Vertically standing Ge nanowires on GaAs(110) substrates

The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380?°C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the [Formula: see text] direction. Using this unique property, vertical [Formula: see text] Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the [Formula: see text] directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.     

Anisotropic interface induced formation of Sb nanowires on GaSb(111)A substrates

The growth of Sb nanowires on GaSb(111)A substrates is studied by in?situ azimuthal scan reflection high-energy electron diffraction (ARHEED). Bulk and layer contributions can be distinguished in the ARHEED transmission pattern through the Sb nanowires. The three-dimensional structure of the growing Sb nanowires is identified by post-growth atomic force microscopy (AFM) and x-ray diffraction (XRD). The lattice match of the Sb crystal along the [Formula: see text] and the GaSb crystal along [Formula: see text] directions lead to a preferential orientation of the Sb nanowires. The Sb adsorption and desorption kinetics is studied by thermal desorption spectroscopy.     

Longer InN phonon lifetimes in nanowires

We present a Raman scattering study of the anharmonic phonon decay of the [Formula: see text], [Formula: see text] and E(1)(LO) phonons in InN nanowires over the 80-400?K temperature range. While the temperature-dependent anharmonic decay in the nanowires is similar to that found for bulk InN, the background contribution to the phonon lifetime is strongly reduced as a result of the improved crystalline quality. High-resolution measurements reveal a remarkably long lifetime of the [Formula: see text] mode. From the comparison between the [Formula: see text] frequencies measured in the nanowires with those of the thin film we obtain the deformation potentials for the [Formula: see text] mode.     

Unique electronic band structures of hydrogen-terminated [Formula: see text] silicon nanowires

Band structure mutation from an indirect to a direct gap is a well-known character of small hydrogen-terminated [Formula: see text] and [Formula: see text] silicon nanowires (SiNWs), and suggests the possible emission of silicon. In contrast, we show that hydrogen-terminated [Formula: see text] SiNWs consistently present indirect band gaps even at an extremely small size, according to our calculations using density functional theory. Interestingly, the band gap of [Formula: see text] SiNWs shows a quasi-direct feature as the wire size increases, suggesting the possibility of using medium SiNWs in optoelectronic devices. This result also indicates that the electronic structures of SiNWs are strongly orientation dependent.     

Analogous mechanical behaviors in [Formula: see text] and [Formula: see text] directions of Cu nanowires under tension and compression at a high strain rate

This study analyzes the plastic deformation on the atomic scale of Cu nanowires (NWs) with [Formula: see text] and [Formula: see text] orientations during uniaxial tension and compression, using a molecular dynamic simulation. The maximum local stress (MLS) method is employed to evaluate mechanical behavior during deformation. Following yielding, the flow stress strongly depends on the variation in the degree of orientation caused by twinning. Both the tension of the [Formula: see text] NW and the compression of the [Formula: see text] NW cause twin deformation and consequent geometrical softening. In contrast, the compression of the [Formula: see text] NW and the tension of the [Formula: see text] NW form twin bands and cause geometrical hardening. These behaviors result in the stress-strain curves that reveal the pseudo-skew-symmetry characteristic. With respect to the difference between the critical resolved shear stress (τ(c)) associated with the distinct orientations, τ(c) depends strongly on the surface critical resolved stress (τ(sc)). Under tension, τ(sc) depends on the degree of lattice distortion. A larger lattice distortion (pre-tensile stress) corresponds to higher τ(sc). However, under compression, a geometrical factor can be used to describe the difference in τ(sc) between the different orientations. A larger geometrical factor corresponds to a larger τ(sc).     

Ag-assisted CBE growth of ordered InSb nanowire arrays

We present growth studies of InSb nanowires grown directly on [Formula: see text] and [Formula: see text] substrates. The nanowires were synthesized in a chemical beam epitaxy (CBE) system and are of cubic zinc blende structure. To initiate nanowire nucleation we used lithographically positioned silver (Ag) seed particles. Up to 87% of the nanowires nucleate at the lithographically pre-defined positions. Transmission electron microscopy (TEM) investigations furthermore showed that, typically, a parasitic InSb thin film forms on the substrates. This thin film is more pronounced for InSb((111)B) substrates than for InAs((111)B) substrates, where it is completely absent at low growth temperatures. Thus, using InAs((111)B) substrates and growth temperatures below 360?°C free-standing InSb nanowires can be synthesized.     

