Metal-semiconductor Hybrid Research Articles

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.

Structural and optical modifications of CdS properties in CdS-Au thin films prepared by CBD

In this work highlights the modifications of the crystalline structure and properties of the nanostructured cadmium sulphide (CdS) thin films, including gold quantum dots grown to form a hybrid metal-semiconductor system. By means of chemical bath deposition (CBD) good quality thin films are obtained, with the advantage of doping in-synthesis with no required additional steps. Film thicknesses around 100 nm were determined by ellipsometry. The binding energies of elements in the CdS and CdS-Au samples were measured by X-ray photoelectronic spectroscopy (XPS). The materials crystalline structure was studied by X-ray diffraction (XRD). A change from disperse like-single-crystalline (highly oriented) CdS NP’s films to monodisperse CdS NP’s–Au QD’s like-single-crystalline samples was observed. This behaviour was confirmed by TEM micrographs. The samples properties pass from an intermediate quantum confinement regime (IQCR) in CdS to a strong one (SQCR) in CdS-Au. The Raman vibrational spectra allowed the analysis of the phonon emissions in CdS, where the Raman shift gives structural information and confirms the effects of IQCR in CdS and the SQCR in CdS-Au. The optical UV–Vis characterization describes the effect of the structural modifications, with an optical band gap shift at higher energy in the CdS-Au sample related to the SQCR. Photoluminescence (PL) measured at room-temperature shows a decrease of PL intensity in CdS-Au film due to via defects-recombination, with respect to that in CdS. This fact is interpreted as an effect of a decreasing of surface defects due to passivation by the Au presence.

Visualizing Ultrafast Electron Transfer Processes in Semiconductor-Metal Hybrid Nanoparticles: Toward Excitonic-Plasmonic Light Harvesting.

Recently, it was demonstrated that charge separation in hybrid metal–semiconductor nanoparticles (HNPs) can be obtained following photoexcitation of either the semiconductor or of the localized surface plasmon resonance (LSPR) of the metal. This suggests the intriguing possibility of photocatalytic systems benefiting from both plasmon and exciton excitation, the main challenge being to outcompete other ultrafast relaxation processes. Here we study CdSe-Au HNPs using ultrafast spectroscopy with high temporal resolution. We describe the complete pathways of electron transfer for both semiconductor and LSPR excitation. In the former, we distinguish hot and band gap electron transfer processes in the first few hundred fs. Excitation of the LSPR reveals an ultrafast (<30 fs) electron transfer to CdSe, followed by back-transfer from the semiconductor to the metal within 210 fs. This study establishes the requirements for utilization of the combined excitonic–plasmonic contribution in HNPs for diverse photocatalytic applications.

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.

Magnetic sensors and geometrical magnetoresistance: A review

Magnetic sensing devices are of the extremely significant kind of detectors, that are used several important and useful applications. Geometrical extraordinary magnetoresistance (EMR) is the geometrical kind of magnetoresistance associated with the non-magnetic semiconductor-metal hybrid structure and influenced by geometrical shape. As a result of Lorentz force, the current path change from metal (in absence of magnetic field) to semiconductor (under the subjection of the magnetic field) in semiconductor-metal hybrid structure is the key of EMR phenomena, i.e. once the metal is placed in a semiconductor, it works as a short circuit with the majority of applied current moving through metallic inhomogeneity and the almost whole resistance of semiconductor-metal hybrid structure drops to value smaller than that of homogeneous semiconductor in absence of magnetic field, in other hands, applying of magnetic field alters the current route to be around the metallic inhomogeneity where it works as an open circuit and the whole resistance turns into a quite high magnitude relies on the geometrical form of a device. The variables govern these phenomena are metal and semiconductor conductivity, semiconductor charge carriers mobility, and device geometry. Within this review, EMR phenomena history, variables governed it, materials, and applications of EMR devices are overviewed.

Modified Absorption and Emission Properties Leading to Intriguing Applications in Plasmonic–Excitonic Nanostructures

AbstractHybrid nanostructures composed of metal and semiconducting nanocrystals have drawn tremendous attention owing to their extraordinary absorption and emission properties. The energy transfer in metal–semiconductor hybrids as a result of synergetic plasmon–exciton interactions leads to numerous applications in the field of solar energy harvesting, photocatalytic reactions, imaging, photonics, sensing, and many more. Various breakthroughs in advanced characterization techniques over the past decade have disclosed several factors affecting the energy transfer processes leading to modified absorption and emission properties in metal–semiconductor hybrids. Herein, various theoretical and experimental methodologies are summarized, covering numerous advancements in metal–semiconductor nanostructures with greater emphasis on identifying parameters such as nanostructure size, shape, surface, distance between nanocrystals, and many more, which influence the optical properties of these hybrid nanocrystals. In addition, the impact of these parameters on the performance of various applications is also illuminated. Several challenges and future opportunities in the field are discussed, that will not only increase the performance and practicality of current applications, but also pave the way for future applications.

