Spatially resolved electronic inhomogeneities of graphene due to subsurface charges

We probe the local inhomogeneities in the electronic properties of exfoliated graphene due to the presence of charged impurities in the SiO₂ substrate using a combined scanning tunneling and electrostatic force microscope. Contact potential difference measurements using electrostatic force microscopy permit us to obtain the average charge density but it does not provide enough resolution to identify individual charges. We find that the tunneling current decay constant, which is related to the local tunneling barrier height, enables one to probe the electronic properties of graphene distorted at the nanometer scale by individual charged impurities. We observe that such inhomogeneities do not show long-range ordering and their surface density obtained by direct counting is consistent with the value obtained by macroscopic charge density measurements. These microscopic perturbations of the carrier density significantly alter the electronic properties of graphene, and their characterization is essential for improving the performance of graphene based devices.     

Electrostatic transparency of graphene oxide sheets

The interaction of graphene oxide of varying reduction degrees with dielectric and metallic surfaces is probed in this study, in order to assess the influence that the supporting substrate has on the electronic properties of as-produced graphene oxide and its reduced form. Lateral inhomogeneities in the distribution of substrate trapped charged impurities are found to affect the electronic properties of reduced graphene oxide, giving rise to significant in-plane variations of the local electrostatic potential on reduced one-layer sheets supported on dielectric substrates. On the contrary, no such surface potential fluctuations are identified on as-produced graphene oxide sheets, or on graphene oxide layers deposited on a metallic substrate. Thicker, two-layer reduced graphene oxide sheets show effective screening of the electrostatic effects caused by charge impurities trapped in the substrate. The current study provides a useful account of the limitations that device performance could face when attempting to tune the electronic structure of graphene oxide via functionalization, highlighting the role of substrate-related disorder affecting the behaviour of nanodevices. The role of the substrate is particularly important for applications where electronic properties of graphene oxide are especially targeted, such as transparent conducting films, sensors and electrochemical devices.     

Theoretical study of the role of metallic contacts in probing transport features of pure and defected graphene nanoribbons

Understanding the roles of disorder and metal/graphene interface on the electronic and transport properties of graphene-based systems is crucial for a consistent analysis of the data deriving from experimental measurements. The present work is devoted to the detailed study of graphene nanoribbon systems by means of self-consistent quantum transport calculations. The computational formalism is based on a coupled Schrödinger/Poisson approach that respects both chemistry and electrostatics, applied to pure/defected graphene nanoribbons (ideally or end-contacted by various fcc metals). We theoretically characterize the formation of metal-graphene junctions as well as the effects of backscattering due to the presence of vacancies and impurities. Our results evidence that disorder can infer significant alterations on the conduction process, giving rise to mobility gaps in the conductance distribution. Moreover, we show the importance of metal-graphene coupling that gives rise to doping-related phenomena and a degradation of conductance quantization characteristics.     

Sublattice imbalance of substitutionally doped nitrogen in graphene

Motivated by the recently observed sublattice asymmetry of substitutional nitrogen impurities in CVD grown graphene, we show, in a mathematically transparent manner, that oscillations in the local density of states driven by the presence of substitutional impurities are responsible for breaking the sublattice symmetry. While these oscillations are normally averaged out in the case of randomly dispersed impurities, in graphene they have either the same, or very nearly the same, periodicity as the lattice. As a result, the total interaction energy of randomly distributed impurities embedded in the conduction-electron-filled medium does not vanish and is lowered when their configuration is sublattice-asymmetric. We also identify the presence of a critical concentration of nitrogen above which one should expect the sublattice asymmetry to disappear. This feature is not particular to nitrogen dopants, but should be present in other impurities.     

Point Defects and Sintering of Lead Zirconate-Titanate

The crystal chemistry of point defects in lead zirconate-titanate is discussed. The results are used to interpret sintering and grain-growth behavior. Lattice vacancies are created thermally, by substitutional impurities with incorrect valences, and by changes in stoichiometry. Charged O vacancies are introduced when Al³⁺ replaces Ti⁴⁺, and charged Pb vacancies occur when Nb⁵⁺ replaces Ti⁴⁺. These vacancies are believed to be associated with the impurity ions and cause them to be adsorbed at grain boundaries. This behavior retards grain growth and thereby expedites densification. Aluminum ions (deficient valence) compensate for niobium ions (excess valence). These "paired" defects are not associated with vacancies and are not adsorbed; thus, they do not impede grain growth. Sintering follows Coble's model of bulk diffusion of vacancies from pores to grain boundaries. Oxygen vacancies are believed to be the slowest-moving species. Aluminum: niobium compensation is confirmed by ferroelectric measurements. Doping with Al decreases the mobility of ferroelectric domain boundaries, whereas Nb increases it. Doping with both ions produces ferroelectric properties similar to those of the undoped material.     

