Heterointerface Screening Effects between Organic Monolayers and

Heterointerface Screening Effects between Organic Monolayers and Monolayer Transition Metal Dichalcogenides Yu Jie Zheng,†,¶ Yu Li Huang,†,‡,¶ Yifeng Chen,†,§,¶ Weijie Zhao,† Goki Eda,†,§,∥ Catalin D. Spataru,⊥ Wenjing Zhang,# Yung-Huang Chang,⊗ Lain-Jong Li,○ Dongzhi Chi,‡ Su Ying Quek,*,†,§,◆ and Andrew Thye Shen Wee*,†,§ †

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore Institute of Materials Research & Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore § Centre for Advanced 2D Materials, National University of Singapore, Block S14, Level 6, 6 Science Drive 2, Singapore 117546, Singapore ∥ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore ⊥ Sandia National Laboratories, Livermore, California 94551, United States # SZU-NUS Collaborative Innovation Center for Optoelectronic Science & Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, Shenzhen University, Shenzhen 518060, China ⊗ Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan ○ Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ◆ Institute of High Performance Computing, Agency for Science Technology and Research, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore ‡

S Supporting Information *

ABSTRACT: The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics etc. The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in “van der Waals (vdW) heterostructures” and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe2), bare graphite, and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO−LUMO gap on the electronic screening effects from the substrate. The monolayer WSe2 interlayer provides substantial, but not complete, screening at the organic/ inorganic interface. Our results lay a foundation for the exploitation of the complex interfacial properties of hybrid systems based on TMD materials. KEYWORDS: two-dimensional transition metal dichalcogenides, organic−inorganic interface, screening effects, energy level alignment, scanning tunneling microscopy/spectroscopy, first principle calculations response,5,6 and valleytronics phenomena.7,8 Other unique physical phenomena are also of great interest. For example, large exciton binding energies for single-layer semiconducting

T

wo-dimensional (2D) transitional metal dichalcogenides (TMDs) have attracted significant attention as promising materials for next-generation optoelectronics technology due to their novel electronic and optical properties. Intensive effort has been devoted to the exploitation of their remarkable properties such as indirect-to-direct bandgap crossover,1,2 field-induced transport,3,4 strong photovoltaic © XXXX American Chemical Society

Received: November 20, 2015 Accepted: January 20, 2016

A

DOI: 10.1021/acsnano.5b07314 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Atomic structure and electronic properties of SL-WSe2 on graphite. (a) A large-scale STM image of a monolayer WSe2 on graphite (80 × 80 nm2, VTip = −1.3 V). The height of monolayer WSe2 is ∼7 Å as revealed by the lateral profile in the inset. (b) High-resolution STM image of the SL WSe2 showings a moiré pattern with an angle of 29° between the moiré pattern and the WSe2 lattice (6 × 6 nm2, VTip = −1.0 V). (c) STS spectrum reveals a 1.95 eV band gap for monolayer WSe2 (set point: VTip = 1.3 V, ITip = 64 pA). (d) PL spectra acquired at 300 K with emission peak centered at 1.64 eV, shifting to 1.70 eV at 77 K.

TMDs have been theoretically predicted9,10 and experimentally verified.11−13 2D TMDs have also been incorporated with organic molecules14−16 or other 2D materials6,13 to form van der Waals (vdW) heterostructures. However, the complex interactions of 2D TMD materials with adsorbates are still not well understood, and such heterointerfaces play critical roles in determining the functionalities of hybrid systems. The energy level alignment (ELA), which describes how the energy levels of the different materials are aligned relative to one another, has a key impact on device performance by controlling the electron/hole transport barriers. Fundamentally, the electronic screening effect of the underlying substrate on the adsorbate has significant influence on the energy level positions and hence the charge transport properties. Physisorption on a metallic surface reduces the molecular HOMO−LUMO (HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital) gap significantly:17 the HOMO level relative to the vacuum level measures the energy required to remove an electron from the molecule; the image charge formed due to electronic screening in the metal stabilizes the added hole, making it easier to remove the electron, shifting the HOMO up. Similarly, the

added electron is stabilized in the case of the LUMO, shifting the LUMO level down. Therefore, the HOMO−LUMO gap of the adsorbed molecule strongly depends on the extent of screening in the underlying substrate.18 2D TMDs differ from metallic substrates in two ways: first, they are 2D rather than three-dimensional, and second, many 2D TMDs are semiconductors. It is thus far unclear how 2D TMD interlayers would screen an organic−inorganic interface, and how they would affect ELA and transport barriers. In this paper, we present a systematic experimental and theoretical study of the ELA at a prototypical organic/inorganic heterostructure, constructed by a self-assembled monolayer of PTCDA physisorbed on 2D single-layer (SL) tungsten diselenide (WSe2) on graphite. Comparative studies were performed for PTCDA directly adsorbed on graphite, as well as on the Au(111) surface. Scanning tunneling microscopy (STM) reveals that PTCDA forms a well-ordered herringbone structure on all the substrates considered. The HOMO− LUMO gap of PTCDA on WSe2/graphite is measured to be 3.73 eV. This gap becomes smaller on the semimetallic graphite substrate, and is further narrowed on the metallic Au substrate. Our first-principles calculations show that these differences in B


