Progress Report for Subproject 4
Characterization and manipulation of the basic building blocks
of advanced materials
Interplay between the electronic structures of Ag nanopucks
and Pb quantum islands
邱雅萍、林欣瑜、黃立維、傅祖怡、張嘉升、鄭天佐
中文摘要
銀奈米顆粒可以自組式地成長在二
維鉛量子島表面的週期性圖案上。該週期
性圖案的起源是和島嶼中量子化的電子有
關，有別於一般因為晶格常數不同所造成
的結構性週期圖案。由於島嶼層之排列順
序不同，會影響其間之電子，造成兩種島
嶼上之圖案明顯不一樣。我們發現所形成
的銀量子點，很明確的反映了襯底的電子
特性。另外，測量這些銀量子點發現，侷
限在銀量子點中的電子能階很明確地量子
化，也促成了進一步探討銀量子點與基底
之間的作用。
Abstract
Ag nanopucks are found to
self-organizedly grow on the template of 2D
lead (Pb) quantum islands. This template of
periodic patterns originates from the lattice
mismatch occurring at the interface of Pb
islands and the Si substarte. These patterns
exhibit the distinct oscillatory electronic
contrast in two types of islands, which differ
in stacking sequence, thus are novel from
traditional
structure-driven
templates.
Both the size distribution and spatial
arrangement of the Ag nanopucks are
analyzed and found to be commensurate
with the characteristics of the template
island, which exhibits a bi-layer oscillatory
behavior. Further electronic measurements
on these Ag nanopucks show the lateral
quantization of electronic density states. In
turns, these states shed new light on
investigating the interplay between the Ag
nanopucks and Pb quantum islands.
1. Introduction
It is well known that as the physical
size of a structure is comparable with the de
Broglie wavelength of Fermi electrons
confined in the structure, many of the
original bulk properties fail. More
significantly, the quantum confinement of
electrons in such nanometer scale will
possess a host of interesting and novel
properties.
Take the system of Pb
deposited on Si as an example, from the
previous theoretical calculations and
experimental results, the thickness of the
metal thin films or islands can affect their
surface potentials and work functions, and
oscillations are found in all these quantities
[1-4]. The characteristics of electronic
properties are proved to result from the
quantum size effect of the electronic wave
function confined in the direction normal to
the plane (defined as the z direction).
However, it is believed that if the size of the
structure is further reduced in its width, the
lateral confinements (x and y directions) will
result as well, and then the electronic feature
should possess a strong size and shape
dependence. As the adatoms grown on a
metal surface, the discrete state is known to
spread into broad resonances [5-7]. The
electronic structures and the orbital
geometry of these resonances play an
important role on most major properties,
such as binding energy, work function, and
equilibrium separation distance of the
adatoms [5-8]. Relying on the ability of
scanning
tunneling
microscopy,
experimental evidence of the size-dependent
electronic properties are quantitative derived
from both inelastic [9] and elastic [10,11]
electron tunneling spectroscopy on various
adsorbates on semiconductor or oxide
surfaces [9-15]. It is conceived that the
shape of a nanostructure is also a decisive
parameter on its electronic properties.
Nevertheless, the correlation between
geometric shapes and electronic properties
has still been rarely investigated. At the
present work, we will study the
shape-dependent characteristics on the
electronic structures of Ag nanopucks.
Since they are grown on a template substrate,
we can further explore the interplay between
the electronic structures of Ag nanopucks
and those of Pb island substrates.
are located at the fcc-stacked site of Type I
Pb quantum islands in a periodic
arrangement with a saturated coverage of
~0.2ML at 100K. The numbers in the Fig.
1(a) indicate the Pb layer thickness above
the interface between Pb and Si. These Ag
nanostructures are of one layer in height and
around 2nm in diameter. By differentiating
the STM topographic images in Fig. 1(a),
the shapes of these Ag nanopucks can be
made apparent and displayed in Fig. 1(b).
