Mechanochemistry at the Single Bond Limit: Towards 'Deterministic Epitaxy'
Lead Research Organisation:
King's College London
Department Name: Physics
Abstract
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Publications
Abbasi-Pérez D
(2019)
Controlling the preferential motion of chiral molecular walkers on a surface.
in Chemical science
Abbasi-Pérez D
(2021)
Cyclic Single Atom Vertical Manipulation on a Nonmetallic Surface.
in The journal of physical chemistry letters
| Description | We have attempted a very detailed theoretical study of an ability of an AFM tip to vertically manipulate an atom on a crystal surface. Specifically, and probably for the first time, we attempted to consider a complete manipulation cycle comprising of two steps: (i) pick up an atom from the surface, and then (ii) place this atom back on the surface at a different location fully restoring the original tip ready to repeat this kind of manipulation cycle again. First if all, we have developed special computational tools to perform this kind of study. These tools enable us to consider, semi-automatically, the potential energy surface of a tip at different lateral positions on the surface and at different heights. The peculiarity of our method is that we are not primarily interested in the 3D tip force calculations which became routine, but in understanding the ability of the tip to pick up or deposit an atom. This means that in our method, at each lateral position, we have to lower the tip to a particular height above the surface and then retract it recording any substantial changes in the surface (or tip) geometry. Then the tip is lowered more, and the retraction process is repeated. The whole process would include many such approach-retraction cycles, to different tip heights above the surface, with the lowest tip position, even possibly corresponding to a significant indentation of the tip into the surface. This way one can try to test various tip structures upon their ability to perform a complete detachment/attachment cycle so that the tip would perform as a 'crane'. First of all, we found that detaching an atom from a terrace on this surface is energetically too costly and hence cannot be considered as a possible experimental strategy. Then a very large number of tips have been considered consisting of Ga, As, and Au atoms and their combinations at the tip apecies, interacting with the GaAs (110) surface. The goal was to understand whether these tips can deposit one or more of their atoms on the surface. These calculations enabled us to construct a lateral map for each tip tried (more than 10 tip structures) in their ability to: (i) deposit their atom(s), and (ii) leave the surface structure minimally destroyed. Au tips were found to be too soft for the job, easily destroyable by the GaAs surface. However, for one of the tips we found that an exchange mechanism is in place when at a certain lateral position of the tip the apex Au atom is replaced by the surface As atom. GaAs based tips are stiffer and can deposit between 1 to 3 their apex atoms, and, depending on the level of indentation, may only slightly or considerably destroy the initial surface structure. Then, we come across a set of simulations with one particular pyramid GaAs tip that is able to deposit just one Ga apex atom on the surface leaving it as an adatom. The rest of the blunt tip was found to be able to pick up that atom again at a slightly different lateral position fully restoring the original tip. These calculations essentially mean that this blunt tip can indeed act as a crane by picking up and then depositing the Ga adatom performing the complete vertical manipulation cycle with Ga adatoms, any number of times, repeatedly. Then, we find that the same tip can also pick up and deposit, repeatedly, As adatoms as well at different lateral positions above the surface. Hence, this work demonstrated, as a proof of principle, that it is possible to have a tip such that would be able to work as a crane on the GaAs (110) surface by picking up and depositing anywhere on the surface both Ga and As atoms. Attempts have also been made to verify if this blunt tip is able of performing such repetitive deposition/puck-up cycles for other atoms, such as Au, Sb, In, Al and P. Surprisingly, we have found that, apart from P, Sb and In, it can! Inspired by this breakthrough, we then considered, using the Nudged Elastic Band (NEB) method, the energy barriers for both parts of the cycle in each case (retraction/deposition) at different tip heights. These calculations should open up a new avenue in our future work related to studying kinetics of the vertical manipulation with an oscillating AFM tip that could be done using kinetic Monte Carlo (kMC) approach. Encouraged by these exciting results, we started searching for other possible tips that would be able to perform similar job. We have found a GaAs tip with several Au atoms at its apex that are capable to pick up and then deposit (without changing the tip structure) a Au atom on the GaAs surface, also repeatedly. Even a deposition of two Au atoms next to each other was demonstrated. Hence, our simulations proved that tips that can pick up and deposit a Ga, As, Al and Au atom on the GaAs surface do indeed exist, and hence experimental effort should be directed towards finding them. Our Nottingham partners are aware of that work and have started relevant experiments (they experienced some technical difficulties before, but now the situation has changed to the better). This work was reported in an oral talk at NC AFM 2018 conference in Finland (17-21 September). We have prepared the first publication on these results that is theory only. However, it was decided to refrain from submitting this paper and wait for experimental results to support our findings as this way a higher impact of this work would be expected. Unfortunately, the funding is now stoped and it is not longer possible to continue this work by Dr. Abbasi. Our plans for future work would be to see if immersing tips into clusters of atoms would give useful clues as to the "best" tip structures. Our further direction would also involve (i) simulating building of nano-structures, (ii) finding more tips, (iii) continue NEB simulations for picking up and depositing go atoms, and (iv), when this is done, performing kMC simulations of the vertical manipulation kinetics. Added on the 1st of March 2021: Over the last year our experimental partners at Nottingham tried to obtain results that would confirm the results of our theoretical calculations. However, because of carious problems with their AFM machine and because of the pandemic, no results emerged and it was recently decided to go on with the theory-only paper. The manuscript is now ready for submission and will be submitted by the 10th of March 2021 after a final review. |
| Exploitation Route | We believe that our work may shed light on what is important in searching for the "best" AFM tips that are able to perform the complete retraction/deposition cycle. Our methodology can be easily adopted to other systems (tips and surfaces). |
| Sectors | Aerospace, Defence and Marine,Chemicals,Education,Electronics,Energy,Environment,Pharmaceuticals and Medical Biotechnology |
| Description | Oviedo University |
| Organisation | University of Oviedo |
| Country | Spain |
| Sector | Academic/University |
| PI Contribution | The design of the project, the methods used, calculations performed and the results obtained. |
| Collaborator Contribution | Prof J. Manuel Recio contributed to discussions and manuscript writing. |
| Impact | We were able to identify GaAs-based AFM tips that are capable of performing a full vertical manipulation cycle like a crane. |
| Start Year | 2019 |
| Description | NC-AFM conference presentation |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | International |
| Primary Audience | Professional Practitioners |
| Results and Impact | The 21st International Conference on Non-contact Atomic Force Microscopy, 17-21 September, Porvoo, Finland. This is the main NC-AFM International conference with over 100 participants from all over the world. |
| Year(s) Of Engagement Activity | 2017 |