WP2: In situ Raman and supportive Raman studies

  1. Ex situ studies of 2DM growth on LMCat using the Raman spectrometer and CVD growth system in UP
  2. Design and assembly of the in situ Raman spectroscopy module
  3. Preliminary tests of the in situ Raman module in Leiden
  4. In situ study of the catalytic properties of LMCat and kinetics of 2DM formation using the Raman module
  5. Performing simultaneous Raman spectroscopy and x-ray diffraction measurements during 2DM formation

To study the catalytic properties of LMCats and the formation of 2DMs, the availability of a chemically sensitive technique capable of detecting molecules and chemical bonds is of crucial importance. Among the spectroscopic techniques, Raman spectroscopy, that is used to observe vibrational and rotational modes of a system, has been used extensively for studying nanomaterials in general and graphene (on solid substrates) in particular. This technique can provide information about the chemical structure on the surface with a spatial resolution in the order of a few hundreds of nanometers. However, when performing in situ Raman spectroscopy at high temperatures (T > 1400 K) the black body radiation has a serious influence on the Raman intensity. Heating and blackbody radiation may be by itself so efficient that is impossible to observe Raman spectra. A hot object emits electromagnetic radiation, and an appreciable proportion of this radiation is in the visible region of the spectrum at high temperatures. However, in situ Raman measurements can be achieved by a) spatial filtering provided by confocal optics to block the background thermal radiation of surface regions surrounding the surface spot, onto which the laser is shone, b) using laser sources with higher frequencies (close to the UV region of the optical spectrum) to shift the Raman-scattered light further away from the thermal radiation spectrum, c) utilizing the anti-Stokes scattering signal instead of the standard Stokes signals, as they lay at frequencies further away from the thermal spectrum, and their intensity becomes stronger at higher temperatures due to the higher occupancy of the 2DM vibrational states, and d) utilizing multi-photon approaches which can increase the efficiency and hence the intensity of the Raman signal significantly. Coherent anti-Stokes Raman spectroscopy has already been used extensively for flame and combustion studies. This can provide the opportunity of using lower input power (preventing beam damage on 2DMs), overcoming detector noise, and increasing the imaging speed for known substances. By implementing some of the above-mentioned procedures, Raman experiments have been performed successfully on samples with temperatures up to 2200 K, and on graphene on SiO2 substrates up to 1450 K.

The most important application of the Raman technique in this project is to detect the formation of 2DMs and to quantify their growth kinetics on LMCat surfaces. For that, the Raman probe should be able to: (a) recording G and 2D Stokes Raman bands and anti-Stokes Raman (at least the G Raman band) in a short acquisition time, (b) spatial mapping for characterizing graphene crystals during growth, (c) spectrum collection/acquisition times adapted to the time scale of the growth process, and (d) diversity of excitation lines (325 nm, 532 nm, and 632 or 785 nm). These options are commercially available (see e.g. Streamline concept developed by Renishaw). Moreover, with Raman spectroscopy we would be able to detect the traces of the precursor adsorbates, intermediate reaction species, and final 2DM products on the LMCat surface.. This includes the investigation of the graphene internal structure and its defects such as: a) number of graphene layers, b) density of vacancy defects, c) whether compounds of graphene with e.g. O2 and/or H2 are forming. In the microscopy mode, this technique would provide us with information about the shape evolution of the 2DM domains and the growth speed of domain boundaries. In situ Raman spectroscopy in combination with isotope signalling can provide deeper insight into graphene nucleation and growth mechanisms. These data are of central importance in this project, as they bridge the macroscopic surface-averaged data of x-ray scattering experiments on one side, and the atomic-scale and micro-kinetic models explored in the theoretical simulations on the other side.