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4 Feb 2008

Volume 92, Issue 5, Articles (05xxxx)

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Appl. Phys. Lett. 92, 053301 (2008); http://dx.doi.org/10.1063/1.2838457 (3 pages)

Michael S. Arnold, Gregory J. McGraw, Stephen R. Forrest, and Richard R. Lunt
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Hugoniot measurement of gold at high pressures of up to 580 GPa

Manabu Yokoo, Nobuaki Kawai, Kazutaka G. Nakamura, and Ken-ichi Kondo

Appl. Phys. Lett. 92, 051901 (2008); http://dx.doi.org/10.1063/1.2840189 (3 pages) | Cited 9 times

Online Publication Date: 4 February 2008

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Hugoniot for Au was measured over a pressure range from 190 to 580 GPa using a two-stage light-gas gun. The accuracy for impactor velocity was 0.2% and that for shock velocity ranged from 1.0% to 2.3%. Symmetric-impact experiments were performed to obtain shock compression data that are independent of those of other materials. The relationship between the shock and the particle velocity shows that our data for shock velocity deviate upward from the previously established relationship by up to 2.7% at up = 3.5 km/s. Moreover, we also report data obtained by impedance matching using Ta and Cu.
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64.30.Ef Equations of state of pure metals and alloys
62.50.Ef Shock wave effects in solids and liquids

Superheating and melting of nanocavities

G. Ouyang, X. L. Li, and G. W. Yang

Appl. Phys. Lett. 92, 051902 (2008); http://dx.doi.org/10.1063/1.2833258 (3 pages) | Cited 3 times

Online Publication Date: 4 February 2008

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Melting behaviors of nanocavities in matrix is investigated on the basis of thermodynamics in terms of continuum mechanics at the nanometer scale. An analytical model is developed to elucidate the void shrinking kinetics and the void melting behavior. It is found that the shrinkage of nanocavities exhibits a pronounced nonlinear kinetic character when the nanocavity’s size goes into several nanometer scales. The giant superheating of the nanocavity with the small size appears when the temperature is higher than the melting point of the matrix. The size dependence of the inner surface energy of nanocavities seems responsible for these anomalous melting behaviors.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
64.70.Nd Structural transitions in nanoscale materials
64.70.dj Melting of specific substances

Correlation of In–Ga intermixing with band-tail states in InAs/GaAs quantum dots

Hung-Chin Chung, Yi-Feng Lai, Chuan-Pu Liu, Yen-Lin Lai, Yu-Ching Fang, and Li Hsu

Appl. Phys. Lett. 92, 051903 (2008); http://dx.doi.org/10.1063/1.2839376 (3 pages) | Cited 7 times

Online Publication Date: 4 February 2008

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We have investigated the shape and composition profiles of buried and surface InAs/GaAs Stranski–Krastanov quantum dots (QDs) by using the spectrum-imaging (SI) method with energy-filtered transmission electron microscopy (EFTEM). Indium maps from EFTEM SI reveal lens and truncated pyramid shapes for the surface and buried QDs, with an increase in composition variations for the buried QDs. Photoluminescence measurements reveal an emission at 1.075 eV, associated with confined states in the buried QDs, along with a high energy shoulder, associated with band-tail states due to In–Ga intermixing in the vicinity of the buried QDs.
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68.65.Hb Quantum dots (patterned in quantum wells)
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors

Inhomogeneous and anisotropic equilibrium state of a swollen hydrogel containing a hard core

Xuanhe Zhao, Wei Hong, and Zhigang Suo

Appl. Phys. Lett. 92, 051904 (2008); http://dx.doi.org/10.1063/1.2840158 (3 pages) | Cited 16 times

Online Publication Date: 4 February 2008

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A polymer network can imbibe water from environment and swell to an equilibrium state. If the equilibrium is reached when the network is subject to external mechanical constraint, the deformation of the network is typically anisotropic and the concentration of water inhomogeneous. Such an equilibrium state in a network constrained by a hard core is modeled here with a nonlinear differential equation. The presence of the hard core markedly reduces the concentration of water near the interface and causes high stresses.
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61.25.hp Polymer swelling, cross linking
82.70.Gg Gels and sols
62.10.+s Mechanical properties of liquids

Proposal of mechanics of liquid crystals and development of liquid crystalline microactuators

Shigeomi Chono and Tomohiro Tsuji

Appl. Phys. Lett. 92, 051905 (2008); http://dx.doi.org/10.1063/1.2840673 (3 pages) | Cited 4 times

