{'fraction_non_alphanumeric': 0.03319362142891555, 'fraction_numerical': 0.022353904706845885, 'mean_word_length': 4.704314372080241, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Dynamic strain in gold nanoparticle supported graphene induced by focused laser irradiation

Graphene on noble-metal nanostructures constitutes an attractive nanocomposite with possible applications in sensors or energy conversion. In this work we study the properties of hybrid graphene/gold nanoparticle structures by Raman spectroscopy and Scanning Probe Methods. The nanoparticles (NPs) were prepared by local annealing of gold thin films using focused laser beam. The method resulted in a patterned surface, with NPs formed at arbitrarily chosen microscale areas. Graphene grown by chemical vapour deposition was transferred onto the prepared, closely spaced gold NPs. While we found that successive higher intensity (6 mW) laser irradiation increased gradually the doping and the defect concentration in SiO2 supported graphene, the same irradiation procedure did not induce such irreversible effects in the graphene supported by gold NPs. Moreover, the laser irradiation induced dynamic hydrostatic strain in the graphene on Au NPs, which turned out to be completely reversible. These results can have implications in the development of graphene/plasmonic nanoparticle based high temperature sensors operating in dynamic regimes.


Introduction
Graphene has been in the focus of unceasing interest for both fundamental and applied research due to its unique material properties 1 . However, assigning functionality and integrating graphene into nanodevices has been challenged by the necessity of reliable and accessible preparation and characterization methods. Scanning probe microscopy and Raman spectroscopy are frequently used characterization methods for graphene and other two-dimensional (2D) materials. Moreover, Raman-spectroscopy has been settled as a standard tool 2, 3 to measure the quality of graphene, such as the number of layers 4 , the doping 5 , the strain 6,7,8,9,10 in graphene, etc. As a standard, it has been considered a non-invasive method, which is true only for low power measurements. The laser power can alter the samples in various ways 11,12 , especially if used in ambient conditions, for example by laser induced thermal oxidation 13,14,15,16 or increase in chemical doping 17 . On the other hand, when determining a safe power density, it should be taken into account that recording Raman spectra with too low laser power leads to bad signal-to-noise ratio. Oxidation might occur even at very low laser power densities, which are often regarded non-invasive. According to Kraus et al. 11 , this occurs on a time scale of few hours for cleaved graphene, while only a few tens of seconds for graphene grown by chemical vapour deposition (CVD) 12 . The laser induced changes during Raman spectroscopy investigations of graphene has been investigated comprehensively mostly with SiO2 as substrate.
Although this is a common substrate used in the field, it is of immediate importance to reveal the possible effects that can occur on other prevalent supporting materials 18 , now including different nanoparticles (NPs).
Graphene on noble-metal nanostructures has been demonstrated to be an attractive nanocomposite with many possible applications 19,20,21 . The surface-enhanced Raman spectroscopy (SERS) on noble-metal NPs is widely used to measure very low amounts of molecules, in some cases even single molecules can be detected 22 . Graphene-enhanced Raman scattering, which is considered to be based on a chemical enhancement mechanism 23 , also has significant potential in microanalysis 24, 25 . Combining the two processes, i.e. the SERS of the noble-metal NPs and the chemical enhancement of graphene could even improve the possibilities, either by depositing plasmonic structures onto graphene 26,27,28,29 , or by transferring graphene onto the metal NPs 30,31,32 . In this latter case the graphene shields the metal NPs from chemical interactions 33 , while its chemisorption activity increases due to the corrugated nature of these substrates 34,35 . The Raman signal of graphene itself can also be amplified by annealing, as the suspended graphene regions fill better the space between the NPs where the plasmonic enhancement of the electric field is the highest. As graphene follows the shape of the NPs, strain will emerge, and the improved adherence will modify also the electrostatic doping. This process is traceable with Raman spectroscopy and atomic force microscopy (AFM) measurements 31 . In this work we study the effects of optically induced heating of graphene-gold hybrid nanostructures. We produce gold NPs locally by focused laser irradiation of gold thin films. We show that the shift of Raman peaks of graphene transferred onto the NPs can be attributed mainly to strain, which is switched on and off by the applied laser.

Experimental
Graphene-gold hybrid nanostructures were prepared as follows: gold grains of 99.99999 % purity were applied as source material for evaporation, at a background pressure of 3×10 -8 mbar. A thin gold film of 5 nm was deposited onto a SiO2(285 nm)/Si substrate by an electron-beam evaporation system (AJA) at a rate of 0.2 nm s −1 . Gold nanoparticles were formed on areas of 5×5 µm 2 by local laser heating of the gold layer using a confocal Raman microscope (WITec) and a focused, 6 mW laser-power @633 nm. The laser beam was focused into a wavelength-wide spot and scanned over the selected 5×5 µm 2 regions with 20×20 points and a dwelling time of 3 seconds in each point.
