Electro-and magnetotransport properties of disordered carbon phases synthesized from C60 fullerite at moderate pressures P syn < 1.5 GPa

Charge transport has been investigated in new disordered carbon phases synthesized from C₆₀ at “moderate” pressures P _syn < 1.5 GPa and high temperatures T _syn > 1000 K. Under these synthesis conditions, a structure transition from nanographite phases to new amorphous modifications is induced when pressure decreases. The transition from hopping transport to diffusive electron transport typical of “dirty” metals is observed in a number of samples synthesized at various pressures and temperatures in the indicated ranges. The magnetotransport properties of these phases under intermediate synthesis conditions can be described by diffusive electron transport.     

Phase transformations in amorphous fullerite C60 under high pressure and high temperature

First phase transformations of amorphous fullerite C₆₀ at high temperatures (up to 1800 K) and high pressures (up to 8 GPa) have been investigated and compared with the previous studies on the crystalline fullerite. The study was conducted using neutron diffraction and Raman spectroscopy. The amorphous fullerite was obtained by ball-milling. We have shown that under thermobaric treatment no crystallization of amorphous fullerite into С₆₀ molecular modification is observed, and it transforms into amorphous-like or crystalline graphite. A kinetic diagram of phase transformation of amorphous fullerite in temperature–pressure coordinates was constructed for the first time. Unlike in crystalline fullerite, no crystalline polymerized phases were formed under thermobaric treatment on amorphous fullerite. We found that amorphous fullerite turned out to be less resistant to thermobaric treatment, and amorphous-like or crystalline graphite were formed at lower temperatures than in crystalline fullerite.     

不同退火温度下晶化硅薄膜的电学输运性质

采用等离子体化学气相沉积技术制备氢化非晶硅薄膜,经过不同温度下的热退火处理,使薄膜由非晶结构向晶化结构转变,得到含有纳米晶粒的晶化硅薄膜.在晶化过程中,采用Raman技术对样品的结构进行表征.通过变温电导率的测试,对薄膜的电学输运性质进行了分析.研究结果表明：退火温度为700 ℃时,样品中开始有纳米晶形成,随着退火温度的增加,样品的晶化比增大,在1000 ℃时,薄膜的晶化比达到90%以上.在700 ℃退火时,薄膜中晶化成分较低,载流子的传输特性主要受到与硅悬挂键有关的缺陷态影响,表现为带尾定域态的跳跃电导 关键词： 氢化非晶硅 退火 纳米硅 电输运     

Specific heat and thermal conductivity of low-stress amorphous Si-N membranes

We present values of the specific heat and thermal conductivity from 3-300 K of low-stress amorphous silicon-nitride thin-films determined from measurements using a membrane-based microcalorimeter. The thermal conductivity has a temperature dependence often seen in amorphous solids, but the magnitude is large, with the expected plateau occurring at significantly higher temperatures than seen in other amorphous systems. Specific heat measurements show that the expected ‘peak’ in the vibrational spectrum also occurs at relatively high temperatures. The estimated phonon mean-free-path at 300 K is ≈5 Å, comparable to the inter-atomic spacing, as seen in other amorphous solids. Below ≈ 20 K the mean free path is comparable to or exceeds the thickness of the membrane, indicating that surface scattering dominates the thermal transport. This surface scattering is found to be either specular or diffuse, depending on details of the membrane processing, which affects both the thermal conductivity and specific heat below 10 K.     

Creep strain recovery of Fe-Ni-B amorphous metallic ribbon

Creep strain recovery and structural relaxation of the amorphous metallic glass Fe₄₀Ni₄₁B₁₉ after longtime loading at different annealing temperatures below the glass transition temperature have been studied using anisothermal differential scanning calorimetry (DSC) and dilatometry (TMA). It has been demonstrated that structural relaxation effects depend on the stress-annealing temperature of the amorphous ribbon. The structural relaxation states of the amorphous ribbon annealed at different temperatures under and without applied stress have been compared. The activation energy spectra were calculated from the anisothermal dilatometric measurements using the modern method based on the Fourier transformation technique. The influence of the annealing temperature on the shape of creep strain recovery spectra has been analyzed.      

