Carbon Nanotube Bundles Research Articles

Crosstalk Delay and Reliability Analysis of Carbon Nanotube On-Chip Interconnects: Modelling and Optimization using Metaheuristic Algorithm

In the field of VLSI technology, the miniaturization of ICs poses significant challenges to overall interconnect performance, particularly due to the rising concerns of crosstalk-induced reliability and durability issues. This work explores graphene-based randomly mixed carbon nanotube bundle (RMCB) interconnects as a viable solution for advanced reliable VLSI applications. Employing metaheuristic algorithms (Maximum Hole Degree, Particle Swarm Optimization, Stoyan and Yaskov Algorithm), this study seeks to optimize RMCB structures, maximizing carbon nanotube density within a fixed area. Notably, this study explores the effectiveness of algorithms’ performance in optimizing RMCB structures at a nano-technology node. Extensive signal integrity and reliability assessments, considering both rugged and pristine substrates, reveal that Stoyan and Yaskov (SY)-based optimization excels over PSO and MHD-based counterparts in terms of on-chip interconnect performance and reliability. The SY structure significantly dominates MHD-based (and PSO) counterparts by reducing crosstalk delay by 50% (30%), enhancing the average failure rate by 40% (37%), and improving electromigration reliability by 170% (63%).

Analysis of Delay and Dynamic Crosstalk in Spatially Arranged Mixed CNT Bundle Interconnects at Different Technology Nodes

In the present nanoscale regime, mixed carbon nanotube bundles (MCBs) are considered to be highly promising interconnect options. This research paper introduces a spatially arranged mixed carbon nanotubes (CNTs) bundle (MCB), wherein single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) occupy equal halves in the MCB. An equivalent single conductor (ESC) model for MCB is employed to analyze the interconnect performances in terms of signal transmission delay and dynamic crosstalk delay at different technology nodes (i.e., 32nm, 22nm, and 16 nm). Encouragingly, a significant reduction in signal transmission delay and dynamic crosstalk induced delay are observed at 32 nm technology node. It is observed that at 32 nm technology node, the propagation delay and crosstalk induced delay significantly improves by 29.40% and 55.53%, respectively, compared to 22 nm technology node and 187.88% and 185.94%, respectively, compared to 16 nm technology node. The improvement in interconnect performances can primarily be attributed to the improvement in the number of conducting channels inside the MCB at 32 nm, which greatly impacted the interconnect parasitics such as quantum resistance, quantum capacitance, kinetic inductance etc.

Nanocarbon and medicine: polymer/carbon nanotube composites for medical devices

AbstractIn view of wide-ranging application to the biomedical field, this work investigates the mechanical and electrical properties of a composite made of Single Wall Carbon Nanotubes (SWCNT) bundles self-grafted onto a poly-dimethyl-siloxane (PDMS) elastomer, particularly Sylgard 184, that has well assessed biocompatible properties and is commonly used in prosthetics. Due to the potential risks associated with the use of carbon nanostructures in implanted devices, we also assess the viability of cells directly grown on such composite substrates. Furthermore, as the stability of conductive, stretchable devices made of such composite is also crucial to their use in the medical field, we investigate, by different experimental techniques, the grafting of SWCNT bundles deep into PDMS films. Our findings prove that penetration of SWCNT bundles into the polymer bulk depends on heating time and carbon nanotubes can be seen beyond 150 μm from the surface. This is confirmed by direct electron microscopy observation of large bundles as deep as about 20 μm. The composites exhibit reliable mechanical and electrical responses that are more suitable to large and repeated deformation of the polymer with respect to thermoplastic based composites, suggesting a wide potential for their application to stretchable biomedical devices. Aiming at the proposed application of artificial bladders, a bladder prototype made of poly-dimethyl siloxane endowed with a printed SWCNT-based strain sensor was developed.

