Charge Transfer, Phase Separation, and Mott–Hubbard Transition in

Article pubs.acs.org/JPCC

Charge Transfer, Phase Separation, and Mott−Hubbard Transition in Potassium-Doped Coronene Films Xuefeng Wu,† Chaoqiang Xu,† Kedong Wang,‡ and Xudong Xiao*,† †

Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, P. R. China Department of Physics, South University of Science and Technology of China, Shenzhen, Guangdong 518055, P. R. China

‡

ABSTRACT: Scanning tunneling microscopy and spectroscopy have been applied to study the pristine and potassium-doped coronene films at the molecular level with the doping ratio x ranging from 0 to 4. Both the morphology and the electronic property were found to be strongly governed by the amount of charge transfer. Certain phases of K3Coronene and K4Coronene could be fabricated at the corresponding K doping ratios. While in K4Coronene a doubly degenerate LUMO state of the undoped coronene became fully occupied, in K3Coronene that state was observed to cross the Fermi level and split into an upper unoccupied part and a lower occupied part, implying the presence of electronic correlation effect and Mott−Hubbard transition in the system. The observed four different phases of K3Coronene further showed different strengths of electronic correlation effect and led us to speculate that the superconductivity transition with different Tc in potassium-doped coronene observed elsewhere might have an origin from the electronic correlation effect with different strengths. hydrocarbon films at the molecular scale. For simplicity, a 2 ML thick coronene film on Au(111) was used as the starting material. By gradually increasing the potassium doping level, the evolution of structures and electronic properties was systematically investigated. Upon K doping, the interaction between coronene molecules and K atoms gave rise to a series of changes both in film structures and in electronic states. Particularly, a splitting of the lowest unoccupied molecular orbital (LUMO) state was observed when the LUMO was brought to the Fermi level, giving a direct evidence for electronic correlation effect in this system. Our results should contribute to a better understanding of the interaction between coronene molecules and potassium atoms and may shed some light on understanding the superconductivity properties of this system.

1. INTRODUCTION Charge transfer is a very effective method to modify the electronic properties of organic semiconductors1,2 and is widely used in many fields, including light-emitting devices,3 organic field effect transistors,4 and photovoltaic cells.5 Interactions in the systems are fundamental to understand the properties of doped organic semiconductors and improve the performance of organic devices. Scanning tunneling microscopy (STM) combined with scanning tunneling spectroscopy (STS) is very suitable to characterize the doping effect and reveal the interacting details from a molecular point of view.6,7 Since 2010, superconducting transitions induced by electron doping in picene, 8 coronene, 9 and other hydrocarbon molecules,10−12 which were called hydrocarbon superconductors, have been discovered. The hydrocarbon superconductors have relatively simple structures, theoretically calculable electronic properties, and strong electron−phonon coupling.13 Hence, they can be served as a special platform to study the mechanisms of organic superconductivity and open a new route to search for higher Tc superconductors.14 However, theoretical attempts to understand the superconductivity mechanism in this class of materials are still hindered by the limited experimental information available.15−18 Especially, the measured occupied electronic states of alkali metal doped hydrocarbon films give very contradictory results.19−23 This gives rise to the debate on the electronic correlation effect which may play an important role in the superconducting mechanism of hydrocarbon superconductors.13,24,25 In our experiment, STM and STS were applied to detect both the structures and the electronic properties of the doped © XXXX American Chemical Society

2. EXPERIMENTAL DETAILS We have carried out the experiment using an Omicron lowtemperature scanning tunneling microscope (LT-STM) in ultrahigh vacuum with a base pressure better than 1 × 10−10 mbar. A clean Au(111) surface was prepared as the substrate via cycles of Ar+ sputtering and 500 °C annealing. While keeping the substrate at room temperature (RT), coronene molecules were first deposited from an evaporation source at a rate of ∼0.3 ML/min to form pristine coronene films, and K atoms were subsequently deposited from commercial potassium SAES Received: April 11, 2016 Revised: June 26, 2016

