凝態科學研究中心 - 林麗瓊


  My lab (Advanced Materials Laboratory) is specialized in microwave plasma enhanced chemical vapor deposition (PECVD), electron-cyclotron-resonance (ECR) PECVD, ion beam sputtering, magnetron sputtering, as well as ion-beam assisted processes. All these techniques involve synthesizing materials in conditions far away from equilibrium process; therefore, they offer an unprecedented opportunity for forming metastable phases and novel structures that can not be produced by the conventional techniques. In addition to utilizing these energized process techniques for fabricating advanced materials and devices, we are also competent in using electron microscopies along with a variety of characterization and spectroscopic tools to study the structural and physical properties of these materials. Moreover, based on our past experience on semiconductors, electronic and optoelectronic materials, we have been extending our efforts to molecular sensing and energy related research over the last three years.

Thru strong support from and intense collaboration with other research groups within the CCMS, NTU, as well as the Institute of Atomic and Molecular Sciences, Academia Sinica, we have already established a solid foundation in research and development of novel nano-systems for next-generation optoelectronic and energy applications. Some major research achievements of the past few years are summarized below.

(1) carbon nanotubes for field emission and electrochemical energy applications

For carbon nanotubes (CNTs), significant efforts have been devoted to their functionalization and device fabrication using arrays of CNTs [Adv. Func. Mater. 12, 687 (2002)]. Si-based wafer techniques were employed for fabricating CNTs-based integrated devices such as gated and TFT-controlled field emission devices. Both vertically and horizontally configured field emitters with good mission I-V characters have been successfully demonstrated. Electron injection from the back electrodes into emitters was found to play an important role in their field emission performance [Phys. Rev. B 68, 125322 (2003)].

Recently, an efficient and simple route to attach Pt and Ru nanoparticles (NPs), ca. 2-3 nm in diameter, on the sidewalls of CNT arrays has been developed [Chem. of Mater. 17, 3749 (2005)]. It was found that nitrogen incorporation, while causing some deformation, facilitates nucleation and growth of catalyst on CNT with site and size controllability [J. Am. Chem. Soc. 128, 8368 (2006)]. Utilizing these nitrogen-containing carbon nanotubes (CNx NTs) grown directly on carbon cloth (CC) as supports for Pt–Ru NPs, a composite electrode is formed and has shown a much higher electrocatalytic activity than its counterpart using bare CC. This can be attributed to substantial increase in surface area, fast electron-transfer kinetics, and low interfacial barriers of the CNx NTs/CC-Pt-Ru NPs composite electrode, providing an efficient e-transport route for direct methanol fuel cell (DMFC). Notably, our CNx NTs-Pt-Ru nanocomposite is highly efficient in loading precious metal, as similar power performance is obtained with only one tenth of loading, in comparison to that of the conventional method, thus, offering a significant cost-reduction in fuel cells [Electrochimica Acta 52, 1612 (2006); J. Power Sources 171, 55 (2007)]. Similar idea was employed for RuO2 NPs dispersed CNx NTs, showing ultra-fast charging/discharging capacitive property [Nanotechnology 18, 485716 (2007); J. Electrochem. Soc. 155, K15 (2008)].

(2) Novel One-dimensional III-Nitride and Oxide Nanostructures

Our works on nitride/oxide nanorods and nanowires have been quite solid, as evidenced by the outstanding citations, including two listed as top 1% most cited papers in the field of Chemistry and Physics [J. Am. Chem. Soc. 123, 2791 (2001) and Appl. Phys. Lett. 81, 1312 (2002)] in recent years. These low dimensional nanostructures often exhibit novel electronic and optoelectronic properties unmatched by their counterparts in the forms of thin film or bulk. For instance, InN nanobelts, while exhibiting strong surface band bending and photo-elastic effects, showed photoluminescence in IR with a peak width of 14 meV, the sharpest reported to date for InN [Adv. Func. Mater. 16, 537 (2006); Adv. Mater. 19, 4524 (2007)]. The rectangular cross-sectional nanobelt has the potential to serve as an effective Fabry-Perot microcavity for lasing process [Appl. Phys. Lett. 90, 123109 (2007)]. Unlike the “epitaxial” film, wherein lattice mismatch between the film and the substrate often gives rise to very high density of extended defects, these free-standing nanostructures are nearly defect-free, as exemplified by weak temperature quenching in the photoluminescence property of InN nanotips [Appl. Phys. Lett. 87, 203103 (2005)], implying high quantum efficiency. Interestingly, contrary to the quantum confinement effect commonly believed to occur at a particle size or wire diameter below Bohr radius, we have observed anomalous blue shift in the III-N and ZnO nanowires, with diameters far beyond the quantum confinement regime [Appl. Phys. Lett. 88, 241905 (2006)]. More recently, we have also observed a very strong size effect from single-wire conductivity (both dark and photo) measurements. With decreasing wire diameter, ultrahigh responsivity (R >106 A/W) and gain (G >107) in photoconduction are observed in GaN nanowires, which are several orders of magnitude higher than their thin film counterparts [Appl. Phys. Lett. 91, 223106 (2007) and Small (in press, 2008)].

