DFT Study of Ligand Binding to Small Gold Clusters

The influence of ligands on electronic structure of small gold clusters (Au₂, Au₄) has been investigated by density functional theory (DFT). Specifically, we study the effect of bonding of four donor ligands (NH₃, NMe₃, PH₃, and PMe₃) on cluster geometries and energetics in gas phase and in solution. Performance of five generations of DFT functionals and five different basis sets is assessed. Our results benchmark the importance of the DFT functional model and polarization functions in the basis set for calculations of ligated gold cluster systems. We obtain NMe₃ ≈ NH₃ < PH₃ < PMe₃ order of ligand binding energies and observe shallow potential energy surfaces in all molecules. The latter is likely to lead to a conformational freedom in larger clusters with many ligands in solution at ambient conditions. The study suggests appropriate quantum-chemical methodology to reliably model small noble metal clusters in a realistic ligand environment typically present in experiments

Ligand Effects on Optical Properties of Small Gold Clusters: A TDDFT Study

Ligand influence on the excited state structure of small neutral gold clusters (Au₂ and Au₄) has been investigated using Time Dependent Density Functional Theory. We study in detail the absorption profile of bare and ligated small gold clusters in solution modeled with Polarizable Continuum Model. Performance of CAM-B3LYP and TPSS DFT functionals combined with TZVP basis set has been assessed. We found that ligands substantially modify the excited state structure of clusters by eliminating low-lying optically inactive excited states. Depending on the ligand environment, the cluster may gain significant fluorescence efficiency. Our results suggest that small gold clusters ligated with amines will have better fluorescence potential compared to those ligated with phosphine or thiol ligands, in agreement with preliminary experimental data. TPSS fails to describe excited state structure of ligated clusters due to spurious charge-transfer states, thus highlighting the necessity of choosing appropriate quantum-chemistry model for correct excited state description

Surface Ligands Increase Photoexcitation Relaxation Rates in CdSe Quantum Dots

Understanding the pathways of hot exciton relaxation in photoexcited semiconductor nanocrystals, also called quantum dots (QDs), is of paramount importance in multiple energy, electronics and biological applications. An important nonradiative relaxation channel originates from the nonadiabatic (NA) coupling of electronic degrees of freedom to nuclear vibrations, which in QDs depend on the confinement effects and complicated surface chemistry. To elucidate the role of surface ligands in relaxation processes of nanocrystals, we study the dynamics of the NA exciton relaxation in Cd₃₃Se₃₃ semiconductor quantum dots passivated by either trimethylphosphine oxide or methylamine ligands using explicit time-dependent modeling. The large extent of hybridization between electronic states of quantum dot and ligand molecules is found to strongly facilitate exciton relaxation. Our computational results for the ligand contributions to the exciton relaxation and electronic energy-loss in small clusters are further extrapolated to larger quantum dots

Quality Factor of Luminescent Solar Concentrators and Practical Concentration Limits Attainable with Semiconductor Quantum Dots

Luminescent solar concentrators (LSCs) can be utilized as both large-area collectors of solar radiation supplementing traditional photovoltaic cells as well as semitransparent “solar windows” that provide a desired degree of shading and simultaneously serve as power-generation units. An important characteristic of an LSC is a concentration factor (<i>C</i>) that can be thought of as a coefficient of effective enlargement (or contraction) of the area of a solar cell when it is coupled to the LSC. Here we use analytical and numerical Monte Carlo modeling in addition to experimental studies of quantum-dot-based LSCs to analyze the factors that influence optical concentration in practical devices. Our theoretical model indicates that the maximum value of <i>C</i> achievable with a given fluorophore is directly linked to the LSC quality factor (<i>Q</i>_LSC) defined as the ratio of absorption coefficients at the wavelengths of incident and reemitted light. In fact, we demonstrate that the ultimate concentration limit (<i>C</i>₀) realized in large-area devices scales linearly with the LSC quality factor and in the case of perfect emitters and devices without back reflectors is approximately equal to <i>Q</i>_LSC. To test the predictions of this model, we conduct experimental studies of LSCs based on visible-light emitting II–VI core/shell quantum dots with two distinct LSC quality factors. We also investigate devices based on near-infrared emitting CuInSe_<i>x</i>S_2–<i>x</i> quantum dots for which the large emission bandwidth allows us to assess the impact of varied <i>Q</i>_LSC on the concentration factor by simply varying the detection wavelength. In all cases, we find an excellent agreement between the model and the experimental observations, suggesting that the developed formalism can be utilized for express evaluation of prospective LSC performance based on the optical spectra of LSC fluorophores, which should facilitate future efforts on the development of high-performance devices based on quantum dots as well as other types of emitters

