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Biomedical Instrumentation > Quiz 2 > Flashcards

Quiz 2 Flashcards

1
Q

What is the main benefit to AFM?

A

It can scan 3 dimensions, other methods can only give 2D information

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2
Q

What are the steps of AFM imaging in the system?

A

laser on the back of the cantilever by projection and reflected onto a photodetector, a differential amplifier. When the cantilever bends, the light path is changed causing the output electronics to change

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3
Q

What is the AFM modeled equation-wise?

A

It uses a spring constant, it is a force measurement, delta q represents the distance between the tip and the sample

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4
Q

What are other measuring tools in the SPM family?

A

Near field optical microscope,
Scanning tunnel microscope

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5
Q

Examples of Contact mode AFM:

A

Lateral Force, Shark, Lithography, Scanning thermal

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6
Q

Examples of vibrating mode AFM:

A

Close contact, intermittent contact, magnetic force, electric force

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7
Q

Different modes of AFM and the difference between:

A

Contact modes use the deflection in the tip force while non-contact/vibrating is when vibrations of constant amplitude and frequency are distorted due to interactions with the surface

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8
Q

When is topography measured vs force field?

A

Topography is measured when the probe is close to the object, force field is measured when it is far away

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9
Q

What is a limiting characteristic of AFM?

A

It cannot measure features larger than 100 micrometers

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10
Q

Other than 3D what is another thing that AFM allows for that EM does not support?

A

Nanomanipulation using lithography

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11
Q

What is the difference in the type of samples that can be measured between SEM/TEM to AFM?

A

SEM/TEM can only use conductive samples while AFM can use either insulating or conductive samples

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12
Q

Does SEM/TEM have a good or depth of field or contrast on flat samples?

AFM?

A

SEM/TEM has good depth of field and bad contrast on flat samples, AFM has the opposite, AFM also takes 1-5 min while SEM/TEM take between .1-1 minute

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13
Q

AFM vs Electron Microscope(SEM, TEM)

Which is better with mechanical properties and which is better for chemical composition measurement?

A

AFM is good for measuring mechanical properties while EM is good for chemical composition.

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14
Q

Which is more expensive and time consuming to prepare a sample, AFM or EM methods?

A

SEM and TEM take more time and money for sample preparation than AFM does

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15
Q

What are Mica?

A

Silicate materials that are flaky and can be split into really small thin elastic plates, they are often used in AFM images under the object of interest because they are free from contamination and can be extremely flat.

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16
Q

Flow chart for AFM imaging procedure:

A

Prepare sample
Place sample in stage
Replace probe
align laser
Probe approach
optimize feedback
scan sample
Zoom on feature
tip retract

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17
Q

What are important for sample preparation AFM?

A

The sample needs to be clean, adhered to the surface, have realistic dimensions, rigidly mounted to the AFM stage

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18
Q

How does shark mode work?

A

Electrical conductivity maps are measured by monitoring the current flow between the probe and the surface

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19
Q

What is AFM lithography?

A

Nanoindentation made with an AFM probe in PMMA surface

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20
Q

What information can be gathered with AFM?

A

roughness, conductivity, magnetic profile, force fields on surface of the sample, shape and morphology of the sample, topography of the sample, dynamic behavior of the sample, and sample hardness using nano-indentation

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21
Q

In mechanical measurements, what type of forces may the probe interact with?

A

Hard forces, repulsive regime.

22
Q

What is the primary method for making mechanical measurements?

A

The force-distance curve

23
Q

What happens when the probe is placed firmly into the surface?

A

Nano-indentation may occur

24
Q

What angle should the probe be when approaching the plate ideally?

A

Vertical, in reality it is at an angle

25
Q

What are the steps of the force/distance measurement?

A

The probe is lowered until hard forces occur

The probe is pulled into the surface by capillary forces, the probe jumps into contact, the position depends on the contamination layer

The probe moves further down until it enters the repulsive regime

when is it retracting the adhesion causes the probe to stick to the surface

the probe then disengages and continues to retract until it does not have force any longer

26
Q

What forces are present when the tip is approaching the plate?

A

Electrostatic, long range interactions with absorbed molecules

27
Q

What forces are present when the tip is close to the surface?

A

Van der waals
capillary forces
DLVO/ screened electrostatics (in aqueous solutions)
Chemical potential
magnetic
solvation forces (water layering)

28
Q

What forces are present when the tip is indenting the sample?

A

Stiffness/youngs modulus
viscoelastic response
measurement of active forces

29
Q

When might F/D artifacts occur?

A

When the laser spot is not centered, hydrodynamic drag, large indentation

30
Q

What is an artifact?

A

Something that is observed but not present

31
Q

Primary sources of artifacts:

A

Probes, scanners, image processing and vibrations

32
Q

What may make the sharpness of the probe a crucial parameter?

A

the aspect ratio

33
Q

What are common artifacts?

A

features appearing too large, small, strangely shaped objects, or repeating patterns

34
Q

What is normally accurate in a measurement?

A

The height as measured by a line profile

35
Q

What are the artifacts caused by tips?

A

Double tip, tip dilation , sharp feature in substrate, wide tip

36
Q

What artifact may be caused by a double tip?

A

substrate feature repeated image

37
Q

What artifact may be caused by a tip dilation?

A

image of feature in substrate broadened by tip radius

38
Q

What artifact may be caused by a sharp feature in the substrate?

A

substrate features image tip

39
Q

What artifact may be caused by a wide tip?

A

depth of feature in substrate is inaccurate

40
Q

How can artifacts be tested for?

A

Repeat scan, reverse scan direction, rotate scan direction, change scan size, change scan speed

41
Q

What does STM stand for?

A

Scanning Tunneling Microscopy, it is an electrical measurement

42
Q

What does SPM utilize?

A

controlled combination of feedback loops and detectors with a raster motion of piezoelectric actuator, it investigates direct interactions of atomic and nanoscale phenomena.

43
Q

What happens to some type of tip sample interaction?

A

It is held constant in ‘z’ while the tip undergoes an x-y raster motion creating a surface map

44
Q

What does STM use, what is selectivity induced by?

A

tunneling current between the tip and the sample, conservation of energy and momentum, STM has the highest resolution in SPM

45
Q

What is apparent topography demonstrated by in STM?

A

local density of electronic states

46
Q

How does STM interaction work?

A

For a thin enough barrier, electrons can tunnel through at the expense of a reduced wave function
Tunneling current is a quantum mechanical phenomenon.
Electrons on the surface of the sample faces a barrier (air, vacuum or liquid), which is then crossed(wave nature) via the applied bias – a tunneling current is recorded from the sample to the tip.
The tunneling current (nA to pA) depends sensitively on the distance (tens of nm) between the tip andthe sample (with special preamplifiers, currents on the order of 10-14A can be detected).

47
Q

Difference between Physisorbed and Chemisorbed?

A

check slides

48
Q

Difference between constant height and constant current?

A

Height: feedback off, moving current not path

Current is the opposite

49
Q

What does albumin do?

A

protein made by liver, helps keep fluid from leaking out of blood vessels

50
Q

Challenges of STM

A

Both substrate choice & interactions with the tip can influence theconformation of the biomolecules and whether its locallyconductive portions are positioned appropriately to produce atunneling current

The signature bases in the double helix of a DNA are enclosed ina sugar – phosphate backbone and only partially exposed makingit challenging to image them

Tunneling current is sensitive to the local density of electronstates for e.g. on an insulating surface.

Biomedical Instrumentation (3 decks)

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