Ultrafast hole extraction from photoexcited colloidal CdSe quantum dots coupled to nitroxide free radicals.

Organic free radicals related to the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical are known as photoluminescence-quenchers when coupled to group II-chalcogenide colloidal quantum dots (QDs), but the mechanism responsible for this phenomenon has so far remained unresolved. Using a combination of time-resolved photoluminescence and transient absorption spectroscopies, we demonstrate that photoexcited colloidal CdSe QDs coupled to 4-amino-TEMPO undergo highly efficient reductive quenching, that is, hole transfer from the valence band of the quantum dot to the organic paramagnetic species. Interestingly, the process is shown to occur on a subpicosecond time scale for bound 4AT; such a large rate constant for the extraction of holes from photoexcited CdSe QD by a molecular species is rare and underlines the potential that TEMPO derivatives can play in mediating efficient redox processes involving colloidal CdSe QDs.

Orientational Dynamics of Water at an Extended Hydrophobic Interface.

We report on the orientational dynamics of water at an extended hydrophobic interface with an octadecylsilane self-assembled monolayer on fused silica. The interfacial dangling OH stretch mode is excited with a resonant pump, and its evolution followed in time by a surface-specific, vibrationally resonant, infrared-visible sum-frequency probe. High sensitivity pump-probe anisotropy measurements and isotopic dilution clearly reveal that the decay of the dangling OH stretch excitation is almost entirely due to a jump to a hydrogen-bonded configuration that occurs in 1.61 ± 0.10 ps. This is more than twice as fast as the jump time from one hydrogen-bonded configuration to another in bulk H2O but about 50% slower than the reported out-of-plane reorientation at the air/water interface. In contrast, the intrinsic population lifetime of the dangling OH stretch in the absence of such jumps is found to be >10 ps. Molecular dynamics simulations of air/water and hexane/water interfaces reproduce the fast jump dynamics of interfacial dangling OH with calculated jump times of 1.4 and 1.7 ps for the air and hydrophobic interfaces, respectively. The simulations highlight that while the air/water and hydrophobic/water surfaces exhibit great structural similarities, a small stabilization of the OH groups by the hydrophobic interface produces the pronounced difference in the dynamics of dangling bonds.

Biexciton Binding of Dirac fermions Confined in Colloidal Graphene Quantum Dots.

We present transient absorption measurements and microscopic theory of biexciton binding in triangular colloidal graphene quantum dots consisting of 168 sp(2)-hybridized C atoms. We observe optical transitions from the lowest orbitally dark singlet exciton states to states below the energy of an unbound dark+bright singlet-exciton pair. Through microscopic calculations of the low-energy exciton and biexciton states via tight-binding, Hartree-Fock, and configuration interaction methods, the spectra reveal a biexciton consisting primarily of a dark-bright singlet-pair bound by ∼0.14 eV.

Biexciton auger recombination in colloidal graphene quantum dots.

We measure biexciton Auger recombination (AR) in colloidal graphene quantum dots (GQDs) by transient absorption spectroscopy. AR is reflected in GQDs with 132 and 168 sp2-hybridized C atoms as a decay with a ∼0.3 ps time constant and an amplitude depending superlinearly on pump fluence. Despite the orders-of-magnitude difference in size between GQDs and carbon nanotubes, their biexciton AR rates are similar. This similarity is a result of strong carrier interactions in sp2-hybridized carbon nanostructures and suggests that GQDs may hold promise in the context of carrier multiplication.

Single-exciton optical gain in semiconductor nanocrystals.

Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size--owing to the quantum-confinement effect--and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron-hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant ( approximately 100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons.

Triplet States and electronic relaxation in photoexcited graphene quantum dots.

Electronic relaxation in photoexcited graphenes is central to their photoreactivity and their optoelectrical applications such as photodetectors and solar cells. Herein we report on the first ensemble studies of electronic energy relaxation pathways in colloidal graphene quantum dots with uniform size. We show that the photoexcited graphene quantum dots have a significant probability of relaxing into triplet states and emit both phosphorescence and fluorescence at room temperature, with relative intensities depending on the excitation energy. Because of the long lifetime and reactivity of triplet electronic states, our results could have significant implications for applications of graphenes.

Apparent versus true carrier multiplication yields in semiconductor nanocrystals.

