Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition

We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede electrical transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.     

Experimental study of polarization bremsstrahlung from small-grained polycrystals

The spectrum of polarization Bremsstrahlung is measured in backscattering geometry for 7-MeV electrons and a polycrystalline Ni foil without texture. The average size of grains in the foil is 300 nm. The results show the possibility of using a new method in the diagnostics of the atomic structure of polycrystalline materials with nanosized grains. The method is based on measuring the coherent component of polarization Bremsstrahlung.     

Features of the crystal structure and electrical properties of sodium chloride at pressure 20–50 GPa

The electrical properties of polycrystalline samples of sodium chloride are studied at direct and alternating current in a wide range of frequencies at high pressure and room temperature. Graphic analysis of the experimental data [1–3] in the view of equivalent circuits allowed us to separate the contributions to conductivity caused by grains and intergrain boundaries. Features of impedance at pressure up to 37 GPa are in good agreement with earlier data and structural changes. It is shown that in the studied materials the electrical resistance of grains is much greater than the resistance of intergrain boundaries.     

纳米微晶材料的结构和性质

纳米微晶材料是纳米量级晶粒所构成的多晶物质,其晶界区域中存在与长程有序晶态和短程有序非晶态结构不相同的“气体状”的结构。本文讨论了纳米微晶材料的制备方法、结构特点和奇异性质及其在材料科学中的应用。     

Complex impedance spectroscopic analysis of Mn-modified Pb(Zr0.65Ti0.35)O3 electroceramics

The polycrystalline samples of Pb(Zr_0.65−xMn_xTi_0.35)O₃ (PZMT) (x=0, 0.05, 0.10, 0.15) were prepared by a high-temperature solid-state reaction technique. Detailed studies on the effect of compositional variation of manganese (Mn) on the electrical behavior (complex impedance Z*, complex modulus M*, electrical conductivity and relaxation mechanisms) of the PZMT systems have been carried out by a nondestructive complex impedance spectroscopy (CIS) technique at 400 °C. The Nyquist plots suggest that the grains only are responsible in the conduction mechanism of the materials. The occurrence of single arc in the complex modulus spectrum of all the compositions of Mn confirms the single-phase characteristics of the PZMT compounds, and also confirms the presence of non-Debye type of multiple relaxation in the material.     

Impedance spectroscopy of Gd-doped BiFeO3 multiferroics

The polycrystalline Bi_1?x Gd _x FeO₃ (BGFO) (x=0.0, 0.05, 0.10, 0.15, 0.20) materials were synthesized by a solid-state reaction (mixed oxide) technique. Preliminary X-ray structural analysis of the compounds confirmed the formation of single-phase polycrystalline samples. Room temperature scanning electron micrographs of the materials revealed the size, type and distribution of grains on the surface of samples. Studies of impedance, electrical modulus and electric conductivity of the materials in a wide frequency (10–1000 kHz) and temperature (30–500 ^°C) range using a complex impedance spectroscopy technique have provided considerable vital information on contribution of grains, grain boundary and interface in these parameters. A strong correlation between these electrical parameters and microstructures (bulk, grain boundary, nature of charge carrier, etc.) of the materials was established. The frequency dependence of electric modulus and impedance of the material shows the presence of non-Debye type of relaxation.     

Fatigue failure mechanism of polycrystalline materials at the mesoscopic level

Systematic studies of the mesoscopic mechanisms of deformation of polycrystalline materials of lead and its alloys have been carried out under conditions of sign-alternating bending at room temperature. It has been shown that fatigue failure is due to the evolution of vortices of mesoscopic substructures. Multiple slip separated in adjacent grains is the basis for this kind of deformation. This causes extremely strong localization of the displacement in individual favorably oriented grains and self-organization of these grains in agreement with regular structural levels of deformation. In polycrystalline lead, the mesoscopic substructure has a block character, with each block containing several grains. The elements of such substructures are nucleated in stress mesoconcentrator zones which arise at the grain boundaries under conditions of intense grain boundary slippage. In the course of cycling they gradually propagate through the whole transverse cross section of the sample, which completes its failure. Alloying substantially changes the character of the mesoscopic substructures which are formed. We have considered the different types of vortex mesoscopic substructures and studied their connection with cyclical endurance of the alloy. Recommendations for increasing the fatigue endurance of plastic polycrystalline materials are given. Institute of the Physics of hardening and Materials Science, Siberian Section, Russian Academy of Sciences. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 40–57, June, 1996.     

