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How To Obtain High-Quality and High-Stability Interfacial Organic Layer: Insights from the PTCDA Self-Assembly Yinghe Zhao† and Jinlan Wang*,†,‡ †

School of Physics, Southeast University, Nanjing 211189, China Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China

‡

ABSTRACT: Organic/inorganic interfaces play dominant roles in the formation of organic thin films and the performance of organic electronic devices. Preparing a high-quality and highstability interfacial organic layer on the inorganic substrate is currently challenging, and understanding the self-assembly mechanism of the interfacial layer (IL) in depth at a molecular level is thus essential. In this work, by studying the self-assembly of perylene-3,4,9,10tetracarboxylic dianhydride IL on graphene, we unveil that intermolecular H-bonds can considerably widen the nucleation area, heighten the stability−metastability critical temperature, promote the nucleation speed, and guide the nucleation direction. We further demonstrate that such positive effects on the IL self-assembly are of generality. Moreover, it is found that IL can transform into a well-ordered crystal from an amorphous state through suitable thermal treatment or molecule coverage control. Our work highlights that fabricating H-bond networks is desirable for the synthesis of robust and high-quality IL and points out feasible routes to improve the quality of poor IL.

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INTRODUCTION Organic electronic devices have attracted enormous attention1−6 owing to their unique advantages of flexibility, light weight, and low cost. Nevertheless, the fabrication of highperformance devices is still extremely challenging to date, mainly arising from the difficulty in the preparation of highquality and high-stability organic thin films (OTFs).5,6 Currently, the mobility of well-ordered OTFs is about 10 cm2/(V s),5,7,8 which is even better than amorphous silicon (1 cm2/(V s)).5 While for poor OTF devices, the mobility is only 10−4 cm2/(V s) or even lower.5 Among all organic layers of OTFs, the layer with the underlying inorganic substrate, i.e., the interfacial layer (IL), is the most important because it dominantly influences the device performance and represents a growth template for other organic layers.9−13 More recently, two-dimensional (2D) organic molecular crystals on the basis of IL and 2D inorganic materials have been successfully synthesized and show a promising application in transistors,8,14−17 where IL, linking 2D organic crystals and 2D inorganic materials, plays a crucial role in both crystal growth8,15 and charge transport.16,17 Besides, IL itself has wide potential applications, such as passivation layers,18−21 molecular electronics, 22−24 molecular machines,25,26 gas sensors,27,28 electrocatalysts,29 organic ferroelectrics,30,31 photonic devices,32,33 and graphene-based devices.34−36 Therefore, it is crucial and urgent to understand the physical mechanisms on how organic molecules self-assemble into a stable and wellordered IL on the inorganic substrate. Perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) molecules have been experimentally demonstrated to have capacity to self-assemble into a high-quality and high-stability IL on many inorganic substrates,18−20,37−41 whereas the © XXXX American Chemical Society

intrinsic mechanism behind remains elusive at present. Herein, we aim to explore why PTCDA has such good capacity, thereby obtaining general insights toward the IL preparation. The research is built on molecular dynamics (MD) simulation that has proven to be a powerful tool for exploring the initial growth process of OTFs.42−49 We takes PTCDA on graphene as the example which has been widely reported in theory and experiment18,19,37−40,50 and systematically study the nucleation phase diagram and the nucleation time. The phase diagram is depicted at the cost of huge computations in which hundreds of simulations are performed, and the duration time of each simulation is between 0.1 and 1 μs. Our study shows that the intermolecular hydrogen-bonds (H-bonds) play a key and positive role in the IL self-assembly. They can significantly broaden the nucleation area, raise the stability−metastability critical temperature, accelerate the nucleation process, and guide the nucleation direction. Such positive effects are also applicable to the tetracyanoquinodimethane (TCNQ) IL, demonstrating the generality. On this basis, we further propose two practical routes for experimentalists to improve the IL quality if synthesizing poor one.

