Yang-fang Chen - Academia.edu (original) (raw)
Papers by Yang-fang Chen
Scientific reports, Jan 11, 2015
A new approach is proposed to light up band-edge stimulated emission arising from a semiconductor... more A new approach is proposed to light up band-edge stimulated emission arising from a semiconductor with dipole-forbidden band-gap transition. To illustrate our working principle, here we demonstrate the feasibility on the composite of SnO2 nanowires (NWs) and chicken albumen. SnO2 NWs, which merely emit visible defect emission, are observed to generate a strong ultraviolet fluorescence centered at 387 nm assisted by chicken albumen at room temperature. In addition, a stunning laser action is further discovered in the albumen/SnO2 NWs composite system. The underlying mechanism is interpreted in terms of the fluorescence resonance energy transfer (FRET) from the chicken albumen protein to SnO2 NWs. More importantly, the giant oscillator strength of shallow defect states, which is served orders of magnitude larger than that of the free exciton, plays a decisive role. Our approach therefore shows that bio-materials exhibit a great potential in applications for novel light emitters, which...
8th International Conference on Power Electronics - ECCE Asia, 2011
ABSTRACT Transportation electrification is one of the most promising ways to limit the CO2 emissi... more ABSTRACT Transportation electrification is one of the most promising ways to limit the CO2 emission from the transportation sector so as to avoid catastrophic climate changes in the future. However, a large-scale integration of electric vehicles will introduce new challenges to electric power systems operation, especially at distribution network level. This paper aims to analyze possible impacts of large-scale electric vehicle charging system on the distribution network, propose corresponding control methods for impact mitigation, and investigate potential benefits of system performance improvement. In this paper, simulation model development of electric vehicle charging system is described firstly. After that simulation studies for both steady-state and dynamic performance analysis is introduced respectively. Simulation results show that with properly designed control strategies, the impact of large-scale charging system on power grid can be mitigated effectively; furthermore, distribution network can also benefit from electric vehicles due to the improved controllability of active and reactive power.
Nano Letters, 2003
... Synthesis and Characterization of Core−Shell GaP@GaN and GaN@GaP Nanowires. ...
Scientific Reports, 2014
Electrochemical metallization memories based on redox-induced resistance switching have been cons... more Electrochemical metallization memories based on redox-induced resistance switching have been considered as the next-generation electronic storage devices. However, the electronic signals suffer from the interconnect delay and the limited reading speed, which are the major obstacles for memory performance. To solve this problem, here we demonstrate the first attempt of light-emitting memory (LEM) that uses SiO 2 as the resistive switching material in tandem with graphene-insulator-semiconductor (GIS) light-emitting diode (LED). By utilizing the excellent properties of graphene, such as high conductivity, high robustness and high transparency, our proposed LEM enables data communication via electronic and optical signals simultaneously. Both the bistable light-emission state and the resistance switching properties can be attributed to the conducting filament mechanism. Moreover, on the analysis of current-voltage characteristics, we further confirm that the electroluminescence signal originates from the carrier tunneling, which is quite different from the standard p-n junction model. We stress here that the newly developed LEM device possesses a simple structure with mature fabrication processes, which integrates advantages of all composed materials and can be extended to many other material systems. It should be able to attract academic interest as well as stimulate industrial application.
