Nanotribological Study of Supramolecular Template Networks Induced

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Nanotribological Study of Supramolecular Template Networks Induced by Hydrogen Bond and van der Waals Force Hongyu Shi, Xinchun Lu, Yuhong Liu, Jian Song, Ke Deng, Qingdao Zeng, and Chen Wang ACS Nano, Just Accepted Manuscript • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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Nanotribological Study of Supramolecular Template Networks Induced by Hydrogen Bond and van der Waals Force Hongyu Shi,1,2 Xinchun Lu,1 Yuhong Liu,1,* Jian Song, 1 Ke Deng, 2,* Qingdao Zeng,2,* and Chen Wang2,*

1

2

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center

for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China.

ABSTRACT: Nanotribology has been given more and more attention by researchers for pursuing the nature of friction. In the present work, an approach which combines the supramolecular assembly and nanotribology is introduced. Herein, the nanotribological study was carried out on seven supramolecular template networks, namely, hydrogen bond induced tricarboxylic acids and van der Waals force induced hexaphenylbenzene (HPB) derivatives. The template networks, as well as the host-guest assemblies of template molecules induced by different forces, were constructed on the highly oriented pyrolytic graphite (HOPG) surface and explicitly characterized using scanning tunneling microscopy (STM). Meanwhile,

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the nanotribological properties of the template networks were measured using atomic force microscopy (AFM). Together with the theoretical calculation using density functional theory (DFT) method, it was revealed that the friction coefficients were positively correlated with the interaction strength. The frictional energy dissipation mainly derives from both the intermolecular interaction energy and the interaction energy between molecules and the substrate. The efforts not only help us gain insight into the competitive mechanisms of hydrogen bond and van der Waals force in supramolecular assembly, but also shed light on the origin of friction and the relationship between assembly structures and nanotribological properties at the molecular level.

KEYWORDS: supramolecular template, self-assembly, nanotribology, interaction energy, structure-property relationship


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In the current highly industrialized world, tribology has attracted more and more attention for saving energy and enhancing machine performance.1 In nature, the tribological behaviors, namely friction, lubrication and wear, which occur between rubbing material surfaces, are actually a macro manifestation of molecular behaviors.2-5 Therefore, in order to uncover the nature of tribology, it is essential to conduct the study from the molecular perspective. Moreover, with the development of microelectromechanical system (MEMS) devices, their tribological performance including friction, wear and adhesion becomes a non-negligible parameter, whereas the tribological theories in macroscale are inapplicable in this case.6-8 As a result, it is of great significance to promote nanotribology to investigate the intermolecular interaction mechanisms and explore the relationship between interfacial structures and properties.9 Self-assembly is an autonomous organization of components into regular patterns or structures, which has been applied in various fields like surface modification, optoelectronic materials, nanofabrication and life science.10-14 Based on the driven forces, interfacial self-assembly can be divided into two types, molecular and supramolecular assembly.15 For molecular assembly, the assembled molecules are chemisorbed on the surface via covalent bonds which are derived from the chemical reactions, for example, sulfurated compounds on Au and silanes on hydroxylation surfaces.16-18 In contrast, for supramolecular assembly, the assembled molecules are physisorbed on the surface. And the interactions among molecules are driven by a range of non-covalent bonds, such as hydrogen bond, van der Waals force and metal-ligand interaction.19 Scanning tunneling microscopy (STM) has proven itself as an


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ideal technique to gain direct insight into the supramolecular assembly structures with a subnanometer resolution.20 In short, self-assembly could endow the surfaces with regular molecular arrangements and functional properties, which provides an avenue for nanotribological studies at the interface. Nowadays, the researches on the tribological properties of self-assembled monolayers (SAMs) mainly focus on the molecular assembly. During the tribological processes, the shearing occurs between the assembled molecules instead of the friction pair surfaces, so that the interfacial tribological properties could be improved.21,22 Due to the high strength of the covalent bond, the assembled monolayers could resist destruction in some degree when shearing. However, the molecular assembly process is irreversible which makes the structures hard to be controlled. Also, the assembly systems are relatively limited, thus leading to the confinement in the scope of tribological studies. In comparison, the supramolecular assembly rightly makes up these disadvantages. Because of the non-covalent interactions among physisorbed assembled molecules, the assembly systems can achieve self-healing performance to reduce structural defects, and the assembly process can be regulated by external factors like temperature, light and electricity.23-25 Moreover, the range of the assembly systems is considerably extensive. A large amount of supramolecular assemblies of organic molecules (alkanes, porphyrin and phthalocyanine derivatives, peptides, etc.) have been constructed on the surfaces of carbon-based materials and specific metals.26-30 Therefore, although supramolecular assembly is not favorable for building “permanent” architectures, it could serve as a model to explore the intermolecular interaction mechanisms and


