Thickness-Dependent Air-Exposure-Induced Phase Transition of

Feb 3, 2015 - ABSTRACT: Highly ordered organic crystals are important building blocks for future high- performance organic nanodevices. Here we report...
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Thickness-Dependent Air-Exposure-Induced Phase Transition of CuPc Ultrathin Films to Well-Ordered One-Dimensional Nanocrystals on Layered Substrates Lei Zhang,† Yingguo Yang,‡ Han Huang,*,†,§ Lu Lyu,† Hong Zhang,† Ningtong Cao,† Haipeng Xie,† Xingyu Gao,‡ Dongmei Niu,†,§ and Yongli Gao*,†,§,∥ †

Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, P. R. China ‡ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 239 Zhangheng Road, Pudong New Area, Shanghai 201204, P. R. China § Hunan Key Laboratory for Super-microstructure and Ultrafast Process, Central South University, 932 South Lushan Road, Changsha, Hunan 410083, P. R. China ∥ Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, United States ABSTRACT: Highly ordered organic crystals are important building blocks for future highperformance organic nanodevices. Here we report a feasible way to produce arrays of wellordered 1D copper phthalocyanine (CuPc) nanocrystals by using molybdenum disulfide (MoS2) or highly oriented pyrolytic graphite (HOPG) as substrates. The growth behaviors of CuPc on MoS2(0001) as well as on HOPG and corresponding effects of air exposure were systematically investigated by means of in situ photoemission spectroscopy (PES) and lowenergy electron diffraction (LEED), combined with ex situ atomic force microscopy (AFM), surface X-ray diffraction (SXRD), and Raman spectroscopy. PES and LEED results show that CuPc molecules adopt a face-on configuration at thickness up to 4.8 nm, while AFM and SXRD results show that they adopt an edge-on configuration to form 1D nanocrystals in films thicker than 2.4 nm. Detailed analyses show that the formation of these 1D nanocrystals is closely related to air exposure, thicknesses, and growth temperature. Such 1D CuPc nanocrystals can be further optimized by tuning growth conditions and may have great potential for use in high-performance organic devices.

1. INTRODUCTION In the past few decades, organic π-conjugated semiconducting molecules and polymers have attracted much attention due to their promising applications in low-cost, lightweight, flexible, and large-area electronic devices compared with inorganic counterparts. However, it is a great challenge to fabricate highly ordered organic films for the development of high-performance organic devices. As is well known, the micromorphology of organic layers as well as the corresponding molecular orientation and packing play key roles in improving charge-carrier mobility and injection, which determine the device performances.1−3 For instance, Wang et al. used para-sexiphenyl (p-6P) ultrathin film as an inducing layer to successfully fabricate highly ordered zinc phthalocyanine (ZnPc) films with a significantly improved charge mobility of 0.32 cm2·V−1·s−1 (ref 4). Wu et al. used Au film as a template to prepare highly dense single-crystalline copper phthalocyanine (CuPc) nanowires, which demonstrated excellent photoresponse properties.5 Thus, modulating the growth of organic molecules for highly ordered films is both scientifically challenging and of great practical significance. CuPc, a common and important organic π-conjugated semiconductor whose chemical structure is shown in Figure 1a, has demonstrated diverse applications in optical, electronic, and © XXXX American Chemical Society

Figure 1. (a) Molecular structure of CuPc and (b) schematic of preparation of CuPc film with different thickness steps on one MoS2 substrate.

photoelectronic devices, such as gas sensors, recordable disks, organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field-effect transistors (OFETs),6−10 due to its chemical stability as well as outstanding optical and electronic Received: December 18, 2014 Revised: February 2, 2015

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The Journal of Physical Chemistry C

Figure 2. LEED patterns of (a) clean MoS2 substrate and (b) 0.3 and (c) 4.8 nm thick CuPc films on it at electron beam energies of 125, 15, and 27 eV, respectively. Squares in white dotted-dashed, red dotted, and green dashed lines in panel b represent three equivalent domains. The white dotted-dashed square in panel c highlights square symmetry.

