Article pubs.acs.org/JPCC
Noncovalent Molecular Heterojunction: Structure Determination and Property Characterization using Scanning Tunneling Microscopy/ Spectroscopy and Theoretical Calculations Xiao-Ling Duan, Rui-Hua Zhao, Zhi-Fei Liu, Zhi-Yong Yang,* and Zhi-Xiang Wang School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, 19A Yuquanlu, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: Noncovalent molecular heterojunctions (MHJs) formed by the stacking of p-type (electron donor) and n-type (electron acceptor) compounds are essentially important for various optoelectronics as well as molecular devices. Herein we report the construction of the fluorinated copper(II) phthalocyanine (F16CuPc; n-type, acceptor)−polymer NTZ12 (p-type, donor) MHJ via annealing of the film of F16CuPc and NTZ12. Scanning tunneling microscopy and density functional theory calculations validate the formation of the F16CuPc−NTZ12 MHJ at the single molecular level. The constructed MHJ shows a distinctive rectifying effect in the statistical data of scanning tunneling spectroscopy because of the different barriers in two directions of electron tunneling. In the proof-of-concept photocurrent tests, the F16CuPc−NTZ12 MHJ produces much higher current under radiation than in the dark due to its well-organized donor−acceptor interface formed by annealing, whereas the current of the unannealed samples shows almost no response to radiation. The unveiled efficient construction approach, structure, and electronic properties of the MHJ in this study could greatly help the development of molecular devices. More importantly, our results provide direct evidence at the single molecular level for probing the intrinsic mechanism of improving the performances of varied photovoltaic cells by heating treatment. This mechanism is not completely understood as yet; there is a lack of understanding especially at the molecular level. The results in this paper fill this gap very well and, thus, are of significant importance for the development of related organic devices.
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transferring effect.8 Although several systems have been reported, the studies on MHJs are still rather limited, mainly because the construction of MHJs is far from rational. Here, we successfully fabricated the fluorinated copper(II) phthalocyanine (F16CuPc; n-type)−polymer NTZ12 (p-type) MHJ via annealing of the as-cast samples. The structure of the F16CuPc−NTZ12 MHJ was confirmed at the molecular level through STM and density functional theory (DFT) calculations. The fabricated MHJ demonstrates a prominent rectification effect in the statistical data of STS. In the proofof-concept photocurrent tests, the F16CuPc−NTZ12 MHJ produces much higher current under radiation than in the dark, whereas the current of the unannealed samples shows almost no response to radiation, indicating the superior optoelectronic nature of the formed F16CuPc−NTZ12 MHJ in the annealed samples. Numerous reports have shown that heating treatment can greatly improve the performances of organic photovoltaic devices.9,10 Thus, the findings in this study provide important evidence for understanding the influences of heating treatment on the performances of these devices. Our study also suggests that annealing could be an effective way of constructing MHJs,
INTRODUCTION The noncovalent molecular heterojunctions (MHJs) formed by p-type (electron donor) and n-type (electron acceptor) molecules are crucial units in various organic devices and molecular electronics such as solar cells, light-emitting diodes, ambipolar field-effect transistors, molecular rectifiers, and so on, because they have vital influences on the properties of charge carriers, including charge carrier production, separation, recombination, transportation, etc.1−3 To develop highperformance organic devices as well as molecular electronics, it is important to construct favorable MHJs and characterize their structures and electronic properties at the molecular level. The pentacene−C60 heterojunction on Cu(111) is determined by cryogenic scanning tunneling microscopy (STM), and the collected scanning tunneling spectroscopy (STS) curves demonstrate that the rectification ratio can be larger than 1000.4 Besides pentacene, C60 can form heterojunctions with double-decker porphyrins as well. On the basis of their STS data, the authors inferred that the coupling between the toplayer C60 and Ag(111) substrate is largely reduced.5 In addition to the vacuum environment, the C60−cyclothiophene and C60− shape-persistent macrocycle MHJs were also constructed at the interface of the solution/graphite substrate.6,7 In an early report, the anthraquinone−anthracene MHJ was shown to act as a nanogate of a single molecular device because of its charge© XXXX American Chemical Society
Received: March 20, 2017 Revised: May 19, 2017
