Formation of Occupied and Unoccupied Hybrid Bands at Interfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Formation of Occupied and Unoccupied Hybrid Bands at Interfaces between Metals and Organic Donors/Acceptors David Gerbert, Oliver T. Hofmann, and Petra Tegeder J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09606 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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The Journal of Physical Chemistry

Formation of Occupied and Unoccupied Hybrid Bands at Interfaces between Metals and Organic Donors/Acceptors David Gerbert,† Oliver T. Hofmann,‡ and Petra Tegeder∗,† †Ruprecht-Karls-Universit¨ at Heidelberg, Physikalisch-Chemisches Institut, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ‡Technische Universit¨ at Graz, Institut f¨ ur Festk¨ orperphysik, NAWI Graz, Petersgasse 16, 8010 Graz, Austria E-mail: [email protected] Phone: +49 (0) 6221 548475 Abstract Efficient charge transport in organic semiconductors and at their interfaces with electrodes is crucial for the performance of organic molecule-based electronic devices. Band formation fosters effective transport properties and can be found in organic single crystals of large π-stacking aromatic molecules. However, at molecule/metal interfaces hybrid band formation and band dispersion is a rarely observed phenomenon. Using angle-resolved two-photon photoemission supported by density functional theory calculations we demonstrate such band formation for two different molecule/metal systems, namely tetrathiafulvalene (TTF)/Au(111) and tetrafluoro-tetracyanoquinodimethane (F4 TCNQ)/Au(111), in the energy region of occupied as well as unoccupied electronic states. In both cases strong adsorbate/substrate interactions result in formation of

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interface states due to hybridization between localized molecular states and delocalized metal bands. These interface states exhibit significant dispersions. Our study reveals that hybridization in combination with an extended well-ordered adsorption structure of the π-conjugated organic molecules is a striking concept to receive and experimentally observe band formation at molecule/metal interfaces.

Introduction Band formation at molecule/metal interfaces has attracted much interest since efficient charge transport at those interfaces plays a crucial role in any kind of organic moleculebased (opto)electronic device. 1–6 While metallic states are extended leading to delocalized charge carriers with high mobilities, molecular electronic states are localized due to the lack of adequate intermolecular interactions within a thin (disordered) molecular film. Contrary, in organic single crystals of large π-stacking aromatic molecules band formation and dispersion has been found. 7–10 However, it has been suggested that the interaction between delocalized metal and localized molecular wave functions can give rise to new delocalized molecule/metal hybrid states at the interface (see e.g. the Anderson-Grimley-Newns model 11–14 ). 15 From the experimental point of view it is mandatory that the delocalized hybrid states possess a clear energy dispersion parallel to the surface (k ) indicating a lateral delocalization of electrons or holes in order to prove band formation for instance in an angle-resolved photoemission experiment. For several adsorbate/substrate systems the formation of interfacial hybrid states due to the interaction between an unoccupied molecular state (i.e., the lowest unoccupied molecular orbital, LUMO) with the Shockley surface state (SS) of a single-crystal noble metal surface have been proposed. Thereby the resulting interfacial state is energetically upshift compared to the energetic position of the undisturbed SS even above the Fermi level of the substrate. 16–27 Here, the degree of dispersion of the interface states are strongly dominated by the property of the SS, viz. possessing a similar effective electron mass. As demonstrated 2


