Jane P. Chang

University of California, Los Angeles, Los Ángeles, California, United States

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Publications (99)236.09 Total impact

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    ABSTRACT: Beyond-CMOS devices concepts are greatly dependent on new functional materials to provide inspiration and innovation beyond the silicon status quo. Here, we propose a material framework specifically for beyond-CMOS devices. In doing so, material system examples and data points presented are taken from the Center on Functional Accelerated Nanomaterials Engineering, the STARnet Center of Excellence.
    No preview · Article · Dec 2015
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    ABSTRACT: A radical-enhanced atomic layer deposition (RE-ALD) process is described for the synthesis of BiFeO3. Metalorganic precursors β-diketonate, tris(2,2,6,6-tetramethyl-3,5-heptanedionato) iron(III) (Fe(TMHD)3), and Bi(TMHD)3 are coreacted with oxygen radicals (produced by a coaxial microwave cavity) as the oxidation source. Stoichiometric BiFeO3 films were deposited on SrTiO3 (001) substrates at 210 °C and crystallized from 450 to 750 °C. The crystallinity and phases in the films were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The BiFeO3 was determined to be phase-pure and epitaxial when annealed at 650 °C. To study the functional properties, ferroelectric switching was demonstrated using piezoresponse force microscopy (PFM), while weak ferromagnetic behavior was measured using superconducting quantum interference device (SQUID) magnetometry. The films have properties comparable to those of films grown by other methods-growth rates are superior in comparison to other ALD methods, demonstrating the potential of RE-ALD for the synthesis of multiferroic complex-oxide thin films.
    Full-text · Article · Oct 2015 · Chemistry of Materials
  • Jea Cho · Taeseung Kim · Trevor Seegmiller · Jane P. Chang
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    ABSTRACT: A study of surface reaction mechanism on atomic layer deposition (ALD) of aluminum silicate (Al xSiyO) was conducted with trimethylaluminum (TMA) and tetraethoxysilane (TEOS) as precursors and H2O as the oxidant. In-situ Fourier transform infrared spectroscopy (FTIR) was utilized to elucidate the underlying surface mechanism that enables the deposition of Al xSiyO by ALD. In-situ FTIR study revealed that ineffective hydroxylation of the surface ethoxy (–OCH2CH3) groups prohibits ALD of SiO2 by TEOS/H2O. In contrast, effective desorption of the surface ethoxy group was observed in TEOS/H2O/TMA/H2O chemistry. The presence of Al-OH* group in vicinity of partially hydroxylated ethoxy (–OCH2CH3) group was found to propagate disproportionation reaction, which results in ALD of Al xSiyO. The maximum thickness from incorporation of SiOx from alternating exposures of TEOS/H2O chemistry in Al xSiyO was found to be ∼2 Å, confirmed by high resolution transmission electron microscopy measurements.
    No preview · Article · Sep 2015 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
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    ABSTRACT: The etch rate of HfxLayOz films in Cl2/BCl3 plasmas was measured in-situ in an inductively coupled plasma reactor using a quartz crystal microbalance and corroborated by cross-sectional SEM measurements. The etch rate depended on the ion energy as well as the plasma chemistry. In contrast to other Hf-based ternary oxides, the etch rate of HfxLayOz films was higher in Cl2 than in BCl3. In the etching of Hf0.25La0.12O0.63, Hf appeared to be preferentially removed in Cl2 plasmas, per surface compositional analysis by x-ray photoelectron spectroscopy and the detection of HfCl3 generation in mass spectroscopy. These findings were consistent with the higher etch rate of Hf0.25La0.12O0.63 than that of La2O3.
    No preview · Article · May 2015 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
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    ABSTRACT: A reactive ion etching process with alternating Cl2 and H2 exposures has been shown to chemically etch CoFe film that is an integral component in magnetoresistive random access memory (MRAM). Starting with systematic thermodynamic calculations assessing various chemistries and reaction pathways leading to the highest possible vapor pressure of the etch products reactions, the potential chemical combinations were verified by etch rate investigation and surface chemistry analysis in plasma treated CoFe films. An ∼20% enhancement in etch rate was observed with the alternating use of Cl2 and H2 plasmas, in comparison with the use of only Cl2 plasma. This chemical combination was effective in removing metal chloride layers, thus maintaining the desired magnetic properties of the CoFe films. Scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy showed visually and spectroscopically that the metal chloride layers generated by Cl2 plasma were eliminated with H2 plasma to yield a clean etch profile. This work suggests that the selected chemistries can be used to etch magnetic metal alloys with a smooth etch profile and this general strategy can be applied to design chemically based etch processes to enable the fabrication of highly integrated nanoscale MRAM devices.