Continuous Tuning of Au–Cu2O Janus Nanostructures for Efficient Charge Separation

AbstractIn photocatalysis, the Schottky barrier in metal–semiconductor hybrids is known to promote charge separation, but a core–shell structure always leads to a charge build‐up and eventually shuts off the photocurrent. Here, we show that Au–Cu2O hybrid nanostructures can be continuously tuned, particularly when the Cu2O domains are single‐crystalline. This is in contrast to the conventional systems, where the hybrid configuration is mainly determined by the choice of materials. The distal separation of the Au–Cu2O domains in Janus nanostructures leads to enhanced charge separation and a large improvement of the photocurrent. The activity of the Au–Cu2O Janus structures is 5 times higher than that of the core–shell structure, and 10 times higher than that of the neat Cu2O nanocubes. The continuous structural tuning allows to study the structure–property relationship and an optimization of the photocatalytic performance.

Continuous Tuning of Au-Cu2 O Janus Nanostructures for Efficient Charge Separation.

In photocatalysis, the Schottky barrier in metal-semiconductor hybrids is known to promote charge separation, but a core-shell structure always leads to a charge build-up and eventually shuts off the photocurrent. Here, we show that Au-Cu2 O hybrid nanostructures can be continuously tuned, particularly when the Cu2 O domains are single-crystalline. This is in contrast to the conventional systems, where the hybrid configuration is mainly determined by the choice of materials. The distal separation of the Au-Cu2 O domains in Janus nanostructures leads to enhanced charge separation and a large improvement of the photocurrent. The activity of the Au-Cu2 O Janus structures is 5 times higher than that of the core-shell structure, and 10 times higher than that of the neat Cu2 O nanocubes. The continuous structural tuning allows to study the structure-property relationship and an optimization of the photocatalytic performance.

Interfacial facet engineering on the Schottky barrier between plasmonic Au and TiO2 in boosting the photocatalytic CO2 reduction under ultraviolet and visible light irradiation

Hybrid photocatalytic nanostructures composed of plasmonic metal and wide-band-gap semiconductor components have been widely developed, in which metal not only acts as a cocatalyst to trap the photogenerated electrons from semiconductor for improved charge separation and provide highly active sites for accelerated reaction kinetics, but also serves as a light-harvesting antennae to extend the light absorption region based on the injection of plasmonic hot electrons into the semiconductor. In both circumstances, rational design of metal/semiconductor interface is highly desirable to smooth the migration of electrons and promote the separation of carriers. Herein, based on the deposition of Au on TiO2 nanocrystals with different exposed facets, it is found that the formation of Au/TiO2(101) interface lowers the height of Schottky barrier in comparison with Au/TiO2(001) interface, enhancing the transfer of conduction band (CB) electrons from TiO2 to Au cocatalysts under ultraviolet light irradiation and promoting the hot electron injection from plasmonic Au into the CB of TiO2 with the excitation of Au by visible light. The more efficient interfacial charge transfer and separation enable more electrons participating in the conversion of CO2 to CO and CH4. As a result, at both excitation wavelengths, the Au-TiO2 sample with exclusive Au/TiO2(101) interfaces significantly ameliorates the photocatalytic activities in CO and CH4 production compared to other samples containing Au/TiO2(001) interfaces. The interfacial facet engineered Schottky barrier may open a new window to rationally designing metal–semiconductor hybrid structures for photocatalysis.

