2D ANISOTROPIC BISMUTH MATERIALS AND METHOD FOR OBTAINING SAME USING COLLOIDAL SYNTHESIS

Abstract

A 2D bismuth material, also called bismuthene, has a sandwich-like sheet structure with at least two outer layers formed by organic molecules containing sulphur atoms that form BiS bonds and at least one inner layer formed by a crystalline network of Bi(0) atoms. These materials are useful in electronic, optoelectronic, catalytic applications or in energy storage and transformation. Furthermore, a process produces this material from a bismuth salt that reacts with an amine and subsequently with a thiol by effect of the application of radiation and a subsequent reduction. This process has a colloidal approach to producing Bi(0) crystals based on a photocatalytic reduction of a soluble Bi(III) organometallic complex leading to the generation of the crystals.

Claims

1. A material comprising two-dimensional crystals of Bi(0) is formed by: at least two outer layers comprising organic molecules R.sub.2SH, with R.sub.2 being substituted linear or branched C.sub.1-C.sub.18 alkyl or substituted C.sub.5-C.sub.10 aryl, and at least one inner layer between the outer layers comprising metal Bi(0) atoms forming a crystal structure, wherein S atoms of the organic molecules R.sub.2SH of the outer layers are covalently bonded to the adjacent Bi(0) atoms of the inner layer and wherein said outer layers and said inner layer are arranged forming a sandwich-like sheet structure with anisotropic order on a stacking axis.

2. The material according to claim 1, wherein the outer layers have a thickness of between 0.5 to 3 nm.

3. The material according to claim 2, wherein the outer layers have a thickness of 1.5 nm.

4. The material according to claim 1, wherein the inner layer has a thickness of between 5 to 10 nm.

5. The material according to claim 4, wherein the inner layer has a thickness of 7 nm.

6. The material according to claim 1, wherein the Bi(0) of the inner layer has a rhombohedral crystal structure (PDF 44-1246).

7. The material according to claim 1, wherein said material forms crystals of hexagonal morphology.

8. The material according to claim 7, wherein the crystals have a diameter greater than 1,000 nm.

9. The material according to any claim 1, wherein the crystals have a thickness of less than 20 nm.

10. The material according to claim 1, wherein the crystals have an aspect ratio greater than 500.

11. The material according to claim 1, wherein the outer layers are functionalised with sulphur atoms that form BiS bonds and the inner layer is rhombohedral bismuth.

12. A process for producing the material according to claim 1, which comprises the following stages: a) preparing a solution of a bismuth salt with the following formula:
[R.sub.1COO.sup.]Bi.sup.3+ (I) wherein R.sub.1 is a linear or branched C.sub.1-C.sub.18 alkyl, in an organic solvent with the following formula:
CH.sub.3(CH.sub.2).sub.mCHCH.sub.2 (II) where m is an integer value selected from 1 to 20, and add an amine with the following formula:
NH.sub.2(CH.sub.2).sub.n1CHCH(CH.sub.2).sub.n2CH.sub.3 (III) where n1 and n2 are integer values independently selected from between 1 and 10, b) increasing the temperature of the reaction mixture obtained in (a) to a temperature of between 150-250 C., subjecting to vacuum and applying radiation with a wavelength of between 430 and 530 nm, c) breaking the vacuum of the reaction medium of (b) by the introduction of an inert gas, d) adding to the reaction mixture a reducing agent with the following formula:
R.sub.2SH (IV) wherein R.sub.2 is selected from substituted linear or branched C.sub.1-C.sub.18 alkyl or substituted C.sub.5-C.sub.10 aryl, e) stopping the reaction and separating the product obtained.

13. The process according to claim 12, wherein R.sub.1 in the bismuth salt of formula (I) is a linear or branched C.sub.8 alkyl.

14. The process according to claim 13, wherein the bismuth salt of formula (I) is as follows: ##STR00004##

15. The process according to claim 12, wherein m in the organic solvent of formula (II) is an integer value selected from 10 to 18.

16. The process according to claim 1, wherein the organic solvent of formula (II) is the following:
CH.sub.3(CH.sub.2).sub.15CHCH.sub.2

17. The process according to claim 12, wherein n1 and n2 in the amine of formula (III) are an integer value independently selected from between 5 and 8.

18. The process according to claim 1, wherein the amine of formula (III) is the following: ##STR00005##

19. The process according to claim 12, wherein the temperature in step (b) is 200 C.

20. The process according to claim 12, wherein the radiation of step (b) is applied for a time of between 10 and 30 minutes.

