Photoexcited Carrier Concentration Research Articles

Plasma-assisted annealing of Pt-doped rutile TiO2 nanoparticles for enhanced decomposition and bacterial inactivation under general lighting

Enhanced photocatalytic activity of rutile-based TiO2 materials under general lighting is practically desired. O2 plasma-assisted annealing (PAA) effects on Pt-doped rutile TiO2 nanoparticles were clarified along with its visible-light-driven photocatalytic activity enhancement. The PAA-treated samples were mainly analyzed using optical spectroscopy and x-ray photoelectron spectroscopy (XPS). The photocatalytic activity was assessed by decomposing methylene blue dye and inactivating Bacillus subtilis under general lighting. The PAA treatment changed the O 1s, Ti 2p, and Pt 4f spectra of XPS from those of the pristine sample. This change indicated that the PAA treatment introduced more oxygen deficiency or oxygen vacancies and more oxygen groups adsorbed on the surface. The introduced oxygen vacancies and adsorbed oxygen groups would change the band structure, which primarily narrowed the bandgap energy or broadened the valence band edge, increased the number of electron-trapping sites from the shallow to midgap levels, and enhanced the upward band-bending at the surface. The PAA-induced change in the band structure enhanced the decomposition and bacterial inactivation because it facilitated the separation and concentration of photoexcited carriers. The findings provide a new perspective on enhancing the photocatalytic activities of rutile-based TiO2 nanoparticles under general lighting.

Controlling the properties of PbTiO3/SrTiO3 superlattices by photoexcited carriers

The effect of light on the properties of ${\mathrm{PbTiO}}_{3}/{\mathrm{SrTiO}}_{3}$ superlattices is studied using first-principles calculations in their paraelectric phase $p$ as well as in the, $c, r$, and $aa$ phases that are defined by polarization lying along the [001], $[11v]$, and [110] directions, respectively. While the $r$ phase is the ground state in dark conditions, illumination triggers a rotation of the polarization towards the out-of-plane direction, which results in the $r$ phase transforming into a $c$ phase at a large concentration of photoexcited carriers. We further reveal that a space charge region of free carriers builds up at the interface under illumination, implying that these superlattices can be thought as acting as $p\ensuremath{-}i\ensuremath{-}n$ junctions under light. These free charges screen the polarization discontinuity at the interface, effectively reducing the electrostatic depolarizing field.

Photoexcited carrier dynamics in semi-insulating 4H-SiC by Raman spectroscopy

The photoexcited carrier dynamics of high-purity (HPSI) and vanadium-doped semi-insulating (VDSI) 4H-SiC irradiated by lasers with different wavelengths and powers were investigated. Raman spectra were measured at room temperature and the photoexcited carrier concentrations were extracted from the Raman line shape analysis of longitudinal optical phonon–plasmon coupled mode. It was found that the longitudinal optical (LO) peaks of HPSI and VDSI did not shift with laser power variations, due to a low concentration of photoexcited carriers, when a 532- nm laser was used. However, when a 355- nm laser was adapted, the relationship between the photoexcited carrier concentrations and the laser power was found to be nonlinear because of the dominance of trap-assisted Auger (TAA) recombination. The coefficient of TAA recombination was laser power–dependent. The proposed carrier dynamic model deepens the understanding of the physical mechanism of semi-insulating SiC irradiated by nanosecond laser and provides an insight into the interpretation of experimental phenomena related to laser energy in optoelectronic devices.

