The sp3-hybridized group 14 graphane analogues are a unique family of 2D materials in which every atom requires a terminal ligand for stability. Consequently, the optical, electronic, and thermal properties of these materials can be manipulated via covalent chemistry. Herein, we review the methodologies for preparing these materials, and compare their functionalization densities to Si/Ge(111) surfaces and other covalently terminated 2D materials. We discuss how the electronic structure, optical properties, and thermal stability of the 2D framework can be broadly tuned with the ligand identity and framework element. We highlight their recent application in electronics, optoelectronics, photocatalysis, and batteries. Overall, these materials are an intriguing regime in materials design in which both surface functionalization and solid-state chemistry can be uniquely exploited to systematically design properties and phenomena.

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The graphane analogues of group 14 are a unique family of 2D materials due to the necessity of a terminal ligand for stability. Here we highlight how changing the surface ligand can lead to nonobvious interactions with other chemical species. We show using XRD, FTIR, and TGA that GeCH3 reversibly absorbs water into the van der Waals space, whereas GeH does not intercalate water. Molecular dynamics and density functional theory simulations predict that water datively interacts with the Ge–C σ* pocket on the Ge framework, resulting in local structural distortions. Surprisingly, these distortions give rise to an intense above band gap luminescence state of 1.87 eV, with an average lifetime of hundreds of picoseconds. This work opens potential applications for exploiting surface functionalization chemistry of 2D materials to create membrane and separation technologies.

Methyl-substituted germanane is an emerging material that has been proposed for novel applications in optoelectronics, photoelectrocatalysis, and biosensors. It is a two-dimensional semiconductor with a strong above-gap fluorescence associated with water intercalation. Here, we use time-resolved photoluminescence spectroscopy to understand the mechanism causing this fluorescence. We show that it originates from two distinct exciton populations. Both populations recombine exponentially, accompanied by the thermally activated transfer of exciton population from the shorter- to the longer-lived type. The two exciton populations involve different electronic levels and couple to different phonons. The longer-lived type of exciton migrates within the disordered energy landscape of localized recombination centers. These outcomes shed light on the fundamental optical and electronic properties of functionalized germanane, enabling the groundwork for future applications in optoelectronics, light harvesting, and sensing.

The development of thermally robust, air-stable, exfoliatable twodimensional van der Waals ferromagnetic materials with high transition temperatures is of great importance. Here, we establish a family of magnetic alloys, Cr x Pt 1−x Te 2 (x ≤ 0.45), that combines the stability of the late transition metal dichalcogenide PtTe 2 with magnetism from Cr. These materials are easily grown in crystal form from the melt, are stable in ambient conditions, and have among the highest concentrations of magnetic element substitution in transition metal dichalcogenide alloys. The highest Cr-substituted material, Cr 0.45 Pt 0.55 Te 2 , exhibits ferromagnetic behavior below 220 K, and the easy axis is along the c-axis of the material, as determined using a combination of neutron diffraction and magnetic susceptibility measurements. These materials are metallic, with appreciable magnetoresistance below the Curie temperature. Single-crystal and powder diffraction measurements indicate Cr readily alloys onto the Pt site and does not sit in the van der Waals space, allowing these materials to be readily exfoliated to the few-layer regime. In summary, this air-stable, exfoliatable, high transition temperature ferromagnet shows great potential as building block for future 2D devices.