Carbon Nanotube Devices Research Articles

Structure-preserving analysis on chaotic characteristics of transverse vibration for embedded double-walled carbon nanotube

Abstract Analysing on the ultra-high frequency vibrational characteristics of carbon nanotubes, especially on the chaotic characteristics, is a key scientific problem in the dynamic design of the carbon nanotube devices. Considering the van der Waals force between the inner layer and the outer layer of the embedded double-walled carbon nanotube, and the effects of the elastic medium as well as the effects of the simple harmonic external excitation, the coupling dybamic model describing the transverse vibration of the embedded double-walled carbon nanotube is presented. The generalized multi-symplectic formulations with an explicit multi-symplectic structure residual are deduced by introducing the dual momenta. The Preissmann approach, which has been proved to be a structure-preserving method that can be used to reproduce the chaotic characteristics of carbon nanotubes, is employed to discrete the generalized multi-symplectic formulations. The numerical results imply that, the transverse vibration of the embedded double-walled carbon nanotube subjected to the external excitation larger than the critical external excitation will enter the chaotic state through a period-doubling bifurcation path. In addition, the critical external excitation for the chaos of the inner layer carbon nanotube’s transverse vibration is larger than that of the outer layer carbon nanotube’s transverse vibration. The above findings reported in this paper provide some guidance for the dynamic design of the carbon nanotube devices directly.

GPU-Enabled Exascale Quantum Transport Modeling of Carbon Nanotube Devices

The work will describe our efforts to develop an exascale electrostatic-quantum transport framework, Quantum-eXstatic (https://github.com/AMReX-Microelectronics/eXstatic), currently supporting the modeling of carbon nanotube field-effect transistors (CNTFETs). The framework is capable of modeling realistic three-dimensional structures, efficiently, using CPU/GPU heterogeneous architectures of state-of-the-art supercomputers. It is built on top of the AMReX library, which is a GPU-enabled software library containing functionality for writing particle-mesh applications on structured grids. The Quantum-eXstatic framework comprises three major components: the electrostatic module, the quantum transport module, and the part that self-consistently couples the two modules. The electrostatic module using a finite-volume multigrid solver to compute the electrostatic potential induced by charges on the surface of carbon nanotubes, as well as by source, drain, and gate terminals, which can be modeled using an embedded boundary approach to allow for intricate shapes. The quantum transport module uses the Nonequilibrium Green's Function (NEGF) method to model induced charge. Currently, it supports coherent (ballistic) transport, mode-space approximation for subbands of nanotube, contacts modeled as semi-infinite leads, and Hamiltonian representation using the tight-binding approximation. Special features are being included for accurate modeling of long nanotubes, such as adaptive refinement of integration contours to resolve van Hove singularities in long channels. Particular attention is paid to overlap computations with communication to optimize GPU performance. The self-consistency between the two modules is achieved using Broyden's modified second algorithm, which is parallelized using CPUs and GPUs. Preliminary scaling studies show that the code exhibits ~77% weak-scaling efficiency on 512 NVIDIA A100 GPUs of Perlmutter supercomputer for modeling gate-all-around CNTFETs with a channel length of 13.5 microns at equilibrium conditions. This case involved computations of electrostatic potential in 14.5 billion computational cells and Green’s function computation for a Hamiltonian of size 131,072 x 131,072. For problems of this large size, the code takes about 5 min to reach self-consistency. Figure 1

Inner Doping of Carbon Nanotubes with Perovskites for Ultralow Power Transistors.

Semiconducting carbon nanotubes (CNTs) are considered as the most promising channel material to construct ultrascaled field-effect transistors, but the perfect sp2 C─C structure makes stable doping difficult, which limits the electrical designability of CNT devices. Here, an inner doping method is developed by filling CNTs with 1D halide perovskites to form a coaxial heterojunction, which enables a stable n-type field-effect transistor for constructing complementary metal-oxide-semiconductor electronics. Most importantly, a quasi-broken-gap (BG) heterojunction tunnel field-effect transistor (TFET) is first demonstrated based on an individual partial-filling CsPbBr3/CNT and exhibits a subthreshold swing of 35mV dec-1 with a high on-state current of up to 4.9µA per tube and an on/off current ratio of up to 105 at room temperature. The quasi-BG TFET based on the CsPbBr3/CNT coaxial heterojunction paves the way for constructing high-performance and ultralow power consumption integrated circuits.

