C. Wang, W-C Yang, R. Sharma
National Institute of Standards and Technology,
Keywords: environmental transmission electron microscopy, Raman, cathodoluminescence, EELS
Summary:Environmental transmission electron microscopes (ETEM) and TEM holders with windowed reaction cells have allowed atomic-resolution in situ studies of gas-solid and/or liquid-solid interactions. [1-3] However, answers to many important questions related to these reaction processes remain elusive due to the complexity of the reaction kinetics, as well as the limited sampling capability of the TEM technique. The complexity arises from the dynamic interplay between the nanoparticle and the reactive environment, different aspects of which may require different characterization techniques to analyze. Sometimes measurements at different scales are also required. It is therefore important for the instrumental capability to combine an ensemble of techniques in one single experiment where the reactive environmental conditions remain unchanged, such that the results from the different techniques are comparable. In order to compensate for the lack of global information available from the limited field of view in TEM measurements, we have designed and built a unique platform that incorporates an optical system within an ETEM, which allows us to perform Raman spectroscopy on larger areas and also enables the collection of Cathodoluminescence (CL) signals, in addition to the imaging, electron spectroscopy and diffraction analysis capabilities of the microscope. [4, 5] This home-built hybrid transmission electron microscope combines optical and TEM measurement techniques in one instrument, which allows us to concurrently measure atomic-scale and micro-scale changes occurring in samples subjected to identical reactive environmental conditions. We have used this correlative microscopy platform to i) perform optical irradiation experiments, ii) measure the temperature of the sample from a ≈100 µm2 area using Raman shifts, iii) investigate localized surface plasmon resonance (LSPR) promoted chemical reactions, iv) perform concurrent optical and electron spectroscopy combining CL, electron energy-loss spectroscopy (EELS) and Raman. In these experiments, the crystallographic and chemical change of the sample is monitored using atomic-resolution movies and time-resolved EELS, while the temperature and vibrational information is acquired utilizing Raman spectroscopy. In light of the complexity of the reaction dynamics, we have developed an automatic image processing scheme to measure atomic positions from high-resolution images, within 0.015 nm uncertainty, to follow dynamic structural changes using a combination of algorithms publicly available and developed at NIST. This method has been proven to capture the crystal structure fluctuations within a catalyst nanoparticle during the growth of a single-walled carbon nanotube (SWCNT). This combined approach, made available by the hybrid transmission electron microscope, enables us to decipher various aspects of heterogeneous reactions, and provides time-resolved, multi-scale information on the reaction kinetics for an improved understanding of the reaction dynamics. Details of the design, function, and capabilities of the optical spectrum collection platform and image processing scheme will be illustrated with results obtained during in situ measurements.