bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nanostructured Ultrathin NbN Film as a Terahertz Hot-Electron Bolometer Mixer
Authors: Gol'tsman Grigory , Maslennikov Sergey , Finkel Matvey , Antipov Sergey , Kaurova Natalia , Grishina Elisaveta , Polyakov Stanislav , Vachtomin Yury , Svechnikov Sergey , Smirnov Konstantin , Voronov Boris 
Published: April 2011

ABSTRACTPlanar spiral antenna coupled and directly lens coupled NbN HEB mixer structures are studied. An additional MgO buffer layer between the superconducting film and Si substrate is introduced. The buffer layer enables us to increase the gain bandwidth of a HEB mixer due to better acoustic transparency. The gain bandwidth is widened as NbN film thickness decreases and amounts to 5.2 GHz. The noise temperature of antenna coupled mixer is 1300 and 3100 K at 2.5 and 3.8 THz respectively. The structure and composition of NbN films is investigated by X-ray diffraction spectroscopy methods. Noise performance degradation at LO frequencies more than 3 THz is due to the use of a planar antenna and signal loss in contacts between the antenna and the sensitive NbN bridge. The mixer is reconfigured for operation at higher frequencies in a manner that receiver's noise temperature is only 2300 K (3 times of quantum limit) at LO frequency of 30 THz.
Terahertz Resonances in the Dielectric Response Due to Second Order Phonons in a GaSe Crystal
Authors: Yu Baolong L. , Altan Hakan , Zeng Fanang , Kartazayev Vladimir , Alfano Robert , Mandal Krishna C. 
Published: April 2011

ABSTRACTThe dielectric function and momentum relaxation time of carriers for a single-crystal GaSe were investigated using terahertz time-domain spectroscopy over the frequency range from 0.4 to 2.4 THz. The key parameters determined from THz data using the Drude model are: the plasma frequency ωp = 6.1 ± 0.5 THz, the average momentum relaxation time <τ> = 51 +± 6 fs for electrons. The THz absorption spectrum showed resonance structures attributed to the difference frequency combinations associated with acoustical and optical phonons. This paper has already been published, see B. L. Yu, F. Zeng, V. Kartazayev, and R. R. Alfano, “Terahertz studies of the dielectric response of and second order phonons in a GaSe crystal,” Appl. Phys. Lett. 86, 182104 (2005).
SiNx barrier layers deposited at 250°C on a clear polymer substrate
Authors: Cherenack Kunigunde , Kattamis Alex , Long Ke , Cheng I-Chun , Wagner Sigurd , Sturm James C. 
Published: April 2011

ABSTRACTInterest is widespread in flexible thin-film transistor backplanes made on clear polymer foil, which could be universally employed for a variety of applications. All ultralow process temperatures, plastic compatible thin film transistor (TFT) technologies battle short or long term device instabilities. The quality and stability of amorphous silicon thin-film transistors (a-Si:H TFTs) improves with increasing process temperature. TFT stacks deposited at less than 250°C by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) exhibit higher threshold voltage shifts after gate bias stressing than stacks deposited at ∼300°C in the active matrix liquid-crystal display (AMLCD) industry [1]. Therefore, optically clear plastic (CP) substrates are desired that tolerate high process temperatures. The first step in a-Si:H TFT fabrication on a polymer is the deposition of a planarizing barrier and adhesion layer. For this purpose we have been using silicon nitride (SiNx) grown by PECVD.This paper discusses the substrate preparation and SiNx deposition for the process temperature of 250°C. We study the mechanical strain in the SiNx film on the CP substrate, as a function of RF power. Earlier work has shown that SiNx films deposited at low RF power are under tensile strain, and become increasingly compressed as the deposition power is raised [2]. Additionally, at very high deposition power the substrate is bombarded at the beginning of film growth to achieve good film adhesion. The goal is to identify the correct processing conditions at which the total mismatch strain between the film and the substrate is minimized, to keep the film/substrate composite flat and avoid mechanical fracture as well as peeling due to poor adhesion. Optimal deposition conditions were identified to obtain crack-free SiNx barrier layers. The barrier layers were tested by fabricating a-Si:H TFTs on them at 250°C.
Transparent, Conductive and Flexible Carbon Nanotube Films and Their Application in Organic Light Emitting Diodes
Authors: Ryu Koungmin , Zhang Daihua , Liu Xiaolei , Polikarpov Evgueni , Tompson Mark , Zhou Chongwu 
Published: April 2011

