Direct Observation of Plasmon-Induced Interfacial Charge Separation in Metal/Semiconductor Hybrid Nanostructures by Measuring Surface Potentials

A retinomorphic neuron for artificial vision and iris accommodation

The iris of an eye automatically optimizes the amount of light that strikes the retina by accommodating the intensity of ambient light. Here, we describe a retinomorphic neuron using neuromorphic photoreceptors for artificial vision and iris accommodation that mimics the biological structure and processing functions of retinal neurons for light sensing and signal transduction. The system consists of a neuromorphic photoreceptor, an electrochromic device as a light filter, and a spike-generation unit. In particular, the Au nanoparticle (NP) decorated ITO fiber photoreceptor with a well-aligned array structure is able to rely on its own light-tunable synaptic plasticity and the plasmon-enhanced light absorption. Therefore, it allows real-time feedback about light intensity, emits a higher-frequency electrical stimulus to stronger light, flash, or prolonged light illumination time, and drives the electrochromic filter to work, allowing mild light to pass through. Compared with traditional artificial irises or artificial photoreceptors, our design introduces neural pathways and neuromorphic devices, which are closer to biological functions in simulation. To our knowledge, this is the first time that a retinal neuron with neuromorphic photoreceptors has been used for artificial iris vision. Furthermore, we demonstrate direct and consensual pupillary light reflexes. The design of artificial iris vision has potential applications in biomimetic engineering, smart interaction, and visual prostheses.

Observation of Charge Separation Enhancement in Plasmonic Photocatalysts under Coupling Conditions

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Band Alignment Enabling Effective Charge Transfer for the Highly Enhanced Raman Scattering and Fluorescence of Metal-Nanoparticle-Decorated Conjugated Polymer Nanowires

The charge transfer (CT) process has attracted much attention due to its contribution to the improvement of spectroscopic phenomena such as Raman scattering and fluorescence. A current challenge is understanding what factors can influence CT. Here, it is demonstrated that the enhancement factor (EF) of CT (∼2000) can reach the level of electromagnetic enhancement (∼1680) when resonant CT is carried out by (Fermi level energy) band alignment between a metal nanoparticle (NP) and conjugated polymer (polypyrrole (PPy)) nanowire (NW). This band alignment results in an on- or off-resonant CT. As a proof of concept for CT based surface enhanced Raman scattering (SERS) template, the Ag NPs-decorated PPy NW is utilized to effectively enhance the Raman signal of rhodamine 6G (EF of 5.7 × 10⁵). Hence, by means of our demonstration, it is proposed that controlling the band alignment should be considered an important parameter for obtaining a large EF of spectroscopic phenomena.

Invariable resistance of conductive nanocomposite over 30% strain

The dependence of the electrical resistance on materials' geometry determines the performance of conductive nanocomposites. Here, we report the invariable resistance of a conductive nanocomposite over 30% strain. This is enabled by the in situ-generated hierarchically structured silver nanosatellite particles, realizing a short interparticle distance (4.37 nm) in a stretchable silicone rubber matrix. Furthermore, the barrier height is tuned to be negligible by matching the electron affinity of silicone rubber to the work function of silver. The stretching results in the electron flow without additional scattering in the silicone rubber matrix. The transport is changed to quantum tunneling if the barrier height is gradually increased by using different matrix polymers with smaller electron affinities, such as ethyl vinyl acetates and thermoplastic polyurethane. The tunneling current decreases with increasing strain, which is accurately described by the Simmons approximation theory. The tunable transport in nanocomposites provides an advancement in the design of stretchable conductors.

