Carbon Nanotube Intramolecular Junctions Research Articles

Enhanced oxygen reduction reaction activity by utilizing carbon nanotube intramolecular junctions

DFT calculations are performed to study the enhanced oxygen reduction reaction (ORR) activity by utilizing carbon nanotube intramolecular junctions (IMJs). The obtained results suggest that the adsorption of ORR species on the basal carbon of the pristine nanotubes (armchair and zigzag structures) is too weak, but significant enhancement of adsorption energy could be found on the same position of two types of typical IMJ structures (straight and bent junctions), with the values closer to Pt(111). Furthermore, ORR curves catalyzed by two IMJs all become downhill in the relative energy diagram, suggesting the spontaneous nature of the catalytic process. These obtained results indicate that IMJs possess higher catalytic activity towards ORR. Total electron density and deformation electron density analysis reveal that the improved adsorption of ORR species could be caused the electrostatic interaction between the adsorbed ORR species and IMJs, where the electrons transfer from the latter to the former.

Tutorial: Concepts and numerical techniques for modeling individual phonon transmission at interfaces

At the nanoscale, thermal transport across the interface between two lattice insulators can be described by the transmission of bulk phonons and depends on the crystallographic structure of the interface and the bulk crystal lattice. In this tutorial, we give an account of how an extension of the Atomistic Green’s Function (AGF) method based on the concept of the Bloch matrix can be used to model the transmission of individual phonon modes and allows us to determine the wavelength and polarization dependence of the phonon transmission. Within this framework, we can explicitly establish the relationship between the phonon transmission coefficient and dispersion. Details of the numerical methods used in the extended AGF method are provided. To illustrate how the extended AGF method can be applied to yield insights into individual phonon transmission, we study the (16,0)/(8,0) carbon nanotube intramolecular junction. The method presented here sheds light on the modal contribution to interfacial thermal transport between solids.

Growth modes and chiral selectivity of single-walled carbon nanotubes

Chemical vapor deposition synthesis of single-walled carbon nanotubes, using an Fe catalyst, and alternating methane and carbon monoxide as carbon feedstocks, leads to the reversible formation of junctions between tubes of different diameters. Combined with an atomistic modeling of the tube/catalyst interface, this shows that the ratio of diameters of the tube and its seeding particle, denoting the growth mode, depends on the carbon fraction inside the catalyst. With carbon monoxide, nanoparticles are strongly carbon enriched, and tend to dewet the tube, in a perpendicular growth mode. Cross-checking our results with the available reports from the literature of the last decade strongly suggests that these latter conditions should favor the near armchair chiral selectivity observed empirically.

Direct Preparation of Carbon Nanotube Intramolecular Junctions on Structured Substrates.

Leveraging the unique properties of single-walled carbon nanotube (SWNT) intramolecular junctions (IMJs) in innovative nanodevices and next-generation nanoelectronics requires controllable, repeatable, and large-scale preparation, together with rapid identification and comprehensive characterization of such structures. Here we demonstrate SWNT IMJs through directly growing ultralong SWNTs on trenched substrates. It is found that the trench configurations introduce axial strain in partially suspended nanotubes, and promote bending deformation in the vicinity of the trench edges. As a result, the lattice and electronic structure of the nanotubes can be locally modified, to form IMJs in the deformation regions. The trench patterns also enable pre-defining the formation locations of SWNT IMJs, facilitating the rapid identification. Elaborate Raman characterization has verified the formation of SWNT IMJs and identified their types. Rectifying behavior has been observed by electrical measurements on the as-prepared semiconducting-semiconducting (S-S) junction.

ZnO quantum dots decorated on optimized carbon nanotube intramolecular junctions exhibit superior field emission properties

Sensitive optical and superior field emission properties can be attained through the use of ZnO selective quantum well heterostructures grown on carbon nanotubes to fabricate carbon-zinc-oxide (CZO) nanotubes.

