Photophysical properties of Eu3+ complexes approaching electronic contact to a metal surface

Abstract

The application of rare-earth complexes in electrically driven light sources poses a series of challenges that require specific optimization of the molecular photophysical properties. Here, we present a report on films of three different Eu3+ complexes characterized in terms of emission spectra and fluorescence decay. We compare molecular complexes in powder form and sublimed films, in films on glass and on a metal surface, and in films of thicknesses down to less than 3 nm (< 3 ML), approaching electrical coupling. Our photoluminescence experiments supported by scanning tunneling microscopy of sub-monolayers indicate that Eu3+(trensal) complexes are less affected by sublimation and more stable on the metal surface than typical beta diketonate complexes, making them promising candidates for electroluminescence devices.

Figures (4)

• Figure 1: Structure representation of [Eu(tta)$_3$(bpy)], [Eu(btfa)$_3$(bpy)] and [Eu(trensal)] complexes.
• Figure 2: Schematic drawings of the photoluminescence (PL) set-ups. a) Two optical fibers positioned at an angle and about 0.5 mm from the sample for excitation and collection of the PL response. b) Single optical fiber positioned perpendicular to the sample at a distance of about 30 $\upmu$m.
• Figure 3: Photophysical properties of thin films of [Eu(btfa)$_3$(bpy)], [Eu(tta)$_3$(bpy)] and [Eu(trensal)] under excitation with $\lambda=375$ nm. a)-c) PL spectra of powder and films sublimed onto glass and Ag(111) with a density of $\sim$ 22.5 molecules/nm$^2$ at RT. The intensity of the PL spectra has been normalized to the integrated intensity of the $^5D_0\rightarrow\,^7F_1$ transition. Graphs shifted for clarity. d)-f) Decay of the intensity of the $^5D_0 - ^7F_2$ transition (recorded using a bandpass filter of 610 $\pm$ 10 nm) in powder and thin films on glass and Ag samples. The $\tau_{obs}$ obtained by fitting the sum of two exponential decays are included. The colors correspond to the different samples; red, orange: powder; cyan, blue: 22.5 molecules/nm$^2$ film on glass at RT, gray: 22.5 molecules/nm$^2$ film on Ag(111) at 4.5 K. The inset in f) shows an STM topography image of a [Eu(trensal)] thin film on Ag(111) scanned at -10 V, 1 pA, indicating relatively stable conductivity.
• Figure 4: (a) Close-up STM image of lattice formation on a film of [Eu(tta)$_3$(bpy)] on Ag(111) sample with unit cell indicated in red. (2.0 V, 1 pA). (b) STM image of [Eu(tta)$_3$(bpy)] on Au(111) sublimed at 185 °C. (2.0 V, 10 pA). (c) Height profile across island of [Eu(tta)$_3$(bpy)] on Ag(111) sublimed at 185 °C after degassing the crucible at 150 °C for $\sim$ 10 h. (2.0 V, 10 pA). (d) Large scale STM overview of [Eu(btfa)$_3$(bpy)] close to 1 ML of on Ag(111). (2.0 V, 10 pA) (e) Close-up STM image of lattice formation on a [Eu(btfa)$_3$(bpy)] on Ag(111) sample with unit cell indicated in red. (2.0 V, 10 pA). (f) Height profile across island of [Eu(btfa)$_3$(bpy)] (2.0 V, 1 pA). (g) Large scale STM overview of [Eu(trensal)] on Au(111). (2.0 V, 1 pA). (h) Height profile across a molecule on a [Eu(trensal)] on Au(111) sample with corresponding STM image. (2.0 V, 1 pA).