CNT-based Field-effect Transistors Research Articles

Achieving High-Performance Polymer-Wrapper-Free Aligned Carbon Nanotube Field-Effect Transistors Through Degradable Polymer Wrapping and Efficient Removal Techniques.

Semiconducting carbon nanotubes (s-CNTs) have emerged as a promising alternative to traditional silicon for ultrascaled field-effect transistors (FETs), owing to their exceptional properties. Aligned s-CNTs (A-CNTs) are particularly favored for practical applications due to their ability to provide higher driving current and lower contact resistance compared with individual s-CNTs or random networks. Achieving high-semiconducting-purity A-CNTs typically involves conjugated polymer wrapping for selective separation of s-CNTs, followed by self-assembly techniques. However, the presence of the polymer wrapper on A-CNTs can adversely impact electrical contact, gating efficiency, carrier transport, and device-to-device variations, necessitating its complete removal. While various methods have been explored for polymer removal, accurately characterizing the extent of removal remains a challenge. Traditional techniques such as absorption spectroscopy and X-ray photoelectron spectroscopy (XPS) may not accurately depict the remaining polymer content on A-CNTs due to their inherent detection limits. Consequently, the performance of FETs based on pure polymer-wrapper-free A-CNTs is unclear. In this study, we present an approach for preparing high-semiconducting-purity and polymer-wrapper-free A-CNTs using poly[(9,9-dioctylfluorenyl-2,7-dinitrilomethine)-(9,9-dioctylfluorenyl-2,7-dimethine)] (PFO-N-PFO), a degradable polymer, in conjunction with a modified dimension-limited self-alignment process (m-DLSA). Comprehensive transmission electron microscopy (TEM) characterizations, complemented by absorption and XPS characterizations, provide robust evidence of the successful near-complete removal of the polymer wrapper via a cleaning procedure involving acidic degradation, hot solvent rinsing, and vacuum annealing. Furthermore, top-gated FETs based on these high-semiconducting-purity and polymer-wrapper-free A-CNTs exhibit good performance metrics, including an on-current (Ion) of 2.2 mA/μm, peak transconductance (gm) of 1.1 mS/μm, low contact resistance (Rc) of 191 Ω·μm, and negligible hysteresis, representing a significant advancement in the CNT-based FET technology.

Toward the Commercialization of Carbon Nanotube Field Effect Transistor Biosensors.

The development of biosensors based on field-effect transistors (FETs) using atomically thick carbon nanotubes (CNTs) as a channel material has the potential to revolutionize the related field due to their small size, high sensitivity, label-free detection, and real-time monitoring capabilities. Despite extensive research efforts to improve the sensitivity, selectivity, and practicality of CNT FET-based biosensors, their commercialization has not yet been achieved due to the non-uniform and unstable device performance, difficulties in their fabrication, the immaturity of sensor packaging processes, and a lack of reliable modification methods. This review article focuses on the practical applications of CNT-based FET biosensors for the detection of ultra-low concentrations of biologically relevant molecules. We discuss the various factors that affect the sensors' performance in terms of materials, device architecture, and sensor packaging, highlighting the need for a robust commercial process that prioritizes product performance. Additionally, we review recent advances in the application of CNT FET biosensors for the ultra-sensitive detection of various biomarkers. Finally, we examine the key obstacles that currently hinder the large-scale deployment of these biosensors, aiming to identify the challenges that must be addressed for the future industrialization of CNT FET sensors.

Recent trends in carbon nanotube (CNT)-based biosensors for the fast and sensitive detection of human viruses: a critical review.

The current COVID-19 pandemic, with its numerous variants including Omicron which is 50-70% more transmissible than the previously dominant Delta variant, demands a fast, robust, cheap, and easily deployed identification strategy to reduce the chain of transmission, for which biosensors have been shown as a feasible solution at the laboratory scale. The use of nanomaterials has significantly enhanced the performance of biosensors, and the addition of CNTs has increased detection capabilities to an unrivaled level. Among the various CNT-based detection systems, CNT-based field-effect transistors possess ultra-sensitivity and low-noise detection capacity, allowing for immediate analyte determination even in the presence of limited analyte concentrations, which would be typical of early infection stages. Recently, CNT field-effect transistor-type biosensors have been successfully used in the fast diagnosis of COVID-19, which has increased research and commercial interest in exploiting current developments of CNT field-effect transistors. Recent progress in the design and deployment of CNT-based biosensors for viral monitoring are covered in this paper, as are the remaining obstacles and prospects. This work also highlights the enormous potential for synergistic effects of CNTs used in combination with other nanomaterials for viral detection.

