A Molecular Switch by Adsorbing Au6 Cluster on Single-Walled

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A Molecular Switch by Adsorbing Au Cluster on Single-Walled Carbon Nanotubes: Role of Many-Body Effects ofSociety. vdW is published by the American Chemical 1155Forces Sixteenth Street N.W., Washington, DC 20036

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A Molecular Switch by Adsorbing Au6 Cluster on Single-Walled Carbon Nanotubes: Role of ManyBody Effects of vdW Forces Yun Chen, Wang Gao* and Qing Jiang Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130022, China. KEYWORDS: molecular switches, bistable states, nanoscale adsorption, many-body dispersion forces, density functional theory.

ABSTRACT: Advanced molecular switches are essential for constructing the basic components of nanodevices. Herein, a molecular-scale switchable system based on the bistable configurations of Au6 cluster on N-doped single-walled carbon nanotube is proposed, by using density functional theory method with many-body dispersion (MBD) forces. The delicate balance among Pauli repulsion, chemical binding, electrostatic interactions and MBD forces is found to be critical for achieving this molecular switch, while the many-body effects of dispersion forces are identified to be controllable by the former two interactions. These results demonstrate that chemical binding and Pauli repulsion transform the many-body effects of dispersion forces in low-dimensional adsorption systems from negative to positive, which are adjustable by adsorption distance, atomic


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volume, and anisotropy as well as adsorption configuration of adsorbates. These findings provide a means for tuning the stability of a given complex and promise a rational design of nanodevices.

1. INTRODUCTION Advanced molecular-scale electronics and mechanics are the cornerstone in nanotechnology, they are essential to nanoelectromechanical systems (NEMS), nanobiotechnologies as well as information technologies1–5. In particular, reversible molecular switches, depending on interconverting their multi-stable states by external stimuli, are the vital constituents in future ultrafast and ultra-dense functional molecular devices5–8. Many promising molecular switches have been proposed based on organic molecules, because their flexibility and diverse functional groups allow to achieve bistable states for simpler switching9–18. However, these switch systems based on adsorption always have to deposit the organic molecules on active metals or be connected with metal electrodes with the size of a few hundred nanometers at least. Hence, the switches at solely molecular scale are urgently needed for facilitating the ultimate miniaturization of nanodevices. Single-walled carbon nanotubes (SWNTs) have attracted tremendous interests for potential application in nano(opto)electronics and NEMS, because of their one-dimensional shape accompanied with unique electronic, mechanical and optical properties20–23. Through both postgrowth treatments and direct controlled synthesis24–27, SWNTs with finite length are easily achieved, and can serve as promising substrate for molecular switches. However, owing to the chemical inertness of pure SWNTs, both a reactive adsorbate and heteroatom doping have to be considered to enhance the adsorption activity for the switch system. For the reactive adsorbate, an anisotropic geometry is desired to induce the multi-stable states by distinct configurations. Owing to the intrinsic reactivity, the planar Au6 cluster with a D3h symmetry can chemisorb or physisorb


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on SWNTs with the distinguishable configurations28, it thus can serve as an ideal adsorbate in the molecular switch. Therefore, a systematic study of adsorption behavior of Au6 cluster on SWNTs is critical for exploring SWNT-based switches at a completely molecular level. In essence, atomic-scale adsorption is dominated by Pauli repulsion, electrostatic interactions, chemical binding and van der Waals (vdW) forces, which rely on the nature of adsorbate and substrate crucially. Owing to the low-dimensional nature of nanosystems, many-body effects of vdW forces play a crucial role in these adsorption processes. With nonlocal MBD method29,30, Ambrosetti et al. have identified a growth of stabilizing many-body effects of vdW forces with increasing separation between nanostructures at distance > ~1 nm, yielding the MBD forces decay slower relative to the pairwise treatment of vdW forces31. They attribute it to the nontrivial coupling of delocalized dipole fluctuations between nanomaterials. Moreover, Chattopadhyaya et al. indicated that the MBD forces can be tuned by nanostructure environment, leading to either stabilization or destabilization of a given system and accordingly suggesting a potential application for designing innovative nanomaterials32. However, these studies did not elucidate the determining factors that control the many-body effects of vdW forces around equilibrium adsorption distance (< 1 nm), which are crucial for understanding adsorption behavior and are thus more interesting for applications. In this contribution, we adopted density-functional calculations with nonlocal many-body dispersion forces (DFT+MBD) 29,30 to systematically explore the adsorption mechanisms of Au6 cluster on SWNTs. A completely molecular-scale switch is proposed, which is realized by converting the bistable lying and standing configurations of Au6 cluster on N-doped SWNT. We find that the delicate balance among Pauli repulsion, chemical binding, electrostatic interactions and vdW forces is critical for achieving the molecular switch, while the many-body effects of vdW


