Transition Energy Shifts Research Articles

A detailed study of the effects of rapid thermal annealing on the luminescence properties of InAs sub-monolayer quantum dots

We studied the effects of post-growth annealing on InAs sub-monolayer samples with different numbers of quantum dot stacks embedded in InGaAs wells. Four samples, with 4, 6, 8, and 10 dot stacks, were grown with solid phase, molecular beam epitaxy under invariant conditions. Increasing the number of stacks created a gradual shift in the photoluminescence ground-state transition energy of the samples from 1.195 to 1.111eV. Subjecting the samples to post-growth thermal annealing at 650, 700, 750, and 800°C produced a typical blue-shift. The full-width-half-maxima of the emission spectrum narrowed with an increase in annealing temperature. All samples exhibited significant enhancement in their activation energies when annealed at 650°C, suggesting that annealing healed defects created during sample growth.

Tailoring the electronic structure in bilayer molybdenum disulfide via interlayer twist.

Molybdenum disulfide bilayers with well-defined interlayer twist angle were constructed by stacking single-crystal monolayers. Varying interlayer twist angle results in strong tuning of the indirect optical transition energy and second-harmonic generation and weak tuning of direct optical transition energies and Raman mode frequencies. Electronic structure calculations show the interlayer separation changes with twist due to repulsion between sulfur atoms, resulting in shifts of the indirect optical transition energies. These results show that interlayer alignment is a crucial variable in tailoring the properties of two-dimensional heterostructures.

Photosensitivity, substituent and solvent-induced shifts in UV-visible absorption bands of naphthyl-ester liquid crystals: a comparative theoretical approach

The substituent- and solvent-induced shifts in UV-visible absorption bands of naphthyl-ester nematic liquid crystals, viz., 4-octylphenyl-6-octyloxy-2-naphthoate (NAPHE1) and 6-octyloxy-2-naphthylyl-4-octyloxybenzoate (NAPHE2), have been investigated using the DFT, CNDO/S and INDO/S methods. A correlation has been made between molecular charge distribution and phase stability based on Mulliken, Loewdin, AM1, PM3, MNDO, CNDO/S and INDO/S methods. The observed π→π* and n→π* electronic transitions have been reported. The substituent- and solvent-induced shifts in absorption bands, transition energies and energy gaps have been discussed. The photosensitivity of the molecules has been analysed based on these shifts. It has been observed that the substituent has a dominant role on both absorption maxima and energy band gap, whereas the solvent has a dominant role only on absorption maxima, and no effect has been observed on the energy gap. These shifts may provide beneficial consequences in determining the end use of compounds.

Infrared-active spin-orbit transitions of halogen atom dopants in solid parahydrogen: The role of trapping site geometry

We present theoretical calculations of the (2)P(1/2) ← (2)P(3/2) spin-orbit transition of Cl dopants embedded as substitutional impurities in solid parahydrogen (pH2) matrices. In the lower-energy (2)P(3/2) spin-orbit level, the Cl atom's electron density distribution is anisotropic, and slightly distorts the geometry of the atom's trapping site. This distortion leads to a blue shift in the spin-orbit transition energy; the blue shift is enhanced when we account for the large-amplitude zero point motions of the pH2 molecules surrounding the Cl dopant. We also show that the intensity of the transition depends on the geometry of the trapping site. In the gas phase, the (2)P(1/2) ← (2)P(3/2) atomic transition is electric dipole forbidden. However, when the Cl atom resides in trapping sites that mimic the hexagonal close packed morphology of pure solid pH2, the transition becomes electric dipole allowed through interaction-induced transition dipole moments. These transition dipole moments originate in the anisotropic electron density distribution of the lower-energy (2)P(3/2) spin-orbit level.