The Journal of Physical Chemistry C, 2012
ABSTRACT We offer a novel approach to improve the performance of P3HT/TiO2 hybrid photovoltaic de... more ABSTRACT We offer a novel approach to improve the performance of P3HT/TiO2 hybrid photovoltaic devices by incorporating either hydroxyl- or amino-functionalized silica nanodots (SND–OH or SND–NH2) into the hole transport layer of the PEDOT:PSS. The SNDs serve as screens between conducting polymer and ionomer PSS to improve the phase separation and charge transport of the PEDOT:PSS hole transport layer. The power conversion efficiency (PCE) was thus improved by 1.45 and 2.61 fold for devices fabricated with PEDOT:PSS containing 1 wt % of SND–OH (SND–OH device) and 1 wt % of SND–NH2 (SND–NH2 device), respectively, when compared with the devices fabricated by neat PEDOT:PSS. The increase in PCE arises from an increase in short circuit currents, which are affected by the phase separation of PEDOT:PSS with possessing incorporated SNDs. The low surface potential of hydroxyl-functionalized SNDs (SND–OH) is easily aggregated in the PEDOT:PSS solution and forms large-sized phase separation in the PEDOT:PSS film. The aggregation of SND–OH causes slight decreases in the resistance of PEDOT:PSS thin film from (61 ± 1 to 69 ± 4)× 106 Ohm/square and a decrease in the shielding effects of the SNDs. In contrast, the high surface potential of amino-functionalized SNDs are dispersed uniformly in the PEDOT:PSS solution and form morphologies with small-sized domains in the PEDOT:PSS film. As a result, the sheet resistance of PEDOT:PSS thin films is decreased from (61 ± 1 to 46 ± 3) × 106 Ohm/square. Therefore, the SND–NH2 device exhibits greater performance over the SND–OH devices.
Journal of Raman Spectroscopy, 2013
ABSTRACT Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic prop... more ABSTRACT Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic properties associated with their dimensionality. We here investigate the effect of size‐induced changes on phonon localization and explain the enhanced Raman response. The occurrence of normally forbidden transitions in the photoluminescence spectra provides evidence for the predicted localization effect. Spatially resolved Raman spectroscopy reveals a continuous change of the silicon Raman peak position and peak width along the nanotip that is attributed to a smooth change between bulk properties at the base to size‐induced phonon confinement in the apex of the nanotip. This approach allows to exclude heating effects that normally overwhelm the phonon confinement signature. The Raman spectra are in excellent agreement with the spatial correlation model and the extracted correlation length is comparable to the tip dimensions. The observed phonon confinement coincides with an enhancement of the Raman scattering efficiency at the tip apex and results in a 40‐fold increase of the sample's Raman intensity compared with bulk silicon. These results provide a step toward the integration of Si based optoelectronic devices. Copyright © 2012 John Wiley & Sons, Ltd.
Journal of Materials Chemistry, 2008
We have investigated the effect of polymer molecular weight on the morphology and performance of ... more We have investigated the effect of polymer molecular weight on the morphology and performance of poly(3-hexylthiophene)/TiO 2 nanorod hybrid photovoltaic devices by using scanning near field optical microscopy (SNOM), atomic force microscopy (AFM) and confocal Raman microscopy. From the topography and absorption mapping images, it is found that the rod-like structure of the low molecular weight polymer hybrid film consists of a large amount of grain boundaries and has a less continuous absorption mapping image. In contrast, the larger domain structure of the high molecular weight polymer hybrid film exhibits a continuous absorption mapping image, as a result of enhanced polymer stacking and electronic delocalization. The nanoscale morphology of the hybrid samples with different molecular weights also reveals the nature of photovoltaic performance and carrier transport behavior investigated by the time-of-flight technique.
Journal of Biomedical Optics, 2010
Sun, Tzu-Lin et al. "Ex vivo imaging and quantification of liver fibrosis using second-harmonic g... more Sun, Tzu-Lin et al. "Ex vivo imaging and quantification of liver fibrosis using second-harmonic generation microscopy." Journal of Biomedical Optics 15.3 (2010): 036002-6.
