PHYSICAL REVIEW B, VOLUME 64, 235310

Fine electronic structure of the buckled dimers of Si„100… elucidated by atomically resolved
scanning tunneling spectroscopy and bias-dependent imaging
Kenji Hata,*,† Yasunori Shibata, and Hidemi Shigekawa‡
Institute of Applied Physics and CREST, Japan Science and Technology Corporation (JST), University of Tsukuba, Tennodai 1-1-1,
Tsukuba 305-8573, Japan
共Received 22 January 2001; revised manuscript received 3 June 2001; published 16 November 2001兲
We have combined spatially resolved scanning tunneling spectroscopy 共STS兲 measurements with biasdependent scanning tunneling microscope imaging of the buckled dimers of Si共100兲, both carried out at an
unprecedented spatial resolution. By combing both aspects, in a very unambiguous way, we can elucidate thedetail of the electronic structure of the Si共100兲 surface that has not been addressed or has been misunderstood
before. Our measurements clearly show that the third peak 共located at ⬃⫹1.5 V兲 in the STS spectra cannot be
attributed to the upper edge of the ␲ * surface band as previously believed. Analyses of our data suggest thatthe third peak could be attributed to the dimer bond (D 1 ,D i* ) states. Theoretically predicted charge transfer
between the atoms of the dimer, which is the key factor determining both the atomic and electronic structureof the dimer, was directly confirmed for the first time. Our results imply that the empty ␲ * surface state islocalized closer to the Fermi level than previously understood, a point that should be treated with great care in
future surface science researches.
DOI: 10.1103/PhysRevB.64.235310

PACS number共s兲: 68.35.Bs, 68.37.Ef

I. INTRODUCTION

the STS spectrum of the buckled dimer. Specifically, we conclude that the third peak 共located at ⬃⫹1.5 V兲 in the STS
spectra cannot be attributed to the ␲ * surface band as previously believed.11,12 Our results show that the ␲ * surface
band is localized closer to the Fermi level than understoodby STM researchers previously, a point that would be important for future surface science researches. Atomically resolved STS measurements of the buckled dimer enabled toaddress the fine modulation of the electronic structure induced by buckling. We confirm the theoretically predictedcharge transfer between the lower and upper atom of thebuckled dimer13 and point out that a previous STS attempt5
carried out on an apparent symmetric dimer at room temperature must have misjudged the strong spatial dependenceof the third peak as the charge transfer between the dimer
atoms.

Scanning tunneling microscopy 共STM兲 and its variants
such as scanning tunneling spectroscopy 共STS兲 stands as a
powerful tool with outstanding spatial resolution to study theelectronic properties of the surface. For Si共100兲, a number of
STM and STS researches have been conducted from which
many fundamental issues concerning the atomic configuration and electronic structure of the surface dimers have been
elucidated.1–11
However, we point out here that previous researches were
inadequate to resolve the fine electronic structure of thebuckled dimer, particularly features that can be addressed
only through spatially resolved experiments. Many of theprevious STM and STS researches1–5 have been carried out
at room temperature on a flip-flopping apparent symmetricdimer. However, in order to address to the fine and intrinsic
characteristics of the dimer, it is necessary to carry out experiments at low temperatures below 200 K where thedimers are buckled.6 This is especially true when studying
electronic features associated with buckling. Nevertheless,no spatially resolved STS measurements have been reported
on the buckled dimer. Moreover, most of the existing STM
observations had a strong tendency to concentrate on filledstate imaging at a fixed bias.1–5 Only recently it became
realized that probing the surface with various biases is important, and bias-dependent STM observations suggested
that tunneling from surface resonances and bulk states seriously contribute to the STM images within usual tunnelingcondition.1,7Here we report results of spatially resolved STS measurements combined with bias-dependent STM imaging of thebuckled dimers of Si共100兲 at 80 K, carried out at an unprecedented spatial resolution. Analyses of the spatial variationsof the spectroscopic features in the STS spectra and biasdependent STM images lead us to give a new assignment for
0163-1829/2001/64共23兲/235310共6兲/$20.00

