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Article Cite This: ACS Omega 2018, 3, 6874−6879
Solid-to-Solid Crystallization of Organic Thin Films: Classical and Nonclassical Pathways Zhixian Wei,† Jihui Fan,† Chenghu Dai,‡ Zhiyong Pang,*,‡ and Shenghao Han*,‡ †
School of Physics, State Key Laboratory of Crystal Materials and ‡School of Microelectronics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
ACS Omega 2018.3:6874-6879. Downloaded from pubs.acs.org by 46.148.120.127 on 07/02/18. For personal use only.
S Supporting Information *
ABSTRACT: The solid-to-solid crystallization processes of organic molecules have been poorly understood in view of the complexity and the instability of organic crystals. Here, we studied the crystallization of a π-conjugated small molecular semiconductor, bis-(8-hydroxyquinoline) copper (CuQ2), by annealing the thin films at different temperatures. We observed a classical film-to-nanorods crystallization at 80 °C, a coexistence of classical and nonclassical nucleation and particle growth at 120 °C, and a nonclassical crystal growth at 150 °C. We found that the growth of the crystals followed the following processes: particle nucleation, particle growth, particle migration, nondirectional particle attachment, and structure reconstruction. We notice that the growth of CuQ2 particles follows an outside-to-inside process. More interestingly, our experiments suggest that the submicron CuQ2 particles are able to migrate dozens of micrometers at 150 °C. (OLEDs) reported by Tang et al. in 1987,29 8-hydroxyquinoline-based complexes have been widely used in OLEDs, organic photovoltaics, organic field-effect transistor, and other organic electronic or optoelectronic devices.30,31 More interestingly, 8-hydroxyquinoline-based complexes have shown great potential in the new research field of organic spintronics owing to their extremely long spin relaxation times.32,33 In this work, we study the solid-to-solid crystallization of a πconjugated small molecular semiconductor, bis-(8-hydroxyquinoline) copper (CuQ2, C18H12N2O2Cu), by one-step heat treatment. Figure 1 shows the molecular structure of CuQ2. CuQ2 is not only an optoelectronic material but also an organic magnet with spontaneous spin polarization that is attractive for organic spintronics.34 Interesting crystallization process has been found recently in CuQ2-based materials, for example, the single-crystal-to-single-crystal transformation in CuQ2−tetracyanoquinodimethane.35−37 However, the crystallization process in pure CuQ2 is poorly understood. Here, we observed a classical film-to-nanorods crystallization at low temperature (80 °C) and a nonclassical film-to-crystal crystallization at high temperature (120−150 °C). Highquality CuQ2 crystals have been grown by annealing the thin films at 150 °C. The crystallization mechanism of CuQ2 crystals was discussed, and, more interestingly, we found that the submicron CuQ2 particles were able to migrate dozens of micrometers at 150 °C.
1. INTRODUCTION Crystal growth process is basic and essential in the preparation of high-quality single crystals or low-dimensional materials and contributes to the study of the intrinsic properties of functional materials.1−3 Classical crystallization theory assumes that the growth units are individual atoms, ions, or molecules.4−7 However, the recently discovered nonclassical solid-to-solid crystallization, involving aggregation of nanoparticles or prenucleation clusters, challenges the classical growth mechanisms.8−10 For example, the classical and nonclassical crystallization of zinc oxide nanoparticles and the nonclassical crystallization of copper hydroxide acetate have been recently reported.11,12 Compared with inorganic materials, the crystallization processes of organic crystals are more complex in view of the complexity of the soft organic molecules and the instability of the organic crystals.13,14 Organic crystals have enhanced performance, and, more interestingly, offer some novel properties not commonly seen in amorphous films, such as self-healing ability,15 superelasticity,16,17 terahertz emission,18 ambipolar charge-transport behavior,19,20 ferroelectricity,21 and multiferroics.22 Recently, nonclassical solid-to-solid crystallizations of [Ni(quinolone-8-thiolate)2] and hydroxyapatite have been observed.23,24 However, in view of the complexity and the instability of organic crystals, the solid-tosolid crystallization process of organic molecules is still poorly understood. 8-Hydroxyquinoline-based complexes are an interesting and versatile class of materials in view of their excellent thermal stability, adequate charge-carrier mobility, and high luminescence efficiency.25−28 Since the first tris-(8-hydroxyquinoline) aluminum (AlQ 3 )-based organic light-emitting diodes © 2018 American Chemical Society
