- Email: [email protected]
Surface enhanced Raman spectroscopy of carbon nanostructures
Surface Science 600 (2006) 3723–3728 www.elsevier.com/locate/susc
Surface enhanced Raman spectroscopy of carbon nanostructures E. Perevedentseva a
a,b
, A. Karmenyan c, P.-H. Chung a, Y.-T. He a, C.-L. Cheng
a,*
Department of Physics, National Dong-Hwa University, 1, Sec. 2, Da-Hsueh Road, Shoufeng, Hualien 974, Taiwan, ROC b P.N. Lebedev Physics Institute RAS, Moscow, Russia c Institute of Biophotonics Engineering, National Yang-Ming University, Taipei, Taiwan Available online 17 April 2006
Abstract Surface enhanced Raman scattering (SERS) of diamond nanocrystals and fullerene was investigated. Ag and Au films were used as SERS-active agent. In the first series of experiments SERS were prepared with sputtered island metals on the nanoparticles surfaces. In the second one the nanoparticles were positioned on silver surface using laser acceleration method. SERS is a powerful method to analyze the light accelerated nanoparticles patterning for their spatial distribution and structure. The enhancement in Raman spectral intensity of graphite-like phases and blinking effect are observed. Additional bands due to selective enhancement from cluster surface with different arrangement are generated. For SERS of diamond crystal phase as well as for fullerenes the characteristic shift and asymmetry of Raman bands are also observed. The investigation can be of interest both for the understanding of carbon nanostructures’ physics and for their applications, especially for bio-detection, as well as for the studying of SERS mechanism. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Surface enhanced Raman; SERS; Carbon nanostructure; Nanodiamond
1. Introduction Identifying different bonding configuration (sp2, sp3) in various carbon systems is used for the study of low-dimensional objects, such as crystal and onion-like carbon nanoparticles, nanotubes, fullerenes, etc., and for the study of interfacial phenomena, including interaction between carbon nanostructure and substrate surface. One of the advantages of Raman spectroscopy is its capability to provide highly resolved vibrational information and not to suffer rapid photobleaching. However, Raman scattering is not an efficient process; to improve the detection sensitivity, signal amplification can be obtained by exploiting the surface enhanced Raman effect [1]. Surface enhanced Raman scattering (SERS) is a very powerful Raman technique, being able to provide a spectral intensity enhancement by orders of magnitude. The phenomenon occurs when adsorbing the target molecules onto nanometer-sized *
Corresponding author. E-mail address: [email protected] (C.-L. Cheng).
0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.01.074
roughened metal (Ag, Au, Cu and some others) surface, or in colloids of metals. Nanoscale surface roughness supports the electromagnetic resonance, which is the dominant mechanism of enhancement. These electromagnetic resonances can increase the scattered intensity by several orders of magnitude. A second mechanism is chemical, connected with the forming of new electronic states due to adsorbate– substrate bonding interactions. It has been estimated by Kambhampati et al. [2,3] that the chemical mechanism can also greatly enhance the scattering cross-section. Although advantages of the SERS are most useful in the study of organic molecules [4–6], SERS from nanodiamond films and diamond-like carbon films was investigated by several groups [7–12], and from nanodiamond powder [13]. Some new information concerning structures and the physical properties has been provided for diamond-like materials [14], and for fullerenes [15,16]. SERS is extremely sensitive to local structural properties of carbon nanostructures. It is applicable to the analysis of spatial distribution of different types of hybridization and, correspondingly, different carbon phases in nanoscale.
3724
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728
We compare here the SERS from the nanodiamond (nanocrystal diamond particles with average sizes 100 nm and 5 nm) and fullerene C60samples, prepared with two different methods. In the first experimental series the island metal films were sputtered on the nanoparticles samples. This method has been used before for the samples preparation to study SERS of carbon structures [7–11]. In the second series the nanoparticles were positioned on the metal surface by laser acceleration [17–19]. In the laser acceleration experiment, the nanoparticles suspended in distillated water were accelerated by high focused near-infrared laser beam and attached onto Ag surface, which serves as the target. The laser acceleration allows the nanoparticles to be positioned and ordered on the surface and to penetrate deeply in metal layer. This method allows the patterning of nanoparticles [20], which has valuable applications for biosensing and bioprobing. In this method, the interaction between nanoparticles and metal surfaces can be adjusted by varying the laser acceleration parameters. Strong interaction between the nanodiamond or fullerene and metal is observed from the analysis of SERS spectra. Our main objective is to compare the SERS produced by two methods, the sputtering and the laser acceleration methods. The advantage of laser acceleration is to allow positioning nanodiamonds in a desired pattern as reported in our earlier publication [19]. 2. Materials and methods Synthetic diamond powders with average sizes 100 nm (Kay Industrial Diamond, USA) and 5 nm (UltraFine Diamond, Russia) in diameters were used. The nanodiamonds after acid treatment (incubation of the powder portion in a mixture of concentrated H2SO4 and HNO3 with ratio 9:1 at 65 °C for several hours followed by three times washing in distilled water) were suspended in distillated water in concentration of 1–10 mg/ml. Fullerene (C60, Aldrich) was dissolved in toluene in concentration 0.8 mg/ml. The suspensions were dropped on Si surface and dried. Islands Au film with center thickness 20–30 nm was sputtered with ion sputter (E-1010 Hitachi Ion Sputter JEOL). The Raman spectra were measured before and after sputtering with a micro-Raman spectrometer (Renishaw 1000B), complete with microscope (Leica), and with a diode pumped solid-state CW laser (DPSS, Coherent) at 532 nm as excitation wavelength. Spectra were measured with low laser beam power at 1–10 mW in focal spot to avoid laser damage to the samples. For the laser acceleration deposition, water suspensions of nanodiamonds as well as fullerene were used for the patterning of nanoparticles on the Ag and Au films. Water suspension of fullerene was prepared by stirring or sonicating fullerene powder in distilled water with following sedimentation and separation of the largest aggregates. In such suspension the C60 molecular clusters or micro and nanocrystals with wide size dispersion exist [21–23]. A femtosecond Ti:Sapphire laser (Mira-900, Coherent) with
