Carbon Nanotube Bridges Research Articles

Sensitive ${\rm NH}_{3}{\rm OH}$ and HCl Gas Sensors Using Self-Aligned and Self-Welded Multiwalled Carbon Nanotubes

We present the design and fabrication of sensitive and responsive and HCl sensors based on multiwalled carbon nanotubes (MWCNT). The sensors consisted of self-aligned and self-welded MWCNT bridges grown between 18 mum-high silicon posts with 2-6 mum gaps using metal-catalyzed chemical vapor deposition technique (CVD). The 5-10 MWCNTs spanning the gap showed excellent ohmic electrical contacts and mechanical bonding strengths in excess of 0.1 muN/CNT. The conductivity of the MWCNTs changes as oxidizing and reducing gases are physically adsorbed on their large surface-to-volume ratio structures. Since carbon atoms in CNTs do not have any dangling bonds, the interaction between CNTs and gas molecules does not lead to chemical reactions and the process is reversible. Provided that the starting MWCNTs have high resistivity, they have nearly exponential sensitivity in detecting gases and owing to their very small volume and reversible interaction with gases, they can have a very fast response time. In our experiment, upon exposure to ammonia and hydrochloric gases at room temperature, the electrical conductivity of the self-welded carbon nanotube bridges exhibited a dramatic change. The growth process, test results, and the gas sensitivities will be discussed.

Gas damping of carbon nanotube oscillators

Previous experimental results suggest that gas damping is a significant loss mechanism for carbon-nanotube-based mechanical resonators operating in low vacuum conditions. Using free-molecular flow theory for the gas and a continuum-based model for the nanotube resonator, a model for gas damping of single-wall carbon nanotube bridges and cantilevers is proposed. When compared to experimental data for a single-wall carbon nanotube bridge oscillator, the model agrees to within the limits of the known experimental geometry.

Telescoping self-aligned metal-catalyzed carbon nanotube piezoresistors as strain gauges

Multiwalled carbon nanotube (MWCNT) bridges, grown using low-pressure, metal-catalyzed, chemical vapour deposition (CVD) technique between silicon posts, were used as tiny piezoresistors to monitor vibration and bending/deformation of silicon cantilever beams. The weld strength of the CNTs, measured using atomic force microscopy (AFM), was larger than 100 nN/CNT and their full-scale resistance change was larger than 105 ω. The effective longitudinal piezoresistivity of these CNTs was larger than 4×10−8 Pa−1, which is larger than that of π44 in silicon.

Synthesis of carbon nanotubes: Controlled fabrication of intraconnects

The authors have successfully fabricated carbon nanotubes bridges (intraconnects) between predetermined layouts of metal tip electrodes, taking advantage of the tips as leading catalytic points. Overall, the authors have demonstrated a yield of 15%–30% depending on the process parameters and electrode layout. They used two deposition methods: chemical vapor deposition (CVD) and plasma enhanced CVD (PECVD). Bridges could be fabricated with only CVD. The authors used carbon monoxide (CO) for the CVD process; the PECVD, however, required a mixture of CO and H2. Ethanol was successfully used with both deposition methods.

Quantized current in carbon nanotube quantum dots by surface acoustic waves

Single charge effects attract much attention with respect to possible applications as future transistor structures or for quantum cryptography. Here, surface acoustic wave mediated single charge transport through quantum dots (QDs) formed in a carbon nanotube (CNT) is presented. The CNT bridges metallic source and drain contacts on a piezoelectric host substrate, employing an acoustic alignment technique. The metal electrodes in contact with the nanotube provide tunnel barriers to the quasi one-dimensional electron system, hence forming a few QD in series due to the presence of defects within the CNT. The dynamic piezoelectric field associated with the surface wave leads to an acoustoelectric current through the system which turns out to be quantized like in a turnstile device.

Synthesis of individual single-walled carbon nanotube bridges controlled by support micromachining

Single-walled carbon nanotubes (SWNTs) were directly grown onto poly-crystalline silicon grids by catalytic thermal chemical vapour deposition. We demonstrate that simple micromachining of the catalyst-covered support can influence the number, location and alignment of suspended SWNTs. Sharp apexes formed by over-etching circular microstructures enable the scalable, cost-efficient formation of mostly individual, straight SWNT bridges, as verified by Raman scattering and electron diffraction.

