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6 Mar 2006

Volume 88, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 88, 103107 (2006); http://dx.doi.org/10.1063/1.2182096 (3 pages)

A. B. Djurišić, Y. H. Leung, K. H. Tam, L. Ding, W. K. Ge, H. Y. Chen, and S. Gwo
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Dielectric tunability of SrTiO3 thin films in the terahertz range

P. Kužel, F. Kadlec, H. Němec, R. Ott, E. Hollmann, and N. Klein

Appl. Phys. Lett. 88, 102901 (2006); http://dx.doi.org/10.1063/1.2183370 (3 pages) | Cited 18 times

Online Publication Date: 6 March 2006

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We have developed an interdigited electrode structure for applying a static electric field to a ferroelectric thin film, which enables nearly full transmission of a perpendicularly polarized terahertz wave. This approach has been used to determine the electric field dependence of the complex permittivity of a SrTiO3 thin film on a sapphire substrate up to about 2 THz employing time-domain terahertz spectroscopy. We have demonstrated up to 10% variation of the film permittivity at 300 GHz at room temperature induced by an applied electric field of 100 kV/cm. No dielectric dispersion is observed between 1 MHz and about 500 GHz. The field-induced changes are attributed to soft mode hardening.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.80.-e Ferroelectricity and antiferroelectricity
77.22.Ch Permittivity (dielectric function)
84.40.-x Radiowave and microwave (including millimeter wave) technology

Magnetocapacitance without magnetoelectric coupling

G. Catalan

Appl. Phys. Lett. 88, 102902 (2006); http://dx.doi.org/10.1063/1.2177543 (3 pages) | Cited 210 times

Online Publication Date: 7 March 2006

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The existence of a magnetodielectric (magnetocapacitance) effect is often used as a test for multiferroic behavior in new material systems. However, strong magnetodielectric effects can also be achieved through a combination of magnetoresistance and the Maxwell-Wagner effect, unrelated to true magnetoelectric coupling. The fact that this resistive magnetocapacitance does not require multiferroic materials may be advantageous for practical applications. Conversely, however, it also implies that magnetocapacitance per se is not sufficient to establish that a material is multiferroic.
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75.80.+q Magnetomechanical effects, magnetostriction
75.47.Gk Colossal magnetoresistance
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation

Final thermally stimulated discharge current in nonpolar materials

Eugen R. Neagu and Aglaia Vassilikou-Dova

Appl. Phys. Lett. 88, 102903 (2006); http://dx.doi.org/10.1063/1.2182014 (3 pages)

Online Publication Date: 8 March 2006

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The final thermally stimulated discharge current technique is used to study the molecular movements in nonpolar dielectrics. The main peak is an apparent peak being very sensitive to the experimental conditions. The experimental conditions can be chosen so that the thermal activation energy W, a characteristic parameter which has to be independent of the experimental conditions, can be determined. Five relaxations have been identified for Teflon FEP (a copolymer of tetrafluoroethylene with hexafluoropropylene) in the temperature range from 30 to 320 K. W is in the range from 0.06 to 0.53 eV.
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77.84.Jd Polymers; organic compounds
77.22.Ej Polarization and depolarization
77.22.Gm Dielectric loss and relaxation

Highly sensitive gas sensor based on integrated titania nanosponge arrays

A. S. Zuruzi, A. Kolmakov, N. C. MacDonald, and M. Moskovits

Appl. Phys. Lett. 88, 102904 (2006); http://dx.doi.org/10.1063/1.2185247 (3 pages) | Cited 33 times

Online Publication Date: 9 March 2006

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Highly sensitive gas sensors were fabricated using nanostructured titania pad arrays. Nanostructured titania (NST) formed is sponge-like consisting of interconnected nanoscale wires and walls, which are made up of anatase nanocrystals. Electrical characterization indicates that contacts were ohmic and NST was highly sensitive to O2. Variations of hundreds of oxygen molecules over a 20 μm NST square pad sensing element were detected at 250 °C. The NST-based sensor operates at lower temperatures, has fast response time, and superior sensitivity relative to oxygen sensors based on porous undoped titania reported in the literature.
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82.80.-d Chemical analysis and related physical methods of analysis
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Microwave dielectric dispersion process in perovskite ferroelectric systems

