Metal-semiconductor Hybrid Nanostructures Research Articles

The influence of the size of a one-faced metallic head in Janus nanoparticles as a co-catalyst on the photocatalytic efficiency of hydrogen evolution under vis light irradiation

Janus nanoparticles (NPs) consisting of MoOxSy nanospheres and silver (Ag) head, successfully developed by a simple, controlled method were in the first time they were applied as a co-catalysts in photocatalytic hydrogen generation reaction under vis light irradiation (λ > 420 nm). The MoOxSy-Ag as a co-catalysts were deposited on the obtained ZnIn2S4 microspheres (ZIS) using physical absorption method. The influence of the size of a one-faced metallic head (Ag) in JNPs as a co-catalyst on the photocatalytic efficiency of hydrogen evolution has been investigated. Sample 2.5-Ag-MoOxSy@ZIS with the co-catalyst with the smallest head in the range of about 61–71 nm was characterized by 34 times higher photocatalytic hydrogen generation activity compared to pure ZIS. JNPs as a co-catalyst are characterized as asymmetric structure and precisely adjusted interface and thus, they influence on a very good electron mobility in hybrid metal-semiconductor nanostructures. JNPs can be a new alternativity to metal co-catalysts used in a photocatalytic hydrogen production reaction.

Tuning Geometric Chirality in Metallic and Hybrid Nanostructures by Controlled Nanoscale Crystal Symmetry Breaking.

Understanding and controlling chirality in inorganic crystalline materials at the nanoscale is crucial in elucidating fundamental chirality-dependent physical and chemical processes as well as advancing new technological prospects, but significant challenges remain due to the lack of material control. Here, we have developed a facile and general bottom-up synthetic strategy for achieving chiral plasmonic Au nanostructures, including nanocubes and nanorods with fine chirality control. The underlying chiral mechanism enabled by the chiral boundary morphology is substantiated by theoretical modeling and finite element method (FEM) simulation. Because of the robustness of induced handedness and their small size, these as-synthesized chiral nanostructures can be further employed as building blocks toward the formation of complex chiral nanostructures. We have demonstrated a new class of chiral hybrid metal-semiconductor nanostructures that can allow integration of chirality with other properties and functionalities. All of these together have paved the way to engineer nanoscale inorganic chirality and thus study various emerging chirality-entangled effects with practical technological applications.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications.

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

A comparative study of photocatalytic activity of Na+-Ag+ ion-exchanged glass-ceramics with metallic Ag, semiconductor AgBr, and hybrid Ag-AgBr nanoparticles.

Photocatalytic glass-ceramics doped with metallic Ag, semiconductor AgBr, and hybrid metal-semiconductor Ag-AgBr nanostructures were synthesized via low-temperature Na+-Ag+ ion exchange. The spectral features of the nanostructures in the silicate glass matrix as well as their photocatalytic performance were studied in detail. Glass-ceramics containing hybrid metal-semiconductor nanostructures were shown to possess one order of magnitude higher photocatalytic activity compared to their counterparts with metallic and semiconductor nanostructures. Hybrid metal-semiconductor nanostructures allow enhancement of the net density of photogenerated hot electrons.

Plasmon-Exciton Interaction in Perspective Hetero-Systems

Surface plasmons and excitons have been widely studied experimentally and theoretically for various material systems. However, a number of aspects require further deeper study and understanding, among which the connection of these quasi-particles occupies an important place. New physical effects arise when plasmons and excitons in nanostructures begin to be localized at certain small distances, as a result, we can talk about their coupling. Complex systems containing the excitation of plasmons and excitons, as well as their coupling, show interesting optical properties that they cannot exhibit individually. In this type of system, the plasmon enhances the coupling between the system and the external field, and the exciton controls certain spectral properties, which opens up new possibilities for tuning their optical response. The transferred energy between plasmons and excitons becomes an important factor affecting their interaction when the resonance frequency of the localized plasmon is very close to the molecular energy transition frequency. Two types of coupling can occur depending on the ratio between the strength of the coupling and the energy losses of individual components in the system, namely strong and weak. In addition to the mutual coupling between the plasmon and the exciton, their different linewidths and ability to couple to an external field provide a variety of means to tune the optical properties of hybrid systems. Thus, it enables precise control of light at the nanometer scale, opening up possibilities for new electronics and photonics applications. In this review, we highlight the features of weak and strong modes of plasmon-exciton coupling, modern trends, and perspectives in the study of hetero-systems semiconductor–metal, metal–2D material, semiconductor–molecule, etc. Semiconductor-metal hybrid nanostructures open up exciting opportunities for the study of quantum phenomena, optical processes, and multiparticle interactions and confidently lead to application in new photonics devices.

Length-Dependent Photocatalytic Activity of Hybrid Ag-CdS Nanorods

Hybrid metal–semiconductor nanostructures are promising photocatalysts for a wide span of reactions. Determining the relationship of their geometry with catalytic efficiency is critical for optimization of the catalysts. In this work, Ag-CdS nanorods with five different lengths (from 25.0 to 106.5 nm) have been synthesized using a seed-mediated growth method. High-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS) studies confirmed that the formation of a metal tip and semiconductor body part and they absorb strongly and broadly in the UV and visible wavelength range. The nanorods have been employed as photocatalysts for methyl orange degradation and their catalytic efficiency exhibited length-dependence. Specifically, the highest efficiency was observed in the rods of intermediate length (72.1 nm). Time-dependent photoluminescence decay revealed that the Ag-CdS rods with high catalytic efficiency have a higher probability to go through charge-separation-recombination pathway than the rods with low catalytic efficiency. Understanding the physical process in these hybrid structures provides insight into fine-tuning their geometry to improve the charge transfer efficiency from the metal to the semiconductor domain. Thus, better hybrid nanomaterials can be designed and fabricated for photocatalytic and other applications.

Flow synthesis of photocatalytic semiconductor-metal hybrid nanocrystals.

Semiconductor-metal hybrid nanostructures are promising materials for photocatalytic applications, providing high efficiencies compared to their composing counterparts. So far, the synthesis of such hybrid nanoparticles was limited to batch reactors, achieving tunability while demonstrating how each of the nanocrystals' characteristics affects photocatalytic performances. Yet, new methodologies should be established to increase the synthetic yield while maintaining high control over the resulting structures. Herein, scalable advanced flow techniques are introduced, yielding ZnSe-metal hybrid nanoparticles either in a thermal growth or photo-induced growth regime. Firstly, thermal gold growth in the flow reactor is achieved with good control over the metal tip size and the nanoparticle morphology. We address the dependence of the reaction on temperature, the precursor to nanorod molar ratios, and additional parameters. Additionally, light-induced growth by the flow reactor is demonstrated for platinum clusters. The quality of the resulting hybrids is directly demonstrated by their functionality in photocatalytic hydrogen generation by water reduction, displaying enhanced activity compared to bare ZnSe nanorods. The fairly straightforward adaptation of such powerful flow-reaction techniques to scale-up photocatalytic hybrid nanoparticle syntheses takes them one step forwards towards the realization of their potential in real-life application scenarios.

Tuning of the Optical Properties of Monolayer Blue Phosphorene

Blue and black phosphorus are the most stable allotropes of phosphorus family. As a new potential semiconductor material, blue phosphorene shows the advantage of its wide indirect bandgap. We investigate fundamental optical properties and tunability of the bandgap of monolayer blue phosphorene based on density functional theory. Our comparative analysis reveals the bandgap adjustability of blue P along the polarization direction and that the plasmons resonance mode of blue phosphorene in different polarization directions. Besides, we report a unique 2D rectangular semiconductor-metal hybrid nanostructures, the monolayer rectangle blue phosphorene inserted into the Au nanowires. Owing to surface plasmon polaritons (SPPs), the absorption spectra of blue phosphorene is broadened and shifted to the infrared region, in which strong characteristic peaks appear and the existence of the coupling of single electron motion and coherent vibration is proved. The results provide the novle principle for optical modulation of blue phosphorene and possible applications in nanoscale plasma devices.

Metal-semiconductor heterostructures for surface-enhanced Raman scattering: synergistic contribution of plasmons and charge transfer.

