Bumpy Hollow Gold Nanospheres for Theranostic Applications: Effect of Surface Morphology on Photothermal Conversion Efficiency

The combination of hollow core and rugose surface morphology is highly attractive for photoactive near-infrared (NIR) nanomaterials. Here, we present a facile pH modification to hollow gold nanosphere (HGN) synthesis to enable controlled tuning of the surface morphology from smooth to very bumpy. Unlike other methods, the synthetic protocol does not require harsh surfactants, secondary reducing agents, or organic solvents. The resultant bumpy HGNs (bHGNs) are highly monodisperse with little variation in protrusion length from particle to particle. Mechanistic studies suggest that surface rugosity is mainly controlled by the presence of free OH– ions in solution. We also present the first systematic investigation into the effect of surface morphology on the photothermal conversion efficiency (PCE) of bumpy as well as smooth HGNs, with a maximum PCE reaching 99%. Although expected to have a higher scattering component, the bHGNs retain the excellent PCE of their smooth counterparts, which may be due to efficient reabsorption of scattered light

Novel Chiral CsPbBr₃ Metal Halide Perovskite Magic-Sized Clusters and Metal Halide Molecular Clusters with Achiral Ligands

We have synthesized inherently chiral cesium lead halide perovskite magic-sized clusters (PMSCs) and ligand-assisted metal halide molecular clusters (MHMCs) using the achiral ligands octanoic acid (OCA) and octylamine (OCAm). UV–vis electronic absorption was used to confirm characteristic absorption bands while circular dichroism (CD) spectroscopy was utilized to determine their chiroptical activity in the 412–419 and 395–405 nm regions, respectively. In contrast, the larger sized counterpart of PMSCs, namely, perovskite quantum dots (PQDs), do not show chirality. The inherent chirality of the clusters is tentatively attributed to a twisted chiral layered structure, defect-induced chiral structure, or twisted Pb–Br octahedra

Ultrafast Study of Excited State Dynamics of Amino Metal Halide Molecular Clusters

The excited state dynamics of ligand-passivated PbBr2 molecular clusters (MCs) in solution have been investigated for the first time using femtosecond transient absorption spectroscopy. The results uncover a transient bleach (TB) feature peaked around 404 nm, matching the ground state electronic absorption band peaked at 404 nm. The TB recovery signal can be fitted with a triple exponential with fast (10 ps), medium (350 ps), and long (1.8 ns) time constants. The medium and long time constants are very similar to those observed in the time-resolved photoluminescence (TRPL) decay monitored at 412 nm. The TB fast component is attributed to vibrational relaxation in the excited electronic state while the medium component with dominant amplitude is attributed to recombination between the relaxed electron and hole. The small amplitude slow component is assigned to electrons in a relatively long-lived excited electronic state, e.g., triplet state, or shallow trap state due to defects. This study provides new insights into the excited state dynamics of metal halide MCs

Rational Codoping as a Strategy to Improve Optical Properties of Doped Semiconductor Quantum Dots

Doping is a powerful and convenient technique for rationally altering the electronic, magnetic, and optical properties of materials including nanomaterials such as quantum dots (QDs) or nanocrystals (NCs). Most doping involves introduction of an impurity element or ion, into the crystal lattice of the host material, which tends to result in lattice distortion and/or charge imbalance when the dopant charge does not match the charge of the host ion replaced. One solution to such problems is codoping with another element or ion that helps to reduce lattice distortion or charge imbalance, which can stabilize the primary dopant in the host lattice and substantially improve the photoluminescence (PL) of the primary dopant. Furthermore, interaction between the codopant and primary dopant can be used to tune the PL properties by altering energy levels related to donor–acceptor pair recombination

Investigation of Electron Delocalization and Ultrafast Studies of Ru^II/Os^II Dyads with Ethynyl/Butadiynyl-Bridged Polyphosphines

The redox characteristics, electronic absorption, steady-state emission, nanosecond laser flash photolysis, and a femtosecond laser spectroscopic study have been carried out for a series of monomeric, homobimetallic, and heterobimetallic complexes with M(bpy)2Cl-based moieties (M = RuII and OsII) and ethynyl- and butadiynyl-bridged polyphosphines, namely Ph2PC⋮CPPh2 (C2P2) and Ph2PC⋮CC⋮CPPh2 (C4P2). These complexes were synthesized by reactions of the spacers with cis-M(bpy)2Cl2 or by coupling reaction between two [Cl(bpy)2M(Ph2PC⋮CH)](PF6) (M = RuII, OsII) molecules. Electronic communication through polyphosphine/polyyne spacers is found to decrease upon increase of the carbon chain length, and the comproportionation constant Kc was calculated as 14−18 for species with C2P2 and ca. 4 for the ones with C4P2. In addition, fast intramolecular energy transfer from the RuII-based donor to the OsII-based acceptor, with rate constant of (2.4−2.5) × 109 s-1, occurs within heterobimetallic complexes via a Dexter-type mechanism and an attenuation factor (β) of 0.02 Å-1

Exciton Dynamics of CdS Thin Films Produced by Chemical Bath Deposition and DC Pulse Sputtering

