Fabrication of Defective Single Layers of Hexagonal Boron Nitride on

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Fabrication of Defective Single Layers of Hexagonal Boron Nitride on Various Supports for Potential Applications in Catalysis and DNA Sequencing Roland Kozubek, Philipp Ernst, Charlotte Herbig, Thomas Michely, and Marika Y. Schleberger ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00903 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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ACS Applied Nano Materials

Fabrication of Defective Single Layers of Hexagonal Boron Nitride on Various Supports for Potential Applications in Catalysis and DNA Sequencing Roland Kozubek,† Philipp Ernst,† Charlotte Herbig,‡ Thomas Michely,‡ and Marika Schleberger∗,† †Fakultät f¨ ur Physik and CeNIDE, Universit¨ at Duisburg-Essen, D-47048 Duisburg, Germany ‡II. Physikalisches Insitut, Universität zu Köln, Z¨ ulpicher Straße 77, 50937 Köln, Germany E-mail: [email protected] Phone: +49 (0)203 379-1600. Fax: +49 (0)203 379-2334 Abstract Two-dimensional hexagonal boron nitride (hBN) has been shown to be a suitable substrate and gate material for high performance graphene electronics. This study explores how highly charged ions can be used for defect engineering, i.e. to locally induce modifications in single layers of hBN. For this we irradiated single layers of hBN on SiO2 , Mo-foil, and Ir(111) with highly charged Xeq+ ions of different charge states (up to q = 40) at a fixed kinetic energy of 260 keV. The ion-induced nanoscaled modifications are analyzed as a function of the charge state using scanning force microscopy in the friction force mode and secondary ion mass spectrometry. The data shows that a charge state of more than q = 28 corresponding to a minimum potential energy of

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(15 - 17) keV is sufficient to achieve defect creation via electronic excitation. This is higher in comparison to graphene on SiO2 (' 12 keV) which opens up the possibility of selective defect creation in graphene/hBN heterostructures. Our results are further corroborated by two-temperature model calculations showing a good agreement to the experiment. Based on our results we propose that the intense electronic excitation induced by the highly charged ion leads to sublimation of material from nanometer sized regions of the single layer hBN sheet.

Keywords ion irradiation, highly charged ions, hexagonal boron nitride, 2D material, AFM, thermal spike, two temperature model

1

Introduction

Heterostructures made up from two-dimensional materials are currently envisaged to be used for all kinds of applications, e.g. in photovoltaics or nanoplasmonics. 1 In the case of hexagonal boron nitride (hBN), heterostructures with graphene where hBN is used as substrate or gate material, show promising potential for next generation electronics. 2–8 But also the material itself has gained an anormous amount of interest due to its proposed suitability in quantum technologies, 9–11 in DNA sequencing, 8,12,13 and for catalysis. 14–16 An important issue when it comes to the realization of such applications is the presence and nature of defects and how to introduce defects in a reliable way. An established strategy to investigate the role of defects is a quantitative analysis of artificially created defects, e.g. by particle irradiation. While defect engineering in graphene has been studied intensively in this way, 17–22 similar strategies for defect creation in hBN by single ion impacts are not equally well studied. In particular, for hBN no studies yet exist with respect to defect creation by electronic excitation which occurs under irradiation with e.g. highly charged ions (HCI). 23

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These ions deposit their so-called potential energy (sum of the ionization energies of all stripped electrons) predominantly in the electronic system of the solid and thus allow for manipulating 2D materials in a unique way which cannot be achieved otherwise. Because the deposition of potential energy Epot proceeds via charge exchange and electronic excitation, defect creation will depend strongly on the electronic properties of the respective material. Graphene with its linear dispersion and its high mobility can effectively thermalize and distribute excited electrons. As a consequence any energy stored in the electronic system should be quickly dissipated preventing processes such as e.g. Coulomb explosion 24 or thermal spikes. 25 This is in line with the observation that by HCI irradiation of (amorphous, non-conducting) carbon nano-membranes pores of nanometer size are created 26 while in freestanding graphene membranes no defects at all are observed after HCI irradiation. 27 For supported graphene, defects manifesting as nanometer sized areas of enhanced friction have been reported, 28 but no evidence for the removal of atoms has yet been given. The latter experiments clearly show a threshold, i.e. at a fixed kinetic energy the ion has to carry a minimum potential energy (corresponding to a certain charge state) in order to create such a defect, the nature of which has yet to be resolved. Experimental studies on the interaction of HCI with 2D materials other than graphene are still surprisingly sparse, see e.g. 29 With a lattice constant very similar to the one of graphene but being an insulator, single layer (SL) hBN represents an ideal 2D material to study defect production mechanisms by electronic excitation. The motivation for this work is thus twofold: (i) revealing the defect production mechanisms for HCI irradiation as a basis for (ii) establishing defect engineering strategies for reliable and selective introduction of defects into ultrathin hBN. In this way prospective applications of the material may be enabled. In this paper we present clear experimental evidence that nanoscaled modifications can be induced in single layers of hBN on SiO2 by HCI irradiation. The modifications are detected using atomic force microscopy (AFM) in friction force mode (FFM) in which the modifica-

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tions exclusively appear above a certain potential energy. Our FFM data is complemented by secondary ion mass spectrometry (SIMS) to proof the ejection of boron atoms from hBN supported by metal substrates. We compare our experimental data with simulations and solve the problem of the unknown interaction depth by exploiting the naturally given and exactly known thickness of one atomic layer. This allows for a direct comparison of simulation and experimental data. In this way we can identify the relevant mechanism for defect creation in this particular 2D material as the heated zone around the ion impact site, which gives rise to sublimation from a nanometric volume. From our data the threshold for this nanosublimation to occur can be narrowed down to a potential energy of 15-17 keV.