Excitonic analysis of many-body effects on the1s-2pintraband transition in semiconductor systems

We present a detailed study of many-body effects associated with the intraband $1s$-$2p$ transition in two- and three-dimensional photoexcited semiconductors. We employ a previously developed excitonic model to treat effects of exchange and phase space filling (PSF). In this work, we extend the model to include intraband transitions and static free-carrier screening. The exciton transition energies are renormalized by many-body interactions, and the excitonic dynamical equations provide simple expressions for the individual contributions of screening, PSF and exchange. The excitonic model correctly predicts the blue shift and bleaching of the $1s$ exciton resonance due to exchange and PSF. Free-carrier screening is found to enhance these effects by lowering the binding energy of the $1s$ exciton. In contrast, the effects of free-carrier screening on the $1s$-$2p$ transition energy are subtler. For a coherent exciton system, in the absence of free-carrier screening, exchange and PSF lead to a blue shift of the transition energy. However, screening decreases the $1s$ binding energy faster than the $2p$ binding energy, which in turn decreases the transition energy. Thus screening effects oppose exchange and PSF, and the overall magnitude and sign of the $1s$-$2p$ transition energy shift depends on the free-carrier density. Specifically, for low to moderate excitation densities, exchange and PSF can be dominated by screening, leading to a net redshift of the transition energy. The results for two- and three-dimensional systems are qualitatively similar, although the magnitude of the shift is much smaller in three dimensions.

Effective approaches for realizing quantitative analyses and high lateral resolution images on highly insulating samples by Auger electron spectroscopy

The analysis of insulating materials by Auger electron spectroscopy (AES) remains difficult to achieve because of the accumulation of charges in the sample. Residual charges trapped in the specimen can lead to numerous spectra modifications such as an energy shift of the Auger transitions, enlargement, splitting and deformation of the peaks, intense or lack of secondary and Auger electrons emission.Among the different approaches already developed to circumvent this issue, two of them were considered in this study to carry out AES experiments on a SiAlON insulating ceramic: the charge compensation method and the thin film method. For both of these methods, a systematic approach was used, combining a specific sample preparation with optimized analytical settings to mitigate the charge effects to enable quantitative analysis and high lateral resolution images.The charge compensation method, dedicated to bulk samples, is based on the control of the total secondary electron yield (TSEY) during analysis by optimizing the intensity, the energy and the incident angle of the primary electron beam. Its combination with the metallization of the surface sample and the use of low energy Ar+ ions to compensate the charges allowed the determination of the elemental composition of the three sub-micrometric phases of the ceramic. High lateral resolution (70nm) Auger maps were also acquired, demonstrating therefore the long-time stability of the surface charge during acquisitions even for small analysis areas. A second method, consisting in thinning the sample down to less than a hundred of nanometres and analyzing it with a high energy electron beam, was implemented too. The results (quantification and imaging) are in good agreement with the analysis of the sample as a bulk.

Tungsten L transition line shapes and energy shifts resulting from ionization in warm dense matter

Spectra of the W L transitions in the energy range 8–12 keV from warm dense plasmas generated by the Naval Research Laboratory's Gamble II pulsed power machine were recorded by a newly developed high-resolution transmission-crystal X-ray spectrometer with ±2 eV accuracy. The discharges have up to 2 MV voltage, 0.5 MA current, and produce up to 2.4 MJ/cm−3 energy density. The plasma-filled rod pinch (PFRP) diode produces a plasma with Ne ≈ 1022 cm−3 and Te ≈ 50 eV during the time of maximum X-ray emission. By analyzing the line shapes, it was determined that the Lβ2 inner-shell transition from the 4d5/2 level was shifted to higher energy by up to 23 eV relative to nearby Lβ transitions from n = 3 levels. In addition, the Lβ2 transition was significantly broader and asymmetric compared to the n = 3 transitions. The energy shift of the Lβ2 transition results from the ionization of electrons outside the 4d shell that perturbs the transition energies in the ions to higher values. The increased line width and asymmetry result from unresolved transitions from a range of ionization states up to +28. The ionization distribution was determined by comparison of the measured energy shifts and widths to calculated transition energies in W ions, and the ionization was correlated with Gamble discharge parameters such as the anode type and the high voltage delay time. This work demonstrates a new hard X-ray spectroscopic diagnostic technique for the direct measurement of the ionization distribution in warm dense plasmas of the heavy elements W through U that is independent of the other plasma parameters and does not require interpretation by hydrodynamic, atomic kinetics, and radiative simulation codes.

