Seven Strategies to Suppress the Ambipolar Behaviour in CNTFETs: a Review

Silicon - Ambipolar behaviour is the cloud that surrounds the desired nanoelectronics device, the carbon nanotube field effect transistor (CNTFET). Despite all the advancements in the fabrication...     

Specific biosensing using carbon nanotubes functionalized with gold nanoparticle-antibody conjugates

This study demonstrates the electrical detection of protein binding by the introduction of Au nanoparticle-antibody conjugates in a carbon nanotube field effect transistor (CNTFET), in which the nanoparticle-functionalized carbon nanotube serves as the electrical conducting channel. Antibody (anti-horseradish peroxidase) and antigen (horseradish peroxidase) binding events lead to the amplitude change in the drain current, which can be sensitively detected by FET measurements. The sensor shows negligible response to mismatched proteins such as Immunoglobulin G (IgG), confirming the specificity of the biosensor. The reported CNTFET-based biosensor could be adapted to detect a variety of proteins for in vitro diagnostics.     

Growth of self-aligned carbon nanotube for use as a field-effect transistor using cobalt silicide as a catalyst

The growth of carbon nanotube (CNT) using cobalt silicide as a catalyst and source/drain electrode is proposed to explore its feasibility for fabricating integrated-circuit process compatible, self-aligned CNT field-effect transistors (CNTFET). The silicide nanoparticles formed in the Ti/Co/poly-Si source/drain stack were used as a catalyst for CNT growth. Results show that single-walled CNTs have been synthesized between pre-defined catalytic cobalt silicide source/drain pairs by chemical vapor deposition at 800-900 °C. Preliminary transistor characteristics of the CNTFETs have also been achieved.     

Understanding the electrical response and sensing mechanism of carbon-nanotube-based gas sensors

Gas sensors based on carbon nanotube field effect transistors (CNFETs) have outstanding sensitivity compared to existing technologies. However, the lack of understanding of the sensing mechanism has greatly hindered progress on calibration standards and customization of these nano-sensors. Calibration requires identifying fundamental transistor parameters and establishing how they vary in the presence of a gas. This work focuses on modeling the electrical response of CNTFETs in the presence of oxidizing (NO₂) and reducing (NH₃) gases and determining how the transistor characteristics are affected by gas-induced changes of contact properties, such as the Schottky barrier height and width, and by the doping level of the nanotube. From the theoretical fits of the experimental transfer characteristics at different concentrations of NO₂ and NH₃, we find that the CNTFET response can be modeled by introducing changes in the Schottky barrier height. These changes are directly related to the changes in the metal work function of the electrodes that we determine experimentally, independently, with a Kelvin probe. Our analysis yields a direct correlation between the ON – current and the changes in the electrode metal work function. Doping due to molecules adsorbed at the carbon-nanotube/metal interface also affects the transfer characteristics.     

Vertically aligned carbon nanotube field-effect transistors

Vertically aligned carbon nanotube field-effect transistors (CNTFETs) have been developed using pure semiconducting carbon nanotubes. The source and drain were vertically stacked, separated by a dielectric, and the carbon nanotubes were placed on the sidewall of the stack to bridge the source and drain. Both the effective gate dielectric and gate electrode were normal to the substrate surface. The channel length is determined by the dielectric thickness between source and drain electrodes, making it easier to fabricate sub-micrometer transistors without using time-consuming electron beam lithography. The transistor area is much smaller than the planar CNTFET due to the vertical arrangement of source and drain and the reduced channel area.     