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Results and discussion

For the HCI irradiation experiment, SL hBN was mechanically exfoliated from a hBN crystal. In fig. 1(a) a typical hBN flake on SiO2 with SL hBN areas marked by a white box is shown. As the optical contrast of SL-hBN is very low, we used Raman spectroscopy to unambiguously identify SL hBN. The dotted line in fig. 1(b) shows a typical Raman spectrum for few layer hBN (FL hBN) with the characteristic D-band at ≈ 1367 cm−1 . For SL hBN, this D-band shows a decreased intensity and a shift to higher wave numbers, as shown in the Raman spectrum by the solid line. The position of the D-band for SL hBN with 1370 cm−1 is in good agreement with values reported by Gorbachev et al. 30 In order to check the surface quality of the hBN prior to HCI irradiation, we employed tapping mode AFM. In tapping mode the forces on the sample are considerably lower compared to contact mode AFM so that damage due to scanning is avoided. In fig. 1(c) one can see the pristine SL hBN flake, showing no signs of contamination or defects. The line profile marked in (c) and shown in fig. 1(d) reveals the typical step height for SL hBN of about 4 ˚ A±2˚ A, which is again in good agreement with previous findings. 30 The HCI irradiation took place at the Duisburg beamline HICS using an electron beam

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ion trap (EBIT). 31 To determine the threshold for defect creation by HCI projectiles, we irradiated the same sample with Xeq+ (pure isotope with z = 129) of different charge states: q =40, 28, 33, and 37 (corresponding to potential energies of 38.5 keV, 12 keV, 21.2 keV, and 30.3 keV) at a constant kinetic energy of EKin = 260 keV under perpendicular incidence. The charge states are listed in the chronological order of the irradiation steps. The fluence for each irradiation step was adjusted to be around 5·109 ions/cm2 which corresponds to 50 ions/µm2 for each irradiation. Note, that this subsequent irradiation is a crucial detail in our experiment. First, by investigating the surface after each irradiation step, it can be identified which defect belongs to which charge state. Second, it allows to investigate the persistance of ion induced features within the scan experiments. This is important because it has been reported that HCI features in highly oriented pyrolitic graphite (HOPG) might be erased by continued scanning. 32 In addition, by measuring the modifications of the previous irradiation step again, the condition of the tip can be monitored to exclude artefacts due to tip changes. After the first irradiation step of hBN/SiO2 with Xe40+ (38.5 keV) the sample was measured by means of AFM. We found that tapping mode as well as conventional contact mode measurements are not suitable to detect ion induced changes in the topographic images from the hBN surface, which could mean that the hBN lattice is either unaffected or that induced defects are too flat to be identified in the noise of the AFM measurement. We therefore switched to the friction force mode AFM (FFM-AFM). 33 Here, contact mode AFM is employed with the fast scanning direction perpendicular to the cantilever in order to detect its lateral deflection. Defects created by HCI can be identified in this way because in friction images they appear either as hillocks or pits in a forward trace or backward retrace, respectively. The results before and after the irradiation are presented in fig. 2(a) and (b), repectively. The rupture of the monolayer hBN in the bottom of the figure results from the interaction of the AFM tip with the 2D material. Otherwise, the topography seems to be unaffected by the irradiation. No noticeable changes like pits or hillocks nor any other topo-

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graphical features could be identified on SL-hBN. Also the surface roughness of the SL-hBN remains at a constant value of σRMS ≈ 0.27 nm (this is true even after the final irradiation step). In contrast, in the friction images one can clearly see hillocks in the trace scan which appear as pits in the retrace scan. This behaviour is characteristic for local changes in friction, which are often explained in terms of modified chemical bonds. 34 The defect density agrees well with the adjusted fluence of ≈ 50 ions/µm2 applied for this first irradiation step. Note, that no HCI induced modifications could be identified on the SiO2 substrate. The results of the subsequent irradiation steps with Xe28+ , Xe33+ and Xe37+ are presented in fig. 3(a) - (c). Again, the induced modifications do not show up in the topography channel. We therefore omitted the height images and present only friction measurements instead. As mentioned before, all the irradiations have been done on the same sample. The scratches at the bottom of figures (b) and (c) come from the enlargement of the previously mentioned ruptured area, which results from the subsequent scanning of the AFM tip. To distinguish between features induced by ions of different charge states, we measured the same area on the sample after each irradiation and marked new defects with another colour in the respective FFM-AFM images. Fig. 3(a) shows the friction images after irradiation with Xe28+ in trace and retrace scan direction. In comparison with fig. 2, which was taken before this irradiation, one can see, that there is just one additional defect on SL hBN, marked with a yellow circle. Obviously, the defect density is much lower than the fluence of ≈ 50 ions/µm2 . All other features, encircled in black, had already been detected before (see fig. 2) and are therefore not attributed to the Xe28+ irradiation. Looking at fig. 3(b), which shows the surface after the irradiation step with Xe33+ , one can clearly identify several additional defects, which are encircled in blue. Note, that the defect density detected by AFM is still much lower than the nominal fluence, i.e. roughly only every third ion induces a noticeable feature. Finally, we look at fig. 3(c). Here one can see the single layer after irradiation with Xe37+ . Again, additional features, marked in green, could be identified on the sample. The number of defects per area for this irradiation step is in fair agreement with the adjusted fluence.

