Covalently Functionalized Germananes

Warren L. B. Huey, Joshua E. Goldberger

Ohio State University

Introduction

Germananes are a family of layered van der Waals 2D materials that consist of a puckered honeycomb sp³ Ge lattice in which every Ge atom is terminated with a covalent ligand, alternatingly above or below the framework. Their properties readily depend on the identity of the terminating ligands, allowing these materials to be readily tailored for a broad array of applications.¹ Here, we summarize the synthesis, properties, and potential uses of germanane materials.

Synthesis and Properties of Covalently-Terminated Germanane

The synthesis of covalently-terminated germanane is achieved through the topotactic deintercalation of CaGe₂ with electrophilic reagents such as HCl or organohalides (Figure 1). This reaction retains the puckered honeycomb Ge network, while providing a surface terminating ligand on each Ge atom.^{2, 3} When this ligand is small, such as -H, -methyl, -dimethyl ether, or -allyl, it is possible to achieve complete surface termination with a single ligand. With larger bulkier ligands, often a mixture of -ligand and -H termination is observed.^{4, 5} GeH and GeCH₃ are air and water stable as crystals, powders, and when exfoliated onto substrates.

Figure 1. Synthesis of germanane materials: GeH. CaGe2 (left) is reacted with HCl to make GeH (middle), which has a puckered honeycomb framework of Ge atoms (right). Ge atoms are shown in red, Ca atoms are blue, and H atoms are brown.

The chemical identity of the surface ligand impacts the properties of germanane, including thermal stability, electronic structure, band gap, and dispersibility. For instance, GeH starts to amorphize at 75 °C and loses hydrogen completely at 200 °C, whereas GeCH₃ remains stable up to 300 °C.^2,3,6 Second, research shows that the interplay between ligand size and electronegativity can tune the semiconducting band gap from 1.4–1.7 eV (or 700–1000 nm absorption onset) (Figure 2).⁵ In general, larger and more electron-withdrawing ligands, such as -dimethyl ether and -allyl lead to smaller band gaps, while more electron donating ligands, i.e., CH₃, show larger band gaps. The above band gap extinction coefficients are typically ~10³ cm^-1. Finally, the preparation of long-term stable dispersions consisting of single and few-layer sheets with lengths of 10–1000 nm can be prepared via ultrasonication and centrifugation in various solvents dependent on the surface ligand. GeH forms stable dispersions in isopropanol, whereas GeCH₃ and GeCH₂CH₃ form stable dispersions in chlorobenzene and 1,2-dichlorobenzene.^7–9

Figure 2. Semiconducting band gaps of germanane covalently terminated with different ligands. Larger and more electron withdrawing ligands result in smaller band gaps.⁵

Applications of Germanane

Germanane and its covalent terminations offer much utility for applications in membranes, transistors, batteries, photocatalysis, and conductive amorphous Ge coatings:

Organic terminated germanane has enough interlayer van der Waals space to enable the rapid interlayer absorption and permeation of small molecules such as H₂O and NH₃. For instance, GeCH₃ can intercalate 0.37 equivalents of H₂O per mole of GeCH₃. The presence of H₂O in the van der Waals space of GeCH₃ also induces an intense above-band gap red-orange photoluminescence at 1.87 eV.⁶ Such properties allow for the design of germanane membranes for small molecule separations, sensing, and desalination.

Electronic transistors based on GeH have started to attract considerable interest. Typically, ambipolar transport behavior is observed, indicative of populating both the electron and hole channels via gating. Measured electron and hole Hall mobilities at 120K of 6500 and 570 cm² V^-1 s^-1, respectively, have been reported.¹⁰

Studies show GeH is a promising anode material for lithium ion batteries. The fully lithiated Li₁₅Ge₄ phase has a theoretical energy density of 1384 mAh/g. Anodes constructed from GeH approach this limit, with a reversible Li⁺ capacity of 1108 mAh/g when cycled between 0.1-2V vs Li/Li⁺.⁷ In contrast, the large volume expansion of Ge nanoparticles with intercalated Li results in the degradation of the anode during the charging cycles. The GeH anode is also more stable at maintaining its capacity at high cycling rates for greater than 100 cycles.⁷

Germanane materials have also shown excellent promise for photoelectrocatalysis, such as the hydrogen evolution reaction in H₂O/CH₃OH_­ mixtures. For instance, photoelectrodes comprised of GeCH₃/1 wt% Pt nanoparticle readily generate H₂ under visible light irradiation. Here, upon absorption of above band gap light, the photogenerated electrons rapidly transfer to Pt, while the photogenerated holes can be rapidly transferred to CH₃OH, hindering the recombination process.¹¹

Post-synthesis processing also enables hydrogen-terminated germanane to be used to generate conductive amorphous germanium thin films. GeH is intrinsically insulating. However, thermal annealing above the dehydrogenation temperature transforms it into a material with a highly conducting metallic state and resistivity of ~1.6 x 10^-7 Ω m at room temperature.¹²

Conclusion

The tunable nature of covalently-terminated germanane materials allows for their use in a wide array of applications. The terminating ligands provide significant air and water stability while allowing for their exfoliation into ultra-thin layers. The terminating ligand influences the band gap, conductivity, and chemical interactions allowing specific tailoring of these materials for each application.