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by Armbrust et al. 26 the dispersing states result from a modified SS of noble metal surfaces caused by the adsorption of the organic compounds. On the other hand and so far the most prominent examples, metal-molecule hybrid states with lateral delocalization originating from the hybridization of the LUMO wave function with the delocalized metal sp-states have been observed in the case of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) adsorbed on Ag(110). 28,29 However, while hybridization between molecular and metal states arise in many organic/metal systems, an experimental evidence for band formation via the dispersion of the electronic states at these interfaces is rarely found. 28,29 In this article, we demonstrate interfacial band formation via experimentally detected dispersion of hybrid states for two molecule/metal systems, namely tetrathiafulvalene (TTF) and tetrafluoro-tetracyanoquinodimethane (F4 TCNQ) adsorbed on Au(111), respectively, using angle-resolved two-photon photoemisson spectroscopy (AR-2PPE). We choose these molecules, since they are known to strongly interact with the Au(111) substrate, which results in hybridization between molecular and metal states. 30–34 However, so far it has not been demonstrated whether these states retain a more molecular, localized character (no dispersion), or whether true hybrid bands are formed (dispersing states). TTF is a prototype electron-donating molecule and in combination with tetracyanoquinodimethane (TCNQ) one of the original initiators of organic electronics due to the exceptional high electronic conductivity in formed charge-transfer crystals. 35,36 At the TTF/Au(111) interface a molecule to metal electron transfer type interaction takes place. 30,31 In contrast, F4 TCNQ is a strong electron acceptor. Adsorption on the Au(111) surface leads to a metal to molecule electron transfer. 32–34 Supported by density functional theory (DFT) calculations, we find for both systems not only strongly dispersing occupied electronic states, but also unoccupied states. In the case of the occupied hybrid bands we determine energetic positions and effective masses which differ significantly from the values known for the SS of the bare Au(111) surface and the

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modified SS due to adsorption of organic molecules. Thus, we elucidate that in the formation of occupied as well as unoccupied bands the gold sp-bands are involved. Additionally, for F4 TCNQ/Au(111) Au adatoms play a major rule for the formation of hybrid bands. Our results suggest that strong adsorbate/substrate interactions promoting hybridization between localized molecular states and delocalized metal states in combination with highly ordered extended adsorption structures of the π-conjugated organic molecules is a general guide to achieve and experimentally prove band formation at molecule/metal interfaces.

Methods Sample preparation. The Au(111) single crystal was prepared by Ar+ sputtering and annealing at 800 K. TTF or F4 TCNQ molecules were evaporated from a Knudsen cell onto the substrate held at liquid nitrogen temperature (ca. 100 K) in the case of TTF and at room temperature (ca. 300 K) for F4 TCNQ deposition. The film thickness was adjusted and measured via temperature-programmed desorption (TPD) which allows thermally activated self-organization and provides samples with full monolayer coverage on a Au(111) surface. Figure 1 presents TPD spectra as a function of TTF (see Fig. 1a) and F4 TCNQ (see Fig. 1b) coverage, respectively, recorded with a heating rate of 1 K/s. Both molecules show two separate desorption peaks labeled as α1 and α2 . Thereby α1 can be associated with desorption from multilayers, while α2 corresponds to desorption of the monolayer (molecules in direct contact with the gold substrate). Annealing the sample to 370 K for F4 TCNQ and 300 K for TTF leads to the desorption of multilayers, leaving behind a well-defined sample with a monolayer coverage. Photoemission experiments. 2PPE allows investigation of occupied and unoccupied electronic states 37–45 in a pump-probe scheme, while the variation of the detection angle provides insights into the dispersion (k ), i.e., the degree of localization/delocalization of occupied and unoccupied electronic states. The tunable femtosecond laser system which

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Figure 1: Temperature-programmed desorption spectra. Series of TPD spectra for different initial coverages of (a) TTF and (b) F4 TCNQ adsorbed on the Au(111) surface. The desorption signals labeled as α1 and α2 can be assigned to multilayer and monolayer desorption, respectively. delivers laser pulses over a wide range of photon energies, and the 2PPE setup is described elsewhere. 46 DFT calculations. All calculations were performed with the FHI-aims code 47 using ”tight” default numerical settings as shipped with the code. We tested explicitly that these settings yield converged total energies and work function shifts to 10 meV or better. A Γ-centered k-grid was used with a sampling of 7 × 7 × 1 k-points for both TTF and Au. The interfaces were modelled using the periodic slab approach, allowing for at least 30 ˚ Avacuum between the periodic replicas in z-direction and using a self-consistently determined dipole correction to avoid spurious electrostatic interactions. 48 The self-consistent field cycle was converged until the total energy changed by less than 10−6 eV and the sum of eigenvalues changed by less than 10−2 eV. Geometry optimizations were performed relaxing all but the A. bottom three metal layers until the residual force was smaller than 10−3 eV/˚