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Thus, almost every ion impact creates an area of enhanced friction in the single layer. Next, we analyze the dimension of these features. For this we choose the last AFM image with defects from every irradiation to ensure that the tip condition, which has a great effect on the measured defect size, is exactly the same for all features. Since the lateral force strongly depends on the loading force as well as on the shape of the tip (which we cannot determine exactly), we omit a quantitative analysis of this force. The defect diameters are determined as the full width half maximum (FWHM) of a Gaussian fit to line scans taken from fig. 3(c). We find: (10.3±2.5) nm for Xe40+ , (8.3±1.9) nm for Xe37+ , and (7.4±1.9) nm for Xe33+ . The observed difference in friction between the defect spots and the surrounding areas gets weaker after AFM measurement, whereas the size remains constant within the range of the error bars. In some cases, which are marked with dashed circles in fig. 3, we could even observe a total disappearance of features already in the retrace scan. It should be pointed out here, that the area had to be imaged two to three times between each irradiation step, since the exact same area on the sample surface had to be found for this analysis. The effect of erasing modifications by scanning was observed for defects of every charge state. However, the effect was significantly more pronounced for features originating from the irradiation with Xe33+ ions (4 out of 7), while just one erasure (1 out of 20) could be measured for the charge state of q = 40. Due to the limited spatial resolution from the AFM data alone it is not possible to judge, whether material is truly emitted from the 2D layer or if only chemical changes are at the origin of the enhanced friction. Therefore, we complemented our FFM data with SIMS taken from hBN grown per chemical vapour deposition on metallic substrates Ir(111) and Mo (for details see Methods section). For all charge states measured here sputtered boron atoms are detected as can be seen in fig. 4. This is a clear proof of atoms ejected from the 2D material due to HCI bombardment as boron is not present anywhere else. The origin of the features observed in the FFM images are thus not only chemically modified regions but consist of vacancies as well. In addition we observe a clear increase in the sputter yield for the higher

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charge states in our mass spectra, i.e. also here we observe an onset of a new mechanism at a potential energy of '17 keV, which fits well with the threshold determination in the AFM measurements. We find that linear fits describe our SIMS data quite well if we fit data points below (green line in fig. 4) and above (red line) the onset value of ' 17 keV separately and with different slopes. The results presented above unambiguously show that structural defects can be created in SL hBN by individual HCI impacts. From our data we infer that up to a charge state of q = 28 defect creation is a result of the kinetic energy of the ions because no defects are detected by AFM but clearly a SIMS signal is seen. The intensity corresponds to the number of particles sputtered directly from the 2D material as well as indirect sputtering due to interaction processes of the hBN with the sputtered atoms coming from the substrate. The slight increase of the intensity with the potential energy may be related to an enhanced nuclear stopping because of the ionic charge. 35 The significant increase in defect size and particle emission beyond a charge state of q = 28 must be attributed to the potential energy. In order to elucidate the role of the potential energy for the defect creation in SL-hBN, we begin with a brief analysis of the efficiency of defect creation. The number of defects in the case of Xe40+ and Xe37+ agrees well with the fluence. This means that every ion impact results in a defect. For the lower charge states this is no longer the case. While for irradiation with Xe33+ only one third of the expected number of defects is detected, this value further decreases down to just 5% for Xe28+ . The statistics in our experiment is not sufficient to make a truly quantitative statement here, but the trend is apparent. A reduced efficiency in defect creation is a typical indicator for a threshold. If a certain amount of deposited energy is required for defect creation and the potential energy of the projectile is close to that value, one can expect that the probability to create a defect decreases. Minor differences in the amount of energy deposited into the material due to different impact parameters or small differences in the local condition of the 2D material (strain, contaminations, etc.) may become significantly more important for the defect creation efficiency. Also, defects might

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be undetectable due to the limited spatial resolution of the AFM which would underestimate the efficiency and overestimate the threshold. The existence of a threshold for Epot at fixed Ekin is a clear sign for a defect creation related to the electronic excitation triggered by the highly charged ion. From the experimental data presented here, it seems that the potential energy required to create a defect with an efficiency of one is close to 17 keV. This is rather large in comparison with bulk materials such as CaF2 36 and KBr, 37 as well as in comparison with graphene. For the latter a threshold potential energy of Epot =12 keV at the same kinetic energy of Ekin = 260 keV was found. Since both 2D materials were supported by SiO2 , a substrate effect cannot explain this surprising finding which needs to be further investigated. In general the interaction of a solid surface with a highly charged ion can be described by √ 2q(7+) 38 the so-called over-the-barrier model. At a critical distance Rc = (+1)·W the approaching ion begins to neutralize by resonant electron capture and Auger ionization. 39 Electrons from the target are caught in highly excited states of the HCI resulting in a hollow atom. For hBN irradiated with Xe33+ the critical distance is Rc = 2.6 nm with a constant dielectric function of = 7.04 AsV−1 m−1 taken from ref. 40 and a work function of W = 5.5 eV (sum of the band gap 5.2 eV 41 and the electron affinity of boron 0.3 eV 42 ). The approaching and finally impacting hollow atom causes collisional electron-electron processes which lead to the emission of inner-shell electrons form the target atoms. 43,44 The dense electronic excitation due to the HCI impact may set different mechanisms into effect (for a review see Arnau et al. 45 ), namely Coulomb explosion, Desorption Induced by Electronic Transitions (DIET), and the thermal spike. The idea behind a Coulomb explosion is, that the neutralisation of the highly charged ion through hollow atom formation implies a local electron depletion in the h-BN surface and thus a repulsion of these atoms. The ionized atoms repel each other and are eventually emitted from the surface. 24 The other process describes the desportion of ions and neutrals from the 2D material after the electronic excitation to an antibonding state. As a result the electronic excitation is converted into a

