Robert A. Sayer

Toward surround gates on vertical single-walled carbon nanotube devices

The one-dimensional, cylindrical nature of single-walled carbon nanotubes (SWCNTs) suggests that the ideal gating geometry for nanotube field-effect transistors (FETs) is a surround gate (SG). Using vertical SWCNTs templated in porous anodic alumina, SGs are formed using top-down processes for the dielectric/metal depositions and definition of the channel length. Surround gates allow aggressive scaling of the channel to 25% of the length attainable with a bottom-gate geometry without incurring short-channel effects. The process demonstrated here for forming SGs on vertical SWCNTs is amenable for large-scale fabrication of multinanotube FETs.The one-dimensional, cylindrical nature of single-walled carbon nanotubes (SWCNTs) suggests that the ideal gating geometry for nanotube field-effect transistors (FETs) is a surround gate (SG). Using vertical SWCNTs templated in porous anodic alumina, SGs are formed using top-down processes for the dielectric/metal depositions and definition of the channel length. Surround gates allow aggressive scaling of the channel to 25% of the length attainable with a bottom-gate geometry without incurring short-channel effects. The process demonstrated here for forming SGs on vertical SWCNTs is amenable for large-scale fabrication of multinanotube FETs.

Improved Efficiency of Dye-Sensitized Solar Cells Using a Vertically Aligned Carbon Nanotube Counter Electrode

Dye-sensitized solar cells (DSSCs) offer many advantages in comparison to their Si-based counterparts, including lower cost of raw materials, faster manufacturing time, and the ability to be integrated with flexible substrates. Although many advances have been made in DSSC fabrication over recent years, their efficiency remains lower than commercially available Si photovoltaic cells. Here we report improved efficiency of TiO 2 /anthocyanin dye solar cell using vertically aligned arrays of carbon nanotubes (CNTs) as a counter electrode. Dense vertically oriented CNT arrays are grown directly on the counter electrode using microwave plasma chemical vapor deposition and a trilayer (Ti/Al/Fe) catalyst. The resulting arrays are 30 μm in height and have a number density of approximately 5×10 8 /mm 2 . By directly growing the CNTs on the counter electrode substrate, electrical interface conductance is enhanced. The performance of both as-grown and N-doped (using a nitrogen plasma) CNT arrays is reported. The fabricated DSSCs are tested under AM1.5 light. Increased short-circuit current is observed in comparison to graphite and Pt counter electrodes. We attribute this improvement to the large surface area created by the 3D structure of the arrays in comparison to the planar geometry of the graphite and Pt electrodes, as well as the excellent electrical properties of the CNTs.

Vertical Carbon Nanotube Devices With Nanoscale Lengths Controlled Without Lithography

Vertical single-walled carbon nanotubes (v-SWCNTs) are synthesized within highly ordered porous anodic alumina (PAA) templates supported on Si substrates. A process for obtaining thin-film PAA with long-range ordered nanopores is presented in this paper. Each nanopore contains at most one v-SWCNT that is supported by a dielectric and addressed by electrochemically formed Pd nanowire source contacts and evaporated Pd drain contacts. Characteristics of these completely vertical, two-terminal nanotube devices are presented. Control of the v-SWCNT length is demonstrated using a straightforward etching process with lengths of less than 100 nm achieved without the need for complex/expensive lithography. This effective nanoscale length control of highly ordered v-SWCNTs provides a practical basis for the realization of CNT-based nanoelectronics.

Thermal and Electrical Conductivities of Nanocrystalline Nickel Microbridges

DC electrical self-heating (Joule heating) is exploited to characterize the thermal behavior of Ni microbridges. The temperature rise of the devices due to self-heating is monitored using an infrared microscope for current densities up to 105 A/cm2. The obtained temperature profiles reveal significant heating at the bases of the microbridges. Simulations are performed in order to extract the thermal conductivity of the electroplated Ni thin film from the experimental data. The thermal conductivity is found to be 78.8 W/m · K or 13% less than that of bulk Ni. As current flows through the microbridges, they deflect upward, significantly changing the system response and pull-in voltage required for actuation. Additionally, the electrical resistivity and specific electrical contact resistances between the microbridges and the anchor points are reported. The electroplated Ni is found to have an electrical resistivity of 9.7 μΩ · cm which agrees with other values in the literature for thin-film Ni. By combining the electrical and thermal measurements, it is possible to determine the phonon and electron contributions to thermal conductivity. Although demonstrated on Ni films, this technique can be applied to any metallic film without modification. Such characterization of transport properties of constituent materials is important in the modeling of microelectromechanical systems and enables device performance to be predicted with improved accuracy.

