Browsing by Author "Yu, P."

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Above-bandgap voltages from ferroelectric photovoltaic devices
(2010-02-01)
Yang, S. Y.
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Seidel, J.
;
Byrnes, S. J.
;
Shafer, P.
;
Yang, C. -H.
In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.
Broadband and omnidirectional antireflection from conductive indium-tin-oxide nano-columns prepared by glancing-angle deposition with nitrogen (vol 94, 051114, 2009)
(2013-11-11)
Chang, C. H.
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Yu, P.
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Yang, C. S.
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光電工程學系
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Department of Photonics
Chemical vapour deposition growth and Raman characterization of graphene layers and carbon nanotubes
(2016-01-01)
Lai, Y-C
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Rafailov, P. M.
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Vlaikova, E.
;
Marinova, V.
;
Lin, S. H.
Single-layer graphene films were grown by chemical vapour deposition (CVD) on Cu foil. The CVD process was complemented by plasma enhancement to grow also vertically aligned multiwalled carbon nanotubes using Ni nanoparticles as catalyst. The obtained samples were characterized by Raman spectroscopy analysis. Nature of defects in the samples and optimal growth conditions leading to achieve high quality of graphene and carbon nanotubes are discussed.
Chemical vapour deposition growth of graphene layers on metal substrates
(2014-01-01)
Lai, Y-C
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Yu, S-C
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Rafailov, P. M.
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Vlaikova, E.
;
Valkov, S.
Graphene layers were grown by chemical vapour deposition (CVD) on Si wafers covered by a SiO2 substrate layer and a Ni interlayer, and on copper and nickel foil. The obtained graphene layers were characterized by Raman spectroscopy. The films grown on SiO2/Ni substrate and Ni foil comprise mainly multilayer defect-rich graphene, while those on Cu foil exhibit the spectroscopic fingerprint of relatively defect-free single-layer graphene due to the low carbon solubility in copper and the suitably chosen substrate position in a quasi-closed volume. Optimal growth conditions and the nature of defects in the layers are discussed.
Conduction at domain walls in oxide multiferroics
(2009-03-01)
Seidel, J.
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Martin, L. W.
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He, Q.
;
Zhan, Q.
;
Chu, Y. -H.
Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO(3). The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features.
Conversion Efficiency Enhancement of GaN/In(0.11)Ga(0.89)N Solar Cells With Nano Patterned Sapphire and Biomimetic Surface Antireflection Process
(2011-09-15)
Wang, H. W.
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Chen, H. C.
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Chang, Y. A.
;
Lin, C. C.
;
Han, H. W.
In this study, p-i-n double-heterojunction GaN/In(0.11)Ga(0.89)N solar cells grown by metal-organic chemical vapor deposition on pattern sapphire substrate are presented. The solar cell with standard process has a conversion efficiency of 3.1%, which corresponds to a fill factor of 58%, short circuit current density of 2.86 mA/cm(2), and open circuit voltage of 1.87 V under AM1.5G illumination. To further improve the conversion efficiency of the GaN/In(0.11)Ga(0.89)N solar cells, two-dimensional polystyrene nanospheres were deposited and self-organized as mask in the anisotropic inductively coupled plasma reactive ion etching process to form a biomimetic surface roughing texture. The surface morphology of the solar cell shows a periodically hexagonal bead pattern and the beads are formed in a diameter of 160 nm with a period of 250 nm. An increase of 15% in short circuit current density is found, thus improving the conversion efficiency to 3.87%. If we optimize the structure for 180 nm of the height and 375 nm of the period, a 10% gain can be expected when compared to the current structure.
Conversion Efficiency Enhancement of GaN/In0.11Ga0.89N Solar Cells With Nano Patterned Sapphire and Biomimetic Surface Antireflection Process
(2011-09-15)
Wang, H. W.
;
Chen, H. C.
;
Chang, Y. A.
;
Lin, C. C.
