Bias-dependent STM image contrast study of phenyloctadecyl ethers physisorbed onto highly oriented pyrolytic graphite

A homologous series of para-substituted phenyloctadecyl ethers (X-POEs, X = H, Cl, Br, and I) was prepared using Williamson's ether synthesis. The str ucture and purity of the ethers were confirmed using H-1 NMR and GC/MS. Sca nning tunneling microscopy (STM) images were acquired from monolayers of th e ethers which formed at the surface of highly oriented pyrolytic graphite from a 2.5 wt % solution in phenylhexane. For all four ethers, the monolaye rs displayed a contrast which varied as a function of tip-sample bias. A co mparison of STM images of the adsorbed ether molecules with electron densit y contours calculated using HyperChem suggested a bias-dependent participat ion to tunneling of individual bonding molecular orbitals (MOs). For exampl e, at biases of -0.26 to -0.70 V, STM images of I-POE resembled the highest occupied molecular orbital (HOMO) exhibiting two bright spots for the pair of lobes of the phenyl ring and one bright spot for the halogen atom. At h igher biases, contrast was observed for the phenyl ring alone (-0.8 to -1.2 V) and for the alkyl tail (-1.0 to -1.8 V) which was similar to that of th e HOMO-1 and HOMO-4 contours, respectively. Owing to the measurement of an enhanced tunneling current simultaneous with the acquisition of atomically resolved bias-dependent STM images of the X-POE adsorbates, a resonance tun neling mechanism between the tip and substrate via MOs of molecular adsorba tes adjacent to and including the HOMO is proposed.