Electron transport properties of ordered networks using carbon nanotubes

Large tunability in the mechanical and thermal properties of carbon nanotube-fullerene hierarchical monoliths

Carbon based materials have attracted much attention as building blocks in technologically relevant nanocomposites due to their unique chemical and physical properties. Here, we propose a new class of hierarchical carbon based nano-truss structures consisting of fullerene joints attached with carbon nanotubes as the truss forming a three-dimensional network. Atomistic molecular dynamics simulations allow us to systematically demonstrate the ability to simultaneously control the mechanical and thermal properties of these structures, elucidating their unique physical properties. Specifically, we perform uniaxial tensile and compressive loading to show that by controlling the length of the carbon nanotube trusses, the mechanical properties can be tuned over a large range. Furthermore, we utilize the Green-Kubo method under the equilibrium molecular dynamics simulations framework to show that the thermal conductivities of these structures can be manipulated by varying the densities of the overall structures. This work provides a computational framework guiding future research on the manipulation of the fundamental physical properties in these organic-based hierarchical structures composed of carbon nanotubes and fullerenes as building blocks.

Developing nanotube junctions with arbitrary specifications

Experimentally synthesized carbon nanotube (CNTs) junctions (either single or with 2D/3D CNT network topology) are expected to have random orientation of defect sites (non-hexagonal rings) around the junction. This random and irregular nature of the junction topology and defect characteristics is expected to affect their strength and durability as well as impact the associated mesoscopic and macroscopic properties. On the contrary, theoretical and computational studies often investigate structure-property relationships of pristine and regular junctions of carbon nanostructures. In this study, we developed a computational framework to model a variety of junction structures between CNTs with arbitrary spatial (orientation and degree of overlap) and intrinsic (chirality) specifications. The developed computational model also has the ability to tune the degree of topological defects around the junction via a variety of defect annihilation approaches. Our method makes use of the primal/dual meshing concept, where the development and manipulation of the junction nodes occur using triangular meshes (primal mesh), which is eventually converted to its dual mesh (honeycomb mesh) to render a fully covalently bonded CNT junction. Here each carbon atom has 3 bonded neighbors (mimicking sp² hybridization). Under a given set of CNT orientation, overlap and chirality specifications, the approach creates a number of CNT junction configurations with varying degrees of energetic stability, offering an opportunity to investigate the effect of topological arrangement of defects around the junction on mechanical, electrical and thermal properties. In addition, it is shown via few examples that the discussed methodology can easily be extended to create multi-junction nanotube clusters, multi-wall nanotube junctions, as well as true 3D random network structures.

Molecular dynamics simulations of thermal transport in porous nanotube network structures

Carbon nanotube based 3D nanostructures have shown a lot of promise towards designing next generation of multi-functional systems, such as nano-electronic devices. Motivated by their recent successful experimental synthesis as well as characterization, and realizing that thermal dissipation is an important concern in proposed devices because of ever-increasing power density, we have investigated the phononic thermal transport behavior in 3D porous nanotube network structures using reverse non-equilibrium molecular dynamics simulations. Based on our study, the length scale associated with the distance between nanotube junctions emerges as the most dominating parameter that governs phonon scattering (hence the characteristic mean free path) and the heat flow in these nanostructures at molecular length scales. However, because of their spatial inhomogeneity, we show that the aerial density of carbon nanotubes (normal to heat flow) is also of critical importance in determining their system-level thermal conductivity. Based on our findings, we postulate that both parameters should be considered while designing nano-devices where thermal management is relevant.

Electronic transport properties of carbon nanotoroids

We investigate the electronic transport properties of carbon nanotori covalently connected to external electrodes made up of carbon nanotubes of various chiralities. The study is based on computing ballistic transport characteristics within the framework of Green's function theory using a simple π-orbital tight-binding model. The calculations focus on the effect of the relative angle made by the electrodes as they are placed at different positions along the nanoring. The conductance behavior is found to depend on the details of the atomic structure of the torus but also on the positions of the electrodes. Our findings are rationalized using an elementary quantum mechanical interference model, which reproduces well the main features of the numerical data.

Guiding electrical current in nanotube circuits using structural defects: a step forward in nanoelectronics

Electrical current could be efficiently guided in 2D nanotube networks by introducing specific topological defects within the periodic framework. Using semiempirical transport calculations coupled with Landauer-Buttiker formalism of quantum transport in multiterminal nanoscale systems, we provide a detailed analysis of the processes governing the atomic-scale design of nanotube circuits. We found that when defects are introduced as patches in specific sites, they act as bouncing centers that reinject electrons along specific paths, via a wave reflection process. This type of defects can be incorporated while preserving the 3-fold connectivity of each carbon atom embedded within the graphitic lattice. Our findings open up a new way to explore bottom-up design, at the nanometer scale, of complex nanotube circuits which could be extended to 3D nanosystems and applied in the fabrication of nanoelectronic devices.