Highly Stable Monocrystalline Silver Clusters for Plasmonic Applications

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Highly stable monocrystalline silver clusters for plasmonic applications Sergey M Novikov, Vladimir N. Popok, Andrey B. Evlyukhin, Muhammad Hanif, Per Morgen, Jacek Fiutowski, Jonas Beermann, Horst-Guenter Rubahn, and Sergey I Bozhevolnyi Langmuir, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Highly stable monocrystalline silver clusters for plasmonic applications Sergey M. Novikov,*,† Vladimir N. Popok,‡ Andrey B. Evlyukhin,║,# Muhammad Hanif,§ Per Morgen,¤ Jacek Fiutowski,⸹ Jonas Beermann,† Horst-Günter Rubahn⸹ and Sergey I. Bozhevolnyi† †

Centre for Nano Optics, University of Southern Denmark, Campusvej 55, DK-5230 Odense,

Denmark. ‡ Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, DK9220 Aalborg, Denmark.║ Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany. # Laboratory “Nanooptomechanics”, ITMO University, 49 Kronversky Ave., 197101 St. Petersburg, Russia. § Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus, Denmark. ¤ Department of Chemical Engineering, Biotechnology and Environmental Technology, University of Southern Denmark, Campusvej 55, DK-5230 Odense, Denmark. ⸹Mads Clausen Institute, University of Southern Denmark, NanoSYD, Alsion 2 DK-6400 Sønderborg

ABSTRACT: Plasmonics sensor configurations utilizing localized plasmon resonances in silver nanostructures typically suffer from rapid degradation of silver in ambient atmospheric conditions. In this work, we report on the fabrication and detailed characterization of ensembles of monocrystalline silver nanoparticles (NPs), which exhibit a long-term stability of optical properties under ambient conditions without any protective treatments. Ensembles with different

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densities (surface coverages) of size-selected NPs (mean diameters of 12.5 and 24 nm) on quartz substrates are fabricated using the cluster-beam technique and characterized by linear spectroscopy, two photon-excited photoluminescence, surface-enhanced Raman scattering microscopy as well as transmission electron, helium-ion and atomic force microscopies. It is found that the fabricated ensembles of monocrystalline silver NPs preserve their plasmonic properties (monitored with optical spectroscopy) and strong field enhancements (revealed by surface enhanced Raman spectroscopy) at least 5 times longer as compared to chemically synthesized silver NPs with similar sizes. The obtained results are of high practical relevance for the further development of sensors, resonators and metamaterials utilizing plasmonic properties of silver NPs.

1. INTRODUCTION. Light interaction with nanostructures, especially those fabricated of noble metals, gives rise to various fascinating optical phenomena 1,2. One of the current research directions in nano-optics is the development of metal nanostructures that efficiently interconvert propagating and localized optical fields and thereby facilitate the generation of strongly enhanced local electrical fields 3,4. Field enhancements (FEs) occur due to resonantly excited surface plasmons, representing collective electron oscillations in metals coupled to electromagnetic fields in neighboring dielectrics.5, 6 Strong FEs are extremely important for practical applications for example in sensing and catalysis as well as in surface-enhanced Raman scattering (SERS)7-10 Noble metal nanostructures are also very promising for the formation of surface plasmon resonators, which have demonstrated enormous enhancement of the fluorescence of quantum emitters11, 12, thus facilitating the development of ultra-bright and stable


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single-photon sources.. In addition, they are can be used as real-time sensors, plasmonic solar cells, building blocks for light-energy guiding devices and metamaterials with unique optical properties, etc9,10. Silver exhibiting excellent plasmonic properties5 can advantageously be used in the majority of the above-mentioned applications13-16. There are many strategies for the fabrication of plasmonic structures using silver. Different approaches such as particle formation by evaporation or sputtering, high-fluency ion implantation, chemical and photo reduction, production of colloid nanoparticles (NPs), etc. were suggested17-21. Many of them allow formation of nanomaterials with an attractive efficiency13 15. However, rapid oxidation /sulfidation from the ambient atmosphere dramatically decreases all bonuses of this metal22,23 and causes complications in terms of practical applications. For example, the sensors containing Ag usually require storage in inert gases and the operational time of such devices in the ambient atmosphere is rather limited. One existing solution is to make a protection layer24-26, which can prevent the Ag structures from the degradation due to reactions with environmental species. However, these treatments are not always applicable or rather complex to be used on an industrial scale. A possible approach to solve this problem could be the formation of very pure particles with perfect crystalline structure. They should be more stable against the above-mentioned degradation phenomena. A very promising approach for preparation of such NPs is aggregation of metal atoms into clusters from very pure sources in vacuum27,28 . These clusters can be collimated into beams, size selected and deposited or implanted on/in different substrates, which provides great capabilities for controlling the structure and properties of materials on the nanoscale29-31.