Commercially available CVD graphene from Graphenea Inc. was transferred onto the substrate (gold layer and gold NPs) using thermal release tape (TRT) method as described elsewhere 31 . For comparison, we prepared a sample where graphene was transferred directly to a standard, SiO2(285 nm)/Si substrate.
The samples were investigated by confocal Raman microscopy (WITec) using excitation lasers of 488 and 633 nm. Low laser powers (0.6 mW) were used to characterize the same areas both before and after local heating. Raman maps were recorded in order to study the spatial distribution of spectral peaks. In this technique the excitation laser is scanned in a defined geometry and a complete Raman spectrum is recorded in every stepping point. AFM measurements were performed on a MultiMode 8 (Bruker) operating in tapping mode with SuperSharpSilicon™ probes (NANOSENSORS, tip radius ~2 nm). The sample with graphene on the 5 nm gold film was studied by scanning tunnelling microscopy (STM) and tunnelling spectroscopy (STS), using a DI Nanoscope E operating under ambient conditions. The STM measurements were performed in constant current mode with mechanically-cut Pt/Ir (90/10%) tips.

Results and discussion
The thin gold film (5 nm) evaporated on the SiO2(285 nm)/Si substrate was locally transformed into dome-like gold NPs by systematically annealing 36 with a focused laser beam, as described above.
The transformation can be easily observed optically, since the reflectance of the irradiated layer changed, as shown in Fig. 1a. The AFM image in Fig. 1b shows both the as-evaporated gold layer (left side) and one of the irradiated parts (right side) with the borderline in the middle. Dome-like gold NPs formed with diameter of 32 nm and height of 11 nm in average. The distribution of the maximum height of more than 2000 particles can be seen in Fig. 1c (red). Here, the height distribution of the asevaporated layer with a sharp peak at 5 nm is also shown (black).

Figure 1. a) Optical image after 6mW laser annealing. The irradiated areas (blue) can be well distinguished from the non-irradiated areas (purple). b) AFM image showing the border of asevaporated (left) and annealed region (right). c) Height distribution of the non-irradiated area of b) (black line) and the maximum height distribution of 2460 gold NPs formed on the irradiated area in b) (red line).
We transferred CVD-grown graphene onto the prepared gold nanoparticles. Fig. 2a shows an AFMimage of a typical laser irradiated, 5×5 µm 2 region and its surroundings, after the transfer process. Here graphene covers most of the area. Several wrinkles can be observed and also a discontinuity in graphene (darker contrast) as a result of breaking during the transfer. Multiple Raman-spectroscopy measurements were performed on this region and on the surroundings with low (0.6 mW) and high (6 mW) laser power, as described later. Note that the higher laser power caused the formation of the nanoparticles from the evaporated gold layer. We selected two regions to demonstrate the effect of the high laser power on the sample: the first is near a corner-like graphene edge ( eventually no difference can be found between the two regions initially separated by the borderline (Fig. 2e). Note that the large graphene wrinkle is ironed out during the process, and graphene mimics the corrugation of the underlying NPs. In the following we demonstrate the effect of laser irradiation by analysing the Raman peaks of both graphene on standard SiO2 substrate and on gold NPs. For this, we used correlation diagrams of the spectral positions of graphene G and 2D peaks, suggested by Lee et al. 37 to separate the mechanical strain from charge doping in graphene. It has to be noted, that in the original approach the initial strain configuration (uniaxial or biaxial) and the doping type (p-or n-type) of graphene have to be known in order to obtain a good separation. Nevertheless, in the recent work of N. S. Mueller et al. 3 , the authors introduce a new method where one can separate the effect of the hydrostatic and shear strain from doping without any initial assumption on the strain configuration. We plotted the slopes of the pure hydrostatic strain and the pure p-type doping with gradients of (Δω2D/ΔωG)ε hydrostatic =2.21 3 and (Δω2D/ΔωG)n hole =0.55 38 , respectively. We used the Grüneisen parameter of γG=1.8 given by Zabel et al. 39 , which is in good agreement with theoretical calculations 6 . We want to underline that the hydrostatic strain is twice as large as the corresponding biaxial strain. For hole doping, the dependence of the G peak position is taken as ΔPos(G)/ ΔEF ≈ 39 cm -1 /eV 5  area with 20×20 pixels. The excitation laser power was measured right before the experiments. Low (0.6 mW) and high (6 mW) power measurements were performed alternately. The higher laser power was used explicitly to anneal locally the samples, while with low powers we characterized the effect of the applied heating. It is important to note that the laser annealing always takes place in areas of only about 0.5 × 0.5 µm 2 (spots) at the same time, were the laser is focused. As the laser beam is scanned above the sample, previously irradiated spots quickly cool down back to room temperature.
