Our research was supported by the Director, Office of Science, Office of Basic Energy Sciences of the U.S. Department of Energy sp2 Program under contract no. DE-AC02-05CH11231 (STM instrumentation development and device integration); the Office of Naval Research (device characterization), and NSF award no. CMMI-1235361 (dI/dV imaging). STM data were analyzed and rendered using WSxM software.33&#160;D. W. and A.J.B. were supported by the Department of Defense (DoD) through the National Defense Science &#38; Engineering Graduate Fellowship (NDSEG) Program, 32 CFR 168a.

1. Electrochemical Polishing of a Cu Foil22,23
Note: Electrochemical polishing exposes bare Cu surface for graphene growth by removing the protective surface coating and controls the growth seed density.
<ol>
	<li>Prepare an electrochemical polishing solution by mixing 100 ml ultra-pure water, 50 ml ethanol, 50 ml phosphoric acid, 10 ml isopropanol, and 1 g urea.</li>
	<li>Cut Cu foil into multiple 3 cm by 3 cm foils. Note: Each foil serves as either an anode or a cathode.</li>
	<li>Set up the anode/cathode by clipping a Cu foil vertically with a holder and connecting it to the appropriate terminal of the power supply. 
	Note: The anode (positive terminal) will be electrochemically polished for graphene growth.</li>
	<li>Before the polishing begins, set constant voltage of 4.8 V in the power supply.</li>
	<li>Turn on the power supply as soon as the anode and the cathode are simultaneously dipped into the electrochemical polishing solution. Separate the electrodes by about 2 cm. Check that the resulting current is between 1-2 A.</li>
	<li>Stop electrochemical polishing after 2 min by turning off the power supply. Take out the anode and immediately rinse it separately with ultra-pure water, acetone, and isopropanol.</li>
	<li>Blow-dry the rinsed Cu foil with N2 gas and store it in a dry container.</li>
</ol>
2. Chemical Vapor Deposition (CVD) of Graphene on a Cu Foil22-25
<ol>
	<li>Place a quartz tube in a CVD furnace and connect the tube to the rest of the gas line with KF fittings.</li>
	<li>Place an electrochemically polished Cu foil on top of a quartz boat. Using a long rod, push the quartz boat into the quartz tube until the Cu foil is placed at the center of the CVD furnace. Enclose the system with KF fittings.</li>
	<li>Pump down the system with a roughing pump. Purge the system with H2.</li>
	<li>Ramp up the temperature to 1,050 &#176;C with 200 sccm of H2. Anneal at 1,050 &#176;C with the same gas flow for 2 hr. 
	Note: The temperature is measured via built-in Type K thermocouple in a tube furnace.</li>
	<li>Cool down to 1,030 &#176;C with the same gas flow. Grow graphene for 10 min with 40 sccm CH4 and 10 sccm H2.</li>
	<li>As soon as the growth is over, open up the furnace hood to cool rapidly down toRT. Keep the same gas flow.</li>
	<li>Once the temperature is below 100 &#176;C, turn off the gas flow.</li>
	<li>Turn off the pump. Vent the system with N2 gas by slowly opening the metering valve between the gas line and the N2 gas cylinder.</li>
	<li>Take out the sample. Cut the Cu foil into pieces with desired dimensions.</li>
	<li>Store the sample in a dry container inside a desiccator.</li>
</ol>
3. Mechanical Exfoliation19-21 of h-BN onto a SiO2 Chip
<ol>
	<li>Clean an SiO2 chip. Note: The SiO2 chip consists of a roughly 285 nm thick SiO2 layer on top of a bulk Si.
	<ol>
		<li>Cut a SiO2 wafer into about 1 cm by 1 cm chips using a diamond scribe.</li>
		<li>Rinse the SiO2 chip with water and isopropanol. Keep the SiO2 surface covered with water/isopropanol after each rinse.</li>
		<li>Place the SiO2 chip on the spin coater to remove liquid from its surface. Spin the chip with 3,000 rpm for 15 sec.</li>
		<li>Check the cleanliness of the chip under an optical microscope with dark field setting. Note: Under dark field setting, solid impurities appear as bright particles.</li>
	</ol>
	</li>
	<li>Place a clean SiO2 chip, exfoliation tape, and h-BN crystal on a clean table with an optical microscope.</li>
	<li>Prepare two strips of tape: a parent tape and a second tape. Place both on the table with their sticky sides up. Place a h-BN crystal on the sticky side of the parent tape.</li>
