Electron-hole Recombination Process Research Articles

Sensitive and Robust Ultraviolet Photodetector Array Based on Self-Assembled Graphene/C60 Hybrid Films.

Graphene has been widely investigated for use in high-performance photodetectors due to its broad absorption band and high carrier mobility. While exhibiting remarkably strong absorption in the ultraviolet range, the fabrication of a large-scale integrable, graphene-based ultraviolet photodetector with long-term stability has proven to be a challenge. Here, using graphene as a template for C60 assembly, we synthesized a large-scale all-carbon hybrid film with inherently strong and tunable UV aborption. Efficient exciton dissociation at the heterointerface and enhanced optical absorption enables extremely high photoconductive gain, resulting in UV photoresponsivity of ∼107 A/W. Interestingly, due to the electron-hole recombination process at the heterointerface, the response time can be modulated by the gate voltage. More importantly, the use of all-carbon hybrid materials ensures robust operation and further allows the demonstration of an exemplary 5 × 5 (2-dimensional) photodetector array. The devices exhibit negligible degradation in figures of merit even after 2 month of operation, indicating excellent environmental robustness. The combination of high responsivity, reliability, and scalable processability makes this new all-carbon film a promising candidate for future integrable optoelectronics.

Electrochemical Study of Copper Ferrite as a Catalyst for CO2 Photoelectrochemical Reduction

In this work, p-type CuFe2O4 was synthesized by sol gel method. The prepared CuFe2O4 was used as photocathode catalyst for photoelectrochemical (PEC) CO2 reduction. The XRD, UV-Visible Spectroscopy (UV-Vis), and Mott-Schottky (MS) experiments were done to characterize the catalyst. Linear sweep voltammetry (LSV) was employed to evaluate the visible light (λ>400 nm) effect of this catalyst for CO2 reduction. The band gap energy of the catalyst was calculated from the UV-Vis and was found 1.30 eV. Flat band potential of the prepared CuFe2O4 was also calculated and found 0.27 V versus Ag/AgCl. Under light irradiation in the CO2-saturated NaHCO3 solution, a remarkable current development associated with CO2 reduction was found during LSV for the prepared electrode from onset potential -0.89 V with a peak current emerged at -1.01 V (vs Ag/AgCl) representing the occurrence of CO2 reduction reaction. In addition, the mechanism of PEC was proposed for the photocathode where the necessity of a bias potential in the range of 0.27 to ~ -1.0 V vs Ag/AgCl was identified which could effectively inhibit the electron-hole (e-/h+) recombination process leading to an enhancement of CO2 reduction reactions.

Interfacial carrier dynamics of graphene on SiC, traced by the full-range time-resolved core-level photoemission spectroscopy

Carrier dynamics through a heterointerface of a Dirac material and a semiconductor was studied by the measurement of the full-range time-resolved core-level photoemission spectroscopy using synchrotron radiation. The electron-hole recombination process during relaxation of the surface photovoltage effect was delayed in a graphene layer due to the bottleneck effect of Dirac cones. When an intermediate buffer layer exists, the recombination mainly takes place at the interfacial Si dangling-bond sites and the relaxation time shortens by one-order of magnitude. The present result demonstrates unusual carrier dynamics at a semiconductor surface, terminated by a layer of the Dirac material.

The influence of conjugated alkynyl(aryl) surface groups on the optical properties of silicon nanocrystals: photoluminescence through in-gap states

Developing new methods, other than size and shape, for controlling the optoelectronic properties of semiconductor nanocrystals is a highly desired target. Here we demonstrate that the photoluminescence (PL) of silicon nanocrystals (SiNCs) can be tuned in the range 685–800 nm solely via surface functionalization with alkynyl(aryl) (phenylacetylene, 2-ethynylnaphthalene, 2-ethynyl-5-hexylthiophene) surface groups. Scanning tunneling microscopy/spectroscopy on single nanocrystals revealed the formation of new in-gap states adjacent to the conduction band edge of the functionalized SiNCs. PL red-shifts were attributed to emission through these in-gap states, which reduce the effective band gap for the electron–hole recombination process. The observed in-gap states can be associated with new interface states formed via (–Si–C≡C–) bonds in combination with conjugated molecules as indicated by ab initio calculations. In contrast to alkynyl(aryl)s, the formation of in-gap states and shifts in PL maximum of the SiNCs were not observed with aryl (phenyl, naphthalene, 2-hexylthiophene) and alkynyl (1-dodecyne) surface groups. These outcomes show that surface functionalization with alkynyl(aryl) molecules is a valuable tool to control the electronic structure and optical properties of SiNCs via tuneable interface states, which may enhance the performance of SiNCs in semiconductor devices.