Enhanced photocurrent in ferroelectric Bi0.5Na0.5TiO3 materials via ferro-pyro-phototronic effect

The photocurrent generated by ferroelectric materials via the photovoltaic effect has latent applications in opto-electronic devices due to the introduction of a polarization-induced charge carrier separation and transport, which is absent in semiconducting p-n junctions. However, the precise mechanism of photocurrent production in photoelectric devices by a range of ferroelectric materials remains a topic of continued interest. Here we report a significant enhancement in the photocurrent of a ferroelectric Bi0.5Na0.5TiO3-based photoelectric device by ferro-pyro-phototronic effect. When compared with the generated photocurrent under illumination alone, the peak and platform photocurrents induced by the simultaneous use of both light and heating can be increased by 131% and 57%, respectively. The increase in photocurrent is found to increase with temperature, where the mechanism of enhancement is associated with the increase in pyroelectric coefficient, photoexcited carrier concentration and carrier mobility at higher operating temperatures. These factors also lead to a rapid response time of 84.2 ms. This work provides guidelines for photocurrent promotion in ferroelectric devices, pushing forward the potential applications of ferroelectric materials for self-powered photodetectors.

Femtosecond laser-produced plasma driven nanoparticle formation in gold aqueous solution

We report an experimental study of laser-induced plasma in a gold aqueous solution (H[AuCl4]) and we explore the correlation between plasma properties and rate of the synthesis of gold nanoparticles (Au NPs). The concentration of photo-excited carriers produced by a Ti-sapphire laser (wavelength 800 nm, pulse duration 120 fs) was probed in a extremely wide range of laser energies (∼0.2–200μJ) using a pump–probe and scattering microscopy techniques. The experimental data that we obtained enabled us (i) to identify the multiphoton and avalanche regimes of the ionization process in liquid water and (ii) to link the mechanisms of laser ionization with the possibility of NPs synthesis.

Photocatalytic Activity Enhancement of Anatase/Rutile‐Mixed Phase TiO2 Nanoparticles Annealed with Low‐Temperature O2 Plasma

Photodecomposition and photobactericidal activities of anatase/rutile‐mixed phase TiO2 nanoparticles annealed with low‐temperature O2 plasma are clarified by comparing them with those annealed in ambient air. The photocatalytic activities of plasma‐assisted‐annealed sample greatly enhance as compared with the untreated sample, under not only UV irradiation but also visible‐light irradiation. The photocatalytic activities of air‐annealed samples do not enhance under UV irradiation but enhance under visible‐light irradiation. The enhanced photocatalytic activities due to the plasma‐assisted annealing (PAA) originate from the increased photoexcited carrier concentration. This enhancement is discussed from PAA‐induced characteristic factors. PAA facilitates the phase transformation to anatase, contributing directly to extending the photoexcited carrier lifetime. PAA introduces more oxygen vacancies, contributing to trapping more photogenerated electrons. PAA also introduces more bridging/terminal oxygen groups adsorbed on the surface, increasing the upward band‐bending, the depletion layer width at the surface, and the charge transfer from rutile to anatase. These two introductions contribute to facilitating the separation of photoexcited carriers. Furthermore, PAA reduces the aggregate size of TiO2 nanoparticles formed on the surface, contributing to increasing optical absorptions. More reactive oxygen species produced from the bridging/terminal oxygen groups by the photoexcited carriers will also enhance the photocatalytic activities.

Two-dimensional WSe2 flakes under high power optical excitation

Quasi-2D layers of transition metal dichalcogenides are promising candidates for creating saturable absorbers for pulsed lasers. However, the peculiarities of intense electromagnetic radiation’s influence on such structures have not been thoroughly studied. This paper explores the dynamics of photoexcited carriers in WSe2 flakes through experimental studies. These studies found that WSe2 flakes significantly change their optical properties under the influence of a high-power optical pump, allowed estimating the thermalization time of these structures (about 2 ps), and found that full relaxation takes more than 10 ps. The concentration of carriers in the semiconductor surface layer was estimated to be about 1028 m–3. It was found that standard description models of the optical response based on exciton resonances and absorption by free carriers could not adequately describe the experiments’ results. Thus, for an accurate description of the optical response, it was necessary to consider the effects associated with Coulomb screening that are caused by the high concentration of photo-excited carriers of the optical pumping densities used in this experiment.