properties.11,12 Tuning the molecular packing is a challenge for optimizing the quality of organic thin films. The growth behaviors of CuPc on various substrates have been studied. CuPc molecules prefer to form thin films in an edge-on configuration on inert substrates, such as silicon dioxide (SiO2), indium tin oxide (ITO), p-6P/SiO2, and fullerene (C60)/ Ag(111),4,13−16 while they form thin films in a face-on configuration on other substrates, such as highly oriented pyrolytic graphite (HOPG), molybdenum disulfide (MoS2), Ag(111), and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA),13,17−20 depending on the interplay between both molecule−molecule and molecule−substrate interactions.17,21,22 However, the molecular orientation is not fixed but changes with the growth conditions such as growth temperature, deposition rate, and film thickness. For example, CuPc molecules on graphene or HOPG adopt a face-on configuration and a densely packed structure in the initial layers because of the π−π interfacial coupling and gradually tilt up with increasing thickness.18,23,24 The elevated growth temperature enables the molecules to maintain a face-on configuration through a larger range of thickness up to 50 nm.21,23,25,26 Besides the molecular orientation, the dewetting behavior is also important for applications of organic thin films.27,28 Although dewetting effect is considered to be a problem affecting the growth of smooth films and leading degradation of organic devices, sometimes it is a good way to obtain arrays of nanostructures with controlled shape and high crystalline.29−31 Recently, we studied the van der Waals heterojunction formed by depositing CuPc on MoS2(0001) at room temperature (RT) using photoemission spectroscopy (PES), low-energy electron diffraction (LEED), and atomic force microscope (AFM).32 Well-ordered 1D rodlike CuPc nanocrystals are observed. However, the underlying mechanism is not further studied. Using CuPc/MoS2(0001) as a model system, the detailed evolution of morphology as a function of film thicknesses upon air exposure and the effect of growth temperature are investigated by means of LEED, AFM, surface X-ray diffraction (SXRD), and Raman spectroscopy. In situ LEED patterns show that CuPc molecules aggregate into well-ordered small multiple domains at thickness of 0.3 nm and into larger domains in micrometer size at thickness of 4.8 nm with the molecular plane (quasi-)parallel to the MoS2(0001) substrate. Ex situ AFM results show an obvious change of CuPc morphology from smooth surface via dispersed nanoclusters to well-ordered 1D rod-like nanocrystals dependent on the thickness, the same on HOPG. Remarkably, the directions of the 1D rods are strongly aligned to the MoS2 substrate crystallographic axes, indicating an epitaxial relationship. Ex situ SXRD data demonstrate that the CuPc molecules in such 1D rod-like crystals adopt an edge-on

configuration, indicating a change of molecular orientation upon air-exposure. Raman signals of CuPc on MoS2(0001) are rather weaker than on HOPG, which may be attributed to the difference in electronic properties of substrates. Contrary to this, CuPc remains in face-on configuration for the samples prepared at growth temperature higher than 450 K, even if exposed to air. Our research results give a promising way to tuning organic molecular orientation.