	<li>Place the sticky side of the second tape over the h-BN crystal on the parent tape (so that sticky sides of the second and parent tapes are touching). Rub over the crystal gently to remove trapped air bubbles.</li>
	<li>Peel the second tape off to exfoliate the h-BN crystal onto the second tape. Store the parent tape for future use.</li>
	<li>Fold the second tape onto itself, rub gently over the crystal, and peel off the tape. Repeat this process 10 times, folding the second tape so that the h-BN crystals are transferred to a fresh new region of the second tape each time.</li>
	<li>Place a clean SiO2 chip under the microscope. Stick the region of the second tape containing the h-BN crystal onto the SiO2 chip. Ensure that the tape also sticks to the microscope stand for securing the chip during the upcoming exfoliation step.</li>
	<li>Slowly peel off the tape, monitoring the process under the microscope. As the tape is nearly peeled off, use a tweezer to hold the SiO2 chip in place. Note: Peeling too fast will result in less h-BN flakes on the chip.</li>
	<li>Once the tape is peeled off, place the chip inside a CVD furnace. Anneal the chip in air at 500 &#176;C for 2 hr.</li>
</ol>
4. Poly(methyl methacrylate) (PMMA)26-28 Transfer of Graphene onto h-BN/SiO2
<ol>
	<li>Put one drop of PMMA (A4) on the graphene/Cu/graphene foil. Spin-coat the foil with 3,000 rpm in 30 sec. Note: As FeCl3 (aq) etches away the Cu layer during the upcoming etching step, the backside graphene will fall off while the PMMA/graphene layer will remain unreacted (Figure 2).</li>
	<li>Using a FeCl3 resistant spoon, let the spin-coated Cu foil float on the following solutions in this order: 1.5 min on FeCl3 (aq), 5 min on ultra-pure water, 1 min on FeCl3 (aq), 5 min on ultra-pure water, 15 min on FeCl3 (aq), 5 min on ultra-pure water, 5 min on ultra-pure water, and 30 min on ultra-pure water. Prepare each ultra-pure water bath in a separate beaker.</li>
	<li>Fish the PMMA/graphene sample with a h-BN/SiO2 chip. Place it on a hot plate at 80 &#176;C for 10 min to remove water and at 180&#160;&#176;C for 15 min to relax27 PMMA film.</li>
	<li>Place the PMMA/graphene/h-BN/SiO2 chip in CH2Cl2 O/N to dissolve the PMMA layer.</li>
</ol>
5. Ar/H2 Annealing29,30
<ol>
	<li>Place the quartz tube on the CVD furnace and connect the tube to the rest of the gas line with KF fittings.</li>
	<li>Place the graphene/BN/SiO2 chip on a quartz boat. Using a long rod, push the quartz boat into the quartz tube until the chip is placed at the center of the CVD furnace. Enclose the system with KF fittings.</li>
	<li>Pump down the system with a roughing pump. Purge the system with H2 and Ar.</li>
	<li>Build up the pressure to 1 atm with 100 sccm H2 and 200 sccm Ar with a metering valve. Once the pressure reaches 1 atm, adjust the aperture size of the metering valve to stabilize the pressure at 1 atm.</li>
	<li>Ramp up the temperature to 350&#160;&#176;C and anneal for 5 hr with the same gas flow.</li>
	<li>Cool down to RT with the same gas flow.</li>
	<li>Once the temperature is below 100 &#176;C, turn off the gas flow. Close all valves.</li>
	<li>Turn off the pump. Vent the system with N2 gas by slowly opening the metering valve between the gas line and the N2 gas cylinder.</li>
	<li>Take out the sample. Check the number of graphene layers and defect level with Raman spectroscopy32. Check its cleanliness/uniformity under optical microscope. Scan multiple areas that appear clean under the optical microscope with atomic force microscope (AFM) to ensure that the sample appears clean/uniform at a small length scale (&#60;500 nm) as well.</li>
	<li>Store it in a dry container inside a desiccator.</li>
</ol>
6. &#160;Assembling a Gate-tunable Graphene Device for STM Measurement2-5
<ol>
	<li>Evaporate a 50 &#181;m by 50 &#181;m Au/Ti contact pad onto the Ar/H2-annealed CVD graphene/h-BN/SiO2 sample.
	<ol>
		<li>Tape the sample onto a stage on top of a micromanipulator.</li>
		<li>While monitoring with optical microscope, align a stencil mask with graphene using the micromanipulator so that the Au/Ti contact will be deposited near the region of interest without covering the surface.</li>
		<li>Transfer the sample with stencil mask to an e-beam evaporator. Evaporate 10 nm of Ti at 3 &#197;/sec. Cover up the Ti layer by evaporating 30-50 nm of Au at 3 &#197;/sec in the same evaporation session without breaking vacuum. Note: Alternatively, 1 nm of Cr is a good substitute for the 10 nm of Ti.</li>
