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15 Aug 2010

Volume 108, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 108, 041901 (2010); http://dx.doi.org/10.1063/1.3474648 (2 pages)

Sergei V. Kalinin, Nava Setter, and Andrei L. Kholkin
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back to top Nanoscale Science and Design

High-quality CdTe nanowires: Synthesis, characterization, and application in photoresponse devices

Y. Ye, L. Dai, T. Sun, L. P. You, R. Zhu, J. Y. Gao, R. M. Peng, D. P. Yu, and G. G. Qin

J. Appl. Phys. 108, 044301 (2010); http://dx.doi.org/10.1063/1.3474991 (4 pages) | Cited 8 times

Online Publication Date: 16 August 2010

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High-quality straight and multiply kinked CdTe nanowires (NWs) were synthesized by the facile chemical vapor deposition method at 600 °C. The as-synthesized NWs were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and photoluminescence (PL) spectroscopy. The straight CdTe NWs have single crystalline zinc blende structure with growth direction along the 〈111〉 direction. Their PL spectra consist only sharp near band edge emission around 824.3 nm. The multiply kinked CdTe NWs contain one or more fixed ( ∼ 125.2°) angle joints; each arm of the kinked NWs is single crystalline with similar selected area electron diffraction pattern as that of the straight CdTe NWs. The two growth directions in the multiply kinked CdTe NWs are 〈200〉 and 〈111〉. Single straight and kinked CdTe NW photoresponse devices were fabricated and testified to have high photocurrent decay ratio, high responsivity, fast response time, and no decay tail under 633 nm He–Ne laser illumination. These straight and multiply kinked CdTe NWs may open up potential applications in the bottom-up integrated nanoelectronic and nanophotonic systems, such as photovoltaic and multiterminal nanodevices etc.
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78.66.Hf II-VI semiconductors
81.05.Dz II-VI semiconductors
72.20.-i Conductivity phenomena in semiconductors and insulators
78.55.Et II-VI semiconductors
61.46.Np Structure of nanotubes (hollow nanowires)
68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)

Reduction in resistivity of 50 nm wide Cu wire by high heating rate and short time annealing utilizing misorientation energy

Jin Onuki, Khyoupin Khoo, Yasushi Sasajima, Yasunori Chonan, and Takashi Kimura

J. Appl. Phys. 108, 044302 (2010); http://dx.doi.org/10.1063/1.3474663 (7 pages) | Cited 1 time

Online Publication Date: 17 August 2010

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The resistivities and microstructures for 50 nm Cu wires fabricated by high heating rate (3 K/s) and short time (1 min) annealing using infrared rapid thermal annealing equipment have been investigated as a function of annealing temperature and compared to those properties for wires fabricated by a slow heating rate (0.08 K/s), long time (30 min) conventional H2 annealing process. The resistivity of wires annealed by the new process decreased substantially with increasing annealing temperature from 573 to 773 K. The resistivity had its lowest value between 773 and 873 K, and it increased rapidly with annealing temperature above 923 K. The average ρ value was 2.98 μΩ cm for 773 K new process wires, whereas average ρ values were about 3.55 μΩ cm for 573 K and 3.42 μΩ cm for 673 K conventionally H2 annealed wires. This resistivity value for the new process wires was about 16% lower than the value for wires annealed at 573 K and 13% lower than the value for the wires annealed at 673 K by the conventional H2 annealing process. The substantial resistivity decrease in the new process Cu wires is mainly attributed to uniform grain size coarsening and high (111) orientation effects by the high temperature and high rate heating, while the resistivity increase at higher heating temperatures above 923 K for new process wires is mainly attributed to the reaction between Cu and Ta/TaN barriers; the greater the extent of the reaction, the higher the resistivity.
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85.40.Ls Metallization, contacts, interconnects; device isolation
81.16.-c Methods of micro- and nanofabrication and processing
73.63.Nm Quantum wires
61.72.Cc Kinetics of defect formation and annealing

III-nitride nanopyramid light emitting diodes grown by organometallic vapor phase epitaxy

Isaac H. Wildeson, Robert Colby, David A. Ewoldt, Zhiwen Liang, Dmitri N. Zakharov, Nestor J. Zaluzec, R. Edwin García, Eric A. Stach, and Timothy D. Sands