ABSTRACTWe have successfully used a transfer printing technique to directly transfer vacuum-filtered nanotube film to glass and plastic substrates. Our typical SWNT-film has a transparency of ∼80% and a sheet resistance around 400 Ohm/square. Further improvement to the nanotube film includes SOCl2 doping and PEDOT passivation, which significantly improve the sheet conductance and surface quality of the nanotube films. We have applied the optimized SWNT films as hole injection electrodes to demonstrate OLEDs on both rigid glass and flexible substrates.
Optical studies of Poly[p-(2,5-didodecylphenylene)ethynylene] in Thin Films
Authors: Rozanski Lynn , Bout David A. Vanden , Bunz Uwe H.F. 
Published: April 2011

ABSTRACTUnlike many other conjugated polymers popular for use in LEDs, poly[p-(2,5-didodecylphenylene)ethynylene] (DPPE), shows less interchain interaction between polymer chains after annealing to an ordered film. Evidence of this decreased interchain interaction can be seen in the emission spectra of pristine and annealed films. Thin films of pristine DPPE show a broad featureless green emission, characteristic of an excimer-like state, whereas annealed films blue-shift and become structured, resembling dilute solution emission. DPPE packs into ordered domains after annealing, shown by birefringence, polarization anisotropy NSOM, and x-ray crystallography data. The lack of chromophore interaction in the ordered domains can be explained through a brick-wall-type packing motif, where the polymer backbone is insulated by the side chains of offset neighboring polymers. Electroluminescence spectra of pristine and annealed LEDs are nearly identical, both having green emission, unlike their different photoluminescence spectra. This is evidence that light emission in LEDs comes only from excimer sites within the film, which would indicate a decreased performance for the annealed devices compared to pristine devices. Preliminary data of pristine and annealed LEDs supports this theory, with most pristine devices having a slightly higher electroluminescence intensity compared to annealed devices.
Morphology of Polythiophene Films Produced via Surface Polymerization by Ion-Assisted Deposition: A Combined Experimental and Computational Study
Authors: Tepavcevic Sanja , Hsu Wen-Dung , Sinnott Susan B , Hanley Luke 
Published: April 2011

ABSTRACTControl of film morphology is widely recognized as one of the limiting factors in the development of conjugated polymers for photonic and electronic applications. Surface polymerization by ion-assisted deposition (SPIAD) is shown to drive film morphology. A wide variety of structures form in the SPIAD polythiophene films, including islands, lamellar structures, nanoscale crystallites, and fractal-like growth patterns. Density functional theory-molecular dynamic simulations are utilized to illustrate the manner in the incident ions affect polymerization, bond dissociation, and other chemical events in SPIAD. These ion-assisted events mediate thermal processes such as neutral deposition, sublimation, diffusion, and dewetting. However, these thermal processes are on longer timescales that cannot be directly studied by the computational techniques reported here.
Failure Mode Investigation for Blue Polymer EL Device
Authors: Kim Mu-Gyeom , Kim Sang Yeol , Lee Tae Woo , Park Sang Hun , Park Ju Cheol , Park Jong Jin , Pu Lyong Sun 
Published: April 2011

ABSTRACTWe report the failure mode observed in polymer EL blue device. Such modes were analyzed by non-destructive and destructive ways. As a non-destructive way, investigation of mobility changes of hole and electron, measuring transient EL corresponding to life curve, had been done. We also observed compositional and morphological variation using TEM-EDX, STM, FT-IR, TOF-SIMS and reverse engineering method as a destructive way. Electron mobility has a higher dependency on life curve, which fact could be reflected on the formation of insoluble layer inside emitting layer on anode side. And such insoluble layer showed relatively ordered surface morphology, and might be a cross-linked layer through C-O-C bond cleavage process while EL operation. But, contrary to sulfur migration mechanism into insoluble layer insisted by CDT, we did confirm no obvious difference of sulfur composition between insoluble and emitting layers. Rather, there's some degree of Ba diffusion into emitting layer from decomposition of BaF2, but, which dose not have a major effect on device degradation.
Magnetoresistance and Magnetic Field Effect on Electroluminescence in Polyfluorene OLEDs
Authors: Veeraraghavan Govindarajan , Nguyen Tho Duc , Sheng Yugang , Mermer Omer , Wohlgenannt Markus 
Published: April 2011

