Dependent Carrier Research Articles

Unraveling the cation dependent carrier cooling and transient mobility in lead-free A3Sb2I9 perovskites.

Lead halide perovskites (LHPs) have gained prominence for their exceptional photophysical properties, holding promise for applications in high-end optoelectronic devices. However, the presence of lead is one of the major obstacles to the commercialization of LHPs in the field of photovoltaics. To address this, researchers have explored environment friendly lead-free perovskite solar cells by investigating non-toxic perovskite materials. This study explores the enhancement of photophysical properties through chemical engineering, specifically cation exchange, focusing on the crucial photophysical process of hot carrier cooling. Employing femtosecond transient absorption spectroscopy and optical pump terahertz probe spectroscopy, we have probed the carrier relaxation dynamics in A3Sb2I9 with cesium and rubidium cations. This study unravels that the carrier relaxation is found to be slower in Rb3Sb2I9; along with this, the transient mobility decay is found to be retarded. Overall, this study suggests that an antimony-based Rb3Sb2I9 perovskite could be a substantial lead-free perovskite in photovoltaics. These findings provide valuable insights into cation engineering strategies, aiming to improve the overall performance of lead-free-based photovoltaic devices.

Size dependent photogeneration charge carrier density of hollow ZnIn2S4 microspheres for enhanced photocatalytic activity

The effect of the morphology of semiconductors on the photocatalytic performance is commonly observed but ambiguous due to the other structural changes usually occur simultaneously along with morphology variation. In this work, we have successfully constructed a series of ZnIn2S4 hollow microspheres (named ZIS-x, x = 0.6, 0.2, 0.15, 0.1, 0.06.) with systematically decreased size in both shell thickness and diameter of the sphere and comparable band structures, degree of crystallinity and density of defects as confirmed by extensive characterizations. Taking advantage of the above features, the decoupled effect of size on photocatalytic activity is investigated and established, which displays a volcano-type relationship. The best performed ZIS-0.1 displays a hydrogen evolution rate of 3.76 mmol g-1h−1, which is 2.66 times that of the worst performed ZIS-0.6. Further mechanistic studies disclose that the observed size dependent hydrogen evolution rate originates from the ability of charge carrier generation under light, which increases with the light harvesting ability of the hollow microspheres. This work explicitly reveals the effect of the morphology on the photocatalytic activity of semiconductors, which highlights the route to improve the photocatalytic performance morphologically.

Localization effect in single crystal of RuAs2

We report the magnetotransport and thermal properties of RuAs2 single crystal. RuAs2 exhibits semiconductor behavior and localization effect. The crossover from normal state to diffusive transport in the weak localization (WL) state and then to variable range hopping (VRH) transport in the strong localization state has been observed. The transitions can be reflected in the measurement of resistivity and Seebeck coefficient. Negative magnetoresistance (NMR) emerges with the appearance of localization effect and is gradually suppressed in high magnetic field. The temperature dependent phase coherence length extracted from the fittings of NMR also indicates the transition from WL to VRH. The measurement of Hall effect reveals an anomaly of temperature dependent carrier concentration caused by localization effect. Our findings show that RuAs2 is a suitable platform to study the localized state.

Organic cation dependent hot carrier relaxation dynamics in two-dimensional perovskites

Organic cation dependent hot carrier relaxation dynamics in two-dimensional perovskites

