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Overlapping of Frontier Orbitals in Well-Defined DNTT and Picene Monolayers Yuri Hasegawa, Yoichi Yamada, Takuya Hosokai, Rasika Koswattage, Masahiro Yano, Yutaka Wakayama, and Masahiro Sasaki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06838 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

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The Journal of Physical Chemistry

Overlapping of Frontier Orbitals in Well-Defined DNTT and Picene Monolayers Yuri Hasegawa1, Yoichi Yamada1*, Takuya Hosokai2, Kaveenga Rasika Koswattage3, Masahiro Yano4, Yutaka Wakayama5, Masahiro Sasaki1 1 University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8571, JAPAN, 2 National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, 305-8560 JAPAN, 3 Faculty of Applied Science Sabaragamuwa University of Sri Lanka, Sabaragamuwa University of Sri Lanka., P.O. Box 02, Belihuloya, Sri Lanka, 4 Japan Atomic Energy Agency, Shirane Shirakata 2-4, Tokai-mura, Ibaraki, 319-1195, Japan 5 National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan *Corresponding author Yoichi Yamada TEL:+81-29-853-5284 FAX:+81-29-853-5038 E-MAIL: [email protected]

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Abstract Well-defined monolayers with single-crystalline-like molecular arrangements of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene(DNTT) and picene, which are a new class of organic semiconductors with enhanced intermolecular interactions, were fabricated and characterized. Although both molecules initially form a loosely packed monolayer with flat-lying molecule, they undergo phase transition into a densely packed monolayer with single-crystalline-like molecular arrangements with increasing molecular density. Upon the phase transition of the monolayer, the highest occupied molecular orbital (HOMO) level of these molecules splits into two features, as suggested both from the ultraviolet photoelectron spectroscopy and density functional theory calculations. The observed splitting of the HOMO level was observed to be similar to that expected for the molecular arrangement in the single crystal. This splitting, which has not been observed in the polycrystalline film, suggests a substantial overlap of the HOMO in the well-ordered monolayers.

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1. Introduction Small organic compounds with strong intermolecular interaction, such as rubrene, picene and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT), have been attracting attention as promising materials for high-performance organic field-effect transistors because of an efficient overlapping of their molecular orbitals; these materials are also expected to exhibit enhanced carrier mobility. In this study, we focus on DNTT and picene molecules, whose molecular structures are relatively simple among these materials as shown in Fig.1(a) and Fig.5 (a), respectively. DNTT is highly π–extended heteroarene comprising six fused aromatic rings including two sulfur atoms at central heteroaromatic rings. Picene is an aromatic hydrocarbon made of five fused benzene rings in an armchair configuration. Both molecules show higher chemical stability and a larger band gap than conventional organic semiconductor molecules such as pentacene.1,2 Single crystals of these molecules are both monoclinic, consisting of a molecular layer in which molecules are packed in herringbone fashion with the π -stacking forming an efficient 2D network for carrier transport. Indeed, their transport property has shown excellent carrier mobilities as large as that of amorphous silicon.1–3 The clear band dispersion has also been confirmed by angle-resolved ultraviolet photoelectron

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spectroscopy (ARUPS) of rubrene and picene single crystals, in addition to the Hall effect.4–7 These observations provide clear indication of band-like transport due to significant hybridization between molecular orbitals. Thus, controlling the overlap of the molecular orbitals of these materials can substantially modify their electronic properties. Indeed, the electronic properties of these molecules, such as their carrier mobility and optical gap, have been demonstrated to be significantly altered when external pressure is applied.8,9 The structural control of the thin film of these materials are therefore of great importance. However, the methods of controlling the structure of thin films of these materials have not been well established and the deposition of the highly ordered thin films remains challenge. Indeed, well-defined layer-by-layer growth of films of pentacene, which has been one of the most extensively studied molecules, has only recently been realized.10 We speculated that the realization of well-defined and controlled films of the organic semiconductor molecules would be more feasible in the case of molecules with enhanced intermolecular interactions, such as DNTT and picene. In this paper, we therefore determined the molecular arrangement of monolayers of DNTT and picene at the molecular scale by means of scanning tunneling microscopy (STM). We observed that both monolayers undergo a similar structural transition from a loosely packed phase with flat-lying molecules to a densely packed

