High resolution imaging of molecular assemblies on Si(111):B by non-contact AFM and Kelvin probe spectroscopy at low temperature
Résumé
We studied by non-contact AFM (nc-AFM) the formation of molecular self-assemblies on the passivated surface of boron doped silicon B-Si(111)-(V3xV3) R30°. The investigated molecule (1-(4’cyanophenyl)-2,5-bis(decyloxy)-4-(4’-iodophenyl)benzene) possesses two aliphatic chains attached to a triphenyl core ended with two different terminations (either iodine or cyano group). The use of a passivated semiconductor substrate enables creating regular and extended structures without significant change in electronic properties of molecules. Scanning tunneling microscopy and nc-AFM imaging have been performed using a low-temperature (AFM/STM (JT AFM/STM, SPECS) operated at T=4K with high stiffness Kolibri sensors (k=540 kN/m, f0=1 MHz). The growth of a periodic molecular network is observed, formed by parallel lines made by molecule aromatic cores and interdigitated aliphatic chains placed between adjacent rows (see Figure 1). We obtain submolecular resolution in the constant height f images without intentional tip functionalization, but only by conditioning the tip on the silicon surface [4]. We will discuss the high-resolution AFM imaging, as well as the conformation of the molecules in the observed assemblies (e.g. changes in the interdigitated aliphatic chains ordering), and in particular the role of the molecule dipoles. Kelvin probe spectroscopy images with sub-molecular resolution will be presented.