Action on Surfaces: Moving Molecules and Nano Particles on Surfaces
28 Dec, 2007 02:13 pm
To build devices at molecular scale, we need to control the position, orientation and movement of individual molecules on surfaces. Surfaces have been studied in great detail over several decades using ultra-high vacuum environment, scanning probe microscopy and a variety of spectroscopic techniques. What about moving individual molecules or nano-particles on surfaces? This field of research is rather intriguing. Is it a matter of simply scaling down or is the nano-world quite different to the macro-world? Fascinating research gives more and more insight into this new realm never studied before with such powerful tools.
Moving individual molecules on surfaces, molecular transport: What about moving individual molecules or nano-particles on surfaces? In order to move molecules on surfaces, they have to be weakly bound to the surface. Molecules with legs or even wheels reduce friction with the surface. The design and synthesis of such molecules has been the research goal of a number of very active research groups. James M. Tour’s (Rice University) group synthesized molecules with four spherical molecules as wheels. They claim to have observed the rotation of the spherical molecules (wheels) and the molecules move preferentially along on symmetry axis of the molecule. The researchers speak of molecular ‘nanocars’. The movement of the ‘nanocar’ molecules several nanometers in size depend temperature (2). They don’t move at low temperatures and as the temperature is increased they move across the surface. Although the molecules consist of more than one hundred atoms they take a random walk very much like the diffusion of atoms on a surfaces demonstrating that molecules can move on surfaces relatively easily by design. It would be great if the molecule’s movement would not depend only on temperature but have its own motor. Actually in molecular biology one finds a large variety of molecular motors in liquid environment. The motor is one thing but what about the energy to run the motor? One way to go around this problem is in using the microscope itself to influence the position of atoms and molecule on surfaces. Atoms are routinely imaged with scanning probe instruments. The probe itself can be used to pick up and place atoms and molecules or nano-particles. By being able to move molecules on surfaces one can also use the molecules to transport several atoms. In this case the molecules have the function of a transport vehicle on surfaces; some call it ‘nanotruck’ (3). Apart from translational motion, there is the rotational motion of molecules required for the construction of molecular machines. The rotation of star shaped molecule on step edges have been recently demonstrated (4) by the group of Ch. Joachim (CEMES, Univ. Toulouse).
Nanoworld? This field of research is rather intriguing. There is the engineering aspect of this vision suggesting that one can build more complex systems molecule by molecule like in the macroscopic world. But there are simple facts like the Brownian motion of molecules, the constant random movement of small objects directly related to their temperature and the strong interaction forces. This suggests that the molecular world is quite different to the macroscopic world. This issues have been fiercely debated (5) by the late R. Smalley and K. E. Drexler and can also be found in the distinction by R.A.L. Jones (6) of a radical vision of nanotechnology and a vision of nanotechnology which is more cautious about what is achievable in the nanoworld.
(1) GA Somorjai, JY Park Physics Today 10/2007
(2) Y. Shirai et al. Chem. Soc. Rev., 2006, 35, 1043–1055
(3) L. Gross et al, Nature 4 (2005) 892
(4) F. Chiaravalloti et al , Nature (2007) 30
(6) ‘Soft Machines: nanotechnology and life’ by Richard A L Jones, Oxford University Press (2004)