Reordering and Disordering of the Copper ... - ACS Publications

Sep 29, 2014 - Potassium (K)-induced structural and electronic modifications in the copper ... 礼 宋. Journal of Advances in Physical Chemistry 2015...
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Reordering and Disordering of the Copper Hexadecafluorophthalocyanine (F16CuPc) Monolayer by K Doping Yuri Hasegawa, Yoichi Yamada,* and Masahiro Sasaki Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8577, Japan ABSTRACT: Potassium (K)-induced structural and electronic modifications in the copper hexadecafluorophthalocyanine (F16CuPc) monolayer were investigated by scanning tunneling microscopy and photoemission spectroscopy. The adsorption of K was found to cause a rearrangement of the molecular ordering in the F16CuPc monolayer, depending on the K concentration. For doping with one to two K atoms per molecule, K adsorbs in the intermolecular region adjacent to the aza-bridging nitrogen, forming new ordered phases. A further increase in K coverage causes the on-top adsorption of K, which in turn disorders the monolayer. The photoemission spectrum of the K-doped phase exhibits a small density of states immediately below the Fermi level. Both the formation of the new states and the K-induced structural ordering or disordering of the molecular layer may be responsible for the reported alteration of the macroscopic electronic conductivity.

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modification depends on the K concentration.2 A significant increase in the conductivity has been found in the initial doping stage with K concentrations ranging from one to three atoms per molecule, while further doping causes a rapid decrease in conductivity. These observations have led to the assumption of the insulator−metal−insulator transition.2 On the other hand, X-ray photoemission spectroscopy (XPS) investigations have not revealed any evidence for the formation of the metallic state by the doping of alkali metals at any doping level.3−6 This contradiction indicates that the mechanism by which the conductivity is altered in K-doped Pcs has not yet been clarified. It can be speculated that an undetermined structural change of the Pc films upon K doping affects the conductivity. Therefore, in this study, we conducted molecular-level structural measurements of the Pc film upon K doping combined with photoemission measurements utilizing a monolayer of Pc on an inert substrate. As a model for an electron-accepting Pc molecule, we used copperhexadecafluorophthalocyanine (F16CuPc), which shows high electron affinity7 and is therefore widely used as an n-type organic semiconductor.8 It is also known that the molecular distortion of F16CuPc is very small, even on a metal surfaces.9 Therefore, we expect that the molecular orbitals of F16CuPc in the monolayer would not differ significantly from those in a thick film if the substrate is sufficiently inert. In this case, a dopantinduced reaction of Pc molecule can be monitored without being affected by undesired effects from the substrate. In the present study, a Cu3Au(001) surface with a square lattice was used as a substrate. The (001) surface of Cu3Au consists of Au and Cu atoms at a 1:1 ratio.10 This surface is relatively inert

INTRODUCTION To modify the electronic properties of organic semiconductor materials, carrier injection by metal or molecular doping plays a key role as in the case of inorganic semiconductor materials. In particular, alkali-metal-doped organic compounds have been studied extensively because they exhibit a variety of interesting electronic properties such as superconductivity1 and insulator− metal−insulator transition.2 However, in contrast to inorganic systems, the doping mechanism of organic materials has not yet been clarified. A crucial reason for this lack of understanding is the inadequate structural information with respect to doping. In general, the electronic properties of organic compounds depend strongly on the molecular arrangement, which is usually determined by weak intermolecular interactions. The doping of foreign materials can easily alter the molecular arrangement. Therefore, a molecular-scale determination of the structure of organic compounds together with the determination of the electronic structure is essential for understanding the doping mechanism. Structural aspects with respect to doping, however, have seldom been investigated, mainly due to the difficulties of the measurements. In particular, the structural inhomogeneity of organic films can lead to an inhomogeneous distribution of dopants, hindering the detailed understanding of the relation between structural and electronic properties. Therefore, a wellordered model system of the doped organic material is required in order to investigate the doping mechanism. In this study, we demonstrated that a monolayer system weakly supported on a substrate can be utilized as a good model for studying doping. Herein, we focus on potassium (K)-doped phthalocyanine (Pc) and show the correlation between the change in molecular arrangement and electronic structure using a well-ordered monolayer system on an inert metal substrate. According to previous reports, the electric conductivity of metal Pcs can be drastically modified by means of doping with K atoms, and the © 2014 American Chemical Society

