Transport Of Photoexcited Carriers Research Articles

Ultra-Low Threshold Resonance Switching by Terahertz Field Enhancement-Induced Nanobridge.

Ongoing efforts spanning decades aim to enhance the efficiency of optical devices, highlighting the need for a pioneering approach in the development of next-generation components over a broad range of electromagnetic wave spectra. The nonlinear transport of photoexcited carriers in semiconductors at low photon energies is crucial to advancements in semiconductor technology, communication, sensing, and various other fields. In this study, ultra-low threshold resonance mode switching by strong nonlinear carrier transport beyond the semi-classical Boltzmann transport regime using terahertz (THz) electromagnetic waves are demonstrated, whose energy is thousands of times smaller than the bandgap. This is achieved by employing elaborately fabricated 3D tip structures at the nanoscale, and nonlinear effects are directly observed with the THz resonance mode switching. The nanotip structure intensively localizes the THz field and amplifies it by more than ten thousand times, leading to the first observation of carrier multiplication phenomena in these low-intensity THz fields. This experimental findings, confirmed by concrete calculations, shed light on the newly discovered nonlinear behavior of THz fields and their strong interactions with nanoscale structures, with potential implications and insights for advanced THz technologies beyond the quantum regime.

Recent advances in ternary Z-scheme photocatalysis on graphitic carbon nitride based photocatalysts.

Due to its excellent photocatalytic performance over the last few years, graphitic-like carbon nitride (g-C3N4) has garnered considerable notice as a photocatalyst. Nevertheless, several limitations, including small surface area, the rates at which photo-generated electrons and holes recombine are swift, and the inefficient separation and transport of photoexcited carriers continue to impede its solar energy utilization. To overcome those limitations in single-component g-C3N4, constructing a heterogeneous photocatalytic system has emerged as an effective way. Among the various studies involving the incorporation of hetero composite materials to design heterojunctions, among the most promising approaches is to assemble a Z-scheme photocatalytic configuration. The Z-scheme configuration is essential because it facilitates efficient photocarrier separation and exhibits superior redox ability in separated electrons and holes. Moreover, ternary composites have demonstrated enhanced photocatalytic activities and reinforced photostability. Ternary Z-scheme heterostructures constructed with g-C3N4 possess all the above-mentioned merits and provide a pioneering strategy for implementing photocatalytic systems for environmental and energy sustainability. A summary of the latest technological advancements toward design and fabrication in ternary all-solid-state Z-scheme (ASSZ) and direct Z-scheme (DZ) photocatalysts built on g-C3N4 is presented in this review. Furthermore, the review also discusses the application of ternary Z-scheme photocatalytic architecture established on g-C3N4.

Optical Pump-Terahertz Probe Diagnostics of the Carrier Dynamics in Diamonds.

Diamond is a promising material for terahertz applications. In this work, we use a non-invasive optical pump-terahertz probe method to experimentally study the photoinduced carrier dynamics in doped diamond monocrystals and a new diamond-silicon composite. The chemical vapor deposited diamond substrate with embedded silicon microparticles showed two photoinduced carrier lifetimes (short lifetime on the order of 4 ps and long lifetime on the order of 200 ps). The short lifetime is several times less than in boron-doped diamonds and nitrogen-doped diamonds which were grown using a high temperature-high pressure technique. The observed phenomenon is explained by the transport of photoexcited carriers across the silicon-diamond interface, resulting in dual relaxation dynamics. The observed phenomenon could be used for ultrafast flexible terahertz modulation.

Heterostructured CdS Buffer Layer for Sb2Se3 Thin Film Solar Cell

The antimony selenide thin film solar cells technology becomes promising due to its excellent anisotropic charge transport and brilliant light absorption capability. Especially, the device performance heavily relies on the vertically oriented Sb2Se3 grain to promote photoexcited carrier transport. However, crystalline orientation control has been a major issue in Sb2Se3 thin film solar cells. Herein, a new strategy has been developed to tailor the crystal growth of Sb2Se3 ribbons perpendicular to the substrate by using the structural heterostructured CdS buffer layer. The heterostructured CdS buffer layer is formed by a dual layer of CdS nanorods and nanoparticles. The hexagonal CdS nanorods passivated by a thin cubic CdS nanoparticle layer can promote [211] and [221] directional growth of Sb2Se3 ribbons using a close space sublimation approach. The improved buffer/absorber interface, reduced interface defects, and recombination loss contribute to the improved device efficiency of 7.16%. This new structural heterostructured CdS buffer layer can regulate Sb2Se3 nanoribbons crystal growth and pave the way to further improve the low‐dimensional chalcogenide thin film solar cell efficiency.

