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

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

Molecular Adsorption Mechanism of Elemental Carbon Particles on Leaf Surface

Plant leaves can effectively capture and retain particulate matter (PM), improving air quality and human health. However, little is known about the adsorption mechanism of PM on leaf surface. Black carbon (BC) has great adverse impact on climate and environment. Four types of elemental carbon (EC) particles, carbon black as a simple model for BC, graphite, reduced graphene oxide, and graphene oxide, and C₃₆H₇₄/C₄₄H₈₈O₂ as model compounds for epicuticular wax were chosen to study their interaction and its impact at the molecular level using powder X-ray diffraction and vibrational spectroscopy (infrared and Raman). The results indicate that EC particles and wax can form C–H···π type hydrogen bonding with charge transfer from carbon to wax; therefore, strong attraction is expected between them due to the cooperativity of hydrogen bonding and London dispersion from instantaneous dipoles. In reality, once settled on the leaf surface, especially without wax ultrastructures, BC with extremely large surface-to-volume ratio will likely stick and stay. On the other hand, BC particles can lead to phase transition of epicuticular wax from crystalline to amorphous structures by creating packing disorder and end-<i>gauche</i> defects of wax molecular chain, potentially causing water loss and thereby damage of plants

Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr₃ Perovskite Quantum Dots

Exciton–phonon interactions elucidate structure–function relationships that aid in the control of color purity and carrier diffusion, which is necessary for the performance-driven design of solid-state optical emitters. Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) reveal that thermally activated exciton–phonon interactions originate from structural distortions related to vibrations in cubic CsPbBr3 perovskite quantum dots (PQDs) at room temperature. Exciton–phonon interactions cause performance-degrading PL line width broadening and slower electron–hole recombination. Structural distortions in cubic PQDs at room temperature exist as the bending and stretching of the PbBr6 octahedra subunit. The PbBr6 octahedral distortions cause symmetry breaking, resulting in thermally activated longitudinal optical (LO) phonon coupling to the photoexcited electron–hole pair that manifests as inhomogeneous PL line width broadening. At cryogenic temperatures, the line width broadening is minimized due to a decrease in phonon-assisted recombination through shallow traps. A fundamental understanding of these intrinsic exciton–phonon interactions gives insight into the polymorphic nature of the cubic phase and the origins of performance degradation in PQD optical emitters

Preparation and Photoelectrochemical Properties of CdSe/TiO₂ Hybrid Mesoporous Structures

We report on the design and synthesis of a novel CdSe/TiO₂ hybrid mesoporous structure and its implementation as a photoanode for photoelectrochemical (PEC) application. The CdSe/TiO₂ hybrid mesoporous structure was produced by assembling CdSe quantum dots (QDs) and TiO₂ nanocrystals into CdSe/TiO₂ hybrid colloidal spheres, followed by calcination to remove the capping ligands between CdSe and TiO₂. Compared to the system involving CdSe QDs directly linked to TiO₂ through molecular linkers, this CdSe/TiO₂ hybrid mesoporous structure affords the advantage of better interfacial coupling between CdSe and TiO₂ due to closer contact. As a result, the CdSe/TiO₂ hybrid mesoporous structure exhibits significantly improved photoresponse as a photoanode, as demonstrated successfully in comparative PEC studies. This study illustrates the importance of fundamental structural control in influencing PEC properties of hybrid assembled nanostructures

Tunable Photoluminescent Core/Shell Cu⁺‑Doped ZnSe/ZnS Quantum Dots Codoped with Al³⁺, Ga³⁺, or In³⁺

Semiconductor quantum dots (QDs) with stable, oxidation resistant, and tunable photoluminescence (PL) are highly desired for various applications including solid-state lighting and biological labeling. However, many current systems for visible light emission involve the use of toxic Cd. Here, we report the synthesis and characterization of a series of codoped core/shell ZnSe/ZnS QDs with tunable PL maxima spanning 430−570 nm (average full width at half-maximum of 80 nm) and broad emission extending to 700 nm, through the use of Cu⁺ as the primary dopant and trivalent cations (Al³⁺, Ga³⁺, and In³⁺) as codopants. Furthermore, we developed a unique thiol-based bidentate ligand that significantly improved PL intensity, long-term stability, and resilience to postsynthetic processing. Through comprehensive experimental and computational studies based on steady-state and time-resolved spectroscopy, electron microscopy, and density functional theory (DFT), we show that the tunable PL of this system is the result of energy level modification to donor and/or acceptor recombination pathways. By incorporating these findings with local structure information obtained from extended X-ray absorption fine structure (EXAFS) studies, we generate a complete energetic model accounting for the photophysical processes in these unique QDs. With the understanding of optical, structural, and electronic properties we gain in this study, this successful codoping strategy may be applied to other QD or related systems to tune the optical properties of semiconductors while maintaining low toxicity

Understanding and Mitigating the Effects of Stable Dodecahydro-<i>closo</i>-dodecaborate Intermediates on Hydrogen-Storage Reactions