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structure-property relationship at the molecular level.19 Nevertheless, to the best of our knowledge, studies on the tribological behaviors of supramolecular assemblies are exceedingly rare.31-33 In our previous reported work, the friction forces of the saccharic acid/pyridine (co-assembled) system were measured to be larger than those of the pristine saccharic acid (non-assembled) system, which revealed the effect of intermolecular interactions on the nanotribological properties of assembly systems.34 However, the detailed relationship between friction forces and interactions with different bond energies still remains obscure. In the present work, we employed the supramolecular templates driven by different non-covalent bonds as the research objects. Supramolecular templates could enable the immobilization and isolation of functional guest molecules to form regular host-guest assemblies, thus providing a pathway to fabricate certain molecular devices.35,36 Herein, the template networks of two tricarboxylic acids (Scheme 1a and 1b) induced by hydrogen bond and five hexaphenylbenzene (HPB) derivatives (Scheme 1c) induced by van der Waals force were constructed on the highly oriented pyrolytic graphite (HOPG) surface. Their uniform and regular assembly structures were precisely revealed by STM. Moreover, the microscopic friction forces were measured on these template networks by atomic force microscopy (AFM). In combination with density functional theory (DFT) calculations, the origin of the friction force and the structure-property relationship were deeply explored.


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a

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b

c

Scheme 1. Chemical structures of (a) TMA, (b) BTB and (c) HPB derivatives.

RESULTS AND DISCUSSION

Template networks induced by hydrogen bond: TMA vs. BTB Due to the directionality and selectivity, hydrogen bond is considered as one of the most predominant interactions to enable the formation of supramolecular self-assemblies.25,37 TMA is a kind of tricarboxylic acid with three carboxyl groups symmetrically distributed around one benzene ring. It is an ideal building block for hydrogen bond induced template networks. First of all, the topography of TMA network on the HOPG surface was characterized by AFM. As shown in Figure S1a, no regular structures can be observed due to the resolution limitation of AFM. The large-scale STM image of the assembly structure of TMA network at the heptanoic acid/HOPG interface is shown in Figure S2a. It is revealed that TMA molecules assembled into a periodic honeycomb pattern, which is in good agreement with


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previously reported results.38-40 In the high-resolution STM image (Figure 1a), the bright spots measured to be 0.5 ± 0.1 nm are attributed to the benzene rings of TMA molecules. It can be figured out that each hexagon of the honeycomb network is composed of six TMA molecules, as illustrated in the molecular model calculated by DFT method in Figure 1b. Neighboring TMA molecules can form hydrogen bonds through the carboxyl groups, as marked with the red circle in Figure 1b. BTB is another three-fold symmetric tricarboxylic acid with three carboxyl groups connecting to the central π-conjugate part which consists of four benzene rings. The topography of BTB network (Figure S1b) characterized by AFM displays no obvious differences with those of TMA network and the pristine HOPG surface. From the high-resolution STM image of BTB network in Figure 1c (large-scale STM image in Figure S2b), it can be observed that the assembly structure of pristine BTB molecules adopts an oblique hexagonal pattern. The triangular elements were measured to be 1.2 ± 0.1 nm in length, which coincides well with the theoretical size of BTB. Corresponding molecular model was calculated based on the STM observation, as shown in Figure 1d. Although BTB is different from TMA in network pattern, they have the assembly motif in common. Two BTB molecules could interact with each other by forming double hydrogen bonds, as marked with the red circle. Such dimers can be arranged into a ladder via hydrogen bonding between the end carboxyl groups, as marked with the blue circle. Furthermore, the ladders form the zigzag network along the ladder edge. The parameters of the unit cells overlaid on the STM images of TMA and BTB networks were measured as follows: a1 = 1.6 ± 0.1 nm, b1 = 1.6 ± 0.1 nm, α1 = 60 ± 1°; a2 = 1.8 ± 0.1 nm, b2 = 2.9 ± 0.1 nm, α2 = 76 ± 1°.


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b b1 α1 a

1

5 nm

c

d b2 α2 a2

5 nm

Figure 1. High-resolution STM images of the assembly structures of (a) TMA and (c) BTB networks at the heptanoic acid/HOPG interface. Tunneling conditions: (a) Iset = 180.1 pA, Vbias = 846.9 mV; (c) Iset = 85.45 pA, Vbias = 678.7 mV. Calculated molecular models for (b) TMA and (d) BTB networks.

The nanotribological properties of these two template networks induced by hydrogen bond were investigated under ambient conditions by AFM. As mentioned in the Experimental Section, the measurements were conducted after the solvent was evaporated, so that the effect of the solvent on friction forces could be excluded. It should be noted that the drying process may have an effect on the assembly structures owing to the potential shear flow and defects.41,42 To avoid such effect, the samples were characterized at the gas-solid interface by STM before friction measurements. Experimental results show that the structures of TMA and BTB networks are with no observable changes after the solvent evaporation. Therefore, the correspondence between the STM and AFM results can be guaranteed. In Figure 2, friction forces of the TMA network, BTB network and HOPG are presented as a function of


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the applied load. For these three systems, friction forces increased with the load linearly. By comparison, the friction forces of the TMA network are about twice larger than those of the BTB network, while the friction values of pristine HOPG are the smallest. In order to obtain the friction coefficient, the friction-load data was linearly fitted. Then the friction coefficient can be defined as the slope of the linear fitting curve. One thing that should be paid attention to is that the fitting lines are above the original point, which is derived from the adhesion force between the AFM tip and the surface. Figure 2 displays the friction coefficients of the TMA network, BTB network and HOPG. It can be observed that the similar result as the friction force can be concluded, where the friction coefficient of the TMA network is about twice larger than that of the BTB network. In order to explore the underlying friction mechanism, the topographies of the TMA and BTB networks were further characterized by AFM before and after the friction measurements. Comparing the AFM images in Figure S3a and Figure S3b, no obvious topographical differences can be observed (such as defects and wrinkles) and the roughness values are also close to each other. Moreover, as shown in Figure S3c, there is no specific changes in the central area scanned by AFM probe compared with the surrounding area. Therefore, it can be inferred that the TMA and BTB networks could remain intact during the friction processes.


