Laser Nanostructuring for Diffraction Grating Based Surface Plasmon-Resonance Sensors

nanomaterials

Article Laser Nanostructuring for Diffraction Grating Based Surface Plasmon-Resonance Sensors

Iaroslav Gnilitskyi 1,2,*, Sergii V. Mamykin 3, Christina Lanara 4,5, Ihor Hevko 1, Mykhaylo Dusheyko 6, Stefano Bellucci 7 and Emmanuel Stratakis 4,5

1 NoviNano Lab LLC, 79015 Lviv, Ukraine; [email protected] 2 Department of Photonics, Lviv Polytechnic National University, 79021 Lviv, Ukraine 3 V. E. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 02000 Kyiv, Ukraine; [email protected] 4 Foundation for Research and Technology-Hellas (F.O.R.T.H.), Institute of Electronic Structure and Laser (I.E.S.L.), 70013 Heraklion, Greece; [email protected] (C.L.); [email protected] (E.S.) 5 Department of Materials Science and Technology, University of Crete, 70013 Heraklion, Greece 6 Kiev Polytechnic Institute, National Technical University of Ukraine, 03056 Kyiv, Ukraine; [email protected] 7 INFN-Laboratori Nazionali di Frascati, 00044 Frascati, Italy; [email protected] * Correspondence: [email protected]

Abstract: The surface plasmon resonance properties of highly regular laser-induced periodic surface structures (HR-LIPSSs) on Si, functionalized with Au nanoparticles (NPs), were investigated. In particular, the spectral dependencies of polarized light reﬂectance at various angles of incidence were measured and discussed. It is found that the deposition of Au NPs on such periodically textured

substrates leads to signiﬁcant enhancement of the plasmon resonance properties, compared to that measured on planar ones. This effect can be used to improve the efﬁciency of localized-plasmon-

Citation: Gnilitskyi, I.; Mamykin, resonance-based sensors. S.V.; Lanara, C.; Hevko, I.; Dusheyko, M.; Bellucci, S.; Stratakis, E. Laser Keywords: ultrafast laser ablation; silicon; laser-induced periodic surface structures; HR-LIPSS; laser Nanostructuring for Diffraction nanostructuring; femtosecond laser; surface plasmon resonance; plasmonic nanoparticles Grating Based Surface Plasmon- Resonance Sensors. Nanomaterials 2021, 11, 591. https://doi.org/ 10.3390/nano11030591 1. Introduction Many advanced optoelectronic devices contain periodically corrugated surface optical Academic Editor: Ion N. Mihailescu elements, i.e., phase diffraction gratings. Plasmon-active metals, which are placed on the respective gratings, like a silver or gold, enable the excitation of a few surface electromagnetic Received: 19 January 2021 modes. Such modes can be applied as a basis for sensing and photovoltaic applications. Accepted: 18 February 2021 Published: 26 February 2021 For example, depositing metal ﬁlms and loosely packed metal nanoparticles may enable excitation of surface plasmon polaritons (SPPs), whereas excitation of surface plasmon (SP)

Publisher’s Note: MDPI stays neutral may be noticed in experiment with individual metal NPs [1,2]. Based on the respective with regard to jurisdictional claims in SP [3] and SPP [4], sensors are already utilized to spot little changes in effective refractive published maps and institutional afﬁl- index of the environment on account of various chemical or biological reactions [5]. For iations. certain geometry, the simultaneous excitation of SP and SPP may appear [6]. Generally, the manufacturing of diffraction gratings via lithography processes takes a long time and is a complex and high-cost procedure. A promising alternative can be via application of femtosecond lasers, due to their unique non-thermal material ablation property [7], that enables avoiding heat-induced Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. damage [8]. Femtosecond pulses have also been exploited to induce grating-like periodic This article is an open access article surface structures in various materials, such as metals. In principle, the scattered light distributed under the terms and from surface defects interfering with the incoming laser beam causes a periodic inten- conditions of the Creative Commons sity pattern. Such an intensity pattern results in periodic surface structures known as Attribution (CC BY) license (https:// laser-induced periodic surface structures (LIPSSs) [9]. Laser-induced periodic structures creativecommons.org/licenses/by/ have been demonstrated on metals [10–12], semiconductors [13], dielectric surfaces [14], 4.0/). and polymers [15] and have been used in various applications [16], including hydrogen

Nanomaterials 2021, 11, 591. https://doi.org/10.3390/nano11030591 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x 2 of 11