A

DOI: 10.1021/acs.jpcc.6b03686 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Structure change of coronene film upon K doping. (a) Coronene film on Au(111) with a coverage of 1.5 ML (image size: 10 × 10 nm2; imaging condition: Vsample = 2 V, It = 6 pA). (b) dI/dV/(I/V) spectra measured on 1 and 2 ML coronene films at 77 K. Inset is the molecular structure of coronene. The curve of 2 ML film is shifted vertically for clarity. Dashed line indicates the shifted zero line. (c) Coexistence of potassium doped and undoped area. The sample was made by doping potassium atoms onto 2 ML coronene film with a doping ratio of x = 0.1 (image size: 7 × 20 nm2; imaging condition: Vsample = 2 V, It = 6 pA). (d) Structural details of doped area in (c) (image size: 7 × 7 nm2; imaging condition: Vsample = 2 V, It = 6 pA).

between the top sites and the bridge sites on the first layer. As shown in Figure 1b, the electronic structure of the first layer coronene has been strongly affected by the Au(111) substrate; with being buffered by the first molecular layer, intrinsic electronic structures of coronene prevail from the second molecular layer on. Therefore, we hereafter choose 2 ML coronene films as a base to examine K doping effect to minimize the influence of the substrates. It is believed that K atoms can intercalate into the lattice of coronene with an accompanied electron transfer to the coronene film, leading to the filling of unoccupied molecular orbital states of coronene. Thus, the structure and electronic properties of the coronene film are expected to be significantly modified after being doped. At a very low doping ratio x, the left part and the right part of Figure 1c appear very differently in morphology. The left part is identified as undoped region since it has both identical morphology and electronic states as the pristine film. As shown in the better resolved image in Figure 1d, significantly different from the undoped region, the phase in the right part exhibits elongated protrusions arranged into a face-to-edge herringbone structure and has a rectangular unit cell with a size of 1.2 × 1.2 nm2. By increasing x, the area of this phase expands proportionally and eventually covers the entire film at x = 0.2, which is examined by scanning large areas at different positions on the surface. Because of the still small x, the new phase must come from a change in coronene packing symmetry induced by K doping, rather than a K layer covering the coronene molecular film. According to the coronene molecule appearance in the K-doped coronene film and the similarity to herringbone structure in the ab plane of the bulk coronene,27 we speculate that the coronene molecules alter their arrangement from flat-lying to up-standing on Au(111) surface after being doped. A similar phenomenon has been reported in K-doped 1 ML coronene film based on the polarization dependence of XAS spectroscopy.28

Getters onto coronene films. To ensure the reaction between the sequentially deposited potassium atoms and coronene molecules to form well-ordered K doped coronene phases, a 30 min annealing at 350 K was applied. The deposition rate of K atoms was calibrated by directly counting the number of K atoms deposited on Si(111) in a given duration and further assumed to remain the same when deposited on coronene films. The adsorption rate of K atoms on Si(111) and coronene films was found to be independent of the substrate temperature which indirectly validated that the sticking coefficients on both substrates are approximately equal to 1 although the adsorption mechanisms may vary. The K doping level was characterized by the ratio x between the number of K atoms and the number of coronene molecules. For a 2 ML coronene film, different K doping ratios x ranging from 0 to 4 could be achieved by changing the sample exposure time to the K source. During the measurements, the W tip was carefully protected to avoid touching the sample surface to eliminate tip contamination by the sample materials. The imaging was taken at various bias voltages with relatively small tunneling current of ∼10 pA, and the density of state (DOS) was presented as dI/dV/(I/V) spectra by differentiating the acquired I−V spectra and then dividing by I/V spectra.

3. RESULTS AND DISCUSSION 3.1. Structural Evolution with K Doping. First, the structural evolution of the KxCoronene films as a function of K doping ratio x was studied. At x = 0 for the pristine film, coronene molecules assemble into a well-ordered structure and follow a layer-by-layer growth process. Figure 1a shows an STM image of 1.5 ML coronene film Au(111) with a streaky step formed between the second layer and the first layer. The first coronene layer possesses a 4 × 4 structure in which the molecules take a flat-lying configuration, in agreement with previous study.26 The molecules in the second layer assemble into the same structure as the first layer and are located B

DOI: 10.1021/acs.jpcc.6b03686 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Four different phases of K3Coronene. (a) Image of potassium-doped coronene at x = 2.6 showing the coexistence of the herringbone structure and the hexagonal structure of K3Coronene (image size: 20 × 20 nm2; imaging condition: Vsample = 1 V, It = 15 pA). The K3Coronene phase in the figure is phase 2. (b) Image of potassium-doped coronene at x = 3.3 (image size: 20 × 20 nm2; imaging condition: Vsample = 1 V, It = 15 pA). Two K3Coronene phases, phase 1 and phase 4, coexist in the image, which clearly demonstrates the phase separation of K3Coronene. The height difference is mainly due to an Au(111) step. (c−f) Images of totally four different phases of K3Coronene with various sample biases (image size: 7 × 7 nm2). Thermal drift is carefully corrected in all the images.

