Yi-Feng Lai

National Cheng Kung University, 臺南市, Taiwan, Taiwan

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Publications (13)37.03 Total impact

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    ABSTRACT: Multifunctional nanodevices integrated with excellent optoelectric and thermoelectric properties are highly demanded for developing next generation self-powered optoelectronics. In the work, we demonstrate the viability of a coupled structure of homologous In2O3(ZnO)m with In-doped ZnO into heterojunction belts synthesized by alloy-evaporation deposition, offering multiple functions with strong confined optical emissions and high power factors. Energy-filter secondary electron images reveal a five-layer contrast in the width direction of the belts due to different ionization energies corresponding to In2O3(ZnO)m/In:ZnO/ZnO/In:ZnO/In2O3(ZnO)m, as confirmed by transmission electron microscopy analysis. The indium-doped ZnO channels confined in two sides behave like quantum wells through band alignment, and become predominant in the main UV emission at 385 nm, as revealed by cathodoluminescence spectroscopic imaging. More intriguingly, this novel heterostructure provides an ideal pathway to enhance electron conduction through the indium doped ZnO layer, and the homologous In2O3(ZnO)m layer contains numerous interfaces to impede phonon transportation. In this way, the power factor of the heterostructure is greatly enhanced to be 2.07 × 10–4 W m–1 K–2 at room temperature, as compared to about 1.03 × 10–5 W m–1 K–2 for the undoped ZnO nanowires. This unique heterostructure achieved by a facile one-step growth process offers a new concept for designing devices by manipulating photons via coupling with electrons and phonons.
    The Journal of Physical Chemistry C. 11/2013; 117(48):25778–25785.
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    ABSTRACT: This study reports the growth of Zn–ZnO nanocables grown along the [112¯0] (a-plane) and [0 0 0 1] (c-plane) directions. The a-plane-oriented nanocables were synthesized via one-step thermal evaporation and condensation, whereas the highly ordered c-plane-oriented nanocables were synthesized via a vapor–solid–solid growth method with Sn catalysts. Photoluminescence (PL) measurements show that the near-band-edge emissions of both types of Zn–ZnO nanocables are blue-shifted compared to those of ZnO nanotubes without Zn cores, which is caused by compressive stress. Temperature-dependent PL spectra reveal that there are two dominant emissions for each type of nanocables at low temperatures. The emission attributed to excitons bound to neutral donors is common to both types of nanocables and persistent at room temperature. The other emission involves different excitonic transitions, which are related to different surface planes.
    Journal of Alloys and Compounds 03/2013; 554:115–121. · 2.73 Impact Factor
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    ABSTRACT: Non-polar ZnO thin film with high crystal quality is grown on a glass substrate using one-step oblique-angle deposition. Cross-sectional transmission electron microscopy images and selected area electron diffraction patterns reveal that the film is constructed as a stack of grains from the bottom to the top with the [0 0 0 2] axis gradually titled from a vertical to a nearly horizontal orientation with respect to the substrate. The (0 0 0 2) pole figure exhibits a continuous angle distribution in the ψ direction with the most concentration at approximately ψ = 18 0 and φ = 0 0 . Strong anisotropic effects in local electronic structure were observed for the highly oriented ZnO surface rod by angle-dependent X-ray bsorption near-edge structure measurements. The structure also exhibits polarization that depends on Raman scattering.
    J. Eur. Ceram. Soc. 01/2013; 33(10):1809-1814.
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    ABSTRACT: Be microstructure and optical properties of InAs quantum dots (QDs) grown on a GaAs buffer with a 30 nm thick AlAs insertion layer are investigated and compared with those grown on a plain GaAs buffer by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The former InAs QDs exhibit larger dot sizes of 20 nm and higher aspect ratios of 0.4, compared to 15 nm and 0.2, respectively, for the latter. Temperature-dependent PL spectra of the larger dots show that the main emission is dominated by band-tail state transitions at low temperatures and ground state transitions at high temperatures. The ground state transition energy in such quantum dots is significantly red-shifted compared to the smaller InAs QDs. Lower thermal activation energy is also observed for the larger QDs with an AlAs layer. All of the phenomena are caused by different In–Ga intermixing behavior occurring during capping, which is discussed in detail.