Photoanode for solar water oxidation based on titania and hematite films

Nanostructured hematite (α-Fe2O3) film deposited on mesoporous titania (TiO2) underlayer was described as a photoanode for photoelectrochemical water splitting. Mesoporous TiO2 layers were prepared by spin coating a paste on conductive fluorine-doped tin oxide glass followed by an annealing process. Hematite film was prepared after electrochemical and annealing processes. The TiO2/Fe2O3 photoanodes achieve photocurrent densities up to 0.5 mA/cm2 at 1.23 V versus reversible hydrogen electrode. This value is greater than that for photocurrent density obtained with pristine hematite photoanode. Impedance measurements showed that prepared heterostructure TiO2/Fe2O3 contributes to diminution of electron-hole recombination process.

Unstrained PbSe/CdSe core shell nanostructures for broad band absorber and narrow band IR emitters

The purpose of this work is to establish structural, morphological and optical properties of PbSe quantum dots (QDs) and PbSe/CdSe core/shell nanostructures (C/S NSs) for application as broad band absorber and narrow band emitters. These NSs were synthesized using chemical bath deposition technique. The structural properties were studied using X-ray diffraction technique with Rietveld refinement and transmission electron microscopy (TEM). The surface morphology of synthesized thin film was established using atomic force microscopy (AFM) and scanning electron microscope (SEM). The optical properties were explained using UV–Vis absorption and photoluminescence (PL) spectroscopy with electron–hole recombination process. A comparison of estimated crystallite size from XRD, high resolution AFM and TEM images was resulted in good agreement as ~ 6.2 ± 0.5 and ~ 6.5 ± 0.5 nm for PbSe QDs and PbSe/CdSe C/S NSs respectively.These obtained properties of self-assembled PbSe/CdSe C/S NSs are expected to be useful in the development of broad band absorber and narrowband IR emitters and solar cells.

Nickel oxide decorated zinc oxide composite nanorods: Excellent catalyst for photoreduction of hexavalent chromium

In light of the growing interest and ability to search for new materials, we have synthesized Nickel oxide (NiO) nanoparticles decorated Zinc (ZnO) nanorods composite (NiO/ZnO) nanostructure. The NiO/ZnO heterostructure formation was confirmed by X-ray powder diffraction and high-resolution transmission electron microscopy (HRTEM). The fabricated environmental friendly NiO/ZnO composite nanostructure shows a well-defined photoreduction characteristic of hexavalent Chromium (Cr) (VI) to tri-valent Chromium (Cr) (III) under UV-light. Such an enhanced photoreduction property is attributed due to the decreased electron-hole recombination process which was proved by photoluminescence (PL) spectroscopy, photocurrent study, and electrochemical impedance spectroscopy. Furthermore, the photocatalytic activity rate of the NiO decorated ZnO nanorods was much higher than that of bare ZnO nanorods for the reduction of chromium (VI) and the rate is found to be 0.306 min−1. These results have demonstrates that suitable surface engineering may open up new opportunities in the development of high-performance photocatalyst.

Improved visible-light photocatalytic activity of TiO2 co-doped with copper and iodine

Cu-I-co-doped TiO2 photocatalysts active to visible light absorption were prepared by hydrothermal method and calcined at various temperatures (350 °C, 450 °C, and 550 °C). The co-doped powders at 350 °C displayed the highest experimental Brunauer–Emmett–Teller surface area and lowest photoluminescence intensity, which demonstrated that a decrease in electron–hole recombination process. The synthesis of co-doped TiO2 was performed at this optimized temperature. In the co-doped sample, the Cu2+ doped TiO2 lattice created a major “red-shift” in the absorption edge due to the presence of the 3d Cu states, whereas the amount of red-shift from the I5+ doping in the TiO2 lattice was minor. Interestingly, the presence of Cu2+ species also boosted the reduction of I5+ ions to the lower multi-valance state I− in the TiO2 lattice by trapping the photogenerated electrons, which resulted in effective separation of the photogenerated charges. The Cu-I-co-doped TiO2 was able to degrade methyl orange dye under visible-light irradiation with improved photocatalytic activity compared with the single metal-doped TiO2 and pure TiO2 because of the strong visible light absorption and effective separation of photogenerated charges caused by the synergistic effects of Cu and I co-dopants.

