Surface Science 600 (2006) 1604–1607
www.elsevier.com/locate/susc
STS investigations of temperature dependence of Au(1 1 1)
surface state energy position
P. Kowalczyk
b
a,*
, W. Kozlowski a, W. Olejniczak a, P.K. Datta
b
a
Department of Solid State Physics, University of Lodz, Pomorska 149/153, 90-236 Lodz, Poland
Advanced Materials Research Institute (AMRI), University of Northumbria, Ellison Building, Ellison Place, Newcastle upon Tyne, NE1 8ST, UK
Available online 3 February 2006
Abstract
High temperature scanning tunneling spectroscopy (HT-STS) was used to investigate the electronic structure of Au(1 1 1) at different
temperatures in the energy range 0–1 eV below the Fermi level. We concentrated on the influence of temperature on the Shockley surface
state (SS) appearing on noble metals surface due to a surface projected bulk bang gap in [1 1 1] direction. The influence of temperature on
the projected band gap edge (BE) was also investigated. The experiment was carried out in the temperature range 294–580 K. As the
result of the experiment a delicate shift of the SS and the BE in direction of the Fermi level was reported.
2006 Elsevier B.V. All rights reserved.
Keywords: High temperature scanning tunneling spectroscopy; Electronic structure; Gold; Surface state
1. Introduction
Gold belongs to a group of materials which have the
Shockley surface state (SS) lying in the projected bulk band
gap in the C–L direction [1]. In the past the SS was extensively studied by the use of ultraviolet photoemission spectroscopy (UPS) [2–4] and scanning tunneling spectroscopy
(STS) [5–9]. In the recent years it was found that the SS on
Au(1 1 1) surface was split in momentum space which was
interpreted as the presence of a spin structure of the surface
state [10–12]. The influence of size effects on the electronic
structure was also investigated on gold vicinial surfaces
[13–15], clusters [16,17] and atomic chains [18]. Also a lifetime of noble metals surface electrons was investigated
both experimentally [19–22] and theoretically [21–24].
Moreover scanning tunneling microscopy (STM) and spectroscopy were used to show distortions from a parabolic
dispersion relation of noble metal surface electrons [25].
Up to now investigations of the electronic structure of
Au(1 1 1) were performed mainly at temperatures from
*
Corresponding author. Tel.: +48 42 6355694; fax: +48 42 6790030.
E-mail address: [email protected] (P. Kowalczyk).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2005.11.044
4 K up to room temperature. In our experiment we made
an effort to investigate the local density of states (LDOS)
of Au(1 1 1) surface at elevated temperatures in the range
294–580 K. We decided to use a high temperature STS
(HT-STS) technique which was successfully used in the
investigations of metal oxides [26–29]. Our main goal was
to investigate the influence of temperature on energy position of the local maximum originating from the SS. Temperature dependence of energy position of the SS was
investigated previously at low and elevated temperatures
by the use of UPS [4]. According to our knowledge no such
investigations were carried out by the use of STS up to
now. Therefore our results are the complement to work
of Paniago et al. [4].
2. Experimental Details
HT-STS investigations were carried out in UHV by the
use of Omicron VT-AFM/STM system equipped with
LEED/Auger spectrometer and a sputtering gun. The tip
used in this experiment was prepared by mechanical cutting
from 90%Pt–10%Ir ally wires. All spectroscopy data were
P. Kowalczyk et al. / Surface Science 600 (2006) 1604–1607
recorded in the current imaging tunneling spectroscopy
mode (CITS). The influence of transmission coefficient
was reduced by applying an arsinh(I) function on the tunneling current [30]. The first derivative of the tunneling
current with respect to voltage dI/dV(V) was calculated.
Additional normalization was performed by calculating
(dI/dV)/(I/V)(V) [31]. Each curve presented in this paper
is the result of averaging approximately 300 separate
curves.
HT-STM/STS investigations were carried out at five
temperatures 296 K, 373 K, 433 K, 483 K and 580 K with
one tip. The temperature was increased for 2–3 h between
two adjacent experimental steps and stabilized for approximately 12 h.
The Au(1 1 1) surface (gold evaporated on mica substrate) was prepared by repeated cycles of Ar+ sputtering
and annealing.
3. Results and discussion
Fig. 1 shows two normalized differential tunneling conductance curves #1 and #2 recorded at 294 K and 580 K
respectively. The curves were shifted along Y-scale for the
sake of clarity. It is clearly seen that a general shape of
curves #1 and #2 is different which is the result of the tip
changes during the experiment (especially at elevated temperatures a tip is changed relatively often). It has to be
remembered that STS is very sensitive to the tip DOS especially when the occupied states of the sample are investigated [32]. We believe that the change of the tip DOS is
responsible for the change of the shape of presented curves.
