Metallic single-walled silicon nanotubes

Chiraltube, rolling 2D materials into chiral nanotubes

Carbon nanotubes (NTs) are graphene sheets rolled into a 1D material, with a specific chirality that defines its structure and properties. Graphene has triggered the development of thousands of 2D materials, which in principle could also be rolled into 1D NTs. However, most of these NTs have not been proposed due to difficulties in the generation of atomic coordinates for chiral NTs from 2D materials with a non-hexagonal lattice or multi-layered materials. In this paper we present Chiraltube, an open-source Python code that allows the quick generation of a complete NT with any chirality from the unit cell of its original 2D material. We explain the inner workings of the code as well as the theoretical background on which it is built, generalizing concepts from the construction of chiral and achiral carbon NTs to work on any other 2D material. We show various examples of the resulting chiral NT structures built from phosphorene, MoS2 and Ti3C2, and present some analysis on the interatomic distortion in the outermost layers of these NTs, as well as the results of ab initio electronic structure calculations on a set of phosphorene NTs generated by the program, showing the immediate practicality and usefulness of the program. We also explore some limitations and details of the tool as well as further work to be done.

Single-walled silicon nanotube as an exceptional candidate to eliminate SARS-CoV-2: a theoretical study

In this work, computational chemistry methods were used to study a silicon nanotube (Si₁₉₂H₁₆) as possible virucidal activity against SARS-CoV-2. This virus is responsible for the COVID-19 disease. DFT calculations showed that the structural parameters of the Si₁₉₂H₁₆ nanotube are in agreement with the theoretical/experimental parameters reported in the literature. The low energy gap value (0.29 eV) shows that this nanotube is a semiconductor and exhibits high reactivity. For nanomaterials to be used as virucides, they need to have high reactivity and high inhibition constant values. Therefore, the adsorption of ³O₂ and H₂O on the surface of Si₁₉₂H₁₆ (Si₁₉₂H₁₆@O₂-H₂O) was performed. In this process, the formation and activation energies were -51.63 and 16.62 kcal/mol, respectively. Molecular docking calculations showed that the Si₁₉₂H₁₆ and Si₁₉₂H₁₆@O₂H-OH nanotubes bind favorably on the receptor-binding domain of the SARS-CoV-2 spike protein with binding energy of -11.83 (Ki = 2.13 nM) and -11.13 (Ki = 6.99 nM) kcal/mol, respectively. Overall, the results obtained herein indicate that the Si₁₉₂H₁₆ nanotube is a potential candidate to be used against COVID-19 from reactivity process and/or steric impediment in the S-protein.Communicated by Ramaswamy H. Sarma.

On the structural stability and optical properties of germanium-based schwarzites: a density functional theory investigation

Since graphene was synthesized the interest in building new 2D and 3D structures based on carbon allotropes has been growing every day. One of these 3D structures is know as carbon schwarzites. Schwarzites consist of carbon nanostructures possessing the shape of Triply-Periodic Minimal Surfaces (TPMS), which is characterized by a negative Gaussian curvature introduced by the presence of carbon rings with more than six atoms. Some examples of schwarzite families include: primitive (P), gyroid (G) and diamond (D). Previous studies considering different element species of schwarzites have investigated the mechanical, electrical and thermal properties. In this work, we investigated the stability of germanium (Ge) schwarzites using density functional theory with the GGA exchange-correlation functional. We chose one structure of each family (P8bal), (G688) and (D688). It was observed that regions usually flat in carbon schwarzites acquire buckled configurations as previously observed on silicene and germanene monolayers. The investigated structures presented a semiconducting bandgap ranging from 0.13 to 0.27 eV. We also performed calculations of optical properties within the linear regime, where it was shown that Ge schwarzite structures absorb light from infrared to ultra-violet frequencies. Therefore, our results open new perspectives of materials that can be used in optoelectronics device applications.