ABSTRACTWe report on the experimental observation of large magnet field effect (MFE) on current and electroluminescence (EL) in polyfluorene organic light emitting diodes (OLEDs). We provide a comprehensive overview of the observed MFE. To the best of our knowledge, the mechanism causing these MFE is currently not known. Moreover, we show that these experiments do not allow determination whether the MFE acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in hole-only OLEDs and show that the MFE acts on the carrier mobility rather than carrier density.
Pentacene Thin-film Transistors Fabricated with a Simple Solution Process
Authors: Natsume Yutaka , Minakata Takashi 
Published: April 2011

ABSTRACTWe have succeeded in developing a simple solution process of pentacene thin films without particular precursor materials. High crystallinity and large plate-like grains of the solution-processed thin films were observed with several analyses. The solution-processed pentacene thin-film transistors (TFTs) were also fabricated and exhibited good transfer characteristics with maximum carrier mobility above 1 cm2/Vs. The solution-processed TFTs also indicated a steep subthreshold swing and high stability of the threshold voltage against the storage in the atmosphere. The trap states and the bulk carrier density in the films were evaluated from the transfer characteristics by using the analytical model. We considered that these good properties could be attributed to the high crystallinity and the large grains of the solution-processed thin films.
Optical Characterization of CdSe Colloidal Quantum Dot/ MEH-PPV Polymer Nanocomposites Spin-cast on GaAs Substrates
Authors: Stiff-Roberts Adrienne D. , Gupta Abhishek , Zhao Zhiya 
Published: April 2011

ABSTRACTThe motivation and distinct approach for this work is the use of intraband transitions within colloidal quantum dots for the detection of mid- (3-5 μm) and/or long-wave (8-14 μm) infrared light. The CdSe colloidal quantum dot/MEH-PPV conducting polymer nanocomposite material is well-suited for this application due to the ∼1.5 eV difference between the corresponding electron affinities. Therefore, CdSe colloidal quantum dots embedded in MEH-PPV should provide electron quantum confinement such that intraband transitions can occur in the conduction band. Further, it is desirable to deposit these nanocomposites on semiconductor substrates to enable charge transfer of photogenerated electron-hole pairs from the substrate to the nanocomposite. In this way, optoelectronic devices analogous to those achieved using Stranski-Krastanow quantum dots grown by epitaxy can be realized. To date, there have been relatively few investigations of colloidal quantum dot nanocomposites deposited on GaAs substrates. However, it is crucial to develop a better understanding of the optical properties of these hybrid material systems if such heterostructures are to be used for optoelectronic devices, such as infrared photodetectors. By depositing the nanocomposites on GaAs substrates featuring different doping characteristics and measuring the corresponding Fourier transform infrared absorbance, the feasibility of these intraband transitions is demonstrated at room temperature.
Incorporation of Colloidal PbSe Quantum Dots into 2-D Photonic Crystal Structures
Authors: Lee Yun-Ju , Subramania Ganapathi , Hernandez-Sanchez Bernadette A. , Niehaus Michael K. , Boyle Timothy J. , Cesarano Joseph , Clem Paul G. 
Published: April 2011

ABSTRACTWe demonstrate the functionalization of 2-D photonic crystal structures operating at ∼ 1.5 μm with colloidal PbSe quantum dots and examine the modified photoluminescence from the functionalized photonic crystal. Using spin coating and airbrushing, monodisperse PbSe quantum dots were deposited from hexanes on lithographically patterned GaAs photonic crystal substrates. The effectiveness of patterning the PbSe quantum dots via standard liftoff process was examined. The near-IR photoluminescence spectra of quantum dot-functionalized photonic crystals were studied. We found that the photoluminescence peak became attenuated by approximately a factor of five and exhibited a narrow peak width (50 nm vs. 120 nm) compared to PbSe deposited on unpatterned GaAs, suggesting that there is some coupling between the quantum dots and the photonic crystal. Future work to improve the coupling and detection efficiency is proposed.
Synthesis and Optical Characterization of Er-Doped GaN Low-Dimensional Structures
Authors: Carvajal Joan , Aguilo Magdalena , Diaz Francesc , Rojo J. Carlos 
Published: April 2011

