Y. Gu

Blue-emitting β-SiC fabricated by annealing C60 coupled on porous silicon

C60 molecules were chemically coupled in the pores of porous Si through a coupling agent and then coated with a layer of Si, and followed by N2 annealing. X-ray diffraction results indicate that the fabricated samples contain β-SiC particles which may exist in the pores, in addition to Si, SiO2, and graphite. The photoluminescence (PL) spectra show an asymmetrical broadband, which can be Gaussian divided into two bands at 380 (3.26 eV) and 454 (2.73 eV) nm. Spectral analyses and the experimental results from infrared spectroscopy and PL excitation measurements suggest that the 380 nm PL band is related to oxygen-vacancy defects in the SiO2 matrix, whereas the blue PL band is closely connected with the β-SiC particles. Our experiments provide a way for fabricating stable blue-emitting β-SiC materials.

Ultraviolet and blue emission from crystalline SiO2 coated with LiNbO3 and LiTaO3

Crystalline SiO2 (α quartz) coated with LiNbO3 (LN) or LiTaO3 (LT) films emits two strengthened luminescence bands at 368 and 468 nm compared with those of pure α-SiO2, when excited with 280 nm light of a Xe lamp. These two bands strongly depend on the coating: the LN-coated α-SiO2 has a much stronger photoluminescence (PL) than the LT-coated α-SiO2. On the other hand, the two bands are different from those of α-SiO2 in that their excitation spectra do not have any noticeable band in the range 200–320 nm. They could be understood based on excitons in the α-SiO2 surface, which are induced by the photorefractive effect of the coated films. The 368 nm PL band is attributed to the optical transitions of the E′ defect pairs and the 468 nm PL band to the radiative recombination of the self-trapped excitons.

Controlling sound transmission with density-near-zero acoustic membrane network

We demonstrate a design of two-dimensional density-near-zero (DNZ) membrane structure to control sound transmission. The membrane structure is theoretically modeled as a network of inductors and capacitors, and the retrieved effective mass density is confirmed to be close to zero at the resonance frequency. This scheme proposes a convenient way to construct the unit cell for achieving DNZ at the designed frequency. Further simulations clearly demonstrate that the membrane-network has the ability to control sound transmission such as achieving cloaking, high transmission through sharp corners, and high-efficient wave splitting. Different from the phononic-crystal-based DNZ materials, the compact DNZ membrane-network is in deep subwavelength scale and provides a strong candidate for acoustic functional devices.

Self-organized growth and optical emission of silicon-based nanoscale β-SiC quantum dots

Si-based β-SiC quantum dots (QDs) were fabricated for exploring efficient blue emission from β-SiC nanostructures. Microstructural observations and x-ray photoemission spectroscopy reveal that the β-SiC QDs with sizes of 5–7 nm are embedded in the SiO2 and graphite matrices, displaying a locally tetragonal symmetry. Photoluminescence spectral examinations show two narrow blue-emitting bands at 417 and 436 nm, which are determined by both quantum confinement and surface structure of the β-SiC QDs. Electron spin resonance investigation demonstrates that the photoexcited carriers partially come from the β-SiC QD core with a widened band gap, whereas the radiative recombination occurs in Si excess defect centers at the β-SiC QD surface. A theoretical calculation about electronic states caused by the vacancy defects in the gap of balls formed with excess Si atoms at the surfaces of the β-SiC QDs supports our assignment to the two blue-emitting origin.

Acoustic planar hyperlens based on anisotropic density-near-zero metamaterials

Based on anisotropic density-near-zero metamaterials, we demonstrate a planar hyperlens with resolution beyond the diffraction limit in both one and two lateral dimensions. In contrast to the cylindrical hyperlens with elliptical dispersions of finite anisotropy, the proposed planar hyperlens is designed with flat near-zero dispersion that supports wave tunneling with extremely high phase velocity for infinite large transverse wave vectors. Therefore, the acoustic evanescent waves immediately concentrate into the designed oblique path till the output surface, leading to a subwavelength resolution. Prototype hyperlens is constructed with a membrane-network by means of equivalent lumped-circuit model, and the subwavelength magnifying performance for a pair of one-dimensional line objects as well as the complex two-dimensional structure is demonstrated. This method provides diverse routes to construct hyperlens operating without the limitation on imaging region in practical applications.

