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20 Jul 2009

Volume 95, Issue 3, Articles (03xxxx)

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Appl. Phys. Lett. 95, 033502 (2009); http://dx.doi.org/10.1063/1.3178556 (3 pages)

Akihito Ikedo, Takahiro Kawashima, Takeshi Kawano, and Makoto Ishida
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Band engineered epitaxial Ge–SixGe1−x core-shell nanowire heterostructures

K. M. Varahramyan, D. Ferrer, E. Tutuc, and S. K. Banerjee

Appl. Phys. Lett. 95, 033101 (2009); http://dx.doi.org/10.1063/1.3173811 (3 pages) | Cited 11 times

Online Publication Date: 20 July 2009

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We report the growth of germanium (Ge)—silicon-germanium (SixGe1−x) epitaxial core-shell nanowire (NW) heterostructures, with tunable Si and Ge shell content. The Ge NWs are grown using the vapor-liquid-solid growth mechanism, and the SixGe1−x shells are grown in situ, conformally on the Ge NWs using ultrahigh vacuum chemical vapor deposition. We use transmission electron microscopy to demonstrate epitaxial shell growth, and scanning energy dispersive x-ray spectroscopy to determine the shell thickness and content. The Si and Ge shell content can be tuned depending on the SiH4 and GeH4 partial pressures during the shell growth, enabling band engineered core-shell NW heterostructures.
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81.16.Be Chemical synthesis methods
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.80.Ej X-ray, Mössbauer, and other γ-ray spectroscopic analysis methods
81.07.Vb Quantum wires
68.55.ag Semiconductors

Si nanopillar array optimization on Si thin films for solar energy harvesting

Junshuai Li, HongYu Yu, She Mein Wong, Gang Zhang, Xiaowei Sun, Patrick Guo-Qiang Lo, and Dim-Lee Kwong

Appl. Phys. Lett. 95, 033102 (2009); http://dx.doi.org/10.1063/1.3186046 (3 pages) | Cited 72 times

Online Publication Date: 20 July 2009

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In this letter, Si thin film (800 nm thick) with nanopillar array decorated surface is studied via simulation for its solar energy absorption characteristics. It is found that the light absorption is significantly enhanced due to the adding of the Si nanopillar (SiNP) array to the Si thin film. The absorption characteristics of the SiNP structure would be approximately optimum (especially at ∼ 2.5 eV, the high energy density region in the solar spectrum) when the periodicity of SiNP array is set as ∼ 500 nm, which can be explained when comparing the incident light wavelength with the periodicity of SiNP array.
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84.60.Jt Photoelectric conversion
68.55.jd Thickness
61.46.-w Structure of nanoscale materials
78.66.Db Elemental semiconductors and insulators

Low-frequency electronic noise in the double-gate single-layer graphene transistors

G. Liu, W. Stillman, S. Rumyantsev, Q. Shao, M. Shur, and A. A. Balandin

Appl. Phys. Lett. 95, 033103 (2009); http://dx.doi.org/10.1063/1.3180707 (3 pages) | Cited 28 times

Online Publication Date: 20 July 2009

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The authors report the results of an experimental investigation of the low-frequency noise in the double-gate graphene transistors. The back-gate graphene devices were modified via addition of the top gate separated by ∼ 20 nm of HfO2 from the single-layer graphene channels. The measurements revealed low flicker noise levels with the normalized noise spectral density close to 1/f (f is the frequency) and Hooge parameter αH ≈ 2×10−3. The analysis of noise spectral density dependence on the top and bottom gate biases helped to elucidate the noise sources in these devices. The obtained results are important for graphene electronic and sensor applications.
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73.63.Fg Nanotubes
72.70.+m Noise processes and phenomena

Transition from ferromagnetism to diamagnetism in undoped ZnO thin films

Mukes Kapilashrami, Jun Xu, Valter Ström, K. V. Rao, and Lyubov Belova

Appl. Phys. Lett. 95, 033104 (2009); http://dx.doi.org/10.1063/1.3180708 (3 pages) | Cited 39 times

