Temperature-dependent Transport Research Articles

Predicting the structural, optoelectronic, dynamical stability and transport properties of Boron-doped CaTiO3: DFT based study

Undoped and B-doped CaTiO3 Semiconductor Perovskite is investigated by the Density Functional Theory (DFT) and Boltzman transport theory (BoltzTraP) using full potential linearized augmented plane wave (FP-LAPW) method with GGA-PBE approximation. By incorporating B into CaTiO3, the electrical band gap is effectively reduced, and adjusting the substitution atom type may regulate the degree of band gap reduction. As a result, the visible light absorption ability is increased. Our results indicate that all doped structures are highly absorbent and productive, with optical transition energy of between 2 and 4 eV. Temperature-dependent transport characteristics are also determined, which favors undoped CaTiO3 at room temperature and B-doped CaTiO3 at elevated ones.

Reduced dopant-induced scattering in remote charge-transfer-doped MoS2 field-effect transistors.

Efficient doping for modulating electrical properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is essential for meeting the versatile requirements for future electronic and optoelectronic devices. Because doping of semiconductors, including TMDCs, typically involves generation of charged dopants that hinder charge transport, tackling Coulomb scattering induced by the externally introduced dopants remains a key challenge in achieving ultrahigh mobility 2D semiconductor systems. In this study, we demonstrated remote charge transfer doping by simply inserting a hexagonal boron nitride layer between MoS2 and solution-deposited n-type dopants, benzyl viologen. A quantitative analysis of temperature-dependent charge transport in remotely doped devices supports an effective suppression of the dopant-induced scattering relative to the conventional direct doping method. Our mechanistic investigation of the remote doping method promotes the charge transfer strategy as a promising method for material-level tailoring of electrical and optoelectronic devices based on TMDCs.

Temperature-dependent electron spin relaxation at the metal-to-insulator transition in n -type GaAs

We present a detailed study of the temperature-dependent electron spin relaxation rate in $n$-type bulk GaAs in the regime of the metal-to-insulator transition at vanishing magnetic fields. The high-accuracy measurements reveal the longest spin relaxation time for a doping concentration slightly below the metal-to-insulator transition at a finite temperature of $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. This global minimum of the electron spin relaxation rate results from a delicate interplay of hyperfine interaction, variable range hopping, and the Dyakonov-Perel mechanism. At higher doping densities, the Dyakonov-Perel mechanism becomes dominant at all temperatures changing with temperature gradually from the degenerate to the nondegenerate regime. A theoretical model including temperature-dependent transport data yields not only quantitative agreement with the experimental data but reveals additionally the gradual change from percolation-based large angle momentum scattering to ionized impurity small angle scattering. A simple interpolation of all available data allows to extract a maximal-possible spin relaxation time in $n$-doped, bulk GaAs for negligible external magnetic fields of $\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{s}$.

Structural, elastic, electronic, optical and thermoelectric response of lead-free double perovskite Rb2TlInX6 (X=Cl, I) for energy storage devices: DFT+SOC investigations

Double perovskites Rb2TlInX6 (X = Cl, I) have been simulated with the help of density functional theory to determine their structural, elastic, electronic, optical and thermal properties. The full-potential linearized augmented plane wave (FP-LAPW) method is used through the WIEN2K code. Exchange-correlation potential is described by using the well-organized modified Becke-Johnson (mBJ) with the combination of spin-orbit coupling (SOC). Structural optimization and formation energy calculations justify the stability of both compounds. The elastic parameters are calculated to measure the mechanical stuff, such as shear modulus (G), Poisson ratio (ν) and anisotropic factor (A). The studied compounds showing semiconductor nature with a bandgap of 3.51eV and 3.47eV for Rb2TlInCl6 and 1.43eV and 1.40eV for Rb2TlInI6 with mBJ and mBJ+SOC potentials, respectively. The density of states also exposes the bandgap and semiconductor nature of the materials. The optical properties of the compounds have been examined in terms of the dielectric function, refractive index, absorption, reflection, and energy loss. We also investigated the temperature-dependent transport parameters for both compounds. High electrical and low thermal conductivity with good ZT values for both materials make them potential candidates for thermoelectric applications.

