Metal-semiconductor Hybrid Nanostructures Research Articles

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.

Hot-electron transfer from the semiconductor domain to the metal domain in CdSe@CdS{Au} nano-heterostructures.

Semiconductor-metal hybrid nanostructures are recognized as great materials due to their high level of light-induced charge separation, which has direct relevance in photocatalysis and solar energy conversion. To understand the mechanism of charge separation processes, hybrid CdSe@CdS{Au} nano-heterostructures containing Au nanoparticles (NPs) with different sizes were synthesized, and the ultrafast charge-transfer dynamics were monitored using femtosecond transient absorption spectroscopy. Steady-state optical absorption studies suggest the formation of charge-transfer complexes between core shell nanocrystals (NCs) and Au NPs. Steady-state and time-resolved luminescence spectroscopy suggest electron transfer from the photo-excited CdSe@CdS core shell QDs NCs to the Au NPs within the heterostructure. The ultrafast interfacial electron-transfer dynamics in the heterostructures were monitored by femtosecond transient absorption spectroscopy. The results revealed that both hot and thermalized electrons are transferred from the core shell QDs to the metal NPs with time constants of 150 and 300 fs, respectively. Hot-electron transfer from QDs to Au NPs was found to take place predominantly in the heterostructures depending on the sizes of the metal NPs. The photo-degradation of rhodamin B in the presence of the CdSe@CdS{Au} heterostructures under visible-light radiation suggests that the hot electrons in the heterostructures play a major role in photocatalytic degradation.

Nanoscale Strategies for Light Harvesting.

Recent advances and the current status of challenging light-harvesting nanomaterials, such as semiconducting quantum dots (QDs), metal nanoparticles, semiconductor-metal heterostructures, π-conjugated semiconductor nanoparticles, organic-inorganic heterostructures, and porphyrin-based nanostructures, have been highlighted in this review. The significance of size-, shape-, and composition-dependent exciton decay dynamics and photoinduced energy transfer of QDs is addressed. A fundamental knowledge of these photophysical processes is crucial for the development of efficient light-harvesting systems, like photocatalytic and photovoltaic ones. Again, we have pointed out the impact of the metal-nanoparticle-based surface energy transfer process for developing light-harvesting systems. On the other hand, metal-semiconductor hybrid nanostructures are found to be very promising for photonic applications due to their exciton-plasmon interactions. Potential light-harvesting systems based on dye-doped π-conjugated semiconductor polymer nanoparticles and self-assembled structures of π-conjugated polymer are highlighted. We also discuss the significance of porphyrin-based nanostructures for potential light-harvesting systems. Finally, the future perspective of this research field is given.

Hybrid Dot–Disk Au-CuInS2 Nanostructures as Active Photocathode for Efficient Evolution of Hydrogen from Water

The synthesis of hybrid 0D-2D dot–disk Au-CIS heterostructures is enabled through nucleating wurtzite ternary I–III–VI CuInS2 (CIS) semiconductor nanostructures on cubic Au particles via thiol-activated interface reactions. Chemistry of formation of these unique hybrid metal–semiconductor nanostructures is established by correlating successive X-ray diffraction patterns and microscopic images. Furthermore, these nanostructures are explored as an efficient photocathode material for photoelectrochemical (PEC) production of hydrogen from water. Although CIS nanostructures are extensively used as PEC active materials for solar-to-hydrogen conversion, the coupled structures with Au for their exciton–plasmon coupling is observed in producing a higher photocurrent with efficient evolution of hydrogen. In the comparison of materials properties, it is observed that the cathodic photocurrent, onset potential, and the half-cell solar-to-hydrogen efficiency (HC-STH) are recorded to be superior to all CIS-based photoc...

Metal hybrid nanoparticles for catalytic organic and photochemical transformations.

