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14 Dec 2009

Volume 95, Issue 24, Articles (24xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 95, 242102 (2009); http://dx.doi.org/10.1063/1.3272858 (3 pages)

W. H. Lim, F. A. Zwanenburg, H. Huebl, M. Möttönen, K. W. Chan, A. Morello, and A. S. Dzurak
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Depth resolved luminescence from oriented ZnO nanowires

R. A. Rosenberg, M. Abu Haija, K. Vijayalakshmi, J. Zhou, S. Xu, and Z. L. Wang

Appl. Phys. Lett. 95, 243101 (2009); http://dx.doi.org/10.1063/1.3275000 (3 pages) | Cited 11 times

Online Publication Date: 14 December 2009

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We have utilized the limited penetration depth of x-rays to study the near-surface properties of vertically aligned ZnO nanowires. For an energy of 600 eV the penetration depth varies between 3 and 132 nm as the incidence angle changes from 2° to 33°. Thus, by obtaining optical luminescence spectra as a function of incidence angle, it is possible to probe the near-surface region with nanometer-scale resolution. We will present angle dependent optical luminescence data from oriented ZnO nanowires. By fitting the results to a simple model, we extract a depth for the surface defect regions of ∼ 14 nm.
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78.55.Et II-VI semiconductors
78.67.Uh Nanowires

Self-organized quantum dot arrays: Kinetic mapping of adatom capture

I. Levchenko and K. Ostrikov

Appl. Phys. Lett. 95, 243102 (2009); http://dx.doi.org/10.1063/1.3273369 (3 pages) | Cited 7 times

Online Publication Date: 14 December 2009

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Deterministic synthesis of self-organized quantum dot arrays for renewable energy, biomedical, and optoelectronic applications requires control over adatom capture zones, which are presently mapped using unphysical geometric tessellation. In contrast, the proposed kinetic mapping is based on simulated two-dimensional adatom fluxes in the array and includes the effects of nucleation, dissolution, coalescence, and process parameters such as surface temperature and deposition rate. This approach is generic and can be used to control the nanoarray development in various practical applications.
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68.65.Hb Quantum dots (patterned in quantum wells)
64.75.Bc Solubility
61.46.-w Structure of nanoscale materials

Inducing chalcogenide phase change with ultra-narrow carbon nanotube heaters

Feng Xiong, Albert Liao, and Eric Pop

Appl. Phys. Lett. 95, 243103 (2009); http://dx.doi.org/10.1063/1.3273370 (3 pages) | Cited 10 times

Online Publication Date: 14 December 2009

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Carbon nanotube (CNT) heaters with sub-5 nm diameter induce highly localized phase change in Ge2Sb2Te5 (GST) chalcogenide. A significant reduction in resistance of test structures is measured as the GST near the CNT heater crystallizes. Effective GST heating occurs at currents as low as 25 μA, significantly lower than in conventional phase change memory with metal electrodes (0.1–0.5 mA). Atomic force microscopy reveals nucleation sites associated with phase change in GST around the CNT heater. Finite element simulations confirm electrical characteristics consistent with the experiments, and reveal the current and phase distribution in GST.
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68.55.ag Semiconductors
68.37.Ps Atomic force microscopy (AFM)
85.35.Kt Nanotube devices
73.61.Jc Amorphous semiconductors; glasses

Time dependent thermoelectric performance of a bundle of silicon nanowires for on-chip cooler applications

Gang Zhang, Qing-Xin Zhang, Devi Kavitha, and Guo-Qiang Lo

Appl. Phys. Lett. 95, 243104 (2009); http://dx.doi.org/10.1063/1.3273869 (3 pages) | Cited 5 times

Online Publication Date: 14 December 2009

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With finite element simulation, the time dependent thermoelectric performance of silicon nanowires (SiNWs) is studied systematically. Short response time has been observed in SiNW cooler which decreases with increasing of the number of SiNWs. Moreover, the impacts of inhomogeneous thermal conductivity distribution in one bundle on the cooling temperature have been studied. The cooling temperature decreases due to the existing of unexpected high thermal conductivity SiNW. This impact can be suppressed in large system. Our results provide a comprehensive performance analysis of SiNWs for on-chip thermoelectric cooler applications.
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72.20.Pa Thermoelectric and thermomagnetic effects
66.70.Df Metals, alloys, and semiconductors
73.63.-b Electronic transport in nanoscale materials and structures
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)

Influence of metal work function on the position of the Dirac point of graphene field-effect transistors

Noejung Park, Bum-Kyu Kim, Jeong-O Lee, and Ju-Jin Kim

Appl. Phys. Lett. 95, 243105 (2009); http://dx.doi.org/10.1063/1.3274039 (3 pages) | Cited 6 times

