Scanning Tunneling Microscopy to study Catalysis 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 tunneling, surface must be conducting electrons. Bias (V) between sample and tip –> wave functions of atoms on tip and sample overlap –> tunneling
(d between 5-10 Å)
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 thin tip
- Scanner to raster sample surface (piezoelectric ceramics/motor)
- System for coarse position of tip
- Vibration isolation from external vibrations
- Feedback electronics (for control of tip-sample distance)
1) Constant tunneling current mode: x-y varied, I, V kept constant, z measured (d) as a function of tip position (DOS must be assumed constant over sample)
2) Constant height mode: x-y varied, z(d),V kept constant, I measured as function of tip position (high speed, req. flat surface)
3) Scanning tunneling spectroscopy mode: summing electronic states given by the set bias –> chemical information of surface species, band gap measurements
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?
fdsjklf
What do we mean with the terms “pressure gap”, “temperature gap” and “materials gap”?
- Pressure gap: Difference in structure due to pressure difference between “real life” and the measured one
- Same’o
- Not real life <=)
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 your choices (exam bebbeh)
fjdskl
What is a Moire pattern?
Two superimposed pictures (patterns) on top of each other, creating a new, uniqe pattern. Used in STM and when growing a thin film on a subtrate (for identification).
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 (Co for Fischer-Tropsch)?
fjdskl
Is TiO2 the correct formula for this crystal when operated in the STM?
Explain why TiO2 at vacuum conditions is suited for STM (band theory). Discuss if it would be easy to mimic a Pt/Al2O3 catalyst in the STM.
How could you possibly compromise this issue?
fdsjkl
Explain the process of growing graphene on Rh(111)
VT-STM (Variable temp.) - strategy: nucleate graphene at LT, growth at HT at reactant limiting cond.
- Feed C2H4 at low partial pressures; 300K and anneal at 975K to form small particles of most stable form (graphene)
- Complete growth at 975K at reactant limiting cond.
- Avoid segregation of dissolved carbon to form graphene upon cooling