Effect of annealing on the structure of quenched isotactic polypropylene

Smectic isotactic polypropylene films were annealed at different temperatures for different times. Density, wide-angle x-ray scattering (WAXS), differential scanning calorimetry, and the transport properties of methylene chloride vapor were used to follow the transformation of the smectic phase into the monoclinic phase. The results of WAXS diffractograms indicate different saturation levels of crystallization with the maximum order attained progressively more quickly at higher temperatures. Density measurements, however, show that, after a first saturation level, a further increase of this parameter occurs after long annealing times; and this second step is lower, the higher the temperature. Sorption and diffusion, analyzed to investigate the amorphous component, show that annealing the smectic phase at 50°C results in a decrease of amorphous fraction without transformation of the smectic phase. The transition from smectic to monoclinic phase occurs at temperatures higher than 60°C. During the annealing at 80°C there is first a decrease of amorphous fraction corresponding to the first saturation level of the density, and then a further decrease of amorphouse phase corresponding to the final increase of density after 24 hr of annealing.     

Quantum emission efficiency of nanocrystalline and amorphous Si quantum dots

The paper presents the comparison of emission efficiencies for crystalline Si quantum dots (QDs) and amorphous Si nanoclusters (QDs) embedded in hydrogenated amorphous (a-Si:H) films grown by the hot wire-CVD method (HW-CVD) at the variation of technological parameters. The correlations between the intensities of different PL bands and the volumes of Si nanocrystals (nc-Si:H) and/or an amorphous (a-Si:H) phase have been revealed using X-ray diffraction (XRD) and photoluminescence (PL) methods. These correlations permit to discuss the PL mechanisms in a-Si:H films with embedded nc-Si QDs. The QD parameters of nc-Si:H and a-Si:H QDs have been estimated from PL results and have been compared (for nc-Si QDs) with the parameters obtained by the XRD method. Using PL and XRD results the relations between quantum emission efficiencies for crystalline (η_cr) and amorphous (η_am) QDs have been estimated and discussed for all studied QD samples. It is revealed that a-Si:H films prepared by HW-CVD with the variation of wire temperatures are characterized by better passivation of nonradiative recombination centers in comparison with the films prepared at the variation of substrate temperatures or oxygen flows.     

Electrical transport of spin-polarized carriers in disordered ultrathin films

Slow, nonexponential relaxation of electrical transport accompanied by memory effects has been induced in quench-condensed ultrathin amorphous Bi films by the application of a parallel magnetic field. This behavior, which is very similar to space-charge limited current flow, is found in extremely thin films well on the insulating side of the thickness-tuned superconductor-insulator transition. It may be the signature of a collective state that forms when the carriers are spin polarized at low temperatures and in high magnetic fields.     

Elastic properties of amorphous water at low temperatures

We have studied the elastic properties of thin films of amorphous water (a-H₂O), prepared at 4.4 K by quench condensation. At low temperatures acoustic anomalies are observed, which are characteristic for amorphous solids. Annealing, even at moderate temperatures, reduces the number of tunneling states. A strong increase of the acoustic absorption is observed in the vicinity of the annealing temperatures. This indicates that structural rearrangements are possible in quenched condensed films at temperatures well below the glass transition.     

A composite model of the plastic flow of amorphous covalent materials

A model based on the data available in the literature on the computer simulation of amorphous silicon has been proposed for describing the specific features of the plastic flow of amorphous covalent materials. The mechanism of plastic deformation involves homogeneous nucleation and growth of inclusions of a liquidlike phase under external shear stress. Such inclusions experience plastic shear, which is modeled by glide dislocation loops. The energy changes associated with the nucleation of these inclusions at room and increased temperatures have been calculated. The critical stress has been found, at which the barrierless nucleation of inclusions becomes possible. It has been shown that this stress decreases with an increase in temperature. According to the calculations, the heterogeneous (homogeneous) plastic flow of an amorphous material should be expected at relatively low (high) temperatures. Above the critical stress, the homogeneous flow is gradually replaced by the heterogeneous flow.     

