Synergic effect: temperature-assisted electric-field-induced

May 23, 2019 - Using trimesic acid (TMA) as a model system by means of scanning tunneling microscope (STM) equipped with a temperature controller, her...
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Interface-Rich Materials and Assemblies

Synergic effect: temperature-assisted electric-field-induced supramolecular phase transitions at the liquid/solid interface Ayyaz Mahmood, Muhammad Saeed, Yue Chan, Awais Siddique Saleemi, Jiangtao Guo, and Shern-Long Lee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00569 • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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Langmuir

Synergic effect: temperature-assisted electric-field-induced supramolecular phase transitions at the liquid/solid interface Ayyaz Mahmood,†,‡,+ Muhammad Saeed,†, ‡,+ Yue Chan,† Awais Siddique Saleemi,†,‡ Jiangtao Guo,† and Shern-Long Lee†,* †

Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China 518060 Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Provence, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China 518060 ‡

+Authors

contributed equally to this work

KEYWORDS: phase transition • scanning tunneling microscopy • surface chemistry • monolayer assembly • stimuli-responsive systems ABSTRACT: Using trimesic acid (TMA) as a model system by means of scanning tunneling microscope (STM) equipped with a temperature controller, here we report a temperature-assisted method to cooperatively control electric-field-induced supramolecular phase transitions at the liquid/solid interface. Octanoic acid (OA) is used as a solvent due to its good solubility for TMA in OA as well as its less complicated pattern formed under negative STM bias. (e.g., only chicken-wire polymorphs existing. At positive substrate bias, STM revealed that TMA assembly based on temperature modulations underwent phase transitions from a porous (22 °C) to a flower (45 °C), and further to a zigzag (68 °C) structure. The transitions are ascribed to the partial deprotonation of the carboxyl groups of TMA. Both temperature and the electrical polarity of the substrate are crucial, i.e. the transitions only take place at positive substrate bias and elevated temperatures. Molecular mechanics simulations were carried out to calculate the temperature and electric-field dependence of the adsorption enthalpy and free energy of the chicken-wire assembly of TMA on the two layers of graphene surface. The calculated decrease in adsorption enthalpy with the increase of temperature and electric-field values that causes the TMA chicken-wire assembly to be less stable is proposed to promote the occurrence of the phase transition observed by STM. This study paves the way toward program-controlled supramolecular phase switching via the synergic effect of electrical and thermal stimuli.

1. INTRODUCTION Stimuli-responsive phenomena present an event in which changes occur in a system due to external stimuli applied.1 Such a phenomena has become an integral part of design concepts in material sciences and supramolecular chemistry. Stimuli-induced supramolecular phase switching has been extensively explored because of its potential applications in sensing, molecular electronics, and surface modifications, to name a few.2 To date, despite its importance, full control of (supra)molecular self-assembly remains a challenge.3-4 Programmable control is an important concept in our daily life; for instance, in computer science, action commands can be programmed that order the arithmetic operations. For supramolecular self-assembly taking place at a liquid/solid interface, programmable molecular patterning relies on a delicate equilibrium of molecule-molecule, molecule-solvent and molecule-substrate interactions. In this field, an important topic is to develop effective methods for triggering reversible supramolecular phase transitions. To this end, external stimuli are needed; they include factors such as light, heat, flow, ions, electric field, and electrochemical potentials.5-9 These studies are of great importance because such artificial surfaces hold great promises for developing smart devices. Among these examples of efficiently controlled reversible changes, the scenario where the phase transition is induced by an electric field provided by the STM tip is an attractive technique. Using

