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13 Sep 2004

Volume 85, Issue 11, pp. 1871-2145

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 1895 (2004); http://dx.doi.org/10.1063/1.1792802 (3 pages)

Markus Deubel, Martin Wegener, Artan Kaso, and Sajeev John
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Electrochemical growth of Co nanodots on patterned Si substrates

M. V. Rastei, R. Meckenstock, J. P. Bucher, E. Devaux, and Th. Ebbesen

Appl. Phys. Lett. 85, 2050 (2004); http://dx.doi.org/10.1063/1.1787597 (3 pages) | Cited 9 times

Online Publication Date: 17 September 2004

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A silicon substrate, prestructured by a focused ion beam, is used as a nano-electrode template to perform a selective electrodeposition of cobalt nanodots. It is found that the shape of the magnetic nanostructures determines to a large extent their magnetic properties. Using magnetic force microscopy, a transition from in-plane to out-of-plane single domain magnetization is observed when the diameter of the 400 nm height dots is reduced from 400 to 200 nm. Additional ferromagnetic resonance measurements confirm that the magnetic anisotropy is imposed mainly by the shape of the cylindrical dots.
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81.15.Pq Electrodeposition, electroplating
82.45.Aa Electrochemical synthesis
82.45.Fk Electrodes
82.45.Qr Electrodeposition and electrodissolution
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
75.30.Gw Magnetic anisotropy
75.50.Cc Other ferromagnetic metals and alloys

Probing ion transport at the nanoscale: Time-domain electrostatic force spectroscopy on glassy electrolytes

A. Schirmeisen, A. Taskiran, H. Fuchs, B. Roling, S. Murugavel, H. Bracht, and F. Natrup

Appl. Phys. Lett. 85, 2053 (2004); http://dx.doi.org/10.1063/1.1790034 (3 pages) | Cited 6 times

Online Publication Date: 17 September 2004

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We have carried out time-domain electrostatic force spectroscopy on two different ion-conducting glasses using an atomic force microscope. We compare the electrostatic force spectroscopic data obtained at different temperatures with macroscopic electrical data of the glasses. The overall consistency of the data shows that electrostatic force spectroscopy is capable of probing the ion dynamics and transport in nanoscopic subvolumes of the samples.
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73.63.Bd Nanocrystalline materials
61.43.Fs Glasses
61.46.-w Structure of nanoscale materials
66.30.H- Self-diffusion and ionic conduction in nonmetals
82.45.Gj Electrolytes
68.37.Ps Atomic force microscopy (AFM)

Strain-sensitive size modulations in ZnSe∕ZnS quantum dots grown on GaAs substrates

Y. G. Kim, Y. S. Joh, J. H. Song, E. D. Sim, K. S. Baek, S. K. Chang, and J. I. Lee

Appl. Phys. Lett. 85, 2056 (2004); http://dx.doi.org/10.1063/1.1790035 (3 pages) | Cited 3 times

Online Publication Date: 17 September 2004

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Strain effects on sizes and emission energies of ZnSe∕ZnS quantum dots (QDs) have been investigated. The initial strain in the ZnSe QD layer was altered by adjusting the thickness of the ZnS buffer. Consistent blueshifts of the ground-state emission from 4 monolayer ZnSe QDs were observed with increasing the thickness of the ZnS buffer from 20 to 40 nm. Atomic-force microscopy revealed that the blueshifts are due to a continuous QD size reduction. We estimated the emission energy as a function of the initial strain in the ZnSe QD layer, which shows that the band-gap engineering is possible through the strain modification.
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81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.67.Hc Quantum dots
78.55.Et II-VI semiconductors
68.37.Ps Atomic force microscopy (AFM)

Evidence of linear lattice expansion and covalency enhancement in rutile TiO2 nanocrystals

Guangshe Li, Juliana Boerio-Goates, Brian F. Woodfield, and Liping Li

Appl. Phys. Lett. 85, 2059 (2004); http://dx.doi.org/10.1063/1.1790596 (3 pages) | Cited 54 times

