⇦Chinese Physics Letters, 2020, Vol. 37, No. 2, Article code 027101 Adsorption of Perylene on Si(111)($7 \times 7$) * Dandan Guan (管丹丹)1,2**, Xinwei Wang (王欣伟)3, Hongying Mao (毛宏颖)4, Shining Bao (鲍世宁)5, Jin-Feng Jia (贾金锋)1,2 Affiliations 1Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), Shenyang National Laboratory for Materials Science, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240 2Tsung-Dao Lee Institute, Shanghai Jiao Tong University, Shanghai 200240 3School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 4Department of Physics, Hangzhou Normal University, Hangzhou 310036 5Department of Physics, Zhejiang University, Hangzhou 310027 Received 15 November 2019, online 18 January 2020 *Supported by the National Natural Science Foundation of China under Grant Nos. U1632102, 11521404, 11634009, 11674222, 11674226, 11790313, 11574202, 11874256, 11861161003 and 11874258, the National Key Research and Development Program of China (Grant Nos. 2016YFA0300403 and 2016YFA0301003); in part by the Key Research Program of the Chinese Academy of Science (Grant No. XDPB082), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB28000000).
**Corresponding author. Email: ddguan@sjtu.edu.cn Citation Text: Guan D D, Wang X W, Mao H Y, Bao S N and Jia J F et al 2020 Chin. Phys. Lett. 37 027101    Abstract We investigate the adsorption of organic molecular semiconductor perylene on ($7 \times 7$) reconstructed Si(111) surface by ultraviolet photoemission spectroscopy. It is observed that seven features that derive from the organic material are located at 0.71, 2.24, 4.0, 5.9, 7.46, 8.65 and 9.95 eV in binding energy. The theoretical calculation results reveal the most stable adsorption geometry of organic molecule perylene on Si(111) ($7 \times 7$) substrates is at the beginning of deposition. DOI:10.1088/0256-307X/37/2/027101 PACS:71.15.Mb, 68.43.-h, 73.20.At © 2020 Chinese Physics Society Article Text In the last five decades, electronic devices constructed with organic materials have received considerable interest.[1] The properties of organic materials offer enormous advantages compared to the traditional inorganic semiconductors (Si- and Ge-based devices) and conductors, mainly in the area of low cost processing, lightweight, and mechanical flexibility for applications of flexible displays, organic field-effect transistors, organic light-emitting diodes, and photodetectors.[2–8] There are many theoretical and experimental studies on organic materials adsorbed on solid surfaces to have deeper understanding of electronic, optical, chemical and transport properties of both simple $\pi$ conjugated organic systems and complicated polymers adsorbed on suitable substrates.[9–15] Since silicon remains the most popular materials in microelectronic industry, it is important to study in detail the interaction between organic molecules and silicon substrates, which can improve its advanced applications. The underlying mechanisms of organic/inorganic interfaces are still not fully understood. However, there is a common realization that performance of organic electronic devices is strongly influenced by the geometries and electronic structures of organic compounds, substrates and complex interactions between adsorbates and substrates. The most frequently studied organic materials on molecule-silicon interfaces are with linear aromatic structures, such as pentacene[16–18] and tetracene.[19–23] In our previous work, we studied the interfacial electronic properties of tetracene on Si(111) $7 \times 7$ substrates using ultraviolet photoemission spectroscopy (UPS) and density functional theory (DFT) calculations.[21] However, the structure of Si(111) $7 \times 7$ surface is very complex, and there are 19 dangling bonds in a unit cell. This indicates that there will be various adsorption configurations for different types of organic molecules. Perylene has a planar molecular structure consisting of two naphthalene residues joined to each other through the peri-positions. It is one of the simplest and most promising organic semiconductors for the potential application in optical and electronic devices, such as organic light emitting diodes or solar cells. Furthermore, the derivatives of perylyne possess high field-effect mobility and a promising performance in air,[24] which are very useful in realizing the electronic devices. To address the adsorption structure and the interaction between organic materials and silicon surfaces, we present a fundamental study on the electronic states and geometric structure of perylene adsorbed on the Si(111) $7 \times 7$ surface combined by UPS experiments and DFT calculations in this work. The Si (111) substrate degassed at 600 K for more than 4 h in ultrahigh vacuum systems with base pressure better than $8\times 10^{-10}$ mbar, and then flashed up to 1100 K and annealed at 900 K for several cycles to obtain clean ($7 \times 7$) reconstructed surface.[25] The surface cleanliness was checked by UPS and low energy electron diffraction (LEED) (ADES-400, VG).[26] Perylene powder (Sigma, 99%) was then deposited on the substrate using a quartz tube with resistively heating, and the thickness of the organic film was monitored by a quartz crystal oscillator. The UPS measurements were completed using He-I$_\alpha$ light (21.2 eV) in the lab at room temperature, and a $-5$ V bias was applied on the samples. All of the measurements were carried out in situ. First-principle calculations were performed using the density functional theory (DFT) from the Dmol$^{3}$, a DFT package of the Materials Studio of Accelrys Inc.[27] Generalized gradient approximation (GGA) and Perdew–Wang (PW91) functional were used throughout. In the calculation, force convergence criteria of 0.1 eV/Å were used for optimization process, and the $k$-points were obtained from the Monkhorst–Pack scheme. The Si (111) substrate was built by periodic slabs of five-layer silicon atoms, and separated by 1.5 nm in vacuum. The ($7 \times 7$) surface was modeled with the dimer-adatom-stacking faulted model.[28] The bottom two layers of silicon atoms were fixed at bulk lattice positions, while the top three layers were free to optimize.