Generation of multiple electron-hole pairs (excitons) by single photons, known as carrier multiplication (CM), has the potential to appreciably improve the performance of solar photovoltaics. In semiconductor nanocrystals, this effect usually has been detected using a distinct dynamical signature of multiexcitons associated with their fast Auger recombination. Here, we show that uncontrolled photocharging of the nanocrystal core can lead to exaggeration of the Auger decay component and, as a result, significant deviations of the apparent CM efficiencies from their true values. Specifically, we observe that for the same sample, apparent multiexciton yields can differ by a factor of approximately 3 depending on whether the nanocrystal solution is static or stirred. We show that this discrepancy is consistent with photoinduced charging of the nanocrystals in static solutions, the effect of which is minimized in the stirred case where the charged nanocrystals are swept from the excitation volume between sequential excitation pulses. Using side-by-side measurements of CM efficiencies and nanocrystal charging, we show that the CM results obtained under static conditions converge to the values measured for stirred solutions after we accurately account for the effects of photocharging. This study helps to clarify the recent controversy over CM in nanocrystals and highlights some of the issues that must be carefully considered in spectroscopic studies of this process.

A reduction pathway in the synthesis of PbSe nanocrystal quantum dots.

Colloidal nanocrystal quantum dots (NQDs) of narrow band gap materials are of substantial general interest because of their unparalleled potential as infrared fluorophores. While PbSe NQDs are a promising class of infrared-active nanocrystals due to high emission quantum yields and a wide useful spectral range, typical synthetic methods are sensitive to a variety of factors, including the influence of solvent/ligand impurities that render reproducibility difficult. In this work, we specifically examine the effects of diphenylphosphine and 1,2-hexadecanediol, as surrogates for putative trioctylphosphine-based reducing impurities, on the synthesis of PbSe NQDs. Specifically, we compare their influence on NQD size, chemical yield, and photoluminescence quantum yield. While both additives substantially increase the chemical yield of the synthesis, they demonstrate markedly different effects on emission quantum yield of the product NQDs. We further examine the effects of reaction temperature and oleic acid concentration on the diol-assisted synthesis. Increased oleic acid concentration led to somewhat higher growth rates and larger NQDs but at the expense of lower chemical yield. Temperature was found to have an even greater effect on growth rate and NQD size. Neither temperature nor oleic acid concentration was found to have noticeable effects on NQD emission quantum yield. Finally, we use numerical simulations to support the conjecture that the increased yield is likely a result of faster monomer formation, consistent with the activation of an additional reaction pathway by the reducing species.

New aspects of carrier multiplication in semiconductor nanocrystals.

One consequence of strong spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) is a significant enhancement in carrier-carrier Coulomb interactions. This effect leads to a number of novel physical phenomena including ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination and high-efficiency generation of mutiexcitons by single photons via carrier multiplication (CM). Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing, as well as potential applications of CM in solar-energy conversion. The focus of this Account is on CM. In this process, the kinetic energy of a "hot" electron (or a "hot" hole) does not dissipate as heat but is, instead, transferred via the Coulomb interaction to the valence-band electron, exciting it across the energy gap. Because of restrictions imposed by energy and translational-momentum conservation, as well as rapid energy loss due to phonon emission, CM is inefficient in bulk semiconductors, particularly at energies relevant to solar energy conversion. On the other hand, the CM efficiency can potentially be enhanced in zero-dimensional NCs because of factors such as a wide separation between discrete electronic states, which inhibits phonon emission ("phonon bottleneck"), enhanced Coulomb interactions, and relaxation in translational-momentum conservation. Here, we investigate CM in PbSe NCs by applying time-resolved photoluminescence and transient absorption. Both techniques show clear signatures of CM with efficiencies that are in good agreement with each other. NCs of the same energy gap show moderate batch-to-batch variations (within approximately 30%) in apparent multiexciton yields and larger variations (more than a factor of 3) due to differences in sample conditions (stirred vs static solutions). These results indicate that NC surface properties may affect the CM process. They also point toward potential interference from extraneous effects such as NC photoionization that can distort the results of CM studies. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02-0.03 photons absorbed per NC per pulse) reveal that both the electron-hole creation energy and the CM threshold are reduced compared with those in bulk solids. These results indicate a confinement-induced enhancement in the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex (for example, shaped-controlled or heterostructured) NCs that allow for facile manipulation of carrier-carrier interactions, as well as single and multiexciton energies and dynamics.

Biochemical and histopathological evaluation of changes in sled dog paw skin associated with physical stress and cold temperatures.

