Surface Microscopy and Spectroscopy Group

STM is an extremely surface sensitive technique and can be used to visualize surfaces and electronic density of states at the atomic scale. The image above is a STM topography image of the 7x7 reconstructed surface of Si(111). The STM was invented in the early 1980’s by Gerd Binnig and Heinrich Rohrer [Angew. Chemie Int. Ed. English 1987, 26, 606–614; Helv. Phys. Act. 1982, 55, 726–735]. They set out to develop an instrument with the ability to measure electronic properties at atomic resolution. The need for this particular type of instrument originated from a surface science problem relating to the details behind the surface reconstruction of Si(111). They showed that the reconstructed surface of the Si(111) resulted in a 7x7 patterning as in the STM image above, further details can be seen in [Phys. Rev. Lett. 1983, 50, 120–123]. Later in 1986 Binnig and Rohrer received the Nobel prize in Physics for their work on Si(111) 7x7 and the STM [Angew. Chemie Int. Ed. English 1987, 26, 606–6141.] STM/S can be used to measure the local density of states and thus provide a view of the real space electronic band structure. The STM/S is the main tool utilized by members of the SMSG for exploring the nanoscale topography and electronic density of states.

Cross-sectional -STM (XSTM):

XSTM is used to expose fresh uncontaminated surfaces across interfaces of heterostructures such as thin-film/substrate interfaces. The sample is put into the ultra-high vacuum (UHV) chamber for in-situ fracturing. The fracture occurs, as shown in the figure above, with an initial dice mark serving as the starting point for a controlled fracture or crack. The fracturing process is performed by holding the bottom half of the sample with clamps, while pushing the top portion, above the dice, against a rigid metal cleaver. This results in the top portion breaking away from the bottom portion. The freshly exposed top layer surfaces on the bottom portion of the fractured sample are used for STM measurements. Shown in the figure below is an XSTM image of a hybrid halide perovskite [ACS Appl. Materials and Interfaces 8, 29110-29116 (2016)], the density of states mapping (red-blue) is overlaid on the topography (orange-red-yellow) data. The SMSG is skilled at XSTM/S and has investigated the evolution of the density of states across many heterostructure interfaces.

ARPES is a surface sensitive technique which can be used to explore the occupied band structure of molecular crystals, ordered molecular adlayers, and highly ordered materials. The band structure can be defined as the energy dependence of the electron states as a function of the wave vector [Handbook of Thin Films Vol 2, pp. 61-114]. The measured band structure is resolved in reciprocal space, i.e. momentum space, and as such is useful for investigating the wave-like nature of the electron. Electronic band dispersion in the case of ARPES refers to the relationship between the kinetic energy of the emitted electron and the wave vector k. The figure above shows the band dispersion for a 2D material along different high symmetry directions in momentum space [Appl. Phys. Lett. 112(5), 052102 (2018)]. The band dispersion in the figure above indicates anisotropy is present in the valence band. The kinetic energy is directly related to the binding energy so the band dispersion can be viewed as a variation of the molecular binding energy as a function of wave vector. The SMSG travels to user facilities around the world to use synchrotron facilities equipped with ARPES and nanoARPES in order to measure the band structure of various materials.