Direct evidence for the blue luminescence of gold nanoclusters encapsulated inside hydroxyl-terminated polyamidoamine (PAMAM) dendrimers was provided by spectroscopic studies as well as by theoretical calculations. Steady-state and time-resolved spectroscopic studies showed that the luminescence of the gold nanoclusters consisted largely of two electronic transitions. Theoretical calculations indicate that the two transitions are attributed to the different sizes of the gold nanoclusters (Au8 and Au13). The luminescence of the gold nanoclusters was clearly distinguished from that of the dendrimers.

The photophysical properties of CuInxGa1-xS2 (CIGS) thin films, prepared by solution-based coating methods, are investigated to understand the correlation between the optical properties of these films and the electrical characteristics of solar cells fabricated using these films. Photophysical properties, such as the depth-dependent band gap and carrier lifetime, turn out to be at play in determining the energy conversion efficiency of solar cells. A double grading of the band gap in CIGS films enhances solar cell efficiency, even when defect states disturb carrier collection by non-radiative decay. The combinational stacking of different density films leads to improved solar cell performance as well as efficient fabrication because a graded band gap and reduced shunt current increase carrier collection efficiency. The photodynamics of minority-carriers suggests that the suppression of defect states is a primary area of improvement in CIGS thin films prepared by solution-based methods.

Size-dependent luminescence is one of the most promising properties in semiconductor nanocrystals.1 -3 Such sizedependent luminescence is also found in various metal nanocrystals like the gold nanoclusters.4 -6 For example, blue luminescence was observed from Au 8 , which was stabilized and encapsulated by dendrimers. The color of the luminescence was extended to near-IR, as the size of the gold nanoclusters increased, 7,8 which was attributed to the confinement of the cluster sizes comparable to the Fermi wavelength of electrons (approximately 0.7 nm). 9 We recently studied the luminescence of the gold nanoclusters, especially the blue luminescence of Au 8 , 10 which provided a clue to the origin of luminescence in the gold nanoclusters. 11 -13 In this work, we examined the electronic transitions of Au 8 to elucidate the photoluminescence spectrum of the gold nanoclusters in more detail. The calculated geometries and energies of the isomers in Au 8 indicated that one isomer was more stable than the others, implying that a single isomer was the main contributor to the photoluminescence spectrum of Au 8 . The calculated spectrum of the most stable isomer agreed with the experimental spectrum, suggesting that several excited states of the single isomer contributed to the photoluminescence spectrum, whose shape was broad and asymmetric.The photoluminescence of the gold nanoclusters encapsulated by dendrimers was previously reported. 10 The emission maximum at 460 nm (2.7 eV) matched the emission energy of Au 8 (2.75 eV) in the spherical jellium approximation (Figure 1 (a)).8 On the other hand, the photoluminescence band was broad and asymmetric, which was ascribed to the spectral overlapping of emission bands.10 Indeed, the photoluminescence spectrum could be deconvoluted by two Gaussian functions. The low-energy band at 535 nm (2.32 eV) happened to agree with the emission energy of Au 13 (2.34 eV) in the spherical jellium model. 8 In this regard, another nanocluster was previously assigned as Au 13 , 10 because Au 13 was also the stable size of the gold nanoclusters. 8 For better understanding of the broad and asymmetric shape, the electronic transitions of Au 8 were calculated using the GAUSSIAN program.14 At first, the stable geometries and energies of Au 8 were obtained, by analytical gradients with full optimization using ab initio method such as the Møller-Plesset perturbation theory (MP2) and the density functional theory (DFT) such as Becke three-parameter Lee-Yang-Parr (B3LYP) and Perdew-Wang 1991 (B3PW91) gradient corrected exchange and correlation functional at two basis sets (CEP-4G and LANL2DZ). These levels of calculation have been frequently employed to obtain the stable geometries and energies of small gold nanoclusters. 15 -17 Since the emission energy of the dendrimer-encapsulated Au 8 matched the spherical jellium model, three-dimensional geometries were mainly examined, although the most stable geometry was known to be two-dimensional in the gas phase.15 -17 Among the optimized three-dimensio...

A trimer of dicyanoaurate has been studied as a model system of the covalent chemical bond formation. Here, we report the dynamics of dicyanoaurate trimer in water upon photoexcitation by femtosecond time-resolved luminescence (TL) and luminescence spectra at cyrogenic temperature. Temperature was varied as a means to control the medium flexibility as well as the population of isomers. A unique parallelism between the luminescence spectrum vs. time and vs. temperature was observed, which enables unambiguous luminescence band assignments and facilitates investigation of the dynamics. Upon photoexcitation to S 1 , intersystem crossing proceeds in an ultrafast manner within 20 fs due to the large spin−orbit coupling followed by a structural change from a loose bent to a tight linear form in 1.5 ps. Higher oligomerization occurs above the melting temperature. TL reveals a strong coherent excitation of the symmetric Au−Au stretching vibration at 74 cm −1 through the non-Condon effect.