Device and circuit-level performance of carbon nanotube field-effect transistor with benchmarking against a nano-MOSFET

ABSTRACT: The performance of a semiconducting carbon nanotube (CNT) is assessed and tabulated for parameters against those of a metal-oxide-semiconductor field-effect transistor (MOSFET). Both CNT and MOSFET models considered agree well with the trends in the available experimental data. The results obtained show that nanotubes can significantly reduce the drain-induced barrier lowering effect and subthreshold swing in silicon channel replacement while sustaining smaller channel area at higher current density. Performance metrics of both devices such as current drive strength, current on-off ratio (Ion/Ioff), energy-delay product, and power-delay product for logic gates, namely NAND and NOR, are presented. Design rules used for carbon nanotube field-effect transistors (CNTFETs) are compatible with the 45-nm MOSFET technology. The parasitics associated with interconnects are also incorporated in the model. Interconnects can affect the propagation delay in a CNTFET. Smaller length interconnects result in higher cutoff frequency.     

A pH sensor based on electric properties of nanotubes on a glass substrate

We fabricated a pH-sensitive device on a glass substrate based on properties of carbon nanotubes. Nanotubes were immobilized specifically on chemically modified areas on a substrate followed by deposition of metallic source and drain electrodes on the area. Some nanotubes connected the source and drain electrodes. A top gate electrode was fabricated on an insulating layer of silane coupling agent on the nanotube. The device showed properties of an n-type field effect transistor when a potential was applied to the nanotube from the top gate electrode. Before fabrication of the insulating layer, the device showed that the p-type field effect transistor and the current through the source and drain electrodes depend on the buffer pH. The current increases with decreasing pH of the CNT solution. This device, which can detect pH, is applicable for use as a biosensor through modification of the CNT surface.     

Electron transport characteristics of organic molecule encapsulated carbon nanotubes

One-dimensional carbon nanotube (CNT) junctions with interesting device characteristics have been designed by encapsulating p- and n-type organic molecules into CNTs with electrophilic tetracyano-p-quinodimethane (TCNQ) and nucleophilic tetrakis(dimethylamino)ethylene (TDAE) molecules in order to explore the effect of encapsulation of organic molecules and rectifying behaviors of the designed one-dimensional CNT p-n junctions. Our results show that p- and n-type doping of CNTs and their associated charge transfer play an important role in determining the electron transport characteristics and lead to materials with unique properties, p-n junction diode, i.e. Zener-like diode. Furthermore, we show that the operational device characteristics of non-covalently doped CNT junctions originate from the distinct response of intrinsic transmission peaks of pure CNTs according to the type of dopant and the applied bias. We believe that the results give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.     

Memory effects of carbon nanotube-based field effect transistors

In this paper, we report the study on the non-volatile memory effects of carbon nanotube-based field effect transistors (CNTFETs), in which semiconducting single-wall carbon nanotubes (SWNTs) bridge the gold electrodes and the doped silicon substrate acts as the back gate. We find that our CNTFETs exhibit good performance with on/off ratio of more than 10⁴ and they also show strong memory effects. Hysteretic behaviors of the drain current as a function of the gate voltage are clearly observed at room temperature. The threshold voltage shift increases with increasing the sweeping range of the gate voltage. The CNTFET memory effects show good charge retention capability with the data storage time of around 7 days at ambient condition. Besides, the threshold voltage shift of the as-prepared CNTFETs is found to decrease with time and saturate after around 3 days. Water and alcohol molecules adsorbed on the carbon nanotube are suggested to be the origin of the phenomena. It is also observed that the threshold voltage shift in “top-contact” structures is larger than those in “bottom-contact” structures at the same gate voltage sweeping range.     

Charge-induced conductance modulation of carbon nanotube field effect transistor memory devices

We effectively tailored the charge trapping and transport behavior of a carbon nanotube field effect transistor memory device using charge interaction with underlying Ge nanoparticles in a HfO₂ high-κ dielectric. We also suggest a new route for modulating the Schottky barrier at the nanotube-electrode interface with localized charge trapping in discrete nanoparticles. This modification leads to an effective increase in the read-out conductance ratio of two to three orders magnitude under low voltage operation, associated with a large memory window of ∼5.3 V. Furthermore, we achieved a more controllable and reliable memory effect due to stable charge storage in deep nanoparticle traps, as compared to shallow HfO₂ defect states.     

