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We report here the crystallization of spherical, 4.7-nm-diameter Ag nanocrystals, passivated with a fatty-acid surface coating, into well-facetted supercrystals up to 500-µm across and about 1-µm thick.

We report the optical nonlinear response of the confined excitons in nanostructures induced by external stress in a layered crystal BiI3. The absorption intensity of the confined excitons is saturated by intense laser excitation. We clarify the size dependence of the saturation density which reflects the nanostructure size. The saturation density of the confined exciton in large domains is lower than the the case of small domains. By pump-probe measurements using a sub-picosecond laser pulse, we examine the dynamical process of the confined excitons in the disks with various sizes. From the temporal behavior of the absorption saturation, it becomes clear that the exciton life times in small disks are shorter than the large ones. A new nonlinearly growing luminescence due to the process by a inelastic recoiling of excitons is detected.

The effect of hydrostatic pressure up to 1.44 GPa upon the luminescence bands associated with self–trapped excitons (STE's) and impurity (Tl+) in KI has been measured at various temperatures. A sapphire anvil cell with helium as a pressure medium is used. The STE luminescence bands show strong blue shift (0.43 eV/GPa for the σ band), which is much larger than the blue shift of the 1s exciton energy (0.16 eV/GPa). The intensities and half-widths of the STE luminescence bands decrease with pressure. These results are discussed on the basis of the exciton-lattice interaction strength depending on hydrostatic pressure.

Quantum confined exciton states are analyzed from the absorption, luminescence and Raman spectra under hydrostatic pressure in SbI3 and BiI3 crystals. In SbI3, a new transition line series of excitons suddenly appears below the fundamental absorption edge under the pressure higher than ∼1.4 GPa. In BiI3, the similar line series appears even at atmospheric pressure. The energies of the lines converge to the low energy side revealing a feature of the quantum confinement. The pressure induced quantum confined states are discussed in terms of a phase transition of electronic structure due to the molecular rearrangement.

A hot wall technique has been applied to a layered compound material, BiI3, for growth of thin-film single crystals, utilizing van der Waals epitaxy on the selected layered-material substrates of PbI2 and CdI2. The thickness of the films ranged from 100 to 1000 Å. The quality of the films prepared under different growth conditions was evaluated by optical absorption spectra of a band-edge exciton, which turned out to be quite sensitive to the growth conditions of this material. The optimized conditions were established to obtain the best quality thin films, and consequently, it was found that crystal growth took place at very low temperatures around 50∼60°C.

The secondary emission, due to recombination of excitons in the vicinity of the indirect band bottom is investigated, where excitons relax their kinetic energy through multiple scatterings with acoustic phonons and a nonradiative decay process. In this case, the secondary emission can be separated into the two parts: the Raman scattering (RS) and the hot luminescence (HL). The spectral lineshape of the HL directly reflects the exciton distribution near the band bottom. The distribution has been calculated by the Fokker-Planck equation with the source term of exciton supply through the indirect absorption process. It is shown that the excitation spectrum of the RS line and the spectral structure of the HL strongly depend on the two quantities: the kinetic energy of an exciton with the sound velocity and the nonradiative decay rate. General features of RS, HL and their relationship are studied for a system with material constants appropriate to BiI3, since the experiments show that this is an ideal system for the authors' purpose.

For pt.I see ibid., vol.16, p.4719 (1983). Experimental and theoretical studies are presented on the secondary emission (SE) due to recombination of excitons near the indirect band bottom in BiI3. The authors have made systematic measurements to observe the excitation energy, temperature and sample dependences of SE spectra, and obtain a consistent interpretation on these experimental results applying the theory developed in preceding work. The observed SE spectra consist of the Raman scattering (RS) lines, involving resonant higher-order Raman processes, and the hot luminescence (HL) bands in the scheme. Under the excitation near the indirect absorption edge, the spectral structure of SE reflects the relaxation processes of excitons which are governed by the intraband scattering by acoustic phonons and a non-radiative decay due to crystal imperfections. It is shown that the two parameters characterising these two processes are uniquely determined by fitting the calculated intensities of RS lines to the observed ones, and the computed spectral lineshape of HL using these parameter values reproduces experimental results satisfactorily. The relationship between RS and HL intensities is also discussed by comparing the observed and calculated intensity ratios.

Magneto-absorption spectra around the indirect exciton absorption edge of BiI3 single crystals were investigated in pulsed high magnetic fields up to 35T. A remarkable anisotropy of the indirect exciton transition with respect to the magnetic field direction was observed in the spectra. For the first time, another indirect exciton transition induced by magnetic fields was observed. Magnetic field effects on the indirect excitons including the magnetic-field-induced indirect exciton can be interpreted by the cationic exciton model based on the 6s2-6s6p transition in the cation Bi.