Three-dimensional nanoporous heterojunction of CdS/np-rGO for highly efficient photocatalytic hydrogen evolution under visible light

Properly engineered heterojunction photocatalysts warrant a more effective way to facilitate the separation of photoexcited electron-hole pairs and consequently bolster the photocatalytic hydrogen evolution performance. Herein, 3D bicontinuous nanoporous reduced graphene oxide (np-rGO) is served as a free-standing supporting matrix for the anchorage of CdS nanoparticles to obtain the highly efficient photocatalysis system for hydrogen production. The bicontinuous internal ligament and open nanopores in 3D nanoporous heterojunction are conducive to multiple reflecting and scattering of the incident light, leading to improved light absorption and utilization capacity. Moreover, the construction of p-n heterojunction with staggered band alignment is achieved in CdS/np-rGO to accelerate the transport of photo-excited carriers. The constructed CdS/np-rGO p-n heterojunction achieves a corresponding hydrogen generation rate of 2171.23 μmol g−1 h−1, which is 3.6 times in comparison with bare CdS. DFT calculations also indicate the preferable photocatalytic performance of CdS/np-rGO could be assigned to the ideal interfacial charge rearrangement on the heterointerface, which optimizes the H* adsorption kinetic energies and leads to a remarkable enhancement in the charge separation efficiency. It is anticipated this work could provide new sight for making the best usage of sunlight to produce solar fuel.

Self‐Powered Photodetector Based on Perovskite/NiOx Heterostructure for Sensitive Visible Light and X‐Ray Detection

AbstractSelf‐powered halide perovskite X‐ray detectors have evoked busting interests because they present outstanding properties such as low power consumption, high sensitivity, and integrability without external power source. Nevertheless, existing self‐powered X‐ray detectors exhibit low sensitivity due to the insufficient charge separation and transport. Herein, a self‐powered photodetector for sensitive X‐ray and photodetection based on single‐crystalline perovskite FAPbBr3/NiOx heterostructure is demonstrated. The built‐in field of FAPbBr3/NiOx heterojunction provides a driving force for separation and transport of photoexcited carriers. Without an external power supply, the photodetector presents excellent photoresponse performance under 450 nm light illumination, including high photoresponsivity (123.6 mA W−1), good detectivity (D = 7.1 × 1011 Jones), and ultrafast response/recovery time (10/200 ns). Simultaneously, the device achieves photovoltaic effect of 0.80 V and a large mobility–lifetime product of 1.1 × 10−3 cm2 V−1, which enable a superior X‐ray sensitivity of 402 µC Gyair−1 cm−2 at zero bias voltage. These results imply that the simple combination of perovskite single crystal and a hole transporting layer provides a flexible platform for the realization of energy‐saving self‐powered photodetector in visible light and X‐rays.

Photo-excited carrier transport and secondary phases of Na-engineered kesterite flexible thin films

Kesterite [Cu 2 ZnSn(S,Se) 4 (CZTSSe)], which is one of the most promising solar cell materials, can be used as a light absorber owing to its suitable optical and electrical properties. In this study, the optimization process of NaF layers was performed in the deposition of the precursor stacks of Sn/Cu/Zn/Mo to form CZTSSe thin films. The distribution of the secondary phases on the CZTSSe surfaces strongly depends on the insertion of the NaF layer. In particular, the Cu 2-x S binary phase increases with the closeness of the insertion location of the NaF layer to the front side. Moreover, the grain boundary properties can be altered by varying the deposition conditions since Na segregation and defect formation affect the surface electrical properties and carrier transport mechanism. Photo-assisted conductive-atomic force microscopy was used to measure the local variation in the surface photocurrent flow into the CZTSSe grains, and the incorporation of sodium increased the surface current formation. This study exhibits the effects of Na on the optimal growth of kesterite formation and carrier transport of the relevant photovoltaic devices. • Na incorporation was engineered by controlling the insertion location of NaF doping layer. • Formation of binary and ternary phases was affected by the location of NaF doping layer. • Distribution of Cu 2-x S binary phase increased as the insertion location of NaF close to the surface. • Generation of photocurrent was enhanced in Na doped CZTSSe.

