Ag(111) Ultrathin Films

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Structural and Vibrational Properties of CuPc/Ag(111) Ultrathin Films Sebastian Thussing, and Peter Jakob J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02837 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 22, 2016

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The Journal of Physical Chemistry

Structural and Vibrational Properties of CuPc/Ag(111) Ultrathin Films Sebastian Thussing and Peter Jakob∗ Fachbereich Physik und Wissenschaftliches Zentrum f¨ ur Materialwissenschaften der Philipps-Universität Marburg, Renthof 5, D-35032 Marburg, Germany E-mail: [email protected] Abstract The initial stages of copper-II-phthalocyanine (CuPc) thin film growth on Ag(111) have been investigated using Fourier Transform Infrared Absorption Spectroscopy (FTIRAS), Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) and Thermal Desorption Spectroscopy (TDS). Starting at (sub-)monolayer coverages up to 5 monolayers (ML) a number of ordered overlayers are found. Vibrational spectroscopy shows characteristic spectroscopic signatures for the individual submonolayer phases, as well as for the bilayer, trilayer and multilayers. Highly asymmetric line shapes of the in-plane vibrational modes of submonolayer CuPc provide unequivocal evidence for interfacial dynamical charge transfer between the metal electronic states and CuPc molecular orbitals, indicative for a partially filled LUMO at the Fermi energy as well as the prevalence of severe non-adiabaticity in the electron-vibron coupling. Growth of the second and third CuPc layers proceeds in a layer by layer fashion (Frank van der Merwe growth). Higher layers deposited at 300 K, on the other hand, transform into 3D crystallites on top of the CuPc trilayer upon annealing. For CuPc/Ag(111) monolayers thermal desorption spectra reveal intact CuPc desorption for coverages above 0.9 ML. At lower coverages an alternative reaction path involving partial dissociation

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of the CuPc molecules is found, with 14 Pc as the desorbing stable product species. In this study IR absorption spectra have been obtained at an exceptionally high spectral resolution of 0.5 cm−1 , which allows a spectral discrimination of molecular species with unprecedented detail. Specifically, CuPc mono-, bi-, and tri-layers, as well as the bulk-like crystalline phase of CuPc have been discriminated based on clearly resolved, closely spaced vibrational bands.

Introduction The growth of organic thin films on metal substrates has become a fascinating field of research in the last decades. The operation and functioning of novel organic devices is largely determined by the structural arrangement (→ molecular orientation, bonding mechanism) and growth behavior of the various organic compounds. Thereby, the geometric and electronic structure of deposited thin films, as well as their thermal stability represent key factors determining the performance of organic devices and which may be controlled by careful adjustment of growth conditions. Due to their favorable electronic properties aromatic molecules are commonly used in organic devices such as solar cells, 1 gas sensors, 2 organic field effect transistors 3 and organic light emitting diodes. 4 Especially, the interface between the metal surface and the first molecular layer represents a crucial section for their proper functioning. A detailed understanding of such interfaces is essential for tayloring and tuning organic films for future applications. 5 Enhanced efficiencies in photovoltaic cells by combining donor-acceptor type molecules 6 underline the need and prospects of investigations in this field of research to improve our understanding of organic metal interfaces. Metalphthalocyanines (MePc) and especially CuPc represent widely used donor type of molecules in such hetero-organic systems. Their high thermal and chemical stability further illustrate their relevance in organic electronics. Ultrathin CuPc molecular layers have been investigated in the past using SPA-LEED, Normal Incidence X-ray Standing Wave Absorption (NIXSW), Ultraviolet Photoemission 2 ACS Paragon Plus Environment

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Spectroscopy (UPS), Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) calculations. 7–9 IRAS and X-Ray Diffraction (XRD) data, on the other hand, are available for CuPc thin films in excess of 11 nm only. 10 Specifically, the 11 nm CuPc films display a parallel orientation with respect to the Ag(111) surface and the inner structure of the CuPc crystalline phase corresponds to the α phase of CuPc. 10 A comprehensive analysis and discussion of this particular polymorph of CuPc has recently been presented by Hoshino et al. 11 According to SPA-LEED observations the arrangement of CuPc on Ag(111) is governed by repulsive intermolecular interactions leading to a 2D-gas phase at low coverages (ΘCuP c ≤ 0.7 ML). 7 Such behavior is commonly observed for many MePc on (111) noble metal surfaces 12 and is in contrast to the attractive intermolecular interactions and island growth behavior, e.g. of NTCDA and PTCDA. For CuPc the intermolecular repulsion at submonolayer coverages has been associated with the net charge transfer between CuPc and Ag(111) which is only insufficiently counteracted by the weak (attractive) van der Waals forces; for NTCDA and PTCDA strong electrostatic quadrupole moments are held responsible for the distinct tendency towards formation of ordered 2D islands. 13,14 In addition, H-bonding between neighboring molecules contribute to the intermolecular attraction at short distances. 15 For more densely packed CuPc/Ag(111) layers a commensurate phase (0.75 - 0.9 ML) and a point-on-line (p.o.l.) phase (0.9 - 1 ML) is formed. 7 NIXSW results show only minor molecular distortions within the first CuPc/Ag(111) monolayer. Upon increasing the coverage (0.5 → 1.0 ML) a slight increase in adsorption height by about 0.1 ˚ A has been reported. 7 This finding might indicate a reduction in adsorption energy as a result of repulsive intermolecular forces close to saturation of the monolayer. In UPS data an electronic band has been detected at EB = -0.15 eV, that is, slightly below the Fermi energy. 7 This partially filled band has been ascribed to the CuPc lowest unoccupied molecular orbital (LUMO), shifted downward in energy upon adsorption, indicating a non-negligible chemical



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interaction of CuPc on Ag(111), in accordance with very recent theoretical calculations. 9 In this study we present results on the initial growth of CuPc layers on Ag(111) in the 0.3 - 5 ML coverage range. Fourier-transform infrared absorption spectroscopy (FT-IRAS) has been used to derive molecular orientations of (planar) adsorbates with respect to the substrate surface as well as to identify intramolecular distortions due to the interaction with the substrate. Due to the fingerprint character of vibrational modes identification of different molecular species is straightforward. Discrimination of different layers or phases of the same species, e.g. of bilayer, trilayer or multilayer species, on the other hand, is more demanding; in order to detect the associated vibrational lineshifts of only a few wavenumbers, an unusually high spectral resolution (0.5 - 2 cm−1 ) was selected. The high penetration depth of IR light for organic films along with its non-destructiveness thereby facilitates the probing of buried layers which are inaccessible to truly surface sensitive techniques. Spot-profile-analysis low energy electron diffraction (SPA-LEED) data have been obtained in parallel to IRAS, first, to decide on the structural arrangement of the grown layers, and second, to allow for a direct comparison to previous studies in the literature. Adsorbate binding energies and thermal stability of the layers as well as dissociation of CuPc on Ag(111) has been examined using thermal desorption spectroscopy (TDS).