    No preview · Article · Mar 2015 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
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    ABSTRACT: Lithium (Li)-ion batteries have drawn much attention for their outstanding performance in portable electronic applications. These batteries have the potentials to function as miniaturized power source for microelectromechanical (MEMS) devices through the fabrication of 3-dimensional configurations. To fabricate a fully functional 3D Li-ion microbattery, however, an ultra-thin and highly conformal electrolyte layer is required to coat the 3D electrodes. The solid oxide Li-ion conductor, lithium aluminosilicate (LixAlySizO, LASO), synthesized by atomic layer deposition (ALD) is a promising electrolyte material for 3D battery applications owing its adequate ionic conductivity as well as improved electrode stability. The self-limiting characteristic of ALD allows for precise control of thickness and composition of complex oxides and results in a highly conformal and pinhole-free coating even on highly complex structures such as high aspect ratio 3D electrodes. The metal precursors, lithium t-butoxide (LTB), trimethylaluminum (TMA), tris(tert-butoxy)silanol (TTBS), and tetraethylorthosilicate (TEOS) were used to form LixAlySizO via ALD. In-situ FTIR was implemented to study the incubation time and growth mechanisms of each constituent oxide on the other to improve the controllability of deposited films. In-situ FTIR studies revealed that the growth mechanism of silicon oxide is strongly affected by the underlying oxide layer, exhibiting different surface reaction mechanisms during the incubation stage. Li-ion conductivities and the activation energy of as-deposited LASO/LAO/LSO films with respect to lithium contents and the film thickness were studied. The LASO ALD coating on 3D carbon array posts were confirmed to be conformal and uniform using transmission electron microscopy (TEM) imaging. A Li-ion half-cell consisting of LASO coated on 3D carbon array electrode has shown promising improvement in efficiency when compared to bare electrode. Lithiation cycling tests of thin LASO/LAO/LSO films were found to be functions of both composition and thickness. The reversibility and kinetics of insertion as well as the effect on the cycling stability from the direct deposition of LASO/LAO/LSO on potential anode materials, SiNWs were also investigated using in-situTEM observations during lithiation.
    No preview · Conference Paper · Nov 2014
  • Ya-Chuan Perng · Taeseung Kim · Jane P. Chang
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    ABSTRACT: An epitaxial AlN film was synthesized on 4H-SiC using alternate doses of trimethyl aluminum (TMA) and anhydrous ammonia (a-NH3) at 540 degrees C. The growth per cycle (GPC) at the temperature was limited to 0.27 angstrom/cycle under the minute amount of residual H2O. The formation of an inert Al-O-Al bond generated by the recombinative desorption of two Al-OH species to produce H2O has been proposed as a possible reason for the low deposition rate. By eliminating any present moisture in the reactor system, the GPC increased to 1.5 angstrom/cycle. The suppression of the OH group generation at 540 degrees C made it possible to deposit nearly stoichiometric AlN film which has the crystallinity with the preferred orientation along the (0 0 0 1) direction on 4H-SiC templates. The measured RMS surface roughness was comparable to that achieved by molecular beam epitaxy (MBE). (C) 2014 Published by Elsevier B.V.
    No preview · Article · Sep 2014 · Applied Surface Science
  • Taeseung Kim · Jack Kun-Chieh Chen · Jane P. Chang
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    ABSTRACT: A thermodynamic analysis of etch chemistries for Co, Fe, and Ni using a combination of hydrogen, oxygen, and halogen gases suggested that a single etchant does not work at 300 K; however, a sequential exposure to multiple etchants results in sufficiently high partial pressure of the reaction products for the process to be considered viable. This sequential dose utilized the two reactions, a surface halogenation followed by the secondary etchant exposure. (MX2 (c) + 3Y →MY(g) + 2XY(g), where M = Co, Fe, Ni; X = F, Cl, Br; Y = O, H) The volatilization reaction induced by sequential plasma exposure changed the equilibrium point, increasing the partial pressure of the etch product. Amongst all combinations, Cl2 or Br2 plasmas followed by H2 plasma were the most effective. From both the gas phase diagnostics and surface composition analysis, H2 plasma alone could not etch metallic Co, Fe, and Ni films but alternating doses of Cl2 and H2 plasmas resulted in more effective removal of chlorinated metals and increased the overall etch rate.
