Publications

@article{doi:10.1063/1.4725514,

title = {Superradiance from one-dimensionally aligned ZnO nanorod multiple-quantum-well structures},

author = {Takashi Yatsui and Akira Ishikawa and Kiyoshi Kobayashi and Akira Shojiguchi and Suguru Sangu and Tadashi Kawazoe and Motoichi Ohtsu and Jinkyoung Yoo and Gyu-Chul Yi},

doi = {10.1063/1.4725514},

year  = {2012},

date = {2012-05-01},

journal = {Applied Physics Letters},

volume = {100},

number = {23},

pages = {233118},

abstract = {Using one-dimensionally aligned ZnO nanorod multiple-quantum-well structures (MQWs), we observed a superradiance, i.e., a cooperative spontaneous emission. We confirmed that the excitation power dependence of the emissions from the MQWs originated from the coherent coupling of the QWs due to the well organization at nanoscale. We identified two QWs with cooperative emission. Additionally, we evaluated the number of coherently coupled QWs sets of four that resulted in the superradiance. Our findings provide criteria for designing nanoscale synergetic devices without the use of an external cavity.},

keywords = {First, nanorod, Superradiance, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1063/1.3663632,

title = {Optical near-field induced visible response photoelectrochemical water splitting on nanorod TiO2},

author = {Thu Hac Huong Le and Kazuma Mawatari and Yuriy Pihosh and Tadashi Kawazoe and Takashi Yatsui and Motoichi Ohtsu and Masahiro Tosa and Takehiko Kitamori},

url = {https://doi.org/10.1063/1.3663632},

doi = {10.1063/1.3663632},

year  = {2011},

date = {2011-11-01},

journal = {Applied Physics Letters},

volume = {99},

number = {21},

pages = {213105},

abstract = {Here we report a way to induce the visible response of non-doped TiO2 in the photocatalytic electrochemical water splitting, which is achieved by utilizing the optical near-field (ONF) generated on nanorod TiO2. The visible response is attributed to the ONF-induced phonon-assisted excitation process, in which TiO2 is excited by sub-bandgap photons via phonon energy. Our approach directly gets involved in the excitation process without chemical modification of materials; accordingly it is expected to have few drawbacks on the photocatalytic performance. This study may offer another perspective on the development of solar harvesting materials.},

keywords = {nanorod, Near-field effect, TiO2, Water splitting},

pubstate = {published},

tppubtype = {article}

}

@article{Kitamura_2010,

title = {Site-selective deposition of gold nanoparticles using non-adiabatic reaction induced by optical near-fields},

author = {Kokoro Kitamura and Takashi Yatsui and Tadashi Kawazoe and Masakazu Sugiyama and Motoichi Ohtsu},

doi = {10.1088/0957-4484/21/28/285302},

year  = {2010},

date = {2010-06-01},

journal = {Nanotechnology},

volume = {21},

number = {28},

pages = {285302},

publisher = {IOP Publishing},

abstract = {In this paper, we report on site-selective deposition of metal nanoparticles 

using a non-adiabatic photochemical reaction. Photoreduction of gold 

was performed in a silica gel membrane containing tetrachloroaurate (AuCl4 − ) ions, using ZnO nanorods as the sources of optical near-field light, resulting in deposition 

of gold nanoparticles with an average diameter of 17.7 nm. The distribution of 

distances between the gold nanoparticles and nanorod traces revealed that the gold 

nanoparticles were deposited adjacent to the ZnO nanorods, reflecting the attenuation of 

the optical near-fields in the vicinity of the ZnO nanorods. We found that the 

emission wavelength from the ZnO nanorods was longer than the absorption edge 

wavelength of the tetrachloroaurate. Additionally, from the intensity distribution 

obtained by a finite-difference time-domain method, the gold deposited around the 