Experimental All experiments were performed in an ultrahigh vacuum (UHV) chamber at a base pressure of P = 5 × 10−11 mbar. The sample was a Ag single crystal (5N purity) oriented in the (111) direction, with 10 mm in diameter and a thickness of 2 mm, and mounted to a liquid N2 or He cooled cryostat. Temperature measurements were performed using a K-type thermocouple welded at the edge of the Ag(111) sample. Sample annealing was achieved by direct current heating using a computer controlled power supply to ensure linear heating rates. The fourier transform infrared absorption spectrometer was a Bruker IFS 66v/s with evacuable optics.


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A liquid N2 cooled Mercury-Cadmium-Telluride (HgCdTe) detector was used, allowing for measurements in the spectral range of 600 - 5000 cm−1 . Polarized IR radiation is produced by a water cooled black body source (Globar) in conjunction with a wire grid polarizer on a KRS5 substrate. Infrared spectra were taken at a spectral resolution of 0.5 - 2 cm−1 with typically 1000 - 2000 scans coadded. SPA-LEED measurements have been carried out using an Omicron Specta-LEED system at a beam energy of 30 eV. The sample temperature during IRAS and LEED measurements was 80 K unless specified otherwise. For desorption experiments several masses of our quadrupole mass spectrometer (Pfeiffer, QMG 700, mass range 0 - 1024 u) were collected in parallel while applying linear heating rates of 1 K/s. CuPc was deposited onto Ag(111) at a sample temperature Tsample = 300 K using a homebuilt evaporator held at 500 K, leading to typical deposition rates of 0.2 ML/min. While depositing CuPc the background pressure remains below ≤ 8 × 10−11 mbar. Sample cleaning was achieved by Ar+ sputtering (0.7 kV, Isample = 1 µA, Tsample = 380 K, ∆t = 30 min) and annealing to 780 K for about 5 min.

Results and Discussion CuPc thin film growth on Ag(111): monolayer regime Figure 1 shows a series of SPA-LEED images (panels a - c) and IR spectra (panel d) associated with increasing amounts of CuPc on Ag(111) in the coverage regime ΘCuP c = 0.3 - 1.1 ML, with 1 ML denoting a fully covered monolayer of parallel oriented CuPc. In accordance with Kröger et al. 7 three different (sub-)monolayer phases of CuPc on Ag(111) are found. At low CuPc densities a disordered phase (panel a), characterized by a ring-like LEED pattern, is observed. Such patterns can be attributed to an arrangement of CuPc molecules with no azimuthal ordering but with well defined intermolecular distances. For increasing ΘCuP c the radius of the ring grows, indicative of a decreasing average distance between neighboring CuPc. Such behavior is commonly observed if repulsive interactions between adsorbed 5 ACS Paragon Plus Environment


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( a ) 0 .6 M L

( b ) 0 .8 7 M L 7 2 0 .5

3 .0 0 5 .0 0

6 .0 0 0 .0 0

ΘC

7 6 9 .6

7 1 8 .7

2 .6 0 5 .6 0 4 .7 5 -0 .2 5

(c ) 1 M L

u P c

= 0 .3 0 M L

7 6 8 .3 0 .6 0 M L

7 1 3 .1

7 6 3 .4 0 .7 0 M L 0 .8 5 M L 1 .0 0 M L 1 .1 0 M L

7 6 5 .2

7 7 7 .4

7 1 9 .0 0 .2 %

(d )

7 3 3 .3 7 0 0

7 5 0

8 0 0

F re q u e n c y [c m

-1

8 5 0

]

Figure 1: SPA-LEED images (Ekin = 30 eV) of CuPc monolayer phases on Ag(111): (a) disordered phase, (b) commensurate phase (c-phase), and (c) p.o.l. phase. The unit cells in (b) and (c) are depicted in red and contain one CuPc molecule; the vertical arrows thereby denote the direction of an Ag(111) lattice diffraction spot. Panel (d) shows a series of infrared absorption spectra of CuPc/Ag(111) (spectral region 660 - 870 cm−1 ) for CuPc coverages 0.3 - 1.1 ML. During CuPc evaporation the sample temperature was held at 300 K. For better clarity of presentation the spectra are shifted vertically. All data were obtained at T = 80 K.


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molecules prevail (for a detailed discussion see Stadler et al. 12 and Kröger et al. 7 ). The corresponding IR spectrum shows two distinct vibrational features at 720.5 cm−1 and 769.6 cm−1 . These bands are due to C - H out-of-plane bending vibrations with A2u symmetry; 16 note that the irreducible representations specified in this paper refer to the D4h symmetry group of isolated CuPc. Upon increasing the coverage both vibrational modes are subject to minor shifts of the order of a few wavenumbers towards lower frequencies. Since dynamic dipole coupling should give rise to a frequency increase (∆νdyn > 0), we conclude that substrate mediated chemical interactions are responsible for the observed line shifts. We note that both lateral coupling interactions can be disentangled only if isotopic substitution experiments were conducted, as has been demonstrated in several instances for small molecular species, e.g. for CO on various transition metal surfaces. 17–19 In the present case the irrelevance of ∆νdyn is not so much due to a weakness of the CuPc vibrational mode oscillator strength (which is actually similar to a typical νC−O stretching mode of adsorbed CO) but rather to the extended areal size of the flat lying molecules and accordingly large intermolecular distances. In the coverage regime ΘCuP c = 0.7 - 0.9 ML a commensurate phase of CuPc on Ag(111) is formed (c-phase, panel b) which undergoes an order - disorder phase transition when the sample is annealed to room temperature, in accordance with reports in the literature. 7 In the infrared spectrum this new phase is represented by two prominent modes at 713.1 cm−1 and 763.4 cm−1 ; in parallel to their rise, the corresponding vibrational features of the (disordered) 2D gas phase vanish. Above 0.9 ML more CuPc molecules squeeze into the c-phase which then converts to a p.o.l. phase until the CuPc monolayer is completed (panel c). The matrices specified in figs. 1(a - c) represent the corresponding epitaxy matrices derived from the experimental LEED data and are in accordance with those reported by Kröger et al. 7 The vibrational bands associated with the saturated monolayer phase are located at 719 cm−1 and 765.2 cm−1 ; note that small amounts of bilayer CuPc are present as well (vibrational bands at 733.2 and 765.2 cm−1 ), possibly due to a slightly inhomogeneous gas flow across the



