Surfaces and Interfaces Village

It has long been known that many functional properties of a material are linked to its specific structure. These relationships may occur at different length scales, for example, some rely on the microstructure, such as how the size of polycrystalline grains in engineering materials can affect their strength. However, many properties are linked to the atomic structure of the material and this becomes even more important at the surface of a material or at an interface between components where the environment is different from the interior of the sample. A number of different phenomena can occur, including surface relaxation or reconstruction, where the atoms in the outer layers move in directions either perpendicular or parallel to the surface (or both). The delicate balance of energy at the surface or interface can lead to other significant changes such as surface alloying (from constituents that do not readily form bulk alloys), faceting or changes in the chemical state of the surface. Such changes at the surface have led to a wide range of real technological devices; for example, the enhancement of the magnetoresistance in thin films has led to the development of very sensitive read heads for magnetic recording, while improved catalysts have been developed by tailoring the size of particles on a specific support. Diamond has a number of techniques available to provide information about the structure of the surface or interface at the atomic scale or for nanostructures supported on a sample, where the high surface area to volume ratio can have a significant effect on the properties.

Imaging techniques such as Photoelectron Emission Microscopy (PEEM) provide a real space view of nanostructures with a resolution of typically 30 nm, for samples held in ultrahigh vacuum (UHV). PEEM uses a complex series of electron optics to form an image of the emitted photoelectrons, resulting from the impinging soft X-ray beam. The data collected provides a direct link between the structure and the spectroscopic information from the distribution of photoelectrons providing, for example, chemical sensitivity in the distribution of elements at a surface. The measurements available at beamline I06 allow additional versatility as the X-rays are provided by an Apple-II undulator which enables the polarisation to be selected (e.g. horizontal linear, vertical linear or circular). Contrast mechanisms, such as dichroism in the difference spectra, can be used to form images that show magnetic domains and their response to external stimulus. The PEEM on the Nanoscience beamline (I06) has been used to study the local magnetisation in nickel films on ferroelectric BaTiO¹ and combined spectroscopy and microscopy on ultrathin ceria films², amongst many others.

The village also has one facility under construction. B07, the Versatile Soft X-ray (VERSOX) beamline, will complement the range of experiments that can be undertaken by enabling spectroscopic measurements on samples held in non-vacuum environments. This will bring a deeper understanding to science areas such as catalysis, pharmaceuticals and biomaterials under realistic conditions or atmospheric chemistry. Construction work has started and the first users are expected by the end of the year. Concurrently, one of the end stations is being constructed and commissioned on the soft X-ray branch of I09 to enable a fast transfer to B07 when available.

The areas of science covered by the village have continued to grow; in particular there has been a significant increase in the number of studies on oxide samples, including complex transition metal oxides or actinide oxides. Transition metal dichalcogenide samples also remain much studied due to the interesting electronic effects that they exhibit, in particular when doped with other elements. The village has also welcomed users interested in areas as diverse as organic photovoltaics, phase changes in rechargeable battery materials, protein interaction at the lipid-water interface or in situ studies of molecular adsorption on single crystal samples. This year, the highlighted publications show a wide variety of techniques and materials with results all published in high quality journals. A brief summary of those reports is given below.

Beamline I05 has produced many high quality publications since becoming fully operational this year, including the highlighted work from Phil King's group in St. Andrews. They report on the layered semiconductor WSe₂, which demonstrates an interesting spin-orbit coupling within each layer. The overall symmetry of the system is maintained by a 180° rotation between the layers that is confirmed by the close agreement of the photon energy-dependent ARPES measurements with the theoretically calculated electronic structure. The authors discuss the results in the context of spintronic devices, where control of the electron spin could be used to create faster, or more energy efficient, devices.

I06 was the first operational beamline in the village and continues to provide a number of different techniques using the polarised soft X-ray sources, including time resolved pump-probe methods using the lasers available at the beamline. Foerst (Max Planck Institute for the Structure and Dynamics of Matter) et al have performed resonant soft X-ray diffraction experiments at I06 to study the link between superconductivity and charge order in the high temperature superconductor YBa₂Cu₃O_6.6. They show how the superconducting state can be enhanced at temperatures just higher than the superconducting critical temperature (T_c), by excitation using mid-infrared femtosecond pulsed light. This is explained in terms of the excitation of particular vibrational modes in the crystal lattice and melting of the charge stripe order. Together with data recorded at the Linac Coherent Light Source (LCLS) in California, details of the dynamics of this process are presented.

Beamline I07 is generally used to determine very accurate structural parameters about well-ordered reconstructions or layered surfaces and interfaces using a number of grazing incidence hard X-ray scattering measurements. Ian Robinson's (UCL) group have now added Bragg Coherent Diffraction Imaging to this list by exploiting the coherent properties of the Diamond source, to image nanoparticles of ~300 nm diameter. They were able to link the technique and the growth capabilities of the ultrahigh vacuum chamber at I07 to study in situ alloying by depositing copper on top of gold nanoparticles, together with subsequent annealing. They show how the copper alloys with the gold by insertion through channels in the nanoparticles. In particular, phase loops were observed indicating the high degree of strain that occurred at the corners and edges of the faceted gold particles.

The SISA beamline (I09) is unique at Diamond in that it enables experiments using both soft X-rays (100eV-2.1 keV) and hard X-rays (2.1-20 keV) to be used on samples held in the same environment and at the same position. This enables combined studies of the atomic structure (using for example X-ray standing waves or photoelectron diffraction) and the electronic structure (e.g. soft or hard X-ray photoelectron spectroscopy). The highlight shows how Alexander Gerlach's (Universität Tübingen) team have used I09 to study the adsorption of large molecules (in this case porphyrin) onto single crystal metal surfaces. Using X-ray standing waves, they were able to establish the adsorption height of the carbon and nitrogen constituents in the adsorbed molecule, showing a slight distortion of the non-metal containing porphyrin. The process of self-metalation, where modest annealing causes incorporation of a metal atom from the substrate into the adsorbate, was investigated showing how the geometry develops by flattening the molecule.

Science related to spintronics is also highlighted in the article from Adriana Figuera-Garcia (Diamond) and co-workers, who have used X-ray magnetic circular dichroism (XMCD) at beamline I10 together with X-ray absorption spectroscopy at beamline B18 to measure the electronic and magnetic structure due to the presence of a dopant (chromium) in a thin film topological insulator (Bi₂Se₃). The dopant is found to substitutionally replace Bi atoms in the structure, causing a slight distortion in the surrounding lattice. This leads to an unexpected valence in the Cr, which is explained by the covalent nature of the Cr-Se bond. The XMCD measurements highlight differences between the electronic and magnetic structures at the surface of the film and in the bulk.

The Surfaces and Interfaces village continually develops its range of techniques and measurement facilities in consultation with the users. Please get in touch if you would like to discuss any future developments that you would like to see on the beamlines or in any of the extensive offline facilities that are available to users. If you have any questions regarding possible measurements at Diamond or would like advice on how the Surfaces and Interfaces village resources may help with your science programme then please contact chris.nicklin@diamond.ac.uk