Received: January 24, 2018 Accepted: June 14, 2018 Published: June 25, 2018 6874
DOI: 10.1021/acsomega.8b00153 ACS Omega 2018, 3, 6874−6879
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nanorods”, i.e., nanograins, indicating that the mechanism of the film-to-nanorods transformation is a classical crystallization mechanism rather than a nonclassical particle-attachment pathway. The CuQ2 shows different crystallization behaviors at high temperatures. From the SEM images of CuQ2 films, after heat treatment at 120 and 150 °C for 2 h, crystalline particles rather than nanorods can be found, indicating that the CuQ2 grains (120 and 150 °C) have different molecular packing modes compared with the rods (80 °C). This is confirmed by the Xray diffraction (XRD) spectra of the CuQ2 sample (Figure S2), in which the samples annealed at 120 and 150 °C have different diffraction peaks than those of the samples annealed at 80 °C. When annealed at 120 and 150 °C, three new peaks emerged at 8.5, 11.9, and 13°, which correspond to (100), (002), and (002̅) facets of the β phase. This means that the CuQ2 crystal transforms from the α-phase to the β-phase when the temperature is above 120 °C. The CuQ2 migrated, nucleated, and grew into particles. As shown in Figure 2c, most of the particles have regular or irregular hexagonal structures from the top view, indicating an oriented growth perpendicular to the Si substrate. In the plane of the substrate, the particles have no obvious orientation. The hexagonal particles distributed densely on most of the areas of the Si substrate, and their diameters are about 2 μm or less. Surrounding these particles are empty areas where nearly no CuQ2 pieces were left. The distance between two particles is about several hundred nanometers. There are also areas where no hexagonal particles but only CuQ2 pieces nucleated (dashed-line ellipses), showing the inhomogeneity of the nucleation process. For the samples heated at 150 °C, crystals having sizes of several microns were obtained. The smooth surfaces and regular shapes reflect that the quality of the crystals grown directly from thin films is quite high. Surrounding the crystals are large empty areas where most of the CuQ2 molecules have migrated out to the crystals close by. From the SEM image with lower
Figure 1. Molecular structure of CuQ2.
2. RESULTS AND DISCUSSION Figure 2 shows the scanning electron microscopy (SEM) images of CuQ2 thin films before and after heat treatment. The as-evaporated CuQ2 samples were thin films with nanograins distributed randomly on the surface. The surface morphology of the samples had no obvious change, as the heat treatment temperature was below 80 °C. As the heat treatment temperature increased to 80 °C, film-to-nanorods transformation occurred. From the SEM image of CuQ2 films, after heat treatment at 80 °C for 2 h, randomly distributed nanorods with their lengths about 1 μm can be clearly seen. The CuQ2 nanorods have smooth surfaces and no signs of particle attachment or grain agglomerates were found, exhibiting that the growth units are individual molecules. The CuQ2 molecules acquired energy from the annealing process, migrated on the surface under the van der Waals force, hydrogen bonding, and π−π bond stacking interactions,38,39 and then piled up in specific direction on the base of the “seed
Figure 2. SEM images of (a) the as-evaporated CuQ2 thin films and the samples after heat treatment at (b) 80 °C, (c) 120 °C, and (d) 150 °C. The insets in (a−c) show images with higher resolutions. The inset in (d) shows the image with lower magnification. The yellow circles show that the diameters of the empty areas can be up to ∼100 μm. 6875
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Figure 3. SEM images of the CuQ2 samples at different nucleation stages. The samples are obtained by annealing the CuQ2 thin films at 120 °C for different times.