Prediction of an extremely long exciton lifetime in a Janus-MoSTe monolayer.

The electron-hole separation efficiency is a key factor that determines the performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and devices. Therefore, searching for novel 2D TMD materials with a long timescale of carrier lifetime has become one of the most important topics. Here, based on time-domain density functional theory (TD-DFT), we propose a brand new TMD material, namely Janus-MoSTe, which exhibits a strong built-in electric field. Our results show that in the Janus-MoSTe monolayer, the exciton consisting of an electron and hole has a relatively wide spatial extension and low binding energy. In addition, a slow electron-hole recombination process is observed, with a timescale on the order of 1.31 ns, which is even comparable to those of van der Waals (vdW) heterostructures. Further analysis reveals that the extremely long timescale for electron-hole recombination could be ascribed to the strong Coulomb screening effect as well as the small overlap of wavefunctions between electrons and holes. Our findings establish the built-in electric field as an effective factor to control the electron-hole recombination dynamics in TMD monolayers and facilitate their future applications in light detection and harvesting.

Hierarchically Structured Porous SnO2/TiO2 Materials with Design Controllable Phases and Enhanced Photocatalytic Activity

To increase the photogenerated carrier separation and reduce the electron-hole recombination process for photocatalysts performance. In this article, hierarchically macroporous structured TiO2/SnO2 materials with controllable composition and phases have been successfully synthesized by using a typical process. This method involves several sequential preparation steps: (1) preparation of core-shell structure SnO2/PS submicrospheres by hydrothermal method; (2) preparation of TiO2/SnO2/PS submicrospheres by assembling amorphous TiO2 on surface of SnO2/PS submicrospheres; (3) direct calcination of TiO2/SnO2/PS submicrospheres to eventually produce hierarchically structured TiO2/SnO2 materials with pores-in-pores. With rutile phases TiO2, TiO2/SnO2 macroporous materials show significantly enhanced catalytic activity when used as photocatalysts for the degradation of Rhodamine B under UV irradiation. The photodegradation ratio of Rhodamine B was 96% for 25 min.

Synthesis of Polyaniline-TiO2 Nanocomposites and Their Application in Photocatalytic Degradation

Photocatalyst involving conducting polymer doped with titanium dioxide, TiO2 hold a great efficiency for photocatalytic application. In present study, polyaniline (PAni) with different TiO2 content (10%, 20%, and 40%) has been successfully synthesized through template free method. Ultraviolet-visible (UV-vis), Fourier Transform Infrared (FTIR), Raman, and X-ray Powder Diffraction (XRD) characterizations of PAni-TiO2 nanocomposites confirmed the chemical structure of polymer composites was intact after doped with TiO2. Field Emission Scanning Electron Microscope (FESEM) investigation of PAni-TiO2 nanocomposites revealed the formation of nanorod/nanotube. PAni-TiO2(10%) showed the highest conductivity 2.48 × 10−2 S/cm as compared to those with 20% and 40% of TiO2 content. Photocatalytic properties of PAni-TiO2 were examined by degrading Reactive Black 5 (RB5) dye under visible light irradiation. PAni-TiO2(10%) showed the greatest degradation (96%) than that of TiO2 (10%). Due to synergistic effect between PAni and TiO2, it is capable of absorbing visible light more efficiently and decreasing the process of electron hole recombination. Photoluminescence (PL) analysis proved low efficiency of electron hole recombination of PAni-TiO2.

