Adsorption and self-assembly of alkanethiols on GaAs (001) surface

O. Voznyy, J.J. DubowskiDepartment of Electrical and Computer Engineering

Research Center for Nanofabrication and Nanocharacterization Université de Sherbrooke, Sherbrooke, Québec J1K 2R1

Canada

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Outline1. Motivation

2. How thiol connects to GaAs

3. Binding energy

4. Influence of surface termination and reconstructions

5. Packing of thiols on GaAs(001) surface

6. Summary

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• 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…

Motivation

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Bonding nature

Molecular orbitals in 1 eV energy window below Fermi level.

• S saturates As dangling bond• Sulfur 3s and 3p orbitals do not hybridize

As dangling bond

As 4pz

• As dangling bonds create surface states

• Adsorption of thiolate removes them

HC

H

• Steric repulsion of CH2 from the surface determines tilt angle and direction

As

Ga

• Short bond and small charge transfer – strong covalent bonding

2.28

AO.Voznyy, J.J.Dubowski,J.Phys.Chem.B 110, iss.45 (2006)

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S

As

Ga

As

• 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)

Physisorbed precursor

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-2

-1

0

1

2

3

4

freethiol

thio

late

H

chemisorption

alk

yl (

cle

ave

S-C

)

alk

an

e

(re

co

mb

ine

with

H)

H2

thio

late

(cle

ave

S-A

s)

cle

ave

S-H

E,

eV

As-rich (2x1)

physisorption

th

iol

(re

co

mb

ine

with

H)

Adsorption energetics at low coverage

• 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.

Experimental desorption pathwaysN.Singh, D.Doran. Surf.Sci. 422 pp.50-64. (1999)

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• 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

Dependence on surface reconstruction

-2

-1

0

1

2

3

4

Eb=

2.8

eV

H2 desorption

chemisorption

thio

l

Ga-rich (4x2)

E, e

V

thio

late

H

H

thio

late

thio

late

-2

-1

0

1

2

3

4

free thiol

H

thio

late

thio

late

H2

deso

rptio

nth

iola

te c

hem

isor

ptio

n

chemisorption

As-rich (2x4)

E,

eV

H

thio

late

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Interactions between free thiols

* In our calculations LDA was used to account for van der Waals interactions

Ulman, Langmuir 5 (1989), p.1147MM2 force field

Distance and energy vs angle Tilting and interlocking

4.57 A

5.41 A 4.08 A

tilt lean

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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

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SAM on surface

5.5A

51Not densely packedTilt 51

Tilt estimated from experiments ~57

Densely packed thiols without surface - tilt 62

View along chains

At least 5-carbon chains needed for interaction between thiols

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Summary1. Adsorption geometry of thiol is dictated by direction of As dangling

bond, sulphur 3p orbital and first CH2 unit repulsion from the surface.

2. Binding of thiolate to GaAs is comparable or stronger than that of thiols on metal surfaces.

3. Binding energy is generally higher for Ga-rich surface (up to 2.8 eV) and depends strongly on reconstruction and coverage.

4. For some As-rich reconstructions thiol adsorption is not possible at all.

5. Hydrogen on surface reduces the energy for thiol desorption to ~1eV.

6. Thiols cannot fit on every surface As (or Ga), this requires longer chains for ordering and leads to tilt ~57°.

SupportCanadian Institutes for Health Research Canada Research Chair ProgramRéseau Québécois de Calcul de Haute Performance

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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.

a) b)

As

Ga

S

Ga

As

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Low sticking

• 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

Ga

As

S