RAPID COMMUNICATIONSPHYSICAL REVIEW B

VOLUME 62, NUMBER 24

15 DECEMBER 2000-II

Modification of surface-state dispersion upon Xe adsorption: A scanning tunnelingmicroscope study
Ji-Yong Park, U. D. Ham, S.-J. Kahng, and Y. Kuk*
Department of Physics and Center for Science in Nanometer Scale, Seoul National University, Seoul, 151-742, Korea

K. Miyake,1 K. Hata,1 and H. Shigekawa1,21Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,
Tsukuba, 305-8573, Japan2
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo,
Tokyo, 113-8656, Japan
共Received 24 October 2000兲
The modification of the surface electronic structure by an adsorbate is measured quantitatively with scanningtunneling microscopy and spectroscopy for the first time. The standing wave of the Cu surface-state electrons
is utilized to probe the subtle change in the electronic structure on the Xe-covered Cu共111兲. The observed
Fermi wavelength on the Xe-covered surface is longer by 15% than on the bare Cu surface. The change upon
Xe adsorption is explained with the observed modified dispersion of the Cu surface state; upward shift by(130⫾20) meV with almost same effective mass.

There has been interest recently in the surface state electrons, localized on the 共111兲 surfaces of noble metals and the共0001兲, (101̄0) surfaces of Be.1–3 The electrons are known
to play as important a role as the bulk electrons in surfacerelated phenomena, such as adsorption, surface magnetism,surface and step energetics, and catalysis.4 They reveal
nearly two-dimensional 共2D兲 characteristics with a nearly
free-electron-like dispersion.5 The electrons in the surface
states, therefore, form standing waves of quantum interference on the surfaces when there is a potential well or a barrier, created by a point defect or a line defect1,2 or even aquantum corral.6 This long range modulation of local densityof states 共LDOS兲 is explained with many electron effect likeFriedel oscillation, change in the electronic state density near
the Fermi level by electron screening of the defect potential.
Direct imaging in real space of the modulation has been possible only with a scanning tunneling microscope 共STM兲.1,2
Since the observation of the electron confinement in
nanometer-scale structures,6–8 understandings in several
surface-related phenomena, such as the shape of Fermi
contour,3,9,10 surface state dispersions,1,2,11 surface state
lifetime,12,13 and surface Kondo temperature,14 have been
greatly advanced.
It is well known that adsorption of foreign atoms or molecules modifies the geometric structure, the work function,and the surface electronic state of a clean surface. In manycases, the adsorption removes the surface electronic state andleaves the bulk electronic state. In the case of alkali metals
and rare gases, however, their adsorptions modify the surface
electronic structure so slightly that the surface states are
persistent.4 The changes of the surface states and their dispersions with adsorbates have been often studied with photoemission spectroscopy, including two photon photoemission spectroscopy 共2PPES兲. Though the surface state
dispersions on clean surfaces have been successfully measured with STM and scanning tunneling spectroscopy 共STS兲,0163-1829/2000/62共24兲/16341共4兲/$15.00

