Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge
Fullerene C₆₀ films were grown using physical vapor deposition on Si substrates at room temperature. Then chemical modification with cross-linking these films was performed using the reaction with 1,8-octanediamine (DA) or octane-1,8- dithiol (DT). These chemically cross-linked C₆₀ films are capa...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2010
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nasplib_isofts_kiev_ua-123456789-1182342025-06-03T16:26:26Z Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge Dmitruk, N.L. Borkovskaya, O.Yu. Havrylenko, T.S. Naumenko, D.O. Petrik, P. Meza-Laguna, V. Basiuk, E.V. Fullerene C₆₀ films were grown using physical vapor deposition on Si substrates at room temperature. Then chemical modification with cross-linking these films was performed using the reaction with 1,8-octanediamine (DA) or octane-1,8- dithiol (DT). These chemically cross-linked C₆₀ films are capable of stable binding the Ag or Au nanoclusters. Optical properties of the obtained nanostructured hybrid films were investigated by both reflectance spectroscopy and spectral ellipsometry within the spectral range 1.55 to 5.0 eV at various angles of incidence. From the spectral dependences of the extinction coefficient in the region of optical absorption edge, the physical nature of the fundamental allowed direct band-gap transitions between HOMOLUMO states Eg, the optical absorption edge near the intrinsic transition Eo, and exponential tail of the density-of-states caused by defects have been determined. Influence of chemical modification and decoration of metal nanoparticles on the above mentioned parameters has been analyzed. Financial support from the National Autonomous University of Mexico (grant DGAPA IN 103009) and from the National Council of Science and Technology of Mexico (grant CONACYT 56420) is greatly appreciated. V. M.-L. is grateful to DGAPA for a postdoctoral fellowship. 2010 Article Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge / N.L. Dmitruk, O.Yu. Borkovskaya, T.S. Havrylenko, D.O. Naumenko, P. Petrik, V. Meza-Laguna, E.V. Basiuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 180-185. — Бібліогр.: 21 назв. — англ. 1560-8034 PACS 78.40.Ri https://nasplib.isofts.kiev.ua/handle/123456789/118234 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Fullerene C₆₀ films were grown using physical vapor deposition on Si
substrates at room temperature. Then chemical modification with cross-linking these
films was performed using the reaction with 1,8-octanediamine (DA) or octane-1,8-
dithiol (DT). These chemically cross-linked C₆₀ films are capable of stable binding the
Ag or Au nanoclusters. Optical properties of the obtained nanostructured hybrid films
were investigated by both reflectance spectroscopy and spectral ellipsometry within the
spectral range 1.55 to 5.0 eV at various angles of incidence. From the spectral
dependences of the extinction coefficient in the region of optical absorption edge, the
physical nature of the fundamental allowed direct band-gap transitions between HOMOLUMO
states Eg, the optical absorption edge near the intrinsic transition Eo, and
exponential tail of the density-of-states caused by defects have been determined.
Influence of chemical modification and decoration of metal nanoparticles on the above
mentioned parameters has been analyzed. |
| format |
Article |
| author |
Dmitruk, N.L. Borkovskaya, O.Yu. Havrylenko, T.S. Naumenko, D.O. Petrik, P. Meza-Laguna, V. Basiuk, E.V. |
| spellingShingle |
Dmitruk, N.L. Borkovskaya, O.Yu. Havrylenko, T.S. Naumenko, D.O. Petrik, P. Meza-Laguna, V. Basiuk, E.V. Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Dmitruk, N.L. Borkovskaya, O.Yu. Havrylenko, T.S. Naumenko, D.O. Petrik, P. Meza-Laguna, V. Basiuk, E.V. |
| author_sort |
Dmitruk, N.L. |
| title |
Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge |
| title_short |
Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge |
| title_full |
Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge |
| title_fullStr |
Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge |
| title_full_unstemmed |
Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge |
| title_sort |
effect of chemical modification of thin c₆₀ fullerene films on the fundamental absorption edge |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2010 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118234 |
| citation_txt |
Effect of chemical modification of thin C₆₀ fullerene films
on the fundamental absorption edge / N.L. Dmitruk, O.Yu. Borkovskaya, T.S. Havrylenko, D.O. Naumenko, P. Petrik, V. Meza-Laguna, E.V. Basiuk // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2010. — Т. 13, № 2. — С. 180-185. — Бібліогр.: 21 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-11-26T20:19:56Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 180-185.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
