Synthesis and Stability of Two-Dimensional Ge/Sn Graphane Alloys

There has been considerable interest in the germanium and tin graphane analogues due to their potential as optoelectronic building blocks, and novel topological materials. Here, we have synthesized for the first time alloyed germanium/tin graphane analogues from the topochemical deintercalation of CaGe_{2–2<i>x</i>}Sn_2<i>x</i> (<i>x</i> = 0–0.09) in aqueous HCl. In these two-dimensional alloys, the germanium atom is terminated with hydrogen while tin is terminated with hydroxide. With greater tin incorporation, the band gap systematically shifts from 1.59 eV in GeH down to 1.38 eV for Ge_0.91Sn_0.09H_0.91(OH)_0.09, which allows for more sensitive photodetection at lower energies. In contrast to germanane’s oxidation resistance, the Ge and Sn atoms in these graphane alloys rapidly oxidize upon exposure to air. This work demonstrates the possibility of creating functional tin-incorporated group IV graphane analogues

Improved Topotactic Reactions for Maximizing Organic Coverage of Methyl Germanane

The topotactic transformation of Zintl phases such as CaGe₂ into organic-terminated germanium graphane analogues using haloalkanes is a powerful route for generating new 2D optoelectronic and spintronic building blocks. However, uniform ligand coverage is necessary for optimizing the properties and stability of these single-atom-thick frameworks. Here, we compare the effectiveness of different topochemical methods to maximize methyl-termination in GeCH₃. We show that a previously developed CH₃I/H₂O phase transfer route produces a small percentage of partially oxidized germanane. The partially oxidized termination is readily removed upon HCl treatment, which leads to Ge–Cl termination, but rapidly reoxidizes after exposure to the ambient atmosphere. We then show that a one-pot route with CH₃I in distilled CH₃CN solvent and at least six equivalents of H₂O results in no oxidation. The GeCH₃ prepared from this one-pot route also has an increased −CH₃/–H ratio of termination from ∼90:10 to ∼95:5, is air-stable, has greater thermal stability, has a sharper absorption onset, and has more narrow band edge photoluminescence, all of which are signatures of a less defective semiconductor

Single Quasi-1D Chains of Sb₂Se₃ Encapsulated within Carbon Nanotubes

The realization of stable monolayers from 2D van der Waals (vdW) solids has fueled the search for exfoliable crystals with even lower dimensionalities. To this end, 1D and quasi-1D (q-1D) vdW crystals comprising weakly bound subnanometer-thick chains have been discovered and demonstrated to exhibit nascent physics in the bulk. Although established micromechanical and liquid-phase exfoliation methods have been applied to access single isolated chains from bulk crystals, interchain vdW interactions with nonequivalent strengths have greatly hindered the ability to achieve uniform single isolated chains. Here, we report that encapsulation of the model q-1D vdW crystal, Sb2Se3, within single-walled carbon nanotubes (CNTs) circumvents the relatively stronger c-axis vdW interactions between the chains and allows for the isolation of single chains with structural integrity. High-resolution transmission electron microscopy and selected area electron diffraction studies of the Sb2Se3@CNT heterostructure revealed that the structure of the [Sb4Se6]n chain is preserved, enabling us to systematically probe the size-dependent properties of Sb2Se3 from the bulk down to a single chain. We show that ensembles of the [Sb4Se6]n chains within CNTs display Raman confinement effects and an emergent band-like absorption onset around 600 nm, suggesting a strong blue shift of the near-infrared band gap of Sb2Se3 into the visible range upon encapsulation. First-principles density functional theory calculations further provided qualitative insight into the structures and interactions that could manifest in the Sb2Se3@CNT heterostructure. Spatial visualization of the calculated electron density difference map of the heterostructure indicated a minimal degree of electron donation from the host CNT to the guest [Sb4Se6]n chain. Altogether, this model system demonstrates that 1D and q-1D vdW crystals with strongly anisotropic vdW interactions can be precisely studied by encapsulation within CNTs with suitable diameters, thereby opening opportunities in understanding dimension-dependent properties of a plethora of emergent vdW solids at or approaching the subnanometer regime

