The PL quenching phenomena elucidated in this study will give us useful information about the click here dynamics of photo-excited carriers, such as carrier separation and transport, when we apply these Si NDs
to solar cells and high-speed photonic devices. Methods The high-density (7 × 1011 cm−2) Si ND arrays were fabricated from polycrystalline Si thin films deposited on thermally oxidized surfaces of Si substrates under ultra-high vacuum. Bio-nano-templates consisting of ferritin supramolecules containing Fe cores were used to prepare two-dimensional closely packed alignments of the Fe cores as etching masks on the surfaces of Si thin films. The size and interspacing of the Fe cores were intentionally designed by protein engineering for the ferritin supramolecules. The Si NDs were fabricated by forming SiO2 barriers around the Si NDs masked by the Fe cores using the NB etching and subsequent mTOR inhibitor oxidation processes. Details of the fabrication process are described elsewhere [15–17]. The diameter, find more thickness, and interspacing distance of the Si NDs mainly used in this study were designed at 10, 4, and 2 nm, respectively, by the abovementioned
ferritin-protein engineering. The capping and barrier layers of SiO2 were removed with NF3 treatment. Then, a 5-nm-thick SiC layer was finally deposited on the Si ND array under a high vacuum by sputtering. The samples of the Si ND array were placed on a cold finger cooled by a closed He compressor in a vacuum cryostat with quartz windows. The time-resolved PL spectra were observed
at various temperatures by combining the excitation of second harmonic femtosecond pulses with the wavelength of 400 nm, pulse width of 150 fs, and repetition rate of 76 MHz of a mode-locked Ti-sapphire laser, with the detection of a synchroscan streak camera (Hamamatsu Photonics, Hamamatsu, Japan). A spot diameter of the laser light focused on the sample surface was 100 μm. The excitation power density was 8.4 mJ Thalidomide cm−2. The number of electron–hole pair generated per one ND was calculated to be less than 1, taking the sheet density of ND into account. Therefore, the multiple exciton generation or Auger process were not induced. The time width of the instrumental response curve was less than 15 ps, and the time resolution of 5 ps was obtained after deconvolution with the instrumental response. Results and discussion Time-integrated PL spectra of the Si ND array at various temperatures are shown in Figure 1a. PL emission bands with the wavelengths of 655 nm (1.89 eV, E 1 band) and 564 nm (2.22 nm, E 2 band) are visible for the whole temperature range. The observed PL cannot be attributed to the indirect bandgap emission affected by a quantum confinement effect, which was often reported in small Si NCs with diameters of 2 to 5 nm. These confined emission energies increased up to 1.
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