Evaluating the potential energy landscape over single molecules at room temperature with lateral force microscopy

Abstract

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.

A Fourier method for estimating potential energy and lateral forces from frequency-modulation lateral force microscopy data

“…In the FM-LFM image, shown in figure 3(D), the magnitude of the Δf is much higher and a new feature can be seen in the center. This additional sharp decrease in Δf in the center is a signature of a repulsive feature [21].…”

“…In the simultaneously-acquired FM-LFM image the CO appears as a dark-brightdark feature along the direction of the tip oscillation. This signature is indicative of attractive interaction between the tip and sample [21].…”

“…The problem with models based, e.g. on Morse interactions [17] or on a sum of Gaussian functions [21] is that they are not generally applicable. This has prompted us to implement a force and energy deconvolution method that is applicable for potential energy curves that are periodic.…”

Biaxial atomically resolved force microscopy based on a qPlus sensor operated simultaneously in the first flexural and length extensional modes

“…There, it was easier to control the LFM mode with the tip in the interaction range to the surface, and thus, it was possible to realize 1F-FM-LFM measurements. We prepared a Si(111)-7 × 7 surface by a standard cleaning procedure of flashing up to 1350 ○ C for a few seconds with an electron beam heater while keeping the pressure during heating Typically, LFM images show a more complex structure 35,36 as shown in Fig. 6(c).…”

Determining amplitude and tilt of a lateral force microscopy sensor

“…Also in 2002, Giessibl and co-workers performed LFM using a qPlus sensor as shown in Figure 1b [ 8 ]. In our group, we have used this method to quantify molecular stiffness at low temperature [ 9 ] and to evaluate the potential energy landscape above a molecule at room temperature [ 10 ]. More recently, we used LFM with a CO-terminated tip to investigate the internal structure of a molecular adsorbate [ 11 – 12 ].…”