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In this work, we utilize a cluster beam deposition technique based on magnetron sputtering32 as a method of fabrication of ensembles of monocrystalline Ag NPs with well-controlled sizes. Due to these properties they demonstrate excellent stability of plasmon band intensity (it decreases only for about 20% after 30 days) and strong FE which is also preserved over rather long time period at room temperature and ambient atmosphere. We anticipate that the obtained results are very promising for sensor applications and in surface-plasmon resonator technologies.

2. EXPERIMENTAL SECTION 2.1 Ag NPs by a cluster beam deposition technique. A magnetron sputtering cluster apparatus (MaSCA)32 utilizing a commercial source, NC200U from Oxford Applied Research, was used for cluster production from silver targets of 99.99% purity. In the source, the target material was sputtered by an Ar plasma and was condensed into clusters of various sizes in the aggregation region with the help of He as a buffer gas. Thereafter, the clusters were expanded through a nozzle and collimated into a beam. Clusters from a beam were selected using an electrostatic quadrupole mass selector (EQMS) similar to that described elsewhere33. Only charged particles were selected and steered toward deposition on 10x10 mm2 quartz substrates in the vacuum with a background pressure of approximately 5 × 10-7 mbar. The size-selection was performed at two different EQMS voltages of 300 and 1600 V. According to earlier studies, these voltages correspond to mean particle diameters of 12.5±1.1 and 24.0±2.0 nm, respectively34. Clusters of these sizes are further referred to as small and large NPs. The size-selected silver clusters were deposited in a so-called soft-landing regime securing that the cluster kinetic energy is much smaller than the cohesive energy of atoms leading to no or very small distortion of the cluster shape, which is supposed to be close to spherical in a free cluster beam34. Samples were prepared


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with several different cluster coverages (surface densities of NPs) ranging between approximately 300-600 clusters/µm2 for small NPs (samples DN1-DN3) and 70-300 clusters/µm2 for large NPs (samples DN4-DN6). 2.2. Ag NPs by chemical synthesis. For comparison of properties, Ag NPs were also produced by standard chemical means in the following way: 2 ml of a 0.001M AgNO3 solution was added dropwise (1drop /sec) to an ice cold vigorously stirred solution of 0.002 M NaBH4 (30ml) causing the reaction: AgNO3 + NaBH4 → Ag + ½ H2+½ B2H6 +NaNO3, which leads to the formation of Ag NPs in the solution. After the reaction, polyvinylpyrrolidone (PVP) was added to the solution under stirring. PVP is an organic polymer that coats the particle surface and acts as steric stabilizer, preventing NPs from aggregation. Thereafter, the particles were deposited in a drop on a quartz substrate and dried. The size of NPs is found to be 21±6 nm by atomic force microscopy (AFM). 2.3. Transmission electron, atomic force and helium ion microscopies. For high-resolution transmission electron microscopy (TEM) analysis, the clusters were deposited on standard copper TEM grids with 40 nm thick amorphous carbon layer. TEM measurements were carried out using a FEI-Talos microscope operating at 200 kV. The surface morphology of the samples with the NPs deposited on quartz was studied by AFM and Helium Ion Microscopy (HIM). AFM was operated in tapping mode using an Ntegra Aura nanolaboratory (from NT-MDT), utilizing standard commercial silicon cantilevers. HIM was carried out by an Orion NanoFab helium ion microscope (Carl Zeiss) at 30 keV beam energy, with a probe current ranging from 0.5 to 1.1 pA. No conductive coatings were applied to the samples prior to imaging, in order to preserve the


