G. S. Huang

In situ synthesis of Mn-doped ZnO multileg nanostructures and Mn-related Raman vibration

Mn-doped ZnO multileg nanostructures were synthesized via in situ thermal oxidation of Zn and MnO2 powder. Spectroscopic measurements show that Mn ions have been doped into the lattice positions of Zn ions, which strongly induce growth of the observed ZnO multileg nanostructure. It is revealed that the growth mechanism of this kind of multileg ZnO:Mn nanostructure is different from the traditional vapor–solid or vapor–liquid–solid nucleation model of ZnO nanostructures. A possible mechanism is discussed on the basis of the growth process of a tetrapod ZnO nanostructure. Furthermore, we report the observation of an additional Raman peak. This peak is considered to have an origin related to Mn dopant in the ZnO nanostructure. This Raman feature can be regarded as an indicator for the incorporation of Mn ions into the lattice positions of the multileg ZnO nanostructure.

Excitation wavelength dependence of the visible photoluminescence from amorphous ZnO granular films

Amorphous ZnO granular films were fabricated by anodizing zinc sheet in 0.5M oxalic acid solution under direct current voltage. The photoluminescence spectrum of the as-anodized sample shows a very broad visible emission band, which can be Gaussian divided into two subbands at 525 and 600nm. Based on their annealing behavior and the growing mechanism of the ZnO films, the two subbands are attributed to optical transitions in oxygen vacancies and oxygen interstitials, respectively. Obvious redshifts of the two subbands were observed with increasing excitation wavelength. Spectral analyses suggest that the excitation wavelength dependences of the two subbands are due to the quantum confinement on the amorphous ZnO nanoparticles mainly with sizes of ∼10nm. This work provides a good understanding of the photoluminescence behavior of amorphous ZnO particles.

Luminescent silicon carbide nanocrystallites in 3C-SiC∕polystyrene films

We report optical emission of SiC nanocrystallite films, which clearly shows the quantum confinement effect. Bulk polycrystalline 3C-SiC was first electrochemically etched and then the fabricated porous silicon carbide was ultrasonically treated in water or toluene suspension to disperse into colloidal nanoparticles. Transmission electron microscopy images clearly show that the colloidal nanoparticles have 3C-SiC lattice structure with sizes varying from about 6nm down to below 1nm. The suspension of 3C-SiC nanocrystallites exhibits ultrabright emission with wavelengths ranging from 400to520nm when the excitation wavelength varies from 250to480nm, in accordance with the quantum confinement effect. By adding polystyrene to the toluene suspension containing SiC nanoparticles and coating the mixing solution onto a Si wafer, we obtain the SiC∕polystyrene films that luminesce.

Formation, structure, and phonon confinement effect of nanocrystalline Si1-xGex in SiO2-Si-Ge cosputtered films

Using magnetron cosputtering of SiO2, Ge, and Si targets, Si-based SiO2:Ge:Si films were fabricated for exploring the influence of Si target proportion (PSi) and annealing temperature (Ta) on formation, local structure, and phonon properties of nanocrystalline Si1−xGex (nc‐Si1−xGex). At low PSi and Ta higher than 800°C, no nc‐Si1−xGex but a kind of composite nanocrystal consisting of a Ge core, GeSi shell, and amorphous Si outer shell is formed in the SiO2 matrix. At moderate PSi, nc‐Si1−xGex begins to be formed at Ta=800°C and coexists with nc‐Ge at Ta=1100°C. At high PSi, it was disclosed that both optical phonon frequency and lattice spacing of nc‐Si1−xGex increase with raising Ta. The possible origin of this phenomenon is discussed by considering three factors, the phonon confinement, strain effect, and composition variation of nc‐Si1−xGex. This work will be helpful in understanding the growth process of ternary GeSiO films and beneficial to further investigations on optical properties of nc‐Ge1−xSix ...

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.

Low-frequency Raman scattering of Ge and Si nanocrystals in silica matrix

Ge and Si nanocrystals (nc-Ge and nc-Si) with average sizes in the range of 2–7nm, embedded in silica matrix, were fabricated for investigating their acoustic-phonon vibrational properties. The freely elastic sphere theory was found to be unsuitable for explaining the low-frequency phonon vibration character of both nc-Ge and nc-Si in our current experiments. We have assigned the observed low-frequency Raman peaks to “LA-like mode” and “TA-like mode” in terms of their polarization and depolarization behaviors. In addition, it is revealed that the lattice contraction phenomenon exists in nc-Ge and nc-Si with sizes smaller than 4nm, which leads to a contrary effect against matrix traction on the phonon vibrational frequencies.

Optical emission from nano-poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] arrays

Using an anodic alumina membrane with an ordered nanopore array as a template, we have fabricated a two-dimensional light-emitting nanopolymer array by embedding the luminescence polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) into the nanopores. It is revealed experimentally that the conformation of the MEH-PPV chains in the nanopores is in the form of a bunch of polymer chains. The number of the polymer chains in a bunch depends on the concentration of the polymer solution and the diameters of nanopores in the template. Investigation on the photoluminescence spectra of the nano-MEH-PPV array annealed under different temperatures shows that the nanopores can effectively hinder the relaxation of MEH-PPV, which leads to its high thermal stability.

Synthesis of ZnO eggshell-like hollow spheres via thermal evaporation at low temperature

Hollow zinc oxide microspheres have been successfully synthesized on silicon substrate via simple thermal evaporation of pure zinc powder without any catalyst or template material at a relatively low temperature of 550 °C. The morphologies, chemical composition and crystal structure are characterized using x-ray powder diffraction, transmission electron microscopy and scanning electron microscopy. A smooth surface and rough ones with many nanopores are observed. The difference in the structures of the shells is discussed and a possible growth mechanism is proposed in detail. The room-temperature photoluminescence spectrum reveals a peak at 380 nm corresponding to free exciton emission and a strong green emission at ~ 525 nm associated with defect-related emission.

Fabrication and optical properties of C/β-SiC/Si hybrid rolled-up microtubes

C/β-SiC/Si hybrid microtubes have been fabricated by releasing prestressed C/Si bilayer structures and treating with a postannealing process. Detailed characterization reveals the synthesis of β-SiC via a solid phase reaction at the C/Si interface. Remarkably, the production of β-SiC is promoted in the tube wall by rolled-up bonding of adjacent windings, which increases the area of the C/Si interface by a factor of 2. The Raman spectra acquired from the hybrid microtubes disclose peaks pertaining to the optical phonon modes of β-SiC that exhibit obvious downshifts due to surface effects on the SiC nanoparticles. Moreover, two light emission bands are detected from a hybrid microtube and their origin is discussed based on spectral analyses.

Ordered amorphous silicon nanoisland arrays and reflection spectral dependence on nanoisland geometrical parameters

We report an anodic porous alumina mask technique for fabricating orderly hemispherical silicon nanoisland arrays with island diameters of 30–120 nm. Reflection spectral measurements show that the phase transition of incident light can periodically be transferred with changes in geometrical parameters of the ordered arrays. Thus the redshift of the reflection peak with decreasing both sizes of the nanoislands and incident angle of light wave can be explained well on the basis of a modified rigorous vector coupled-wave theory. The simple fabrication technique and reflection spectral redshift of the ordered arrays have potential applications in optical devices and solar cells.