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21 Sep 2009

Volume 95, Issue 12, Articles (12xxxx)

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Appl. Phys. Lett. 95, 121104 (2009); http://dx.doi.org/10.1063/1.3231448 (3 pages)

E. H. Khoo, I. Ahmed, and E. P. Li
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Selecting a single orientation for millimeter sized graphene sheets

R. van Gastel, A. T. N’Diaye, D. Wall, J. Coraux, C. Busse, N. M. Buckanie, F.-J. Meyer zu Heringdorf, M. Horn von Hoegen, T. Michely, and B. Poelsema

Appl. Phys. Lett. 95, 121901 (2009); http://dx.doi.org/10.1063/1.3225554 (3 pages) | Cited 15 times

Online Publication Date: 21 September 2009

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We have used low energy electron microscopy and photo emission electron microscopy to study and improve the quality of graphene films grown on Ir(111) using chemical vapor deposition (CVD). CVD at elevated temperature already yields graphene sheets that are uniform and of monatomic thickness. Besides domains that are aligned with respect to the substrate, other rotational variants grow. Cyclic growth exploiting the faster growth and etch rates of the rotational variants, yields films that are 99% composed of aligned domains. Precovering the substrate with a high density of graphene nuclei prior to CVD yields pure films of aligned domains extending over millimeters. Such films can be used to prepare cluster-graphene hybrid materials for catalysis or nanomagnetism and can potentially be combined with lift-off techniques to yield high-quality, graphene based, electronic devices.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.65.Cf Surface cleaning, etching, patterning
68.55.A- Nucleation and growth
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
81.05.ub Fullerenes and related materials

Stress influence on band-edge luminescence properties of 4H-AlN

Y. C. Cheng, X. L. Wu, S. H. Li, and Paul K. Chu

Appl. Phys. Lett. 95, 121902 (2009); http://dx.doi.org/10.1063/1.3232218 (3 pages)

Online Publication Date: 21 September 2009

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The band-edge luminescence properties of 4H-AlN under biaxial and uniaxial stress are studied using the first-principle method. In equilibrium, excitons B and C have energies 92 and 14 meV lower than exciton A, respectively. When the uniaxial tensile strain exceeds ∼ 0.95%, the top valence band is the B exciton state. Its band-edge emission is no longer prohibited and changes to E⊥c. The shift in the A exciton energy is similar to that of 2H-AlN under stress of −5 to 5 GPa. When the compressive biaxial strain is larger than 5 GPa, the band structure of 4H-AlN becomes indirect.
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78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors
71.35.-y Excitons and related phenomena
71.20.Nr Semiconductor compounds
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
71.15.-m Methods of electronic structure calculations
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Time-resolved photoluminescence analysis of two-peak emission behavior in Sr2Si5N8:Eu2+

Kee-Sun Sohn, Sangjun Lee, Rong-Jun Xie, and Naoto Hirosaki

Appl. Phys. Lett. 95, 121903 (2009); http://dx.doi.org/10.1063/1.3233972 (3 pages) | Cited 13 times

Online Publication Date: 21 September 2009

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Sr2Si5N8:Eu2+, one of the most recently developed phosphors for use in white light emitting diodes, exhibits a two-peak emission. Namely, the emission band of Sr2Si5N8:Eu2+ is deconvoluted into two Gaussian peaks irrespective of the Eu2+ concentration. We examined the two-peak emission of Sr2Si5N8:Eu2+ by analyzing the time-resolved photoluminescence spectra. We revealed that the two-peak emission was closely associated with the energy transfer taking place between Eu2+ activators located at two different crystallographic sites in the Sr2Si5N8 structure. The experimental results coincided well with the rate equation model involving the crystallographic information of the host.
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78.55.Hx Other solid inorganic materials
61.72.up Other materials
78.47.D- Time resolved spectroscopy (>1 psec)
78.47.jd Time resolved luminescence
85.60.Jb Light-emitting devices

Lightweight sodium alanate thin films grown by reactive sputtering

M. Filippi, J. H. Rector, R. Gremaud, M. J. van Setten, and B. Dam

Appl. Phys. Lett. 95, 121904 (2009); http://dx.doi.org/10.1063/1.3236525 (3 pages) | Cited 3 times