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phase with single-crystalline-like molecular arrangement. This change in molecular arrangement in the monolayer caused a substantial modification in the electronic states. The dense phase exhibited splitting of the highest occupied molecular orbital (HOMO) levels because of extensive overlapping of the molecular orbitals, and the width of the splitting was observed to be comparable to that expected for the single-crystalline phase. Therefore, these results demonstrate the formation of a single-crystalline-like monolayer. This characteristic electronic feature has not been recognized so far in thin films on the polycrystalline Au. Thus, we considered that the electronic structure of the film of these materials are quite sensitive to the uniformity of the molecular arrangement and therefore the well ordered monolayer as realized in the present study will be of importance in the future application of these molecules in the functional devices.

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2. Experiments All the experiments were performed in an ultra-high vacuum environment at room temperature. The Au(111) surface was cleaned by conventional cycles of Ar+-ion sputtering followed by annealing. The cleanliness of the substrate was examined by STM and ARUPS measurement. DNTT and picene monolayers were then fabricated by vacuum deposition using home-made Knudsen-cell. The amount of deposited molecules was checked using quartz crystal microbalance. The molecular arrangements and structural transition were confirmed by STM observations and low-energy electron diffraction (LEED) experiments. Ultraviolet photoelectron spectroscopy (UPS) was conducted using the synchrotron facility at UVSOR(BL2B), where a hemispherical electron analyzer (R3000, Sienta VG) is equipped with a custom built apparatus. Valence band spectra were collected using light of 28 eV and emission angle of 6° and 36° for DNTT and picene, respectively. Total energy resolution determined by the Fermi level of Au(111) substrate was 80 meV. The beam current at the sample was less than 100 pA and the damage of the molecular layer was negligible during the acquisition of one spectrum. Note that the spatial distribution of the photoelectron intensity would be sensitive to the molecular configurations and thus the emission angles which give maximum

photoelectron

intensity

are

different

between

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DNTT

and

picene

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monolayers.11 Density Functional theory (DFT) calculations were performed with Gaussian code using the B3LYP functional with the 6-31Gd basis set.

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3. Results and Discussion The STM measurements revealed that the DNTT and picene monolayers undergo a similar phase transition from a loosely packed phase to densely packed phase in the monolayer region with increasing of amount of supplied molecules. Fig. 1 shows STM images of the loosely packed DNTT monolayer on a Au(111) surface. In the loose phase, two types of molecular arrangements (arrangements A and B), as shown in Fig.1(b) and (c), coexist. There also exist several rotational domains of these arrangement. Enlarged images of both arrangements are shown in Fig 1(d) and (e), respectively. In both cases, N-shaped individual DNTT molecules are clearly observed, indicating that the monolayer comprises molecules with face-on adsorption. Two protrusions at the center of each molecule are due to sulfur atoms. The HOMO of DNTT has already been demonstrated by calculation to localize around the sulfur atoms.12 In both arrangements, both sulfur atom of each molecule point toward the benzene ring of a neighboring molecule in a molecular row as depicted in inset of Fig.1 (d) and (e). This arrangement indicates that the molecular orientation is stabilized by the electrostatic interaction between the positively charged hydrogen atom on the benzene ring and the negatively charged sulfur atom. The dimensions of the unit cells of the arrangement A and B is 1.0 × 1.3 and 0.8 × 3.1 nm2 , respectively. Note that the DNTT molecule