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COMPUTATIONAL METHODS MD Simulations. All simulations are performed by combing Gromacs-4.5.4 package51 with all-atom Amber99sb force field52,53 in the NVT (number of molecules, volume, and temperature are constant) ensemble with the time step of 1 fs. The force field files of PTCDA, TCNQ, and graphene can be Received: January 19, 2017 Revised: February 14, 2017 Published: February 14, 2017 A

DOI: 10.1021/acs.jpcc.7b00606 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Molecular structure of PTCDA. Red, gray, and white balls represent oxygen, carbon, and hydrogen atoms, respectively. (b) Initial configuration of 60 PTCDA molecules on graphene. (c, d) Self-assembled structures with and without H-bond interaction at t = 50 and 350 ns. (e, f) Schematics of the unit cells for the structures in (c) and (d).

found in our previous work.50 The graphene substrate consists of 3840 atoms, and its size is 10.26 nm × 9.88 nm. A spherical cutoff of 1.5 nm for van der Waals (vdW) and electrostatic interactions is used throughout. Outside 1.5 nm, the vdW interaction is not considered, and the electrostatic interaction is dealt with by employing the smoothed particle mesh Ewald sum.54 Periodic boundary conditions are applied, and the Berendsen thermostat is utilized to control the temperature. The temperature in the nucleation area is determined according to whether a disordered IL can self-assemble into an ordered structure within the microsecond time scale, and the IL is still ordered during most of the subsequent time. At certain coverage, we first roughly determine the nucleation temperature range by performing 10−20 simulations in a wide range of temperatures. On the basis, we further carry out 10−20 simulations to gradually narrow the nucleation temperature range until the nucleation area is finally determined. Treatment of the H-Bond Interaction. Intermolecular interactions consist of the vdW and electrostatic interactions. They are calculated according to Coulomb’s law and 12−6 Lennard-Jones potential, respectively. The vdW interaction does not change throughout the process, and the H-bond strength is controlled by adjusting the electrostatic interaction. For example, “the H-bonds are artificially cancelled” represents the electrostatic interaction is removed, and the system only depends on the vdW interaction. For this case, PTCDA molecules will self-assemble into a one-dimensional (1D) line structure (see Figure 1f) rather than a herringbone structure (see Figure 1e). The radial distribution functions of intermolecular O−H, O−O, and H−H in the 1D line and the herringbone arrangements are recorded in the dashed lines of Figures 2a and 2b, respectively. From Figure 2a, we can see that intermolecular O−H distribution is much less than the O− O and H−H, which is distinct from Figure 2b where the O−H distribution plays a dominant role. The sharp difference shows that H-bonds disappear with the removal of the electrostatic interaction. Similarly, the 0.25 strength represents that a quarter of the electrostatic interaction is kept. Treatment of the Graphene Substrate. Note that graphene is assumed as a flat surface in our simulations. In

Figure 2. Radial distribution functions of intermolecular O−H, O−O, and H−H without (a) and with (b) H-bond interaction, respectively. The solid lines represent the distribution during the initial 0.5−1.0 ns. The dashed lines represent the distribution that PTCDA molecules finally self-assemble into ordered structures, more specifically, (a) the 1D line structure (see Figure 1f) and (b) the herringbone structure (see Figure 1e).

experiment, PTCDA and TCNQ are often deposited onto epitaxial graphene grown on bulk materials, and the epitaxial graphene has periodic ripples whose height depends on the underlying material. The ripple height of graphene/Ir(111) can be controlled to 0.02 nm that is much less that the molecule− graphene distance (about 0.33 nm),36 so it should be an excellent choice for verifying the nucleation phase diagrams. Moreover, the strong assembly capacity of PTCDA is essentially independent of graphene because PTCDA also can self-assemble into a stable and well-ordered IL on other B


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Figure 3. (a−f) Six structures with different PTCDA coverage: (a) 0.33 × 10−6, (b) 0.50 × 10−6, (c) 0.66 × 10−6, (d) 0.83 × 10−6, (e) 1.00 × 10−6, and (f) 1.16 × 10−6 mol/m2. (g−i) Three structures with different TCNQ coverage: (a) 0.32 × 10−6, (b) 0.98 × 10−6, and (c) 1.64 × 10−6 mol/m2.

inorganic substrates.20,41 Therefore, the assumption of flat graphene will not change the main conclusions and insights toward the IL preparation obtained here.