Augmented Photoelectrochemical Efficiency of ZnO/TiO2 Nanotube Heterostructures

ZnO/TiO2 nanotube heterostructures have been fabricated by electrodeposition of ZnO microcrystals over electrochemically anodized TiO2 nanotube arrays. The resulting ZnO/TiO2 nanotube heterostructures showed enhanced photocurrent density of 5.72 mA cm−2, about 1.5 times the value of 3.68 mA cm−2 shown by bare compact TiO2 nanotubes. This enhanced photocurrent density of the ZnO/TiO2 nanotube heterostructures is due to high electron mobility in the ZnO crystals, thereby decreasing the electron–hole recombination process, good interfacial quality between the ZnO and TiO2 structures, and a proposed smooth charge-transfer mechanism due to band bending at the interface. The morphological features of the as-prepared heterostructures were determined by field-emission scanning electron microscopy (FESEM). The crystallinity and phase purity of the samples were confirmed by x-ray diffraction (XRD) analysis. The light absorption properties of the prepared samples were investigated by ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS). The photoelectrochemical efficiency of bare and ZnO-modified TiO2 nanotube heterostructures was determined by electrochemical analyzer.

Thickness-dependent carrier and phonon dynamics of topological insulator Bi2Te3 thin films.

As a new quantum state of matter, topological insulators offer a new platform for exploring new physics, giving rise to fascinating new phenomena and new devices. Lots of novel physical properties of topological insulators have been studied extensively and are attributed to the unique electron-phonon interactions at the surface. Although electron behavior in topological insulators has been studied in detail, electron-phonon interactions at the surface of topological insulators are less understood. In this work, using optical pump-optical probe technology, we performed transient absorbance measurement on Bi2Te3 thin films to study the dynamics of its hot carrier relaxation process and coherent phonon behavior. The excitation and dynamics of phonon modes are observed with a response dependent on the thickness of the samples. The thickness-dependent characteristic time, amplitude and frequency of the damped oscillating signals are acquired by fitting the signal profiles. The results clearly indicate that the electron-hole recombination process gradually become dominant with the increasing thickness which is consistent with our theoretical calculation. In addition, a frequency modulation phenomenon on the high-frequency oscillation signals induced by coherent optical phonons is observed.

Self-integrated β-Bi2O3/Bi2O2.33@Bi2O2CO3 ternary composites: Formation mechanism and visible light photocatalytic activity

Bi2O2CO3 with a narrow band gap (Eg∼2.93eV) was self-assembled by Bi2O2CO3 nanosheets via a hydrothermal method, and the β-Bi2O3/Bi2O2.33@Bi2O2CO3 composites were further fabricated using Bi2O2CO3 as template by slow thermal decomposition for 4h, described as: Bi2O2CO3→β-Bi2O3/Bi2O2.33@Bi2O2CO3 (300°C)→β-Bi2O3/Bi2O2.33 (320°C)→β-Bi2O3(350°C). Deep investigation towards the generation and existence of β-Bi2O3 and Bi2O2.33 on the surfaces and interfaces of Bi2O2CO3 were also carefully conducted by XRD, HRTEM and XPS analysis. In comparison with pure Bi2O2CO3, β-Bi2O3/Bi2O2.33@Bi2O2CO3 displayed superior photocatalytic degradation efficiency and stability to cationic dyes under visible-light irradiation. Especially, the as-prepared BOC–300°C with relatively high specific surface area (12.11m2g−1) could entirely remove Rh-B and MB in 45min and 60min, which were approximately 4.7 and 6.6 times as high as that of pure Bi2O2CO3, respectively. Further study demonstrated that the enhanced photocatalytic degradation efficiency of β-Bi2O3/Bi2O2.33@Bi2O2CO3 is surely owing to the well matched band gap and p-n heterojunction, which made it possible for much faster charge transfer rather than electron-hole pairs recombination process.

High-order optical nonlinearities in nanocomposite films dispersed with semiconductor quantum dots at high concentrations

We describe the nonlinear optical properties of inorganic-organic nanocomposite films in which semiconductor CdSe quantum dots as high as 6.8 vol.% are dispersed. Open/closed Z-scan measurements, degenerate multi-wave mixing and femtosecond pump-probe/transient grating measurements are conducted. It is shown that the observed fifth-order optical nonlinearity has the cascaded third-order contribution that becomes prominent at high concentrations of CdSe QDs. It is also shown that there are picosecond-scale intensity-dependent and nanosecond-scale intensity-independent decay components in absorptive and refractive nonlinearities. The former is caused by the Auger process, while the latter comes from the electron-hole recombination process.