Role of oxygen vacancy inducer for graphene in graphene-containing anodes

Currently, graphene is only considered as a conductive additive and expansion inhibitor in oxides/graphene composite anodes. In this study, a new graphene role (oxygen vacancy inducer) in graphene/oxides composites anodes, which are treated at high-temperature, is proposed and verified using experiments and density functional theory calculations. During high-temperature processing, graphene forms carbon vacancies due to increased thermal vibration, and the carbon vacancies capture oxygen atoms, facilitating the formation of oxygen vacancies in oxides. Moreover, the induced oxygen vacancy concentrations can be regulated by sintering temperatures, and the behavior is unaffected by oxide crystal structures (crystalline and amorphous) and morphology (size and shape). According to density functional theory calculations and electrochemical measurements, the oxygen vacancies enhance the lithium-ion storage performance. The findings can result in a better understanding of graphene’s roles in graphene/oxide composite anodes, and provide a new method for designing high-performance oxide anodes.     

Room-temperature ferromagnetism in graphitic petal arrays

We report room-temperature ferromagnetism of graphitic petal arrays grown on Si substrates by microwave plasma chemical vapor deposition without catalyst. The samples have been characterized by Raman and X-ray photoelectron spectroscopy to confirm the absence of possible ferromagnetic impurities. The petals exhibit ferromagnetic hysteresis with saturation magnetization of ～4.67 emu cm(-3) and coercivity of ～105 Oe at 300 K, comparable to the reported behavior of few-layer graphene. Upon O2 annealing the saturation magnetization and coercivity decreased to 2.1 emu cm(-3) and ～75 Oe respectively. The origin of ferromagnetism is believed to arise from the edge defects and vacancies in the petals.     

Electron Spin Resonance Analysis of Lattice Defects in Polycrystalline Aluminum Nitride

Electron spin resonance was used in the present study to detect lattice defects in an aluminum nitride lattice. The ESR spectra were obtained from polycrystalline AlN with various thermal conductivities. Measurement of the AlN g values clearly indicated that the obtained ESR signals arose from electrons trapped by nitrogen vacancies. The ESR study revealed that thermal conductivity increases with an increase in the number of electrons trapped by nitrogen vacancies. The explanation for that phenomenon is that the thermal conductivity of AlN increases with a decreasing concentration of oxygen impurities incorporated into the AlN lattice, and the concentration of nitrogen vacancies changes inversely with the concentration of oxygen impurities.     

Probing substrate influence on graphene by analyzing Raman lineshapes

We provide a new approach to identify the substrate influence on graphene surface. Distinguishing the substrate influences or the doping effects of charged impurities on graphene can be realized by optically probing the graphene surfaces, included the suspended and supported graphene. In this work, the line scan of Raman spectroscopy was performed across the graphene surface on the ordered square hole. Then, the bandwidths of G-band and 2D-band were fitted into the Voigt profile, a convolution of Gaussian and Lorentzian profiles. The bandwidths of Lorentzian parts were kept as constant whether it is the suspended and supported graphene. For the Gaussian part, the suspended graphene exhibits much greater Gaussian bandwidths than those of the supported graphene. It reveals that the doping effect on supported graphene is stronger than that of suspended graphene. Compared with the previous studies, we also used the peak positions of G bands, and I_2D/I_G ratios to confirm that our method really works. For the suspended graphene, the peak positions of G band are downshifted with respect to supported graphene, and the I_2D/I_G ratios of suspended graphene are larger than those of supported graphene. With data fitting into Voigt profile, one can find out the information behind the lineshapes.     

