Applied Science and Convergence Technology 2023; 32(4): 89-92
Published online July 30, 2023
Copyright © The Korean Vacuum Society.
aBeamline Research Division, Pohang Accelerator Laboratory, Pohang 37673, Republic of Korea
bDepartment of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
cDepartment of Smart Fabrication Technology, Sungkyunkwan University, Suwon 16419, Republic of Korea
dDepartment of Physics, University of Ulsan, Ulsan 44610, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
Based on surface charge states, the thermodynamics of pentacene molecules exhibit intriguing structural transitions with various temperatures. On an SiO2 surface, the mean tilt angle decreases with increasing temperature, whereas three different structural regimes form on the Au surface. To understand these contrasting temperature dependencies of molecular orientation on the two surfaces, a model wherein the tilt angle α has two stable states is proposed in this study. Consequently, the pentacene crystals undergoes a phase transition at a relatively low temperature of 350 K, thus indicating a hidden factor governing the morphology of molecular crystals, and providing insights into the thermal behavior of physisorbed organic molecules.
Keywords: Surface melting, Phase transition, Organic crystal, Pentacene
Physically adsorbed organic films have attracted considerable attention owing to their potential applications as inhibitor layers for area-selective deposition (ASD) [1–5] and electronic devices [6,7]. Intermolecular and molecule-substrate interactions are crucial in determining the degree of crystallinity of organic films, which is directly related to the area selectivity in ASD or charge mobility in electronic devices [8,9]. However, because these phenomena are based on weak van der Waals interactions, the crystallinity of molecular film formed on the surface is affected by small external stimuli and sensitive changes. Particularly, the surface charge density is a sensitive determinant of the crystallinity of a molecular film, and its understanding is crucial for improving the selectivity of area-selective film formation.
Pentacene is a model system for organic crystal films based on van der Waals interactions. Depending on their substrate, pentacene molecular films exhibit dramatic variations in their structural ordering. The charge-carrier mobility is directly dependent on the crystallinity of the grown organic film, whose reported value is in the range of 0.1–58.0 cm2V–1 s–1 . Studies on the structural ordering of pentacene films under various conditions have demonstrated that the initial arrangement of pentacene molecules on the surface acts as a nucleation seed. [11–13]. On the crystal structure surface, the pentacene molecule exhibits two orientations, i.e., standing-up or lying-down, which are observed when the angle α between the longitudinal axis of pentacene molecule and surface is greater than 70° or less than 10°, respectively .
In the early stage of pentacene growth, the standing-up orientation is important for obtaining highly ordered crystalline structures, owing to the lower surface energy of (001) crystal plane compared with that of the other planes [14,15]. Therefore, the growth directions of (122), (011), and (121) planes can be varied to (001) .
On the substrate, pentacene molecules interact with surface electrons and neighboring molecules with energies of
At a given temperature during the early stage of crystallization, the energy difference between
In this study, the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to analyze the temperature dependence of low-coverage pentacene molecules on silicon dioxide (SiO2) and gold (Au) surfaces. To explain the observed variations in α, a thermodynamic model was developed for the two orientations (standing-up and lying-down) of pentacene. Using this model, the intermolecular interactions were abruptly weakened and induced by a phase transition of the pentacene crystal from solid to quasi-liquid at 350 K.
The sample preparation and spectroscopic measurements were performed at the 4D NEXAFS beamline of the Pohang Accelerator Laboratory. Si with a thermal oxide layer (100 nm) and polycrystalline Au of dimensions 5 × 10 cm2 were simultaneously mounted on a grounded molybdenum holder, which was thermally linked to a resistive heater wrapped in phosphor-boron-nitride. For degassing, the substrates were annealed in an ultrahigh vacuum chamber (base pressure: 4 × 10−10 Torr) at 670 K for 48 h. Prior to sample heating, surface contaminants were cleaned using ion sputtering for 10 min (Ar+, 3 KeV), and the cleanliness of the substrate surfaces was confirmed by using X-ray photoemission spectroscopy (XPS).
Pentacene (Aldrich, purified by gradient sublimation) was thermally evaporated onto the SiO2 and Au surfaces at room temperature (RT) and deposition rate of 0.05 Ås−1. The deposition rate and thickness of the pentacene layer were monitored using a crystal-oscillator thickness monitor (Inficon XTC). The temperature of the sample was regulated within ±0.1 K. The partial electron yield (PEY)-mode NEXAFS was measured by counting the carbon KLL Auger electrons.
