Argonne National Laboratory

Synchrotron X-ray Scanning Tunneling Microscopy

Schematic illustration of the synchroton X-ray scanning tunneling microscopy (SX-STM) technique, where a smart tip is used as the detector and the tip-sample junction is illuminated by monochromatic synchrotron X-rays. 

The smart tip consists of a metallic core that tunnels over the sample surface, an insulating film to reduce the photoelectron background, and a conductive shield to prevent charging of the tip. (b) Simultaneously obtained topography and elemental contrast SX-STM characterizations of a nanoscale nickel (Ni) cluster on copper (Cu). [Image courtesy of SPIE]

Schematic illustration of the synchroton X-ray scanning tunneling microscopy (SX-STM) technique, where a smart tip is used as the detector and the tip-sample junction is illuminated by monochromatic synchrotron X-rays. The smart tip consists of a metallic core that tunnels over the sample surface, an insulating film to reduce the photoelectron background, and a conductive shield to prevent charging of the tip. (b) Simultaneously obtained topography and elemental contrast SX-STM characterizations of a nanoscale nickel (Ni) cluster on copper (Cu). [Image courtesy of SPIE]

Specialized smart tips and topographic filters are used to fully achieve high spatial resolution and high chemical sensitivity imaging with synchrotron X-ray scanning tunneling microscopy.

Synchrotron light sources contribute uniquely to many research fields, including physics, chemistry, materials science, biology, medicine, geology, and even the arts. Synchrotron X-ray scanning tunneling microscopy (SX-STM) is an imaging technique combining the best of two worlds: the exceptional chemical, magnetic, and structural sensitivity of X-rays combined with the unparalleled ability of scanning probe microscopy to resolve and manipulate surfaces at the single-atom level. A joint effort between the Center for Nanoscale Materials and the Advanced Photon Source recently developed the technique of synchrotron X-ray scanning tunneling microscopy with innovations including specialized smart tips and topographic filters.

With these advances, we have demonstrated direct elemental imaging on the atomic scale. For instance, a lateral resolution of 2 nm and a vertical resolution of a single atomic layer (i.e., the ultimate limit) at room temperature has been achieved. Polarized X-rays also can be employed to simultaneously probe the magnetic, elemental, chemical, and topographic properties of a surface.

To make the SX-STM technique available to the wider scientific community, the CNM and APS are constructing a dedicated synchrotron beamline branch at Sector 4 of the APS. This branch will be known as XTIP and will become operational in 2018. Until then, we provide limited beamtime to users for early-science SX-STM experiments. These experiments will focus on the study of chemical and magnetic properties of nanoscale materials using SX-STM at photon energies between 500 to 2500 eV.

In addition, a newly developed low-temperature SX-STM will provide researchers access to a one-of-a-kind instrument. By pairing the capabilities of synchroton X-ray analysis with extremely precise microscopy in XTIP, we will offer a new way to simultaneously determine the physical structure, chemical makeup, and magnetic properties of materials at close-to-atomic scales. In turn, this will open up unique ways of exploring novel phenomena and the fabrication of next-generation nanoscale materials.