Electronic Decoupling of Organic Layers by a Self-Assembled

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Cite This: J. Phys. Chem. Lett. 2019, 10, 4297−4302

Electronic Decoupling of Organic Layers by a Self-Assembled Supramolecular Network on Au(111) Zhonghua Liu,† Kewei Sun,† Xuechao Li, Ling Li, Haiming Zhang,* and Lifeng Chi* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu, PR China

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S Supporting Information *

ABSTRACT: A cyanuric acid and melamine (CA·M) supramolecular network, prepared via the drop-casting method under ambient conditions, can be utilized as a spacer layer to decouple electronic interactions between upper organics and the metal substrate. Typical semiconducting organics 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and C60 are deposited on the CA·M network under ultrahigh vacuum conditions, forming an organics/CA·M/metal heterosystem. Both geometric and electronic structures of the upper organics are characterized by using scanning tunneling microscopy/spectroscopy (STM/STS). On the CA·M network, PTCDA molecules form a well-ordered herringbone structure in submonolayer patterns, whereas C60 molecules aggregate into multilayered islands. STS spectra reveal that the energy gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO − LUMO) is 3.6 eV for PTCDA and 3.8 eV for the first layer of C60 on CA·M. The remarkable bandgap broadening compared with the metal−organic contact indicates successful electronic decoupling of the upper molecules from the metal surface due to the CA·M network.

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layers of trimesic acid and metal−organic complexes, also exhibit relatively high thermal stability and have been well characterized by scanning tunneling microscopy/spectroscopy (STM/STS).20−22 However, for potential candidates of electronic decoupling, most of them suffer either from the difficulty in preparation or the lower HOMO−LUMO gap which may interfere with the upper adsorbates.23,24 The CA·M network contains 1:1 cyanuric acid and melamine molecules held together by triple hydrogen bonds (see Scheme 1a). A well-ordered CA·M network over a large scale cannot be obtained under ultrahigh vacuum (UHV) by the organic molecular beam epitaxy (OMBE) method because of the strong intermolecular interactions (the triple hydrogen bonds),25 but it has been successfully achieved under ambient conditions by the drop-casting method on Au(111),26 HOPG,27 and some two-dimensional van der Waals materials.28 In this work, a large-scale CA·M supramolecular network is first prepared via drop-casting on Au(111) as reported previously.26 Reappearing the CA·M network in UHV thereafter, the upper organic layers of 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) or C60 are fabricated by the OMBE method, as displayed in Scheme 1b. Both geometric and electronic structures of the upper organic layers are characterized by STM/STS, demonstrating that the CA·M network can serve as a good spacer layer for electronic decoupling of the upper organics from metal substrate.

etal−organic contact plays an essential role in organic devices, including organic light emitting diodes (OLEDs), field effect transistors (FETs), photovoltaic cells (PVCs), and sensors.1−3 When molecules physically adsorb on metal surfaces, electronic coupling between adsorbates and metals can significantly reduce the HOMO−LUMO (HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital) energy gap.4−6 To recover and utilize the intrinsic properties of organic molecules, many strategies have been proposed for electronic decoupling, e.g., electronic selfdecoupling via molecular design7 and intercalating ultrathin films of graphene,8−10 h-BN,11 NaCl,12 and WSe2.13 However, these methods are restricted in practical applications, due to the complexity in molecular design for self-decoupling7 or in the preparation of 2D ultrathin films.14 Developing general and convenient methods for electronic decoupling of organic layers from metal surfaces still holds one of the most challenging topics in organic electronics. Molecular self-assembly developed in supramolecular chemistry provides a promising way to prepare organic monolayers on various substrates.15,16 The self-assembled monolayers (SAMs) can serve as a spacer layer for electronic decoupling of the second layer from metal surfaces. Relatively high thermal and structural stability of the spacer layer is required to avoid competitive adsorption of the second layer molecules.17 Although SAMs of chemisorbed thiols are the most widely used monolayers in applications, the main drawbacks in gold substrate dependence and randomly distributed etch pits hold it back as an ideal layer for electronic decoupling.18,19 Other monolayers stabilized by lateral noncovalent interactions, for example, the self-assembled mono© XXXX American Chemical Society