2. EXPERIMENTAL DETAILS Vapor deposition of CuPc and in situ LEED as well as PES analysis were performed in the organic molecular beam deposition (OMBD) chamber and the main chamber of a multifunctional ultrahigh vacuum (UHV) system with base pressures of 8 × 10−9 and 2 × 10−10 mbar, respectively. The main chamber is equipped with a SPECS PHOIBOS 150 hemispherical energy analyzer as well as a monochromatic SPECS XRMF microwave X-ray source (Al Kα = 1486.7 eV) and a SPECS microwave UV light source (He I = 21.2 eV), a SPECS ErLEED optics.32 The freshly cleaved MoS2 substrate (9 × 5 mm2, 2Dsemiconductors) was transferred immediately into the OMBD chamber and degassed overnight at a temperature close to 700 K, whose quality was verified by X-ray photoelectron spectroscopy (XPS) and LEED. CuPc thin films with specific thicknesses were prepared on a MoS2 substrate before characterization, and the corresponding step widths were controlled by pulling away a substrate baffle over a certain distance, as shown in Figure 1b. The deposition rate was maintained at 2 Å/min, monitored by a quartz crystal microbalance. The substrate was kept at RT unless otherwise specified. Ex situ synchrotron-based SXRD experiments were performed under ambient conditions at the BL14B1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) using Xrays with energy of 10 keV. Two dimensional SXRD (2D-SXRD) patterns were acquired by a MarCCD area detector mounted vertically at a distance ∼230 mm from the sample. The incidence angle of X-ray was 3.0°. Lanthanum hexaboride was used to calibrate instrumental broadening of diffraction peaks. The 2DSXRD patterns were analyzed using the Fit2D software and displayed in scattering vector q coordinates, where q = 4π sin θ/λ, θ is half of the diffraction angle, and λ is the X-ray wavelength of 1.2387 Å. Ex situ X-ray diffraction (XRD) measurements on samples prepared at higher growth temperature were performed on a Rigaku D/Max 2500 diffractometer that was operated with Cu Kα radiation with linear focus. AFM imaging and Raman measurements were conducted under ambient conditions with Agilent 5500AFM/SPM and LabRAM HR800 system. For AFM imaging, silicon probes with a 10 nm radius of curvature were used in the tapping mode. For Raman measurements, a 488 nm B

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The Journal of Physical Chemistry C Ar+ laser with a spot diameter of close to 2 μm was used. The RT sample preparation was divided into two steps due to the limited size of MoS2: one with CuPc thicknesses of 0.3, 0.9, 1.2, and 2.4 nm and the other with CuPc thicknesses of 4.8, 12.0, and 30.0 nm.

3. RESULTS AND DISCUSSION 3.1. In Situ LEED Patterns. To investigate the molecular packing of CuPc on MoS2(0001), we implemented in situ LEED measurements. Figure 2a shows the LEED pattern of clean MoS2(0001). The sharp diffraction spots are in a hexagonal pattern, in accord with previous studies,33,34 confirming its high quality. Figure 2b shows the LEED pattern of 0.3 nm thick CuPc film with 12 broader diffraction spots in the inner circle, indicating that long-range ordered superstructures of CuPc with three equivalent domains (white dotted-dashed, red dotted, and green dashed squares) have formed along the three next-nearest neighboring axes of MoS2(0001), respectively, consistent with previous reports.33,34 Figure 2c shows the LEED pattern of 4.8 nm thick CuPc on MoS2(0001) in a square symmetry (as indicated by white dotteddashed square), arising mainly from one out of the three domains in Figure 2b. This means that with the thickness increases from 0.3 to 4.8 nm, the average domain size may be enlarged and the molecular plane remains (quasi-)parallel to the substrate. Otherwise, for CuPc in an edge-on configuration, the LEED pattern should be in a rectangular symmetry. It is in good agreement with previous studies of CuPc on layered substrates such as MoS2, HOPG, and graphene by theoretical calculations,17,21 scanning tunneling microscopy (STM),18,20,35 noncontact AFM,36 PES,23,24,32 and high-resolution electron energyloss (HREEL) spectra.37 It should be noted that all of these studies were performed under UHV conditions. The airexposure influence on CuPc films on layered substrates remains unclear. 3.2. Morphology Evolution upon Air-Exposure. The morphology evolutions of CuPc films as a function of thicknesses upon air exposure were studied by AFM in ambient. The freshly cleaved MoS2 is atomically flat, as shown in Figure 3a. Figure 3b is the AFM image from the 0.3 nm thick CuPc film. The crosssectional profile corresponding to the red line shows the film thickness of ∼0.2 nm, far less than the interlayer distance of ∼1.0 nm for edge-on CuPc,16,27 indicating that the face-on configuration of monolayer CuPc on MoS2 is not affected upon air-exposure due to the existence of molecule−substrate interaction. When the thickness increases via 0.9 to 1.2 nm, the substrate is covered by nucleated spherical crystallites of CuPc (shown in Figure 3c,d), in contrast with the widely accepted layer-by-layer growth mode of few nanometers thick CuPc on layered substrates.17,20 It can be attributed to that for thicker (0.9 and 1.2 nm thick) CuPc in the thin-film phase, the molecule− molecule interaction being perturbed by air-exposure results in CuPc nucleation. Whether the interface layer is affected is unclear. The distribution of equivalent disc radii in the top right corner of Figure 3c shows that the radii of the nuclei are mainly in the range of 15 to 40 nm with a peak value of 28 nm. Considering that the heights of them are ∼4.5 nm, the volumes of such nuclei are then estimated to be in the range of 3.2 × 103 to 2.26 × 104 nm3 with a peak value of 1.11 × 104 nm3. The corresponding magnified images (0.5 μm × 0.5 μm) shown in the top left corner of Figure 3c,d clearly display the coalescence of nuclei. The alignment of elongated nuclei and domain structures emerge in

