Distance- and Voltage-Dependence of Molecular Ion Resonance Maps Using a CO-Functionalized Tip

In scanning probe methods, knowledge about the tip apex is indispensable for reliable identification of the structure and electronic properties of surfaces and adsorbed molecules [1,2]. In particular, well-defined, CO-terminated tips are widely used for atomic resolution in atomic force microscopy (AFM) [2]. In scanning tunneling microscopy (STM), the defined geometry and electronic states [3] of a CO-tip apex enable high-resolution mapping of molecular ion resonances [4]. The measured maps reflect a mix of s- and p-wave tip symmetries which vary with both tip-sample distance [5-7] and bias voltage [8]. In this work, we study the appearance of molecular ion resonance maps of pentacene on bilayer NaCl on Cu(111) as a function of the tip-sample distance and bias voltage. With AFM, we quantify tip-sample distances [9] accessing experimentally and independently from the STM channel, this parameter that is crucial for the STM contrast. Constant-height CO-tip STM imaging reveals a transition from p- to s-wave dominated tunneling upon increasing the tip-sample distance. The contrast change is explained with the symmetry-dependent decay lengths of the respective tunneling matrix elements. A fixed s/p-wave tip ratio used in the simulations reproduces the observed distance-dependence and is well in agreement with the measurement. Our findings allow for more reliable interpretation of the observed CO-tip contrast of a molecule's electronic states. [1] P. Hapala et al., Phys. Rev. B 90, 085421 (2014). [2] L. Gross et al., Science 325, 1110 (2009). [3] A. Gustafsson et al., Phys. Rev. B 93, 115434 (2016). [4] L. Gross et al., Phys. Rev. Lett. 107, 086101 (2011). [5] M. Rohlfing et al., Phys. Rev. B 76, 115421 (2007). [6] J. Martinez-Castro et al., Commun. Mat. 3, 57 (2022). [7] N. Néel et al., J. Phys. Chem. Lett. 14, 16 (2023). [8] N. Pavlicek et al., Phys. Rev. Lett. 110, 136101(2013). [9] L. Gross et al., Science 337, 6100 (2012).