Elucidating slipping behaviors between carbon nanotubes: Using nitrogen doping and electron irradiation to suppress slippage

The slipping phenomenon of carbon nanotubes (CNTs) and nitrogen-doped CNTs (NCNTs) compromises their mechanical strength and potential applications. Elucidation of this microscopic phenomenon is essential for applying CNTs as high-strength materials. This study investigates the slip mechanisms in CNTs and NCNTs, with the aim of enhancing their mechanical properties. By combining various analytical techniques, we elucidated the adhesion and slipping behaviors of CNT bundles under various conditions. van der Waals forces were found to dominate the stick–slip phenomenon in stable CNT stacks. The introduction of amorphous carbon and subsequent electron irradiation led to the formation of stronger covalent bonds between the tubes, enhancing the mechanical resilience. Notably, NCNTs exhibited a higher frequency of covalent bond excitation by electron irradiation than CNTs. These findings indicate the crucial role of electron irradiation in strengthening the covalent bonds within CNT and NCNT bundles, marking a significant contribution to the mechanical applications of nanotechnology.

Investigation of Optical Interconnects for nano-scale VLSI applications

The relentless trend toward miniaturization has brought interconnects to the brink of communication bottlenecks, operating at or near their ampacity (current carrying capacity) limits. This degradation in performance necessitates novel technologies with increased ampacity. This study explores optical interconnect (OI) as a potential future technology, offering high-bandwidth communication and low latency. This study models and simulates OI, integrating recent optical device advancements with modest modulator and detector capacitance (50 fF). Performance metrics including signal delay, power dissipation, and power-delay-product (PDP) are compared across copper (Cu) and single-wall carbon nanotube bundle (SWCNT-B) interconnects at 22 nm and 14 nm technology nodes. Results consistently show OI, with an active voltage current feedback (AVCF) based regulated gain cascode (RGC) transimpedance amplifier (TIA) receiver, outperforms Cu and SWCNT-B interconnects at global lengths and beyond. For instance, at 1000 μm and 22 nm, OI exhibits 88.47 % and 62.15 % delay improvements over Cu and SWCNT-B, respectively. This trend persists at 14 nm, with OI showing 93.68 % and 84.29 % delay improvements, respectively. Additionally, OI surpasses Cu and SWCNT-B in power efficiency beyond a critical length. As interconnects extend to global scales and beyond, the differences in delay, power, and PDP between OI and electrical interconnect increases, highlighting OI's advantages for future VLSI IC applications.

Pragmatic structure optimization: Achieving optimal crosstalk delay and gate oxide reliability of randomly mixed CNT bundle interconnects

This study explores the potential of randomly mixed carbon nanotube bundle (RMCB) as a viable on-chip interconnect. Achieving high-quality carbon nanotubes (CNTs) with uniform diameters is challenging for the current framework of enhanced fabrication techniques. The Stoyan and Yaskov technique is employed to optimize CNT arrangement within a specified rectangular area. This method accounts for statistical variation in CNT diameters, offering a more realistic and fabrication-focused approach to designing CNT bundle interconnects. Eight such practical RMCB structures (RMCB-50 to RMCB-350) are selected using this technique, each characterized by distinct CNT counts and variable diameters. Comprehensive average crosstalk-delay and reliability assessments are conducted by comparing different CNT bundle interconnects with the best-optimized RMCB (O-RMCB) interconnect, placed on various dielectric substrates such as SiO2, SiC, BN. The study unequivocally indicates that O-RMCB produces highly favorable results and stands as the most suitable future solution for VLSI circuits. Additionally, the thickness optimization of O-RMCB interconnect is explored, yielding in improvements in both performance and reliability compared to other well-known CNT bundled interconnects.