Received: July 4, 2014 Revised: September 22, 2014 Published: September 29, 2014 24490

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Figure 1. (a) STM image of the F16CuPc monolayer on Cu3Au(001) observed with Vsample = +1.90 V and It = 85.8 pA. (b) Molecular arrangement illustrated in the enlarged STM image of the pristine monolayer.

deposition amounts of these materials determined from QCM measurements. The structural changes in the F16CuPc monolayer with K doping were observed by STM. XPS measurements were conducted at the synchrotron facility of the Photon Factory, KEK (BL-3B). Valence band spectra were obtained with an incident light of 40 eV. For measurement of the work function, the high-binding energy cutoff of the photoelectron spectrum was measured with an excitation energy of 60 eV, while a direct current bias of 20.0 eV was applied to the sample.

because the topmost potential energy surface is mainly dominated by Au atoms, which have larger atomic radii. In addition, the 4-fold symmetry of the surface is suitable for fabricating a well-ordered monolayer of F16CuPc with 4-fold symmetry. Indeed, it is known that the mismatch of symmetries between the Pc molecule and substrate causes structural imperfections within the monolayer such as domain boundaries and defects.11−14 In this paper, we show that K deposition onto a well-ordered F16CuPc monolayer on a Cu3Au(001) substrate induces a rearrangement of the molecular ordering. While rearrangements into new ordered phases were found in the initial doping stage of one to two K atoms per molecule, further adsorption of K caused a disordering of the monolayer. Scanning tunneling microscopy (STM) observations suggest that the rearrangements into ordered phases are induced by K adsorption adjacent to the aza-bridging nitrogen atoms of F16CuPc, while the on-top adsorption of K atoms leads to disordering. This observation of structural ordering or disordering seems to coincide with the previously reported increase or decrease in conductivity of the Pc films, respectively. Furthermore, photoemission spectroscopy of the ordered phase revealed that a new density of states appears near, but not on, the Fermi level. This observation indicates no metallicity is present in the doped monolayer, as reported for multilayer systems.3,6 Therefore, the molecular-level structural modification should affect electron transport, possibly in combination with the formation of the new states near the Fermi level.

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RESULTS AND DISCUSSION Figure 1(a) shows an STM image of a F16CuPc monolayer on the Cu3Au(001) surface. The F16CuPc molecule consists of four F-substituted peripheral benzene rings, which are often termed “lobe”, located around a central Cu atom (Figure 1(b)). Each lobe is bridged by a nitrogen atom, which is termed the aza-bridging nitrogen. In the STM image, the F16CuPc molecules show four leaves with a central depression originating from the four peripheral benzene rings and the central Cu atom. The molecule is seen to adsorb with its molecular plane parallel to the Cu3Au(001) surface. The observed molecular arrangement of the F16CuPc monolayer is 4-fold symmetry with every lobe of the molecule facing an azabridging nitrogen atom in a neighboring molecule (Figure 1(b)). This arrangement indicates that the molecular orientation is stabilized by the dipole interaction between the fluoride atom on the benzene ring and the aza-bridging nitrogen atom. The F16CuPc monolayer on the Cu3Au(001) surface forms a large domain with a small population of domain boundaries. This is likely due to the 4-fold symmetrical structure of the Cu3Au(001) surface, which matches the symmetry of the monolayer. This scenario is in contrast to the case of 6-fold symmetrical surfaces such as highly ordered pyrolytic graphite (HOPG)11 or (111) planes of face-centered cubic metals,12−14 for which relatively small domains with a notable number of domain boundaries in the Pc monolayers have been reported. The low energy electron diffraction (LEED) pattern of the F16CuPc monolayer indicated the formation of two rotated domains that are not along the symmetrical direction of the substrate, further indicating the weak molecule−substrate interaction. After K deposition on the F 16 CuPc monolayer, a reconstruction of the monolayer occurs, followed by disordering with increasing K coverage. Two new ordered phases were

