Electronic Properties of Tetraazaperopyrene Derivatives
on Au(111): Energy-Level Alignment and Interfacial Band Formation
N-heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field-effect transistors. Here, we investigated the electronic properties of 1,3,8,10-tetraazaperopyrene derivatives adsorbed on Au(111) using a complementary experimental approach, namely, scanning tunneling spectroscopy and two-photon photoemission combined with state-of-the-art density functional theory. We find signatures of weak physisorption of the molecular layers, such as the absence of charge transfer, a nearly unperturbed surface state, and an intact herringbone reconstruction underneath the molecular layer. Interestingly, molecular states in the energy region of the sp- and d-bands of the Au(111) substrate exhibit hole-like dispersive character. We ascribe this band character to hybridization with the delocalized states of the substrate. We suggest that such bands, which leave the molecular frontier orbitals largely unperturbed, are a promising lead for the design of organic–metal interfaces with a low charge injection barrier.
Introduction<!><!>Introduction<!>Methods<!>2PPE Experiments<!>STM/STS Experiments<!>DFT Calculations<!>Results and Discussion<!>Adsorption Structure of
TAPP-CF3 on the Au(111)<!><!>Adsorption Structure of
TAPP-CF3 on the Au(111)<!>Electronic Structure of
TAPP-CF3 on Au(111)<!><!>Electronic Structure of
TAPP-CF3 on Au(111)<!><!>Electronic Structure of
TAPP-CF3 on Au(111)<!><!>Electronic Structure of
TAPP-CF3 on Au(111)<!><!>Electronic Structure of
TAPP-CF3 on Au(111)<!>Electronic
Structure of Further TAPP Derivatives on Au(111)<!><!>Electronic
Structure of Further TAPP Derivatives on Au(111)<!>Band Formation at the TAPP/Au(111)
Interfaces<!><!>Band Formation at the TAPP/Au(111)
Interfaces<!>Conclusions<!>
<p>Organic
electron-transporting (n-channel) semiconductors are of
particular interest for their implementation in field-effect transistors.1 Replacing carbon atoms by nitrogen atoms in a
π-conjugated aromatic molecular backbone typically leads to
an energetic stabilization of the frontier orbitals, i.e., the electron
affinity (EA) and the ionization potential (IP) increase, while the
optical gap size is almost uninfluenced.2−5 N-heteropolycyclic aromatic molecules are
thus expected to be promising candidates for n-channel semiconductors.
Regardless of n- or p-channel semiconducting behavior in a transistor,
the energetic positions of the electron affinity level or the ionization
potential are the relevant quantities for device performance. The
IP can be determined by photoemission spectroscopy (e.g., ultraviolet
photoemission spectroscopy (UPS),6−9 two-photon photoemission (2PPE)10−13) or scanning tunneling spectroscopy (STS).14 For the identification of affinity levels, inverse photoemission,15,16 2PPE, or STS can be used. All methods imply that a former unoccupied
molecular state is populated with an electron, creating a transient
negative ion resonance. The difference between the IP and EA is the
transport gap (IP – EA = Etransp.), which is different from the optical gap (highest occupied molecular
orbital–lowest unoccupied molecular orbital (HOMO–LUMO)
transition leading to exciton formation).17 The latter can be determined in surface-adsorbed molecules with
2PPE,10−13,18 differential reflectance spectroscopy,19−22 or high-resolution electron energy loss spectroscopy.18,23−29</p><p>The electronic structure of free (gas phase) molecules is
strongly
affected in the condensed phase and in particular when the molecules
are adsorbed on metal surfaces. The adsorption geometry of the organic
compound at the hybrid organic/metal interface has a pronounced influence
on the interfacial electronic structure.6−9,20,30−38 The interfacial electronic structure, i.e., the wave function mixing
(hybridization) between localized molecular electronic states and
metal bands, is particularly important for the interfacial charge
injection properties and accordingly crucial for the performance of
organic field-effect transistors. Hybridization is a necessary prerequisite
for interfacial band formation, which can be identified via dispersing
states in angle-resolved photoemission experiments. Interfacial band
formation has been shown in the case of strong electron acceptors
(tetrafluoro-tetracyanoquinodi-methane (F4TCNQ)/Au(111),39,40 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA)/Ag(110),41 and 1,4,5,8-naphthalenetetracarboxylic-dianhydride
(NTCDA)/Ag(111)41,42) and a donor (tetrathiafulvalene
(TTF)/Au(111)40) adsorbed on noble-metal
surfaces. Recently, we demonstrated interfacial band formation for
an N-heteropolycyclic molecule, a 1,3,8,10-tetraazaperopyrene derivative
(TAPP-CF3, see Figure 1) adsorbed on Au(111) in the energy regime of occupied
and unoccupied electronic states occurred.43 TAPPs belong to the class of N-heteropolycyclic aromatics, which
have shown promising results as organic semiconductors in thin-film
transistors.44,45 The TAPP-H/Cu(111) system has
been studied with respect to thermally activated and surface-assisted
reactions using scanning tunneling microscopy (STM).46,47</p><!><p>1,3,8,10-Tetraazaperopyrene
(TAPP) derivatives investigated in
the present study.</p><!><p>In this study, we used
complementary experimental techniques, namely,
STS and 2PPE in combination with density functional theory (DFT) to
determine the electronic properties of a 1,3,8,10-tetraazaperopyrene
derivative (TAPP-CF3, see Figure 1) adsorbed on Au(111). The adsorption structure
of TAPP-CF3 on Au(111) is analyzed by STM and DFT calculations.
