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13 Dec 2004

Volume 85, Issue 24, pp. 5819-6053

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Appl. Phys. Lett. 85, 5968 (2004); http://dx.doi.org/10.1063/1.1830083 (3 pages)

A. Urbieta, P. Fernández, and J. Piqueras
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Growth and luminescence properties of micro- and nanoneedles in sintered CdSe

A. Urbieta, P. Fernández, and J. Piqueras

Appl. Phys. Lett. 85, 5968 (2004); http://dx.doi.org/10.1063/1.1830083 (3 pages) | Cited 16 times

Online Publication Date: 9 December 2004

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Sintering CdSe powder under argon flow at temperatures in the range 800–900 °C produces the formation of needles on the sample surface. Bundles of parallel needles of a diameter of about 50 nm give rise to a domain-like appearance in the sample. In addition, rods and needles with a wide range of dimensions up to several microns appear distributed in the surface. The influence of ball milling of the starting powder on the formation of the needles is investigated. Cathodoluminescence in the scanning electron microscope has been used to characterize the sintered samples.
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81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
61.46.-w Structure of nanoscale materials
78.60.Hk Cathodoluminescence, ionoluminescence
61.43.Gt Powders, porous materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Origin of ultraviolet photoluminescence in ZnO quantum dots: Confined excitons versus surface-bound impurity exciton complexes

Vladimir A. Fonoberov and Alexander A. Balandin

Appl. Phys. Lett. 85, 5971 (2004); http://dx.doi.org/10.1063/1.1835992 (3 pages) | Cited 112 times

Online Publication Date: 9 December 2004

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We have theoretically investigated the origin of ultraviolet photoluminescence (PL) in ZnO quantum dots with diameters from 2 to 6 nm. Two possible sources of ultraviolet PL have been considered: excitons confined in the quantum dot and excitons bound to an ionized impurity located at the quantum-dot surface. It is found that depending on the fabrication method and surface passivation technique, the ultraviolet PL of ZnO quantum dots can be attributed to either confined excitons or surface-bound ionized acceptor-exciton complexes. The exciton radiative lifetime is shown to be very sensitive to the exciton localization and can be used as a tool to discriminate between these two sources of PL.
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73.21.La Quantum dots
78.67.Hc Quantum dots
78.55.Et II-VI semiconductors
71.55.Gs II-VI semiconductors
81.65.Rv Passivation

A growth pathway for highly ordered quantum dot arrays

Jianyu Liang, Hailin Luo, Rod Beresford, and Jimmy Xu

Appl. Phys. Lett. 85, 5974 (2004); http://dx.doi.org/10.1063/1.1834987 (3 pages) | Cited 21 times

Online Publication Date: 9 December 2004

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To realize the desired zero-dimensional behavior of a quantum dot ensemble, the ability to fabricate quantum dots with a high packing density and a high degree of size, shape, and spacing uniformity is crucial. Here we report highly ordered InAs nanodot arrays grown by molecular-beam epitaxy on nonlithographically nanopatterned GaAs. Approximately 20 billion dots are grown in a 1 cm2 area with the smallest size dispersion ever reported and forming a lateral superlattice in hexagonal dense packing form. These techniques presage a pathway to controlled growth of periodic quantum dot superstructures, which offer macroscopic spatial coherence in the interaction of quantum dots with radiation.
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81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.46.-w Structure of nanoscale materials

Growth and electrical properties of multidimensional tungsten nano-buliding blocks

Yun-Hi Lee, Dong-Ho Kim, Chang-Hoon Choi, Yoon-Taek Jang, and Byeong-Kwon Ju

Appl. Phys. Lett. 85, 5977 (2004); http://dx.doi.org/10.1063/1.1829156 (3 pages) | Cited 2 times