From Fig. 1(b), it is obvious that there are
multiple kinds of sizes and shapes of Ag
nanopucks. Since the sizes of these Ag
nanopucks are comparable to the electronic
Fermi wavelength, the lateral electronic
confinement of Ag nanopucks should occur.
Besides the size factor, another interesting
question is that what will the electronic
structures of Ag nanopucks vary with
different shapes? To closely probe the
correlation between the shapes or sizes of
Ag nanopucks and the commensurate
electronic structures of the substrate, three
different sizes of Ag nanopucks with
hexagonal or triangular forms are selected to
examine.
To avoid the interaction among Ag
nanopucks, the number density of the Ag
nanopuck is reduced by depositing a little
amount of Ag (~0.05ML) on Pb islands.
According to the previous work [16, 17],
electronic structures with a phase shift are
existed on not only between Type I and Type
II, but also on each fcc and hcp site.
Therefore, in the present work, we only
consider the Ag nanopucks nucleated at the
Type I fcc site. The shape of a Ag nanopuck
is identified by the differential STM image,
and the size of the Ag nanopuck is obtained
from analyzing its area.
Subsequently,
based on the connection between sizes and
shapes of these Ag nanopucks, the atomic
number (n) of Ag atoms in a nanopuck is
thus obtained.
We herein name the
hexagonal Ag nanopuck: HAgn, and
triangular Ag nanopuck: TAgn.
For
instance, the cluster situated at the
bottom-right site of Fig. 2(a) is recognized
in a hexagonal shape, and is named HAg37.
As the deposition temperature increases
(~150K), it is found that nanopucks are
2. Experimental
The experiments were carried out in a
UHV chamber where the base pressure was
less than 5 × 10-11 torr. The chamber was
also equipped with a variable temperature
scanning tunneling microscope and a
well-collimated e-beam evaporator for
depositing high purity Pb atoms. A clean
Si(111)-7 × 7 surface was prepared by
flashing the sample to 1200°C and annealing
at 900°C for a few minutes, then slowly
cooling down to room temperature. Over
one monolayer of Pb was first evaporated
onto the 7×7 at room temperature, followed
by annealing at 480°C for a few seconds to
generate the stripe incommensurate phase
(SIC). The sample was then cooled to 200
K and an extra amount of lead was further
added to form Pb quantum islands of various
thicknesses. To form nanoclusters on the
Pb quantum islands, we deposited a suitable
amount of Ag while the sample was held at
different temperatures. STM observations
and measurements were carried out after the
deposition.
The
local
electronic
characterizations
of
individual
Ag
nanopucks are all measured at 100K. By
taking the first derivation of the tunneling
current as a function of sample bias (dI/ dV),
the local density of states (LDOS) for a Ag
nanopuck is measured.
3. Results and discussion
Figure 1(a) shows that Ag nanopucks
2
preferred to grow in the triangular form at a
higher temperature as shown at the
bottom-right site of Fig. 2(b), where the
nanopuck is in a triangular form and is
denoted TAg36.
The electronic properties of these Ag
nanopucks were determined by STS, which
detects the tunneling current as a function of
sample bias. The tunneling conductance
(dI/dV) gives a measure of the local density
of states (LDOS) [18].
A set of
conductance spectra for HAgn and TAgn
nanopucks are acquired. In Fig. 2(c), the
spectrum taken on the fcc site of Type I
3ML Pb islands (L3 curve) before Ag
deposition is displayed as a reference. The
electronic signature of the substrate Pb
quantum islands with 3-layer thickness
occurs near 1.6V above Fermi level.
However, the dI/ dV spectra taken on Ag
nanopucks have some additional small
undulations. It strongly implies that the
lateral confinement of Ag nanopucks has
taken place. The correlation between the
electronic signature and the appearance of a
Ag nanopuck is further pursued with an
analysis of dI/dV spectra peaks. Electronic
configurations by marking the peak
positions of averaged dI/dV spectra for
TAg21, TAg28, Tag36, HAg37, HAg61, and
HAg91 nanopucks are plotted in Fig. 3. In
Fig. 3, those gray lines are dI/dV peaks from
experimental measurements, and the
standard deviations of these dI/dV peaks
positions are indicated with the error bars.