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shows a 2D chirality, and the domain A comprises molecules of an enantio-pure phase and domain B comprises a racemic molecular row. Note that the homochiral domain with the other chirality also exists (not shown). DNTT molecules in arrangements A and B should have different adsorption sites. However, both phases do not appear to strongly interact with the Au substrate, because the herringbone reconstruction of the Au(111) surface underneath the monolayer is visible in the STM image. Therefore we speculate that the electronic structures of these phases do not differ significantly. After further deposition of DNTT molecules, molecular arrangement of the whole monolayer changed into a denser packing, as shown in Fig. 2(a). The amount of further supplied DNTT molecule is approximately same as that required for the formation of the loose phase. The STM image of the dense phase reveals one-dimensionality in the molecular arrangement; it comprises of the molecular rows aligned in the 〈0 1 1〉 direction of the Au(111) substrate. Although the size of the domain seems to increased compared to the loose phase, we also found the rotated domains and chiral domains (not shown). Each molecular row comprises bright and dim molecules arranged alternately, as shown in Fig. 2(b). In this ordering, two types of molecules appear to be dimerized as illustrated in Fig. 2(c). The molecular arrangement in the densely packed monolayer appears similar to the arrangement in the (110) plane

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of bulk crystal of DNTT.

The lattice parameters of this dense phase of the DNTT

monolayer ( 1.1 × 1.6 nm2) are indeed similar to the lattice constants of the (110) plane (1.0 × 1.6 nm2), although the shape of the unit cell of the dense layer differs slightly from that of the single crystal. The density of the molecule is approximately twice as much as that for the loose phase. Therefore, we consider that this phase is also monolayer, with densely-packed molecules. In order to check macroscopic structural features of the dense phase, we examined STM images of different positions throughout in the sample. We did not find any indication of the second layer formation or aggregation of DNTT. Therefore, we concluded that the system is densely-packed monolayer throughout in the sample and the STM image shown in Fig. 2 represents the average structure of the dense monolayer. It is noted that the transition from the loose to the dense phase are clearly visible in LEED as shown in Fig. 3, suggesting that the transition is macroscopic. For both phases, the LEED pattern consists from the two chiral unit cells shown as solid and dotted lines. Each unit cell exhibits 6-rotational domains, resulting in 12 domains. The multi-domain structure with chiral molecules revealed by LEED is also compatible to the STM image shown in Fig.1 and 2. It is important to note that the unit cells deduced from the STM image corresponds well to those deduced from the LEED images.

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However, in the case of the DNTT molecule, the size and shape of the unit cell of the loose and dense phases are rather similar, yielding similar LEED pattern. Nevertheless, we can distinguish these two from the pattern. Here we note that the diffraction spots due to the arrangement B in the loose phase are not clear in LEED. A picene monolayer undergoes a similar structural transition on Au(111), as we have already reported.13 Fig. 4 shows STM images of two types of picene monolayers on Au(111) surfaces. In the loose phase shown in Fig.4 (b), w-shaped molecules are clearly observed, indicating that picene molecules are flat-lying on Au(111). The dimension of the unit cell is 0.8 × 1.5 nm2. Further deposition of picene molecules on the loose phase results in the formation of one-dimensional molecular packing, i.e., the dense phase, as shown in Fig. 4(c). In the molecular row, a bright-dim molecular arrangement is observed. The unit cell is 1.1 × 1.5 nm2. Molecular arrangement in the dense phase resembles the (110) layer of the single crystal of picene as shown in Fig. 4(d). However, the local arrangement of the molecules in the dense phase differs slightly from that in the single-crystalline phase. Notably, as with the DNTT phase, we also observed herringbone reconstruction of the Au(111) surface in the STM image, indicating a weak interactions between molecules and the substrate. The LEED patterns corresponding to the loose and dense phases of the picene monolayer are shown in Fig. 5 (a) and (b). In

