Interface States in Gate Stack of Carbon Nanotube Array Transistors.

A deep understanding of the interface states in metal-oxide-semiconductor (MOS) structures is the premise of improving the gate stack quality, which sets the foundation for building field-effect transistors (FETs) with high performance and high reliability. Although MOSFETs built on aligned semiconducting carbon nanotube (A-CNT) arrays have been considered ideal energy-efficient successors to commercial silicon (Si) transistors, research on the interface states of A-CNT MOS devices, let alone their optimization, is lacking. Here, we fabricate MOS capacitors based on an A-CNT array with a well-designed layout and accurately measure the capacitance-voltage and conductance-voltage (C-V and G-V) data. Then, the gate electrostatics and the physical origins of interface states are systematically analyzed and revealed. In particular, targeted improvement of gate dielectric growth in the A-CNT MOS device contributes to suppressing the interface state density (Dit) to 6.1 × 1011 cm-2 eV-1, which is a record for CNT- or low-dimensional semiconductors-based MOSFETs, boosting a record transconductance (gm) of 2.42 mS/µm and an on-off ratio of 105. Further decreasing Dit below 1 × 1011 cm-2 eV-1 is necessary for A-CNT MOSFETs to achieve the expected high energy efficiency.

Multi-Walled Carbon Nanotube Array Modified Electrode with 3D Sensing Interface as Electrochemical DNA Biosensor for Multidrug-Resistant Gene Detection.

Drug resistance in cancer is associated with overexpression of the multidrug resistance (MDR1) gene, leading to the failure of cancer chemotherapy treatment. Therefore, the establishment of an effective method for the detection of the MDR1 gene is extremely crucial in cancer clinical therapy. Here, we report a novel DNA biosensor based on an aligned multi-walled carbon nanotube (MWCNT) array modified electrode with 3D nanostructure for the determination of the MDR1 gene. The microstructure of the modified electrode was observed by an atomic force microscope (AFM), which demonstrated that the electrode interface was arranged in orderly needle-shaped protrusion arrays. The electrochemical properties of the biosensor were characterized by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). Chronocoulometry (CC) was used for the quantitative detection of the MDR1 gene. Taking advantage of the good conductivity and large electrode area of the MWCNT arrays, this electrochemical DNA sensor achieved a dynamic range from 1.0 × 10-12 M to 1.0 × 10-8 M with a minimal detection limit of 6.4 × 10-13 M. In addition, this proposed DNA biosensor exhibited high sensitivity, selectivity, and stability, which may be useful for the trace analysis of the MDR1 gene in complex samples.

Synergistic enhancement on flexible solid-state supercapacitor based on redox-active Fe3+ions/natural spidroin modified vertically aligned carbon nanotube arrays.

The conductive skeleton and aligned carbon nanotube array (CNTA) structure can greatly shorten the ion transfer path and promote the charge transfer speed, which makes the CNTA an ideal electrode material for energy storage application. However, poor mechanical stability and low specific capacitance greatly impede its practical utilization. Here, we introduce a promising flexible electrode material based on the natural spider silk protein (SSP) modified CNTA(SSP/CNTA) with improved hydrophilicity and mechanical flexibility. The redox-active Fe3+doped SSP/CNTA flexible solid-state supercapacitor (FSSC) device with superior energy storage performance was assembled in a symmetric 'sandwich-type' structure. The synergetic interaction between Fe3+ions and the SSP are proved to greatly enhance the electrochemical performance especially the long-term cyclic stability. The Fe3+doped SSP/CNTA FSSCs device achieves an ultra-high volumetric capacitance of 4.92 F cm-3at a sweep speed of 1 mV s-1. Meanwhile it exhibited an excellent cycling stability with an increased capacitance by 10% after 10 000 charge-discharge cycles. As a control, a Fe3+doped CNTA composite device without SSP will lose over 74% of the capacitance after 10 000 cycles. The energy storage mechanism analysis confirms the dominated capacitive behavior of the device, which explained a considerable power density and rate performance. Our method thus provides a promising strategy to build up highly-efficient redox-enhanced FSSCs for next generation of wearable and implantable electronics.

MoSe2 nanoflakes-decorated vertically aligned carbon nanotube film on nickel foam as a binder-free supercapacitor electrode with high rate capability.

