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Synthesis, structure, and optical properties of manganese phthalocyanine thin films and nanostructures
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Original Research
Synthesis, structure, and optical properties of manganese phthalocyanine thin films and nanostructures☆ Lu Menga, Kai Wanga, Yuyan Hanb, Yi Yaoa, Pin Gaoa, Chao Huanga, Wenhua Zhanga, ⁎ Faqiang Xua, a b
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China
A R T I C L E I N F O
A BS T RAC T
Keywords: MnPc Nanostructure OMBD Substrate temperature Optical property
Manganese phthalocyanine (MnPc) nanostructures with different morphologies were prepared on porous anodic alumina oxide (AAO) at different substrate temperature (Ts=50 ℃, 80 ℃, 120 ℃, 180 ℃, 240 ℃) in an organic molecular beam deposition (OMBD) system. The nanostructures morphologies were studied using scanning electron microscopy (SEM) and the results showed that the nanostructures morphologies could be modulated by the control of Ts, as a result, the continuous film was obtained at 50 ℃, whereas the nanorods (NRs), nanoribbons (NBs), nanowires (NWs), nanosheets (NSs) and nanoparticles (NPs) were facilely generated as Ts increased. At the same time, the density and the uniformity of the nanostructures decreased. The results of X-ray diffraction (XRD) indicated that only the β-phase polymorph formed throughout the growth process irrelevant to the Ts. Additionally, the ultraviolet visible (UV–Vis) absorption spectra demonstrated that the main absorption bands of MnPc nanostructures showed a remarkable band broadening as the Ts was increased.
1. Introduction
A recent work demonstrates that NRs, NBs, NWs, NSs and NPs of perylene-3,4,9,10-tetracarboxylic dianhydride have been prepared on porous anodic alumina oxide (AAO) [8]. There are numerous active sites with smaller curvature radius on the AAO surface which can provide nanostructures with high surface energy (Es). And the proposed “site-selective” mechanism has been proved to be suitable for the synthesis of organic nanostructures. In this paper, what we concern most is the modulation of morphology and optical properties corresponding to MnPc nanostructures. Immediately following the synthesis of MnPc NRs, NBs, NWs, NSs, and NPs, the internal correlation among the morphologies, optical properties and Ts is investigated so that a method of preparing MPcs nanostructures with controllable structures is developed. Simultaneously, it has been found that the MnPc nanostructures have excellent crystallinity and enhanced optical properties, which will open new possibilities for the application of organic nanostructures.
In recent years, the functional organic molecules, especially their nanostructures, have attracted considerable attention motivated by their unique excellent optical and electrical properties [1,2]. Organic nanostructures possess some more competitive properties such as relative ease of chemical doping [3,4], good processability [4], high reactivity [5], and high flexibility [6,7] contrast with the inorganic materials due to their weak intermolecular forces (π–π conjugation, van der Waals forces, H-bands and charge-transfer interactions) [8]. Metal phthalocyanine (MPc), one of the most typical functional organic molecules, has become an extremely attractive option for applications in light-emitting diode (OLED) [9–13], organic field-effect transistor (OFET) [14–19], organic solar cells (OSCs) [20–24], and optical waveguides [25], etc. Looking back on the previous works, a good deal of researches had been made on MPcs films to fulfill the requirements for high-performance devices, while few of works focused on the synthesis and modulation of the MPcs nanostructures. Nevertheless, MPcs nanostructures are still the best objects for fulfilling the organic semiconductor devices properties owing to their absence of grain boundaries, perfect molecular order and minimized concentration of charge traps [26].
