Optimization of carbon nanotube ultracapacitor for cell design

We report a methodology to optimize vertically grown carbon nanotube (CNT) ultracapacitor (CNU) geometrical features such as CNT length, electrode-to-electrode separation, and CNT packing density. The electric field and electrolyte ionic motion within the CNU are critical in determining the device performance. Using a particle-based model (PBM) based on the molecular dynamics techniques we developed and reported previously, we compute the electric field in the device, keep track of the electrolyte ionic motion in the device volume, and evaluate the CNU electrical performance as a function of the aforementioned geometrical features. We show that the PBM predicts an optimal CNT density. Electrolyte ionic trapping occurs in the high CNT density regime, which limits the electrolyte ions from forming a double layer capacitance. In this regime, the CNU capacitance does not increase with the CNT packing density as expected, but dramatically decreases. Our results compare well with existing experimental data and the PBM methodology can be applied to an ultracapacitor built from any metallic electrode materials, as well as the vertical CNTs studied here

Monte Carlo simulation of scanning electron microscopy bright contrast images of suspended carbon nanofibers

The authors present a Monte Carlo study of previously observed bright contrast from carbon nanofibers suspended over the underlying substrate using scanning electron microscopy (SEM). The analysis shows that the origin of the bright contrast is mainly the increase in the secondary electron signal from the substrate when a gap between the nanofiber and substrate exists. The SEM signal dependence on the gap height is well reproduced by Monte Carlo simulation as well as a derived analytical expression. The bright contrast prevails when the SEM beam size is much smaller than the nanofiber diameter

Length dependence of current-induced breakdown in carbon nanofiber interconnects

Current-induced breakdown is investigated for carbon nanofibers (CNF) for potential interconnect applications. The measured maximum current density in the suspended CNF is inversely proportional to the nanofiber length and is independent of diameter. This relationship can be described with a heat transport model that takes into account Joule heating and heat diffusion along the CNF, assuming that breakdown occurs when and where the temperature reaches a threshold or critical value

Change in carbon nanofiber resistance from ambient to vacuum

The electrical properties of carbon nanofibers (CNFs) can be affected by adsorbed gas species. In this study, we compare the resistance values of CNF devices in a horizontal configuration in air and under vacuum. CNFs in air are observed to possess lower current capacities compared to those in vacuum. Further, Joule heating due to current stressing can result in desorption of gas molecules responsible for carrier trapping, leading to lower resistances and higher breakdown currents in vacuum, where most adsorbed gaseous species are evacuated before any significant re-adsorption can occur. A model is proposed to describe these observations, and is used to estimate the number of adsorbed molecules on a CNF device

Improved contact for thermal and electrical transport in carbon nanofiber interconnects

We study the performance and reliability of carbon nanofiber (CNF) interconnects under high-current stress by examining CNF breakdown for four test configurations, suspended/supported with/without tungsten deposition. The use of W is to improve the CNF-electrode contact. The supported cases show a larger current density just before breakdown than the suspended ones, suggesting an effective heat dissipation to the substrate. The W-deposited contacts reduce the initial total resistance from megaohm range without W to kilo-ohms. High-current stress does not change the total resistance of the test structures with W unlike those without W deposition

Contact resistance in carbon nanostructure via interconnects

We present an in-depth electrical characterization of contact resistance in carbon nanostructure via interconnects. Test structures designed and fabricated for via applications contain vertically aligned arrays of carbon nanofibers (CNFs) grown on a thin titanium film on silicon substrate and embedded in silicon dioxide. Current-voltage measurements are performed on single CNFs using atomic force microscope current-sensing technique. By analyzing the dependence of measured resistance on CNF diameter, we extract the CNF resistivity and the metal-CNF contact resistance

Tunneling between carbon nanofiber and gold electrodes

In a carbon nanofiber (CNF)-metal system such as a bridge between two gold electrodes, passing high current (current stressing) reduces the total resistance of the system (CNF resistance RCNF plus contact resistance Rc) by orders of magnitude. The role of current stressing is modeled as a reduction in the interfacial tunneling gap with transport characteristics attributed to tunneling between Au and CNF. The model predicts a reduction in Rc and gradual disappearance of the nonlinearity in the current-voltage (I-V) characteristics as Rc decreases. These results are consistent with measured I-V behavior

Achieving higher photoabsorption than group III-V semiconductors in silicon using photon-trapping surface structures

The photosensitivity of silicon is inherently very low in the visible electromagnetic spectrum, and it drops rapidly beyond 800 nm in near-infrared wavelengths. Herein, we have experimentally demonstrated a technique utilizing photon-trapping surface structures to show a prodigious improvement of photoabsorption in one-micrometer-thin silicon, surpassing the inherent absorption efficiency of gallium arsenide for a broad spectrum. The photon-trapping structures allow the bending of normally incident light by almost ninety degrees to transform into laterally propagating modes along the silicon plane. Consequently, the propagation length of light increases, contributing to more than an order of magnitude improvement in absorption efficiency in photodetectors. This high absorption phenomenon is explained by FDTD analysis, where we show an enhanced photon density of states while substantially reducing the optical group velocity of light compared to silicon without photon-trapping structures, leading to significantly enhanced light-matter interactions. Our simulations also predict an enhanced absorption efficiency of photodetectors designed using 30 and 100-nanometer silicon thin films that are compatible with CMOS electronics. Despite a very thin absorption layer, such photon-trapping structures can enable high-efficiency and high-speed photodetectors needed in ultra-fast computer networks, data communication, and imaging systems with the potential to revolutionize on-chip logic and optoelectronic integration.Comment: 24 pages, 4 figure