Compression-Induced Modification of Boron Nitride Layers: A

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Compression Induced Modification of Boron Nitride Layers: a Conductive Two-Dimensional BN Compound Ana P. M. Barboza, Matheus J.S. Matos, Helio Chacham, Ronaldo Junio Campos Batista, Alan B. de Oliveira, Mario S. C. Mazzoni, and Bernardo R. A. Neves ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01911 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Compression Induced Modification of Boron Nitride Layers: a Conductive Two-Dimensional BN Compound Ana P. M. Barboza1#, Matheus J.S. Matos1#, Helio Chacham2, Ronaldo Junio Campos Batista1, Alan B. de Oliveira1, Mario S. C. Mazzoni2* and Bernardo R. A. Neves2*

1

Departamento de Física, Universidade Federal de Ouro Preto, 35400-000, Ouro Preto, MG, Brazil

2

Departamento de Física, Universidade Federal de Minas Gerais, CP 702, 31270-901, Belo Horizonte, MG, Brazil

# - These authors equally contributed to this work. * - Corresponding authors e-mail: [email protected]; [email protected]

Abstract The ability of creating materials with improved properties upon transformation processes applied to conventional materials is the keystone of materials science. Here, hexagonal boron nitride (h-BN), a large bandgap insulator, is transformed into a conductive two-dimensional (2D) material – bonitrol – that is stable at ambient conditions. The process, which requires compression of at least two h-BN layers and hydroxyl ions, is characterized via scanning probe microscopy experiments and ab initio calculations. This material and its creation mechanism represent an additional strategy on the transformation of known 2D materials into artificial advanced materials with exceptional properties.

Keywords: 2D materials, h-BN, re-hybridization, DFT, SPM

TOC Figure

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Hexagonal boron nitride (h-BN) is a large and direct bandgap1 two-dimensional (2D) material with current interest due to its use as a flat insulating substrate for graphene,2 enabling exotic transport features such as Hofstadter butterfly3,4 and fractional fractal quantum Hall effect.3,5 Additionally, its optical properties enable the fabrication of deep UV light sources6 and room-temperature single-photon quantum emitters.7 Among all properties of h-BN, the nearconstancy of its large bandgap, regardless its related 2D8 and 1D9 forms, stands out as a cornerstone. However, there is a class of structural transformations in graphitic structures, which may be characterized by a rearrangement of chemical bonds, that lead to all-sp3 hybridized materials with improved mechanical properties.10-12 Recently, the route for the synthesis of a 2D compound has been reported: under compression by a scanning probe microscopy (SPM) tip, the two topmost layers of few-layer graphene were shown to generate an ultra-thin

diamond-like compound

with semiconductor

properties,

the

diamondol.13

Measurements in a pressurized Raman vessel have provided spectroscopic evidences for the diamondol formation, generalizing its concept to include the diamondization in presence of other chemical species, such as H, which leads to a structure named diamondene.14 Since h-BN is structurally similar to graphene, albeit with strikingly different electronic properties, several questions raise on how hypothetically transformed structures would behave, if they could indeed be formed in a similar experiment, and how stable they would be. Therefore, in the present work, we report on the compression-induced modification of few-layered h-BN into a conductive boron nitride 2D material that is stable at ambient pressure. The full phenomenology of this material´s electronic properties obtained by scanning probe techniques is consistent with a proposed ab initio model of a hydroxylated, two-dimensional sp3-bonded BN material that we named bonitrol. In our calculations, pressure-induced sp2-sp3 re-hybridization between the two uppermost BN layers in the presence of hydroxyl chemical groups lead to formation of such material. Both experiments and theory indicate that bonitrol is a conductive material with a large work function. Results/Discussion Figure 1 shows a series of SPM experiments addressing the above questions by both inducing the re-hybridization and electrically characterizing the resulting compound.13,15 Figure 1a shows an atomic force microscopy (AFM) image of a typical sample of exfoliated BN flakes (see sample preparation details in Materials and Methods). Monolayer (ML-BN) and few-layer (FL-BN) h-BN flakes can be identified atop the silicon oxide (SiOx) substrate in this image (some line profiles in the image help to identify the flake thickness). Some wrinkles are observed on the ML-BN and FL-BN flakes.16 Initially, a biased EFM tip is employed to simultaneously apply forces and inject charges on all entities of fig. 1a (SiOx, ML-BN and FLBN) and subsequent EFM imaging monitors transferred charges13,15 (see details in Materials and 2 ACS Paragon Plus Environment