From experimental data, it is obvious that
both the shape and size of a Ag nanopuck
have a vital influence on its electronic
structure. How great an effect of the shape
on the electronic structure can be closely
examined from TAg36 and HAg37. It is
evident that the tunneling spectra of Ag
nanopucks possess the shape-dependent
characteristics. Apart from the shape
-dependent influence, the next question is
what kind of effect the substrate has.
Compare the theoretical calculations of the
local density of states for free-standing Ag
nanopucks, depicted as the black lines in Fig.
3, with experimental results, it shows that, in
addition to the theoretically predicted
resonance states, some extra states are also
detected. These states exist not only at the
distinguished peaks of 3-layer Pb islands
(about +1.6V and -0.7V), but also at +0.5V
and -1.5V. Those states are close to the
corresponding peak positions of 4-layer Pb
islands (Type I (fcc) L4 curve in Fig.2 (c)).
It implies that the electronic properties of Ag
nanopucks have coupled with the substrate
but preserved original characteristic features.
However, due to the limitation in the energy
resolution of the current STS, the coupling
strength in quantitative term should be
further studied.
In summary, we have investigated the
correlation of the electronic structures of Ag
nanopucks with their size and shape.
Employing the electronic Morie pattern on a
Pb quantum island as a template, Ag
nanopucks can be grown in various
geometric shapes spontaneously by properly
adjusting the experimental parameters.
This system also renders the possibility to
study the interaction between a supported
nanocluster with its substrate.
3
(a)
(b)
3
SIC
1
Fig. 1
(b)1
5/10
(a)
(c)
dI / dV
/ Type I (fcc) L3
TAg21/ Type I (fcc) L3
HAg37/ Type I (fcc) L3
/ Type I (fcc) L4
0
1
Sample Bias (V)
Fig. 2
4
2
1.5
1.5
1.0
1.0
0.5
0.5
eV0.0
0.0
-0.5
-0.5
-1.0
-1.0
-1.5
-1.5
TAg21
TAg
28
Theo.
TAg
H Ag
36
Exp.
37
Theo.
Exp.
HAg
61
Theo.
Exp.
HAg91
Fig. 3
Reference:
1. C. Marliere, Vacuum 41, 1192 (1900)
2. T. Miller, A. Samsavar, G. E. Franklin,
and T. C. Chiang, Phys. Rev. Lett. 61,
1404 (1988).
3. D. A. Evans, M. Alonso, R. Cimino,
and K. Horn, Phys. Rev. Lett. 70, 3483
(1993)
4. W. B. Su, S. H. Chang, W. B. Jian, C. S.
Chang, L. J. Chen, and Tien T. Tsong,
Phys. Rev. Lett. 86,(2001) 5116.
A. Zangwill, Physics at Surfaces
(Cambridge University Press,
Cambridge, 1988).
5. J. W. Gadzuk, Phys. Rev. B 1 (1970)
2110.
6. N. D. Lang and A. R. Williams, Phys.
Rev. B 18 (1978) 616.
7. N. D. Lang, Phys. Rev. Lett. 46, 842
(1981)
8. G. V. Nazin, X. H. Qiu, and W. Ho,
Phys. Rev. Lett. 90, 216110(2003).
9. M. F. Crommie, C. P. Lutz, and D. M.
10.
11.
12.
13.
14.
15.
16.
17.
5
Eigler, Phys. Rev. B 48, 2851(1993).
N. Nilius, T. M. Wallis, and W. Ho,
Science 297, 1853 (2002).
R. M. Feenstra, Phys. Rev. Lett. 63,
1412 (1989).
P. N. First, J. A. Stroscio, R. A.
Dragoset, D. T. Pierce and R. J. Celotta,
Phys. Rev. Lett. 63, 1416 (1989)
.I.-W. Lyo and P. Avouris, Science 245,
1369 (1989).