Figure 3. AFM images (5 μm × 5 μm) of (a) 0, (b) 0.3, (c) 0.9, and (d) 1.2 nm thick CuPc films on MoS2(0001). Left-corner insets of panels c and d are the corresponding magnified images (0.5 μm × 0.5 μm). The histogram in panel c shows the radius distribution of spherical crystallites. The cross-sectional profiles along the red lines in panels b and d are shown right above them, respectively.

Figure 3d, indicating the substrate influence on it. The crosssectional profile corresponding to the red line of the inset of Figure 3d shows the nuclei in heights of 7 ± 1 nm. Thus, the volume of the elongated nuclei is an order of magnitude larger than that of Figure 3c. This is consistent with the growth process of thin film: more source molecules lead nuclei to coalesce into larger islands. Figure 4 shows the evolutions of the CuPc morphology with thicknesses >2 nm. When the thickness increased to 2.4 nm (Figure 4a), the domain structures are more obvious. The

Figure 4. AFM images (10 μm × 10 μm) of (a) 2.4, (b) 4.8, (c) 12.0, and (d) 30 nm thick CuPc films on MoS2(0001). The insets in the top left corners are the corresponding magnified images (1 μm × 1 μm). The cross-sectional profiles corresponding to the red lines of panels a and b are shown, respectively. C

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The Journal of Physical Chemistry C magnified image (the inset of Figure 4a, 1 μm × 1 μm) shows that the crystallites in each domain are elongated parallel to each other in a particular direction and maintain a width of 90 ± 10 nm and a height of 7.7 ± 0.4 nm. It should be noted that there are still a few layers of CuPc molecules existing in the spaces between those rod-like crystals, which can be seen clearly in the inset of Figure 4a and are confirmed by the more sensitive amplitude image (not shown), suggesting the interfacial few layer CuPc existed. For the 4.8 nm thick CuPc film (Figure 4b), a compact film is formed with the average domain area of ∼10 μm2, consistent with the previously mentioned in situ LEED results. The 1D rodlike crystals are ∼1 μm in length (some are as long as 3 μm) and 115 ± 5 nm in width. Another feature is that these rod-like crystals have flat surfaces and sharp edges, as seen from the crosssectional profile of the inset of Figure 4b, indicating a better crystalline. As the thickness increases from 1.2 to 4.8 nm, the heights of the nuclei/nanocrystals have almost no change (see the cross-sectional profiles in Figures 3d and 4a,b), indicating a diffusion-controlled lateral growth preferred. On the contrary, more molecules or higher growth temperature are required to grow CuPc into rod-like crystals directly on hydrogenterminated Si(111) (Si(111)-H) or SiO2.22,38,39 One possible reason is the relatively small diffusion barrier for CuPc on layered substrates (8.7 meV on HOPG),21,22,40−42 which facilitates efficient lateral diffusion of the molecules over the surface. For thicker CuPc films shown in Figure 4c (12.0 nm thick) and Figure 4d (30.0 nm-thick), the rod-like crystals display the tendency of coalescing side by side. However, the coalescence between domains does not occur. Figure 5a is a representative zoomed-out AFM image (20 μm × 20 μm) to show the domain structure of 4.8 nm thick CuPc

