Atomic layer deposition (ALD) has attracted interest because of its extraordinary ability to deposit ultrathin films of defined conformality even on complex three-dimensional structures. Textile electronics is one of the fields considered to profit from this new technology. So far, progress has however been hampered by the fact that the deposition of metal films requires comparatively high temperatures, destroying or damaging the thermally weak substrates such as plastics, cellulose papers and polymeric textiles in the ALD process. Low-temperature metal ALD on thermally weak substrates has so far only been achieved using highly reactive counter reactants such as plasma-generated radicals or O3, which in turn severely damage the substrate.
These drawbacks have been overcome by a new method proposed by researchers from Yonsei and Incheon National Universities, South Korea. They found and proved an effective ALD process for Pt achieved at temperatures as low as 180 °C using [(1,2,5,6-η)-1,5-hexadiene]dimethylplatinum(II) (HDMP) as a precursor and O2 as counter reactant. By using low-temperature ALD for Pt, highly conformal Pt layers were deposited onto various thermally weak substrates, such as a strand of hair, papers and cotton fibers, delivering substrates with excellent electrical properties. As a proof of concept, a high-performance capacitive textile pressure sensor was successfully fabricated by crossing two conductive cotton fibers coated with poly(dimethylsiloxane) (PDMS) perpendicularly. The textile pressure sensor provided high performances such as a high sensitivity, a high stability and an extremely low detection limit.
In their study, the researchers compared the deposition characteristics of their newly proposed HDMP precursor with the widely used MeCpPtMe3 precursor. Theoretical calculations where used to elucidate and quantify the experimental findings. The adsorption mechanism of the two precursors in the ALD was investigated by density functional theory calculations, respectively, using the ORCA program package. It turns out that the difference in molecular geometry of the two precursors, HDMP and MeCpPtMe3, results in a significant difference in their activation energies (22.6 kcal mol− 1 for HDMP versus 40.8 kcal mol− 1 for MeCpPtMe3). The transition states of both precursors assume a square pyramidal geometry that imposes steric repulsion between the ligands. Although the binding configuration of η4-1,5-hexadinene for HDMP is similar for both the molecular precursor and the transition state, η5-MeCp of MeCpPtMe3 suffers a large hindrance upon the fifth coordination to Pt and becomes η1 in the transition state, losing its aromaticity and thus exhibiting a larger endothermicity. On the other hand, the adsorption energy of the CH4-eliminated Pt fragments was shown to be only marginally more exothermic for HDMP (−11.1 kcal mol− 1) than for MeCpPtMe3 (−6.3 kcal mol − 1). This is what one would expect as the same bonds were cleaved (Pt-C and O-H) and formed (Pt-O and C-H) in either reaction. Thus, the far higher nucleation rate and shorter nucleation incubation period of HDMP as compared to MeCpPtMe3 can be attributed to the intrinsically higher reactivity of the HDMP molecule upon dissociative adsorption.