2. Material and methods 2.1. Synthesis MnPc powder (Sigma-Aldrich, 97%) was purified three times before
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (F. Xu). http://dx.doi.org/10.1016/j.pnsc.2017.04.010 Received 19 December 2016; Received in revised form 10 April 2017; Accepted 14 April 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Meng, L., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.04.010
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
L. Meng et al.
Fig. 1. (a) Cross-sectional schematic diagram of the experiment apparatus. (b) Schematic structure of MnPc molecule.
an ultra-thin layer of Au to avoid charging effect and increase the conductivity of the MnPc nanostructures. The X-ray diffraction (XRD) measurements were carried out on a Rigaku-TTR3, with a Cu Kα source (λ=1.541 Å) to characterize the crystal structure. The ultraviolet visible (UV–Vis) absorption spectra were recorded on a Shimadzu DUV-3700 spectrophotometer. And all the substrate signals have been subtracted during data processing.
the use. The purchased AAO template was selected as the substrate with the pore diameter and depth of 50 nm and 60 µm, respectively. The synthesis of MnPc film and nanostructures were conducted by OMBE method inside an ultrahigh vacuum chamber, in which the base pressure was about 5.0×10−10 Torr. The cross-sectional schematic diagram of the experiment apparatus is shown in Fig. 1(a). First of all, the AAO substrate was transported into the ultrahigh vacuum chamber and annealed at 350 ℃ for 30 min in order to remove the moisture and impurities on the AAO surface, followed by cooled down to the selected Ts. Next, the MnPc powder placed in the evaporator was heated to the evaporation temperature (about 200 ℃). And the MnPc film and nanostructures were deposited on AAO at different Ts, i.e., 50 ℃, 80 ℃, 120 ℃, 140 ℃, 180 ℃ and 200 ℃. The deposition rate was monitored by a quartz crystal microbalance at about 6 Å/min and the working vacuum was maintained at about 2.0×10−8 Torr during the deposition process. The growth time of all samples was 2 h.
3. Results and discussion 3.1. Morphology analysis by SEM The molecular structure of MnPc is demonstrated in Fig. 1(b). The MnPc molecule possesses planar geometry and a very stable πconjugated macrocyclic ligand. An unsaturated transition metal-manganese ion with +2 oxidation state is at the center of the molecule. Fig. 2 shows the SEM images of the MnPc nanostructures prepared at different Ts. The continuous and uniform MnPc film grown at 50 °C was composed of many nanograins with an average dimension of about 70 nm. There were significant similarities between the morphologies of MnPc film and AAO according to Fig. 2(a). Fig. 2(b) shows that when Ts increased to 80 °C, the short NRs with an average diameter of 35 nm
2.2. Characterization The morphology of MnPc nanostructure was examined by scanning electron microscopy (SEM) using a FEI NanoLab 600i SEM/FIB dualbeam system. Prior to the SEM detection the samples were coated with
Fig. 2. Schematic structure of MnPc film and nanostructures formed on AAO at (a) Ts=50 °C, (b) Ts=80 °C, (c) Ts=120 °C, (d) Ts=180 °C, and (e) Ts=240 °C. A partially enlarged image of nanostructures produced at 180 °C is presented in (f). The insets in (a)-(e) are the enlarged images. And in order to observe the morphology feature more clearly, images of (b) and (e) were taken with a 45° tilted angle.
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began to appear, while the average length of the NRs was mere about 150 nm. From the SEM images in Fig. 2(c), we note that many NBs with the length more than 2 µm formed at Ts=120 °C. The NBs had flexible and twisted morphologies and distribute on the AAO densely with uniform width of 60 nm and thickness of 30 nm approximately. It is rather remarkable that NWs, NSs formed and dispersed unevenly on AAO substrate except for some NBs as Ts increases to 180 ℃ according to Fig. 2(d). The length of the NBs was more than 2.5 µm, contrastively, the length of the NWs and NSs ranged from hundreds to thousands of nanometers. Further increasing the substrate temperature to 240 °C which was much higher than the evaporation temperature, no NRs, NBs, NWs, or NSs could be seen on the AAO surface as shown in Fig. 2(e). However, we notice that a few NPs formed at the pore openings of AAO according to the enlarged image. The average diameter of the NPs was about 40 nm. We also take a partially enlarged image of nanostructures produced at 180 °C, in which the nanostructures distributed sparsely as shown in Fig. 2(f) in order to observe the nucleation sites intuitively. It is shown in Fig. 2(a), (b) and (f) that the edge of pore openings, the connection areas