The preparation of graphene decorated with manganese dioxide nanoparticles by electrostatic adsorption for use in supercapacitors

Graphene decorated with manganese dioxide nanoparticles are prepared by electrostatic adsorption. The manganese dioxide is synthesized by a microemulsion route using the cationic surfactant hexadecyltrimethyl ammonium bromide, which dispersed in water is converted to be positively charged. The surface charge of graphene in water is negative, allowing two forms of manganese dioxide-decorated graphene to be synthesized by electrostatic adsorption: (a) free in situ synthesis and (b) layer-by-layer self-assembly. By electrochemical analysis, the specific capacitances of two materials are found to be about 40% and 250% larger than that of manganese dioxide. The improvement is because of the tighter contact between graphene and manganese dioxide, and the higher conductive and capacitive characteristics of graphene.     

Study on the Characteristics of Gas Molecular Mean Free Path in Nanopores by Molecular Dynamics Simulations

This paper presents studies on the characteristics of gas molecular mean free path in nanopores by molecular dynamics simulation. Our study results indicate that the mean free path of all molecules in nanopores depend on both the radius of the nanopore and the gas-solid interaction strength. Besides mean free path of all molecules in the nanopore, this paper highlights the gas molecular mean free path at different positions of the nanopore and the anisotropy of the gas molecular mean free path at nanopores. The molecular mean free path varies with the molecule’s distance from the center of the nanopore. The least value of the mean free path occurs at the wall surface of the nanopore. The present paper found that the gas molecular mean free path is anisotropic when gas is confined in nanopores. The radial gas molecular mean free path is much smaller than the mean free path including all molecular collisions occuring in three directions. Our study results also indicate that when gas is confined in nanopores the gas molecule number density does not affect the gas molecular mean free path in the same way as it does for the gas in unbounded space. These study results may bring new insights into understanding the gas flow’s characteristic at nanoscale.     

Electronic states of disordered grain boundaries in graphene prepared by chemical vapor deposition

Perturbations of the two dimensional carbon lattice of graphene, such as grain boundaries, have significant influence on the charge transport and mechanical properties of this material. Scanning tunneling microscopy measurements presented here show that localized states near the Dirac point dominate the local density of states of grain boundaries in graphene grown by chemical vapor deposition. Such low energy states are not reproduced by theoretical models which treat the grain boundaries as periodic dislocation-cores composed of pentagonal–heptagonal carbon rings. Using ab initio calculations, we have extended this model to include disorder, by introducing vacancies into a grain boundary consisting of periodic dislocation-cores. Within the framework of this model we were able to reproduce the measured density of states features. We present evidence that grain boundaries in graphene grown on copper incorporate a significant amount of disorder in the form of two-coordinated carbon atoms.     

Charging of unfunctionalized graphene in organic solvents

Unfunctionalized graphene is positively or negatively charged when it is dispersed in organic solvents. The charging is negative in solvents with high electron donor numbers and positive in those with low donor numbers. We suggest that the charging originates from electron transfer between graphene surfaces and solvent molecules, and the stable dispersion of unfunctionalized graphene in organic solvents is mainly controlled by electrostatic repulsion between the charged graphene surfaces.     

Impurity effects on polaron dynamics in graphene nanoribbons

The impurity effects on the dynamics of polarons in armchair graphene nanoribbons are numerically investigated in the scope of a two-dimensional tight-binding approach with lattice relaxation. The results show that the presence of an impurity changes significantly the net charge distribution associated to the polaron structure. Moreover, the interplay between external electric field and the local impurities plays the role of drastically modifying the polaron dynamics. Interestingly, nanoribbons containing mobile polarons are noted to take place even when considering high impurity levels, which is associated with the highly conductive character of the graphene nanoribbons. This investigation may enlighten the understanding of the charge transport mechanism in carbon-based nanomaterials.     

Defect Chemistry of Undoped and Acceptor-Doped BaPbO3

The equilibrium defect chemistry of polycrystalline, undoped, and acceptor-doped BaPbO₃ was studied by measurement of the equilibrium electrical conductivity as a function of temperature, 800°–900°C, and oxygen activity, 10⁻¹⁸–1 atm. Both equilibrium electrical conductivity data of undoped and acceptor-doped samples were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, lead vacancies, holes, and acceptor impurities. The results in low and midrange of oxygen activity are dominated by acceptor impurities, whether deliberately added or not. Only in the highly oxidized condition is the conductivity independent of impurity content, confirming that this region represents the intrinsic behavior of BaPbO₃.     

