We use a nanofabricated scanning tunneling microscope tip as a detector to investigate local X-ray induced tunneling and electron emission from a single cobalt nanocluster on a Au(111) surface. The tip-detector is positioned a few angstroms above the nanocluster, and ramping the incident X-ray energy across the Co photoabsorption K-edge enables the detection of element specific electrons. Atomic-scale spatial dependent changes in the X-ray absorption cross section are directly measured by taking the X-ray induced current as a function of X-ray energy. From the measured sample and tip currents, element specific X-ray induced current components can be separated and thereby the corresponding yields for the X-ray induced processes of the single cobalt nanocluster can be determined. The detection of element specific synchrotron X-ray induced electrons of a single nanocluster opens an avenue for materials characterization on a one particle at-a-time basis.

Synchrotron X-rays are widely used in many disciplines including physics, materials science, chemistry, biology, and medicine.1–9 X-ray induced element specific electron detection is the key for many synchrotron experiments. However, spatial resolution in most synchrotron measurements is limited because it is typically determined by the size of the X-ray beam on the sample. Here, we use a synchrotron X-ray scanning tunneling microscope (SX-STM) that enables extracting pure X-ray induced electron emission current independent of the X-ray beam size. In SX-STM, X-ray excited electrons are generated when the tip-sample junction is illuminated by synchrotron X-rays. Some of these electrons are ejected from the sample, while others can modulate the tunneling current when their energies match those of unoccupied states between Fermi level and the work function of the sample [Fig. 1(a)].10–14 

FIG. 1.

(a) In SX-STM, X-ray excited electrons populate unoccupied states and then tunnel into the tip. Higher energy electrons are ejected and some are captured by the tip while others are lost to the vacuum. (b) A nanofabricated SX-STM tip. Inset shows a nanofabricated smart tip with its coating layers and a small exposed tip-apex. (c) STM image shows isolated Co nanoclusters on Au(111) surface. (d) Corresponding line profile of (c). (e) Current measured at the tip increases when the X-ray energy exceeds Co K-edge (marked by a dashed line). The change in current, ΔI, is related to the absorption cross section of Co.

FIG. 1.

(a) In SX-STM, X-ray excited electrons populate unoccupied states and then tunnel into the tip. Higher energy electrons are ejected and some are captured by the tip while others are lost to the vacuum. (b) A nanofabricated SX-STM tip. Inset shows a nanofabricated smart tip with its coating layers and a small exposed tip-apex. (c) STM image shows isolated Co nanoclusters on Au(111) surface. (d) Corresponding line profile of (c). (e) Current measured at the tip increases when the X-ray energy exceeds Co K-edge (marked by a dashed line). The change in current, ΔI, is related to the absorption cross section of Co.

Close modal

Experiments were performed at room temperature using an ultrahigh vacuum (UHV) SX-STM system15 at beamline ID-26 of the Advanced Photon Source and Center for Nanoscale Materials at Argonne National Laboratory.16 Au(111) single crystal surface was cleaned by repeated Ar+ ion sputtering and annealing cycles, and then a very low coverage of Co nanoclusters was deposited via vacuum evaporation. For the X-ray experiments, a monochromatic X-ray beam of 100 μm × 500 μm (vertical × horizontal) was illuminated the tip-sample junction. The beam passed through an optical chopper operating at 50% duty cycle with a frequency ω = 3000 Hz. In order to collect local X-ray induced electrons, a coaxial “smart” tip14 fabricated by a sharp PtIr wire coated with layers of SiO2, Ti, and Au. Only a small area of the tip-apex was exposed [Fig. 1(b)]. Both tip and sample currents were simultaneously measured, and all data shown here were collected with a bias of −1 V with respect to the sample, and a conventional STM tunneling current of 0.3 nA.