Abstract Twenty-six Alaskan sled dogs were used to study the biochemical and histopathological changes which occur when dog paws are exposed to cold temperatures and physical stress. They were separated into a running group of 20 dogs and a control group of six non-running dogs. Over 2 1/2 days, the running group ran in their natural environment for 170 miles and environmental parameters were recorded. Following the run, an 8-mm díameter skin biopsy specimen was taken from the lateral aspect of the right fore and hind paws of the running and non-running dogs. The skin was evaluated for histopathological changes and the présence of 2, 3-dinor thromboxane B2 (2, 3-dinor TxB2 ). No significant histopathological changes were noted in any of the biopsy specimens. Based on measured elevation of 2, 3-dinor TxB2 , the forepaws experienced more physical stress than the hind paws. Wet snow at higher environmental temperatures caused more paw stress than hard crusted snow at lower environmental temperatures. Resumen Se emplearon veintiséis perros trineo de Alaska para estudiar las alteraciones bioquimicas e histopatológicas ocurridas durante la exposición de las patas a las bajas temperaturas y al estrés fisico. Se dividieron entre un grupo de 20 perros corredores y un grupo control de seis perros no corredores. Durante dos días y medio el grupo de corredores corrió en su ambiente natural 170 millas. Durante la carrera se tomó una biopsia por punch de 8 mm del aspecto lateral de la pata derecha delantera y trasera de los perros corredores y no corredores. Se evaluaron las alteraciones histopatológicas cutáneas y la presencia de tromboxano B2 2, 3-dinor (2, 3-dinor TxB2 ). No se observaron alteraciones histopatológicas significativas en ninguna de las muestras de biopsia. Según la elevación detectada en 2.3-dinor TxB2 , las patas delanteras sufrieron un estrés fisico mayor que las traseras. La nieve húmeda en temperaturas ambientales elevadas causó un estrés mayor que la nieve dura en temperaturas ambientales más bajas. [Bradley, D.M., Swaim, S.F., Vaughn, D.M., Powers, R.D., McGuire, J.A., Reinhart, G.A., Burr, J., Swenson, R.A. Biochemical and histopathological evaluation of changes in sled dog paw skin associated with physical stress and cold temperatures. (Estudio bioquimico e histopatologico de las alteraciones en las patas de perros de trineo asociadas al estrés fisico y por las bajas temperaturas). Veterinary Dermatology 1996; 7: 203-208.] Résumé Vingt six chiens de traineau de l'Alaska fürent utilisés dans une étude évaluant les modifications biochimiques et histologiques survenant lorsque les pattes sont exposées à des températures froides et des stress physiques. lls fürent divisés en un groupe de vingt chiens de course et un groupe de six chiens ne courant pas. Après deux jours et demi, le groupe de course avait parcouru 170 miles dans son environnement naturel, dont les paramètres fürent enregistrés. Après la course, des biopsies de 8 mm de díamètre fürent prises sur les faces latérales des pattes antérieure et postérieure droites des chiens d'attelage et de ceux qui étaient restés au repos. L'évaluation de l'état cutané s'est faite sur les modifications histologiques et la présence de 2, 3-dinor thromboxane B2 (2, 3-dinor TxB2 ). Aucune modification histologique significative n'a été notée dans aucune des biopsies réalisées. Jugé sur l'élévation du 2, 3 TxB2 , les pattes antérieures subissent un stress physique supérieur aux pattes postérieures. La neige humide à temperatures plus élevées cause plus d'agression que la neige dure et crouteuse à des températures plus basses. [Bradley, D.M., Swaim, S.F., Vaughn, D.M., Powers, R.D., McGuire, J.A., Reinhart, G.A., Burr, J., Swenson, R.A. Biochemical and histopathological evaluation of changes in sled dog paw skin associated with physical stress and cold temperatures. (Evaluation des modifications biochimiques et histologiques induites par les stress physiques et le froid au niveau de la peau des pattes de chiens de traineau). Veterinary Dermatology 1996; 7: 203-208.] Zusammenfassung Sechsundzwanzig Alaska-Schlittenhunde wurde für eine Studie über biochemische und histopathologische Veränderungen herangezogen, die an den Pfotenballen auftreten, wenn sie kalten Temperaturen und physischem Streß ausgesetzt sind. Die Hunde wurden in eine Renngruppe von 20 Hunden und eine Kontrollgruppe von sechs Nicht-Rennhunden eingeteilt. Über einen Zeitraum von zweieinhalb Tagen rannte die Renngruppe in lhrer natürlichen Umgebung 170 Mellen, wobei die Umgebungsparamenter aufgezeichnet wurden. Nach dem Rennen wurden Biopsien mit einem Durchmesser von 8 mm von der Lateralseite der rechten Vorder-und Hinterpfote bei den Renn-und Nichtrennhunden entnommen. Die Haut wurde auf histopathologische Veränderungen und die Anwesenheit von 2, 3-dinor-Thromboxan BA (2, 3-dinor TxB2 ) untersucht. Es wurden keine signifikanten histopathologischen Veränderungen bei einer der Biopsien festgestellt. Auf der Basis der gemessenen Erhöhung von 2, 3-dinor TxB2 erfuhren die Vorderpfoten mehr physischen Streß als die Hinterpfoten. Nasser Schnee und höhere Umgebungstemperaturen verursachten mehr Pfotenstreß als harter, verkrusteter Schnee bei niedrigeren Umgebungstemperaturen. [Bradley, D.M., Swaim, S.F., Vaughn, D.M., Powers, R.D., McGuire, J.A., Reinhart, G.A., Burr, J., Swenson, R.A. Biochemical and histopathological evaluation of changes in sled dog paw skin associated with physical stress and cold temperatures (Biochemische und histopathologische Untersuchung von Hautveränderungen an den Pfoten von Schlittenhunden in Verbindung mit physischem Streß und Kälte). Veterinary Dermatology 1996; 7: 203-208.].