Thick-Shell CuInS₂/ZnS Quantum Dots with Suppressed “Blinking” and Narrow Single-Particle Emission Line Widths

Quantum dots (QDs) of ternary I–III–VI₂ compounds such as CuInS₂ and CuInSe₂ have been actively investigated as heavy-metal-free alternatives to cadmium- and lead-containing semiconductor nanomaterials. One serious limitation of these nanostructures, however, is a large photoluminescence (PL) line width (typically >300 meV), the origin of which is still not fully understood. It remains even unclear whether the observed broadening results from considerable sample heterogeneities (due, e.g., to size polydispersity) or is an unavoidable intrinsic property of individual QDs. Here, we answer this question by conducting single-particle measurements on a new type of CuInS₂ (CIS) QDs with an especially thick ZnS shell. These QDs show a greatly enhanced photostability compared to core-only or thin-shell samples and, importantly, exhibit a strongly suppressed PL blinking at the single-dot level. Spectrally resolved measurements reveal that the single-dot, room-temperature PL line width is much narrower (down to ∼60 meV) than that of the ensemble samples. To explain this distinction, we invoke a model wherein PL from CIS QDs arises from radiative recombination of a delocalized band-edge electron and a localized hole residing on a Cu-related defect and also account for the effects of electron–hole Coulomb coupling. We show that random positioning of the emitting center in the QD can lead to more than 300 meV variation in the PL energy, which represents at least one of the reasons for large PL broadening of the ensemble samples. These results suggest that in addition to narrowing size dispersion, future efforts on tightening the emission spectra of these QDs might also attempt decreasing the “positional” heterogeneity of the emitting centers

Enhanced Luminescent Stability through Particle Interactions in Silicon Nanocrystal Aggregates

Close-packed assemblies of ligand-passivated colloidal nanocrystals can exhibit enhanced photoluminescent stability, but the origin of this effect is unclear. Here, we use experiment, simulation, and <i>ab initio</i> computation to examine the influence of interparticle interactions on the photoluminescent stability of silicon nanocrystal aggregates. The time-dependent photoluminescence emitted by structures ranging in size from a single quantum dot to agglomerates of more than a thousand is compared with Monte Carlo simulations of noninteracting ensembles using measured single-particle blinking data as input. In contrast to the behavior typically exhibited by the metal chalcogenides, the measured photoluminescent stability shows an enhancement with respect to the noninteracting scenario with increasing aggregate size. We model this behavior using time-dependent density functional theory calculations of energy transfer between neighboring nanocrystals as a function of nanocrystal size, separation, and the presence of charge and/or surface-passivation defects. Our results suggest that rapid exciton transfer from “bright” nanocrystals to surface trap states in nearest-neighbors can efficiently fill such traps and enhance the stability of emission by promoting the radiative recombination of slowly diffusing excited electrons

Photoluminescence Dynamics of Aryl sp³ Defect States in Single-Walled Carbon Nanotubes

Photoluminescent defect states introduced by sp³ functionalization of semiconducting carbon nanotubes are rapidly emerging as important routes for boosting emission quantum yields and introducing new functionality. Knowledge of the relaxation dynamics of these states is required for understanding how functionalizing agents (molecular dopants) may be designed to access specific behaviors. We measure photoluminescence (PL) decay dynamics of sp³ defect states introduced by aryl functionalization of the carbon nanotube surface. Results are given for five different nanotube chiralities, each doped with a range of aryl functionality. We find that the PL decays of these sp³ defect states are biexponential, with both components relaxing on time scales of ∼100 ps. Exciton trapping at defects is found to increases PL lifetimes by a factor of 5–10, in comparison to those for the free exciton. A significant chirality dependence is observed in the decay times, ranging from 77 ps for (7,5) nanotubes to >600 ps for (5,4) structures. The strong correlation of time constants with emission energy indicates relaxation occurs <i>via</i> multiphonon decay processes, with close agreement to theoretical expectations. Variation of the aryl dopant further modulates decay times by 10–15%. The aryl defects also affect PL lifetimes of the free <i>E</i>₁₁ exciton. Shortening of the <i>E</i>₁₁ bright state lifetime as defect density increases provides further confirmation that defects act as exciton traps. A similar shortening of the <i>E</i>₁₁ dark exciton lifetime is found as defect density increases, providing strong experimental evidence that dark excitons are also trapped at such defect sites