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DOI: 10.1021/acs.jpcc.7b02650 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DFT Calculations. DFT calculations were performed at the levels of B3LYP/6-31g** (NTZ12) and B3LYP/6-31g* (F16CuPc, pseudopotential LANL2DZ; F16CuPc−NTZ12 heterojunction, pseudopotential LANL2DZ and empirical dispersion correction) with the Gaussian 09 package.20 The monomer of NTZ12 was first optimized, and the conformation with lowest energy was used to construct the polymer models. To reduce the computing cost to an acceptable level, (1) the side alkyl chains of NTZ12 were replaced with methyl groups, (2) the graphite substrate was not involved in the optimization, and (3) NTZ12 (n = 2) was used to construct the models of the F16CuPc−NTZ12 heterojunction. Photocurrent Tests. Fluorine-doped tin oxide (FTO) glass covered the top of the respective sample. The fiber end of the light source (output light power 400 mW/cm2, CEL-TCX250, Ceaulight Co.) was fixed about 1 cm above the top surface of the FTO-covered samples. All of the photocurrent tests were performed using a Keithley 2612B source meter and a probe station with a shielding box (ZFT-50-T, Titanspower Co.) at room temperature in ambient conditions. Silver paste was used as the electric contact between the FTO and metal probe. A 2 mV potential was applied to the graphite substrate during the current tests. The data collection time for a single curve is 2.4 s, except the time of the light-on−off curve is 24 s. Each curve given in the main text was produced by averaging 4−5 single testing curves, but the light-on−off curve was not averaged.
which is significant support for boosting the investigations on MHJs and other molecular electronics in the future.
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EXPERIMENTAL AND CALCULATION METHODS STM Experiments. NTZ12 was synthesized according to the previous report.11 Chlorobenzene and n-type F16CuPc were purchased from Sigma-Aldrich Co. and used as received. For the samples containing both NTZ12 and F16CuPc, these two components were codissolved in chlorobenzene under the assistance of sonication. The unannealed samples were prepared by depositing the chlorobenzene solution on the surface of freshly cleaved highly oriented pyrolytic graphite (HOPG; SPI Co.). After chlorobenzene vaporized completely at room temperature, the unannealed samples were first examined by STM. Then they were annealed for 30 min at 190 °C in a tube furnace under the protection of N2 and cooled to room temperature naturally before being taken out for further STM/STS experiments.11 All of the STM and STS data were collected in ambient conditions with a Picoscan 2100 (Agilent Co.) scanning tunneling microscope. Bias is applied to the mechanically cut Pt/Ir (90%/10%, Goodfellow Co.) tip during the experiments. WSxM 4.0 was used to analyze the acquired STM images.12,13 STS Experiments. The STM feedback loop was turned off when the current (I)−voltage (V) data were collected. The (dI/dV)−V curve was acquired through differentiation of the original I−V spectroscopy in Origin software and was smoothed by averaging nearby points to reduce random noises or artificial mathematical errors.14,15 The process of determining the inflections of each (dI/dV)−V curve can be found in previous reports.16,17 When the STS data of the F16CuPc−NTZ12 heterojunction were collected, only the central parts of ordered domains were used because the central parts are more stable than the areas of the boundaries or near boundaries during the STS measurements. To prepare the samples for taking the STS data of the NTZ12 backbones, only NTZ12 was dissolved in chlorobenzene. The NTZ12 samples were prepared and annealed according to the above-mentioned process before the STM/ STS experiments were performed. The large-ordered NTZ12 domains produced through heating treatment make the STS measurements more reliable. The features of the NTZ12 assembly after annealing were given in our previous report.11 To prepare the samples for collecting the STS data of F16CuPc, octadecanethiol (Sigma-Aldrich Co.) and F16CuPc were codispersed in chlorobenzene. Octadecanethiol was used to stabilize the F16CuPc domains as reported in our previous research.18 The preparation and annealing processes of the F16CuPc samples were the same to those of the NTZ12 samples. The typical close-packed domains of F16CuPc in the annealed samples are shown in Figure S1 (Supporting Information). Besides the STS data of the F16CuPc−NTZ12 heterojunction, F16CuPc, and NTZ12, the I−V curves of pristine graphite were also obtained as the control experiment. The I−V curves of pristine graphite show no or an extremely small (usually narrower than 100 mV) zero-current range, and the corresponding (dI/dV)−V curves are of a parabola shape.16,17 When the STS results of the F16CuPc−NTZ12 heterojunction, F16CuPc, and NTZ12 were analyzed, those showing graphite features were discarded reasonably.19
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RESULTS AND DISCUSSION Although F16CuPc (Figure 1a, top, n-type molecule) and NTZ12 (Figure 1a, bottom, p-type molecule) were codeposited
Figure 1. (a) Structural model of F16CuPc (top) and NTZ12 (bottom). (b) STM image of the unannealed samples. Tunneling conditions: Vbias = −1.3 V, Itip = 0.027 nA.