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Figure 2. Submonolayer PTCDA self-assembled on SL WSe2. (a) STM image of the PTCDA island on the WSe2/graphite interface with a herringbone packing structure (45 × 45 nm2, VTip = −1.5 V). (b) Atomically resolved STM image of the PTCDA island boundary (6 × 12 nm2, VTip = 1.3 V). (c) Height profile corresponding to the dark line in panels a revealing the apparent height of the PTCDA layer to be 3.5 Å. (d and e) Bias-dependent STM image corresponds to the occupied and unoccupied states of PTCDA molecule, respectively. The corresponding calculated HOMO and LUMO orbitals are shown in each inset (d, 4 × 4 nm2, VTip = 2.5 V; e, 4 × 4 nm2, VTip = −1.3 V).

depending on the relative stacking orientations (see Supporting Information, Figure S3). The electronic properties of SL WSe2 are determined by STS (Figure 1c). The valence band maximum (VBM) corresponds to a tip bias of 0.88 ± 0.03 V and the conduction band minimum (CBM) corresponds to −1.07 ± 0.03 V relative to the Fermi level (0 V). The quasi-particle bandgap is deduced to be 1.95 ± 0.05 eV, consistent with previous reports.12,20 The WSe2/graphite interface has a relatively smooth electronic potential without pronounced variance over the surface. With the use of photoluminescence spectroscopy, the optical bandgap of SL-WSe2 on graphite was determined to be 1.64 eV at 300 K and 1.70 eV at 77 K (Figure 1d), consistent with previous reports.12,21,22 The exciton binding energy is thus deduced to be 0.25 eV for SL WSe2 on graphite (at 77 K), slightly smaller than the 0.37 eV binding energy reported for SL WSe2 monolayer on SiO2/Si surface (at room temperature).12 The smaller exciton binding energy on graphite than on SiO2/ Si can be attributed to increased screening from graphite. Large exciton binding energies have been reported for other SL TMDs,11,13 and have been attributed to the substantially reduced dielectric screening in these TMD materials which enhances the Coulomb interaction. After 0.6 ML PTCDA was deposited onto the WSe2/graphite surface, well-ordered monolayer PTCDA islands with a herringbone molecular packing structure are formed on both the WSe2 adlayer (Figure 2a,b) and the bare graphite surface (Figure S4). The lateral profile in Figure 2c corresponds to the black line in Figure 2a, revealing an apparent height of 3.5 Å for the PTCDA monolayer, consistent with a flat-lying configuration, i.e., planar PTCDA molecules lie flat with their molecular π-plane parallel to the surface. Figure 2b reveals an atomically resolved STM image recorded at 1.3 V tip bias, whereby both the atomic structure of WSe2 (left) and individual PTCDA molecules (right) are visible. There does not appear to be orientational alignment between the PTCDA adlayer and WSe2 substrate, which is expected for weak interfacial interactions. The unit cell of the PTCDA layer is marked by a black rectangle in Figure 2d, which is composed of two molecules. The lattice parameters of the unit cell are a = 14.2 ± 0.5 Å and b = 21.2 ± 0.5 Å with an internal angle of 93° ± 3°. Bias-dependent STM images in Figure 2d,e are recorded at tip bias voltages (VT) of 2.5 V and −1.3 V, corresponding to the occupied and unoccupied states of the PTCDA molecules, respectively. The fine structures of the PTCDA molecule are

gaps can be explained only after electronic screening effects from the substrate are taken into account explicitly. The HOMO−LUMO gap of PTCDA therefore depends strongly on the surface environment as a result of different screening effects. We find that surface dielectric effects are important in graphite and, furthermore, that the WSe2 monolayer provides substantial screening at the organic−inorganic interface. These findings highlight the importance of substrate selection as well as the layer packing sequence for the design of future hybrid devices, e.g., FETs and LEDs based on heterostructures,14−16,19 as such electronic screening would play a critical role in determining the ELA and transport barriers, thus influencing the device performance.

RESULTS AND DISCUSSION The WSe2/graphite interface (see sample preparation in Materials and Methods and Supporting Information, Figure S1), which serves as the substrate for molecular adsorption, is first investigated. Graphite is selected as the substrate as it has relatively weak electronic coupling with the adsorbate/adlayer. Figure 1a shows a typical STM image of the SL WSe2 film. The graphite surface is not yet fully covered, and the SL WSe2 flake can be easily distinguished due to the irregular and brighter edge features.13 The apparent height of the WSe2 flake is ∼7 Å (Figure 1a inset), corresponding to the thickness of a single WSe2 layer. Unknown atomic/molecular contaminations are adsorbed at the edges, which are difficult to remove even after increasing the degassing temperature because of the high chemical reactivity of the edges.13 The clean WSe2 surface is imaged away from the edges, as revealed by the high resolution STM image in Figure 1b. The WSe2 unit cell is marked by a blue parallelogram with a lattice constant of 3.4 ± 0.1 Å. A moiré pattern due to the lattice and orientation mismatch between the WSe2 adlayer and graphite substrate is clearly visible. As highlighted by the green parallelogram in Figure 1b, the dimensions of the moiré superstructure are a = b = 10.0 ± 0.3 Å with an interior angle of 60° ± 2°. The orientation difference between the moiré pattern and the WSe2 lattice is about 29° ± 2° as denoted by the green and blue arrows (a possible structural model see Supporting Information, Figure S2). As both the WSe2 adlayer (a continuous film) and graphite are polycrystalline, a variety of moiré superstructures with different dimensions and intersection angles can be observed over the whole sample, C


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Figure 3. HOMO and LUMO levels of monolayer PTCDA on different substrates. (a) STS spectra reveal the HOMO−LUMO gap varies with the substrate, namely 3.73 eV on SL-WSe2/graphite (set point: VTip = 2.2 V, ITip = 60 pA), 3.49 eV on bare graphite (set point: VTip = 2.2 V, ITip = 73 pA), and 3.10 eV on Au(111) (set point: VTip = 1.9 V, ITip = 100 pA). (b) Theoretical predictions of the HOMO and LUMO levels of PTCDA on different substrates, obtained by applying the corrections, ∑0,H + PH and ∑0,L + PL (see Figure 5), to the DFT PBE HOMO and LUMO positions for PTCDA on the respective substrates. These levels are artificially broadened using a Gaussian function.

