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31 Aug 2009

Volume 95, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 95, 091901 (2009); http://dx.doi.org/10.1063/1.3212896 (3 pages)

Noy Bassik, George M. Stern, and David H. Gracias
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Confinement, overflow, and emission of holes on SiGe surface with Ge dots: Heterogeneous hole redistribution and its application to virtual dot manipulation

Masashi Ishii, Sarnjeet S. Dhesi, and Bruce Hamilton

Appl. Phys. Lett. 95, 093101 (2009); http://dx.doi.org/10.1063/1.3220065 (3 pages) | Cited 1 time

Online Publication Date: 31 August 2009

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The hole confinement in Ge dots fabricated on a wetting layer in Stranski–Krastanov (S-K) growth was directed by an applied bias. At medium bias voltage, the holes overflowed from the small dots, indicating a moderate potential barrier without a notch at the boundary. The electrostatic force of the confined holes attracted excessive holes to the wetting layer. The system was energetically stabilized by the formation of a “virtual dot” in an open space enclosed by dots. At a high bias voltage, the virtual dot disappeared since the holes in the wetting layer were emitted from the surface.
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73.21.La Quantum dots
73.63.Kv Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
72.20.Ee Mobility edges; hopping transport
72.20.Fr Low-field transport and mobility; piezoresistance

Conformation-induced self-assembly of rubrene on Au(111) surface

Li Wang, Huihui Kong, Xiu Chen, Xinli Du, Feng Chen, Xiaoqing Liu, and Hongming Wang

Appl. Phys. Lett. 95, 093102 (2009); http://dx.doi.org/10.1063/1.3213563 (3 pages) | Cited 5 times

Online Publication Date: 31 August 2009

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Large -area well-ordered monolayer of rubrene has been fabricated on Au(111) surface by gradually annealing a multilayer of rubrene in vacuum. Scanning tunneling microscopy clearly demonstrates that the rubrene monolayer experiences a disorder-order transition with increase of the annealing temperature and the molecules form dimer rows with opposite directions for two neighbored rows. The π-π and π-H interactions between the molecules induced by conformation of the molecules in monolayer account for the main driving forces to form this kind of ordered layer.
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68.55.-a Thin film structure and morphology
68.65.Ac Multilayers
81.40.Gh Other heat and thermomechanical treatments
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
64.70.K- Solid-solid transitions

Interrogating vertically oriented carbon nanofibers with nanomanipulation for nanoelectromechanical switching applications

Anupama B. Kaul, Abdur R. Khan, Leif Bagge, Krikor G. Megerian, Henry G. LeDuc, and Larry Epp

Appl. Phys. Lett. 95, 093103 (2009); http://dx.doi.org/10.1063/1.3211851 (3 pages)

Online Publication Date: 31 August 2009

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We have demonstrated electrostatic switching in vertically oriented carbon nanofibers synthesized on refractory metallic nitride substrates, where pull-in voltages Vpi ranged from 10 to 40 V. A nanoprobe was used as the actuating electrode inside a scanning-electron microscope and van der Waals interactions at these length scales appeared significant, suggesting such structures are promising for nonvolatile memory applications. A finite element model was also developed to determine a theoretical Vpi and results were compared to experiment. Nanomanipulation tests also revealed tubes synthesized directly on Si by dc plasma-enhanced chemical-vapor deposition with ammonia and acetylene were electrically unsuitable for dc nanoelectromechanical switching applications.
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81.16.-c Methods of micro- and nanofabrication and processing
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
77.65.-j Piezoelectricity and electromechanical effects
81.05.U- Carbon/carbon-based materials
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Gate tunable photoconductivity of p-channel Se nanowire field effect transistors

Zhi-Min Liao, Chong Hou, Qing Zhao, Li-Ping Liu, and Da-Peng Yu

Appl. Phys. Lett. 95, 093104 (2009); http://dx.doi.org/10.1063/1.3216846 (3 pages) | Cited 6 times

Online Publication Date: 31 August 2009

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We have investigated the photoelectrical properties of p-type selenium nanowire field effect transistors. The hole concentrations are estimated to be 4.1×1017 and 2.5×1018 cm−3 for the device under dark and illumination, respectively. The photoelectrical on/off ratio can be tunable from 4 to 160 as the gate voltage decreases from 20 to −20 V.
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85.30.Tv Field effect devices

Semiconductor quantum dots through conversion of micelle-generated metal clusters

D. Wahl, A. Ladenburger, M. Feneberg, W. Schoch, K. Thonke, and R. Sauer

Appl. Phys. Lett. 95, 093105 (2009); http://dx.doi.org/10.1063/1.3216849 (3 pages)

Online Publication Date: 31 August 2009

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We introduce a method of producing semiconductor quantum dots that is essentially based on the conversion of metal clusters into semiconductor clusters. In the first step, metal dots are generated with the help of self-organizing diblock-copolymers. Then, in the second step, these metal dots are converted into traditional III-V or II-VI semiconductor quantum dots by adding the nonmetallic component. Unlike the commonly used Stranski–Krastanow growth method, our approach completely avoids the formation of an underlying wetting layer. It works basically for any combination of substrate and dot materials being independent of differences in the lattice constants of substrate and dots as are essential for the Stranski–Krastanow growth. Also, the self-ordering of the diblock copolymers is imposed on the metal dots and is retained during their conversion to the semiconductor dots. The dots form an array with average sizes and distances controlled by the partial length of the polymer chains allowing high flexibility for future applications.
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68.65.Hb Quantum dots (patterned in quantum wells)
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
61.41.+e Polymers, elastomers, and plastics
68.08.Bc Wetting
81.07.Ta Quantum dots

Directed self-assembly of monodispersed platinum nanoclusters on graphene Moiré template

Yi Pan, Min Gao, Li Huang, Feng Liu, and H.-J. Gao

Appl. Phys. Lett. 95, 093106 (2009); http://dx.doi.org/10.1063/1.3223781 (3 pages) | Cited 22 times

Online Publication Date: 3 September 2009

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Monodispersed crystalline platinum nanoclusters (NCs) have been grown on a template of graphene Moiré pattern formed on Ru(0001). The Pt NCs are directed to nucleate at a unique site in the Moiré unit cell, and grow in a layer-by-layer mode up to 4-atomic-layer height without coalescence at room temperature. The size of Pt NCs can be controlled by tuning the coverage. This system may find application in the study of Pt nanocatalyst, and the graphene Moiré pattern may be generally applied as template to direct self-assembled growth of metallic or nonmetallic NCs.
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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)
61.46.Hk Nanocrystals
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Pentacene as protection layers of graphene on SiC surfaces

Hae-geun Jee, Jin-Hee Han, Han-Na Hwang, Bongsoo Kim, Hee-seob Kim, Young Dok Kim, and Chan-Cuk Hwang

Appl. Phys. Lett. 95, 093107 (2009); http://dx.doi.org/10.1063/1.3224833 (3 pages) | Cited 6 times

Online Publication Date: 4 September 2009

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We report that pentacene can be used as a protection layer of graphene using synchrotron radiation-based photoemission spectroscopy. When pentacene was deposited on a single layer graphene, molecular states of pentacene were clearly observed, yet no change in the band structure of graphene could be identified. Unique electronic properties of graphene can be preserved in the presence of pentacene layers, and this finding can be exploited for fundamental research as well as application of graphene in electronic devices. After exposing the pentacene-covered graphene to air followed by a subsequent annealing under vacuum, band structure of graphene was completely maintained.
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73.20.At Surface states, band structure, electron density of states
79.60.Bm Clean metal, semiconductor, and insulator surfaces
61.72.Cc Kinetics of defect formation and annealing
81.40.Gh Other heat and thermomechanical treatments
71.20.Tx Fullerenes and related materials; intercalation compounds
81.05.ub Fullerenes and related materials

Fabrication of Cu2O/TiO2 nanotube heterojunction arrays and investigation of its photoelectrochemical behavior

Y. Hou, X. Y. Li, Q. D. Zhao, X. Quan, and G. H. Chen

Appl. Phys. Lett. 95, 093108 (2009); http://dx.doi.org/10.1063/1.3224181 (3 pages) | Cited 25 times

Online Publication Date: 4 September 2009

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Cu2O/TiO2 nanotube heterojunction arrays have been prepared by photoreduction depositing Cu2O nanoparticles on TiO2 nanotube arrays. The heterojunction arrays show uniformly distributed Cu2O nanoparticles, and high crystallinity of anatase and cubic from the TiO2 and Cu2O, respectively. The asymmetry of the current-voltage plot for the material reveals that a heterojunction has been formed between TiO2 and Cu2O. Enhanced charge separation efficiency and improved photoconversion capability are confirmed by electrochemical impedance spectroscopy and photocurrent measurement.
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81.16.-c Methods of micro- and nanofabrication and processing
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
82.80.Fk Electrochemical methods
82.45.Rr Electroanalytical chemistry
82.50.-m Photochemistry
72.40.+w Photoconduction and photovoltaic effects

Raman investigation of oxidation mechanism of silicon nanowires

L. Z. Liu, X. L. Wu, Z. Y. Zhang, T. H. Li, and Paul K. Chu

Appl. Phys. Lett. 95, 093109 (2009); http://dx.doi.org/10.1063/1.3224183 (3 pages) | Cited 6 times

Online Publication Date: 4 September 2009

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Raman spectra are acquired from Si nanowires (NWs) with diameters of 2–15 nm oxidized for different time durations. The Si TO optical phonon peak downshifts asymmetrically finally becoming an amorphous Si peak after a long oxidation time. The spectral changes cannot be correlated using the phonon confinement model of cylindrical NWs. Microstructural observations disclose that the strain induced by oxidization breaks the NWs into small nanocrystals. By considering the morphological transformation, we adopt the phonon confinement models on wires and dots to explain very well the Raman spectra acquired from Si NWs with different diameters.
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78.67.Lt Quantum wires
63.22.Gh Nanotubes and nanowires
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
78.30.Am Elemental semiconductors and insulators
81.65.Mq Oxidation
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