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forces are controllable by the former two interactions. Our results demonstrate that chemical binding and Pauli repulsion can transform the many-body effects of vdW forces in lowdimensional systems from negative to positive. Meanwhile, these effects depend strongly on anisotropy, atomic volume, and adsorption configuration of adsorbates. Our study reveals the mechanism of dynamic response of interactions in adsorption systems, promising the rational design of miniaturized devices in nanotechnology. 2. METHODS All calculations were carried out by the FHI-aims code with a “tight” basis set33. Perdew–Burke– Ernzerhof (PBE)34 augmented with TS35 and MBD29,30 methods were employed to obtain the geometric and energetic details. Based on electron density, TS method adopts a pairwise-additive description for the long-range vdW correlation, in which the atomic polarizabilities and C6 coefficients are derived from the local hybridization effects. Apparently, the pairwise approach cannot capture the long-range collective many-body response of vdW forces, which is pronounced in the low-dimensional systems, due to the highly anisotropic polarizability. Beyond TS, the MBD method enables to account for the collective many-body vdW forces based on the adiabatic connection fluctuation-dissipation theorem in a RPA-like treatment. Depending on the coupled fluctuating dipole model in the MBD framework, the valence electronic response takes an accurate quantum mechanical parameterization, which makes the systems obtain a chemically accurate treatment. The adsorption energy 𝐸𝑎𝑑 is calculated as 𝐸𝑎𝑑 = 𝐸adsorbate/SWNT ― 𝐸𝑆𝑊𝑁𝑇 ― 𝐸adsorbate


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where 𝐸adsorbate/SWNT is the total energy of adsorption system, while 𝐸SWNT and 𝐸adsorbaterepresent the energies of isolated SWNT and adsorbate, respectively. To better understand adsorption properties, the total adsorption energy is separated into the long-range vdW contribution, EMBD and ETS, and the PBE contribution EPBE that includes Pauli repulsion, electrostatic interactions, chemical binding and the short-range correlation. We used a vacuum width of 100 Å to separate periodic images and a k-point grid of 1 × 1 × 2 for 1 × 1 × 5 supercell of SWNTs, which converge adsorption energy to a meV/atom level. In addition, the diffusion barriers of Au6 cluster and the activation barriers for configuration transition are calculated by the linear/quadratic synchronous transit method36, as implemented in the CASTEP code37. Constant-current scanning tunneling microscopy (STM) simulation is achieved to characterize the adsorption state of Au6 cluster on SWNTs by the Tersoff-Hamann approximation38. 3. RESULTS AND DISCUSSION The key for a molecular switch is bistability and reversibility4, which require two equally stable states and demand feasible activation barriers for switching. Meanwhile, the substrates should be able to prohibit the diffusion of adsorbates. We find that the planar Au6 cluster and a substrate with particular chemical reactivity can meet these requirements. To achieve the bistable adsorption states, we modulate the substrates by choosing the certain curvature SWNTs, since the large curvature generally corresponds to great chemical reactivity39. Hence, we begin with exploring the adsorption of Au6 cluster on pure zigzag SWNTs with radii of 3.9 –7.8 Å, namely SWNTs (10,0), (14,0), (17,0) and (20,0), to demonstrate the bistability concept. The adsorption configurations and energies are summarized in Figure 1 and Table S1 in Supporting Information. Au6 cluster is found to take two optimal configurations on SWNTs — lying and standing with comparable stability in


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Figures 1(a)-(c). This indicates a possibly reversible standing-to-lying transition in Au6/SWNT systems, suggesting a potential molecular switch system.

Figure 1. (a) Total adsorption energies of Au6 cluster lying/standing (L/S) on pure and N-doped SWNTs, obtained by PBE+MBD. (b)[(c)] Schematic illustration of lying (standing) configuration of Au6 cluster on pure SWNT (14,0) in top view. The insets are the simulated STM spectra for both modes at a positive bias voltage of 1 V. (d) Relative energy diagrams of the bistable states transition on pure and N-doped SWNTs (14,0). Potential energies of the initial states in both cases are set to zero for comparison. (e) The ratios of EMBD/ETS for the cases shown in (a).

