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20 Sep 2010

Volume 97, Issue 12, Articles (12xxxx)

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Appl. Phys. Lett. 97, 123101 (2010); http://dx.doi.org/10.1063/1.3490637 (3 pages)

Mark W. Licurse and Peter K. Davies
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Photoluminescence from colloids containing aluminum hydroxide nanocrystals with uniform size

T. H. Li, L. Z. Liu, X. L. Wu, J. C. Shen, F. Gao, and Paul K. Chu

Appl. Phys. Lett. 97, 121901 (2010); http://dx.doi.org/10.1063/1.3491161 (3 pages) | Cited 2 times

Online Publication Date: 20 September 2010

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Aluminum hydroxide nanocrystals consisting of an amorphous shell and crystalline core are fabricated by pulsed laser ablation of an aluminum target in water. The colloid consisting of nanocrystals with a uniform size exhibits a size-independent photoluminescence (PL) band at ∼ 383 nm. According to the PL excitation spectra and time-resolved PL decay analysis, this PL band originates from oxygen vacancies in the amorphous shell and Förster energy transfer occurs between the oxygen vacancy levels in the crystalline core and amorphous shell. These phenomena are found to alter the PL excitation spectra.
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81.16.-c Methods of micro- and nanofabrication and processing
82.70.Dd Colloids

The effect of underlayer thin films on the surface-enhanced Raman scattering response of Ag nanorod substrates

Qin Zhou, Yongjun Liu, Yuping He, Zhengjun Zhang, and Yiping Zhao

Appl. Phys. Lett. 97, 121902 (2010); http://dx.doi.org/10.1063/1.3489973 (3 pages) | Cited 11 times

Online Publication Date: 21 September 2010

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The effect of underlayer thin films on the sensitivity of Ag nanorod surface-enhanced Raman scattering (SERS) substrates was studied both theoretically and experimentally. With the same Ag nanorod film, different materials (Ag, Al, Si, and Ti) with different thicknesses (25, 100, and 400 nm) were used as underlayers to alter the reflectivity systematically. The SERS intensity was found to increase linearly with the underlayer reflectivity, which can be explained by a modified Greenler’s model due to the contribution of reflected electric field from the substrate. This finding can be used to design high enhancement SERS substrates.
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78.30.Er Solid metals and alloys
82.80.Gk Analytical methods involving vibrational spectroscopy
78.67.Qa Nanorods
61.46.-w Structure of nanoscale materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

The shock-front structure of nanocrystalline aluminum

Wen Ma, Wenjun Zhu, and Fuqian Jing

Appl. Phys. Lett. 97, 121903 (2010); http://dx.doi.org/10.1063/1.3490643 (3 pages) | Cited 1 time

Online Publication Date: 21 September 2010

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The shock-front structures of nanocrystalline Al are investigated in detail by exploring the relationship between the evolution of stress, particle velocity distributions, and the atomistic structures through molecular dynamics simulations. It is found that in nanocrystalline Al the contribution of grain boundary-mediated plasticity to the shock-front width is significant in comparison with dislocation-mediated plasticity. Due to different deformation mechanisms and time sequences, the shock front can be separated into following three stages: elastic, grain boundary dominated plasticity, and dislocation emission and propagation.
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81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity
61.72.Bb Theories and models of crystal defects
62.20.F- Deformation and plasticity
61.46.Hk Nanocrystals
61.72.Mm Grain and twin boundaries
62.25.-g Mechanical properties of nanoscale systems

Persistent two-dimensional growth of (110) manganite films

R. Bachelet, D. Pesquera, G. Herranz, F. Sánchez, and J. Fontcuberta

Appl. Phys. Lett. 97, 121904 (2010); http://dx.doi.org/10.1063/1.3490713 (3 pages) | Cited 9 times

Online Publication Date: 21 September 2010

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We have deposited La2/3Sr1/3MnO3(110) thin films by pulsed laser deposition, changing the ratio of surface diffusivity to deposition flux (D/F) by adjusting substrate temperature and laser repetition rate. We show that persistent two-dimensional layer-by-layer growth, at least up to 30 nm, can be achieved by exploiting diffusion-limited growth (small D/F ratio), giving rise to atomically-flat epitaxial films. The results are of interest in spintronic applications such as tunnel magnetoresistance devices, and the approach presented here can be extended to other functional films of high surface energy.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Dd Nonmetallic ferromagnetic materials
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.35.Fx Diffusion; interface formation
81.15.Fg Pulsed laser ablation deposition
68.55.at Other materials