Synergistic Combination of Charge Carriers and Energy-Transfer Processes in Plasmonic Photocatalysis

Important efforts are currently under way in order to develop further the nascent field of plasmonic photocatalysis, striving for improved efficiencies and selectivities. A significant fraction of such efforts has been focused on distinguishing, understanding, and enhancing specific energy-transfer mechanisms from plasmonic nanostructures to their environment. Herein, we report a synthetic strategy that combines two of the main physical mechanisms driving plasmonic photocatalysis into an engineered system by rationally combining the photochemical features of energetic charge carriers and the electromagnetic field enhancement inherent to the plasmonic excitation. We do so by creating hybrid photocatalysts that integrate multiple plasmonic resonators in a single entity, controlling their joint contribution through spectral separation and differential surface functionalization. This strategy allows us to create complex hybrids with improved photosensitization capabilities, thanks to the synergistic combination of two photosensitization mechanisms. Our results show that the hot electron injection can be combined with an energy-transfer process mediated by the near-field interaction, leading to a significant increase in the final photocatalytic response of the material and moving the field of plasmonic photocatalysis closer to energy-efficient applications. Furthermore, our multimodal hybrids offer a test system to probe the properties of the two targeted mechanisms in energy-related applications such as the photocatalytic generation of hydrogen and open the door to wavelength-selective photocatalysis and novel tandem reactions.

Direct observation of carrier migration in heterojunctions to discuss the p-n and direct Z-scheme heterojunctions

Type II p-n heterojunction and direct Z-scheme heterojunction are identical staggered band alignments, but were reported ambiguously in many composite photocatalysts because their carriers migrate in opposite directions. In this research, metal oxides CuO, NiO and Co₃O₄-based heterojunctions with Na_0.9Mg_0.45Ti_3.55O₈(NMTO) were synthesized via a simple hydrothermal method. The CuO/NMTO heterojunction was demonstrated as a direct Z-scheme heterojunction, whereas the NiO/NMTO and Co₃O₄/NMTO heterojunctions showed type II p-n band alignment, distinguished by the direct observation of carrier migration under light illumination, and confirmed by the x-ray photoelectron spectroscopy, Mott-Schottky measurements, ultraviolet photoelectron spectra and capture experiments. These all heterojunctions enjoyed better photocatalytic performance to degrade methylene blue and antibiotics (Enrofloxacin, Metronidazole and tetracycline) than the pure NMTO, attributed to their effective separation of the photoinduced electron-hole pairs owing to the staggered band alignment. Prominently, the NiO/NMTO and Co₃O₄/NMTO p-n heterojunctions exhibited superior degradation ability to the CuO/NMTO Z-scheme heterojunction. The initial relative Fermi position of two semiconductors is the prerequisite to determine whether the p-n heterojunction or direct Z-scheme heterojunction is built because the electrons diffuse from one semiconductor with a higher Fermi level to another with a lower Fermi level while the holes diffuse reversely until a united Fermi level when they combine. The built-in electric field at the heterojunction interface is determined by the difference in the initial Fermi levels or work functions of two semiconductors, regulating the separation ability of photogenerated electrons and holes to affect the photocatalytic performance. Thus, the high difference in the initial Fermi levels of semiconductors is crucial in the development of heterojunctions with staggered band alignment to obtain high performance in photocatalytic reactions.

Interface engineering enhanced near-infrared electroluminescence in an n-ZnO microwire/p-GaAs heterojunction

Interface engineering in the fabrication of low-dimensional optoelectronic devices has been highlighted in recent decades to enhance device characteristics such as reducing leakage current, optimizing charge transport, and modulating the energy-band structure. In this paper, we report a dielectric interface approach to realize one-dimensional (1D) wire near-infrared light-emitting devices with high brightness and enhanced emission efficiency. The light-emitting diode is composed of a zinc oxide microwire covered by a silver nanolayer (Ag@ZnO MW), magnesium oxide (MgO) buffer layer, and p-type gallium arsenide (GaAs) substrate. In the device structure, the insertion of a MgO dielectric layer in the n-ZnO MW/p-GaAs heterojunction can be used to modulate the device features, such as changing the charge transport properties, reducing the leakage current and engineering the band alignment. Furthermore, the cladding of the Ag nanolayer on the ZnO MW can optimize the junction interface quality, thus reducing the turn-on voltage and increasing the current injection and electroluminescence (EL) efficiency. The combination of MgO buffer layer and Ag nanolayer cladding can be utilized to achieve modulating the carrier recombination path, interfacial engineering of heterojunction with optimized band alignment and electronic structure in these carefully designed emission devices. Besides, the enhanced near-infrared EL and improved physical contact were also obtained. The study of current transport modulation and energy-band engineering proposes an original and efficient route for improving the device performances of 1D wire-type heterojunction light sources.