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Figure 2. Friction force as a function of applied load of TMA network, BTB network and HOPG.

In order to explore the relationships between nanotribological properties and intermolecular interactions, it is essential to evaluate the interaction strength from a global perspective. Obviously, each TMA molecule can form six complete hydrogen bonds with neighboring molecules as shown in Figure 1b. While, for each BTB molecule, only two complete hydrogen bonds can be formed and the other hydrogen bonds are less stable due to the certain angle between the O−H···O as shown in Figure 1d. Therefore, qualitatively speaking, the TMA network should be more stable than the BTB network. Combined with the friction results, it could be concluded that the TMA network with stronger intermolecular interaction strength has the larger friction force and friction coefficient.

Template networks induced by van der Waals force: HPB derivatives Van der Waals force is another common intermolecular interaction in supramolecular assemblies, especially for the molecules with alkyl chains.26,43 Compared with hydrogen bond, van der Waals force is non-directional and its average bond energy is lower. HPB is a series


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of molecules with different alkyl chain lengths. Seven benzene rings constitute the central π-conjugate part with high density of electronic states, from which six alkyl chains stretch out. Accordingly, HPB derivatives are good candidates for the formation of van der Waals force induced

template

networks.

Herein,

five

kinds

of

HPB

derivatives

(HPB-C8/C10/C12/C14/C16) were chosen as the assembly elements. Similar to TMA and BTB networks, the topographies of HPB-C8/C10/C12/C14/C16 networks (Figure S4) characterized by AFM display no regular structures. Figure 3a/3c/3e/3g/3i present the high-resolution STM images of HPB-C8/C10/C12/C14/C16 networks at the heptanoic acid/HOPG interface, respectively (large-scale STM images in Figure S5). It is worth noting that the six peripheral benzene rings tend to rotate by certain angle from the central plane to decrease the steric hindrance, and accordingly the central benzene ring is lower than the peripheral benzene rings. That would explain why the central π-conjugate part appears as a bright hollow circle in the STM images. The average diameter of the circles was measured to be 1.5 ± 0.1 nm, which corresponds to the previously published result.44 Comparing the STM images of the HPB networks, it can be observed that HPB-C8/C10/C12 molecules could assemble into a snowflake-like structure. The darker areas between neighboring circles are attributed to the alkyl chains. The only difference of their structures lies in the distances between neighboring circles. The molecular models were calculated based on the measured structural parameters, as shown in Figure 3b/3d/3f. It can be observed that the six alkyl chains of these three HPB molecules are fully stretched out and form an angle of 60° with respect to each other. Each alkyl chain interacts with the chain of the neighboring molecule


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through van der Waals force. The triangular cavities can be used to accommodate and immobile guest molecules.45 For the HPB-C14 network, most molecules still adopt the snowflake-like structure, as shown by the molecular model in Figure 3h. While in Figure 3g and Figure S5d, some molecules are arranged in parallel and more closely. It is interesting that a total different network for HPB-C16 appears in Figure S5e. From the STM image in Figure 3i, it can be clearly distinguished that all the alkyl chains are arranged in parallel. The calculated molecular model in Figure 3j further confirmed the observation. The six alkyl chains are evenly distributed on the two sides of the central conjugated backbone of HPB-C16 molecule. Similarly, the assembly motif derives from the van der Waals force between alkyl chains of neighboring molecules. Therefore, it can be concluded that with the increase of the alkyl chain length of HPB derivatives, their assembly structures would transform from the snowflake-like pattern to the linear array, and HPB-C14 should be the critical point between the two structures. The parameters of the unit cells overlaid on the STM images of HPB-C8/C10/C12/C14/C16 networks were measured as follows: a3 = 2.6 ± 0.1 nm, b3 = 2.6 ± 0.1 nm, α3 = 60 ± 1°; a4 = 2.8 ± 0.1 nm, b4 = 2.8 ± 0.1 nm, α4 = 60 ± 1°; a5 = 3.1 ± 0.1 nm, b5 = 3.1 ± 0.1 nm, α5 = 60 ± 1°; a6 = 3.4 ± 0.1 nm, b6 = 3.4 ± 0.1 nm, α6 = 60 ± 1°; a7 = 3.5 ± 0.2 nm, b7 = 2.4 ± 0.2 nm, α7 = 69 ± 2°.


