Carrier Relaxation Time Research Articles

Structural and electrical properties of pure and doped lanthanum oxide

In this communication, structural and electrical properties of rare earth oxides La2O3 (LO) and LaNdO3 (LNO) have been studied. To understand the structural properties of the LO and LNO samples, X-ray diffraction (XRD) measurement was carried out at room temperature. The XRD patterns have been analyzed by Rietveld refinement to confirm the single-phase nature of both the samples. The crystal structures of studied samples were created from the derived parameters of Rietveld parameters. The crystal size and lattice strain have been estimated using Williamson–Hall (W–H) plot analysis. Frequency-dependent dielectric constant and loss tangent have been studied for a frequency range of 20 Hz to 2 MHz. To estimate the relaxation time and contribution of the charge carriers in the studied samples, relaxation mechanism and universal dielectric response (UDR) model have been employed. The ac conductivity measurements were carried out for the same frequency range (i.e., 20 Hz to 2 MHz) which has been understood on the basis of Jonscher’s power law. The barrier height has been calculated by fitting the power law. Frequency-dependent impedance behavior has been discussed in the context of grains and grain boundaries for both the samples under study.

Alternately Intercalated Heterostructures of Carbon and MoS2 Monolayers as Saturable Absorber for Ultrashort Femtosecond Mode‐Locked Lasers

Abstract2D van der Waals (VDWs) heterostructures have attracted much attention due to their exotic electronic and optical properties, which provide a new platform for material science and device fabrication. In this work, by coupling atomic interface engineering, an alternately intercalated superstructure of carbon and MoS2 monolayers (m‐C/MoS2) is designed as a novel saturable absorber (SA) for mode‐locked ultrafast laser generation. The novel intercalated structures exhibit excellent properties including wide bandgap, fast inter‐ and intra‐band carrier relaxation time, and superior saturable absorption characteristic, etc. Consequently, a continuous wave mode‐locked Yb:CALGO bulk laser with pulse width as short as 92 fs is achieved by using the prepared m‐C/MoS2 SA mirror, which is much shorter than those obtained with other reported 2D materials. This study provides a valuable strategy for the development of ultrashort pulse femtosecond lasers. The novel VDWs heterostructures also open new avenues toward advanced photonic devices.

Germanium Nanosheets with Dirac Characteristics as a Saturable Absorber for Ultrafast Pulse Generation.

Bulk germanium as a group-IV photonic material has been widely studied due to its relatively large refractive index and broadband and low propagation loss from near-infrared to mid-infrared. Inspired by the research of graphene, the 2D counterpart of bulk germanium, germanene, has been discovered and the characteristics of Dirac electrons have been observed. However, the optical properties of germanene still remain elusive. In this work, several layers of germanene are prepared with Dirac electronic characteristics and its morphology, band structure, carrier dynamics, and nonlinear optical properties are systematically investigated. It is surprisingly found that germanene has a fast carrier-relaxation time comparable to that of graphene and a relatively large nonlinear absorption coefficient, which is an order of magnitude higher than that of graphene in the near-infrared wavelength range. Based on these findings, germanene is applied as a new saturable absorber to construct an ultrafast mode-locked laser, and sub-picosecond pulse generation in the telecommunication band is realized. The results suggest that germanene can be used as a new type of group-IV material for various nonlinear optics and photonic applications.

Hot-carrier optoelectronic devices based on semiconductor nanowires

In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs is generated as so-called hot carriers with an excess kinetic energy that is typically lost as heat. The long-standing aim to harvest this excess energy to enhance device performance has proven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on increasing the hot carrier relaxation time and on tailoring heterostructures that allow for hot-carrier extraction on short time and length scales. Recently, semiconductor nanowires have emerged as a promising system to achieve these aims, because they offer unique opportunities for heterostructure engineering as well as for potentially modified phononic properties that can lead to increased relaxation times. In this review we assess the current state of theory and experiments relating to hot-carrier dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a foundation, we begin with a brief overview of the fundamental processes involved in hot-carrier relaxation and how these can be tailored and characterized in nanowires. We then analyze the advantages offered by nanowires as a system for hot-carrier devices and review the status of proof-of-principle experiments related to hot-carrier photovoltaics. To help interpret existing experiments on photocurrent extraction in nanowires we provide modeling based on non-equilibrium Green's functions. Finally, we identify open research questions that need to be answered in order to fully evaluate the potential nanowires offer toward achieving more efficient, hot-carrier based, optoelectronic devices.

Achievement of strain-driven ultrahigh carrier mobility in β-TeO2

Layered crystals of β-TeO2 with high carrier mobility and wider bandgap could be a promising candidate for nanoelectronics applications. Suitable mechanical flexibility permits strain-engineering in such layered structures to introduce anisotropy associated with electron and hole mobility. In this context, we have used the ab-initio based density functional theory (DFT) method along with the Boltzmann transport equation (BTE) to explore the strain-induced charge carrier anisotropy in mobility for β-TeO2. Furthermore, the lone pair associated with tellurium atoms leads to additional transport anisotropy in electron and hole mobility. Interestingly, the application of critical uniaxial compressive strain results in sudden amplification of carrier mobility, which has been discussed based on carrier scattering information and anisotropy in carrier effective mass derived from electronic dispersion curve. The strong electron and hole anisotropy in mobility can also be realized in terms of higher carrier relaxation time under uniaxial strain. Finally, all these findings make layered β-TeO2 an emerging candidate for power electronics applications and advanced transport devices.

Investigation into thermoelectric properties of M (M=Hf, Zr) X2(X=S, Se, Te) nanotubes using first-principles calculation

This study investigates the electronic and thermoelectric properties of (5, 0) single-wall M (M = Hf, Zr) X2(X = S, Se, Te) nanotubes by using first-principles calculations and semi-classical Boltzmann theory. Effective mass (m∗), Deformation potential (DP), Stretching modulus (C) Relaxation time (τ) and mobility of charge carriers (μ), for each nanotube were calculated using the deformation potential theory. The tubes, possess indirect bandgap varying between 1.12 and 0.075 eV and it is found that band gap was reduced upon increasing the chalcogen atomic size. Electrical conductivity (σ), Seebeck coefficient (S), electronic thermal conductivity (κe) and electronic part of Figure of merit (ZT) as a function of doping level were obtained for each nanotube. Our systematic study can recommend that a good thermoelectric material with a high Seebeck coefficient and merits be chosen.

Thermal conductivity and enhanced thermoelectric performance of SnTe bilayer

Tin chalcogenides (SnS, SnSe, and SnTe) are found to have improved thermoelectric properties upon the reduction of their dimensionality. Here we found the tilted AA + s stacked two-dimensional (2D) SnTe bilayer as the most stable phase among several stackings as predicted by the structural optimization and phonon transport properties. The carrier mobility and relaxation time are evaluated using the deformation potential theory, which is found to be relatively high due to the high 2D elastic modulus, low deformation potential constant, and moderate effective masses. The SnTe bilayer shows a high Seebeck coefficient, high electrical conductivity, and ultralow lattice thermal conductivity. High TE figure of merit (ZT) values, as high as 4.61 along the zigzag direction, are predicted for the SnTe bilayer. These ZT values are much enhanced as compared to the bulk as well as monolayer SnTe and other 2D compounds.

Experimental evidence of polarons in silver doped ZnO films

Pure and silver (0.27 at.%)-doped zinc oxide (Ag:ZnO) films were deposited on sapphire substrates by e-beam evaporation technique at 290 °C and post-annealed at 350 °C for 1 h under ambient air. The XRD analysis confirms the formation of wurtzite hexagonal structure. From the UV–Vis absorption spectra, the incorporation of Ag into the ZnO involves a slight decrease in the optical band gap as compared to that of undoped ZnO film. Infrared reflection spectral data of the Ag:ZnO films indicate hole polaron states placed in the range of 0.136–0.439 eV above the valence band maximum. According to the calculated electron–phonon coupling strength (1.04) and binding energy (− 76 meV), the polaron of large radius is defined. The polaron optical conductivity and carrier relaxation time are determined from the experimental spectra using Kramers–Kronig relations.

