Submolecular Insights into Interfacial Water by Hydrogen-Sensitive Scanning Probe Microscopy

Exploring in-plane interactions beside an adsorbed molecule with lateral force microscopy

Atomic force microscopy with a CO-functionalized tip can be used to directly image the internal structure of a planar molecule and to characterize chemical bonds. However, hydrogen atoms usually cannot be directly observed due to their small size. At the same time, these atoms are highly important, since they can direct on-surface chemical reactions. Measuring in-plane interactions at the sides of PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) molecules with lateral force microscopy allowed us to directly identify hydrogen atoms via their repulsive signature, which we confirmed with a model incorporating radially symmetric atomic interactions. Additional features were observed in the force data and could not be explained by H-bonding of the CO tip with the PTCDA sides. Instead, they are caused by electrostatic interaction of the large dipole of the metal apex, which we verified with density functional theory. This calculation allowed us to estimate the strength of the dipole at the metal tip apex. To further confirm that this dipole generally affects measurements on weakly polarized systems, we investigated the archetypical surface adsorbate of a single CO molecule. We determined the radially symmetric atomic interaction to be valid over a large solid angle of 5.4 sr, corresponding to 82°. We therefore find that in both the PTCDA and CO systems, the underlying interaction preventing direct observations of H-bonding and causing a collapse of the radially symmetric model is the dipole at the metal apex, which plays a significant role when approaching closer than standard imaging heights.

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (diw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T1) of the OH stretching mode at 3200 cm-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine diw. By investigating the T1 values at the SiO2/NaCl solution interface, we established a time-space (T1-diw) relationship. We find that water has a T1 lifetime of ≥0.5 ps for diw ≤ 3 Å, while it displays bulk-like dynamics with T1 ≤ 0.2 ps for diw ≥ 9 Å. T1 decreases as diw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK15β protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T1 values.