Control of Charge Carriers Trapping and Relaxation in Hematite by Oxygen Vacancy Charge: Ab Initio Non-adiabatic Molecular Dynamics.

Ultrafast charge recombination in hematite (α-Fe2O3) severely limits its applications in solar energy conversion and utilization, for instance, in photoelectrochemical water splitting. We report the first time-domain ab initio study of charge relaxation dynamics in α-Fe2O3 with and without the oxygen vacancy (Ov) defect, using non-adiabatic molecular dynamics implemented within time-dependent density functional theory. The simulations show that the hole trapping is the rate-limiting step in the electron-hole recombination process for both neutral and ionized Ov systems. The electron trapping is fast, and the trapped electron are relatively long-lived. A similar asymmetry is found for the relaxation of free charge carriers: relaxation of photoholes in the valence band is slower than relaxation of photoelectrons in the conduction band. The slower dynamics of holes offers an advantage to water oxidation at α-Fe2O3 photoanodes. Notably, the neutral Ov defect accelerates significantly the charge recombination rate, by about a factor of 30 compared to the ideal lattice, due to the stronger electron-vibrational coupling at the defect. However, the recombination rate in the ionized Ov defect is decreased by a factor of 10 with respect to the neutral defect, likely due to expansion of the local iron shell around the Ov site. The Ov defect ionization in α-Fe2O3 photoanodes is important for increasing both electrical conductivity and charge carrier lifetimes. The simulations reproduce well the time scales for the hot carrier cooling, trapping and recombination available from transient spectroscopy experiments, and suggest two alternative mechanisms for the Ov-assisted electron-hole recombination. The study provides a detailed atomistic understanding of carrier dynamics in hematite, and rationalizes the experimentally reported activation of α-Fe2O3 photoanodes by incorporation of Ov defects.

Fabrication and Photoelectrochemical Properties of a Cu2O/CuO Heterojunction Photoelectrode for Hydrogen Production from Solar Water Splitting

We report on the fabrication and characterization of a novel Cu2O/CuO heterojunction structure with CuO nanorods embedded in Cu2O thin film as an efficient photocathode for photoelectrochemical (PEC) solar water splitting. A CuO nanorod array was first prepared on an indium-tin-oxide-coated glass substrate via a seed-mediated hydrothermal synthesis method; then, a Cu2O thin film was electrodeposited onto the CuO nanorod array to form an oxide semiconductor heterostructure. The crystalline phases and morphologies of the heterojunction materials were examined using X-ray diffraction and scanning electron microscopy, as well as Raman scattering. The PEC properties of the fabricated Cu2O/CuO heterojunction photocathode were evaluated by photocurrent conversion efficiency measurements under white light illumination. From the observed PEC current density versus voltage (J-V) behavior, the Cu2O/CuO photocathode was found to exhibit negligible dark current and high photocurrent density, e.g. −1.05 mA/cm2 at −0.6 V vs. Hg/HgCl2 in 1 mM Na2SO4 electrolyte, revealing the effective operation of the oxide heterostructure. The photocurrent conversion efficiency of the Cu2O/CuO photocathode was estimated to be 1.27% at −0.6 V vs. Hg/HgCl2. Moreover, the PEC current density versus time (J-T) profile measured at −0.5 V vs. Hg/HgCl2 on the Cu2O/CuO photocathode indicated a 3-fold increase in the photocurrent density compared to that of a simple Cu2O thin film photocathode. The improved PEC performance was attributed to a certain synergistic effect of the bilayer heterostructure on the light absorption and electron-hole recombination processes.

