Metal-Organic Heterojunction: Thiophenthiolate Chemisorbed on Au(111)

The metal-organic interface plays a critical role since it determines, to a large extent, the properties and performances of devices. Understanding the energetic of metal/molecular film interfaces (metal work function, ionization potential and electronic affinities) is the key to control the behavior of the devices and to suggest the chemical modification of organic molecules required to lower the energy barrier for charge injection and increase the device performances.

In general, various self assembled monolayers (SAMs) formed by oligo- and poly-thiophenes are widely used as organic semiconductor since they have shown advantageous physical properties for organic field effect transistor (OFET) applications.

In collaboration with Dr. Andrea di Matteo we evaluated the interaction between a simple thiophene unit chemisorbed (by thiol derivate) or physisorbed on Au(111) layer and to monitor the shift of work function (WF) associated to adsorption process. We explored, using density functional theory (DFT), the role played by different contributions to the surface dipole of thiophene and thiophenethiolate on gold. The computational scheme is based on a pseudopotential planewave method using PWSCF code as implemented in the QUANTUM-ESPRESSO package.

Thiophenthiolate chemisorbed over gold (111) surface and tilting angle theta.

We have calculated the optimized geometries for the case of chemisorbed systems where the S atom in the starting configuration was constrained to lie above 4 sites that have been chosen to be representative chemical anchoring. These sites were the on-top, bridge, fcc-hollow (S directly above a 3 fold hollow site with no atom in the second layer just below the site) and hcp-hollow (S directly above a 3 fold hollow site with an atom in the second layer just below the site). We have found that not all of these sites are stable for anchoring and in particular we will see that the hollow sites are moved toward the bridge site.

Binding sites.

Surface Dipole

The behavior of the WF can be partly correlated to theta tilting angles. A large tilt angle gives a small normal molecular dipole component and small shift of WF with respect to the clean Au(111). In addition, the nature of the bonding between the molecules and the surface, that is different for the several adsorption sites, contributes to the WF shift. Indeed it is easy to show that the change of the WF, Delta_W (given by the difference between the WF of the clean gold and that of the gold monolayer-terminated), is related to the difference between the dipoles formed on the gold monolayer-terminated surface and the clean gold surface, dAu-SAM and dAu respectively, by means of the relation (in atomic units):

Delta_W= 4pi/A∙(dAu-SAM,z – dAu,z)     (1)

where dz is the projection of d in the z direction and A is the gold surface area in the unit cell.

In the same way we can write the relation connecting the energy step Delta_ESAM along the z direction, perpendicular to the metal slab, to the dipole dmol of the isolated molecule in the SAM (we removed the gold atoms):

Delta_ESAM = 4pi/A∙dmol,z      (2)

where dmol,z is given by:

dmol,z = dmol∙cos(theta)         (3)

with theta tilting angle.

If we set Delta_d = dAu-SAMdAu , we can define the following dipole:

dchem = Delta_ddmol         (4)

and exploiting the proportionality relations (1) and (2) we can write also

Delta_Echem = Delta_W – Delta_ESAM  (5).

The dipole dchem and the corresponding energy step Delta_Echem can be interpreted as the dipole and the energy step associated to the charge reorganization due to the formation of chemical bonds between the metal surface and the adsorbate molecule.

Starting from the above mentioned sites as starting positions we found one top and three bridge optimized geometries for chemisorbtion. The bridge sites are different for the tilting angle. Also physisorbtion in top and bridge sites has been investigated. The results are resumed in the following table:

System

WFa

DEchema

Phys. top

4.55

0.629

Phys. bridge

4.50

0.521

Chem. top

4.84

-0.194

Chem. bridge-2

4.58

-0.108

Chem. bridge

4.59

-0.234

Chem. bridge-3

4.55

-0.323

a) Energy is measured in eV; Phys. = physisorbtion, Chem. = chemisorbtion.

In conclusion by calculating Delta_W and Delta_ESAM  we have, from the equation (5), the energy associated to the charge reorganization on the slab and we can quantify how much the slab is affected from the organic/inorganic interaction.

The full article “Energetic of molecular interface at metal-organic heterojunction: the case of thiophenethiolate chemisorbed on Au(111) ” can be found on Theoretical Chemistry Accounts 124, 217 (2009).

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