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influence of the adsorbates on the surface states has not been
tried with STM. It has been believed that the surface stateelectrons may not tunnel through the adsorbate layer.
In this Rapid Communication, we report a way to quantitatively measure the change of the surface electronic state byan adsorbate on a metal surface with STM and STS for the
first time. We measure the standing waves of the surface
state electrons not only on the bare Cu共111兲 surface but also
on the surface with one monolayer 共ML兲 Xe. The smallchange in the Fermi wavelength 共15%兲 and the modified dispersion relation of the surface state measured by STS areused to explain the proposed redistribution of the charge density with the adsorption of Xe.15
Our experiments are performed in a low temperature
STM16 at the base pressure of ⬍1⫻10⫺10 torr. It works at
the temperature down to ⬃5 K. A Cu共111兲 single crystal iscleaned by repeated cycles of Ar⫹ ion sputtering and annealing up to 900 K. High purity Xe gas is dynamically dosed
through a precision leak valve and a stainless steel tube withthe diameter of 1/16 inch to the cold sample surface. This
dynamic supply ensures proper partial pressure of Xe gas
around the sample surface, but not in the whole STM chamber. The sample can be annealed by controlling the liquid
helium supply or using a heater attached on the back of thesample.
Xe atoms nucleate at step edges and defect sites on the Cu
terrace at the coverage of ⬃0.7 ML dosed at 10 K. The Xelayer can be agglomerated by annealing at ⬎30 K 共Ref. 17兲
and a flat Xe layer of 1 ML height can be obtained. The
surface is, then, partially 共⬃70%兲 covered with 1 ML highXe patches as shown in Fig. 1共a兲. Bright small, hexagonalislands are Xe islands of 2 ML height, which are pinned
around defects even after annealing. Using the Cu lattice
image as in Fig. 1共b兲, the length scale can be self-calibrated
exactly. Figure 1共c兲 is a high-resolution image of the Xelayer, showing the perfect crystallinity. The nearest neighbordistance of the Xe adlayer is determined to be (4.25
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FIG. 2. 共a兲 A Fourier-transformed 共logarithm of the power spectrum兲 image of pair correlation of Fig. 1共d兲 (1.2 Å ⫺1 ⫻1.2 Å ⫺1 ).
Two peaks at either side originate from different wavelengths of thestanding waves on the Xe-covered and the bare Cu surface. 共b兲
Cross section along white line in 共a兲. A difference of ⬃15% isvisible. k F,Xe and k F,Cu denote the Fermi wave vectors for the 1 ML
Xe-covered and the bare Cu共111兲, respectively.

ingly, we observe the standing wave patterns even on the
Xe-covered surface. With the step-flow growth of the Xelayer at the lower step edge at high temperature,17 standing
waves are imaged both on the 1 ML Xe-covered surface on
the lower terrace and on the bare Cu surface on the upperterrace simultaneously at the region, marked by a square in
Fig. 1共a兲, as shown in Fig. 1共d兲. With this image, any possibility of artifacts due to the tip change can be excluded. The
slight difference in height is due to the difference in LDOS
on the Xe-covered and the bare Cu surface at the bias voltage. The white protrusions along the boundary are Xe atoms
pinned at upper step edges of Cu during annealing process.
The wavelength of the standing waves on Xe layer is longer
by 15% than that on Cu surface.
The periodic structures can be revealed more clearly if theimage is presented in the Fourier space. This technique wasalready applied to determine the Fermi contour with the electron standing waves.9,10 In order to visualize the two different periodicities, we take the Fourier transformation 共FT兲 ofa positional pair-correlation function, in which the positional
pair-correlation function, G(r), is defined as
G 共 r兲 ⫽FIG. 1. Constant current STM image of 共a兲 ⬃0.7 ML Xecovered Cu共111兲 surface with V t ⫽0.4 V and I t ⫽0.4 nA at
5 K(2000⫻2000 Å 2 ). The image is shown as if a light source is atright side. Some regions are marked for clarity 共A: Cu terrace, B:
Xe layer, dotted line: original Cu step兲. Atomically resolved imageof 共b兲 Cu terrace and 共c兲 Xe layer in the region marked by A and B
in 共a兲, respectively 关 I t ⫽1 nA, 38⫻38 Å 2 , V t ⫽10 mV 共b兲 and 0.8 V
共c兲兴. 共d兲 A perspective view of the region marked by square in 共a兲.
Left side is Xe monolayer covering lower Cu terrace and right side
is upper Cu terrace 关V t ⫽10 mV and I t ⫽0.4 nA at 5 K(150⫻150 Å 2 )]. The inset shows an image obtained on Xe layer whichshows both standing waves and Xe lattice 关V t ⫽10 mV, I t⫽1.5 nA (75⫻75 Å 2 )].