A Compact Model of Carbon Nanotube Field-Effect Transistors for Various Sizes with Bipolar Characteristics

Carbon nanotubes have excellent electrical properties and can be used as a new generation of semiconductor materials. This paper presents a compact model for carbon nanotube field-effect transistors (CNTFETs). The model uses a semi-empirical approach to model the current–voltage properties of CNTFETs with gate lengths exceeding 100 nm. This study introduces an innovative approach by proposing physical parametric reference lengths (Lref), which facilitate the integration of devices of varying sizes into a unified modeling framework. Furthermore, this paper develops models for the bipolar properties of carbon nanotube devices, employing two distinct sets of model parameters for enhanced accuracy. The model offers a comprehensive analysis of the different capacitances occurring between the electrodes within the device. The simulation of the model shows good agreement with the experimental measurements, confirming the model’s validity. The model is implemented in the Verilog-A hardware description language, with the circuit being subsequently constructed and subjected to simulations via the HSPICE tool. The CNTFET-based inverter exhibits a gain of 7.022 and a delay time of 16.23 ps when operated at a voltage of 1.2 V.

Infrared Light-Emitting Diodes Based on Chirality-Sorted Carbon Nanotube Films.

An important goal in carbon nanotube optoelectronics is to achieve a high-performance near-infrared light source. But there are still many challenges such as the purity of single-walled carbon nanotube (SWCNT) chirality, nonradiative defects, thin-film quality, and device structure design. Here, we realize infrared light-emitting diodes (LEDs) based on chirality-sorted (10, 5) SWCNT network films, which operate at a low bias voltage and emit at a telecom O band of 1290 nm. Asymmetric palladium (Pd) and hafnium (Hf) contacts are used as electrodes for hole and electron injection, respectively. However, the large Schottky barrier at the interface of the SWCNTs and the Hf electrode, primarily resulting from the polymer wrapped on the nanotube surface during the sorting process, leads to inefficient electron injection and thus a low electroluminescence efficiency. We find that the efficiency of electron injection can be improved by the local doping of the nanotubes with dielectric layers of YOX-HfO2, which reduces the Schottky barrier at the SWCNT/Hf interface. Accordingly, the (10, 5) SWCNT film-based LED achieves an external quantum efficiency of larger than 0.05% without any optical coupling structure. With further improvement, we expect that such an infrared light source will have great application potential in the carbon nanotube monolithic optoelectronic integrated system and on-chip optical interconnection, especially in the field of short-distance optical fiber communications and data center.

Peptide-Functionalized Carbon Nanotube Chemiresistors: The Effect of Nanotube Density on Gas Sensing.

Biorecognition element (BRE)-based carbon nanotube (CNT) chemiresistors have tremendous potential to serve as highly sensitive, selective, and power-efficient volatile organic compound (VOC) sensors. While many research groups have studied BRE-functionalized CNTs in material science and device development, little attention has been paid to optimizing CNT density to improve chemiresistor performance. To probe the effect of CNT density on VOC detection, we present the chemiresistor-based sensing results from two peptide-based CNT devices counting more than 60 different individual measurements. We find that a lower CNT density shows a significantly higher noise level and device-to-device variation while exhibiting mildly better sensitivity. Further investigation with SEM images suggests that moderately high CNT density with a stable connection of the nanotube network is desirable to achieve the best signal-to-noise ratio. Our results show an essential design guideline for tuning the nanotube density to provide sensitive and stable chemiresistors.

Additive Influence of Top Metal Contact and Alumina Deposition on the Threshold Voltage of Suspended Carbon Nanotube Field-Effect Transistors.

One-dimensional nanostructures such as carbon nanotubes offer excellent properties useful for applications in gas sensors, piezoresistive devices, and radio frequency resonators. Considering their nanoscale form factor, carbon nanotubes (CNTs) are highly sensitive to surface adsorbents. This study presents the fabrication flow of CNT devices with extended passivated areas around electrical contacts between the CNT and source and drain electrodes. These types of structures could help in understanding the intrinsic CNT response by eliminating the analyte impact on the Schottky barrier regions of the CNT field-effect transistors (CNTFETs). The influence of multiple processing conditions on the electronic properties of CNTFETs with a suspended individual CNT used as the CNTFET channel is presented. Our findings show a threshold voltage shift in CNT ISD-Vg characteristics following the metal deposition and alumina atomic layer deposition.

Directional Activated Exciton Highway via Fractal Electric Field Modulation for Ultrasensitive Carbon Nanotube-Based Sensors.