180
PACS 78.40.Ri
Effect of chemical modification of thin C60 fullerene films
on the fundamental absorption edge
N.L. Dmitruk1, O.Yu. Borkovskaya1, T.S. Havrylenko1, D.O. Naumenko1, P. Petrik2, V. Meza-Laguna3,
E.V. Basiuk (Golovataya-Dzhymbeeva)3
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
2Research Institute for Technical Physics and Materials Science,
Budapest 114, P.O. Box 49, H-1525, Hungary
3Centro de Ciencias Aplicadas y Desarrollo Tecnológico,
Universidad Nacional Autónoma de México, Circuito Exterior C. U., 04510 México D.F., Mexico
Abstract. Fullerene C60 films were grown using physical vapor deposition on Si
substrates at room temperature. Then chemical modification with cross-linking these
films was performed using the reaction with 1,8-octanediamine (DA) or octane-1,8-
dithiol (DT). These chemically cross-linked C60 films are capable of stable binding the
Ag or Au nanoclusters. Optical properties of the obtained nanostructured hybrid films
were investigated by both reflectance spectroscopy and spectral ellipsometry within the
spectral range 1.55 to 5.0 eV at various angles of incidence. From the spectral
dependences of the extinction coefficient in the region of optical absorption edge, the
physical nature of the fundamental allowed direct band-gap transitions between HOMO-
LUMO states Eg, the optical absorption edge near the intrinsic transition Eo, and
exponential tail of the density-of-states caused by defects have been determined.
Influence of chemical modification and decoration of metal nanoparticles on the above
mentioned parameters has been analyzed.
Keywords: fullerene C60 films, chemical modification, optical parameters.
Manuscript received 05.03.10; accepted for publication 25.03.10; published online 30.04.10.
1. Introduction
The advancement of modern electronics, especially
optoelectronics and photovoltaics, demands new
technologies for manufacturing advanced materials.
Various fullerene-based materials, due to their
special optical and electronic properties, have a great
potential for application in many nanoelectronic devices.
In the recent decade, there has been a great interest to
modification of C60 fullerene films on various substrates,
especially on silicon substrates due to their wide
applications in microelectronic industry. Optical
properties of thin fullerite films are very dependent on
technology of preparation as well as structure of
substrate material. For example, it has been shown in [1]
that the optical properties of the C60 films are deeply
influenced by the nature of substrate (amorphous or
crystalline), temperature and deposition rate. Namely,
the larger film deposition rate results in the higher
structural and compositional disorder and in the lower
value of the optical density.
Modification of C60 fullerene films on various
substrates by using special chemical treatment with 1,8-
diaminooctane (or octane-1,8-dithiol) that simultaneously
react with neighboring fullerene molecules acting as a
cross-linking agents changes solubility of fullerene films
and influences on their physical and chemical
characteristics [2, 3]. In the work [4], fullerene C60 films
were exploited as molecular templates for metal cluster
superstructure. Chemical bonding of metal nanoparticles
to fullerene support reduced considerably the undesirable
coalescence effects. Using an aliphatic bifunctional amine
or thiol as linker, the immobilized silver and gold
nanoparticles have been obtained. The physical properties
of C60 films decorated with Ag and Au nanoparticles
depend considerably on the interface configuration.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 180-185.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
181
Formation of metal nanoparticles on the surface of
C60 film covering silicon substrate is of considerable
interest as related with: (1) possibility to enhance
transmittance of light into semiconductor due to plasmon
resonance excitation in nanoparticles; (2) the local
electromagnetic field enhancement near the metal
surface and penetration of it into semiconductor base
with following generation of non-equilibrium electron-
hole pairs [5]; (3) the light scattering by large metal
nanoparticles [6]; and (4) the enhancement of optical
absorption of incident photons due to increase in
duration of interaction between near field and
semiconductor.
Besides, using a thin absorbing layer between the
conducting emitter and photoactive base, one can change
the spectral region of solar cell photosensitivity. So,
controlling the fundamental absorption spectral region of
fullerene thin films on silicon substrate is very
important.