2PPE is applied to study the electronic properties of further TAPP
derivatives (TAPP-H and TAPP-C3F7, see Figure 1). For all TAPP derivatives,
we determined the energetic position of several affinity levels as
well as the ionization potential. Hybrid band formation in the energy
region of occupied and unoccupied delocalized states is identified
via angle-resolved 2PPE.</p><!><p>STS and 2PPE experiments
were carried out with two different setups
under ultrahigh-vacuum (UHV) conditions. In both cases, a clean Au(111)
substrate was prepared by a standard procedure of sputtering–annealing
cycles. The TAPP molecules were deposited from an effusion cell held
at a temperature of 520 K, while the Au(111) surface was kept at room
temperature. For the STM experiments, the deposited coverage was below
one monolayer (ML). This provides clean areas of Au(111) for tip treatment
and reference measurements. To prepare a well-defined ML coverage
for the 2PPE experiments, a multilayer coverage was deposited and
afterward annealed to 500 K. The desorption of the multilayer was
monitored by temperature-programmed desorption measurements (see the Supporting Information).</p><!><p>2PPE measurements enable the quantitative
determination of the energetic position of occupied initial and unoccupied
intermediate or final electronic states of the adsorbate/substrate
system in a pump–probe scheme.11,12,23,48−53 Detailed photon-energy-dependent measurements are needed to assign
the observed photoemission peaks to occupied or unoccupied states.
Dispersion relations of the electronic states can be obtained by variation
of the detection angle (α) as ℏk∥ = (2me·Ekin)1/2 sin α, where me denotes the free electron mass and k∥ = 0 corresponds to electrons detected
along the surface normal (i.e., α = 0). The tunable femtosecond
laser system, which delivers laser pulses over a wide range of photon
energies, and the 2PPE setup are described elsewhere.54</p><!><p>STM and STS
measurements have been
carried out on a submonolayer coverage of TAPP-CF3 on Au(111)
at a temperature of T = 4.5 K under UHV conditions.
The STM tip was prepared by indentation into the clean Au crystal.
Differential conductance (dI/dV)
spectra and maps were recorded using a lock-in amplifier at a frequency
of f = 937 Hz with a modulation amplitude of VRMS = 5 mV. The metallic properties of the tip
were checked by recording dI/dV spectra
on clean areas of the Au(111) substrate, where only in the energy
regime of the occupied Au(111) band structure, the Shockley-type surface
state, the sp- and d-bands were detected while the spectrum was featureless
otherwise. Additional peaks in the dI/dV spectra can thus be associated with molecular resonances.</p><!><p>State-of-the-art DFT calculations
of surface-bound molecules were performed with the FHI-aims package55 using the PBE exchange–correlation functional
together with van der Waals (vdW) corrections based on the TSsurf method.56 FHI-aims employs
numeric atom-centered orbitals. For this work, we used the default
tight settings for the basis set, except for the onset of the cutoff
potential for the gold basis functions, which was increased to 5 Å
to get a more accurate description of the surface dipole. The unit
cell was sampled with a 7 × 7 generalized Monkhorst–Pack
grid and a Gaussian smearing of 0.1 eV. The interface was modeled
using periodic boundary conditions, using a five-layer slab for the
metal and approximately 80 Å of vacuum. To electrostatically
decouple the slabs perpendicular to the surface, a dipole correction
was employed.57 To find the optimal adsorption
position, the molecule as well as the two topmost Au layers were allowed
to fully relax until the forces fell below 0.02 eV/Å.</p><p>For
the analysis of the vibronic nature of side bands in the dI/dV spectra, we calculated the relaxed
structures of the neutral and negatively charged free molecule (gas
phase), using the B3PW91 functional and the 6-31g(d,p) basis set as
implemented in the Gaussian 09 package.58</p><!><p>We first focus on the TAPP-CF3/Au(111) system and elucidate
the adsorption structure by means of STM and DFT since it strongly
influences the interfacial electronic structure. We then analyze the
electronic properties of a (sub)monolayer coverage of TAPP-CF3 molecules on the Au(111) surface in detail using STS and
2PPE, and complement these findings with DFT calculations. Subsequently,
