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Transient absorption spectra (TAS) of lead halide perovskites can provide important insights into the nature of the photoexcited state dynamics of this prototypical class of materials. Here, we perform ground and excited state molecular dynamics (MD) simulations within a restricted open shell Kohn-Sham (ROKS) approach in order to interpret the characteristic features of the TAS of CsPbBr3. Our results reveal that properties such as the finite temperature band gap, the Stokes shift, and therefore, also the TAS are strongly size-dependent. Our TAS simulations show an early positive red-shifted feature on the fs scale that can be explained by geometric relaxation in the excited state. As excited-state processes can crucially affect the electronic properties of this class of photoactive materials, our observations are an important ingredient for further optimization of lead halide based optoelectronic devices.

In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to reveal that organic-inorganic lead-bromide perovskites host Mahan excitons at room temperature. Persistence of the Wannier exciton peak and the enhancement of the above-bandgap absorption are observed at all achievable photoexcitation densities, well above the Mott density. This is supported by the solution of the semiconductor Bloch equations, which confirms that no sharp transition between the insulating and conductive phase occurs. Our results demonstrate the robustness of the bound states in a regime where exciton dissociation is otherwise expected, and offer promising perspectives in fundamental physics and in room-temperature applications involving high densities of charge carriers., The Mahan exciton, exotic quasiparticle predicted in 1967, had never been found in room temperature semiconductors. With ultrafast optics and many-body theory, Palmieri et al. show that methylammonium lead bromide perovskites are ideal platforms to unveil Mahan exciton physics at room temperature.

In this work we demonstrate a novel approach to achieve efficient charge separation in dimensionally and dielectrically confined two-dimensional perovskite materials. Two-dimensional perovskites generally exhibit large exciton binding energies that limit their application in optoelectronic devices that require charge separation such as solar cells, photo-detectors and in photo-catalysis. Here, we show that by incorporating a strongly electron accepting moiety, perylene diimide organic chromophores, on the surface of the two-dimensional perovskite nanoplatelets it is possible to achieve efficient formation of mobile free charge carriers. These free charge carriers are generated with ten times higher yield and lifetimes of tens of microseconds, which is two orders of magnitude longer than without the peryline diimide acceptor. This opens a novel synergistic approach, where the inorganic perovskite layers are combined with functional organic chromophores in the same material to tune the properties for specific applications., Functionalizing two-dimensional (2D) hybrid perovskites with organic chromophores is a novel approach to tune their optoelectronic properties. Here, the authors report efficient charge separation and conduction in 2D hybrid perovskite nanoplatelets by incorporating an electron acceptor chromophore.

Selenium (Se) nanowires have generated much interest both for the fundamental understanding of crystal formation and growth and for technological applications in optoelectronics, imaging, piezoelectricity, catalysis, and energy harvesting and storage. Several methods have been established to synthesize Se nanowires, but they require sophisticated fabrication steps, are energy intensive, and may involve complex chemical reactions. Moreover, despite an increasing interest, little is known regarding photocarrier dynamics of Se nanowires. Here, we investigate a solution-based approach for the facile synthesis of single-crystal Se nanowires over the large scale where nanowires are directly grown from an amorphous bulk in a solution at room temperature without any chemical reaction. We study the nanowire nucleation and growth mechanism via electron microscopy. We also investigate, for the first time, the charge carrier dynamics and mobility of Se nanowire meshes by means of ultrafast transient absorption spectroscopy, nanosecond flash photolysis, and time-resolved terahertz spectroscopy. These contact-free and noninvasive approaches reveal a lifetime on the picosecond scale for free carriers and on the microsecond scale for trapped carriers, both of which are limited by trap-assisted recombination and a free carrier mobility of similar to 3.0 cm(2) V-1 s(-1). Our work for the first time reveals the rationale behind the excellent properties of some Se NW-based optoelectronic devices. It also highlights the simplicity and robustness of the novel Se nanowire synthesis scheme and paves the way toward the simple fabrication of advanced nanowire-based electronic and optoelectronic nanodevices.