Nanomaterials 2021and, 11, 591have been used in various applications [16], including hydrogen sensors [17], plasmon- 2 of 11 ics [18], colorizing metals [19], wettability [20], and tribology [21] applications. Most of the LIPPS fabrication methods reported to date lack the long-range ordered coverage required for large area applications, such as display technologies [22] and also sensors [17], plasmonics [18], colorizing metals [19], wettability [20], and tribology [21] the high degreeapplications. of periodicity demanded by photonics and photovoltaics applications [23]. This is due to theMost absence of the LIPPSof a feedbac fabricationk mechanism methods reported capable toof date guaranteeing lack the long-range the trans- ordered lational invariancecoverage of surface required structures. for large area Recently, applications, highly such regular as display LIPSSs technologies (HR-LIPSSs) [22] and also were developedthe via high a degree low-cost of periodicity single-step demanded maskless by photonicsprocess and and photovoltaicsindustrially applicationsaccepted [23]. speed productionThis [24]. is due It tois thebased absence on femtos of a feedbackecond laser mechanism superheating capable and of guaranteeing ablation mech- the transla- anism, enablingtional the invariancecoverage of surfacelarge areas structures. in many Recently, materials highly with regular highly LIPSSs uniform (HR-LIPSSs) peri- were odic structuresdeveloped [25]. In the via present a low-cost work, single-step we report maskless the creation process of andoptically industrially resonant accepted pho- speed production [24]. It is based on femtosecond laser superheating and ablation mechanism, tonic structures on Si substrate via the use of HR-LIPSS. The application of the fabricated enabling the coverage of large areas in many materials with highly uniform periodic HR-LIPSS in thestructures enhancement [25]. In of the the present surfac work,e plasmon we report resonance the creation of Au of NPs, optically which resonant are pho- chemically bondedtonic structuresto the laser on textured Si substrate Si gratings, via the use is of demonst HR-LIPSS.rated The and application discussed. of the fabricated HR-LIPSS in the enhancement of the surface plasmon resonance of Au NPs, which are 2. Materials andchemically Methods bonded to the laser textured Si gratings, is demonstrated and discussed.

2.1. Fabrication2. of MaterialsHR-LIPSS and on MethodsSi Substrates For this work,2.1. Fabrication single crystalline of HR-LIPSS undoped on Si Substrates Si <111> wafers with 4 Ohm·cm resistivity and 300 µm thicknessFor were this work, used. single crystalline undoped Si <111> wafers with 4 Ohm·cm resistivity A schematicand of 300 theµm different thickness steps were used.required for the fabrication of Au NPs-based Si gratings is presentedA schematicin Figure of1. For the differentthe fabrication steps required of HR-LIPSS for the, Yb:KGW fabrication chirped-pulse of Au NPs-based Si application lasergratings system is presented (model PHAROS in Figure1 .P- For20 thefrom fabrication Light Conversions of HR-LIPSS, Ltd.) Yb:KGW was chirped-pulse used. All experimentsapplication were carried laser out system by using (model la PHAROSser wavelength P-20 from 1030 Light nm Conversions and pulse duration Ltd.) was used. 266 fs. To accelerateAll experiments the processing were carried speed, out a galvanometric by using laser wavelength scanning head 1030 nm(ScanLab) and pulse was duration 266 fs. To accelerate the processing speed, a galvanometric scanning head (ScanLab) was applied. Alignment of linearly polarized laser light was controlled by a half-wave plate. applied. Alignment of linearly polarized laser light was controlled by a half-wave plate. The surface treatmentThe surface was treatment performed was in performed air at room in air temperature at room temperature by scanning by scanning laser beam laser beam over sample surface.over sample The laser surface. beam The was laser focused beam was by an focused F-theta by lens an F-theta with focal lens withlength focal 56 length mm that produced56 mm an that approximate produced an diameter approximate of the diameter irradiation of the spot irradiation on the sample spot on of the 10.4 sample of µm (1/e2 of peak10.4 intensity).µm (1/e2 Atofpeak these intensity). parameters, At these the peak parameters, fluence the on peak the ﬂuencesurface on was the equal surface was to ~0.76 J/cm2. equal to ~0.76 J/cm2.

Figure 1. SchematicFigure 1. ofSchematic the experimental of the experimental process followed process the fabrication followed of the Aufabrication nanoparticles of the (NPs)-based Au nanoparticles Si gratings. Flat Si substrate(NPs)-based (a); nanopatterned Si gratings. by Flat highly Si substrate regular laser-induced (a); nanopatterned periodic by surface highly structure regular (HR-LIPSS) laser-induced substrate peri- of Si during ultrashortodic surface pulsed structure laser processing(HR-LIPSS) (b substrate); functionalization of Si during procedure ultrashort of nanopatterned pulsed laser processing Si surfaces and(b); Aufunc- NPs immobilizationtionalization procedure procedure onto nanopatterned of nanopatterned Si surfaces Si (c surf). aces and Au NPs immobilization procedure onto nanopatterned Si surfaces (c).