Hall effect and symmetry breaking in the nonmagnetic metal LuB12 with dynamic charge stripes

A comprehensive study of magnetoresistance and Hall effect has been performed for the set of the single crystals of non-magnetic metal LuB12 with the Jahn-Teller instability of the boron cage and dynamic charge stripes forming along <110> direction. Anomalous positive contribution to Hall effect for particular direction of magnetic field H//[001] is found in the single crystals of LuB12 of the highest quality. This contribution arising at T~ 150 K is shown to increase drastically when approaching the disordered ground state below 60 K. The Hall effect anomaly is shown to appear in combination with the peak of magnetoresistance. The various scenarios allowing for the topology of Fermi surface, anisotropy of relaxation time for charge carriers and the filamentary structure of fluctuating charge stripes are proposed to explain the features of magnetotransport in this metal with inhomogeneous distribution of electron density. The origin of SdH oscillations, which are observed in this non-equilibrium metal with electron phase separation and strong charge carrier scattering, is discussed.

Nanosecond dynamics in intrinsic topological insulator Bi2−xSbxSe3 revealed by time-resolved optical reflectivity

$\text{Bi}_2\text{Se}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets as compared to $n$-type bulk-metallic $\text{Bi}_2\text{Se}_3$ at low temperatures, which approaches $3.3 \text{ ns}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in $n$-type $\text{Bi}_2\text{Se}_3$, namely the slow bimolecular recombination of bulk carriers.

Ta2NiSe5 nanosheets as a novel broadband saturable absorber for solid-state pulse laser generation

Two-dimensional (2D) ternary chalcogenides have attracted great attentions because of their novel chemical and physical properties arising from the synergistic effect and stoichiometric variation with the additional third element compared with their binary counterparts. Here, high-quality 2D tantalum nickel selenide (Ta2NiSe5) nanosheets are successfully fabricated by a liquid-phase exfoliation (LPE) method. The ultrafast excited carrier relaxation time and nonlinear optical absorption response are investigated and reveal that the prepared 2D Ta2NiSe5 nanosheets have excellent broadband saturable absorption properties, which are further illustrated by three passively Q-switched (PQS) all-solid-state lasers operating at 1.0, 2.0 and 2.8 µm with the Ta2NiSe5 nanosheet-based saturable absorber (SA). Furthermore, mode-locked laser operation with the pulse width as short as 356 fs is also realized at 1.0 µm. This work not only demonstrates the excellent nonlinear optical proprieties and optical modulation performance of Ta2NiSe5, but also paves the way for exploring the photonic and optoelectronic proprieties of ternary chalcogenide materials.

Improved room-temperature thermoelectric characteristics in F4TCNQ-doped CNT yarn/P3HT composite by controlled doping

Abstract High room-temperature thermoelectric performance is important for low-grade waste heat power generation as there are plenty of heat thrown away uselessly in our daily life, most of which are below 100 °C. However, most of the thermoelectric materials are limited to high temperature application. In this work, room-temperature thermoelectric power factor of carbon nanotube (CNT) yarn is improved by controlled doping, which is achieved by making composite with poly 3-hexylthiophene −2, 5-diyl (P3HT) followed by doping with 2, 3, 5, 6-tetrafluo-7, 7, 8, 8-tetracyanoquinodimethane (F4TCNQ). The temperature-dependent Seebeck coefficient based on power–law model suggests that P3HT shifts the Fermi energy of CNT yarn towards the valence band edge, and reduces the ionic scattering and carrier relaxation time. As a result, the Seebeck coefficient is increased while the variation of Seebeck coefficient with temperature is reduced, and hence, the room-temperature thermoelectric power factor is improved. With controlled doping, the power factor of CNT yarn/P3HT composite reaches to 1640–2160 μW m−1K−2 at the temperature range of 25–100 °C, which is higher than that of CNT yarn alone.