Double-walled silicon nanotubes: an ab initio investigation

The synthesis of silicon nanotubes realized in the last decade demonstrates multi-walled tubular structures consisting of Si atoms in [Formula: see text] and the [Formula: see text] hybridizations. However, most of the theoretical models were elaborated taking as the starting point [Formula: see text] structures analogous to carbon nanotubes. These structures are unfavorable due to the natural tendency of the Si atoms to undergo [Formula: see text]. In this work, through ab initio simulations based on density functional theory, we investigated double-walled silicon nanotubes proposing layered tubes possessing most of the Si atoms in an [Formula: see text] hybridization, and with few [Formula: see text] atoms localized at the outer wall. The lowest-energy structures have metallic behavior. Furthermore, the possibility to tune the band structure with the application of a strain was demonstrated, inducing a metal-semiconductor transition. Thus, the behavior of silicon nanotubes differs significantly from carbon nanotubes, and the main source of the differences is the distortions in the lattice associated with the tendency of Si to make four chemical bonds.

Magnetism, structures and stabilities of cluster assembled TM@Si nanotubes (TM = Cr, Mn and Fe): a density functional study

Four different types (Type 1 to Type 4) of empty and transition metal (Cr, Mn and Fe) doped silicon nanotubes have been studied. The calculated band structures and DOS assigned them as metallic, semiconductor, semi-metallic and half-metallic depending upon the combination of the type of nanotube and the transition metal doping.

One-dimensional silicon and germanium nanostructures with no carbon analogues

In this work we report new silicon and germanium tubular nanostructures with no corresponding stable carbon analogues. The electronic and mechanical properties of these new tubes were investigated through ab initio methods. Our results show that these structures have lower energy than their corresponding nanoribbon structures and are stable up to high temperatures (500 and 1000 K, for silicon and germanium tubes, respectively). Both tubes are semiconducting with small indirect band gaps, which can be significantly altered by both compressive and tensile strains. Large bandgap variations of almost 50% were observed for strain rates as small as 3%, suggesting their possible applications in sensor devices. They also present high Young's modulus values (0.25 and 0.15 TPa, respectively). TEM images were simulated to help in the identification of these new structures.

A nickel-gold bilayer catalyst engineering technique for self-assembled growth of highly ordered silicon nanotubes (SiNT)

We report the growth of vertically aligned high-crystallinity silicon nanotube (SiNT) arrays on silicon substrate by means of a Ni-Au bilayer catalyst engineering technique. Nanotubes were synthesized through solid-liquid-solid method as well as vapor-liquid-solid. A precise evaluation utilizing atomic force microscopy and lateral force microscopy describes that the gold profile in Ni regions leads to the construction of multiwall SiNTs. The agreement of the structural geometry and stiffness of the obtained SiNTs with previous theoretical predictions suggest sp(3) hybridization as the mechanism of tube formation. Apart from scanning electron and transmission electron microscopy techniques, photoluminescence spectroscopy (PL) has been conducted to investigate the formation of nanostructures. PL spectroscopy confirms the evolution of ultrafine walls of the silicon nanotubes, responsible for the observed photoemission properties.

Two-dimensional boron monolayer sheets

Boron, a nearest-neighbor of carbon, is possibly the second element that can possess free-standing flat monolayer structures, evidenced by recent successful synthesis of single-walled and multiwalled boron nanotubes (MWBNTs). From an extensive structural search using the first-principles particle-swarm optimization (PSO) global algorithm, two boron monolayers (α(1)- and β(1)-sheet) are predicted to be the most stable α- and β-types of boron sheets, respectively. Both boron sheets possess greater cohesive energies than the state-of-the-art two-dimensional boron structures (by more than 60 meV/atom based on density functional theory calculation using PBE0 hybrid functional), that is, the α-sheet previously predicted by Tang and Ismail-Beigi and the g(1/8)- and g(2/15)-sheets (both belonging to the β-type) recently reported by Yakobson and co-workers. Moreover, the PBE0 calculation predicts that the α-sheet is a semiconductor, while the α(1)-, β(1)-, g(1/8)-, and g(2/15)-sheets are all metals. When two α(1) monolayers are stacked on top each other, the bilayer α(1)-sheet remains flat with an optimal interlayer distance of ~3.62 Å, which is close to the measured interlayer distance (~3.2 Å) in MWBNTs.

