Bismuth Single Crystals Research Articles

Optical properties of single crystal Bi4Ge3O12 from the infrared to ultraviolet

Optical properties in the form of the complex dielectric function spectra (ε = ε1 + iε2) of commercially produced single crystal bismuth germanate (Bi4Ge3O12) have been studied using spectroscopic ellipsometry from 0.033 to 6.478 eV. Transmission measurements have also been performed in order to more accurately obtain low values of the absorption coefficient close to the band edge. Critical point parameters have been determined by analyzing the interband transitions. Infrared extended measurements yield vibrational modes corresponding to chemical bonding environments and the lattice structure. The overall analysis yields ε from 0.033 to 6.478 eV and provides information about the crystal such as the energy of the band gap at 4.16 ± 0.01 eV; interband transitions at 4.49, 4.75, 4.81, 5.08, 5.59, and 6.08 eV; transverse optical phonon modes at 283, 364, 395, 447, 702, 729, and 778 cm−1 and longitudinal optical phonon modes at 292, 386, 445, 458, 710, 779, and 817 cm−1.

Evidence of topological two-dimensional metallic surface states in thin bismuth nanoribbons.

Understanding the exotic quantum phenomena in bulk bismuth beyond its ultraquantum limit remains controversial and gives rise to renewed interest. The focus of the issues is whether these quantum properties have a conventional bulk nature or just the surface effect due to the significant spin-orbital interaction and in relation to the Bi-based topological insulators. Here, we present angular-dependent magnetoresistance (AMR) measurements on single-crystal bismuth nanoribbons of different thicknesses with magnetic fields up to 31 T. In thin nanoribbons with thickness of ∼40 nm, a two-fold rational symmetry of the low field AMR spectra and two sets of 1/2-shifted (i.e., γ = 1/2) Shubnikov-de Haas (SdH) quantum oscillations with exact two- dimensional (2D) character were obtained. However, when the thickness of the ribbon increases, a 3D bulk-like SdH oscillations with γ = 0 and a four-fold rotational symmetry of the AMR spectra appear. These results provided unambiguous transport evidence of the topological 2D metallic surface states in thinner nanoribbons with an insulating bulk. Our observations provide a promising pathway to understand the quantum phenomena in Bi arising from the surface states.

Study on lattice vibrational properties and Raman spectra of Bi2Te3 based on density-functional perturbation theory

We present a variational density-functional perturbation theory (DFPT) to investigate the lattice dynamics and vibrational properties of single crystal bismuth telluride material. The phonon dispersion curves and phonon density of states (DOS) of the material were obtained. The phonon dispersions are divided into two fields by a phonon gap. In the lower field, atomic vibrations of both Bi and Te contribute to the DOS. In the higher field, most contributions come from Te atoms. The calculated Born effective charges and dielectric constants reveal a great anisotropy in the crystal. The largest Born effective charge generates a significant dynamic charge transferring along the c axis. By DFPT calculation, the greatest LO—TO splitting takes place in the infrared phonon modes and reaches 1.7 THz in the Brillouin zone center. The Raman spectra and peaks corresponding to respective atomic vibration modes were found to be in good agreement with the experimental data.

Approach for extracting complex dielectric function spectra in weakly-absorbing regions

Ellipsometric spectra have been analyzed using a procedure whereby the complex dielectric function (ε=ε1+iε2) is parameterized using different, physically realistic models in isolated regions of a single spectrum while layer thicknesses are kept common. The isolated spectral ranges correspond to regions where the material of interest is either non-absorbing or heavily absorbing and physically realistic models describing ε are available. Common structural parameters (thicknesses) obtained by this divided spectral range approach are used to extract ε by numerical inversion over the full spectral range. The divided spectral range analysis has been applied to study amorphous hydrogenated silicon, nanocrystalline zinc oxide, and epitaxial bismuth stannate thin films, as well as single crystal bismuth germanate. Layer thicknesses obtained from the divided range analysis are compared to those using continuous, but not necessarily physically supported, parameterizations of ε over the full spectral range. The divided spectral range analysis yields inverted spectra in ε which are free of discontinuities beyond noise present in experimental data and whose determination does not require any assumption to be made about the line shape of ε in weakly absorbing regions.