Comparison between modulations of contact and channel potential in nitrogen dioxide gas response of ambipolar carbon nanotube field-effect transistors

Carbon nanotubes (CNTs) are promising materials for gas sensing because of their large specific area and high sensitivity to charge differentiation. In CNT-based field-effect transistors (FETs) for gas sensing, both CNT potential modulation in the channels and Schottky barrier height modulation at the CNT/metal electrode contact influence the current properties. However, researchers have not used Schottky barrier height modulation for gas detection. To investigate and compare the effects of Schottky barrier height modulation and CNT channel potential modulation on NO2 gas exposure, we fabricated ambipolar CNT FETs by the dielectrophoretic assembly. We exposed CNT FET gas sensors to N2 gas containing 100-ppb NO2 and observed two different responses in the electric properties: a steady current shift in the positive direction in the hole-conduction region because of the channel potential modulation, and an abrupt decrease in transconductance in the electron-conduction region because of the Schottky barrier modulation. The CNT channels and CNT/metal contact both contributed to the sensor response, and the modulation rate of the Schottky barrier was higher than that of the CNT potential shift in the channel.

Sub-Nanometer Interfacial Oxides on Highly Oriented Pyrolytic Graphite and Carbon Nanotubes Enabled by Lateral Oxide Growth.

A new generation of compact and high-speed electronic devices, based on carbon, would be enabled through the development of robust gate oxides with sub-nanometer effective oxide thickness (EOT) on carbon nanotubes or graphene nanoribbons. However, to date, the lack of dangling bonds on sp2 oriented graphene sheets has limited the high precursor nucleation density enabling atomic layer deposition of sub-1 nm EOT gate oxides. It is shown here that by deploying a low-temperature AlOx (LT AlOx) process, involving atomic layer deposition (ALD) of Al2O3 at 50 °C with a chemical vapor deposition (CVD) component, a high nucleation density layer can be formed, which templates the growth of a high-k dielectric, such as HfO2. Atomic force microscopy (AFM) imaging shows that at 50 °C, the Al2O3 spontaneously forms a pinhole-free, sub-2 nm layer on graphene. Density functional theory (DFT) based simulations indicate that the spreading out of AlOx clusters on the carbon surface enables conformal oxide deposition. Device applications of the LT AlOx deposition scheme were investigated through electrical measurements on metal oxide semiconductor capacitors (MOSCAPs) with Al2O3/HfO2 bilayer gate oxides using both standard Ti/Pt metal gates as well as TiN/Ti/Pd gettering gates. In this study, LT AlOx was used to nucleate HfO2 and it was shown that bilayer gate oxide stacks of 2.85 and 3.15 nm were able to achieve continuous coverage on carbon nanotubes (CNTs). The robustness of the bilayer was tested through deployment in a CNT-based field-effect transistor (FET) configuration with a gate leakage of less than 10-8 A/μm per CNT.

Buried-Gate MWCNT FET-Based Nanobiosensing Device for Real-Time Detection of CRP.

C-reactive protein (CRP), an acute-phase protein synthesized in the liver in response to inflammation, is one of the biomarkers used for the detection of several diseases. Sepsis and cardiovascular diseases are two of the most important diseases for which detection of CRP at very early stages in the clinical range can help avert serious consequences. Here, a CNT-based nanobiosensing system, which is portable and reproducible, is used for label-free, online detection of CRP. The system consists of an aptameric CNT-based field-effect transistor benefiting from a buried gate geometry with Al2O3 as a high dielectric layer and can reflect the pro-cytokine concentration. Test results show that the device responds to CRP changes within 8 min, with a limit of detection as low as 150 pM (0.017 mg L–1). The device was found to have a linear behavior in the range of 0.43–42.86 nM (0.05–5 mg L–1). The selectivity of the device was tested with TNF-α, IL-6, and BSA, to which the nanosensing system showed no significant response compared with CRP. The device showed good stability for 14 days and was completely reproducible during this period. These findings indicate that the proposed portable system is a potential candidate for CRP measurements in the clinical range.

Carbon Nanotube Field-Effect Transistor-Based Chemical and Biological Sensors.

