RSC thanks the support of the National Science Council (NSC) of Taiwan under Project NSC 102-2112-M-011-001-MY3. YSH acknowledges the support of the NSC of Taiwan under Project NSC 100-2112-M-011-001-MY3.

1. Structural Characterization of MoSe2 Layer Crystals (See Step 1 in Figure 1)
<ol>
	<li>XRD Measurement Procedure
	<ol>
		<li>Mount a MoSe2 layer crystal (with the size range of 5 x 5 x 0.1-10 x 10 x 0.5 mm3) or crystal powder (which was mixed with quartz powder and binder and was smeared on the slide glass) on the holder.</li>
		<li>Press the holder by a slide glass to ensure layer crystal surface parallel to the holder surface.</li>
		<li>Load the sample holder into the diffractometer.</li>
		<li>Close the doors of diffractometer.</li>
		<li>Calibrate beam line accordingly to manufacturer&#8217;s instructions.</li>
		<li>Input measurement parameters such as the 2 scan range (10-80&#176;), the increment (0.004&#176;), and the dwell time (0.1 sec).</li>
		<li>Start the DIFFRAC.Measurement Center program on the computer attached to the diffractometer and then save data and name data file according to manufacturer&#8217;s protocol.</li>
		<li>Analyze the XRD pattern by identifying the positions of the diffraction peaks using the software and then compare with the standard data from JCPDS card database to confirm the single out-of-plane orientation and single-crystalline quality of the MoSe2 layer crystals32,33.</li>
	</ol>
	</li>
	<li>Micro-Raman Measurement Procedure
	<ol>
		<li>Perform the Raman equipment calibration using a silicon wafer as the standard sample. The measurement of the silicon wafer is the same as the procedure described below for the interested MoSe2 layer crystal.</li>
		<li>Mount a MoSe2 layer crystal on the slide glass.</li>
		<li>Load the slide glass on the holder of optical microscope and focus the sample surface with a white light source.</li>
		<li>Switch the light source from a white light to a laser beam (wavelength at 514 nm).</li>
		<li>Input measurement parameters such as the wavenumber scan range (150-500 cm-1), the integration time (10 sec), and the number of scan times (10-30 times).</li>
		<li>Start the program on computer attached to the Raman spectrometer and then save data and name data file according to manufacturer&#8217;s protocol.</li>
		<li>Analyze the Raman spectrum by identifying their peak widths and positions using the software and then compare with standard data from references to confirm the crystalline structure type and quality of the MoSe2 layer crystals34,35.</li>
	</ol>
	</li>
</ol>
2. Fabrication of MoSe2 Layer Nanocrystal Devices
<ol>
	<li>Mechanical Exfoliation of Layer Crystals
	<ol>
		<li>Clean tweezers with acetone and alcohol.</li>
		<li>Pick the MoSe2 layer crystals (4 to 8 pieces) with a shiny surface (i.e. mirror-like crystal face) and an area size larger than 0.5 x 0.5 mm2 with the tweezers and put them on the dicing tape with an area size of 20 x 60 mm2.</li>
		<li>Fold the tape in half to exfoliate the layer crystal and repeat the action approximately twenty times. Usually layer crystals can be stripped into many micrometer-sized crystals in width (see Step 2 in Figure 1).</li>
		<li>Load the dicing tape with the layer nanocrystal powder into the SEM chamber to observe the sizes and morphologies of these stripped MoSe2 layer microcrystal. If the width distributions of the layer nanocrystal are at 1-20 &#956;m, the nanocrystal powder can meet the criteria for the device fabrication.</li>
	</ol>
	</li>
	<li>Dispersion of the Layer Nanocrystals on the Device Template
	<ol>
		<li>Place the dicing tape with the layer nanocrystal powder upside-down on the device template. The template is SiO2 (300 nm)-coated silicon substrate with sixteen pre-patterned Ti (30 nm)/Au (90 nm) electrodes on the SiO2 surface (see Step 4 in Figure 1). The area size of the template is 5 x 5 mm2.</li>
		<li>Tap the dicing tape lightly to make some nanocrystals (roughly 10 to 100 pieces) fall on the template.</li>
		<li>Check the number density and dispersion condition of the nanocrystal on the template by optical microscope or sometimes by SEM if the dispersed nanocrystals can be not observed by optical microscope. Usually 2 to 5 pieces of nanocrystals (area size larger than 2 x 2&#160;&#956;m2) dispersed on the center square (with area of 80 x 80 &#956;m2) of the template without overlapping to each other are the better condition for the next FIB processing.</li>