Understanding parameters affecting field emission properties of directly spinnable carbon nanotube webs

The electron field emission (FE) properties of highly aligned carbon nanotube webs (CNTWs) spun directly from carbon nanotube forests are elucidated in this study. By controlling the synthesis parameters, a series of CNTWs with different structural properties are synthesized and the effect of web areal density and the length of the constituent nanotubes on the field emission property studied. An empirical/analytical factor (Tip Factor, T) is developed which relates the structural properties of the web to their effect on the effective concentration of free nanotube ends, and hence on FE. The validity of T as a measure of tip concentration is further demonstrated by measuring the pull-off adhesive forces using an AFM-based technique. Both FE and mechanical adhesion are linearly related to T. These results suggest that in order to achieve the highest field emission or dry adhesion webs, features desired include short nanotubes, with dense and even coverage of the surface.     

Application of carbon nanotubes to field emission displays

Large-area field emission displays were fabricated with single-wall carbon nanotube emitters. A carbon nanotube paste was prepared and screen-printed to form an electron emission layer on a glass-based substrate. Carbon nanotube-based field emission displays fabricated by thick film processing were successfully integrated to demonstrate moving color images. They revealed excellent field emission characteristics of a threshold electric field of approximately 2 V/μm. We have also investigated triode-type field emission display structures to achieve high-gray scale and high brightness. In the triode structure, it was observed that electron emission from carbon nanotube emitters was controlled by modulation of gate voltages.     

Increasing the semiconducting fraction in ensembles of single-walled carbon nanotubes

The carbon source and growth conditions for single-walled carbon nanotube (SWCNT) growth in hot-wall chemical vapor deposition affect the chirality of the SWCNT ensemble produced. Raman spectroscopy elucidates the trends of the SWCNT semiconducting percentage grown under different conditions. Field-effect transistors using few SWCNTs per transistor were fabricated to allow for a semiconducting SWCNT enumeration and to confirm these trends. The semiconducting SWCNT percent in isopropanol-based devices peaked at 800 °C with 85% semiconducting. 2-Butanol-based and methane-based devices were 70% and 32% semiconducting, respectively.     

Improved field emission from multiwall carbon nanotubes with nano-size defects produced by ultra-low energy ion bombardment

Multiwalled carbon nanotubes were irradiated with ultra-low energy (few eV) nitrogen and hydrogen ions using a microwave discharge. These ultra-low energy plasma-ions remain confined to the nanotube walls, transferring their maximum energy to the carbon atoms, and produce extraordinary structural changes to the carbon nanotube pillars as well as within the carbon nanotubes. Conical shaped emitters and nanotube structures with nano-defects are produced that exhibit remarkable field emission with ultra-low turn-on electric field (∼0.16 V/μm) and a >300-fold increase in the maximum emission current density compared to non-irradiated nanotubes. Doping of nitrogen is also identified due to such irradiation processes.     

Horizontally aligned carbon nanotube field emitters having a long term stability

Horizontally aligned carbon nanotube (CNT) field emitters fabricated by electrophoresis deposition and fissure formation techniques show good field emission properties such as high current density, low turn-on voltage and long-term stability. Horizontally aligned multi-walled carbon nanotube (MWCNT) field emitters show an unusual very long-term stability, much better stability than the single-walled carbon nanotube (SWCNT) ones. The cause of the degradation is due to the heat generated by the resistance of CNTs. We were able to prevent effectively the degradation of the horizontally aligned field emitters by using MWCNTs and an additional deposition of aluminum on the CNT films, and the required time for 10% degradation is very long, 121 h.     

