First-principles simulations of chemical reactions in an HCl molecule embedded inside a C or BN nanotube induced by ultrafast laser pulses

Selecting Carbon Nanotubes with Diameters of Less than 1 nm by Laser Pulses: An Ab Initio Exploration

Despite the thermal instability of carbon nanotubes (CNTs) with diameters of less than 1 nm, first-principles simulations indicate the possibility of selecting such narrow CNTs using laser pulses. The simulations suggested the possibility of selecting CNTs narrower than 1 nm under pulsed laser irradiation with a full width at half-maximum of 10 fs, a wavelength of 800 nm, and a maximum field intensity ranging from 4.5 to 5 V/Å when the polarization vector was set perpendicular to the CNT axis. This result was common to both zigzag and armchair CNTs, suggesting that the preferential survival of narrow CNTs is independent of the chirality. The mechanisms underlying the preferential survival of narrower CNTs are discussed from analogous simulations of graphene nanoribbons under various polarization directions of the laser field, and the possibility of selecting CNTs with subnanometer diameters is evaluated on the basis of the simulation results.

Photoinduced Dynamics in Carbon Nanotube Aggregates Steered by Dark Excitons

Only optically active excitons can be identified by transient absorption spectroscopy, and the actual mechanisms of exciton relaxation in nanoscale systems remain unknown as dipole-forbidden transitions and charge-transfer states are not accounted for. Focusing on interacting (6,4) and (8,4) carbon nanotubes (CNTs), we show that dark excitons largely determine the relaxation pathways for photogenerated excitons in CNT bundles. New channels appear involving asymmetric electron-hole excitations within the same CNT and charge-transfer states, in which the electron and hole are confined to separate CNTs. The energy and charge transfers are facilitated by coupling to both low- and high-frequency phonons. Radial breathing modes are particularly important because they distort the CNT geometry, induce crossings of electronic states, and modulate coupling between CNTs. The time domain simulations reported herein uncover the quantum states and phonon modes that contribute to exciton relaxation in a CNT cluster, elucidating the complete relaxation mechanism. The established role of optically dark states pertains to nonequilibrium dynamics in nanoscale materials in general.

Influence of point defects on the structural and electronic properties of SiC nanotubes

We have performed studies of the structural and electronic properties of functionalized single wall silicon carbide nanotubes (SW-SiCNTs) with chirality (5,5). Our first principles studies are done using density functional theory. The exchange-correlation energies are modeled according to the Hamprecht-Cohen-Tozer-Handy functional in the generalized gradient approximation (HCTH-GGA) and the DNP basis function with double polarization is applied. To determine the most stable geometry, we have applied the minimum energy criterion considering several initial configurations of the hydroxyl (OH) functional group interacting with the single wall SiCNT. In particular, we tested different orientations of the OH adsorbed on the nanotube surface (oriented towards the Si or C atoms) and at the end of the nanotube. Results indicate that the most favorable geometry corresponds to OH adsorption (chemisorption) at the end of the nanotube. The polarity increases yielding better conditions for solubility and dispersion. The work function of the SW-SiCNT-OH is reduced, which in turn favors conditions for field emission properties (FEPs). Finally, the presence of silicon or carbon mono-vacancies in the functionalized nanotubes suggests this may be a good structure to fabricate semiconductor devices

Pulse-induced nonequilibrium dynamics of acetylene inside carbon nanotube studied by an ab initio approach

Nanoscale molecular confinement substantially modifies the functionality and electronic properties of encapsulated molecules. Many works have approached this problem from the perspective of quantifying ground-state molecular changes, but little is known about the nonequilibrium dynamics of encapsulated molecular system. In this letter, we report an analysis of the nonequilibrium dynamics of acetylene (C(2)H(2)) inside a semiconducting carbon nanotube (CNT). An ultrashort high-intense laser pulse (2 fs width and 10(15) W/cm(2) intensity) brings the systems out of equilibrium. This process is modeled by comprehensive first-principles time-dependent density-functional simulations. When encapsulated, acetylene dimer, unlike a single acetylene molecule, exhibits correlated vibrational dynamics (C-C bond rotation and H-C-C bending) that is markedly different from the dynamics observed in the gas phase. This result highlights the role of CNT in modulating the optical electric field within the tube. At longer simulation timescales (> 20 fs) in the largest-diameter tube studied here [CNT(14,0)], we observe synchronized rotation about the C-C axes in the dimer and ultimately ejection of one of the four hydrogen atoms. Our results illustrate the richness of photochemical phenomena in confined geometries.

Water boiling inside carbon nanotubes: toward efficient drug release

We show using molecular dynamics simulation that spatial confinement of water inside carbon nanotubes (CNTs) substantially increases its boiling temperature and that a small temperature growth above the boiling point dramatically raises the inside pressure. Capillary theory successfully predicts the boiling point elevation down to 2 nm, below which large deviations between the theory and atomistic simulation take place. Water behaves qualitatively different inside narrow CNTs, exhibiting transition into an unusual phase, where pressure is gas-like and grows linearly with temperature, while the diffusion constant is temperature-independent. Precise control over boiling by CNT diameter, together with the rapid growth of inside pressure above the boiling point, suggests a novel drug delivery protocol. Polar drug molecules are packaged inside CNTs; the latter are delivered into living tissues and heated by laser. Solvent boiling facilitates drug release.