STM images show two-atomic layer high17 Co nanoclusters sparsely populated on Au(111) surface [Figs. 1(c), 1(d)]. To confirm the presence of Co nanoclusters, the tip is initially placed ∼400 nm above the surface static, and the X-ray energy is varied from 7696 to 7717 eV while the tip current is monitored. When the X-ray energy exceeds the Co K-edge energy of 7709 eV, a sudden increase in X-ray induced emission current is observed [Fig. 1(e)], thereby confirming the presence of Co on the surface. Since the tip is located in the far-field, i.e., it is not in the electron tunneling distance, the current increase is caused by the electrons emitted from an ensemble average of Co on the surface. Such current versus energy (I-E) spectroscopy is related to the X-ray-absorption cross section of the material, and it is akin to conventional X-ray absorption spectroscopy (XAS)9,18 except that an SX-STM tip is used here as a local detector.

To detect X-ray induced electrons from a single nanocluster, the tip is positioned just a few Angstroms above (near-field) the sample surface. Now, electrons from the surface can tunnel into the tip, and the measured current is the convolution of the X-ray induced currents and the conventional STM tunneling current. We have developed a topological filter that enables separation of the X-ray induced currents and the STM tunneling current.19 Because the X-ray beam is chopped, the X-ray induced currents in the tip (or sample) can be extracted by lock-in amplifiers (LIAs), and the resulting LIA outputs are comprised exclusively of X-ray induced current components. In the near-field, the X-ray induced currents include both emission and X-ray excited electron tunneling components (Fig. 2). The X-ray energy range used for the experiments, from 7680 eV to 7756 eV, is just below and above the Co K-absorption edge energy of 7709 eV, respectively, which does not match other absorption edge energies of the tip and sample materials (supplementary material). Therefore, only electrons originated from Co will cause a sudden increase in current when the X-ray energy exceeds the Co K-edge. Two energy regimes with respect to the Co K-edge, pre-edge (7680 eV < E < 7709 eV) and post-edge (7709 eV < E < 7756 eV), are considered. For each energy regime, the sample and tip currents are analyzed on the bare Au(111) surface [Fig. 2(a)] and on the Co nanocluster [Fig. 2(b)].

FIG. 2.

(a) When the tip is on Au(111), ISample is produced by emitted electrons (1,2), received electrons from the tip (3), and X-ray excited tunneling of background electrons (4). ITip is generated by emitted electrons (4) and received electrons (1,4). (b) When the tip is above a Co nanocluster, element specific X-ray excited tunneling (5) occurs above the K-edge. Electrons emitted from the Co nanocluster are also captured (6). (c) In “I-E” spectroscopy, the tip is fixed above the surface and records the current while the X-ray energy is varied. I increases when the X-ray energy exceeds the K-edge. (d) In “I-d” spectroscopy, the X-ray energy is fixed and the tip records local current changes across a lateral distance. Below K-edge energy, the Ipre edge may or may not change when crossing the Au-Co boundary. Above K-edge energy, Ipost edge increases when crossing the Au-Co boundary.

FIG. 2.

(a) When the tip is on Au(111), ISample is produced by emitted electrons (1,2), received electrons from the tip (3), and X-ray excited tunneling of background electrons (4). ITip is generated by emitted electrons (4) and received electrons (1,4). (b) When the tip is above a Co nanocluster, element specific X-ray excited tunneling (5) occurs above the K-edge. Electrons emitted from the Co nanocluster are also captured (6). (c) In “I-E” spectroscopy, the tip is fixed above the surface and records the current while the X-ray energy is varied. I increases when the X-ray energy exceeds the K-edge. (d) In “I-d” spectroscopy, the X-ray energy is fixed and the tip records local current changes across a lateral distance. Below K-edge energy, the Ipre edge may or may not change when crossing the Au-Co boundary. Above K-edge energy, Ipost edge increases when crossing the Au-Co boundary.

