While preserving many of the unusual features of single-layer graphene, few-layer graphene (FLG) provides a richness and flexibility of electronic structure that render this set of materials of great interest for both fundamental studies and applications. Essential for progress, however, is an understanding of the evolution of the electronic structure of these materials with increasing layer number. In this report, the evolution of the electronic structure of FLG, for N = 1 - 8 atomic layers, has been characterized by measurements of the optical conductivity spectra. For each layer thickness N, distinctive peaks are found in the infrared range, with positions obeying a simple scaling relation. The observations are explained by a unified zone-folding scheme that generates the electronic structure for all FLG materials from that of bulk graphite.
The precise value of the $g$-factor in graphene is of fundamental interest for all spin-related properties and their application. We investigate monolayer graphene on a Si/SiO2 substrate by resistively detected electron spin resonance (ESR). Surprisingly, the magnetic moment and corresponding g-factor of 1.952+/-0.002 is insensitive to charge carrier type, concentration, and mobility.
We use confocal Raman microscopy and modified vector analysis methods to investigate the nanoscale origin of strain and carrier concentration in exfoliated graphene-hexagonal boron nitride (hBN) heterostructures on silicon dioxide (SiO2). Two types of heterostructures are studied: graphene on SiO2 partially coved by hBN, and graphene fully encapsulated between two hBN flakes. We extend the vector analysis methods to produce spatial maps of the strain and doping variation across the heterostructures. This allows us to visualise and directly quantify the much-speculated effect of the environment…
Inherent symmetries of a system lead to multiple degeneracies of its energy spectra. Introducing individual atomic impurities can locally break these symmetries, which is expected to lift the degenerate degrees of freedom around the impurities. Although central to our understanding of the fundamental properties of solids, the broken-symmetry states induced by individual atomic impurities have so far eluded observation. Here, we report nanoscale probing of the broken-symmetry states in graphene induced by two types of individual atomic impurities, i.e., isolated nitrogen dopants and isolated hy…
The light-like dispersion of graphene monolayer results in many novel electronic properties in it1, however, this gapless feature also limits the applications of graphene monolayer in digital electronics2. A rare working solution to generate a moderate bandgap in graphene monolayer is to cut it into one-dimensional (1D) nanometre-wide ribbons3-13. Here we show that a wide bandgap can be created in a unique 1D strained structure, i.e., graphene-nanoribbon-like (GNR-like) structure, of a continuous graphene sheet via strong interaction between graphene and the metal substrate, instead of cutting…
The dynamics of photo-generated electron-hole pairs in solids are dictated by many-body interactions such as electron-electron and electron-phonon scattering. Hence, understanding and controlling these scattering channels is crucial for many optoelectronic applications, ranging from light harvesting to optical amplification. Here we measure the formation and relaxation of the photo-generated non-thermal carrier distribution in monolayer graphene with time- and angle-resolved photoemission spectroscopy. Using sub 10fs pulses we identify impact ionization as the primary scattering channel, which…
Gas molecules trapped between graphene and various substrates in the form of bubbles are observed experimentally. The study of these bubbles is useful in determining the elastic and mechanical properties of graphene, adhesion energy between graphene and substrate, and manipulating the electronic properties via strain engineering. In our numerical simulations, we use a simple description of elastic potential and adhesion energy to show that for small gas bubbles ($\sim 10$ nm) the van der Waals pressure is in the order of 1 GPa. These bubbles show universal shape behavior irrespective of their …
Probing charge scattering mechanisms in suspended graphene by varying its dielectric environment
具有高载流子迁移率μ的石墨烯对于石墨烯基电子器件和石墨烯的狄拉克费米子的基本性质的研究都是必需的。人们普遍认为，石墨烯中的迁移率限制因素是驻留在石墨烯上或底层衬底中的带电杂质的库仑散射。这对于SiO2衬底上的传统石墨烯器件和最近报道的高迁移率悬浮和支撑器件都是如此。减少这种散射的有吸引力的方法是将石墨烯放置在具有高静态介电常数κ的环境中，这将有效地屏蔽由于杂质引起的电场。然而，到目前为止，实验仅报告了高κ环境对迁移率的适度影响。在这里，我们通过研究悬浮在液体中的多终端石墨烯器件中的电输运来研究石墨烯的介电环境的影响，其中κ范围从1.9到33。对于非极性液体（κ<5），我们观察到μ随着κ快速增加，并报告了石墨烯器件在苯甲醚（κ=4.3）中的室温迁移率高达约60，000 cm 2/Vs，而在极性液体（κ>18）中，我们观察到迁移率急剧下降。我们证明，非极性液体通过筛选吸附在石墨烯上的带电杂质来增强流动性，而极性液体中的带电离子引起所观察到的流动性抑制。此外，使用分子动力学模拟，我们建立了平面外弯曲声子的散射，在室温下在真空中悬浮石墨烯中的主要散射机制，被液体的存在抑制。我们希望我们的研究结果可以提供控制和减少未来石墨烯电子器件中载流子散射的途径。
In this paper we analyze the effects of nonlocality on the optical properties of a system consisting of a thin metallic film separated from a graphene sheet by a hexagonal boron nitride (hBN) layer. We show that nonlocal effects in the metal have a strong impact on the spectrum of the surface plasmon-polaritons on graphene. If the graphene sheet is shaped into a grating, we show that the extinction curves can be used to shed light on the importance of nonlocal effects in metals. Therefore, graphene surface plasmons emerge as a tool for probing nonlocal effects in metallic nanostructures, inclu…
Black phosphorus (BP) has recently emerged as a promising narrow band gap layered semiconductor with optoelectronic properties that bridge the gap between semi-metallic graphene and wide band gap transition metal dichalcogenides such as MoS2. To date, BP field-effect transistors have utilized a lateral geometry with in-plane transport dominating device characteristics. In contrast, we present here a vertical field-effect transistor geometry based on a graphene/BP van der Waals heterostructure. The resulting device characteristics include high on-state current densities (> 1600 A/cm2) and curre…
We perform extensive analysis of graphene Josephson junctions embedded in microwave circuits. By comparing a diffusive junction at 15 mK with a ballistic one at 15 mK and 1 K, we are able to reconstruct the current-phase relation.