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sample surface information. Charge compensation was ensured through a low energy electron beam (flood gun, 600 eV) directed at the sample. 2.4. Optical and X-ray photoelectron spectroscopies. Optical transmission spectra of the deposited Ag clusters on quartz substrates were measured with a double beam Perkin Elmer High-Performance Lambda 1050 Spectrometer in a standard configuration. The obtained spectra were normalized by subtraction of the spectrum obtained on bare quartz and the transmittance was converted into extinction. 2.5. XPS analysis. Samples of the Ag nanoparticles deposited on Si (100) wafers with a native surface oxide layer were studied with XPS. The samples were fixed on a Ta foil and this sandwich of samples and foil attached to the sample holder with double sided carbon tape and inserted in an ultrahigh vacuum system containing a SPECS PHOIBOS analyzer. Two sets of samples with different densities of particles were analyzed. The spectra were recorded with Mg x-rays and at a fixed pass energy of 50 eV in the analyzer giving an effective resolution (FWHM) of around 2.4 eV of the peaks. This resolution ensures a high transmission, so the sensitivity to the identification of doublets, and shifts in the positions and intensities of the peaks in the spectra, is optimized. The samples were kept in a room ambient between the measurements, which were done with delays of 5, 24 and 66 days after the deposition. 2.6. Two photon-excited photoluminescence (TPL) microscopy. The setup35 consists of a scanning optical microscope in reflection geometry built on the base of a commercial microscope and a computer-controlled translation stage. The linearly polarized light beam from a modelocked pulsed (pulse duration ~200 fs, repetition rate ~80 MHz) Ti- Sapphire laser (wavelength λ = 730 – 860 nm, δλ ~10 nm, average power ~300 mW) is used as a source of sample illumination at the fundamental harmonic (FH) frequency and are recorded TPL photons within


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the transmission band of 350-550 nm. The FH and TPL resolution at full-width-half-maximum with a Mitutoyo infinity-corrected objective (×100, N.A.=0.70) is ~0.75 µm and ~0.35 µm, respectively, which means no individual clusters will be resolved in the TPL images. In this work, we used the following scan parameters: the integration time (at one point) of 50 ms, speed of scanning (between the measurement points) of 20 µm/s, scan area of 10 × 10 µm2, and scanning step size of 350 nm. We adjusted the incident power P within the range 0.3-1 mW in order to obtain significant TPL signals and record the TPL signal dependence on the incident powers. For simplicity we kept the excitation wavelength fixed at 740 nm. 2.7. Surface enhanced Raman scattering microscopy. The SERS measurements where done by the commercially available confocal scanning Raman microscope (Alpha300R) from Witec and measurements were obtained using linearly polarized excitation of wavelength 532 nm, 600 lines/mm diffraction grating, and ×100 objective (N.A. = 0.90), whereas we use unpolarized detection in order to have a significant signal to noise ratio. Detailed SERS images were formed by mapping the spatial dependence of SERS intensity integrated around the main Raman peaks within the shift range 1560-1650 cm-1 for each of the 28 × 28 points (step size 350 nm, area of scan 10 × 10 µm2) in the scan, integration time of 500 ms at each point and incident power P~ 0.13 mW (for the cluster), and P~ 0.3 mW (for the colloidal NPs). These scan-parameters were selected as a compromise between minimum damage/bleaching of the dye molecules (Crystal Violet – CV) and significant signal to noise ratios. This Raman active dye is chosen due to the well-known and well-characterized properties. Moreover, CV is not resonant at the excitation wavelength (absorption line ~590 nm) used in these experiments and it is relatively stable. The non-resonant Raman characterization helps to prevent the strong fluorescence which can sometimes completely dominate the Raman spectra and influence the enhancement estimation.