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a

b

b3 a α3 3

5 nm

c

d

b4 α4 a4

5 nm

e

f

b5 α5 a5

5 nm

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g b6 α6

a6

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j

b7

a7 α7

5 nm

Figure 3. High-resolution STM images of the assembly structures of (a) HPB-C8, (c) HPB-C10, (e) HPB-C12, (g) HPB-C14 and (i) HPB-C16 networks at the heptanoic acid/HOPG interface. Tunneling conditions: (a) Iset = 271.6 pA, Vbias = 649.7 mV; (c) Iset = 207.5 pA, Vbias = 634.2 mV; (e) Iset = 299.1 pA, Vbias = 699.8 mV; (g) Iset = 213.6 pA, Vbias =


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690.0 mV; (i) Iset = 216.7 pA, Vbias = 693.1 mV. Calculated molecular models for (b) HPB-C8, (d) HPB-C10, (f) HPB-C12, (h) HPB-C14 and (j) HPB-C16 networks.

Furthermore, we have investigated the nanotribological properties of van der Waals force induced HPB networks using AFM. Before the measurements, we have characterized the samples at the gas-solid interface by STM, obtaining the identical nanostructures with the results in Figure 3. The friction forces of HPB-C8/C10/C12/C14/C16 networks and HOPG under various load conditions are presented in Figure 4. Similar to TMA and BTB, the adhesion forces between the AFM tip and the surfaces also affect the friction values. To make a more reasonable comparison, the corresponding friction coefficients were obtained via the slopes of the linear fitting curves of the friction-load data, as labeled in Figure 4. Results show that both the friction values and the friction coefficient of the HPB-C16 network are larger than those of other HPB networks. Although the friction values of HPB-C8/C10/C12 networks are larger than those of the HPB-C14 network, their friction coefficients are close to each other. As illustrated above, the HPB-C16 network adopts the linear array instead of the symmetric snowflake-like pattern. To further explore the possible effect of the scan direction, the friction forces of the HPB-C16 network with different sample angles were measured. It should be mentioned that because the scan angle is set to the fixed value (90°) when measuring friction forces by AFM, the experiments were conducted by turning the sample into different angles. As shown in Figure S6, under three different applied loads, the sample angle has a slight effect on the friction forces. However, because the resolution of AFM is not high enough to obtain the legible assembly structures, it is hard to confirm the exact scan


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direction of the AFM probe relative to the orientation of the HPB-C16 network. Despite the effect of the scan direction, the result that the friction values of the HPB-C16 network are larger than those of other HPB networks is not affected. In addition, comparing the AFM images of the HPB networks before and after the friction measurements as shown in Figure S7, there are no obvious changes in both topographies and roughness values, which indicates that the assembled networks could remain intact during the friction processes.

Figure 4. Friction force as a function of applied load of HPB-C8/C10/C12/C14/C16 networks and HOPG.

Obviously, the longer the alkyl chains in the molecules are, the stronger the van der Waals forces arising in the network would be. Furthermore, compared with the snowflake-like HPB-C8/C10/C12/C14 networks, the linear HPB-C16 network is more compact, indicating that more van der Waals forces arise among HPB-C16 molecules. Consequently, the intermolecular interactions among HPB-C16 molecules are the strongest. Combined with the friction results, it could be concluded that the HPB-C16 network with the strongest


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intermolecular interaction strength has the largest friction force and friction coefficient. From HPB-C8 to HPB-C14 networks, although the alkyl chains are longer, the unit cells are larger and their structures are less compact. Hence, it is unreliable to compare their intermolecular interaction strengths without any quantitative calculation.

Template networks induced by different forces: tricarboxylic acids vs. HPB derivatives In order to investigate the competitive relationship of different driven forces, tricarboxylic acid solutions and HPB derivative solutions were mixed and then deposited onto the HOPG surface for STM characterization. For the TMA/HPB co-assembled systems, results show that these two components assemble individually, as displayed in Figure S8. Clearly, due to the size mismatch, it is impossible for HPB molecules with long alkyl chains to be introduced into the TMA network, and TMA molecules were not observed to be trapped in the cavities formed by HPB molecules either. In comparison, when BTB and HPB molecules were co-adsorbed on the surface, results show complete difference. The BTB/HPB-C8 and BTB/HPB-C10 co-assembled structures at the heptanoic acid/HOPG interface are shown in the high-resolution STM images (Figure 5a and Figure 5c). Strikingly, it can be distinguished that the triangular BTB molecules have escaped the hydrogen bonding and been trapped into the cavities formed by HPB-C8 and HPB-C10 molecules as the guest. The parameters of the unit cells overlaid on the STM images of BTB/HPB-C8 and BTB/HPB-C10 host-guest assembly structures were measured as follows: a8 = 3.3 ± 0.1 nm, b8 = 3.3 ± 0.1 nm, α8 = 60 ± 1°; a9 = 3.4 ± 0.1 nm, b9 = 3.4 ±