Decorating InSe Surface by Gold Species for Improved Carrier Transport and Efficient Sunlight Harvesting Toward High‐Performance Flexible Photodetectors

AbstractTwo‐dimensional (2D) materials have aroused widespread interest due to the high potential in modern photoelectronics. The strategy for improving the stability of 2D materials in the air, reinforcing formation, and transport of photoexcited carriers would open up promising routes toward flexible facilities. In this paper, surface engineering is executed on 2D InSe by decorating Au species for a lower bandgap allowing for efficient sunlight harvesting and decreased barrier with the substrate for improved electron transport. Moreover, hot electrons produced by Au nanoparticles under light irradiation pour into InSe for boosting photocurrent. Au nanoparticles also serve as conducting bridges in InSe−Au photoanode, where the contact resistance is two orders of magnitude lower than that of InSe electrode. Compared with InSe and other 2D counterparts, InSe−Au flexible photoelectrochemical detectors behave with outstanding performances under sunlight irradiation, including responsibility 55.4 µA W−1, detectivity 4.18 × 109 Jones. Importantly, the working electrode shows excellent ON/OFF switching stability after bending for 5000 times (3 months of storage in the air). This surface engineering provides a general strategy to tailor 2D materials for wearable photoelectronic devices in the future.

Enhanced photoelectrochemical performance of carbon nanotubes-modified black TiO2 nanotube arrays for self-driven photodetectors

Self-driven ultraviolet photodetectors based on wide-bandgap semiconductors have been well investigated but are still being explored for further performance enhancement. Here we report a self-driven electrochemical ultraviolet photodetector (EUVPD) using a three-dimensional nanostructured photoanode based on defect-engineered TiO 2 nanotube arrays (TNAs) modified with single-walled carbon nanotubes (SWCNTs) to enhance photoelectric performance. The EUVPDs based on Ar-annealed TNAs modified with 0.1 mg/mL of SWCNTs were found to have higher responsivity (∼60 mA/W), higher on/off current ratio (∼4.3 × 10 3 ), the faster response time (4 ms of rise-time and 27 ms of decay-time) when compared with unmodified and air-annealed ones. This performance enhancement is attributed to the highly efficient separation and transport of photoexcited carriers through the electrochemical redox reaction in the SWCNTs network anchored on the defect-engineered TNAs/electrolyte heterojunction nanostructure. • SWCNTs-modified TNAs were used to prepare the self-driven EUVPDs. • 3-D TNAs enabled an enhanced light-trapping effect inside the nanotubes. • TiO 2 /electrolyte heterojunction made a high-efficient separation of the electron-hole pairs. • SWCNTs anchored on 3-D TNAs enhanced the photoelectrochemical redox reaction.

High-polycondensation and porous carbon nitride nanosheets for highly efficient photocatalytic hydrogen evolution

The degree of polycondensation (DP) has significantly affected the physical and chemical performance of polymeric graphitic carbon nitride (PCN), especially in optimizing the electronic structure and surface chemical properties, thus regulating DP is a promising strategy for improving the photocatalytic performance of graphitic carbon nitride. This investigation supplies a facile procedure, step-by-step polymerization, to synthesize porous carbon nitride nanosheets (PCNNs) with different DP. The PCNNs revealed an uplifted visible-light-driven (λ ˃ 420 nm) photocatalytic hydrogen evolution rate of 415.6 μmol/h/g, which is approximately 7.3-fold higher than that of PCN. The increased activity of PCNNS would be originated from the porous structure with good hydrophilicity and the higher DP of the heptazine unit facilitating the separation and transport of photoexcited carriers. This work successfully demonstrates a prospective post-synthesis strategy effective for boosting the performance of graphitic carbon nitride.