After 45 years of its first observation, surface-enhanced Raman spectroscopy (SERS) has become an ultrasensitive tool applied in chemical analysis, materials science, and biomedical research. SERS-active nanomaterials, such as noble metals, transition metals, and semiconductors, have undergone extensive development. The hybridization of semiconductors with plasmonic metal nanomaterials is highly effective in boosting light harvesting and conversion, which enables the rapid growth of metal-semiconductor hybrid nanostructures in SERS-based research fields. With the combination of the unique photoelectric properties and giant SERS signals attributed to the synergistic contribution of plasmons and change transfer (CT), metal-semiconductor heterostructures allow diverse and novel applications of SERS in CT investigations for the rational design of photovoltaic devices and ultrasensitive chemical or biological sensing. In this review, we specifically discuss SERS-active metal-semiconductor heterostructures including their building blocks, enhancement mechanisms, and applications. Moreover, we highlight the current challenges and opportunities for future research in this field based on our recent studies and other related research.

Ultrafast and Hot Electron Transfer in CdSe QD–Au Hybrid Nanostructures

Recently, much attention has been paid to semiconductor–metal hybrid nanostructures (HNs) in terms of efficient electron transfer from the semiconductor to the metal and semiconductor–metal HNs are...

Tailoring the Third-Order Nonlinear Optical Property of a Hybrid Semiconductor Quantum Dot-Metal Nanoparticle: From Saturable to Fano-Enhanced Absorption.

Semiconductor-metal hybrid nanostructures present an exotic class of nonlinear optical materials due to their potential optoelectronic applications. However, most studies to date focus on their total optical responses instead of contributions from individual nonlinear orders. In this Letter, we present a theoretical study on the third-order nonlinear optical absorption of a hybrid colloidal semiconductor quantum dot (SQD)-metal nanoparticle (MNP) system. We develop a novel analytic treatment based on the nonlinear density matrix equation and derive a closed-form expression for the optical susceptibility. Our study identifies the parameter space that governs the system's optical transition from being a saturable absorber to a Fano-enhanced absorber. We attribute this transition to the plasmon-mediated self-interaction of the SQD. The findings provide a valuable guideline for optimized designs of functional nanophotonic devices based on SQD-MNP hybrid structures.

Regulation of electron-hole recombination kinetics on uniform metal-semiconductor nanostructures for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution has garnered considerable attention as a potential technology for the conversion of solar energy to chemical energy to replace fossil fuels with the development of hydrogen energy infrastructure. Semiconductors have been intensively studied as photocatalysts due to their tunable bandgap, eco-friendly reaction mechanism, photochemical stability, and ease of reusability. To achieve highly efficient photocatalysts, regulation of exctions, which are photoinduced electrons and holes in photocatalysts, is necessary. Semiconductor nanoparticles have been applied in this purpose because of their confined exciton pathways and differentiated catalytic characteristics depending on their size, shape, and morphology. In addition, metal cocatalysts have been decorated with semiconductor nanoparticles because the metal cocatalyst not only provides efficient shuttling of photoinduced electrons and proper reaction sites for the hydrogen evolution but also controls exciton pathways via fast electron transfer kinetics from semiconductor to metal. This research update reviews recent advances in representative metal-semiconductor hybrid nanostructures of core-shell and tipped nanorods for photocatalysts with a focus on the exciton pathways. The metal at semiconductor core-shell nanostructures has shown extraordinary photocatalytic stability via passivation of the metal by a semiconductor. In photocatalytic hydrogen evolution, the semiconductor shell hinders electron transfer to water. Hence, various core-shell related metal-semiconductor nanostructures such as yolk-shell, core-island shell, and double shell hollow structures have been proposed in efforts to overcome the electron transfer barrier to water. Metal tipped nanorods are another versatile nanostructure to control and monitor exciton pathways. The correlation between exciton pathways and photocatalytic efficiencies was demonstrated by monitoring metal tipped semiconductor nanorods with different composition, morphology, and surface structure. The insights reported here suggest a rational and versatile design strategy of metal-semiconductor hybrid nanostructures for developing highly efficient photocatalysts for hydrogen evolution.