Close modal

The X-ray beam simultaneously illuminates the tip and the sample and hence both eject electrons generate background currents (supplementary material). However, the X-ray excited tunneling currents, ItunnelAu and ItunnelCo, are generated only locally when the tip is above the bare Au(111) surface [Fig. 2(a)] and above a Co nanocluster [Fig. 2(b)], respectively. In the pre-edge regime, X-ray induced emission and X-ray excited tunneling are mainly originated from background excitations. The sample currents when the tip is on Au(111) and on the Co nanocluster can be described as

Isample,Au(pre)=Ibackgroundsample+ItunnelAu,
(1)
Isample,Copre=Ibackgroundsample+ItunnelCo.
(2)

Here, Ibackgroundsample is the background emission current in the pre-edge regime. In the post-edge regime, where the X-ray energy is above the Co K-edge, element specific electrons are generated by the Co nanoclusters. Because the X-ray energy is ramped over the Co K-edge, there are almost no changes in ItunnelAu. However, when the tip is above the Co nanocluster, an additional current channel (ItunnelCo*) is now opened due to the X-ray excited tunneling. The sample currents in the post-edge regime can be described as

Isample,Au(post)=Ibackgroundsample*+ItunnelAu,
(3)
Isample,Co(post)=Ibackgroundsample*+ItunnelCo+ItunnelCo*,
(4)

where “*” denotes the involvement of element specific electrons. ΔI for the sample current when the tip is above Au(111) can be determined by subtracting (1) from (3), which cancels out ItunnelAu, as

ΔIsample,Au=Ibackgroundsample*Ibackgroundsample.
(5)

By subtracting (2) from (4), ΔI for the sample current when the tip is on the Co nanocluster is obtained

ΔIsample,Co=Ibackgroundsample*Ibackgroundsample+ItunnelCo*,
(6)

Subtracting (5) from (6) cancels all the background currents and exclusively gives the element specific X-ray excited tunneling current originated from the Co nanocluster, ItunnelCo*

ItunnelCo*=ΔIsample,CoΔIsample,Au.
(7)

Similarly, the change in the tip currents between the pre-edge and post-edge regimes measured on Au(111) and on the Co nanocluster (supplementary material) can be expressed as

ΔItip,Au=Ibackgroundtip*Ibackgroundtip.
(8)

When the tip is above the Co nanocluster, in addition to the X-ray excited tunneling, the tip also captures X-ray induced electrons ejected by the cluster, IcapCo*

ΔItip,Co=Ibackgroundtip*IbackgroundtipItunnelCo*IcapCo*.
(9)

The negative signs represent the currents produced by the electrons that arrive at the tip. Subtracting (9) from (8) eliminates all the background tip currents and yields ItunnelCo* +IcapCo*. Since ItunnelCo* is known from the measured sample currents, IcapCo* can be determined.

To measure the X-ray excited element specific electrons, we use two experimental schemes, “I-E” and “I-d.” The “I-E” spectroscopy determines the tip/sample currents as a function of photon energy [Fig. 2(c)], from which ΔI is deduced. The “I-d” spectroscopy measures the tip/sample currents as a function of lateral distance [Fig. 2(d)] reflecting local changes in current components depending on the type of materials. For the measurements, an isolated Co cluster on the Au(111) surface is chosen [Fig. 3(a)]. The spectroscopic measurements are performed along a line [Fig. 3(b)] crossing the boundary between the Au(111) and Co nanocluster with a step size of ∼6.25 Å. During the experiment, the tip is maintained at a constant height (constant current scanning mode) from the surface (∼0.5 nm) utilizing the topological filter. Figure 3(c) presents an “I-E” spectroscopy data for the tip currents measured on Au(111) surface and on the Co nanocluster as a function of energy. As expected, the tip current on Au(111) does not change by varying photon energy from 7680 eV to 7756 eV. However, the tip current on the Co nonocluster increases when the X-ray energy exceeds the Co K-edge. From “I-E” spectroscopic data acquired at each location [Fig. 3(c)], ΔItip and ΔIsample are determined. Both ΔItip and ΔIsample rapidly increase in the Co nanocluster with ΔItip having a larger increase than ΔIsample [Fig. 3(d)]. After the tip passing the Co nanocluster edge, the tip height continues to increase [Fig. 3(a) and supplementary material]. Since STM measures the local density of states (LDOS), the increase in the tip height here is due to an increase in the LDOS. Consequently, more local states are available for X-ray excited tunneling process. Accordingly the X-ray excited tunneling current increases as the tip moves from the edge to the center on top of the nanocluster in Fig. 3(d). As shown in Eq. (7), the change in ΔIsample over the Co nanocluster (∼6% increase) is generated by the X-ray induced tunneling, ItunnelCo*, and it is measured to be 3.1 ± 0.4 pA. The increase in ΔItip is given by IcapCo*+ItunnelCo*, and it is determined as 4.0 ± 0.5 pA. Thus IcapCo* is ∼0.9 pA. From these measured currents, it is apparent that a large portion of the X-ray induced current is provided by X-ray excited tunneling of element specific electrons. Therefore X-ray excited tunneling is a crucial process for the elemental contrast in SX-STM.