In the following we briefly review the properties of DFT, and how they affect the direct comparison between theory and experiment. First, it should be emphasized that by construction, DFT is a ground state method, designed to yield the correct electron density. 5



Thus, DFT in general (and dispersion-corrected PBE in particular) yields generally good results for covalently bonded (i.e., hybridized) systems. With the advent of van-der-Waals corrections, it now reproduces geometries of systems where such contributions play a major role, 49 such as the ones under investigation here. PBE also performs well for work-function of metallic systems, 50 and the literature available indicates that is performs similarly well for adsorption-induced work-function modifications. 51 That being said, the orbital energies obtained within the DFT framework do not have a strict physical meaning. Still, they can be - and commonly are - taken as good approximation to (photoemission) ionization energies. 52 However, due to the so-called many-electron self-interaction error, 53 occupied states are always predicted too high in energy, while the energies of unoccupied states are always found at too low energies (a property often referred to as the band gap problem 54 ). In the context of the present paper, it should be emphasized that the magnitude of the deviation depends strongly on the localization of the state in question: More localized states suffer more strongly from this effect than delocalized states. In other words, compared to delocalized states, localized states are always found too close to the Fermi energy. Despite this principle shortcoming, a recent survey of several molecules on metal substrates showed that the occupied part of the density of states/band structure is generally well reproduced within DFT, with only minor deviations. Whether the same observation also holds for the unoccupied part of the spectrum (as probed with 2PPE) is, however, not a priori clear. This is because a fundamental shortcoming of the orbital picture is that it does not discriminate between charged and neutral excitations, i.e. it neglects the exciton binding energy, that lowers the optical band gap compared to the fundamental band gap. In organic materials, this exciton binding energy can easily reach up to 1eV and more. This makes a direct comparison to 2PPE-Experiments, where an unoccupied state is populated through an excitation from either the metal or within the molecule, far from straightforward. In summary, from the DFT calculation one should generally expect a good representation of the geometry and the adsorption-induced work function modification. Also the hybridization of the molecu-

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lar/metallic states and the resulting band dispersion should be well captured, even if the energies of the states may be off by a few 100 meV compared to the experiment.

Results and Discussion

a

b

E-EF EVac

D = 1.35 eV

F = 5.50 eV

IPS 3.65 eV UIS1 3.20 eV

EF

OIS1 -0.10 eV

SS

OMS -1.20 eV OIS2 -1.60 eV

sp

Au(111)

hn = 6.08 eV

OIS2 OMS OIS1

-2.0

1 ML TTF

-1.5

hn = 4.03 eV

-0.5

0.0

8.0 -1.3

30°

-1.0

EBind - EF [eV]

d

hn = 6.08 eV m*OIS = 0.84 ± 0.06 me

EBind - EF [eV]

2

0°

-1.5

7.6

-1.6 7.4

-1.7

UIS1 OIS1

7.0

7.5

7.8

-1.4

hn = 4.03 eV

-10°

m*UIS = 1.15 ± 0.05 me

-1.8 6.5

22°

-10°

c 2PPE Intensity [normalized]

UPS Intensity [normalized]

Band formation at the TTF/Au(111) interface. Figure 2a shows an energy level

8.0

8.5

EFinal - EF [eV]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

-0.1 0.0

EFinal - EF [eV]

7.2

0.1 0.2 0.3 0.4 -1 kII [Å ]

Figure 2: Electronic structure and angle-resolved photoemission data of 1 monolayer TTF adsorbed on Au(111). (a) Energy level diagram of the experimentally observed electronic states measured at k = 0. EF denotes the Au(111) Fermi level, IPS an image potential state, UIS an unoccupied interface state, OIS an occupied interface state, and OMS an occupied molecular state. (b) Angle-resolved UPS data obtained with hν = 6.08 eV. (c) Angle-resolved 2PPE data recorded with hν = 4.03 eV. (d) Dispersion of the OIS2 and UIS around the Γ-point (k = 0). The OIS2 is shown with its binding energy with respect to the Fermi level (right axis), UIS is displayed with the final state energy above the Fermi level (left axis). Parabolic fits yield the effective masses of both states around the Γ-point.