AlGaN High Electron Mobility Transistor for High Temperature Logic

We report on AlGaN HEMT-based logic development, using combined enhancement- and depletion-mode transistors to fabricate inverters with operation from room temperature up to 500°C. Our development approach included: (a) characterizing temperature dependent carrier transport for different AlGaN HEMT heterostructures, (b) developing a suitable gate metal scheme for use in high temperatures, and (c) over-temperature testing of discrete devices and inverters. Hall mobility data revealed the GaN-channel HEMT experienced a 6.9× reduction in mobility, whereas the AlGaN channel HEMTs experienced about a 3.1x reduction. Furthermore, a greater aluminum contrast between the barrier and channel enabled higher carrier densities in the two-dimensional electron gas for all temperatures. The combination of reduced variation in mobility with temperature and high sheet carrier concentration showed that an Al-rich AlGaN-channel HEMT with a high barrier-to-channel aluminum contrast is the best option for an extreme temperature HEMT design. Three gate metal stacks were selected for low resistivity, high melting point, low thermal expansion coefficient, and high expected barrier height. The impact of thermal cycling was examined through electrical characterization of samples measured before and after rapid thermal anneal. The 200 nm tungsten gate metallization was the top performer with minimal reduction in drain current, a slightly positive threshold voltage shift, and about an order of magnitude advantage over the other gates in on-to-off current ratio. After incorporating the tungsten gate metal stack in device fabrication, characterization of transistors and inverters from room temperature up to 500°C was performed. The enhancement-mode (e-mode) devices’ resistance started increasing at about 200°C, resulting in drain current degradation. This phenomenon was not observed in depletion-mode (d-mode) devices but highlights a challenge for inverters in an e-mode driver and d-mode load configuration.

Temperature dependent charge carrier dynamics in 2D ternary Cu2MoS4 nanoflakes: An effect of electron-phonon coupling.

Two-dimensional transition metal chalcogenides (2D TMCs) like MoS2, WS2 etc., have established significant dominance in the field of nanoscience and nanotechnology, owing to their unique properties like strong light-matter interaction, high carrier mobility, large photo-responsivity etc. Despite the widespread utilization of these binary TMCs, their potential in the advancement of the optoelectronic research is limited due to the constraints in band tuning and charge carrier lifetime. To overcome these limitations, ternary transition metal chalcogenides have emerged as promising alternatives. Although, the optical properties of these materials have never been explored properly. Herein, we have investigated one such promising member of this group, Cu2MoS4 (CMS) using both steady state and time-resolved spectroscopic techniques. The material exhibits a broad range of visible light absorption, peaking at 576nm. Photoluminescence spectroscopy confirmed the presence of both band gap emission and trap state-mediated emissions. Transient absorption spectroscopy unraveled the excited state charge carrier dynamics of CMS in sub-ps timescale, upon irradiation of visible light. We found significant influence of the trap mediated recombination, while Auger process being dominant at high charge density. We extended our study in a wide temperature range (5-300K), which reveals the impact of electron-phonon coupling strength on the band gap and charge carrier dynamics of this material. This detailed study would draw more attention toward the unexplored optical properties of ternary 2D chalcogenides and will open new avenues for the construction of 2D material-based optical devices.

New Ternary Blend Strategy Based on a Vertically Self-Assembled Passivation Layer Enabling Efficient and Photostable Inverted Organic Solar Cells.

Herein, a new ternary strategy to fabricate efficient and photostable inverted organic photovoltaics (OPVs) is introduced by combining a bulk heterojunction (BHJ) blend and a fullerene self-assembled monolayer (C60 -SAM). Time-of-flight secondary-ion mass spectrometry - analysis reveals that the ternary blend is vertically phase separated with the C60 -SAM at the bottom and the BHJ on top. The average power conversion efficiency - of OPVs based on the ternary system is improved from 14.9% to 15.6% by C60 -SAM addition, mostly due to increased current density (Jsc ) and fill factor -. It is found that the C60 -SAM encourages the BHJ to make more face-on molecular orientation because grazing incidence wide-angle X-ray scattering - data show an increased face-on/edge-on orientation ratio in the ternary blend. Light-intensity dependent Jsc data and charge carrier lifetime analysis indicate suppressed bimolecular recombination and a longer charge carrier lifetime in the ternary system, resulting in the enhancement of OPV performance. Moreover, it is demonstrated that device photostability in the ternary blend is enhanced due to the vertically self-assembled C60 -SAM that successfully passivates the ZnO surface and protects BHJ layer from the UV-induced photocatalytic reactions of the ZnO. These results suggest a new perspective to improve both performance and photostability of OPVs using a facial ternary method.

A specialized metabolic pathway partitions citrate in hydroxyapatite to impact mineralization of bones and teeth.