wavelength 780 nm, time duration 150 fs, average power up to 1000 mW, and repetition rate of 76 MHz pumped by a solid-state laser (Verdi, Coherent), served as the light source for particles’ acceleration. Laser was connected with a modular microscope (Leica DM IRB) and a programmed XYZ-scanning stage (SGSP (MS) 20–85). The scanning of target realizes the positioning of the accelerated nanodiamond on the target according to predetermined program. The automatic monitoring and steering was realized with stage controller (Mark-204-MS) with a fully closed-loop control system. Water suspended nanoparticles were applied on the metal surface and treated by laser beam trough microscopic objective, scanning with fixed rate along the programmed patterns. The treated area for every sample was typical 2–4 mm2. The obtained samples were investigated using scanning electron microscope (SEM, JEOL JSM6500F). SERS from different kind of samples was analyzed and compared. 3. Results and discussion In Fig. 1, Raman spectra of 5 nm (a) and 100 nm (b) nanodiamonds before and after Au sputtering are shown, measured in different points of the sample. The observed characteristic Raman bands for nanodiamond are: 1332 cm 1 for carbon sp3 bonding (the diamond band); the E2g symmetry allowed G-band at 1590 cm 1 (graphite band); the disordered allowed 1350 cm 1 D-band; and the 1150 cm 1 and 1450 cm 1 bands recently assigned by Ferrari and Robertson [24] as trans-polyacetylene bands. In Fig. 1a(1) and b(1), for 100 nm nanodiamond the band 1332 cm 1 distinctly predominates whereas for 5 nm nanodiamond D- and G-bands are more intensive comparable to the diamond band. The strong D- and G-band, especially in the 5 nm diamond nanoparticles, are quite different compared to the spectra obtained from nanodiamond films. Our samples were nanodiamond particles, 5 and 100 nm in sizes, and were subject to acid treatment prior to the SERS investigation. During the acid treatment process, the surface could have covered with an extended graphitic structure or hydrogenated carbon surface as depicted in our normal Raman spectra shown in Fig. 1. We did not observe the expected 1240 cm 1 peak that was assigned to the nanodiamond phase with the size between 2 and 40 nm by Roy et al. [11]. The 1332 cm 1 band was also slightly asymmetric. The hydrogenated surface could form trans-polyacetylene chains according to Ferrari and Robertson [24]. This reflects in the m1 and m3 bands in normal Raman or SERS spectra (Fig. 1). The fact sp2 related bands dominated the spectra as compared to the diamonds films used by Roy et al. [11] should come from the graphitic structure on surface and the large surface area in our diamond nanoparticles. Also, the sp2 Raman cross-section is 50–200 times larger than the sp3 carbons should also contribute to our spectra [7]. In Fig. 1 both for 5 and 100 nm nanodiamonds the increasing of the Raman signal intensity after Au sputtering is more striking for graphite and partic-
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728
3725
G
5 nm
ν3
G
D 4000
20000
2 3000
D
100 nm
ν1
Intensity (a.u.)
Intensity (a.u.)
ν1
ν3
diamond
2
diamond
10000
1
1200
a
1400
1600
1
1800
Raman shift (cm-1)
1200
1400
b
1600
1800
Raman shift (cm-1)
Fig. 1. Raman spectra of 5 nm (a) and 100 nm (b) nanodiamond before (1) and after (2) Au film sputtering; the spectra were measured in different points of the samples.
5 nm
8000
a Intensity (a.u.)
ularly for amorphous carbon, in agreement with Ilie et al. [12] and Veres et al. [14]. No additional bands as result of selective enhancement are clearly observed at such sample configuration. This shows that the interface with Au suffers strong modification compared to the un-treated nanodiamonds, which could be consistent with sp2 promotion and increased interfacial disorder. The enhancement of absolute signal intensity for diamond band at 1332 cm 1 was not observed. Positioning of nanodiamond on Ag metal film by laser beam acceleration allows observation of more effects, characteristic for SERS from nanodiamond and diamond-like carbon materials (DLC). In Figs. 2 and 3 the Raman spectra are plotted for 5 nm and 100 nm nanodiamonds positioned on the Ag surface by laser acceleration. In every set (a or b) the spectra are measured sequentially in the same point of the sample. Both for 5 and 100 nm nanodiamond attached on Ag, additional bands relative to the regular bands of carbon structure are observed in nearby range (Figs. 2 and 3a). The preferred enhancement of non-diamond bands (D- and G-bands as well as trans-polyacetylene bands etc.) are observed for nanodiamond positioned on silver surface by laser acceleration of the nanoparticles similar to nanodiamond with sputtered metal film (Fig. 1). Complication of the spectra and repeated increasing and decreasing of Raman signal intensity during sequential measurements, which possess selective enhancement of some composite band intensity and blinking effect, are characteristic features of SERS of carbon structures [14]. Blinking effect consisting of considerable fluctuations in both signal intensities and frequencies has been observed
4000
b
0 1200
1400
1600
1800
Raman shift (cm-1)
Fig. 2. Raman spectra from 5 nm nanodiamond, attached on the Ag by laser beam acceleration at laser power 0.5 mW in focal spot. The time of measurement of each spectrum is 10 s, data acquisition time is 100 s. Spectra (a) are measured in one point of the sample and spectra (b) are measured in other point of the same sample.
previously for single-molecule fluorescent spectroscopy at SERS. It was studied in particular by Nie et al. [4,5] and Bjerneld et al. [6], who have shown that blinking effect is observed for SERS under conditions where many
3726
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728 100 nm
100 nm
8000
Intensity (a.u.)
Intensity (a.u.)
4000
2000
4000 0
0
0 1200
a
1400 1600 Raman shift (cm-1)
1200
1800
b
1400 1600 Raman shift (cm-1)
1800
Fig. 3. Raman spectra of 100 nm, attached on the Ag film by laser acceleration at laser power 0.5 mW in focal spot. The time of measurement each spectrum is 10 s, data acquisition time is 100 s. Spectra (a) are measured in one point of the sample and spectra (b) are measured in other point of the same sample.
molecules are adsorbed on SERS-active substrate (in contrast to the one for single-molecule fluorescence) due to that the ‘‘many-molecule’’ spectra can be dominated by single molecules interacting with SERS-active site, especially if these molecules can jump around between different metastable chemisorption states with different spectral characteristics. In our case, laser acceleration may create structural changes for the nanoparticle that may cause the blinking effect observed for many-molecular system. Briefly, SERS blinking effect can be explained by the assumption that the G- and D-bands are inhomogeneously broadened [25,26]. The intensity of the Gaussian broadened composite bands is determined by the concentration and the Raman cross-section of the structural unit related to it. This is the case of conventional Raman scattering, and the line broadening is a convolution of composite bands. If some composite atomic group (any structural unit) gets in very close contact to the silver surface, its Raman signal is amplified and enhanced corresponding peaks appear in the spectrum. In particular, often accompanying D- and G-bands at 1140–1150 cm 1 and near 1450 cm 1, attributed as transpolyacetylene chains on diamond surface [9,27,28] are selectively enhanced in some spectra in Figs. 2 and 3a. These peaks are not intense because non-hydrogenated nanodiamonds were investigated; the hydrogenation was only from the acid wash process used to prepare the samples. Also hydrogenated fragments of hexagonal and tetragonal carbon clusters can give rise to the group of spectral lines in the range 1300–1350 cm 1 [28]. The splitting of the G-band, 1580 cm 1, and 3–4 lines in range 1518–1590 cm 1 [29–31] can arise from the forming of convolute graphite nanostructures [31]; both the frequencies and intensities of additional bands depend on the number of graphite layers, curvature and symmetry of tube-like nanostructure [30,31].
