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9 Feb 2004

Volume 84, Issue 6, pp. 837-1024

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 975 (2004); http://dx.doi.org/10.1063/1.1646453 (3 pages)

Xiang Yang Kong and Zhong Lin Wang
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Resonant Raman scattering in nanoscale pentacene films

Rui He, Irene Dujovne, Liwei Chen, Qian Miao, Cyrus F. Hirjibehedin, Aron Pinczuk, Colin Nuckolls, Christian Kloc, and Arza Ron

Appl. Phys. Lett. 84, 987 (2004); http://dx.doi.org/10.1063/1.1646756 (3 pages) | Cited 25 times

Online Publication Date: 4 February 2004

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Resonant Raman scattering intensities from nanoscale films of pentacene display large resonant enhancements that enable observation of vibrational modes in monolayer cluster films. The resonant enhancements occur when the outgoing photon energy overlaps the free exciton optical transitions observed in luminescence. The results point to the significant potential of resonant Raman methods in the characterization of nanoscale structures of organic molecular semiconductors. © 2004 American Institute of Physics.
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78.30.Jw Organic compounds, polymers
78.66.Qn Polymers; organic compounds
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
78.55.Kz Solid organic materials

In situ observations of carbon nanotube formation using environmental transmission electron microscopy

Renu Sharma and Zafar Iqbal

Appl. Phys. Lett. 84, 990 (2004); http://dx.doi.org/10.1063/1.1646465 (3 pages) | Cited 50 times

Online Publication Date: 4 February 2004

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Environmental transmission electron microscope is used for in situ observations of the growth mechanism and reaction conditions of carbon nanotubes. Chemical vapor deposition was performed by flowing propylene or acetylene gas (precursor) over Ni or Co catalyst heated to 450 °C and 700 °C. We are reporting the in situ observations of the growth process of carbon nanotubes. Multi-wall nanotubes formed at temperatures as low as 450 °C while only single-wall carbon nanotubes formed at higher temperatures (700 °C and above). At lower temperatures, a cubic phase was also observed to form on the walls of the multi-wall carbon nanotubes. © 2004 American Institute of Physics.
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81.07.De Nanotubes
61.46.-w Structure of nanoscale materials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.40.Gh Other heat and thermomechanical treatments

Fabrication of ordered array of nanowires of La0.67Ca0.33MnO3 (x = 0.33) in alumina templates with enhanced ferromagnetic transition temperature

K. Shantha Shankar, Sohini Kar, A. K. Raychaudhuri, and G. N. Subbanna

Appl. Phys. Lett. 84, 993 (2004); http://dx.doi.org/10.1063/1.1646761 (3 pages) | Cited 25 times

Online Publication Date: 4 February 2004

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We report fabrication of array of crystalline nanowires (average diameter of 65 nm) of colossal magnetoresistive oxide La0.67Ca0.33MnO3 (LCMO, x = 0.33) within anodized alumina templates by filling the pores with a sol that allows formation of LCMO phase at the relatively low temperature of 600 °C. The crystalline nanowires with correct stoichiometry stabilize in the orthorhombic phase at room temperature. The nanowires are ferromagnetic at room temperature and exhibit enhanced ferromagnetic transition temperature well in excess of 300 K, which is substantially higher than that of single crystalline LCMO. This enhancement we attribute to the size induced lattice contraction in the nanowires. © 2004 American Institute of Physics.
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75.50.Dd Nonmetallic ferromagnetic materials
81.07.Bc Nanocrystalline materials
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities
61.66.Bi Elemental solids
61.66.Dk Alloys
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Controllable quantum spin precession by Aharonov–Casher phase in a conducting ring

Shun-Qing Shen, Zhi-Jian Li, and Zhongshui Ma

Appl. Phys. Lett. 84, 996 (2004); http://dx.doi.org/10.1063/1.1644914 (3 pages) | Cited 31 times

Online Publication Date: 4 February 2004

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We investigate quantum spin transport in a structure of a conducting ring embedded in a textured electric field with two leads, and obtain an exact solution for the problem. The spin precession induced by the Aharonov–Casher phase is studied. It is shown that the spin-polarized current and its polarization orientation can be controlled by the electric field. As a result the modulated polarization orientation is a function of the geometric phase which originates from spin–orbital interaction in the ring. © 2004 American Institute of Physics.
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72.25.-b Spin polarized transport
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
73.23.-b Electronic transport in mesoscopic systems

Multilevel conductivity and conductance switching in supramolecular structures of an organic molecule

Anirban Bandyopadhyay and Amlan J. Pal

Appl. Phys. Lett. 84, 999 (2004); http://dx.doi.org/10.1063/1.1644611 (3 pages) | Cited 36 times