a comparison to other TAPP derivatives (TAPP-H and TAPP-C3F7, see Figure 1) will be drawn by 2PPE measurements.</p><!><p>STM images reveal that
TAPP-CF3 molecules form densely
packed extended islands with mono-domain structures up to 100 nm diameter
(Figure 2a). We observe
no preferential orientation of the islands with respect to the underlying
Au(111) substrate, suggesting a weakly physisorbed state. This scenario
is further supported by the herringbone reconstruction remaining intact
below the molecular layer. A close-up view on the molecular arrangements
reveals a flat adsorption geometry of the individual molecules (Figure 2b) and allows for
a structural model of the molecular layer (Figure 2c).</p><!><p>STM topography of a submonolayer coverage TAPP-CF3 on
Au(111). (a) Large-scale image revealing densely packed islands with
the size limited by monoatomic steps of the Au(111) surface. Vbias = 300 mV, It = 300 pA. (b) Close-up view of the monolayer showing the individual
molecules. Vbias = −300 mV, It = 300 pA. (c) Close-up view of the superimposed
structure model and unit cell of the molecules periodic lattice. The
unit cell size is a = 1.23(5) nm × b = 1.17(5) nm with an angle of 104(3)° between a and b, resulting in a unit cell area
of 1.39(5) nm2. Vbias = −300
mV, It = 300 pA. (d) Theoretical structure
model (see text).</p><!><p>To understand the formation
of this self-assembled structure, we
performed dispersion-corrected DFT calculations. In a first step,
we calculated the interaction energy between two parallel aligned
TAPP-CF3 molecules in the gas phase at different relative
positions and distances (see the Supporting Information). The energetically beneficial relative positions obtained with
this approach qualitatively agree very well with the experimental
unit cell (Figure 2d). In a second step, we determined the adsorption site of TAPP-CF3 by running several full geometry optimizations, with the
initial position of the TAPP-CF3 molecule on the four different
high-symmetry points of the Au(111) surface (top, bridge, hexagonal
close-packed (hcp), and face-centered cubic (fcc) hollow). The energetically
most favorable adsorption site is found when the center of the molecule
is above a bridge position. The other adsorption sites are between
10 (top) and 50 (both hollow sites) meV energetically less favorable.
From this, it can be concluded that the monolayer structure is mostly
determined by intermolecular interactions (as opposed to molecule–substrate
interactions), in line with previous conclusion (see above) that the
molecule is mostly physisorbed. The adsorption energy (at the bridge
site) has an overall value of −2.585 eV, which can be split
into a vdW contribution of −2.852 eV and an electronic contribution
(+0.262 eV). The latter has a positive value, indicating that the
adsorption is mainly governed by vdW forces.</p><!><p>To resolve the electronic
states of the TAPP-CF3 molecules,
we recorded differential conductance spectra (dI/dV) on the molecular layer. The spectra of the occupied (negative
bias voltage) states are shown in Figure 3a (red and orange spectra) and compared to
a background spectrum on the bare Au(111) (yellow). First, we note
that the broad peak at −1.0 eV is also present in the spectrum
on the bare surface and can be associated with the sp band of Au.
Second, the surface state of the Au(111) substrate is shifted toward
the Fermi level. Third, we observe an additional resonance at −1.8
eV, followed by a broader satellite structure. These features are
only present on the molecular layer and may be associated with tunneling
through occupied molecular-derived states. Spectra in the regime of
the unoccupied states reveal two resonances at 1.2 and 1.4 V, and
two resonances around 2.35 and 2.55 V that only appear on the molecules.
Notably, the spectra shown in Figure 3a,b are recorded on and in close vicinity of the CF3 group and exhibit variations in the relative intensity of
the peaks.</p><!><p>(a, b, d) Tunneling spectra recorded at negative (a) and positive
(b) bias voltages at different positions on an individual TAPP-CF3 molecule. (d) Zoom-in view of the region around the LUMO
resonance around 1.0–1.7 V. The background spectrum of the
bare Au(111) surface is displayed in yellow. The insets show STM images
with color markers to indicate where the individual spectra were taken.
