Fano spectroscopy of impurities and clusters on solid surfaces

Journal of Electron Spectroscopy and Related Phenomena 124 (2002) 195–210www.elsevier.com/ locate /elspec

Fano spectroscopy of impurities and clusters on solid surfacesa , a b a*Marina Pivetta , Francois Patthey , Bernard Delley , Wolf-Dieter Schneider

a ` ´ ´Institut de Physique de la Matiere Condensee, Universite de Lausanne, CH-1015 Lausanne, SwitzerlandbPaul Scherrer Institut, CH-5232 Villigen, Switzerland

Received 14 August 2001; received in revised form 5 November 2001; accepted 7 November 2001

Abstract

When a discrete electronic level interferes with a broad continuum of states a Fano resonance phenomenon occurs givingrise to characteristically asymmetric peaks in excitation spectra. Here we report on recent observations of such spectroscopicsignatures in: (i) normal emission ultraviolet photoelectron spectra (UPS) of Cu impurities embedded in noble gas-monolayers on highly oriented pyrolytic graphite (HOPG), and in (ii) low-temperature scanning tunneling spectra (STS) ofindividual Ce adatoms and of small Ce clusters on Ag(111). These findings provide a new and promising avenue for thedetection and characterization of magnetic and non-magnetic impurities and small clusters at surfaces. Furthermore, (iii) ashort account is given on new developments towards photoemission at atomic-scale resolution using the tip of a scanningtunneling microscope (STM) as local electron analyzer. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Fano resonance; Kondo resonance; Scanning tunneling spectroscopy; Local photoemission

1. Introduction also reported in surface physics. For example, inangular resolved photoelectron spectra of Cu and Ag

In order to observe a Fano resonance [1] with its single crystal surfaces, taken in normal emissioncharacteristic lineshape in an excitation spectrum, geometry, the asymmetric line shape of the direct-transitions from both, the localized and the extended transition peak derived from the nearly free-electron-states need to be allowed and the localized state has like Ag sp-states has been interpreted to originateto couple weakly with a broad continuum of states. from an interference between the outgoing electronSuch resonances have been found in atomic physics, waves from the direct-transition and surface emissione.g. in the photo-ionization continuum [2] and in the in the final state, a phenomenon which has beenshakeup satellite electron spectrum [3] of rare gases, tentatively associated with the physics underlying aand in solid state physics, e.g. in resonant photo- Fano resonance [5,6]. Because the parameters gov-emission of rare earths [4]. More recently, indica- erning the lineshape of the resonance are intimatelytions for such an interference phenomenon have been linked to the nature of the quantum interference, i.e.

the ratio of the transition probabilities between thediscrete state and the band of unperturbed continuum

*Corresponding author. Tel.: 141-21-692-3661; fax: 141-21-states [1], such studies have contributed considerably692-3635.to a detailed understanding of the electronic prop-E-mail address: [email protected] (W.-D.

Schneider). erties of gas phase and condensed matter.

0368-2048/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 02 )00054-3

196 M. Pivetta et al. / Journal of Electron Spectroscopy and Related Phenomena 124 (2002) 195 –210

Recently, the first measurement has been reported electrons [22] still constitutes a major challenge inwhich reveals the manifestation of a typical Fano condensed matter physics.resonance profile in valence band photoemission Recently, for the first time, two consecutive STS/spectra [7]. This situation has been realized by STM experiments spatially resolved the electronicembedding isolated atoms of Cu or Ag in a com- properties of individual magnetic adatoms displayingmensurate Xe monolayer (ML) on HOPG and by the Kondo effect. In particular, the observed Fanotaking advantage of the Brillouin zone folding which lineshape of the Kondo resonance reveals unambigu-couples the Xe-overlayer electronic structure with ously the details of the quantum mechanical interfer-the one of the HOPG surface. In contrast to Fano’s ence between the localized orbital and the conduc-paper [1] where the localized state is in the final tion electrons on an atomic length scale [8,9,23,24].state, here the localized Cu 4s and Ag 5s initial state In the light of the above experiments scanningorbitals mix with the continuous band of states tunneling spectroscopy (STS) combining atomicfollowed by a transition into the continuum of states scale lateral resolution with high energy resolutionof the outgoing photoelectron. These conditions [25] may appear to be superior to photoemissionresult in a relatively sharp resonance with a pro- spectroscopy which has achieved comparable energynounced Fano lineshape. resolution [21,26] but is actually far from the spatial