P. Bedrossian, D. M. Chen, K.
Mortensen and J. A. Golovchenko,
Nature 342, 258(1989).
W. B. Jian, W. B. Su, C. S. Chang and T.
T. Tsong, Phys. Rev. Lett. 90, 196603
(2003).
H.Y. Lin, Y.P. Chiu, L.W. Huang, Y.W.
Chen, T.Y. Fu, C.S. Chang,and Tien T.
Tsong, Phys. Rev. Lett. 94,
136101(2005).
R. J. Hamers, R. M. Tromp, and J. E.
Demuth, Phys. Rev. Lett. 56,
1972(1986).
Progress Report for Subproject 4
Characterization and manipulation of the basic building blocks
of advanced materials
Transmission Resonance and Quantum Bound States by Low-Temperature
Scanning Tunneling Spectroscopy on Thin Ag Films
蘇維彬、呂欣明、施華德、蔣季倫、張嘉升、鄭天佐
一、中文摘要
resonance, and quantum bound states. In
terms of analysis for spectra, the spectral
intensity around transmission resonance is
equivalent to the electron transmittivity, and
the spectral intensity of quantum bound state
is correlated to electron reflectivity.
Therefore, the distribution of spectral
intensity is essentially constrained by the
fact that the transmittivity plus the
reflectivity is equal to one. Due to that the
intensity or energy levels of the
image-potential
states,
transmission
resonance and quantum bound states vary
with the film thickness, the spectroscopy of
each
thickness
exhibits
a
unique
characteristic like a fingerprint. Therefore,
these characteristic spectra can be used to
identify the thickness. Therefore, we can
utilize this location-dependent spectroscopy
to
probe
the
interface
structure
nondestructively.
銀單晶薄膜在低溫下可以成長在矽半
導體表面上。我們利用低溫掃描穿隧顯像
與能譜技術(STM&STS)觀察並量測不同
厚度的銀薄膜的電性。每個能譜都包含三
種量子特徵：像位能態、共振穿透、量子
束縛態。更仔細的分析顯示，在共振穿透
附近的態密度可以對等於自由電子的穿透
機率；量子束縛態的狀態密度與電子的反
射率有明顯的對等關係；而能譜所代表的
總電子態密度對應於穿透機率與反射機率
的總和。由於像位能態、共振穿透、及量
子束縛態的強度會隨著薄膜厚度變化。因
Keywords: UHV low-temperature scanning
tunneling
microscopy,
silver
film,
image-potential
state,
transmission
resonance,
quantum
bound
state,
thickness-dependent
characteristic
spectroscopy
此，可由電子的能譜來訂出薄膜的厚度，
並且用來探測薄膜與基底介面的特性。
關鍵詞:掃描穿隧顯像與能譜術，銀薄膜，
像位能態，共振穿透，量子束縛態
I. Introduction
When the thickness of a metal film is
comparable to the electron de Broglie
wavelength, electrons in the film as well as
those transmitting through the film can both
manifest the quantum size effect (QSE). For
the former, electrons are confined in a
quantum well of the metal film to form
quantized standing wave states in the
surface normal. For the latter, the electron
QSE appears above the vacuum level, and
can be explained to be due to an interference
of electron waves that are reflected from the
Abstract
It is known that flat silver crystalline
film can be grown on Si(111)77 surface.
We use low-temperature scanning tunneling
microscopy and spectroscopy to probe the
electronic structure of the film of different
thickness. Each spectroscopy contains
signals originated from three kinds of
quantum
phenomena.
They
are
image-potential
state,
transmission
6
film surface and the film-substrate interface.
The QSE results in the electron transmission
spectra of the metal film to reveal
resonances [1, 2, 3], i.e., electron can
penetrate the film easily at some specific
energy.
It is known that flat silver films with the
(111) face can be grown on Si(111)7×7 at
room
temperature
[4].
