Applied Surface Science 161 Ž2000. 508–514
www.elsevier.nlrlocaterapsusc
Observations of local electron states on the edges of the circular
pits on hydrogen-etched graphite surface by scanning
tunneling spectroscopy
Z. Klusek a,) , Z. Waqar b, E.A. Denisov c , T.N. Kompaniets c , I.V. Makarenko b,
A.N. Titkov b, A.S. Bhatti d
a
b
Department of Solid State Physics, UniÕersity of Lodz Pomorska 149 r 153, 90-236 Lodz, Poland
A.F. Ioffe Physical-technical Institute, Russian Academy of Sciences 26 Politekhnicheskaya, 194021 St. Petersburg, Russian Federation
c
Solid State Electronics Department, Research Institute of Physics, St. Petersburg UniÕersity, UlianoÕskaya 1, PetrodÕorets,
198904 St. Petersburg, Russian Federation
d
Dipartimento Di Fisica (G29), UniÕersita Degli Studi Di Roma La Sapienza, Piazzale Aldo Moro 2, 00185 Rome, Italy
Received 9 March 2000; accepted 28 April 2000
Abstract
Scanning tunneling microscopy ŽSTM. and spectroscopy ŽSTS. are used to study electronic states at the edges of the
circular pits on the hydrogen-etched graphite surface. The edge surface state revealed by tunneling spectroscopy appears as
the maximum of the local density of states ŽLDOS. in the energy range of 90–250 meV above the Fermi level. The
dispersion of the energy state is explained by the band broadening in AB stacked H-terminated graphite. The magnitude of
the edge state decreases with the distance from the pit edge as theoretically predicted. q 2000 Elsevier Science B.V. All
rights reserved.
PACS: 61.16.Ch; 61.72.Ff; 73.20.At
Keywords: Local electron states; Hydrogen-etched graphite surface; Scanning tunneling spectroscopy
1. Introduction
The electronic structure of graphite has been the
subject of many theoretical and experimental studies
as reviewed in Refs. w1–4x. Recently, the attention
has been drawn to tunneling spectroscopy investigations of the graphite p bands splitting w5x, electronic
structure of graphite surface with steps and edges
)
Corresponding author. Tel.: q48-42-6355704; fax: q48-426790030.
E-mail address: [email protected] ŽZ. Klusek..
w6–11x, and electronic structure of graphite disclination centres w12,13x. In particular, the tight binding
bands calculations were performed on the single-layer
graphite ribbon with edges of two shapes, zigzag and
armchair w6–9x. The ribbon zigzag edge shows the
localisation of the electrons near the edge and peak
in the density of states ŽDOS. at the Fermi level, i.e.
the localised edge state. The localised state stems
from the topology of the p electron networks with
the zigzag edge and does not appear in the armchair
edge at all w7–9x. As a result, the electronic structure
of the armchair edge is almost the same as in the
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 7 4 - 3
Z. Klusek et al.r Applied Surface Science 161 (2000) 508–514
case of a semi-infinite two-dimensional Ž2D. graphite
sheet and makes no contribution to the edge state.
Since the real edge structure is rather irregular,
having a mixture of zigzag and armchair sites, the
conditions under which the localised edge state survives are important. Fortunately, the theoretical investigations have shown that even a small number of
zigzag sites in armchair sequence generate nonnegligible edge state w8x. Then, the peculiar electronic structure of the edge can also be observed on
less developed morphological systems.
The influence of the localised state on the global
electronic properties considered in terms of DOS is
rather small when the ribbon width is large. However, in this case, the localised state can be observed
on the local density of states ŽLDOS. measured in
the vicinity of the zigzag ribbon edge. Furthermore,
the predicted edge state in a single-layer ribbon is
well reproduced in the zigzag edges of the multi-layer
AB stacking ribbons w11,14x. It suggests that the
localised edge state can be observed on a more
realistic surface, like AB stacked bulk graphite, instead of a single-layer graphite ribbon. The only
discrepancy which can be expected is the shift of the
surface state relative to the Fermi level. It is because
of the presence of small band dispersion around this
level.
The experimental observation of the localised state
might be difficult since its appearance depends on
the edge shape. Then, specific surface preparations
and techniques are required to give us spectroscopic
information on the LDOS at atomic level. In a
previous paper w11x, we suggested thermal oxidation
of the Ž0001. basal plane of highly oriented pyrolytic
graphite surface ŽHOPG. as a preparation method.