Received: April 23, 2019 Accepted: June 19, 2019 Published: June 19, 2019 4297

DOI: 10.1021/acs.jpclett.9b01167 J. Phys. Chem. Lett. 2019, 10, 4297−4302

Letter

The Journal of Physical Chemistry Letters

Maintaining the structure of organic monolayers after degassing under UHV is one of the challenges for transferring from the atmosphere into the UHV system. Contaminants in the atmosphere (mainly H2O), if any, can be fixed on the substrate and visualized by STM at low temperature. The transferred sample is annealed at 120 °C for 1 h to eliminate adsorbed gases and small molecules like water on the surface as well as the sample holder. This is a safe temperature examined by a series of thermal annealing at various temperatures, where structural deformation appears at 130 °C (as depicted in the Supporting Information, Figure S1). Figure 1b presents an STM image of the CA·M SAMs under UHV at 77 K. The honeycomb structure together with the herringbone-like reconstructed ridges of Au(111) is similar to that observed under ambient conditions (Figure 1a). A closer view of the CA·M SAMs depicts more structural details (see the inset of Figure 1b). The unit composition, lattice parameters (a = b = 0.98 ± 0.01 nm, α = 120°), and long axis direction of the unit cell are in line with those observed in the atmosphere. Successful transfer of the large-scale CA·M network from ambient conditions to vacuum provides a suitable template for building an organics/CA·M/metal heterosystem. Typical organic semiconductors, PTCDA and C60, are selected as representative molecules for their different geometry but similar nucleation mode of epitaxial growth on Au(111).29−32 STM images of PTCDA and C60 on CA·M/ Au(111) demonstrate the structural relationship between the second layers and the CA·M network. Displayed in Figure 2a is an overview STM image (25 × 25 nm2) of self-assembled PTCDA with a submonolayer coverage on the CA·M network. No multilayers of PTCDA are observed at randomly selected spots on the sample, corresponding well to the epitaxial growth model of PTCDA.32 The self-assembled structure of PTCDA with respect to the CA·M network is shown in the highresolution STM image obtained on the boundary of a PTCDA island (see Figure 2b). The unit cells of CA·M and PTCDA are highlighted by a white rhombus and rectangle, respectively. The cell parameters of PTCDA are a = 1.25 ± 0.01 nm, b = 1.95 ± 0.02 nm, and β = 90°. The dashed lines marked in the image represent the lattice of CA·M. Obviously, PTCDA molecules are located randomly with respect to the lattice of the CA·M network. An incommensurate structural model of PTCDA on the CA·M network is therefore proposed in Figure 2c.

Scheme 1. (a) Chemical Structures of Cyanuric Acid, Melamine, and the CA·M Network and (b) Schematic Diagram for the Combined Preparation of the Organics/ CA·M/Metal Heterosystema

a The CA·M network was first prepared by the drop-casting method under ambient conditions (left). After thermal annealing at 120 °C under UHV, the sample of the organics/CA·M/metal heterosystem is built by the organic molecular beam evaporation technique on the CA·M/Au(111) surface under UHV (right).

Shown in Figure 1a is a large-scale STM image of the CA·M sample prepared under ambient conditions. A well-ordered two-dimensional (2D) honeycomb structure appears together with herringbone-like reconstructed ridges of the Au(111). The inset of Figure 1a is a high-resolution image, presenting the morphology of the honeycomb structure in detail. The unit cell (highlighted by white lines) of the honeycomb structure is composed of two triangular shapes with different sizes, and the cell parameters are a = b = 0.99 ± 0.01 nm, α = 120°. Both STM images and the cell parameters are well consistent with previously reported results,26 confirming that high quality CA· M SAMs are prepared in this work.

Figure 1. (a) Typical STM image (34 × 34 nm2) of a large-scale CA·M network prepared under ambient conditions on Au(111). (b) STM image (30 × 30 nm2) of the CA·M network under UHV at 77 K. 4298

DOI: 10.1021/acs.jpclett.9b01167 J. Phys. Chem. Lett. 2019, 10, 4297−4302

Letter

The Journal of Physical Chemistry Letters

Figure 2. Hetero-organic layers on Au(111). (a) A hetero-organic bilayer formed by depositing PTCDA on CA·M (25 × 25 nm2). (b) Highresolution STM image obtained on the boundary of a PTCDA island, showing structural correlations with respect to CA·M (10 × 10 nm2). The white dashed lines indicate the lattice of the CA·M network. (c) Proposed structural model of PTCDA on the CA·M network. (d) STM image of C60 multilayers on CA·M (39 × 39 nm2). (e) A closer view of the C60 multilayers on CA·M (10 × 10 nm2). The white dashed lines indicate the lattice of the CA·M network. (f) A structural model of C60 molecules on CA·M. Each C60 molecule adsorbs on the center of a CA·M lattice, the position on top of the amine group of a melamine molecule.