the herringbone structure was imaged by scanning tunneling microscopy. 18,19,37−40 When the H-bond interaction is artificially canceled, PTCDA will self-assemble into a onedimensional (1D) line structure (see Figure 1d,f). Influences of H-Bonds on the IL Self-Assembly. The comparison between Figures 1c and 1d suggests that H-bonds can strongly influence the IL morphology. Naturally, it is expected that H-bonds would play an important role in the IL self-assembly as well. Next we explore what the influences of Hbonds are. We consider six initial disordered configurations with different PTCDA coverages varying from 0.33 × 10−6 to 1.16 × 10−6 mol/m2 as shown in Figure 3. The first structure presents a sparse distribution of PTCDA, and the last one represents PTCDA molecules almost overspreading the underlying graphene. On this basis, we draw the coveragetemperature nucleation phase diagrams in Figure 4a,b where the blue space records the nucleation area. Outside the blue area PTCDA molecules are out of order, more specifically, in an amorphous solid below and a disordered liquid above. For the case without H-bond interaction, it is obvious that the temperature window for the nucleation is rather narrow (see Figure 4a). This means that the adsorbed molecules can deviate from the nucleation area easily. The crystal nucleus determines the subsequent growth as it represents a growth template and core. As a result, there is less probability for the self-assembly into a highly ordered IL. In contrast, the nucleation area is fairly wide when H-bond interaction is included (see Figure 4b). The striking difference shows that the participation of H-bonds significantly increases the opportunity of the self-assembly into a high-quality IL.

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RESULTS AND DISCUSSION Self-Assembly of PTCDA IL on Graphene. Figure 1a describes the PTCDA molecular structure that consists of a perylene backbone and carboxylic acid anhydride side groups. Oxygen and hydrogen atoms are around the outside and have opposite partial charges.50 The repulsion among like charges and the attraction among opposite charges result in the formation of H-bonds among PTCDA molecules (C−H···O). The self-assembly from disorder to order is shown in Figure 1b,c, from which we can observe that PTCDA molecules selfassemble into a highly ordered herringbone structure. Figure 1e depicts the schematic of the unit cell, and the lattice parameters a and b from simulation are 2.05 and 1.25 nm. The experimental values from two different reports are 2.17 ± 0.1 and 1.26 ± 0.1 nm as well as 1.8 ± 0.2 and 1.2 ± 0.1 nm, respectively.19,40 The good agreement justifies the reliability of our MD simulations. Besides the herringbone structure, PTCDA is able to selfassemble into a square structure.50 This structure has never been observed on graphene in experiment as far as we know, but it has been seen on some metal substrates, such as Au(111).55 Not surprisingly, polymorphism is common for 2D supramolecular self-assembly driven by H-bond interaction.56−59 It is noteworthy that according to simulations PTCDA molecules arrange in a herringbone pattern in the vast majority of cases. This coincides with the experiment in which C


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The state in the nucleation area is essentially metastable, and its stability is between stable solid and unstable liquid. Note that an ordered IL is quite stable in the amorphous-solid area of the phase diagram, and it will not transform into an amorphous state. As shown in Figure 4a, the stability−metastability critical temperature (see the boundary between amorphous solid and nucleation area) is very low, even under a high coverage of 1.16 × 10−6 mol/m2. The lower temperature means weaker capacity to resist thermal disturbance. By contrast, the critical temperature is considerably higher for the system with Hbond interaction (see Figure 4b). The distinct phase-transition temperature suggests that H-bonds can strongly enhance the thermal stability of IL. This is easy to understand since the Hbonds strengthen the intermolecular interaction, and more energies are thus needed to break them. We further investigate the impact of H-bonds on the nucleation time. Considering the calculation cost, the simulation is built on the structure in Figure 3a because its atom number is the fewest. Figure 4c records the distribution of the nucleation time without H-bond interaction. The time distribution almost totally concentrates in the periods of 5−50 or ≥50 ns in the absence of H-bonds. In contrast, the nucleation time less than or equal to 5 ns accounts for 70% for the case with H-bond interaction (see Figure 4d). Such a sharp difference suggests that the presence of H-bonds remarkably speeds up the nucleation process. The above discussion clearly shows that the H-bonds are beneficial to the self-assembly of PTCDA IL. Are the positive influences of H-bonds on the IL self-assembly common? To verify this, we perform further calculations for TCNQ IL. Figure 5a,b displays the structure and unit cell of TCNQ IL in an ordered arrangement, which agrees well with that from experiment.35,36,60 Figure 5c depicts the TCNQ coveragetemperature nucleation phase diagram with H-bond interaction. The nucleation temperature window is about 100 K for a given coverage. The nucleation time mainly distributes in periods ≤5 and 5−50 ns (see Figure 5d). With the removal of H-bond interaction, it is found that the self-assembly of TCNQ IL from disorder to order is very difficult to achieve. For example, we