Physical properties of industrially produced carbon nanotube yarns for use in structural nanocomposites

Industrially-produced carbon nanotube (CNT) networks are a promising base for high-performance, continuous CNT nanocomposites, motivating widespread use in research and application development. Floating catalyst chemical vapor deposition (FCCVD) is a scalable, widely-adopted approach for synthesis of CNT networks; however, FCCVD can yield networks with organic impurities which complicate their characterization and processing. Herein, we analyze two varieties of FCCVD-synthesized commercial CNT yarn to explore the effects of densification and purification as they relate to forming infiltrated polymer composites. First, the fluid permeabilities and porosities of neat roving and chemically-stretched CNT yarns (CSYs) are estimated and compared to gas adsorption studies, which suggest high-tenacity CSYs are largely impermeable, leading to dry-core composites when polymer infiltration is attempted. Next, the presence and effects of oxygen-rich amorphous carbon impurities in the CSYs are identified and found to exist at the scale of CNT bundles, wherein the amorphous carbon behaves like a hygroscopic polymer matrix in a composite. Removal of the amorphous carbon phase in a 130%-increase in specific surface area, as quantified by BET analysis, and a statistically significant reduction in tensile properties. We discuss challenges in estimating yarn alignment and crystallinity resulting from these factors, and place our results in context of past studies analyzing intrinsic organic coatings in FCCVD-synthesized CNT networks.

Frequency-domain analysis of CMOS-driven interconnects utilizing doped multilayer graphene nanoribbons and mixed carbon nanotube bundles

A frequency-domain model is developed to analyze isolated interconnects of multilayer graphene-nanoribbon (MLGNR) and mixed carbon-nanotube bundle (MCB) driven by CMOS gates. The model derived is founded on an equivalent-single-conductor model of MLGNR and MCB that takes thermal considerations into account (i.e. TD-ESC). The model includes the derivation of transfer function of interconnect to estimate its delay and bandwidth performance. The attained results, reveals that among the neutral MLGNR (N-MLGNR), intercalation doped MLGNR (ID-MLGNR) intercalated with FeCl3, MCB and Cu interconnects, FeCl3 ID-MLGNR achieves the best bandwidth efficiency. At a global interconnect length of 1 mm, FeCl3 ID-MLGNR outperforms N-MLGNR, MCB, and Cu in terms of bandwidth with an improved bandwidth value of 12.2 GHz, 7 GHz, and 61.4 GHz, respectively. Further, employing the proposed CMOS-gate-driven model, for FeCl3 ID-MLGNR, bandwidth is improved by nearly 7.52 × at global length (∼1 mm) in relation to the linear resistance model. Additionally, TD-ESC dependency of the proposed model reveals that FeCl3 ID-MLGNR becomes more stable as interconnect resistance increases.

Synthesis of Ultrahigh-Purity (6,5) Carbon Nanotubes Using a Trimetallic Catalyst.

Chirality-controlled synthesis of carbon nanotubes (CNTs) is one of the ultimate goals in the field of nanotube synthesis. At present, direct synthesis achieving a purity of over 90%, which can be called single-chirality synthesis, has been achieved for only two types of chiralities: (14,4) and (12,6) CNTs. Here, we realized an ultrahigh-purity (∼95.8%) synthesis of (6,5) CNTs with a trimetallic catalyst NiSnFe. Partial formation of Ni3Sn crystals was found within the NiSnFe nanoparticles. The activation energy for the selective growth of (6,5) CNTs decreased owing to the formation of Ni3Sn crystals, resulting in the high-purity synthesis of (6,5) CNTs. Transmission electron microscopy (TEM) reveals that one-dimensional (1D) crystals of periodic strip lines with 8.8 Å spacing are formed within the as-grown ultrahigh-purity (6,5) CNTs, which are well-matched with the simulated TEM image of closely packed 37 (6,5) CNTs with 2.8 Å intertube distance, indicating the direct formation of chirality-pure (6,5)-CNT bundle structures. The photoluminescence (PL) lifetime increases more than 20 times by the formation of chirality-pure bundle structures of (6,5) CNTs compared to that of isolated (6,5) CNTs. This can be explained by exciton delocalization or intertube excitons within bundle structures of chirality-pure (6,5) CNTs.

Nanoscale Heat Transport of Vertically Aligned Carbon Nanotube Bundles for Thermal Management Applications.