HREELS and ex situ AFM measurements. More direct evidence is required to understand the contradiction. 3.3. SXRD. To further confirm the molecular orientation of CuPc in such 1D nanocrystals, we performed SXRD experiments at an incidence angle of 3.0°. Figure 6a,b shows the synchrotron-

Figure 6. 2D-SXRD patterns of (a) 1.2 and (b) 4.8 nm thick CuPc films at an incidence angle of 3.0°. (c) Corresponding out-of-plane SXRD spectra from panels a and b at qxy = 0. (d) Radially integrated intensity plots along the rings at qz ≈ 5.11 nm−1 from panels a and b, which are assigned to the α-phase (100) plane of CuPc.

based 2D-SXRD patterns obtained from 1.2 and 4.8 nm thick CuPc films, respectively. The strong and sharp diffraction patterns imply good crystallization and large crystal size. Only out-of-plane diffraction vectors can produce interference fringes, indicating that the CuPc molecules display a layer-by-layer stacking. The diffraction maximum positioned at qz = 9.67 nm−1, which could be indexed to an interplane distance of 6.5 Å, comes from the reflection of MoS2(0002). Another strong diffraction maximum at qz = 5.11 nm−1 is assigned to the α-phase (100) plane of CuPc, suggesting that the CuPc molecules orient perpendicularly to MoS2(0001).21 Thus, the inconsistency between in situ LEED and ex situ AFM results can be attributed to a phase transition accompanied by molecular reorientation upon air exposure. Figure 6c shows the corresponding out-of-plane SXRD spectra of 1.2 and 4.8 nm thick CuPc films around the α-phase (100) peak. It is sharper for the latter one, indicating a better crystalline. In addition, as indicated in Figure 6d, radially integrating intensity plots along the rings at qz = 5.11 nm−1 from the corresponding 2D-SXRD patterns in Figure 6a,b show that the diffraction spots of the 1.2 and 4.8 nm thick CuPc films concentrate in the ranges of 77 to 103° and 86 to 92°, respectively, implying that CuPc molecules in thicker films adopt a more strict edge-on configuration with much higher orientation order. Thus, film thickness plays an important role in the airexposure-induced phase transition. Figure 7 shows a tentative growth model of CuPc on MoS2(0001) at RT and the effect of the following air exposure. For monolayer, because of the van der Waals interaction with MoS2, CuPc molecules maintain the face-on configuration even after air exposure (Figure 7a). For as-deposited thicker films, micrometer flat islands are formed on top of the monolayer with the CuPc molecules in face-on configuration, as shown in Figure 7b, which has been proved by Fukuma et al.36 However, the decreased influence of the substrate and thickness-dependent

Figure 5. (a) AFM image (20 μm × 20 μm) of a 4.8 nm thick CuPc film. (b) Corresponding statistical distribution of domain orientations relative to the horizontal direction.

film. Statistically, the domain (1D nanocrystals) orientations relative to the horizontal direction (fast scanning direction) are concentrated around 39 ± 5, 99 ± 5, and 159 ± 5°, and the corresponding separation between them is ∼60°, as shown in Figure 5b. This can be attributed to the C3v symmetry of MoS2(0001) and an epitaxial relationship between them. In general, CuPc molecules stack parallel to each other and have a strong tendency to grow into slender crystals elongated along the columnar direction (namely, b axis) if there are no obvious interface interactions,38,43−45 for instance, on Si at the growth temperature of ≥410 K where phthalocyanines adopt an edge-on configuration with the b axis parallel to the substrate plane.22 Thus, CuPc molecules in such 1D rod-like crystals, which were observed in ambient, possibly adopt an edge-on configuration in contrast with the previously mentioned LEED results. Although it is reported that iron phthalocyanine (FePc) in similar 1D nanorods and domains on graphene/Ni(111)37 is in face-on configuration, it faces the same problem of in situ D

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Figure 7. Schematic of molecular packing orientations of monolayer (a) and multilayer CuPc on MoS2(0001) before (b) and after (c) air exposure.