A new type of composite electrode material, MoSe2 nanoflakes grown on the vertically aligned carbon nanotube array film (VACNTF) with binder-free nickel foam as current collector (VACNTF@MoSe2/NF), was fabricated by a simple spraying chemical vapor deposition method combined with the solvothermal technique. Owing to the introduction of the VACNTF with ordered channels and appropriate intertube spacing, which facilitate electrolyte ions quickly transferring and alleviate the volume changes in the electrochemical measurements, the VACNTF@MoSe2/NF sample presents superior electrochemical performance compared to pure MoSe2/NF sample. The VACNTF@MoSe2/NF sample exhibits high specific capacitance of 435 F·g-1 at 1 A·g-1, remarkable cycling stability (92% of the original capacitance maintaining over 5000 cycles) and especially excellent rate capability (84.1% capacitance retention with the current density changed from 1 to 15 A·g-1). Moreover, the VACNTF@MoSe2/NF based asymmetric supercapacitor exhibits a high energy density with 22 Wh·kg-1 for a power density of 330 W·kg-1. This paper offers a new strategy to prepare transition metal dichalcogenides based electrode materials with high rate performance.

An Integrated Structural Air Electrode Based on Parallel Porous Nitrogen-Doped Carbon Nanotube Arrays for Rechargeable Li-Air Batteries.

The poor discharge and charge capacities, and the cycle performance of current Li-air batteries represent critical obstacles to their practical application. The fabrication of an integrated structural air electrode with stable parallel micropore channels and excellent electrocatalytic activity is an efficient strategy for solving these problems. Herein, a novel approach involving the synthesis of nitrogen-doped carbon nanotube (N-CNT) arrays on a carbon paper substrate with a conductive carbon-black layer for use as the air electrode is presented. This design achieves faster oxygen, lithium ion, and electron transfer, which allows higher oxygen reduction/evolution reaction activities. As a result, the N-CNT arrays (N/C = 1:20) deliver distinctly higher discharge and charge capacities, 2203 and 186 mAh g-1, than those of active carbons with capacities of 497 and 71 mAh g-1 at 0.05 mA cm-2, respectively. A theoretical analysis of the experimental results shows that the parallel micropore channels of the air electrode decrease oxygen diffusion resistance and lithium ion transfer resistance, enhancing the discharge and charge capacities and cycle performance of Li-air batteries. Additionally, the N-CNT arrays with a high pyridinic nitrogen content can decompose the lithium peroxide product and recover the electrode morphology, thereby further improving the discharge-charge performance of Li-air batteries.

Quasi-ballistic carbon nanotube array transistors with current density exceeding Si and GaAs.

Carbon nanotubes (CNTs) are tantalizing candidates for semiconductor electronics because of their exceptional charge transport properties and one-dimensional electrostatics. Ballistic transport approaching the quantum conductance limit of 2G 0 = 4e (2)/h has been achieved in field-effect transistors (FETs) containing one CNT. However, constraints in CNT sorting, processing, alignment, and contacts give rise to nonidealities when CNTs are implemented in densely packed parallel arrays such as those needed for technology, resulting in a conductance per CNT far from 2G 0. The consequence has been that, whereas CNTs are ultimately expected to yield FETs that are more conductive than conventional semiconductors, CNTs, instead, have underperformed channel materials, such as Si, by sixfold or more. We report quasi-ballistic CNT array FETs at a density of 47 CNTs µm(-1), fabricated through a combination of CNT purification, solution-based assembly, and CNT treatment. The conductance is as high as 0.46 G 0 per CNT. In parallel, the conductance of the arrays reaches 1.7 mS µm(-1), which is seven times higher than the previous state-of-the-art CNT array FETs made by other methods. The saturated on-state current density is as high as 900 µA µm(-1) and is similar to or exceeds that of Si FETs when compared at and equivalent gate oxide thickness and at the same off-state current density. The on-state current density exceeds that of GaAs FETs as well. This breakthrough in CNT array performance is a critical advance toward the exploitation of CNTs in logic, high-speed communications, and other semiconductor electronics technologies.

Carbon nanotube array inducing osteogenic differentiation of human mesenchymal stem cells.

Carbon nanotubes (CNTs) are a kind of nanomaterials which have been shown a promising application for biomedicine. There are a lot of studies to use CNTs to induce the differentiation of mesenchymal stem cells (MSCs). However, the cellular behavior of MSCs on the top layer of CNT array was still not well understood. In this study, we evaluated the morphology, the gene expressions of the osteogenic differentiation related markers, and the gene expressions of collagen type II (Col II, a marker of chondrogenesis), PPARγ (a marker of adipogenesis) and scleraxis (SCX, a marker of tenogenesis) in human mesenchymal stem cells (hMSCs) cultured on multi-walled carbon nanotube (MWCNT) array. The effect of MWCNT array on the mineralization of hMSCs which were cultured in osteogenic differentiation medium (ODM) was further assayed. Our results showed that the hMSCs cultured on MWCNT array spread well, formed numerous spiral shaped cell colons and showed perinuclear morphology. Compared to hMSCs cultured on dish, the gene expression of osteocalcin (OCN) was increased while the gene expressions of collagen type II (Col II), PPARγ and scleraxis (SCX) were decreased in hMSCs which were cultured on MWCNT array without any differentiation factors. Furthermore, compared with hMSCs on dish, the gene expressions of collagen type I (Col I), osteocalcin (OCN), osteopontin (OPN) and RUNX2, and the mineralization of hMSCs on MWCNT array were enhanced when they were cultured in osteogenic differentiation medium (ODM). Our results indicated that MWCNT array was able to promote the osteogenesis of hMSCs.

