H. T. Chen

Intrinsic Dipole-Field-Driven Mesoscale Crystallization of Core−Shell ZnO Mesocrystal Microspheres

Novel uniform-sized, core-shell ZnO mesocrystal microspheres have been synthesized on a large scale using a facile one-pot hydrothermal method in the presence of the water-soluble polymer poly(sodium 4-styrenesulfonate). The mesocrystal forms via a nonclassical crystallization process. The intrinsic dipole field introduced by the nanoplatelets as a result of selective adsorption of the polyelectrolyte on some polar surfaces of the nanoparticles acts as the driving force. In addition, it plays an important role throughout the mesoscale assembly process from the creation of the bimesocrystalline core to the apple-like structure and finally the microsphere. Our calculation based on a dipole model confirms the dipole-field-driven mechanism forming the apple-like structure.

Tin Oxide Nanoribbons with Vacancy Structures in Luminescence-Sensitive Oxygen Sensing

Vacancy structures in tin oxide nanoribbons fabricated via thermal evaporation and post-processing are probed by luminescence spectroscopy, and interesting properties that bode well for oxygen sensing are observed. Besides a broad 620-nm band, the fabricated tin oxide nanoribbons show a photoluminescence band at 480 nm when the measurement temperature is <100 K. The blue band appears from nanoribbons synthesized under high oxygen pressure or annealed under oxygen. The dependence suggests that the oxygen interstitial and vacancy densities determine the electronic states that produce the blue band. Calculation of the electron structures based on the density functional theory shows that decreased oxygen vacancies or increased oxygen interstitials enhance the 480-nm band but suppress the 620-nm band. The results reported here indicate that the tin oxide nanoribbons with vacancy structures have potential applications in luminescence-sensitive oxygen sensing.

Anodic alumina template on Au/Si substrate and preparation of CdS nanowires

Abstract A layer of thin gold film was sandwiched between a silicon substrate and an Al film to form the Al/Au/Si structure. Subsequent anodization leads to formation of a Si-based anodic aluminum oxide (AAO) template (AAO/Au/Si structure) with ordered nanopores. This kind of template has unique electrodeposition properties and can bond well with the deposited materials. The anodic process of the Al/Au/Si structure was investigated in detail by in situ monitoring the current–time curve. As an application, CdS nanowires were fabricated on the silicon substrate using this kind of AAO templates. Light-emitting property from the CdS nanowires was observed. This kind of Si-based light-emitting nanowires are expected to have practical applications in optoelectronic integration.

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.

ZnO nanoparticles prepared by thermal decomposition of β-cyclodextrin coated zinc acetate

Abstract Uniform ZnO nanoparticles have been prepared via a convenient thermal decomposition approach, in which β-cyclodextrin (β-CD) is selected to coat the precursor of zinc acetate. The decomposition process of this system is investigated by thermogravimetric and differential thermal analysis (TG–DTA). TEM or AFM studies reveal that ZnO nanoparticles and the corresponding film doped on the silicon substrate by this method present weak agglomeration and regular size distribution. The possible formation mechanism of ZnO nanoparticles under the effects of β-CD coating is also discussed.

Photoluminescence spectra of C60 molecules embedded in porous Si

The photoluminescence (PL) spectra of C60 molecules embedded in porous Si through both physical deposition and chemical coupling were measured. In addition to the PL peak of porous Si, a peak at 730 nm caused by perfect C60 molecules and other peaks at 620 and 630 nm caused by imperfect C60 molecules were observed. The peak at 620 nm measured in the sample with physically deposited C60 is induced by C60 adsorbed on the Si atoms of the pore wall, while the peak at 630 nm measured in the sample with chemically coupled C60 molecules is caused by the coupled C60 molecules. At room temperature, the PL intensity of C60 embedded in the porous Si is obviously enhanced, and the transfer of carriers from porous Si grains into adjacent C60 is considered to be responsible for the PL enhancement.

Oxygen-related surface states and their role in photoluminescence from porous Si

Abstract During spontaneous oxidation and NH4OH treatment the photoluminescence spectra of porous Si shift to their respective pinning wavelength ranges defined by the surface states associated with the structural groups –O2SiH and –O3SiH. It is proposed that the emission energy is defined by the difference between band-edge and surface states and thus sensitive to both porosity of porous Si and post processing.

Semiconducting Ge–Si–Fe alloy grown on Si(100) substrate by reactive deposition epitaxy

In this letter, we report a semiconducting Ge–Si–Fe alloy thin film grown on Si(100) by reactive deposition epitaxy using high vacuum evaporation technique. This work is based on the idea that the band structure of β‐FeSi2 will be changed with part of the Si atoms in the lattice replaced by Ge atoms. An iron film was first deposited on a SiGe/Si(100) structure, then the alloy was formed during an annealing process. Auger electron spectroscopy and x‐ray diffraction results indicate that the new alloy film can be regarded as a distorted β‐FeSi2 thin film with the participation of Ge. The direct band gap of the Ge–Si–Fe alloy is determined to be 0.83 eV by optical transmission measurements, which indicate a redshift of the band gap with regard to that of β‐FeSi2 (Eg=0.87 eV) thin films.

Structural characterization of Ta/Al multilayer films

Ta/Al multilayer films with modulation wavelength 4.6 and 9.5 nm were fabricated by magnetron sputtering. The structure of the multilayer was studied by x‐ray diffraction. The combination of Ta and Al forms a metallic superlattice with Ta(110) and Al(111) textures. The x‐ray diffraction patterns were simulated using the model for the modulated composition multilayer. The coherence length perpendicular to the film is about 100 nm. The composition modulation is approximately a trapezoidal wave. The thickness of the transition layer is about 1.00 nm for the multilayer with a modulation wavelength of 4.6 nm. The fluctuation of atomic planes in each period is about 5%.

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.