Online Publication Date: 21 September 2009

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We report the preparation of sodium alanate, a promising hydrogen storage material, in a thin film form using cosputtering in a reactive atmosphere of atomic hydrogen. We study the phase formation and distribution, and the hydrogen desorption, with a combination of optical and infrared transmission spectroscopy. We show that the hydrogen desorption, the phase segregation, and the role of the dopants in these complex metal hydrides can be monitored with optical measurements. This result shows that a thin film approach can be used for a model study of technologically relevant lightweight metal hydrides.
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81.15.Cd Deposition by sputtering
68.55.aj Insulators
84.60.-h Direct energy conversion and storage
68.43.Nr Desorption kinetics
81.05.-t Specific materials: fabrication, treatment, testing, and analysis
64.75.St Phase separation and segregation in thin films
78.66.Nk Insulators
78.30.Hv Other nonmetallic inorganics

The strong influence of heat losses on the accurate measurement of thermal diffusivity using lock-in thermography

Agustín Salazar, Arantza Mendioroz, and Raquel Fuente

Appl. Phys. Lett. 95, 121905 (2009); http://dx.doi.org/10.1063/1.3236782 (3 pages) | Cited 3 times

Online Publication Date: 23 September 2009

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In modulated photothermal experiments the lateral thermal diffusivity can be obtained from the slope of the linear relation between the phase of the surface temperature and the distance to the heating spot. However, this slope is greatly affected by heat losses so that the measured thermal diffusivity is overestimated, especially for thin samples of poor thermal conducting materials. In this paper we definitely identify the physical mechanism responsible for the overestimation of the diffusivity as heat conduction to the surrounding gas. Accurate measurements of the thermal diffusivity using the “slope method” have been obtained by keeping the sample in vacuum.
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07.20.-n Thermal instruments and apparatus
07.60.Dq Photometers, radiometers, and colorimeters
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

Study of the free energy of the La1−xCaxMnO3 manganites based on the temperature dependence of the crystal cell volume

G. D. Tang, S. P. Liu, X. Zhao, Y. G. Zhang, D. H. Ji, Y. F. Li, W. H. Qi, W. Chen, and D. L. Hou

Appl. Phys. Lett. 95, 121906 (2009); http://dx.doi.org/10.1063/1.3233934 (3 pages) | Cited 4 times

Online Publication Date: 23 September 2009

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A study of the free energy of the perovskite manganites was performed by fitting experimental results for the temperature dependence of the crystal cell volume using a detailed semiclassical model of the free energy. The fitted results coincide with the experimental results for the samples La1−xCaxMnO3 (x = 0.2, 0.25, 0.3, and 0.5). The free energy includes the energy corresponding to thermal vibration of the crystal lattice, the ionic cohesive energy, and additional energies produced by electrical carriers from the double exchange mechanism and the thermal activation of small polarons.
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63.20.-e Phonons in crystal lattices
65.40.G- Other thermodynamical quantities
75.50.Dd Nonmetallic ferromagnetic materials
72.80.Sk Insulators
72.20.Fr Low-field transport and mobility; piezoresistance
61.50.Lt Crystal binding; cohesive energy

In-depth resolved Raman scattering analysis of secondary phases in Cu-poor CuInSe2 based thin films

X. Fontané, V. Izquierdo-Roca, L. Calvo-Barrio, J. Álvarez-Garcia, A. Pérez-Rodríguez, J. R. Morante, and W. Witte

Appl. Phys. Lett. 95, 121907 (2009); http://dx.doi.org/10.1063/1.3236770 (3 pages) | Cited 8 times

Online Publication Date: 24 September 2009

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Raman scattering analysis of Cu-poor CuInSe2 layers shows the coexistence of the ordered vacancy compound (OVC), CuAu–CuInSe2 and chalcopyrite (CH) CuInSe2 phases as function of the Cu/In content ratio x. In-depth resolved measurements from layers with x ≤ 0.57 show a strong inhibition in the relative intensity of the CH-CuInSe2 mode at the back region. Micro-Raman spectra directly measured at different regions from the layers with 0.66 ≤ x ≤ 0.71 also suggest a higher content of the OVC phase at this back region. These data suggest an enhancement in the formation of OVC at this region in the layers.
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78.30.Hv Other nonmetallic inorganics
78.66.Li Other semiconductors
81.05.Hd Other semiconductors

