Why force applying mechanosynthesis should work in brief

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★ Relates to a sub challenge of the direct path & mixed path.
★ Is regarding concerns about atom-by-atom pick-and-place assembly aka piezochemical mechanosynthesis.

Legend:
★ ED … experimental demonstration
★ TA … theoretical analysis

Experimental

ED – scanning in 3D (high vertical ledges) is now possible

Unmentioned scanning height record: Notice the vertically standing tetramantane molecule (b)? The authors managed to scan the very top of this molecule (crystolecule) which is higher up than the length of the for scanning to the SPM tip asdorbed CO molecule. This may well be a height above surface record for atomically resolved structure, aside nanotubes maybe. The STM tip apex atop the short height scanning CO molecule is more like a blunt big ball thus constant frequency shift scanning (scanning over the surface keeping contact at all times) would just crash the tip into the very high standing molecules from the side (this is a suspicion, not mentioned in the paper) and make them stick to the SPM tip on the side. Thus the actual method they were using is first taking a STM height reference, then moving up to save height, and then carefully doing constant height scans going down scan-plane by scan-plane in not too big steps till images appear. A very slow process. – Surprising gentleness of scanning: The bottom is only very weak vdW bonded via the hydrogen atoms to the gold surface, yet the scanning of the top (that bent the probing CO a bit as usual) did not slide it along. Note that "tipping over" is not a concern so long not dragged over an obstructing step-edge as forces from gravitative mass are way too small to matter at this scale. The SPM needle tip trajectory does not care if it is a strong covalent bond or a weak vdW bonds. Given its FAPP infinite inertia it is a constant speed source mowing everything down in its path if the path isn't carefully chosen. – Brightness interpretation The brightness of the image id the qPlus sensors frequency shift which can be interpreted roughly as a map of stiffness. Not force!
The (unlike STM) very directly bonding structure revealing scanning probe technique of qPluc nc-AFM has exceptionally good sensitivity (i.e. change of frequency shift signal over height). The problematic flip side of the coin is that in constant height mode (needed to be used to not crash into higher ledge molecules fro the side - see above) only a very limited layer of depth is imaged. A very crude analogy is the limited depth of field in optical microscopy. The authors managed to image lower lying hydrogen atoms by scanning down a ramp. This points to a way of overcoming this limitation. That is automating and making much smarter the ramping. Optical microscopy does "focus stacking" or "focus fusion". This would be an analogy.

See main page: Scanning probe microscopy upwards into 3D

Coping with high vertical ledges demonstrated possible:
The diamond like surface of a tetramantane molecule (of natural origin)
was subatomically resolvingly scanned while standing upright.
They managed to do this despite the tetramantane molecule being higher
than the picked up CO molecule that was used for imaging.
The method: They took an STM height reference on the gold surface and then
did constant height nc-AFM scanning to get a picture of the top.

Why constant frequency shift scanning is not an option here (and generally rarely):
★ the frequency shift signal of nc-AFM is even more height sensitive than STM
(i.e. nc-AFM features an extremely shallow "depth of fields" in optical analogy terms)
★ coming from the side at surface scanning level would likely
– crash the side of the big fat SPM needle tip apex into the molecule (feedback too slow) or even if no crash …
– pick up the molecule to the side of the SPM needle tip apex due to vdW forces or …
– less likely: scan part of the SPM needle tip apex tip with the tetramantane molecule (reverse imaging situation).
(wiki-TODO: Add a sketch.)

Side-note: The frequency shift in nc-AFM (with qPlus brand sensor) is roughly proportional to
surface stiffness for low excitation amplitudes. Not force as the name AFM would confusingly suggest.
Historic accident. That adding atop there actually being contact possible (rare but possible) despite "noncontact". OUCH!

Remaining challenge: Grater depth of field with focus stacking fusion:
nc-AFM (qPlus brand sensor) has extremely shallow "depth of field" (in optical analogy terms)
Even for a fraction of an atomic step they needed to ramp down the height to see some hidden hydrogen atoms.
This not being automated they where very glad and proud (title of the paper) that they not had to do it again.
Their trick of identification of molecuee species by slant is a special case though.
Needed is a general capability. Needed is automation of things like this height ramping.
More generally we really want automated SPM control to be able to do something that could be called
nc-AFM focus stacking fusion (in optical analogy terms).

Remaining challenge: Stronger attached tool that can not only image but also manipulate:
Note that the standing tetramantane molecule was not picked up, sled around too much for imaging, or tossed over.
That despite there only being a weak vdW bond between three hydrogen atoms and the gold surface.
The scanning is extremely gentle.
For covalent manipulations a picked up CO molecule very weakly bonded to the SPM needle tip apex would not work at all.
Other much more strongly bonded "adapter molecules" are needed. Rabbit hole of design constraints.
Also challenging the current SPM usage philosophy of disposable tip SPM needle tip apex structure.

ED – strong covalent nano-molecular manipulation in 3D

2020 Three-dimensional graphene nanoribbons as a framework for molecular assembly and local probe chemistry
https://www.science.org/doi/10.1126/sciadv.aay8913

Covalent fusion of a C60 buckyball molecule onto two upwards facing
carbon radicals provided by a graphene nanoribbon that has vertically upward facing parts.
Demonstration that strong covalent bonds can be formed in deeply in 3D way beyond the height of a mono-atomic step.