Electronic and optical characterization of bulk single crystals of cubic boron nitride (cBN)

Cubic boron nitride (cBN) is a relatively less studied wide bandgap semiconductor despite its many promising mechanical, thermal, and electronic properties. We report on the electronic, structural, and optical characterization of commercial cBN crystal platelets. Temperature dependent transport measurements revealed the charge limited diode behavior of the cBN crystals. The equilibrium Fermi level was determined to be 0.47 eV below the conduction band, and the electron conduction was identified as n-type. Unirradiated dark and amber colored cBN crystals displayed broad photoluminescence emission peaks centered around different wavelengths. RC series zero phonon line defect emission peaks were observed at room temperature from the electron beam irradiated and oxygen ion implanted cBN crystals, making this material a promising candidate for high power microwave devices, next generation power electronics, and future quantum sensing applications.

In Situ Synthesis of Multivariate Zeolitic Imidazolate Frameworks for C2 H4 /C2 H6 Kinetic Separation.

Herein, a new approach for the in situ synthesis of zeolitic imidazolate framework (ZIF) nanoparticles with triple ligands, referred to as Sogang ZIF-8 (SZIF-8), is reported for enhanced C2 H4 /C2 H6 kinetic separation. SZIF-8 consists of tetrahedral zinc metals coordinated with tri-butyl amine (TBA), 2,4-dimethylimidazole (DIm), and 2-methylimidazole (MIm). SZIF-8(x) with different DIm contents in x (up to 23.2 mol%) are synthesized in situ because TBA preferably deprotonates DIm ligands due to the much lower pKa of DIm over MIm, allowing for the Zn-DIm coordination. The Zn-DIm coordination reduces the window size of ZIF-8 with suppressed linker flipping motion due to bulky DIm ligands and simultaneously enhances the interfacial interaction between 6FDA-DAM polyimide (6FDA) and SZIF-8 via electron donor-acceptor interactions. Consequently, 6FDA/SZIF-8(13) mixed matrix membrane exhibits an excellent C2 H4 permeability of 60.3 Barrer and C2 H4 /C2 H6 selectivity of 4.5. The temperature-dependent transport characterization reveals that such excellent C2 H4 /C2 H6 kinetic separation is attained by the enhancement in size discrimination-based energetic selectivity. Our hybrid multi-ligand approach can offer a useful tool for the fine-tuning of molecular structures and textural properties of other metal organic frameworks.

Stability of combination of rarefaction waves with viscous contact wave for compressible Navier-Stokes equations with temperature-dependent transport coefficients and large data

Abstract In this article, we study the large-time behavior of combination of the rarefaction waves with viscous contact wave for a one-dimensional compressible Navier-Stokes system whose transport coefficients depend on the temperature. It is shown that if the adiabatic exponent γ is suitably close to 1, the unique solution global in time to ideal polytropic gas exists and asymptotically tends toward the combination of a viscous contact wave with rarefaction waves under large initial perturbation. New and subtle analysis is developed to overcome difficulties due to the smallness of γ – 1 to derive heat kernel estimates. Moreover, our results extend the studies in a previous work [F. M. Huang, J. Li, and A. Matsumura, Arch. Ration. Mech. Anal. 197 (2010), no. 1, 89–116].

The effect of heat treatment on temperature-dependent transport and magnetoresistance in polyacrylonitrile-based carbon fibers

The effects of heat treatment temperatures from 1000 to 2700 °C on the structural, chemical, and mechanical properties of polyacrylonitrile-based carbon fibers (CFs) were investigated. Structural changes affected mechanical properties, and in particular, a higher sp3 content induced a higher tensile strength below 1600 °C. For a comprehensive study on the heat treatment effect and the mechanism for developing high-strength CFs, electrical transport properties were measured for the CFs. The transport behaviors in temperature-dependent conductivity and magnetoresistance were also significantly modified, which led to crossover from strong localization to weak localization due to sp2 carbon ordering. Interestingly, in samples that were subjected to intermediate heat treatment temperatures ranging from 1100 to 1600 °C, anomalous metallic temperature-dependent behavior was observed at low temperatures of below 40 K. This unusual charge transport was analyzed further with a pertinent heterogeneous model employing three-dimensional variable range hopping, weak localization, and linear metallic transport. The results suggest that mixed noncrystalline and crystalline structures with sp2/sp3 carbons, oxygen, and quaternary N groups in CFs enabled the transition behavior at such low temperatures.

Optical Visualization of Photoexcitation Diffusion in All-Inorganic Perovskite at High Temperature.

