Playing with carbon and silicon at the nanoscale

SiC nanocrystals: high-rate deposition and nano-scale control by thermal plasma

SiC nanocrystals were fabricated at a high rate with SiCl₄ as the Si source by using thermal-plasma-enhanced chemical vapor deposition through the assembly of precursor atoms.

Investigations on phosphorous doped hydrogenated amorphous silicon carbide thin films deposited by a filtered cathodic vacuum arc technique for photo detecting applications

This paper reports the growth and properties of phosphorous doped hydrogenated amorphous silicon carbide thin films deposited by a filtered cathodic vacuum arc technique using P doped solid silicon target as a cathode in the presence of acetylene gas.

Fullerenes generated from porous structures

Structural and electronic properties of macromolecules, which have the architecture of fullerenes, were studied using reactive and density functional-based methods.

Thermal spin filtering, thermal spin switching and negative-differential-resistance in thermal spin currents in zigzag SiC nanoribbons

A thermoelectric heterojunction device based on zigzag silicon carbide nanoribbons can serve as a perfect thermal spin filter and switcher.

From pure C₃₆ fullerene to cagelike nanocluster: a density functional study

The geometrical structures, energetics properties, and aromaticity of C(₃₆-n) Si(n) (n ≤ 18) fullerene-based clusters were studied using density functional theory calculations. The geometries of C(₃₆-n) Si(n) clusters undergo strong structural deformation with the increase of Si substitution. For the most energy favorable structures of C(₃₆-n) Si(n) , the silicon and carbon atoms form two distinct homogeneous segregations. Subsequently, the binding energy, HOMO-LUMO energy gap, vertical ionization potential, vertical electron affinity, and chemical hardness for the energetic favorable C(₃₆-n) Si(n) geometries were computed and analyzed. In addition, the aromatic property of C(₃₆-n) Si(n) cagelike clusters was investigated, and the result demonstrate that these C(₃₆-n) Si(n) cagelike structures possess strong aromaticity.

When epitaxy meets plasma: a path to ordered nanosheets arrays

The possibility of a controlled assembly of 2-dimensional (2D) nanosheets (NSs) into ordered arrays or even more sophisticated structures offers tremendous opportunities in the context of fabrication of a variety of NSs based devices. Reports of such ordered NSs are rare and all conventional "top-down" methods typically led to coarse structures exhibiting only limited surface quality. In this work, we demonstrate a path to directly synthesis ordered NSs arrays in a plasma activated chemical vapor deposition technique utilizing planar defects formed during hetero-epitaxial growth of crystals featuring a close-packed lattice. As an example, the synthesis of 3C-SiC NSs arrays with well-defined orientation on (001) and (111) Si substrates is shown. A detailed analysis identifies planar defects and the plasma environment as key factors determining the resulting 2D NSs arrays. Consequently, a "planar defects induced selective growth" effect is proposed to elucidate the corresponding growth mechanism.

Photoluminescent SiC tetrapods

Recently, significant research efforts have been made to develop complex nanostructures to provide more sophisticated control over the optical and electronic properties of nanomaterials. However, there are only a handful of semiconductors that allow control over their geometry via simple chemical processes. Herein, we present a molecularly seeded synthesis of a complex nanostructure, SiC tetrapods, and report on their structural and optical properties. The SiC tetrapods exhibit narrow line width photoluminescence at wavelengths spanning the visible to near-infrared spectral range. Synthesized from low-toxicity, earth abundant elements, these tetrapods are a compelling replacement for technologically important quantum optical materials that frequently require toxic metals such as Cd and Se. This previously unknown geometry of SiC nanostructures is a compelling platform for biolabeling, sensing, spintronics, and optoelectronics.

Tunable synthesis and in situ growth of silicon-carbon mesostructures using impermeable plasma

In recent years, plasma-assisted synthesis has been extensively used in large scale production of functional nano- and micro-scale materials for numerous applications in optoelectronics, photonics, plasmonics, magnetism and drug delivery, however systematic formation of these minuscule structures has remained a challenge. Here we demonstrate a new method to closely manipulate mesostructures in terms of size, composition and morphology by controlling permeability at the boundaries of an impermeable plasma surrounded by a blanket of neutrals. In situ and rapid growth of thin films in the core region due to ion screening is among other benefits of our method. Similarly we can take advantage of exceptional properties of plasma to control the morphology of the as deposited nanostructures. Probing the plasma at boundaries by means of observing the nanostructures, further provides interesting insights into the behaviour of gas-insulated plasmas with possible implications on efficacy of viscous heating and non-magnetic confinement.

Observation of intermediate template directed SiC nanowire growth in Si-C-N systems

SiC nanowires (NWs) are commonly prepared in a Si-C-N system, but its formation mechanism is not fully understood. High resolution transmission electron microscopy and electron energy loss spectroscopy observation recorded the growth process of how Si(3)N(4) NWs were transformed into SiC NWs, and demonstrated the validity of an intermediate template directed SiC NW growth via carbothermal reduction of intermediate Si(3)N(4) NWs in a Si-C-N system. Based on this discovery, an intermediate-template growth mechanism of SiC NWs was proposed.

