Concentration,mobility and 1/f noise of electrons and holes in thin bismuth films

We have studied bismuth films evaporated to a thickness t in the range of 300– 14000 Å. The resistivity ?, Hall coefficient R_H, magnetoresistance Δ?/(?B²) and the 1/f noise were measured as functions of thickness at room temperature. With the help of ?, R_H, Δ?/(?B²) and literature data for the thermoelectric power P of bismuth, the electron and hole concentrations n and p, and the mobilities μ_n and μ_p were calculated as functions of the thickness. The results are compared with values from the literature. The sensitivity of calculated values of n, p, μ_n and μ_p to measurement errors have been investigated. The magnitude of the 1/f noise can be characterized by the parameter α. It has been found experimentally that α decreases with thickness.     

Cyclotron resonance of electrons and holes in graphene monolayers

We report studies of cyclotron resonance in monolayer graphene. Cyclotron resonances are detected by observing changes in the photoconductive response of the sample. An electron velocity at the Dirac point of 1.093 x 10(6) m s(-1) is obtained, which is the fastest velocity recorded for all known carbon materials. In addition, a significant asymmetry exists between band structure for electrons and holes, which gives rise to a 5% difference between the velocities at energies of 125 meV away from the Dirac point.     

The avalanche multiplication of electrons and holes in cadmium telluride

It is experimentally demonstrated that the process of impact ionization in cadmium telluride crystals is stimulated by holes. The ratio of the impact ionization coefficients of holes (α_p) and electrons (α_n) amounts to α_p/α_n ≈ 30–40.     

Magnetic multi-lens focusing optical system

A magnetic focusing system called B-channel is introduced. Three methods of ion optical calculation are presented and a comparison with experimental results is shown. The properties of B-channel are discussed in comparison with a classical solenoid.     

Transitions of electrons and holes drive diffusion in crystals,glasses and melts

Diffusion of atoms or molecules (generally: particles) is driven by differences and gradients of the chemical potential of the particles in their accessible space. If the difference of the chemical potential is due to differences of concentrations alone, one arrives at the diffusion equations of Fick. The diffusion coefficients are described in known models by vibrations of atoms in condensed matter which cause the exchange of preferentially neutral particles with neighbouring particles, impurities, interstitial places and vacancies near or on surfaces, grain boundaries, dislocation lines and in the homogeneous bulk. The rates of electronic transitions, however, increase also in melts and solids of chemically bonded particles with increasing temperature. Such transitions cause large fluctuating deviations of the local energy, the charge distribution and the local chemical and electrical potentials. The fluctuating deviations interact with the core ions and drive particles to interchange. This mechanism that supplements the known mechanisms of diffusion has not yet found adequate attention in the literature until now. Foundations, experimental results, evidence and consequences for diffusion are discussed.     

Magnetic properties of bismuth at high fields

The magnetizationM of bismuth is calculated as a function of the magnetic fieldH atT=0 for binary, bisectrix, and trigonal orientations in the range of highH, from beyond the last de Haas-van Alphen oscillation up to 400 kG. The method of calculation is to sum the energies of the occupied energy levels and then to differentiate with respect toH; the energy levels are based on the ellipsoidal but nonparaboloidal model of the band structure. In qualitative and almost quantitative agreement with experiment, considerable departures M are found from the proportionality betweenM andH which holds in the limit of lowH, and the results are presented in the form of tables and graphs of M as a function ofH; the field dependence of the Fermi energy is also presented. An important contribution to the nonproportionality M at highH proves to come from the electrons in the filled band and this contribution is calculated by a special method involving a cutoff parameter; the value chosen for this cutoff parameter affects only the proportional part ofM, but not M.Supported in part by the U.S. National Science Foundation.     

Modeling of columnar recombination for high-energy photon generated electrons and holes in amorphous selenium

An analytical model is developed to study the mechanisms of X-ray generated free Electron–hole pair (EHP) creation energy in amorphous selenium (a-Se) at high electric fields. The model is presented to show the electric field and temperature dependence of the charge extraction yield limited by the columnar recombination for the materials that have widely unequal drift mobility for electrons and holes, such as a-Se. The model is compared with Jaffe’s columnar recombination model with widely varying field and temperature. In addition, the free EHP creation energy is calculated by incorporating the initial charge extraction yield and the charge collection efficacy of the free carriers. Also, the results of this model are compared with the recently published experimental results on EHP creation energy. The analysis of the results confirm that the proposed model gives the best possible explanation to the columnar recombination mechanisms in a-Se and the free EHP creation mechanisms at diagnostic X-ray exposures can be described by the columnar recombination.     

Confinement of holes and electrons in blue organic light-emitting diodes with additional red emissive layers

We used various emissive layer (EML) structures with ultrathin red EMLs to enhance the charge carrier balance and carrier recombination rate in blue PHOLED devices. These EML materials have different energy gaps between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. The ultrathin red EMLs, which were inserted in between the blue EMLs, effectively confined the charge carriers in EML, and increased the carrier recombination rate. The thickness of the individual EML was optimized, under 30 nm of the total thickness of EML. The blue PHOLEDs with ultrathin red EMLs achieved a luminous efficiency of 19.24 cd/A, which was 28.7% higher than those without ultrathin red EMLs, and the maximum external quantum efficiency was 11.81% at 500 cd/m².     