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All samples under the study are covered by an ethanol 10-6 M solution of CV and dried under ambient conditions. For the NPs obtained from cluster beams the covering was performed only once: 8 days after the deposition and just prior the first SERS measurement. For the chemically synthesized NPs, the samples were covered by CV 3, 24, 32, and 48 hours after the drop-casting and then immediately used for SERS study. 2.8. Numerical method. For theoretical analysis, we calculated the scattering cross-sections of silver nanoparticles located in free space or on a glass substrate using the method of discrete dipole approximation based on the Green tensor approach36, 37. 3. 3. RESULTS AND DISCUSSION In-plane TEM and HIM images show that individual NPs of both sizes are round in shape (Figure 1a,b). However, due to surface diffusion, some neighboring particles tend to agglomerate while preserving individual crystalline structure. From the HIM and TEM images we estimated that the majority (over 50%) of the deposited NPs are individual ones with either spherical or slightly oblate shape. Other clusters represent closely located NPs forming dimers or trimers (their ratio is increasing function of coverage) and the remaining particles originate larger agglomerates. HIM and TEM imaging is also compared with AFM studies demonstrating distribution of NPs of both sizes and different coverages on larger surface areas (see Figures 2a,b).


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Figure 1. (a) HIM image of deposited large silver NPs (24 nm). (b) TEM in-plane image of deposited small silver NPs (12.5 nm). (c) TEM in-plane images of two neighboring NPs, with corresponding fast Fourier plot (top insert) and high-resolution TEM (bottom insert) demonstrating (111) atomic planes of FCC structure.

Comparison of mean diameter measured by TEM with mean height obtained by AFM gives evidence that NPs are slightly oblate in vertical direction yielding, for example, a ratio q=/ ≈ 1.25-1.30 for small NPs. Oblation effect can also be seen in the images obtained by HIM for large NPs (Figure 1a). This flattening is most probably originated from the very large difference in surface tension between silver (γ ≈ 1200 mJ/m2) and quartz (γ < 100 mJ/m2)38. To minimise the difference in Gibbs free energy at the interface a particle has a tendency to deform in order to increase the contact area that results in an oblate shape. High resolution TEM shows (Figure 1c) that the NPs have monocrystalline structure with facecentered cubic (FCC) lattice of bulk silver[39]. Monocrystalline nature of NPs leads to long-term stability of localized surface plasmon resonances (LSPRs) as described in more detail below. After the deposition, the samples are characterized by linear spectroscopy. The experimental optical spectra recorded for the small and large NPs and different surface densities are presented in Figure 2c. In the case of small NPs (dashed lines), there are two resonance bands near the wavelengths of 385-390 nm and around 460-505 nm. The intensity of the first resonance is gradually increased with the NPs coverage while the band position on the wavelength scale is stable. The resonance near 460-505 nm is less pronounced, broader and strongly dependent on the cluster coverage.


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Figure 2. AFM images of quartz substrates with silver NPs of (a) 12.5 nm and a coverage of ≈300 µm-2 and (b) 24 nm and a coverage of ≈70 µm-2. (c) Experimental extinction spectra for the ensembles of small (dashed lines) and large (solid lines) Ag NPs with three different surface densities. (d) Evolution of relative extinction of a sample with small NPs as a function of time.

Intensity of this band is significantly increased and the maximum is blue shifted with surface density of NPs. For the ensembles of large NPs (solid lines), there are two bands at 366-374 nm and 480-520 nm with the behavior similar to that found for small clusters. Additionally, one can see a third resonance with increasing intensity and blue shifting from 620 to 580 nm with cluster coverage. As mentioned in the introduction, single-crystalline nature of NPs is expected to slow down degradation of LSPRs due to possible reactions of silver with environmental species. Indeed, in mono-crystalline NPs with FCC structure, only the atoms present at the surface would