Nanotribology at high temperatures

Recent molecular dynamics simulation results have increased conceptual understanding of the grazing and the ploughing friction at elevated temperatures, particularly near the substrate's melting point. In this commentary we address a major constraint concerning its experimental verification.

Spin-lattice relaxation in aluminum-doped semiconducting 4H and 6H polytypes of silicon carbide

NMR spin-lattice relaxation efficiency is similar at all carbon and silicon sites in aluminum-doped 4H- and 6H-polytype silicon carbide samples, indicating that the valence band edge (the top of the valence band), where the holes are located in p-doped materials, has similar charge densities at all atomic sites. This is in marked contrast to nitrogen-doped samples of the same polytypes where huge site-specific differences in relaxation efficiency indicate that the conduction band edge (the bottom of the conduction band), where the mobile electrons are located in n-doped materials, has very different charge densities at the different sites. An attempt was made to observe (27)Al NMR signals directly, but they are too broad, due to paramagnetic line broadening, to provide useful information about aluminum doping.

Theoretical predictions of a bucky-diamond SiC cluster

A study of structural relaxations of Si(n)C(m) clusters corresponding to different compositions, different relative arrangements of Si/C atoms, and different types of initial structure, reveals that the Si(n)C(m) bucky-diamond structure can be obtained for an initial network structure constructed from a truncated bulk 3C-SiC for a magic composition corresponding to n = 68 and m = 79. This study was performed using a semi-empirical Hamiltonian (SCED-LCAO) since it allowed an extensive search of different types of initial structures. However, the bucky-diamond structure predicted by this method was also confirmed by a more accurate density functional theory (DFT) based method. The bucky-diamond structure exhibited by a SiC-based system represents an interesting paradigm where a Si atom can form three-coordinated as well as four-coordinated networks with carbon atoms and vice versa and with both types of network co-existing in the same structure. Specifically, the bucky-diamond structure of the Si(68)C(79) cluster consists of a 35-atom diamond-like inner core (four-atom coordinations) suspended inside a 112-atom fullerene-like shell (three-atom coordinations).

Near-Infrared Fluorescent Nanoprobes for in Vivo Optical Imaging

Near-infrared (NIR) fluorescent probes offer advantages of high photon penetration, reduced light scattering and minimal autofluorescence from living tissues, rendering them valuable for noninvasive mapping of molecular events, assessment of therapeutic efficacy, and monitoring of disease progression in animal models. This review provides an overview of the recent development of the design and optical property of the different classes of NIR fluorescent nanoprobes associated with in vivo imaging applications.

Influence of microstructures on mechanical behaviours of SiC nanowires: a molecular dynamics study

The tensile behaviours of [111]-oriented SiC nanowires with various microstructures are investigated by using molecular dynamics simulations. The results revealed the influence of microstructures on the brittleness and plasticity of SiC nanowires. Plastic deformation is mainly induced by the anti-parallel sliding of 3C grains along an intergranular amorphous film parallel to the plane and inclined at an angle of 19.47° with respect to the nanowire axis. Our study suggests that the wide dispersion of mechanical properties of SiC nanowires observed in experiments might be attributed to their diverse microstructures.

Measuring Si-C60 chemical forces via single molecule spectroscopy

We measure the short-range chemical force between a silicon-terminated tip and individual adsorbed C(60) molecules using frequency modulation atomic force microscopy. The interaction with an adsorbed fullerene is sufficiently strong to drive significant atomic rearrangement of tip structures.

Boron and nitrogen impurities in SiC nanoribbons: an ab initio investigation

Using ab initio calculations based on density-functional theory we have performed a theoretical investigation of substitutional boron and nitrogen impurities in silicon carbide (SiC) nanoribbons. We have considered hydrogen terminated SiC ribbons with zigzag and armchair edges. In both systems we verify that the boron and nitrogen atoms energetically prefer to be localized at the edges of the nanoribbons. However, while boron preferentially substitutes a silicon atom, nitrogen prefers to occupy a carbon site. In addition, our electronic-structure calculations indicate that (i) substitutional boron and nitrogen impurities do not affect the semiconducting character of the armchair SiC nanoribbons, and (ii) the half-metallic behavior of the zigzag nanoribbons is maintained in the presence of substitutional boron impurities. In contrast, nitrogen atoms occupying edge carbon sites transform half-metallic zigzag nanoribbons into metallic systems.

Theoretical Study of Carbon Clusters in Silicon Carbide Nanowires

Using first-principles methods we performed a theoretical study of carbon clusters in silicon carbide (SiC) nanowires. We examined small clusters with carbon interstitials and antisites in hydrogen-passivated SiC nanowires growth along the [100] and [111] directions. The formation energies of these clusters were calculated as a function of the carbon concentration. We verified that the energetic stability of the carbon defects in SiC nanowires depends strongly on the composition of the nanowire surface: the energetically most favorable configuration in carbon-coated [100] SiC nanowire is not expected to occur in silicon-coated [100] SiC nanowire. The binding energies of some aggregates were also obtained, and they indicate that the formation of carbon clusters in SiC nanowires is energetically favored.

