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.
Highlights
Vetting graphene as a candidate for advanced electronics requires examination of both intrinsic and extrinsic influences on its material properties, as well as a nuanced characterization of how graphene responds in different heterogeneous configurations
Graphene films were grown by chemical vapor deposition (CVD) on copper foil ‘enclosures’[22] at 1030 °C using mixtures of H2/CH4 and subsequently transferred via wet chemical etching and a PMMA support film to SiO2 (100 nm)/Si substrates
We note that each additional graphene layer grows beneath the continuous first layer[24], forming an inverse ‘step pyramid’ type structure, which is preserved during the transfer process
Summary
Vetting graphene as a candidate for advanced electronics requires examination of both intrinsic and extrinsic influences on its material properties, as well as a nuanced characterization of how graphene responds in different heterogeneous configurations. In the simplest case, stacking two graphene layers to form twisted bilayer graphene (TBG) already leads to measureable differences in interlayer screening[4], optical absorption[5,6], chiral charge carriers[7], and chemical reactivity[8]. Once a twist is introduced, a moiré pattern forms from the overlapping lattices This moiré structure can itself induce a periodic potential that influences the electronic structure[9,10], a feature unique to atomically-thin systems. Since the difference in work function between graphene and, for example, a KPFM tip defines the contact potential (or relative surface potential (ΔΦ)), ΔΦ is directly influenced by changes in EF (or n). We present the first results mapping the surface potential of graphene layers with varying twist angle and thickness. Variations can influence Φ by the same magnitude as that found for changes in layer thickness, which reinforces that θ must be carefully considered in designing 2D multilayer systems
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