Seeking borophene on Ni₃Al(111): an experimental characterization of boron segregation and oxidation

Deposition and growth of boron on the Ni₃Al(111) surface was accomplished by choosing the substrate temperature and the B deposition rate so to promote ordering. Deposition of 2 ML of B at 500 K maintains the sharp p(2 × 2) symmetry of the LEED pattern (figures S1(a)–(c)), associated with the surface chemical and structural order, which is therefore preserved. Only through energy-dependent (I–V) closer insight (figure S1(c)) it becomes clear that the surface structure has been altered without affecting the periodicity, nor compromising the local surface order (width of the diffraction spots). Figure 1 reports the XPS core level spectra collected in UHV at room temperature for three selected growth conditions (0.7 and 1.6 ML at 500 K, 0.7 ML at 415 K) compared with the bare substrate case. As a general consideration, while several B-induced spectroscopic features appear in the Ni 2p_3/2, B 1s, and Al 2p core level spectra, their binding energies are scarcely affected by the growth conditions (B coverage and surface temperature), at variance with their intensities, which are strongly influenced instead. In detail, we observe that the interaction of B with the alloy termination is associated with the growth of a Ni 2p_3/2 component (Ni₂—green) shifted by 0.45 eV to higher binding energy with respect to the main bulk feature (Ni₁—grey) at 852.24 eV (figure 1(a)). The plasmonic resonance at +6.14 eV shifts to +6.61 eV and up to + 7.45 eV, depending on the synthesis conditions of the B ad-layer. Similar to the Ni case (figure 1(c)), the Al 2p core level spin-orbit doublet (Al₁—cyan, separation 0.405 eV, Al 2p_3/2 position at 72.14 eV) gives in intensity to a B-induced doublet at +0.24 eV (Al₂—red). A hardly visible shoulder appears at +1.38 eV due to the presence of minor contaminant Al oxide species accumulating at the surface during the longest B deposition (Al₃—yellow). Summarizing, both Al and Ni core level spectra show a single, B-induced additional component. Instead, already at first sight, the B 1s spectrum (figure 1(b)) appears quite complex, showing a variety of features with different relative intensities, depending on the surface preparation recipe. More specifically, a dominant component grows at 187.68 eV (B₂—dark green) for the lowest B surface coverage and growth temperature, accompanied by three other peaks at −0.35 (B₁—cyan), +0.25 (B₃—light green), and +0.48 eV (B₄—red), respectively. A small shoulder (B₅—magenta) appears at +1.43 eV, associated with minor O-induced surface contamination due to the long B deposition times (0.07 ML min⁻¹, up to 30 min for the samples of figure 1). While the core level shifts do not vary, the synthesis conditions of the B ad-layer affect the relative intensities of the spectroscopic features. From an absolute intensity point of view, we find that the overall B coverage (extrapolated from a comparison between the cross-section-weighted B 1s, Al 2p, and Ni 2p peak areas at different photon energies) is not proportional to the amount of the evaporated boron. When deposited at 500 K (last row of figure 1), >2 ML of B yield an actual surface coverage of only 1.6 ML, strongly supporting partial dilution of B into the bulk of Ni₃Al. Focusing on the relative intensities, it is observed that the B_2–3 components behave in line with the Ni₂ peak, being more intense when depositing 0.7 ML of B at 415 K than at 500 K, and in the case of higher B loadings (1.6 ML). We thus relate B₂ and B₃ contributions to B–Ni bonds. Furthermore, as a general rule based on the initial state of the photoemission process, it is accepted that higher B coordination numbers reflect in higher B 1s binding energies [16], following a trend observed also for C atoms in graphene [32]. Following this assumption, we assign B atoms contributing to B₃ to more coordinated bonding geometries with respect to those contributing to B₂. Analogously, the intensity behavior of B₄ well-correlates with the Al₂ features, suggesting a relation with B–Al bonds. In fact, we observe weak dependence on the growth temperature, and a strong relation with the B loading. B₄ is the most shifted component, thus indicating highest coordination. B₁ is then associated with a low coordination geometry of the B ad-atoms, in line with its intensity decrease for higher B coverage.