All-optical control and visualization of ultrafast two-dimensional atomic motions in a single crystal of bismuth

In a bulk solid, optical control of atomic motion provides a better understanding of its physical properties and functionalities. Such studies would benefit from active control and visualization of atomic motions in arbitrary directions, yet, so far, mostly only one-dimensional control has been shown. Here we demonstrate a novel method to optically control and visualize two-dimensional atomic motions in a bulk solid. We use a femtosecond laser pulse to coherently superpose two orthogonal atomic motions in crystalline bismuth. The relative amplitudes of those two motions are manipulated by modulating the intensity profile of the laser pulse, and these controlled motions are quantitatively visualized by density functional theory calculations. Our control-visualization scheme is based on the simple, robust and universal concept that in any physical system, two-dimensional particle motion is decomposed into two orthogonal one-dimensional motions, and thus it is applicable to a variety of condensed matter systems.

First-principle calculation and quasi-harmonic Debye model prediction for elastic and thermodynamic properties of Bi2Te3

The influence of temperature and pressure on the mechanical and thermodynamic properties of single crystal Bismuth Telluride material was investigated by the first-principle calculation and quasi-harmonic Debye model. The computation results indicated that the lattice constant in the c axis increases nonlinearly and the elastic constants Cij and bulk module B0 increase linearly with increasing the pressure. The Young modulus of the Bi2Te3 crystal in the x and y axes are 28.6% larger than that in the z axis. The Poisson’s ratios along the three axes range from 0.2752 to 0.3853. When the temperature is less than 150K, the lattice specific heat capacity of Bi2Te3 depends on the temperature. The thermal expansion coefficients of Bi2Te3 vary remarkably with the temperature and pressure. The chemical balance bonding of Bi2Te3 is predicated to be Bi2+0.15Te-0.14Te2-0.08 based on the density functional theory, revealing the evident natural bonding interaction between Bi–Te and Te–Te. Those calculations are in good agreement with previous experimental data.

Focused ion beam processing to fabricate ohmic contact electrodes on a bismuth nanowire for Hall measurements

Ohmic contact electrodes for four-wire resistance and Hall measurements were fabricated on an individual single-crystal bismuth nanowire encapsulated in a cylindrical quartz template. Focused ion beam processing was utilized to expose the side surfaces of the bismuth nanowire in the template, and carbon and tungsten electrodes were deposited on the bismuth nanowire in situ to achieve electrical contacts. The temperature dependence of the four-wire resistance was successfully measured for the bismuth nanowire, and a difference between the resistivities of the two-wire and four-wire methods was observed. It was concluded that the two-wire method was unsuitable for estimation of the resistivity due to the influence of contact resistance, even if the magnitude of the bismuth nanowire resistance was greater than the kilo-ohm order. Furthermore, Hall measurement of a 4-μm-diameter bismuth microwire was also performed as a trial, and the evaluated temperature dependence of the carrier mobility was in agreement with that for bulk bismuth, which indicates that the carrier mobility was successfully measured using this technique.PACS81.07.Gf

Time- and momentum-resolved probe of heat transport in photo-excited bismuth

We use time- and momentum-resolved x-ray scattering to study thermalization in a photo-excited thin single crystal bismuth film on sapphire. The time-resolved changes of the diffuse scattering show primarily a quasi-thermal phonon distribution that is established in ≲100ps and that follows the time-scale of thermal transport. Ultrafast melting measurements under high laser excitation show that epitaxial regrowth of the liquid phase occurs on the time-scale of thermal transport across the bismuth-sapphire interface.

Temperature Coefficient of Electrical Resistivity in Individual Single-Crystal Bismuth Nanowires

The temperature dependence of electrical resistivity and the crystal orientation of single-crystal bismuth nanowires each encapsulated in a quartz template were studied. The electrical resistivities of four bismuth nanowires with diameter of 356 nm, 376 nm, 622 nm, and 633 nm were measured in the temperature range from 4.2 K to 300 K. The temperature coefficient of resistivity of 376-nm- and 633-nm-diameter nanowires was negative in the low-temperature region. On the other hand, a positive temperature coefficient appeared in 356-nm- and 622-nm-diameter nanowires. The positive temperature coefficient was not explained by carrier mean free path limitation. Thus, the crystal orientation of the bismuth nanowires was observed by x-ray diffraction measurements to study the relationship between electrical resistivity and crystal orientation. It was confirmed that the temperature dependence of electrical resistivity strongly depended on the crystal orientation of the bismuth nanowire.

