Shengjun Yuan

Fluorographene: A Two-Dimensional Counterpart of Teflon

A stoichiometric derivative of graphene with a fluorine atom attached to each carbon is reported. Raman, optical, structural, micromechanical, and transport studies show that the material is qualitatively different from the known graphene-based nonstoichiometric derivatives. Fluorographene is a high-quality insulator (resistivity >10(12) Ω) with an optical gap of 3 eV. It inherits the mechanical strength of graphene, exhibiting a Youngs modulus of 100 N m(-1) and sustaining strains of 15%. Fluorographene is inert and stable up to 400 °C even in air, similar to Teflon.

Resonant Scattering by Realistic Impurities in Graphene

We develop a first-principles theory of resonant impurities in graphene and show that a broad range of typical realistic impurities leads to the characteristic sublinear dependence of the conductivity on the carrier concentration. By means of density functional calculations various organic groups as well as adatoms such as H absorbed to graphene are shown to create midgap states within ±0.03  eV around the neutrality point. A low energy tight-binding description is mapped out. Boltzmann transport theory as well as a numerically exact Kubo formula approach yield the conductivity of graphene contaminated with these realistic impurities in accordance with recent experiments.

Production of Highly Monolayer Enriched Dispersions of Liquid-Exfoliated Nanosheets by Liquid Cascade Centrifugation

While liquid exfoliation is a powerful technique to produce defect-free nanosheets in large quantities, its usefulness is limited by broad nanosheet thickness distributions and low monolayer contents. Here we demonstrate liquid processing techniques, based on iterative centrifugation cascades, which can be designed to achieve either highly efficient nanosheet size-selection and/or monolayer enrichment. The resultant size-selected dispersions were used to establish quantitative metrics to determine monolayer volume fraction, as well as mean nanosheet size and thickness, from standard spectroscopic measurements. Such metrics allowed us to design and optimize centrifugation cascades to enrich liquid exfoliated WS2 dispersions up to monolayer contents of 75%. Monolayer-rich dispersions show relatively bright photoluminescence with narrow line widths (<35 meV) indicating the high quality of the nanosheets. The enriched dispersions display extinction spectra with distinct features, which also allow the direct estimation of monolayer contents.

Modeling electronic structure and transport properties of graphene with resonant scattering centers

We present a detailed numerical study of the electronic properties of single-layer graphene with resonant (hydrogen) impurities and vacancies within a framework of noninteracting tight-binding model on a honeycomb lattice. The algorithms are based on the numerical solution of the time-dependent Schrodinger equation and applied to calculate the density of states, quasieigenstates, ac and dc conductivities of large samples containing millions of atoms. Our results give a consistent picture of evolution of electronic structure and transport properties of functionalized graphene in a broad range of concentration of impurities (from graphene to graphane), and show that the formation of impurity band is the main factor determining electrical and optical properties at intermediate impurity concentrations, together with a gap opening when approaching the graphane limit.

Landau Level Spectrum of ABA- and ABC-stacked Trilayer Graphene

We study the Landau level spectrum of ABA- and ABC-stacked trilayer graphene. We derive analytic low energy expressions for the spectrum, the validity of which is confirmed by comparison to a �-band tight-binding calculation of the density of states on the honeycomb lattice. We further study the effect of a perpendicular electric field on the spectrum, where a zero-energy plateau appears for ABC stacking order, due to the opening of a gap at the Dirac point, while the ABAstacked trilayer graphene remains metallic. We discuss our results in the context of recent electronic transport experiments. Furthermore, we argue that the expressions obtained can be useful in the analysis of future measurements of cyclotron resonance of electrons and holes in trilayer graphene.

Optical transmittance of multilayer graphene

We study the optical transmittance of multilayer graphene films up to 65 layers thick. By combing large-scale tight-binding simulation and optical measurement on CVD multilayer graphene, the optical transmission through graphene films in the visible region is found to be solely determined by the number of graphene layers. We argue that the optical transmittance measurement is more reliable in the determination of the number of layers than the commonly used Raman Spectroscopy. Moreover, optical transmittance measurement can be applied also to other 2D materials with weak van der Waals interlayer interaction.