In order to understand heterogeneous catalytic reactions, model catalysts such as a single crystalline surface have been widely studied for many decades. However, catalytic systems that actually advance the reactions are three-dimensional and commonly have multiple components including active metal nanoparticles and metal oxide supports. On the other hand, as nanochemistry has rapidly been developed and been applied to various fields, many researchers have begun to discuss the impact of nanochemistry on heterogeneous catalysis. Metal hybrid nanoparticles bearing multiple components are structurally very close to the actual catalysts, and their uniform and controllable morphology is suitable for investigating the relationship between the structure and the catalytic properties in detail. In this Account, we introduce four typical structures of metal hybrid nanoparticles that can be used to conduct catalytic organic and photochemical reactions. Metal@silica (or metal oxide) yolk-shell nanoparticles, in which metal cores exist in internal voids surrounded by thin silica (or metal oxide) shells, exhibited extremely high thermal and chemical stability due to the geometrical protection of the silica layers against the metal cores. The morphology of the metal cores and the pore density of the hollow shells were precisely adjusted to optimize the reaction activity and diffusion rates of the reactants. Metal@metal oxide core-shell nanoparticles and inverted structures, where the cores supported the shells serving an active surface, exhibited high activity with no diffusion barriers for the reactants and products. These nanostructures were used as effective catalysts for various organic and gas-phase reactions, including hydrogen transfer, Suzuki coupling, and steam methane reforming. In contrast to the yolk- and core-shell structures, an asymmetric arrangement of distinct domains generated acentric dumbbells and tipped rods. A large domain of each component added multiple functions, such as magnetism and light absorption, to the catalytic properties. In particular, metal-semiconductor hybrid nanostructures could behave as effective visible photocatalysts for hydrogen evolution and CO oxidation reactions. Resulting from the large surface area and high local concentration of the reactants, a double-shell hollow structure showed reaction activities higher than those of filled nanoparticles. The introduction of plasmonic Au probes into the Pt-CdS double-shell hollow particles facilitated the monitoring of photocatalytic hydrogen generation that occurred on an individual particle surface by single particle measurements. Further development of catalysis research using well-defined metal hybrid nanocatalysts with various in situ spectroscopic tools provides a means of maximizing catalytic performances until they are comparable to or better than those of homogeneous catalysts, and this would have possibly useful implications for industrial applications.

Fabrication of ZnO nanoflowers on gold coated pillars

We fabricated flower-like shaped ZnO nanorods on the apexes of hollow gold nanopillars by Focused Ion Beam lithography and hydrothermal growth. The hollow nanopillar geometry was defined on a photoresist by ion-beam induced inversion and subsequently metalized. Then the bottom section of the pillars was protected by a second resist coating, exposing only the nanopillar tips for hydrothermal growth of ZnO. The wurtzite crystalline structure of the ZnO nanorods that composes the flowers was confirmed by Raman spectroscopy. Such novel 3D hybrid metal semiconductor nanostructures are promising materials for catalysis and energy harvesting.

Ultrafast Carrier Transfer and Hot Carrier Dynamics in PbS–Au Hybrid Nanostructures

Semiconductor–metal hybrid nanostructures (HNs) are promising materials for photovoltaics and photocatalysis because of their efficient charge separation in the excited state. However, analysis of their carrier dynamics has been limited to only a handful of HNs, such as CdSe–Au and CdS–Au. Herein, we synthesized and characterized PbS–Au HNs and examined their carrier dynamics by femtosecond near-IR pump–probe spectroscopy. A decrease in band-edge (1S) bleach yields was clearly observed in PbS–Au HNs as compared to PbS quantum dots (QDs) under 1S excitation, suggesting the existence of ultrafast carrier transfer from the 1S states of PbS QDs to Au nanoparticles (NPs) that was faster than the instrumental response function. A rise time analysis of 1S bleach dynamics determined that no hot carrier transfer was present. In addition to ultrafast carrier transfer, picosecond-scale carrier transfer was observed, with the rate constants increasing with Au NPs diameter. This difference between ultrafast and picosecond-scale carrier transfer is likely due to a difference in rate constants between electron and hole transfer.

Morphology control synthesis of Au–Cu2S metal-semiconductor hybrid nanostructures by modulating reaction constituents

Hybrid nanoparticles containing solid solution Au–Cu and Cu2S phases.

Optical excitations of hybrid metal-semiconductor nanoparticles

We theoretically investigate Coulomb coupling effects in hybrid metal-semiconductor nanostructures, whose optical response is governed by plasmonic and excitonic effects (plexcitons). The plasmonic response of the nanoparticle is modeled within the framework of Maxwell’s equations, using a suitable dielectric function for the metal, and the excitonic response is described through the Schrodinger equation and the semiconductor Bloch equations. Our approach accounts for the quantum confinement of carriers in the semiconductor, for static screening in the formation of the exciton, and for a dynamic coupling between plasmons and excitons in the optical absorption or scattering. We apply our model to a prototypical CdS-based matchstick structure and investigate the importance of the various Coulomb coupling effects.

Probing the nanoscale Schottky barrier of metal/semiconductor interfaces of Pt/CdSe/Pt nanodumbbells by conductive-probe atomic force microscopy.

