# Borophene bottom-up syntheses: a critical review

Neil Richard Innis, Catherine Marichy, Catherine Journet, Colin Bousige

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Neil Richard Innis, Catherine Marichy, Catherine Journet, Colin Bousige. Borophene bottom-up syntheses: a critical review. 2D Materials, 2025, 12 (2), pp.022005. 10.1088/2053-1583/adafbf.hal-04938009v2

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# Borophene bottom-up syntheses: a critical review

Neil R. Innis, Catherine Marichy*, Catherine Journet, Colin Bousige*

Université Claude Bernard Lyon 1, CNRS, LMI UMR 5615, Villeurbanne, F-69100, France

*catherine.marichy@univ-lyon1.fr; colin.bousige@cnrs.fr

## Abstract

The ever-expanding portfolio of two-dimensional (2D) materials now also includes metallic boron sheets, known as borophene. Initially predicted by simulation in 1997, borophene was first synthesized in 2015. This 2D form of boron exists under various polymorphic structures, the most observed being $\beta_{12}$ and $\chi_3$. Borophene fills a crucial gap in the 2D material repertoire thanks to its intrinsic metallicity and relative stability in air. However, in view of practical applications including borophene, it is crucial to master its synthesis to enable controlling its structure and thus tuning its properties. This review focuses on its current bottom-up syntheses, i.e., physical vapor deposition and chemical vapor deposition. Indeed, borophene's structure is different from the bulk boron one, which excludes its top-down synthesis. The impact of substrate characteristics, temperature, pressure conditions, and precursor molecules on borophene's phase and domain size is analyzed, along with key characterization techniques. The review extends to hydrogenated borophene and bilayer borophene for a comprehensive understanding of boron bonding and substrate interactions. Given the importance of ambient stability and transferability of 2D boron layers for device applications, a thorough comprehension of borophene oxidation is imperative. The review thus also discusses potential solutions to mitigate oxidation-related challenges.

## 1. Introduction

*Borophene, a new 2D material.* The exploration into crystalline monolayers began with the first successful exfoliation of a monoatomic semi-metallic honeycomb carbon sheet from graphite called graphene in 2004¹. This continued with other elements into what we now refer to as 2D materials. Such materials show exceptional electronic and mechanical properties due to their large surface area and quantum confinement effects². Two decades of intensive research in this domain of material chemistry yielded a variety of interesting 2D compounds ranging from insulators, like hexagonal boron nitride (hBN), to semiconducting and metallic transition-metal dichalcogenides (TMDs), such as $MoS_2$ and $NbSe_2$, respectively. However, the last decade was a key period in the discovery of other mono-elemental 2D sheets comprising of $tin^2$ (stanene), $phosphorus^{3,4}$ (phosphorene), $silicon^{5,6}$ (silicene), $germanium^{7,8}$ (germanene), and $boron^{9,10}$ (borophene). While most of these elemental sheets are either insulating¹¹ or semiconducting, only borophene⁹ stands out as a polymorphic¹² 2D material where most of its crystalline structures have been proven metallic.
**Stacking 2D layers for new properties.** Another interest lies in the layer-by-layer assembly of various 2D materials – the so-called van der Waals heterostructures (VDWH)¹³,¹⁴. Stacking these weakly interacting 2D materials can indeed result in an entirely new structure that can display physics and functionalities different from the ones of each individual constituents¹³,¹¹,². This 2D construction game opens avenues for the design of materials, where theoretical predictions and the desired physicochemical properties precede and orient the VDWH fabrication for practical applications¹⁵⁻¹⁸. VDWH are thus excellent candidates for electronics and optoelectronic devices, such as transistors and diodes, light-harvesting and detection devices as well as photon emitters¹⁹,². To best exploit the possibilities offered by VDWH, it is essential to have a pool of available constituents with the most diverse properties. However, the list of stable or partially stable materials in ambient conditions that are gathered in Table 1 is rather limited, and most VDWH are usually constituted of graphene, which is a semi-metal, hBN an insulator and semiconducting TMDs such as MoS₂ and MoSe₂. There are indeed a few metallic TMDs such as NbSe₂ and 1T phase of WS₂²⁰,²¹, however, until recently, no stable mono-elemental metallic 2D material had been identified. However, the synthesis in 2015 of partially stable monoatomic two-dimensional boron crystal, borophene⁹, has brought the missing metallic piece into the 2D materials pool – other such Xene were synthesized since, like beryllene²².

**Intrinsic borophene properties.** With its numerous polymorphs, borophene has tremendous potential in flexible²³⁻²⁵ and transparent electronics²⁶, optoelectronics²⁷, catalysis²⁸, gas detection²⁹⁻³¹, biosensing³²,³³, energy storage and conversion³⁴⁻³⁷, and a plethora of new possibilities if included in VDWH. It should be mentioned that borophene cannot properly be called a van der Waals material due to stronger interactions with its environment³⁸, but it can nevertheless be included in VDWH. The reports on borophene are rather scarce in comparison to the likes of other 2D materials such as graphene and hBN. At the time of writing this review, the bottom-up syntheses of 11 borophene polymorphs on six metallic substrates are reported by only a few laboratories⁹,¹⁰,³⁹⁻⁴⁵.

<table><caption>Table 1. List of 2D materials that have been isolated up to now, gathered according to their structure and electronic properties. The ones marked in bold have demonstrated at least partial stability in ambient conditions³,¹¹,²⁰,²¹,⁴⁶⁻⁴⁸. Borophene is one of the only monoatomic metals on the list that has demonstrated partial stability at ambient conditions²²,⁴⁹.</caption><thead><tr><th>Electronic Nature</th><th>Monoatomic</th><th>Graphene family</th><th>Transition Metal Dichalcogenides</th><th>Oxides</th></tr></thead><tbody><tr><td>Insulators</td><td></td><td>hBN, BCN, Fluorographene</td><td></td><td>Micas, Graphene Oxide</td></tr><tr><td>Topological Insulators</td><td>Stanene, Silicene, Germanene</td><td></td><td></td><td></td></tr><tr><td>Semi-metals</td><td><strong>Graphene</strong></td><td></td><td></td><td></td></tr><tr><td>Semiconductors</td><td>Phosphorene</td><td></td><td>MoS<sub>2</sub>, MoSe<sub>2</sub>, WS<sub>2</sub>, WSe<sub>2</sub>, MoTe<sub>2</sub>, ZrS<sub>2</sub>, ZrSe<sub>2</sub></td><td>WO<sub>3</sub>, MoO<sub>3</sub></td></tr></tbody></table>
<table><tr><td>Metals</td><td>Borophene, Beryllene</td><td>NbSe<sub>2</sub>, NbS<sub>2</sub>, TaS<sub>2</sub>, TiS<sub>2</sub>, NiSe<sub>2</sub></td></tr></table>

Ultimately, to unlock the full potential of this new material it is imperative to be able to obtain large surface area (µm²) and tuneable crystalline borophene samples that can easily be handled and characterized. Furthermore, the synthesis method steers the crystallinity, conformality, coverage, and thickness of the prepared samples: thus, depending on the application, a particular synthesis may be preferred over the others.

To expand the reach of this fascinating 2D material, this review aims at facilitating the understanding of precursors, techniques, and mechanisms for borophene syntheses. It will assess its various synthesis methods while putting them into perspective in terms of sample quality in a broad sense – sample size, crystallinity, allotrope selectivity, ambient stability, and transferability. The first part of the review gives a historical perspective on the theoretical predictions of borophene structures that have preceded its synthesis. It also describes its polymorphism and corresponding nomenclature, as well as its predicted properties and the expected role of metal substrates. The second part assesses the experimental fabrication of borophene allotropes, with physical vapor deposition (PVD), and chemical vapor deposition (CVD). Finally, the last part will focus on the stability and transferability of the obtained borophene layers while suggesting passivation methods that may extend the ambient stability of borophene. Perspectives on the potential future directions for borophene research are then discussed.

## 2. What is borophene?

### 2.1 Structure, polymorphism, and nomenclature

In 1997, Boustani⁵⁰ predicted by *ab-initio* calculations that small-sized boron clusters of n=2 to 14, n being the number of boron atoms, would be formed along a triangular network geometry and should moreover exhibit quasi-planar structures. Later, this was proved experimentally by two independent research groups⁵¹,⁵². Further, it was predicted that the planar B₃₆ cluster⁵³ would be stabilized by a hexagonal vacancy and could be indefinitely extended to form a crystalline monoatomic layer of boron, the so-called borophene. This structure is later referred to as the α-sheet. These findings show that the hexagonal hole vacancies and their concentration in the triangular lattice possibly impact the stability of boron nanoclusters.

Various crystalline structures of boron sheets, in Fig. 1, have since been predicted by *ab-initio* and density functional theory (DFT) computations¹²,²⁴,⁵⁴⁻⁵⁹. Boron does not favor a two-center honeycomb lattice because there is an electron deficiency compared to the interatomic graphene bonding, both in σ (sp²) and π (p_z) bonds. However, the incompatibility of p orbital symmetry with planar triangular lattice results in buckled boron sheets thus reinforcing the binding energy of B atoms. As a result, a sheet of a triangular lattice is more stable than a honeycomb one.
<figure><img src="image_2.png" /><figcaption>Figure 1. Polymorphism in free-standing borophene extends from unstable triangular and honeycomb lattices to more stable structures with periodic holes. The value displayed under each structure represents the concentration “η” of hollow hexagons.</figcaption></figure>

The deficiency of electrons in honeycomb boron sheets and the excess of electrons in flat triangular sheets can form a donor-acceptor-like structure of borophene sheets which consists of triangular grids with hollow hexagons (HHs)⁵²,⁶⁰⁻⁶³. The HHs concentration η (also noted ν in some papers) is defined by:

$$ \eta = \frac{m}{N} $$

where m is the number of HHs in a supercell of N triangular lattice sites⁶⁴. For example, an empty honeycomb (graphene-like) lattice has the highest hole density, i.e., η=1/3, while an all-boron triangular lattice has η=0 (Fig. 1). Systematic calculations¹² using DFT and the cluster expansion method have shown that the most stable structures possess η values between 0.1 and 0.15.

However, the parameter η is rather global and insufficient to distinguish between the polymorphs as it does not give information on the spatial arrangements of the HHs. Hence, the nomenclature⁶⁵ for borophene structures, some of which can be seen in Fig. 2, also considers the different coordination numbers (CN) of boron atoms in each monolayer: (i) α-type with CN=5 and 6, (ii) β-type with CN= 4, 5 and 6 (iii) χ-type with CN= 4 and 5 (iv) ψ-type with CN= 3, 4 and 5 (v) δ-type with a single value of CN.
<figure><img src="image_3.png" /><figcaption>Figure 2. Top view of various low-energy structures of (a) δ-, (b) χ-, (c) α-, and (d) β-type boron monolayer sheets. Red and yellow balls denote boron atoms moving outward or inward from the plane, respectively, resulting in buckled boron sheets. Reproduced with permission from ref.⁶⁵.</figcaption></figure>

Further, Wu *et al.*⁶⁵ have also provided values of n - number of boron atoms per unit cell, and E<sub>c</sub> - cohesive energy per atom value for predicted boron sheets. A higher E<sub>c</sub> value corresponds to a more energetically favorable structure. Subscripts are assigned to each type of borophene structure as a function of their E<sub>c</sub>, η, and CN values. For instance, in case of a δ sheet, its CN is represented in the subscript where δ<sub>3</sub> is a planar honeycomb sheet with CN=3, and δ<sub>6</sub> is a buckled triangular sheet with CN=6. As compiled in Table 2, there is just one ψ sheet reported in calculations⁶⁵ while several sheets exist for other types.