Nanoscale localization of electromagnetic fields near metallic nanostructures underpins the fundamentals and applications of plasmonics. The unavoidable energy loss from plasmon decay, initially seen as a detriment, has now expanded the scope of plasmonic applications to exploit the generated hot carriers. However, quantitative understanding of the spatial localization of these hot carriers, akin to electromagnetic near-field maps, has been elusive. Here we spatially map hot-electron-driven reduction chemistry with 15 nm resolution as a function of time and electromagnetic field polarization for different plasmonic nanostructures. We combine experiments employing a six-electron photo-recycling process that modify the terminal group of a self-assembled monolayer on plasmonic silver nanoantennas, with theoretical predictions from first-principles calculations of non-equilibrium hot-carrier transport in these systems. The resulting localization of reactive regions, determined by hot-carrier transport from high-field regions, paves the way for improving efficiency in hot-carrier extraction science and nanoscale regio-selective surface chemistry., Quantitative understanding of the spatial localization of hot carriers has been elusive. Here Cortes et al. spatially map hot-electron-driven reduction chemistry with 15 nm resolution as a function of time and electromagnetic field polarization for different plasmonic nanostructures.

The origin of the long carrier-lifetime in Lead halide perovskites is still under debate and, among different hypothesis, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. In this work, using state of the art ab initio calculations, we report a systematic study of the polaron formation process in metal halides perovskites focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs and Cs:MA), B-site cations (Pb, Sn and Pb:Sn) and X-site anions (Br, I). Our study confirms the stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A-cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy than the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is non monotonic in mixed Sn:Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn:Pb perovskites depending on the relative Sn percentage.

In this work, lanthanum orthoniobates doped with either antimony, calcium, or both have been synthesized and studied. The water uptake of the investigated materials has been analyzed by means of thermogravimetric studies. The results show the difference between the thermodynamics of hydration between the lanthanum orthoniobate system and other proton conducting ceramics. The relation between the water uptake and effective acceptor doping for the investigated system has been found, and the energetics of the water uptake relation are discussed.

For semiconductors used in photovoltaic devices, the effective mass approximation allows calculation of important material properties from first-principles calculations, including optical properties (e.g. exciton binding energies), defect properties (e.g. donor and acceptor levels) and transport properties (e.g. carrier mobilities). The conduction and valence bands of semiconductors are commonly approximated as parabolic around their extrema, which gives a simple theoretical description, but ignores the complexity of real materials. In this work, we use density functional theory to assess the impact of band non-parabolicity on four common thin-film photovoltaic materials - GaAs, CdTe, Cu$_2$ZnSnS$_4$ and CH$_3$NH$_3$PbI$_3$ - at temperatures and carrier densities relevant for real-world applications. First, we calculate the effective mass at the band edges. We compare finite-difference, unweighted least-squares and thermally weighted least-squares approaches. We find that the thermally weighted least-squares method reduces sensitivity to the choice of sampling density. Second, we employ a Kane quasi-linear dispersion to quantify the extent of non-parabolicity, and compare results from different electronic structure theories to consider the effect of spin-orbit coupling and electron exchange. Finally, we focus on the halide perovskite CH$_3$NH$_3$PbI$_3$ as a model system to assess the impact of non-parabolicity on calculated electron transport and optical properties at high carrier concentrations. We find that at a concentration of 10$^{20}$ cm$^-3$ the optical effective mass increases by a factor of two relative to the low carrier-concentration value, and the polaron mobility decreases by a factor of three. Our work suggests that similar adjustments should be made to the predicted optical and transport properties of other semiconductors with significant band non-parabolicity., Updated after review process

Understanding exciton formation is of fundamental importance for emerging optoelectronic materials, like hybrid organic-inorganic perovskites, as excitons are the lowest-energy photoexcitations in semiconductors, are electrically neutral, and do not directly contribute to charge transport, but can emit light more efficiently than free carriers. However, despite the increasing attention toward these materials, experimental results on the processes of formation of an exciton population in perovskites are still elusive. Here, an ultrafast differential photoluminescence technique is presented that is able to track the kinetics of exciton formation and dissociation in CH3NH3PbBr3. Data show the presence of geminate excitons, i.e., primary excitons directly created upon photon absorption, and their dissociation into free electron-hole pairs. The formation is demonstrated of a secondary exciton phase through pairing of the initial population of free carriers. The analysis of the generation of secondary excitons provides an estimate of the Langevin factor, the parameter governing the charge-pairing rate. Understanding and controlling the formation of a bright exciton population instead of a highly conductive free carrier population may help to design new hybrid perovskite materials with tailored optoelectronic functionalities.