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motion of the nuclei. 46 Yet another possibility is the so-called thermal spike model in which the electronic excitation is transferred via electron-phonon-coupling to the target lattice. 25 In the following we focus on the thermal spike as it has already been successfully employed to describe HCI induced modifications in bulk insulators such as CaF2 . 47 As the excited region is confined to an atomically thin layer, the lattice temperature in this region can easily exceed e.g. the melting temperature of the material. However, CaF2 has a rather low melting temperature (TCaF2 = 1700 K 48 ), whereas hBN does not melt but undergoes sublimation at an extremely high temperature of TSubl = 3000 K. 49 In general one would have to take both the above surface as well as the subsurface neutralization processes, i.e. the excitation of the electrons above and below the surface, into account, as was demonstrated by Aumayr et al. 50 However, for simplicity we assume here that the potential energy of the incoming ion is deposited exclusively into the electronic system of the 2D material, i.e. within a radially confined area which we have chosen to be of a Gaussian profile. The energy then spreads through the solid due to electron transport, while the lattice is heated via the electron-phonon coupling mechanism. The latter is modelled by a set of coupled differential equations:

Ce (Te )

∂Te (~r, t) = ∇ · (κe (Te )∇Te (~r, t)) − ∂t − g · (Te (~r, t) − Tl (~r, t)) + S(~r, t),

Cl (Tl )

(1)

∂Tl (~r, t) = ∇ · (κl (Tl )∇Tl (~r, t)) + ∂t + g · (Te (~r, t) − Tl (~r, t)).

(2)

T (~r, t) is the temperature, while C(T ) and κ(T ) are the heat capacity and thermal conductivity for electrons (subscript e) and phonons (subscript l), respectively. The electron-phonon coupling parameter g is used to parametrize the transfer of energy from the electronic to the phononic system. The term S(~r, t) represents the energy deposited by the ion and is

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calculated as follows:

S(~r, t) = bEPot G(t)F (~r) .

(3)

b is a normalization constant while G(t) and F (~r) are the temporal and spatial evolution of the energy deposition, respectively. For modelling the spatial distribution F (~r) for the HCI irratiation, we used a hemispherical volume described by |~r|2 F (~r) = exp − √ 4 2ln2A2

! .

(4)

with ~r being the distance from the impact site and A being the width of the Gaussian energy profile. Eventually, the lattice temperature Tl may exceed the target’s sublimation temperature, which we attribute to an induced defect. Neglecting the substrate in this model is justified by the following reasoning: In particular for the metallic substrates used for our SIMS measurements any energy deposited in the substrate will dissipate quickly. Therefore HCI yield in general no structural changes in metals. 45 In case of a SiO2 substrate there could in principle be some contribution but as we could not detect any ion-induced changes neither by AFM nor FFM in the SiO2 substrate even for the highest charge state, we infer that the substrate can have only a minor effect. Thus the threshold values are obtained by comparing the calculated lattice temperatures with the sublimation temperature of hBN of TSubl = 3000 K. The only remaining parameters in our calculations are the width of the Gaussian energy profile A and the electron-phonon coupling parameter g. The electron-phonon coupling parameter is calculated based on the band gap of hBN (Eg = 5.2 eV 41 ) as suggested by Toulemonde et al., 51 which results in g = 1, 1 · 1019 W/m3 K. While all other parameters are kept fixed (Table 1), we treat the width of the Gaussian energy profile A as a fitting parameter. We found that a value of A = 4.2 nm fits our experimental data best. 11



The results of the TTM calculations are depicted in the array of curves in fig. 5(a). Here, one can see the spatial evolution of the lattice temperature for different times after the deposition of the potential energy of a Xe40+ ion into the electronic system in the range of 1 ps ≤ t ≤ 35 ps and in time steps of ∆t = 1 ps. The red curve for t = 1 ps shows a very high lattice temperature of ≈ 5700 K at the impact site, which decreases rapidly with increasing distance r. The decrease in temperature near the impact site r . 4 nm with the simultaneous increase of TL for larger distances r & 6 nm shows the dissipative behaviour of the system. For t ≈ 6 ps, represented by the green curve, the area of sublimated material is found to be largest. The corresponding temperature and radius are marked by dashed lines. This radius has been determined for all potential energies and the values have been plotted by the red line in fig. 5(b) for comparison with our experimental data. The comparison shows that the sublimation temperature of hBN can be reached with HCI projectiles carrying a potential energy of more than 15 keV. This agrees well with our experimental data (black squares) and in particular, the threshold value is correctly reproduced by the TTM model. The experimentally determined diameter values for the modifications are slightly higher then the ones calculated by TMM. This can be explained by tip convolution effects caused by the AFM tip which is most likely larger than the measured modifications. The inset shows a snapshot (at the time when the sublimation area reaches its maximum extension) of the spatial extension of the heated zone for different charge states. For Xe28+ the temperature is always below the dotted line which denotes the sublimation temperature of hBN. The sublimation temperature is reached from Xe33+ onwards at a radius of 2.8 nm and the calculated temperature can be as high as 4200 K high for Xe40+ . Reaching the sublimation temperature does not necessarily mean, that all the target atoms within the heated area ejected into the vacuum. In fact, it is more likely that the area consists of a combination of vacancies and modifications of the lattice like changes form sp2 to sp3 -hybridization and chemisorption of adatoms and substrate atoms all of which have theoretically been shown to be stable. 52–54 When chemical modifications occur

