Investigation of the interactions between molecules of β-Carotene, Vitamin A and CNTs by MD simulations

Two typical phenomena (wrapping and filling), mainly about the interactions between biological molecules and carbon nanotubes (CNTs), were investigated by performing molecular dynamics (MD) simulations. We calculated the center of mass (COM) distance and the interaction energy between the biological molecules and single-walled nanotubes (SWNTs). The influence of nanotube wall number, chirality, radius and temperature was also investigated by a series of MD simulations. The results indicated that Vitamin A and β-Carotene were two promising biomaterials for decoration of CNTs. The interactions between biological molecules and CNTs could be influenced by those four factors. The general conclusions derived from this study may be of importance in medical and biological areas.     

Tuning the conductance of a molecular switch

The ability to control the conductance of single molecules will have a major impact in nanoscale electronics. Azobenzene, a molecule that changes conformation as a result of a trans/cis transition when exposed to radiation, could form the basis of a light-driven molecular switch. It is therefore crucial to clarify the electrical transport characteristics of this molecule. Here, we investigate, theoretically, charge transport in a system in which a single azobenzene molecule is attached to two carbon nanotubes. In clear contrast to gold electrodes, the nanotubes can act as true nanoscale electrodes and we show that the low-energy conduction properties of the junction may be dramatically modified by changing the topology of the contacts between the nanotubes and the molecules, and/or the chirality of the nanotubes (that is, zigzag or armchair). We propose experiments to demonstrate controlled electrical switching with nanotube electrodes.     

Modelling peptide nanotubes for artificial ion channels

We investigate the van der Waals interaction of D,L-Ala cyclopeptide nanotubes and various ions, ion-water clusters and C(60) fullerenes, using the Lennard-Jones potential and a continuum approach which assumes that the atoms are smeared over the peptide nanotube providing an average atomic density. Our results predict that Li(+), Na(+), Rb(+) and Cl(-) ions and ion-water clusters are accepted into peptide nanotubes of 8.5 ? internal diameter whereas the C(60) molecule is rejected. The model indicates that the C(60) molecule is accepted into peptide nanotubes of 13 ? internal diameter, suggesting that the interaction energy depends on the size of the molecule and the internal diameter of the peptide nanotube. This result may be useful for the design of peptide nanotubes for drug delivery applications. Further, we also find that the ions prefer a position inside the peptide ring where the energy is minimum. In contrast, Li(+)-water clusters prefer to be in the space between each peptide ring.     

Imaging the passage of a single hydrocarbon chain through a nanopore

Molecular transport through nanoscale pores in films, membranes and wall structures is of fundamental importance in a number of physical, chemical and biological processes. However, there is a lack of experimental methods that can obtain information on the structure and orientation of the molecules as they pass through the pore, and their interactions with the pore during passage. Imaging with a transmission electron microscope is a powerful method for studying structural changes in single molecules as they move and for imaging molecules confined inside carbon nanotubes. Here, we report that such imaging can be used to observe the structure and orientation of a hydrocarbon chain as it passes through nanoscale defects in the walls of a single-walled carbon nanotube to the vacuum outside, and also to study the interactions between the chain and the nanopore. Based on experiments at 293 K and 4 K we conclude that the major energy source for the molecular motions observed at 4 K is the electron beam used for the imaging.     

Spectroscopic evidence and molecular simulation investigation of the pi-pi interaction between pyrene molecules and carbon nanotubes

The pi-pi interaction between pyrene molecules and single-walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs) was studied by fluorescence, FTIR, Raman spectroscopy and molecular simulation. The carbon nanotubes were incubated in pyrene solution and dried for characterization. A broadband fluorescence emission at 463 nm of the incubated samples was observed, which is similar to that of pyrene excimers but shifts to shorter wavelength. The typical FTIR bands of pyrene shift to lower wavenumbers in the incubated samples. D- and G-bands in Raman spectra of SWNTs also shift to low frequencies. All these spectroscopic evidences reveal the stronger pi-pi stacking interaction between the nanotubes and pyrene molecules over the pyrene dimers, which leads to the formation of pyrene-carbon nanotube complexes. The systems of SWNTs and pyrene molecules were also studied with molecular simulation. It was found from the binding energy calculation that a stronger interaction presents between the pyrene molecule and the nanotube. In addition, the simulation gives some structural information about the pyrene-nanotube complex, such as the staggered conformation of pyrene on nanotube. The effect of defects in carbon nanotube sidewall was also discussed.     

