Ab-initio study of self-assembled monolayers of thiols on (001) GaAs 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 1.Motivation 2.Model 3.Thiol adsorption at low coverage 4.Formation of self-assembled monolayer 5.Summary

3 Self-assembled monolayers (SAMs) of organosulfur compounds on solid surfaces attract a lot of interest from both fundamental perspective and their potential applications: development of precursors for the growth of II-VI materials, creation of transition layers for ohmic contacts and Schottky diodes, passivation of GaAs surfaces, nanolithography, electrochemical applications and biosensing. In contrast to alkanethiols on gold there are no theoretical studies of thiols on GaAs. Motivation

4 Model Calculations were done using a density functional theory (DFT) approach based on pseudopotentials and numerical localized atomic orbitals as basis sets (4x2) surface unit cell (16x8 Å 2 ) with dimerized As Different exchange-correlation functionals were tested Our test results: LDAPBEBLYPRPBEExpt S-H length, Å E(S-H), eV H-H length, Å E(H-H), eV (no ZPE) a(GaAs), Å Unit cell used in calculations As Ga S C As

5 Optimized geometries at low coverage 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. Strong dependence of total energy on As-S-C angle. Bridge and hollow site positions are not favorable. a) b) Top view Side view As Ga S

6 Molecular orbitals Molecular orbitals in 1 eV energy window below Fermi level. Sulfur 3s and 3p orbitals do not hybridize forcing C-S-As angle to be close to 90º. S 3p x As dangling bond As 4p z Top layer As 4p orbitals dehybridize and create states close to valence band maximum. H C H Steric repulsion of the first CH2 unit from the surface prevents this and forces the tilt in the direction of hollow between As As

7 Bonding nature Regions of loss (light blue) and gain (red) of electron density induced by adsorption of thiolate on surface. Isodensity surfaces correspond to ±0.006 a.u. Only electrons around sulfur and S-C bond are involved in bonding. Thus, binding energy doesnt depend on chain length. Very small charge transfer of 0.05e from thiolate to surface, in comparison with transfer to thiol of 0.4e from gold surface and 0.7e from copper. Shorter S-As bond length (2.28 Å in comparison with 2.5 Å for S-Au and 2.31Å for S-Cu) All factors indicate a highly covalent nature of the bonding and stronger binding of thiolate to GaAs than to metal surfaces. As S Ga Loss of electrons from As dangling bond Formation of covalent bond Redistribution around S As

8 Passivation effect of thiol As 4p z Molecular orbitals in 1 eV energy window above Fermi level.

9 Adsorption energetics at low coverage Calculated binding energy of 2.1 eV is bigger than 1.7 eV for thiols on gold and 2.03 eV for thiols on copper. Hydrogen stays adsorbed on the adjacent As near adsorbed thiolate. At high temperatures hydrogen participates in recombinative desorption with creation of molecular hydrogen, thiol and penthane. N.Singh, D.Doran. Surf.Sci. 422 pp (1999)

10 Interactions between free thiols LDA reasonably reproduces van der Waals interactions between chains (energy and optimal distance) while GGA scheme cant reproduce attraction at all. Ulman, Langmuir 5 (1989), p.1147 MM2 force field E(4.5Å), kcal/mol d optimal, Å MM2 (Ulman) LDA74.4 PBE-3- RPBE-10- BLYP-7- Expt4.57 Our data

11 SAM on surface 5.5A 51 Not densely packed Tilt 51 Experimental tilt ~57 Densely packed thiols without surface - tilt 62

12 Summary 1.As-S bond is highly covalent. 2.Adsorption geometry is dictated by direction of As dangling bond, S 3p orbital and first CH 2 unit repulsion from the surface. 3.Thiol molecule cannot lie flat on the surface once it has been chemisorbed. 4.Barrier for thiol diffusion on the surface is about 1eV 5.Thiol passivates GaAs surface (removes surface states from the bandgap of GaAs). 6.Binding energy is 2.1eV and is stronger than that of thiols on metal surfaces. 7.Hydrogen plays important role in adsorption/desorption of thiol 8.Calculated tilt angle of densely packed monolayer of free thiols is 62° (experimentally observed value for thiols on GaAs is 57° ± 3°) Support Canadian Institutes for Health Research Canada Research Chair Program Réseau Québécois de Calcul de Haute Performance