II. EXPERIMENT

All measurements were performed on the commercial
JEOL 4500-XT system built in an ultrahigh vacuum chamberwith a base pressure of 3⫻10– 9 Pa. N-type Si samples
phosphorus-doped with a conductivity of 0.01 ⍀ cm wereused.14 After ultrasonic cleaning for 10 min in acetone,
Si共001兲 sample were loaded into the vacuum chamber andprebaked at ⬃600 °C for 12 h with a vacuum pressure below
1.0⫻10⫺7 Pa. After prebaking, the sample was once flashedto 1200 °C for 10 s to remove the oxidized layers. After the
sample and sample holder are completely cooled, an additional flashing was carried out at 1200 °C for a very shorttime 共⬃5 s兲 to reduce the density of defects. Description of
the complete fabrication process of the Si共100兲 clean surface
has been published elsewhere.15 The STM tips were preparedby the electrochemical etching of tungsten wires.
Spectroscopic measurements were carried out in the current imaging tunneling spectroscopy 共CITS兲 mode16 at 80 K.
An I-V curve consists of 128 I-V values in the range of ⫺2

64 235310-1©2001 The American Physical Society

KENJI HATA, YASUNORI SHIBATA, AND HIDEMI SHIGEKAWA

PHYSICAL REVIEW B 64 235310

FIG. 1. Averaged STS spectra of a 128⫻128 CITS data set
acquired on a defect free 4⫻4 nm. surface area of buckled dimersof Si共100兲 at 80 K. Tunneling conditions were established at V s⫽⫹0.6 V, I t ⫽1 nA. The shaded region represents the surface
bandgap where the tunneling noise 共⬃several pA兲 is seriously emphasized by the normalization procedure.

to ⫹2 V, thus the energy resolution is 0.031 V. I-V measurements were made on 128⫻128 positions of the scanned surface area. Measurements were carried out on regions thathave the least defects in the surroundings to avoid any possible influence from them. Tip-sample separations during theI-V measurement were established at a set surface bias V s of
⫹0.6 V, and a set tunneling current I t of 1.0 nA, thoughsimilar results were obtained for negative set surface bias
V s ⫽⫺1.0 and other cases, e.g., V s ⫽⫹1.0 V. For the 4⫻4 nm surface area imaged in the case of Fig. 2, the geometric resolution is limited to about one spectrum per
0.09Å2. An average tunneling spectrum is presented in Fig. 1that was calculated from 16384 I-V curves of a CITS data
set of which a part is shown in Fig. 2. The similarity between
the STS spectrum of Fig. 1 and that obtained on apparent
symmetric dimers at room temperature by Hamers et al.2 ensures that the obtained STS spectra are not seriously devalued by any possible electronic structure of the tunnelingtip.17 From the measured I-V curves, the normalized tunneling conductivities 关STS spectra, (dI/dV)/(I/V) versus V兴
were numerically calculated as described in Ref. 18.
III. RESULTS AND DISCUSSION

FIG. 2. Spatial mapping of the STS spectra of the buckled
dimers of Si共100兲 at 80 K. 共a兲 A part of the topographic STM image
共V s ⫽⫹0.6 V, I t ⫽1.0 nA兲 taken simultaneously to the spectra. In
each area surrounded by the black grids, 3⫻3 tunneling spectroscopy measurements were carried out. Registry of the buckled
dimers was given by considering the symmetry of the image following the procedure in Ref. 7. White grids overlapped with the
STM image show the 2⫻1 units of the dimers. They are slightly
distorted as a result of thermal drift. This slight distortion is compensated or taken into consideration in the analyses. The large andsmall circles indicate the location of the upper and lower atoms ofthe dimer where their coordinates against the unit cell grids were
provided by first principle calculations in Ref. 13. 共b兲 A set of 9
⫻9 normalized tunneling spectra taken from the surface area enclosed by the box in Fig. 2共a兲. Each STS spectrum represents anaverage of 3⫻3 STS spectra. For clarity, the empty state STM
image is overlapped.

A tunneling spectrum of the dimers is characterized bythree peaks; one pronounced peak in the filled states at⫺0,7⬃⫺0.8 V 共first peak兲 and two rather small peaks in theempty states at ⬃⫹0.5 V 共second peak兲 and at ⬃⫹1.5 V
共third peak兲, with a surface bandgap of ⬃0.5 V as shown in
Fig. 1. Assignment of the two peaks in the empty states
seriously conflicts among researchers. Based on first principle calculations of the buckled dimers, Kageshima andTsukada insisted that the empty state peaks at ⬃⫹0.5 and⬃⫹1.5 V corresponds to the lower and upper edge of the ␲ *surface state 共refer to Fig. 8 and explanation in Ref. 12兲.
Actually, this assignment has been used to interpret sometunneling spectroscopy data.11 Their interpretation agrees
with elementary band theory where a one-dimensional 共1D兲band is taught to have a large local density of states at bothedges.19 First principle calculations13 and angle resolved ul-