The electrical vapor sensor based on carbon nanotubes (CNTs) has attracted wide attention due to its excellent conductivity, stable interfacial structure, and low dimensional quantum effects. However, the conductivity and contact interface activity were still limited by the random distribution of coated CNTs, which led to limited performance. We developed a new strategy to unify the CNT directions with image fractal designing of the electrode system. In such a system, directional aligned CNTs were gained under a well-modulated electric field, leading to microscale CNT exciton highways and molecule-scale host-guest site activation. The carrier mobility of the aligned CNT device is 20-fold higher than that of the random network CNT device. With excellent electrical properties, such modulated CNT devices based on fractal electrodes behave as an ultrasensitive vapor sensor for methylphenethylamine, a mimic of illicit drug methamphetamine. The detection limit reached as low as 0.998 ppq, 6 orders of magnitude sensitive than the reported 5 ppb record based on interdigital electrodes with random distributed CNTs. Since the device is easily fabricated in wafer-level and compatible with the CMOS process, such a fractal design strategy for aligned CNT preparation will be widely applied in a variety of wafer-level electrical functional devices.

Gate-Controlled Quantum Interference Effects in a Clean Single-Wall Carbon Nanotube p-n Junction.

The precise control and deep understanding of quantum interference in carbon nanotube (CNT) devices are particularly crucial not only for exploring quantum coherent phenomena in clean one-dimensional electronic systems, but also for developing carbon-based nanoelectronics or quantum devices. Here, we construct a double split-gate structure to explore the Aharonov-Bohm (AB) interference effect in individual single-wall CNT p-n junction devices. For the first time, we achieve the AB modulation of conductance with coaxial magnetic fields as low as 3T, where the flux through the tube is much smaller than the flux quantum. We further demonstrate direct electric-field control of the nonmonotonic magnetoconductance through a gate-tunable built-in electric field, which can be quantitatively understood in combination with the AB phase effect and Landau-Zener tunneling in a CNT p-n junction. Moreover, the nonmonotonic magnetoconductance behavior can be strongly enhanced in the presence of Fabry-Pérot resonances. Our Letter paves the way for exploring and manipulating quantum interference effects with combining magnetic and electric field controls.

Converting Ambipolar Double‐Walled Nanotube Bundles to Unipolar Carbon Nanotube Field‐Effect Transistor Devices by Inner Functionalization

Carbon nanotube (CNT)‐based transistor devices have been demonstrated and are seen as the next‐generation alternative to replace the current generation electronic devices based on silicon MOS technology. Their limitations, however, are related to their instability in ambient conditions and technical challenges in scaling up production of individual CNT devices. Herein, a simple technique to convert ambipolar (both n‐ and p‐carrier) individual CNT‐based transistors into unipolar (p‐type carrier) transistors that carry large currents ≈±7 mA before saturation is demonstrated. Such large current‐carrying capability from an individual or single‐bundle CNTs will be ideal for applications that require high currents such as output stages of radio frequency (RF) and audio amplifiers. This work highlights the ability to tune CNT‐based transistor devices to suit specific applications, which is essential for the development of next generation of nanoelectronics.

Insights into the Charge Storage Mechanism of Binder-Free Electrochemical Capacitors in Ionic Liquid Electrolytes

Electrochemical capacitors (synonymously supercapacitors) working under an electrochemical double-layer charge storage mechanism (EDLC) are widely investigated because of their excellent power density and cycle life; however, their energy density is lower than those of lithium-ion batteries. Ionic liquids (ILs) are of great interest as electrolytes for EDLCs due to their wide operational voltage window. Here, we provide a systematic investigation on the influence of anions of ILs on the charge storage mechanism and electrochemical stability of EDLC electrodes. Two IL electrolytes, viz., [1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)], having similar cations but different anions and carbon nanotube (CNT) electrodes are chosen for this study. The CNT//BF4:TFSI//CNT-based device showed superior electrochemical performance (∼69 F·g–1 gravimetric specific capacitance, ∼949 W·kg–1 power density, and ∼139 Wh·kg–1 energy density at 0.5 A·g–1) to CNT//EMIMBF4//CNT and CNT//EMIMTFSI//CNT devices. The device using a mixture of BF4:TFSI (1:0.5) electrolytes has an operating voltage of 0–3.8 V and specific capacitance retention of ∼45% at 0.5 A·g–1 after 500 cycles. In the case of the IL mixture (BF4:TFSI), the combined anion structure and their properties play very crucial part in the improvement of the electrochemical performance of the CNT//BF4:TFSI//CNT device. The assembled Teflon Swagelok-type cell could light up green (3.3 V) and red (2.1 V) light-emitting diodes for more than 5 min.

Outstanding Robust Photo- and Thermo-Electric Applications with Stabilized n-Doped Carbon Nanotubes by Parylene Coating.