CuPc appear. Modes at 1106, 1343, and 1452 cm−1 are assigned to in-plane diag symmetric N−M−N bend, in-plane full symmetric N−C stretch and ring C−C stretch, and in-plane ring symmetric outer ring C−C stretch, respectively.51,52 These peaks are more observable in the Raman spectrum from 4.3 nm thick CuPc on HOPG in Figure 8a. The mode at 1583 cm−1 is assigned to the G band of HOPG.53 Obviously, the intensity of the Raman signals of the 4.3 nm thick CuPc film on HOPG is far stronger than the corresponding peak on MoS2 and in commensurate with that of the 30 nm thick CuPc on MoS2. The corresponding 10 μm × 10 μm AFM image shows that CuPc molecules aggregate into similar rod-like nanocrystals on HOPG (Figure 8b), implying that the phase transition on MoS2 is a common phenomenon for CuPc on layered substrates. The lack of orientation preference is ascribed to the stacking disorder of the HOPG substrate whose LEED pattern is ring-shaped.54 Strong charge transfer and interface dipole−dipole interactions between CuPc and substrate are the two main origins of Raman enhancement effect of the layered substrates.51 The much weaker Raman signals observed on MoS2 for CuPc is mainly attributed to the weaker charge transfer between CuPc and MoS2 caused by its ∼1.3 eV band gap and the weaker dipole−dipole interactions compared with other 2D materials like hexagonal boron nitride (h-BN).51,55 Besides, the stronger reflection of the laser on MoS2 with respect to on HOPG should be taken into consideration.51,55,56 3.5. Influence of Growth Temperature. We also studied the influence of growth temperature on the growth behaviors of CuPc film. Figure 9a−c shows the morphology of 2.4 nm thick CuPc films grown on MoS2 at 330 ± 2, 370 ± 2, and 450 ± 2 K, respectively. CuPc molecules still form rod-like crystals arranged quasi-parallel to each other at 330 K, but the crystals are much shorter (mainly in 0.3 to 0.6 μm) and higher (10−20 nm) compared with those prepared at RT. When the growth temperature is ∼370 K (Figure 9b), two main types of film morphology coexist: rectangular flat islands (left) and irregular porous structure (right). Only a few rod-like crystals exist. The

Ehrlich−Schwoebel barrier as well as the increased roughness (and thus lower mobility and increased density of nucleation sites) jointly lead to a metastable structure.46−50 Upon exposure to ambient air, CuPc molecules, including those in the interface layer, collectively rearrange into edge-on configuration to enhance the π−π intermolecular interactions. The metastable islands further dewet into stable rod-like nanocrystals to release accumulated strain. Because of the strong interaction between MoS2 and first layer of CuPc, the wetting layer with CuPc in the face-on configuration remains on MoS2 without 1D CuPc nanocrystals (Figure 7c). The mechanism on how ambient air causes this phase transition requires further theoretical investigation. 3.4. Raman Spectra. Figure 8a shows the Raman spectra of the MoS2 substrate, 12 and 30 nm thick CuPc films on

Figure 8. (a) Raman spectra of MoS2, 12 and 30 nm thick CuPc films on it, and 4.3 nm thick CuPc film on HOPG. (b) AFM image (10 μm × 10 μm) of the 4.3 nm thick CuPc film on HOPG.

MoS2(0001), as well as 4.3 nm thick CuPc film on HOPG in a range from 1000 to 1700 cm−1. When the thickness of CuPc on MoS2 is