All-inorganic halide perovskites are promising candidates for optoelectronic and photovoltaic devices because of their good thermal stability and remarkable optoelectronic properties. Among those properties, carrier transport properties are critical as they inherently dominate the device performance. The transport properties of perovskites have been widely studied at room and lower temperatures, but their high-temperature (i.e., tens of degrees above room temperature) characteristics are not fully understood. Here, the photoexcitation diffusion is optically visualized by transient photoluminescence microscopy (TPLM), through which the temperature-dependent transport characteristics from room temperature to 80 °C are studied in all-inorganic CsPbBr3 single-crystalline microplates. We reveal the decreasing trend of diffusion coefficient and the almost unchanged trend of diffusion length when heating the sample to high temperature. The phonon scattering in combination with the variation of effective mass is proposed for the explanation of the temperature-dependent diffusion behavior.

Charge. transport, conductivity and Seebeck coefficient in pristine and TCNQ loaded preferentially grown metal-organic framework films

This investigation on metal-organic framework (MOF) HUKUST-1 films focuses on comparing the undoped pristine state and with the case of doping by TCNQ infiltration of the MOF pore structure. We have determined the temperature dependent charge transport and p-type conductivity for HKUST-1 films. Furthermore, the electrical conductivity and the current–voltage characteristics have been characterized in detail. Because the most common forms of MOFs, bulk MOF powders, do not lend themselves easily to electrical characterization investigations, here in this study the electrical measurements were performed on dense, compact surface-anchored metal-organic framework (SURMOF) films. These monolithic, well-defined, and (001) preferentially oriented MOF thin films are grown using quasi-liquid phase epitaxy (LPE) on specially functionalized silicon or borosilicate glass substrates. In addition to the pristine SURMOF films also the effect of loading these porous thin films with TCNQ has been investigated. Positive charge carrier conduction and a strong anisotropy in electrical conduction was observed for highly oriented SURMOF films and corroborated with Seebeck coefficient measurements. Van der Pauw four-point Hall sample measurements provide important insight into the electrical behavior of such porous and hybrid organic–inorganic crystalline materials, which renders them attractive for potential use in microelectronic and optoelectronic devices and thermoelectric applications.

Temperature-dependent transition of charge transport in core/shell structured colloidal quantum dot thin films: From Poole–Frenkel emission to variable-range hopping

Solution-processed core/shell quantum dot films are of great significance for light-emitting diodes. It is well known that the operation of core/shell quantum dot-based light-emitting diodes largely relies on charge transport. However, the charge transport mechanism in quantum dot films is still under debate and inconclusive. Herein, the temperature-dependent charge transport properties of light-emitting core/shell colloidal quantum dot thin films are characterized and analyzed across a wide temperature range, and the charge transport mechanism is studied. The results reveal that Poole–Frenkel emission conduction is applicable in the high-temperature range. With the decrease in the temperature, the measured current can be described by the Efros–Shklovskii variable-range hopping model. It is worth noting that, in both cases, trap states and disorders in the quantum dot film play a very important role in charge transport. These findings are of great importance for optimizing quantum dot light-emitting diodes and understanding the effects of charge transport on the device performance.

Micropore Filling and Temperature Dependent Electrical Transport Aspects of Poly(3,4‐ethylenedioxythiophene) Polymerized Zr‐Based Metal–Organic Framework

This work reports the polymerization of poly(3,4‐ethylenedioxythiophene) (PEDOT) within a metal–organic framework (MOF), namely UiO‐66 containing pores of tetrahedral and octahedral geometry. Partial filling of micropores of UiO‐66 with PEDOT and occupancy up to 29.6% has brought up conformational changes, vibrational responses as well as altered carrier transport phenomena characterized through different techniques. X‐ray diffraction (XRD) and Brunner–Emmett–Teller (BET) surface area analysis have revealed the structural stability of the MOF after the inclusion of PEDOT while microporous nature was maintained with a primary filling of octahedral micropores. Interestingly, the PEDOT polymerization is believed to draw a conformational change from benzoid to quinoid structure as substantiated through the Raman studies. Moreover, a noticeable improvement in electrical conductivity has been realized (typically, ≈10−3 to 10−2 S cm−1) for the composite while its temperature dependency followed a semiconducting feature. Controlled insertion, limited space‐filling, and occupancy can strengthen the insights on soft matter models and reptation theory and would find scope in applications where porosity, stability, and conductivity are desired.

