Showing papers on "Photoexcitation published in 2013"

Photoexcitation cascade and multiple hot-carrier generation in graphene

Abstract: The efficiency of carrier–carrier scattering in graphene is now experimentally demonstrated. The dominance of this mechanism over phonon-related scattering means that a single high-energy photon could create two or more electron–hole pairs in graphene; an effect useful for optoelectronic applications.

The quantum coherent mechanism for singlet fission: experiment and theory

Abstract: The absorption of one photon by a semiconductor material usually creates one electron–hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron–hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit wi...

Observation of a transient decrease in terahertz conductivity of single-layer graphene induced by ultrafast optical excitation.

Abstract: We have measured the terahertz frequency-dependent sheet conductivity and its transient response following femtosecond optical excitation for single-layer graphene samples grown by chemical vapor deposition. The conductivity of the unexcited graphene sheet, which was spontaneously doped, showed a strong free-carrier response. The THz conductivity matched a Drude model over the available THz spectral range and yielded an average carrier scattering time of 70 fs. Upon photoexcitation, we observed a transient decrease in graphene conductivity. The THz frequency-dependence of the graphene photoresponse differs from that of the unexcited material but remains compatible with a Drude form. We show that the negative photoconductive response arises from an increase in the carrier scattering rate, with a minor offsetting increase in the Drude weight. This behavior, which differs in sign from that reported previously for epitaxial graphene, is expected for samples with relatively high mobilities and doping levels. T...

Imaging photoelectron circular dichroism of chiral molecules by femtosecond multiphoton coincidence detection

Abstract: Here, we provide a detailed account of novel experiments employing electron-ion coincidence imaging to discriminate chiral molecules. The full three-dimensional angular scattering distribution of electrons is measured after photoexcitation with either left or right circular polarized light. The experiment is performed using a simplified photoelectron-photoion coincidence imaging setup employing only a single particle imaging detector. Results are reported applying this technique to enantiomers of the chiral molecule camphor after three-photon ionization by circularly polarized femtosecond laser pulses at 400 nm and 380 nm. The electron-ion coincidence imaging provides the photoelectron spectrum of mass-selected ions that are observed in the time-of-flight mass spectra. The coincident photoelectron spectra of the parent camphor ion and the various fragment ions are the same, so it can be concluded that fragmentation of camphor happens after ionization. We discuss the forward-backward asymmetry in the photoelectron angular distribution which is expressed in Legendre polynomials with moments up to order six. Furthermore, we present a method, similar to one-photon electron circular dichroism, to quantify the strength of the chiral electron asymmetry in a single parameter. The circular dichroism in the photoelectron angular distribution of camphor is measured to be 8% at 400 nm. The electron circular dichroism using femtosecond multiphoton excitation is of opposite sign and about 60% larger than the electron dichroism observed before in near-threshold one-photon ionization with synchrotron excitation. We interpret our multiphoton ionization as being resonant at the two-photon level with the 3s and 3p Rydberg states of camphor. Theoretical calculations are presented that model the photoelectron angular distribution from a prealigned camphor molecule using density functional theory and continuum multiple scattering X alpha photoelectron scattering calculations. Qualitative agreement is observed between the experimental results and the theoretical calculations of the Legendre moments representing the angular distribution for the two enantiomers. The electron-ion coincidence technique using multiphoton ionization opens new directions in table-top analytical mass-spectrometric applications of mixtures of chiral molecules.

Coherence and Uncertainty in Nanostructured Organic Photovoltaics

Abstract: The effects of fundamental uncertainty present a compelling rationale for a highly delocalized photoexcitation on ultrafast time scales. This delocalized photoexcitation enables an immediate probability of charge-transfer over distances compatible with the uncertainty principle. We perform transient absorption measurements on organic bulk heterojunction solar cells to investigate charge-transfer dynamics in a variety of materials. A startling generality emerges indicating that the majority of charge carriers are generated at times within the temporal resolution of our instrument (∼100 fs).

