Molecular layer deposition (MLD) for lightwave control and extended applications

Abstract

Molecular layer deposition (MLD) is a process to form pure organic and organic-inorganic hybrid materials in monomolecular growth steps. MLD produces ultra-thin/conformal tailored films, in which molecules/atoms are artificially assembled in designated arrangements, on surfaces of particles, holes, trenches and rough/porous/tortuous structures, as well as flat surfaces. MLD started in the late 1980s as a process to fabricate photonic/optoelectronic (OE) materials and devices such as nonlinear optical materials and integrated optical circuits, which are required for controlling lightwaves in high-performance computers and optical communication systems, and later, further applications of MLD for solar energy conversion (sensitization, photovoltaics, photosynthesis) and biomedical/cancer therapy (molecular targeted drug delivery, laser surgery, photodynamic therapy) were proposed. After the middle of the 2000s, MLD produced organic-inorganic hybrid materials and the application fields were extended to microelectronics (lithography, diffusion barriers, insulators, semiconductors, conductors) and molecular sieving/catalysis (batteries, gas separation/water purification, dry reforming, photocatalysis). In the present paper, the fundamentals of MLD and photonic/OE applications are reviewed, with discussions on future challenges, and extended applications are summarized. Particularly, the following subjects are emphasized. For the fundamentals of MLD, to perform MLD with increased tolerance for the substrate temperature, strategy to widen the MLD window is formulated based on configurational coordinate diagrams. For photonic/OE applications, the correspondence between photon electric field E(x) and electron wavefunction ψ(x) is clarified, and it is predicted that lightwaves can be controlled by manipulating shapes/dimensionality of ψ(x) via artificial molecular arrangements produced by MLD, enabling us to fabricate high-performance nonlinear optical materials such as electro-optic (EO) polymer multiple quantum dots (MQDs). The organic MQDs are also promising in the two-photon sensitization via two-step excitation for photovoltaics/photosynthesis and the photo-assisted cancer therapy with in-situ synthesis at cancer cell sites.