Alkali metal borohydrides can reversibly store hydrogen; however, the materials display poor cyclability, oftentimes linked to the occurrence of stable <i>closo</i>-polyborate intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of metal borohydrides, several alkali metal dodecahydro-<i>closo</i>-dodecaborate salts were isolated in anhydrous form and characterized by diffraction and spectroscopic techniques. Mixtures of Li₂B₁₂H₁₂, Na₂B₁₂H₁₂, and K₂B₁₂H₁₂ with the corresponding alkali metal hydrides were subjected to hydrogenation conditions known to favor partial or full reversibility in metal borohydrides. The stoichiometric mixtures of MH and M₂B₁₂H₁₂ salts form the corresponding metal borohydrides MBH₄ (M = Li, Na, K) in almost quantitative yield at 100 MPa H₂ and 500 °C. In addition, stoichiometric mixtures of Li₂B₁₂H₁₂ and MgH₂ were found to form MgB₂ at 500 °C and above upon desorption in vacuum. The two destabilization strategies outlined above suggest that metal polyhydro-<i>closo</i>-polyborate species can be converted into the corresponding metal borohydrides or borides, albeit under rather harsh conditions of hydrogen pressure and temperature

Dependence of Interfacial Charge Transfer on Bifunctional Aromatic Molecular Linkers in CdSe Quantum Dot Sensitized TiO₂ Photoelectrodes

Quantum dot (QD) sensitization of TiO₂ is a powerful method to improve its performance as a photoanode material in solar energy conversion. The efficiency of sensitization depends strongly on the rate of interfacial electron transfer (ET) from the QDs to TiO₂. To understand the key factors affecting the ET, arene-substituted (ortho, meta, and para) bifunctional linkers with single or double aromatic rings were employed to link CdSe QDs to TiO₂ and control the strength of their interaction as well as the rate of interfacial ET. Interestingly, the para-substituted aromatic linker, 4-mercaptobenzoic acid (4MBA) with the longest distance between the carboxyl and thiol groups, shows the best photoelectrochemical (PEC) performance, when compared to those of ortho-subtituted (2-mercaptobenzoic acid, 2MBA) and meta-substituted (3-mercaptobenzoic acid, 3MBA) aromatic linkers. Two other bifunctional linkers with double aromatic rings, 4′-mercapto-[1,1′-biphenyl]-4-carboxylic acid (4M1B4A) and 6-mercapto-2-naphthioc acid (6M2NA), were also studied for comparison. Ultrafast transient absorption (TA) spectroscopy was used to study the exciton dynamics in CdSe QDs and determine the interfacial ET rate constant (<i>k</i>_ET). The <i>k</i>_ET results are consistent with the trend of PEC measurements in that 4MBA shows the highest <i>k</i>_ET. To gain further insight into the ET mechanism, we performed density functional theory (DFT) calculations to examine the intrinsic properties of the linkers. The results revealed that the favorable wave function distribution of the molecular orbitals of 4MBA and 4M1B4A are responsible for the higher interfacial ET rate and PEC performance due to better interfacial coupling, a factor that dominates over distance. The present study provides important new insight into the mechanism of interfacial ET using aromatic bifunctional linkers, which is useful in designing QD sensitized semiconductor metal oxide nanostructures for applications including photovoltaics and solar fuel generation

Coherent Vibrational Oscillations of Hollow Gold Nanospheres

Ultrafast pump−probe spectroscopy was used to characterize coherent vibrational oscillations of hollow gold nanospheres (HGNs) composed of a polycrystalline Au shell and a hollow, solvent-filled interior. Different HGN samples show heavily damped radial breathing mode oscillations with a period ranging from 28 ± 2 to 33 ± 3 ps. We theoretically modeled the oscillation period of HGNs while varying both the shell thicknesses and particle radii. Creation of a hollow cavity was predicted to increase the oscillation period relative to solid gold nanoparticles, and this result was verified experimentally. Our theoretical predictions of oscillation period are significantly lower than the experimental measurements. We propose that this difference is due to the polycrystalline nature of HGNs that softens the vibration of the lattice compared with a single-crystalline shell. We compare our system to solid Au nanoparticles and Au nanoparticle aggregates and find a general trend of longer oscillation period with increasing particle polycrystallinity

Ultrafast Exciton Dynamics in Silicon Nanowires

Ultrafast exciton dynamics in one-dimensional (1D) silicon nanowires (SiNWs) have been investigated using femtosecond transient absorption techniques. A strong transient bleach feature was observed from 500 to 770 nm following excitation at 470 nm. The bleach recovery was dominated by an extremely fast feature that can be fit to a triple exponential with time constants of 0.3, 5.4, and ∼75 ps, which are independent of probe wavelength. The amplitude and lifetime of the fast component were excitation intensity-dependent, with the amplitude increasing more than linearly and the lifetime decreasing with increasing excitation intensity. The fast decay is attributed to exciton–exciton annihilation upon trap state saturation. The threshold for observing this nonlinear process is sensitive to the porosity and surface properties of the sample. To help gain insight into the relaxation pathways, a four-state kinetic model was developed to explain the main features of the experimental dynamics data. The model suggests that after initial photoexcitation, conduction band (CB) electrons become trapped in the shallow trap (ST) states within 0.5 ps and are further trapped into deep trap (DT) states within 4 ps. The DT electrons finally recombine with the hole with a time constant of ∼500 ps, confirming the photophysical processes to which we assigned the decays