cpl-37-2-027101-fig1.png
Fig. 1. (a) UPS spectra with varying surface coverage, (b) cut-off energy, and (c) work function of Si (111) surface with different layers of the perylene film. The spectra in (a) and (b) are shifted vertically for a clearer view.
Figure 1(a) shows the UPS spectra of different perylene layers on Si(111) $7 \times 7$ surface. During the measurements, the incidence light was kept along the surface normal, and emission electrons were accepted at 30$^{\circ}$ of the surface normal. The spectrum at the bottom in Fig. 1(a) indicates a clean Si(111) $7 \times 7$ surface. After growing the perylene molecule, we can see that the intensity of the features originating from the organic materials increases, while the emission features of the Si(111) substrate (dash-dotted lines in Fig. 1(a)) progressively decrease. The emission features of the monolayer (ML) perylene film appear at binding energies 2.64, 4.16, 5.9, 7.98 and 9.95 eV (labeled as a, b, c, d and e in Fig. 1(a)). With further increase of the coverage, peaks a and b shift towards lower binding energy and peak d splits into two peaks located at 7.46 and 8.65 eV, respectively (labeled as g and h). The coverage-dependent shift in binding energy can attribute to the charge redistribution near the interface because the substrate has less influence on the multilayer films than monolayer. These phenomena indicate that there exists a weak interaction between the molecular orbitals and the substrate at the interface. When the coverage reaches 6.0 ML, a new feature emerges at 0.71 eV below the Fermi level (labeled as f), which is in good agreement with the calculated density of states (DOS) (grey solid curve in the top of Fig. 1(a)) both in position and intensity. However, due to the molecule-molecule interaction in the multilayer perylene film, the peaks originating from the perylene molecules could be broadened or shift in binding energy, which results in the low discrepancy between the calculated DOS and UPS spectra. The work function of the surface is determined by the secondary cutoff of the UPS spectra (as shown in Fig. 1(b)). The values of work function are 4.20, 4.12, 4.11, 4.09, 4.08, 4.01, 4.09, 4.10, 4.11 and 4.11 eV for the thicknesses of 0.0, 0.1, 0.2, 0.4, 0.6, 1.0, 2.5, 3.5, 4.5 and 6.0 ML, respectively (as shown in Fig. 1(c)). The work function reaches the minimum at 1 ML, and then increases slightly with further deposition. The decrease in work function can be assigned to the formation of interface dipole caused by charge transfer from the substrate to the organic film at the beginning of the deposition. The change in work function occurs in many interfaces between organic semiconductors and metals/Si-based substrates. The molecular orbitals of gas-phase perylene are obtained by the DFT calculation, and summarized in Table 1, in comparison with the photoemission from ML and multilayer on Si(111) surface. Since the peaks of calculation are relative to vacuum energy level, the experimental data are corrected by the corresponding work function of 4.01 and 4.11 eV for ML and multilayer films, respectively. From the data in Table 1, we can conclude that the typical emission features from the multilayer film located at 0.71, 2.24, 4.0, 5.9, 7.46, 8.65 and 9.95 eV can be considered as deriving from the same orbits of perylene in the gas-phase located at 4.71, 6.35, 7.92, 10.02, 11.53, 12.85, 14.12 eV, respectively. The orbit located at 4.71 eV (peak f in Fig. 1(a), 0.71 eV below the Fermi level), corresponding to the highest occupied molecule orbital (HOMO), is well resolved because there is no other state close to this energy.
Table 1. Theoretical and experimental results of the molecular orbitals of perylene molecules.
Level Type Monolayer Multilayer Gas-phase
(eV) (eV) (eV)
1 $\pi$ 4.82 4.71
2 $\pi$ 6.65 6.35 6.35
3 $\pi + \sigma$ 8.17 8.11 7.92
4 $\pi$ 9.91 10.01 10.02
5 $\sigma$ 11.99 11.57 11.53
6 $\pi$ 12.76 12.85
7 $\sigma$ 13.96 14.06 14.12
cpl-37-2-027101-fig2.png
Fig. 2. The charge distribution of the main orbitals of peaks: HOMO(f), a, b, c, g, h and e.