Online Publication Date: 9 December 2004

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We report the demonstration of tungsten nanoblocks such as nanorods, nanoblocks, and nanocylinders through a simple thermal process of sputtered W films. The formation of various types of tungsten nanostructures was based on the difference in the magnitude of the residual film stress and in its spatial distribution, which depends on the deposition pressure during the sputter deposition of self-catalytic W thin film. The resulting shapes displayed well-controlled structures that have characteristic dimensionalities such as zero-dimensional dot, one-dimensional wire, and two-dimensional nanosheet with the deposition pressure. The results demonstrate the possibility of construction of in situ multidimensional achitectures through the one-step thermal process which uses a self-catalytic function of each thin film.
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81.07.Bc Nanocrystalline materials
73.63.Bd Nanocrystalline materials
61.46.-w Structure of nanoscale materials
81.15.Cd Deposition by sputtering
68.60.Bs Mechanical and acoustical properties

Theoretical study of coated spherical metallic nanoparticles for single-molecule surface-enhanced spectroscopy

Hongxing Xu

Appl. Phys. Lett. 85, 5980 (2004); http://dx.doi.org/10.1063/1.1833570 (3 pages) | Cited 19 times

Online Publication Date: 9 December 2004

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We investigate the possibility to use coated metallic nanoparticles for single-molecule surface-enhanced spectroscopy by extending the Mie theory. The aggregates of metallodielectric spheres of Au and Ag show a strong local intensity enhancement in the cavity between particles, which can result in up to a 1014-fold increase for surface-enhanced Raman scattering. We also examine the thickness of the coating layer, the size of the core, and the incident wavelength that determine the enhancement.
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78.30.Er Solid metals and alloys
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.55.-g Dielectric thin films

Shockley partial dislocations to twin: Another formation mechanism and generic driving force

Jian Wang and Hanchen Huang

Appl. Phys. Lett. 85, 5983 (2004); http://dx.doi.org/10.1063/1.1835549 (3 pages) | Cited 42 times

Online Publication Date: 9 December 2004

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We report a mechanism of twin formation from two Shockley partial dislocations on non-neighboring glide planes. Further, we correlate the driving force of twin nucleation with the local stress of large magnitude. Using the embedded atom method potential for atomic interactions, our molecular dynamics simulations of Cu nanograins and a single crystal reveal the formation mechanism and identify the driving force. Stacking faults expand under high stress, increasing their energy and making their transformation to a twin energetically favorable. Although this study focuses on only Shockley partial dislocations on non-neighboring glide planes, the driving force—the stress of large magnitude—is expected to apply also to the twin formation from Shockley partial dislocations on neighboring glide planes.
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61.46.-w Structure of nanoscale materials
61.72.Mm Grain and twin boundaries
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Nn Stacking faults and other planar or extended defects
61.72.Bb Theories and models of crystal defects

Thermoelectric figure of merit of II–VI semiconductor nanowires

N. Mingo

Appl. Phys. Lett. 85, 5986 (2004); http://dx.doi.org/10.1063/1.1829391 (3 pages) | Cited 19 times

Online Publication Date: 9 December 2004

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The room temperature thermoelectric figure of merit and power factor are calculated for bulk and nanowires made of CdTe, ZnTe, ZnSe, and ZnS, of the zincblende structure. To this end, an iterative solution of the Boltzmann transport equation is used to obtain the electronic transport properties. The lattice thermal conductivity is estimated using a full dispersion transmission function approach. A saturation to finite values is predicted for the power factor in the small thickness limit. The figure of merit increase upon thickness reduction is largest for CdTe, followed by ZnTe, ZnSe, and ZnS. II–VI nanowires show smaller increases of the figure of merit as compared with III–V nanowires, despite their bulk values are of comparable magnitude in the two groups. The reasons for this are explained.
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73.63.Bd Nanocrystalline materials
72.20.Pa Thermoelectric and thermomagnetic effects
72.20.Fr Low-field transport and mobility; piezoresistance