Modulating the performance of carbon nanotube field-effect transistors via Rose Bengal molecular doping

A simple, reliable, and large scale ambient environment doping method for carbon nanotubes is a highly desirable approach for modulating the performance of nanotube based electronics. One of the major challenges is doping carbon nanotubes to simultaneously offer a large shift in threshold voltage and an improved subthreshold swing. In this paper, we report on modulating the performance of carbon nanotube field-effect transistors (CNTFETs) by rationally selecting doping molecules. We demonstrated that Rose Bengal sodium salt (RB-Na) molecular doping can effectively shift the threshold voltage (ΔVth) of CNTFETs up to ～6 V, decrease the subthreshold swing down to 130 mV/decade, and increase the effective field-effect mobility to 5 cm2 V(-1) s(-1). It is also shown that CNTFETs doped with Rose Bengal lactone (RBL) show a smaller variation in ΔVth (～2 V) and subthreshold swing than those doped by RB-Na, which can be attributed to the difference in their molecular structures. The observed right-shift of the threshold voltage is attributed to the positive charge doping of the nanotube conduction channel from Rose Bengal molecules. The resultant lowering of the subthreshold swing is due to the reduced Schottky barrier at the CNT/metal/molecule interface. This room temperature chemical doping approach provides an efficient, simple, and cost-effective method to fabricate highly reliable and high-performance nanotube transistors for future nanotube based electronics.     

Tuning the conductance of carbon nanotubes with encapsulated molecules

It was recently shown that a molecule encapsulated inside a carbon nanotube can be used to devise a novel type of non-volatile memory element. At the heart of the mechanism for storing and reading information is the new concept of a molecular gate where the molecule acts as a passive gate that hinders the flow of electrons for a given position relative to the nanotube host. By systematically exploring the effects of encapsulation of an acceptor molecule in a series of carbon nanotubes, we show that the reliability of the memory mechanism is very sensitive to the interaction between the nanotube host and the molecule guest.     

Adhesion between two radially collapsed single-walled carbon nanotubes

Continuum mechanics analysis and molecular mechanics simulations are performed to study adhesion between two identical, radially collapsed single-walled carbon nanotubes. Not only the inter-adhesion energy between nanotubes but also the inner adhesion energy in a nanotube is considered. A closed-form solution to the adhesion configuration is achieved, which is well consistent with our molecular mechanics simulation. Comparing the potential energy of the adhesion structures formed by two identical single-walled carbon nanotubes, three types of configurations, i.e., circular, deformed, and collapsed shape, will be formed with increasing carbon nanotubes radius and separated by two critical radii of the single-walled carbon nanotube. Furthermore, it is found that the collapsed adhesion structure possesses the highest interfacial adhesion energy. The results demonstrate that, as a potential application in carbon nanotube reinforced composites, arrays formed by collapsed carbon nanotubes will be optimal due to the strong interface strength.     

Chemisorption of hydrogen molecules on carbon nanotubes: charging effect from first-principles calculations

We report a systematic investigation of the charging effect on hydrogen molecule chemisorption on (3, 3), (5, 5), (5, 0), and (8, 0) carbon nanotubes by first-principles calculations. The influence of injected charge on the chemisorption energy barriers is found to be sensitive to the nanotube diameter and chirality. The calculated results also indicate that electron injection is more effective in lowering the energy barrier for armchair carbon nanotubes while hole injection is more effective for zigzag nanotubes. The origin of these interesting trends and systematics can be understood by a close examination of the underlying electronic structure and the electron transfer between the hydrogen molecules and the nanotubes.     

Atomically resolved mechanical response of individual metallofullerene molecules confined inside carbon nanotubes

The hollow core inside a carbon nanotube can be used to confine single molecules and it is now possible to image the movement of such molecules inside nanotubes. To date, however, it has not been possible to control this motion, nor to detect the forces moving the molecules, despite experimental and theoretical evidence suggesting that almost friction-free motion might be possible inside the nanotubes. Here, we report on precise measurements of the mechanical responses of individual metallofullerene molecules (Dy@C82) confined inside single-walled carbon nanotubes to the atom at the tip of an atomic force microscope operated in dynamic mode. Using three-dimensional force mapping with atomic resolution, we addressed the molecules from the exterior of the nanotube and measured their elastic and inelastic behaviour by simultaneously detecting the attractive forces and energy losses with three-dimensional, atomic-scale resolution.     

