Nanocrystals Size-Tunable UV-Luminescent Silicon Nanocrystals** Naoto Shirahata, * Tsuyoshi Hasegawa, Yoshio Sakka, and Tohru Tsuruoka Colloidal semiconductor quantum dots (QDs) are frequently called ‘‘artificial atoms’’, and are undoubtedly one of the prominent light sources. In a colloidal QD, the electronic motion is confined in all spatial directions to give a discrete energy level. Hence, the precise size control of the QDs leads to highly efficient light emission with a narrow full width at half maximum (FWHM). As well as visible-light sources, light emitters in the ultraviolet (UV) region have recently been attracting research attention owing to their potential optics applications, including compact UV lasers, laser diodes, convenient light sources suitable for white-light illumination and biomedical tagging, and photodetectors. [1] For example, the shortening of the emission wavelength means that the light is focused more sharply, thus leading to its application in high- density information storage devices such as optical and magnetic discs. A near-UV laser is also suitable for medical care of heat-labile areas in the human body, for example the cornea, because, unlike IR light, such a short wavelength does not produce heat radiation. However, current rapid develop- ments of these devices have been entirely focused on enhancing the solid-state emitters. Unlike such semiconductor com- pounds, soft materials, for example organic phosphor mole- cules, provide many advantages: mechanical flexibility, light weight, excellent processability, compactness, and cost effectiveness. Due to limited types of p-conjugated molecules and their inherent photoinstability, [2] the development of UV- emitting soft materials has not yet been pursued aggressively. Bulk silicon (Si), a platform for large-scale integrated circuits, has a significantly poorer optical performance due to its indirect bandgap character. The discovery of red photolumi- nescence (PL) from porous Si with a 1% quantum yield (QY), which is 10 000 times higher than that of bulk Si, has therefore created an opportunity for its optical functionality, and opened a long-running debate on its PL origin at the same time. [3] Modification of the nonoxidized structure of crystalline Si such that its three-dimensional physical size does not exceed the bulk exciton Bohr radius for the crystal (5 nm) leads to the appearance of the effect of quantum confinement (QC) imposed upon the charge carriers, which results in widening of the bandgap from 1.14 eV of the bulk to 3.26 eV (380 nm). [4] Some of theoretical calculations have highlighted similar remarkable changes in the optical transition for other nonoxidized configurations, such as clusters and amorphous structures. [5] Although the question of the PL origin has not yet been satisfactorily answered, we can see a consensus that the QC effect of Si creates discrete sizes with distinct fluorescence emission wavelengths in the visible region, and enhances the light-emission efficiencies. A 1–7% QY has been estimated from nonoxidized nanoparticles (NPs) due to hydride passiva- tion. [6,7] Unlike such passivated surfaces, the optical transition in NPs covered with silica is dominated by defect states at the Si/ silica interface, but the presence of the oxide shell leads to improvements in emission efficiency (0.5  QY  15%). [8] QYs of more than 10% have also been estimated from organically capped NPs. [9] To date, the feasibility of UV-light emission from Si has been hinted at in a few literature reports. In concrete terms, Wilcoxon et al. produced a continuous size distribution of hydrogen-passivated nanoclusters (d  5 nm), which give inhomogeneously broadened PL spectra across the wide wavelength range of 300–700 nm when excited at 256 nm. [7] Kauzlarich et al. reported UV–blue PL spectra centered between 360 and 390 nm from alkyl-terminated nanoclusters of diameter d ¼ 1–5 nm. [10] Tilley et al. synthesized blue PL NPs (d ¼ 1.8  0.2 nm), which have an emission maximum at 335 nm when excited at 290 nm. [11] More recently, we described a broad PL signal evolving between 300 and 650 nm from diamond- structured Si nanocrystals (NCs) passivated with hydrocarbon chains. [12] These studies suggest the potential use of nano- structured Si in UV-light sources, but it is still a challenging theme from the viewpoint of the precise control of such a unique [  ] Dr. N. Shirahata National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) and PRESTO, Japan Science and Technology Agency (JST) 4-1-8 Honcho Kawaguchi, Saitama 332-0012 (Japan) E-mail: SHIRAHATA.naoto@nims.go.jp Dr. T. Tsuruoka, Dr. T. Hasegawa World Premier International Research Center Initiative for Materials Nanoarchitronics, NIMS 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) and CREST, JST 4-1-8 Honcho Kawaguchi, Saitama 332-0012 (Japan) Dr. Y. Sakka NIMS 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) [  ] We thank the Bio-Organic Materials Facility, Nanotechnology Inno- vation Center, for the use of facilities. This work was supported in part by the JST PRESTO program and a Grant-in-Aid for Young Scientists from the MEXT, Japan. : Supporting Information is available on the WWW under http:// www.small-journal.com or from the author. DOI: 10.1002/smll.200902236 small 2010, 6, No. 8, 915–921 ß 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 915