Photoexcited carrier dynamics in a GaAs photoconductive switch under nJ excitation

In this article, the bunched transport of photoexcited carriers in a GaAs photoconductive semiconductor switch (PCSS) with interdigitated electrodes is investigated under femtosecond laser excitation. Continuous outputs featuring high gain are obtained for single shots and at 1 kHz by varying the optical excitation energy. An ensemble three-valley Monte Carlo simulation is utilized to investigate the transient characteristics and the dynamic process of photoexcited carriers. It demonstrates that the presence of a plasma channel can be attributed to the bunching of high-density electron–hole pairs, which are transported in the form of a high-density filamentary current. The results provide a picture of the evolution of photoexcited carriers during transient switching. A photoinduced heat effect is analyzed, which reveals the related failure mechanism of GaAs PCSS at various repetition rates.

Enhanced Efficiency and Stability in Sb2S3 Seed Layer Buffered Sb2Se3 Solar Cells

AbstractAntimony selenide (Sb2Se3) has excellent directional optical and electronic behaviors due to its quasi‐1D nanoribbons structure. The photovoltaic performance of Sb2Se3 solar cells largely depends on the orientation of the nanoribbons. It is desired to grow these Sb2Se3 ribbons normal to the substrate to enhance photoexcited carrier transport. Therefore, it is necessary to develop a strategy for the vertical growth of Sb2Se3 nanoribbons to achieve high‐efficiency solar cells. Since antimony sulfide (Sb2S3) and Sb2Se3 are from the same space group (Pbnm) and have the same crystal structure, herein an ultrathin layer (≈20 nm) of Sb2S3 has been used to assist the vertical growth of Sb2Se3 nanoribbons to improve the overall efficiency of Sb2Se3 solar cell. The Sb2S3 thin layer deposited by the hydrothermal process helps the Sb2Se3 ribbons grow normal to the substrate and increases the efficiency from 5.65% to 7.44% through the improvement of all solar cell parameters. This work is expected to open a new direction to tailor the Sb2Se3 grain growth and further develop the Sb2Se3 solar cell in the future.

Electron-coupled enhanced interfacial interaction of Ce-MOF/Bi2MoO6 heterostructure for boosted photoreduction CO2

To achieve the task of “Carbon neutrality” in China, the photoreduction of CO2 was applied over the solid-phase synthesized Ce-MOF/Bi2MoO6 heterostructure. The inexhaustible driven force of sunlight was adopted and the value-added chemicals of methane, methanol and formic acid were obtained during photoreduction. The firstly successful construction of Ce-MOF/Bi2MoO6 heterostructure was generated by their intense contact via the electron-coupled structure, which could take full advantage of the high surface area of Ce-MOF and the efficient electrons transfer of Bi2MoO6. The segregation and transport of photoexcited carriers were heightened by the enhanced interfacial interaction. Moreover, the generation of HCOOH appeared only over the composites, which was caused by the inhibition of the fast interaction between the photoexcited electrons and CO2-reduced intermediates. As a result, the CO2 photoreduction performance of electrons-induced heterostructured of Ce-MOF/Bi2MoO6 composites exhibited higher CH4, CH3OH and HCOOH yield amount than the neat Ce-MOF and BMO. The optimized photocatalyst reached up to 113.87 μmol·h−1·g−1 of CH4, 40.59 μmol·h−1·g−1 of CH3OH and 73.48 μmol·h−1·g−1 of HCOOH. The mechanism of CO2 photoreduction process was gained by ESR and in-situ DRIFTS techniques.

Out-of-plane dipole-modulated photogenerated carrier separation and recombination at Janus-MoSSe/MoS2 van der Waals heterostructure interfaces: an ab initio time-domain study.

Out-of-plane mirror symmetry-breaking provides a powerful tool for engineering the electronic properties and the exciton behavior of two-dimensional materials. Here, by combining time-domain density functional theory with nonadiabatic dynamics, we investigate the underlying mechanism of how the vertical dipole moment modulates the photoexcited carrier transport and the electron-hole recombination dynamics in polar Janus MoSSe/MoS2 stacked heterostructures. It is shown that the stronger nonadiabatic coupling, interlayer-state delocalization and the built-in electric field caused by charge redistribution facilitate a more rapid photocarrier separation across the interface in the S/S stacked bilayer compared with the S/Se bilayer, explaining the experimentally observed stronger photoluminescence quenching effect in the S/S heterostructure. We also found that the photocarrier recombination of the heterostructure with the S/Se interface has a timescale up to nanoseconds, which is ∼4 times longer than that of the S/S bilayer. Such a prolonged recombination time originates from the dipole-weakened nonadiabatic coupling between occupied and unoccupied states instead of quantum coherence and the band gap effect. Overall, Janus MoSSe/MoS2 heterostructures exhibit superior photocatalytic activity, reflecting the ultrafast photocarrier separation triggered by the built-in electric field, suppressed carrier recombination, high solar-to-hydrogen conversion efficiency and the strong absorption coefficient expanding from visible-light to near-infrared-light. The above atomistic and time-domain findings reveal the intrinsic dipole as an effective freedom to regulate the nonadiabatic photocarrier dynamics in Janus-based 2D heterostructures for efficient energy harvesting and optoelectronic applications.