FIG. 3.

(a) A 3-D STM image of a Co nanocluster used to investigate X-ray induced currents. (b) I-E spectroscopic data are measured along the dashed line. (c) I-E data measured at fixed locations on Au(111) (blue dots) and on Cu nanocluster (red dots). An increased current is observed on the Co nanocluster when the energy exceeds the Co K-edge. (d) ΔI as a function of distance corresponds to the sample (orange) and tip (green) currents. Here, “0” is the edge of Co nanocluster, and the negative distances are on Au(111) while the positive distances correspond to the tip on the Co nanocluster.

FIG. 3.

(a) A 3-D STM image of a Co nanocluster used to investigate X-ray induced currents. (b) I-E spectroscopic data are measured along the dashed line. (c) I-E data measured at fixed locations on Au(111) (blue dots) and on Cu nanocluster (red dots). An increased current is observed on the Co nanocluster when the energy exceeds the Co K-edge. (d) ΔI as a function of distance corresponds to the sample (orange) and tip (green) currents. Here, “0” is the edge of Co nanocluster, and the negative distances are on Au(111) while the positive distances correspond to the tip on the Co nanocluster.

Close modal

For SX-STM investigations of nanostructures, it is desirable to determine the X-ray induced electron yields. The measured photon flux of the synchrotron X-ray beam in our experiment is 2.5 × 1013 photons s−1 mm−2. From the Co nanocluster area of 25.6 nm2 with two atomic-layer height, the number of Co atoms in the cluster can be estimated. Then, the rate of photons incident on a Co atom is estimated as 1.35 photons/s. From the measured ItunnelCo* and IcapCo*, the yields for the element specific X-ray excited tunneling electrons, and X-ray induced ejected electrons are determined to be 1.4 × 104 and 4.2 × 103 electrons/photon per Co atom, respectively. Note here that not all of the X-ray photons hitting a Co atom are being adsorbed on the specific atom. The large number of x-ray induced electrons demonstrates that not just primary electrons, but also a cascade of secondary electrons is responsible for generating the currents that enable SX-STM measurements although it is not possible to distinguish these two components.

In conclusion, we have employed a nanofabricated STM tip to detect synchrotron X-ray induced currents from a single Co nanocluster on a Au(111) surface at room temperature. Spatial dependent X-ray induced absorption is measured at the atomic scale between a Au(111) surface and a Co nanocluster boundary with a tip detector located ∼5 Å above the surface. By tuning the incident X-ray photon energies to core level electron binding energies, we are able to observe element specific X-ray induced currents. Moreover, from simultaneously measured tip and sample currents, we are able to separate individual current components corresponding to X-ray induced emission and X-ray excited tunneling. The ability to measure X-ray induced currents from a single nanoscale cluster opens up material characterization using synchrotron X-rays to study quantum processes occurring at individual nanoclusters including quantum confinement effects. The study of single molecules and even single atoms may become a possibility in the near future.

See supplementary material for X-ray induced tip and sample current components, elemental specific X-ray induced currents, SX-STM tip, topological filter, and extraction of currents and the height profile of the Co cluster.

This work was funded by the Office of Science Early Career Research Program through the Division of Scientific User Facilities, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant No. SC70705. H.K. and S.W.H. acknowledges the support of U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-FG02-02ER46012 for the SX-STM spectroscopic data analysis. Use of the Advanced Photon Source and the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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Supplementary Material