Online Publication Date: 17 September 2004

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Lattice variations and bonding characteristics in rutile TiO2 nanocrystals were examined by x-ray diffraction and x-ray photoelectron spectroscopy. With a reduction in the physical dimensions, rutile TiO2 nanocrystals show a linear lattice expansion and an anomalous covalency enhancement in apparent contradiction to the ionicity increase in BaTiO3 and CuO nanocrystals as reported recently by S. Tsunekawa et al. [Phys. Rev. Lett. 2000, 85, 3440] and V. R. Palkar et al. [Phys. Rev. B 1996, 53, 2167]. A surface defect dipole model is proposed to explain these physical phenomena in terms of the strong interactions among the surface dipoles that produce an increased negative pressure. The covalency enhancement is interpreted according to the critical properties of the increased Ti�O bond lengths in the expanded lattice.
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81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
61.50.Lt Crystal binding; cohesive energy
79.60.Jv Interfaces; heterostructures; nanostructures

Dependence of oxide thickness on O2 pressure and experimental determination of Gibbs free energies of ultrathin oxide films in vacuum

L. Bouzidi and A. J. Slavin

Appl. Phys. Lett. 85, 2062 (2004); http://dx.doi.org/10.1063/1.1785862 (3 pages)

Online Publication Date: 17 September 2004

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Ultrathin oxide films grown in a vacuum are important in many industrial processes, with the values of Gibbs free energies of the gas and films determining the oxide type that grows on a surface, and its thickness. A high-stability quartz-crystal microbalance is used to provide quantitative experimental data on these free energies, for the model case of oxidation of a lead film on a gold substrate. The surface oxide, PbO, forms a single molecular layer at 10−6 Torr of O2, but thickens abruptly to two layers at 1×10−4 Torr, and becomes even thicker by 0.33 Torr. This is explained using film free energies.
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65.40.G- Other thermodynamical quantities
81.65.Mq Oxidation
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.60.Dv Thermal stability; thermal effects
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.55.A- Nucleation and growth

Thermionic emission from defective carbon nanotubes

D. C. Cox, R. D. Forrest, P. R. Smith, and S. R. P. Silva

Appl. Phys. Lett. 85, 2065 (2004); http://dx.doi.org/10.1063/1.1790597 (3 pages) | Cited 9 times

Online Publication Date: 17 September 2004

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Using a nanomanipulation system contained within a scanning electron microscope we investigate the thermionic electron emission from multiwall carbon nanotubes. Peak emission currents of 65 nA are measured. The carbon nanotubes being grown at low temperature by the chemical vapor deposition method are defective with poor thermal conductivity. We believe it is crucial for the thermal conductivity to be poor in order to obtain significant thermionic emission from the carbon nanotubes. This allows for the carbon nanotube during electron emission to be at high temperatures, and thus give higher emission efficiencies. At the highest emission current levels we estimate the temperature of the nanotubes to be approximately 2900 K.
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81.07.De Nanotubes
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
79.40.+z Thermionic emission
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Light-induced instability of PbO-filled single-wall carbon nanotubes

Martin Hulman, Hans Kuzmany, Pedro M. F. J. Costa, Steffi Friedrichs, and Malcolm L. H. Green

Appl. Phys. Lett. 85, 2068 (2004); http://dx.doi.org/10.1063/1.1790603 (3 pages) | Cited 11 times

Online Publication Date: 17 September 2004

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We investigated single-wall carbon nanotubes filled with lead oxide, PbO, by transmission electron microscopy and Raman spectroscopy. It is concluded that PbO crystallizes in the orthorombic phase forming nanowires inside the nanotubes. The positions of the PbO Raman lines are downshifted as compared to the bulk material as a result of the reduced dimensionality. As a consequence of the filling, nanotubes become sensitive to the laser irradiation. At higher laser power densities, they oxidize and the free PbO nanowires are left in the sample.
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61.82.Rx Nanocrystalline materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
61.46.-w Structure of nanoscale materials
78.67.Ch Nanotubes
78.30.Na Fullerenes and related materials

Size- and shape-controlled GaN nanocrystals grown on Si(111) substrate by reactive epitaxy

Chung-Lin Wu, Li-Jen Chou, and Shangjr Gwo

Appl. Phys. Lett. 85, 2071 (2004); http://dx.doi.org/10.1063/1.1787947 (3 pages) | Cited 7 times