Figure 3. Structure of K4Coronene. (a) Coexistence of K3Coronene and K4Coronene at x = 3.3 (image size: 20 × 20 nm2; imaging condition: Vsample = 1 V, It = 15 pA). The K3Coronene phase in the figure is phase 2. (b, c) Structural details of K4Coronene taken at Vsample = 1 V, It = 15 pA and Vsample = −0.5 V, It = 15 pA, respectively (image size: 7 × 7 nm2). K4Coronene exhibits a √3 × √3 superstructure in which two types of molecules reverse their relative brightness at positive and negative bias.

As the doping ratio x further increases from 0.2 to 2.30, the doped coronene film retains the same herringbone structure with an unchanged lattice distance. The entire KxCoronene surface is uniformly covered by the identical phase with interruption only at Au(111) steps and does not show any phase separation at any doping ratio in the range 0.2 < x < 2.3. We checked the uniform phase by scanning areas with micro scale at different positions and different doping ratios. Additional K doping will lead to the formation of another phase with a hexagonal structure in which the molecules turn back to the flat-lying configuration.29 This structural change is well demonstrated by the coexistence of the two very different phases (Figure 2a). The conversion into the hexagonal phase is completed as the doping ratio is close to x = 3, so the hexagonal phase is believed to be K3Coronene phase. Interestingly, there is not the only one K3Coronene phase. Coexistence of more than one phase is frequently observed. Figure 2b shows the image of K-doped coronene film at x = 3.3 in an area which is covered by two highly ordered phases labeled as phase 1 and phase 4. Molecules in phase 4 appear as circular spots while molecules in phase 1 arrange into a striped structure. Because the doping ratio is directly related to the positions of electronic states, the identical positions of the LUMO and LUMO+1 state

(Figure 5) imply that the two phases belong to the same stoichiometric K3Coronene. Moreover, we have carefully checked the lattice constants of these phases and found they are different from each other. Totally, there are four different K3Coronene phases which are displayed in Figure 2c−f with various sample biases. The possible origin of the four isomers of K3Coronene might come from different locations of K atoms with respect to coronene molecules in the K3Coronene compound. As the doping ratio increases beyond x = 3, K4Coronene phase starts to emerge and gradually expands its coverage. As shown in Figure 3a on the same sample as in Figure 2b, phase 2 of K3Coronene and K4Coronene phase coexist at x = 3.3. The structural analysis demonstrates a √3 × √3 superstructure with a central molecule surrounded by the ones which appear brighter at positive bias but darker at negative bias. K4Coronene is the stable phase obtained with the highest doping ratio in our experiments. Further K doping will induce disorders on the film. 3.2. Evolution of Electronic Properties with K Doping. Corresponding to the structural evolution, the electronic properties of the K-doped cononene film also vary with the K doping ratio. As shown in Figure 4, the LUMO and LUMO C

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Figure 4. dI/dV/(I/V) curves of potassium-doped coronene with the doping ratios x < 2.3. The doping ratios are labeled in the figure. All of these STS spectra were measured at 77 K. The curves are shifted vertically for clarity.

Figure 6. dI/dV/(I/V) curves of K4Coronene and K3Coronene (phase 2) measured at corresponding positions in Figure 3a. K4Coronene shows obvious band shift. Previous LUMO state has been fully filled. The curve of K3Coronene is shifted vertically for clarity. Dashed line indicates the shifted zero line.

+1 states at x = 0.2 are slightly shifted from their corresponding peaks at x = 0. With the K doping ratio x increasing from 0.2 to 2.3, the LUMO and LUMO+1 states progressively and rigidly shift toward the Fermi level. Quantitatively, the LUMO state continuously shifts from 2.1 eV above the Fermi level to 0.6 eV above the Fermi level and the LUMO+1 state shifts by a similar amount. The continuous shift of the unoccupied electronic states is accompanied by the absence of phase separation in this doping range. Even the doping ratio increases a bit (like from 1.55 to 1.69), the slight shift in electronic state can be detected. However, the uniform phase with herringbone structure is not interrupted. This tendency of continuous energy shift is broken by an abrupt structure change and the formation of certain K3Coronene and K4Coronene phase. At x = 3, the LUMO and LUMO+1 states of phase 1 to phase 4 appear at identical positions (Figure 5), implying that they belong to the same stoichiometric K3Coronene. At x = 4, as shown in Figure 6, the whole band structure shifts downward further with LUMO fully filled and LUMO+1 remained empty. The bandgap between LUMO and LUMO+1 keeps identical to their previous energy difference.

As summarized in Figure 7, the shift rate of LUMO and LUMO+1 states is ∼0.7 eV per K atom which is almost

Figure 7. Summary of the energy positions of HOMO, LUMO, and LUMO+1 state with x from 0 to 4. For K3Coronene, phase 2 is selected for the plot since it is the most common one among the four phases. The two points around the Fermi level at x = 3 represent the two peaks of the splitting LUMO state. Errors of the data point in the figure are not larger than 0.1 eV.

independent of the structure of KxCoronene. The shift of the highest occupied molecular orbital (HOMO) state deviates from that of unoccupied states at very low and high doping ratio regions. Compared to the undoped film, HOMO state of KxCoronene at x = 0.2 is greatly pushed down from −1.7 to −2.7 eV. The different shift rate in occupied and unoccupied states is possibly due to the change of molecular binding state because of the turning of molecular configuration from flatlying to up-standing. The upshift of HOMO for K3Coronene and K4Coronene is attributed to the relaxation to the previous HOMO state, in analogue to alkali metal doped picene21 and other organic semiconductors.30 The electronic structure of K3Coronene deserves special discussions. Different from other doping ratio, its LUMO state is brought to the Fermi level and partially filled. However, this LUMO state splits into the occupied and unoccupied parts and develops an energy gap between the two parts, which prohibits the formation of metallic state (as shown in Figure 5). Our result confirms the insulating state of alkali metal doped hydrocarbon molecular film observed in previous PES experiments.21−23 Without the information on unoccupied state

Figure 5. dI/dV/(I/V) spectra of the four phases of K3Coronene. LUMO state splits into occupied part and unoccupied part when it is brought to the Fermi level. The four K3Coronene phases possess different gap sizes at the Fermi level. The curves are shifted vertically for clarity. Lattice distances and gap sizes are inserted in the figure. D