⫾0.1) Å, indicating that the Xe layer is incommensuratewith the Cu substrate at ⬍10 K. The standing waves of thesurface state electrons are observed near defects or steps on
clean Cu共111兲 surface at 5 K as reported earlier.1 Surpris-冕h 共 r⬘ 兲 •h 共 r⬘ ⫹r兲 dr⬘ ,共1兲

where h(r) is the height in STM image at position r. Figure
2共a兲 is an example of the FT of the pair-correlation. Twopeaks are visible at both sides, confirming the existence of
two standing waves with different wavelengths; those on the
Xe-covered and the bare Cu surface. The features at the center and along the axis are artifacts that are originated fromnoise and insufficient sampling at small wave vectors. Figure
2共b兲 is the cross section along the line from center throughthese peaks, revealing two peaks with the instrumental andthe thermal broadening. The wavelength on the Xe-covered
surface is again found to be 15% longer than that on the bareCu surface as deduced from this cross section.
At first sight, it is difficult to imagine how the standing
waves can be observed on the Xe-covered surface, because
共i兲 the Xe layer does not have a surface state near the Fermi
level, 共ii兲 the wavelength is too large to be the atomic corrugation of Xe共atomic radius⫽4.3 Å). In the case of Xe atoms,
the highest occupied atomic orbital, 5p, and the lowest un-

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MODIFICATION OF SURFACE-STATE DISPERSION . . .

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On the Cu surface with 1 ML Xe, the onset is shifted towardFermi level by (130⫾20) meV as shown in Fig. 3共b兲. A shiftof ⬃100 meV was obtained in a 2PPES experiment.20 In thecase of alkali adsorbates, they change the surface electronic
structure such that lowering of the work function and downward shift of the surface state are expected.21 Although thework function is lowered on 1 ML Xe on Cu共111兲,20 the shift
is upward, i.e., opposite direction. The dI/dV images wereacquired simultaneously with constant current image. The
dispersion of the surface states can also be determined byanalyzing dI/dV images at varying bias voltages. We fit thedI/dV line scans perpendicular to a step by LDOS of the 2D
nearly free electrons of effective mass m * when they arescattered off the one-dimensional barrier 共step兲. The LDOS,
␳ s , is given by
FIG. 3. Experimental dI/dV spectra taken from 共a兲 Cu共111兲 and共b兲 1 ML Xe-covered Cu共111兲 at 5 K. 共c兲 dI/dV plot perpendicular
to a step at E⫽⫺200 meV 共solid circle兲. The line is a fit by Eq. 共2兲.
Left side 共negative distance兲 is the Xe-covered surface and rightside 共positive distance兲 is the bare Cu surface. 共d兲 Dispersion relation of the clean 共solid circle兲 and Xe-covered 共open circle兲 Cu共111兲
surface state at 5 K. The solid lines are quadratic fits to data.

