Carbon Nanotube Junctions Research Articles

A Mechanism of Resistance Switching in CNT Based Memory Devices

Carbon nanotubes (CNTs) show promise for high energy efficiency electronic devices due to their high electrical conductivity. One possible application is in the fabrication of resistance-change computer memory, where the application of an external electrical bias can use a CNT layer as the switching element in a RAM cell. Memory cells of this type have been extensively demonstrated, where the electrical resistance of a switching layer of CNTs can be reversibly controlled by the application of external bias [1].To better understand these devices, we have developed a nanoscale model of their electrical switching. Towards this end, first-principles calculations of the electrical conductivity of CNT-CNT junctions were performed using the Density Functional Tight Binding (DFTB) in combination with the Non-Equilibrium Greens functions (NEGF) approach, enabling an improved understanding of the tunnelling of electrons across a range of CNT junctions [2]. These results demonstrate that the conductivity of CNT contacts depends on the overlap area between nanotubes and exponentially on the distances between the carbon atoms of the interacting CNTs. Secondly, we employed coarse-grained molecular dynamics [3,4] to produce nanoscale dense CNT film models like those used in the devices. We devise a set of metrics for characterising the structural properties of CNT films, and apply them to a set of model films designed to mimic those used in microelectronics devices. In order to understand how the geometrical properties of CNTs affect the film structure, we analyse films with different CNT chirality and length distributions. Unlike previous studies that focused on relatively low-density systems below ∼0.4 g/cm3, we also consider films with density of ∼0.6 g/cm3, as employed in experimentally fabricated CNT-based NRAM devices. By adding amorphous carbon to the film models, in varying amounts, we are then able to assess its effects. Finally, these two elements were combined to produce electrical simulations of the conductivity of the CNT films connected to TiN electrodes. These electrodes are shown to be partially oxidized during the device fabrication. The electrical conductivity in a device is characterized by formation of conduction paths formed by CNTs between the electrodes and connected via the CNT junctions. The conductivity is modulated by charging of CNTs and amorphous carbon present in the film.Using these simulations, we propose a resistance switching mechanism in TiN/CNT/TiN devices based on the physical movement of CNTs. During the initial operation of the device, oxygen is released from the oxidized bottom electrode and this readily reacts with individual CNTs to form charge trapping sites. Once these sites have become charged, they enable controllable movement of CNTs at the bottom electrode under the application of electric field. By reversibly connecting and disconnecting such CNTs from the bottom electrode, the electrical conductivity through the CNT film as a whole can be controlled.

On comparative analysis of graph entropies of symmetrical carbon nanotube Y-junctions

Entropy is crucial in statistical mechanics, thermodynamics, and information theory as it measures a system’s level of randomness or disorder. Entropy is widely used in mathematical chemistry and computational physics to predict the behavior of a system under various conditions. Among numerous carbon nanotube structures, three-terminal carbon nanotube junctions are important structures not only for electrical but also for mechanical appliances. Recently, significant attention has been given to the understanding of carbon nanotube junctions. This research paper focuses on calculating graph entropies based on Zagreb indices for symmetrical single-walled armchair carbon nanotube Y − junctions and comparing the index-entropies of these junctions. The study aims to demonstrate the behavior of the nanotube Y − junctions by adding atoms at the end of the tubes.

Role of the Carbon Nanotube Junction in the Mechanical Performance of Carbon Nanotube/Polyethylene Nanocomposites: A Molecular Dynamics Study.

Carbon nanotube (CNT)-based networks are promising reinforcements for polymer nanocomposites without the issue of CNT agglomeration. In this study, the CNT junction, a vital and representative structure of CNT-based networks, was applied as the reinforcement of the polyethylene (PE) matrix. The tensile properties of the CNT-junction/PE nanocomposite were investigated via molecular dynamics (MD) simulations and compared with those of pure PE matrix and conventional CNT/PE nanocomposites. The CNT junction was found to significantly increase the mechanical properties of the PE matrix. The Young's modulus, yield strength, and toughness rose by 500%, 100%, and 200%, respectively. This mechanism is related to the enhanced interfacial energy, which makes the polymer matrix denser and stimulates the bond and angle deformations of the polymer chains. Furthermore, the CNT junction demonstrated a more profitable reinforcement efficiency compared to conventional straight CNTs in the PE matrix. Compared to the ordinary CNT/PE model, the improvements in the Young's modulus and toughness induced by the CNT junction were up to 60% and 25%. This is attributed to the reduced mobility induced by the geometry of the CNT junction and stronger interfacial interactions provided by the Stone-Wales defects of the CNT junction, slowing down the void propagation of the nanocomposite. With the understanding of the beneficial reinforcing effect of the CNT junction, this study provides valuable insights for the design and application of CNT-based networks in polymer nanocomposites.

