Jianfeng Wu

Portland State University, Portland, OR, United States

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Publications (9)

  • [Show abstract] [Hide abstract] ABSTRACT: Nanostructures are considered the critical component in a wide range of potential nanoscale device applications. Yet a procedure to fabricate them with both controllable results and in bulk quantities must be developed in order to achieve their commercialization at reduced cost. In this report, we introduce an improved vapor-liquid-solid method that is capable of preparing high yield, high quality CdS nanowires and nanobelts in a turf-like configuration. To increase yield, we placed gold-coated substrates in a ceramic boat partially covered with a glass slide to form a gas trap. Only a small opening was provided to allow the CdS vapor to escape from the trap. This arrangement increases catalyst exposure to CdS vapor flow in comparison to conventional CVD methods. This allowed the CdS vapor to deposit densely over the substrate at a predetermined temperature range of 501°C-630°C inside the quartz tube. These conditions results in synthesis of various morphologies on both quartz and tungsten substrates including an intertwined-like structure not previously reported. Electron microscopy and microanalysis techniques were utilized in characterizing these morphologies, internal structures and elemental compositions. Electron field emission properties were investigated in an ultra high vacuum chamber set up with a base pressure of ∼1E-9 torr.
    Article · Jan 2011 · MRS Online Proceeding Library Archive
  • [Show abstract] [Hide abstract] ABSTRACT: Fabricating horizontally aligned single wall carbon nanotubes (CNTs) with controlled properties has been one of the significant challenges for field-effect transistor (FET) applications. This report demonstrates a novel procedure for the fabrication of horizontally aligned single walled CNTs using the focused ion beam (FIB) and chemical vapor deposition (CVD). This method allows the morphologies, internal structures, and elemental compositions of CNTs to be directly analyzed in the scanning electron microscope (SEM) and transmission electron microscope (TEM) and avoids any sample preparation procedures that might alter the structure of the CNTs. The techniques of electron beam and ion beam induced deposition (EBID and IBID) of Pt electrodes to the CNT ends were compared and both were found to produce metal contamination around the target area. The fabrication of large area electrodes to assist in testing the CNT's electronic properties, including contact resistance and I-V characteristics was investigated. Using this fabrication technique we were able to perform an I-V sweep on a CNT circuit as well as detect the metal contamination on the CNTs which occurred as a result of electrode deposition.
    Article · Jan 2011 · MRS Online Proceeding Library Archive
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    [Show abstract] [Hide abstract] ABSTRACT: We fabricated an antireflective hybrid nanostructure using anatase TiO2 nanobelts synthesized by an alkaline hydrothermal route and a ZnO nanowire array grown via a low-temperature solution-phase process. The replacement of TiO2 nanoparticles with TiO2 nanobelts improved the electron transport in the TiO2 porous film. Importantly, rigorous coupled-wave analysis and reflectance measurements indicate that the well-designed composite of TiO2 nanobelt-ZnO nanowire array acted as an efficient antireflection coating. The photoanode made of this hybrid nanostructure may enhance the performance of dye-sensitized solar cells by minimizing the electron hole recombination-related and reflection-induced energy loss.
    Full-text Article · Jul 2010 · The Journal of Physical Chemistry C
  • Jianfeng Wu · Madeline Wyse · Jun Jiao
    Article · Jul 2009 · Microscopy and Microanalysis
  • Jianfeng Wu · Madeline Wyse · Devon McClain · [...] · Jun Jiao
    [Show abstract] [Hide abstract] ABSTRACT: We report here an effective method for the fabrication of a large number of triode-type microgated carbon nanotube field emitter arrays. Our technique combines dual-beam focused ion beam technology and plasma-enhanced chemical vapor deposition, avoiding the tedious lithography and wet chemistry procedures conventionally used to fabricate such structures. Field emission testing revealed that increasing gate voltage by as little as 0.3 V had significant impact on the local electric fields, lowering the turn on and threshold fields by 3.6 and 3.0 V/microm, respectively. The field enhancement factor of the emitter arrays was also increased from 149 to 222. A quantum mechanical model for such triode-type field emission indicates that the local electric field generated by a negatively or positively biased gate directly impacts the tunneling barrier thickness and thus the achievable emitter current.
    Article · Feb 2009 · Nano Letters
  • Congkang Xu · Steven Youkey · Jianfeng Wu · Jun Jiao
    [Show abstract] [Hide abstract] ABSTRACT: Large channel length field effect transistors (FETs) based on Pt contacts to ferromagnetic BiMn-codoped ZnO bicrystal nanobelts have been fabricated using dielectrophoresis and a focused ion beam. Electrical transport studies show n-type behavior of the ferromagnetic ZnO nanobelts. The current−voltage characteristics of the FETs exhibit Schottky barrier behavior. The contact resistances and the Pt diffusion are responsible for the reduction of the conductance and the threshold shift. The reduction of the mobility can be attributed to the enhanced interface scattering at Pt electrodes/nanobelt contact regions after Pt deposition. The devices are also found to be strongly dependent on the channel length.
    Article · Jul 2007 · The Journal of Physical Chemistry C
  • Devon McClain · Jianfeng Wu · Noel Tavan · [...] · David F. Bahr
    [Show abstract] [Hide abstract] ABSTRACT: As the morphologies of patterned carbon nanotube (CNT) field emission arrays become more complex, it is critical to examine the interactions of individual emitters in close proximity to one another. The number of active emission sites in dense arrays can be severely inhibited by electrostatic shielding. To explore this effect, we have fabricated an array of microscale CNT bundles with varying diameters. Field enhancement was found to increase with decreasing bundle diameter. High-resolution scanning electron microscopy revealed a corresponding increase in the number of stray CNT tips as the bundle diameter decreased. These CNT tips were not as heavily shielded, resulting in improved emission current density. Our results confirm that electrostatic shielding significantly affects the field enhancement of dense arrays, particularly in terms of the number of emitters actively participating in the field emission process.
    Article · May 2007 · The Journal of Physical Chemistry C
  • Jun Jiao · Devon McClain · Jianfeng Wu · Joshua M. Green
    [Show abstract] [Hide abstract] ABSTRACT: Field emission from carbon nanotubes (CNTs) at low applied electric fields has been demonstrated by a number of groups. However, the emission behavior reported for CNTs varies widely. Most of this variation is attributed to differences in the nanotube morphologies and contents of measured samples. We report here the effect of fabrication techniques including thermal chemical vapor deposition (CVD) and plasma enhanced CVD on the formation of CNTs. The post-deposition process of using atomic layer deposition (ALD) of ZnO on CNTs was also investigated. The results suggest that numerous ZnO "nanobeads" formed on the surface of the CNTs resulting in dramatically improved electron field-emission
    Article · Jul 2006
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    Micah Eastman · Devon Mcclain · Jianfeng Wu · [...] · Ken Teo
    [Show abstract] [Hide abstract] ABSTRACT: A three month intensive collaborative effort has been conducted by research teams from Portland State University and the University of Cambridge. During this time, preliminary measurements and analyses have been carried out for a series of long-term research projects. These include two investigations of metal gated carbon nanotube field emitter arrays: a characterization and exploration of the reduction of nearest-neighbor shielding effects of emitters using an externally applied gate potential, and the fabrication of emitter arrays with an integrated gate layer. Additional efforts have been put forth to analyze methods to improve carbon nanotube field effect transistors, particularly the improvement of contacts between metal electrodes and semiconducting nanotubes.
    Full-text Article ·