Identification of halogen atoms in scanning tunneling microscopy images of substituted phenyl octadecyl ethers.

Abstract

A homologous series of para-substituted phenyl octadecyl ethers (X-POEs, where X = H, Cl, Br, I) was prepared using the Williamson ether synthesis and characterized by 1H NMR and GC/MS. Scanning tunneling microscopy images acquired from these ethers revealed a bias-dependent contrast corresponding to electron density contours of various X-POE molecular orbitals. Images reflecting the electron density contour of the highest occupied molecular orbital exhibited four bright spots--one for the halogen atom, two representing the pair of lobes of the phenyl ring, and one for the oxygen/alpha-carbon atoms. For each X-POE, the intensities (Zmax) of the spots for the halogen atom and the lobe of the phenyl ring closest to the halogen were measured and their ratio was calculated (Cl-POE 0.49 +/- 0.06; Br-POE 0.59 +/- 0.06; I-POE 0.75 +/- 0.07). Analysis of variance at the 95% confidence level revealed that the intensity ratios were consistent from molecule to molecule, image to image, and day to day. According to the Student t-test, the average Zmax ratios for Cl-POE, Br-POE, and I-POE are different at the 95% confidence level. In addition, they follow a trend that corresponds favorably with that of the atomic radii of the halogens. The probability of classifying a single X-POE molecule as Cl-POE, Br-POE, or I-POE is variable and depends on the magnitude of the Zmax ratio.