Single-walled Boron Nitride Nanotubes Research Articles

On the Determination of Elastic Properties of Single-Walled Nitride Nanotubes Using Numerical Simulation.

In recent years, tubular nanostructures have been related to immense advances in various fields of science and technology. Considerable research efforts have been centred on the theoretical prediction and manufacturing of non-carbon nanotubes (NTs), which meet modern requirements for the development of novel devices and systems. In this context, diatomic inorganic nanotubes formed by atoms of elements from the 13th group of the periodic table (B, Al, Ga, In, Tl) and nitrogen (N) have received much research attention. In this study, the elastic properties of single-walled boron nitride, aluminium nitride, gallium nitride, indium nitride, and thallium nitride nanotubes were assessed numerically using the nanoscale continuum modelling approach (also called molecular structural mechanics). The elastic properties (rigidities, surface Young's and shear moduli, and Poisson's ratio) of nitride nanotubes are discussed with respect to the bond length of the corresponding diatomic hexagonal lattice. The results obtained contribute to a better understanding of the mechanical response of nitride compound-based nanotubes, covering a broad range, from the well-studied boron nitride NTs to the hypothetical thallium nitride NTs.

Structural, electronic, phonon, and elastic properties of zigzag single-walled and double-walled boron nitride nanotubes

The study used Density Functional Theory to simulate the properties of single-walled and double-walled boron nitride nanotubes. The cohesive energy calculations suggest that double-walled nanotubes are more stable than single-walled nanotubes. Double-walled nanotubes have a narrower bandgap than single-walled nanotubes. The smaller diameter of the nanotubes reduces available energy for vibrational modes. Double-walled nanotubes have larger Young’s modulus, shear modulus, and bulk modulus than single-walled nanotubes. Both single-walled and double-walled nanotubes perform like auxetic materials in some directions. The group velocity of acoustic phonons in SWBNNTs (6, 0) is higher than in SWBNNTs (12, 0).

Investigation of drug delivery capability of single-walled carbon and boron-nitride nanotubes, boron-nitride (B16N16), and C32 fullerenes as nanocarriers of captopril drug; DFT study

Density functional theory (DFT) calculations were performed on M062X/6-31G (d) and M062X/6-311G (d, p) surfaces to investigate the interaction of captopril molecules with single-walled carbon nanotubes (SWCNT) and C32 nanoclusters. In order to enhance adsorption, a doping method was employed, and structures of single-walled boron nitride nanotubes (SWBNNT) and boron nitride nanoclusters (B16N16) were selected for improved drug delivery purposes in this study. The adsorption energy of captopril on the nanostructures was calculated in both phases. In both phases, captopril adsorption energy on nanostructures was calculated. The negative values of adsorption energy showed that the interaction between captopril and nanostructures is exothermic. The adsorption energy values were higher in the gas phase compared to the aqueous phase, indicating a stronger interaction in the gas phase. The density of states (DOS) study was employed to evaluate the effect of molecular adsorption on the electronic properties of nanostructures. The results revealed that the B16N16 nanocluster approached the Fermi level more closely after drug adsorption compared to other nanostructures. Quantum Theory of Atoms in Molecules (QTAIM) calculations showed that there is a weak interaction between captopril and both nanostructures. Based on the adsorption energy values obtained from both methods, the interaction between captopril and SWCNT was found to be stronger than that between captopril and SWBNNT, whereas the interaction in the B16N16 nanocluster was found to be equal to that in C32-fullerene. Significant changes in ΔEg values were observed for Captopril@SWCNT and Captopril@B16N16-fullerene in each method, indicating the conductivity of these two structures increased more after captopril adsorption compared to other nanostructures. Therefore, SWBNNT and B16N16-fullerene can serve as captopril nanosensors.

