Katsuhiro Nose

Size dependent optical band gap of ternary I-III-VI2 semiconductor nanocrystals

The size dependent optical band gap of the less-toxic ternary I-III-VI2 chalcopyrite-type semiconductor quantum dots (QDs), CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInSe2, AgGaS2, and AgGaSe2, were evaluated using the finite-depth-well effective mass approximation calculation. From the comparison of the calculation result with the experimental values for the CuInS2 case, it was shown that the calculation was highly valid to predict the size dependent optical gap of the ternary semiconductor QDs. The optical band gap of the above seven I-III-VI2 QDs covers a wide wavelength range from the near-infrared to ultraviolet. It has been shown that the I-III-VI2 semiconductor QDs have a significant potential as alternatives to the highly toxic cadmium-containing II-VI semiconductor QDs and they are applicable to the wide range of light emitting devices and solar cells.

Electronic transition responsible for size-dependent photoluminescence of colloidal CuInS2 quantum dots

Colloidal CuInS2 quantum dots with an average size of 2.9–4.1 nm were synthesized by the one-pot heating of an organometallic solution. The size-dependent optical gaps and photoluminescence (PL) emission energies were analyzed based on the finite-depth well effective mass approximation. The PL emission was attributed to the recombination of electrons in the donor level and holes in the quantized hole state. Indium atom defects at copper sites or sulfur vacancies were proposed to be the defects acting as donors at ∼0.1 eV below the conduction band.

Wide band gap semiconductor alloy: x(LiGaO2)1∕2–(1−x)ZnO

Oxide semiconductor alloys of x(LiGaO2)1∕2–(1−x)ZnO were synthesized by a standard solid state reaction. The wurtzite-type single phases were obtained in the wide composition range of x⩽0.38 because the β‐LiGaO2 possesses a wurtzite-derived structure and approximately the same lattice constants with ZnO. The resulting alloys showed characteristics of wide band gap oxide n-type semiconductor; the optical transparency reached over to the ultraviolet (UV) light. The electrical conductivity and energy band gap for 0.38(LiGaO2)1∕2–0.62ZnO were 8.2Ω−1cm−1 and 3.7eV, respectively, at room temperature. The alloy is a good candidate for the barrier material of the ZnO-based heterostructures and UV-transparent electrodes.

Photoluminescence of CuInS2-based semiconductor quantum dots; Its origin and the effect of ZnS coating

The origin of the size-dependent photoluminescence (PL) of CuInS2-ZnS alloyed quantum dots and the effect of a ZnS thin layer coating on PL properties were studied. The PL emission band shifted to the shorter wavelength region with a decrease in the crystal size. A time-resolved measurement indicated that the PL lifetime for the lower energy component of the emission band was longer than that of the higher energy component. Although this is a characteristic feature of the donor-acceptor pair (DAP) recombination, the size dependent shift of the band was too large to be attributed to DAP recombination. The emission was attributed to an electronic transition from the quantum confined conduction band to an acceptor level. By coating with a ZnS thin layer the PL quantum yield increased up to 31%.

Zn2LiGaO4, Wurtzite-Derived Wide Band Gap Oxide

The phase that appeared in the pseudo-binary LiGaO2–ZnO system was investigated by powder X-ray diffraction (XRD), selected area electron diffraction by transmission electron microscopy (TEM-SAD) and Raman spectroscopy, especially focusing on the 0.5(LiGaO2)1/20.5ZnO composition. A new quaternary wurtzite-derived Zn2LiGaO4 phase was found in this system. The TEM-SAD indicated that the phase possesses an incommensurately modulated ordering. The optical energy band gap of Zn2LiGaO4 was determined to be ~4.0 eV from its diffuse reflectance and photoluminescence spectra; and it was suggested that the Zn2LiGaO4 was a direct semiconductor.

UV luminescent organic-capped ZnO quantum dots synthesized by alkoxide hydrolysis with dilute water

A novel synthesis route to organic-capped and colloidal ZnO quantum dots (QDs) has been developed. Specifically, zinc-di-t-butoxide and zinc-di-n-butoxide are hydrolyzed by very dilute water (400-600 mass ppm) in hydrophilic benzylamine and polymerized to ZnO by dehydration and/or a butanol elimination reaction. Growth of the ZnO QDs and exchange of the surface capping ligand from the hydroxyl groups and/or benzylamine to the oleylamine occur by heating the colloidal solution after addition of the oleylamine at 100-180°C. The final ZnO QDs with diameters in the range of 3-7 nm are highly dispersible in various organic solvents. The ZnO QDs exhibit the quantum size effect upon UV emission; it was controlled between 3.39 and 3.54 eV in the present study. The defect-related Vis emission decreased and the UV emission becomes dominant when zinc-di-n-butoxide with a 99.99% zinc purity is used as the starting material. The intensity of the photoluminescence UV emission is 1.5 times higher than that of the Vis emission.