Combined low-temperature scanning tunneling/atomic force microscope for atomic resolution imaging and site-specific force spectroscopy

Abstract

We present the design and first results of a low-temperature, ultrahigh vacuum scanning probe microscope enabling atomic resolution imaging in both scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) modes. A tuning-fork-based sensor provides flexibility in selecting probe tip materials, which can be either metallic or nonmetallic. When choosing a conducting tip and sample, simultaneous STMMC-AFM data acquisition is possible. Noticeable characteristics that distinguish this setup from similar systems providing simultaneous STM/NC-AFM capabilities are its combination of relative compactness (on-top bath cryostat needs no pit), in situ exchange of tip and sample at low temperatures, short turnaround times, modest helium consumption, and unrestricted access from dedicated flanges. The latter permits not only the optical surveillance of the tip during approach but also the direct deposition of molecules or atoms on either tip or sample while they remain cold. Atomic corrugations as low as 1 pm could successfully be resolved. In addition, lateral drifts rates of below 15 p d h allow long-term data acquisition series and the recording of site-specific spectroscopy maps. Results obtained on Cu(1 1 1) and graphite illustrate the microscope's performance.

Simultaneous Measurement of Multiple Independent Atomic-Scale Interactions Using Scanning Probe Microscopy: Data Interpretation and the Effect of Cross-Talk

“…38 All measurements have been performed at Let us mention at this point that the term "cross-talk" is frequently used in the scanning probe microscopy community to refer to an electrical coupling between the tunneling current and frequency shift data channels during simultaneous STM/ NC-AFM measurements in certain tuning-fork based microscope designs as a result of stray capacitance, 39 which is avoided when separate electrical connections for the collection of tunneling current and frequency shift are employed. 37 As such, it should be emphasized that, in this work, we strictly refer to topography-feedback-induced cross-talk between data channels, rather than the specific electrical coupling issue mentioned above. Moreover, the "phantom force" effect has also been observed to result in a significant influence of the tunneling current on detected frequency shift values during simultaneous STM/NC-AFM measurements performed on samples with limited conductivity, for example, semiconductors, which is thus not relevant for our sample surface.…”

“…36 Combined STM/NC-AFM measurements in the dynamic STM mode (i.e., at constant average tunneling current) have been carried out using a home-built low-temperature microscope operating in UHV described in detail elsewhere. 37 Electrochemically etched Pt/Ir and W tips attached at the end of quartz tuning forks have been employed to collect the tunneling current and frequency shift data. 38 All measurements have been performed at Let us mention at this point that the term "cross-talk" is frequently used in the scanning probe microscopy community to refer to an electrical coupling between the tunneling current and frequency shift data channels during simultaneous STM/ NC-AFM measurements in certain tuning-fork based microscope designs as a result of stray capacitance, 39 which is avoided when separate electrical connections for the collection of tunneling current and frequency shift are employed.…”

3D Force Field Spectroscopy

“…Specifically, measurements performed at room temperature as opposed to low temperatures (corresponding to liquid nitrogen or helium temperatures) may typically feature drift rates of several Å per minute. On the other hand, operation at, e.g., liquid helium temperatures may severely reduce drift rates (as low as a few Å per day, [23]). Despite the impractically large drift rates typically experienced by scanning probe microscopes operating at room temperature, the associated problems have been largely overcome in recent years by the introduction of atom-tracking and feedforward positioning methods.…”

“…As such, the technique requires either low temperature operation [23] or the utilization of atom-tracking/feedforward methodologies [24,25] for proper execution.…”

“…While the DFS technique allows the acquisition of single interaction curves on defined lattice sites as indicated, converting the method into a comprehensive tool capable of collecting full three-dimensional (volumetric) maps of interaction forces and energies with atomic resolution took considerable effort due to limitations in terms of various experimental factors including tip and sample stability, as well as thermal drift. Eventually, thanks to advancements in instrumentation and experimental methodology such as low temperature operation [23] and atom-tracking/feedforward methods [24,25], the method of three-dimensional atomic force microscopy (3D-AFM) was established, implementation of which has now successfully been demonstrated by various research groups on a variety of sample surfaces [26][27][28][29][30][31][32][33][34][35][36][37]. Originally developed for vacuum conditions, the method has recently been extended to operation under liquid environments, thereby opening up tremendous possibilities for high-resolution investigation of biological material under close-to-natural conditions [38][39][40].…”