J. de Los S. Guerra, M. H. Lente, and J. A. Eiras

Appl. Phys. Lett. 88, 102905 (2006); http://dx.doi.org/10.1063/1.2172072 (3 pages) | Cited 12 times

Online Publication Date: 10 March 2006

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The microwave dielectric dispersion process was investigated in relaxor and normal ferroelectric compositions of Pb1−xLaxTiO3 ceramics. The results revealed the following remarkable features: (i) the mechanism responsible for the microwave dielectric dispersion process in perovskite ferroelectric systems may be associated to the vibration of the boundaries of polar regions, independently of their size and dynamical properties; (ii) the characteristic frequency, fR, is governed by the ratio between the force constant and the effective mass of such boundaries; (iii) the temperature dependence of fR above the temperature of the maximum of the dielectric permittivity directly reflects the ferroelectric-type phase transition.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ch Permittivity (dielectric function)
77.80.B- Phase transitions and Curie point

Thermodynamic and electrostatic analysis of threading dislocations in epitaxial ferroelectric films

I. B. Misirlioglu, S. P. Alpay, M. Aindow, and V. Nagarajan

Appl. Phys. Lett. 88, 102906 (2006); http://dx.doi.org/10.1063/1.2178194 (3 pages) | Cited 18 times

Online Publication Date: 10 March 2006

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The role of threading dislocations on the electrical properties of epitaxial ferroelectric films is analyzed using a thermodynamic formalism and basic electrostatics. The modeling is carried out for a 300 nm thick (001) PbZr0.2Ti0.8O3 on (001) SrTiO3 which displays a large population of threading dislocations as determined by transmission electron microscopy. Results show that although the phase transformation characteristics of ferroelectric films containing threading dislocations are altered such that the transformation is “smeared” over a temperature interval due to local strain variations, these defects do not have as profound an effect on the electrical properties as the misfit dislocations.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
73.61.Ng Insulators
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
65.40.G- Other thermodynamical quantities
77.80.B- Phase transitions and Curie point

Characterization of domain walls in BaTiO3 using simultaneous atomic force and piezo response force microscopy

Christian Franck, Guruswami Ravichandran, and Kaushik Bhattacharya

Appl. Phys. Lett. 88, 102907 (2006); http://dx.doi.org/10.1063/1.2185640 (3 pages) | Cited 14 times

Online Publication Date: 10 March 2006

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In this letter a method to simultaneously measure the physical and the polarization thickness of a 90° domain wall in a ferroelectric perovskite is presented. This method combines accurate atomic force microscopy and piezoresponse force microscopy scans of the same area with little drift and an analysis of the entire scanned area. It is found that the physical thickness is significantly narrower (about seven and a half times) than the polarization thickness in a 90° domain wall in BaTiO3. Evidence of the trapping of defects at such domain walls is also found.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization

Polarization fatigue and frequency-dependent recovery in Pb(Zr,Ti)O3 epitaxial thin films with SrRuO3 electrodes

L. Pintilie, I. Vrejoiu, D. Hesse, and M. Alexe

Appl. Phys. Lett. 88, 102908 (2006); http://dx.doi.org/10.1063/1.2186074 (3 pages) | Cited 16 times

Online Publication Date: 10 March 2006

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Fatigue is investigated in epitaxial Pb(Zr,Ti)O3 films grown on SrRuO3/SrTiO3 substrates with Pt or SrRuO3 (SRO) top electrodes. It was experimentally determined that fatigue occurs irrespective of whether the top electrode is Pt or SRO. The fatigue behavior is strongly dependent on the frequency. A polarization recovery was observed for both types of top electrodes, but the recovery is almost complete for a SRO top electrode and only about 40% from the initial polarization value for Pt top electrodes. The results are tentatively explained by the frequency response of the deep traps and by migration of oxygen vacancies.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
81.05.-t Specific materials: fabrication, treatment, testing, and analysis
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
77.22.Ej Polarization and depolarization
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Influence of oxidizing ambient to tetraethylorthosilicate thin films containing solid-state reaction silicon nanocrystals