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the case of picene, the unit cell of the two phases differs significantly providing clearly different LEED patterns. Here also, the unit cell deduced from the STM image can well explain the LEED pattern. Just as in the case of the DNTT, LEED seems to be consisting of multiple domains with two chirality whose unit cells are indicated with solid and dotted lines. Note that, to our knowledge, the this type of the reconstruction in the monolayer has not been observed in other small organic semiconductors, including pentacene14. Therefore characteristic reconstruction in the monolayer of DNTT and Picene molecules as observed above appears to be common for this class of molecules with enhanced intermolecular interaction. Upon reconstruction of the monolayers of DNTT and picene, their electronic structures also change, as observed in the UPS results. We first discuss the case of the DNTT monolayer. Fig. 6 (a) shows the UPS spectrum collected from the clean Au(111) surface and two phases (i.e., the loose and the dense phases) of the DNTT monolayer on Au(111). In the case of the loose phase, the density of states derived from the HOMO appears at a shoulder of the d-band of the Au(111) substrate. Although molecular arrangements A and B coexist in this phase, we considered the difference in the electronic states between these two arrangements to be small, as we will discuss later. However,

in

the

dense

phase,

a

new

density

of

12


states

appears

at

the

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lower-binding-energy side. To check the intrinsic spectral shapes of the DNTT layer, we subtracted the intensity of the Au substrate from the spectra of monolayers, as shown in Fig. 6 (b). The HOMO was then clearly observed and was found to be split into two features in the dense phase. To confirm this analysis, we also took the second derivative of the spectrum without subtraction of the substrate signal, as shown in the Fig. 6 (c). Note that we multiplied by -1 to match the peak positions of the raw and the second-derivative spectra. It is seen that the peak positions observed in the second derivative and the subtracted spectra well coincide. Thus, the splitting of the HOMO in the case of the dense phase is firmly recognized. The energy splitting of the HOMO in the dense phase is approximately 0.5 eV. Here we note that the Au(111) substrate exhibits a relatively large d-band and is thus not a very ideal substrate for examining the frontier orbitals of the organic molecules. Nevertheless, we were able to analyze the details of the changes in the HOMO via careful measurements and analyses. These results are important for further investigating the electronic states of the organic semiconductors because Au is one of the useful substrate especially in STM measurements. We also performed UPS measurements of picene monolayer which also undergoes a structural transition from a loose to a dense phase, similar to the case of

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DNTT monolayer. UPS spectra of the picene monolayers are shown in Fig. 6 (d), together with the spectrum of the Au(111) substrate. Note that the take-off angle of the photoelectrons in this case is different from that in the case of DNTT; thus the spectral shape of the d-band of the Au(111) substrate differs from that in Fig. 6 (a). The HOMO in the loose phase is observed at 1.8 eV, which is shallower than the HOMO in the case of the DNTT and is therefore more clearly observed. In the dense phase, a new intensity was clearly observed at lower binding energies as shown in Fig. 6 (e) and (f). Therefore, we consider that the HOMO of picene in the dense phase became split by approximately 0.4 eV. Therefore, we observed that both DNTT and picene monolayers, which undergo similar phase transition from the loose phase into the dense phase, exhibit splitting of the HOMO in the densely packed phase of the monolayers. We speculate that this splitting is due to the enhanced overlapping of the frontier orbitals in the dense phase. To understand the mechanism of the observed alteration of the HOMO level upon the structural transition of the DNTT and picene monolayers, we performed simple DFT calculations using Gaussian code. In our simple calculations, we took the pair of molecules as a model of each phase and evaluated their electronic structures. The distance and arrangement of the pair of molecules are determined based on the STM and LEED measurement, and we did not relax the confihgurations of the

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molecular pair during the calculations. Fig. 7 (a) and (b) shows the calculated orbital energies of the HOMOs for DNTT and picene, respectively, for the molecular pairs with different configurations. As a model of the molecular configuration of the A and B ordering in the loose phase of DNTT, we used a pair of molecules with an intermolecular distance of 1.0 nm and 0.8 nm, respectively, that are the observed values in the STM image. In contrast, we utilized a pair of molecules in the (110) plane of singe-crystalline DNTT as a model for the dense phase. This is because the fact that we cannot fully clarify the detailed molecular configuration in the dence phase only from the STM images. We observed that the orbital energy of model A is quite similar to that of the isolated single molecule. In contrast, subtle splitting of the HOMO level was observed in model B, possibly reflecting the smaller intermolecular distance. However, the splitting was as small as 50 meV and it could not be observed in the UPS spectra collected under the room temperature conditions used in this work. In the case of the dense phase, greater splitting of HOMO level as large as 0.3 eV was resulted. Note that the splitting of the HOMO is asymmetrical with respect to the HOMO position of the monomer, i.e., the anti-bonding orbital is largely unstabilized whereas the stabilization of the bonding state is as small as approximately 50 meV. The contour surface of the wave function of the HOMO for each model is also depicted in Fig. 7 (a). The amplitude of the isosurfaces

