Atomic layer deposition (ALD) is a thin-film deposition technique with applications in microelectronics and catalysis. More recently, its accessibility in academic and industrial labs is leading to rapid development of novel materials and synthesis methods. We have made significant contributions to the field with our systematic (cycle by cycle) computational study of atomistic mechanisms of metal-oxide growth on a variety of surfaces. We have proposed new ALD processes to grow atomic clusters with spatial precision of a single atom! Two examples are discussed below.

Growth of ZnO on Graphene (JACS 2013)

Interfacing the inert basal plane of graphene with other materials has limited the development of graphene-based devices. We have developed a new ALD process to grow ZnO nanoparticles on epitaxial graphene (EG). The nanoparticles are obtained by sequential exposure of EG to atomic oxygen (AO) and diethyl-zinc (DEZ) precursors under ultrahigh vacuum conditions. Atomic force microscopy shows that this process creates regularly sized nanoparticles on the surface of EG, while X-ray photoelectron spectroscopy confirms the chemical identity of these nanoparticles as ZnO. DFT calculations provide the following mechanism for nanoparticle growth. First, the exposure to AO leads to epoxide functionalities on EG. Then, exposure to DEZ leads to two subsequent oxygen atom abstractions by DEZ from the surface to form diethoxy-zinc (DEoZ) that either docks to remaining epoxy sites or reacts with other precursors. In further AO and DEZ cycles, the epoxy functionalization of EG and oxygen atom abstraction by DEZ continues, in addition to the growth of existing ZnO clusters. The structural integrity of the underlying EG upon ZnO growth is confirmed by Raman spectroscopy. The nanoparticle growth can be controlled using AO exposure (it determines the amount of available oxygen on the surface of EG) or temperature (an optimum growth temperature is expected: higher temperature is required for faster oxygen abstraction, whereas lower temperature suppresses precursor/intermediate desorption from surface). The method can be generalized to a wide variety of related surface reactions on graphene.

Growth of ZnO on Gold (Unpublished)

Atomic clusters of well-defined elemental composition and structure can be synthesized by ALD. To achieve this feat, gold substrates are functionalized with alkane-thiol monolayers to create substrates that are largely unreactive. Functional groups can be introduced on a substrate to provide nucleation sites for ALD growth. DFT reliably predicts the growth, structure, electronic and catalytic properties of clusters. Considering the gamut of available ALD precursors, we are able to computationally design ALD processes that can immediately be tested experimentally.