Monday, October 3, 2016, 2:00 pm

Hosted by: 'Percy Zahl'

Despite the evolution of scanning probe microscopy (SPM) into a powerful set of techniques that image surfaces and map their properties down to the atomic level, significant limitations in both imaging and mapping persist. Currently, typical SPM capabilities qualitatively record only one property at a time and at a fixed distance from the surface. Furthermore, the probing tip's apex is chemically and electronically undefined, complicating data interpretation. To overcome these limitations, we have started to integrate significant extensions to existing SPM approaches. First, we expanded noncontact atomic force microscopy with atomic resolution to three dimensions by adding the capability to quantify the tip-sample force fields near a surface with picometer and piconewton resolution [1, 2]. Next, we gained electronic information by recording the tunneling current simultaneously with the force interaction [3] and introduced a new operating scheme called tuned-oscillator atomic force microscopy that substantially improved imaging robustness and therefore sample throughput and user friendliness [4]. Finally, we will illustrate how the tip chemistry, tip asymmetry, and tip-sample distance influence the recorded interactions – and thus the information one can gain from images –, ultimately allowing to selectively image specific atomic species [3, 5]. During the talk, applications to various model systems including oxides, metals, ionic crystals, and layered materials will be presented. [1] B. J. Albers et al., Nature Nanotechnology 4, 307 (2009). [2] M. Z. Baykara et al., Advanced Materials 22, 2838 (2010). [3] M. Z. Baykara et al., Physical Review B 87, 155414 (2013). [4] O. E. Dagdeviren et al, Nanotechnology 27, 065703 (2016). [4] H. Mönig et al., ACS Nano 7, 10233 (2013). Host: Percy Zahl

Thursday, October 6, 2016, 11:00 am

Hosted by: ''Eric Stach''

Metal-semiconductor hybrid nanoparticles (NPs) have synergistic properties that have been exploited in photocatalysis, electrical, and optoelectronic applications. Rational design of hybrid NPs requires the knowledge of the underlying mechanisms of diffusion of the metal species through the nanoscale semiconductor lattice. One extensively studied process of diffusion of two materials across the nanoparticle surface is known as the nanoscale Kirkendall effect. There, an atomic species A with the lower diffusion rate enters the nanocrystal slower than the B species diffusing from the nanocrystal outward. As a result, voids are formed in B, providing an interesting avenue for making hollow nanocrystals. We used time-resolved X-ray absorption fine-structure spectroscopy, X-ray diffraction and electron microscopy to monitor the diffusion process of Au atoms through InAs nanocrystals in real time. In this system the diffusion rate of the inward diffusing species (Au) is faster than that of the outward diffusion species (InAs), which results in the formation of a crystalline metallic Au core surrounded by an amorphous, oxidized InAs shell with voids in it. These observations indicate that in hybrid Au-InAs NPs the rarely observed "reversed nanoscale Kirkendall effect" is in play. It presents a potentially new way to synthesize unique nanoscale core-shell structures.

Thursday, October 6, 2016, 4:00 pm

Hosted by: '''Mircea Cotlet'''

Monday, November 7, 2016, 4:00 pm

Hosted by: ''Mircea Cotlet''

Understanding the elementary steps in acid-base and metal catalyzed organic transformations is a key for sustainable chemical conversions. Solid acids and bases with nano-pores such as zeolites act as solid Brønsted and Lewis acids, widely used as catalysts with well-defined acid-base sites and a well-defined reaction space around the sites. Within the pores of molecular sieves reacting molecules are constrained in a reaction space, which can be subtly adjusted via direct synthesis, as well as via the addition of cations, oxidic clusters or organic fragments. The impact of such changes on mono- and bimolecular reactions such as elimination reactions of alcohols, cracking and alkylation of hydrocarbons are discussed for gas and liquid phase reactions. Experimental methods to define the state of the reacting molecules combined with detailed kinetic analysis and theory will be used to explain the principal contributions of the interactions and the confinement to determine reaction rates. It will be discussed how reaction rates and pathways can be tailored using the space available for a transition state and the chemical constituents around the active site.