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Methods – it should be stressed that the 300nm-thick layer of SiOx eliminates any possibility of tip-induced oxidation of the Si substrate). The result of this experiment, shown in Fig. 1b, is a plot of injected charges as a function of applied force for SiOx, ML-BN and FL-BN, which may indicate changes in electronic properties of these materials with pressure.13,15 It is clear that the amount of injected charges does not depend on the applied tip force for monolayer h-BN and SiOx (orange and red symbols, respectively). The observed difference in their injected charge magnitudes may be ascribed solely to differences in their electronic structures.

Figure 1. Fabrication and characterization of bonitrol. (a) AFM image of monolayer (MLBN) and few-layer (FL-BN) hexagonal boron nitride flakes mechanically exfoliated onto a SiOx substrate. Line profiles of both ML-BN and FL-BN flake edges are shown as blue lines. (b) Injected charge as a function of applied force for fixed tip bias (V = -12V) for ML-BN, FL-BN and SiOx. (c) Dependence of injected charges on bias voltage for small (F = 50nN) and large (F = 1100nN) forces applied to ML-BN, FL-BN and SiOx. (d) AFM image of a FL-BN flake atop a SiOx substrate. The blue-dashed square indicates the region of the compression experiment. (e) SKPM mapping of the surface potential before the compression experiment. A surface potential profile is shown by a solid blue line. (f) SKPM mapping of the surface potential after the compression experiment on the marked square region. A surface potential profile is shown by solid blue line. In all SPM images, scale bars indicate their respective dimensions and dashed lines indicate where the profiles (topography or surface potential) were acquired. In all graphs, dashed lines are linear fits of their respective data.


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However, the most important feature of Fig. 1b is a completely distinct behavior for few-layer h-BN (green symbols): the amount of injected charges increases with the applied force, almost doubling its value from F=50 nN to F=500 nN, which suggests a progressive structural change accompanied by a metallization process. A variation of this experiment corroborates and expands these results: for a given applied force, the charge injection q in any material must obviously depend on bias V (as in a capacitive system: q = CV). Therefore, keeping the capacitance C fixed, the amount of injected charge should increase linearly with bias V. That is indeed what is shown in Fig. 1c, where the amount of injected charges is plotted as a function of tip bias for SiOx and FL-BN (red and green symbols, respectively) at two different force regimes: small force (50nN – open symbols) and large force (1100nN – full symbols). Any variation on the slope of such graph indicates a variation on the effective capacitance C. Since the capacitor geometry does not change, such slope variation indicates a modification on the dielectric properties of the capacitor. For SiOx, there is no variation on the slope with force (either small or large), as there is no modification of this material with force. For FL-BN flakes, however, there is a noticeable variation of the slope with force. For small forces, no modifications are expected on the FL-BN flakes and, thus, the slope should be somewhat similar to the bare silicon oxide case (just another thin layer of dielectric material is being added to the capacitor – in this case, the FL-BN flake thickness is 8nm). When a large force is applied, a significant variation on the slope is observed, indicating a modification in the electrical properties of the capacitor (transformation of part of the BN flake into a material with different dielectric constant). In other words, charge injection efficiency increases in FL-BN as the applied force increases. A second set of experiments was conducted to characterize the stability of the FL-BN flake modification upon pressure application. Initially, Fig. 1d shows an AFM image of a FL-BN flake atop SiOx and the square region, indicated by dashed blue lines, is the region in which the compression experiment will take place. Simultaneously, surface potential mapping of this sample prior to any modification was carried out via SKPM and is shown in Fig. 1e. As shown in the image and its surface potential profile, before any compression, there is a significant contrast (~ 40meV) between the SiOx (green) and FL-BN (red) regions, which is associated with their distinct work functions.17 The region, indicated by the blue square in fig. 1d, was then continuously scanned for 20 minutes with a significant compression applied by the SPM tip (F=300nN – Nominal tip radius=5nm). Subsequently (several minutes after pressure application), the SKPM imaging was repeated in the same region. The result, shown in Fig. 1f, may be viewed as an experimental verification of a permanent (irreversible) modification of FLBN when subjected to pressure: the red square region has now a surface potential ~30 meV