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MATERIALS AND METHODS All experiments were carried out under ultrahigh vacuum (UHV) conditions at room temperature. The Cu3Au(001) substrate was prepared by several cycles of sputtering and annealing. Subsequently, further annealing was performed at 650 K which is below the phase transition temperature of 664 K,10 to order the Cu3Au(001) surface. The cleanliness of the Cu3Au(001) surface was verified by STM measurements. F16CuPc, purchased from Aldrich (98% dye content), was degassed prior to use. A F16CuPc monolayer was prepared by vacuum deposition using a homemade Knudsen cell. K was deposited onto the F16CuPc monolayer from a SAES getter source. The deposition rate was monitored using a quartzcrystal microbalance (QCM). In this paper, the doping level x indicates the number of K atoms with respect to the number of F16CuPc molecules. The value x is estimated from the 24491

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Figure 2. STM image of the K-doped F16CuPc monolayer on Cu3Au(001) with (a) 1:1 phase, (b) mixture of 1:1 and 1:2 phases, and (c) 1:2 phase. Vsample = +1.74 V, It = 88.1 pA.

proposed that a stable adsorption site for K atoms exists near the aza-nitrogen atom, even for thick film.3,6 In the present case, therefore, the stable adsorption of a K atom on the monolayer can also be considered to occur at aza-bridging nitrogen sites. The incorporation of a single K atom at an azabridging nitrogen site induces anisotropy of the molecule, which then stabilizes the one-dimensional ordering of the composites. Figure 2(d) shows an STM image of the F16CuPc monolayer with a K coverage of approximately 1.3 atoms per molecule. At this coverage, further reordering starts to coexist with the 1:1 phase. In the upper right part of Figure 2(d), the remaining 1:1 phase is visible, while a new ordering of the molecule with a square unit cell can also be observed in the upper left part. The region with square ordering increases with increasing K coverage, and the entire surface is eventually converted to square ordering when the doping level reaches approximately two atoms per molecule. We term this new ordering as the “1:2 phase”. In this phase, every molecular lobe points to the neighboring molecular lobes; this particular arrangement of molecules in the square phase indicates the further adsorption of K atoms next to unoccupied aza-bridging nitrogen atoms in addition to those in the 1:1 phase. This additional adsorption of K atoms into the aza-bridging nitrogen sites may bring about further reordering of the molecular arrangement between the rows, resulting in the square arrangement of the F16CuPc. Indeed, the detailed molecular-level STM image of the 1:2 phase shown in Figure 2(d) shows a slight increase in the intensity beside every aza-bridging nitrogen atom of the molecule (marked by circles). This observation confirms the

observed depending on the K concentration at a doping level below two K atoms per one F16CuPc molecule (Figure 2). Figure 2(a) shows an STM image of the F16CuPc monolayer after K deposition of approximately 0.6 atoms per molecule, as determined from the deposited amount. In contrast with the nondoped monolayer, a new ordering consisting of anisotropic molecular rows was formed; we term this new phase the “1:1 phase”. At this coverage of K atoms, below 1 K/molecule, the 1:1 phase coexists with the nondoped phase. We found that the best conditions for STM imaging are different for those two phases; therefore, we focus on the area fully covered with the 1:1 phase. The detailed STM image of the 1:1 phase is shown in Figure 2(b), and a schematic illustration of the molecular arrangement is shown in Figure 2(c). In the 1:1 phase, the lobes of one molecule point to those of a neighboring molecule in the same row, resulting in a unidirectional connection of the molecules. On the other hand, when we look at the inter-row connection, the lobes of the molecules in one row point toward the aza-bridging nitrogen of the molecule in the adjacent row, as in the case of the pristine monolayer. In this phase, it can be speculated that K adsorption occurs in the intermolecular region, i.e., aside the aza-bridging nitrogen. Indeed, the STM image reveals a small intensity between the lobes of neighboring molecules, as marked by circles in Figures 2(b) and (c), possibly indicating the presence of a K atom at this position. This observation suggests that molecules in the row are “connected” via the K atoms in between. Recently, lowtemperature STM experiments and density functional theory calculations revealed that the stable adsorption of an alkali (Li) atom occurs next to an aza-bridging nitrogen atom of the single CuPc molecule on the Ag(100) surface.15 It has also been 24492

