Simulated vibrational sum frequency generation from a multilayer thin film system with two active interfaces

Abstract

In the field of surface-specific vibrational sum frequency generation spectroscopy (VSFG) on organic thin films, optical interferences combined with the two-interface problem presents a challenge in terms of qualitative assessment of the data and quantitative modeling. The difficulty is amplified when considering systems comprised of more than a single material thin film layer. Recently, in our lab we have developed a generalized model that describes thin film interference in interface-specific nonlinear optical spectroscopies from arbitrary multilayer systems. Here, we apply the model to simulate VSFG spectra from the simplest multilayer: a system of two thin films, one of which is an organic small molecule and the other is a dielectric layer on a semiconductor substrate system where we idealize that the organic interfaces are equally VSFG active. Specifically, we consider the molecule N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) deposited on a silicon wafer with a thermally grown oxide dielectric. We present results for the four polarization experiments that sample the nonzero nonlinear susceptibility elements of macroscopically centrosymmetric materials (ssp, sps, pss, and ppp) and in two mIR frequency windows (the imide carbonyl stretches around 1680 cm(-1) and the alkyl stretches around 2900 cm(-1)) as a function of both thin film thicknesses with fixed input beam angles. We use frequency dependent refractive indices for all materials. The goal is to illustrate some of the intricacies contained in the intensity data of such systems. Of particular interest is the effect of the relative polar orientation of modes at the interfaces and the possibility of designing a system where the collected signal is exclusively attributable to a single interface. Our calculations indicate that in order to unambiguously identify the relative polar orientation one must experimentally vary an additional system parameter such as thin film thickness or input beam angle and for quantitative modeling one cannot ignore either interfacial contribution. The results show that proper modeling of thin film interference effects is essential for accurate data analysis and should include the frequency dependent refractive indices, especially for modes with larger mIR absorption cross sections, even when absorptive losses are small.

Experimental evidence for an optical interference model for vibrational sum frequency generation on multilayer organic thin film systems. I. Electric dipole approximation

“…By using the thin film interference model, we were able to show that the thicknesses of the thin films should have a profound effect on the detected signal intensity. 58,59 Herein, we present experimental evidence that demonstrates that the multilayer thin film model provides a good description of optical interference effects in VSFG spectra of thin film systems. We continue emphasis on our model system: the organic semiconductor N,N ′ -dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C 8 ) vapor deposited as a thickness gradient on silicon wafer substrates (Fig.…”

“…The resulting optical constants agreed with previous results for this material in this spectral range. 59 The thin film thicknesses are shown in Table I. These were used in the multilayer VSFG model.…”

“…59 In that study, we discussed the feasibility of quantifying molecular structure at the buried interface of an organic thin film by specifically considering the two-interface problem applied to the simplest multilayer, a system comprised of two thin films in which an organic layer is vapor deposited onto a thermally grown oxide dielectric on a silicon wafer substrate. By using the thin film interference model, we were able to show that the thicknesses of the thin films should have a profound effect on the detected signal intensity.…”

“…In the case of OFETs, which are almost always multilayered, these questions are prominent issues that we have focused on addressing with a generalized multilayer interference model. 58,59 For any interface-specific nonlinear optical spectroscopy, the detected signal will depend on the local electric field amplitudes at each active interface as well as on the emitted waves, all of which are subject to thin film optical interference effects. For a system comprised of only a single thin film, the description of optical interference in VSFG data is tractable by considering the Fresnel formulas for reflection and transmission at interfaces.…”

Rearrangement of the molecular orientation of Alq3 in organic light-emitting diodes under constant current aging investigated using sum frequency generation spectroscopy

“…This signal strength difference corresponds well with the difference in the thickness of Alq 3 layer in the OLEDs. Note that to evaluate the precise thickness dependence of the SFG signal intensity variations, multiple reflection effects should be taken into account [11]. However, neglecting the effects of multiple reflection, this result clearly indicates that the intensities of the SFG signal strengths of Alq 3 are correlated with the thickness of the Alq 3 layer, and it does not come from the orientation of the molecules at the interface.…”