traviolet photoelectron spectroscopy20,21 show that the bandsof the ␲ and ␲ * surface state are dispersive along the dimer
row 共the ⌫-J ⬘ direction in the surface Brillouin zone兲 whilea nearly flat band is obtained in the perpendicular direction(⌫-J direction兲. This means that the correlation among dimerrows is substantial weak, and that the surface states of the
dimer exhibit a one-dimensional 共1D兲 character. In fact, the1D nature of the ␲ * surface state was directly demonstrated
by the excellent match between the dispersion of the ␲ *surface state deduced from the standing-wave patterns
formed by electrons scattered by a 1D potential Al barriersand calculated from the 1D-Schrödinger equation.22
Qin and Lagally1 proposed a different assignment. Basedon a set of voltage dependent STM observations of the
dimers focused on low surface biases V s ⫽⫹1.4 and below,
they implied that the low-bias STM images dominantly re-235310-2

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PHYSICAL REVIEW B 64 235310

flect the ␲ * surface state, while the conventional high-bias
images 共typically 2.0 V兲 reflect mixed states, with the surface
state contribution not prominent. Studies carried out at low
temperatures 共80 K兲 reinforced this concept.7 These results
suggest that the third peak does not originate from the ␲ *surface state.
A spatially resolved STS measurement combined withbias-dependent imaging of the buckled dimers at low temperature are ideal means to address to this controversy. A partof the STM image taken simultaneously to a CITS measurement is shown in Fig. 2共a兲. Five bright rows running throughthe diagonal of the image are the Si dimer rows. A set of 9
⫻9 STS spectra taken from the surface area enclosed by thebox in Fig. 2共a兲 is displayed in Fig. 2共b兲. Each STS spectrum
represents an average of 3⫻3 STS spectra. For clarity, theempty state STM image is overlapped. Every STS spectrum
is composed from the three main peaks, though central to thefocus of this section is the spatial dependence of intensity ofthe third peak. At the first sight, it is clear that the intensity ofthe third peak shows an eminent atomic scale spatial variation. STS spectra of which the intensity of the third peak was
similar to that of the second peak were colored black, while
STS spectra that had a stronger third peak were coloredwhite. Comparison of the colored STS spectra and the overlapped STM image immediately leads to a conclusion thatthe third peak has a strong intensity at regions in between the
dimer rows. This result is in excellent agreement with therecently studied voltage dependence of the empty state STM
images of the dimers.1,7 At low biases 共below ⫹1.4 V兲, tunneling current from the ␲ * dangling bond localized at thelower atom of the buckled dimer dominates, hence the dimer
row is observed in the STM images. On the other hand, when
the bias is increased from a low to a high bias 共above ⫹1.4
V兲, a phase shift occurs in the STM images, and regions
between the dimer rows are observed at high biases.7 Since
the third peak of the STS spectra is located 共in energy兲 above
the bias where the phase shift is observed and is localized 共inspace兲 at regions in between the dimer, we conclude thattunneling from the electronic state that represents the third
peak in the STS spectra overwhelms tunneling from the state
associated with the second peak and is mainly observed in
the STM images at high biases. From a different point ofview, imaging the high bias empty state by STM could be
used as a sensitive and convenient method to map out the
spatial variation of the third peak of the STS spectra.
We utilize the spatial dependence of the high bias STM
images to seek the origin of the third peak of the STS spectra. The first possible origin that comes to mind is to attributethe third peak to the upper edge of the ␲ * surface state, aprevailed view previously proposed as mentioned in the introduction. Physics of chemical bonding tells that the node ofan antibonding state extends spatially with bias. A moderate
extension of the node could cause the phase shift observed athigh biases, a mechanism we nominate as the ␲ * node extension model in the followings. Figures 3共a兲–3共d兲 show
schematics of the predicted STM images based on the ␲ *node extension model at low and high biases. The left column shows cases of the buckled dimers 共at low temperatures
below 200 K兲, the right shows cases of the apparent symmet-