Mechanically generated surface chirality: Control of chiral strength

Sameh Ferjani, Joel Pendery, and Charles Rosenblatt

Appl. Phys. Lett. 97, 121905 (2010); http://dx.doi.org/10.1063/1.3491157 (3 pages) | Cited 1 time

Online Publication Date: 21 September 2010

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A substrate coated with an achiral polyimide alignment layer was scribed with the stylus of an atomic force microscope having a line-to-line force profile FAFBFCFAFBFC…. The strength of the resulting chiral surface was examined using the nematic liquid crystal electroclinic effect induced by the surface. The magnitude of the electroclinic effect was found to increase with increasing scribing force, which suggests a method for controlling the chiral strength. Additionally, the electroclinic magnitude divided by the rms surface roughness was approximately constant with scribing force, suggesting that the azimuthal anchoring strength coefficient is nearly independent of the scribing force.
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61.30.-v Liquid crystals

Theory of a three-dimensional nanoporous silicon lattice with unsaturated bonding

Feng Zhang, Dragan S. Stojkovic, and Vincent H. Crespi

Appl. Phys. Lett. 97, 121906 (2010); http://dx.doi.org/10.1063/1.3491240 (3 pages)

Online Publication Date: 21 September 2010

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Several molecules are known to contain stable silicon double or triple bonds that are sterically protected by bulky side groups. Through first-principles computation, we demonstrate that well-defined π bonds can also be stabilized in a prototypical crystalline Si structure: Schwarzite Si-168, when modest negative pressures are applied to a nanoscale porous framework. The sp2-bonded Si-168 is thermodynamically preferred over diamond silicon at a negative pressure of −2.5 GPa. Ab-initio molecular dynamics simulations of Si-168 at 1000 K reveal significant thermal stability. Si-168 is metallic at P = 0 in density functional theory, but a gap (between π-like and π∗-like bands) opens around the Fermi level at the transition pressure of −2.5 GPa. Alternatively, a band gap buried below the Fermi level at P = 0 can be accessed via hole doping in semiconducting Si144B24.
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61.50.Lt Crystal binding; cohesive energy
73.20.At Surface states, band structure, electron density of states
61.43.Bn Structural modeling: serial-addition models, computer simulation

Impact of ion implantation boundary dimensionality on boron transient diffusion in submicron scale patterns

Wei Yen Woon (溫偉源) and Chia Ling Chen (陳佳伶)

Appl. Phys. Lett. 97, 121907 (2010); http://dx.doi.org/10.1063/1.3492840 (3 pages) | Cited 1 time

Online Publication Date: 22 September 2010

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We investigate two-dimensional boron transient diffusion in sub-micron scale patterns by plane view scanning capacitance microscopy (SCM). Submicron long strips and squares ion implantation windows of systematically varying sizes have been designed and fabricated. Boron ion implantation and spike annealing were followed to activate the dopant and cause diffusion. Square opening windows show more enhanced diffusion than the long strip counterparts, especially at larger length scales. We explain the observation and fit the experimental data by a nonlinear logistics model. The implication to modern microelectronic circuit design and conventional dopant profiling methodology are discussed.
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85.40.Ry Impurity doping, diffusion and ion implantation technology
81.05.Cy Elemental semiconductors
61.72.uf Ge and Si
61.72.Cc Kinetics of defect formation and annealing
68.35.Fx Diffusion; interface formation
61.72.jj Interstitials

Carbon nanotube-induced chirality in an achiral liquid crystal

Rajratan Basu, Krysta A. Boccuzzi, Sameh Ferjani, and Charles Rosenblatt

Appl. Phys. Lett. 97, 121908 (2010); http://dx.doi.org/10.1063/1.3492844 (3 pages) | Cited 4 times

Online Publication Date: 22 September 2010

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A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal (LC), and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the LC’s smectic-A phase, which showed significant pretransitional behavior on approaching the smectic-A–smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the LC.
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61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
61.25.-f Studies of specific liquid structures
64.70.M- Transitions in liquid crystals
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures

Structural and optical properties of GaInP grown on germanium by metal-organic chemical vapor deposition

W. He, S. L. Lu, J. R. Dong, Y. M. Zhao, X. Y. Ren, K. L. Xiong, B. Li, H. Yang, H. M. Zhu, X. Y. Chen, and X. Kong

Appl. Phys. Lett. 97, 121909 (2010); http://dx.doi.org/10.1063/1.3492854 (3 pages) | Cited 7 times

Online Publication Date: 22 September 2010

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Structural and optical properties of Si-doped as well as nominally undoped GaInP epilayers grown on Germanium substrates by metal-organic chemical vapor deposition have been investigated by high resolution transmission electron microscope and photoluminescence (PL). Si incorporation results in an increased inner band PL transition and a blue shift of PL energy with increasing temperature, which arises from the trapping states around Ge-GaInP interface due to Ge diffusion to GaInP epilayer as well as Si doping. For the inter band PL transition, a competition between the emission processes near the band edge and in the ordered GaInP domains is responsible for the inverted S shape temperature dependence of PL peaks. By analyzing the time-resolved PL results, we attribute this emission near the ordered states to the localized states due to the potential fluctuation, which is induced by the compositional inhomogeneity of Ga and In in the partially ordered GaInP.
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78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors
78.55.Cr III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.U- Doping and impurity implantation
68.55.ag Semiconductors

Observation of coherent phonons in metallic carbon nanotubes

Keiko Kato, Katsuya Oguri, Atsushi Ishizawa, Hideki Gotoh, Hidetoshi Nakano, and Tetsuomi Sogawa

Appl. Phys. Lett. 97, 121910 (2010); http://dx.doi.org/10.1063/1.3491220 (3 pages) | Cited 1 time

Online Publication Date: 22 September 2010

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We investigated the ultrafast dynamics of coherent phonons with sub-10 fs laser pulses in metallic single-walled carbon nanotubes (m-SWCNTs). We identified radial breathing mode (RBM), D-mode, and G-mode coherent phonons in m-SWCNTs. We also resolved G−- and G+-mode coherent phonons, which correspond to the longitudinal and transverse optical phonon modes, respectively. The frequencies of the RBM coherent phonons shift with increases in laser intensity, suggesting that photocarriers modify the phonon frequency.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
63.20.kd Phonon-electron interactions

The transition in hydrogen sensing behavior in noncontinuous palladium films

Thomas Kiefer, Luis Guillermo Villanueva, Frédéric Fargier, Frédéric Favier, and Jürgen Brugger

Appl. Phys. Lett. 97, 121911 (2010); http://dx.doi.org/10.1063/1.3491263 (3 pages) | Cited 5 times

Online Publication Date: 22 September 2010

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The morphological transition in ultrathin palladium (Pd) films around the percolation threshold and the related transition in hydrogen sensing behavior is investigated. We find that besides the transition from continuous to discontinuous Pd, an intermediate - semicontinuous-state must be considered. It shows hydrogen sensing features of both continuous and discontinuous film types, simultaneously. This study focuses on the discontinuous-semicontinuous transition. Experimental evidence is supported by studying the evolution of the electrical resistance with temperature, under hydrogen exposure and after thermal annealing. The results are highly relevant for the optimization of nanogap based hydrogen sensors.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis

Congruent evaporation temperature of GaAs(001) controlled by As flux

Z. Y. Zhou, C. X. Zheng, W. X. Tang, D. E. Jesson, and J. Tersoff

Appl. Phys. Lett. 97, 121912 (2010); http://dx.doi.org/10.1063/1.3491552 (3 pages) | Cited 1 time

Online Publication Date: 22 September 2010

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The congruent evaporation temperature Tc is a fundamental surface characteristic of GaAs and similar compounds. Above Tc the rate of As evaporation exceeds that of Ga during Langmuir (free) evaporation into a vacuum. However, during molecular beam epitaxy (MBE) there is generally an external As flux F incident on the surface. Here we show that this flux directly controls Tc. We introduce a sensitive approach to measure Tc based on Ga droplet stability, and determine the dependence of Tc on F. This dependence is explained by a simple model for evaporation in the presence of external flux. The capability of manipulating Tc via changing F offers a means of controlling congruent evaporation with relevance to MBE, surface preparation methods, and droplet epitaxy.
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64.70.kg Semiconductors
64.70.fh Boiling and bubble dynamics
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
47.55.dk Surfactant effects

