1. Adsorption and self-assembly of alkanethiols on GaAs (001) surface
O. Voznyy, J.J. Dubowski
Department of Electrical and Computer Engineering
Research Center for Nanofabrication and Nanocharacterization
Université de Sherbrooke, Sherbrooke, Québec J1K 2R1
Canada

2. Outline Motivation How thiol connects to GaAs Binding energy
Influence of surface termination and reconstructions
Packing of thiols on GaAs(001) surface
Summary

3. Motivation
Verify the robustness of the GaAs-thiol interface - the weakest link in our biosensor architecture.
Understand the atomic level processes to improve our thiol deposition technique.
Due to semiconductor-specific properties, very little information from thiols on metals can be used.
Fundamental interest in self-assembly process.
Potential applications: passivation of GaAs surfaces, transition layers for Schottky diodes, nanolithography…

4. Bonding nature S saturates As dangling bond
Sulfur 3s and 3p orbitals do not hybridize
Steric repulsion of CH2 from the surface determines tilt angle and direction H
As dangling bonds create surface states
Adsorption of thiolate removes them C As
dangling bond H
2.28 A
As 4pz As
Short bond and small charge transfer – strong covalent bonding Ga
O.Voznyy, J.J.Dubowski,
J.Phys.Chem.B 110, iss.45 (2006)
Molecular orbitals in 1 eV energy window below Fermi level.

5. Physisorbed precursor
Energy of S-H bond is 3.75 eV
Due to thermal vibrations S and H can appear close to surface atoms and bind to them, which requires only ~0.4eV (thermal energy at ~150K) As As S Ga

6. Adsorption energetics at low coverage
Experimental desorption pathways
N.Singh, D.Doran. Surf.Sci. 422 pp (1999)
Thiolate binding energy is 2.3 eV
Higher than 1.7 eV for thiols on gold and 2.03eV for thiols on copper.
Hydrogen stays adsorbed nearby
It can recombine and desorb as H2, thiol or alkane.

7. Dependence on surface reconstruction
Binding energy depends strongly on surface termination, reconstruction, thiol coverage and H site (electron counting rules)
In some cases thiol adsorption is not favorable at all
H2 desorption is favorable or requires little energy

8. Interactions between free thiols
Distance and energy vs angle
Tilting and interlocking
4.57 A
5.41 A
4.08 A
Ulman, Langmuir 5 (1989), p.1147
MM2 force field
tilt
lean
* In our calculations LDA was used to account for van der Waals interactions

9. Surface constraints
On GaAs(001) S-As bond predetermines the tilt angle and direction.
Thiols cannot fit on both As atoms in dimer
5.65 A

10. SAM on surface
5.5A
51
View along chains
Not densely packed
Tilt 51
Tilt estimated from experiments ~57
Densely packed thiols without surface - tilt 62
At least 5-carbon chains needed for interaction between thiols

11. Summary
Adsorption geometry of thiol is dictated by direction of As dangling bond, sulphur 3p orbital and first CH2 unit repulsion from the surface.
Binding of thiolate to GaAs is comparable or stronger than that of thiols on metal surfaces.
Binding energy is generally higher for Ga-rich surface (up to 2.8 eV) and depends strongly on reconstruction and coverage.
For some As-rich reconstructions thiol adsorption is not possible at all.
Hydrogen on surface reduces the energy for thiol desorption to ~1eV.
Thiols cannot fit on every surface As (or Ga), this requires longer chains for ordering and leads to tilt ~57°.
Support
Canadian Institutes for Health Research
Canada Research Chair Program
Réseau Québécois de Calcul de Haute Performance

12. Optimized geometries at low coverageb) S As Ga Ga As
Optimized geometries of pentanethiol on As-rich GaAs (001) surface obtained from relaxation of
(a) thiolate lying flat to the surface and (b) thiolate standing upright.

13. Low sticking S As Ga
If thiol is not physisorbed H and S cannot be both close to As at the same moment
when H feels steric repulsion of the surface it easily rotates around S-C bond