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0.1 nm, α9 = 60 ± 1°. Compared with the pristine HPB-C8 and HPB-C10 networks, it can be inferred that the triangular cavities formed by HPB molecules have been enlarged with the inclusion of BTB molecules. As marked by red circles in the corresponding calculated molecular models (Figure 5b and Figure 5d), the interacted alkyl chains have been separated to some extent. Besides, the effect of component ratio on the co-assembled structure has been investigated further, as shown in Figure S9. When one component is excessive, the host-guest assemblies would co-exist with the pristine assembly (Figure S9a and Figure S9c). While, when two components are relatively balanced, the host-guest assemblies are fully formed (Figure S9b). It can be noticed that despite the different component ratios, the host-guest assemblies are distributed on the surface scatteredly instead of in large size. As mentioned above, the separation between interacted alkyl chains would cause the assembly systems to be less stable. Therefore, it is inferred that the small domain sizes and scattered distribution of the host-guest assemblies mainly derive from the low structural stability. However, when BTB molecules were mixed with other HPB molecules, namely HPB-C12/C14/C16, no host-guest assemblies could be observed. Figure S10 shows that BTB and HPB molecules assemble individually, similar to the TMA/HPB co-assembled systems. Therefore, it can be concluded that the competitive relationship between hydrogen bond and van der Waals force plays an important role in the tricarboxylic acid/HPB derivative co-assembled structures. In order to explore the competitive mechanism, quantitative calculation of their interaction strengths is necessary.


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b

b8 α8

a8

5 nm

c

d

b9 α9 a 9 5 nm

Figure 5. High-resolution STM images of (a) BTB/HPB-C8 and (c) BTB/HPB-C10 co-assembled structures at the heptanoic acid/HOPG interface. Tunneling conditions: (a) Iset = 33.57 pA, Vbias = 1072 mV; (c) Iset = 198.4 pA, Vbias = 678.7 mV. Calculated molecular models for (b) BTB/HPB-C8 and (d) BTB/HPB-C10 co-assembled structures.

The nanotribological properties of the template networks induced by different forces were compared, as shown in Figure 6. It can be observed that the error bars of friction values of BTB and HPB-C8/C10 networks are overlapped, indicating that the nanotribological properties of these three systems are close to each other.


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Figure 6. Friction force as a function of applied load of BTB network, HPB-C8/C10 networks and HOPG.

Theoretical calculation Through the comparisons of the template networks induced by hydrogen bond and van der Waals force, it can be concluded that there seems to be a correlation between the interaction strength and nanotribological property. To be specific, the network with stronger interaction strength mostly displays larger friction forces and friction coefficient. In contrast with the previous work which compared the co-assembled and non-assembled systems,34 the present work further compares the assembled systems induced by different intermolecular interactions. To further confirm this hypothesis, the interaction energies of template networks and host-guest assemblies were calculated with the DFT method based on the STM observations. Table 1 lists the calculated unit cell parameters of these systems, which agree well with the corresponding experimental results. The interaction energies between molecules were calculated, as listed in the first column in Table 2. It can be found that the intermolecular interaction energy of TMA (-89.407 kcal mol-1) is lower than that of BTB (-54.461 kcal mol-1), and the energy of HPB-C16 (-40.409 kcal mol-1) is the lowest among HPB derivatives. Herein, the lower energy indicates that the interaction is stronger. Therefore, the calculated results necessarily confirmed the above analyses about the intermolecular interaction strengths of template networks. Combined with the friction results, it seems that the friction force and friction coefficient are negatively correlated with the interaction energy between


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molecules (equal to positively correlated with the intermolecular interaction strength). However, this is not the case when comparing networks induced by different forces. In other words, the intermolecular interaction energy of BTB (-54.461 kcal mol-1) is lower than those of HPB-C8 (-21.479 kcal mol-1) and HPB-C10 (-23.744 kcal mol-1), while the friction values and friction coefficients of these three networks are close to each other (Figure 6).

Table 1. Experimental (Expt.) and calculated (Cal.) unit cell parameters of template networks and host-guest assemblies on the HOPG surface Unit cell parameters a (nm)

b (nm)

α (°)

Expt.

1.6 ± 0.1

1.6 ± 0.1

60 ± 1

Cal.

1.62

1.62

60.0

Expt.

1.8 ± 0.1

2.9 ± 0.1

76 ± 1

Cal.

1.70

3.05

76.0

Expt.

2.6 ± 0.1

2.6 ± 0.1

60 ± 1

Cal.

2.58

2.58

60

Expt.

2.8 ± 0.1

2.8 ± 0.1

60 ± 1

Cal.

2.83

2.83

60.0

Expt.

3.1 ± 0.1

3.1 ± 0.1

60 ± 1

Cal.

3.12

3.12

60

Expt.

3.4 ± 0.1

3.4 ± 0.1

60 ± 1

Cal.

3.35

3.35

60

Expt.

3.5 ± 0.2

2.4 ± 0.2

69 ± 2

Cal.

3.65

2.55

68.0

HPB-C16-fake

Cal.

3.65

3.65

60

BTB/HPB-C8

Expt.

3.3 ± 0.1

3.3 ± 0.1

60 ± 1

TMA

BTB

HPB-C8

HPB-C10

HPB-C12

HPB-C14

HPB-C16


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Cal. BTB/HPB-C10 Expt. Cal.