The intensity of the molecular resonances varies strongly across the
molecule. The feedback parameters for the STM images were Vbias = 0.3 V and It = 300 pA. The spectra were recorded in constant-current mode with
a tunneling current of It = 1 nA. (c)
Huang–Rhys factors (blue dots) and simulated Franck–Condon
dI/dV spectra (green lines) of TAPP-CF3 obtained from gas-phase DFT simulations (for details, see
the Methods section and ref (59)). Different broadenings
were used to simulate different lineshapes. A half-width at half-maximum
(HWHM) broadening of 65 meV produces lineshapes that agree well with
the experimental lineshapes of HOMO, LUMO, and LUMO + 1.</p><!><p>A better perception of the variations can be gained from
differential
conductance maps at the respective energies. These are shown in Figure 4. The maps at 1.2
and 1.4 eV show the largest intensity all over the TAPP-CF3's backbone, in agreement with the intensity of these peaks
decaying quickly away from the molecule. Interestingly, both maps
are very similar. In contrast, the maps at 2.35 and 2.55 eV exhibit
the largest intensity at the CF3 terminations. Again, both
maps are similar. The distinct spatial intensities can be ascribed
to tunneling through different molecular orbitals. Tentatively, we
assigned these to the lowest unoccupied molecular orbital (LUMO at
1.2 eV) and LUMO + 1 (at 2.35 eV). This assignment will be corroborated
by DFT calculations below. The appearance of satellite peaks with
the same spatial extent as the main peak is typically ascribed to
vibronic states, which arise from the simultaneous excitation of electronic
states and vibrational modes in the tunneling process.60−66 Further indication of this process is given by the similar spacing
of the satellite peaks in LUMO and LUMO + 1.</p><!><p>STM images (gray) and
differential conductance maps (color) recorded
in the same area as the corresponding STM images. The conductance
maps were recorded in constant-height mode at the energies of the
resonance peak positions, as indicated in the maps. (a) Conductance
map at the energy of the molecular HOMO. (b) Conductance maps at the
energy of the LUMO. (c) Conductance maps at the energies of the molecular
LUMO + 1 and LUMO + 2. The setpoints for the STM images and before
taking the maps were for (a, c) Vbias =
0.3 V, It = 300 pA and for (b) Vbias = 0.5 V, It = 300 pA.</p><!><p>To sustain the interpretation
of the vibronic nature of the side
bands, we calculated the expected dI/dV lineshape within the Franck–Condon picture. For this, we
considered a free molecule in the gas phase and calculated the relaxed
structures of the neutral and negatively charged molecules. From these,
we derived the vibrational modes and their corresponding Huang–Rhys
factor,59 which mimics the electron-vibration
coupling strength. Figure 3c shows the Huang–Rhys factors for all modes. There
are several modes with a large Huang–Rhys factor, most notably
at 22, 50, 118, 162, 172, 177, and 205 meV. To simulate the dI/dV spectrum, we considered individual
and coupled modes with a significant Huang–Rhys factor, i.e.,
higher harmonics of individual and coupling of different modes and
their harmonics ("progression of progressions"). Then,
the intensities were convoluted with a Lorentzian lineshape.59,67 A set of spectra with different Lorentzian widths is shown in Figure 3c (green). The best
agreement with the experimental lineshape of the LUMO and LUMO + 1
(consisting of a broad peak and a shoulder) is obtained when applying
a Lorentzian width of 65 mV (half-width at half-maximum). We note
that the simulations reveal an additional shoulder at ∼400
meV above the main resonance, which can also be found as a faint signal
in the experimental spectra (see close-up view on the LUMO in Figure 3d). The strong similarity
of the spectral lineshape of calculated vibronic states and experimental
dI/dV spectra, together with the
similar spatial extent of the main peak and satellite, strongly suggest
a vibronic origin of the satellite peak structure. However, we note
that fine details in the relative intensity of main resonance and
satellite peak vary across the molecule (compare red and orange spectra
in Figure 3b,c). This
behavior is not captured in the Franck–Condon picture. We suggest
that vibration-assisted tunneling can account for these intensity
variations.68,69</p><p>We briefly note that the
resonance at −1.8 eV is also followed
by a satellite structure, albeit with even less resolution. We suggest
that also in this case vibronic peaks contribute to the spectral intensity
as the energy spacing matches the one at positive bias voltages.</p><p>Complementary insight into the electronic structure of TAPP-CF3 molecules on Au(111) can be gained by 2PPE measurements.