<table><caption>Table 2. Boron sheet types with the corresponding η, CN, and E<sub>c</sub> values⁶⁵.</caption><thead><tr><th>Boron sheet</th><th>η</th><th>CN</th><th>E<sub>c</sub> (eV)</th></tr></thead><tbody><tr><td>α</td><td>1/9</td><td rowspan="7">5 & 6</td><td>5.760</td></tr><tr><td>α'</td><td>1/9</td><td>5.762</td></tr><tr><td>α<sub>1</sub></td><td>1/8</td><td>5.732</td></tr><tr><td>α<sub>2</sub></td><td>1/8</td><td>5.744</td></tr><tr><td>α<sub>3</sub></td><td>1/9</td><td>5.735</td></tr><tr><td>α<sub>4</sub></td><td>1/11</td><td>5.696</td></tr><tr><td>α<sub>5</sub></td><td>1/12</td><td>5.629</td></tr><tr><td>β</td><td>1/8</td><td rowspan="3">4, 5 & 6</td><td>5.651</td></tr><tr><td>β<sub>1</sub></td><td>1/8</td><td>5.746</td></tr><tr><td>β<sub>2</sub></td><td>1/7</td><td>5.743</td></tr></tbody></table>

<table><thead><tr><th>Boron sheet</th><th>η</th><th>CN</th><th>E<sub>c</sub> (eV)</th></tr></thead><tbody><tr><td>χ<sub>1</sub></td><td>3/17</td><td rowspan="6">4 & 5</td><td>5.740</td></tr><tr><td>χ<sub>2</sub></td><td>1/6</td><td>5.740</td></tr><tr><td>χ<sub>3</sub></td><td>1/5</td><td>5.723</td></tr><tr><td>χ<sub>4</sub></td><td>1/6</td><td>5.660</td></tr><tr><td>χ<sub>5</sub></td><td>-</td><td>5.638</td></tr><tr><td>ψ</td><td>1/4</td><td>3, 4 & 5</td><td>5.483</td></tr><tr><td>δ<sub>3</sub></td><td>1/3</td><td>3</td><td>4.877</td></tr><tr><td>δ<sub>4</sub></td><td>1/4</td><td>4</td><td>5.384</td></tr><tr><td>δ<sub>5</sub></td><td>1/7</td><td>5</td><td>5.684</td></tr><tr><td>δ<sub>6</sub></td><td>0</td><td>6</td><td>5.662</td></tr></tbody></table>
<table><tr><td>β<sub>3</sub></td><td>1/6</td><td>5.737</td></tr><tr><td>β<sub>4</sub></td><td>1/8</td><td>5.747</td></tr><tr><td>β<sub>5</sub></td><td>2/15</td><td>5.740</td></tr><tr><td>β<sub>6</sub></td><td>1/6</td><td>5.734</td></tr><tr><td>β<sub>7</sub></td><td>1/7</td><td>5.736</td></tr><tr><td>β<sub>8</sub></td><td>1/9</td><td>5.724</td></tr><tr><td>β<sub>9</sub></td><td>1/6</td><td>5.719</td></tr><tr><td>β<sub>10</sub></td><td>1/8</td><td>5.718</td></tr><tr><td>β<sub>11</sub></td><td>1/6</td><td>5.712</td></tr><tr><td>β<sub>12</sub></td><td>1/6</td><td>5.712</td></tr><tr><td>β<sub>13</sub></td><td>1/6</td><td>5.702</td></tr></table>

All the predicted free-standing polymorphs of borophene possess high surface energy and may partially react with ambient oxygen on prolonged exposure to form some borophene oxide particles which may deter its properties and possible applications. Theoretical studies suggest borophene passivation in the form of polymorphic “borophane” - two-dimensional boron hydride sheets<sup>66–69</sup>, and bilayer borophene<sup>70</sup>. We will therefore also consider these two cases below.

## 2.2. Properties and Applications

Borophene is expected to exhibit anisotropy along with excellent mechanical, optical, electronic, and thermal properties. However, some of these theoretically deduced properties could differ with the polymorphic structure. This makes the 2D boron a compelling material to explore. In the following, we will discuss some of borophene’s most prominent properties and their linked applications. More details can be found in the numerous reviews available on borophene applications<sup>64,71–76</sup> – which are outside the scope of this review article.

**Anisotropy.** The physical properties of borophene have been predicted to vary with temperature, polymorphic structure, and along the armchair (a₁), and zigzag (a₂) directions of propagation represented in Fig. 3. The Poisson’s ratio, i.e., lateral expansion when stretched, is calculated to be negative for both in-plane directions, thus verifying borophene’s anisotropic nature<sup>24</sup> which may be further tuned by using mechanical strain<sup>77</sup>. The polymorphism of borophene with its intricate web of triangular grids and HHs makes the anisotropy even more diverse.
<figure><img src="image_4.png" /><figcaption>Figure 3. The crystal structure of δ₆ borophene in top and side views. Atoms in different planes are distinguished by different colors.</figcaption></figure>

**Optical.** A transmittance of up to 100% in visible light is predicted for a borophene monolayer²⁷, making it a probable candidate for transparent conductive materials. The reflectivity of borophene is < 30% in direction a₁ and > 40% in direction a₂⁷⁸. It presents an exotic 1D plasmon excitation with near infra-red or even visible light response (2-3 eV) and large wave-vector (1-2 Å⁻¹) that can make it a promising candidate for applications in nano-photonics and optoelectronics working at optical frequencies²⁶,⁷⁹. Further, borophene shows several absorption bands around 3.65 eV and 10.36 eV along the a₁ direction, and three main absorption bands at 1.09 eV, 8.29 eV, and 10.31 eV along the a₂ direction. Borophene oxide on the other hand has lower optical conductivity in the visible region than pristine borophene⁸⁰.

**Electronic.** Confined electron conductivity of triangular borophene sheet along the a₁ direction is theoretically known⁸⁰. This anisotropic metallic behavior is attributed to the buckling along the a₂ direction as it opens indirect bandgaps of 9.66 eV and 4.34 eV in the structure⁸⁰. The I-V curves show a calculated anisotropy of 2.1 in current at 100 mV⁸¹. For β₁₂ and χ₃ sheets with HHs, a decrease in anisotropic behavior is speculated when compared to the triangular sheet⁷⁷. Negligible anisotropy is observed in I-V curves for β₁₂, however, an anisotropy value of 1.5 is obtained for χ₃. Further, χ₃ shows a higher anisotropy in the a₂ direction with alternating HHs. Hence, HHs alone may not lead to anisotropy, a structural synergy among them is required. Computational evidence shows that borophene, being metallic, can provide electrons to 2D semiconductors⁸² with large bandgaps (E_g > 2.0 eV) to reduce the Schottky barrier, thus, resulting in ohmic contacts. This predicted behavior may be used in the future to replace traditional noble metal electrodes while improving the electronic performance of 2D semiconductors⁸². Na and Li ions⁸³ incorporations in χ₃ and β₁₂ borophene have been suggested to form conductive systems that can be a promising electrode material for high-performance rechargeable metal-ion batteries⁸²,⁸⁴⁻⁸⁶. Therefore, borophene has been envisaged for energy storage⁸⁷,⁸⁸ and supercapacitor⁸⁹ applications. The χ₃ and β₁₂ phases of borophene can act as excellent hydrogen evolution catalysts with a ΔG≈0 eV⁹⁰.
**Catalysis.** Thanks to its metallicity and its abundant presence of electrocatalytic active sites (such as unsaturated boron atoms with lower coordination number$^{91}$), borophene is a candidate of choice for hydrogen evolution reaction (HER)$^{91-93}$. Calculations show an influence of the metal substrate and borophene allotrope on borophene's catalytic performances$^{93}$. In addition, the hexagonal hole present in the structure of some allotropes can act as an anchoring site for metal adsorption (Ti, V, Mn, Fe, Co, Ni): the obtained metal-supported borophene can be used as a catalyst$^{93-95}$. In addition, borophene can be used as catalyst for $O_2^{96}$, $CO_2^{97}$ or $N_2^{98,99}$ reduction reactions. While the pristine sheets present poor performances by themselves, the electron transfer resulting from their metal-decoration allow tuning their overpotentials and hence their catalytic properties$^{94,100-103}$. The interested reader can refer to reviews on the subject$^{93,104}$. Moreover, the hydrogenated counterpart of borophene, i.e., borophane, decorated with single atom transition metals from 3d, 4d, and 5d series like Mn, Co, Zr, and Hf is predicted to be an excellent catalyst for oxygen evolution and oxygen reduction reactions$^{28,98}$.

**Mechanical.** Borophene, unlike other 2D materials, is predicted not to undergo structural failure when stretched beyond critical strain, but rather to undergo a phase transition that further strengthens the material$^{23,105-107}$. The B-B bond contributes significant strength to the boron sheets while the metallic multicenter bonds provide potential fluxional behavior. It is reported via computational calculations$^{23,105-107}$ that borophene sheets exhibit negative Poisson's ratio and high in-plane Young's modulus, i.e., only slight changes in their shape when under elastic or recoverable loads. The two-dimensional Young's modulus for various boron sheets at room temperature$^{23}$ is found to vary from 63 to 136 N.m$^{-1}$ and 12 to 19 N.m$^{-1}$, along $a_1$ and $a_2$ directions, respectively. A decrease in Young's modulus with increasing $\eta$ is suggested. Strains at tensile strength vary from 11 to 19% at 27 $^{\circ}$C$^{19,66}$ and a reduction in the toughness of borophene sheets is reported when temperature is increased from 1 K to 600 K. At room temperature, $\eta=2/15$ shows a calculated toughness of 1 J.m$^{-2}$ and 1.8 J.m$^{-2}$ in $a_1$ and $a_2$ directions, respectively. Some first principle calculations suggest that borophene can withstand stress up to 20.26 N.m$^{-1}$ and 12.98 N.m$^{-1}$ in $a_1$ and $a_2$ directions, respectively$^{106}$. The HHs in borophene are mechanically softer to applied strain than the filled hexagons leading to its high in-plane elasticity. In addition to this, borophene is super stretchable and flexible compared to graphene due to the alignment and spacing of HHs and their fluxional behavior$^{24,107}$. An assessment on flexibility$^{24}$ of boron sheets is done by calculating the Foppl-von Karman number per unit area where a higher value corresponds to higher flexibility. For $\eta=1/6$ borophene it is found to be 568 N.m$^{-2}$ and decreases to 377 N.m$^{-2}$ for $\eta=1/5$, and finally, 34 N.m$^{-2}$ for $\eta=0$. Such versatile properties of borophene polymorphs can find potential applications in composites$^{109}$ and flexible devices$^{110}$, provided they can be synthesized in a controlled manner.

**Thermal.** The thermal conductivity of borophene is rather peculiar and less studied because of the numerous polymorphs and their respective isotropic and anisotropic behaviors. On considering both heat
carriers – phonons and electrons –, study<sup>59</sup> reveals that the phonon heat capacity and thermal conductance tend to decrease with increasing η of boron sheets. This is due to a decrease in the interatomic vibrations as the sheets become less dense with increasing η. The thermal conductance ratio in the a₁ and a₂ directions gives an anisotropy value of around 1.55 for δ<sub>6</sub>, and nearly 1 for χ<sub>3</sub> at T > 1227 °C. The electron thermal conductivity, however, remains unaffected. It is also suggested<sup>111</sup> that strong phonon-phonon scattering along the armchair and zig-zag directions leads to a low lattice thermal conductivity. Furthermore, borophene exhibits an intrinsic phonon-mediated superconductivity below a critical temperature (T<sub>c</sub>) in the range 10-20 K<sup>112,113</sup>.

**Other applications.** Apart from the above-cited properties and their related applications, borophene is envisaged as a good candidate for bio-sensing applications<sup>33,114-116</sup> thanks to the presence of atomic ridges and dangling bonds in its structure. Borophene could also be a good candidate for gas-capture<sup>117</sup>. Some studies would also make use of borophene for water filtration, but since borophene is highly sensitive to oxidation we would think that this is quite unrealistic.

The future of borophene will depend on its characteristics. For instance, pristine borophene, i.e., borophene in its original, pure, and unoxidized form, does not possess high enough adsorption energies for hydrogen storage<sup>118</sup> and is prone to react with hydrogen to form boranes. However, metal decoration could be a possible solution<sup>37</sup>. DFT calculations for Li<sup>34-37</sup> and Ca<sup>37,119</sup> decorated borophene show excellent hydrogen storage capabilities. Borophene derivatives such as borophane and bilayer borophene may enable tuning borophene properties such as its bandgap, thus making it desirable for the semiconductor industry. Simulations have predicted that using borophene as an anode for Li- or Na-ion battery applications would be interesting. However, for such an application, a large quantity of borophene flakes is necessary, which is not compatible with the current bottom-up syntheses. Finally, for optoelectronics, the grain size of the polymorph, direct growth on desired substrate or its transferability onto required support would play a major role.