3.33

3.33

60

3.4 ± 0.1

3.4 ± 0.1

60 ± 1

3.35

3.35

60

Table 2. Total energies and energies per unit area of template networks and host-guest assemblies on the HOPG surface. The total energy includes the interaction energies between molecules and the interaction energies between molecules and substrate Interactions

Interactions between

Total energy

Total energy per

between molecules

molecules and substrate

(kcal mol-1)

unit area

(kcal mol-1)

(kcal mol-1)

TMA

-89.407

-41.415

-130.822

-0.576

BTB

-54.461

-94.575

-149.036

-0.296

HPB-C8

-21.479

-143.855

-165.334

-0.287

HPB-C10

-23.744

-173.929

-197.673

-0.285

HPB-C12

-24.458

-212.336

-236.794

-0.281

HPB-C14

-28.462

-220.268

-248.73

-0.256

HPB-C16

-40.409

-231.533

-271.942

-0.315

HPB-C16-fake

-30.706

-241.824

-272.53

-0.236

BTB/HPB-C8

-21.910

-189.627

-211.537

-0.220

BTB/HPB-C10 -35.645

-213.434

-249.079

-0.256

(kcal mol-1 Å-2)

In fact, for the surface supramolecular assembly systems, both the interactions between assembled molecules and the interactions between molecules and the substrate play an important role in the assembly processes. As shown in the second column in Table 2, the interactions between assembled molecules and the substrate of HPB networks are stronger than those of tricarboxylic acid networks, which mainly come from the van der Waals force


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between the alkyl chains of HPB molecules and HOPG.45 From the tribological view, this kind of interaction may have an impact on the friction results. Therefore, the total energies were calculated as the sum of these two kinds of interaction energies, as displayed in Table 2. In general, the total energy can be viewed as a parameter to evaluate the thermodynamic stability of the assembly systems with the same unit cell. However, for the template networks with different unit cells, it is more reasonable to use the total energy per unit area to evaluate their thermodynamic stabilities, which have been presented in the last column in Table 2. Firstly, it can be noticed that the total energy per unit area of TMA (-0.576 kcal mol-1 Å-2) is the lowest, suggesting that it is the most thermodynamically stable among these template networks. This could explain why TMA molecules were not observed to escape the hydrogen bonding and fall into the HPB networks in the TMA/HPB co-assembly systems (Figure S8). Secondly, for HPB networks, the total interaction energies decreased with the increase of the alkyl chain length of HPB molecules. However, from HPB-C8 to HPB-C14, it is interesting that the total energy per unit area becomes higher, which is due to the enlargement of the unit cell size. That is why the assembly structures transform from the loose snowflake-like pattern to the compact linear array with the increase of the alkyl chain length (Figure 3). As for HPB-C16, the total energy per unit area of the whole linear pattern is -0.315 kcal mol-1 Å-2, much lower than those of the other HPB template networks. As a comparison, the snowflake-like pattern of HPB-C16 was constructed as shown in Figure S11 and its total energy per unit area was calculated to be -0.236 kcal mol-1 Å-2. It is indicated that the linear HPB-C16 network is more energetically stable, which is consistent with the experimental


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results. Thirdly, as for the BTB/HPB-C8 and BTB/HPB-C10 co-assemblies, the intermolecular interactions include not only the interactions between HPB molecules but also the interactions between HPB and BTB molecules, which could explain the lower intermolecular interaction energies of these two co-assemblies than HPB assemblies. Similarly, due to the added interaction between BTB molecules and the substrate, the molecule-substrate interaction energies of the co-assemblies are lower than those of HPB assemblies. However, because of the enlargement of the unit cell sizes, the total energies per unit area of both BTB/HPB-C8 (-0.220 kcal mol-1 Å-2) and BTB/HPB-C10 (-0.256 kcal mol-1 Å-2) co-assemblies are higher than those of HPB assemblies. This result provides quantitative evidence for the decreased stability after the inclusion of BTB molecules, which accounts for the small domain sizes and scattered distribution of the co-assemblies as illustrated above. In addition, for BTB and HPB-C16, a dynamic transformation process was observed during STM characterization. Figure 7 shows that the BTB network was gradually replaced by the HPB-C16 network, indicating that the latter is more energetically stable and has an advantage in the competitive adsorption process. Such structural transformation confirms our theoretical calculation, in which the energy per unit area of HPB-C16 is much lower than that of BTB.

a

b

c HPB-C16

BTB

HPB-C16

HPB-C16

BTB

20 nm

20 nm

20 nm

Figure 7. Dynamic transformation of the BTB/HPB-C16 co-assembled structures at the


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heptanoic acid/HOPG interface. For clarity, the red dotted lines were drawn to distinguish the two assembled domains in (a) and (b). Tunneling conditions: Iset = 85.45 pA, Vbias = 678.7 mV.

To better understand the relationship between the interaction strength and the nanotribological property, the measured friction coefficients and calculated total energies per unit area of TMA, BTB and HPB-C8/C10/C12/C14/C16 template networks were summarized in Figure 8. It can be clearly observed that the friction coefficient is positively correlated with the total energy per unit area (absolute value). That is to say, the network with stronger interaction energy would display larger friction coefficient. In addition, the adhesion forces of the seven template networks were measured by AFM, as shown in Figure S12. Results show that the adhesion forces mainly derive from the capillary force at the gas-solid interface and have no specific connection with assembly structures or interaction strength.

