The QPlus sensor allows for atomic resolution non-contact AFM while maintaining the user-friendliness and known performance level of the LT STM. Benchmark measurements on single crystal NaCl(100), KBr(001), Si(111)7x7 and Au(111) prove that resolution is competitive with the best cantilever-based AFM results. Atomic resolution in genuine df feedback and at temperatures of below 5 K can be maintained with oscillation amplitudes below 100 pm. All measurements and NaCl amd KBr have been carried out on single crystalline samples to fundamentally prove the AFM performance.
The ease of use of the QPlus sensor now makes atomic resolution AFM a routine experiment. The sensor employs a quartz tuning fork for force detection in non-contact AFM operation. One prong of the tuning fork is fixed, while the SPM probe tip is mounted to the second prong. It thus acts as a quartz lever transforming its oscillation into an electrical signal as a result of the piezoelectric effect. The feedback signal is based on the frequency shift originating from the tipsample force interaction. A dedicated and unique preamplification technique ensures distance control based on the pure AFM signal.
Wet-chemically etched tungsten tips are employed for QPlus AFM to enable its simultaneous or alternative use as a high-resolution STM probe. The ability to simultaneously monitor frequency shift and tunnelling current and to freely select one or both as the feedback signal strongly enhances experimental flexibility and opens up a variety of new experiments.