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higher than that of the unmodified BN flake. It is interesting to note that no topographic modifications are observed in this modified region of the FL-BN flake (data not shown). The conductive character of these modified regions on FL-BN flakes may also be inferred by EFM charge injection experiments shown in Figure 2. Fig. 2a presents the AFM image of a pristine FL-BN flake, where a 1.5µm-square region was compressed (F = 300nN – blue dashed square). Punctual charge injection was subsequently performed at two different locations indicated by blue disks in Fig. 2a: right in the middle of the compressed region and in a pristine FL-BN region above it (Vtip = -4V – contact force F = 10nN). Then, EFM imaging allowed a charge distribution mapping in these locations, as shown in Fig. 2b. As a consequence of its insulating character, pristine FL-BN concentrates the received charge in the circular spot in which it was injected (upper dark blue spot on Fig. 2b). On the other hand, Fig. 2b clearly shows the delocalized behavior of the injected charge in the compressed region. Indeed, the surface is uniformly covered by the charge, which may be considered as a signature of its conductive behavior.

Figure 2. Electrical conductivity of bonitrol. (a) AFM image of a pristine FL-BN flake, where a region (blue dashed square) was transformed into bonitrol. The blue dots indicate the position of identical and punctual charge injection procedures (F = 10nN, VTip = -4V). (b) EFM image of the same region in (a) mapping the injected charges. While the charge injected on the insulating FL-BN surface remains localized, it immediately spreads over the entire bonitrol surface, evidencing its large conductivity.

The experiments above indicate a rich phenomenology of deformation-induced electronic modifications of BN layers which can be further elaborated by means of first principles calculations based on the DFT formalism as shown in Figure 3. A theoretical protocol to 5 ACS Paragon Plus Environment

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describe the structural transformation of BN layers and to predict the required threshold pressure may be formulated in the following terms:18,19 a supercell describing a BN bilayer (or few-layer) is built with a given interlayer distance d. Hydroxyl groups, representing chemical radicals present in experimental conditions, are placed on top of each boron atom in the uppermost BN layer. This configuration is illustrated on the top row of Fig. 3 - (a) bilayer in the AB stacking case; (b) multilayer in AB stacking; and (c) bilayer in the AA stacking case. It is used as a starting point for a constrained geometry optimization. During relaxation, the z (vertical) coordinates of BN atoms of the bottommost layer are not allowed to be smaller than a given z1 value, while the z coordinates of oxygen atoms cannot be greater than a z2 value, that is, the relaxation takes place between “hard walls”. In an analogy with the experiment, such “hard walls” can be associated with the “substrate” and “tip”, respectively, which are schematically depicted in Fig. 3.

Figure 3. Calculated structural and electronic properties of bonitrol. (a) Structural transformation of a BN bilayer (AB stacking) in presence of -OH groups. On the top row, left and right panels show the initial and final relaxed configurations (side views), respectively. On the bottom row, the band structure is shown, with the Fermi level set to zero. Blue and red lines correspond to the two spin components. The inset shows the top view of the relaxed structure. (b) and (c): the same as in (a), but for a four-layer and bilayer (AA stacking), respectively. In all cases, the initial interlayer distance d is 2.9Å. In all structures, pink, blue, red and white spheres represent boron, nitrogen, oxygen and hydrogen atoms, respectively. z1 and z2 define the “hard-wall” positions (in analogy to tip and substrate positions in the experiment).


