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We utilise chemical vapour deposition (CVD) processes to synthesise carbon nanomaterials and nanostructures with high precision and accuracy using dedicated CVD furnaces. The lab currently hosts a range of furnaces for the production of graphene, transition metal dichalcogenides (TMDs) and other nanomaterials. These include two dedicated graphene furnaces, one of which is wafer scale, as well as custom-built, two-zone sulfurisation and selenisation furnaces for the growth of different TMDs. At CRANN and AMBER, we employ state-of-the-art microscopic and spectroscopic techniques to characterise our nanostructures, these include HRSEM, HRTEM and He-Ion microscopy as well as Raman, UV-Vis, photoluminescence and X-ray photoelectron spectroscopy. We also study the electrical and optical properties of these novel nanostructures using various probe station setups.
We combine top-down structuring techniques with the in-situ synthesis of novel functional nanostructures. The creation of such hybrid structures is aimed at the fabrication of new adaptive devices based on the bottom-up growth of nanomaterials with unique functionality. Using this hybrid approach the precision and scaling capabilities of silicon structuring technology will be maintained. We also utilise 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 nanostructures but also inorganic nanowires and thin films. The following sections will outline the different materials synthesised within our group and our approaches to implementing these novel materials in adaptive devices.
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.
Initial 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 CVD growth of graphene on 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 doping 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 has also been investigated.
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.
TMDs can be described as hexagonal layers of metal atoms between two layers of chalcogen atoms, with an MX2 stoichiometry (M=transition metal, X=chalcogen, eg S, Se, Te). The materials' properties can vary from metallic to insulating.
Although these materials have been studied for decades, progress in materials characterisation and device fabrication at the nanoscale have opened up new opportunities for two-dimensional layers of thin TMDs in nanoelectronics and optoelectronics. TMDs such as MoS2, MoSe2, WS2 and WSe2 have sizeable bandgaps that change from indirect to direct in single layers, allowing for potential applications as transistors, photodetectors and electroluminescent devices. In addition, superconductivity and charge-density wave effects have been observed in some TMDs. This versatility makes them potentially useful in many areas of electronics.
Mechanical exfoliation is the most reliable method to produce high-quality flakes of these materials, but suffers from low yield and non-scalable production rates. We are currently working on optimising the growth of these materials via CVD and high temperature sulfurisation. We also characterise the materials extensively and examine their suitability for use in a range of applications, including integrated devices for ICT, photodetectors, gas sensing, and catalysis.
Figure 3 Top left: Schematic of TMD crystal structure. Right: Experimental setup for vapour phase sulfurisation. Bottom left: Different MoS2 film thicknesses on fused quartz
Figure 4 (Clockwise from top left): (1) Schematic of experimental setup for chemical vapor deposition. (2) Schematic of microreactor growth. (3) Diffraction pattern of highly crystalline CVD MoS2 monolayers. (4) HRTEM image of MoS2 monolayers with hexagonal lattice clearly evident. (5) Optical image of MoS2 monolayers grown in their distinctive triangular shape.
Raman spectroscopy is a widely used technique in materials science and can be used to study molecular vibrations in 2D materials. This can reveal a wealth of information on material properties in a fast and non-destructive manner. In the case of graphene, and other 2D materials, Raman spectroscopy can be used to investigate the number and relative orientation of individual atomic layers, and can provide information on defect levels, strain and doping. Raman spectroscopy is used within the group for characterisation and assessment of material properties, as well as fundamental studies on the crystal properties. Raman measurements are performed on a group tool, a Witec alpha 300R with 532 nm and 633 nm excitation laser capabilities. The Witec alpha 300R has also been fitted with a Rayshield Coupler to detect low frequency Raman lines close to the Rayleigh line at 0 cm-1.
Figure 5 (a) Optical image of MoSe2 crystals synthesised by chemical vapor deposition. (b) Map of max A’1/A1g (~ 239 cm-1) high wavenumber Raman mode (c) Map of max E’/Eg (~ 287 cm-1) Raman mode (d) High-frequency Raman spectra of 1, 2, 3, 4, and 5+ L MoSe2 (e) Witec alpha 300R scanning Raman microscope with additional Rayshield Coupler fitted (f) Schematic of Raman active vibration modes in MoSe2 with labels and positions below (g) Position map of maxima of low-frequency Raman modes in MoSe2, allowing fast identification of layer number (h) Low-frequency Raman spectra of shear and layer-breathing modes of 1-5+L MoSe2
Figure 5 (a) Optical image of exfoliated graphene flake showing regions of a different number of layers (b) Raman spectra corresponding to each region, 2D FWHM values included for each (c) Intensity profile maps for G, 2D and D bands and a 2D FWHM map for a fitted Lorentzian peak.
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 flakes and films of 2D materials, including graphene, carbon nanotubes, TMDs, and black phosphorous. Selective functionalisation of such contacted regions can also be performed through the use of selective spotting.
Figure 6 (a) Contacted graphene flake (b) Patterned graphene ribbons (w = 4 μm) contacted with Ni contacts (c) Selective "spotting" of active region on graphene based device (d) Optical images of a Au nanoparticle decorated transparent and flexible SWCNT network film before (inset) and after electrical contacts deposition, for use as gas sensors (e) Optical image, sensing behaviour, and schematic of high performance sensors based on MoS2 grown by vapour phase sulfurisation (f) Sensing behaviour and optical images of WS2 thin film gas sensors manufactured by low temperature plasma assisted synthesis of WO3 films.
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 7 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.
ALD is a thin film production technique used in materials science. It utilises gaseous precursors reacting in sequence to deposit films on a surface in a controllable method. Within the ASIN group, ALD is used to deposit high-k dielectrics on 2D materials for device applications, for patterning graphene ribbons and also to study fundamental properties of 2D material/high-k dielectric composites.
Figure 8 (A) Schematic of pre-growth patterning of Cu for graphene devices: (i) pattern definition on Cu using conventional UV lithography; (ii) Al2O3 deposition via ALD; (iii) photoresist removal; (iv) graphene growth; (v) transfer to SiO2/Si. (B) Optical micrograph of patterned graphene stripes used as parallel FET channels. The metal electrodes are visible at the top and the bottom. (C) SEM micrograph of an individual channel. The width of the graphene channel is 10 μm. The dark spots indicate graphene multilayers.
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 9 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.
Figure 10 Schematic of fabrication process for MoS2 based HER electrodes on PyC films.
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 11 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.