	</ol>
	</li>
	<li>Electrode Fabrication by FIB
	<ol>
		<li>Mount templates on the FIB holder using conducting copper foil tape. Typically, the area of conducting tape of 3 x 2.4 cm2 was required for mounting 6-8 templates.</li>
		<li>Load the holder into the FIB chamber.</li>
		<li>Evacuate the chamber to the vacuum degree down to 10-5 mbar by clicking the button &#8220;Pump&#8221;.</li>
		<li>Set the electron beam current (41 pA) and acceleration voltage (10 kV) for the SEM mode.</li>
		<li>Set the ion beam current (0.1 nA) and acceleration voltage (30 kV) for the FIB mode.</li>
		<li>Warm up the ion beam system and gas-injection-system (GIS) by clicking the button &#8220;beam on&#8221; and the button &#8220;Cold&#8221; in the &#8220;Gas Injection&#8221; block, respectively.</li>
		<li>Turn on the electron-beam by clicking the button &#8220;Beam On&#8221; and focus the image at a low magnification of 100X.</li>
		<li>Set the z-axial working distance (WD) at 10 mm for SEM mode.</li>
		<li>Set the magnification at 5,000X and focus.</li>
		<li>Set the tilt angle of the holder to 52 degrees by clicking the button &#8220;Navigation&#8221; and input the tilt angle &#8220;52&#8221;.</li>
		<li>Select a MoSe2 layer nanocrystal with a certain thickness (ranging from 5 to 3,000 nm) and a rectangular and square shape for the electrode fabrication.</li>
		<li>Take the SEM images at different magnifications (from 1,000X to 10,000X) of the targeted pristine material before electrode fabrication by clicking the button &#8220;Snapshot&#8221;.</li>
		<li>Switch to FIB mode and take an FIB image by the snapshot mode to reduce the exposure time of the targeted material under ion beam bombardment.</li>
		<li>Define the electrode deposition area, select the &#8220;Pt deposition&#8221; mode, and input the thickness (0.2-1.0 &#956;m) value of the deposited Pt electrode.</li>
		<li>Introduce the capillary of GIS into the chamber by clicking the box &#8220;Pt dep&#8221; in the &#8220;Gas Injection&#8221; block.</li>
		<li>Take an image by the snapshot mode again and modify the position of the electrodes if the originally defined pattern shifts slightly.</li>
		<li>Turn on the FIB deposition by clicking the button &#8220;Start Patterning&#8221;.</li>
		<li>After deposition, draw the capillary of GIS back by unclicking the box &#8220;Pt dep&#8221; in the &#8220;Gas Injection&#8221; block.</li>
		<li>Switch to SEM mode and check the result of the deposited Pt electrodes on the layer nanocrystal.</li>
		<li>Take the SEM images at different magnifications of the completed devices with two or four electrodes (see Step 3 in Figure 1).</li>
		<li>Set the tilt angle of the holder return to 0 degrees by clicking the button &#8220;Navigation&#8221; and input the tilt angle &#8220;0&#8221;.</li>
		<li>Take the top-viewed SEM images at different magnifications for the estimations of the material width and electrode inter-distance by clicking the button &#8220;Snapshot&#8221;.</li>
		<li>Turn off the electron-beam and ion beam systems and cool down GIS system by clicking the button &#8220;Beam Off&#8221; and the button &#8220;Warm&#8221; in the &#8220;Gas Injection&#8221; block, respectively.</li>
		<li>Vent the chamber by introducing nitrogen gas by clicking the buttons &#8220;Vent&#8221; and then take the holder out of the chamber. It typically takes 5 to 10 min to finish the venting process.</li>
		<li>Close the chamber door and evacuate the chamber.</li>
	</ol>
	</li>
</ol>
3. Characterizations of the MoSe2 Layer Nanocrystal Devices
<ol>
	<li>Thickness Measurement of the Layer Nanocrystals by AFM
	<ol>
		<li>Install the AFM cantilever to the probe holder.</li>
		<li>Turn on AFM program and select &#8220;ScanAsyst&#8221; mode.</li>
		<li>Load the probe holder and connect it with the laser diode head of AFM station.</li>
		<li>Perform the calibration to align the incident laser beam position and the cantilever according to manufacturer&#8217;s protocol.</li>
		<li>Mount the sample (the template chip with FIB-fabricated layer nanocrystal devices) on the sample holder by Cu foil tape.</li>
		<li>Load the sample holder to the AFM station.</li>
