Nanomechanical Investigations of Crystals of Copper Nanocluster Isomorphs: Enhanced Hardness of the Low-Density Analogue

Atomically precise cluster crystals, with constituent units composed of tens to hundreds of atoms, are important for the construction of miniaturized solid-state devices. Understanding the mechanical characteristics of such crystals is crucial for these applications. In this study, we focused on the nanomechanical properties of crystals of two isomorphic copper nanoclusters (Cu4L4), protected by ortho-carborane-9-thiol, Cu4(oCBT)4, and meta-carborane-9-thiol, Cu4(mCBT)4. These two clusters possess identical square planar Cu4 cores embedded in butterfly-shaped Cu4S4 staples. Load–displacement measurements indicated that the crystals of Cu4(oCBT)4 (hardness of ∼534.31 MPa) were harder than those of Cu4(mCBT)4 (hardness of ∼335.49 MPa). Despite their lower density, crystals of Cu4(oCBT)4 demonstrated increased hardness, owing to the presence of locked slanted layers that efficiently interacted with each other through various short contact supramolecular interactions. During indentation studies, multiple “pop-in” events were observed for the crystals of both clusters, suggesting the dislocation of molecular layers within the crystal lattice. Dynamic mechanical analysis conducted at different loading frequencies indicated that crystals of Cu4(oCBT)4 have a higher storage modulus than Cu4(mCBT)4. Both the crystals are thermally robust, as evident from thermogravimetric analysis and attenuated total reflection-IR analysis. Using density functional theory, we calculated Young’s modulus (Er) for both crystals at 1 and 2% strain and found that the high-density isomorph had a lower Er, consistent with experimental data showing Er of Cu4(oCBT)4 and Cu4(mCBT)4 to be 9.79 and 8.54 GPa, respectively. These findings highlight the significant role of subtle structural differences in governing the nanomechanical behavior of isomorphic cluster crystals, paving the way for their rational design in advanced solid-state device applications.