on the graphite surface from solution, only NTZ12 was observed in the repeating STM experiments of the unannealed samples, as shown by the typical image of Figure 1b. The small and defective NTZ12 domains revealed here are similar to those we reported previously.11 When imaging the unannealed samples, we tried to use parameters similar to those of the images of the F16CuPc−NTZ12 MHJ in Figure 2 (see the following results). In this case, the STM tip is intended to be set in a position very far from the surface because of the extreme low current set point and large bias, which avoids the concern that the top F16CuPc in the unannealed films is not imaged or swiped off because the tip is too close to the surface. However, the extremely low current set point and large bias deteriorate the quality of Figure 1b heavily since the STM tip is too far away from the surface. B
DOI: 10.1021/acs.jpcc.7b02650 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C After the samples were annealed at 190 °C for 30 min, the surface features unveiled by the typical STM image (Figure 2a)
polymers.21 In the ordered F16CuPc domains, the average distance between the neighbor molecular chains is determined to be 2.4 ± 0.2 nm, roughly equal to that of two neighbor NTZ12 backbones given in our previous report.11 Thus, the STM results disclose two facts: first, F16CuPc chains follow the folding path and assembling periodicity of the NTZ12 backbones, and second, NTZ12 is not observed under the imaging conditions similar to those of Figure 2a. On the basis of these two facts, we deduced that F16CuPc adsorbs on the top of the NTZ12 backbones, forming the F16CuPc−NTZ12 MHJ. To verify the proposed MHJ, both the top F16CuPc and bottom NTZ12 were recorded in one STM image through altering tip− sample bias. In Figure 2b, the imaging bias was reduced from −1.3 to −0.75 V abruptly at the position marked by the black arrow. The F16CuPc domains unveiled with high bias are similar to those in Figure 2a, whereas in the area of low bias the bright strips in the linear or varied bent shapes are definitely the NTZ12 skeletons, indicating that NTZ12 is indeed the bottom layer. Then the only criterion left to confirm the suggested MHJ is to show that F16CuPc sits on the backbones, rather than the side alkyl chains of NTZ12. At the position of the bias jumping, the interface of the top F16CuPc and bottom NTZ12 is recorded clearly in Figure 2b. Carefully examining the ordered F16CuPc chains and the revealed NTZ12 backbones near the interface (marked with the dashed rectangle), it can be found that the center of each F16CuPc chain faces the backbone of the underlying NTZ12 exactly. Therefore, the stacking of the F16CuPc−NTZ12 MHJ is proved initially. Due to the extreme significance of the stacking position, further evidence was acquired to corroborate the proposed MHJ. The NTZ12 backbones covered by the isolated F16CuPc chains (L1−L3 in Figure 2c) are exposed clearly in the lower part of Figure 2d. C1 and C2 in Figure 2d mark two F16CuPc circles recorded with higher bias (the bottom NTZ12 in C1 and C2 is screened), whereas C3 in the area of low scanning bias is the circle of the NTZ12 backbone. In Figure 2e, the screened NTZ12 skeletons at C1 and C2 are uncovered with lower bias. Meanwhile, the top F16CuPc molecules at C3 are imaged with higher bias. The processes shown in Figure 2b−e validate the formation of the