Online Publication Date: 20 July 2009

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We report a systematic study of the film thickness dependence (0.1–1 μm) of room-temperature ferromagnetism in pure magnetron-sputtered ZnO thin films wherein a sequential transition from ferromagnetism to paramagnetism and diamagnetism as a function of film thickness is observed. The highest saturation magnetization (MS) value observed is 0.62 emu/g (0.018 μB/unit cell) for a ∼ 480 nm film. On doping the ZnO film with 1 at. % Mn enhances the MS value by 26%. The ferromagnetic order in ZnO matrix is believed to be defect induced. In addition, on doping with Mn hybridization between the 2p states of O and the 3d states of Mn occurs.
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68.55.ag Semiconductors
61.72.uj III-V and II-VI semiconductors
75.30.Cr Saturation moments and magnetic susceptibilities
75.50.Pp Magnetic semiconductors
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.70.-i Magnetic properties of thin films, surfaces, and interfaces

Multiferroic behavior observed in highly orientated Mn-doped BaTiO3 thin films

Yuan-Hua Lin, Jiancong Yuan, Songyin Zhang, Yi Zhang, Jing Liu, Yao Wang, and Ce-Wen Nan

Appl. Phys. Lett. 95, 033105 (2009); http://dx.doi.org/10.1063/1.3182793 (3 pages) | Cited 14 times

Online Publication Date: 20 July 2009

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Highly orientated Mn-doped BaTiO3 thin films have been prepared on Nb-doped SrTiO3 substrate by pulsed laser deposition. Structural and electrical studies indicate that pure and Mn-doped BaTiO3 thin films show single phase character and good ferroelectric performance. All of the Mn-doped BaTiO3 thin films show obvious room-temperature ferromagnetism and the magnetization increases with the growing concentration of doped Mn. A bound magnetic polaron model was proposed to understand the observed ferromagnetic behavior of resistive Mn-doped BaTiO3 thin films. Our results demonstrate that ferromagnetism can be induced in the ferroelectric BaTiO3 film, which makes it a promising single-phase multiferroic.
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75.80.+q Magnetomechanical effects, magnetostriction
75.70.Ak Magnetic properties of monolayers and thin films
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.15.Fg Pulsed laser ablation deposition
75.50.Dd Nonmetallic ferromagnetic materials
68.55.-a Thin film structure and morphology
77.55.-g Dielectric thin films
77.80.-e Ferroelectricity and antiferroelectricity
71.38.-k Polarons and electron-phonon interactions
73.61.Ng Insulators

Highly efficient nonradiative energy transfer using charged CdSe/ZnS nanocrystals for light-harvesting in solution

Evren Mutlugün, Sedat Nizamoğlu, and Hilmi Volkan Demir

Appl. Phys. Lett. 95, 033106 (2009); http://dx.doi.org/10.1063/1.3182798 (3 pages) | Cited 4 times

Online Publication Date: 20 July 2009

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We propose and demonstrate highly efficient nonradiative Förster resonance energy transfer (FRET) facilitated by the use of positively charged CdSe/ZnS core-shell nanocrystals (NCs) for light-harvesting in solution. With rhodamine B dye molecules used as the acceptors, our time-resolved photoluminescence measurements show substantial lifetime modifications of these amine-functionalized NC donors from 18.16 to 1.88 ns with FRET efficiencies >90% in solution. These strong modifications allow for light-harvesting beyond the absorption spectral range of the acceptor dye molecules.
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78.55.Et II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.46.Hk Nanocrystals
78.47.jd Time resolved luminescence
78.47.D- Time resolved spectroscopy (>1 psec)

Thermal rectification in asymmetric graphene ribbons

Nuo Yang, Gang Zhang, and Baowen Li

Appl. Phys. Lett. 95, 033107 (2009); http://dx.doi.org/10.1063/1.3183587 (3 pages) | Cited 40 times

Online Publication Date: 20 July 2009

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In this paper, heat flux in graphene nanoribbons has been studied by using molecular dynamics simulations. It is found that the heat flux runs preferentially along the direction of decreasing width, which demonstrates significant thermal rectification effect in the asymmetric graphene ribbons. The dependence of rectification ratio on the vertex angle and the length are also discussed. Compared to the carbon nanotube based one-dimensional thermal rectifier, graphene nanoribbons have much higher rectification ratio even in large scale. Our results demonstrate that asymmetric graphene ribbon might be a promising structure for practical thermal (phononics) device.
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73.40.Ei Rectification
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
73.63.Fg Nanotubes
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems

Microevaporators with accumulators for the screening of phase diagrams of aqueous solutions

P. Moreau, J. Dehmoune, J.-B. Salmon, and J. Leng

Appl. Phys. Lett. 95, 033108 (2009); http://dx.doi.org/10.1063/1.3159811 (3 pages) | Cited 3 times

Online Publication Date: 21 July 2009

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We design near-autonomous microfluidic devices for concentrating aqueous solutions steadily over days in a very controlled manner. We combine suction pumps that drive the solution and concentrate it steadily, with a nanoliter-sized storage pool where the solute accumulates. The fine balance between advection and diffusion in the pump and diffusion alone in the accumulation pool yields several filling regimes. One of them is universal as being steady and independent of the solute itself. It results a specific equivalence between time and concentration, which we use to build the phase quantitative diagram of a ternary aqueous solution on nanoliter scale.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
66.10.C- Diffusion and thermal diffusion
47.61.-k Micro- and nano- scale flow phenomena
68.03.Fg Evaporation and condensation of liquids

Ab initio design of Ca-decorated organic frameworks for high capacity molecular hydrogen storage with enhanced binding

Y. Y. Sun, Kyuho Lee, Yong-Hyun Kim, and S. B. Zhang

Appl. Phys. Lett. 95, 033109 (2009); http://dx.doi.org/10.1063/1.3182796 (3 pages) | Cited 19 times

Online Publication Date: 21 July 2009

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Ab initio calculations show that Ca can decorate organic linkers of metal-organic framework, MOF-5, with a binding energy of 1.25 eV. The Ca-decorated MOF-5 can store molecular hydrogen (H2) in both high gravimetric (4.6 wt %) and high volumetric (36 g/l) capacities. Even higher capacities (5.7 wt % and 45 g/l) can be obtained in a rationally designed covalent organic framework system, COF-α, with decorated Ca. Both density functional theory and second-order Møller–Plesset perturbation calculations show that the H2 binding in these systems is significantly stronger than the van der Waals interactions, which is required for H2 storage at near ambient conditions.
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84.60.-h Direct energy conversion and storage
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Statistical interpretation of “femtomolar” detection

Jonghyun Go and Muhammad A. Alam

Appl. Phys. Lett. 95, 033110 (2009); http://dx.doi.org/10.1063/1.3176017 (3 pages) | Cited 2 times

Online Publication Date: 22 July 2009

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We calculate the statistics of diffusion-limited arrival-time distribution by a Monte Carlo method to suggest a simple statistical resolution of the enduring puzzle of nanobiosensors: a persistent gap between reports of analyte detection at approximately femtomolar concentration and theory suggesting the impossibility of approximately subpicomolar detection at the corresponding incubation time. The incubation time used in the theory is actually the mean incubation time, while experimental conditions suggest that device stability limited the minimum incubation time. The difference in incubation times—both described by characteristic power laws—provides an intuitive explanation of different detection limits anticipated by theory and experiments.
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87.85.Rs Nanotechnologies-applications
07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
87.15.Vv Diffusion
87.15.ak Monte Carlo simulations

Controlled free-form fabrication of nanowires by dielectrophoretic dispension of colloids

Niklas C. Schirmer, Timo Schwamb, Brian R. Burg, Nico Hotz, and Dimos Poulikakos

Appl. Phys. Lett. 95, 033111 (2009); http://dx.doi.org/10.1063/1.3186789 (3 pages) | Cited 4 times