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2.2. Deposition of Gold Nanoparticles onto the Nano-Structured Si Substrates During the experiment, we thermally oxidized at 1000 ◦C for 30 min, in air, the ﬂat and nanopatterned Si substrates. Such oxidization brings forth a conformal silicon oxide layer. After that, the substrates were activated by immersion in Piranha solution (i.e., H2SO4:H2O2 = 3:1 (v/v)), for 30 min, at room temperature (RT). Then we rinsed samples with nano-pure water and dried with nitrogen gas. Following the above stage, we dipped the substrates into MPTMS solution in dry toluene (1.85% (v/v)), for 3 h, at room temperature. Then, the substrates were gargled in toluene and ethanol (two times), and the procedure was completed by drying the substrates with nitrogen gas and thermal annealing at 100 ◦C for 30 min. It should be noted that, in order to bind the Au NPs onto the MPTMS-functionalized surfaces, we used the evaporation from solution. To reach that, a drop of 70 µL was placed onto the MPTMS-functionalized silicon surfaces, they were allowed to evaporate for 16 h, and then we rinsed the sample with nano-pure water.

2.3. Morphological Characterization All the surfaces were morphologically characterized by scanning electron microscopy (SEM), on a JEOL 7000 ﬁeld emission scanning electron nanoscope (FESEM; JEOL 7000) (JEOL (Europe) BV, Zaventem, Belgium), or a FEI Nova NanoSEM 450, Billerica, MA, USA). equipped with energy-dispersive X-ray spectroscopy (EDS), model Bruker QUANTAX-2 (Billerica, MA, USA).). In order to calculate the statistical parameters for the nanopatterned and Au NPs-functionalized substrates, as well as the Au NPs density (Table1), an image- processing algorithm (ImageJ, National Institutes of Health, Bethesda, MD, USA) was implemented.

Table 1. Au NPs’ density distribution on both ﬂat and nanostructured Si substrates (per um2).

Au NPs Density (per µm2) Au NPs Shape Flat Si Substrates Nanostructured Si Substrates Spheres 13 nm diameter 522 100 Spheres 80 nm diameter 14 25 Rods 3:1 aspect ratio 60 55

2.4. Optical Characterizations Once the respective experiment was finalized, we researched the optical properties of the samples by applying measurements of spectral and angular dependence of P- and S-polarized light reflection in the 0.4–1.1 µm wavelength range and 10–70◦ angles of incidence. The halogen tungsten lamp, the mechanical light chopper, a monochromator with Glan prism at the exit, and a rotary table for samples were used for the measurements of the samples [2]. The light reflection intensity was measured by a silicon photodetector, the signal of which was applied to the input of analog-to-digital converter. The dimension possibilities of such a setup permitted us to construct the dispersion curves of excited optical modes and to determine their types.

3. Results and Discussiondone 3.1. Nanopatterned Silicon Substrates Decorated with Au NPs Typical SEM images of the laser-patterned Si surfaces, presented in Figure2, show the formation of highly regular and homogeneous linear LIPSS with practically no bifurcations over the entire treated area (~10 µm wide). Nanoparticles observed onto the LIPSS indicate possible back-deposition of the ablation products. Nanomaterials 2021, 11, x 4 of 11 Nanomaterials 2021, 11, 591 4 of 11

FigureFigure 2. SEM2. SEM image image of flat (left)of flat and (left) HR-LIPSS and generated HR-LIPSS on Si generated surface. on Si surface. Following the Au NPs deposition protocol (Figure1), the Au NPs were successfully attachedFollowing to the surfaces, the asAu confirmed NPs deposition by SEM imaging. protocol Using (Figure the protocol 1), the of dropletAu NPs were successfully evaporationattached to after the surface surfaces, functionalization, as confirmed a plethora by ofSE AuM NPsimaging. with various Using shapes the protocol of droplet and sizes, including spheres and rods, were successfully deposited (Figure3) . Thus, sphere-shapedevaporation Au after NPs surface with 13 nm functionalization, diameter (Figure3a,b) a ,plethora sphere-shaped of Au Au NPs NPs withwith various shapes and 80sizes, nm diameter including(Figure spheres3c,d) , and and rod-shaped rods, were Au NPs successfully with 53 nm length,deposited aspect (Figure ratio 3). Thus, sphere- 3:1shaped(Figure 3Aue,f) ,NPs were with chemically 13 nm attached diameter to both (Figur flat ande nanopatterned 3a,b), sphere-shaped Si substrates. Au NPs with 80 nm Remarkably, the Au NPs could conformably disperse over the entire surface area of the nanostructureddiameter (Figure substrates, 3c,d), covering and bothrod-shaped the “ripples” Au (Figure NPs3 b,d,f)with and 53 thenm surrounding length, aspect ratio 3:1 (Figure flat3e,f), area were (Figure chemically3a,c,e). Au NPs attached were in the to form both of singleflat and particles nanopatterned but also in the form Si substrates. of Remarkably, smallthe Au clusters NPs (Figure could3c). conformably Finally, to improve disperse the sphere-shaped over the Au entire NPs’ (13surface nm diameter) area of the nanostructured dispersion onto the functionalized surfaces, Au NPs solution was diluted in ethanol before thesubstrates, drop evaporation covering step. both the “ripples” (Figure 3b,d,f) and the surrounding flat area (Fig- ureElemental 3a,c,e). Au analysis NPs with were energy-dispersive in the form X-ray of single spectroscopy particles (EDS) but of the also nanopat- in the form of small clus- ternedters (Figure Si substrates 3c). showed Finally, that, to apart improve from the Sithe peak sp (Figurehere-shaped4b), carbon Au (C) andNPs’ Oxygen (13 nm diameter) disper- (O) elements are observed in respect to untreated Si, where we observed the only peak of Sision (Figure onto4a). the functionalized surfaces, Au NPs solution was diluted in ethanol before the drop evaporation step.