		<li>Move the sample holder to the position approximately underneath the laser beam or AFM cantilever.</li>
		<li>Lower down AFM cantilever to the focus position by focusing the optical microscope image of the layer nanocrystal.</li>
		<li>Input scan parameters such as the scan area (6 x 6-30 x 30 &#956;m2), the frequency (0.5-1.5 Hz), and the resolution (256-512 lines).</li>
		<li>Start the program and save data according to the manufacturer&#8217;s protocol.</li>
		<li>Raise the AFM cantilever and take the sample holder out.</li>
		<li>Load the second sample and repeat the measurement procedure described above if need.</li>
		<li>Estimate the thickness of layer nanocrystals by analyzing the AFM image and height profile using the software &#8220;NanoScope Analysis&#8221;. Select a lateral height profile from the AFM image and determine the average thickness value by the flatten area of the profile. (See Figure 2d and 2e)</li>
	</ol>
	</li>
	<li>Current versus voltage (I-V) measurement of the layer nanocrystals
	<ol>
		<li>Mount the sample (the template chip with FIB-fabricated layer nanocrystal devices) on the mica substrate by Cu foil tape.</li>
		<li>Bond the enameled wires or Cu wires on the electrodes of the chip by Ag paste. (See Step 4 in Figure 1.)</li>
		<li>Load the completed sample in the probe station chamber and fix it on the sample holder by Cu foil tape. The cryogenic probe station was located in the dark environment. (See Step 5 in Figure 1.)</li>
		<li>Solder the electrical wires of the sample and the metal electrodes of the probes one by one.</li>
		<li>Cap the chamber top and evacuate the chamber down to 10-4 mbar. Cool down sample to the 77 K by introducing liquid nitrogen into the probe station. Set the temperature range (usually from 80 to 320 K), interval, and dwell time for the temperature control. (Necessary only for temperature-dependent measurement).</li>
		<li>Set the applied voltage sweeping range (typically from -1 to 1 V), voltage interval (0.01 V), and the limited maximal current (10 or 100&#160;&#956;A) in an ultrahigh-impedance multifunctional electrometer for the two-terminal I-V measurement. For four-terminal measurement, set the applied current sweeping range (typically from -100 to 100 &#956;A) and current interval (1 &#956;A).</li>
		<li>Start the program and save the I-V data at room temperature or at different temperatures.</li>
		<li>Open the chamber cap if necessary and take the sample out of the chamber.</li>
		<li>Load the second sample if need and repeat the procedure described above.</li>
		<li>Analyze the I-V curve by plotting the measured current versus applied voltage data using the software. Fit the I-V curve by selecting the Linear Fitting function. Check the linearity of the I-V curve and obtain the slope value (i.e. conductance value). (See Step 6 in Figure 1.)</li>
		<li>Repeat the Step 3.2.10 for the I-V curves measured at different temperatures if need.</li>
		<li>Calculate the conductivity (&#963;) value according to the equation &#963;= G(t/tw) by adopting the parameters obtained by I-V, SEM, and AFM measurements including conductance (G), thickness (t), width (w) and length (l) of the layer nanocrystal.</li>
		<li>Plot the curves of the conductance and conductivity values versus thickness of layer nanocrystals.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Wilson, J. A., Yoffe, A. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+transition+metal+dichalcogenides+discussion+and+interpretation+of+the+observed+optical,+electrical+and+structural+properties.">The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties.</a> Adv. Phys. 18 (73), 193-335 (1969).</li><li>Ataca, C., Sahin, H., Ciraci, S. <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+MX2+transition-metal+oxides+and+dichalcogenides+in+a+honeycomb-like+structure.">Single-layer MX2 transition-metal oxides and dichalcogenides in a honeycomb-like structure.</a> J. Phys. Chem. C. 116 (16), 8983-8999 (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> Nature Nanotech. 7 (11), 699-712 (2012).</li><li>Mak, K. F., Lee, C., Hone, J., Shan, J., Heinz, T. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomically+thin+MoS2:+A+new+direct-gap+semiconductor.">Atomically thin MoS2: A new direct-gap semiconductor.</a> Phys. Rev. Lett. 105 (13), 136805 (2010).