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be reactive due to the reduced coordination numbers and presence of dangling bonds. In the case of poly-crystalline particles, there is higher probability for reactive species to attack silver atoms at the grain boundaries, thus, converting them into compounds in expense of pure metal. It is also worth mentioning that presence of lattice defects and boundaries in a particle leads to additional scattering of conduction electrons causing large optical losses decreasing the plasmonic efficiency [40, 41]. Thus, single-crystallinity is an important factor for the extinction efficiency. To prove that the monocrystalline nature of the particles can significantly slow down the degradation of LSPR, the optical spectra of the same sample with small NPs have been measured over a period of 30 days (Figure 2d) while keeping it in ambient atmosphere at room temperature. Intensity and spectral position of the first plasmon band are monitored. After one day, the resonance band becomes slightly “red” shifted and its intensity is increased. After 7 days the intensity starts slowly decreasing and the band continues the “red” shift. After 30 days, the band intensity is decreased for less than 20%, thus, demonstrating a good stability of the optical properties. To explain the reasons for the observed spectral evolution on the time scale we need first to start with the analysis of different LSPR bands. In order to explain the obtained optical results (Figure 2 c,d) the numerical modeling using the Green tensor approach42,43, Mie theory44, and the images method43 is carried out. The dielectric function of silver was taken from the literature 45. Results of these simulations are presented in Figures 3 and S1. For the resonance on an individual cluster, the real part of the particle dielectric function is related to the dielectric constant of the media as Re [εm(ω)] = −2εd which is called the Fröhlich condition for the quasi-static approximation. For a spherical silver NP in air the resonance position is calculated to be 360 nm (Figure 3a). As can be seen in the experimental microscopy images, along with individual clusters there is a considerable number of


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agglomerates made of two particles forming elongated features (Figures 1a,b). Therefore, such structures are also modelled taking into account different sizes and spatial orientations of the elongates as well as cases for extinction and scattering (Figure 3a). By comparing the simulations with experiments one can conclude that the second resonance band at longer wavelengths (Figure 2c) is a result of the longitudinal dipolar mode of these nanostructures. The similar position of the second resonance on the wavelength scale for the small and large NPs can be explained by preservation of the aspect ratio for the elongated nanostructures formed by both small and large NPs. Our conclusions concerning existence of two resonances for small and large NPs, are justified from the HIM and TEM images in Figure 1 a,b. They demonstrate that single NPs and large NPs were formed by aggregating two single NPs, resulting thereby in only weak dispersion due to careful size-selection of individual particles. Furthermore, the relative ratio of elongated ensembles made of three particles is low compared to that of dimers or individual particles. Since the optical spectra are obtained from relatively large areas (3 mm in diameter), the response is averaged from all-possible orientations and aspect ratio dispersion of agglomerates yielding broader bands compared to the calculations. With an increase of the cluster surface density, the number of paired NPs enlarges causing an increase in the band intensity. A small blue shift could be connected with an increase in plasmonic coupling between Ag clusters. However, we should stress that it could be one of the possible explanations and we do not have satisfactory explanation of this shift. The resonance at around 500 nm (Figure 3a) in the modelling allows to suggest that the third band in the experimental spectra (Figure 2c) is related to the longitudinal dipolar mode of three agglomerated NPs. However, due to their aspect ratio dispersion this experimental band is very broad and the agreement with the simulations is only qualitative. The low intensity of this band compared to the other two resonances is related


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to a lower probability for the formation of such agglomerates. It should be stressed that the plasmonic coupling between ellipsoids does not play an important role in the modelled resonance peaks. Also, we would like to emphasize that these theoretical demonstrative results are used only for a support of our idea, which could qualitatively explain the experimental results. Now, we would like to come back to the first LSPR and address the experimentally observed red shifts of nearly 20-25 nm for small particles and about 10 nm for large ones, compared to the calculated value in air (compare Figure 2c and Figure 3a). One of the most probable reasons is the influence of the quartz substrate, in particular, of its dielectric function which is higher than that of air. Taking into account a partial contribution of εquartz into the calculation of the resonance condition gives a red shift of the plasmon band maximum (Figure 3b), due to interaction of the real particles with their images, as it follows from the electric image theory. This shift is expected to be higher for the small NPs. Further red shift of the first plasmon band in the experiments can be related to the change of the particle shape towards oblate (ellipsoidlike) which is observed by the microscopy studies (described above) and predicted by the simulations presented in Figure 3b demonstrating it for different aspect ratios q (length to width of an ellipsoid). The time evolution of the spectra presented in Figure 2d can now be explained as following. After the first day, the resonance band becomes slightly red shifted and its intensity is increased. We believe that this spectral change is related to the deformation of the particle from spherical towards oblate (ellipsoid-like) shape which is confirmed by the microscopy studies and in line with the simulations presented in Figure 3b predicting both the red shift and rise of intensity. One can argue that the formation of an oxide shell due to interaction with atmospheric oxygen can be another possible reason for the red shift.


