Perovskite and Ionic Liquid Thin Films

2D IR spectroscopy experiments on monolayers or thin films are experimentally challenging. These very thin samples have small signals, which are orders of magnitude smaller than typical bulk samples. In the traditional 2D IR spectroscopy based on BOXCARS geometry, such a small signal can be sensitively detected by interfering the signal field with a small external local oscillator field. However, the data acquisition rate is limited by the slow mechanical motion of the delay lines. In the 2D IR spectroscopy based on pump-probe geometry, the data acquisition rate is remarkably faster and the acquired data is phase-stable; the disadvantage is that the local oscillator field in this geometry is much larger than the signal field and cannot be adjusted relative to the signal field, as the third probe pulse itself is a local oscillator. Consequently the interference in the local oscillator field caused by a small signal from a thin layer is very small, making it hard to detect.

Figure 1. Near-Brewster’s angle reflection pump-probe geometry. When the incident angle for the probe beam is close to Brewster’s angle, the reflected probe is low in amplitude, and can be regarded as a heavily attenuated local oscillator. As a result, the signal-to-local oscillator ratio is enhanced.

Typically, 2D IR experiments based on pump-probe geometry are carried out in the transmission mode because for thick samples that is the only direction in which the signals from different depths in the sample will constructively interfere. However, for thin samples, there is also constructive interference in the reflected direction (Figure 1). As a result, 2D IR experiments on thin samples can be carried out in the reflection mode. The reflection mode provides an opportunity for enhancing the signal that is not possible in the transmission mode. The amplitude of the reflected signal is similar to the amplitude of the transmitted signal. However, the amplitude of the reflected local oscillator can be adjusted by making the probe beam p-polarized and adjusting the incident angle. As the incident angle approaches Brewster’s angle, the amplitude of the reflected local oscillator decreases toward zero; by choosing an incident angle close to Brewster’s angle, the local oscillator amplitude can be significantly reduced. This results in a greater modulation of the local oscillator by the signal field, leading to an enhanced signal. Using this technique, the modulation can be enhanced by a factor exceeding 50 (Figure 2).

Figure 2. 2D IR spectra of C11-Re(phen)(CO)3Cl monolayer acquired in transmission mode (above) and near-Brewster’s angle reflection mode (bottom). The data quality in the reflection geometry is greatly enhanced. In the p-polarized reflection geometry, the sign of the signal depends on whether the incident angle is larger or smaller than Brewster’s angle.

We have used this technique to study the dynamics in thin films; one such sample is the hybrid organic-inorganic perovskite (CH3NH3)2Pb(SCN)2I2. Perovskites are interesting materials because they have potential applications as photovoltaics. Thin films of perovskite were prepared by spin-coating onto SiO2-coated CaF2 windows. The thiocyanate ion (SCN-) is a suitable vibrational probe, and so the dynamics of the perovskite structure were investigated directly by probing the thiocyanate ions in the structure.

We are also studying thin films of room temperature ionic liquids (RTILs). RTILs are ionic salts that are liquids at ambient temperatures. RTILs have good solvation properties, as well as low vapor pressures and good thermal stability, which make them useful for various chemical applications. An interesting question is how thin a layer of an RTIL must become in order for its structure and dynamics to significantly differ from that of the bulk RTIL. To investigate this, we are studying thin films of the RTIL BmimNTf2 (mixed with small amounts of BmimSeCN so that SeCN- can be used as the vibrational probe). Thin films of BmimNTf2 with various thicknesses in the range 50-200 nm were prepared by spin-coating; however, this method was successful only after the SiO2-coated CaF2 substrate was chemically modified to attach an ionic monolayer with a structure very similar to the RTIL’s. We are studying how the dynamics of the RTIL depend on the thickness of the film, and in the future we plan to compare different RTILs to see if they respond differently to being confined in thin layers.

Relevant Publications

460. "Enhanced Nonlinear Spectroscopy for Monolayers and Thin Films in Near-Brewster’s Angle Reflection Pump-probe Geometry," Jun Nishida, Chang Yan, and Michael D. Fayer J. Chem. Phys. 146, 094201 (2017). [SI]