</li><li>Splendiani, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emerging+photoluminescence+in+monolayer+MoS2.">Emerging photoluminescence in monolayer MoS2.</a> Nano Lett. 10 (4), 1271-1275 (2010).</li><li>Leb&#232;gue, S., Eriksson, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electronic+structure+of+two-dimensional+crystals+from+ab+initio+theory.">Electronic structure of two-dimensional crystals from ab initio theory.</a> Phys. Rev. B. 79 (11), 115409 (2009).</li><li>Kuc, A., Zibouche, N., Heine, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+quantum+confinement+on+the+electronic+structure+of+the+transition+metal+sulfide+TS2.">Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2.</a> Phys. Rev. B. 83 (24), 245213 (2011).</li><li>Yoffe, A. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Layer+compounds.">Layer compounds.</a> Annu. Rev. Mater. Sci. 3, 147-170 (1993).</li><li>Chen, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Tunable+band+gap+photoluminescence+from+atomically+thin+transition-metal+dichalcogenide+alloys.">Tunable band gap photoluminescence from atomically thin transition-metal dichalcogenide alloys.</a> ACS Nano. 7 (5), 4610-4616 (2013).</li><li>Radisavljevic, B., 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=Mobility+engineering+and+a+metal-insulator+transition+in+monolayer+MoS2.">Mobility engineering and a metal-insulator transition in monolayer MoS2.</a> Nature Mater. 12 (9), 815-820 (2013).</li><li>Zhang, Y., Ye, J., Matsuhashi, Y., Iwasa, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ambipolar+MoS2+thin+flake+transistors.">Ambipolar MoS2 thin flake transistors.</a> Nano Lett. 12 (3), 1136-1140 (2012).</li><li>Liu, H., Neal, A. T., Ye, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Channel+length+scaling+of+MoS2+MOSFETs.">Channel length scaling of MoS2 MOSFETs.</a> ACS Nano. 6 (10), 8563-8569 (2012).</li><li>Ghatak, S., Pal, A. N., Ghosh, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nature+of+electronic+states+in+atomically+thin+MoS2+field-effect+transistors.">Nature of electronic states in atomically thin MoS2 field-effect transistors.</a> ACS Nano. 5 (10), 7707-7712 (2011).</li><li>Ong, Z. Y., Fischetti, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mobility+enhancement+and+temperature+dependence+in+top-gated+single-layer+MoS2.">Mobility enhancement and temperature dependence in top-gated single-layer MoS2.</a> Phys. Rev. B. 88 (16), (2013).</li><li>Hwang, W. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparative+study+of+chemically+synthesized+and+exfoliated+multilayer+MoS2+field-effect+transistors.">Comparative study of chemically synthesized and exfoliated multilayer MoS2 field-effect transistors.</a> Appl. Phys. Lett. 102 (4), 165316 (2013).</li><li>Park, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+environmental+and+passivation+effects+on+molybdenum+disulfide+field+effect+transistors.">Oxygen environmental and passivation effects on molybdenum disulfide field effect transistors.</a> Nanotechnology. 24 (9), 095202 (2013).</li><li>Wu, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+mobility+and+high+on/off+ratio+field-effect+transistors+based+on+chemical+vapor+deposited+single-crystal+MoS2+grains.">High mobility and high on/off ratio field-effect transistors based on chemical vapor deposited single-crystal MoS2 grains.</a> Appl. Phys. Lett. 102 (14), 142106 (2013).</li><li>Larentis, S., Fallahazad, B., Tutuc, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Field-effect+transistors+and+intrinsic+mobility+in+ultra-thin+MoSe2+layers.">Field-effect transistors and intrinsic mobility in ultra-thin MoSe2 layers.</a> Appl. Phys. Lett. 101 (22), 223104 (2012).</li><li>Bolotin, K. I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrahigh+electron+mobility+in+suspended+graphene.">Ultrahigh electron mobility in suspended graphene.</a> Solid State Commun. 146 (9), 351-355 (2008).</li><li>Novoselov, K. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-dimensional+gas+of+massless+Dirac+fermions+in+graphene.">Two-dimensional gas of massless Dirac fermions in graphene.</a> Nature. 438 (7065), 197-200 (2005).</li><li>Zhang, Y., Tan, Y. W., Stormer, H. L., Kim, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+observation+of+the+quantum+Hall+effect+and+Berry's+phase+in+graphene.">Experimental observation of the quantum Hall effect and Berry's phase in graphene.