Jifa Qi

Viral assembly of oriented quantum dot nanowires

The highly organized structure of M13 bacteriophage was used as an evolved biological template for the nucleation and orientation of semiconductor nanowires. To create this organized template, peptides were selected by using a pIII phage display library for their ability to nucleate ZnS or CdS nanocrystals. The successful peptides were expressed as pVIII fusion proteins into the crystalline capsid of the virus. The engineered viruses were exposed to semiconductor precursor solutions, and the resultant nanocrystals that were templated along the viruses to form nanowires were extensively characterized by using high-resolution analytical electron microscopy and photoluminescence. ZnS nanocrystals were well crystallized on the viral capsid in a hexagonal wurtzite or a cubic zinc blende structure, depending on the peptide expressed on the viral capsid. Electron diffraction patterns showed single-crystal type behavior from a polynanocrystalline area of the nanowire formed, suggesting that the nanocrystals on the virus were preferentially oriented with their [001] perpendicular to the viral surface. Peptides that specifically directed CdS nanocrystal growth were also engineered into the viral capsid to create wurtzite CdS virus-based nanowires. Lastly, heterostructured nucleation was achieved with a dual-peptide virus engineered to express two distinct peptides within the same viral capsid. This work represents a genetically controlled biological synthesis route to a semiconductor nanoscale heterostructure.

Spectroscopy of individual silicon nanowires

Photoluminescence (PL) spectroscopy of individual silicon nanowires has been investigated. A narrow emission band (85 meV) was observed associated with a fast luminescence decay in the picosecond region and is considered due to the recombination relaxation of confined electronic states. The optical anisotropy was found in the individual nanowires. When a wire was excited by linearly polarized light, the maximum intensity of linearly polarized PL was along the axis direction of the wire, and the maximum degree of polarization was determined to be 0.5. The value agrees well with the calculated one, which suggests that the polarization arise from the dielectric contrast between the crystalline cores and the silicon oxide sheathes of the nanowires.

Optical spectroscopy of silicon nanowires

Abstract Silicon nanowires (SiNWs) were prepared by laser ablation at high temperature and studied by electron microscopy and optical spectroscopy. As-synthesized SiNWs are found orderly aligned on the silica substrates, exhibiting uniform shape with a silicon crystalline core and an amorphous silicon oxide sheath. Asymmetrically broadened Raman spectral peaks downshifted from 520 cm −1 were observed, which related to the confinement effects of optical phonon by nanowire boundaries. The SiNWs showed strong photoluminescence (PL) bands peaked at 455 and 525 nm, which quenches rapidly with an increase in temperature and may arise from the defects surrounding the silicon nanowire crystalline core.

Comparison of energy levels of Mn2+ in nanosized- and bulk-ZnS crystals

Abstract The 3d5 multiplet level structure of the manganese ion is determined for two ZnS : Mn 2+ nanoparticle samples, whose average crystallite sizes are different, by measuring the photoluminescence excitation spectra in the ultraviolet- and visible-regions. The peak positions of the spectra of both samples are found to be almost the same as those observed for the bulk crystal, showing that the multiplet energies of Mn2+ are almost independent of the ZnS host crystal size. This result suggests that the degree of the mixing between the s–p state of ZnS and the 3d orbitals of Mn2+ does not change significantly with the reduction in the host crystallite diameter down to a few nanometers.

Europium silicate thin films on Si substrates fabricated by a radio frequency sputtering method

Europium silicate thin films have been fabricated on Si(100) substrates by a radio frequency magnetron sputtering method and characterized by x-ray photoelectron spectroscopy and x-ray diffraction spectroscopy. The constituents of the films, mixtures of europium silicates, silicon and europium oxides, are sensitive to the stoichimetry of sputter targets, working gases and thermal annealing conditions. Electroluminescent devices based on the europium silicate thin films exhibit large-area uniform broad-band electroluminescence with an external quantum efficiency of about 0.1% at room temperature and a low operating threshold voltage (about 6 V).

Optical properties of undoped and Mn2+-doped CdS nanocrystals in polymer

We prepare a polyvinyl alcohol film containing undoped and Mn 2+ -doped CdS nanocrystals by a chemical method, and investigate the photoluminescence properties. The luminescence of Mn 2+ of this sample shows long decay times of about 1.1 and 0.2 ms, unlike the previous report on ZnS : Mn 2+ nanocrystals. Under the interband excitation, the temperature quenching of the Mn 2+ luminescence is found to be remarkably weak in comparison with the bulk crystals of CdS: Mn 2+ . Probable causes of this difference are discussed in terms of the electron-hole pair (or exciton)-phonon interaction and the thermally induced dissociation of the electron-hole pair. Further, we measure site-selective luminescence spectra and discuss the origin of the green luminescence of the undoped CdS nanocrystals.

Electroluminescence of europium silicate thin film on silicon

We report an electroluminescent device fabricated by a europium silicate layer on a silicon substrate. The device exhibits uniform-intense white color electroluminescence with an external quantum efficiency about 0.1% at room temperature, a low operating threshold voltage (about 6 V) and a fast response to the modulation signal at the frequency of 1 MHz.

Time Response of One-Dimensional Photonic Crystals with a Defect Layer Made of Semiconductor Quantum Dots

The effective optical nonlinearity of semiconductor quantum dots can be enhanced in one-dimensional photonic crystal structures. We have fabricated a one-dimensional photonic crystal with a structural defect which is made of CdSe quantum dots. Besides the enhancement of degenerate four-wave mixing efficiency, a fast response of transient population grating in an order of a picosecond was obtained in the one-dimensional photonic crystal. This structure can be used as a future photonic device with high nonlinearity and fast response.

Characterizations of Simultaneously Fabricated Silicon and Silicon Monoxide Nanowires

Silicon and silicon monoxide (SiO) nanowires have been simultaneously fabricated by simply sublimating a pressed silicon powder plate (Si and Fe mixtures) at 1200°C in a flowing argon gas environment. The length of the SiO wire exceeds 0.8 mm and its average diameter varies from 70 nm to 1.35 µm, while the length of the silicon nanowire reaches 5 micrometers, with its average diameter varying from 20 to 40 nm. Both wires possess uniform diameters throughout the entire lengths, and smooth surfaces.

Copper silicide nanocrystals in silicon nanowires

We report the formation of a new type of nanosized composite of copper silicide nanocrystals embedded in a silicon nanowire that obtained by laser ablation of Si/metal mixture targets at 1200°C in an argon gas flow. Like the bulk phase of crystal silicon, the copper atoms were found have a high solubility in the silicon nanowires and precipitate in the nanowires. The silicon wires are smooth in surfaces and straight in length, while the copper silicide nanocrystals are sphere-shaped. The diameters of the nanocrystals increase with that of the host nanowire. Because of Si nanowires and copper silicide nanocrystals have distinctive electronic and optical properties, the composite nanowires may possess unique properties arising from both the size and the interface effects that are of fundamental and application interests.