The relatively weak peak in range 1240–1280 cm 1 often is observed from crystallites with nanometer size [8– 11,13,27], with decreasing of the size and amorphisation of surface this peak widens [11,32]. It can be ascribed to the maximum in the vibrational density of states in diamond, which take place in nano-sized crystals [10] or to amorphous sp3 bonded carbons [13]. The peaks in range 1470–1490 cm 1 and above 1600 cm 1 (1600–1630 cm 1) arise due to defects [32]: the peaks near 1470–1490 cm 1 are assigned to a vacancy or divacancy with conjugated single and double bonds; in the vicinity of a vacancy, regions of mixed single and double bonds are formed. When a vacancy is introduced into an otherwise pristine diamond lattice, new vibrational modes appear at 1470–1490 cm 1. While the 1630 cm 1 peak is attributed to the [1 0 0] split interstitial defect, which consists of an isolated sp2-bonded carbon pair occupying the position of one carbon atom in a normal diamond lattice. The spectra of Fig. 3b, diamond peak at 1332 cm 1 for 100 nm nanodiamond looks slightly asymmetric and exhibits a tail towards lower wavenumbers. This asymmetric diamond peak is often observed in SERS spectra. The asymmetry in the shape of the peak at 1332 cm 1 has been ascribed by Lopez-Riyos et al. and others [10,11] to the non-conserved momentum phonon excitations, which take place in nanometer-sized crystals. Observed enhancement of signals can be achieved both for 100 nm and 5 nm nanodiamond up to two orders of magnitude. From the analysis of SERS spectra, one can conclude that the high intensity of SERS for graphitic and trans-polyacetylene as well as additional bands in comparison with the diamond band both for 5 nm and 100 nm nanodiamond is connected not only with higher efficiency of the Raman scattering for them, but also with transfor-
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728 Ag2
Hg7
1000
Intensity (a.u.)
mation of nanodiamond particles at laser beam treatment and mechanical percussion with Ag target. Gogotsi et al. [33] observed the diamond–graphite transformation at high contact compression on diamond. High temperature at the laser acceleration also can create condition for local structural transformation in diamond, arising changes in Raman spectra [32,34]. Therefore, structural and shape transformations connected with deformation can also be observed in our case, which needs in further investigation of physical mechanism of these transformations as well as of necessary conditions to produce the patterns with predictable properties as the laser acceleration can be a destructive method. The SERS spectra of 5 nm and 100 nm nanodiamond at their high variability look more similar to each other, than the corresponding regular Raman spectra and the difference in SERS observed for the samples prepared by two different methods is very significant. The reason obvious lies in the difference in carbon–metal interaction occurring in both kinds of samples prepared with metal film sputtering and laser acceleration. Some minor difference in the observed SERS can also be contributed to the use of different metal films, Ag and Au [15]. But the difference observed by Chase et al. [15] is negligible in comparison with what observed in our work. In the same time no significant difference was observed in SERS from other kind of investigated carbon nanostructures, fullerene, for samples prepared by two discussed methods. We observed SERS from C60 for the samples prepared by metal sputtering on the fullerene film on Si as well as by laser acceleration of fullerene with metal film targeting. In both cases we should speak not about sepa˚ , but rate fullerene C60 molecules/particles with size 7–6 A about nano and microcrystals or clusters. Such structures formed on the Si surface at the sample preparation (solution drying) and predominated in water suspension of fullerene. In pristine C60 the most frequently studied Raman mode is the Ag2 intramolecular tangential pentagonal pinch mode at 1469 cm 1. This mode shows a strong shift to lower wave numbers when C60 is doped with metals or when it is polymerized to different structures [15]. Also the oxidized C60 displays a shift of the Ag2 mode [35,36]. It has been found [15,35,37] that the Ag2 mode is sensitive to a charge transfer of C60 cage as a result of strong electromagnetic field at the metal–C60 interface, inducing surface plasmons, as well as to changes in symmetry [15,35] and formation of metal–C60 hybrid bonds, with their own vibrational wavenumbers [37]. The downshift can be accounted for by the elongated C–C bonds in the negatively charged molecule. The Ag2 mode usually is accompanied by satellite with wavelength 1447 cm 1 [35], which also is shifted for metal–C60 system. In Fig. 4 Raman spectra of fullerene on Si, after Au film sputtering, and fullerene positioned on Ag by laser acceleration are compared. The enhancement of the relative intensity of Ag2 mode is observed for both fullerene–metal samples. The changing of relative intensities of these bands
3727
Hg8
c
500
b
a 0 1300
1350
1400 1450 1500 Raman shift (cm-1)
1550
Fig. 4. Raman spectra of fullerene on Si (a); fullerene on Si after Au film sputtering (b); fullerene positioned on Ag film by laser acceleration (c). Laser power is 0.5 mW in focal spot. The time of measurement of each spectrum is 10 s, data acquisition time is 100 s. The fitting of spectra was done with Lorenzian algorithm.