ZnO nanorods was found to be due to a non-adiabatic photochemical reaction.},

keywords = {Nanophotonic fabrication, nanorod, Near-field effect, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{2009kitamuraAPB,

title = {Observation of quantum confinement in ZnO nanorods fabricated using a two-temperature growth method},

author = {Kokoro Kitamura and Takashi Yatsui and Motoichi Ohtsu},

doi = {10.1007/s00340-009-3654-1},

year  = {2009},

date = {2009-12-01},

journal = {Applied Physics B},

volume = {97},

number = {4},

pages = {825-828},

publisher = {Springer Nature},

abstract = {We observed a quantum confinement effect in vertically well-aligned ultrafine ZnO nanorods using polarized excitation photoluminescence measurements. Room-temperature and low-temperature photoluminescence spectra revealed that free excitons were confined in the nanorods. The magnitude of the energy shift due to the quantum confinement in the ultrafine ZnO nanorods was 6 meV at room temperature, which corresponded to the luminescence from ZnO nanorods 12.8 nm in diameter. The diameter estimated from the spectra was comparable to the value measured from SEM images.},

keywords = {nanorod, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1063/1.3090491,

title = {Nanophotonic energy up conversion using ZnO nanorod double-quantum-well structures},

author = {Takashi Yatsui and Suguru Sangu and Kiyoshi Kobayashi and Tadashi Kawazoe and Motoichi Ohtsu and Jinkyoung Yoo and Gyu-Chul Yi},

url = {https://doi.org/10.1063/1.3090491},

doi = {10.1063/1.3090491},

year  = {2009},

date = {2009-02-01},

journal = {Applied Physics Letters},

volume = {94},

number = {8},

pages = {083113},

abstract = {We report on near-field spectroscopy of ZnO/ZnMgO nanorod double-quantum-well structures (DQWs) for a nanometer-scale photonic device. We observed energy up conversion assisted by the absorption of phonons generated in the ZnO nanorod DQWs. Theoretical calculation of the coherent excitonic excitation of the population with incoherent phonon coupling agrees well with the obtained experimental power dependence of the up conversion efficiency. It should result in an increase in the efficiency of devices, such as photodetectors, solar cells, and so on.},

keywords = {First, nanorod, Non-uniform optical near field, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{Kitamura_2008,

title = {Optical and Structural Properties of ZnO Nanorods Grown on Polyimide Films},

author = {Kokoro Kitamura and Takashi Yatsui and Motoichi Ohtsu},

doi = {10.1143/apex.1.081202},

year  = {2008},

date = {2008-08-01},

journal = {Applied Physics Express},

volume = {1},

number = {8},

pages = {081202},

publisher = {IOP Publishing},

abstract = {We report the direct growth of vertically well-aligned single-crystal ZnO nanorods on a polyimide film by low-temperature catalyst-free metal organic chemical vapor deposition at 450 °C. The polyimide film maintained its flexibility after the nanorods were grown. The growth direction of the nanorods was along the c-axis. They exhibited excellent photoluminescence characteristics. The spectral full-width at half-maximum measured at room temperature was found to be comparable to that of ZnO nanorods grown on sapphire substrate.},

keywords = {nanorod, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{Kitamura_2008b,

title = {Fabrication of vertically aligned ultrafine ZnO nanorods using metal–organic vapor phase epitaxy with a two-temperature growth method},

author = {Kokoro Kitamura and Takashi Yatsui and Motoichi Ohtsu and Gyu-Chul Yi},

doi = {10.1088/0957-4484/19/17/175305},

year  = {2008},

date = {2008-03-01},

journal = {Nanotechnology},

volume = {19},

number = {17},

pages = {175305},

publisher = {IOP Publishing},

abstract = {We report the fabrication of vertically aligned ultrafine ZnO nanorods using metal–organic 

vapor phase epitaxy and applying a two-temperature growth method. First, thick nanorods 

were grown vertically on the substrate at a lower temperature. Then, ultrafine ZnO 

nanorods with an average diameter of 17.7 nm were grown from the tips of the thick 

nanorods at a higher temperature. The direction of the ultrafine ZnO nanorods followed 

that of the preformed vertically aligned thick nanorods. Electron microscopy revealed that 