Electron-Deficiency Aromaticity in Silicon Nanoclusters

Aromaticity in silicon-containing molecules has been a controversy for more than a century. Combining molecular dynamics simulations with ab initio calculations, we show here that it is possible to obtain aromatic-like behavior with pure hydrogenated silicon clusters without the need for multiple bonds. To this end, we exploit the natural tendency of silicon toward overcoordination to construct electron-deficient molecules with ring structures. Even without the incorporation of any protective bulky substituents the resulting structures are more stable than any other known hydrogenated silicon nanoparticles of this size and exhibit aromatic-like properties due to strong electron delocalization.

From crystalline germanium-silicon axial heterostructures to silicon nanowire-nanotubes

One-dimensional (1D) nanostructures have attracted considerable attention as a result of their exceptional properties and potential applications. Among them, 1D axial heterostructures with well-defined and controlled heterojunctions between different nanomaterials or between different 1D nanostructures (i.e., nanowire-nanotube heterojunctions) have recently become of particular interest as potential building blocks in future high-performance nano-optoelectronic and nanoelectronic devices. Here, we report on the preparation and characterization of crystalline silicon nanowire-nanotube (SiNW-NT) heterostructures with controlled geometry, kinked and unkinked, and composition using germanium-silicon nanowire heterostructures with abrupt heterojunctions (~2 nm wide) as a template via the VLS-CVD mechanism.

Ultrastable silicon nanocrystals due to electron delocalization

We report a new nanocrystalline form of silicon that gives birth to pure hydrogenated silicon nanocrystals that absorb light in the ultraviolet, visible, and infrared spectral region despite their small size of only 1 nm and without the need for expensive or toxic metal atoms. On the basis of first-principles calculations, we demonstrate that those pure, but overcoordinated silicon nanocrystals are more stable than any other known silicon nanocrystals due to electron delocalization and that they form spontaneously via self-assembly. Therefore, we predict their immediate application in fields ranging from photovoltaic and light-emitting devices to photothermal cancer treatment.

Controlled synthesis of germanium nanowires and nanotubes with variable morphologies and sizes

We report on the controlled growth of germanium (Ge) nanostructures in the form of both nanowire (NW) and nanotube (NT) with ultrahigh aspect ratios and variable diameters. The nanostructures are grown inside a porous anodic aluminum oxide (AAO) template by low-temperature chemical vapor deposition (CVD) assisted by an electrodeposited metal nanorod catalyst. Depending on the choice of catalytic metals (Au, Ni, Cu, Co) and germane (GeH(4)) concentration during CVD, either Ge NWs or NTs can be synthesized at low growth temperatures (310-370 °C). Furthermore, Ge NWs and NTs with two or more branches can be grown from the same stem while using AAO with branched channels as templates. Transmission electron microscopy studies show that NWs are single crystalline and that branches grow epitaxially from the stem of NWs with a crystalline direction independent of diameter. As-grown NTs are amorphous but can crystallize via postannealing at 400 °C in Ar/H(2) atmosphere, with a wall thickness controllable between 6 and 18 nm in the CVD process. The yield and quality of the NTs are critically dependent on the choice of the catalyst, where Ni appears the best choice for Ge NT growth among Ni, Cu, Co, and Au. The synthesis of structurally uniform and morphologically versatile Ge nanostructures may open up new opportunities for integrated Ge-nanostructure-based nanocircuits, nanodevices, and nanosystems.

Wall-selective chemical alteration of silicon nanotube molecular carriers

Recently, there has been significant interest in the synthesis and potential applications of semiconductor nanotubes (NTs). In this context, many efforts have been invested in developing new routes to control and engineer their surface chemistry. We report herein on a simple route to differentially and selectively functionalize the inner and outer surfaces of silicon nanotubes (SiNTs) with organic molecular layers containing different functional groups and hydrophobicity/hydrophilicity chemical nature, via covalent binding, to give nanotubular structures with dual chemical properties. Significantly, our unique synthetic approach can be further extended to directly form hollow crystalline nanotubular structures with their inner/outer surfaces independently and selectively altered chemically. Additionally, SiNTs inner and/or outer walls can be selectively decorated with metal nanoparticles. Both inner and outer walls can be individually and separately modified with the same metal nanoparticles, with different metal NPs in the inside and outside walls or with a combination of metal NPs decoration and molecular layers, if so required. Furthermore, the dually modified nanotubes were then exploited as phase extraction nanocarriers to demonstrate their potential in future chemical and biological separation, extraction, and filtering applications.

