Yields and images of secondary ions from organic materials by different primary Bi ions in time-of-flight secondary ion mass spectrometry

Rationale Bi cluster ions are used as a source of primary ions for time-of-flight secondary ion mass spectrometry (TOF-SIMS), and it has been recognized that secondary ion yields of macromolecules are higher with Bi cluster ions than with monomer ions or other cluster ions such as Cs⁺, Ga⁺ and Au_n⁺. However, the analysis conditions of Bi cluster TOF-SIMS are not sufficiently established. This study provides information on the secondary ion yields, damage cross-section and spatial resolution obtained with different primary Bi ions. Methods We investigated the secondary ion yields, damage cross-section and spatial resolution using three different primary Bi ions in TOF-SIMS. The primary ions selected were Bi₁⁺, Bi₃⁺ and Bi₃²⁺ that were accelerated with 25 kV and the positively charged secondary ions were analyzed. The samples were 1, 2-distearoyl-sn-glycero-3-phosphocholine (C₄₄H₈₈NO₈P, DSPC), which is a typical lipid, and N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (C₄₄H₃₂N₂, NPD) and 4,4',4"-tris[2-naphthyl(phenyl)amino]triphenylamine (C₆₆H₄₈N₄, 2-TNATA), which are organic functional materials. Results Although the secondary ion yields of DSPC were highest when measured with Bi₃⁺, the spatial resolution obtained from all DSPC analyses could not be evaluated because of the low intensity of the secondary molecular ions. On the other hand, for both NPD and 2-TNATA, the secondary ion yields were highest when imaged with Bi₃²⁺. Also, we obtained the highest spatial resolution using Bi₃²⁺. In the analysis of all molecules, the damage cross-section obtained with Bi₃²⁺ was also the highest. Conclusions When secondary ions were sensitively detected, images of the high spatial resolution were obtained by using Bi₃²⁺. On the other hand, when the secondary ion sensitivity was low, the spatial resolution depended on the yields of secondary ions, implying that the selection of the primary ion species is crucial for SIMS analysis of large molecules.