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diagram of 1 monolayer (ML) TTF adsorbed on Au(111), which is based on detailed photon energy dependent 2PPE measurements (see Supporting Information, SI). Deposition of 1 ML TTF leads to a work function decrease of 1.35 eV compared to the bare Au(111) surface and induces an occupied interface state (OIS1 ) at a binding energy of -0.1 eV. Since it is known that TTF gets positively charged at the Au(111) surface, we assign the OIS1 to a hybrid state arising from the highest occupied molecular orbital (HOMO). This is in agreement with earlier DFT calculations 30,31,55 and the DFT calculations in this work. Figure 2b displays angle-resolved ultraviolet photoemission spectroscopy (UPS) data in which the electron-like dispersion of the occupied interface states OIS1 and, additionally, a second interface state OIS2 at a binding energy of -1.60 eV can be clearly observed. Note that the occupied molecular state (OMS) is localized. Furthermore, we find a dispersing unoccupied interface state (UIS), for which the 2PPE data are shown in Figure 2c. While a reliable analysis of the dispersion of OIS1 is not possible, since it crosses the Fermi-energy, we can determine the effective masses of both, the OIS2 (m∗ = 0.84 ± 0.06 me ) and the UIS1 (m∗ = 1.15±0.05 me ), as shown in Figure 2d. Theoretical modeling of the TTF/Au(111) interface. In order to corroborate our experimental results, we performed DFT calculations for the TTF/Au(111) surface. All geometries have been optimized using the PBE functional 56 augmented by the vdW surf scheme 57 to account for the missing long-range van-der-Waals forces (for details see Method section). The unit cell for TTF/Au(111) determined in Ref. 55 has been used. This cell contains two molecules in an herringbone arrangement as shown in Fig. 3a. Following the procedure proposed in Ref., 28 i.e., to separate between intermolecular (lateral) and substratemolecule interactions as well as charge transfer, we first perform calculations on a hypothetical, free-standing TTF monolayer in the same geometry as it adopts on the Au(111) surface. The results are shown in Fig. 3b. We find that already in the absence of substrate-mediated interactions, the two HOMOs of the system (consisting of the bonding and antibonding linear combination of the HOMOs of the two non-equivalent molecules and hence labelled

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as HOMO and HOMO* in Fig. 3b) exhibit a noticeable dispersion of ca. 50 meV. Importantly, the HOMO exhibits an electron-like dispersion, while the HOMO* shows a hole-like dispersion. Consequently, the split between the bonding and antibonding linear combination is largest at the Γ point, decreases on its path toward the K point and is smallest at the Brillouin-zone boundary.

Figure 3: Modeling the TTF/Au(111) interface. (a) Structure of the TTF/Au interface used for the calculation. The two molecules in the unit cell are highlighted. (b) Dispersion of the states of the hypothetical, free-standing monolayer. (c) Surface band structure along the Γ − K-direction, projected onto the molecule. Areas which carry greater weights on the molecule are more red. The white area above 2.0 eV represents a very strong molecular character. The yellow dotted lines indicate the energetic position of the experimentally observed interface states. (d) Density of states (integrated over all k-points that were sampled) of the TTF/Au interface, projected onto the molecular orbitals. Note that since there are two molecules in the unit cell, all orbitals (HOMO-1, HOMO, LUMO) appear twice. As a next step, we calculated the electronic structure of the TTF monolayer when adsorbed on a five-layered Au(111) slab. We find that the work function decreases by -1.4 eV, which is in excellent agreement with the experiment (see Fig. 2a). Figure 3c displays the surface band structure along the ΓK-direction. Between EF and EF + 1 eV, we find a