Ag2, Hg7, Hg8 was observed previously [37], and it is not explained yet. The shift of this band from 1468 cm 1 to 1458–1460 cm 1 as discussed above, as well as of satellite band 1447 cm 1 also are observed as well as some splitting, which can be explained by non-homogeneous enhancement of signal from surface and bulk C60 molecules in crystal or cluster or by enhancement of satellite Ag2 bond 1447 cm 1. Also some enhancement as well as shift of mode Hg7 from 1430 cm 1 to 1423–1425 cm 1 was observed. Raman spectral lines of fullerene are shifted but do not transform into graphite spectrum, as was observed by Stenzel et al. [36], so the method of laser acceleration is not destructive for fullerene; can be used for patterning, including for SERS investigations. 4. Conclusion We compared surface enhanced Raman scattering of carbon nanostructures of diamond nanoparticles with size 100 and 5 nm and fullerene C60 at different conditions of their interaction with SERS active surface. Sputtering of the metal film together with deposition of nanodiamond on metal surface (rather, nanostructured metal surface) as well as deposition of metal nanoparticles on the surface of carbon sample [14] is often used for the observation of SERS. Every method has its own advantages and can reveal some characteristics of the investigated object as well as the SERS mechanism. In addition to the method of the metal film sputtering on the nanoparticle samples we have investigated the method of positioning nanoparticles on metal surface by laser acceleration. This method allows reaching strong carbon–metal interaction for SERS
3728
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728
analyzing of the obtained structures; also some structural rearrangement of graphite and amorphous structures in diamond as well as transformation of the nanodiamond into graphite as result of diamond crystal deformation can occur. Laser acceleration method of the sample preparation can not be called non-destructive, but it opens the new possibilities for nanotechnologies and also for some structural investigations, as well as for the investigation of SERS. In contrast to nanodiamond, for fullerene the difference in SERS observed for the samples prepared by two different methods is insignificant and noninvasive patterning is possible. Only some changes in the molecular symmetry and some charge transfer are observed for fullerene molecules interacted with metal independently on ways to provide this interaction. Acknowledgments The authors appreciate the financial support of this research by National Science Council of Taiwan, ROC under Grant No. NSC-94-2120-M-259-002. References [1] Selected papers on surface-enhanced Raman scattering, in: Milton Kerker (Ed.), SPIE Milestone Series, MS 10, 1990. [2] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241. [3] P. Kambhampati, C.M. Child, M.C. Foster, A. Campion, J. Chem. Phys. 108 (12) (1998) 5013. [4] S. Nie, S.R. Emory, Science 275 (1997) 1102. [5] W.E. Doering, S. Nie, J. Phys. Chem. B 106 (2002) 311. [6] (a) E.J. Bjerneld, P. Johansson, M. Ka¨ll, Single Mol. 1 (2000) 239; (b) H. Xu, E.J. Bjerneld, M. Kall, L. Borjesson, Phys. Rev. Lett. 83 (1999) 4357. [7] D.S. Knight, E. Weiner, L. Pilione, Appl. Phys. Lett. 56 (14) (1990) 1320. [8] D. Roy, Z.H. Barber, T.W. Clyne, J. Appl. Phys. 91 (9) (2002) 6085. [9] T. Lo´pez-Ry´ios, E´. Sandre´, J. Raman Spectrosc. 29 (1998) 733. ´ . Sandre´, S. Leclercq, E ´ . Sauvain, Phys. Rev. Lett. [10] T. Lo´pez-Ry´ios, E 76 (26) (1996) 4935. [11] M. Roy, V.C. George, A.K. Dua, P. Raj, S. Schulze, D.A. Tenne, G. Salvan, D.R.T. Zahn, Diam. Relat. Mater. 11 (2002) 1858.
[12] A. Ilie, C. Durkan, W.I. Milne, M.E. Welland, Phys. Rev. B 66 (2002) 045412. [13] S. Praver, K.W. Nugent, D.N. Jamieson, J.O. Orwa, L.A. Bursill, J.L. Peng, Chem. Phys. Lett. 332 (2000) 93. [14] M. Veres, M. Fule, S. Toth, M. Koos, I. Pocsik, Diam. Relat. Mater. 13 (2004) 1412. [15] S.J. Chase, W.S. Bacsa, M.G. Mitch, L.J. Pilione, J.S. Lannin, Phys. Rev. B 46 (12) (1992) 7873. [16] Y. Mo, G. Mattei, M. Pagannone, Appl. Phys. Lett. 66 (19) (1995) 2591. [17] A. Ashkin et al., Phys. Rev. Lett. 24 (1970) 156. [18] S.E. Skipetrov, S.S. Chesnokov, S.D. Zakharov, M.A. Kazaryan, V.A. Shcheglov, JETP Lett. 67 (1998) 635. [19] E. Perevedentseva, A. Karmenyan, P.-H. Chung, C.-L. Cheng, J. Vac. Sci. Technol. B 23 (2005) 1980. [20] Y. Xia, J.A. Rogers, K.E. Paul, G.M. Whitesides, Chem. Rev. 99 (1999) 1823. [21] I.D. Fortner, D.Y. Lyon, C.M. Sayes, A.M. Boyd, J.C. Falkner, E.M. Hotze, L.B. Alemany, Y.J. Tao, W. Guo, K.D. Ausmann, V.L. Colvin, J.B. Hughes, Environ. Sci. Technol. 39 (11) (2005) 4307. [22] G.V. Andrievsky, V.K. Klochkov, A.B. Bordyuh, G.I. Dovbeshko, Chem. Phys. Lett. 364 (2002) 8. [23] G.V. Andrievsky, V.K. Klochkov, E.L. Karyakina, N.O. MchedlovPetrossyan, Chem. Phys. Lett. 300 (1999) 392. [24] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [25] I. Po´csik, M. Hundhausen, M. Koo´s, L. Ley, J. Non-Cryst. Solids 227–230 (1998) 1083. [26] I. Po´csik, M. Koo´s, Diam. Relat. Mater. 10 (2001) 161. [27] A.C. Ferrari, J. Robertson, Phys. Rev. B 60 (12) (2001) 121405(R). [28] D. Zhang, R.O. Zhang, J. Phys. Chem. B 109 (2005) 9006. [29] G.S. Duesberg, W.J. Blau, H.J. Byrne, J. Muster, M. Burghard, S. Roth, Chem. Phys. Lett. 310 (1999) 8. [30] A. Mews, F. Koberling, T. Basche, G. Philipp, G.S. Guesberg, S. Roth, M. Burghard, Adv. Mater. 12 (16) (2000) 1210. [31] A. Kasuya, Y. Sasaki, Y. Saito, K. Tohji, Y. Nishina, Phys. Rev. Lett. 78 (23) (1997) 4344. [32] J.O. Orwa, K.W. Nugent, D.N. Jamieson, S. Prawer, Phys. Rev. B 62 (9) (2000) 5461. [33] Y. Gogotsi, A. Kailer, K.G. Nickel, Nature 401 (1999) 663. [34] H. He, T. Sekine, T. Kobayashi, Appl. Phys. Lett. 81 (4) (2002) 810. [35] R.L. Garrell, T.M. Herne, C.A. Szafranski, F. Diederich, F. Ettl, R.L. Whetted, J. Am. Chem. Soc. 113 (1991) 6302. [36] O. Stenzel, A. Stendal, D. Drews, T. Werninghaus, M. Falke, D.R.T. Zahn, C. von Borczyskowski, Appl. Surf. Sci. 108 (1887) 71. [37] R. Manaila, A. Belu-Marian, D. Macovei, G. Brehm, D.T. Marian, I. Baltog, J. Raman Spectrosc. 30 (1999) 1019.