Role of Photoexcitation and Field Ionization in the Measurement of Accurate Oxide Stoichiometry by Laser-Assisted Atom Probe Tomography

Abstract: The addition of pulsed lasers to atom probe tomography (APT) extends its high spatial and mass resolution capability to nonconducting materials, such as oxides. For a prototypical metal oxide, MgO, the measured stoichiometry depends strongly on the laser pulse energy and applied voltage. Very low laser energies (0.02 pJ) and high electric fields yield optimal stoichiometric accuracy. Correlated APT and aberration-corrected transmission electron microscopy (TEM) are used to establish the high density of corner and terrace sites on MgO sample surfaces before and after APT. For MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O2. The observed trends are best explained by the relative field-dependent ionization of photodesorbed O or O2 neutrals. These results emphasize the importance of considering electronic excitations in APT analysis of oxide materials.

Direct imaging of free carrier and trap carrier motion in silicon nanowires by spatially-separated femtosecond pump-probe microscopy.

Abstract: We have developed a pump–probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of ∼104 cm/s. The free carrier dynamics are followed by trap carrier migration on slower time scales.

Photoluminescence study of Y2O3:Er3+-Eu3+-Yb3+ phosphor for lighting and sensing applications

Abstract: The Er3+, Eu3+, and Yb3+ codoped Y2O3 phosphors have been synthesized by combustion synthesis process. For the structural information, the XRD analysis of the developed phosphor has been done. The frequency upconversion (UC) emissions in the codoped Y2O3 phosphor on excitation with 980 nm diode laser in the visible region have been performed and explained on the basis of excited state absorption and energy transfer process. The mechanism responsible in UC emissions was observed to involve two photon absorption and efficiency of the UC luminescence is significantly enhanced by introducing the Yb3+ ions. The tunability in colour of emitted radiation has been visualized by chromaticity diagram on increasing power of excitation source. The temperature sensing behaviour of developed phosphor material has been investigated using fluorescence intensity ratio technique.

Ultrafast manipulation of near field coupling between bright and dark modes in terahertz metamaterial

Abstract: We demonstrate ultrafast optical control of near field coupling between bright and dark mode resonances in metamaterials. The meta-molecule design consists of two orthogonally twisted resonators tightly coupled through near fields. We place ion implanted silicon patch with ultrafast carrier lifetime inside dark resonator split gap to achieve active control of its fundamental resonance that determines the near field coupling in the meta-molecule. Upon near infrared photoexcitation, we observed ultrafast dynamical transition of near field coupling between bright and dark resonators allowing the meta-molecule to change its state from coupled to decoupled and eventually back to the coupled state.

Electronic origin of ultrafast photoinduced strain in BiFeO3.

Abstract: Above-band-gap optical excitation produces interdependent structural and electronic responses in a multiferroic ${\mathrm{BiFeO}}_{3}$ thin film. Time-resolved synchrotron x-ray diffraction shows that photoexcitation can induce a large out-of-plane strain, with magnitudes on the order of half of one percent following pulsed-laser excitation. The strain relaxes with the same nanosecond time dependence as the interband relaxation of excited charge carriers. The magnitude of the strain and its temporal correlation with excited carriers indicate that an electronic mechanism, rather than thermal effects, is responsible for the lattice expansion. The observed strain is consistent with a piezoelectric distortion resulting from partial screening of the depolarization field by charge carriers, an effect linked to the electronic transport of excited carriers. The nonthermal generation of strain via optical pulses promises to extend the manipulation of ferroelectricity in oxide multiferroics to subnanosecond time scales.

Dark and photo-conductivity in ordered array of nanocrystals.

Abstract: A theory of photo- and dark-band conductivities in semiconductor supercrystals consisting of nanocrystals is developed by assuming scattering by structural defects in the supercrystals. A new proposed mechanism of photoexcitation, which is triggered by an efficient Auger ionization of charged nanocrystals, provides explanation for the measured photocurrent being 2–3 orders of magnitude larger than the dark current. For dark conductivity, the metal–insulator transitions and temperature dependence of mobility in the metal phase are considered.