such an approach, STM can visualize the evolution process of molecular assembling and control these changes by varying the electrical polarity of the substrate simultaneously.10-12 Elegant examples of the so-called “STM-induced” phase transitions of molecular packing include molecular assemblies constituted by charged polycyclic aromatic hydrocarbons (PAHs),13 asymmetric tris(phthalocyaninato) lutetium tripledecker complex,14 and 1,3,5-tris(4-carboxyphenyl)-benzene (BTB).12,15-16 All the molecules bear their own intrinsic intramolecular dipole moment. Recently, an interesting paper reported that such phase switching can occur for the chickenwire assembly of trimesic acid (TMA) which is an analogue to BTB (Figure 1a).17 This finding is intriguing to this field because TMA is neither dipole nor charged. Moreover, this result is inconsistent with some literature reports demonstrating that the porous networks of TMA (Figure 1b) can be used as a stable template for trapping guest molecules, regardless of the electrical polarity of a substrate. For example, Wan et al. utilized the chicken-wire monolayer of TMA to assist the growth of bilayer11 and 3D self-assemblies.18 In this field, thus far the effective approach suggested for triggering the phase transformations of TMA is to combine an electrochemical potential controller with STM, so-called an EC-STM method.1920 Such an approach has been implemented for at least a decade in a wide variety of molecular systems for tailoring supramolecular assemblies on solid surfaces.21-27

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Figure 1. Chemical structure of TMA and BTB, the molecular model and STM bias-related experiments of the chicken-wire assembly of TMA. a) Chemical structure of TMA and BTB; b) the model of the chicken-wire self-assembly of TMA; c) a typical STM image for TMA at the octanoic acid/graphite interface; d) The STM bias-related experiment of the chicken-wire network of TMA. Unitcell parameters of a, b, and 𝛼:1.7 (±0.3) nm, 1.7 (±0.2) nm, 60°(± 2°). Imaging conditions (Ebias, itunneling) for c) −0.85 V, 100 pA; for d) the tunneling current is 100 pA. The polarity and magnitude of the STM bias applied to the surface are noted on the image. The blue arrows indicate the lattice directions of the HOPG underneath. For all experiments the sample is a 3-µL droplet of 1.0 mM put onto surface and scanned by STM at the OA/graphite interface.

Due to its fundamental importance as well as being an elegant model system in many research domains,28-29 further exploration of TMA self-assembly behavior merits a special attention. Here, we report a deeper insight into the topic of the phase transition of this important system. The temperature and electric-field stimuli have previously been reported to induce the phase transition of TMA; however, this work reports for the first time the combined use of electric and thermal effect to induce such transitions. The utilization of such synergic (electro-thermal) effect to control the phase transitions in molecular assemblies is a novel approach that can be of great importance and benefit for researchers working in different research fields. Briefly, we found that the combination of thermal and electrical effect can readily trigger the reversible phase transformations of 2D polymorphs of TMA, which would not be easily accessible.

2. EXPERIMENTAL SECTION The STM experiments were performed at an octanoic acid (OA)/highly oriented pyrolytic graphite (HOPG) interface using a Keysight 5500 operating in the sample-biased and constant-current mode at either room temperature (ca. 22 C) or substrate-temperature modulated conditions via a temperature controller. The experiments were repeated several times. STM tips were mechanically cut Pt/Ir wires (80%/20%, diameter 0.25 mm). HOPG was purchased from Advanced Ceramics (ZYB grade, Advanced Ceramics Inc.). Imaging conditions of Vbias and Iset were ranged from −1.50 to 1.50 V (sample bias) and