Preferred growth orientation of metallic fcc nanowires under direct and alternating electrodeposition conditions

Gold and copper nanowires were generated through electrochemical deposition into nanoporous polymeric templates. Depending on the growth conditions, such wires exhibited a distinct textured structure as evidenced by x-ray diffraction. The preferred growth orientation is explained by applying the broken-bond model in combination with surface-energy anisotropy and energy minimization. During the growth process, the aspect ratio of the cylindrical nanowire and thus the area of the mantle surface and its contribution to the total surface energy increase. Under direct current deposition conditions, [Formula: see text] textured metallic fcc nanowires represent the configuration of lowest surface energy at aspect ratios above?1. Under alternating current deposition conditions, {110} nanowire base surfaces vanish due to their high surface energy, leading to successive development of a [Formula: see text] texture as the configuration of lowest energy at aspect ratios above 5.     

Adsorption, manipulation and self-assembling of TBrPP-Co molecules on a Ag/Si(111) surface by scanning tunnelling microscopy

Individual adsorption and two-dimensional assembling of 5,10,15,20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecules on a Si(111)-[Formula: see text] Ag reconstructed surface have been studied using low-temperature scanning tunnelling microscopy (STM). All the isolated molecules are observed in a planar shape with slight distortion. The isolated molecules can be controllably rotated with an STM tip to the orientation along the trigonal lattice ([Formula: see text] direction) of the substrate. With an increased coverage (0.07?ML) and appropriate annealing, the molecules assemble to form three types of ordered phase. The long-range ordered structures, however, disappear at higher coverage (0.75?ML).     

Fabrication, structural characterization and photoluminescence of single-crystal Zn(x)Cd(1-x)S zigzag nanowires

Large-scale synthesis of ternary Zn(x)Cd(1-x)S zigzag nanowires was achieved in a one-step metal-organic chemical vapour deposition (MOCVD) process with co-fed single precursors of ZnS and CdS. Their morphologies, structures and optical properties were characterized and confirmed by scanning electron microscopy, high-resolution transmission electron microscopy, x-ray spectroscopy, and photoluminescence. The Zn(x)Cd(1-x)S zigzag nanowires are single crystalline, with axis [001], by changing the growth direction from [Formula: see text] to [Formula: see text]. Regarding the formation of zigzag nanowires, we suggest that the shear strain and slight fluctuation of the reaction conditions may be the major factors that make the nanowires change growth direction. In addition, because of the lower temperature and versatility, this new fabrication method might present a new and facile way to form other ternary nanomaterials. Furthermore, the green emission of the nanowires may have potential applications in electronic/optical nanodevices.     

Modelling the formation of high aspect CdSe quantum wires: axial-growth versus oriented-attachment mechanisms

Following the recent low temperature synthesis of high quality and single crystal CdSe quantum nanowires, we have used a thermodynamic model to investigate the plausibility of axial-growth and oriented-attachment formation mechanisms. Using surface energies for clean and alkylamine-passivated CdSe surfaces reported elsewhere by Manna et al (2005 J. Phys. Chem. B 109 6183), we have compared equilibrium and metastable shapes of CdSe nanowires as a function of aspect ratio and axial orientation for different degrees of surface passivation. In general, the theoretical results support the oriented-attachment of low aspect quantum dots or nanorods, followed by coalescence to form high aspect [Formula: see text] quantum wires.     

Size effects on Raman spectra of small CdSe nanoparticles in polymer films

The results of a resonant Raman scattering (RRS) study of polymer-stabilized colloidal CdSe nanoparticles (NPs) are reported. The size-selective nature of the RRS is demonstrated by analysing the NP ensembles with different average size [Formula: see text] and size distribution Δd using a set of excitation wavelengths. The effect of size selection on the estimation of [Formula: see text] and Δd values from the RRS spectra is discussed, as well as some peculiarities of RRS on surface optical phonons. From the experimentally observed small variation of the I(2LO)/I(LO) ratio for 2-5?nm NPs a minor effect of [Formula: see text] on the electron-phonon coupling strength in this [Formula: see text] range is supposed.     

