Mechanical properties of van der Waals interactions at compositionally graded interfaces between metals and organic semiconductors

Interfacial bonding plays an important role in energy transfer (e.g., electrical conduction, thermal transport, or acoustic coupling) in composite materials as well as electronic and optoelectronic devices. In this work we use an ultrafast laser to excite vibrational modes in a thin aluminum film that is in contact with a small molecular organic semiconductor (copper phthalocyanine, CuPc). From the measured acoustic dynamics, we derive the fundamental mechanical properties of the van der Waals bonding at the Al/CuPc interface, and further study how these mechanical properties change in a series of samples as the interface is compositionally graded. The implications of these results are discussed in the contexts of interfacial thermal resistance, organic optoelectronic devices, and thermoelectric energy conversion.