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In figure 2 we plot a selection of STM images obtained at liquid helium temperature after depositing 0.7 ML of B (0.07 ML min⁻¹, 10 min) on the sample held at 500 K. Panels (a) and (b) of the figure show large scale areas of the surface, from which it is clear at first glance that no long-range ordered superstructure is formed, apparently. Few B clusters are uniformly and randomly widespread over the surface, with a broad size distribution centered at about 1 nm diameter, not exceeding 10 nm, and a preference for seeding larger clusters at step edges. However, the surface shows large areas (tens of nm) appearing with different (dark and bright) contrast upon imaging (best evident in figure 2(b)), with a very different local structure, as evidenced by closer inspection (figure 2(c)). More specifically, the darker areas (black arrow) of each terrace correspond to residual clean portions of the Ni₃Al(111) termination, in agreement with previous observations [27, 33, 34], where achievement of atomic resolution and clear cut imaging of the p(2 × 2) phase is quite puzzling due to the presence of distorting charge density waves and chemical disorder. Nevertheless, the 5.03 Å periodicity of the surface unit cell could be observed and measured locally, confirming the known Al–Al distance (the unit cell is depicted in figure 2(d)). In addition to the clean surface pattern, the bright areas (white arrow in figure 2(c)) of the surface show instead at least three B-induced local structures, embedded in the topmost surface layer (figure 2(d)) or decorating the termination as ad-structures (panels (e) and (f)), matching the p(2 × 2) periodicity and thus supporting the observed LEED pattern. The bright protrusions visible in figure 2(d) are in phase and in plane with the surface Al atoms sites at the corners of the p(2 × 2) cell and can be associated with substitutional B atoms in the terminal surface layer. While the measured corrugation induced by the Al atoms of the clean Ni₃Al(111) surface amounts to 5 pm, the substitutional B atoms induce a corrugation of 20 pm. The triangular shapes evidenced in panels (e) and (f), instead, can be associated with ad-structures, with height profiles well-exceeding 1 Å and a side length equal to the p(2 × 2) lattice vector. Moreover, we can distinguish between two non-equivalent triangles with the tip pointing downwards (panel (e)) and upwards (panel (f)) in the respective images. At variance with the former, in the latter case internal resolution could be achieved, showing the presence of three self-assembled protrusions.

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On the basis of the spectroscopy and microscopy information collected up to here, we can put forward tentative structural models that are compatible with the observations. In figure 3 we reproduce the STM imaging of selected areas (panels (a) and (c)) associated with their proposed modeling (panels (b) and (d)) and then extend the models to a locally higher B coverage (panels (e)–(g) and (f)–(h), respectively), which is simply obtained with a higher density of the triangular units. This is inferred from the relative intensities measured for the B_1–4 components in the corresponding B 1s XPS core level spectra reported in figure 1(b). For better direct comparison, the plots have been merged in figure S2. The structure nucleation sites consist in the p(2 × 2) of embedded B atoms (dark green) that are substitutional to Al in (at least) the first Ni₃Al layer, directly imaged by STM and associated with the B₂ component of the B 1s core level. B trimers nucleate at these sites, reasonably stabilized by a coordinating Al ad-atom (yellow). The underlying B atom of the surface, acting as a seed, increases its coordination (light green) thus contributing now to the B₃ component in the B 1s spectra, while the trimer B ad-atoms are associated with the B₁ (cyan, low coordination) and B₄ (red, high coordination) components, respectively. The two non-equivalent triangular strictures can be obtained from the re-orientation of the B trimers originating from the direct coordination to the Al ad-atoms. A role could also be played by adsorption at the hcp or fcc sites of the p(2 × 2)-Al structure of the substrate, respectively. Following this model, the B–Ni and B–Al coordination evidenced in the Ni and Al 2p core level spectra is also reproduced.

