Variation Of Carrier Concentration Research Articles

Effect of pressure on superconductivity in the indium-doped topological crystalline insulator SnTe

We report on the impact of hydrostatic pressure on the superconductivity of optimally (indium)-doped SnTe which is established to be derived from a topological crystalline insulating phase. Single crystals of Sn1−xInxTe were synthesized by a modified Bridgman method that exhibited maximum superconducting Tc of 4.4 K for x = 0.5. Hydrostatic pressure up to 2.5 GPa was applied on the crystals of Sn0.5In0.5Te, and electrical resistivity as a function of temperature and pressure was measured. We observed a decrease in the onset superconducting transition temperature from 4.4 K to 2.8 K on increasing pressure from ambient to 2.5 GPa. The normal state resistivity also decreased abruptly by an order of magnitude at 0.5 GPa but for higher pressures, it decreased marginally. From onset, offset and zero resistivity values, dTc/dP of ∼ −0.6 K GPa−1 was confirmed. The low temperature normal state resistivity followed T2 dependence suggesting Fermi liquid behaviour both for ambient and high pressure data. This increase in metallic characteristics accompanied by normal state Fermi liquid behaviour is in accordance with a ‘dome structure’ for Tc variation with varying carrier concentration.

Dependence of the Structural, Electrical and Optical Properties of PbSe Nanomaterial Prepared by Hydrothermal Method on the Polyethylene Glycol Content

PbSe was prepared by hydrothermal method using 20, 60, 80, 100 mg contents of polyethylene glycol (PEG) as a surfactant. PbSe films with a thickness of about 120 nm and constant rate of evaporation were prepared on glass and quartz substrates using thermal evaporation technique for studying their electrical and optical properties, respectively. The prepared films were identified by X-ray diffraction, transmission electron microscope and energy dispersive X-rays. The variations of resistivity, carrier concentration and carrier mobility of the films with temperature and PEG content were investigated. The optical properties of the films were studied by using spectrophotometric measurements of the transmittance and reflectance in the wavelength range 500–2500 nm. The optical band gap of the films was calculated and it is found to increase with increasing PEG content.

On the effect of carbon nanotubes on the thermoelectric properties of n-Bi2Te2.4Se0.6 made by mechanical alloying

Composite materials based on n-Bi2Te2.4Se0.6 with commercial multi walled carbon nanotubes (MWCNTs) have been synthesized via high energy ball milling and spark plasma sintering. The effect on the transport properties of the presence of MWCNTs during the ball billing step is studied. It is demonstrated that MWCNTs, even in small quantity, change dramatically the transport properties. Seebeck coefficient, electrical resistivity, thermal diffusivity and carrier concentration are measured and their variation discussed. The presence of MWCNTs during the ball milling step is discussed as it clearly induces three competing effects, the presence of tellurium vacancy, the decreased grain size of Bi2Te2.4Se0.6 either promoted during the ball milling or result of the a prevented grain growth during the sintering and a n-type doping. The large carrier concentration variation caused by the MWCNTs represents an alternative way of tuning the transport properties.

Drug composition matters: the influence of carrier concentration on the radiochemical purity, hydroxyapatite affinity and in-vivo bone accumulation of the therapeutic radiopharmaceutical 188Rhenium-HEDP

Introduction188Rhenium-HEDP is an effective bone-targeting therapeutic radiopharmaceutical, for treatment of osteoblastic bone metastases. It is known that the presence of carrier (non-radioactive rhenium as ammonium perrhenate) in the reaction mixture during labeling is a prerequisite for adequate bone affinity, but little is known about the optimal carrier concentration. MethodsWe investigated the influence of carrier concentration in the formulation on the radiochemical purity, in-vitro hydroxyapatite affinity and the in-vivo bone accumulation of 188Rhenium-HEDP in mice. ResultsThe carrier concentration influenced hydroxyapatite binding in-vitro as well as bone accumulation in-vivo. Variation in hydroxyapatite binding with various carrier concentrations seemed to be mainly driven by variation in radiochemical purity. The in-vivo bone accumulation appeared to be more complex: satisfactory radiochemical purity and hydroxyapatite affinity did not necessarily predict acceptable bio-distribution of 188Rhenium-HEDP. ConclusionsFor development of new bisphosphonate-based radiopharmaceuticals for clinical use, human administration should not be performed without previous animal bio-distribution experiments. Furthermore, our clinical formulation of 188Rhenium-HEDP, containing 10μmol carrier, showed excellent bone accumulation that was comparable to other bisphosphonate-based radiopharmaceuticals, with no apparent uptake in other organs. Advances in knowledgeRadiochemical purity and in-vitro hydroxyapatite binding are not necessarily predictive of bone accumulation of 188Rhenium-HEDP in-vivo. Implications for patient careThe formulation for 188Rhenium-HEDP as developed by us for clinical use exhibits excellent bone uptake and variation in carrier concentration during preparation of this radiopharmaceutical should be avoided.