Novel Enhancement Opportunities in the Thermal and Electrical Conductivity of α-Al-CNTs+AgNPs Nanocomposites

In this study, effort was made to develop novel, cutting-edge composite materials consisting of conducting Al-CNTs and green synthesis silver nanoparticles (AgNPs). Spark plasma sintering (SPS) and very intense ball milling were used to develop the composites. The nanocomposites' microstructure, thermal and electrical conductivity were determined. Al-4%CNTs was refined into finer grains when AgNPs are present. The Al-4%CNTs+2%Ag.NPs composite produces a higher dislocation density because of the production of sub-grain. Al-AgNPs + CNTs can be used to make conductors with a high aspect ratio and lower contact resistance at the CNT junctions. It was established that enhanced electrical and thermal conductivity can be obtained using the developed AgNPs from sustainable materials to increase the dispersion of CNTs in Al for the production of high tensile conductors.

CP-AFM Molecular Tunnel Junctions with Alkyl Backbones Anchored Using Alkynyl and Thiol Groups: Microscopically Different Despite Phenomenological Similarity.

In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of n-alkyl chains anchored with acetylene groups (CnA; n = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (I-V) characteristics of CnA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance GAg < GAu < GPt is found to follow the same ordering as the metal work function ΦAu < ΦAu < ΦPt, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CnT) to which we refer in view of some similarities. Furthermore, unlike in CnT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CnA an additional MO shift, whose independence of size (n) rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CnA and CnT junctions different. What ultimately makes CnA unique at the microscopic level is a salient difference never reported previously, namely, that CnA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CnA from the CnT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.

Controlling the electronic transport through the carbon nanotube junction via the local electronic environment

The nonvertical edges of SWNT-4CH greatly hinder the electronic signature. Removing and replacing edge positions can significantly increase the current. The current through SWNTs is twenty times greater than that through SWNT-4CH.

Thermal Transport Study in a Strained Carbon Nanotube and Graphene Junction Using Phonon Wavepacket Analysis

This study investigates single-mode phonon scattering from a junction structure consisting of a (6,6) single-walled carbon nanotube (SWCNT) and graphene, subject to mechanical deformation, using phonon wavepacket analysis. Results show that longitudinal acoustic (LA) and transverse acoustic (TA) phonons at low frequencies are transmitted more effectively through the SWCNT–graphene junction when the junction is deformed. As low-frequency phonons in LA and TA modes are major energy carriers, it is expected that thermal transport across the SWCNT–graphene junction will be more efficient when the junction is deformed. Interfacial thermal resistance across the SWCNT-graphene junction was calculated using reverse nonequilibrium molecular dynamics (RNEMD). The RNEMD results show that the interfacial thermal resistance decreases when the structure is elongated, deforming the junction between the SWCNT and graphene. However, there was no notable difference in the transmission of twisting (TW) and flexural (FO) phonons when the junction was deformed. The study also showed that the transmission of phonon energy through the SWCNT–graphene junction has a slight dependence on the group velocity of phonons, with phonons having higher group velocities transmitting the junction more effectively. The findings of this research will play a significant role in advancing the development of futuristic electronics by providing a tool for developing 3D carbon nanostructures with high thermal performance under mechanical deformation.

A Hybrid Nanocomposite Based on the T-Shaped Carbon Nanotubes and Fullerenes as a Prospect Material for Triple-Value Memory Cells.