Online Publication Date: 14 December 2009

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We studied the effect of metal contact on the position of the Dirac point by means of transport measurements. To determine the sole effect of metal contact, we prepared more than 100 graphene devices following the same fabrication procedure and with a device layout that differed only in the type of metal electrode used. By measuring the peak position of the resistance, the Dirac points (VgDirac) were recorded in the gate response. The work function of metal-graphene complex was found to be a fair phenomenological indicator of the location of VgDirac in the transfer response.
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85.30.Tv Field effect devices

Site-controlled InAs quantum dots grown on a 55 nm thick GaAs buffer layer

T. J. Pfau, A. Gushterov, J. P. Reithmaier, I. Cestier, G. Eisenstein, E. Linder, and D. Gershoni

Appl. Phys. Lett. 95, 243106 (2009); http://dx.doi.org/10.1063/1.3265918 (3 pages) | Cited 5 times

Online Publication Date: 14 December 2009

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We present site-controlled low density InAs quantum dots grown by molecular beam epitaxy with a template based overgrowth technique allowing enlarged buffer layers upto 55 nm. Growing a seeding layer of InAs quantum dots in etched holes reduces closing of the holes, so that a second layer of InAs quantum dots can be aligned to the holes after a buffer layer overgrowth. Confocal microphotoluminescence measurements show a significant decrease of the low temperature photoluminescence linewidth of the quantum dots to an average value of ∼ 500 μeV and a minimum width of 460 μeV. This is to be compared to 2 to 4 meV of quantum dots grown on thin buffer layers. This improvement is due to the enlarged distance to residual defects at the overgrown surface.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors

Independent tuning of quantum dots in a photonic crystal cavity

Hyochul Kim, Susanna M. Thon, Pierre M. Petroff, and Dirk Bouwmeester

Appl. Phys. Lett. 95, 243107 (2009); http://dx.doi.org/10.1063/1.3275002 (3 pages) | Cited 4 times

Online Publication Date: 15 December 2009

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One of the main obstacles to coupling two quantum dots (QDs) to a single nanocavity mode in a cavity quantum electrodynamics system is the ability to independently tune the QD frequencies. We demonstrate that in a GaAs photonic crystal membrane structure with two embedded QD layers, the QD emission frequencies of one QD layer can be tuned independently of the other by applying a voltage across only one of the QD layers.
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42.70.Qs Photonic bandgap materials
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Strong size-dependent characteristics of carrier injection in quantum-confined silicon nanocrystals

Chang-Hee Cho, Sang-Kyun Kim, Baek-Hyun Kim, and Seong-Ju Park

Appl. Phys. Lett. 95, 243108 (2009); http://dx.doi.org/10.1063/1.3273861 (3 pages) | Cited 2 times

Online Publication Date: 15 December 2009

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We report the strong size-dependent carrier injection process in quantum-confined silicon nanocrystals embedded in silicon nitride films. As the diameter of silicon nanocrystals increases, the threshold voltage for carrier injection decreases whereas the number of injected carriers increases due to the quantum size effect. The tunneling time for the carrier injection is decreased by two orders of magnitude when the diameter of silicon nanocrystals is increased from 3.4 to 5.0 nm, and this is attributed to the enhanced nonresonant tunneling in the larger silicon nanocrystals.
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81.05.Cy Elemental semiconductors
84.32.Tt Capacitors
73.40.Gk Tunneling
73.61.Cw Elemental semiconductors
72.20.-i Conductivity phenomena in semiconductors and insulators
81.07.Bc Nanocrystalline materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Carbon nanofiber supercapacitors with large areal capacitances

James R. McDonough, Jang Wook Choi, Yuan Yang, Fabio La Mantia, Yuegang Zhang, and Yi Cui

Appl. Phys. Lett. 95, 243109 (2009); http://dx.doi.org/10.1063/1.3273864 (3 pages) | Cited 22 times

Online Publication Date: 15 December 2009

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We develop supercapacitor (SC) devices with large per-area capacitances by utilizing three-dimensional (3D) porous substrates. Carbon nanofibers (CNFs) functioning as active SC electrodes are grown on 3D nickel foam. The 3D porous substrates facilitate a mass loading of active electrodes and per-area capacitance as large as 60 mg/cm2 and 1.2 F/cm2, respectively. We optimize SC performance by developing an annealing-free CNF growth process that minimizes undesirable nickel carbide formation. Superior per-area capacitances described here suggest that 3D porous substrates are useful in various energy storage devices in which per-area performance is critical.
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84.60.Ve Energy storage systems, including capacitor banks
84.32.Tt Capacitors
81.05.Rm Porous materials; granular materials
81.05.U- Carbon/carbon-based materials

