S.K. Chang

Self-Assembly and Dynamics of [2]- and [3]Rotaxanes with a Dinuclear Macrocycle Containing Reversible Os−N Coordinate Bonds

With a dinuclear macrocycle 2 that contains weak reversible OsVI-N coordinate bonds, self-assembly and equilibrium dynamics of [2]- and [3]rotaxanes have been investigated. When the macrocycle 2 was mixed together with threads 4a-e, which all contain an adipamide station but different sizes of end groups, [2]pseudorotaxane- and rotaxane-like complexes were immediately formed with large association constants of >7 x 103M(-1) in CDCl3 at 298 K. Exchange dynamics, explored by 2D-EXSY experiments, suggest that assembly and disassembly of complexes occur through two distinct pathways, slipping or clipping, and this depends on the size of the end groups. The slipping pathway is predominant with smaller end groups that give pseudorotaxane-like complexes, while the clipping pathway is observed with larger end groups that yield rotaxane-like complexes. Under the same conditions, exchange barriers (deltaG++) were 14.3 kcalmol(-1) for 4a and 16.7 kcalmol(-1) for 4d, and indicate that the slipping process is at least one order of magnitude faster than the clipping process. Using threads 13a and 13b that contain two adipamide groups, more complicated systems have been investigated in which [2]rotaxane, [3]rotaxane, and free components are in equilibrium. Concentration- and temperature-dependent 1H NMR spectroscopic studies allowed the identification of all possible elements and the determination of their relative distributions in solution. For example, the relative distribution of the free components, [2]rotaxane, and [3]rotaxane are 30, 45, and 25 %, respectively, in a mixture of 2 (2mM) and 13a (2mM) in CDCl3 at 10 degrees C. However, [3]rotaxane exists nearly quantitatively in a mixture of 2 (4 mM) and 13 a (2 mM) in CDCl3 at a low temperature - 10 degrees C.

Exciton transfer processes in ZnSe1-xTex

Abstract Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of ZnSe 1− x Te x crystals has been investigated. At low Te concentration, x = 0.005, a free exciton peak as well as a broad emission band having phonon structure are observed in the PL spectra. The intensity of the free exciton luminescence increases with increasing temperature up to 50 K and above this temperature it decreases, while that of the blue band decreases monotonically. These features are interpreted in terms of the extrinsic self-trapping of excitons induced by Te atoms. In particular, the temperature behavior of the luminescence is explained by the back transfer of excitons from a self-trapped state to a free exciton state. The results are discussed by using configurational coordinate diagrams.

Optical properties of ZnSxSe1−x (x<0.18) random and ordered alloys grown by metalorganic atomic layer epitaxy

ZnS x Se 1-x (x < 0.18) random alloy and ordered alloy were grown on GaAs (001) substrate by metalorganic chemical vapor deposition in ALE mode (MOCVD-ALE) using dimethylzinc, H 2 Se and H 2 S as source materials. In order to investigate the strain effect of ZnS x Se 1 -x epilayers, heavy hole (hhx) and light hole (lhx) exciton peaks in PL spectra were monitored as a function of S composition. The identification of hhx and lhx peaks was confirmed by photoreflectance spectroscopy. The S composition at which lattice matching took place was determined to be 5.6%. The full-width at half-maximum (FWHM) of hhx peak increased and deep-level emission appeared in the PL spectra as the S composition increased. For the ordered alloy, the FWHM of exciton peak was narrower than that for the random alloy and the deep level emission was suppressed. An analysis of broadening of the exciton line show that the optical properties of random alloy are influenced by alloy potential and that the alloy potential fluctuation is significantly constrained in the ordered alloy.

High purity ZnSe epilayers grown by atmospheric double zone metalorganic atomic layer epitaxy

Abstract ZnSe epilayers were grown on semi-insulating GaAs (100) substrates by metalorganic atomic layer epitaxy (MOALE) using dimethylzinc (DMZn) and hydrogen selenide (H 2 Se) source gases with a substrate temperature of 350°C. The MOALE system is modified from atmospheric metalorganic vapor phase epitaxy (MOVPE) by rotating susceptor to allow successive exposure to streams of gases. Also long gas inlets were introduced in the pre-heating zone of the reactor for purifying the source gases. The photoluminescence spectra of the ZnSe epilayers show dominant heavy-hole and light-hole exciton emissions for thicknesses below and above a critical thickness. The epilayer thickness was examined for the different operating cycles and was found to depend only on the number of substrate rotation cycles. These results reflect that the growth of high purity ZnSe epilayers are achieved by MOALE.