Batch fabrication of nanotubes suspended between microelectrodes

We report a fabrication method, which uses standard UV-lithography to pattern the catalyst for the chemical vapour deposition (CVD) of suspended double clamped single walled carbon nanotubes. By using an aqueous solution of Fe(NO 3) 3 the patterning of the catalyst material onto microelectrodes can be done with a simple lift-off process with standard photolithographic resist. An applied electric field is sustained between the microelectrodes during CVD to guide the nanotube growth. Comparison with simulations shows that the location and the orientation of the grown carbon nanotubes (CNT) correspond to the regions of maximum electric field, enabling accurate positioning of a nanotube by controlling the shape of the microelectrodes. The CNT bridges are deflected tens of nm when a DC voltage is applied between the nanotube and a gate microelectrode indicating that the clamping through the catalyst particles is not only mechanically stable but also electrical conducting. This method could be used to fabricate nanoelectromechanical systems based on suspended double clamped CNTs depending only on photolithography and standard Cleanroom processes.

Surface Electron Spin Resonance Study on Ruby Crystal Using Evanescent Microwave Microscopy Techniques

As the silicon technology approaches to its physical limit, the future electronic devices will depend on behaviors of a few electrons. This study is to explore the possibility of detecting a single electron spin transition by using nondestructive evanescent microwave microscopy (EMM) techniques. To enhance the RF magnetic field and minimize the dielectric losses, the sample is placed at the center of the conventional electron spin resonance (ESR) microwave cavity that does not have scanning image capabilities. In this paper, a magnetic dipole probe (MDP) is presented that not only has the advantages of the microwave cavity, but is also capable of surface scanning at high speeds. At present, the minimum detectable electron spin transitions are 20 000 on the ruby crystal (Cr3+ doped in Al2O3) surface, whereas the commercially available ESR microwave cavity has a resolution of 106 minimum detectable spins limit. Three ESR energy absorption spikes were detected at 3.77 and 3.73 GHz with the ruby crystal placed inside and outside of the MDP conductor loop, respectively. The measured ESR energy absorption spectra are consistent with theoretical analysis and the conventional ESR experimental results. The current MDP sensor has a 500-mum spatial resolution with a 1-mm radius conductor loop made by 150-mum copper wires. The nondestructive and noninvasive natures of the EMM microscopy are suitable for many biomedical applications, such as DNA sequencing, Alzheimer, and other biological tissue studies. Future efforts will be focused on integration of the MDP on the atomic force microscopy with carbon-nanotube bridges

One by one control of the exact number of carbon nanotubes formed by chemical vapor deposition growth: A digital growth process

The authors have succeeded in precisely controlling the exact number of individual bridging carbon nanotubes (CNTs) between two electrodes during thermal chemical vapor deposition (CVD) by counting the number of steps in a monitoring current. A direct current bias was applied between electrodes and the resulting current was monitored during the CVD process. The monitoring current showed several steplike increases, each of which corresponded to bridging by a single CNT. After thermal CVD, the number of bridging CNTs was confirmed by scanning electron microscopy observation and by the electrical breakdown process: the results coincided exactly with the number of current steps monitored during the CVD growth.

Metal-Catalyzed Carbon Nanotube Piezoresistors with Very Large Longitudinal Piezoresistivity

Multi-walled carbon nanotube (MWCNT) bridges, grown using low- pressure metal-catalyzed chemical vapor deposition technique between silicon posts, were used as tiny piezoresistors to monitor vibration and bending/deformation of silicon cantilever beams. The weld strength of CNTs measured using atomic force microscope was larger than 100 nN/CNT and their full-scale resistance change was larger than 1e5 Ohm.The effective longitudinal piezoresistivity of these CNTs was larger than 4x1e-8 1/Pa which is larger than that of Pi44 in silicon.

Electromechanical analysis of suspended carbon nanotubes for memory applications

A nanoelectromechanical model based on atomistic simulations including chargetransfer was investigated. Classical molecular dynamics simulations combined withcontinuum electric models were applied to a carbon-nanotube nanoelectromechanicalmemory device that was characterized by carbon-nanotube bending performance.For a suspended (5, 5) carbon-nanotube bridge with a length of 11.567 nm(LCNT) and a trenchdepth of 0.9–1.5 nm (H), molecular dynamics results showed that the threshold voltage increased linearly asH increasedand the transition time decreased exponentially at each trench depth as the applied bias increased.When H/LCNT was below 0.13, the carbon-nanotube nanoelectromechanical memories acted as nonvolatilememory devices, whereas they were volatile memory or switching devices whenH/LCNT was above 0.14.