Surface enhanced Raman spectroscopy of carbon nanostructures E. Perevedentseva a
a,b
, A. Karmenyan c, P.-H. Chung a, Y.-T. He a, C.-L. Cheng
a,*
Department of Physics, National Dong-Hwa University, 1, Sec. 2, Da-Hsueh Road, Shoufeng, Hualien 974, Taiwan, ROC b P.N. Lebedev Physics Institute RAS, Moscow, Russia c Institute of Biophotonics Engineering, National Yang-Ming University, Taipei, Taiwan Available online 17 April 2006
Abstract Surface enhanced Raman scattering (SERS) of diamond nanocrystals and fullerene was investigated. Ag and Au films were used as SERS-active agent. In the first series of experiments SERS were prepared with sputtered island metals on the nanoparticles surfaces. In the second one the nanoparticles were positioned on silver surface using laser acceleration method. SERS is a powerful method to analyze the light accelerated nanoparticles patterning for their spatial distribution and structure. The enhancement in Raman spectral intensity of graphite-like phases and blinking effect are observed. Additional bands due to selective enhancement from cluster surface with different arrangement are generated. For SERS of diamond crystal phase as well as for fullerenes the characteristic shift and asymmetry of Raman bands are also observed. The investigation can be of interest both for the understanding of carbon nanostructures’ physics and for their applications, especially for bio-detection, as well as for the studying of SERS mechanism. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Surface enhanced Raman; SERS; Carbon nanostructure; Nanodiamond
1. Introduction Identifying different bonding configuration (sp2, sp3) in various carbon systems is used for the study of low-dimensional objects, such as crystal and onion-like carbon nanoparticles, nanotubes, fullerenes, etc., and for the study of interfacial phenomena, including interaction between carbon nanostructure and substrate surface. One of the advantages of Raman spectroscopy is its capability to provide highly resolved vibrational information and not to suffer rapid photobleaching. However, Raman scattering is not an efficient process; to improve the detection sensitivity, signal amplification can be obtained by exploiting the surface enhanced Raman effect [1]. Surface enhanced Raman scattering (SERS) is a very powerful Raman technique, being able to provide a spectral intensity enhancement by orders of magnitude. The phenomenon occurs when adsorbing the target molecules onto nanometer-sized *
Corresponding author. E-mail address: [email protected] (C.-L. Cheng).
0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.01.074
roughened metal (Ag, Au, Cu and some others) surface, or in colloids of metals. Nanoscale surface roughness supports the electromagnetic resonance, which is the dominant mechanism of enhancement. These electromagnetic resonances can increase the scattered intensity by several orders of magnitude. A second mechanism is chemical, connected with the forming of new electronic states due to adsorbate– substrate bonding interactions. It has been estimated by Kambhampati et al. [2,3] that the chemical mechanism can also greatly enhance the scattering cross-section. Although advantages of the SERS are most useful in the study of organic molecules [4–6], SERS from nanodiamond films and diamond-like carbon films was investigated by several groups [7–12], and from nanodiamond powder [13]. Some new information concerning structures and the physical properties has been provided for diamond-like materials [14], and for fullerenes [15,16]. SERS is extremely sensitive to local structural properties of carbon nanostructures. It is applicable to the analysis of spatial distribution of different types of hybridization and, correspondingly, different carbon phases in nanoscale.
3724
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728
We compare here the SERS from the nanodiamond (nanocrystal diamond particles with average sizes 100 nm and 5 nm) and fullerene C60samples, prepared with two different methods. In the first experimental series the island metal films were sputtered on the nanoparticles samples. This method has been used before for the samples preparation to study SERS of carbon structures [7–11]. In the second series the nanoparticles were positioned on the metal surface by laser acceleration [17–19]. In the laser acceleration experiment, the nanoparticles suspended in distillated water were accelerated by high focused near-infrared laser beam and attached onto Ag surface, which serves as the target. The laser acceleration allows the nanoparticles to be positioned and ordered on the surface and to penetrate deeply in metal layer. This method allows the patterning of nanoparticles [20], which has valuable applications for biosensing and bioprobing. In this method, the interaction between nanoparticles and metal surfaces can be adjusted by varying the laser acceleration parameters. Strong interaction between the nanodiamond or fullerene and metal is observed from the analysis of SERS spectra. Our main objective is to compare the SERS produced by two methods, the sputtering and the laser acceleration methods. The advantage of laser acceleration is to allow positioning nanodiamonds in a desired pattern as reported in our earlier publication [19]. 2. Materials and methods Synthetic diamond powders with average sizes 100 nm (Kay Industrial Diamond, USA) and 5 nm (UltraFine Diamond, Russia) in diameters were used. The nanodiamonds after acid treatment (incubation of the powder portion in a mixture of concentrated H2SO4 and HNO3 with ratio 9:1 at 65 °C for several hours followed by three times washing in distilled water) were suspended in distillated water in concentration of 1–10 mg/ml. Fullerene (C60, Aldrich) was dissolved in toluene in concentration 0.8 mg/ml. The suspensions were dropped on Si surface and dried. Islands Au film with center thickness 20–30 nm was sputtered with ion sputter (E-1010 Hitachi Ion Sputter JEOL). The Raman spectra were measured before and after sputtering with a micro-Raman spectrometer (Renishaw 1000B), complete with microscope (Leica), and with a diode pumped solid-state CW laser (DPSS, Coherent) at 532 nm as excitation wavelength. Spectra were measured with low laser beam power at 1–10 mW in focal spot to avoid laser damage to the samples. For the laser acceleration deposition, water suspensions of nanodiamonds as well as fullerene were used for the patterning of nanoparticles on the Ag and Au films. Water suspension of fullerene was prepared by stirring or sonicating fullerene powder in distilled water with following sedimentation and separation of the largest aggregates. In such suspension the C60 molecular clusters or micro and nanocrystals with wide size dispersion exist [21–23]. A femtosecond Ti:Sapphire laser (Mira-900, Coherent) with