Orientation control and self-assembled nanopyramid structure of LaFeO3 films epitaxially grown on SrTiO3(001) substrates

Lei Bi, Hyun-Suk Kim, Gerald F. Dionne, C. A. Ross, Hanjong Paik, and Yun Chang Park

Appl. Phys. Lett. 95, 121908 (2009); http://dx.doi.org/10.1063/1.3222912 (3 pages) | Cited 2 times

Online Publication Date: 24 September 2009

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Epitaxial films of LaFeO3 (LFO) were grown on SrTiO3(001) (STO) substrates using pulsed laser deposition. Two epitaxial orientations were observed: α where LFO(110)∥STO(100) and β where LFO(001)∥STO(100). By controlling the deposition conditions, LaFeO3 films could be grown with just the α orientation or with simultaneous α and β orientations in which the film consisted of a self-assembled array of nanoscale β pyramids embedded in a matrix of α. The microstructure and growth mechanism of the films and their exchange-bias with a Co overlayer are discussed.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Cc Other ferromagnetic metals and alloys
75.50.Ee Antiferromagnetics
81.15.Fg Pulsed laser ablation deposition
81.16.Dn Self-assembly

A superlens for the deep ultraviolet

Alina Schilling, Joerg Schilling, Carsten Reinhardt, and Boris Chichkov

Appl. Phys. Lett. 95, 121909 (2009); http://dx.doi.org/10.1063/1.3226101 (3 pages) | Cited 5 times

Online Publication Date: 24 September 2009

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A superlens based on a single aluminum layer operating at deep ultraviolet (UV) wavelengths is investigated both, theoretically and experimentally. Using a wavelength of 157 nm double slits with a center-to-center separation of only 70 nm were experimentally resolved and simulations indicate a possible resolution down to 29 nm with experimentally feasible arrangements. The results demonstrate the significance of aluminum as plasmonic material for the deep UV.
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68.55.-a Thin film structure and morphology
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
42.79.Bh Lenses, prisms and mirrors
42.70.-a Optical materials

Nitridating r-plane sapphire to improve crystal qualities and surface morphologies of a-plane GaN grown by metalorganic vapor phase epitaxy

Bei Ma, Weiguo Hu, Hideto Miyake, and Kazumasa Hiramatsu

Appl. Phys. Lett. 95, 121910 (2009); http://dx.doi.org/10.1063/1.3237164 (3 pages) | Cited 5 times

Online Publication Date: 25 September 2009

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Effects of the nitridation of the r-plane sapphire were investigated on the growth of a-plane GaN. Surface morphology and crystal quality were very sensitive to the nitridated time. A high quality a-plane GaN with a pit free-surface was obtained with nitridation at 1100 °C for 5 min, compared with under- or overnitridation. Nitridated layer were identified as AlN grains with 〈11math0〉 preferred orientation, which acted as the nuclearation layers for a-plane GaN growth. Moreover, the qualities improvements were attributed to enhancing grain uniformity and size with 5 min nitridation.
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81.10.Bk Growth from vapor
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.47.Fg Semiconductor surfaces
68.35.bg Semiconductors
61.66.Fn Inorganic compounds

Structural relaxation and crystallization of NiTi thin film metallic glasses

Xu Huang and A. G. Ramirez

Appl. Phys. Lett. 95, 121911 (2009); http://dx.doi.org/10.1063/1.3236544 (3 pages) | Cited 6 times

Online Publication Date: 25 September 2009

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This letter demonstrates the effects of structural relaxation on the crystallization and phase transformation behavior of NiTi thin films. Heat treatments below the glass transformation temperature produce films with a greater hardness than as-deposited films. The reduction in free volume occurring during film relaxation plays a role. Using scanning electron microscopy, structural relaxation was found to decrease the overall crystallization time and increase the nucleation rate, thus modifying the resulting microstructures. Structural relaxation had little effect on the phase transformation temperatures of fully crystallized films, but slightly increased the resulting actuation force during transformation.
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61.43.Fs Glasses
68.55.-a Thin film structure and morphology
62.20.Qp Friction, tribology, and hardness
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
64.70.kj Glasses
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
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