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To investigate boron oxidation on the Ni₃Al(111) termination, we adopted three complementary approaches: (i) boron deposition in UHV and post oxidation by exposure to O₂, (ii) boron deposition in O₂ background, and (iii) boron deposition on the ultra-thin Al₂O₃/Ni₃Al(111) oxide film. In the latter case, the oxide film was synthesized by high temperature oxidation of the bare alloy single crystal surface in O₂ background following established recipes, obtaining an extremely sharp characteristic LEED pattern (figure S1(e)) showing the typical (∏67 × ∏67)-R12.2° unit cell [35–43]. When depositing B on this film, no B-induced surface order was observed in LEED, while degradation of the diffraction pattern of the superstructure was evident instead (figure S1(e')). Interestingly, procedures (i) and (ii) led to boron oxidation, at variance with what was observed for the AlB₂ layer on Al(111), where the metal substrate was selectively oxidized [25]. For low exposures (of the order of 10 L), the B/Ni₃Al(111) layer maintains the p(2 × 2) symmetry, in spite of a progressive degradation of the diffraction pattern (figures S1(d), (d') and (d'')). The effects of oxidation are already evident from the Auger spectroscopy measurements (figure S3), showing that Ni is not involved, while both B and Al surface oxide phases are formed. Post-exposure of 2 ML B/Ni₃Al(111) at room temperature to increasing amounts of background O₂ up to a cumulative dose of 36 L induces according shifts in the Auger KLL transitions witnessing oxidation [44–47]: from 65 to 50 eV for Al, from 178 to 168 and 152 eV for B, accompanied by the growth of atomic oxygen KLL modulations at 510 (KVV ¹D) and 488 (KL₁V ¹P) eV. The oxidation process is best evident from the XPS core level spectra, providing finer details. In figure 4 we report the direct comparison between the Ni 2p_3/2 (panel (a)), O 1s (panel (b)), B 1s (panel (c)), and Al 2p (panel (d)) core level spectra collected on the pristine B/Ni₃Al(111) layer (nominal 2 ML of B, effective 1.6 ML of surface B, first row), after post-oxidation with 18 L O₂ at 400 K (central row), and upon deposition of B in O₂ background at 500 K (total O₂ dose 18 L). Already at a first glance we can catch two relevant pieces of information from the spectra: (i) nickel is not oxidized (figure 4(a)) at variance with both boron and aluminum, confirming the Auger spectroscopy data; (ii) oxygen induces surface segregation of B, as evident from the B 1s peak areas in panel (c) of the figure (the spectrum in the topmost panel is multiplied by a factor of 5). Moreover, the intensity ratio of the Ni₁ and Ni₂ components is reversed in the cases of post-oxidation and B deposition in O₂ background, suggesting B segregation is further favored in the former case. The O 1s spectrum (figure 4(b)) reveals the presence of two broad, mostly gaussian features, associated with surface inhomogeneity broadening, at 531.1 and 532.2–532.4 eV. According to literature [43, 45], the low energy peak can be attributed to interfacial Al–O bonds of an Al oxide film and to a ternary O–Al–B phase, while the high energy peak originates from terminal Al–O species and boron oxide [48]. The Al 2p spectrum (figure 4(d)) confirms oxidation of the Ni₃Al(111) termination towards the initial formation of an ultra-thin alumina oxide film [43], with two new Al 2p spin orbit doublets growing at +1.66 and +2.59/+2.81 eV from the bulk reference, associated with interface and terminal Al–O species, respectively. The B 1s spectrum reveals the presence of two additional components with respect to B_1-5 that can be observed upon growth of the bare B system. Specifically (figure 4(c)), a minor feature appears at 190.5 eV (violet), while a very intense peak dominates at 192.6–192.8 eV (yellow), associated with ternary Al–B–O and B oxide phases, respectively [25, 48].

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