Interfacial Structure of Naphthalenetetracarboxylic Dianhydride at the Silicon Dioxide Surface

The molecular structures at buried and outer interfaces of thin films of 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) were characterized by vibrational sum frequency generation spectroscopy (VSFG). NTCDA was deposited at different thicknesses on a silicon wafer with a native oxide layer, and VSFG was measured using three different polarization combinations. Global fitting of the data set was used to determine the second-order susceptibility for molecules at both interfaces. Using the asymmetric stretching modes, NTCDA was determined to be nearly face-on at the buried silica interface with an angle of approximately 68° from the surface normal. Atomic force microscopy measurements revealed an increasing distribution of heights with increased film thicknesses that resulted in highly variable second-order susceptibilities for NTCDA at the outer (air) interface. The face-on structure of the first layer of NTCDA is in contrast to the herringbone structure in bulk crystals, and provides evidence of an interphase layer. This could hinder charge transfer at the buried interface when the semiconductor is used in field-effect transistor applications.