;
Han, H. W.
In this study, p-i-n double-heterojunction GaN/In0.11Ga0.89N solar cells grown by metal-organic chemical vapor deposition on pattern sapphire substrate are presented. The solar cell with standard process has a conversion efficiency of 3.1%, which corresponds to a fill factor of 58%, short circuit current density of 2.86 mA/cm(2), and open circuit voltage of 1.87 V under AM1.5G illumination. To further improve the conversion efficiency of the GaN/In0.11Ga0.89N solar cells, two-dimensional polystyrene nanospheres were deposited and self-organized as mask in the anisotropic inductively coupled plasma reactive ion etching process to form a biomimetic surface roughing texture. The surface morphology of the solar cell shows a periodically hexagonal bead pattern and the beads are formed in a diameter of 160 nm with a period of 250 nm. An increase of 15% in short circuit current density is found, thus improving the conversion efficiency to 3.87%. If we optimize the structure for 180 nm of the height and 375 nm of the period, a 10% gain can be expected when compared to the current structure.
Correlation between nanoscale and nanosecond resolved ferroelectric domain dynamics and local mechanical compliance
(2011-05-01)
Polomoff, N. A.
;
Rakin, A.
;
Lee, S.
;
Palumbo, V.
;
Yu, P.
The local dynamics of ferroelectric domain polarization are uniquely investigated with sub-20-nm resolved maps of switching times, growth velocities, and growth directions. This is achieved by analyzing movies of hundreds of consecutive high speed piezo force microscopy images, which record domain switching dynamics through repeatedly alternating between high speed domain imaging and the application of 20-nanosecond voltage pulses. Recurrent switching patterns are revealed, and domain wall velocities for nascent domains are uniquely reported to be up to four times faster than for mature domains with radii greater than approximately 100 nm. Switching times, speeds, and directions are also shown to correlate with local mechanical compliance, with domains preferentially nucleating and growing in compliant sample regions while clearly shunting around locations with higher stiffness. This deterministic switching behavior strongly supports a defect-mediated energy landscape which controls polarization reversal, and that can therefore be predicted, modeled, and even manipulated through composition, processing, and geometry. Such results have important implications for the practical performance of ferroelectric devices by enabling guided optimization of switching times and feature densities, while the methods employed provide a new means to investigate and correlate dynamic functionality with mechanical properties at the nanoscale. (C) 2011 American Institute of Physics. [doi:10.1063/1.3581205]
Critical thickness and orbital ordering in ultrathin La(0.7)Sr(0.3)MnO(3) films
(2008-09-01)
Huijben, M.
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Martin, L. W.
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Chu, Y. -H.
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Holcomb, M. B.
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Yu, P.
Detailed analysis of transport, magnetism, and x-ray absorption spectroscopy measurements on ultrathin La(0.7)Sr(0.3)MnO(3) films with thicknesses from 3 to 70 unit cells resulted in the identification of a lower critical thickness for a nonmetallic nonferromagnetic layer at the interface with the SrTiO(3) (001) substrate of only three unit cells (similar to 12 angstrom). Furthermore, linear-dichroism measurements demonstrate the presence of a preferred (x(2)-y(2)) in-plane orbital ordering for all layer thicknesses without any orbital reconstruction at the interface. A crucial requirement for the accurate study of these ultrathin films is a controlled growth process, offering the coexistence of layer-by-layer growth and bulklike magnetic/transport properties.
Critical thickness and orbital ordering in ultrathin La0.7Sr0.3MnO3 films
(2008-09-01)
Huijben, M.
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Martin, L. W.
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Chu, Y. -H.
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Holcomb, M. B.
;
Yu, P.