The typical spectroscopic signature of a Fano resolution obtained in STS. There have been already,resonance has been found recently also in a different however, attempts in the past to perform photo-context, in scanning tunneling spectra of a single emission on a local scale [27,28]. Gimzewski et al.magnetic atom on a metal surface [8–10]. The [27] measured photoemission characteristics fromfascinating many-body physics involved in the inter- cesiated Au(110) surfaces illuminated by photonsaction of a single magnetic impurity with the con- using the tip of a scanning tunneling microscope as aduction electrons of its non-magnetic metallic host is local probe. The photons were generated by a quartzreflected in unconventional phenomena in magnet- halogen lamp, coupled to a fiber optic cable (coldism, transport properties and the specific heat. The light source) and focussed onto the tip–surfacepicture that has emerged to explain these observa- region. An analysis of the tip current as a function oftions is that the local moment is gradually screened applied bias permitted the determination of theby the conduction electrons of the metal host as the maximum kinetic energy of impinging photoelec-temperature approaches T 5 0 K and a many-body trons whose value was in good agreement with annon-magnetic singlet ground state is formed close to independent analysis based on macroscopic measure-the Fermi energy. These low-energy excitations with ments of the surface workfunction. The authorsa characteristic width of d 5 kT (T is the Kondo obtained at rather large tip–sample distances, i.e. inK K

temperature and k the Boltzmann constant), known the field emission range, a photoelectron micrographas Abrikosov–Suhl [11,12] or Kondo-resonance [13], of cesiated gold with only moderate sub-micrometercontain all the pertinent information on the quantum lateral resolution. Based on these preliminary resultsphysics involved in correlated electron systems [14– the authors suggested schemes for local probe photo-18]. Experimentally, the Kondo resonance has been electron analysers for use with synchrotron or tun-observed for a number of homogeneous dense Kondo able laser illumination to obtain a lateral spatialcompounds using high-resolution photoelectron spec- resolution in the nanometer range.troscopy [19–21] which averages over a typical These suggestions motivated Gray [28] to build a

2surface area of 1 mm . However, the measurement similar experimental setup based on a 20 mW 4881of the unique properties of the Kondo resonance nm (2.54 eV) Ar ion laser. In his work the STM tip

created by the interaction of a single magnetic atom was used also in the tunneling regime to detectwith a non-magnetic metallic host, its predicted photoelectrons emitted from a Rb-saturated Al(111)temperature dependence leading to its collapse for surface. Conductivity spectra clearly show sharptemperatures above T , its predicted splitting in an thresholds which were found to be related to the tipK

external magnetic field [18], and the spatial exten- and sample workfunctions and the incident photonsion of the Kondo screening cloud of the conduction energy. Maps of the photocurrent showed features

M. Pivetta et al. / Journal of Electron Spectroscopy and Related Phenomena 124 (2002) 195 –210 197

that are related to the topography of the surface and with a hemispherical electron analyzer (HA 150are only 2–3 nm in size. Both, the spectral and from VSW). The total instrumental energy resolutionspatial resolution of this technique represent signifi- was set to 50 meV and the angular resolution wascant advances on previous forms of photoelectron 628. The workfunction was determined by applyingemission microscopy (PEEM) [29,30] or workfunc- a negative voltage to the sample and by measuringtion microscopy. Based on these promising results the secondary electron onset E . Knowing theonset

we have recently implemented local photoemission Fermi level position E , we define DE 5 E 2 EF F onset

spectroscopy in our existing low-temperature STM as the amplitude of the photoemitted spectra. The[31,32] and we present a few preliminary results in workfunction of the sample is then obtained bythis communication. ef 5 hn 2 DE, where hn is the photon energy. The

The application of local photoemission spectros- HOPG crystal was cleaved in air and subsequentlycopy and microscopy in nanoscale surface physics transferred into the vacuum chamber on a cold fingermay provide new insight into the electronic prop- held at 50 K by a closed-cycle helium refrigerator. Aerties of nanostructures. For example, local photo- retractable electron gun allowed for rapid heating toemission with an STM has the potential to spatially 1200 K of the substrate for cleaning and annealingscale down traditional occupied state spectroscopies, [34]. The sample temperature of T550(5) K waslike photoelectron spectroscopy, to the atomic scale. measured with a precision of 65% using a W–Thus measurements of local workfunction differ- 26%Re/W–5%Re thermocouple spot-welded to theences become possible at unprecedented lateral sample holder. Xe was introduced into the vacuumresolution. In addition, the possibility to compare by a leak valve and was adsorbed at low temperaturetwo quantum mechanically different effects, tunnel- on the clean HOPG substrate. The thickness of theing and photoemission, on the same scale might Xe (Kr, Ar) film was determined from the charac-allow us to obtain new insights into these phenomena teristic Xe 5p (Kr, Ar) photoemission signalsand, on the other hand, to understand more deeply [35,36]. Subsequently Ag or Cu impurities werethe physical and chemical properties of individual deposited from a directly heated tungsten filament ornanoscale objects, like supported atoms, molecules by using a mass-selected cluster beam [37] where theand clusters. Moreover, the realisation of local amount of the deposited material was determined byphotoemission with the tip of an STM as an atomic- monitoring the beam current on the sample.scale analyzer may be considered as a first steptowards future experiments using synchrotron radia- 2.2. Scanning tunneling spectroscopytion.