Interfacial modulation and plasmonic effect mediated high-brightness green light sources in a single Ga-doped ZnO microwire based heterojunction

Heterostructure manufacturing has been extensively studied as indispensable footstones in the progressive semiconductor optoelectronic devices due to their constituent materials, interfacial states and electronic transport capabilities, thus enabling competitive candidates...

Zero→Two-Dimensional Metal Nanostructures: An Overview on Methods of Preparation, Characterization, Properties, and Applications

Metal nanostructured materials, with many excellent and unique physical and mechanical properties compared to macroscopic bulk materials, have been widely used in the fields of electronics, bioimaging, sensing, photonics, biomimetic biology, information, and energy storage. It is worthy of noting that most of these applications require the use of nanostructured metals with specific controlled properties, which are significantly dependent on a series of physical parameters of its characteristic size, geometry, composition, and structure. Therefore, research on low-cost preparation of metal nanostructures and controlling of their characteristic sizes and geometric shapes are the keys to their development in different application fields. The preparation methods, physical and chemical properties, and application progress of metallic nanostructures are reviewed, and the methods for characterizing metal nanostructures are summarized. Finally, the future development of metallic nanostructure materials is explored.

Hot Electrons in TiO2-Noble Metal Nano-Heterojunctions: Fundamental Science and Applications in Photocatalysis

Plasmonic photocatalysis enables innovation by harnessing photonic energy across a broad swathe of the solar spectrum to drive chemical reactions. This review provides a comprehensive summary of the latest developments and issues for advanced research in plasmonic hot electron driven photocatalytic technologies focusing on TiO2-noble metal nanoparticle heterojunctions. In-depth discussions on fundamental hot electron phenomena in plasmonic photocatalysis is the focal point of this review. We summarize hot electron dynamics, elaborate on techniques to probe and measure said phenomena, and provide perspective on potential applications-photocatalytic degradation of organic pollutants, CO2 photoreduction, and photoelectrochemical water splitting-that benefit from this technology. A contentious and hitherto unexplained phenomenon is the wavelength dependence of plasmonic photocatalysis. Many published reports on noble metal-metal oxide nanostructures show action spectra where quantum yields closely follow the absorption corresponding to higher energy interband transitions, while an equal number also show quantum efficiencies that follow the optical response corresponding to the localized surface plasmon resonance (LSPR). We have provided a working hypothesis for the first time to reconcile these contradictory results and explain why photocatalytic action in certain plasmonic systems is mediated by interband transitions and in others by hot electrons produced by the decay of particle plasmons.

Atomic-Level Charge Separation Strategies in Semiconductor-Based Photocatalysts

Semiconductor-based photocatalysis as a productive technology furnishes a prospective solution to environmental and renewable energy issues, but its efficiency greatly relies on the effective bulk and surface separation of photoexcited charge carriers. Exploitation of atomic-level strategies allows in-depth understanding on the related mechanisms and enables bottom-up precise design of photocatalysts, significantly enhancing photocatalytic activity. Herein, the advances on atomic-level charge separation strategies toward developing robust photocatalysts are highlighted, elucidating the fundamentals of charge separation and transfer processes and advanced probing techniques. The atomic-level bulk charge separation strategies, embodied by regulation of charge movement pathway and migration dynamic, boil down to shortening the charge diffusion distance to the atomic-scale, establishing atomic-level charge transfer channels, and enhancing the charge separation driving force. Meanwhile, regulating the in-plane surface structure and spatial surface structure are summarized as atomic-level surface charge separation strategies. Moreover, collaborative strategies for simultaneous manipulation of bulk and surface photocharges are also introduced. Finally, the existing challenges and future prospects for fabrication of state-of-the-art photocatalysts are discussed on the basis of a thorough comprehension of atomic-level charge separation strategies.

