Copper phthalocyanine (CuPc) is a small molecule often used in organic light emitting diodes where it is deposited on a conducting electrode. Previous scanning tunneling microscopy (STM) studies of CuPc on Cu(111) have shown that inelastic tunneling events can cause CuPc to switch between a ground state and two symmetrically equivalent metastable states in which the molecule is rotated. We investigated CuPc on Cu(111) and Ag(111) with STM and lateral force microscopy (LFM). Even without inelastic events, the presence of the tip can induce rotations and upon closer approach, causes the rotated states to be favored. Combining STM measurements at various temperatures and LFM measurements, we show that the long-range attraction of the tip changes the potential energy landscape of this molecular switch. We can also determine the geometry of the rotated and ground states. We compare our observations of CuPc on Cu(111) to CuPc on Ag(111). On Ag(111), CuPc appears flat and does not rotate. Stronger bonding typically involves shorter bond lengths, larger shifts of energy levels, and structural stability. Although the binding of CuPc to Cu(111) is stronger than that on Ag(111), the nonplanar geometry of CuPc on Cu(111) is accompanied by two metastable states which are not present on the Ag(111) surface.
One of the challenges of AFM, in contrast to STM, is that the measured signal includes both long-range and short-range components. The most accurate method for removing long-range components is to measure both on and off an adsorbate and to subtract the difference. This on-off method is challenging at room temperature due to thermal drift. By moving to a non-contact scheme in which the lateral component of the force interaction is probed, the measurement is dominated by short-range interactions. We use frequency-modulation lateral force microscopy to measure individual PTCDA molecules adsorbed on Ag/Si(111)-(3×3). By fitting the data to a model potential, we can extract the depth and width of the potential. When the tip is closer to the sample, a repulsive feature can be observed in the data.
Sliding friction is a nonconservative force in which kinetic energy is dissipated via various phenomena. We used lateral force microscopy to measure the energy loss as a tip oscillates laterally above a surface with sub-Angstrom amplitudes. By terminating the tip with a single molecule, we ensure the tip ends in a single atom. We have reported that energy is dissipated as a CO molecule at the tip apex is oscillated over pairs of atoms. This is a result of the CO being bent in different directions as the tip moves in one direction and then in the other. We confirm this with a model that describes the CO on the tip as a torsional spring. Surprisingly, we only observe dissipation within a small range of tip heights. This allows us to determine the necessary components to model friction and shows how sensitive friction is to the local potential energy landscape.
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