Figure 4. (a, b) Coverage-temperature nucleation phase diagrams of PTCDA on graphene without (a) and with (b) H-bond interaction, and they are drawn according to the original data in Table 1. (c) Distribution of the nucleation time without H-bond interaction in periods ≤5, 5−50, and ≥50 ns. The statistics are built on 50 independent simulations of the initial structure in Figure 3a (0.33 × 10−6 mol/m2 coverage) at 155 K. Similarly, (d) records the distribution with H-bond interaction at 325 K. The simulation temperatures 155 and 325 K are just in the middle of the nucleation temperature window.

Table 1. Boundary Temperatures in Figure 4a,ba coverage (10−6 mol/m2) T1 T2 T3 T4

(K) (K) (K) (K)

0.33

0.50

0.66

0.83

1.00

1.16

140 170 230 420

170 195 310 500

190 220 360 550

195 230 390

210 240 400

220 250 430

a

T1 and T2 represent the temperatures at lower and upper boundaries in Figure 4a, respectively. Similarly, T3 and T4 stand for those in Figure 4b.

Figure 5. (a) Self-assembled TCNQ structure on graphene from a disordered arrangement in Figure 3i. (b) Schematic of the unit cell of the structure in (a). (c) Nucleation phase diagram of TCNQ on graphene in the coverage-temperature plane, and it is obtained based on the three structures in Figure 3g−i. The original data are placed in Table 2. (d) Distribution of the nucleation time in periods ≤5, 5−50, and ≥50 ns, and it is built on 50 independent simulations of the structure in Figure 3g at 310 K. (e, f) Self-assembled TCNQ structure from the structure in Figure 3g without H-bond interaction and the corresponding unit cell. D


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nucleation area highly depends on the H-bond strength. The expansion of the nucleation area stems from the enhanced intermolecular interaction by the H-bonds. The nucleation time is recorded in Figure 6b for the strength at 0.25. It is almost as long as that at the H-bond strength of 1 (see Figure 4d). This suggests that the H-bond strength is not responsible for accelerating the nucleation process. Besides, Figure 6c records the nucleation time for the H-bond strength at 0.25 at 155 K. Obviously, the nucleation time in Figure 6c is closer to those in Figures 4d and 6b and is much shorter than that in Figure 4c, showing that the nucleation temperature plays a minor role. Thus, the acceleration effect must be derived from the guidance of H-bonds for the nucleation process. To verify this, we further record the radial distribution functions of intermolecular O−H, O−O, and H−H with and without H-bonds in Figure 2. For the system with H-bond interaction, the distribution during the initial 0.5−1 ns (solid lines in Figure 2b) is similar to the final distribution (dashed lines in Figure 2b). This shows that from the start the H-bonds can guide the IL to self-assemble toward the final structure well. In contrast, for the system without Hbond interaction, its initial distribution (solid lines in Figure 2a) is distinct from its final one (dashed lines in Figure 2a). Now, we can conclude that the positive roles that the H-bonds play in the IL self-assembly are (i) widening the nucleation area and raising the critical temperature through H-bonds strengthened intermolecular interaction and (ii) promoting the nucleation ascribed to the guidance of H-bonds. We believe that these conclusions are generally applicable to a broad class of organic molecules.

perform 20 simulations based on the structure in Figure 3g with the temperatures from 100 to 200 K, and only one shows the successful self-assembly into an ordered structure (see Figure 5e,f). This demonstrates that similar to the PTCDA system the H-bonds are also highly favorable to the self-assembly of TCNQ IL. Table 2. Boundary Temperatures in Figure 5ca coverage (10−6 mol/m2) T1 (K) T2 (K)

0.32

0.98

1.64

270 350

330 430

370 510

a

T1 and T2 represent the temperatures at lower and upper boundaries, respectively.