QUANDO L’INSIEME È MEGLIO DELLA SOMMA: NANOMATERIALI IBRIDI ARGENTO-TITANIA ED ORO-TITANIA PER SENSORI ELETTROANALITICI

The interest of the scientific community for nanotechnologies and nanomaterials was born in 1959, on the occasion of the annual meeting of the American Physical Society, when a new approach began with which to consider and manipulate matter on a micro- and nano-metric scale. The subsequent increase in the use of nanomaterials in scientific research is due to their peculiar characteristics: increased surface / volume ratio, new optical and physical properties, high active surface, increased or modified material transport, increased selectivity. All these features make nanomaterials extremely suitable for a wide range of applications, from energy, to catalysis and sensors. In recent decades, apart from the application of nanomaterials characterized by a single component, the use of “hybrid” nanomaterials, composed of two or more components in “intimate” contact, is also rapidly growing. This is connected to the fact that, since in a hybrid material the establishment of synergistic properties is confined to the contact region between the different components, in nanomaterials, given the small size, these effects can become a dominant factor of the entire structure. The result is precisely the establishment of new and unexpected properties (often desired and interesting) which are not the simple sum of the ownership of individual contributions. Given all this, hybrid materials are excellent candidates for applications in the most varied fields, including electroanalysis, which can greatly benefit from these systems. In this context, this work aims to present the construction of devices based on hybrid metal-semiconductor nanomaterials (based on silver or gold nanoparticles and titanium dioxide). In these systems, the presence of metal nanoparticles is exploited for the electroanalytical determination of analytes of medical / diagnostic or environmental interest, while the presence of TiO2 is essential for overcoming the problems of fouling and passivation of the electrode surface following the determination of the molecule under examination. . The possibility of renewing the electrode surface simply by irradiating the device with ultraviolet light, allows you to use the devices remotely for a long time, without their continuous recovery. In the case of the Ag-TiO2 system, an “intimate contact” interphase between the silver nanoparticles and titanium dioxide was also found, which makes the hybrid a real new material, with different characteristics and with interesting applications compared to its precursors.

Molecular Fingerprint Detection Using Portable Water‐Compatible Electronic Tunneling Spectroscopy Device

AbstractReal‐time and label‐free recognition of molecules is highly desired in numerous fields; however, most existing detection schemes require specific functionalization of the device suited for the molecule to be detected and is often complicated by nonselective adsorption. Current detection methodologies that deliver detailed molecular‐level information typically require elaborate analytical instrumentation to overcome these limitations, rendering those costly, not scalable, and nonportable techniques. Herein, a detection scheme is presented that rely on measurement of the tunneling currents through molecules under ambient conditions by employing a robust, scalable, portable, and water‐compatible device. High aspect ratio semiconductor–metal hybrid nanosystems, which feature a metal tip (Au) that functions as a nanoelectrode, are utilized to create self‐formed nanojunctions. Analyte molecules intercalate between the nanoscopic gold tip and a counter electrode, resulting in distinctive tunneling currents. Characteristic dI/dV fingerprint traces, unique to the density of states of the molecular species embedded in the liquid medium, are detectable. Resetting is demonstrated by desorption of adsorbed molecules by applying a reset voltage bias. This study paves the way toward low‐cost, on‐site, and real‐time information‐rich molecular detectors.

Insights into the Effect of Reactive Oxygen Species Regulation on Photocatalytic Performance via Construction of a Metal-Semiconductor Heterojunction.

Reactive oxygen species (ROS), as primary intermediates formed during photocatalytic reactions, are critical for a photocatalyst to realize high activity. The major objective of this study was to investigate the regulation of ROS involved in the photocatalytic degradation process via constructing a typical metal-semiconductor hybrid heterojunction using Ag nanoparticles/anatase TiO₂ as an example. Based on radical capturing, electron paramagnetic resonance, and electron spin resonance experiments, an increase in the available ROS can be achieved in the Ag/TiO₂ heterojunction due to the fast separation of photogenerated carriers. In addition, due to the change in the electron transfer pathway, superoxide radicals (·O-₂) are the dominant reactive species responsible for dye degradation using Ag/TiO₂ rather than hydroxyl radicals (·OH) as the main free radicals in pristine TiO₂. This study offers fresh insight into the regulation of ROS in photodegradation via the construction of a Ag/TiO₂ heterojunction.

Enhancing ZnO nanowire gas sensors using Au/Fe2O3 hybrid nanoparticle decoration

Heterojunctions are an important strategy for designing high performance electrical sensor materials and related devices. Herein, a new type of metal-semiconductor hybrid nanoparticle has been successfully used to remarkably sensitize the surface of ZnO nanowires for detecting NO2 with high responses over a broad temperature window ranging from room temperature to 600 °C. These hybrid nanoparticles are comprised of iron oxide nanowires with well dispersed single crystalline Au nanoparticles. The hybrid nanoparticle decorated ZnO nanowires have achieved a giant response, as high as 74 500 toward NO2 gas, about 42 times that of Au decorated ZnO nanowire sensors. This dramatic enhancement may be attributed to the efficient charge transfer across the Au–Fe2O3 Schottky and Fe2O3–ZnO semiconductor heterojunction interfaces. Due to the incorporation of thermally-stable Fe2O3 nanoparticles as the support of Au nanoparticles, the working temperature of nanowire sensors was successfully extended to higher temperatures, with an increase of 200 °C, from 400 °C to 600 °C. Such a combination of semiconductor heterojunction and semiconductor-metal Schottky contact presents a new strategy for designing high performance electrical sensors with high sensitivity, stability, selectivity, and wide operation temperature window, which are potentially suitable for advanced energy systems such as automotive engines and power plants.