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topography also changes - however, the corresponding changes in height are typically too small to be detected via AFM measurements. Assuming that these chemical modifications are mainly responsible for the enhancement in friction, the observed erasure of the defects can be attributed to the invasive character of the AFM measurement. This hypothesis however needs to be verified by additional measurements like scanning transmission electron microscopy. Note that we had to fit the area of energy deposition for the TTM model, and therefore the exact numbers should not be taken as face value. Nevertheless, the good agreement corroborates our hypothesis, that indeed a thermal process resulting in sublimation on the nanoscale is taking place giving rise to ejection of ions from the 2D layer. Furthermore, it indicates that a major part of the potential energy is deposited within the 2D material itself. Despite the good agreement between the experimental data and the results from the TTM model, we like to point out that other mechanisms as Coulomb explosion or DIET cannot be ruled out. To distinguish between the different mechanisms one would need to experimentally determine other quantities, e.g. the energy distribution of the sputtered particles and/or their charge states as well as the emission characteristics of other particles such as electrons and photons. This is however clearly beyond the scope of this paper.

2.1

Conclusions

We have successfully applied a novel defect engineering strategy to single layer hBN. Our results obtained from different samples show that the irradiation with highly charged ions of different charge state can be used to create nanoscaled defects of controllable size. Experimentally we find a threshold for defect creation which indicates that indeed electronic excitations are at the origin of the modifications. A two-temperature model could be used to verify the existence of a threshold and the theoretically determined threshold showed a good agreement with the experimentally determined value of 17 keV. At this energy the temperature of an area of a few nanometer in radius rises high enough for sublimation to 13



set in. These defective hBn sheets, in particular the metal supported ones, could be used as electrocatalysts for the reduction of oxygen or the oxidation of CO. 14–16 To enable other advanced applications such as the ultra-sensitive detection of forces via the defect spin qubit or DNA sequencing 8,11–13 hBN membranes with defects are required. We are planning to meet the challenge of preparing freestanding hBN to investigate the 2D material on the atomic scale using scanning transmission electron microscopy to further verify this pore creation process and to investigate the bare interaction between a highly charged ion and the hBN monolayer without any influence of an underlying substrate. Our findings underline the versatility of HCI as a tool for defect engineering applicable even for materials with extremely high phase transition temperatures. Because the threshold for hBN on SiO2 is higher than the one for graphene on SiO2 (12 keV), selective defect creation of heterostructures is indeed feasible. For example, HCI irradiation of such a heterostructure with a potential energy of 12 keV could be used to introduce defects into graphene without significantly affecting the hBN layer. This would enable to study the influence of defects in high-mobility hBN-encapsulated graphene field effect transistors.

3 3.1

Methods Sample Preparation

Exfoliation: A single crystal from ”HQ graphene” (Netherlands, Groningen) was used for mechanical exfoliation with scotch tape. As substrates we used 90 nm thick SiO2 on a silicon substrate. The mechanical exfoliation, as it is common in the preparation of graphene, is much more difficult for hBN. This has two reasons: (i) the number of hBN crystallites which are only one layer thick, is pronouncedly smaller compared to graphene and (ii) these monolayers are much more difficult to observe through an optical microscope due to an imperceptible optical contrast. Our strategy for finding hBN monolayers was to search for thin hBN in the optical microscope 14


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(3 layers and thicker) and search for monolayers in the immediate vicinity using AFM. This of course is very time-consuming and severely limited the number of samples available for this study. CVD hBN on Ir(111): The hBN single layer on Ir(111) was grown in a variable-temperature STM system in Cologne with a base pressure below 1 × 10−10 mbar range. Ir(111) was prepared by cycles of noble gas sputtering and flash annealing to ≈1500 K. The quality of the clean Ir(111) surface was verified by LEED and STM. A complete hBN layer was grown by dosing borazine at 1270 K with a pressure around 10−6 mbar for around 100 s. This recipe results in well-aligned hBN layers with an orientation scatter for the densely packed rows of hBN with respect to those of Ir(111) of approximately 1◦ as checked by LEED and STM. After growth the sample was transferred to the beamline at the UDE where it was heated to remove contaminations from the transfer between the two UHV-systems before irradiations and measurements took place. CVD hBN on Mo: A thin PMMA film was spin coated onto commercially available hBN CVD-grown on a copper foil (Graphene Supermarket). To remove the supporting copper foil, the sample was placed onto the surface of an ammonium persulphate solution. After approximately 24 hours the etchant was substituted by deionised water. The 100 µm thick molybdenum foil was immersed in the water and carefully pulled out again, depositing the hBN/PMMA stack onto the Molybdenum surface. The remaining PMMA is dissolved in an acetone bath for 1 hour. Then the samples were transferred to the beamline where they were heated at 250o C for 12 hrs to remove remaining contaminations.

3.2

Raman Spectroscopy

The Raman measurements were done using a Renishaw InVia Raman Spectrometer with λ=532 nm and a spotsize of ca. 1 µm. The power was kept below 0.5 mW to prevent heating effects which have been observed to produce damage in the case of other 2D materials.

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3.3

Friction Force Mode Atomic Force Microscopy

The FFM measurements were done with a VEECO DI 3100 AFM in air using constant force mode in perpendicular scan direction with Nanosensors PPP-CONTR cantilevers (nominal tip radius r < 10 nm) and typical loading forces of ≈ 30 nN. When the AFM tip is scanned over an area with an enhanced friction, the tip bends due to the lateral friction force. The direction of this friction induced bending depends on the scan direction. This is why the defects appear as hillocks/pits in trace/retrace direction. While this nominal tip radius will change during the scan we omit the quantitative analysis of the loading and thus of the lateral force. Tapping mode measurements were performed at the same device using Nanosensors PPP-NCHR cantilevers with nominal tip radius of r < 10 nm. Note that the thickness of single layers may vary depending on the interfacial water layer. In our samples this layer was nearly absent. Images are treated using Gwyddion.

3.4

Time-of-flight Secondary Ion Mass Spectrometry

In our ToF-SIMS set-up, the ions ejected from the surface are accelerated towards the ToF spectrometer entrance by a pulsed extraction potential (6 µs, 1 keV). After passing deflectors and a focusing lenses within the ToF the ions enter a field-free drift zone at the end of which they are reflected by a constant potential (yielding an improved mass resolution). After passing through a second field-free drift zone they are due to different flight times detected mass separated by a chevron micro channel plate. Within the expected kinetic energy distribution of the sputtered particles, the detection probability is independent from the particles kinetic energy. The SIMS spectra for hBN/Mo was normalized to the ion current, whereas the hBN/Ir measurement was normalized to the potassium (an ubiquitous contamination) peak of the mass spectrum due to the geometry of the sample holder. This leads to bigger error bars for the latter sample system.

16


Page 16 of 36

Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.5

Two-Temperature-Model Calculations

The following parameters were used for the TTM calculations: Table 1: TTM parameters Parameter Width of Gaussian A Target atomic density ρ Target molar mass M Electron-phonon-coupling g Lattice diffusivity DL Electron diffusivity De Target Debye temperature TD Target sublimation temperature Ts Target energy of sublimation ∆HSub

Value 4.2 nm 2.25 g/cm3 24.83 g/mol 1.1 · 1019 W/m3 K 0.03 ˚ A/fs 1˚ A/fs 1900 K 3000 K 500 J/g

Acknowledgement We thank M. El Kharrazi and O. Osmani for developing the software package for the TTM calculations. This work has been financially supported by the DFG within the SFB 1242: ”Non-Equilibrium Dynamics of Condensed Matter in the Time Domain”, project C5 as well as through project MI581/22-1 ”Low energy ion irradiation of 2D materials”.

17



(a) FL-hBN

SL-hBN

SiO2

Intensity [arb. u.]

(b)

D mode 1367 cm-1

SL hBN FL hBN

D mode 1370 cm-1

10 µm

1350 1400 Raman shift [cm-1 ]

(c)

16

(d)

Line profile

12

Z [Å]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

8

SL-hBN SiO2

4Å

1 µm 4

0

3

X [Å]

6

Figure 1: (a) Optical microscope image of a SL hBN flake. The white box marks the location of the SL hBN, which is barely visible in the optical microscope. The solid and dashed dots show the location where Raman spectroscopy has been performed (b) Raman spectra of FL hBN (blue) and SL hBN (black) where SL hBN shows a decreased intensity and a shift towards higher wave numbers. (c) AFM topography (tapping mode) of the pristine SL hBN. The left inset shows the whole flake. (d) Line profile of the white line marked in (c) revealing a SL hBN height of about (4 ± 2) ˚ A.

18


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

pristine

4.4 nm

(b) irradiated with Xe40+ 4.4 nm Topography

Topography

no modifications

no modifications

500 nm FFM trace

500 nm

0.0 nm FFM retrace

0.0 nm FFM retrace

FFM trace

4.4 nm

Pits

Hillocks 500 nm

Figure 2: AFM messurement in friction force mode of the SL hBN/SiO2 sample (a) before and (b) after irradiation with Xe40+ . The surface topography at the top seems to be unaffected while in the FFM images in the bottom features after the irradiation with Xe40+ appear as hillocks in one and as pits in the other scan direction. This is typical for frictional changes of the surface.

19



(a)

Xe40+ Xe28+

(b)

Xe33+

Page 20 of 36

(c)

Xe37+

trace

trace

trace

retrace

retrace

retrace

Figure 3: Friction images of a SL hBN in trace and retrace direction after the irradiation with Xe28+ (a), Xe33+ (b) and Xe37+ (c). The defects appear as frictional changes which one can recognise by different appearences in trace (hillocks) and retrace scans (pits). The features are marked with circles of different colors to be able to distinguish between defects due to the irradiation with Xe40+ (black), Xe28+ (yellow), Xe33+ (blue) and Xe37+ (green). Circles with dashed lines indicate preassigned features, that could only be detected in the previous measurement.

20


Page 21 of 36

1 .8

8

D h B h B N e a r fit e a r fit

N o n o n m lo w h ig h

M o -fo il o n o c ry s ta llin e Ir

X e

4 0 +

q

1 .6 q 3 7 +

X e

1 .4 6

X e 4

X e 2

0

X e 5

3 3 +

1 .2

2 8 +

2 6 +

X e 1 0

1 5

2 0

2 5

3 0

q +

@

2 6 0 k e V 3 5

In te n s ity (n o rm a liz e d )

C V S L lin lin

1 0

In te n s ity (n o rm a liz e d )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 .0

4 0

P o te n tia l E n e rg y (k e V ) Figure 4: Results from the SIMS measurements. Shown is the intensity of the B11 signal in the SIMS spectra for Xeq+ irradiation with different charge states and at a fixed kinetic energy of 260 keV. The black dots show data obtained from CVD hBN on a Mo-foil as substrate. The data is normalized to the total current of the incoming ions, left y-scale. The blue dots represent data obtained from hBN grown on a Ir(111) single crystal. Linear fits with slopes of (0.12 ± 0.01)/keV below q = 28 and of (0.29 ± 0.12)/keV above q = 28 describe the data quite well. Due to the set-up of the sample holder a normalization of the SIMS spectra to the ion current was not possible. Instead the data was normalized to the potassium (an ubiquitous contamination) peak of the mass spectrum, right y-scale.