Crystalline silicon nanotubes and their connections with gold nanowires in both linear and branched topologies

Silicon, being in the same group in the periodic table as carbon, plays a key role in modern semiconductor industry. However, unlike carbon nanotube (NT), progress remains relatively slow in silicon NT (SiNT) and SiNT-based heteroarchitectures, which would be the fundamental building blocks of various nanoscale circuits, devices, and systems. Here, we report the synthesis of linear and branched crystalline SiNTs via porous anodic aluminum oxide (AAO) self-catalyzed growth and postannealing, and the connection of crystalline SiNTs and gold nanowires (AuNWs) via a combinatorial process of electrodepositing AuNWs with predesired length and location in the channels of the AAO template and subsequent AAO self-catalyzed and postannealing growth of SiNTs in the remaining empty channels adjacent to the AuNWs. Using the approach, a large variety of two-segment AuNW/SiNT and three-segment SiNT/AuNW/SiNT heteronanostructures with both linear and branched topologies have been achieved, paving the way for the rational design and fabrication of SiNT-based nanocircuits, nanodevices, and multifunctional nanosystems in the future.

The geometric structure of single-walled nanotubes

In this paper, we survey a number of existing geometric structures which have been proposed by the authors as possible models for various nanotubes. Atoms assemble into molecules following the laws of quantum mechanics, and in general computational approaches to predicting the molecular structure can be arduous and involve considerable computing time. Fortunately, nature favours minimum energy structures which tend to be either very symmetric or very unsymmetric, and which therefore can be analyzed from a geometrical perspective. The conventional rolled-up model of nanotubes completely ignores any effects due to curvature and the present authors have proposed a number of exact geometric models. Here we review a number of these recent developments relating to the geometry of nanotubes, including both the traditional rolled-up models and some exact polyhedral constructions. We review a number of formulae for four materials, carbon, silicon, boron and boron nitride, and we also include results for the case when the bond lengths may take on distinct values.

An idealized polyhedral model and geometric structure for silicon nanotubes

In this paper, we introduce an idealized model of silicon nanotubes comprising an exact polyhedral geometric structure for single-walled silicon nanotubes. The silicon nanotubes considered here are assumed to be formed by sp(3) hybridization and thus the nanotube lattice is assumed to comprise only squares or skew rhombi. Beginning with the three postulates that all bond lengths are equal, all adjacent bond angles are equal, and all atoms are equidistant from a common axis of symmetry, we derive exact formulae for the geometric parameters such as radii, bond angles and unit cell length. We present asymptotic expansions for these quantities to the first two orders of magnitude. Because of the faceted nature of the polyhedral model we may determine a perceived inner radius for the nanotube, from which an expression for the wall thickness emerges. We also describe the geometric properties of some ultra-small silicon nanotubes. Finally, the values of the diameters for the polyhedral model are compared with results obtained from molecular dynamics simulations and some limited numerical calculations are undertaken to confirm the meta-stability of the proposed structures.

Relative stability of planar versus double-ring tubular isomers of neutral and anionic boron cluster B20 and B20-