Ab initio study of noncovalent sidewall functionalization of carbon nanotubes

Seokho Lim and Noejung Park

Appl. Phys. Lett. 95, 243110 (2009); http://dx.doi.org/10.1063/1.3274041 (3 pages) | Cited 5 times

Online Publication Date: 16 December 2009

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We investigated noncovalent molecular adsorptions onto carbon nanotubes (CNTs), using density-functional theory methods including recently developed hybrid functionals. Planar aromatic molecules have greater binding strength than fully saturated nonaromatic molecules, and adsorption geometries are consistent with the features of intermolecular π−π stacking. We show that modifications with alcohol or a thiol terminal group increase the adsorption strength consistently throughout the considered molecules. This result may be used in experiments involving noncovalent sidewall functionalizations of CNTs.
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68.43.Mn Adsorption kinetics
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Tx Fullerenes and related materials; intercalation compounds
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems

Chemical composition anomalies of nanotubes stretched from Au-Si(Ge) liquid alloy by an intense electric field

P. Joyes, J. Van de Walle, and R.-J. Tarento

Appl. Phys. Lett. 95, 243111 (2009); http://dx.doi.org/10.1063/1.3275011 (3 pages)

Online Publication Date: 17 December 2009

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Nanotubes are stretched by intense electric fields (few volts per angstrom) supplied by an eutectic Au0.73Ge0.27 or Au0.82Si0.18 alloy liquid metal ion source. Chemical composition anomalies are observed in the stretched nanotubes. In the Au-Ge system, nanotubes are the smaller two-shell pure gold nanowires. In the Au-Si system, nanotubes have same geometric structures as previously but now silicon atoms can substitute gold atoms. Nanotubes are mainly formed by SiAu3 alloy zones separated by smaller pure gold parts. Our results are consistent with local modifications of the chemical composition of the alloy liquid in the nanotube vicinity submitted to intense electric field gradients.
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61.46.Np Structure of nanotubes (hollow nanowires)
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Gas sensing properties of single crystalline porous silicon nanowires

Kui-Qing Peng, Xin Wang, and Shuit-Tong Lee

Appl. Phys. Lett. 95, 243112 (2009); http://dx.doi.org/10.1063/1.3275794 (3 pages) | Cited 18 times

Online Publication Date: 18 December 2009

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We demonstrate that the porous silicon nanowires (SiNWs) prepared by metal-assisted chemical etching method could impart sensitivity of nanowire electrical properties to gaseous nitrogen oxide (NO) at room temperature, thus are suitable for sensing NO and air monitoring. Particularly, the sensors made from the porous SiNWs assembly showed fast response and excellent reversibility to subparts per million NO concentrations. The excellent sensing performance coupled with scalable synthesis of porous SiNWs could open up opportunities in scalable production of sensor chips working at room temperature.
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81.16.Be Chemical synthesis methods
81.65.Cf Surface cleaning, etching, patterning
73.63.Nm Quantum wires
81.07.Gf Nanowires
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
81.05.Cy Elemental semiconductors

Design guidelines of periodic Si nanowire arrays for solar cell application

Junshuai Li, HongYu Yu, She Mein Wong, Xiaocheng Li, Gang Zhang, Patrick Guo-Qiang Lo, and Dim-Lee Kwong

Appl. Phys. Lett. 95, 243113 (2009); http://dx.doi.org/10.1063/1.3275798 (3 pages) | Cited 35 times

Online Publication Date: 18 December 2009

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In this letter, optimum periodic Si nanowire (SiNW) arrays are designed via simulation for solar cell application, in terms of the structural parameters, e.g., the array periodicity (P) and SiNW diameter (D). It is found that the more efficient light absorption compared to that of the Si thin film with the same thickness could be realized when P is between 250 and 1200 nm. Further, the ratio of D to P should be >0.5 (or more specifically ∼ 0.8) for the optimized solar energy harvesting. The underlying physics is also discussed in this work.
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81.07.Gf 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)
88.40.jj Silicon solar cells
84.60.-h Direct energy conversion and storage
81.16.-c Methods of micro- and nanofabrication and processing

Coherent electronic grating cavity modes in corrugated ultrathin metal films

Y. Liu, T. Miller, and T.-C. Chiang

Appl. Phys. Lett. 95, 243114 (2009); http://dx.doi.org/10.1063/1.3275865 (3 pages)

Online Publication Date: 18 December 2009

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Stepped semiconductor substrates can serve as a template for growing metal films with a corrugated structure at the nanoscale. The corrugation modulates the electron motion and thus modifies the electronic properties of the system. This work is a study of corrugated films of Ag and Pb using angle-resolved photoemission. Our measurements show that the valence electrons form coherent grating cavity modes defined by the corrugation geometry and the photoemission patterns are governed by quantum interference.
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71.20.Be Transition metals and alloys
79.60.Dp Adsorbed layers and thin films
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