Swastik Kar

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).

Tunable Graphene–Silicon Heterojunctions for Ultrasensitive Photodetection

We present the photodetection properties of graphene/Si heterojunctions both in the photocurrent and photovoltage modes. Monolayer graphene/Si junctions were found to be excellent weak-signal detectors with photovoltage responsivity exceeding 10(7) V/W and with noise-equivalent-power reaching ∼1 pW/Hz(1/2), potentially capable of distinguishing materials with transmittance, T = 0.9995 in a 0.5 s integration time. In the photocurrent mode, the response was found to remain linear over at least six decades of incident power (P), with tunable responsivity up to 435 mA/W (corresponding to incident photon conversion efficiency (IPCE) > 65%) obtained by layer thickening and doping. With millisecond-scale responses and ON/OFF ratios exceeding 10(4), these photodiodes are highly suitable for tunable and scalable broadband (400 < λ < 900 nm) photodetectors, photometers, and millisecond-response switching, spectroscopic and imaging devices, and further, and are architecturally compatible with on-chip low-power optoelectronics.

Contact transfer of aligned carbon nanotube arrays onto conducting substrates

The authors demonstrate the fabrication of different architectures of carbon nanotubes on conducting substrates via contact transfer of nanotubes using low temperature solders. Lithographically patterned multiwalled carbon nanotube arrays grown on silica substrates by chemical vapor deposition methods are transferred onto solder coated substrates. Both negative and positive patterns can be obtained by changing the printing parameters. Good wetting and electrical contacts are confirmed by measuring their field emission properties. This method can be used to construct nanotube structures of different shapes and dimensions over large areas on substrates of choice and could be a feasible process to integrate nanotubes into various devices.

Optical and Sensing Properties of 1-Pyrenecarboxylic Acid-Functionalized Graphene Films Laminated on Polydimethylsiloxane Membranes

We present fabrication and characterization of macroscopic thin films of graphene flakes, which are functionalized with 1-pyrenecarboxylic acid (PCA) and are laminated onto flexible and transparent polydimethylsiloxane (PDMS) membranes. The noncovalently (π-stacked) functionalization of PCA allows us to obtain a number of unique optical and molecular sensing properties that are absent in pristine graphene films, without sacrificing the conducting nature of graphene. The flexible PCA-graphene-PDMS hybrid structure can block 70-95% of ultraviolet (UV) light, while allowing 65% or higher transmittance in the visible region, rendering them potentially useful for a number of flexible UV absorbing/filtering applications. In addition, the electrical resistance of these structures is found to be sensitive to the illumination of visible light, atmospheric pressure change, and the presence of different types of molecular analytes. Owing to their multifunctionality, these hybrid structures have immense potential for the development of versatile, low-cost, flexible, and portable electronic and optoelectronic devices for diverse applications.

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...

Superconducting NbSe2 nanostructures

Nanotubes of NbSe 2 in admixture with nanorods have been produced by the thermal decomposition of NbSe 3 and investigated by electron microscopy and other techniques. The nanostructures have outer diameters in the 35-100 nm range, with lengths in excess of several hundred nanometers. The nanostructures are metallic and become superconducting at 8.3 K.

A General Synthetic Approach to Interconnected Nanowire/Nanotube and Nanotube/Nanowire/Nanotube Heterojunctions with Branched Topology

Heterojunctions between nanotubes (NTs) and nanowires (NWs) could provide building blocks for nanoelectronics and nanophotonics, with other applications in barcodes, optical readout, biomolecular separation, catalysis, selfassembly, and magnetic manipulation. Although hybrid NWs (metal/polymer, semiconductor/semiconductor, 9] metal/semiconductor, and metal/metal ), hybrid NTs (metal/metal), NT/NW heterojunctions, and tree-like nano-heterojunctions have beenmade, the corresponding studies demonstrated limited control over the geometry and complexity of the nano-heterojunctions, which ultimately are central to the design of building blocks for nanocircuits, nanodevices, and nanosystems. Herein we show a general synthetic approach to various branched two-segment NW/NT and three-segment NT/NW/NT heterojunctions, based on a combinatorial process of electrodepositing NWs within the branched channels of anodic aluminum oxide (AAO) templates, selectively etching part of the electrodeposited NWs, and growing NTs on the ends of the NWs. The NWs can be metallic or semiconducting, while the NTs can consist of carbon, silicon, and silica; the two NT segments in threesegment NT/NW/NT nanoarchitectures can comprise either the same or different materials. This approach enables excellent control over the geometry, chemical composition, and complexity of the hetero-nanoarchitectures that can be the framework for nanoscale devices and systems. Figure 1 shows schematic depictions of the basic heteronanoarchitectures we have made, which consist of various NT and NW segments placed combinatorially in a Y-shaped topology. The synthesis scheme follows a typical buildingblock concept in which a set of different nanoscale components (NTs and NWs of different materials with distinct properties, in linear and branched topologies) can be connected in a predetermined fashion inside the branched

Highly Aligned Scalable Platinum-Decorated Single-Wall Carbon Nanotube Arrays for Nanoscale Electrical Interconnects

We present the fabrication and characterization of nanoscale electrical interconnect test structures constructed from aligned single-wall carbon nanotubes using a template-based fluidic assembly process. This CMOS-friendly process enables the formation of highly aligned parallel nanotube interconnect structures on SiO(2)/Si substrates of widths and lengths that are limited only by lithographical limits and, hence, can be easily integrated onto existing Si-based platforms. These structures can withstand current densities of approximately 10(7) A.cm(-2), comparable or better than copper at similar dimensions. Both the nanotube alignment and failure current density improve with decreasing structure width. In addition, we present a novel Pt nanocluster decoration method that drastically decreases the resistivity of the test structures. Ab initio density functional theory calculations indicate that the increase in conductivity of the nanotubes is caused by an increase in conduction channels close to their Fermi levels due to the platinum nanocluster decoration, with a possible conversion of the semiconducting single-wall carbon nanotubes into metallic ones. These results reflect a huge step toward the proposed replacement of copper-based interconnects with carbon nanotubes at gigascale integration levels.

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.