Ice-assisted transfer of carbon nanotube arrays.

Decoupling the growth and the application of nanomaterials by transfer is an important issue in nanotechnology. Here, we developed an efficient transfer technique for carbon nanotube (CNT) arrays by using ice as a binder to temporarily bond the CNT array and the target substrate. Ice makes it an ultraclean transfer because the evaporation of ice ensures that no contaminants are introduced. The transferred superaligned carbon nanotube (SACNT) arrays not only keep their original appearance and initial alignment but also inherit their spinnability, which is the most desirable feature. The transfer-then-spin strategy can be employed to fabricate patterned CNT arrays, which can act as 3-dimensional electrodes in CNT thermoacoustic chips. Besides, the flip-chipped CNTs are promising field electron emitters. Furthermore, the ice-assisted transfer technique provides a cost-effective solution for mass production of SACNTs, giving CNT technologies a competitive edge, and this method may inspire new ways to transfer other nanomaterials.

Binder-free carbon nanotube electrode for electrochemical removal of chromium.

Electrochemical treatment of chromium-containing wastewater has the advantage of simultaneously reducing hexavalent chromium (CrVI) and reversibly adsorbing the trivalent product (CrIII), thereby minimizing the generation of waste for disposal and providing an opportunity for resource reuse. The application of electrochemical treatment of chromium is often limited by the available electrochemical surface area (ESA) of conventional electrodes with flat surfaces. Here, we report the preparation and evaluation of carbon nanotube (CNT) electrodes consisting of vertically aligned CNT arrays directly grown on stainless steel mesh (SSM). We show that the 3-D organization of CNT arrays increases ESA up to 13 times compared to SSM. The increase of ESA is correlated with the length of CNTs, consistent with a mechanism of roughness-induced ESA enhancement. The increase of ESA directly benefits CrVI reduction by proportionally accelerating reduction without compromising the electrode's ability to adsorb CrIII. Our results suggest that the rational design of electrodes with hierarchical structures represents a feasible approach to improve the performance of electrochemical treatment of contaminated water.

Conformal atomic layer deposition of alumina on millimeter tall, vertically-aligned carbon nanotube arrays.

Atomic layer deposition (ALD) can be used to coat high aspect ratio and high surface area substrates with conformal and precisely controlled thin films. Vertically aligned arrays of multiwalled carbon nanotubes (MWCNTs) with lengths up to 1.5 mm were conformally coated with alumina from base to tip. The nucleation and growth behaviors of Al2O3 ALD precursors on the MWCNTs were studied as a function of CNT surface chemistry. CNT surfaces were modified through a series of post-treatments including pyrolytic carbon deposition, high temperature thermal annealing, and oxygen plasma functionalization. Conformal coatings were achieved where post-treatments resulted in increased defect density as well as the extent of functionalization, as characterized by X-ray photoelectron spectroscopy and Raman spectroscopy. Using thermogravimetric analysis, it was determined that MWCNTs treated with pyrolytic carbon and plasma functionalization prior to ALD coating were more stable to thermal oxidation than pristine ALD coated samples. Functionalized and ALD coated arrays had a compressive modulus more than two times higher than a pristine array coated for the same number of cycles. Cross-sectional energy dispersive X-ray spectroscopy confirmed that Al2O3 could be uniformly deposited through the entire thickness of the vertically aligned MWCNT array by manipulating sample orientation and mounting techniques. Following the ALD coating, the MWCNT arrays demonstrated hydrophilic wetting behavior and also exhibited foam-like recovery following compressive strain.

A facile and general approach for the direct fabrication of 3D, vertically aligned carbon nanotube array/transition metal oxide composites as non-Pt catalysts for oxygen reduction reactions.

A simple and effective strategy involving nebulized ethanol assisted infiltration for the general synthesis of 3D structure-based vertically aligned carbon nanotube arrays (VACNTs) uniformly and deeply decorated with various transition-metal oxide (MOs) has been developed. Furthermore, it is demonstrated that the 3D structure-based VACNTs with decorated MnO2 can exhibit superior electrocatalytic oxygen reduction reaction activity, long-term stability, and an excellent resistance to crossover effects compared to the commercial Pt/C catalyst.