    No preview · Article · Jul 2014 · Journal of Vacuum Science & Technology A Vacuum Surfaces and Films
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    ABSTRACT: Thin films of a solid electrolyte, lithium aluminosilicate, were synthesized by atomic layer deposition (ALD) for potential applications in 3D Li-ion microbatteries. The films were deposited at 290 °C via alternating the ALD growth of the constituents, LiOH, Al2O3 and SiO2. Manipulation of the cation composition and thickness was achieved through well-controlled surface reactions during each precursor pulse cycle. Various compositions were obtained by changing the number of pulse cycles for each precursor, which enabled lithium aluminate (LixAlyO), lithium aluminosilicate (LixAlySizO) and stoichiometric LiAlSiO4 materials to be prepared. The as-deposited ALD films were amorphous and formed conformal coatings over Si nanowires. Films as thin as 6 nm were found to be free of pinholes. Complex impedance measurements confirmed that the films were ionic conductors with the room temperature conductivity in the range of 10−7 to 10−9 S cm−1 and an activation energy between 0.46 and 0.84 eV, depending upon the film composition.
    Preview · Article · Jun 2014
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    ABSTRACT: The effect of oxygen addition to chlorine plasma during shallow trench isolation etching is quantified in this work. Specifically, the electron density and the electron temperature in an electron cyclotron resonance reactor were characterized by a Langmuir probe and were found to remain relatively constant upon O2 addition. The silicon etching rates were found to increase with the square root of the ion energy, suggesting the etching reaction is limited by the momentum transfer from ions to the surface. A relatively small amount of oxygen addition (<10%) to the chlorine plasma simultaneously changes the reactor wall conditions and surface kinetics, since oxygen becomes actively involved in the surface reactions. The change in the chamber wall conditions and surface kinetics leads to the change in both the amount of etch products and the etched feature profile. The incorporation of oxygen on the surface results in a significant change of the etched surface morphology and its composition. This work suggests a small amount of O2 addition to Cl2 plasmas in shallow trench isolation etching changes the etching behavior primarily through modifying the kinetics on etched surfaces. A multiscale etch model consisting of translating mixed layer and Monte Carlo modules for bulk and feature scale etching, respectively, was successfully applied to this case, demonstrating good agreement with the experimental results.
    No preview · Article · Jul 2013 · Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society
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    ABSTRACT: We demonstrated that incorporation of octadecyltrimethoxysilane (OTMS) functionalized, spectrally tuned, gold/silica (Au/SiO2) core/shell nanospheres and nanorods into the active layer of an organic photovoltaic (OPV) device led to an in increase photo conversion efficiency (PCE). A silica shell layer was added onto Au core nanospheres and nanorods in order to provide an electrically insulating surface that does not interfere with carrier generation and transport inside the active layer. Functionalization of the Au/SiO2 core/shell nanoparticles with the OTMS organic ligand was then necessary in order to transfer the Au/SiO2 core/shell nanoparticles from an ethanol solution into an OPV polymer-compatible solvent, such as dichlorobenzene (DCB). The OTMS-functionalized Au/SiO2 core/shell nanorods and nanospheres were then incorporated into the active layers of two OPV polymer systems: a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCB60M) OPV device and a poly[2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4-dione] (PBDTT-DPP:PC60BM) OPV device. For the P3HT:PC60BM polymer with a band edge of ~700 nm, the addition of the core/shell nanorods with an aspect ratio (AR) of ~2.5 (extinction peak ~670 nm) resulted in a 7.1% improvement in PCE, while for the PBDTT-DPP:PC60BM polymer with a band edge ~860 nm, the addition of core/shell nanorods with an AR of ~4 (extinction peak ~830 nm) resulted in a 14.4% improvement in PCE. The addition of Au/SiO2 core/shell nanospheres to the P3HT:PC60BM-polymer resulted in a 2.7% improvement in PCE, while their addition to a PBDTT-DPP:PC60BM polymer resulted in a 9.1% improvement. The PCE and Jsc enhancements were consistent with external quantum efficiency (EQE) measurements and the EQE enhancements spectrally matched the extinction spectra of Au/SiO2 nanospheres and nanorods in both OPV polymer systems.
