Ferroelectric-carbon nanotube memory devices

One-dimensional ferroelectric nanostructures, carbon nanotubes (CNT) and CNTinorganic oxides have recently been studied due to their potential applications for microelectronics. Here, we report coating of a registered array of aligned multi-wall carbon nanotubes (MWCNT) grown on silicon substrates by functional ferroelectric Pb(Zr,Ti)O 3 (PZT) which produces structures suitable for commercial prototype memories. Microstructural analysis reveals the crystalline nature of PZT with small nanocrystals aligned in different directions. First-order Raman modes of MWCNT and PZT/MWCNT/n-Si show the high structural quality of CNT before and after PZT deposition at elevated temperature. PZT exists mostly in the monoclinic Cc/Cm phase, which is the origin of the high piezoelectric response in the system. Lowloss square piezoelectric hysteresis obtained for the 3D bottom-up structure confirms the switchability of the device. Currentvoltage mapping of the device by conducting atomic force microscopy (c-AFM) indicates very low transient current. Fabrication and functional properties of these hybrid ferroelectriccarbon nanotubes is the first step towards miniaturization for future nanotechnology sensors, actuators, transducers and memory devices. © 2012 IOP Publishing Ltd.

Carbon nanotube growth for through silicon via application.

Through silicon via (TSV) technology is key for next generation three-dimensional integrated circuits, and carbon nanotubes (CNT) provide a promising alternative to metal for filling the TSV. Three catalyst preparation methods for achieving CNT growth from the bottom of the TSV are investigated. Compared with sputtering and evaporation, catalyst deposition using dip-coating in a FeCl2 solution is found to be a more efficient method for realizing a bottom-up filling of the TSV (aspect ratio 5 or 10) with CNT. The CNT bundles grown in 5 min exceed the 50 μm length of the TSV and are multi-wall CNT with three to eight walls. The CNT bundles inside the TSV were electrically characterized by creating a direct contact using a four-point nanoprober setup. A low resistance of the CNT bundle of 69.7 Ω (297 Ω) was measured when the CNT bundle was contacted midway along (over the full length of) the 25 μm deep TSV. The electrical characterization in combination with the good filling of the TSV demonstrates the potential use of CNT in fully integrated TSV applications.

Structurally programmed capillary folding of carbon nanotube assemblies.

We demonstrate the fabrication of horizontally aligned carbon nanotube (HA-CNT) networks by spatially programmable folding, which is induced by self-directed liquid infiltration of vertical CNTs. Folding is caused by a capillary buckling instability and is predicted by the elastocapillary buckling height, which scales with the wall thickness as t(3/2). The folding direction is controlled by incorporating folding initiators at the ends of the CNT walls, and the initiators cause a tilt during densification which precedes buckling. By patterning these initiators and specifying the wall geometry, we control the dimensions of HA-CNT patches over 2 orders of magnitude and realize multilayered and multidirectional assemblies. Multidirectional HA-CNT patterns are building blocks for custom design of nanotextured surfaces and flexible circuits.

Corrugated carbon nanotube microstructures with geometrically tunable compliance

Deterministic organization of nanostructures into microscale geometries is essential for the development of materials with novel mechanical, optical, and surface properties. We demonstrate scalable fabrication of 3D corrugated carbon nanotube (CNT) microstructures, via an iterative sequence of vertically aligned CNT growth and capillary self-assembly. Vertical microbellows and tilted microcantilevers are created over large areas, and these structures can have thin walls with aspect ratios exceeding 100:1. We show these structures can be used as out-of-plane microsprings with compliance determined by the wall thickness and number of folds. © 2011 American Chemical Society.

Fabrication of high-aspect-ratio polymer microstructures and hierarchical textures using carbon nanotube composite master molds

Scalable and cost effective patterning of polymer structures and their surface textures is essential to engineer material properties such as liquid wetting and dry adhesion, and to design artificial biological interfaces. Further, fabrication of high-aspect-ratio microstructures often requires controlled deep-etching methods or high-intensity exposure. We demonstrate that carbon nanotube (CNT) composites can be used as master molds for fabrication of high-aspect-ratio polymer microstructures having anisotropic nanoscale textures. The master molds are made by growth of vertically aligned CNT patterns, capillary densification of the CNTs using organic solvents, and capillary-driven infiltration of the CNT structures with SU-8. The composite master structures are then replicated in SU-8 using standard PDMS transfer molding methods. By this process, we fabricated a library of replicas including vertical micro-pillars, honeycomb lattices with sub-micron wall thickness and aspect ratios exceeding 50:1, and microwells with sloped sidewalls. This process enables batch manufacturing of polymer features that capture complex nanoscale shapes and textures, while requiring only optical lithography and conventional thermal processing. © 2011 The Royal Society of Chemistry.

Non-destructive characterization of structural hierarchy within aligned carbon nanotube assemblies

Understanding and controlling the hierarchical self-assembly of carbon nanotubes (CNTs) is vital for designing materials such as transparent conductors, chemical sensors, high-performance composites, and microelectronic interconnects. In particular, many applications require high-density CNT assemblies that cannot currently be made directly by low-density CNT growth, and therefore require post-processing by methods such as elastocapillary densification. We characterize the hierarchical structure of pristine and densified vertically aligned multi-wall CNT forests, by combining small-angle and ultra-small-angle x-ray scattering (USAXS) techniques. This enables the nondestructive measurement of both the individual CNT diameter and CNT bundle diameter within CNT forests, which are otherwise quantified only by delicate and often destructive microscopy techniques. Our measurements show that multi-wall CNT forests grown by chemical vapor deposition consist of isolated and bundled CNTs, with an average bundle diameter of 16 nm. After capillary densification of the CNT forest, USAXS reveals bundles with a diameter 4 m, in addition to the small bundles observed in the as-grown forests. Combining these characterization methods with new CNT processing methods could enable the engineering of macro-scale CNT assemblies that exhibit significantly improved bulk properties. © 2011 American Institute of Physics.