A detailed analysis of the observed peaks and their photon energy
dependency has been published in ref (43). Here, we compile these results together with
the data from the tunneling spectra in Figure 5. We find a very good agreement between the
two experimental techniques. In summary, deposition of 1 ML TAPP-CF3 leads to a work function (Φ) decrease of 0.4 eV compared
to the bare Au(111) surface (Φ = 5.50 eV). Again, we find that
the LUMO of TAPP-CF3 is energetically far above EF (1.20 eV) and the HOMO far below EF (−1.73 eV). Thus, no Fermi-level pinning occurs,
and accordingly no charge transfer takes place. Both the LUMO (electron
affinity level) and HOMO (ionization potential) are transport states;
hence, the transport gap is 2.93 eV. Notably, the vibronic contributions
related to the LUMO are also observed in 2PPE. The higher-lying affinity
level, LUMO + 1 (2.35 eV), is not seen in 2PPE. This might be due
to a weak wave function overlap (transition dipole moment). Two further
unoccupied intermediate states are found in 2PPE at 3.19 and 3.80
eV, an energy region in which STS measurements are not feasible. In
addition, an unoccupied final state at 5.65 eV is observed. Notably,
an occupied interfacial hybrid band (OIHB) and an unoccupied interfacial
hybrid band (UIHB) have been identified as discussed in detail in
ref (43) (see below).</p><!><p>Energy-level
diagram of TAPP-CF3 adsorbed on Au(111)
based on STS and 2PPE43 measurements in
the sub- to monolayer regime. Dashed lines indicate vibronic transition. EF denotes the Au(111) Fermi level, and Φ
denotes the work function. UMO refers to an unoccupied molecular orbital,
OIHB denotes an occupied interfacial hybrid band, and UIHB denotes
an unoccupied interfacial hybrid band. SS′ refers to the shifted
surface state, and IPS denotes an image potential state.</p><!><p>To corroborate our interpretation of the molecular-derived
resonances,
we employed DFT calculations to the surface-adsorbed molecule. First
of all, our calculations show that upon adsorption of TAPP-CF3, the work function of the interface is reduced by 0.24 eV
compared to the pristine Au(111) surface. This is in excellent agreement
with the experiment, for which a work function reduction of 0.40 eV
was found. The reduction in work function can be explained by an induced
interface dipole. Computationally, the interface dipole can be separated
into a molecular contribution (which is obtained by calculating the
potential jump for the hypothetical, freestanding monolayer in the
geometry it adopts on the surface) and a contribution from the interaction
between metal and molecule (often termed "bond dipole").
Here, the molecular contribution amounts to +0.26 eV, arising from
the fact that the CF3 groups bend out of the molecular
plane during adsorption (see the Supporting Information). The bond dipole consequently amounts to −0.50 eV. We note
in passing that the computational method we employ here (PBE + TSsurf) has a tendency to induce molecular bending upon adsorption,70 thus overestimating the molecular dipole, while
underestimating the adsorption distance.71−73 As a consequence,
the effect of Pauli pushback is overestimated. The latter describes
the effect that the electron density spilling out from the surface
is pushed back by the π conjugated electron system of the molecular
adsorbate layer. As a result, it requires less energy to remove electrons
from the substrate and the work function of the system is lowered.74,75 The slight underestimation of the work function reduction by theory
(compared to the experiment) is thus fully consistent with expectations.</p><p>Remarkably, both the theoretical and experimental interface dipole
values are relatively small. They are smaller than the work function
reductions usually found for systems that interact only via Pauli
pushback, which for inert organic molecules on gold often amounts
to up to 1 eV.76,77 An almost zero net interface
dipole has been observed for the low-coverage phase of hexaazatriphenylene-hexanitrile
(HATCN) on Ag(111).78 However, in contrast
to HATCN/Ag(111), for TAPP-CF3/Au(111), we find that the
molecule is ≈0.2 e positively charged (when
applying the Mulliken charge partitioning scheme, see the Supporting Information), while the LUMO remains
empty (see Figure 6a). We therefore explain the relatively small interface dipole by
the fact that the bulky CF3 group slightly lifts the molecule
from the surface, resulting in a relatively large adsorption height
and, correspondingly, in a small dipole as the pushback is reduced.