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In the end of the relaxation, the pressure can be estimated by adding up the constrained z force components in either the oxygen atoms or the BN bottom atoms. The application of this protocol to a bilayer in the AB stacking led to a complete re-hybridization between the two BN layers when the initial interlayer distance was set to d = 2.9Å, as shown in Fig. 3a. For a slightly larger distance, d = 3.0Å, the re-hybridization does not take place, which allows the estimation of a threshold pressure of ~ 6.7GPa for the restructuring process. The side and top views of the re-hybridized structure may be viewed in the right panel (top row) and inset (bottom row) of Fig. 3a, respectively. They are somewhat similar to the semi-fluorinated BN bilayer recently reported.20 After releasing the constraints, this structure remains stable, that is, given the availability of -OH groups to guarantee an optimal coverage, the process is irreversible and leads to a stable compound: the bonitrol (boron-nitride-ol). The top layer atoms are found making sp3 bonds, with all nitrogen atoms covalently bonded to boron atoms of the bottom layer, leaving the bottom nitrogen atoms with a dangling bond in a sp3 tetrahedral conformation. This should have a profound effect in the electronic structure, since it is expected to behave as an array of dangling bonds whose signature in the band structure must be a defective (centered in the bottom nitrogen atoms) and dispersive band (due to proximity of adjacent dangling bonds). The DFT calculations, shown in the bottom of Fig. 3a, confirm this idea, indicating that the material becomes conductive because the aforementioned band is found crossing the Fermi level (set to zero in the plot). Magnetic moments localized in the bottom nitrogen atoms are responsible for a net magnetization of 1 mB per primitive cell. Also, from band structure calculations, the bonitrol work function may be estimated as 5.5eV, which is slightly smaller than that of bilayer BN (5.8eV). This is in very good qualitative agreement with the results shown in Fig. 1f, which assigns a larger surface potential (or lower work function) for bonitrol when compared to pristine BN.20 The same phenomenology occurs for multi-layered systems: up to seven-layer structures (AB stacking) were tested and, in all of them, the re-hybridization is restricted to the two uppermost layers when the initial interlayer distance is set to 2.9Å. Fig. 3b illustrates this situation for the four-layer system. The threshold pressures change only slightly with the number of layers: 6.5, 7.6, 7.5, 7.9 and 8.2 GPa for three-, four-, five-, six- and seven layers, respectively. By decreasing the initial distance d, the re-hybridization process may be extended to other layers. For instance, the third layer becomes re-hybridized if the distance d is set to 2.4Å (2.6Å) in the three-layer (four-layer) system; for a re-hybridization involving all layers of the four-layer structure the initial distance has to be significantly reduced to 2.2Å. It is important to mention that these results refer to the AB stacking. Generally, exfoliated hexagonal BN flakes may also adopt the AA configuration. Therefore, we also performed calculations up to the seven-layer structure in the AA stacking, which confirmed the phenomenology previously


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described, as shown in Fig. 3c. The two topmost layers become re-hybridized, this time in a wurtzite-like structure, also leading to metallic states originated in the array of dangling bonds. In the AA stacking, the pressure thresholds are slightly larger: 8.2GPa and 9.3GPa for the twoand four-layer systems, and 9.0GPa for all others. Also, the structures have higher total energies per primitive cell compared with the AB case. For the bilayer, this difference is 0.09eV. From the experimental viewpoint, the bilayer case must be considered as distinct from the few- and many-layer cases, since the absence of a third layer leaves the array of dangling bonds free to interact with the environment, which may result in its chemical saturation. This would render an insulating character to the structure, which would prevent its identification with the experiments shown in Figs. 1 and 2. Finally, considering that water provides both hydroxyl and hydrogen ions, it is also worth analyzing the hypothesis that the same phenomenology could appear due to the bonding of H atoms atop either B or N atoms. Therefore, we carried out further ab initio calculations to establish the relative stabilities of these possible outcomes of the experiment which could result in distinct structures. The case of H atoms atop B atoms turned out not to be stable. Upon relaxation, the BN sheets became flat and separated by the van der Waals distance. We did find a converged sp3 structure with metallic character for the second case - H atoms atop N atoms, rendering a re-hybridized structure similar to bonitrol (see both geometry and band structure in Fig. S1 in the Supporting Information). However, considering an energetic balance, in which this resulting structure plus a -OH group is compared with bonitrol plus a -H atom, we found that bonitrol is more stable than this structure by more than 2.2eV per primitive cell. A hypothesis may now be formulated to explain the experimental phenomenology: the compression of water molecules, either from ambient air or a contamination layer, against the BN sample by the SPM tip favors the formation of -OH groups close to the surface, which induces the re-hybridization process and leads to a stable ultra-thin sp3 BN structure. The resulting compound – bonitrol – is stable, conductive and, with its formation, the charge injection process becomes more efficient. There are several experimental tests that may to be carried out to validate this hypothesis: i) Is the observed phenomenology resulting from a simple contamination layer removal from the FL-BN flake? In other words, is the AFM tip sweeping material across the FL-BN surface upon scanning? ii) Are there any tribocharging effects21 on the FL-BN surface during the bonitrol formation procedure? In other words, is the difference in surface potential between bonitrol and BN due to triboelectric charging21 of the BN surface? iii) The presence of water molecules over the sample (or covering the SPM tip) is crucial to activate the re-hybridization process. Therefore, if the charge injection EFM experiment is repeated in a high temperature and dry atmosphere, the charge injection increase must be inhibited; iv) given that the re-hybridization process takes place within the first two topmost layers, the behavior of charge injection with force should not depend on the number n


