March 01, 2013
C 2013 American Chemical Society
Digitized Charge Transfer Magnitude Determined by Metal Organic
Coordination Number
Hung-Hsiang Yang,†Yu-Hsun Chu,†Chun-I Lu,†Tsung-Han Yang,†Kai-Jheng Yang,†Chao-Cheng Kaun,‡ Germar Hoffmann,†and Minn-Tsong Lin†,§,*
†Department of Physics, National Taiwan University, Taipei, Taiwan and‡Research Center for Applied Sciences and§Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
E
lectron transport through metal organic heterostructures in organic electronics strongly depends on the interface formed between the metal atom and the organic molecule in terms of geo- metry and electronic configuration.1In par- ticular, the charge redistribution within the metalorganic interface plays a crucial role for electrical, optical, and magnetic proper- ties which are relevant in technological applications such as organic light-emitting diodes, field-effect transistors,2,3 organic solar cells,4,5and organic spin valves.6To improve the performance of organic de- vices requires more detailed understanding of the local mechanisms responsible for the charge transfer between metals (small ioni- zation potential) and molecules (larger elec- tron affinity with respect to metals). Metalorganic complexes have attracted wide- spread interest in the past decade,7particu- larly for their electronic and magnetic pro- perties.8,9The growth of predefined metal
organic structures has been achieved by the appropriate choice of head groups and molecular ligands.10,11Thus, various coordi- nation geometries can be engineered in
such metalorganic structures,12which pro- vide different chemical environments for molecules. A nonlocal approach measure- ment (X-ray photoemission spectroscopy) combined with preliminary scanning tun- neling microscopy (STM) topography shows various oxidation states of both metal atoms and organic molecules in coordina- tion networks.13
PTCDA (chemical structure in Figure 1a) is a large π-conjugated organic molecule, which has been used in the fabrication of organic thin-film transistors,14organic light- emitting diodes,15and organic spin valves.16 Self-assembled structures on various sub- strates such as Ag(111) and Au(111) are reported.1722 Molecules remain nondis- torted on Au(111) which refers to a physi- sorption. Weak interaction between PTCDA and Au(111) substrate results in high mo- lecular mobility even at reduced tempera- tures. Therefore, Au(111) is a suitable substrate for PTCDA to form metalorganic complexes. In a recent work,23the appear- ance of self-assembled PTCDA in con- tact with a metallic substrate [Au(111)]
was investigated by STM. Furthermore,
* Address correspondence to [email protected].
Received for review January 23, 2013 and accepted March 1, 2013.
Published online 10.1021/nn4003715 ABSTRACT Well-ordered metalorganic nanostructures of
Fe-PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) chains and networks are grown on a Au(111) surface. These structures are investigated by high-resolution scanning tunneling microscopy. Digitized frontier orbital shifts are followed in scanning tunneling spectroscopy. By comparing the frontier energies with the molecular coordination environments, we conclude that the specific coordination affects the magnitude of charge transfer onto each
PTCDA in the Fe-PTCDA hybridization system. A basic model is derived, which captures the essential underlying physics and correlates the observed energetic shift of the frontier orbital with the charge transfer.
KEYWORDS: scanning tunneling microscopy . scanning tunneling spectroscopy . metalorganic coordination . self-assembled nanostructure . charge transfer
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Méndez et al. made an interesting observation when they additionally introduced Fe adsorbates. Metal
organic networks, such as Fe-PTCDA chains24and two- dimensional structures,25 were controllably formed.
These room-temperature STM experiments revealed a modification of the electronic structure. Accompany- ing density functional theory (DFT) calculations sug- gested a charge transfer between Fe atoms and PTCDAs.25To gain deeper insight into how the molec- ular electronic structure is modified by the formation of a moleculemetal interface and the peculiar role of the Fe, precise and laterally resolved spectroscopy data are required. In this article,first low-temperature STM and scanning tunneling spectroscopy (STS) results are pre- sented, which reveal the effect of the local environ- ment on the charge state on a single-molecule scale.
Wefind digitized charge states of PTCDAs which are determined by the coordination number of Fe atoms and neighboring molecules in self-assembled networks.
RESULTS AND DISCUSSION
Figure 1b shows a representative STM image of an Fe-PTCDA chain structure as acquired at0.5 V. The intramolecular structure of PTCDAs in the chain shows mirror symmetry along both the short and long axis of the molecule, revealing two lobes for the outer and three for the inner part. This orbital structure has been identified as the lowest unoccupied molecular orbital (LUMO) of gas phase PTCDA in DFT calculations (Figure 1c). In the following, the LUMO is called frontier orbital (frontier orbital is defined as the state which is
the nearest to the Fermi level and relevant for chemical reactivity26) while discussing the LUMO shifting from positive to negative energy. Molecules in the inset of Figure 1b show no obvious internal structure atþ1.8 V.