Photoexcited carrier dynamics and impact-excitation cascade in graphene

Abstract: In materials with strong electron-electron interactions, photoexcitation can trigger a cascade in which multiple particle-hole excitations are generated. Here we analyze the cascade of impact-excitation processes in graphene in which many hot carriers are generated by a single absorbed photon. We show that the number of generated carriers has a strong dependence on doping (gate tunability). Linear scaling with photon energy is predicted for the number of pairs and for the duration of the cascade. These dependencies, along with a sharply peaked angular distribution of excited carriers, provide clear experimental signatures of hot carrier multiplication.

Influence of Site-Dependent Pigment−Protein Interactions on Excitation Energy Transfer in Photosynthetic Light Harvesting

Abstract: A site-dependent spectral density system-bath model of the Fenna-Matthews-Olsen (FMO) pigment-protein complex is developed using results from ground-state molecular mechanics simulations together with a partial charge difference model for how the long-range contributions to the chromophore excitation energies fluctuate with environmental configuration. A discussion of how best to consistently process the chromophore excitation energy fluctuation correlation functions calculated in these classical simulations to obtain reliable site-dependent spectral densities is presented. The calculations reveal that chromophores that are close to the protein-water interface can experience strongly dissipative environmental interactions characterized by reorganization energies that can be as much as 2-3 times those of chromophores that are buried deep in the hydrophobic protein scaffolding. Using a linearized density matrix quantum propagation method, we demonstrate that the inhomogeneous system-bath model obtained from our site-dependent spectral density calculations gives results consistent with experimental dissipation and dephasing rates. Moreover, we show that this model can simultaneously enhance the energy-transfer rate and extend the decoherence time. Finally, we explore the influence of initially exciting different chromophores and mutating local environments on energy transfer through the network. These studies suggest that different pathways, selected by varying initial photoexcitation, can exhibit significantly different relaxation times depending on whether the energy-transfer path involves chromophores at the protein-solvent interface or if all chromophores in the pathway are buried in the protein.

Charge trapping dynamics in PbS colloidal quantum dot photovoltaic devices.

Abstract: The efficiency of solution-processed colloidal quantum dot (QD) based solar cells is limited by poor charge transport in the active layer of the device, which originates from multiple trapping sites provided by QD surface defects. We apply a recently developed ultrafast electro-optical technique, pump-push photocurrent spectroscopy, to elucidate the charge trapping dynamics in PbS colloidal-QD photovoltaic devices at working conditions. We show that IR photoinduced absorption of QD in the 0.2–0.5 eV region is partly associated with immobile charges, which can be optically detrapped in our experiment. Using this absorption as a probe, we observe that the early trapping dynamics strongly depend on the nature of the ligands used for QD passivation, while it depends only slightly on the nature of the electron-accepting layer. We find that weakly bound states, with a photon-activation energy of 0.2 eV, are populated instantaneously upon photoexcitation. This indicates that the photogenerated states show an int...

Tracking the evolution of electronic and structural properties of VO2 during the ultrafast photoinduced insulator-metal transition

Abstract: We present a detailed study of the photoinduced insulator-metal transition in VO${}_{2}$ with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When we excite VO${}_{2}$ above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground-state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in the lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the nonequilibrium transition, we observe that the electronic properties of the material evolve on a different, slower time scale. This separation of time scales suggests that the early state of VO${}_{2}$, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase.