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Figure 3. (a) STM image of a K-doped F16CuPc monolayer on Cu3Au(001) with doping level slightly larger than 2 atoms per molecule. Vsample = −2.06 V, It = 193 pA. (b) Magnified image of a bright spot on the molecular lobe. (c) Magnified image of a bright molecule. (d) STM image of the disordered K-doped F16CuPc monolayer. Vsample = −2.00 V, It = 81 pA.

examined by photoelectron spectroscopy. Figure 4 shows the change in the work function as a function of K concentration.

assumption that all the aza-bridging nitrogen sites are occupied with K atoms in the 1:2 phase, as illustrated in Figure 2(f). When K coverage exceeds two K atoms per molecule, disordering of the molecular arrangement begins to occur (Figure 3(a)). When the K coverage exceeds two atoms per molecule, K atoms are considered to be adsorbed on top of the molecule since the adsorption sites adjacent to the aza-bridging nitrogen atoms are already occupied. In this case, the STM image shows bright protrusions imposed on the 1:2 phase, indicating the on-top adsorption of K atoms. Two types of ontop adsorption-induced features are identified, reflecting different adsorptions (Figure 3(b) and (c)). Figure 3(b) shows the STM image of a molecule with a bright spot at the lobe site, and Figure 3(c) shows the entirely bright molecule. Several adsorption sites for alkali metals on the molecular plane of Pc have been reported. For Li adsorption on a single CuPc molecule, stable Li adsorption sites have been proposed to exist on top of the lobe and at the center metal.15 Figures 3(b) and (c) are considered to correspond to K adsorption on top of the lobe sites and on the central Cu atom, respectively. These ontop adsorptions have already been observed in separate investigations, and the STM features corresponding to these adsorptions have already been addressed.15 The bright molecule, which corresponds to the molecule with alkali metals on top of the central Cu, is rotated with respect to its original orientation in the 1:2 phase. This adsorption-induced molecular rotation causes disordering of the monolayer. Indeed, domain boundaries were found to appear around these bright molecules. K adsorption on top of the central Cu site has been assumed to cause significant charge transfer to the molecule.15 This charge transfer may reduce the interaction between the molecule and substrate; therefore, the K adsorption at the center site may facilitate a further decoupling of the molecule from the substrate, resulting in the disordering of the monolayer. Further K doping increases the population of bright molecules. This causes further disordering possibly due to Coulomb repulsion between the molecules with excess charges. Finally, the surface became fully covered with disordered bright molecules at an elevated K coverage of approximately 4.2 K atoms per molecule (Figure 3(d)). Here, we note that although a particular ordering of the molecules with on-top alkali metal can exist at lower temperature,12 it seems to be absent in this study at room temperature. Next, we discuss the electronic modification upon the reconstruction of the alkali-metal-doped F16CuPc monolayer, as

Figure 4. Work function change in the K-doped F16CuPc monolayer on Cu3Au(001) as a function of the K coverage.

