Valence Excitons Research Articles

Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core–hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe–Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides─TiO2, α-Cr2O3, β-MnO2, α-Fe2O3, Co3O4, NiO, CuO, and ZnO─are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core–valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

ZnO core-triggered nitrogen-deficient carbonaceous g-C3N4 shell enhances the visible-light-driven disinfection

• A solid-state synthesis of ZnO@g-C 3 N 4 core-shell nanocomposites is reported. • A ZnO-triggered nitrogen-deficient shell is formed up to 20 wt.% g-C 3 N 4 loading. • This shell imparts stability and enhanced photocatalytic efficacy to the core. • The 5 wt.% g-C 3 N 4 loading exhibits high valence band intensity and exciton lifetime. • This optimal catalyst displays high disinfection performance under visible light. We report a solid-state synthesis of ZnO@g-C 3 N 4 core-shell nanocomposites, in which melamine—a precursor of graphitic carbon nitride (g-C 3 N 4 )—is thermally decomposed over a pre-formed ZnO core. The thermolysis has led to the formation of a nitrogen-deficient carbonaceous shell over the ZnO core—reported for the first time—when the g-C 3 N 4 loading has been sequentially increased up to 20 wt.%. With a further increase in loading to 50 and 80 wt.%, the signature of the g-C 3 N 4 has been observed in the shell. The quantitative studies using TGA have revealed the excessive decomposition of melamine over the surface of ZnO, whilst the nitrogen-deficiency of the carbonaceous shell has been unearthed by the XPS studies. The core-shell nanocomposite with 5 wt.% g-C 3 N 4 loading has been found to possess enhanced photocatalytic performance towards methylene blue degradation and visible-light-driven water disinfection. The enhanced photocatalytic activity has been attributed to the thin carbonaceous shell imparting high valence band intensity and exciton lifetime to the ZnO core. The nanocomposites have been found to generate an excess of hydroxyl radicals in comparison to g-C 3 N 4 or ZnO. Furthermore, the toxicity studies performed with the fibroblast feeder cells have revealed the biocompatibility of the materials for practical applications.

Valence and core excitons in solids from velocity-gauge real-time TDDFT with range-separated hybrid functionals: An LCAO approach

An atomic-orbital basis set framework is presented for carrying out velocity-gauge real-time time-dependent density functional theory (TDDFT) simulations in periodic systems employing range-separated hybrid functionals. Linear optical response obtained from real-time propagation of the time-dependent Kohn-Sham equations including nonlocal exchange is considered in prototypical solid-state materials such as bulk Si, LiF and monolayer hexagonal-BN. Additionally core excitations in monolayer hexagonal-BN at the B and N K-edges are investigated and the role of long-range and short-range nonlocal exchange in capturing valence and core excitonic effects is discussed. Results obtained using this time-domain atomic orbital basis set framework are shown to be consistent with equivalent frequency-domain planewave results in the literature. The developments discussed lead to a time-domain generalized Kohn-Sham TDDFT implementation for the treatment of core and valence electron dynamics and light-matter interaction in periodic solid-state systems.

Quasiparticle Lifetime Broadening in Resonant X-ray Scattering of NH4NO3.

It has been previously shown that two effects cause dramatic changes in the x-ray absorption and emission spectra from the N K edge of the insulating crystal ammonium nitrate. First, vibrational disorder causes major changes in the absorption spectrum, originating not only from the thermal population of phonons, but, significantly, from zero-point motion as well. Second, the anomalously large broadening (~ 4 eV) of the emission originating from nitrate σ states is due to unusually short lifetimes of quasiparticles in an otherwise extremely narrow band. In this work we investigate the coupling of these effects to core and valence excitons that are created as the initial x-ray excitation energy is progressively reduced toward the N edge. Using a GW/Bethe-Salpeter approach, we show the extent to which this anomalous broadening is captured by the GW approximation. The data and calculations demonstrate the importance that the complex self-energies (finite lifetimes) of valence bands have on the interpretation of emission spectra. We produce a scheme to explain why extreme lifetimes should appear in σ states of other similar compounds.