Intrinsic Mechanisms. It has been shown that H-bonds can widen the nucleation area, increase the stability− metastability critical temperature, and speed up the nucleation process. However, the intrinsic mechanism behind remains elusive. As aforementioned, the increase of the critical temperature is easy to understand because H-bonds enhance the intermolecular bonding strength. Therefore, we next focus on why the nucleation area is broadened and the nucleation time is shortened in the presence of H-bonds. We take the structure in Figure 3a as the example to minimize the computational cost. Figure 6a depicts the nucleation area as a function of the H-bond strength scaled from 0 to 1. With the strength enhancement, the nucleation temperature gradually ranges from 140−170 to 230−420 K. This means that the

Table 3. Boundary Temperatures in Figure 6aa coverage (10−6 mol/m2) T1 (K) T2 (K)

0.00

0.25

0.50

1.00

140 170

150 250

180 320

230 420

a

T1 and T2 represent the temperatures at lower and upper boundaries, respectively.

Insights for Improving the IL Quality. In experiment, it is very difficult, if not impossible, to obtain a well-ordered sample every time. A poor sample is often abandoned, which is undoubtedly a waste of resources. Probing how to improve the quality, therefore, is very necessary. From the nucleation phase diagram in Figure 4a,b, we can deduce that a poor sample is a result of the deviation from the nucleation area in a large degree. For example, the self-assembly from Figure 7a,b is represented by the red dot in Figure 4b. Obviously, PTCDA molecules self-assemble into an amorphous solid rather than an ordered crystal. For the structure in Figure 7b, there exist two typical routines back to the nucleation area, i.e., increasing temperature and decreasing coverage (see the two red lines in Figure 4b). Indeed, the amorphous solid in Figure 7b evolves into an ordered crystal in Figure 7c at elevated temperature to 400 K. Similarly, decreasing the PTCDA coverage is also practical (see Figure 7b,d,e). Very interestingly, a recent experiment suggests that the quality of C60 IL can be improved by postdeposition annealing.61 We believe the elevated temperature leads to the phase transition of C60 IL from an amorphous state to an ordered crystal, similar to the process presented in Figure 7b,c. The good agreement between theory and experiment suggests that the strategies proposed here have

Figure 6. (a) H-bond strength−temperature nucleation phase diagram of the structure in Figure 3a, and it is drawn according to the original data in Table 3. (b, c) Distribution of the nucleation time for the strength at 0.25 under the conditions of (b) 200 K (just in the middle of the temperature window) and (c) 155 K. E


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Figure 7. (a, b) Self-assembly of 50 PTCDA molecules (0.83 × 10−6 mol/m2 coverage) on graphene at 350 K. The structure in (b) is taken from the trajectory at t = 100 ns. (c) Evolution of (b) with the temperature increased to 400 K at t = 50 ns. (d) Transformation of (b) with the coverage decreased to 0.50 × 10−6 mol/m2. (e) Evolution of (d) at 350 K after 10 ns.

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potential to be common and powerful techniques to improve the quality of poor IL.

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CONCLUSIONS

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AUTHOR INFORMATION

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In conclusion, by exploring the nucleation processes of PTCDA and TCNQ on graphene, we reveal the positive roles of Hbonds in the IL self-assembly. It is found that intermolecular Hbonds can significantly broaden the nucleation area and heighten the stability−metastability critical temperature. Also, we unveil that H-bonds enable the nucleation process to proceed more smoothly, which is mainly ascribed to the guidance of H-bonds. More importantly, we propose two practical routes along which IL in an amorphous state can transform into a well-ordered crystal by suitable thermal treatment or molecular coverage control. Our work highlights that fabricating H-bond networks deserves our great efforts toward the preparation of high-quality and high-stability IL and provides valuable insights into how to improve the IL quality once synthesizing poor one in experiment, thereby avoiding waste.

Corresponding Author

*E-mail [email protected] (J.W.). ORCID

Jinlan Wang: 0000-0002-4529-874X Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

This work is supported by the NSFC (21525311, 21173040, 21373045) and jiangsu 333 project (BRA2016353) and NSF of Jiangsu (BK20130016) and SRFDP (20130092110029) and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1669) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin. F


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