Design and Synthesis of Heterostructured Quantum Dots with Dual Emission in the Visible and Infrared

The unique optical properties exhibited by visible emitting core/shell quantum dots with especially thick shells are the focus of widespread study, but have yet to be realized in infrared (IR)-active nanostructures. We apply an effective-mass model to identify PbSe/CdSe core/shell quantum dots as a promising system for achieving this goal. We then synthesize colloidal PbSe/CdSe quantum dots with shell thicknesses of up to 4 nm that exhibit unusually slow hole intraband relaxation from shell to core states, as evidenced by the emergence of dual emission, <i>i</i>.<i>e</i>., IR photoluminescence from the PbSe core observed simultaneously with visible emission from the CdSe shell. In addition to the large shell thickness, the development of slowed intraband relaxation is facilitated by the existence of a sharp core–shell interface without discernible alloying. Growth of thick shells without interfacial alloying or incidental formation of homogeneous CdSe nanocrystals was accomplished using insights attained <i>via</i> a systematic study of the dynamics of the cation-exchange synthesis of both PbSe/CdSe and the related system PbS/CdS. Finally, we show that the efficiency of the visible photoluminescence can be greatly enhanced by inorganic passivation

Two-Photon Absorption in CdSe Colloidal Quantum Dots Compared to Organic Molecules

We discuss fundamental differences in electronic structure as reflected in one- and two-photon absorption spectra of semiconductor quantum dots and organic molecules by performing systematic experimental and theoretical studies of the size-dependent spectra of colloidal quantum dots. Quantum-chemical and effective-mass calculations are used to model the one- and two-photon absorption spectra and compare them with the experimental results. Currently, quantum-chemical calculations are limited to only small-sized quantum dots (nanoclusters) but allow one to study various environmental effects on the optical spectra such as solvation and various surface functionalizations. The effective-mass calculations, on the other hand, are applicable to the larger-sized quantum dots and can, in general, explain the observed trends but are insensitive to solvent and ligand effects. Careful comparison of the experimental and theoretical results allows for quantifying the range of applicability of theoretical methods used in this work. Our study shows that the small clusters can be in principle described in a manner similar to that used for organic molecules. In addition, there are several important factors (quality of passivation, nature of the ligands, and intraband/interband transitions) affecting optical properties of the nanoclusters. The larger-size quantum dots, on the other hand, behave similarly to bulk semiconductors, and can be well described in terms of the effective-mass models

Fluorescent Carbon Nanotube Defects Manifest Substantial Vibrational Reorganization

Fluorescent defects have opened up exciting new opportunities to chemically tailor semiconducting carbon nanotubes for imaging, sensing, and photonics needs such as lasing, single photon emission, and photon upconversion. However, experimental measurements on the trap depths of these defects show a puzzling energy mismatch between the optical gap (difference in emission energies between the native exciton and defect trap states) and the thermal detrapping energy determined by application of the van ’t Hoff equation. To resolve this fundamentally important problem, here we synthetically incorporated a series of fluorescent aryl defects into semiconducting single-walled carbon nanotubes and experimentally determined their energy levels by temperature-dependent and chemically correlated evolution of exciton population and photoluminescence. We found that depending on the chemical nature and density of defects, the exciton detrapping energy is 14–77% smaller than the optical gap determined from photoluminescence. For the same type of defect, the detrapping energy increases with defect density from 76 to 131 meV for 4-nitroaryl defects in (6,5) single-walled carbon nanotubes, whereas the optical gap remains nearly unchanged (<5 meV). These experimental findings are corroborated by quantum-chemical simulations of the chemically functionalized carbon nanotubes. Our results suggest that the energy mismatch arises from vibrational reorganization due to significant deformation of the nanotube geometry upon exciton trapping at the defect site. An unexpectedly large reorganization energy (on the order of 100 meV) is found between ground and excited states of the defect tailored nanostructures. This finding reveals a molecular picture for description of these synthetic defects and suggests significant potential for tailoring the electronic properties of carbon nanostructures through chemical engineering