Citrate is a critical metabolic substrate and key regulator of energy metabolism in mammalian cells. It has been known for decades that the skeleton contains most (>85%) of the body's citrate, but the question of why and how this metabolite should be partitioned in bone has received singularly little attention. Here, we show that osteoblasts use a specialized metabolic pathway to regulate uptake, endogenous production, and the deposition of citrate into bone. Osteoblasts express high levels of the membranous Na+-dependent citrate transporter solute carrier family 13 member 5 (Slc13a5) gene. Inhibition or genetic disruption of Slc13a5 reduced osteogenic citrate uptake and disrupted mineral nodule formation. Bones from mice lacking Slc13a5 globally, or selectively in osteoblasts, showed equivalent reductions in cortical thickness, with similarly compromised mechanical strength. Surprisingly, citrate content in mineral from Slc13a5-/- osteoblasts was increased fourfold relative to controls, suggesting the engagement of compensatory mechanisms to augment endogenous citrate production. Indeed, through the coordinated functioning of the apical membrane citrate transporter SLC13A5 and a mitochondrial zinc transporter protein (ZIP1; encoded by Slc39a1), a mediator of citrate efflux from the tricarboxylic acid cycle, SLC13A5 mediates citrate entry from blood and its activity exerts homeostatic control of cytoplasmic citrate. Intriguingly, Slc13a5-deficient mice also exhibited defective tooth enamel and dentin formation, a clinical feature, which we show is recapitulated in primary teeth from children with SLC13A5 mutations. Together, our results reveal the components of an osteoblast metabolic pathway, which affects bone strength by regulating citrate deposition into mineral hydroxyapatite.

A realistic model of temperature dependent carrier diffusion constant in MAPbI3 films

The charge carrier mobility in perovskites is known to be affected by temperature. However, at low temperatures the perovskite film bears interfacial strains by deformation between the film and its substrate with difference coefficients of thermal expansion. Herein we provide a more realistic temperature-dependent carrier dynamics model involving both interfacial strain effect and intrinsic optoelectronic properties. The interfacial strain of a MAPbI3 film is confirmed through in-situ temperature-dependent X-ray diffractions. The carrier dynamics at different temperatures are unraveled using carrier dynamics fittings to time-resolved photoluminescence decays. An abnormal drop in carrier mobility is revealed with continuously decreasing temperature. The strain affects the trap density in films thus declines the carrier mobility at low temperatures. The carrier losses at the buried surface become significant at low temperatures due to the interfacial strain. This work provides a reminder that substrates should be carefully considered in temperature-related applications of perovskites.

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.

Structure and properties of the Sr2In1-xSnxSbO6 double perovskite

A series of n-type oxide double perovskite semiconductors, Sr2In1-xSnxSbO6 (0 ​≤ ​x ​≤ ​0.3) has been synthesized; Sn4+ partially substitutes for In3+. 121Sb and 119Sn Mössbauer spectroscopy are employed to investigate the B-site cation ordering because this issue cannot be resolved by conventional diffraction techniques alone. Rigid ordering between In3+/Sn4+ and Sb5+ sites is revealed by the spectroscopic method, and hence in combination with the structural parameters extracted from the XRD structural refinements, the crystallographic structure of this series of compounds is depicted. The temperature dependent magnetic susceptibilities, band gaps, and carrier type are characterized, and the calculated band structure is presented.

Magneto-tunable photoresponse in ZnO-rGO/La0.7Sr0.3MnO3/ITO heterostructure: An opto-spintronic phenomenon

We have investigated both red light (660 nm) and magnetic field (H) dependent carrier transport properties of ZnO-rGO/La0.7Sr0.3MnO3(LSMO)/ITO heterostructure at room temperature. H dependent current-voltage (I-V) response shows positive magnetoresistance behavior over the entire applied bias region. This phenomenon is attributed to larger spin dependent scattering of electrons at t↓2g band of LSMO depletion region with increase in H. Enhancement in device current with light intensity, at positive bias region and zero applied H, is supposed to be associated with dissociation of excitons at ZnO-rGO granular interface, ZnO-rGO/LSMO depletion region and LSMO active layer of the device. Device current is suddenly observed to decrease with light intensity at higher constant applied H of 1 kOe. Simultaneous light and magnetic field dependent modification of ZnO-rGO/LSMO depletion region is supposed to be the origin of such reversal of light dependent device current behavior at high magnetic field.

