Presentation on theme: "Adsorption and self-assembly of alkanethiols on GaAs (001) surface"— Presentation transcript:
1Adsorption and self-assembly of alkanethiols on GaAs (001) surface O. Voznyy, J.J. DubowskiDepartment of Electrical and Computer EngineeringResearch Center for Nanofabrication and NanocharacterizationUniversité de Sherbrooke, Sherbrooke, Québec J1K 2R1Canada
2Outline Motivation How thiol connects to GaAs Binding energy Influence of surface termination and reconstructionsPacking of thiols on GaAs(001) surfaceSummary
3MotivationVerify 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…
4Bonding nature S saturates As dangling bond Sulfur 3s and 3p orbitals do not hybridizeSteric repulsion of CH2 from the surface determines tilt angle and directionHAs dangling bonds create surface statesAdsorption of thiolate removes themCAsdangling bondH2.28 AAs 4pzAsShort bond and small charge transfer – strong covalent bondingGaO.Voznyy, J.J.Dubowski,J.Phys.Chem.B 110, iss.45 (2006)Molecular orbitals in 1 eV energy window below Fermi level.
5Physisorbed precursor Energy of S-H bond is 3.75 eVDue 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)AsAsSGa
6Adsorption energetics at low coverage Experimental desorption pathwaysN.Singh, D.Doran. Surf.Sci. 422 pp (1999)Thiolate binding energy is 2.3 eVHigher than 1.7 eV for thiols on gold and 2.03eV for thiols on copper.Hydrogen stays adsorbed nearbyIt can recombine and desorb as H2, thiol or alkane.
7Dependence 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 allH2 desorption is favorable or requires little energy
8Interactions between free thiols Distance and energy vs angleTilting and interlocking4.57 A5.41 A4.08 AUlman, Langmuir 5 (1989), p.1147MM2 force fieldtiltlean* In our calculations LDA was used to account for van der Waals interactions
9Surface constraintsOn GaAs(001) S-As bond predetermines the tilt angle and direction.Thiols cannot fit on both As atoms in dimer5.65 A
10SAM on surface5.5A51View along chainsNot densely packedTilt 51Tilt estimated from experiments ~57Densely packed thiols without surface - tilt 62At least 5-carbon chains needed for interaction between thiols
11SummaryAdsorption 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°.SupportCanadian Institutes for Health ResearchCanada Research Chair ProgramRéseau Québécois de Calcul de Haute Performance
12Optimized geometries at low coverage b)SAsGaGaAsOptimized 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.
13Low stickingSAsGaIf thiol is not physisorbed H and S cannot be both close to As at the same momentwhen H feels steric repulsion of the surface it easily rotates around S-C bond