High carrier mobility is often invoked to justify the exceptionally long diffusion length in CH3NH3PbI3 perovskites. Using a combination of an ab initio band structure and scattering models, we present clear evidence that large electrical and Hall mobilities are crucially related to the low scattering rate of carriers with polar optical phonons, which represents the dominant mobility-limiting mechanism at room temperature. With a charge-injection regime at room temperature, we obtained carrier relaxation times (tau(rel)) of similar to 10 fs, which are typical of polar inorganic semiconductors, and electrical mobilities (mu) as high as similar to 60 cm(2) V-1 s(-1) and 40 cm(2) V-1 s(-1) for electrons and holes, respectively, which were robustly independent on the injected carrier density in the range of n similar to 10(14) cm(-3) to 10(20) cm(-3). In the absence of a significant concentration of trapping centers, these mobilities foster diffusion lengths of similar to 10 mu m for the low injection density regime (n similar to 10(15) cm(-3)), which are in agreement with recent measurements for highly pure single-crystal perovskites.

The present work describes ultrafast thermalized and hot hole transfer processes from photo-excited CdSe quantum dots (QDs) and CdSe/ZnS core–shell QDs (CSQDs) to newly synthesized thiols. Three thiols namely 2-mercapto-N-phenylacetamide (AAT), 3-mercapto-N-phenylpropanamide (APT) and 3-mercapto-N-(4-methoxyphenyl) propanamide (ADPT) were synthesized and their interaction with both CdSe QDs and CdSe/ZnS CSQDs was monitored. Steady state absorption study suggests the exciton delocalization from CdSe QDs in the presence of the thiols. However similar features were not observed in the presence of a ZnS shell over a CdSe core, instead a broadening in the excitonic peak was observed with both APT and ADPT but not with AAT. This exciton delocalization and broadening in the excitonic peak was also confirmed by ultrafast transient absorption studies. Steady state and time resolved emission studies show hole transfer from photo-excited QDs and CSQDs to the thiols. A signature of hot hole extraction was observed in transient absorption studies which was confirmed by fluorescence upconversion studies. Both hot and thermalized hole transfer rates from CdSe QDs and CdSe/ZnS CSQDs to the thiols were determined using the fluorescence up-conversion technique. Experiments with different ZnS shell thicknesses have been carried out which suggest that hole transfer is possible till 2.5 monolayer of the ZnS shell. To the best of our knowledge we are reporting for the first time the extraction of hot holes from CdSe/ZnS type I CSQDs by a molecular adsorbate.

The present study investigates the response of NO2 gas sensing devices, based on bis(phthalocyaninato)titanium thin films, by combined electrical and time-resolved energy-dispersive X-ray reflectometry (EDXR) analysis. Samples of various thicknesses were exposed to a NO2 gas flux and their electrical response was recorded, during interaction with the oxidising molecules. At the same time, the changes induced in the film thickness and roughness, produced by the 'breathing like' mechanism, which characterized the diffusion of the gas in the film bulk, were monitored by EDXR. Comparing the two results, the first direct correspondence between the morphological changes and the electrical response of the sensor was found. This also demonstrates that the morphological characteristics of the films are actually related to their sensing behaviour and can therefore be used, as an alternative to the electrical response, to follow the gas-film interaction process. (C) 2007 Elsevier B.V. All rights reserved.

Organic–inorganic metal halide perovskites have demonstrated high power conversion efficiencies in solar cells and promising performance in a wide range of optoelectronic devices. The existence and stability of bound electron–hole pairs in these materials and their role in the operation of devices with different architectures remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modelling as a function of the degree of polycrystallinity and temperature, that the electron–hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycrystalline disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphology strongly influence perovskite behaviour, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.

Cathodoluminescence spectroscopy is profitably exploited to study energy transfer mechanisms in Au and Pt/black TiO2 heterostructures. While Pt nanoparticles absorb light in the UV region, Au nanoparticles absorb light by surface plasmon resonance and interband transitions, both of them occurring in the visible region. The intra-bandgap states (oxygen vacancies) of black TiO2 play a key role in promoting both hot electron transfer and plasmonic resonant energy transfer from Au nanoparticles to the TiO2 semiconductor with a consequent photo-catalytic H2 production increase. An innovative criterion is introduced for the design of plasmonic composites with increased efficiency under visible light.