The temperature-dependent desorption patterns of 100 nm pentacene films grown on SiO2 and Au were observed using optical microscopy. As the temperature increased, the inhomogeneous and homogenous desorption of pentacene film was observed on SiO2 and Au surfaces, respectively (Fig. 1). In the former case, the desorption was initiated by the defect sites denoted by the red arrows in Fig. 1b, which continued to evolve gradually around the defect centers [Figs. 1(c) and 1(d)]. Therefore, the intermolecular interaction energy (
Compared with the SiO2 surface, the Au surface contains a larger number of crystal defects; hence, the pentacene molecules exhibit homogeneous desorption from the Au surface . This can be attributed to the interaction energies of pentacene molecules formed on Au, which are in the order
To analyze the molecular orientation of sub-monolayer, a 7 Å-thick layer (approximately 0.5 molecular layers) of pentacene molecules was simultaneously deposited on cleaned SiO2 and Au. Furthermore, NEXAFS measurements were performed in vacuum [14,15].
The NEXAFS spectra of an as-prepared sample were recorded at incident angles θ
Because the π* orbital is strongly localized perpendicular to the plane of the carbon ring of pentacene molecule, the π* resonance peak strongly depends on the polarization of electric field. Therefore, the angle-dependent NEXAFS spectra can be used to probe the orientation of pentacene molecules [Fig. 2(a)]. The peak intensity
To evaluate the average ⟨α⟩ values at each temperature, the NEXAFS spectra at θ
Figures 2(d)-(f) show three different dichroic responses observed on the Au surface. Compared with the pentacene molecule on Si surface, the
At RT, the intensity of
To describe the thermodynamic behavior of ⟨α⟩ pentacene molecules on SiO2 and Au surfaces, a model with the following two stable states of α was considered: standing-up (α = 74°) with
Thereafter, the averaged tilt angle of ⟨α⟩ can be obtained as
Detailed thermal behavior of ⟨α⟩ were acquired using the angular dichroism of the NEXAFS spectra of pentacene molecules at 7 Å on the SiO2 and Au surfaces [Figs. 4(a) and 4(b)].
The experimental results differed significantly from the smooth transitions predicted by calculations (Fig. 3), thus indicating the absence of an unknown geometric state of α because the addition of new states in the calculation does not cause the abrupt transition observed in the experimental results. However, the abrupt variation in Δ
The best fit of the temperature behavior of ⟨α⟩ for the two substrates is indicated by the solid blue lines [Figs. 4(a) and 4(b)]. Notably, the Δ
On the Au surface, excluding the contribution of β near RT, ⟨α⟩ was estimated to be approximately 0° [Fig. 4(b)] . As the temperature increased, ⟨α⟩ increased until 380 K and sharply decreased to approximately zero [Fig. 4(b)] owing to the desorption of pentacene adlayer molecules, which is consistent with the findings of a previous report . Above 380 K, the carbon 1s intensity decreased, thus facilitating the desorption of molecule.
After the desorption of adlayer molecules, the NEXAFS spectrum revealed the information regarding the direct binding of pentacene molecules to the Au surface. The desorption of a molecule from the adlayer exposes a flat-lying molecule in the Au monolayer with β = 0. The binding energy of a pentacene molecule bonded directly to the Au surface is 1.82 eV, and such molecules exhibit no desorption in the temperature range of this experiment . Similar to the SiO2 surface, the fitted curve was derived by adjusting the value of Δ
This study investigated the thermal behaviors of pentacene molecules on SiO2 and Au surfaces using soft X-ray absorption spectroscopy. The initial configuration of these molecules can be explained using two main interactions: intermolecular and molecular-surface. Because the SiO2 and Au surfaces have significant variations in valence electron densities, they exhibit opposite initial molecular arrangements. Furthermore, a model with two energy states was proposed to adequately describe the thermodynamic behavior of pentacene molecules on the two surfaces by considering an additional phase transition. These results describe the morphologies of pentacene observed at various temperatures. The energy difference Δ
This study was supported by the National Research Foundation of Korea (NRF) (Grant No. NRF-2018R1D1A1B07043155, 2022R1F1A1 068192) from the Ministry of Science, ICT, and Future Planning, Korea. The experiments at the 4D beamline of the PLS–II were supported by MSIP, R. O., Korea.
The authors declare no conflicts of interest.