The experiments were performed with a home-built ultrahigh vacuum (UHV) low-temperature STM[31]. The Ag(111) surface was prepared by Ar ion

2. Experimental bombardment and annealing cycles. Isolated Ceatoms and, for comparison, isolated Ag atoms on

2.1. k-Resolved photoemission Ag(111) were deposited by evaporation from atungsten filament onto the Ag substrate held at T 5 5

The experiments were performed in an ultra-high K. Upon deposition at T 5 50 K Ce cluster formationvacuum (UHV) chamber with a base pressure of is observed [38,39]. A measure of the LDOS of the

2101 3 10 mbar employing in situ characterization of Ag(111) surface and of the adsorbed impurities wasthe sample by low energy electron diffraction obtained from measurements of the differential con-(LEED) and UPS. Satellite-free photoelectron spec- ductance dI /dV versus the sample bias voltage Vtra were measured using the He I (21.2 eV) and He II performed under open feedback loop conditions withline (40.8 eV) of a high-intensity gas discharge lamp lock-in detection (243 Hz, 2–4 mV sinusoidalrms

(GAMMADATA) combined with a homebuilt mono- modulation added to the bias). These tunnelingchromator [33]. All spectra are obtained in normal spectra were measured using three electrochemicallyemission geometry by collecting the photoelectrons etched W tips which were prepared in UHV by


heating and Ar ion bombardment. Each of these tips obtained which are necessary to develop local photo-emission at atomic resolution.was repeatedly and intentionally modified by apply-

ing voltage pulses and by bringing it into contactwith the Ag surface. The spectroscopic features

3. Results and discussiondiscussed below were observed with all these tipsover a range of tip–sample distances defined by the

3.1. Brillouin zone folding and Fano resonances intunneling parameters (500 mV.V .2500 mV, 100angle resolved photoemissionpA,I,2 nA) used prior to opening the feedback

loop.3.1.1. Brillouin zone folding

When suitable coupling is provided by commensu-2.3. Local photoemission with a scanningrate adsorbate lattices, folding between the two-tunneling microscopedimensional Brillouin zones of the overlayer and ofthe substrate surface occurs. As a consequenceThe experiments were performed using our UHVcertain high-symmetry points of the overlayer arelow temperature STM [31,32]. The tunneling tip wasidentical to other high-symmetry points of the sub-electrochemically etched Ir, cleaned in UHV by Arstrate surface Brillouin zone. Such a phenomenonion sputtering. The photon source was an intracavityhas been first observed in electron energy lossfrequency doubled ion laser system (Innova 300Cspectra (EELS) of surface phonons on Ni(111)FRED from Coherent) that uses BBO to producecovered with an ordered (232) adsorbate lattice ofSecond Harmonic Generation (SHG) output at six

] ]Œ ŒC H , and with p(232) and ( 3 3 3)R308 latticedifferent wavelengths from 229 to 264 nm. Addition- 2 2

of oxygen, respectively [40]. The low energy elec-ally, singe-line and multi-line visible, fundamentaltron diffraction (LEED) reflexes of the p(232) andUV (333–364 nm), and single-frequency perform-

]] ] ] ]Œ Œance is available. This constitutes a reasonable ( 3 3 3)R308 structures fall on M and K. G points] ]

energy window to scan over the workfunction differ- of the overlayers are therefore identical to M and Kences between sample and tip and to disentangle of the substrate for the two adsorbate layers, respec-density of state signals from workfunction onsets. tively [40].The laser intensity (maximum power 200 mW) was Recently, in ultraviolet photoelectron spectra

] ]Œ Œcontrolled by adjusting the power level and by (UPS) of a commensurate ( 3 3 3)R308 Xe-over-defocussing the laser beam. The unfocussed laser layer on graphite manifestations of the same physicalbeam passes through a sapphire window of the STM- phenomenon have been reported [7]. In this case the

]]chamber and reaches the tip sample region at an overlayer couples K point electrons with G point

]angle of 458 with respect to the sample normal. The electrons of pure graphite rendering the former Ktypical spot diameter at the tip position is about 1 point states of graphite observable under normal

]mm. The photocurrent excited by the laser light was photoemission at the G point of the substrate leadingcollected by the STM tip and was typically 10 to considerable emission intensity at the Fermi level.nA/mW corresponding to a photoionization cross Fig. 1(a) and (b) are LEED images of the clean

24section of 10 . The differential conductance of the HOPG substrate displaying its characteristic hexa-photoelectron current was measured with lock-in gonal structure [41] and the substrate after adsorptiondetection (2 kHz, 10 mV sinusoidal modulation of 1 ML Xe taken at T550(5) K [36], respectively.rms

added to the tip bias). For spatial maps of the Heating the sample above T565(5) K causes aphotocurrent a mechanical chopper modulates the sudden extinction of the inner spots indicatinglaser beam at typically 3 kHz and a lock-in amplifier desorption of the Xe film from the surface [42]. The

] ]Œ Œis used to discriminate this component from the commensurate ( 3 3 3)R308 Xe adlayer ontunnel current. The strengths of the present ex- graphite implies that the reciprocal lattice vector GG

perimental setup are the tunable light source and the associated with the Xe adlayer matches exactly theachievement of low-temperatures (4 K). In this way vector GK of the graphite Brillouin zone (see Fig.stable and reproducible experimental conditions are 1(c)), a very important property for what follows.