Electronic devices continue to shrink in size while increasing in performance, making excess heat dissipation challenging. Traditional thermal interface materials (TIMs) such as thermal grease and pads face limitations in thermal conductivity and stability, particularly as devices scale down. Carbon nanotubes (CNTs) have emerged as promising candidates for TIMs because of their exceptional thermal conductivity and mechanical properties. However, the thermal conductivity of CNT films decreases when integrated into devices due to defects and bundling effects. This study employs a novel cross-sectional approach combining high-vacuum scanning thermal microscopy (SThM) with beam-exit cross-sectional polishing (BEXP) to investigate the nanoscale morphology and thermal properties of vertically aligned CNT bundles at low and room temperatures. Using appropriate thermal transport models, we extracted effective thermal conductivities of the vertically aligned nanotubes and obtained 4 W m-1 K-1 at 200 K and 37 W m-1 K-1 at 300 K. Additionally, non-negligible lateral thermal conductance between CNT bundles suggests more complex heat transfer mechanisms in these structures. These findings provide unique insights into nanoscale thermal transport in CNT bundles, which is crucial for optimizing novel thermal management strategies.

Materials design using genetic algorithms informed by convolutional neural networks: Application to carbon nanotube bundles

Carbon nanotube (CNT) composites are promising but complex material systems whose mechanical response is influenced by many features, including CNT bundle microstructures. It is intractable to design CNT bundle microstructures using experiments and/or simulations alone. Thus, we implement a machine learning-based tool comprising a genetic algorithm (GA) (which aids in selecting features for bundle microstructures) and a convolutional neural network (CNN) (which rapidly predicts mechanical performance of bundle microstructures). Training data for the CNN are obtained from micromechanical finite element simulations. CNN predictions achieve R2>0.96 for elastic and shear moduli and R2>0.83 for Poisson’s ratios. Microstructures identified by the CNN-informed GA outperform between 79% and 100% of solutions found using a brute-force search and are found in less than 5% of the time, providing an efficient tool for informed materials design of CNT bundle microstructures.

(Invited) Collective Radial Breathing Modes in Homogeneous Nanotube Bundles

Carbon nanotubes (CNTs) exist in many atomic structures typically grow as mixtures of of many chiralities with a vast variety of properties. Identical CNTs that are arranged into two-dimensional hexagonal lattices, their vibrational properties have been predicted to change with additional low-frequency modes appearing in the Raman spectrum. [1] Up to now, the experimental study of collective vibrations has been limited by a lack of pure homogeneous chirality bundles. We overcome this challenge by employing a self-coil mechanism of a very long CNT that loop into itself during the growth process resulting in a tightly packed hexagonal stack [2]. This lattice is perfectly homogeneous in terms of diameter, chiral twist, and even handedness of the tube. By characterizing and comparing the physical properties of the coil with respect to its tails, the bundling effects are clearly visible. We report on two breathing-like modes for quasi-infinite bundles, compared to the single radial breathing mode characteristic for isolated tubes. The exciton-phonon coupling in these modes is probed with resonant Raman spectroscopy, revealing the same resonance energy for both breathing-like peaks. Additionally, we study the dependence of vibrational coupling on the tube diameter by analysing different tube’s diameter coils and other bundling geometries. Our experimental findings align well with previously reported theoretical studies, demonstrating a 1/d scaling for all modes, as well as confirming the relative shift of the modes dependent on intertube interaction. These vibrations provide insight into the role of intertube lattice dynamics in two-dimensional THz-range phononic crystals.[1] Popov, V. N.; Henrard, L. Evidence for the existence of two breathinglike phonon modes in infinite bundles of single-walled carbon nanotubes. Phys. Rev. B 2001, 63, 233407[2] Nakar, D.; Gordeev, G.; Machado, L. D.; Popovitz-Biro, R.; Rechav, K.; Oliveira, E. F.; Kusch, P.; Jorio, A.; Galvao, D. S.; Reich, S.; others Few-wall Carbon Nanotube Coils. Nano Letters 2019, 20, 953–962.