Low threshold, single-mode laser based on individual CdS nanoribbons in dielectric DBR microcavity

Abstract Single-mode lasers with low threshold are attractive for their potential applications in many areas, such as optical communication, signal processing and displays. Here we report a nanoscale single-mode laser with CdS nanoribbons (NRs) sandwiched between two dielectric distributed Bragg reflectors (DBRs). Under optical pumping, the band edge emission of CdS ribbons can be effectively confined and give lasing in the DBR microcavity, with the lasing threshold as low as ~8 μJ/cm 2 , which is one order lower than those of bare CdS ribbons for comparison. More importantly, the CdS-DBR laser can realize single-mode emission, for the length of the resonance cavity can support a mode spacing larger than the bandwidth of the optical gain. Light polarization measurements demonstrate the single-mode lasing has a polarization degree as high as 97.4%. Time-resolved photoluminescence (TRPL) measurements further reveal that the lasing action comes from the electron-hole plasma recombination process. Moreover, the wavelength of the lasing mode can be broadly tuned by changing the thickness of CdS NRs. This low-threshold single-mode laser may find applications in highly integrated photonics devices and systems.

Hieratical Cu2SnS3 (CTS) Nanostructure As a Highly Efficient Visible Light Photocatalyst for CO2 Reduction to Solar Fuels

The conversion of greenhouse CO2 gas into value added chemicals using solar energy or photocurrent derived from visible light is known as artificial photosynthesis. Recently, the production of solar fuel through artificial photosynthesis has been gained tremendous attention due to their fascinating potential in environmental and energy-related application. In CO2 photoreduction technology, the major challenge is the low efficiency of the photocatalyst. Several wide band gap and hybrids carbonaceous materials have been demonstrated as photocatalyst for CO2 reduction. However, in real application these photocatalyst materials are far away from the actual requirement due to the low quantum efficiency and lack of selectivity. Thus, it is a great challenge to develop a potential low-cost photocatalyst for a high CO2 reduction efficiency and produce desire solar fuel under visible light. Last few years several metal sulfides like MoS2, CuS, Cu2S, ZnS and their composite have been attracted considerable attention as a photocatalyst due to their good optical property, high absorption coefficient. However, the application of these perfect semiconductors for photocatalytic CO2 reduction to solar fuels may be limited due to their high interfacial electron-hole recombination process. Recently, Cu-Sn-S ternary semiconductor system has been attracted for its potential contribution as a light absorber layer in photovoltaic application. Cu2SnS3 (CTS) is a ternary p-type semiconductor with a high absorption coefficient. The observed optical band edge of CTS is around 1.4 eV, nearly ideal for solar-energy harvesting. In this study, CTS nanostructure has been demonstrated as a photocatalyst for photocatalytic CO2 reduction to solar fuels under visible light. The ternary CTS nanostructure was obtained from the hydrothermal process and characterized by XRD, Raman spectroscopy, UV-Vis spectroscopy and SEM technique. The photocatalytic activity of the CTS nanostructure hybrid with different S composition was measured in the gas phase CO2 photoreduction under visible-light irradiation. Ethanol has been found to be major product as a high selectivity and efficiency from the photocatalytic CO2 reduction. The observed ethanol formation rate is around 5.6 μmol/g-cat.hr. Detailed investigation on the optimum phase composition, its nanoarchitectures and possible mechanism for the selective solar fuel generation will be discussed. References Nanoscale, 5, 262, (2013) Nano letters, 14, 6097, (2014)

Aminosilicate sol-gel supported zinc oxide-silver nanocomposite material for photoelectrocatalytic oxidation of methanol

A (3-aminopropyl) triethoxysilane supported zinc oxide-silver nanocomposite material [APS/(ZnO-Ag)NCM] was prepared by a simple chemical reduction method and their optical, structural, morphology, elemental and surface properties were studied by UV-DRS, PL, XRD, SEM, TEM, EDX and XPS analysis. The photoelectrocatalytic activity of APS/(ZnO-Ag)NCM modified electrode was investigated towards methanol oxidation. The modification of ZnO nanomaterial with silver nanoparticle resulted in improvement of interfacial charge transfer and minimized the electron-hole charge recombination process. This process significantly enhanced the photoelectrocatalytic activity and performance due to synergic effect of silver. The aminosilicate sol-gel support improved the deposition of silver metal on ZnO surface and it promotes efficient charge transfer in photoelectrocatalysis. The APS/(ZnO-Ag) modified electrode showed four-fold enhancement in photocurrent generation toward photoelectrocatalytic oxidation of methanol compared with ZnO. This aminosilicate sol-gel stabilized APS/(ZnO-Ag)NCM is an efficient photoelectrode for the photoelectrochemical applications.