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Article Plasmonic Photomobile Films

Riccardo Castagna 1,*, Massimo Rippa 1, Francesco Simoni 1,2,*, Fulvia Villani 3, Giuseppe Nenna 3 and Lucia Petti 1 1 Institute of Applied Sciences and Intelligent Systems, CNR, 80078 Pozzuoli (Napoli), Italy; [email protected] (M.R.); [email protected] (L.P.) 2 Dipartimento SIMAU, Università Politecnica delle Marche, 60131 Ancona, Italy 3 and Devices , ENEA, 80055 Portici (Napoli), Italy; [email protected] (F.V.); [email protected] (G.N.) * Correspondence: [email protected] (R.C.); [email protected] (F.S.)

 Received: 10 June 2020; Accepted: 17 July 2020; Published: 1 August 2020 

Abstract: In this work, we introduce the approaches currently followed to realize photomobile polymer films and remark on the main features of the system based on a biphasic structure recently proposed. We describe a method of making a plasmonic nanostructure on the surface of photomobile films. The characterization of the photomobile film is performed by means of Dark Field Microscopy (DFM), Scanning Electron Microscopy (SEM), and (AFM). Preliminary observations of the -induced effects on the Localized Surface Resonance are also reported.

Keywords: plasmonics; photomobile polymer films; localized surface plasmonic resonance; flexible substrates

1. Introduction The direct conversion of light into mechanical work [1,2] could play an important role in harvesting [3,4], allowing further development of the sector by giving different alternatives to the energy storage [5]; at the same time, the light-induced actuation of the would be able to add additional properties to previously fabricated nanostructures. In particular, it would be of great interest to have nanostructures with plasmonic properties that can be modulated by photoactuation. To this aim, different scientific problems linked to two different technologies have to be solved. One concerns the approach chosen to obtain a photomobile film, and the other concerns how to associate the nanostructure to it. It is well known that different have been proposed to get photo-induced actuation. In the recent decade, Marangoni’s effect [6] induced by light has been proposed as a basic mechanism to induce and control the motion of macro- or micro-objects floating on a surface [7–10]. Its working mechanism is based on the surface tension gradient induced by light close to the object that should be moved. Photochromic capillary effects are also widely reported and studied [11,12]. However, the most popular way to obtain the direct conversion of light into mechanical work is based on the use of a particular kind of liquid crystalline polymer with the inclusion of azobenzenes moieties. The first realization of such a compound has to be attributed to Angeloni et al. [13], while the realization of very efficient azobenzene-based photomobile polymer (PMP) films was reported for the first time by Ikeda’s group [2]. These allow the direct conversion of light energy into mechanical work: the PMP film bends under the illumination of light with the specific characteristics and bending efficiency dependent on the intensity, polarization, and wavelength of the incident light. In this case, the mechanism responsible for the light-induced bending of azo--based liquid PMP films (azo-LC-PMP) is the cis-trans photo-isomerizations leading to the film at a macroscopic level, while the configuration of the system allows the alignment of