This is obviously good enough for whole molecule manipulation in a one off demonstration.
Automated reliable usage and more challenging manipulation of single atoms (not counting attached hydrogen atoms)
will need adapter/tool molecules helping in solving solving the problem of the unstable ever changing SPM needle tip apex surface.

Also: 2025 Inverted-Mode Scanning Tunneling Microscopy for Atomically Precise Fabrication
https://arxiv.org/abs/2512.24431

Purely mechanical abstraction of a single hydrogen atom.
No STM current involved.

ED – SPM manipulations being automateable and scaleable

This was a quite early work.
Still STM only and Cl ad-atons on large scale atomically flat Cu(100) metal.
It did demonstrate automateability and scalability.
The underlying process needs to be switched to new capabilities in
– working up in deep 3D and
– working with strong covalent bonds

ED – elemental identification of element types via force curves

2007 Chemical identification of individual surface atoms by atomic force microscopy
https://www.nature.com/articles/nature05530
download via researchgate.net
read access via academia.edu

To place the right elements at the right spots
one obviously needs to find out what kind of elements are available where for the picking.

In this paper by analysis of the nc-AFM frequency shift curves
they managed to identify the elements {Si, Sn, Pb}
(i.e. the entirety of group14 except C & Ge; group of high interest)
present as mixed cover of ad-atoms on the surface Si(111)√3×√3R30°

(wiki-TODO: Add reference to and discussion of paper(s).)
(wiki-TODO: More details later …)

EDs – progress in atomically resolving nc-AFM on AP nanographenes & less flat structures

Very flat:
Graphene sp2 carbon nanoribbons:

Generally many very impressive larger scale subatomically resolving nc-AFM images in this field.

Less flat:
Aliphatic sp3 carbon too under special circumstances:

Beyond the limit 3D:
Less flat DNA seems too non.flat to ne iageable sith subatomic resolution.
Especially with something like nc-AFM focus stacking not yet being developed.

GNR synthesis:
Nanoribbons have a problem with termination control in their bottom up synthesis.
But there is progress on this front too:

ED – preserving of tip apex structure during transfer to a different macroscale sample (and back)

(wiki-TODO: Add reference to best fitting COFI paper.)

COFI method: SPM needle tip apex structure has been
– reverse imaged by a CO on a Au sample
– macroscale transferred over to other sample to be used for imaging there
– macroscale transferred back to check that no tip changes occurred in the meantime

Seems A picked up CO molecule was macroscopically transferred
from Au sample over to Si sample too as an alternate method.
(wiki-TODO: Find that paper and add reference here.)

Relevant for ex-situ tip fictionalization.
Escaping local heterogeneity limits.

(wiki-TODO: More details later …)

ED – progress with ultra flat surfaces

ED – progress with potential "adapter molecules"

(wiki-TODO: Add reference to and discussion of papers.)

  • several papers
  • transfer paper & conductivity paper

ED – progress in covalent bond formation control (2D for now)

ED – progress in STM control

(wiki-TODO: Add reference to and discussion of papers.)

Early ED (on silicon)

Extraction an re-deposition a a single silicon atom (at 78K) was experimentally demonstrated. This gives experimental evidence for piezochemical mechanosynthesis working.

It was possible to experimentally demonstrate mechanosynthesis of silicon.
Abd that even even with today's still very crude means (meaning blunt tips).
See: Silicon mechanosynthesis demonstration paper or more generally: Experimental demonstrations of single atom manipulation

  • Silicon is a relevant material quite similar in covalent character to diamond.
  • This has been done an reasonable temperatures (meaning not liquid helium but liquid nitogen)

ED – 4K cryo codeposition of various molecularspecies "garden of molecules"

Many different molecules and atoms can be co-deposited and at deep cryo even higly vlatile species stay stuck. This can potentially serve as feedstock for mechanosynthesis in early systems.

2015 – Atomic Resolution on Molecules with Functionalized Tips – Leo Gross at al.
https://www.researchgate.net/publication/282314727_Atomic_Resolution_on_Molecules_with_Functionalized_Tips
https://link.springer.com/chapter/10.1007/978-3-319-15588-3_12

The big STM images in Figure1 looks a bit like a "garden of molecules".
Molecules/atoms deposited include: CO, CoPc, pentacene, C60, TPP, PTCDA, Ag, Au

Standard format citation: Gross, Leo & Schuler, Bruno & Mohn, Fabian & Moll, Nikolaj & Repp, Jascha & Meyer, Gerhard. (2015). Atomic Resolution on Molecules with Functionalized Tips. NanoScience and Technology. 97. 223-246. 10.1007/978-3-319-15588-3_12.

Theoretical

TA – Highly meticulous theoretical analysis (with carbon, a complete system)

It has been shown that the infamous finger problems like …
– the sticky finger problem and
– the fat finger problem
… are not valid.

See: A Minimal Toolset for Positional Diamond Mechanosynthesis (paper)

TA – older theoretical analysis on silicon mechanosynthesis

(wiki-TODO: Add reference to and discussion of papers.)

Related

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