Online Publication Date: 4 February 2004

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We have demonstrated conductance switching between multilevel states in devices based on Rose Bengal molecules embedded in supramolecular matrices. Two mechanisms, namely electroreduction and conformational change of the molecules, resulting in conjugation modification have been proposed to be applicable in these devices. In a low voltage region, reverse-bias induced electroreduction of Rose Bengal facilitated conjugation restoration in the backbone of the molecule and, hence, switching to a high-conducting state. At high biases, the two perpendicular planes present in Rose Bengal, which have permanent dipole moments, allowed forward-bias induced conformation change to occur, and results in conductance switching. We have demonstrated how the devices can switch between two pair of conducting states for random-access memory and read-only memory applications for several hours. © 2004 American Institute of Physics.
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71.20.Rv Polymers and organic compounds
72.80.Le Polymers; organic compounds (including organic semiconductors)
85.65.+h Molecular electronic devices

AlGaN-based ultraviolet light-emitting diodes grown on bulk AlN substrates

Toshio Nishida, Toshiki Makimoto, Hisao Saito, and Tomoyuki Ban

Appl. Phys. Lett. 84, 1002 (2004); http://dx.doi.org/10.1063/1.1646454 (2 pages) | Cited 14 times

Online Publication Date: 4 February 2004

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We have confirmed the potential of a bulk AlN substrate for high current operation of nitride ultraviolet-light-emitting diodes (UV-LEDs). For the high flux UV extraction from nitride UV-LEDs, transparency and high thermal conductivity of the substrates are important issues. The bulk AlN is one of the best candidates, because it satisfies requirements above, and has the same crytallographic symmetry with those of AlGa(In)N families, which is beneficial to the high-quality crystal growth of the nitride device structures. We formed AlGaN-based UV-LEDs on a bulk AlN substrate and compared its performance with that of a reference device grown on an AlN-template grown on a sapphire substrate. The output power linearly increases with a saturation injection current of 300 mA, which is two times higher than that of the reference device. The emission spectrum under high current injection is much more stable than that of conventional substrate. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices

Improving organic transistor performance with Schottky contacts

Raoul Schroeder, Leszek A. Majewski, and Martin Grell

Appl. Phys. Lett. 84, 1004 (2004); http://dx.doi.org/10.1063/1.1645993 (3 pages) | Cited 57 times

Online Publication Date: 4 February 2004

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Organic field-effect transistors (OFETs) with non-Ohmic contacts, e.g., pentacene with gold electrodes, exhibit a linearly growing threshold voltage with increased film thickness due to tunnel injection [R. Schroeder et al., Appl. Phys. Lett. 83, 3201 (2003)]. In this letter, we demonstrate gold/pentacene OFETs with a low threshold voltage independent of pentacene thickness. By doping the pentacene in the contact area with FeCl3 (iron-III-chloride), the metal-insulator-type tunneling barrier was changed to a metal-semiconductor Schottky barrier. Since the injection through a Schottky barrier depends on the potential and not on the electric field, the threshold voltage is no longer a function of the semiconductor thickness. Through selective doping of the area under the electrode, the channel remains undoped, and large on/off ratios are retained. © 2004 American Institute of Physics.
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85.30.Tv Field effect devices
73.30.+y Surface double layers, Schottky barriers, and work functions
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Thickness and low-temperature conductivity of DNA molecules

A. Yu. Kasumov, D. V. Klinov, P.-E. Roche, S. Guéron, and H. Bouchiat

Appl. Phys. Lett. 84, 1007 (2004); http://dx.doi.org/10.1063/1.1644909 (3 pages) | Cited 35 times

Online Publication Date: 4 February 2004

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We argue that interaction between molecules and substrate is a key parameter which determines the conducting or insulating behavior of DNA molecules. In this letter, we show that strongly deformed DNA molecules deposited on a substrate, whose thickness is less than half the native thickness of the molecule, are insulating, whereas molecules keeping their native thickness are conducting down to very low temperature with a non-ohmic behavior characteristic of a 1D conductor with repulsive electron–electron interactions. © 2004 American Institute of Physics.
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87.14.G- Nucleic acids
87.64.Dz Scanning tunneling and atomic force microscopy
68.37.Ps Atomic force microscopy (AFM)
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
72.80.Le Polymers; organic compounds (including organic semiconductors)

Digital holographic method for tomography-image reconstruction

Cheng Liu, Changchun Yan, and Shumei Gao

Appl. Phys. Lett. 84, 1010 (2004); http://dx.doi.org/10.1063/1.1646224 (3 pages)