Ga-GaP nanowire hybrid optical system for enhanced coupling, focusing and steering of light

Nanoparticles (NPs) with different geometry have been extensively investigated for several decades owing to their unique optical properties which provide efficient focusing, absorption, scattering, waveguiding and directivity at the nanoscale. The development of an efficient photonic device requires good antenna, as well as resonator properties, that is where hybrid metal-semiconductor nanostructure comes in to play. This work is devoted to the numerical modeling of the optical properties of GaP nanowire (NW) with Ga plasmonic droplet. The modeling results demonstrate that such a self-assembled hybrid nanostructure provides polarization switchable focusing of light at the metal–semiconductor interface, effective beam steering, and emission confinement. Numerical simulation estimates that the field can be focused tenfold at the metal–semiconductor interface providing pathway for excitation of the quantum emitter or hot electron injection. The hybrid structure can also be used to turn the light efficiently without the need to bend the NW acting as a waveguide providing four times more efficient beam steering compare to the NW without the Ga NP. We demonstrate that the light intensity varies 16 times at the interface for S and P polarization which can be utilized for optical switching in photonic or data processing applications. Finally, we discuss the architecture of the devices for the future applications based on the investigated hybrid structure.

Exploration of the duality between generalized geometry and extraordinary magnetoresistance

We outline the duality between the extraordinary magnetoresistance (EMR), observed in semiconductor-metal hybrids, and non-symmetric gravity coupled to a diffusive $U(1)$ gauge field. The corresponding gravity theory may be interpreted as the generalized complex geometry of the semi-direct product of the symmetric metric and the antisymmetric Kalb-Ramond field: ($g_{\mu\nu}+\beta_{\mu\nu}$). We construct the four dimensional covariant field theory and compute the resulting equations of motion. The equations encode the most general form of EMR within a well defined variational principle, for specific lower dimensional embedded geometric scenarios. Our formalism also reveals the emergence of additional diffusive pseudo currents for a completely dynamic field theory of EMR. The proposed equations of motion now include terms that induce geometrical deformations in the device geometry in order to optimize the EMR. This bottom-up dual description between EMR and generalized geometry/gravity lends itself to a deeper insight into the EMR effect with the promise of potentially new physical phenomena and properties.

Controlled Synthesis of Au Nanocrystals-Metal Selenide Hybrid Nanostructures toward Plasmon-Enhanced Photoelectrochemical Energy Conversion

A simple method for the controllable synthesis of Au nanocrystals–metal selenide hybrid nanostructures via amino acid guiding strategy is proposed. The results show that the symmetric overgrowth mode of PbSe shells on Au nanorods can be precisely manipulated by only adjusting the initial concentration of Pb2+. The shape of Au–PbSe hybrids can evolve from dumbbell-like to yolk-shell. Interestingly, the plasmonic absorption enhancement could be tuned by the symmetry of these hybrid nanostructures. This provides an effective pathway for maneuvering plasmon-induced energy transfer in metal–semiconductor hybrids. In addition, the photoactivities of Au–PbSe nanorods sensitized TiO2 electrodes have been further evaluated. Owing to the synergism between effective plasmonic enhancement effect and efficient interfacial charge transfer in these hybrid nanostructures, the Au–PbSe yolk-shell nanorods exhibit an outstanding photocurrent activity. Their photocurrent density is 4.38 times larger than that of Au–PbSe dumbbell-like nanorods under light irradiation at λ > 600 nm. As a versatile method, the proposed strategy can also be employed to synthesize other metal–selenide hybrid nanostructures (such as Au–CdSe, Au–Bi2Se3 and Au–CuSe).