		<li>Transfer the sample with stencil mask back to the micromanipulator for removing the stage.</li>
	</ol>
	</li>
	<li>Mount the sample onto an ultra-high-vacuum (UHV) sample plate.
	<ol>
		<li>Place a thin piece of sapphire on a UHV sample plate. Note: The sapphire acts as an insulating layer that prevents electrical contact between the Si and STM ground. Furthermore, sapphire is an excellent thermal conductor for the purpose of sample annealing.</li>
		<li>Place the sample on top of the sapphire. Place another thin piece of sapphire on the sample. Make sure that the sapphire does not cover the graphene surface.</li>
		<li>Secure the sapphire/sample/sapphire structure with a metallic clamp. Ensure that the entire structure is rigid or else it will vibrate inside a STM.</li>
	</ol>
	</li>
	<li>Wire-bond the UHV sample plate terminals to appropriate contacts on the graphene device. Note: The deposited Au/Ti electrodes are wire-bonded to ground while the Si bulk is wire-bonded to the gate electrode (Figure 1).
	<ol>
		<li>Place the mounted sample on a grounded wire-bonding stage.</li>
		<li>Identify locations of the Au/Ti contacts using an optical microscope.</li>
		<li>Turn on a wire-bonder. If the wire-bonder is pneumatic, turn on N2 gas. Set the wire-bonder to make two bonds.</li>
		<li>Using a wire-bonding arm, place the tip of the wire bonder on top of the appropriate terminal on the UHV sample plate. Move down and gently press the tip of the wire bonder onto the terminal until the wire-bonder indicates that the bonding is finished.</li>
		<li>Thread a wire to the Au/Ti contact on the graphene device. Move down and gently press the tip of the wire bonder onto the Au/Ti contact until the wire-bonder indicates that the bonding is finished.</li>
		<li>Use a diamond scribe to scratch off some SiO2 on the edge of the SiO2 chip to expose the Si that will be used as a back-gate. Repeat the two-bonding process for the exposed Si.</li>
		<li>Insert the sample inside a UHV (10-10 Torr) prep chamber and anneal the wire-bonded graphene device at around 300 &#176;C. Transfer the device to a STM chamber. 
		Note: The graphene device should be annealed until its surface appears clean under STM (see Sec. 8 of the Protocol). The annealing time will vary depending on the initial cleanliness of the device.</li>
	</ol>
	</li>
</ol>
7.&#160;STM Tip Calibration on Au(111) Surface31
<ol>
	<li>Anneal/sputter an Au(111) sample on the heater stage in a UHV chamber to clean/flatten the Au surface. Anneal for 5 min at 375&#160;&#176;C, and sputter for 5 min with Ar+ beam accelerated to 500 V. Transfer the Au(111) sample to the STM scanning stage.</li>
	<li>Approach an Au(111) surface with a STM tip. Apply 10 V pulses on the STM tip until an Au(111) herringbone reconstruction is clearly visible.</li>
	<li>Calibrate the tip by adjusting the tip shape and comparing the differential conductance dI/dV spectrum to the standard Au(111) dI/dV spectrum.31
	<ol>
		<li>Scan the Au(111) surface with 40 nm by 40 nm frame to identify a clean/flat area. If the surface has a high density of step edges, move to a new area for scanning.</li>
		<li>Gently crash the STM tip 0.4 to 1.0 nm into a clean region of the Au(111) surface; this controlled crashing is referred to as a &#8220;poke.&#8221; Turn off the feedback and click &#8220;Take Spectroscopy&#8221; button to take a dI/dV spectrum via lock-in onto the current response of an ac modulated voltage (6 mV and 613.7 Hz) added to the tip bias. 
		Note: Once ac modulated voltage (6 mV and 613.7 Hz) is provided on the tip, the resulting tunneling current goes into the lock-in amplifier, which isolates the component of the current with the same frequency and returns dI/dV signal. By recording this signal as the sample bias is swept from -1.0 to 1.0 V, dI/dV spectrum is generated. Because this spectroscopy program is home-written, the instruction for taking the spectroscopy will vary among different programs.</li>
		<li>Check the obtained dI/dV curve against a standard Au(111) dI/dV spectrum (Figure 4). Make sure the Au(111) surface state is present in the dI/dV curve and that the spectrum is absent of any anomalous feature. If the measured dI/dV curve is not acceptable, repeat the poke in step 7.3.2 until the dI/dV curve looks like the one shown in Figure 4.</li>
