Overview
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We utilise a Chemical Vapour Deposition (CVD) processes to synthesize carbon nanostructures with high precision and accuracy, using a dedicated CVD furnaces. Currently, the lab hosts a state of art cold wall PECVD chamber (Oxford Plasmalab 800), a hot wall tube furnace with RF plasma and a custom built rapid heating furnace. For the growth of nanotubes and nanowires, thin metal films are used as a template. These metal films are produced using sputtering and chemical techniques. For the growth of graphene thick metallic films and Cu foils are used as growth templates. At CRANN, we employ state of art microscopic and spectroscopic techniques to characterise our nanostructures, these include HRSEM, HRTEM and He-Ion microscopy as well as Raman and X-ray Photoelectron spectroscopy. We also study the electrical and optical properties of these novel nanostructures.
We combine top-down structuring techniques with the in-situ synthesis of novel functional nano-structures. The creation of such hybrid structures is aimed at the fabrication of new adaptive devices based on the bottom up growth of nano-materials with unique functionality. Using the hybrid approach the precession and scaling capabilities of silicon structuring technology will be maintained. We also utilize state-of the art lithographic structuring and self-assembly techniques to obtain mesoscopic preforms such as AAO to integrate functional nano-materials. In particular, we aim to implement not only graphitic nano-structures such carbon nanotubes and graphene but also inorganic nanowires. The following sections will outline the different materials synthesised within our group and our approaches to implementing these novel materials in adaptive devices.
Graphene
Graphene can be considered as an isolated graphitic monolayer and was first discovered by Geim and coworkers in the University of Manchester in 2004. The 2D nature of graphene infers a number of interesting properties. In pristine samples electron and hole mobilities have been shown to exceed 15,000 cm2/Vs. The charge carriers satisfy Dirac's equation in quantum mechanics and are known as massless Dirac Fermions. This unique situation arises due to interactions with the periodic potential of the honeycomb lattice. These massless Dirac Fermions can be considered as electrons which have lost their rest mass. This in combination with other novel effects such as the room temperature quantum Hall effect, high thermal conductivity and tunable band gaps make graphene potentially useful for innovative approaches to electronic devices and other applications.
Initlal graphene samples were produced by the mechanical exfoliation technique. This produces high quality samples but has a very low throughput and can only produce isolated flakes (as opposed to large scale films). The recently reported growth of graphene on Ni and Cu substrates marked a huge step forward in terms of graphene integration, uniformity and scalability. CVD methods have a relatively low growth temperature, are compatible with existing semiconductor processing steps and also allow for dopin in the gas phase.
The growth of graphene by CVD is one of the primary areas of research in the ASIN group. Initial studies centred on growth optimisation of few layer graphene (FLG) on Ni substrates and monolayer graphene on Cu substrates. The use of Raman spectroscopy and XPS in tandem with assorted forms of electron microscopy demonstrated the high crystallinity (and low defect levels) of films grown.
Recently extensive studies have been carried out on post growth patterning and processing steps using assorted masking, etching and transferring processes. These studies have assisted with current work on the incorporation of graphene into different functional devices including gas and biosensors. The use of plasma treatments to clean and functionalise graphene is also under investigation.
Figure 1 Top Left: HRTEM image of monolayer graphene. Top Right: Raman spectrum indicating monolayer graphene. Bottom Left: Schematic representation of graphene. Bottom Right: HRSEM of FLG.
Figure 2 Scanning Raman maps of CVD graphene grown in the ASIN lab and transferred onto SiO2.
Novel Sensing Platforms
The nanostructured materials fabricated in the ASIN labs are ideally suited for incorporation into various sensing applications including gas sensors and biological sensors. In the case of graphene adsorbed gas or bio-molecules lead to a shift in the Dirac point, potentially changing the carrier type, the charge carrier density and the conduction regime. Selectivity in such a device can be attained through functionalisation of the active region or the addition of a mediation layer.
The first steps in the production of such a device entail the processing, patterning and contacting of the active layer. Extensive work in the group has been performed in the area of contacting individual graphene flakes and also in contacting patterened graphene films and ribbons. Selective functionalisation of such contacted regions can be performed through the use of slective spotting.
Figure 3 Left: Contacted graphene flake. Middle: Patterned graphene ribbons (w = 4 μm) contacted with Ni contacts. Right: Selective "spotting" of active region on graphene based device.
The electrical response of graphene devices to adsorbents acts as the sensing element, with shifts in the Dirac point and resistance observed. Desorption of adsorbed species restores graphene to its normal state thus recovering the sensor. The sensitivity of graphene devices to both biomolecules and assorted gaseous species has been demonstrated within the ASIN group. Current research is focused on optimising the performance of such devices and extending applicability through functionalisation.
Figure 4 Top: Dirac point for graphene based device in air. Bottom: Dirac point shifts of graphene device on exposure to different concentrations of the biomolecule LB.