occupied atomic orbital, 6s, are far from the Fermi level.15
But according to a local density approximation calculation,15
the Fermi-level conduction electrons extend further out into
the vacuum in the presence of Xe adsorbates than on clean
metal surface. Previous experiments also suggested thepile-up of the conduction electron density at the Fermi levelin the core of adsorbed rare gas atoms18 and reduced tunneling barrier of electrons through the adsorbed Xe layer.19 We
conclude that the observed standing waves are caused byelectrons in the Cu surface state of which the charge density
extends through 1 ML Xe layer with a longer decay lengththan on the bare surface. No sign of standing waves can befound on 2 ML Xe patches such as bright islands in Fig. 1共a兲,
indicating that the surface state electrons on Cu surface cancontribute to the tunneling process only through the 1 ML
Xe.
There can be several mechanisms, which can result in
longer wavelengths on Xe-covered layer. 共i兲 The confinement of the surface state electrons on a narrow terrace canresult in different wavelength in the STM image.7,8 As our
experimental results are obtained on wide terraces 共⬎200 Å兲,the confinement effect can be neglected. 共ii兲 It was shownthat the bulk electrons also contribute to the formation of thestanding waves with different wavelength.10 But this contribution near the step was found to be negligible10 and our
results do not support this possibility since most of our images are obtained near steps. 共iii兲 The longer wavelength may
be explained with the modified dispersion of the surface statewith 1 ML Xe. In order to verify this scenario, we perform
STS on both the Xe-covered and the bare Cu surface. The
measured dI/dV versus sample voltage dependence at one
point is approximately proportional to the product of LDOS
and the tunneling probability at the bias. It is obtained underopen feedback condition with a lock-in amplifier. By measuring the local dI/dV spectra on both surfaces, the onsets ofsurface states can be determined as in Figs. 3共a兲 and 3共b兲.
Well-known onset of the surface state at (440⫾10) meV below Fermi level is visible on clean Cu surface in Fig. 3共a兲.

␳ s ⫽L 0 关 1⫺rJ 0 共 2k 0 x 兲兴 ,共2兲

where J 0 is the zero-order Bessel function, L 0 ⫽m * /( ␲ ប 2 ),x is the distance from the step, k 2 ⫽2m * E/ប 2 , and r is thereflectance of the step. Figure 3共c兲 is one example of such a
fit at E⫽⫺200 meV. The results are summarized in Fig. 3共d兲on the Xe-covered and the bare Cu surfaces. Since the dI/dV
image represents the convolution of the topography and theLDOS, the topographic contribution can be minimized bytaking the image at a large tunneling gap.22 The measured
dispersion on the bare Cu surface is similar to the one reported earlier.1 Again it is confirmed that the dispersion ofCu surface with 1 ML Xe is shifted by (130⫾20) meV andthe effective electron mass is hardly changed within the parabolic band approximation 关(0.40⫾0.02)m e and (0.42⫾0.03)m e ]. The change of other characteristics can be deduced from our data. For example, the onset of the surface
state is broadened on the Xe-covered surface 关compare Figs.
3共a兲 and 3共b兲兴, suggesting the reduced lifetime of the surface
state,12 possibly due to the opening of a new decay channel
resulting from phonons on Xe overlayer. We find that theshift of the surface state persists on 1 ML Xe islands as small
as 20 nm2, discriminating the effect of impurity.
Our results clearly demonstrate that significant parts ofthe signal from adsorbate-covered surface in STM can comefrom the substrate through the adsorbates, especially which
do not have much DOS near the Fermi level. We also believethat our findings can be extended to other adsorbate systems,
such as alkali metals. Our result can be utilized such that thesurface state may be intentionally modified with rare gas
atoms to investigate whether the depopulation or populationof the surface state influences growth behaviors and others,23
which is in controversy recently.24
In summary, we show that the effect of adsorbate on surface state can be successfully investigated with STM for the
first time. The surface state electrons of Cu near Fermi levelare still visible with the presence of 1 ML Xe layer due to theredistribution of charge density in such a way that it extends
further at Xe adsorbates. It is believed that the longer wavelength of standing waves on the Xe-covered surface compared with the bare Cu surface is due to the shift of surface
state dispersion of Cu with Xe adsorption.
After submission of this paper, we realize similar experiments are in progress on the alkali-covered noble metal
surfaces.25

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The authors would like to thank N. D. Lang, J. Ihm, and J.
Yu for carefully reading this manuscript. This work was partially supported by Korean Ministry of Science and Technology through Creative Research Initiative Program and Ko-

rean Ministry of Education, BSRI-98-2416. Support fromGrant-in-Aid for Scientific Research from the Ministry of
Education, Science, Sport and Culture of Japan is also acknowledged.

*To whom correspondence should be addressed; Electronic ad-

12dress: ykuk@phya.snu.ac.kr
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