Coherently-enabled environmental control of optics and energy transfer pathways of hybrid quantum dot-metallic nanoparticle systems

It is well-known that optical properties of semiconductor quantum dots can be controlled using optical cavities or near fields of localized surface plasmon resonances (LSPRs) of metallic nanoparticles. In this paper we study the optics, energy transfer pathways, and exciton states of quantum dots when they are influenced by the near fields associated with plasmonic meta-resonances. Such resonances are formed via coherent coupling of excitons and LSPRs when the quantum dots are close to metallic nanorods and driven by a laser beam. Our results suggest an unprecedented sensitivity to the refractive index of the environment, causing significant spectral changes in the Förster resonance energy transfer from the quantum dots to the nanorods and in exciton transition energies. We demonstrate that when a quantum dot-metallic nanorod system is close to its plasmonic meta-resonance, we can adjust the refractive index to: (i) control the frequency range where the energy transfer from the quantum dot to the metallic nanorod is inhibited, (ii) manipulate the exciton transition energy shift of the quantum dot, and (iii) disengage the quantum dot from the metallic nanoparticle and laser field. Our results show that near meta-resonances the spectral forms of energy transfer and exciton energy shifts are strongly correlated to each other.

Electron tunneling from electronically excited states of isolated bisdisulizole-derived trianion chromophores following UV absorption

Photoelectron spectra of isolated [M-BDSZ](3-) (BDSZ = bisdisulizole, M = H, Li, Na, K, Cs) triply charged anions exhibit a dominant constant electron kinetic energy (KE) detachment feature, independent of detachment wavelengths over a wide UV range. Photoelectron imaging spectroscopy shows that this constant KE feature displays an angular distribution consistent with delayed rather than direct electron emission. Time-resolved pump-probe (388 nm/775 nm) two-colour photoelectron spectroscopy reveals that the constant KE feature results from two simultaneously populated excited states, which decay at different rates. The faster of the two rates is essentially the same for all the [M-BDSZ](3-) species, regardless of M. The slower process is associated with lifetimes ranging from several picoseconds to tens of picoseconds. The lighter the alkali cation is, the longer the lifetime of this state. Quantum chemical calculations indicate that the two decaying states are in fact the two lowest singlet excited states of the trianions. Each of the two corresponding photoexcitations is associated with significant charge transfer. However, electron density is transferred from different ends of the roughly chain-like molecule to its aromatic center. The energy (and therefore the decay rate) of the longer-lived excited state is found to be influenced by polarization effects due to the proximal alkali cation complexed to that end of the molecule. Systematic M-dependent geometry changes, mainly due to the size of the alkali cation, lead to M-dependent shifts in transition energies. At the constant pump wavelength this leads to different amounts of vibrational energy in the respective excited state, contributing to the variations in decay rates. The current experiments and calculations confirm excited state electron tunneling detachment (ESETD) to be the mechanism responsible for the observed constant KE feature. The ESETD phenomenon may be quite common for isolated multiply charged anions, which are strong fluorophores in the condensed phase - making ESETD useful for studies of the transient response of such species after electronic excitation.

Structural characterization of small Xe clusters using their 5s correlation satellite electron spectrum

The Xe 5s photoelectron spectrum and 5p(4)nl correlation satellites have been studied in small Xe clusters of an average size of about 15 atoms. The satellite structures are interpreted with the help of the atomic Xe lines. Transition energy shifts between the atomic and the corner/edge/face/bulk components in clusters are divided into polarization screening and exchange interaction energy. Interestingly enough, the ratios between corner/edge/face/bulk polarization screening and exchange interaction energies are found to reflect the ratios of the coordination numbers of corner/edge/face/bulk atoms in these small icosahedral cluster structures.

Reversible modulation and ultrafast dynamics of terahertz resonances in strongly photoexcited metamaterials

We demonstrate an ultrafast reversible modulation of resonant terahertz (THz) response in strongly photoexcited metamaterials. The transient spectral-temporal response of the dipole transition $\ensuremath{\sim}$1.6 THz exhibits a distinct nonmonotonic variation as a function of pump fluence. The transition energy shift, strength, spectral width, and density-dependent ultrafast relaxation manifest a remarkable reemergence of the transmission dip after initial quenching. Our simulations, incorporating the first-order diffraction from the photoinduced transient grating, reproduce the salient features, providing a new avenue for designing nonlinear and frequency-agile THz modulators.