Electron-hole correlation effects in core-level spectroscopy probed by the resonant inelastic soft x-ray scattering map of C60

We have employed a unique spectroscopic approach, a resonant inelastic soft x-ray scattering (RIXS) map, to identify and separate electron-hole correlation effects in core-level spectroscopy. With this approach, we are able to derive a comprehensive picture of the electronic structure, separating ground state properties (such as the HOMO-LUMO separation) from excited state properties (such as the C 1s core-exciton binding energy of C(60)). In particular, our approach allows us to determine the difference between core- and valence exciton binding energies in C(60) [0.5 (±0.2) eV]. Furthermore, the RIXS map gives detailed insight into the symmetries of the intermediate and final states of the RIXS process.

Resonant inelastic x-ray scattering and UV–VUV luminescence at the Be 1s edge inBeO

We carried out a combined study of UV–VUV luminescence and resonant x-ray emissionfrom BeO single crystals with incident photon energies in the vicinity of the Be 1sabsorption edge. The x-ray emission spectra show that at the Be 1s photoabsorption edgethe lattice relaxation processes in the excitation site take place already on the timescale ofthe radiative decay of the core excitation. Comparison of the x-ray emission andthe luminescence spectra indicates that the maximum energy loss of the processof lattice relaxation during the decay of inner-shell holes is similar to the lossthat occurs in the self-trapping process of valence excitons. The possible decaychannels of core excitations have been discussed and the mechanism for the creationof 5.2 eV luminescence at the photoabsorption resonances has been suggested.

Photostimulated detection of radiation defects produced by VUV light in BaF2

Abstract Production of radiation defects in a widely used scintillation material BaF 2 has been studied by means of a combination spectroscopy of synchrotron radiation (SR) and laser, in which defects produced by SR irradiation are sensitively detected by observing the luminescence stimulated by laser light. The photostimulated luminescence arises from the recombination of self-trapped holes (V K centers) with electrons released from trapped centers by laser light. The obtained result reveals that the production efficiency of radiation defects is drastically dependent on the excitation photon energy of valence or core excitons.

Core and valence exciton formation in x-ray absorption, x-ray emission andx-ray excited optical luminescence from passivated Si nanocrystals at the SiL2,3 edge

Resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopyand x-ray excited optical luminescence (XEOL) have been used tomeasure element specific filled and empty electronic states over the SiL2,3 edge of passivated Si nanocrystals of narrow size distribution (diameter2.2 ± 0.4 nm). These techniques have been employed to directly measure absorption and luminescencespecific to the local Si nanocrystal core. Profound changes occur in the absorptionspectrum of the nanocrystals compared with bulk Si, and new features are observed in thenanocrystal RIXS. Clear signatures of core and valence band exciton formation, promotedby the spatial confinement of electrons and holes within the nanocrystals, areobserved, together with band narrowing due to quantum confinement. XEOL at 12 Kshows an extremely sharp feature at the threshold of orange luminescence (i.e., at∼1.56 eV (792 nm)) which we attribute to recombination of valence excitons, providing a lowerlimit to the nanocrystal band gap.

Photoemission, resonant photoemission, and x-ray absorption of a Ru(II) complex adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition

An experimental study of the bonding geometry and electronic coupling of cis-bis(isothiocyanato)bis(2,2(')-bipyridyl-4,4(')-dicarboxylato)-ruthenium(II) (N3) adsorbed on rutile TiO(2)(110) is presented, along with supporting theoretical calculations of the bonding geometry. Samples were prepared in situ using ultrahigh vacuum electrospray deposition. Core-level photoemission spectroscopy was used to characterize the system and to deduce the nature of the molecule-surface bonding. Valence band photoemission and N 1s x-ray absorption spectra were aligned in a common binding energy scale to enable a quantitative analysis of the bandgap region. A consideration of the energetics in relation to optical absorption is used to identify the photoexcitation channel between the highest occupied and lowest unoccupied molecular orbitals in this system, and also to quantify the relative binding energies of core and valence excitons. The core-hole clock implementation of resonant photoemission spectroscopy is used to reveal that electron delocalization from N3 occurs within 16 fs.

