Unveiling the nature of 2D boron at atomic scale

Abstract

Boron, a group 13 element, has been a fascinating subject for scientists for decades due to its electron deficiency, which gives rise to a complex array of structural variations, including several distinct allotropes and polymorphs. The discovery of boron’s superconductivity under high pressure and in magnesium diboride (MgB2) has further heightened interest in this element. The superconductivity in MgB2 is attributed to the planarity of boron within its structure. Additionally, theoretical predictions suggest that boron clusters exhibit greater stability, particularly those corresponding to individual icosahedra. These factors collectively enhance the interest in exploring the two-dimensional (2D) counterpart of boron. The landscape of materials science has been fundamentally transformed by the advent of 2D materials. Among the non-van der Waals 2D materials, boron stands out as one of the most complex elements predicted to form a 2D counterpart. Boron is renowned for its diverse chemistry and it possesses only three valence electrons, yet it demonstrates a fascinating capability to engage in electron-sharing through various mechanisms, resulting in the formation of two-electron, multicenter (2e-nc) bonds. The exploration of boron cluster stability is pivotal for evaluating the practicality of borophene, a 2D material counterpart of boron. Theoretical studies have shown that different boron clusters exhibit stability when supported on metallic substrates, and these clusters extend to form borophene and serve as foundational elements for various borophene polymorphs. Borophene was successfully synthesized on silver, marking a significant milestone that has subsequently led to various other research groups producing different phases of borophene on a range of substrates. The exfoliation of non-van der Waals materials into 2D structures was initially considered unlikely. However, successful exfoliation of materials such as tungsten trioxide (𝛼-WO3) and hematite (Fe2O3) has significantly strengthened confidence in the potential to isolate new 2D non-van der Waals materials. The successful synthesis of freestanding borophene using a top-down approach, specifically through liquid-phase exfoliation (LPE), paves the way for the production of substrate-free borophene. This study investigates 2D boron at the atomic scale using scanning tunneling microscopy (STM) for both liquid-phase exfoliated (LPE) and beam-grown samples. Additionally, high-resolution transmission electron microscopy (HRTEM) was used to explore the LPE-synthesized 2D boron. We investigated the adsorption behavior of freestanding borophene on Au(111), including atomic-scale structural and electronic characterization of LPE borophene on the Au(111) substrate. Using experimental techniques such as scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It was complemented by density functional theory (DFT) calculations, we identified the phase as 𝛽12 borophene. Our study aims to elucidate the atomic structure of LPE borophene and its interaction with the gold substrate. Subsequently, we examined a denser coverage of borophene, resulting in a complete monolayer of the 𝛽12 borophene phase on the Au(111) surface. Following this, a spin-coating cycle led to the self-assembly of boron clusters across the entire substrate. Wherein the borophene monolayer served as a template for the self-assembly of clusters. The interaction of boron clusters with the Au (111) substrate was investigated, and the boron clusters with the 𝛽12 monolayer phase were examined using STM measurements. STS measurements were employed to determine the metallicity of both the clusters and the 𝛽12 monolayer. While multiple reports document polymorphisms in borophene when grown on substrates, there has been no single experimental study that has reported the evolution of various phases in liquid-phase exfoliated borophene. We provide compelling evidence from HRTEM to establish the evolution of 𝛽12, 𝜒3, striped phases, as well as several phases with 4-fold, 6-fold, and mixed-fold symmetries, and various borophene phases with complex geometries. Additionally, density functional theory band structure calculations of prominent borophene phases. Following the investigation of LPE borophene and clusters, we examined electron beam-grown borophene and clusters on a silver substrate. Previous studies on borophene growth predominantly employed electron beam evaporation with a boron rod. We used boron powder in place of a boron rod to grow borophene. By employing electron beam evaporation, we achieved the formation of single-phase borophene, rather than a polymorphic phase. We employed STM, and STS to analyze the properties and phase of the synthesized borophene. Additionally, we studied the stability of the synthesized borophene, observing the evolution of clusters. The metallicity of the grown borophene and clusters was determined through STS measurements. These investigations will be pivotal in unraveling the fundamental properties of 2D boron and clusters. The interaction between borophene and its substrates constitutes the initial step toward elucidating the behavior of Borophene when it interacts with various other materials. Such insights are critical for applications spanning sensing, catalytic activities, battery applications, and beyond, where interactions play a crucial role.