Ab-initio study of self-assembled monolayers of thiols on (001) GaAs

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. Motivation2. Model3. Thiol adsorption at low coverage4. Formation of self-assembled monolayer5. Summary

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• 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

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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 Å2) with dimerized As

• Different exchange-correlation functionals were tested

Our test results:

SIESTA

LDA PBE BLYP RPBE Expt

S-H length, Å 1.361 1.359 1.360 1.359 1.35

E(S-H), eV 4.27 3.78 3.75 3.7 3.73

H-H length, Å 0.768 0.752 0.748 0.749 0.742

E(H-H), eV 4.935 4.567 4.769 4.609 4.75 (no ZPE)

a(GaAs), Å 5.6 5.75 5.9 5.8 5.65Unit cell used in calculations

As

Ga

SC

As

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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.

• 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

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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 3px

As dangling bond

As 4pz

• Top layer As 4p orbitals dehybridize and create states close to valence band maximum.

HC

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

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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 doesn’t 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

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Passivation effect of thiol

-14 -12 -10 -8 -6 -4 -2 00,0

0,2

0,4

0,6

0,8

1,0

surf

ace

stat

es

dehy

brid

izat

ion

of p

z

stat

es in

the

gapSlab total

As 4sAs 4p

x

As 4py

As 4pz

Astop

(non-saturated)

PD

OS

E, eV

-14 -12 -10 -8 -6 -4 -2 00,0

0,2

0,4

0,6

0,8

1,0

surf

ace

stat

es

mix

ing

with

S o

rbita

ls

hybr

idiz

atio

n of

p o

rbita

ls

elim

inat

ion

of s

tate

s ne

ar b

andg

ap

Slab totalAs 4sAs 4p

x

As 4py

As 4pz

Astop19

(bonded to S)

PD

OS

E, eV

As 4pz

Molecular orbitals in 1 eV energy window above Fermi level.

9-2

-1

0

1

2

3

4

thiolate+H/surface

desorption pathways observed experimentally

S/surf+C

5H

12

S/surf+H/surf+C

5H

11

thiolate/surf+1/2H

2

thiolate/surf+H

thiolate/surf+H/surface

thiolate+H+surface

E, e

V

thiol+surface

Eb=2.1 eV

adsorption

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.50-64. (1999)

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Interactions between free thiolsLDA reasonably reproduces van der Waals interactions between chains (energy and optimal distance) while GGA scheme can’t reproduce attraction at all.

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

E(4.5Å), kcal/mol

doptimal,

Å

MM2 (Ulman)

4.8 4.24

LDA 7 4.4

PBE -3 -

RPBE -10 -

BLYP -7 -

Expt 4.57

Our

dat

a

11

SAM on surface

5.5A

51 Not densely packedTilt 51

Experimental tilt ~57

Densely packed thiols without surface - tilt 62

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Summary1. As-S bond is highly covalent.2. Adsorption geometry is dictated by direction of As dangling bond, S

3p orbital and first CH2 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 1eV5. 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 thiol8. Calculated tilt angle of densely packed monolayer of free thiols is 62°

(experimentally observed value for thiols on GaAs is 57° ± 3°)

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