</a> Nature. 438 (7065), 201-204 (2005).</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> Nature Nanotech. 6 (3), 147-150 (2011).</li><li>Chang, C. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrical+transport+properties+of+single+GaN+and+InN+nanowires.">Electrical transport properties of single GaN and InN nanowires.</a> J. Electro. Mater. 35 (4), 738-743 (2006).</li><li>Calarco, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Size-dependent+photoconductivity+in+MBE-grown+GaN-nanowires.">Size-dependent photoconductivity in MBE-grown GaN-nanowires.</a> Nano. Lett. 5 (5), 981-984 (2005).</li><li>Soci, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ZnO+nanowire+UV+photodectors+with+high+internal+gain.">ZnO nanowire UV photodectors with high internal gain.</a> Nano Lett. 7 (4), 1003-1009 (2007).</li><li>Nam, D. T., Fischer, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Disorder+effects+in+focused-ion-beam-deposited+Pt+contacts+on+GaN+nanowires.">Disorder effects in focused-ion-beam-deposited Pt contacts on GaN nanowires.</a> Nano Lett. 5 (10), 2029-2033 (2005).</li><li>Chen, R. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anomalous+quantum+efficiency+for+photoconduction+and+its+power+dependence+in+metal+oxide+semiconductor+nanowires.">Anomalous quantum efficiency for photoconduction and its power dependence in metal oxide semiconductor nanowires.</a> Nanoscale. 5 (15), 6867-6873 (2013).</li><li>Chen, R. S., Tang, C. C., Shen, W. C., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thickness-dependent+electrical+conductivities+and+ohmic+contacts+in+transition+metal+dichalcogenides+multilayers.">Thickness-dependent electrical conductivities and ohmic contacts in transition metal dichalcogenides multilayers.</a> Nanotechnology. 25 (41), (2014).</li><li>Huang, Y. H., Peng, C. C., Chen, R. S., Huang, Y. S., Ho, C. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transport+properties+in+semiconducting+NbS2+nanoflakes.">Transport properties in semiconducting NbS2 nanoflakes.</a> Appl. Phys. Lett. 105 (9), (2014).</li><li>Hu, S. Y., Liang, C. H., Tiong, K. K., Lee, Y. C., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+and+characterization+of+large+niobium-doped+MoSe2+single+crystals.">Preparation and characterization of large niobium-doped MoSe2 single crystals.</a> J. Crystal Growth. 285 (3), 408-414 (2005).</li><li>Das, S., Appenzeller, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Where+does+the+current+flow+in+two-dimensional+layered+systems.">Where does the current flow in two-dimensional layered systems.</a> Nano Lett. 13 (7), 3396-3402 (2013).</li><li>Cullity, B. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Elements of X-ray Diffraction. , (1978).</li><li>Jadczak, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Composition+dependent+lattice+dynamics+in+MoSxSe(2-x)+alloys.">Composition dependent lattice dynamics in MoSxSe(2-x) alloys.</a> J. Appl. Phys. 116 (19), 193505 (2014).</li><li>Weber, W. H., Merlin, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Raman Scattering in Materials Science. , (2000).</li><li>Tonndorf, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photoluminescence+emission+and+Raman+response+of+monolayer+MoS2,+MoSe2,+and+WSe2.">Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2.</a> Opt. Express. 21 (4), 4908-4916 (2013).</li><li>Hu, S. Y., Liang, C. H., Tiong, K. K., Lee, Y. C., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+and+characterization+of+large+niobium-doped+MoSe2+single+crystals.">Preparation and characterization of large niobium-doped MoSe2 single crystals.</a> J. Crystal Growth. 285 (3), 408-414 (2005).</li><li>Hu, S. Y., Liang, C. H., Tiong, K. K., Huang, Y. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+Re+dopant+on+the+electrical+and+optical+properties+of+MoSe2+single+crystals.">Effect of Re dopant on the electrical and optical properties of MoSe2 single crystals.</a> J. Alloys Compounds. 442 (1-2), 1-2 (2007).</li><li>Bougouma, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+and+characterization+of+large,+high+quality+MoSe2+single+crystals.">Growth and characterization of large, high quality MoSe2 single crystals.</a> J. Crystal Growth. 363, 122-127 (2013).</li></ol>