Analysis of longitudinal vibration band gaps in periodic carbon nanotube intramolecular junctions using finite element method

The longitudinal vibration band gaps in periodic (n, 0)–(2n, 0) single-walled carbon nanotube(SWCNT) intramolecular junctions(IMJs) are investigated based on the finite element calculation. The frequency ranges of band gaps in frequency response functions(FRF) simulated by finite element method (FEM) show good agreement with those in band structure obtained by simple spring-mass model. Moreover, a comprehensive parametric study is also conducted to highlight the influences of the geometrical parameters such as the size of unit cell, component ratios of the IMJs and diameters of the CNT segments as well as geometric imperfections on the first band gap. The results show that the frequency ranges and the bandwidth of the gap strongly depend on the geometrical parameters. Furthermore, the influences of geometrical parameters on gaps are nuanced in IMJs with different topological defects. The existence of vibration band gaps in periodic IMJs lends a new insight into the development of CNT-based nano-devices in application of vibration isolation.

Wave propagation in carbon nanotube intramolecular junctions: finite element calculations

The dispersion of longitudinal and transverse waves in (n,0)–(2n,0) intramolecular junctions (IMJs) are investigated using an atomistic finite element method (FEM). The transient responses of IMJs with different connection types subjected to harmonic incident wave were modelled using three-dimensional elastic beams of carbon bonds and point masses. The linkage between the force-field constants of molecular mechanics and input parameters of beam and mass elements was established through the molecular structural mechanics approach. The wave dispersion simulated by FEM shows good agreement with that of the non-local elastic model in a wide frequency range up to the terahertz region. It is shown that both the microstructure of conical part (connection part) and the coupling of longitudinal vibration and transverse vibration brought by the conicity play important roles in the dispersion of longitudinal and transverse wave in a single-walled IMJ. The amplitude decay of longitudinal wave depended on the distance propagating; the wavelength and the structure in connection part are examined. The results show that the dispersion of the decay of the wave amplitude in IMJ with less pentagon–heptagon defects has a better agreement with analytical results of macroscopic conical shell.

First-principles study of mechanical properties of one-dimensional carbon nanotube intramolecular junctions

The mechanical properties of single-walled carbon nanotubes with one-dimensional intramolecular junctions (CNT-IMJs) are investigated, using first-principles density functional theory calculations. The effects of pentagon–heptagon (5–7) defects (a pair of five- and seven-membered rings) at a junction on the Young’s modulus, tensile strength, and breaking strain of CNT-IMJs are discussed from the viewpoint of charge density and interatomic distance. Our calculations indicate that the deformation concentration on a seven-membered ring causes the strength and elongation of CNT-IMJs to decrease. It is found that the tensile strength and breaking strain of CNT-IMJs depend on the position of 5–7 defects, while these properties of CNT-IMJs are not significantly affected by the number of 5–7 defects. The applicability of the AIREBO classical interatomic potential in the simulation of the tensile deformation in CNT-IMJs is also discussed.

Separation of Hydrogen Using a Size-Changeable Nanochannel

On the basis of a carbon nanotube intramolecular junction and a C60, a molecular sieve for H2 is proposed. The small interspace formed between C60 and the junction provides a size changeable nanochannel for the permselectivity of H2 while blocking Ne and Ar. The sieving mechanism is discussed in terms of soft repulsion of H2 and the size of the nanochannel.

Design of a one-way nanovalve based on carbon nanotube junction and C60

We report the conceptual construction of a one-way nanovalve based on a carbon nanotube intramolecular junction and a C60. Using molecular dynamics simulations, we demonstrate the flow behavior of helium through the nanovalve as controlled by the pressure balance around C60. The van der Waals interaction between C60 and carbon nanotube keeps C60 close to the junction, while the pressure balance around C60 can juggle it between two adsorption sites, which in turn closes or opens the nanovalve.