Stabilization techniques for n-doped carbon nanotubes (CNTs) are essential for the practical use of CNT devices. However, none of the reported n-dopants have sufficient robustness in a practical environment. Herein, we report a highly stable technique for fabricating n-doped CNT films. We elucidate the mechanism by which air stability can be achieved by completely covering CNTs with n-dopants to prevent oxidation; consequently, the stability is lost when exposed to scratches or moisture. Therefore, we introduce parylene as a protective layer for n-doped CNTs and achieve air stability for more than 365 d. Moreover, we demonstrate outstanding robust thermo-electric power generation from strong acids, alkalis, and alcohols, which cannot be realized with conventional air-stable n-dopants. The proposed stabilization technique is versatile and can be applied to various n-dopants. Thus, it is expected to be a key technology in the practical application of CNT devices.

Exploration of Charge Storage Behavior of Binder-Free EDL Capacitors in Aqueous Electrolytes.

Charge storage in electrochemical double-layer capacitors (EDLCs) is via the adsorption of electrolyte counterions in their positive and negative electrodes under an applied potential. This study investigates the EDLC-type charge storage in carbon nanotubes (CNT) electrodes in aqueous acidic (NaHSO4), basic (NaOH), and neutral (Na2SO4) electrolytes of similar cations but different anions as well as similar anions but different cations (Na2SO4 and Li2SO4) in a two-electrode Swagelok-type cell configuration. The physicochemical properties of ions, such as mobility/diffusion and solvation, are correlated with the charge storage parameters. The neutral electrolytes offer superior charge storage over the acidic and basic counterparts. Among the studied ions, SO4 2- and Li+ showed the most significant capacitance owing to their larger solvated ion size. The charge stored by the anions and cations follows the order SO4 2- > HSO4 - > OH- and Li+ > Na+, respectively. Consequently, the CNT//Li2SO4//CNT cell displayed outstanding charge storage indicators (operating voltage ∼0-2 V, specific capacitance ∼122 F·g-1, specific energy ∼67 W h·kg-1, and specific power ∼541 W·kg-1 at 0.5 A·g-1) than the other cells, which could light a red light-emitting diode (2.1 V) for several minutes. Besides, the CNT//Li2SO4//CNT device showed exceptional rate performance with a capacitance retention of ∼95% at various current densities (0.5-2.5 A·g-1) after 6500 cycles. The insights from this work could be used to design safer electrochemical capacitors of high energy density and power density.

All-Aerosol-Jet-Printed Carbon Nanotube Transistor with Cross-Linked Polymer Dielectrics.

The printability of reliable gate dielectrics and their influence on the stability of the device are some of the primary concerns regarding the practical application of printed transistors. Major ongoing research is focusing on the structural properties of dielectric materials and deposition parameters to reduce interface charge traps and hysteresis caused by the dielectric-semiconductor interface and dielectric bulk. This research focuses on improving the dielectric properties of a printed polymer material, cross-linked polyvinyl phenol (crPVP), by optimizing the cross-linking parameters as well as the aerosol jet printability. These improvements were then applied to the fabrication of completely printed carbon nanotube (CNT)-based thin-film transistors (TFT) to reduce the gate threshold voltage (Vth) and hysteresis in Vth during device operation. Finally, a fully aerosol-jet-printed CNT device was demonstrated using a 2:1 weight ratio of PVP with the cross-linker poly(melamine-co-formaldehyde) methylated (PMF) in crPVP as the dielectric material. This device shows significantly less hysteresis and can be operated at a gate threshold voltage as low as -4.8 V with an on/off ratio of more than 104.

Ultrastrong coupling between electron tunneling and mechanical motion

The ultrastrong coupling of single-electron tunneling and nanomechanical motion opens exciting opportunities to explore fundamental questions and develop new platforms for quantum technologies. We have measured and modeled this electromechanical coupling in a fully suspended carbon nanotube device and report a ratio of ${g}_{\text{m}}/{\ensuremath{\omega}}_{\mathrm{m}}=2.72\ifmmode\pm\else\textpm\fi{}0.14$, where ${g}_{\text{m}}/2\ensuremath{\pi}=0.80\ifmmode\pm\else\textpm\fi{}0.04\phantom{\rule{0.16em}{0ex}}\mathrm{GHz}$ is the coupling strength and ${\ensuremath{\omega}}_{\mathrm{m}}/2\ensuremath{\pi}=294.5\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$ is the mechanical resonance frequency. This is well within the ultrastrong coupling regime and the highest among all other electromechanical platforms. We show that, although this regime was present in similar fully suspended carbon nanotube devices, it went unnoticed. Even higher ratios could be achieved with improvement on device design.