Since
the
transmission resonances have been observed
in the Ag/W(110) system [1], it can be
expected that they would also appear in the
Ag/Si(111)7×7 system. We utilize the
scanning tunneling spectroscopy (STS) to
investigate the electronic structure of Ag
film of different thickness at the energy
range of 2~9 eV above the Fermi level. Our
results demonstrate that the transmission
resonances indeed can be observed by STS.
Besides the transmission resonance,
however, sharp peak features are also found
in the spectra, which are quantized states
related to reflected electrons confined in the
triangular potential well between the tip and
sample. We term them quantum bound states
(QBS).
2(a). There arrows mark the bump features
appearing in the spectra of 9~11-layer thick
film (indicated by number in the
parenthesis). It is obvious that the energy
separation between the bump features
decreases with increasing film thickness. In
addition, the energy levels of these bump
features are all located above the vacuum
level, refering to the work function of the Ag
film on Si(111) being 4.41 eV [5]. These
properties guide us to think that the bump
features is due to the QSE above the vacuum
level.
According to quantum mechanics, the
probability for an electron transmitting
through a square potential well obey the
following equation [6]
1/T=1+V2sin2(kt)/4E(E+V)
(1)
where T is the transmission probability, E is
the energy of incident electrons, V is the
depth of the potential well, t is the width of
the well, and ħ2k2/2m=E+V. It is plausible to
assume that Ag film has a similar square
potential well in the surface normal. Figure
2(b) shows calculated curves of the
transmission probability as a function of
electron energy for 9~11-layer thick films
by using Eq.(1) with the the parameters V is
8 eV [1] and t is equal to layer number ×2.5
Å. Each calculated curve exhibits an
oscillatory aspect, indicating that both
transmission and reflection can occur for
any energy except at certain energy levels
(marked by dash lines) electrons can
penetrate the film totally, which are termed
the transmission resonance. The energy
levels of transmission resonance move
toward the vacuum level with increasing
film thickness. This is consistent with the
bump features shown in Fig. 2(a). The
calculated (Cal.) values of the energy
separation
between
the
first
two
transmission resonances are tabulated in Fig.
2(a). They decreases with increasing film
thickness and agree with the experimental
(Exp.) measurements. Because of these
similarities, we thus conclude that the bump
features are resulted from to the
transmission resonance.
When we acquired the spectra on films
of the same thickness, we often observed
II. Results and Discussion
Figure 1(a) shows a typical STM
topography image of the Ag film grown at
room temperature. Low-temperature STS is
used to take Z-V spectra on films of
different thickness. The red curve in Fig. 1(b)
shows such a spectrum taken on a film of 9
atomic layers above the silicon substrate.
For comparison, the spectrum is also
acquired on the crystal Ag(111) surface,
drawn as the black curve in Fig. 1(b). Both
curves are similar and reveal step-like
features that were interpreted as the Stark
shift image-potential states in previous
studies. They correspond to peak features in
dZ/dV-V curves, as shown in Fig. 1(c).
Besides these peaks, two extra bumps
marked by two black downward arrows are
also observed in the curve of 9-layer thick
film. However, they do not appear in that of
crystal Ag, indicating that the bump feature
is specific to the Ag thin films. These peak
and bump features can also appear in the
spectra obtained by lock-in technique with
the feedback kept active, as shown in Fig.
7
that the spectral intensity changes slightly
with the measured location. Figure 3(a)
shows two spectra acquired at two locations
on the 5-layer thick film, revealing visible
intensity differences at energy levels of
transmission resonance (marked by an
arrow), the end (marked by green dash line)
and the maximum of the first QBS (marked
by 1). These subtle variations are real
because they can manifest in the spatial
mapping of the spectral intensity. Figures
3(b) and (c) show the mappings of energy
levels at the maximum and the end,
respectively. Crosses mark the locations
where the spectra are acquired, and their
colors correspond to that of spectra in Fig.