Online Publication Date: 5 February 2008

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We propose a research field “mechanics of liquid crystals,” in which liquid crystals are studied from a mechanical viewpoint. The unsteady behaviors of a liquid crystal between parallel plates under an electric field are investigated. The imposition of the electric field on the liquid crystal induces flows, whose profile and magnitude strongly depend on the twist angle of the director at the plates. A visualization experiment confirms the generation of flows. The mechanism of such generation can be explained by considering that the rotation of molecules generated by the imposition of the electric field induces a local velocity gradient.
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83.80.Xz Liquid crystals: nematic, cholesteric, smectic, discotic, etc.
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)

Tunable energy transfer efficiency based on the composite of mixed CdSe quantum dots and elastomeric film

C. W. Chen, C. H. Wang, Y. F. Chen, C. W. Lai, and P. T. Chou

Appl. Phys. Lett. 92, 051906 (2008); http://dx.doi.org/10.1063/1.2841047 (3 pages) | Cited 4 times

Online Publication Date: 5 February 2008

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We demonstrate a facile and general approach to investigate the dependence of energy transfer on the separation distance between proximal mixed-size quantum dots. Without varying the mixed concentrations, the tunable energy transfer efficiency is achieved based on the composite of mixed quantum dots and elastomeric film by utilizing the inherent nature of the flexibility of elastomeric film. To demonstrate our working principle, the composite of mixed-size CdSe quantum dots and poly-dimethylsiloxane has been studied. The results clearly show that the energy transfer process between proximal quantum dots follows the Förster resonance energy transfer, in which the dependence of the transfer efficiency E as a function of the donor-acceptor distance R obeys E = 1/[1+(R/R0)6].
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78.67.Hc Quantum dots
78.66.Qn Polymers; organic compounds
78.55.Et II-VI semiconductors
78.55.Kz Solid organic materials

In situ observation of gas reemission and blister rupture during helium implantation in silicon

Klaus Wittmaack

Appl. Phys. Lett. 92, 051907 (2008); http://dx.doi.org/10.1063/1.2839601 (3 pages) | Cited 3 times

Online Publication Date: 5 February 2008

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Gas phase ionization mass spectrometry is shown to provide means for exploring the processes that control the retention of 10 keV He+ implanted in Si at 45°. Blistering, observed in situ, was preceded by a long period of bombardment (70% of the critical fluence for blistering) during which implanted He was released at a rather high rate ( ∼ 33% of the incident He flux). Assisted by the stress due to the growing bubbles, bombardment induced detrapping allows a large fraction of the implanted He atoms to migrate to the nearby surface and escape into vacuum.
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61.72.U- Doping and impurity implantation
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Localized and free exciton spin relaxation dynamics in GaInNAs/GaAs quantum well

S. L. Lu, L. F. Bian, M. Uesugi, H. Nosho, A. Tackeuchi, and Z. C. Niu

Appl. Phys. Lett. 92, 051908 (2008); http://dx.doi.org/10.1063/1.2841041 (3 pages) | Cited 2 times

Online Publication Date: 6 February 2008

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We have investigated the exciton spin relaxation in a GaInNAs/GaAs quantum well. The recombination from free and localized excitons is resolved on the basis of an analysis of the photoluminescence characteristics. The free exciton spin relaxation time is measured to be 192 ps at 10 K, while the localized exciton spin relaxation time is one order of magnitude longer than that of the free exciton. The dependence of the free exciton spin relaxation time on the temperature above 50 K suggests that both the D’yakonov–Perel’ and the Elliot–Yafet effects dominate the spin relaxation process. The temperature independence below 50 K is considered to be due to the spin exchange interaction. The ultralong spin relaxation time of the localized excitons is explained to be due to the influence of nonradiative deep centers.
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73.61.Ey III-V semiconductors
78.55.Cr III-V semiconductors
71.35.-y Excitons and related phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Defect reduction by epitaxial lateral overgrowth of nanorods in ZnO/(0001) sapphire films

D. Cherns and Y. Sun

Appl. Phys. Lett. 92, 051909 (2008); http://dx.doi.org/10.1063/1.2841707 (3 pages) | Cited 5 times

Online Publication Date: 7 February 2008

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It is shown that epitaxial ZnO/(0001) sapphire films grown by pulsed laser deposition have defect-free Zn-polar nanorods protruding from a continuous O-polar underlayer containing high densities of threading dislocations (TDs). By continuing the ZnO growth hydrothermally, the nanorods grew laterally over the O-polar layer. TDs in the underlayer were thereby blocked and only formed in the overlayer when nanorods coalesced. This epitaxial lateral overgrowth produced continuous Zn-polar films with TD densities reduced to 1×109/cm2 from 7×1010/cm2 in the underlayer. It is noted that the same growth mode can be achieved in GaN/(0001) sapphire films.
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81.15.Fg Pulsed laser ablation deposition
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Catalytic effect of mixed Zr–Fe additives on the hydrogen desorption kinetics of MgH2

