Dual Transient Bleaching of Au/PbS Hybrid Core/Shell Nanoparticles

Obtaining Ligand-Free Aqueous Au-Nanoparticles Using Reversible CsPbBr3 ↔ Au@CsPbBr3 Nanocrystal Transformation

Water-hexane interfacial preparation of photostable Au@CsPbBr3 (Au@CPB) hybrid nanocrystals (NCs) from pure CsPbBr3 (CPB) NCs is reported, with the coexistence of exciton and localized surface plasmon resonance with equal dominance. This enables strong exciton-plasmon coupling in these plasmonic perovskite NCs where not only the photoluminescence is quenched intrinsically due to ultrafast charge separation, but also the light absorption property increases significantly, covering the entire visible region. Using a controlled interfacial strategy, a reversible chemical transformation between CPB and Au@CPB NCs is shown, with the simultaneous eruption of larger-size ligand-free aqueous Au nanoparticles (NPs). An adsorption-desorption mechanism is proposed for the reversible transformation, while the overgrowth reaction of the Au NPs passes through the Au aggregation intermediate. This study further shows that the plasmonic Au@CPB hybrid NCs as well as ligand-free Au NPs exhibit clear surface enhanced Raman scattering (SERS) effect of a commercially available probe molecule. Overall, the beautiful interfacial chemistry delivers two independent plasmonic materials, i.e., Au@CPB NCs and ligand-free aqueous Au NPs, which may find important implications in photocatalytic and biomedical applications.

Polaron States as a Massive Electron-Transfer Pathway at Heterojunction Interface

For heterojunction semiconductor photoelectrodes, efficient charge separation is localized in the junction-induced electric field region and charge transfer follows a band-to-band charge-transfer pathway. Here, we found that polaron states at the heterojunction interface have a function of storing and transferring electrons. As a successful demonstration, we verified that the polaron states (Ti³⁺OH) on TiO₂ are not passivated when used to create a CdS/TiO₂ heterojunction and function as an efficient pathway for massively capturing, storing, and transferring the electrons from conduction bands of both TiO₂ and CdS, thus effectively enhancing the charge separation efficiency of the heterojunction photoanode. The electron throughput of polaron states remains a positive correlation with polaron state density. Interfacial electron transfer through the TiO₂ surface polaron states has great potential application in the development of high-performance heterojunction devices based on TiO₂.

Gold nanoparticle on semiconductor quantum dot: Do surface ligands influence Fermi level equilibration

Semiconductor-metal heterojunction nanostructures possess an ability to store electrons upon photoexcitation through Fermi level equilibration. The unique role of capping ligands in modulating the equilibration of Fermi level in CdSe-Au heteronanostructures is explored by taking alkyl thiols and alkyl amines as examples. Alkyl thiol having its highest occupied molecular orbital (HOMO) above the valence band of the heterojunction nanostructure inhibits the exciton recombination by scavenging the photogenerated hole. This leads to the elevation in the Fermi level of Au and equilibration with the conduction band of CdSe. The Fermi level equilibrated electrons are further transferred to an acceptor molecule such as methyl viologen, demonstrating the potential of heterojunction nanostructures capped with hole accepting ligands for charge transport application in photovoltaics. In contrast, alkyl amine being a non-hole acceptor ligand with its HOMO placed below its valence band promotes rapid Au mediated exciton recombination, limiting its usefulness in charge transport application. Thus, the energetics of ligands on heterojunction nanostructures plays a decisive role in Fermi level equilibration.

Ternary Metal Chalcogenides: Into the Exciton and Biexciton Dynamics

Intra-band-gap state-induced low-toxicity colloidal I-III-VI ternary metal chalcogenide nanocrystals (NCs) have emerged as promising alternatives to the toxic Cd- and Pb-chalcogenides for different optoelectronic and bioimaging applications. In this Perspective, we provide the primary understanding of the intra-band-gap state-induced photoluminescence (PL) of I-III-VI NCs, specifically CuInS₂ and AgInS₂, as a function of particle size and composition and correlated with time-resolved PL measurements. The intra-band-gap state-induced ultrafast exciton and biexciton dynamics are discussed in detail to unravel the subpicosecond carrier relaxation dynamics through transient absorption measurement. Furthermore, ultrafast dissociation of the biexciton on Au@CuInS₂ hybrid NCs has been revealed to be due to the presence of Au, which has direct relevance to the improvement of the solar cell efficiency. The proper fundamental insight of the ultrafast exciton and biexciton dynamics of these materials will enable utilization of ternary metal chalcogenides in photovoltaic as well as light-emitting devices.

