SCANNING TUNNELING MICROSCOPY TO STUDYCATALYSIS AND GROWTH PROCESSES Flashcards
What is the principles behind STM? How is the tunneling current correlated to the distance between sample and tip?
Based on quantum mechanical tunneling, which is an effect that allows particles to tunnel through a classical forbidden region (energy barrier). STM requires the surface to be conducting.
Bias between tip and sample will make electrons tunnel through if the distance is not too large (5-10 Å). Current is inverse exponentially proportional to distance (I = -Vexp(-kd).
What are the main hardware components in the STM and which three main modes can a STM be operated in? What gives contrast when the STM is operated in “constant current mode”?
Atomically sharp tip, piezo electrin mover, datasysytem, vibration canceller.
Modes: Constant tunneling current mode, Constant height mode, Scanning Tunneling Spectroscopy (SPS).
Density of states and topography gives contrast.
Explain in brief main challenges when constructing a high temperature variable STM machine. Explain in brief how this method is used to study nucleation and growth of graphene on Rh(111), and discuss the optimal synthesis approach. Justify for your choses (note – at the exam –you must expect to be familiar to this study to a similar extent as we discussed in the lectures).
Drift in images due to change in thermal expansion/contraction (slide 12). Ideally all such effects could be zeroed out. In addition –piezoelectric motor cannot be heated above it transition temperature.
Synthesis:
- Feed carbon source C2H4 at low temp and pressure, and then anneal at 975K to form small particles of the most stable form (graphene).
- Complete growth at 975K at reactant limiting conditions.
- Avoid segregation of dissolved carbon to form graphene upon cooling.
What is a Moire pattern? Explain.
Two different patterns create a third pattern when superimposed. Of relevance for STM and when growing a film on a substrate.
What do you understand with the term “operando”? In this course we have discussed two ways to operate the STM in an operando mode. Explain briefly. What do we gain by having the possibility to perform such experiments?
Operando – performed the analysis of the material when it is performing the chemical reaction (or perform by other means) at the same time as you analyze products formed or other properties in action. Such experiments allow us to investigate the nature of the surface when in operation. Often very different from UHV; and not the same as before and after chemical reaction.
Explain in brief main challenges when constructing a high pressure STM suited for studying chemical reactions. Do you have an example from the lecture notes on how reactor STM can give new insight into the interaction of reactants and the catalytic active surface (hint –Co for Fischer-Tropschsynthesis? Explain.
Chemical resistance in the STM machine. Above 1 bar, need a specific cell inside the vacuum chamber so the windows in the STM machine will break.
Operando STM to study accumulation of long hydrocarbons