High-level ab initio molecular-orbital methods have been employed to determine the relative stability among four neutral and anionic B20 isomers, particularly the double-ring tubular isomer versus three low-lying planar isomers. Calculations with the fourth-order Moller-Plessset perturbation theory [MP4(SDQ)] and Dunning's correlation consistent polarized valence triple zeta basis set as well as with the coupled-cluster method including single, double, and noniteratively perturbative triple excitations and the 6-311G(d) basis set show that the double-ring tubular isomer is appreciably lower in energy than the three planar isomers and is thus likely the global minimum of neutral B20 cluster. In contrast, calculations with the MP4(SDQ) level of theory and 6-311+G(d) basis set show that the double-ring anion isomer is appreciably higher in energy than two of the three planar isomers. In addition, the temperature effects on the relative stability of both 10B20- and 11B20- anion isomers are examined using the density-functional theory. It is found that the three planar anion isomers become increasingly more stable than the double-ring isomer with increasing the temperature. These results are consistent with the previous conclusion based on a joint experimental/simulated anion photoelectron spectroscopy study [B. Kiran et al., Proc. Natl. Acad. Sci. U.S.A. 102, 961 (2005)], that is, the double-ring anion isomer is notably absent from the experimental spectra. The high stability of the double-ring neutral isomer of B20 can be attributed in part to the strong aromaticity as characterized by its large negative nucleus-independent chemical shift. The high-level ab initio calculations suggest that the planar-to-tubular structural transition starts at B20 for neutral clusters but should occur beyond the size of B20- for the anion clusters.

Structures and stabilities of small lead oxide clusters PbmOn (m=1–4,n=1–2m)

The structures and stabilities of small lead oxide clusters PbmOn with m=1-4, n=1-2m are systematically studied using density functional theory. It is found that the lowest-energy structures of all these clusters can be obtained by the sequential oxidation of small "core" lead clusters. For Pb-rich clusters (oxygen-to-lead ratio<1), oxygen atoms favor bridge sites for Pb2On and Pb3On and surface sites for Pb4On. The lead-monoxide-like clusters (PbO)i (i=1-4) have great stability because of their significant dissociation energies and highest occupied molecular orbital-lowest unoccupied molecular orbital gaps. This suggests that they could be adopted as the building blocks of cluster-assembled materials. For O-rich clusters (oxygen-to-lead ratio>1), the grouping of oxygen atoms usually appears. It is found that the structures with a grouping of more than two oxygen atoms are unstable.

Molecular dynamics simulations of self-organized polyicosahedral Si nanowire

A novel polyicosahedral nanowire is spontaneously formed in a series of annealing molecular dynamics simulations of liquid Si inside a nanopore of 1.36 nm in diameter. The polyicosahedral Si nanowire is stable even in a vacuum up to about 77% of the melting temperature of bulk Si. Our structural energy calculations reveal that the polyicosahedral nanowire is energetically advantageous over the pentagonal one for a wire whose diameter is less than 6.02 nm, though the latter has been recently proposed as the lowest energy wire. These results suggest the possibility of the formation of a new stable polyicosahedral Si nanowire.

Magic structures of h-passivated 110 silicon nanowires

We report a genetic algorithm approach combined with ab initio calculations to determine the structure of hydrogenated 110 Si nanowires. As the number of atoms per length increases, we find that the cross section of the nanowire evolves from chains of six-atom rings to fused pairs of such chains to hexagons bounded by {001} and {111} facets. Our calculations predict that hexagonal wires become stable starting at about 1.2 nm diameter, which is consistent with recent experimental reports of nanowires with diameters of about 3 nm.

Investigation of Possible Structures of Silicon Nanotubes via Density-Functional Tight-Binding Molecular Dynamics Simulations and ab Initio Calculations

We show, computationally, that single-walled silicon nanotubes (SiNTs) can adopt a number of distorted tubular structures, representing respective local energy minima, depending on the theory used and the initial models adopted. In particular, "gearlike" structures containing alternating sp(3)-like and sp(2)-like silicon local configurations have been found to be the dominant structural form for SiNTs via density-functional tight-binding molecular dynamics simulations (followed by geometrical optimization using Hartree-Fock or density function theory) at moderate temperatures (below 100 K). The gearlike structures of SiNTs deviate considerably from, and are energetically more stable than, the smooth-walled tubes (the silicon analogues of single-walled carbon nanotubes). They are, however, energetically less favorable than the "string-bean-like" SiNT structures previously derived from semiempirical molecular orbital calculations. The energetics and the structures of gearlike SiNTs are shown to depend primarily on the diameter of the tube, irrespective of the type (zigzag, armchair, or chiral). In contrast, the energy gap is very sensitive to both the diameter and the type of the nanotube.