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Nanoscale Surface Dynamics

Methods available for surface dynamics measurements
Comparison of experimental techniques for studying aperiodic transport and diffusion on surfaces. Our technique (QHAS, yellow box) is the only method which can simultaneously access both nanometre length scales and picosecond timescales.
Surface Dynamics on the Nanoscale

The dynamics of surface atoms, and of atoms and molecules adsorbed on surfaces is of both technological and fundamental importance. Processes as diverse as the fabrication of self assembled materials, molecular beam epitaxy and even heterogeneous catalysis are all controlled by surface dynamics. At present, rather little is known about many processes in the nanoscale-nanosecond regime. We focus on using our unique methods (Spin-Echo, QHAS) to gain insights into the fundamental physics controlling these important applications.

At present we have a particular focus on:

  • Hydrogen dynamics & surface transport by quantum tunneling
  • The dynamics of water on surfaces
  • Dynamics of graphitic systems & graphene
  • Novel modes of surface transport
  • Motion in nanostructured systems

 


Quasi-elastic helium atom scattering (QHAS)

Helium atom scattering, a gentle and inert experimental technique, can be applied to study surface diffusion. The way of doing so, known as quasie-elastic helium atom scattering, or QHAS [1]. QHAS makes use of the fact that when a beam of helium atoms scatter from diffusing particles, they undergo a Doppler like energy broadening. By measuring this broadening with scattering angle it is possible to build up a complete description of the dynamics of the surface species.

Unlike real space imaging techniques, which are limited to framing rates of milliseconds at best, QHAS allows the true dynamics of the adsorbate to be revealed - over the picosecond to nanosecond timescales that are characteristic of two atoms passing each other by.

Conventionally QHAS was difficult to apply, since the Doppler like energy changes were masked by the resolution of time-of-flight helium spectrometers. However, the spin echo apparatus, with its vastly superior energy resolution, has extended the time scales available to by three orders of magnitude. This increased time range means that a huge range of systems which were moving too slow to be detected with time of flight experiments can now be studied: we are venturing into a new field of measurement.

In this section we present a few examples of some of the diffusion systems we have studied to date:

 

CO Diffusion on Cu(001)

The CO/Cu(001) measurements, performed with our machine [2,3], make full use of this extended time range. By following the sub micro eV broadenings of the quasielastic peak, we were able to extract the microscopic diffusion mechanism of the CO molecule on the surface, along with a potential energy surface describing the motion.

A more detailed description of this work is available in Science and in Phys. Rev. Lett.

 

Alkali Metal Dynamics on Cu(001)

Sodium on Cu(001) was the first adsorbate to be studied using QHAS, by using Time-of-Flight techniques. Considerable work was performed on the analysis and consequently, Na on Cu(001) has become 'the' reference system for the QHAS technique. We have revisited the system and have extended the early measurements considerably. The high resolution of our spectrometer allowed us to distinguish for the first time a coverage dependent vertical motion of the adsorbate [4] which has prompted further theoretical calculations [5].

Our improved resolution has also allowed us to study the dynamics of several other alkali metals: Li, K and Cs, systems which have been of interest in benchmarking new density functional theory studies [6,7].

 

Molecular Diffusion

A key goal for the spectrometer is to move beyond the simple systems studied so far to more complex dynamical systems. Initial studies of molecular adsorbates have revealed a variety of phenomena: uncorrelated motion in a system previously modelled with strong pairwise forces [8]; the modification of an existing experical potential to describe the dynamics of an adsorbed alkane [9]; measurement of single-molecule frictional dissipation in a previously unmeasured high-friction regime [10].

A more detailed summary of some of these systems may be found in our recent review paper [11].

 

References

  1. A. P. Jardine, J. Ellis and W. Allison, "Quasi-elastic helium-atom scattering from surfaces: experiment and interpretation", J. Phys.: Condens. Matter 14, 6173-6191 (2002). (Invited Review)
  2. A.P. Jardine, S. Dworski, P. Fouquet, G. Alexandrowicz, G.Y.H. Lee, D.J. Riley, J. Ellis, W. Allison, "Ultrahigh resolution spin-echo measurement of surface potential energy landscapes", Science 304, 1790-1793 (2004).
  3. G. Alexandrowicz, A.P. Jardine, P. Fouquet, S. Dworski, W. Allison and J. Ellis, "Observation of Microscopic CO Dynamics on Cu(001) using 3He Spin-Echo Spectroscopy", Phys. Rev. Lett. 93, 156103 (2004).
  4. G. Alexandrowicz, A.P. Jardine, H. Hedgeland, W. Allison and J. Ellis, " Onset of 3D Collective Surface Diffusion in the Prescence of Lateral Inter\ actions: Na/Cu(100) ", Phys. Rev. Lett. 97, 156103 (2006).
  5. G. Fratesi, G. Alexandrowicz, M.I. Trioni, G.P. Brivio, and W. Allison, "Crucial electronic contributions to measures of surface diffusion by He a\ tom scattering", Phys. Rev. B 77, 235444 (2008).
  6. A.P. Jardine, G. Alexandrowicz, H. Hedgeland, R.D. Diehl, W. Allison and J. Ellis, " Vibration and diffusion of Cs atoms on Cu(001)", J. Ph\ ys.: Condens. Matter 19, 305010 (2007).
  7. H. Hedgeland, P.R. Kole, H.R. Davies, A.P. Jardine, G. Alexandrowicz, W. Allison, J. Ellis, G. Fratesi and G.P. Brivio, "The surface dynamics and f\ riction of K/Cu(001) characterized by helium-3 spin-echo and density functional theory", submitted (2009).
  8. G. Alexandrowicz, P. Kole, E. Lee, H. Hedgeland, R. Ferrando, A.P. Jardine, W. Allison and J. Ellis, "Observation of uncorrelated microscopic motion \ in a strongly interacting adsorbate system ", J. Am. Chem. Soc. 130 (21)6789-6794 (2008).
  9. A.P. Jardine, H. Hedgeland, D. Ward, Y. Xioaqing, W. Allison, J. Ellis and G. Alexandrowicz, "Probing molecule-surface interactions through ultra-fas\ t adsorbate dynamics: Propane/Pt(111)", New J. Phys. 10, 125026 (2008).
  10. H. Hedgeland, P. Fouquet, A.P. Jardine, G. Alexandrowicz, W. Allison and J. Ellis, "Measurement of single-molecule frictional dissipation in a prototypical nanoscale system", Nature Phys., advanced online publication (2009).
  11. A.P. Jardine, G. Alexandrowicz, H. Hedgeland, W. Allison and J. Ellis,"Studying the microscopic nature of diffusion with helium-3 spin-echo", <\ b>Phys. Chem. Chem. Phys. 11, 3355 (2009).