wavelength 780 nm, time duration 150 fs, average power up to 1000 mW, and repetition rate of 76 MHz pumped by a solid-state laser (Verdi, Coherent), served as the light source for particles’ acceleration. Laser was connected with a modular microscope (Leica DM IRB) and a programmed XYZ-scanning stage (SGSP (MS) 20–85). The scanning of target realizes the positioning of the accelerated nanodiamond on the target according to predetermined program. The automatic monitoring and steering was realized with stage controller (Mark-204-MS) with a fully closed-loop control system. Water suspended nanoparticles were applied on the metal surface and treated by laser beam trough microscopic objective, scanning with fixed rate along the programmed patterns. The treated area for every sample was typical 2–4 mm2. The obtained samples were investigated using scanning electron microscope (SEM, JEOL JSM6500F). SERS from different kind of samples was analyzed and compared. 3. Results and discussion In Fig. 1, Raman spectra of 5 nm (a) and 100 nm (b) nanodiamonds before and after Au sputtering are shown, measured in different points of the sample. The observed characteristic Raman bands for nanodiamond are: 1332 cm 1 for carbon sp3 bonding (the diamond band); the E2g symmetry allowed G-band at 1590 cm 1 (graphite band); the disordered allowed 1350 cm 1 D-band; and the 1150 cm 1 and 1450 cm 1 bands recently assigned by Ferrari and Robertson [24] as trans-polyacetylene bands. In Fig. 1a(1) and b(1), for 100 nm nanodiamond the band 1332 cm 1 distinctly predominates whereas for 5 nm nanodiamond D- and G-bands are more intensive comparable to the diamond band. The strong D- and G-band, especially in the 5 nm diamond nanoparticles, are quite different compared to the spectra obtained from nanodiamond films. Our samples were nanodiamond particles, 5 and 100 nm in sizes, and were subject to acid treatment prior to the SERS investigation. During the acid treatment process, the surface could have covered with an extended graphitic structure or hydrogenated carbon surface as depicted in our normal Raman spectra shown in Fig. 1. We did not observe the expected 1240 cm 1 peak that was assigned to the nanodiamond phase with the size between 2 and 40 nm by Roy et al. [11]. The 1332 cm 1 band was also slightly asymmetric. The hydrogenated surface could form trans-polyacetylene chains according to Ferrari and Robertson [24]. This reflects in the m1 and m3 bands in normal Raman or SERS spectra (Fig. 1). The fact sp2 related bands dominated the spectra as compared to the diamonds films used by Roy et al. [11] should come from the graphitic structure on surface and the large surface area in our diamond nanoparticles. Also, the sp2 Raman cross-section is 50–200 times larger than the sp3 carbons should also contribute to our spectra [7]. In Fig. 1 both for 5 and 100 nm nanodiamonds the increasing of the Raman signal intensity after Au sputtering is more striking for graphite and partic-
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728
3725
G
5 nm
ν3
G
D 4000
20000
2 3000
D
100 nm
ν1
Intensity (a.u.)
Intensity (a.u.)
ν1
ν3
diamond
2
diamond
10000
1
1200
a
1400
1600
1
1800
Raman shift (cm-1)
1200
1400
b
1600
1800
Raman shift (cm-1)
Fig. 1. Raman spectra of 5 nm (a) and 100 nm (b) nanodiamond before (1) and after (2) Au film sputtering; the spectra were measured in different points of the samples.
5 nm
8000
a Intensity (a.u.)
ularly for amorphous carbon, in agreement with Ilie et al. [12] and Veres et al. [14]. No additional bands as result of selective enhancement are clearly observed at such sample configuration. This shows that the interface with Au suffers strong modification compared to the un-treated nanodiamonds, which could be consistent with sp2 promotion and increased interfacial disorder. The enhancement of absolute signal intensity for diamond band at 1332 cm 1 was not observed. Positioning of nanodiamond on Ag metal film by laser beam acceleration allows observation of more effects, characteristic for SERS from nanodiamond and diamond-like carbon materials (DLC). In Figs. 2 and 3 the Raman spectra are plotted for 5 nm and 100 nm nanodiamonds positioned on the Ag surface by laser acceleration. In every set (a or b) the spectra are measured sequentially in the same point of the sample. Both for 5 and 100 nm nanodiamond attached on Ag, additional bands relative to the regular bands of carbon structure are observed in nearby range (Figs. 2 and 3a). The preferred enhancement of non-diamond bands (D- and G-bands as well as trans-polyacetylene bands etc.) are observed for nanodiamond positioned on silver surface by laser acceleration of the nanoparticles similar to nanodiamond with sputtered metal film (Fig. 1). Complication of the spectra and repeated increasing and decreasing of Raman signal intensity during sequential measurements, which possess selective enhancement of some composite band intensity and blinking effect, are characteristic features of SERS of carbon structures [14]. Blinking effect consisting of considerable fluctuations in both signal intensities and frequencies has been observed
4000
b
0 1200
1400
1600
1800
Raman shift (cm-1)
Fig. 2. Raman spectra from 5 nm nanodiamond, attached on the Ag by laser beam acceleration at laser power 0.5 mW in focal spot. The time of measurement of each spectrum is 10 s, data acquisition time is 100 s. Spectra (a) are measured in one point of the sample and spectra (b) are measured in other point of the same sample.
previously for single-molecule fluorescent spectroscopy at SERS. It was studied in particular by Nie et al. [4,5] and Bjerneld et al. [6], who have shown that blinking effect is observed for SERS under conditions where many
3726
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728 100 nm
100 nm
8000
Intensity (a.u.)
Intensity (a.u.)
4000
2000
4000 0
0
0 1200
a
1400 1600 Raman shift (cm-1)
1200
1800
b
1400 1600 Raman shift (cm-1)
1800
Fig. 3. Raman spectra of 100 nm, attached on the Ag film by laser acceleration at laser power 0.5 mW in focal spot. The time of measurement each spectrum is 10 s, data acquisition time is 100 s. Spectra (a) are measured in one point of the sample and spectra (b) are measured in other point of the same sample.
molecules are adsorbed on SERS-active substrate (in contrast to the one for single-molecule fluorescence) due to that the ‘‘many-molecule’’ spectra can be dominated by single molecules interacting with SERS-active site, especially if these molecules can jump around between different metastable chemisorption states with different spectral characteristics. In our case, laser acceleration may create structural changes for the nanoparticle that may cause the blinking effect observed for many-molecular system. Briefly, SERS blinking effect can be explained by the assumption that the G- and D-bands are inhomogeneously broadened [25,26]. The intensity of the Gaussian broadened composite bands is determined by the concentration and the Raman cross-section of the structural unit related to it. This is the case of conventional Raman scattering, and the line broadening is a convolution of composite bands. If some composite atomic group (any structural unit) gets in very close contact to the silver surface, its Raman signal is amplified and enhanced corresponding peaks appear in the spectrum. In particular, often accompanying D- and G-bands at 1140–1150 cm 1 and near 1450 cm 1, attributed as transpolyacetylene chains on diamond surface [9,27,28] are selectively enhanced in some spectra in Figs. 2 and 3a. These peaks are not intense because non-hydrogenated nanodiamonds were investigated; the hydrogenation was only from the acid wash process used to prepare the samples. Also hydrogenated fragments of hexagonal and tetragonal carbon clusters can give rise to the group of spectral lines in the range 1300–1350 cm 1 [28]. The splitting of the G-band, 1580 cm 1, and 3–4 lines in range 1518–1590 cm 1 [29–31] can arise from the forming of convolute graphite nanostructures [31]; both the frequencies and intensities of additional bands depend on the number of graphite layers, curvature and symmetry of tube-like nanostructure [30,31].