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d is o r d e r e d - p h a s e ω0 = 1 5 1 6

ω0 = 1 3 8 8 . 5

ω0 = 1 1 2 1 . 5 ωτ = 0 . 7 5 γ= 7

ωτ = 1 . 1 γ= 1 4

ωτ = 0 . 9 γ= 1 2

ω0 = 1 3 8 5

ω0 = 1 5 1 6

c -p h a s e

ω0 = 1 1 1 9

ωτ = 1 . 2 5 γ= 2 7

ωτ = 0 . 9 5 γ= 2 3

ω0 = 1 1 2 0 . 5

ω0 = 1 3 8 7 . 5

ω0 = 1 5 1 8

ωτ = 0 . 7 5 γ= 1 3

ωτ = 1 . 1 γ= 2 5

ωτ = 0 . 8 5 γ= 9

0 .0 5 %

p .o .l.- p h a s e

1 1 0 0

1 2 0 0

1 3 0 0


1 4 0 0 -1

ωτ = 0 . 9 5 γ= 2 0

1 5 0 0

]

Figure 2: Infrared absorption spectra of CuPc/Ag(111) (sub-)monolayer phases in the spectral region of 1020 - 1590 cm−1 . Top, center and bottom spectra refer to the disordered phase, the commensurate phase, and the point-on-line phase, respectively (black lines). The curve fitting of the Fano-type lineshapes used the formula derived by Langreth 22 (red lines); the values for ω0 and γ are given in units of cm−1 . For better clarity of presentation the spectra are shifted vertically. surface. It is apparent that each phase is defined by characteristic vibrational bands. Similar sequences of overlayer structures, i.e. the appearance of a disordered, a commensurate and an incommensurate phase has already been reported for various MePc and H2 Pc molecular species on various (111) noble metal surfaces. 7,12,20,21 Figure 2 shows infrared absorption spectra (black lines) of the three (sub-)monolayer phases (disordered, commensurate, and p.o.l. phase) of CuPc/Ag(111) in the spectral range of 1020 - 1590 cm−1 . For each spectrum three distinct vibrational modes at about 1120 cm−1 , 1390 cm−1 and 1520 cm−1 are observed, all of them characterized by a highly asymmetric line shape. DFT calculations for free CuPc molecules 16 assigned all vibrational bands with frequencies above 1000 cm−1 to in-plane modes. This seems to be a rather general signature of planar organic molecules (see also reports on PTCDA or NTCDA 23–25 ) which is due to the weaker restoring forces of out-of-plane bending mode displacements (and accordingly lower frequencies), as compared to the in-plane bending and stretching of bonds within the


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molecular plane. For parallel oriented CuPc molecules on Ag(111) the number of vibrational modes which can be excited by IR absorption is reduced substantially due the surface selection rule for metals. Disregarding additional symmetry reductions related to the specific coordination of CuPc on Ag(111), modes visible in IR spectra should belong to the A1g or A2u type of irreducible representations of the D4h symmetry group (free CuPc). For CuPc on Ag(111), that is, for parallel oriented molecules, in-plane A1g type of vibrations may gain a nonzero dynamical dipole moment perpendicular to the surface in two different ways: (i) an out-of-plane deformation of the molecular structure, induced by the bonding of the molecule to the substrate may give rise to an additional vertical component in the displacement pattern, even for ’in-plane’ modes. The possibility of a warped molecular plane of adsorbed CuPc, however, has been ruled out on the basis of NIXSW data 7 which confirm the parallel orientation of CuPc with negligible (vertical) molecular distortions (this finding is in contrast to findings of PTCDA and NTCDA on Ag(111) which display a pronounced downward bending of the carboxyl groups due to a chemical bonding to Ag substrate atoms 25–27 ). Note that Eu type of in-plane vibrational modes (isolated CuPc molecule), 16 while dipole active for gas phase CuPc, are invisible due to the (metal) surface selection rule for parallel oriented adsorbates. (ii) vibrational motion may lead to interfacial dynamical charge transfer (IDCT) between the molecular orbitals and the substrate electronic levels. A partially filled LUMO located at the Fermi edge of the metal is thereby required to initiate such dynamical processes. Langreth and Crljen 22,28 derived an expression for the vibrational line shapes influenced by IDCT and, in addition, subject to non-adiabatic electron-phonon coupling. Specifically, such vibrational bands exhibit Fano - type line shapes characterized by a damping constant γ (associated with the lifetime of the vibrational mode) and an asymmetry parameter ωτ which accounts for the phase delay between the resulting total dynamic dipole moment



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and the contribution thereof associated with nuclear motion alone. Note that for strongly chemisorbed species such oscillatory motion of charge carriers between molecule and substrate proceeds instantaneously, i.e. there will be no or a negligible phase shift with respect to vibrational motion (adiabatic approximation). In fig. 2 the vibrational bands have been fitted using Langreth’s expression 22 (red curves, parameters as given in the figure). Some residual baseline instabilities notwithstanding, the experimentally observed spectra are perfectly reproduced. The used parameters ωτ and γ, defining the theoretical line shapes, seem to follow a clear trend. While the derived values display some distinct variations when comparing the individual modes, a change in CuPc coverage leads to only slight changes (see fig. 2); the linewidths at low coverages, i.e. for isolated CuPc molecules, seems to be notably smaller, though (for each of the modes). Interestingly, the asymmetry parameters are quite large (close to one) for all three prominent modes in fig. 2, independent of CuPc coverage. This indicates that the molecular level involved in IDCT is identical for these modes. Interfacial dynamical charge transfer and associated asymmetric line shapes have been reported for various large organic molecules on noble metal surfaces either using IRAS, 24 or high resolution electron energy loss spectroscopy (HREELS). 29,30 For PTCDA/Ag(111) it has been invoked that the central carbon ring of PTCDA plays a key role in the adsorbate substrate dynamical charge transfer. 31,32 In a recent work on NTCDA/Ag(111) this idea has been questioned since NTCDA has no central carbon ring; 24 instead it has been suggested that each individual molecule has different functional groups which are linked to dynamical charge transfer. Calculations of CuPc vibrational modes (free molecule) 16 yield vibrational bands very close to those depicted in fig. 2, with all of them belonging to the A1g irreducible representation and corresponding to vibrations of the pyrrole ring of the CuPc molecule. Interestingly, the LUMO of free CuPc molecules which defines the acceptor part of the substrate-molecule charge transfer is also located mainly within the pyrrole ring. 33 Based on DFT calculations


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1 .1

n o r m a liz e d in te n s ity [a .u .]