Analyze the temperature-dependent elastic properties of single-walled boron nitride nanotubes by a modified energy method

The elastic properties of boron nitride nanotubes (BNNTs) were investigated utilizing an enhanced energy method. By considering small deformations and applying the principle of minimum potential energy, the variations in atomic bonds and bond angles within the nanotube structure were determined. The modified model incorporated the contribution of inversion energy to the overall potential energy of the system, leading to the derivation of analytical expressions for the Young's modulus, shear modulus, and strain energy of both armchair and zigzag BNNTs under varying temperatures. The results indicate that compared to zigzag BNNTs, the impact of inversion energy on the elastic constants of armchair BNNTs is more significant, especially at small diameters (<1nm). In thermal environment, this study demonstrates that the change in Young's modulus of BNNTs is lower than that of carbon nanotubes (CNTs), confirming the superior thermal stability of BNNTs over CNTs. Furthermore, molecular structure mechanics (MSM) and continuum mechanics models were employed to analyze the strain energy of BNNTs. The effects of different bonds, bond angles, and inversion angles on strain energy were analyzed in a thermal environment, revealing distinct differences between the two types of BNNTs. These findings provide more accurate theoretical guidance for thermal applications based on the stretching of BNNTs.

Predicting Quantum-Mechanical Partial Charges in Arbitrarily Long Boron Nitride Nanotubes to Accurately Simulate Nanoscale Water Transport.

Single-walled boron nitride nanotubes (BNNTs) have been explored for various applications, ranging from water desalination to osmotic power harvesting. However, no simulation work so far has modeled the changes in the partial charge distribution when a flat sheet is rolled into a tube, hindering the ability to perform accurate molecular dynamics (MD) simulations of water flow through BNNTs. To address this knowledge gap, we employ electronic density functional theory (DFT) calculations to precisely estimate quantum-mechanically derived partial charges on boron (B) and nitrogen (N) atoms in BNNTs of varying lengths and diameters. We observe a spatially varying charge distribution inside both armchair and zigzag nanotubes of finite lengths. Performing DFT calculations for longer BNNTs is computationally intractable, even with state-of-the-art computing resources. To solve this issue, we devise a charge assignment scheme to predict partial charges for longer BNNTs using DFT data for shorter nanotubes, thus overcoming the need to perform more expensive DFT calculations. We show that these charges reproduce the electrostatic potential predicted from first-principles simulations. Subsequently, we carried out MD simulations to predict the effect of the charge distribution inside BNNTs on water flow enhancement via them. We find that using uniform charges leads to an underprediction in flow enhancement, as compared to using quantum-mechanical charges for both armchair and zigzag BNNTs. We also incorporate atomic vibrations into our simulations and show that these vibrations lead to a reduction in the water flow through aperiodic BNNTs. Our work demonstrates the requirement of a quantum-mechanical charge assignment scheme for BNNTs and evolves a framework to assign charges to nanotubes of arbitrary length, thus allowing realistic MD simulations of long BNNTs using accurate partial charges.

First-principle study of CrO2-BNNT-CrO2 based MTJ device using EHTB model and its application in a MRAM circuit

In this work we investigated the performance of Single-Walled Boron Nitride Nanotube (SWBNNT) sandwiched between CrO2 based Magnetic Tunnel Junction (MTJ) structures with atomistic simulations. MTJ based device-level simulation was performed using the Extended Huckel based Tight Binding (EHTB) model. The SWBNNT as a dielectric layer and CrO2 as Ferromagnetic (FM) layers based Magnetic Tunnel Junction (MTJ) heterostructure was computed. The proposed MTJ device operates at the nano-Ampere (nA) range depicting its potential application in a low-powered nanoelectronics device. It also shows a promising Tunnel Magnetoresistance (TMR) effect. The practical circuit-level implementation of the proposed MTJ device in Magnetic Random-Access Memory (MRAM) was validated using LTspice. Hence, the calculated static power consumption and writing energy per bit was minimal for S1 and S2 based MRAM circuit. The perfect switching operation was depicted by the proposed MTJ device based MRAM.

Study of Interactions of Nucleoside Anticancer Drugs, Capecitabine and Gemcitabine, with SWNT and BNNT using Molecular and Quantum Mechanical Calculations

