My name is Alex Marsden and I started a PhD in Physics at the University of Warwick in October 2011. The project was on graphene, a relatively new material only one atom thick that has huge potential for future technologies.
I joined the Microscopy Group at Warwick with the aim of using the range of different microscopy techniques available. However, to first make the graphene I wanted to study, I had to learn a lot in areas outside of microscopy, especially chemistry and surface science, where growing thin films is well established. Already my training had started to cross traditional boundaries.
Alex is flanked on the left by a scanning photoemission microscopy (SPEM) image of graphene supported on a metal grid normally used for TEM, and on the right by a scanning tunnelling microscopy (STM) image of graphene on copper, obtained in Chemistry here at Warwick.
Microscopy answered some of the questions we had, but it quickly became apparent that there were other techniques that could help. Surface science-based techniques such as LEED, XPS and ARPES helped examine graphene on length scales that are not accessible in microscopy and to get information beyond what microscopy can provide. In particular, they helped look at the interaction between the graphene and the metals we were growing it on.
As I started to use the surface science techniques, more questions began to arise about what was actually happening to the graphene. Fortunately, I had my training in microscopy, so I could then go back and study the same materials using these techniques, and better understand the results. This collaboration is helping us to understand how individual atoms attached to graphene affect its properties. We have obtained facility time at two leading syncrotron radiation facilities, Elettra in Italy, and Soleil in France, to do nano-spot ARPES. So I have also gained valuable experience in synchrotron radiation science as part of a PhD originally focused on electron and scanning probe microscopy.
Overall, the most robust research is done when the problem is approached using the fullest range of techniques possible. If you can perform all of these experiments yourself, you have direct access to the solution, quickly.
I joined the surface science group in Sept 2013 to begin my experimental PhD. My area of research is half metallic ferromagnets, materials which can be used for spintronics, and I have been using molecular beam epitaxy (MBE) to grow single-crystal ferromagnetic MnSb thin films on top of GaAs. Choosing to study at Warwick was a great decision for me, as I have access to many techniques to study the samples that I grow. Some sample characteristics are impossible to discover in the lab and I am fortunate to be able to attend international facilities where this data can be taken.
Steph at the ILL in Grenoble - one nice aspect of working at ESRF or ILL in Grenoble is the great food, such as cassoulet!
At the beginning of my Ph.D. I firstly had to learn how to use the MBE, and special cleaning techniques required to prepare the GaAs substrates for growth. During deposition of MnSb it is possible to use reflective high energy electron diffraction (RHEED) to gather information about the growth quality of the material by diffracting electrons from the surface. This can also give atomic layer sensitivity. Once grown, it is important to find the properties of the sample and I can perform these experiments by using the facilities available at Warwick.
SEM and AFM can be used to view small sections of the sample surface to look for roughness or characteristics of the growth method. It is important to grow good quality films with smooth surfaces as the diffraction experiments are sensitive to this. X-ray diffraction (XRD) shows how the lattice parameters vary within the sample. We aim to grow perfect crystals, but strain due to the mismatch of the substrate can cause differences in the crystal structure and this can be seen in the data obtained from XRD. Magnetometry has been used to measure the magnetic behaviour of the thin films. X-ray reflectivity (XRR) can also be measured at Warwick, and gives the layer by layer structure of the thin film including thicknesses and roughness. Polarised neutron reflectivity (PNR) is a technique available at international facilities such as ISIS, UK and ILL, France. PNR gives structural and magnetic layer information about the thin film. Properties from previously mentioned experiments are used as a preliminary fit to the neutron data and the magnetic profile is then adjusted, giving accurate layer by layer details.
The combination of all these techniques can provide full characterisation of my materials and I have been able to use all of these them independently after receiving the training required. This is important to me as I am free to do the research I need to complete my Ph.D.
I first learned about graphene during my MSc by Research with the Surface, Interface & Thin Film Group in 2010. The project lasted a year but the skills I developed would later prove to be vital for my Materials Physics Doctorate based in the Superconductivity & Magnetism Group which I started in 2012.
Mo working at the ISIS neutron facility in Oxfordshire, and an example SEM image of a nanowire topological insulator.
During my MSc I worked closely with the Microscopy Group, and this two pronged approach utilising surface science techniques alongside microscopy ensured we had the right tools to grow and characterise graphene. At the time, this new material was making headlines all over the world as a new "wonder material". We were able to grow high quality graphene using CVD growth methods and then explore the growth behaviour from ambient pressures right down to ultra-high vacuum. What’s great about learning to use various scientific techniques is the speed at which the research can progress. We were able to quickly find out more about our materials without having to wait for somebody else to perform experiments.
By the end of my MSc, I had learned how to obtain and interpret XPS, LEED, Hall, SEM, EDX, EBSD and AFM data. This ensured that I hit the ground running for my PhD project, which is on topological insulators and making nanomaterials of these. These materials also grew using CVD methods, and understanding the growth of these new materials with the tools I had learned during my MSc made the challenges easier. My PhD work has opened up another array of scientific techniques to explore our materials. I have learned how to use SQUID magnetometry, muon spin rotation, electrical transport measurements, Laue diffraction and XRD, which when brought together ensures the most efficient and effective way to do research.