3(a). The maximum in the red curve is
higher than that in the black curve whereas
the intensity at the end shows a reverse
situation. It is consistent with that the
contrast at the location of the red cross is
brighter than that at the black cross in Fig.
3(b), whereas in Fig. 3(c) the contrast is
reversed. In addition, Fig. 3(b) shows a clear
hexagonal pattern with a period of about 27
Å, in agreement with that of the Si(111)7×7
reconstruction. Therefore, variations of
spectral intensity with locations are
originated from the Ag/Si(111)7×7 interface
property. That is, the local variation of
potential barrier affects the electron
reflection phase at the buried interface,
causing the transmittivity of electron to vary
with the location.
with the spectra in the data base.
III. Conclusions
In summary, we have observed the
transmission resonance of thin Ag films
formed on Si(111)7×7 by STS. This
observation implies that the issues of
electron scattering in the unbound system
can also be studied by STS. In addition, STS
spectra exhibit some thickness-dependent
and location-dependent characteristics. They
allow us to determine unambiguously the
film thickness and also to probe the
Ag/Si(111)-7×7 interfacial structure. We
believe that these characteristics generally
appear in spectra of flat metal films on the
semiconductor substrate, therefore the
techniques we presented here should be
useful in technological applications.
IV. Reference
[1] B.T. Jonker, N.C. Bartelt, and R.L. Park,
Surf. Sci. 127, 183 (1983).
[2] E. Bauer, Rep. Prog. Phys. 57, 895
(1994).
[3] M.S. Altman, W.F. Chung, and C.H. Liu,
Surf. Rev. Lett. 5, 1129 (1998).
[4] P. Sobotík, I. Ošťádal, J. Mysliveček, T.
Jarolímek, and F. Lavický, Surf. Sci.
482-485, 797 (2001).
[5] A. Thanailakis, J. Phys. C: Solid State
Phys., 8, 655 (1975).
[6] Richard L. Liboff, Introductory
Quantum Mechanics, Addison-Wesley,
1980.
Figure 4 show the dZ/dV-V spectra
acquired on the Ag film of 3~9 layers. The
numbers at the right hand side of each curve
mark the film thickness. Each spectroscopy
also contains three quantum bound states.
Due to the fact that the intensity or energy
levels of the image-potential states,
transmission resonance and quantum bound
states vary with the film thickness, as one
can see, the spectroscopy of each thickness
exhibits a unique characteristic like a
fingerprint. Figure 4 can be used as a data
base of the fingerprints for determining the
film thickness. Therefore, besides measuring
the film thickness directly from height
distribution of STM images, one can also
distinguish the thickness by acquiring
dZ/dV-V spectra on films and compare them
8
Fig. 1 (a) The growth of flat Ag films on
Si(111)77 surface at room temperature at
the coverage of 3.6 ML. Image size is
150×150 nm2. (b) Z-V spectroscopy
measured on the 9-layer thick Ag film (red
curve) and crystal Ag(111) surface (black
curve). (c) dZ/dV-V curves directly
differentiated from Z-V spectra in (b).
Fig. 2 (a) spectra acquired on 9~11-layer
thick films by lock-in technique with the
feedback kept active. Number in parenthesis
indicates film thickness. (b) Calculation
curves of transmission probability as a
function of electron energy for 9~11-layer
thick films. Dash lines indicate energy
levels of transmission resonance.
9
Fig. 3 (a) Spectra revealing visible
difference, acquired at two locations on the
5-layer thick film. The blue and green dash
lines mark the onset and end of the first
QBS in black and red curves, respectively.
The arrow marks the transmission resonance.
(b) and (c) show the mappings of energy
levels at the maximum and the end,
respectively.
9
3.6
8
3.0
dZ/dV
7
2.4
6
1.8
5
1.2
4
3
0.6
1
2
3
4
5
6
7
8
9
10
Sample bias (V)
Fig. 4 Spectra acquired on the Ag film of
different thickness. Numbers at the right side
of spectra mark atomic layers of the
thickness.
10