Ming Wei

Bio-molecule-conjugated fluorescent organically modified silica nanoparticles as optical probes for cancer cell imaging

Organically modified silica nanoparticles doped with Nile Red were synthesized and characterized. Silica encapsulation is relatively transparent for light and can protect hydrophobic Nile Red against denaturalization induced by the extreme bio-environment, making the entire nanoparticle hydrophilic and possess stable optical properties. The nanoparticles were conjugated with bio-molecules (such as apo-transferrin and folic acid), and our in vitro experiments revealed that these functionalized nanoparticles can serve as effective optical probes for specific targeting of cancer cells. As a preliminary study for future in vivo animal experiment, ORMOSIL nanoparticles were further co-conjugated with polyethyleneglycol (PEG) and apo-transferrin and the conjugates were also very good for in vitro targeting of HeLa cells. These bio-molecule functionalized ORMOSIL nanoparticles may serve as a robust tool for early diagnosis/therapy of cancer and other diseases.

Bandgap engineering of sol-gel synthesized amorphous Zn1−xMgxO films

Amorphous Zn1−xMgxO (α-Zn1−xMgxO) ternary alloy thin films across the full compositional range were synthesized by a low-cost sol-gel method on quartz substrates. The amorphous property of the α-Zn1−xMgxO films was verified by x-ray diffraction, and atomic force microscopy revealed a smooth surface with sub-nanometer root-mean square roughness. The current phase segregation issue limiting application of crystalline Zn1−xMgxO with 38% < x < 75% was completely eliminated by growing amorphous films. Optical transmission measurements showed high transmissivity of more than 90% in the visible and near infrared regions, with optical bandgap tunability from 3.3 eV to more than 6.5 eV by varying the Mg content.

Deep-ultraviolet photodetectors from epitaxially grown NixMg1−xO

Deep-ultraviolet (DUV) photodetectors were fabricated from high quality NixMg1−xO epitaxially grown by plasma-assisted molecular beam epitaxy on an approximately lattice matched MgO ⟨100⟩ substrate. A mid-range Ni composition (x=0.54) NixMg1−xO film was grown for DUV (λpeak<300 nm) photoresponse and the film was characterized by reflected high-energy electron diffraction, Rutherford backscattering spectroscopy, x-ray diffraction, and optical transmission measurements. Photoconductive detectors were then fabricated by deposition of symmetric interdigitated contacts (10 nm Pt/150 nm Au) with contact separations of 5, 10, and 15 μm. The detectors exhibited peak responsivities in the DUV (λpeak≈250 nm) as high as 12 mA/W, low dark currents (Idark<25 nA), and DUV:visible rejection ratio of approximately 800:1.

The effect of oxygen flow rate and radio frequency plasma power on cubic ZnMgO ultraviolet sensors grown by plasma-enhanced molecular beam epitaxy

Cubic Zn1−xMgxO thin films were produced by Plasma-Enhanced Molecular Beam Epitaxy. Oxygen flow rate and applied Radio-Frequency (RF) plasma power were varied to investigate the impact on film growth and optoelectronic device performance. Solar-blind and visible-blind detectors were fabricated with metal-semiconductor-metal interdigitated Ni/Mg/Au contacts and responsivity is compared under different growth conditions. Increasing oxygen flow rate and RF plasma power increased Zn incorporation in the film, which leads to phase segregation at relatively high Zn/Mg ratio. Responsivity as high as 61 A/W was measured in phase-segregated ZnMgO visible-blind detectors.

High response solar-blind MgZnO photodetectors grown by molecular beam epitaxy

High quality w-MgxZn1-xO thin films were grown epitaxially on c-plane sapphire substrates by plasma-assisted Molecular Beam Epitaxy. ZnO thin films with high crystalline quality, low defect and dislocation densities, and subnanometer surface roughness were achieved by applying a low temperature nucleation layer. By tuning Mg/Zn flux ratio, wurtzite MgxZn1-xO thin films with Mg composition as high as x=0.46 were obtained without phase segregation. Metal- Semiconductor-Metal (MSM) photoconductive and Schottky barrier devices with interdigitated electrode geometry and active surface area of 1 mm2 were fabricated and characterized. Resultant devices showed ~100 A/W peak responsivity at wavelength of ~260nm. We also report on cubic rock salt c-MgxZn1-xO thin films, following a non-traditional approach on MgO substrates, to demonstrate solar-blind photoresponse in MSM photodetectors, realizing a peak responsivity of 460 A/W (@ 250 nm) and 12.6 mA/W (@ 240nm) for mixed phase and single crystal films, respectively. A specific focus of the work is on identifying the impact of various growth parameters on the performance of the c- MgZnO detectors.

Growth of high quality ZnO thin films with a homonucleation on sapphire

ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373 nm for 1 μm × 1 μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking...

Impact of growth conditions on ZnO homoepitaxial films on ZnO substrates by plasma-assisted molecular beam epitaxy

ZnO thin films were epitaxially grown on Zn-polar (0001) ZnO substrates by plasma-assisted molecular beam epitaxy. Surface root mean square (rms) roughness below 0.3 nm was achieved on a large range of growth temperatures by growing on ZnO substrates with 0.5 degree miscut angle toward [11¯00] axis. Surface treatment with acid etching and ozone exposure was required to remove contamination such as silica residual and carboxyl and carbonate groups on the surface. Removal of these surface impurities reduces the likelihood of extrinsic defect migration into the epitaxial films. High growth temperature (> 640°C) and oxygen rich conditions were required for films with terrace steps, but resulted in a very low growth rate (~30nm/h) and low photoluminescence (PL) lifetimes of lower than 50 ps. With moderate growth temperature (~610°C), higher growth rate and higher PL lifetime with up to 380 ps were achieved. EIT was used for the oxygen plasma to reduce reactive oxygen species etching of the surface, resulting in a higher growth rate and fewer defects in the films. Good crystalline quality was evident in Xray rocking curves with consistent narrow full width at half maximum (FWHM) of (0002), (101¯2) and (202¯1) peaks, indicating low threading dislocations. Both room-temperature and low-temperature photoluminescence indicated high optical quality of the resultant films with few non-radiative recombination centers.