ELECTRONIC TRANSPORT PROPERTIES (LOW TEMPERATURE HALL EFFECT MEASUREMENT) OF WSe2 SINGLE CRYSTAL

WSe2 crystals grown by Direct Vapour Transport (DVT) technique. The structural properties of as grown crystals have been measured by X-ray diffraction analysis. The temperature dependence Hall coefficient study revealed that the grown crystal shows mixed conduction behavior in the temperature region 300K to 10K. The variation of mobility with temperature, also show that the WSe2 possesses a mixed scattering mechanism. The Hall scattering factor calculated from the temperature dependent Hall coefficient and carrier density. Detailed analysis of temperature dependent Hall effect data thus yields information concerning the electronic transport properties of tungsten diselenide semiconducting layered materials.

Excitation density dependent carrier dynamics in a monolayer MoS2: Exciton dissociation, formation and bottlenecking

Research on Molybdenum Disulfide (MoS2) monolayer at high excitation densities is critical due to its rising demand in optoelectronic applications such as lasers, compact optical parametric amplifiers, and high power detectors. We show that at high excitation densities, measurements on a single monolayer MoS2 reveal a transient response rich in physical processes. Despite the fact that carriers are excited straight to the A-excitonic state, the time and fluence dependence of the observed optical response indicates exciton dissociation followed by hot-carrier generation. In the picosecond time scale, these hot-carriers form excitons, which have a longer lifetime at higher excitation densities, implying a bottlenecking process. For the use of MoS2 at high-excitation densities, the fluence dependency of the relaxation mechanisms detailed here is essential.

Improved electrical resistivity-temperature characteristics of insulating epoxy composites filled with polydopamine-coated ceramic particles with positive temperature coefficient

Incorporating positive temperature coefficient (PTC) ceramic particles into polymer matrices provides a promising solution to suppress the distortion of DC electric field within polymeric insulation caused by the negative temperature coefficient (NTC) of electrical resistivity. However, along with the suppressed NTC effect comes the decrease of dielectric strength due to the agglomeration of PTC particles. Here, we addressed this issue in the epoxy resin system by coating polydopamine onto PTC particles to improve the compatibility with matrix, thus achieving a maintained DC breakdown strength. The improved evenness of PTC particles dispersion also provided sufficient interface to generate interface charge traps, which contributed to the increase of electrical resistivity at elevated temperature. Combined with temperature dependent charge carrier amount affected by PTC filler, NTC effect of epoxy composites was further mitigated and the maximum DC electric field was reduced by 49% within insulation assessed by the simulation of HVDC bushing. The method proposed in this paper is crucially important for the reliable and economic application of composite dielectrics under harsh conditions.

Temperature dependent carrier transport in few-layered MoS2: from hopping to band transport

Understanding the carrier transport mechanisms is critical for electronic devices based on 2D semiconductors. Here, using a two-terminal device configuration, we show that the carrier transport behaviours in chemical vapour deposited few-layer MoS2 transition from resonant tunnelling to hopping, and eventually to band transport as the temperature increases from 5 K to 370 K. Specifically, the transport in the channel is dominated by resonant tunnelling when T < 30 K is reflected in the temperature-independent conductance. At 50 K < T < 110 K, the channel conductance exhibits a dependence of exp(T 1/2), a signature of Efros–Shklovskii type variable range hopping (VRH). At 110 K < T < 160 K, carrier transport behaves in a transition region with potential attribution to Mott-type VRH. At 160 K < T < 210 K, the nearest neighbour hopping mechanism is confirmed by the linear dependence from the resistance curve derivative analysis. For VRH, the localization length, hopping distance and energy, Coulomb gap energy and density of states are extracted. At T > 210 K, the carrier transport is dominated by thermally activated band transport based on AC conductance and mobility analysis. These findings are significant for revealing the material properties for future 2D semiconductor device applications.