J. Appl. Phys. 108, 044303 (2010); http://dx.doi.org/10.1063/1.3466998 (8 pages) | Cited 6 times

Online Publication Date: 18 August 2010

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Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six {1math01} semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to ∼ 20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650–780 °C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs.
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85.60.Jb Light-emitting devices

Fabrication and photoluminescence of ZnS:Mn2+ nanowires/ZnO quantum dots/SiO2 heterostructure

Jinghai Yang, Jian Cao, Lili Yang, Yongjun Zhang, Yaxin Wang, Xiaoyan Liu, Dandan Wang, Maobin Wei, Ming Gao, and Jihui Lang

J. Appl. Phys. 108, 044304 (2010); http://dx.doi.org/10.1063/1.3467762 (7 pages) | Cited 1 time

Online Publication Date: 18 August 2010

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In this paper, we demonstrated the encapsulation of ZnS:Mn2+ nanowires (NWs) and ZnO quantum dots (QDs) with a layer of mesoporous SiO2 shell for the purpose of integrating dual emission property into one common nanostructure. The average diameter of ZnS:Mn2+ NWs, ZnO QDs, and ZnS:Mn2+/ZnO@SiO2 heterostructure was about 10 nm, 6 nm, and 22 nm, respectively. Within ZnS:Mn2+/ZnO@SiO2 nanocomposites, the intensity of the yellow-orange emission contributed by ZnS:Mn2+ NWs and the UV emission contributed by ZnO QDs was three and ten times higher than their individual components, respectively. The fluorescence intensity ratio of the dual emission can be tuned by adjusting the hydrolysis time of tetraethyl orthosilicate. The peak energy of the yellow-orange and UV emission showed blueshift and redshift as increasing the temperature, respectively. The anomalous enhancement of the integrated intensity for the UV emission with the temperature indicated that the high surface state density existing in ZnO QDs can overrun the influence of temperature quenching and even alter the photoluminescent properties.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
78.55.Hx Other solid inorganic materials
81.40.Gh Other heat and thermomechanical treatments
73.20.At Surface states, band structure, electron density of states
78.40.Ha Other nonmetallic inorganics
78.67.Sc Nanoaggregates; nanocomposites

Defects in gallium nitride nanowires: First principles calculations

Zhiguo Wang, Jingbo Li, Fei Gao, and William J. Weber

J. Appl. Phys. 108, 044305 (2010); http://dx.doi.org/10.1063/1.3476280 (6 pages) | Cited 2 times

Online Publication Date: 18 August 2010

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Atomic configurations and formation energies of native defects in an unsaturated GaN nanowire grown along the [001] direction and with (100) lateral facets are studied using large-scale ab initio calculation. Cation and anion vacancies, antisites, and interstitials in the neutral charge state are all considered. The configurations of these defects in the core region and outermost surface region of the nanowire are different. The atomic configurations of the defects in the core region are same as those in the bulk GaN, and the formation energy is large. The defects at the surface show different atomic configurations with low formation energy. Starting from a Ga vacancy at the edge of the side plane of the nanowire, a N–N split interstitial is formed after relaxation. As a N site is replaced by a Ga atom in the suboutermost layer, the Ga atom will be expelled out of the outermost layers and leaves a vacancy at the original N site. The Ga interstitial at the outmost surface will diffuse out by interstitialcy mechanism. For all the tested cases N–N split interstitials are easily formed with low formation energy in the nanowires, indicating N2 molecular will appear in the GaN nanowire, which agrees well with experimental findings.
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71.55.Eq III-V semiconductors
82.60.Cx Enthalpies of combustion, reaction, and formation
73.20.Hb Impurity and defect levels; energy states of adsorbed species
61.72.jd Vacancies
61.72.jj Interstitials