(3) Nanostructure-enhanced photoresponse: gold nanoparticle-embedded dielectric nanowires and core-shell semiconductor nanowires

We have demonstrated the synthesis of self-organized growth of Au nanopeapodded silica nanowires using microreactor approach by MPECVD. The observed axially anisotropic shape deformation of Au nanoparticles was correlated with the formation of strain in the silica nanowires as confirmed by transmission electron microscopy & cathodoluminescence analyses. Pronounced surface plasmon absorption (532 nm) was observed from the hybrid nanowires. Wavelength-dependent and reversible photoresponse behavior based on the hybrid nanowire ensemble devices were demonstrated, which may be ascribed to the surface plasmon resonance (SPR). We believe that this approach could be extended to prepare other metal/dielectric peapod-type nanowires with various properties for different applications. Meanwhile, core-shell nanowire, be it formed by inherent surface band bending or intentional post-growth treatment, possesses unique optical and transport properties, such as ultrahigh gain in photocurrent, unmatched by their thin film counterparts, could also be used as functional building blocks for constructing solar cells. [Adv. Mater. 14, 1847 (2002); Nano Lett. 3, 537 (2003); Nature Mater. 5, 102 (2006)]

Quite honorably, our Nature Mater. 2006 paper on nanopeapod has even been identified by ISI Web of Knowledge, Essential Science Indicators, as a Fast Breaking Paper in Material Science. Fast breaking papers represent recent scientific contributions that are just beginning to attract the attention of the scientific community. According to the ISI, only one paper is selected among the top 1% of the most cited papers in a field having the largest percentage increase in citations from one bimonthly update to the next.


(4) Improved Broadband and Wide-angle Antireflection Properties with Biomimetic Silicon Nanostructure

We have successfully demonstrated the broadband, wide angle and polarization independent antireflection properties in biomimetic silicon nanotips (SiNTs) fabricated on a 6-inch Si wafer using electron-cyclotron-resonance (ECR) plasma enhanced CVD. This patented ECR-CVD process for producing wafer-scale arrayed nanotips is generally applicable to a wide range of materials, including metals, semiconductors and dielectrics. The extremely high density arrayed nanotips (1012 cm-2) could serve as a template for further deposition/dispersion of Ag nanoparticles, which in turn, with properly controlled particle-to-particle distance, demonstrated excellent surface enhancement in Raman scattering. Meanwhile, the large area subwavelength structure that mimicked a moth eye demonstrated a low hemispherical reflectance of <1% from ultraviolet to the infrared region and show significant suppression of specular reflection in the far-infrared to terahertz region. The specular reflection is nearly unaffected by varying the angle of incidence up to 70° of s- and p- polarized light. A refractive index profile for the SiNT system can be generated using the gradient index of refraction approach, which in turn, can simulate the reflectance results as a function of incident angle for the s- and p-polarized light at wavelength 632.8nm. The remarkably high and featureless optical absorption in the aperiodic SiNTs, as against the periodic sub wavelength structures, promises enhanced performance in solar cell applications. [Nano Lett. 4, 471 (2004); Chem. of Mater. 17, 553 (2005); Nanotechnology 17, 2542 (2006); Nature Nanotech. 2, 170 (2007)]

(5) Enhanced charge separation by sieve-layer mediation in high efficiency inorganic-organic solar cell

We have demonstrated the enhanced performance of an inorganic-organic heterojunction photovoltaic cell by the use of an electronic sieve layer. A thin electronic sieve layer of LiF or SiOx (x~0.4) was shown to increase the cell efficiency to 3.4 and 6.04 %, respectively. The purpose of the sieve layer was to preferentially block the hole diffusion into the acceptor layer of the cell and thereby decreasing carrier recombination. This technique may be extended to enhance the charge collection that translates into better performance of the all-organic planar heterojunction solar cells. [Adv. Mater., accepted]

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