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

Summary

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.

Abstract

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’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's electronic structure in the presence of a charged impurity. 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’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.

Introduction

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 c....

Protocol

1. Electrochemical Polishing of a Cu Foil^22,23

Note: Electrochemical polishing exposes bare Cu surface for graphene growth by removing the protective surface coating and controls the growth seed density.

1. 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.
2. Cut Cu foil into multiple 3 cm by 3 cm foils. Note: Each foil serves as either an anode or a cathode.
3. 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.
   N....

Results

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.......

Discussion

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 “gate leakage”), and 4) depositing individual Coulomb impurities.

The first goal is governed by the CVD process, during which graphene grows on a Cu foil. Altho.......

Materials

List of materials used in this article

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