It has been well established that in organic thin film transistors (OTFTs), charge transport occurs within the first few monolayers of the semiconductor at the semiconductor/dielectric interface. Understanding and engineering the semiconductor-dielectric is therefore critical. Large discrepancies in performance, even with seemingly identical surface treatments, indicate that additional surface parameters must be identified and controlled in order to optimize OTFTs. Here, we used the Langmuir-Blodgett technique to study the effect of an octadecylsilane dielectric modification layer on OTFT performance. We found a crystalline, dense OTS monolayer promotes two-dimensional growth in a variety of organic semiconductors. Mobilities as high as 5.3 cm²/Vs and 2.2 cm²/Vs were demonstrated on crystalline OTS for C ₆₀ and pentacene, respectively. Finally, we also developed a simple, scalable spin-coating method to produce crystalline OTS. This work represents a significant step towards a general approach for morphological control of organic semiconductors which is directly linked to their thin film transistor performance.