H. W. Lau and O. K. Tan

Appl. Phys. Lett. 88, 103101 (2006); http://dx.doi.org/10.1063/1.2182071 (3 pages)

Online Publication Date: 7 March 2006

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In this work, we present a systematic study on the oxidation of solid-state reaction silicon nanocrystals (Si nc) embedded in tetraethylorthosilicate (TEOS) thin films. The 12.8 nm Si nc are spun coated in TEOS thin film and hard baked in O2 ambient at 900 °C with varying times. The resulting grain size is investigated using x-ray diffraction and transmission electron microscopy. Si nanoclusters are observed. Upon 900 °C oxidation with varying times, three main regions of grain size transition are identified. With short oxidation time, sintering with oxidation at the rim of the agglomerate dominates; at medium oxidation time, full sintering in the agglomerate with pure oxidation retarded by interfacial stress and strain dominates; at long oxidation time, self-limiting effect prevails. Study has been carried out using a combination of classical sintering model and Kao’s two dimensional oxidation model considering the decrease of reaction rate with increasing stress perpendicular to the Si surface. In our developed model, the critical stress for self-limiting oxidation is found to be 2.9×109 Pa.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
61.46.Hk Nanocrystals
81.65.Mq Oxidation
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation

Arrays of microdots of gold nanoparticles immobilized above gold surface probed by optical second-harmonic microscopy

Kazuma Tsuboi and Kotaro Kajikawa

Appl. Phys. Lett. 88, 103102 (2006); http://dx.doi.org/10.1063/1.2181273 (3 pages) | Cited 3 times

Online Publication Date: 7 March 2006

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Gold nanoparticle arrays are fabricated on a planar gold surface obtained from UV-photopatterned self-assembled monolayer (SAM) films. The arrays are composed of microdots in which spherical gold nanoparticles are immobilized above a gold surface with a gap distance of a few nanometers supported by SAM films used as a spacer. Two kinds of preparation methods were examined: (1) to pattern a spacer SAM film that makes cross-linkage between the gold surface and the gold nanoparticles, and (2) to form ordered microholes in a SAM film, followed by deposition of the spacer SAM film that makes the cross-linkage in the microholes. Both methods provide us with the ordered microdots of gold nanoparticles on a gold surface. The formation of the microdot arrays were probed by optical second-harmonic microscopy.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.16.Dn Self-assembly
81.16.Rf Micro- and nanoscale pattern formation
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Design and control of spin gates in two quantum-dot arrays

Gonzalo Usaj and C. A. Balseiro

Appl. Phys. Lett. 88, 103103 (2006); http://dx.doi.org/10.1063/1.2181275 (3 pages)

Online Publication Date: 7 March 2006

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We study the spin-spin interaction between quantum dots coupled through a two-dimensional electron gas with spin-orbit interaction. We show that the interplay between transverse electron focusing and spin-orbit coupling allows to dynamically change the symmetry of the effective spin-spin Hamiltonian. That is, the interaction can be changed from Ising-like to Heisenberg-like and vice versa. The sign and magnitude of the coupling constant can also be tuned.
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73.21.La Quantum dots
73.63.Kv Quantum dots
75.10.Dg Crystal-field theory and spin Hamiltonians
75.10.Hk Classical spin models
75.30.Et Exchange and superexchange interactions
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
72.25.-b Spin polarized transport

Self-organization of nanoneedles in Fe/GaAs (001) epitaxial thin film

Y. Z. Huang, S. G. Wang, C. Wang, Z. B. Xie, D. J. H. Cockayne, and R. C. C. Ward

Appl. Phys. Lett. 88, 103104 (2006); http://dx.doi.org/10.1063/1.2182017 (3 pages) | Cited 4 times