Free-carrier relaxation and lattice heating in photoexcited bismuth

We report ultrafast surface pump and interface probe experiments on photoexcited carrier transport across single crystal bismuth films on sapphire. The film thickness is sufficient to separate carrier dynamics from lattice heating and strain, allowing us to investigate the time scales of momentum relaxation, heat transfer to the lattice, and electron-hole recombination. The measured electron-hole ($e\ensuremath{-}h$) recombination time is 12--26 ps and ambipolar diffusivity is 18--40 cm${}^{2}$/s for carrier excitation up to $\ensuremath{\sim}{10}^{19}{\text{cm}}^{\ensuremath{-}3}$. By comparing the heating of the front and back sides of the film, we put lower limits on the rate of heat transfer to the lattice, and by observing the decay of the plasma at the back of the film, we estimate the time scale of electron-hole recombination. We interpret each of these time scales within a common framework of electron-phonon scattering and find qualitative agreement between the various relaxation times observed. We find that the carrier density is not determined by the $e\ensuremath{-}h$ plasma temperature after a few picoseconds. The diffusion and recombination become nonlinear with initial excitation $\ensuremath{\gtrsim}{10}^{20}{\text{cm}}^{\ensuremath{-}3}$.

Self-organization of structures on a bismuth single-crystal surface in an atmosphere of atomic hydrogen

The morphology of a cleavage surface of single-crystal bismuth has been studied by atomic force microscopy after treatment in atomic hydrogen. It has been ascertained that the surface relief changes due to the formation of micro- and nanocrystalline structures.

The coalescence of twins in bismuth single crystals

The process of the coalescence of twin boundaries in bismuth single crystals has been studied. It has been established that the twin coalescence can occur without the formation of an interface between them or with the formation of a double twin boundary, depending on the direction of twin growth and orientation of twinning planes.

Determination of magneto-optical quality and refractive index of bismuth germanium oxide single crystals grown by Czochralski technique

Bi12GeO20 single crystals were grown by the Czochralski technique. The critical diameter and the critical rate of rotation were calculated. Suitable polishing and etching solutions were determined. As a result of our experiments, the pale yellow and black single crystals were produced. The structure of the Bi12GeO20 has been investigated by X-ray diffraction, Raman and IR spectroscopy. The atomic ratio Bi/Ge was determined for the both crystals. The refractive indexes of both Bi12GeO20 crystals were determined by the spectroscopic ellipsometry method. Important properties of Bi12GeO20 for sensing applications, optical activity, Faraday rotation and absorption were measured and magneto optical quality was calculated and compared.

Large thermal Hall coefficient in bismuth

We present a systematical study of thermal Hall effect on a bismuth single crystal by measuring resistivity, Hall coefficient, and thermal conductivity under magnetic field, which shows a large thermal Hall coefficient comparable to the largest one in a semiconductor HgSe. We discuss that this is mainly due to a large mobility and a low thermal conductivity comparing theoretical calculations, which will give a route for controlling heat current in electronic devices.

Thermostimulated surface autosegregation in bismuth ferrite single crystals

The chemical and phase composition and morphological features of the surface nanostructure have been studied by the methods of high-resolution scanning electron microscopy, X-ray microprobe analysis, and atomic force microscopy in bismuth ferrite single crystals. This structure was formed as a result of the thermostimulated surface segregation after annealing in air or vacuum at the pressure of 10−4 Pa. It has been experimentally found that, at temperatures less than 500°C, Bi2F4O9 nanoparticles were formed due to the selective diffusion of iron atoms to the surface. Starting from 300°C in vacuum and 450°C in air, the segregating atom type changed and nanophases with high bismuth concentration (sillenites Bi26 − xFexO39 and Bi2O3 appeared in some regions. The partial orientation of new phases has been observed in some surface regions. A probable mechanism of the described phenomenon that represents a combination of selective intrinsic mass-transport of atoms from the bulk to the surface and their thermal evaporation has been discussed.