Figure 4. Images of CuQ2 samples heat-treated at 150 °C for (a, b) 20 min; (c, d) 40 min; (e, f) 60 min; and (g, h) 120 min. The left column and the right column have different magnifications.
annealed at 180 °C and above, the CuQ2 molecules volatilized and nothing was left on the substrate. For a better understanding of the nucleation process of the CuQ2 particles, Figure 3 shows the SEM image of the CuQ2 samples at different nucleation stages. The samples were obtained by annealing the CuQ2 thin films at 120 °C for different times. At the first stage, the substrate was covered by cuboid nanograins and no particle nucleation can be observed.
magnification, as shown in inset of Figure 2d, it can be seen that the diameters of the empty areas can be up to 100 μm, indicating that the migration distance of the CuQ2 is extremely long (dozens of microns), not only far longer than the wellknown Ostwald ripening by the migration of molecules in surface science,5,40 but also longer than the reported longrange migration of organic nanoparticles (∼1.6 μm) during the nonclassical growth of molecular crystal.23 As the CuQ2 films 6876
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Figure 5. (a) Quantity of CuQ2 crystal nucleus for different stages at 120 °C. The data are collected from Figure 3. (b) The quantity of CuQ2 crystals and average sizes for different stages at 150 °C. The data are collected from the left column of Figure 4.
observation of “oriented attachment” crystallization pathway that the nanoparticles adjust their orientation and attach to a large (compared with the particle) crystal, in this experiment, the attached particles have similar sizes and no evidence of oriented attachment has been found. The in-plane orientations of the attached particles seem to be random. The dashed-line circle labels a particle formed from three attached particles. It can be clearly seen that the particles are distorted to adjust their orientation, revealing that the particles reconstruct their structures after particle attachment.41,42 The SEM images in Figure 4 shows the crystallization process of CuQ2 at 150 °C. First, CuQ2 particles were nucleated via molecular migration (20 min). Different from the particles that nucleated at 120 °C, the CuQ2 particles that nucleated at 150 °C have complete single crystalline shapes and no cavity can be found on their topside (Figure 4), showing that the CuQ2 molecules acquired enough energy at 150 °C and had the ability of completing the molecular migration and structure adjusting process at short time. Once nucleated, nearby particles moved from their original positions and attached to each other (40 min). Figure 4d shows an aggregate of CuQ2 particles. It can be clearly seen that the aggregate is formed by nondirectional attachment of several microparticles. The aggregates underwent a structure reconstruction process and then grew into big particles (∼3 μm, yellow circles). Subsequently, the new formed big particles agglomerated into larger particle groups (60 min) and then underwent the structure reconstruction process again. As shown in Figure 4f, in this stage, the attachment of big particles is also nondirectional. The irregular curve labeled by the red arrow shows a particle boundary under structure reconstruction. Different from the previous observation that organic crystal grows via oriented attachment of small particles to a large crystal, in this experiment, the crystals are formed via nondirectional attachment of particles with similar sizes and large-scale structure reconstruction. Figure 4g shows the crystals growth by above process (120 min). The crystals have sizes of several microns and complete crystalline shapes, showing that the qualities of the crystals grown from this process are quite high. We note that the CuQ2 particles or crystals have excellent migration ability. As labeled by the dash−dot circles, it can be seen that the diameters of the empty areas can be up to dozens of microns, indicating that the migration distance of the CuQ2 is extremely long.