DOI: 10.1021/acs.jpcc.6b03686 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C previously in the PES measurement, a number of effects were suggested as possible explanations for the insulating state, for example, electronic correlation effect,21 polaronic effect,22 and mixture of band insulating phases x = 2 and x = 4 because of the instability of the x = 3 phase.23 Our direct observation of a splitting of LUMO state on a single phase by STS implies that electronic correlation effect is a plausible explanation. In fact, electronic correlation effect was also included in previous theoretical calculations of the electronic states of alkali metal doped hydrocarbon molecules.18,21,24,31,32 Using Hubbard model, at U/W > 1 (U is the electron Coulomb repulsion energy and W is the intermolecular hopping related bandwidth), the strong electron−electron interaction clearly splits the electronic state into the occupied lower Hubbard band and the unoccupied upper Hubbard band and leads to the formation of a Mott−Hubbard insulating state.21,33 In addition to the emergence of the gap at the Fermi level, the peak-to-peak gap size of K3Coronene seems to broaden up from 0.29 eV for phase 1 to 0.64 eV for phase 4 (Figure 5) as the average lattice distance expands. To find out the reason for this phenomenon, the screening effect is taken into consideration.34,35 Especially, the screening from the neighboring molecules is strongly dependent on the intermolecular distance as δUp = nαe2/(4πϵ0R4) where δUp is the reduction of Coulomb repulsion, n is the number of nearest neighboring molecules, α is the molecular polarizability, and R is the intermolecular distance. The lattice expansion leads to the weakening of the screening effect from the neighboring molecules and thus the enhancement of the effective Coulomb repulsion. By using the method given in ref 34, the effective U (Ueff) can be roughly estimated as about 1.1 eV for K3Coronene phase 1, which is larger than W (W ∼ 0.5 eV) of LUMO as shown in Figure 5 and is comparable to the value given in ref 34. Assuming other screening effects remain unchanged for the four K3Coronene phases, lattice expanding reduces δUp from ∼0.3 eV for phase 1 to ∼0.1 eV for phase 4 (for coronene α = 46.3 Å3).36 Accordingly, there will be a 0.2 eV increase of Ueff. Therefore, phase 4 is expected to have a wider gap at the Fermi level than phase 1. In this calculation, α for coronene is adopted. Generally, K3Coronene should have a larger α, although this value is not known.36,37 This will lead to a larger difference of Ueff between phase1 and phase 4, which is in better agreement with our experimental result of gap size change. In this way, a widening of different K3Coronene phases can be understood, although the charged levels of the four phases barely differ. The relation between the gap size and the lattice distance further supports the presence of electronic correlation effect in this system. Above all, the splitting of LUMO state at the Fermi level provides a direct evidence for strong electronic correlation in this system. Moreover, the four different phases of K3Coronene differ in lattice constant and exhibit different electronic correlation strengths. In organic superconductors, the electronic correlation may participate into the mechanism of superconductivity transition for K-doped hydrocarbon superconductors.13,24,25 Previous calculation results indicate that electronic correlation can enhance or weaken superconducting properties.38 We therefore speculate that the various Tc of Kdoped coronene observed in previous experiments may come from the different K3Coronene phases with different electronic correlation strengths.24

Besides the electronic correlation effect, the Jahn−Teller effect is possible to exist in this system.39,40 As being doped, the high symmetry of coronene is susceptible to be broken by a Jahn−Teller distortion, which will remove the degeneracy of LUMO state. In this case, both effects contribute to the splitting of LUMO state. The gap in LUMO state will be determined by the sum of Ueff and Jahn−Teller gap ΔJ−T.34 Another issue to be discussed is the gradual shift of the unoccupied states upon K doping in the range 0.2 < x < 2.3. According to the simple band-filling model, electrons are transferred from the donors to molecules and fill the LUMO state. As long as there are electrons effectively transferred to coronene, Kxcoronene is expected to have a partially filled LUMO state. However, in our experimental results, LUMO of KxCoronene for 0.2 < x < 2.3 is completely empty. The unfilled LUMO state of KxCoronene can be explained in a manner of interface dipoles. When coronene is adsorbed on the substrate, the difference in the vacuum level results in a charge transfer at coronene−Au interface and then forms of interface dipoles. Finally, the Fermi level of the substrate is pinned within the energy gap (as shown in Figure 1b).41−43 At this condition, the substrate will participate in the charge transfer when the coronene films are doped by K atoms. At the initial doping range, with the K s-electron transferred to the substrate, the interface dipole increases and establishes an interface electric field pointing toward the substrate. Therefore, the energy states of the coronene film with respect to the Fermi level of the substrate is downshifted by the potential difference ΔV across the interface.43 With increased K doping ratio, the interface electric field enhances and so do the potential difference. Our observed gradually shift of electronic states is in agreement with this interface dipole enhanced by charge transfer from K to the substrate and consistent with the monotonic decrease of work function versus doping ratio observed in ref 28. In this picture, there is effectively no charge transfer from the K atoms to the coronene molecules in the K doping range of x = 0.2−2.3. When the tail of the LUMO state starts to be filled, there accompanies a structural change of the coronene molecules from up-standing to flat-lying configuration. This changes the binding condition between KxCoronene and the substrate and leads to a sudden charge transfer to the coronene molecules. So, K3Coronene has a partially filled LUMO state.