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1 .0 0 .9 0 .8

1 1 2 1 c m 1 3 8 8 c m 1 5 1 6 c m

0 .7

-1 -1 -1

0 .6 0 .5

0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

C u P c c o v e ra g e [M L ]

Figure 3: Coverage dependent intensities (per molecule) of various CuPc vibrational modes normalized to the respective low coverage values of CuPc/Ag(111). Huang et al. 9 showed that upon adsorption on Ag(111) charge is accumulated in the inner part of the π-system of CuPc. Therefore, we suggest that the CuPc-Ag(111) dynamic charge transfer and electron-phonon coupling primarily concerns vibrational modes with large amplitudes in these sections of the CuPc molecule, i.e. the vibrational bands at about 1120, 1390 and 1520 cm−1 . In fig. 3 the intensities of the prominent in-plane vibrational modes of CuPc on Ag(111) are plotted as a function of coverage. The intensities have been scaled according to the number of adsorbed molecules to yield the oscillator strength per molecule. We find that all three prominent in-plane modes of CuPc/Ag(111) follow a similar trend; specifically, their dynamic dipole moments stay constant up to 0.5 ML and then start decreasing as the layer approaches saturation. At present, it must remain open whether this is simply due to the closer intermolecular distances, or due to the formation of long range ordered overlayer structures with well defined coordination of CuPc on Ag(111) and/or between neighboring CuPc. As outlined above the intensities of (totally symmetric) in-plane vibrational modes largely rely on a partially filled LUMO and associated IDCT. It is therefore feasible that the reduced intensities of these modes with increasing coverage are associated with a shift in energy of the (former) LUMO of CuPc which alters the density of states at F and thus affect the 11 ACS Paragon Plus Environment


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amplitude of dynamic charge oscillations (IDCT). In addition, simple electronic screening of the dynamic dipole moments by neighboring molecules may contribute to the observed behavior. For CO on various transition metal surfaces a similar attenuation of absorption intensities per molecule has been observed in IRAS and HREELS spectra as the layers approach saturation. 17,34 A similar but slightly different trend has been observed for in-plane vibrational modes of NTCDA/Ag(111) which display a constant dynamical dipole moment per molecule up to coverages close to saturation of the monolayer. 24 Specifically, the observed in-plane mode intensities increase linearly with coverage reaching a maximum at 0.9 ML (relaxed monolayer phase) and thereafter loose substantially in intensity as the layer converts to the compressed monolayer phase at 1 ML. For this particular system the constant µdyn per molecule can be ascribed to the pronounced island growth behavior of NTCDA on Ag(111) at submonolayer coverages, which yields an identical local environment of NTCDA (r-ML phase) over an extended coverage range.

CuPc thin film growth on Ag(111): Multilayers In fig. 4 various amounts (1 - 5 ML) of CuPc have been deposited on Ag(111) at 300 K and vibrational spectra thereof displayed in the spectral region of out-of-plane (left panel), as well as in-plane vibrational modes (right panel). For CuPc in excess of one monolayer the vibrational bands in fig. 4 show minor but distinct frequency shifts. In order to identify such minute shifts a high spectral resolution of 0.5 cm−1 turned out to be essential. As an all optical method, IR spectroscopy is lacking intrinsic surface selectivity and due to its penetrating nature this method represents a highly efficient tool for investigating multilayer and interface systems of molecular thin films. With increasing CuPc coverages beyond the first monolayer new modes start growing at 733.6 and 777.3 cm−1 . Upon completion of the bilayer these two modes experience slight frequency shifts (∆ν = 1 − 2 cm−1 ) to 734.9 cm−1 and 779.2 cm−1 and reach their maximum 12 ACS Paragon Plus Environment

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7 1 8 .9

7 3 3 .6

7 6 5 .2

1 .1 M L C u P c

7 7 7 .3

x 5

7 7 9 .2

1 .4 M L x 2

x 5

7 3 4 .7 7 3 2 .8

2 M L x 5

7 3 4 .9

7 7 2 .8

3 M L

7 3 4 .8

7 6 9 .7

4 M L

7 3 2 .7

7 7 2 .8

7 6 9 .6

5 M L

7 7 2 .4

1 0 9 2 .0

8 7 2 .8 9 0 1 .7

1 1 6 7 .2

1 1 2 1 .7

9 5 1 .6

1 4 2 2 .8 ≈1 3 4 0

1 5 1 0 .5 1 5 2 4

9 4 1 .2

5 %

1 2 8 7

0 .2 %

7 3 2 .4 7 2 0

7 3 0

7 4 0

7 5 0

7 6 0


7 7 0 -1

7 8 0

8 0 0

]

9 0 0

1 0 0 0

1 1 0 0

1 2 0 0


1 3 0 0 -1

1 4 0 0

1 5 0 0

1 6 0 0

]

Figure 4: Infrared absorption spectra of CuPc adsorbed on Ag(111) for coverages up to 5 monolayer in the spectral regions of 710 - 790 cm−1 (left panel) and 790 - 1600 cm−1 (right panel). In the right panel infrared active in-plane and out-of-plane vibrational modes of multilayer CuPc are marked with a dot and an open square, respectively (after Liu et al. 16 ). During evaporation the sample temperature was held at 300 K. The spectra were taken at 80 K, using an instrumental resolution of 0.5 cm−1 . For better clarity of presentation the spectra are shifted vertically. intensity at ΘCuP c = 2 ML. For the strong band at 733 - 735 cm−1 a contribution to the observed blue shift due to dynamic dipole coupling among bilayer CuPc molecules appears feasible; the second band at about 778 cm−1 , on the other hand, is much too weak to experience a noticeable effect from this type of lateral coupling. Due to the rather weak interaction of second layer CuPc to the Ag(111) substrate we exclude through metal electronic coupling, i.e. chemical shifts of these vibrational modes. A closer look at these two vibrational bands, e.g. the ×5 enlarged sections in the left panel, reveals that each of them is actually composed of two components; the one is prevailing at low coverages of the CuPc bilayer, while the other is exclusively observed for the complete bilayer. They are, therefore, attributed to a CuPc 2D gas phase and 2D islands thereof on top of the CuPc/Ag(111) monolayer. According to fig. 4 (left panel), the linewidths of mono- and bilayer out-of-plane modes display notable differences. For the bilayer exceptionally narrow linewidths of 0.7 cm−1 (mode at 734.9 cm−1 ) and 1.1 cm−1 (779.2 cm−1 ) have been observed which are substantially