## 2.3. The influence of metal substrates

Borophene synthesis requires looking into suitable aspects of substrates such as charge transfer, commensurability, lattice mismatch concerning polymorphs, reactivity towards boron, diffusion, and solubility of boron at a given temperature and pressure. In 2013, Yakobson *et al.*<sup>57</sup> theoretically explored the synthesis of various borophene structures on metal substrates, particularly (111) surfaces of face-centered cubic (fcc) Au, Cu, and Ag. These metal substrates offer low solubility of B atoms, no boride formation, and the best lattice match with borophene structure. Three-dimensional boron clusters have high nucleation energy, therefore, it was suggested that the low solubility and fast diffusion of B atoms on metal substrates could favor the growth of 2D sheets<sup>120</sup>. The calculated chemical potential, μ, is represented in Fig. 4, for various boron sheets at 0 K. It concludes that borophene layers are more stable on these metals than their free-standing counterparts. Another computational study<sup>120</sup> points out that it
is chemical hybridization, and not electron transfer, at the boron-metal interfaces that guides the structure of two-dimensional boron sheets. The buckled structures of borophene arise due to the poor overlap between Au 5d orbital and B p<sub>z</sub> orbital. In contrast, planar borophene structures on Ag, Cu, and Ni result from substantially better overlap between metal and B orbitals. The most stable borophene structures on these substrates are predicted, for instance, η=1/9 on Au, and η=1/6 on Cu, Ag, and Ni. Therefore, the choice of a metallic substrate with its charge transfer capabilities, largely defined by its work function, and lattice mismatch may contribute to varying the coverage, stability, and formation of specific borophene polymorphs. The influence of the substrate on resulting atomic structures arises from boron atom's mobility, and a contest between the strain and orbital hybridization of boron with the catalyst surface<sup>120</sup>. Recently, efforts have been made in developing a Machine Learning potential for borophene on silver substrate, yielding the same evolution of borophene energy as a function of hole density as was obtained with DFT at 0 K, but on much larger structures and for molecular dynamics at 27 °C<sup>121</sup>.

<figure><img src="image_5.png" /><figcaption>Figure 4. (a) Evolution of the chemical potential, μ, of B sheets as a function of vacancy concentration, x, in a vacuum and on different substrates. Reproduced with permission from ref. <sup>57</sup>. (b) Borophene potential energy for various allotropes on Ag, as predicted from using machine learned interaction potential. Reproduced with permission from ref. <sup>121</sup>.</figcaption></figure>

In summary, theoretical calculations have shown that the borophene structure composed of HHs and triangular patterns possess greater cohesive energies than those without them. These findings help to understand the role of metal substrates in the selectivity and stability of the borophene polymorphs, thus, acting as markers for the experimental exploration of the material. The reported synthesis methods and characterizations of borophene layers will be discussed in the following section.

### 3. Experimental realizations of borophene

2D materials are usually obtained either via a *top-down* or a *bottom-up* approach. In the top-down techniques, the bulk multi-layer sheets are exfoliated to obtain the 2D material. Mechanical exfoliation of graphite using scotch tape to yield graphene¹, and also its chemical exfoliation<sup>122,123</sup> are representatives of this approach. Such approaches treat the bulk material as a 3D template having a layered crystalline structure identical to the targeted 2D monolayer. Bulk boron powder, however,
commonly possesses a non-layered rhombohedral or tetragonal structure. Therefore, a graphene-like exfoliation process for borophene cannot be envisaged. Some authors$^{124-137}$ do claim obtaining borophene via liquid exfoliation of such boron powders. Such exfoliated flakes are non-planar, inhomogeneous in thickness and crystallinity. Hence, this review focuses on the *bottom-up* approaches. The two commonly used vapor deposition techniques$^{138}$ for this approach are PVD and CVD. These have been widely applied to obtain borophene and have provided consistent results of the obtained 2D boron layers.

## 3.1. Physical vapor deposition

In physical vapor deposition$^{139}$, the source material, generally solid, is subjected to physical processes like thermal evaporation or bombardment by an energetic beam (ions, photons, or electrons) to vaporize its atoms/molecules, which get deposited on a substrate. It is operated under ultra-high vacuum (UHV) to avoid film contamination. Molecular beam epitaxy (MBE)$^{138}$ is a representative technique of PVD and has led to the deposition of several borophene polymorphs, bilayer borophene, and borophane onto metal substrates.

### 3.1.1. Borophene on noble metals

The synthesis of pristine borophene has witnessed significant developments, particularly in the mode and ease of synthesis on metal substrates. A thorough study of the prepared borophene samples is necessary, especially polymorphism, purity, oxidation or other contamination. This can generally be assessed via X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). XPS is crucial in revealing the atomic bonding in borophene. Boron atoms around the positively charged hexagonal holes, and in the negatively charged triangular grids in borophene result in two unique B-B contributions near 187 eV and 188 eV, respectively$^{49}$. These B-B peaks are a little red shifted in energy compared to pure boron B-B bonds$^{140,141}$ due to borophene's 2D nature and interfacial metal interactions. Borophene oxidation can also be monitored via the higher binding energy values for the B-O bond peak at ~192 eV$^{141}$. Furthermore, common impurities of carbon, oxygen, or nitrogen in borophene can be detected and quantified by XPS. STM imaging, with its lateral resolution of up to 0.1 nm, greatly helps to identify the borophene polymorphs – although correlations with DFT simulations of STM images could be necessary. STM topographical images are also a good way to see oxidation (or adatoms) in borophene and often depict oxygen atoms as bright spots on borophene$^{49,142}$. Electron diffraction techniques such as reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) have been utilized to determine the symmetry and crystallinity of the obtained borophene samples$^{143}$. Borophene was experimentally realized for the first time in 2015 by Mannix et al.$^{9}$, via MBE$^{139}$ on Ag(111) substrate, a form of PVD operated under ultra-high vacuum. Fig. 5(a) shows that Ag(111) is kept between 450 °C and 700 °C and pure boron is evaporated by an electron beam in UHV$^{9}$. Nanometer-sized borophene islands (Fig. 5(b)) are thus obtained at 550 °C, which consist of two STM-resolved polymorphs. After an extensive STM study$^{56}$, these polymorphs are confirmed to be a
striped β₁₂, buckled on a reconstructed silver surface, and a homogeneous χ₃. Further, β₁₂ is found to be the dominant phase when using lower boron flux of 0.01 monolayer (ML) per minute and higher growth temperatures of 700 °C. χ₃ on the other hand is favored at low temperatures of 450 °C and higher boron fluxes of 1 ML/min, thus making it a metastable phase. Phase mixing, the co-existence of more than one polymorph in the borophene domains, is brought to light here. This could have been caused by the weak Ag-B interactions that lead to borophene structures with E_c very close to each other. Also, the charge transfer values of 0.01e⁻ and 0.08 e⁻ from silver to boron atoms in β₁₂ and χ₃, respectively⁶⁰, are quite close. It is interesting to note that upon annealing borophene sample at 377 °C, most of the β₁₂ sheets could be transformed into χ₃⁴⁹. Later, an α-sheet buckled at 1.1 Å, along with a β₁₂-sheet rotated at 30° (β₁₂-R30) relative to the silver lattice, are also reported. All these four polymorphs on Ag(111) are metallic⁴⁰. Reports on buckled β and β₈ borophene nanoribbons on Ag(110)⁴¹ suggest that borophene polymorphs are highly selective not just to the flux of boron atoms and substrate temperature, but also to the substrate’s crystal orientation.

<figure><img src="image_6.png" /><figcaption>Figure 5. (a) Scheme of Borophene deposition on Ag(111) by PVD and (b) polymorphs imaged via STM. Reproduced with permission from ref.⁹.</figcaption></figure>

These first borophene samples⁹ show signs of oxidation after two hours, as depicted in the XPS spectrum in Fig. 6(a), while a 2 to 4 nm thick amorphous Si capping could protect it for up to three weeks. The chemical state of boron atoms in β₁₂ and χ₃ polymorphs is further investigated¹⁴⁴ via X-ray standing wave (XSW)-excited XPS (Fig. 6(b)) with sub-Å spatial resolution measuring the distribution of boron atoms relative to the Ag(111) lattice. XSW-excited XPS demonstrates that as-deposited borophene is vertically separated by 2.4 Å from the silver substrate. This value is comparable to hBN on metal substrates. Thus, a single layer B atom distribution on Ag(111) with strong in-plane B-B bonds and no Ag-B bonding, just weak interactions, is obtained. At 300 °C substrate temperature, one observes that β₁₂ (R) has three intense B 1s peak contributions - R₄, R₅, and R₆, at 187.3 eV, 187.7 eV, and 189.5 eV, respectively, attributed to the 4, 5 and 6 coordinated boron atoms. In comparison, the χ₃ (D) polymorph only shows two low-intensity peaks - D₄ and D₅ for 4 and 5 coordinated B atoms at 187.6 eV and 188.4 eV, respectively. However, the situation is inversed for depositions at 400 °C, where χ₃ peaks (D₄ and
$D_5$) are much higher in intensity than $\beta_{12}$ peaks (R$_4$, R$_5$, and R$_6$). The intensity ratios of the peaks for $\beta_{12}$ and $\chi_3$, respectively, are in good agreement with the theoretical studies$^{9,58,120}$. This work experimentally shows the discrete, planar, and polymorphic nature of borophene.

<figure><img src="image_7.png" /><figcaption>Figure 6. (a) XPS spectra of B 1s comparing the oxidation of borophene with and without Si capping after different times. (b) High-resolution XPS of Boron 1s core-level spectra for borophene samples grown at 300 and 400 °C. $\beta_{12}$ and $\chi_3$ sheets are labelled R and D due to their respective primitive rectangular and diamond STM image shapes. Reproduced with permission from ref.$^{9,144}$.</figcaption></figure>

Perhaps the most distinguishing and fascinating property of borophene among all other flat materials is its metallicity$^{58}$. Scanning tunneling spectroscopy (STS), an extension of STM, reveals the density of electrons as a function of energy in the material, enabling observation of the borophene's metallic properties$^{143}$. STS analysis (Fig. 7) on borophene sample$^{9,40}$ shows the metallic nature of borophene through dI-dV graphs typical of metals. However, with silver being highly conducting in nature, the substrate could easily dominate any contribution from atomically thin borophene and hence a technic with a better depth resolution is preferred to better distinguish the contributions of the boron layer from the one of its substrate.

<figure><img src="image_8.png" /><figcaption>Figure 7. Metallic nature observed via STS dI/dV spectra for $\beta_{12}$ (rectangular and striped phases) and $\chi_3$ (homogenous phase) on Ag(111) (silver reference spectrum inset). Reproduced with permission from ref.$^9$</figcaption></figure>

Thus, Feng *et al.*$^{145}$ have performed angle-resolved photoemission spectroscopy (ARPES) where distinct metallic boron bands, B1 to B6, are found to be well preserved on the bulk sp bands of Ag(111) substrate (Fig. 8).
<figure><img src="image_9.png" /><figcaption>Figure 8. (a-e) ARPES images of as-deposited borophene on Ag(111). Metallic bands of borophene, labelled B1 through B6, could be clearly distinguished from the metallic sp band of bulk silver (f) The constant energy contours at the Fermi surface shows three spectral features (B1, B2, and B2'), clearly indicating that the β₁₂ boron layer is metallic. Reproduced with permission from ref.¹⁴⁵.</figcaption></figure>

Other techniques for elemental analyzes of borophene include Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS), which are more sensitive to boron detection and quantification in comparison to energy dispersive X-ray spectroscopy (EDS). Borophene growth is demonstrated via AES¹⁴³ in Fig. 9(a) with a distinct boron K edge signal at 180 eV on Ag(111) substrate post-boron deposition. The crystallinity is assessed by RHEED¹⁴³ (Fig. 9(b)) where new lines corresponding to crystalline boron in χ₃ borophene are pointed out. EELS, in Fig. 9(c), shows three small energy loss peaks at 23.1 meV, 62.5 meV, and 88.0 meV attributed to B-B bonds in borophene, and a weak signal at 160.8 meV suggesting B-O bonds¹⁴³.

<figure><img src="image_10.png" /><figcaption>Figure 9. (a) AES spectra showing the characteristic B KVV peak after borophene growth, in red, on Ag(111), in black. (b) RHEED patterns taken at 15 keV before (left) and after (right) borophene growth on Ag(111) show the signals from borophene and Ag substrate represented by red and black lines, respectively. (c) EELS spectrum showing the three characteristic borophene peaks. Reproduced with permission from ref.¹⁴³.</figcaption></figure>

Raman spectroscopy has proven to be an important tool in distinguishing various bulk boron polymorphs such as α-, β- and γ-B¹⁴⁶,¹⁴⁷. Sheng et al.¹⁴⁸ extended this polymorph resolving power of Raman to
borophene and acquired the distinct spectra presented in Fig. 10, for β₁₂ and χ₃ polymorphs on Ag(111). Far-field Raman and tip-enhanced Raman spectroscopy (TERS) of the samples are carried out under vacuum at 78 K using a 532 nm laser and no sample damage is reported. Far-field Raman reveals the in-plane vibrations of borophene, while TERS enhanced the out-of-plane vibrations. Also, due to the low Raman cross-section of boron atoms, the far-field Raman spectroscopy took 50 min for a discernible β₁₂ spectrum, while TERS produced a decent spectrum in 50 s. For β₁₂, several low-frequency vibration modes are found in the far-field Raman spectrum that are distinct from that of bulk β rhombohedral boron powder, used as B source for the deposition. The β₁₂ far-field Raman and TERS spectra are very different from that of χ₃, particularly in high-frequency zones as seen in Table 3. The spectra also corroborate that the striped and rectangular β₁₂ are inherently the same structures- the differences arise because of the strain and lattice mismatch of Ag(111) and β₁₂ borophene. The energy shifts between the expected and observed Raman modes should be carefully noted as they can occur due to substrate-induced interactions and molecular/ion doping leading to a change in pristine borophene's chemical environment¹⁴⁹. Borophene also possesses strong electron-phonon interactions, and the calculated electron-phonon coupling responsible for superconductivity in borophene, combined with the McMillan equation gives a T_c of 7.1 K and 9.8 K for the metallic β₁₂ and χ₃ sheets, respectively.