The effect of tetravalent Zr substitution (≤2 at%) at the Cu site in T′-type Nd 2-x Ce x CuO 4-δ (NCCO), x = 0.12, 0.14, 0.15, on the superconducting (SC) behavior is investigated. EDX analyses of the samples clearly reveal the presence of only ~0.5 at% Zr in the T′-phase. Irrespective of the Ce content (0.12 ≤ x ≤ 0.15), T c is found to depress with Zr. A fall in T c of ~2 K is observed due to 0.5 at% Zr 4+ in x = 0.14 samples, while the NCCO (x = 0.12) on substitution of Zr (expected to dope 2 electrons per Zr 4+ and induce superconductivity) remains non-SC. The reported conjecture regarding T c inducement due to the carrier addition of 0.03 (per formula unit) arising from 3 at% In 3+ (or Ga 3+) substitution for Cu in Nd 1.88 Ce 0.12 CuO 4-δ [Ayoub et al., PhysicaC170, 211 (1990); Felner et al., Phys. Rev.B40, 11366 (1989)] is not applicable for the Zr-substituted NCCO.

We report core level and valence band photoemission results obtained for Nd2-xCexCuO4 (x = 0.15) single crystals and films by using both soft and hard x rays, hence, with tunable depth sensitivity. When using hard x rays only, we observe distinct and energy separated structures in the main 2p(5)3d(9)L peak of Cu 2p(3/2) and 2p(1/2) core levels, including the well screened features located at the high kinetic energy side, which were recently reported by Taguchi et al. [Phys. Rev. Lett. 95, 177002 (2005)]. By varying the photoelectron takeoff angle, we analyze the difference in the screening properties between surface and bulk, and we demonstrate the depth dependence of the electronic properties by following the evolution of the bulk-related peak. The possible influence of the surface conditions on the Cu 2p spectral features is also discussed.

Effects of the substitution of divalent A (Ca, Sr or Ba) ion in Ce excess non-superconducting Nd1.82Ce0.18CuO4.09 compound, leading to revival of superconductivity, are studied by measuring absorption edges of all the cations with synchrotron radiation. It is found that in these NACCO samples, Cu exists mainly in the 2+ state; the weak pre-edge feature near the Cu+ position is marginally affected due to A(2+) ions. Except for the case of Pa doping, a fall in the intensity of the white line at the Cu K-edge is observed. This is attributed to a decrease in lattice oxygen content (oxygen vacancy creation) leading to an increase in electron carriers. It is noted that when the effective charge on Cu is less than or equal to 1.88, the T'-type samples are superconducting.

Thermoelectric power ( S ) studies on the nominally electron-doped Nd 1.82− z Sr z Ce 0.18 CuO y (NSCCO) superconductor system in the temperature range 35 K to 250 K are reported are. S of the reduced z =0.09 sample has negative sign, indicating electron conduction, and has small magnitude, showing high metallicity. The sign of S for the z =0.18 sample however shows a cross-over below 75 K, from negative to positive, conflicting perhaps with electronic conduction. This sample on oxygenation exhibits broadened positive peak and apparently gets carrier concentration enhanced. There is seemingly a competition between electron and hole-like conduction in these co-doped enhanced. There is seemingly a competition between electron and hole-like conduction in these co-doped compounds also.

Thermoelectric power (S) studies on a Nd1.82-xSrxCe0.18CuOy:x less than or equal to 0.18 superconducting system in the temperature range 35-250 K are reported here. In the x=0.09 sample, synthesized in the reduced environment, the small magnitude of S is highly metalliclike and its sign is negative, a characteristic of electron conduction. The sign of S for the x=0.18 sample shows a crossover below 75 K from negative to positive, in apparent conflict with electronic conduction. Interestingly, after oxygenation this sample exhibits a broadened but positive phonon draglike peak. This oxygenated sample shows overcompensation of the carrier (electron) concentration. Critical analysis of the data suggests that Sr doping seemingly causes a competition between electron- and holelike conduction. The slope dS/dT is, in general, negative suggesting that the main contribution is coming from the diffusive part. The observed thermopower features seem to fall in line with the theoretical curves of Durczewski and Ausloos [Z. Phys. B 85, 59 (1991); Phys. Rev. B 53, 1762 (1996)] based on the inelastic scattering of quasifree electrons by phonons. (C) 1996