FIG. 3. Stick and ball models showing the predicted STM images. The left column shows cases of the buckled dimers 共at low
temperatures below 200 K兲, and the right column shows cases of
the apparent symmetric 共flip-flopping兲 dimers. 共a兲 to 共d兲 Predicted
schematics of the STM images based on the ␲ * node extension
model at low and high biases. 共e兲,共f兲 Schematics of the actual STM
image presented in Fig. 4.

ric 共flip-flopping兲 dimers. The ␲ * node extension model predicts STM images where zigzagged rows localized at the
dimer rows at low biases are observed 关Fig. 3共a兲兴, and this isexactly what is observed in the actual STM images 关Fig.
4共a兲兴. At high biases, a phase shift is predicted, and(2⫻1)-like features should be observed in between the
dimer rows adjacent to the lower atom of the buckled dimer
关Fig. 3共c兲兴.
STM images 共80 K兲 of a particular region that has a domain of apparent symmetric 共flip-flopping兲 dimers in the
middle and two buckled dimer domains at the sides areshown in Fig. 4. Coexistence of the buckled and apparent
symmetric dimers in the same image gives a unique opportunity to simultaneously study the voltage dependence of the
STM images of both kinds of dimers excluding any possible
ambiguity. Figure 4共a兲 is a low bias 共⫹0.6 V兲 STM image,and Fig. 4共b兲 is an image of the same region where the bias
was switched from a low 共⫹0.6 V兲 to a high bias 共⫹1.5 V兲 atthe location indicated by the arrow. Absolute registry of thehigh bias image against the low bias image can be given byoverlapping the two images. An enlarged high bias image
with the registry of the lower atom of the dimer is shown in
Fig. 4共c兲. Schematics of the low and high bias STM images
are shown in Fig. 3共e兲 共apparent symmetric dimer兲 and Fig.
3共f兲 共buckled dimer兲. Rows observed in the high bias imagesof the buckled dimer are localized in between the dimer row
and regions adjacent to the upper atom have the strongest
intensity. This experimental result is opposite to the predictions of the node extension model 关compare Fig. 3共c兲 and
Fig. 3共e兲兴. It should be noted that this discrepancy between

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PHYSICAL REVIEW B 64 235310

FIG. 5. STS spectra of the lower 共dashed line兲 and upper
共real line兲 atoms of the buckled dimer. The shaded region represents
the energy window of the surface band gap where the tunnelingnoise is emphasized by the normalized procedure.
FIG. 4. Empty state STM images of a particular surface region
where both buckled and apparent symmetric dimers domains coexist taken with different surface biases to study the dependence
共80 K兲 of the STM images on the surface bias. 共a兲 An empty state
STM image taken at a low bias (V s ⫽⫹0.6 V). 共b兲 The same
region though the bias was switched from a low (V s ⫽⫹0.6 V) to ahigh bias (V s ⫽⫹1.5 V) at the location indicated by the arrow.
共c兲 Enlarged image of the boundary. Upper region probed with a
low bias (V s ⫽⫹0.6 V) and the lower region with a high bias
(V s ⫽⫹1.5 V). Registry of the 2⫻1 units of the dimers represented
by the white grids and that of the lower atoms shown as circles
were provided from the region probed with a low bias based by aprocedure described in Ref. 7.

