AVS2013 Session IS+AS+SP-FrM: Evolving In Situ Microscopic and Spectroscopic Techniques and Applications

Cryogenic sample environments allow investigations in the X-ray ‘water window’ of 284 – 540 eV which is of tremendous interest for microscopy and coherent scattering due to the high contrast of organic matter with respect to the aqueous background. In addition, no fixatives and stains are required and thus real in-situ analysis becomes possible. We present coherent diffractive imaging and X-ray micrographs of bacteria and compare the data against small angle X-ray scattering on large ensembles. Most measurements have been conducted at the synchrotron BESSY II, U49 PGM2 with our dedicated scattering chamber HORST. As example for the approach, ultrastructure changes in bacteria induced by stress conditions such as different environmental conditions and biocides. We show that imaging and scattering provide complementary results as latter provides information averaged over thousands of cells and can thus provide insights of a very high statistical quality.

We present a newly designed electrochemical Surface Forces Apparatus (EC-SFA) that allow control of surface potential and interfacial electrochemical reactions with simultaneous measurements distances between apposing surfaces (with 0.1 nm resolution), normal interaction forces (with nN resolution), and friction forces (with μN resolution). We will describe three applications of the EC-SFA:

(1) Oxide growth of a gold surface. Applying a high positive electrical potential on a metal surface will lead to an oxide growth. Using the EC-SFA we compared the measured thickness of the anodic gold oxide layer and the charge consumed for generating this layer which allowed the identification of its chemical structure as a hydrated Au(OH)_3­ phase formed at the gold surface at high positive potentials.

(2) The evolution of the friction forces at a metal-ceramic contact as a function of the applied electrochemical potential.

(3) The phenomenon of “pressure solution”. We have found an intimate relationship between dissolution rate and the surface potential difference across interfaces of dissimilar materials that are immersed in brine solution. For example, using the EC-SFA we have visualized and measured the dissolution of silica glass surfaces close to a gold electrode surface, which is on the order of 0.1 nm/hr. This is similar to geological samples of sandstones.

References:

1. Valtiner M, Banquy X, Kristiansen K, Greene GW, and Israelachvili JN, Langmuir, 28 (2012) 13080-13093.
2. Kristiansen K, Valtiner M, Greene GW, Boles JR, and Israelachvili JN, Geochimica et Cosmochimica Acta, 75 (2011) 6882-6892.
3. Valtiner M, Kristiansen K, Greene GW, and Israelachvili JN, Advanced Materials, 23 (2011) 2294-2299.

Advanced technology arises from the understanding in basic science, and both rest in large on in-situ/operando characterization tools for observing related physical and chemical processes directly at the places where and while reactions occur. In energy science, experimental insight into physical and chemical processes has been largely limited to information obtain with in a framework of thermodynamic and kinetic concepts or atomic and nanoscale. In many important energy systems such as energy conversion, energy storage and catalysis, advanced materials and fundamental phenomena play crucial roles in device performance and functionality due to the complexity of material architecture, chemistry and interactions among constituents within. To understand and ultimately control the interfaces in energy conversion and energy storage application calls for in-situ/operando characterization tools. Soft x-ray spectroscopy may offer some unique features. This presentation reports the development of in-situ reaction cells for soft x-ray spectroscopic towards the studies of photosynthesis and catalytic reactions in recent years. The challenge has been that soft x-rays cannot easily peek into a high-pressure catalytic cell or a liquid photoelectrochemical cell (PEC). The unique design of the in-situ cell has overcome the burden. Some of the instrumentation design and fabrication principle are to be presented, and a number of experimental studies of nanocatalysts are given as the examples, also the recent experiment performed for studying the hole generation in a specifically designed photoelectrochemical cell under operando conditions.