21. The process according to claim 12, wherein the inert gas of step (c) is a non-oxidising gas.

22. The process according to claim 12, wherein R.sub.2 in the reducing agent of formula (IV) is selected from a linear C.sub.8-C.sub.12 alkyl or a phenyl.

23. The process according to claim 1, wherein the reducing agent of formula (IV) is selected from dodecanethiol or thiophenol.

24. The process according to claim 12, wherein the reaction is stopped in step (e) by suddenly reducing the temperature of the mixture obtained in (d).

25. The process according to claim 12, wherein the product is separated in step (e) by centrifugation or filtration.

26. A method of using the material according to claim 1, comprising storing or generating energy.

27. A method of using the material according to claim 1, comprising catalysis.

28. A method of using the material according to claim 1, comprising applying in photonics.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0050] FIG. 1.(a) Shows the synthetic procedure for obtaining a few layers of two-dimensional bismuthene (2D). A solution of BiNeo and OA in ODE is degassed at 200 C., then DDT is added to the reaction mixture. The formation of the organometallic complex Bi-DDT is demonstrated by the yellow colour of the solution. Subsequently, the Bi(III) is reduced to Bi(0) while the thiol is oxidised to disulfide. (b) BF-TEM of a single crystal of Bi(0) with a diameter greater than 1.5 m, (c) electron diffraction (SAEP) that allows observing the rhombohedral ordering along the axis [001]. (d) High-resolution TEM image. (e) AFM performed in tapping mode indicating the scale on the z-axis. (f) PXRD pattern that enables corroborating in general terms the obtaining of Bi(0) with rhombohedral structure (PDF #44-1246). (g) Single crystal Raman spectrum (inset shows optical micrograph), recorded with red laser (633 nm), exclusively showing peaks E.sub.9 and A.sub.1g.

[0051] FIG. 2.a) Dependence of the reaction rate on DDT concentration. The black curve, 3 equivalents of DDT with respect to Bi(III), while the grey curve has an excess of DDT (1000 mM). Photographs of the solutions at the beginning (yellow. Bi-DDT) and end (black. Bi(0) NPs) of the reaction. b) TEM image that enables observing the presence of Bi(0) NPs with spherical morphology.

[0052] FIG. 3.Dependence of light and temperature on producing Bi(0) by photochemical reduction. Representative BF-TEM images of the nanomaterials obtained for the different temperatures evaluated using white LED light of different intensity: 807 lm (a) and 2000 lm (b). Dependence of reaction rate on temperature and light intensity or brightness (c). Characteristic emission spectra of the LED lamps used: 807 lm (grey) and 2000 lm (black) (d). The grey box indicates the region of the electromagnetic spectrum responsible for the photocatalytic reaction, 450-550 nm.

[0053] FIG. 4.AFM image of a hexagonal particle (a) wherefrom the thickness profile (b) is extracted. Raman spectrum typically obtained with the signals E.sub.9 and A.sub.1g (c).

[0054] FIG. 5.BF-STEM image (a) and integrated EDS signals for Bi (b), S (c) and O (d). EDS spectrum signalling lines Bi-M, Bi-L and SK (e). Adjustment of the EDS signal in the region of energy comprised for the Bi-M and SK lines considering the sulphur signal.

[0055] FIG. 6.XPS spectrum of the material obtained by the synthesis process where a clear characteristic signal of S 2p is observed that is superimposed with the Bi 4f signals. (a). After removal of this S-containing surface layer by Ar.sup.+ pickling, no S signals are observed and instead Bi(0) signals can be observed exclusively (b).

[0056] FIG. 7.AC-HAADF-HRSTEM image of Bi(0) where a defect-free surface can be observed on a scale greater than 1000 nm.sup.2. An FFT of the image is shown in the box.

[0057] FIG. 8.Characterisation of the organometallic precursor of Bi(III): PXRD pattern. Box: increase in the 2-10 range. The first three peaks can be indexed with a 00I group with an interlamellar distance of 31.7 (A). ATR-FTIR spectroscopy. The absence of signals at 2500 cm.sup.1 (SH vibrations) ensures the absence of free thiol, suggesting BiS bonds (B). The diffuse UV-Vis reflectance spectrum has an absorption maximum at 417 nm (C). Thermogravimetric analysis carried out in an inert atmosphere (He, 20 mL/min) at 10 C./min, enables observing a single decomposition step at 305 C. (D).

