1. Introduction

ZnO-based materials in the low dimensional regime have attracted considerable attention owing to a wide bandgap of 3.4 eV, large excitonic binding energy of 60 meV and their size-dependent optical and electronic properties, based on the quantum confinement effect of the electronic states [1–4]. This effect also changes the vibrational properties of material. Theoretical [4,5] and experimental [5,6] studies of the phonon modes in ZnO wurtzite nanosized crystals were reported.

Plasmonic enhancement of semiconductor nanocrystals (NCs) secondary radiation was established for two decades [7–11], but studies of the surface-enhanced fluorescence prevailed. The interest in this field is connected with increasing use of quantum dots as fluorescent labels for biological analyses and cell imaging due to their enhanced resistance to photobleaching and tunable radiative properties [12,13]. However Raman scattering allows to investigate the internal properties of material, phonon modes, and electron-phonon interactions [14]. Since the discovery that Raman signals are enhanced at nanostructured metal surface, surface enhanced Raman scattering (SERS) found different applications including ultrasenstitve analysis of organic and inorganic compounds [15,16], biosensing, monitoring microorganisms [13,17], etc... The first work on SERS of colloidal nanocrystals was published in 1993 [10] and the area is still developing [18–21].

Surface enhanced Raman scattering of light by ZnO nanostructures was previously reported by several groups [5,18,21,22]. Under resonant conditions spectra show the enhanced A1 longitudinal optical (LO) mode [5,15,21]. An increase of Raman scattering for nonresonant excitation was obtained for ZnO NCs coupled with colloidal silver nanoparticles [18], ZnO nanorods and nanocrystals covered with silver clusters by vacuum deposition [5]. The enhanced modes were A₁ (LO) and E₂ (high). In view of possible applications of SERS of ZnO NCs in ultrasensitive sensing, it is important to know the behavior of the Raman spectra of ZnO nanocrystals in the vicinity of the silver and gold nanoparticles in different configuration with metal and semiconductor counterparts. In this paper, we report on significant surface enhanced nonresonant Raman scattering of light by ZnO NCs coupled with metallic nanostructures.

2. Experimental

Colloidal ZnO nanocrystals solution in ethanol were synthesized by hydrolysis of zinc acetate (reagent grade) with sodium hydroxide (pure) at 0 °C, as described in more details elsewere [18]. After the synthesis solution was kept at 55–60 °C for 2 h to promote ZnO particles crystallization. The molar zinc oxide concentration in solution was 0.01 M corresponding to about 10⁻⁵ M in terms of NC concentration [18]. Thus, studied solution was ZnO in ethanol, minor components that remained in the system after the synthesis were residuals of sodium acetate and zinc acetate.

Raman scattering experiments were performed on ZnO NCs deposited by drop-casting onto the glass substrate. As the source of plasmonic enhancement a nanostuctured silver and gold thin films (ca. 5 nm) were deposited by means of the vacuum evaporation technique. Two sorts of samples were prepared: with gold or silver film deposited on glass before ZnO NCs casting (Ag-ZnO and Au-ZnO samples) or by deposition of the nanostructured metal film on top of the ZnO NCs preliminary applied onto the pure glass surface (ZnO-Ag and ZnO-Au samples). In both cases, deposition of ZnO NC film was performed by dropping a 10 μL of ZnO NCs solution onto the substrate and drying for at least 1 hour. The reference sample was prepared in the same way but without metal deposition. To ensure stability of the analyte and to exclude possible side effects, Raman spectra were taken for multiple spots on a surface, measurements were repeated several times during the period of one month (the representative spectra are shown). Raman spectra were collected in backscattering geometry using Jobin-Yvon Labram confocal spectrometer with He-Ne laser source (λ_exc = 632.8 nm). Hitachi S3500 microscope was used for scanning electron microscopy (SEM).