Probing ultrafast dynamics in photoexcited pyrrole: timescales for 1πσ* mediated H-atom elimination

Abstract: The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250–200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 11πσ* (1A2) state; and (ii) initial photoexcitation to the higher energy 1ππ* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 11πσ* (1A2) at 250 nm, we observe a single time-constant of 126 ± 28 fs for N–H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 11πσ* (1A2) surface in the vertical Franck–Condon region, followed by non-adiabatic coupling through a 11πσ*/S0 conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N()) together with H-atoms. At 238 nm, direct excitation to, and N–H dissociation along, the 11πσ* (1A2) surface is observed to occur with a time-constant of 46 ± 22 fs. Upon initial population of the 1ππ* (1B2) state at 200 nm, a rapid 1ππ* (1B2) → 11πσ* (1A2) → N–H fission process takes place within 52 ± 12 fs. In addition to ultrafast N–H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S0 ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 ± 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C–H bond cleavage.

Ring-closing reaction in diarylethene captured by femtosecond electron crystallography.

Abstract: The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low...

Influence of Site-Dependent Pigment–Protein Interactions on Excitation Energy Transfer in Photosynthetic Light Harvesting B

Abstract: A site-dependent spectral density system–bath model of the Fenna–Matthews–Olsen (FMO) pigment–protein complex is developed using results from ground-state molecular mechanics simulations together with a partial charge difference model for how the long-range contributions to the chromophore excitation energies fluctuate with environmental configuration. A discussion of how best to consistently process the chromophore excitation energy fluctuation correlation functions calculated in these classical simulations to obtain reliable site-dependent spectral densities is presented. The calculations reveal that chromophores that are close to the protein–water interface can experience strongly dissipative environmental interactions characterized by reorganization energies that can be as much as 2–3 times those of chromophores that are buried deep in the hydrophobic protein scaffolding. Using a linearized density matrix quantum propagation method, we demonstrate that the inhomogeneous system–bath model obtained from our site-dependent spectral density calculations gives results consistent with experimental dissipation and dephasing rates. Moreover, we show that this model can simultaneously enhance the energy-transfer rate and extend the decoherence time. Finally, we explore the influence of initially exciting different chromophores and mutating local environments on energy transfer through the network. These studies suggest that different pathways, selected by varying initial photoexcitation, can exhibit significantly different relaxation times depending on whether the energy-transfer path involves chromophores at the protein–solvent interface or if all chromophores in the pathway are buried in the protein.

Charge photogeneration in donor-acceptor conjugated materials: influence of excess excitation energy and chain length.

Abstract: We investigate the role of excess excitation energy on the nature of photoexcitations in donor–acceptor π-conjugated materials. We compare the polymer poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b;3,4-b′]dithiophene)-4,7-benzo[2,1,3]thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest th...

Photocurrent induced by two-photon excitation in ZnTeO intermediate band solar cells

Abstract: Intermediate band (IB) solar cell structures based on ZnTeO highly mismatched alloy were examined to demonstrate a photocurrent induced by a two-photon excitation (TPE) process. Two types of the devices, with and without a blocking layer for the IB, are prepared. The device with a blocked IB exhibits small external quantum efficiency (EQE) in photon energy range in which electron transitions from valence band (VB) to IB take place, implying the electron accumulation in IB. The enhancement of EQE is observed in TPE experiments as a result of electron transition from VB to conduction band via IB.

Correlating the excited state relaxation dynamics as measured by photoluminescence and transient absorption with the photocatalytic activity of Au@TiO2 core–shell nanostructures

Abstract: The spectroscopic and photocatalytic properties of a series of Au@TiO2 core–shell nanostructures are characterized. The crystallinity of the TiO2 shells was varied by changing the etching and calcination conditions. Measurements of the photoluminescence, transient absorption, and H2 production rate permit us to look for correlations between the spectroscopic and catalytic behaviors. We found that there is a strong effect of crystallinity on the H2 production rate and also the stretched exponential lifetime of the photoluminescence created by short-wavelength (266 and 300 nm) photoexcitation. As the TiO2 crystallinity is increased, the photoluminescence lifetime increases from 22 to 140 ps in a 1 ns detection window, while the H2 production rate increases by a factor of ∼4. There is no discernible effect of crystallinity on the photoluminescence dynamics excited at 350 or 430 nm, or on the electronic dynamics measured by femtosecond transient absorption after excitation at 300 nm. We hypothesize that high-energy photons create reactive and emissive charge-separated states in parallel, and that both species are subject to similar electron–hole recombination processes that depend on sample crystallinity. Based on our observations, it can be concluded that the photoluminescence dynamics may be used to evaluate the potential performance of this class of photocatalysts.