Interface states in carbon nanotube junctions: Rolling up graphene

We study the origin of interface states in carbon nanotube intramolecular junctions between achiral tubes. By applying the Born-von Karman boundary condition to an interface between armchair- and zigzag-terminated graphene layers, we are able to explain their number and energies. We show that these interface states, costumarily attributed to the presence of topological defects, are actually related to zigzag-edge states, as those of graphene zigzag nanoribbons. Spatial localization of interface states is seen to vary greatly and may extend appreciably into either side of the junction. Our results give an alternative explanation to the unusual decay length measured for interface states of semiconductor nanotube junctions and could be further tested by local probe spectroscopies.

Comparison of the Properties of Bent and Straight Single-Walled Carbon Nanotube Intramolecular Junctions.

The properties of four finite-length bent and straight intramolecular junctions (IMJs) connecting two armchair and zigzag single-walled carbon nanotube segments, viz. (3,3)-(6,0) and (4,4)-(8,0), were investigated. Their structures were calculated using the density functional theory (DFT) methods at the B3LYP/6-31G(d) level of theory. The results indicate that the bent junctions are more stable than the straight ones due to the energetically favored defect structures. Remarkable differences of the HOMO and LUMO orbitals appear between the straight and the bent IMJs. The spin-unrestricted calculations at the same level of theory were also performed to obtain the antiferromagnetic-type ground state, suggesting that the spin polarizations mainly occur on the zigzag edge and the defect rings of the straight (4,4)-(8,0) IMJ and induce marked changes of the electronic structures. Additionally, the energy band structures of the four junctions with periodic boundary conditions were calculated based on DFT calculations using generalized gradient approximation with the Perdew and Wang function. The calculated band gaps suggest that the conductance of the straight IMJs is higher than the bent ones.

Electronic Structure of Straight and T-Shape Singled-Wall Carbon Nanotube Junctions: A Molecular Quantum Mechanics Study

In this paper, the straight and T-shape single-walled carbon nanotube intramolecular junction (SWCNT-IMJ) were studied theoretically. The geometries of topological defect can be constructed by fusing two nanotubes having different helicities and diameters. For straight IMJ, the armchair (5,5) and (8,8) nanotubes segments were fused with the zigzag (n,0) segment varying from (6,0) to (10,0). T-junctions were constructed by joining a zigzag (n,0) tube, varying from n = 5 to 10, onto a fixed zigzag (9,0) tube sidewall. These junctions are composed of one or more pentagon and heptagon rings, and mixed pentagon-heptagon pairs as defects in the perfect hexagonal lattice. The study shows that the occurrence of defects geometries on SWCNT-IMJ and T-shape structures depends on the variation of tube helicity and diameter of combining nanotubes. The HOMO-LUMO energy levels exhibit the even-odd “quantum size” oscillation. The decreasing energy gap depends on variation of tube helicity and diameter of carbon nanotube. This study can be helpful as a knowledge base in the field of carbon nanotube molecular electronics.

Thermal rectifiers from deformed carbon nanohorns

We study thermal rectification in single-walled carbon nanohorns (SWNHs) by using anon-equilibrium molecular dynamics (MD) method. It is found that the horns with thebigger top angles show larger asymmetric heat transport due to the larger structuralgradient distribution. This kind of gradient behavior can be further adjusted byapplying external strain on the SWNHs. After being carefully elongated along theaxial direction, thermal rectification in the elongated SWNHs can become moreobvious than that in undeformed SWNHs. The maximum rectification efficiency ofSWNHs is much bigger than that of carbon nanotube intramolecular junctions.

Transition from direct tunneling to field emission in carbon nanotube intramolecular junctions

Transport measurements through metal-semiconductor carbon nanotube intramolecular junctions were carried out at high gate voltages in which regime the influence of Schottky barrier to charge transport is weak. The I-Vds curves exhibit an inflection point in the form of ln(I∕Vds2)−1∕Vds, showing a transition of transport mechanism from direct tunneling to field emission. The findings are interpreted in terms of quantum tunneling through a rectanglelike barrier at the junction, with a barrier width of ∼4nm, in good agreement with that observed on pentagon-heptagon defects at nanotube junctions via scanning tunneling spectroscopy.

