Infrared laser deposition toward exploration of thin films and new materials

Abstract

[Introduction]

There has been growing interest in “soft materials”, which have potential to solve global energy and environmental problems facing our daily life. They include dye-compounds, organic semiconductors and inorganic-organic hybrid perovskites for solar cell devices, polymer gels and ionic liquids as electrolyte for secondary batteries, and metal borohydrides as hydrogen storage media. Since most of these soft materials are implemented in a thin film form into the devices, a new vacuum deposition process lowering the “activation barrier” against the first attempt to fabricate thin films of soft-materials, particularly those of newly-synthesized ones, is highly required in the R&D levels. This background is behind a recent attention to “infrared (IR) laser deposition”, a vacuum deposition process that has been newly developed in our research group for soft-materials deposition [1].

[Infrared laser deposition]

The first plot type of our IR laser deposition system appeared around in the early 2000s, whose deposition is based on the thermal process. At that time, there was the other vacuum deposition system with IR laser by a different group, which was called “IR laser deposition” as well, but more precisely, “picosecond resonant IR laser deposition”, used for polymer deposition, and rather based on the ablation-like process [2]. Figure shows a schematic of our IR laser deposition system with ellipsometer to monitor the deposition amount. The set-up is almost the same as that of pulsed laser deposition with UV laser (UV-PLD), which is popularly used for deposition of ceramics films, such as oxides and nitrides. Compared with resistive heating-type evaporators such as Knudsen cells, the IR laser deposition in the similar way to the UV-PLD allows its deposition to be efficiently controlled by digital (on/off) operation of the laser and source materials to be easily changed by “target” exchange without breaking the vacuum.

[Applications]

IR laser deposition has been found useful for deposition of various kinds of soft-materials, spanning low molecular weight organic semiconductor and EL compounds [1, 3], inorganic-organic hybrid perovskites [4] and metal borohydrides [5, 6], as well as ionic liquids [7, 8]. The IR laser deposition is also applicable to the deposition of ionic solids such as alkali-halides [9] and halide perovskites [10]. Furthermore, the compact nature of the system has led to the development of various in situ IR laser deposition systems combined with synchrotron XRD [11] and IR absorption spectroscopy [12, 13].

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