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smaller than for the corresponding bands of the CuPc/Ag(111) monolayer. Taking the instrumental resolution of our spectrometer (0.5 cm−1 ) into account, we derive linewidths of 0.3 - 0.4 cm−1 (mode at 734.9 cm−1 ) and 0.9 cm−1 (mode at 779.2 cm−1 ). This is a clear indication of a substantially weaker interaction between Ag(111) and second layer CuPc as compared to the monolayer species. The differences in vibrational linewidth thus make it very easy to distinguish between monolayer and bilayer species in IR spectroscopy. We suspect that monolayer CuPc experiences a chemical bonding to Ag(111) while bilayer CuPc is purely physisorbed. This latter interpretation is supported by vibrational line positions close to values of isolated molecules for second and higher layer CuPc. Comparing bilayer and monolayer intensities for the two dominant C-H bending modes, distinct differences are found. For the modes at 734.9 cm−1 (bilayer) and 718.8 cm−1 (monolayer species) integrated intensities yield an intensity ratio of 3 (bilayer with respect to the monolayer) while only minor variations occur for the bands at 779.2 cm−1 (bilayer) and 765.2 cm−1 (monolayer species). For the bilayer, the intensity ratio of these two out-of-plane bending modes amounts to 6.5:1 which is very similar to values reported for thick bulk-like films (6:1), or for CuPc in KBr pellet samples (6.7:1). 10,35 In accordance with the indicated numbers this ratio is much lower for monolayer CuPc/Ag(111) and equals 2:1 which points at a pronounced reduction of the dynamic dipole moment of the 718.8 cm−1 band of monolayer CuPc. As mentioned above, this mode yields considerable vibrational amplitude at the Pyrrole-ring, that is, the preferred location of charge transfer between CuPc and Ag(111) upon adsorption (→ partial filling of the CuPc LUMO). We suggest that the total dynamical dipole moment of this mode got reduced due to dynamic charge oscillations within the molecules as well as across the adsorbate substrate interface. For the 765.2 cm−1 band the contribution arising from the altered occupancy of the CuPc molecular orbitals seems much lower, most probably due to a displacement pattern which does not correlate well with the LUMO. At ΘCuP c = 3 ML a set of new distinct modes in the spectral region of out-of-plane


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modes is identified at 732.8 cm−1 and 772.7 cm−1 . As more CuPc is deposited (see fig. 4) these vibrational bands increase in intensity with very minor additional shifts in frequency. In parallel, the characteristic bilayer vibrational mode at 735 cm−1 gradually turns into a shoulder of the ever growing multilayer band at 732 - 733 cm−1 . We note that the observed frequency shifts for molecular films in excess of the bilayer are, in general, only very minor, in accordance with the fading influence of the substrate; thereby, vibrational line positions in fig. 4 (multilayer spectrum, bottom curve) agree favorably with those of CuPc bulk crystals 35 and of thin CuPc films (11nm) on Ag(111). 10 Moreover, the observed vibrational features for ΘCuP c ≥ 2 ML are adequately reproduced in calculated CuPc vibrational frequencies (free CuPc molecule). 16,36 In addition to vibrational frequency shifts, we noticed a slight intensity loss of the bilayer vibrational modes upon further deposition of CuPc, which we attribute to a screening of dynamic dipole moments of buried species by the capping third and higher layers. The vanishing influence of the Ag substrate on CuPc molecular films is likewise reflected in CuPc highest occupied molecular orbital (HOMO) binding energies which differ significantly for mono-, bi-, and trilayers and which Kröger et al. ascribed to a weakening of the coupling of the photoelectron holes to the electronic system of the substrate. 7 For CuPc film thicknesses beyond 3 ML this shift converges, which the authors have interpreted as full electronic decoupling of these layers from the substrate. 7 In the spectral region ν > 800 cm−1 (right panel fig. 4), only minor changes between mono- and bilayer spectra have been found. For the complete bilayer very weak extra bands are discernible, in addition to those already present due to monolayer CuPc. The absence of any intense in-plane vibrational modes confirms the parallel orientation of the CuPc bilayer. We note that IDCT, which lead to considerable IR intensities of in-plane vibrational modes for monolayer CuPc, is not operative for bi- and higher layers as they are lacking orbital overlap with substrate electronic bands. Upon completion of the third CuPc layer various (weak) in-plane vibrational modes



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are clearly discernible and they further gain intensity as more CuPc is deposited. The question arises as to the origin of these in-plane mode IR absorption intensities. For strictly parallel oriented CuPc, those molecules deposited on top of the CuPc monolayer should display negligible in-plane mode intensity. Two explanations as to their emergence appear conceivable: The one possibility is a breakdown of the surface selection rule for layers ≥ 3 ML which has been suggested by Tautz et al. in the context of a vibrational analysis of PTCDA multilayers using HREELS. 23 Specifically, for PTCDA on Ag(110) and Ag(111) in-plane modes of presumably parallel oriented PTCDA molecules gain more and more intensity in specular spectra (dipole scattering geometry) for layer thicknesses of 3 ML and above. The suggested explanation, however, seems to be specific to HREELS only: impinging electrons supposedly create dipolar fields which are oriented perpendicular to the metal surface at the surface mirror plane only (strictly speaking), while they should expose some parallel component at a distance. For incident IR radiation the model of an electron located above a conducting surface plane, along with its dipolar field distribution is not applicable. Screening of parallel dipoles should therefore be active for much thicker layers due to the much longer wavelength of the incident light. Much more likely, CuPc in the second and higher layer experience a reduction of CuPc molecular symmetry D4h → C4v → C2v which renders a fair number of in-plane vibrational modes dipole active, even considering the massive screening of parallel dipoles due to the metal substrate. For example, a very slight deviation from the 100% parallel orientation of the CuPc molecular plane in the third (and higher) layers could give rise to a dynamic dipole moment perpendicular to the surface for Eu type of in-plane modes. A related explanation would be a distortion of the molecular plane (i.e. loss of the CuPc planarity) due to the coordination of CuPc within the crystalline solid, i.e. with respect to neighboring molecules in the layers above and below. A reduction of CuPc molecular symmetry (in conjunction with a slight distortion of the molecular plane, or a minor tilt) would be in accordance with the prevalence of the α-type polymorph of CuPc when grown on Ag(111). This phase is characterized by a parallel stacking of CuPc in consecutive layers