Investigating the key role of carrier transport mechanism in SnSe nanoflakes with enhanced thermoelectric power factor

A novel SnSe nanoflake system is explored for its thermoelectric properties from both experiments and ab initio study. The nanoflakes of the low temperature phase of SnSe (Pnma) are synthesized employing a fast and efficient refluxing method followed by spark plasma sintering at two different temperatures. We report an enhanced power factor (12–67 μW mK− 2 in the temperature range 300–600 K) in our p-type samples. We find that the prime reason for a high PF in our samples is a significantly improved electrical conductivity (1050–2180 S m−1 in the temperature range 300–600 K). From our ab initio band structure calculations accompanied with the models of temperature and surface dependent carrier scattering mechanisms, we reveal that an enhanced electrical conductivity is due to the reduced carrier-phonon scattering in our samples. The transport calculations are performed using the Boltzmann transport equation within relaxation time approximation. With our combined experimental and theoretical study, we demonstrate that the thermoelectric properties of p-type Pnma-SnSe could be improved by tuning the carrier scattering mechanisms with a control over the spark plasma sintering temperature.

Wavelength dependent photoinduced charge carrier dynamics of heterojunction materials: The case of CuO/ZnO

Heterojunction materials composed of narrow and wide band-gap semiconductors possess different photoelectric responses under different wavelength incident light, and show distinct photogenerated charge carrier dynamics. In this work, taking the hydrothermal method fabricated CuO nanosheets / ZnO nanorods heterojunction materials as example, the wavelength dependent photoinduced charge carrier dynamics in this heterojunction material have been carefully studied. Transient surface photovoltage (TSPV) measurements reveal that the photogenerated holes gather at the surface of the fabricated CuO / ZnO with 355 nm light illumination, but photogenerated electrons gather at the surface under 532 nm light. Photocatalytic degradation experiments suggest that CuO / ZnO heterojunction material has better performance under ultraviolet light than that under visible light.

Temperature Dependent Carrier Transport in Hydrogenated Amorphous Semiconductors for Thin Film Memristive Applications

This paper demonstrates the transport of electron and hole carriers in two distinct hydrogenated amorphous semiconductor materials at different temperatures. Compared to crystalline materials, the amorphous semiconductors differ structurally, optically and electrically, hence the nature of carrier transport through such amorphous materials differ. Materials like hydrogenated amorphous silicon and amorphous IGZO have been used for the study of temperature dependent carrier transport in this paper. Simulation results have been presented to show the variation of free electron and hole concentration, trapped electron and hole concentration with energy at 300K for both the materials. The change in mobility with a change in the Fermi level has been plotted for different temperatures. The effect of temperature on Brownian motion mobility of electrons and holes in hydrogenated amorphous silicon and amorphous IGZO has been demonstrated towards the end of this paper.

Insight into morphology dependent charge carrier dynamics in ZnSe-CdS nanoheterostructures.

Semiconductor nanoheterostructures (NHSs) are being increasingly used for the photocatalytic conversion of solar energy in which photo-induced charge separation is an essential step and hence it is necessary to understand the effect of various factors such as size, shape, and composition on the charge transfer dynamics. Ultrafast transient absorption spectroscopy is used to investigate the nature and dynamics of photo-induced charge transfer processes in ZnSe-CdS NHSs of different morphologies such as nanospheres (NSs), nanorods (NRs), and nanoplates (NPs). It demonstrates the fast separation of charge carriers and localization of both charges in adjacent semiconductors, resulting in the formation of a charge-separated (CS) state. The lifetime of the charge-separated state follows the order of NSs < NPs < NRs, emphasizing the effect of morphology on the enhancement of photo-induced charge separation and suppression of backward recombination. The separated charge carriers have been utilized in visible light driven hydrogen production and the hydrogen generation activity follows the same order as that for the lifetime of the CS state, underlining the role of charge separation efficiency. Therefore, the variation of the morphology of NHSs plays a significant role in their charge carrier dynamics and hence the photocatalytic hydrogen production activity.