Crystals 2020, 10, 660; doi:10.3390/cryst10080660 www.mdpi.com/journal/crystals Crystals 2020, 10, 660 2 of 12 the containing the azo-benzene groups during the thermally induced that brings formation to the film. Since photo-isomerization is strongly dependent on the wavelength and on the polarization state of the incident light, the same dependence is found in the bending process. The different properties obtained with different compounds in the last decades are substantially based on the different lengths of the chains of the forming the final polymer film. In short, two monomers are present in the system: one with the azobenzene moiety linked to two acrylate functions, while the other one has the azobenzene moiety linked to one acrylate . The consequence of the photo-isomerization of all the components of the film is the light-induced bending. UV-light is used to get cis-trans isomerization leading to bending, while visible light induces the opposite process, allowing a quick recovery of the initial condition. A further development brought to the realization of azo-LC-PMP is its ability to oscillate under continuous illumination at a frequency of up to about 26 Hz with a lifetime of about 2.5 h, as reported by White et al. in 2008 [14]. These materials are very interesting because of the large bending angle and pretty high oscillation frequency induced by light under different conditions of wavelengths and polarization states. In order to be active, these materials require a pre-exposure step using a specific light polarization necessary to align the molecules in the system. In general, the molecular structure of suitable for realizing PMPs should be mainly based on linear or ramified chains, but never reticulated ones. This is a typical feature of materials where the polymer chains are linked together through secondary bonds (van der Waals forces and hydrogen bridge bonds) that can be easily broken by mechanical, thermal, or photothermal (in this case) excitation [15]. Recently, a new approach has been proposed for the realization of PMP films based on a biphasic architecture [16]. This configuration can be considered as an emerging novel research direction for the fabrication of efficient PMPs. The actuation mechanism is based on the difference in expansion coefficients (under illumination) of the two polymeric layers, resulting in the bending of the PMP film. In this case, there is no need of an additional light beam to restore the initial position of the film: when the light is switched on, the film bends; when the light is switched off, the film goes back to its initial condition. Since this approach has been chosen to obtain a polymeric film with plasmonic properties, we will briefly review its properties in the next section. Concerning the realization of nanostructures with plasmonic properties, different patterning techniques have been developed; however, the most popular and efficient one is based on Electron Beam Lithography (EBL). It allows fabricating high-quality nanostructures using different geometries, realizing large area 2D crystals, which are often called “plasmonic crystals”, when plasmon resonance is achieved by the proper choice of materials (usually gold on silica ), the shape of the nanoelements, and the crystal lattice. Among the few examples where an ordered structure, i.e., a 2D crystal made by micropillars/ microwells, is modified by the action of light, we remind of the one recently reported by Pirani et al.[17,18]. In addition in this case, the photo-isomerization of azobenzene moieties is exploited. Nevertheless, this structure did not include plasmonic properties. On the other hand, the realization of PMP films that are able to host plasmonic crystals is a real challenge for Material Science. In the past two decades, advanced technologies based on plasmonics have been developed such as Localized Surface Plasmonic Resonance (LSPR) [19–21] and Surface Enhanced Raman Scattering (SERS) [22–26] allowing the detection of monomolecular layers and measurements of refractive index changes at the nanoscale. The implementation of these techniques is usually obtained by realizing metallic nano-elements (nanopillars or nanocavities) on rigid substrates [27,28]. The fabrication of these structures on flexible supports might provide new potential applications for these technologies (e.g., by adapting plasmonic devices for topical investigations) [29,30]. Some advances in this field are represented by plasmonics structures realized on flexible and extensible soft polymers used for molding and the transfer of previously realized nanopatterns [31–34]. Crystals 2020, 10, 660 3 of 12

However, the realization of a plasmonic nanostructure on a photomobile polymeric surface has not been reported yet, which is probably because those structures are realized by a lithographic process that requires solvents (developers and removers), critically affecting their implantation on PMP films. The aim of the present work is to give a demonstration that by using our unconventional approach for the realization of a photomobile polymer film, it is possible to transfer on it a gold nanostructure that keeps its plasmonic properties; then, it is possible to realize a plasmonic PMP film. In the next section, we briefly review the properties of the photomobile material before describing the focus and the achievements of this research in Section3.

2. A New Approach: Modulation of the Surface Tension at a Biphasic Interface We have recently proposed a novel, fast, and inexpensive approach to processing PMP films [16]. In this case, PMP is obtained by the photopolymerization of an organic mixture sandwiched between two slide . The mixture components have different polymerization rates; they are di-pentaerythritol-penta/hexa-acrylate (DPEPA), 1-vinyl-2-pyrrolidinone (NVP), and 4-aminophenol (4-AP) (Sigma-Aldrich, St. Louis, MO, USA). Bis (2,4,6-trimethylbenzoyl) phosphines (CIBA, Basel, Switzerland) is used as a photoinitiator. Details are reported in Castagna et al. [16]. In short, an 8:1 molar ratio of NVP to DPEPA and illumination by one side of the sample allows getting an asymmetric photopolymerization process; therefore, the fabricated film results in a flexible system that is composed by a harder DPEPA-co-NVP network and a softer doped-NVP part, in which a presence of is identified. The Scanning Electron Microscopy (SEM) (Raith 150, Dortmund, Germany) images show a brush-like morphology in the DPEPA-co-NVP side, when dissolving the film in N-methyl-pirrolidinone (Sigma-Aldrich, St. Louis, MO, USA); while a viscous component is on the other side of the film. The basic film structure can be seen as made by two different inter-digitized parts: one more rigid, but flexible, (the reticulated one) acting as a sort of supporting skeleton for the system, while the second acts as the effective light-activated motor of the system. The separation process, occurring in the bulk during photopolymerization, is responsible for the observed bristles structure. Actually, the resulting observed morphology is reminiscent of the one obtained by phase separation in holographic polymer dispersed liquid crystals (HPDLCs), where an alternating sequence of liquid crystal-rich and polymer-rich regions is obtained [35–37]. Here, soft doped-NVP plays the same role of inert LC in HPDLCs. Thus, in our case, as mentioned, the doped-NVP-richer part is the actual motor of the PMP film. Some preliminary observations [16] suggested that 4-AP-doped NVP could act as a driving motor for the light-induced motion of a composite film when included in the PMP mixture, as a result of the light-induced changes of the surface tension at the interface between the DPEPA skeleton and the NVP-rich side. The reduction of surface tension gradient increases the tendency of NVP to escape when illuminated by light. As a consequence, the thin highly reticulated film of DPEPA tends to bend. By using in place of NVP a different similar compound—for instance, N-methyl-pyrrolidinone (NMP), which has a structure similar to NVP, but does not allow the formation of oligomers/polymers—no movement is observed under illumination, underlining the need to include NVP in the system. Switching-off the incident beam results in restoring the original shape of the material and, as a consequence, of the whole PMP film. In this way, the viscoelastic properties of the 4-AP-doped NVP together with the re-adaptation of the harder polymeric network of DPEPA brings the film to the original position. The working mechanism of the described PMP film is depicted in Figure1, in which the initial position (Figure1a) is obtained when the light is switched-o ff, while the bending (Figure1b) under light illumination. Crystals 2020, 10, x FOR PEER REVIEW 4 of 12