Chemical and biological sensors have attracted great interest due to their importance in applications of healthcare, food quality monitoring, environmental monitoring, etc. Carbon nanotube (CNT)-based field-effect transistors (FETs) are novel sensing device configurations and are very promising for their potential to drive many technological advancements in this field due to the extraordinary electrical properties of CNTs. This review focuses on the implementation of CNT-based FETs (CNTFETs) in chemical and biological sensors. It begins with the introduction of properties, and surface functionalization of CNTs for sensing. Then, configurations and sensing mechanisms for CNT FETs are introduced. Next, recent progresses of CNTFET-based chemical sensors, and biological sensors are summarized. Finally, we end the review with an overview about the current application status and the remaining challenges for the CNTFET-based chemical and biological sensors.

Carbon nanotubes advance next-generation electronics

Abstract Single-walled carbon nanotubes (SWCNTs) are potential candidates for next-generation computing to meet the requirement of device scaling. Here we discuss recent breakthroughs in the CNT-based field-effect transistors (FETs) and highlight key ongoing topics in purification and assembly. A few key challenges that capitalize on the chemical vapor deposition (CVD) of SWCNTs are discussed in which surface science and model systems can enable advances in high-purity synthesis. A full description of the CVD growth and chemical sorting of SWCNTs falls beyond the scope of this opinion, but we focus on the framework of SWCNTs in FETs and draw connections to their requirements for high-quality materials that highlight the underlying mechanisms.

Effective surface emissivity and heat dissipation among integrated bamboo-like super-black vertical carbon nanotube array electrodes in silicon via holes

Recently, carbon nanotube (CNT)-based micro-devices have attracted extensive attention at the frontiers of intelligent electronics, but the practical application of these devices are seriously limited by the low CNT package rates, poor heat dissipation, and intricate in-situ growth processes. Here, microelectrodes with low impedance based on in-situ-grown super-black vertical bamboo-like multi-walled CNT arrays (∼70 μm) were fabricated inside silicon via holes using face-down catalytic plasma enhanced chemical vapor deposition. Interestingly, the unique CNT array exhibited uniformly high absorption efficiency with a broad range of wavelength (exceeding 99.65% in the UV-NIR band and 99% in the mid-infrared region). Moreover, the excellent cooling performance of CNT array microelectrodes based on in-situ-grown bamboo-like CNT arrays within silicon via holes was demonstrated. Our study highlights the benefits of CNT-based micro functional devices, which indicates the integrated in-situ-grown bamboo-like CNT arrays will have potential applications in advanced microelectrodes, CNT-based field-effect transistors, and portable terahertz inspection devices.

Electrochemical functionalization of single-walled carbon nanotubes with amine-terminated dendrimers encapsulating Pt nanoparticles: Toward facile field-effect transistor-based sensing platforms

We present a method for functionalization of single-walled carbon nanotubes (CNTs) with amine-terminated dendrimers encapsulating catalytic nanoparticles, and demonstrate the feasibility of functionalized CNTs as efficient field-effect transistor (FET)-sensing platforms. As a model system, we synthesized Pt dendrimer-encapsulated nanoparticles (DENs) using amine-terminated sixth-generation poly(amidoamine) dendrimers, and functionalized CNT-based FET channels with Pt DENs via electro-oxidative grafting of the terminal amines of dendrimers onto the surface of CNTs. We also demonstrated spatially-controlled surface functionalization of CNTs and subsequent modification of the functionalized CNT-based FET channels with glutamate oxidases for efficient FET-based sensing of glutamates. Our approach could facilitate the functionalization of CNTs with a variety of catalytic nanoparticles and biologically active materials, enabling the development of sensitive and selective FET-based sensor devices.

Scalable Preparation of High-Density Semiconducting Carbon Nanotube Arrays for High-Performance Field-Effect Transistors.

