点缺陷石墨烯的电导

Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%–34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%–62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2–3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.     

Graphene Defects in Saccharide Carbon Dots Govern Electrochemical Sensitivity

Galactose (Gal), lactose (Lac), and glucose (Glu) derived carbon dots (CDs) were evaluated for their utility as electrochemical sensing composites using acetaminophen (APAP) as a probe molecule. The goal of this work is to ascertain the role of graphene defects on electrochemical activity. Higher sp²-to-sp³ hybridized carbon ratios (in parentheses) in the CDs correlated with higher sensitivity in the order according to measured Raman I_G/I_D intensities: GluCDs (6.53)<LacCDs (9.30)<GalCDs (10.18). A dynamic measurement in the 0–2.0 mmol dm⁻³ APAP range at pH=7.0 was achieved, suitable for practical APAP toxicity monitoring. Defect density within the GalCDs provided the highest sensitivity.     

Surface Enhanced Raman Scattering in Graphene Quantum Dots Grown via Electrochemical Process

Graphene Quantum dots (GQDs) are used as a surface-enhanced Raman substrate for detecting target molecules with large specific surface areas and more accessible edges to enhance the signal of target molecules. The electrochemical process is used to synthesize GQDs in the solution-based process from which the SERS signals were obtained from GQDs Raman spectra. In this work, GQDs were grown via the electrochemical process with citric acid and potassium chloride (KCl) electrolyte solution to obtain GQDs in a colloidal solution-based format. Then, GQDs were characterized by transmission electron microscope (TEM), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy, respectively. From the results, SERS signals had observed via GQDs spectra through the Raman spectra at D (1326 cm⁻¹) and G (1584 cm⁻¹), in which D intensity is defined as the presence of defects on GQDs and G is the sp² orbital of carbon signal. The increasing concentration of KCl in the electrolyte solution for 0.15M to 0.60M demonstrated the increment of Raman intensity at the D peak of GQDs up to 100 over the D peak of graphite. This result reveals the potential feasibility of GQDs as SERS applications compared to graphite signals.     

双空位缺陷双层石墨烯储钠性能的第一性原理研究

采用基于密度泛函理论(DFT)的色散修正方法,研究了Na吸附和嵌入在双空位缺陷(DV)双层石墨烯(BLG)体系中的形成能、电荷转移、电极电势和扩散行为。形成能计算表明,无论单个Na原子在BLG表面吸附还是层间嵌入,均在DV空位中心处更稳定。电荷密度分布和Bader电荷计算表明Na与BLG的结合方式表现出离子性。Na嵌入DV缺陷BLG层间,缺陷浓度增加使BLG由AB堆垛向AA堆垛转变过程推迟;使Na在DV缺陷BLG的表面和层间能够稳定储钠的容量之和增至262.75 mAh?g~(-1),对应浓度Na与C摩尔比为2:17,储钠浓度继续增加,Na在BLG表面吸附容易产生枝晶或团簇。当层间嵌入Na原子时,表面Na原子向DV缺陷中心方向扩散能垒减小、表面Na原子沿相反方向的扩散能垒增加,DV缺陷的存在提高了BLG表面捕获Na的能力。     

Cu3BiS3: Two-Dimensional Coordination Induces Out-of-Plane Phonon Scattering Enabling Ultralow Thermal Conductivity

The discovery of compounds with low thermal conductivity and the understanding of their microscopic mechanisms are of great challenges and scientific significance. Herein, we report a unique ternary sulfide compound, Cu₃BiS₃, in which all Cu atoms are coordinated within a two-dimensional [CuS₃] triangle plane. This local coordination leads to efficient out-of-plane phonon scattering and an ultralow thermal conductivity. Through DFT phonon spectrum calculations and analyses, we reveal that the lowest vibration frequency decreases from 2 THz for high-dimensional [CuS₄] tetrahedral coordinated Cu atoms in CuBiS₂ (CN=4, with an average Cu−S bond length of 2.328 Å) to 1.5 THz for low-dimensional [CuS₃] triangular coordinated Cu atoms in Cu₃BiS₃ (CN=3, with a shorter Cu−S bond length of 2.285 Å). This is due to the out-of-plane thermal vibration of the Cu atoms in the latter. Consequently,Cu₃BiS₃ exhibits one of the lowest values of κ_lat (0.32 W/m K) among its peer, with a 36 % reduction compared to CuBiS₂ (0.50 W/m K). This groundbreaking discovery highlights the significant role of 2D local coordination in reducing thermal conductivity through characteristic out-of-plane phonon scattering, while also contributing to a large Grüneisen parameter (2.06) in Cu₃BiS₃.     