This hypothesis is also consistent with the observation that the TAPP-H
derivative shows a larger (more negative) and the C3F7 derivative shows a smaller (less negative) interface dipole
experimentally (see below).</p><!><p>(a) Calculated density of states (DOS) projected
on the adsorbed
molecule. (b) Computed tunnel element maps for the orbitals HOMO –
LUMO + 2. The elements were obtained as the overlap integral between
the respective eigenstate of a freestanding molecule and a Au 6s state
(representing the tip). The Au state was centered 8 Å above the
substrate (corresponding to approximately 3 Å above the F atoms
and <5 Å above the carbon backbone) and moved in the x–y-plane to probe different tip
positions. All maps are normalized to their maximum intensity.</p><!><p>Inspection of the density of states (DOS) projected
onto the adsorbed
molecule (Figure 6a)
reveals that the LUMO is located close to the Fermi energy (at +0.43
eV). This may easily be mistaken as an indication for Fermi-level
pinning, which is not the case. The LUMO peak is more than 3 times
the broadening value (0.1 eV) due to the Fermi energy. Hence, the
LUMO is not populated, which would be a prerequisite for Fermi-level
pinning. Rather, the proximity of LUMO and Fermi energy is coincidental
and partly due to inherent shortcomings of the DFT methodology, as
we discuss below. Besides the LUMO at +0.43 eV, our projected DOS
shows strong molecular peaks at +1.73 and +2.36 eV, originating from
LUMO + 1 and LUMO + 2, respectively. The DOS is thus qualitatively
in agreement with the observed experimental values (e.g., the LUMO/LUMO
+ 1 separation of 1.3 eV), although the energetic position of the
states is consistently at too low energies. The fact that there is
no perfect agreement between theory and experiment is, per se, not
surprising. Technically, Kohn–Sham orbitals are not physical
observables, but they can be associated with ionization energies under
the right circumstances.79 More importantly,
DFT consistently underestimates the energies of empty states, but
it also misses the stabilization of ions near a surface through image
charge effects.80 At metal–organic
interfaces, the latter two effects often cancel each other out, although
not perfectly. There are elaborate schemes to achieve better qualitative
agreement (e.g., via the use of hybrid functionals and image charge
correction schemes81), but these are computationally
extremely expensive.</p><p>Instead, to verify the experimental assignment,
we computed the
expected dI/dV maps for the HOMO
and the first three unoccupied states. To this end, we calculated
the overlap integrals between a hypothetical Au tip, consisting of
a 6s orbital, and the eigenstate densities of the molecular monolayer
(after removing the substrate for computational effort). In Figure 6b, we display maps
of these tunnel elements for the HOMO and for LUMO – LUMO +
2 when scanning the tip across the surface at a height of 8 Å
above the substrate (3 Å above the fluorine atoms). The map of
the HOMO agrees well with the map at −1.8V, thus corroborating
its previous assignment in experiment (see Figure 4). The theoretical map of the LUMO exhibits
a rich structure of nodal planes along the molecular backbone. Experimentally
we could not resolve these details, but the overall intensities at
the molecules' center in the dI/dV maps at 1.2 and 1.4 V is consistent with the interpretation as being
LUMO-derived. LUMO + 1 has the largest tunnel matrix element at the
CF3 group, similar to the experimental maps at 2.35 V.
