I. Tranca and K. Schroeder
Institut für Festkörperforschung, Forschungszentrum Jülich GmbH, Jülich, Germany
2,5 PDA molecule adsorbed on Cu(011) surface 2,5 PDA molecule adsorbed on Cu(011) surface 2,5 PDA molecule in vacuum
2,5 PDA molecule in vacuum
Objectives of the study:
Objectives of the study:
Conformations of adsorbed molecules: which type of further reactions at the vacuum exposed COOH group is possible
Identification of physical properties relevant for electronic devices: work function, electron injection barrier (EIB), hole injection barrier (HIB)
Experimental evidences for different conformations: work function, infrared (IR) spectrum
Infrared spectrum Infrared spectrum
W
e have identified the vibrational pattern. symmetry determines frequency degeneracy and IR intensities
IR spectroscopy can be used to distinguish between various
conformers
DFT study of 2,5 pyrazine di-carboxylic acid (PDA) in DFT study of 2,5 pyrazine di-carboxylic acid (PDA) in
vacuum and adsorbed on Cu(011) surface vacuum and adsorbed on Cu(011) surface
Schottky barriers Schottky barriers
HIB (Hole injection barrier) ~ 1.00 eVEIB (Electron injection barrier) ~ 1.5 eV
at its surface-molecule interface the Cu- PDA system behaves as a p-type organic semiconductor. Good candidate as anode in an organic-based (opto)-electronicdevice, e.g. OLED, OFET.
Reflexion Absorption Reflexion Absorption
Infrared Spectrum (RAIRS) Infrared Spectrum (RAIRS)
I I
C
hange of frequency and infra-red intensities upon adsorption due to the lowering of the symmetry and molecule- molecule interactions.
Non-planar adsorbed PDA with the H atom (of COOH) down- or up- oriented found. H atom rotation yields work function variation of 2.6 eV.
With a small HIB and with HIB < EIB, Cu-PDA is a p-type organic semiconductor.
Expect distinction of H-up from H-down structures in RAIRS and STM.
Conclusions:
Conclusions:
Work function variation Work function variation
The work function of the Cu-PDA system increases by 2.6 eV when the H atom belonging to the vacuum exposed COOH group rotates from up to down (and the oxygen lone pairs are displaced). Important for the in-situ variation of the system work function. A possible alternative to atom substitution approach.
Φ
tot=Φ
Cu+Φ
SAM+Φ
interface (Left) work function variation with the adsorbed PDA conformer; (Middle) charge difference variation with the adsorbed PDA conformer; (Right) 3D representation
of the charge difference for the most stable adsorbed structure.
10 possible vacuum conformers. The four most stable are given below.
0.000 eV 0.109 eV 0.162 eV 0.230 eV
All conformers are planar.
Intra-molecular hydrogen bonds (OH···N, OH···O=C, CH···O=C) could appear.Conformers Conformers
Electron Localization Function (ELF) Electron Localization Function (ELF)
U
sed to obtain a visually simple understanding of the chemical bonds present, of the lone pair distribution and of the existing hybridization.3D ELF plots. Offer a clear view of the O and N lone pairs, on the chemical bonds present. Helpful
in determining the hybridization type.
2D ELF plot. Used to probe the presence of the OH···N
hydrogen bond.
3D charge density plot.
No lone pairs visible, no peak at the bonds.
Possible configurations Possible configurations
20 possible adsorbed configurations tested. 2 of the most stable are given below. Most s
table configurations (with Eads ~ -2.96 eV) have non-planar PDA radicals.
Inter-molecular hydrogen bonds OH···O=C replace -for H-down PDA structures- the intra- molecular OH···N hydrogen bonds.H-down configuration H-down configuration. Unit cell (2 x 1) in the black rectangle. The upper COOH
groups are aligned along the [011]-
direction.
H-up configuration. The upper COOH groups are aligned along the diagonal of the unit cell.
HOOC−COOH HOOC−C6H 4−COOH HOOC−C5 H 3 N 1−COOH HOOC−C 4 H 2 N 2−COOH
HOMO-LUMO gap HOMO-LUMO gap
The N atoms from the aromatic ring influence the HOMO-LUMO gap of the molecule. The more the N atoms in the aromatic ring the smaller the HOMO-LUMO gap. Consequently, N hetero-atomic aromatic structures are good models for basic studies on electron transfer phenomena as well as potential candidates for molecular opto-electronic applications.~ 3.5 eV ~ 3.0 eV
~ 3.5 eV ~ 2.5 eV
HOMO-LUMO gap for related molecules.
H – light gray O – orange C – dark blue N – light blue
N – light blue
