Optimal Carrier Density Research Articles

Thermoelectric response of Janus monolayer M2P2S3Se3 (M = Zn and Cd).

Thermoelectric transport properties of Janus monolayers M2P2S3Se3 (M = Zn and Cd) are investigated by the first-principles based transport theory. The Zn2P2S3Se3 and Cd2P2S3Se3 monolayers are indirect-gap semiconductors. The high polarizability of M-Se/S bonds in the MS3Se3 distorted octahedrons leads to anharmonic phonon behavior, which produces an intrinsic lattice thermal conductivity (κl) as low as 1.06 and 1.99W m-1 K-1 at 300K for Zn2P2S3Se3 and Cd2P2S3Se3 monolayers, respectively. The lower κl of the Zn2P2S3Se3 monolayer is mainly attributed to more pronounced flat modes of the phonon dispersion in a frequency range of 1-1.7THz caused by the softer Zn-Se/S bonds. The polar optical phonon scattering of carriers surprisingly plays a dominant role in carrier transport of both the monolayers, which greatly suppresses the electrical conductivity and thereby the power factor by about an order of magnitude. The predicted figure of merit (zT) increases monotonically with the temperature at the optimal carrier density, and at the operating temperature of 1200K, it reaches an optimal value of 0.86 at an optimal electron density of ∼1.5×1013 cm-2 for the n-type Zn2P2S3Se3 monolayer and 0.30 at an optimal electron density of ∼7×1012 cm-2 for the n-type Cd2P2S3Se3 monolayer.

Transport characteristics and lattice dynamics with phonon topology accentuation in layered CuTlX (X: S, Se)

In recent years, numerous Cu-based compounds have attracted a great deal of interest for enhanced thermoelectric energy conversion. Here, we demonstrate that CuTlX (X: S, Se), a layered semiconductor, exhibits low lattice thermal conductivity (κ l ) and a high thermoelectric figure of merit (ZT), using density functional theory calculations and Boltzmann transport theory beyond relaxation time approximation. To evaluate the absolute values of thermoelectric coefficients, different scattering mechanisms such as acoustic deformation potential scattering, impurity phonon scattering, and polar optical phonon scattering are analysed. This low lattice thermal conductivity, which is complemented by a low group velocity and a low phonon lifetime, accounts for the remarkable thermoelectric efficiency in these compounds. In CuTlS, the contribution of the in-plane optical phonon mode to κ l results in a decrease in its value, which might be attributed to the occurrence of Dirac-like crossings with non-trivial topological characteristics, as corroborated by the non-zero Berry curvature value. Overall, the thermoelectric behavior of both compounds is favorable at ambient temperature. Specifically, the out-of-plane direction in CuTlSe presents elevated thermoelectric performance with a high value for the thermoelectric figure of merit, with 1.08 and 1.16 for holes and electrons, respectively, at 300 K at the optimal carrier density of 1019 cm−3 , which well aids in both the electron and phonon transport. We also undertook monolayer examinations of these compounds due to the existence of van der Waals interactions, which predicted strong thermoelectric performance for both carrier concentrations at 300 K. As a result, our study presents a theoretical prediction on transport phenomena that requires experimental verification and should motivate additional research into prospective thermoelectric materials in the same crystal family for device applications.

Efficient thermoelectricity in Sr2Nb2O7 with energy-dependent relaxation times

We evaluate theoretically the thermoelectric efficiency of the layered perovskite Sr$_2$Nb$_2$O$_7$ via calculations of the electronic structure and transport coefficients within density-functional theory and Bloch-Boltzmann relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$, computed with energy-, chemical potential- and temperature-dependent relaxation times, has one component increasing monotonically from around 0.4 at room temperature to 2.4 at 1250 K at an optimal carrier density around 2$\times$10$^{20}$ cm$^{-3}$, while the other components are small. The Seebeck coefficient is about 250 to 300 $\mu$V/K at optimal doping, and reaches 800 $\mu$V/K at lower doping. We provide a {\tt python} code implementing various approximations to the energy-dependent relaxation time transport, which can be used to address different systems with an appropriate choice of material parameters.

