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We have previously predicted CuBiW2O8 (CBTO) as a novel quaternary metal oxide semiconductor with favorable band gap for applications in photocatalysis and photovoltaics. Here, we report the synthesis of CBTO and the first characterization of its optical, electrical and photoelectrical properties. We demonstrate a Cu-rich solid state synthesis method that enables the synthesis of CBTO powders and continuous thin films. The CBTO contains bulk Bi2WO6 impurity that is not removed by annealing at higher temperatures, as well as CuO surface impurity. Density Functional Theory (DFT) calculations show that Bi2WO6 co-exists with CBTO due to reasons of thermodynamic stability and has a type II band offset that may result in electron trapping. CBTO is measured to have a direct band gap of ∼1.9–2.0 eV (which is smaller than existing oxides Cu2O and BiVO4), and optical absorption coefficient of 104 to 105 cm−1 for visible-wavelength photons. DFT calculations match these results and show that Cu-vacancies are responsible for p-type conductivity, which was also measured by Hall effect. In addition, Hall effect, time-resolved photoluminescence (TRPL), and optical pump – THz probe spectroscopy (OPTP) measurements reveal that both slow hopping transport of trapped carriers (with 0.32 cm2 V−1 s−1 mobility) over nanosecond timescales and fast band-like motion of free carriers (with ∼150 cm2 V−1 s−1 mobility) over picosecond timescales result in comparable diffusion lengths of ∼10 nm. However, because this carrier diffusion length is shorter than the optical absorption depth (100–200 nm), nanostructured heterojunctions will likely be needed to achieve efficient solar energy conversion.

Replacing some Bi by In in Bi2Se3 transforms it from a topological insulator to a band insulator. Here, we have used time-resolved terahertz spectroscopy to investigate photoexcited carrier dynamic...

MXenes are a recently discovered family of two-dimensional transition metal carbides, nitrides, and carbonitrides with electronic properties that can be tuned by their chemistry and structure. Herein THz spectroscopy was used to investigate the microscopic conductivity and photoexcited charge carrier dynamics in two Mo-based MXenes: Mo2Ti2C3Tz and Mo2TiC2Tz. We find that both have high intrinsic carrier densities (∼1020 cm–3 in Mo2Ti2C3Tz and ∼1019 cm–3 in Mo2TiC2Tz) and mobilities and exhibit high conductivities within individual nanosheets. Optical excitations result in a transient conductivity increase in both compositions, in stark contrast with the most studied member of the MXene family, Ti3C2Tz, where photoexcitation suppresses the conductivity for nanoseconds. Deintercalation of water, and other species, from between the nanosheets by mild vacuum annealing at 200 °C further improves the long-range, internanosheet transport of the photoexcited carriers and increases their lifetime. High, and long-l...

High electrical conductivity and strong absorption of electromagnetic radiation in the terahertz (THz) frequency range by metallic 2D MXene Ti3C2Ty make it a promising material for electromagnetic interference shielding, THz detectors, and transparent conducting electrodes. Here, we demonstrate that ultrafast optical pulses with wavelengths straddling the visible range (400 and 800 nm) induce transient broad-band THz transparency in the MXene that persists for nanoseconds. We demonstrate that optically induced transient THz transparency is independent of temperature from 95 to 290 K. This discovery opens new possibilities for development of switchable electromagnetic interference shielding materials and devices that can be rendered partially transparent on demand for transmitting THz signals, or for designing new THz devices such as sensitive optically gated detectors.

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Replacing some of the Bi atoms with In transforms Bi2Se3 from a topological to a band insulator. We have used time-resolved terahertz spectroscopy to study carrier dynamics in (Bi1-xInx)2Se3 films across this transition.

One route to a brighter global energy future may be through enhancing the efficiency and stability of perovskite solar cells (PSCs), which depends on the level of defects in the photoactive absorber and along the interfaces of the multilayered structure. Here, we use a combined experimental and theoretical approach to study the effects of pressure-induced compaction of microvoids and closure of cracks on the power conversion efficiency (PCEs) and stability of formamidinium-rich PSCs. A range of mechanical pressures was applied to the PSCs to reduce pre-existing grain-boundary voids and interfacial cracks within the devices. The PCEs of the PSCs increased from ~19.5% to ~ 23.5% for applied pressures between ~ (0 – 7) MPa. Unlaminated device stability increased by 33%, falling to 80% of initial PCE in 1800 hrs without compression, as compared to 2400 hrs with compression. The implications of this study are discussed in light of possible future manufacturing processes.

We investigate properties of reduced graphene oxide films in the THz frequency range. We demonstrate tuning their conductivity and THz attenuation by reduction as well as by intercalation of metals. These findings suggest that graphene oxide holds promise for electromagnetic interference shielding and other THz device applications.

Intense THz pulses have been shown to induce non-thermal biological effects at multiple scales of structural organization, including differential gene expressions in human skin tissue. Epidermal differentiation is a calcium-regulated process, and an excellent candidate for studying genetics of THz-induced biological effects due to the strong over-representation of THz-affected genes in these processes. Results indicate application of THz for targeted inhibition of mitotic activity in skin, and additionally suggest membrane permeation as a key mechanism of THz-induced biological effects. Preliminary experiments investigating fluorescence detection of membrane permeability and techniques of field enhancement in THz-exposed cell lines will be discussed.

Electronic properties of 2D MXenes can be tuned by their chemistry and structure. We report on THz conductivity and ultrafast photoexcited carrier dynamics in several members of MXene family with high intrinsic free carrier densities and carrier mobility. We find that MXenes of different chemistries respond to photoexcitation in dramatically different ways, exhibiting behaviors from conductivity suppression to long-lived photoconductivity. We discuss applications of MXenes in a variety of electro-optical and THz devices.

Metallic 2D Ti 3 C 2 T y MXene shows high electrical conductivity and strong absorption of electromagnetic radiation in the THz frequency range. We demonstrate that optical pulses (400nm and 800nm) induce transient broadband THz transparency in this MXene, which lasts for nanoseconds and is independent of temperature from 95 to 290 K. This optically controlled THz electromagnetic interference shielding material could be exploited in future THz communication systems.

MXenes are 2D transition metal carbides and nitrides with electronic properties that can be tuned by their chemistry and structure. Three members of MXene family, Ti3C2Tz , Mo2Ti2C3Tz and Mo2TiC2Tz are all intrinsically metallic, with high free carrier densities and high carrier mobility within individual nanosheets. However, they respond to photoexcitation in dramatically different ways: while photoexcitation suppresses conductivity in Ti3C2Tz, it results in a long-lived positive photoconductivity in both Mo2Ti2C3Tz and Mo2TiC2Tz. Those responses suggest applications of MXenes in a variety of electro-optical and THz devices.

MXenes are 2D transition metal carbides and nitrides with electronic properties that can be tuned by their chemistry and structure. Three members of MXene family, Ti3C2Tz , Mo2Ti2C3Tz and Mo2TiC2Tz are all intrinsically metallic, with high intrinsic free carrier densities and high carrier mobility within individual nanosheets. However, they respond to photoexcitation in dramatically different ways: while photoexcitation suppresses conductivity in Ti3C2Tz, it results in a long-lived positive photoconductivity in both Mo2Ti2C3Tz and Mo2TiC2Tz. Those responses suggest applications of MXenes in a variety of electro-optical and THz devices.

MXenes are a new family of two-dimensional transition metal carbides, nitride and carbonitrides with high conductivity and versatile chemical structures. Here we have used THz spectroscopy to study microscopic conductivity and photoinduced carrier dynamics in two Mo-based MXenes, Mo2Ti2C3Tz and Mo2TiC2Tz. Both exhibit high intrinsic carrier densities (~ 1020 cm-3 in Mo2Ti2C3Tz, and ~ 1019 cm-3 in Mo2TiC2Tz), mobilities, and high conductivities within individual nanosheets. We also observe that optical excitation increases their conductivity, unlike Ti3C2Tz, in which photoexcitation suppresses conductivity for nanoseconds. Vacuum annealing improves the long-range transport of photoinduced carriers and further increases their lifetime, as it results in de-intercalation of water and other species from van der Waals gaps between the nanosheets in the films. High and long-lived photoinduced conductivity suggests Mo-based MXenes a promising candidate for optoelectronic, sensing and photoelectrochemical applications.

We use time-resolved THz spectroscopy to study microscopic conductivity and photoinduced carrier dynamics in MBE-grown 100 nm thick (Bi1-xInx)2Se3 thin films with indium concentration varying from x=0 to x=0.5. Both intrinsic and photoinduced conductivity in Bi2Se3 is significantly higher compared to the films with x=0.25 and x=0.50, with carriers that are not constrained by the twin domain boundaries and exhibit high mobility of 1100 cm2/Vs. We find that introducing indium with concentration of x=0.25 and higher, above the threshold for a topological to trivial transition, suppresses both intrinsic and photoinduced conductivity by over an order of magnitude and reduces the lifetime of photoexcited carriers. These findings demonstrate that controlling indium concentration in (Bi1-xInx)2Se3 films provides an avenue to design (Bi1- xInx)2Se3 films with desired properties for high-speed optoelectronic devices.

Germanium sulfide (GeS) is a 2D semiconductor with high carrier mobility and a moderate band gap (~1.5 eV for multilayer crystals), which holds promise for high-speed optoelectronics and energy conversion. Here, we use time resolved THz spectroscopy to investigate how intercalation of Au, Cu, and Sn impacts the photoexcited carrier dynamics and transient photoconductivity of GeS nanoribbons. We find that zero-valent metals affect the photoexcited carrier lifetime and mobility in different ways. Intercalation of GeS with Cu reduces the lifetime of carriers from ~ 120 ps to 60 ps, while Au and Sn intercalation do not. At the same time, intercalation with Cu, Sn and Au significantly enhances the scattering time of photoexcited carriers (~120 fs vs ~65 fs without intercalation), highlighting the potential of zero-valent metal intercalation as a tool for engineering the optoelectronic properties of GeS nanostructures for application in high-speed electronic devices.

Group-IV monochalcogenides is a family of layered van der Waals materials that includes GeS, GeSe, SnS and SnSe. Its members exhibit carrier mobility that rivals single crystalline silicon, exceptional thermoelectric properties and band gaps in the visible and near-infrared range. Highly anisotropic distorted orthorhombic lattice structure of group-IV monochalcogenides results in anisotropic electronic and optical properties, and in the limit of a single monolayer is predicted to produce a large spontaneous ferroelectric polarization and prominent second order nonlinear optical effects. Experiments suggest that such effects also occur at the surface of multilayer GeS and GeSe, paving the wave for applications of these materials in nonlinear optical devices, terahertz sources and solar energy conversion devices based on bulk photovoltaic effects. In this chapter, we provide an overview of the structure, electronic, linear and nonlinear optical properties of layered group-IV monochalcogenides and discuss current approaches to their fabrication.

High electrical conductivity and strong absorption of electromagnetic radiation in the terahertz (THz) frequency range by metallic 2D MXene Ti

The strong interaction between terahertz (THz) radiation and biological systems has motivated the development of several biomedical technologies, including imaging and spectroscopy applications with promising potential for improved disease diagnosis. This interaction mechanism also implies that external excitation with intense pulses of THz energy could couple to important biological structures and induce significant downstream phenotypic effects. In this study, we expose human skin tissue models to a prolonged train of high-intensity THz pulses and measure the resulting global differential gene expression. From these data, signal pathway perturbation analysis identified pathways that are predicted to be significantly dysregulated, including the cytokine-cytokine receptor interaction and glioma pathways, and further identified the gene-level mechanisms predominantly responsible. These results indicate that induction of an inflammatory-like response and suppression of division/differentiation in cancer are possible. These effects could be further explored and characterized in different types of normal and cancerous tissues to determine potential novel clinical applicability of intense THz pulses.

Annealing is one of the processing methods that are used for the fabrication of defect-free, photoactive perovskite films with compact grains in highly efficient and stable perovskite solar cells (PSCs). Thus, the annealing temperature is a key parameter for the control of the interdiffusion (of constituent elements) in photoactive films. In this paper, we present the results of a systematic study of the effects of annealing on the interdiffusion of constituent elements in efficient formamidinium-based PSCs. We also explore the effects of annealing-induced interdiffusion on layer microstructures, local strains, and the optoelectronic properties of perovskite films. We observe a dramatic upward diffusion of tin (Sn) and titanium (Ti) from fluorine-doped tin oxide and titanium dioxide (TiO2) to the perovskite films. We also observe a downward diffusion of lead (Pb) and iodine (I) from the perovskite films to the mesoporous layer of the electron transporting layer (ETL), after annealing at temperatures between 100 and 150 °C. The diffused I substitutes for Ti in the ETL, which improves the optoelectronic properties of the films, for annealing temperatures between 100 and 130 °C. The annealing-induced interdiffusion that occurs at higher temperatures (between 140 and 150 °C) results in higher levels of interdiffusion, along with increased local strains that lead to the nucleation of pores and cracks. Finally, the implications of the results are discussed for the design of PSCs with improved photoconversion efficiencies and stability.

Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with band gaps in the visible range as candidates for so-called shift current photovoltaics with efficiencies not constrained by the Schockley–Queisser limit. Recent theoretical works have predicted that two-dimensional IV–VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation, and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third-generation photovoltaics ...

Bi2S3 is a non-toxic n-type semiconductor, which has been commonly synthesized in the form of quantum dots or nanocrystalline films by solution deposition methods. Despite a favorable optical band gap of ∼1.3 eV, such films have not achieved high solar energy conversion efficiencies to date. We hypothesize that this is in part due to the presence of sulfur vacancies that, according to our density functional theory calculations, form a deep trap state in the band gap of Bi2S3, which can act as a strong recombination channel for photoexcited charges. Here, we report a microcrystalline Bi2S3 thin-film synthesized by annealing solution-deposited nanocrystalline Bi2S3 in a sulfur vapor environment at 445 °C, which simultaneously increases the grain size and phase purity of Bi2S3, fills in sulfur vacancies, and improves optical absorption. Time-resolved terahertz spectroscopy (TRTS) reveals that sulfur annealing increases the photoexcited carrier lifetime from sub-picosecond to ∼30 picoseconds, while the internal quantum efficiency of a photoelectrochemical solar cell device is increased 4-fold from ∼10% to ∼40%. In addition, TRTS reveals that the intra-grain carrier mobility in the sulfur-annealed films is ∼165 cm2 V−1 s−1 and the long-range mobility is ∼111 cm2 V−1 s−1 at short times, indicating that carriers are able to hop across grain boundaries. These results indicate that annealing in sulfur vapor can produce simultaneously high light absorption and charge separation efficiencies by achieving carrier diffusion length that is comparable to the light absorption depth, leading to high solar energy conversion efficiencies in Bi2S3.

Intense pulses of THz radiation significantly alter gene expression and dysregulate phenotypic endpoints of cancer-related signaling pathways, including activation of an inflammatory response and suppression of pro-mitotic signaling in human skin. Of 1681 potential candidates, the predicted cancer-related dysregulation is nearly entirely localized to a subset of only 42 key driver genes belonging to 7 gene families/superfamilies. The individual and concerted regulatory roles of these genes within the relevant cancer-related pathways are elucidated and discussed in the context of gene-specific response to THz exposure and potential for clinical application.

Terahertz (THz) radiation has shown unique advantages in biomedical applications for novel diagnostic technologies due to the high sensitivity to molecular structure and chemical concentration. However, emerging evidence shows that intense pulses of THz radiation can induce significant non-thermal biological effects that must be characterized. In human skin exposed to intense THz pulses, relatively large responses characterized by differential gene expression profiles are observed. These data are analyzed by signaling pathway perturbation analysis to predict phenotypic endpoints and dysregulatory effects on cancer-related processes. The activities of several important pathways that drive the initiation, development, and progression of many human cancers are predicted to be suppressed, and this effect is intensity-dependent. Some affected pathways are targets for current and emerging anti-cancer therapies. In particular, the activity of the Ras signaling and Calcium signaling pathways is predicted to be significantly inhibited. These results indicate the possibility of an additional therapeutic mechanism of intense THz pulses, due to the potential for targeted suppression of pro-mitotic activity in diseased tissue.

This paper presents the results of experimental and theoretical studies of the effects of pressure and thermal annealing on the photo-conversion efficiencies (PCEs) of polymer solar cells with active layers that consist of a mixture of poly(3-hexylthiophene-2,5-diyl) and fullerene derivative (6,6)-phenyl-C61-butyric acid methyl ester. The PCEs of the solar cells increased from ∼2.3% (for the unannealed devices) to ∼3.7% for devices annealed at ∼150 °C. A further increase in thermal annealing temperatures (beyond 150 °C) resulted in lower PCEs. Further improvements in the PCEs (from ∼3.7% to ∼5.4%) were observed with pressure application between 0 and 8 MPa. However, a decrease in PCEs was observed for pressure application beyond 8 MPa. The improved performance associated with thermal annealing is attributed to changes in the active layer microstructure and texture, which also enhance the optical absorption, mobility, and lifetime of the optically excited charge carriers. The beneficial effects of applied pressure are attributed to the decreased interfacial surface contacts that are associated with pressure application. The implications of the results are then discussed for the design and fabrication of organic solar cells with improved PCEs.

We explore ultrafast carrier dynamics and interactions of photoexcited carriers with lattice vibrational modes in BiVO4 photoanode material using time-resolved terahertz spectroscopy and first-principles phonon spectrum calculations. We find that photoexcited holes form bound polaron states by introducing lattice distortion that changes phonon spectrum and suppresses the Ag phonon mode associated with opposite motion of Bi and VO4 molecular units. At excitation fluence higher than 1 mJ cm−2 (or 2 × 1015 cm−2 per pulse), lattice distortion due to self-localized holes alters the lattice symmetry and vibrational spectrum, resulting in bleaching of THz absorption by Ag phonons. Concurrently, we observe a short lived population of free carriers which exhibit Drude conductivity with mobility on the order of 200 cm2 V−1 s−1, orders of magnitude higher than typical carrier mobility in BiVO4. The anomalously high carrier mobilities are explained in the framework of a Mott transition. This demonstration of enhanced transport suggests how engineering BiVO4 photoanodes to take advantage of free carrier transport under high excitation conditions may in the future significantly enhance performance of photoelectrochemical devices.

We use time-resolved THz spectroscopy to study the effects of photoexcitation on conductivity and ultrafast carrier in 2D layered materials with different electronic properties: metallic transition metal carbides (MXenes) [1] and semiconducting germanium sulfide (GeS) [2].

The biological effects of intense THz pulses on human skin tissue models are investigated by applying gene ontology and pathway perturbation analyses to measured global differential gene expression profiles. Biological processes and cellular structures that regulate epidermal differentiation are highly overrepresented by THz-affected genes, and several signaling pathways that regulate the development of human cancer are predicted to be dysregulated. In particular, the calcium signaling pathway is predicted to be suppressed by THz exposures, and this effect increases with THz intensity. Mechanisms for the predicted suppression are identified and discussed in the context of potential clinical considerations of intense THz pulses.

We show how intense terahertz pulses can modulate photoluminescence lineshape and efficiency in direct gap semiconductors through hot-carrier-enhancement of high energy PL and diffusion induced quenching of low energy photoluminescence. We compare these effects in the GaAs and InP.

The role of surface species in the optical properties of silicon nanocrystals (SiNCs) is the subject of intense debate. Changes in photoluminescence (PL) energy following hydrosilylation of SiNCs with alkyl-terminated surfaces are most often ascribed to enhanced quantum confinement in the smaller cores of oxidized NCs or to oxygen-induced defect emission. We have investigated the PL properties of alkyl-functionalized SiNCs prepared using two related methods: thermal and photochemical hydrosilylation. Photochemically functionalized SiNCs exhibit higher emission energies than the thermally functionalized equivalent. While microsecond lifetime emission attributed to carrier recombination within the NC core was observed from all samples, much faster, size-independent nanosecond lifetime components were only observed in samples prepared using photochemical hydrosilylation that possessed substantial surface oxidation. In addition, photochemically modified SiNCs exhibit higher absolute photoluminescent quantum y...

We investigate the biological effects of intense THz pulses by measuring global differential gene expression profiles of THz-exposed human skin tissue models. The onset of significant differences in expression was found to be energy-dependent. Differentially expressed genes were subsequently analyzed in the context of known roles and interaction properties of the associated proteins. Protein signaling pathways that are often mutated/over-active in cancer (MAPK, RAS, and PI3K-AKT) were found to be significantly dysregulated, suggesting the potential for both tumor suppressing and oncogenic effects induced by exposures to intense THz pulses. Predictions are offered for the overall phenotypic endpoint that could result from the observed gene expression profiles, and potential targets for novel applications in cancer therapeutics are identified.

The Drude-Smith model has been used extensively in fitting the THz conductivities of nanomaterials with carrier confinement on the mesoscopic scale. Here, we show that the conventional 'backscattering' explanation for the suppression of low-frequency conductivities in the Drude-Smith model is not consistent with a confined Drude gas of classical non-interacting electrons and we derive a modified Drude-Smith conductivity formula based on a diffusive restoring current. We perform Monte Carlo simulations of a model system and show that the modified Drude-Smith model reproduces the extracted conductivities without free parameters. This alternate route to the Drude-Smith model provides the popular formula with a more solid physical foundation and well-defined fit parameters., 18 pages, 8 figures, accepted to PRB, various changes and clarifications

We have observed emission of terahertz radiation from photoexcited GeS nanosheets without external bias. We attribute the origin of terahertz pulse emission to the shift current resulting from inversion symmetry breaking in ferroelectric single- or few-layer GeS nanosheets.

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3-4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies.

Ultrafast studies1,2 of excitations on the nanometre scale are essential for guiding applications in nanotechnology. Efforts to integrate femtosecond lasers with scanning tunnelling microscopes (STMs)3 have yielded a number of ultrafast STM techniques4,5,6,7,8,9,10,11,12,13,14, but the basic ability to directly modulate the STM junction bias while maintaining nanometre spatial resolution has been limited to ∼10 ps (refs 7,8) and has required specialized probe or sample structures. Here, without any modification to the STM design, we modulate the STM junction bias by coupling terahertz pulses to the scanning probe tip of an STM and demonstrate terahertz-pulse-induced tunnelling in an STM. The terahertz STM (THz-STM) provides simultaneous subpicosecond (

BiVO 4 is a promising material for solar energy conversion applications. However, it suffers from low mobility due to strong carrier-phonon coupling, which favors formation of polarons. To elucidate carrier transport mechanisms in BiVO 4 , we apply time-resolved THz spectroscopy to probe dynamics of photexcited carriers and their coupling to lattice vibrational modes at early times (1–20 ps) after photoexcitation. We find that at high excitation fluences, trap states are saturated and a short-lived population of highly mobile residual carriers exists. At the same time, saturation of hole trap states manifests itself in changes in the low-frequency (1.92 THz) seesaw phonon mode between the Bi and VO 4 units.

Temperature-dependent polarized microphotoluminescence measurements of single GaAsAlGaAs core-shell nanowires are used to probe their electronic states. The low-temperature emission from these wires is strongly enhanced compared with that observed in bare GaAs nanowires and is strongly polarized, reflecting the dielectric mismatch between the nanowire and the surrounding air. The temperature-dependent band gap of the nanowires is seen to be somewhat different from that observed in bulk GaAs, and the PL rapidly quenches above 120 K, with an activation energy of 17 meV reflecting the presence of nonradiative defects. © 2006 American Institute of Physics.