Selective area growth of high quality InP on Si (001) substrates

G. Wang, M. R. Leys, R. Loo, O. Richard, H. Bender, N. Waldron, G. Brammertz, J. Dekoster, W. Wang, M. Seefeldt, M. Caymax, and M. M. Heyns

Appl. Phys. Lett. 97, 121913 (2010); http://dx.doi.org/10.1063/1.3491554 (3 pages) | Cited 4 times

Online Publication Date: 22 September 2010

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In this work, we demonstrate the selective area growth of high quality InP layers in submicron trenches on exactly (001) oriented Si substrates by using a thin Ge buffer layer. Antiphase domain boundaries were avoided by annealing at the Ge surface roughening temperature to create additional atomic steps on the Ge buffer layer. The mechanism of Ge surface atomic step formation and the corresponding step density control method are illustrated. The elimination of antiphase boundaries from the optimized Ge buffer layer, together with the defect necking effect, yield defect-free top InP layers inside the trenches.
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68.37.-d Microscopy of surfaces, interfaces, and thin films
68.55.ag Semiconductors
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
81.40.Lm Deformation, plasticity, and creep
73.40.Ns Metal-nonmetal contacts
68.47.Fg Semiconductor surfaces

Atomic structure of misfit dislocations in nonpolar ZnO/Al2O3 heterostructures

H. Zhou, M. F. Chisholm, P. Pant, H. J. Chang, J. Gazquez, S. J. Pennycook, and J. Narayan

Appl. Phys. Lett. 97, 121914 (2010); http://dx.doi.org/10.1063/1.3489687 (3 pages) | Cited 7 times

Online Publication Date: 22 September 2010

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Understanding dislocation core structures at the atomic level is of significant theoretical and technological importance because of the role dislocations play in the electronic/optical properties of materials. In this paper, we report our aberration-corrected scanning transmission electron microscopy study on misfit dislocation core structures at non-polar (11math0)ZnO/(1math02)Al2O3 (a-ZnO/r-Al2O3) interface. The atomic configuration of the core structure is found to be closely related to the preferred interfacial bonding configuration. A significant number of these misfit dislocations have undergone a core structure modification involving the incorporation of Zn in the Al2O3 side of the dislocation.
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71.20.Nr Semiconductor compounds
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.05.Dz II-VI semiconductors
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.20.-e Optical properties of bulk materials and thin films

Disentangling the role of linear transition dipole in band-edge emission from single CdSe/ZnS quantum dots: Combined linear anisotropy and defocused emission pattern imaging

Austin Cyphersmith, Kevin Early, Artem Maksov, Jeremy Graham, Yikuan Wang, and Michael Barnes

Appl. Phys. Lett. 97, 121915 (2010); http://dx.doi.org/10.1063/1.3488669 (3 pages) | Cited 1 time

Online Publication Date: 23 September 2010

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Combined linear anisotropy and defocused wide-field fluorescence imaging of individual CdSe/ZnS quantum dots reveal an unambiguous contribution of a linear transition dipole polarized along the hexagonal (c-) symmetry axis of the nanocrystal which grows with increasing tilt angle of the c-axis with respect to the surface normal. These results offer some insights into quantum dot luminescence polarization dynamics, and provide a basis for unambiguously assigning the two Euler angles describing the c-axis orientation from defocused emission patterns for multidipole systems.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.07.Ta Quantum dots
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
82.70.Dd Colloids

Magnetocaloric effect in manganites: Metamagnetic transitions for magnetic refrigeration

M. Quintero, J. Sacanell, L. Ghivelder, A. M. Gomes, A. G. Leyva, and F. Parisi

Appl. Phys. Lett. 97, 121916 (2010); http://dx.doi.org/10.1063/1.3491175 (3 pages) | Cited 7 times