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of BN layers for n > 2; v) for the same reason, a monolayer BN flake (n = 1) should not suffer any re-hybridization (or increase in charge injection efficiency); vi) since the bonitrol band structure suggests a high asymmetry between electron and hole states, the injection of charges should depend on the bias voltage polarity; vii) The ab initio calculations predict a nonnegligible magnetization of bonitrol. Can this predicted magnetic behavior of bonitrol be detected? Figs. 4a to 4h show the results of additional experiments carried out to answer these questions (except item (v), which is already demonstrated in Fig. 1b) and they all corroborate the bonitrol hypothesis.

Figure 4. Morphological and tribocharging investigations; effects of flake thickness, humidity, bias polarity and magnetic characterization of bonitrol. (a) AFM image of a FLBN flake after the formation of bonitrol on the region indicated by the dashed square. A yellow line shows the topography profile acquired in the region indicated by the dashed yellow line. (b) EFM image of the same region acquired at zero tip bias. The white dashed square indicates the bonitrol region. (c) SKPM image acquired at the same region of (a) and (b). (d) Influence of BN flake thickness on charge injection efficiency (applied force: 500nN – bias: -12V). The inset shows the temperature dependence of charge injection in a FL-BN flake in a dry N2 atmosphere. (e) Graph showing the rectifying character of bonitrol, where charge is efficiently injected for a given bias polarity only (shown at the inset) (applied force: 400nN). The black dashed line is 9 ACS Paragon Plus Environment

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just a guide for the eye. (f) MFM image acquired at the same region of (a) with the tip magnetic dipole pointing inward the sample surface. (g) MFM of the same region, but with the tip magnetic dipole pointing outward the sample surface. (h) MFM image showing the subtraction of images (f) minus (g). In all SPM images, scale and color bars indicate the appropriate values and units.

Figure 4a shows an AFM image of a BN flake where bonitrol was formed in the region indicated by the white dashed square. A line profile (yellow line), acquired in the region indicated by the yellow dashed line, shows the topography variation across a length encompassing BN-bonitrol-BN regions. Two immediate results are seen: 1) There is no visible pile-up of material at the edges of the bonitrol region (bonitrol-BN edges). In other words, there is no contamination layer which is being removed, or swept, from the BN surface. 2) Both bonitrol and pristine BN surfaces show identical sub-nanometric height variations. In other words, from a topographic analysis alone, it is not possible to detect any modification on the sample surface. Figure 4b shows an EFM image acquired simultaneously with 4a and its homogenous coloring shows that there are no electrical charges on the region corresponding to bonitrol (indicated by a white dashed square).15, 22 Therefore, the triboelectrification hypothesis is not verified. As a consequence, the surface potential contrast shown in figure 4c, between the bonitrol and BN regions, is solely due to their work function difference and not due to a tribocharging mechanism. Fig. 4d indicates the independence of the effect on the number of layers: apart from monolayer and bilayer, any multi-layered BN flake shows a similar and substantial charge injection efficiency upon compression. In the inset of Fig. 4d, the temperature dependence of the effect clearly shows a sharp decrease in the charge injection efficiency beginning at ~ 65°C in a dry N2 environment. At such temperatures and in a dry environment, water molecules start to desorb from both BN flake and SPM tip surfaces, precluding the formation of bonitrol. Figure 4e shows the inhibition of charge transfer for negative sample bias and positive tip bias (configurations which have the resulting electric field vector pointing to exactly the same direction). In other words, electron removal from the sample is prevented, which is consistent with the high work function found for bonitrol. As indicated by the scheme in the inset of Fig. 4e, the electron transfer occurs preferentially if the electric field direction is from the sample to the tip, favoring electron transfer to the re-hybridized BN structure – bonitrol. This is consistent with the band structures shown in Fig. 3: indeed, the Fermi level crosses the dispersive bands, characteristic of the bonitrol electronic structure, implying the existence of states which are available to receive electrons.

