Reduced Heterogeneity of Electron Transfer into Polycrystalline TiO₂ Films: Site Specific Kinetics Revealed by Single-Particle Spectroscopy

The presenting surface of TiO₂ is one of the key factors that influence the photoinduced charge injection process from covalently bound chromophores. However, the dependence of electron transfer (ET) on TiO₂ surface properties (structure, defects, and facets) remains poorly understood due to the difficulties of deconvoluting the signal from a multitude of surface binding sites in highly heterogeneous ET systems. In an effort to correlate TiO₂ surface features with ET, we compare the photoinduced ET dynamics from single quantum dots (QDs) to polycrystalline TiO₂ thin films (pc-TiO₂) grown by atomic layer deposition (ALD) with that of porous TiO₂ nanoparticle films (np-TiO₂) by utilizing single-particle fluorescence spectroscopy. Unlike the broad distribution of ET rates (deduced from fluorescence lifetimes) on np-TiO₂, QDs on pc-TiO₂ exhibit two narrowly distributed ET rates that we attribute to reduced site heterogeneity. Variable temperature pc-TiO₂ annealing studies suggest that the double-peaked distribution of ET rates is related to TiO₂ surface defects, where QDs undergo more rapid ET. Further modification of pc-TiO₂ with a submonolayer of Al₂O₃ enables the selective exclusion of the more rapid ET pathway. More generally, this study provides insight into the role of surface defects in photoinduced ET into crystalline semiconductor oxides

Unusual Solvent Effects on Optical Properties of Bi-Icosahedral Au₂₅ Clusters

Temperature-dependent and time-resolved absorption measurements were carried out to understand the influence of solvent hydrogen bonding on the optical properties of bi-icosahedral [Au₂₅(PPh₃)₁₀(C₆S)₅Cl₂]²⁺ (bi-Au₂₅) clusters. Theoretical calculations have shown a low energy absorption maximum that is dominated by the coupling of the two Au₁₃ icosahedra, as well as a high energy absorption arising from the individual Au₁₃ icosahedra that make up the bi-Au₂₅ clusters. Temperature-dependent absorption measurements were carried out on bi-Au₂₅ in aprotic (toluene) and protic (ethanol and 2-butanol) solvents to elucidate the cluster–solvent hydrogen bonding interactions. In toluene, both the low and high energy absorption bands shift to higher energies consistent with electron–phonon interactions. However, in the protic solvents, the low energy absorption shows a zigzag trend with decreasing temperature. In contrast, the high energy absorption in protic solvents shifts monotonically to higher energy similar to that of toluene. Also at the temperature where the zigzag trend was observed, new absorption peaks emerged at higher energy region. The results are attributed to the hydrogen bonding of the solvent with Au–Cl leading to a disruption of the coupling of icosahedra, which is reflected in unusual trends at the low energy absorption. However, at the transition temperature, the hydrogen bonding solvents distort the icosahedrons so much so that the symmetry of Au₁₃ icosahedron is lifted leading to new absorption peaks at high energy. The transition happens at the dynamic crossover temperature where the solvent attains high density liquid status. Femtosecond time-resolved absorption measurements have shown similar dynamics for bi-Au₂₅ in ethanol and toluene with slower vibrational cooling in ethanol. However, the nanosecond transient measurements show significantly longer lifetime for bi-Au₂₅ in ethanol that suggest the solvent does have an influence on the exciton recombination

Photon Upconversion Using Baird-Type (Anti)Aromatic Quinoidal Naphthalene Derivative as a Sensitizer

A naphtho-<i>p</i>-quinodimethane (QDM) exhibiting Baird’s 4<i>n</i> – π antiaromaticity was used as green photons-harvesting chromophore to sensitize perylene (Per) leading to upconverted blue photoluminescence. The solution phase QDM → Per triplet energy transfer (TET) could not be unraveled via the Stern–Volmer method, but transient absorption measurements revealed that the kinetics of T₁ → T<i>_n</i> for QDM (τ = 1.4 μs) was 1 order of magnitude reduced (τ = 0.17 μs) as a result of ³(Per)* formation. Furthermore, we demonstrated that incident light with power densities in the microwatt regime is sufficient to perform photon upconversion using the present set of molecular systems

Ultrafast Charge Separation from Highly Reductive ZnTe/CdSe Type II Quantum Dots

The low electron affinity of ZnTe quantum dots (QDs) makes it of interest for critically important photocatalytic processes. However, studies of charge transfer from such materials to adsorbates have not been reported. Here, the ultrafast internal and external charge separation dynamics in ZnTe/CdSe core/shell type II QDs have been studied through femtosecond transient absorption spectroscopy. The internal electron transfer time from a ZnTe core to a CdSe shell was found to be 0.67 ps, while the external electron transfer time from QDs to adsorbed molecules was found to be <0.2 ps. Such a fast external charge separation time is due to the extraordinarily high conduction band energy potential of this QD composition. This study indicates the preservation of high photodriven reductive abilities in the ZnTe/CdSe core/shell type II heterostructures

Energy Transfer from Quantum Dots to Metal–Organic Frameworks for Enhanced Light Harvesting