4. CONCLUSIONS In summary, the structures and electronic properties of coronene films have been studied as a function of potassium doping ratios. At the very initial doping stage, adsorption of potassium atoms on the 2 ML coronene film induced the molecular orientation change from flat-lying to up-standing configuration to form a herringbone structure, which remained unchanged up to a K doping ratio of 2.3. In this doping range the charge transfer practically occurred between the K atoms and the substrate and produced a gradual downward shift of the electronic states of coronene molecules. With higher K doping level, the molecules returned back to the flat-lying configuration, and the electrons of K atoms were transferred to the coronene molecules, leading to the formation of certain Kdoped coronene phases. The doubly degenerate LUMO state of undoped coronene film was fully filled in K4Coronene. For K3Coronene, the splitting of LUMO state provides a direct evidence for the presence of the electronic correlation effect. Moreover, there were four structures observed for K3Coronene E

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(14) Kubozono, Y.; Goto, H.; Jabuchi, T.; Yokoya, T.; Kambe, T.; Sakai, Y.; Izumi, M.; Zheng, L.; Hamao, S.; Nguyen, H. L.; et al. Superconductivity in Aromatic Hydrocarbons. Phys. C 2015, 514, 199−205. (15) Casula, M.; Calandra, M.; Profeta, G.; Mauri, F. Intercalant and Intermolecular Phonon Assisted Superconductivity in K-Doped Picene. Phys. Rev. Lett. 2011, 107, 137006. (16) Kim, M.; Min, B. I.; Lee, G.; Kwon, H. J.; Rhee, Y. M.; Shim, J. H. Density Functional Calculations of Electronic Structure and Magnetic Properties of the Hydrocarbon K3Picene Superconductor near the Metal-Insulator Transition. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 214510. (17) Teranishi, K.; He, X. X.; Sakai, Y.; Izumi, M.; Goto, H.; Eguchi, R.; Takabayashi, Y.; Kambe, T.; Kubozono, Y. Observation of Zero Resistivity in K-Doped Picene. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 060505. (18) Giovannetti, G.; Capone, M. Electronic Correlation Effects in Superconducting Picene from Ab Initio Calculations. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 134508. (19) Okazaki, H.; Wakita, T.; Muro, T.; Kaji, Y.; Lee, X.; Mitamura, H.; Kawasaki, N.; Kubozono, Y.; Yamanari, Y.; Kambe, T.; et al. Electronic Structure of Pristine and K-Doped Solid Picene: Nonrigid Band Change and Its Implication for Electron-IntramolecularVibration Interaction. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 195114. (20) Okazaki, H.; Jabuchi, T.; Wakita, T.; Kato, T.; Muraoka, Y.; Yokoya, T. Evidence for Metallic States in Potassium-Intercalated Picene Film on Graphite. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 245414. (21) Ruff, A.; Sing, M.; Claessen, R.; Lee, H.; Tomic, M.; Jeschke, H. O.; Valenti, R. Absence of Metallicity in K-Doped Picene: Importance of Electronic Correlations. Phys. Rev. Lett. 2013, 110, 216403. (22) Mahns, B.; Roth, F.; Knupfer, M. Absence of Photoemission from the Fermi Level in Potassium Intercalated Picene and Coronene Films: Structure, Polaron, or Correlation Physics? J. Chem. Phys. 2012, 136, 134503. (23) Caputo, M.; Di Santo, G.; Parisse, P.; Petaccia, L.; Floreano, L.; Verdini, A.; Panighel, M.; Struzzi, C.; Taleatu, B.; Lal, C.; et al. Experimental Study of Pristine and Alkali Metal Doped Picene Layers: Confirmation of the Insulating Phase in Multilayer Doped Compounds. J. Phys. Chem. C 2012, 116, 19902−19908. (24) Capone, M.; Fabrizio, M.; Castellani, C.; Tosatti, E. Strongly Correlated Superconductivity. Science 2002, 296, 2364−2366. (25) Huang, Z. B.; Zhang, C.; Lin, H. Q. Magnetic Instability and Pair Binding in Aromatic Hydrocarbon Superconductors. Sci. Rep. 2012, 2, 922. (26) Yoshimoto, S.; Narita, R.; Itaya, K. Highly Ordered Coronene Adlayer on Au (111) Surface Formed in Benzene Solution: In Situ Scanning Tunneling Microscopy Study. Chem. Lett. 2002, 356−357. (27) Kosugi, T.; Miyake, T.; Ishibashi, S.; Arita, R.; Aoki, H. Ab Initio Electronic Structure of Solid Coronene: Differences from and Commonalities to Picene. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 020507. (28) Yano, M.; Endo, M.; Hasegawa, Y.; Okada, R.; Yamada, Y.; Sasaki, M. Well-Ordered Monolayers of Alkali-Doped Coronene and Picene: Molecular Arrangements and Electronic Structures. J. Chem. Phys. 2014, 141, 034708. (29) Chen, W.; Huang, H.; Thye, A.; Wee, S. Molecular Orientation Transition of Organic Thin Films on Graphite: The Effect of Intermolecular Electrostatic and Interfacial Dispersion Forces. Chem. Commun. 2008, 4276−4278. (30) Koch, N.; Chan, C.; Kahn, A.; Schwartz, J. Lack of Thermodynamic Equilibrium in Conjugated Organic Molecular Thin Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 195330. (31) Giovannetti, G.; Casula, M.; Werner, P.; Mauri, F.; Capone, M. Downfolding Electron-Phonon Hamiltonians from Ab Initio Calculations: Application to K3 Picene. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 115435.

