Modulation of Surface Plasmon in Nanocrystalline Gold/DLC
Composite Films Prepared by Plasma CVD Technique
R. Paul and A K Pal
Department of Instrumentation Science, USIC Building, Jadavpur University,
Kolkata -700 032, India
ABSTRACT
Composite films containing gold nanoparticles embedded in diamond-like carbon
(Au-DLC) matrix were deposited on glass and Si (100) substrates by using capacitatively
coupled plasma (CCP) chemical vapour deposition technique (CVD). Particle size and
metal volume fraction varied between 2.7-3.5 nm and 0.04-0.7, respectively with the
amount of argon in the methane+argon gas mixture in the plasma. Bonding environment
in these films were obtained from XPS, Raman and FTIR studies. Microstructural studies
were carried out by SEM, XRD and TEM studies. Blue-shift of the surface plasmon
resonance peak (located ~540-561 nm) in the optical absorbance spectra of the films
could be associated with the reduction of the particle size while red shift was associated
with the increase in volume fraction of metal particles. The experimental results have
been discussed in light of the core-shell model.
1. INTRODUCTION
Nanoparticulate gold films encapsuled in different matrix has been reported by a
number of groups [1-10] due to their technological importance in many fields. Sizedependent electrical, optical and catalytic properties, biological assays, and surfaceenhanced Raman scattering (SERS) have been the subject of studies in recent past
[11,12]. Several researchers [5-7,9,10,13,14] have reported their studies on optical
properties of gold or silver nanoparticles embedded in dielectric matrix by using a variety
of techniques. The choice of the dielectric matrix was limited mainly to SiO2, ZnO,
TiO2, Al2O3 and PMMA[13-17]. The main hurdle to the widespread application of metal
nanoparticles is their agglomeration due to high surface energy [18,19]. Several
techniques have been proposed for the deposition of metal nanoparticles containing DLC
films, which include electrochemical [17,20], sputter deposition [21,22], plasmaenhanced chemical vapor deposition [23], etc. Capacitatively coupled plasma chemical
vapour deposition (CCP-CVD) technique being simple, scalable and cost competitive has
been widely used to deposit DLC coatings. Since the DLC films are large band gap
material, metal inclusion in nanocrystalline form should also reveal interesting optical
properties like surface plasmon resonance (SPR) in these classes of composite materials
which have not yet been explored systematically.
In this communication, we present our studies on composite material consisting of
diamond-like carbon and embedded gold nanocrystallites having different grain sizes
prepared by CCP-CVD technique. Films were characterized by TEM, SEM and XRD.
Raman and FTIR studies were carried out to obtain information on the bonding
environment in these films. SPR properties in these films were studied critically.
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2. EXPERIMENTAL DETAILS
Composite films of nanocrystalline gold/DLC films (Au-DLC) were deposited by
cracking premixed methane+argon gas mixture by r.f. power (13.56 MHz; 100 W).
Chamber pressure during deposition was ~0.25 mbar. Plasma was generated between two
circular aluminium discs. The top aluminium disc (2.54 cm in diameter) was used to hold
a gold disc of same diameter and connected to the r.f power while the bottom one served
as the substrate holder. Premixed gas mixture containing argon in four different
proportions was fed to the stainless steel CVD chamber through appropriate flow meter.
Films were deposited onto glass and Si (100) substrates. The deposition time was kept
constant at ~ 10 min. Nanocrystalline gold content in the films was controlled by the
amount of argon ions bombarding the top gold disc and as such higher percentage of
argon in the plasma would culminate in higher number density of gold nanocrystallites in
the DLC matrix. A pure DLC film without gold incorporation was also deposited by
replacing the top disc with a pure aluminium disc of similar dimension.
3. RESULTS AND DISCUSSION
3.1. Microstructural studies
Pure DLC film without gold incorporation had very smooth surface as examined
under SEM. The films were more or less featureless and amorphous in nature as indicated
by the XRD trace (inset of Fig.1). Changes in morphology of gold incorporated DLC film
as depicted by SEM studies (Fig.1b). Signature of dispersed gold nanoparticles is quite
visible. The corresponding XRD trace is shown in the inset of Fid.1b which clearly
indicated peaks arising for reflections from (111), (200), (220), (311) and (400) planes of
gold. TEM pictures (Fig.2(a-d)) show increased gold incorporation resulting from
increased amount of argon ions in the plasma. The histograms, shown in the left insets of
the TEM pictures, indicated a narrow distribution of the gold nanocrystallites in the DLC
matrix for films deposited with lower argon concentration (Fig.2a) and became broader as
the gold content in the films were increased. Average crystallite size varied between 2.7
nm and 3.5 nm. Electron diffraction patterns (inverted) of the corresponding films are
shown in the right hand insets of Fig.2a-d. Reflections from (311) and (400) planes of
gold could be observed along with a halo arising out of embedding DLC matrix.