Design of a Two-State Shuttle Memory Device

In this study, we investigate the mechanics of a metallofullerene shuttle memory device, comprising a metallofullerene which is located inside a closed carbon nanotube. The interaction energy for the system is obtained from the 6-12 Lennard-Jones potential using the continuum approximation, which assumes that a discrete atomic structure can be replaced by an average atomic surface density. This approach shows that the system has two equal minimum energy positions, which are symmetrically located close to the tube extremities, and therefore it gives rise to the possibility of being used as a two-state memory device. On one side the encapsulated metallofullerene represents the zero information state and by applying an external electrical field, the metallofullerene can overcome the energy barrier of the nanotube, and pass from one end of the tube to the other end, where the metallofullerene then represents the one information state. By appropriately selecting different nanotube geometries, the memory device can be designed to have various data transfer rates. In particular, design parameters are presented for the optimization of the data transfer rates and the stabilization of the data storage. The former involves optimization of the nanotube length and the applied electric field, while the latter involves the nanotube radius and the choice of metallofullerene.     

The effect of nanotube radius on the constitutive model for carbon nanotubes

We investigate the effect of nanotube radius on the constitutive model of single wall carbon nanotubes. We adopt a modified Cauchy–Born rule to incorporate the interatomic potential into the continuum analysis, and such an approach ensures the equilibrium of atoms. It is shown that the nanotube radius has little effect on the mechanical behavior of single wall carbon nanotubes subject to simple tension or pure torsion, while the nanotube orientation has somewhat larger influences.     

Interaction of molecular and atomic hydrogen with single-wall carbon nanotubes

Density functional calculations are performed to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Molecular physisorption is predicted to be the most stable adsorption state, with the molecule at equilibrium at a distance of 5-6 a.u. from the nanotube wall. The physisorption energies outside the nanotubes are approximately 0.07 eV, and larger inside, reaching a value of 0.17 eV inside the (5,5) nanotube. Although these binding energies appear to be lower than the values required for an efficient adsorption/desorption operation at room temperature and normal pressures, the expectations are better for operation at lower temperatures and higher pressures, as found in many experimental studies. A chemisorption state with the molecule dissociated has also been found, with the H atoms much closer to the nanotube wall. However, this state is separated from the physisorption state by an activation barrier of 2 eV or more. The dissociative chemisorption weakens carbon-carbon bonds, and the concerted effect of many incoming molecules with sufficient kinetic energies can lead to the scission of the nanotube.     

Effect of elliptical deformation on molecular polarizabilities of model carbon nanotubes from atomic increments

The interacting induced dipole polarization model implemented in our program POLAR is used for the calculation of the dipole-dipole polarizability alpha. The method is tested with single-wall carbon nanotube models as a function of nanotube radius and elliptical deformation. The results for polarizability follow the same trend as reference calculations performed with our version of the program PAPID. For the zigzag tubes, the polarizability is found to follow a remarkably simple law, that is, it varies as the inverse of the radius. A dramatic effect is also found with elliptical deformation. It is found that the polarizability and related properties can be modified continuously and reversibly by the external radial deformation. These results suggest an interesting technology in which mechanical deformation can control chemical properties of the carbon nanotubes. POLAR calculations differentiate more effectively than PAPID computations among single-wall nanotube models with increasing radial deformation. Different effective polarizabilities are calculated for the atoms at the highest and lowest curvature sites. POLAR calculations discriminate more efficiently than PAPID computations between the effective polarizabilities of the highest and lowest curvature sites. This remarkable and significant tunable polarizability can have important implications for metal coverage of metals on nanotubes and selective adsorption and desorption of foreign atoms and molecules on nanotubes and can lead to a wide variety of technological applications, such as catalysts, hydrogen storage, magnetic tubes, etc.     

First-principles study of physisorption of nucleic acid bases on small-diameter carbon nanotubes

We report the results of our first-principles study based on density functional theory on the interaction of the nucleic acid base molecules adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U), with a single-walled carbon nanotube (CNT). Specifically, the focus is on the physisorption of base molecules on the outer wall of a (5, 0) metallic CNT possessing one of the smallest diameters possible. Compared to the case for CNTs with large diameters, the physisorption energy is found to be reduced in the high-curvature case. The base molecules exhibit significantly different interaction strengths and the calculated binding energies follow the hierarchy G>A>T>C>U, which appears to be independent of the tube curvature. The stabilizing factor in the interaction between the base molecule and CNT is dominated by the molecular polarizability that allows a weakly attractive dispersion force to be induced between them. The present study provides an improved understanding of the role of the base sequence in deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) in their interactions with carbon nanotubes of varying diameters.     

