Name: fabrication of gate-tunable graphene devices for scanning tunneling microscopy studies with coulomb impurities
Uploaded: 2015-07-24T15:00:00
Description: Watch this Scientific Journal Video about Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities at JoVE.com

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>
	<...

<ol>
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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 ...

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