Heterobilayers of 2D materials as a platform for excitonic superfluidity

Excitonic condensate has been long-sought within bulk indirect-gap semiconductors, quantum wells, and 2D material layers, all tried as carrying media. Here, we propose intrinsically stable 2D semiconductor heterostructures with doubly-indirect overlapping bands as optimal platforms for excitonic condensation. After screening hundreds of 2D materials, we identify candidates where spontaneous excitonic condensation mediated by purely electronic interaction should occur, and hetero-pairs Sb2Te2Se/BiTeCl, Hf2N2I2/Zr2N2Cl2, and LiAlTe2/BiTeI emerge promising. Unlike monolayers, where excitonic condensation is hampered by Peierls instability, or other bilayers, where doping by applied voltage is required, rendering them essentially non-equilibrium systems, the chemically-specific heterostructures predicted here are lattice-matched, show no detrimental electronic instability, and display broken type-III gap, thus offering optimal carrier density without any gate voltages, in true-equilibrium. Predicted materials can be used to access different parts of electron-hole phase diagram, including BEC-BCS crossover, enabling tantalizing applications in superfluid transport, Josephson-like tunneling, and dissipationless charge counterflow.

Research on high robustness 4.5kV FRD chip

A robustness 4500V/100A FRD with was designed by simulation and verified by experiment. The chip composed with optimized carrier density cell area and ruggedness terminal area. The cell area composed of P-body/N-sub/N+ layers, has good static and dynamic trade off characteristics by carefully design. The simulation show that there was no difference between conventional multi-deep P-ring terminal and multi-deep P-ring plus inner field plate terminal. Both terminals have the same width and almost the same breakdown voltage, low electric field and etc.by simulation.The experiment show that different terminal chips have almost the same trade off characters but different in reliability test, the inner field plate terminal pass 1000H HTRB test while the conventional multi-deep P-ring terminal failed at 168H HTRB test.

UiO-67 metal-organic gel material deposited on photonic crystal matrix for photoelectrocatalytic hydrogen production.

Robust UiO-67 metal–organic framework nanoparticles have been precisely and uniformly anchored on the surface of a photonic crystal via metal–organic gelation, resulting in a nanoscale UiO-67 composite. Mott–Schottky measurements indicate that UiO-67/B is an n-type semiconductor with electron conduction, and the band gap significantly decreases with the assistance of the photonic crystal matrix with a band gap of 0.75 eV. Benefiting from the abundant photoelectrons trapped from the photonic crystal, good hydrogen evolution reaction performance is achieved under light irradiation. The current density increases from 3.2 to 7.0 mA cm−2 at −0.6 V (vs. RHE) for UiO-67/B. The optimized carrier density obtained from UiO-67/B is apparently increased 2.15 times under light irradiation for 30 min. This work provides a rational strategy to address the photo-capture and energy transfer issues of metal–organic frameworks under visible light irradiation for H2 production in artificial photosynthesis.

Synergistically optimizing charge and phonon transport properties in n-type PbTe via introducing ternary compound AgSb(Se, Te)2

PbTe-based thermoelectric materials are considered to be a medium-temperature thermoelectric candidates and the high-performance n-type PbTe is urgently to be developed to match its p-type counterpart on account of practical application. Herein, we firstly optimized the carrier density of n-type PbTe through Sb doping, and obtained a maximum ZT value of ∼1.0. Then, based on the optimized carrier density, we introduced ternary compounds AgSb(Se, Te)2 to suppress the lattice thermal conductivity. Minimum lattice thermal conductivity of n-type PbTe was reduced as low as ∼0.53 W m−1K−1 and ∼0.62 Wm−1K−1 through introducing AgSbSe2 and AgSbTe2, respectively. Combining the maintained electrical properties with suppressed thermal conductivity, maximum ZT values ∼1.2 and ∼1.3 were obtained in PbTe+2%AgSbSe2 and PbTe+2%AgSbTe2, respectively. This work provides an effective strategy of synergistically balancing charge and phonon transports in n-type PbTe with ternary AgSb(Se, Te)2 compounds.