Study of optical and thermoelectric properties of double perovskites Cs2KTlX6 (X = Cl, Br, I) for solar cell and energy harvesting

The double perovskites have emerging optical properties and extensively studied solar cells and renewable energy. In this work, we have investigated the optical, electronic and transport properties of Cs2KTlX6 (X = Cl, Br, I) using density functional theory. The thermodynamic stability is determined from the calculation of enthalpy of formation and structural stability is probed by computing the tolerance factor. Energy-volume optimization graphs reveal that lattice constant increased (11.31–12.80 Å) and bulk modulus decreased (20.46–15.90 GPa) with the increasing size of halogens. The optical properties are computed using TB-mBJ which reveals direct bandgaps (3.10, 2.0, 0.96) eV for (Cl, Br, I). In addition, the dielectric constant, refractive index, and reflectivity has been discussed in detail in the energy range of 0–6 eV. The absorption bands are recorded in infrared, visible and ultraviolet regions for Cl, Br and I based DPs. The temperature and chemical potential dependent transport properties are computed using BoltzTrap code. The appropriate values of thermoelectric parameters with high figure of merit at room temperature show the potential of these DPs for thermoelectric applications.

Evaluation on Temperature-Dependent Transient VT Instability in p-GaN Gate HEMTs under Negative Gate Stress by Fast Sweeping Characterization.

In this work, temperature-dependent transient threshold voltage (VT) instability behaviors in p-GaN/AlGaN/GaN HEMTs, with both Schottky gate (SG) and Ohmic gate (OG), were investigated systematically, under negative gate bias stress, by a fast voltage sweeping method. For SG devices, a concave-shaped VT evolution gradually occurs with the increase in temperature, and the concave peak appears faster with increasing reverse bias stress, followed by a corresponding convex-shaped VT recovery process. In contrast, the concave-shaped VT evolution for OG devices that occurred at room temperature gradually disappears in the opposite shifting direction with the increasing temperature, but the corresponding convex-shaped VT recovery process is not observed, substituted, instead, with a quick and monotonic recovery process to the initial state. To explain these interesting and different phenomena, we proposed physical mechanisms of time and temperature-dependent hole trapping, releasing, and transport, in terms of the discrepancies in barrier height and space charge region, at the metal/p-GaN junction between SG and OG HEMTs.

High-Temperature Electronic Transport Properties of PEDOT:PSS Top-Contact Molecular Junctions with Oligophenylene Dithiols

In this study, we investigated the high-temperature electronic transport behavior of spin-coated PEDOT:PSS top-contact molecular ensemble junctions based on self-assembled monolayers (SAMs) of oligophenylene dithiols. We observed irreversible temperature-dependent charge transport at the high-temperature regime over 320 K. The effective contact resistance and normalized resistance decreased with increasing temperature (320 to 400 K), whereas the tunneling attenuation factor was nearly constant irrespective of temperature change. These findings demonstrate that the high-temperature transport properties are not dominated by the integrity of SAMs in molecular junctions, but rather the PEDOT:PSS/SAMs contact. Transition voltage spectroscopy measurements indicated that the contact barrier height of the PEDOT:PSS/SAMs is lowered at elevated temperatures, which gives rise to a decrease in the contact resistance and normalized resistance. The high-temperature charge transport through these junctions is also related to an increase in the grain area of PEDOT cores after thermal treatment. Moreover, it was found that there was no significant change in either the current density or normalized resistance of the annealed junctions after 60 days of storage in ambient conditions.

Role of defect states in hopping transport and photoconductivity properties of WSe2-FeS2 nanocomposite thin films

Transition metal dichalcogenides (TMDs) are one of the promising materials in the field of electronics and optoelectronics. The multifunctional TMD based nanocomposites such as WSe 2 -FeS 2 can also become a potential candidate for these applications. In this work, WSe 2 -FeS 2 nanocomposite thin films have been fabricated and studied for electrical transport and photoconductivity studies for the first time. Structural and optical analysis has been done, probing nanocomposite formation and defect states present in the deposited film. XPS analysis of WSe 2 -FeS 2 shows solid interfacial bonding in the deposited films. Raman analysis shows the presence of E 1 2 g mode for WSe 2 and E g , A g , T g modes for FeS 2 and shift in Raman peaks corresponds to the disorder in the film. Also, the blue (red) shift of FeS 2 (WSe 2 ) PL emission probes the presence of defect/trap states. Temperature-dependent (173–350 K) resistivity results reveal the presence of more than one type of carrier transport mechanism - thermally activated conduction and hopping transport. The chalcogenide vacancies in WSe 2 -FeS 2 affect the formation of trap states responsible for forming localized states. These localized states play a crucial role in the variable range hopping conduction at low temperatures. Whereas photoconductivity study yields a non-significant effect of light on photocurrent due to entrapment of carriers in trap states up to 250 K. Afterwards, a significant photocurrent increase was observed. The detailed mechanism considering all these aspects is explained in this work. It provides substantial new understandings for the carrier transport in WSe 2 -FeS 2 nanocomposite thin films. • Structural, morphological and optical analysis of WSe 2 - FeS 2 nanocomposite thin films. • Temperature dependent carrier transport studies by thermally activated and hopping conduction mechanism. • Persistent photoconductivity in WSe 2 - FeS 2 nanocomposite film due to entrapment of carriers in mid band gap trap states.