Alterations in the local structure of the Co/SiO2 dispersed carbon nanotubes induced by CO molecules during microwave irradiation

This study presents a detailed structural investigation of Co/SiO2 catalysts dispersed carbon nanotubes (CNTs) treated by microwave in Ar and CO ambient, using X-ray absorption spectroscopy and Raman scattering. For the local structure of the cobalt site in the Co/SiO2 catalysts dispersed CNTs treated by microwave in Ar, the coordination number (NCo–Co) of the first shell is 10.8 with a Co–Co bond length of 2.496 Å, corresponding to the Co–Co bonding in cubic cobalt metal. After the microware treatment in CO, partial amounts of the cobalt species in the Co/SiO2 catalysts are translated to Co2+. In terms of the local structure of carbon site, the π orbital in CNTs are more delocalized than those in graphite and the defects are significant in the case of treating by microwave in Ar. For the microwave treatment in CO, the C–C π* transition energy shifts to that of graphite and the C–C σ* increases, indicating that the defect density is reduced by the microwave treatment in CO ambient. This detailed knowledge of the local structural translation is useful to the modification of the structure of CNTs.

Energy shifts of Kα,β,γ X-ray satellites for atoms with 13 ⩽ Z ⩽ 90: A systematic calculation and a new analytical description

The transition energy shifts of the Kα, Kβ and Kγ X-rays resulted from addition vacancies in L-shell for atoms with 13⩽Z⩽90 have been calculated systematically, by using the Flexible Atomic Code (FAC), in which the Breit interaction, QED corrections and nuclear finite mass effects are also considered. It is shown by means of concrete figures that the overall agreement among the present results, theoretical results of other authors as well as a large body of measured ones was excellent. Contrary to other discussions, we found that the energy shifts of the Kα and Kβ satellites did not varies absolutely linearly with Z, the contributions of the Z2 term cannot be neglected, and the energy differences between neighboring L satellites were not equal. Three simple empirical formulae for estimating the energy shifts of the Kα, Kβ and Kγ X-ray satellites for any atom with any number of L-shell vacancies were put forward. They will be useful in qualitative interpretation of various K X-ray spectra and better understanding of the collision processes.

Absorption and Fluorescence Spectra of Heterocyclic Isomers from Long-Range-Corrected Density Functional Theory in Polarizable Continuum Approach

Long-range-corrected (LC) DFT/TDDFT methods may provide adequate description of ground and excited state properties; however, accuracy of such an approach depends much on a range separation (exchange screening) representing adjustable model parameter. Its relation to a size or specific of molecular systems has been explored in numerous studies, whereas the effect of solvent environment is usually ignored during the evaluation of state properties. To benchmark and assess the quality of the LC-DFT/TDDFT formalism, we report the optical absorption and fluorescence emission energies of organic heterocyclic isomers, DPIPQ and PTNA, calculated by LC-BLYP DFT/TDDFT method in the polarizable continuum (PCM) approach. The calculations are compared with the optical absorption and fluorescence spectra measured in organic solvents of different polarity. Despite a considerable structural difference, both dyes exhibit quite similar range separations being somewhat different for the optical absorption and fluorescence emission processes. Properly parametrized LC-BLYP xc-potential well reproduces basic features of the optical absorption spectra including the electronic transitions to higher excited states. The DFT/TDDFT/PCM analysis correctly predicts the solvation trends although solvatochromic shifts of the electronic transition energies appear to be evidently underestimated in most cases, especially for the fluorescence emission. Considering the discrepancy between the experiment and theory, evaluated state dipole moments and solvation corrections to the exchange screening are analyzed. The results of the present study emphasize the importance of a solvent-dependent range separation in DFT/TDDFT/PCM calculations for investigating excited state properties.

Tuning the Raman Resonance Behavior of Single-Walled Carbon Nanotubes via Covalent Functionalization

We present a systematic Raman study over a range of excitation energies of arc discharge single-walled carbon nanotubes (SWCNTs) covalently functionalized according to two processes, esterification and reductive alkylation. The SWCNTs are characterized by resonance Raman spectroscopy at each step of the functionalization process, showing changes in radial breathing mode frequencies and transition energies for both semiconducting and metallic tubes. Particular attention is given to a family of tubes clearly identified in the Kataura plot for which we continuously tune the excitation energy from 704 to 752 nm. This allows us to quantify the energy shift occurring in the spacing of the van Hove singularities. We demonstrate that, independently of the functionalization technique, the type of chain covalently bound to the tubes plays an important role, notably when oxygen atoms lie close to the tubes, inducing a larger shift in transition energy as compared to that of other carbonaceous chains. The study shows the complexity of interpreting Raman data and suggests many interpretations in the literature may need to be revisited.