Abstract: Using single-wall carbon nanotubes (SWCNTs) and boron nitride nanotubes (BNNTs), this study evaluated the interactions between Capecitabine (CAP) and Gemcitabine (GEM). Molecular mechanics and quantum mechanics were used in the analysis. : The interaction between CP and GM with SWCNTs and BNNTs under various solvents was analyzed using the self-consistent reaction field (SCRF) as a model and DFT as the analytical method. Additionally, the effect of temperature on the stability of the molecule interactions during the experiment was examined. The thermodynamic properties of the title compounds were analyzed based on theoretical calculations. It included the calculation of Frontier Molecular Orbitals (FMOs) and Total Density of States (DOS). In addition to studying the ionization potential (I), we also examined the other molecular properties of the structures, such as electrophilicity (ω), electron affinity (A), chemical hardness (η), and electronic chemical potential (μ). To investigate the interaction between CAP and GEM with SWCNTs and BNNTs, molecular mechanics methods (MM) including AMBER, OPLS, CHARMM, and MM+ force fields, were employed. Monte Carlo simulation techniques were used to calculate the results at different temperatures. : The effects of the liquid phase and mixed solvent media with varying dielectric constants (Water, DMSO, Methanol, and Ethanol) on the interaction between CAP and GEM using the force fields described above were examined in this study. The correlation between data generated by Monte Carlo, Quantum Mechanics, and Molecular Mechanics was demonstrated. There was a striking similarity in the thermodynamic properties and conformer populations of all three materials.

DFT Studies of the Photocatalytic Properties of MoS2-Doped Boron Nitride Nanotubes for Hydrogen Production.

This study investigated the photocatalytic properties of MoS2-doped boron nitride nanotubes (BNNTs) for overall water splitting using popular density functional theory (DFT). Calculations of the structural, mechanical, electronic, and optical properties of the investigated systems were performed using both the generalized gradient approximation and the GW quasi-particle correction methods. In our calculations, it was observed that only (10, 10) and (12, 12) single-walled BNNTs (SWBNNTs) turned out to be stable toward MoS2 doping. Electronic property calculations revealed metallic behavior of (10, 10)-MoS2-doped SWBNNTs, while the band gap of (12, 12) SWBNNT was narrowed to 2.5 eV after MoS2 doping, which is within the obtained band gaps for other photocatalysts. Hence, MoS2 influences the conduction band of pure BNNT and improves its photocatalytic properties. The water-splitting photocatalytic behavior is found in (12, 12) MoS2-doped SWBNNT, which showed higher water oxidation (OH-/O2) and reduction (H+/H2) potentials. In addition, optical spectral calculations showed that MoS2-doped SWBNNT had an optical absorption edge of 2.6 eV and a higher absorption in the visible region. All of the studied properties confirmed MoS2-doped SWBNNT as a better candidate for next-generation photocatalysts for hydrogen evolution through the overall water-splitting process.

The effect of increasing temperature on simulated nanocomposites reinforced with SWBNNs and its effect on characteristics related to mechanics and the physical attributes using the MDs approach

This study examines the effect of increasing temperature (300, 350, 400, 450 and 500 K) on simulated nanocomposites reinforced with exploration of the impact of single-walled boron nitride nanotubes (SWBNNTs) on both the mechanical properties (including Young's modulus, Poisson's ratio, shear modulus, and bulk modulus) and the physical property of density, achieved through molecular dynamics (MDs) simulations. MDs utilized to simulate nanocomposite models consisting of five case studies of SWBNNs with different chiralities (5, 0), (10, 0), (15, 0), (20, 0), and (25, 0) as the reinforcement and using thermoplastic polyurethane (TPU) as the common matrix. The results reveal that with increasing temperature and chiralities of SWBNNTs, the density and Poisson's ratio increase dramatically, and Young's, shear, and bulk moduli decrease continuously. At a consistent temperature, there is a noteworthy trend in the mechanical properties of SWBNNTs with various chiralities. This includes the increase in Young's modulus, Poisson's ratio, shear modulus, and bulk modulus in the simulated nanocomposite, ranging from SWBNNTs (5, 0) to (25, 0). Similarly, the physical property of density exhibits an increasing trend from SWBNNTs (5, 0) to (20, 0) and then decreases at SWBNNTs (25, 0). To validate the accuracy of these findings, a Radial Distribution Function (RDF) diagram is generated using Materials Studio software.

Covalent modification of single-walled boron nitride nanotube (BNNT) with amino acids: Ab initio method

Based on the First principles density functional theory (DFT), a pristine single-walledBoron Nitride nanotube (BNNT) is chosen and functionalized with amino acids (glutamine,glycine and serine)of increased concentrations. The structures were perturbed randomly to obtain the minimum value of the total energy difference and binding energy to predict their stability. Our results, predict that the energy bandgap for (5, 5) BNNT is found to be 4.46 eV, which shows a wide bandgap semiconducting behavior. The chemical potential of the pristine system is found to be -3.81 eV. Chemical potential analysis indicates a red shift in the functionalized nanotubes. Upon chemical modification, the increase in partial charges ≥ 0.4 e and p character ≥ 70% shows a stable covalent interaction and an increase in the ionicity of BN bonds. The presence of amino acids tunes the polarity of nanotubes to be used as possible nanocarriers in drug delivery applications.