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Figure 3. SEM images of spherical Au NPs with 13 nm si sizeze diameter, covalently bound onto flatflat (a) and nanopatterned Si substrates (b) (scale bars: 100 nm) diluted in 40% ethanol; spherica sphericall Au NPs with 80 nm diamet diameter,er, covalently bound onto flatflat (c) andand nanopatternednanopatterned (d) Si substratessubstrates (scale bars: 1 µµm)m) deposited viavia drop evaporation protocol diluted in 40% of ethanol; rod-shapedrod-shaped withwith 5353 nmnm lengthlength Au Au NPs NPs and and aspect aspect ratio ratio 3:1, 3:1, covalently covalently bound bound onto onto flat flat (e )(e and) and nanopatterned nanopatterned (f) (f) Si substratesSi substrates (scale (scale bar: bar: 100 100 nm) nm) deposited deposited via via drop drop evaporation evaporation protocol protocol diluted diluted in 40%in 40% of ethanol.of ethanol.

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FigureFigure 4. EDS 4. EDS spectrums spectrums of Si ofsubstrates: Si substrates: (a) untreated (a) untreated and (b and) treated (b) treated by HR-LIPSS by HR-LIPSS Si substrates. Si substrates.

3.2.Elemental Spectroscopy analysis Results with ofenergy-dispersive the Hierarchical Nano-Patterned X-ray spectroscopy Si Substrates (EDS) of the nanopatterned Si substrates showed that, apart from the Si peak (Figure 4b), carbon (C) and Oxy- The thickness of the SiO2 layer on the oxidized Si substrate was determined from genreflectance (O) elements measurements are observed in to respect be equal to un totreated 80, 93, andSi, where 94 nm we for observed the samples the only with peak deposited of SiAu (Figure NPs 134a). nm, Au nanorods 53 nm, and Au NPs 80 nm respectively. To account for the optical properties of the different samples prepared, the spectra of light reflection R ,R at 3.2. Spectroscopy Results of the Hierarchical Nano-Patterned Si Substrates p s different incidence angles for samples with vertically oriented diffraction gratings (grating ripplesThe thickness perpendicular of the SiO to2 light layer incidence on the oxidized plane), Si and substrate for flat was oxidized determined Si were from measured reflectance measurements to be equal to 80, 93, and 94 nm for the samples with deposited for p- and s-polarizations. The corresponding maps of the Rp/Rs ratio are presented on Au FiguresNPs 13 5nm,–7. Au On nanorods these maps, 53 nm, the and dispersion Au NPs 80 curves nm respectively. for SPP waves To account for two for boundaries, the opticalair/gold properties (±1, 2,of andthe 3)different and gold/silica samples prepared, (±1s, 2s, andthe 3s),spectra are shownof light byreflection lines for R thep, R differents at different incidence angles for samples with vertically oriented diffraction gratings (grat- diffraction orders 1, 2, and 3 (shown on Figure5. The spectra of the R p/Rs ratio enable us ing ripples perpendicular to light incidence plane), and for flat oxidized Si were measured emphasize the effect of excitation of plasmonic modes in case they exist. Positions of SPP for p- and s-polarizations. The corresponding maps of the Rp/Rs ratio are presented on wave in coordinates of angle of light incidence can be calculated from the phase-matching Figures 5–7. On these maps, the dispersion curves for SPP waves for two boundaries, condition [26,27]: air/gold (±1, 2, and 3) and gold/silica (±1s, 2s, and 3s), are shown by lines for the different k sinθ + mG = kPP (1) diffraction orders 1, 2, and 3 (shown on figure 5. The spectra of the Rp/Rs ratio enable us emphasizewhere kthe= effect 2π/(λ of/n excitati)—waveon vectorof plasmonic of the modes incident in case radiation they exist. with Positions a wavelength of SPP λ in a wavevacuum; in coordinatesθ—angle of ofangle incidence; of light minciden—an integerce can be (m calculated10) and denotes from the the phase-matching diffraction order; G = condition2π/a—reciprocal [26,27]: vector of grating with a period of a, ε = n2—permittivity and refractive index of the environment; kPP—wave vector of SPP. The SPP wave vector is assumed to be k sinθ + mG = kPP (1) the same as for a smooth metal film [2,26]: where k = 2π/(λ/n)—wave vector of the incident radiation with a wavelength λ in a vac- θ 1 1/2 uum; — angle of incidence; kmPP—an= ± integer(2π/(λ (/mn))0) [andεMe εdenotes/(εMe + theε)] diffraction order; G = (2) 2π/a - reciprocal vector of grating with a period of a, ε = n2- permittivity and refractive 0 2 indexwhere of thek PPenvironment;have “+” sign kPP—wave at m > vector 0, and of “-” SPP. at Them < SPP 0. Here waveε vectorMe = ε isMe assumed+ iε”Me to= be (n + ik) — the complexsame as for permittivity a smooth metal of the film metal [2,26]: at the wavelength λ. For calculations, the optical constants of gold from Reference [27] were used, and the refractive index of the substrate kPP = ± (2π/(λ/n)) [εMeε/(εMe + ε)]1/2 (2) was taken equal to ns = 1.48, i.e., that of SiO2. For a given gratings period and optical whereconstants, kPP have the “+” excitation sign at m >0, of modesand “-” with at m m<0.= Here +1 (1, εMe 1s) = andε’Me +m iε=’’Me− 1= (−n +1, ik−)21s) - complex are possible at permittivitythe interface of the air/gold metal at (1, the− 1)wavelength and gold/substrate λ. For calculations, (1s, −1s). the optical constants of gold from Reference [27] were used, and the refractive index of the substrate was taken equal to ns = 1.48, i.e., that of SiO2. For a given gratings period and optical constants, the excitation of modes with m = +1 (1, 1s) and m = −1 (−1, −1s) are possible at the interface air/gold (1, −1) and gold/substrate (1 s, −1s). Light reflectance for flat oxidized Si substrate (Figure 5) measured as reference to show the effect of surface texturization complemented with Au NPs. As it can be seen, no special features are observed for bare Si, but for oxidized Si (flat and with diffraction grating), there is a strong maximum at 0.5 um and large angles 40–70°. This maximum is due to interference in thick SiO2 layer on Si substrate. For the sample with LIPSS diffraction