As summarized in Table 1, the PTCDA HOMO−LUMO gap is substrate dependent, and is reduced in size from

well resolved, and are consistent with the computed HOMO and LUMO of the free molecule (shown in the inset of each panel). These results point to the weak interactions between PTCDA and the WSe2 surface, similar to other chemically inert surfaces such as graphite and Au(111).23−29 STS spectra were measured to determine the energy positions of the PTCDA LUMO and HOMO states. The dI/ dV curve for monolayer PTCDA on WSe2/graphite is shown at the bottom of Figure 3. A wide gap is observed, with LUMO and HOMO states at tip biases of −1.16 ± 0.03 and 2.57 ± 0.03 V, respectively, corresponding to a gap of 3.73 eV. This gap is smaller than the isolated PTCDA molecule gap, which we compute to be 5.0 eV, in agreement with previous reports.30,31 To further evaluate the impact of the WSe2 interlayer on the PTCDA HOMO−LUMO gap, comparative STM and STS measurements of submonolayer PTCDA on bare graphite and Au(111) were performed. Similar to the WSe2/graphite substrate, the PTCDA molecules physisorb into herringbone arrangements on both graphite and Au(111) surfaces (Figure S4). In addition, the lattice parameters for these herringbone arrangements are very similar for the different substrates (Table S1). However, their HOMO−LUMO gaps are distinctly different. Although the shape of the dI/dV curve taken on bare graphite is similar to that on WSe2/graphite, a narrowing of the gap to 3.49 eV is observed, as both the LUMO and HOMO states move slightly toward the Fermi level (Figure 3). Compared to the 3.73 eV band gap for PTCDA on WSe2/ graphite surface, it is clear that the insertion of the atomically thin semiconductor WSe2 monolayer widens the HOMO− LUMO gap by 0.24 eV. The changes are more pronounced on Au(111), where the PTCDA adlayer has a gap of 3.10 eV (Figure 3). Moreover, compared to the WSe2/graphite and bare graphite substrates, where the background in the dI/dV curves are low and smooth in the gap region, several features are observed in the background (see more discussion in Figure S7) of the dI/dV spectrum on Au(111), consistent with a larger density of states near the Fermi level for the metallic substrate.24 The reduction of the gap is consistent with stronger electronic screening expected on the metallic Au(111) surface.

Table 1. Summary of Calculated Geometry and Electronic Properties of Adlayer/Substrate Systems, Compared with Experimental Values for the Band Gapsa

Au(111) Graphite

WSe2/graphite

vdW correction

d (Å)

optB86b-vdW PBE-D2 optB86b-vdW PBE-D2

3.35 3.09 3.27 3.24 10.18 3.40 3.36

optB86b-vdW PBE-D2

z0 (Å)

cal. gap (eV)

exp. gap (eV)

0.9

2.93 2.79 2.99 2.97 4.29 3.29 3.28

3.10

0.7

0.1

3.49

3.73

a d is the distance between the PTCDA layer and the first atomic layer of the substrates. z0 is the computed distance of the substrate image plane from the first atomic layer of the substrates. Graphite d = 10.18 Å refers to the case where the WSe2 is removed, keeping the molecule and graphite in their original positions in the PTCDA SL-WSe2/ graphite system. The calculated gaps in this table are obtained using the two-step approach in Figure 5, starting from the ΔSCF gap value in the gas phase.

semiconducting to semimetallic and to metallic substrates. In the following, theoretical analysis is presented to better understand the fundamental mechanisms determining the PTCDA HOMO−LUMO gaps on the different substrates, and to probe the extent to which the SL-WSe2 participates in electronic screening. To model WSe2/graphite, we choose a low strain, high symmetry superlattice with a 3 × 3 WSe2 lattice on top of a 4 × 4 graphite lattice. The relaxed structures for PTCDA (Figure 4a) on WSe2/graphite are shown in Figure 4b,c. These results have been obtained using the optB86-vdW32,33 functional to account for vdW interactions within density functional theory (DFT) (see Materials and Methods). With the use of this approach, the equilibrium binding distance of PTCDA on Au(111) (Table 1) is very close to the experimental value (3.27 Å).23 Equilibrium binding distances computed using both optB86-vdW and PBE-D234 are shown in Table 1. Neither the imposed strain on the PTCDA layer/WSe2 nor the relative D