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from 50 to 200 pA, respectively. The reported STM images were calibrated by the unit-cell vectors of the underlying HOPG using an SPIP software (scanning probe image processor, Image Metrology ApS). The chemicals were commercially available and were used as received, including octanoic acid (OA,  99%), trimesic acid (TMA,  98%), and coronene (COR,  98%). TMA molecules were weighed and dissolved in OA and for all the experiments the sample concentration is 1.0 mM. It should be noted OA is used as the solvent here due to its excellent solubility for TMA and less-complex pattern formed under negative STM bias (e.g., only chicken-wire polymorphs existing). In our work, a commercial temperature controller was used to control our experimental temperature. A 10 µL sample solution in a liquid-cell was used. Typically, it costs 20 minutes to heat a substrate to targeted temperature. Only when the thermal drift was not significant, we start to perform the STMbias-related investigations. We note that the sample volume lost is ca. 10~20 % for one run of experiments (1 hour). The modelled molecular packing structures were obtained using HyperChemTM Professional 7.5 program based on the latticestructure parameters. First, a molecular model for a single molecule was built, and then this model was duplicated. We constructed the model of the entire monolayer via placing the molecules in accordance with the intermolecular distances and angles obtained from calibrated STM images. The experimental errors were derived from five STM images. The imaging parameters are indicated in the figure caption: sample bias (Ebias) and tunneling current (itunneling).

3. RESULTS AND DISCUSSION 3.1. Self-Assemblies of TMA Formed Under Controlled Temperature and Electric Field. Figure 1c exhibits a typical STM image of the chicken-wire network of TMA. The chickenwire assembly of TMA was tested by frequent switching of electrical polarity of substrate (STM bias); it was observed upon switching of the substrates electrical polarity that the assembly does not undergo phase transition at room temperature (Figure 1d). The chicken-wire motif stably exists, suggesting that the TMA network is insensitive to the change in the substrate electrical polarity. Additionally, an increase of the magnitude of STM sample bias from 0.85 V to 1.2 V did not influence the assembly (Figure 1d). Next, the systems temperature was modulated to see if any possible phase transitions were triggered. At the liquid/solid interface, heating of the substrate is expected to improve the solubility as well as concentration of TMA in liquid phase, which may induce closely packed assemblies to form on the surface. Thinking along this line, a temperature controller in the STM experiments was incorporated. STM revealed that the electrically induced phase switching of TMA took place at elevated temperatures under positive STM bias. Under negative STM bias, the chicken-wire network stably exists even at elevated temperatures. However, switching the substrate polarity from negative to positive at the elevated temperatures induces the occurrence of phase transition from a chicken-wire (22 °C) to a flower and a zigzag polymorph30 at 45 °C and 68 °C, respectively (Figure 2a,b). The mixture of the two phases appear at 60 °C (Figure S1 in the Supporting Information). The transitions of these polymorphs controlled by temperature and the electrical polarity of the substrate are fully reversible and highly reproducible. This suggests the temperature-assisted

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Langmuir strategy is highly efficient to promote electric-field-induced supramolecular patterning at the interface. These specific phases presented in Figure 2 can be selectively formed under the experimental conditions of the corresponding temperature and the electric polarity of substrate. However, if the surface is negatively charged (e.g., negative sample bias with STM set-up) and under the experimental condition where temperature ranges from 30 to 60 °C, no phase transitions occur and the surface remains the chicken-wire TMA motif. Further increase of temperatures lead to an empty surface observed (e.g., more than 100 °C).

Nevertheless, one can note that the packing density of TMA on the surface increases when the STM polarity is switched from negative to positive. The TMA molecules under positive STM bias are likely to be negatively charged (deprotonated) because they come to the heated surface and form patterns promptly. The periodic (bright and dark) pattern arising referred to as moiré pattern may result from commensurate and incommensurate electronic coupling between TMA and HOPG.32,33 3.2. Computational Simulations of Adsorption Energies. Figure 3 shows the proposed different orientations of TMA molecules adsorbed on graphite and their adsorption energy derived by MM calculations. The values suggest their stability i.e. more negative the adsorption energy the higher the stability. Consistently, the face-on orientations are more stable than the standing-up ones. The top orientation is the most stable one, followed by bridge, cross, and center types. The higher stability of face-on orientations can be ascribed to the strong π–π stacking interaction between TMA molecules and graphite. To compare, in Figure 3 we also present the edge-on fashion. The situation of two carboxyl groups (COOH) of TMA facing the surface is calculated to be more stable than the orientation with one carboxyl facing the surface. This result can be ascribed to stronger molecule-substrate interactions (Figure 3e–f). The orientation of TMA on the graphite surface presented in Figure 3f is also expected to exist.19 However, this type is less stable because the TMA-graphite attraction is weaker, compared with the other two types.