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with a molecular offset between layers of about 1.6 ˚ A along the CuPc wings. 11 Alternative polymorphs of CuPc bulk samples comprise herringbone-type of molecular arrangements which we can definitely exclude, based on the weakness of in-plane vibrational modes of our 5 ML CuPc on Ag(111) samples. Despite our observation of various prominent in-plane modes at 1000 - 1500 cm−1 , the intensity of the C-H out-of-plane bending mode at 732.4 cm−1 for multilayer CuPc is still substantially more intense, in accordance with a parallel or only slightly inclined orientation of CuPc molecules on Ag(111). Strong out-of-plane modes and weak in-plane modes were commonly observed by HREELS for MePc on noble metals and associated with a parallel orientation of the molecules. 30,37 Following these geometrical arguments the tilt angle of CuPc bi-, tri-, and multilayers on the Ag(111) surface should amount to a few degrees only. Comparing the two bands at 1092 cm−1 and 732 - 733 cm−1 (which display about similar intensities in KBr pellet samples, i.e. µ1092 ' µ732 ), gives an intensity ratio of 0.01 for our 3 - 5 ML thin films. According to

0.01 =

µ⊥ 2 µ1092 · sinα 2 I1092 ∼ ( 1092 ) = (tanα)2 ) =( ⊥ I732 µ732 µ732 · cosα

this ratio may be converted to a ’tilt’ of α ' 5◦ as an upper limit. The derived value is corroborated by observations of cobalt-II-phthalocyanine (CoPc) molecules on Au(111) which seem to be tilted by 3 − 4◦ for second and third layer species, as deduced from STM images. 38 A prominent example demonstrating the effect of an inclined adsorption geometry is NTCDA on Ag(111) which experiences a conversion from parallel to an all perpendicular, standing geometry upon special processing; specifically, a dramatic increase of in-plane vibrational modes in conjunction with a fading of out-of-plane bands has been observed in IR absorption spectra. 39



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7 3 2 .8

7 3 4 .8

3 M L C u P c

7 7 2 .8

(a )

(a )

5 M L

7 7 2 .4

(b )

(b )

(c )

(c )

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7 3 2 .4

3 M L 5 M L + T S

→4 8 0 K

(d )

(d )

7 7 1 .5

7 3 4 .8 - 3 M L C u P c

7 6 9 .6

7 3 2 .8

(e )

(e )

7 7 1 .5

7 3 1 .1

7 2 0

7 3 0

7 4 0

7 5 0

1 0 9 2 .0

1 1 6 7 .7

7 7 0

7 8 0

8 0 0

9 0 0

1 0 0 0

]

1 1 0 0

1 5 1 0 .7

0 .3 %

9 5 2 .0

-1

1 3 3 6 .4 1 4 2 2 .2

1 1 2 2 .0

9 4 0 .8

7 6 0


1 5 2 4

9 0 2 .0 8 7 2 .8

5 %

1 4 8 0 .8

1 2 8 7 .7

1 2 0 0

1 3 0 0


1 4 0 0 -1

1 5 0 0

1 6 0 0

]

Figure 5: Infrared absorption spectra of CuPc multilayers on Ag(111) in the spectral region of 710 - 790 cm−1 (left panel) and 790 - 1600 cm−1 (right panel). Curves (a) and (b) correspond to 3 ML and 5 ML of CuPc and they are identical to the respective curves in fig. 4. For the bottom spectrum (e) the 3 ML spectrum (c) was subtracted from the annealed 5 ML spectrum (d). The spectra were taken at 80 K, using a spectral resolution of 0.5 cm−1 . For better clarity of presentation the spectra are shifted vertically.

Thermal evolution of CuPc/Ag(111) molecular films Vibrational Spectroscopy In fig. 5 (left panel IR spectra of 3 and 5 ML CuPc on Ag(111) are shown in the spectral region of 710 - 790 cm−1 ). For 3 ML of CuPc (curve a) two vibrational modes are observed at 734.8 and 732.8 cm−1 which can be ascribed to the CuPc bilayer and third layer, respectively. As discussed in fig. 4, these latter bands shift to slightly lower frequencies (∆ν = 0.4 cm−1 ) upon adding two more layers. We have now annealed the 5 ML CuPc/Ag(111) molecular film in (b) to 480 K which creates a new distinct peak at 731.1 cm−1 (d); annealing of 3 ML CuPc/Ag(111) does not induce any spectral changes, i.e. a sample temperature of 300 K is sufficient to attain thermodynamic equilibrium at 3 ML thickness. Interestingly, the characteristic vibrational bands of the bilayer and third layer which became somewhat indistinct for the 5 ML film before annealing (curve b) are perfectly recovered and they exhibit a virtually identical spectral signature as the pure 3 ML film in (a).


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Subtraction of the 3 ML spectrum (red line, curve c) from the annealed 5 ML spectrum (curve d) then yields the vibrational bands exclusively due to the newly formed CuPc multilayer phase, formed above the CuPc trilayer (bottom curve e). Hence, the observed structural transformation concerns only molecules on top of the third layer and leaves the underlying layers untouched. SPA-LEED images of 3 ML, 5 ML, as well as 5 ML annealed to 480 K correspond to long range ordered phases which are very similar (but not identical) to the CuPc bilayer (for details see Supporting Information, Figure S1). Thus, a structural phase transformation within the topmost layers induced by annealing can be excluded. Rather we believe that the rearrangement induced by annealing to 480 K is associated with a dewetting of the grown film and formation of CuPc bulk crystallites. As a consequence the layer transforms into a virtually undistorted trilayer (i.e. not affected by extra layers on top) with few scattered 3D crystallites on top of it. 40 These observations suggest that for CuPc on Ag(111) not only mono and bilayers represent thermodynamically stable structures (stable with respect to the formation of bulk crystallites), but the same applies to the parallel oriented CuPc trilayer. A similar dewetting behavior and the formation of nanocrystalites has been recently suggested for CuPc adsorbed on Cu(111). 41 Thereby, CuPc has been found to grow in a layer by layer fashion up to the third layer and, with more CuPc deposited at room temperatures, 3D-islands emerge (identified by unstable imaging conditions in STM and damage of tips). For PTCDA thin films on various noble metals causes a conversion of PTCDA layers into 3D-crystallites already for layer thicknesses in excess of 2 ML. 42 The main difference with respect to the CuPc system is therefore that the dewetting includes the third layer as well. Both molecular systems (and probably many others) can therefore be classified by a generalized Stranski-Krastanov growth behavior, with n ≥ 1 thermodynamically stable layers. In our IR spectra we find that the linewidth of the peak at 731.1 cm−1 due to CuPc 3D cluster is quite narrow; nonetheless, its inhomogeneous broadening is notably larger



