Work Function Variations in Twisted Graphene Layers

Abstract

By combining optical imaging, Raman spectroscopy, kelvin probe force microscopy (KFPM), and photoemission electron microscopy (PEEM), we show that graphene’s layer orientation, as well as layer thickness, measurably changes the surface potential (Φ). Detailed mapping of variable-thickness, rotationally-faulted graphene films allows us to correlate Φ with specific morphological features. Using KPFM and PEEM we measure ΔΦ up to 39 mV for layers with different twist angles, while ΔΦ ranges from 36–129 mV for different layer thicknesses. The surface potential between different twist angles or layer thicknesses is measured at the KPFM instrument resolution of ≤ 200 nm. The PEEM measured work function of 4.4 eV for graphene is consistent with doping levels on the order of 1012cm−2. We find that Φ scales linearly with Raman G-peak wavenumber shift (slope = 22.2 mV/cm−1) for all layers and twist angles, which is consistent with doping-dependent changes to graphene’s Fermi energy in the ‘high’ doping limit. Our results here emphasize that layer orientation is equally important as layer thickness when designing multilayer two-dimensional systems where surface potential is considered.

Manipulation of Charge Transfer in FeP@Fe2O3 Core–Shell Photoanode by Directed Built-In Electric Field

“…As discussed above, to further understand the mechanism of simultaneously enhanced charge separation and oxidation kinetics, kelvin probe force microscopy (KPFM) was used to study the surface potentials ( V surface ) of Fe 2 O 3 and FeP@Fe 2 O 3 . , Panels a and b of Figure show the KPFM and topology images of the as-prepared samples, indicating the similar structure of the two types of photoanodes on the FTO substrate. Interestingly, the surface potential images of various samples reveal the V surface of Fe 2 O 3 and FeP@Fe 2 O 3 -20 are 125 and 78 mV (Figure c), respectively.…”

Noninvasive Subsurface Electrical Probe for Encapsulated Layers in van der Waals Heterostructures

“…In this article, we present electrostatic force microscopy (EFM) as an effective tool to spatially map and study the local surface potential variations in encapsulated vdW heterostructures. In the past, surface potential profiles in isolated 2D layered materials have been arXiv:1906.02413v1 [cond-mat.mes-hall] 6 Jun 2019 investigated using EFM [15][16][17][18][19][20] and Kelvin probe force microscopy (KPFM) [21][22][23][24][25][26]. These studies were found to be particularly useful in studying doping [18], substrate [24] and thickness dependences [15,16] and contact resistances [17,[21][22][23]25] of graphene flakes supported on SiO 2 /Si [15,18,21,22,25], copper [26] or SiC [16,17,23] substrates.…”

“…In the past, surface potential profiles in isolated 2D layered materials have been arXiv:1906.02413v1 [cond-mat.mes-hall] 6 Jun 2019 investigated using EFM [15][16][17][18][19][20] and Kelvin probe force microscopy (KPFM) [21][22][23][24][25][26]. These studies were found to be particularly useful in studying doping [18], substrate [24] and thickness dependences [15,16] and contact resistances [17,[21][22][23]25] of graphene flakes supported on SiO 2 /Si [15,18,21,22,25], copper [26] or SiC [16,17,23] substrates. Extension of this technique to vdW heterostructures involving multiple 2D layered materials has been limited [27,28].…”

Direct growth of mm-size twisted bilayer graphene by plasma-enhanced chemical vapor deposition

“…Kelvin probe force microscopy (KPFM) is a technique that enables nanoscale spatially resolved measurements of the surface potential difference between an atomic force microscopy (AFM) tip and the sample [80][81][82]. The contact potential differences (CPD) measured by the strength of the electrostatic forces between a conductive probe and the sample in the KPFM can reflect on the surface potential difference between two materials, which can be directly transformed into the work function [83].…”