The orbital types in Table 1 are determined by the charge distribution of the corresponding orbitals, as shown in Fig. 2. It is clear that the HOMO with $\pi$ bonding located at the benzene ring of the molecule is dominated by the states with $b_{1g}$ symmetry. The schematic diagram of the electron orbits with its polarization perpendicular to the molecular plane is shown. Due to the resolution limit in the UPS measurements, the charge distribution of orbitals in Fig. 2 is combined with several nearby orbitals except the HOMO. The calculation results reveal that the emissions a, c, and h present $\pi$ characters, while the emissions g and e show $\sigma$ bonding clearly. The emission b possibly has both contributions from $\pi$ and $\sigma$ states.
cpl-37-2-027101-fig3.png
Fig. 3. The calculated most stable adsorption configuration of the perylene molecule: (a) top view and (b) side view.
Finally, we performed the calculations using the model of one perylene molecule adsorbed on a periodic Si(111) $7 \times 7$ surface, as the same as our previous work.[21] By comparing the adsorption energies for all the adsorption configurations, the most stable adsorption structure at the beginning of the deposition is that the center of the perylene molecule locates on the hollow site with the longer axis along the $[1\bar{1}0]$ azimuth, as shown in Fig. 3. The longer axis of the molecule is parallel to the substrate surface, and the angle between the shorter axis of the molecule and the substrate is about 12.81$^{\circ}$, which indicates that perylene molecules adsorb in almost flat configuration on Si(111) $7 \times 7$ surface. With further deposition, the intermolecular interaction should take into account. Since the interaction between the Si substrate and perylene are relatively weak, the self-assembled monolayer structures will have complex patterns with the perylene molecules connected with each other. The perylene molecules with two dimer adsorption structures on the Ag substrate have been found by scanning tunneling microscopy (STM) and DFT calculation.[29] The interaction between perylene molecules and the Si substrate is even weaker than those on the Ag substrate, thus the monolayer adsorption structures are probably dimer structures, which need further studies by STM. In summary, we have studied the adsorption properties of the perylene molecule on the clean Si(111) $7 \times 7$ surface together with the UPS measurements and DFT calculations. The slight changes in peaks' positions and work function during the deposition of perylene indicate that a weak interaction occurs between the organic molecule and the Si substrate. According to the DFT calculation, the adsorption structure is most stable when the long axis of the adsorbed molecule is along the $[1\bar{1}0]$ azimuth with the center at the hollow site for the sub-monolayer case. Our results provide important information for exploring the adsorption configuration and interfacial interaction between the derivatives of perylene and silicon substrates. References The path to ubiquitous and low-cost organic electronic appliances on plasticLight-Emitting Field-Effect Transistor Based on a Tetracene Thin FilmComparative study of the photoresponse from tetracene-based and pentacene-based thin-film transistorsA bright future for organic field-effect transistorsStretchable active matrix inorganic light-emitting diode display enabled by overlay-aligned roll-transfer printingLight-Emitting Devices Based on Electrochemiluminescence: Comparison to Traditional Light-Emitting Electrochemical CellsCharge-Transport Regime of Crystalline Organic Semiconductors: Diffusion Limited by Thermal Off-Diagonal Electronic DisorderElectron-Phonon Coupling in Crystalline Pentacene FilmsHOMO band structure and anisotropic effective hole mass in thin crystalline pentacene filmsStudy on the adsorption of fluorescein on Ag(110) substrateStrain relaxation and epitaxial relationship of perylene overlayer on Ag(110)True perylene epitaxy on Ag(110) driven by site recognition effectChemical vapor deposition graphene as structural template to control interfacial molecular orientation of chloroaluminium phthalocyanineAdsorption of pentacene on a silicon surfaceThe chemisorption of pentacene on Si(001)-2×1The chemisorption of polyaromatic hydrocarbons on Si(100)H dangling bondsDeposition of Tetracene on GaSe Passivated Si(111)Studies of Chemisorbed Tetracene on Si(111)-7×7 †Interfacial electronic states of tetracene deposited on Si(111)The chemisorption of tetracene on Si(100)-2×1 surfaceScanning tunnelling microscopy of tetracene on Si(100)-2 × 1Air Stable n -Channel Organic Semiconductors for Thin Film Transistors Based on Fluorinated Derivatives of Perylene DiimidesSurface preparation of Si substrates for epitaxial growthThe ordered thin-film growth of organic semiconductor on Ag(110)From molecules to solids with the DMol3 approachStructure analysis of Si(111)-7 × 7 reconstructed surface by transmission electron diffractionAggregation of organic molecules on silver surface with the balance between molecule–substrate interaction and intermolecular interaction
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