been also observed in the flake. The carrier density shows the large increase in large VG region, indicating the intercalation process. (e) Temperature dependence of resistance of WS2 NT after intercalation process. Superconducting transition has been observed at 5.8 K. (f) Temperature dependence of resistance of WS2 flake after intercalation process. Superconducting transition has been observed at 8 K. All the figures have been reproduced and modified&#160;from Qin, F. et al. and Shi, W. et al.38,50 <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56862/56862fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating at JoVE.com

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

Here, we present a protocol to control the carrier number in solids by using the electrolyte.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape ...

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11.3K Views.  The University of Tokyo. Here, we present a protocol to control the carrier number in solids by using the electrolyte.

Video: Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

Development of Whispering Gallery Mode Polymeric Micro-optical Electric Field Sensors

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The optical modes are frequently referred to as "whispering gallery modes" (WGM) or "morphology dependent resonances" (MDR) and exhibit high optical quality factors. Some proposed applications of micro-cavity optical resonators are in spectroscopy1, micro-cavity laser technology2, optical communications3-6 as well as sensor technology. The WGM-based sensor applications include those in biology7, trace gas detection8, and impurity detection in liquids9. Mechanical sensors based on microsphere resonators have also been proposed, including those for force10,11, pressure12, acceleration13 and wall shear stress14. In the present, we demonstrate a WGM-based electric field sensor, which builds on our previous studies15,16. A candidate application of this sensor is in the detection of neuronal action potential.
The electric field sensor is based on polymeric multi-layered dielectric microspheres. The external electric field induces surface and body forces on the spheres (electrostriction effect) leading to elastic deformation. This change in the morphology of the spheres, leads to shifts in the WGM. The electric field-induced WGM shifts are interrogated by exciting the optical modes of the spheres by laser light. Light from a distributed feedback (DFB) laser (nominal wavelength of ~ 1.3 &mu;m) is side-coupled into the microspheres using a tapered section of a single mode optical fiber. The base material of the spheres is polydimethylsiloxane (PDMS). Three microsphere geometries are used: (1) PDMS sphere with a 60:1 volumetric ratio of base-to-curing agent mixture, (2) multi layer sphere with 60:1 PDMS core, in order to increase the dielectric constant of the sphere, a middle layer of 60:1 PDMS that is mixed with varying amounts (2% to 10% by volume) of barium titanate and an outer layer of 60:1 PDMS and (3) solid silica sphere coated with a thin layer of uncured PDMS base. In each type of sensor, laser light from the tapered fiber is coupled into the outermost layer that provides high optical quality factor WGM (Q ~ 106). The microspheres are poled for several hours at electric fields of ~ 1 MV/m to increase their sensitivity to electric field.

A high-sensitivity photonic micro sensor was developed for electric field detection. The sensor exploits the optical modes of a dielectric sphere. Changes in the external electric field perturb the sphere morphology leading to shifts in its optical modes. The electric field strength is measured by monitoring these optical shifts.

Optical modes of dielectric micro-cavities have received significant attention in recent years for their potential in a broad range of applications. The ...

Gain-compensation Methodology for a Sinusoidal Scan of a Galvanometer Mirror in Proportional-Integral-Differential Control Using Pre-emphasis Techniques

Galvanometer mirrors are used for optical applications such as target tracking, drawing, and scanning control because of their high speed and accuracy. However, the responsiveness of a galvanometer mirror is limited by its inertia; hence, the gain of a galvanometer mirror is reduced when the control path is steep. In this research, we propose a method to extend the corresponding frequency using a pre-emphasis technique to compensate for the gain reduction of galvanometer mirrors in sine-wave path tracking using proportional-integral-differential (PID) control. The pre-emphasis technique obtains an input value for a desired output value in advance. Applying this method to control the galvanometer mirror, the raw gain of a galvanometer mirror in each frequency and amplitude for sine-wave path tracking using a PID controller was calculated. Where PID control is not effective, maintaining a gain of 0 dB to improve the trajectory tracking accuracy, it is possible to expand the speed range in which a gain of 0 dB can be obtained without tuning the PID control parameters. However, if there is only one frequency, amplification is possible with a single pre-emphasis coefficient. Therefore, a sine wave is suitable for this technique, unlike triangular and sawtooth waves. Hence, we can adopt a pre-emphasis technique to configure the parameters in advance, and we need not prepare additional active control models and hardware. The parameters are updated immediately within the next cycle because of the open loop after the pre-emphasis coefficients are set. In other words, to regard the controller as a black box, we need to know only the input-to-output ratio, and detailed modeling is not required. This simplicity allows our system to be easily embedded in applications. Our method using the pre-emphasis technique for a motion-blur compensation system and the experiment conducted to evaluate the method are explained.