F16CuPc−NTZ12 MHJ definitely. Very occasionally, F16CuPc molecules were observed sitting on the alkyl chains of NTZ12, as the STM image of Figure S2a (Supporting Information) shows. In this kind of structure, the backbones of NTZ12 and chains of F16CuPc can be recorded easily with the same set of tunneling conditions, and two kinds of molecular lines appear in the STM images alternatively. Thus, our conclusion that the structure shown in Figure 2a is the F16CuPc−NTZ12 MHJ is supported by Figure S2a from the other side. To further understand the obtained heterojunction, we carried out DFT calculations. F16CuPc and NTZ12 (n = 2) were optimized separately (Figure S2b, Supporting Information) before they were used to construct the MHJ models. There are three types of aromatic groups (naphthalene, thiophene, and thiazolothiazole) in NTZ12; accordingly, three initial adsorbing positions were considered by centering F16CuPc on these aromatic units (Figure S2c, top, naphthalene; middle, thiophene; bottom, thiazolothiazole). DFT calculations demonstrate that all of these initial arrangements lead to the corresponding stable structures, as shown in the top, middle, and bottom of Figure 2f (the large-size version is given in Figure S2d). F16CuPc is about 0.32 nm away from NTZ12 in the top and middle structures of Figure 2f. The distance
Figure 2. (a) STM image of the F16CuPc−NTZ12 MHJ. Tunneling conditions: Vbias = −1.30 V, Itip = 0.027 nA. Scale bar = 20 nm. (b−e) STM image of the F16CuPc−NTZ12 MHJ. Imaging the top F16CuPc and bottom NTZ12 simultaneously through changing the tunneling parameters. (b) Scale bar = 14 nm. (c−e) Scale bar = 20 nm. Tunneling conditions: (b) upper part, Vbias = −1.30 V, Itip = 0.039 nA; lower part, Vbias = −0.75 V, Itip = 0.039 nA; (c) Vbias = −1.30 V, Itip = 0.033 nA; (d) upper part, Vbias = −1.30 V, Itip = 0.033 nA; lower part, Vbias = −0.75 V, Itip = 0.033 nA; (e) upper part, Vbias = −0.75 V, Itip = 0.033 nA; lower part, Vbias = −1.30 V, Itip = 0.033 nA. (f) Three DFToptimized structures of the F16CuPc−NTZ12 MHJ. The top F16CuPc is colored violet for clarity. The large-size version of (f) is provided in Figure S2d (Supporting Information).
are entirely different from those of the unannealed samples, indicating a revolutionary change in the surface structures. Small dots are the basic units of the surface film, rather than the long strips shown in Figure 1b. Considering the geometrical frames of the two components in our system (Figure 1a), these dots can be safely assigned to the F16CuPc molecules.18 The size of the ordered F16CuPc domains varies in the range of tens of nanometers. The space between the well-patterned domains is usually taken by the randomly adsorbed molecules or isolated short chains of F16CuPc. Some F16CuPc chains are bent to the hairpin or circle conformation, as pointed out by the solid and dashed arrows in Figure 2a, respectively. These two folding styles are the typical bendings of NTZ1211 and other C
DOI: 10.1021/acs.jpcc.7b02650 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 3. Typical I−V curves (a), corresponding (dI/dV)−V curves (b), and inflection histograms (c) of the F16CuPc−NTZ12 MHJ, F16CuPc, and NTZ12. (d) Illustration of the process of electron tunneling through the F16CuPc−NTZ12 MHJ under the negative and positive tip biases. The linear solid arrow marks the tendency of electron donation from NTZ12 to F16CuPc.