The authors have nothing to disclose.

The accurate determination of the &#963; value and its dimension dependence in the layer nanocrystals is highly dependent on the quality of the electrical contacts. The FIB deposition method used for metal electrode deposition played a crucial role throughout the study. According to electrical, structural, and composition analyses, the fabrication of stable and highly reproducible ohmic contacts, using FIB deposition method, in the MoSe2 or MoS2 devices was facilitated by the formation of t...

Transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2, have an interesting two-dimensional (2D) layer structure and semiconducting properties1-3. Scientists have recently discovered that the monolayer structure of MoS2 shows a substantially enhanced light-emitting efficiency because of the quantum confinement effect. The finding of the new direct-bandgap semiconductor material has attracted substantial attention4-7. In addition, the easily stripped layer structure of TMDs is an excellent platform for studying the fundamental properties of 2D materials. Unlike metallic graphene without the bandgap, TMDs have inherent semiconducting characteristics and have a bandgap in the range of 1-2 eV1,3,8. The 2D structures of the ternary compounds of TMDs9 and the possibility of the integration of these compounds with graphene provide an unprecedented opportunity to develop ultrathin and flexible electronic devices.
Unlike graphene, the room temperature electron mobility values of 2D TMDs are at a moderate level (1-200 cm2V&#8722;1sec&#8722;1 for MoS210-17; approximately 50 cm2V&#8722;1sec&#8722;1 for MoSe218). The optimal mobility values of graphene have been reported to be higher than 10,000 cm2V&#8722;1sec&#8722;1.19-21 Nevertheless, semiconducting TMD monolayers exhibit excellent device performance. For instance, the MoS2 and MoSe2 monolayers or multilayer field-effect transistors exhibit extremely high on/off ratios, up to 106-109 10,12,17,18,22. Therefore, it is crucial to understand the fundamental electrical properties of the 2D TMDs and their bulk materials.
However, studies of the electrical properties of the layer materials have been partially hampered because of the difficulty in forming good ohmic contact on the layer crystals. Three approaches, shadow mask deposition (SMD)23, electron beam lithography (EBL)24,25, and focused-ion beam (FIB) deposition,26,27 have been used to form electrical contacts on nanomaterials. Because SMD typically involves the use of a copper grid as the mask, the spacing between two contact electrodes is mostly larger than 10 &#956;m. Unlike EBL and FIB deposition, metal deposition of electrode arrays on a substrate is performed without targeting or selecting nanomaterials of interest in the SMD method. This approach cannot guarantee that the metal patterns are correctly deposited on individual nanomaterials as the electrodes. The result of the SMD method has an element of chance. The EBL and FIB deposition methods are used in the scanning electron microscope (SEM) system; nanomaterials can be directly observed and selected for electrode deposition. In addition, EBL can be used to easily fabricate metal electrodes with a line width and a contact electrode spacing smaller than 100 nm. However, the residual resist on the nanomaterial surface left during lithography inevitably results in the formation of an insulating layer between the metal electrode and the nanomaterial. Thus, EBL leads to high contact resistance.
The main advantage of electrode fabrication through FIB deposition is that it leads to low contact resistance. Because metal deposition is performed by the decomposition of an organometallic precursor by using an ion beam at the defined area, metal deposition and ion bombardment occur simultaneously. This could destroy the metal&#8211;semiconductor interface and prevent the formation of Schottky contact. Ion bombardment can also eliminate surface contaminants such as hydrocarbons and native oxides, which decreases contact resistance. Ohmic contact fabrication through FIB deposition has been demonstrated for different nanomaterials27-29. In addition, the entire fabrication procedure in the FIB deposition approach is simpler than that in EBL.
As layer semiconductors typically show highly anisotropic electrical conduction, the conductivity in the layer-to-layer direction is several orders of magnitude lower than that in the in-plane direction30,31. This characteristic increases the difficulty of fabricating ohmic contacts and determining electrical conductivity. Therefore, in this study, FIB deposition was used for studying the electrical properties of layer semiconductor nanostructures.