Orange-green emission from porous Si coated with Ge films: The role of Ge-related defects

Photoluminescence (PL) spectra of porous Si (PS) coated with Ge films were examined using the 514.5 nm line of Ar+ laser. A new orange-green PL band, centered at 2.25 eV, was observed with full-width at half-maximum of ∼0.1 eV, in addition to the reported PL bands at 3.1, 2.0, and 1.72 eV. With increasing the thickness of Ge layer coated, the new PL band remains unchanged in peak energy but drops abruptly in intensity. Spectral analysis and some experimental results from Raman scattering and x-ray diffraction indicate that Ge-related defects at the interfaces between PS and the Ge nanocrystals embedded in the pores are responsible for the orange-green PL band.

The observation of the low-frequency acoustic phonon torsional modes in nanocrystalline silicon

Low-frequency Raman spectra of the porous structure formed on C+-implanted silicon were examined using the 514.5 and 488 nm lines of Ar+ laser. The sharp low-frequency Raman peaks were observed with narrow linewidth and large intensity. According to usual vibration theory of the elastic body and related symmetry analysis, they are identified as the localized acoustic phonon torsional modes resulting from the surface vibrations of quasifree Si nanocrystals with nonspherical shapes.

Enhanced and stable photoluminescence from partially oxidized porous Si coated with Si thin films

Photoluminescence (PL) spectra of partially oxidized porous Si (POPS) coated with Si thin films were examined using the 488 nm line of Ar+ laser. The obtained PL is stable, peaks at 1.763 eV with a blueshift of ∼60 meV, and its maximal intensity is seven times larger than that of the POPS. Spectral analysis and the experimental results from infrared spectroscopy and electron spin resonance suggest that the enhanced and stable PL arises from optical transitions in the nonbridging oxygen hole centers (NBOHCs). Si coating mainly leads to introduction of the NBOHCs defects and thus makes the PL intensity enhanced. The blueshift of ∼60 meV is a result of the local equilibrium of NBOHCs defects under high temperature.

On the origin of strong visible photoluminescence in a Ge/porous Si structure

We have studied the origin of strong visible photoluminescence (PL) in a Ge/porous Si (PS) structure in terms of infrared spectroscopy and electron spin resonance (ESR). Spectral analyses indicate that the enhanced PL cannot arise from both the quantum confinement on Ge nanocrystals embedded in the pores and the chemical compound of Ge, O, and H at the surface of the porous Si formed during Ge deposition. The experimental result from ESR strongly suggests that optical transitions in the oxygen-related defect centers (nonbridging oxygen hole centers) at the interface between PS and the Ge layer are responsible for the enhanced PL.

Photoluminescence properties of alternating nanocrystalline silicon/amorphous silicon multilayers

Abstract Photoluminescence (PL) properties of alternating hydrogenated nanocrystalline silicon/amorphous silicon multilayers were investigated at room temperature and low temperature (77 K). The room-temperature PL spectra clearly show two peaks at about 1.54 and 1.7 eV. The ∼1.7 eV peak has no crystallite size dependence of the peak energy. The low-temperature PL spectra can be decomposed into three bands peaked at 1.53, 1.58 and 1.65 eV. The 1.53 eV band is related to the growth process of our samples. The 1.58 eV band is believed to be related to a-Si : H tissue. For the 1.65 eV band, spectral analysis indicates that the radiative recombination of photogenerated carriers in the near-surface region of nanocrystallites is responsible for the visible PL band.