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

Figure 10. Raman spectra recorded at 10 mW and 50 s for (a) boron powder used to grow (b) β₁₂ borophene. (c) TERS spectra recorded at 10 mW and 50 s for striped and rectangular β₁₂ borophene, denoted as β₁₂-M, and β₁₂-S. (d) In black and red, Raman and TERS spectra for χ₃ borophene, respectively. TERS was done at 14 mW and 100 s and Raman at 25.7 mW and 50 s. Reproduced with permission from ref. ¹⁴⁸.
Another Raman study¹⁵⁰ on metallic α borophene/Ag(111) experimentally demonstrates its slightly buckled nature. The nanometer-sized α domains in β₁₂ dominant growth conditions⁴⁹ hinder the characterization of this borophene polymorph. Samples are analyzed using STM-TERS under vacuum (10⁻¹⁰ mbar) at 77 K and with a 532 nm laser. As the STM tip traverses from β₁₂ to α in Fig. 11(a), the characteristic B₃g² peak of β₁₂ disappears and a characteristic 116.8 cm⁻¹ peak for α borophene is observed (Fig. 11(b)). Significant enhancements of the out-of-plane vibrations in α borophene can be seen in Fig. 11(c) as the TERS tip is brought closer to the sample. Such behavior is not observed in β₁₂ or χ₃ due to the presence of only in-plane vibrations. This study demonstrates the atomic resolution of TERS analysis and its significance in distinguishing borophene polymorphs.

<figure><img src="image_15.png" /><figcaption>Figure 11. (a) STM image with TERS line crossing β₁₂ and α borophene phases on Ag(111) with corresponding (b) TERS (50 s, 10 mW) spectra, and (c) Tip-sample distance dependent (50 s, 10 mW) spectra for α borophene. Reproduced with permission from ref. ¹⁵⁰.</figcaption></figure>

<table><caption>Table 3. Theoretical and experimental Raman modes for χ₃¹⁴⁸, β₁₂¹⁴⁸, and α¹⁵⁰ borophene using a 532 nm laser. Spectra are acquired at 78 K for χ₃ and β₁₂, and at 77 K for α.</caption><thead><tr><th>Polymorph</th><th>Modes</th><th>Theoretical (cm<sup>-1</sup>)</th><th>Experimental (cm<sup>-1</sup>)</th><th>FWHM (cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td rowspan="5">β<sub>12</sub></td><td>B<sub>3g</sub><sup>2</sup></td><td>186</td><td>188</td><td>17.5</td></tr><tr><td>B<sub>1u</sub><sup>1</sup>(X)</td><td>255</td><td>264</td><td>29.4</td></tr><tr><td>B<sub>1u</sub><sup>2</sup></td><td>292</td><td>296</td><td>27.5</td></tr><tr><td>B<sub>1g</sub><sup>2</sup></td><td>417</td><td>372</td><td>13.6</td></tr><tr><td>B<sub>3g</sub><sup>1</sup></td><td>495</td><td>490</td><td>17.7</td></tr></tbody></table>
<table><tbody><tr><td></td><td>A<sub>g</sub><sup>2</sup></td><td>863</td><td>856</td><td>25.1</td></tr><tr><td></td><td>A<sub>g</sub><sup>2</sup>(S)</td><td>900</td><td>906</td><td>20.9</td></tr><tr><td></td><td>B<sub>1g</sub><sup>1</sup></td><td>1026</td><td>1060</td><td>31.9</td></tr><tr><td></td><td>A<sub>g</sub><sup>1</sup></td><td>1155</td><td>1148</td><td>43.7</td></tr><tr><td></td><td>-</td><td>-</td><td>108</td><td>19.9</td></tr><tr><td></td><td>-</td><td>-</td><td>148</td><td>13.0</td></tr><tr><td rowspan="7">χ<sup>3</sup></td><td>B<sub>g</sub><sup>2</sup></td><td>172</td><td>181</td><td>19</td></tr><tr><td>B<sub>g</sub><sup>2</sup>(X')</td><td>256</td><td>260</td><td>28.3</td></tr><tr><td>A<sub>u</sub>(Y)</td><td>306</td><td>291</td><td>23.2</td></tr><tr><td>B<sub>g</sub><sup>1</sup></td><td>409</td><td>415</td><td>31.7</td></tr><tr><td>B<sub>g</sub><sup>1</sup>(Y)</td><td>460</td><td>445</td><td>12.9</td></tr><tr><td>A<sub>g</sub><sup>4</sup></td><td>482</td><td>486</td><td>13.7</td></tr><tr><td>A<sub>g</sub><sup>3</sup></td><td>769</td><td>745</td><td>48.6</td></tr><tr><td rowspan="7">α</td><td>A<sub>g</sub><sup>2</sup></td><td>1091</td><td>1077</td><td>41.2</td></tr><tr><td>A<sub>g</sub><sup>1</sup></td><td>1244</td><td>1244</td><td>44.9</td></tr><tr><td>-</td><td>-</td><td>119</td><td>20.5</td></tr><tr><td>-</td><td>-</td><td>148</td><td>13.7</td></tr><tr><td>E<sup>7</sup></td><td>107.85</td><td>116.8</td><td>-</td></tr><tr><td>E<sup>6</sup></td><td>188.6</td><td>157.3</td><td>-</td></tr><tr><td>A<sub>1</sub><sup>4</sup></td><td>352.94</td><td>339</td><td>-</td></tr><tr><td>A<sub>1</sub><sup>3</sup></td><td>425.2</td><td>406.4</td><td>-</td></tr><tr><td>A<sub>2</sub><sup>3</sup></td><td>507.83</td><td>446.8</td><td>-</td></tr><tr><td>E<sup>4</sup></td><td>684.32</td><td>702.6</td><td>-</td></tr><tr><td>E<sup>3</sup></td><td>907.16</td><td>920.4</td><td>-</td></tr></tbody></table>

Single phase growth of $v_{1/12}$ borophene islands is reported on Au(111)$^{42}$, highlighting strong Au-B interactions that avoid phase mixing, thus, leading to the selective growth of monophasic borophene. A surface segregation growth mechanism (Fig. 12(a)) is suggested, where the dissolution of boron into the Au substrate at 550 °C is followed by its segregation into borophene islands on cooling. This hypothesis is supported by the fact that at temperatures lower than 550 °C only 3-D boron clusters are observed on the surface because of the limited solubility of boron in the gold substrate$^{42}$. XPS analysis in Fig. 12(b) shows that the B 1s peak is absent on a clean Au(111) herringbone surface, and an intense boron signal is observed on depositing 3D boron clusters at room temperature. When depositing boron on Au(111) kept at 550 °C, boron nuclei are formed onto the nodes of reconstructed trigonal gold surface, resulting in a low intensity B 1s peak. On further increasing the boron dosage, this intensity of B 1s peak does not increase, suggesting a lack of 3D growth, but rather a dissolution of excess boron into gold. However, on cooling, these nanoscale boron nuclei act as templates and generate larger borophene islands of tens of nanometers, thanks to the surface segregated boron from bulk gold. STS spectra suggest the metallic nature of these borophene islands$^{42}$ and correlate well with borophene grown on Ag(111)$^{9}$.
<figure><img src="image_16.png" /><figcaption>(a)</figcaption></figure>

Figure 12. Borophene deposition on Au(111) (a) depicting the growth, and (b) XPS spectra of B 1s. Reproduced with permission from ref. 42.

<figure><img src="image_17.png" /><figcaption>(b)</figcaption></figure>

Borophene islands are also reported when boron is deposited on an Ir(111)³³ crystal kept between 300-600 °C. The structure of χ₆ borophene is established in Fig. 13(a) by the well aligned high resolution STM image of borophene with the DFT generated structure. LEED patterns obtained post boron deposition in Fig. 13(b), highlight yellow spots due to boron atoms in the crystalline χ₆ borophene structure while the predominant purple spots correspond to Ir(111). Bader charge analysis suggests a marginal hole doping by borophene sheet where a charge of about 0.08 e⁻ is transferred from B atom to Ir atom. On prolonged boron exposure, 3D boron clusters are obtained instead of a continuous 2D monolayer. However, no elemental data is provided³³, so it is difficult to comment upon the chemical environment of the boron and iridium atoms.

<figure><img src="image_18.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_19.png" /></figure>

Figure 13. (a) High-resolution STM image of χ₆ borophene on Ir(111) overlaid with DFT structure. (b) LEED patterns highlighting the borophene with yellow spots and iridium crystal with purple. Reproduced with permission from ref. 39.

While the growth mechanism of borophene on these three substrates is still not fully understood, it appears that there is a strong correlation between substrate orientation, lattice mismatch and the synthesized allotrope. As such, it would be interesting to try growing borophene on other noble metal substrates like Pt(111) as it would roughly have a ~1% compressive lattice mismatch compared to a ~3% extensive one for Au(111) and Ag(111). One should also distinguish noble metal substrates that are
able to form borides<sup>151,152</sup> (Ir, Pt, Rh, Ru, Pd, Os) from the others (Ag, Au), as the growth mechanisms in both cases is expected to be completely different – the temperature of the substrate is an important parameter here. Moreover, within the metal-forming borides, the boride structure may also have an influence, as in some cases it contains 2D boron layers which appear favorable to borophene formation<sup>153</sup>. Using well-established substrates for other 2D materials growths is thus not straightforward for borophene as additional parameters must be considered.

The experimental realization of borophene onto noble metal substrates such as silver and gold lays the foundation for understanding borophene growth and characterizations. These substrates don't form borides and the low electron charge transfer from these metal atoms to boron atoms is sufficient to stabilize the borophene structures. Iridium appears as a special case since it can form borides, and the growth mechanism should thus be different than the one on Ag and Au. It is however clear that the choice of substrate, its orientation and temperature, boron fluxes, and post-annealing methods have demonstrated the polymorphic nature of borophene. Various metallic polymorphs, such as α, β, β<sub>8</sub>, β<sub>12</sub>, χ<sub>3</sub> and χ<sub>6</sub> are structurally resolved and the nature of chemical bonds in borophene is also elucidated. This begs the question if cheaper, more reactive metal substrates, such as aluminum or copper can be used to grow borophene? Will the higher charge transfer capabilities from such metal atoms to boron atoms help achieve monophasic borophene? The case of Al is particularly interesting since it should form borides with 2D boron layers<sup>152</sup>.

### 3.1.2. Borophene on reactive metals

The role of metal-borophene interactions for polymorph selectivity is also evidenced on cheaper non-noble single-crystal substrates of aluminum or copper. The planar honeycomb borophene ($\delta_3$) in Fig. 14 is epitaxially realized on Al(111) at about 227 °C<sup>10</sup> and supposed stable. Although predicted to be unstable in its free-standing state, the $\delta_3$ phase, proven by STM, stabilized itself due to a high electron transfer of about 1 e<sup>-</sup> from Al atoms to B atoms, and the nearly perfect lattice match between Al(111) and the honeycomb phase<sup>154</sup>. However, no study of the stability over time was performed to confirm this.

<figure><img src="image_20.png" /><figcaption>Figure 14. STM image of honeycomb borophene on Al(111). Reproduced with permission from ref. <sup>10</sup>.</figcaption></figure>
Soon after, copper entered this list of substrates with the synthesis of borophene on Cu(111)⁴⁴ at 497 °C. The χ₃-like structure is elucidated via STM imaging and ab-initio calculations. The XPS spectrum of B 1s core-shell recorded after an hour of ambient exposure, yields a tiny B-B peak at 187.5 eV along with a large B-O peak at 191.8 eV that roughly accounts for 80% of boron. It should be noted that this B-O contribution is found to be only 25% in borophene deposited on Ag(111)⁴⁴. Nonetheless, the sheets are relatively larger, up to 100 µm² in size, stabilized by a charge transfer of 0.23 e⁻ from Cu to B atoms.