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is 0.01 , where is Bohr radii; different colors indicate different sign of the wave function. For the HOMO state in the monomer of DNTT, the wave function concentrates around the sulfur atoms as already proposed.12

From the plot of the wave function, the

HOMO level splitting in the case of the dense phase is clearly observed to be caused by the formation of the bonding and antibonding type of wave functions. Also, the shape of the wave function of the split HOMO levels observed in dimer clearly differ from that of HOMO-1 in monomer, as shown in Fig. 7 (a). Therefore, we firmly confirmed the splitting in the HOMO orbitals. Simultaneously, a substantial overlapping of the wave function of the HOMO is clearly observed in the plot. These results indicate that the large splitting of the HOMO in the dense phase is due to the significant overlapping of molecular

orbitals

of

the

HOMO

around

sulfur

atoms

and

the

resultant

bonding-antibonding splitting of the HOMO orbitals. Similar HOMO splitting was also reproduced by calculation in the case of picene. The calculated orbital energy and isosurface of HOMO wave function for each model are shown in Fig. 7 (b). Here we used a pair of molecules with an intermolecular distance of 0.8 nm as a model of the loose phase and a molecular pair in the single crystal as the model of the dense phase. As in the case of DNTT, a subtle split of the HOMO into bonding and anti-bonding orbitals is observed in the loose phase, and the

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energy splitting becomes large in the dense phase. These calculated results are essentially similar to the case of DNTT and explain the experimental results qualitatively. Therefore, we conclude that the modification of the HOMO upon the structural reconstruction of the monolayer is due to the enhanced overlap of the frontier orbitals of molecules. Note that the experimentally observed energy splitting of the HOMO in the dense phase of the monolayer is observed to be comparable to or even larger than the calculated width. Because the calculation utilizes the molecular arrangement in the single-crystalline phase, these results suggest the realization of a single crystalline-like electronic structure in the dense phase of the monolayer of these molecules. Split in HOMO level as demonstrated above using well-controlled film, has not been recognized for neither molecules in previous studies using thin films deposited in vacuum onto the polycrystalline Au substrates.16,17 As previously suggested by experimental and theoretical investigations, the splitting of the HOMO level is very sensitive to the molecular ordering that provides sufficient overlapping of molecular orbitals.18 We thus speculate that the molecular arrangement of thin films fabricated on polycrystalline substrates in previous study was not sufficiently uniform, resulting in broadening of the HOMO. Therefore, as in the case of this study, a well-defined system

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such as monolayer is useful for elucidating the intrinsic electronic state of condensed organic semiconductor system. We would like to note here that, in the case of the dense phase, we might have to consider the effect of the poor screening of the photo-hole due to electron of underlying Au substrate, which basically shifts the HOMO levels of the upper molecule in the dense phase to higher-binding-energy side compared to that of the bottom molecules in the dense phase, resulting in the apparent splitting of the HOMO.15 However, in such a case, a new density of state should appear at the lower-binding-energy side. Therefore, we consider that the screening effect is minor in the present case, possibly because of rather large molecule-substrate distance for both molecules in the monolayer. The stronger intermolecular interaction in the dense phase reduces the molecule-substrate interaction, elongating the molecule-substrate distance in the dense phase. Therefore the screening effect, which depends on the distance from the substrate, is not significantly differed in the two inequivalent molecules in the dense phase. In addition, we might also have to consider the change of ionization potential which depends on molecular orientation. Molecular orientation in the dense phase of DNTT and picene is tilting, and these orientations contribute to occurrence of quadruple which modifies the ionization potential. This mechanism may shift the