3.2. Absorption studies
The interaction of the small metal particles with the external electromagnetic field
induced by light would result in a coherent oscillation of the conduction (free) electrons
(mainly within the surfaces), called the surface plasmon resonance. This surface plasmon
resonance largely depends on the particle size, shape, and of course the metallic material
and its surrounding environment i.e. DLC here. The surface plasmon spectra for the AuDLC films are shown in Fig.3a. The optical absorbance spectra are dominated by single
absorption peak corresponding to dipolar interaction between the particles. It may be
noted here that the absorption spectra for pure DLC film (Fig.3a, curve-1) did not
indicate any peak but resembled the spectra for a typical semiconductor
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Fig.1. SEM micrograph of : (a) pure DLC film and (b) Au-DLC film deposited with 50%
argon. Insets show the corresponding XRD traces.
(
b
)
Fig.2. TEM micrographs and corresponding histograms of four representative Au-DLC
films deposited with different amount of argon in methane+argon gas mixtures: (a) 50%,
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(b) 60%, (c) 70% and (d) 80%. Left hand insets show the corresponding histograms while
right hand insets show the diffraction patterns of the same film.
The absorption peaks related to the surface plasmon resonance are red-shifted
(Table-I) with the increase in gold volume fraction (i.e. with increasing argon
concentration in the plasma). With the increase in the number density of the gold
nanocrystallites (Fig.2c and Fig.2d), proximity of the two adjacent nanocrystallites
increased. This would facilitate increased coalescence in the films. Therefore, the
observed red-shift of the plasmon band could be associated with the increase in crystallite
size and is a characteristic feature of the composite films having large volume fraction of
gold nanoparticles. The above shift may be ascribed to the increased electromagnetic
interaction between the particles [19,24] and change in the effective permittivity of the
surrounding matrix. Thus, the detailed description of the DLC as the dielectric
embedding medium would be important. In general, diamond-like carbon (DLC) films
consist of graphitic clusters linked by sp2 ( bonding) or sp3 bonding ( bonding)
forming an extended carbon network and the relative amounts of the sp2 and sp3 bonded
carbon present in the network dictate the physical properties. With the incorporation of
metal nanocrystallites, the relative amount of sp2/sp3 bonded carbon would be modulated
which in turn would modulate the permittivity of the dielectric medium and as such the
surface plasmon resonance phenomena in nanocrystalline Au-DLC composite films. The
relative amount of sp2/sp3 bonded carbon could be estimated from the FTIR, Raman and
XPS studies which are presented below.
3.3. FTIR spectra
The FTIR spectra for the Au-DLC films deposited with 50%, 60%, 70% and 80%
argon in the plasma were recorded in the range 4000-400 cm-1. The relevant portion
related to sp2 and sp3 bonded carbon for the above representative films are shown in
Fig.3(b-e). The FTIR spectrum for a representative pure DLC film deposited with 50%
argon is shown in Fig.3f for comparison. Deconvolution of the peaks around 2900 cm-1
indicated the presence of three major peaks related to different characteristic sp3 and sp2
C-H bonds with different stretch area under individual peak. With increasing argon ion
concentration, the peaks at ~ 2850 cm-1 and ~2920 cm-1 related to sp2 -CH olefinic group
and sp3-CH symmetric modes, respectively became distinct (Fig.3e). The shoulder at
~2950 cm-1 arising due to sp3-CH symmetric stretching vibrations also became
prominent. It may be observed that sp2/sp3 ratio in the Au-DLC films increased from
0.075 for the film with lesser gold content to 0.345 for films with higher gold content in
the DLC matrix. Pure DLC films also showed similar trend and the sp2/sp3 ratio increased
from 0.038 to 0.081 with the increase in argon content in the plasma (Table-I). The
values obtained as above for all the films are shown in Table-I. It may thus be inferred
that the DLC films became sp2 rich with increased incorporation of gold nanocrystallites.
3.4. Raman spectra
Typical Raman spectra, shown in Fig.5, indicate the presence of two broad peaks
located at ~1382 cm-1 (D-line) and ~1583 cm-1 (G-line). Raman spectra for a
representative pure DLC film deposited with 50% argon is shown in the inset of Fig.3g.