Relying on empirical and quantum chemical methods, a hybrid nanocomposite based on the T-shaped carbon nanotube (CNT) junction and internal fullerene C60 is proposed as a potential triple-value memory cell. The T-shaped CNT provides three potential wells where the internal fullerene can be located. The fullerene can move between these wells under the periodic external electric field, whose strength and frequency parameters are identified. The process of the fullerene's motion control corresponds to the memory cell write operation. The read operation can be realized by determining the fullerene's position inside the CNT by estimation of the charge transfer between a fullerene and the CNT's walls. Calculations took into account such external factors as temperature and air environment.

Computational analysis of tunneling conduction in piezoresistive carbon nanotube polymer composites

Carbon nanotube polymer composites exhibit piezoresistivity that can be utilized for flexible strain sensors. Piezoresistive modeling is critical for improving sensor performance, but remains a challenging subject due to the complex CNT network microstructure and the various effects occurring at both the micro and nano scales. Tunneling conduction between CNTs is the primary mechanism underlying piezoresistivity in carbon nanotube polymer composites. Existing models have employed a myriad of assumptions on the CNT network kinematics and the tunneling conduction behavior, but their physical basis and effects have not been thoroughly studied. In this paper, a detailed evaluation on these modeling assumptions is presented with the goal of establishing an approach without ad hoc assumptions and improved predictive accuracy. A computational model that is capable of tracking the history of tunneling separation of each CNT junction is first constructed. The effects of CNT extensibility, tunneling cross-section, conduction channels are studied via statistical analysis of the tunneling conduction and comparison of computed piezoresistive response. It is found that CNT inextensibility leads to higher predicted piezoresistivity, the tunneling cross-section area in the Simmons approximation is a highly influential parameter, and the Landauer formula provides a more physically based approach that accounts for CNT properties.

Phosphorus and nitrogen codoped entangled carbon nanotubes forming spongy materials: Synthesis and characterization

Carbon sponges are of great interest of the scientific community given their potential in advanced applications such as water cleaning, catalysis, energy storage, and mechanical reinforcement. These carbon nanomaterials improve their attributes for unexpected properties and applications by doping with foreign atoms. Nitrogen‑phosphorus-codoped and functionalized carbon nanotube sponges (NP-CNTSs) were synthesized, using the aerosol-assisted chemical vapor deposition (AACVD) method. We used acetonitrile and triphenylphosphine (TPP) as nitrogen and phosphorus precursors. According to our results, the NP-CNTSs were formed by entangled multi-walled carbon nanotubes (MWCNTs) of diameters ranging between 60 and 300 nm with Fe2P and Fe3C nanoparticles in the walls and inside the carbon nanotube sponges (CNTSs) respectively. The Fourier transform infrared (FTIR) characterization revealed the vibrational modes of aromatic and aliphatic CH, PO, PH, and OCN bonds. The NP-CNTSs undergo a quasi-reversible hydroquinone/quinone redox process as revealed by cyclic voltammetry studies. They also efficiently retain organic solvents, absorbing up to 40 times their weight for dichlorobenzene. The XPS and Raman characterizations show the presence of pyridinic nitrogen, which might be responsible for the formation of cavities in the wall of the CNTSs and the entanglement of carbon nanotubes, which promotes the growth of the NP-CNTSs. To study the role of nitrogen and phosphorus in the electronic properties of carbon materials, we carried out calculations proposed in the density functional theory (DFT) for phosphorus‑nitrogen codoping bilayer graphene and carbon nanotube junctions containing pentagonal, hexagonal, and heptagonal carbon rings.

Atomistic Modeling of the Electrical Conductivity of Single‐Walled Carbon Nanotube Junctions

Carbon nanotubes (CNTs) have many interesting properties that make them a focus of research in a wide range of technological applications. In CNT films, the bottleneck in charge transport is typically attributed to higher resistance at CNT junctions, leading to electrical transport characteristics that are quite different from individual CNTs. Previous simulations confirm this; however, a systematic study of transport across junctions is still lacking in the literature. Herein, density functional tight binding (DFTB) theory combined with the nonequilibrium Green's functions (NEGF) method is used to systematically calculate current across a range of CNT junctions. A random sampling approach is used to sample an extensive library of junction structures. The results demonstrate that the conductivity of CNT contacts depends on the overlap area between nanotubes and exponentially on the distances between the carbon atoms of the interacting CNTs. Two models based solely on the atomic positions of carbon atoms within the nanotubes are developed and evaluated: a simple equation using only the smallest C–C separation and a more sophisticated model using the positions of all C atoms. These junction current models can be used to predict transport in larger‐scale simulations where the CNT fabric structure is known.