The relatively weak peak in range 1240–1280 cm 1 often is observed from crystallites with nanometer size [8– 11,13,27], with decreasing of the size and amorphisation of surface this peak widens [11,32]. It can be ascribed to the maximum in the vibrational density of states in diamond, which take place in nano-sized crystals [10] or to amorphous sp3 bonded carbons [13]. The peaks in range 1470–1490 cm 1 and above 1600 cm 1 (1600–1630 cm 1) arise due to defects [32]: the peaks near 1470–1490 cm 1 are assigned to a vacancy or divacancy with conjugated single and double bonds; in the vicinity of a vacancy, regions of mixed single and double bonds are formed. When a vacancy is introduced into an otherwise pristine diamond lattice, new vibrational modes appear at 1470–1490 cm 1. While the 1630 cm 1 peak is attributed to the [1 0 0] split interstitial defect, which consists of an isolated sp2-bonded carbon pair occupying the position of one carbon atom in a normal diamond lattice. The spectra of Fig. 3b, diamond peak at 1332 cm 1 for 100 nm nanodiamond looks slightly asymmetric and exhibits a tail towards lower wavenumbers. This asymmetric diamond peak is often observed in SERS spectra. The asymmetry in the shape of the peak at 1332 cm 1 has been ascribed by Lopez-Riyos et al. and others [10,11] to the non-conserved momentum phonon excitations, which take place in nanometer-sized crystals. Observed enhancement of signals can be achieved both for 100 nm and 5 nm nanodiamond up to two orders of magnitude. From the analysis of SERS spectra, one can conclude that the high intensity of SERS for graphitic and trans-polyacetylene as well as additional bands in comparison with the diamond band both for 5 nm and 100 nm nanodiamond is connected not only with higher efficiency of the Raman scattering for them, but also with transfor-
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728 Ag2
Hg7
1000
Intensity (a.u.)
mation of nanodiamond particles at laser beam treatment and mechanical percussion with Ag target. Gogotsi et al. [33] observed the diamond–graphite transformation at high contact compression on diamond. High temperature at the laser acceleration also can create condition for local structural transformation in diamond, arising changes in Raman spectra [32,34]. Therefore, structural and shape transformations connected with deformation can also be observed in our case, which needs in further investigation of physical mechanism of these transformations as well as of necessary conditions to produce the patterns with predictable properties as the laser acceleration can be a destructive method. The SERS spectra of 5 nm and 100 nm nanodiamond at their high variability look more similar to each other, than the corresponding regular Raman spectra and the difference in SERS observed for the samples prepared by two different methods is very significant. The reason obvious lies in the difference in carbon–metal interaction occurring in both kinds of samples prepared with metal film sputtering and laser acceleration. Some minor difference in the observed SERS can also be contributed to the use of different metal films, Ag and Au [15]. But the difference observed by Chase et al. [15] is negligible in comparison with what observed in our work. In the same time no significant difference was observed in SERS from other kind of investigated carbon nanostructures, fullerene, for samples prepared by two discussed methods. We observed SERS from C60 for the samples prepared by metal sputtering on the fullerene film on Si as well as by laser acceleration of fullerene with metal film targeting. In both cases we should speak not about sepa˚ , but rate fullerene C60 molecules/particles with size 7–6 A about nano and microcrystals or clusters. Such structures formed on the Si surface at the sample preparation (solution drying) and predominated in water suspension of fullerene. In pristine C60 the most frequently studied Raman mode is the Ag2 intramolecular tangential pentagonal pinch mode at 1469 cm 1. This mode shows a strong shift to lower wave numbers when C60 is doped with metals or when it is polymerized to different structures [15]. Also the oxidized C60 displays a shift of the Ag2 mode [35,36]. It has been found [15,35,37] that the Ag2 mode is sensitive to a charge transfer of C60 cage as a result of strong electromagnetic field at the metal–C60 interface, inducing surface plasmons, as well as to changes in symmetry [15,35] and formation of metal–C60 hybrid bonds, with their own vibrational wavenumbers [37]. The downshift can be accounted for by the elongated C–C bonds in the negatively charged molecule. The Ag2 mode usually is accompanied by satellite with wavelength 1447 cm 1 [35], which also is shifted for metal–C60 system. In Fig. 4 Raman spectra of fullerene on Si, after Au film sputtering, and fullerene positioned on Ag by laser acceleration are compared. The enhancement of the relative intensity of Ag2 mode is observed for both fullerene–metal samples. The changing of relative intensities of these bands
3727
Hg8
c
500
b
a 0 1300
1350
1400 1450 1500 Raman shift (cm-1)
1550
Fig. 4. Raman spectra of fullerene on Si (a); fullerene on Si after Au film sputtering (b); fullerene positioned on Ag film by laser acceleration (c). Laser power is 0.5 mW in focal spot. The time of measurement of each spectrum is 10 s, data acquisition time is 100 s. The fitting of spectra was done with Lorenzian algorithm.