Catalytic chemical vapor deposition (C-CVD), using a transition metal catalyst (Ni, Fe, Co, etc.) on an SiO₂, Al₂O₃, or MgO support and a carbon-containing precursor (C₂H₂, C₂H₄, CH₄, CO, etc.), is commonly employed for large-scale synthesis of carbon nanotubes (CNTs). However, synthesis of CNTs with the desired structure and morphology for a specific application has still not been demonstrated. Understanding the atomic-scale interplay between catalyst structure and CNT nucleation will aid us in determining the reaction conditions suitable for selective synthesis, especially for single walled CNTs (SWCNTs). During the last decade, the environmental scanning transmission electron microscope (ESTEM) has been successfully employed to reveal the structural, chemical and morphological changes occurring in catalyst nanoparticles during CNT growth. However, the mechanisms of CNT cap formation are yet to be revealed under normal growth conditions: the SWCNT nucleation and growth process is too fast to be captured at currently available video frame rates (30 s^-1). We have successfully addressed this problem by slowing the kinetics of the process using a Co-Mo/MgO catalyst system and low pressures of acetylene (C₂H₂) and ethanol (C₂H₅OH) as carbon precursors. Our direct observations show that the CNT cap preferentially nucleates on certain surfaces and first finds two surfaces as suitable anchor points before lift-off. The detailed interplay of catalyst surface structure, cap formation, incubation period, and lift-off will be presented using atomic-resolution videos recorded under these novel CVD conditions. Our observations provide direct insight into the mechanisms of SWCNT growth and open up possibilities for diameter and chirality control.

In situ imaging, using an environmental scanning transmission electron microscope (ESTEM), has been successfully used to reveal and better understand the crucial chemical and physical processes occurring at the nanoscale, e.g. oxidation/reduction, coalescence, Ostwald ripening, surface reconstruction, substrate/catalyst interaction… However, the relevance of such ETEM studies can be diminished if the following two questions cannot be satisfactorily answered: i) Do high energy electrons affect the reaction mechanism? In other words: is the probed area representative of what is happening on the whole sample? ii) What is the sample temperature in the gaseous environment? Here, we present unique instrumentation that helps to solve these two issues by collecting Raman data during ETEM observation. We can now combine and compare the structural information and kinetics obtained from large (micrometer-scale) areas by the Raman spectrometer with the local information collected by the ETEM at the nanoscale. This system also enables us to simultaneously monitor the actual temperature of the probed material by analyzing shifts in Raman peak frequency. Moreover, this versatile optical setup can be used i) to investigate light/matter interactions (the current 532 nm laser can be easily replaced by any IR/Vis/UV wavelength) ii) as a heating source: the sample can be heated up to 1000°C at 15 mW with a 532 nm laser, iii) for general spectroscopy (absorption, photoluminescence, cathodoluminescence…).

This combined approach is made possible by the insertion of a parabolic mirror in between the sample holder and the lower pole piece of the microscope (Fig1. in Supp Info). It focuses a laser on the sample and collects the scattered Raman photons. A set of optics then carries the Raman signal to the spectrometer.

Micro-Raman systems have become very popular tools for analysing novel materials in a wide variety of application areas. Their ease of use has driven them to be employed routinely with carbon materials (graphene, carbon nanotubes, diamond, diamond-like-carbon (DLC) ); semiconductors (silicon, germanium, III-V and II-VI materials); photovoltaics (silicon, CIGS, CdTe, CZTS); Chalcogenides (MoS₂, Bi₂Te₃, Bi₂Se₃); as well as oxides, nitrides and carbides. This technology enables researches to probe the molecular structure for phase identification, stress and crystal quality. It can also be used for reaction and catalysis monitoring.

New demands are now being made that require the use of in-situ techniques with very high sensitivity. The ability to conduct measurements inside the vacuum chamber has been enabled with highly efficient optical probes and spectrometer technology to give the required performance in these demanding applications. This talk will cover the basic concepts of Raman spectroscopy, the instrumentation required for these measurements, and highlight some of the applications where in-situ Raman and photoluminescence can increase the understanding of vacuum processes and how they can be optimised to improve quality and yield.