Crystalspolymeric2020, stripe10, 660 is not dependent on the illumination side (Figure 1b), while the amount and rate4 of of 12 bending increase with the light intensity and depend on the used wavelength.

Figure 1. Sketch of the working mechanism of photomobile polymer (PMP) based on di-pentaerythritol-pentaFigure 1. Sketch of the/hexa-acrylate working (DPEPA)-co-NVPmechanism of photomobile (1-vinyl-2-pyrrolidinone) polymer (PMP) film: based (a) before on lightdi-pentaerythritol-penta/hexa-acrylate irradiation; (b) under light irradiation (DPE (fromPA)-co-NVP either side (1-vinyl-2-pyrrolidinone) of the film). film: (a) before light irradiation; (b) under light irradiation (from either side of the film). The demonstration of the photomobile properties of this film has been given in a simple experiment reportedThe byadvantages Castagna of et al.this [16 approach]. It is remarkable over the thatothe ther methods bending to direction realize PMPs of the polymericrelies on the stripe very is noteasy dependent and extremely on the cheap illumination way to obtain side (Figurethese films.1b), while As a matter the amount of fact, and the rate PMP of film bending is obtained increase in withone step: the light the intensityphotopolymerization and depend on synthesis, the used wavelength.using commercially available compounds. Large bendingThe advantagesangles are obtained, of this approach and the over film the is othereasily methods activated to realize(low energy PMPs doses relies onare the required very easy to andmove extremely the film). cheap The drawback way to obtain is th thesee slow films. relaxation As a matter time (the of fact, time the required PMP film to isrestore obtained the ininitial one step:position the of photopolymerization the film), being governed synthesis, by usingdiffusion commercially and capillary available forces. compounds. At the present Large state bending of the anglesart, this are prevents obtained, the andachievement the film is of easily high activatedoscillation (low frequencies. energy doses are required to move the film). The drawbackOne basic isproperty the slow of relaxation the used timemixture (the is time a stro requiredng shrinking to restore effect the un initialder photopolymerization position of the film), being[38–46] governed with the by potential diffusion to and act capillaryas a glue forces. [47–54]. At As the mentioned, present state the of thePMP art, film this is prevents made by the a achievementdouble-layered of highstructure oscillation realized frequencies. by an asymmetric photopolymerization process [16]. As a consequence,One basic propertythe PMP of film the usedcomponent mixture iswith a strong a high shrinkinger degree effect of under polymerization photopolymerization is the first [38 –one46] withinteracting the potential with tothe act asincoming a glue [47 polymerizing–54]. As mentioned, light the(as PMP confirmed film is made by byAttenuated a double-layered Total structureReflectance/Fourier realized by Transform an asymmetric Infra-Red photopolymerization (ATR/FT-IR) measurements process [16 ].[16]). As aThis consequence, is mainly made the PMP by filmthe polymerized, component with highly a higher cross-linked, degree of multi-acryla polymerizationte present is the first in the one mixture. interacting In withthese the conditions, incoming polymerizingwhen polymerized, light multi-acrylate (as confirmed is by able Attenuated to act as a Totalglue. The Reflectance polymer/Fourier shrinkage Transform is well assessed Infra-Red in (ATRacrylate/multi-acrylate/FT-IR) measurements systems [16 ]).[38–46]: This it is is mainly due to made the conversion by the polymerized, of sp2 highly to carbon cross-linked, double multi-acrylatebonds to sp3 presentcarbon–carbon in the mixture. bonds under In these polymeri conditions,zation. when In polymerized,the first case, multi-acrylate the intermolecular is able todistances act as a are glue. in the The range polymer of approximately shrinkage is well 4–5 assessedÅ, while inin acrylatethe second/multi-acrylate case, they are systems reduced [38 (C-C–46]: itbonds is due length to the is conversion approximately of sp2 1.4 carbon Å). This to carbon proper doublety makes bonds the to material sp3 carbon–carbon suitable for bonds realizing under a polymerization.plasmonic photomobile In the first film, case, as described the intermolecular in the next distances section. are in the range of approximately 4–5 Å, while in the second case, they are reduced (C-C bonds length is approximately 1.4 Å). This property makes3. Plasmonic the material Photomobile suitable forFilms realizing a plasmonic photomobile film, as described in the next section. The basic idea to achieve the goal of realizing a plasmonic nanostructure on a photomobile film 3. Plasmonic Photomobile Films is based on exploiting the “glue” property of our PMP film in order to pull up gold from nanopillars-basedThe basic idea toplasmonic achieve thenanost goalructures. of realizing We a plasmonicshow that nanostructure this process, on in a photomobileour experimental film is basedconditions, on exploiting gives rise the to “glue” a complementary property of our gold PMP nanocavities-based film in order to pull pattern up gold nestled from nanopillars-basedon the surface of plasmonicthe PMP film, nanostructures. as explained Wein the show following. that this process, in our experimental conditions, gives rise to a complementaryThe nanostructure gold nanocavities-based is previously fabricated pattern nestled on Indium on the surfaceTin of the (ITO)-glass PMP film, asslides explained by a instandard the following. Electron Beam Lithography (EBL) technique onto a polymeric electronic resist followed by gold Theevaporation nanostructure [25,26]: is previouslydue to the fabricatedlift-off process on Indium of the Tinresist, Oxide the (ITO)-glassnanopattern slides is formed by a standard by gold Electronnanopillars Beam on the Lithography substrate. The (EBL) mixture technique to prep ontoare aPMP polymeric is made electronic by approximately resist followed 65% of bymixture gold evaporationA (composed [ 25by,26 NVP]: due (90%) to theand lift-o oxidized-4-APff process of(10%)), the resist, 35% of the mixture nanopattern B (composed is formed by byDPEPA gold nanopillars(approximately on the 97%) substrate. and Irg819 The mixture(approximately to prepare 3%)) PMP blended is made together. by approximately The fabrication 65% ofprocess mixture of