Because of their efficient energy-transport properties, porphyrin-based metal–organic frameworks (MOFs) are attractive compounds for solar photochemistry applications. However, their absorption bands provide limited coverage in the visible spectral range for light-harvesting applications. We report here the functionalization of porphyrin-based MOFs with CdSe/ZnS core/shell quantum dots (QDs) for the enhancement of light harvesting via energy transfer from the QDs to the MOFs. The broad absorption band of the QDs in the visible region offers greater coverage of the solar spectrum by QD–MOF hybrid structures. We show through time-resolved emission studies that photoexcitation of the QDs is followed by energy transfer to the MOFs with efficiencies of more than 80%. This sensitization approach can result in a >50% increase in the number of photons harvested by a single monolayer MOF structure with a monolayer of QDs on the surface of the MOF

Photoexcited Carrier Dynamics of In₂S₃ Thin Films

Indium sulfide (In₂S₃) is a promising absorber base for substitutionally doped intermediate band photovoltaics (IBPV); however, the dynamics of charge carriers traversing the electronic density of states that determine the optical and electronic response of thin films under stimuli have yet to be explored. The kinetics of photophysical processes in In₂S₃ grown by oxygen-free atomic layer deposition are deduced from photoconductivity, photoluminescence (PL), and transient absorption spectroscopy. We develop a map of excited-state dynamics for polycrystalline thin films including a secondary conduction band ∼2.1 eV above the first, plus sulfur vacancy and indium interstitial defect levels resulting in long-lived (∼100 ns) transients. Band-edge recombination produces PL and stimulated emission, which both intensify and red-shift as deposition temperature and grain size increase. The effect of rapid conduction band electron relaxation (<30 ps) and deep defect levels on IBPV employing In₂S₃-based absorbers is finally considered

Kinetics of J-Aggregate Formation on the Surface of Au Nanoparticle Colloids

The kinetics of J-aggregate assembly on the surface of noble metal nanoparticle colloids are described, providing new mechanistic insight into the interaction of the dye molecules with the surface of metal nanoparticles. We specifically studied the J-aggregation of a thiacyanine dye (TC, 3,3′-disulfopropyl-5,5′-dichlorothiacyanine sodium salt) on the surface of gold nanoparticle colloids. The hybrid J-aggregate–Au colloidal dispersions were characterized by UV–vis spectrophotometry, fluorescence measurements, zeta potential measurements, and TEM analysis. Kinetic measurements were carried out using a stopped-flow method, indicating that the J-aggregate formation on the surface occurs via a two-step process. The first step includes adsorption of the initial dye layer, followed by an order of magnitude slower growth of consecutive layers. Activation parameters determined from the fluorescence measurements yielded further details about the nature of the interaction

Photoexcited Carrier Dynamics of Cu₂S Thin Films

Copper sulfide is a simple binary material with promising attributes for low-cost thin film photovoltaics. However, stable Cu₂S-based device efficiencies approaching 10% free from cadmium have yet to be realized. In this Letter, transient absorption spectroscopy is used to investigate the dynamics of the photoexcited state of isolated Cu₂S thin films prepared by atomic layer deposition or vapor-based cation exchange of ZnS. While a number of variables including film thickness, carrier concentration, surface oxidation, and grain boundary passivation were examined, grain structure alone was found to correlate with longer lifetimes. A map of excited state dynamics is deduced from the spectral evolution from 300 fs to 300 μs. Revealing the effects of grain morphology on the photophysical properties of Cu₂S is a crucial step toward reaching high efficiencies in operationally stable Cu₂S thin film photovoltaics

Distance-Engineered Plasmon-Enhanced Light Harvesting in CdSe Quantum Dots

Improvement of light harvesting in semiconductor quantum dots (QDs) is essential for the development of efficient QD-based solar energy conversion systems. In this study, plasmon-enhanced light absorption in CdSe QDs sensitized on silver (Ag) nanoparticle (NP) films was examined as a function of interparticle (QD to Ag NP) distance. Up to 24-fold plasmonic enhancement of fluorescence from QDs was observed when the particle separation distance was ≥5 nm. The enhancement effect was observed to largely sustain the exciton lifetimes in QDs and to strongly depend on the incident photon wavelength following the plasmon resonant strength of Ag NPs, confirming that the enhanced photoluminescence was mainly due to the enhancement in photoabsorption in CdSe QDs by the plasmon of Ag NPs. This study suggests applications of Ag NPs in QD-based solar energy conversion for significantly improving light harvesting in QDs

Size Dependence of the Plasmonic Near-Field Measured via Single-Nanoparticle Photoimaging

Plasmonic nanostructures are being exploited for optical and photovoltaic applications, particularly where field enhancement of optical processes is desirable. Extensive work has focused on the optimization of plasmonic near-fields by geometric tuning and interparticle coupling, but the size tunability of near-fields has received less attention. We used single-nanoparticle photochemical imaging to characterize the near-field intensity around a plasmonic nanoparticle as a function of size. The measured near-field intensity increases with nanoparticle size, reaching a maximum at a size of 50 nm, followed by a decrease at larger sizes. An electrodynamic model explains both the measured size dependence and the optimum size for field enhancement. Whereas intrinsic damping is size-independent, the smallest nanoparticles exhibit weak fields due to surface damping of electrons. On the other end, larger nanoparticles show low field enhancement due to strong radiative scattering. The measured volcano trend, however, most closely mirrors the size dependence of electromagnetic retardation. Above 50 nm size, retardation causes damping, but below a size of 50 nm, it surprisingly reduces nonradiative dissipation, a previously unknown effect. The size dependence of plasmonic field intensity described here can guide design of plasmonic nanostructures for applications in spectroscopy, photovoltaics, photocatalysis, and lithography