Surface Depletion Effects in Bromide-Ligated Colloidal Cadmium Selenide Nanoplatelets: Toward Efficient Emission at High Temperature.

The colloidal semiconductor nanoplatelet (NPL) with broad ligand-semiconductor interface is an ideal system for surface science investigation, but the study regarding depletion effects in NPLs remains lacking. Herein we explore such effects in colloidal CdSe NPLs through Br ligation. Apart from improved brightness and red-shifted optical features, we also experimentally observed abnormal negative thermal quenching phenomena in bromide-ligated CdSe NPLs over 200 K under a cryogenic environment and up to 383 K under an ambient environment, which was absent in pristine NPLs. We speculate that the surface depletion effect shall account for these anomalous phenomena due to the susceptibility of the surface depletion region on photoexcited carrier concentration and surface condition. The existence of the depletion layer in NPLs is also validated quantitatively with k·p simulation. Besides offering an alternative explanation on the red-shifted optical properties of CdSe NPLs by Br-ligation, our findings pave the new route toward solution-processed NPLs-based optoelectronics with boosted thermal resistance.

Topological phonons in oxide perovskites controlled by light.

Perovskite oxides exhibit a rich variety of structural phases hosting different physical phenomena that generate multiple technological applications. We find that topological phonons-nodal rings, nodal lines, and Weyl points-are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (BaTiO3, PbTiO3, and SrTiO3), and external conditions (photoexcitation, strain, and temperature). In particular, in the tetragonal phase of these compounds, all types of topological phonons can simultaneously emerge when stabilized by photoexcitation, whereas the tetragonal phase stabilized by thermal fluctuations only hosts a more limited set of topological phonon states. In addition, we find that the photoexcited carrier concentration can be used to tune the topological phonon states and induce topological transitions even without associated structural phase changes. Overall, we propose oxide perovskites as a versatile platform in which to study topological phonons and their manipulation with light.

Carrier recombination mechanism in CsPbBr3 revealed by time-resolved photoluminescence spectroscopy

We systematically investigate the recombination mechanism of photogenerated charge carriers in bulk $\mathrm{CsPbB}{\mathrm{r}}_{3}$ by means of time-resolved photoluminescence (TR-PL) spectroscopy at low temperature and various laser excitation powers. A dynamic recombination model is proposed to describe the TR-PL that predicts the time-dependent exciton and free-charge populations. It provides a clear representation of competing mono- and bimolecular recombination processes. A decrease in carrier lifetime with increasing laser intensity was observed that was attributed to exciton-exciton scattering. A bimolecular recombination coefficient of $\ensuremath{\sim}{10}^{\ensuremath{-}7}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$ was obtained for exciton recombination. As the concentration of photoexcited carriers increases, stronger exciton-exciton annihilation occurs. The exciton-exciton annihilation rate for $\mathrm{CsPbB}{\mathrm{r}}_{3}$ is $3.63\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ at 10-mW laser power. Notably, the exciton-exciton annihilation rate in bulk material is comparable to that obtained for photoexcited $\mathrm{CsPbB}{\mathrm{r}}_{3}$ nanoscale quantum dots.

Influence of the effect of two-photon interband absorption saturation on the photoexcitation of monocrystalline Si by intense femtosecond laser pulse

Theoretical analysis of the influence of the effect of two-photon interband absorption saturation on space–time dynamics of the concentration of electron-hole plasma photoexcited under the action of a femtosecond laser pulse in monocrystalline silicon was carried out in this work. It was shown that the effects of the depletion of electrons in the valence band and the filling of conduction band may lead to a twofold decrease of the two-photon absorption coefficient and may also affect the concentration of photoexcited carriers achieved in the femtosecond laser treatment process.