    No preview · Article · Apr 2013 · ACS Nano
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    Ju H. Choi · Feng Zhang · Ya-Chuan Perng · Jane P. Chang
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    ABSTRACT: The incubation time during atomic layer deposition (ALD) of lead oxide, zirconium oxide, and titanium oxide on each other was quantified in order to precisely control the composition of lead zirconate titanate (PZT). The desired stoichiometry of Pb:Zr:Ti=2:1:1, which yields the desired ferroelectricity, was found to depend strongly on the ALD sequence, the substrate of choice, as well as the postdeposition annealing temperature. With the desired stoichiometry, the ferroelectric and piezoelectric properties of the PZT films were validated by polarization–voltage hysteresis loop and piezoresponse force microscopy, respectively, demonstrating that ALD method is a viable technique for ultra thin ferroelectric films for device applications.
    Full-text · Article · Jan 2013 · Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures
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    ABSTRACT: Lithium (Li)-ion batteries have drawn much attention for their outstanding performance in portable electronics applications with the potentials to function as a power source for further minizaturized devices, including micro-systems through the utilization of 3-dimensional electrodes based on high aspect ratio pillars. To realize this potential, an ultra-thin and highly conformal electrolyte layer is needed to coat the 3D electrode array. The ionic conductor lithium aluminosilicate (LiAlSiO4) synthesized by atomic layer deposition (ALD) is a promising electrolyte material for 3D battery applications not only due to its high ionic conductivity along its c-axis resulting from channels formed by the alternating tetrahedra of aluminum-oxygen (Al-O) and silicon-oxygen (Si-O), but also expected to provide similar improved cell cyclability, as reported in the preliminary studies of ultra-thin metal-oxide ALD coatings on electrodes. The self-limiting characteristic of ALD allows for precise control of thickness and composition of complex oxides and results in a highly conformal and pinhole-free coating suitable in 3D micro-battery applications or electrolyte surface coatings. The metal precursors used in this work are tetraethyl orthosilicate (TEOS), trimethylaluminum (TMA) and lithium t-butoxide (LTB). These precursors, along with water vapor as the oxidant, were used to deposit SiO2, Al2O3 and Li2O, with the deposition rates in the range of 0.8~2Å/cycle, respectively. The deposition rate of stoichiometric LiAlSiO4 was ~20Å/cycle at a temperature of 290°C. The concentration of each metal element in LixAlySizO (LASO) thin films was found to correlate closely to ALD cycles and the associated incubation times. The crystallinity of the films after post-deposition rapid thermal annealing (RTA) was a function of cation atomic percentage. Li-ionic conductivities and the activation energy of as-deposited LASO films with respect to lithium contents as well as their relation to the film thickness were studied. The LASO ALD coating on 3D features, such as NWs and nanopaticles (NPs), were confirmed to be conformal and uniform by transmission electron microscopy (TEM) imaging. The cell performance as well as cyclability enhancement from LixAlySizO was investigated for a silicon-nanowire 3D microbattery, where SiNW was used as an anode, to explore the potentials of a solid-state SiNW battery with a solid-oxide electrolyte.
    No preview · Conference Paper · Oct 2012
  • Nathan Marchack · Jane P Chang
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    ABSTRACT: The invention of the transistor followed by more than 60 years of aggressive device scaling and process integration has enabled the global information web and subsequently transformed how people communicate and interact. The principles and practices built upon chemical processing of materials on silicon have been widely adapted and applied to other equally important areas, such as microfluidic systems for chemical and biological analysis and microscale energy storage solutions. The challenge of continuing these technological advances hinges on further improving the performance of individual devices and their interconnectivity while making the manufacturing processes economical, which is dictated by the materials' innate functionality and how they are chemically processed. In this review, we highlight challenges in scaling up the silicon wafers and scaling down the individual devices as well as focus on needs and challenges in the synthesis and integration of multifunctional materials.
    No preview · Article · Jul 2012 · Annual Review of Chemical and Biomolecular Engineering
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    ABSTRACT: Rare-earth (RE = Eu, Dy, Tb, and Ce) ion doped core and core–shell LaPO4 phosphors were synthesized to elucidate the effect of spatial distribution of dopants on the emission spectra. The core–shell architecture was designed as a single particle that can be excited by a single wavelength and yield a balanced white light appearance with long emission lifetimes. Specifically, a multishell architecture was employed to separate the Eu3+ and Tb3+ within the phosphor to circumvent the energy transfer between them, passivate the surface quenching sites, and control Ce3+ doping to sensitize other RE ions. To assess the effectiveness of these core–shell phosphors, the International Commission on Illumination (CIE) coordinates and luminescence lifetimes are quantified as the figures of merit. The Eu3+:LaPO4|Ce3+,Dy3+:LaPO4|Tb3+:LaPO4 layering resulted in CIE coordinates of (0.34, 0.35) using 365 nm excitation, nearly at center of the white light regime at (0.35, 0.35). Finally, the emission lifetimes were measured to be 0.85, 4.34, and 3.26 ms and resulted in a total increase of 31, 36, and 16% over the RE3+:LaPO4 reference phosphors, where RE = Dy, Tb, and Eu, respectively. The synthesized phosphor material has high-quality white light with improved emission lifetimes, suitable for application in white light LED devices.