Applied Physics Letters, 2010
The enhancement of electroluminescence from the various direct current aged InGaN-based light-emi... more The enhancement of electroluminescence from the various direct current aged InGaN-based light-emitting diodes (LEDs) is presented. It is found that the light output intensity of the aged LED shows an enhancement of about 150% at low driving current density compared with that of the original LEDs. The efficiency increases and the peak-efficiency-current shifts toward lower magnitude of the aged LEDs
2010 International Symposium on Computer, Communication, Control and Automation (3CA), 2010
In this work, both of ZnO based metal-semiconductor-metal photodetectors with and without ZnO cap... more In this work, both of ZnO based metal-semiconductor-metal photodetectors with and without ZnO cap layer were fabricated on flexible substrates (polyethylene terephthalate, PET) for comparative analysis. The ZnO films were prepared by a low temperature sputtering process. The photodetector with ZnO cap layer (stack structure: ZnO/Ag/ZnO/PET) shows a much higher UV-to-visible rejection ratio of 1.56×103 than one without. It can be attributed to the photocurrents significantly enhanced under UV light illumination in such a novel structure. With an incident wavelength of 370 nm and 3 V applied bias, the responsivities of both photodetectors with and without ZnO cap layer are 3.80×10-2 and 2.36×10-3 A/W, respectively, which correspond to quantum efficiencies of 1.13 % and 0.07 %. The Schottky barrier height on Ag/ZnO interfaces is also obtained to be 0.782 eV.
Proceedings of SPIE - The International Society for Optical Engineering, 2009
Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD a... more Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD and thermal CVD will be presented, along with their relation and difference to thin film counterparts. While there is a growing acceptance of the low band gap (0.6-0.7 eV) of InN, the optical properties of the actual samples still suffered, presumably due to the difficulty in obtaining high-quality samples and/or controlling their defect and carrier concentrations. However, the free-standing nanobelts can be nearly defect-free, allowing an excellent opportunity for fundamental investigations on unique dimensionality. InNNBs show photoluminescence (PL) in IR with peak width of 14 meV, the sharpest reported to date for InN. Interestingly, with increasing excitation intensity, InNNBs reveal an anomalously large blueshift in PL, compared to thin films; along with a decrease in the phonon frequencies as evident by Raman measurements. Surface band bending, converse piezoelectric effect, and photoelastic effects are employed to explain these behaviors. As for InGaN NWs, both In-rich and Ga-rich ternary nanowires have been synthesized by simply varying growth temperature. Morphological and structural characterizations reveal a phase-separated microstructure wherein the isovalent heteroatoms are self-aggregated, forming self assembled quantum dots (SAQDs) embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich nanowires, which has been explained by proposing a multi-level band schema.
ABSTRACT Multiphoton autofluorescence and second harmonic generation (SHG) microscopy are useful ... more ABSTRACT Multiphoton autofluorescence and second harmonic generation (SHG) microscopy are useful in respectively identifying elastin and collagen fibers within the skin dermis. In this study, we attempt to characterize the degree of skin thermal damage by using multiphoton microscopy to characterize the thermal changes to collagen and elastin fibers. We found that autofluorescence and SHG imaging behave differently in skin dermis treated with different temperatures and that an index reflecting the relative changes in autofluorescence and SHG intensity is useful to identify the degree of dermal thermal damage to the skin. With additional development, our approach can be used to identify the extent of thermal damage in patients.
The Journal of Physical Chemistry B, 2010
A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3H... more A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The λ max of the film absorption is blue-shifted from 520 to 470 nm, and its λ max of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration.
Solar Energy Materials and Solar Cells, 2009
We have fabricated a photovoltaic (PV) device based on the polymer blends of (poly(3-hexylthiophe... more We have fabricated a photovoltaic (PV) device based on the polymer blends of (poly(3-hexylthiophene) (P3HT)/polymethylmethacrylate (PMMA)) and inorganic TiO 2 nanorod bulk heterojunction. The optimized photovoltaic device with 1.6 wt% PMMA concentration has a power conversion efficiency of 0.65% under simulated AM 1.5 illumination (100 mW/cm 2 ), which is 38% more efficient than the device without the incorporation of PMMA. Furthermore, the PMMA-included device gives a shortcircuit current density of 2.57 mA/cm 2 , an open-circuit voltage of 0.53 V, and a fill factor of 0.48. Our studies have shown that having optimal PMMA concentration in the photovoltaic devices helps to smoothen the surface of the hybrid thin film, broaden the absorption spectrum, and improve the electrical conductivity. The results implying improvement in cell performance can be illustrated using atomic force microscopy (AFM), a UV/vis spectrophotometer and electrical measurements.