Figure 8. Friction coefficients and total energies per unit area (absolute value) of TMA, BTB and HPB-C8/C10/C12/C14/C16 networks.

According to the above discussion, it could conceivably be hypothesized that there is a


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positive correlation between the interaction strength and nanotribological property. Herein, the interaction is actually a combination of the intermolecular interaction and the molecule-substrate interaction. During the sliding on the template networks, the AFM probe needs to overcome these physical interactions, which give rise to the torsion of the cantilever. As mentioned above, the topography images in Figure S3 and Figure S7 indicate that the assembled networks could remain intact during the friction processes, which means that the molecules can reassemble rapidly after the disturbance by the AFM probe, so that a dynamic equilibrium state can be achieved.19 Therefore, the friction process could be considered as a continuous alternation of the rupture and regeneration of the physical bonds among molecules and between molecules and the substrate. From the energy point of view, the frictional energy dissipation is closely related to the interaction energy which includes the intermolecular interaction energy and the interaction energy between molecules and the substrate. The origin of friction derives from the process of continuously overcoming the energy barrier on the surface.

CONCLUSIONS In summary, the assembly structures and nanotribological properties of template networks induced by hydrogen bond and van der Waals force were studied at the molecular level. High-resolution STM images precisely revealed the regular nanostructures of pristine networks, as well as the host-guest assembly behavior of template molecules induced by different forces, and enabled the exploration of the competitive mechanisms of hydrogen bond and van der Waals force. More importantly, the nanotribological properties were


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measured on the constructed template networks using AFM. Based on the qualitative structural analyses and quantitative theoretical calculation, it was concluded that the network with stronger interaction strength (sum of the intermolecular interaction and the molecule-substrate interaction) would display larger friction coefficient. The frictional energy dissipation process is actually an alternation of the rupture and regeneration of the physical bonds, which corresponds to continuously overcoming the energy barrier on the surface. This work enhances our understanding of the nature of friction and provides a promising way to exploring the structure-property relationship. From another point of view, on the basis of the established relationship between interaction and friction, not only the interaction strength can be used to predict the nanotribological property of supramolecular assembly system, but also the nanotribological property can function as a quantization parameter to describe the interaction strength of the supramolecular self-assembly, which is an effective approach to solving chemical problems from the tribological perspective.

EXPERIMENTAL SECTION Materials and sample preparation The two tricarboxylic acids, TMA (Scheme 1a) and BTB (Scheme 1b), were purchased from J&K Chemical Ltd. (Beijing, China). The five HPB derivatives (Scheme 1c) with different alkyl chain lengths, HPB-C8/C10/C12/C14/C16, were synthesized according to the reported method.44 Heptanoic acid (98%) used as the solvent was purchased from J&K Chemical Ltd. (Beijing, China). All the chemicals above were used without any further purification.


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The solid-state chemicals were dissolved in heptanoic acid with the concentration less than 10-4 M. Subsequently, the samples were prepared by depositing a droplet of the corresponding solutions (0.1 µL) onto the freshly cleaved HOPG (grade ZYA, NTMDT, Russia) surface for STM characterization of the template networks.

STM characterization The samples were characterized using a Nanoscope IIIA system (Bruker, USA) under ambient conditions. In order to achieve better imaging resolution of the template networks, the measurements were carried out at the liquid-solid interface by immersing the STM tip directly into the solution. All the images were captured with the mechanically made from Pt/Ir (80/20) wires under constant-current mode. The figure captions have included the specific tunneling conditions (i.e. tunneling current and bias). The drift for all the images was calibrated using an atomic-resolution HOPG lattice as a reference.

Friction force measurements The microscopic friction forces of different template networks were measured using an MFP-3D AFM (Asylum Research, USA) at room temperature. To eliminate the solvent effect on the friction forces, the measurements were conducted at the gas-solid interface after the solvent was evaporated. Before each measurement, the samples were characterized by STM to confirm the network structures in case of the impact of the drying process. The CSG10 probe with a rectangular cantilever (nominal normal spring constant 0.1 N m-1) was used. It should be mentioned that the genuine normal and lateral factors need to be calibrated prior to each measurement. The normal photodetector sensitivity was calculated from the slope of the


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force curve obtained on a hard substrate. The normal spring constant was estimated from the power spectral density of thermal noise fluctuations under ambient condition.46 By scanning a commercially available TGF11 silicon grating (MikroMasch), the lateral factor was calculated based on an improved wedge calibration method, so that the voltage results can be transformed to the force values.47,48 The microscopic friction forces were measured by scanning in the perpendicular direction to the cantilever in the contact mode. The scan size was 150 × 150 nm2 and the scan rate was 1 Hz. During the measurements, the feedback gains should be appropriate to make sure that the noise of the deflection signal was minimal while the height signal was tracked well. At least three positions were measured for each sample. For the data processing, friction values were calculated as half of the difference between the trace and retrace signals, so that the topography effect on friction can be excluded.