21



6000 5000

(b) 1 ps D t = 1 ps

14 AFM results TTM calculation

12

40+

37+

3000

35 ps 2000 1000 0

0

33+

8

Xe

6

5000

4 2

28+

Xe

4

6

8

10

3000 2000 1000

0 2

40+ 37+ 33+ 28+

sublimation

4000

T L [K]

4000

Xe

Xe

10 Diameter [nm]

(a) Lattice temperature TL [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

00

10

15

Radius r [nm]

no sublimation 2

4

6

Radius [nm]

20 25 30 Potential energy [keV]

35

8

10

40

Figure 5: (a) The array of curves represent the spatial evolution of the lattice temperature at different times after the impact of a Xe40+ ion in the range of 1 ps ≤ t ≤ 25 ps and with time steps of ∆t = 1 ps. The green curve (t = 6 ps) represents the time where the area of sublimated material is found to be largest. The corresponding temperature and radius are marked by dashed lines. (b) Comparison between the diameter of the HCI induce modifications obtained from the friction force images (black squares) and the results of the TTM calculation (red line). The inset depicts the calculated lattice temperature at the time where the sublimation area has reached its maximum extension in dependence of the potential energy for a Gaussian profile of A = 4.2 nm.

22


Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1) Novoselov, K.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D Materials and Van Der Waals Heterostructures. Science 2016, 535, 461. (2) Decker, R.; Wang, Y.; Brar, V. W.; Regan, W.; Tsai, H.-Z.; Wu, Q.; Gannett, W.; Zettl, A.; Crommie, M. F. Local Electronic Properties of Graphene on a BN Substrate via Scanning Tunneling Microscopy. Nano Lett. 2011, 11, 2291–2295. (3) Tien, D. H.; Park, J.-Y.; Kim, K. B.; Lee, N.; Choi, T.; Kim, P.; Taniguchi, T.; Watanabe, K.; Seo, Y. Study of Graphene-based 2D-Heterostructure Device Fabricated by All-Dry Transfer Process. ACS Appl. Mater. Interfaces 2016, 8, 3072–3078. (4) Goossens, A. M.; Driessen, S. C. M.; Baart, T. A.; Watanabe, K.; Taniguchi, T.; Vandersypen, L. M. K. Gate-Defined Confinement in Bilayer Graphene-Hexagonal Boron Nitride Hybrid Devices. Nano Lett. 2012, 12, 4656–4660. (5) Petrone, N.; Dean, C. R.; Meric, I.; van der Zande, A. M.; Huang, P. Y.; Wang, L.; Muller, D.; Shepard, K. L.; Hone, J. Chemical Vapor Deposition-Derived Graphene with Electrical Performance of Exfoliated Graphene. Nano Lett. 2012, 12, 2751–2756. (6) Britnell, L.; Gorbachev, R. V.; Jalil, R.; Belle, B. D.; Schedin, F.; Katsnelson, M. I.; Eaves, L.; Morozov, S. V.; Mayorov, A. S.; Peres, N. M. R.; Castro Neto, A. H.; Leist, J.; Geim, A. K.; Ponomarenko, L. A.; Novoselov, K. S. Electron Tunneling through Ultrathin Boron Nitride Crystalline Barriers. Nano Lett. 2012, 12, 1707–1710. (7) Lee, K. H.; Shin, H.-J.; Lee, J.; Lee, I.-y.; Kim, G.-H.; Choi, J.-Y.; Kim, S.-W. LargeScale Synthesis of High-Quality Hexagonal Boron Nitride Nanosheets for Large-Area Graphene Electronics. Nano Lett. 2012, 12, 714–718. (8) Shukla, V.; Jena, N. K.; Grigoriev, A.; Ahuja, R. Prospects of Graphene-hBN Het-

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(25) Toulemonde, M.; Dufour, C.; Paumier, M. E. Transient Thermal Process after a HighEnergy Heavy-Ion Irradiation of Amorphous Metals and Semiconductors. Nucl. Instrum. Methods Phys. Res., Sect. B 1992, 46, 14362. (26) Ritter, R.; Wilhelm, R. A.; Stöger-Pollach, M.; Heller, R.; M¨ ucklich, A.; Werner, U.; Vieker, H.; Beyer, A.; Facsko, S.; Gölzhäuser, A.; Aumayr, F. Fabrication of Nanopores in 1 nm Thick Carbon Nanomembranes with Slow Highly Charged Ions. Appl. Phys. Lett. 2013, 102, 063112. (27) Gruber, E.; Wilhelm, R. A.; Pétuya, R.; Smejkal, V.; Kozubek, R.; Hierzenberger, A.; Bayer, B. C.; Aldazabal, I.; Kazansky, A. K.; Libisch, F.; Krasheninnikov, A. V.; Schleberger, M.; Facsko, S.; Borisov, A. G.; Arnau, A.; Aumayr, F. Ultrafast Electronic Response of Graphene to a Strong and Localized Electric Field. Nat. Commun. 2016, 7, 13948. (28) Hopster, J.; Kozubek, R.; Ban-d’Etat, B.; Guillous, S.; Lebius, H.; Schleberger, M. Damage in Graphene due to Electronic Excitation Induced by Highly Charged Ions. 2D Mater. 2014, 1, 011011. (29) Hopster, J.; Kozubek, R.; Krämer, J.; Sokolovsky, V.; Schleberger, M. Ultra-Thin MoS2 Irradiated with Highly Charged Ions. Nucl. Instrum. Methods Phys. Res., Sect. B 2013, 317, 165–169. (30) Gorbachev, R. V.; Riaz, I.; Nair, R. R.; Jalil, R.; Britnell, L.; Belle, B. D.; Hill, E. W.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K.; Blake, P. Hunting for Monolayer Boron Nitride: Optical and Raman Signatures. Small 2011, 7, 465–468. (31) Peters, T.; Haake, C.; Hopster, J.; Sokolovsky, V.; Wucher, A.; Schleberger, M. HICS: Highly Charged Ion Collisions with Surfaces. Nucl. Instrum. Methods Phys. Res., Sect. B 2009, 267, 687–690.