of adjacent pores were the prior nucleation sites which are believed to possess high surface energy (Es). This is consistent with the studies in previous reports [27,28]. As discussed above, the morphologies of the MnPc nanostructures are primarily dependent on the substrate temperature and the surface energy. At the lower Ts=50 °C, the possibilities of deposition are equal to all MnPc molecules regardless of the restrictions of Es. As a consequence, the MnPc molecules tend to nucleate easily on the surface of AAO, simultaneously, the newly formed small crystal nucleus grown together to form a uniform and continuous film which duplicates the morphologies of AAO in a great degree. With an increase in Ts, the vapored MnPc molecules prefer to deposit on the active sites of AAO and nucleate there to become the new active site so that the newly vapored molecules can grow on them. As Ts=80 °C, the MnPc molecules nucleate on the active sites and extend to form the short NRs gradually. While as Ts=120 °C, the size of crystal nucleus increases corresponding to the rise of Ts. The adjacent crystal nucleus coalesce consequently and grow into NBs. As Ts further increases to 180 °C, the active sites with higher Es become the preferential choice for MnPc molecules to nucleate. Meanwhile, the amount of crystal nucleus decrease which results in a consequence that only a handful of crystal nucleus can grow into sparse nanostructures and disperse unevenly on AAO substrate. As Ts=240 °C, the formation of the NPs decrease the curvature radius of the active sites so that the newly vapored MnPc molecules can not deposit on them and further extend to form the NRs, NBs, NWs, or NSs.
Fig. 3. XRD patterns of MnPc film and nanostructures prepared at different Ts. The background intensity is mainly caused by the X-ray scattering of AAO substrate.
3.3. Optical property analysis of the MnPc nanostructures The UV–Vis absorption spectra were measured with the purpose of researching the optical properties of the MnPc film and nanostructures with multiple morphologies synthesized at different Ts and the results are presented in Fig. 4. All of the samples had three absorption peaks at about 300 nm, 500 nm and 750 nm, respectively. The absorption bands in the range of 250–350 nm are called the B or Soret bands originated from the π → π* electron transitions, specifically the a2u(π) → eg(π*) transitions. And it can easily be seen that the B bands at different Ts are located at constant energy positions, which indicates the a2u(π) → eg(π*) electron transition is the intrinsic properties of MnPc and independent from the morphologies of the nanostructures. Analogously, the predominant absorption bands ranged from 710 nm to 890 nm are called Q bands [33] and arose from the a1u(π) → eg(π*) electron transitions. We notice that the Q bands displayed a remarkable broadening as the Ts is increased from 50 °C to 240 °C and showed a significant red-shift compared with the H2Pc and most of the MPcs molecules whose Q bands of absorption are located below 686 nm [34]. The absorption bands located in the region between 470 nm and 570 nm are induced by the charge-transfer (CT) exciton owing to the existence of unsaturated manganese ions at the molecular center, and
3.2. Crystal structure analysis by XRD The crystal structure of the MnPc film and nanostructures grown at different Ts were characterized by XRD, with a Cu Kα source, and the results are shown in Fig. 3. From the XRD spectra it may be concluded that the prepared MnPc nanostructures can be indexed as the β-phase of MnPc based on the previous reports [29–31]. All of the samples have diffraction peaks located at 5.98°, which are associated with (10-2) plane of the β-phase MnPc crystals. As Ts increased to 180 °C, the (213) diffraction peaks appear with the peak broadening at 2θ=15.00°, 15.48°, respectively. The peak broadening was caused by the decreased lateral dimension of the nanostructures as Ts increased. Concurrently, another stronger (10-2) diffraction peak was observed at 2θ=6.88°, suggesting that (10-2) plane is the preferential orientation lattice plane for MnPc nanostructures at 180 °C. And the appearance of newly induced peaks corresponds to the formation of MnPc nanostructures shown in Fig. 2(d). Upon enhancing Ts further to 240 °C, the (21-3) diffraction peaks disappeared and the intensity of the (10-2) diffraction peak weakened, which accords well with the results shown in Fig. 2(e). In summary, the XRD observations manifest that the MnPc nanostructures prepared at different Ts have relatively good crystallinity and the β-phase polymorph is the thermodynamically stable structure for MnPc, which is consistent with the opinion of Uyeda and Ashida [32].
