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Ar Implantation at the hBN/Rh(111) Nanomesh by ab initio Molecular Dynamics Marcella Iannuzzi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06774 • Publication Date (Web): 28 Aug 2015 Downloaded from http://pubs.acs.org on September 5, 2015

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The Journal of Physical Chemistry

Ar Implantation at the hBN/Rh(111) Nanomesh by ab initio Molecular Dynamics Marcella Iannuzzi∗ Institut für Chemie, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland E-mail: [email protected]

Abstract The controlled intercalation of selected species underneath graphene or hexagonal boron nitride (hBN) on a substrate opens new ways for the functionalization and the tuning of properties of these systems. In this work, the case of hBN on rhodium exposed to a low energy Ar-ion beam [Cun et al., Nano Letters 2013] is further considered theoretically. With the help of ab initio molecular dynamics, the structural rearrangements induced by the interaction between the impacting Ar ion and the substrate are investigated. It is shown that the ion can be intercalated and trapped between the metal and hBN by breaking some BN bonds and penetrating the over layer. The resulting defective structure relaxes quickly while Ar moves to a stable site where a protrusion appears on top of the characteristic super honeycomb lattice of the nanomesh. The presented results provide a first atomistic description of the complex processes leading to the formation of the, so called, boron nitride nanotents.

Keywords: hexagonal boron nitride, implantation, thermal stability, nanomesh, nanotent, density functional theory, molecular dynamics

∗ To

whom correspondence should be addressed

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Introduction The growth of hexagonal boron nitride (hBN) on various transition metals leads to formation of highly periodic superstructures 1 that have been extensively studied 2–8 in the past few years. Of particular relevance is the variation of the hBN interaction with the underlaying metal, which is responsible for the topographic corrugation of the monolayer, the spatial modulation of the BN bond length as well as of the electronic structure. Not only these features can be exploited to template self-assembly of molecules 9–11 and selective adsorption processes, 5,12–16 but also specific chemical reaction and directed functionalization have been proved to be feasible. 17–19 Recently, the intercalation of single atoms (H 20 , Ar 21–23 , O 24 , Co 25 , Si 26 ) and small molecules (CO 27–29 ,O2 30,31 ,H2 O 32 ) under sp2 hybridized hBN and graphene layers grown on transition metals has been achieved. In most of the cases, intercalation is thought to occur under sufficient gas pressure by diffusion through extended defects (e.g., grain boundaries, island edges). The success of such processes depends on the competing binding of the adsorbate with the metal in comparison to the strength of the interaction between the metal and the 2D film. Thereby, the intercalation changes the structural and electronic properties of the system, often decoupling the sp2 layer from the metallic substrate 20,29,33 . On the other hand, the removal of the intercalated species is in general also possible upon annealing. Another investigated mechanism is the functionalization by low energy ion implantation. After exposing to “low energy” Ar ion beam the hexagonal boron nitride nanomesh 1 (NM) at room temperature, scanning tunnelling spectroscopy (STM) reveals the presence of protrusions on top of the characteristic super honeycomb structure. These have been interpreted as Ar atoms intercalated between the sp2 bonded boron nitride layer and the rhodium substrate. 22 Similar implantation phenomena have also been obtained with Ne 22 and Rb 34 atoms, and are expected to be possible for other atoms as well. These new features decorating the NM have been named nanotents. They are found at distinct sites in the 3.2 nm super honeycomb unit cell, are stable at room temperature, and survive exposure to air. Irradiation by ion beam has been applied also to graphene supported on Ir(111) 35,36 and Ru(0001) 21 leading to the controlled creation of defects and the implantation 2 ACS Paragon Plus Environment