		<li>Once tip shape and dI/dV spectrum are optimized, wait 15 to 30 min; if the STM tip is unstable, dI/dV spectrum will change during this time interval. Retake a dI/dV spectrum at a different location to confirm whether the STM tip is stable or not.</li>
		<li>Repeat the poke in step 7.3.2 if dI/dV curve has changed. Proceed to scan graphene if dI/dV curve is unchanged.</li>
	</ol>
	</li>
</ol>
8. Scanning Graphene
<ol>
	<li>Transfer the graphene device to a STM scanning stage.</li>
	<li>Use a long-distance optical microscope to see the STM tip and graphene device. After laterally aligning the tip and h-BN flake of interest, approach the graphene.</li>
	<li>Start scanning a 2 nm by 2 nm area. Slowly enlarge the scan window to 5 nm by 5 nm, 10 nm, by 10 nm, 15 nm by 15 nm, 20 nm by 20 nm, etc. If a large impurity (&#62;100 pm in height) is encountered, withdraw the tip and move to a different area.</li>
	<li>Take a dI/dV spectrum and compare to the standard dI/dV spectrum on graphene/h-BN substrate (see Ref. 21). If the spectrum is not comparable, recalibrate the tip on an Au(111) surface (see Sec. 7 of the Protocol).</li>
	<li>Scan multiple areas to get a sense of how frequently large impurity (&#62;100 pm in height) is encountered. Based on these statistics, deduce the cleanliness of the sample.</li>
</ol>
9. Depositing Coulomb Impurities on a Graphene Surface2-4 
<ol>
	<li>Obtain a Ca source. Calibrate the evaporation of Ca atoms with a residual gas analyzer (RGA) and quartz crystal microbalance (QCM) in an UHV test chamber. Note: RGA evaluates the purity of Ca deposit while QCM measures a Ca deposition rate.
	<ol>
		<li>Run current through the Ca source.</li>
		<li>Increase the current until a Ca partial pressure of 10-10 Torr is detected in the RGA mass spectrum. Be aware of the fact that Ca and Ar have the same mass and therefore are indistinguishable in the RGA.</li>
		<li>Measure the Ca deposition rate (layer/sec) with the QCM.
		<ol>
			<li>Input the density (e.g., 1.55 g/cm3 for Ca ion) of the deposit to convert frequency shift value to deposition rate.</li>
			<li>The deposition rate monitor for the QCM reads the deposition rate in /s; convert this to layer/sec by assuming that the thickness of a monolayer is equal to the ionic diameter (e.g.,&#160;0.228&#160;&#197; for Ca ion) of the deposit.&#160;</li>
		</ol>
		</li>
		<li>Determine the optimal current by adjusting the current until the deposition rate monitor indicates a desired deposition rate (e.g., 3.33 x 10-5 layer/sec).&#160;</li>
	</ol>
	</li>
	<li>Given the deposition rate (e.g., 3.33 x 10-5 layer/sec) in step 9.1.3, calculate the time (sec) for depositing the desired amount (e.g., 0.01 monolayer) of Ca. In the STM, deposit Ca on a Cu(100) surface in situ with the optimal current setting from step 9.1.3. Check the Ca coverage and cleanliness of the post-deposition Cu(100) surface with STM (see step 8.5); recalibrate the current setting until the Cu(100) surface condition under STM appears as expected. 
	Note: Deposition parameters are optimized on Cu(100) first to minimize the risk of contaminating the graphene device with poorly controlled deposition.</li>
	<li>Transfer the graphene device to a STM for in situ deposition at 4 K.</li>
	<li>Deposit charged Ca atoms onto the graphene surface.
	<ol>
		<li>Before depositing Ca on graphene, outgass the Ca source. Slowly increase the current on the source by 0.25 A every 5-10 min until the desired current in step 9.2 is reached. Close a shutter between the graphene and Ca source to prevent outgassed contaminants from reaching the graphene.</li>
		<li>Let the evaporation flux stabilize for 20 min before opening the shutter.</li>
		<li>Open the shutter and deposit desirable (e.g., 0.01 monolayer) amount of Ca ions onto the graphene surface. Ensure that the graphene device has line-of-sight with the Ca source. Ensure that the STM tip is out of line-of-sight of the Ca source to prevent Ca atoms from sticking to the STM tip.</li>
	</ol>
	</li>
	<li>Check the Ca coverage and cleanliness of the post-deposition graphene surface with STM (see Step 8.5). Refer to Ref. 2, 3, and 4 for further protocol of the study of Coulomb impurities on graphene.</li>
</ol>