As LUMO + 2 is found 200 meV above LUMO + 1, we suggest that it overlaps
with the vibronic peaks of LUMO + 1. The map at 2.55 V is thus most
probably a convolution of vibronic states and LUMO + 2.</p><!><p>In the following,
we study the influence of the side chains on
the interfacial electronic structure, in particular with respect to
interfacial band formation of the other two TAPP derivatives, namely,
TAPP-H and TAPP-C3F7 (see Figure 1) adsorbed on Au(111) by means of 2PPE.</p><p>Figure 7a displays
a 2PPE spectrum of 1 ML TAPP-H/Au(111) recorded with a photon energy
of 4.3 eV. The spectrum is fitted by an exponential background and
Gaussian-shaped peaks. Several 2PPE features are observed. We use
the characteristic behavior of photon-energy-dependent measurements
to assign peaks to occupied or unoccupied electronic states (Figure 7b). Additional 2PPE
data are presented in the Supporting Information. Apart from occupied and unoccupied electronic TAPP-H-derived states,
the Au(111) d-bands, the shifted surface state (SS′), as well
as the first (n = 1) image potential state (IPS)
are observed. Moreover, a peak deriving from an OIHB is identified
(see below). The observed energy levels are compiled in Figure 8, right column. Analogous measurements
and their analysis have been applied to a monolayer of TAPP-C3F7 on Au(111) (data can be found in the Supporting Information). The results are also
shown in Figure 8.</p><!><p>(a) 2PPE
spectrum recorded with a photon energy of hÎ½
= 4.3 eV at 1 ML TAPP-H/Au(111). The energy axis reveals
the final state (EFinal) of photoemitted
electrons with respect to the Fermi energy EF (EFinal – EF = Ekin + Φ); thus,
the low-energy cutoff corresponds to the work function (Φ) of
the adsorbate/substrate system. The spectrum is fitted by an exponential
background and Gaussian-shaped peaks. (b) Photon-energy-dependent
peak positions to assign the observed photoemission signals to occupied,
unoccupied intermediate or final electronic states. A slope of 1 suggests
that a peak originates from an unoccupied intermediate state, a slope
of zero suggests that it is from an unoccupied final state (located
above the vacuum level), and a slope of 2 is related to peaks originating
from occupied states. LUMO* indicate vibronic contributions related
to the LUMO.</p><p>Energy-level diagram of 1 ML TAPP-R (R = −H,
−CF3, −C3F7) adsorbed
on Au(111). EF denotes the Au(111) Fermi
level, and Φ
denotes the work function. UMO refers to an unoccupied molecular orbital,
OIHB denotes an occupied interfacial hybrid band, and UIHB denotes
an unoccupied interfacial hybrid band. SS′ refers to the shifted
surface state, and IPS denotes an image potential state. Dashed lines
indicate vibronic transitions. Exc. names an excitonic state.</p><!><p>Comparison of the three derivatives allows us to
identify the following
similarities and differences: Adsorption of a monolayer of all of
these molecules leads to a shift of the Au(111) surface state (SS′)
of around 200 meV toward the Fermi level. Similar shifts have been
observed for many organic molecules on Au(111), where charge transfer
to the substrate is negligible and ascribed to a modification of the
image charge and work function (Φ).13,53,82−87 Indeed, we find a reduction of the work function for all three molecules,
the largest one for TAPP-H by 0.74 eV.</p><p>While the work functions
for 1 ML TAPP-CF3/Au(111) (Φ
= 5.10 ± 0.05 eV) and 1 ML TAPP-C3F7 (Φ
= 5.17 ± 0.05 eV) are very similar, the work function possesses
a smaller value for 1 ML TAPP-H/Au(111) (Φ = 4.76 ± 0.05
eV). STM results indicate an almost equal unit cell area for TAPP-CF3 and TAPP-C3F7 (see Figure 2c and the Supporting Information); thus, a similar Φ value is
expected, when considering a similar pushback effect. In the absence
of the fluorinated alkyl side chains, i.e., for TAPP-H, the packing
density is most likely increased, leading to a stronger reduction
of Φ. Since the image potential states (IPS) are pinned to the
vacuum level, their energetic position follows the work function shift.</p><p>The HOMO of all three species is located at −1.73 eV, while
the unoccupied states vary between the molecules. In the case of TAPP-H/Au(111),
we observe vibronic contributions belonging to the LUMO (at 1.36 and
1.53 eV) similar to TAPP-CF3/Au(111). In addition, an unoccupied
state at 1.96 eV is observed, which may be attributed to an excitonic
state related to the LUMO + 1, observed by STS in TAPP-CF3/Au(111) at 2.4 eV. Some states in TAPP-H/Au(111), e.g., the UMO4 and UMO5, and in TAPP-C3F7/Au(111), the vibronic states associated with the LUMO are not detected.