Plasma-stimulated penetrability of hydrogen as a tool for studying phase transitions at grain boundaries

It is well known that the diffusion of hydrogen atoms through the intrinsic defects of a crystal lattice has characteristics different from those of bulk diffusion and, at certain parameters for some polycrystalline metals, ensures the determining contribution to the transfer of hydrogen atoms through the material. Grain boundaries (and dislocations) are the most important and shortest paths, the diffusion through which is much faster than bulk diffusion through a crystal lattice. It is particularly important to take into account this diffusion in materials with grains having sizes of about several nanometers. The possibility of using the method of the plasma-stimulated penetrability of hydrogen to analyze phase transitions at the grain boundaries is demonstrated on the example of polycrystalline niobium foils. In contrast to the existing methods, this method proposed for studying grain-boundary diffusion and phase transitions is simple and ensures control over the surface. The temperature characteristics of the diffusion of hydrogen atoms through niobium grain boundaries have been measured.     

Laser crystallization for large-area electronics

Laser crystallization is reviewed for the purpose of fabrication of polycrystalline silicon thin film transistors (poly-Si TFTs). Laser-induced rapid heating is important for formation of crystalline films with a low thermal budget. Reduction of electrically active defects located at grain boundaries is essential for improving electrical properties of poly-Si films and achieving poly-Si TFTs with high performances. The internal film stress is attractive to increase the carrier mobility. Recent developments in laser crystallization methods with pulsed and continuous-wave lasers are also reviewed. Control of heat flow results in crystalline grain growth in the lateral direction, which is important for fabrication of large crystalline grains. We also report an annealing method using a high-power infrared semiconductor laser. High-power lasers will be attractive for rapid formation of crystalline films over a large area and activation of silicon with impurity atoms.     

Structural and impedance spectroscopic studies of samarium modified lead zirconate titanate ceramics

The polycrystalline samples of Pb_1−xSm_x(Zr_0.60Ti_0.40)_1−x/4O₃ (PSZT) where x=0.00, 0.03, 0.06 and 0.09 were prepared by a high-temperature solid-state reaction technique. The preliminary structural analysis using X-ray diffraction (XRD) data collected at room temperature has confirmed the formation of single-phase compounds in tetragonal crystal system. The morphological study of each sample using scanning electron microscope (SEM) has revealed that the grains are uniformly distributed through out the surfaces of the samples. Using complex impedance spectroscopy (CIS) technique, the electrical impedance and modulus properties of the materials were studied in a wide range of temperatures at different frequencies. The impedance analysis indicates the presence of bulk resistive contributions in the materials which is found to decrease on increasing temperature. The nature of variation of resistances with temperature suggests a typical negative temperature coefficient of resistance (NTCR) type behavior of the materials. The complex modulus plots clearly exhibits the presence of grain boundaries along with the bulk contributions in the PSZT materials. The presence of non-Debye type of relaxation has been confirmed by the complex impedance analysis. The variation of dc conductivity (bulk) with temperature demonstrates that the compounds exhibit Arrhenius type of electrical conductivity.     

Utilizing broadband X‐rays in a Bragg coherent X‐ray diffraction imaging experiment

A method is presented to simplify Bragg coherent X‐ray diffraction imaging studies of complex heterogeneous crystalline materials with a two‐stage screening/imaging process that utilizes polychromatic and monochromatic coherent X‐rays and is compatible with in situ sample environments. Coherent white‐beam diffraction is used to identify an individual crystal particle or grain that displays desired properties within a larger population. A three‐dimensional reciprocal‐space map suitable for diffraction imaging is then measured for the Bragg peak of interest using a monochromatic beam energy scan that requires no sample motion, thus simplifying in situ chamber design. This approach was demonstrated with Au nanoparticles and will enable, for example, individual grains in a polycrystalline material of specific orientation to be selected, then imaged in three dimensions while under load.     