2-81
(a)
(b)
(c)
(d)
(e)
(f)
(h)
(g)
3
2
r1
1
r2
Fig. 3. (a) Absorption spectra of pure DLC and Au-DLC films, Deconvoluted FTIR peak
~2900 cm-1 for four representative Au-DLC films: (b) 50 %, (c) 60%, (d) 70 %, (e) 80 %
argon in the plasma and (f) for pure DLC film of 50% argon, (g) Raman spectra for four
2-82
representative Au-DLC films. Inset shows Raman spectra for pure DLC film deposited
with 50% argon in the plasma. (h) Schematic diagram of the core-shell structure.
The G (graphite) peak is mainly sensitive to the configuration of sp2 sites because
of their higher cross section [25]. The G-lines indicated a shift in the peak positions
towards higher wavenumber with the incorporation of nanocrystalline gold in them
(Fig.3g). This shift may be due to the confinement of electrons in short chains. Thus,
ID/IG would be a measure of the size of sp2 phase organized in rings. But the full width at
half maximum (FWHM) of the G-peak is mainly sensitive to structural disorder, which
arises from bond angle and bond length distortions. FWHM of the G-peak would be small
if the clusters were defect free, unstrained or molecular. The ID/IG values for the Au-DLC
films decreased when the films were deposited with increasing amount of argon in the
plasma. Pure DLC film also indicated a decrease in ID/IG values with increased argon
content in the plasma. FWHM of the G-peak for both the Au-DLC and DLC films
decreased when deposited with increasing amount of argon in the plasma. Values of
FWHM of Au-DLC films were significantly lower than the corresponding values of pure
DLC films indicating that inclusion of gold nanoparticles in DLC matrix would render
the resultant films more sp2 rich. Thus, inclusion of nanocrystalline gold in DLC matrix
would culminate structural change and render the DLC matrix to be more graphitic in
nature. This observation is in conformity with the findings from FTIR studies as well.
3.5. Surface plasmon phenomena
A number of theoretical models were proposed [19,24,26-28] and several
experiments were carried out to understand the effect of particle size on the surface
plasmon effect and to justify the observed peak shift (either blue-shift or red-shift) and its
broadening with decreasing size. As the films under investigation are composite in
nature, the Au nanoparticles may be looked at as core surrounded by DLC as shell (shown
subsequently, Fig.3h) and hence, would obey the core-shell model theory [29-31]. The
dielectric constant would be modulated significantly from that of Au nanocrystallites or
DLC films and may not follow the linear composition of the weighted average. Thus, the
core-shell model would offer a better description of the observed surface plasmon
phenomena in these films.
In our analysis, we have assumed that the particle diameter is much less than the
wavelength of the incident field and that the particle is subjected to uniform field. The
particle then oscillates like a simple dipole with polarization proportional to the incident
field. The quasi-static approach postulated by Averitt et al. [30] can then be employed to
calculate the absorption cross-section from the
polarizability [31].
In the geometry of the core-shell structure used as above (Fig.3h), region 1 is the
core and is characterized by a radius r1and dielectric function 1. The shell has a thickness
t = r2-r1 and a dielectric function 2. 3 is the dielectric constant of the surrounding
medium i.e.air. Each of the dielectric functions has their real and imaginary components.
The geometry becomes that of a dielectric nanoshell when 2 becomes that of a dielectric
and the core becomes purely metallic with 1 being that of a metal. This would mean that
the metal nanocore with radius r1 and consisting of a nanometer-scale metal particles
2-83
would be embedded in the dielectric shell of thickness r2-r1. Metal nanocore would
exhibit a geometric tenability, characterized by the ratio of the core radius to the total
radius (r1/r2), and effectively that would extend the optical properties of the metal
nanoparticles from the visible to the infrared region. The geometry of core-shell structure
as shown below may be considered to evaluate the absorption cross-section from eqn.(1):
α abs 
2
24 2 R 31i 2
Im( ) 
 0
[(1r  2 2 ) 2  1i 2
(1)
The above simulations done by assuming a core-shell model for the Au/DLC
nanocrystallites is similar to the Drude Model which consists of an alloying sphere of
radius R in a suspending medium, 1, 2 are the dielectric functions for the nanosphere
and the embedding regions, respectively. Both 1 and 2 have real and imaginary
frequency-dependent components. The frequency dependent complex dielectric constant
can be written as [32]:
     r  i i   b   
p2 /2
1  1/  
2
2
i
p2 /2
 (1  1 /   )
2
(2)
2
where b() is the dielectric function of bulk metal which is due to inter-band transition,
p denotes the plasmon frequency of the bulk metal,  is the relaxation time and  is the
frequency of the electromagnetic wave.