The thermal oxidation process of the HOPG leads to
the removal of monoatomic carbon layers from the
surface and to the formation of monolayer and multilayer circular pits on the exposed plane w10,11,15,16x.
The circular geometry of the pit makes it possible to
find the proper orientation of the step edge Žrunning
around the pit, see Fig. 1. and to detect the localised
state by use of the scanning tunneling spectroscopy
ŽSTS. technique.
Our spectroscopic data manifested the appearance
of the localised edge state near the Fermi level. The
state is caused by the presence of the zigzag regions,
which form the pit edge. However, the thermal oxi-
509
Fig. 1. Development of nearly circular pits with zigzag and
armchair edge shapes.
dation process leads to the appearance of small
amounts of covalently bounded surface heterogroups
ŽC`OH, C`OOH. at the pit edge w10,11x. The existence of the heterogroups affects the p and p )
bands of the graphite located close to the Fermi level
w17x. Then, the circumspection is needed when comparing the tunneling spectra with the theoretical studies of the stepped graphite surface.
The exposure of the Ž0001. basal plane of HOPG
to the atomic hydrogen leads to the formation of
circular pits and steps on the graphite exposed plane
w18x. The advantage of this method, in comparison
with the thermal oxidation, is the lack of covalently
bounded oxygenated groups ŽC`OH, C`OOH. at
the pit edge. The atomic hydrogen treatment leads
only to the appearance of hydrogen-terminated carbon atoms at the edge. Fortunately, the C`H bond is
the srs ) states and does not mix with prp )
states of graphite. Then, the influence of the C`H
bonds makes no contribution to the electronic states
near the Fermi level w6,8,9,14x. As in the case of
thermal oxidation, the circular geometry of the pit
Žcreated in hydrogen treatment. makes it possible to
find the zigzag regions, which generate the localised
edge state near the Fermi level.
The purpose of this study is to obtain a detailed
understanding of the tunneling spectra near the pit
edge created by the atomic hydrogen treatment. In
this way, we can determine whether the edge state
near the Fermi level exists. Since the specific surface
preparation will be used, we exclude convolution of
the pit edge LDOS near the Fermi level with other
510
Z. Klusek et al.r Applied Surface Science 161 (2000) 508–514
contributions. The obtained results will make it possible to understand better the electronic structure of
graphite, the electronic structure of stepped graphite
surface, and can be useful in interpreting spectra
obtained from the physically adsorbed molecules on
the defected graphite substrate w19x.
2. Experimental
Pyrolytical graphite samples of 0.15 = 1 = 40
mm3 sizes are cleaved by an adhesive tape and
mounted inside the vacuum chamber. For 2 h, the
temperature is raised to 14008C with a base pressure
of 10y8 Torr to purge impurity gases from the
surfaces. Then the samples are cooled down to room
temperature and reference samples are taken out
from the vacuum chamber for scanning tunneling
microscopy ŽSTM. investigations. For hydrogen exposition, the chamber is filled with purified molecular hydrogen to a base pressure of 10y2 Torr. Dissociation of molecular hydrogen into atomic hydrogen
is made by a 100-mm diameter tungsten wire heated
to 25008C and placed parallel to the sample at a
distance of 6 mm. The hydrogen atom flux targeting
the basal plane of the graphite sample is of the
5 = 10 13 atomsrcm2 s order. Different samples are
exposed by atomic hydrogen for different time intervals. After the treatment, the samples are cooled
down to room temperature and thermal desorption
ŽTD. is used to measure the amount of the sorbed
hydrogen by the graphite. No TD signal of the
hydrogen is observed after an exposure of the samples to molecular hydrogen, even for long time
intervals, whereas for the samples exposed to atomic
hydrogen, the TD of hydrogen is observed in the
amounts depending on the time of exposure and flux
of the hydrogen atoms. It confirms that only atomic
hydrogen interacts with the graphite surface. The TD
measurements show the amount of sorbed hydrogen
from 7.5 = 10 15 Žthe 2.5 min irradiated sample. up to
3.75 = 10 17 atomsrcm2 Žthe 125 min irradiated sample.. Finally, the samples subjected to different hydrogen dozes and the reference samples Žwithout
hydrogen treatment. are studied by STM.