3. Results and discussion

Electron microscope image of ZnO nanocrystals on glass is shown in Fig. 1(a). The mean NCs size was about 4 nm. Figures 1(b)–1(d) presents the electron microscope images of gold- and silver-covered ZnO NCs film (ZnO-Au, ZnO-Ag) and ZnO-covered silver film (Ag-ZnO). The different morphology of the films can be explained by different adhesion for particular combination of matrials which are different in each sample. For both Ag- and Au-containing samples the λ_exc = 632.8 nm used for exciting Raman spectra was within the plasmon resonance band – at its long-wavelength tail for Ag (extinction maximum at 550 nm) and at the short-wavelength tail for Au (extinction maximum at 700 nm). Strongly different plasmon peak position in our samples is obviously determined by the nature of the metals, because the film morphology is not very different – the average grain size is 20-25 nm for Au (Fig. 1(b)) and 35-40 nm for Ag (Fig. 1(c)).

 

Fig. 1 (a) SEM pictures of the ZnO nanocrystals on glass, (b) gold-covered ZnO NCs film (ZnO-Au), (c) silver-covered ZnO NCs film (ZnO-Ag), (d) ZnO-covered silver-island film (Ag-ZnO).

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Hexagonal wurtzite ZnO (space group C⁴_6ν) is known to exhibit 12 phonon branches grouped into 2E₂ + 2E₁ + 2A₁ + 2B₁ normal modes. Only E₁, E₂, A₁ modes are Raman active [23]. The A₁ and E₁ modes are polar. A₁ (LO) dominate the spectra at resonance excitation [4,24,25], while at off-resonance conditions, when excitation energy falls well below the absorption onset, as in our case, all Raman-active modes can be expected in the spectrum. Note, however, that at the quantities and concentration of ZnO NCs used in the experiments, no detection of Raman lines is possible without applying the plasmonic enhancement provided by the nanostructured metal film (Fig. 2 bottom curve).

 

Fig. 2 Raman spectra of ZnO NCs on a Ag-coated glass substrate, ZnO NCs beneath a Ag-coating, reference spectra for ZnO NCs on glass substrate and for glass.

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The Raman spectra of the ZnO NCs on and beneath the silver coating is shown in Fig. 2. As compared to the former spectrum, the latter one is featured by a higher intensity of the peaks and has an additional peak at about 370 cm⁻¹. The frequency range of 350–650 cm⁻¹, where the main Raman peaks of ZnO are expected [23,25], was fitted for both above SERS spectra, and results are shown in Fig. 3. The frequency positions of the components obtained by fitting are 491 cm⁻¹ and 564 cm⁻¹ for Ag-ZnO sample and 373 cm⁻¹, 500 cm⁻¹, 562 cm⁻¹ for ZnO-Ag sample. The highest frequency component at about 560 cm⁻¹ can be assigned to the A1-LO mode, which is expected to be at 590 cm⁻¹ in bulk crystals at room temperature [23], and can shift downwards by up to 10 cm^–1 due to phonon confinement effect in the NCs [25]. The confinement effect is, however, not sufficient to explain the appearance of this mode near 560 cm⁻¹ in our case. Two explanations can be suggested therefore: (i) the local heating of the NCs in the vicinity of metal nanostructures resonantly absorbing laser radiation; (ii) the phonon mode in question is not the “bulk” LO phonon but one of the surface phonons allowed for a semiconductor NC of a spherical shape [26]. The latter assumption is corroborated by some of the earlier works where only the surface phonons were observed in the SERS of ZnO nanostructures [5]. The dominance of the surface phonons in the SERS spectrum may be understood in view of the very short-range efficiency of the plasmon enhancement – not exceeding a few nm or even less outside the metal nanostructure. Hence, according to the results of Ref [5], the modes at 490–500 cm⁻¹ and 560 cm⁻¹ can be attributed to the surface optical (SO) phonons formed from phonons of the symmetry A₁ or the symmetry E₁. In Ref [5]. such surface modes were observed for ellipsoidal NCs – the modes with quantum numbers m = 0, l = 1 and m = 1, l = 2 can be responsible for the peak at 560 cm⁻¹, whereas the peak at 490–500 cm⁻¹ may correspond to the m = 0, l = 2 mode [4,5].