Investigation of the strain induced optical transition energy shift of the GaN nanorod light emitting diode arrays

Strain in the semiconductor light emitting layers has profound effect on the energy band structure and the optical properties of the light emitting diodes (LEDs). Here, we report the fabrication and characterization of GaN nanorod LED arrays. We found that the choice of nanorod passivation materials results in the variation of strain in the InGaN/GaN quantum wells, and thus the corresponding change of light emission properties. The results were further investigated by performing Raman measurement to understand the strain of nanorods with different passivation materials and by calculating the optical transition energy of the devices under the influence of strain-induced deformation potential and the piezoelectric polarization field.

Bandgap modifications by lattice deformations in β-FeSi 2 epitaxial films

The modifications of direct transition energies by lattice deformations were investigated in β-FeSi 2 epitaxial films, polycrystal films and single crystal, systematically. The lattice deformations depending on thermal annealing temperature ( T a ) were observed in β-FeSi 2 epitaxial films. In photoreflectance (PR) measurements, the direct transition energies of the epitaxial films shifted to lower energies as the T a increased. The polycrystal films did not show the lattice deformation and the shift of direct transition energies. These results show that the direct bandgap is modified by the lattice deformation originating from the lattice mismatch at the hetero-interface of β-FeSi 2/Si.

Implementation of the analytic energy gradient for the combined time-dependent density functional theory/effective fragment potential method: Application to excited-state molecular dynamics simulations

Excited-state quantum mechanics/molecular mechanics molecular dynamics simulations are performed, to examine the solvent effects on the fluorescence spectra of aqueous formaldehyde. For that purpose, the analytical energy gradient has been derived and implemented for the linear-response time-dependent density functional theory (TDDFT) combined with the effective fragment potential (EFP) method. The EFP method is an efficient ab initio based polarizable model that describes the explicit solvent effects on electronic excitations, in the present work within a hybrid TDDFT/EFP scheme. The new method is applied to the excited-state MD of aqueous formaldehyde in the n-π* state. The calculated π*→n transition energy and solvatochromic shift are in good agreement with other theoretical results.

Optical characterization using ellipsometry of Si nanocrystal thin layers embedded in silicon oxide

ABSTRACTWe have developed optical models for the characterization of grain size in nanocrystal thin films embedded in SiO2 and fabricated using low pressure chemical vapor deposition of Si from silane on a quartz substrate, followed by thermal oxidation. The as-grown nanocrystals thin film on quartz was composed of a two-dimensional array of Si nanocrystals (Si-NC) showing columnar structure in the z-direction and touching each other in the x-y plane. The nanocrystal size in the z-direction was equal to the Si nanocrystal film thickness, changing by the deposition time, while their x-y size was almost equal in all the samples, with small size dispersion. After high temperature thermal oxidation, a thin silicon oxide film was formed on top of the nanocrystals layer. The aim of this work was to measure the grain size and the nanocrystallinity of the Si nanocrystal thin films, a quantity related to the change of the dielectric function. We used a definition for the nanorcystallinity that is related to the effective medium analysis (EMA) of the material. The optical technique used for the investigations was spectroscopic ellipsometry. To measure the above sample properties the thickness and composition of several layers on a quartz substrate had to be determined by proper modeling of this complex system. We found that the nanocrystallinity (defined as the ratio of nc-Si/(c-Si+nc-Si) decreases systematically with increasing the Si-NC layer thickness. Using this approach we are sensitive to the lifetime broadening of electrons caused by the scattering on the grain boundaries, and not to the shift of the direct interband transition energies due to quantum confinement.

Bose-Einstein condensate near a surface: Quantum field theory of the Casimir-Polder interaction

We derive an expression for the collective Casimir-Polder interaction of a trapped gas of condensed bosons with a plane surface through the coupling of the condensate atoms with the electromagnetic field. A systematic perturbation theory is developed based on a diagrammatic expansion of the electromagnetic self-energy. In the leading order, the result for the interaction-energy is proportional to the number of atoms in the condensate mode. At this order, atom-atom interactions and recoil effects lead to corrections compared to the single-atom theory, through shifts of the atomic transition energies. We also discuss the impact of the spatial delocalization of the condensate mode.