贵金属原子与点缺陷石墨烯的键增强作用

通过密度泛函理论研究了Ag、Au、Pt原子在完美和点缺陷(包括N掺杂、B掺杂、空位点缺陷)石墨烯上的吸附以及这些体系的界面性质.研究表明Ag、Au不能在完美的石墨烯上吸附,N、B掺杂增强了三种金属与石墨烯之间的相互作用.而空位点缺陷诱发三种金属在石墨烯上具有强化学吸附作用.通过电子结构分析发现,N掺杂增强了Au、Pt与C形成的共价键,而Au、Ag与B形成了化学键.空位点缺陷不仅是金属原子的几何固定点,同时也增加了金属原子和碳原子之间的成键.增强贵金属原子和石墨烯相互作用的顺序是:空位点缺陷>>B掺杂>N掺杂.     

含氮-空位缺陷锯齿状石墨烯纳米条带中负微分电阻和自旋过滤效应

采用第一性原理和非平衡格林函数方法,系统研究了含氮空位缺陷锯齿状石墨烯纳米条带的自旋极化输运特性．理论计算结果表明边界非对称的这类石墨纳米条带的基态具有铁磁性,由其构建的分子结中负微分电阻效应具有鲁棒性,是电极局域的态密度及依赖偏压的散射区-电极耦合作用结果．此外,在特定偏压区域还观察到几乎完美的自旋过滤效应．     

Characterization of Nano-Scale Parallel Lamellar Defects in RDX and HMX Single Crystals by Two-Dimension Small Angle X-ray Scattering

Nano-scale crystal defects extremely affect the security and reliability of explosive charges of weapons. In this work, the nano-scale crystal defects of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) single crystals were characterized by two-dimension SAXS. Deducing from the changes of SAXS pattern with sample stage rotating, we firstly found the parallel lamellar nano-scale defects in both RDX and HMX single crystals. Further analysis shows that the average diameter and thickness of nano-scale lamellar defects for RDX single crystal are 66.4 nm and 19.3 nm, respectively. The results of X-ray diffraction (XRD) indicate that the lamellar nano-scale defects distribute along the (001) in RDX and the (011) in HMX, which are verified to be the crystal planes with the lowest binding energy by the theoretical calculation.     

多晶铂电极表面硫吸附物种的原位表面增强拉曼光谱研究(英文)

本工作首次报道了在酸性或碱性电解质中高粗糙度多晶铂电极表面硫物种的吸附、电氧化过程的原位表面增强拉曼光谱研究.分别在两种情况下采集了粗糙铂电极的表面增强拉曼光谱:(a)循环伏安处理后控制一定电位(0.2V)下;(b)逐步控制不同电位.酸性电解质条件下,两种情况均观察到位于300cm-1的Pt—S振动.Pt—S键较强,阻止了氢在铂电极表面的吸附,需多次循环伏安处理才能完全将表面吸附的硫去除.同时还观察到位于470cm-1处的少量多聚S物种(S8或者其他类型).这些多聚S物种仅仅微弱地键合在铂电极的表面,可以容易地除去.在碱性电解质中,同样也在(a)和(b)情况下观察到位于310cm-1的Pt—S振动吸收.本工作了获得了有关硫在粗糙多晶铂电极表面吸附和电氧化重要信息,证明了原位表面增强拉曼光谱在研究铂表面的适用性.     

The Origin of Improved Electrical Double‐Layer Capacitance by Inclusion of Topological Defects and Dopants in Graphene for Supercapacitors

Low‐energy density has long been the major limitation to the application of supercapacitors. Introducing topological defects and dopants in carbon‐based electrodes in a supercapacitor improves the performance by maximizing the gravimetric capacitance per mass of the electrode. However, the main mechanisms governing this capacitance improvement are still unclear. We fabricated planar electrodes from CVD‐derived single‐layer graphene with deliberately introduced topological defects and nitrogen dopants in controlled concentrations and of known configurations, to estimate the influence of these defects on the electrical double‐layer (EDL) capacitance. Our experimental study and theoretical calculations show that the increase in EDL capacitance due to either the topological defects or the nitrogen dopants has the same origin, yet these two factors improve the EDL capacitance in different ways. Our work provides a better understanding of the correlation between the atomic‐scale structure and the EDL capacitance and presents a new strategy for the development of experimental and theoretical models for understanding the EDL capacitance of carbon electrodes.     

Structure and Spin-Polarized Transport of Co Atomic Chains on Graphene with Topological Line Defects

We carry out density functional theory calculations to explore the nucleation growth of Co atoms absorbed on graphene with extended linear defect (LD@Gr). Based on the analysis of optimized structures, binding energies and diffusion barriers, we predict a patterned growth of linear arranged Co clusters along the LD. With the increase of cluster size, Co chain forms gradually and results in the construction of a quasi-one-dimensional (1D) heterostructure (Co/LD@Gr). Moreover, the transport properties of the Co/LD@Gr are investigated by using the non-equilibrium Green’s function method combined with density functional theory. We find that the spin current paralleling to the LD is polarized. The spin-resolved transmission pathways and eigenchannels indicate that there is high spin injection from Co chain into graphene. As predetermined location and direction for the LD in graphene have been realized experimentally, this quasi-1D Co/LD@Gr heterostructure would be a compelling and feasible candidate for future spintronic related applications.     