Temperature-mediated growth of single-walled carbon-nanotube intramolecular junctions

Single-walled carbon nanotubes (SWNTs) possess superior electronic and physical properties that make them ideal candidates for making next-generation electronic circuits that break the size limitation of current silicon-based technology. The first critical step in making a full SWNT electronic circuit is to make SWNT intramolecular junctions in a controlled manner. Although SWNT intramolecular junctions have been grown by several methods, they only grew inadvertently in most cases. Here, we report well-controlled temperature-mediated growth of intramolecular junctions in SWNTs. Specifically, by changing the temperature during growth, we found that SWNTs systematically form intramolecular junctions. This was achieved by a consistent variation in the SWNT diameter and chirality with changing growth temperature even though the catalyst particles remained the same. These findings provide a potential approach for growing SWNT intramolecular junctions at desired locations, sizes and orientations, which are important for making SWNT electronic circuits.

Raman characteristic peaks induced by the topological defects of carbon nanotube intramolecular junctions

The vibrational modes of some single-wall carbon nanotube (SWNT) intramolecular junctions (IMJs) have been calculated using the latest Brenner reactive empirical bond order (REBO) potential, based on which their nonresonant Raman spectra have been further calculated using the empirical bond polarizability model. It is found that the Raman peaks induced by pentagon defects lie out of the $G$-band of the SWNTs; thus, the high-frequency part of the Raman spectra of the SWNT IMJs can be used to experimentally determine their detailed geometrical structures. Also, the intensity of the Raman spectra has a close relation with the number of pentagon defects in the SWNT IMJs. Following the Descartes-Euler polyhedral formula, the number of heptagon defects in the SWNT IMJs can also be determined. The first-principles calculations are also performed, verifying the results obtained by the REBO potential. The $G$ band width of the SWNT IMJ can reflect the length of its transition region between the pentagon and heptagon rings.

A simple way to CN x/carbon nanotube intramolecular junctions and branches

Large quantities of CN x /carbon nanotube intramolecular junctions were successfully synthesized on a silicon substrate via the chemical vapor deposition method by pyrolysis of ferrocene and melamine. The nanotubes have two apparently different sections, one made of carbon with an empty hollow cylinder structure, and the other made of carbon nitride with a bamboo-like structure. Three- and four-terminal CN x /carbon nanotube intramolecular branches were also observed, and such multi-terminal structures offer potential applications for future nanodevices. A simple model is suggested for the synthesis of these CN x /carbon nanotube intramolecular junctions and branches.

Quantum interference in carbon nanotube intramolecular junction

The quantum conductance oscillations (QCOs) of the intramolecular junction (IMJ) composed of two single-wall carbon nanotubes (SWNTs) have been studied by using a π-orbital only tight-binding (TB) model and a Green's function technique. It is found that in the IMJs in addition to the rapid QCO frequencies corresponding to the constituent tubes there exist also their sum frequencies. The slow QCO frequencies of the IMJ will be different from those of its corresponding two perfect tubes if they have different chiral angles. PACS. 73.40.Jn Metal-to-metal contacts - 73.63.Fg Nanotubes - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals

Raman Spectral Imaging of a Carbon Nanotube Intramolecular Junction

Confocal Raman spectral imaging results are presented for a metal-to-semiconductor intramolecular junction (IMJ) on an isolated carbon nanotube. Spectra observed at the junction are consistent with the symmetry lowering expected from the occurrence of pentagon-heptagon defects to produce the chirality shift. The IMJ transition zone is sharp and preserves the nanotube diameter. These results have significant implications for the future use of IMJs as electronic devices, including how prevalent these structures are and how their growth may be rationally targeted. Raman imaging has been demonstrated to be a powerful tool for IMJ studies and provides a more accessible method for further studies of IMJ structure and growth.