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of ions in the interface region. Also in these cases, the modulation of the interaction between the metal and graphene characteristic of the Moiré-like superstructure determines the observed patterns of defects. The implantation process is believed to occur by Ar creating local damage on an otherwise pristine hBN over layer and being trapped underneath. Indeed, the structural analysis carried out after the exposure shows an intact sample, where the NM is still recognisable. The only indication of damage are some relatively small irregularities that sometime appear in the vicinity of the protrusions and have been associated to vacancy defects formed during the dosing. 23 Moreover, processing the sample by several annealing cycles, the NM structure is not further deteriorated, but rather the dynamics of the nanotents, by hopping and clustering, can be observed over an otherwise regular super-honeycomb lattice. 37 Better understanding of this type of implantation process is of great relevance because the controlled functionalization of the sp2 layers on transition metals might improve the templating character of the NM, for example to induce molecular assembly or specific modulation patterns of electronic and magnetic properties. In the previous works on the nanotents, density functional theory (DFT) calculations have been used to study the structural properties of the intercalated Ar atom. The site selectivity observed in experiment has been reproduced by the calculations. 22 Moreover, it has been confirmed that nanotents formed by more Ar atoms aggregated in a cluster are favoured. The stability of single and pair vacancies in the hBN layer has also been investigated by DFT, 23,38 concluding that the interaction with Rh favours the stabilisation of the dangling bonds. This work is a first attempt to describe the dynamical processes that lead to the implantation of Ar and the successive stabilisation of the generated defects and of the nanotent itself. DFT calculations are applied in combination with molecular dynamics (MD) to shed light on the microstructural evolution of the NM in the few femtosecond following the impact of Ar, and in the few picosencods following the effective implantation, leading to the stabilisation of the nanotent.



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Computational Details The simulations reported in the present work are based on electronic structure calculations using Kohn–Sham density functional theory within the Gaussian and plane waves (GPW) formalism as implemented in the Quickstep module in the CP2K program package. 39 Dual-space pseudopotentials 40 41 are used to describe the interaction of valence electrons with atomic cores. Exchange and correlation are calculated with the Perdew-Burke-Ernzerhof (PBE) 42,43 GGA exchange-correlation (XC) functional. Long-range dispersion interactions are been computed using DFT-D3 formalisms 44 (more details in SI). We employ a slab model that consists of four 12×12 Rh(111) atomic layers, covered on one side by the 13×13 hBN over layer. The computational set-up used for this work reproduces the NM properties in good agreement with experiment. 15,38 The optimisation leads to the corrugation of hBN and the formation of the NM pore where the registry corresponds to (N-top,B-fcc). The pore is defined as the flat area where hBN lays between 2.2 and 2.6 Å over the Rh surface. In this model the pore occupies more than 40% of the NM unit cell. Here, the interaction with the substrate is dominated by the charge redistribution at the N centers, which is polarised towards the Rh surface. In order to optimise this interaction, the BN bonds in the pore are stretched up to two percent more than the equilibrium bond length. The stretching strain is compensated by the compression of the BN bonds in the surrounding region, the wire. Here, the interaction with the metal is mainly dominated by the dispersion forces. The transition region, where hBN bends to connect pore and wire, is called the rim. The Tersoff-Hamann approximation 45,46 for STM simulations (THSTM) is used to reproduce the iso-current topography above the nanomesh. The THSTM image calculated from the described model reproduces very well size and shape of the pore region. The slight over-estimation of the corrugation along the density profile is a common feature of the this approximation. Figure 1 summarises some results obtained with the described NM model. Panel a is a top view of the replicated NM unit cell, where the color code represents the height of the BN bonds over Rh(111). The wire presents two non equivalent crossing sites differing for registry. WXA is where (B-hcp,N4 ACS Paragon Plus Environment

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fcc), and it is slightly higher (0.2 Å) over Rh than WXB where (B-top,N-hcp). The deviation from the equilibrium BN bond length (1.453 Å) versus the height over Rh is plotted in panel b. The distribution among pore, rim and wire is clearly distinguished, they correspond to 40%, 20% and 36% of the NM unit cell, respectively. The THSTM image obtained for this NM model is reported in panel c together with the topographic profile as calculated along the red line running over the unit cell through the pore (Fig. 1d). Ab initio molecular dynamics 47 (AIMD) simulations are carried out with the described NM model, always within the spin polarised density functional approximation. The temperature is controlled by coupling the system with an external thermostat. 48 The integration time step is tuned on the fly, by setting a maximum value of 0.8 fs, and reducing it to smaller values when needed, in order to avoid that atomic displacements become larger than 0.1 Å per integration step. This expedient is particularly important when simulating Ar impacting against the NM with a starting kinetic energy of tens of eV. In order to more efficiently observe the possible relaxation processes occurring after the implantation of Ar, the metadynamics approach (MTD) 49,50 is also used. MTD accelerates the exploration of the accessible phase space, thus allowing the simulation of rare events that otherwise would require unfeasibly long simulation times. It is based on the definition of a few collective variables (CVs), which are conveniently selected to describe the most relevant structural changes. In the present work, the CVs need to be associated with the formation or the healing of damage in the hBN mesh. A natural choice of CV is the coordination number (related to the number of BN bonds) of those B and N atoms involved in the defective structure. The nudged elastic band method 51 (NEB) is applied in order to estimate the potential energy barrier for the diffusion of the implanted Ar atom. The initial and final states are the optimised configurations of Ar trapped at wire crossing sites. The minimum energy pathway describes the diffusion of Ar over a distance of about 20 Å and it is described by 48 NEB beads.