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Mater. 10 (4), 282-285 (2011).</li><li>Decker, 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=Local+electronic+properties+of+graphene+on+a+BN+substrate+via+scanning+tunneling+microscopy.">Local electronic properties of graphene on a BN substrate via scanning tunneling microscopy.</a> Nano Lett. 11 (6), 2291-2295 (2011).</li><li>Yan, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Toward+the+synthesis+of+wafer-scale+single-crystal+graphene+on+copper+foils.">Toward the synthesis of wafer-scale single-crystal graphene on copper foils.</a> ACS Nano. 6 (10), 9110-9117 (2012).</li><li>Zhang, B., 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=Low-temperature+chemical+vapor+deposition+growth+of+graphene+from+toluene+on+electropolished+copper+foils.">Low-temperature chemical vapor deposition growth of graphene from toluene on electropolished copper foils.</a> ACS Nano. 6 (3), 2471-2476 (2012).</li><li>Li, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large-area+synthesis+of+high-quality+and+uniform+graphene+films+on+copper+foils.">Large-area synthesis of high-quality and uniform graphene films on copper foils.</a> Science. 324 (5932), 1312-1314 (2009).</li><li>Zhang, Y., Zhang, L., Zhou, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Review+of+chemical+vapor+deposition+of+graphene+and+related+applications.">Review of chemical vapor deposition of graphene and related applications.</a> Acc. Chem. Res. 46 (10), 2329-2339 (2013).</li><li>Reina, 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=Transferring+and+identification+of+single-and+few-layer+graphene+on+arbitrary+substrates.">Transferring and identification of single-and few-layer graphene on arbitrary substrates.</a> J. Phys. Chem. C. 112 (46), 17741-17744 (2008).</li><li>Suk, J. 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=Transfer+of+CVD-grown+monolayer+graphene+onto+arbitrary+subsrates.">Transfer of CVD-grown monolayer graphene onto arbitrary subsrates.</a> ACS Nano. 5 (9), 6916-6924 (2011).</li><li>Regan, 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=A+direct+transfer+of+layer-area+graphene.">A direct transfer of layer-area graphene.</a> Appl. Phys. Lett. 96 (11), 113102 (2010).</li><li>Ishigami, M., Chen, J. H., Cullen, W. G., Fuhrer, M. S., Williams, E. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+structure+of+graphene+on+SiO2.">Atomic structure of graphene on SiO2.</a> Nano Lett. 7 (6), 1643-1648 (2007).</li><li>Dan, Y., Lu, Y., Kybert, N. J., Luo, Z., Johnson, A. T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intrinsic+response+of+graphene+vapor+sensors.">Intrinsic response of graphene vapor sensors.</a> Nano Lett. 9 (4), 1472-1475 (2009).</li><li>Chen, W., Madhavan, V., Jamneala, T., Crommie, M. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Scanning+tunneling+microscopy+observation+of+an+electronic+superlattice+at+the+surface+of+clean+gold.">Scanning tunneling microscopy observation of an electronic superlattice at the surface of clean gold.</a> Phys. Rev. Lett. 80 (7), 1469-1472 (1998).</li><li>Saito, R., Hofmann, M., Dresselhaus, G., Jorio, A., Dresselhaus, 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=Raman+spectroscopy+of+graphene+and+carbon+nanotubes.">Raman spectroscopy of graphene and carbon nanotubes.</a> Adv. Phys. 60 (3), 413-550 (2011).</li><li>Horcas, I., Fernandez, R., Gomez-Rodriguez, J. M., Colchero, J., Gomez-Herrero, J., Baro, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=WSXM:+A+software+for+scanning+probe+microscopy+and+a+tool+for+nanotechnology.">WSXM: A software for scanning probe microscopy and a tool for nanotechnology.</a> Rev. Sci. Instrum. 78 (1), 013705 (2007).</li></ol>