Charge transfer in SnS2/Na0.9Mg0.45Ti3.55O8 heterojunction in photocatalytic process

SnS₂/Na_0.9Mg_0.45Ti_3.55O₈ (SNMTO) composite photocatalyst was synthesized by a hydrothermal method. The chemical combination in lattice scale between SnS₂ and Na_0.9Mg_0.45Ti_3.55O₈ (NMTO) was observed by high-resolution transmission electron microscopy, indicating that heterojunctions were obtained between SnS₂ and NMTO. The photocatalytic activity of SNMTO heterojunctions was improved in comparison with that of pure NMTO and SnS₂ for the photocatalytic degradation of methylene blue and Rhodamine B. Electrons were excited in n-type semiconductors NMTO and SnS₂ under light illumination, and a part of them moved to the interface, determined with the surface potential reduction observed directly by Kelvin probe force microscopy. The charge redistribution in the composite illustrates a high density of interface states between SnS₂ and NMTO, which attract lots of photoelectrons, as a result enhancing the photocatalytic performance. This finding is very different from the speculation that the photogenerated electrons and holes migrate from one part to another because it is difficult for charge carriers to travel through the interface with high energy.

Rapid fabrication of silver–cuprous oxide core–shell nanowires for visible light photocatalysts

This work reports a novel method that enables facile fabrication of Ag–Cu₂O core–shell nanowires (Ag@Cu₂O NWs) for visible light photocatalysts.

Plasmon expedited response time and enhanced response in gold nanoparticles-decorated zinc oxide nanowire-based nitrogen dioxide gas sensor at room temperature

A highly sensitive and rapidly responsive nitrogen dioxide (NO₂) gas sensor based on gold (Au) nanoparticles (NPs)-decorated zinc oxide (ZnO) nanowires (NWs) is presented. The Au NPs decoration was conducted onto ZnO NWs with and without a (3-aminopropyl)triethoxysilane (APTES) layer on their surface by using the electrostatic force. The samples without the APTES layer exhibited high NO₂ gas sensitivity (i.e. expedited response time and enhanced gas response) due to localized surface plasmon resonance (LSPR) of the Au NPs; in particular, the NO₂ gas response and the response time were increased by three times and shortened by 86%, respectively, compared with the undecorated ZnO NWs. The presence of the APTES layer improved the Au NPs attachment, but hindering the gas adsorption on the ZnO NWs surface, as proven by the observed photocurrent and gas response. Our findings imply that the response time of semiconductor gas sensors can be remarkably expedited by the LSPR effect, which is useful for developing practical gas sensors.

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

Hybrid nanostructures, composed of multi-component crystals of various shapes, sizes and compositions are much sought-after functional materials. Pairing the ability to tune each material separately and controllably combine two (or more) domains with defined spatial orientation results in new properties. In this review, we discuss the various synthetic mechanisms for the formation of hybrid nanostructures of various complexities containing at least one metal/semiconductor interface, with a focus on colloidal chemistry. Different synthetic approaches, alongside the underlying kinetic and thermodynamic principles are discussed, and future advancement prospects are evaluated. Furthermore, the proved unique properties are reviewed with emphasis on the connection between the synthetic method and the resulting physical, chemical and optical properties with applications in fields such as photocatalysis.

Advanced space- and time-resolved techniques for photocatalyst studies

Nanoparticle photocatalysts present the obvious characteristic of heterogeneity in structure, energy, and function at spatial and temporal scales.

Single-Crystalline Metal Oxide Nanostructures Synthesized by Plasma-Enhanced Thermal Oxidation

To unravel the influence of the temperature and plasma species on the growth of single-crystalline metal oxide nanostructures, zinc, iron, and copper foils were used as substrates for the study of nanostructure synthesis in the glow discharge of the mixture of oxygen and argon gases by a custom-made plasma-enhanced horizontal tube furnace deposition system. The morphology and microstructure of the resulting metal oxide nanomaterials were controlled by changing the reaction temperature from 300 to 600 °C. Experimentally, we confirmed that single-crystalline zinc oxide, copper oxide, and iron oxide nanostructures with tunable morphologies (including nanowires, nanobelts, etc.) can be successfully synthesized via such procedure. A plausible growth mechanism for the synthesis of metal oxide nanostructures under the plasma-based process is proposed and supported by the nanostructure growth modelling. The results of this work are generic, confirmed on three different types of materials, and can be applied for the synthesis of a broader range of metal oxide nanostructures.