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PTCDA molecule (namely, ∑0,H and ∑0,L) are computed. GW within the one-shot approximation (G0W0) gives a HOMO− LUMO gap of 4.7 eV, consistent with other reports.30,36 However, G0W0 has been found to underestimate the HOMO− LUMO gap of molecules such as pentacene.36 We use instead ΔSCF to predict the gas phase gap, because the ΔSCF method is known to be accurate for predicting the HOMO−LUMO gaps of isolated molecules.31,43−45 The ΔSCF gap value for isolated PTCDA is 5.0 eV, similar to previous reports30,31 and close to the 5.1 eV gap value obtained using the G1W1 approximation.36 In the second step, correlation effects from the substrate are approximated using an image charge model, giving corrections PH and PL to the HOMO and LUMO positions (see formulas (1−2) in the Supporting Information). We focus first on the case of PTCDA on Au(111), because Au(111) is a metallic substrate for which the image plane position z0 can be unambiguously predicted using state-of-theart theories that include exchange-correlation effects.40,46 The HOMO−LUMO gap for PTCDA on Au(111) is thus computed to be ∼2.8−2.9 eV, much closer to the experimental value of 3.1 eV (Table 1) compared to the DFT gap. We note that while the above two-step approach has been used in the literature for similar molecule/substrate systems,40 the approach in fact considers the limit of an isolated molecule on a substrate and also ignores the polarizability of the molecule (molecular layer). Since PTCDA is a large molecule and forms an ordered layer on Au(111), PTCDA/Au(111) provides a benchmark system to understand how the polarizability of the molecular layer can change the predicted gaps. We found the G0W0 gap of the PTCDA monolayer to be 4.1 eV, 0.6 eV smaller than that computed for the isolated PTCDA molecule. Bringing substrates into the picture complicates the screening problem. We explicitly include the substrate image charge screening effects via an embedding procedure47 in the G0W0 calculation for the PTCDA layer, where the metallic substrate is modeled as a semi-infinite electron gas and its screening properties are included via a renormalization of the bare Coulomb potential (see Materials and Methods). Applied to PTCDA on Au(111), we obtain a HOMO−LUMO gap of 2.9 eV within the G0W0 approximation. Taking into account the intrinsic underestimation of the PTCDA HOMO−LUMO gap within G0W0 (we find that using G1W1 on the isolated PTCDA monolayer increases the gap by 0.3 eV compared to the G0W0 value), this gap value is in very good agreement with experiment (3.1 eV). Setting the polarizability of the molecular layer to be zero in the embedding calculation decreases the predicted gap to 2.4 eV. Our results are consistent with other reports that the polarizability of the adsorbate increases the HOMO−LUMO gap from that predicted by the image charge model.47,48 Applying the two-step approach (Figure 5) to graphite, using the image plane position of z0 = 0.7 Å obtained using the method in ref 40, we obtain a PTCDA HOMO−LUMO gap of ∼3.0 eV. This value is 0.1−0.2 eV larger than our predicted HOMO−LUMO gap for PTCDA on Au(111) using the same approach (Table 1). In contrast, the HOMO−LUMO gap measured for PTCDA on graphite is ∼0.4 eV larger than that measured on Au(111). To understand this difference, we note that graphite does not behave as a perfect metallic substrate (an assumption used in our prediction above). Figure 6 plots the difference in electrostatic potential between a slab with an external electric field applied normal to the surface, and a slab with no external electric field. For a metal such as Au, the

Figure 4. Atomic structures obtained from calculations. (a) PTCDA molecule. (b) Top view of a unit cell of the PTCDA herringbone array on WSe2 commensurate with graphite substrate (lattice parameters a = 17.08 Å and b = 19.72 Å). (c) Side view of the PTCDA layer on SL−WSe2 on graphite.

orientation of WSe2 on graphite has significant impact on the computed binding distances (see Supporting Information). The DFT PBE35 HOMO−LUMO gap for the PTCDA layer on SL-WSe2/graphite is ∼1.45 eV, much smaller than the experimental value. This large discrepancy is consistent with the literature where DFT relying on standard local or semilocal approximations to the exchange-correlation potential underestimate the HOMO−LUMO gap.36 Furthermore, the DFT PBE HOMO−LUMO gap for PTCDA is the same on all substrates considered, as well as in the gas phase. This is in sharp contrast to the substrate dependence of the HOMO− LUMO gap observed in experiments, and indicates that charge transfer effects, which are captured within DFT, cannot account for the observed substrate dependence of the PTCDA HOMO−LUMO gap. In contrast to DFT, the GW approximation,37,38 which takes into account nonlocal exchange and correlation effects missing in DFT (so-called self-energy corrections), can predict gaps quantitatively comparable to experiment.39 However, GW calculations on PTCDA/substrate systems are computationally prohibitive. Here we take into account self-energy corrections by using a two-step process (Figure 5) that has been previously applied to predict the HOMO−LUMO gaps for similar molecules on metallic substrates.17,40−42 First, the self-energy corrections to the HOMO and LUMO levels in the isolated

Figure 5. Two-step approach for predicting the HOMO−LUMO gap of PTCDA. ∑0,L and ∑0,H are electron self-energy corrections to the LUMO and HOMO levels, respectively. PL and PH are the energy renormalizations to LUMO and HOMO, respectively, computed using the image charge model. E


