User:GERphysicist/Synchrotron x-ray scanning tunneling microscopy



Synchrotron X-ray Scanning Tunneling Microscopy (SXSTM) enables imaging of nanoscale structures with electronic, chemical, and magnetic contrast by combining scanning tunneling microscopy (STM) with synchrotron radiation. It takes advantage of core level excitations in a sample, which can be generated by high-brilliance X-ray radiation from synchrotron source facilities for the purpose of obtaining high-resolution, site-specific information. While the scanning probe provides spatial resolution, the x-ray absorption directly yields chemical, electronic, and magnetic sensitivity. The strength of x-rays is the ability to excite core electrons of a specific level by tuning the incident photon energy to the binding energy. Hence, specific excitations allow discrimination between different chemical species. The technique is based on the concept of quantum tunneling. When a conducting tip is brought very near to the surface to be examined, a bias (voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. Simultaneous irradiation of a sample by intense monochromatic X-ray beams with known energy causes characteristic current contributions to the SXSTM current signal, which modulate the conventional tunnel current.

The Development of Synchrotron Radiation-Enhanced Scanning Tunneling Microscopy
Already few years after the invention of STM, the idea of combining this technique with photon excitations was proposed. Walle et al. [21] presented the first photoassisted STM measurements utilizing a HeNe laser and a halogen lamp in 1987. The team demonstrated the feasibility of using photoexcitation on semi-insulating material for the application in STM. In 1995, exactly 100 years after Wilhelm Röntgen discovered what he later called X-rays, Tsuji et al. presented the first measurement of X-ray-excited current using STM equipment.

Concept
Proper understanding of nanoscale systems requires tools with both the ability to resolve nanometer structure and to provide detailed information about chemical, electronic, and magnetic state. Scanning probe microscopies achieve the requisite high spatial resolution; however, direct elemental determination is not easily accomplished with scanning tunneling microscopy or other scanning probe variants. Only in very specific cases do various effects lead to chemical contrast. X-ray microscopies, on the other hand, provide elemental selectivity, but currently have spatial resolution of typically only tens of nanometers. Here I propose to utilize a radically different concept for high-resolution microscopy that utilizes the detection of local x-ray interactions by a scanned probe, in which the scanning probe provides spatial resolution and x-ray absorption directly yields chemical, electronic, and magnetic sensitivity. The strength of x-rays is the ability to excite core electrons of a specific level by tuning the incident photon energy to the binding energy. Hence, specific excitations allow discrimination between different chemical species. A schematic view of the participating density of states is presented in Figure 1. The three relevant energy levels for synchrotron x-ray scanning tunneling microscopy (SXSTM) are the core-level Energy EC, the work function EΦ, and the Fermi energy EF. The sign of the applied bias Ugap determines the direction of the tunnel process. Figure 1 shows tunneling from the sample into the tip causing a tunneling current. If during tunneling the sample is simultaneously illuminated with monochromatic x-rays, characteristic absorption will arise. If the excitation energy is tuned to a specific core level energy EC, a jump-wise increase in the number of excited electrons occurs. Electrons that are excited into unoccupied levels close to EF may consequently contribute an x-ray enhanced tunnel current. In addition, excitations will also generate secondary electrons with energies larger than EF. These will eventually escape from the sample. The tunnel current then increases by  due to photoejected electrons that reach the tip. Electrons that escape from the sample without detection carry a current  away. Because of the close proximity of tip and sample, x-rays will also illuminate the tip. This causes currents  and , which describe electrons that leave the tip and reach the sample, and electrons that escape into the vacuum without detection at the sample, respectively. The resulting signal for SXSTM for a negatively biased sample is therefore:. Utilization the polarization of x-rays allows further for probing the magnetic properties of matter with nanoscale resolution. This capability has the potential to broaden and deepen the general understanding of nanomagnetism.