Fig. 2 presents UP spectra taken with He IIradiation from the clean HOPG substrate (a) andcovered with a Xe ML (b). The observation (a) of aweak conduction band below the Fermi level (E ), aF

s band at 4.5 eV, a p band at 8 eV and another s

band at 20 eV confirms earlier experimental andtheoretical findings [43–45]. After adsorption of Xe(b), the Xe 5p states between ¯ 6 and 7.5 eV,characteristic for the presence of exactly one ML ofadsorbed Xe [46,35,47], as well as, emission fromthe atomic Xe 5s states at about 17 eV bindingenergy [48], become visible. In addition to the nowslightly reduced emission (due to the Xe overlayer)from the HOPG valence band states (a), three new

Fig. 2. UP spectra (He II) of (a) clean HOPG and (b) afteradsorption of 1 ML Xe at T550(5) K. Positions of the s and p

Fig. 1. Hexagonal LEED (E 5 184 eV) patterns at T550(5) K of bands after Ref. [44]; lower indices d (bzf) for direct (Brillouinp ] ]Œ Œ(a) HOPG and (b) a commensurate ( 3 3 3)R308 Xe ML on zone folded) transition corresponding to emission from the G

HOPG. (c) Resulting Brillouin zones for Xe (thin) and HOPG (K)-point of the first Brillouin zone of graphite. (c) Inset: HOPG(bold line). After Ref. [7]. band structure from Ref. [44]. After Ref. [7].


spectral features can be discerned at ¯ 1.8, 11 and and Xe, respectively. The same additional spectral14 eV binding energy in (b) where the spectral intensity increase is observed for all rare gas over-intensity changes in a spectacular way between E layers in identical energy regions [49].F

and about 2 eV binding energy. This additional The considerable increase in spectral density nearintensity near E after Xe adsorption is also clearly E after adsorption of a Xe (Ar,Kr) monolayerF F

visible in the photoemission spectra obtained with cannot be caused by Xe because the nearest levelHe I excitation (Fig. 4(a)). The inset (d) shows the (6s) is located above E . Even if this level may beF

Xe 5p excitations and, for comparison at the same partly occupied due to interaction with the substratevertical scale (e), the featureless emission from [50] this effect is far too small to explain ouruncovered HOPG. observations. Rather, the geometry of the commensu-

These investigations have been extended to com- rate Xe adlayer on graphite contains the clue to themensurate adlayers of Ar and Kr which confirm the experimental observations. Fig. 1(c) shows that theexperiments with Xe/HOPG. Fig. 3 displays UP reciprocal lattice vector, GG, associated with the Xespectra taken with He II radiation from the clean adlayer matches exactly the vector GK of theHOPG substrate and covered with a ML of Ar, Kr, graphite Brillouin zone. In other words, the Bril-

louin-zone folding resulting from the relatively weak] ]Œ Œperturbation by the Xe ( 3 3 3)R308 overlayer

couples K-point with G-point electrons of puregraphite (Fig. 2). This symmetry argument explainswhy states with wavevector around the K-point ofthe unperturbed graphite may become observableunder normal emission (k 5 0) with UPS. If thei

overlayer is looked at as a perturbation, one may saythat bound Bloch waves as well as photoelectronsemitted from graphite with k ¯ K can be scatteredi

into k 5 0. A comparison between the energy posi-i

tions of the newly observed spectral features with theknown band structure of graphite [44] (see inset ofFig. 2) confirms this reasoning and explains theobserved sensitivity of the spectra with respect tooff-normal emission. Because of their dispersionnormal to the graphite layers, the p bands (bzf forbzf

Brillouin zone folding) near E give rise to smoothF

emission in the energy range between E and 2 eV.F

3.1.2. Fano resonancesDeposition of 0.1 ML Cu atoms impinging from a

cluster beam with 664 eV kinetic energy on theXe-film or, alternatively, Cu vapour deposition leadsto the second surprising observation in the normalemission spectra, the appearance of a clear Fano lineprofile at 250 meV below E (Fig. 4(b)). We note thatF

the Fano signal is present for He I and also at asomewhat weaker intensity for He II excitation

Fig. 3. UP spectra (He II) of clean HOPG and after adsorption of pointing to an initial state effect. The resonance1 ML Ar, Kr, and Xe at T550(5) K. Positions of the s and p

signal increases with Cu coverage becoming firstbands after Ref. [44]; lower indices d (bzf) for direct (Brillouinvisible at about 2% ML Cu. A concomitant broaden-zone folded) transition corresponding to emission from the G

(K)-point of the first Brillouin zone of graphite. After Ref. [49]. ing and a binding energy shift of the resonance from


the Xe-film and subsequent thermal desorption (T $

65 K) of the Xe monolayer.Fano resonances have been also observed in the

normal emission UP spectra of Cu impurities onKr/HOPG but not on Ar/HOPG. Fig. 5(a) showsagain normal emission UP spectra in an energy rangeclose to the Fermi level measured for 1 ML Xe onHOPG (bottom) and after deposition of Cu (equiva-lent of 0.05 ML Xe) (top). Fig. 5(b) and (c) show theUP spectra for Cu on Kr and Cu on Ar, respectively,before and after Cu deposition. For Cu on Kr theresonance is already less developed while for Cu onAr no Fano resonance is observed at all. Ourworkfunction measurements and their analysis withinthe Topping model [54,55] give an interesting clue tothis astonishing behaviour. In Fig. 6 we show theresults of a fit of this model to our work functiondata as a function of the Cu-coverage on the Xelayer. The dashed curve models an adatom super-structure of isolated Cu atoms and disagrees clearlywith the experimental data. The dotted curve corre-sponds to a fully random distribution of the adsor-bates and it follows already closely the experimentaldata. The full solid line represents the best fit to thedata differing only slightly (14%) from the valueappropriate for the random distribution scenario.Thus the Cu atoms are practically immobile and theyare randomly distributed on the Xe layer. TheFig. 4. UP spectra (He I) T550(5) K of (a) 1 ML Xe on HOPG;