Bridging Growth of Single-Walled Carbon Nanotubes on Nanostructures by Low-Pressure Hot-Filament Chemical Vapor Deposition

Suspended single-walled carbon nanotubes (SWNTs) bridging between Si- or SiO2 nano-pillars were synthesized at low pressure using hot-filament chemical vapor deposition. The relation between the bridging probability of SWNTs and growth conditions was analyzed statistically. The bridging probability of the SWNTs strongly depends on pillar configuration, including pillar spacing, but is not significantly affected by the pressure, growth method, pillar material, and gas flow. These results indicate that environment in the gas phase has little influence on the suspended growth of SWNTs, and they support our vibratory growth model, in which the vibration of growing nanotubes plays a crucial role in bridging to an adjacent pillar.

Selective growth of horizontally-oriented carbon nanotube bridges on patterned silicon wafers by electroless plating Ni catalysts

The main purpose of this research was to develop an Integrated Circuit compatible process to grow the horizontally-oriented carbon nanotubes (CNTs) across the trenches of the patterned Si wafer, which was produced by conventional photolithography technique. The selectivity of the process is based on the difference in electrical conductivity between amorphous silicon (a:Si) and silicon nitride (Si 3N 4), where the catalyst can be much easier deposited by electroless plating on the a:Si part of the pattern. The selectivity is also based on greater chemical reactivity of the catalyst with a:Si to form silicides, instead of with Si 3N 4. Furthermore, the Si 3N 4 barrier layer of the pattern was designed on top of the a:Si layer to guide the growth of CNTs in horizontal direction to bridge the trenches of the pattern. The as-deposited catalysts were examined by Auger electron spectroscopy (AES). The catalyst-coated pattern was pretreated in hydrogen plasma and followed by CNT growth in a microwave plasma chemical vapor deposition (MPCVD) system. The CNT bridges were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and I–V measurements. Under the present deposition conditions, TEM and HRTEM examinations indicate that the deposited nanostructures are bamboo-like multiwalled carbon nanotubes (MWNTs) with a wall thickness of 20∼30 graphene layers. Electrical conductivity of the as-deposited MWNTs can be greatly improved by subjecting to 760 °C heat treatment under nitrogen atmosphere. The results demonstrate that the amounts of CNTs and bridges are tunable with the Ni catalyst plating time. Under the present experimental configuration and at a catalyst plating time of 20 s, countable numbers of bridges can be obtained, which are selectively and horizontally grown on the areas of the pattern with Ni catalyst. This process can be a step approaching the application of CNTs in electronic devices.

Microstructure and mechanical properties of hot-pressed carbon nanotubes compacted by spark plasma sintering

Bulk carbon nanotube samples were prepared by spark plasma sintering. The as-prepared bulk carbon nanotube material exhibited brittle fracture similar to that of common ceramics. Its fracture toughness was around 4.2 MPa m 1/2 while flexural strength was 50 MPa due to the weak bonding between carbon nanotubes. Obvious carbon nanotube bridging was found during the development of the crack induced by an indenter, which provides a possibility of carbon nanotube tough material.

Electrical conductivity measurements of a multi‐walled carbon nanotube

AbstractWe present a new method for fabricating a multi‐walled carbon nanotube (CNT) probe using a low‐melting‐point alloy as the connecting material. In particular, we constructed a CNT bridge structure connected by the alloy and measured current–voltage (I–V) characteristics exhibiting a resistance of <450 kΩ, which is low enough to compare with the tunnelling gap resistance of scanning tunnelling microscopy. Furthermore, to investigate the electrical conductivity of the CNT bridge structure in detail, we assembled a special holder to manipulate the CNT within a scanning transmission electron microscope. We then fabricated a CNT bridge structure and measured its I–V characteristics in the microscope using TiNi nanoparticles (with an average diameter of 100 nm) as the connecting material between the CNT and the electrode. The resistance of the CNT bridge structure showed an ideal quantum resistance, indicating that TiNi nanoparticles can be considered as one of the ideal connecting materials. This success demonstrates the usefulness of the holder that we assembled. We believe that this holder will contribute to the fabrication of conductive CNT probes and the realization of CNT‐based electrical devices. Copyright © 2005 John Wiley & Sons, Ltd.