The reasons are not obvious. However, in all TAPP-R/Au(111) systems,
we find interfacial occupied and unoccupied hybrid bands (OIHB and
UIHB), which show pronounced dispersions as we will demonstrate by
angle-resolved 2PPE in the next section.</p><!><p>Recently,
we observed the dispersion of the UIHB and OIHB for the TAPP-CF3 derivative on Au(111).43 Since
a different packing or ordering behavior is expected for the three
TAPP derivatives due to the varying size of the side groups, we studied
the influence of the side chains with respect to interfacial band
formation of the other two TAPP derivatives, namely, TAPP-H and TAPP-C3F7 (see Figure 1). To be able to conclude that band formation has occurred
and to measure dispersion in angle-resolved photoemission, lateral
extended interfacial hybrid states (bands) must be formed. This is
only feasible if the molecules on the surface display well-defined
long-range order. To check for similar interfacial bands, we recorded
angle-resolved 2PPE spectra also for the two other derivatives (Figure 9a–c). A compilation
of the dispersions is shown in Figure 9d, evidencing a hole-like dispersion of both the OIHBs
and UIHBs.</p><!><p>(a–c) Angle-resolved 2PPE measurements at the TAPP-R/Au(111)
interfaces. (d) Hole-like dispersion of the OIHBs, UIHBs, and a state
labeled as B (see text) around the Γ-point (k∥ = 0). Parabolic fits yield the effective masses
(m*) of the states around the Γ-point.</p><!><p>As already demonstrated in our previous study for
the TAPP-CF3/Au(111) interface, the effective masses (m*) of the OIHB and UIHB are m* = −1.18
±
0.05 me and −0.39 ± 0.06 me, respectively.43 For the TAPP-H/Au(111) interface, the OIHB possesses an effective
mass of m* = −1.76 ± 0.18 me. In the case of the TAPP-C3F7/Au(111)
system, the UIHB exhibits an effective mass of m*
= −0.14 ± 0.03 me. The dispersion
of the OIHB is visible, but the peak positions cannot be precisely
identified, since the 2PPE peak is merged into the broad contribution
of the nearby d-bands. Thus, a determination of m* is impossible. Additionally, we observe two dispersing states labeled
as A and B (Figure 9c). Unfortunately, these peaks overlap with other 2PPE features,
in particular under normal emission (k∥ = 0). This renders an assignment to occupied or unoccupied states
via photon-energy-dependent measurements difficult. In the case of
state B, extrapolating an energy EFinal – EF = 5.9 eV from the parabolic
fit at k∥ = 0 would result in an
energetic position of an unoccupied state at 1.4 eV or for an occupied
state at −3.1 eV with respect to EF. This state has an effective mass of m* = −0.86
± 0.24 me.</p><p>While hybridization
between molecular and metal states occurs in
many organic/metal systems, experimental evidence for interfacial
band formation via dispersing electronic states is rarely observed.
Only for strong electron acceptors (F4TCNQ/Au(111),39,40 PTCDA/Ag(110),41 NTCDA/Ag(110)42) or a donor (TTF/Au(111)40), dispersing interface bands have been proposed. In all
cases, a charge transfer between molecular states and the surface
state of the metallic substrate appears (Fermi-level pinning). In
contrast, for the TAPP derivatives adsorbed on Au(111), no charge
transfer occurs, since the HOMO is far below EF (−1.73 eV) for all three adsorbate/substrate systems
and the LUMO of TAPP-CF3/Au(111) (1.20 eV) as well as the
UMO1 (1.36 eV) of TAPP-H/Au(111) are energetically far
above EF. As discussed in detail for TAPP-CF3/Au(111),43 interfacial band formation
is only possible if wave function mixing of localized molecular states
with delocalized metal bands takes place. The OIHBs are energetically
located in the Au(111) d-band region.88−90 Thus, we suggest that
occupied TAPP states hybridize with the d-band, leading to the emergence
of the OIHBs. The UIHBs lie within the energy regime of the unoccupied
metal sp-band.88,90 Hence, we propose a hybridization
between the unoccupied molecular state and this band. Note that due
to the planar adsorption geometry of the TAPP-R molecules on the Au(111)
surface in the monolayer regime (see the Supporting Information and refs (26) and (43)), intermolecular band formation can be excluded.</p><!><p>In summary, we have investigated the electronic structure of three
tetraazaperopyrene derivatives on Au(111) in great detail by complementary
experimental techniques and density functional theory. We find strong
similarities between the derivatives, suggesting common adsorption
properties. Most notably, all of these derivatives develop interfacial
hybrid bands, which we ascribe to hybridization of higher-lying molecular
states with the sp- and d-bands of the substrate. Band formation with
the substrate is rather surprising as the energy-level alignment of
all molecules indicates the absence of charge transfer. Indeed, we
find several indications that the molecules are weakly physisorbed
on a Au(111) surface, such as an intact herringbone reconstruction
below the molecular layer and the persistence of the surface state.
Interfacial band formation while molecular properties are preserved
can be considered to be beneficial for charge injection into the organic
semiconductor.</p><!><p>Temperature-programmed
desorption data for TAPP-R (R:
H, CF3, C3F7), calculated molecular
interactions of TAPP-CF3, two-photon photoemission data
for TAPP-H and TAPP-C3F7, and scanning tunneling
microscopy images of TAPP-C3F7 (PDF)</p><p>jp1c04217_si_001.pdf</p><p>The authors declare no
competing financial interest.</p>