Strain-renormalized energy spectra of electrons and holes in InAs quantum dots in the InAs/GaAs heterosystem

Analytical expressions describing the energy spectrum of electrons and holes are obtained for a quantum dot (QD) occurring in a self-consistent strain field created by an array of coherently stressed QDs. A method of taking into account the lattice mismatch at the QD-matrix interface is proposed that allows for the dependence of the mismatch parameter on the QD size and the matrix layer thickness. It is shown that the internal elastic strain arising at the QD-matrix interface influences the energy spectrum of electrons more significantly than the spectrum of holes.     

Magnetic, magnetocapacitance and dielectric properties of Cr doped bismuth ferrite nanoceramics

BiFe_1−xCr_xO₃ (x = 0, 0.04, 0.06 and 0.08) nanoceramics were prepared by sol-gel method. Nanoceramics were calcined at 450 °C. Calcined powders were leached in diluted nitric acid to get single phase. TEM analysis shows the particle size to be ∼80 nm. Thermogravimetric analysis of as prepared powder indicates that the single phase is formed at around 450 °C. Magnetization was found to increase as the concentration of Cr was increased. Dielectric constant and dielectric loss were found to decrease with increase in frequency for all the compositions. Magnetocapacitance was found to increase with magnetic field. For BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, the change of dielectric constant induced by magnetic field may be well approximated by Δ?/? = γM², here, γ (magnetoelectric interaction) is small and positive. A linear fit gave the value of γ of ∼18.4 × 10⁻², 12.3 × 10⁻² and 3.3 × 10⁻² for BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, respectively.     

Local investigation of the optical properties of subwavelength rectangular holes with a focused beam of electrons

The optical properties of rectangular subwavelength holes in a gold film are investigated using the light generated when a focused beam of electrons impinges on the sample close to the hole. Using this technique, multi-spectral maps of the holes are obtained with a resolution beyond the optical diffraction limit. The results show the influence of hole shape on the spectrum of locally scattered light. Rectangular holes of varying shape and size are investigated, and the spatial distribution of the polarization of the observed light is measured. The influence of neighbouring holes is investigated by measuring small clusters of holes.     

Magnetic and optical properties of multiferroic bismuth ferrite nanoparticles by tartaric acid-assisted sol-gel strategy

Pure BiFeO₃ nanoparticles have been successfully synthesized through the tartaric acid-assisted sol-gel method at relatively low temperature. The as-prepared nanoparticles were characterized by a variety of techniques. The success in preparing pure BiFeO₃ may be attributed to the formation of heterometallic polynuclear complexes in the tartaric acid system. The ferroelectric phase transition (T_C = 851 °C) was determined, revealing the ferroelectric nature of the as-prepared BiFeO₃ nanoparticles. The result of magnetic measurement indicates the weak ferromagnetic order of BiFeO₃ nanoparticles at room temperature, which may be ascribed to the size confinement effect. The observed strong absorption in the UV region will enable BiFeO₃ nanoparticles to be potentially used as promising photocatalytic decomposition material.     

The precision polishing of bismuth silicate and bismuth germanate

A recipe is given for the polishing of precise surfaces oncrystals of bismuth silicate (BSO) and bismuth germanate(BGO). Using this recipe, crystals having surface figure betterthan 1/10 wave, roughness of 20-50 ? rms, and laserquality parallelism (10 arcsec or better) were obtained withabout 1-2 hours effort.     

Ambipolar two-dimensional bismuth nanostructures in junction with bismuth oxychloride

Despite the unique properties of bismuth(Bi),there is a lack of two-dimensional(2D)heterostructures between Bi and other functional 2D materials.Here,a coherent strategy is reported to simultaneously synthesize rhombohedral phase Bi nanoflakes and bismuth oxychloride(BiOCI)nanosheets.The delicate balance between several reactions is mediated mainly for the reduction and chlorination in the chemical vapor transport(CVT)process.The Bi-BiOCI lateral heterostructures have been constructed via the coalescence of the two different 2D nanostructures.The characteristics of ambipolar conducting Bi and insulator-like BiOCI are elaborated by scanning microwave impedance microscopy(sMIM).This work demonstrates a way to construct a 2D Bi nanostructure in junction with its oxyhalide.     

Determination of the purity of bismuth and bismuth oxide by atomic absorption spectrometry

To enable high-speed analyses in the preparation of high-purity bismuth and bismuth oxide, we have developed an atomic absorption technique which ensures Ag, Cr, Cu, Fe, Mg, Mn, Ni, Pb, and Te detection limits in the range 2 × 10⁻⁷ to 2 × 10⁻⁵ wt %. The technique was used to assess the purity of bismuth and bismuth oxide in metal refining and oxidation steps.