Authors have nothing to disclose.

For STM characterization, critical goals of the graphene device fabrication include: 1) growing monolayer graphene with a minimal number of defects, 2) obtaining a large, clean, uniform, and continuous graphene surface, 3) assembling a graphene device with high resistance between the graphene and the gate (i.e., no &#8220;gate leakage&#8221;), and 4) depositing individual Coulomb impurities.
The first goal is governed by the CVD process, during which graphene grows on a Cu foil. Altho...

Graphene is a two-dimensional material with a unique linear band structure, which gives rise to its exceptional electrical, optical, and mechanical properties.1,9-16 Its low-energy charge carriers are described as relativistic, massless Dirac fermions15, whose behavior differs significantly from that of non-relativistic charge carriers in traditional systems.15-18 Controlled deposition of a variety of impurities onto graphene provides a simple yet versatile platform for experimental studies of the response of these relativistic charge carriers to a range of perturbations. Investigations of such systems reveal that graphene impurities can shift the chemical potential6,7, alter the effective dielectric constant8, and potentially lead to electronically mediated superconductivity9. Many of these studies6-8 employ electrostatic gating as a means to tuning the properties of the hybrid impurity-graphene device. Electrostatic gating can shift the electronic structure of a material with respect to its Fermi level without hysteresis.2-5 Moreover, by tuning the charge2 or molecular5 states of such impurities, electrostatic gating can reversibly modify the properties of a hybrid impurity-graphene device.
Back-gating a graphene device provides an ideal system for investigation by scanning tunneling microscopy (STM). A scanning tunneling microscope consists of a sharp metal tip held a few angstroms away from a conductive surface. By applying a bias between the tip and the surface, electrons tunnel between the two. In the most common mode, constant current mode, one can map the topography of the sample surface by raster-scanning the tip back and forth. Additionally, the local electronic structure of the sample can be studied by examining a differential conductance dI/dV spectrum, which is proportional to local density of states (LDOS). This measurement is often termed scanning tunneling spectroscopy (STS). By separately controlling the bias and back-gate voltages, the response of graphene to impurities can be studied by analyzing the behavior of these dI/dV spectra.2-5
In this report, the fabrication of a back-gated graphene device decorated with Coulomb impurities (e.g., charged Ca atoms) is outlined. The device consists of elements in the following order (from top to bottom): calcium adatoms and clusters, graphene, hexagonal boron nitride (h-BN), silicon dioxide (SiO2), and bulk silicon (Figure 1). h-BN is an insulating thin film, which provides an atomically flat and electrically homogeneous substrate for the graphene.19-21 h-BN and SiO2 act as dielectrics, and bulk Si serves as the back-gate.
To fabricate the device, graphene is first grown on an electrochemically polished Cu foil22,23, which acts as a clean catalytic surface for the chemical vapor deposition (CVD)22-25 of graphene. In a CVD growth, methane (CH4) and hydrogen (H2) precursor gases undergo pyrolysis to form domains of graphene crystals on the Cu foil. These domains grow and eventually merge together, forming a polycrystalline graphene sheet.25 The resulting graphene is transferred onto the target substrate, an h-BN/SiO2 chip (prepared by mechanical exfoliation19-21 of h-BN onto an SiO2/Si(100) chip), via poly(methyl methacrylate) (PMMA) transfer.26-28 In the PMMA transfer, the graphene on Cu is first spin-coated with a layer of PMMA. The PMMA/graphene/Cu sample then floats on an etchant solution (e.g., FeCl3 (aq)28), which etches away the Cu. The unreacted PMMA/graphene sample is fished with an h-BN/SiO2 chip and subsequently cleaned in an organic solvent (e.g., CH2Cl2) and Ar/H2 environment29,30 to remove the PMMA layer. The resulting graphene/h-BN/SiO2/Si sample is then wire-bonded to electrical contacts on an ultra-high-vacuum (UHV) sample plate and annealed in an UHV chamber. Finally, the graphene device is deposited in situ with Coulomb impurities (e.g., charged Ca atoms) and studied by STM.2-5

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Cu foil</td>
			<td>Alfa Aesar</td>
			<td>CAS # 7440-50-8</td>
			<td>99.8% Cu</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td>Lot # F22X029</td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td>Stock # 13382</td>
			<td></td>
		</tr>
		<tr>
			<td>Scotch Magic Tape</td>
			<td>Scotch&#174;</td>
			<td>N/A</td>
			<td>for exfoliation of hBN</td>
		</tr>
		<tr>
			<td>PMMA</td>
			<td>Micro Chem</td>
			<td>M23004 0500L 1GL</td>
			<td>A4</td>
		</tr>
		<tr>
			<td>FeCl3 resistant spoon</td>
			<td>Bel-Art ScienceWare</td>
			<td>367300015</td>
			<td>PTFE coated double ended&#160;</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>chemical spoon, 15 cm length</td>
		</tr>
		<tr>
			<td>FeCl3 (aq)</td>
			<td>Ricca Chemical</td>
			<td>3127-16</td>
			<td>40% w/v</td>
		</tr>
		<tr>
			<td>SiO2/Si(100) Chip</td>
			<td>NOVA Electric Materials</td>
			<td>HS39626-OX</td>
			<td>n/a</td>
		</tr>
		<tr>
			<td>h-BN</td>
			<td>K. Watanabe and</td>
			<td>Contact the group</td>
			<td>hexagonal Japanese BN (JBN)</td>
		</tr>
		<tr>
			<td></td>
			<td>T. Taniguchi Group</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Au(111)</td>
			<td>Agilent Technologies</td>
			<td>N9805B-FG</td>
			<td>Au(111) epitaxially grown on mica</td>
		</tr>
		<tr>
			<td>Sapphire</td>
			<td>Precision Ferrites &#38; Ceramic, Inc.</td>
			<td>Contact vendor</td>
			<td>P/N Sapphire Chips</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>0.22 X 0.125 X 0.015&#34;</td>
		</tr>
		<tr>
			<td>Ca source</td>
			<td>Trace Sciences International Corp.</td>
			<td>AS-3-Ca-5-S</td>
			<td>n/a</td>
		</tr>
		<tr>
			<td>Cu(100)</td>
			<td>Princeton Scientific</td>
			<td>Contact vendor</td>
			<td>Cu(100) single crystal</td>
		</tr>
		<tr>
			<td>Methane</td>
			<td>Praxair, Inc.</td>
			<td>ME 5.0RS-K</td>
			<td>Graphene growth precursor gas</td>
		</tr>
		<tr>
			<td>Hydrogen</td>
			<td>Praxair, Inc.</td>
			<td>HY 6.0RS-K</td>
			<td>Graphene growth precursor gas</td>
		</tr>
	</tbody>
</table>

fabrication of gate-tunable graphene devices for scanning tunneling microscopy studies with coulomb impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