Stacking Fault-Induced Minimized Lattice Thermal Conductivity in the High-Performance GeTe-Based Thermoelectric Materials upon Bi2Te3 Alloying.

Materials with low lattice thermal conductivity (κlat) are crucial for the applications of thermal insulation and thermoelectric (TE) energy conversion. Stacking fault (SF)-induced phonon scattering within interfaces has been put forward theoretically by Klemens in 1950s. However, unlike other traditional defects such as point defects, grain boundaries, and dislocations, the role of SF for reducing κlat remains poorly understood and is yet to be revealed experimentally. The layered Bi2Te3 with a van der Waals gap shows different stacking structures than the nonlayered GeTe, which is used to introduce SFs into the GeTe-based alloys in this work. On the basis of the experimental and theoretical modeling results, this paper reveals the significant contribution of SF phonon scattering for minimizing the κlat. Besides the achieved extremely low κlat (∼0.39 W m-1 K-1 at 573 K), optimized carrier density and band convergence are also realized in the GeTe-based alloys upon Bi2Te3 alloying, leading to a significant high TE figure of merit ZT > 2 at 773 K and an averaged ZT > 1.4 within 300-773 K. This SF engineering strategy provides a different avenue to reduce the κlat for enhancing the performance of thermal insulation and TE materials.

Highly enhanced thermoelectric performance in BiCuSeO ceramics realized by Pb doping and introducing Cu deficiencies

AbstractIn recent years, BiCuSeO oxyselenides have been developed as a promising thermoelectric material. In this article, PbxBi1−xCu1−ySeO (x = y = 0, 0.02, 0.04, 0.06, and 0.08) are prepared by solid‐state reaction method and spark plasma sintering (SPS), and the combinatorial effects of Pb doping and Cu deficiencies on thermoelectric properties are investigated systematically. The transport properties are significantly enhanced due to the optimized carrier density, majorly contributing to the promotion of ZT values. As a result, the maximum ZT of 0.77 at 873 K and average ZT (from 300 to 873 K) of 0.50 are obtained for Pb0.06Bi0.94Cu0.94SeO sample. The values are 0.4 and 1.2 times, respectively, higher than that of pristine sample.

High thermoelectric figure of merit and thermopower in layered perovskite oxides

We predict high thermoelectric efficiency in the layered perovskite La$_2$Ti$_2$O$_7$, based on calculations (mostly ab-initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit $ZT$ computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around 10$^{20}$ cm$^{-3}$. The Seebeck thermopower coefficient is between 200 and 300 $\mu$V/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the potential of this material is due to its lattice thermal conductivity of order 1 W/(K m); using a model based on ab initio anharmonic calculations, we interpret this low value as due to effective phonon confinement within the layered-structure blocks.

Thermo-electric figure of merit of quasi-two-dimensional Bi2Te3 nano-structures

Quasi-two dimensional nano-structures are characterized by radical changes in electron and phonon properties. Electron behavior in Q-2D nano-structures is modified by the presence of multiple sub-bands. For well widths smaller than 50 Å simplified model calculations based on the assumption of electrons occupying the lowest sub-band appears to be quite reasonable. For larger width others, parabolic sub-bands need to be considered. The present paper investigates the thermoelectric figure-of-merit of bismuth telluride which is five layered (Te-Bi-Te-Bi-Te) crystal with hexagonal symmetry with six equivalent valleys, in the range 40–300 Å and observe that for 100 Å well width three sub-band model and for 200 Å well width six sub-band model are good approximation at room temperature. Further it is observed that optimum carrier density for 200 Å well width is and maximum of ZT shifts towards lower concentration; and discusses the acceptability of single band approximation.

