Surface Exciton States Research Articles

Polarizability of germanium quantum dots with spatially separated electrons and holes

In the framework of dipole approximation, it is shown that the quantities (the oscillator strengths of transitions, the dipole moments for transitions, and the polarizability) describing optical absorption on surface exciton states with spatially separated electrons and holes (the hole moves in the germanium quantum dot and the electron is localized over the spherical interface of the silicon quantum dot matrix) assume giant values considerably exceeding the typical values of the corresponding quantities for semiconductors under the action of low-intensity light.

Polarizability of germanium quantum dots with spatially separated electrons and holes in Ge/Si heterostructures

ABSTRACTIn the framework of dipole approximation, it is shown that the quantities (the oscillator strengths of transitions, the dipole moments for transitions, and the polarizability) describing optical absorption on surface exciton states with spatially separated electrons and holes (the hole moves in the germanium quantum dot and the electron is localised over the spherical interface of the silicon quantum dot matrix) assume giant values considerably exceeding the typical values of the corresponding quantities for semiconductors under the action of low-intensity light.

Exciton spectroscopy with spatially separated electron and hole in Ge/Si heterostructure with germanium quantum dots

It is shown that taking into account the centrifugal energy in the Hamiltonian of an exciton with spatially separated electron and hole (the hole moves in the germanium quantum dot and the electron is localized over the spherical interface of the silicon quantum dot matrix) leads to the appearance of quasistationary states in the zone of surface exciton states, which transform into stationary states with increasing quantum dot radius. It is determined that the absorption spectra of the nanosystem interband consist of energy bands that are formed by electron transitions between quasistationary and stationary states, and the intraband absorption spectra consist of zones formed by electron transitions between stationary states.

Optical spectroscopy of excitons with spatially separated electrons and holes in nanosystems containing dielectric quantum dots

It is shown that in the potential energy of an exciton of spatially separated electrons and holes [hole moves in the amount of quantum dots (QDs), the electron is localized on a spherical surface section (QD—dielectric matrix)], taking into account centrifugal, energy gives rise band of the quasistationary surface exciton states that with the increase of the radius of QD becomes stationary state. The mechanisms of formation of the spectra of interband and intraband absorption (emission) of light in nanosystems containing aluminum oxide QDs, placed in the matrix of vacuum oil, are presented. It is shown that the electron transitions in the area of the surface exciton states cause significant absorption in the visible and near-infrared wavelengths and cause the experimentally observed significant blurring of the absorption edge.

Exciton Spectroscopy of Spatially Separated Electrons and Holes in the Dielectric Quantum Dots

It is shown that in the potential energy of an exciton of spatially separated electrons and holes (hole moves in the amount of quantum dots (QDs), and the electron is localized on a spherical surface section (QD—dielectric matrix)) taking into account centrifugal energy gives rise band of the quasi-stationary surface exciton states that with the increase of the radius of QD becomes stationary state. The mechanisms of formation of the spectra of interband and intraband absorption (emission) of light in nanosystems containing aluminum oxide QDs, placed in the matrix of vacuum oil, are presented. It is shown that the electron transitions in the area of the surface exciton states cause significant absorption in the visible and near infrared wavelengths, and cause the experimentally observed significant blurring of the absorption edge.

Surface States and Band Gap Correlation in Silicon Nanoclusters

Tuning the visible emission of Si nanomaterials by modifying their size and shape is one of the key issue in optoelectronics. The observed optical gain in Si-nanoclusters (NCs) has given further impulse to nanosilicon research. We develop a phenomenological model by combining the effects of surface passivation, exciton states and quantum confinement (QC). The size and passivation dependent band gap, oscillator strength, radiative lifetime and photoluminescence (PL) intensity for NCs with diameter ranging from 1.0 to 6.0 nm are presented. By controlling a set of fitting parameters, it is possible to tune the optical band gap, PL peak and intensity. In case of pure clusters, the band gap is found to decrease with increasing NC size. Furthermore, the band gap increases on passivating the surface of the cluster with hydrogen and oxygen respectively in which the effect of oxygen is more robust. Both QC and surface passivation in addition to exciton effects determine the optical and electronic properties of silicon NCs. Visible luminescence is due to radiative recombination of electrons and holes in the quantum-confined NCs. The role of surface states on the band gap as well as on the HOMO-LUMO states is also examined and a correlation is established. Our results are in conformity with other observations. The model can be extended to study the light emission from other nanostructures and may contribute towards the development of Si based optoelectronics.