5 2 0 K

Q M S s ig n a l [a .u .]

C u P c /A g (1 1 1 ) 5 .0 2 .6 1 .6 1 .0

m a s s 5 7 5 u

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M L M L M L M L

5 6 0 K 6 5 5 K

5 0 0

6 0 0

7 0 0

8 0 0

T e m p e ra tu re [K ]

Figure 6: Thermal desorption spectra (heating rate 1 K/s) for various amounts of CuPc (1 - 5 ML) deposited on Ag(111) at 300 K. as compared to the peaks of the CuPc bilayer (734.8 cm−1 ) and third layer (732.8 cm−1 ). This may indicate that CuPc is experiencing slightly different local environments, e.g. the formation of nonumiformly sized CuPc 3D crystals, or, admixtures of non-equivalent surface species covering the crystallites (as has been detected for benzene multilayers on Ru(0001) using IRAS 43 by means of isotopic substitution experiments).

Thermal Desorption Spectroscopy Figure 6 shows a series of thermal desorption spectra associated with 1 - 5 ML CuPc on Ag(111). CuPc desorption yields three features associated with mono-, bi-, and multilayers. For coverages close to completion of the monolayer a rather weak signal is observed at about 600 - 700 K which is ascribed to desorption of monolayer CuPc. Desorption from the bilayer proceeds at distinctly lower temperatures (500 - 600 K). The integrated monolayer signal is substantially smaller than the corresponding value of the bilayer, indicating incomplete desorption of CuPc in direct contact with the Ag(111) surface and the prevalence of a competing process such as dissociation; specifically, we have direct evidence for a thermally activated decay of adsorbed CuPc and desorption of fragments thereof (see below). At temperatures slightly below second layer desorption, i.e. at T = 480 - 530 K, desorption of CuPc multilayers is observed (fig. 6). In accordance with our finding of a thermodynamically 20 ACS Paragon Plus Environment

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1 .6 M L C u P c /A g (1 1 1 ) [C u ]

Q M S s ig n a l [a .u .]

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+

[C u P c ]

+

[1 /4 P c + C u ]

+

[1 /4 P c ]

5 0 0

6 0 0

+

7 0 0

8 0 0

T e m p e ra tu re [K ]

Figure 7: Thermal desorption spectra (dT/dt = 1K/s) of different CuPc fragments of 1.6 ML of CuPc desorbing from the Ag(111) surface. The individual ionic fragments were [Cu]=63 u, [1/4 Pc]=128 u, [1/4 Pc + Cu]=191 u and intact [CuPc]=575 u) stable parallel oriented bilayer the CuPc bilayer desorption trace is largely unaffected by additional layers grown on top of it. According to the identical rising edges of the desorption traces for 2.6 and 5 ML CuPc films, zero’th order desorption kinetics is suggested. Using the Arrhenius equation (leading edge analysis) a desorption energy of 2.20 eV (±0.05 eV) and a preexponential factor of 1.5 ·1020 s−1 is extracted for CuPc multilayer desorption. Thereby the difference in bi- and multilayer desorption temperatures gives an estimate of the residual influence of the Ag substrate on the CuPc bilayer adsorption energy: Assuming identical prefactors this extra contribution to Eads amounts to 150 - 200 meV. Our results are close to the values found for MePc multilayer films on graphene/Ir(111), with activation energies for molecular desorption ranging between 2.2 and 2.4 eV and assuming a prefactor of 1018 s−1 . 44 In fig. 7 a CuPc coverage of 1.6 ML has been deposited on Ag(111) and the desorption signals of various CuPc fragments monitored. The individual thermal desorption traces were collected ’simultaneously’ during linear heat-up with a delay time of about 200 ms between each mass. The mass values of the various fragments were calibrated via a mass scan of the CuPc molecular beam, evaporated from our Knudsen cell. The ion signals detected in addition to the dominant mass of intact desorbing CuPc (575 u) are assigned to the



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following fragments, according to Achar et al.: 45 63 u = [Cu]+ ; 128 u = [1/4P c]+ ; 191 u = [1/4P c + Cu]+ (see fig. 7). In the temperature range 480 - 600 K virtually identical desorption traces are found for all detected fragments which we attribute to intact CuPc desorption (bilayer peak). Relative intensities match those observed for the CuPc molecular beam of the Knudsen cell. The parallel detection of the other ionic signals with different mass values is ascribed to e-beam ionization and fragmentation in the quadrupole mass spectrometer (QMS). In the temperature range of CuPc/Ag(111) monolayer desorption, that is, above 600 K, the signals of desorbing CuPc are generally quite weak. Surprisingly, the mass 128 u ionic fragment shows a second bump (maximum at 677 K) even though the QMS signals of intact CuPc (including its fragments) are continuously dropping towards zero for T > 600 K. Apparently, a competing process, namely CuPc dissociation takes effect, in parallel to intact CuPc desorption. More precisely, 1/4 Pc species are produced in the course of CuPc decay on Ag(111) and most probably desorb from the surface right after formation; Cu atoms, on the other hand, seem to remain on the surface. Comparison to CuPc bilayer signals (integral was set to 1 ML) by means of integration of the TDS traces allows us to estimate the amount of intact desorbing monolayer CuPc which gave 0.1 ML. A similar value has been reported in the literature for weakly chemisorbed endcapped oligothiophenes ECnt (n=3-6) on Ag(111) (at T > 600 K). 46 STM experiments of CuPc on Ag(111) and followed by extended annealing to 780 K show minute amounts of leftover CuPc species, forming dendrite-like chains. 8 While they appear seemingly intact (four-lobed appearance of CuPc), they look as if they are linked together, which has been ascribed to dehydrogenation and subsequent dimerization. In this respect it is interesting that annealing to 720 K in our experiment still yielded a (sharp) c-phase LEED pattern, supporting these STM findings and the presence of ’intact’ CuPc molecules on Ag(111) after such thermal treatment. In IR spectra the vibrational features very much resembled those of a proper CuPc c-phase; individual bands, however, displayed