<table>
	<tbody>
		<tr>
			<td>HRTEM&#38;SEAD</td>
			<td>FEI (http://www.fei.com/products/tem/tecnai-g2/?ind=MS)</td>
			<td>Tecnai&#8482; G2 F-20</td>
			<td></td>
		</tr>
		<tr>
			<td>SEM&#38;EDS</td>
			<td>HITACHI (http://www.hitachi-hitec.com/global/em/sem/sem_index.html)</td>
			<td>S-3000H</td>
			<td></td>
		</tr>
		<tr>
			<td>FIB</td>
			<td>FEI (http://www.fei.com/products/dualbeam/versa-3d/)</td>
			<td>Quanta 3D FEG</td>
			<td></td>
		</tr>
		<tr>
			<td>AFM</td>
			<td>BRUKER (http://www.bruker.com/products/surface-analysis/atomic-force-microscopy/dimension-icon/overview.html)</td>
			<td>Dimension Icon</td>
			<td></td>
		</tr>
		<tr>
			<td>XRD</td>
			<td>Bruker (https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/d2-phaser/learn-more.html)</td>
			<td>D2 PHASER X-ray Diffractometer</td>
			<td></td>
		</tr>
		<tr>
			<td>Raman</td>
			<td>Renishaw (http://www.renishaw.com/en/renishaw-enhancing-efficiency-in-manufacturing-and-healthcare--1030)</td>
			<td>inVia Raman microscope system</td>
			<td></td>
		</tr>
		<tr>
			<td>Keithley-4200</td>
			<td>keithley (http://www.keithley.com.tw/products/dcac/currentvoltage/4200scs)</td>
			<td>4200scs</td>
			<td></td>
		</tr>
		<tr>
			<td>ultralow current leakage cryogenic probe station</td>
			<td>Lakeshore Cryotronics (http://www.lakeshore.com/)</td>
			<td>TTP4</td>
			<td></td>
		</tr>
		<tr>
			<td>copper foil tape</td>
			<td>3M (http://solutions.3m.com/wps/portal/3M/en_US/Electronics_NA/Electronics/Products/Product_Catalog/~/3M-Copper-Foil-Shielding-Tape-1182?N=4294300025+5153906&#38;&#38;Nr=AND%28hrcy_id%3A8CQ27CX0WMgs_F2LMWMM6M6_N2RL3FHWVK_GPD0K8BC31gv%29&#38;rt=d)</td>
			<td>1182</td>
			<td></td>
		</tr>
		<tr>
			<td>Ag paste</td>
			<td>Well-Being (http://www.gredmann.com/about.htm)</td>
			<td>MS-5000</td>
			<td></td>
		</tr>
		<tr>
			<td>Cu wire</td>
			<td>Guv Team (http://www.guvteam.com)</td>
			<td>ICUD0D01N</td>
			<td></td>
		</tr>
		<tr>
			<td>dicing tape</td>
			<td>Nexteck (http://www.nexteck-corp.com/tw/product-tape.html)</td>
			<td>contact vender</td>
			<td></td>
		</tr>
		<tr>
			<td>mica</td>
			<td>Centenary Electronic (http://100y.diytrade.com/sdp/307600/4/pl-1175840/0.html)</td>
			<td>T0-200</td>
			<td></td>
		</tr>
		<tr>
			<td>enamel wire</td>
			<td>Light-Tech Electronics (http://www.ltc.com.tw/product_info.php/products_id/57631)</td>
			<td>S.W.G #38</td>
			<td></td>
		</tr>
	</tbody>
</table>

ohmic contact fabrication using a focused-ion beam technique and electrical characterization for layer semiconductor nanostructures

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

The determined values of the electrical conductance (G) and conductivity (&#963;) of layer nanomaterials with different thicknesses are highly dependent on the quality of the electrical contacts. The ohmic contacts of the FIB-deposition-fabricated two-terminal MoSe2 devices are characterized by measuring the current&#8211;voltage (I&#8211;V) curve. The room temperature I&#8211;V curves for the two-terminal MoSe2 nanoflake devices with different t...

Watch this Scientific Journal Video about Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures at JoVE.com

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

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

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

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12.1K Views.  National Taiwan University of Science and Technology. We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described. 