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HOMO level of the one molecule toward lower binding energy side. However, it is difficult to discuss the molecular orientation induced change in ionization potential, since we do not know precise tilting angle. Furthermore, since the experiments rather fit to the calculation without considering the modulations in the ionization potential, we consider that the bonding-antibonding splitting of HOMO is the main cause of the modification of the electronic state in the dense phase. Then, it is considered that the strongly overlapped monolayers of these molecules exhibit band like transport of the carrier with band dispersion. In the present system, however, the domain size in the dense phase is not very large (approximately several tens of nanometers) and many rotational domains coexist, precluding the ideal band-like transport in the monolayer and the clear band dispersion in the photoemission spectra. Fabrication of the single domain phase of densely packed molecules will be very important for applications of the well-ordered thin films of these molecules, and this may be possible utilizing anisotropic surface such as (110) surface of fcc crystal. Realization of the band-like transport would then become feasible, and such a trial is currently underway.

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4. Conclusion STM measurements revealed that both DNTT and picene monolayers undergo a similar phase transition from a loosely packed phase consisting of face-on molecules to a densely packed phase with single crystalline-like molecular arrangements. Thus, the observed reconstruction appears to be common for this class of molecules with strong intermolecular interactions. Upon the reconstruction into the dense phase, the electronic states of the HOMO levels are also significantly altered. The HOMO level splits into two features, bonding and anti-bonding orbitals, because of substantial overlap of the HOMO in the dense phase. This scenario was also confirmed by DFT calculation. The splitting width of the HOMO level in the monolayer was observed to be similar to the calculated results, which were based on the molecular arrangement in the bulk single crystal. Therefore, these observations demonstrate that a well-ordered single-crystalline-like monolayer was successfully formed. Since this characteristic electronic feature has not been recognized so far in thin films on the polycrystalline Au, it is considered that the electronic structure of the film of these materials are quite sensitive to the uniformity of the molecular arrangement. Therefore, the well ordered film of these class of molecules with enhanced intermolecular interaction, as realized in the present study, will be of importance in

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the applications of these molecules in the next generation organic electronics.

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Acknowledgement This work was performed with the approval of UVSOR (Proposal Number 28-509) and Photon Factory (Proposal No. 2014G170) . This work was supported by JSPS KAKENHI Grant Number 15J05607, 16K13678, 24760023, 26286011, and 25870034.

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Xin, Q.; Duhm, S.; Bussolotti, F.; Akaike, K.; Kubozono, Y.; Aoki, H.; Kosugi, T.; Kera, S.; Ueno, N. Accessing Surface Brillouin Zone and Band Structure of Picene Single Crystals. Phys. Rev. Lett. 2012 2012, 108 , 226401.

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Okada, Y.; Sakai, K.; Uemura, T.; Nakazawa, Y.; Takeya, J. Charge Transport and Hall Effect in Rubrene Single-Crystal Transistors under High Pressure. Phys.

Rev. B 2011, 2011 84 , 245308. (7)

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Sakai, K.; Okada, Y.; Kitaoka, S.; Tsurumi, J.; Ohishi, Y.; Fujiwara, A.; Takimiya, K.; Takeya, J. Anomalous Pressure Effect in Heteroacene Organic Field-Effect Transistors. Phys. Rev. Lett. 2013, 2013 110, 96603.

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Fanetti, S.; Citroni, M.; Malavasi, L.; Artioli, G. A.; Postorino, P.; Bini, R. High-Pressure Optical Properties and Chemical Stability of Picene. J. Phys.

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Zhang, Y.; Qiao, J.; Gao, S.; Hu, F.; He, D.; Wu, B.; Yang, Z.; Xu, B.; Li, Y.; Shi, Y.; et al. Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit. Phys. Rev. Lett. 2016, 2016 116 , 16602.

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Liu, Y.; Ikeda, D.; Nagamatsu, S.; Nishi, T.; Ueno, N.; Kera, S. Impact of 23



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Figure captions FIG. 1 (a) Molecular structure of DNTT. STM images of loosely packed DNTT monolayer comprising (b, d) enantiopure and (c, e) racemic-mixture arrangement on Au(111).