Capacitance response of solar cells based on amorphous Titanium dioxide (A-TiO&lt;sub&gt;2&lt;/sub&gt;) semiconducting heterojunctions

In the present study, we studied the capacitance frequency response of amorphous titanium dioxide (A-TiO2)/poly(3-hexylthiophene) (P3HT) solar cells. The capacitance was measured to provide information on interfacial layer between the two materials. At a low frequency, the capacitance increased because the frequency was lower than the relaxation time of the charge carriers, thus providing evidence of the formation of a depletion region at the P3HT/A-TiO2 interface. The loss tangent was measured for applied voltage ranging from 0 to 1.5 V and the frequencies from 20 Hz and 1 MHz. Peaks in the loss tangent appeared as a function of the applied voltage due to changes in the transport and accumulation mechanisms of charge at the interface and the presence of oxygen molecules in the TiO2. The resulting C-V curves were used to calculate dopant concentration and the barrier's potential, which was found to 1017 cm−3 and 0.6 V, respectively. This confirmed the presence of a depletion region placed in the P3HT region and the validated barrier's potential effect on the open circuit voltage value. It was also shown that the light J-V characteristics of the A-TiO2/P3HT solar cells were independent of the ambient conditions because the conductivity of P3HT and the depletion region were not affected.

Boron quantum dots all-optical modulator based on efficient photothermal effect

All-optical devices without external electronic components have drawn extraordinary attentions in all-optical communication. In this work, boron quantum dots (BQDs) were synthesized by a facile liquid-phase exfoliation method. The as-prepared BQDs showed good structural homogeneity and crystallinity, broadband optical absorption as well as excellent photothermal properties. Femtosecond-resolved transient absorption further revealed the short carrier relaxation time of BQDs. Inspired by the outstanding photothermal properties and ultrafast carrier dynamic of BQDs, we fabricated BQDs-based all-optical modulator. The phase shift with a slope efficiency of 0.032 π/mW and response time of 0.97 ms can be achieved. The modulator was used in laser resonance cavity to achieve all-optical actively Q-switched laser operation with control repetition rate. This prototypical BQDs-based all-optical modulator shows a great potential to be applied in all-optical information processing and communication.

Determining Quasiparticle Bandgap of Two-Dimensional Transition Metal Dichalcogenides by Observation of Hot Carrier Relaxation Dynamics.

Using excitation-energy-scanning ultrafast infrared microspectroscopy, the excess energy-dependent hot carrier relaxation dynamics in atomically thin two-dimensional transition metal dichalcogenides (2D TMDs) after femtosecond photoexcitation was directly monitored. A good linear relationship between the carrier relaxation time and the excitation wavelength is observed for all measured monolayer (ML) and bilayer (BL) TMD samples, which allows us to determine their quasiparticle bandgaps as well as corresponding exciton binding energies. A carrier-optical-phonon scattering-mediated cascading-relaxation model is proposed, which can perfectly describe all the measured dynamics. As a consequence, the quasiparticle bandgaps of ML MoSe2, ML MoS2, BL MoSe2, and BL WSe2 are determined to be 2.07, 2.11, 1.67, and 1.81 eV, respectively. Our work reveals a general picture for the hot carrier relaxation dynamics in atomically thin TMDs and offers an effective experimental approach in probing the bandgaps of TMDs under ambient conditions.

High thermoelectric performance in two dimensional chalcogenides systems: GaSe and GaTe

Among the group-III chalcogenides, the two-dimensional (2D) GaSe and GaTe materials have been synthesized, but recent theoretical studies have raised controversial results regarding their thermoelectric (TE) properties. Hereby, systematically investigated the temperature and carrier concentration dependent TE properties of 2D GaSe and GaTe. We found that the GaSe had an indirect band gap of 2.94 eV while the GaTe had an indirect band gap of 1.88 eV. Both materials had almost the same Seebeck coefficients, but the p-type GaTe had the longest carrier relaxation time. We obtained the largest electrical conductivity over the thermal conductivity ratio in p-type GaTe compared with all other systems. This results in a very high p-type ZT of 0.91. Moreover, this high ZT performance is only changed by approximately 7% in a wide range of temperatures (300–700 K) and carrier concentration (1011–1013 hole cm−2). Compared with previously reported results, we find that it is necessary to consider the carrier relaxation time and spin–orbit coupling effect for determining reliable TE property. Overall, we propose that the p-type GaTe have outstanding TE property, and it can be utilized for potential TE device applications.