A direct evidence is obtained for the local oxygen non-stoichiometry in T ′-type Nd 2− x Ce x CuO 4−δ using a high resolution analytical SEM technique. A correlation between the oxygen non-stoichiometry and superconductivity is established. It is found that the oxygen content increases from the grain boundary towards the interior of a superconducting grain, suggesting a gradual fall in T c towards the core. Our results reveal that critical levels of carriers and oxygen vacancies are essential for the T ′-type cuprates to exhibit superconductivity.

The oxygen vacancy parameter, delta, in electron-doped superconducting Nd1.85Ce0.15CuO4-delta, has been varied by changing the sample-processing conditions. A T(c)-onset value of 19 K was obtained by quenching alone. T(c) onset as high as 24 K, T(c)(R = 0) = 17 K and DELTAT(90%-10%) = 3 K have been achieved for delta = 0.05 in argon-annealed single-phase Nd1.85Ce0.15CuO4-delta samples.

Despite numerous attempts to explain the phenomenon of transport and magnetoresistance in manganites based on Jahn- Teller phonon coupling and double exchange mechanisms, satisfactory results could not be reached. The small polaron transport mechanism, based on the valence exchange arising from Madelung energy, is shown to account for the resistivity and magnetoresistance of the doped manganite La(2/3)Ca(1/3)MnO(3) in the ferromagnetic range. The transport equation for the correlated polaron based on the Holstein Hamiltonian shows the well known transition from the low temperature band to the localized polaron hopping conductivity at high temperature in agreement with experiment. The present attempt is to demonstrate that the complexity of the problem that involves the interplay between the spin and charge order on one hand and itinerant and localized behavior on the other for a simple ferromagnetic metal system of manganites is best described by the correlated small polaron model leaving the more complicated systems for future studies. (C) 2009 [DOI: 10.1063/1.3123764]

New photoemission data are presented for a series of Ce-doped or undoped, reduced or nonreduced, Nd cuprates. The data, taken with high signal-to-noise level in the constant-initial-state mode, demonstrate the presence of Cu+, in contrast with previous photoemission and electron-energy-loss studies.

The organic cations in hybrid organic-inorganic perovskites rotate rapidly inside the cuboctahedral cavities formed by the inorganic lattice, influencing optoelectronic properties. Here, we provide a complete quantitative picture of cation dynamics for formamidinium-based perovskites and mixed-cation compositions, which are the most widely used and promising absorber layers for perovskite solar cells today. We use 2H and 14N quadrupolar solid-state NMR relaxometry under magic-angle spinning to determine the activation energy (Ea) and correlation time (tau c) at room temperature for rotation about each principal axis of a series of organic cations. Specifically, we investigate methylammonium (MA+), formamidinium (FA+), and guanidinium (GUA+) cations in current state-of-the-art single-and multi-cation perovskite compositions. We find that MA+, FA+, and GUA+ all have at least one component of rotation that occurs on the picosecond timescale at room temperature, with MA+ and GUA+ also exhibiting faster and slower components, respectively. The cation dynamics depend on the symmetry of the inorganic lattice but are found to be insensitive to the degree of cation substitution. In particular, the FA+ rotation is invariant across all compositions studied here, when sufficiently above the phase transition temperature. We further identify an unusual relaxation mechanism for the 2H of MA+ in mechanosynthesized FAxMA1-xPbI3, which was found to result from physical diffusion to paramagnetic defects. This precise picture of cation dynamics will enable better understanding of the relationship between the organic cations and the optoelectronic properties of perovskites, guiding the design principles for more efficient perovskite solar cells in the future.

New photoemission data are presented for a series of Ce-doped or undoped, reduced or nonreduced, Nd cuprates. The data, taken with high signal-to-noise level in the constant-initial-state mode, demonstrate the presence of Cu+, in contrast with previous photoemission and electron-energy-loss studies.