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For the SL-WSe2/graphite system, we use a definition of the image plane that is different from ref 40 which is valid for metal surfaces only. Specifically, we use the classical definition of the image plane position, following ref 42 where the image charge correction was computed for a molecule on a substrate consisting of a NaCl bilayer on top of Ag(100). The two-step image charge approach then predicts a PTCDA HOMO− LUMO gap of 3.3 eV on SL-WSe2/graphite, 0.3 eV larger than that for graphite, and 0.4−0.5 eV larger than that for Au (Table 1). Considering that we are making a further approximation that the SL-WSe2/graphite substrate screens homogeneously, our computed gap is in reasonably good agreement with experiment. In Figure 3b, we further show the predicted PTCDA HOMO and LUMO levels on the different substrates, by applying the corrections, ∑0,H + PH and ∑0,L + PL (see Figure 5), to the DFT PBE HOMO and LUMO positions for PTCDA on the respective substrates. These predictions are in reasonable agreement with the STS spectra in Figure 3a. Can the WSe2 interlayer effectively screen the PTCDA/ graphite heterointerface? Similar to experiment, we see a clear increase in the PTCDA HOMO−LUMO gap when moving from the graphite substrate to the SL-WSe2/graphite substrate, corresponding to reduced electronic screening in the latter, as captured indicatively in the calculations by an image plane position closer to the surface atomic layer (Table 1). Similarly, we found that adding SL-WSe2 on Au(111) moves the image plane closer to the surface atomic layer by 0.5 Å (see Supporting Information, part 5). On the other hand, from Figure 6c, we can see that the applied external electric field is still screened significantly within SL-WSe2. If we ignore the effect of screening in WSe2 and treat it as a vacuum-like spacer (corresponding to PTCDA distance d = 10.18 Å from graphite as shown in Table 1), we predict a HOMO−LUMO gap of 4.3 eV, much larger than the measured gap value. Taken together, our results show that the 2D semiconductor, SL-WSe2, can substantially (albeit not completely) screen the organic− inorganic interface, and this electronic screening in turn has a notable impact on the HOMO−LUMO gap of the molecular adsorbate. Therefore, while excitons confined within the 2D semiconductor experience significantly less electronic screening than excitons in the bulk system,9,10 we show here that the 2D semiconductor, SL-WSe2, can effectively screen the quasiparticles formed in the adsorbed molecule at an organic−inorganic interface. Figure 6. Difference in electrostatic potential (ΔV) between the slabs with and without an external electric field of 0.5 V/Å; (a) Au(111), (b) graphite, (c) WSe2/graphite slabs. Specifically, ΔV = Vex − V0, where Vex is the electrostatic potential, averaged in the x− y plane, for the system in the presence of the external electric field, and V0 is the corresponding electrostatic potential in the absence of any external electric fields. Black dashed lines are Au (a) or C (b and c) atomic layers; blue dashed lines are Se and W atomic layers. The black dotted ellipse in (b) indicates the graphite surface region, where the external field is only partially screened. Similar plots are obtained for electric fields of different magnitudes.

CONCLUSIONS In summary, we have investigated the atomic and electronic structure of an organic/2D TMD system, PTCDA on SLWSe2/graphite. Although the atomic structure of PTCDA on SL-WSe2/graphite is very similar to that on Au(111) and graphite, the measured PTCDA HOMO−LUMO gap on SLWSe2/graphite is 0.6 eV larger than that on Au(111) and 0.2 eV larger than that on graphite. Our first-principles calculations show that these differences in HOMO−LUMO gaps can only be understood when substrate electronic screening effects are taken into account. Using state-of-the-art methods that include substrate static polarization (image charge) effects, we obtain a PTCDA gap on Au(111) in good agreement with experiment. The differences in measured gaps among the different substrates can be attributed to different electronic screening effects in the substrates. We further show that the 2D semiconductor, SL-WSe2, substantially (but not completely) screens the organic−inorganic interface, reducing the PTCDA

external electric field is completely screened within the slab, corresponding to the horizontal line within the slab in Figure 6a for Au(111). In contrast, the electric field in the graphite slab is not fully screened at the graphite surface (Figure 6b). This reduced screening at the graphite surface thus appears to be important in determining the PTCDA HOMO−LUMO gap on graphite. F


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the isolated molecule was used to eliminate the spurious Coulomb interaction between its periodic images. The ΔSCF method to compute the HOMO and LUMO levels for isolated PTCDA was performed using total energy calculations in the Gaussian code with the B3LYP56 exchange-correlation functional. To compute the image plane position, we used the method described in in ref 40 for metallic surfaces, applied to Au(111) and graphite. We note that since WSe2/graphite clearly cannot be approximated by a metallic surface, we obtain the image plane position z0 of 0.1 Å by applying external electric fields42 (using a standard sawtooth potential), corresponding to a classical interpretation of the image plane position. The GW calculation within the embedding approach was performed using a method similar to the one described in ref 47, with generalization to 2D periodic systems by utilizing a cell slab Coulomb truncation. The Generalized Plasmon Pole model37 was used to take into account finite-frequency effects of the dielectric screening properties of the combined PTCDA−metal system. The optB86bvdW geometry was used in this calculation.

HOMO−LUMO gap by 1.0 eV compared to the case where the WSe2 interlayer is replaced by vacuum. 2D TMD layers are thus not only suited as a template for forming well-ordered organic layers, but also can participate actively in hybrid organic−inorganic devices with tailored structures and properties.