Thermal conductivity modeling of circular-wire nanocomposites

Tse-Yang Hsieh and Jaw-Yen Yang

J. Appl. Phys. 108, 044306 (2010); http://dx.doi.org/10.1063/1.3457230 (9 pages) | Cited 5 times

Online Publication Date: 18 August 2010

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A phonon Boltzmann equation solver using multiblock-structured grid system is developed and applied to study transverse thermal transport in silicon–germanium circular-wire nanocomposite (silicon nanowires embedded in germanium host matrix). Past studies usually assume geometric simplification for the circular-wire nanocomposite, so the heat transfer is actually modeled in a square-wire nanocomposite. To demonstrate geometry effect, phonon transport in both the circular-wire and square-wire nanocomposites are investigated with various wire spacings, volume fractions, and dimensions. In ballistic phonon transport, due to the smoothness of circular shape, the circular wire imposes less thermal resistance than the square wire. Nevertheless, in the geometric simplification, the wire spacing of the square-wire nanocomposite is larger than that of the circular-wire nanocomposite. The usual geometric simplification can overestimate the thermal conductivity of the circular-wire nanocomposite. The obtained results can provide essential information for the development of bulk-nanostructured thermoelectric devices.
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66.70.Df Metals, alloys, and semiconductors
61.46.-w Structure of nanoscale materials
47.27.te Turbulent convective heat transfer
72.20.Ht High-field and nonlinear effects

Contact voltage-induced softening of RF microelectromechanical system gold-on-gold contacts at cryogenic temperatures

D. Berman, M. J. Walker, and J. Krim

J. Appl. Phys. 108, 044307 (2010); http://dx.doi.org/10.1063/1.3459893 (8 pages) | Cited 5 times

Online Publication Date: 19 August 2010

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A series of experiments were performed in vacuum environments to investigate the impact of rf micromechanical system switch contact voltage versus resistance for gold-on-gold contacts at cryogenic temperatures. The purpose of this work was twofold as follows: (1) to examine whether asperity heating models already validated for high temperature contacts were also applicable at cryogenic temperatures and (2) to explore the implications and validity of prior suggestions that contact temperatures between 338 and 373 K are high enough to dissociate adsorbed film and/or push them aside but low enough to prevent asperities from becoming soft and adherent. Measurements on two distinct switch types, fabricated at independent laboratories, were performed in the temperature range 79–293 K and for contact voltages ranging from 0.01 to 0.13 V. Contact resistance values at all temperatures were observed to be lower for higher contact voltages, consistent with the aforementioned asperity heating models, whereby increased contact currents are associated with increased heating and softening effects. In situ removal of adsorbed species by oxygen plasma cleaning resulted in switch adhesive failure. Switches that had not been cleaned meanwhile exhibited distinct reductions in resistance at contact temperatures close to 338 K, consistent with suggestions that films begin to desorb, disassociate, and/or be pushed aside at that temperature.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.32.Dd Connectors, relays, and switches
73.40.Cg Contact resistance, contact potential
68.43.Mn Adsorption kinetics
68.43.Nr Desorption kinetics
52.77.Bn Etching and cleaning

Green backlighting for TV liquid crystal display using carbon nanotubes

Gabriel Delepierre, Rami Mahfouz, Francisco J. Cadete Santos Aires, and Jean Dijon

J. Appl. Phys. 108, 044308 (2010); http://dx.doi.org/10.1063/1.3448021 (8 pages) | Cited 1 time

Online Publication Date: 19 August 2010

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A methodology to evaluate the emission characteristics of carbon nanotube layers in the context of liquid crystal display backlighting has been elaborated. Carbon nanotube layers with emission characteristics compatible with backlighting have been demonstrated for growth temperature as low as 400 °C, thanks to the use of plasma pretreatment before growth. This very low growth temperature allows to use soda lime glass for the backlight unit and thus to expect very low cost and very low power consumption devices with this technology.
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42.79.Kr Display devices, liquid-crystal devices
85.35.Kt Nanotube devices

Temperature dependence of Raman scattering in filled double-walled carbon nanotubes

