Y. M. Yang

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.

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 ...

Origin of the 370-nm luminescence in Si oxide nanostructures

The nature of the ∼370-nm (3.35-eV) photoluminescence (PL) in Si oxide and nanostructures, which has a PL excitation band at ∼260nm, is studied experimentally and theoretically. It is revealed that the PL occurs at the interface between Si structure and its oxide and is closely associated with a characteristic infrared absorption band at ∼1250cm−1. Spectral analyses suggest that the ∼370-nm PL originates in the –SiO3 group, which bonds to Si structural surface. The density functional theory calculations are consistent with our experiments. This work clarifies some controversies regarding the ∼370-nm PL mechanisms in a number of Si oxide and nanostructures.

Formation mechanism of alumina nanotubes and nanowires from highly ordered porous anodic alumina template

Alumina nanotube (ANT) and nanowire (ANW) arrays were produced from highly ordered porous anodic alumina (PAA) by etching in phosphoric acid and subsequent thermal treatment. The domain structures that have small sizes provide more boundaries and break the hexagonal symmetry of the PAA template, thereby inducing star-like splitting modes. Using very thin PAA films, ANTs and their arrays are fabricated, whereas ANWs and their arrays are produced when using thick films. This is due to the thin wall between nanopores. Our results provide some mechanistic and fundamental information pertaining to the fabrication of nanostructures via a PAA template.

High piezoelectricity of Pb(Zr,Ti)O3-based ternary compound thin films on silicon substrates

Pb(Zr,Ti)O3 (PZT)-based ternary compound thin films, 0.06PMnN-0.94PZT(50/50) (PMnN-PZT), are deposited on Si-based heterostructures by rf magnetron sputtering system. The intrinsic PZT(50/50) thin films are also deposited on the same kind of substrates for comparison. The PMnN-PZT thin films show the similar polycrystalline structures as those of PZT with highly (111) oriented perovskite phase. The PMnN-PZT thin films show excellent piezoelectricity and ferroelectricity which are distinctly better than those of PZT thin films prepared with the same deposition conditions. Besides, the cantilevers of PMnN-PZT thin films on the heterostructure substrates also exhibit higher sensitivities than the PZT thin film cantilevers.

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.

Light-emitting mechanism conversion in C60-coupled porous Si systems

Abstract We reveal two kinds of light-emitting origins in two typical porous Si (PS) samples by carefully examining the photoluminescence (PL) and PL excitation (PLE) spectra. The interfacial oxygen-related defect states and SiO binding states at the surfaces of the two samples are considered to be responsible for the two emissions, which is useful in clarifying some problems in controversy related to the emission mechanisms of PS. After the two kinds of PS samples were coupled with C 60 molecules, the PL from the surface states is replaced by the emission associated with the oxygen-related defects. The coupling process plays an important role in conversion of the light-emitting mechanism. We suggest that this PL conversion is due to destruction of original SiO binding states caused by C 60 coupling and therefore the PL from the interfacial defect states becomes main emission process.

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.

Size-independent low-frequency Raman scattering in Ge-nanocrystal-embedded SiO2 films.

The peak position and linewidth of the low-frequency Raman mode observed from amorphous silica films embedded with Ge nanocrystals doped with Si show a size-independent behavior. Spectral analysis reveals the formation of a thin amorphous GeSi layer on the surface of the Ge nanocrystal. Theoretical calculation based on a modified three-region model discloses that the acoustic impedance of the interfacial GeSiO layer is responsible for the size-independent behavior. During high-temperature annealing, Ge atoms are segregated from the interface into the core, and the GeSiO interface layer is converted to SiO(2), leading to disappearance of the size-independent vibration mode.

Local vibration at the surface of a Ge nanocrystal embedded in a silicon oxide matrix

A low-frequency Raman vibration mode, whose peak position and linewidth are independent of the sizes of Ge nanocrystals and the polarization configuration of incident excitation light, was observed in silicon oxide films embedded with Ge nanocrystals which were prepared using magnetron cosputtering of SiO2–Ge–Si targets. The peak position of the Raman mode is sensitive to the content of Si in the matrix. After the sample is annealed above a special temperature that increases with the content of Si, the Raman mode disappears. Microstructural observations and spectral analyses disclose that this low-frequency Raman mode arises from a local structure which is positioned at the surfaces of Ge nanocrystals and consists of Ge, Si, and O atoms. High-temperature annealing leads to the removal of Ge atoms from the local structure. As a result, the local vibration mode vanishes.