In this sense, the electronic

structure of bonitrol is similar to a highly p-doped semiconductor, which, in contact with a metal (the SPM tip), would result in a junction with a Schottky-type diode behavior.23 Finally, 10 ACS Paragon Plus Environment

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the magnetic characterization of bonitrol is shown in figures 4f, 4g and 4h via magnetic force microscopy (MFM) images of the same region of fig. 4a. MFM employs an SPM probe covered by a magnetic cobalt alloy film. Due to the thickness of the Co-film and the tip geometry, the MFM tip behaves as a magnetic dipole, whose direction can be tuned by the use of a powerful magnet nearby (normally, a rare-earth magnet).22 Thus, using the north and south poles of a SmCo alloy magnet, the MFM tip magnet dipole can be oriented at opposing directions (pointing inward – fig. 4f – or outward – fig. 4g – the sample surface). Both MFM images in figs. 4f and 4g show a significant magnetic signal coming from bonitrol (light-shaded regions), though with different magnitudes. Due to the metallic nature of the MFM tip, it is also sensitive to electric interaction and even non-specific long range (van der Waals) interactions.22 Therefore, in order to remove such eventual spurious influences from an MFM image, a simple approach is to subtract two images of the same region acquired with different tip magnetization orientations. All other parameters kept constant, only the magnetic signal would appear on such subtracted image. Therefore, fig. 4h shows the subtracted image (Fig. 4f minus Fig. 4g), where a non-negligible magnetic behavior of bonitrol is clear. This figure also suggests some sort of ferromagnetic, or ferrimagnetic, order on bonitrol, but a detailed study of its exact nature is beyond the capabilities of the employed MFM setup. Conclusions In summary, 2D materials seem to have an endless capability of producing unexpected and interesting properties and processes. The creation of another 2D material from pressureinduced re-hybridization of two monolayers with completely different electronic properties from its parent material is one of them. It was initially found for carbon (in its 2D form – graphene – a zero-gap semiconductor), which produced 2D diamond-like materials (diamondol and diamondene13,14), which are insulators. This is now extended for another compound, hexagonal boron nitride – an insulator – which generated bonitrol – with conductive character. Diamondol and bonitrol are probably just the tip of an iceberg: there is no fundamental reason the same process should not occur for other monoatomic 2D materials, like phosphorene or germanene, and for other diatomic 2D materials, like metallic dichalcogenides and chalcogenides. In fact, even hybrid materials, formed from two distinct monolayered-materials, may be envisaged. In conclusion, the present work may mark the consolidation of a different strategy in the discovery of exclusively man-made materials, resulting in the rise of uncountable and yet undescribed 2D materials with attractive properties.

Methods/Experimental


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Sample Preparation: The h-BN samples were prepared by the mechanical exfoliation of boron nitride powder onto a 300nm-thick Si oxide layer covering the p-doped Si substrate. Mono-, biand few-layer flakes were initially identified using optical microscopy. SPM characterization: SPM measurements were carried out with a Bruker MultiMode 8 SPM in controlled humidity atmospheres with the help of a homemade environmental control chamber (from 0% to 60% RH). Heating experiments were carried out with a commercial hot stage AFM setup (High Temperature Heater accessory for Bruker MultiMode SPM). During the heating experiments, both sample and tip are heated to avoid any tip contamination from condensation of desorbed material from the sample. In some experiments, a larger k (spring constant) probe is used to create bonitrol (the compression experiments) and, then, the probe is switched to a low k one for electrical measurements. This is the case for the SKPM experiments in Figs. 1 and 4: NSC35 probes, from Mikromasch (k ~ 10N/m), are employed for the compression and CSC37 Cr-Au-coated probes (k ~ 0.5N/m; from Mikromasch) for the SKPM characterization. In such cases, optical microscopy and fiduciary marks on the sample enable the localization of the same region again after switching probes. This process normally takes 5-10 minutes. For all the EFM characterization, only a single probe is used (either Asyelec-01, from Asylum Research (k ~ 3N/m), or NSC35 from Mikromasch) – data shown in Figs. 1b, 1c, 2 and 4. Both the Asyelec-01 and NSC35-type probes achieve the force range required for the compression and, at the same time, are sensitive enough for the subsequent EFM characterization of the injected charge. Probe switching is only required when there is no charging involved (SKPM characterization does not require any charging, as it measures surface potential differences due to different work functions). As a consequence, in all experiments where charging is involved – including the temperature-dependent EFM experiments, a single probe is used. Thus, there are no time and environment issues related to probe switching. During a punctual-compression-and-chargingEFM experiment, charge injection is carried out at the peak compression force with a dwell time ~ 100ms. The EFM imaging starts as soon as the microscope is set back to EFM mode (~ tens of seconds) and an image normally requires several minutes (5 to 20 minutes). Both PeakForce QNM24 and AFM-contact modes can be used to make the areal compression shown in Figs. 1, 2 and 4. PeakForce QNM mode was mostly used, as it provides a better control of parameters, but similar results were observed when Contact-AFM was used to create bonitrol. Finally, MFM images were acquired in LiftMode using MESP probes from Bruker (k~3N/m – lift height = 50nm). The magnetization of MFM probes was tuned using the different poles of a strong SmCo alloy magnet placed ~1mm atop the tip apex. All SPM images were processed with either WSXM or Gwyddion software packages.