Electric transport measurement of a multi-walled carbon nanotube in scanning transmission electron microscope

We used a carbon nanotube (CNT) manipulator in scanning transmission electron microscope (STEM). The manipulator was composed of two tungsten heater stages facing each other. One was fixed and the other stage could be moved. Titanium nickel (TiNi) particles whose average size was about 100 nm were put on the tungsten heaters. Finally, we constructed multi-walled CNT bridge structure by heating TiNi in STEM. Furthermore, we measured the electric transport of the CNT bridge in STEM. The current–voltage characteristics of the CNT bridge showed that the maximum current was 157 μA at 2.6 V and the current density was estimated to be 2×10 11 A/m 2 . The resistance showed a constant of 13 kΩ . This quite coincided with the quantum conductance of G 0=2 e 2/ h. This means that the CNT bridge using TiNi particles can reproducibly function as a ballistic conductor.

An approach to a multi-walled carbon nanotube based mass sensor

We propose an approach to a nanoscale mass sensor based on a gold electrode structure, on which a multi-walled carbon nanotube (MWCNT) bridge can be placed and soldered. The structure is comprised of three electrodes with a width of 2 or 4 μm. Two outer electrodes with a length of 10 or 15 μm serve as source and drain electrodes for the MWCNT bridge, whereas an inner electrode with a length of 8 or 13 μm is for electrostatic excitation of the CNT. Some structures have an extra pair of outer electrodes, which may deflect the inner electrodes and thereby be used for stretching or compressing the bridging nanotube. The free standing MWCNTs were fabricated by chemical vapour deposition of Fe(II) phthalocyanine. A nanomanipulator with an x− y− z translation stage was used for placing the MWCNTs across the source–drain electrodes. The nanotubes were soldered onto the substrate by electron beam induced deposition of an organometallic compound.

Selective Growth of Straight Carbon Nanotubes by Low-Pressure Thermal Chemical Vapor Deposition

Carbon nanotubes (CNTs) were grown by low-pressure thermal chemical vapor deposition using pure ethylene. It was found that straight CNTs, which were composed of bundled single- or double-wall CNTs and multiwall CNTs, were preferentially bridged between Fe nanoparticles under a low pressure of 100 Pa. Moreover, utilizing this method, we attempted to grow CNT bridges between patterned Ta electrodes. By nitriding the surface of the Ta electrodes, Fe nanoparticles with a moderate size were effectively formed, resulting in bridging CNTs between the electrodes.

Layer-by-Layer Electrostatic Self-Assembly of Polyelectrolyte Nanoshells on Individual Carbon Nanotube Templates

Carbon nanotubes have been featured prominently in the nanotechnology research for some time, yet robust strategies for noncovalent chemical modification of the nanotube surface are still missing. Such strategies are essential for the creation of functional device architectures. Here, we present a new general procedure for carbon nanotube modification based on polyelectrolyte layer-by-layer assembly. We have built multilayer structures around individual carbon nanotube bridges by first modifying the nanotube surface with a pyrene derivative followed by layer-by-layer deposition of polyelectrolyte macroions on the nanotube. Transmission electron microscopy and scanning confocal fluorescence microscopy images confirm the formation of nanometer-thick amorphous polymer nanoshells around the nanotubes. These multilayer polyelectrolyte shells on individual carbon nanotubes introduce nearly unlimited opportunities for the incorporation of various functionalities into nanotube devices, which, in turn, opens up the possibility of building more complex multicomponent structures.

Fabrication and mechanical properties of SiO 2 matrix composites reinforced by carbon nanotube

Carbon nanotubes (CNTs) and SiO 2 glass powder were mixed by ultrasonication in ethanol solution. CNT–SiO 2 composites with improved mechanical properties were fabricated by hot pressure sintering in 1300 °C 25 MPa, N 2 atmosphere). Effects of CNTs content on the relative density and mechanical properties were investigated. The bending strength and fracture toughness of the 5vol.%CNT–SiO 2 composite, compared with the monolithic SiO 2, were enhanced 65 and 100%, respectively. The main reinforcing mechanisms include CNTs pulling-out, crack deflxion, bridging and CNTs rupture. The defects and aligning direction of CNTs in the matrix are also the affecting factors on the mechanical properties of composites as prepared.