Figure 2. Temperature- and electric-polarity-related polymorphs. a,b) the switching of the chicken-wire into flower and zigzag structures of TMA at the given experimental conditions; c) illustration of the phase formations of TMA by thermal and electrical stimuli. Imaging conditions (Ebias, itunneling): ±0.85 V, 100 pA. The polarity of STM bias and the environmental temperature for STM imaging are noted on the image. Unit-cell parameters of a, b, and 𝛼 for chicken-wire motif (see Figure 1), for flower: 2.7 (±0.2) nm, 2.8 (±0.3) nm, 60°(±2°), and for compact: 1.0 (±0.2) nm, 1.8 (±0.2) nm, 87°(±3°). The blue arrows indicate the latticestructure directions of graphite underneath. The packing density for chicken-wire, flower, and zigzag is 0.78, 0.98, and 1.13 molecules/nm2, respectively.

Below the mechanism of the phase transition is discussed. It is first worth noting that the thermal and electrical stimuli have previously been proposed to result in the partial deprotonation of the carboxyl groups of TMA. 17,31 With this in mind, the mechanism of the phase switching can be attributed to a cooperative induction of partial deprotonation of TMA by both stimuli. Consequently, the higher temperature offered here, gives a higher density packing of TMA, stabilized by moleculesubstrate attractions, appears on the surface. Note that “partial deprotonation” might be a plausible hypothesis compared with “deprotonation” since in our STM observations, the stimuliinduced phase transformations are fully reversible and take place promptly. While STM can distinguish the topological difference of the TMA polymorphs, in our liquid-solid-interface experiments, it is difficult to distinguish whether the TMA molecules within the assemblies are charged or partially deprotonated.

Figure 3. Expected adsorption orientations of TMA molecule on the graphite surface and their adsorption energy derived by MM calculations. These adsorption energies are calculated for the most stable adsorption site. (a–d) The face-on, cross, center and bridge orientation; (e, f) two types of the edge-on orientations.

To have further insight into the mechanism of the bi-stimuli system, molecular mechanics (MM) simulations using universal force field (UFF) were performed using Gaussian. The details of computational procedures used are presented in Supporting Information. Our simulations aim to explore the variations in the adsorption enthalpy and the free energy for the TMA chicken-wire assembly in the presence of thermal and electrical stimuli. Figure 4 presents the plots of the calculated adsorption enthalpies as a function of temperature and electricfield. The calculated free energy is presented in Figure S3 in the Supporting information. The adsorption enthalpies were calculated using the equation as follows.