The electrical nature of the nanoscale contact between metal nanodots and semiconductor rods has drawn significant interest because of potential applications for metal-semiconductor hybrid nanostructures in energy conversion or heterogeneous catalysis. Here, we studied the nanoscale electrical character of the Pt/CdSe junction in Pt/CdSe/Pt nanodumbbells on connected Au islands by conductive-probe atomic force microscopy under ultra-high vacuum. Current-voltage plots measured in contact mode revealed Schottky barrier heights of individual nanojunctions of 0.41 ± 0.02 eV. The measured value of the Schottky barrier is significantly lower than that of planar thin-film diodes because of a reduction in the barrier width and enhanced tunneling probability at the interface.

Converting Ag₂S-CdS and Ag₂S-ZnS into Ag-CdS and Ag-ZnS nanoheterostructures by selective extraction of sulfur.

A mild three-step solution strategy is developed to prepare Ag-MS (M=Zn, Cd) nanoheterostructures composed of MS nanorods with silver tips. First, Ag2S-MS heterostructures are synthesized by following a solution-liquid-solid mechanism with Ag2S nanoparticles as catalysts, then the Ag2S sections of the heterostructures are converted into silver nanoparticles by selective extraction of sulfur. Notably, for the prepared Ag-CdS heterostructures, the localized surface plasmon resonance of silver remarkably intensifies the photoluminescence of CdS by enhancing the excitation light absorption, which is beneficial for potential applications of CdS nanoparticles in the fields of biolabeling, light-emitting diodes, and so forth. The strategy reported herein would be useful for designing and fabricating other metal-semiconductor hybrid nanostructures with desirable performances.

Charge Transfer Induced Encapsulation of Si Quantum Dots by Atomically Larger and Highly Lattice-Mismatched Au Nanoparticles

Semiconductor–metal hybrid nanostructures are extremely promising candidates for exploring fundamental physics and chemistry of small systems and have multifarious potential applications. However, most of the synthesis techniques result in the formation of core–shell nanostructures with the atomically larger entity comprising the core, and allows moderate lattice-mismatch between the materials of the core and the shell. In this work we demonstrate the synthesis of a stable hybrid core–shell nanocomposite, consisting of Si quantum dots encapsulated by atomically larger and highly lattice-mismatched Au nanoparticles, induced by charge transfer between the two species. The semiconductor–metal hybrid system exhibits strong and stable room temperature photoluminescence with a substantially high quantum yield of 17.1%. Structural and spectroscopic investigations confirm the encapsulation of light emitting Si quantum dots by several Au nanoparticles, which consequently restrict surface oxidation of the quantum dots and effectively stabilizes the emission spectrum. These luminescent heteronanostructures have huge potential applications ranging from energy harvesting to bioimaging.

Remarkable thermal conductivity reduction in metal-semiconductor nanocomposites

Remarkable reduction in thermal conductivity, by ∼2 orders of magnitude compared to the bulk counterpart, is observed in a metal-semiconductor nanocomposite consisting of silver (Ag) and silicon (Si) nanostructures. The variation of thermal conductivity with temperature and with volume fraction of metallic inclusion exhibits counter-intuitive behavior. Contrary to bulk composites, thermal conductivity decreases with the increase in the volume fraction of Ag nanocrystals (at least till 0.067 experimented) and increases with temperature over the range of 303-473 K. This remarkable reduction in the thermal conductivity of the nanocomposite is due to the interplay of size-dependent reduction in thermal conductivity of the individual nanostructures, increased contribution of phonon scattering at the interfaces between nanoparticles, and electron-phonon coupling inside metallic nanocrystals and across metal-semiconductor interface. Such hybrid metal-semiconductor nanostructures with reduced thermal conductivity offer immense potential for developing high efficiency thermoelectric materials.

Selective Growth of Gold onto Copper Indium Sulfide Selenide Nanoparticles

Hybrid nanostructures are interesting materials for numerous applications in chemistry, physics, and biology, due to their novel properties and multiple functionalities. Here, we present a synthesis of metal-semiconductor hybrid nanostructures composed of nontoxic I-III-VI semiconductor nanoparticles and gold. Copper indium sulfide selenide (CuInSSe) nanocrystals with zinc blende structure and trigonal pyramidal shape, capped with dodecanethiol, serve as an original semiconductor part of a new hybrid nanostructure. Metallic gold nanocrystals selectively grow onto vertexes of these CuInSSe pyramids. The hybrid nanostructures were studied by transmission electron microscopy, energy dispersive X-ray analysis, X-ray diffraction, and UV-Vis-absorption spectroscopy, which allowed us conclusions about their growth mechanism. Hybrid nanocrystals are generated by replacement of a sacrificial domain in the CuInSSe part. At the same time, small selenium nanocrystals form that stay attached to the remaining CuInSSe/Au particles. Additionally, we compare the synthesis and properties of CuInSSe-based hybrid nanostructures with those of copper indium disulfide (CuInS2). CuInS2/Au nanostructures grow by a different mechanism (surface growth) and do not show any selectivity.