Crystals 2020, 10, x FOR PEER REVIEW 5 of 12 the plasmonic PMP film is sketched in Figure 2. The PMP mixture is placed by capillarity in a sandwichCrystals 2020 formed, 10, 660 by two glass slides separated by 100 µm Mylar spacers. The internal surface of5 one of 12 glass slide is made by the previously fabricated nanostructure based on gold nanopillars through the EBLA (composed (steps (1), by (2) NVP in Figure (90%) 2) and asoxidized-4-AP similarly described (10%)), in 35% previo ofus mixture B (composed[25,26]. Two by types DPEPA of plasmonic(approximately quasi-crystals 97%) and Irg819were fabricated: (approximately (1) an 3%)) octagonal blended pattern together. with The gold fabrication nanopillars process with of the a diameterplasmonic d PMP= 750 filmnm and is sketched minimum in Figureinterparticles2. The PMPdistance mixture a = 180 is placednm; and by (2) capillarity a dodecagonal in a sandwich pattern withformed gold by nanopillars two glass slides with separated d = 500 nm by 100andµ am = Mylar 1000 spacers.nm. In both The internalstructures, surface the height of one glassof the slide gold is nanopillarsmade by the is previously approximately fabricated 60 nm. nanostructure based on gold nanopillars through the EBL (steps (1), (2) inThen, Figure step2) as 3) similarly in Figure described 2, the photopolymerization in previous papers [ is25 induced,26]. Two by types irradiation of plasmonic with a quasi-crystals UV-A lamp (Philipswere fabricated: TL-D 18W (1) BLB) an lasting octagonal 40 min. pattern During with the gold illumination, nanopillars the with distance a diameter betweend =the750 lamp nm andand theminimum sample interparticles was set at distance2 cm. aAt= 180the nm;end and of (2)the a dodecagonalprocess, the patternsandwich with is gold opened nanopillars from withthe non-structuredd = 500 nm and side a = and1000 left nm. in In aerobic both structures, conditions the for height a week. of theAt goldthis stage, nanopillars the PMP is approximately film is easily removable60 nm. from the glass surface by the use of a cutter. The strong shrinkage of multi-acrylate gives rise to the exfoliation of the gold nanostructures, producing finally the plasmonic PMP.