[0058] FIG. 9.UV-Vis absorption spectra for solutions containing mixtures of Bi-Neo, DDT and amine: pure reagents (A) and mixture thereof (B).

[0059] FIG. 10. Cross-section inspection of hexagonal particles obtained by the method of the invention. A sandwich structure based on the surface layer of ca. 1.5 nm and a core section of approx. 7 nm. Experiment (left), simulation (centre) and model (right).

[0060] FIG. 11. Localised surface plasmons of colloidally synthesised few-layer bismuthene. a) STEM-EELS experimental data: a1) HAADF-STEM data with the positions indicated for the extraction of EEL spectra. a2)-a5) integrated EELS intensities showing 4 distinct plasmonic modes in the indicated energy ranges. b) Simulation of the EELS intensities of individual hexagons.

[0061] FIG. 12. Detailed schematic representation of the bismuth/bismuthene sandwich hybrid structure. Electron diffraction patterns for each of the indicated regions. (lower right panel) Inspection of the cross-section by means of HRTEM compared to its respective simulation.

[0062] FIG. 13. Inspection of the cross-section of hexagonal particles obtained by the method of the invention by STEM coupled with EDS/EDX probe. Upper panel: The lines represent the mass normalised percentage for the Bi-M, SK, CK and OK signals. Note: the SK signal was increased by a factor of 10 to expedite viewing. The lines corresponding to the signals of each element are extracted from the HAADF image (lower panel), then allowing a visual inspection of the atomic distribution.

EXAMPLES OF THE INVENTION

Example 1: Synthesis of Bi2D

[0063] In this process, a soluble organic precursor of Bi(III) was reduced under the influence of light and temperature, using dodecanethiol as a reducing agent. The obtained material was separated from the reaction mixture by centrifugation and washed with chloroform.

[0064] As a first step, a stock solution of 100 mM bismuth neodecanate (BiNeo) in 1-octadecene (ODE) was prepared by dissolving 3.614 g BiNeo in 25 mL ODE. In the particular case of colloidal synthesis, 125 mol (1.25 mL) of this solution were pipetted and placed in a 50 mL two-necked flask. Subsequently, 22.22 mL of ODE and 1.235 mL of oleylamine (3.75 mmol, 30 eq.) were added. The system was subjected to vacuum, while being heated in oil bath (200 C.) under constant magnetic stirring (400 RPM) and continuously illuminated using an LED lamp. After 13 minutes of degassing, it was changed to argon atmosphere, while the reaction mixture remained at 200 C. for a further 2 minutes. Through a septum, and using a syringe, 0.3 mL (1.25 mmol, 10 eq.) of dodecanethiol (DDT) was injected. The resulting mixture was allowed to react for 30-40 seconds. Upon addition of DDT, the reaction mixture immediately turned yellow, indicating formation of Bi-DDT. The organometallic complex of Bi(III) was photochemically reduced to Bi(0) while DDT was oxidised to disulfide. The reduction of Bi(III) to Bi(0), and consequently the end of the reaction, was confirmed by the appearance of a black colour (see colour evolution in the scheme of FIG. 1a), showing a marked nematic texture (a first evidence of the high anisotropy of crystals, i.e.: two-dimensional (2D) crystals). The reaction was stopped by immersing the flask in ice bath for 5 minutes. The solution was then centrifuged (10k RPM, 10 minutes) in an argon dry box to avoid oxidation of the Bi(0) crystals.

[0065] After separation of the 2D crystals of Bi(0), the solid was redispersed in chloroform and washed by centrifugation 3 times (10 k RPM, 10 minutes). In this way, we sought to exchange the remaining ODE with chloroform, thus allowing the solid to dry more easily.

[0066] The final product was allowed to dry in the dry box, or stored as a suspension in chloroform, to be used in subsequent characterisations. In the case of microscopies, the suspensions were sonicated before use (TEM, SEM, AFM, Raman), while in other assays the final dry solid was used (PXRD, TGA).

TABLE-US-00001 TABLE 1 Chemical reagents, molecular formulas and codes. Name Formula Code Bismuth neodecanoate Bi(C.sub.10H.sub.19O.sub.2).sub.3 Bi(neo).sub.3 Bismuth Bi(C.sub.12H.sub.25S).sub.3 Bi(C.sub.12S).sub.3 dodecanethiolate Dodecanethiol C.sub.12H.sub.25SH C.sub.12SH Dodecyldisulfide (C.sub.12H.sub.25S).sub.2 (C.sub.12S).sub.2 Oleylamine C.sub.18H.sub.35NH.sub.2 C.sub.18NH.sub.2 Octadecene C.sub.18H.sub.36 C.sub.18 Note: Bismuth dodecanethiolate is the organometallic complex of Bi(III) referred to throughout the text.