MATERIALS AND METHODS Sample Preparation. WSe2 layers were grown by chemical vapor deposition (CVD) on sapphire and then transferred onto graphite. The as-grown WSe2 film is continuous and homogeneous over 5 × 5 mm2. More details about the sample preparation (growth, characterization and transfer) can be found in Supporting Information and previous reports.49 The WSe2/graphite sample was degassed overnight at 350 °C before STM/STS measurements. Au(111) surfaces were prepared by repeated cycles of Ar+ sputtering at 1000−1500 kV followed by annealing at 430 °C. Clean graphite was freshly cleaved by scotch tape and degassed in the UHV chamber overnight at 430 °C. Before PTCDA deposition, the cleanliness of Au(111) and graphite were verified by STM imaging. PTCDA (Sigma-Aldrich, 99.9%) molecules were thermally deposited in stitu onto WSe2/graphite, bare graphite, and Au(111) substrates from a Knudsen Cell (MBEKomponenten, Germany) in ultrahigh vacuum (10−9 mbar). During deposition, the substrates were kept at room temperature and the deposition rate was ∼0.25 ML/min. STM/STS Measurements. STM/STS measurements were performed in a custom-built multichamber system housing an Omicron LT-STM operating at ∼77 K under ultrahigh vacuum conditions (10−10 mbar). All STM images were recorded in constant current mode with tunneling current in the range 50−100 pA. Differential conductance dI/dV or STS were acquired by a lock-in amplifier with a sinusoidal modulation of 40 mV at 625 Hz. Note that the bias voltage (V) is applied on the STM tip; hence, negative values correspond to conduction bands and positive values correspond to valence bands. Each curve was obtained by averaging hundreds of individual spectra acquired at random locations. All measurements were obtained with electrochemically etched tungsten tips. PL Measurements. Photoluminescence (PL) spectra were collected using a micro-Raman system in a back scattering geometry. The intensities of the 532 nm excitation laser were kept below 1 mW to avoid sample damage, as confirmed with repeatable PL results. A liquid nitrogen-flow cryostat with an optical window was used for the temperature dependent PL measurements, and samples were in a high vacuum (∼10−5 Torr) during PL measurements. Theoretical Details. All geometry optimization calculations were performed with the VASP package50,51 employing the projector augmented wave (PAW) potential and within the GGA-PBE35 approximation. For PTCDA on substrates, 3 × 3 × 1 Γ-centered Monkhorst−Pack k-points grids were used to sample the Brillouin Zone. Plane-wave cutoff energies were set at 400 eV and at least 15 Å of vacuum was used to avoid spurious interactions between neighboring slabs. A force convergence criterion of 0.05 eV/Å was used. Van der Waals (vdW) interactions were included using the PBED2 functional34 and optB86-vdW density functional.32,33 We obtained the binding distances d by finding the minimum adsorption energies from distance-dependent adsorption energy curves. With the equilibrium adsorption distances, the PTCDA layer and top two layers of substrates (top three layers for WSe2/graphite) were allowed to relax (we found no change in the equilibrium adsorption distance). The G0W0 calculations for the PTCDA molecule and layer were performed using the BerkeleyGW code.52 The starting DFT-LDA eigenvectors and eigenvalues were obtained by the Quantum ESPRESSO53 code. Here, LDA and Perdew−Zunger norm-conserving pseudo potentials54 were employed. A wave function kinetic energy cutoff of 60 Ry and a large supercell of 39.8 × 27.0 × 15.1 bohr3 were used. A total of 3430 unoccupied bands were included and the GW dielectric matrix cutoff was set at 15 Ry. To accelerate the convergence of the self-energy with respect to the number of unoccupied bands, the static remainder correction was employed.55 Cell-box truncation for

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b07314. Detailed information for sample preparation; STM images of moiré patterns of SL-WSe2 on graphite and proposed structural model corresponding to Figure 1b; STM image of a PTCDA/WSe2/graphite step. STM images of PTCDA on graphite and Au(111); structures and results of ab initio calculations; detailed information about the image charge model; additional states in the STS of PTCDA/Au(111) within the HOMO−LUMO gap (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ¶

Yu Jie Zheng, Yu Li Huang, and Yifeng Chen contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank Zhuo Wang and Qixing Wang for helping us with transferring the CVD-WSe2 samples and checking the sample quality, Prof. Satoshi Kera and Kyushu Synchrotron Light Research Center (Japan) for ARPES mapping of the clean Au(111) surface, as well as Xin Luo, Kapildeb Dolui, Suchun Li and Zijing Ding for discussions. A.T.S.W. acknowledges support from MOE Grant R-144-000-321-112. S.Y.Q. and Y.C. acknowledge support from Grant NRF-NRFF2013-07 from the National Research Foundation, Singapore. G.E. acknowledges support from Grant NRF-NRFF2011-02 from the National Research Foundation, Singapore. Computations were performed on the NUS Graphene Research Centre cluster. We acknowledge support from the Singapore National Research Foundation, Prime Minister’s Office, under its medium-sized centre program. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Co., for the U.S. DOE under contract DEAC04-94AL85000. G


