Vibrational sum frequency generation (VSFG) spectroscopy is used to probe the polymer−silica interface of poly(3-hexylthiophene) (P3HT) organic field-effect transistors (oFETs) in situ during device operation. The VSFG spectra exhibit dramatic changes upon charge accumulation at the buried interface. Proper modeling of the data reveals that the changes in the spectroscopic features are almost exclusively due to changes in the amplitude and relative phase of the nonresonant signal, while the P3HT alkyl CH3 and CH2 vibrations remain unperturbed. We interpret the spectroscopic data in light of vibrationally resonant, electronically resonant, and electric field dependent enhancements that occur upon oxidative doping of the P3HT, as measured by visible to mid-IR spectroelectrochemistry. Notably, we observe electric field enhancement of the VSFG signals at both positive and negative gate biases despite unipolar current−voltage responses, which we attribute to the trapping of electrons at the dielectric interface.

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.

We develop a generalized model to describe thin film interference in interface-specific nonlinear optical spectroscopies of ideal isotropic stratified systems that enables the separation of this effect from the individual interfacial nonlinear responses. The model utilizes a property of the transfer matrix formalism that allows for simplification of an arbitrary layered system to a single layer with newly defined coefficients of reflection and transmission. In addition to the already well known internal transfer coefficients that relate incident fields to internal fields, we define external transfer coefficients that describe how internally generated fields propagate out of the system. By applying the usual boundary conditions we are able to analytically describe the local and induced fields immediately adjacent to an arbitrary interface, followed by transfer of the generated fields out of the system. The model provides a complete and easily implemented approach to calculating the observables from interface-specific spectroscopies on arbitrary layered thin film systems in a concise way.

Vibrational sum frequency generation (VSFG) spectroscopy was used in conjunction with steady-state IR spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry to characterize organic semiconductor thin films that were vapor deposited on silica- and trimethoxy(octadecyl)silane (ODTMS)-functionalized silica surfaces. The growth of perylene derivative N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C(8)) was found to proceed differently on simple glass slides relative to that of native oxide on silicon and fused quartz slides. VSFG was applied to these samples to isolate structural changes that occurred specifically at the buried interface between the organic semiconductor and the silica dielectric upon thermal annealing. A model was introduced to globally fit the imide carbonyl symmetric and asymmetric interfacial spectra that included contributions from both inner and outer interfaces. The fits to the VSFG data and AFM topographic images revealed significant reordering at the outer interface on all substrates upon thermal annealing. Within the model, the spectroscopic data reported that the inner interfacial PTCDI-C(8) monolayer reoriented to a more reclined phase on bare substrates after annealing but remained essentially unchanged on ODTMS monolayers. Electrical characterization of PTCDI-C(8) field-effect transistors indicated that electron mobilities were higher on bare substrate devices but could be improved by a factor of 2 on both surface types by thermal annealing. The mobility effects were attributed to the annealing-driven coalescence of PTCDI-C(8) grain boundaries. Consistent with previous structural reports, the molecular rearrangements of the first monolayer of PTCDI-C(8) on bare substrates that were reported by VSFG spectroscopy had a noticeable impact on the device performance.

In the field of vibrational sum frequency generation spectroscopy (VSFG) applied to organic thin film systems, a significant challenge to data analysis is in the accurate description of optical interference effects. Herein, we provide experimental evidence that a model recently developed in our lab provides an accurate description of this phenomenon. We studied the organic small molecule N,N'-dioctyl-3,4,9,10-perylenedicarboximide vapor deposited as a thickness gradient on silicon wafer substrates with two oxide thicknesses and two surface preps. VSFG data were obtained using the ssp and the sps polarization combinations in the imide carbonyl stretching region as a function of organic thickness. In this first of two reports, the data are modeled and interpreted within the ubiquitous electric dipole approximation for VSFG. The intrinsic sample responses are parameterized during the fitting routines while optical interference effects are simply calculated from the model using known refractive indices, thin film thicknesses, and beam angles. The results indicate that the thin film model provides a good description of optical interferences, indicating that interfacial terms are significant. Inconsistencies between the fitting results within the bounds of the electric dipole response motivate deliberation for additional effects to be considered in the second report.