Borophene growth on a square lattice Cu(100)⁴⁵ kept at 480 – 550 °C is also reported. Unlike the χ₃ phase on Cu(111)⁴⁴, an STM-resolved β₁₃ borophene is obtained here. This emphasizes the role of copper substrate symmetry in the growth of borophene phases. A slight decrease in charge transfer values is observed when compared to Cu(111): with 0.21 e⁻ charge donated from Cu atoms to B atoms, slightly weaker Cu-B interactions exist in Cu(100), thus making borophene relatively flatter. This weak Cu-B charge transfer can also be exploited for the transfer of borophene from copper onto other surfaces for device fabrication. The borophene islands on Cu(100) are indicated to coalesce into a monolayer on further boron deposition. This self-limiting borophene growth is true for both copper (111) and (100) surfaces. XPS analysis is in favor of this theory. The B 1s core-shell displays a peak at 186.6 eV assigned to B-B bonds with no signs of oxidation. A broadening is observed in this B 1s peak if excess boron is supplied after borophene layer formation. This results into a higher binding energy boron contribution at 187.2 eV, attributed to 3D boron clusters.

Other recent boron depositions on Cu(111)¹⁵⁵⁻¹⁵⁷ claim the formation of a 2D copper boride¹⁵⁵ - Cu₈B₁₄, instead of borophene. These studies employ angle-resolved XPS, ARPES, and STM, along with DFT calculations to negate the formation of borophene on copper. Copper boride is reported to grow on Cu(111) at 467 °C¹⁵⁵,¹⁵⁷, and is also obtained on-post annealing¹⁵⁵ (≈ 227 °C) boron deposited on copper at room temperature. In Fig. 15(a), in-situ angle-resolved XPS measurements¹⁵⁷ of Cu 2p³/₂ core level binding energy peak at 932.7 eV for the Cu(111) substrate shows no apparent change in lowering grazing emission angles. However, post-boron deposition, the Cu 2p³/₂ core level peak broadens and increases to 933.3 eV. On lower emission angles, an increase in binding energy highlights the chemical bonding of surface copper atoms with boron atoms. On the other hand, B 1s core level has a constant binding energy of 187.8 eV throughout the range of emission angles that indicates just one chemically bonded state of boron. In Fig. 15(b), the P1 contribution for Cu-B bonds in Cu 2p³/₂ spectra increases on lowering emission angles from 75° to 10° whereas the P2 contribution for bulk copper decreases. Such observations indicate copper boride formation, thus making it possible to differentiate borophene on copper from copper boride. Similarly, one should note that quite a few authors claim to have synthesized borophene in their published works, but special attention needs to be paid to the reported characterizations of thickness, structure, elemental composition and bonding state to be sure that the resulting samples are not simply thin layers of bulk boron or boron oxide clusters.
Regarding the reactive substrates, only the use of aluminum and copper have been reported up to now for PVD growth. Similarly to noble metal supports, the crystalline orientation and the lattice mismatch may strongly impact the resulting borophene structure. While Cu should not lead to boride formation, Al is known to form lamellar borides, which seems favorable for borophene formation. Therefore, the boron dissolution within its substrate must be considered for the growth. In addition to the parameters to be considered for noble metals, reactive substrates induce a strong electron transfer with the borophene layer – for Cu it is roughly one order of magnitude larger than for Ag or Au. In the end, using well-established substrates for graphene growth like nickel may be considered, as similar phenomena may be expected.

<figure><img src="image_21.png" /><figcaption>Figure 15. Angle-resolved XPS spectra for (a) from top to bottom, clean Cu(111), Cu post-boron deposition, and B 1s at various emission angles. (b) Corresponding Cu 2p<sup>3/2</sup> spectra showing the two contributions P1 and P2 at 75°, 45° and 10° emission angles. Reproduced with permission from ref. <sup>157</sup>.</figcaption></figure>

3.1.3. Borophene derivatives

3.1.3.1. Bilayer borophene

Bilayer (BL) α borophene islands via MBE on Ag(111) are first reported in 2021<sup>158</sup>. The Ag(111)
substrate is held between 350 and 450 °C during the growth. BL borophene's stability is mainly
attributed to the self-doping of B atoms between the two borophene layers. Islands of BL-α nucleate at
intersections of multiple β<sub>12</sub> (η=1/6) and χ<sub>3</sub> (η=1/5) borophene domains. The simulated structures
correspond to two covalently bonded α (η=1/9) layers. This is confirmed with CO-molecule-
functionalized STM and atomic force microscopy (AFM) images. It should be noted that monolayer α
borophene is predicted to be unstable on Ag(111). The STS analysis proves a metallic behavior,
however, a slightly higher density of states (DOS) in the negative bias of BL-α is noticed when compared
to β<sub>12</sub>.
$\beta_{12}$-like BL islands are obtained via MBE on Cu(111)$^{159}$ at 327 $^\circ$C. A redistribution of charge occurs between copper and borophene that stabilizes the BL. Almost complete coverage of BL on the copper substrate is reported. The $\eta$ values are calculated to be 5/36 and 1/6 for the first and second borophene layers, respectively. In *ex-situ* XPS analysis, B 1s peak at 191.5 eV, assigned to oxidized boron, contributes to only 25% of boron atoms for BL, in comparison monolayer borophene appears fully oxidized. As the $\beta_{12}$-like BL almost covers the entire substrate, some oxidation could arise due to small amounts of monolayer borophene. In addition, STS demonstrates the metallic nature of $\beta_{12}$-like BL.

Such stable and metallic bilayer borophene could be suitable for device applications. Can such chemical robustness be achieved for monolayer borophene, particularly towards aerial oxidation, and without altering too much its inherent properties? Hydrogenation might be an answer.

### 3.1.3.2. Borophane

The borophane structures (Fig. 16(a)) possess two-center-two-electron (2c-2e) B-H bonds and three-center-two-electron (3c-2e) B-H-B bonds$^{160}$, however, all B atoms may not be hydrogenated. Borophane growth is demonstrated on Ag(111)$^{160}$ by controlled exposure of atomic hydrogen onto four previously grown borophene polymorphs$^{161}$: $\beta_{12}$, $\chi_3$, and their 30° rotated domains. Interestingly, eight distinct STM-resolved borophane structures are derived from the hydrogenation of these four borophene polymorphs. This shows that borophane is polymorphic and its structures, explained by DFT and STM image correlation$^{160}$, are a result of different hydrogen atom arrangements and bonding onto the borophene polymorphs. The final hydrogen proportion, however, is not reported. Both borophane and borophene samples are exposed to ambient environment and are monitored via XPS in conjunction with AFM imaging. XPS analysis of B 1s core-shell (Fig. 16(b,c,d)) shows B-B bond contributions at 187.8 eV and 189.1 eV. After an hour, a B-O bond contribution at 192.3 eV can already be seen (Fig. 16(b)) for borophene with corresponding AFM image showing several white, presumably, boron oxide particles. Although a few white particles are seen on borophane (Fig. 16(c,d)), a tiny B-O bond contribution only appears after a week of ambient exposure.

These results experimentally corroborate with predicted hydrogen passivation of borophene. An important aspect is the reported reversibility of these borophane polymorphs to pristine borophene on annealing at 300 $^\circ$C$^{160}$. Hence, hydrogenation can be temporarily done to prolong the shelf life of borophene based devices.
<figure><img src="image_22.png" /><figcaption>Figure 16. (a) A borophane structure with B-B and B-H-B bonds. XPS analysis of B 1s core-shell and AFM images for (b,c) borophene and borophane grown on Ag(111) after 1 hour of ambient exposure. (d) borophane showing signs of oxidation after one week of ambient exposure. Reproduced and adapted with permission from ref. ¹⁶⁰.</figcaption></figure>

### 3.1.4 Summary and reflection

The PVD of borophene is a steppingstone to exploring this fascinating material experimentally. It has allowed for the first time the demonstration of the correlation between synthesis parameters – such as substrate type and orientation or operating and annealing temperatures – and polymorph selectivity. The list of all published PVD synthesis conditions and resulting borophene characteristics is gathered in Table 4.

<table><caption>Table 4. List of all 11 PVD-obtained borophene polymorphs using pure boron as the precursor. Dominant polymorph is in bold.</caption><thead><tr><th>Substrate</th><th>Deposition and annealing temperatures (°C)</th><th>Polymorph</th><th>Lateral area</th><th>Oxidized boron in XPS</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="2">Ag(111)</td><td>207</td><td rowspan="2">β<sub>12</sub>, χ<sub>3</sub></td><td rowspan="2">nm<sup>2</sup></td><td>-</td><td>9</td></tr><tr><td>297</td><td>yes</td><td>49</td></tr></tbody></table>
<table><thead><tr><th></th><th></th><th>β<sub>12</sub>, β<sub>12</sub>-R30°, α</th><th></th><th></th><th></th></tr></thead><tbody><tr><td></td><td>β<sub>12</sub> annealed at 376</td><td>χ<sub>3</sub></td><td></td><td>-</td><td>40</td></tr><tr><td></td><td>300</td><td>β<sub>12</sub>, χ<sub>3</sub></td><td></td><td>no</td><td>144</td></tr><tr><td></td><td>400</td><td>β<sub>12</sub>, χ<sub>3</sub></td><td></td><td>-</td><td>40</td></tr><tr><td></td><td>407</td><td>χ<sub>3</sub></td><td></td><td>-</td><td>148</td></tr><tr><td></td><td>β<sub>12</sub> annealed at 427</td><td>β<sub>12</sub>, χ<sub>3</sub></td><td></td><td>yes</td><td>9</td></tr><tr><td></td><td>450</td><td>β<sub>12</sub>, χ<sub>3</sub></td><td></td><td>yes</td><td>9</td></tr><tr><td></td><td>700</td><td>χ<sub>3</sub></td><td></td><td>-</td><td>41</td></tr><tr><td>Ag(110)</td><td>296.85</td><td>χ<sub>3</sub>, β, β<sub>8</sub></td><td></td><td>-</td><td>42</td></tr><tr><td>Au(111)</td><td>550</td><td>ν<sub>1/12</sub></td><td></td><td>no</td><td>10</td></tr><tr><td>Al(111)</td><td>549.85</td><td>δ<sub>3</sub></td><td></td><td>-</td><td>10</td></tr><tr><td>Ir(111)</td><td>300 – 600 and annealed at 1200</td><td>χ<sub>6</sub></td><td></td><td>-</td><td>39</td></tr><tr><td>Cu(111)</td><td>497</td><td>χ<sub>3</sub>-like</td><td rowspan="2">μm<sup>2</sup></td><td>yes</td><td>44</td></tr><tr><td>Cu(110)</td><td>480 – 550</td><td>β<sub>13</sub></td><td>no</td><td>45</td></tr></tbody></table>

So far, successful PVD growth of borophene have been reported on five different metals with different crystalline orientations. As far as the borophene growth mechanism is understood, PVD synthesis necessitates metallic single-crystal substrates, both to promote borophene growth, either by epitaxy or by segregation upon cooling, and to stabilize the borophene layer via charge transfer and/or hybridization. It appears that the synthesized allotropes are correlated with the nature and surface orientation of the substrate, and with its temperature. However, up to now, only metals have been investigated as substrates. Many other supports could be targeted for their small lattice mismatch with borophene or their ability to form lamellar borides. In particular, it would be interesting to target insulator substrates, such as sapphire, as it would allow better assessing the intrinsic electronic properties of the deposited borophene. Another approach would be the direct growth of borophene on another 2D material such as hBN, graphene or TMDs, to construct lamellar heterostructures with targeted properties. Mastering borophene deposition is now subjected to in-depth investigations of the growth mechanism on different type of substrates (boride- and non-boride-forming substrates, metals, insulators...).

Considerations should be also given to experimental conditions: so far, only UHV conditions and evaporation temperature of 2000 °C have been used. Laser-assisted evaporation could be an alternative to the high temperatures required for boron sublimation. Low-pressure conditions instead of UHV could also be explored to decrease the energy consumption of the process. Using a small pressure of hydrogen radicals during synthesis could also lead to more uniformly hydrogenated borophane – thus better passivated.

