HAXPES of novel charge transfer compounds K. Medjanik 1 , A. - - PowerPoint PPT Presentation

HAXPES of novel charge transfer compounds

• K. Medjanik1, A. Gloskovskii2, D. Chercka3,
• M. Baumgarten3, K. Müllen3 and G. Schönhense1

(1) Inst. f. Physik, Johannes Gutenberg Universität, SFB/TR49, D-55099 Mainz, Germany (2) Inst. f. Inorganic and Analytical Chemistry, Johannes Gutenberg Universität, D-55099 Mainz, Germany (3) Max-Planck Institute for Polymer Research, D-55021 Mainz, Germany

Outline

• 1. Introduction
• 2. Behaviour at metal-organic interface
• 3. HAXPES results on charge transfer salts

Functionalized polycyclic aromatic hydrocarbons (PAHs) this talk Organic (super-) conductors (kationic radical salts) Poster # 35

• 4. Summary and Conclusions

Motivation

• Search for novel charge transfer (CT) complexes with

designed donor and acceptor molecules

• Explore UHV-codeposition for the production of compounds

that cannot be grown from solution

• Systematic study of changes of the electronic structure upon

formation of CT complexes

• First HAXPES-results for such CT compounds

Electron acceptor Electron donor TMP TCNQ

d q

Charge transfer complexes

The total energy per donor-acceptor pair E (d q)= (ED

ion - EA aff)d q - a (e2/a)(d q)2

HMP-TCNQ

d q

TMP C20H18O4 TCNQ C24H12N4

Optical microscope AFM

Dark-looking crystallites: higher absorption in the visible

10 mm

• K. Medjanik et al., Phys. Rev. B 82 (2010) 245419

UHV co-deposited thin films of TMP-TCNQ-complex

UHV co-deposited thin films of TMP-TCNQ-complex

d q

Tetramethoxy

• Pyrene

TMP C20H18O4 TCNQ C24H12N4

• K. Medjanik et al., Phys. Rev. B 82 (2010) 245419

TMP+ TMP0

HAXPES on thin films of TMP-TCNQ-complex

e-

Oxygen 1s HAXPES core level spectra for a co-deposited thin film of the CT complex TMP-TCNQ grown in UHV (a), together with the spectrum of a thin film of pure donor material TMP (b).

O1s

Neutral donor in complex Ionic donor

+ d q

Ionic acceptor

• d q

Mixed-stack arrangement (from XRD) No band-formation but local dimers, unlike k-systems (Poster #35) Neutral acceptor in complex

TMP+ TMP0

Pure TMP: 26% Relative abundance

• f ionic TMP in complex:

61%

C D B B + +

C D C +

TMP+ TMP0

HAXPES on thin films of TMP-TCNQ-complex

Oxygen 1s HAXPES core level spectra for a co-deposited thin film of the CT complex TMP-TCNQ grown in UHV (a), together with the spectrum of a thin film of pure donor material TMP (b).

e-

O1s

Optical microscopy

1mm

d q

CT complex TCNQ HMP Microcrystals of same complexes grown from solution

• CT-complexes HMP - TCNQ (HMPx-TCNQy, X:Y=1:1, 1:2, 2:1)
• Grown via vapour diffusion

TCNQ C24H12N4 HMP C22H22O6

e- HMP O1s

C D B B + + C D C +

Pure HMP: 20% 42% 73% Relative abundance

• f ionic HMP in complex:

64% 63% 34% HMP+ Oxygen 1s HAXPES core level spectra for three microcrystal fractions of the CT complex HMP-TCNQ grown in solutions with stoichiometries HMPx:TCNQy (X:Y= 1:2, 2:1, 1:1) together with the spectrum of pure donor material HMP. HMP0

Summary and Conclusions

• HAXPES core level spectra avoid the problem of surface

preparation for solution-grown organic crystals.

• Oxygen 1s signal provides a local spectroscopic probe at donor

(Oxygen is only contained in the methoxy group).

• The degree of charge transfer in HMPx-TCNQy (x:y=1:1, 1:2, 2:1)

microcrystals and TMP-TCNQ thin films is about 0.65.

Institut für Physik, Johannes Gutenberg- Universität, Dmytro Kutnyakhov, Sergej Nepijko, Hans-Joachim Elmers Institut für Analytische und Anorganische Chemie, J. Gutenberg- Universität (theory) Shahab Naghavi, Claudia Felser

Thanks:

In the framework of Transregio SFB TR49 (Frankfurt, Mainz, Kaiserslautern) And Graduate School of Excellence MAINZ DESY, Hamburg Wolfgang Drube, Sebastian Thiess, Heiko Schulz-Ritter

Thank you!

Dependence of the HOMO and LUMO positions and the gap on the intermolecular distance for HMP and TCNQ

• sandwiched in different local configurations
• dashed line denotes the interlayer spacing in HMP-TCNQ as determined from X-ray diffraction
• Gaussian 03 with B3LYB hybrid functional

N 1s region high-resolution X-ray photoelectron spectra of TCNQ.

TCNQ C24H12N4

John M. Lindquistt and John C. Hemminger, J. Phys. Chem. 92 (1988) 1394-1396.

Pure TCNQ: 40% 22% Nitrogen 1s HAXPES core level spectra for two fractions of solution- grown microcrystals of the CT complexes HMPx:TCNQy (X:Y= 1:2, 1:1) (a,b) together with the spectrum of the pure acceptor material TCNQ (c). TCNQ- TCNQ0 Relative abundance

• f ionic TCNQ in

complex: 33% 69% e- N1s

Organics-based flexible display Organic LEDs

Figure Energy scheme of the photoemission transitions observed . i > and f > denote the initial states and the final states.

Top: CN stretching vibration in a TCNQ film on Au Bottom: Same for the mixed phase TCNQ - TMP on Au at low coverage Centre: Same at higher coverage (evidence of coexistence with pure TCNQ phase) CN vibration

{3] D. L. Harms, Anal. Chem., 1953, 25 (8), 1140-1155 [4] X. Chi et al., Chem. Mater., 2004, 16, 5751

Formation of the complex softens the CN – vibration Mode [3,4].

IR-spectra

• K. Medjanik et. al., Phys. Rev. B 82 (2010) in print

7cm-1 red shift