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grating (DG), this maximum is a little bit shifted to the blue range, due to a slightly higher SiO2 thickness of 100 nm (Figure 5c).

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Nanomaterials 2021, 11, 591 7 of 11 grating (DG), this maximum is a little bit shifted to the blue range, due to a slightly higher SiO2 thickness of 100 nm (Figure 5c).

(a) (b) (c)

Figure 5. Map of Rp/Rs ratio of light reflection for bare Si with native oxide (a), flat oxidized Si substrate (b), and oxidized Si with vertically oriented LIPSS diffraction gratings (c).

As shown in Figure 6, the deposition of 13 nm Au NPs does not influence the Rp/Rs ratio for flat oxidized Si; however, for samples with DG, some additional features ap- (a) peared. Firstly, the maximum(b around) 0.5 µm at 40–70° becomes more(c) intense. Secondly, there is an enhancement of the Rp/Rs ratio along the dispersion curve corresponding to FigureFigure 5.5. MapMap ofof RRpp/R/Rss ratioratio mainof of light light +1 reflection reﬂection order of for forthe bare SPP Si wave with nativethat appeared oxide (a), ﬂatonflat the oxidizedoxidized boundary SiSi substratesubstrate with air((bb),), (see andand oxidized+1oxidized dispersion SiSi withwith verticallyvertically orientedoriented LIPSSLIPSSblack diffractiondiffraction solid on gratings Figuregratings 6b). ( c(c).).

As shown in Figure 6, the deposition of 13 nm Au NPs does not influence the Rp/Rs ratio for flat oxidized Si; however, for samples with DG, some additional features appeared. Firstly, the maximum around 0.5 µm at 40–70° becomes more intense. Secondly, there is an enhancement of the Rp/Rs ratio along the dispersion curve corresponding to main +1 order of the SPP wave that appeared on the boundary with air (see +1 dispersion black solid on Figure 6b).

(a) (b)

FigureFigure 6. 6.Comparison Comparison between between samples samples with with 13 nm13 Aunm NPsAu NPs on oxidized on oxidized ﬂat Si flat (a) andSi (a on) and oxidized on oxidized Si with verticallySi with vertically aligned Nanomaterials 2021, 11, x 8 of 11 diffractionaligned diffraction grating (DG) grating (b). Dispersion (DG) (b). curvesDispersion (b) for curves surface (b plasmon-polariton) for surface plasmon-polarito waves for twon boundaries,waves for two air/gold boundaries, (±1, 2, andair/gold 3) and (±1, gold/silica 2, and 3) (and±1s, gold/silica 2s, and 3s), (±1s, are 2s, shown and for3s), differentare shown diffraction for different orders diffraction 1, 2, and orde 3 (shownrs 1, 2, on and the 3ﬁgures). (shown on the figures). R /R for Au 53nm nanorods, vertical DG p s Figure 7 presents the maps70 of the Rp/Rs ratio for oxidized Si samples8.500 with deposited (a) -3 (b) Au nanorods. The effect of nanorods is+3s almost not observed for flat Si samples (Figure 7a); 60 Figure 6. Comparison betweenthere samples is only with an R13p /Rnms maximum Au NPs on present,oxidized whicflat Sih (isa) relatedand on oxidizedto the interference Si with vertically with SiO2 aligned diffraction grating (DG) (b). Dispersion curves (b) for surface plasmon-polariton waves for6.375 two boundaries, layer, which is also observed for50 flat oxidized Si-3s without Au NPs. Apart from this maxi- air/gold (±1, 2, and 3) and gold/silicamum, the (±1s, samples 2s, and with 3s), are DG sh alownso show for different a maximum diffraction around orde+1s rs0.8 1, µm,2, and 0–30° 3 (shown for vertical on the DG. figures). 40 Compared to samples with 13 nm Au+2 NPs,+2s the nanorods sample shows4.250 more pronounced -2