In this work, we study the near fundamental
absorption edge for five types of pristine and chemically
functionalized fullerene C60 films on n-Si substrate:
1) pristine fullerene films (C60/Si); 2) 1,8-
diaminooctane-cross-linked C60 films ( DA/SiC60 );
3) cross-linked films decorated with Ag nanoparticles
( Ag/SiDTC60 ); 4) octane-1,8-dithiol-cross-
linked C60 films ( DT/SiC60 ); 5) cross-linked films
decorated with Au nanoparticles ( Au/SiDTC60 ).
2. Samples and experimental technique
Deposition of fullerene C60 films onto Si was performed
in a vacuum chamber at pressure of Torr106.2 -6 ,
without heating the substrates. The average film
thickness obtained was about 80 to 120 nm.
For gas-phase functionalization, the samples
(C60/Si) were taken out from the vacuum sublimation
chamber and immediately degassed at low vacuum at
120 C for 30 min using a custom-made Pyrex manifold
[7]. Then, 5 mg of 1,8-dimainooctane were added
directly to the reactor bottom, avoiding direct contact
with the sample. The gas-phase reaction was carried out
at a pressure of ca. 1 Torr and temperature of about
150 C for 2 h, as was described previously [4]. The
high derivatization temperature not only facilitates the
reaction, but also enables to minimize the amount of
diamine and humidity physically adsorbed on the sample
( DA/SiC60 ) surface.
Deposition of Ag nanoparticles on chemically
modified C60 films (samples C60-DA-Ag/Si) was based
on the reduction of AgNO3 with citric acid, as described
elsewhere [4, 8]. In a typical experiment, the fullerene
samples were placed into 10 ml of 2-propanol. Then two
solutions, one of 8.5 mg of AgNO3 in 10 ml of 2-
propanol and another one of 19.2 mg of citric acid in
10 ml 2-propanol, were simultaneously added dropwise,
while the reaction system was vigorously stirred at room
temperature for 30 to 60 min. After finishing the
deposition process, the samples were washed with 2-
propanol, dried and stored under vacuum at room
temperature.
Functionalization of C60 films with octane-1,8-
dithiol was performed by means of the gas-phase
reaction at a pressure of ca. 1 Torr and 140 C
( DT/SiC60 ). Au nanoparticles decoration of
modified C60 films (C60-DT-Au/Si) was carried out as a
result of the reducing chemical reaction between HAuCl4
and citric acid solutions in 2-propanol, simultaneously
dropped onto C60 film surfaces [9].
Morphology and structure of fullerite films were
investigated by means of atomic force microscopy
(AFM) using Nanoscope IIIA, Digital Instruments USA
and of high-resolution transmission electron microscopy
(HRTEM), using JEOL 2010, Japan, respectively.
For optical characterization of investigated films,
the reflectance spectra were measured within the energy
range hν = 1.1-3.1 eV at variable angles of incidence for
p- and s-polarized light. The fullerene layer thickness
value and its optical parameters (refractive index n, and
extinction coefficient k in the complex refractive index
iknn ~ ) were determined by fitting experimental
dependences with theoretical ones, calculated within the
framework of one-layer model for pristine or
functionalized C60 films. To confirm these results, the
ellipsometric measurements were performed using a
Woollam M2000DI rotating compensator ellipsometer
within the spectral range 1.55 to 5.0 eV at the angles of
incidence 45°, 60°, 65° and 75°.
The parameters of fullerite films on semiconductor
substrates at the fixed wavelength = 632.8 nm, the
refraction index (n) and extinction coefficient (k) were
estimated using the multi-angle-of-incidence (MAI)
ellipsometry with the laser ellipsometer LEF-3M and
He-Ne laser as a source of monochromatic light.
Measurements of the polarization angles and were
carried out at the angles of incidence = 45 to 83. The
angles and were measured using the double-zone
method. The optical parameters n and k have been
calculated from dependences of the polarization angles
and Δ on the incidence angle [10].
The determined parameters were used for
calculation of the absorption coefficient = 4k/. The
comprehensive analysis of the (hν) spectra using
various coordinate systems allowed us to explain the
absorption edge and its modification with both chemical
polymerization and metal nanoparticles decoration.
