Network Transistors Research Articles

Conjugated polymer-controlled selective dispersion of single-walled carbon nanotubes and fabrication of network transistors

A family of polymers based on fluorene including homopolymers, alternating copolymers and two fourarmed polymers are synthesized by polycondensation. Their ability to selectively wrap specific (n, m) species using HiPco single-walled carbon nanotubes (SWNTs) as a raw material is investigated. Fluorescence, absorption and Raman spectroscopies are used to analyze SWNT species after addition to polymer suspensions. The results show separation of (n, m) SWNTs can be realized using an optimized polymer structure. The optimized polymer contains fluorene in the main chain to preferentially separate semiconducting nanotube species and hydrofluoric acid (HF)-degradable disilane groups so that it can be removed from the SWNTs after separation. We used the enriched SWNTs to prepare a solution-processed field-effect transistor with a random nanotube network in the active channel to demonstrate the selectivity of the polymer for (n, m) SWNTs. The device exhibits stable p-type semiconductor behavior in air with an on/off current ratio of 121 and hole mobility of 3.2 cm2V−1s−1. Open image in new window

High-performance carbon nanotube network transistors fabricated using a hole punching technique

We develop a unique approach for fabricating high-performance carbon nanotube network transistors based on lithographic density control via a hole punching technique. In an as-grown dense network of single-walled carbon nanotubes (SWNTs), approximately two-thirds of the tubes are semiconducting and one-third are metallic, leading to a poor on/off ratio ( 10 000, with a high yield (>85%) and a large carrier mobility (20 cm2 V−1 s−1). The on current degradation is not severe, unlike the degradation in networks prepared with a homogeneous density. The hole punching technique introduced here may be applied to network SWNT field effect transistor applications and provide new opportunities for controlling the properties of one-dimensional nanostructured percolating systems.

Supported Lipid Bilayer Nanopore Protein Gated All Semiconducting Nanotube Network Devices

We demonstrate an integrated system in which all-semiconducting nanotube network transistors are coated directly with lipid bilayers which contain transmembrane ion channel proteins gramicidin A (gA) and α-Hemolysin (α-HL). Dynamic opening and closing of the pores is observed through measurement of the current from the nanotube network, through the nanopores, and into solution. The pores investigated pass either only cations (gA) or both anions and cations (a-HL), allowing a study of the effect of both species on the threshold voltage and mobility of the nanotube network. Blocking of the ion channel currents is demonstrated to occur with PEG, indicating potential applications in nanopore based sequencing technologies. The all-semiconducting nanotube network devices are compatible with low cost printed electronics, opening a window for massively parallel manufacturing of nanotechnology for a variety of applications in electrophysiology and biosensors.

Impact of thermal boundary conductances on power dissipation and electrical breakdown of carbon nanotube network transistors

We study the impact of thermal boundary conductance (TBC) at carbon nanotube (CNT)-substrate interfaces and CNT junctions on power dissipation and breakdown in CNT network based thin film transistors (CN-TFTs). Comparison of our results from an electro-thermal transport model of CN-TFTs to experimental measurements of power dissipation and temperature profiles allows us to estimate the average CNT-SiO2 TBC as g ∼ 0.16 Wm−1 K−1 and the TBC at CNT junctions as GC ∼ 2.4 pWK−1. We find the peak power dissipation in CN-TFTs is more strongly correlated to the TBC of the CNT-substrate interface than to the TBC at CNT junctions. Molecular dynamics simulations of crossed CNT junctions also reveal that the top CNT is buckled over ∼30 nm lengths, losing direct contact with the substrate and creating highly localized hot-spots. Our results provide new insights into CNT network properties which can be engineered to enhance performance of CN-TFTs for macro and flexible electronics applications.

Complementary Logic Gate Arrays Based on Carbon Nanotube Network Transistors

An efficient technique of fabricating high performance n- and p- type single-walled carbon nanotube (SWNT) network field-effect transistors (NET-FETs) is successfully demonstrated. Complementary inverters, NOR, NAND, OR, AND logic gates have been achieved from integrating these p- and n-type SWNT-NET-FETs. The processing technique described here is fully compatible with conventional silicon microelectronic technologies and it is readily suitable for scalable integration.

