1、2-78Modulation of Surface Plasmon in Nanocrystalline Gold/DLC Composite Films Prepared by Plasma CVD TechniqueR. Paul and A K PalDepartment of Instrumentation Science, USIC Building, Jadavpur University, Kolkata -700 032, India ABSTRACTComposite films containing gold nanoparticles embedded in diamon
2、d-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+
3、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 fil
4、ms 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. INTRODUCTIONNanoparticulate gold films encapsuled in different matri
5、x has been reported by a number of groups 1-10 due to their technological importance in many fields. Size-dependent electrical, optical and catalytic properties, biological assays, and surface-enhanced Raman scattering (SERS) have been the subject of studies in recent past 11,12. Several researchers
6、 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 PMMA13-17. The main hurdle to the widespread applicat
7、ion 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, plasma-enhanced chemical vapor deposition 23, etc.
8、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 prope
9、rties 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 pre
10、pared 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. 2-792. EXPERIMENTAL DETAILSComposite films of nanocrystalline gold
11、/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 o
12、f 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.
13、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.
14、 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 DISCUSSION3.1. Microstructural studiesPure DLC film without gold incorporation had very smooth surface as examined under SEM. The films were more or
15、 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 w
16、hich 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, indicat
17、ed 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)
18、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 f
19、ield 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 envi
20、ronment i.e. DLC here. The surface plasmon spectra for the Au-DLC 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,
21、curve-1) did not indicate any peak but resembled the spectra for a typical semiconductor 2-80Fig.1. SEM micrograph of : (a) pure DLC film and (b) Au-DLC film deposited with 50% argon. Insets show the corresponding XRD traces.Fig.2. TEM micrographs and corresponding histograms of four representative
22、Au-DLC films deposited with different amount of argon in methane+argon gas mixtures: (a) 50%, (b)2-81(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 pla
23、smon 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
24、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
25、 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
26、 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 ca
27、rbon 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
28、 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 s
29、pectrum 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.
30、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
31、 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 pla
32、sma (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 loc
33、ated 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.2-82Fig. 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
34、%, (c) 60%, (d) 70 %, (e) 80 % argon in the plasma and (f) for pure DLC film of 50% argon, (g) Raman spectra for four r1r2213(a) (b)(c) (d)(e) (f)(g) (h)2-83representative Au-DLC films. Inset shows Raman spectra for pure DLC film deposited with 50% argon in the plasma. (h) Schematic diagram of the c
35、ore-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 d
36、ue 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-
37、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 plas
38、ma. 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
39、 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
40、 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
41、 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 film
42、s 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 fiel
43、d 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 polarizabil
44、ity 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
45、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 nanomete
46、r-scale metal particles 2-84would 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 nanoparticle
47、s 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): The above simulations done by assuming a core-shell model for the Au/DLC nanocrystallites is similar to the Drude Model which consis
48、ts 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: wh
49、ere 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
