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15 Jul 2007

Volume 102, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 102, 021101 (2007); http://dx.doi.org/10.1063/1.2759200 (24 pages)

Yuri B. Melnichenko and George D. Wignall
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Room-temperature ferromagnetism of Mn doped ZnO aligned nanowire arrays with temperature dependent growth

J. J. Liu, K. Wang, M. H. Yu, and W. L. Zhou

J. Appl. Phys. 102, 024301 (2007); http://dx.doi.org/10.1063/1.2753589 (6 pages) | Cited 10 times

Online Publication Date: 16 July 2007

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Aligned one-dimensional diluted magnetic semiconductor Zn1−xMnxO nanowires were synthesized from a reaction of metallic Zn foil and MnCl2⋅6H2O under oxygen environment at variant temperatures between 750 and 950 °C by a chemical vapor deposition method. The c-axis preferentially grown nanowire arrays are single crystalline wurtzite structure, of which the growing temperature has a significant influence on both morphology and magnetic ordering. Nanowires with the highest aspect ratios were grown at 850 °C, whereas nanowires presenting largest room-temperature ferromagnetism were formed at 950 °C. More Mn2+ substitution in the ZnO lattice was observed at 950 °C, resulting in strong room-temperature ferromagnetism with a saturation magnetization of 0.25 emu/g. At synthesis temperatures of 750 and 850 °C, formation of a ZnMn2O4 room-temperature paramagnetic second phase was found. The nanostructures with different aspect ratios were obtained with the variation of synthesis temperature. The temperature dependent growth of aligned Zn1−xMnxO nanowires reveals strong room-temperature ferromagnetism occurs in the nanowire arrays synthesized at high temperature. The nanowires with strong room temperature have great potential in spintronic nanodevice application.
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75.75.-c Magnetic properties of nanostructures
75.50.Dd Nonmetallic ferromagnetic materials
75.50.Pp Magnetic semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

Comparative study of the effects of thermal treatment on the optical properties of hydrogenated amorphous silicon-oxycarbide

Spyros Gallis, Vasileios Nikas, Mengbing Huang, Eric Eisenbraun, and Alain E. Kaloyeros

J. Appl. Phys. 102, 024302 (2007); http://dx.doi.org/10.1063/1.2753572 (9 pages) | Cited 11 times

Online Publication Date: 17 July 2007

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Findings are presented from a systematic study of the effects of postdeposition thermal treatment on the optical characteristics of hydrogenated amorphous silicon-oxycarbide (a-SiCxOyHz) materials. Three different classes of a-SiCxOyHz films: SiC-like (SiC1.08O0.07H0.21), Si-C-O (SiC0.50O1.20H0.22), and SiO2-like (SiC0.20O1.70H0.24), were deposited by thermal chemical vapor deposition. The effects of thermal annealing on the compositional and optical properties of the resulting films were characterized using Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, nuclear reaction analysis, and spectroscopic ultraviolet-visible ellipsometry. As the Si-C-O system evolved from a SiC-like to SiO2-like matrix, its refractive index and optical absorption strength decreased, while its optical band gap increased. Thermal annealing between 500 and 1100 °C resulted in hydrogen desorption from and densification of the a-SiCxOyHz films. Concurrently, thermally induced changes were also observed for the optical properties of the films, as evidenced by an increase in film refractive index and an accompanying decrease in optical gap. These changes are analyzed in the context of the underlying physical processes, particularly modifications in the electronic configuration (bonding) and hydrogen desorption mechanisms. Furthermore, based on the observed structural and optical properties of the thermally treated a-SiCxOyHz films, the Si-C-O matrix was employed in the successful development of an Er-doped Si-C-O system with efficient Er excitation and strong room-temperature photoluminescence emission around 1540 nm within a broad (460–600 nm) excitation band. As such, a-Si-C-O represents a material system that provides considerably efficient energy transfer mechanisms at the same Er concentration level than previously investigated Si-based materials.
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78.66.-w Optical properties of specific thin films
78.55.Qr Amorphous materials; glasses and other disordered solids
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.40.Pg Disordered solids
78.35.+c Brillouin and Rayleigh scattering; other light scattering
81.40.Tv Optical and dielectric properties related to treatment conditions