Online Publication Date: 23 September 2010

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We present a study of the magnetocaloric effect in La5/8−yPryCa3/8MnO3 (y = 0.3) and Pr0.5Ca0.09Sr0.41MnO3 manganites. The low temperature state of both systems is the result of a competition between the antiferromagnetic and ferromagnetic phases. The samples display magnetocaloric effect evidenced in an adiabatic temperature change during a metamagnetic transition from an antiferromagnetic to a ferromagnetic phase. As additional features, La5/8−yPryCa3/8MnO3 exhibits phase separation characterized by the coexistence of antiferromagnetic and ferromagnetic phases and Pr0.5Ca0.09Sr0.41MnO3 displays inverse magnetocaloric effect in which temperature decreases while applying an external magnetic field. In both cases, a significant part of the magnetocaloric effect appears from nonreversible processes. As the traditional thermodynamic description of the effect usually deals with reversible transitions, we developed an alternative way to calculate the adiabatic temperature change in terms of the change of the relative ferromagnetic fraction induced by magnetic field. To evaluate our model, we performed direct measurement of the sample’s adiabatic temperature change by means of a differential thermal analysis. An excellent agreement has been obtained between experimental and calculated data. These results show that metamagnetic transition in manganites play an important role in the study of magnetic refrigeration.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Ee Antiferromagnetics
75.50.Dd Nonmetallic ferromagnetic materials

Flux-mediated diffuse mismatch model

G. C. Loh, B. K. Tay, and E. H. T. Teo

Appl. Phys. Lett. 97, 121917 (2010); http://dx.doi.org/10.1063/1.3491210 (3 pages) | Cited 8 times

Online Publication Date: 23 September 2010

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The diffuse mismatch model (DMM) is modified to account for the effect of thermal flux on phonon transmission at interfaces. This new model, the flux-mediated diffuse mismatch model (FMDMM) takes a slightly different approach in its formulation, and does not employ the principle of detailed balance. Two competing processes—an increase in the flux coefficient, and a decrease in the rest of the transmission term, may result in either a rise or fall in thermal boundary resistance when thermal flux is increased. This might partially explain the large disparities between experimental, theoretical, and simulated results of thermal boundary resistance.
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44.20.+b Boundary layer heat flow
05.60.-k Transport processes

Photoluminescence characteristics of polar and nonpolar AlGaN/GaN superlattices

Z. Vashaei, C. Bayram, P. Lavenus, and M. Razeghi

Appl. Phys. Lett. 97, 121918 (2010); http://dx.doi.org/10.1063/1.3493185 (3 pages) | Cited 4 times

Online Publication Date: 23 September 2010

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High quality Al0.2Ga0.8N/GaN superlattices (SLs) with various (GaN) well widths (1.6 to 6.4 nm) have been grown on polar c-plane and nonpolar m-plane freestanding GaN substrates by metal-organic chemical vapor deposition. Atomic force microscopy, high resolution x-ray diffraction, and photoluminescence (PL) studies of SLs have been carried out to determine and correlate effects of well width and polarization field on the room-temperature PL characteristics. A theoretical model was applied to explain PL energy-dependency on well width and crystalline orientation taking into account internal electric field for polar substrate. Absence of induced-internal electric field in nonpolar SLs was confirmed by stable PL peak energy and stronger PL intensity as a function of excitation power density than polar ones.
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81.05.Ea III-V semiconductors
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.55.Cr III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.37.Ps Atomic force microscopy (AFM)
78.20.Jq Electro-optical effects

Negative refraction of zero order flexural Lamb waves through a two-dimensional phononic crystal

J. Pierre, O. Boyko, L. Belliard, J. O. Vasseur, and B. Bonello

Appl. Phys. Lett. 97, 121919 (2010); http://dx.doi.org/10.1063/1.3491290 (3 pages) | Cited 5 times

Online Publication Date: 23 September 2010

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We investigate experimentally the refraction of antisymmetric Lamb waves at frequencies in the second band of a two-dimensional phononic crystal. The heterostructure is made of a square lattice of air inclusions in a silicon plate. Our experimental scheme is based on the selective generation of narrow band elastic waves at a few megahertz. Both positive and negative refractions are observed. The comparison of our data with computed dispersion curves shows very good agreement.
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62.30.+d Mechanical and elastic waves; vibrations
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