Tunable plasmonic core-shell heterostructure design for broadband light driven catalysis

Considerable effort has been devoted to manipulating the optical absorption of metal nanostructures for diverse applications. However, it still remains a challenge to develop a general and flexible method to promote broadband absorption of metal nanostructures without changing their size and shape. Here, we report a new strategy of hybridizing two conceptually different optical models to realize broadband absorption enhancement of metal nanoparticles (NPs), which is enabled by constructing a core-shell heterostructure, consisting of a spherical dielectric core covered by a metal NPs interlayer and tunable semiconductor shell. This approach integrates the interfacial photon management, photoexcitation of metal NPs and injection of hot charge carriers into the semiconductor shell, and results in distinctly enhanced hot charge carrier generation and transfer, thereby boosting the broad-spectrum light driven catalysis. The structure-plasmon-catalysis interplay of the heterostructure is comprehensively studied and optimized. This proof-of-concept proves to be generally feasible by varying the type of both metal NPs and support medium, opening a new avenue to control the optoelectronic properties of materials.

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.

Silicon nanocrystal-noble metal hybrid nanoparticles

We report a novel and facile self-limiting synthesis route of silicon nanocrystal (Si NC)-based colloidally stable semiconductor-metal (gold, silver and platinum) hybrid nanoparticles (NPs). For the formation of hybrid NPs, we employ ligand-free colloidal Si NCs with heavily boron (B) and phosphorus (P) doped shells. By simply mixing B and P codoped colloidal Si NCs with metal salts, hybrid NPs consisting of metal cores and Si NC shells are spontaneously formed. We demonstrate the synthesis of highly uniform and size controllable hybrid NPs. It is shown that codoped Si NCs act as a reducing agent for metal salts and also as a protecting layer to stop metal NP growth. The process is thus self-limiting. The development of a variety of Si NC-based hybrid NPs is a promising first step for the design of biocompatible multifunctional NPs with broad material choices for biosensing, bioimaging and solar energy conversion.

Au Nanowire-Striped Cu3P Platelet Photoelectrocatalysts

A stripy pattern of continuous epitaxial growth of thin Au nanowires on plasmonic Cu3P platelets is reported. The obtained Au-Cu3P heterostructures retain their wide area interfacial heterojunction, which is typically not observed in metal-semiconductor heterostructures. This is performed by phosphine-mediated in situ reduction of Au ions on specific facets of Cu3P platelets. The intriguing stripy movements of nanowires are regulated by strong surface binding ligands. Because this is a dual plasmon heterostructure with wide visible absorption window, these are further explored as a photoelectrocatalyst for efficient hole transfer and sensing of an important biomolecule, nicotinamide adenine dinucleotide (NADH). The observed anodic photocurrent was 30 times higher in the presence of NADH, and this proves that the heterostructured material is an ideal photosenser and an efficient catalyst for solar energy conversion.

Ab Initio Analysis of Auger-Assisted Electron Transfer

Quantum confinement in nanoscale materials allows Auger-type electron-hole energy exchange. We show by direct time-domain atomistic simulation and analytic theory that Auger processes give rise to a new mechanism of charge transfer (CT) on the nanoscale. Auger-assisted CT eliminates the renown Marcus inverted regime, rationalizing recent experiments on CT from quantum dots to molecular adsorbates. The ab initio simulation reveals a complex interplay of the electron-hole and charge-phonon channels of energy exchange, demonstrating a variety of CT scenarios. The developed Marcus rate theory for Auger-assisted CT describes, without adjustable parameters, the experimental plateau of the CT rate in the region of large donor-acceptor energy gap. The analytic theory and atomistic insights apply broadly to charge and energy transfer in nanoscale systems.