 

Fig. 3 Raman spectra of the structure with ZnO NCs on a silver-coated glass substrate (a) and beneath a silver coating (b). Red line represents the results of the fitting of the spectra by Lorentzian profiles (dashed lines).

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The mode at 373 cm⁻¹ presents when silver covers quantum dots. It is by 5 cm⁻¹ “blue”-shifted from the bulk A₁ transversal optical (TO) phonon of ZnO [24]. Having a closer look at the full width at half maximum (FWHM) of the Raman peaks can help us to shed light on the nature of underlying phonon modes. The FWHM of the bands at 373 and 490–500 cm⁻¹ is similary large, 50-100 cm⁻¹, and at least twice as large as a width of the component at 560 cm⁻¹ (Fig. 3). The above large widths are typical for the SO phonons and have been reported for different semiconductor NCs, exceeding corresponding “bulk” phonon FWHM by 2-3 times [25,27,28]. Notable downward phonon peaks shifts due to NCs heating under laser beam were usually observed at the resonant Raman excitation, when absorption of electromagnetic energy in each NC is higher than can be dissipated in its volume [29,30]. Nevertheless, similar effect under non-resonant excitation of NCs but resonant excitation of the plasmon in SERS was already noted recently for TiO₂ NCs [31].

In the SERS spectra (Fig. 2) series of less intense features present with maxima at 689 cm⁻¹, 853 cm⁻¹, 923 cm⁻¹, 1060 cm⁻¹, 1314 cm⁻¹, 1434 cm⁻¹ for ZnO-Ag, and at 846 cm⁻¹, 923 cm⁻¹, 1054 cm⁻¹, 1320 cm⁻¹, 1434 cm⁻¹ for Ag-ZnO. Peaks at 689 cm⁻¹, 923 cm⁻¹, 1060 cm⁻¹, 1314-1320 cm⁻¹ and 1434 cm⁻¹ can be attributed to zink acetate, for which the following assignment can be found: O–C–O symmetric stretch (693 cm⁻¹), C–C symmetric stretch (923 cm⁻¹), CH₃ rocking vibration (1020 – 1080 cm⁻¹), CH₃ symmetric bend (1360 cm⁻¹) and C–O symmetric stretch (1453 cm⁻¹) [32,33]. They are more pronounced for ZnO deposited on the silver layer. The C–O stretch of acetate is red-shifted by 19 cm⁻¹ compared to that of free zinc acetate. This shift can be connected with the bonding with the ZnO surface [32]. Other modes are close to those of the free zinc acetate.

Surface-enhanced Raman scattering has been also observed for samples with gold in both configuration – deposition of ZnO NCs on Au-coated glass substrates and otherwise deposition of gold film on ZnO NCs. The spectra in both cases have only one pronounced maximum near 561 cm⁻¹ (Fig. 4). Nevertheless, the low-frequency asymmetry of this band clearly indicates that the mode at 490–500 cm⁻¹, distinctly observed for Ag-containing samples, is also present for samples with Au but is not well resolved, most probably due to stronger broadening of the peaks. Note that the overall intensity of Raman peaks was comparable in all SERS spectra, allowing for equally efficient usage of both enhancement configuration employed here – with nanostructured metal film beneath or on top of the ZnO NC film.

 

Fig. 4 Raman spectra of the structure with ZnO NCs on a Au-coated glass substrate and beneath a Au-coating. Both spectra have maximum at 561 cm⁻¹.

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4. Conclusions

We have investigated non-resonant surface enhanced Raman scattering by optical phonons of ZnO nanocrystals in two different configurations – with the nanostructured silver and gold deposited beneath or on top of the ZnO NC film. Both SERS configurations demonstrated comparable enhancement efficiency. Some spectral differences were observed between the two sample configurations and between two noble metal used in the same configuration. The latter differences can be related to the morphology of each particular sample, as well as to structure of the interface between the semiconductor and metal NCs. These structural factors can be of primary importance for the enhancement efficiency of different phonon modes, especially those related with the surface of semiconductor NCs. This assumption is corroborated by the different intensity of the residual zinc acetate peaks in different samples.