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6 Feb 2006

Volume 88, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 88, 063509 (2006); http://dx.doi.org/10.1063/1.2171834 (3 pages)

M. Feng, N. Holonyak, R. Chan, A. James, and G. Walter
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Influence of atomic hydrogen on step stability during homoepitaxial growth on vicinal GaAs surfaces

P. Tejedor, M. L. Crespillo, and B. A. Joyce

Appl. Phys. Lett. 88, 063101 (2006); http://dx.doi.org/10.1063/1.2171793 (3 pages) | Cited 3 times

Online Publication Date: 6 February 2006

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We have employed a combination of reflection high-energy electron diffraction intensity oscillations and atomic force microscopy to investigate the effect of atomic hydrogen on the step bunching instability that arises during homoepitaxial growth from molecular beams of Ga and As4 on vicinal GaAs(110) surfaces due to the asymmetric incorporation rates of adatoms to upper and lower step edges. We discuss the variations observed in the GaAs growth mode and in the terrace width distribution of the bunched-step morphology in terms of H-induced changes in the reaction pathway and in the kinetics of adatom migration and incorporation at steps.
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68.55.-a Thin film structure and morphology
68.47.Fg Semiconductor surfaces
68.37.Ps Atomic force microscopy (AFM)
66.30.J- Diffusion of impurities

Functionalization of carbon nanotubes with proteins and quantum dots in aqueous buffer solutions

R. Prakash, Rich Superfine, S. Washburn, and Michael R. Falvo

Appl. Phys. Lett. 88, 063102 (2006); http://dx.doi.org/10.1063/1.2171802 (3 pages) | Cited 4 times

Online Publication Date: 6 February 2006

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We report here on a method of suspending carbon nanotubes (CNTs) in aqueous buffer solutions and functionalizing CNTs with a molecule that is “sticky” to proteins. The specific bifunctional molecule used in this study is 1-pyrene butanoic acid succidymidyl ester (1-pbase). We report successful protein and quantum dot functionalization of the CNTs, using 1-pbase as a linking agent.
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87.80.-y Biophysical techniques (research methods)
87.14.E- Proteins

Static dielectric constant of isolated silicon nanocrystals embedded in a SiO2 thin film

C. Y. Ng, T. P. Chen, L. Ding, Y. Liu, M. S. Tse, S. Fung, and Z. L. Dong

Appl. Phys. Lett. 88, 063103 (2006); http://dx.doi.org/10.1063/1.2172009 (3 pages) | Cited 11 times

Online Publication Date: 6 February 2006

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The static dielectric constant of isolated silicon nanocrystals (nc-Si) embedded in a SiO2 thin film synthesized by Si+ implantation has been determined from capacitance measurement based on the Maxwell–Garnett effective medium approximation and the stopping and range of ions in matter simulation. For the nc-Si with a mean size of ∼ 4.5 nm, the dielectric constant so determined is 9.8, being consistent with a theoretical prediction. This value is significantly lower than the static dielectric constant (11.9) of bulk crystalline Si, indicating the significance of nc-Si size effect. The information of nc-Si dielectric constant is not only important to the fundamental physics but also useful to the design and modeling of nc-Si-based memory devices.
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77.22.Ch Permittivity (dielectric function)
61.46.Hk Nanocrystals
61.72.up Other materials

Synthesis and room temperature ferromagnetism of FeCo-codoped ZnO nanowires

L. Q. Liu, B. Xiang, X. Z. Zhang, Y. Zhang, and D. P. Yu

Appl. Phys. Lett. 88, 063104 (2006); http://dx.doi.org/10.1063/1.2168510 (3 pages) | Cited 35 times

Online Publication Date: 6 February 2006

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FeCo-codoped ZnO nanowires were synthesized by in situ doping with iron and cobalt using a chemical vapor deposition method. Structure and compositional analysis indicated that the as-prepared nanowires were single- crystalline and FeCo codoped. No FeCo clusters and other secondary phases were found in the nanowires. A Curie temperature higher than 300 K was observed from the as-doped nanowires. The room temperature ferromagnetic properties of the synthesized Zn1−x(FeCo)xO nanowire make it useful in future spintronic devices.
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81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
81.16.-c Methods of micro- and nanofabrication and processing
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Stacking and polarization control of wavelength-tunable (1.55 μm region) InAs/InGaAsP/InP (100) quantum dots