Solar Energy Materials and Solar Cells, 2009
We have studied the effect of polymer molecular weight on the performance of poly(3-hexylthiophen... more We have studied the effect of polymer molecular weight on the performance of poly(3-hexylthiophene):TiO 2 hybrid photovoltaic device using atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). The atomic force microscopic studies show the nanoscale morphology of the hybrid film changes from small domain size rod-like structure to large domain nodule-like structure with increasing the molecular weight of poly(3-hexylthiophene). The studies of SNOM of hybrid film reveal that the large domain structure of the high-molecular-weight P3HT hybrid film exhibits continuous absorption mapping as opposite to the discontinuous absorption mapping of the lowmolecular-weight P3HT hybrid film. Both results suggest the improvement in device efficiency from high-molecular-weight P3HT is due to the formation of large domain structure with increased carrier mobility and light harvesting.
In the past few years, we have fabricated nanoscale La0.7Sr0.3MnO3 periodic arrays with unique op... more In the past few years, we have fabricated nanoscale La0.7Sr0.3MnO3 periodic arrays with unique optical and magnetic properties successfully. These periodic patterns were made by La0.7Sr0.3MnO3 resist that can be developed under a nontoxic and environmental friendly manner using pure water. The resist is also capable to exhibit both positive and negative resist behaviors depending on the electron beam dosage. Thus, these special characteristics are used to fabricate and tune periodic structure thin film having controlled optical reflectance properties in the wavelength of 300 nm to 800 nm with one fixed design electron beam pattern by simply changing the electron beam dosage only. Additionally, the magnetization of La0.7Sr0.3MnO3 patterns can be enhanced by post sintering the sample at 900 °C after electron beam irradiation. Therefore, our study provides a one-step, simple, and convenient alternative technique for the fabrication of tunable optical structure and nanoscale magnetic pa...
Optics Express, 2008
Large-area multiphoton laser scanning microscopy (LMLSM) can be applied in biology and medicine f... more Large-area multiphoton laser scanning microscopy (LMLSM) can be applied in biology and medicine for high sensitivity and resolution tissue imaging. However, factors such as refractive index mismatch induced spherical aberration, emission/excitation absorption and scattering can result in axial intensity attenuation and lateral image heterogeneity, affecting both qualitative and quantitative image analysis. In this work, we describe an image correction algorithm to improve three-dimensional images in LMLSM. The method consists of multiplying the measured nonlinear signal by a three-dimensional correction factor, determined by the use of twophoton images of the appropriate specimens and specimen absorption and scattering properties at the excitation and emission wavelengths. The proposed methodology is demonstrated in correcting multiphoton images of objects imbedded in uniform fluorescent background, lung tissue, and Drosophila larva.
Nanotechnology, 2011
An on-chip approach for fabricating ferromagnetic/semiconductor-nanotip heterojunctions is demons... more An on-chip approach for fabricating ferromagnetic/semiconductor-nanotip heterojunctions is demonstrated. The high-density array of Si nanotips (SiNTs) is employed as a template for depositing La(0.7)Sr(0.3)MnO(3) (LSMO) rods with a pulsed-laser deposition method. Compared with the planar LSMO/Si thin film, the heterojunction shows a large enhancement of room temperature magnetoresistance (MR) ratio up to 20% under 0.5 T and a bias current of 20 µA. The MR ratio is found to be tunable, which increases with increasing external bias and the aspect ratios of the nanotips. Electric-field-induced metallization, in conjunction with nanotip geometry, is proposed to be the origin for the giant MR ratio.