Efficient direct electron transfer via band alignment in hybrid metal-semiconductor nanostructures toward enhanced photocatalysts

Plasmonic metal nanostructures can improve the photon-charge conversion efficiency in metal/semiconductor hybrid systems through direct electron transfer (DET), resonance energy transfer (RET), and accelerated exciton separation. These enhancement mechanisms of photon-charge conversion have been independently investigated, and it remains unclear which is the most efficient process for the conversion. Herein, DET is proposed as the most efficient pathway due to its ability to facilitate charge separation along with the extended band bending at the interface between the metal and the semiconductor under light irradiation. This investigation on plasmon-induced charge separation was carried out with metal-nanoparticle-decorated p-type semiconductor nanowires by wavelength-tunable light-irradiated Kelvin probe force microscopy and photocurrent measurements. In addition, the band alignment between the metal and the semiconductor was characterized by density functional theory calculations. DET was shown to be superior to RET by an approximated ratio of 8:2 with the coexistence of both plasmonic and excitonic excitations. This observation suggests a straightforward way to develop efficient photocatalysts – engineering individual pathways of plasmon-induced charge separation and transfer.

Heavy-Metal-Free Colloidal Semiconductor Nanorods: Recent Advances and Future Perspectives.

Quasi-1D colloidal semiconductor nanorods (NRs) are at the forefront of nanoparticle (NP) research owing to their intriguing size-dependent and shape-dependent optical and electronic properties. The past decade has witnessed significant advances in both fundamental understanding of the growth mechanisms and applications of these stimulating materials. Herein, the state-of-the-art of colloidal semiconductor NRs is reviewed, with special emphasis on heavy-metal-free materials. The main growth mechanisms of heavy-metal-free colloidal semiconductor NRs are first elaborated, including anisotropic-controlled growth, oriented attachment, solution-liquid-solid method, and cation exchange. Then, structural engineering and properties of semiconductor NRs are discussed, with a comprehensive overview of core/shell structures, alloying, and doping, as well as semiconductor-metal hybrid nanostructures, followed by highlighted practical applications in terms of photocatalysis, photodetectors, solar cells, and biomedicine. Finally, challenges and future opportunities in this fascinating research area are proposed.

Separation of charge carriers and generation of reactive oxygen species by TiO2 nanoparticles mixed with differently-coated gold nanorods under light irradiation

Combinations of semiconductor nanoparticles (NPs) with noble metal NPs enable an increase in the photoactivity of semiconductor NPs into the visible and near-infrared regions. The design rationale of the semiconductor-metal hybrid nanostructures for the optimization of charge carrier separation and reactive oxygen species (ROS) generation remains unclear. In this study, the interactions of Au nanorods (AuNRs) with TiO2 NPs were modulated by controlling their surface charges. Positively charged AuNRs formed aggregates with the negatively charged TiO2 NPs (AuNR@CTAB/TiO2) upon mixing, suggesting that Schottky junctions may exist between Au and TiO2. In contrast, negatively charged AuNRs (AuNR@PSS) remained spatially separated from the TiO2 NPs in the mixed suspension (AuNR@PSS/TiO2), owing to electrostatic repulsion. We used electron spin resonance (ESR) spectroscopy to detect the separation of charged carriers and ROS generation in these two mixtures under simulated sunlight irradiation. We also explored the role of dissolved oxygen in charge carrier separation and ROS generation by continuously introducing oxygen into the AuNR@CTAB/TiO2 suspension under simulated sunlight irradiation. Moreover, the generation of ROS by the AuNR@CTAB/TiO2 and AuNR@PSS/TiO2 mixtures were also examined under 808 nm laser irradiation. Our results show that the photogenerated electrons of excited semiconductor NPs are readily transferred to noble metal NPs simply by collisions, but the transfer of photogenerated hot electrons from excited AuNRs to TiO2 NPs is more stringent and requires the formation of Schottky junctions. In addition, the introduction of oxygen is an efficient way to enhance the photocatalytic activity of semiconductor NPs/noble metal NPs system combinations.