Mariana Sendova, Emmanuel Flahaut, and Thomas Hartsfield

J. Appl. Phys. 108, 044309 (2010); http://dx.doi.org/10.1063/1.3463400 (6 pages) | Cited 1 time

Online Publication Date: 20 August 2010

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Four types of filled double-walled carbon nanotubes (DWNTs) (Se@DWNT; Te@DWNT; HgTe@DWNT; and PbI2@DWNT) have been studied by high-resolution transmission electron microscopy and micro-Raman spectroscopy in the temperature interval from 80 to 700 K employing 785 nm excitation wavelength. The temperature dependence of the dominant bands (D-band, G-band, and the (2D)-band) are analyzed in terms of the model developed by Klemens, Hart, Agraval, Lax, and Cowley and extended by Balkanski for anharmonic decay of optical phonons. The quasiharmonic frequencies and the anharmonicity constants were obtained from the temperature dependences of the analyzed Raman bands. The findings were compared to analogous study for empty DWNTs. The strength of the van der Waals interaction between the guest material and the carbon nanotube (CNT) estimated through the quasiharmonic frequencies was found to decrease in the following order: Se@DWNT; Te@DWNT; PbI2@DWNT, and HgTe@DWNT. In agreement with this, the anharmonicity due to the phonon–phonon interactions was found to decrease in the same order.
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78.30.Hv Other nonmetallic inorganics
63.20.kg Phonon-phonon interactions
78.67.Ch Nanotubes

Polymer-assisted conformal coating of TiO2 thin films

E. S. Gillman, D. Costello, M. Moreno, A. Raspopin, R. Kasica, and L. Chen

J. Appl. Phys. 108, 044310 (2010); http://dx.doi.org/10.1063/1.3443574 (5 pages) | Cited 2 times

Online Publication Date: 23 August 2010

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Conformal coating of nanofabricated structures with a high-index dielectric material is a common problem for a diverse set of integrated photonic and plasmonic devices such as planar waveguides, on-chip spectrometers, gratings, flat panel displays, optical sensors, and integrated optical devices. In this paper we were recently able to demonstrate an alternate method for conformally coating photonic nanostructures using a low cost, polymer-assisted deposition (PAD) process for the metal-oxide TiO2. In a PAD process a thermally curable, hybrid high refractive index polymer solution is spin-coated onto a substrate. The polymer controls the viscosity and binds the metal ions, resulting in a homogeneous distribution of the precursor in solution. When cured at elevated temperature, the hybrid polymer coating decomposes to form a metal oxide-rich film that has a high refractive index that conformally fills the voids in nanofabricated structures. The resulting films have refractive indices higher than 1.83 in the visible region and film thicknesses between 250–500 nm depending on the level of metal-oxide loading, cure temperature, and number of coatings.
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81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.07.Bc Nanocrystalline materials
78.66.Nk Insulators
68.55.aj Insulators

Nanopatchwork cathodes: Patch-fields and field emission of nanosize parallel e-beams

Vu Thien Binh, R. Mouton, Ch. Adessi, V. Semet, M. Cahay, and S. Fairchild

J. Appl. Phys. 108, 044311 (2010); http://dx.doi.org/10.1063/1.3475343 (6 pages) | Cited 3 times

Online Publication Date: 23 August 2010

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A numerical calculation of the patch-field distribution across nanopatchwork surfaces has been developed. Results show that the low work-function nanosize zones are intrinsically protected by an electrostatic screen, which is induced by the surrounding area having a higher work function. In presence of an applied field, during field emission, a preferential opening of the surface barrier above the nanopatches induces a field emission array of parallel e-beams whose geometrical distribution is defined by the positions of the nanopatches.
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79.70.+q Field emission, ionization, evaporation, and desorption
73.30.+y Surface double layers, Schottky barriers, and work functions

Thermally enhanced mechanical properties of arc evaporated Ti0.34Al0.66N/TiN multilayer coatings

A. Knutsson, M. P. Johansson, L. Karlsson, and M. Odén

J. Appl. Phys. 108, 044312 (2010); http://dx.doi.org/10.1063/1.3463422 (7 pages) | Cited 16 times

Online Publication Date: 23 August 2010

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Cubic metastable Ti0.34Al0.66N/TiN multilayer coatings of three different periods, 25+50, 12+25, and 6+12 nm, and monoliths of Ti0.34Al0.66N and TiN where grown by reactive arc evaporation. Differential scanning calorimetry reveals that the isostructural spinodal decomposition to AlN and TiN in the multilayers starts at a lower temperature compared to the monolithic TiAlN, while the subsequent transformation from c-AlN to h-AlN is delayed to higher temperatures. Mechanical testing by nanoindentation reveals that, despite the 60 vol % TiN, the as-deposited multilayers show similar or slightly higher hardness than the monolithic Ti0.34Al0.66N. In addition, the multilayers show a more pronounced age hardening compared to the monolith. The enhanced hardening phenomena and improved thermal stability of the multilayer coatings are discussed in terms of particle confinement and coherency stresses from the neighboring TiN-layers.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
68.65.Ac Multilayers
62.20.Qp Friction, tribology, and hardness
81.40.Cd Solid solution hardening, precipitation hardening, and dispersion hardening; aging
68.35.Gy Mechanical properties; surface strains
81.70.Bt Mechanical testing, impact tests, static and dynamic loads