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∆𝐻𝑎𝑑𝑠 = ∆𝐻𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒…(𝑇𝑀𝐴)6 − ∆𝐻𝑠𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 − ∆𝐻(𝑇𝑀𝐴)6 It can be observed from Figure 4 that the adsorption enthalpy decreases as the temperature increases from 0 °C to 100 °C. The calculated decrease in adsorption enthalpy is from –236.93 kcal mol–1 at 0 °C to –98.70 kcal mol–1 at 100 °C (black line). Note that the sign here is negative due to reason that the chicken-wire assembly on HOPG surface is lower in energy than the separated reactants, e.g.,TMA assembly and HOPG surface. The adsorption enthalpy decreases 138.23 kcal mol –1 upon a 100 °C increase of temperature in the absence of an electric field i.e. a decrease of 1.38 kcal mol–1 for each degree increase in temperature. Therefore, when the temperature is elevated from 22 to 45 °C (experimental value in Figure 2a), it can be derived by calculus (1.38 ×(45–22)) that there is a 31.70 kcal mol–1 drop in adsorption energy. This decrease in adsorption enthalpy is due to the fact that the enthalpy of reactants and products increases with the increase of temperature. However, the enthalpy increase is calculated to be larger for the reactant i.e. HOPG-TMA complex as compared to the separated products i.e. HOPG and TMA chicken-wire assembly. Indeed, the contributions to enthalpy increase come from the increased vibrational, rotational, translational and electronic motions as a result of temperature increase. On the other hand, we have approximated the gap between the STM tip and the substrate as 1 nm.34 Based on this, we simulated the system to which an external electric-field is applied. The electric-field value used in simulations was obtained by dividing the voltage value applied in the experiments by the distance between STM tip and substrate. The values used are equal to the experimental value (e.g., ±9.0 × 108 Vm–1). The simulations reveal that the presence of negative bias electricfield causes an increase in adsorption energy. For instance, a negative bias of 0.9 V/nm increases the adsorption by ~10 kcal mol–1. This result is in line with literature reports.35 On the other hand, it was calculated that switching the polarity of the electric field (± 0.9 V) decreases the enthalpy of the adsorption by ca. 17.50 kcal mol–1 at all temperatures (compare the red lines in Figure 4). These results agree with the previous literature reports i.e. a negative bias electric-field causes the adsorption to increase while a positive bias electric-field does the opposite although it may also depend on the molecules dicussed.35

Figure 4. The enthalpy of adsorption plotted as a function of temperature for the TMA chicken-wire assembly. The temperature was raised from 0 to 100 °C at 0 (no electric-field, black line), and

+ and – bias 9.0 × 108 Vm–1 (experimental value of electric-field). The black, and red lines belong to plots at electric-field 0 and at ±9.0 ×108 Vm–1, respectively.

Significantly, the simulation results provide the quantitative information for STM experiments, i.e., switching STM bias (e.g., ± 0.9 V) cannot trigger the phase transition of TMA and the minimum elevated temperature for driving the phase transformation is 45 °C. For the adsorption enthalpy decrease of the chicken-wire motif of TMA, now it can be quantitatively known that the electric-field stimulus is 17.50 kcal mol–1 and the thermal one is 31.70 kcal mol–1. The Gibbs free energy for the formation of chicken-wire assembly was also calculated from Gaussian simulations and plotted as a function of temperature, presented in Figure S3 of Supporting Information. The free energy a calculated to be higher than the adsorption enthalpy, however, it shows a similar trend to that of adsorption enthalpy in response to the change of temperature and electricfield. The presence of negative bias electric-field increases the free energy by ca. 9.98 kcal mol–1 at all temperatures. The change of polarity from negative to positive causes a decrease in free energy by ca. 19.98 kcal mol–1 at any temperature. The free energy is less effected than the enthalpy by the temperature stimuli. The free energy decreases from –372.97 kcal mol–1 at 0 °C to –350.41 at 100 °C i.e. a 100 °C increase of temperature causes a 22.56 kcal mol–1 drop in free energy for the formation of assembly. The sign of the free energy is negative because the self-assembly on HOPG has a lower free energy than the separated reactants i.e. the chicken-wire assembly and the HOPG surface. The change in free energy caused by temperature increase is quite lower than the adsorption enthalpy change. This is due the T∆S product in the equation ∆G = ∆H − T∆S which cancels out some of the enthalpy change effect. The temperature stimulus accounts for 5.18 kcal mol–1 decrease in free energy for a 23 °C increase of temperature (from 22 to 45 °C that causes the phase change) while the electric-field one is 19.98 kcal mol–1 (change of STM polarity). Indeed, both free for the formation of self-assembly and adsorption enthalpy show a similar trend in response to the temperature and electric-field stimuli. The thermal stimuli have been proposed to cause the partial deprotonation of the molecules. However, the protons of the partially deprotonated molecules remain in the vicinity of TMA molecules. At a negative substrate bias, these protons are likely to move between the negatively charged substrate surface and COO– giving rise to high adsorption enthalpies. Nevertheless, at a positive substrate bias, the deprotonation is unlikely to occur. The protons are pushed away from the equilibrium positions with respect to surface which causes the adsorption enthalpies to decrease. The decrease in adsorption enthalpy with the increase of temperature also explains well the occurrence of partial deprotonation of the molecules at higher temperature. 3.3. Applications in Host-Guest Chemistry. From the discussions presented above, we conclude that the strategy of heating in STM experiments is decisive for driving the phase transformation of TMA to occur. The chicken-wire assembly constituted by TMA molecules cannot undergo phase switching via merely changing the electric-field orientations in STM. We thus emphasize that the electric-field-induced phase transitions of TMA were cooperatively assisted by the thermal effect. To the best of our knowledge, the pioneers in this research field thus far have not discussed such combination strategy for