Ag2, Hg7, Hg8 was observed previously [37], and it is not explained yet. The shift of this band from 1468 cm 1 to 1458–1460 cm 1 as discussed above, as well as of satellite band 1447 cm 1 also are observed as well as some splitting, which can be explained by non-homogeneous enhancement of signal from surface and bulk C60 molecules in crystal or cluster or by enhancement of satellite Ag2 bond 1447 cm 1. Also some enhancement as well as shift of mode Hg7 from 1430 cm 1 to 1423–1425 cm 1 was observed. Raman spectral lines of fullerene are shifted but do not transform into graphite spectrum, as was observed by Stenzel et al. [36], so the method of laser acceleration is not destructive for fullerene; can be used for patterning, including for SERS investigations. 4. Conclusion We compared surface enhanced Raman scattering of carbon nanostructures of diamond nanoparticles with size 100 and 5 nm and fullerene C60 at different conditions of their interaction with SERS active surface. Sputtering of the metal film together with deposition of nanodiamond on metal surface (rather, nanostructured metal surface) as well as deposition of metal nanoparticles on the surface of carbon sample [14] is often used for the observation of SERS. Every method has its own advantages and can reveal some characteristics of the investigated object as well as the SERS mechanism. In addition to the method of the metal film sputtering on the nanoparticle samples we have investigated the method of positioning nanoparticles on metal surface by laser acceleration. This method allows reaching strong carbon–metal interaction for SERS
3728
E. Perevedentseva et al. / Surface Science 600 (2006) 3723–3728
analyzing of the obtained structures; also some structural rearrangement of graphite and amorphous structures in diamond as well as transformation of the nanodiamond into graphite as result of diamond crystal deformation can occur. Laser acceleration method of the sample preparation can not be called non-destructive, but it opens the new possibilities for nanotechnologies and also for some structural investigations, as well as for the investigation of SERS. In contrast to nanodiamond, for fullerene the difference in SERS observed for the samples prepared by two different methods is insignificant and noninvasive patterning is possible. Only some changes in the molecular symmetry and some charge transfer are observed for fullerene molecules interacted with metal independently on ways to provide this interaction. Acknowledgments The authors appreciate the financial support of this research by National Science Council of Taiwan, ROC under Grant No. NSC-94-2120-M-259-002. References [1] Selected papers on surface-enhanced Raman scattering, in: Milton Kerker (Ed.), SPIE Milestone Series, MS 10, 1990. [2] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241. [3] P. Kambhampati, C.M. Child, M.C. Foster, A. Campion, J. Chem. Phys. 108 (12) (1998) 5013. [4] S. Nie, S.R. Emory, Science 275 (1997) 1102. [5] W.E. Doering, S. Nie, J. Phys. Chem. B 106 (2002) 311. [6] (a) E.J. Bjerneld, P. Johansson, M. Ka¨ll, Single Mol. 1 (2000) 239; (b) H. Xu, E.J. Bjerneld, M. Kall, L. Borjesson, Phys. Rev. Lett. 83 (1999) 4357. [7] D.S. Knight, E. Weiner, L. Pilione, Appl. Phys. Lett. 56 (14) (1990) 1320. [8] D. Roy, Z.H. Barber, T.W. Clyne, J. Appl. Phys. 91 (9) (2002) 6085. [9] T. Lo´pez-Ry´ios, E´. Sandre´, J. Raman Spectrosc. 29 (1998) 733. ´ . Sandre´, S. Leclercq, E ´ . Sauvain, Phys. Rev. Lett. [10] T. Lo´pez-Ry´ios, E 76 (26) (1996) 4935. [11] M. Roy, V.C. George, A.K. Dua, P. Raj, S. Schulze, D.A. Tenne, G. Salvan, D.R.T. Zahn, Diam. Relat. Mater. 11 (2002) 1858.
[12] A. Ilie, C. Durkan, W.I. Milne, M.E. Welland, Phys. Rev. B 66 (2002) 045412. [13] S. Praver, K.W. Nugent, D.N. Jamieson, J.O. Orwa, L.A. Bursill, J.L. Peng, Chem. Phys. Lett. 332 (2000) 93. [14] M. Veres, M. Fule, S. Toth, M. Koos, I. Pocsik, Diam. Relat. Mater. 13 (2004) 1412. [15] S.J. Chase, W.S. Bacsa, M.G. Mitch, L.J. Pilione, J.S. Lannin, Phys. Rev. B 46 (12) (1992) 7873. [16] Y. Mo, G. Mattei, M. Pagannone, Appl. Phys. Lett. 66 (19) (1995) 2591. [17] A. Ashkin et al., Phys. Rev. Lett. 24 (1970) 156. [18] S.E. Skipetrov, S.S. Chesnokov, S.D. Zakharov, M.A. Kazaryan, V.A. Shcheglov, JETP Lett. 67 (1998) 635. [19] E. Perevedentseva, A. Karmenyan, P.-H. Chung, C.-L. Cheng, J. Vac. Sci. Technol. B 23 (2005) 1980. [20] Y. Xia, J.A. Rogers, K.E. Paul, G.M. Whitesides, Chem. Rev. 99 (1999) 1823. [21] I.D. Fortner, D.Y. Lyon, C.M. Sayes, A.M. Boyd, J.C. Falkner, E.M. Hotze, L.B. Alemany, Y.J. Tao, W. Guo, K.D. Ausmann, V.L. Colvin, J.B. Hughes, Environ. Sci. Technol. 39 (11) (2005) 4307. [22] G.V. Andrievsky, V.K. Klochkov, A.B. Bordyuh, G.I. Dovbeshko, Chem. Phys. Lett. 364 (2002) 8. [23] G.V. Andrievsky, V.K. Klochkov, E.L. Karyakina, N.O. MchedlovPetrossyan, Chem. Phys. Lett. 300 (1999) 392. [24] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [25] I. Po´csik, M. Hundhausen, M. Koo´s, L. Ley, J. Non-Cryst. Solids 227–230 (1998) 1083. [26] I. Po´csik, M. Koo´s, Diam. Relat. Mater. 10 (2001) 161. [27] A.C. Ferrari, J. Robertson, Phys. Rev. B 60 (12) (2001) 121405(R). [28] D. Zhang, R.O. Zhang, J. Phys. Chem. B 109 (2005) 9006. [29] G.S. Duesberg, W.J. Blau, H.J. Byrne, J. Muster, M. Burghard, S. Roth, Chem. Phys. Lett. 310 (1999) 8. [30] A. Mews, F. Koberling, T. Basche, G. Philipp, G.S. Guesberg, S. Roth, M. Burghard, Adv. Mater. 12 (16) (2000) 1210. [31] A. Kasuya, Y. Sasaki, Y. Saito, K. Tohji, Y. Nishina, Phys. Rev. Lett. 78 (23) (1997) 4344. [32] J.O. Orwa, K.W. Nugent, D.N. Jamieson, S. Prawer, Phys. Rev. B 62 (9) (2000) 5461. [33] Y. Gogotsi, A. Kailer, K.G. Nickel, Nature 401 (1999) 663. [34] H. He, T. Sekine, T. Kobayashi, Appl. Phys. Lett. 81 (4) (2002) 810. [35] R.L. Garrell, T.M. Herne, C.A. Szafranski, F. Diederich, F. Ettl, R.L. Whetted, J. Am. Chem. Soc. 113 (1991) 6302. [36] O. Stenzel, A. Stendal, D. Drews, T. Werninghaus, M. Falke, D.R.T. Zahn, C. von Borczyskowski, Appl. Surf. Sci. 108 (1887) 71. [37] R. Manaila, A. Belu-Marian, D. Macovei, G. Brehm, D.T. Marian, I. Baltog, J. Raman Spectrosc. 30 (1999) 1019.