All-<i>in Situ</i> Molecular-Templated Atomic Layer Deposition for Volatile Organic Compound Sensors

Molecular-templated atomic layer deposition (ALD) is defined as a deposition process in the presence of target molecules on surfaces. The resulting nanocavities after removal of the template molecules have unique molecular recognition abilities and are promising for various applications, including volatile organic compound sensors. However, few studies have investigated the nanocavity formation process, mainly due to its complexity. In particular, the complicated reconstructions of metal oxide surfaces in solution-phase processes have hindered facile control of the molecular template formation process. Here, we developed a molecular-templated ALD system performed entirely in the gas phase. The key to this system is a QCM chip covered by a metal oxide (ZnO) nanowire array, which amplifies the QCM signals to enable minute amounts of molecular monitoring. The suppression of metal oxide (TiO2) deposition in the ALD low cycle region (less than 20 cycles) was confirmed by QCM, indicating that the molecule significantly affects the formation of metal oxide nanosurfaces. The effect of the template molecule on the nanosurface formation in this region was also suggested by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Furthermore, the molecular capture ability after removing the template molecule was investigated by exposure to hexanal vapor. The amount of molecular adsorption was dependent on the ALD cycle number, with the highest value obtained near the cycle number, where the template molecule influenced the ALD growth mode. These results suggest that TiO2 grew around the template molecule to form a nanostructure influenced by the presence of the molecule, enhancing its ability to capture molecules of similar size as the template molecules. The technology developed in this study is expected to pave the way for the development of molecular sensors that can selectively adsorb and detect specific substances in gas mixtures.