The Study of Interaction of Melphalan with SWCNT- BNNT Through Force Fields Molecular Mechanics and Quantum Calculations in Different Solvents and Temperatures

Abstract:In this study, the interaction of Melphalan, which is an anti-cancer medicine, with Singlewall carbon nanotubes (SWCNTs) and Boron nitride nanotubes (BNNTs), was investigated. Calculations were performed by using two methods of quantum mechanics and molecular mechanics. Thermodynamic parameters and Frontier Molecular Orbitals (FMOs) of the title compounds were evaluated by using the Density Functional Theory (DFT) calculations. The Quantum Mechanics calculations proved that BNNTs are more suitable carriers for Melphalan. Moreover, the interaction of Melphalan with SWCNTs and BNNTs at different temperatures was evaluated by Monte Carlo calculations. The MM+ force field was chosen as the most efficient field, and the HCl solvent has the lowest amount of energy and proven to be the most stable solvent for the simulation. The most significant finding obtained from this study is that the results of all types of calculations are in line with each other regarding both thermodynamic properties and conformers.

Investigating the thermoelectric properties of the (6, 6) two sided-closed single-walled boron nitride nanotubes ((6, 6) TSC-SWBNNTs) due to the impurity of a single carbon atom and temperature changes

In this research, the thermoelectric properties of the (6, 6) two sided-closed single-walled boron nitride nanotube ((6, 6) TSC-SWBNNT) was investigated in the state without impurity and carbon atom impurity instead of boron and nitrogen atoms in the center, left, and right the nanotube. The test conditions were the energy range of −5.5 to 5.5 eV and temperatures of 200, 300, 500, 700, 900, 1100, and 1300 K. Based on the obtained results, with an increase in temperature and the creation of impurities, the band gap is affected and becomes noticeably smaller. At the temperature of 1300 K, the band gap shows the greatest decrease and the peak height shows the least decrease. With increasing temperature, the number of peaks has decreased, suggesting an increase in the mobility of electrons and holes and a decrease in their localization. The Seebeck coefficient figures also changed by replacing carbon atoms with boron and nitrogen atoms in different parts of the nanotube. In addition, the height of the heat conduction peaks increased with increasing temperature. However, the heat conduction values are generally in the range of 9–10 nm, which are small values. With the increase in temperature, ZT values increased such that the highest values corresponded to the temperature of 1300 K. The ZT values higher than 1, especially at high temperatures, show that (6, 6) TSC-SWBNNT nanotubes are suitable candidates for thermoelectric materials.

The influence of single carbon atom impurity on the electronic transport of (6, 3) two side-closed single-walled boron nitride nanotubes

In this study, the electronic transport of (6, 3) two side-closed single-walled boron nitride nanotubes ((6, 3) TSC-SWBNNTs) located between two electrodes of (5, 5) conductive carbon nanotubes is investigated.Introducing carbon impurities instead of nitrogen and boron atoms in different locations of two side-closed (6, 3) SWBNNTs would change the transmission spectrum and reduce the gap. As the bias voltage increases, the peaksof the transmission spectrum become sharper and narrower. Substituting carbon impurities instead of nitrogenatoms leads to larger currents than substituting carbon impurities instead of boron. For the carbon impurity insteadof N atoms, the current in the center is observed to be larger than currents on the left and right sides. In addition,negative resistance can be seen in current-voltage diagrams, which are used in the construction of high-speedelectronic switches in electrical circuits. Due to the larger number of atoms, the study of structures by the common approach is time-consuming and sometimes impossible. Hence, in this research, the Quantum ATK simulation software is used to investigate the characteristics of electronic transport. For this purpose, the Slater-Koster method and the tight-closure approximation are used. In this method, the non-equilibrium Green function (NEGF) approach is employed.

