Size-Dependent Band Gaps in Metal-Organic Framework Nanoparticles

Decades of research into size-dependent semiconductor optical gaps have focused on quantum confinement as the dominant mechanism. Emerging reports indicate that lattice strain—intentional or incidental—can impart optical shifts similar or greater in magnitude. Here, we report evidence of optical absorption and photoluminescence spectra of M(1,2,3- triazolate)2 M = Mg, Cr, Mn, Fe, Co, Cu, Zn, Cd nanoparticles that blueshift from bulk values with decreasing particle sizes in a manner that defies explanation by conventional quantum confinement. The phenomenon persists for particle sizes as large as 200 nm, whereas quantum confinement generally ceases beyond 20-30 nm diameters, and follows a weaker dependence on particle radius. Computational simulations and crystallographic analysis suggest this behavior arises from size-dependent changes to metal-linker bonding that manifests in strain values comparable to literature reports of straininduced optical shifts in other classes of materials. Taken together, these results represent one of the few examples of sizedependent strain in crystalline materials and reinforce the emerging view that MOFs become softer materials when isolated as nanoparticles.