which varied not only in morphology but also in electronic correlation strength. Our results may shed some light on the understanding of the superconductivity properties of this system.

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AUTHOR INFORMATION

Corresponding Author

*(X.D.X.) E-mail [email protected]; phone (852) 39434388. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Research Grant Council of Hong Kong (Grant. No. 404613) and the National Natural Science Foundation of China (Grant No. 11574128).

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REFERENCES

(1) Craciun, M. F.; Rogge, S.; den Boer, M.-J. L.; Margadonna, S.; Prassides, K.; Iwasa, Y.; Morpurgo, A. F. Electronic Transport through Electron-Doped Metal Phthalocyanine Materials. Adv. Mater. 2006, 18, 320−324. (2) El-Sayed, A.; Borghetti, P.; Goiri, E.; Rogero, C.; Floreano, L.; Lovat, G.; Mowbray, D. J.; Cabellos, J. L.; Wakayama, Y.; Rubio, A.; et al. Understanding Energy-Level Alignment in Donor-Acceptor/ Metal Interfaces from Core-Level Shifts. ACS Nano 2013, 7, 6914− 6920. (3) Shaheen, S. E.; Kippelen, B.; Peyghambarian, N.; Wang, J.-F.; Anderson, J. D.; Mash, E. A.; Lee, P. A.; Armstrong, N. R.; Kawabe, Y. Energy and Charge Transfer in Organic Light-Emitting Diodes: A Soluble Quinacridone Study. J. Appl. Phys. 1999, 85, 7939−7945. (4) Videlot-Ackermann, C.; Ackermann, J.; Fages, F. Charge Transfer Effects in Organic Field-Effect Transistors Containing a Donor/ Acceptor Heterojunction. Synth. Met. 2007, 157, 551−557. (5) Deibel, C.; Strobel, T.; Dyakonov, V. Role of the Charge Transfer State in Organic Donor−Acceptor Solar Cells. Adv. Mater. 2010, 22, 4097−4111. (6) Robles, R.; Lorente, N.; Isshiki, H.; Liu, J.; Katoh, K.; Breedlove, B. K.; Yamashita, M.; Komeda, T. Spin Doping of Individual Molecules by Using Single-Atom Manipulation. Nano Lett. 2012, 12, 3609−3612. (7) Krull, C.; Robles, R.; Mugarza, A.; Gambardella, P. Site- and Orbital-Dependent Charge Donation and Spin Manipulation in Electron-Doped Metal Phthalocyanines. Nat. Mater. 2013, 12, 337− 343. (8) Mitsuhashi, R.; Suzuki, Y.; Yamanari, Y.; Mitamura, H.; Kambe, T.; Ikeda, N.; Okamoto, H.; Fujiwara, A.; Yamaji, M.; Kawasaki, N.; et al. Superconductivity in Alkali-Metal-Doped Picene. Nature 2010, 464, 76−79. (9) Kubozono, Y.; Mitamura, H.; Lee, X.; He, X.; Yamanari, Y.; Takahashi, Y.; Suzuki, Y.; Kaji, Y.; Eguchi, R.; Akaike, K.; et al. MetalIntercalated Aromatic Hydrocarbons: A New Class of Carbon-Based Superconductors. Phys. Chem. Chem. Phys. 2011, 13, 16476−16493. (10) Wang, X. F.; Liu, R. H.; Gui, Z.; Xie, Y. L.; Yan, Y. J.; Ying, J. J.; Luo, X. G.; Chen, X. H. Superconductivity at 5 K in Alkali-MetalDoped Phenanthrene. Nat. Commun. 2011, 2, 507. (11) Xue, M. Q.; Cao, T. B.; Wang, D. M.; Wu, Y.; Yang, H. X.; Dong, X. L.; He, J. B.; Li, F.; Chen, G. F. Superconductivity above 30 K in Alkali-Metal-Doped Hydrocarbon. Sci. Rep. 2012, 2, 389. (12) Artioli, G. A.; Hammerath, F.; Mozzati, M. C.; Carretta, P.; Corana, F.; Mannucci, B.; Margadonna, S.; Malavasi, L. Superconductivity in Sm-Doped [N] Phenacenes (N= 3, 4, 5). Chem. Commun. 2015, 51, 1092−1095. (13) Subedi, A.; Boeri, L. Vibrational Spectrum and Electron-Phonon Coupling of Doped Solid Picene from First Principles. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 020508. F