Mobility of dislocations in carbon nanotube bundles

Horizontally aligned carbon nanotubes (CNTs) have a unique combination of mechanical and physical properties and find numerous applications. The shear deformation of a horizontally aligned CNT bundle is studied under plane-strain conditions using the method of molecular dynamics. The CNTs in the bundle are not perfectly packed, and it is important to see how defects alter the mechanisms of their deformation under applied load. The defects in the form of edge dislocations are introduced into the bundle. It is found that the dislocation core structure depends on the CNT diameter, as the deformability of the CNT cross section increases with the diameter. Different deformation mechanisms (dislocation sliding or cracking) are revealed in the bundles with CNTs of different diameters. The results presented describe how the dislocation core structure affects the dislocation mobility in CNT bundles and the mechanisms of their deformation.

Epitaxial growth of highly atomically ordered Pt-Fe nanoparticles from carbon nanotube bundles as durable oxygen reduction electrocatalysts

Intermetallic Pt-based nanoparticles have displayed excellent activity for the oxygen reduction reaction (ORR) in fuel cells. However, it remains a great challenge to synthesize highly atomically ordered Pt-based nanoparticle catalysts because the formation of an atomically ordered structure usually requires high-temperature annealing accompanied by grain sintering. Here we report the direct epitaxial growth of well-aligned, highly atomically ordered Pt3Fe and PtFe nanoparticles (<5 nm) on single-walled carbon nanotube (SWCNT) bundles films. The long-range periodically symmetric van der Waals (vdW) interactions between SWCNT bundles and Pt-Fe nanoparticles play an important role in promoting not only the alignment ordering of inter-nanoparticles but also the atomic ordering of intra-nanoparticles. The ordered Pt3Fe/SWCNT catalyst showed enhanced ORR catalytic performance of 2.3-fold higher mass activity and 3.1-fold higher specific activity than commercial Pt/C. Moreover, the formation of an interlocked interface and strong vdW interaction endow the Pt-Fe/SWCNT catalysts with extreme long-term stability in potential cycling and excellent anti-thermal sintering ability.

CNT array-induced nanobubble assembly, nanodisk fabrication and enhanced spectral detection of CNT bundle density

CNT array-induced nanobubble assembly, nanodisk fabrication and enhanced spectral detection of CNT bundle density

Nucleation of disclinations in carbon nanotube bundle structures under twisting loads

Twisting is a simple and effective method for assembling individual carbon nanotubes (CNTs) into high-strength, tough, and continuous CNT yarns. However, the unexpectedly low stretching performance of the resulting carbon nanotube bundle (CNTB) structures is a problem that must be resolved. In this study, we investigated the impact of twisting on the structure and stretching performance of CNTBs through molecular dynamics (MD) simulations while considering the presence of disclinations within the CNTB structure. We found that disclinations nucleate in twisted CNTBs, and the longer the disclination line in CNTBs, the lower the Young's modulus. The presence of disclination lines in the twisted CNTBs may be an important reason for the decrease in the tensile properties of the CNT yarns. Although the theoretical equations in textile field can be applied to describe the mechanical properties of CNT yarns, there are significant relative errors between the equations and the MD simulation results presented herein. Therefore, we revised the equations based on the simulation results to reduce the relative error for seven-layer CNTBs to less than 10 %. This paper provides new insights into the low tensile performance of CNT yarns and offers potential guidance for the production of high-performance CNT yarns through twisting.

Introducing a nano-scale surface morphology parameter affecting fracture properties of CNT nanocomposites