Temperature dependent charge transport in ferroelectrically gated graphene far from the Dirac point

Charge transport in ferroelectric (FE) gated graphene far from the Dirac point (DP) was studied in the temperature range 300 K < T < 350 K. A non-monotonic/monotonic/non-monotonic behavior in the conductivity [σ(T)] was observed as one moved away from the DP. As the gate polarization increased, additional impurity charges were compensated, which reduced charge scattering. The uncompensated charges doped graphene and σ(T) switched to a monotonic increase with increasing T. However, far from the DP, the polarization reached saturation, which resulted in still lower impurity charge scattering. The carrier concentration increased, and a non-monotonic response in σ(T) reappeared, which was attributed to phonon scattering. A theoretical model is presented that combined impurity charge and phonon scattering conduction mechanisms. The top gate polarizable FE provided a novel approach to investigate charge transport in graphene via controlled compensation of impurity charges, and in the process revealed non-monotonic behavior in σ(T) not previously seen in SiO2 back gated graphene devices.

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy.

The controlled introduction of covalent sp3 defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp3 defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp3 functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp3 defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks.

Josephson vortices and intrinsic Josephson junctions in the layered iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5

Modulated electronic state due to the layered crystal structures brings about moderate anisotropy of superconductivity in the iron-based superconductors and thus Abrikosov vortices are expected in the mixed state. However, based on the angular and temperature dependent transport measurements in iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5 with T c ≃ 12 K, we find clear evidences of a crossover from Abrikosov vortices to Josephson vortices at a crossover temperature T* ≃ 7 K, when the applied magnetic field is parallel to the superconducting FeAs layers, i.e., the angle between the magnetic field and the FeAs layers θ = 0°. This crossover to Josephson vortices is demonstrated by an abnormal decrease (increase) of the critical current (flux-flow resistance) below T*, in contrast to the increase (decrease) of the critical current (flux-flow resistance) above T* expected for Abrikosov vortices. Furthermore, when θ is larger than 0.5°, the flux-flow resistance and critical current have no anomalous behaviors across T*. These anomalous behaviors can be understood in terms of the distinct transition from the well-pinned Abrikosov vortices to the weakly-pinned Josephson vortices upon cooling, when the coherent length perpendicular to the FeAs layers ξ ⊥ becomes shorter than half of the interlayer distance d/2. These experimental findings indicate the existence of intrinsic Josephson junctions below T* and thus quasi-two-dimensional superconductivity in Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5, similar to those in the cuprate superconductors.

Demonstration of Molecular Tunneling Junctions Based on Vertically Stacked Graphene Heterostructures

We demonstrate the fabrication and complete characterization of vertical molecular tunneling junctions based on graphene heterostructures, which incorporate a control series of arylalkane molecules acting as charge transport barriers. Raman spectroscopy and atomic force microscopy were employed to identify the formation of the molecular monolayer via an electrophilic diazonium reaction on a pre-patterned bottom graphene electrode. The top graphene electrode was transferred to the deposited molecular layer to form a stable electrical connection without filamentary damage. Then, we showed proof of intrinsic charge carrier transport through the arylalkane molecule in the vertical tunneling junctions by carrying out multiprobe approaches combining complementary transport characterization methods, which included length- and temperature-dependent charge transport measurements and transition voltage spectroscopy. Interpretation of all the electrical characterizations was conducted on the basis of intact statistical analysis using a total of 294 fabricated devices. Our results and analysis can provide an objective criterion to validate molecular electronic devices fabricated with graphene electrodes and establish statistically representative junction properties. Since many of the experimental test beds used to examine molecular junctions have generated large variation in the measured data, such a statistical approach is advantageous to identify the meaningful parameters with the data population and describe how the results can be used to characterize the graphene-based molecular junctions.