Scientific reports, Jan 11, 2015
A new approach is proposed to light up band-edge stimulated emission arising from a semiconductor... more A new approach is proposed to light up band-edge stimulated emission arising from a semiconductor with dipole-forbidden band-gap transition. To illustrate our working principle, here we demonstrate the feasibility on the composite of SnO2 nanowires (NWs) and chicken albumen. SnO2 NWs, which merely emit visible defect emission, are observed to generate a strong ultraviolet fluorescence centered at 387 nm assisted by chicken albumen at room temperature. In addition, a stunning laser action is further discovered in the albumen/SnO2 NWs composite system. The underlying mechanism is interpreted in terms of the fluorescence resonance energy transfer (FRET) from the chicken albumen protein to SnO2 NWs. More importantly, the giant oscillator strength of shallow defect states, which is served orders of magnitude larger than that of the free exciton, plays a decisive role. Our approach therefore shows that bio-materials exhibit a great potential in applications for novel light emitters, which...
8th International Conference on Power Electronics - ECCE Asia, 2011
ABSTRACT Transportation electrification is one of the most promising ways to limit the CO2 emissi... more ABSTRACT Transportation electrification is one of the most promising ways to limit the CO2 emission from the transportation sector so as to avoid catastrophic climate changes in the future. However, a large-scale integration of electric vehicles will introduce new challenges to electric power systems operation, especially at distribution network level. This paper aims to analyze possible impacts of large-scale electric vehicle charging system on the distribution network, propose corresponding control methods for impact mitigation, and investigate potential benefits of system performance improvement. In this paper, simulation model development of electric vehicle charging system is described firstly. After that simulation studies for both steady-state and dynamic performance analysis is introduced respectively. Simulation results show that with properly designed control strategies, the impact of large-scale charging system on power grid can be mitigated effectively; furthermore, distribution network can also benefit from electric vehicles due to the improved controllability of active and reactive power.
Nano Letters, 2003
... Synthesis and Characterization of Core−Shell GaP@GaN and GaN@GaP Nanowires. ...
Scientific Reports, 2014
Electrochemical metallization memories based on redox-induced resistance switching have been cons... more Electrochemical metallization memories based on redox-induced resistance switching have been considered as the next-generation electronic storage devices. However, the electronic signals suffer from the interconnect delay and the limited reading speed, which are the major obstacles for memory performance. To solve this problem, here we demonstrate the first attempt of light-emitting memory (LEM) that uses SiO 2 as the resistive switching material in tandem with graphene-insulator-semiconductor (GIS) light-emitting diode (LED). By utilizing the excellent properties of graphene, such as high conductivity, high robustness and high transparency, our proposed LEM enables data communication via electronic and optical signals simultaneously. Both the bistable light-emission state and the resistance switching properties can be attributed to the conducting filament mechanism. Moreover, on the analysis of current-voltage characteristics, we further confirm that the electroluminescence signal originates from the carrier tunneling, which is quite different from the standard p-n junction model. We stress here that the newly developed LEM device possesses a simple structure with mature fabrication processes, which integrates advantages of all composed materials and can be extended to many other material systems. It should be able to attract academic interest as well as stimulate industrial application.