Modeling and characterization of the electrical conductivity on metal nanoparticles/carbon nanotube/polymer composites

Flexible conductive films have good deformability and conductivity, and are expected to be used in flexible electronic devices. In this paper, four kinds of flexible conductive films were successfully prepared by compounding nano-sized metal (Ni, Cu, Au or AuCu alloy) particles to CNT surface and then dispersing to polydimethylsiloxane matrix. Experiment results show that the conductivity of these prepared films are almost two orders of magnitude higher than that of CNT/polydimethylsiloxane films with the same CNT loadings. A simulation model based on percolation network theory and Monte Carlo technology is introduced to study the influence of nanoparticles on the composite conductivity. Results confirmed that the introduction of nanoparticles effectively reduces the effective resistance of CNT and the tunnelling resistance at CNT junctions. The intrinsic conductivity and the length diameter ratio of CNT, the intrinsic conductivity, the size and the coverage ratio of nanoparticles are the core parameters affecting the conductivity of composite. Compared with CNT/polydimethylsiloxane films, the optimized theoretical conductivity of these nano-sized particles enhanced composites can be further improved.

Comparative Study of Y‐Junction Nanotubes with Vertex‐Edge Based Topological Descriptors

The current results of various forms of carbon nanostructures and its applications in different areas attract the researchers. In pharmaceutical, medicine, industry and electronic devices they used it by its graphical invariants. The detection of different types of carbon nanotubes junctions enhanced the attention and interest for forthcoming devices like transistors and amplifiers. A topological index plays a very important role in the study of physicochemical properties of biological and chemical structures. In this paper, we determine results of ve‐degree topological indices for various type of carbon nanotubes Y‐junctions and their comparisons. The particular indices called as The first ve‐degree Zagreb β index, the second ve‐degree Zagreb index, ve‐degree Randic index, ve‐degree atom‐bond connectivity index, ve‐degree geometric‐arithmetic index, ve‐degree harmonic index and ve‐degree sum‐connectivity index.

Achieved negative differential resistance behavior of Si/B-substituted into a C6 chain sandwiched between capped carbon nanotube junctions.

Electronic transport properties of a pristine C6 chain and Si/B-substituted into the C6 chain sandwiched between two (5, 5) capped carbon nanotube electrodes were investigated through first-principles calculations based on non-equilibrium Green's functions (NEGF) conjugated with density functional theory (DFT). Si and B substitutions will affect the I–V curve of a pristine C6 chain. In the I–V characteristics, multi negative differential resistance (NDR) with large peak to valley ratio (PVR) and rectifying actions were observed. The NDR behavior originates from the joining and moving of conduction orbitals inside and outside of the bias window at a certain bias voltage. Furthermore, the assessment of transmission coefficient and distribution of molecular orbitals reveals that the rectifying performance is the result of the asymmetric distribution of the frontier molecular orbitals in the central region and their coupling with the electrodes. Multi NDR behavior of B substitution under very low bias voltage is a unique property of our proposed devices. Moreover, the CNT|C–(B–C)2–C|CNT molecular device shows a high PVR up to 31.8, which demonstrates that the proposed devices can be useful for molecular switching in nanoelectronic devices.

Enhanced Electron Transfer Kinetics of Covalent Carbon Nanotube Junctions

Enhanced Electron Transfer Kinetics of Covalent Carbon Nanotube Junctions

Multi-Terminal Nanotube Junctions: Modeling and Structure-Property Relationship

Carbon nanotube based multi-terminal junction configurations are of great interest because of the potential aerospace and electronic applications. Multi-terminal carbon nanotube junction has more than one carbon nanotube meeting at a point to create a 2D or 3D structure. Accurate atomistic models of such junctions are essential for characterizing their thermal, mechanical and electronic properties via computational studies. In this work, computational methodologies that uses innovative Computer-Aided Design (CAD) based optimization strategies and remeshing techniques are presented for generating such topologically reliable and accurate models of complex multi-terminal junctions (called 3-, 4-, and 6-junctions). This is followed by the prediction of structure-property relationship via study of thermal conductivity and mechanical strength using molecular dynamics simulations. We observed high degradation in the thermal and mechanical properties of the junctions compared to pristine structures which is attributed to high concentration of non-hexagonal defects in the junction. Junctions with fewer defects have better thermal transport capabilities and higher mechanical strengths, suggesting that controlling the number of defects can significantly improve inherent features of the nanostructures.

