August 21, 2008

Philip Moriarty discusses Molecular Nanotechnology Validation experiment plans

On the Center for Responsible Nanotechnology blog comments, Philip Moriarty discusses his plans for testing the viability of positionally-controlled atom-by-atom fabrication of diamondoid materials as described in the Freitas-Merkle minimal toolset theory paper.

A combination of low temperature tuning fork (Qplus) AFM, STM, and tunnelling spectroscopy (dI/dV and d2I/dV2, i.e. inelastic tunnelling spectroscopy) will be used to implement and/or characterise scanning probe-driven mechanosynthesis reactions on diamond (C(100)) in UHV and at temperatures in the 4 K - 300K range. Initially we will need to demonstrate atomic resolution on C(100). We will then explore some of the ideas in Freitas and Merkle's "minimal toolset" paper in order to extract hydrogen from a H-passivated C(100) surface and subsequently add a carbon dimer.

As regards verification, a key goal is for theory and experiment to run in parallel, one reinforcing the other. For example, we will aim to reproduce experimental force-distance spectra (measured as a tip approaches a diamond surface during a mechanosynthesis reaction) using DFT calculations. You ask whether the research includes "seeking out work-arounds". Yes, most definitely! There's an interesting quote from a recent international review of UK materials research that should be printed in bold capital letters on the front of all documentation produced by funding bodies, viz.: "Research is always about risk taking, no matter whether the risk involves failure to meet a certain set of expectations or failure to create truly new, significant knowledge or understanding of a problem. To be clear, if the outcome of the effort can be anticipated, it is highly questionable whether this effort should be called research."

When the project gets going I will aim to set up a blog that will report on progress.

Philip also had a comment about diamond versus graphene nanotechnology.

It's important to note that the diamond mechanosynthesis proposal focuses specifically on diamond and, indeed, on a particular challenge which I first raised in my debate with Chris Phoenix a few years back: scanning probe "epitaxy" of a row of carbon dimers using purely force-driven reactions on hydrogen-passivated diamond. Rob Freitas and Ralph Merkle's recent minimal toolset paper has been particularly important in defining the plan and objectives of the proposal and I want to stay focused on this, rather than move to graphene.

Graphene is, of course, a very interesting system and it's possible that we may explore this in the course of the five year mechanosynthesis grant. My suspicion, however, is that achieving basic "mechanoepitaxy" on diamond will take at the very least five years!

As regards [Jim Moore] points:

1. Being able to hold a sheet of graphene away from another surface may well be useful for longer term mechanosynthetic work but the primary objective of the EPSRC-funded work is to demonstrate the validity of a small number of mechanosynthesis reactions - which have been explored by Freitas and Merkle via DFT calculations using very many thousands of CPU hours - on a bulk diamond surface.

2. This is actually a rather challenging way of detecting a successful operation. It will be more straight-forward to use the scanning probe itself - through force-distance, I(V) and d2I/dV2 (inelastic) tunnelling spectroscopy - to monitor a successful mechanosynthesis reaction event.

3. The metastability of diamond with respect to graphite/graphene is not really an issue here. The H:C(100) surface represents an excellent platform for site specific scanning probe-driven chemistry. Drexler understood this very well - his choice of diamond(oid) in Nanosystems was very well-informed.

4. Hmmmm. Yes, graphene is certainly a well-funded area but it may not always be a good idea to chase current trends in order to secure money for research!

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