resonance at 0.53 µm. Angle, degs Figure 7 presents the maps 30of the Rp/Rs ratio for oxidized Si samples with deposited 2.125 Au nanorods. The effect of nanorods is almost+1 not observed-2s for flat Si samples (Figure 7a); 20 there is only an Rp/Rs maximum present,+3 which is related to the interference with SiO2

layer, which is also observed for10 flat oxidized Si without Au NPs. Apart0.000 from this maximum, the samples with DG also show0.40.60.81.0 a maximum around 0.8 µm, 0–30° for vertical DG. Wavelength, μm Compared to samples with 13 nm Au NPs, the nanorods sample shows more pronounced resonance(a) at 0.53 µm. (b) Figure 7. Comparison between samples with 53 nm Au nanorods on oxidized flat ﬂat Si ( a) and on oxidized Si with vertically aligned DG (b).). Dispersion curvescurves ((bb)) forfor surface-plasmon-polaritonsurface-plasmon-polariton waves waves for for two two boundaries, boundaries, air/gold air/gold ((±1,±1, 2, and 3) and gold/silica (±1s, (±1s, 2s, 2s, and and 3s), 3s), are are shown shown for for different different di diffractionffraction orders orders 1, 1, 2, 2, and and 3 3 (shown (shown on on the the figures). ﬁgures).

Figure 8 shows the Rp/Rs ratio for oxidized Si samples with deposited 80 nm Au NPs. It can be observed that the flat oxidized Si with Au NPs do not exhibit any difference compared with sample without NPs; in particular, the same maximum at 0.5 µm, 50–70° is observed in both cases. For the vertically aligned DG two maxima are observed at 0.5 µm, 30–70° and 0.75 µm, 10–30°. This sample is found to be the most suitable for SPP wave excitation because the presence of larger Au NPs increases the range of near field interaction. The dispersion curve corresponding to main +1 order of SPP wave also fits well to Rp/Rs maximum observed.

R /R , 80 nm Au NPs on SiO /Si, vertical DG p s 2 70 4.500 -3 +3s 60 3.375 50 -3s +1s

40 +2 +2s 2.250 -2 Angle, degs Angle, 30 1.125 +1 -2s 20 +3

10 0.000 0.4 0.6 0.8 1.0 Wavelength, μm (a) (b) Figure 8. Comparison between samples with 80 nm Au NPs on oxidized flat Si (a) and on oxidized Si with vertically aligned DG. Dispersion curves (b) for surface-plasmon-polariton waves for two boundaries, air/gold (±1, 2, and 3) and gold/silica (±1s, 2s, and 3s), are shown for different diffraction orders 1, 2, and 3 (shown on the figures).

In conclusion, the LIPSS structures comprising the DG behave similarly to flat oxidized Si. As a consequence, the DG itself introduces a small effect into the respective optical spectra. On the contrary, the addition of plasmon-active metal NPs dramatically changes the nature of the reflection spectra. More important, this is not evident in the case of NPs deposited onto planar oxidized silicon. Figure 9 shows the reflectivity spectra Rp/Rs for structures with flat and diffraction grating surface with and without Au NPs measured at a 45° angle of light incidence to emphasize the joint effect of Au NPs deposited on dif-

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Light reflectance for flat oxidized Si substrate (Figure5) measured as reference to show the effect of surface texturization complemented with Au NPs. As it can be seen, R /R for Au 53nm nanorods, vertical DG no special features are observed70 forp bares Si, but for oxidized Si (flat and8.500 with diffraction -3 ◦ grating), there is a strong maximum at+3s 0.5 um and large angles 40–70 . This maximum is 60 due to interference in thick SiO2 layer on Si substrate. For the sample with LIPSS diffraction grating (DG), this maximum is a little bit shifted to the blue range, due6.375 to a slightly higher 50 -3s SiO2 thickness of 100 nm (Figure5c). +1s 40 As shown in Figure6, the deposition+2 of+2s 13 nm Au NPs does not influence4.250 the R p/Rs ratio for flat oxidized Si; however, for samples with-2 DG, some additional features appeared. Angle, degs Firstly, the maximum around 0.530µm at 40–70◦ becomes more intense. Secondly, there is 2.125 an enhancement of the Rp/Rs ratio along the+1 dispersion-2s curve corresponding to main +1 20 order of the SPP wave that appeared+3 on the boundary with air (see +1 dispersion black