Tilted Dirac cone gapped due to spin-orbit coupling and transport properties of a 3D metallic system CaIr2Ge2

We present results of electrical transport (resistance, magnetoresistance, Hall effect) and heat capacity measurements performed on single crystals of the tetragonal compound, CaIr2Ge2. Their analysis is supported by the electronic structure data (band dispersion, density of states, Fermi surface), calculated for this three-dimensional (3D) system from first principles, using the full-potential local-orbital code. Interestingly, we have found the highly anisotropic Dirac cone at the Fermi level, in the bulk band structure, being gapped due to the strong p-d hybridization and spin-orbit coupling effects. However, this feature seems to have insignificant influence on the transport properties studied. The compound appears to be metallic-like with a rather low Sommerfeld coefficient (3.23 mJ mol−1 K−2) and non-superconducting even down to 0.1 K. In turn, the transverse magnetoresistance curves do not saturate with magnetic field up to 9 T revealing sub-quadratic scaling but relatively small values (up to 11 % in 2 K) as for Dirac semimetals. We may ascribe these properties to small values of the estimated relaxation time of charge carriers (~10−13 s) and, therefore, small electronic mobilities. Moreover, the angular magnetoresistance exhibits very small anisotropy (~ 1.7 %), in line with weakly anisotropic large 3D Fermi surface sheets, predicted by our calculations.

Role of inter- and intraconfigurational transitions of Pr3+ ion in nonequilibrium carrier relaxation in garnet-type scintillators

Abstract Transient optical absorption induced by selective photoexcitation is exploited for a subpicosecond study of the electronic excitation relaxation in Pr3+ acting as an activator ion in garnet-type scintillating materials. Both intra- and interconfigurational (4f2–4f15d1) channels are shown to be important. The latter one limits the rate of population of the emitting level of Pr3+ ion and, consequently, the luminescence rise time. The relaxation time of nonequilibrium carriers to the radiating level of 4f15d1 configuration after selective excitation of Pr3+ ion to higher excited levels is found to be ~4 ps. The energies of the Pr3+ levels involved in the excitation relaxation are estimated in (Lu–Y)3Al5O12:Pr. The obtained results show that the relaxation rate might be decreased by engineering of the crystal field in the vicinity of activator ion.

Temperature dependence of the relaxation time in scattering of elementary excitations in graphene

This work primarily analyses the temperature dependence of the relaxation time of elementary excitations – quasi-particles (electrons) in graphene. Various mechanisms of relaxation (electrons collisions with impurities, with phonons, with vacancies, etc.) are essential in transport processes. Therefore, for some of the most characteristic and most common collisions a relaxation time is obtained. Regardless of which method (for example, solving the Boltzmann equation with the adequate initial boundary conditions and similar) is used for studying the transport characteristics, these relaxation times play a fundamental role. Their role is particularly significant in obtaining the temperature dependence of coefficients of electronic and thermal conductivity, as measurable macroscopic transport characteristics. In this text, relaxation times have been obtained by analytical analysis of the scattering process in the characteristic processes mentioned above. However, the temperature dependence of the relaxation time, including the relevant transport values, is not possible to obtain without the adequate law of scattering of elementary excitations. Here in this work, it was obtained by the method of Green's functions and it was applied to determine the temperature dependence of the relaxation time of charge carriers in the main scattering processes in graphene.