the ultrafine nanorods were single crystals and the growth direction was along the 

c 

axis. Excellent photoluminescence characteristics of the nanorods were confirmed.},

keywords = {nanorod, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{Yatsui_2007,

title = {Low-temperature (~270 oC) growth of vertically aligned ZnO nanorods using photoinduced metal organic vapour phase epitaxy},

author = {Takashi Yatsui and Jungshik Lim and Tooru Nakamata and Kokoro Kitamura and Motoichi Ohtsu and Gyu-Chul Yi},

doi = {10.1088/0957-4484/18/6/065606},

year  = {2007},

date = {2007-02-01},

journal = {Nanotechnology},

volume = {18},

number = {6},

pages = {065606},

publisher = {IOP Publishing},

abstract = {We successfully produced a drastic decrease in the required growth temperature 

of single-crystalline ZnO nanorods, and enabled successful growth of vertically 

aligned ZnO nanorods on a Si(100) substrate using photoinduced metal organic 

vapour phase epitaxy (MOVPE). We introduced 325 nm light during the MOVPE 

growth, and achieved vertical growth of single-crystalline ZnO nanorods 

with a hexagonal crystal structure on Si(100) at a growth temperature of 

270 °C. The successful low-temperature growth of ZnO nanorods on the Si(100) substrate 

described here is a promising step toward designing nanoscale photonic and electronic 

devices required by future systems.},

keywords = {First, nanorod, Near-field effect, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{2006yatsuiOptRev,

title = {Evaluating the quantum confinement effect of isolated ZnO nanorod single-quantum-well structures using near-field ultraviolet photoluminescence spectroscopy},

author = {Takashi Yatsui and Motoichi Ohtsu and Sung Jin An and Jinkyoung Yoo and Gyu-Chul Yi},

doi = {10.1007/s10043-006-0218-z},

year  = {2006},

date = {2006-07-01},

journal = {Optical Review},

volume = {13},

number = {4},

pages = {218-221},

publisher = {Springer Nature},

abstract = {Using low-temperature near-field spectroscopy, we obtained spatially and spectrally resolved photoluminescence (PL) images of individual ZnO nanorod single-quantum-well structures (SQWs) with a spatial resolution of 20 nm. We observed the dependence of the quantum confinement effect of the PL peak on the well width (L aw), from which the linewidths of near-field PL spectra of ZnO nanorod SQWs (L aw = 2.5 and 3.75 nm) were determined to be as narrow as 3 meV. However, near-field PL spectra of individual SQWs with L aw = 5.0 nm exhibited two PL peaks, presumably due to strains or defects in the ZnMgO in the nanorod SQWs. Since the exciton in a quantum structure is an ideal two-level system with long coherence times, our results provide criteria for designing nanophotonic devices.},

keywords = {First, nanorod, Near-field spectroscopy, ZnO},

pubstate = {published},

tppubtype = {article}

}

@article{doi:10.1063/1.1776338,

title = {Evaluation of the discrete energy levels of individual ZnO nanorodsingle-quantum-well structures using near-field ultraviolet photoluminescence spectroscopy},

author = {Takashi Yatsui and Jungshik Lim and Motoichi Ohtsu and Sung Jin An and Gyu-Chul Yi},

doi = {10.1063/1.1776338},

year  = {2004},

date = {2004-08-01},

journal = {Applied Physics Letters},

volume = {85},

number = {5},

pages = {727-729},

abstract = {Spatially and spectrally resolved photoluminescence imaging of individual ZnO∕ZnMgO nanorod single-quantum-well structures (SQWs) with a spatial resolution of 55nm was performed using the optical near-field technique with a metallized UV fiber probe. Using excitation power density-dependent photoluminescence spectra of a ZnO∕ZnMgO SQW nanorod, we observed the discrete energy levels in a ZnO quantum-well layer.},

keywords = {First, nanorod, Near-field spectroscopy, QW, ZnO},

pubstate = {published},

tppubtype = {article}

}