solid on Figure6b). 10 0.000 Figure7 presents the maps of the0.40.60.81.0 R /R ratio for oxidized Si samples with deposited p Wavelength,s μm Au nanorods. The effect of nanorods is almost not observed for flat Si samples (Figure7a) ; (a) (b) there is only an Rp/Rs maximum present, which is related to the interference with SiO2 Figure 7. Comparison betweenlayer, samples which with is also53 nm observed Au nanorods for flat on oxidized flat Si Si without (a) and Auon oxidized NPs. Apart Si with from vertically this maxi- ◦ aligned DG (b). Dispersion curvesmum, ( theb) for samples surface-plasmon-polariton with DG also show waves a maximum for two boundaries, around 0.8 air/goldµm, 0–30 (±1, 2,for and vertical 3) and DG. gold/silica (±1s, 2s, and 3s), Comparedare shown for to different samples di withffraction 13 nm orders Au 1, NPs, 2, and the 3 nanorods(shown onsample the figures). shows more pronounced resonance at 0.53 µm. Figure 88 shows the R Rp/Rp/Rs ratios ratio for foroxidized oxidized Si samples Si samples with with deposited deposited 80 nm 80 Au nm NPs. Au ItNPs. can It be can observed be observed that thatthe theflat flatoxidized oxidized Si with Si with Au Au NPs NPs do do not not exhibit exhibit any any difference compared with sample without NPs; in part particular,icular, the same maximum at 0.5 µµm,m, 50–7050–70°◦ is observed observed in in both both cases. cases. For For the the vertically vertically aligned aligned DG DG two two maxima maxima are are observed observed at 0.5 at µm,0.5 µ 30–70°m, 30–70 and◦ and 0.75 0.75µm, µ10–30°.m, 10–30 This◦. Thissample sample is found is found to be tothe be most the mostsuitable suitable for SPP for wave SPP waveexcitation excitation because because the presence the presence of larger of Au larger NPs Au increases NPs increases the range the of range near field of near interac- field tion.interaction. The dispersion The dispersion curve corresponding curve corresponding to main to +1 main order +1 of order SPP wave of SPP also wave fits also well fits to wellRp/Rs to maximum Rp/Rs maximum observed. observed.

R /R , 80 nm Au NPs on SiO /Si, vertical DG p s 2 70 4.500 -3 +3s 60 3.375 50 -3s +1s

40 +2 +2s 2.250 -2 Angle, degs Angle, 30 1.125 +1 -2s 20 +3

10 0.000 0.4 0.6 0.8 1.0 Wavelength, μm (a) (b)

Figure 8.8. ComparisonComparison between between samples samples with with 80 80 nm nm Au Au NPs NPs on oxidizedon oxidized ﬂat Si flat (a) Si and (a on) and oxidized on oxidized Si with Si vertically with vertically aligned DG.aligned Dispersion DG. Dispersion curves (b curves) for surface-plasmon-polariton (b) for surface-plasmon-polarito waves forn twowaves boundaries, for two boundaries, air/gold (± ai1,r/gold 2, and (±1, 3) and 2, and gold/silica 3) and (gold/silica±1s, 2s, and (±1s, 3s), 2s, are and shown 3s), are for shown different for diffraction different di ordersffraction 1, 2, orders and 3 (shown1, 2, and on 3 (shown the ﬁgures). on the figures).

In conclusion,conclusion, thethe LIPSS LIPSS structures structures comprising comprising the the DG DG behave behave similarly similarly to flat to oxidizedflat oxi- dizedSi. As Si. a consequence,As a consequence, the DG the itselfDG itself introduces introduces a small a small effect effect into into the respectivethe respective optical op- ticalspectra. spectra. On the On contrary, the contrary, the addition the addition of plasmon-active of plasmon-active metal NPs metal dramatically NPs dramatically changes changesthe nature the of nature the reflection of the reflection spectra. spectra. More important, More important, this is not this evident is not evident in the case in the of NPscase ofdeposited NPs deposited onto planar onto planar oxidized oxidized silicon. silicon. Figure Figure9 shows 9 shows the reflectivity the reflectivity spectra spectra R p/R Rspfor/Rs forstructures structures with with flat flat and and diffraction diffraction grating grating surface surface with with and and without without Au Au NPsNPs measuredmeasured at a 45° angle of light incidence to emphasize the joint effect of Au NPs deposited on dif-