Transport properties of electrons in energy band tails

Transport properties of electrons in energy band tails of disordered semiconductors are studied experimentally using a material system in which (i) the width and shape of the band-tail are approximately known and (ii) the Fermi energy is controllable. The material is heavily-doped, closely-compensated, crystalline n-GaAs whose compensation ratio can be made arbitrarily close to unity by the use of two techniques that are described in detail. This control of the Fermi level through compensation permits the measurement of the transport properties of electrons at various energies in the band-tail.

Using band tails having a width of ～50 meV, measurements have been made of the temperature dependence of the d.c. conductivity and Hall coefficient, the frequency dependence of the a.c. conductivity and the electric field dependence of the d.c. conductivity (the last two at low temperatures).

The evidence demonstrates the progressively greater localization of states deeper in the tails. No sign is found of a sharp mobility edge. There is a number of close similarities to the properties of amorphous semiconductors but some significant differences. The frequency dependence of the a.c. conductivity at low temperatures is essentially identical with that of amorphous semiconductors in accord with the general interpretation that conductivity at low temperatures takes place by electron hopping among localized states near the Fermi energy. The detailed temperature dependence of the d.c. conductivity at low temperatures is log σ=σ ₀ exp [?(T ₀/T)^1/2], thus disagreeing with a theoretical expectation that the exponent for low temperature hopping conduction should be 1/4. At low temperatures, the electric field dependence of the conductivity shows a variation as σ～exp (bF/T) over a considerable range extending down to field strengths close to 1 V/cm. This closely resembles recent observations on amorphous semiconductors but the range of field strengths here is lower by several orders of magnitude.     

Effect of quenching temperature on the structure of isotactic polypropylene films

Films of isotactic polypropylene (iPP) were quenched at different temperatures. Wide-angle x-ray diffractograms for these samples show the presence of the smectic form of iPP at low quenching temperatures and the appearance of mono-clinic form on increasing the thickness of the substrate and the quenching temperature. A quenching temperature higher than 80°C produces only the monoclinic form of iPP, whereas at intermediate temperatures we obtain three-phase amorphous-smectic-crystalline systems. Except for the two-phase amorphous-crystalline system obtained at high temperatures, density values alone do not allow us to obtain the three-phase fractions. We studied the transport properties, sorption, and diffusion of CH₂Cl₂ vapor in these systems to investigate the thermodynamic state of the amorphous component. The behavior of the amorphous component with respect to the diffusion of CH₂Cl₂, which proved identical in all the samples, led us to conclude that at low penetrant activity the smectic phase is not permeable, and therefore we were able to obtain the amorphous fraction in each sample. This value together with density values allowed the determination of the complete composition in terms of three phases for every sample. The crystalline fraction expressed as % mono-clinic form is very well correlated with the reciprocal of the half-height broadening of the (110) diffraction peak at about 2e = 14°.     

Dispersive transport and trap saturation in doped hydrogenated amorphous silicon

Transient photocurrent measurements in arsenic-doped hydrogenated amorphous silicon show trap-controlled dispersive transport interrupted by trap saturation in an exponential band tail. A nonlinear temperature dependence of the dispersion parameter α is observed at low temperatures, indicating that the density of shallow traps (～0.1 eV below the mobility edge) is only weakly energy dependent.     

Origin of nonlinear transport across the magnetically induced superconductor-metal-insulator transition in two dimensions

We have studied the effect of perpendicular magnetic fields and temperatures on nonlinear electronic transport in amorphous Ta superconducting thin films. The films exhibit a magnetic field-induced metallic behavior intervening the superconductor-insulator transition in the zero temperature limit. We show that the phase-identifying nonlinear transport in the superconducting and metallic phases arises from an intrinsic origin, not from an electron heating effect. The nonlinear transport is found to accompany an extraordinarily long voltage response time.     

