Formation of an intermolecular charge-transfer compound in UHV codeposited tetramethoxypyrene and tetracyanoquinodimethane

K. Medjanik, S. Perkert, S. Naghavi, M. Rudloff, V. Solovyeva, D. Chercka, M. Huth, S. A. Nepijko, T. Methfessel, C. Felser, M. Baumgarten, K. Müllen, H. J. Elmers, and G. Schönhense

Phys. Rev. B 82, 245419 – Published 13 December 2010

Abstract

Ultrahigh vacuum (UHV)-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane $({TMP}_{1} {-TCNQ}_{1})$ on gold have been studied using ultraviolet photoelectron spectroscopy (UPS), x-ray diffraction (XRD), infrared (IR) spectroscopy, and scanning tunneling spectroscopy (STS). The formation of an intermolecular charge-transfer (CT) compound is evident from the appearance of new reflexes in XRD ( $d_{1} = 0.894 nm$ and $d_{2} = 0.677 nm$ ). A softening of the CN stretching vibration (redshift by $7 {cm}^{- 1}$ ) of TCNQ is visible in the IR spectra, being indicative of a CT on the order of $0.3 e$ from TMP to TCNQ in the complex. Characteristic shifts in the electronic level positions occur in UPS and STS that are in reasonable agreement with the prediction of density-functional theory (DFT) calculations (GAUSSIAN03 with hybrid functional B3LYP). STS reveals a highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of the CT complex of about 1.25 eV being much smaller than the gaps $(> 3.0 eV)$ of the pure moieties. The electron-injection and hole-injection barriers are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of the HOMOs determined by UPS and STS are discussed in terms of the different information content of the two methods.

Received 25 August 2010

DOI:https://doi.org/10.1103/PhysRevB.82.245419

Authors & Affiliations

K. Medjanik¹, S. Perkert¹, S. Naghavi², M. Rudloff³, V. Solovyeva³, D. Chercka⁴, M. Huth³, S. A. Nepijko¹, T. Methfessel¹, C. Felser², M. Baumgarten⁴, K. Müllen⁴, H. J. Elmers¹, and G. Schönhense^1,*

¹Institut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, D-55128 Mainz, Germany
²Institut für Analytische und Anorganische Chemie, Johannes Gutenberg-Universität, Staudingerweg 9, D-55128 Mainz, Germany
³Physikalisches Institut, Goethe-Universität, Max-von-Laue-Str. 1, D-60438 Frankfurt am Main, Germany
⁴Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany

^*FAX: +49-6131-3923807; schoenhe@uni-mainz.de

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Vol. 82, Iss. 24 — 15 December 2010

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Images

Figure 1
Plot of the (a) molecular structures, (b) AFM images, and (c) x-ray diffractograms of thin films of the mixed phase ${TMP}_{1} {-TCNQ}_{1}$ . Image (i) in (b) shows the coexistence of a smooth multilayer covered with 3D crystallites. Image (ii), taken at higher coverage, shows a thick 3D film with crystallite sizes in the micrometer range. The x-ray diffraction patterns (c) for TMP-TCNQ films grown on Si and Au/Si substrates reveal new diffraction peaks (marked by arrows) in comparison with the reference diffraction patterns for pure TMP and TCNQ, see text.Reuse & Permissions
Figure 2
IR spectra of the CN stretching vibration in a TCNQ film on Au and films of the mixed phase ${TMP}_{1} {-TCNQ}_{1}$ on Au at (i) low coverage and (ii) high coverage. In the mixed phase the vibration frequency is redshifted by $Δ でるた = 7 {cm}^{- 1}$ , being indicative of a CT on the order of $0.3 e$ .Reuse & Permissions
Figure 3
Coverage series of He I UPS spectra of (a) TMP and (b) TCNQ on Au. For comparison, spectra calculated on the basis of $Δ でるた SCF$ binding energies are included (top) that agree well with the measured spectra. In order to align the HOMO, the theoretical spectra have been rigidly shifted by 0.52 eV and 0.48 eV to lower binding energy for TMP and TCNQ, respectively. Full and dotted bars denote the binding energy positions of $π ぱい$ - and $σ しぐま$ -like orbitals, respectively. The insets show the corresponding work function-vs-coverage curves (coverage in monolayers, ML), numbers correspond to the spectra numbers. ${Δ でるた}_{h}$ denotes the hole-injection barriers.Reuse & Permissions
Figure 4
Coverage series of UPS spectra taken during the growth of the TMP-TCNQ mixed phase on Au. The inset shows the corresponding work function curve, numbers correspond to the spectra numbers. For comparison, a theoretical spectrum is shown (top), calculated on the basis of $Δ でるた SCF$ binding energies. In order to align the HOMO with the experimental signal A the theoretical spectrum has been rigidly shifted by 0.97 eV to higher binding energy.Reuse & Permissions
Figure 5
Differential conductivity $d I / d U$ spectrum measured at room temperature on an island of TMP-TCNQ, compared with a spectrum from the clean W(110) substrate. Data shown result from an average of individual spectra measured on homogeneous topographically elevated areas of the sample (for clarity, the TMP-TNCQ spectrum was shifted upwards by 0.2 arbitrary units). The sample voltage modulation was 50 mV and the tip was stabilized at $U = 0.7 V$ and $I = 0.35 nA$ . The inset shows a processed topographical STM image of the corresponding region where the spectra were measured. Full (dashed) arrows indicate monoatomic substrate (molecular complex) steps indicating an initial step flow growth mode. ${Δ でるた}_{h}$ and ${Δ でるた}_{e}$ denote the hole- and electron-injection barriers, respectively.Reuse & Permissions
Figure 6
Energy scheme of the TMP-TCNQ complex (center) in comparison with the pure phases of TCNQ (left) and TMP (right). In order to compare with theory, the binding energy scale is referenced to the vacuum level. Thick, thin, and dotted lines denote the experimental UPS, STS, and CV results, chain and dashed lines represent theoretical molecular-orbital eigenvalues and $Δ でるた SCF$ binding energies, respectively. ${Δ でるた}_{opt}$ is the experimental value (Ref. 21) for the optical gap of TCNQ, adapted at the STS LUMO level (Ref. 29). ${Δ でるた}_{exp}$ denote the gaps determined by STS for TMP and the complex.Reuse & Permissions

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