Example 2: Characterisation of 2D Bismuth Crystals

[0067] FIG. 1 shows the microscopic and structural characterisation of the 2D hexagonal crystals of Bi(0) obtained. In FIG. 1b) a low magnification image of BF-TEM enables observing crystals of around 1.5 m in diameter exhibiting hexagonal morphology. In FIG. 1c), the electron diffraction pattern (SAEP) provides clear evidence of a rhombohedral crystal structure, where the crystallite size is oriented on axis [001], along the stacking axis of the lamellar crystal. HRTEM (d) and AFM (e) confirm the hexagonal morphology of the obtained lamellar crystals. The PXRD pattern in (f) can be perfectly indexed with the crystal structure reported for rhombohedral Bi (PDF #44-1246), providing a final test of the overall rhombohedral structure of the crystals obtained.

[0068] The organometallic complex Bi-DDT was identified as the active species (precursor) in the reduction reaction from Bi(III) to Bi(0), which can be monitored even at room temperature (see Example 3 below). In FIG. 2a) the dependence of the reaction rate on the concentration of DDT is observed. When using the stoichiometric amount of DDT (3 equivalents with respect to Bi(III), black curve) the reaction is considerably slower than when using an excess of 1000 mM of DDT (grey curve). Beyond the concentration of DDT used, in all cases evaluated, spherical nanoparticles smaller than 20 nm in size are obtained at room temperature (see FIG. 2b). It is important to mention that the reaction indispensably needs visible light for the reduction of Bi-DDT to Bi(0) to take place. If the Bi-DDT precursor, or the reaction mixture, is stored in the dark the reduction does not occur.

[0069] Taking into account the behaviour at room temperature, the dependence of the reaction rate and the resulting morphology on both the light intensity and the reaction temperature was also evaluated. The results are shown in FIG. 3. Overall, a clear trend between reaction rate and temperature can be observed. At higher temperatures, the time needed to reduce Bi(III) to Bi(0) decreases considerably, as can be seen in FIG. 3c) (note that the time axis is on a logarithmic scale). In addition to the effect of temperature on reaction rate, a marked increase in reaction rate can also be observed when brighter/intense LED lamps are used (black squares, 2000 lm vs. grey circles, 807 lm). FIG. 3d) shows the emission spectrum of the lamps where, beyond the change in intensity, it can be observed that both emissions are similar, thus ruling out the effect of changes in the emission spectrum. In turn, experiments were performed using red and blue light, which allow us to conclude that only the blue emission is responsible for increasing the reaction rate (grey box, FIG. 3d). Interestingly, red light can even slightly slow down the reaction rate.

[0070] As can be seen in FIG. 3a), temperature plays a key role in the morphology of Bi(0) crystals. While spheroidal particles prevail at low temperatures, the increase in spheroidal particles favours the faceting of the particles that eventually develop into highly anisotropic 2D materials at temperatures of 200 C. or higher. Interestingly, in the case where the maximum temperature and the most intense light are used, the size of the crystals decreases again, observing a growth along the most pronounced stacking axis (thickness).

[0071] After this analysis, the parameters 200 C. and 2000 lm were chosen as the optimised synthetic conditions. FIG. 4 shows an AFM image a) indicating the sector where the thickness measurement b) was taken. The Raman spectrum of an 8 nm thick crystal is shown in FIG. 4c), where the typical double degenerate in-plane and out-of-plane modes can be observed with symmetries E.sub.9 and A.sub.1g with Raman displacements of 74.8 and 99.8 cm.sup.1, respectively. Spectra were measured using a HeNe laser with an emission wavelength of 632.8 nm. The characterisation performed by Raman microscopy enables ruling out the presence of Bi.sub.2O.sub.3 (signals around 320 cm-1), guaranteeing the absence of oxidation in the Bi(0) crystals.