Carbon Nanotube (CNT) particles and membranes improve interlaminar fracture toughness of nanocomposites. The novelty of this study is to introduce the effect of a new nanoscale parameter - CNT network (bundle) size to interphase thickness ratio - on modes I and II interlaminar fracture toughness of CNT Polymer Nanocomposites (PNCs). This investigation includes the stochastic distribution function of CNT bundle size, interphase thickness, and modes I and II fracture toughness of PNCs based on an extensive experimental study at the nano- and macro-scales. The Atomic Force Microscopy PeakForce Quantitative Nanomechanics Mapping technique was used to collect 500 datasets for a low-weight percentage of CNT PNC and 180 datasets for a high-weight percentage of CNT PNC. Each dataset included CNT bundle size and interphase thickness at the nanoscale. Double cantilever beam and end-notched flexure experiments were conducted to collect 600 modes I and II fracture toughness data. Two-, three-, and four-parameter Weibull models were used to simulate the nano- and macro-scale material properties of CNT PNCs. Results indicate that (i) a lower CNT bundle size to interphase thickness ratio improves fracture toughness, and (ii) a four-parameter Weibull model is the most efficient model for simulated data.

Eco-friendly aqueous spinning of robust and porous carbon nanotube/graphene hybrid microelectrodes: The graphene oxide size effect

An eco-friendly, cost-effective, and scalable aqueous spinning of robust and porous hybrid fibers consisting of carbon nanotubes (CNTs) and graphene has been developed. We focus on the design of nanoarchitecture within CNT fibers involving the precise control of CNT bundle size and alignment, porosity, and the interaction between CNTs and Graphene. The effect of graphene oxides (GO) sizes on the quality of CNT/GO aqueous dispersions and the structures/properties of the as-spun fibers was investigated, and nano-sized GO was found to be most effective in reducing CNT bundle size, improving CNT alignment, and strengthening the fibers. After high-temperature treatment, the specific strength and specific conductivity of these CNT/graphene hybrid fibers reached to 1.02 N/tex and 1202.01 S m2/g, which were 7.39 and 1.21 times higher than those of pure CNT fibers. Meanwhile, the CNT/graphene hybrid fibers also have a high specific capacity of 168 F g−1. The combination of high mechanical, electrical, and electrochemical performances makes the CNT/graphene hybrid fibers promising materials for wearable electronics.

Approaching Technological Limit for Wet‐Pulling Technique

Carbon nanotube fibers (CNTFs) are a promising economical replacement material for contemporary metallic wired conductors due to their lightweight and advanced electromechanical properties. The wet‐pulling technique for CNTF manufacturing is highly versatile, adaptable for both laboratory as well as industrial‐scale production, and can be optimized for the maximum enhancement of electrophysical properties. Herein, a mechanosolvent‐based postfabrication approach for maximizing densification and improving electrical properties of wet‐pulled CNTFs is examined. The experimental process results in fibers achieving 60% of the theoretically maximum density and conductivity of maximally densified metallic single‐walled carbon nanotube bundles. The technique allows a corresponding increase of ≈700% in fiber density (from 100 to 704 kg m−3), a simultaneous ≈530% increase in electrical conductivity (from 748 to 3990 S cm−1), and reduced volume defects from 18% to 2%. The approach was combined with a step‐wise microstructure monitoring using focused ion beam–scanning electron microscopy to determine the mechanisms behind the optimized structures. This work is the first to provide an experimental and theoretical base for the postfabrication optimization of wet‐pulled CNTFs and lays the foundation for further enhancement with techniques such as chemical doping, fiber compounding, and combined infiltration/densification mechanisms.

Estimation of the Band Gap of Carbon Nanotube Bundles.

The electronic structure of carbon nanotube bundles (CNTBs) can be a tough task for the routine first-principle calculation. The difficulty comes from several issues including the atomic structure, the boundary condition, and above all the very large number of atoms that makes the calculation quite cumbersome. In this work, we estimated the band gap of the CNTBs based on the results from single-walled carbon nanotubes (SWCNTs) under different deformations. The effects of squeezing, stretching, and torsion on the bands of SWCNTs were investigated through first-principle calculations, from which the band gaps of bundles were analyzed because the effects of these deformations were qualitatively independent when the distortions were small. Specifically, the gaps of (4,4) and (8,0) CNTBs under a reasonable torsional strength were predicted, wherein we were able to see metal-semiconductor and semiconductor-metal transitions, respectively. Such reversible mechanical modification of the conductivity may be helpful to the future band-gap engineering in nanoscale circuits.