The work function is deduced from the high-binding energy cutoff of the photoelectron spectra. A rapid decrease in work function was observed in the low K-coverage region, while the change became saturated at higher coverages. Although the transition between the two regions should be in the doping level of 3 to 4 K atoms/molecule, it is not clearly determined due to certain error in the measurement. However, the initial rapid change in work function is attributable to the structural rearrangement of the monolayer, which may drastically alter its surface dipole layer. The apparent saturation of the work function, on the other hand, seems to correspond to the disordering of the layer. Next we investigated the change in the valence band structure. Figure 5 shows the valence band photoemission spectra of the doped F16CuPc monolayer on the Cu3Au(001) surface as a function of the doping level x. The bottom spectrum in Figure 5 is taken from the Cu3Au(001) surface. The peaks in the spectrum of the clean surface located at 2−4 eV and 6−7 eV represent the Cu- and Au-derived states, respectively, while the small peak located at 5 eV is reportedly due to the mixed state of Cu and Au.16 The spectrum of the clean Cu3Au(001) surface agrees with the results from a previous ultraviolet photoelectron spectroscopy (UPS) study, confirming the cleanliness of the substrate. After the formation of the F16CuPc monolayer, new components located between 13 and 8 eV were observed; these should originate from the molecular orbitals. Note here that the emission from the 24493

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Figure 6. (a) Photoelectron spectra around the Fermi level of F16CuPc/Cu3Au(001) as a function of the K coverage. (b) Difference spectra obtained by subtracting the spectrum of the monolayer from that of the doped phase.

Figure 5. Valence-band and K 3p spectra measured for the F16CuPc monolayer on the Cu3Au(001) surface as a function of the K coverage.

increasing K coverage taken with an emission angle of 60° to increase the surface sensitivity. The bottom spectrum is taken from the nondoped F16CuPc monolayer. Note that the Fermi edge of the substrate is not clearly observed for the case of the grazing emission. After K deposition, a new density of states (DOS) appears immediately below the Fermi level and forms Fermi edge-like features. To check the intrinsic spectral shape of the doped F16CuPc, we subtracted the monolayer spectrum from those of the doped phases (as shown in Figure 6(b)). Although this leaves a small DOS at the Fermi energy, the maximum intensity is located between 0.2 and 0.4 eV below the Fermi level. This observation suggests that the apparent Fermi edge results from the tail of the new DOS near the Fermi level. This observation leads us to the conclusion that the system is not metallic, which is in agreement with previous studies.3−6 However, even without an onset of the metallicity, the formation of the new DOS immediately below the Fermi level can affect the electronic transport; therefore, this observation is important to understand the conductivity of this system. Note that the appearance of the new DOS near the Fermi level has not been previously observed. It is considered that this feature is sensitive to molecular arrangements in the layer. Therefore, we suggest that the well-ordered monolayer of the doped phase realized in this study can provide reliable information about the electronic properties of alkali-doped Pcs. Although the present study does not support the onset of the metallicity in K-doped F16CuPc, both the change in molecular ordering and the formation of new states near the Fermi level are considered to be related to the previously reported drastic alteration in the conductivity (a significant increase in conductivity at a doping level of 1−3 K atoms per molecule followed by a rapid decrease in conductivity with increasing K concentration2). This phenomenon appears to be related to the reordering of the molecular layer observed in the monolayer in the present study. Even without the absence of the metallicity, the formation of the ordered phases of F16CuPc linked via alkali metals revealed herein may enhance the conductivity by enhancing the hopping probability. This linking may involve the hybridization of the wave functions of the F16CuPc molecule and alkali metals, possibly forming the new DOS near the Fermi level, as observed in the photoemission spectra. We also note that the formation of this state can reduce the carrier injection barrier from the metal substrate, which also