(b) Cu on the Xe ML; (c) gas phase spectrum of Cu after Ref. presence and the intensity of the Fano resonance[57]. Inset: (d) Xe 5p excitations and (e) uncovered HOPG in a appears to be directly linked to the geometry of thewider binding energy range. After Ref. [7]. Cu impurities on the rare gas substrate. For Cu on Kr

our analysis indicates Cu adatom diffusion and islandabout 100 to 250 meV below E is observed which formation at the surface, i.e. only a fraction of theF

implies an interaction with the Xe-matrix [51]. This adsorbed Cu atoms remains isolated and participatesfinding is corroborated by the fact that the Xe 5p in the Fano resonance. For Cu on Ar an extreme caselevels also broaden and shift with increasing Cu is realized, where the Cu atoms diffuse freely on theconcentration. After deposition of Ag or Cu the rare gas surface, form islands and, consequently, noXe-overlayer structure persists in the observed LEED Fano resonance is observed. Thus, we can safelypattern. Rotating the sample by 108 from the surface conclude that the Fano resonance signal is related tonormal leads to the extinction of both, the additional the presence of isolated Cu impurities in the ad-intensity in the Xe covered HOPG spectrum near E sorbed Xe film [54] and that the outermost boundF

and the Fano resonance. The Fano signal is only electrons of the impurity atoms play a decisive roleobservable when Cu or Ag are deposited after Xe in the formation of the resonance.adsorption on HOPG. Deposition of Cu(Ag) directly Gas phase photoemission spectra of Ag and Cuon HOPG leads to a spectrum characteristic for the [56,57] indicate that after adsorption on a substrate ametallic state [52,53] indicating island formation due binding energy of the 5s (4s) levels just below EF

to the high mobility of Cu on HOPG even at 50 K. can be expected [58]. In Fig. 4(c) the 4s level in theThe same behaviour is observed after deposition on Cu gas phase spectrum [57] has been aligned with


Fig. 5. UP spectra (He I, normal emission, T550 K) of: (a) 1 ML Xe (lower) and after Cu evaporation on the Xe layer (upper); (b) 1 ML Krand same amount of Cu evaporated on 1 ML Kr; (c) 1 ML Ar and Cu evaporated on 1 ML Ar. The Cu concentration is about 5% of a XeML on HOPG. The spectra after Cu evaporation have been displaced vertically for clarity. The spectra in (b) and (c) have been multiplied bya factor of 5. After Ref. [54].

the Fano resonance showing also that the atomic Cu excitation spectra. A Fano line shape analysis of the3d-excitations contribute weakly to emission be- narrow spectral structure, including a Fermi functiontween 3 and 4 eV in the spectrum of Fig. 4(b). The cut-off, captures the essential physics of this reso-remarkable amplification of the 4s signal by roughly nance phenomenon (see Fig. 7). We obtain a reso-a factor of 100 indicates an interesting and new Fano nance energy E 5 0.25 eV and a Fano parameter0

resonance phenomenon observed with photoemission q 5 1.5, i.e. the ratio of the transition matrix ele-techniques. ments between the modified, localized states and the

In the present case the Cu 4s and Ag 5s atomic unperturbed extended states which is of the order oflevels couple relatively weakly with this p band one. The coupling strength or hybridization betweenbzf]and the direct band from K point states. These localized and extended states is reflected in the widthconditions necessarily lead to quantum interference of the resonance [1]. The magnitude of the totalwhich results in the photoemission intensity I from cross section depends on how the interaction redistri-the zone-folded and the atomic-like transitions, butes the oscillator strength density in the region of

the resonance. For this reason, the total cross sectionfolded atomic 2I ~ uM 1 M u (1) is not necessarily the cross section that would be

measured in the absence of an interaction with theand which causes a pronounced Fano profile in the discrete state [2]. We note that in contrast to Ref. [1],


in the present experiment the localized orbital whichmixes with the continuous band of states occurs inthe initial state and is followed by a transition is intothe continuum of states of the outgoing photoelec-tron.

This study has shown the presence of two interest-ing effects in angle resolved photoemission spectraof layered systems. Firstly, Brillouin zone foldingbetween adjacent commensurate layers may createunexpected spectral intensities in a certain energyrange of the valence band spectra. Secondly, interfer-ence with localized levels of impurities present in thesame energy window may then lead, provided favor-able coupling between extended and localized statesoccurs in the initial state, to a characteristic Fanoprofile in photoemission spectra taken at fixedphoton energies. These findings may bear somerelevance for the study of low-dimensional systems,

Fig. 6. Measured WF change as a function of Cu concentration on of heavy Fermions, and of high-T materials, andc1 ML Xe on HOPG ( + ) and curves calculated with the Toppingmay provide a new and promising avenue for a Fanomodel. The dashed curve corresponds to a superstructure ofspectroscopy of impurities and small clusters atisolated Cu atoms, the dotted curve to a random distribution of Cusurfaces.atoms; the thick solid line represents the best fit. After Ref. [54].

