Early prototyping of piezochemical mechanosynthesis
Up: Direct path
(wiki-TODO: add illustrative image)
Contents
Challenges
There are several hard hurdles where progress seems slow. Like:
- relatively slow progression of miniaturization of SPM and consequently ...
- ... little increase in reliability at which scanning probe microscopes (SPMs) with positional atomic precision can operate.
- ... little increase of speed at which scanning probe microscopes (SPMs) with positional atomic precision can operate.
- except very view hard to find instances there there are barely any attempts of miniaturizing UHV vacuum systems (wiki-TODO: add links to UHV miniaturization attempts)
- Due to very little control of the tip apex structure and even less of the rest of the tips conical widening behind multi-tip-interactions (anything going beyond surface-to-tip-interaction like tip-tip or tip-surface-tip or tip-tip-tip) is still far out of reach.
- very poor handling of steps bigger than a single atom (slow response z control + z drift) => crashes in up-steps - "shadows" in down-steps
Beside of hardware the limitations of the control electronics are a significant factor too.
One needs to look at the entire transfer function find bottlenecks and improve on various spots.
Low speed, relatively bad vacuum and high error rate leads to the necessity of keeping the produced atomically precise (precise in position not only topology) diamondoid designs rather small.
- Restriction to minimal hydrocarbon designs that exclude further chemical elements. See the Discussion of proposed nanofactory designs.
- Possibly a hydrosilicon design instead of a hydrocarbon design if silicon becomes mechanosynthesizable before carbon.
Current experimental "high temperature" (70K...300K) mechanosynthesis demonstrations all use silicon (update more low temp ~4K research by 2024). Carbon is still stuck in theory (See papers linked on the page about mechanosynthesis).
Tip functionalization / decoration
The idea here is to pick up specially designed molecules and
to then use them as tools to manipulate other feed-stock molecules more reliably.
These molecules shall act kind of like adapter pieces between …
- the usually poorly defined labile SPM tip apex surface and
- the very small feed-stock molecules (that may be not more than an atom capped with hydrogen or halogen atoms).
Desired properties of these adapter molecules include:
- stiff molecule structure
- stiff or sufficiently stretched legs
- tripod like (or more legs) to define a (single) stable pose in space
- only one single clearly most stable state in known pose (position and orientation)
- sufficiently strong binding to the surface (to not come off or change pose during mechanosynthesis)
- symmetry (matching surface symmetry)
Benefits of these adapter molecules:
- feedstock ending up at a defined position on the SPM tip after pickup
- feedstock attached to the SPM tip with a defined known energy and force
- effectively a much more pointy SPM tip reducing unintended reactions on the side
Callenges with adapter molecules'
- STM currents may damage the molecules.
- Molecules may reduce conductivity for STM operation leading to lower signal to noise STM imaging.
- Not getting several metastable poses that the molecule easily can jump between
Energy / binding strength cascade
The idea is to transfer carbon from germanium (Ge bearing adatper molecule on surface)
to silicon (Si bearing adapter tool on SPM tip) to carbon (on target structure inder construction).
Successively stronger binding giving some sort of reliable >>kT "click mechanochemistry".
It may perhaps be possible to shift this up one period in the periodic table for handing of silicon.
I.e. transfer form tin (Sn) to germanium (Ge) to silicon (Si).
Problems may include a need to switch to different adapter moleculed for then hoding an Sn core.
Also for bigger weaker bonding directionality of binding decreases which may make reactions more unrelaible.
It may bot be alwas the case taht atoms want to go from higher period atoms to lower period atoms.
E.g. halogens may energetically want to go from C to Si rather than fro Si to C.
All things to investigate experimentally eventually.
Build surface / Buildplates
A lot of options here. Concerns include:
- avoiding large lattice mismatch to the structure built atop
- having the final product separable from the surface rather than non-removable fused
- industry use prevalence (silicon)
- avoiding surface reconstructions (reason to avoid pure silicon?)
- research use prevalence (gold)
- various 2D materials atop base materials
- annealability of surfaces (diamond is problematic, thermodynamic minimum is graphite)
- neither binding too strongly nor too weakly
Focus of material
- Carbon (challenge with small atom sites and sp2 hybridization as lowest energy state)
- Silicon (challenge of dropping it onto silicon from a weaker binding tool)
- Silicon carbide ?
- building blocks of pre-made stiff molecules
- other …