Online Publication Date: 7 March 2006

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Well-aligned nanoneedles are found to form in a Fe/GaAs (001) epitaxial thin film when irradiated with a Ga+ ion beam normal to the sample surface at an ion dose of 3×1017/cm2 in a focused ion beam. These nanoneedles, with their axes perpendicular to the original sample surface, are composed of GaAs nanorods with Fe nanoislands situated at the top. The GaAs nanorods, formed from the GaAs substrate, are crystalline and in the same crystal orientation as the GaAs substrate. The crystalline nature of the Fe thin film is disrupted by the ion sputtering, becoming nanocrystalline Fe islands. It is the thin Fe layer that is responsible for the formation of the nanoneedles. The ion sputtering of the Fe layer roughens the sample surface due to the uneven sputtering rate of the Fe layer. The removal of the Fe layer exposes the substrate to the incident ions, which then sputter the substrate leading to the formation of the nanoneedles.
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81.07.Bc Nanocrystalline materials
81.16.Dn Self-assembly
61.46.-w Structure of nanoscale materials
81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning

X-ray generation using carbon-nanofiber-based flexible field emitters

T. T. Tan, H. S. Sim, S. P. Lau, H. Y. Yang, M. Tanemura, and J. Tanaka

Appl. Phys. Lett. 88, 103105 (2006); http://dx.doi.org/10.1063/1.2182022 (3 pages) | Cited 23 times

Online Publication Date: 7 March 2006

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Carbon nanofibers were grown on flexible polyimide substrates using an ion-beam sputtering technique. Field emission measurement showed a fairly low threshold voltage of 1.5 V/μm with a current density of 1 μA/cm2. The field enhancement factor was determined to be 4400. The emitter showed resilience when exploited as a high voltage electron source for x-ray generation. The x-ray generated by the flexible emitter is capable of delivering fine images of biological samples with superior sharpness, resolution, and contrast.
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79.70.+q Field emission, ionization, evaporation, and desorption
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors

Synthesis of ordered large-scale ZnO nanopore arrays

G. Q. Ding, W. Z. Shen, M. J. Zheng, and D. H. Fan

Appl. Phys. Lett. 88, 103106 (2006); http://dx.doi.org/10.1063/1.2182025 (3 pages) | Cited 19 times

Online Publication Date: 7 March 2006

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An effective approach is demonstrated for growing ordered large-scale ZnO nanopore arrays through radio-frequency magnetron sputtering deposition on porous alumina membranes (PAMs). The realization of highly ordered hexagonal ZnO nanopore arrays benefits from the unique properties of ZnO (hexagonal structure, polar surfaces, and preferable growth directions) and PAMs (controllable hexagonal nanopores and localized negative charges). Further evidence has been shown through the effects of nanorod size and thermal treatment of PAMs on the yielded morphology of ZnO nanopore arrays. This approach opens the possibility of creating regular semiconducting nanopore arrays for the application of filters, sensors, and templates.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Rm Porous materials; granular materials
81.05.Dz II-VI semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
81.15.Cd Deposition by sputtering
61.43.Gt Powders, porous materials

Green, yellow, and orange defect emission from ZnO nanostructures: Influence of excitation wavelength

A. B. Djurišić, Y. H. Leung, K. H. Tam, L. Ding, W. K. Ge, H. Y. Chen, and S. Gwo

Appl. Phys. Lett. 88, 103107 (2006); http://dx.doi.org/10.1063/1.2182096 (3 pages) | Cited 84 times