FIG. 2 (a, b) STM image of a densely packed DNTT monolayer on Au(111). (c) The model for molecular arrangement in the dense phase.

FIG. 3 LEED images of (a) loose phase and (c) dense phase of DNTT monolayer. Schematic of the reciprocal lattices of (b) the loose phase consisting of molecular arrangement A and (d) that of dense phase. Yellow solid and blue dashed oblique represent two chiral unit cell.

FIG. 4 (a) Molecular structure of picene. STM images of (b) the loose and (c) the dense phases of picene monolayers on an Au(111) surface. (d) Model for molecular arrangement of the dense phase.

FIG. 5 LEED images of (a) the loose and (c) the dense phases of picene monolayers. (b)

25



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reciprocal lattices of (b) the loose and (d) the dense phases. Yellow solid and blue dashed oblique represent two chiral unit cells.

FIG. 6 (a) UPS spectra of an Au(111) surface(black) and of loose(blue) and dense(red) phases of DNTT and (d) those of picene monolayer on Au(111) surface. (b, e) Substrate-subtracted intensity of the UPS spectra of DNTT and picene monolayers. (c, d) Second derivative of the UPS spectra of DNTT and picene monolayers.

FIG. 7 (a)Left side: Calculated orbital energy and plotted isosurface of the wave function of the HOMO of the DNTT. Right side: Change in the HOMO positions observed in UPS. (b) Left side: Calculated orbital energy and plotted isosurface of the wave function of Picene. Right side: Change in the HOMO positions observed in UPS.

26


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Figures

FIG. 1 Y. Hasegawa et. al.

FIG.2 Y. Hasegawa et. al.

27



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28


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29



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TOC

31



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Fig. 1 (a)

Carbon Sulfur Hydrogen

Loose phase A 𝟏𝟏𝟐

(b)

Loose phase B 𝟏𝟏𝟐

(c)

(e)

(d)

0.8 nm 1.3nm

3.1 nm

1.0nm


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Fig. 2

(a)

𝟏𝟏𝟐

(c)

(b) 1.6nm 1.1 nm



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

Fig. 3

24.3eV

(a)

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19.1eV

(c)

(d)

(b)

Loose phase

Dense phase


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Fig. 4 (a)

Carbon Hydrogen

(b)

(c)

(d)

0.8 nm 1.5 nm

1.5 nm 1.1 nm



Fig. 5 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

(a)

(c)

(b)

(d)

Loose phase

Dense phase ACS Paragon Plus Environment

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Picene/Au(111)

DNTT/Au(111)

(a)

(d) D phase

Intensity (a. u.)

Intensity (a. u.)

D phase

L phase(A.B)

L phase

Au(111) D phase

L phase(A.B)

(c)

Au(111)

(e)

(a. u.) (a.u.) Intensity of Intensity Difference

(b)

(a. u.) (a.u.) Intensity of Intensity Difference

D phase

L phase

(f) D phase

L phase(A.B)

D phase

(a. u.) Intensity u.) -d2I/dE2(a.

Fig. 6

2I/dE2(a. u.) (a.u.) -dIntensity

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


L phase

Au(111) -2.0

-1.5

-1.0

Binding Energy (eV)

Au(111) -2.0

-2.5

-1.5

-1.0

Binding Energy (eV)



Fig. 7 (a) DNTT

Orbital Energy(eV)

monomer

DFT calculations L phase A L phase B

UPS

D phase

L phase

D phase

-5.0

-1.5

HOMO -5.5

-2.0

Binding Energy (eV)

HOMO-1

(b) Picene Orbital Energy (eV)

-5.0

monomer

DFT calculations L phase

DNTT

UPS D phase

L phase

D phase -1.5

-5.5

HOMO HOMO -2.0

HOMO-1 -6.0 Picene ACS Paragon Plus Environment

Binding Energy (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

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TOC

 DNTT

 Picene

L phase

D phase


Overlapping of Frontier Orbitals in Well-Defined ... - ACS Publications

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