PERMITTIVITY IN PERTURBED MOLECULAR NANOFILMS

A microscopic theory of dielectric properties of thin molecular films, i.e., quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green's functions. It has been shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Conditions for the appearance of localized or unoccupied exciton states have been especially analyzed.

Selective IR absorption in molecular nanofilms

We have formulated a microscopic theory of optical properties of ultrathin molecular films (nanofilms), i.e. quasi 2D systems parallel to XY planes bounded by two surfaces. Exposure of nanofilms to the external electromagnetic fields has result in creation of excitons – but different than bulk ones. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green’s functions. It has been shown that two types of optical excitations can occur: bulk and surface exciton states. Exciton energy dispersion law shows discrete behavior with non-zero values. Analysis of the dielectric properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. In addition, permittivity shows very narrow and discrete dependence of external electromagnetic field frequency, which is a consequence of both resonance and quantum size effects. Influences of boundary conditions on optical characteristics (through analyses of dynamical absorption coefficient) of these nanostructures were specially and in details explored.

Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.

Boundary Influence on Permittivity in Molecular Films

A microscopic theory of optical properties of thin molecular films, i.e. quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green’s functions. It was shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties (i.e. dielectrical permittivity) of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Influences of boundary conditions on optical characteristics of these nanostructures were especially analyzed.

Theory of dielectrically induced surface excitonic states in spherical quantum dots

The formation of quantum dot (QD) excitonic surface states induced by dielectric mismatch is theoretically explored in spherical nanocrystals embedded in very high and in very low permittivity media. It is found that the transition from volume to surface exciton states $(V\ensuremath{\rightarrow}S)$ always parallels a sudden drop of exciton brightness if the QD is embedded in low dielectric constant media. This is not the case of a QD buried in high permittivity media. In this case, the $V\ensuremath{\rightarrow}S$ transition is monitored by a reduction in exciton brightness or not depending on the ${m}_{h}^{*}∕{m}_{e}^{*}$ ratio between the effective masses of electron and hole. The presence of a hydrogenic donor impurity at the QD center can drastically reduce the electron-hole density overlap and thus the excitonic binding energy and the drop of brightness that parallels the formation of surface states.

Permittivity in Molecular Nanofilms

AbstractA microscopic theory of dielectrical properties of thin molecular films, i.e. quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green's functions. It has been shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Conditions for the appearance of localized exciton states have been especially analyzed.

Laser Control of Desorption through Selective Surface Excitation

We review recent developments in controlling photoinduced desorption processes of alkali halides. We focus primarily on hyperthermal desorption of halogen atoms and show that the yield, electronic state, and velocity distributions of desorbed atoms can be selected using tunable laser excitation. We demonstrate that the observed control is due to preferential excitation of surface excitons. This approach takes advantage of energetic differences between surface and bulk exciton states and probes the surface exciton directly. We demonstrate that desorption of these materials leads to controlled modification of their surface geometric and electronic structures. We then extend the exciton mechanism of desorption, developed for alkali halides, to metal oxide surfaces, in particular magnesium oxide. In addition, these results demonstrate that laser desorption can serve as a solid-state source of halogen and oxygen atoms, in well-defined electronic and velocity states, for studying chemical processes in the gas phase and at surfaces.

Electronic excitations of CO adsorbed on MgO(001)

We report ab initio calculations of the quasiparticle band structure and the optical excitation spectrum of bulk MgO, the MgO(001) surface, and CO molecules adsorbed on MgO(001). Many-body exchange and correlation effects are included within the GW approximation of the electron self-energy operator and the corresponding electron–hole interaction. The excited electron–hole states are obtained from the Bethe–Salpeter equation. At the clean MgO(001) surface exciton states are found with binding energies that are significantly stronger than in the bulk. The exciton spectrum of the adsorbate system CO : MgO is dominated by charge-transfer excitons, which couple strongly to the molecular excitations of CO.