Lead Phthalocyanine Films by Near Edge X-ray Absorption Fine Structure Spectroscopy

We measured NEXAFS spectra on both C and N K-edges of an ordered monolayer of lead phthalocyanine adsorbed onto an InSb(001) 4 × 2/c(8 × 2) substrate. We compared the spectra with the calculated density of unoccupied states of an isolated molecule. We demonstrate that molecules lay flat on the surface, determine the energy position of the lowest excited state, and show that it is site dependent. Finally, we compare the core exciton structure with the valence exciton structure previously measured by two different valence spectroscopic techniques and discuss their differences in terms of electron−nuclear and electron−hole Coulomb interactions.

Adsorption of Metal-Free Phthalocyanine on InSb and InAs(001)-4 × 2/c(8 × 2) Surface

We have studied the adsorption of metal-free phthalocyanine on 4 x 2/c(8 x 2) surface reconstruction of both InSb(001) and InAs(001) surfaces, by means of low-energy electron diffraction and high-resolution electron energy loss spectroscopy. We show that the molecule-surface interaction is stronger in the case of the InAs substrate as compared to the InSb one. Indeed, we observed that at full monolayer coverage, when the molecules are adsorbed on the InAs, and only in that case, there are substantial energy shifts of both the substrate surface plasmon and the first valence exciton of the molecules. These shifts are evidence of charge transfer from the molecules toward the substrate responsible for a strong molecule-substrate interaction such as chemisorption. In spite of the difference between the two substrates, at monolayer completion, H 2 Pc on InAs and InSb adopts a common (4n x 3) surface arrangement. The same that was already reported for the nonplanar lead and tin phthalocyanines (respectively, PbPc and SnPc) adsorbed on InSb(001).

Resonant inelastic X-ray scattering at the Be 1s edge in BeO

We present the resonant inelastic X-ray scattering (RIXS) spectra of BeO at the Be 1s photoabsorption edge. The RIXS spectra excited in the vicinity of the core exciton resonance show two principal features: the scattering on a valence exciton (which at higher excitation energies verges into the characteristic K α emission), and a remarkably strong energy loss side-band to the elastic scattering peak, which has a maximum width of 6 eV and has intensity, at resonance, comparable to the valence scattering. The energy loss side-band appears to result from rapid lattice relaxation at the absorption site on the X-ray scattering time-span.

Exciton induced photodesorption in rare gas solids

This paper reviews our progress on the desorption induced by electronic transitions(DIET) in rare gas solids by selective excitation of valence excitons. Observation ofmetastable atoms desorbed by excitonic excitation gives us direct information onthe exciton-induced desorption processes in rare gas solids. The validity of threedesorption mechanisms, cavity ejection, excimer dissociation, and internal sputtering, isdemonstrated by systematic measurements of kinetic energies and angular distributions ofdesorbed particles. The absolute yield of total and partial desorption was measured,which can lead us to the quantitative understanding of exciton-induced desorptionprocesses.

Comparison of the size of excitonic effects in molecular π systems as measured by core and valence spectroscopies

In aromatic molecules characterised by extended π-systems valence and core excitons can have very similar binding energies. This is shown for a number of compounds, and in each case the core exciton binding energies are analysed in terms of the Coulomb screening and rehybridisation contributions. The latter semiquantitatively explain the binding energy differences between the valence and core excitons. This suggests that core and valence excitons have a similar spatial extent in such systems. In favourable cases core level techniques can thus be used to elucidate valence excited state properties.

Simulation of optical and X-ray sum frequency generation spectroscopy of one-dimensional molecular chain

The time-resolved sum frequency generation signal of an optical and X-ray pulse is calculated for a one-dimensional molecular chain. We show how the valence exciton motion induced by the first optical light pulse can be probed with atomic scale precision by tuning the X-ray frequency across a core excitation of a particular atom. This technique provides a novel real-time, real-space probe for dynamics of valence excitation in a single molecule.