ABSTRACTEr-doped GaN rod-shaped low-dimensional structures have been synthesized on the surface of a silicon (001) substrate by the direct reaction of Ga, NH3 and Er using a catalyst-assisted chemical vapor deposition technique. The low-dimensional nanostructures were characterized spectrocopically analyzing the hypersensitive4G11/2and2H11/2bands of Er3+located at 375 and 425 nm, respectively. Green and near-infrared (IR) emission produced by these Er-doped GaN low-dimensional structures has been observed under excitation at 488 and 543 nm in a confocal microscope.
Interplay Between Strain and Effective Electron Mass on the Absorption Strength of Dilute Nitride Semiconductor Quantum Wires
Authors: Feltrin Andrea , Alemu Andenet , Freundlich Alexandre 
Published: April 2011

ABSTRACTWe have investigated the absorption spectrum and strength of InyGa1−yAs1−xNx quantum wires. We show that compounds with varying fractions of indium and nitrogen, but similar band gaps have different absorption patterns. This behavior is related to the interplay between different effects as strain, which mainly affects the band offsets, and the increased electron mass in dilute nitride III-V semiconductors. We also study how the influence of these parameters changes with the optical band gaps associated to common optical telecommunication wavelengths. Our model calculations are performed in the parabolic band approximation and include excitonic effects.
Which states contribute to the tunneling current for large barrier thicknesses?
Authors: Heiliger Christian , Zahn Peter , Mertig Ingrid 
Published: April 2011

ABSTRACTThe transport properties of planar Fe/MgO/Fe tunnel junctions are investigated theoretically by means of ab initio calculations. In particular, the k||-resolved conductance in dependence on the barrier thickness, the interface structure, and the magnetic configuration is studied. The results show that the number of states in the k||-space contributing significantly to the overall current is decreasing with increasing barrier thickness as expected. In contrast to simple parabolic band models the contribution of states in the vicinity of k||=0, however, is only involved for a few considered configurations of the system Fe/MgO/Fe.
Development of Block Co-Polymers as Self-Assembling Templates for Magnetic Media and Spin-Valves
Authors: Warke Vishal Vitthal , Redden Chris , Bakker Martin , Nikles David , Hong Kunlun , Mays Jimmy , Britt Phil 
Published: April 2011

ABSTRACTPoly(styrene)-Poly(methylmethacrylate) block co-polymers (PS-b-PMMA) of appropriate block length and PS to PMMA ratio self-assemble into a 2-D hexagonal phase in which the PS majority phase is continuous and surrounds cylinders of the minority, PMMA phase. By UV irradiation and washing with acetic acid it is possible to remove the minority phase to leave empty channels. It is also possible to rearrange the PMMA phase with acetic acid to leave somewhat smaller pores. For most substrates the interactions between the polymer and the substrate surface are such that one block is preferentially adsorbed to the substrate resulting in alignment of the PMMA domains parallel to the substrate surface. It is possible to orient the polymer perpendicular to the surface by first adding a thin film of a random PS-PMMA co-polymer before applying the PS-b-PMMA block co-polymer. However thin films of the random PS-PMMA do not give good surface coatings, and thicker films are generally too thick for the pores in the PS-b-PMMA block co-polymer to be propagated to the substrate surface. For a few substrates, thin PS-b-PMMA films naturally adopt a perpendicular orientation after annealing, washing with acetic acid produces arrays of pores of diameter as small as 3 nm. For a number of other substrates the interaction between the polymer blocks and the surface is such that upon annealing the polymer rearranges to form micron sized domains which are not polymer coated, surrounded a areas which have a thicker polymer coating. We have observed this behavior with both carbon coated substrates and with ITO glass substrates. In both cases the areas of polymer are perpendicularly oriented, and upon washing with acetic acid give rise to pores that extend completely through the polymer film. In some cases films on ITO glass are continuous even after annealing. After washing with acetic acid it was possible to electrodeposit nickel into the pores to give nickel nano-pillars of 18 nm diameter.
Synthesis and Control of Nano-Scale CN Particles and its Distributions
Authors: Feng Peter x. , Shi Y. C. 
Published: April 2011