Transmission mode of a single layer graphene and its performance in the detection of the vibration spectrum of gas molecular

Since its successful preparation in 2004, graphene has attracted a great deal of attention, and the sensing application is an important research field. But nearly all the researches about graphene sensors focus on low frequency band, of which the mechanism is mainly dependent on the detection of charge carrier concentration and conductivity variation induced by the absorption of molecules. However, due to the fact that most of the molecules absorbed on the surface of graphene will induce the change of conductivity, this method is incapable of distinguishing different molecules. Transmission mode of a single molecular layer is studied based on Kubo formula and combined with a numerical method. The relation between transmission properties and effective mode index is analyzed, and the broadband localization capability of the waveguide mode is demonstrated. Meanwhile, the variation of the transmission intensity which is due to the interaction between the first order waveguide mode and the gas is adopted to retrieve the vibration spectrum of molecules. Taking the sensing of SO2, CO and C7H8 as examples, the effectiveness of this method is verified based on eigenmode analysis. Results show that the transmission spectrum is consistent with the variation spectrum of gas molecules; besides, in the transmission direction, the larger the interaction range, the greater the attenuation of mode transmission intensity will be. This study has provided a theoretical foundation for the realization of the detection and identification of gas moleculan fingerprints.

On the effect of δ-doping in self-switching diodes

Lowering the carrier concentration is presented as a way to considerably improve the performance of self-switching diode (SSD) detectors. A physics-based theoretical model was used to derive an expression for the responsivity of SSDs as a function of carrier concentration, mobility, and design parameters. Monte Carlo simulations confirmed the modeled effect of varying carrier concentration and channel width. SSDs were fabricated in InAs heterostructures with different δ-doping levels. Radio frequency (RF) characterization at 50 GHz reproduced the modeled trends. By reducing the carrier concentration in InAs SSDs with 40 nm wide channels from 2.7 × 1012 cm−2 to 1.5 × 1012 cm−2 (−44%), the noise equivalent power (NEP) improved from 130 pW/Hz½ to 87 pW/Hz½ (−33%).

Cr6+ ion doping effects in layered Ca3Co4O9 system

The effects of Cr doping on structural, magnetic, electrical and thermal transport properties of Ca3Co4−xCrxO9 (0⩽x⩽0.3) have been investigated systematically. Based on the analysis of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) data, the Cr ions are suggested to be doped at Co4+-sites in CoO2 layers in the form of Cr6+. The introduction of the nonmagnetic Cr6+ ions results in the decrease of the average effective magnetic moment, which confirming that the ferrimagnetic state of Ca3Co4O9 system can be well suppressed by Cr doping. The magnitude of resistivity and the metal–insulator-transition temperature increase monotonously with increasing x, indicating the enhanced carrier localization and the more stable spin-density-wave state in these Cr-doped samples. As Cr6+ ions are doped into system, the thermopower increases obviously. Correspondingly, the room-temperature thermopower reaches the maximum value of 140μV/K for x=0.3 sample. These results of magnetism and electrical/thermal transport are mainly originated from the variations of carrier concentration and electronic correlation resulted from the partial substitution of Cr6+ for Co ions.

Structure and transport properties in Ca3Co4−xMxO9 (M=Re and Pt) ceramics

The effect of Re and Pt doping on the structural, electrical and thermal transport properties of Ca3Co4−xMxO9 (0≤x≤0.2, M=Re and Pt) samples have been investigated systematically. As Re7+ or Pt4+ ions are doped into Ca3Co4O9 lattice, the resistivity and the metal–insulator-transition temperature increase monotonously with increasing x, indicating that both Re- and Pt-substitution can result in more stable spin-density-wave state. For Re-doped samples, the thermopower increases with increasing x, while the thermopower of Pt-doped samples decreases monotonously. Although the decreased thermal conductivity of both Re- and Pt-doped samples is beneficial to the thermoelectric performance, their ZT value decreases with increasing x. The result shows that both Re and Pt doping may indeed exert a significant influence in the structural and transport properties of Ca3Co4O9 system, which are suggested to originate from the variations of carrier concentration and electronic correlation induced by Re or Pt doping.

