Morphological Control of PbS Grown on Functionalized Self-Assembled Monolayers by Chemical Bath Deposition

We have investigated the chemical bath deposition (CBD) of PbS on functionalized alkanethiolate self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The deposition mechanism involves both cluster-by-cluster and ion-by-ion growth. The dominant reaction pathway and the chemical composition and morphology of the deposited layer are dependent on both the SAM terminal group and the experimental conditions. On −COOH-terminated SAMs, three types of crystallites are observed: nanocrystals formed by heterogeneous ion-by-ion growth, larger needle-like particles, and ∼2 μm particles deposited by homogeneous cluster-by-cluster deposition. The nanocrystals nucleate at Pb²⁺-carboxylate surface complexes, and so strongly adhere to the substrate. On −OH- and −CH₃-terminated SAMs, only the micrometer-sized particles are formed by a cluster-by-cluster deposition mechanism. These particles do not adhere strongly to the SAM surface and can be easily removed. SIMS and XPS analyses indicate that the larger needle-like crystals and micrometer-sized particles are composed of oxidized lead sulfide and lead oxides, while the nanocrystals are composed of ≥85% PbS. Using sonication-assisted CBD, we demonstrate that PbS is deposited by ion-by-ion growth alone on −COOH-terminated SAMs. The deposited film is more compact with a smaller grain size and is >90% PbS.