Figure 3. Differential conductance (dI/dV) spectra of PTCDA and C60 on CA·M/Au(111). (a) dI/dV spectrum of PTCDA (blue) adsorbed on CA·M. Two prominent peaks are observed at bias voltages of −1.8 and 1.8 V. STS curves obtained on top of CA·M (black) and clean Au(111) (red) are provided as reference curves for clarity. The PTCDA and CA·M curves are offset vertically for a better view. (b) dI/dV spectrum of C60first on CA·M/Au(111). Two resonances centered at −2 and 1.8 V represent the tunneling into the HOMO and LUMO of C60.

It is interesting to explore how the existence of the CA·M network affects the film growth of functional molecules atop. There are two typical phases of PTCDA reported on Au(111): loosely packed square phase and closely packed herringbone phase.31 This diversity is reduced to herringbone phase only when decreasing adsorbate−substrate interactions, such as in the case of depositing NaCl thin films on Au(111).33 The constraint is also observed in our PTCDA/CA·M/Au(111) system, implying weaker adsorbate−substrate interactions between PTCDA and Au(111). Similar conclusions can also be drawn from the self-assembled pattern of C60 on CA·M/ Au(111). Due to the strong interaction between C60 and Au(111), epitaxial growth of C60 prevails and no multilayer islands are observed when the coverage of C60 is less than one monolayer.30 However, multilayer islands of C60 (mainly

Displayed in Figure 2d is the STM image of C60 multilayers on the CA·M network. Fullerene molecules accumulate on the CA·M and form islands with various heights, indicating a different growth mode with respect to its epitaxial growth on Au(111).30 More structural details are distinguished in the hexagonal close-packed structure shown in Figure 2e. The lattice parameters (a = b = 1.03 ± 0.02 nm, β = 120°) of the first layer fullerene (hereafter, C60first, the layer adsorbed on the CA·M) are similar to those of the second layer fullerene (hereafter, C60second, the layer adsorbed on the C60first). The white dashed lines highlight the lattice of the CA·M network. From the extension part of the dashed lines, each fullerene molecule in C60first locates precisely in the center of the CA·M lattice. A structural model of adsorbed C60 on the CA·M network is depicted in Figure 2f. 4299

DOI: 10.1021/acs.jpclett.9b01167 J. Phys. Chem. Lett. 2019, 10, 4297−4302

Letter

The Journal of Physical Chemistry Letters Table 1. HOMO−LUMO Gaps Determined by STS for PTCDA and C60 on CA·M or Au(111) PTCDA adsorption system HOMO−LUMO gap gas-phase

Au(111) PTCDA/Au(111) 2.9 eV34 3.3 eV34 HOMO−LUMO gap: 5.0 eV38

C60 CA·M/Au(111) 3.6 eV (this work)

Au(111) C60/Au(111) 2.2 eV35 3.5 eV35 HOMO−LUMO gap: 4.91 eV38

CA·M/Au(111) 3.8 eV (this work)

from the CA·M network should result from weaker molecule− molecule interactions between the adsorbates and the CA·M network, as π−π interactions provide stronger molecule− molecule interactions between layers for both PTCDA and C60. This hypothesis can be corroborated from the adsorption geometry of adsorbates on the CA·M network. In fact, PTCDA molecules form incommensurate structures on the CA·M network and C60 molecules adopt an island growth mode, evidencing the weaker π−π interactions between adsorbates and the CA·M network. To summarize, a general and convenient strategy to fabricate hetero-organic bilayer structures was developed by combining the drop-casting method under ambient conditions and molecular vapor deposition under UHV conditions. The growth and electronic properties of PTCDA and C60 molecules adsorbed on CA·M/Au(111) are characterized by STM/STS. The experimental results indicate that the existence of an organic spacer layer CA·M remarkably weakens the interaction and electronic coupling between the upper adsorbates and metal substrate. Regarding the recent development in building supramolecular heterostructures on CA·M28 and the advances in scanning tunneling microscopy induced luminescence (STML),39 this finding would enable a thorough understanding of the intrinsic electronic structures of organic molecules and shed new light on the design of organic electronics that requires electronic decoupling.