3. Results and discussion
It is well known that C60 in the solid state is a typical
molecular material with weak bonds between
molecules. According to AFM data, the thin films of
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 180-185.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
182
fullerene C60 have granular (cluster or microcrystallite)
structure with the average grain diameter of about
50 nm. Chemical modification of C60 films with
different organic cross-linkers (diamine, dithiol)
resulted in formation of covalently linked hybrid
superstructures, capable of strong binding and
immobilization of noble metals. However, the grains of
modified films became bigger, their RMS-roughness
changes weakly (for example, from 1.22 to
1.08 nm [11]). The treatments with DA and DT reduce
solubility of C60 films in toluene, indicating the
transformation of C60 into different solid phases of
polymeric nature. Chemical cross-linking the C60
molecules removes degeneracy of electron energy
levels in the initial C60 monomers. Ag nanoparticles on
the C60-DA-Ag/Si sample present uniform coverage
and are about 5 nm in diameter. For octane-1,8-dithiol
functionalized films, the Au nanoparticle sizes
distribution exhibit a narrow dispersion with the
average diameter 3 nm.
However, the additional chemical treatment leads
to changes of surface particles in C60 cluster film, and
position/intensity of absorption peaks and fundamental
absorption edge especially.
Spectra of optical constants, n and k, for pristine
and functionalized C60 films in the wide spectral range
200-1100 nm are shown in Figs 1a and 1b. The main
features of k(h) spectrum for C60 film are in
agreement with the absorption spectra reported in the
literature [1, 12, 13]. We observed the characteristic
structure with the absorption maxima at 2.5, 3.6, 4.5
and 5.5 eV. Here, the latter three ones correspond to
dipole allowed intramolecular transitions identified as
hg,ggt1u, huhg and hg,gyt2u, respectively, and the
absorption range below 3 eV is caused by the solid-
state specific absorption. It is seen that the most
essential differences between the spectra of optical
constants for pristine and functionalized C60 films are
observed for the k and α = 4πk/λ values. Therefore, in
Fig. 2 spectra of the light absorption coefficient α are
shown within the spectral range h from 1.1 to 3.2 eV
for these 5 types of investigated fullerene films:
pristine C60, functionalized by diamine or dithiol,
DAC60 , DTC60 , and fullerite films with the
immobilized gold or silver nanoparticles, C60-DT-Au,
C60-DA-Ag. The spectral range of (h) dependences
may be separated into four main parts: I – the part of
the more intensive increase of (h > 2.2 eV), II – the
optical absorption edge (h from 1.8 to 2.1 eV), which
may be analyzed using the Tauc model [14], III – the
absorption tail caused by the exponentially distributed
states extending into gap of the films, which exhibit
characteristic exponential (Urbach) form (h from 1.6
to 1.9 eV), and IV – the low energy absorption band
caused by the structure defects and/or impurities, in
particular by oxygen [15] (h < 1.6 eV). The further
treatments of these spectra in various coordinates are
shown in Figs 3 to 5.
2 3 4 5 6
1.2
1.6
2.0
2.4
R
ef
ra
ct
iv
e
in
de
x,
n
Energy, eV
1
3
2
a
2 3 4 5 6
0.0
0.4
0.8
1.2
E
xt
in
ct
io
n
co
ef
fic
ie
nt
, k
Energy, eV
3 1
2
b
Fig. 1. Spectra of the refractive index (a) and of the extinction
coefficient (b) for 60C (1), DAC60 (2) and
AuDTC60 (3) films on Si substrate, as determined
from the spectral ellipsometric measurements.
The (h)2 vs h dependences are shown in Figs
3a and 3b. The cutoffs of their linear parts on h axis
determine the direct band gap values for pristine and
modified C60 films (Eg), which are presented in Table.
As it is seen, in the frameworks of experimental errors,
the used modifications of C60 films do not change their
Eg values that coincide with the band gap (2.3 0.1) eV
determined in [16] and the mobility gap values of [17].
The alternative interpretation of Eg as intermolecular
charge-transfer (CT) exciton energy value [18] is also
trustworthy.
Table. The optical absorption edge parameters for pristine
and functionalized C60 thin films.
