Deposition of Chiral Heptahelicene Molecules on Ferromagnetic Co and Fe Thin-Film Substrates

Nanomaterials 12 (2022) 3281

Mohammad Reza Safari, Frank Matthes, Karl-Heinz Ernst, Daniel E. Bürgler and Claus M. Schneider

We study the deposition of chiral heptahelicene by sublimation under ultra-high vacuum onto bare Cu(111), Co bilayer nanoislands on Cu(111), and Fe bilayers on W(110) by low-temperature spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS).

In all cases, the molecules remain intact and adsorb with the proximal phenanthrene group aligned parallel to the surface. Three degenerate in-plane orientations on Cu(111) and Co(111), reflecting substrate symmetry, and only two on Fe(110), i.e., fewer than symmetry permits, indicate a specific adsorption site for each substrate. Heptahelicene physisorbs on Cu(111) but chemisorbs on Co(111) and Fe(110) bilayers, which nevertheless remain for the sub-monolayer coverage ferromagnetic and magnetized out-of-plane. We are able to determine the handedness of individual molecules chemisorbed on Fe(110) and Co(111), as previously reported for less reactive Cu(111). The demonstrated deposition control and STM/STS imaging capabilities for heptahelicene on Co/Cu(111) and Fe/W(110) substrate systems lay the foundation for studying CISS in ultra-high vacuum and on the microscopic level of single molecules in controlled atomic configurations.

Deposition on Co(111)

(a) Constant-current topographic image of heptahelicene on Co nanoislands on Cu(111). (b) Simultaneously with (a) measured dI/dV map at -600mV (V_bias = -600 mV, I_t = 900 pA, V_mod = 20 mV, f_mod = 752 Hz, 5 K, Co-functionalized W tip). (c) High-resolution topographic image of the red marked area in (a) after subsequent Gaussian high-pass filtering. Dashed blue and red circular arrows indicate the handedness and in-plane adsorption orientation of the [7]H molecules (V_bias = 1000 mV, I_t = 100 pA, 5 K).

Deposition on Fe(110)

(a) Constant-current topographic image of heptahelicene on Fe/W(110). (b) The dI/dV map measured simultaneously with (a) reveals that out-of-plane magnetized domains occur in bilayer regions also in the presence of heptahelicene. (c) High-resolution topographic image of heptahelicene on an unperturbed Fe bilayer region after applying Gaussian high-pass filtering. From these data, the molecular handedness and adsorption orientation can be determined, as exemplified by dashed blue and red circular arrows in the inset. Data have been measured at V_bias = 50 mV, I_t = 300 pA, V_mod = 10 mV, f_mod = 875 Hz, 5 K, and by means of a Fe-functionalized W tip for (a,b) and a non-magnetic W tip for (c).