Carbon Nanotube Devices for Quantum Technology.

Carbon nanotubes, quintessentially one-dimensional quantum objects, possess a variety of electrical, optical, and mechanical properties that are suited for developing devices that operate on quantum mechanical principles. The states of one-dimensional electrons, excitons, and phonons in carbon nanotubes with exceptionally large quantization energies are promising for high-operating-temperature quantum devices. Here, we discuss recent progress in the development of carbon-nanotube-based devices for quantum technology, i.e., quantum mechanical strategies for revolutionizing computation, sensing, and communication. We cover fundamental properties of carbon nanotubes, their growth and purification methods, and methodologies for assembling them into architectures of ordered nanotubes that manifest macroscopic quantum properties. Most importantly, recent developments and proposals for quantum information processing devices based on individual and assembled nanotubes are reviewed.

Field emission energy distribution studies of single multi-walled carbon nanotube emitter with gas adsorptions

The field emission electron energy distribution (FEED) investigation on the individual carbon nanotube (CNT) is a powerful approach to study the intrinsic electronic properties, including the work function and field emission energy level, for CNT devices. We fabricated the single multi-walled carbon nanotube (MWNT) emitter by the solution evaporation dielectrophoretic process after the chemical vapor deposition (CVD) synthesis of the MWNT film. The FEED properties of the MWNT emitter in hydrogen and nitrogen environments were investigated. The FEED peaks shifted up gradually in both hydrogen and nitrogen ambiences, indicating the effective work function reduction effect with gas adsorptions. The amounts of spectrum displacements increased with raising gas partial pressures, with larger displacements of up to 0.65 eV for hydrogen between 10−7 Pa and 10−4 Pa. The double peak spectra imply that emission electrons could be from the hexagonal and the pentagonal carbons. Meanwhile, high carbon content was detected in the ultimate vacuum, attributed highly to the MWNT etching from the emission. This investigation suggests that the work function variation plays key role for the field emission instability and the pressure sensing effect of CNT emitters.

Improvement in laser-based micro-processing of carbon nanotube film devices

Carbon nanotubes (CNTs) are considered remarkable low-dimensional materials that can overcome the problems associated with bulk semiconductors. The development of CNT processing technology complementing conventional processing technology is required to advance research on CNT devices. We assessed the potential and application of laser-based micro-processing for patterning micrometre-thick CNT films. We discovered that the laser ablation method can pattern micrometre-thick CNT films in micrometre spatial resolution and can directly pattern the CNT films inside the protective film—features not achievable by competing processing technologies. Thus, the laser ablation method is uniquely advantageous and applicable in manufacturing functional CNT devices.

Recent advances in printable carbon nanotube transistors for large-area active matrices

Active-matrices serve as the backplane circuitry for large-area display technologies and distributed sensors. Recently, there has been significant interest in developing flexible, additively manufactured active matrices for the burgeoning flexible electronics industry. Carbon nanotubes (CNTs) are prime candidate materials for semiconducting elements of transistors due to their solution processability, carrier mobility, and mechanical flexibility. There have been many recent accomplishments in the development of CNT inks and in their deposition via printing that enable their use in thin-film transistors (TFTs), and their appropriate performance make them suitable for use in large-area active matrices. In this review, we provide an overview of the field, with a specific focus on recent advancements in CNT sorting, ink preparation, and printing techniques. We also provide a benchmarking study of printed CNT devices presented in literature after 2017. Next, we discuss printable CNT-TFTs used for active-matrix applications. Finally, we provide a concluding perspective on the outlook and challenges for printed CNT-TFT active matrices.

Generation-recombination and 1/f noise in carbon nanotube networks

The low-frequency noise is of special interest for carbon nanotubes devices, which are building blocks for a variety of sensors, including radio frequency and terahertz detectors. We studied noise in as-fabricated and aged carbon nanotube networks (CNNs) field-effect transistors. Contrary to the majority of previous publications, as-fabricated devices demonstrated the superposition of generation-recombination (GR) and 1/f noise spectra at a low-frequency range. Although all the devices revealed identical current–voltage characteristics, GR noise was different for different transistors. This effect is explained by the different properties and concentrations of trap levels responsible for the noise. Unexpectedly, exposure of these devices to the atmosphere reduced both the resistance and GR noise due to nanotube's p-doping by adsorbed water molecules from the ambient atmosphere. The presence of the generation recombination noise and its dependences on the environment provides the basis for selective gas sensing based on the noise measurements. Our study reveals the noise properties of CNNs that need to be considered when developing carbon nanotubes-based selective gas sensors.