Marcel J. Rost

Pushing the limits of SPM

In the two decades since the invention of the scanning tunneling microscope (STM) 1 , the family of local probing techniques known as scanning probe microscopy (SPM) has come to full maturity. Nowadays, the quality with which nanoscale images can be obtained and local spectroscopic information acquired using these instruments is spectacular. In addition, the ease of use of these machines has improved so much that they have found their way into the laboratories, not just of physicists, but also chemists, biologists, and engineers.

Scanning probe microscopes go video rate and beyond

In this article we introduce a, video-rate, control system that can be used with any type of scanning probe microscope, and that allows frame rates up to 200images∕s. These electronics are capable of measuring in a fast, completely analog mode as well as in the more conventional digital mode. The latter allows measurements at low speeds and options, such as, e.g., atom manipulation, current–voltage spectroscopy, or force–distance curves. For scanning tunneling microscope (STM) application we implemented a hybrid mode between the well-known constant-height and constant-current modes. This hybrid mode not only increases the maximum speed at which the surface can be imaged, but also improves the resolution at lower speeds. Acceptable image quality at high speeds could only be obtained by pushing the performance of each individual part of the electronics to its limit: we developed a preamplifier with a bandwidth of 600kHz, a feedback electronics with a bandwidth of 1MHz, a home-built bus structure for the fas...

Scanning probe microscopy at video-rate

Recent results have demonstrated the feasibility of video-rate scanning tunneling microscopy and video-rate atomic force microscopy. The further development of this technology will enable the direct observation of many dynamic processes that are impossible to observe today with conventional Scanning Probe Microscopes (SPMs). Examples are atom and molecule diffusion processes, the motion of molecular motors, real-time film growth, and chemical or catalytic reactions. Video-rate scanning probe technology might also lead to the extended application of SPMs in industry, e.g. for process control. In this paper we discuss the critical aspects that have to be taken into account for improving the imaging speed of SPMs. We point out the required instrumentation efforts, give an overview of the state of the art in high-speed scanning technology and discuss the required future developments for imaging at video-rates.

Subsurface atomic force microscopy: towards a quantitative understanding.

Recent experiments in the field of subsurface atomic force microscopy have demonstrated that it is possible to nondestructively image micro- and even nanoparticles that are embedded significantly deep within the bulk of a sample. In order to get insights into the contrast formation mechanism, we performed a finite element analysis and an analytical study, in which we calculated the amplitude and phase variation on the surface of an ultrasound wave that has traveled through the sample. Our calculations were performed as closely as possible to the situation in the experiments to enable a (future) comparison based on our predictions. We show that Rayleigh scattering of acoustic waves accounts for the measured contrast and we verify the characteristic Rayleigh dependences. The numerical results show that the contrast is independent of the depth at which a particle is buried, whereas the analytical study reveals a 1/depth dependence. In addition, we find a large deviation in the width of the particle in the contrast at the surface when applying the numerical or the analytical calculation respectively. These results indicate the importance of both the reflections of sound waves at the sample interfaces and bulk damping, as both are treated differently in our two models.

Structural Accelerating Effect of Chloride on Copper Electrodeposition

Under the microscope: In situ, video-rate scanning-tunneling-microscopy imaging during Cu electrodeposition reveals a profound structural accelerating effect of Cl(-) on the deposition process. This effect could be present in systems with different metals and different additives. The structural accelerating effect is important for the fundamental understanding of electrodeposition phenomena and for applications in industry.

Plug “n” play scanning probe microscopy

A new concept of scanning probe microscopy allows the investigation of arbitrarily positioned and oriented, possibly curved locations situated at large and immobile objects, which cannot be isolated from the environment. The concept is based on the beetle type scanning probe microscope and uses, as a key element, magnetic forces which increase the pressure at the contacts of microscope and object. The magnetic forces are shown to greatly decrease the sensitivity of the microscope to vibrations and acoustic noise from the environment. Sufficiently large magnetic forces make the microscope operation independent from orientation and thereby relieve a decisive constraint for imaging application. The capabilities of the new concept are exemplified for a plug “n” play scanning tunneling microscope.