Scanning probe microscopy for real-space observations of local chemical reactions induced by a localized surface plasmon

Localised surface plasmon (LSP) resonance has attracted considerable attention in recent years as an efficient driving force for chemical reactions. The chemical reactions induced by LSP are classified into two types, namely, redox reactions based on plasmon-induced charge separation (PICS) and chemical reactions induced by the direct interaction between LSP and molecules (plasmon-induced chemical reactions). Although both types of reactions have been extensively studied, the mechanisms of PICS and plasmon-induced chemical reactions remain unexplained and controversial because conventional macroscopic methods can hardly grasp the local chemical reactions induced by LSP. In order to obtain mechanistic insights, nanoscale observations and investigations are necessary. Scanning probe microscopy (SPM) is a powerful experimental tool to investigate not only the surface morphology but also the physical and chemical properties of samples at a high spatial resolution. In this perspective review, we first explain SPM combined with optical excitation, and then, review the recent studies using SPM techniques for real-space observations of the chemical reactions induced by LSP.

Optically Governed Dynamic Surface Charge Redistribution of Hybrid Plasmo-Pyroelectric Nanosystems

Though plasmonic effect is making some headway in the energy harvesting realm, its fundamental charge transfer mechanism to a large extent is attributed to the hot-carrier generation at the contact interface. Herein this work attempts to elucidate the physical origin of light induced plasmo-pyroelectric enhancement based on charge density manipulation on surface state in the vicinity of the metal-ferroelectric contact interface. More importantly, by tuning the band bending, it is shown that the charge density on the surface state of a hybrid plasmo-pyroelectric (BaTiO₃ -Ag) nanosystem can be manipulated and largely increased under the resonant blue light illumination (363 nm). It is also demonstrated that owing to this effect, the spatial pyroelectric activity of a hybrid plasmo-pyroelectric nanosystem governs 46% enhancement in pyroelectric coefficient. This research highlights the optically regulated charge density in plasmo-pyroelectric nanosystems, which could pave a new avenue for energy harvesting/conversion devices with distinguished advantages in wireless, photonic-controlled, localized, and dynamic stimulation.

Plasmon-enhanced photocatalytic activity of Na0.9Mg0.45Ti3.55O8 loaded with noble metals directly observed with scanning Kelvin probe microscopy

The noble metals Au, Ag and Pt were loaded onto Na_0.9Mg_0.45Ti_3.55O₈ (NMTO) using a chemical bath deposition method devised in our recent work for the first time. The composite photocatalysts exhibit more effective photodegradation of methylene blue, due to the Schottky barrier built between NMTO and noble metal. Hot electrons generated during localized surface plasmon processes in metal nanoparticles transfer to the semiconductor, manifesting as a depression of surface potential directly detectable by scanning Kelvin probe microscopy. The key factor responsible for the improved ability of semiconductor-based photocatalysts is charge separation. The most effective weight concentrations of Au, Ag and Pt loaded onto NMTO were found to be 5.00%, 12.6% and 5.55% respectively. NMTO loaded with noble metals shows good photostability and recyclability for the degradation of methylene blue. A possible mechanism for the photodegradation of methylene blue over NMTO loaded with noble metals is proposed. This work highlights the potential application of NMTO-based photocatalysts, and provides an effective method to detect localized surface plasmons.

Imaging photogenerated charge carriers on surfaces and interfaces of photocatalysts with surface photovoltage microscopy

Recent advances in imaging and characterizing charge separation on surfaces and interfaces of photocatalysts by surface photovoltage spectroscopy were reviewed and highlighted.