Crystallographic tilt in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition

We report on the studies of crystallographic tilt induced by miscut of the Si (111) substrate in GaN-on-Si (111) heterostructures grown by metal–organic chemical vapor deposition. By employing high-resolution X-ray diffraction, we found that the onset of crystallographic tilt occurred at the interface between the AlN nucleation layer and the Si (111) substrate. The orientation of the GaN overlayer always follows that of the AlN nucleation layer irrespective of its quality and miscut of the substrates. The resultant GaN [0002] is tilted toward GaN (11?20) and (10?10) atomic planes for the miscuts of Si (111) toward Si [1?10] and [11?2], respectively. In both cases, the misorientation of GaN (0002), i.e., the tilt of GaN [0002] from the surface normal direction, is in the same direction of the miscut of Si (111). The misorientation angle of the GaN epilayer is generally smaller than the miscut angle of the substrate. However, the crystallographic tilt, i.e., the angle formed between GaN [0002] and Si [111], is always much larger than the Nagai tilt. These observations are attributable to misfit dislocations that are anisotropically generated at the AlN/Si (111) interface. This mechanism is discussed based on recent microscopic observations of in-plane misfit dislocations at the interface near the atomic step edges.     

A novel method for preparing vertically grown single-crystalline gold nanowires

A surfactant-free, template-less and seed-less method, namely the thermal-assisted photoreduction (TAP) process, has been developed to synthesize vertically grown Au nanowires (30-80?nm in diameter and about 2?μm in length) on the surface of thin film titanium dioxide (TiO(2)), which is locally excited by blackbody radiation. The Au nanowires thus produced are single-crystalline with a preferred [Formula: see text] growth direction. The electrical behavior investigated using a nanomanipulation device indicates that the Au nanowires possess an excellent electrical resistivity of about 3.49 × 10(-8)?Ω?m.     

Effect of interfacial reaction rate on the morphogenesis of nanostructured coatings in a simulated electrodeposition process

Brownian dynamics simulations (BDSs) are performed to investigate the influence of interfacial electrochemical reaction rate on the evolution of coating morphology on circular fibres. The boundary condition for the fluid phase concentration, representing the balance between the rates of interfacial reaction and transport of ions by bulk diffusion, is incorporated into the BDS by using a reaction probability, P(s). Different modes of growth, ranging from diffusion limited ([Formula: see text]) to reaction controlled [Formula: see text], are studied. It is found that, consistent with experimental observations, two distinct morphological regimes exist, with a dense and uniform structure for [Formula: see text] (reaction limited deposition (RLD)) and an open and porous one as [Formula: see text] (diffusion limited deposition (DLD)). An analysis of the fractal dimension indicates that this morphological transition occurs at P(s)≈0.3. Long-time power-law scalings for the evolution of thickness [Formula: see text] and roughness (ξ) of the coating exist, i.e.?[Formula: see text] with 0.86≤α≤0.91 and 0.56≤β≤0.93 for 0.01≤P(s)≤1. These values are different from those reported for sequential, pseudo-time lattice simulations on planar surfaces, signifying the importance of multiparticle dynamics and surface curvature. The internal structure and porosity of the coating are characterized quantitatively by the radial density profile, pair correlation function, two-point probability function, void distribution function and pore area distribution. For RLD the radial density, ρ(n), remains nearly constant, while for DLD ρ(n) follows a power law, [Formula: see text]. The coating exhibits short ranged order in the RLD regime while a long range order is created by DLD. The void distribution function becomes broader with increasing P(s), indicating that in the RLD regime the coating consists of small and spherical pores, while in the DLD regime large and elongated pores are obtained. The pore area distribution shows narrower distributions in DLD for small pores, while the area of the largest pore increases by nearly three orders of magnitude as one moves from the RLD to the DLD regime. Such morphological diversity could be potentially exploited for applications such as percolation, catalysis and surface protection.     

