We examined the formation mechanism of active sites on Cu/ZrO2 specific toward CO2-to-methanol hydrogenation. The active sites on Cu/a-ZrO2 (a-: amorphous) were more suitable for CO2-to-methanol hydrogenation than those on Cu/t-ZrO2 (t-: tetragonal) and Cu/m-ZrO2 (m-: monoclinic). When a-ZrO2 was impregnated with a Cu­(NO3)2·3H2O solution and then calcined under air, most of the Cu species entered a-ZrO2, leading to the formation of a Cu–Zr mixed oxide (Cu a Zr1‑a O b ). The H2 reduction of the thus-formed Cu a Zr1‑a O b led to the formation of Cu nanoparticles on a-ZrO2, which can be dedicated to CO2-to-methanol hydrogenation. We concluded that the selective synthesis of Cu a Zr1‑a O b , especially amorphous Cu a Zr1‑a O b , is a key feature of the catalyst preparation. The preparation conditions of the amorphous Cu a Zr1‑a O b specific toward CO2-to-methanol hydrogenation is as follows: (i) Cu­(NO3)2·3H2O/a-ZrO2 is calcined at low temperature (350 °C in this study) and (ii) the Cu loading is low (6 and 8 wt % in this study). Via these preparation conditions, the characteristics of a-ZrO2 for the catalysts remained unchanged during the reaction at 230 °C. The latter preparation condition is related to the solubility limit of Cu species in a-ZrO2. Accordingly, we obtained the amorphous Cu a Zr1‑a O b without forming crystalline CuO particles.

Efficient conversion of CO2 into useful chemicals, exemplified by the development of methods for low-temperature hydrogenation of CO2 to form methanol (CH3OH), is a highly attractive research target. Herein, we report that Pt nanoparticles, loaded on MoO x /TiO2 as a support (Pt(3)/MoO x (30)/TiO2; Pt 3 wt %, MoO3 30 wt %), promote selective hydrogenation of CO2 to produce CH3OH in 73% yield under mild conditions (T = 150 °C; t = 48 h; p CO2 = 1 MPa; p H2 = 5 MPa). It is significant that the observed yield is almost the equilibrium yield of CH3OH expected under the current reaction conditions. In terms of both the yield and selectivity for CH3OH production, the performance of Pt(3)/MoO x (30)/TiO2 is better than that of other metal catalysts supported on MoO x (30)/TiO2 and of Pt catalysts on other supports. Moreover, the results of an investigation of the reaction mechanism using in situ X-ray absorption fine structure (XAFS) suggest that reduced MoO x species are responsible for the progress of this efficient reaction.

Optically engineered ultrafast pulses for controlled rotations of exciton qubits in semiconductor quantum dots J. Appl. Phys. 112, 014313 (2012) Hot electron extraction from CdTe quantum dots via beta carotene molecular energy levels Appl. Phys. Lett. 100, 261110 (2012) Effect of proton bombardment on InAs dots and wetting layer in laser structures Appl. Phys. Lett. 100, 261105 (2012) Enhancement of the Purcell effect for colloidal CdSe/ZnS quantum dots coupled to silver nanowires by a metallic tip Appl. Phys. Lett. 100, 253110 (2012) Additional information on J. Appl. Phys. The control of the emission energy from self-assembled InAs quantum dots has been demonstrated by using indium flush. The low-temperature indium-flush method was found to control the emission energy preserving the high structural uniformity attributed to the slow dot growth. In the standard indium-flush method, where the substrate temperature was raised up from the dot-growth temperature, blueshift larger than the shift by the low-temperature indium flush was observed and was explained reasonably by the enhanced In/Ga-interdiffusion. Also, the effect of AlGaAs capping layer before the indium-flush step was studied.

We have investigated the influence of postgrowth thermal annealing on the optical properties of InAs/GaAs self-assembled quantum dots. Upon annealing, the emission from InAs QDs shifts toward higher energies with a narrowing of the photoluminescence spectra of QD ensemble. The time-integrated PL spectra show also the nonradiative recombination rate does not increase by annealing process. In addition, from the timeresolved PL, the exciton radiative recombination rate increases with increasing the annealing temperature and agrees well with the atomic-like radiative recombination. These results indicate that interdiffusion of Ga and In atoms into and out of the QDs leads to an increase of the average dot size and concurrently a decrease in the confinement potentials of the QDs preserving their optical quality and zero-dimensional density of states after interdiffusion in the studied annealing temperature range.Semiconductor self-assembled quantum dots (QDs) exhibit a variety of confinement-related optical and electronic properties useful for opto-electronic device applications such as QD lasers and detectors. In particular, broad efforts are currently underway to develop new techniques for optical controlling spin degrees of freedom in QDs [1]. Some proposals rely upon the energy selectivity in the optical excitation processes of spins and therefore the precise control of electronic or excitonic energies in a QD is required. Standing on this perspective, until now we have investigated the self-assembled InAlAs/AlGaAs quantum dots (QDs) in terms of the carrier spin depolarization via tunnelling [2], the exciton spin relaxation [3], and the nuclear spin polarization [4,5] via the photoluminescence (PL) from ensemble QDs and a single QD by using various techniques. InAlAs QDs have the emissions from the exciton ground state in ∼780-nm region where the detectors with high sensitivity are available. This is a great advantage for the single QD spectroscopy, however, the fine control of the exciton energy levels is found to be difficult with retaining their optical quality, in particular, for the coupled QDs. In order to perform more sophisticated experiments with the coupled QDs, both high sample quality and high detection sensitivity are necessary. It is therefore the purpose of this work to investigate the effect of thermal annealing for the self-assembled InAs/GaAs QDs where the various growth techniques are well stored. In InAs/GaAs self-assembled QDs [6,7], several control techniques of the size and exciton energy levels of QDs have been established such as Indium-flush [8,9], atomic force microscopy oxidation [10], rapid thermal annealing [11][12][13][14][15], and closely-stacking [16].The InAs QDs were grown on a GaAs (120 nm)/AlAs(2 nm)/GaAs (300 nm) buffer layer on semiinsulating (001) GaAs substrates by a molecular beam epitaxy by using a Riber-MBE32P. The growth temperature was 620• C for the buffer layer and 475• C for the InAs QDs. The substrate temperature was lowered gradually during the growth of the buffe...