DOI: 10.1021/acs.jpcc.6b03686 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (32) Kim, M.; Choi, H. C.; Shim, J. H.; Min, B. I. Correlated Electronic Structures and the Phase Diagram of Hydrocarbon-Based Superconductors. New J. Phys. 2013, 15, 113030. (33) Georges, A.; Kotliar, G.; Krauth, W.; Rozenberg, M. J. Dynamical Mean-Field Theory of Strongly Correlated Fermion Systems and the Limit of Infinite Dimensions. Rev. Mod. Phys. 1996, 68, 13−125. (34) Wang, Y. Y.; Yamachika, R.; Wachowiak, A.; Grobis, M.; Crommie, M. F. Tuning Fulleride Electronic Structure and Molecular Ordering Via Variable Layer Index. Nat. Mater. 2008, 7, 194−197. (35) Rotenberg, E.; Enkvist, C.; Brühwiler, P. A.; Maxwell, A. J.; Mårtensson, N. Local-Field Effects on Photoemission of C60. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, R5279−R5282. (36) Miller, K. J.; Savchik, J. A New Empirical Method to Calculate Average Molecular Polarizabilities. J. Am. Chem. Soc. 1979, 101, 7206− 7213. (37) Rayane, D.; Antoine, R.; Dugourd, P.; Benichou, E.; Allouche, A. R.; Aubert-Frécon, M.; Broyer, M. Polarizability of KC60: Evidence for Potassium Skating on the C60 Surface. Phys. Rev. Lett. 2000, 84, 1962− 1965. (38) Nomura, Y.; Nakamura, K.; Arita, R. Ab Initio Derivation of Electronic Low-Energy Models for C60 and Aromatic Compounds. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 155452. (39) Kato, T.; Yoshizawa, K.; Yamabe, T. Jahn−Teller Effects in the Coronene Anions and Cations. J. Chem. Phys. 1999, 110, 249−255. (40) Filatov, A. S.; Sumner, N. J.; Spisak, S. N.; Zabula, A. V.; Rogachev, A. Y.; Petrukhina, M. A. Jahn-Teller Effect in Circulenes: XRay Diffraction Study of Coronene and Corannulene Radical Anions. Chem. - Eur. J. 2012, 18, 15753−15760. (41) Topham, B. J.; Kumar, M.; Soos, Z. G. Profiles of Work Function Shifts and Collective Charge Transfer in Submonolayer Metal-Organic Films. Adv. Funct. Mater. 2011, 21, 1931−1940. (42) Amsalem, P.; Niederhausen, J.; Wilke, A.; Heimel, G.; Schlesinger, R.; Winkler, S.; Vollmer, A.; Rabe, J. P.; Koch, N. Role of Charge Transfer, Dipole-Dipole Interactions, and Electrostatics in Fermi-Level Pinning at a Molecular Heterojunction on a Metal Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 035440. (43) Monti, O. L. A. Understanding Interfacial Electronic Structure and Charge Transfer: An Electrostatic Perspective. J. Phys. Chem. Lett. 2012, 3, 2342−2351.

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DOI: 10.1021/acs.jpcc.6b03686 J. Phys. Chem. C XXXX, XXX, XXX−XXX