Nonlinear vibration analysis of curvy single-walled boron nitride nanotube using mathematical modeling for dynamic responses

This work explores the feasibility of nonlinear behavior of doubly clamped single-walled boron nitride nanotube (SW-BNNT)-based nanoresonator. A nonlinear mathematical model of wavy SW-BNNT has been developed for analyzing the geometrical nonlinearity. Dynamic responses of nonlinear model have been analyzed for different waviness factors varying from 0.01 to 0.06 using various tools like time series, phase space, Poincaré map and Fast Fourier Transforms (FFTs). For the analysis, 20[Formula: see text]nm length of SW-BNNT has been considered. It has been observed from nonlinear analysis, that for responses with a lower value of waviness (e.g., 0.01) for 20[Formula: see text]nm long BNNT, the system’s nature loses its periodicity and shows onset of chaos with dense spectrum in Poincaré maps and irregular pattern in time response. Thus, it is concluded that chaotic response with a less strange attractor has been observed when waviness is 0.01. It is also concluded that, with increase in waviness factor from 0.02 to 0.06, the system showed the multi-periodic response with 2-T, 3-T and 4-T periods. The dynamic responses with varying waviness showed that the system behavior is changing from chaotic to periodic. This change in periodicity is one of the characteristics of chaotic solution.

Dynamic analysis of boron nitride nanotube using different boundary conditions under influence of vacancy defect: Insights from finite element method

In this paper, the effect of defects such as atomic vacancies (single atom vacancies like boron atom(Vb) or nitrogen atom vacancy(Vn) and di-atomic vacancies(VB-N) corresponds to vacancy of one boron and adjacent one nitrogen atom) on the vibrational behaviour of a single-walled boron nitride nanotube (SWBNNT) are investigated using finite element modelling in context of their applicability as mass sensors. The change in resonance frequency produced by vacancy defects at various points throughout the span when a mass is connected to the tip has also been examined. Space frames with three-dimensional components and point masses have been used to explain the cantilevered armchair (5, 5) and zigzag (10, 0) SW-BNNT with changing vacancy defects. The position of atomic vacancy has been varied (i.e., at 25 %, 50 % and 90 % of length from fixed end) to investigate the effect of location of defect along the length on the resonance behaviour of SW-BNNT. Further defective BNNTs have been analysed in such a way that the same can be used as nanomechanical resonator for detecting small mass of the order of femtogram level. For this,a small mass (varying from 10-20 to 10−25kg) is attached at the tip of cantilevered SW-BNNT. Substantial drop in frequency shift is observed when the position of defect moves toward the free end. the excitation frequency of defective BNNT is larger near the free end as compared to pristine one.

The potentials of boron-doped (nitrogen deficient) and nitrogen-doped (boron deficient) BNNT photocatalysts for decontamination of pollutants from water bodies.

This work investigates the structural, elastic, electronic, and photoabsorption properties of boron- (N-deficient) and nitrogen- (B-deficient) doped single-walled boron nitride nanotube (SWBNNT) for photocatalytic applications for the first time. All calculations of the optimized systems were performed with DFT quantum simulation codes. The results of the structural analysis showed that SWBNNT is stable to both B and N dopants. It was also observed that the photodecomposition activity of the B-doped nanotube improved significantly under the condition of slight compressive stress, while it decreased for the N-doped nanotube. Therefore, N-doped SWBNNT showed poor performance under external pressure. Both B and N-doped systems could narrow the wide band gap of SWBNNT to the photocatalytic region below 3 eV, therefore this material can be used as photocatalysts in water splitting for hydrogen evolution, dye degradation, wastewater treatment, etc. Analysis of the optical properties revealed that B-doped SWBNNT absorbs more photons in the visible range than the N-doped SWBNNT and can therefore be considered as a more efficient photocatalyst. In addition, it was found that all doped nanotubes are anisotropic since the absorption in one direction of nanotube axes is worse than the other.

Single-wall boron nitride nanotubes encapsulating conjugated bithiophene molecule: Raman analysis

In this paper, we investigate the hybrid systems based on single-wall boron nitride nanotubes (SWBNNTs) encapsulating the bithiophene molecule (2T) by a combination of molecular dynamics, density functional theory (DFT), and spectral moments method (SMM). The structural stability of the hybrid systems 2T@SWBNNTs is explored using Lennard-Jones potential. We computed the optimal SWBNNTs diameter for which the resulting hybrids 2T@SWBNNTs are stable. Then, we report the calculated Raman spectra of the 2T molecule and the (7,7) SWBNNT before and after encapsulation. The structural stability and the occurrence (or not) of a possible charge transfer in the hybrid 2T@ (7,7) are investigated by analyzing the encapsulation effects on the Raman active modes of the (7,7) nanotube, as well as on those of the 2T molecule.