In Fig. 3a we plotted the 2D-G peak positions, where we used different colours for the subsequent measurements on the same area. The average (G,2D) position from the first, low-power measurement (black dots) exhibit a small hole doping of -18±13 meV and a compressive hydrostatic strain of -0,06±0,01%, which are reasonable values for CVD graphene on SiO2 substrate. The second (red dots) and the forth (light blue dots) measurements were performed with high laser power, while the third (blue dots) and fifth (green dots) were again low-power measurements. The corresponding average strain and doping values are -0,07±0,01%; -140±34 meV and -0,07±0,01%; -238±19 meV for the third and the fifth measurement, respectively. A significant increase in the p-type doping is clearly seen from the correlation plot, the shift of the averages follow the pure p-doping slope of 0.55 very well. The average spectra from the first and fifth measurements are shown in Fig. 3b, where one can notice -apart from the peak shifts -the lowered intensity ratio of the G-2D peaks, which is also a fingerprint of the increased doping. The defect related D peak slightly increased, which shows that the high power laser irradiation not only heats the sample but also introduces defects. These defects, as well as the structural deformations induced by laser annealing, capture more easily airborne contaminants 16 , giving a plausible explanation for the observed overall increase in doping.  Supplementary Information). In Fig. 4 we plotted the 2D-G correlation plots (a, c) and the average spectra (b, d) for laser excitations of 633 and 488 nm, respectively. For clarity, we normalised the spectra to the Si peak (at 520 1/cm) for the spectra measured with 488 nm excitation, and to the graphene 2D peak for the spectra measured with 633 nm excitation. The first noticeable difference is the presence of the photoluminescent (PL) background of the metal nanoparticles. First we discuss this phenomenon, because it provides a standalone evidence on the temperature and the volume change, and will be helpful in the interpretation of graphene Raman spectra. When the excitation was performed with 2.54 eV photons (488 nm), we measured a complete fluorescence spectrum, while in the case of 1.95 eV photons (633 nm) we observed only the lower, decreasing part of the spectrum. This is due to the fact that the red laser only captures the low energy part of the induced interband transitions 41 . The temperature increase due to the laser irradiation is clearly seen both on the redshift and on the lowered intensity of the fluorescent background. The corresponding maximum of 1861 1/cm (536 nm) shifts to 2380 1/cm (552 nm) when the laser power is switched from low to high, as shown in Fig. 4d. Such redshifts and lowered intensity in PL and surface plasmon resonance frequencies is attributed to thermal dilatation, which also means a decrease of electron density 41,42,43 . On the average spectra one can notice the redshift of the graphene Raman peaks for high laser power.
More insights can be revealed from the analysis of the correlation plots. In the case of the 633 nm excitation (Fig. 4a) the average of the first low power measurement was at (ωG; ω2D)=(1582.2; 2631.9), which corresponds to a small tensile strain ε hidr 1 = 0.08% and p-doping of EF1 = -89 meV. The following high power measurements (no. 2 and 4) show a significant shift of both G and 2D peaks, with the averages (ωG; ω2D)=(1572.1; 2607.4) and (ωG; ω2D)=(1572.7; 2609) for measurement no. 2 and 4, respectively. Note that the effect is reproducible, the peak shifts are very similar in the two cases. Furthermore, the low power measurements are also very reproducible, the no. 3 and 5 have averages close to the first one. For the blue laser we found the same behaviour. On average, neither the strain nor the doping was changed significantly, as shown in Fig. 4c. We should note, however, that here the standard deviation of the G peak position was very large, probably due to the high PL background. In order to analyse the G peak shifts ∆ in Figs. 4a and 4c, one should consider the contributions from thermal expansion of the lattice (∆ ), anharmonic effect (∆ ), and strain induced by the thermal expansion coefficient (TEC) mismatch between the substrate and graphene (∆ ) 44,45 : where is the temperature of the sample induced by high power laser irradiation. The contribution from strain can be written as where β is the biaxial strain coefficient of the G band 9 , ( ) and ℎ ( ) are the temperature dependent TECs of the gold NPs and graphene, respectively. A prominent feature observed on the correlation plots (Figs. 4a and 4c) is the nearly parallel shift of the Raman-peaks with respect to the pure hydrostatic strain slope. We fitted the average values for each laser and found the slopes to be 2.37±0.02 and 1.72±0.07 for the 633 nm and for the 488 nm laser, respectively, which are close to 2.2 (the "strain" slope) 38 . This clearly shows that the main origin of the shift is due to hydrostatic strain from the underlying Au NPs, expressed by Eq.