Online Publication Date: 4 February 2004

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A digital holographic method for three-dimensional reconstruction of tomography images is demonstrated theoretically and experimentally. In this proposed method, a numerical hologram is first computed by calculating the total diffraction field of all transect images of a detected organ. Then, the numerical hologram is transferred to the usual recording medium to generate a physical hologram. Last, all the transect images are reconstructed in their original position by illuminating the physical hologram with a laser, thereby forming a three-dimensional transparent image of the organ detected. Due to its true third dimension, the reconstructed image using this method is much more vivid and accurate than that of other methods. Potentially, it may have great prospects for application in medical engineering. © 2004 American Institute of Physics.
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81.70.Tx Computed tomography
42.30.Wb Image reconstruction; tomography
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Effect of thermal variations on the Knudsen forces in the transitional regime

A. L. Lereu, A. Passian, R. J. Warmack, T. L. Ferrell, and T. Thundat

Appl. Phys. Lett. 84, 1013 (2004); http://dx.doi.org/10.1063/1.1644916 (3 pages) | Cited 7 times

Online Publication Date: 4 February 2004

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When objects are maintained at different temperatures and separated by distances of the order of the mean free path of the surrounding host gas molecules, gas kinetic forces called Knudsen forces may be involved. The understanding of this effect may result in some improvements in microelectromechanical devices and measurement systems. We present the thermal dependence of these forces in the transitional regime for different gases. In this mode, the Knudsen effect can be significant and, therefore, become a problem in microscale devices. For this study, a silicon microcantilever, mounted close to a substrate, is used and changes in temperature are observed by measuring bending of the microcantilever. © 2004 American Institute of Physics.
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47.45.Dt Free molecular flows
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Simulation of rapid startup in microwave magnetrons with azimuthally varying axial magnetic fields

M. C. Jones, V. B. Neculaes, W. White, Y. Y. Lau, and R. M. Gilgenbach

Appl. Phys. Lett. 84, 1016 (2004); http://dx.doi.org/10.1063/1.1646225 (3 pages) | Cited 18 times

Online Publication Date: 4 February 2004

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A method is proposed whereby the startup of a magnetron may be hastened. For a N-cavity magnetron operating in the pi mode, the formation of the N/2 electron spokes is much more rapid when an azimuthally varying axial magnetic field of N/2 periodicity is employed. Electromagnetic particle-in-cell code simulations are presented that show electron prebunching by the azimuthally varying axial magnetic field, long before the pi mode is excited. © 2004 American Institute of Physics.
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84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)

Reversing granular flow on a vibratory conveyor

R. Grochowski, P. Walzel, M. Rouijaa, C. A. Kruelle, and I. Rehberg

Appl. Phys. Lett. 84, 1019 (2004); http://dx.doi.org/10.1063/1.1646219 (3 pages) | Cited 9 times

Online Publication Date: 4 February 2004

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Experimental results are presented for the transport properties of granular materials on a vibratory conveyor. For circular oscillations of the shaking trough nonmonotonous dependence of the transport velocity on the normalized acceleration Γ is observed. Two maxima are separated by a regime where the granular flow is much slower and, in a certain driving range, even reverses its direction. Similar behavior is found for a single solid body with a low coefficient of restitution, whereas an individual glass bead of 1 mm diameter is propagated in the same direction for all accelerations. © 2004 American Institute of Physics.
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45.70.Mg Granular flow: mixing, segregation and stratification
81.05.Rm Porous materials; granular materials
46.40.-f Vibrations and mechanical waves

Carbon nanotube field emitter arrays having an electron beam focusing structure

Jun Hee Choi, Andrei R. Zoulkarneev, Yong Wan Jin, Young Jun Park, Deuk Seok Chung, Byung Kwon Song, In Taek Han, Hang Woo Lee, Sang Hyun Park, Ho Suk Kang, Ha Jong Kim, Jung Woo Kim, Jae Eun Jung, Jong Min Kim, Hong Gu Baek, et al.

Appl. Phys. Lett. 84, 1022 (2004); http://dx.doi.org/10.1063/1.1645315 (3 pages) | Cited 20 times

Online Publication Date: 4 February 2004

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An electron beam focusing structure was incorporated into the gated field emitter arrays where the emitters were screen-printed carbon nanotubes. The focusing structure was comprised of 8-μm-thick bulky SiOx focus gate insulator and Cr focus gate, and exhibited negligible leakage between the gate and the focus gate. In current–voltage measurements, it is found that the anode current strongly depends on both the focus gate and the anode bias voltages. Electron beams were focused well at the anode with a slight overfocusing effect, which is due to the wide electron beam divergence from carbon nanotubes. A new focusing structure based on the simulation is proposed to overcome the overfocusing. © 2004 American Institute of Physics.
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85.45.Db Field emitters and arrays, cold electron emitters
78.67.Ch Nanotubes
41.85.Lc Particle beam focusing and bending magnets, wiggler magnets, and quadrupoles
85.35.Kt Nanotube devices
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