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Langmuir triggering supramolecular phase transformations. This simple method, namely, the synergic effect for controlling the supramolecular phase switching is useful in a range of research domains (e.g., host-guest chemistry). Examples include controlled capture and release of guest molecules (e.g., coronene and fullerene) incorporated in the chicken-wire assembly of TMA.36 Figure 5 shows a result using the synergiceffect approach toward controlled accommodation and release of coronene molecules incorporated in the chicken-wire assembly of TMA. In general, the capture of coronene takes place at negative voltage bias and room temperature while the release can be controlled to occur at positive voltage bias and an elevated temperature.

Figure 5. STM images and the corresponding scheme of the synergic-effect approach toward controlled accommodation and release of coronene incorporated in the chicken-wire assembly of TMA. The capture of coronene takes place at negative voltage bias and room temperature while the release can be controlled to occur at positive voltage bias and elevated temperature. Imaging conditions (Ebias, itunneling): ±0.90 V, 50 pA. The polarity of STM bias and the environmental temperature for STM imaging are noted on the image. The blue arrows indicate the lattice-structure directions of graphite underneath. For this experiment the sample is a 3-µL droplet of 1.0 mM TMA mixed with 1.0 mM coronene put onto surface and scanned by STM at the OA/graphite interface.

Although the temperature-related 2D molecular polymorphs including TMA have been abundantly studied in recent years, our present work provides significant results. First of all, it has been reported that TMA molecules can constitute various patterns at specific substrate temperatures. 31 However, the TMA phases in these studies are static. They are formed by a procedure in which TMA self-assembly occurs at a substrate pre-heated prior to molecular depositions. Moreover, these STM experiments were performed at room temperature. Hence, they can be considered as an ex-situ heating method. On the other hand, for the ultra-high vacuum (UHV) conditions, while the temperature at which molecular self-assembly takes place can be tuned, the assemblies formed under such conditions are irreversible in general.37 The manipulation of reversible supramolecular phase transitions requires a dynamic interface, for example the liquid/solid interface, and at least an external stimulation is needed and applied to the system in situ.38 Based on the viewpoints, our procedure is significant. In our present work, the environmental temperature of the systems is