Computing and comparative analysis of topological invariants of Y‐junction carbon nanotubes

AbstractRecent findings of diverse forms of carbon nanostructures have encouraged research on their applications in several areas. They are used as a structural material in electronic and optical devices, involving transistors, photodetectors, plastic, biological sensors, pharmaceutical and medicine, and drug delivery systems. Since the discovery of Y‐shaped carbon nanotubes (CNTs) junctions, they have drawn interest for future electron devices, for example, three‐terminal transistors, amplifiers, and switches. A topological index is a number that is used to study the physicochemical properties of chemical structures. In this paper, we are providing topological properties of CNT Y‐junctions. The results obtained for different types of Y‐junction graphs appeared with comparisons. According to the best of our knowledge and literature, this is the first report about topological indices on this class of CNT junctions.

Thermoelectric Performance Enhanced by Destructive Quantum Interference in Nanoporous Carbon Nanotube Based Junctions

Aided by density functional theory with nonequilibrium Green's functions simulations, the thermoelectric (TE) properties of nanoporous carbon nanotube junctions formed by covalently binding carbon nanotubes via benzene bridges with either meta‐ or para‐connections are investigated. The results show that the TE performances of meta‐connected nanoporous carbon nanotube junctions are significantly improved due to the influence of destructive quantum interference (DQI) effect. Moreover, the TE properties of meta‐connected carbon nanotube junctions can be further improved by substitution of nitrogen atom and gate voltage. The reason is that the quantum interference patterns can be transformed from DQI to constructive quantum interference (or Fano resonance). The theoretical analysis shows that the TE figure of merit (ZT) for meta‐connected carbon nanotube junctions at room temperature is close to 0.5 near the Fermi level, ≈20 times higher than that of the perfect carbon nanotube junction. These results reveal that quantum interference effect can indeed be used to enhance the TE performance of molecular junctions.

Tri-molybdenum phosphide (Mo3P) and multi-walled carbon nanotube junctions for volatile organic compounds (VOCs) detection

Detection and analysis of volatile organic compounds' (VOCs) biomarkers lead to improvement in healthcare diagnosis and other applications such as chemical threat detection and food quality control. Here, we report on tri-molybdenum phosphide (Mo3P) and multiwalled carbon nanotube (MWCNT) junction-based vapor quantum resistive sensors (vQRSs), which exhibit more than one order of magnitude higher sensitivity and superior selectivity for biomarkers in comparison to pristine MWCNT junctions based vQRSs. Transmission electron microscope/scanning tunneling electron microscope with energy dispersive x-ray spectroscopy, x-ray diffraction, and x-ray photoelectron spectroscopy studies reveal the crystallinity and the presence of Mo and P elements in the network. The presence of Mo3P clearly enhanced the performance of vQRS as evidenced in sensitivity and selectivity studies. The vQRSs are stable over extended periods of time and are reproducible, making them a potential candidate for sensing related applications.

Reduction of atomistic ice tensile stress by graphene–carbon nanotube coating

The accumulation of ice has adverse effects on human activities on the ground and in the air while the dynamic mechanism of the interaction force between the ice and wall surface has not been fully explored. Herein, molecular dynamics (MD) simulations are conducted to investigate the underpinning physics of ice adhesion at the molecular level. The graphene and carbon nanotube junctions (G-CNTs) are proposed for the first time to be sandwiched between the ice and the hydrophilic (i.e., silver and copper) and hydrophobic (i.e., silicon) substrates. The results show that the ice adhesion stresses on these three substrates can be reduced by 91.80%, 91.59% and 58.75% due to the geometrical structure, buckling phenomenon and hydrophobicity of the G-CNTs, respectively, and the reduction can be further strengthened when there is a water lubrication layer. This work sheds light on the design of the icephobic wall surface and the development of de-icing techniques.