We propose a method to extend the corresponding frequency by using a pre-emphasis technique. This method compensates for the gain reduction of a galvanometer mirror in sine-wave path tracking using proportional-integral-differential control.

Galvanometer mirrors are used for optical applications such as target tracking, drawing, and scanning control because of their high speed and accuracy. ...

Microfluidic Imaging Flow Cytometry by Asymmetric-detection Time-stretch Optical Microscopy (ATOM)

Scaling the number of measurable parameters, which allows for multidimensional data analysis and thus higher-confidence statistical results, has been the main trend in the advanced development of flow cytometry. Notably, adding high-resolution imaging capabilities allows for the complex morphological analysis of cellular/sub-cellular structures. This is not possible with standard flow cytometers. However, it is valuable for advancing our knowledge of cellular functions and can benefit life science research, clinical diagnostics, and environmental monitoring. Incorporating imaging capabilities into flow cytometry compromises the assay throughput, primarily due to the limitations on speed and sensitivity in the camera technologies. To overcome this speed or throughput challenge facing imaging flow cytometry while preserving the image quality, asymmetric-detection time-stretch optical microscopy (ATOM) has been demonstrated to enable high-contrast, single-cell imaging with sub-cellular resolution, at an imaging throughput as high as 100,000 cells/s. Based on the imaging concept of conventional time-stretch imaging, which relies on all-optical image encoding and retrieval through the use of ultrafast broadband laser pulses, ATOM further advances imaging performance by enhancing the image contrast of unlabeled/unstained cells. This is achieved by accessing the phase-gradient information of the cells, which is spectrally encoded into single-shot broadband pulses. Hence, ATOM is particularly advantageous in high-throughput measurements of single-cell morphology and texture &#8211; information indicative of cell types, states, and even functions. Ultimately, this could become a powerful imaging flow cytometry platform for the biophysical phenotyping of cells, complementing the current state-of-the-art biochemical-marker-based cellular assay. This work describes a protocol to establish the key modules of an ATOM system (from optical frontend to data processing and visualization backend), as well as the workflow of imaging flow cytometry based on ATOM, using human cells and micro-algae as the examples.

Microfluidic Imaging Flow Cytometry by Asymmetric-detection Time-stretch Optical Microscopy ATOM

This protocol describes the implementation of an asymmetric-detection time-stretch optical microscopy system for single-cell imaging in ultrafast microfluidic flow and its applications in imaging flow cytometry.

Scaling the number of measurable parameters, which allows for multidimensional data analysis and thus higher-confidence statistical results, has been the ...

Study of Siphon Breaker Experiment and Simulation for a Research Reactor

Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous outward flow of water. To prevent this outflow, a control device is required. A siphon breaker is a type of safety device that can be utilized to control the loss of coolant water effectively.
To analyze the characteristics of siphon breaking, a real-scale experiment was conducted. From the results of the experiment, it was found that there are several design factors that affect the siphon breaking phenomenon. Therefore, there is a need to develop a theoretical model capable of predicting and analyzing the siphon breaking phenomenon under various design conditions. Using the experimental data, it was possible to formulate a theoretical model that accurately predicts the progress and the result of the siphon breaking phenomenon. The established theoretical model is based on fluid mechanics and incorporates the Chisholm model to analyze two-phase flow. From Bernoulli&#39;s equation, the velocity, quantity, undershooting height, water level, pressure, friction coefficient, and factors related to the two-phase flow could be obtained or calculated. Moreover, to utilize the model established in this study, a siphon breaker analysis and design program was developed. The simulation program operates on the theoretical model basis and returns the result as a graph. The user can confirm the possibility of the siphon breaking by checking the shape of the graph. Furthermore, saving the entire simulation result is possible and it can be used as a resource for analyzing the real siphon breaking system.
In conclusion, the user can confirm the status of the siphon breaking and design the siphon breaker system using the program developed in this study.

