Variations in the work function of doped single- and few-layer graphene assessed by Kelvin probe force microscopy and density functional theory

Abstract

We present Kelvin probe force microscopy measurements of single-and few-layer graphene resting on SiO 2 substrates. We compare the layer thickness dependency of the measured surface potential with ab initio density functional theory calculations of the work function for substrate-doped graphene. The ab initio calculations show that the work function of single-and bilayer graphene is mainly given by a variation of the Fermi energy with respect to the Dirac point energy as a function of doping, and that electrostatic interlayer screening only becomes relevant for thicker multilayer graphene. From the Raman G-line shift and the comparison of the Kelvin probe data with the ab initio calculations, we independently find an interlayer screening length in the order of four to five layers. Furthermore, we describe in-plane variations of the work function, which can be attributed to partial screening of charge impurities in the substrate, and result in a nonuniform charge density in single-layer graphene.

Open loop Kelvin probe force microscopy with single and multi-frequency excitation

“…These Raman signatures are consistent with the presence of a single graphene crystal with a smaller graphene crystal residing at its centre being twisted with respect to the bottom layer by roughly 10°. A higher SP of 128 ± 10 mV was observed on multilayer graphene than the surrounding single layer graphene, which can be understood in terms of a shift of the Fermi level with respect to the Dirac point as a result of substrate induced doping and interlayer screening [52,53]. While we are confident that changes observed in both Raman spectroscopy and CL-KPFM measurements are related to the changes in the electronic structure due to stacking of graphene layers, unambiguous determination of layer thickness is beyond the scope of this work.…”

Characterization and physical modeling of MOS capacitors in epitaxial graphene monolayers and bilayers on 6H-SiC

“…This suggests a ratio n acc /n t ot (0) of 469 (36) in monolayer(bilayer) material suggesting that charge control should be much more effective in the monolayers. However, in the presence of surface potential fluctuations, the RMS carrier densities become 0.51(2.86)·10 12 cm −2 such that a reduced ratio of 15(3.6) is expected at 77K.…”

“…As graphene is sensitive to atmospheric and polymer contaminants, it is necessary to perform the KPFM measurements in a controlled atmosphere. [34][35][36] Prior to scanning, the chamber was evacuated and then subsequently filled with N 2 at room temperature. Finally, the atmosphere was saturated to a relative humidity of 30% to approximate ambient conditions.…”

“…The mean work function of φ g = 4.8 eV for graphene estimated from the KPFM measurements is in relative agreement with literature values. 36,45,46 The estimation of the φ g from KPFM is calibrated relative to the work function of the Au contact metalization.…”

Band excitation Kelvin probe force microscopy utilizing photothermal excitation

“…Since its inception in 1991, 1 Kelvin probe force microscopy (KPFM) has had extraordinary success studying local electrochemical, 2,3 electronic, 4,5 and transport properties [6][7][8] of materials and devices with nanometer resolution. KPFM has been particularly useful for characterizing materials and devices ranging from metals, 1 semiconductors, 8,9 and ferroelectrics, 10,11 to self-assembled monolayers, 12 polymers, 13 and biomolecules.…”