Comparative DMFT study of theeg-orbital Hubbard model in thin films

Heterostructures of transition-metal oxides have emerged as a new route to engineer electronic systems with desired functionalities. Motivated by these developments, we study a two-orbital Hubbard model in a thin-film geometry confined along the cubic [001] direction using the dynamical mean-field theory. We contrast the results of two approximate impurity solvers (exact diagonalization and one-crossing approximation) to the results of the numerically exact continuous-time quantum Monte Carlo solver. Consistent with earlier studies, we find that the one-crossing approximation performs well in the insulating regime, while the advantage of the exact-diagonalization-based solver is more pronounced in the metallic regime. We then investigate various aspects of strongly correlated ${e}_{g}$-orbital systems in thin-film geometries. In particular, we show how the interfacial orbital polarization dies off quickly a few layers from the interface and how the film thickness affects the location of the interaction-driven Mott transition. In addition, we explore the changes in the electronic structure with varying carrier concentration and identify large variations of the orbital polarization in the strongly correlated regime.

Strengthening of Thermoelectric Performance via Ir Doping in Layered Ca3 Co4 O9 System

The effects of Ir doping on thermoelectric (TE) as well as transport and magnetic properties of Ca3Co4−xIrxO9 (0 ≤ x ≤ 0.4) series samples have been investigated systematically. Based on the analysis of X-ray photoelectron spectroscopy data, the valence state of the doped Ir ions is suggested to be +4. As Ir ions are doped into system, both the resistivity and the metal-insulator transition temperature increase till to x = 0.3, indicating the more stable spin-density-wave state in these Ir-doped samples. The thermopower increases monotonously with increasing x. For x = 0.4 sample, its room-temperature thermopower S300K reaches 166.2 μV/K, which is 36% larger than that of the undoped sample. The results show that a proper Ir doping may be an effective route to strengthen the TE performance of Ca3Co4O9 system, which is suggested to originate from the variations of carrier concentration and lattice disharmony induced by Ir doping. At the same time, Ir doping suppresses the low-temperature ferrimagnetic state and also introduces a spin-glass behavior at low temperatures.

Controllable Transformation from Rhombohedral Cu1.8S Nanocrystals to Hexagonal CuS Clusters: Phase- and Composition-Dependent Plasmonic Properties

Because of the rich polymorphs and lower diffusion energy barriers of copper chalcogenide systems, the phase transformation of colloidal Cu2–xS (0 ≤ x ≤ 1) nanocrystals is critical for understanding their fundamental properties and designing convenient synthetic routes. In this work, high quality digenite Cu1.8S nanocrystals with rhombohedral structure were synthesized at gram-scale. The as-prepared colloidal nanocrystals undergo an in situ phase transformation from rhombohedral Cu1.8S nanocrystals to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. The observed transformation was explored by a combination of structural and spectroscopic analyses, including powder X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. A possible mechanism is proposed and thoroughly discussed. We further determined the evolution of plasmonic absorption spectra during the transformation. The Cu1.8S nanocrystals and CuS clusters exhibit composition-dependent local surface plasmon resonance absorption (LSPR) in the near-infrared region, which are in good agreement with calculated extinction spectra based on Mie-Drude model. Combined experimental and theoretical analyses demonstrated that both the phase induced dielectric constant change and the composition induced carrier concentration variation account for the spectroscopic evolution.

Temperature dependent charge transport in organic field-effect transistors with the variation of both carrier concentration and electric field

We present experimental evidence of combined effects of temperature, carrier concentration, electric field as well as disorder on charge transport in an organic field-effect transistor (OFET). Transfer characteristics of an OFET based on sexithiophene active layer were measured from 80 to 300 K. Thermally activated carrier mobility followed Arrhenius law with two activation energies. Carrier density variation led to finite extrapolated Meyer–Neldel (MN) temperature (780 K) at low fields. Negative electric field-dependent mobility was observed in available field range. MN temperature shifted towards higher temperature when the electric field increased, and did not retain its finite character above the field of 4 × 103 V cm−1.

Temperature-dependent electroluminescence and voltages of multi-junction solar cells

The spectral electroluminescence of a monolithic triple-junction solar cell reveals the sub-cell open-circuit voltages under variation of temperature and carrier concentration. We present an electroluminescence set-up that is able to acquire a full spectrum in less than 10 s and give insight into the voltage-extraction process. The sum of the sub-cell voltages is in excellent agreement to open-circuit voltages acquired under concentrated illumination. The temperature-induced voltage losses are investigated and explained by a bandgap dependent increase in intrinsic carrier concentration. Finally, the accurate extraction of the sub-cell bandgaps helps to identify a temperature rise with increasing current density. Copyright © 2013 John Wiley & Sons, Ltd.

Carrier concentration modulation by hot pressing pressure in n-type nanostructured Bi(Se)Te alloy

We demonstrate experimentally that an optimal hot pressing pressure is required for high thermoelectric power factor in different n-type Bi(Se)Te alloys for a given processing temperature. This phenomenon is attributed to the variations in carrier concentration, which changes the Seebeck coefficient and therefore the power factor. The variations could arise from the difference in the concentration of charged antisite defects as their formation energy changes with pressures. Furthermore, modifications of the energy gap resulting from the lattice distortions at high pressure also likely play a role.