Tb nanocrystalline array assembled directly from alloy melt

Y. T. Wang, X. K. Xi, Y. K. Fang, D. Q. Zhao, M. X. Pan, B. S. Han, W. H. Wang, and W. L. Wang

Appl. Phys. Lett. 85, 5989 (2004); http://dx.doi.org/10.1063/1.1836863 (3 pages) | Cited 5 times

Online Publication Date: 9 December 2004

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A two-dimensional (2D) thin film composed of Tb nanocrystals with uniform orientations is fabricated directly from Tb65Fe25Al10 alloy melt upon quenching. The Tb nanocrystals with vertical height of 15–30 nm and lateral width of 10–20 nm are assembled in the nanocrystalline array on an amorphous substrate. The formation mechanism for the aligned Tb nanocrystals is discussed. The single-step formation method may provide a new and flexible alternative to fabricate nanostructured films or arrays used for submicron devices.
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81.05.Bx Metals, semimetals, and alloys
81.07.Bc Nanocrystalline materials
81.16.Dn Self-assembly
68.55.A- Nucleation and growth
81.40.Gh Other heat and thermomechanical treatments

Impacts of metal electrode and molecule orientation on the conductance of a single molecule

W. T. Geng, Jun Nara, and Takahisa Ohno

Appl. Phys. Lett. 85, 5992 (2004); http://dx.doi.org/10.1063/1.1836872 (3 pages) | Cited 11 times

Online Publication Date: 9 December 2004

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We present first-principles investigation of electrical conductance of a benzene-1,4-dithiolate (�S�C6H4�S�) molecule bridging the (111) surfact of Pt and Au carried out using the Lippmann–Schwinger scattering method combined with the density functional theory. We show that Pt makes better electrodes than noble metals, due to a closer positioning of the transmission resonance to the Fermi level. Interestingly, we find that the peak transmission corresponding to the highest occupied molecular orbital decreases with the increasing of the tilting angle of the benzene dithiolate. Moreover, the flattening comes together with a widening of the peak, and consequently, the transmission at the Fermi level is enhanced.
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73.20.At Surface states, band structure, electron density of states
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
68.43.Mn Adsorption kinetics

Transplanting carbon nanotubes

Tarek A. El-Aguizy, Jeung-hyun Jeong, Yong-Bae Jeon, W. Z. Li, Z. F. Ren, and Sang-Gook Kim

Appl. Phys. Lett. 85, 5995 (2004); http://dx.doi.org/10.1063/1.1836865 (3 pages) | Cited 9 times

Online Publication Date: 9 December 2004

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Current efforts of guided growth of carbon nanotubes alone cannot make large-scale and directed assembly of them. The nanopelleting concept overcomes this limitation by embedding carbon nanotubes in microscale pellets that can be transplanted readily. This technique includes vertical growth of carbon nanotubes, pellet casting, planarization, pellet separation, and transplantation. A specific manufacturing process is developed and tested with favorable results. This technology will enable directed assembly of carbon nanotubes in a long-range order.
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81.07.De Nanotubes
81.16.Dn Self-assembly
81.65.-b Surface treatments
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Silicon oxide colloidal∕polymer nanocomposite films

Haifeng Wang, Wenwu Cao, Q. F. Zhou, K. Kirk Shung, and Y. H. Huang

Appl. Phys. Lett. 85, 5998 (2004); http://dx.doi.org/10.1063/1.1836018 (3 pages) | Cited 5 times

Online Publication Date: 9 December 2004

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The quarter-wavelength (λ∕4) acoustic matching layer, a vital component in medical ultrasonic transducer, can bridge the large acoustic impedance mismatch between the piezoelectric material and the human body. Composite materials are widely used as matching materials in order to cover the wide acoustic impedance range that cannot be accomplished by using a single-phase material. At high frequencies (>50 MHz), the λ∕4 matching layers become extremely thin so that the fabrication of homogeneous composite material matching layers becomes very challenging. A method is reported in this letter to fabricate sol-gel silicon oxide colloidal∕polymer composite film on silicon substrate, in which the particle size of silicon oxide colloidal is between 10 and 40 nm. The acoustic impedance of the nanocomposite films versus aging temperature has been measured at the desired operating frequency.
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81.05.Mh Cermets, ceramic and refractory composites
81.05.Qk Reinforced polymers and polymer-based composites
81.07.Bc Nanocrystalline materials
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
81.40.Cd Solid solution hardening, precipitation hardening, and dispersion hardening; aging
81.70.Cv Nondestructive testing: ultrasonic testing, photoacoustic testing

Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

D.-A. Borca-Tasciuc, G. Chen, A. Prieto, M. S. Martín-González, A. Stacy, T. Sands, M. A. Ryan, and J. P. Fleurial

Appl. Phys. Lett. 85, 6001 (2004); http://dx.doi.org/10.1063/1.1834991 (3 pages) | Cited 27 times

Online Publication Date: 9 December 2004

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Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40 nm diameter Bi2Te3 nanowires∕alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10−7 m2 s−1 at 150 K to 6.9×10−7 m2 s−1 at 300 K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.
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68.65.La Quantum wires (patterned in quantum wells)
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
66.30.Xj Thermal diffusivity

Vertical nanowire light-emitting diode

R. Könenkamp, Robert C. Word, and C. Schlegel

Appl. Phys. Lett. 85, 6004 (2004); http://dx.doi.org/10.1063/1.1836873 (3 pages) | Cited 172 times

Online Publication Date: 9 December 2004

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We report room-temperature, white-color electroluminescence in vertically oriented ZnO nanowires. Excitonic luminescence around 380 nm is observed as a shoulder on a broader defect-related band covering all of the visible range and centered at 620 nm. The ZnO nanowires are grown in a low-temperature process on SnO2-coated glass substrates, employing a technique that is suitable for large-area applications. The nanowires are robustly encapsulated in a thin polystyrene film deposited from high-molecular-weight solutions. Electron injection occurs through the transparent SnO2 layer, while hole injection is mediated by a p-doped polymer and an evaporated Au contact. Stable device operation is observed at ambient conditions on the time scale of 1 h.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
61.46.-w Structure of nanoscale materials

Laser writing of semiconductor nanoparticles and quantum dots

M. F. Bertino, R. R. Gadipalli, J. G. Story, C. G. Williams, G. Zhang, C. Sotiriou-Leventis, A. T. Tokuhiro, S. Guha, and N. Leventis

Appl. Phys. Lett. 85, 6007 (2004); http://dx.doi.org/10.1063/1.1836000 (3 pages) | Cited 13 times

Online Publication Date: 9 December 2004

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Silica aerogels were patterned with CdS using a photolithographic technique based on local heating with infrared (IR) light. The solvent of silica hydrogels was exchanged with an aqueous solution of the precursors CdNO3 and NH4OH, all precooled to a temperature of 5°C. Half of the bathing solution was then replaced by a thiourea solution. After thiourea diffused into the hydrogels, the samples were exposed to a focused IR beam from a continuous wave, Nd-YAG laser. The precursors reacted in the spots heated by the IR beam to form CdS nanoparticles. We lithographed features with a diameter of about 40 μm, which extended inside the monoliths for up to 4 mm. Samples were characterized with transmission electron microscopy and optical absorption, photoluminescence, and Raman spectroscopies. Spots illuminated by the IR beam were made up by CdS nanoparticles dispersed in a silica matrix. The CdS nanoparticles had a diameter in the 4–6 nm range in samples exposed for 4 min to the IR beam, and of up to 100 nm in samples exposed for 10 min.
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81.05.Dz II-VI semiconductors
81.07.Ta Quantum dots
81.07.Bc Nanocrystalline materials
81.16.Nd Micro- and nanolithography
82.70.Gg Gels and sols
78.55.Et II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.67.Hc Quantum dots
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Rx Nanocrystalline materials
61.82.Fk Semiconductors
68.37.Lp Transmission electron microscopy (TEM)
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