The Journal of Physical Chemistry C, 2012
ABSTRACT We offer a novel approach to improve the performance of P3HT/TiO2 hybrid photovoltaic de... more ABSTRACT We offer a novel approach to improve the performance of P3HT/TiO2 hybrid photovoltaic devices by incorporating either hydroxyl- or amino-functionalized silica nanodots (SND–OH or SND–NH2) into the hole transport layer of the PEDOT:PSS. The SNDs serve as screens between conducting polymer and ionomer PSS to improve the phase separation and charge transport of the PEDOT:PSS hole transport layer. The power conversion efficiency (PCE) was thus improved by 1.45 and 2.61 fold for devices fabricated with PEDOT:PSS containing 1 wt % of SND–OH (SND–OH device) and 1 wt % of SND–NH2 (SND–NH2 device), respectively, when compared with the devices fabricated by neat PEDOT:PSS. The increase in PCE arises from an increase in short circuit currents, which are affected by the phase separation of PEDOT:PSS with possessing incorporated SNDs. The low surface potential of hydroxyl-functionalized SNDs (SND–OH) is easily aggregated in the PEDOT:PSS solution and forms large-sized phase separation in the PEDOT:PSS film. The aggregation of SND–OH causes slight decreases in the resistance of PEDOT:PSS thin film from (61 ± 1 to 69 ± 4)× 106 Ohm/square and a decrease in the shielding effects of the SNDs. In contrast, the high surface potential of amino-functionalized SNDs are dispersed uniformly in the PEDOT:PSS solution and form morphologies with small-sized domains in the PEDOT:PSS film. As a result, the sheet resistance of PEDOT:PSS thin films is decreased from (61 ± 1 to 46 ± 3) × 106 Ohm/square. Therefore, the SND–NH2 device exhibits greater performance over the SND–OH devices.
Journal of Raman Spectroscopy, 2013
ABSTRACT Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic prop... more ABSTRACT Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic properties associated with their dimensionality. We here investigate the effect of size‐induced changes on phonon localization and explain the enhanced Raman response. The occurrence of normally forbidden transitions in the photoluminescence spectra provides evidence for the predicted localization effect. Spatially resolved Raman spectroscopy reveals a continuous change of the silicon Raman peak position and peak width along the nanotip that is attributed to a smooth change between bulk properties at the base to size‐induced phonon confinement in the apex of the nanotip. This approach allows to exclude heating effects that normally overwhelm the phonon confinement signature. The Raman spectra are in excellent agreement with the spatial correlation model and the extracted correlation length is comparable to the tip dimensions. The observed phonon confinement coincides with an enhancement of the Raman scattering efficiency at the tip apex and results in a 40‐fold increase of the sample's Raman intensity compared with bulk silicon. These results provide a step toward the integration of Si based optoelectronic devices. Copyright © 2012 John Wiley & Sons, Ltd.
Journal of Materials Chemistry, 2008
We have investigated the effect of polymer molecular weight on the morphology and performance of ... more We have investigated the effect of polymer molecular weight on the morphology and performance of poly(3-hexylthiophene)/TiO 2 nanorod hybrid photovoltaic devices by using scanning near field optical microscopy (SNOM), atomic force microscopy (AFM) and confocal Raman microscopy. From the topography and absorption mapping images, it is found that the rod-like structure of the low molecular weight polymer hybrid film consists of a large amount of grain boundaries and has a less continuous absorption mapping image. In contrast, the larger domain structure of the high molecular weight polymer hybrid film exhibits a continuous absorption mapping image, as a result of enhanced polymer stacking and electronic delocalization. The nanoscale morphology of the hybrid samples with different molecular weights also reveals the nature of photovoltaic performance and carrier transport behavior investigated by the time-of-flight technique.
Journal of Biomedical Optics, 2010
Sun, Tzu-Lin et al. "Ex vivo imaging and quantification of liver fibrosis using second-harmonic g... more Sun, Tzu-Lin et al. "Ex vivo imaging and quantification of liver fibrosis using second-harmonic generation microscopy." Journal of Biomedical Optics 15.3 (2010): 036002-6.
