Excitons in nanoscale systems

Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.     

Semi-empirical simulations of the electron centers in MgO crystal

Semi-empirical quantum chemical simulations of 125-atom clusters have been undertaken to obtain the self-consistent atomic and electronic structure of the two basic electron defects in MgO crystals — F⁺ and F centers (one and two electrons trapped by an O vacancy). The calculated absorption and luminescence energies agree well with the experimental data, the excited states of both defects are found to be essentially delocalized over nearest-neighbour cations. The mechanism of the F⁺ → F photoconversion is discussed.     

Clusters stuffed inside frameworks: Electronic structure theory

Summary A zeolite, or other framework material, can be used as a template to form regular 3D arrays of nanosize clusters stuffed inside the framework cages. The aluminosilicate zeolite framework materials have wide electronic bandgaps, so that the stuffed species can produce new active electronic states within that bandgap. We discuss several real and model examples of such systems. We start with a framework of pure silicon, which is the silicon clathrate, and discuss its properties when stuffed with a single alkali atom in each cage. Caged structures of clathrates are similar to those of zeolites and can house alkali metal atoms in a periodic fashion with spacing varying from 5 to 15 Å Next we investigate a model system of semiconductor clusters (Si) in the SiO₂ framework in the sodalite geometry. Sodalite contains \-cages which are building blocks common to the structure of many important zeolites. We investigate theoretically atomic geometries, energetics and electronic properties of small semiconductor clusters in silica-sodalite. We also consider metal atom Na clusters in naturally occurring sodalite. We find that introducing Si semiconductor clusters within the cages of zeolites gives rise to flat electronic bands in the bandgap of the host, while alkali metal clusters give broad bands. The electronic properties are found in many cases to be dominated by many-body electronic correlation effects.     

Atomically precise cluster catalysis towards quantum controlled catalysts

Catalysis of atomically precise clusters supported on a substrate is reviewed in relation to the type of reactions. The catalytic activity of supported clusters has generally been discussed in terms of electronic structure. Several lines of evidence have indicated that the electronic structure of clusters and the geometry of clusters on a support, including the accompanying cluster-support interaction, are strongly correlated with catalytic activity. The electronic states of small clusters would be easily affected by cluster–support interactions. Several studies have suggested that it is possible to tune the electronic structure through atomic control of the cluster size. It is promising to tune not only the number of cluster atoms, but also the hybridization between the electronic states of the adsorbed reactant molecules and clusters in order to realize a quantum-controlled catalyst.     

One- and Two-Electron Bubbles in Superfluid 4He

Properties of one- and two-electron bubbles in superfluid ⁴He at 0 K were studied by density functional theory. The model allows for accurate treatment of both the electronic and liquid degrees of freedom and as such, enables accurate calculation of bubble energetics for the ground and excited electronic states. The obtained results were compared against the earlier “bubble model” calculations and the limits and accuracy of the bubble model were established. The calculations were carried out in 3-D space and the non-spherical solvation structures for the 1P and 1D excited states were calculated. The 1P state was found to be stable within the radiative lifetime and no plausible non-radiative relaxation channels were found. Finally, a coupled boson and fermion density functional theory was used to show that two-electron bubbles are unstable in both the singlet and triplet electronic states.      

Quantum dots as dynamical systems

Quantum dots show a range of time-dependent behaviours. We show that the polarity of II-VI nanoparticles has important dynamical implications for electronic, vibrational and other phenomena. Polarity-dependent phenomena are found even for nearly spherical stoichiometric clusters of ZnO and ZnS in studies based on interatomic potentials or on a plane-wave density-functional approach. We find a substantial dipole moment for free nanoparticles, whether of the zinc blende or wurtzite structure. This dipole causes a highly non-uniform spin-density distribution on electronic excitation or after a change in the dot's electronic charge state. The spin density of the triplet exciton shows that the dipole aligns so as to reduce the dipole moment in the electronically excited state. The polarity of II-VI dots also affects their vibrational properties. High- and low-frequency tails of the vibration density of states arise from modes strongly localized at surface atoms, near the poles of the dipole. These features, first noted for free clusters, also hold for particles embedded in a wide-gap dielectric, a-SiO(2). We present the results of molecular dynamics of the ZnS particle embedded into the silica glass, and consider the role played by the soft modes in energy-dissipation processes such as dephasing during non-radiative recombination of excitons, and energy transfer from the dot to the matrix.     