[0072] STEM-EDS mappings reveal the elemental composition of the hexagonal crystals of Bi(0) with a small amount of sulphur (S) on the surface. FIG. 5b-d) shows the mapping of the elements Bi, S and O. While Bi and S are equally distributed, the absence of O in the crystal guarantees the purity of the crystals obtained (native surface without oxidation). FIG. 5e) shows the integrated EDS spectrum indicating the SK line. Since the sulphur signal overlaps considerably with the Bi-M lines, an adjustment of the EDS signal was made in the energy range 2.20-2.65 keV. Results are shown in FIG. 5f-g. In this way it is clear that a quantitative adjustment of the EDS signal can only be made if the intensity of the SK line is considered.

[0073] XPS measurements confirm the presence of sulphur on the surface of the Bi crystals.

[0074] Analogous to the case of EDS measurements, the energies of Bi 4f and S 2p overlap, preventing an unequivocal characterisation of the surface chemistry of the crystals (FIG. 6a). However, after performing a pickling using Ar+ ions to remove the first surface layers, the characteristic Bi(0) signal can be unequivocally observed (FIG. 6b). This first surface layer (s) is rich in sulphur and acts as a barrier against oxidation, preserving the identity of the material.

[0075] FIG. 7 shows an image of HAADF-HRSTEM with aberration correction, where in turn, a fast Fourier transform, FFT (Fast Fourier Transformation) is observed. As noted, the material is a single crystal without considerable defects on a large scale (>1000 nm.sup.2).

Example 3: Surface Chemistry

[0076] In particular, more detailed research was performed using spectroscopic techniques, namely XPS and STEM-EDS in combination with Raman spectroscopy. In FIG. 5, the STEM-EDS mapping shows the integrated Bi-M, SK, and OK signals next to a BF-STEM image. The higher magnification TEM data contained in ED show a thin amorphous edge around the Bi crystallites that are indicative of the thickness of the coating layer.

[0077] From the EDS it follows that Bi and S are equally distributed, while the oxygen and carbon signal can be clearly correlated with the underlying support film of the TEM. As shown in the inset of FIG. 5e, only the additional contribution of sulphur allows a correct adjustment of the obtained integrated signal. The surface sensitivity of X-ray photospectroscopy helps to clarify the presence of a sulphur-rich surface layer, as seen in FIG. 6. Interestingly, the strong contribution of S 2p between 165-170 eV points towards strongly oxidised sulphur species (BiS bond test), indicative of oxygen protection.

[0078] Under vacuum conditions in the XPS, a brief sputtering of Ar (30 s) was performed to remove this coating layer with the result shown in FIG. 6 a and b. After etching, an almost complete loss of the XPS intensities related to carbon (285 eV) and oxygen (532 eV) can be followed, as well as the disappearance of the S 2p contribution between 165-170 eV (FIG. 6b). By focusing on the Bi 4f region, a shift of both 4f.sub.7/2 and 4f.sub.5/2 contributions is evident, giving rise to a pure Bi(0) surface. Therefore, Ar sputtering allows simple and effective removal of the protective layer and gives easy access to pure non-oxidised Bi(0).

[0079] The combination of surface sensitive XPS and spatially resolved EDS thus allows concluding the equally distributed surface coating with the protective sulphur species which, while separating Bi from environmental degradation, can be easily removed by ion beam sputtering.

[0080] Finally, a cross-sectional inspection of the hexagonal microparticles has been performed to unequivocally demonstrate the sulphur-based protective layer. FIG. 10 shows the TEM image recorded on a hexagonal particle. As can be seen, the material shows an ordering of the atoms throughout the inspected area. Interestingly, two different sections can be observed, namely the surface exhibits a different atomic distribution compared to the inner part. While in the core of the microparticle the structure corresponds to the rhombohedral Bi (as demonstrated by PXRD in FIG. 1), the surface consists of a highly ordered arrangement of the BiS atoms, responsible for the lack of surface oxidation (FIG. 5d).

[0081] Further analysis of this sandwich structure was performed by electron diffraction (ED). FIG. 12 shows a more detailed description of the bismuth/bismuth hybrid structure with the ED simulation for each layer. The perfect correlation between the obtained ED patterns and the simulation reinforce the hypothesis about obtaining the structure that protects the present invention. Finally, an inspection of the cross-section of this hexagonal material obtained by the method of the invention was carried out by means of stem coupled with an EDS/EDX probe (FIG. 13), which allows spatially locating each type of element. As can be seen in the upper panel, while in the centre of the particles it is only composed of Bi, it is on the faces where the presence of S and C atoms, typical of the DDT molecule, is observed. It is important to note that no O atoms are observed either inside or on the surface of the material, thus being able to rule out the formation of bismuth oxides of Bi.sub.xO.sub.y type. Therefore, these results confirm the obtaining of a sandwich structure composed of a rhombohedral Bi core and two protective outer layers of functionalised Bi covalently bound to the DDT molecule (BiS bonds).