As a consequence of the tip DOS change we decided to
analyze positions of the local maxima on the averaged
curves rather than the shape of separate curves.
Fig. 1. Experimental differential tunneling conductance curves #1 and #2
recorded at temperatures 294 K and 580 K respectively. Dashed lines
indicate maxima of tunneling conductance. Bold dashed lines denoted by
SS and BE indicate maxima originating from the Shockley surface state
and the bulk band gap edge projected on the surface.
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After analysis of our spectroscopic data we denoted
eight of the LDOS maxima present at all temperatures.
For a better illustration of their position in Fig. 1 a dashed
line was added. It is clearly seen that the maxima positions
on plot #2 are shifted toward the Fermi level in comparison to plot #1. It has to be emphasized that the observed
shift was reversible with temperature. What is more, the
shifts of some of the observed maxima were weaker than
others. Particularly a very small shift was observed for
three out of eight observed maxima. These maxima were
located at the energy of 0.2 eV, 0.3 eV and 0.5 eV below
the Fermi level (as recorded at room temperature). In our
opinion these maxima are related to the tip DOS which
influences HT-STS measurements below the Fermi level
[32]. The tip temperature is lower than the sample temperature because the tip is heated by the sample radiation and
the effective area of heat transport to the tip apex is very
small. Assuming the influence of temperature on maxima
energy positions it is clear that the shift of the maxima originating from the tip should be weaker. We believe that
other maxima observed originate from the Au(1 1 1) surface
electronic structure. These maxima were located at the energy of 0.43 eV, 0.63 eV, 0.71 eV, 0.81 eV and 0.91 eV below the Fermi level. In our opinion two of these maxima
located at 0.43 eV and 0.91 eV below the Fermi level (as
read from curve #1) denoted by SS and BE in Fig. 1 are
related to the Shockley surface state [9–12,21,22,33] and
the surface projected bulk band edge (BE) [3,4,33] respectively. The origin of the rest of the maxima located at the
energy of 0.63 eV, 0.71 eV, 0.81 eV below the Fermi level
(as recorded at room temperature) are currently unknown
and will become the subject of further investigations.
The change of the SS and the BE energy positions with
temperature was shown in Fig. 2. Five curves recorded close
to the SS state at different temperatures in the range 294–
580 K denoted by #3–#7 are shown in Fig. 2(a). Curves denoted by #8–#12 recorded close to the BE state in the range
294–580 K were shown in Fig. 2(b). All curves were shifted
vertically for the sake of clarity. The SS as well as the BE
maxima were joined with dashed lines in Fig. 2(a) and (b)
which indicates the shift of maxima energy position. It is
noticeable that with the increase of the temperature the
SS maximum shifts towards the Fermi level (Fig. 2(a)). Energy position of SS equals 0.43 eV, 0.41 eV, 0.40 eV, 0.39 eV
and 0.37 eV below the Fermi level for temperatures 294 K,
373 K, 433 K, 483 K and 580 K respectively. For the BE
maximum we observed a similar shift toward the Fermi level. The energy position of the BE maxima equals 0.93 eV,
0.91 eV, 0.89 eV, 0.88 eV and 0.84 eV for temperatures
294 K, 373 K, 433 K, 483 K and 580 K, respectively.
In our opinion the most important physical effect
responsible for the observed states shift is enlargement of
Au atomic unit at elevated temperatures. The SS energy
position in noble metals is closely related to the energy position of the upper and the lower bulk band gap edges in
[1 1 1] direction. However, the change of the atomic unit
has a direct influence on the material electronic structure.
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P. Kowalczyk et al. / Surface Science 600 (2006) 1604–1607
Fig. 2. Experimental differential tunneling conductance curves recorded in the vicinity of the Shockley surface state (a) and the surface projected bulk
band gap edge (b) at different temperatures. Dashed lines in (a) and (b) indicate energy position of the SS and the BE maximum at all temperatures.
Therefore the increase of temperature of the Au(1 1 1) surface leads to a change in the gold dispersion relation and as
a consequence to a shift of the lower and the upper bulk
band gap edges towards the Fermi level [4]. Thus, the decrease of the bulk band gap leads to a change of the SS energy position. As a result the shift of the SS towards the
Fermi level is expected [4]. The BE shift is a direct consequence of the lower bulk band edge position at elevated
temperature. In summary, the change of the atomic unit
size due to sample heating leads to the SS and the BE maxima shift towards the Fermi level.
