Elemental Fingerprinting of Materials with Sensitivity at the Atomic Limit

Abstract

By using synchrotron X-rays as a probe and a nanofabricated smart tip of a tunneling microscope as a detector, we have achieved chemical fingerprinting of individual nickel clusters on a Cu(111) surface at 2 nm lateral resolution, and at the ultimate single-atomic height sensitivity. Moreover, by varying the photon energy, we have succeeded to locally measure photoionization cross sections of just a single Ni nanocluster, which opens new exciting opportunities for chemical imaging of nanoscale materials.

Hard X-ray beam damage study of monolayer Ni islands using SX-STM

“…However, such small changes of the island shape can also occur due to room temperature thermal diffusion of Ni atoms at island step edges without X-ray illumination. In addition to the topographic images, chemical contrast images [12] were obtained simultaneously. Figure 2f shows a chemical contrast image at a photon energy of 8.55 eV, obtained after 10 hours and 58 minutes of continuous illumination.…”

“…When the sample is illuminated by synchrotron X-rays, photoelectrons can be generated [14]. Because photoejected electrons from the sample can interfere with the conventional tunneling conditions of the STM [15], a nanofabricated "smart tip" was utilized, covered with layers of insulating and conductive coatings [12]. The smart tip focuses electron detection solely near the apex region of the scanning tip where the main interaction with the sample occurs, minimizing the background electrons collected at the sidewalls of the tip [16,17].…”

“…Recently, synchrotron X-ray scanning tunneling microscopy (SX-STM) showed the capability to measure twodimensional real space information of monolayer thick metal islands during X-ray illumination [12]. Here we present a study of potential hard X-ray beam-induced damage of monolayer Ni islands on a Cu(111) substrate using SX-STM.…”

Ultra-high vacuum compatible optical chopper system for synchrotron x-ray scanning tunneling microscopy

“…Recently, elemental contrast with 2 nm lateral resolution and sensitivity at the single atomic layer limit has been presented [8]. The contrast in SX-STM is obtained as a result of the differences in photo-absorption between different materials on a surface.…”

“…A specialized sharp tip [9,10] that is rastering over the sample surface during x-ray illumination detects x-ray enhanced currents. These currents can provide localized magnetic [11,12] and elemental contrast [8,13], because the specialized tip limits the detection of photo-ejected [6,14] and tunneling [3] electrons to a nanoscale region at the tip apex. Additionally, an electronic filter (topo filter) is used to separate the x-ray induced current signal and the conventional tunneling current signal into two separate channels simultaneously [15].…”