double layers) are the dominant pattern on CA·M/Au(111), suggesting that molecular interactions between C60 molecules are stronger than those between C60 and CA·M. While the molecule−molecule interaction of C60 is less than the molecule−substrate interaction on Au(111), we can conclude that C60−CA·M interaction should be weaker than C60− Au(111) interaction. The CA·M decoupling layer fabricated on Au(111) allows for the STS characterization of intrinsic properties of the top layer adsorbates. Both PTCDA and C60first on CA·M/Au(111) have been investigated on their electronic structures with STS, as displayed in Figure 3. Differential conductance (dI/dV) spectra on bare Au(111) and the CA·M network are first collected as reference spectra (see Figure 3a). Apart from the decayed surface state of Au(111), the spectrum of the CA·M network appears featureless within bias voltage ranging from −2 to 2 V. In contrast, the dI/dV curve obtained on top of a PTCDA molecule displays two prominent peaks centered at −1.8 and 1.8 V, arising from tunneling into the HOMO and LUMO of PTCDA, respectively. The surface state of Au(111) (a broad bump starting at −0.5 V) is still visible in the dI/dV curve of the CA·M network (black curve with triangle), but it has been completely suppressed in the spectrum of PTCDA (blue curve). Similar suppression of the Au(111) surface state appears in the spectrum of C60first on CA·M/Au(111) (see Figure 3b). No obvious features are detected in the gap between the resonances at −2 and 1.8 V. Considering different heights of PTCDA and C60 in the vertical direction of the surface, the similar suppression of the surface state in the spectra of PTCDA and C60first suggests that the existence of the CA·M network can remarkably impede electronic interactions between the upper organic layers and the Au(111). The HOMO−LUMO gap of PTCDA (3.6 eV) and C60 (3.8 eV) on the CA·M network is consistently larger than that of molecules adsorbed directly on Au(111) substrate34,35 and molecules on intercalated graphene, such as graphene/Pt(111)36 and graphene/Cu(111).37 The large HOMO−LUMO gap also is in fact comparable with that of molecules on semiconductor WSe2,13 indicating the well electronic decoupling by the CA·M network. Since the electronic decoupling from the adsorbate itself has been revealed in multilayer PTCDA and C60 on Au(111),34,35 it is worth comparing the decoupling effect from either molecule itself or the spacer layer CA·M network. When PTCDA and C60 adsorb on Au(111), the HOMO−LUMO gap of the second layer adsorbates is 3.3 eV for PTCDA34 and 3.5 eV for C60,35 consistently lower than that of adsorbates on the CA·M network, i.e., 3.6 eV for PTCDA and 3.8 eV for C60 (see Table 1). Such an increment in HOMO−LUMO gap on the CA·M network suggests that the electronic decoupling from the CA·M network is better than that from the adsorbates themselves, which can also be confirmed by the fact that the HOMO−LUMO gaps of organics in this work are closer to the gas-phase HOMO−LUMO gaps obtained by first-principles GW calculations.38 In consideration of the conjugated structure of PTCDA and C60, the better electronic decoupling

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EXPERIMENTAL METHODS Cyanuric acid (CA) and melamine (M) (≥98% in purity; Fluka) were used directly without further purification. The CA· M networks were prepared by casting a droplet of CA and M aqueous mixture (CA, 1 mM, and M, 1 mM, ca. 350 K) on a freshly prepared reconstructed Au(111) surface (ca. 390 K). The preparation method was previously used for preparing large-scale CA·M SAMs on Au(111)26 and highly oriented pyrolytic graphite.27 A mechanically sharpened Pt/Ir tip (80/ 20) was used for scanning tunneling microscopy (STM) characterization (BRUKER, NanoScope V) under ambient conditions. 3,4,9,10-Perylene-tetracarboxylic-dianhydride (PTCDA, ca. 560 K) and C60 (ca. 600 K) molecules were deposited on the CA·M/Au(111) surface by sublimation in an ultrahigh vacuum (UHV). A low temperature UHV-STM system (Omicron, base vacuum