Excitation‐Wavelength‐Dependent, Ultrafast Photoinduced Electron Transfer in Bisferrocene/BF2‐Chelated‐Azadipyrromethene/Fullerene Tetrads

Abstract: Donor-acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron-transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor-acceptor dyads, revealing excitation-wavelength-dependent photochemical properties demands multimodular, photosynthetic-reaction-center model compounds. Here, we successfully demonstrate donor- acceptor excitation-wavelength-dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF2 -chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured "closely" and "distantly" positioned donor-acceptor systems. The near-IR-emitting BF2-chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation-wavelength-dependent, photoinduced, electron-transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc(+)-ADP-C60(.-) charge-separated state upon C60 excitation, and Fc(+)-ADP(.-)-C60 formation upon ADP excitation is demonstrated.

Mapping multidimensional excited state dynamics using pump-impulsive-vibrational-spectroscopy and pump-degenerate-four-wave-mixing

Abstract: Pump-impulsive vibrational spectroscopy (pump-IVS) is used to record excited state vibrational dynamics following photoexcitation of two carotenoids, β-carotene and lycopene, with <30 fs temporal resolution, and covering the full vibrational spectrum of the investigated chromophores. The results record the course of S2–S1 internal conversion, followed by vibrational relaxation and decay to the electronic ground state. This interpretation is corroborated by comparison with pump-degenerate-four-wave-mixing (pump-DFWM) experiments on the same systems. The results demonstrate the potential of both time-domain spectroscopic techniques to resolve photochemical dynamics, including fingerprint frequencies which directly reflect changes in bonding and structure in the nascent sample. The exclusive strengths and limitations of these two methods are compared with those presented by the frequency-domain Femtosecond Stimulated Raman Scattering (FSRS) technique, highlighting the complementary nature of the three, and the benefits of using them in concert to investigate vibrational dynamics in reactive species.

X-ray Absorption Spectroscopy of Ground and Excited Rhenium–Carbonyl–Diimine Complexes: Evidence for a Two-Center Electron Transfer

Abstract: Steady-state and picosecond time-resolved X-ray absorption spectroscopy is used to study the ground and lowest triplet states of [ReX(CO)3(bpy)]n+, X = Etpy (n = 1), Cl, or Br (n = 0). We demonstrate that the transient spectra at both the Re L3- and Br K-edges show the emergence of a pre-edge feature, absent in the ground-state spectrum, which is associated with the electron hole created in the highest occupied molecular orbital following photoexcitation. Importantly, these features have the same dynamics, confirming previous predictions that the low-lying excited states of these complexes involve a two-center charge transfer from both the Re and the ligand, X. We also demonstrate that the DFT optimized ground and excited structures allow us to reproduce the experimental XANES and EXAFS spectra. The ground-state structural refinement shows that the Br atom contributes very little to the latter, whereas the Re–C–O scattering paths are dominant due to the so-called focusing effect. For the excited-state spe...