The microscope structures of amorphous phosphorus

The microscopic structures of thin films of amorphous phosphorus have been investigated by Raman scattering. The thin films were deposited by vacuum evaporation at substrate temperatures T_s between 300 and 510K. The Raman spectra of the films were compared with vibrational spectra of different allotropes of phosphorus: orthohombic black P, bulk amorphous red P and Hittorf's P. The structure of the films with T_s ? 300K is built up of double layers similar to those of orthorhombic black P. For T_s?510K the thin films have a structure similar to bulk amorphous red P based on the pentagonal tubes of Hittorf's P. A continuous transformation from one structure to the other is indicated by the Raman spectra of the films at intermediate T_s. The Raman measurements support early determinations of the structures of amorphous phosphorus inferred from radial distribution functions.     

非晶态超导体的转变温度和霍耳系数之间的经验关系

在大量的非过渡金属和合金中,非晶态的超导转变温度T_c与其霍耳系数R_H以及与相应的液态金属霍耳系数R_HL之间存在着一个经验关系,并由这个经验关系可以判断,到目前为止已报道过的无序金属Be,Tl,Zn,Pb和Hg还没有真正做到非晶态。 关键词：     

A microscopy study of the effect of heat treatment on the structure and properties of anodised TiO2 nanotubes

Titanium oxide (TiO₂) nanotubes prepared by anodisation of titanium in an aqueous electrolyte and glycerol have been heat treated in the temperature range 200-600 °C to control the conversion of the amorphous structure to nano-crystalline anatase and rutile. The phase changes have been monitored are observed at lower temperatures (100 °C or more) than previously reported. The sensitivity of the different techniques, each of which depends on the size of the crystalline phase, can explain the discrepancy with previous results. Transmission electron microscopy (TEM) has shown the phase changes which have occurred and which have been reported in an earlier publication; phenomena such as the collapse of the structures are explained. The TEM results are consistent with the Raman and XRD data, apart from the transformation temperatures, and also shed light on the nature of an amorphous phase found on the surfaces of the nanotubes.     

Influence of cumulenic chains on the vibrational and electronic properties of s p-s p2 amorphous carbon

We report the production and characterization of a form of amorphous carbon with s p-s p(2) hybridization (atomic fraction of sp hybridized species > or =20%) where the predominant sp bonding appears to be (=C=C=)(n) cumulene. Vibrational and electronic properties have been studied by in situ Raman spectroscopy and electrical conductivity measurements. Cumulenic chains are substantially stable in high vacuum conditions for temperatures lower than 250 K and they influence the electrical transport properties of the s p-s p(2) carbon through a self-doping mechanism by pinning the Fermi level closer to one of the mobility gap edges. Upon heating above 250 K the cumulenic species decay to form graphitic nanodomains embedded in the s p(2) amorphous matrix thus reducing the activation energy of the material. This is the first example of a pure carbon system where the s p hybridization influences bulk properties.     

聚苯胺的低温比热研究

测量了聚苯胺的中间氧化态以及掺杂盐酸后的样品在1.8至45K范围内的比热，比热数据可以用非晶态和晶态两部分的贡献来解释.利用声子-分形子模型成功地解释了低温下比热高于德拜理论趋势的反常现象.从本文的实验数据来看，没有观察到电子对比热容的贡献. 关键词： 低温比热 聚苯胺 分形     

Ultrasonic investigation of amorphous carbon in various frequencies at low temperatures from 2.1 to 300 K

Using pulse echo overlap measurement, the elastic behavior of amorphous carbon has been studied at ambient and low temperatures. The smaller ratio B/G of the bulk modulus to shear modulus and smaller Poisson's ratio σ at room temperature indicate that there is an intrinsic stiffening of transverse acoustic phonons in the amorphous carbon. The acoustic velocity and attenuation for longitudinal modes have been measured between 2.1 and 300 K at three frequencies of 7, 21 and 35 MHz, respectively. Their frequency and temperature dependence are observed. The elastic constant C₁₁ increases with decreasing temperature and show enhanced stiffening at low temperatures. In the 130-220 K region, the abnormal change and effect of longitudinal velocity and attenuation with temperature and frequency, and a phase transition associated with structure relaxations are discussed.