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Implantation process by AIMD The implantation occurs by sputtering ions with sufficiently large kinetic energy, such that the hBN can be penetrated, but small enough to minimise the damage on the substrate. The initial kinetic energy, the impact angle and the impact site on the NM are expected to determine the success of the implantation process. Ab initio MD is applied to reproduce the first events after the impact of an Ar ion onto the NM. The choice of such a computational demanding methodologies makes it difficult to provide extended sampling and large statistics, which would be required to adequately describe the physics of the low-energy ion beam interacting with the sample. However, the goal of this study is to shine light on the microstructure evolution occurring after the first impact of the ion, addressing in particular the ability of bond breaking and reforming, the relaxation of the electronic structure at defective sites, the role of the metal below hBN and the charge redistribution. Therefore, simulations based on electronic structure theory are better suited. First the system has been thermalised at 100 K, by a short simulation in the canonical ensemble (NVT). After the equilibration, one Ar ion is added above the hBN surface. A series of 23 simulations has been started always changing the initial position (rAr ) and velocity (vAr ) of the ion. These variables determine approximately where Ar is going to strike the NM and with which impact angle θ , i.e., the angle between vAr and the normal to the Rh(111) surface. We distinguish among impact sites on the wire (W), the pore (P), and the rim (R). Ar can be directed against one boron atom (B), one nitrogen atom (N), one bond center (BN), or the center of one BN hexagonal ring (hex). The sketch in figure 2 illustrates three examples of approaching directions. Hence, the varying parameters are the Ar kinetic energy KAr , measured when the ion is at about 2 Å from the closest atom on hBN, the impact angle θ , and the impact site. KAr is chosen between 40 and 105 eV. These energies are most probably in the energy spectrum of the beam used for the experiments resulting in the formation of nanotents. 22 These runs are not meant to be exhaustive of all the possible events occurring in experiment during the ion beam exposure, but, changing the initial conditions, an approximate picture of the effects on the NM is provided. The implantation attempt is not successful when Ar is reflected back 7 ACS Paragon Plus Environment



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recovered. This is true in particular when the impact occurs in the pore, where the hBN is flat, between 2.2 and 2.5 Å above Rh(111), and interact strongly with Rh, leaving not much space for the inwards deformation. Actually, of the seven attempts carried out sending Ar to the pore, none has resulted in a successful implantation. When Ar is directed against the wire, larger effects are observed. The wire is slightly curved, with height between 3.6 and 4.4 Å above Rh(111), the BN bonds are predominantly shorter than the equilibrium bond length. The low stiffness of the entire wire/rim region, which promotes the topographic adaptation of the layer, has been also demonstrated by measuring the response to strain. 52 It turns out that, on the wire, Ar penetrates more easily the sp2 layer, but, in most of the cases, it is reflected back by Rh. The effective trapping occurs only when the outgoing direction brings Ar in an intercalated region, where the hBN is still intact. After the first impact, the kinetic energy of Ar is not anymore sufficient to break again the sp2 layer, and it remains trapped, rebouncing between Rh and hBN until it relaxes into an equilibrium intercalated position. Figure 3 summarises one run started with KAr = 100 eV and θ = 40◦ , where Ar has been directed towards the center of one BN-hexagon on the wire (run-4 in SI-1). The analysed MD trajectory is only 200 fs long, and it shows the impact of Ar, followed by its penetration and trapping below the hBN membrane. Approaching the substrate, the ion first pushes away a B atom of the wire, which moves inwards into the interlayer region (coloured black starting from the top view taken at 30 fs). While Ar breakes through the hBN layer, also a N atom, is knocked out and moves outwards. Its trajectory is also indicated in black in the figure. The hBN lattice in the vicinity of the generated hole is strongly distorted. A second N moves to the Rh layer (50-80 fs), while Ar pushes the layer outwards giving rise to a protrusion. The potential energy of the whole system is plotted in panel d of figure 3. It increases by several eV, due to the formation of the defect. Meanwhile the Ar kinetic energy quickly decreases (Fig. 3e). In such a short simulation time, the thermostat cannot efficiently dissipate the excess heat, hence locally the system is still hot due to the interaction with Ar (3f). After 200 fs, the structure around the impact site is still defective. Ar is trapped, the nanotent has formed, the knocked-out B atom is in the interlayer