The STMrSTS experiments are performed at
room temperature in ambient air conditions. A high
stability STM with spectroscopic capabilities is used
for measurements w20x. The tips are obtained by the
mechanical cut from the 90% Pt–10% Ir alloy wires.
In spectroscopic mode, IrV curves are recorded
simultaneously with a constant current image by the
interrupted-feed-back-loop technique. At every point
in the STM X-Y scan, the feedback-loop is stopped
and within a limited time interval Ž37 ms for both
polarities., the IrV characteristic is recorded for 40
discrete voltages in the range from y1 to q1 V.
Since a large number of data is collected, the scan
takes about 6 min to finish. Then, some thermal drift
can be expected during the scanning process. Nevertheless, the strength of the method is that the topographic image and spectroscopic data are correlated
because they are taken simultaneously. The IrV
curves obtained in the way described above are
stored in a laboratory computer and their voltage
derivative is obtained. After each acquisition sequence, the surface is scanned again in order to
observe the influence of spectroscopy measurements.
In the case of visible surface damages, the spectroscopy data are not taken into account.
3. STM results
The STM studies show that all samples exposed
to atomic hydrogen demonstrate etch-pits formation
on their surfaces. When increasing a doze, the pits
become larger and their density gets higher. However, the STS results at the pit edges are quite similar
for all the samples.
The STM image of the surface before the exposition and the AFM image after the exposition are
shown in Fig. 2a and b, respectively. It should be
mentioned that it is not possible to record clear STM
images of the graphite surface immediately after
exposition by atomic hydrogen. In our opinion, it is
due to the hydrogen accumulation between the upper
graphite layers leading to large increase of the tunneling current noise.
However, keeping sample in air tends to flatten
the surface due to the gain of minimum energy for
atoms redistributed by irradiation process. The STM
images recorded after 4 weeks show smooth graphite
etched surface with nearly circular pits distributed
randomly over the terraces. They have different diameters and appear on different levels of the basal
Z. Klusek et al.r Applied Surface Science 161 (2000) 508–514
Fig. 2. Ža. 1000=1000 nm STM image of fresh cleaved graphite
surface. Žb. 1000=1000 nm AFM image recorded immediately
after the atomic hydrogen irradiation process. Žc. 280=280 nm
STM image of the atomic hydrogen etched pit. Žd. 763=763 nm
STM image of the thermally oxidised etched pits.
plane. A detailed structure of the surface can be seen
by STM, as presented in Fig. 2c. The depth of the pit
Ždenoted by an arrow. estimated from the height
profile is 0.38 nm, which is roughly the spacing
between the graphite layers. Multilayer pits are also
observed. The typical atomic structure for the
graphite plane can be seen away from the pit edges
both on the terraces and the pit bottoms. The created
pits are very similar to those obtained for thermally
oxidised graphite surface w9,10x. This is presented in
Fig. 2d, where monolayer circular pits created at
8008C in air for 3 min are shown.
The obtained STM results show that on graphite
surface, the atomic hydrogen irradiation process leads
to the appearance of well-defined nearly circular pits.
Then, circular geometry of the pit makes it possible
to find the zigzag regions at the edge and to detect
the edge state near the Fermi level by the use of the
STS technique.
4. STS results
The main idea of STS is to measure the dependence of the tunneling current on the applied voltage,
511
i.e. IrV. Then, the first derivative of the tunneling
current with respect to voltage, i.e. d IrdV, gives the
LDOS of the sample affected by the transmission
coefficient and the electronic structure of the tip w21x.
Even though the electronic structure of the tip is
unknown, it is both constant and independent of
spatial location. Hence, the electronic structure of the
tip can be treated as a constant background to the
d IrdV. The effect of the voltage dependence of the
transmission coefficient is often minimised by the
use of normalised first derivative of the tunneling
current with respect to voltage, i.e. Žd IrdV .rŽ IrV .
w22,23x.
It should be mentioned here that as the case may
be the methods of the spectroscopic data, presentation can be addressed in several ways. In our studies,
we focus on the d IrdV vs. voltage plots. It is due to
the fact that the d IrdV quantity can be presented in
real units ŽnArV. instead of arbitrary units typical of
the Žd IrdV .rŽ IrV .. Then, not only qualitative but
also quantitative comparisons can be performed. Furthermore, as it has been found during the experiments, the influence of the transmission coefficient
on the strength of the spectroscopic features is negligible. Consequently, the use of d IrdV instead of
Žd IrdV .rŽ IrV . is justified.