S. Anantathanasarn, R. Nötzel, P. J. van Veldhoven, F. W. M. van Otten, T. J. Eijkemans, and J. H. Wolter

Appl. Phys. Lett. 88, 063105 (2006); http://dx.doi.org/10.1063/1.2172729 (3 pages) | Cited 29 times

Online Publication Date: 6 February 2006

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Stacking and polarization control of wavelength-tunable InAs quantum dots (QDs) embedded in lattice-matched InGaAsP on InP (100) grown by metalorganic vapor-phase epitaxy is demonstrated. Wavelength control over the 1.55 μm region at room temperature is achieved by inserting ultrathin GaAs interlayers underneath the QDs and adjusting the amount of InAs. For widely stacked QDs with a 40 nm separation layer, the linear dependence of the emission wavelength on the GaAs interlayer thickness coincides with that of single QD layers revealing the reproduction of identical QD layers. For closely stacked QDs with 4 nm separation layer, the emission wavelength as a function of the GaAs interlayer thickness is systematically redshifted and the linewidth is reduced indicating vertical electronic coupling which is proven by the linear polarization of the cleaved-side luminescence changing from in-plane to isotropic.
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81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)

Intersublevel polaron laser with InAs/GaAs self-assembled quantum dots

S. Sauvage and P. Boucaud

Appl. Phys. Lett. 88, 063106 (2006); http://dx.doi.org/10.1063/1.2169919 (3 pages) | Cited 10 times

Online Publication Date: 7 February 2006

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We propose a three-level scheme to achieve intersublevel population inversion, optical gain, and intersublevel lasing effect in n-doped InAs/GaAs self-assembled quantum dots under optical pumping. The proposed Ruby-type laser scheme uses the natural splitting of the s-p polaron intersublevel transitions around 25 μm wavelength. The relaxation time engineering, which leads to optical gain in the system, relies (i) on the slow polaron relaxation from the P− state down to the S ground state, governed by the specific strong coupling regime for the electron-phonon Fröhlich interaction and (ii) on the fast nonradiative relaxation of the polaron between the P+ and P− levels through the irreversible emission of acoustic phonons. TE-polarized optical gain up to 330 cm−1 is calculated for 80 quantum dot planes in an in-plane monomode waveguide geometry and a laser pump intensity threshold as low as 930 W/cm2, two orders of magnitude smaller than for quantum wells, is predicted.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
71.38.Fp Large or Fröhlich polarons

Contrast in scanning probe microscopy images of ultrathin insulator films

Andreas Klust, Qiuming Yu, Marjorie A. Olmstead, Taisuke Ohta, Fumio S. Ohuchi, Markus Bierkandt, Carsten Deiter, and Joachim Wollschläger

Appl. Phys. Lett. 88, 063107 (2006); http://dx.doi.org/10.1063/1.2172397 (3 pages)

Online Publication Date: 8 February 2006

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The contrast in scanning probe microscopy images of ultrathin CaF2 films epitaxially grown on Si(111) is studied using scanning tunneling microscopy (STM) and scanning force microscopy (SFM). Material contrast between CaF2 and the underlying Si–CaF interface layer can be achieved by multiple scan modes. STM images of CaF2/Si(111) exhibit a distinct contrast depending on the bias voltage. Using SFM, CaF2 and CaF can be distinguished by the contact potential difference (CPD) determined by measuring the bias voltage dependence of the cantilever resonance frequency. The CPD also causes a material contrast in the dissipation signal.
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68.55.-a Thin film structure and morphology
73.61.Ng Insulators
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
73.40.Cg Contact resistance, contact potential

Template-free synthesis of conducting-polymer polypyrrole micro/nanostructures using electrochemistry

S. Gupta

Appl. Phys. Lett. 88, 063108 (2006); http://dx.doi.org/10.1063/1.2168688 (3 pages) | Cited 5 times

Online Publication Date: 8 February 2006

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Controlled synthesis of conducting-polymer polypyrrole microcontainers by electrogenerated H2 gas bubbles act as template from a surfactant cum electrolyte (2-naphthalene sulfonic acid; β-NSA) on a stainless steel working electrode, followed by the electrochemical polymerization of pyrrole around the micelles is reported. The films consist of round grains and cup/bowl-like structures, which became lantern-like with increasing cycles. Sodium chlorate (NaClO3) was used as a reference electrolyte. The bowl diameter and room temperature conductivity ranged 50–2000 nm and 1–50 S cm−1, respectively. An analysis of these films was complemented with Raman spectroscopy to identify the oxidized PPy, polaron, and bipolarons.
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82.35.Cd Conducting polymers
73.61.Ph Polymers; organic compounds
82.45.-h Electrochemistry and electrophoresis
61.46.-w Structure of nanoscale materials
68.55.-a Thin film structure and morphology
78.66.Qn Polymers; organic compounds

Single metal nanoparticle absorption spectroscopy and optical characterization

O. L. Muskens, N. Del Fatti, F. Vallée, J. R. Huntzinger, P. Billaud, and M. Broyer

Appl. Phys. Lett. 88, 063109 (2006); http://dx.doi.org/10.1063/1.2172143 (3 pages) | Cited 32 times

Online Publication Date: 8 February 2006

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Optical absorption spectra of small single metal nanoparticles are measured using a far-field technique combining a spatial modulation microscope with a broadband light source. Quantitative determination of the spectral and polarization dependencies of the absorption cross section of individual gold nanoparticles permits precise determination of their geometrical properties in excellent agreement with transmission electron microscopy measurements.
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78.40.Kc Metals, semimetals, and alloys
78.30.Er Solid metals and alloys
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

ZnO quantum dots synthesized by a vapor phase transport process

J. G. Lu, Z. Z. Ye, J. Y. Huang, L. P. Zhu, B. H. Zhao, Z. L. Wang, and Sz. Fujita

Appl. Phys. Lett. 88, 063110 (2006); http://dx.doi.org/10.1063/1.2172154 (3 pages) | Cited 22 times

Online Publication Date: 8 February 2006

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A vapor phase transport growth process has been developed to synthesize ZnO quantum dots (QDs) on Si substrates. The characteristics were investigated for as-prepared ZnO QDs without any additional treatment. The formation of ZnO QDs with 6 nm in height and 15 nm in diameter is confirmed by scanning electron microscope and atomic force microscopy. Room-temperature photoluminescence reveals that the as-prepared ZnO QDs exhibit a predominant ultraviolet emission at 3.32 eV while the low energy defect-related blue-green emission is significantly quenched. The band gap of ZnO QDs is determined to be 3.41 eV, which evidently indicates the quantum confinement effects.
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81.05.Dz II-VI semiconductors
81.07.Ta Quantum dots
78.67.Hc Quantum dots
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
78.55.Et II-VI semiconductors
78.40.Fy Semiconductors

Modulation of photoemission spectra of In2O3 nanowires by the variation in Zn doping level

C. L. Hsin, J. H. He, and L. J. Chen

Appl. Phys. Lett. 88, 063111 (2006); http://dx.doi.org/10.1063/1.2172219 (3 pages) | Cited 13 times

Online Publication Date: 8 February 2006

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The growth of In2O3 nanowires on Si substrate with different zinc doping levels has been achieved by a vapor transport and condensation method. The atomic percentage in doping level is approximately proportional to the weight of ZnO powder added in the source. The ultraviolet (UV) and green light photoemissions of the In2O3 nanowires are depressed and enhanced, respectively, with the doping level of Zn impurity. The UV and green light peaks are attributed to oxygen vacancy and zinc impurity energy levels. Similar tuning by other impurities can be expected and will be beneficial for possible optoelectronic applications.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
79.60.Bm Clean metal, semiconductor, and insulator surfaces
78.40.Ha Other nonmetallic inorganics
61.46.-w Structure of nanoscale materials
71.55.Ht Other nonmetals

Silicon nanomechanical resonators with a double-triangle cross section leading to an enhanced mass sensitivity

I. De Vlaminck, K. De Greve, L. Lagae, and G. Borghs

Appl. Phys. Lett. 88, 063112 (2006); http://dx.doi.org/10.1063/1.2172236 (3 pages) | Cited 3 times

Online Publication Date: 8 February 2006

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We report on a silicon nanomechanical resonator fabricated with a double-triangle cross section, leading to an enhanced mass sensing capabilities. We analytically derive the dependence of the mass detection limit on, respectively, the moment of inertia and the cross section of a beam resonator. We conclude that our scaling compatible fabrication scheme can yield an enhancement of as much as 2.3 with regard to regular, rectangular resonators of the same size. We experimentally confirm the shape dependence of the resonant frequency and maximum drive energy, supporting our model for the mass detection limit.
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07.10.Cm Micromechanical devices and systems
06.30.Dr Mass and density
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Electrical characteristics of amorphous carbon nanotube and effects of contacts

Y. F. Hu, X. L. Liang, Q. Chen, L.-M. Peng, and Z. D. Hu

Appl. Phys. Lett. 88, 063113 (2006); http://dx.doi.org/10.1063/1.2172237 (3 pages) | Cited 4 times

Online Publication Date: 8 February 2006

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Amorphous-carbon nanotube (a-CNT) based devices have been fabricated and transport measurements on these devices have been carried out. It was found that the Schottky barrier formed between the metal contact and a-CNT plays a significant role in the transport of a-CNT devices, and electrical characteristics of these devices may be engineered by varying the characteristics of the contact. In particular for asymmetrically contacted a-CNT devices it was demonstrated that the introduction of hydrogen may increase the current by more than 14 times at a positive bias of 3 V.
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85.35.Kt Nanotube devices
73.40.Ns Metal-nonmetal contacts

Shape and spatial correlation control of InAs-InAlAs-InP (001) nanostructure superlattices

W. Lei, Y. H. Chen, P. Jin, X. L. Ye, Y. L. Wang, B. Xu, and Z. G. Wang

Appl. Phys. Lett. 88, 063114 (2006); http://dx.doi.org/10.1063/1.2172288 (3 pages) | Cited 8 times

Online Publication Date: 8 February 2006

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The control of shape and spatial correlation of InAs-InAlAs-InP(001) nanostructure superlattices has been realized by changing the As overpressure during the molecular-beam epitaxy (MBE) growth of InAs layers. InAs quantum wires (QWRs) are obtained under higher As overpressure (1×10−5 Torr), while elongated InAs quantum dots (QDs) are formed under lower As overpressure (5×10−6 or 2.5×10−6 Torr). Correspondingly, spatial correlation changes from vertical anti-correlation in QWR superlattices to vertical correlation in QD superlattices, which is well explained by the different alloy phase separation in InAlAs spacer layers triggered by the InAs nanostrcutures. It was observed that the alloy phase separation in QD superlattices could extend a long distance along the growth direction, indicating the vertical correlation of QD superlattices can be kept in a wide range of spacer layer thickness.
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68.65.Cd Superlattices
68.65.La Quantum wires (patterned in quantum wells)
68.65.Hb Quantum dots (patterned in quantum wells)
61.46.-w Structure of nanoscale materials
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
64.75.-g Phase equilibria

Propagation of surface plasmons on Ag and Cu extended one-dimensional arrays on silicon substrates

Sri Priya Sundararajan, Jennifer Marie Steele, and Naomi J. Halas

Appl. Phys. Lett. 88, 063115 (2006); http://dx.doi.org/10.1063/1.2172289 (3 pages) | Cited 5 times

Online Publication Date: 8 February 2006

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Propagating surface plasmon waves can be supported by Cu and Ag periodic array, or grating, structures on silicon substrates. The plasmon dispersion characteristics, such as group velocity and bandgap associated with these structures are measured experimentally. In the infrared region of the spectrum (1.3–1.6 μm) the properties of surface plasmons supported by these Ag and Cu periodic structures are virtually indistinguishable. The plasmon dispersion can be modified by varying either the grating period or the plasmon order. The plasmonic bandgap for this array geometry increases with increasing plasmon order.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.68.+m Optical properties of surfaces
78.30.Er Solid metals and alloys
71.20.Be Transition metals and alloys

Nanogaps by direct lithography for high-resolution imaging and electronic characterization of nanostructures