The lattice Boltzmann Peierls Callaway equation for mesoscopic thermal transport modeling

The lattice Boltzmann Peierls Callaway (LBPC) method is a recent development of the versatile lattice Boltzmann formalism aimed at a numerical experiment on mesoscale thermal transport in a multiphase phonon gas. Two aspects of mesoscopic thermal transport are discussed: the finite phonon mean free path and the interface thermal resistance. Based on the phonon momentum screening length measured in the LBPC computational apparatus, the validity of the Umklapp collision relaxation time in the Callaway collision operator is examined quantitatively. The discrete nature of the spatio-temporal domain in the LBPC method, along with the linear approximation of the exponential screening mechanism in the Callaway operator, reveals a large discrepancy between the effective phonon mean free path and the analytic phonon mean free path when the relaxation time is small. The link bounce back interface phonon collision rule is used to realize the interface thermal resistance between phonon gases with dissimilar dispersion relations. Consistent with the Callaway collision operator for the bulk phonon dynamics, the interface phonon collision process is regarded as a linear relaxation mechanism toward the local pseudo-equilibrium phonon distribution uniquely defined by the energy conservation principle. The interface thermal resistance is linearly proportional to the relaxation time of the proposed phonon interface collision rule.     

未掺杂铅酸钡陶瓷的缺陷补偿机理

用高温平衡电导法研究了未掺杂铅酸钡(BaPbO3)陶瓷的缺陷化学,确定了未掺杂BaPbO3材料的缺陷化学模型,同时,从BaPbO3材料的缺陷结构的角度讨论了热处理气氛对材料室温电导率的影响.BaPbO3材料在实验氧分压范围内呈现p型电导,在高氧分压下,铅离子(Pb4 )空位和空穴占主导,材料表现出本征缺陷行为.在中氧分压下,受主杂质成为主导缺陷,电荷补偿缺陷为空穴.在低氧分压下,受主杂质的电荷补偿缺陷转变为氧离子空位,还原反应成为电荷补偿缺陷的主要来源.材料室温电导率的变化完全是由于在不同的氧分压环境下材料中主导缺陷的转变和缺陷浓度的变化而引起的.     

Evidence for Suppression of Radiation Damage in Li-Doped MgO

The net production of stable intrinsic defects in MgO, such as anion vacancies and divacancies, produced by electron and neutron irradiations is shown to be suppressed by doping with Li impurities.     

Effect of reduced graphene oxide functionalization by sulfanilic acid on the mechanical properties of poly(styrene‐co‐acrylonitrile)/reduced graphene oxide composites

The sulfonation of reduced graphene oxide (SRGO) by the aryl diazonium salt of sulfanilic acid was focused to examine the enhancement effect on the mechanical properties of poly styrene‐acrylonitrile (SAN). The SAN was prepared by surfactant‐free emulsion copolymerization using a cationic initiator. By mixing sulfonated RGO (SRGO) into the SAN polymer matrix, positively‐charged SAN particles were attracted to the negatively‐charged surfaces of SRGO sheets via electrostatic interactions. The storage modulus of SAN‐SRGO increased to 46% at 4 wt% SRGO loading. This improvement is attributed to strong interactions between sulfonated groups on the surface SRGO and the nitrile groups of SAN. POLYM. COMPOS., 44–50, 2016. © 2014 Society of Plastics Engineers     

Evidence for formation of multi-quantum dots in hydrogenated graphene

ABSTRACT: We report the experimental evidence for the formation of multi-quantum dots in a hydrogenated single-layer graphene flake. The existence of multi-quantum dots is supported by the low-temperature measurements on a field effect transistor structure device. The resulting Coulomb blockade diamonds shown in the color scale plot together with the number of Coulomb peaks exhibit the characteristics of the so-called 'stochastic Coulomb blockade'. A possible explanation for the formation of the multi-quantum dots, which is not observed in pristine graphene to date, was attributed to the impurities and defects unintentionally decorated on a single-layer graphene flake which was not treated with the thermal annealing process. Graphene multi-quantum dots developed around impurities and defect sites during the hydrogen plasma exposure process.