Other effect which could be responsible for the shift of the
maxima energy position at elevated temperature is dependence of the tunneling current on temperature. However,
we believe that in the first approximation the effects related
to the tunneling current dependence on temperature could
be neglected because of a weak influence of the Fermi–Dirac
function on tunneling current in that range of temperatures.
Finally, the position shift could be observed as a result
of the tip DOS changes. However, the average procedure
used for our experimental data generated almost a constant
value of the maximum energy position. The additional effect of averaging was broadening of the peek halfwidth.
However, the shape of the curves was not analyzed. In
our opinion the method we chose minimizes the effect of
the tip DOS influence.
Our results were summarized in Fig. 3 where the SS and
the BE maxima energy position versus temperature are
shown. Open square symbols were used to denote the SS
maxima energy positions at different temperatures and
open circle symbols were used for the BE maxima. A closer
examination of Fig. 3 reveals a linear dependence of the SS
and the BE energy position on temperature. Our observations resemble those previously reported by Paniago et al.
[4]. Following these results we assumed a linear dependence
of the energy position of both states on temperature and
plotted linear regression curves on the figure. We obtained
the following dependences describing the SS and the BE
energy position versus temperature ESS = 0.49 + 2.02 ·
10 4 Æ T and EBE = 1.02 + 2.86 · 10 4 Æ T, respectively.
Fig. 3. The SS and the BE energy position dependence on temperature.
An open circle symbol indicates the BE positions and an open square the
SS positions. Dashed lines were used to show linear approximation curves
fitted to experimental data. Dependences of the SS energy position and the
BE energy position on temperature denoted by ESS(T) and EBE(T),
respectively were shown.
The first parameter in the above equations represents the
energy position of both states at 0 K equal 0.49 eV and
1.02 eV below the Fermi level for the SS and the BE respectively. It is well known that the SS for Au(1 1 1) is located at
the energy of 0.5 eV below the Fermi level [9–12,21,22,33]
and the BE 1.0 eV below the Fermi level [3,4,33]. Our results are well comparable with the previous estimations.
Approximation of our results to 0 K allows us to estimate
the energy position of the SS and the BE maxima at that
temperature. It is clear that those values are different than
the ones measured at room temperature and equal 0.43 eV
and 0.93 eV below the Fermi level respectively. It gives the
difference in order of 10–15% to the estimation of the peek
position at room temperature. In very precise STM/STS
experiments performed at room temperature one has to
remember about that difference and should take it into consideration during data analysis.
P. Kowalczyk et al. / Surface Science 600 (2006) 1604–1607
The second parameter in both equations called temperature dependent energy shift coefficient was equal for the
SS and the BE maxima 2.02 · 10 4 eV/K and 2.86 ·
10 4 eV/K, respectively. The coefficients we obtained in
our experiment are well comparable to earlier results of
Paniago et al. and Christensen and equal 1.4 · 10 4 eV/K
[4] and 2.6 · 10 4 eV/K [4,34] for the SS and the BE,
respectively.
4. Conclusions
Variable temperature spectroscopic investigations were
done on Au(1 1 1) surface at five temperatures 294 K,
373 K, 433 K, 483 K and 580 K. Spectroscopic curves recorded in the energy range 0–1 eV below the Fermi level
showed a few local maxima which energy position was
dependent on temperature. The shift of all maxima toward
the Fermi level with the increase of temperature was observed. What is more, the observed behavior was reversible
with temperature. In our opinion some of the observed
maxima originated from the tip DOS, others from the electronic structure of gold. Maxima originating from the
Shockley surface state and the surface projected bulk band
gap edge were observed at all temperatures. Temperature
dependence of energy position of the SS and the BE maxima was estimated, then energy position of both maxima at
0 K were found. Hence the method of maximum energy position estimation at any temperature could be given. First,
one has to measure the maximum energy position at two
different temperatures. Then it is necessary to perform a
linear approximation which lets estimate the maximum energy position at desired temperature particularly at 0 K. Finally, a temperature dependent energy shift coefficient for
the SS and the BE states was found. More investigations
are necessary to understand the described phenomena. In
the future the emphasis will be put on investigations at
low temperatures and in a wider energy range.
Acknowledgements
This work was carried out at the Advanced Material Research Institute, University of Northumbria, supported by
grants from the European Regional Department Founding
and the University Innovation Centre for Nanotechnology.
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