Chemical vapour deposition is a scalable, industry-compatible technique for the growth of thin films. My initial experience with this technique was for the growth of carbon nanotube (CNT) forests and networks during my PhD studies under the guidance of Prof. Werner Blau. During my time in Prof. Georg Duesberg's group I have used CVD methodologies to grow an array of 2D materials including graphene and assorted transiton metal dichalcogenides (TMDs), such as MoS2, WS2, MoSe2 and WSe2. These materials show promise for a wide range of applications in electronics, optoelectronics and catalysis. Additionally, CVD methods have been used for wafer-scale growth of pyrolytic carbon (PyC), which has been used for the production of electrochemical electrodes and Schottky barrier diodes.
Figure 1 (a) General schematic for TMD synthesis by CVD. (b) Large-area MoS2 (primarily monolayer) grown by CVD on a SiO2/Si subnstrate. (c) MoSe2 crystals of various thicknesses grown by CVD on a quartz substrate. (d) MoS2 films of different thicknesses grown by CVD. (e) PyC films of different thicknesses grown by CVD, numbers indicate dwell time in minutes. (f) CVD-grown multilayer graphene transferred onto PET. (g) A forest of multiwalled CNTs grown by CVD.
Raman spectroscopy is a powerful and non-destructive characterisation technique, based on the inelastic scattering of light (the Raman effect, named after Sir C. V. Raman), which is ideally suited to the characterisation on nanomaterials. I first started using Raman spectroscopy back in 2005 to characterise CVD-grown CNTs where it provided information on material quality/purity, as well as tube type, diameter and chirality. I've subsequently used Raman spectroscopy for the characterisation of CVD-grown 2D materials. In the case of graphene and 2D TMDs, Raman analysis can give information on layer number and defect levels as well as doping and strain. Through Raman mapping this information can be acquired over large areas and visualised in the form of maps (of peak intensity, position, width etc.), allowing localised features to be identified and the uniformity of samples to be assessed. Additionally, I have used Raman spectroscopy to characterise assorted liquid-phase exfoliated 2D materials including graphene, assorted TMDs, MoO3 and black phosphorus.
Figure 2 (a) Optical image of graphene crystal grown by CVD. (b) Spectra extracted from regions marked in (a). (c) D band, (d) G band and (e) 2D band intensity Raman maps of the area outlined in (a).
Figure 3 (a) MoSe2 crystals grown by CVD. (b) Raman map of A1g mode intensity. (c) Spectra for different regions marked in (a) corresponding to flakes of different thickness. (d) Raman spectra for different CVD-grown TMD films.
Niall talks to SFI about his current research