Detailed analysis of transport, magnetism, and x-ray absorption spectroscopy measurements on ultrathin La0.7Sr0.3MnO3 films with thicknesses from 3 to 70 unit cells resulted in the identification of a lower critical thickness for a nonmetallic nonferromagnetic layer at the interface with the SrTiO3 (001) substrate of only three unit cells (similar to 12 angstrom). Furthermore, linear-dichroism measurements demonstrate the presence of a preferred (x(2)-y(2)) in-plane orbital ordering for all layer thicknesses without any orbital reconstruction at the interface. A crucial requirement for the accurate study of these ultrathin films is a controlled growth process, offering the coexistence of layer-by-layer growth and bulklike magnetic/transport properties.
Efficiency Enhancement in Single-Junction InGaP Solar Cells by Using Self-Assembled Nanospheres
(2011-06-01)
Chen, H. C.
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Lin, C. C.
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Wang, H. W.
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Tsai, M. A.
;
Tseng, P. C.
A single-junction InGaP solar cell with polystyrene (PS) nanospheres on the surface has been developed. The self-assembly of PS nanospheres is one of the simplest and the fastest methods with which to build a 2-D closely packed periodic structure. Due to the scattering of the PS nanospheres, the light path length can be increased when compared to the InGaP solar cells without PS nanospheres on the top. An increase short-circuit current from 8.93 to 10.31 mA is improved when a single-junction InGaP solar cell is coated with PS nanospheres. The conversion efficiency measured can also be improved from 8.79% to 9.71%. The single-junction InGaP solar cell with PS nanospheres was achieved.
Electric modulation of conduction in multiferroic Ca-doped BiFeO(3) films
(2009-06-01)
Yang, C. -H.
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Seidel, J.
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Kim, S. Y.
;
Rossen, P. B.
;
Yu, P.
Many interesting materials phenomena such as the emergence of high-T(c) superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO(3) through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of similar to 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO(3).
Electric modulation of conduction in multiferroic Ca-doped BiFeO3 films
(2009-06-01)
Yang, C. -H.
;
Seidel, J.
;
Kim, S. Y.
;
Rossen, P. B.
;
Yu, P.
Many interesting materials phenomena such as the emergence of high-T-c superconductivity in the cuprates and colossal magnetoresistance in the manganites arise out of a doping-driven competition between energetically similar ground states. Doped multiferroics present a tantalizing evolution of this generic concept of phase competition. Here, we present the observation of an electronic conductor-insulator transition by control of band-filling in the model antiferromagnetic ferroelectric BiFeO3 through Ca doping. Application of electric field enables us to control and manipulate this electronic transition to the extent that a p-n junction can be created, erased and inverted in this material. A 'dome-like' feature in the doping dependence of the ferroelectric transition is observed around a Ca concentration of similar to 1/8, where a new pseudo-tetragonal phase appears and the electric modulation of conduction is optimized. Possible mechanisms for the observed effects are discussed on the basis of the interplay of ionic and electronic conduction. This observation opens the door to merging magnetoelectrics and magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom already present in the multiferroic BiFeO3.
Electrically controllable spontaneous magnetism in nanoscale mixed phase multiferroics
(2011-03-01)
He, Q.
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Chu, Y. -H.
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Heron, J. T.
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Yang, S. Y.
;
Liang, W. I.
Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO(3). Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.
Emergent phenomena at multiferroic heterointerfaces
(2012-10-28)
Yu, P.
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Chu, Y. H.
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Ramesh, R.
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材料科學與工程學系
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Department of Materials Science and Engineering
The coupling and reconstruction of electronic degrees of freedom (such as charge, spin and orbital) at a heterointerface can lead to unexpected and exotic states of matter. In this study, using model systems consisting of multiferroic BiFeO3 and ferromagnetic La0.7Sr0.3MnO3, we review the current understanding of a novel interfacial magnetic state formed at the interface, and highlight some possible mechanisms responsible for this interesting phenomenon and identify open questions for future studies.
Enhance current density and light trapping effect in a-Si thin film solar cells by flexible textured PDMS film
(2013)
Han, H. V.