A novel semi-metallic 1T′-MoReS3 co-catalyst

In this paper, we for the first time report that semi-metallic 1T′-MoReS3 acts as cocatalyst loading on the g-C3N4 nanotubes (NTs) through a solvothermal reaction process for photocatalytic hydrogen evolution. 1T′-MoReS3/g-C3N4 composites with 7 wt% 1T′-MoReS3 exhibit an expressively enhanced photocatalytic hydrogen evolution activity (2671 μmol·g−1·h−1), better than g-C3N4 NTs (61 μmol·g−1·h−1), 1T′-MoS2/g-C3N4 composites (2207 μmol h−1 g−1) and 1T′-ReS2/g-C3N4 composites (1600 μmol h−1 g−1). According to the DFT calculation and experimental data, the enhanced photocatalytic activity of 1T′-MoReS3/g-C3N4 composites is ascribed to the advantages as follows: 1) The porous g-C3N4 NTs with hollow structure possess short charge transfer distance and large surface area, promoting the light scattering and the transport of photoexcited carriers; 2) 1T′-MoReS3 as cocatalysts show a synergistic effect of 1T′-MoS2 and 1T′-ReS2 with better electrical conductivity, boosting the electron transfer process; 3) After loading 1T′-MoReS3 cocatalysts, 1T′-MoReS3/g-C3N4 composites show an improved light absorption and photoelectrochemical performance.

High-valence-state Ni–Fe bimetallic phosphonate nanoribbons catalyst for enhanced photocatalytic and electrocatalytic oxygen production

The design and synthesis of economy and efficiency materials for oxygen evolution reaction (OER) have been a continuous hot spot in the field of scientific study. Herein, a high-valence-state two-dimension (2D) NiFe phosphonate-based (NiFeP) nanoribbons catalyst has been constructed through a one-step solvothermal process. The NiFeP nanoribbons exhibit highly active in both photocatalytic and electrocatalytic water oxidation due to the 2D nanoribbons and high-valence Ni3+ sites. The 2D nanoribbons not only provide more reactive sites for OER but also shorten bulk diffusion distance with better photoexcited carrier transport from the interior to the surface. Meanwhile, the existence of high-valence Ni3+ could be acted as an efficient redox site to reduce the overpotential and facilitate the catalytic reaction. In consequence, the NiFeP nanoribbons catalyst demonstrates a superior O2 yield of 65.7% and O2 production rate of 25.97 umol s−1 g−1, which are comparable or even much higher than those other reported transition metal oxide photocatalysts. At last, the possible proton-coupled electron transfer mechanism is also proposed. This study not only demonstrates the potential of a low-cost metal phosphonate OER catalyst but also provides a referential system for the fabrication of high activity and stability catalysts toward replacing noble metals for energy storage and conversion. High-valence-state 2D NiFe phosphonate-based nanoribbons catalyst has been constructed for the first time through a one-step solvothermal process without any organic additives or template agent. The as-prepared catalyst of NiFeP nanoribbons exhibits highly active in both photocatalytic and electrocatalytic water oxidation.

Evaluating the ratio of electron and hole mobilities from a single bulk sample using Photo-Seebeck effect

When a semiconductor is under photoexcitation, the voltage response to a temperature gradient is the photo-Seebeck effect. Here we study this effect, focusing on the contribution from transport of photo-excited carriers. We demonstrate that by combining photo-Seebeck with photoconductivity measurements, one can determine the ratio between electron and hole mobilities, and hence both of them when one is known. This is found for the case of defect-free samples, where no detail on the absorbance, carrier lifetime or recombination is necessary. Our method reported here does not require chemical doping, which could introduce defects and is often not feasible. It applies to both thin film and bulk samples. Experiment-wise, photo-Seebeck effect is relatively easy to implement, or added to existing systems. In a broader context, for semiconductors with significant influence from defects, our result suggests that the photo-Seebeck behavior can still be understood. In this case another photo-transport property is necessary, in order to identify the mobilities of carriers and information regarding the defects. This framework integrates the information from photoexcitation and thermal gradients to provide a general method to determine fundamental electronic properties of materials.