Impurity-free, mechanical doping for the reproducible fabrication of the reliable n-type Bi2Te3-based thermoelectric alloys

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi2Te3 alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material's performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides, combining a cold deformation and a hot extrusion. The cold deformation enables controlling the electron density in the range of ∼1019/cm3 via the formation of intrinsic point defects, and the hot extrusion allows texturing the microstructure to enhance the electrical conductivity, hence a large power factor of >5 × 10−3 W-m−1-K−2. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Using this method, we can decouple the relationship between bandgap, carrier density, and composition to improve the high-temperature thermoelectric property. Moreover, we demonstrate the fabrication of high-performance thermoelectric materials from low-graded, raw materials by modifying the degree of the mechanical deformation to reach an optimum carrier density. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way.

4.5kV FRD Development for high current power modules

A 4.5kV/100A FRD was designed by simulation, which had optimized carrier density distribute cell and ruggedness terminal. The cell was composed of P-body/N-sub/N+ layers, when the P-body doping concentration is lower, the carrier density distribution on the P-body/N-sub is lower; when carrier density di stribution on the P-body/N-sub side is lower than that on the N-sub/N+ side, the FRD has soft recovery but bad surge-current capability. So the P-body doping concentration needs trade-off consideration. Lifetime control technology was also used to optimize the carrier density distribution and trade-off characteristics. The terminal has high breakdown voltage, low electric field and large process window, which means more ruggedness and high reliability. The experiment results show that the design chip and competitor chip has nearly the same trade-off characteristics, the design chip has larger dynamic loss but lower static loss. The design chip has high surge current, the surge current is 13 times as much as the rate current.

A Novel Silicon Carbide Accumulation Channel Injection Enhanced Gate Transistor With Buried Barrier Under Shielding Region

A silicon carbide (SiC) injection enhanced gate transistor with accumulation channel (AC-IEGT) is proposed in this letter, which has a barrier layer with small windows under the p+ shielding region. The new structure can enhance electron injection through accumulation channel. Thus, an optimized carrier density can be obtained in the emitter side. Compared with conventional SiC insulated gate bipolar transistor and carrier stored trench bipolar transistor, static simulation results show that the static characteristics of the device are improved significantly. The figure of merit ( $V_{\text {BR}}^{2}/R_{\text {on,sp}})$ of AC-IEGT increases by 81.96 % and 17.78 %, respectively. The mix-mode circuit simulation results show that the proposed could be more suitable for ultra-high voltage application.

Electrodeposition of tin oxide thin film from nitric acid solution: the role of pH

Tin oxide (SnO2) thin films were synthesized by electrodeposition method from nitric acid medium with SnCl2 precursor and NaNO3 supporting electrolyte at 70 °C. The effect of pH electrolyte in the properties of SnO2 thin films was investigated. Cyclic voltammetry (CV) was utilized for the determination of the selective potential for electrodeposition of pure SnO2. Mott–Schottky (M–S) plots and electrochemical impedance spectroscopy (EIS) techniques were carried out to estimate the electrical characteristics of the SnO2 thin films deposited at different pH electrolytes. These latter techniques show n-type conductivity for all the samples with optimal carrier density of 8.41 × 1020/cm3 and high value of conductivity for the films electrodeposited at pH ~ 1.10. Atomic force microscopy (AFM) observations showed that the SnO2 thin films obtained at different pH are more homogenous in appearance and present lower surface roughnesses at lower pH value. Structural analysis of the SnO2 thin films was performed by X-ray diffraction (XRD) which exhibit a polycrystalline structure for pH ~ 1.25 and amorphous one for another pH values. From the optical study, the transmittance and the gap energy were founded to be depending to the pH of electrolyte.

Appealing Perspectives of Hybrid Lead–Iodide Perovskites as Thermoelectric Materials

The fundamental properties of lead halide perovskites, rivaling those of conventional semiconductors, make these systems attractive not just for solar cells but also for a broader playground of energy and nanotechnology applications. The recently measured ultralow thermal conductivity of the perovskites suggests the possibility of high thermoelectric efficiency and the possible use of the perovskites for solar-thermoelectric generation capable to capture both above-gap and below-gap sun illumination. Here we explore this possibility presenting a theoretical analysis of the thermoelectric behavior of CH3NH3PbI3 for a wide range of temperatures and carrier concentrations. For electron doping, we find optimal carrier density n ∼ 1019 cm–3, at which this material displays room-T power factor σS2 ∼ 0.8 × 10–3 W/mK2, derived by moderate electrical conductivity σ and robust thermopower, with Seebeck coefficient S of approximately hundreds of μV/K, typical of polar insulating perovskites. In combination with a me...