Probing Intrinsic Elastic Properties of Multilayer Graphene -- a New Mechanical Constant
Korea Institute for Advanced Study（韩国高等研究院）；Department of Physics, Pukyong National University（釜山国立大学物理系）；Center for Theoretical Physics of Complex Systems, Institute for Basic Science（基础科学研究院复杂系统理论物理中心）
The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative DC bias for both kinds of tunnel barriers. The analysis of the bias dependence for…
Here we report a facile method to generate a high density of point defects in graphene on metal foil and show how the point defects affect the electronic structures of graphene layers. Our scanning tunneling microscopy (STM) measurements, complemented by first principle calculations, reveal that the point defects result in both the intervalley and intravalley scattering of graphene. The Fermi velocity is reduced in the vicinity area of the defect due to the enhanced scattering. Additionally, our analysis further points out that periodic point defects can tailor the electronic properties of gra…
The epitaxial-graphene/graphene-oxide junction, an essential step towards epitaxial graphene electronics
已经沉积氧化石墨烯（GO）薄片以桥接两个外延石墨烯电极之间的间隙以产生全石墨烯装置。电学测量表明，由于GO中的带隙，在石墨烯/氧化石墨烯结处存在肖特基势垒（SB）。发现势垒高度约为0.7 eV，并且在180 $^\circ$ C退火后降低，这意味着可以通过改变氧化程度来调节间隙。发现GO迁移率的下限为850 cm $^2$ /Vs，与硅相当。已经实现了图案化外延石墨烯的{ \it In situ}局部氧化。
The fabrication of atomically precise structures with designer electronic properties is one of the emerging topics in condensed matter physics. The required level of structural control can either be reached through atomic manipulation using the tip of a scanning tunneling microscope (STM) or by bottom-up chemical synthesis. In this review, we focus on recent progress in constructing novel, atomically precise artificial materials: artificial lattices built through atom manipulation and graphene nanoribbons (GNRs) realized by on-surface synthesis. We summarize the required theoretical background…
Graphene functionalization by hydrogen and fluorine has been proposed as a route to modulate its reactivity and electronic properties. However, until now, proposed systems present degradation and limited hydrogen adsorption capacity. In this study, combining first-principles calculations based on density functional theory (DFT) and reactive molecular dynamics, we analyze the tuning of hydrogen adsorption and electronic properties in fluorinated and hydrogenated monolayer graphenes. Our results indicate that fluorine adsorption promotes stronger carbon$-$hydrogen bonds. By changing the concentr…
Electronic Transport in Chemical Vapor Deposited Graphene Synthesized on Cu: Quantum Hall Effect and Weak Localization
我们报告的电子性质的石墨烯合成的化学气相沉积（CVD）铜，然后转移到SiO2/Si。已经合成了晶圆级（高达4英寸）石墨烯膜，如光谱拉曼映射所示，主要由单层石墨烯组成。在由这种CVD石墨烯制造的微型器件上进行低温输运测量，显示双极场效应（开/关比约为5，载流子迁移率高达约3000 cm ^2/Vs）和“半整数”量子霍尔效应，这是单层石墨烯的固有电子性质的标志。我们还观察到弱本地化和提取信息的相位相干性和散射的载波。
The recent discovery of two-dimensional magnetic insulators has generated a great deal of excitement over their potential for nanoscale manipulation of spin or magnetism. One intriguing use for these materials is to put them in contact with graphene, with the goal of making graphene magnetic while maintaining its unique electronic properties. Such a system could prove useful in applications such as magnetic memories, or could serve as a host for exotic states of matter. Proximity to a magnetic insulator will alter the spin transport properties of graphene, and the strength of this interaction …
