Chemical and structural stability of liquid-phase-exfoliated (LPE) two-dimensional (2D) materials
Creators: name: Ingle, Aviraj
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340307
subject: Structure and dynamics of materials

description: Atomically thin two-dimensional (2D) materials have gathered massive attention owing to their unique properties that could benefit both the conventional semiconductor industry and wide-ranging nanotechnology-enable applications. At the atomically thin regime, 2D materials exhibit an array of unusual physiochemical, optical, and electronic phenomena enabling sustainable, high-performing, and faster devices, a prerequisite for future electronic and optoelectronic technologies. The emerging repertoire of 2D materials offers great flexibility in tuning these properties by allowing controlled fabrication of predetermined phases, compositions, and thicknesses. These properties are remarkably different from their bulk counterparts. These advancements have resulted in a large portfolio of 2D materials ranging from metals to semiconductors and insulators, offering possibilities to engineer and manoeuvre them for a specific purpose. 

Largely, the properties of a 2D material depend on its crystal structure, composition and surface characteristics that authorise it to behave in a certain manner. Their atomistic nature makes them ultrasensitive to ambient conditions which influences their physico-chemistry and therefore affecting their performance. Hence, the topic of chemical and structural stability in 2D materials has undoubtedly become an unsettling question and receiving notable traction. The work presented in this thesis attempts to investigate the irregularities associated with the crystal structure and chemistry of materials at atomically thin regimes through their careful synthesis and characterisation. This is achieved by focussing on three technologically promising 2D materials, including black phosphorus (BP - phosphorene), lead monoxide (PbO), and lead (Pb - plumbene), which are discussed in three subsequent chapters, after discussing the state-of-the-art in the field in Chapter 1.

Chapter 2 introduces “few-layer black phosphorus (FLBP)”, a technologically promising 2D material with tuneable bandgap across the visible-infrared region. A major drawback with FLBP is its atmospheric instability as it has a high propensity to interact with its surrounding environment and rapidly degrade. Blocking direct exposure of FLBP to its environment remains a key method to achieve its protection against ambient oxidation. The work presented in this chapter proposes an alternative ambitious approach that not only ensures its protection but also allows to repair its surface to pristine conditions, if oxidised. This work has taken inspiration from naturally occurring processes in plants that produce antioxidant pigments such as β-carotene to defend them against photo-oxidative stresses. A similar protective and reparative mechanism of β-carotene is demonstrated to protect FLBP against photooxidation. The concept is demonstrated by first fabricating FLBP through a liquid-phase exfoliation (LPE) technique, followed by studying the interaction of FLBP with β-carotene molecules. β-carotene displayed exceptional protective ability, as illustrated by various microscopic and spectroscopic investigations, along with computational studies and biochemical assays. The outcomes from this study have created a potential opportunity to harness the strengths of naturally available antioxidant molecules in stabilising technologically important, yet environmentally sensitive 2D materials against ambient oxidation. This should enhance the opportunities for their deployment for various applications.

Chapter 3 of this thesis investigates PbO as a model system to evaluate the structural stability of 2D materials at atomically thin regimes. In contrast to FLBP investigated in the previous chapter that faces chemical instability, many other materials may undergo crystal phase transformations when reduced to atomically thin sizes. Despite the crystal structure al a material being a key determinant of its properties, an aspect that remains rather elusive is the influence of 2D crystal thickness on the structural stability of 2D materials. In particular, polymorphic crystals such as PbO that have a metastable crystal structure, make them an interesting candidate to study thickness-dependent phase transformations. To study these aspects, the work first developed an innovative reactive oxygen species (ROS) sequestration-mediated strategy to fabricate layered PbO crystals from a Pb foil. The thicker orthorhombic (β) PbO nanosheets obtained from this strategy were observed to gradually transform to tetragonal (α) PbO as the nanosheet thickness was reduced towards atomically thin layers by deploying an LPE method. These experimental findings from a range of microscopy and spectroscopy techniques were also supported by computational studies that indicated that a monolayer of α-PbO is thermodynamically more stable than β-PbO. An interesting observation was co-assembly of [001]-oriented α-PbO regions with [100]-oriented β-PbO regions within a single ultrathin nanosheet, suggesting the possibility to create interesting crystal structures at atomically thin regimes. Since the crystal structure of a material is the key determinant of its properties, the outcomes from this study offer opportunities to controllably create new 2D materials with unique crystal structures that have not been realised previously. Further, the polymorphic transformation of 2D PbO observed in this work highlights the need to carefully evaluate any potential changes in the crystal structure of materials at atomically thin levels.

Chapter 4 focuses on investigating the synthesis and stability of monolayer and few-layer thin lead (FLPb), commonly referred as “plumbene”. Free-standing plumbene has been proposed as an interesting 2D material to realise quantum spin hall phenomenon, which is critical for spintronics applications. While plumbene has been previously grown epitaxially on palladium surface, and a plethora of theoretical work has emphasised upon the importance of free-standing (substrate-free) plumbene, FLPb and plumbene have remained unrealised in the form of free-standing nanosheets. To address this gap, in this chapter, a cryo-mediated LPE strategy is discussed to produce FLPb by exfoliation of bulk Pb, a non-layered, non-van der Waal crystal. The cryogenic temperature along with mechanical forces (grinding and sonication) induced a ductile-to-brittle transition in Pb crystal and created higher DMF binding sites, favouring its cleaving between the (100) planes. The obtained FLPb nanosheets were found preferentially oriented along the [z00] direction. The strategy could achieve production of free-standing plumbene monolayers with a thickness of ~0.6 nm and average lateral dimension of ~450 nm. The work also revealed that FLPb is prone to surface oxidation, leading to the formation of lead oxides on its surface. In particular, radiation-induced surface oxidation of FLPb was noted during spectroscopic analysis of FLPb, suggesting that electrons, x-rays, and even visible light may oxidise plumbene during analysis and characterisation. This poses challenges in confidently determining whether the as-synthesised FLPb was free of surface oxides or was inherently stabilised by an oxide layer. Nevertheless, the access to free-standing plumbene to the 2D materials community is likely to offer opportunities to explore this exciting material for emerging applications.

Chapter 5 discusses the overall conclusions from this thesis and propose directions for future research. Overall, the investigations made during this thesis have enhanced our understanding of the chemistry of materials at atomically thin regimes by selecting three model systems, phosphorene, lead oxide, and plumbene. Phosphorene represents a material class that can undergo complete oxidative degradation, lead oxide is representative of materials that can exhibit remarkable crystal transformations, and plumbene belongs to those 2D materials whose surface can perhaps be controllably oxidised to create a shield that protects them from further deterioration. The knowledge generated from this thesis is likely to influence further studies in this area, where efforts will be made to understand the chemistry of 2D materials deeply and carefully. The learnings from these studies will expedite the technological translation of 2D materials.