The siphon breaking phenomenon was investigated experimentally and a theoretical model was proposed. A simulation program based on the theoretical model was developed and the results of the simulation program were compared with experimental results. It was concluded that the results of the simulation program matched the experimental results well.

Under the design conditions of a research reactor, the siphon phenomenon induced by pipe rupture can cause continuous outward flow of water. To prevent ...

Measurements of Soil Carbon by Neutron-Gamma Analysis in Static and Scanning Modes

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of gamma rays created when neutrons interact with soil elements. The main parts of the INS system are a pulsed neutron generator, NaI(Tl) gamma detectors, split electronics to separate gamma spectra due to INS and thermo-neutron capture (TNC) processes, and software for gamma spectra acquisition and data processing. This method has several advantages over other methods in that it is a non-destructive in situ method that measures the average carbon content in large soil volumes, is negligibly impacted by local sharp changes in soil carbon, and can be used in stationary or scanning modes. The result of the INS method is the carbon content from a site with a footprint of ~2.5 - 3 m2 in the stationary regime, or the average carbon content of the traversed area in the scanning regime. The measurement range of the current INS system is &#62;1.5 carbon weight % (standard deviation &#177; 0.3 w%) in the upper 10 cm soil layer for a 1 hmeasurement.

Here, we present the protocol for in situ measurement of soil carbon using the neutron-gamma technique for single point measurements (static mode) or field averages (scanning mode). We also describe system construction and elaborate data treatment procedures.

The herein described application of the inelastic neutron scattering (INS) method for soil carbon analysis is based on the registration and analysis of ...

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques to create an application-based microelectromechanical systems (MEMS) filter and how to use it efficiently (i.e., tuning the tuning-capability and avoiding both failure and stiction). LDV enables crucial measurements that are impossible with the network analyzer, such as higher mode detection (highly sensitive biosensor application) and resonance measurement for very small devices (fast prototyping). Accordingly, LDV was used to characterize the frequency tuning range and resonance frequency at different modes of the MEMS filters built for this study. This wide range frequency tuning mechanism is based simply on Joule heating from the embedded heaters and relatively high thermal stress with respect to the temperature of a fixed-fixed beam. However, we demonstrate that another limitation of this method is the resulting high thermal stress, which can burn the devices. Further improvement was achieved and shown for the first time in this study, such that the tuning capability was increased by 32% through an increase in the applied DC bias voltage (25 V to 35 V) between the two adjacent beams. This important finding eliminates the need for the extra Joule heating at the wider frequency tuning range. Another possible failure is through stiction and requirement of structure optimization: We propose a simple and easy technique of low frequency square wave signal application that can successfully separate the beams and eliminates the need for the more sophisticated and complicated methods given in the literature. The above findings necessitate a design methodology, and so we also provide an application-based design.

A protocol for a fixed-fixed beam design using a laser Doppler vibrometer (LDV), including the measurement of frequency tuning, modification of tuning capability, and avoidance of device failure and stiction, is presented. The superiority of the LDV method over the network analyzer is demonstrated due to its higher mode capability.

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Generation and Coherent Control of Pulsed Quantum Frequency Combs

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional states on-chip in a practical way have remained elusive due to the increasing complexity of the quantum circuitry needed to prepare and process such states. Here, we outline how high-dimensional, frequency-bin entangled, two-photon states can be generated at a stable, high generation rate by using a nested-cavity, actively mode-locked excitation of a nonlinear micro-cavity. This technique is used to produce pulsed quantum frequency combs. Moreover, we present how the quantum states can be coherently manipulated using standard telecommunications components such as programmable filters and electro-optic modulators. In particular, we show in detail how to accomplish state characterization measurements such as density matrix reconstruction, coincidence detection, and single photon spectrum determination. The presented methods form an accessible, reconfigurable, and scalable foundation for complex high-dimensional state preparation and manipulation protocols in the frequency domain.

