Photonic crystal slabs (PCSs), which generally consist of two-dimensional arrays of nanoholes in the top layer of a dual layer dielectric film, have been demonstrated as a promising platform for optical biosensing. Both the Fano resonance in a perfect PCS and the Lorentzian resonance in a micro-cavity resulted from an introduced defect in PCS have been studied. While, the use of resonance peak shift for detecting molecules owing to the change of the refractive index is a nonspecific biosensing technique. Biorecognition molecules, such as antibodies that can specific bond to interesting molecules, are conjugated on the PCS to improve the detection specificity. It is a widely adopted assumption that the conjugated molecules form into a uniform nanofilm in the PCS based biosensors, which covers either the entire surface of the dielectric layer or the entire sidewalls of nanoholes. However, the actual device performance is much lower than that obtained based on this assumption, which suggests the over-simplicity of the hypothesis above. It is of keen interest to reveal the actual arrangement and distribution of molecules on PCS for designing high-performance PCS biosensors. Here, we propose models and analysis of the distribution of nanofilms on PCS. We employed Raman scattering technique to experimentally reveal the actual various configurations of nanofilms, which support our theoretical modeling. The results obtained in this research can be essential for designing high-performance PCS based nanobiosensors.