Figure 1 illustrates a schematic of a back-gated graphene device. Wire-bonding Au/Ti contact to an UHV sample plate grounds graphene electrically, while wire-bonding Si bulk to an electrode that connects to an external circuit back-gates the device. By back-gating a device, a charge state of a Coulomb impurity at a given sample bias (which is controlled by the STM tip) can be tuned to a different charge state.2-4
Figure 2 outlines the steps for fabr...

Watch this Scientific Journal Video about Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities at JoVE.com

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

fabrication-gate-tunable-graphene-devices-for-scanning-tunneling

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15.3K Views.  University of California at Berkeley. This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Video: Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...

Scanning Light Scattering Profiler (SLPS) Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light scattering from intraocular lenses (IOLs) using goniophotometer principles. This protocol describes the SLSP platform and how it employs a 360&#176; rotational photodetector sensor that is scanned around an IOL sample while recording the intensity and location of scattered light as it passes through the IOL medium. The SLSP platform can be used to predict, non-clinically, the propensity for current and novel IOL designs and materials to induce light scatter. Non-clinical evaluation of light scattering properties of IOLs can significantly reduce the number of patient complaints related to unwanted glare, glistening, optical defects, poor image quality, and other phenomena associated with the unintended light scattering. Future studies should be conducted to correlate SLSP data with clinical results to help identify which measured light scatter is most problematic for patients that have undergone cataract surgery subsequent to IOL implantation.

Scanning Light Scattering Profiler SLPS Based Methodology to Quantitatively Evaluate Forward and Backward Light Scattering from Intraocular Lenses

This protocol describes the scanning light scattering profiler (SLSP) that enables the full-angle quantitative evaluation of forward and backward scattering of light from intraocular lenses (IOLs) using goniophotometer principles.

The scanning light scattering profiler (SLSP) methodology has been developed for the full-angle quantitative evaluation of forward and backward light ...

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution, which motivates the use of scanning tunneling microscopy (STM). However, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming, including all-electronic time-resolved STM, which is used in this study to examine dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

We demonstrate an all-electronic method to observe nanosecond-resolved charge dynamics of dopant atoms in silicon with a scanning tunneling microscope.

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new ...

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

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

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

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

Fabrication of Magnetic Nanostructures on Silicon Nitride Membranes for Magnetic Vortex Studies Using Transmission Microscopy Techniques

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared on transparent membranes which are very fragile and difficult to manipulate. We present processes for the fabrication of samples with magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for Lorentz transmission electron microscopy and magnetic transmission x-ray microscopy studies. The samples are prepared on silicon nitride membranes and the fabrication consists of a spin coating, UV and electron-beam lithography, the chemical development of the resist, and the evaporation of the magnetic material followed by a lift-off process forming the final magnetic structures. The samples for the Lorentz transmission electron microscopy consist of magnetic nanodiscs prepared in a single lithography step. The samples for the magnetic x-ray transmission microscopy are used for time-resolved magnetization dynamic experiments, and magnetic nanodiscs are placed on a waveguide which is used for the generation of repeatable magnetic field pulses by passing an electric current through the waveguide. The waveguide is created in an extra lithography step.

A protocol for the fabrication of magnetic micro- and nanostructures with spin configurations forming magnetic vortices suitable for transmission electron microscopy (TEM) and magnetic transmission x-ray microscopy (MTXM) studies is presented.

Electron and x-ray magnetic microscopies allow for high-resolution magnetic imaging down to tens of nanometers. However, the samples need to be prepared ...

Fabrication of Refractive-index-matched Devices for Biomedical Microfluidics

The use of microfluidic devices has emerged as a defining tool for biomedical applications. When combined with modern microscopy techniques, these devices can be implemented as part of a robust platform capable of making simultaneous complementary measurements. The primary challenge created by the combination of these two techniques is the mismatch in refractive index between the materials traditionally used to make microfluidic devices and the aqueous solutions typically used in biomedicine. This mismatch can create optical artifacts near the channel or device edges. One solution is to reduce the refractive index of the material used to fabricate the device by using a fluorinated polymer such as MY133-V2000 whose refractive index is similar to that of water (n = 1.33). Here, the construction of a microfluidic device made out of MY133-V2000 using soft lithography techniques is demonstrated, using O2 plasma in conjunction with an acrylic holder to increase the adhesion between the MY133-V2000 fabricated device and the polydimethylsiloxane (PDMS) substrate. The device is then tested by incubating it filled with cell culture media for 24 h to demonstrate the ability of the device to maintain cell culture conditions during the course of a typical imaging experiment. Finally, quantitative phase microscopy (QPM) is used to measure the distribution of mass within the live adherent cells in the microchannel. This way, the increased precision, enabled by fabricating the device from a low index of refraction polymer such as MY133-V2000 in lieu of traditional soft lithography materials such as PDMS, is demonstrated. Overall, this approach for fabricating microfluidic devices can be readily integrated into existing soft lithography workflows in order to reduce optical artifacts and increase measurement precision.