Metal/Semiconductor hybrid nanostructures for plasmon-enhanced applications

Hybrid nanostructures composed of semiconductor and plasmonic metal components are receiving extensive attention. They display extraordinary optical characteristics that are derived from the simultaneous existence and close conjunction of localized surface plasmon resonance and semiconduction, as well as the synergistic interactions between the two components. They have been widely studied for photocatalysis, plasmon-enhanced spectroscopy, biotechnology, and solar cells. In this review, the developments in the field of (plasmonic metal)/semiconductor hybrid nanostructures are comprehensively described. The preparation of the hybrid nanostructures is first presented according to the semiconductor type, as well as the nanostructure morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then elucidated. Lastly, possible future research in this burgeoning field is discussed.

Enhanced lifetime of excitons in nonepitaxial Au/CdS core/shell nanocrystals

The ability of metal nanoparticles to capture light through plasmon excitations offers an opportunity for enhancing the optical absorption of plasmon-coupled semiconductor materials via energy transfer. This process, however, requires that the semiconductor component is electrically insulated to prevent a "backward" charge flow into metal and interfacial states, which causes a premature dissociation of excitons. Here we demonstrate that such an energy exchange can be achieved on the nanoscale by using nonepitaxial Au/CdS core/shell nanocomposites. These materials are fabricated via a multistep cation exchange reaction, which decouples metal and semiconductor phases leading to fewer interfacial defects. Ultrafast transient absorption measurements confirm that the lifetime of excitons in the CdS shell (τ ≈ 300 ps) is much longer than lifetimes of excitons in conventional, reduction-grown Au/CdS heteronanostructures. As a result, the energy of metal nanoparticles can be efficiently utilized by the semiconductor component without undergoing significant nonradiative energy losses, an important property for catalytic or photovoltaic applications. The reduced rate of exciton dissociation in the CdS domain of Au/CdS nanocomposites was attributed to the nonepitaxial nature of Au/CdS interfaces associated with low defect density and a high potential barrier of the interstitial phase.

Tuning the observability of surface plasmon in silica-gold raspberry shaped nanoparticles using cuprous oxide shell

A raspberry shaped silica-gold nanoparticle system has been coated with a cuprous oxide shell using a simple wet chemical approach. The optical properties of such particles depend on thin dielectric shell material, and we calculate far-field scattering and extinction of cuprous oxide coated silica-gold composite. In accordance with our theoretical findings, for ultrasmall gold nanoparticles (AuNPs < 5 nm) attached over silica, the localized surface plasmon resonance (LSPR) peak is completely suppressed after Cu2O coating. The cloaking (nonobservability) of the LSPR peak in extinction spectra has been explained via calculation of contribution from absorbance (<10%) and scattering (>90%) in the composite nanostructure. For larger particles (>5 nm), the traditional red-shift of the plasmon peak (from 532 to 588 nm) is still significant due to the large dielectric constant (approx. 8.0 @ 600 nm) of cuprous oxide (Cu2O) coating. A complete and controlled suppression of LSPR in small sized gold nanoparticles due to high dielectric refractory oxide shell could play a significant role in plasmon derived applications.

Plasmon dynamics in colloidal Au₂Cd alloy-CdSe core/shell nanocrystals

Metal-semiconductor nanocrystal heterostructures are model systems for understanding the interplay between the localized surface plasmon resonances in the metal domain and the relaxation of the excited carriers in the semiconductor domain. Here we report the synthesis of colloidal Au₂Cd (core)/CdSe (shell) nanocrystal heterostructures, which were characterized extensively with several structural and optical techniques, including time-resolved fluorescence and broad-band transient absorption spectroscopy (both below and above the CdSe band gap). The dynamics of the transient plasmon peak was dominated by the relaxation of hot carriers in the metal core, its spectral shape was independent of the pump wavelength, and the bleaching lifetime was about half a picosecond, comparable with the value found in the AuCd seeds used for the synthesis.