Nanostructured electrodes for thermionic and thermotunnel devices

Avto N. Tavkhelidze

J. Appl. Phys. 108, 044313 (2010); http://dx.doi.org/10.1063/1.3464256 (7 pages) | Cited 1 time

Online Publication Date: 23 August 2010

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Recently, distinctive quantum features have been studied in the area of ridged quantum wells (RQWs). Periodic ridges on the surface of the quantum well layer impose additional boundary conditions on the electron wave function and reduce the quantum state density. Electrons, rejected from forbidden quantum states, have to occupy the states with higher energy. As a result, Fermi energy in RQW increases and work function (WF) decreases. We investigate low WF electrode, composed from a metal RQW layer and a base substrate. The substrate material was selected so that electrons were confined to the RQW. The WF value depends on ridge geometry and electron confinement. We calculate WF in the metal RQW films grown both on a semiconductor and metal substrates. In the case of semiconductor substrate, wide band gap materials are preferable as they allow more reduction in RQW WF. In the case of metal substrate, low Fermi energy materials are preferable. For most material pairs, the WF was reduced dramatically. Such structures, can serve as electrodes for room temperature thermionic and thermotunnel energy converters and coolers.
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73.40.Cg Contact resistance, contact potential
73.21.Fg Quantum wells
79.40.+z Thermionic emission

Local intermixing on Ge/Si heterostructures at low temperature growth

H. H. Cheng, W. P. Huang, V. I. Mashanov, and G. Sun

J. Appl. Phys. 108, 044314 (2010); http://dx.doi.org/10.1063/1.3478718 (5 pages) | Cited 1 time

Online Publication Date: 24 August 2010

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We report experimental investigation on a series of strained Ge/Si heterostructure with various Ge thicknesses (the order of nanometers) grown at low temperatures (260 °C). In addition to the conventional uniform intermixing at the Ge/Si interface for structures with thin Ge layer, local intermixing characterized by a pattern structure is observed for structures with thick Ge layer. The pattern is formed beneath the Ge layer with an island shape and exhibits a Ge concentration dependent profile. From the analysis, it shows that the growth temperature and the strain energy stored in the Ge layer play a dominated role for the observation. In comparison with the conventional high temperature growth, this investigation shows that the self-assembly process is suppressed at low growth temperatures and intermixing plays a dominant role for strain relaxation.
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68.55.A- Nucleation and growth
78.30.Am Elemental semiconductors and insulators

Desalination of water by vapor-phase transport through hydrophobic nanopores

Jongho Lee and Rohit Karnik

J. Appl. Phys. 108, 044315 (2010); http://dx.doi.org/10.1063/1.3419751 (11 pages) | Cited 5 times

Online Publication Date: 24 August 2010

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We propose a new approach to desalination of water whereby a pressure difference across a vapor-trapping nanopore induces selective transport of water by isothermal evaporation and condensation across the pore. Transport of water through a nanopore with saline water on one side and pure water on the other side under a pressure difference was theoretically analyzed under the rarefied gas assumption using a probabilistic framework that accounts for diffuse scattering from the pore walls as well as reflection from the menisci. The analysis revealed that in addition to salinity, temperature, and pressure difference, the nanopore aspect ratio and the probability of condensation of a water molecule incident on a meniscus from the vapor phase, known as the condensation coefficient, are key determinants of flux. The effect of condensation coefficient on mass flux becomes critical when the aspect ratio is small. However, the mass flux becomes independent of the condensation coefficient as the pore aspect ratio increases, converging to the Knudsen flux for long nanopores. For design of a nanopore membrane that can trap vapor, a minimum aspect ratio is derived for which coalescence of the two interfaces on either side of the nanopore remains energetically unfavorable. Based on this design criterion, the analysis suggests that mass flux in the range of 20–70 g/m2 s may be feasible if the system is operated at temperatures in the range of 30–50 °C. The proposed approach further decouples transport properties from material properties of the membrane, which opens the possibility of engineering membranes with appropriate materials that may lead to reverse osmosis membranes with improved flux, better selectivity, and high chlorine resistance.
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68.03.Fg Evaporation and condensation of liquids