Fig. 4. UV–Vis absorption spectra of MnPc film and nanostructures prepared at Ts=50 °C, 80 °C, 120 °C, 180 °C and 240 °C. The dashed lines are given to label the position of the absorption bands.
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References
they exhibited the band broadening and red-shift effect with the increase of Ts. As a common sense, the CT states can be critically affected by the size and crystal quality of the molecular aggregates, and the virtual orbital overlap between the adjacently stacked molecules [8]. As the Ts increases, the crystal quality of the nanostructures is improved and the intermolecular overlap of MnPc molecules is enhanced, which results in the band broadening and red-shift effect of the absorption bands. As we can see, the main absorption bands of MnPc nanostructures were located in the range of 710–890 nm. And it is well known that more than sixty percent of the total solar photon flux is at wavelengths beyond 600 nm with approximately fifty percent at wavelengths of 600–1000 nm [35]. Therefore the MnPc nanostructures with relatively good crystallinity and enhanced optical properties should be of great significance for the better application of photodetectors and photovoltaic cells, which can make the most of solar photons.
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4. Conclusions MnPc film and nanostructures with relatively good crystallinity on AAO substrate have been prepared using OMBD method, and the morphologies modulated by controlling the substrate temperature. The continuous film is formed at about 50 ℃, whereas NRs, NBs, NWs, NSs and NPs are sequentially generated as Ts increases from 80 ℃ to 240 ℃. The nanostructures distribute on the substrate sparsely and unevenly with the gradual increase of Ts as a result of the “siteselective” nucleation mechanism. All the nanostructures exhibit only the β-phase polymorph as shown in XRD spectra. The UV–Vis absorption spectra demonstrate that the main absorption bands of MnPc film and nanostructures are originated from π → π* electron transition and show a remarkable broadening as the Ts is increased. Additionally, the absorption bands induced by CT exciton also demonstrate a band broadening and a slight red-shift effect accompanied with Ts going up. It has been shown that the MnPc nanostructures may offer significant advantages in devices of photodetectors and photovoltaic cells with eminent optical properties, and will open new possibilities for the application of organic nanostructures. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (grant numbers: U1232137, 11575187) , National Key Research and Developmemnt Program (grant number: 2016YFB0700205) and Scientific Research Grant of Hefei Science Center of CAS 2015SRG-HSC032.
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Contents lists available at ScienceDirect
HOSTED BY
Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Original Research
Synthesis, structure, and optical properties of manganese phthalocyanine thin films and nanostructures☆ Lu Menga, Kai Wanga, Yuyan Hanb, Yi Yaoa, Pin Gaoa, Chao Huanga, Wenhua Zhanga, ⁎ Faqiang Xua, a b
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, China
A R T I C L E I N F O
A BS T RAC T
Keywords: MnPc Nanostructure OMBD Substrate temperature Optical property
Manganese phthalocyanine (MnPc) nanostructures with different morphologies were prepared on porous anodic alumina oxide (AAO) at different substrate temperature (Ts=50 ℃, 80 ℃, 120 ℃, 180 ℃, 240 ℃) in an organic molecular beam deposition (OMBD) system. The nanostructures morphologies were studied using scanning electron microscopy (SEM) and the results showed that the nanostructures morphologies could be modulated by the control of Ts, as a result, the continuous film was obtained at 50 ℃, whereas the nanorods (NRs), nanoribbons (NBs), nanowires (NWs), nanosheets (NSs) and nanoparticles (NPs) were facilely generated as Ts increased. At the same time, the density and the uniformity of the nanostructures decreased. The results of X-ray diffraction (XRD) indicated that only the β-phase polymorph formed throughout the growth process irrelevant to the Ts. Additionally, the ultraviolet visible (UV–Vis) absorption spectra demonstrated that the main absorption bands of MnPc nanostructures showed a remarkable band broadening as the Ts was increased.
