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Based on a proven low temperature scanning tunneling microscope (STM) platform, we have integrated a QPlus sensor, which employs a quartz tuning fork for force detection in non-contact atomic force microscopy (AFM). For combined STM operation, this sensor has key advantages over conventional sensors. For quantitative force spectroscopy on insulating thin films or semiconductors, decoupling of the tunneling current and the piezo-electrically induced AFM signal is important. In addition, extremely low signals require the first amplification stage to be very close to the sensor, i.e. to be compatible with low temperatures. We present atomic resolution imaging on single-crystal NaCl(100) with oscillation amplitudes below 100 pm (peak-to-peak) and operation at higher flexural modes in constant frequency shift (df) imaging feedback. We also present atomic resolution measurements on MgO(100) and Au(111), and first evaluation measurements of the QPlus sensor in Kelvin probe microscopy on Si(111) 7 x 7.

Two-dimensional atomically flat sheets with a high mechanical flexibility are very attractive as ultrathin membranes but are also inherently challenging for microscopic investigations. We report on a method using Scanning Tunneling Microscopy (STM) under ultra-high vacuum conditions for non-indenting low-force spectroscopy on micrometer-sized freestanding graphene membranes. The method is based on applying quasi-static voltage ramps with active feedback at low tunneling currents and ultimately relies on the attractive electrostatic force between the tip and the membrane. As a result a bulge-test scenario can be established. The convenience and simplicity of the method relies on the fact that the loading force and the membrane deflection detection are both provided simultaneously by the STM. This permits the continuous measurement of the stress-strain relation. Electrostatic forces applied are typically below 1 nN and the membrane deflection is detected at sub-nanometer resolution. Experiments on single-layer graphene membranes with a strain of 0.1% reveal a two-dimensional elastic modulus E = 220 N m.

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Two-dimensional atomically flat sheets with a high flexibility are very attractive as ultrathin membranes but are also inherently challenging for microscopic investigations. We report on a method using Scanning Tunneling Microscopy (STM) under ultra-high vacuum conditions for large-scale mapping of several-micrometer-sized freestanding single and multilayer graphene membranes. This is achieved by operating the STM at unusual parameters. We found that large-scale scanning on atomically thin membranes delivers valuable results using very high tip-scan speeds combined with high feedback-loop gain and low tunneling currents. The method ultimately relies on the particular behavior of the freestanding membrane in the STM which is much different from that of a solid substrate.