Fabrication of Nanometer-Scale Sensors on Scanning-Probe Microscope Tips

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New classes of scanning sensors and probes, with minimum feature sizes of approximately 10 nm, will be made on the tips of scanning-probe microscopes using a new stencil technique, which allows the direct deposition of nanostructures onto non-planar surfaces. These sensors will be used to image the nm-scale functional properties of electronic devices and materials with greatly improved resolution and sensitivity, as compared with existing techniques. They should also allow several new forms of scanning microscopy, providing images of quantities not currently accessible. The new probes will be supplied to and optimized in cooperation with several of the leading young researchers pursuing scanned-probe microscopy. Examples of the types of sensors and the research subjects that will be investigated include: (1) magnetic field sensors. Single ferromagnetic nanoparticles will be deposited on the tips of atomic-force-microscope probes, for use in magnetic force microscopy (MFM). Compared to conventional MFM probes, they will provide improved spatial resolution and less magnetic perturbation on the sample, in local studies of magnetic and superconducting devices. Giant-magnetoresistance sensors composed of a ferromagnetic / non-magnetic / ferromagnetic trilayer will also be made with 10-nm feature sizes for imaging the spatial distributions of magnetic fields. (2) bow-tie antennas for near-field optical microscopy. Metallic pads in a bow-tie configuration will be deposited at the tips of optical fibers, to serve as an antenna for optical photons, and thereby make a nm-scale optical source for use in ear-field scanning optical microscopy. This is predicted to allow greatly improved optical transmission efficiency (10's of % efficiency, as compared to 10 -3 for aluminum-coated tapered-optical-fiber sources), thus permitting improved spectroscopy and time-resolved measurements. At the same time, these antennas should provide improved spatial resolution, on the order of 10 nm rather than approximately 50 nm for conventional sources. (3) scanning single-electron-transistor electrometers. A metal particle approximately 10 nm in diameter will be deposited on the tip of a scanning tip, in tunneling contact with 2 electrodes, to make a single-electron tunneling device that will serve as a sensitive electrometer. These will be employed to probe electric-field configurations of semiconductor and metal devices while they are in operation, the effects that impurities on metal surfaces have in producing local changes in the work function, and the microscopic origin of offset charges and charge noise in single-electron tunneling devices. (4) gated scanning-tunneling-microscope tips. Two electrically separate metal electrodes will be deposited in close proximity on the same insulating tip. One will be used for conventional scanning tunneling microscopy, while an independent voltage can be applied to the second electrode so that it acts as a gate to shift the local electrostatic potential of the sample under study. This will provide a new capability for characterizing the quantum-mechanical levels participating in electron tunneling, within quantum dots and single-molecule devices. Other new types of sensors or probes that may be fabricated on the 10 nm scale include scannable thermometers, heaters, and refrigerators, sources and detectors of microwave radiation, and electronic spin-filters for use in spin-resolved scanning tunneling microscopy.