Nicola Bazzanella, Riccardo Checchetto, and Antonio Miotello

Appl. Phys. Lett. 92, 051910 (2008); http://dx.doi.org/10.1063/1.2840180 (3 pages) | Cited 5 times

Online Publication Date: 7 February 2008

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Mg films containing Fe–Zr mixed additives were prepared by rf sputtering and their hydrogen sorption kinetic was studied by Sievert technique. We observed that mixed Fe–Zr additives improve the H2 desorption kinetics better than singly Fe or Zr. X-ray diffraction analysis evidences that mixed additives optimize the Fe distribution that forms smaller clusters than in Mg sample with single Fe catalyst. The presence of mixed additives, with Fe having larger diffusivity than Zr, contributes to maintain the atomic Zr dispersion in the Mg matrix, thus, favoring the presence of a greater number of nucleation sites for MgH2 to Mg transformation.
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68.43.Nr Desorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Blue-light emission from GaN/Al0.5Ga0.5N quantum dots

T. Huault, J. Brault, F. Natali, B. Damilano, D. Lefebvre, L. Nguyen, M. Leroux, and J. Massies

Appl. Phys. Lett. 92, 051911 (2008); http://dx.doi.org/10.1063/1.2841825 (3 pages) | Cited 12 times

Online Publication Date: 7 February 2008

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The growth by molecular beam epitaxy and the optical properties of GaN/Al0.5Ga0.5N quantum dots on (0001) sapphire substrates are reported. The quantum dots are spontaneously formed via a two dimensional to three dimensional transition upon growth interruption. Photoluminescence over the blue range (435–470 nm) is obtained at room temperature by varying the GaN nominal thickness. A weak temperature dependence of the integrated photoluminescence intensity between low temperature and room temperature is observed indicating strong carrier localization in the quantum dots.
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68.65.Hb Quantum dots (patterned in quantum wells)
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Anomalous two-photon generation of excitons in CuCl pellets

J. I. Jang, M. A. Anderson, J. B. Ketterson, and R. P. H. Chang

Appl. Phys. Lett. 92, 051912 (2008); http://dx.doi.org/10.1063/1.2841043 (3 pages) | Cited 3 times

Online Publication Date: 7 February 2008

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We report on an anomalous excitation of excitonic matter in pressed CuCl pellets. In these random materials, we find that excitons are initially created for an excitation energy in the range of 1892–2843 meV, which is far below the bandgap of this semiconductor. The excitation-power dependence from the population dynamics of excitonic matter strongly indicates an unusual two-photon generation of ground-state excitons. Considering this off-resonance effect, excitons are probably generated from a continuum of impurity-induced levels excited by two-photon pumping.
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71.35.-y Excitons and related phenomena
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Broad forbidden bands in parallel-coupled locally resonant ultrasonic metamaterials

Y. Cheng, J. Y. Xu, and X. J. Liu

Appl. Phys. Lett. 92, 051913 (2008); http://dx.doi.org/10.1063/1.2839401 (3 pages) | Cited 12 times

Online Publication Date: 8 February 2008

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The authors demonstrate that a class of ultrasonic metamaterial, which is composed of subwavelength resonant units built up by parallel-coupled Helmholtz resonators with identical resonant frequency, possesses broad locally resonant forbidden bands. The bandwidths are strongly dependent on the number of resonators in each unit. The broadening of bands is ascribed to the change of effective acoustic impendence. The coupling effects on the wave vector and negative dynamic modulus are discussed. Numerical simulations by finite element method further confirm the theoretical results.
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62.65.+k Acoustical properties of solids
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Mechanical stress impact on thin Pd1−xFex film thermodynamic properties

Stefan Wagner and Astrid Pundt

Appl. Phys. Lett. 92, 051914 (2008); http://dx.doi.org/10.1063/1.2841636 (3 pages) | Cited 8 times

Online Publication Date: 8 February 2008

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Thermodynamic properties of thin films deviate strongly from those of bulk. The deviations are reported to originate from microstructure and from mechanical stress, whereas the contribution of both is unknown in particular. Focussing on the mechanical stress contribution and by using Pd1−xFex–H as a model system, it is shown that mechanical stress strongly changes phase transition pressures. The measured loading pressures shift up to 400 mbars in contrast to 18 mbars for bulk. These shifts relate to the film bonding to the substrate and can be affected by film detachment.
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68.60.Bs Mechanical and acoustical properties
65.40.G- Other thermodynamical quantities
64.70.kd Metals and alloys
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