1. Introduction
A recent work demonstrates that NRs, NBs, NWs, NSs and NPs of perylene-3,4,9,10-tetracarboxylic dianhydride have been prepared on porous anodic alumina oxide (AAO) [8]. There are numerous active sites with smaller curvature radius on the AAO surface which can provide nanostructures with high surface energy (Es). And the proposed “site-selective” mechanism has been proved to be suitable for the synthesis of organic nanostructures. In this paper, what we concern most is the modulation of morphology and optical properties corresponding to MnPc nanostructures. Immediately following the synthesis of MnPc NRs, NBs, NWs, NSs, and NPs, the internal correlation among the morphologies, optical properties and Ts is investigated so that a method of preparing MPcs nanostructures with controllable structures is developed. Simultaneously, it has been found that the MnPc nanostructures have excellent crystallinity and enhanced optical properties, which will open new possibilities for the application of organic nanostructures.
In recent years, the functional organic molecules, especially their nanostructures, have attracted considerable attention motivated by their unique excellent optical and electrical properties [1,2]. Organic nanostructures possess some more competitive properties such as relative ease of chemical doping [3,4], good processability [4], high reactivity [5], and high flexibility [6,7] contrast with the inorganic materials due to their weak intermolecular forces (π–π conjugation, van der Waals forces, H-bands and charge-transfer interactions) [8]. Metal phthalocyanine (MPc), one of the most typical functional organic molecules, has become an extremely attractive option for applications in light-emitting diode (OLED) [9–13], organic field-effect transistor (OFET) [14–19], organic solar cells (OSCs) [20–24], and optical waveguides [25], etc. Looking back on the previous works, a good deal of researches had been made on MPcs films to fulfill the requirements for high-performance devices, while few of works focused on the synthesis and modulation of the MPcs nanostructures. Nevertheless, MPcs nanostructures are still the best objects for fulfilling the organic semiconductor devices properties owing to their absence of grain boundaries, perfect molecular order and minimized concentration of charge traps [26].
2. Material and methods 2.1. Synthesis MnPc powder (Sigma-Aldrich, 97%) was purified three times before
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (F. Xu). http://dx.doi.org/10.1016/j.pnsc.2017.04.010 Received 19 December 2016; Received in revised form 10 April 2017; Accepted 14 April 2017 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Meng, L., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.04.010
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
L. Meng et al.
Fig. 1. (a) Cross-sectional schematic diagram of the experiment apparatus. (b) Schematic structure of MnPc molecule.
an ultra-thin layer of Au to avoid charging effect and increase the conductivity of the MnPc nanostructures. The X-ray diffraction (XRD) measurements were carried out on a Rigaku-TTR3, with a Cu Kα source (λ=1.541 Å) to characterize the crystal structure. The ultraviolet visible (UV–Vis) absorption spectra were recorded on a Shimadzu DUV-3700 spectrophotometer. And all the substrate signals have been subtracted during data processing.
the use. The purchased AAO template was selected as the substrate with the pore diameter and depth of 50 nm and 60 µm, respectively. The synthesis of MnPc film and nanostructures were conducted by OMBE method inside an ultrahigh vacuum chamber, in which the base pressure was about 5.0×10−10 Torr. The cross-sectional schematic diagram of the experiment apparatus is shown in Fig. 1(a). First of all, the AAO substrate was transported into the ultrahigh vacuum chamber and annealed at 350 ℃ for 30 min in order to remove the moisture and impurities on the AAO surface, followed by cooled down to the selected Ts. Next, the MnPc powder placed in the evaporator was heated to the evaporation temperature (about 200 ℃). And the MnPc film and nanostructures were deposited on AAO at different Ts, i.e., 50 ℃, 80 ℃, 120 ℃, 140 ℃, 180 ℃ and 200 ℃. The deposition rate was monitored by a quartz crystal microbalance at about 6 Å/min and the working vacuum was maintained at about 2.0×10−8 Torr during the deposition process. The growth time of all samples was 2 h.