In parallel with the PVD method, a chemical approach has also been explored in recent years.
## 3.2. Chemical Vapor Deposition

In a chemical vapor deposition process¹⁶², the film-constituting species or precursors are vaporized and undergo chemical reactions near or on the surface of the heated substrate, resulting in the deposition of the desired material. Processes often exploit the thermal decomposition of precursors, and catalytic properties of the substrate. CVD can be operated in versatile pressure conditions, ranging from UHV to atmospheric pressure conditions.

### 3.2.1. Borophene

The first CVD of borophene is reported by Mazheri et al.⁴³ in early 2021 via thermal decomposition of diborane (B₂H₆), and ever since, more reports¹⁴⁹,¹⁶³⁻¹⁶⁵ have been cropping up. Up to date, only diborane and borazine (B₃N₃H₆) have been employed as precursors to obtain borophene. Almost all groups⁴³,¹⁴⁹,¹⁶³,¹⁶⁴ exploit the thermal decomposition of the precursors to enrich the metal substrate with boron followed by slow cooling or annealing – to expel the impurities, and to finally recrystallize boron into two-dimensional boron sheets. The thermal decomposition of diborane releasing H₂ at ≥ 557 °C to yield boron has been known for almost half a century¹⁶⁶. The dissolution of this boron in a polycrystalline quasi-liquid aluminum layer on top of SiO₂/Si at 657 °C results in borophene sheets upon slow cooling, as seen in scanning electron microscopy (SEM) image⁴³ (Fig. 17(a)). The XPS data recorded from the diborane-obtained borophene in Fig. 17(b) shows the characteristic peaks at 187.9 eV and 189.2 eV arising from the 2D bonding in boron atoms with a significant contribution from oxidized boron at 191.1 eV. The as-prepared borophene sheets are transferred (cf. section 4.3) onto carbon-coated copper grids for high-resolution transmission electron microscopy (HR-TEM). The image thus obtained in Fig. 17(c) highlights an inter-strip distance of 4.16 Å in the boron sheet. This correlates well with χ₃ borophene having a calculated 4.2 Å distance between parallel atomic boron strip regions. Similar approach is used to determine the in-set as α₁ polymorph. Comparing the experimental data to theoretical predictions of the structure allows to conclude the dominant phase for borophene growth above 527 °C is indeed χ₃, as seen with PVD results on Ag(111)⁴⁹ and Cu(111)⁴⁴. One must note that a mixture of mono-, bi-, and tri-layer borophene with µm² coverage is obtained, as depicted by the statistical measurements by AFM in Fig. 17(d).
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>Figure 17. (a) SEM image of borophene on Al/SiO₂/Si with corresponding (b) XPS analysis of B 1s narrow scan showing B-B peaks at 187.9 eV and 189.2 eV, respectively, and B-O at 191.9 eV. (c) Top-view HRTEM image of borophene showing inter-strip distance in χ₃ polymorph, and α₁ polymorph in the in-set (d) AFM evaluation of the statistical distribution of borophene coverage and thickness after transfer on SiO₂/Si. Reproduced with permission from ref. ⁴³.</figcaption></figure>

Borazine¹⁶³ is reported to catalytically decompose onto the Ir(111) substrate at 1100 °C to obtain borophene. However, at slightly lower temperatures, a mix of hBN and borophene can be obtained. STM image in Fig. 18(a) shows hBN domains as bright spots due to their insulating nature while borophene appears as well-defined stripes. On further lowering the temperature to 850 °C only hBN is obtained. Additionally, ex-situ XPS analysis (Fig. 18(b)), at 850 °C shows B 1s peak at 190 eV and N 1s centered at 397.3 eV corresponding to hBN. At 950 °C, an additional B-O bond, attributed to ambient borophene oxidation, is observed at 191.4 eV, also signifying the inter-mixing with hBN. On further increasing the temperature to 1100 °C, hBN peaks of boron and nitrogen disappear while the B-O bond is sustained, but no B-B bonds are seen. One should note that borazine is a conventional precursor to obtain hBN¹⁶⁷ as it already possesses a planar hexagonal B-N skeleton. hBN is very stable against ambient oxidation¹⁶⁸ so the B-O bond cannot arise due to it. This theorizes the borophene growth is by elemental decomposition of borazine into boron, hydrogen, and nitrogen atoms at 1100 °C. Nitrogen and hydrogen atoms are desorbed from the substrate as N₂ and H₂ gases, and boron atoms dissolve into iridium. On cooling, segregation of boron atoms favors the epitaxial growth of borophene and is monitored via low energy electron microscopy (LEEM) imaging³⁹. The χ₆ monolayer structure is elucidated via high-resolution STM images and LEED patterns³⁹.
<figure><img src="image_27.png" /><figcaption>Figure 18. (a) STM image of borophene stripes (red square) with hBN domains (circles) at 990°C deposition temperature and subsequent annealing to 1100°C (b) XPS analysis of B 1s and N 1s peaks at different growth temperatures. Reproduced with permission from ref. $^{163}$. (c) An AFM image showing phase mixing of triangular hBN and circular borophene domains on Ir(111). (d) Correlation between calculated (in orange) and measured Raman spectra (in blue) of Borophene/Ir and Borophene/Si. The green vertical columns highlight peaks present in all cases while orange vertical columns are only for peaks present in experimentally obtained borophene. Reproduced with permission from ref. $^{149}$.</figcaption></figure>

Similar approach is reported to produce millimeter-sized χ₆ borophene on Ir(111)¹⁴⁹. The deposition is carried out at 1200 °C and the sample is cooled at a rate of 2-3 °C·s⁻¹ to 850 °C to obtain borophene. The χ₆ borophene is then transferred onto SiO₂/Si for further characterizations. The traces of triangular hBN islands with borophene as seen in Fig. 18(c) AFM image question the selectivity of the route. On speculation, this borophene monolayer with a calculated height of 3.1 nm, i.e., an order of magnitude larger than expected values, could be heavily oxidized, or picked up some impurities during or after the transfer to SiO₂/Si wafer. The lack of XPS or time of flight secondary ion mass spectrometry (ToF-SIMS) data to confirm the bonding and elemental composition in the film begs to question the purity of the samples. Borophene oxidation¹⁶⁹ and organic impurities¹⁷⁰ are reported to affect the Raman spectra, and one should also consider the temperature increase due to Raman scattering that can alter peak positions of substrate-supported 2D materials¹⁷¹. The Raman spectra in Fig. 18(d) attempt to correlate the DFT generated signals of borophene with the experimentally deposited borophene on Ir, followed by a transfer onto silicon. Indeed, the majority of borophene peaks calculated by DFT, and marked by green vertical columns, are present in all cases with minor shifts. Experiment-only peaks marked with vertical orange columns appear in addition. The shifts and peaks could be attributed to the change of borophene-substrate interactions and the adsorbates originating from the polymethyl methacrylate (PMMA) transfer process.
Another borazine CVD approach in Fig. 19(a) probably results in triangular δ₆ borophene on Ru(0001)¹⁶⁴. hBN is deposited at 950 °C from borazine and then flash annealed with an ongoing borazine flux to 1200 °C, dissociating hBN into boron and nitrogen. This step led to the desorption of nitrogen from ruthenium surface and the supersaturation of boron into the newly vacant interstitial sites. Borophene is thus obtained via segregation of boron atoms on cooling to 850 °C as seen in the LEEM image of Fig. 19(b). Yet again, the ex-situ XPS analysis (Fig. 19(c)) of this borazine-obtained borophene possesses a B-O peak and lacks the characteristic borophene bonds. Interestingly, on prolonged cooling to about 700 °C, complete bilayer borophene coverage is demonstrated in Fig. 19(d) LEEM image and supported by LEED patterns in Fig. 19(e). However, no further characterizations are reported on this bilayer.

<figure><img src="image_28.png" /><figcaption>Figure 19. (a) Borophene CVD growth mechanism on Ru(0001) demonstrates (i) hBN deposition followed by (ii) desorption of nitrogen and (iii) supersaturation of boron leading to (iv) borophene formation. (b) Real-time LEEM image after 20 s of borophene growth showing bright borophene domains on ruthenium surface and their expansion along the direction of colored lines until full coverage is achieved. (c) XPS of B 1s core-shell recorded from the obtained borophene on Ru(0001) demonstrating oxidized boron and absence of characteristic hBN peak. (d) LEEM image showing bilayer borophene coverage on further cooling the sample to 700 °C. (e) LEED pattern of bilayer borophene shows a set of patterns typical of the parent borophene monolayer along with rows of aligned spots. The exact crystal structure is not elucidated at the time of writing this article. Reproduced with permission from ref ¹⁶⁴.</figcaption></figure>
A recent study<sup>165</sup> (Fig. 20(a)) demonstrating the selective deposition of borophene and hBN suggests that diborane uniquely provides borophene while borazine yields hBN. These 2D materials are obtained on Ir(111) and Cu(111) substrates kept at 960 °C and 500 °C, respectively. Borophene growth, unlike segregation reports<sup>39,149,163</sup>, is largely credited to the boron species formed due to the dehydrogenation of diborane and the catalytic metal surface. DFT and high resolution STM are employed to assess the structure as χ<sub>6</sub> on Ir(111)<sup>39,163</sup> and χ<sub>3</sub>-like on Cu(111)<sup>44</sup>. In-situ XPS analysis of χ<sub>6</sub>/Ir(111) in Fig. 20(b) shows the B 1s core-shell resolved into two B-B contributions at 188.2 eV and 188.9 eV; no B-O is observed. Nitrogen is not detected in N 1s core-shell scan, thus negating the presence of hBN. Further, borophene-hBN heterostructures on Ir(111) are obtained by altering borazine and diborane precursor dosing.

<figure><img src="image_29.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_30.png" /></figure>

Figure 20. (a)STM imaging of χ<sub>6</sub> borophene, in red, and hBN, in green, on Ir(111) (b) XPS analysis of χ<sub>6</sub> borophene on Ir(111) with B 1s core-shell showing B-B bonds labelled B<sub>4</sub> and B<sub>5</sub>, and the absence of nitrogen in N 1s core-shell scan. Reproduced with permission from ref <sup>165</sup>.

### 3.2.2. Borophane

Hydrogenated borophene, i.e. borophane, is also fabricated via thermal CVD of borohydrides, namely, sodium borohydride<sup>172</sup> (NaBH<sub>4</sub>), and bis(triphenylphosphine) copper borohydride ((Ph<sub>3</sub>P)<sub>2</sub>Cu(BH<sub>4</sub>))<sup>173</sup>. Borophane<sup>172</sup> is deposited on non-metallic fluoro phlogopite mica (KMg<sub>3</sub>AlSi<sub>3</sub>O<sub>10</sub>F<sub>2</sub>, also called F-mica) substrate with NaBH<sub>4</sub> as B source. NaBH<sub>4</sub> is annealed at 490 °C for 2 hours to form metastable boron clusters (with NaH as byproduct) that are carried onto the F-mica substrate at 690 °C by a 10 sccm H<sub>2</sub> flux for an hour. The lateral epitaxial growth of borophane is explained by the small migration energy barrier of boron atoms on the mica surface. These metallic borophane layers, as low as 1.7 nm thickness, are also transferred onto a silicon substrate using PMMA-based approach. This solvent-based transfer technique adds organic impurities to the borophane as seen in XPS analysis of C 1s core-shell (Fig. 21(a)) that shows C-C, C-O, and O-C=O contributions at 248.8 eV, 285.6 eV, and 288.4 eV, respectively. However, B 1s core-shell (Fig. 21(b)) reveals two B-B contributions at 187.2 eV and 188.5 eV arising from borophane, B-C and B-O bonds are notably absent. Fig. 21(c) shows Raman modes, A<sub>1g</sub> at 828.8 cm<sup>-1</sup> and A<sub>1g</sub>+E<sub>g</sub> at 985.4 cm<sup>-1</sup> from B-B vibrations and a B-H vibration mode at 2331.9 cm<sup>-1</sup>. Additionally, FTIR spectrum (Fig. 21(d)) confirms B-B absorption peaks at 936 cm<sup>-1</sup> and 1066 cm<sup>-1</sup>, B-H stretching vibrations at 771 cm<sup>-1</sup> and 2322 cm<sup>-1</sup>, and C-C and C-H vibrations at 2212 cm<sup>-1</sup> and 2926 cm<sup>-1</sup>. Thus, the borophane obtained contains B-B and B-H chemical bonds with significant carbon impurities. The crystal structure is resolved with DFT calculations, TEM imaging and selected area
electron diffraction (SAED) patterns (Fig. 21(e,f)) showing an interplanar distance of 4.41 Å and 4.39 Å corresponding to α'-2H-borophane. The borophane¹⁷² is then explored for photodetection applications across LED wavelengths of 370 nm, 625 nm, and 940 nm using a Borophane-Si Schottky junction, and is found to be best at 625 nm LED. The photoresponsivity (R), specific detectivity (D*), and external quantum efficiency values (EQE) for the junction are found to be 1.04 A.W⁻¹, 1.27 x10¹¹ Jones (Jones = cm Hz¹/².W⁻¹) and 119%, respectively, under an illumination of a 625 nm wavelength LED and 4 V reverse bias. The R value for borophane is remarkably higher¹⁷² than graphene (6.1 x 10⁻³ A.W⁻¹) and TMDs such as WS₂ (4 x 10⁻³ A.W⁻¹), MoSe₂ (0.27 A.W⁻¹), MoS₂ (0.12 A.W⁻¹), SnS2 (9 x 10⁻³ A.W⁻¹), and CrSe (0.18 A.W⁻¹).