ABSTRACTMicro-scale to nano-scale carbon nitride (CN) particles were prepared by using plasma sputtering deposition technique. The preferred orientation of nanoscale CN particle distributions was obtained. Particles have been examined by using both Scanning Electron Microscopy (SEM) and Raman scattering (RS) spectroscopy. Setting bias voltage up to 5 kV, plasma-sputtering deposition gave rise to several ring (diameters: 2.4, 3.2, and 4.4 mm) patterns of particle distributions where many small groups of nanoscale particles were observed. Each group of these particles appeared in a sunflower type of distribution, in which the biggest (85 nm) particle at the center was surrounded by many small sizes (30 nm) of CN particles. Disk type of the particles with a diameter of 10 μm was also observed at different experimental conditions. Typical G, D bands and C=N band in the Raman spectra of the samples were identified. The intensity of the D bands obviously varied at the different deposition conditions.
In-Situ Polymerization of Poly (Methyl Methacrylate)/MgAl Layer Double Hydroxides Nanocomposites with High Dispersion and Enhanced Physics Properties
Authors: Tsai Tsung-Yen , Lu Shau-Wen , Li Fu-Shou 
Published: April 2011

ABSTRACTPoly(methyl methacrylate)/MgAl layer double hydroxides (PMMA/MgAl LDH) nanocomposites were prepared by in situ free radical polymerization with the organic modified MgAl-K2 LDH in the methyl methacrylate monomer and initiator of benzoyl peroxide. The morphology of MgAl-K2 LDH and PMMA/MgAl-K2 LDH nanocomposites were investigated by transmission electron microscopy (TEM) and powder X-ray diffraction, indicated that the MgAl-K2 LDH were dispersed in PMMA matrix to form PMMA/MgAl-K2 LDH nanocomposites with heterogeneous morphology comprising both exfoliated and intercalated PMMA/MgAl-K2 LDH nanocomposites . The thermal and mechanical characterization were carried out by thermogravimetric analysis (TGA) ¡Bdifferential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The properties of PMMA/MgAl-K2 LDH nanocomposites exhibit the better thermal and mechanical properties.
Preparation of High Surface Area Ni-SDC Cermets Using a Surfactant-Assisted Co-Precipitation Method
Authors: Park Sang Joon , Eom Tae Wook , Oh Jae Eun , Yang Hae Kwang , Kim Kyung Hwan 
Published: April 2011

ABSTRACTA surfactant-assisted co-precipitation method was employed for obtaining high surface area Ni-SDC with improved structural properties for SOFC applications. In the work, a cationic surfactant, cetyltrimethylammonium bromide(CTAB) was employed with NiCl2, SmCl3 and CeCl3 as precursors and NH4OH as mineralizer. The elimination of surfactants upon calcination gives rise to the formation of high surface area NiO-SDC. When calcined at 600°C, the powders with surface area of 249 m2/g, were obtained and the pore size was 14.45 nm. The powders consist of two phases, the cubic NiO and SDC confirmed with X-ray diffraction identification.
Criticality of Electron and Hole Escape Sequence in Nano-Structured Photovoltaic Devices
Authors: Freundlich Alex , Coaquira J.A.H. , Freundlich Alex 
Published: April 2011

ABSTRACTIt has been empirically established that for quantum confined p-i-n solar cells a high electric field across the i-region is necessary for an optimal extraction of carriers from the well. This restriction imposes an upper limit for the total thickness of the i-region beyond which severe performance degradations are reported. For a given material system, the best efficiency trade-off is often achieved in the vicinity of this critical i-region thickness where the Voc degradation remains minimal and a higher photocurrent is afforded by the larger number of wells. But, even for devices that satisfy this condition, occasionally a severe Voc degradation occurs. Here, we show that this degradation is directly correlated to the carrier escape sequence from the wells and that a careful engineering of hole and electron confining potentials can be used to alleviate this shortcoming.
Triple and Quadruple Junctions Thermophotovoltaic Devices Lattice Matched to InP
Authors: Bhusal Lekhnath , Freundlich A 
Published: April 2011

ABSTRACTPower conversion in thermophotovoltaic (TPV) or any other photovoltaic device can be increased by implementing monolithically series connected multi-bandgap structure in the device. The main concern for multi-bandgap material is the availability of different band gaps for the optimal operation of the device. Based on the recent work, GaAsN/InAsN superlattice lattice matched to InP has shown the potential of achieving band gaps in the range of 0.65-0.35eV at 300K, which is technologically important range for the TPV structure due to the availability of the photon energies in this range from the heat source. In this work, we will present the calculation details and results to find the maximum power generated by the multi-bandgap monolithically series connected devices. Optimized band gaps for p-i-n junction subcells were estimated by finding the optimal current to provide the maximum power through the series-connected double, triple and quadruple junction cells for 1350K blackbody radiation as an incident flux.