Kitu Kumar

Out-of-plane growth of CNTs on graphene for supercapacitor applications

This paper describes the fabrication and characterization of a hybrid nanostructure comprised of carbon nanotubes (CNTs) grown on graphene layers for supercapacitor applications. The entire nanostructure (CNTs and graphene) was fabricated via atmospheric pressure chemical vapor deposition (APCVD) and designed to minimize self-aggregation of the graphene and CNTs. Growth parameters of the CNTs were optimized by adjusting the gas flow rates of hydrogen and methane to control the simultaneous, competing reactions of carbon formation toward CNT growth and hydrogenation which suppresses CNT growth via hydrogen etching of carbon. Characterization of the supercapacitor performance of the CNT-graphene hybrid nanostructure indicated that the average measured capacitance of a fabricated graphene-CNT structure was 653.7 μF cm(-2) at 10 mV s(-1) with a standard rectangular cyclic voltammetry curve. Rapid charging-discharging characteristics (mV s(-1)) were exhibited with a capacitance of approximately 75% (490.3 μF cm(-2)). These experimental results indicate that this CNT-graphene structure has the potential towards three-dimensional (3D) graphene-CNT multi-stack structures for high-performance supercapacitors.

On the growth mode of two-lobed curvilinear graphene domains at atmospheric pressure.

We demonstrate the chemical vapor deposition (CVD) growth of 2-lobed symmetrical curvilinear graphene domains specifically on Cu{100} surface orientations at atmospheric pressure. We utilize electron backscattered diffraction, scanning electron microscopy and Raman spectroscopy to determine an as-yet unexplored growth mode producing such a shape and demonstrate how its growth and morphology are dependent on the underlying Cu crystal structure especially in the high CH4:H2 regime. We show that both monolayer and bilayer curvilinear domains are grown on Cu{100} surfaces; furthermore, we show that characteristic atmospheric pressure CVD hexagonal domains are grown on all other Cu facets with an isotropic growth rate which is more rapid than that on Cu{100}. These findings indicate that the Cu-graphene complex is predominant mechanistically at atmospheric pressure, which is an important step towards tailoring graphene properties via substrate engineering.

A Study on Carbon-Nanotube Local Oxidation Lithography Using an Atomic Force Microscope

In this paper, nanoscale anodic oxidation lithography using an atomic force microscope (AFM) is systematically studied on carbon nanotubes (CNTs). Trends between the produced feature size and the corresponding process parameters, such as applied voltage, water meniscus length, tip speed during oxidation (hold time), and humidity are observed. By methodically varying these process parameters, the appropriate working ranges have been found to create features of various sizes based on the oxidation of the CNT structure. We have obtained feature sizes down to 58 nm by setting oxidation time per pixel to 20 ms corresponding to a tip speed of 1.50 μ m/s. Optimizing the tip speed during line scans was found to be critical in maintaining the presence of the water meniscus, which was often found to break above the tip speeds of 1 μm/s. In addition, a comparison of the results from employing this technique between CNT and graphene patterning is illustrated. Other factors affecting the reproducibility of the results are addressed in an endeavor to make the oxidization process more robust and repeatable.

Nanoscale Graphene and Carbon Nanotube Lithography Using an Atomic Force Microscope

In this work, we demonstrate the use of a voltage-applied Atomic Force Microscopy (VAFM) local anodic oxidation nanolithography process to precisely fabricate small (<20 nm) structures from graphene and carbon nanotube material. These graphitic materials have exceptional electrical properties which give them a niche in emerging nanoelectronics applications requiring quantum structures. While several methods for nanoscale patterning of these materials exist, the VAFM nanolithography technique has lately been shown to address the fabrication issues of graphitic nanodevices on the order of tens of nanometers [1]. If the tip is raised sufficiently from the substrate, in high atmospheric humidity, a water meniscus forms between the two (Fig 1). Application of an appropriate electric field between the tip and substrate dissociates the H2O molecules into H+ and OH-. The H+ ions rush towards the negatively charged tip and the OH-ions gather near the positively substrate. The oxygen reacts with the carbon in the graphitic material to form volatile or nonvolatile carbon oxides depending on the voltage applied. This oxidation, coupled with the x-y scanning capability of the AFM allows for thin structure patterning ability. Depending on such process parameters as applied voltage, pulse width, tip dimensions, contact force, and humidity, the oxidation of the graphitic material into carbon oxides enables the formation of insulating trenches or bumps to make any structure or morphology conceivable [2]. This technique can also be performed in the ambient environment, eliminating several fabrication steps, such as the poly(methyl methacrylate) (PMMA) processing required in conventional electron-beam lithography process. We have used the VAFM technique in preliminary studies to cut few layer graphene and “draw” insulating patterns on highly ordered pyrolyzed graphite (HOPG). A negative bias of 10V applied to the AFM tip with no feedback in a high humidity atmosphere created 0.5 nm deep trenches spaced 27 nm apart. Preliminary experiments have also been conducted on 50 nm diameter multi-walled carbon nanotubes. A negative bias of 5V to the AFM tip pulsed for 100 ms segmented the multi-walled nanotube at selected points. Single wall carbon nanotubes were grown using chemical vapor deposition. Graphene was mechanically exfoliated and prepared using methods described elsewhere [3] on 300 nm SiO2 on Si substrate. The samples were connected electrically to ground and placed in an AFM system (Pacific Nanotechnology NANO-I) with environmental control. The samples were imaged in contact mode with an electrically conductive sharp AFM tip after which humidity was raised to 40–45%. Once the humidity was sufficiently raised, the tip was raised from the desired location on either the Carbon nanotubes or graphene/graphite and feedback was turned off. Patterns were drawn by the tip in this configuration with applied tip voltage running anywhere from −5V to −10V. See Figs. 2 and 3 for results on graphene and carbon nanotubes. Currently, a parametric study on AFM lithography on graphene and carbon nanotubes is underway. By varying voltage, humidity, tip speed, dwell time, and tip-substrate distance, we will determine the optimal conditions required to accomplish precise patterning of graphene and controlled segmentation of carbon nanotubes. In conclusion, we have demonstrated a voltage-applied technique utilizing an atomic force microscope tip to pattern nanoscale features on graphitic materials. A systematic study on oxidation parameters is forthcoming.Copyright

Acoustic noise synthesis for duct systems

Studies on acoustical noise from duct systems are important in their design and performance. A methodology is presented for simulation of acoustical noise from a duct system which can be modeled in terms of source, duct element and termination. In this methodology first the transfer function of the given duct system is evaluated using the four pole parameters of the duct system, source strength, source acoustic and termination acoustic impedances. Then the impulse response of the predicted transfer function is convolved with source signal to obtain the acoustic noise output of the duct system in frequency domain. In order to verify this methodology, the simulated noise is compared with experimentally measured noise for two types of duct systems namely a straight pipe and a simple expansion chamber. The results show good agreement between the simulated and measured noise spectra. The methodology presented in this work provides the capability of simulating the noise of a duct system from its design before it is actually built. Also sound quality studies can be based on design that would assist in reaching the final design to be fabricated