highest occupied molecular orbital (HOMO), which should usually be at approximately 1.9 eV below the Fermi level, is not seen possibly due to significant overlap with the strong signal from the substrate. Upon the formation of the F16CuPc monolayer, the Cu-derived peaks of the substrate were primarily modified. However, since the shift in substrate peaks is negligible after the formation of the monolayer, the interaction between the F16CuPc and substrate is considered to be weak. After the deposition of K on the F16CuPc monolayer, the K 3p peak appeared at a binding energy of 17.5 eV, and its intensity increased with the level of deposition. A slight shift of this peak is observed at higher coverages. This indicates that different chemical conditions exist for the K atoms depending on the coverage, which is consistent with the STM observations. At the initial stages of K deposition, a significant modification of the shape of molecular states and their shift toward higher binding energy were observed. These behaviors indicate the significant interaction between the molecule and the K atoms accompanied by the charge transfer. Note here that the substrate peaks remain unchanged with K doping, although their intensity decreased. This indicates that the deposited K atoms do not interact with the substrate but instead form compounds with the molecules. We also note that the direction of the chemical shift of the molecular (acceptor) peaks upon K doping is opposite to the usual shift direction of the acceptor, which typically shows a shift toward lower binding energy. This apparently opposite chemical shift is typical for a weakly interacting overlayer system on an inert substrate.17,19 In addition, the initial position of the binding energy of K 3p is smaller than the binding energy of the neutral K atom. This indicates an apparent chemical shift toward the lower binding energy side, which is also opposite to the case of the usual donor. These observations suggest that the present overlayer system is not pinned to the Fermi level of the substrate, but to the vacuum level, reflecting the weak interaction between the molecular layer and the substrate. Therefore, it is confirmed that the present monolayer system is well decoupled from the substrate and is thus suitable for modeling the alkali doping of Pc molecules. Finally, we discuss the electronic modifications around the Fermi level upon the doping. Figure 6(a) shows the evolution of the photoelectron spectra around the Fermi level with 24494

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2012G756 and 2014G170). This work was supported by JSPS KAKENHI Grant Numbers 23360019 and 24760023.

contributes to the enhancement of the electric conductivity. On the other hand, a significant decrease in the electric conductivity reported for doping with excess K atoms might be related to the disordering of the F16CuPc monolayer. The valence band spectrum does not change much during doping, suggesting that the structural properties at the molecular level have a much stronger effect on the electric conductivity than on the electronic structure. Therefore, as demonstrated in this study, the structural determination at the molecular level is highly significant in understanding transport in this material. However, it is notable that the molecular packing in the F16CuPc monolayer on the Cu3Au(001) surface is significantly different from the thicker film grown on the hydrogenterminated silicon surface used in the transport measurements.2 In the thicker film system, molecules are in the upright configuration with respect to the surface, forming the column structure. Therefore, the applicability of the structural information deduced in the monolayer system to the discussion of the thick film system is not straightforward. However, some similarities can be already found in the structural features with respect to the doping of the monolayer and the thicker layers with respect to the column structure. In the column structure, K atoms are reported to intercalate and to adsorb around the aza-bridging nitrogen atoms of the molecule.19 Although the change in the ordering of the column film upon doping has not been thoroughly investigated, reordering into a new ordered phase would be possible. On the other hand, the alkali-induced disordering of the thicker Pc layers has already been suggested for ZnPc, H2Pc, and FePc, although indirectly, for doping levels above four K atoms per molecule.4−6 Therefore, the weakly coupled F16CuPc monolayer presented in this study can model the properties of thicker film systems to a certain extent.

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CONCLUSION

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AUTHOR INFORMATION

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REFERENCES

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We have reported on the K-induced structural and electronic modification of the F16CuPc monolayer on a Cu3Au(001) surface. STM measurements revealed that K doping alters the molecular ordering because of K adsorption aside aza-bridging nitrogen atoms in the low K coverage region, whereas further K deposition causes disordering of the monolayer due to K adsorption on top of the molecular surface. These observations suggest that K-induced structural reordering and disordering lead to an increase and decrease of the electronic conductivity, respectively. Furthermore, photoemission spectroscopy revealed that K doping resulted in the formation of a new DOS very close to the Fermi level. This indicates a lack of metallicity of the K-doped monolayer, as in multilayer systems. The new DOS close to the Fermi level may be attributed to the alteration in electronic conductivity in combination with structural reordering.

Corresponding Author

*Tel.: +81-298-535038. Fax: +81-298-535038. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed with the approval of the Photon Factory Program Advisory Committee (Proposal No. 24495

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(19) Margadonna, S.; Prassides, K.; Iwasa, Y.; Taguchi, Y.; Craciun, M. F.; Rogge, S.; Morpurgo, A. F. Potassium Phthalocyanine, KPc: One-Dimensional Molecular Stacks Bridged by K+ Ions. Inorg. Chem. 2006, 45, 10472−10478.

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