Piezoelectric and dielectric constants of topologically defected boron nitride nanotubes.

Boron nitride nanotubes (BNNTs) form a promising low-dimensional piezoelectric material that may be used for multifunctional energy-harvesting nanopiezoelectronic devices. The piezoelectric and dielectric constants of a single-walled BNNT are determined via a molecular dynamics simulation with several Tersoff-like potential models and Born effective charges. The effect of Stone-Wales (SW) defects on the electroelastic behavior of BNNTs is considered, owing to their importance in determining the multifunctionality of BNNTs. Both solid and hollow cylinder structures are considered as equivalent continuous tubular structures that represent the BNNT in terms of electroelasticity. Direct and reverse piezoelectricity of the BNNTs are simulated by applying elastic strain and a constant electric field along the longitudinal direction of each tube, respectively. The initial polarization of the BNNTs changes, owing to the rotation of boron and nitrogen atoms. The Tersoff potential model considered herein predicts an increase in the dielectric constant with the SW defect, which is attributable to the opposite electric displacement of nitrogen and boron atoms under an electric field. It is also observed that the elastic modulus of BNNTs is degraded by the SW defect. However, the piezoelectric constants of the BNNTs either increase or decrease as the SW defect accumulates, exhibiting a strong dependency on the applied Tersoff potential model. The performance of each Tersoff potential model in describing the geometry of the SW defect and the effect of a change in polarization and electric displacement on the electroelasticity of BNNTs is discussed. The results herein offer a deep insight into the application of BNNTs to nanopiezoelectronics and a guideline to designing the optimal BNNT composition and topology for multifunctional energy-harvesting nanocomposites.

Axial and torsional free vibrations of restrained single-walled boron nitride nanotube (SWBNNT) embedded in an elastic medium via nonlocal strain gradient theory

The effect of the surrounding elastic matrix on the axial and torsional vibrations of embedded single-walled boron nitride nanotube (SWBNNT) is studied in this work. The SWBNNT is modeled as a nanorod and the nonlocal strain gradient theory is utilized to derive the size-dependent equation of motion. Also, the surrounding elastic matrix is simulated by employing a one-parameter foundation model. Stokes’ transformation method as an accurate and efficient mathematical tool in conjunction with the Fourier trigonometric series solution is utilized to discretize the equation of motion. These two mathematical methods create two different eigenvalue problems, which can solve any boundary conditions for torsional and axial vibrations. Calculated Eigen frequencies are validated with earlier works in the literature. The effects of the surrounded elastic matrix parameter together with the other deformable boundary coefficients and also the small scale parameter on the axial and torsional vibration frequencies are researched and some conclusions are outlined.

Thermoatomic analysis of monovacancy defected single-walled boron nitride nanotube under quasi-static strain: Insights from molecular dynamics

The structural and thermal stability of single-walled boron nitride nanotubes (SWBNNTs) under strained conditions are essential in pursuit of their applications that are subjected to high temperature processing and/or working environment. However, there are dearth of high temperature (>1000 K) studies on SWBNNTs and other forms of hexagonal boron nitride (h-BN) structures, either with or without defects, due to experimental difficulties. In this paper, an atomistic approach is adopted to perform uniaxial tensile and torsional quasi-static straining of pristine and monovacancy defected SWBNNTs at different temperatures while the thermal stability of monovacancy defected SWBNNT structures were predicted from its mean square displacement during equilibration up to 2400 K temperature. During uniaxial tensile and torsional straining, Young's modulus, Poisson's ratio, and shear modulus values of monovacancy defected SWBNNTs decreases on increasing the temperature condition compared to high temperature. For instance, in case of monovacancy concentration of 0.2% in SWBNNTs, the increase in temperature from 300 K to 2400 K reduced its Young's, Poisson's ratio, axial strain and shear moduli by 14%, 19%, 50.5%, and 87.28%, respectively. Findings of this work may have future implication in extreme temperature applications of BNNT-based structural materials.