With considering the great success of scanning tunnelling microscopy (STM) studies of graphene in the past few years, it is quite surprising to notice that there is still a fundamental contradiction about the reported tunnelling spectra of quasi-free-standing graphene monolayer. Many groups observed V-shape spectra with linearly vanishing density-of-state (DOS) at the Dirac point, whereas, the others reported spectra with a gap of 60 meV pinned to the Fermi level in the quasi-free-standing graphene monolayer. Here we systematically studied the two contradicted tunnelling spectra of the quasi-free-standing graphene monolayer on several different substrates and provided a consistent interpretation about the result. The gap in the spectra arises from the out-of-plane phonons in graphene, which mix the Dirac electrons at the Brillouin zone corners with the nearly free-electron states at the zone center. Our experiment indicated that interactions with substrates could effectively suppress effects of the out-of-plane phonons in graphene and enable us to detect only the DOS of the Dirac electrons in the spectra. We also show that it is possible to switch on and off the out-of-plane phonons of graphene at the nanoscale, i.e., the tunnelling spectra show switching between the two distinct features, through voltage pulses applied to the STM tip.
Tunnelling Characteristics of Stone-Wales Defects in Monolayers of Sn and Group-V Elements
在多层膜的合成和加工过程中，常常会形成拓扑缺陷，从而严重影响其电子性能。在本文中，我们调查的作用，斯通-威尔士（SW）缺陷在修改的电子性质的单分子膜的Sn和V族元素。计算结果发现锡烯（Sn原子单层）的电子性质强烈依赖于SW缺陷的浓度，当缺陷浓度为2.2 x 10^13 cm^-2时，有缺陷的锡烯的带隙几乎为零（约0.03 eV），当缺陷浓度为3.7 x 10^13 cm^-2时，带隙最大为0.2 eV。相反，SW-缺陷出现诱导导电状态的半导体单层的V族元素。这些导电状态作为电子隧穿的通道，和计算的隧穿特性示出了最高的微分电导的负偏压与不对称的电流-电压特性。在另一方面，最高的微分电导被发现的正偏锡烯。锡烯和V族元素单分子膜的STM形貌模拟图像显示出明显不同的横截面和距离-高度轮廓特征，这些特征可以作为实验中识别IV族和V族元素单分子膜中拓扑缺陷的指纹。
Recent experiment demonstrated the formation of free-standing Au monolayers by exposing Au-Ag alloy to electron beam irradiation. Inspired by this discovery, we used semi-empirical effective medium theory simulations to investigate monolayer formation in 30 different binary metal alloys composed of late d-series metals Ni, Cu, Pd, Ag, Pt, and Au. In qualitative agreement with the experiment, we find that the beam energy required to dealloy Ag atoms from Au-Ag alloy is smaller than the energy required to break the dealloyed Au monolayer. Our simulations suggest that similar method could also be…
Bulk amorphous materials have been studied extensively and are widely used, yet their atomic arrangement remains an open issue. Although they are generally believed to be Zachariasen continuous random networks, recent experimental evidence favours the competing crystallite model in the case of amorphous silicon. In two-dimensional materials, however, the corresponding questions remain unanswered. Here we report the synthesis, by laser-assisted chemical vapour deposition, of centimetre-scale, free-standing, continuous and stable monolayer amorphous carbon, topologically distinct from disordered…
Determination of the Spacing Between Hydrogen-Intercalated Quasi-Free-Standing Monolayer Graphene and 6H-SiC(0001) Using Total-Reflection High-Energy Positron Diffraction
On the Mechanical and Thermal Stability of Free-standing Monolayer Amorphous Carbon
最近（C. T. Toh等人，Nature 577，199（2020）），实现了自支撑单层无定形碳（MAC）的首次合成。MAC是由五个、六个、七个和八个随机分布的原子环组成的纯碳结构。MAC被证明是令人惊讶的稳定和高度抗断裂性。其电子性质与氮化硼相似。在这项工作中，我们采用全原子反应分子动力学模拟研究了MAC模型的力学性能和热稳定性。为了比较的目的，将结果与类似尺寸的原始石墨烯（PG）模型进行对比。我们的研究结果表明，MAC和PG表现出不同的力学行为和断裂动力学模式。而PG后的临界应变阈值直接从弹性到脆性政权，MAC这两个政权之间表现出不同的弹性阶段。值得注意的是，MAC是热稳定的高达3600 K，这是接近PG的熔点。这些特殊的物理性能使MAC基材料成为新技术的候选者，如柔性电子产品。
We investigate a two-dimensional (2D) heterostructure consisting of few-layer direct bandgap ReS2, a thin h-BN layer and a monolayer graphene for application to various electronic devices. Metal-insulator-semiconductor (MIS)-type devices with two-dimensional (2D) van-der-Waals (vdW) heterostructures have recently been studied as important components to realize various multifunctional device applications in analogue and digital electronics. The tunnel diodes of ReS2/h-BN/graphene exhibit light tuneable rectifying behaviours with low ideality factors and nearly temperature independent electrical…
In graphene, charged defects break the electron-hole symmetry and can even give rise to exotic collapse states when the defect charge exceeds a critical value which is proportional to the Fermi velocity. In this work, we investigate the electronic properties of twisted bilayer graphene (tBLG) with charged defects using tight-binding calculations. Like monolayer graphene, tBLG exhibits linear bands near the Fermi level but with a dramatically reduced Fermi velocity near the magic angle (approximately 1.1°). This suggests that the critical value of the defect charge in magic-angle tBLG should al…
The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab-initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der…
The impact of an applied electric field on the exchange coupling parameters has been investigated based on first-principles electronic structure calculations by means of the KKR Green function method. The calculations have been performed for a Fe film, free-standing and deposited on two different substrates, having 1 monolayer (ML) thickness to minimize the effect of screening of the electric field typical for metallic systems. By comparing the results for the free-standing Fe ML with those for Fe on the various substrates, we could analyze the origin of the field-induced change of the exchang…
Experimental Realization of the Topologically Nontrivial Phase in Monolayer Si$_2$Te$_2$
Universität Würzburg（维尔茨堡大学）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area (Guangdong)（粤港澳大湾区量子科学中心（广东））；Fuzhou University（福州大学）；The University of Tokyo（东京大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；Spallation Neutron Source Science Center（散裂中子源科学中心）
Graphene-based nanostructured systems and van-der-Waals heterostructures comprise a material class of growing technological and scientific importance. Joining materials with vastly different properties, polymer-graphene heterosystems promise diverse applications in surface- and nanotechnology, including photovoltaics or nanotribology. Fundamentally, molecular adsorbates are prototypical systems to study confinement-induced phase transitions exhibiting intricate dynamics, which require a comprehensive understanding of the dynamical and static properties on molecular time and length scales. Here…