Although chemical vapor deposition (CVD)-grown carbon nanotube (CNT) arrays are considered ideal materials for constructing high-performance field-effect transistors (FETs) and integrated circuits (ICs), a significant gap remains between the required and achieved densities and purities of CNT arrays. Here, we develop a directional shrinking transfer method to realize up to 10-fold density amplification of CNT array films without introducing detectable damage or defects. In addition, the method improves the film uniformity while retaining the perfect alignment and high carrier mobility of 1600 cm2 V-1 s-1 of CVD-grown CNT arrays. By combining the density amplification method with the thermocapillary flow method developed by Rogers et al., semiconducting CNT arrays with high densities and high qualities are obtained. High-performance FETs with a channel length of 200 nm are demonstrated using these high-density semiconducting CNT arrays, yielding a record-high on-state current density of 150 μA/μm, a peak transconductance of 80 μS/μm, and a current on/off ratio of more than 104 among the CVD-grown CNT-based FETs.

Bonding characteristics and electronic structures of the contact interfaces between group 13 metals and carbon nanotubes

The electronic density of states (DOS) in carbon nanotubes (CNTs) was recently measured using a structure comprising a single CNT-based field effect transistor (FET) device with an indium (In) electrode. Here, we report the bonding characteristics and electronic structure of the contact interface between Group 13 metals (In, Ga, Al) and (8,0) semiconducting CNTs using first-principles density functional theory (DFT) calculations. Our calculation results showed that CNTs in contact with Al, Ga, or In surfaces display lower binding energies to their contact metal surfaces than that of Pd surface while also preserving their original electronic properties. These results originate from the absence of d-orbitals in the Group 13 metals near the Fermi level, the large equilibrium distance, and the deficiencies in electron charge density at the metal surfaces. This study deepens our understanding of the contact properties between Group 13 metals and CNTs.

Analytical modelling of sensing performance of carbon nanotubes for gas sensing

Carbon nanotubes (CNTs) have recently captured attention as novel materials for nanoelectronic sensors due to their ability to achieve better sensitivity and accuracy in these devices. Up until now, the majority of investigations have focused on experimental studies for sensors. Therefore, there is lack of analytical models comparative to experimental investigations. The proposed model employs a CNT-based field effect transistor (CNT-FET) scheme to model the transport parameters of the CNT-based gas sensor. The CNT conductance is affected on exposure to the target analyte – NH3 gas molecules. The NH3 gas concentration is absorbed on the CNT surface after a chemical reaction between the CNT and NH3. This modulates the current-voltage (I-V) characteristics and conductance of the CNT-based gas sensor. The I-V characteristics of the CNT-based sensor have been proposed as a criterion to detect the effect of gas absorption. Finally, the proposed model was compared with experimental investigation for validation.

Analytical modelling of sensing performance of carbon nanotubes for gas sensing

Carbon nanotubes (CNTs) have recently captured attention as novel materials for nanoelectronic sensors due to their ability to achieve better sensitivity and accuracy in these devices. Up until now, the majority of investigations have focused on experimental studies for sensors. Therefore, there is lack of analytical models comparative to experimental investigations. The proposed model employs a CNT-based field effect transistor (CNT-FET) scheme to model the transport parameters of the CNT-based gas sensor. The CNT conductance is affected on exposure to the target analyte – NH3 gas molecules. The NH3 gas concentration is absorbed on the CNT surface after a chemical reaction between the CNT and NH3. This modulates the current-voltage (I-V) characteristics and conductance of the CNT-based gas sensor. The I-V characteristics of the CNT-based sensor have been proposed as a criterion to detect the effect of gas absorption. Finally, the proposed model was compared with experimental investigation for validation.

Review of Recent Advances in Carbon Nanotube Biosensors Based on Field-Effect Transistors

The extraordinary physiochemical properties of carbon nanotubes (CNTs) stimulated their wide application in biosensing research. Nanotube characteristics of fast electron transport, large surface area, high strength, excellent catalytic activity and good chemical stability contribute to ultrasensitive, highly selective and stable CNT biosensors. Among the various CNT biosensors, the field-effect transistor (FET) architecture has received tremendous attention due to the advantages of high performance, miniaturization, and capability for mass production. In this paper, we address recent advances in the development of CNT biosensors based on FETs. The synthesis and properties of CNTs are discussed, along with their integration into biosensors. Recent progress in device fabrication, including CNT functionalization, attachment, and bioreceptor immobilization in CNT-based FET biosensors are highlighted. Examples in medical, food and environmental fields are illustrated.

