Semiconductor quantum dot (QD) superlattices, which are periodically ordered three-dimensional (3D) array structures of QDs, are expected to exhibit novel photo-optical properties arising from the resonant interactions between adjacent QDs. Since the resonant interactions such as long-range dipole-dipole Coulomb coupling and short-range quantum resonance strongly depend on inter-QD nano space, precise control of the nano space is essential for physical understanding of the superlattice, which includes both of nano and bulk scales. Here, we study the pure quantum resonance in the 3D CdTe QD superlattice deposited by a layer-by-layer assembly of positively charged polyelectrolytes and negatively charged CdTe QDs. From XRD measurements, existence of the periodical ordering of QDs both in the lamination and in-plane directions, that is, the formation of the 3D periodic QD superlattice, was confirmed. The lowest excitation energy decreases exponentially with decreasing the nano space between the CdTe QD layers and also with decreasing the QD size, which is apparently indicative of the quantum resonance between the QDs rather than a dipole-dipole Coulomb coupling. The quantum resonance was also computationally demonstrated and rationalized by the orbital delocalization to neighboring CdTe QDs in the superlattice.
We have fabricated Cu2Sn1-xGexS3 thin-film solar cells by cosputtering deposition of Cu and Sn followed by sulfurization in S and GeS2 vapors. The conversion efficiency was significantly improved to be as high as 6.0% compared with the values of Cu2SnS3 solar cells similarly fabricated. Scanning electron microscopy observation revealed that alloying with Ge accelerated the grain growth during the sulfurization process, contributing to the improvement in the conversion efficiency. The bandgap energy of Cu2Sn0.83Ge0.17S3 was about 1.0 eV, which is suitable for bottom cells used in double-junction solar cells.
We demonstrate that iodine-doping into single-walled carbon nanotubes (SWCNTs) can be effectively done using an electrochemical method. It is shown by in situ Raman measurements that the iodine-doping level can be easily and finely controlled because de-doping is also possible by changing the polarity. In situ synchrotron XRD measurements reveal that iodine molecules are mainly inserted into the hollow core of SWCNTs. The dispersion state of the iodine-doped SWCNTs in water as a function of temperature is also investigated. It is shown that the iodine-doped SWCNTs can be homogeneously dispersed in water at low temperature (ca. <15 °C).
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