Online Publication Date: 17 September 2004

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We report the investigation of coherent GaN nanocrystals spontaneously formed by nitrogen-plasma-assisted reactive epitaxy with Ga droplets supported on the single crystalline Si3N4(0001)∕Si(111) surface. The distribution of grown GaN nanocrystals, as revealed by scanning electron microscopy and cross-sectional transmission electron microscopy, is very uniform in size (∼16 nm) and shape and the distribution width is significantly narrower than that of Ga droplets deposited in the Volmer–Weber mode. By using high-resolution electron microscopy, the shape and crystalline structure of the self-assembled GaN nanocrystals can be determined to be truncated triangular pyramids formed by the facets of the GaN wurtzite lattice.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
61.66.Fn Inorganic compounds
61.46.-w Structure of nanoscale materials
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)
52.77.Dq Plasma-based ion implantation and deposition

Anomalous change in electron density at nuclear sites in nanosize zinc ferrite

C. Upadhyay and H. C. Verma

Appl. Phys. Lett. 85, 2074 (2004); http://dx.doi.org/10.1063/1.1786368 (3 pages) | Cited 7 times

Online Publication Date: 17 September 2004

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Nanosize samples of zinc ferrite with different particle sizes synthesized by citrate precursor route have been studied by x-ray diffraction, transmission electron microscopy and Mössbauer spectroscopy. The observations suggest that the electronic processes depend sensitively on size, and a big variation occurs in the pattern of temperature dependence of isomer shift on changing the size from 5 nm to 6 nm and from 6 nm to 7 nm. The electron density at the nuclear sites first increases, then decreases and again increases as the temperature of the nanosize samples is increased from 12 K to 400 K. The electric-field gradient varies considerably as the size is changed, but remains almost independent of temperature for a particular size. It is concluded that different aspects of hyperfine interactions in nanophase become significant at different sizes.
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75.50.Gg Ferrimagnetics
81.07.Wx Nanopowders
61.46.-w Structure of nanoscale materials
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
71.70.Jp Nuclear states and interactions
76.80.+y Mössbauer effect; other γ-ray spectroscopy

Defect-free InP nanowires grown in [001] direction on InP (001)

U. Krishnamachari, M. Borgstrom, B. J. Ohlsson, N. Panev, L. Samuelson, W. Seifert, M. W. Larsson, and L. R. Wallenberg

Appl. Phys. Lett. 85, 2077 (2004); http://dx.doi.org/10.1063/1.1784548 (3 pages) | Cited 64 times

Online Publication Date: 17 September 2004

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We report on [001] InP nanowires grown by metalorganic vapor phase epitaxy directly on (001) InP substrates. Characterization by scanning electron microscopy and transmission electron microscopy reveals wires with nearly square cross sections and a perfect zinc-blende crystalline structure that is free of stacking faults. Photoluminescence measurements of single [001] nanowires exhibit a narrow and intense emission peak at approximately 1.4 eV, whereas 〈111〉B grown reference wires show additional broad luminescence peaks at lower energy. The origin of this uncommon wire growth direction [001] is discussed as a means of controlled formation of [00l]-oriented nanowires on (001) substrates.
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81.07.Bc Nanocrystalline materials
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.46.-w Structure of nanoscale materials
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Lp Transmission electron microscopy (TEM)
61.72.Nn Stacking faults and other planar or extended defects

Porous nanotubes of Co3O4: Synthesis, characterization, and magnetic properties

R. M. Wang, C. M. Liu, H. Z. Zhang, C. P. Chen, L. Guo, H. B. Xu, and S. H. Yang

Appl. Phys. Lett. 85, 2080 (2004); http://dx.doi.org/10.1063/1.1789577 (3 pages) | Cited 48 times