maintained while STM is visualizing the process of molecular assembling at the liquid/solid interface. This enables the realtime observation and manipulation for the reversible switching of TMA polymorphs. Secondly, while temperature and fieldinduced reversible phase transformations of different molecules have been reported, the present work for the first time reports how the switching of molecular polymorphs in 2D can be controlled by synergic effect. The novelty of the present work is to combine the thermal and electrical stimuli for cooperatively controlling interfacial supramolecular patterning in an advance manner. 3.4. Discussion. Finally, some viewpoints to clarify the diverse experimental results of TMA found in different research groups on this subject. From literature survey, the porous chicken-wire TMA network has been widely used as a stable template for host-guest applications, unaffected by the electrical polarity of a substrate.18 This is consistent with the data presented in Figure 1, demonstrating that the TMA assembly is less sensitive to the changes of the electrical polarity of a substrate. Apart from the electrical polarity effect of a substrate, it has also been reported that concentration effect of TMA resulting from the treatment of long-time sonication can induce TMA to form various polymorphs on an HOPG surface.31 Such sonication processes can involve thermal stimuli that induce TMA deprotonations.32 The higher concentrations of the sample solutions lead to higher density packing formed on solid surfaces. In literature, the electric-field induced phase transitions of TMA have also been observed at room temperature. However, this phenomenon takes place rather sporadically as concluded by the authors. 16 Such sporadic occurrence may result from the ppm-level water molecules existing in organic solvents, thereby promoting the transition to occur at room temperature under positive STM bias.12 Elevated temperatures can improve the solubility and thus the concentration of TMA. The phase transition of chicken-wire and flower motifs takes place upon electrical polarity changes of substrate, indicating the existence of electrostatic attraction of TMA and the positively charged substrate. We note that it is also reported that the flower motif can be formed either due to solvent solvation effect (e.g., using heptanoic acid)39 or concentration effect (5 hours sonication). 31,40 Therefore, the formation of flower structure may or may not involve TMA deprotonation.31,39,40 Detailed studying of TMA polymorphs formed under positive STM bias is under progress. The significance of the present work is that the combination modulations of temperature and the electrical polarity of a substrate allow the phase transitions of TMA readily triggered, demonstrating that the cooperative effect can be applied for controlling molecular (re)assembly at the liquid/solid interface.

4. CONCLUSION In conclusion, exemplifying the elegant system of TMA we have reported a temperature-modulated method for programming the field-induced supramolecular phase transitions. We have shown using STM that specific polymorphs can be formed by the combined means of tuning an externally orientated electric-field and substrate temperature. In general, TMA molecules constitute a chicken-wire motif at room temperature on a graphite surface. At an elevated temperature and negative substrate bias, the phase remains porous. It can further be controlled and turn into flower and

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zigzag structures at positive substrate bias when the

temperatures are maintained at 45 °C and 68 °C, respectively. We attribute such phase switching to the partial deprotonation of the carboxyl acids of TMA. By MM simulations, we concluded that the adsorption enthalpy of TMA assembly decreases as a function of the increase of temperature and electric field values, which appreciably lowers the stability of the chicken-wire assembly of TMA. This may in turn promote the occurrence of the phase transition observed experimentally by STM. The free energy results obtained from a similar calculation is consistent with our STM observations. Overall, this study presents a synergic-effect strategy for advanced controls of interfacial supramolecular self-assembling processes.

ASSOCIATED CONTENT Supporting Information Supporting Information includes the STM and the simulation method as well as adding STM images. The Supporting Information is available free of charge on the ACS Publications website as word file.

AUTHOR INFORMATION Corresponding Author: Shern-Long Lee

E-mail: [email protected] Author Contributions +Authors

contributed equally to this work

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors thank Shenzhen University, Guangdong government (2018A030313467), Shenzhen city (1. the oversea talent set-up funding; 2. project: exploring multicomponent supramolecular systems, JCYJ20180305124732178); 3. Shenzhen University and National Taipei University of Technology jointed funding (2019008).

ABBREVIATIONS STM, scanning tunneling microscope; TMA, trimesic acid; BTB, 1,3,5-tris(4-carboxyphenyl)-benzene; OA, octanoic acid; EC-STM, electrochemical scanning tunneling microscope; UHV, ultra-high vacuum; MM, molecular mechanics; UFF, universal force field; 2D, 2-dimensional.

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SYNOPSIS TOC Synergic effect in supramolecular phase switching refers to a system where at least two types of external stimuli are introduced simultaneously for cooperatively affecting molecular self-assembly behavior. Here, we combine the thermal and electrical stimulations for con-trolling on-surface supramolecular patterning in a programmable fashion.

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