(2). As we discussed above, the thermal expansion of the gold nanoparticles can be observed from the shift of the PL background.
Taking this into account together with the negative thermal expansion coefficient of graphene 44,46 it is straightforward that a tensile strain will emerge in graphene. This result is in agreement with recent findings showing the dominant role of thermal strain in the temperature dependence of graphene Raman peaks 45,47 . Nevertheless, the temperature dependence of phonon properties remains a challenging topic, as it is highly affected by the supporting materials and their TECs [ 45,47 ]. In Fig. 4, if we assume p-type doping during the whole low-high laser power cycle, the strain is dynamically increased from 0.08±0.01% to 0.48±0.06%, and from 0.07±0.02% to 0.46±0.11% during red and blue laser irradiation, respectively. Each spectral point from the measurements no. 2 and 4 (for both lasers) are aligned along the pure hydrostatic strain slope, which shows that the emerging strain varies in a very wide scale as a result of the relatively broad distribution of nanoparticle heights and diameters.
Another interesting aspect of the measurements is that no observable doping or damaging occurred in graphene, even after several power cycles, unlike on the SiO2 substrate (Fig. 3), which is rather surprising. The 2D peak width analysis confirms this finding, there is no significant change in the full width at half maximum (FWHM) distributions corresponding to the successive low power measurements (see Supplementary Information, Figs. S3 and S4). Most of the reports from the literature 5,11-16 which investigate the resistance of graphene to radiation defects used standard Si/SiO2 substrate and found similar behaviour in terms of increased damaging and doping. It was pointed out that the presence of water can seriously affect the photo-oxidation of graphene 15 . It is known that a thin water layer can be trapped between the silicon-dioxide and the transferred graphene, and in most of the cases a water film could form on the top of graphene as well 48,49,50 ,although there is a debate on the wetting properties of graphene 51,52,53,54 . We suppose that the graphene on Au is more hydrophobic than graphene on SiO2 51 , and suggest that the absence of water layer (graphene on Au NPs) helps the graphene to resist the high power laser irradiation. Further investigations on the role of the supporting material regarding the resistance of graphene to photoradiation would be useful.
Finally, the structure and local electronic properties of graphene on the continuous Au film were investigated by STM and STS. An STM image recorded on a fully graphene covered region of the 5 nm thin gold layer is shown in Fig. 5a. Interestingly, the corrugation of graphene is quite similar to the initial corrugation of graphene transferred onto Au NPs (Fig. 2b). This is related to the morphology of the as deposited 5 nm gold layer (Fig. S2, Supplementary Information), on which graphene is partially suspended. Extended graphene ripples can be observed, similar to the ripples observed on nonannealed graphene by AFM (Fig. 2b). STS measurements were performed in several arbitrary points to investigate the local doping of graphene. Fig. 5b shows four average spectra corresponding to four different spots on the sample. The spectra show slightly varying doping, with the Dirac point observed between 66 meV and 94 meV. This is in very good agreement with the result obtained from the Raman spectroscopy of Au NPs supported graphene (89 meV, black dashed line in Fig. 5b). Gold nanoparticles were produced locally by focused laser irradiation of thin Au films evaporated on SiO2/Si substrate. The properties of graphene transferred onto these NPs were studied by AFM, STM/STS, and confocal Raman spectroscopy. We demonstrated that dynamic strain could be induced in the Au NP supported graphene by high power (6 mW) laser irradiation, an effect which was completely reversible upon switching off the laser. We also showed that -while similar laser irradiation induced increased doping and damage in SiO2/Si supported graphene -no change in doping or defect concentration was observed on Au NP supported graphene, even after several irradiation cycles. Our findings can gain importance in local-heat assisted applications like plasmonic sensors, spasers, or photothermal therapy with nanoparticles or graphene/nanoparticle hybrids. Since the high temperature associated with high power laser irradiation increases the strain in graphene, which in turn enhances its sensing properties, these results can have implications in the development of graphene/plasmonic nanoparticle based high temperature sensors as well, operating either in steadystate or dynamic regimes.

Conflicts of interest
There are no conflicts of interest to declare.