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Bottom-up Nanowires as Scanning Multifunctional Sensors

Final Report Summary - NWSCAN (Bottom-up Nanowires as Scanning Multifunctional Sensors)

NWScan has developed nanowires (NWs) as sensors in two wew types of scanning probe microscopy.

NWs are extremely tiny filamentary crystals which are built-up molecule by molecule from various materials and which are now being very actively studied by scientists all around the world because of their exceptional properties. They normally have a diameter of 100 nanometers and therefore possess only about one thousandth of a hair thickness. Because of this tiny dimension, they have a very large surface in comparison to their volume. This fact, their small mass and flawless crystal lattice make them very attractive in a variety of nanometer-scale sensing applications, including as sensors of biological and chemical samples, and as pressure or charge sensors.

In this project, we demonstrated that nanowires can also be used as force sensors in atomic and magnetic force microscopes (AFM/MFM). Based on their special mechanical properties, nanowires vibrate along two perpendicular axes at nearly the same frequency. By measuring changes in the perpendicular vibrations of a NW scanning over a sample, we can image the sample in new ways. Essentially, we can use the nanowires like tiny mechanical compasses that point out both the direction and size of the surrounding forces. This allows us to map the two-dimensional force field above a sample due to various types of small forces, including electric, magnetic, and van der Waals. Today AFM, MFM, and other force microscopies are widely used in different fields, including solid-state physics, materials science, biology, and medicine. The various different types of AFM are most often carried out using cantilevers made from crystalline Si as the mechanical sensor. Moving to much smaller NW sensors may now allow for even further improvements on an already amazingly successful technique.

We also demonstrated the use of quantum dots (QD), embedded in NWs as a scanning probes for electric fields. In proof-of-principle experiments, we read out the electric field generated by a pair of nanometer-scale electrodes using a measurement of the quantum-confined Stark. This form of scanning QD microscopy had never been demonstrated before and is promising for the mapping of single charges on surfaces, measuring individual tunneling events, and monitoring charging dynamics in few-electron and mesoscopic systems.