Phonon dispersion in polyguanylic acid

A study of the normal modes of vibration and their dispersion in polyguanylic acid along the chain axis (tetrad form) based on Urey-Bradley force field and Raman and Fourier transform infrared is reported. It has been greatly assisted by the vibrational analysis of guanine tetrad and spectral information for telomeric DNA d(G₁₂). Spectral relationship for polyG, telomeric DNA d(G₁₂) and characteristic features of dispersion curves such as regions of high density-of-states, repulsion and character mixing are discussed. Predictive value of heat capacity as a function of temperature is reported.     

Phonon dispersion in polycytidilic acid

A study of the normal modes of vibration and their dispersion in polycytidilic acid based on the Urey–Bradley force field and Raman and Fourier transform infrared spectroscopy is reported. Characteristic features of dispersion curves such as regions of high density of states, repulsion, and character mixing of dispersive modes are discussed. The predictive value of the heat capacity as a function of temperature is reported. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 504–516, 2006     

Silver Nanostructures on Graphene Oxide as the Substrate for Surface-Enhanced Raman Scattering (SERS)

Nanosized surface-enhanced Raman scattering (SERS) substrates fabricated by the controlled growth of metal nanostructures on water-dispersed two-dimensional nanomaterials can open a new avenue for SERS analysis of liquid samples in biological fields. In this work, regular and uniform Ag nanostructures were grown on the surface of graphene oxide (GO) through a microwave-assisted hydrothermal method. Polyamidoamine (PAMAM) dendrimers were assembled on the surface of GO to form GO/PAMAM templates for growing Ag nanostructures, which are primarily comprised of Ag dimers and trimers. The prepared Ag/GO nanocomposites are highly dispersed and stable in aqueous solution and may be used as substrates for enhanced Raman detection of rhodamine 6?G (R6G) in aqueous solution. This special substrate provides high-performance SERS and suppresses R6G fluorescence in aqueous solution and is promising as a nanosized material for the enhanced Raman detection of liquid samples in biological diagnostics.     

氧化石墨烯/金银纳米粒子复合材料的制备及其SERS效应研究

近年来,氧化石墨烯/金银纳米粒子复合材料由于其优异的表面增强拉曼散射（SERS）性能引起了人们极大的关注,在污染物检测、化学传感和癌症诊断等领域具有重要的应用价值。本文综述了氧化石墨烯片层上修饰金银纳米粒子、氧化石墨烯包覆金银纳米粒子、氧化石墨烯附着在金银纳米粒子层三种氧化石墨烯/金银纳米粒子复合材料的制备方法,对其SERS效应进行了详细介绍。SERS研究表明,结合了金银纳米粒子与氧化石墨烯两种材料各自在SERS研究与应用中的优势,氧化石墨烯/金银纳米粒子复合材料的SERS性能比单纯金银纳米粒子更加优异。氧化石墨烯在其中起到了化学增强、分子富集、钝化保护、荧光猝灭的重要作用。氧化石墨烯/金银纳米粒子复合材料在表面增强拉曼光谱中具有广阔的应用前景。     

Growth Mechanism of Graphene on Graphene Films Grown by Chemical Vapor Deposition

We report an approach for the synthesis of mono‐ or bilayer graphene films by atmospheric‐pressure chemical vapor deposition that can achieve a low defect density through control over the growth time. Different heating ramp rates were found to lead to variation in the smoothness and grain size of the Cu foil substrate, which directly influenced the density of the graphene domains. The rough Cu surface induced by rapid heating creates a high density of graphene domains in the initial stage, ultimately resulting in a graphene film with a high defect density due to an increased overlap between domains. Conversely, a slow heating rate resulted in a smooth and flat Cu surface, thereby lowering the density of the initial graphene domains and ensuring a uniform monolayer film. From this, we demonstrate that the growth mechanism of graphene on existing graphene films is dependent on the density of the initial graphene domains, which is affected by the heating ramp rate.     

Tuning 2D Light Upconversion Emission by Modulating Phonon Relaxation

The photonic upconversion in rare earth atoms is widely used to convert “invisible” near infrared photons to “visible” photons with continuous wave light. By using a patterned substrate, upconversion become a route for creating new information‐incorporating security codes. The amount of information in the cipher increases in proportion to the number of emission colors as well as the pattern structure. Subsequently, changing the chemical composition of upconversion phosphors on 2 D substrates is required to manufacture information‐rich upconversion cryptography. In this study, we exploited temperature‐controlled thermal reaction on upconversion films deposited on a quartz substrate to prepare security information codes. Multiple color emission was generated from upconversion films as the result of inserting high‐frequency molecular oscillators into the film structures. Fourier‐transform infrared (FTIR) and time‐resolved study corroborated the mechanism of spectral variation of upconversion films.