Structures d, nm Eg, eV
Eo, eV
“Tauc
gap”
EU, eV
Urbach tail
parameter
C60/Si 120 2.36±0.01 1.57±0.01 0.26±0.01
C60-DA/Si 122 2.36±0.01 1.59±0.01 0.25±0.01
C60-DA-Ag/Si 122 2.33±0.01 1.48±0.01 0.16±0.01
C60/Si 80 2.33±0.01 1.68±0.01 –
C60-DT/Si 82 2.35±0.01 1.62±0.01 0.29±0.01
C60-DT-Au/Si 90 2.32±0.01 1.53±0.01 0.23±0.01
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 180-185.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
183
1.0 1.5 2.0 2.5 3.0
2
4
6
8
10
12
ln
(
)
Energy, eV
3
1
2
a
1.0 1.5 2.0 2.5 3.0
6
8
10
12
ln
(
)
Energy, eV
1 2
3
b
Fig. 2. Semilogarithmic plots of the absorption coefficients
spectra for two sets of pristine and modified C60 films on Si sub-
strate: (a) – 60C (1), DAC60 (2), AgDAC60 (3),
(b) – 60C (1), DTC60 (2), AuDTC60 (3).
The (h)1/2 vs h dependences, shown in Figs 4a
and 4b, allow to determine the optical gap Eo via the
Tauc model expression [14] often used for
noncrystalline semiconductors
2)( oEhch (1)
assuming a parabolic density of states and a constant
matrix element of interaction.
It was found that both DA and DT chemical
treatments of C60 films change Eo value (Table), but the
most essential change (decrease of Eo) is observed for
chemically modified films decorated with metal
nanoparticles (C60-DA-Ag and C60-DT-Au). According
to the data of [19], by varying the deposition conditions
of C60 thin films a continuum of structure types ranging
from crystalline to amorphous can be obtained. In the
same way, the fundamental optical absorption band gap
can be changed from 1.3…1.6 eV for crystalline films to
2.4…2.6 eV for amorphous films. It was shown in [20]
that both the extinction coefficient and electron spectrum
near the absorption edge are related with differences in
the film structure and with suppression of charge-
transfer (CT) excitons. The position and intensity of
absorption peaks for films grown in argon are changed
as compared with the film grown in vacuum [21],
without no evidence of chemical changes. Besides, in
[21] it was revealed the existence of a mixture of face-
centred cubic and hexagonal close-packed phases. All
these mean, that intermediate values of optical band gap
may be obtained depending on the degree of film
crystallinity, which depends in its turn on the additional
chemical treatment by diamine or dithiol.
Fig. 5a and 5b demonstrate the magnified parts of
lnα vs h dependences for the spectral region III, where
they exhibit the characteristic Urbach form:
UEEhh 10 exp)( . (2)
Here, α0 and E1 are constants, and EU is the Urbach
tail parameter that characterizes the energy extending of
the density-of-states into the forbidden gap below the
absorption edge (Eo). As it has been shown in [1], the
larger the value of EU, the greater is the compositional,
topological or structural disorder of C60 films, which
depends both on the C60 deposition rate and substrate
temperature during deposition. So, the known EU values
range from 0.036 eV [15] for C60 film, prepared at the
substrate (quartz) temperature of 250 C, to 0.84 eV and
even 1.71 eV for the films deposited with the rates 0.14
and 0.44 Å/s, respectively, on the unheated substrates
[1]. The EU values obtained for the investigated films are
summarized in Table. It should be noted that the spectral
range where the linear parts of dependences shown in
Fig. 5 may be considered as those of Urbach type is
narrowed in the case of considerable absorption, induced
by impurities (oxygen) of region IV. Therefore, the EU
value is omitted for the C60 film from the second set of
structures.