Tunable Conductivity and Half Metallic Ferromagnetism in Monolayer Platinum Diselenide: A First-Principles Study

On basis of the first-principles calculations, we have studied the effects of hole doping and biaxial tensile strain on the electronic and magnetic properties of monolayer of platinum diselenide (PtSe2). Due to the large density of states near the valence band edge, this nonmagnetic monolayer semiconductor switches to a ferromagnetic half metal within a small range of hole doping. With an increase of hole density, average magnetic moment per carrier also increases and reaches at its maximum value over a specific range of carrier density, while the system remains in a half metal state before the magnetic moment abruptly begins to fall. We also predict a critical value of biaxial tensile strain (5%) for doped monolayer PtSe2, after which the optimal carrier density becomes constant, while the magnetic moment/carrier gradually increases and the ferromagnetic state of the system becomes more stable with increasing values of strain. This work paves a possible way to engineer the magnetic properties of the two-...

Rationally Designing High-Performance Bulk Thermoelectric Materials.

There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of the electron and phonon transport through coherent interface, matrix/precipitate electronic bands alignment, and compositionally alloyed nanostructures. Finally, recent discoveries of new compounds with intrinsically low thermal conductivity are summarized, where SnSe, BiCuSeO, MgAgSb, complex copper and bismuth chalcogenides, pnicogen-group chalcogenides with lone-pair electrons, and tetrahedrites are given particular emphasis. Future possible strategies for further enhancing ZT are considered at the end of this review.

Dependence of sheet resistance on dispersion of multi-walled carbon nanotubes grown using size-controlled catalyst particles

Abstract Catalyst-assisted synthesis of multi-walled carbon nanotube (MWCNT) networks was successfully realized by controlling the catalyst particle size with and without post annealing. The sizes of ternary composite catalyst synthesized from spray pyrolysis method varied with different partial pressures and post-annealing. Various-sized MWCNT networks corresponding to individual catalyst sizes were produced through chemical vapor deposition technique. To investigate the dispersion of MWCNTs with respect to catalyst particle size, particle size analyses, scanning electron microscopy, and transmission electron microscopy were performed. In addition, the sieve shaker with 250 mesh were also used for evaluation of correlation between the MWCNTs networks and catalyst sizes. For electrical properties of the samples, MWCNT sheet resistance values were estimated with a four-point probe. These values were used to establish the optimal dispersion of MWCNTs needed to yield optimal carrier density and mobility.

Theoretical study of the optical gain characteristics of a Ge1−xSnx alloy for a short-wave infrared laser**Project supported by the Major State Basic Research Development Program of China (Grant No. 2013CB632103), the National High-Technology Research and Development Program of China (Grant No. 2012AA012202), and the National Natural Science Foundation of China (Grant Nos. 61177038 and 61176013).

Optical gain characteristics of Ge1−xSnx are simulated systematically. With an injection carrier concentration of 5 × 1018/cm3 at room temperature, the maximal optical gain of Ge0.922Sn0.078 alloy (with n-type doping concentration being 5 × 1018/cm3) reaches 500 cm−1. Moreover, considering the free-carrier absorption effect, we find that there is an optimal injection carrier density to achieve a maximal net optical gain. A double heterostructure Ge0.554Si0.289Sn0.157/Ge0.922Sn0.078/Ge0.554Si0.289Sn0.157 short-wave infrared laser diode is designed to achieve a high injection efficiency and low threshold current density. The simulation values of the device threshold current density Jth are 6.47 kA/cm2 (temperature: 200 K, and λ = 2050 nm), 10.75 kA/cm2 (temperature: 200 K, and λ = 2000 nm), and 23.12 kA/cm2 (temperature: 300 K, and λ = 2100 nm), respectively. The results indicate the possibility to obtain a Si-based short-wave infrared Ge1−xSnx laser.