<figure><img src="image_31.png" /><figcaption>Figure 21. (a, b) XPS spectra for C 1s (a) and B 1s (b) core-shells of α'-2H-borophane. (c) Raman and (d) FTIR spectra showing the B-B, B-H, C-C and C-H vibrations. (e, f) TEM image with corresponding SAED pattern (inset) where (f) HRTEM carried out in the yellow square reveals the crystal structure of borophane. Reproduced with permission from ref ¹⁷²</figcaption></figure>

From thermal decomposition of bis(triphenylphosphine) copper tetrahydroborate, metallic β₁₂-borophane¹⁷³, i.e., β₁₂-borophene bonded with two hydrogen atoms (B₅H₂) are obtained on polycrystalline copper. The copper foil is annealed at 1000 °C to obtain a smoother surface with larger grain boundaries and then kept at 650 °C. The boron precursor is heated to 120 °C and undergoes the following decomposition:

$$ (\mathrm{Ph_3P})_2\mathrm{Cu}(\mathrm{BH_4})(\mathrm{s}) \rightarrow \mathrm{Cu(s)} + \frac{1}{2}\mathrm{H_2(g)} + \mathrm{Ph_3P(g)} + \mathrm{Ph_3P.BH_3(g)}. $$

The formed species except Cu, are carried with H₂ gas close to the copper foil substrate to generate borophane¹⁶⁶. XPS analysis on Cu shows: C 1s core-shell (Fig. 22(a)) contains three C-C contributions at 283.2 eV, 283.9 eV, and 284.8 eV, and the B 1s spectrum (Fig. 22(b)) is fitted into a B-B contribution at 187.9 eV and a tiny B-O peak at 190.4 eV. FTIR spectrum (Fig. 22(c)) shows absorption bands for B-H vibrations at 519 cm⁻¹ and 2325 cm⁻¹, and B-B vibrations at 1012 cm⁻¹ and 1180 cm⁻¹. This indeed
suggests a hydrogen containing borophane, with carbon impurities arising from the $Ph_3P$ decomposition species. ToF-SIMS analysis (Fig. 22(d)) of the sample displays an intense $B^+$ and $H^+$ peaks along with low intensity $BH^+$ and $BH_2^+$, thus confirming boron hydrogenation. HRTEM and corresponding SAED pattern (Fig. 22(e,f)) show a crystalline structure with inter-layer distances of about 3 Å and 5 Å that correlates well with DFT calculated values for $\beta_{12}$-borophane$^{173}$. The metallic nature is established by STS. The optimized 30-minutes deposition results in up to 4 monolayers, which are then transferred to investigate their performance for photodetection. The borophane-Si detector is explored for wavelengths of 365, 395, 460, 525, 590, 600, 735 and 850 nm, and the best performance$^{173}$ is found under the illumination of 395 nm with 5 V reverse bias. The R, D*, and EDE are found to be 0.50 A.W$^{-1}$, 5.2 x 10$^{11}$ Jones, and 158%, respectively.

<figure><img src="image_32.png" /><figcaption>Figure 22. Analysis of $\beta_{12}$-borophane/copper. (a,b) XPS spectra of C 1s (a) and B 1s (b) core-shells (c) FTIR spectrum (d) ToF-SIMS spectrum (e) HRTEM images of borophane transferred onto TEM grid with corresponding SAED pattern (inset) (f) High magnification HRTEM of the red square revealing the $\beta_{12}$-borophane crystal structure. Reproduced with permission from ref. $^{173}$.</figcaption></figure>

### 3.2.3 Summary and reflection

Regarding the chemical approach, only CVD has been investigated so far. At the time of writing this review, only five studies have been published – they are gathered in Table 5.

<table><caption>Table 5. List of all five CVD-obtained borophene polymorphs. Oxidized boron is reported via XPS in all cases while no data is provided for $\chi_3$-like borophene on Cu(111)$^{165}$.</caption><thead><tr><th>Substrate</th><th>Precursor</th><th>Deposition temperatures (°C)</th><th>Operating pressure (mbar)</th><th>Polymorph</th><th>Lateral area</th><th>Reference</th></tr></thead><tbody><tr><td>Al/SiO<sub>2</sub>/Si</td><td rowspan="2">Diborane</td><td>557</td><td>10<sup>-5</sup> to 10<sup>-1</sup></td><td>$\chi_3$, $\alpha_1$</td><td rowspan="2">µm<sup>2</sup></td><td>43</td></tr><tr><td>Cu(111)</td><td>500</td><td>&lt;10<sup>-7</sup></td><td>$\chi_3$-like</td><td>165</td></tr></tbody></table>
<table><tr><td>Ir(111)</td><td rowspan="4">Borazine</td><td>960</td><td rowspan="4">Triangular<br>(δ<sub>6</sub>)</td><td rowspan="2">χ<sup>6</sup></td><td rowspan="2">mm<sup>2</sup></td><td>163</td></tr><tr><td>Ru(111)</td><td>1100</td><td>149</td></tr><tr><td></td><td>1200</td><td rowspan="2">μm<sup>2</sup></td><td rowspan="2">164</td></tr><tr><td></td><td>950 annealed at<br>1200</td></tr></table>

Among the four distinct substrates tested for CVD, one should note the use of Ru(111) for borophene deposition, extending the variety of metallic supports. The 2D boron polymorphs obtained by CVD on Cu(111) and Ir(111) agree with the ones using PVD, thereby proving that the nature and symmetry of the substrate indeed play a role in the resulting structure. In the case of the Al/SiO<sub>2</sub>/Si substrate, two distinct polymorphs are observed, which are different from those obtained by PVD on Al(111), in accordance with the previous statement. Indeed, in this specific case, the CVD occurs on quasi-liquid substrate rather than on monocrystalline one. This discrepancy could also be explained by a different growth mechanism induced by the quasi-liquid support. Moreover, it should be highlighted that all reported CVD substrates in Table 5 are metals, and all except Cu may form boride and thus dissolve boron.

As discussed in the PVD section, it would be valuable to explore non-boride-forming substrates to investigate potential new growth pathways. Once again, it is crucial to gain a deeper understanding of the underlying growth mechanisms in order to control the deposition process. Besides, only two precursors have been reported up to now. (i) Boranes only bring boron and hydrogen atoms to the reaction chamber – thus allowing for high purity borophene. However, they are highly toxic. (ii) Borazine present a very stable B-N bond, which would rather argue against its use as a precursor. The reported works on borazine-obtained borophene have indeed failed to show evidence of characteristic B-B bond peaks in XPS, and have rather shown evidence of the presence of oxidized boron. Therefore, processes using new precursors need to be developed to enable sustainable and reliable large-scale processes. For example, borophane has been reported on F-mica and polycrystalline Cu using various borohydrides. Annealing of borophane may then lead to pure borophene<sup>160</sup>.

All in all, the CVD approach has opened new possibilities to relatively large films<sup>39</sup>, up to μm<sup>2</sup>, by thermally decomposing boron-rich precursors instead of evaporating pure boron sources. In all the synthesis approaches discussed so far, we have seen that borophene is prone to oxidation. We will now try to shed some light on this behavior.
## 4. Borophene stability and transferability

The stability of a 2D material in ambient conditions and its transferability onto other surfaces is imperative for device applications.

### 4.1 Oxidation

The oxidation issue of borophene is raised in the experimental section and needs to be understood in its entirety: is the oxygen chemisorbed or physiosorbed? Is the oxidation of the borophene uniform, meaning, simultaneously on the basal plane and edges, or is it limited to the edges and/or defects? Are all borophene polymorphs equally susceptible to oxidation? What are the properties of the resulting borophene – is oxidation detrimental to applications? Addressing all these intricacies is necessary to overcome the oxidation problem by passivation and/or improving the synthesis method.

Although predicted to be stable<sup>61</sup>, the oxidation of borophene is a major challenge since the first reported borophene synthesis<sup>9</sup> for studying its properties and future applications. Its oxidation decreases the density of states in borophene, making it significantly less metallic<sup>142</sup>, nudging it towards the insulating nature of boron oxide<sup>49</sup>. The χ<sub>3</sub> and β<sub>12</sub> borophene phases are the most frequently reported ones, and they have been subjected to several experimental oxidation studies. PVD grown χ<sub>3</sub>/Ag(111) samples<sup>49</sup> are exposed to air for 10 min, as well as to controlled O<sub>2</sub>, up to 1620 Langmuir (L)<sup>1</sup>, in UHV. STM images show bright oxidation spots on the edges of borophene islands in both ambient, and controlled oxygen exposures. The atoms inside the islands, i.e. basal plane, remain mostly inert. The under-coordinated B atoms at the edges may display higher reactivity than fully coordinated basal atoms, thus undergoing quicker oxidation. A similar behavior is observed for other 2D materials such as hBN<sup>174</sup>.

A deeper understanding of borophene oxidation comes from two recent independent studies<sup>142,169</sup>. The first is the interaction of oxygen with β<sub>12</sub>-R30<sup>142</sup> and χ<sub>3</sub> borophene on Ag(111). *In-situ* STM and XPS analyzes of the as-deposited borophene<sup>142</sup> (Fig. 23(a)) supported by DFT calculations suggest that it starts to oxidize merely after a minute of ambient exposure as evident by B-O signal in Fig. 23(d). A rather controlled O<sub>2</sub> exposure under UHV at room temperature hints that the dioxygen molecule first thermodynamically dissociates on the χ<sub>3</sub> borophene surface, followed by diffusion and chemisorption to both edges and basal plane. The oxidation of the basal plane is visualized by B-O peak appearing in XPS (Fig. 23(e)) from 10 L onwards dosing, and as bright white spots of insulating boron oxide<sup>49</sup> in STM images (Fig. 23(b,c)). The basal plane oxidation is found to be much less compared to the disordered edge oxidation that increases with increasing O<sub>2</sub> dosage. As visible in Fig. 23(f), the edge oxidized borophene suffers a decrease in its density of states hence proving to be less metallic than pristine borophene. The β<sub>12</sub>-R30 polymorph shows similar behavior, however, certain straight edges were resistant to oxidation, particularly the ones parallel to the hollow hexagon rows. A probable

<footnote n="1">¹ A Langmuir (L) corresponds to $10^{-6}$ torr during one second</footnote>
explanation is that these rows undergo a double-unit cell construction that lowers their chemical reactivity. Annealing these samples at 500 °C leads to irreversible oxidation, hinting at the high stability of oxidized borophene.

