A review and outlook for an anomaly of scanning tunnelling microscopy (STM): superlattices on graphite

Abstract

Since its invention in 1981, scanning tunnelling microscopy (STM) is well-known for its supreme imaging resolution enabling one to observe atomic-scale structures, which has led to the flourishing of nanoscience. As successful as it is, there still remain phenomena which are observed using STM but are beyond our understanding. Graphite is one of the surfaces which have been most extensively studied using STM. However, there are a number of unusual properties of graphite surfaces. First reported in the 1980s, superlattices on graphite have since been observed many times and by many groups, but as yet our understanding of this phenomenon is quite limited. Most of the observed superlattice phenomena are widely believed to be the result of a Moiré rotation pattern, arising from the misorientation between two graphite layers, as verified experimentally. A Moiré pattern is a lattice with larger periodicity resulting from the overlap of two lattices with smaller periodicities. As graphite layers are composed of hexagonal lattices with a periodicity of 0.246 nm, as observed using STM, when there are misoriented graphite layers overlapping each other, a Moiré pattern with larger periodicity, depending on the misorientation angle, will be produced and appear as a superperiodic hexagonal structure on top of the graphite atomic lattice of the topmost surface layer. It is important to study graphite superlattices because, firstly, knowledge of this phenomenon will enable us to properly interpret STM images; secondly, it helps us to understand the correlation between electronic structures and atomic-structure rearrangement of graphite which is of tremendous aid for engineering material properties; thirdly, and perhaps most importantly, the observation of the phenomenon exhibits the capability of STM to produce images indicating the nature of internal defects which are below the surface. Over recent years, experimental and modelling techniques have been developed to study this anomalous regime of STM; however, there is a lack of a systematic classification of this scattered information. This review article thus serves the purpose of organizing all these results so as to enable a more comprehensive understanding of this phenomenon. We review the discovery of graphite superlattices, the observation of the associated properties, and the research efforts on this subject. An effort is made to envision the future experimental and theoretical research possibilities to unveil the mystery of this anomaly of STM. Applications of graphite superlattices are also proposed.

Unraveling the Intrinsic and Robust Nature of van Hove Singularities in Twisted Bilayer Graphene by Scanning Tunneling Microscopy and Theoretical Analysis

“…For intermediate angles (1 15 ), it is predicted that, while the linear dispersion persists in the vicinity of the Dirac points of both layers, the band velocity is depressed and two saddle points appear in the band structure, giving rise to two logarithmic van Hove singularities (vHs) in the density of states (DOS) [9,[13][14][15][16][17][18]. For smaller angles ( 1 ) weakly dispersive bands appear at low energy [19,20] with sharp DOS peaks very close to the Dirac point [17,18].Twisted graphene layers are commonly found on different substrates, such as metals [13,21,22], the C face of SiC [23][24][25], or graphite surfaces [26,27]. Transfer techniques yielding large domains of twisted bilayers over a macroscopic sample [28] and quantitative, fast, Raman characterization tools [29,30] have recently been proposed.…”

“…Twisted graphene layers are commonly found on different substrates, such as metals [13,21,22], the C face of SiC [23][24][25], or graphite surfaces [26,27]. Transfer techniques yielding large domains of twisted bilayers over a macroscopic sample [28] and quantitative, fast, Raman characterization tools [29,30] have recently been proposed.…”

“…STM images [40] reveal coherent domains with lateral size around 100 nm. Within these domains, the rotations between the surface graphene layers give rise to superstructures with period P in the nanometer range, which are identified as moiré patterns (MP) [23,[25][26][27]. The STM or STS experiments have been made in a separate UHV system using a homemade low temperature (5 K) STM [41].…”

Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene

“…Bernal-stacked graphene shows quadratic energymomentum dispersion due to strong interlayer interaction and a bandgap opening due to symmetry breaking. Twisted graphene (TG) rotated by an arbitrary angle is commonly found in graphene grown on metals 3,9,10 , C-face SiC 11 or bulk graphite surfaces 12,13 , and is still under intensive study to identify its properties. Ab initio calculations showed that the Dirac cones and Fermi velocity of each layer preserved those of SLG due to decoupling between layers 14 .…”

X-ray micro-modulated luminescence tomography (XMLT)

“…However, imaging depth of optical microscopy has been fundamentally limited to millimeter or sub-millimeter due to multiple scattering of light in a biological sample. Conventional microscopy techniques utilize visible light or electron sources [12][13][14]. Optical microscopy divides into transmission (i.e., wide-field microscopies for snap-shot of 2D images in terms of light absorption, phase contrast, or dark-field signals) and emission modes (i.e., wide-field fluorescence microscopy, confocal laser scanning microscopy, and two-photon fluorescence microscopy).…”