3.2. Scanning tunneling spectroscopy of singlemagnetic impurities

Magnetic Ce adatoms on a single crystal Ag(111)surface were chosen to represent an idealized Kondosystem [8,20,17,21]. Isolated Ce atoms and, forcomparison, isolated Ag atoms on Ag(111) weredeposited by evaporation from a tungsten filamentonto the Ag substrate at T 5 5 K. Subsequently thesurface was imaged to detect the isolated adatoms onthe Ag(111) terraces. In Fig. 8 the adatoms appear as

˚protrusions with ¯ 0.9 and ¯ 1.2 A height for Ag˚and Ce, respectively, with typical widths of ¯ 15 A

in constant current topographs taken at a tunnelingresistance of a few hundred MV. Fig. 9(b) displays adI /dV tunneling spectrum measured on top of anisolated Ce adatom. A clear depression or antireso-nance is observed in dI /dV around the Fermi energy.When the tip is moved laterally away from theimpurity atom this antiresonance vanishes already at

˚a lateral distance of 5 A from the center of themagnetic adatom and on the Ag(111) substrate theFig. 7. He I excited UP spectra (dots) and Fano line shapetypical spectrum of the Ag(111) surface state isanalysis (full curve) of the resonance in a narrow region below the

Fermi energy (see text). After Ref. [7]. observed (Fig. 9(e)), with the surface state band edge


Fig. 8. Pseudo three-dimensional constant-current image (low-pass filtered) of 5 Ag adatoms on Ag(111) at a temperature of T 5 5 K(tunneling parameters V5 200 mV, I 5 1 nA, area56.536.5 nm). After Ref. [38].

at E 5 2 70 meV and a surface band at higher observed reflecting a decreased hot-hole lifetime dueenergies [59,60]. At intermediate distances (Fig. 9(c) to scattering of surface state electrons at the impurityand (d)) broadening of the surface state edge is [61]. The weak spectral features above the onset of

the surface state are due to standing wave patternscaused by scattering at the adatoms [62,59]. Theseenergy-resolved Friedel oscillations are also visiblein space in the constant current image of Fig. 8 asweak circular waves around the adatoms. For com-parison, tunneling spectra of a single adsorbed Agatom serving as a prototype for a non-magnetic atomwere taken. The spectrum taken on top of a Agadatom (Fig. 9(a)) is featureless throughout the rangefrom 2 200 to 200 mV which is attributed toquenching of the Ag(111) surface state [59]. Thecomparison between Fig. 9(a) and (b) clearly indi-cates that the dip or antiresonance near the Fermienergy E is indeed characteristic of the magnetic CeF

adatom.To interpret these spectroscopic observations

model calculations for a single magnetic 4f impurityadsorbed on a non-magnetic metallic host were

Fig. 9. dI /dV spectra on (a) a single Ag adatom and (b)–(e) on performed using the zero temperature Gunnarsson–and near a single Ce adatom at T 5 5 K. The lateral distance

¨Schonhammer theory [15] and the slave bosonbetween the tip position and the center of the adatom is indicated.approach in the Non-Crossing-Approximation (NCA)Before opening the feedback loop the tunnel parameters were

V5 200 mV, I 5 0.1 nA. After Ref. [39]. [63,64]. For parameters in the range of published


values for CeAg alloys [65] Kondo peaks appear onan energy scale which matches that of the observedantiresonance.

A closer inspection of the tunneling process led us[8] to an explanation for the existence of an antireso-nance in the tunneling spectra as opposed to theresonance predicted by theory [66] and measured byphotoemission for similar systems [64,21]. Theprobing of a localized state immersed in a continuumby the tunneling microscope resembles the spec-troscopy of a discrete autoionized state, which hasbeen elucidated by Fano [1]. An electron tunnelingfrom the STM tip to, e.g. the empty states of the

Fig. 10. dI /dV spectra of a Ce impurity on Ag(111) for an energyKondo resonance above E (sample voltage V $ 0)Frange around the Fermi level. Squares: measurement (V5 100 mV,can occupy two energetically degenerate possibleI 5 0.1 nA before opening the feedback loop). Solid line: Calcula-

final states: the modified (hybridized) localized state tion using the Anderson single impurity model (see text). Afterand the continuum of band states. This situation Ref. [8].leads to quantum interference. In the limit where thetransition matrix element for the localized stateapproaches zero (q ¯ 0 case in Ref. [1]), the au-toionization spectrum shows a dip or antiresonance, on Au(111) [9] and on Cu(111) [10] which also giveas a consequence of the interference effects at the rise to a typical Fano-lineshape. Moreover, this Fanosite with the localized state. The width of the lineshape is also obtained in recent theoretical treat-antiresonance is on the same scale as that of the ments of the Kondo effect of a single magneticresonance. Indeed, at large tip to surface distances, adatom [23,24,73]. We note that the re-appearance ofthe STM is known to probe sp wave functions rather the surface state onset in the tunneling spectra at a