CuS-modified ZnO rod/reduced graphene oxide/CdS heterostructure for efficient visible-light photocatalytic hydrogen generation

Solar energy utilization is a promising strategy for the photocatalytic generation of H2 from water. Herein, a CuS-modified ZnO rod/reduced graphene oxide (rGO)/CdS heterostructure was fabricated via Cu-induced electrochemical growth with Zn powder at room temperature. The resulting powder revealed good interfacial bonding and promoted photoexcited carrier transport. The CuS nanoparticles played a pivotal role in enhancing visible-light responses and demonstrated excellent catalytic performance. A high visible-light photocatalytic H2 generation rate of 1073 μmol h−1 g−1 was obtained from the CuS–ZnO/rGO/CdS heterostructure containing 0.23% CuS and 1.62% CdS. Increased photoexcited electron lifetimes, improved carrier transport rates, and decreased fluorescence intensities confirmed the synergistic effects of each of the components of the heterostructure. This study provides an innovative strategy for constructing multi-component heterostructures to achieve efficient visible-light H2 evolution.

Electronic Structure, Optical Properties, and Photoelectrochemical Activity of Sn-Doped Fe2O3 Thin Films

Hematite (Fe2O3) is a well-known oxide semiconductor suitable for photoelectrochemical (PEC) water splitting and industry gas sensing. It is widely known that Sn doping of Fe2O3 can enhance the device performance, yet the underlying mechanism remains elusive. In this work, we determine the relationship between electronic structure, optical properties, and PEC activity of Sn-doped Fe2O3 by studying highly crystalline, well-controlled thin films prepared by pulsed laser deposition (PLD). We show that Sn doping substantially increases the n-type conductivity of Fe2O3, and the conduction mechanism is better described by a small-polaron hopping (SPH) model. Only 0.2% Sn doping significantly reduces the activation energy barrier for SPH conduction from at least 0.5 eV for undoped Fe2O3 to 0.14 eV for doped ones. A combination of X-ray photoemission, X-ray absorption spectroscopy, and DFT calculations reveals that the Fermi level gradually shifts toward the conduction band minimum with Sn doping. A localized Fe2+-like gap state is observed at the top of the valence band, accounting for the SPH conduction. Interestingly, different from the literature, only 0.2% Sn doping in Fe2O3 significantly improves the PEC activity, while more Sn decreases it. The improved PEC activity is partially attributed to an increased band bending potential which facilitates the charge separation at the space charge region. The reduced activation energy barrier for SPH will facilitate the transport of photoexcited carriers for the enhanced PEC, which is of interest for further carrier dynamics study.

ZnO rod/reduced graphene oxide sensitized by α-Fe2O3 nanoparticles for effective visible-light photoreduction of CO2

ZnO as a potential semiconductor photocatalyst is applied in photoelectrochemistry, photodegradation and photocatalytic hydrogen evolution. However, the lack of visible light absorption and unsatisfactory photocatalytic activity restrict the potential applications of ZnO. In the study, a novel in-situ electrochemical growth strategy was developed to construct α-Fe2O3-ZnO rod/reduced graphene oxide (rGO) heterostructure for extending visible-light absorption ability and improving the photoexcited carrier transport process. The electrochemical growth strategy can also be used to design other heterostructure photocatalytic materials. The α-Fe2O3-ZnO/rGO heterostructure can not only exhibit enhanced photoelectrochemical performance but can also effectively capture CO2 and reduce CO2 to CH3OH under visible light. The interface coordination effect between ZnO and α-Fe2O3 are considerably enhanced via a heterojunction constructed at the interface region. The heterostructure might be applied in the photoelectrochemical water splitting and artificial photosynthesis. The electrochemical growth strategy can be also used to design other heterostructure photocatalytic materials.