Lateral Force Microscopy Reveals the Energy Barrier of a Molecular Switch

Abstract

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.

Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors

“…2 shows LFM images that were obtained with sensor 1 at the torsional eigenmode and with a lateral oscillation amplitude of 28 pm for four different average tip-substrate distances. The dependence of the LFM contrast on the average tip-substrate distance 56 is rather strong as illustrated by the images in Fig. 2ad, which use the same contrast scaling.…”

“…It delivers highquality submolecular resolution images that are in good agreement with the results obtained with special qPlus sensors designed for LFM (sensor and tip rotated by 90°). 55,56 Here, the advantage is that one can easily switch between conventional bond imaging and lateral bond imaging by using either the fundamental or the torsional eigenmode of the same qPlus sensor without changing the sensor. In case of sensor 2, we observe that the image contrast of the 2 nd flexural eigenmode is affected by lateral tip oscillations.…”

Measuring sliding friction at the atomic scale

“…One example is a single copper phthalocyanine molecule on the Cu(111) surface. 17) Nominally, this molecule sits with one of its four lobes aligned to a high-symmetry direction of the underlying copper atoms. When the tip approaches, however, the molecule jumps to a rotated state.…”

Tracking the Interaction between a CO-Functionalized Probe and Two Ag-Phthalocyanine Conformers by Local Vertical Force Spectroscopy

“…The flexibility of CO was previously identified as the cause for the AFM contrast mechanism ,− as well as for the modification of apparent bond lengths , in the Pauli repulsion range of probe–adsorbate separations. The close proximity of the probe was likewise shown to impact the geometry of the adsorbate complex, such as the tilting of molecular adsorbates in the vicinity of a metal tip or different preferred orientations of an adsorbed molecule depending on the distance to a CO-terminated probe . At bonding distances, attractive forces between the tip and surface become strong enough to even irreversibly distort the structural integrity of the junction and transform it into a hybrid object. , Hence, taking into account accurate relaxations of the junction geometry as modeled within density functional theory (DFT) is crucial for the interpretation of distance-dependent interactions, , in particular for junction widths in the Pauli repulsion distance range.…”