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(49) Eremets, M.; Takemura, K.; Yusa, H.; Golberg, D.; Bando, Y.; Blank, V.; Sato, Y.; Watanabe, K. Disordered State in First-Order Phase Transitions: Hexagonal-to-Cubic and Cubic-to-Hexagonal Transitions in Boron Nitride. Phys. Rev. B 1998, 57, 5655– 5660. (50) Aumayr, F.; Winter, H. Electron Emission Induced by Slow Highly Charged Ions on a Clean Metal Surface. Nucl. Instrum. Methods Phys. Res., Sect. B 1994, 90, 523–532. (51) Toulemonde, M.; Dufour, C.; Meftah, A.; Paumier, E. Transient Thermal Processes in Heavy Ion Irradiation of Crystalline Inorganic Insulators. Nucl. Instrum. Methods Phys. Res., Sect. B 2000, 166-167, 903–912. (52) Shevlin, S. A.; Guo, Z. X. Hydrogen Sorption in Defective Hexagonal BN Sheets and BN Nanotubes. Phys. Rev. B 2007, 76, 7862. (53) Hao, R.; Shi, J.; Zhu, L.; Ji, L.; Sun, T.; Feng, S. A First-Principle Study on Adsorption of Atomic Hydrogen on the Two-Dimensional Hexagonal Boron Nitride Monolayer. Superlattices Microstruct. 2017, 111, 696–703. (54) Kumar, R.; Parashar, A. Fracture Toughness Enhancement of h-BN Monolayers via Hydrogen Passivation of a Crack Edge. Nanotechnology 2017, 28, 165702.

29



Graphical TOC Entry

ron

n

highly charged ion de itri

electronic excitation

o .b

nano sublimation

x

he

defect formation

Silico n Dioxid e

30


Page 30 of 36

highly charged Page 31 of 36ACS Applied Nano Materials electronic ion

de

itri

n ron

1 2 3 4

o .b

excitation

nano sublimation

x

he

Silico

n

defect formation

DioxidParagon Plus Environment ACS e

(a)

SL-hBN

1367 cm

Intensity [arb. u.]

FL-hBN

SiO2

Page 32 of 36

SL hBN FL hBN

-1

D mode 1370 cm-1

10 µm

1350 1400 Raman shift [cm-1 ] 16

(d)

Line profile

12

Z [Å]

1 2 3 4 5 6 7 8 9 10 11 12 13 (c) 14 15 16 17 18 19 20 21 22 23 24 SiO2 25 26 27

(b)Materials ACS Applied Nano D mode

SL-hBN

4Å 8

1ACS µmParagon Plus Environment 4

0

3

X [Å]

6

Page 33 of 36

(a)

pristine


4.4 nm

Topography

Topography 1 2 3 4 5 6 7 8 9 10 11 12 13 14 FFM 15 16 17 18 19 20 21 22 23 24 25 26 27 28

(b) irradiated with Xe40+ 4.4 nm no modifications

no modifications

500 nm trace

0.0 nm FFM retrace

500 nm 0.0 nm FFM retrace

FFM trace

4.4 nm

Pits ACS Paragon Plus Environment

Hillocks 500 nm

(a) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Xe37+ Page 34 of 36

Xe40+ Xe28+

(b) ACS AppliedXe Nano Materials

trace

trace

trace

retrace

retrace

retrace

33+


(c)

Page 35 of 36

1.8 CVD hBN on Mo-foil SL hBN on monocrystalline Ir linear fit low q linear fit high q

8

Xe40+ 1.6

Xe37+ 1.4

6

Xe33+

4

1.2

Xe28+ 2

0

Xe26+ 5

10

Xeq+ @ 260 keV 15

20

25

30

Potential Energy (keV) ACS Paragon Plus Environment

35

40

1.0

Intensity (normalized)

10

Intensity (normalized)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


5000

(b) ACS Applied Nano Materials

Page 36 of 36

14

1 ps D t = 1 ps

AFM results TTM calculation

12

40+

4000 3000

35 ps 2000

37+

1000 0

0

33+

8

Xe

6

5000

28+

Xe

4

6

8

Radius r [nm]

10

2000

00


10

3000

1000

0 2

40+ 37+ 33+ 28+

sublimation

4000

4 2

Xe

Xe

10

T L [K]

Lattice temperature TL [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

6000

Diameter [nm]

(a)

15

no sublimation 2

4

6

Radius [nm]

20 25 30 Potential energy [keV]

35

8

10

40

Fabrication of Defective Single Layers of Hexagonal Boron Nitride on

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