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Chen, H. C.
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Tsai, Y. L.
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Lin, C. C.
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Kuo, H. C.
The light trapping effect of a textured polydimethylsiloxane (PDMS) film on the suppression of surface reflection in a-Si:H solar cell is investigated. The angular dependence of scattered light from this PDMS film was measured and a symmetric scattering intensity peak was found at +/- 20(0). The enhancement of power conversion efficiency in the a-Si:H solar cell with textured PDMS film can reach 16.37% compared to the a-Si:H solar cells without textured PDMS film. This improvement is mainly attributed to the enhanced efficiencies of light trapping and scattering. Such a design concept and fabrication process can be widely adapted to a variety of thin film solar cells.
Enhancement of Light Harvesting and Power Conversion Efficiency in GaAs solar cells using flexible textured PDMS film
(2013-01-01)
Han, H. V.
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Chen, H. C.
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Lin, C. C.
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Tsai, Y. L.
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Yeh, Y. L.
Light trapping of textured polydimethylsiloxane films for antireflection is investigated. The efficiency of GaAs solar cell with textured PDMS film reach 28% enhancement. Numerical analysis successfully rebuilds the surface profile close to the real situation. (C) 2013 Optical Society of America
Fabrication of aligned single wall carbon nanotube absorbers for high power passive mode-locked Nd:GdVO(4) laser
(2012-01-01)
Cheng, S. Y.
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Wang, Y. G.
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Yu, P.
;
Cheng, Y. J.
;
Tang, J.
Using the vertical evaporation technique we fabricated saturable absorbers by transferring the water-soluble single wall carbon nanotubes (SWCNT) onto a hydrophilic quartz substrate. The fast recovery times of the absorber were measured to be 136 and 790 fs. The modulation depth of the absorber was about 2%. Passive mode-locked Nd:GdVO(4) laser using such an absorber was demonstrated. The continuous wave mode-locked pulses with the pulse duration of 12.4 ps and the repetition of 120 MHz were achieved. The maximum average output power of the mode-locked laser is 2.4 W at the pump power of 13 W. Such kind of absorber has potential to be put into practical use for high power solid-state laser mode locking.
Fabrication of aligned single wall carbon nanotube absorbers for high power passive mode-locked Nd:GdVO4 laser
(2012-01-01)
Cheng, S. Y.
;
Wang, Y. G.
;
Yu, P.
;
Cheng, Y. J.
;
Tang, J.
Using the vertical evaporation technique we fabricated saturable absorbers by transferring the water-soluble single wall carbon nanotubes (SWCNT) onto a hydrophilic quartz substrate. The fast recovery times of the absorber were measured to be 136 and 790 fs. The modulation depth of the absorber was about 2%. Passive mode-locked Nd:GdVO4 laser using such an absorber was demonstrated. The continuous wave mode-locked pulses with the pulse duration of 12.4 ps and the repetition of 120 MHz were achieved. The maximum average output power of the mode-locked laser is 2.4 W at the pump power of 13 W. Such kind of absorber has potential to be put into practical use for high power solid-state laser mode locking.
Ferromagnetic Enhancement of CE-Type Spin Ordering in (Pr, Ca)MnO(3)
(2011-05-04)
Zhou, S. Y.
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Zhu, Y.
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Langner, M. C.
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Chuang, Y. -D.
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Yu, P.
We present resonant soft x-ray scattering results from small bandwidth manganites (Pr, Ca)MnO(3), which show that the CE-type spin ordering (SO) at the phase boundary is stabilized only below the canted antiferromagnetic transition temperature and enhanced by ferromagnetism in the macroscopically insulating state (FM-I). Our results reveal the fragility of the CE-type ordering that underpins the colossal magnetoresistance effect in this system, as well as an unexpected cooperative interplay between FM-I and CE-type SO which is in contrast to the competitive interplay between the ferromagnetic metallic state and CE-type ordering.