A protocol is presented for the practical generation and coherent manipulation of high-dimensional frequency-bin entangled photon states using integrated micro-cavities and standard telecommunications components, respectively.

We present a method for the generation and coherent manipulation of pulsed quantum frequency combs. Until now, methods of preparing high-dimensional ...

Generation and Control of Electrohydrodynamic Flows in Aqueous Electrolyte Solutions

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed electric body force has to be induced by ionic motions in liquid. On the other hand, positive and negative charges attract each other, and electroneutrality is maintained everywhere in equilibrium conditions. Furthermore, an increase in an applied voltage has to be suppressed to avoid water electrolysis, which causes the solutions to become unstable. Usually, EHD flows can be induced in non-aqueous solutions by applying extremely high voltages, such as tens of kV, to inject electrical charges. In this study, two methods are introduced to generate EHD flows induced by electrical charge separations in aqueous solutions, where two liquid phases are separated by an ion-exchange membrane. Due to a difference in the ionic mobility in the membrane, ion concentration polarization is induced between both sides of the membrane. In this study, we demonstrate two methods. (i) The relaxation of ion concentration gradients occurs via a flow channel that penetrates an ion-exchange membrane, where the transport of the slower species in the membrane selectively becomes dominant in the flow channel. This is a driving force to generate an EHD flow in the liquid. (ii) A long waiting time for the diffusion of ions passing through the ion-exchange membrane enables the generation of an ion-dragged flow by externally applying an electric field. Ions concentrated in a flow channel of a 1 x 1 mm2 cross-section determine the direction of the liquid flow, corresponding to the electrophoretic transport pathways. In both methods, the electric voltage difference required for an EHD flow generation is drastically reduced to near 2 V by rectifying the ion transport pathways.

The rectification of ion transport pathways is an effective method to generate one-directional ion-dragged electrohydrodynamic flows. By setting an ion-exchange membrane in a flow channel, an electrically polarized condition is generated and causes a liquid flow to be driven when an electric field is externally applied.

To drive electrohydrodynamic (EHD) flows in aqueous solutions, the separation of cation and anion transport pathways is essential because a directed ...

Controllable Nucleation of Cavitation from Plasmonic Gold Nanoparticles for Enhancing High Intensity Focused Ultrasound Applications

In this study, plasmonic gold nanoparticles were simultaneously exposed to pulsed near-infrared laser light and high intensity focused ultrasound (HIFU) for the controllable nucleation of cavitation in tissue-mimicking gel phantoms. This in vitro protocol was developed to demonstrate the feasibility of this approach, for both enhancement of imaging and therapeutic applications for cancer. The same apparatus can be used for both imaging and therapeutic applications by varying the exposure duration of the HIFU system. For short duration exposures (10 &#181;s), broadband acoustic emissions were generated through the controlled nucleation of inertial cavitation around the gold nanoparticles. These emissions provide direct localization of nanoparticles. For future applications, these particles may be functionalized with molecular-targeting antibodies (e.g. anti-HER2 for breast cancer) and can provide precise localization of cancerous regions, complementing routine diagnostic ultrasound imaging. For continuous wave (CW) exposures, the cavitation activity was used to increase the localized heating from the HIFU exposures resulting in larger thermal damage in the gel phantoms. The acoustic emissions generated from inertial cavitation activity during these CW exposures was monitored using a passive cavitation detection (PCD) system to provide feedback of cavitation activity. Increased localized heating was only achieved through the unique combination of nanoparticles, laser light and HIFU. Further validation of this technique in pre-clinical models of cancer is necessary.

This protocol demonstrates the controllable nucleation of cavitation in gel phantoms, through simultaneous exposure to both near-infrared pulsed laser light and high intensity focused ultrasound (HIFU). The cavitation activity can then be used for enhancing imaging and/or therapeutic uses of HIFU.

In this study, plasmonic gold nanoparticles were simultaneously exposed to pulsed near-infrared laser light and high intensity focused ultrasound (HIFU) ...