Ultrafast carrier dynamics in Ge by ultra-broadband mid-infrared probe spectroscopy

In this study, we carried out 800-nm pump and ultra-broadband mid-infrared (MIR) probe spectroscopy with high time-resolution (70 fs) in bulk Ge. By fitting the time-resolved difference reflection spectra [ΔR(ω)/R(ω)] with the Drude model in the 200–5000 cm−1 region, the time-dependent plasma frequency and scattering rate have been obtained. Through the calculation, we can further get the time-dependent photoexcited carrier concentration and carrier mobility. The Auger recombination essentially dominates the fast relaxation of photoexcited carriers within 100 ps followed by slow relaxation due to diffusion. Additionally, a novel oscillation feature is clearly found in time-resolved difference reflection spectra around 2000 cm−1 especially for high pump fluence, which is the Lorentz oscillation lasting for about 20 ps due to the Coulomb force exerted just after the excitation.

Active guiding of Dirac plasmons in graphene

The Dirac plasmon propagation in active pristine graphene with the carrier population inversion created by the diffusion of the photoexcited carriers from a semiconductor substrate is studied theoretically. It is shown that an order of magnitude smaller pump power can be used for the diffusion pumping as compared to direct optical pumping of graphene for obtaining the same plasmon gain in graphene. We find that the field of the amplified plasmons remains strongly confined in the vicinity of graphene similarly to the case of the attenuated plasmons. Remarkably, the diffusion pumping is characterized by low insertion losses due to small photoexcited carrier concentration in the carrier-supplying semiconductor substrate in the region of the plasmon field confined near graphene.

Threshold of terahertz population inversion and negative dynamic conductivity in graphene under pulse photoexcitation

We present a theoretical study of population inversion and negative dynamic conductivity in intrinsic graphene in the terahertz (THz) frequency range upon pulse photoexcitation at near-/mid-infrared wavelengths. The threshold pulse fluence required for population inversion and negative dynamic conductivity can be orders of magnitude lower when the pulse photon energy is lower, because of the inverse proportionality of the photoexcited carrier concentration to the pulse photon energy and because of the weaker carrier heating. We also investigate the dependence of dynamic conductivity on momentum relaxation time. Negative dynamic conductivity takes place either in high- or low-quality graphene, where Drude absorption by carriers in the THz frequency is weak.

Spatiotemporal dynamics of photogenerated carriers in GaAs/AlGaAs multiple quantum wells

The ambipolar diffusion coefficient of (110) GaAs/AlGaAs multiple quantum wells was measured by the transient spin grating technique.The ambipolar diffusion coefficient and carrier life time,which are Da=13.0 cm2/s and τR=1.9 ns,were obtained directly by this technique under carrier concentration nex=3.4×1010/cm2 at room temperature.The measured Da keeps almost a constant value when the photoexcited carrier concentration is increased up to 1.2×1011/cm2.

Influence of long lifetime absorption process on the measurement of ultrafast carrier dynamics

The wavelength dependence of ultrafast relaxation dynamics of photoexcited carriers in bulk GaAs is studied using femtosecond time-resolved pump-probe transmission spectroscopy under the same photoexcited carrier concentration and the ratio of pump to probe in intensity. Negative and oscillating time-delayed signals are observed at central wavelengths of 760 nm and 780 nm and judged to be incorrect physically by comparson with a simulated computation result. It is found by waveform analysis that they are caused by the phase reversal of the output waveforms from a photodetector, while the phase reversal originates from a long lifetime absorption process existing in GaAs sample. It is pointed out that the phase reversal of the waveform can be corrected by raising the ratio of pump to probe in intensity at the photodetector, so that right transient traces can be obtained. However, raising the ratio is incompatible with the viewpoint that the scattered contribution to the photodetector from pump light should be filtered as much as possible. This result has an important reference value for the acquisition of correct ultrafast dynamics using time-resolved pump-probe spectroscopy.