Cu2O–Au nanowire field-effect phototransistor for hot carrier transfer enhanced photodetection

In metal-semiconductor hybrid nanostructures, metal absorbs incident photons and generates hot carriers. The hot carriers are injected into the adjacent semiconductor and subsequently contribute to photocurrent. This process increases the conversion efficiency of optoelectronic devices and provides a new path of photodetectors. In this work, we report an enhanced photodetector by hot holes transfer, which is based on Au nanoparticles decorated p-type Cu2O nanowires. The photodetector achieves an enhanced photo-responsivity up to 0.314 A W−1, a higher detectivity of 3.7 × 1010 Jones. The response time and external quantum efficiency of the Cu2O–Au nanowires photodetector are 3.7 times faster and 18.2 times higher than that of the Cu2O nanowires, respectively. The findings indicate that Cu2O–Au nanowires would be a promising candidate in developing novel plasmonic hot carrier devices.

Plasmonic Enhancement of the Photoluminescence in Hybrid Structures with SiGe Quantum Dots and Ag Nanoislands

Plasmonic enhancement of the photoluminescence in hybrid structures with SiGe quantum dots and Ag nanoislands was found. Ag nanoislands grown on the top of the multilayer structures with SiGe quantum dots (QDs) support a surface plasmon resonance that can be tuned to the QD emission wavelength by changing of Ag nanoparticle parameters. Photoluminescence measurements of the hybrid metal-semiconductor nanostructures revealed a fourfold increase of the integral intensity of SiGe QD emission in the spectral range from 0.8 to 1 eV.

The optical response of self-organized Ag-CdTe metal-semiconductor hybrid nanostructures: Change in interaction vs number density variation

Using experimental results and a theoretical model, we establish a relation for the optical response of a mixed nanocolloid. We show that colloids prepared by mixing metal and semiconductor nanocolloids in various ratios result in a mixture composed of a particular metal-semiconductor hybrid nanostructure and unattached original particles. Thus, the properties of hybrid samples prepared by such self-organized growth are decided by the variation in number density of the hybrid and the unattached particles and not due to a continuous change in the interaction between metal nanoparticles and semiconductor quantum dots as was assumed earlier. Using these results, a method to prepare a single type of hybrid nanostructure by measuring only its optical response is presented.

Engineering Reaction Kinetics by Tailoring the Metal Tips of Metal-Semiconductor Nanodumbbells.

Semiconductor-metal hybrid nanostructures are one of the best model catalysts for understanding photocatalytic hydrogen generation. To investigate the optimal structure of metal cocatalysts, metal-CdSe-metal nanodumbbells were synthesized with three distinct sets of metal tips, Pt-CdSe-Pt, Au-CdSe-Au, and Au-CdSe-Pt. Photoelectrochemical responses and transient absorption spectra showed that the competition between the charge recombination at the metal-CdSe interface and the water reduction on the metal surface is a detrimental factor for the apparent hydrogen evolution rate. For instance, a large recombination rate (krec) at the Pt-CdSe interface limits the quantum yield of hydrogen generation despite a superior water reduction rate (kWR) on the Pt surface. To suppress the recombination process, Pt was selectively deposited onto the Au tips of Au-CdSe-Au nanodumbbells in which the krec was diminished at the Au-CdSe interface, and the large kWR was maintained on the Pt surface. As a result, the optimal structure of the Pt-coated Au-CdSe-Au nanodumbbells reached a quantum yield of 4.84%. These findings successfully demonstrate that the rational design of a metal cocatalyst and metal-semiconductor interface can additionally enhance the catalytic performance of the photochemical hydrogen generation reactions.

Structural and electronic investigation of metal-semiconductor hybrid tetrapod hetero-structures

This article highlights the new electronic properties of tetrapod hetero-structures with metal Au core and semiconductor CdSe arms, which is one of the new classes of hybrid metal-semiconductor nanostructures. From the analysis of XRD, HRTEM, HAADF-STEM images, and EDAX line-scan studies, the growth mechanism of all these hetero-structures is proposed. These findings are important from the basic fundamental aspects of understanding the shape control of hetero-structures. Scanning tunneling spectroscopic study confirms the coulomb staircase-like features near Au which is characteristic of Au nanoparticles and the gap increases as we move the tip towards CdSe. Analysis suggests that the resonance tunneling occurs between valance band edge (conduction band edge) of CdSe and coulomb stairs of Au dot. These tetrapod hetero-structures could pave the way for designing new optical-based materials for developing new challenging photonic devices.