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Figure 3. The theoretical calculation of (a) extinction and scattering cross sections of three ellipsoidal silver nanostructures located in air: case 1 - three differently-oriented ellipsoids with the same size parameters of 60x30x30 nm and case 2 - three differently-oriented ellipsoids with size parameters of 60x30x30 nm for two ellipsoids and 80x30x30 nm for the third (middle) one. The ellipsoid orientations correspond to the x-axis (first ellipsoid), 45 degrees with respect to xaxis (middle ellipsoid), and y-axis (last ellipsoid). The incident electric field was polarized along the x-axis. (b) Resonance position on the wavelength scale for the spherical (diameter of 20 nm and aspect ratio q = 1) and ellipsoidal (q = 1.24 and 1.37) silver particles on quartz substrate.

The results of simulations for core/shell (Ag/AgO) structures (Figure S1) show that growth of this shell leads to a red shift of the band. However, a reference sample kept after the cluster deposition in an inert N2 atmosphere shows the same tendency in change of spectrum (Figure S2) one day after the deposition as for that stored in ambient air, thus, allowing one to disregard any contribution of environmental factors and, in particular, oxidation. As mentioned above, after 7 days the LSPR intensity starts gradually decreasing and the band continues the red shift (Figure 2d). Contrary to this tendency, the spectrum of the sample stored in N2 is stable (Figure S2) allowing one to conclude that interaction of NPs with environmental species is the main reason


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for the temporal degradation of their plasmonic properties which is also in agreement with findings in the literature 23,46. In order to clarify the situation with oxidation of the deposited silver clusters, we performed XPS analysis on a time scale of over 2 months (Figure 4).

Figure 4. (a) XPS spectra of one sample of Ag NPs obtained 5, 24 and 66 days after deposition. (b) Zoom for Ag 3d peaks showing stable position and intensities. The spectra were recorded with identical parameters but the displays are shifted in intensity for clarity of presentations.

According to the literature47,48, the oxide formation should lead to small but detectable “negative” shift (for about 0.3-0.4 eV) of the Ag 3d peak. The spectra taken 5, 24 and 66 days after the cluster deposition do not show any measurable difference in the peak positions or intensities (Figure 4b). Thus, one can allow for a thin oxide shell to have formed after taking the samples from the vacuum chamber into the ambient atmosphere but there is no evidence for any following oxidation on the long time scale. These results lead us to the conclusion that the degradation of LSPR of silver NPs with time shown in Figures 2d and S2 is most probably related to the interaction with other atmospheric species than oxygen.


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In order to estimate the FE and applicability of the samples for SERS applications, both SERS and TPL characterizations are performed. The measurements are carried out on the samples kept in the air for 8 days after the fabrication. All samples are covered by an ethanol 10-6 M solution of CV dye and dried under ambient conditions. As shown elsewhere, the oxidation of silver NPs or other reactions with species present in air typically lead to significant decrease or completely disappearing SERS signal on a relatively short time scale of a few days 48-450. The typical SERS spectra are shown in Figure 5 and they demonstrate considerable or even high intensity depending on particle size and surface coverage. SERS images show rather homogeneous signal distribution across the samples (Figure S3) except a few fluctuations in intensity related to cluster diffusion and aggregation around natural surface defects of quartz. As can be seen from the spectra presented in Figure 5a, the SERS intensity increases with increasing cluster density from DN1 to DN3 and from DN4 to DN6.

Figure 5. SERS spectra (a) for the samples DN1-DN6 obtained 8 days after the deposition of small and large Ag clusters on the glass substrate (cluster surface density increases from DN1 to DN3 for the small NPs (12.5 nm) and from DN4 to DN6 for the large NPs (24 nm)), (b) for


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the sample with cluster surface density DN3 and different time durations after coverage by analyte.