Subnanometer surface roughness of dc magnetron sputtered Al films

D. L. Rode, V. R. Gaddam, and Ji Haeng Yi

J. Appl. Phys. 102, 024303 (2007); http://dx.doi.org/10.1063/1.2756039 (8 pages) | Cited 4 times

Online Publication Date: 18 July 2007

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The surface roughness of dc magnetron sputtered Al films has been characterized as a function of (a) substrate deposition temperature from 54 to 223 °C, (b) argon sputtering pressure from 1.0 to 9.0 mTorr, and (c) dc magnetron power from 140 to 3590 W corresponding to (d) deposition rates ranging from 0.28 to 7.7 nm/s. The surface roughness of Al films ranging in thickness from 440 to 650 nm has been analyzed by use of atomic force microscopy. Over the entire range of deposition conditions, the mean surface roughness varies from as large as 99.6 nm to as small as 0.58 nm. The films range in visual appearance from “milky” to completely specular to the unaided eye under high-intensity normally incident white-light illumination. Optimization of sputtering conditions resulted in the deposition of very smooth 460 nm thick films with mean surface roughness of 0.58 nm for a deposition temperature of 57 °C, 1.0 mTorr argon sputtering pressure, and 3590 W dc magnetron power. The results reported here represent some of the smallest mean surface roughness values ever reported for dc magnetron sputtered Al thin films.
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68.55.A- Nucleation and growth
81.15.Cd Deposition by sputtering
81.05.Bx Metals, semimetals, and alloys
68.55.-a Thin film structure and morphology
68.37.Ps Atomic force microscopy (AFM)

First-principles study of the effects of polytype and size on energy gaps in SiC nanoclusters

X.-H. Peng, S. K. Nayak, A. Alizadeh, K. K. Varanasi, N. Bhate, L. B. Rowland, and S. K. Kumar

J. Appl. Phys. 102, 024304 (2007); http://dx.doi.org/10.1063/1.2756047 (5 pages) | Cited 10 times

Online Publication Date: 18 July 2007

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We have studied the band-gap variation and stability energy in silicon carbide (SiC) nanoclusters of different polytypes using density functional theory (DFT) based on a gradient-corrected approximation. We have obtained a series of spherical SiC nanoclusters with dimensions up to 2 nm from bulk 2H, 3C, and 4H polytype crystals. All clusters with diameters smaller than 1 nm exhibit similar energy-gap-size variations, while energy gaps for clusters larger than 1 nm show a distinct size dependence with different polytypes and approach their bulk gaps with an increase in cluster size. In contrast to their bulk behavior, the binding energy difference between polytypes of clusters within the diameter range 0.5 nm−2 nm is found to be negligible, suggesting that the problems associated with the synthesis of polytypes of SiC in bulk may disappear for small clusters. The convergence of the energy gap and binding energy with different polytypes at small size clusters and the transition between the clusters to bulk behavior in SiC systems could be exploited for making future nano-optoelectronics devices.
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73.22.-f Electronic structure of nanoscale materials and related systems
61.46.-w Structure of nanoscale materials
71.15.Nc Total energy and cohesive energy calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Scanning thermal imaging of an electrically excited aluminum microstripe

Benjamin Samson, Lionel Aigouy, Rossella Latempa, Gilles Tessier, Marco Aprili, Michel Mortier, Jérôme Lesueur, and Danièle Fournier

J. Appl. Phys. 102, 024305 (2007); http://dx.doi.org/10.1063/1.2756088 (8 pages) | Cited 4 times

Online Publication Date: 19 July 2007

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We study the Joule heating of a 1.25 μm wide aluminum microstripe excited by an electrical current. The temperature changes are measured with a scanning thermal microscope that uses a small fluorescent particle as a sensor. The lateral resolution observed for this sample is better than 300 nm. We have compared the temperature distribution in the stripe with a simple analytical model of heat propagation in the wire and the substrate. A good qualitative agreement is observed, although the measured temperature is much smaller than the estimated one, showing that the heat transfer between the hot wire and the fluorescent probe is not fully efficient.
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47.27.te Turbulent convective heat transfer
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
78.55.Hx Other solid inorganic materials