Computational details Theoretical calculations were performed using DFT provided by a DMol3 code.49 We used the periodic boundary conditions (PBC) to describe the 2D periodic structure on the graphite in this work. The Perdew and Wang parameterization of the local exchange correlation energy was applied in local spin density approximation (LSDA) to describe exchange and correlation.50 All-electron spin-unrestricted Kohn–Sham wave functions were expanded in a local atomic orbital basis. For the large system, the numerical basis set was applied. All calculations were all-electron ones, and performed with the medium mesh. A self-consistent field procedure was performed with a convergence criterion of 10−5 au on the energy and electron density. Combined with the experimental data, we have optimized the unit cell


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parameters and the geometry of the adsorbates in the unit cell. When the energy and density convergence criterion are reached, we could obtain the optimized parameters and the interaction energy between the adsorbates. To evaluate the interaction between the adsorbates and HOPG, we design the model system. In our work, adsorbates consist of π-conjugated benzene-ring. Since adsorption of benzene on graphite and graphene should be very similar, we have performed our calculations on infinite graphene monolayers using PBC. In the superlattice, graphene layers were separated by 40 Å in the normal direction and represented by orthorhombic unit cells containing two carbon atoms. When modeling the adsorbates on graphene, we used graphene supercells and sampled the Brillouin zone by using a 1 × 1 × 1 k-point mesh. The interaction energy, Einter, of adsorbates with graphite is given by Einter = Etot(adsorbates/graphene) − Etot(isolated adsorbates in vacuum) − Etot(graphene).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. AFM images and large-scale STM images of the assembly structures of TMA, BTB, HPB-C8/C10/C12/C14/C16 networks at the heptanoic acid/HOPG interface. Friction force as a function of sample angle of HPB-C16 network under different applied loads. STM images of TMA/HPB-C10, TMA/HPB-C12, BTB/HPB-C10, BTB/HPB-C12, BTB/HPB-C14, BTB/HPB-C16 co-assembled structures at the heptanoic acid/HOPG interface. Calculated molecular model for the snowflake-like HPB-C16 network (HPB-C16-fake). Adhesion of


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TMA, BTB, HPB-C8/C10/C12/C14/C16 networks and HOPG. The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

ACKNOWLEDGEMENTS

This work was financially supported by the National Natural Science Foundation of China (Nos. 51522504, 21472029, 21773041, 51527901) and the National Basic Research Program of China (Nos. 2016YFA0200700, 2017YFA0205001). Also, the authors gratefully acknowledge financial supports by State Key Laboratory of Tribology (No. SKLT2016D06).

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40. MacLeod, J. M.; Lipton-Duffin, J.; Fu, C.; Taerum, T.; Perepichka, D. F.; Rosei, F. A 2D Substitutional Solid Solution through Hydrogen Bonding of Molecular Building Blocks. ACS Nano 2017, 11, 8901-8909. 41. Abdeljalil, E.; Keita, B.; Nadjo, L.; Contant, R. STM and AFM Characterization of Thin Metal Oxide Films Electrodeposited from [P2Mo18O62]6-. J. Solid State Electr. 2001, 5, 94-106. 42. Wang, Z. G.; Zhou, C. Q.; Wang, C.; Wan, L. J.; Fang, X. H.; Bai, C. L. AFM and STM Study of Beta-Amyloid Aggregation on Graphite. Ultramicroscopy 2003, 97, 73-79. 43. Zhang, X.; Xu, S.; Li, M.; Shen, Y.; Wei, Z.; Wang, S.; Zeng, Q.; Wang, C. Photo-Induced Polymerization and Isomerization on the Surface Observed by Scanning Tunneling Microscopy. J. Phys. Chem. C 2012, 116, 8950-8955. 44. Zhang, R.; Wang, L.; Li, M.; Zhang, X.; Li, Y.; Shen, Y.; Zheng, Q.; Zeng, Q.; Wang, C. Heterogeneous Bilayer Molecular Structure at a Liquid-Solid Interface. Nanoscale 2011, 3, 3755-3759. 45. Chang, S.; Liu, R.; Wang, L.; Li, M.; Deng, K.; Zheng, Q.; Zeng, Q. Formation of Ordered Coronene Clusters in Template Utilizing the Structural Transformation of Hexaphenylbenzene Derivative Networks on Graphite Surface. ACS Nano 2016, 10, 342-348. 46. Hutter, J. L.; Bechhoefer, J. Calibration of Atomic-Force Microscope Tips. Rev. Sci. Instrum. 1993, 64, 1868-1873. 47. Ogletree, D. F.; Carpick, R. W.; Salmeron, M. Calibration of Frictional Forces in Atomic Force Microscopy. Rev. Sci. Instrum. 1996, 67, 3298-3306.


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48. Varenberg, M.; Etsion, I.; Halperin, G. An Improved Wedge Calibration Method for Lateral Force in Atomic Force Microscopy. Rev. Sci. Instrum. 2003, 74, 3362-3367. 49. Delley, B., From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756-7764. 50. Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation-Energy. Phys. Rev. B 1992, 45, 13244-13249.


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Table of Contents TMA

HPB-C10

BTB

HPB-C16


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Nanotribological Study of Supramolecular Template Networks Induced

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