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Tartakovskii, A. Light-Emitting Diodes by Band-Structure Engineering in Van der Waals Heterostructures. Nat. Mater. 2015, 14, 301. (20) Chiu, M.-H.; Zhang, C.; Shiu, H.-W.; Chuu, C.-P.; Chen, C.-H.; Chang, C.-Y. S.; Chen, C.-H.; Chou, M.-Y.; Shih, C.-K.; Li, L.-J. Determination of Band Alignment in the Single-Layer MoS2/WSe2 Heterojunction. Nat. Commun. 2015, 6, 7666. (21) Liu, H.; Jiao, L.; Xie, L.; Yang, F.; Chen, J.; Ho, W.; Gao, C.; Jia, J.; Cui, X.; Xie, M. Molecular-Beam Epitaxy of Monolayer and Bilayer WSe2: A Scanning Tunneling Microscopy/Spectroscopy Study and Deduction of Exciton Binding Energy. 2D Mater. 2015, 2, 034004. (22) Wang, G.; Marie, X.; Bouet, L.; Vidal, M.; Balocchi, A.; Amand, T.; Lagarde, D.; Urbaszek, B. Exciton Dynamics in WSe2 Bilayers. Appl. Phys. Lett. 2014, 105, 182105. (23) Henze, S. K. M.; Bauer, O.; Lee, T. L.; Sokolowski, M.; Tautz, F. S. Vertical Bonding Distances of PTCDA on Au(111) and Ag(111): Relation to the Bonding Type. Surf. Sci. 2007, 601, 1566−1573. (24) Nicoara, N.; Román, E.; Gómez-Rodríguez, J. M.; Martín-Gago, J. A.; Méndez, J. Scanning Tunneling and Photoemission Spectroscopies at the PTCDA/Au(111) Interface. Org. Electron. 2006, 7, 287− 294. (25) Burrows, P. E.; Zhang, Y.; Haskal, E. I.; Forrest, S. R. Observation and Modeling of Quasi-Epitaxial Growth of a Crystalline Organic Thin-Film. Appl. Phys. Lett. 1992, 61, 2417−2419. (26) Wang, Q. H.; Hersam, M. C. Room-Temperature MolecularResolution Characterization of Self-Assembled Organic Monolayers on Epitaxial Graphene. Nat. Chem. 2009, 1, 206−211. (27) Martínez-Galera, A. J.; Nicoara, N.; Martínez, J. I.; Dappe, Y. J.; Ortega, J.; Gómez-Rodríguez, J. M. Imaging Molecular Orbitals of PTCDA on Graphene on Pt(111): Electronic Structure by STM and First-Principles Calculations. J. Phys. Chem. C 2014, 118, 12782− 12788. (28) Huang, H.; Chen, S.; Gao, X.; Chen, W.; Wee, A. T. S. Structural and Electronic Properties of PTCDA Thin Films on Epitaxial Graphene. ACS Nano 2009, 3, 3431−3436. (29) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. STM Investigations of PTCDA and PTCDI on Graphite and MoS2 - A Systematic Study of Epitaxy and STM ImageContrast. Z. Phys. B: Condens. Matter 1994, 93, 365−373. (30) Dori, N.; Menon, M.; Kilian, L.; Sokolowski, M.; Kronik, L.; Umbach, E. Valence Electronic Structure of Gas-Phase 3,4,9,10Perylene Tetracarboxylic Acid Dianhydride: Experiment and Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 195208. (31) Martínez, J.; Abad, E.; Flores, F.; Ortega, J.; Brocks, G. Barrier Height Formation for the PTCDA/Au (111) Interface. Chem. Phys. 2011, 390, 14−19. (32) Klimeš, J.; Bowler, D. R.; Michaelides, A. Van der Waals Density Functionals Applied to Solids. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 195131. (33) Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B. I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. (34) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−99. (35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (36) Sharifzadeh, S.; Biller, A.; Kronik, L.; Neaton, J. B. Quasiparticle and Optical Spectroscopy of the Organic Semiconductors Pentacene and PTCDA from First Principles. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 125307. (37) Hybertsen, M. S.; Louie, S. G. Electron Correlation in Semiconductors and Insulators: Band Gaps and Quasiparticle Energies. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34, 5390. (38) Hedin, L.; Lundqvist, S. Effects of Electron-Electron and Electron-Phonon Interactions on the One-Electron States of Solids. Solid State Phys. 1970, 23, 1−181. (39) Sharifzadeh, S.; Tamblyn, I.; Doak, P.; Darancet, P. T.; Neaton, J. B. Quantitative Molecular Orbital Energies Within a G0W0 Approximation. Eur. Phys. J. B 2012, 85, 1−5.