    No preview · Article · Jun 2012 · The Journal of Physical Chemistry C
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    ABSTRACT: Rare-earth (RE) (Er3+ and Yb3+, Er3+)-doped yttrium oxide (Y2O3) core–shell particles were synthesized in this work using a two-step process where the cores were formed by molten salt synthesis while the shell was deposited by a sol–gel process. The cores were 100–150 nm, and a shell layer, up to 12 nm thick, was controllable based on the mass ratio between the RECl3 salts and the Er3+:Y2O3 (1 mol %) particles. A passive Y2O3 shell layer, at an optimal thickness around 8 nm, passivated the surface quenching sites and resulted in a 53% increase in photoluminescence lifetimes and visible separation in Stark splitting. Optically active shell layers, such as Yb2O3 and Yb3+:Y2O3, not only passivated the quenching sites but also facilitated energy transfer between the spatially controlled RE ions. Furthermore, the effect of surface passivation on the upconversion luminescence was determined through the purposed dynamic processes to corroborate the effect of the hydroxyl groups on energy dissipation. The addition of a passive shell layer or a sensitizer reduced the upconversion to a two-photon process due to a decreased branching ratio at the 4I11/2 energy level. Yb2O3 is deemed the most effective shell material due to the largest increase photoluminescence intensity at 1535 nm as a function of the pump power and the lifetime of the 4S3/2 radiative transition, important in upconversion luminescence. The increased lifetime and low pump power achieved with Er3+:Y2O3|Yb2O3 core–shell phosphors hold promise in lighting devices for improved overall device efficiency.
    No preview · Article · Apr 2012 · The Journal of Physical Chemistry C
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    ABSTRACT: To predict and optimize luminescence efficiency of rare-earth ion doped (RE) nanophosphors, a relationship between the RE-concentration and the luminescent parameters is often obtained by Judd-Ofelt analysis, where the quality factor (χ = Ω4/Ω6) depends on the Er interactions with other RE elements in the second nearest neighboring shell. In this work, a detailed analysis of the local bonding environment by extended x-ray absorption fine structure (EXAFS) analyses is shown as effective as the Judd-Ofelt analysis to quantify the Er↔RE interaction in the second nearest neighboring shell (ρN = IREr↔RE2/IREr↔RE1). As the physical basis of ρN is consistent to that of χ, the EXAFS analysis becomes a viable alternative to replace Judd-Ofelt analysis to predict the optimum dopant concentration. This approach was corroborated based on analysis of Er3+:Y2O3 and core-shell Er3+:Y2O3|Y2O3 (5 nm shell) nanoparticles (NPs), with Er3+ concentrations up to 20 mol %. The ρN ratio from EXAFS analysis was shown to strongly correlate to the lifetimes extracted from the Judd-Ofelt analysis, both predicting the optimal dopant concentrations to be at 5 mol % and 2 mol % for the Er3+:Y2O3 and core-shell NPs, respectively. This confirms that EXAFS analysis can be used as a more time efficient method to achieve the same outcome typically obtained by Judd-Ofelt analysis, enabling the optimization of the luminescent lifetimes of RE doped nano-phosphors.
    No preview · Article · Apr 2012 · Journal of Applied Physics
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    ABSTRACT: Di-block copolymer synthesized Co/Al2O3 core-shell nanocrystal (NC) capacitors were fabricated in order to study the temperature-dependent electron transport. The capacitance-voltage memory window is shown to increase proportionally with the substrate temperature, saturating at 3.5 V, at 175 °C. At elevated operating temperatures, the tunneling of electrons increases, resulting in large flatband voltage shift. Furthermore, the electron leakage of the NCs at high temperature is faster than the leakage at room temperature due to thermally assisted tunneling. The activation energy is determined by exponentially fitting the thermally dependent retention performance, which was then used to model the occupied energy levels and further elucidate the electron transport within the NC memory.