Thermoelectric properties for single crystal bismuth nanowires using a mean free path limitation model

We present a mean free path limitation model to describe the temperature dependence of both resistivity and Seebeck coefficient for bismuth nanowire. Since the mobility of carriers for bismuth nanowire was limited due to dominant collision at wire boundary, the effective mobility for each carrier was estimated using cyclotron mass, appropriate band structure, and temperature dependence of Fermi energy from 4 to 300 K. Then, the resistivity and the Seebeck coefficient were calculated by using carrier density reported for bulk single crystal. In addition, an individual single-crystal bismuth nanowire sample (725 nm diameter and 2.37 mm length) grown into a quartz template was prepared to estimate the model, and the measurements were also performed. The temperature dependences of not only resistivity, but also Seebeck coefficient were quantitatively and qualitatively in very good agreement in the whole temperature region by using its crystal orientation measured from Laue measurement. We conclude that the mean free path limitation model proposed made us understand the temperature dependences of single-crystal bismuth nanowire without a finite size effect.

Tip-enhanced photovoltaic effects in bismuth ferrite

Photoelectric properties of multiferroic BiFeO3, such as large photovoltages exceeding several times the bandgap or switchable photocurrents, have recently generated much interest. This is mostly because of potential applications in photovoltaic or photocatalytic devices. Although the fundamentally different (from classical semiconductors) polarization-related charge separation mechanisms allow unlimited photovoltages, these materials have never been considered a viable alternative to classical semiconductor-based photovoltaics because of a very low light conversion efficiency of less than 10−4. Here we investigate the anomalous photovoltaic effect in bismuth ferrite BiFeO3 single crystals, using photoelectric atomic force microscopy and piezoresponse force microscopy. We show that using a nanoscale top electrode, the photoexcited carriers are efficiently collected and the external quantum efficiency is enhanced by up to seven orders of magnitude. This enhancement might provide a viable alternative for the effective use of the anomalous photovoltaic effect in photovoltaic applications.

Measuring pulsed heat flux by anisotropic thermoelement

The relationship between the electric response signal of a thermal sensor and the temperature change caused by pulsed thermal load on the working surface is established within the framework of a one-dimensional model of the anisotropic thermoelement. A signal from the gradient heat-flux sensor based on an anisotropic bismuth single crystal that occurs on a steel plate exposed in a pulsed supersonic flow is analyzed in terms of the model. The results of the heat flux calculation using the temperature sensor response are compared to the data of direct measurements.

Fern-shaped bismuth dendrites electrodeposited at hydrogen evolution potentials

Easy fabrication of nanostructured materials with high porosity and high specific surface area has been drawing much scientific interest. Here, we describe a direct electrodeposition of fern-shaped bismuth dendrites at hydrogen evolution overpotentials without any additive and foreign template. A predeposition step was adopted to adjust and control the nucleation and growth of hydrogen bubbles. Subsequently, the simultaneous electrochemical reductions of Bi3+ and H+ ions were performed at a constant overpotential. Scanning electron microscopy and transmission electron microscopy characterizations showed that the resultant microporous netlike films consisted of fern-shaped single-crystal bismuth dendrites. The formation mechanism was discussed and ascribed to a “stagnant” template effect of hydrogen bubbles, which limited the lateral growth of fern-shaped dendrites. Such a two-step electrodeposition could also be extended to the nanostructure fabrication of other metals and alloys with high hydrogen overpotentials.

Characterization of bismuth tri-iodide single crystals for wide band-gap semiconductor radiation detectors

Bismuth tri-iodide is a wide band-gap semiconductor material that may be able to operate as a radiation detector without any cooling mechanism. This material has a higher effective atomic number than germanium and CdZnTe, and thus should have a higher gamma-ray detection efficiency, particularly for moderate and high energy gamma-rays. Unfortunately, not much is known about bismuth tri-iodide, and the general properties of the material need to be investigated. Bismuth tri-iodide does not suffer from some of the material issues, such as a solid state phase transition and dissociation in air, that mercuric iodide (another high-Z, wide band-gap semiconductor) does. Thus, bismuth tri-iodide is both easier to grow and handle than mercuric iodide. A modified vertical Bridgman growth technique is being used to grow large, single bismuth tri-iodide crystals. Zone refining is being performed to purify the starting material and increase the resistivity of the crystals. The single crystals being grown are typically several hundred mm 3 . The larger crystals grown are approximately 2 cm 3 . Initial detectors are being fabricated using both gold and palladium electrodes and palladium wire. The electron mobility measured using an alpha source was determined to be 260±50 cm 2 /Vs. An alpha spectrum was recorded with one of the devices; however the detector appears to suffer from polarization.