properties of NTZ12. Thus, the STS results of F16CuPc and NTZ12 validate that the STS measurements in our experiments indeed disclose the intrinsic properties of the tested sample. This validation is the key basis for the use of STS to investigate our MJH. For the F16CuPc−NTZ12 heterojunction, Figure 3c unveils that the left inflection is located at −1.9 eV and the right one is around 1.6 eV. The STS gap of the F16CuPc−NTZ12 MHJ is larger than that of F16CuPc or NTZ12. A similar phenomenon was also found in a former study on the C60− cyclothiophene heterojunction.6 In that research, the STS gap of the C60−cyclothiophene heterojunction is also larger than that of the cyclothiophene molecules. More experimental and theoretical works need to be done in the future to figure out the intrinsic tunneling mechanism of varied noncovalent MJHs (but are beyond the scope of this paper). Meanwhile, the STS data of the F16CuPc−NTZ12 MHJ in Figure 3c imply that the turn-on voltage in the region of positive tip bias is smaller, which suggests that the preferred path of electron flow in the F16CuPc−NTZ12 MHJ is from the substrate to the donor molecule NTZ12, to the acceptor molecule F16CuPc, and to the tip. In a previous study,25 the heterojunction was constructed with a substrate-adsorbed porphyrin and a tip-attached fullerene. The STS results of that study indicate that the favorable direction of electron flow is from the donor compound porphyrin to the acceptor molecule fullerene as well. The features displayed in the STS curves of the F16CuPc− NTZ12 MHJ can be understood qualitatively with the schematic illustrations in Figures 3d and S3b. On the basis of the information given by the DFT calculations, the energy levels of the frontier orbitals of NTZ12 (n = 2) and F16CuPc are aligned in Figure S3b, which clearly demonstrates the
between the two components in the bottom structure is a little shorter, roughly 0.31 nm. Accordingly, the energy of this structure is slightly lower than that of the other two. After validating the structure of the F16CuPc−NTZ12 MHJ in the annealed samples, we next investigated its electronic properties through STS experiments. One typical I−V curve of the F16CuPc−NTZ12 MHJ is given in Figure 3a, showing an asymmetrical feature, as it can be seen that the tunneling current increases more rapidly when the tip is positively biased. For comparison, I−V curves were also collected on the samples only containing F16CuPc or NTZ12. The representative curves of F16CuPc and NTZ12 are included in Figure 3a as well. F16CuPc produces a larger current in the area of negative tip bias, whereas NTZ12 behaves reversely. In contrast to the spectra of F16CuPc and NTZ12, the MJH curve shows a wider zero-current range. The original I−V curves were differentiated to generate (dI/dV)−V data. According to the previous analysis on the (dI/dV)−V data of assorted molecules,14,22−24 the inflection in the range of negative tip bias indicates the edge of the lowest unoccupied molecular orbital (LUMO), whereas the one in the range of positive tip bias is the edge of the highest occupied molecular orbital (HOMO).16,17 The (dI/dV)−V data of three systems in Figure 3b demonstrate features very different from those of the pristine graphite (Figure S3a, Supporting Information). On the basis of the statistical data in Figure 3c, the LUMO and HOMO edges of F16CuPc are estimated at −1.1 and 1.5 eV correspondingly. For NTZ12, its LUMO is at −1.6 eV and that for the HOMO is at 1.2 eV. The STS-determined LUMO and HOMO positions of F16CuPc properly reflect its n-type semiconducting properties, and those of NTZ12 also excellently match the p-type semiconducting D
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NTZ12 MHJ constructed through annealing displays superior photovoltaic properties, which, at the molecular level, reveals the possible mechanism of improving the performances of organic photovoltaic cells by heating treatment.