Self-assembly of rubrene on Cu(111)

We performed an ultra-high vacuum scanning tunneling microscopy (STM) investigation of the self-assembly of rubrene at room temperature on Cu(111), a metal surface with threefold symmetry. Rubrene self-assembles into two different structures called row and trimer. Both are different than the structures already observed on Cu(110) and Cu(100). Row and trimer structures have comparable molecular packing densities and are equally distributed across the surface. In the row structure the molecules are oriented with their backbone along the same high symmetry directions of the surface: [[Formula: see text]], [[Formula: see text]] or [[Formula: see text]]. The trimer structure is composed of units of three rubrene molecules, oriented along the high symmetry surface directions. These units are chiral, as revealed by height profile measurements by STM, and self-assemble in domains containing only one type of enantiomer.     

Orientation and temperature dependence of the tensile behavior of GaN nanowires: an atomistic study

Gallium nitride (GaN) is a high-temperature semiconductor material of considerable interest. It emits brilliant light and has been considered as a key material for the next generation of high frequency and high power transistors that are capable of operating at high temperatures. Due to its anisotropic and polar nature, GaN exhibits direction-dependent properties. Growth directions along [001], [1?10] and [110] directions have all been synthesized experimentally. In this work, molecular dynamics simulations are carried out to characterize the mechanical properties of GaN nanowires with different orientations at different temperatures. The simulation results reveal that the nanowires with different growth orientations exhibit distinct deformation behavior under tensile loading. The nanowires exhibit ductility at high deformation temperatures and brittleness at lower temperature. The brittle to ductile transition (BDT) was observed in the nanowires grown along the [001] direction. The nanowires grown along the [110] direction slip in the {010} planes, whereas the nanowires grown along the [1?10] direction fracture in a cleavage manner under tensile loading.     

Analysis of the magnetic force generated at a hemispherical microelectrode

An analytical expression is presented for the magnetic force generated during steady-state voltammetry at a hemispherical microelectrode immersed in a uniform magnetic field. Diffusion of electrogenerated ions through the magnetic field results in a magnetic force that induces convective solution flow near the electrode surface. The magnetic force per unit volume,[Formula: see text] (i.e., force density), is shown to decrease as r(-)(2), where r is the distance away from the center of the hemispherical electrode. A consequence of the inverse square dependence of[Formula: see text] on r is that the magnetic force is confined to a microscopic solution volume near the electrode surface (e.g., ～2 × 10(-)(9) L for a 12.5-μm-radius hemispherical electrode). The net magnetic force acting on the diffusion layer volume,[Formula: see text] , is computed as a function of magnetic field strength and orientation and used in an approximate analysis of experimental data obtained at an inlaid 12.5-μm-radius Pt microdisk electrode. Enhancements in voltammetric currents are shown to result from magnetic forces as small as 2 × 10(-)(11) N.     

Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni(2)Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni(2)Ge nanowires exceeds 3.5 × 10(7) A cm(-2), and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni(2)Ge/Ge/Ni(2)Ge heterostructure. The interface epitaxial relationships are determined to be [Formula: see text] and [Formula: see text]. Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni(2)Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10(3) and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.     

Wurtzite ZnSe nanowires: growth, photoluminescence, and single-wire Raman properties

Wurtzite ZnSe nanowires were prepared on GaAs substrates in a metal-organic chemical vapour deposition system. Electron microscopy shows that they are smooth and uniform in size. Both transmission electron microscopy and x-ray diffraction reveal the wurtzite structure of the nanowires, which grows along the [Formula: see text] direction. Raman scattering studies on individual nanowires were performed in the back-scattering geometry at room temperature. Besides the commonly observed longitudinal and transverse optical phonon modes, a possible surface mode located at 233?cm(-1) is also observed in the Raman spectrum. A peak located at 2.841?eV was clearly observed in the photoluminescence spectra of the nanowires, which can be assigned to near band edge emissions of wurtzite ZnSe.     

Distribution of cations in nanosize and bulk Co-Zn ferrites

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, M?ssbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.