Photoexcitation cascade and multiple hot carrier generation in graphene

Abstract: For many optoelectronic applications, such as photodetection and light harvesting, it is highly desirable to identify materials in which an absorbed photon is efficiently converted to electronic excitations. The unique properties of graphene, such as its gapless band structure, flat absorption spectrum and strong electron-electron interactions, make it a highly promising material for efficient broadband photon-electron conversion [1]. Indeed, recent theoretical work has anticipated that in graphene multiple electron-hole pairs can be created from a single absorbed photon during energy relaxation of the primary photoexcited e-h pair [2]. A photoexcited carrier relaxes initially trough two competing pathways: carrier-carrier scattering and optical phonon emission. In the former process the energy of photoexcited carriers remains in the electron system, being transferred to secondary electrons that gain energy (become hot), whereas in the phonon emission process the energy is lost to the lattice as heat. While recent experiments have shown that photoexcitation of graphene can generate hot carriers [3], it remains unknown how efficient this process is with respect to optical phonon emission.

Hole-lattice coupling and photoinduced insulator-metal transition in VO 2

Abstract: Photoinduced insulator-metal transition in VO${}_{2}$ and the related transient and multi-time-scale structural dynamics upon photoexcitation are explained within a unified framework. Holes created by photoexcitation weaken the V-V bonds and eventually break V-V dimers in the ${M}_{1}$ phase of VO${}_{2}$ when the laser fluence reaches a critical value. The breaking of the V-V bonds in turn leads to an immediate electronic phase transition from an insulating to a metallic state while the crystal lattice remains monoclinic in shape. The coupling between excited electrons and the 6.0-THz phonon mode is found to be responsible for the observed zigzag motion of V atoms upon photoexcitation and is consistent with coherent phonon experiments.

Type-II Quantum-Dot-Sensitized Solar Cell Spanning the Visible and Near-Infrared Spectrum

Abstract: Type-II heterostructure CdTe/CdSe core/shell nanocrystals (quantum dots, QDs) are explored as sensitizers in a QD-sensitized photoelectrochemical solar cell. These QDs comprise a hole-localizing core and an electron-localizing shell. Among their advantages is the significant red shift of the absorption edge of the heterostructured QD relative to its two constituents due to spatially indirect absorption leading to improved absorption characteristics, intraparticle exciton dissociation upon photoexcitation, and a relatively small content of the less abundant tellurium element. Upon incorporation in a sensitized solar cell utilizing a porous TiO2 and a polysulfide electrolyte, these QDs exhibited efficient charge separation and high internal quantum efficiency despite hole localization in the CdTe core. Monochromatic incident photon-to-current conversion efficiency (IPCE) measurement shows a spectrally broad photoresponse spanning the whole visible spectrum and reaching up to ∼900 nm.

Density-dependent electron scattering in photoexcited GaAs in strongly diffusive regime

Abstract: In a series of systematic optical pump–terahertz probe experiments, we study the density-dependent electron scattering rate in photoexcited GaAs in the regime of strong carrier diffusion. The terahertz frequency-resolved transient sheet conductivity spectra are perfectly described by the Drude model, directly yielding the electron scattering rates. A diffusion model is applied to determine the spatial extent of the photoexcited electron-hole gas at each moment after photoexcitation, yielding the time-dependent electron density, and hence the density-dependent electron scattering time. We find that the electron scattering time decreases from 320 to 60 fs, as the electron density changes from 1015 to 1019 cm−3.

Strong photon energy dependence of the photocatalytic dissociation rate of methanol on TiO2(110).

Abstract: Photocatalytic dissociation of methanol (CH3OH) on a TiO2(110) surface has been studied by temperature programmed desorption (TPD) at 355 and 266 nm. Primary dissociation products, CH2O and H atoms, have been detected. The dependence of the reactant and product TPD signals on irradiation time has been measured, allowing the photocatalytic reaction rate of CH3OH at both wavelengths to be directly determined. The initial dissociation rate of CH3OH at 266 nm is nearly 2 orders of magnitude faster than that at 355 nm, suggesting that CH3OH photocatalysis is strongly dependent on photon energy. This experimental result raises doubt about the widely accepted photocatalysis model on TiO2, which assumes that the excess potential energy of charge carriers is lost to the lattice via strong coupling with phonon modes by very fast thermalization and the reaction of the adsorbate is thus only dependent on the number of electron–hole pairs created by photoexcitation.