Subsurface-AFM: sensitivity to the heterodyne signal

Applying heterodyne force microscopy (HFM), it has been impressively demonstrated that it is possible to obtain subsurface information: 20 nm large gold nanoparticles that were buried 500 nm deep have been imaged. It is the heterodyne signal that contains the subsurface information. We elucidate, both theoretically and experimentally, the sensitivity to the heterodyne signal as a function of the tip-sample distance. This is crucial information for experiments as the distance, and therefore the sensitivity, is tunable. We show that the amplitude of the heterodyne signal has a local maximum in the attractive part of the tip-sample interaction, before it surprisingly reaches an even higher plateau, when the tip-sample interaction is repulsive. This can only be explained by a non-decreasing amplitude of the ultrasonic motion of the tip, although it is in full contact with the surface. We confirm this counterintuitive tip behavior experimentally even on a hard surface like silicon.

Beating beats mixing in heterodyne detection schemes.

Heterodyne detection schemes are widely used to detect and analyse high-frequency signals, which are unmeasurable with conventional techniques. It is the general conception that the heterodyne signal is generated only by mixing and that beating can be fully neglected, as it is a linear effect that, therefore, cannot produce a heterodyne signal. Deriving a general analytical theory, we show, in contrast, that both beating and mixing are crucial to explain the heterodyne signal generation. Beating even dominates the heterodyne signal, if the nonlinearity of the mixing element (mixer) is of higher order than quadratic. The specific characteristic of the mixer determines its sensitivity for beating. We confirm our results with both a full numerical simulation and an experiment using heterodyne force microscopy, which represents a model system with a highly non-quadratic mixer. As quadratic mixers are the exception, many results of previously reported heterodyne measurements may need to be reconsidered.

Thermodynamics of deposition flux-dependent intrinsic film stress

Vapour deposition on polycrystalline films can lead to extremely high levels of compressive stress, exceeding even the yield strength of the films. A significant part of this stress has a reversible nature: it disappears when the deposition is stopped and re-emerges on resumption. Although the debate on the underlying mechanism still continues, insertion of atoms into grain boundaries seems to be the most likely one. However, the required driving force has not been identified. To address the problem we analyse, here, the entire film system using thermodynamic arguments. We find that the observed, tremendous stress levels can be explained by the flux-induced entropic effects in the extremely dilute adatom gas on the surface. Our analysis justifies any adatom incorporation model, as it delivers the underlying thermodynamic driving force. Counterintuitively, we also show that the stress levels decrease, if the barrier(s) for adatoms to reach the grain boundaries are decreased.

Cantilever dynamics in Heterodyne Force Microscopy.

Experiments in Heterodyne Force Microscopy (HFM) show the possibility to image deeply buried nanoparticles below a surface. However, the contrast mechanism and the motion of the cantilever, which detects the subsurface signal, are not yet understood. We present a numerical study of the cantilever motion in different HFM modes using realistic tip-sample interactions. The results provide information on the sensitivity to the heterodyne signal. The parameters in our calculations are chosen as closely as possible to the situation in real experiments to enable (future) comparisons based on our predictions. In HFM both the tip and the sample are excited at slightly different ultrasonic frequencies such that a difference frequency is generated that can contain subsurface information. We calculate the amplitude and phase of the difference frequency generated by the motion of the cantilever. The amplitude shows a local maximum in the attractive Van-der-Waals regime and an even higher plateau in the repulsive regime. The phase shifts 180° or 90°, depending on the mode of operation. Finally, we observe oscillations in both the amplitude and the phase of the difference frequency, which are caused by a shift of the resonance frequency of the cantilever and an involved transient behavior.