Photoabsorption spectroscopy on isolated GaNAsM clusters

Quasiparticle electronic structure and optical absorption of diamond nanoparticles from ab initio many-body perturbation theory

The excited states of small-diameter diamond nanoparticles in the gas phase are studied using the GW method and Bethe-Salpeter equation (BSE) within the ab initio many-body perturbation theory. The calculated ionization potentials and optical gaps are in agreement with experimental results, with the average error about 0.2 eV. The electron affinity is negative and the lowest unoccupied molecular orbital is rather delocalized. Precise determination of the electron affinity requires one to take the off-diagonal matrix elements of the self-energy operator into account in the GW calculation. BSE calculations predict a large exciton binding energy which is an order of magnitude larger than that in the bulk diamond.

Optical properties of GaAs nanocrystals: influence of an electric field

A study of the electronic and optical properties of the hydrogen-terminated GaAs nanocrystals Ga₆₈As₆₈H₉₆ and Ga₉₂As₈₀H₁₀₈ is presented. In this study, their dielectric functions, refractive indices, and absorption coefficients were calculated using density functional theory (DFT). The influence of a uniform external electric field on the optical properties of the nanocrystals was also explored. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for each nanocrystal were studied in the absence and the presence of the uniform external electric field. Our results indicate that the HOMO-LUMO gap decreases with increasing electric field strength. The calculated density of states revealed that the main reason for this shrinking gap is an increase in the delocalization of the gallium π-orbitals under the influence of an increasing external electric field. The permanent dipole moment and the polarizability of the nanocrystals under the induced electric field increased with increasing nanocrystal radius. The induced electric field caused a noticeable redshift in the absorption peaks. The electric field also increased the absorption intensity, particularly when the field strength was >0.25 V/Å.

Optical response of diamond nanocrystals as a function of particle size, shape, and symmetry

The optical spectra of hydrogen-passivated diamond clusters (diamondoids) precisely defined in size and shape have been measured in the gas phase, i.e., under an environment similar to boundary conditions typically assumed by theory. Characteristic optical properties evolve for these wide band-gap semiconductor nanocrystals as a function of size, shape, and symmetry in the subnanometer regime. These effects have not previously been theoretically predicted. The optical response of the tetrahedral-shaped C_{26}H_{32} diamond cluster [1(2,3)4] pentamantane is found to be remarkably similar to that of bulk diamond.

Oscillation of absorption bands of Zn(1-x)Mn(x)S clusters: an experimental and theoretical study

Electroporation of synthetic vesicles is utilized for the preparation of molecular size uncapped Zn(1-x)Mn(x)S clusters. The absence of caps permits (i) continued growth of the Zn(1-x)Mn(x)S clusters formed, (ii) the assessment of their true absorption spectra unaltered by stabilizing ligands, and (iii) the previously inaccessible live observation of the growth of the clusters in the molecular size regime. Upon cluster growth, the UV spectra exhibit novel, time-dependent, oscillation of red and blue shifts of the characteristic absorption band. The structure and electronic properties of Zn(N-1)MnS(N) clusters with N = 1-9 are calculated using the first-principles DMol(3) package. On the basis of similarities between the oscillating trend of the experimentally observed absorption spectra and that of the calculated highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap of Zn(N-1)MnS(N) clusters with N = 1-9, the wavelengths of the sequential spectral peaks can be assigned to Zn(2)MnS(3), Zn(3)MnS(4), Zn(4)MnS(5), Zn(6)MnS(7), and Zn(8)MnS(9), respectively. Our results demonstrate that both the cluster size and the composition can be used to tune the optical properties.

Ab initiofinite field (hyper)polarizability computations on stoichiometric gallium arsenide clusters GanAsn (n=2–9)

We report reliable ab initio finite field (hyper)polarizability values at Hartree-Fock and second order Moller-Plesset perturbation theory (MP2) levels of theory for different geometrical configurations of small gallium arsenide clusters Ga(n)As(n) with n=2-5. We relied on all-electron basis sets and pseudopotentials suitable for (hyper)polarizability calculations. In each case, we used structures that have been established in the literature after we optimized their geometries at B3LYP/cc-pVTZ-PP level of theory. Our results suggest that the first order hyperpolarizability (beta) is much more sensitive to the special geometric features than the second order hyperpolarizability (gamma). For the most stable configurations up to ten atoms the second order hyperpolarizability at MP2 level of theory varies between 15 x 10(4) and 32 x 10(4) e(4)a0 (4)Eh(-3). In addition, we examined the polarizability per atom evolution versus the cluster size for Ga(n)As(n) with n=2-9. Our work extends earlier theoretical studies which were limited to eight atoms and exposes that the polarizability/atom of the most stable stoichiometric configurations up to Ga(9)As(9) continues the monotonic downward trend with increasing size. Lastly, from the methodological point of view, our analysis shows that apart from polarizabilities, augmented pseudopotentials yield reliable first and second hyperpolarizability values as well.

A theoretical investigation of hyperpolarizability for small GanAsm (n+m=4–10) clusters

In this paper, the second and third order polarizabilities of small Ga(n)As(m) (n + m=4-10) clusters are systematically investigated using the time dependent density functional theory (TDDFT)6-311+G* combined with the sum-over-states method (SOSTDDFT6-311+G*). For the static second order polarizabilities, the two-level term (beta(vec.2)) makes a significant contribution to the beta(vec) for all considered Ga(n)As(m) clusters except for the Ga3As4 cluster. And, for the static third order polarizabilities, the positive channel (gamma(II)) makes a larger contribution to gamma(tot) than the negative channel (gamma(I)). Similar to the cubic GaAs bulk materials, the small Ga(n)As(m) cluster assembled materials exhibit large second order (1 x 10(-6) esu) and third order susceptibilities (5 x 10(-11) esu). The dynamic behavior of beta(-2omega; omega, omega) and gamma(-3omega; omega, omega, omega) show that the small Ga(n)As(m) cluster will be a good candidate of nonlinear optical materials due to the avoidance of linear resonance photoabsorption.

Ab initio absorption spectra and optical gaps in nanocrystalline silicon

The optical absorption spectra of Si(n)H(m) nanoclusters up to approximately 250 atoms are computed using a linear response theory within the time-dependent local density approximation (TDLDA). The TDLDA formalism allows the electronic screening and correlation effects, which determine exciton binding energies, to be naturally incorporated within an ab initio framework. We find the calculated excitation energies and optical absorption gaps to be in good agreement with experiment in the limit of both small and large clusters. The TDLDA absorption spectra exhibit substantial blueshifts with respect to the spectra obtained within the time-independent local density approximation.