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Nanomaterials 2021, 11, 591 9 of 11

fraction grating. As can be seen from the figure, the reflectivity spectrum of DG with deposited atAu a NPs 45◦ anglehas narrower of light and incidence more intense to emphasize resonance the around joint effect 0.5 µ ofm. Au Sensors NPs depositedbased on on surfacediffraction plasmon grating. resonance As use can the be dependence seen from the of figure,the spectral the reflectivity position of spectrumthe resonance of DG with on the parametersdeposited of Au the NPs environment. has narrower Therefore, and more it isintense obvious resonance that their ability around to 0.5 measureµm. Sensors resonancebased shift on will surface depend plasmon on the acuity resonance and intensity use the dependence of that resonance. of the Thus, spectral we positionbelieve of the that sensorsresonance based onon thegold parameters nanoparticles of the on environment. a diffraction grating Therefore, will it have is obvious a higher that sensi- their ability tivity. Toto measureoptimize resonance geometrical shift parameters will depend of on the the considered acuity and intensitystructures, of thata theoretical resonance. Thus, model forwe Au believe NPs deposited that sensors on basedan oxidized on gold Si nanoparticlessubstrate with on diffraction a diffraction grating grating will be will have developeda higher in our sensitivity. future studies. To optimize It can therefore geometrical be concluded parameters that of this the methodology considered structures, can a be employedtheoretical to improve model forthe Auefficiency NPs deposited of localized-plasmon-based on an oxidized Si substrate sensors. with diffraction grating will be developed in our future studies. It can therefore be concluded that this methodology can be employed to improve the efficiency of localized-plasmon-based sensors.

4 Au NPs on oxidized Si with DG Au NPs on oxidized Si Oxidized Si DG 3 Oxidized Si

Rp/Rs 2

0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Wavelength, μm

Figure 9. Comparison between samples with and without 80 nm Au NPs on oxidized flat Si and on Figure 9.oxidized Comparison Si with between vertically samples aligned with DG. and without 80 nm Au NPs on oxidized flat Si and on oxidized Si with vertically aligned DG. 4. Conclusions 4. ConclusionsHighly regular LIPSS structures can be obtained on Si, which is suitable for the design Highlyof diffraction-grating-based regular LIPSS structures SPRcan be devices. obtained It ison shown Si, which that is suchsuitable LIPSS for the structures design can be of diffraction-grating-basedeasily functionalized withSPR devices. Au NPs ofIt differentis shown sizes that givingsuch LIPSS rise to structures significant can enhancement be easily functionalizedof the plasmon with resonance Au NPs of features, different compared sizes giving to rise planar to significant Si. This effectenhancement can be used to of the plasmonimprove resonance the efficiency features, of localized-plasmon-resonance-based compared to planar Si. This effect sensors. can be used to improve the efficiency of localized-plasmon-resonance-based sensors. Author Contributions: Conceptualization, I.G. and S.V.M.; methodology, I.G., S.V.M., C.L. and M.D.; formal analysis, S.V.M. and I.H.; investigation, I.G., S.V.M. and C.L.; resources, E.S.; data curation, Author Contributions:C.L.; writing—original Author draftContributions: preparation, Conceptualization, I.G., S.V.M. and C.L.;I.G. and writing—review S.V.M.; methodology, and editing, I.G., I.G., S.V.M.,S.V.M., C.L., C.L., and S.B. M.D.; and formal E.S.; visualization, analysis, S.V.M., S.V.M. and and I.H.; I.H.; investigation, supervision, I.G., S.B. andS.V.M. E.S. and All C.L.; authors have resources,read E.S.; and data agreed curation, to the C.L.; published writing—origin version ofal thedraft manuscript. preparation, I.G., S.V.M., and C.L.; writing—review and editing, I.G., S.V.M., C.L., S.B. and E.S.; visualization, S.V.M. and I.H.; supervision, S.B. and Funding:E.S. All authorsThis project have read has received and agreed funding to the from publishe the EU-H2020d version research of the manuscript. and innovation program under grant agreement No. 654360, having benefitted from the access provided by FORTH (Foundation Funding:for This Research project and has Technology–Hellas) received funding from in Heraklion the EU-H2020 (Crete) research within the and framework innovation of theprogram NFFA-Europe under grantTransnational agreement Access No. 654360, Activity. having This benefitte researchd was from funded the access by the prov Ministryided by of FORTH Education (Foun- and Science dation forof UkraineResearch with and theTechnology–Hellas) project “Nanostructured in Heraklion interfaces (Crete) based within on non-toxic the framework materials of forthe practical NFFA-Europeapplications” Transnational (No. 0120U100675). Access Activity. This research was funded by the Ministry of Educa- tion and Science of Ukraine with the project “Nanostructured interfaces based on non-toxic materials for practicalData Availability applications” Statement: (No. 0120U100675).Not applicable. Acknowledgments: The authors would like to acknowledge Antonios Kanaras and Maria-Eleni Kyriazi from Physics and Astronomy, Faculty of Physical Sciences and Engineering, University of

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Southampton, SO17 1BJ, (United Kingdom), for Au NPs synthesis. They also would like to acknowledge Alexanda Manousaki from University of Crete, Physics Department, Heraklion, Crete, (Greece), for SEM–EDS characterizations. SM acknowledges the support from the grant No. 2020.01/0348, “Smoke sensor based on plasmon-polariton photodetector”, from the National Research Foundation of Ukraine. IG acknowledges the support from the NFFA grant. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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