Saikat Talapatra

Direct growth of aligned carbon nanotubes on bulk metals

There are several advantages of growing carbon nanotubes (CNTs) directly on bulk metals, for example in the formation of robust CNT–metal contacts during growth. Usually, aligned CNTs1,2,3,4,5,6,7,8,9 are grown either by using thin catalyst layers predeposited on substrates1,2,3,4,5,6,7 or through vapour-phase catalyst delivery7,8,9. The latter method, although flexible, is unsuitable for growing CNTs directly on metallic substrates. Here we report on the growth of aligned multiwalled CNTs on a metallic alloy, Inconel 600 (Inconel), using vapour-phase catalyst delivery. The CNTs are well anchored to the substrate and show excellent electrical contact with it. These CNT–metal structures were then used to fabricate double-layer capacitors and field-emitter devices, which demonstrated improved performance over previously designed CNT structures. Inconel coatings can also be used to grow CNTs on other metallic substrates. This finding overcomes the substrate limitation for nanotube growth which should assist the development of future CNT-related technologies.

Stable Aqueous Dispersions of Noncovalently Functionalized Graphene from Graphite and their Multifunctional High-Performance Applications

We present a scalable and facile technique for noncovalent functionalization of graphene with 1-pyrenecarboxylic acid that exfoliates single-, few-, and multilayered graphene flakes into stable aqueous dispersions. The exfoliation mechanism is established using stringent control experiments and detailed characterization steps. Using the exfoliated graphene, we demonstrate highly sensitive and selective conductometric sensors (whose resistance rapidly changes >10,000% in saturated ethanol vapor), and ultracapacitors with extremely high specific capacitance (∼ 120 F/g), power density (∼ 105 kW/kg), and energy density (∼ 9.2 Wh/kg).

Intrinsic carrier mobility of multi-layered MoS2 field-effect transistors on SiO2

By fabricating and characterizing multi-layered MoS2-based field-effect transistors in a four terminal configuration, we demonstrate that the two terminal-configurations tend to underestimate the carrier mobility μ due to the Schottky barriers at the contacts. For a back-gated two-terminal configuration, we observe mobilities as high as 91 cm2 V−1 s−1 which is considerably smaller than 306.5 cm2 V−1 s−1 as extracted from the same device when using a four-terminal configuration. This indicates that the intrinsic mobility of MoS2 on SiO2 is significantly larger than the values previously reported, and provides a quantitative method to evaluate the charge transport through the contacts.

Carbon Nanotube−MoS2 Composites as Solid Lubricants

Solid lubricants (SLs) characterized by low coefficients of friction (mu) and wear rates (w) drastically improve the life span of instruments that undergo extreme frictional wear. However, the performance of SLs such as sputtered or nanoparticulate molybdenum disulfide (MoS(2)), tungsten disulfide (WS(2)), or graphite deteriorates heavily under extreme operational conditions such as elevated temperatures and high humidity. Here, we present our preliminary results, which demonstrate that composites of carbon nanotubes (CNTs) and MoS(2) produced by electrodeposition of MoS(2) on vertically aligned CNT films have low mu ( approximately 0.03) and w (approximately 10(-13) mm(3)/N.mm) even at 300 degrees C, which are about 2 orders of magnitude better than those of nanoparticulate MoS(2)-based coatings. The high load-bearing capacity of CNTs provides a strong enduring support to MoS(2) nanoclusters and is responsible for their ultralow w. The incorporation of these composites in liquid lubricants reduces the friction coefficient of the liquid lubricants by approximately 15%. The technique described here to produce SL coatings with extremely appealing frictional properties will provide valuable solutions for a variety of tribological applications where the coatings encounter high temperature, reduced pressure, and/or low- and high-humidity conditions.

Electrochemical double layer capacitor electrodes using aligned carbon nanotubes grown directly on metals

We report on the fabrication of electrochemical double layer capacitor (EDLC) electrodes with aligned carbon nanotubes (CNTs) grown directly on conductive substrates using an air assisted chemical vapor deposition technique. The fabricated EDLCs showed very small equivalent series resistances (approximately few hundreds of mOmega), a direct consequence of integrating CNTs with metal current collectors. The specific capacitance of the CNTs used for EDLC electrodes increased with decreasing CNT lengths and ranged from 10.75 F g(-1) to 21.57 F g(-1) with maximum energy and power density ranging from 2.3 to 5.4 Wh kg(-1) and 19.6 to 35.4 kW kg(-1), respectively. These results indicate that the integrated CNT electrodes fabricated using a simple single step process hold significant promise in applications related to electrochemical energy storage.

Electrochemical characterization of liquid phase exfoliated two-dimensional layers of molybdenum disulfide.