Cross-sectional scanning tunneling microscopy of biased semiconductor lasers

R. J. Cobley, K. S. Teng, M. R. Brown, and S. P. Wilks

J. Appl. Phys. 102, 024306 (2007); http://dx.doi.org/10.1063/1.2757006 (7 pages) | Cited 2 times

Online Publication Date: 19 July 2007

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Cross-sectional scanning tunneling microscopy is applied to semiconductor lasers which are biased and producing light. Two device structures are investigated—a double quantum well laser and a buried heterostructure device with two-dimensional growth variation. Scanning tunneling microscopy (STM) images are collected as a function of sample drive bias. Changes that occur in the STM image as a result of powering the sample are observed, which are due to changes in the energy band structure and the carrier concentration in the device as it responds to bias. The observed changes are largely reversible and a model is presented which matches and confirms this behavior. Once these effects are confirmed and decoupled, the technique can be used to study device-specific behavior resulting from physical changes in lasers as they are operated.
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42.55.Px Semiconductor lasers; laser diodes
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Dynamic force microscopy study of the Ga-rich c(8×2) and As-rich c(4×4) reconstructions of the GaAs(001) surface

Shigeki Kawai, Franck Rose, Takanori Ishii, Shiro Tsukamoto, and Hideki Kawakatsu

J. Appl. Phys. 102, 024307 (2007); http://dx.doi.org/10.1063/1.2757001 (6 pages) | Cited 1 time

Online Publication Date: 23 July 2007

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As-rich and Ga-rich GaAs(001) surfaces were studied with frequency-modulation dynamic force microscopy. By simply changing the parameters of argon sputtering and annealing during sample preparation, surfaces reconstructed with the As-rich c(4×4) phase or the Ga-rich c(8×2) phase could be obtained. True atomic resolution of the c(8×2) reconstruction is achieved by using constant frequency shift imaging. We show that tip functionalization allows selective species imaging. The presence at the tip apex of empty Ga dangling bonds or of fully filled As dangling bonds leads to selective atomic resolution of the As or Ga sublattices of the c(8×2) reconstructed surface, respectively. Our observations support the ζ model for the c(8×2) reconstruction (but no dimers were found) and the α model for the c(8×2) reconstruction (individual As–As dimers were resolved by dynamic force microscopy).
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68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)

Correlation between surface composition and luminescence of nanocrystalline silicon particles dispersed in pure water

Masaki Hiruoka, Keisuke Sato, and Kenji Hirakuri

J. Appl. Phys. 102, 024308 (2007); http://dx.doi.org/10.1063/1.2756048 (4 pages) | Cited 4 times

Online Publication Date: 23 July 2007

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The stability of the luminescene of nanocrystalline silicon (nc-Si) particles passivated with a number of different elements, including hydrogen, carbon, and oxygen, has been investigated in pure water. Each sample emitted red light with a peak wavelength in the range of 740–800 nm. The intensity of red luminescence decreased after a short period of time when the hydrogen- and/or carbon-passivated samples were immersed in pure water. Further, the peak wavelength concomitantly shifted toward a shorter wavelength. These effects were attributed to the generation of defects (Pb centers), the reduction in particle size due to the desorption of hydrogen and/or carbon atoms, and the replacement of the Si–H and/or Si–C bonds, respectively, with Si-O bonds on the surface of nc-Si particles. On the other hand, the oxygen-passivated samples showed stable luminescence in addition to a slight blueshift of the peak wavelength upon immersion in pure water for 400 h. This stability was attributed to the development of stable surface conditions. These results are a strong indication that the stability of luminescence in pure water can be remarkably improved by oxygen passivation on the surface of nc-Si particles.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Ap Elemental semiconductors
68.35.Dv Composition, segregation; defects and impurities
81.65.Rv Passivation
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
68.43.Mn Adsorption kinetics