Theory of the phonon properties of pure and ion-doped ZnO nanoparticles

J. M. Wesselinowa and A. T. Apostolov

J. Appl. Phys. 108, 044316 (2010); http://dx.doi.org/10.1063/1.3467530 (7 pages) | Cited 4 times

Online Publication Date: 25 August 2010

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Based on the s-d model including electron–phonon and spin–phonon interaction, and using a Green’s function technique we have studied surface, size, and ion doping effects on the phonon properties of ZnO nanoparticles (NPs). We have shown that the electron–phonon and anharmonic phonon–phonon interactions play an important role in pure ZnO NPs, whereas in the transition metal (TM) and rare earth (RE) doped ZnO NPs the spin–phonon interaction must be taken into account in order to explain the experimental data. Due to surface and size effects, the phonon frequency decreases and the phonon damping increases with decreasing of particle size. By TM ion doping we obtain hardening, whereas by doping with Mn or RE ions-softening of the phonon energy with increasing the dopant concentration. This is due to the different radii of the doping ions compared to the host Zn ion radius and to the induced different lattice strain and spin–phonon interactions. The phonon damping is always enhanced compared to the undoped case. The observed results are in qualitative agreement with the experimental data. Our model and theory can be applied to all pure and doped diluted magnetic semiconductor NPs.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
63.20.kd Phonon-electron interactions
63.20.kg Phonon-phonon interactions

Young’s modulus and density measurements of thin atomic layer deposited films using resonant nanomechanics

B. Ilic, S. Krylov, and H. G. Craighead

J. Appl. Phys. 108, 044317 (2010); http://dx.doi.org/10.1063/1.3474987 (11 pages) | Cited 6 times

Online Publication Date: 31 August 2010

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Material properties of atomic layer deposited (ALD) thin films are of interest for applications ranging from wear resistance to high-k dielectrics in electronic circuits. We demonstrate the ability to simultaneously measure Young’s modulus (E) and density (ρ) of 21.2–21.5 nm ALD hafnia, alumina, and aluminum nitride ultrathin films by observing vibrations of nanomechanical cantilever beams. The nanomechanical structures were fabricated from a 250 nm thick single crystal silicon layer with varying length and width ranging from 6 μm to 10 μm and 45 nm to 1 μm, respectively. Our approach is based on an optical excitation and interferometric detection of in-plane and out-of plane vibrational spectra of single crystal silicon cantilevers before and after a conformal coating deposition of an ALD thin film. In conjunction with three-dimensional numerical finite element analysis, measurements of resonance carried out prior to the ALD revealed that while the influence of clamping compliance arising from the undercut of the sacrificial layer is significant for wider beams, the effect is less pronounced for both, narrower cantilevers and the in-plane vibrational response. Following the deposition, higher stiffness alumina films (E>ESi) showed an increase in the resonant frequency whereas lower stiffness (E<ESi) hafnia and aluminum nitride films decreased the natural frequency. From the measured spectral response, material properties were extracted using simple expressions for E and ρ in terms of measured in-plane and out-of-plane frequencies shifts. The derived model was based on an ideally clamped Euler–Bernoulli beam with effective bending stiffness and effective mass per unit length. In-plane and out-of-plane frequency measurements provided two equations that enabled simultaneous extraction of E and ρ. Three-dimensional finite element analysis showed that residual stress, nonideal clamping conditions, and the mismatch in the Poisson’s ratio between the deposited film and the nanomechanical oscillator have minor influence on the determined material properties. Experimental results obtained for the measured films were in excellent agreement with finite element simulations incorporating the geometric undercut caused by release of the suspended structures.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
62.20.dj Poisson's ratio
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
68.55.A- Nucleation and growth
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