3. Results and discussion 3.1. Morphology analysis by SEM The molecular structure of MnPc is demonstrated in Fig. 1(b). The MnPc molecule possesses planar geometry and a very stable πconjugated macrocyclic ligand. An unsaturated transition metal-manganese ion with +2 oxidation state is at the center of the molecule. Fig. 2 shows the SEM images of the MnPc nanostructures prepared at different Ts. The continuous and uniform MnPc film grown at 50 °C was composed of many nanograins with an average dimension of about 70 nm. There were significant similarities between the morphologies of MnPc film and AAO according to Fig. 2(a). Fig. 2(b) shows that when Ts increased to 80 °C, the short NRs with an average diameter of 35 nm
2.2. Characterization The morphology of MnPc nanostructure was examined by scanning electron microscopy (SEM) using a FEI NanoLab 600i SEM/FIB dualbeam system. Prior to the SEM detection the samples were coated with
Fig. 2. Schematic structure of MnPc film and nanostructures formed on AAO at (a) Ts=50 °C, (b) Ts=80 °C, (c) Ts=120 °C, (d) Ts=180 °C, and (e) Ts=240 °C. A partially enlarged image of nanostructures produced at 180 °C is presented in (f). The insets in (a)-(e) are the enlarged images. And in order to observe the morphology feature more clearly, images of (b) and (e) were taken with a 45° tilted angle.
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began to appear, while the average length of the NRs was mere about 150 nm. From the SEM images in Fig. 2(c), we note that many NBs with the length more than 2 µm formed at Ts=120 °C. The NBs had flexible and twisted morphologies and distribute on the AAO densely with uniform width of 60 nm and thickness of 30 nm approximately. It is rather remarkable that NWs, NSs formed and dispersed unevenly on AAO substrate except for some NBs as Ts increases to 180 ℃ according to Fig. 2(d). The length of the NBs was more than 2.5 µm, contrastively, the length of the NWs and NSs ranged from hundreds to thousands of nanometers. Further increasing the substrate temperature to 240 °C which was much higher than the evaporation temperature, no NRs, NBs, NWs, or NSs could be seen on the AAO surface as shown in Fig. 2(e). However, we notice that a few NPs formed at the pore openings of AAO according to the enlarged image. The average diameter of the NPs was about 40 nm. We also take a partially enlarged image of nanostructures produced at 180 °C, in which the nanostructures distributed sparsely as shown in Fig. 2(f) in order to observe the nucleation sites intuitively. It is shown in Fig. 2(a), (b) and (f) that the edge of pore openings, the connection areas of adjacent pores were the prior nucleation sites which are believed to possess high surface energy (Es). This is consistent with the studies in previous reports [27,28]. As discussed above, the morphologies of the MnPc nanostructures are primarily dependent on the substrate temperature and the surface energy. At the lower Ts=50 °C, the possibilities of deposition are equal to all MnPc molecules regardless of the restrictions of Es. As a consequence, the MnPc molecules tend to nucleate easily on the surface of AAO, simultaneously, the newly formed small crystal nucleus grown together to form a uniform and continuous film which duplicates the morphologies of AAO in a great degree. With an increase in Ts, the vapored MnPc molecules prefer to deposit on the active sites of AAO and nucleate there to become the new active site so that the newly vapored molecules can grow on them. As Ts=80 °C, the MnPc molecules nucleate on the active sites and extend to form the short NRs gradually. While as Ts=120 °C, the size of crystal nucleus increases corresponding to the rise of Ts. The adjacent crystal nucleus coalesce consequently and grow into NBs. As Ts further increases to 180 °C, the active sites with higher Es become the preferential choice for MnPc molecules to nucleate. Meanwhile, the amount of crystal nucleus decrease which results in a consequence that only a handful of crystal nucleus can grow into sparse nanostructures and disperse unevenly on AAO substrate. As Ts=240 °C, the formation of the NPs decrease the curvature radius of the active sites so that the newly vapored MnPc molecules can not deposit on them and further extend to form the NRs, NBs, NWs, or NSs.
Fig. 3. XRD patterns of MnPc film and nanostructures prepared at different Ts. The background intensity is mainly caused by the X-ray scattering of AAO substrate.