    No preview · Article · Mar 2012 · Journal of Applied Physics
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    ABSTRACT: Tin (Sn) crystal growth on Sn-based anodes in lithium ion batteries is hazardous for reasons such as possible short-circuit failure by Sn whiskers and Sn-catalyzed electrolyte decomposition, but the growth mechanism of Sn crystals during battery cycling is not clear. Here we report different growth mechanisms of Sn crystal during the lithiation and delithiation processes of SnO(2) nanowires revealed by in situ transmission electron microscopy (TEM). Large spherical Sn nanoparticles with sizes of 20-200nm grew instantaneously upon lithiation of a single-crystalline SnO(2) nanowire at large current density (j>20A/cm(2)), which suppressed formation of the Li(x)Sn alloy but promoted agglomeration of Sn atoms. Control experiments of Joule-heating (j≈2400A/cm(2)) the pristine SnO(2) nanowires resulted in melting of the SnO(2) nanowires but not Sn particle growth, indicating that the abnormal Sn particle growth was induced by both chemical reduction (i.e., breaking the SnO(2) lattice to produce Sn atoms) and agglomeration of the Sn atoms assisted by Joule heating. Intriguingly, Sn crystals grew out of the nanowire surface via a different "squeeze-out" mechanism during delithiation of the lithiated SnO(2) nanowires coated with an ultra-thin solid electrolyte LiAlSiO(x) layer. It is attributed to the negative stress gradient generated by the fast Li extraction in the surface region through the Li(+)-conducting LiAlSiO(x) layer. Our previous studies showed that Sn precipitation does not occur in the carbon-coated SnO(2) nanowires, highlighting the effect of nanoengineering on tailoring the electrochemical reaction kinetics to suppress the hazardous Sn whiskers or nanoparticles formation in a lithium ion battery.
    No preview · Article · Feb 2012 · Micron
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    ABSTRACT: The development of rare-earth (RE) doped phosphors allows for the conversion of photons to energies that are more usable for the desired application. Additionally, these RE phosphors have long lifetimes, on the order of ms, which offer potential in many energy conversion and energy transfer devices. Currently, RE phosphors are used in fiber optics amplifiers, modulated hybrid laser, optical interconnects and switches, optical displays, broad absorption solar cells and various other lighting applications. Energy transfer mechanisms of the excited RE states, such as defect quenching and sensitizer/emitter interactions, must be understood in order to achieve high efficiency energy conversion and propagation for future applications. In order to synthesize high efficiency phosphors, trivalent RE ions are being doped into a core-shell metal oxide host lattice. The role of the core-shell structure reduces the effect of the surface quenching sites by increasing the distance between active ions and the surface hydroxyl groups. Secondly, the luminescent fingerprint can be further controlled with proper doping of the shell structure by either increasing the absorption spectrum or adding additional emission peaks. Primarily, this work focuses on the emission of visible photons through upconversion in Y2O3:Er3+, Yb3+ nanophosphors, making them ideal components in broad absorption solar cells. By spatially controlling the position and concentration of the RE ions within the nanostructure, increased luminescence is observed due to energy transfer between the dopant ions within a critical interatomic distance. Passitvation of surface sites with increasing shell thickness was shown to increase luminescent lifetimes up to 53%, with a critical shell thickness of 8 nm, while lowering the theoretical lifetimes extracted from Judd-Ofelt parameters. The effect of the spatially controlled Yb ions was probed through the extraction of the upconversion photon requirement, showing a statistical decrease in photons from 2.16 to 1.43, or ~30 %. Finally, the effective energy transfer distance and energy transfer coefficients were studied by selectively exciting the Yb 4F5/2-4F7/2 transition with a 904 nm laser diode while probing the Er3+ emission at 1540 nm as a Y2O3 spacer layer is added to the system. Measured results show the occurrence of energy transfer between the ions with a ~3 nm spacer layer, confirming the prediction of the Föester-Dexter theory. Additionally, these results are applied to a RE doped LaPO4 system, producing white light from a single, high efficiency core-shell phosphor.
    No preview · Conference Paper · Oct 2011

Publication Stats

2k Citations
236.09 Total Impact Points


  • 2000-2015
    • University of California, Los Angeles
      • • California NanoSystems Institute
      • • Department of Chemical and Biomolecular Engineering
      Los Ángeles, California, United States
  • 1997
    • Massachusetts Institute of Technology
      • Department of Chemical Engineering
      Cambridge, Massachusetts, United States