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strongly dispersing unoccupied state that has hybridized with the substrate. The region at even higher energies is almost exclusively dominated by states of the organic molecules. In the energy region of occupied states the graph is characterized by a lack of clearly distinct features. To understand this puzzling behavior, we projected onto each molecular orbital separately (see Fig. 3d). It becomes evident that the split HOMO-HOMO* pair, that disperses over ca. 50 meV in the isolated monolayer, has hybridized strongly with substrate states and now covers a range of more than 1 eV. Also most other π-orbitals, in particular those in the region between EF -5 eV and EF , are strongly hybridized, covering a large range of energies and hence showing a very low (molecular) density of states for any given energy interval. Band formation at the F4 TCNQ/Au(111) interface. In analogy to the TTF/Au(111) Fig. 4a summarizes the experimentally determined molecule-derived electronic energy levels for 1 ML F4 TCNQ adsorbed on Au(111). The associated extensive 2PPE data are presented in the SI. The state labeled as OIS involves the LUMO. Since the LUMO is below EF , 32–34,59 electron transfer from the metal to the molecule occurs and the LUMO becomes occupied. Figure 4b and 4c show angle-resolved 2PPE data recorded at two different photon energies in order to resolve the angular dependency of the respective electronic states. The OIS as well as both unoccupied states the UIS and UMB (UMB: unoccupied molecular band) reveal a dispersion. Based on coverage dependent measurements, we recently demonstrated that the dispersion of UMB is due to intermolecular hybridization and band formation. 58 In the case of the OIS (see Fig. 4d) a hole-like dispersion with an effective mass of m∗ = −1.4± 0.1 me is obtained. The variation in energy within the measured angular range is about 200 meV. A recent ARPES study also found this dispersing state, which has been proposed to arise from the F4 TCNQ Au-adatom network. 59 Our calculations presented below will corroborate that Au-adatoms are indeed involved in the occurrence of the OIS. For the unoccupied interface state, UIS, an electron-like dispersion of m∗ = 5.6±1.8 me is detected. The energy as a function of k varies about 50 meV for the UIS.

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a

b

E-EF EVac

D = 0.15 eV

F = 5.50 eV

UMB 4.32 eV UIS 3.64 eV

EF OIS

SS

-0.42 eV

sp

Au(111)

c hn = 4.74 eV

30°

-10°

7.8

7.0 7.5 8.0 EFinal - EF [eV]

8.5

hn = 4.74 eV

hn = 4.00 eV

9.4

m*UIS= 5.6 ± 1.8 me

30°

EFinal - EF [eV]

2PPE Intensity [normalized]

d

8.0

UIS

6.5

1 ML F4TCNQ

hn = 4.00 eV

OIS

7.6

9.3

7.4

9.2

m*OIS = -1.4 ± 0.1 me

9.1

UMB -10°

8.5

9.0

EFinal - EF [eV]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2PPE Intensity [normalized]

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9.5

m*UMB= 1.2 ± 0.1 me

7.2

10.0

-0.2

-0.1

0.0

0.1

-1

0.2

0.3

kII [Å ]

EFinal - EF [eV]

Figure 4: Electronic structure and angle-resolved photoemission data of 1 monolayer F4 TCNQ on Au(111). (a) Energy level diagram of the experimentally observed electronic states measured at k = 0. UMB denotes an unoccupied molecular band, 58 UIS an unoccupied interface state, and OIS an occupied interface state. (b) and (c) Angle-resolved 2PPE data obtained with hν = 4.00 eV and hν = 4.74 eV, respectively. (d) Dispersion of the OIS, UIS, and UMB around the Γ-point (k = 0). Parabolic fits yield the effective masses of the states around the Γ-point.

Theoretical modeling of the F4 TCNQ/Au(111) interface. For the modelling of F4 TCNQ on Au(111), we use the unit cell proposed by Faraggi et al. 34 which is based on STM measurements and supported by low-energy electron diffraction of 1 ML F4 TCNQ/Au(111). 58 The unit cell contains only one F4 TCNQ molecule (in contrast to TTF). The network between the F4 TCNQ molecules is mediated by Au adatoms, as shown in Fig. 5a. To understand the impact of these adatoms, we performed an analysis in three steps. First, a hypothetical free-standing monolayer of only F4 TCNQ (i.e., without adatoms) in the same geometry as it 11



adopts on the surface has been calculated. Due to the relatively large separation between the molecules, none of the orbitals show any appreciable dispersion. The situation changes qualitatively when we also include the adatoms. Most notably, the interfacial band structure (see Fig. 5b) becomes metallic, i.e., the lowest unoccupied state state crosses the Fermi-energy. The band at the EF displays a clear hole-dispersion at the Γ-point, with a bandwidth of approximately 300 meV.

Figure 5: Modeling the F4 TCNQ/Au(111) interface. (a) Structure of the F4 TCNQ/Au interface used for the calculations. The molecule and the surface adatom are hightlighted. (b) Dispersion of the states of the hypothetical, free-standing monolayer, including Au adatoms. (c) Surface band structure along the Γ − K direction, projected onto the molecule. Lighter areas carry greater weights on the molecule. In the F4 TCNQ-Au network (see Fig. 5a) each adatom is surrounded by four (equivalent) F4 TCNQ-molecules. Thereby the distances are not equidistant, viz. the distances from the top left and bottom right F4 TCNQ to the adatom are shorter (therefore called short diagonal hereafter) than the distances from the top right and bottom left F4 TCNQ molecules to the Au adatom (called the long diagonal hereafter). A visual inspection of the state crossing the Fermi-edge indicates that it is derived from the former F4 TCNQ LUMO and reveals the 12


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origin of the hole-like dispersion. The LUMO is inversion symmetric (i.e., is gerade), as is the Au dxz -orbtial that links the F4 TCNQ molecules across the short diagonal. At k = Γ, the linear combination between the Au-state and the F4 TCNQ LUMOs is strongly antibonding with respect to along the short diagonal and non-bonding (i.e., the overall overlap between the LUMOs and the Au-orbital essentially cancels) across the long-diagonal. Any phase factor this lattice-periodic function picks up (i.e., for any k = 0) leads to a decrease of the bonding character, and thus to a reduction in energy (i.e., hole-like character). At ca. EF = -1 eV we find another strongly dispersing state. In contrast to the former LUMO, however, it exhibits a clear electron-like dispersion. A visual inspection of the shape of the orbital shows that this orbital is comprised from the former HOMO and the dz 2 orbital of the Au adatom (see Fig. 5 b). Interestingly, we find also the total bandwidth of this state to be approximately 300 meV (see above). In the energy region of unoccupied states, most states retain molecular orbital character and do not show a k-dependent dispersion exceeding 100 meV, with one notable exception at approx. EF + 2.75 eV. This state clearly possesses an electron-like dispersion. Although this level still shows a pronounced molecular character, it appears not to be associated with low-lying molecular orbitals from the (metalfree) F4 TCNQ molecule. Rather, it is associated with the interaction of F4 TCNQ with the Au adatoms, and thus inherently an interface-induced state. As a final step, we place the F4 TCNQ/Au network onto the Au(111) slab. Our calculations yield a work function reduction of ΔΦ = -0.2 eV compared to the bare Au(111), in very good agreement with the 2PPE result (see Fig. 4a). This corroborates the selected unit cell, including the surface adatoms (without adatoms, the work function increases by 0.2 eV). The surface band structure shown in Fig. 5c shows that the character of the frontier states of the F4 TCNQ-Au network is mostly retained upon contact with the bulk metal. The band corresponding to the former LUMO can now be found directly below EF , indicating that it becomes fully filled upon contact. The energy region of unoccupied states (2.0 – 2.5 eV above EF ) shows a non-dispersive state followed by a state with a weak electron-like

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dispersion. Merging the experimental and theoretical results we clearly find band formation in the energy region of occupied as well as unoccupied electronic states for both the TTF/Au(111) and F4 TCNQ/Au(111) interface. In both systems the molecular electronic structure is strongly modified due to the adsorption on the metal resulting in a hybridization between molecular and metal states. For TTF/Au(111) we experimentally observed an occupied interface state (OIS1 ) close to EF at the Γ-point, which crosses EF for higher detection angles. According to our DFT calculations this state results from a strong hybridization between the TTF HOMO and substrate sp-band. Theory and experiment both show an electron-like dispersion of the OIS1 . At higher binding energies (around -1.6 eV with respect to EF ) a further electron-like dispersing state OIS2 (m∗ = 0.84± 0.06 me ) with a bandwidth of 270 meV is experimentally observed. Based on our calculations we attribute OIS2 to an interfacial hybrid state involving the TTF HOMO-1 and the metal sp-band. In the region of unoccupied electronic states UIS exhibits an electron-like dispersion (m∗ = 1.15± 0.05 me ) with bandwidth of ca. 300 meV. DFT found also a strongly dispersing unoccupied hybrid state at the TTF/Au(111) interface. Although DFT predicts the state at lower energies, this is to be expected as a consequence of the shortcomings of the employed functional as discussed in the SI. We propose that the UIS most likely results from wave function mixing of a TTF LUMO+n and the unoccupied sp-band of the gold substrate. For the F4 TCNQ/Au(111) the occupied interface state (OIS) possessing a hole-like dispersion (m∗ = −1.4± 0.1 me ), our calculations clearly demonstrate that it arises from the interaction of the cyano moieties with the Au adatoms. For this band we calculated a bandwidth of approximately 300 meV, which is in good agreement with our experimental findings. In the energy region above EF an electron-like dispersing unoccupied electronic state UIS is identified in 2PPE. The UIS can most likely be correlated with the calculated interface state found around 2.75 eV above EF (see Fig. 5b). This interface state also arises from the interaction of the molecules with the Au adatoms. For both molecule/metal interfaces we unambiguously demonstrate the formation of

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bands in both, the occupied and unoccupied region of the density of states via dispersion measurements in angle-resolved two-photon photoemission. This is due to wave function mixing of localized molecular states with delocalized metal (sp) bands (hybridization), which clearly differs from the results of former studies on other systems. 16–25 In the latter cases, the dispersing states resulted from a modified SS of the noble metal surfaces due to the adsorption of the organic molecules. These states possess a similar reduced mass as the SS, but upshifted in energy. This is the case for almost all systems as nicely demonstrated in Ref. 26 In order to draw general conclusions about prerequisites needed to achieve and experimentally determine band formation at molecule/metal interfaces on basis of our results we want to stress the following aspects: Strong electronic interactions between the adsorbate and metallic substrate resulting in hybridization of localized molecular states and delocalized metal band are required. In addition, to observe dispersion in angle-resolved photoemission and to be able to conclude that band formation occurred, lateral extended hybrid or interface states have to be formed. This is only possible if the molecules arrange at the surface in a defined and well-ordered extended structure. In both systems TTF/Au(111) and F4 TCNQ/Au(111) the adsorbate forms a well-ordered and -defined structure in the monolayer regime. 34,55,58

Conclusions In summary, we have demonstrated formation of hybrid bands at organic molecule/metal interfaces using angle-resolved two-photon photoemission (AR-2PPE) and density functional theory (DFT) calculations. In two molecule/metal systems, the tetrathiafulvalene (TTF)/Au(111) and tetrafluoro-tetracyanoquinodimethane (F4 TCNQ)/Au(111), we identified band formation via strongly dispersing bands in the energy region of both occupied and unoccupied states. The generation of bands at the interface resulted from hybridization between localized molecular states with delocalized metal (sp) bands. Our study revealed that hybridization in combination with an extended well-ordered adsorption structure of the

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π-conjugated organic molecules are requirements to receive and experimentally observe band formation at molecule/metal interfaces. Following these requirements may open a way to design molecule/metal interfaces allowing efficient charge injection from the metal into the organic layer and effective charge transport within the layer which is important in organic molecule-based electronic devices.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Angle-resolved photoemission data of the bare Au(111), additional 2PPE data of the TTF/Au(111) and F4 TCNQ/Au(111) interface in order to elucidate the electronic structure.

Acknowledgments Funding by the German Research Foundation (DFG) through the project TE 479/3-1 and the collaborative research center SFB 1249 (project B06) is gratefully acknowledged. D.G. acknowledges financial support from the Heidelberg Graduate School of Fundamental Physics. OTH acknowledges financial support from the FWF through the project P27868-N36.

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hn

2

q

hn

Ekin

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Localized

sin q

Ekin

electron-like dispersion

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1

Ekin

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hole-like dispersion

sin q

Figure : TOC Graphic

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Formation of Occupied and Unoccupied Hybrid Bands at Interfaces

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