Fig. 3 presents typical IrV curves recorded on
the hydrogenated graphite sample away from the
monolayer pit edge Žsee Fig. 2c.. The curves show a
little asymmetric shape, the tunneling current being
higher for negative polarisation of the sample than
for a positive voltage of the same value. This type of
asymmetry is typical of the pure graphite surface
w17,24,25x and shows that in these regions, we are
dealing with the unaffected graphite. This conclusion
is also confirmed by STM measurements showing
typical triangular structure of graphite in these regions.
The next stage of our investigations is to measure
the tunneling spectra close to the monolayer pit edge.
The results show the inset in Fig. 3. It is immediately
seen that the IrV curves manifest an abrupt increase
of the tunneling current at small positive bias voltages, which is different from an unaffected graphite
surface. This can be due to the presence of the
localised edge state near the Fermi level. Then, it
seems to be interesting to analyse the spectroscopic
data of the form d IrdV vs. sample bias Žsample bias
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Z. Klusek et al.r Applied Surface Science 161 (2000) 508–514
Fig. 3. The Ir V data recorded on the hydrogenated graphite
sample, away from the mololayer pit edge. The inset shows the
Ir V data recorded close to the pit edge.
corresponds to the energy of the state relative to the
Fermi level., which is related to the LDOS of the
sample.
In Fig. 4, the five d IrdV curves, denoted as Ž a.,
Ž b ., Ž c ., Ž d . and Ž e . are recorded at different distances from the monolayer pit edge. The Ž a. curve is
recorded at 2 nm from the pit edges and the curve
Ž e ., over the edge. The Ž b ., Ž c . and Ž d . curves are
recorded at 1.5, 1 and 0.5 nm from the pit edge,
respectively. The inspection of this figure shows the
appearance of the local maximum of LDOS located
at about 200 meV above the Fermi level Žcurves Ž c .,
Ž d . and Ž e ... In our interpretation, the observed
peaks can be attributed to the localised edge state on
the graphite surface. The observed features are well
visible and cannot be mistaken with other graphite
states. In the low energy range around the Fermi
level Ž"1 eV., it is possible to observe the state at
0.6–0.8 eV below the Fermi level, which is attributed to the point P1yŽ p . in the Brillouin zone
Žsee Ref. w5x and references therein.. It is also possiŽ ) . state located at about 0.8
ble to observe the Py
2 p
eV above the Fermi level Žsee Ref. w5x and references
therein.. Both, theoretical and experimental studies
for unaffected graphite do not predict any features
close to the Fermi level. This means that the interpre-
tation of the peak near 200 meV above the Fermi
level as a localised pit edge state seems to be justified. Additionally, the obtained results show clearly
that the peak attributed to the localised edge state
decreases with the distance from the pit edge. At
about 2 nm from the pit, the d IrdV curve resembles
the curves recorded over the unaffected graphite
basal plane. The observations can be explained in
terms of a decrease of the localised edge state magnitude with an increase in the distance from the pit
edge w6x. Similar behaviour is observed on the pits
edges of thermally oxidised graphite surface and
reported in Ref. w11x.
It should be emphasised here that the d IrdV data
obtained from different points over the pit edges
indicate energetic heterogeneity considered in terms
of the LDOS changes. In particular, the curves with
the Ž a. curve shape, presented in Fig. 4, are frequently seen. The observed differences prove that the
edge is not uniform since it has a mixture of zigzag
and armchair structures. Thus, we can assume that
the localised state near the Fermi level appears only
in the regions in which the edge is of a zigzag shape
Fig. 4. The d IrdV data for occupied and unoccupied electronic
states recorded at the points close to one another along the line
perpendicular to the pit edge. The Ž a. curve is recorded at about 2
nm from the pit edge, while the Ž e . curve is over the edge. The
Ž b ., Ž c . and Ž d . curves are recorded at 1.5, 1 and 0.5 nm from the
pit edge, respectively.
Z. Klusek et al.r Applied Surface Science 161 (2000) 508–514
Žsee Fig. 1.. Unfortunately, by the use of STM, we
are not able to distinguish different types of edge
shapes. In the pit edge regions, an extra contrast of
small bright areas of nearly atomic size is observed
only. It is because the STM does not show an atomic
structure of the surface in crystallographic sense, but
rather, an electronic structure of the surface, which is
strongly affected by the LDOS near the Fermi level.