expected tendency of electron donation from the donor molecule NTZ12 to the acceptor molecule F16CuPc, as the red arrow points out. After building this simple physical model, we can consider the tunneling process when the tip is biased positively or negatively. In the case of applying a positive voltage to the tip (the right process in Figure 3d; the linear solid arrow marks the tendency of electron donation in the F16CuPc−NTZ12 MHJ), tunneling electrons flow from the substrate, passing NTZ12 and F16CuPc in sequence, to the tip. Evidently, the direction of electrons traveling in this process is in line with the tendency of electron donation from NTZ12 to F16CuPc. Thus, a smaller turn-on voltage in the area of positive tip potential is a natural result. On the contrary, when the tip voltage is negative (the left process in Figure 3d), electrons are forced to move from the tip to F16CuPc, to NTZ12, and to the substrate. Obviously, the direction of electrons flowing from the acceptor molecule F16CuPc to the donor compound NTZ12 violates the preferred electron-donating tendency from NTZ12 to F16CuPc in our MHJ. Thus, additional energy barriers have to be overcome, reasonably leading to a larger turn-on voltage. Besides the rectification effect, the donor−acceptor heterojunction is the most important working unit in varied photovoltaic devices. Therefore, we further tested the photocurrent of the F16CuPc−NTZ12 MHJ. Figure 4 combines the
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CONCLUSIONS The noncovalent molecular heterojunction formed by vertically stacking n-type F16CuPc on the backbone of p-type NTZ12 polymers is definitely revealed in the annealed samples through the molecular-resolution images of STM and the DFT calculations. The STS results unveil its apparent rectification effect due to the different barriers in two directions of electron tunneling. Furthermore, the F16CuPc−NTZ12 MHJ shows preferred photovoltaic properties in the photocurrent tests because of its well-organized donor−acceptor interface. Our study implies that annealing could be an efficient method to construct MHJs, which could help boost the studies on MHJ and molecular electronics. More importantly, our results provide visible evidence for understanding the processes of optimizing the properties of organic devices through heating treatment, which is paramount for the development of highperformance organic electronics as well.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02650. STM image of the F16CuPc assembly after annealing, STM image of F16CuPc adsorbing on the alkyl chains of NTZ12, DFT-optimized models of F16CuPc and NTZ12 (n = 2), proposed initial and DFT-optimized models of the F16CuPc−NTZ12 MHJ, typical (dI/dV)−V curve of the pristine graphite, and calculated energy levels of F16CuPc and NTZ12 (n = 2) (PDF)
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Figure 4. Measured current of the F16CuPc−NTZ12 MHJ, unannealed samples, and pristine graphite with/without radiation. The data collection time for a single curve is 2.4 s, except the time of the light-on−off curve is 24 s. Each current curve shown in this graph was produced by averaging 4−5 single testing curves, but the light-on− off curve was not averaged.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhi-Yong Yang: 0000-0002-6079-6763 Zhi-Xiang Wang: 0000-0001-5815-3032 Notes
current values of the F16CuPc−NTZ12 MHJ, unannealed samples, and pristine graphite with/without radiation. The F16CuPc−NTZ12 MHJ produces a significantly larger current under radiation than in the dark, as shown by the remarkable difference between the averaged solid (radiation) and hollow (no radiation) circle lines. The current produced by the F16CuPc−NTZ12 MHJ drops sharply to a very low value when the radiation source is turned off (the green rhombus line in Figure 4), which verifies that the larger current detected in the F16CuPc−NTZ12 MHJ samples under radiation is indeed mainly photogenerated current. For the unannealed samples, the gap between the solid (on radiation) and hollow (in the dark) triangle lines is tiny and almost indiscernible in Figure 4. The current of the pristine graphite shows no response to radiation because the average signal under radiation (the solid square line) is completely overlapped with the one in the dark (the hollow square line). It is widely accepted in the area of photovoltaic research that charge carriers separate more efficiently at the well-defined donor−acceptor interface of heterojunctions. Therefore, the highly ordered F16CuPc−
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported in part by the National Natural Science Foundation of China (Grant 21303200) and Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant 2015360). We also thank Prof. Gui Yu and Dr. Wei-Feng Zhang for providing the NTZ12 sample.
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DOI: 10.1021/acs.jpcc.7b02650 J. Phys. Chem. C XXXX, XXX, XXX−XXX