3.3. Optical property analysis of the MnPc nanostructures The UV–Vis absorption spectra were measured with the purpose of researching the optical properties of the MnPc film and nanostructures with multiple morphologies synthesized at different Ts and the results are presented in Fig. 4. All of the samples had three absorption peaks at about 300 nm, 500 nm and 750 nm, respectively. The absorption bands in the range of 250–350 nm are called the B or Soret bands originated from the π → π* electron transitions, specifically the a2u(π) → eg(π*) transitions. And it can easily be seen that the B bands at different Ts are located at constant energy positions, which indicates the a2u(π) → eg(π*) electron transition is the intrinsic properties of MnPc and independent from the morphologies of the nanostructures. Analogously, the predominant absorption bands ranged from 710 nm to 890 nm are called Q bands [33] and arose from the a1u(π) → eg(π*) electron transitions. We notice that the Q bands displayed a remarkable broadening as the Ts is increased from 50 °C to 240 °C and showed a significant red-shift compared with the H2Pc and most of the MPcs molecules whose Q bands of absorption are located below 686 nm [34]. The absorption bands located in the region between 470 nm and 570 nm are induced by the charge-transfer (CT) exciton owing to the existence of unsaturated manganese ions at the molecular center, and
3.2. Crystal structure analysis by XRD The crystal structure of the MnPc film and nanostructures grown at different Ts were characterized by XRD, with a Cu Kα source, and the results are shown in Fig. 3. From the XRD spectra it may be concluded that the prepared MnPc nanostructures can be indexed as the β-phase of MnPc based on the previous reports [29–31]. All of the samples have diffraction peaks located at 5.98°, which are associated with (10-2) plane of the β-phase MnPc crystals. As Ts increased to 180 °C, the (213) diffraction peaks appear with the peak broadening at 2θ=15.00°, 15.48°, respectively. The peak broadening was caused by the decreased lateral dimension of the nanostructures as Ts increased. Concurrently, another stronger (10-2) diffraction peak was observed at 2θ=6.88°, suggesting that (10-2) plane is the preferential orientation lattice plane for MnPc nanostructures at 180 °C. And the appearance of newly induced peaks corresponds to the formation of MnPc nanostructures shown in Fig. 2(d). Upon enhancing Ts further to 240 °C, the (21-3) diffraction peaks disappeared and the intensity of the (10-2) diffraction peak weakened, which accords well with the results shown in Fig. 2(e). In summary, the XRD observations manifest that the MnPc nanostructures prepared at different Ts have relatively good crystallinity and the β-phase polymorph is the thermodynamically stable structure for MnPc, which is consistent with the opinion of Uyeda and Ashida [32].
Fig. 4. UV–Vis absorption spectra of MnPc film and nanostructures prepared at Ts=50 °C, 80 °C, 120 °C, 180 °C and 240 °C. The dashed lines are given to label the position of the absorption bands.
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References
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4. Conclusions MnPc film and nanostructures with relatively good crystallinity on AAO substrate have been prepared using OMBD method, and the morphologies modulated by controlling the substrate temperature. The continuous film is formed at about 50 ℃, whereas NRs, NBs, NWs, NSs and NPs are sequentially generated as Ts increases from 80 ℃ to 240 ℃. The nanostructures distribute on the substrate sparsely and unevenly with the gradual increase of Ts as a result of the “siteselective” nucleation mechanism. All the nanostructures exhibit only the β-phase polymorph as shown in XRD spectra. The UV–Vis absorption spectra demonstrate that the main absorption bands of MnPc film and nanostructures are originated from π → π* electron transition and show a remarkable broadening as the Ts is increased. Additionally, the absorption bands induced by CT exciton also demonstrate a band broadening and a slight red-shift effect accompanied with Ts going up. It has been shown that the MnPc nanostructures may offer significant advantages in devices of photodetectors and photovoltaic cells with eminent optical properties, and will open new possibilities for the application of organic nanostructures. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (grant numbers: U1232137, 11575187) , National Key Research and Developmemnt Program (grant number: 2016YFB0700205) and Scientific Research Grant of Hefei Science Center of CAS 2015SRG-HSC032.
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