Crystal phase engineering in single InAs nanowires

Achieving phase purity and control in III-V nanowires is a necessity for future nanowire-based device applications. Many works have focused on cleaning specific crystal phases of defects such as twin planes and stacking faults, using parameters such as diameter, temperature, and impurity incorporation. Here we demonstrate an improved method for crystal phase control, where crystal structure variations in single InAs nanowires are designed with alternating wurtzite (WZ) and zinc blende (ZB) segments of precisely controlled length and perfect interfaces. We also demonstrate the inclusion of single twin planes and stacking faults with atomic precision in their placement, designed ZB quantum dots separated by thin segments of WZ, acting as tunnel barriers for electrons, and structural superlattices (polytypic and twin plane). Finally, we present electrical data to demonstrate the applicability of these designed structures to investigation of fundamental properties. From electrical measurements we observe clear signatures of controlled structural quantum dots in nanowires. This method will be directly applicable to a wide range of nanowire systems.

In vitro evaluation of SiC nanoparticles impact on A549 pulmonary cells: cyto-, genotoxicity and oxidative stress

Silicon carbide (SiC) is considered a highly biocompatible material, consequently SiC nanoparticles (NPs) have been proposed for potential applications in diverse areas of technology. Since no toxicological data are available for these NPs, the aim of this study was to draw their global toxicological profile on A549 lung epithelial cells, using a battery of classical in vitro assays. Five SiC-NPs, with varying diameters and Si/C ratios were used, and we show that these SiC-NPs are internalized in cells where they cause a significant, though limited, cytotoxic effect. Cell redox status is deeply disturbed: SiC-NP exposure cause reactive oxygen species production, glutathione depletion and inactivation of some antioxidant enzymes: glutathione reductase, superoxide dismutase, but not catalase. Finally, the alkaline comet assay shows that SiC-NPs are genotoxic. Taken together, these data prove that SiC-NPs biocompatibility should be revisited.

Interaction of C(59)Si with Si based clusters: a study of Janus nanostructures

Using density functional theory with the generalized gradient approximation (GGA), we show that carbon-silicon Janus anisotropic nanostructures can be synthesized by using C(59)Si heterofullerene as a seed where the doped Si atom preferentially attaches to some well-known silicon and silicon based clusters such as Si(10), WSi(12), TiSi(16), and BaSi(20). The interaction energy of these clusters with C(59)Si varies from 0.9 to 1.9 eV. The anisotropy of the resulting carbon-silicon Janus structures produces large dipole moments (4-9 D), anisotropic distributions of electronic orbitals, and the anisotropic reactivity.

Surface relaxation and oxygen adsorption behavior of different SiC polytypes: a first-principles study

The surface relaxations and oxygen adsorptions on C- and Si-terminated 3C-SiC(111) and 2H/4H/6H-SiC(0001) surfaces are systematically studied using density functional theory (DFT) calculations. First, the general surface relaxation trends of different SiC surfaces are explained using the electrostatic interaction and the calculation results of spin density distributions. In the second part of the present work, the relations between adsorption energies and stacking sequence are studied. We find that the adsorption energies of bridge, hollow-3 and T4 configurations on Si-terminated SiC surfaces increase with the increasing of the real number T(I), which is a translation of the polytypic sequence and quantifies the amount of 'h' character of the surface and of the deeper layers, while the energies of the on-top configurations on Si-terminated SiC surfaces and of all configurations on the C-terminated SiC surface seem to depend only on the stacking orientation of the topmost layer and not on the subsequent ones.

Defect-enhanced charge transfer by ion-solid interactions in SiC using large-scale ab initio molecular dynamics simulations

Large-scale ab initio molecular dynamics simulations of ion-solid interactions in SiC reveal that significant charge transfer occurs between atoms, and defects can enhance charge transfer to surrounding atoms. The results demonstrate that charge transfer to and from recoiling atoms can alter the energy barriers and dynamics for stable defect formation. The present simulations illustrate in detail the dynamic processes for charged defect formation. The averaged values of displacement threshold energies along four main crystallographic directions are smaller than those determined by empirical potentials due to charge-transfer effects on recoil atoms.

Synthesis of amorphous silicon carbide nanoparticles in a low temperature low pressure plasma reactor

Commercial scale production of silicon carbide (SiC) nanoparticles smaller than 10 nm remains a significant challenge. In this paper, a microwave plasma reactor and appropriate reaction conditions have been developed for the synthesis of amorphous SiC nanoparticles. This continuous gas phase process is amenable to large scale production use and utilizes the decomposition of tetramethylsilane (TMS) for both the silicon and the carbon source. The influence of synthesis parameters on the product characteristics was investigated. The as-prepared SiC particles with sizes between 4 and 6 nm were obtained from the TMS precursor in a plasma operated at low temperature and low precursor partial pressure (0.001-0.02 Torr) using argon as the carrier gas (3 Torr). The carbon:silicon ratio was tuned by the addition of hydrogen and characterized by x-ray photoelectron spectroscopy. The reaction mechanism of SiC nanoparticle formation in the microwave plasma was investigated by mass spectroscopy of the gaseous products.