Improvement in the I–V characteristics of carbon nanotube network transistors using microwave treatment

This work describes the effects of microwave irradiation on the transfer characteristics of single-walled carbon-nanotube network transistors. The microwave treatment dramatically increases the on-off ratio of the electrical currents by reducing the off-state current. Detailed analyses of Raman spectroscopy data, the thermal effect, and a direct image comparison suggest preferential removal of the metallic paths formed in the transistor channel. This treatment can be utilized as method to repair for electrically failed transistors even after completion of device fabrication.

Comparative study of solution-processed carbon nanotube network transistors

Carbon nanotube networks in thin-film type transistors were studied experimentally, comparing the use of pre-separated semiconducting enriched nanotubes (90% and 99% purity) to examine how topology affects the properties of the devices. Measurements are reported for two deposition methods used for network formation: random and spin-aligned deposition methods. The results show that the thin-film transistors fabricated via spin-aligned deposition demonstrate better electrical uniformity and performance than those produced by the random network deposition method. Our results imply that coverage and alignment are strongly correlated with the properties of the devices and should therefore be simultaneously optimized for improved electrical uniformity and performance.

ZnO nanowire network transistors based on a self-assembly method

Dense, uniform ZnO nanowire (NW) networks are prepared by using a simple and sufficient self-assembly method. In this method, ZnO NWs are modified with aminopropyltriethoxysilane (APTES) to form positively charged amine-terminated surfaces. The modified ZnO NWs are adsorbed on negatively charged SiO2/Si substrates to form ZnO NW networks by the electrostatic interaction in an aqueous solution. Field-effect transistors (FETs) are fabricated and studied based on the ZnO NW networks. For a typical device with an NW density of 2.8 μm−2, it exhibits a current on/off ratio of 2.4 × 105, a transconductance of 336 nS, and a field-effect mobility of 27.4cm2/(V·s).

Effect of the metal work function on the electrical properties of carbon nanotube network transistors

A nearly perfect semiconducting single-walled carbon nanotube random network thin film transistor array was fabricated, and its reproducible transport properties were investigated. The effects of the metal work function for both the source and the drain on the electrical properties of the transistors were systematically investigated. Three different metal electrodes, Al, Ti, and Pd, were employed. As the metal work function increased, p-type behavior became dominant, and the fieldeffect hole mobility dramatically increased. Also, the Schottky barrier of the Ti-nanotube contact was invariant to the molecular adsorption of species in air.

Selective dispersion of high purity semiconducting single-walled carbon nanotubes with regioregular poly(3-alkylthiophene)s

Conjugated polymers, such as polyfluorene and poly(phenylene vinylene), have been used to selectively disperse semiconducting single-walled carbon nanotubes (sc-SWNTs), but these polymers have limited applications in transistors and solar cells. Regioregular poly(3-alkylthiophene)s (rr-P3ATs) are the most widely used materials for organic electronics and have been observed to wrap around SWNTs. However, no sorting of sc-SWNTs has been achieved before. Here we report the application of rr-P3ATs to sort sc-SWNTs. Through rational selection of polymers, solvent and temperature, we achieved highly selective dispersion of sc-SWNTs. Our approach enables direct film preparation after a simple centrifugation step. Using the sorted sc-SWNTs, we fabricate high-performance SWNT network transistors with observed charge-carrier mobility as high as 12 cm(2) V(-1) s(-1) and on/off ratio of >10(6). Our method offers a facile and a scalable route for separating sc-SWNTs and fabrication of electronic devices.

