Localization of the phantom force induced by the tunneling current

The phantom force is an apparently repulsive force, which can dominate the atomic contrast of an AFM image when a tunneling current is present. We described this effect with a simple resistive model, in which the tunneling current causes a voltage drop at the sample area underneath the probe tip. Because tunneling is a highly local process, the areal current density is quite high, which leads to an appreciable local voltage drop that in turn changes the electrostatic attraction between tip and sample. However, Si(111)-7×7 has a metallic surface state and it might be proposed that electrons should instead propagate along the surface state, as through a thin metal film on a semiconducting surface, before propagating into the bulk. In this paper, we first measure the phantom force on a sample that displays a metallic surface state [here, Si(111)-7×7] using tips with various radii. If the metallic surface state would lead to a constant electrostatic potential on the surface, we would expect a direct dependence of the phantom force with tip radius. In a second set of experiments, we study H/Si(100), a surface that does not have a metallic surface state. We conclude that a metallic surface state does not suppress the phantom force, but that the local resistance Rs has a strong effect on the magnitude of the phantom force

Amplitude dependence of image quality in atomically-resolved bimodal atomic microscopy

In bimodal FM-AFM, two flexural modes are excited simultaneously. The total vertical oscillation deflection range of the tip is the sum of the peak-to-peak amplitudes of both flexural modes (sum amplitude). We show atomically resolved images of KBr(100) in ambient conditions in bimodal AFM that display a strong correlation between image quality and sum amplitude. When the sum amplitude becomes larger than about 200 pm, the signal-to-noise ratio (SNR) is drastically decreased. We propose this is caused by the temporary presence of one or more water layers in the tip-sample gap. These water layers screen the short range interaction and must be displaced with each oscillation cycle. Further decreasing the sum amplitude, however, causes a decrease in SNR. Therefore, the highest SNR in ambient conditions is achieved when the sum amplitude is slightly less than the thickness of the primary hydration layer.Comment: 3000 words, 3 Figures, 3 supplimentary figure

Revealing a spatially inhomogeneous broadening effect in artificial quantum structures caused by electron-adsorbate scattering

What defines the lifetime of electronic states in artificial quantum structures? We measured the spectral widths of resonant eigenstates in a circular, CO-based quantum corral on a Cu(111) surface and found that the widths are related to the size of the corral and that the line shape is essentially Gaussian. A model linking the energy dependence with the movement of single surface electrons shows that the observed behavior is consistent with lifetime limitations due to interaction with the corral walls

The effect of sample resistivity on Kelvin probe force microscopy

Kelvin probe force microscopy (KPFM) is a powerful technique to probe the local electronic structure of materials with atomic force microscopy. One assumption often made is that the applied bias drops fully in the tip-sample junction. We have recently identified an effect, the Phantom force, which can be explained by an ohmic voltage drop near the tip-sample junction causing a reduction of the electrostatic attraction when a tunneling current is present. Here, we demonstrate the strong effect of the Phantom force upon KPFM that can even produce Kelvin parabolae of opposite curvature

Identifying the atomic configuration of the tip apex using STM and frequency-modulation AFM with CO on Pt(111)

We investigated the atomic structure of metal tips by scanning individual CO molecules adsorbed on Pt(111) using scanning tunneling microscopy (STM) and frequency-modulation atomic force microscopy (FM-AFM). When scanning very close over a CO molecule, the frontmost atoms of the tip can be individually resolved in both the FM-AFM image and in the STM image. This is in contrast to previous work where CO was adsorbed on a different substrate: Cu(111). In this previous study, individual atoms could not be observed in the raw STM image but only in FM-AFM. We discuss the mechanisms behind the higher spatial resolution in STM. On Cu(111), the occupied surface state plays a large role in STM images near the Fermi level, and as adsorbed CO repels the surface state, it appears as a wide trough in STM images. In contrast, Pt(111) lacks an occupied surface state and an adsorbed CO molecule appears as a peak. We investigate if CO bending strongly influences the STM images, concluding that the atomic resolution of the tip over Pt(111) is due to highly localized through-molecule tunneling and CO bending is insignificant for contrast formation. Modelling the current between the CO and front atoms of the tip supports our findings

From a free electron gas to confined states: A mixed island of PTCDA and copper phthalocyanine on Ag(111)

When perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) is deposited on the Ag(111) surface at submonolayer coverage, it forms islands under which the native Shockley state of the Ag(111) surface can no longer be found. Previous work has shown that this state shifts upwards to form a new interface state starting at 0.6 V above the Fermi level, having properties of a two-dimensional electron gas (2DEG). We investigated mixed islands of PTCDA and copper phthalocyanine (CuPc) to study the change in the electronic state with the addition of an electron donor. We no longer observe a 2DEG state and instead identify states at 0.46 and 0.79 V. While one state appears in dI/dV images as an array of one-dimensional quantum wells, our analysis shows that this state does not act as a free electron gas and that the features are instead localized above individual PTCDA molecules

Revealing buckling of an apparently flat monolayer of NaCl on Pt(111)

Platinum is relatively reactive, compared to silver and copper, which prompted us to study the growth and structure of a thin insulating layer on Pt(111). We grew monolayer islands of NaCl and studied them with scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM images of the islands revealed a square lattice which we confirmed, via density functional theory (DFT) calculations, to be the Cl anions, similar to other surfaces. Surprisingly, however, the AFM images appeared to only resolve approximately two-thirds of the Cl ions. DFT calculations showed that the adsorption heights of the Cl anions above the surface have a bimodal distribution in which they are either approximately 60 pm above the Na ionic plane or 40 pm lower. An electrostatic model shows that this structure reproduces the AFM observations

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

One mode of atomic force microscopy (AFM) is frequency-modulation AFM, in which the tip is driven to oscillate at its resonance frequency which changes as the tip interacts with the surface. Frequency-modulation lateral force microscopy (FM-LFM) is the variant of this technique in which the tip is oscillated along the surface. For an isolated adsorbate on a flat surface, the only signal in FM-LFM is caused by the short-range interaction with the adsorbate. Various deconvolution methods exist to convert the observed frequency shift into the more physically relevant parameters of force and energy. While these methods are often used for FM-AFM data, the high number ofinflection points of FM-LFM data make standard deconvolution methods less reliable. In this article, we present a method based on Fourier decomposition of FM-LFM data and apply it to data taken of an isolated CO molecule on the Pt(111) surface. We probe the potential energy landscape past the potential energy minimum and show how over an adsorbate, the potential energy can be evaluated with a single FM-LFM image