˚than the confined 4f-derived Kondo ground state lateral distance of about 40 A from the magnetic[67]. Thus the dI /dV spectrum should show an impurity may indicate the lateral influence of Kondoantiresonance instead of a f-spectral peak as ob- scattering on a quasi-two-dimensional electron sys-served experimentally. By approximating the Fano tem, i.e. the Ag(111) surface state electrons seem todip shape by the inverse NCA resonance [8] the be strongly involved in the scattering mechanism.lineshape in Fig. 10 (solid curve) is obtained, which The present scanning tunneling spectroscopicis in encouraging agreement with the experimental studies are a first step towards an understanding ofobservations (dots). The width of the Kondo an- magnetic impurity scattering at an atomic level.tiresonance obtained from the NCA calculation for There is still a lot of work ahead of us. For example,the single isolated Ce impurity on Ag(111) (Fig. 10 the spectroscopic investigation of a single magnetic(solid curve)) is d 5 kT 525(5) meV yielding a impurity as a function of temperature in order toK

Kondo temperature of T 5290(50) K which is reveal the evolution of the Kondo resonance belowK

considerably lower than the value of 1000 K found and above T . Moreover, the application of anK

for bulk a-Ce [17,21] and consistent with the external magnetic field should split the resonance byexpected decrease in hybridization due to reduced the Zeeman energy [16] as has been shown for aoverlap of the Ce adatom with its supporting Ag quantum dot in a single electron transistor [68,69].surface atoms. The finding of a change in the width and shape of the

Consequently, the Kondo resonance known from Kondo resonance for the Ce-clusters [8] and thephotoemission appears as a Kondo antiresonance in observation of the insensitivity of the lineshape tothe tunneling spectra due to a Fano interference the proximity of other Co atoms for interatom

˚effect. This interpretation has recently been con- spacings down to 6 A [9] points to exciting possi-firmed by similar STS observations from Co atoms bilities. The atom manipulation capabilities of the


STM [70] should allow us to construct new arrange- magnetic atoms with atomic resolution opens newments of magnetic impurities on a surface and to perspectives for probing magnetic nanostructures.study their mutual interaction via exchange couplingwith the conduction electrons as a function of their 3.3. Local photoemission with an STM-tipinteratomic distances. Atomically fabricated cobaltdimers on Au(111) have been found to show an 3.3.1. Relevant energy levels and thresholdsabrupt disappearance of the Kondo resonance for The experiment is conceptually simple, laser lightcobalt–cobalt separations of less than 0.6 nm [71] is focussed onto the tunnel junction formed by awhereas a trimer of Co-atoms shows again a Kondo- STM tip and a sample and the excited photoelectronsresonance [72]. In this way, e.g. a two-dimensional are detected by the tip in a retracted position or inKondo lattice can be realized [72]. Recently, a tunneling mode. It is instructive to visualizebeautiful experiment of a ‘quantum mirage’ has been schematically the energy levels of sample and tip inperformed, where the Kondo resonance is seen in the the region of the tunnel barrier during illuminationsecond focus of an elliptical quantum corral while and variation of the tunnel bias voltage. This dia-the magnetic Co adatom is placed only at one focal gram is correct for both cases, the tip in theposition [10,73]. These new experiments clearly tunneling mode and the tip in a retracted position. Inshow that detection and spectroscopy of individual the latter situation the barrier width is becoming

Fig. 11. (Top): Schematic energy level diagram in the region of the tunnel barrier during illumination of the sample with photons of energyhn . eF . eF . V5 0: Fermi level E of tip and sample; E : vacuum level of the sample; eF : sample workfunction; eF : tipS T F V S T

workfunction. Characteristic photocurrent thresholds V and V , see text. The tip bias voltage changes from left to right as indicated.1 2

(bottom): Resulting typical conductivity (I(V )) and differential conductivity (dI /dV ) curves. After Ref. [32].


much larger and the slope of the trapezoidal barrier the photoelectrons excited in the sample have suffi-changes accordingly [28]. Fig. 11(top) depicts a cient kinetic energy to overcome the barrier definedcommonly encountered situation, the photon energy by the workfunction of the tip and V is the threshold2

hn is larger than the workfunction of the sample when the vacuum level of the tip (by applying less(eF ) and the tip (eF ). In this case the photoelec- negative bias to the tip) is lowered until the barrierS T

trons can freely enter the tip and the tip voltage has height is only determined by the workfunction of theto be negatively biased to obtain the threshold V . At sample and all the excited photoelectrons from the1

smaller negative tip bias voltage the threshold V sample at the given photon energy can enter the tip,2

should be observable as indicated. The resulting i.e. they do not see any barrier at all. The value oftypical current–voltage (I /V ) and the corresponding the voltage for threshold V is given by eV 5 eF 21 1 T

differential conductance curves (dI /dV ) as a function hn, the one for V by V 5 F 2 F . Lowering the tip2 2 T S

of the tip bias voltage are illustrated schematically in voltage even more would in principle allow us toFig. 11(bottom) showing the characteristic thresholds detect bound states between the vacuum level andV and V [28]. Here V denotes the threshold when the Fermi level of a metallic sample, e.g. unoccupied1 2 1

surface states or image states. In order to observesuch states, however, tunneling through the barrier isnecessary.