This is an obvious tendency because every cluster represents the so-called hot spot, i.e. the place with enhanced local field. Hence, an increase in cluster density gives higher intensity. At the same time, the intensity for the samples with large NPs tends to be much higher compared to the samples with small ones due to the factor of two difference in radius between the small and large NPs yielding 8 times difference in the effective volume for individual particles. Thus, more dye molecules surrounding large clusters become excited and contribute to SERS signal. It is also known that the maximal gain can be expected for specific (efficient) particle size and distances between them51,52. However, study of this phenomenon was not among the scopes of current work. In order to monitor evolution of spectra after covering the silver NPs by the analyte, we performed the measurements each 24 hours. These SERS spectra (see Figure 5b) shows only near 15% decrease of intensity after 72 hours, thus, demonstrating great stability of the cluster ensembles with respect to SERS in ambient atmospheric conditions. The stability for SERS application of Ag NPs obtained by cluster beam deposition technique is compared with that for the NPs obtained by chemical synthesis (see details about sample preparation in section 2.2). For the comparison of the stability of our clusters for SERS application, we used colloidal silver nanoparticles with size 21±6 nm. Several samples were prepared using these colloids by drop-casting on the glass substrate. Each sample consists on 3-4 drops located separately on one glass slide. The samples were covered by CV 3, 24, 32, and 48 hours after the drop-casting, respectively; each with the same concentration as used for clusters, and dried under ambient conditions. The level of SERS signals is found to be strong only for the


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samples covered by the dye 3 and 24 hours after the drop-casting of NPs (Figure S4c). The mapping shows homogeneously strong SERS signals across the whole area selected for the scan (Figure S5). For the sample covered by CV 32 hours after the drop-casting, the intensity of spectral peaks is significantly decreased and they nearly disappear for the sample covered by the dye 48 hours after the particle deposition (Figure S4c). The mapping made for these two samples shows that the SERS signal is present only on some parts of the scanned area. In order to monitor stability of colloidal NPs after covering by the analyte, we performed the measurements each 24 hours, similar to that carried out for the NPs deposited from cluster beams (see Figure 5b). The SERS spectra (Figure S5d) show nearly 3 times decrease of intensity after first 24 hours , thus, demonstrating much faster degradation of FE compared to the cluster beammade particles. To compare a signal increase in SERS, with that in ordinary Raman keeping the same experimental parameters, the analytical enhancement factor (EF expression) is used 53. The average EF is determined by comparing the signals acquired from CV at a concentration of 5×102

M on a glass substrate, with the signals obtained from 10-6 M of CV on the ensembles of Ag

clusters. The following relation is used: EF =

I SERS Cref , I ref CSERS

(1)

where, ISERS and Iref represent the background-subtracted intensities of the 1186 cm-1 band (most intense one) for CV adsorbed on the silver NPs and the glass substrate, respectively, whereas CSERS and Cref represent the corresponding concentrations of CV on these substrates. The average EF are estimated to be ~0.05× 104 (DN1), ~0.46× 104 (DN2), ~1.41× 104 (DN3), ~0.25× 105 (DN4), ~0.52× 105 (DN5) and ~1.39 × 105 (DN6), respectively. We would like to stress that


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in this case we have used non-resonant dye, whereas for resonant cases the enhancement estimation would be one-to-two orders of magnitude higher. In addition, it is worth mentioning that the purpose of this work is not optimization of structures to get the highest SERS signal and finding the minimal resolvable concentration of an analyte but rather to estimate the stability and applicability of the prepared cluster ensembles. A simple change of substrate type, for example, substituting quartz by Au or Ag, which allow to excite surface plasmon modes, could significantly increase the signal. In future work we are going to work on the optimization of cluster sizes and surface density in order to reach all benefits from the proposed method of fabrication and combine the current technique with lithography. Furthermore, it is worth mentioning that the samples demonstrate several resonances covering almost all visible spectral intervals and one can vary the spectral position by changing the cluster size and surface coverage. FE for the samples is also estimated using TPL microscopy 54,55. Each sample is scanned in different places, at least 5 times, for better statistics. The TPL images shows homogeneity across the samples (see as example Figure S6a). The TPL signal intensity on the incident powers exhibits quadratic power dependence (Figure S7). However, for relatively high values (around 1 mW) we observe damaging of the samples (Figure S6) leading to the damping of the quadratic power law. Experimentally, the level of TPL enhancement can be objectively evaluated by taking into account the area and incident power producing the TPL signal56. The intensity enhancement factor can be calculated using the following relation:

α=

S str Pref

2

Aref

S ref Pstr

2

Astr

,

(2)


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where S is the obtained TPL signal, < P > is the used average incident power, and A is the TPL source area within the FH focus spot (diameter ~0.75 µm) producing the enhancement. Using Eq. (1), the intensity enhancement is estimated for the ensembles of Ag clusters as ~25 (DN2), ~32 (DN3), ~68 (DN4), ~90 (DN5), ~110 (DN6), respectively. However, these estimations are rather rough as they do not take into account a more detailed ratio between areas, since for the clusters it is difficult to accurately evaluate the smaller TPL source area, which would lead to higher FE estimates. It is worth mentioning that in the cases of highest cluster densities DN3 and DN6 the area contributing to the TPL signal is almost equal to that of the laser spot (deposited NPs create almost a monolayer with a few gaps in between) but in other cases this area is smaller due to the lower cluster density and many gaps between NPs, thus, leading to lower contribution to TPL signal. This explains why the intensity of TPL from samples DN2, 4 and 5 is not so strong although the local intensity enhancement can be relatively high. Here we should stress, the TPL signal is produced inside of Ag. The TPL skin depth amounts to ~ 5.7.nm at λ = 740 nm. That is why even the presence of 1-1.5 nm shell of oxide is not critically influencing the level of TPL signals. However, at the same time, the presence of even less than 1 nm of oxide shell is critical for the SERS signal and dramatically decreases the level of Raman signal.

CONCLUSIONS In summary, ensembles of size-selected silver nanoparticles on quartz substrates are fabricated using a cluster beam deposition method. In the current series of experiments silver NPs of approximately 12.5 and 24 nm in diameter are used for the formation of samples with different surface coverages. Microscopy studies show that the particles have monocrystalline structure


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and slightly oblate morphology after the deposition on quartz substrates. Increase of coverage favors the formation of dimers and even larger agglomerates due to the surface diffusion of NPs while preserving the individual crystalline structure of the particles in the majority of cases. Individual NPs as well as small agglomerates (dimers and trimers) cause specific LSPR bands. The spectral position of these bands can be controlled by varying the cluster size and surface density, which makes it possible to optimize the plasmonic properties of the ensembles and adopt them for particular applications. The obtained experimental data on the plasmon resonances are found to be in good agreement with theoretical modeling of the optical properties confirming a slightly oblate shape of the particles and the formation of small agglomerates. The ensembles demonstrate great stability of plasmonic properties on the time scale of one month and show considerable field enhancement even after 8 days of keeping them in ambient atmosphere while the field enhancement properties of chemically synthesized Ag NPs made for comparisons degrade within 2 days. The maximum levels of intensity enhancement of ∼110 is found for large NPs with the highest used coverage in these experiments of about 300 clusters/µm2. The maximum analytical enhancement factor of ~1.39 × 105 is obtained for the same sample using CV 10-6 M dye deposited on the ensembles. The demonstrated stability of plasmonic properties is caused by the monocrystalline structure of silver NPs revealing strong resistance to oxidation and other reactions with atmospheric gases or typical contaminants (for example, sulfurous compounds) during the storage in ambient atmosphere at room temperature. The reported results are of high relevance for the fabrication of sensors, resonators and metamaterials utilizing plasmonic properties of silver NPs. This work will be followed by further research in order to optimize parameters of the fabricated ensembles of NPs.


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ASSOCIATED CONTENT Supporting Information: Additional figures for the characterization of the silver clusters and colloidal NPs. AUTHOR INFORMATION Corresponding Author *[email protected] Conflict of Interest: The authors declare no competing financial interest. ACKNOWLEDGMENT. The authors gratefully acknowledge financial support from the University of Southern Denmark (SDU2020 funding), the Russian Science Foundation Grant No. 16-12-10287 and the Deutsche Forschungsgemeinschaft (DFG) EV 220/2-1.


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Highly Stable Monocrystalline Silver Clusters for Plasmonic Applications

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