Carbon Nanotubes (CNTs)
An individual single-wall carbon nanotube (SWNT) can be considered as a graphene sheet rolled into a cylinder and capped at both ends by the introduction of pentagons (similar to half a buckyball). MWNTs are similar but consist of a series of concentric shells with an inter-shell spacing slightly higher than the interlayer spacing in graphite. SWNTs typically have diameters in the range 0.7 - 1.4 nm and lengths of ~ μm. The structure of CNTs engenders unique electrical, mechanical, thermal and optical properties. This in combination with their massive aspect ratio makes nanotubes potentially suitable for a wide range of applications. In some cases, this involves incorporating CNTs into existing processes. In others, exciting new approaches are envisaged which are only possible through the exploitation of nanotube properties.
The growth of carbon nanotubes by CVD is probably the most promising production technique, as it is scalable and allows for growth at predefined locations on substrates through various lithographic methods. Many different catalyst systems and growth recipes are possible depending on the type of nanotubes desired. The facilities in the ASIN lab allow for the production of a wide array of different types of CNTs. Both MWNTs and SWNTs can be grown in both thin films and forest geometries. The quality of these CNTs can be tuned through varying the growth parameters used.
Current CNT research in the ASIN group can be broken down into a number of different areas;
- Growth of MWNT forests on conducting substrates: MWNTs are grown on substrates consisting of a catalyst layer deposited on top of a conducting substrate. This leads to good electrical contact between the substrate and the tubes and is of high interest for usage in vertical interconnects (VIAS), supercapacitors (and other elctrochemical electrodes) and field emission devices.
- Growth of SWNT networks: Films and networks of ultra high purity SWNTs are produced in the ASIN group using a series of novel alloy catalysts. Control over purity and diameter distributions has been demonstrated. These networks have interesting electrical properties and potential applicability in field effect transistors (FETs), network transistors and gas sensors.
- Remote plasma assisted growth: Plasma Enhanced CVD (PECVD) has been used extensively to lower the growth temperature of CNTs. This however typically has a number of drawbacks with ion indced damage being prevalent. The use of remote (downstream) plasma is under invesigation in the ASIN group and the growth of high purity SWNT films as well as MWNT forests has been demonstrated using this technique.
Figure 5 Collage demonstrating different types of CNT growth in the ASIN lab. Examples include, aligned forests of SWNTs and MWNTs, patterned growth, SWNT networks, contacted arrays and growth from holes.
Pyrolytic Carbon (PyC)
Pyrolytic carbon (PyC) can be considered as disordered nanocrystalline graphite and belongs to the family of turbostratic carbons due to slipped or randomly oriented basal planes of crystallites.Pyrolytic carbon can be formed through gas phase dehydrogenation (or pyrolysis) of hydrocarbons and subsequent deposition on surfaces. This is non-catalysed and can be thought of as a pure CVD process. Gas phase deposition means that PyC can be deposited onto many different substrates in a conformal fashion.
Work in the ASIN group focuses on the optimisation of PyC growth. The thickness and roughness of films can be precisely controlled through growth parameter variation. The production of very smooth (Ra < 1 nm) and thin conducting layers (ρ ~ 2 x 10-5 Ωm) of this material has been investigated. Current research is focussed on the use of PyC films as electrochemical electrodes and the improvement of other materials properties through the deposition of thin PyC layers.
Figure 6 Top Left: SEM of PyC film on SiO2 substrate. Top Right: HRTEM of interface between PyC and SiO2. Bottom Left: PyC coating on SWNT film and anodic alumina oxide (AAO). Bottom Right: Different thicknesses of PyC films prepared on 300 nm SiO2 substrates with dwell times in minutes.
Si Nanowires
Silicon based devices form the basis of today's integrated circuitry. To extend such devices into the nano-regime new approaches are required. To this end, research is performed in the ASIN group on the fabrication and subsequent electrical characterisation of silicon nanowires. These Si nanowires have been produced by a number of different etching and oxidation steps in combination with templates made from block copolymers (BCP) or e-beam lithography and hydrogen silsesquioxane (HSQ).
Figure 7 Left: Si NWs produced using a BCP template. Right: Parallel SiNWs produced using a HSQ etch mask formed using e-beam lithography.
Surface Modification and Analysis
The properties of CVD Nanocarbon materials can be altered through the use of chemical or plasma based functionalisation. In the ASIN group particular emphasis has been placed on the use of remote plasma treatments to modify the surface of films. We have already demonstrated a flexible process for improving the electrochemical properties of PyC films through the use of an O2 plasma treatment. In this case XPS analysis proved to be a most powerful tool probing only the surface layers of the material and detecting subtle changes in the chemical makeup.
Currently, the use of different plasma treatments to both clean and fucntionalise CVD grown graphene films is under investigation.
Figure 8 Top: Voltammograms recorded for as grown (dashed) and O2 plasma treated (solid) PyC in 1mM Fe(CN)63-/4- in 1M KCl with a scan rate of 100 mVs-1. Bottom : XPS C1s spectra for PyC films pre and post O2 plasma treatment indicating the incorporation of oxygenated functionalities on the surface.