the experimental observation and the prediction of the node
extended model is impossible to recognize on the apparent
symmetric dimers 关compare Fig. 3共d兲 and Fig. 3共f兲 for schematics, and regions indicated by 共f兲 in Fig. 4共b兲 for actual
STM images兴. Since the first STM observations of the
dimers were carried out at room temperature on an apparent
symmetric dimer, it is now wonder that the node extension
model was proposed at that time.3,4 This result highlights theimportance to carry out experiments at low temperatures
共⬃200 K兲 on buckled dimers to understand the correct characteristics of the dimers. When the voltage dependent STM
images and CITS measurements of buckled dimers are combined, we reach to a conclusion that the third peak of the
STS spectra can not be attributed to the upper edge of the ␲ *surface state, and must originate from other state than the ␲and ␲ * surface states.
Another possible origin of the third peak of the STS spectra is the extended surface resonances and bulk continuum
states located close to the Fermi level and localized close to
the surface. It is a difficult task to estimate these states by theconvenient first principle calculations that are based on the
super cell techniques because these states might extend deep
into the substrate and beyond the boundary of the cell used.
First principle self-scattering theoretical methods are suitedto treat surface resonances extending a long rang into thebulk. Kruger et al. have carried out first principle selfscattering calculations of the buckled dimer showing that the
dimer bond (D 1 ,D *i ) states lie close to the ␲ * surface statein energy.23,24 Tentatively, we assign them as the states observed by STM at high biases, though further detailed theoretical calculations are required to provide a deeper insight
into the origin of the third peak.
Since the third peak is not related to the ␲ and ␲ * surface
states, the first and second peaks should represent the ␲ and␲ * surface states. It is natural to attribute the first 共second兲
peak to the ␲ filled 共␲ * empty兲 surface state. Theories predict a charge transfer from the lower to the upper atom of the
dimer that is induced by buckling because the sp 2 (sp 3 )component of the hybrid Si-Si orbital of the lower 共upper兲atom of the dimer increases which in turn leads to an increase of the p(s) orbital component of the dangling bond.
This charge transfer must be reflected in the STS spectra, andthe first 共second兲 peak is predicted to have a stronger intensity at the upper 共lower兲 atom compared to the second 共first兲
peak. At the first sight of the spatially mapped STS spectra ofFig. 2共b兲, such difference is not readily noticeable. In order
to investigate the influence of the charge transfer on the STS
spectrum, 9⫻9(1 Å 2 ) STS spectra close to the upper andlower atoms of the buckled dimer were averaged of which
the first and second peak are displayed in Fig. 5. It showsthat the upper atom of the buckled dimer has a stronger intensity at the first peak 共␲ surface state兲 than the lower atom,
while the lower atom has a stronger intensity at the second
peak 共␲ * surface state兲 than the upper atom. The excellent
coincidence between the experimental results and theoreticalpredictions confirms the existence of a charge transfer fromthe lower atom to the upper atom.
We would like to note that the difference between the STS
spectra of the lower and upper atom is relatively small, a factthat becomes important when our results are compared to aprevious STS attempt to observe the charge transfer on apparent symmetric dimers at room temperature.5 Munz et al.
observed a very significant difference in spectra of dimers
taken at the right and left side of a dimer row in the empty
state high bias region at room temperature.25 They stressedthat this difference is due to the charge transfer between the
low and upper atom. First, we would point out that the influence of the charge transfer on STS spectra would be averaged out on an apparent symmetric dimer because the rightand left sides of a dimer are identical for a (2⫻1) apparent

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PHYSICAL REVIEW B 64 235310

symmetric dimer, each representing an overlapped state of
the two possible configuration of buckling as a result of theflip-flop motion. A preference for one configuration of thetwo possible configurations of buckling,26 might give aslightly asymmetric appearance of the dimers that was actually observed by Munz et al. Even in this case, the differenceof the STS spectra observed in the right and left sides should
be small, since the intrinsic difference between the STS spectra of the lower and upper atom is small. Therefore, thiseffect cannot account for the observed large difference between the STS spectra taken on the right and left side of the
dimer row. It is very likely that they have observed the strong
spatial dependence of the intensity of the third peak and havemisinterpreted it to be a result of a charge transfer between
dimer atoms.27 This result stresses the importance to carryout experiments at low temperatures 共below 200 K兲 on buckled dimers, not on apparent dimers at room temperature, tounderstand the correct characteristics of the dimers.
IV. CONCLUSION

We have carried out a very high spatial resolution STS
measurement and bias-dependent STM imaging of the buckled dimers of Si共100兲 at 80 K. Results from these high resolution studies enables us to address several key point unsolved issues regarding the Si共100兲 surface. First, mapping*Electronic address: khata@cmliris.harvard.edu†

Present address: Department of Chemistry and Chemical Biology,
Harvard Univsersity, Cambridge, MA 02138.
‡
Electronic address: hidemi@ims.tsukuba.ac.jp, www.http://
dora.ims.tsukuba.ac.jp
1
X. R. Qin and M. G. Lagally, Phys. Rev. B 59, 7293 共1999兲.
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R. M. Tromp, R. J. Hamers, and J. E. Demuth, Phys. Rev. Lett.
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R. J. Hamers, R. M. Tromp, and J. E. Demuth, Phys. Rev. B 34,
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A. W. Munz, Ch. Ziegler, and W. Gopel, Phys. Rev. Lett. 74, 2244
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H. Shigekawa, K. Hata, K. Miyake, M. Ishida, and S. Ozawa,Phys. Rev. B 55, 15 448 共1997兲.
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T. Yokoyama and K. Takayanagi, Phys. Rev. B 61, 5078 共2000兲.
12H. Kageshima and M. Tsukada, Phys. Rev. B 46, 6928 共1992兲.
13
R. Ramstad, G. Brocks, and P. J. Kelly, Phys. Rev. B 51, 14 504
共1995兲.
14
The peak positions of the STS spectra presented in this paper isshifted toward lower energy by approximately 0.2 V comparedto that reported by Hamers et al. 共Ref. 2兲. This is because thesubstrate we use is highly doped to ensure enough conductivity