Chiral selectivity in the charge-transfer bleaching of single-walled carbon-nanotube spectra

Chiral selective reactivity and redox chemistry of carbon nanotubes are two emerging fields of nanoscience. These areas hold strong promise for producing methods for isolating nanotubes into pure samples of a single electronic type, and for reversible doping of nanotubes for electronics applications. Here, we study the selective reactivity of single-walled carbon nanotubes with organic acceptor molecules. We observe spectral bleaching of the nanotube electronic transitions consistent with an electron-transfer reaction occurring from the nanotubes to the organic acceptors. The reaction kinetics are found to have a strong chiral dependence, with rates being slowest for large-bandgap species and increasing for smaller-bandgap nanotubes. The chiral-dependent kinetics can be tuned to effectively freeze the reacted spectra at a fixed chiral distribution. Such tunable redox chemistry may be important for future applications in reversible non-covalent modification of nanotube electronic properties and in chiral selective separations.     

Plumbing carbon nanotubes

Since their discovery, the possibility of connecting carbon nanotubes together like water pipes has been an intriguing prospect for these hollow nanostructures. The serial joining of carbon nanotubes in a controlled manner offers a promising approach for the bottom-up engineering of nanotube structures--from simply increasing their aspect ratio to making integrated carbon nanotube devices. To date, however, there have been few reports of the joining of two different carbon nanotubes. Here we demonstrate that a Joule heating process, and associated electro-migration effects, can be used to connect two carbon nanotubes that have the same (or similar) diameters. More generally, with the assistance of a tungsten metal particle, this technique can be used to seamlessly join any two carbon nanotubes--regardless of their diameters--to form new nanotube structures.     

Carbon nanotube/detergent interactions via coarse-grained molecular dynamics

Detergent interactions with carbon nanotubes are of potential importance in a number of bionanotechnology applications. We investigate the interaction of lysophospholipids with single-walled carbon nanotubes via coarse-grained molecular dynamics. We present compelling evidence that the mechanism of adsorption of these detergents onto a carbon nanotube is dependent upon detergent concentration. Furthermore, the chirality of the carbon nanotube influences the detergent wrapping angle for low detergent concentration. These findings advance our understanding of the mechanism of carbon nanotube solubilization via detergent molecules.     

Interaction of methane with carbon nanotube thin films: role of defects and oxygen adsorption

This paper deals with the dependence of the electrical conductance on the presence of structural defects and of molecular oxygen adsorbates in carbon nanotube (CNT) thin films for gas molecule detection. Our results show that oxygen contamination may be responsible for the reported sensitivity of the electronic and transport properties to methane at room temperature. In particular, the sample exhibits a crossover from decreasing to increasing electrical resistance vs. methane concentration depending on the surrounding atmosphere. The obtained results show that when the nanotube walls contain topological defects, oxygen molecules become chemisorbed. We suggest that the conductivity type of the CNT can be changed from p-type to n-type by adsorption of O₂ acting as an electron and donor doping the CNTs, which has p-type semiconductor character in the outgassed state. The obtained results demonstrate that nanotubes could be used as sensitive chemical gas sensor likewise indicate that intrinsic properties measured on as-grown nanotubes may be severely changed by extrinsic oxidative treatments.     

Supported lipid bilayer/carbon nanotube hybrids

Carbon nanotube transistors combine molecular-scale dimensions with excellent electronic properties, offering unique opportunities for chemical and biological sensing. Here, we form supported lipid bilayers over single-walled carbon nanotube transistors. We first study the physical properties of the nanotube/supported lipid bilayer structure using fluorescence techniques. Whereas lipid molecules can diffuse freely across the nanotube, a membrane-bound protein (tetanus toxin) sees the nanotube as a barrier. Moreover, the size of the barrier depends on the diameter of the nanotube--with larger nanotubes presenting bigger obstacles to diffusion. We then demonstrate detection of protein binding (streptavidin) to the supported lipid bilayer using the nanotube transistor as a charge sensor. This system can be used as a platform to examine the interactions of single molecules with carbon nanotubes and has many potential applications for the study of molecular recognition and other biological processes occurring at cell membranes.