Broad spectral photocurrent enhancement in Au-decorated CdSe nanowires

Metal-semiconductor hybrid nanostructures promise improved photoconductive performance due to plasmonic properties of the metal portions and intrinsic electric fields at the metal-semiconductor interface that possibly enhance charge separation. Here we report gold decorated CdSe nanowires as a novel functional material and investigate the influence of gold decoration on the lateral facets on the photoconductive properties. Gold decorated nanowires show typically an at least ten-fold higher photocurrent as compared to their bare counterparts. Interestingly, the photocurrent enhancement is wavelength independent, although the plasmon resonance related to the gold particles appears in the absorption spectra. Our experiments show that light scattering and Schottky fields associated with the metal-semiconductor interface are at the origin of the photocurrent enhancement.

A facile method for gold decoration of Te@CdTe nanorods in aqueous solution

Colloidal synthesis of metal–semiconductor hybrid nanostructures is mainly achieved in organic solution. In some applications of hybrid nanoparticles relevant in aqueous media, phase transfer of hydrophobic metal–semiconductor hybrid nanostructures is essential. In this work, we present a simple method for direct synthesis of water-soluble gold (Au) decorated Te@CdTe hybrid nanorods (NRs) at room temperature by using aqueous Te@CdTe NRs as templates, which were preformed by using CdTe nanocrystals (NCs) as precursor in the presence of hydrazine hydrate (N2H4). Our results showed that NRs were decorated with Au islands both on tips and along the surface of the NRs. The size and density of Au islands can be controlled by varying the amount of Au precursor (mixture of HAuCl4 and thioglycolic acid (TGA)) and TGA/HAuCl4 ratio. A possible growth mechanism for the Au decoration of Te@CdTe NRs is concluded as three steps: (1) the formation of AuTe1.7 via the substitution reaction of Cd2+ by Au3+, (2) adsorption of Au–TGA complex onto the preformed AuTe1.7 anchors and following reduction by CdTe and N2H4, leading to the formation of small Au NCs, (3) Au NCs grow to bigger ones, followed by reduction of more Au precursor by N2H4.

Generalized Synthesis of Hybrid Metal–Semiconductor Nanostructures Tunable from the Visible to the Infrared

Hybrid superstructures allow a convenient route to the development of materials with multiple functionalities (e.g., sensor, marker, conductor) out of monofunctional (e.g., excitonic, plasmonic) building blocks. This work describes a general synthetic route to the preparation of metal|dielectric|quantum dot hybrid superstructures that have excitonic and plasmonic resonances independently tunable from the ultraviolet to the mid-infrared spectral region. We demonstrate that structural tuning can be used to control intercomponent coupling leading to the emergence of unique optical properties. We illustrate this capability by demonstrating single- and multicolor emission from coupled systems, and a significant enhancement of two-photon absorption cross sections of quantum dots. Such properties in a robust yet dispersible particle can be useful in a number of applications including bioimaging and microscopy, and in optoelectronic devices, as well as serve as a platform for fundamental studies of metal-semiconductor interactions.

Quantum theory of the nonlinear Fano effect in hybrid metal-semiconductor nanostructures: The case of strong nonlinearity

We develop a quantum theory of the field-tunable nonlinear Fano effect in the hybrid metal-semiconductor nanostructures, in which the plasmon (semicontinuous collective intraband excitation) and the exciton (discrete single-particle interband excitation) are treated on the same footing. Our quantum theory shows that the quantum interference due to the plasmon-exciton interaction leads to the nonlinear Fano effect described by a generalized complex field-tunable Fano factor for the systems with strong external field and dephasing. We establish the relation between quantum and semiclassical theories and show that the results of the quantum and semiclassical theories differ both qualitatively and quantitatively in the strongly nonlinear regime---in particular, the quantum theory predicts the absence of nonlinear instability in the hybrid systems with plasmon relaxation.

Self-aligned epitaxial metal-semiconductor hybrid nanostructures for plasmonics

We demonstrate self-alignment of epitaxial Ag nanocrystals on top of low-density near-surface InAs quantum dots (QDs) grown by molecular beam epitaxy. The Ag nanocrystals support a surface plasmon resonance that can be tuned to the emission wavelength of the QDs. Photoluminescence measurements of such hybrid metal-semiconductor nanostructures reveal large enhancement of the emission intensity. Our concept of epitaxial self-alignment enables the integration of plasmonic functionality with electronic and photonic semiconductor devices operating down to the single QD level.