Thickness dependent properties of ZnSe on (100) GaAs grown by atomic layer epitaxy

Abstract ZnSe epilayers were grown on semi-insulating GaAs (100) substrates by ALE (atomic layer epitaxy) modified from CVD (chemical vapor deposition). The optical properties of ZnSe films depending on thickness were studied through micro-Raman and PL (photoluminescence) spectroscopy. The critical thickness was determined to be about 0.1 μm by analyzing the change in the peak shift of LO-phonon modes of ZnSe films. This is confirmed from the increase of the intensities of the deep center band in the PL spectra when the thickness exceeds 0.1 μm.

Growth of ZnSe on (100) GaAs by atomic layer epitaxy

Abstract ZnSe films have been grown on (100) oriented GaAs substrates by atomic layer epitaxy (ALE) using ZnCl2 and H2Se as source materials. The growth has been performed by using a hydride vapor phase epitaxial growth system. The epilayer thickness was examined for different growth conditions and was found to depend only on the number of substrate rotation cycles for the growth temperature near 450°C. Pseudomorphic ZnSe films on GaAs substrates were achieved by ALE for the growth of 1000 operating cycles. These films show good surface morphology and good crystallographic properties.

Energy transfer mechanisms in CaAl12O19:Ce0.063+,Eux2+ phosphors

CaAl12O19:Ce0.063+,Eux2+ phosphors were synthesized through a combustion process, and their optical properties were investigated using cathodoluminescence (CL) measurements. The results for the CL spectra showed that different energy transfer mechanisms exist in CaAl12O19:Ce0.063+,Eux2+ and are dependent on the Eux2+ acceptor concentration. As the Eux2+ acceptor concentration increased, the lifetime of the Ce3+ donor decreased, while that of the donor related to the diffusion increased. The dominant energy transfer at low concentrations of the Eux2+ acceptor in CaAl12O19:Ce0.063+,Eux2+ was attributed to the electric dipole–dipole interaction between the Ce3+ donors and the Eu2+ acceptors, resulting from the diffusion limited energy migration. While a similar energy transfer mechanism at the high concentrations of the Eu2+ acceptor in CaAl12O19:Ce0.063+,Eux2+ dominated in the transient state part, a different energy transfer mechanism appeared in the steady state part of the decay process.

Correlation relationship between conductivity and emission luminance intensity in ZnGa2−xInxO4 phosphors for various indium mole fractions

Abstract The correlation relationship between conductivity and luminance intensity in ZnGa 2−x In x O 4 phosphors for various indium mole fractions was investigated. The cathodoluminescence spectra showed that the tuning range of the color emission was between 340 and 420 nm, and the X-ray diffraction curves indicated that ZnGa 2−x In x O 4 with the indium mole fraction between 0 and 0.0006 formed a single phase. The increase of the conductivity in the ZnGa 2−x In x O 4 enhanced the luminance intensity. The present observation provides information on the optical, structural, and electrical properties useful for improving device efficiencies in the field emission displays fabricated utilizing ZnGa 2−x In x O 4 phosphors.

Interference effects in reflectance line shapes from ZnSe/GaAs epilayers

The reflectance spectroscopy is performed at 77 K on ZnSe epilayers of different thicknesses grown on GaAs substrates. The exciton line shapes of the reflectance spectra change dramatically with the epilayer thickness. This observation suggests that the interference effects between the reflected waves from the surface and from the interface play an important role on the exciton line shape of the reflectance spectra. To analyze the change in the exciton line shapes quantitatively, the reflectance spectra were calculated using a simple oscillator model for a dielectric function and considering a multiple reflection. Calculated line shapes of the reflectance spectra show good agreement with the observations.

Influence of lattice relaxation on the properties of ZnSeZnS single quantum wells by MOVPE

Abstract The growth of ZnSe ZnS single quantum well (SQW) structures was performed by atomic layer epitaxy (ALE) in an atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) system. Through the photoluminescence measurement of ZnSe ZnS SQWs, we observed clear shifts of the excitons to higher energies with decreasing well width, which demonstrate effects of strain and quantum confinement. Transition energies were calculated considering (i) a simple square well potential and (ii) a parabolic potential model under the action of strain effects. From comparison of the exciton energy with the calculated transition energies, we conclude that lattice relaxation induced interdiffusion occurs as the well width becomes thicker than the critical thickness.