Reentrant Kondo effect for a quantum impurity coupled to a metal-semiconductor hybrid contact

Using NRG, we show that a system containing a quantum impurity (QI), strongly coupled to a semiconductor (gap $2 \Delta$) and weakly coupled to a metal, displays a 'reentrant' Kondo stage at low temperatures. The NRG analysis of the corresponding Single Impurity Anderson Model (SIAM) shows that the reentrant stage is characterized by a second sequence of SIAM fixed points: free orbital (FO) > local moment (LM) > strong coupling (SC). In the first stage, the SC fixed point (Kondo temperature $T_{K1}$) is unstable, while the second stage exhibits a much lower Kondo temperature $T_{K2}$, associated to a stable SC fixed point. The results indicate that the reentrant Kondo screening is associated to an effective SIAM, with an effective repulsion $U_{eff}$. This low temperature effective SIAM, which we dub as 'reentrant' SIAM, behaves as a 'replica' of the high temperature (bare) SIAM. The intuitive picture that emerges is that the first Kondo state develops through impurity screening by semiconducting electrons, while the second Kondo state involves screening by metallic electrons, once the semiconducting electrons are out of reach to thermal excitations ($T < \Delta$) and only the metallic spectral weight inside the gap is available for impurity screening. In addition, we analyze a hybrid system formed by a QI `sandwiched' between an armchair graphene nanoribbon (AGNR) and a scanning tunneling microscope (STM) tip, with respective couplings set to reproduce the generic model described above. The energy gap in the AGNR can be externally tuned by an electric-field-induced Rashba spin-orbit interaction. We analyzed this system for realistic parameter values, using NRG, and concluded that the reentrant SIAM, with its associated second stage Kondo, is worthy of experimental investigation.

Electromagnetic induction derived micro-electric potential in metal-semiconductor core-shell hybrid nanostructure enhancing charge separation for high performance photocatalysis

The application of an external electric field is one of the most efficient approaches for photo-induced charge separation. However, the application of the electric-field- enhanced charge separation is limited in powder catalysts. In this study, the electromagnetic induction derived micro-electric potential in metal-semiconductor core-shell hybrid nanostructure was used to enhance charge separation in the shell semiconductor photocatalysts. The efficiency of photocatalytic hydrogen production can be improved around 110% by utilizing a core-shell nanostructure with a permanent magnet that moves under the normal photocatalytic reactor device. This electromagnetic induction derived electric field via the metal-semiconductor core-shell structure shows efficient conversions from relative motion to electric potential that provide a new opportunity to enhance photocatalytic performance with non-contacted interaction.

Construction of immobilized 0D/1D heterostructure photocatalyst Au/CuS/CdS/TiO2 NBs with enhanced photocatalytic activity towards moxifloxacin degradation

Strong absorption in a wide wavelength range and effective charge separation are main factors for improving photocatalytic activity of semiconductor photocatalysts. Herein, immobilized zero-dimensional/one-dimensional (0D/1D) Au/CuS/CdS/TiO2 nanobelts (Au/CuS/CdS/TiO2 NBs) heterostructure photocatalyst with innovative morphology and appropriate band-gap configuration was synthesized for the first time. For this construction, TiO2 NBs provide the 1D structure for a large superficial area and fast electron mobility, 0D CdS and CuS nanoparticles are the visible-light responsive nanomaterials, and Au nanoparticles serve as the surface plasmon resonance effect and electron transfer. The Au/CuS/CdS/TiO2 NBs showed improved photocatalytic performance towards moxifloxacin (MOX) degradation, which can be attributed to the promoted photoabsorption contribution and enhanced separation and transfer of photoexcited electrons and holes. The morphology, constitution, optical and electrochemical properties of the prepared photocatalyst were investigated comprehensively. Moreover, the photocatalytic degradation mechanism and degradation by-products of moxifloxacin were studied. Results showed that the OH, O2− and h+ are the main chemical oxidation species responsible for the degradation of MOX. Eight different photocatalytic degradation products were detected. The stability assessment showed that photodissolution plays the primary role in the photocatalytic performance decrease of Au/CuS/CdS/TiO2 NBs. This work gives a promising strategy to rational design efficient hybrid metal-semiconductor photocatalysts for solar energy conversion.

Carbon Monoxide Oxidation Promoted by Surface Polarization Charges in a CuO/Ag Hybrid Catalyst

Composite structures have been widely utilized to improve material performance. Here we report a semiconductor-metal hybrid structure (CuO/Ag) for CO oxidation that possesses very promising activity. Our first-principles calculations demonstrate that the significant improvement in this system’s catalytic performance mainly comes from the polarized charge injection that results from the Schottky barrier formed at the CuO/Ag interface due to the work function differential there. Moreover, we propose a synergistic mechanism underlying the recovery process of this catalyst, which could significantly promote the recovery of oxygen vacancy created via the M-vK mechanism. These findings provide a new strategy for designing high performance heterogeneous catalysts.