Example 4: Surface Plasmon Modes

[0082] After clarifying the functionalisation/protection of the surface by spectroscopic techniques, monochromatic and aberration-corrected STEM-EELS spectrum images were used to investigate the plasmonic behaviour of the material in the low energy loss regime. Electronic excitations in unoccupied orbitals are usually probed in the EELS. Here the low-energy region of the material was investigated. The observed low energy modes can give access to phononic modes (<1 eV), localised surface plasmon resonances (1-10 eV), as well as volume plasmon modes (20-50 eV). These optical modes indicative of a metal surface are especially interesting for photonics and catalysis.

[0083] The observation of localised surface plasmons is a proof of the high quality of the surface, usually the quality of the plasmonic emission in bismuth systems is deteriorated by the accumulation of defects and distortions in the crystalline order. As shown in FIG. 1, the colloidally synthesised hexagonal bismuth microparticles show no mentionable crystal defects, either locally (HR-STEM) or globally (PXRD), highlighting the advantages of the colloidal approach of the process of the invention.

[0084] This exceptional quality of the material allows viewing different plasmonic modes in an energy range below 10 eV. FIG. 11a shows a STEM image of an investigated hexagon, the resolved modes (a2-a5), as well as the calculated field distributions (b2-b5). The extraordinary surface quality in combination with the next-generation electron microscopy thus allows the direct measurement of these surface modes without the need for any prior treatment.

[0085] Specifically, four different plasmonic modes could be displayed at the indicated energies. Comparison with the simulated results reveals an excellent match of the localisation near the surfaces. To extract the localised EELS spectra, a logarithmic Fourier deconvolution and a subsequent subtraction of the vacuum-acquired deconvoluted ZLP were performed.

Description of the Methodology in the Measurements:

Electron Microscopy:

[0086] Transmission (scanning) electron microscopy and diffraction have been carried out on Bi(0) crystals placed on TEM sample holders covered with a lacey carbon film of a CHCl.sub.3 solution. A JEOL-ARM200F was used for the AC-HRSTEM, operated at 200 kV. TEM and ED measurements were carried out using a Tecnai F20 (200 kV) or a JEOL JEM1010 (100 kV). EDXS mappings were recorded on a Fei Titan Themis300 (300 kV). Scanning Electron Microscopy data were acquired on a Hitachi S4800 FEG, operated at 10 kV. The samples for the AFM, SEM and Raman measurements were prepared by pouring a Bi(0) solution onto silicon wafers with a standard silicon oxide layer and allowing the CHCl.sub.3 to evaporate.

Atomic Force Microscopy:

[0087] Atomic force microscopy was carried out using a Veeco Nanoscope IVa, operated in Si cantilever strike mode.

Raman Spectroscopy:

[0088] Raman spectroscopic measurements were performed with a Horiba LabRam HR Evolution, using a HeNe laser (632.8 nm) with a final power of 7.6 W and a 100 objective (NA 0.9).

Powder X-Ray Diffraction:

[0089] Powder X-ray diffraction (PXRD) patterns were obtained using a PANalytical Empyrean X-ray platform with a capillary platform (diameter: 0.7 mm) and copper radiation (Cu K=1.541 78 ). Measurements were made in triplicate in the 2-theta 2-70 range using a step size of 0.02/step with an integration time of 1 s.

X-Ray Photoelectron Spectroscopy (XPS):

[0090] XPS measurements were recorded on a Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer. Al K X-ray radiation was used as the X-ray source. For all elements, more than 100 spectra were recorded using a 0.1 eV step with a focused point greater than 400 m. XPS data were analysed with Thermo Avantage v5.9912 software.

FIB Cross Section:

[0091] The FIB was carried out with a FEI Helios G4 dual-beam FIB-SEM. A representative crystallite was chosen for the cross-section and a protective layer of Pt was deposited over the crystallite. Two trenches were then cut into the silicon substrate with 30 kV Ga ions. The sheet was attached to the needle of a micromanipulator and lifted from the silicon wafer and welded to an Omniprobe copper grid using Pt. Then, the sheet was thinned until reaching electronic transparency (thickness of about 80 nm) with Ga ions. The foil sample was then investigated on a Philips CM200 FEG TEM, operated at 200 kV.