Competitive Impact of Nanotube Assembly and Contact Electrodes on the Performance of CNT-based FETs

We report the fabrication and characterization of highly dense field-effect-transistor (FET) arrays based on single-walled carbon nanotubes (SWCNTs). The nanotubes were sorted according to the electronic type by using density gradient ultracentrifugation (DGU). By employing dielectrophoresis (DEP), SWCNTs with enriched semiconducting (sc) content were systematically integrated as active elements into FETs. The performance of air-operating FETs was addressed via an extended statistic study involving both electrical and structural analyses. The competitive impact of nanotube purity and assembly as well as the metal electrode composition and a thermal treatment on the final device performance was shown. Regardless of the used sc-content, the device-to-device consistency was improved via employing annealing up to 250 °C for 1 h in a vacuum. The observed clockwise hysteresis, known so far only in connection with CNT–FETs built on ferroelectric substrates as well as electrolyte gated CNT–FETs, was found to reve...

Effect of cleaning procedures on the electrical properties of carbon nanotube transistors—A statistical study

The interface between a carbon nanotube (CNT) and its environment can dramatically affect the electrical properties of CNT-based field-effect transistors (FETs). For such devices, the channel environment plays a significant role inducing doping or charge traps giving rise to hysteresis in the transistor characteristics. Thereby the fabrication process strongly determines the extent of those effects and the final device performance. In CNT-based devices obtained from dispersions, a proper individualization of the nanotubes is mandatory. This is generally realized by an ultrasonic treatment combined with surfactant molecules, which enwrap nanotubes forming micelle aggregates. To minimize impact on device performance, it is of vital importance to consider post-deposition treatments for removal of surfactant molecules and other impurities. In this context, we investigated the effect of several wet chemical cleaning and thermal post treatments on the electrical characteristics as well as physical properties of more than 600 devices fabricated only by wafer-level compatible technologies. We observed that nitric acid and water treatments improved the maximum-current of devices. Additionally, we found that the ethanol treatment successfully lowered hysteresis in the transfer characteristics. The effect of the chemical cleaning procedures was found to be more significant on CNT-metal contacts than for the FET channels. Moreover, we investigated the effect of an additional thermal cleaning step under vacuum after the chemical cleaning, which had an exceptional impact on the hysteresis behavior including hysteresis reversal. The presence of surfactant molecules on CNT was evidenced by X-ray photoelectron and Raman spectroscopies. By identifying the role of surfactant molecules and assessing the enhancement of device performance as a direct consequence of several cleaning procedures, these results are important for the development of CNT-based electronics at the wafer-level.

Microfabrication of Vertical Carbon Nanotube Field-Effect Transistors on an Anodized Aluminum Oxide Template Using Atomic Layer Deposition

This paper presents vertical carbon nanotube (CNT) field effect transistors (FETs). For the first time, the author successfully fabricated vertical CNT-based FETs on an anodized aluminum oxide (AAO) template by using atomic layer deposition (ALD). Single walled CNTs were vertically grown and aligned with the vertical pores of an AAO template. By using ALD, a gate oxide material (Al2O3) and a gate metal (Au) were centrally located inside each pore, allowing the vertical CNTs grown in the pores to be individually gated. Characterizations of the gated/vertical CNTs were carried and the successful gate integration with the CNTs was confirmed.

Nanowelding of carbon nanotube-metal contacts: An effective way to control the Schottky barrier and performance of carbon nanotube based field effect transistors

Schottky barriers formed at carbon nanotube (CNT)-metal contacts have been well known to be crucial for the performance of CNT based field effect transistors (FETs). Through first principles calculations we show that a nanowelding process can drastically reduce the Schottky barriers at CNT-metal interfaces, resulting in significantly improved conductivity of CNT-based FETs. The proposed nanowelding can be realized by either laser local heating or a heating process via a controllable pulse current. Results presented in this paper may have great implications in future design and applications of CNT-based electronics.

Assembly of carbon nanotube devices by tip-induced optical trapping

ABSTRACTFabrication of nanoscale devices by assembling individual carbon nanotubes (CNTs) remains challenging despite enormous effort made in this field. Fulfilling the promise of CNTs requires more efficient assembly techniques. In this study, we have developed an in-situ assembly method for precise and cost-effective integration of CNTs using a laser-assisted chemical vapor deposition (LCVD) process. Results show that CNTs can be trapped between sharp tip-shaped electrodes due to the optical gradient forces around the tip apexes generated by a CO2 laser irradiation. This method enables the precise assembly of CNT-based field-effect transistors (FETs) and paves the way for the successful implementation of the CNT-based nanoelectronics.