Quantitative excited state spectroscopy of a single InGaAs quantum dot molecule through multi-million-atom electronic structure calculations

Atomistic electronic structure calculations are performed to study the coherent inter-dot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum by Krenner et al (2005) Phys. Rev. Lett. 94 057402 is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15-million-atom structures. An excited state spectroscopy technique is applied where the externally applied electric field is swept to probe the ladder of the electronic energy levels (electron or hole) of one quantum dot through anti-crossings with the energy levels of the other quantum dot in a two-quantum-dot molecule. This technique can be used to estimate the spatial electron-hole spacing inside the quantum dot molecule as well as to reverse engineer quantum dot geometry parameters such as the quantum dot separation. Crystal-deformation-induced piezoelectric effects have been discussed in the literature as minor perturbations lifting degeneracies of the electron excited (P and D) states, thus affecting polarization alignment of wavefunction lobes for III-V heterostructures such as single InAs/GaAs quantum dots. In contrast, this work demonstrates the crucial importance of piezoelectricity to resolve the symmetries and energies of the excited states through matching the experimentally measured spectrum in an InGaAs quantum dot molecule under the influence of an electric field. Both linear and quadratic piezoelectric effects are studied for the first time for a quantum dot molecule and demonstrated to be indeed important. The net piezoelectric contribution is found to be critical in determining the correct energy spectrum, which is in contrast to recent studies reporting vanishing net piezoelectric contributions.     

Ground state structures and properties of small hydrogenated silicon clusters

We present results for ground state structures and properties of small hydrogenated silicon clusters using the Car-Parrinello molecular dynamics with simulated annealing. We discuss the nature of bonding of hydrogen in these clusters. We find that hydrogen can form a bridge like Si-H-Si bond connecting two silicon atoms. We find that in the case of a compact and closed silicon cluster hydrogen bonds to the silicon cluster from outside. To understand the structural evolutions and properties of silicon cluster due to hydrogenation, we have studied the cohesive energy and first excited electronic level gap of clusters as a function of hydrogenation. We find that first excited electronic level gap of Sin and SinH fluctuates as function of size and this may provide a first principle basis for the short-range potential fluctuations in hydrogenated amorphous silicon. The stability of hydrogenated silicon clusters is also discussed.     

Free electron lasers applied to VUV and soft X-ray physics of semiconductors

The development of free electron lasers as a spectroscopic source would offer unique research opportunities and open up new avenues of research for the bulk and surface electronic properties of semiconductors. Existing spectroscopic techniques, based on conventional sources or synchrotron radiation, have contributed immensely to our understanding of the electronic structure of filled (photoemission) and empty (inverse photoemission) electronic levels. However, much less is known about the dynamic transient processes of electrons excited into empty states. A detailed understanding of both bulk (e.g., hot electron properties) and surface phenomena (e.g., photochemical reactions) are critically dependent on the transient dynamics of the excited electrons. The main assets of free electron laser radiation would be its high intensity, tunability, and time-structure. The tunability will provide selectivity for the final state energies into which the electrons are excited. Interesting resonance phenomena should be observed where interference occurs between different deexcitation channels. The time-structure will make it possible to study the time evolution of electron-electron, electron-phonon, electron-defect, etc., interactions of photoexcited carriers. The traditional optical probes (e.g., fluorescence) of hot carriers are bulk techniques. The photoelectron spectroscopy techniques are surface sensitive and provide a possible tool to study differences in relaxation phenomena for states at the surface versus states in the bulk. The surface sensitivity also makes it possible to study other dynamical processes on the surface: chemical reactions, chemi- and physi-sorption, and surface reconstructions.     

Relative Stabilities of C74 Isomers

Relative stabilities of six C₇₄ fullerene cages are evaluated: one species obeying the isolated pentagon rule (IPR), three isomers with a pentagon-pentagon junction, two structures with one pentagon-pentagon pair and one heptagon. The computations are carried out using the Gibbs energy in a broad temperature interval. It is shown that the IPR cage (D_3h symmetry) prevails throughout. As low-lying electronic excited states are possible for the cages, their electronic partition functions are included into consideration. It is argued that for the special conditions of the fullerene synthesis/isolation, the electronic partition function based on the singlet excited states only should better reproduce the experimental population findings. The computations indicate that isolation of other C₇₄ cage, in addition to the IPR isomer, is less likely though not impossible.     

"光子-买一赠三”--简评有机三线态电致发光材料与器件进展*

有机电致发光材料与器件的研究已取得了重要进展,但要实现高信息含量的应用,器件的稳定性和效率仍须进一步提高。基于量子统计理论的研究结果表明,只有25%的电子空穴复合能量生成单线态激子,对于一个纯荧光的发光材料,在理论上,其器件效率的上限是光致发光效率的25%。三线态发光材料的应用,理论上可有效利用所有的复合能量,从而大幅度提高器件效率,目前已成为有机电致发光领域的研究热点。综述了有机三线态电致发光材料与器件的进展。     

Electronic excited states in bilayer graphene double quantum dots

We report tunneling spectroscopy experiments on a bilayer graphene double quantum dot device that can be tuned by all-graphene lateral gates. The diameter of the two quantum dots are around 50 nm and the constrictions acting as tunneling barriers are 30 nm in width. The double quantum dot features additional energies on the order of 20 meV. Charge stability diagrams allow us to study the tunable interdot coupling energy as well as the spectrum of the electronic excited states on a number of individual triple points over a large energy range. The obtained constant level spacing of 1.75 meV over a wide energy range is in good agreement with the expected single-particle energy spacing in bilayer graphene quantum dots. Finally, we investigate the evolution of the electronic excited states in a parallel magnetic field.     