Modelling of diffusion and conductivity relaxation of oxide ceramics

A two-dimensional square grain model has been applied to simulate simultaneously the diffusion process and relaxation of the dc conduction of polycrystalline oxide materials due to a sudden change of the oxygen partial pressure of the surrounding gas phase. The numerical calculations are performed by employing the finite element approach. The grains are squares of equal side length (average grain size) and the grain boundaries may consist of thin slabs of uniform thickness. An additional (space charge) layer adjacent to the grain boundary cores (thin slabs) either blocking (depletion layer) or highly conductive for electronic charge carriers may surround the grains. The electronic transport number of the mixed ionic-electronic conducting oxide ceramics may be close to unity (predominant electronic conduction). If the chemical diffusion coefficient of the neutral mobile component (oxygen) of the grain boundary core regions is assumed to be higher by many orders of magnitude than that in the bulk, the simulated relaxation curves for mass transport (diffusion) and dc conduction can deviate remarkably from each other. Deviations between the relaxation of mass transport and dc conduction are found in the case of considerably different electronic conductivities of grain boundary core regions, space charge layers, and bulk. On the contrary, the relaxation curves of mass transport and electronic conductivity are in perfect coincidence, when either effective medium diffusion occurs or the effective conductivity is unaffected by the individual conductivities of core regions and possible space charge layers, i.e. the grain boundary resistivity is negligible.     

多晶金刚石烧结中晶粒表面石墨化的实验研究

 利用X射线微区衍射，X射线光电子谱（XPS）及激光拉曼光谱等分析，对高温高压下以金刚石微粉为原料的多晶金刚石烧结过程中，金刚石晶粒表面石墨化现象进行了考察。结果表明，虽经历了烧结过程，但在没有掺杂剂作用的区域内，没有发现金刚石晶粒表面的石墨化。晶粒表面石墨化的高峰期，处于掺杂剂刚开始液化，但尚未饱和充填金刚石晶粒间空隙的烧结初期阶段，随着液相掺杂剂的饱和充填作用，金刚石晶粒表面的石墨消失，并最终完成多晶金刚石的烧结。     

多晶银纳米线拉伸变形的分子动力学模拟研究

纳米尺度金属Ag以其独特的导电和导热性,广泛应用于微电子、光电子学、催化等领域,特别是在纳米微电极和纳米器件方面的应用.本文采用分子动力学方法模拟了不同晶粒尺寸下多晶银纳米线的拉伸变形行为,详细分析了晶粒尺寸对多晶银纳米线弹性模量、屈服强度、塑性变形机理的影响.发现当晶粒尺寸小于13.49 nm时,多晶Ag纳米线呈现软化现象,出现反Hall-Petch关系,此时的塑性变形机理主要以晶界滑移、晶粒转动为主,变形后期形成五重孪晶;当晶粒尺寸大于13.49 nm时,塑性变形以位错滑移为主,变形后期产生大量的孪晶组织.     

Magneto-transport and ferromagnetic resonance studies of polycrystalline La0.6Pb0.4MnO3 thin films

To understand the nature of grain boundaries in polycrystalline materials, magneto-transport and ferromagnetic resonance measurement have been performed in polycrystalline La_0.6Pb_0.4MnO₃ (LPMO) thin films prepared by pulsed laser deposition. Films are found to undergo a semiconductor to metal transition at 230 K and re-enter into the semiconducting state below 130 K. Microwave absorption measurements carried out as function of applied field show two components of resonant absorption signal. First component is in accordance with ferromagnetic transition of grains at Curie temperature and the second component shows antiferromagnetic transition of grain boundaries at 160 K. An additional non-resonant absorption signal centered at zero field has also been observed that supports transition from conducting to insulating grain boundaries at ∼160 K. Further, temperature dependence of resistance in semiconducting state at low temperatures is in accordance with coulomb blockade model indicating insulating nature of AFM grain boundaries.     

Synthesis, characterization of chemically deposited indium selenide thin films at room temperature

Polycrystalline In₂Se₃ semiconducting thin films were prepared by using relatively simple chemical bath deposition method at room temperature by the reaction between indium chloride, tartaric acid, hydrazine hydrate and sodium selenosulphate in an aqueous alkaline medium. Various preparative conditions of thin film deposition are outlined. The as grown films were found to be transparent, uniform, well adherent and red in color. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, atomic absorption spectroscopy and energy dispersive atomic X-ray diffraction (EDAX). The XRD analysis of the film showed the presence of polycrystalline nature with hexagonal crystal structure. SEM study revels that the grains are homogenous, without cracks or pinholes and well covers the glass substrate. The optical absorption and electrical conductivity was measured. The direct optical band gap value for the films was found to be of the order of 2.35 eV at room temperature and have specific electrical conductivity of the order of 10⁻² (Ω cm)⁻¹ showing n-type conduction mechanism. The utility of the adapted technique is discussed from the view-point of applications considering the optoelectric and structural data.     