Online Publication Date: 7 March 2006

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ZnO commonly exhibits luminescence in the visible spectral range due to different intrinsic defects. In order to study defect emissions, photoluminescence from ZnO nanostructures prepared by different methods (needles, rods, shells) was measured as a function of excitation wavelength and temperature. Under excitation at 325 nm, needles exhibited orange-red defect emission, rods exhibited yellow defect emission, while shells exhibited green defect emission. Obvious color change from orange to green was observed for needles with increasing excitation wavelengths, while nanorods (yellow) showed smaller wavelength shift and shells (green) showed no significant spectral shift. Reasons for different wavelength dependences are discussed.
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78.55.Et II-VI semiconductors
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y2SiO5:Ce nanophosphors

D. W. Cooke, J.-K. Lee, B. L. Bennett, J. R. Groves, L. G. Jacobsohn, E. A. McKigney, R. E. Muenchausen, M. Nastasi, K. E. Sickafus, M. Tang, J. A. Valdez, J.-Y. Kim, and K. S. Hong

Appl. Phys. Lett. 88, 103108 (2006); http://dx.doi.org/10.1063/1.2183737 (3 pages) | Cited 25 times

Online Publication Date: 7 March 2006

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Hydrothermally prepared nanophosphor Y2SiO5:Ce crystallizes in the P21/c structure, rather than the B2/b structure observed in bulk material. Relative to bulk powder, nanophosphors of particle size ∼ 25–100 nm diameter exhibit redshifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and medium-dependent radiative lifetime. Photoluminescence data are consistent with reduced symmetry of the P21/c structure and are not necessarily related to reduced dimensionality of the nanophosphor. In contrast, medium-dependent lifetime and enhanced light output are attributed to nanoscale behavior. Perturbation of the Ce ion electric field is responsible for the variable lifetime.
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78.55.Hx Other solid inorganic materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
61.46.-w Structure of nanoscale materials

Fabrication and characterization of solid-state nanopores using a field emission scanning electron microscope

Hung Chang, Samir M. Iqbal, Eric A. Stach, Alexander H. King, Nestor J. Zaluzec, and Rashid Bashir

Appl. Phys. Lett. 88, 103109 (2006); http://dx.doi.org/10.1063/1.2179131 (3 pages) | Cited 26 times

Online Publication Date: 8 March 2006

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The fabrication of solid-state nanopores using the electron beam of a transmission electron microscope (TEM) has been reported in the past. Here, we report a similar method to fabricate solid-state nanopores using the electron source of a conventional field-emission scanning electron microscope (FESEM) instead. Micromachining was used to create initial pore diameters between 50 nm and 200 nm, and controlled pore shrinking to sub 10 nm diameters was performed subsequently during in situ processing in the FESEM. Noticeably, different shrinking behavior was observed when using irradiation from the electron source of the FESEM than the TEM. Unlike previous reports of TEM mediated pore shrinkage, the mechanism of pore shrinkage when using the FESEM could be a result of surface defects generated by radiolysis and subsequent motion of silicon atoms to the pore periphery.
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81.05.Rm Porous materials; granular materials
81.16.-c Methods of micro- and nanofabrication and processing
61.43.Gt Powders, porous materials
61.46.-w Structure of nanoscale materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
61.82.-d Radiation effects on specific materials

Study of porous glass doped with quantum dots or laser dyes under alpha irradiation

S. E. Létant and T.-F. Wang

Appl. Phys. Lett. 88, 103110 (2006); http://dx.doi.org/10.1063/1.2182072 (3 pages) | Cited 6 times

Online Publication Date: 8 March 2006

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We demonstrate that nanocomposite materials based on semiconductor quantum dots have potential for radiation detection via scintillation. While quantum dots and laser dyes both emit in the visible range at room temperature, the Stokes shift of the dyes is significantly larger. The scintillation output of both systems was studied under alpha irradiation and interpreted using a combination of energy loss and photon transport Monte Carlo simulation models. The comparison of the two systems, which allows the quantification of the role played by the Stokes shift in the scintillation output, opens up exciting possibilities for a new class of scintillators that would take advantage of the limitless assembly of nanocrystals in large, transparent, and sturdy matrices.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.55.-m Photoluminescence, properties and materials
61.43.Gt Powders, porous materials
61.80.Jh Ion radiation effects