Theoretical Calculation of the Low-Density Transport Properties of Monatomic Silver Vapor

Calculations of low-density transport property collision integrals are used to obtain the high-temperature transport properties of silver atoms as a function of temperature. The collision integrals depend on the two-body interaction potentials between silver atoms in various electronic states. Contributions are included from the ground and excited molecular electronic states of the silver dimer that dissociate to two ground-state silver atoms and from the excited molecular state that dissociates to a ground state and an excited state silver atom. Spectroscopic constants are available for these three electronic states, and these spectroscopic constants have been used to determine the Hulburt–Hirschfelder (HH) potentials for these three states. The HH potential is perhaps the best general-purpose potential for representing atom–atom interactions. This potential depends only on the spectroscopic constants, and can be used to calculate the viscosity and diffusion collision integrals for the three molecular electronic states. The collision integrals are then degeneracy averaged over the three states. The heat capacity of silver atoms is also calculated at high temperatures. These results provide the information required to obtain the thermal conductivity, viscosity, and self-diffusion coefficients of silver atoms over a wide temperature range from the boiling point of silver to temperatures at which ionization becomes important.     

Photosynthetic light harvesting: excitons and coherence

Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques.     

Ground state structures and properties of Si<Subscript>3</Subscript>H<Subscript>n</Subscript> (n = 1–6) clusters

The ground state structures and properties of Si ₃H_n (1 ≤n ≤ 6) clusters have been calculated using Car-Parrinello molecular dynamics with simulated annealing and steepest descent optimization methods. We have studied cohesive energy per particle and first excited electronic level gap of the clusters as a function of hydrogenation. Hydrogenation is done till all dangling bonds of silicon are saturated. Our results show that over coordination of hydrogen is favoured in Si3Hn clusters and the geometry of Si₃ cluster does not change due to hydrogenation. Cohesive energy per particle and first excited electronic level gap study of the clusters show that Si₃H₆ cluster is most stable and Si₃H₃ cluster is most unstable among the clusters considered here.     

Complex dynamics variables for molecules and molecular clusters

We describe a method to analyse dynamic properties of molecules and molecular clusters. The model involves the change in lifetimes of collective, quasi-stationary excited states of the system under consideration by low energy electron local excitations. Probabilities of desorption-ionisation processes and cluster decomposition are also estimated. The model is based on the quantum multi-particle theory and the exciton or excimol theory for condensed matter and molecules. The energies of excitation have complex values which permit us to take into account the unharmonic character of bond vibrations and the relaxation process. We apply a perturbation theory for excitation analysis describing the possible deviation of molecules or molecular clusters from regular structures. The wave-functions can be taken in their asymptotic form and the dilatation method is used for the complex energy states.     

Triplet state absorption in carbon nanotubes: a TD-DFT study

We predict properties of triplet excited states in single-walled carbon nanotubes (CNTs) using a time-dependent density-functional theory (TD-DFT). We show that the lowest triplet state energy in CNTs to be about 0.2-0.3 eV lower than the lowest singlet state energies. Like in pi-conjugated polymers, the lowest CNT triplets are spatially localized. These states show strong optical absorption at about 0.5-0.6 eV to the higher lying delocalized triplet states. These results demonstrate striking similarity of the electronic features between CNTs and pi-conjugated polymers and provide explicit guidelines for spectroscopic detection of CNT triplet states.     

Electron propagation along Cu nanowires supported on a Cu(111) surface

We present a joint experimental-theoretical study of the one-dimensional band of excited electronic states with sp character localized on Cu nanowires supported on a Cu(111) surface. Energy dispersion and lifetime of these states have been obtained, allowing the determination of the mean distance traveled by an excited electron along the nanowire before it escapes into the substrate. We show that a Cu nanowire supported on a Cu(111) surface can guide a one-dimensional electron flux over a short distance and thus can be considered as a possible component for nanoelectronics devices.     

Electronic shell structure of cluster quantum mechanical effects in the semiclassical approximation

The electronic shell structure of spherical and circular potential wells of finite depth is examined as an extension of the semiclassical analysis of Balian and Bloch. The resulting analytical expressions for the electronic density of states are in good agreement with results of numerical calculations. For circular discs the oscillating structure in the density of states shows a different supershell pattern than for spheres. This is a consequence of the difference in the effective multiplicity of the closed paths, which is determined by the quantization momentum. The positions of the maxima and minima in the density of states depend strongly on the phase shifts in the particle waves arising for each reflection at the cluster surface. These phase shifts have to be determined from a quantum mechanical calculation. Thus the electronic shell structure of spherical clusters has a close similarity with electromagnetic resonances of dielectric spheres (whispering gallery modes).