There are two distinct size regimes to be considered in the determination of the
electromagnetic properties of metallic nanoparticles [27]. In the extrinsic size regime the
optical properties can be fully described by specification of the nanoparticle radius and by
the use of the bulk frequency dependent dielectric constant (()). For solid gold
nanoparticles this so-called extrinsic regime occurs for diameters greater than ~50nm. In
this regime, the size dependent effects are purely electromagnetic. For example,
contribution from higher-order multipoles and phase retardation effects become important
with increasing size. The relative contribution of scattering to the total cross
2-84
Fig.4. Theoretical plots based on core-shell model for films deposited with: (a) 50%, (b)
60%, (c)70 % and (d)80 % argon in the plasma. (● experimental and __theoretical fit).
section increases and the plasmon resonance broadens. Conversely, in the intrinsic regime
(2r<50nm), dipole plasmon absorption is by far the dominant contributor to the extinction
cross-section. Thus, size dependent effects must be accounted for by employment of a
size dependent dielectric function (,r), where r is the nanoparticle radius. This is
required as information about changes in the electronic and the optical properties are
reflected in the respective changes in (,r). In addition, r<< (wavelength), and the
quasi static approach can be employed in the calculations and the spatial variation of the
electromagnetic field could be neglected keeping the temporal dependence preserved.
This would simplify the calculations.
The scattering cross-section may be obtained by using the scattering theory as [33]:
(2 ) 4
2
 sca 

2 4
60 
128 5 ( 2 r  i 2i   3 )[( 2 r  i 2i   1 ) R1  ( 1  2 r i 2i ) R2 ]  2( 1   2 r  i 2i ) 3 R2

2
34
( 1   2 r  i 2i )( 2 r  i 2i   3 ) R1  ( 1   2 r  i 2i )( 2 r  i 2i   3 )
2
2
2 2
(3)
where, the quantities refer to the core, shell or the surrounding media based on their
suffixes.
We have simulated the absorbance spectra using eqns. (1), (2) and (3) taking Au
nanocrystallites as core and DLC as shell. It may be noted here that the dielectric
functions in eqn.(3) were frequency dependent. The parameters for calculating the
dielectric constant for Au were obtained from Johnson and Christy [34]. The theoretically
simulated spectra using this above core-shell model are shown in Fig.4 for films
deposited at four different percentages of argon in the plasma. The fitting parameters are
shown in Table-II. As the thickness of the films increased for films deposited with
increasing amount of argon in the plasma, one would expect an increase in core diameter
and associated decrease in the shell thickness. This has been reflected in the observed
decrease in shell thickness with the increase in core diameter as indicated in Table-II. It
may be noted here that the core diameter of 2.8 nm to 4.36 nm obtained from the
theoretical fit matched reasonably well.
4. CONCLUSIONS
Au-DLC composite films with various volume fractions of gold were prepared by
plasma CVD technique. Gold nanocrystallite size increased from 2.7 nm to 3.5 nm for
films deposited with increasing amount of gold in the DLC matrix. Relative amount of
sp2/sp3 bonded carbon was ascertained from Raman, FTIR and XPS studies. The
experimental results of optical absorbance spectra have been discussed in light of shellcore-shell model. It may be observed that both the core and shell diameters increased
with increased incorporation of Au nanocrystallites.
2-85
Table-I : Characteristics of gold nanocrystallite embedded DLC films and pure DLC film
Ar %
Thickness
of the film
(nm)
SPR
position
(nm)
sp2/sp3
from
FTIR
FWHM
of G
peak(cm
-1
)
Au-DLC films
0.075
140
G peak
position
(cm-1)
ID/IG
sp2/sp3
from
XPS
1583
0.72
0.67
50
60
540
60
67
545
0.122
137
1584
0.71
0.80
70
90
553
0.307
133
1585
0.70
1.10
80
120
561
0.345
125
1587
0.69
1.50
Pure DLC film
0.038
192
1584
0.595
50
65
-
60
68
-
0.050
190
1592
0.557
70
70
-
0.065
186
1594
0.533
80
75
-
0.081
177
1596
0.523
Table II: Values of the different parameters for Au-DLC films based on the core-shell
model
2-86
Ar %
Volume
fraction
of Au
(nm-2)
InterAu core Shell
particle radius, (DLC)
distance r1 (nm) radius
(nm)
r2
(nm)
12.5
1.4
2.45
50
0.04
60
0.17
6.1
1.69
70
0.39
1.6
80
0.70
0.8
Thickness Surrou
of DLC
nding
shell (r2-r1) medium
(nm)
1.05
air
2.71
1.02
air
2.06
3.05
0.99
air
2.18
3.12
0.94
air
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