Light emission from Si nanoclusters formed at low temperatures

X. D. Pi, O. H. Y. Zalloum, T. Roschuk, J. Wojcik, A. P. Knights, P. Mascher, and P. J. Simpson

Appl. Phys. Lett. 88, 103111 (2006); http://dx.doi.org/10.1063/1.2183813 (3 pages) | Cited 8 times

Online Publication Date: 8 March 2006

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Photoluminescence (PL) from amorphous Si nanoclusters (Si-ncls) formed by thin-film deposition via electron-cyclotron resonance plasma-enhanced chemical vapor deposition followed by annealing at temperatures ⩽ 875 °C has been investigated. We find that Si-ncls grow very slowly after their initial nucleation at low temperatures. An increase in the size of Si-ncls, which can be controlled by the annealing temperature, induces a redshift in the Si-ncl PL peak. While the emitted optical power is more than 100 times smaller than that of Si nanocrystals formed in an identically deposited film, it is increased by a factor of up to approximately four times following hydrogen passivation. The incorporation of hydrogen causes a redshift in the PL peak position, suggesting a partial hydrogenation induced bond distortion of the Si-ncls. This redshift decreases with increasing hydrogen ambient annealing temperature.
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81.05.Cy Elemental semiconductors
81.05.Gc Amorphous semiconductors
78.55.Ap Elemental semiconductors
78.55.Qr Amorphous materials; glasses and other disordered solids
61.43.Dq Amorphous semiconductors, metals, and alloys
52.77.Dq Plasma-based ion implantation and deposition
61.46.-w Structure of nanoscale materials

Microcompression of nanocrystalline nickel

B. E. Schuster, Q. Wei, H. Zhang, and K. T. Ramesh

Appl. Phys. Lett. 88, 103112 (2006); http://dx.doi.org/10.1063/1.2183814 (3 pages) | Cited 17 times

Online Publication Date: 8 March 2006

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Microcompression is a technique that was developed as a means to probe the properties of micrometer-sized specimens using a modification of a conventional nanoindentation system. We use this technique to present the first uniaxial compressive data on electrodeposited nanocrystalline nickel (a material system where the grain size is much smaller than the specimen size). The compression-tension asymmetry of this nanocrystalline material is also discussed.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
62.25.-g Mechanical properties of nanoscale systems
61.46.-w Structure of nanoscale materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials

Effect of oxide overlayer formation on the growth of gold catalyzed epitaxial silicon nanowires

Hemanth Jagannathan, Yoshio Nishi, Mark Reuter, Matthew Copel, Emanuel Tutuc, Supratik Guha, and Rafael P. Pezzi

Appl. Phys. Lett. 88, 103113 (2006); http://dx.doi.org/10.1063/1.2179370 (3 pages) | Cited 8 times

Online Publication Date: 8 March 2006

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A direct dependence between the inadvertent formation of SiO2 on gold films deposited on silicon 〈111〉 substrates, and the nucleation and yield of epitaxial, gold catalyzed, silicon nanowires grown on such substrates is reported. The unintended SiO2 layer formed due to the diffusion of silicon from the underlying substrate through the gold film is observed to be 0.5 nm with medium energy ion scattering after brief exposures of 10–15 min in air. Silicon nanowires grown at 500 °C on such samples show reduced nucleation and growth. A remarkable improvement in nanowire nucleation density and epitaxy is observed on removing the SiO2 overlayer prior nanowire growth.
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81.07.Bc Nanocrystalline materials
81.05.Cy Elemental semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
68.35.Fx Diffusion; interface formation

Quantum size effects manifest in infrared spectra of single bismuth nanowires

T. W. Cornelius, M. E. Toimil-Molares, R. Neumann, G. Fahsold, R. Lovrincic, A. Pucci, and S. Karim

Appl. Phys. Lett. 88, 103114 (2006); http://dx.doi.org/10.1063/1.2183823 (3 pages) | Cited 19 times