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region, not too far from the original site, while N is free above the layer. The two dangling B atoms form a BB bond of 2 Å, well visible in figure 3c, whereas, the two dangling N atoms bend towards the Rh surface, thus strengthening the interaction with the metal. Indeed, the two closest Rh atoms move by 0.3 Å upwards. Due to the presence of the intercalated B, one corner of the pore is slightly lifted, as indicated by the color change of the ball and stick model reported in figure 3a. The Mulliken charges 53 computed along the trajectory show that the positive Ar ion is sharing electrons with the substrate once it is captured in the interlayer region. The charges of the knocked-out B and N atoms undergo larger fluctuations soon after the impact. Afterwards, B turns out to be positively charged, indicating the interaction with the metal, while the N above the surface behaves as a neutral radical. The highest rate of success for the Ar implantation has been obtained by shooting the ion against the rim, taking advantage of the curvature of the layer. When Ar penetrates the BN membrane, it moves towards the wire, where there is more space between hBN and Rh. This occurs also by transferring a relative low kinetic energy. Moreover, the created hole remains smaller and the relaxation of the hBN soon after the impact prevents that Ar moves back into vacuum after being deflected by the Rh surface. In one case of successful implantation (run-19 in SI-1), the MD run has been extended for more than one ps, in order to observe the further relaxation of the structure. At the impact, Ar removes one N and two B atoms from the rim. While the two B atoms quickly come back in place, after bouncing onto the Rh surface, the N atom moves together with Ar under the wire and it is pushed against the over-laying atomic layer. Here, it is grabbed by another B atom. The presence of Ar causes the deformation of the layer and the weakening of the bonds. Hence, the freed N can be inserted in the layer forming a NN bond in a defective geometry. The four top panels of figure 4 show the first femtoseconds of the path of Ar penetrating hBN. After 150 fs, the two B atoms are bonded again, while from below the N atom and Ar push to form the protrusion causing the lifting of another nitrogen, coloured in red in the figure. Finally, the knocked-out N reappears at the surface, bonded with one N and two B atoms. From this state, the simulation is carried on for more


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than one picosecond, setting the thermostat at 300 K. Panel b, c, and d of figure 4 show the potential energy, the Ar temperature, and the temperature of the full system together with the temperature of the B and N subsystems, respectively. Ar looses most of its kinetic energy and stabilises under the protrusion at the wire crossing site. Locally hBN presents an irregular structure with one NN (1.84 Å) and two BB (1.91 and 1.89 Å) bonds, giving rise to 5 and 7 member rings. The defect is stabilised such that all B and N atoms are three fold coordinated (Fig. 4e). At the very top of the protrusion is one N atom, as the apex of a triangular pyramid, below which the Ar atom sits. The simulated STM image obtained from this relaxed structure (Fig. 4f) presents the features observed in experiment, i.e. the protrusion at the wire crossing position and the deformed shape of the pore.

This short simulation time is not sufficient to allow for a complete healing of the defect, and the small hole on the protrusion just above the rim is still present. However, from experiment it is known that by annealing the implanted NM, in a temperature range between 300 and 500 K, the super structure does not deteriorate, on the contrary the number of defects is reduced, and the nanotents are still present. Though, they might diffuse and change in size (becoming larger), suggesting the aggregation of Ar atoms in small clusters.

Lattice reconstruction Whether the knocked-out N and B atom trapped in the interlayer region may contribute to the formation of the protrusions is still an open question. In order to address this point, MD can be used to simulate the processes involving the dangling and freed species generated by the Ar impact. To this purpose, we investigate here the case of an Ar ion impacting to the rim with kinetic energy of about 50 eV, leading to the implantation of the ion. The Ar ion strikes against a BN bond. The B and N atoms are knocked out from hBN and move towards the Rh surface. While the N re-bounces on Rh and soon moves back to bind again with the mesh, the freed B atom is stabilised in the interlayer region at an hollow site over Rh(111), not too far from the vacancy hole left in hBN and from the implanted Ar (at 3.6 Å). After a short equilibration carried out at 12 ACS Paragon Plus Environment

