We have investigated GaAs-based p-i-n quantum dot solar cells (QDSCs) with 10 up to 20 stacked layers of self-assembled InAs quantum dots (QDs) grown by atomic hydrogen-assisted molecular beam epitaxy. The net average lattice strain was minimized by using the strain-compensation technique, in which GaNAs dilute nitrides were used as spacer layers. The filtered short-circuit current density beyond GaAs bandedge was 2.47 mA/cm2 for strain-compensated QDSC with 20 stacks of InAs QD layers, which was four times higher than that for strained QDSC with identical cell structure.

We have developed a technique to fabricate quantum dot (QD) solar cells with direct doping of Si into InAs QDs in GaNAs strain-compensating matrix in order to control the quasi-Fermi level of intermediate QD states. The Si atoms were evenly incorporated into QDs during the assembling stage of growth such that a uniform array of partially filled QDs has been obtained. Nonradiative recombination losses were also reduced by Si doping and a photocurrent increase due to two-step photon absorption was clearly measured at room temperature detected under filtered air-mass 1.5 solar spectrum.

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Time-resolved tunneling current measurement in the subpicosecond range was realized by ultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pair method. A low-temperature-grown GaN x As 1−x ͑x = 0.36% ͒ sample exhibited two ultrafast transient processes in the time-resolved tunnel current signal, whose lifetimes were determined to be 0.653± 0.025 and 55. Smaller and faster are the key words in the progress of current nanoscience and technology. Thus, for further advances, a method of exploring the ultrafast transient dynamics of the local quantum functions in organized small structures is eagerly desired. Ultrashort optical pulse technology in the near-infrared to ultraviolet region has allowed us to observe transient phenomena in the femtosecond range, the optical-monocycle region, 1,2 which, however, has a drawback of a relatively low spatial resolution due to electromagnetic wavelength. On the other hand, scanning tunneling microscopy (STM), although its time resolution is limited by circuit bandwidth ͑ϳ100 kHz͒, enables us to observe spatial dynamics at the atomic level in real space. 3 Therefore, the integration of ultrashort optical technology with STM has been one of the most exciting goals since their invention. [4][5][6][7] Pioneering works were performed by Hamers et al., 4-7 which have attracted the extensive interest of researchers in various fields. However, there remain critical problems which have prevented the achievement of the laser-combined STM measurement, such as the displacement current due to the stray capacitance of the tunneling gap and photoelectrons produced by multiple photoabsorption. [4][5][6][7] In such cases, since a large area is included in the processes, the superior space resolution of STM cannot be utilized. In particular, the thermal expansion of the STM tip by photoillumination causes much large noise in the tunneling current, making the measurement difficult.Here, we show the results of the time-resolved tunneling current measurement in the subpicosecond range, which can advance the development of future research in terms of ultimate temporal and spatial resolutions.A schematic of the measurement system is shown in Fig. 1. We adopted the recently developed shaken-pulse-pairexcited STM (SPPX-STM) method, which realizes highly sensitive measurement free from the thermal expansion effect of the tip and sample. 8 The tunneling junction is directly illuminated by a sequence of laser pulse pairs and average tunneling current, I t ͑t d ͒, is measured as a function of the delay time between the two pulses, t d . To decrease broadband noise, the delay time of the two pulses t d is modulated with a small amplitude ⌬t d at frequency , and the tunneling current is detected by a lock-in amplifier. Since the tunneling current I t responds to the modulation as In the pulse-pair-excited STM measurement, the first laser pulse in each pulse pair acts as the pump pulse to excite and modulate the electronic structure of the sample surface, which might cause d...

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Studies of spin dynamics in low-dimensional systems are important from both fundamental and practical points of view. Spin-polarized scanning tunnelling microscopy allows localized spin dynamics to be characterized and plays important roles in nanoscale science and technology. However, nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be pursued to advance further the investigation of quantum dynamics in functional structures of small systems. Here, we demonstrate the optical pump-probe scanning tunnelling microscopy technique, which enables the nanoscale probing of spin dynamics with the temporal resolution corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was observed with a spatial resolution of ∼ 1 nm. In addition to spin relaxation dynamics, spin precession, which provides an estimation of the Landé g factor, was observed successfully.

We have fabricated and compared the performance of GaAs-based p-i-n quantum dot solar cells with ten multilayer stacked structures of self-assembled InAs quantum dots embedded with GaNxAs1−x strain-compensating spacer layers. Reducing the thickness of the spacer layer, and hence increasing the nitrogen composition in GaNxAs1−x, from 40 nm (x=0.5%) to 15 nm (x=1.5%) thereby fulfilling the net strain-balanced condition, resulted in a steady increase in the short-circuit density, while a decreasing trend for the open-circuit voltage was observed. The observed results can be interpreted in terms of the difference in the quantum confinement structure.

Acyclic diastereoisomers, 1-deoxy-1-(N-acetyl-α-methylbenzylamino)alditol acetates, are readily obtained by reductive amination of sugars with chiral α-methylbenzylamine in the presence of sodium cyanoborohydride. Ten pairs of enantiomers of common monosaccharides are resolved by an adsorption HPLC on the diastereoisomers.