Toward a realistic description of multilayer black phosphorus: From GW approximation to large-scale tight-binding simulations

Weprovideatight-bindingmodelparametrizationforblackphosphorus(BP)withanarbitrarynumberoflayers. The model is derived from partially self-consistent GW0 approach, where the screened Coulomb interaction W0 is calculated within the random phase approximation on the basis of density functional theory. We thoroughly validate the model by performing a series of benchmark calculations, and determine the limits of its applicability. TheapplicationofthemodeltothecalculationsofelectronicandopticalpropertiesofmultilayerBPdemonstrates good quantitative agreement with ab initio results in a wide energy range. We also show that the proposed model can be easily extended for the case of external fields, yielding the results consistent with those obtained from first principles. The model is expected to be suitable for a variety of realistic problems related to the electronic properties of multilayer BP including different kinds of disorder, external fields, and many-body effects.

Spectroscopic metrics allow in situ measurement of mean size and thickness of liquid-exfoliated few-layer graphene nanosheets

Liquid phase exfoliation is a powerful and scalable technique to produce defect-free mono- and few-layer graphene. However, samples are typically polydisperse and control over size and thickness is challenging. Notably, high throughput techniques to measure size and thickness are lacking. In this work, we have measured the extinction, absorption, scattering and Raman spectra for liquid phase exfoliated graphene nanosheets of various lateral sizes (90 ≤ 〈L〉 ≤ 810 nm) and thicknesses (2.7 ≤ 〈N〉 ≤ 10.4). We found all spectra to show well-defined dependences on nanosheet dimensions. Measurements of extinction and absorption spectra of nanosheet dispersions showed both peak position and spectral shape to vary with nanosheet thickness in a manner consistent with theoretical calculations. This allows the development of empirical metrics to extract the mean thickness of liquid dispersed nanosheets from an extinction (or absorption) spectrum. While the scattering spectra depended on nanosheet length, poor signal to noise ratios made the resultant length metric unreliable. By analyzing Raman spectra measured on graphene nanosheet networks, we found both the D/G intensity ratio and the width of the G-band to scale with mean nanosheet length allowing us to establish quantitative relationships. In addition, we elucidate the variation of 2D/G band intensities and 2D-band shape with the mean nanosheet thickness, allowing us to establish quantitative metrics for mean nanosheet thickness from Raman spectra.

Transport and optical properties of single- and bilayer black phosphorus with defects

We study the electronic and optical properties of single- and bilayer black phosphorus with shortand long-range defects by using the tight-binding propagation method. Both types of defect states are localized and induce a strong scattering of conduction states reducing significantly the charge carrier mobility. In contrast to properties of pristine samples, the anisotropy of defect-induced optical excitations is suppressed due to the isotropic nature of the defects. We also investigate the Landau level spectrum and magneto-optical conductivity, and find that the discrete Landau levels are sublinearly dependent on the magnetic field and energy level index, even at low defect concentrations.

Effect of point defects on the optical and transport properties of mos2 and ws2

Introduction. The mobility of current single-layer crystals of transition metal dichalcogenides (TMD) is highly dependent on the screening environment and is limited by the presence of defects in the samples. The existence of defects in the chemical and structural composition of those materials can influence their optical and transport properties, as revealed by recent experimental results. A broad peak at ∼700 nm (∼1.77 eV) in the optical spectrum of bilayer MoS2 has been associated to impurities [1], whereas the mobility of multilayer samples has been shown to highly depend on the substrate and dielectric effects [2]. Vacancy defects in the crystal, which can be created by means of thermal annealing and α-particle [3] or electron beam irradiation [4,5], trap free charge carriers and localize excitons, leading to new peaks in the photoluminescence spectra [3]. Recent experiments [5] show that the density of sulfur vacancies in MoS2 is of the order of 10 13 cm −2 , corresponding to an average defect distance of about 1.7 nm. The existence of line defects, which separate patches or islands where the layer direction is opposite to its surrounding, can lead to changes in the carrier mobility [6], and the importance of short-range disorder has been proposed as the main limitation for the mobility of chemical vapor deposition grown single-layer MoS2 [7,8]. Therefore, it is necessary to understand the effect of impurities in the optical and transport properties of TMD, as a first step to exploit the controlled creation of defects as a route to manipulate their electronic properties. There are several theoretical works which have studied this problem usingabinitio methods [6,9–15]. However, the simulation of realistic disordered samples of TMD with a random distribution of defects is extremely expensive computationally for density functional theory methods, since they require a very large unit cell in the calculation. In this Rapid Communication we follow an alternative route and perform a systematic study of the density of states, optical and dc conductivities of single layers of MoS2 and WS2 in the presence of point defects, by means of a real space tight-binding (TB) model for large systems,