Post Annealing Effect on the Electrical Properties of Top-Gate SWNT Network Transistors

High performance top-gate single walled carbon nanotube network transistors are fabricated with aluminum oxide (Al2O3) layer as a gate dielectric by atomic layer deposition. It exhibits large on/off ratio (>10(4)) due to selective growth of semiconducting tubes by the plasma enhanced chemical vapor deposition. I-V characteristics show p-type or n-type depending on the deposition temperature. We investigate the type dependent characteristics for the carrier polarities with the post annealing effect on the top-gate SWNT network transistors. The dramatic change in the polarity of the top-gate SWNT network transistors, from n-type to p-type due to conversion of I-V characteristics is observed by post-annealing at 350 degrees C for 30 minutes under vacuum. Our observation suggests that competition between electron transfer from the Al2O3 layers to the SWNT surface and electron capture by oxygen molecules adsorbed on the tube walls seems to be the key point for the V(th) change as a function of Al2O3 deposition temperature.

The Geometrical Effect of Single Walled Carbon Nanotube Network Transistors on Low Frequency Noise Characteristics

The noise characteristics of randomly networked single walled carbon nanotubes grown directly by plasma enhanced chemical vapor deposition with field effect transistor. Geometrical complexity due to the large number of tube-tube junctions in the nanotube network is expected to be one of the key factors for the noise power of 1/f dependence. We investigated low frequency noise as a function of channel length (2-10 microm) and found that increased with longer channel length. Percolational behaviors of nanotube network that differs from ordinary semiconducting and metallic materials can be characterized by a geometrical picture with electrical homo- and hetero-junctions. Fixed nanotube density provides a test conditions to evaluate the contributions of junctions as a noise center. Hooge's empirical law is applied to investigate the low frequency noise characteristics of single walled carbon nanotube random network transistors. The noise power shows the dependence of the transistor channel length. It is understood that nanotube/nanotube junctions act as a noise center. However, the differences induced by channel length in the noise power are observed as not so significant. We conclude that tolerance of low frequency noise is important property for SWNT networks as an electronic device application.

Imaging conduction pathways in carbon nanotube network transistors by voltage-contrastscanning electron microscopy

The performance of field-effect transistors based on single-walled carbon nanotube(SWCNT) networks depends on the electrical percolation of semiconducting and metallicnanotube pathways within the network. We present voltage-contrast scanningelectron microscopy (VC-SEM) as a new tool for imaging percolation in a SWCNTnetwork with nano-scale resolution. Under external bias, the secondary-electroncontrast of SWCNTs depends on their conductivity, and therefore it is possibleto image the preferred conduction pathways within a network by following thecontrast evolution under bias in a scanning electron microscope. The experimentalVC-SEM results are correlated to percolation models of SWCNT-bundle networks.

The Perspective of Nanotechnology and Its Convergence with Future Information Technology

We present the current and future of nanotechnology, especially focusing on the convergence of nanotechnology with electronics, photonics, energy, and biology. Nano-electronics aspect addresses the nano carbon and its applications with graphene and carbon nanotubes. These will be covering displays, lighting, THz radiation, network transistors and many applications. As a part of nano-photonics, quantum dot and its related application for LED and displays are addressed. We also discuss nano energy generator for energy harvesting. Concept of e-nose and e-tongue with nano wires is presented. Printable and flexible electronics is discussed with nano materials. The convergence of nanotechnology is covered in details.

Nearly Single‐Chirality Single‐Walled Carbon Nanotubes Produced via Orthogonal Iterative Density Gradient Ultracentrifugation

We describe how iterative, orthogonal density gradient ultracentrifugation (DGU) separations can be used to produce nearly single-chirality (6,5) SWNTs. SWNT network transistors made from these highly pure, ≥98% semiconducting SWNTs simultaneously exhibit high on/off ratios, mobilities, and on-state conductances, suggesting their future application in integrated circuits and near-infrared optoelectronic light emitters and photodetectors.

Imaging dissipation and hot spots in carbon nanotube network transistors

We use infrared thermometry of carbon nanotube network (CNN) transistors and find the formation of distinct hot spots during operation. However, the average CNN temperature at breakdown is significantly lower than expected from the breakdown of individual nanotubes, suggesting extremely high regions of power dissipation at the CNN junctions. Statistical analysis and comparison with a thermal model allow the estimate of an upper limit for the average tube-tube junction thermal resistance, ∼4.4×1011 K/W (thermal conductance of ∼2.27 pW/K). These results indicate that nanotube junctions have a much greater impact on CNN transport, dissipation, and reliability than extrinsic factors such as low substrate thermal conductivity.