This suggests an occupied state for molecules in the chain. Figure 1d presents STS results of Fe-PTCDA (blue curve), PTCDA (red curve), and Au(111) surface (black curve). The spectroscopy curve on Fe-PTCDA shows a peak at around 0.4 V which originates from the frontier orbital. For self-assembled PTCDA, the STS data reveal two peaks atþ1 V and 0.4 V. The former one is the frontier orbital state of self-assembled PTCDA. The latter one is the surface state mediated by the first PTCDA film at reduced intensity and shifted peak position.23Respectively, the frontier orbital is imaged as an unoccupied state in STM (see Figure 1e). After the formation of Fe-PTCDA chains, the formerly unoccu- pied frontier orbital of the PTCDA dominates the appearance of PTCDAs within the chains for negative bias (Figure 1b); that is, the frontier orbital is now an occupied state. This detail was not resolved in previous room-temperature experiments.
To investigate the charge transfer behavior, DFT calculations have been performed for the Fe-PTCDA chain structure adsorbed on a Au(111) surface. In Figure 2a, we verified that the chain structure with two bridging Fe atoms is the energetic minimum geometry (see Methods section). According to the relaxed geometry of the chain structure, Fe atoms Figure 2. Charge transfer map in different cross sections with colors indicating the amount of transferred charges in comparison to the neutral region (green). Red indicates a reduction of electron density and blue an enhancement. (a) In the molecular plane, molecules accept electrons with the amount modulated over the entire molecule. (b) In the Au layer underneath, in the vicinity of the Fe atoms, electrons are accumulated in the Au layer. (c) Perpendicular cross section, Fe donates electrons to its environment.
Figure 1. (a) PTCDA chemical structure. (b) STM image (4 4 nm2) of Fe-PTCDA chain structure with the frontier orbital resolved at0.5 V (inset: 4 4 nm2,þ1.8 V). (c) PTCDA projected frontier orbital. (d) Spectroscopy recorded on gold surface (black curve), for self-assembled PTCDA (red curve) and PTCDA in an Fe-PTCDA chain structure (blue curve). (e) STM image (4 4 nm2) of self-assembled PTCDA with the frontier orbital visible atþ1.0 V. (The main reason for the asymmetry of the appearance of PTCDA in a different row might be due to the asymmetry of the tip state.)
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adsorb on top of the hollow Au(111) site (see Figure 2b).
PTCDA molecules, which coordinate with Fe atoms, are bent and lifted (Figure 2c). As a consequence, Fe atoms are below the plane of the molecules. Figure 2 shows the charge accumulation (blue) and depletion (red) of the hybridized chain structures on Au(111) surface on three planes: the molecular plane (Figure 2a), the Au surface (Figure 2b), and the orthogonal plane contains the Fe atoms (Figure 2c). The results show that the Fe atoms donate electrons to both molecules and the Au(111) surface. Each Fe atom (there are two in every unit cell) gives 0.97 electrons to its adjacent environ- ment, where each molecule accepts 1.11 electrons and the gold surface accepts 0.83 electrons per unit cell.
Combining STS spectroscopy and the appearance of the orbital structure obtained by STM, we can identify the energy shift of the molecular state due to the charging effect of molecules. Furthermore, the DFT results show that the environment of Fe atoms, includ- ing neighboring molecules and Au atoms, is relevant for the charge redistribution.
Depending on the preparation conditions, chain structures (Figure 3a) and network structures (Figure 3b) are created (see the Methods section). Following Mén- dez et al.,24chain structures are stabilized by two iron atoms connecting with dianhydride groups from two PTCDAs, while networks are constructed by several chains interconnected by PTCDAs. In the work of Alvarez et al.,25a perpendicular arrangement of inter- connecting PTCDAs is suggested. From our high- resolution data, a modified structure with rotated PTCDAs can be deduced (Figure 3c). Although a preferentially parallel alignment of interconnecting PTCDAs is ob- served within ordered structures, at a low frequency,
oppositely rotated PTCDAs are identified (see Figure 3d, arrows). For convenience, PTCDA connected to four Fe atoms is called chain-PTCDA and interconnecting PTCDA is called rung-PTCDA. A significant variation of the electronic structure is indicated by the energy at which the frontier orbital is resolved in STM. From STM images (Figure 3d,e), wefind a strong bias-dependent appearance of molecules. In chain-PTCDA, the STM image at0.5 V (Figure 3d) shows the frontier orbital appearance, which appears at þ0.5 V in Figure 3e.
Considering that the binding mechanism is different for the two PTCDA species between the occupied and unoccupied frontier orbitals, we expect that an envi- ronment-dependent charge transfer occurs.