We now study the origin of the two optimal configurations. When Au6 cluster adopts the lying mode, its atomic plane is parallel to the surfaces of SWNTs [Figure 1(b)], by which the vdW interactions between the adsorbate and the substrates are maximized by minimizing the average adsorption distance. On the other hand, the adsorption distances (d = 3.29 - 3.35 Å) are smaller than the sum of vdW radii of C and Au atom (1.7 + 1.66 = 3.36 Å), indicating that Pauli repulsion is also crucial to the lying mode. This is further confirmed by the positive adsorption energies by PBE [Table S1]. In the standing mode, the atomic plane of Au6 cluster is perpendicular to the surfaces of SWNTs [Figure 1(c)] and sits at the top site. Although the contribution to adsorption energy is still mostly from vdW forces (75% at least), the chemical binding between SWNT and Au6 cluster is essential in generating the standing configuration. The weaker chemical binding with respect to vdW forces is attributable to the intrinsic chemical stability of Au6 cluster. In the two modes, the adsorption strength of Au6 cluster exhibits the opposite trends with the change of SWNT size [Figure 1(a)]. In the case of the standing mode, the strength of the chemical binding


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of Au6 cluster to SWNTs is inversely proportional to the SWNT size, because the intrinsic chemical reactivity of SWNTs decreases with increasing size. Consequently, the total adsorption strength of Au6 cluster decays generally with increasing size of SWNTs, since the chemical binding governs the adsorption properties in the standing mode. In terms of the lying mode, the vdW forces dictate the adsorption nature, which are increased markedly with increasing SWNT size due to the decreased average adsorption distances. Accordingly, the total adsorption strength of Au6 cluster is enhanced with the increase of SWNT size. As a result, Au6 cluster prefers the standing mode when the radius of SWNT is smaller than 5.5 Å, but the lying mode when the radius of SWNT is larger than 5.5 Å. Interestingly, a coexistence of the two modes with identical adsorption energies appears when Au6 cluster adsorbs on the pure SWNT with the radius of 5.5 Å, namely SWNT (14,0). This suggests the Au6/SWNT (14,0) system to be a potential molecular switch with a reversible lying-to-standing transition. For practical application, a moderate and feasible configuration transition as well as suppressed surface diffusion of adsorbate are crucial for a switchable system. This can be evaluated from the transition barrier between the lying and standing modes and the surface diffusion barrier of Au6 cluster on SWNT. On pure SWNT (14,0), the activation barrier for configuration transition is about 0.14 eV [Figure 1(d)], which indicates even minor thermal fluctuation is able to switch the two modes randomly and harms the switching performance. In addition, a nearly barrierless diffusion is also found in the lying mode, implying a quick migration of Au6 cluster and the incapacitation of the system for switching. Therefore, one needs to improve the switching and diffusion barriers for Au6 cluster on substrate simultaneously. Because of the similar atomic size compared to C atom and the electron-donor nature, a single N dopant is implemented in pure SWNTs to investigate the means for improving switching


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performance. The N dopant is found to increase the adsorption energies for both modes [Figure 1(a) and Table S1], suggesting the enhanced chemical reactivity of N-doped (N-) SWNTs and thereby the higher stability of Au6/N-SWNT systems compared to pure SWNT systems. Owing to the strong electronegativity of N atom, the substitution of C atom by N atom induces the redistribution of electron density and forms the polar C-N bond in the N-SWNTs. As a result, it triggers the induction interactions between Au6 cluster and the N-SWNTs and enhances the adsorption strength for both modes. For the lying mode, the induction interactions tune the PBE adsorption energy to be negative (see Table S1). In the case of standing mode, the stronger Coulomb interactions and chemical binding are caused in addition to the induction interactions, since more electrons transfer to the adsorbate from the N-SWNTs relative to those on the pure SWNTs (see Table S2 and Figure S2). Interestingly, we find that the adsorption configurations of Au6 cluster on N-SWNTs are the same as those on pure ones [see Figure S1 in Supporting Information], with the lying and standing modes being equally stable on N-SWNT (14,0) [Figure 1(a)]. Moreover, the diffusion barrier of Au6 cluster is increased to 0.19 eV while the activation barrier for configuration transition is raised to ~0.35 eV. It demonstrates such N doped SWNT anchors Au6 cluster effectively, consequently improving the accessibility of adsorbate for switching and preserving the function of switching up to a relatively high temperature. Meanwhile, in consideration of the competition from the rotating of the standing Au6 cluster around the chemical Au-C bond, the rotation barrier is also calculated. As a result, we find a barrier of 0.204 eV to trigger the standing cluster for 90 degrees of rotation, which is smaller than the switching barrier and could affect the switch efficiency when using the heat stimulation. Nevertheless, if using pressure to switch from standing mode to lying one, one could avoid this issue and takes advantage of the switch functionality of Au6/N-SWNT.