Reversible O–O Bond Scission and O₂ Evolution at MOF-Supported Tetramanganese Clusters

The scission of the O–O bond in O2 during respiration and the formation of the O–O bond during photosynthesis are the engines of aerobic life. Likewise, the reduction of O2 and the oxidation of reduced oxygen species to form O2 are indispensable components for emerging renewable technologies, including energy storage and conversion, yet discrete molecule-like systems that promote these fundamental reactions are rare. Herein, we report a square-planar tetramanganese cluster formed by self-assembly within a metal–organic framework that reversibly reduces O2 by four electrons, facilitating the interconversion between molecular O2 and metal-oxo species. The tetranuclear cluster spontaneously cleaves the O–O bond of O2 at room temperature to generate a tetramanganese-bis(μ2-oxo) species, which, in turn, is competent for O–O bond reformation and O2 evolution at elevated temperatures, enabled by the head-to-head orientation of two oxo species. This study demonstrates the viability of four-electron interconversion between molecular O2 and metal-oxo species and highlights the importance of site isolation for achieving multi-electron chemistry at polynuclear metal clusters

Synthesis of 1T, 2H, and 6R Germanane Polytypes

Polytypism, or the ability for materials to crystallize with different stacking sequences, often leads to fundamentally different properties in families of two-dimensional materials. Here, we show that is possible to control the polytype of GeH, a representative two-dimensional material that is synthesized topotactically by first controlling the polytype sequence of the precursor Zintl phase. 1T, 2H, and 6R GeH can be prepared by the topotactic deintercalation of 1T EuGe₂, 2H α-CaGe₂, and 6R β-CaGe₂, respectively. The 6R and 1T GeH polytypes exhibit remarkably similar properties and feature band gaps of 1.63 and 1.59 eV, respectively. However, the 2H CaGe₂ precursor forms due to the incorporation of small amounts of In flux in the germanium lattice, which is retained when converted to GeH. Consequently, 2H GeH has a reduced band gap of 1.45 eV. Finally, temperature dependent diffraction of 6R GeH shows a negative coefficient of thermal expansion along the <i>a</i>-axis and a positive coefficient of thermal expansion along the out-of-plane <i>c</i>-axis

Tailoring the Electronic Structure of Covalently Functionalized Germanane via the Interplay of Ligand Strain and Electronegativity

The covalent functionalization of 2D crystals is an emerging route for tailoring the electronic structure and generating novel phenomena. Understanding the influence of ligand chemistry will enable the rational tailoring of their properties. Through the synthesis of numerous ligand-functionalized germanane crystals, we establish the role of ligand size and electronegativity on functionalization density, framework structure, and electronic structure. Nearly uniform termination only occurs with small ligands. Ligands that are too sterically bulky will lead to partial hydrogen termination of the framework. With a homogeneous distribution of different ligands, the band gaps and Raman shifts are dictated by their relative stoichiometry in a pseudolinear fashion similar to Vegard’s law. Larger and more electronegative ligands expand the germanane framework, thereby lowering the band gap and Raman shift. Simply by changing the identity of the organic ligand, the band gap can be tuned by ∼15%, highlighting the power of functionalization chemistry to manipulate the properties of single-atom thick materials

NaSn₂As₂: An Exfoliatable Layered van der Waals Zintl Phase

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin–orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials. Herein, using NaSn₂As₂ as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt₂Pn₂ (A = group 1 or 2 element, Tt = group 14 tetrel element, and Pn = group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman-active modes of the bulk crystals <i>via</i> polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn₂As₂ samples are resistant toward oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy, density functional theory, and transport on bulk and exfoliated samples, we show that NaSn₂As₂ is a highly conducting 2D semimetal, with resistivities on the order of 10^–6 Ω·m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics