chapter 14 new capabilities at the interface of x-rays and...

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Introduction

Nanoscale structures are at the forefront of fundamental research as well as the keystone for whole new classes of potential applications. Beyond doubt, the ability to manipulate and organize matter at small length scales together with the utiliza-tion of functional structures has enormous potential to change society. It is antici-pated that global research efforts will lead to new medical treatments, more efficient energy production, lighter and stronger materials, advances in agriculture, and powerful nanoelectronics [1], and these are just a few of the more significant ways in which people are discussing the use of nanotechnology. Certainly, nanoscale science and engineering will be an essential component in gaining a better under-standing and control of nature in the next decades. The fascination of nanoscience and nanotechnology is driven by the fact that nanoscale structures often exhibit novel physical, chemical, and biological properties, substantially different from those displayed by bulk materials. The study of these new, emerging phenomena will facilitate an exciting new voyage of discovery.

However, proper understanding of nanoscale systems though requires tools with both the ability to resolve nanometer structure and to provide detailed infor-mation about chemical, electronic, and magnetic state. Scanning probe microsco-pies achieve the requisite high spatial resolution; however, direct elemental determination is not easily accomplished with scanning tunneling microscopy (STM) or other scanning probe variants. Only in very specific cases do various

Chapter 14New Capabilities at the Interface of X-Rays and Scanning Tunneling Microscopy*

Volker Rose, John W. Freeland, and Stephen K. Streiffer

V. Rose (*) Advanced Photon Source Argonne National Laboratory, Argonne, IL 60439, USA e-mail: [email protected]

* The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. DE-AC02-06CH11357 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare deriva-tive works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

S.V. Kalinin and A. Gruverman (eds.), Scanning Probe Microscopy of Functional Materials: Nanoscale Imaging and Spectroscopy, DOI 10.1007/978-1-4419-7167-8_14, © Springer Science+Business Media, LLC 2010

406 V. Rose et al.

effects lead to chemical contrast [2, 3, 4]. X-ray microscopies, on the other hand, provide elemental selectivity, but currently have spatial resolution of typically only tens of nanometers [5, 6].

Here we describe 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 differ-ent chemical species. The combination of the spatial resolution of STM with the energy selectivity afforded by X-ray absorption spectroscopy (XAS) provides a powerful analytical tool.

In Section “Basic Interactions of X-Rays with Matter” of this review, we will first outline the basic interactions of X-rays with matter. The fundamental concepts of electron tunneling under X-ray illumination are discussed in Section “The Physics of X-Ray-Enhanced Scanning Tunneling Microscopy.” Several groups worldwide are working on the integration of STM and synchrotron-based radiation. An historical outline and important achievements are highlighted in Section “The Development of Synchrotron Radiation-Enhanced Scanning Tunneling Microscopy.” The utilization of STM in combination with synchrotron-based radiation necessi-tates the development of specialized insulator-coated tips, which is the subject of Section “Fabrication of Insulator-Coated Smart Tips.” Generally, we distinguish between experiments in which the tip/sample separation lies within the tunneling regime (near field), and separations that exceed the tunneling distance for STM (far field). Photoelectron detection in the far field is discussed in Section “Photoelectron detection using a scanning tunneling microscope”, and considerations about near-field studies are presented in Section “X-ray assisted scanning tunneling micros-copy”. Concluding remarks about the new capabilities at the interface of X-rays and STM are recapitulated in Section “Concluding Remarks.”

Basic Interactions of X-Rays with Matter

There are five basic interactions of X-rays with matter: elastic (coherent) scattering, Compton scattering, the photoelectric effect, pair production, and photodisintegra-tion. The latter two are only possible at very high photon energies (>1.02 and > 8MeV, respectively), and thus, are not further considered here. In an elastic scatter-ing event (Fig. 14.1a), also called Rayleigh scattering, an X-ray photon is elastically scattered, i.e., only the direction of the photon changes. Ionization does not occur. An event in which the incident X-ray photon loses energy (increases its wavelength) and is deflected from its original path by an interaction with an electron is called Compton scattering (Fig. 14.1b). The photon continues its travel on an altered path, while simultaneously the electron containing the remaining energy is ejected. The photoelectric effect is a quantum electronic phenomenon in which electrons are

40714 New Capabilities at the Interface of X-Rays and Scanning Tunneling Microscopy

emitted from matter after the absorption of photons (Fig. 14.2a). The probability of the photoelectric effect occurring depends on a number of factors. Obviously, the incident photon must have energy greater than the binding energy of the electron that gets ejected. The X-ray absorption coefficient for photoabsorption decreases smoothly with increasing photon energy. However, when the photon energy reaches one of the deep inner-shell ionization energies of an atom, a sharp jump (absorption edge) marks the opening of an additional photoabsorption channel. The removal of an inner shell electron produces a vacancy, which either results in an Auger cascade (Fig. 14.2b) or an X-ray emission (Fig. 14.2c) by the following mechanisms.

Fig. 14.1 (a) An incident X-ray photon is elastically scattered. Ionization does not occur. (b) In Compton scattering, an incident X-ray photon is deflected from its original path by an interaction with an electron. An electron is ejected from its orbital position and the X-ray photon loses energy

Fig. 14.2 Schematic representations of the photoelectric effect. (a) An incident photon ejects a photoelectron. The photon must have an energy greater than the binding energy of the electron. Subsequently, there are two competing paths for energy dissipation. (a) The vacancy is filled by a second atomic electron from a higher shell, and a third electron, an Auger electron, escapes car-rying the excess energy. (b) The vacancy is filled by an electron from a higher shell and an X-ray photon removes the energy

408 V. Rose et al.

In an Auger process, a second electron from a higher shell fills the inner-shell vacancy. The energy difference between the two shells must be simultaneously released. Consequently, a third electron, the Auger electron is ejected, carrying the excess energy in a radiationless process; hereby, the excited ion decays into a doubly charged ion. This is the most probable process for low-atomic number elements during which a K-level electron is ejected by the energy absorbed from the X-ray photon, an L-level electron drops into the vacancy, and another L-level electron is ejected. High-atomic number elements have LMM and MNN transitions that are more probable.

In contrast to an Auger process, a characteristic X-ray photon can remove the energy when the inner shell vacancy is filled by a higher-level atomic electron (Fig. 14.2c). The probability of a core hole in the K or L shells being filled via a radiative process, also referred to as fluorescence yield, increases with atomic number [7]. In addition to the primary photoelectrons (from the photoabsorption of the incident X-rays) and primary Auger electrons (from the de-excitation after photoionization), inelastic interactions with valence electrons of atoms can lead to the emission of secondary electrons. Such secondary photoelectrons are caused by photoabsorption of fluorescent radiation in the sample, and secondary Auger electrons from the relaxation of secondary excited atoms. Electron emission is then domi-nated by low-energy (a few electron volts) secondary electrons [8, 9].

Utilizing the polarization of X-rays allows for probing of magnetic properties of matter. Generally, in X-ray absorption an atom absorbs a photon, giving rise to

Fig. 14.3 This simplified model illustrates the origin of X-ray circular dichroism, which is caused by the excitation of spin-orbit split 2p

3/2 and 2p

1/2 levels to empty d valence states. Right circularly

polarized and left circularly polarized photons transfer an opposite angular momentum to the excited photoelectrons, and electrons with opposite spins are created in the two cases


the transition between a core level and an empty state above the Fermi level. X-ray magnetic circular dichroism (XMCD) measures the dependence of X-ray absorp-tion by a magnetic material on the helicity of the X-rays [10, 11]. It is the X-ray equivalent of the Faraday or Kerr effects, which occur in the visible range. The underlying physical mechanism of XMCD can be illustrated in 3d metal systems by a two-step model (Fig. 14.3) proposed by Stohr and Wu [12, 13]. Magnetic properties of 3d transition metals are determined primarily by their d valence elec-trons [14]. The core level is split into p

3/2 and p

1/2 states, where spin and orbit are

coupled parallel and antiparallel, respectively. In the first step, the emission with the light helicity vector parallel (antiparallel) to the 2p orbital moment results in photoelectrons of preferred spin up (down) direction. In the second step, the spin-split valence band acts as a detector for the spin of the excited electrons. Theoretical sum rules relate the integrated difference signal in absorption between left- and right circularly polarized (LCP, RCP) X-rays at the 2p absorption edges to the ground-state magnetic moment of the 3d transition metals [15, 16]. Hence, due to the dipole selection rules, final states with different symmetry can be probed by choosing the initial state.

The Physics of X-Ray-Enhanced Scanning Tunneling Microscopy

The development of the family of scanning probe microscopes was initiated by the original invention of the STM by Gerd Binnig, Heinrich Rohrer, and co-workers from the IBM Research Laboratory at Rüschlikon more than a quarter century ago [17, 18]. The STM is based on a quantum mechanical effect. When a sharp con-ducting tip is brought close to a conductive sample, a bias voltage can allow elec-trons to tunnel through the vacuum gap between them. As the tip is scanned across the sample surface, while a feedback loop keeps the tunneling current constant by adjusting the tip-sample separation. In this fashion, the STM tracks the topography of the sample, assuming that the local density of states related to the tunneling process is independent of topography. The complication is therefore obvious – more precisely, the STM measures a mixture of both topography and the local density of electronic states. The tunnel current STM

tunnelI depends exponentially on both

vacuum gap distance and the local barrier height EF. Hence, the STM delivers real-space information on the morphology and electronic structure of surfaces with atomic resolution [19]. The applied bias voltage between the tip and the sample determines the direction of the electron tunneling. When the sample is negatively biased (Fig. 14.4a), electrons tunnel from the occupied states of the sample into the unoccupied states of the tip. In the case of a positively biased sample (Fig. 14.4b), electrons tunnel from the occupied states of the tip to the unoccupied states of the sample. Only electrons close to the Fermi energy E

F contribute to the tunneling

current. Unfortunately, these electrons do not necessarily carry any direct chemical information about their originating atoms.

410 V. Rose et al.

The combination of STM with synchrotron-based X-ray radiation has the poten-tial to enable element-specific microscopy that goes beyond the well-established topographic contrast. If during tunneling a sample is illuminated with monochro-matic X-rays (Fig. 14.5), characteristic absorption will arise. When the excitation energy is tuned to a core level energy E

C, a jump-wise increase in the number of

excited electrons occurs, and core electrons are excited into unoccupied states above the Fermi level E

F. Depending on the sign of the applied bias between tip and

sample, those excited electrons might increase or decrease the conventional tunnel current STM

tunnelI . When the sample is negatively biased (Fig. 14.5a), STM

tunnelI may be

enhanced by an additional tunneling current X-ray

tunnelI that originates from the tunneling

of excited electrons through the vacuum barrier into the tip. Because the energy of the incident X-rays is well known, the increase in the total tunnel current could potentially directly reveal chemical information of the sample surface. Tunneling is also well localized; thus, high spatial resolution is obtained. When the sample is positively biased (Fig. 14.5b), the excitation of core-level electrons in the sample manifests itself as a decrease of STM

tunnelI . In this case, electrons tunnel from occupied

states of the tip to unoccupied states of the sample. Yet excited electrons are lifted into states close to E

F, diminishing the number of available empty states for tunnel-

ing. Hence, independent of the applied bias the modulation of STM

tunnelI caused by

variations in the energy of the incident X-rays allows a chemical mapping of the surface. Due to the nature of tunneling, atomic resolution may be expected.

Fig. 14.4 Bias dependence for conventional STM. (a) When the tip is positively biased, electrons tunnel from the occupied states of the sample to the unoccupied states of the tip. (b) If the tip is negatively biased, electrons tunnel from the occupied states of tip to the unoccupied states of the sample. Only electrons close to the Fermi energy E

F contribute to the tunneling. The tunnel current

STM

tunnelI depends exponentially on both vacuum gap distance and the local barrier height EF


Although the concept that we outline above takes advantage of the full potential of STM in terms of spatial resolution, so far only excitation of electrons to ener-gies below the work function EF are considered. But if an electron absorbs the energy of a photon and has more energy than EF , it is ejected. In addition, inelastic interactions in the sample can cause the emission of secondary electrons. Photo-ejected electrons will therefore always provide an additional channel for chemical mapping. Figure 14.6 shows a schematic representation for the origin of different contributions to the photocurrent. Because the separation between tip and sample is only a few nm under tunneling condition, the tip always gets illuminated in an actual experiment. Therefore, photoejected electrons are generated at the sample as well as at the tip. The photoelectrons emerge with all velocities from zero up to a more or less sharply defined maximum velocity [20]. Electrons that escape from the sample and are detected at the tip cause a current

sample

passI , while the remaining

electrons that escape without detection carry a current sample

lossI away. Simultaneously,

currents tip

passI and tip

lossI are generated, which describe electrons that leave the tip and

reach the sample, or electrons that escape into the continuum without detection at the sample, respectively. Generally, the number of photoejected electrons exhibits a sharp jump when the photon energy reaches one of the inner-shell ionization energies. This accounts for the chemical sensitivity of the photocurrent channel. It is clear that the number of electrons in each channel depicted in Fig. 14.6 strongly depends on the bias between tip and sample. In particular, secondary electron emission is dominated by low-energy electrons, which are extremely sensitive to an electric field and, hence, can in principle be varied via application of low voltages.

Fig. 14.5 Schematic representation of the local density of states for X-ray-enhanced electron tunneling. (a) Incident X-rays excite core electrons with energy E

C to unoccupied states close to

the Fermi energy EF. The conventional tunnel current STM

tunnelI is then enhanced by X-ray

tunnelI .

(b) When the sample is positively biased, electrons that are excited to states close to the EF reduce

the number of available empty states for tunneling from the tip

412 V. Rose et al.

To sum up, when a sample is illuminated with X-rays, the conventional tunnel current STM

tunnelI experiences a modulation due to an absorption-induced tunnel current X-ray

tunnelI , and

contributions caused by photocurrents. The resulting signal for synchrotron X-ray-enhanced scanning tunneling microscopy (SXSTM) at a negatively biased sample is

= + − − −SXSTM

tip tip sample STM X-raypass loss pass tinnel tunnel .I I I I I I (14.1)

In this convention, currents that arrive at the tip are counted as negative, while currents that leave the tip have a positive sign. When the sample is positively biased, the SXSTM signal is

= + − + −SXSTM

tip tip sample STM X-raypass loss pass tinnel excited ,I I I I I I (14.2)

where X-ray

excitedI denotes the reduction of tunnel current caused by the excitation of elec-

trons into unoccupied sample states close to EF (cf., hatched area in Fig. 14.5b).

After electrons are ejected from either the sample or the tip they are accelerated in the electric field generated by the bias voltage between tip and sample. If the sample is positively biased (Fig. 14.7a) photoelectrons are prone to be recollected by the sample, while the tip due to the relatively negative potential repels those photoelectrons. The situation is reversed when the sample is negatively biased (Fig. 14.7b). Thus, the sign and the size of the bias voltage allow for controlling the

Fig. 14.6 Electrons with energy greater than EF can escape from the sample and generate a pho-tocurrent, which consists of sample

passI (electrons that pass to the tip) and sample

lossI (electrons that

escape into the continuum). Because the tip always gets illuminated in an actual experiment, additional photoelectrons tip

passI and tip

passI are generated at the tip


photoelectron channel. Particularly, the detection of low-energy secondary electron emission can be tuned to meet the experimental needs.

The Development of Synchrotron Radiation-Enhanced Scanning Tunneling Microscopy

The concept of achieving elemental information with ultra-high spatial resolution by combining X-ray absorption and STM is based on two different experimental realizations. In the spectroscopy mode (Fig. 14.8), an STM tip is positioned over a well-defined surface area. Then, the energy E of the incident X-rays is scanned and the tip current I is recorded while the tip remains at the same height over the surface. Equivalently, the change of the tip height z can be recorded when I is kept constant. The spectra obtained on different surface positions directly yield local chemical information. In the imaging mode (Fig. 14.9), a region of interest is first scanned in the conventional constant-current mode. Then, the same region is scanned again under monochromatic X-ray illumination. If the X-ray energy E is tuned to an absorption edge of an elemental species that is present at the surface, the SXSTM current will be modified at those locations as described in Section “The Physics Of X-Ray-Enhanced Scanning Tunneling Microscopy.” In order to keep the current constant, the tip height z is adjusted by a feedback loop. By subtracting the previous obtained topographic scan from the one under X-ray illumination, an element-sensitive image is obtained. In the case of samples with low corrugation, an ele-ment-sensitive image can also be achieved by scanning in the constant-height mode. Here, instead of analyzing the tip height z, the tip current I is studied.

A 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 photoas-sisted STM measurements utilizing a HeNe laser and a halogen lamp in 1987. The team demonstrated the feasibility of using photoexcitation on semi-insulating

Fig. 14.7 The bias between tip and sample determines the number of photoejected electrons that arrive at the tip ( sample

passI ) or at the sample ( tip

passI ). (a) A negatively biased tip repels photoelectrons

from the sample, while (b) a positively biased tip attracts those electrons. Electrons that are ejected from the tip or sample without detection carry a current tip

lossI and sample

lossI away, respectively

414 V. Rose et al.

Fig. 14.8 Spectroscopy mode of SXSTM. The tip is positioned over the sample surface and the energy E of the incident X-rays is varied. X-ray absorption spectra are obtained by recording the tip current I at constant tip height (or tip height z at constant current) as a function of X-ray energy

Fig. 14.9 Schematic representation of the SXSTM imaging mode. A region of interest is first scanned in the conventional STM constant current mode. In a second step, the same area is scanned under X-ray illumination with energy E. The subtraction of both scans yields an element-selective image


material for the application in STM. In their accompanying discussions they already envisioned the possibility of investigating infrared-excited vibrations of adsorbed molecules and electronic states with STM. This optically-assisted spec-troscopy would distinguish between different chemical species on a surface and convey information impossible to obtain solely with STM. This idea was widely adapted in the following years and launched various research activities in the emerging field of photoassisted STM [22, 23]. We would like to mention that ongoing efforts utilizing the combination of a related technique, atomic force microscopy (AFM), and synchrotron-based radiation have likewise let to considerable advances in nanoscale characterizations [24, 25].

In 1995, exactly 100 years after Röntgen discovered what he later called X-rays, Tsuji et al. presented the first measurement of X-ray-excited current using STM equipment [26]. In this experiment, a tip was separated from a sample surface by about 600nm, where tunneling current could not be detected (far field). The sample was then directly irradiated with polychromatic X-rays from an X-ray tube, which caused a tip current that was amplified in air. From that, they concluded that the origin of this current was the electrons emitted from the sample due to the photo-electric effect. The amplification of the tip current was explained by the ionization of air molecules by emitted electrons. The influence of the presence of molecular gases was studied in more detail later on [27, 28]. It has also been shown that it is in principle possible to obtain a regular STM image under X-ray irradiation, when the tip/sample separation lies within the tunneling regime (near field) [29, 30, 31]. However, because the STM tip cannot energy analyze electrons that arrive from the sample, it was first necessary to change the energy of the incident X-rays in order to obtain unambiguous chemical information.

Ten years ago, the first experiments with monochromatic hard X-ray radiation were carried out at the Photon Factory in Tsukuba, Japan [32]. By changing the photon energy from 7.5 to 12keV, the research team was able to obtain extended X-ray absorption fine structure (EXAFS)-like and X-ray absorption near-edge structure (XANES)-like spectra of patterned Au–Ni thin-film samples with an STM tip in the far field. The experiment was performed under normal air pressure. The tip current exhibited a jump-like increase, when the X-ray energy approached the Ni K-edge or the Au L III-edge. The group also identified the important role of the tip properties in the enhancement of spatial resolution. Generally, electrons emitted from the sample surface are detected not only at the tip apex but also at the side of the tip. This is the main reason why the surface area that contributes to the measured tip current was about 2mm2 for an uncoated STM tip [27]. Therefore, in order to make the analyzing region small, the tip was next coated with an insulating polymer film that covered the sidewalls of the tip. The measurements suggested that the tip now collected electrons from a surface area of about only 1mm in diameter. In the following years, the development of “smart” tips, i.e., tips with different kinds of insulating coverage, was recognized as a key task for the success of X-ray-enhanced scanning probes.

In 2004, a group from the University of Tokyo reported on the development and trial measurement of synchrotron-radiation-light-illuminated STM [33].

416 V. Rose et al.

By measuring photoexcited electron current together with the conventional STM current, Si 2p soft XAS were obtained from a Si(111) surface in the near field. The spatial resolution of the XAS measurements by an uncoated tip has been estimated to be of the order of several 10s of microns. To improve the spatial resolution, they repeated the measurements with a polymer-coated tip [34]. A tungsten tip was covered except for an area of around 100 mm from the tip apex. The detected photo-current was reduced to about 1–10% of the current detected by a bare tip. However, measurements on microsize dots suggest that the spatial resolution was above 5 mm.

Yet another option to improve the spatial resolution is to use smaller X-ray spots. Saito et al. attempted elemental analysis by STM using a sample of Ge nanoislands on a Si(111) surface, based on the core-level interaction between highly brilliant X-rays and surface atoms [35, 36]. An uncovered STM tip made from electro-chemically etched tungsten wire was used. It appears that the ideal ambient experi-mental condition is ultra-high vacuum (UHV), rather than gases or air, to prevent the unnecessary excitation of many electrons, which cause undesired noise. A modification of the tunnel current at the Ge absorption edge was detected with a spatial resolution on the order of 10nm utilizing a hard X-ray microbeam. It is nevertheless surprising that the average tip current over the Ge island was smaller than over the Si substrate for an X-ray energy that was slightly above the Ge Ka absorption edge.

Because hard X-rays penetrate through air and beryllium windows, the analysis chamber can be disconnected from the hard X-ray flight path, and thus, the mechanical vibrations from the beamline can be eliminated. Experiments with soft X-rays are more challenging because, due to the short free mean path of the X-rays, the STM chamber has to be directly connected to the beamline. Further, the cross section of photoionization is higher for soft X-rays. Hence, employing smart tips in order to eliminate the undesired background component in the photoinduced current caused by electrons impinging at the side of the tip is essential. Utilizing a tungsten tip covered with a glass layer except for an area of less than 5 mm from the tip apex [37], a spatial resolution of about 14nm was demonstrated with soft X-rays [38]. An array of mm-size Ni dots fabricated by electron-beam lithography was used as a sample. An increase of the tip current of a few pA was observed, when the tip was located over a Ni dot. The sample bias was set to -10V in order to maxi-mize the signal detection; however, this widens the reduced-barrier area and thus deteriorates the spatial resolution. The authors concluded that the bias voltage could be adjusted to optimize both signal detection and spatial resolution when advanced synchrotrons with higher brightness are used.

A spatial resolution of around 10nm was observed using soft X-rays on a pat-terned Ni, Fe, and Au sample [39]. Okuda et al. attributed the high spatial resolu-tion to a local reduction of the surface potential barrier caused by the close proximity of tip and sample. Consequently, secondary electrons that could not overcome the surface barrier when the tip was in the far field might be detected under tunneling condition. Furthermore, excited electrons with lower kinetic energies than the reduced surface barrier might tunnel through the barrier.


Fabrication of Insulator-Coated Smart Tips

In order to achieve high-resolution imaging in STM, the properties of the tip are of particular importance. From the very first experiments with STM it has been known that resolution depends on the sharpness of the tip [17, 40]. A STM tip is usually fabricated with platinum–iridium (Pt–Ir) or tungsten (W) wires. Well-established electrochemical methods result in sharp tips [41, 42]. On the other hand, sharp Pt–Ir tips are also obtained by remarkably simple and unsophisticated means, such as cutting Pt–Ir wire with pliers. The overall shape of the cut tips may not be well defined, but the apex can be very sharp. The Pt/Ir tips do not oxidize as quickly as W, and are therefore superior in ex situ studies.

While the sharpness of the tip is a basic requirement even for conventional STM, the combination with X-ray absorption necessitates some additional processing. Generally, a conducting tip can detect electrons that are ejected from the sample surface not only at the tip apex but also at the sidewalls. Thus, in order to spatially limit the tip active area, a specific nanofabricated tip structure is required. An insu-lating tip coating with conducting apex can limit the probe collection area. This requirement is similar to that for electrochemical STM work, where coated tips with a small exposed area are routinely employed [43, 44]. For studies that combine STM with X-rays, several coatings have been utilized such as nail polish [27], polymers [34], glass [37], or SiO

2 [45]. The fabrication of coated tips can be extremely sophis-

ticated and sometimes consists of multiple steps, including focused ion-beam etch-ing in order to remove insulating material from the tip apex [37, 44]. In contrast, a fairly easy dip-coating technique for the fabrication of boron–nitride (BN)-covered Pt–Ir tips [46] is depicted in Fig. 14.10.Boron nitride is isoelectronic with carbon and exhibits excellent dielectric and thermal properties. A tip is prepared by first cutting a Pt

90Ir

10 wire with a diameter of 250 mm and subsequent mechanical polish-

ing. The tip is then immersed into a solution of BN with ethanol and slowly extracted. After drying (i.e., removal of the volatile alcohol component) the tip exhibits an insulating coverage except at the very end of the tip apex. The dry composition is 97% BN, 2% SiO

2, and 1% MgO. Most likely, the surface tension at the sharp tip

Fig. 14.10 Fabrication of “smart” tips: A Pt–Ir tip is dipped into a boron nitride solution and afterwards dried. Eventually, a thin insulating boron nitride film covers the tip except at the very end of the tip apex

418 V. Rose et al.

apex is responsible for the uncovered cone. In Fig. 14.11, we present scanning electron micrographs of a mechanically polished bare Pt

90Ir

10 tip, and a different tip

after dip-coating in a BN solution. To confirm the formation of a BN coating on the tip surface, energy dispersive X-ray (EDX) microanalysis was performed. The spectra at the bottom of Fig. 14.11 were taken over a BN-coated area and the free uncoated tip apex as schematically indicated in the micrograph. The characteristic peaks of the Pt

90Ir

10 tip get screened over the coated area. In contrast, the uncoated

region at the tip apex does not present peaks that are related to the coating. The size of the uncoated area at the tip apex is estimated to be below 1 mm. An exact quantita-tive assessment of this area is nevertheless relatively difficult. The spatial resolution of EDX is determined by the probe size, beam broadening within the specimen, and the effect of backscattered electrons on the specimen around the point of analysis. This inherently encumbers the quantitative investigation.

To facilitate nanoscale analysis utilizing X-ray-enhanced STM, the passivation of the tip except the apex is indispensable for adequate signal and noise levels. The size reduction of the tip active area down to the nanometer scale is a prerequisite

Fig. 14.11 Scanning electron microscopy images of (a) a mechanically polished Pt–Ir tip and (b) a different Pt–Ir tip after coating with a boron nitride film. The EDX spectra at the bottom were taken over a coated area and over the free uncoated tip apex


for improving measurement accuracy and spatial resolution. Therefore, it is expected that the advancing development of smart tips constitutes an active research field with vital importance for the utilization of synchrotron-based scanning probe microscopy.

Photoelectron Detection Using a Scanning Tunneling Microscope

As discussed in Section “The Physics of X-Ray-Enhanced Scanning Tunneling Microscopy,” both tunneling as well as photoejected electrons contribute to X-ray-enhanced STM measurements. This inherent convolution of the signals makes the understanding and quantification of the involved processes difficult. Therefore, we focus in this section exclusively on the detection of photoejected electrons in the far field, where quantum mechanical tunneling does not take place [45]. Consequently, the tunneling associated with terms in (14.1) and (14.2) vanish, and a tip current

= + −tip tip sampleTIP pass loss passI I I I (14.3)

is obtained. Likewise, total electron yield (TEY) can be measured directly at the sample via the sample-current method. The electrically conductive sample has to be grounded and the sample drain current

= + −sample sample tipTEY pass loss pass ,I I I I (14.4)

i.e., the charge that is needed to compensate for the incurred loss by emitted elec-trons is measured. A schematic view of the experimental configuration is shown in Fig. 14.12.The STM tip can be placed at a variable distance perpendicular to the sample surface using a calibrated one-axis linear piezomotor. Then the sample is irradiated at an angle of 10° with respect to the plane of the sample, while the tip current I

TIP and I

TEY are measured simultaneously. In order to manipulate the ratio

of passed and lost free electrons, a bias Ubias

can be applied to the sample. The sign of I

TIP is positive for currents leaving the tip. Likewise, the positive I

TEY direction

is defined for currents leaving the sample. Due to the close proximity between sample and tip, we have to consider that both are always illuminated in an actual measurement.

All experiments reported in the following were performed at beamline 4-ID-C of the Advanced Photon Source at Argonne National Laboratory using a focused beam size of around 100 × 100mm2 [47]. The circularly polarized undulator delivers a flux of 1 × 1012 photons/s at 1,000eV. Emitted electrons were detected at a BN-coated tip of a modified Omicron cryogenic scanning probe microscope head [48]. A custom-made support with fluoroelastomer damping was utilized in order to reduce the mechanical vibrations of the beamline from the microscope head. The studied sample consisted of a patterned Cu (3-nm)/NiFe (5-nm) multilayer film, which was fabricated on a Si(001) wafer by means of electron beam evaporation and lift-off.

420 V. Rose et al.

The TEY of the sample for X-ray energies close to the Ni L absorption edges is shown in Fig. 14.13a.The BN-coated tip was placed 1,600nm away from the sur-face, and spectra were recorded with different biases U

bias between -5 and +5V

applied at the sample. In all spectra, peaks due to the metallic Ni L absorption edge are clearly visible at 852.4 and 869.6eV. The multiplet structure of the L edges indicates a slight oxidation of the sample. The intensity ratio of the Ni L

2 to Ni L

3

peak amounts to 0.19 ± 0.01, independent of the applied bias. This value is in good agreement with calculations of the expected Ni L peak ratio [49]. The peaks are superimposed on a monotonically increasing background caused by the continuum of photoejected electrons. In order to remove this background, the derivative dI/dE was formed for each spectrum (Fig. 14.13b), and peak-to-peak intensities were obtained for further analysis. In Fig. 14.13c, we show the normalized peak-to-peak intensities of the Ni L

3 and Ni L

2 peaks as a function of applied bias. The peak

intensities decrease from around 1.4 to 0.4 for increasing bias from -5 to +5V. In addition to the variation of the peak intensity, the TEY spectra exhibit a character-istic offset for each spectrum as a function of the applied bias. In Fig. 14.13d, we present the bias dependence of the nonresonant mean TEY, normalized to the spec-trum obtained at 0V. The values are obtained from the mean current for the non-resonant energy range of 860–865eV in each spectrum. The curve progression reflects the trend already found for the peak-to-peak intensities. The mean TEY drops with increasing positive bias. Obviously, the net amount of departing photo-ejected electrons sample sample

pass loss+I I and arriving charge tip

passI gets smaller when the

sample bias is more positive. Nevertheless, ITEY

remains positive, indicating that the number of arriving electrons at the sample is always smaller than the number of ejected electrons.

The simultaneous measured tip current spectra are presented in Fig. 14.14.X-ray energies close to an absorption edge lead to negative peaks in the spectra. It is a

Fig. 14.12 Origin of the photocurrent at the tip (ITIP

) and sample (ITEY

) under X-ray illumination. The positive direction of the measured current ⊕ is indicated. The bias U

bias is applied at the sample

while the tip is always grounded


straightforward proposition to understand the appearance of negative peaks considering the sign of I

TIP. Electrons that leave the tip give rise to a positive current, while

electrons that arrive from the sample yield a negative current. The peaks are super-imposed on a monotonically decreasing background caused by an increasing num-ber of photoejected electrons arriving from the sample. Unlike the TEY spectra, the tip current can change its sign, as presented in Fig. 14.14b. If no bias is applied between tip and sample, I

TIP amounts to around 2pA at 840eV. At this photon

energy, tip tip

pass loss+I I is greater than sample

passI . But at around 850eV the tip current I

TIP

changes its sign due to the emergent pre-edge of the Ni L3 peak. Now sample

passI

becomes greater than tip tip

pass loss+I I . Finally, at 880eV, I

TIP yields a background of

around -2pA. As in the case of the TEY spectra, the intensities of the Ni L3 and L

2

peaks decrease with increasing Ubias

(Fig. 14.14c). However, the bias dependence is much stronger in the case of the tip. With respect to the unbiased situation, the peak-to-peak intensities double for U

bias of -5V. On the other hand, only extremely

small peaks are detectable for a bias of 5V. In addition to the peak intensities, the

Fig. 14.13 (a) Total electron yield of the Cu/NiFe sample with X-ray excitation between 840 and 880 eV as function of applied sample bias U

bias. (b) The derivative of TEY spectra yields peak-to-

peak heights for subsequent analysis. (c) Normalized peak-to-peak intensities of the Ni L2 and Ni

L3 peaks. (d) The normalized nonresonant mean TEY describes the vertical offset of the spectra

as a function of Ubias

422 V. Rose et al.

mean tip current varies as a function of the applied bias, which causes a DC offset of the spectra. The nonresonant mean tip current increases with more positive biases (Fig. 14.14d), which is contrary to the behavior found at the sample (Fig. 14.13d). The nonresonant tip-current background varies between 23pA with a sample bias of 5V, and -28pA at -5V. At the sample, the background TEY is maximal for the -5V bias and amounts to 250pA. If no bias is applied, the back-ground TEY decreases to 190pA and is only 80pA at 5V.

Figure 14.15 shows the tip-to-sample peak-to-peak ratios of the Ni L2 and Ni L

3

absorption edges. The ratio describes how many of the resonant photoejected elec-trons contribute to the signal at the tip. The sensitivity is independent of the sample/tip separation examined here. Data that are obtained in the far field with separations of 400, 800, and 1,600nm can be fitted by one linear function. Generally, the tip/sample ratio decreases with increasing voltage. With U

bias of -5V, around 20% of

Fig. 14.14 (a) Tip current simultaneous recorded with the TEY spectra shown in Fig. 14.13. The tip/sample separation is 1,600 nm and the bias is applied to the sample. (b) The spectrum obtained at 0 V clearly shows that the tip current changes its sign. For photon energies smaller than around 850 eV the tip current is positive, indicating that more electrons leave the tip than arrive. If the amount of arriving electrons exceeds the number of leaving electrons, the net current is negative. (c) Normalized peak-to-peak intensities of the Ni L

2 and L

3 peaks. (d) The nonresonant tip current

increases as a function of the applied bias


the ejected electrons are detected at the tip. The ratio decreases to around 12% at 0V, and only 3% at +5V. Thus, a small bias between tip and sample allows an accu-rate and effective control of the background currents as well as the desired signal obtained at absorption edges.

Measuring X-ray absorption spectra yields local chemical and electronic proper-ties. But in addition to the elemental selectivity, exploiting the polarization of the X-rays allows for elucidation of magnetic properties. The underlying effect is XMCD, which measures the dependence of X-ray absorption on the helicity of the X-ray beam by a magnetic material (cf., Fig. 14.3). The experimental geometry used in the experiment described here is illustrated in Fig. 14.16a.Left and right circularly polarized X-rays probe the averaged magnetization M component of a NiFe film parallel to the direction of the incoming beam. The incidence angle q

i of

the X-ray beam amounts to around 10°. At each energy step, the tip current is mea-sured for LCP and RCP radiation ( LCP

TIPI and RCP

TIPI , respectively). Figure 14.16b

shows a XMCD difference spectrum for a BN-coated tip that is placed around 200nm away from the surface. A bias U

bias=-2V is applied to the sample. The spec-

trum is obtained from the difference of LCP

TIPI and RCP

TIPI , and exhibits a net negative

2p3/2

and positive 2p1/2

peak. The different number of spin-up and spin-down holes available in the 3d electron states causes this. Because the 3d states are the origin of the magnetic properties of this material, the spectrum contains information on spin and orbital magnetic moments. Therefore, the distinctive and unique potential of the combination of X-rays and STM lies not only in the ability to localize spins, but also in the capability to measure their size.

It is obvious that a tip placed in the far field only provides a limited spatial resolu-tion. Recently, Chiu et al. [50] have simulated electron trajectories for photoelectrons

Fig. 14.15 Relative peak-to-peak intensity ratio of the Ni L3 and Ni L

2 peaks at the tip and sample

as a function of applied sample bias. Data are obtained for tip/sample separations of 400, 800, and 1,600 nm

424 V. Rose et al.

escaping from the surface. Assuming a constant photon flux homogenously delivered to a sample area A

ph on a metallic surface, they obtained an effective probing diameter

Dprob

of the tip that is defined as the diameter of a circular region on the surface within which emitted electrons contribute 90% to I

TIP. According to this definition, the abso-

lute value of ITIP

is estimated from the sample current Isample

delivered by the incident X-rays:

�

= prob sample

TIPph

210

9 2

D II

A (14.5)

Generally, decreasing the probing area requires a small tip/sample separation, a smart tip with minimized area of detection, and a strong acceleration field resulting in a large negative U

bias. The best resolving power is achieved when the tip/sample

Fig. 14.16 (a) The magnetization M of a NiFe film is derived from tip current (ITIP

) spectra, one taken with LCP, and one taken with RCP light. The incidence angle of the X-rays is q

i. (b) X-ray

magnetic circular dichroism spectrum measured with the tip


separation lies in the tunneling regime, i.e., in the near field. Chiu et al. were able to show that in this case, the signal that is typically detected in SXSTM measure-ments is too large to be only caused by photoejected electrons. Thus, if the tip/sample separation lies within the conventional tunneling regime, both channels photoejected as well as X-ray-excited tunneling electrons must be considered. The illustration of this regime will be the subject of the following section.

X-Ray-Assisted Scanning Tunneling Microscopy

In order to achieve the ultimate spatial resolution in SXSTM, the separation between the tip and sample obviously has to be minimized. Therefore, unlike in the studies of photoelectron detection in the far field, a separation within the quantum mechanical tunneling regime has to be employed. Consequently, we have to expect two qualitatively different current contributions in the SXSTM signal I

SXSTM, that is to say, the current

caused by photoelectrons as well as the tunneling current. The signal ISXSTM

represents a convolution of those signal channels. While the tunneling current is highly localized and, in principle, is capable of delivering atomic resolution, the photocurrent contribu-tion can be considered as an undesired noise that degrades resolution. Nevertheless, spatial resolutions of 10nm for hard X-rays [35], and 14nm for soft X-rays [38] have already been demonstrated and it is expected that further developments, e.g., improve-ments of high-performance tips, may significantly improve the spatial resolution.

In conventional STM the feedback loop keeps the tunneling current STM

tunnelI con-

stant, so that the consequential adjustment of the tip height (i.e., the z-piezo voltage) in principle yields a topography image of the sample surface. But as shown in Section “The Physics of X-Ray-Enhanced Scanning Tunneling Microscopy,” the illumination with X-rays during scanning produces several additional current con-tributions. Although some of those additional contributions provide the chemical sensitivity of SXSTM, large currents can overstrain the dynamical range of the feed-back loop. The situation is illustrated in Fig. 14.17.The tip was positioned over a NiFe film and stabilized under X-ray illumination at 850eV, a tunneling current of 0.2nA, U

bias=-3.5V, and z-piezo voltage of 5V. Then the X-ray energy was scanned

from 850 to 856eV. An X-ray absorption spectrum was simultaneously obtained using an energy standard (dotted line), which exhibits the Ni L

3 edge. The z-piezo

voltage remains constant up to an X-ray energy of 851.7eV. Then the z-piezo voltage decreases rapidly and reaches -137V at 852.4eV. Here the z-piezo is fully retracted. Generally, the z-piezo voltage ranges between +137V (maximal approach) and -137V (maximal retracted tip). For energies larger than 855.7eV the z-piezo voltage returns to its initial value, i.e., the tip approaches the sample surface again. Although the z-piezo starts to retract the tip for X-ray energies larger than 851.7eV, the SXSTM current I

SXSTM continues to decrease (gets more negative, i.e., the net amount

of charge arriving at the tip still increases). Apparently, the feedback loop is not able to maintain a constant I

SXSTM because the current caused by the interaction with

X-rays is too large. In the case of the fully retracted tip, ISXSTM

appears to mirror the

426 V. Rose et al.

progression of the intensity of the energy standard. Not until ISXSTM

falls below the threshold of -0.11nA does the tip approach the sample again. Although such strong responses still allow the spectroscopy mode of SXSTM, it is undesirable for the imaging mode. In order to maintain sound tunneling conditions during the experi-ment, it is crucial to maintain a somewhat small current that lies within the dynamic range of the STM feedback loop. An easy way to adjust the conditions of an SXSTM experiment is the variation of U

bias. In Fig. 14.18, we present a point spectroscopy

measurement on the same NiFe film with a bias of only -3V. Here, the tip was sta-bilized at 845eV with a z-piezo voltage of 11V. The appearance of the Ni L

3 peak

causes a notable decrease of the z-piezo voltage to -61V. However, tunneling condi-tions are preserved during the energy scan. The Ni L

2 peak does not produce a

response of the z-piezo voltage at all, because the z-piezo voltage reacts only to a normalized intensity of the energy standard larger than 0.6. The Ni L

2 peak exhibits

an intensity of only 0.5. The feedback loop keeps ISXSTM

more or less constant over the whole energy scan, with only small peaks related to the up- and downward slope of the z-piezo voltage. From this spectroscopy experiment we can expect that appro-priate tunneling conditions can enable an imaging mode of SXSTM.

The feasibility of imaging with an STM under synchrotron-based X-ray illumina-tion is demonstrated in Fig. 14.19.At first a sputtered Cu film on a Si substrate was scanned with conventional STM in constant-current mode. The image obtained from the reaction of the z-piezo (Fig. 14.19, left) shows the rough topography of the Cu film featuring elongated elevations. The tip current (Fig. 14.19, top) reflects these structural features, but is nevertheless relatively flat. For comparison, the same sur-face area was scanned under X-ray illumination. The energy was tuned to the Cu L

3

Fig. 14.17 Response of the SXSTM signal and the z-piezo voltage for an energy scan of a NiFe film. The dotted line represents the spectrum simultaneously obtained from a NiFe standard. Between 852.4 and 855.0 eV the z-piezo is fully retracted at -137 V. (U

bias = -3.5 V; I

t = 0.2 nA)


absorption edge at 931.2eV. During the scan, the X-ray shutter was repetitively opened and closed at every 25th scan line. While the topography image provides basically the same information as the one shown on the left side of Fig. 14.19, the tip current image exhibits a pronounced stripe structure. The image shows dark

Fig. 14.18 Response of the z-piezo and the SXSTM current as a function of X-ray energy for a point spectrum over a NiFe film. The z-piezo shows a strong response when the energy approaches the Ni L

3 absorption peak, but does not reach the saturation voltage

Fig. 14.19 Topography image of a Cu film (left) and the associated conventional tunneling cur-rent (top). The bottom image shows the current image for a sequence of illumination at the Cu L

3

absorption edge (bright stripes) and without illumination (dark stripes). The image was obtained with an uncovered Pt-Ir tip (U

t = -2 V; I

t = 1 nA; scan speed was 2,000 nm/s)

428 V. Rose et al.

stripes when the X-ray shutter is closed. By contrast, if the sample is illuminated with X-rays, bright stripes are recorded. Although this is a basic demonstration of the achievability of imaging under X-ray illumination, care has to be taken in the analysis of chemical species in such images. As shown in Section “Photoelectron Detection Using a Scanning Tunneling Microscope,” illumination with X-rays can cause a DC offset of current spectra, which may also lead to a similar response.

So far, we have seen that ISXSTM

as well as the z-piezo voltage can directly provide elemental selectivity, while U

bias largely controls the sensitivity of SXSTM. Further,

the feasibility of imaging under X-ray illumination was demonstrated. In Fig. 14.20, we present finally the experimental justification of the SXSTM mechanism proposed in Section “The Physics of X-Ray-Enhanced Scanning Tunneling Microscopy.” Point energy scans were carried out on a patterned NiFe (20 nm) ring structure over position S as shown in Fig. 14.20a. The sample was fabricated on a Si(001) wafer by electron beam evaporation and lift-off. An X-ray absorption spectrum, which is simultaneously obtained from a NiFe energy standard, is shown in Fig. 14.20b. The peak is caused

Fig. 14.20 (a) The topography scan of patterned NiFe rings. Current spectra were obtained at the indicated sample position S. The feedback loop was switched off for the energy scans. (b) The X-ray absorption spectra of a NiFe standard shows the Ni L

3 peak. (c) The SXSTM signal exhib-

its a decrease when the sample is positively biased at 1.5 V. (d) In the case of tunneling from the sample into the tip (U

bias = -1.5 V), the modulus of the SXSTM signal increases, i.e., the current

gets more negative when the photon energy reaches the Ni L3 transition


by the Ni L3 absorption edge. Before each scan, the tip was stabilized under X-ray

illumination at 845eV, Ubias

=±1.5V, and ISXSTM

of ±0.2nA. Hereby, a positve Ubias

causes tunneling from the tip into the sample, and therewith a positive I

SXSTM.

Likewise, a negative ISXSTM

results from a negative bias. The X-ray energy was scanned from 845 to 858eV, while the feedback loop was switched off. This assures a constant tip/sample separation during the scan. In the case of the postive U

bias, the

SXSTM current drops from around 0.20 to 0.07nA when the energy reaches the Ni L

3 peak (Fig. 14.20c). This can be explained by electrons that are excited to states

close to EF, which reduce the number of available states for tunneling from the tip into

the sample (cf., Fig. 14.5b). Furthermore, the current sample

passI that is caused by photo-

ejected electrons is opposite to the initial tunnel current (cf., Fig. 14.6). The scenario is reversed when the sample is negatively biased. Fig. 14.20d shows the response of I

SXSTM for a bias of -1.5V. The appearance of the Ni L

3 transition causes an increase

of the absolute value of ISXSTM

by about 0.34nA. We would like to remind the reader that the sign of I

SXSTM is only indicative of the current direction, which means that the

more negative ISXSTM

gets, the more charge arrives at the tip. Incident X-rays excite core electrons to unoccupied states close to E

F. Consequently, the conventional tunnel

current is enhanced by X-ray

tunnelI as well as by sample

passI (cf., Figs. 14.5a and 14.6).

Concluding Remarks

Depending on the scientific community asked, the benefits and prospects of SXSTM could be viewed from two different perspectives. On the one hand, the synchrotron community experiences a fundamentally different and unusual approach to high-resolution X-ray microscopy. Currently, all efforts to improve spatial resolution for the X-ray sciences try to modify the beam properties by means of sophisticated optics [6, 51]. In contrast, SXSTM has the potential to achieve those resolutions even with relatively large beams, because the close proximity of the tip, and not the small footprint of an incoming X-ray beam, achieve the high resolution. On the other hand, in the STM community chemical sensitivity has been a dream since the early days of this amazing technique. Additionally, SXSTM allows for direct detecting and quantifying of magnetic properties. Therewith, SXSTM is not just a marginal extension of the established STM. It has the potential to revolutionize the way in which we are able to study nanostructures.

However, several remaining challenges have to be overcome before SXSTM can be put into widespread use. The technique is still in the early stages and a huge effort has to be undertaken to further develop and better understand this novel tool. One example is the development of dedicated electronics. The interaction of X-rays with the sample and the tip introduces currents that are not present in conventional STM experiments. Hence, one must emphasize that care must be taken in the analysis of signals obtained with conventional STM electronics. It is further desirable that electronics take advantage of the fast time resolution of third-generation synchro-trons. A detailed discussion of dedicated electronics for SXSTM would far exceed

430 V. Rose et al.

the subject of this chapter, and therefore is omitted here. A second example is the fabrication of “smart” tips. The size reduction of the tip active area is key to successful nanoscale imaging. So far, several more or less sophisticated approaches have been implemented, but there is still room for further optimization, which would improve the spatial resolution of SXSTM.

Today, the emerging fields of nanoscience and nanotechnology are leading to unprecedented understanding and control over the fundamental building blocks of nature. The combination of STM and synchrotron radiation with nanoscale resolution has the potential to provide a major impact on nanoscale research by enabling funda-mentally new methods of characterization. Further vigorous and diversified develop-ment of the technique is critical to fully realizing its potential for nanotechnology.

Acknowledgements The authors would like to express their gratitude to several people who contributed to this project. Special thanks go to Kenneth Gray for the generous allocation of experimental equipment, which made this work possible in the first place. We thank Vitali Metlushko for the growth and patterning of the studied samples. Curt Preissner is acknowledged for his engineering support and Matthias Bode for several fruitful discussions. This work has been supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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