We report on the electrochemical charge storage behavior of few-layered flakes of molybdenum disulfide (MoS2) obtained by liquid phase exfoliation of bulk MoS2 powder in 1-dodecyl-2-pyrrolidinone. The specific capacitances of the exfoliated flakes obtained using a 6 M KOH aqueous solution as an electrolyte were found to be an order of magnitude higher than those of bulk MoS2 (∼0.5 and ∼2 mF cm(-2) for bulk and exfoliated MoS2 electrodes, respectively). The exfoliated MoS2 flakes also showed significant charge storage in different electrolytes, such as organic solvents [1 M tetraethylammonium tetrafluoroborate in propylene carbonate (Et4NBF4 in PC)] and ionic liquids [1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)]. The values of specific capacitances obtained using Et4NBF4 in PC and BMIM-PF6 were ∼2.25 and ∼2.4 mF cm(-2), respectively. An analysis of electrochemical impedance spectroscopy using an equivalent circuit modeling was performed to understand the charge storage mechanism of these exfoliated MoS2 flakes using different electrolytes. Our findings indicate that liquid phase exfoliation methods can be used to produce large quantities of electrochemically active, two-dimensional layers of MoS2 and can act as an ideal material in several applications related to electrochemistry.

Charge-injection-induced dynamic screening and origin of hysteresis in field-modulated transport in single-wall carbon nanotubes

Gate modulated transport in semiconducting single-wall carbon nanotubes shows significant hysteresis in their transfer characteristics. The origin of this hysteresis is generally attributed to the screening of the gate voltage due to the movement of mobile charges/ions in the inherent presence of a trapping/detrapping mechanism in the adjacent dielectric, as in conventional silicon metal-oxide-semiconductor field-effect transistors. However, recent works have conjectured that the screening charges may originate from the nanotube itself. From an extensive study of the temperature dependence of the hysteresis behavior in nanotube field-effect transistors the authors experimentally establish this alternative mechanism, in which the screening charges are injected from the nanotube itself into the surrounding dielectric. Any detailed trapping/detrapping mechanism does not appear to play a significant role, and all experimental results can be simply explained in terms of a capacitive charging of the surrounding...

Hall and field-effect mobilities in few layered p-WSe2 field-effect transistors

Here, we present a temperature (T) dependent comparison between field-effect and Hall mobilities in field-effect transistors based on few-layered WSe2 exfoliated onto SiO2. Without dielectric engineering and beyond a T-dependent threshold gate-voltage, we observe maximum hole mobilities approaching 350 cm2/Vs at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm2/Vs as T is lowered below ~150 K, indicating that insofar WSe2-based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. The gate capacitance, as extracted from the Hall-effect, reveals the presence of spurious charges in the channel, while the two-terminal sheet resistivity displays two-dimensional variable-range hopping behavior, indicating carrier localization induced by disorder at the interface between WSe2 and SiO2. We argue that improvements in the fabrication protocols as, for example, the use of a substrate free of dangling bonds are likely to produce WSe2-based FETs displaying higher room temperature mobilities, i.e. approaching those of p-doped Si, which would make it a suitable candidate for high performance opto-electronics.

Quantitative analysis of hysteresis in carbon nanotube field-effect devices

The authors present a model to analyze hysteresis in transfer characteristics (TCs) of single-wall carbon nanotube field-effect transistors, based on capacitive charging of the surrounding dielectric by charges injected out of the nanotube. The model identifies the extent and time scale of the hysteresis and correctly describes the dependence of the magnitude of hysteresis on common experimental parameters. The authors propose and experimentally establish a “time-decay” experiment for obtaining accurate device properties in hysteresis-affected devices without actually measuring TCs. The authors further show that values obtained by this method can be used to precisely predict TCs for the same device under different experimental parameters.

Tunable Electronics in Large-Area Atomic Layers of Boron-Nitrogen―Carbon

We report on the low-temperature electrical transport properties of large area boron and nitrogen codoped graphene layers (BNC). The temperature dependence of resistivity (5 K < T < 400 K) of BNC layers show semiconducting nature and display a band gap which increases with B and N content, in sharp contrast to large area graphene layers, which shows metallic behavior. Our investigations show that the amount of B dominates the semiconducting nature of the BNC layers. This experimental observations agree with the density functional theory (DFT) calculations performed on BNC structures similar in composition to the experimentally measured samples. In addition, the temperature dependence of the electrical conductivity of these samples displays two regimes: at higher temperatures, the doped samples display an Arrhenius-like temperature dependence thus indicating a well-defined band gap. At the lowest temperatures, the temperature dependence of the conductivity deviates from activated behavior and displays a conduction mechanism consistent with Motts two-dimensional (2D) variable range hopping (2D-VRH). The ability to tune the electronic properties of thin layers of BNC by simply varying the concentration of B and N will provide a tremendous boost for obtaining materials with tunable electronic properties relevant to applications in solid state electronics.