Then, the presence of zigzag and armchair regions
can only be determined by the use of tunneling
spectroscopy instead of tunneling microscopy. The
recording of energetic heterogeneity seems to be an
important result in our investigations and is the
subject of our further studies.
In Fig. 5, the d IrdV data Žfor the sake of clarity,
we present only three curves. for occupied and unoccupied electronic states are recorded at the same
distance from the pit edge and it equals 0.5 nm. The
distance between the Ž a., Ž b . and Ž c . curves is also
the same and equals 0.5 nm as well. It is seen that
the intensity of the observed peaks does not depend
strongly on the tip location. However, the energetic
position changes considerably. When considering a
smaller distance between measurement points Ži.e.
0.2 nm., the smooth shift of the peaks in the energy
range of 90–250 meV is observed. Moreover, the
Fig. 5. The d IrdV data for occupied and unoccupied electronic
states are recorded at the same distance from the pit edge and it
equals 0.5 nm. The distance between the Ž a., Ž b . and Ž c . curves is
also the same and equals 0.5 nm as well.
513
amplitude of the peaks decreases with the distance
from the pit edge.
In our opinion, peaks in the energy range of
90–250 meV are simply interpreted in terms of
localised edge state. To support this conclusion, we
recall that the localised edge state at the Fermi level
appears only in the case of a single-layer Hterminated graphitic ribbon w6–9,14x. In addition,
when we consider the AB stacked H-terminated
graphite, a small band dispersion around the Fermi
level is expected w14x. Consequently, the energy of
the edge state can move away from the Fermi level.
Since in our experiments, we use hydrogenated
graphite instead of a single-layer H-terminated
graphitic ribbon, the spread of the edge state energy
depending on the local atomic conditions can be
observed. Furthermore, our experimental results show
that both the appearance and the energy range of the
localised edge state do not depend strongly on the
depth of the etched pit. It is in good agreement with
the first-principles study of the edge state in the case
of the AB stacking graphite. However, more research
needs to be done to clarify the influence of pit depth
and local surface defects on the edge states. That is
why here, we present the results on monolayer pits
only.
Finally, it should be mentioned that the existence
of a circular edge in the graphite sheet may lead to
the appearance of fused disclination centres on the
upper terrace as a result of mechanical deformations
w12,13x. Especially, fused disclination centre consisting of five- and seven-membered rings lead to sharp
resonant states with the energy of 0.2T below and
0.2T above the Fermi level ŽT means hopping integral, whose value roughly equals 3 eV for graphite..
However, in our tunneling experiments, we do not
observe distinct electronic states at energy "600
meV around the Fermi level. We may believe that
the observed peaks in the energy range of 90–250
meV should be considered as a state caused by
zigzag edge sites instead of fused disclination centres. The above conclusion is supported by the fact
that it is possible to detect states with energy near
the 600 meV on the other carbon systems. Especially, these states are observed in the pentagon–
heptagon regions close to the end of the carbon
nanotube and in the joint connecting two nanotubes
w26–29x.
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Z. Klusek et al.r Applied Surface Science 161 (2000) 508–514
5. Conclusions
By the use of tunneling spectroscopy, we have
detected the localised state near the Fermi level on
the circular monolayer pit edge in the hydrogenated
graphite surface. In our opinion, the state is caused
by the zigzag edge sites instead of fused disclination
centres. The edge state appears as the maximum of
the LDOS in the energy range of 90–250 meV above
the Fermi level. The state magnitude decreases with
the distance from the pit edge as theoretically predicted.
The shift of the state energy in the range of
90–250 meV is explained by the small band broadening around the Fermi level in AB stacked Hterminated graphite. The influence of the C`H bonds
in hydrogen-etched graphite makes no contribution
to the electronic states near the Fermi level. It is
because the C`H bonds are the srs ) states and do
not mix with the prp ) states of graphite.
The existence of edge state on the multilayer pits
is also observed. However, the obtained results demand thorough discussion on the influence of the pit
depth on magnitude of the edge state and will be
presented in a separate paper.
Acknowledgements
The work was supported by the Lodz University
within Research Grant 505r255, and partly, by the
Russian Foundation for Basic Research.
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