With the increase in the heat treatment time, hexagonal particles (dashed-line squares) nucleated in stage #2 grew from nanometers to about 1−2 μm. At the same time, the shape of the CuQ2 nanograins, especially those that surround the hexagonal particles, changed from regular cuboid to irregular cuboid and further to irregular pieces. This phenomenon was more obvious in stages #3 and #4. The change in the shape of nanograins revealed molecular migration processes of CuQ2 molecules, indicating a classical crystallization pathway in this period. We notice an interesting phenomenon that almost all of the hexagonal particles have a cavity on the topside. It reveals that the growth of the particles is from outside to inside. At first, the CuQ2 molecules migrate to the outside of the particles, diffuse across the surface, and adjust their positions according to the crystal orientation, resulting in an outside-first growth process of the particles. It is also found that the bigger particles have smaller cavities, confirming the outside-to-inside process during particle growth. This process is also reflected by the neat side faces of the hexagonal particles, no matter how small the particles are. Besides the classical molecular migration, attached particles have also been found from the beginning of the particle growth. As labeled by circles in stages #2−5 in Figure 3, aggregation and attachment of two or more CuQ2 particles are clearly shown, exhibiting a coexistence of classical and nonclassical crystal growth pathways at 120 °C. From stage #2 to stage #5, the number of the particles increases significantly; however, the sizes of the particles are limited to about 1−2 μm and there is no obvious increase. At the same time, the amount of the CuQ2 grains (pieces) decreases substantially to an extremely small value in stage #5, exhibiting that classical molecular migration is the main crystal growth mechanism at this temperature. For the nonclassical crystallization pathway via particle migration, attachment, and structure reconstruction, the progress is much slower than that in the classical pathway because particle migration needs more energy than molecular migration. The yellow circles label some particles that are formed by particle migration and attachment of proximate particles. These particles have amalgamated into big particles, although their irregular shapes exhibit that the structure reconstruction process has not yet finished. As labeled by the blue circles, there are also “particle groups” that are composed of two or more CuQ2 particles. These particles are still at the stage of particle aggregation or attachment. Different from the previous 6877
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To clarify the classical and nonclassical solid-to-solid crystallization pathway, the statistical processing of the images has been performed. Figure 5a shows the quantity of CuQ2 crystal nucleus of different stages at 120 °C (Figure 3). The quantity of CuQ2 crystal nucleus of different stages increased drastically with annealing time. These results are in accordance with our analysis of coexistence of classical and nonclassical processes. On the contrary, as shown in Figure 5b, the quantity of CuQ2 crystal nucleus of different annealed stages at 150 °C decreased fast with an increase in time. Furthermore, the average sizes of different stages became bigger and reached 8 μm after 120 min, confirming the existence of the nonclassical crystallization pathway.
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00153. SEM side view of samples; XRD of samples and calculated XRD from α- and β-phase CuQ2; EDS of purchased substance and CuQ2 thin films on Si substrate (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Z.P.). *E-mail:
[email protected] (S.H.).
3. CONCLUSIONS
ORCID
Shenghao Han: 0000-0001-6612-2498
In summary, the solid-to-solid crystallization process of CuQ2 thin film has been investigated by heat treatment. It is found that the film shows different crystallization behaviors at different temperatures: at a low temperature (80 °C), the film grows into nanorods via classical Ostwald ripening; at a middle temperature (120 °C), the film grows into crystals via particle nucleation, particle growth, particle migration, particle attachment, and structure reconstruction. The growth of hexagonal particles is from outside to inside. Classical Ostwald ripening and nonclassical particle-attachment pathways have both been found at this stage; at a high temperature (150 °C), the particles grow into high-quality crystals via several times nondirectional particle-attachment and structure reconstruction processes. It is found that the CuQ2 particles have the ability of migrating dozens of micrometers. Our results may facilitate the understanding of the crystallization of solid-state organic materials and further the development of molecular crystalline materials with controlled structures and properties.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for financial support from the Natural Science Foundation of China (11374184) and the Shandong Provincial Natural Science Foundation, China (ZR2018MF030).
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REFERENCES
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4. EXPERIMENTAL DETAILS CuQ2 was purchased from J&K Scientific in powder form and used as received without further purification. The energy dispersive spectrometer (EDS) analysis of the purchased CuQ2 powder as well as the evaporated thin films have been performed and shown in Supporting Information. The CuQ2 thin films were deposited on precleaned Si substrates by thermally evaporating pure CuQ2 powder at a base pressure of 2.0 × 10−4 Pa in a home-made thermal evaporator. The film thickness was in situ monitored using a quartz crystal thickness monitor. In our experiments, the thickness of the CuQ2 thin films was about 200 nm. The vacuum-deposited CuQ2 thin films were thermally annealed under flowing argon gas for 2 h using the crystal growth apparatus. Heat was applied by electronic resistance furnace, and the temperature rate can be controlled accurately. The flowing rate of high-pure and dry argon gas (99.999%) was 0.4−0.6 L/min, and the working gas pressure was 0.35 Pa. Moreover, to avoid the ambient adverse interference, the annealing chamber was washed by high-pure and dry argon (99.999%) gas with 2.0 L/min for 30 min prior to annealing. The annealing temperature was set for 2 h at temperatures 80, 120, and 150 °C. The surface morphology imaging of the samples were performed by a high-resolution scanning electron microscope (S-4800). 6878
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