Impurity scattering and size quantization effects in a single graphene nanoflake

Abstract

By using Fourier-transform scanning tunneling spectroscopy we measure the interference patterns produced by the impurity scattering of confined Dirac quasiparticles in epitaxial graphene nanoflakes. Upon comparison of the experimental results with tight-binding calculations of realistic model flakes, we show that the characteristic features observed in the Fourier-transformed local density of states are related to scattering between different transverse modes (subbands) of a graphene nanoflake and allow direct insight into the gapped electronic spectrum of graphene. We also observe a strong reduction of quasiparticle lifetime which is attributed to the interaction with the underlying substrate. In addition, we show that the distribution of the on-site energies at flower defects leads to an effectively broken pseudospin selection rule, where intravalley backscattering is allowed. [17,18]. With regard to the electronic transport through graphene nanoribbons, the effect of impurity scattering and edge disorder becomes an important issue. A powerful tool to examine the quasiparticle interference (QPI) effects in graphene due to scattering at defects and edges is scanning tunneling microscopy and spectroscopy [19][20][21][22]. The observed QPI is directly related to modulations in the local density of states (LDOS) [23,24] and provides access to the present scattering vectors and thus to the electronic structure of graphene [25][26][27][28]. Recently, the influence of local scattering centers on the local density of states in graphene nanoribbons has been studied theoretically [29]. The interplay between single impurity scattering and size quantization was shown to generate characteristic spectral features in the Fourier transform (FT) LDOS that can be related to the transverse modes of the nanoribbon.Here we present a comprehensive study of size quantization in epitaxial graphene nanoflakes (GNFs) on Ag(111) upon analysis of QPI by STM and tight-binding simulations of realistic model flakes. We indeed find the characteristic features in the FT-LDOS related to scattering between different transverse modes of a GNF as predicted by theory. Detailed analysis of the scattering features allows one to gain a profound insight into the behavior of charge carriers in graphene flakes, including discrete electronic spectrum and quasiparticle lifetimes, as well as effects of pseudospin.Graphene nanoflakes were initially grown on Ir(111) and decoupled by noble metal intercalation as described elsewhere [13,30]. STM and STS measurements were carried out in an Omicron cryogenic STM setup in ultrahigh vacuum at T = 5−10 K. Differential conductance (dI/dV ) maps were * mikhail.fonin@uni-konstanz.de obtained using a standard lock-in technique with modulation voltages V mod = 3 mV(rms) and at frequencies f mod = 600−800 Hz. Tight-binding calculations were performed using an atomistic recursive Green's function formalism including the effects of trigonal warping in order to account for the relatively large doping level of graphene on A...

Formation of Micro- and Nano-Trenches on Epitaxial Graphene

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Free-standing and supported phosphorene nanoflakes: Shape- and size-dependent properties

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Atomic Valley Filter Effect Induced by an Individual Flower Defect in Graphene

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