This protocol describes the fabrication of microfluidic devices from MY133-V2000 to eliminate artifacts that often arise in microchannels due to the mismatching refractive indices between microchannel structures and an aqueous solution. This protocol uses an acrylic holder to compress the encapsulated device, improving adhesion both chemically and mechanically.

The use of microfluidic devices has emerged as a defining tool for biomedical applications. When combined with modern microscopy techniques, these devices ...

Fabrication of Three-Dimensional Graphene-Based Polyhedrons via Origami-Like Self-Folding

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent properties has been of great interest for the development of novel device applications. Here, fabrication of 3D, microscale, hollow polyhedrons (cubes) consisting of a few layers of 2D graphene or graphene oxide sheets via an origami-like self-folding process is described. This method involves the use of polymer frames and hinges, and aluminum oxide/chromium protection layers that reduce tensile, spatial, and surface tension stresses on the graphene-based membranes when the 2D nets are transformed into 3D cubes. The process offers control of the size and shape of the structures as well as parallel production. In addition, this approach allows the creation of surface modifications by metal patterning on each face of the 3D cubes. Raman spectroscopy studies show the method allows the preservation of the intrinsic properties of the graphene-based membranes, demonstrating the robustness of our method.

Fabrication of Three-Dimensional Graphene-Based Polyhedrons via Origami-Like Self-Folding

Here, we present a protocol for fabrication of 3D graphene-based polyhedrons via origami-like self-folding.

The assembly of two-dimensional (2D) graphene into three-dimensional (3D) polyhedral structures while preserving the graphene's excellent inherent ...

Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy (STM) with spin-polarization techniques, we visualize antiferromagnetic (AFM) domains and their atomic structure in Fe1+yTe samples, the parent compound of iron-based superconductors, and demonstrate how these domains respond to applied uniaxial strain. We observe the bidirectional AFM domains in the unstrained sample, with an average domain size of ~50-150 nm, to transition into a single unidirectional domain under applied uniaxial strain. The findings presented here open a new direction to utilize a valuable tuning parameter in STM, as well as other spectroscopic techniques, both for tuning the electronic properties as for inducing symmetry breaking in quantum material systems.

Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope

Using uniaxial strain combined with spin-polarized scanning tunneling microscopy, we visualize and manipulate the antiferromagnetic domain structure of Fe1+yTe, the parent compound of iron-based superconductors.

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental ...

Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics

Controlled nanoscale manipulation of fluids is known to be exceptionally difficult due to the dominance of surface and viscous forces. Megahertz-order surface acoustic wave (SAW) devices generate tremendous acceleration on their surface, up to 108 m/s2, in turn responsible for many of the observed effects that have come to define acoustofluidics: acoustic streaming and acoustic radiation forces. These effects have been used for particle, cell, and fluid manipulation at the microscale, although more recently SAW has been used to produce similar phenomena at the nanoscale through an entirely different set of mechanisms. Controllable nanoscale fluid manipulation offers a broad range of opportunities in ultrafast fluid pumping and biomacromolecule dynamics useful for physical and biological applications. Here, we demonstrate nanoscale-height channel fabrication via room-temperature lithium niobate (LN) bonding integrated with a SAW device. We describe the entire experimental process including nano-height channel fabrication via dry etching, plasma-activated bonding on lithium niobate, the appropriate optical setup for subsequent imaging, and SAW actuation. We show representative results for fluid capillary filling and fluid draining in a nanoscale channel induced by SAW. This procedure offers a practical protocol for nanoscale channel fabrication and integration with SAW devices useful to build upon for future nanofluidics applications.

We demonstrate fabrication of nanoheight channels with the integration of surface acoustic wave actuation devices upon lithium niobate for acoustic nanofluidics via liftoff photolithography, nano-depth reactive ion etching, and room-temperature plasma surface-activated multilayer bonding of single-crystal lithium niobate, a process similarly useful for bonding lithium niobate to oxides.

Controlled nanoscale manipulation of fluids is known to be exceptionally difficult due to the dominance of surface and viscous forces. Megahertz-order ...