Matrices to enhance the ion yield of OLED molecules in ToF-SIMS: An interesting alchemist solution

Abstract

Since the early days of time of flight secondary ion mass spectrometry (ToF-SIMS), increasing the ion signal has been crucial. It is even more crucial when performing tandem mass spectrometry experiments. To achieve this goal, many developments have been made over the years, which are divided into two categories: instrumental development and sample modification. The latter involves sample metallization, matrix deposition, or changing the temperature of the measurement. In this study, the possibility of using matrices to enhance the signals of organic light emitting device (OLED) molecules was explored. Seven molecules commonly used in OLEDs were separately deposited on Si wafers: Alq3, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile, Ir(mppy)3, N,N′-bis(naphthalene-1-yl)-N,N′-bis(phenyl)benzidine, 2,2′,7,7′-tetra(N,N-ditolyl)-amino-spiro-bifluor (STTB), and tris(4-carbazoyl-9-ylphenyl)amine. Using the same solvent, three different matrices with different thicknesses, common in matrix assisted light desorption ionization time of flight mass spectrometry, α-cyano-4-hydroxycinnamic acid, 2,5-dihydrobenzoic acid (DHB), and dihydrochloride N-(1-Naphthyl)ethylenediamine, were sprayed on these surfaces. Spectra were acquired for all compounds and spraying conditions in static ToF-SIMS experiments for Alq3 chemical imaging and depth profiling were performed. This allowed the investigation of the fragmentation pattern of the chosen matrices in ToF-SIMS and, thus, obtained a reference for these molecules. The results show that matrices can enhance the signal of fragments of the studied molecules, for example, the signal of STTB is increased with DHB spraying. Samples sprayed only with the solvent were also prepared to verify the impact of the matrices on the signal. Spraying with the solvent alone can enhance the signal even more than the matrices up to four times in the case of Alq3. This result opens new possibilities in the field of matrix-enhanced ToF-SIMS in terms of applications and matrix choices.