FigureFigure 2. 2. SketchSketch of of the the plasmonic plasmonic photomobile photomobile po polymerlymer (PMP) (PMP) fabrication fabrication procedure. procedure. ( (11)) Plasmonic Plasmonic crystalcrystal is is fabricated fabricated on on a a rigid rigid glass glass slide slide substr substrateate (plasmonic (plasmonic crystal crystal on on glass glass slide, slide, PCGS); PCGS); ( (22)) PMP PMP mixturemixture is placedplaced inin a a sandwich sandwich consisting consisting of PCGSof PCGS and and another another glass glass slide separatedslide separated by mylar by spacers;mylar spacers;(3) (3) ultraviolet irradiation irradiation on the PCGS on the side; PCGS (4) side; after ( irradiation,4) after irradiation, the PMP the film PMP is peeled film is o ffpeeledfrom off the fromsystem, the resultingsystem, resulting in the plasmonic in the plasmonic PMP structure. PMP structure.

WhenThen, the step PMP (3) in film Figure is peeled-off2, the photopolymerization from the sandwich is, inducedstep (4) in by Figure irradiation 2, a reproduction with a UV-A of lamp the nanopattern(Philips TL-D is 18Wobtained BLB) on lasting the PMP 40 min. surface During film. the Th illumination,is is firstly checked the distance by Dark between Field the Microscopy lamp and (DFM)the sample imaging was (see set at Figure 2 cm. At3, which the end contains of the process, the images the sandwichof the nanostructure is opened from on the the rigid non-structured substrate (a)side and and on left the in PMP aerobic film conditions (b). The same for a pattern week. Atcan this be stage,observed the even PMP if film the is details easily cannot removable be seen from due the toglass the surfacelimited by spatial the use resolution). of a cutter. Details The strong are clea shrinkager in Figures of multi-acrylate 4 and 5, reporting gives rise data to the obtained exfoliation by Scanningof the gold Electron nanostructures, Microscopy producing (SEM) finallyand Atomic the plasmonic Force Microscopy PMP. (AFM) (Digital Instruments, DimensionWhen 3100-Nanoscope the PMP film is peeled-oIV, Veeco,ff from New the York, sandwich, NY, USA). step The (4) inefficiency Figure2 ,of a reproductionthe transfer process of the ofnanopattern the nano-pattern is obtained is investigated on the PMP by surface LSPR; film. in fact, This isthe firstly LSPR checked peak of by the Dark nanopillars Field Microscopy on the ITO–glass(DFM) imaging substrate (see falls Figure in 3the, which red re containsgion, having the images a maxi ofmum the nanostructureextinction at λ on = the663 rigidnm, while substrate the LSPR(a) and peak on for the the PMP nanostructure film (b). The on same the NPV-based pattern can PMP be observed is located even at λ ifapproximately the details cannot 949 nm. be This seen remarkabledue to the limitedshift is spatial due to resolution). the difference Details between are clear the in refractive Figures4 andindices5, reporting of the nanostructures data obtained substrate,by Scanning being Electron the polymer Microscopy refractive (SEM) index and higher Atomic than Force the Microscopy ITO glass slide. (AFM) (Digital Instruments, Dimension 3100-Nanoscope IV, Veeco, New York, NY, USA). The efficiency of the transfer process of the nano-pattern is investigated by LSPR; in fact, the LSPR peak of the nanopillars on the ITO–glass substrate falls in the red region, having a maximum extinction at λ = 663 nm, while the LSPR peak for the nanostructure on the NPV-based PMP is located at λ approximately 949 nm. This remarkable CrystalsCrystals 2020 2020, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 6 6of of 12 12