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Bonitrol Charging Characterization: The h-BN charging experiments were made by controllably touching a given region of the flake with a biased AFM tip during a constant contact time t = 0.1s.15 Upon subsequent imaging in EFM mode, the attractive interaction between charged flakes and the conductive tip shifts the cantilever oscillation frequency to lower values.15 Therefore, this EFM response ∆ω can be directly correlated with the amount of injected charge. The theory of EFM shows that the frequency shift ∆ω, measured in EFM images, is directly related to the gradient of the electrostatic tip-sample force F’ by ∆ω = ω0(2k)1

F’, where ω0 and k are the cantilever resonant frequency and spring constant, respectively.22

Through a model that considers the flake area (carrying a charge density σ per unit area) and the tip effective radius R, the density of unbalanced charges in a given h-BN flake can be estimated from EFM images.13 DFT calculations: Our calculations are based on the spin density functional theory18 within the SIESTA implementation.25 For the exchange-correlation potential, we used the generalized gradient approximation (GGA).26 We make use of norm-conserving pseudopotentials in the Kleinman-Bylander factorized form27, 28 and a double-zeta basis set composed of finite-range numerical atomic pseudofunctions enhanced with polarization orbitals. Integrals in real space are conducted in a grid defined by a mesh cutoff of 450 Ry. All geometries are optimized until the maximum force component in any atom is less than 10 meV/A.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at …: Additional ab initio results.

Acknowledgments We acknowledge financial support from the Brazilian agencies CNPq, FAPEMIG, CAPES and INCT-Nano-Carbono. M. J. S. M. and A. P. M. B. acknowledge support from UFOP – Grant Custeio 2017.

Competing Interests The authors declare no competing interests.


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References 1. Watanabe, k.; Taniguchi, T.;

Kanda, H. Direct-Bandgap Properties and Evidence for

Ultraviolet Lasing of Hexagonal Boron Nitride Single Crystal. Nat. Mater. 2004, 3, 404-409. 2. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726. 3. Dean, C. R.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; Taniguchi, T.; Watanabe, K.; Shepard, K. L.; Hone, J.; Kim, P. Hofstadter’s Butterfly and the Fractal Quantum Hall Effect in Moiré Superlattices. Nature 2013, 497, 598-602. 4. DaSilva, M. A.; Jung, J.; MacDonald, A. H.; Fractional Hofstadter States in Graphene on Hexagonal Boron Nitride. Phys. Rev. Lett. 2016, 117(3), 036802. 5. Wang, L.; Gao, Y.; Wen, B.; Han, Z.; Taniguchi, T.; Watanabe, K.; Koshino, M.; Hone, J.; Dean, C. R. Evidence for a Fractional Fractal Quantum Hall Effect in Graphene Superlattices Science 2015, 350, 1231-1234. 6. Kubota, Y.; Watanabe, K.; Tsuda, O.; Taniguchi, T. Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 2007, 317, 932-934. 7. Tran, T. T.; Bray, K.; Ford, M. J.; Toth, M.; Aharonovich, I. Quantum Emission from Hexagonal Boron Nitride Monolayers. Nat. Nanotechnol. 2016, 11, 37-41. 8. Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-Dimensional Atomic Crystals. Proc. Natl. Acad. Sci. 2005, 102, 1045110453. 9. Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Boron Nitride Nanotubes. Science 1995, 269, 966-967. 10. Khaliullin, R. Z.; Eshet, H.; Kühne, T. D; Behler, J.; Parrinello, M. Nucleation Mechanism for the Direct Graphite-To-Diamond Phase Transition. Nat. Mater. 2011, 10, 693-697. 11. Antipina, L. Y., Sorokin, P. B. Converting Chemically Functionalized Few-Layer Graphene to Diamond Films: A Computational Study. J. Chem. Phys. C 2015, 119, 2828-2836. 12. Pan, Z.; Sun, H.; Zhang, Y.; Chen, C. Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite. Phys. Rev. Lett. 2009, 102, 055503. 13. Barboza, A. P. M.; Guimaraes, M. H. D.; Massote, D. V. P.; Campos, L. C.; Barbosa Neto, N. M.; Cancado, L. G.; Lacerda, R. G.; Chacham, H.; Mazzoni, M. S. C.; Neves, B. R. A.


