Layer-by-Layer Growth on Ge(100) via Spontaneous Urea Coupling Reactions

We have demonstrated the layer-by-layer growth, via a urea coupling reaction between two bifunctional molecules, ethylenediamine and 1,4-phenylene diisocyanate, to form an ultrathin film on Ge(100)-2 x 1 at room temperature under vacuum conditions. The initial adsorption and subsequent growth of each layer was studied with multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy. Ethylenediamine reacts with Ge(100)-2 x 1 to produce a surface-bound amine group which is available for additional reaction. Subsequent exposure of 1,4-phenylene diisocyanate leads to a spontaneous urea coupling reaction between the surface-bound amine and the highly reactive isocyanate functional group. Three bands at 1665, 1512, and 1306 cm(-)(1) are characteristic of a urea linkage and provide evidence of the coupling reaction. The coupling procedure can be repeated in a binary fashion to create covalently bound ultrathin films at room temperature, and in the present work, we demonstrate the successful growth of four layers. In addition, we have found that an initial exposure of 1,4-phenylene diisocyanate to Ge(100)-2 x 1 produces an isocyanate-functionalized surface which, upon exposure to ethylenediamine, also forms urea linkages. This layer-by-layer deposition method provides a strategy with which to design and produce precisely tailored organic materials at semiconductor interfaces.