Fusion of carbon nanotubes for fabrication of field emission cathodes

Consolidated carbonaceous samples prepared by spark plasma sintering of multi-walled carbon nanotubes are analyzed, and the effect of the heating regime on their morphology, density, thermal stability, electron field emission and adhesive behavior studied. The trend in the field emission properties of these samples is explained by the changes in the mobility of the nanotube tips. The effect of such changes in the number of free nanotube tips is also deduced from micro-adhesion data, obtained from pull-off tests using atomic force microscopy.     

Effect of Purity and Substrate on Field Emission Properties of Multi-walled Carbon Nanotubes

Multi-walled carbon nanotubes (MWNT) have been synthesized by chemical vapour decomposition (CVD) of acetylene over Rare Earth (RE) based AB₂ (DyNi₂) alloy hydride catalyst. The as-grown carbon nanotubes were purified by acid and heat treatments and characterized using powder X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Thermo Gravimetric Analysis and Raman Spectroscopy. Fully carbon based field emitters have been fabricated by spin coating a solutions of both as-grown and purified MWNT and dichloro ethane (DCE) over carbon paper with and without graphitized layer. The use of graphitized carbon paper as substrate opens several new possibilities for carbon nanotube (CNT) field emitters, as the presence of the graphitic layer provides strong adhesion between the nanotubes and carbon paper and reduces contact resistance. The field emission characteristics have been studied using an indigenously fabricated set up and the results are discussed. CNT field emitter prepared by spin coating of the purified MWNT–DCE solution over graphitized carbon paper shows excellent emission properties with a fairly stable emission current over a period of 4 h. Analysis of the field emission characteristics based on the Fowler–Nordheim (FN) theory reveals current saturation effects at high applied fields for all the samples.     

Synthesis of tellurium nanotubes by galvanic displacement

Tellurium nanotubes with controlled diameter and wall thickness were synthesized by galvanic displacement of cobalt nanowires and their temperature dependent field effect transistor and magnetoresistance properties were systematically investigated. The nanotube diameter was slightly larger than the sacrificial cobalt nanowire diameter with a wall thickness of range from 15 to 30 nm depending on the diameter of cobalt nanowires. Te nanotubes show p-type semiconducting property with the field effect carrier mobility of approx. 0.01 cm²/V s which is relatively lower than other 1D nanostructure. Low mobility might be attributed to porous morphology with small grain size (<10 nm). Temperature dependent mobility also exhibiting a Conwell-Weisskopf relationship to temperatures below 250 K, indicating that the dominant scattering sites are ionized impurity centers. Unique MR behavior was observed from nanotube with a maximum magnetoresistance ratio of 37% at 260 K.     

有机相变蓄冷材料中碳纳米管添加剂性能实验研究

针对有机相变蓄冷材料导热系数低、传热性能差的缺点,采用向其中添加碳纳米管,通过超声分散法及添加分散剂制备稳定分散液来改善其导热性能。对分散剂的种类、碳纳米管的质量浓度、超声时间和分散剂的浓度对碳纳米管分散稳定性的影响及添加碳纳米管对导热性能的影响进行了实验研究。研究结果表明,分散剂对碳纳米管悬浮液的稳定性具有关键作用,十二烷基苯磺酸钠(SDBS)是一种比较理想的分散剂,碳纳米管稳定分散悬浮液的最佳制备条件为:碳纳米管质量浓度0.4 g/L;分散剂SDBS质量浓度0.2 g/L;超声时间80 min。通过在有机相变蓄冷材料加入碳纳米管可以有效增大其导热系数。     

碳纳米管水泥基复合材料导电特性影响因素研究进展

将适量碳纳米管加入水泥基材料,可使其拥有独特的导电特性,进而实现水泥基材料结构的自感知和智能化.碳纳米管水泥基复合材料的导电特性受到物理场和材料组分等因素的影响和制约,但现有研究对此问题的关注和深度不够.综述了碳纳米管水泥基复合材料导电特性影响因素的研究现状,针对复合材料的电阻率和电流等电学指标,分析了物理场影响复合材...