This reasoning has been tested by performingphotocurrent measurements on a Ag(100) sample asfunction of photon energy (4.82, 5.08, and 5.21 eV)with the tip retracted by 1 nm from the tunnelingposition, i.e. no tunneling electrons are detected. Fig.12 displays the results of these experiments for twodifferent photon energies (4.82 and 5.21 eV). A clearshift of threshold V towards negatively tip bias1

voltages for increasing photon energies is observed.This experimental finding corresponds exactly to themodel situation illustrated in Fig. 11 and confirmsearlier results obtained at different wavelengths andfor different samples [27,28]. From this measurementwe are able to deduce the workfunction of the tip andof the sample. For example, in Fig. 12(bottom), for aphoton energy of 4.82 eV we obtain the photoelec-tron onset V at eV 5 2 0.8960.02 eV ( 5 eF 21 1 T

4.82 eV) yielding eF 5 3.9360.02 eV. Similarly,T

with V 5 0.2560.02 V we derive for the sample2

workfunction eF 5 4.1860.04 eV, which is in theS

range of the values known for Ag [74]. For higherphotonenergies a higher negative tip bias voltage hasto be applied in order to observe the photocurrentonset V , which leads (within the experimental error)1

to the same tip and sample workfunctions. ThisFig. 12. Conductivity (I(V )) and differential conductivity (dI /dV )from a Ag(100) sample at two laser wavelengths (energies). result shows that the Ir tip with a nominal workfunc-Lower: 257 nm (4.82 eV); upper: 238 nm (5.21 eV). The tip has tion of 5.27 eV (value for polycrystalline Ir [74]) isbeen withdrawn by 1 nm to exclude a tunnel current. V and V are1 2 most probably coated by Ag from mechanical con-defined at full width half maximum (FWHM) of the dI /dV curve.

tacts with the sample leading to a considerableThe threshold V clearly shifts in the direction of negative tip bias1lowering of the workfunction. We note that thevoltage with increasing photon energy. dI /dV is determined by

lock-in detection adding 10 mV modulation at 2 kHz to the bias. threshold V does not change with photon energy2


because it depends only on the work function recent work, illustrates the present spatial resolutiondifference between tip and sample. achieved in photocurrent imaging of a Rb covered

1Al(111) surface. In this experiment an Ar ion laser3.3.2. Photocurrent maps was used at a wavelength (energy) of 488 nm (2.54

The measurements of Gray [28] represent the eV). The results of mapping the photoelectron dis-state-of-the-art of the method. Fig. 13, taken from his tribution as a function of tip position is shown in Fig.

Fig. 13. (a) Topograph of a Rb/Al(111) surface showing an area with many domain boundaries (100 3 100 nm, I 5 1.5 nA). (b)t

Simultaneously acquired photocurrent map at a tip bias of 1.55 eV which is 0.05 eV below the photocurrent threshold. (c) Photocurrent mapat a tip bias of 1.65 eV which is 0.05 eV above the photocurrent threshold. From Ref. [28].


13. The tip bias voltage was set at 1.55 eV when troscopy of individual magnetic impurities, the ex-scanning from left to right (Fig. 13(b)), and at 1.65 pected Kondo resonance manifested itself as a FanoeV when scanning from right to left (Fig. 13(c)), i.e. resonance, providing us with new insights intojust 0.05 eV below and 0.05 eV above the threshold correlated electron states of an individual magneticT . The laser was chopped at 3 kHz and the output adatom. In the future, the combination of these two1

from the lock-in amplifier was used to form an image techniques on an atomic-scale may provide a promis-of the photocurrent simultaneously with the topog- ing tool for the detection and characterization ofraphy (Fig. 13(a)). magnetic and non-magnetic impurities and small

The topographs for the forward and backwards clusters on solid surfaces.scans were identical. Fig. 13(a) reveals an area of thesurface with a high density of domain boundaries. InFig. 13(b) no such features are detected in the Acknowledgementsphotocurrent map obtained below the threshold.However, Fig. 13(c), taken above threshold, shows It is a pleasure to acknowledge a very fruitfulan obvious correlation with the topography and, collaboration with R. Berndt, S. Crampin, R. Gaisch,which is the highlight of this experiment, domain J. Li, S. Messerli, and M.-H. Schaffner. The authorsboundaries of 3 nm width are clearly resolved in the would like to express their gratitude to S.M. Gray forphotocurrent map. his stimulating contributions in the beginning of the

The possibility to observe photoelectrons stimu- local photoemission project in our laboratory. Thislated by laser light from low workfunction surfaces work was supported by the Swiss National Scienceusing an STM tip as a detector is very interesting in Foundation.the context of spectroscopy and local workfunctionmeasurements on deposited, size-selected small clus-ters, an ongoing research activity in our laboratory References[75]. Here, the excitation of photoelectrons and, inthe tunneling regime, their interaction with tunneling

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Fano spectroscopy of impurities and clusters on solid surfaces