Carbon nanotube forests growth using catalysts from atomic layer deposition

We have grown carbon nanotubes using Fe and Ni catalyst films deposited by atomic layer deposition. Both metals lead to catalytically active nanoparticles for growing vertically aligned nanotube forests or carbon fibres, depending on the growth conditions and whether the substrate is alumina or silica. The resulting nanotubes have narrow diameter and wall number distributions that are as narrow as those grown from sputtered catalysts. The state of the catalyst is studied by in-situ and ex-situ X-ray photoemission spectroscopy. We demonstrate multi-directional nanotube growth on a porous alumina foam coated with Fe prepared by atomic layer deposition. This deposition technique can be useful for nanotube applications in microelectronics, filter technology, and energy storage. © 2014 AIP Publishing LLC.

Ultra-broadband Polarisers Based on Metastable Free-Standing Aligned Carbon Nanotube Membranes

A carbon nanotube free-standing linearly dichroic polariser is developed using solid-state extrusion. Membrane cohesion is experimentally and numerically demonstrated to derive from inter-tube van der Waals interactions in this family of planar metastable morphologies, controlled by the chemical vapour deposition conditions. Ultra-broadband polarisation (400 nm – 2.5 mm) is shown and corroborated by effective medium and full numerical simulations.

Benzoxazine Resin/Carbon Nanotube Nanostructured Composite's Degradation Kinetic

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Solidly Mounted Resonators with Carbon Nanotube Electrodes for Biosensing Applications

The work reported here shows a direct experimental comparison of the sensitivities of AlN solidly mounted resonators (SMR)-based biosensors fabricated with standard metal electrodes and with carbon nanotube electrodes. SMRs resonating at frequencies around 1.75 GHz have been fabricated, some devices using a thin film of multi-wall carbon nanotubes (CNTs) as the top electrode material and some identical devices using a chromium/gold electrode. Protein solutions with different concentrations were loaded on the top of the resonators and their responses to mass-load from physically adsorbed coatings were investigated. Results show that resonators using CNTs as the top electrode material exhibited higher frequency change for a given load due to the higher active surface area of a thin film of interconnecting CNTs compared to that of a metal thin film electrode and hence exhibited greater mass loading sensitivity. It is therefore concluded that the use of CNT electrodes on resonators for their use as gravimetric biosensors is viable and worthwhile.

Switchable surfactant-assisted carbon nanotube coatings: innovation through pH shift

The inner surface of fused-silica capillaries has been coated with a dense/homogeneous coating of commercial multi-wall carbon nanotubes (MWCNTs) using a stable ink as deposit precursor. Solubilization of the MWCNTs was achieved in water/ethanol/dimethylformamide by the action of a surfactant, which can switch between a neutral or an ionic form depending on the pH of the medium, which thus becomes the driving force for the entire deposition process. Careful control of the experimental conditions has allowed us to selectively deposit CNTs on the inner surface of insulating silica capillaries by a simple, reproducible, and easily adaptable method.

Enhancement of field emission from multistage structure of carbon nanotube arrays

The field emission measurements for the multistage structured nanotubes (i.e., thin-multiwall and single wall carbon nanotubes grown on multiwall carbon nanotubes) were carried out and a low turn-on field of ~0.45 V/ μm, high emission current of 450 μA at a field of IV/μm and a large field enhancement factor of ~26200 were obtained. The thin multiwall carbon nanotubes (thin-MWNTs) and single wall carbon nanotubes (SWNTs) were grown on the regular arrays of vertically aligned multi wall carbon nanotubes (MWNTs) on porous silicon substrate by Chemical Vapor Deposition (CVD) method. The thin-MWNTs and SWNTs grown on MWNTs in this way have a multistage structure which gives higher enhancement of the electric field and hence the electron field emission.

Characterization of commercial double-walled carbon nanotube material : composition, structure, and heat capacity

A purified commercial double-walled carbon nanotube (DWCNT) sample was investigated by transmission electron microscopy (TEM), thermogravimetry (TG), and Raman spectroscopy. Moreover, the heat capacity of the DWCNT sample was determined by temperature-modulated differential scanning calorimetry in the range of temperature between -50 and 290 °C. The main thermo-oxidation characterized by TG occurred at 474 °C with the loss of 90 wt% of the sample. Thermo-oxidation of the sample was also investigated by high-resolution TG, which indicated that a fraction rich in carbon nanotube represents more than 80 wt% of the material. Other carbonaceous fractions rich in amorphous coating and graphitic particles were identified by the deconvolution procedure applied to the derivative of TG curve. Complementary structural data were provided by TEM and Raman studies. The information obtained allows the optimization of composites based on this nanomaterial with reliable characteristics.

Purity evaluation and influence of carbon nanotube on carbon nanotube/graphite thermal stability

High resolution thermogravimetry has been used to evaluate the carbonaceous content in a commercial sample of single-walled carbon nanotube (SWNT). The content of SWNTs in the sample was found to be at least 77mass% which was supported by images obtained with scanning and transmission electron microscopies (SEM and TEM). Furthermore, the influence of SWNT addition on the thermal stability of graphite in mixtures of SWNT/graphite at different proportions was investigated. The graphite stability decreased with the increased of SWNT content in the overall range of composition. This behavior could be due to the close contact between these carbonaceous species as determined by SEM analysis.