Online Publication Date: 22 July 2009

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The vertical growth of conductive gold nanowires in ambient atmosphere through on-demand dielectrophoretically guided deposition of nanoparticle-laden colloids is demonstrated. Accelerated by dielectrophoresis, rapidly evaporating colloidal droplets leave a capillary nozzle (diameter of 500 nm) and are guided through an inhomogeneous electrostatic field. The nanoparticles in the colloidal droplets combine under the action of van der Waals attraction forces on the substrate to form highly flexible vertical nanowires with easily controllable lengths and diameters, here in the range of 150–800 nm. After bending to bridge an electrode pair and after annealing at 300 °C, the sample gold nanowires demonstrate excellent electrical conductivity considering their size.
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81.16.-c Methods of micro- and nanofabrication and processing
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
82.45.Fk Electrodes
73.63.-b Electronic transport in nanoscale materials and structures
81.40.Gh Other heat and thermomechanical treatments
82.70.Dd Colloids

Strain distribution and interface modulation of highly lattice-mismatched InN/GaN heterostructure nanowires

Y. H. Kim, H. J. Park, K. Kim, C. S. Kim, W. S. Yun, J. W. Lee, and M. D. Kim

Appl. Phys. Lett. 95, 033112 (2009); http://dx.doi.org/10.1063/1.3184541 (3 pages) | Cited 5 times

Online Publication Date: 22 July 2009

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The structural properties of InN/GaN heterostructure nanowires (NWs) were studied using transmission electron microscope techniques to determine strain behavior. A great quantity of the misfit strain between InN and GaN was relaxed through the introduction of misfit dislocations along the interface. Geometric phase analysis revealed a strain-concentration phenomenon in the strain map of the out-of-plane components and a gradual lattice recovery in that of the in-plane components over the InN/GaN interface. Interface structures that were modulated at the atomic-scale were observed in several InN/GaN heterostructure NWs. Complex strain distributions were identified in both InN and GaN.
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81.07.Vb Quantum wires
81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
68.35.Ct Interface structure and roughness

In-plane thermal conductivity of nanoscale polyaniline thin films

Jiezhu Jin, Mohan P. Manoharan, Qing Wang, and M. A. Haque

Appl. Phys. Lett. 95, 033113 (2009); http://dx.doi.org/10.1063/1.3184786 (3 pages) | Cited 7 times

Online Publication Date: 23 July 2009

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Thermal characterization of conducting polymers is important in understanding transport phenomena in energy conversion and flexible electronics devices. We present an experimental technique to determine the in-plane thermal conductivity and the thermal contact resistance of thin films on substrates simultaneously. For 20 nm thick polyaniline films on SiO2 substrate, the respective values were measured to be 0.0406 W/m K and 0.806 K m2/W. We also observed thickness dependence of in-plane thermal conductivity, which suggests that heat transfer is governed by phonon-boundary scattering when the film thickness is close to the mean free path.
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66.70.Hk Glasses and polymers
68.55.am Polymers and organics
68.55.jd Thickness
73.61.Ph Polymers; organic compounds
73.40.Cg Contact resistance, contact potential

Translation of nanoantenna hot spots by a metal-dielectric composite superlens

Zhengtong Liu (刘政通), Mark D. Thoreson, Alexander V. Kildishev, and Vladimir M. Shalaev

Appl. Phys. Lett. 95, 033114 (2009); http://dx.doi.org/10.1063/1.3176975 (3 pages) | Cited 7 times

Online Publication Date: 23 July 2009

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We employ numerical simulations to show that highly localized, enhanced electromagnetic fields, also known as “hot spots,” produced by a periodic array of silver nanoantennas can be spatially translated to the other side of a metal-dielectric composite superlens. The proposed translation of the hot spots enables surface-enhanced optical spectroscopy without the undesirable contact of molecules with metal, and thus it broadens and reinforces the potential applications of sensing based on field-enhanced fluorescence and surface-enhanced Raman scattering.
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84.40.Ba Antennas: theory, components and accessories

Tunable magnetic response of metamaterials

Shumin Xiao, Uday K. Chettiar, Alexander V. Kildishev, Vladimir Drachev, I. C. Khoo, and Vladimir M. Shalaev

Appl. Phys. Lett. 95, 033115 (2009); http://dx.doi.org/10.1063/1.3182857 (3 pages) | Cited 37 times