<figure><img src="image_33.png" /><figcaption>Figure 23. (a,b,c) pristine borophene (a) and its oxidation following molecular oxygen exposures of (b) 1000 L and (c) 20,000 L. (d,e) XPS spectra showing the evolution of B 1s and O 1s peaks (d) for pristine borophene and borophene exposed to ambient conditions with increasing duration; (e) as a function of increasing molecular oxygen dosing in UHV. (f) STS heatmaps at the B-Ag interface show a decrease in the density of states of pristine borophene (top) after oxidation (bottom). Reproduced with permission from ref. $^{142}$.</figcaption></figure>

The second study$^{169}$ investigates the oxidation mechanism in β₁₂ and χ₃ borophene on Ag(111), through a tandem technique called STM-TERS. The simultaneous acquisition of STM images and Raman spectra reveals the nature of chemical bonds with Raman spectra in specific regions of interest with STM images: borophene oxidation on the edges and basal plane. Indeed, extensive TERS studies in tandem with STM$^{169}$ imaging reveal characteristic chemical bonds and interactions of O and O₂ species with boron atoms in borophene grown on Ag(111). Controlled atomic and molecular oxidations under UHV are performed on χ₃ and β₁₂. On being exposed to atomic oxygen, Fig. 24(a,c,d) show that the TERS tip while traversing along pristine borophene with its vibration mode at 189 cm⁻¹, abruptly shows a new B-O vibration at 205 cm⁻¹ on the bright spot. The bridge site at the hollow hexagons shown in Fig. 24(b) is found to be the most preferred oxidation site. Upon O-oxidation, the B-B bond cleaves and O binds with the two B atoms. These oxygen-modified Raman modes are found to be delocalized and not only on the O adsorbed sites but also consist of vibrations from single B-O bonds and adjacent B atoms. O₂ exposure at 3600 L leads to inhomogeneous oxidation, forming disordered edges on borophene.
<figure><img src="image_34.png" /><figcaption>Figure 24. (a) TERS line scan across the interface of an oxygen adatom on β₁₂ or η=1/6 borophene along the trace marked with colored pluses in the inset STM image. (b) Top, side, and oblique views of the most energetically stable configuration of single oxygen adatoms (green spheres) on β₁₂ borophene. (c, d) TERS line scan over an oxygen adatom following a green trace show (d) the rise in intensity of the B-O vibration at 205 cm⁻¹ and a decrease in the borophene vibration band at 189 cm⁻¹ as the tip traverses across the oxygen adatom. Intensity scale shows a green zone for borophene and a blue zone for oxygen. Reproduced with permission from ref ¹⁶⁹.</figcaption></figure>

However, observations differ with χ₃, the TERS profile in Fig. 25(a) shows a new peak at 202 cm⁻¹ on an O adatom, while the tip traversed smoothly on pristine borophene with its vibration at 180 cm⁻¹. On the other hand, an O₂ exposure of up to 4800 L on χ₃, shows a distinct morphology and is found to be less prone to oxidation compared to β₁₂. The STM image in Fig. 25(b) highlights the preferential adsorption of oxygen adatoms on the β₁₂ domains and line defects. The uniformly oxygen-decorated χ₃ borophene hints at a weaker B-O bonding, as the two B atoms on the bridge stay covalently bonded while also being bonded to O, as depicted by Fig. 25(c) showing a boron-based epoxide group (B-O-B). Furthermore, as seen in Fig. 25(d) the room-temperature atomic O-oxidation on borophene is reported to be reversed after annealing at 360 °C, while molecular O₂-oxidation appears to lead to a stable oxidized-form of borophene.
<figure><img src="image_35.png" /><figcaption>Figure 25. (a) TERS of atomic oxygen adsorbed on χ₃ or η=1/5 borophene. Inset: arrows mark the tip positions during TERS measurements; scale bar: 1 nm. (b) STM image of χ₃ borophene after exposure to 4800 L of molecular oxygen. A small χ₃ domain and χ₃-line defects are indicated by blue arrows. (c) Oblique, side, and top views of the most energetically stable configuration of an oxygen adatom on χ₃ borophene (d) STM images of O-oxidized borophene before and after annealing at 360 °C. Reproduced with permission from ref. ¹⁶⁹.</figcaption></figure>

Borophene oxidation probed with STM, XPS, and TERS yields valuable information. The controlled oxidation can help to tune borophene properties, while the B-O-B epoxy group could act as surface sites for further chemical functionalization⁷¹. This introduction of epoxy groups could also be used as a passivation tool to reversibly obtain borophene on annealing. Oxidation probability and preference of certain borophene sites and polymorphs can help to explore and design more passivation pathways.

## 4.2 Passivation

The oxidation of borophene hinders its properties such as metallicity, and therefore the quest to stabilize borophene in ambient conditions paved the way for hydrogenated polymorphic borophene – borophane, BL borophene, and VDWH. They have been reported to considerably delay the ambient oxidation of pristine borophene. Mannix *et al.*, after the first borophene synthesis⁹, encountered oxidation and tried to prevent it by depositing a 2-4 nm thick amorphous silicon layer on top. The XPS analysis in Fig. 6(a) revealed a B-O peak, at 192.4 eV, only after three weeks of ambient exposure. Such relatively thick layers may be detrimental for device applications. However, it could be worthwhile to cover borophene with another oxidation-resistant 2D material(s) to preserve its crystal structure and properties, this would also make it a viable component in the heterostructure construction game. A reported hBN-covered χ₆ borophene¹⁶⁵ (metal-insulator) heterostructure (Fig. 26(a)) shows no signs of oxidation in its B 1s core-shell (Fig. 26(b)), after controlled O₂ exposure. In contrast, uncovered χ₆ borophene is readily oxidized
and shows increasing B-O peak as a function of O₂ exposure. Similar two-dimensional sandwiches with other stable 2D materials like graphene or TMDs could extend and enhance the usability of borophene. The production of such sandwiched structures is facilitated by the ability of transferring 2D materials from their synthesis substrate to the desired surface.

<figure><img src="image_36.png" /><figcaption>Figure 26. (a) Atomically resolved STM image of hBN-χ6 borophene heterostructure on Ir(111) (hBN unit cell in green). Inset: LEED pattern acquired at 79 eV (simulated diffraction pattern of hBN in green and borophene in red). (b) XPS spectra of B Is core-shell of hBN-covered borophene (left) in contrast to uncovered borophene (right). Reproduced with permission from *ref.*$^{165}$.</figcaption></figure>

### 4.3 Transferability

All reported borophene syntheses require metal substrates. For practical applications, borophene must be either deposited or transferred onto dielectric materials like silicon. When performing such a transfer, it becomes important to look at any cracks in borophene, adsorption of transferring agents, and unwanted functionalization of borophene during or after the transfer to the desired substrate. The transfer of borophene onto non-metallic substrates is inspired by *wet* and *dry* transfer methods$^{175}$ developed for 2D materials. The idea is to either peel-off the borophene layer from the metal substrate or selectively sacrifice the metal to obtain free-standing borophene which can be transferred onto the substrate of interest. The peel-off and transfer method is an obvious choice over the sacrificial approach when precious metals like silver and gold are involved.

This method has been for example achieved for $\chi_3$ and $\alpha_1$ borophene obtained via CVD of diborane$^{43}$ on aluminum-coated silicon, that need to be transferred onto an unknown insulating substrate to test its conductivity. The process consists of spin-coating PMMA onto the sample, heating it around 87 °C for 5 minutes to strengthen the borophene adhesion to the polymer, and then immersing it in HCl solution for a few seconds, allowing dissolving the top Al layers at the interface with borophene. By their density difference, Al/Si wafer sinks while the PMMA/$\chi_3$ stays afloat and is dip-coated on the insulating substrate. The removal of PMMA is done by acetone dissolution while borophene is left on the substrate. The low-resolution SEM image is not sufficient to comment on the quality and smoothness of the transferred borophene.
Another approach displays the decoupling of borazine-obtained χ₆ borophene¹⁴⁹ from Ir(111) via electrochemical delamination. Pt foil is made the anode and Ir crystal acts as the cathode. In the first electrochemical step, at 1.9 V, a 100:1 solution of tetra-n-octyl ammonium bromide (TOABr) and acetonitrile is used to form an intercalation layer of TOA⁺ ions to weaken Ir-borophene bonds. A complete detachment of borophene is achieved with an additional drop of PMMA on χ₆/Ir so that borophene adheres well to this polymer. The second step is bubbling, the hydrogen bubbles generated on the metal surface in 1M NaOH, at 2.5 V, expand across the interface to detach borophene. This PMMA/χ₆ film is then scooped by a SiO₂/Si wafer and heated at 100 °C for 30 min to reduce wrinkling and chemical residues. Finally, the PMMA is removed by immersing the sample into acetone for 10 min; rinsing with de-ionized water is done after each step. The transferred borophene deteriorates and results in cracks and borophene layer overlaps.

Cheaper metals such as copper are probable candidates to be a sacrificial buffer layer for borophene synthesis on non-metallic substrates like sapphire and silicon. Taking cues from established methods for transferrable graphene films suggests that borophene islands⁶,¹³⁸ on Cu(111) coated sapphire could be transferred onto other substrates. Copper can be dissolved in FeCl₃ solution, removing the borophene/Cu(111) interface and leaving borophene lying on the sapphire or silicon. However, no published works have demonstrated this kind of transfer.

## 5. Conclusions

Borophene became an experimental reality after almost two decades of research, from its first synthesis by PVD to its more recent ones by CVD. The discovery of different polymorphic structures has corroborated theoretical predictions, sparking interest in hydrogenated and bilayer borophene structures, each with their own sets of fascinating properties. This review highlights how polymorphic selectivity and borophene growth are influenced by the substrate type, its temperature and its orientation.

PVD synthesis has been demonstrated on several types of metallic substrates, from noble metals (Ag, Au, Ir) to more reactive ones (Al, Cu). Depending on the support, different phenomena can occur. For example, for boride forming metals (e.g. Ir, Al), boron dissolution may occur, which can be tuned by the process temperature. In some reported processes, the substrate temperature used may result in a high mobility of the surface atoms (e.g. Ag, Au), allowing some boron diffusion within the surface. In addition, strong electronic transfer with the deposited boron layer can occur, especially in the case of reactive metals. In the rare cases where it has been reported, annealing has shown rearrangements from one allotrope to another. However, the effect of annealing on allotrope formation has not been systematically investigated. Overall, it appears that different polymorphs can be obtained depending on the substrate type and surface orientation, as well as the process temperature, which could indicate an epitaxial phenomenon. However, with the variety of cases described above, the question of understanding the growth mechanism of borophene on these substrates remains whole.
The CVD approach to borophene synthesis has been investigated to a limited extent. The resulting polymorphs are highly dependent on the nature and symmetry of the substrate, with metallic substrates such as Ru(111) and Cu(111) proving critical, although non-boride-forming options remain underexplored. In addition, challenges such as precursor toxicity, oxidation, and incomplete B-B bond formation underscore the need for alternative precursors and improved growth mechanisms for scalable and sustainable borophene production.

Most efforts have been devoted to the synthesis of monolayer borophene, which has crystalline domains with surface areas ranging from nm² to µm². However, few studies have reported the fabrication of bilayer borophene with increased stability compared to monolayers while maintaining metallicity. Increased stability has also been observed with hydrogenated borophene (borophane).

## 6. Prospects

Advancements in the metallic 2D boron synthesis methods have enabled the transition from nanometer-scale domains obtained via PVD to micrometers-scale ones achieved by thermal CVD.

**PVD.** The successful PVD growth of borophene has been demonstrated on metallic single-crystal substrates, which seem essential for epitaxy, stabilization, and determination of the synthesized allotrope. Exploration of alternative substrates, such as insulators like sapphire or 2D materials like graphene, could enable new properties and heterostructures. To enable borophene-based devices, it is now necessary to address the current challenges of allotrope selectivity, domain size and stability. It is also necessary to reduce process energy consumption to enable industrial scale-up. Innovations such as laser-assisted evaporation, primary vacuum synthesis, and the use of hydrogen radicals could be explored alongside chemical approaches to improve borophene fabrication techniques. Many questions remain, particularly regarding the growth mechanisms and phase transitions of borophene. For example, is substrate crystallinity a prerequisite for borophene growth? Would annealing an amorphous boron monolayer lead to borophene? Since the α phase is the thermodynamically stable one according to calculations⁶⁵,¹²¹, can annealing lead to phase transitions and yield only the α phase? Or will the resulting phases be driven by the nature and orientation of the substrate? Could it be possible to grow borophene directly on amorphous substrates? A systematic study of the effect of annealing on different substrates would yield very interesting results in this regard.

**CVD.** Since only diborane and borazine are commonly used for CVD, much thought must be given to the precursors that can be explored and exploited to obtain this 2D material. Not to mention that certain borazine-derived borophenes suffer from inadequate characterization evidence that can be addressed. Different versions of CVD, such as plasma-enhanced CVD, could be useful to further reduce deposition temperatures. However, CVD lacks precise thickness control and can result in a mixture of several borophene layers⁴³. In addition, cheap metal substrates such as Ni, Al and Cu need to be used for
deposition processes to make borophene deposition and transfer to other desired substrates cost effective.

***Towards applications.*** The distinct properties resulting from borophene's polymorphism hold promise for a wide range of future applications, positioning borophene as a strong candidate in the lamellar heterostructure game. Reversible hydrogenation and oxidation of borophene results in stable structures that may be useful for transportation as they are less susceptible to oxidation than pristine borophene. Borophane could be an interesting solution to passivate borophene for its use in devices. For example, direct PVD deposition of borophane has never been achieved and could be attempted by introducing a low pressure of hydrogen radicals during growth.

Overall, the potential applications of borophene in electrodes, energy storage, bio-sensing, energy conversion, and optoelectronics make it worthwhile to pursue research to obtain large, uniform, and crystalline borophene.

## Acknowledgements

This work was supported by the French National Research Agency grant ANR-21-CE09-0001-01.

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