Theoretical Analysis of Diamond Mechanosynthesis. Part III. Positional C₂ Deposition on Diamond C(110) Surface Using Si/Ge/Sn-Based Dimer Placement Tools

This paper extends an ongoing computational and theoretical investigation of the vacuum mechanosynthesis of diamond on a clean C(110) diamond surface from carbon dimer (C₂) precursors, using Si-, Ge-, and Sn-substituted triadamantane-based positionally-controlled DCB6 dimer placement tools. Interactions between the dimer placement tools and the C(110) surface are investigated by means of stepwise ab initio molecular dynamics (AIMD) simulations, using Density Functional Theory (DFT) with generalized gradient approximation (GGA), implemented in the VASP software package. The Ge-based tool tip provides better functionality over a wider range of temperatures and circumstances (as compared with the Si or Sn tool tips). The transfer of a single carbon dimer from the Si-based tool tip onto C(110) is not controllable at 300 K but is workable at 80 K; the Ge-based tool remains workable up to 300 K. Geometry optimization suggests the Sn-based tool deposits reliably but the discharged tool is distorted after use; stepwise AIMD retraction simulations (at 300 K for the Sn tip) showed tip distortion with terminating Sn atoms prone to being attracted towards the surface carbon atoms. Stepwise AIMD shows successful placement of a second dimer in a 1-dimer gapped position, and successful intercalation of a third dimer into the 1-dimer gap between two previously deposited dimers, on clean C(110) at 300 K using the Ge tool. Maximum tolerable dimer misplacement error, investigated by stepwise AIMD quantification, is 0.5 Å in x (across trough) and 1.0 Å in y (along trough) for a positionally-correct isolated C₂ deposition, and 1.0 Å in x and 0.3 Å in y for C₂ intercalation between two gapped ad-dimers. Rotational misplacement tolerances for dimer placement are ±30° for the isolated dimer and −10°/+22.5° for the intercalated dimer in the xy plane, with a maximum tolerable "in plane" tip rolling angle of 32.5° and "out-of-plane" tip rocking angle of 15 for isolated dimer. Classical molecular dynamics (MD) analysis of a new Ge tooltip + handle system at 80 K and 300 K found that dimer positional uncertainty is halved by adding a crossbar in the most compliant direction. We conclude that the Si-based and Ge-based tools can operate successfully at appropriate temperatures, including up to room temperature for the Ge-based tool.