Applied Physics Letters, 2010
The enhancement of electroluminescence from the various direct current aged InGaN-based light-emi... more The enhancement of electroluminescence from the various direct current aged InGaN-based light-emitting diodes (LEDs) is presented. It is found that the light output intensity of the aged LED shows an enhancement of about 150% at low driving current density compared with that of the original LEDs. The efficiency increases and the peak-efficiency-current shifts toward lower magnitude of the aged LEDs
2010 International Symposium on Computer, Communication, Control and Automation (3CA), 2010
In this work, both of ZnO based metal-semiconductor-metal photodetectors with and without ZnO cap... more In this work, both of ZnO based metal-semiconductor-metal photodetectors with and without ZnO cap layer were fabricated on flexible substrates (polyethylene terephthalate, PET) for comparative analysis. The ZnO films were prepared by a low temperature sputtering process. The photodetector with ZnO cap layer (stack structure: ZnO/Ag/ZnO/PET) shows a much higher UV-to-visible rejection ratio of 1.56×103 than one without. It can be attributed to the photocurrents significantly enhanced under UV light illumination in such a novel structure. With an incident wavelength of 370 nm and 3 V applied bias, the responsivities of both photodetectors with and without ZnO cap layer are 3.80×10-2 and 2.36×10-3 A/W, respectively, which correspond to quantum efficiencies of 1.13 % and 0.07 %. The Schottky barrier height on Ag/ZnO interfaces is also obtained to be 0.782 eV.
Proceedings of SPIE - The International Society for Optical Engineering, 2009
Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD a... more Growth and luminescence properties of InN nanobelts (InNNBs) and InGaN nanowires (NWs) by MOCVD and thermal CVD will be presented, along with their relation and difference to thin film counterparts. While there is a growing acceptance of the low band gap (0.6-0.7 eV) of InN, the optical properties of the actual samples still suffered, presumably due to the difficulty in obtaining high-quality samples and/or controlling their defect and carrier concentrations. However, the free-standing nanobelts can be nearly defect-free, allowing an excellent opportunity for fundamental investigations on unique dimensionality. InNNBs show photoluminescence (PL) in IR with peak width of 14 meV, the sharpest reported to date for InN. Interestingly, with increasing excitation intensity, InNNBs reveal an anomalously large blueshift in PL, compared to thin films; along with a decrease in the phonon frequencies as evident by Raman measurements. Surface band bending, converse piezoelectric effect, and photoelastic effects are employed to explain these behaviors. As for InGaN NWs, both In-rich and Ga-rich ternary nanowires have been synthesized by simply varying growth temperature. Morphological and structural characterizations reveal a phase-separated microstructure wherein the isovalent heteroatoms are self-aggregated, forming self assembled quantum dots (SAQDs) embedded in NWs. The SAQDs are observed to dominate the emission behavior of both In-rich and Ga-rich nanowires, which has been explained by proposing a multi-level band schema.
ABSTRACT Multiphoton autofluorescence and second harmonic generation (SHG) microscopy are useful ... more ABSTRACT Multiphoton autofluorescence and second harmonic generation (SHG) microscopy are useful in respectively identifying elastin and collagen fibers within the skin dermis. In this study, we attempt to characterize the degree of skin thermal damage by using multiphoton microscopy to characterize the thermal changes to collagen and elastin fibers. We found that autofluorescence and SHG imaging behave differently in skin dermis treated with different temperatures and that an index reflecting the relative changes in autofluorescence and SHG intensity is useful to identify the degree of dermal thermal damage to the skin. With additional development, our approach can be used to identify the extent of thermal damage in patients.
The Journal of Physical Chemistry B, 2010
A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3H... more A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The λ max of the film absorption is blue-shifted from 520 to 470 nm, and its λ max of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration.
Solar Energy Materials and Solar Cells, 2009
We have fabricated a photovoltaic (PV) device based on the polymer blends of (poly(3-hexylthiophe... more We have fabricated a photovoltaic (PV) device based on the polymer blends of (poly(3-hexylthiophene) (P3HT)/polymethylmethacrylate (PMMA)) and inorganic TiO 2 nanorod bulk heterojunction. The optimized photovoltaic device with 1.6 wt% PMMA concentration has a power conversion efficiency of 0.65% under simulated AM 1.5 illumination (100 mW/cm 2 ), which is 38% more efficient than the device without the incorporation of PMMA. Furthermore, the PMMA-included device gives a shortcircuit current density of 2.57 mA/cm 2 , an open-circuit voltage of 0.53 V, and a fill factor of 0.48. Our studies have shown that having optimal PMMA concentration in the photovoltaic devices helps to smoothen the surface of the hybrid thin film, broaden the absorption spectrum, and improve the electrical conductivity. The results implying improvement in cell performance can be illustrated using atomic force microscopy (AFM), a UV/vis spectrophotometer and electrical measurements.
