Stiff cantilever AFM

Stiff cantilever AFM generally known as (subatomically resolving) nc-AFM (rarely FM-AFM)
is a form of scanning probe microscopy that probes forces rather than currents.

old nc-AFM ~vs~ new nc-AFM

Early AFM used soft cantilevers which led to problems that made subatomic resolution impossible.
The cantilevers where so soft that VdW forces, electrostatic forces, and whatever other forces present
led to a bi-stable snap to the surface.
– either these forces pressed the tip to the surface making it scratch over the surface quite roughly (contact mode)
– or the tip was so far away from the surface that subatomic resolution was not posible either (non-concact nc mode)
Noting in-between the two due to the bi-stable snap-to surface.

With the introduction of stiffer cantilevers the bi-stable snap could finally be avoided
and the tip could get close enough without snapping and crudely scratching such that that subatomic resolution with AFM could finally be achieved.
"contact mode" as in "crudely scratching over the surface with large long range press on forces" was no longer a thing.

Force measurement method

Another difference between old and new:
– old nc-AFM (an c-AFM) used deflection of a laser and a four quadrant photo-diode
– new nc-AFM measures detuning of resonant frequency (frequency shift)
time can be measures especially precisely so this helps
Rarely it's called FM-AFM for frequency modulation.

A horrible historic naming accident

Unfortunately the contact vs non-contact distinction (now with the bi-stable snap gone) makes no longer sense.
Rather more sensible would be to distinguish non-contact from contact by the switch from
attractive forces to repulsive forces. Roughly meaning the distance where one crosses under the vdW radii of atoms.
Now we often have a situation where we clearly are within the Pauli-repulsive-regime
(which is as much contact as one can possibly get) yet still the technique is named non-contact.
Reason for why this page is called stiff cantilever AFM rather than nc-AFM.

There is also qPlus and kolibri, but these are brand names of specific stiff cantilever sensors.
Not neutral names. More on these brands later.

STM ~vs~ stiff cantilever nc-AFM

STM was the oldest and first scanning probe microscopy technique that achived subatomic resolution.
AFM was for a long time not capable of comparable resolution due to the afore explained soft-cantilever-bistable-snap problem.
Once that problem was solved though resolution even exceeded the one of STM.
Actually it's rather that smaller electron structures can now be images that are usually easier to interpret by human intuition.

STM gets a great deal of resolution due to the fact that the
tunneling current steeply exponentially declines with distance
making only the foremost atom contribute by far the most.

Stiff cantilever AFM detects Pauli-repulsion-force which too declines sharply with distance.

STM is typically imaging farther away non-bonding orbitals which are bigger and
give less fine details about the shape of the molecule(s) under investigation.
Higher energy molecular orbitals often have node structures that are not really intuitive to interpret.
Leaving only quantum mechanical simulations and comparing these teoretical predictions with experimental images.
Good to do that for AFM too, but before that in case of AFM intuition can already help more.

There is one new STM mode with picked up molecule that
manages to achieve a similar resolution to stiff cantilever nc-AFM.
(wiki-TODO: Add more on that here.)

Amplitudes, frequencies, parameters

qPlus sensor

Originally hacked together from quartz fork resonators from digital clocks.
One side of the fork eventually tied to bulk mass for better results.
Oscillation amplitudes are typically slightly to strongly subatomic.

Quantitative numbers form pentacene paper:

• Apmplitude: 100pm down to 20pm
• Frequency: f0 = 23.165 kHz – Q = 50.000 (Meaning well more than 2s for ringing out after power-off.)
• Stiffness: 1800 N/m

Kolibri sensor

Is is again orders of magbitude stiffer than the qPlus sensor.
As it can oscillate in two directions it can detect sidewards forces too not just vertical ones.
As it is a newer younger design there may bot yet be as much community knowledge and user experience documentation around.
It seems to come withing a metal housing giving it a significant bigger size compared to qPlus.

(wiki-TODO: Add quantitative details.)

Related

• Scanning probe microscopy
• Snapback

External links

Videos

• 08m21s Franz J Giessibl and the Q plus sensor: scanning probe microscopy 30 years on
• 16m31s The genesis of the qPlus sensor - Focus Session Scanning Probe Microscopy with Quartz Sensors ~ by Franz J. Giessibl
• Some relatively recent stiff cantilever nc-AFM work:
  56m33s MIT.nano Seminar Series: Leo Gross

Papers

• 2018 – The qPlus sensor, a powerful core for the atomic force microscope – by Franz J. Giessibl – (link to pdf)
  – this is a thorough overview by the inventor himself
• 2015 – Atomic Resolution on Molecules with Functionalized Tips – by Leo Gross et.al. – (via ResearchGate)
  – Figure1 shows sort of a "garden of molecules" on gold (Au) and bilayer(?) salt (NaCl).
• 2009 – The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy – by Leo Gross et.al. – (link to pdf)
  – this was perhaps the watershed moment paper

Polyynes

• 2023 – On-surface synthesis of a polyynic carbon chain with ~40 alkyne units – by Wenze Gao et.al.
  prepring CC BY 4.0 ~ preprint, direct preview on ResearchGate
• (wiki-TODO: Add other work on synthesis of polyyne rings on NaCl salt.)

Misc

• Graphitic structures (especially including things like Graphene nanoribbons (wikipedia) have become heavily studied by stiff cantilever nc-AFM.

Inventor & developers

• Franz Josef Giessibl (en)
• Franz Josef Gießibl (de)
• Leo Gross (Physiker) (de)