Depth profile investigations of silicon nanocrystals formed in sapphire by ion implantation

S. Yerci, I Yildiz, M. Kulakci, U. Serincan, M. Barozzi, M. Bersani, and R. Turan

J. Appl. Phys. 102, 024309 (2007); http://dx.doi.org/10.1063/1.2756622 (5 pages) | Cited 3 times

Online Publication Date: 24 July 2007

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Depth profiles of Si nanocrystals formed in sapphire by ion implantation and the effect of charging during X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) measurements have been studied. Atomic concentration and the chemical environment of Si, Al, and O have been measured as a function of depth from the sample surface by SIMS and XPS. Both as-implanted and annealed samples have been analyzed to understand the effect of nanocrystal formation on the depth distribution, chemical structure, and the charging effect before and after the formation process. SIMS measurements have revealed that the peak position of the Si concentration shifts to deeper values with implantation dose. This is explained by the fact that the structure of the matrix undergoes a phase transformation from pure sapphire to a Si rich amorphous Al2O3 with heavy dose implantation. Formation of Si nanocrystals has been observed by XPS by an increase in the Si-Si signal and a decrease in Si-O bond concentrations after the annealing. Variation in binding energies of Si and O with Si concentration (i.e., with depth) has been studied in terms of chemical environments and charging effects. It is found that binding energy of these elements shifts to lower values with increasing Si content. This is a result of less charging due to the presence of easy discharge paths in the Si rich regions of the matrix. Nanocrystal formation leads to even less charging which is probably due to the further increase in conductivity with the formation.
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61.46.Hk Nanocrystals
61.80.Jh Ion radiation effects
78.30.Am Elemental semiconductors and insulators
64.75.-g Phase equilibria
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Electron beam machining of nanometer-sized tips from multiwalled boron nitride nanotubes

Ayten Celik-Aktas, James F. Stubbins, and Jian-Min Zuo

J. Appl. Phys. 102, 024310 (2007); http://dx.doi.org/10.1063/1.2757007 (5 pages) | Cited 9 times

Online Publication Date: 24 July 2007

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We report here that high energy electron irradiation of multiwalled boron nitride nanotubes can be used to form sharp, crystalline, conical tips, or to cut boron nitride nanotubes by controlling the electron beam size. Electron beam cutting is observed when a focused electron beam with a diameter much smaller than the tube diameter is used. The tip formation is observed when a shaped, disklike, electron beam is used to irradiate the tube; the diameter of the beam in this case is similar to the tube diameter. In situ electron microscopy observation shows that the tip formation effect is driven by layer peeling and the collapse of the inner walls of the nanotube. This is very different from the formation of nanoarches observed during cutting. The combination of shaping and cutting can be used to fabricate atomically sharp tips for field emitters, nanoimaging, and manipulations.
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81.16.-c Methods of micro- and nanofabrication and processing

GaN quantum dots grown on AlxGa1−xN layer by plasma-assisted molecular beam epitaxy

Y. Hori, O. Oda, E. Bellet-Amalric, and B. Daudin

J. Appl. Phys. 102, 024311 (2007); http://dx.doi.org/10.1063/1.2759168 (6 pages) | Cited 2 times

Online Publication Date: 25 July 2007

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We report on the growth of GaN quantum dots on an AlxGa1−xN layer. Taking advantage of the delayed strain relaxation of AlGaN on an AlN template, we were able to grow GaN quantum dots on an AlGaN layer with Al content as low as 34%. Real-time monitoring of the variations of the in-plane lattice parameter revealed that the growth of the self-organized GaN quantum dots depended not only on the in-plane lattice mismatch but also on the chemical composition of the underlying layer. The morphological properties of the GaN quantum dots were studied by atomic force microscopy. The size distribution of the quantum dots varied from bimodal to monomodal at 80% Al content. Monomodal quantum dots decreased the aspect ratio when the Al content of the AlGaN layer decreased, which is consistent with a reduced elastic relaxation compensated for by a decrease of interfacial energy.
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81.07.Ta Quantum dots
81.05.Ea III-V semiconductors
68.35.Md Surface thermodynamics, surface energies
68.65.Hb Quantum dots (patterned in quantum wells)
81.40.Jj Elasticity and anelasticity, stress-strain relations
82.80.-d Chemical analysis and related physical methods of analysis

Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics

K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand

J. Appl. Phys. 102, 024312 (2007); http://dx.doi.org/10.1063/1.2759195 (6 pages) | Cited 14 times

Online Publication Date: 25 July 2007

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Eu3+-doped transparent nano-glass-ceramics, 32(SiO2)9(AlO1.5)31.5(CdF2)18.5(PbF2)5.5(ZnF2):3.5(EuF3)mol %, have been prepared aiming at the investigation of the site structure of the Eu3+ rare-earth dopant. In this nano-glass-ceramic host, other rare-earth dopants, such as Er3+, Tm3+, Ho3+, and Dy3+, also have served as a dopant; however the Eu3+ ion is a preferably sensitive probe for a rare-earth dopant site structure due to its unique purely magnetic and purely electric dipole moment optical transitions involving the nondegenerate excited 5D0 and ground 7F0 states. The splitting in the respective emission and absorption spectra in the crystalline field of the nano-glass-ceramics and its parent glass has been studied. It has been concluded that the major site of the rare-earth dopant in the nano-glass-ceramics is nearly cubic, with an orthorhombic distortion according to the sequence Oh→D4h→D2h, and the majority of the rare-earth dopants substitute onto the Pb2+ site in the PbF2 nanocrystals embedded in the glass network. A vibronic contribution to the strength of the 5D0→7F2 electric dipole moment transition has been found and the respective dominating contributing vibration mode has been proposed to be and to change as a2u→b1u→au, according to lowering the site symmetry in a sequence Oh→D4h→D2h.
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81.05.Pj Glass-based composites, vitroceramics
81.07.-b Nanoscale materials and structures: fabrication and characterization
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
63.50.-x Vibrational states in disordered systems
75.30.Cr Saturation moments and magnetic susceptibilities
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Surface structures and electronic states of silicon nanotubes stabilized by oxygen atoms

Mingwen Zhao, R. Q. Zhang, and Yueyuan Xia

J. Appl. Phys. 102, 024313 (2007); http://dx.doi.org/10.1063/1.2752115 (5 pages) | Cited 3 times

Online Publication Date: 26 July 2007

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The geometric and electronic structures of silicon nanotubes stabilized by incorporating oxygen atoms were studied using first-principles calculations within density functional theory. The predicted tubes present one-dimensional characters stacked with n-side silicon polygons connected by oxygen atoms. The stable configurations considered in this work include the tubes with varied facet number of the silicon polygons (n) from n = 4 to 28 and of different surface structures. The configurations with n = 5, 12, 15, 18, and 21 were found energetically extremely favorable. All the tubes are narrow-band-gap semiconductors with the band gap varying between 0.17 and 0.84 eV, dependent on the surface structure of the tubes. This study provides an interesting route to stabilize silicon nanotubes and tune their electronic properties.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
73.22.-f Electronic structure of nanoscale materials and related systems
71.20.Mq Elemental semiconductors
61.46.Fg Nanotubes
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Evolution of self-assembled InAs quantum ring formation

Hong-Shi Ling and Chien-Ping Lee

J. Appl. Phys. 102, 024314 (2007); http://dx.doi.org/10.1063/1.2761799 (5 pages) | Cited 17 times

Online Publication Date: 27 July 2007

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The evolution of InAs quantum ring (QR) formation and the corresponding optical properties were investigated by atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. Just like a nanoscale volcanic eruption, the transformation from quantum dots (QDs) to QRs using the capping and annealing process depends on how much InAs is removed from the center of the dots to the surrounding areas. The final structure was found to depend on the annealing temperature and the cap layer thickness. We have investigated the QR formation at different stages using various growth conditions. The findings provided a clear picture on the mechanism of ring formation. We were able to obtain QRs with various geometries by controlling these growth parameters.
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78.67.Hc Quantum dots
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
81.16.Dn Self-assembly