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Hence, proper external stimuli should be applied to trigger molecular switch, such as pressure, forces and so on7. Particularly, an external charge stimuli is likely effective for the vdW- dominant switch system, because it can modulate dispersion forces by up to 35%40. Moreover, the corresponding states can be characterized by a variety of experimental techniques. As shown in Figures 1(b) and 1(c) clearly, two modes exhibit distinct STM spectra. The completely molecular conformation is seen as six spots in STM image for the lying Au6 cluster, while only one unambiguous spot accounting for the top Au atom of adsorbate is observed on STM projection for the standing mode. Therefore, our proposed molecular switch can be monitored easily and effectively. To understand the underlying mechanism of atomic-scale adsorption, we further investigate the interplay of Pauli repulsion, chemical binding, electrostatic interactions, and vdW forces with respect to adsorption distance, by focusing on the many-body effects of vdW forces. In general, pairwise TS method35 misses the long-range collective many-body effects and overestimates vdW forces for low-dimensional adsorption systems. To clarify the role of many-body effects of vdW forces, we thus define a ratio of EMBD/ETS to quantify these effects. These effects are positive when EMBD/ETS > 1 and negative when EMBD/ETS < 1, whereas they are eliminated as neutral around EMBD/ETS = 1. Intriguingly, our results identify the significantly negative effects on the lying mode but nearly neutral effects on the standing case at the equilibrium adsorption distance, for the manybody effects of vdW forces [Figure 1(e)]. To uncover the mechanism behind this phenomenon, we calculated binding curves of Au6 cluster lying/standing on pure SWNT (10,0) and plotted the ratio of EMBD/ETS as a function of adsorption distance in Figure 2(a). There are always minima in the curves, we thus define the range beyond this critical point (but still < 1 nm) as to be Ll-range and that within this critical point to be


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Ls-range. Strikingly, a distance-dependent behavior is observed in many-body effects of vdW forces, in which they are negative within Ll-range while varying from negative to positive within

Figure 2. (a) The ratios of EMBD/ETS for Au6 cluster lying/standing (L/S) on pure SWNT (10,0), as a function of average adsorption distance dave (The nearest distance between Au6 cluster and SWNT in standing mode, dS,Au-C, is also shown as the abscissa on the top.).The related PBE potential energies are shown in (b). (c) MBD eigenvalue spectrum of single pure SWNT (10,0) relating with the 3N collective eigenmodes by MBD model in ascending order. As shown in insets, the predominant polarizability of SWNT is that in the longitudinal direction with the lowest energy, the second is that around the tubular circle, and the one perpendicular to the circumferential surface has the negligible contribution to the total polarizability due to its high energy. At the top left corner, the Cartesian components of total polarizability of single Au6 cluster in atomic unit are also shown. (d) MBD eigenvalue spectrum for Au6 cluster lying/standing (L/S) on pure SWNT (10,0) at different adsorption distance.

Ls-range. These behaviors further reveal the impacts of geometry and interactions on the manybody effects of vdW forces. Within the Ll adsorption range, where the electron density of adsorption systems is still highly close to the isolated adsorbate and substrate, the delocalized dipole fluctuations that stems from the highly anisotropic polarizability of low dimensional


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materials give rise to a destabilizing contribution to adsorption41, accordingly resulting in the negative many-body effects in the Au6/SWNT systems. With the separation decreasing further, the destabilization effect gets more pronounced and continually governs the negative many-body effects of vdW forces. Once adsorption distance is smaller than the critical point, the effective electron density overlap appears between Au6 cluster and SWNT and causes Pauli repulsion (Pauli repulsion and/or chemical binding) in the lying (standing) mode. As shown in Figure 2(b), the critical point of many-body effects locates at the equilibrium position between Pauli repulsion and Coulomb attractive interactions in the lying mode, while in the standing mode it is at the position where the chemical binding between Au6 cluster and substrate forms and the PBE energy curve gets steep abruptly. Besides the local hybridization effects of electron density, Pauli repulsion and chemical binding induce a marked delocalization of the dipole fluctuations across Au6 cluster and SWNT and generate a nontrivial coupling between them. This is verified further by the MBD eigenvalue spectrum analysis31. It is known that the quantity αiωi2, where αi represents the static polarizability of the ith MBD eigenmode and ωi is the corresponding resonant frequency, remains constant for every MBD eigenmode due to the charge conservation. Thereby, a low-energy MBD eigenmode with a high polarizability corresponds to the markedly delocalized dipole fluctuations in single objects and further reflects a nontrivial coupling between them in systems. As depicted in Figure 2(d), the downward tendency of MBD eigenvalue spectrums with decreasing distance indicates the increasingly stronger coupling of delocalized dipole fluctuations between Au6 cluster and SWNTs in both modes. Hence, the positive many-body effects of vdW forces gradually counteract the negative ones, resulting in altering the net effects from negative to positive in the range of Ls.