Solar Energy Materials and Solar Cells, 2009
We have studied the effect of polymer molecular weight on the performance of poly(3-hexylthiophen... more We have studied the effect of polymer molecular weight on the performance of poly(3-hexylthiophene):TiO 2 hybrid photovoltaic device using atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). The atomic force microscopic studies show the nanoscale morphology of the hybrid film changes from small domain size rod-like structure to large domain nodule-like structure with increasing the molecular weight of poly(3-hexylthiophene). The studies of SNOM of hybrid film reveal that the large domain structure of the high-molecular-weight P3HT hybrid film exhibits continuous absorption mapping as opposite to the discontinuous absorption mapping of the lowmolecular-weight P3HT hybrid film. Both results suggest the improvement in device efficiency from high-molecular-weight P3HT is due to the formation of large domain structure with increased carrier mobility and light harvesting.
In the past few years, we have fabricated nanoscale La0.7Sr0.3MnO3 periodic arrays with unique op... more In the past few years, we have fabricated nanoscale La0.7Sr0.3MnO3 periodic arrays with unique optical and magnetic properties successfully. These periodic patterns were made by La0.7Sr0.3MnO3 resist that can be developed under a nontoxic and environmental friendly manner using pure water. The resist is also capable to exhibit both positive and negative resist behaviors depending on the electron beam dosage. Thus, these special characteristics are used to fabricate and tune periodic structure thin film having controlled optical reflectance properties in the wavelength of 300 nm to 800 nm with one fixed design electron beam pattern by simply changing the electron beam dosage only. Additionally, the magnetization of La0.7Sr0.3MnO3 patterns can be enhanced by post sintering the sample at 900 °C after electron beam irradiation. Therefore, our study provides a one-step, simple, and convenient alternative technique for the fabrication of tunable optical structure and nanoscale magnetic pa...
Optics Express, 2008
Large-area multiphoton laser scanning microscopy (LMLSM) can be applied in biology and medicine f... more Large-area multiphoton laser scanning microscopy (LMLSM) can be applied in biology and medicine for high sensitivity and resolution tissue imaging. However, factors such as refractive index mismatch induced spherical aberration, emission/excitation absorption and scattering can result in axial intensity attenuation and lateral image heterogeneity, affecting both qualitative and quantitative image analysis. In this work, we describe an image correction algorithm to improve three-dimensional images in LMLSM. The method consists of multiplying the measured nonlinear signal by a three-dimensional correction factor, determined by the use of twophoton images of the appropriate specimens and specimen absorption and scattering properties at the excitation and emission wavelengths. The proposed methodology is demonstrated in correcting multiphoton images of objects imbedded in uniform fluorescent background, lung tissue, and Drosophila larva.
Nanotechnology, 2011
An on-chip approach for fabricating ferromagnetic/semiconductor-nanotip heterojunctions is demons... more An on-chip approach for fabricating ferromagnetic/semiconductor-nanotip heterojunctions is demonstrated. The high-density array of Si nanotips (SiNTs) is employed as a template for depositing La(0.7)Sr(0.3)MnO(3) (LSMO) rods with a pulsed-laser deposition method. Compared with the planar LSMO/Si thin film, the heterojunction shows a large enhancement of room temperature magnetoresistance (MR) ratio up to 20% under 0.5 T and a bias current of 20 µA. The MR ratio is found to be tunable, which increases with increasing external bias and the aspect ratios of the nanotips. Electric-field-induced metallization, in conjunction with nanotip geometry, is proposed to be the origin for the giant MR ratio.