STM Flashcards
What does STM detect?
Current
Does the probe touch the surface?
Comes close but doesn’t touch
How does the piezoelectric scanner work?
Pressure exerted on non-conducting ceramic crystal -> deforms crystal -> induces p.d.
Apply a bias voltage b/w inner + outer electrodes -> tube will expand/contract -> therefore moves tip up + down based on how p.d. changes
Therefore, crystal attached to probe as a ‘feedback position controller’ which pulls the tip back to prevent it from crashing into the surface.
Describe a piezoelectric scanner
Thin-walled, polarised RADIALLY, electrodes connected to internal + external faces.
How does current tunnelling occur?
Apply bias -> tip approaches surface -> electrons jump via quantum tunnelling
(NOTE: electrons either transferred from tip or taken up by it depending on direction the voltage bias is set)
What are the 2 things current tunnelling depends on?
- tip-surface distance
- overlap b/w electron WAVEFUNCTIONS of tip and sample
Why can STM only characterise CONDUCTING samples?
Because it detects current. The images produced are current maps where inner atoms appear darker as there is less conductivity and so less communication b/w the probe and the surface
Explain the 2 modes of STM operation
Constant height + moves across:
- measures variation in current
-needs current feedback to avoid crashing
Constant current (“topographic method”):
-tip has to jump according to features of surface
-keeps current constant by adjusting height (through feedback loop)
What is one advantage and disadvantage of the constant height mode?
Advantage: can scan fast as not limited by response time of vertical movement of tip
Disadvantage: could crash if don’t know max. possible height of surface.
What is an advantage of the topographic mode?
no danger of crashing tip
(possible disadv: time for scanning)
Give 3 possible methods of performing spectroscopy using STM
- Take topographic images (so constant current) using diff bias voltages and compare them
- Take current images (so constant height) at diff heights and compare them
- Ramping the bias voltage with the tip positioned over a feature of interest while recording the tunnelling current (I-V curve) characteristic of electronic structures at as specific x-y location on the surface.
What kind of spectrographic information can you obtain with a lock-in amplifier?
- dI/dV [conductivity]
- (dI/dV)/(I/V) [electronic density of states]
- dI/dz [work function]
What information does STM spectroscopy provide?
local density of states by detecting wavefunctions of specific atoms
Explain how STM works on a semiconductor surface
In semiconductors, there are 2 types of atoms: one has occupied states, other has unoccupied states. Can either observe either one set or other by changing bias.
Positive sample bias: Current arises from electrons tunneling from OCCUPIED states of the TIP to UNOCCUPIED states of the SAMPLE -> so contour related to spatial distribution of UNOCCUPIED states of sample.
Negative sample bias: (opposite)
Describe the 2 ways to position atoms using STM
Slide individual atoms sideways: Xenon atoms stick to tip, tip drags + moves position, atom released by changing bias
Tip induced metal deposition: metal ions attract onto STM tip -> tip moves up + down (no dragging) -> deposits metal cluster on substrate surface.