    Journal of The Electrochemical Society. 04/2011; 158(5):H540-H544.
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    ABSTRACT: A multi-source/component spray coating process to fabricate the photoactive layers in polymer solar cells is demonstrated. Well-defined domains consisting of polymer:fullerene heterojunctions are constructed in ambient conditions using an alternating spray deposition method. This approach preserves the integrity of the layer morphology while forming an interpenetrating donor (D)/acceptor (A) network to facilitate charge transport. The formation of multi-component films without the prerequisite of a common solvent overcomes the limitations in conventional solution processes for polymer solar cells and enables us to process a wide spectrum of materials. Polymer solar cells based on poly(3-hexylthiophene):[6,6]-phenyl C(61) butyric acid methyl ester spray-coated using this alternating deposition method deliver a power conversion efficiency of 2.8%, which is comparable to their blend solution counterparts. More importantly, this approach offers the versatility to independently select the optimal solvents for the donor and acceptor materials that will deliver well-ordered nanodomains. This method also allows the direct stacking of multiple photoactive polymers with controllable absorption in a tandem structure even without an interconnecting junction layer. The introduction of multiple photoactive materials through multisource/component spray coating offers structural flexibility and tenability of the photoresponse for future polymer solar cell applications.
    ACS Nano 08/2010; 4(8):4744-52. · 12.03 Impact Factor
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    ABSTRACT: We demonstrate a novel defect-induced bending mechanism for a modified oblique-angle deposition (OAD) system, where different defect density was introduced to accommodate the mass difference between the shadowed and exposed surfaces, leading to continuous structural bending. Oblique angle sputtering and hydrothermal processes were employed for growth of inclined ZnO nanowire arrays on ZnO bent columns. Transmission electron microscopy images reveal that a dislocation network was introduced to accommodate the mass difference in bent columns, and the bending angle could be controlled by growth temperature. Nanowires were then grown along the tangent lines of the bent column tips. The bent column curvature and limited space determine the nanowire growth direction. The reflectance measurements demonstrate that the oblique-aligned ZnO nanowire arrays are an excellent candidate for antireflection coatings, showing the significant suppression of reflectance of 87.5% and 90.0% for polished Si under TE and TM polarization, respectively. The interference oscillations of reflectance show the optical anisotropy of oblique-aligned ZnO nanowire arrays, which is dependent on the angle range of nanowire direction.
    Crystal Growth & Design - CRYST GROWTH DES. 07/2010; 10(8).
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    ABSTRACT: The optical properties of focused ion beam-engraved perfectly aligned and spatially controlled multi-quantum-well InGaN-GaN nanopillars were investigated. Crystalline MQW nanopillars with a diameter of 30 to 95 nm and high aspect ratios showed a maximum of three-fold enhancement in emission intensity per unit active area. A light emitting contour map of Taiwan is drawn with a nanopillar structure to demonstrate the site control of the technique adopted in the present study. Raman scattering studies were used to characterize the newly created surfaces. Unknown peaks in GaN and InGaN nanostructures are identified for surface optical (SO) phonon modes with proper assignments of wave vectors using multiple excitations, and the SO mode for the ternary phase is reported for the first time.
    Journal of Raman Spectroscopy 07/2009; 40(12):2044 - 2049. · 2.68 Impact Factor
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    ABSTRACT: We report on negative differential resistance (NDR) from individual GaN nanowires prepared without catalysts by thermal chemical vapor deposition. Conductive atomic force microscopy was used to characterize the electron transport behavior and transmission electron microscopy was employed to characterize the microstructure of the GaN nanowires. The current-voltage curve exhibits two clear NDR regions in the forward bias. The defect assisted inelastic tunneling process resulting in the NDR behavior and the related mechanism for energy band diagram is proposed and discussed.