Online Publication Date: 23 July 2009

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We demonstrate a thermally tunable optical metamaterial with negative permeability working in the visible range. By covering coupled metallic nanostrips with aligned nematic liquid crystals (NLCs), the magnetic response wavelength of the metamaterial is effectively tuned through control of the ambient temperature, changing the refractive index of LC via phase transitions. By increasing the ambient temperature from 20 to 50 °C, the magnetic response wavelength shifts from 650 to 632 nm. Numerical simulations confirm our tests and match the experimental observations well.
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42.70.Df Liquid crystals
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.Dw Liquids
64.70.mj Experimental studies of liquid crystal transitions

Effect of substrate roughness on the apparent surface free energy of sputter deposited superhydrophobic polytetrafluoroethylene thin films

Harish C. Barshilia, D. Krishna Mohan, N. Selvakumar, and K. S. Rajam

Appl. Phys. Lett. 95, 033116 (2009); http://dx.doi.org/10.1063/1.3186079 (3 pages) | Cited 8 times

Online Publication Date: 24 July 2009

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Surface morphology plays an important role in determining the superhydrophobic behavior of solids. To provide insights on the influence of roughness over a wide range, we have carried out a quantitative study of effect of substrate roughness on contact angle and surface free energy (SFE) of sputter deposited superhydrophobic polytetrafluoroethylene thin films. An optimum value for substrate roughness has been derived and a practical methodology (sand blasting) for its fabrication has been proposed. The SFE values were calculated using Zisman method and validated by Fowkes–Girifalco–Good theory.
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68.35.bm Polymers, organics
61.41.+e Polymers, elastomers, and plastics
65.40.gp Surface energy
68.35.Md Surface thermodynamics, surface energies

p-type conduction in arsenic-doped ZnSe nanowires

H. S. Song, W. J. Zhang, G. D. Yuan, Z. B. He, W. F. Zhang, Y. B. Tang, L. B. Luo, C. S. Lee, I. Bello, and S. T. Lee

Appl. Phys. Lett. 95, 033117 (2009); http://dx.doi.org/10.1063/1.3186359 (3 pages) | Cited 13 times

Online Publication Date: 24 July 2009

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Reliable p-type conduction was achieved in ZnSe nanowires (NWs) synthesized by introducing Zn3As2 as a dopant source. The crystal structure and orientation of NWs remained unchanged after doping. The electrical and transport properties of As-doped ZnSe NWs were investigated via the characteristics of NW-based field-effect transistors. The origin of p-type conduction in ZnSe NWs is attributed to the formation of substitutional AsSe and AsZn−2VZn complexes. Arsenic atoms were considered to incorporate into ZnSe lattices partly as As–H pairs; therefore postgrowth annealing could improve p-type conduction by dissociating As–H bonds and activating As acceptors.
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73.63.Nm Quantum wires
61.72.uj III-V and II-VI semiconductors
68.65.La Quantum wires (patterned in quantum wells)
61.72.Cc Kinetics of defect formation and annealing
85.30.Tv Field effect devices
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Memory characteristics of metal-oxide-semiconductor capacitor with high density cobalt nanodots floating gate and HfO2 blocking dielectric

Yanli Pei, Chengkuan Yin, Toshiya Kojima, Masahiko Nishijima, Takafumi Fukushima, Tetsu Tanaka, and Mitsumasa Koyanagi

Appl. Phys. Lett. 95, 033118 (2009); http://dx.doi.org/10.1063/1.3189085 (3 pages) | Cited 11 times

Online Publication Date: 24 July 2009

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In this letter, cobalt nanodots (Co-NDs) had been formed via a self-assembled nanodot deposition. High resolution transmission electron microscopy and x-ray photoelectron spectroscopy analyses clearly show that the high metallic Co-ND is crystallized with small size of ∼ 2 nm and high density of (4–5)×1012/cm2. The metal-oxide-semiconductor device with high density Co-NDs floating gate and high-k HfO2 blocking dielectric exhibits a wide range memory window (0–12 V) due to the charge trapping into and distrapping from Co-NDs. After 10 years retention, a large memory window of ∼ 1.3 V with a low charge loss of ∼ 47% was extrapolated. The relative longer data retention demonstrates the advantage of Co-NDs for nonvolatile memory application.
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84.32.Tt Capacitors
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