Page 15 of 16 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

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Room-Temperature Compression-Induced Diamondization of Few-Layer Graphene. Adv. Mater. 2011, 23, 3014-3017. 14. Martins, L. G. P.; Matos, M. J. S.; Paschoal, A. R.; Freire, P. T. C.; Andrade, N. F.; Aguiar, A. L.; Kong, J.; Neves, B. R. A.; Oliveira, A. B.; Mazzoni, M. S. C.; Filho, A. G. S.; Cançado, L. G.

Raman Evidence for Pressure-Induced Formation of Diamondene. Nat.

Comm. 2017, 8, 96. 15. Barboza, A. P. M.; Gomes, A. P.; Archanjo, B. S.; Araujo, P. T.; Jorio, A.; Ferlauto, A. S.; Mazzoni, M. S. C.; Chacham, H.; Neves, B. R. A. Deformation Induced Semiconductor-Metal Transition in Single Wall Carbon Nanotubes Probed by Electric Force Microscopy. Phys. Rev. Lett. 2008, 100, 256804. 16. Oliveira, C. K.; Gomes, E. F. A.; Prado, M. C.; Alencar, T. V.; Nascimento, R.; Malard, L. M.; Batista, R. J. C.; Oliveira, A. B.; Chacham, H.; de Paula, A. M.; Neves, B. R. A.; CrystalOriented Wrinkles with Origami-Type Junctions in Few-Layer Hexagonal Boron Nitride. Nano Res. 2015, 8, 1680-1688. 17. Nonnenmacher, M.; O’Boyle, M. P.; Wickramasinghe, H. K. Kelvin Probe Force Microscopy. Appl. Phys. Lett. 1991, 58, 2921. 18. Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864. 19. Kohn, W.; Sham, L. J.; Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133. 20. Wang, Y.; Ding, Y. Structural, Electronic, and Magnetic Properties of the Semifluorinated Boron Nitride Bilayer: A First-Principles Study. J. Phys. Chem. C. 2013, 117, 3114-3121. 21. Terris, B. D.; Stern, J. E.; Rugar, D.; and Mamin, H. J. Contact Electrification Using Force Microscopy. Phys. Rev. Lett. 1989, 63, 2669. 22. Bonnell, D. A. Scanning Probe Microscopy and Spectroscopy, Wiley-VCH, NY, 2001; pp 246-250. 23. Bardeen, J. Surface States and Rectification at a Metal Semi-Conductor Contact. Phys. Rev. 1947, 71, 717. 24. Sahin, O.; Erina, N.; High-Resolution and Large Dynamic Range Nanomechanical Mapping in Tapping-Mode Atomic Force Microscopy. Nanotechnology 2008, 19, 445717. 25. Artacho, E.; Anglada, E.; Diéguez, O.; Gale, J. D.; García, A.; Junquera, J.; Martin, R. M.; Ordejón, P.; Pruneda, J. M.; Sánchez-Portal, D.; Soler, J. M. The SIESTA Method; Developments and Applicability. J. Phys.: Condens. Matter 2008, 20, 6.


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Page 16 of 16

26. Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. 27. Troullier, N.; Martins, J. L. Efficient Pseudopotentials for Plane-Wave Calculations. Phys. Rev. B. 1991, 43, 1993. 28. Kleinman, L.; Bylander, D. M. Efficacious Form for Model Pseudopotentials. Phys. Rev. Lett. 1982, 48, 1425.


Compression-Induced Modification of Boron Nitride Layers: A

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