Spectral profile of the two-photon absorption coefficients in CaF2 and BaF2

Two-photon absorption spectra due to the valence excitons in CaF2 and BaF2 have been measured using the second harmonic of the output of a XeCl-excimer-laser pumped dye laser as a light source. The band gap energies of CaF2 and BaF2 are obtained as 11.8 and 10.6 eV at 298 K, respectively. The two-photon absorption coefficient β in the spectral region below the 2P exciton is expressed by an exponential function of the photon energy in both crystals. The values of β at the 2P exciton peak are about 2×10−9 cm/W for CaF2 and 3×10−9 cm/W for BaF2.

Two-photon absorption spectroscopy of valence excitons in alkaline-earth fluorides

The 2 P state energy of the valence exciton in SrF 2 has been obtained to be 11.1 v eV at 298 v K through two-photon absorption (TPA) spectroscopy. The luminescence of self-trapped excitons was monitored as the signal of TPA. The exciton binding energy is estimated to be 0.9 v eV. This value of binding energy is comparable not only to those obtained through TPA in other alkaline-earth fluorides of CaF 2 and BaF 2 , but also to those of alkali fluorides. The TPA coefficient of SrF 2 was about 5 ‐ 10 m 9 v cm/W for 5.55 v eV-photons. The value is in the same order of magnitude as those measured at 2 P state of the exciton in CaF 2 and BaF 2 crystals.

Resonant enhancement effect on two-photon absorption due to excitons in alkaline-earth fluorides excited with synchrotron radiation and laser light

Exciton states in wide-gap materials ${\mathrm{CaF}}_{2}$ and ${\mathrm{BaF}}_{2}$ have been studied with the technique of two-photon spectroscopy using synchrotron radiation and laser. The two-photon excitation spectrum of ${\mathrm{CaF}}_{2},$ and that of ${\mathrm{BaF}}_{2}$ as well, exhibited a clear peak on the high-energy side of the $1S$ exciton energy. The peaks are attributed to the $2P$ states of the valence excitons. According to this assignment, the binding energies of the excitons have been determined to be 0.8 and 0.9 eV for ${\mathrm{CaF}}_{2}$ and ${\mathrm{BaF}}_{2},$ respectively. The signal intensities were larger than that in the case of one-beam excitation by a factor of about 500, which is attributed to a resonant effect on two-photon absorption.

Combination of synchrotron radiation and laser for two-photon spectroscopy of BaF2

Abstract We have carried out nonlinear spectroscopy by making use of tunable VUV light from synchrotron radiation (SR) together with intense light from a laser. The valence excitons of BaF 2 with a large band gap of ∼11 eV were chosen as our target. A single crystal of BaF 2 was irradiated at 15 K by the two light pulses from the SR (6–9 eV) and an Nd : YAG laser (2.33 eV). Self-trapped exciton luminescence induced by two-photon excitation was detected with a time-gated photon-counting system developed for the present study. This zero-method technique is expected to be sensitive compared to early experiments by an Italian group monitoring transmittance changes. The obtained two-photon spectrum shows a distinct band below the band gap energy. This band includes two contributions, a two-step cascade process and a two-photon absorption process. The energy of the 2 P exciton is estimated to be 10.6 eV at 15 K.

Raman vs Rayleigh scattering in the soft-x-ray region

A sharp and intense structure of the core exciton is observed in total photon yield spectra of a cubic BN crystal at 192 eV, below the transition from the boron 1$s$ level to the conduction-band edge. We have discovered, however, that the Raman intensity for leaving behind the valence exciton does not show any enhancement even when the frequency of the incident field crosses the core-exciton level. This comes from the fact that relative magnitudes among the nonlinear channels depend on the relevant frequency. We analyzed the Raman spectra taking into account the strong coupling between the core exciton and radiation fields, i.e., the polariton effect, Rayleigh scattering, and the Auger process.