Study of photoexcited plasma in p-doped GaAs beveled structures by micro-Raman spectroscopy

The generation and properties of photoexcited steady-state plasma of electrons and holes in bevel-shaped p-type GaAs structures were studied by micro-Raman spectroscopy. The best correspondence of theoretical calculations with experimental spectra was obtained by using of photoexcited carrier concentration of 1.1 × 10 17 cm −3 and mobility 600 and 40 cm 2/V s for the photoexcited steady-state electrons and holes, respectively. The analysis of the plasma behavior and its coupling with longitudinal optical phonons at different positions along the bevel shows that the mode resulting from this coupling causes the changes of Raman intensities recorded in frequency positions of transversal (TO) and longitudinal (LO) optical phonon peaks. These changes were further studied and physical interpretation was provided. The dependence of their ratio in the region affected by surface depletion layer can be fitted by linear function very well. The linearity was observed at all studied structures. This behavior on beveled structures prepared by special treatment with very low bevel angle can be used for analysis of the p-type GaAs nanostructures, particularly for measurement and extraction of a doping profile of p-type impurities in GaAs with very high resolution in nm scale.

Energy relaxation of InN thin films

The energy relaxation of InN thin films has been studied by ultrafast time-resolved photoluminescence technique. The obtained carrier cooling curves can be explained by carriers releasing excessive energy through the carrier–LO-phonon interaction. The extracted effective phonon emission times decrease as the photoexcited carrier concentration reduces and come close to the theoretical prediction of 23fs at small carrier concentration. The reduction of energy loss rate at high photoexcited carrier density is attributed to the hot phonon effect.

Photoinduced Hydrophilic Conversion Properties of Titania Nanosheets

Photoinduced hydrophilic conversion properties of titania nanosheets were investigated. A highly hydrophilic state was achieved on the monolayer film surface of titania nanosheets, the thickness of which is less than 1 nm. The hydrophilic conversion rate (k) for the monolayer film of titania nanosheets was proportional to the intensity (I) of irradiated light, suggesting that the hydrophilic conversion of the titania nanosheet surface proceeded under light-limited conditions even with a high concentration of photoexcited carriers. As the number of layers of titania nanosheets increased, the dependence behavior changed from k proportional, variant I(1.0) to k proportional, variant I(0.5), indicating that the recombination processes become dominant in the multilayer films of titania nanosheets. In-plane X-ray diffraction (XRD) analyses showed a very small but reproducible structural change of titania nanosheets upon UV irradiation.

Low-temperature recombination kinetics of photoexcited persistent charge carriers in conjugated polymer/fullerene composite films

The recombination kinetics of long-lived photoexcited charge carriers in a composite of $\mathrm{poly}[2\ensuremath{-}\mathrm{methoxy}{\ensuremath{-}5\ensuremath{-}(3}^{\ensuremath{'}}{,7}^{\ensuremath{'}}\ensuremath{-}\mathrm{dimethyloctyloxy})\ensuremath{-}1,4\ensuremath{-}\mathrm{phenylene}$ vinylene] (MDMO-PPV) and $1\ensuremath{-}(3\ensuremath{-}\mathrm{methoxycarbonyl})\ensuremath{-}\mathrm{propyl}\ensuremath{-}1\ensuremath{-}\mathrm{phenyl}\ensuremath{-}(6,6){\mathrm{C}}_{61}$ (PCBM) at low temperatures $(T=40\mathrm{K})$ are investigated by light induced electron-spin resonance (LESR). These long-lived (persistent) photoinduced charge carriers exhibit recombination times that extend over several hours after cessation of the photoexcitation. These long relaxation times can be explained by nongeminate recombination of randomly distributed carriers assuming charge neutrality. The decay curves fit well to a model in which the recombination mechanism of photoexcited carriers consists of tunneling processes and in which the recombination rate only depends on the intrapair distance between the photoexcited carriers. It is shown that the residual photoexcited carrier concentration after long times tends to be independent of the generation rate. The presented model has already been successful in describing the recombination kinetics of photoexcited carriers in inorganic, amorphous semiconductors, which indicates that the presented recombination mechanism is common to disordered organic and inorganic materials.