Promoting Hot Carrier Extraction in Zn-Ag-In-Te Nanocrystals By Irradiation of High-Energy Light

Colloidal semiconductor nanocrystals (NCs), so-called quantum dots, offer attractive advantages such as low cost, large-scale solution processing, and tunable physicochemical properties depending on their size. Due to these advantages, it is expected to be used for highly efficient and low-cost energy conversion systems such as solar cells and photocatalysts. Since the relaxation rates of carriers in highly excited states are remarkably reduced by the discrete energy levels of semiconductor NCs, unique photoresponses of semiconductor NCs are induced by the absorption of photons with energy higher than the energy gap (Eg) of NCs. It has been reported that by extending the relaxation time of hot carriers, it is possible to transfer excess kinetic energy before releasing it as heat. Thus, next-generation solar cells of outstandingly high efficiency beyond the Shockley-Queisser limit can be developed if photogenerated carriers are harvested from a semiconductor NC without energy loss of incident photons. I-III-VI2 semiconductor-based NCs, such as CuInS2 or AgInS2 and their solid solution with ZnS, have been intensively studied as a visible-light-driven photocatalyst because of their excellent optical properties. Recently, we prepared ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1-x)Te2: ZAITe) NCs with a rod shape, the Eg of which was tunable in near-IR region from 1.20 to 1.60 eV with an increase in Zn content.1 These NCs are expected to act as near-IR photosensitizers and enable the use of NC-generated hot carriers. However, their photoelectrochemical properties and hot carrier behavior have not been investigated to date. In this study, we clarified the behavior of hot carrier in ZAITe NCs by measurering the photoelectrochemical properties of ZAITe NC-immobilized photoelectrodes.2 Furthermore, we prepared ZAITe NC-Methylviologen (MV2+) complexes to evaluate the dynamics of hot electron transfer from ZAITe NCs to MV2+ under the excitation of lights with various photon energies3. ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. TEM observations revealed that rod-shaped NCs with width of ca. 4 nm and length of ca. 11 nm were formed regardless of chemical composition of particles, that is, the x value. We revealed the electronic energy structures of ZAITe NCs with the compositions of x= 1.0, 0.75, and 0.5, the Eg values of which were 1.20, 1.27, and 1.46 eV, respectively. The band-edge potentials of conduction band (ECB) and valence band (EVB) were negatively and positively shifted with an increase in the Eg of NCs, the degree being more remarkable for the change in ECB. The photoelectrochemical behavior of ZAITe NCs immobilized on ITO electrode with thickness of less than one monoparticle, resembling a p-type semiconductor photoresponse, was remarkably different from that of thicker ZAITe NC multilayer films: The onset of cathodic photocurrent was shifted more positively than that of EVB of ZAITe NCs and the photocurrent generation efficiency was significantly enlarged, when photons of energy higher than ca. 2.5 eV were irradiated. The dependence of photocurrent-potential curves on the energy of irradiaited photons for thin ZAITe NC films were investivated. The magnitude of the cathodic photocurrent was increased with a negative shift of the applied pototential, regardless of the energy of excitation photons. Irradiation of photons with lower energy than 2.43 eV gave the onset potential at around 0 V vs. Ag/AgCl, being comparable to the EVB of ZAITe NCs with x = 0.75, -0.05 V vs. Ag/AgCl. In contrast, the increases of photon energy to 2.95 and 3.40 eV gave more positive onset potentials of 0.1 and 0.3 V vs. Ag/AgCl, respectively. we conclude that excitation with photons of > ca. 2.5 eV enabled direct injection of hot holes from ZAITe NCs into ITO electrodes, resulting in enhancement of the photocurrent generetion efficiency of ZAITe NCs. By using transient absorption spectroscopy, the migration of hot carriers in NCs can be directly observed, enabling better clarification of the photochemical properties of ZAITe NCs. Therefore, we also investivated the hot electron transfer from ZAITe NCs to MV2+ by masureing transient absorption spectra of the complexes. The details of this result will be discussed in the presentation. Reference Kameyama, et al., J. Mater. Chem. C, 2018, 6, 2034.Kameyama, et al., ChemNanoMat, 2019, 5, 1028.Kameyama, et al., ACS Adv., 2020, 10, 16361.