Figure 4a demonstrates spectra of molecules in different environments. For all spectra, we find a peak at the energy of the Au(111) surface state. In addition, unoccupied molecular states are identified. Thereby, the energetically lowest state (i.e., the frontier orbital) is easily identified in STM data by its characteristic ap- pearance. As a consequence, the spectra in Figure 4a are sorted by the energy of the frontier orbital. In Figure 4b, the respective positions of acquisition are indicated. Molecules with different energies are re- spectively labeled A, B, C, and D.
A direct correlation between the energetic shift and the environment is striking. The frontier orbital of the molecule at the edge (A) attached to only one Fe has the highest energy (1.23 eV). Molecules denoted by B, B0, and B00give the identical electronic signature. These molecules have two Fe atoms attached but different binding orientations. Thereby, whether the molecule is attached at both ends each to one Fe (B) or at one end to two Fe atoms (B0, B00) does not influence the charge Figure 3. STM image of Fe-PTCDA (a) chain and (b) network structure (each 30 30 nm2,1.0 V) along with a (c) schematic representation. STM images (5.7 5.7 nm2) of the same network structure as recorded at (d)0.5 V and (e) þ0.5 V. Thereby, the frontier orbital is an occupied state for chain-PTCDA and an unoccupied state for rung-PTCDA. The arrows in (d) indicate two rung-PTCDAs which are rotated against each other.
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transfer. However, the environment of the attached Fe atom is relevant. The molecule labeled by C also has two neighboring Fe atoms but differs in energy from molecule B; one Fe connects to three, while the other one only contacts two PTCDAs. Interesting is also the case of the molecule denoted by D (as well as within chains). Spectra from these molecules show no addi- tional peak apart from the surface state of the under- lying Au(111) substrate. However, STM imaging gives evidence that a molecular state is present. This is demonstrated in the insets of Figure 4a. We therefore conclude that the frontier orbital hybridizes with the Au(111) surface state and is pinned at its energy. For molecules A, B, and C, the frontier orbital is unoccupied and recorded in STM at positive bias, whereas for molecule D, the frontier orbital is occupied and the characteristic appearance is not observed at positive but at negative bias.
The shift of the frontier orbital from an unoccupied to occupied state gives direct evidence for a charge transfer process. We interpret the energetic shift for A, B, and C as the consequence of a charge transfer and a respective distribution over the neighboring mo- lecules which is correlated to the coordination. It is instructive tofirst revisit recently obtained results on artificially created metalorganic junctions. Repp et al.27achieved a charging of pentacene molecules by attaching Au atoms by STM manipulation. Electro- nically decoupled from the substrate through an in- sulating layer, the acceptance of one (or more) electron causes the occupation of one (or more) previously unoccupied state. Yamachika et al.28observed a linear shifting of the LUMOþ1 of C60 by controlling the amount of attaching K atoms. The linear shifting originates from the charge transfer from K to the LUMO of C60, resulting from a constant number of electrons provided by each K atom. Different to Repp et al., we do not observe for AC the crossing of the Fermi level
(Figure 4a). Instead, we observe only an energetic shift.
To explain our observation, we estimate that every Fe atom donates 0.56 electrons to the molecule. These donor electrons are equally distributed among the neighboring PTCDAs. Therefore, each Fe atom gives 0.56/n electrons to PTCDA, where n is the number of the neighboring molecules. The isotropic distribution of electrons can be deduced from the comparison of B versus B0, the relevance of the number of attached PTCDA from the comparison of molecules B00 and C.
This heuristically developed picture is applied to the structure presented in Figure 4b. In Figure 4b, wefind that, within a self-assembled network, PTCDAs are attached to one (at the edge) up to four Fe atoms (chain-PTCDA). As a consequence, the total number of molecular accepted electrons R varies and is deter- mined asR = ∑i(N/ni), (i = index of connected Fe atoms and N = 0.56).
Figure 5 presents the digitized energies of the frontier orbital for all observed coordination geome- tries plotted against the total number of accepted electrons R. The graph reveals a linear relation (red line, the slope is0.34 eV/e) between the accepted electron number and the frontier orbital energy. The digitized shifts are in accordance with Yamachika et al., which supports our interpretation that the observed shifts are the response to a charging event. The reason for the digitized shifts is due to electrons partially occupying the broadened molecular state which are due to the hybridization with the substrate and Fe atoms. Therefore, we observed the frontier orbital shifts while the Fermi level is filled up. However, a peculiar involvement of the substrate by the formation of moleculesubstrate hybrid states is also expected.