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low temperature, the local structure around the Ar+vacancy defect does not change anymore. Of the three dangling N atoms at the B vacancy, two bind to the closest Rh atoms underneath, thus inducing the downwards bending of the intact BN bonds. Ar instead pushes upwards, creating the protrusion. These two opposite strains applied to the membrane brake another BN bond near the vacant site. Therefore, in the final configuration there are four N atoms and two B atoms with a reduced coordination due to the breaking of the BN bonds. The first snapshot in figure 5a shows this initial state, where the B and N atoms with reduced coordination are coloured in black. The B atom in the interlayer region is hidden under a N atom of the mesh. Starting from this state, a MD simulation at 300 K is carried out to verify whether the defective sp2 layer easily heals and how the protrusion evolves. Since further processes are expected to occur on a longer time scale, the MD simulation is accelerated by means of metadynamics. Two collective variables are defined: the average coordination number of the four dangling N atoms with respect to all B atoms (CN ), and the average coordination number of two displaced B atoms (one trapped on Rh, the other with one BN-bond less) with respect to all N atoms (CB ). The two coordination numbers have starting values around 2 and 0.9, respectively, and should increase to 3 for the pristine hBN layer. Values smaller than the initial ones are to be expected if the defective area is enlarged and more bonds are broken. Five snapshots of the MTD run taken at 1, 2, 6, 10 and 15 ps are displayed in figure 5a. The plot in panel b, instead, shows the time evolution of the two collective variables, while the MTD potential is activating structural changes. The first rearrangement occurs already within the first 2 ps, leading to a concerted stepwise increase of CN and CB . One BN bond is mended, the Ar atom moves farther from the hole towards the wire crossing site, and the interlayer B atom comes closer to the vacancy site. Three N atoms still form only 2 BN bonds, and only the interlayer B atom is not bonded to any N. During the following 10 ps, the Ar wanders around under the wire, while the B atom does not change substantially its site. After 14 ps, the B atom leaves the interlayer region and binds to the three dangling N atoms, finalising the reconstruction of the nanomesh. The representation of the free energy surface in the reduced space of the two CVs is reported in figure 5c. The contour plot shows that the first minimum is more localised, while the second state, where


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the system remains for longer time, is indeed broader and deeper. The healing process occurs rather quickly crossing free energy barriers of about 1 eV. Soon after the recombination of B with the mesh the simulation has been interrupted. Hence the most stable state, corresponding to the intact nanomesh, has not been sampled.

Implanted species The results of the MD simulations confirm that after implantation and annealing, protrusions are formed at wire crossing positions. Some properties of the implanted structures have been already discussed in previous works. 22,37 Here, some results are summarised, for completeness, and some new features are presented. In particular, details about the structural and electronic features around the implanted species are given, which are considered important to characterise the stabilisation of the defective structure. It should be noticed that the PBE+D3 functional used for this work, which gives better agreement of the NM structure with the experiments, differs from the functional applied for previous simulations reported in Ref. 22,37 This explains some differences in the reported data. Geometry optimisation has been carried out by locating the Ar atom between hBN and Rh(111) at three intercalated sites: WXA, WXB, and center of the pore. In all cases, the protrusion is formed and stabilised, without the formation of any additional defect. The implantation energy is calculated as Eimp = EhBN/Ar/Rh − ENM − EAr ,

(1)

where ENM and EAr are the energies of the pristine NM and of the Ar atom in vacuum, respectively. It turns out that the WX positions are clearly favoured, with a slight preference for WXA (1.38 eV) with respect to WXB (1.45 eV). At the pore center, instead, Eimp increases to 6.62 eV. These results are in agreement with the site selectivity observed in experiment, where protrusions are found only at WX sites, with a prevalence of site A with respect to site B, which depends on coverage and post-annealing treatment. 23 The positive Eimp values indicate that Ar is trapped underneath the mesh. The dominant contributions to this energy are due to the deformation of the hBN layer and 16 ACS Paragon Plus Environment

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its weakened interaction to Rh(111). This latter term is particularly influent when Ar is at the pore, since here the binding interaction to the metal is stronger. Indeed, by running a MD simulation of few ps at 300 K, starting with Ar trapped at the pore, the system relaxes quickly while Ar diffuses to the wire. In a similar MD run, starting with Ar in the WXB position, instead, the atom wanders around, without effectively leaving the initial site. Even though the WXA site would be more stable, the diffusion of Ar underneath the wire requires a rearrangement of the NM atomic structure, which does not occur during such a short sampling. Some additional details on these two MD runs are given in the SI. At WXA, Ar sits on top of a Rh atom, exactly under the center of a BN-hexagon. The energy increase due to the distortion of the NM amounts to 0.67 eV. From the distribution of BN bond length with respect to height (panel a of figure 6) it appears that about 6% of the NM unit cell is involved, and the maximum height from Rh reaches 5.3 Å. The rearrangement of the electronic structure is investigated by means of projected densities of states and the calculation of the electronic density difference, obtained as

∆ρ (r) = ρhBN//Ar/Rh (r) − ρdNM (r) − ρAr (r).

