Jianfeng Wu

Portland State University, Portland, OR, United States

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Publications (6)17.86 Total impact

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    ABSTRACT: We fabricated an antireflective hybrid nanostructure using anatase TiO 2 nanobelts synthesized by an alkaline hydrothermal route and a ZnO nanowire array grown via a low-temperature solution-phase process. The replacement of TiO 2 nanoparticles with TiO 2 nanobelts improved the electron transport in the TiO 2 porous film. Importantly, rigorous coupled-wave analysis and reflectance measurements indicate that the well-designed composite of TiO 2 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. Introduction Photosensitization of TiO 2 by dye molecules is a promising and low-cost way to prepare sunlight-absorbing materials for dye-sensitized solar cells (DSSCs). 1 In a DSSC, the anchored dye molecules generate electron-hole pairs after harvesting photons. The excited electrons are thermodynamically injected into the TiO 2 conduction band and transported through the TiO 2 layer to the transparent conducting glass. The holes diffuse through the electrolyte (usually I -/I 3 -) to the opposing electrode. 2-5 In general, the photoanode of the DSSC is highly porous and made of stacking crystalline TiO 2 nanoparticles (NPs), allowing the photoanode to load adequate dye molecules for sufficient light absorption. 6 To reach the transparent conducting glass, the injected electrons need to encounter on average more than 10 6 TiO 2 NPs along random and complicated pathways in the porous TiO 2 NP film. 7 The excessive electron traps related to the surface defects, which exist at the weak contact between the neighboring TiO 2 NPs and on the TiO 2 NP surface, cause the trap-limited electron diffusion rate to be several orders of magnitude slower than that in TiO 2 single crystals. 7-10 This leads to the exacerbation of the energy depletion connected to the electron-hole recombination across the oxide-electrolyte interface. Adachi et al. aligned and attached TiO 2 NPs to nanowires (NWs), resulting in a high rate of electron transfer through the TiO 2 single-crystal-like anatase NW network. 11 They further reported the use of TiO 2 nanorods (NRs) instead of TiO 2 NPs to decrease the Ohmic loss during transport of the injected electrons. 12 This indicates that one-dimensional TiO 2 nanostructures, such as TiO 2 NRs, NWs, or nanobelts (NBs), are ideal materials for DSSC fabrication due to their electron diffusion length over tens of micrometers in comparison to that of TiO 2 NPs. 13-16 Similarly, ZnO NWs were utilized for DSSCs owing to their substantially larger electron diffusion lengths related to the high electron mobility of 1-5 cm 2 s -1 V -1 along the ZnO crystal c axis and the radial electric field in ZnO NW's crystalline core. 17,18 The vertically aligned ZnO NWs can form continuous electron paths between the dye molecule and the transparent conducting glass. The electron-transport rate in the ZnO NW array-based DSSC measured by Law et al. is 1 or 2 orders of magnitude greater than that in the TiO 2 NP-based DSSC. 19,20 However, the replacement of the tiny NPs with thick NRs, NWs, or NBs leads to a great loss of surface area, which is proportional to the amount of absorbed sensitizers. To overcome the dilemma of having to compromise the DSSC anode's light harvesting for enhanced electron transport or vice versa, we integrated TiO 2 NBs and a ZnO NW array into a hybrid architecture. The network fabricated with anatase TiO 2 NBs demonstrated a lower resistance than that of the TiO 2 NP film. In the network, the TiO 2 NBs may serve as fast electron paths and as efficient dye absorbers due to their thin diameters. The rigorous coupled-wave analysis (RCWA) model and reflectance measurements reveal that, in the hybrid nanostructure of TiO 2 NBs and a ZnO NW array, the TiO 2 NB filled ZnO NW array is an efficient antireflection coating (ARC) between the woven TiO 2 NB network and the transparent conducting glass.
    Journal of Physical Chemistry C - J PHYS CHEM C. 01/2010; 114(26).
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    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.
    Nano Letters 02/2009; 9(2):595-600. · 13.03 Impact Factor
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    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.
    Journal of Physical Chemistry C - J PHYS CHEM C. 07/2007; 111(33).
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    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.
    The Journal of Physical Chemistry C 01/2007; 111(20):7514-7520. · 4.84 Impact Factor
  • J. Jiao, D. McClain, Jianfeng Wu, J. M. Green
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    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
    01/2006;
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    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.