Site-Specific Excitation in Free Krypton Clusters

Site-specific excitation in Kr clusters is investigated by high resolution inner-shell excitation in combination with model calculations, which are based on a core exciton model. Partial cation and total electron yield spectra of variable size Kr clusters are reported for the $\mathrm{Kr}3d$ excitation regime (90--96 eV) using synchrotron radiation. A cluster size-dependent spectral evolution is observed, corresponding to the transition from low lying cluster Rydberg states into surface and bulk exciton states. The results indicate that individual sites in perfectly and imperfectly shaped clusters are clearly distinguished.

The surface and bulk excitons of crystalline LiF. Li +nF −m cluster embedded in an ionic cage

The bulk and surface excitons in the crystalline LiF were theoretically studied by means of a cluster model embedded in the ion cage. The method employed is that of the self-consistent field (SCF) theory, where multi-center exchange potentials as well as integrals are calculated exactly within the accuracy of the basis sets. It will be shown that the bulk exciton band lies 11.3 eV above the valence band at the SCF level approximation while a correlation correction brings the band 13.4 eV above the valence band. The experimental bulk value is 13.5 eV. The SCF approximation gives the surface exciton 9.4 eV above the valence band while the correlation correction brings the band 11.5 eV above. The experimental value is 10.3 eV. The difference of the charge distributions between the surface and bulk exciton states will be also discussed.

ChemInform Abstract: PURE ELECTRONIC, VIBRONIC, AND TWO‐PARTICLE SURFACE EXCITONS ON THE ANTHRACENE CRYSTAL. I. GENERAL THEORETICAL BASIS, EXPERIMENTAL STUDY, AND ANALYSIS OF THE (0,0) REGION

These two papers (papers I and II) present a complete study—theoretical and experimental—of the first singlet–singlet electronic and vibronic transitions of the anthracene crystal. We interpret the new observed structures in terms of pure electronic, vibronic, and two‐particle site shift surface exciton states. The first paper (I) begins by a model description of the bulk and surface excitons in a rigid lattice. We then describe the experimental study of the anthracene crystal reflectivity and surface emission excitation at low temperatures with and without nitrogen coating of the crystal surface. For the first time, we clearly established the following results: (1) The existence and the accurate position of the upper a polarized Davydov component for the pure electronic surface exciton; (2) The equality within our experimental accuracy (≊2 cm−1), of the surface and bulk excitonic Davydov splitting (Δ=223 cm−1) and the translational equivalence (δ=206 cm−1) of the surface structures with their bulk coun...

Pure electronic, vibronic, and two-particle surface excitons on the anthracene crystal. I. General theoretical basis, experimental study, and analysis of the (0,0) region

These two papers (papers I and II) present a complete study—theoretical and experimental—of the first singlet–singlet electronic and vibronic transitions of the anthracene crystal. We interpret the new observed structures in terms of pure electronic, vibronic, and two-particle site shift surface exciton states. The first paper (I) begins by a model description of the bulk and surface excitons in a rigid lattice. We then describe the experimental study of the anthracene crystal reflectivity and surface emission excitation at low temperatures with and without nitrogen coating of the crystal surface. For the first time, we clearly established the following results: (1) The existence and the accurate position of the upper a polarized Davydov component for the pure electronic surface exciton; (2) The equality within our experimental accuracy (≊2 cm−1), of the surface and bulk excitonic Davydov splitting (Δ̄=223 cm−1) and the translational equivalence (δ̄=206 cm−1) of the surface structures with their bulk counterparts; (3) A variation of the surface Davydov splitting when the crystal surface is coated with frozen nitrogen.

Fluorescence from (001) surface and sub-surface excitons in anthracene crystal: Some experimental evidences

Evidences for the existence of surface and sub-surface exciton states are deduced from quasi-simultaneous reflectivity and fluorescence studies of the same anthracene (001) face at low temperature.

Fluorescence from (001) surface and sub-surface exciton states in anthracene crystal

We present quasi-simultaneous reflectivity and fluorescence spectra of crystalline anthracene (001) face, with and without gas surface coating. We show the common origin of these spectra and present evidence for the existence of surface exciton states.