Michael D. Fischbein and Marija Drndić

Appl. Phys. Lett. 88, 063116 (2006); http://dx.doi.org/10.1063/1.2172292 (3 pages) | Cited 27 times

Online Publication Date: 8 February 2006

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We report a method for fabricating nanogaps directly with electron beam lithography (EBL). The primary resolution-limit of EBL, electron back-scattering, is reduced dramatically by using a thin-film as a substrate. We show that this resolution enhancement allows one to fabricate metal electrodes with separation from arbitrarily large to under one nanometer. Furthermore, because these nanogaps are on a thin film, they can be imaged with high-resolution transmission electron microscopy (HRTEM). Using these nanogaps we measured the charge transport through several coupled PbSe nanocrystals and correlated the data with detailed structural information obtained by performing HRTEM on the same device.
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81.16.Nd Micro- and nanolithography
61.46.Hk Nanocrystals
68.37.Lp Transmission electron microscopy (TEM)

Formation of octahedral FePt nanoparticles by alternate deposition of FePt and MgO

T. Shima, K. Takanashi, Y. K. Takahashi, and K. Hono

Appl. Phys. Lett. 88, 063117 (2006); http://dx.doi.org/10.1063/1.2172710 (3 pages) | Cited 22 times

Online Publication Date: 10 February 2006

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Two-dimensional assemblies of octahedral-shaped FePt nanoparticles embedded in a MgO matrix were formed by alternate deposition of FePt and MgO layers on MgO (001) single-crystal substrates with a thermal cycling process. Highly ordered L10 structure with the [001] orientation perpendicular to the plane and large coercivity of more than 60 kOe were obtained. Cross-sectional transmission electron microscopy observation revealed that the equilibrium crystal shape of L10 ordered FePt particles was octahedral structure due to the anisotropy of the surface energy. The {111} plane shows the minimum surface energy in the L10 structure, which is similar to the case of face-centered-cubic structure.
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81.07.Bc Nanocrystalline materials
75.50.Tt Fine-particle systems; nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.37.Lp Transmission electron microscopy (TEM)
68.35.Md Surface thermodynamics, surface energies

Tunable and augmented plasmon resonances of Au/SiO2/Au nanodisks

K. H. Su, Q. H. Wei, and X. Zhang

Appl. Phys. Lett. 88, 063118 (2006); http://dx.doi.org/10.1063/1.2172712 (3 pages) | Cited 38 times

Online Publication Date: 10 February 2006

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The plasmon resonance of Au/SiO2 multilayered nanodisks was studied using light scattering spectroscopy and numerical calculations. Compared to single layered Au nanodisks, multilayered nanodisks exhibit several distinctive properties including significantly enhanced plasmon resonances and tunable resonance wavelengths which can be tailored to desired values by simply varying dielectric layer thickness while the particle diameter is kept constant. Numerical calculations show that slicing one metal layer into metal multilayers leads to higher scattering intensity and more “hot spots,” or regions of strong field enhancement. This tunable and augmented plasmon resonance holds a great potential in the applications of surface-enhanced Raman scattering (SERS).
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.35.+c Brillouin and Rayleigh scattering; other light scattering
78.30.-j Infrared and Raman spectra
78.68.+m Optical properties of surfaces

Dedicated fabrication of silicon-based ensembles of dot molecules with a specific and unique number of dots

M. Hanke, T. Boeck, A. -K. Gerlitzke, F. Syrowatka, and F. Heyroth

Appl. Phys. Lett. 88, 063119 (2006); http://dx.doi.org/10.1063/1.2173216 (3 pages) | Cited 4 times

Online Publication Date: 10 February 2006

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We have performed a two-step liquid phase epitaxy yielding ensembles of SiGe/Si(001) dot molecules with a specific and unique number of dots. An undersaturation of the initial bismuth solution causes strain-induced pits in the epitaxial Si0.985Ge0.015 layer which are effectively preserved during subsequent Si0.68Ge0.32 dot growth at considerably lower temperatures. Since the latter process happens extremely close to thermodynamic equilibrium, we are able to interrupt it after the formation of ensembles of dimers, trimers or quadruplets, respectively. The crosslike ensemble symmetry is discussed in terms of strain energy distribution as revealed by finite element calculations.
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81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
81.05.Hd Other semiconductors
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