Lithography-free fabrication of carbon nanotube network transistors

A novel non-lithographic technique for the fabrication of carbon nanotube thin filmtransistors is presented. The whole transistor fabrication process requires onlyone mask which is used both to pattern transistor channels based on aerosolsynthesized carbon nanotubes and to deposit electrodes by metal evaporationat different angles. An important effect of electrodynamic focusing was utilizedfor the directed assembly of transistor channels with feature sizes smaller thanthe mask openings. This dry non-lithographic method opens up new avenues fordevice fabrication especially for low cost flexible and transparent electronics.

Self-consistent electrothermal analysis of nanotube network transistors

We develop an electrothermal transport model for nanocomposite thin films based on self-consistent solution of drift-diffusion and Poisson equations for electrons coupled with diffusive transport of heat. This model is used to analyze the performance of an electronic display the pixels of which are controlled by carbon nanotube (CNT) network thin-film transistors (TFTs). The effect of electrothermal coupling on device performance and steady state temperature rise is analyzed as a function of key device parameters such as channel length, network density, tube-to-substrate thermal conductance, and tube-to-substrate thermal conductivity ratio. Our analysis suggests that device on-current Ion may reduce by 30% for a 1 μm channel length devices due to self-heating. The temperature rise in such devices can be as high as 500 K in extreme cases due to the thermally insulating substrate and the low tube-to-substrate thermal conductance. These results suggest that an appropriate combination of network density, channel length and width should be selected for CNT-TFTs to avoid device temperature rise above acceptable limits. We analyze the effectiveness of active cooling in reducing the temperature and enhancing the performance of the device. We find that the high thermal spreading resistance between the CNT device and the electronic display reduces the effectiveness of forced convective cooling, necessitating the exploration of alternative designs for viable CNT-FET based display technology.

Effects of Dispersion Conditions of Single-Walled Carbon Nanotubes on the Electrical Characteristics of Thin Film Network Transistors

To facilitate solution deposition of single-walled carbon nanotubes (SWNTs) for integration into electronic devices they need to be purified and dispersed into solutions. The vigorous sonication process for preparing these dispersions leads to large variations in the length and defect density of SWNTs, affecting the resulting electronic properties. Understanding the effects of solution processing steps can have important implications in the design of SWNT films for electronic applications. Here, we alter the SWNTs by varying the sonication time, followed by deposition of sub-monolayer SWNT network films onto functionalized substrates. The corresponding electrical performance characteristics of the resulting field effect transistors (FETs) are correlated with SWNT network sorting and morphology. As sonication exposure increases, the SWNTs shorten, which not only affects electrical current by increasing the number of junctions but also presumably leads to more defects. The off current of the resulting transistors initially increased with sonication exposure, presumably due to less efficient sorting of semiconducting SWNTs as the defect density increases. After extended sonication, the on and off current decreased because of increased bundling and fewer percolation pathways. The final transistor properties are influenced by the nanotube solution concentration, density, alignment, and the selectivity of surface sorting of the SWNT networks. These results show that in addition to chirality, careful consideration of SWNT dispersion conditions that affect SWNT length, bundle diameter, and defect density is critical for optimal SWNT-FET performance and potentially other SWNT-based electronic devices.

Charge conversion effects of carbon nanotube network transistors by temperature for Al2O3 gate dielectric formation

We report on the observation of the electrical characteristics for field effect transistors with random networks of single-walled carbon nanotubes induced by thermal contribution on gate dielectric formation. For the Al2O3 gate dielectric, only the deposition temperature gradually changes the electrical polarity from p-type to n-type. Competition between the electron transfer from the Al2O3 layers to the nanotube surface and the electron capture by the oxygen molecules adsorbed on the tube wall is critical for transport depending on the deposition temperature.