Online Publication Date: 9 March 2006

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Infrared transmission spectroscopy measurements on single bismuth nanowires of various diameters d are presented. The spectra show a strong absorption whose onset is blueshifted proportionally to 1/d2. We ascribe the absorption to interband transitions. The blueshift results from quantum size effects since they lead to the d-dependent splitting of the energy bands and to a respective shift of energy gaps.
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73.63.Nm Quantum wires
72.30.+q High-frequency effects; plasma effects
78.30.Er Solid metals and alloys
78.66.Bz Metals and metallic alloys

Self-organized growth of Mn nanocluster arrays on Si(111)-(7×7) surfaces

Hui Wang and Zhi-Qiang Zou

Appl. Phys. Lett. 88, 103115 (2006); http://dx.doi.org/10.1063/1.2184809 (3 pages) | Cited 12 times

Online Publication Date: 9 March 2006

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We demonstrate the formation of well-ordered arrays of Mn nanoclusters of nearly identical sizes on a Si(111)-(7×7) surface. The Mn atoms are found not to react with the Si substrate at temperatures below ∼ 260 °C. At room temperature, the Mn clusters occupy both halves of the (7×7) unit cell and arrange themselves into a honeycomb structure, whereas at a temperature of 258 °C, they exclusively occupy the faulted half of the unit cell and form an array with hexagonal symmetry. The self-organization process of the nanocluster array of Mn is different from that previously reported for the group-IIIA metals, due to stronger Mn–substrate and Mn–Mn interactions.
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81.16.Dn Self-assembly
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
68.47.Fg Semiconductor surfaces

Surface nanostructuring and optical activation of lithium fluoride crystals by ion beam irradiation

V. Mussi, F. Granone, C. Boragno, F. Buatier de Mongeot, U. Valbusa, T. Marolo, and R. M. Montereali

Appl. Phys. Lett. 88, 103116 (2006); http://dx.doi.org/10.1063/1.2177660 (3 pages) | Cited 7 times

Online Publication Date: 9 March 2006

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We present results on simultaneous nanostructuring and optical activation of lithium fluoride crystals by 800 eV off-normal Ar+ sputtering at different ion doses. The samples were studied by atomic force microscopy and optical spectroscopy. After ion irradiation smoothening of the initial random roughness is achieved and well-defined self-organized ripple structures appear, having a mean periodicity of 30 nm and a mean height of 3 nm. The simultaneous optical activation of the irradiated samples is due to the stable formation of electronic defects with intense photoluminescence in the visible spectral range.
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81.07.Bc Nanocrystalline materials
81.65.-b Surface treatments
81.16.-c Methods of micro- and nanofabrication and processing
61.80.Jh Ion radiation effects
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Monolithic, high surface area, three-dimensional GeO2 nanostructures

S. O. Kucheyev, T. F. Baumann, Y. M. Wang, T. van Buuren, J. F. Poco, J. H. Satcher, and A. V. Hamza

Appl. Phys. Lett. 88, 103117 (2006); http://dx.doi.org/10.1063/1.2182064 (3 pages) | Cited 3 times

Online Publication Date: 10 March 2006

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We report a sol-gel synthesis of monolithic three-dimensional GeO2 nanostructures (aerogels). Transmission electron microscopy shows that these ultralow-density ( ∼ 99.2% porous) nanostructures are formed by a continuous network of elongated amorphous GeO2 nanoligaments, ∼ 15–20 nm in diameter and ∼ 50–200 nm in length. As a result, monoliths exhibit a high surface area of ∼ 60 m2 g−1. A comparison of soft x-ray absorption spectra at Ge L3 and O K edges with theoretical densities of states reveals that the electronic structure of aerogels is similar to that of hexagonal GeO2, indicating a (distorted) tetrahedral environment of Ge atoms in the amorphous nanostructure.
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61.46.-w Structure of nanoscale materials
61.43.Er Other amorphous solids
78.70.Dm X-ray absorption spectra
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