out the spatial dependence of the STS spectra combined with
voltage dependent STM observations show that the third
peak 共located at ⬃⫹1.5 V兲 in the STS spectra cannot be
attributed to the upper edge of the ␲ * surface band as previously believed. Second, our results suggest that the third
peak could be attributed to the dimer bond (D 1 ,D i* ) states.
Third, STS spectra averaged over areas 1 Å square close to
the lower and upper atom of the buckled dimers revealed thetheoretically predicted charge transfer between the atoms ofthe dimer. Conformation of the charge transfer is very important because it is the factor that determines both theatomic and electronic structure of the surface, i.e., it makes
the surface semiconductive and is the driving force of buckling. Our new data advances in a significant way our understanding of the electronic structure of the Si共100兲 surfaces. It
clearly shows that the ␲ * surface state is localized muchcloser to the Fermi level than understood by STM researchers previously, and evokes a care to lower the surface bias
from the typically used value of 2 V in future STM studies. It
also suggests very well defined theoretical studies to be pursued in the future to resolve the remaining questions in thistechnological important surface.
The support of a Grant-in-Aid for Scientific Researchfrom the Ministry of Education, Science, Sports and Cultureof Japan is acknowledged.

to execute STM observations at low temperatures. Use of substrates with different doping density and types 共p-type substrates兲 give similar results, though with a shift of the Fermi
level.
15
K. Hata, T. Kimura, S. Ozawa, and H. Shigekawa, J. Vac. Sci.
Technol. A 18, 1933 共2000兲.
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R. J. Hamers, R. M. Tromp, and J. E. Demuth, Phys. Rev. Lett.
56, 1972 共1986兲.
17
Tip apexes that did no provide STS spectra similar to those presented in Ref. 3 were abandoned.
18
K. Hata, S. Ozawa, Y. Sainoo, K. Miyake, and H. Shigekawa,
Surf. Sci. 447, 156 共2000兲.
19
P. A. Cox, The Electronic Structure and Chemistry of Solids 共Oxford University Press, New York, 1987兲, Chap. 4.
20
Y. Enta, S. Suzuki, and S. Kono, Phys. Rev. Lett. 65, 2704 共1990兲.
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T. Yokoyama, M. Okamoto, and K. Takayanagi, Phys. Rev. Lett.
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P. Kruger, A. Mazur, J. Pollmann, and G. Wolfgarten, Phys. Rev.
Lett. 57, 1468 共1986兲.
24
P. Kruger and J. Pollmann, Phys. Rev. B 38, 10 578 共1988兲.
25
Refer to Fig. 2共b兲 in Ref. 5, note that their STS spectra do not
clearly resolve the three peaks.
26
K. Hata, Y. Sainoo, and H. Shigekawa, Phys. Rev. Lett. 86, 3084
共2001兲.
27
Clear evidence can be found from Fig. 3共a兲 in Ref. 5, where asequence of spectra recorded across three dimer rows is presented. Two dips observed there 共note that the dips are due tosaturation of the preamplifier and hence means high conductivity兲 are located in between the dimer row, and must reflect the235310-5

KENJI HATA, YASUNORI SHIBATA, AND HIDEMI SHIGEKAWA
strong intensity of the third peak at this place and energy. The
influence of the charge transfer should be observed withineach of the three noisy regions adjacent to the dips 共note that
the STS sequence spans three dimer rows兲, which is clearly
beyond the reliability of their measurement. The difference between the noisy region and the dipped region cannot reflect the235310-6

PHYSICAL REVIEW B 64 235310
charge transfer as they insisted because 共1兲 the STS sequencespans three dimer rows thus there should be three dips and noisy
regions instead of the two dips and three noisy region and 共2兲
our results exclusively show that the dip is caused by the third
peak which is shown to have a different origin from the surface
states.