Micromechanical model of deformation-induced surface roughening in polycrystalline materials

A micromechanical model has been developed to describe deformation-induced surface roughening in polycrystalline materials. The three-dimensional polycrystalline structure is taken into account in an explicit form with regard to the crystallographic orientation of grains to simulate the micro- and mesoscale deformation processes. Constitutive relations for describing the grain response are derived on the basis of crystal plasticity theory that accounts for the anisotropy of elastic-plastic properties governed by the crystal lattice structure. The micromechanical model is used to numerically study surface roughening in microvolumes of polycrystalline aluminum and titanium under uniaxial tensile deformation. Two characteristic roughness scales are distinguished in the both cases. At the microscale, normal displacements relative to the free surface are caused by the formation of dislocation steps in grains emerging on the surface and by the displacement of neighboring grains relative to each other. Microscale roughness is more pronounced in titanium, which is due to the high level of elastic-plastic anisotropy typical of hcp crystals. The mesoscale roughness includes undulations and cluster structures formed with the involvement of groups of grains. The roughness is quantitatively evaluated using a dimensionless parameter, called the degree of roughness, which reflects the degree of surface shape deviation from a plane. An exponential dependence of the roughness degree on the strain degree is obtained.     

Dielectric and impedance properties of Bi(Zn2/3V1/3)O3 electronic material

A polycrystalline vanadium doped lead free dielectric material of Bi(Zn_2/3V_1/3)O₃ (BZV) has been prepared using a standard high-temperature solid state reaction technique. Its temperature and frequency dependent capacitive, conductive and resistive characteristics are outlined though experimental investigation. The formation of single phase compound of BZV material with orthorhombic crystal symmetry is identified through X-ray diffraction data analysis, and the homogeneous distribution of grains are realized through scanning electron micrograph. The acquaintance of frequency–temperature dependent electrical parameters with the obtained micrograph provides the experimental evidence of contributions of grain as well as grain boundary in its capacitive and resistive characteristics. The negative temperature coefficient of resistance behaviour of the material is revealed from impedance characteristic, and non-Debye type relaxation has been realized from the Nyquist plot. The charge carriers of this electronic compound have both long & short range order that has been validated from the complex modulus and impedance analysis. The prepared electronic material substantiate some important dielectric features which props up the material as promising component for electronic devices.     

Three-dimensional X-ray diffraction in the diamond anvil cell: application to stishovite

Three-dimensional X-ray diffraction can be used for characterizing the orientation, position, and strain tensor of single grains in a polycrystalline aggregate. Here, we show how the method is well suited for diamond anvil cell data with heterogeneous grain sizes, with an application to two samples of stishovite at 15 and 26 GPa. For each grain, we obtain a well-defined orientation matrix and cell parameters. Center of mass position can also be adjusted to the experimental data, with errors in the present experiment. Finally, strain tensors are adjusted for the individual grains. The stress distribution obtained is in agreement with expectations from the diamond anvil cell geometry and previous measurements of stishovite strength. Advantages and potential for improvement of the method are then discussed.     

Investigation of the domain structure of sintered Nd-Fe-B permanent magnets by Bitter-pattern method

The conventional Bitter-pattern technique and the colloid-scanning electron microscopy (colloid-SEM) method were used to study the domain structure of polycrystalline sintered Nd-Fe-B permanent magnets. In the thermally demagnetized state most of the grains are multidomain and the domain structures resemble those observed in bulk uniaxial crystals with strong magnetocrystalline anisotropy. Investigations of the magnetic microstructure during magnetizing cycle showed that the domain walls can easily be moved within the grains and that the magnetization reversal in sintered Nd-Fe-B magnets occurs predominantly by the nucleation and expansion of reverse domains at structural imperfections near the grain boundaries. It is also shown that the colloid-SEM method is more surface sensitive and reveals the domain structure with better resolution than the conventional Bitter technique. Thanks to the application of digital image processing systems, clear and high contrast domain images were obtained. The work was supported by the Lódź University within Research Grant 505/694 (2004).