Shot Noise Thermometry for Thermal Characterization of Templated Carbon Nanotubes

A carbon nanotube (CNT) thermometer that operates on the principles of electrical shot noise is reported. Shot noise thermometry is a self-calibrating measurement technique that relates statistical fluctuations in dc current across a device to temperature. A structure consisting of vertical, top, and bottom-contacted single-walled carbon nanotubes in a porous anodic alumina template was fabricated and used to measure shot noise. Frequencies between 60 and 100 kHz were observed to preclude significant influence from Vf noise, which does not contain thermally relevant information. Because isothermal models do not accurately reproduce the observed noise trends, a self-heating shot noise model has been developed and applied to experimental data to determine the thermal resistance of a CNT device consisting of an array of vertical single-walled CNTs supported in a porous anodic alumina template. The thermal surface resistance at the nanotube-dielectric interface is found to be 1.5 × 108 K/W, which is consistent with measurements by other techniques.

Length and temperature dependent 1/f noise in vertical single-walled carbon nanotube arrays

We report measurements of temperature- and length-dependent 1/f noise in vertical single-walled carbon nanotube (SWCNT) arrays. Carbon nanotubes are synthesized in a porous anodic alumina template with sub-micrometer channel lengths ranging from 100 to 700 nm. A significant difference is observed in the 1/f noise magnitude of quasi-ballistic and diffusive SWCNT devices, with quasi-ballistic devices exhibiting 1/f noise levels that are one to two orders of magnitude less than diffusively conducting devices. Furthermore, 1/f noise was measured from 90 to 400 K, and the noise prefactor decreased significantly at temperatures below 250 K.

Stochastic Modelling of Drop Coalescence on Non-Porous Substrates for Inkjet Applications

Coalescence between two drops on a substrate is one of the important factors that can affect print quality in inkjet applications. Two stochastic models (constant contant angle mode and constant contact area mode) that consider drop placement error, drop impact, and drop evaporation are proposed for determining the probability of coalescence between adjacently printed drops on nonporous substrates. Experiments are conducted to measure the probability of coalescence with respect to deposition time difference between adjacently printed drops and compared to the predictions of the models. The measured coalescence follows the constant contact angle mode evaporation model during the initial phase of the life of the first drop, which is followed by a mix between the constant contact angle mode and the constant contact area mode models for the remainder of the life of the first drop. This study shows that for probabilities of coalescence between 10 % and 80 % the constant contact angle mode model can be used to determine deposition time difference threshold values for adjacent drops in applications promoting drop coalescence while the constant contact area mode model can be used for applications avoiding drop coalescence. Further efforts are needed to capture the dynamics of the mixed-model evaporation and to more accurately predict larger (greater than 80 %) and smaller (less than 10 %) occurrences of coalescence.Copyright

Low-Frequency Electrical Noise Thermometry for Micro- and Nano-Scale Devices

Electrical noise is inherent in all conductors; the magnitude of this noise is proportional to device temperature in some frequency bands. Shot noise thermometry is a self-calibrating measurement technique that relates statistical fluctuations in DC current across a device to temperature. Historically, low frequency bands that contain 1/f noise have been filtered out in noise thermometry analysis. We report a noise analysis that accounts for 1/f noise in temperature measurements, thus reducing the required bandwidth of the measurement system. Numerical simulation is used to show the efficacy of this approach. Computer-generated noise signals containing Johnson, shot and 1/f noise are randomly generated. Data processing is used to determine device temperature and Fano factor. The impact of important factors such as data averaging, 1/f noise magnitude, bias range, ambient temperature and Fano factor on the accuracy and precision of fit values is investigated. This technique is then applied to experimental measurements on a vertical single-walled carbon nanotube array.Copyright

Improved Efficiency of Dye Sensitized Solar Cells Using Aligned Carbon Nanotubes

Dye sensitized solar cells (DSSCs) offer many advantages in comparison to their Si-based counterparts, including lower cost of raw materials, faster manufacturing time, and the ability to be integrated with flexible substrates. Although many advances have been made in DSSC fabrication over recent years, their efficiency remains lower than commercially available Si photovoltaic cells. Here we report improved efficiency of TiO2 /anthocyanin dye solar cell using aligned arrays of carbon nanotubes (CNTs) as a counter electrode. Dense vertically oriented CNT arrays are grown directly on the counter electrode using microwave plasma chemical vapor deposition and a tri-layer (Ti/Al/Fe) catalyst. The resulting arrays are 30 micrometers in height and have a number density of approximately five hundred million per square millimeter. By directly growing the CNTs on the counter electrode substrate, electrical interface conductance is enhanced. The performance of both as-grown and N-doped (using a nitrogen plasma) CNT arrays is reported. The fabricated DSSCs are tested under AM1.5 light. Increased short circuit current is observed in comparison to graphite and Pt counter electrodes. We attribute this improvement to the large surface area created by the 3D structure of the arrays in comparison to the planar geometry of the graphite and Pt electrodes as well as the excellent electrical properties of the CNTs.Copyright

Shot noise thermometry with carbon nanotubes

A carbon nanotube (CNT) thermometer is reported that operates on the principles of electrical shot noise. Shot noise thermometry is a self calibrating measurement technique that relates statistical fluctuations in dc current across a device to temperature. A self-heating shot noise model has been developed and applied to experimental data to determine the thermal resistance of a CNT device consisting of an array of vertical CNTs supported in a porous anodic alumina template. The thermal resistance is found to be 1.5 *108 K/W.