Influence of TCNQ Loading on the Mechanical Properties of Metal-Organic Framework Films

Abstract

Bulk Metal-organic-framework (MOF) materials are designed scaffold-like compounds that consist of metal ions connected by organic ligands, forming highly ordered porous structures. Bulk MOFs yield typically powders with micron sized crystallites, which limit their applications. Many practical applications require instead suitably uniform MOF films. In recent years, efforts to grow highly oriented Surface Anchored Metal-Organic Framework (SURMOF) films of Cu3(BTC)2 (BTC: benzenetricarboxylic acid), known as HKUST-1 MOF, have been successful by utilizing a liquid phase epitaxy (LPE) spray method on surface functionalized gold coated silicon substrates. Originally MOFs were intended for gas storage due to high storage capacity inside the porous MOF bulk material. New potential device applications in thermoelectrics and microelectronics [1,2] were explored based on the premise that the electrical properties of the otherwise insulating MOF can be custom tailored by loading the porous MOF structure with metal clusters. For modulating the electrical conductivity of MOF films the most effective method is to infiltrate guest molecules inside the porous framework to transport ions, and hence to modulate the electrical properties of the host framework. Early work focused on the mechanical properties of pristine SURMOF films [3,4]. In this study, we have investigated the influence of loading pristine highly oriented MOF films with TCNQ on the mechanical properties by utilizing nanoindentation methods to measure the hardness and modulus of SURMOF films with and without TCNQ loading. Our findings reveal that undoped pristine SURMOF samples are relatively harder with an intrinsic hardness of ~ 1.6 GPa in the upper third of the MOF film. The hardness of the pristine SURMOF samples is steadily increasing as a function of the penetration depth. The hardness is approaching the substrate Si hardness as the indenter tip approaches the substrate. The measured modulus values of the pristine SURMOF samples are also steadily increasing as a function of the indentation depth. However, the TCNQ loaded SURMOF samples differ markedly and do not show a substrate effect. Interestingly the loaded SURMOF films are much softer and exhibit a constant hardness as a function of depth of ~ 0.5 GPa. As the indenter approaches the Si substrate there is no measurable change in the constant hardness and an absence of a substrate effect. The modulus values of TCNQ loaded SURMOF films is also significantly lower compared with the pristine sample and remains constant as a function of indenter penetration depth. Thus, loading the porous scaffolding of SURMOF films with TCNQ did not only alter the electrical conductivity but triggers marked changes in the mechanical properties, i.e., softening effect. This softening effect in TCNQ infiltrated MOF films can be accounted for by a widening of the lattice constant und the concomitant introduction of lattice defects that weaken the crystal and lead to a much lower hardness.