These moleculesubstrate hybrid states might result in states with energy between the LUMO and the Fermi level. Moreover, the frontier orbital of the tungsten tip and the hybrid states is localized, therefore, the hybrid Figure 4. (a) STS curves on molecules in different bonding environments. The frontier orbital shifts to lower energies upon coordination with Fe. Once occupied, the frontier orbital is pinned by the surface state of Au(111), which is mediated through the molecular layer at lower coordination (Note: variation of the detailed line shapes has to be attributed to the usage of different tips; see Supporting Information). Inset: STM images show the appearances of B and D molecules at indicated (red and green circles) bias voltages. (b) Indicates the originating positions. Thereby, labels AD indicate all characteristic structures. An electron donation (blue arrows) from the Fe to neighboring PTCDAs is deduced with the amount of charge accepted by each PTCDA depending on the local geometry (N/2 or N/3, where N = 0.56).
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states cannot be probed above the molecule.29 Furthermore, above acceptance of (8/6)Ne, a satura- tion is observable (blue line). The number of accepted electrons for molecules in D and chains are different but reveal the identical frontier orbital peak center. To explain the saturation point, we expect a peculiar involvement of the substrate as a host of electrons. A saturation of the shifts indicates a limitation of the number of electrons which can be accepted by a PTCDA molecule. Surface states of substrates may determine such a limitation and provide an electron
reservoir. Result from this is that, once the frontier orbital is pinned by the surface state, the energy remainsfixed independent of the environment and the electrons energetically prefer to go into the surface.30 These results are in accordance with our DFT calculations. Electrons are also transferred into the Au(111) surface in the chain structure, which is the heavy saturated system in our analysis.
CONCLUSION
In conclusion, we studied self-assembled Fe-PTCDA networks by STM and STS supported by DFT calcula- tions. Thereby, different metalorganic coordination and charge states of molecules are provided. We identified digitized frontier orbital energy shifts which are related to the local metalorganic coordination. In the assembled structures, the Fe atom plays the role of an electron donor. Donated electrons are equally shared among adjacent molecules. Our data indicate that the characteristic frontier orbital is preserved after the charge transfer process but shifted in en- ergy proportional to the amount of accepted elec- trons. Ourfindings, which are related to the charge redistribution at metalorganic interfaces on the molecular scale, are important to understand the energy level alignment.31As a result, the ability to design the network structure by binding metal atoms with organic molecules offers a unique way to engineer the oxidation state of involved mol- ecules as also relevant for molecular magnetism8 and superconductivity.32
METHODS
Measurements were performed in an Omicron ultrahigh vacuum low-temperature STM. Sample temperatures were maintained at 4.5 and 77.7 K during experiments with equiva- lent results obtained. After cycles of Arþsputtering and anneal- ing processes for Au(111) substrate preparation, PTCDA (rate:
0.01 ML/s, sublimation temperature ∼550 K) and Fe (rate:
0.02 ML/s) were dosed onto the room-temperature sample surfaces.
Upon afinal annealing step to ∼400 K, moleculemetal net- works were formed. Thereby, when PTCDA was prepared after Fe deposition, a dense Fe-PTCDA network was formed; other- wise, chains structures were predominant. For scanning tunnel- ing spectoscopy, a modulation voltage (ΔU = 40 mV, f = 5.9 kHz) was added to the bias voltage and the induced current mod- ulation was recorded via lock-in techniques. Thereby, positive (negative) bias voltages refer to tunneling into unoccupied (out- of occupied) states. Curves on molecules were extracted from two- dimensional data sets. Every STS curve was taken at the center of a molecule and averaged over∼50 spectra (area ∼0.3 nm2).
The geometry structure relaxation was performed with Quan- tum ESPRESSO.33We chose GGA pseudopotential34to get the optimized geometry relaxation. The cut off kinetic energy and charge density were 20 and 240 Ry, respectively. The Brillouin zone was sampled at theΓ reciprocal point.35For the Fe-PTCDA chain on the Au(111) system, there were 1492 electrons per unit cell. The convergence condition is that the force acting on each atom was smaller than 103Ry/a0. In order to calculate the electronic structure, we used Nanodcal36DFT package with LCAO basis. The Brillouin zone was sampled with 5 10 1
reciprocal points.35The spacing between two slabs was 22.5 Å, which ensured that the system was isolated in the z-direction.
We used LDA exchange correlation for electronic structure calculations.
Conflict of Interest: The authors declare no competing financial interest.
Acknowledgment. This work was supported in part by the National Science Council of Taiwan through Grant No. NSC 101- 2120-M-002-012.
Supporting Information Available:Complete STS curves on molecules AD in Figure 3 with different tips. The STS curves taken at different atomic position of molecules C and D00 are included. This material is available free of charge via the Internet at http://pubs.acs.org.
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