    Applied Physics Letters 05/2009; 94(18):182101-182101-3. · 3.52 Impact Factor
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    ABSTRACT: We report the optical properties of a composite structure comprising a type-II GaAs0.7Sb0.3/GaAs quantum well (QW) and an InAs quantum-dot (QD) layer adjacent to the QW. The low-temperature photoluminescence (PL) of the composite structure with a 5-nm-thick GaAs spacer demonstrates a redshift of 44 meV, as compared with that of GaAs0.7Sb0.3/GaAs single QW at low excitation level. The redshift reveals the existence of local potential minimums, induced by the stress exerted by the adjacent QDs, in the type-II QW. At higher temperature, the composite structure shows stronger PL intensity than the GaAs0.7Sb0.3 QW, indicating the potential of the applications to laser diodes.
    Applied Physics Letters 01/2009; 94. · 3.52 Impact Factor
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    ABSTRACT: ZnO seed layers and well-aligned ZnO single-crystalline micro/nanorods were synthesized on bare Si in one step without the assistance of catalysts by chemical bath deposition. Scanning electron microscopy (SEM) images and X-ray diffraction patterns show that the alignment of ZnO rods on Si(100) could be adjusted by varying the substrates’ angles of incline, the reaction temperature, and the precursor concentration. Transmission electron microscopy cross-sectional images demonstrate that a polycrystalline seed layer with (0002) preferred orientation was formed between the well-aligned rods and Si substrate placed vertically while a randomly oriented layer was formed between the randomly aligned rods and Si substrate placed horizontally. The formation of seed layers and alignment of as-synthesized ZnO rods were attributed to the assistance of boundary layers in a chemical bath deposition system.
    Applied Physics A 01/2009; 96(3):775-781. · 1.69 Impact Factor
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    ABSTRACT: Single-crystalline ZnO nanorods emitting two characteristic optical emissions from opposite halves of the nanorods were synthesized by thermal chemical vapor deposition using Zn/Al mixed powders. Energy dispersive x-ray spectra with transmission electron microscopy show a gradually decreasing Zn:O atomic ratio from the root to the top of a nanorod, and the averaged ratios at the two ends are ≈57.2:42.8 and 49.5:50.5. Room-temperature cathodoluminescence measurements show that the nanorods exhibit a sharp ultraviolet emission at 377 nm from one segment and a broad green band at 500 nm from the other, which is attributed to different oxygen concentrations along the nanorods. The luminescence behavior suggests further applications for nano-pixel optoelectric devices.
    Nanotechnology 07/2008; 19(28):285703. · 3.84 Impact Factor
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    ABSTRACT: We have investigated the shape and composition profiles of buried and surface InAs/GaAs Stranski–Krastanov quantum dots (QDs) by using the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075 eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In–Ga intermixing in the vicinity of the buried QDs.
    Applied Physics Letters 02/2008; 92(5):051903-051903-3. · 3.52 Impact Factor
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    ABSTRACT: The authors have synthesized InN quantum dots by ion implantation into a Si (100) substrate followed by a postannealing process. X-ray photoemission spectroscopy data verified the formation of In–N bonding in both as-implanted and postannealed samples. Diffraction patterns from transmission electron microscopy (TEM) confirm that the dots are of cubic crystal (zinc-blende phase) with no presence of wurtzite InN. The silicon matrix provides a constraint for the formation of the InN cubic metastable phase. However, dislocations were revealed by high resolution TEM at the interfaces between the dots and the silicon. In addition, the authors found that as the annealing temperature or time increases, dot size increases and dot density decreases. Furthermore, they demonstrate that the main emission energy of zinc-blende InN dots is about 0.736 eV.
    Applied Physics Letters 08/2007; 91(9):091921-091921-3. · 3.52 Impact Factor