(2)

Here, ρhBN//Ar/Rh is the electronic density of the full system, while ρdNM and ρAr are the densities of the system without Ar and the Ar atom alone, respectively, at exactly the same coordinates as in the full system. The ∆ρ iso-surfaces shown in panel b of figure 6 correspond to the accumulation (red) and the depletion (blue) of electrons at the value of 0.04 el/Å3 . The Ar atom atom appears as compressed between the metal and hBN. Some charge is transferred from Ar to the metal and the remaining positive Ar-center polarises the electronic clouds of the closest B and N forming the protrusion. Hence, the modification remains quite localised at the protrusion site, and it acts as a trapping potential that stabilises the intercalated species. In the plots in figure 6c, the PDOS calculated for the Rh, N, and B atoms directly interacting with Ar are compared to PDOS of the same atoms in the pristine NM. The p and s atomic states of Ar are well below the Fermi energy,



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though some density appears at the Fermi energy due to the re-hybridisation with the d-band of Rh. The Rh-d band has reduced density around the Fermi energy and some features appear at the energy of the Ar p-states (-8 eV). This is another indication of the charge redistribution occurring locally. Also the N-pz and B-pz states distributions are slightly modified. In particular, the broadening of the band towards lower energies indicates the polarisation of the electrons around the positive Ar-center. The experiments show that the functionalised NM survives annealing temperatures up to 600 K. However, the distribution of the protrusions evolves. The number of protrusions decreases but their size increases, suggesting the aggregation of more Ar atoms at the same WX site. 37 The optimisation of the NM structure when two intercalated Ar atoms are present in the unit cell shows that the implantation energy per atom is reduced locating them at the same WX site (structures and STM images are shown in the SI). The main reason is the reduced distortion energy due to the additional strain of the NM, with respect to having two atoms forming two independent protrusions. The implantation energy obtained for different configurations of one and two intercalated Ar are reported in figure 6e. The energy barrier for the diffusion of Ar from WXB to WXA has been also estimated by means on a nudged elastic band simulation. The shortest direct path from B to A runs below the wire and it is about 17 Å long. The computed potential energy barrier is about 1 eV, which is consistent with the fact that experimentally hopping processes are observed. However, the rearrangements of the nanotents is a more complicated process, which most probably involves Ar clusters of larger size and is going to be addressed in future studies.

Conclusions In this work a first description of the dynamic processes leading to the formation of the boron nitride nanotents is presented. By means of AIMD simulations it is possible to follow the local structural evolution induced by the interaction of the hot Ar ion striking against the NM. It is shown that the penetration of Ar through the hBN layer occurs predominantly at the rim and it


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induces a relatively limited damage of the NM, also thanks to the efficient self healing dynamics of the supported sp2 layer. A few B and N atoms happen to be knocked-out from their lattice sites, but in general remain in the vicinity of the defect, because of the interaction with Rh. It is demonstrated that the displaced species can be easily re-located in the hBN mesh by self-structural healing. At higher temperatures it turns out that the hBN layer undergoes fast rearrangements, which facilitate the reconstruction after the damage but also the flexible adaptation of the mesh at the presence of Ar. A clear example is the wave-like movement of hBN that allows the displacement of Ar from less favourable sites (e.g. at the pore) to more stable positions. Finally, the stability and properties of the single-Ar nanotent are quickly reviewed, providing additional details on the electronic structure effects. All these results are consistent with the impressive stability and reversibility of the intercalated-NM as demonstrated in experiments. On the other hand, there is still a lot to understand about the dynamics of the Ar clusters and of the defects, which might be responsible for the formation of the nano voids observed upon annealing to temperatures higher than 600K. Moreover, the intercalation of other species, like He and Rb, also leads to peculiar defect structures that need to be better understood. Hence, we are going to extend the study on the funtionalization of the NM in these directions. Acknowledgements: I would like to thank Prof. Thomas Greber and Dr. Huanyao Cun for fruitful discussions and valuable input from the experimental perspective, and Prof. Jürg Hutter for the advise and support . The Swiss National Supercomputer Centre (CSCS) is acknowledged for the generous allocation of computer time.

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