REFERENCES (1) Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271−1275. (2) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (3) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (4) Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74−80. (5) Bernardi, M.; Palummo, M.; Grossman, J. C. Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials. Nano Lett. 2013, 13, 3664− 3670. (6) Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V.; Grigorenko, A. N.; Geim, A. K.; Casiraghi, C.; Castro Neto, A. H.; Novoselov, K. S. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311−1314. (7) Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802. (8) Jones, A. M.; Yu, H.; Ghimire, N. J.; Wu, S.; Aivazian, G.; Ross, J. S.; Zhao, B.; Yan, J.; Mandrus, D. G.; Xiao, D. Optical Generation of Excitonic Valley Coherence in Monolayer WSe2. Nat. Nanotechnol. 2013, 8, 634−638. (9) Komsa, H.-P.; Krasheninnikov, A. V. Effects of Confinement and Environment on the Electronic Structure and Exciton Binding Energy of MoS2 from First Principles. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 241201. (10) Ramasubramaniam, A. Large Excitonic Effects in Monolayers of Molybdenum and Tungsten Dichalcogenides. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 115409. (11) Ugeda, M. M.; Bradley, A. J.; Shi, S.-F.; Felipe, H.; Zhang, Y.; Qiu, D. Y.; Ruan, W.; Mo, S.-K.; Hussain, Z.; Shen, Z.-X. Giant Bandgap Renormalization and Excitonic Effects in a Monolayer Transition Metal Dichalcogenide Semiconductor. Nat. Mater. 2014, 13, 1091. (12) He, K.; Kumar, N.; Zhao, L.; Wang, Z.; Mak, K. F.; Zhao, H.; Shan, J. Tightly Bound Excitons in Monolayer WSe2. Phys. Rev. Lett. 2014, 113, 026803. (13) Huang, Y. L.; Chen, Y.; Zhang, W.; Quek, S. Y.; Chen, C.-H.; Li, L.-J.; Hsu, W.-T.; Chang, W.-H.; Zheng, Y. J.; Chen, W.; Wee, A. T. S. Bandgap Tunability at Single-Layer Molybdenum Disulphide Grain Boundaries. Nat. Commun. 2015, 6, 6298. (14) Velez, S.; Ciudad, D.; Island, J.; Buscema, M.; Txoperena, O.; Parui, S.; Steele, G. A.; Casanova, F.; van der Zant, H. S.; CastellanosGomez, A.; Hueso, L. E. Gate-Tunable Diode and Photovoltaic Effect in an Organic-2D Layered Material p-n Junction. Nanoscale 2015, 7, 15442−9. (15) Liu, F.; Chow, W. L.; He, X.; Hu, P.; Zheng, S.; Wang, X.; Zhou, J.; Fu, Q.; Fu, W.; Yu, P.; Zeng, Q.; Fan, H. J.; Tay, B. K.; Kloc, C.; Liu, Z. Van der Waals p-n Junction Based on an Organic-Inorganic Heterostructure. Adv. Funct. Mater. 2015, 25, 5865−5871. (16) Jariwala, D.; Howell, S.; Chen, K.-S.; Kang, J.; Sangwan, V. K.; Filippone, S. A.; Turrisi, R.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Hybrid, Gate-Tunable, Van der Waals p-n Heterojunctions from Pentacene and MoS2. Nano Lett. 2016, 16 (1), 497−503. (17) Neaton, J. B.; Hybertsen, M. S.; Louie, S. G. Renormalization of Molecular Electronic Levels at Metal-Molecule Interfaces. Phys. Rev. Lett. 2006, 97, 216405. (18) Garcia-Lastra, J. M.; Rostgaard, C.; Rubio, A.; Thygesen, K. S. Polarization-Induced Renormalization of Molecular Levels at Metallic and Semiconducting Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 245427. (19) Withers, F.; Del Pozo-Zamudio, O.; Mishchenko, A.; Rooney, A.; Gholinia, A.; Watanabe, K.; Taniguchi, T.; Haigh, S.; Geim, A.; H


Article

ACS Nano (40) Egger, D. A.; Liu, Z.-F.; Neaton, J. B.; Kronik, L. Reliable Energy Level Alignment at Physisorbed Molecule−Metal Interfaces from Density Functional Theory. Nano Lett. 2015, 15, 2448−2455. (41) Quek, S. Y.; Venkataraman, L.; Choi, H. J.; Louie, S. G.; Hybertsen, M. S.; Neaton, J. Amine-Gold Linked Single-Molecule Circuits: Experiment and Theory. Nano Lett. 2007, 7, 3477−3482. (42) Samsonidze, G.; Cohen, M. L.; Louie, S. G. First-principles Study of Quasiparticle Energies of a Bipolar Molecule in a Scanning Tunneling Microscope Measurement. Comput. Mater. Sci. 2014, 91, 187−191. (43) Sau, J. D.; Neaton, J. B.; Choi, H. J.; Louie, S. G.; Cohen, M. L. Electronic Energy Levels of Weakly Coupled Nanostructures:C60Metal Interfaces. Phys. Rev. Lett. 2008, 101, 026804. (44) Cappellini, G.; Casula, F.; Yang, J.; Bechstedt, F. Quasiparticle Energies in Clusters Determined via Total-Energy Differences: Application to C60 and Na4. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 3628. (45) Jones, R. O.; Gunnarsson, O. The Density Functional Formalism, Its Applications and Prospects. Rev. Mod. Phys. 1989, 61, 689−746. (46) White, I.; Godby, R.; Rieger, M.; Needs, R. Dynamic Image Potential at an Al(111) Surface. Phys. Rev. Lett. 1998, 80, 4265. (47) Spataru, C. D. Electronic and Optical Gap Renormalization in Carbon Nanotubes Near a Metallic Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 125412. (48) Puschnig, P.; Amiri, P.; Draxl, C. Band Renormalization of a Polymer Physisorbed on Graphene Investigated by Many-Body Perturbation Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 085107. (49) Huang, J. K.; Pu, J.; Hsu, C. L.; Chiu, M. H.; Juang, Z. Y.; Chang, Y. H.; Chang, W. H.; Iwasa, Y.; Takenobu, T.; Li, L. J. LargeArea Synthesis of Highly Crystalline WSe2 Monolayers and Device Applications. ACS Nano 2014, 8, 923−930. (50) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169. (51) Kresse, G.; Furthmüller, J. Efficiency of ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (52) Deslippe, J.; Samsonidze, G.; Strubbe, D. A.; Jain, M.; Cohen, M. L.; Louie, S. G. BerkeleyGW: A Massively Parallel Computer Package for the Calculation of the Quasiparticle and Optical Properties of Materials and Nanostructures. Comput. Phys. Commun. 2012, 183, 1269−1289. (53) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; et al. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (54) Perdew, J. P.; Zunger, A. Self-Interaction Correction to DensityFunctional Approximations for Many-Electron Systems. Phys. Rev. B: Condens. Matter Mater. Phys. 1981, 23, 5048. (55) Deslippe, J.; Samsonidze, G.; Jain, M.; Cohen, M. L.; Louie, S. G. Coulomb-Hole Summations and Energies for GW Calculations with Limited Number of Empty Orbitals: A Modified Static Remainder Approach. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 165124. (56) Stephens, P.; Devlin, F.; Chabalowski, C.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627.

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Heterointerface Screening Effects between Organic Monolayers and

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