Properties of Annealed Indium Tin Tantalum Oxide Films Prepared by Reactive Magnetron Sputtering

ndium tin oxide (ITO) and indium tin tantalum oxide (ITTO) films were deposited on glass substrates at room temperature by magnetron sputtering. Properties of the ITO and ITTO films showed a dependence on annealing treatment. ITTO film deposited at room temperature showed the enhancement in (400) orientation and the increasing in grain size. With an increase in annealing temperature, the intensity of XRD peak increased and the grain size showed an evident increasing. The reflection edge in near-IR range and the absorption edge in near-UV range shifted due to the variation in carrier concentration. Ta-doping improved the carrier concentration of the films and widened the corresponding optical band gap. The variations in optical band gap were due to Burstein-Moss effect. The higher value of figure of merit of ITTO films was observed. The tantalum-doped ITO films could find extensive application in some devices.

Structure, magnetic and transport properties in Ca3Co4−xSbxO9 ceramics

The structure, magnetic, electrical and thermal transport properties of Ca3Co4−xSbxO9 (0⩽x⩽0.2) have been investigated systematically. Base on the analysis of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the valence state of the doped Sb ions is suggested to be +3 in the x=0.05 sample, and its content increases monotonously along with the gradual introduction of Sb5+ ions as x further increases. The resistivity and the metal–insulator-transition temperature increase with increasing x, indicating the more stable spin-density-wave state in the Sb-doped samples. The thermopower decreases first as x=0.05, and then increases monotonously with increasing x till to 0.15. The thermal conductivity decreases obviously due to Sb-doping. As a result, the Ca3Co3.90Sb0.10O9 sample has the largest thermoelectric figure-of-merit ZT value of 0.012 at room temperature, which is about two times larger than that of Ca3Co4O9. The results are suggested to originate from the variations of carrier concentration and electronic correlation via Sb-doping with the different valence states.

Comparison of boron precipitation in p-type bulk nanostructured and polycrystalline silicon germanium alloy

Boron precipitation process and its effect on electronic properties of p-type bulk nanostructured silicon germanium (Si0.8Ge0.2) compared with large grain polycrystalline Si0.8Ge0.2 have been studied. The structures were synthesized and their thermoelectric properties were measured versus temperature during heating and cooling cycles. The experimental data showed stronger temperature variation of Seebeck coefficient, carrier concentration, and conductivity in the nanostructured Si0.8Ge0.2 compared with the polycrystalline form indicating stronger boron precipitation in this structure. The electrical properties of both samples were calculated using a multi-band semi-classical model. The theoretical calculations confirm that the increase of boron precipitation in the nanostructured Si0.8Ge0.2 is responsible for its higher thermal instability. Since the thermoelectric properties of the nanostructured sample degrade as a result of thermal cycling, the material is appropriate only for continuous operation at high temperature without cooling.

Band bowing and Si donor levels in InGaN layers investigated by surface photo voltage spectroscopy

Bowing parameter and intra gap states have been measured on good quality undoped and n-type Si doped InGaN layers, by surface photovoltage spectroscopy (SPS). Bowing parameter calculations have been done with consideration of strain and relaxed nature of InGaN layers. Si donor levels have been measured by SPS on layers with different indium content. Free carrier concentration and Si donor level variations as a function of In content have been analysed. It has been found that the interaction between In and Si plays a fundamental role in Si doping process in InGaN.

Schottky barriers based on metal nanoparticles deposited on InP epitaxial layers

Fabrication of high-quality Schottky barriers on InP epitaxial layers prepared by liquid-phase epitaxy from rare-earth treated melts is reported. The Schottky structures are based on metal nanoparticles and a graphite layer deposited from colloidal solutions onto epitaxial layers with varying carrier concentration. The structures have notably high values of the barrier height and of the rectification ratio giving evidence of a small degree of the Fermi-level pinning. Electrical characteristics of these diodes are shown to be extremely sensitive to the exposure of gas mixtures with small hydrogen content.

Carrier type- and concentration-dependent absorption and photoluminescence of ZnO films doped with different Na contents

The electrical and optical properties of zinc oxide (ZnO) films doped with different Na contents and grown by pulsed laser deposition were investigated. Hall measurements witnessed the conductivity conversion from n-type to p-type with targeted Na doping content increased up to more than 1%. The photoluminescence intensity first decreased as the targeted Na content increased to 1%, while non-degraded and even enhanced PL intensity was observed in p-type ZnO:Na0.02 film. This photoluminescence enhancement was ascribed to enhanced radiative recombination with more acceptor (NaZn) introduced. The band-gap shift of ZnO:Nax films was related to the variation of carrier type and concentration. Band-gap shrinkage was adopted to explain the carrier type- and concentration-dependent band-gap shift of ZnO:Nax films.