Online Publication Date: 17 September 2004

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Stoichiometric Co3O4 porous nanotubes have been synthesized through a simple modified microemulsion method. The structural and the chemical information of the as-grown nanotubes have been investigated by means of x-ray diffraction, electron microscopy, electron energy loss spectroscopy, and dynamic force microscopy. The results reveal that the as-grown materials are formed by concentric stacking of Co3O4 (111) planes or weaved porous nanotubes with diameters ranging from tens to ∼200 nm and sidewall thickness ranging from 2 to ∼20 nm. Magnetic property of the sample demonstrates a magnetic transition temperature at 8.4 K, indicating macroscopic quantum confinement effects from the sidewall thickness of the porous nanotube.
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81.07.De Nanotubes
81.05.Rm Porous materials; granular materials
75.50.Ee Antiferromagnetics
82.70.Kj Emulsions and suspensions
61.46.-w Structure of nanoscale materials
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
79.20.Uv Electron energy loss spectroscopy
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Exciton spin relaxation dynamics in InGaAs∕InP quantum wells

Shunsuke Akasaka, Shogo Miyata, Takamasa Kuroda, and Atsushi Tackeuchi

Appl. Phys. Lett. 85, 2083 (2004); http://dx.doi.org/10.1063/1.1792376 (3 pages) | Cited 10 times

Online Publication Date: 17 September 2004

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We have investigated the exciton spin relaxation mechanism between 13 and 300 K in InGaAs∕InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The exciton spin relaxation time, τs above 40 K was found to depend on temperature, T, according to τs∝T−1.1, although the spin relaxation time is constant below 40 K. The clear carrier density dependence of the exciton spin relaxation time was observed below 40 K, although the carrier density dependence is weak above 40 K. These results imply that the main spin relaxation mechanism above and below 40 K are the D’yakonov–Perel’ process and the Bir–Aronov–Pikus process, respectively.
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73.21.Fg Quantum wells
78.67.De Quantum wells
78.47.-p Spectroscopy of solid state dynamics
71.35.Cc Intrinsic properties of excitons; optical absorption spectra

Comparison of thermal and piezoresistive sensing approaches for atomic force microscopy topography measurements

William P. King, Thomas W. Kenny, and Kenneth E. Goodson

Appl. Phys. Lett. 85, 2086 (2004); http://dx.doi.org/10.1063/1.1787160 (3 pages) | Cited 29 times

Online Publication Date: 17 September 2004

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Atomic force microscope cantilevers with integrated piezoresistive displacement sensors are widely used for nanometer-scale topographic measurement and force sensing. Heated cantilevers used in thermomechanical data storage are a promising alternative for topographic measurement. For both cantilever types, this letter models and predicts cantilever displacement sensitivity and noise-limited displacement resolution. The predictions for the thermal cantilever sensitivity compare well with data. Comparing the thermal cantilever with a similarly sized piezoresistive cantilever, the thermal cantilever provides more than one order of magnitude improved performance in both sensitivity and resolution over the piezoresistive cantilever.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.10.Pz Instruments for strain, force, and torque
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
68.37.Ps Atomic force microscopy (AFM)
81.40.Gh Other heat and thermomechanical treatments

Photoconductivity from PbS-nanocrystal∕semiconducting polymer composites for solution-processible, quantum-size tunable infrared photodetectors

S. A. McDonald, P. W. Cyr, L. Levina, and E. H. Sargent

Appl. Phys. Lett. 85, 2089 (2004); http://dx.doi.org/10.1063/1.1792380 (3 pages) | Cited 47 times

Online Publication Date: 17 September 2004

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We report photoconductivity at infrared wavelengths, 975–1300 nm, from a polymer∕nanocrystal quantum dot composite. Biased films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) sensitized with PbS nanocrystals (∼5 nm diameter) demonstrate photocurrent at wavelengths beyond the response of the polymer and corresponding to the absorption of the nanocrystals. The photocurrent is attributed to absorption in the nanocrystals with subsequent hole transfer to the polymer and had an internal quantum efficiency of ∼5×10−6 to ∼10−5 charges∕photon at 5 V bias.
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72.40.+w Photoconduction and photovoltaic effects
73.50.Pz Photoconduction and photovoltaic effects
78.66.Qn Polymers; organic compounds
78.30.Hv Other nonmetallic inorganics
78.30.Jw Organic compounds, polymers
78.40.Me Organic compounds and polymers
78.40.Fy Semiconductors
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