2.2 2.4 2.6 2.8
0.0
2.0x1010
4.0x1010
6.0x1010
8.0x1010
1.0x1011
(
*h
v)
2
Energy, eV
1
2
3
a
2.2 2.4 2.6 2.8
0.0
2.0x1010
4.0x1010
6.0x1010
(
*h
v)
2
Energy, eV
1
2
3
b
Fig. 3. Spectra of the light absorption coefficients α in
coordinates (h)2 vs h for: (a) – 60C (1), DAC60 (2),
AgDAC60 (3), (b) – 60C (1), DTC60 (2),
AuDTC60 (3) films on Si substrate.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 180-185.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
184
1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
0
50
100
150
200
250
(
*h
v)
1
/2
Energy, eV
1
2
3
a
1.5 1.6 1.7 1.8 1.9 2.0 2.1
0
50
100
150
200
250
(
*h
v)
1
/2
Energy, eV
1
2 3
b
Fig. 4. Spectra of the light absorption coefficients α in
coordinates (h)1/2 vs h for: (a) – 60C (1),
DAC60 (2), AgDAC60 (3), (b) – 60C (1),
DTC60 (2), AuDTC60 (3) films on Si substrate.
1.5 1.6 1.7 1.8 1.9 2.0 2.1
8
9
10
ln
(
)
Energy, eV
1 2
3
a
1.5 1.6 1.7 1.8 1.9 2.0
7
8
9
10
ln
(
)
Energy, eV
1
2
3
b
Fig. 5. lnα vs photon energy dependences for: (a) –
60C (1), DAC60 (2), AgDAC60 (3), (b) –
60C (1), DTC60 (2), AuDTC60 (3) films on Si
substrate.
We observed long-time variations of the optical
absorption edge as well, which are associated with a
change in the concentration of defects forming shallow
tails of the density-of-states.
Thus, the optical parameters of pristine and
chemically modified C60 films, summarized in Table,
demonstrate a certain tendency of their change in
consequence of chemical modification with diamine and
dithiol, and especially after deposition of the metal (Ag
and Au) nanoparticles. These dependences are somewhat
apparently weakened due to the one-layer model used
for computation of optical parameters for modified C60
films. Nevertheless, the determined parameters allow
calculating both the absorption and transmittance spectra
in these films, which is necessary for the analysis of
photoelectric properties of barrier structures including
these layers [3, 7]. It should be noted that diamine and
dithiol treatments of C60 film influence differently on the
Eo value: DA increases and DT decreases it, which
correlates with the observed blue-shift [4] and red-shift
[9], respectively, of the absorption band with the
maximum at the wavelength 380 nm, which authors of
[4, 9] related to the allowed dipole transition hg,ggt1g.
On the other hand, the deposition of metal nanoparticles
(both Ag and Au) onto the C60 films, functionalized with
1,8-diaminooctane or octane-1,8-dithiol, respectively,
changes their optical parameters in a similar manner, i.e.
decreases Eg, Eo and EU. The latter fact is an evidence of
diminishing the structural disorder in C60 films caused
by the stable and homogeneous incorporation of metal
nanoparticles of ca. 5 nm average diameters ensured
with previous chemical modification of C60 film.
4. Conclusions
The determined dispersion of optical parameters for thin
modified C60 films on Si substrate and the detailed
analysis of their absorption coefficient () spectra near
the fundamental absorption edge (by using various
coordinate systems for vs h dependences) allowed us
to distinguish regularities of their change caused by
modification in the framework of the one-layer model
used for their description:
- The direct band gap (or mobility gap) value
(Eg = 2.34 ± 0.02 eV) does not practically change in a
consequence of diaminooctane or dithioloctane
treatment of C60 film with a slight decrease of it after
deposition of the metal (Ag, Au) nanoparticles.
- Both DA and DT gas-phase treatments of C60 film
causing the transformation of pristine C60 into different
solid phases with covalently cross-linked fullerene
molecules change the optical gap value Eo (according to
the Tauc model) in a different way: DA increases, and
DT decreases Eo; moreover the Urbach tail parameters
EU are changed in the opposite direction.
- Deposition of the metal (Ag, Au) nanoparticles
onto chemically modified C60 films decreases all their
optical parameters (Eg, Eo, and EU), which testifies
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2010. V. 13, N 2. P. 180-185.
© 2010, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
185
diminishing the structural disorder in C60 films. So, such
modifications of thin C60 fullerene films may be
promising for their application in energy conversion and
photonic devices.
Acknowledgement
Financial support from the National Autonomous
University of Mexico (grant DGAPA IN 103009) and
from the National Council of Science and Technology of
Mexico (grant CONACYT 56420) is greatly
appreciated. V. M.-L. is grateful to DGAPA for a
postdoctoral fellowship.
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