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In addition, we also find the many-body effects of vdW forces strongly rely on adsorption configuration. As Figure 2(a), at the same adsorption separation, the ratios of EMBD/ETS are always smaller in the lying case than in the standing one. On the one hand, it results from the prominent destabilizing many-body vdW forces in cases where the larger components of polarizability are aligned41. Thereby, the lying mode, with the predominant in-plane polarizability of Au6 cluster aligning with the dominant polarizability of SWNT along the cylindrical surface [Figure 2(c)], accounts for the more negative many-body effects than the standing case. On the other hand, the standing mode takes the stronger response from Pauli repulsion and chemical binding than the lying case within Ls-range. With the lower MBD energy eigenvalues [Figure 2(d)], the standing case produces the more pronounced coupling between the delocalized dipole fluctuations and results in the more positive many-body effects of vdW forces. We now analyze the dependence of many-body effects of vdW forces on adsorbate characteristics, by comparing the adsorption behavior of isotropic atoms Ar, Xe, and planar molecules benzene and Au6 cluster lying on pure SWNT (10,0) [Figure 3]. We find that atomic volume and anisotropy of adsorbates are also crucial in determining the many-body effects of vdW forces. Within Ll-range, the lying planar adsorbates will take the more destabilizing many-body effects relative to the isotropic ones, due to the additional contributions from anisotropy. Therefore, benzene with the most pronounced anisotropy takes the strongest negative effects, while Au6 cluster takes the second place. For isotropic atoms, the small atomic volume with tight binding of extranuclear electrons weakens the response between adsorbate and substrate. Thereby, Ar atom experiences the weakest negative many-body effects, while Xe atom takes the third place. Within Ls-range, the adsorbate with large atomic volume should exhibit the marked positive manybody effects of vdW forces, due to the strong Pauli repulsion with SWNT. However, the additional


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destabilizing many-body effects from the adsorbate anisotropy will abate the positive parts. Hence, Xe atom shows the strongest effects in the positive-effects section, while Au6 cluster takes the second place. In contrast, the most anisotropic benzene with the smallest atomic volume has the weakest positive many-body effects among the adsorbates. Lastly, Ar atom takes the third place in the range of Ls.

Figure 3. The ratios of EMBD/ETS for Ar, Xe, benzene and Au6 cluster lying on pure SWNT (10,0), as a function of average adsorption distance dave.

4. CONCLUSIONS In conclusion, a promising switch at solely molecular scale has been proposed utilizing PBE+MBD method, which relies on converting the lying and standing configurations of Au6 cluster on NSWNT (14,0). We find that the delicate balance among Pauli repulsion, chemical binding, electrostatic interactions and MBD forces is critical for achieving the molecular switch, while the many-body effects of vdW forces are essentially controlled by the former two interactions. Our results demonstrate that chemical binding and Pauli repulsion can transform the many-body effects of vdW forces in low-dimensional systems from negative to positive. In addition, these effects on dispersion forces are found to depend strongly on anisotropy, atomic volume, and adsorption


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configuration of adsorbates. Our findings not only open the routines to realize a completely molecular-scale switch, but also provide the means of tuning the dynamic response of interactions between nanomaterials for rational design of nanodevices.

ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Numerical details of the Au6/(N-)SWNT adsorption systems and the stable adsorption configurations of Au6 cluster on N-SWNT (14,0) as well as the charge density difference analysis. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the support from the Program of the Thousand Young Talents Plan, the National Natural Science Foundation of China (No. 21673095, 51631004), the Program of Innovative Research Team (in Science and Technology) in University of Jilin Province, and the computing resources of High Performance Computing Center of Jilin University and National Supercomputing Center in Jinan, China


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A Molecular Switch by Adsorbing Au6 Cluster on Single-Walled

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