Spectroscopic signatures of photocharging due to hot-carrier transfer in solutions of semiconductor nanocrystals under low-intensity ultraviolet excitation.

We show that excitation of solutions of well-passivated PbSe semiconductor nanocrystals (NCs) with ultraviolet (3.1 eV) photons can produce long-lived charge-separated states in which the NC core is left with a nonzero net charge. Since this process is not observed for lower-energy (1.5 eV) excitation, we ascribe it to hot-carrier transfer to some trap site outside the NC. Photocharging leads to bleaching of steady-state absorption, partial quenching of emission, and additional fast time scales in carrier dynamics due to Auger decay of charged single- and multiexciton states. The degree of photocharging, f, saturates at a level that varies from 5 to 15% depending on the sample. The buildup of the population of charged NCs is extremely slow indicating very long, tens of seconds, lifetimes of these charge-separated states. Based on these time scales and the measured onset of saturation of f at excitation rates around 0.05-1 photon per NC per ms, we determine that the probability of charging following a photon absorption event is of the order of 10(-4) to 10(-3). The results of these studies have important implications for the understanding of photophysical properties of NCs, especially in the case of time-resolved measurements of carrier multiplication.

Effect of air exposure on surface properties, electronic structure, and carrier relaxation in PbSe nanocrystals.

Effects of air exposure on surface properties, electronic structure, and carrier relaxation dynamics in colloidal PbSe nanocrystals (NCs) were studied using X-ray photoelectron spectroscopy, transmission electron microscopy, and steady-state and time-resolved photoluminescence (PL) spectroscopies. We show that exposure of NC hexane solutions to air under ambient conditions leads to rapid oxidation of NCs such that up to 50% of their volume is transformed into PbO, SeO2, or PbSeO3 within 24 h. The oxidation is a thermally activated process, spontaneous at room temperature. The oxidation-induced reduction in the size of the PbSe "core" increases quantum confinement, causing shifts of the PL band and the absorption onset to higher energies. The exposure of NC solutions to air also causes rapid (within minutes) quenching of PL intensity followed by slow (within hours) recovery during which the PL quantum yield can reach values exceeding those observed prior to the air exposure. The short-term PL quenching is attributed to enhanced carrier trapping induced by adsorption of oxygen onto the NC surface, while the PL recovery at longer times is predominantly due to reduction in the efficiency of the "intrinsic" nonradiative interband recombination caused by the increase of the band gap in oxidized NCs. Although the analysis of subnanosecond relaxation dynamics in air-exposed NCs is complicated by a significant enhancement in fast carrier trapping, our picosecond PL measurements suggest that air exposure likely has only a weak effect on Auger recombination and also does not significantly affect the efficiency of carrier multiplication. We also show that the effects of air exposure are partially suppressed in PbSe/CdSe core/shell structures.

Ultrafast vibrational dynamics at water interfaces.

Time-resolved sum-frequency vibrational spectroscopy permits the study of hitherto neglected ultrafast vibrational dynamics of neat water interfaces. Measurements on interfacial bonded OH stretch modes revealed relaxation behavior on sub-picosecond time scales in close resemblance to that of bulk water. Vibrational excitation is followed by spectral diffusion, vibrational relaxation, and thermalization in the hydrogen-bonding network. Dephasing of the excitation occurs in

Electron dynamics of silicon surface states: second-harmonic hole burning on Si(111)-(7 x 7).

We report the first all-optical study of homogeneous linewidths of surface excitations by the spectral-hole-burning technique with surface-specific second-harmonic generation as a probe. Measurement of transient spectral holes induced by a 100 fs pump pulse in excitations of the surface dangling-bond states of Si(111)-(7 x 7) led to a pump-fluence-dependent homogeneous linewidth as broad as approximately 100 meV or a dephasing time as short as 15 fs. The hole-burning spectra also revealed a strong coupling between the localized dangling-bond states and the associated surface phonon mode at 570 cm(-1). Carrier-carrier scattering was responsible for the linear dependence of the dephasing rate on pump fluence, and the carrier screening effect appeared to be weak.