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some broadening and a reduced intensity, in accordance with a partial desorption and dissociation, possibly at Ag(111) steps or defects. We conclude that annealing (weakly chemisorbed) molecular layers to temperatures beyond bilayer desorption always bear the risk of thermally activated dissociation to occur. Moreover, the present results clearly demonstrate the need to detect the major mass of a molecule in thermal desorption spectroscopy; monitoring small ionic fragments of a molecule produces ambiguous evidence regarding the intact desorption of molecular species.

Summary In the present paper the adsorption of CuPc on Ag(111) has been investigated using FTIRAS, SPA-LEED and TDS. The focus was laid on the structural and vibrational properties of CuPc molecular films up to 5 ML thickness. Vibrational spectroscopy revealed characteristic spectroscopic signatures for the (sub-)monolayer phases of CuPc and also for the bi-, triand multilayers. For the parallel oriented CuPc/Ag(111) monolayer repulsive intermolecular interactions lead to uniform distances between neighboring CuPc in the submonolayer regime. When approaching saturation the constrained lateral space in conjunction with minimization of repulsive forces lead to the formation of a commensurate as well as an incommensurate pointon-line phase at 0.7 - 0.9 ML and 1.0 ML, respectively. In vibrational spectra the associated change in the local environment leads to distinct frequency shifts of modes located at the periphery of the CuPc molecule, namely, C-H out-of-plane bending modes. In-plane vibrational modes of (sub-)monolayer CuPc/Ag(111) display substantial intensity despite a parallel orientation of the molecular plane which points at a pronounced interfacial dynamical charge transfer between the metal and the molecular layer. Moreover, these bands are characterized by highly asymmetric line shapes which indicate a severe violation of adiabaticity in electron-vibron coupling. A partially filled LUMO is a prerequisite



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for such a vibrationally induced charge transfer process and has actually been found both in experiments 7 as well as theory. 9 DFT calculations 16 reveal that those vibrations which we found most susceptible to IDCT are located in the pyrrole section of CuPc, i.e. where the LUMO is mainly located. For layers in excess of the (weakly) chemisorbed monolayer the combination of intense out-of-plane and weak in-plane modes are clear indications for parallel oriented molecules growing in a layer by layer fashion at room temperature. The gradual appearance of in-plane modes for CuPc films of 3 ML and beyond is ascribed to slightly inclined arrangement of CuPc for these layers, most likely due to a parallel displaced pi - stacking of CuPc in consecutive layers. Annealing CuPc multilayer films to temperatures close to desorption induces a major rearrangement, with layers in excess of the third layer transforming into 3D-cluster. This latter conclusion relies on the reappearance of the distinct CuPc trilayer spectral features in high resolution IR spectra. We note that the parallel (or close to) orientation of CuPc within the formed 3D crystallites is retained. TD spectra display intact molecular desorption of CuPc bilayer and multilayer species in the temperature range 480 - 600 K. For the more strongly bound CuPc/Ag(111) monolayer decomposition represents an alternative reaction channel besides molecular desorption. Thereby, the central Cu atom is abstracted from the phthalocyanine unit (and remains on the surface), while the residual molecule is fragmented and devided into 1/4 Pc segments at T > 600 K (with the latter desorbing right after formation). The 10 % fraction of intact CuPc desorbing from the saturated monolayer provides strong evidence that this process is aided by significant repulsive intermolecular interactions at high CuPc densities.

Associated Content Supporting Information Available: [Long range order of CuPc molecular layers (coverage regime 0.9 - 10 ML) from SPALEED images.] This material is available free of charge via


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the Internet at http://pubs.acs.org.

Acknowledgments Financial support has been provided by Deutsche Forschungsgemeinschaft through SFB1083.

Author Information Tel.: +49-6421-2824328. Fax: +49-6421-2824218. Email: [email protected]



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References (1) Tang, C. W. Two-Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, 183–185. (2) Wright, J. D. Gas Adsorption on Phthalocyanines and its Effects on Electrical Properties. Prog. Surf. Sci. 1989, 31, 1–60. (3) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Organic Field-Fffect Transistors with High Mobility Based on Copper Phthalocyanine. Appl. Phys. Lett 1996, 69, 3066–3068. (4) Lee, S. T.; Wang, Y. M.; Hou, X. Y.; Tang, C. W. Interfacial Electronic Structures in an Organic Light-Emitting Diode. Appl. Phys. Lett. 1999, 74, 670–672. (5) Barth, J. V.; Costantini, G.; Kern, K. Engineering Atomic and Molecular Nanostructures at Surfaces. Nature 2005, 437, 671–679. (6) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789–1791. (7) Kröger, I.; Stadtm¨ uller, B.; Stadler, C.; Ziroff, J.; Kochler, M.; Stahl, A.; Pollinger, F.; Lee, T.-L.; Zegenhagen, J.; Reinert, F.; et al., Submonolayer Growth of CopperPhthalocyanine on Ag(111). New J. Phys. 2010, 12, 083038. (8) Manandhar, K.; Ellis, T.; Park, K. T.; Cai, T.; Song, Z.; Hrbek, J. A Scanning Tunneling Microscopy Study on the Effect of Post-Deposition Annealing of Copper Phthalocyanine Thin Films. Surf. Sci. 2007, 601, 3623–3631. (9) Huang, Y. L.; Wruss, E.; Egger, D. A.; Kera, S.; Ueno, N.; Saidi, W. A.; Bucko, T.; Wee, A. T. S.; Zojer, E. Understanding the Adsorption of CuPc and ZnPc on Noble Metal Surfaces by Combining Quantum-Mechanical Modelling and Photoelectron Spectroscopy. Molecules 2014, 19, 2969–2992.


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Table of Contents Graphic

Fano-type vibrational line shape of CuPc/Ag(111), introduced by non-adiabatic electronvibron coupling in conjunction with interfacial dynamical charge transfer (IDCT).


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Ag(111) Ultrathin Films

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