Formation and growth of SnO2 nanoparticles in silica glass by Sn implantation and annealing

P. K. Kuiri, H. P. Lenka, J. Ghatak, G. Sahu, B. Joseph, and D. P. Mahapatra

J. Appl. Phys. 102, 024315 (2007); http://dx.doi.org/10.1063/1.2761778 (5 pages) | Cited 9 times

Online Publication Date: 27 July 2007

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Nanocrystalline Sn particles have been formed in silica glass through 50 keV Sn− implantation followed by annealing in N2 at 650 °C for 30 min. Samples prepared this way have been annealed in air for 1 h, separately at four different temperatures, 400, 600, 800, and 1000 °C, each at a given temperature. Annealing at temperatures higher than 400 °C has been found to result in oxidation of the Sn nanoparticles (NPs) and formation of the SnO2 phase as confirmed from optical absorption (OA), transmission electron microscopy, and Raman scattering measurements. For the sample annealed at 600 °C, Raman scattering data showed three bands at about 525, 629, and 771 cm−1, the last two corresponding to the A1g and B2g classical Raman modes of rutile SnO2. Increase in annealing temperature resulted in an increase in the intensities of the A1g and B2g modes showing better crystallinity. Also, the A1g peak shifted toward a higher wave number with a steady decrease in the intensity at 525 cm−1. This is in line with the growth in size of NPs as well as a reduction in the surface disorder. The Urbach tail width derived from the OA data also agrees with this.
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81.07.Bc Nanocrystalline materials
81.05.Hd Other semiconductors
81.16.Pr Micro- and nano-oxidation
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.72.Cc Kinetics of defect formation and annealing
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Room temperature operational single electron transistor fabricated by focused ion beam deposition

P. Santosh Kumar Karre, Paul L. Bergstrom, Govind Mallick, and Shashi P. Karna

J. Appl. Phys. 102, 024316 (2007); http://dx.doi.org/10.1063/1.2761837 (4 pages) | Cited 6 times

Online Publication Date: 27 July 2007

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We present the fabrication and room temperature operation of single electron transistors using 8 nm tungsten islands deposited by focused ion beam deposition technique. The tunnel junctions are fabricated using oxidation of tungsten in peracetic acid. Clear Coulomb oscillations, showing charging and discharging of the nanoislands, are seen at room temperature. The device consists of an array of tunnel junctions; the tunnel resistance of individual tunnel junction of the device is calculated to be as high as 25.13 GΩ. The effective capacitance of the array of tunnel junctions was found to be 0.499 aF, giving a charging energy of 160.6 meV.
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85.35.Gv Single electron devices
81.15.Jj Ion and electron beam-assisted deposition; ion plating
85.40.Sz Deposition technology

Cross linking of thiolated carbon nanotubes: An ab initio study

Igor Vasiliev and Seamus A. Curran

J. Appl. Phys. 102, 024317 (2007); http://dx.doi.org/10.1063/1.2759866 (5 pages) | Cited 4 times

Online Publication Date: 30 July 2007

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We have studied the mechanism of covalent cross linking between carbon nanotubes functionalized with thiocarboxylic and dithiocarboxylic esters. The interconnected nanotube structures were modeled using density functional theory combined with the pseudopotential approximation. Our calculations revealed the important role of surface defects when forming chemical bonds that connect nanotubes to each other. The strength and stability of intertube bonds increased in the vicinity of defect sites. The computed binding energies and potential energy profiles of linked nanotubes were found to be sensitive to the choice of exchange-correlation functional used within the density functional formalism. The observed sensitivity could be explained by a nonuniform distribution of the electronic charge density near defect sites. This result suggests that gradient-corrected functionals are essential for accurate theoretical modeling of functionalized carbon nanotubes and nanotube-based composites.
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61.46.Fg Nanotubes
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