Devon McClain

The coordinated buckling of carbon nanotube turfs under uniform compression.

Complex structures consisting of intertwined, nominally vertical carbon nanotubes (CNTs), referred to as turfs, have unique properties that arise from their complex nanogeometry and interactions between individual CNT segments. For applications such as contact switches for electrical or thermal transfer it is necessary to understand the properties that arise from the collective behavior of an assemblage of CNTs rather than the properties of a single tube. In this study, the mechanical response of turfs bonded to substrates under compressive loading is demonstrated experimentally; coordinated alignment and buckling takes place under uniform loads. The mechanical response of turf structures provides some surprising results regarding parameters that control permanent deformation and buckling in assemblages of nanostructures; buckling of the turf structure is controlled by the height and effective modulus of the turf, but not the aspect ratio of the structure. We present and verify a model which describes the coordinated buckling phenomena relevant for applications such as CNT turfs for thermal transfer media.

Thermocompression bonding of vertically aligned carbon nanotube turfs to metalized substrates

Vertically aligned carbon nanotube turfs (VACNTs), consisting of entwined, nominally vertical carbon nanotubes, are being proposed for use as electrical and thermal contact materials. Issues in their implementation include high contact resistance, the van der Waals interactions of carbon nanotubes, and a low temperature limit during processing. One route for circumventing the 750 degrees C temperatures required for VACNT growth using chemical vapor deposition is for the VACNTs to be grown separately, and then transferred to the device. A method of mechanical transfer, using thermocompression bonding, has been developed, allowing dry mechanical transfer of the VACNTs at 150 degrees C. This method can be used for the construction of both a thermal switch or a permanent conducting channel. The conductivity of the bonded structure is shown to be independent of the imposed strain, up to strains in excess of 100%.

Fabrication and field emission properties of triode-type carbon nanotube emitter arrays.

We report here an effective method for the fabrication of a large number of triode-type microgated carbon nanotube field emitter arrays. Our technique combines dual-beam focused ion beam technology and plasma-enhanced chemical vapor deposition, avoiding the tedious lithography and wet chemistry procedures conventionally used to fabricate such structures. Field emission testing revealed that increasing gate voltage by as little as 0.3 V had significant impact on the local electric fields, lowering the turn on and threshold fields by 3.6 and 3.0 V/microm, respectively. The field enhancement factor of the emitter arrays was also increased from 149 to 222. A quantum mechanical model for such triode-type field emission indicates that the local electric field generated by a negatively or positively biased gate directly impacts the tunneling barrier thickness and thus the achievable emitter current.

Comparative Investigation of the Effect of Oxygen Adsorbate and Electrode Work Function on Carbon-Nanotube Field-Effect Transistors

The effect of electrode work function (WF) on the device resistance of carbon-nanotube field-effect transistors (CNT-FETs) is less significant than that of device surface chemistry, specifically interactions with molecular oxygen adsorbate. Experimental results are based on over 130 individual devices with electrode WFs varying from 4.17 to 5.21 eV that were tested under standard-atmosphere, ultrahigh-vacuum, and pure-oxygen environments. Oxygen decreased device resistance by an average of 56% and altered majority charge carriers regardless of electrode metal. Variations in CNT-FET performance based on electrode material appeared more closely associated with crystallization and oxidation states than WF.

Spatial Identification of Defect Sites in Individual Carbon Nanotubes Using Micro-Raman Spectroscopy

The role of defects in the structural and electrical properties of carbon nanotubes (CNTs) is becoming increasingly important for a number of applications. These defects have been traced to changes in transport behavior (e.g. from metallic to semiconducting) in transistors [1], compressive buckling in thin films [2], and electrochemical sensitivity in sensors [3]. Raman spectroscopy has been successfully used for determining CNT defect density and chirality of bulk samples but it has only recently been applied to individual tubes [4]. The advantages of this technique include its speed, non-destructive nature and ability to characterize disorder in sp 2 carbon materials like CNTs. The disorder-induced D-bands are the principle Raman signal used to determine the presence of defects in nanotubes [5]. However, use of these peaks to map the locations of defects in CNTs is complicated by very weak D-band intensities in single, isolated tubes.

Spatial Variations in the Mechanical Properties and Electrical Properties of Carbon Nanotube Turfs

The mechanical properties of arrays of nominally vertically aligned carbon nanotubes, often referred to as turfs, have been measured using nanoindentation and the electrical properties have been measured using electrical contact resistance (ECR) nanoindentation. The elastic properties do not vary significantly between the top and the bottom of the same carbon nanotube turf. Within a single turf the lateral spatial variation is less than 10% when volumes of μms are probed with the indenter, indicating that each turf can be treated mechanically as continuum on this scale. The electrical properties vary significantly within a single turf on the same scale. This suggests that the use of average mechanical properties for a given vertically aligned turf should be suitable for design purposes without the need to account for spatial variation in structure, and variations in mechanical properties on the micrometer scale are not dependent on spatially distinct defects. However, local contact behavior appears to dominate the electrical behavior on this same length scale.

High Yield Growth of Various CdS Nano-Structures and Their Electron Field Emission Behavior

Nanostructures are considered the critical component in a wide range of potential nanoscale device applications. Yet a procedure to fabricate them with both controllable results and in bulk quantities must be developed in order to achieve their commercialization at reduced cost. In this report, we introduce an improved vapor-liquid-solid method that is capable of preparing high yield, high quality CdS nanowires and nanobelts in a turf-like configuration. To increase yield, we placed gold-coated substrates in a ceramic boat partially covered with a glass slide to form a gas trap. Only a small opening was provided to allow the CdS vapor to escape from the trap. This arrangement increases catalyst exposure to CdS vapor flow in comparison to conventional CVD methods. This allowed the CdS vapor to deposit densely over the substrate at a predetermined temperature range of 501°C-630°C inside the quartz tube. These conditions results in synthesis of various morphologies on both quartz and tungsten substrates including an intertwined-like structure not previously reported. Electron microscopy and microanalysis techniques were utilized in characterizing these morphologies, internal structures and elemental compositions. Electron field emission properties were investigated in an ultra high vacuum chamber set up with a base pressure of ∼1E-9 torr.

Characterization of Nanocones Grown During DC Magnetron Sputtering of an ITO Target

A low temperature method for uniform growth of In 2 O 3 nanostructures on Si wafers that does not require separate catalyst materials or template-assistance is investigated. Nanostructures are uniformly deposited on either bare or SiO 2 thin film coated Si substrates via DC magnetron sputtering at 200-400°C using a 90% In 2 O 3 / 10% SnO 2 (ITO) target. The nanostructures are approximately 500 nm long, sit atop an accompanying underlying 100 nm conductive film, and are conically shaped, with a diameter of about 80 nm at the base, tapering to a point that is capped with a spherical “ball”. X-ray diffraction (XRD) indicates a cubic In 2 O 3 phase. Field emission from the tips is observed at a base pressure of 10-8 Torr with turn-on fields in a range between 45-75 V/cm and threshold fields from 64-105 V/cm. Nanocone growth is investigated with respect to O 2 and Ar flow rates, temperature, power, pressure, wafer rotation, and time.