Exciton dynamics of CdS films have been investigated using ultrafast laser spectroscopy with an emphasis on understanding defect-related recombination. Two types of CdS films were deposited on glass substrates via direct current pulse sputtering (DCPS) and chemical bath deposition (CBD) techniques. The films displayed distinct morphological, optical, and structural properties. Their exciton and charge carrier dynamics within the first 1 ns following photoexcitation were characterized by femotosecond pump probe spectroscopy. A singular value decomposition (SVD) global fitting technique was employed to extract the lifetime and wavelength dependence of transient species. The excited electrons of the DCPS sample decays through 1.8, 8, 65, and 450 ps time constants which were attributed to donor level electron trapping, valence band (VB) → conduction band (CB) recombination, shallow donor recombination, and deep donor recombination, respectively. The CBD sample shows time constants of 6, 65, and 450 ps which were attributed to CB → VB recombination, sulfur vacancy (<i>V</i>_S) recombination, and <i>V</i>_S → oxygen interstitial (O_i) donor–acceptor pair (DAP) recombination, respectively. It was found that the DCPS deposition technique produces films with lower defect density and improved carrier dynamics, which are important for high performance solar cell applications

Synergistic Effect of CdSe Quantum Dot Sensitization and Nitrogen Doping of TiO₂ Nanostructures for Photoelectrochemical Solar Hydrogen Generation

We report the synthesis and photoelectrochemical (PEC) studies of TiO2 nanoparticles and nanowires simultaneously doped with nitrogen and sensitized with CdSe quantum dots (QDs). These novel nanocomposite structures have been applied successfully as photoanodes for PEC hydrogen generation using Na2S and Na2SO3 as sacrificial reagents. We observe significant enhanced photoresponse in these nanocomposites compared to N-doped TiO2 or CdSe QD sensitized TiO2. The enhancement is attributed to the synergistic effect of CdSe sensitization and N-doping that facilitate hole transfer/transport from CdSe to TiO2 through oxygen vacancy states (Vo) mediated by N-doping. The results demonstrate the importance of designing and manipulating the energy band alignment in composite nanomaterials for fundamentally improving charge separation and transport and thereby PEC properties

Electronic Conductivity of Semiconductor Nanoparticle Monolayers at the Air|Water Interface

The electronic conductivity of PbS and CdTe nanoparticle monolayers was examined voltammetrically by using interdigitated array (IDA) electrodes at the air|water interface. Their band gap energies were estimated from the I−V responses and were very consistent with results obtained from optical measurements as well as solution electrochemistry. For CdTe nanoparticles, the I−V responses were analogous to those of a molecular diode with reproducible voltammetric behavior after repeated potential cycling. Interestingly, there appeared to be indications of particle surface trap states in the voltammetric responses that correlated with spectroscopic measurements. In addition, the band gap of the nanoparticle monolayers could be manipulated by the interparticle interactions, red shifting with decreasing interparticle separation. In contrast, the electroactive nature of the PbS particles led to the decomposition of the nanoparticles and hence deposition onto the electrode surface. The resulting voltammetric responses evolved from those typical of the faradaic reactions to a rectifying feature of much larger current scales, which finally became linear (ohmic) because of shorting between neighboring IDA fingers. In these studies, it was found that photoexcitation played an important role in regulating the current responses, providing a mechanistic basis on which to manipulate the electronic/electrical properties of semiconductor nanomaterials. The conductivity of the final interfinger deposits was about 2 orders of magnitude smaller than that for pure metallic lead, indicating some surface contamination and/or less than perfect crystalline structure

Growth and Characterization of Highly Branched Nanostructures of Magnetic Nanoparticles

Magnetite nanoparticles of Fe3O4 have been found to grow into large highly branched nanostructures including nanochains and highly branched nanotrees in the solid state through a postannealing process. By varying the preparation conditions such as annealing time and temperature, the nanostructures could be easily manipulated. Changing the starting concentration of the magnetic nanoparticle solution also caused significant changes of the nanoarchitectures. When the magnetic nanoparticle concentration is low, the nanoparticles formed straight rods mainly with an average diameter of 80 nm and a length of several microns. With increasing concentration of the nanoparticles, treelike structures began to form. With further increase of the concentration, well-ordered nanostructures with the appearance of snowflakes were generated. The driving force for the formation of the highly ordered nanostructures includes interaction between the nanoparticles and interaction through surface-capping molecules. This experiment demonstrates that novel nanostructures can be generated by self-assembly of magnetic nanoparticles under the solid state

Experimental and TD-DFT Study of Optical Absorption of Six Explosive Molecules: RDX, HMX, PETN, TNT, TATP, and HMTD

Time dependent density function theory (TD-DFT) has been utilized to calculate the excitation energies and oscillator strengths of six common explosives: RDX (1,3,5-trinitroperhydro-1,3,5-triazine), β-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), TATP (triacetone triperoxide), HMTD (hexamethylene triperoxide diamine), TNT (2,4,6-trinitrotoluene), and PETN (pentaerythritol tetranitrate). The results were compared to experimental UV–vis absorption spectra collected in acetonitrile. Four computational methods were tested including: B3LYP, CAM-B3LYP, ωB97XD, and PBE0. PBE0 outperforms the other methods tested. Basis set effects on the electronic energies and oscillator strengths were evaluated with 6-31G­(d), 6-31+G­(d), 6-31+G­(d,p), and 6-311+G­(d,p). The minimal basis set required was 6-31+G­(d); however, additional calculations were performed with 6-311+G­(d,p). For each molecule studied, the natural transition orbitals (NTOs) were reported for the most prominent singlet excitations. The TD-DFT results have been combined with the IP_v calculated by CBS-QB3 to construct energy level diagrams for the six compounds. The results suggest optimization approaches for fluorescence based detection methods for these explosives by guiding materials selections for optimal band alignment between fluorescent probe and explosive analyte. Also, the role of the TNT Meisenheimer complex formation and the resulting electronic structure thereof on of the quenching mechanism of II–VI semiconductors is discussed