AA comparative comparative analysis analysis between between the the nanostructur nanostructureses with with octagonal octagonal symme symmetrytry realized realized on on the the glassglass substrate substrate and and the the ones ones obtained obtained on on the the PMP PMP film film is is performed performed by by DFM DFM and and Scanning Scanning Electron Electron MicroscopyMicroscopy (SEM) (SEM) and and shown shown in in Figures Figures 3 3 and and 4. 4. ByBy looking looking at at Figure Figure 4, 4, the the pattern pattern realized realized on on the the solid substrate substrate (see (see Figure Figure 4a) 4a) is is exactly exactly reproducedreproduced on on PMP PMP (Figure (Figure 4b). 4b). In In order order to to investigat investigatee the the details details of of the the copy copy process, process, it it is is useful useful to to increaseincrease the the image image magnification, magnification, as as reported reported in in Figure Figure 4c. 4c. We We notice notice the the apparent apparent “donut” “donut” shape shape Crystalshighlightedhighlighted2020, 10 by ,by 660 the the darker darker part part in in the the center center of of ea eachch single single gold gold element element of of the the pattern. pattern. This This result 6result of 12 pointspoints out out a a refractive refractive index index difference difference between between th thee edge edge (white) (white) and and the the center center (dark) (dark) of of the the rings rings that corresponds to a different gold concentration, which is higher at the edges and lower in the thatshift corresponds is due to the dito ffaerence different between gold theconcentration, refractive indices whichof is thehigher nanostructures at the edges substrate, and lower being in the center. We can explain this observation taking into account that the roughness on the tip of the polymercenter. We refractive can explain index this higher obse thanrvation the ITOtaking glass into slide. account that the roughness on the tip of the pillarspillars does does not not allow allow a a uniform uniform gold gold exfoliation. exfoliation.

FigureFigure 3.3. 3. DarkDark Dark Field FieldField Microscopy MicroscopyMicroscopy (DFM) (DFM)(DFM) of plasmonic of of plasmonic plasmonic aperiodic aperiodic aperiodic crystals crystals crystals (octagonal (octagonal (octagonal symmetry) symmetry) symmetry) placed: (aplaced:) on a rigid (a) substrateon a rigid (glass substrate slide) and (glass (b) itsslide) complementary and (b) its nestledcomplementary on PMP film. nestled Magnification: on PMP 20film.. placed: (a) on a rigid substrate (glass slide) and (b) its complementary nestled on PMP film.× TheMagnification:Magnification: image is collected 20×. 20×. The The by image animage Olympus is is collected collected optical by by microscopean an Olympus Olympus equipped optical bymicroscope a Charge equipped Coupledequipped by Device by a aCharge Charge CCD µ CoupledcameraCoupled connected Device Device CCD CCD to a computer.camera camera connected connected The length to to a scale acomputer. computer. is shown The The by length thelength white scale scale bar is is (150shown shownm) by atby the thethe white bottom white bar bar of (150the(150 figures. µm) µm) at at the the bottom bottom of of the the figures. figures.

(a(a) ) (b(b) ) (c()c )

FigureFigure 4. 4.4. Comparative ComparativeComparative analysis analysis analysis of of of( a()a ()octagonala octagonal) octagonal symmetry symmetry symmetry of of plasmoni plasmoni of plasmonicc cquasi-crystal quasi-crystal quasi-crystal on on glass glass on (SEM glass(SEM (SEMimage;image; image; columns columns columns diameter: diameter: diameter: approximately approximately approximately 750 750 750nm) nm) nm) and and and ( b(b) ( )bthe )the the complementary complementary complementary transferred transferred on onon the the surfacesurface of of the the PMP PMP filmfilm film (DFM; (DFM; Magnification:Magnification: Magnification: 50×). 50 50×).). ( (c(c)c) )Zoom ZoomZoom of ofof the thethe structure structurestructure shown shown shown in in in ( ( bb(b).).). ×

AnAAn comparative additional additional demonstration demonstration analysis between of of the the the result result nanostructures of of th thee exfoliation exfoliation with octagonal method method is symmetryis shown shown in in realizedFigure Figure 5 5 onwhere where the topographyglasstopography substrate (Figure (Figure and the5a–c) 5a–c) ones and and obtained phase phase (Figure (Figure on the 5d) PMP 5d) measurements measurements film is performed obtained obtained by DFM by by AFM andAFM Scanningare are reported reported Electron for for a a plasmonicMicroscopyplasmonic substrate substrate (SEM) and with with shown dodecagonal dodecagonal in Figures symmetry. symmetry.3 and4. AllByAll lookingthese these measurements measurements at Figure4, thehighlight highlight pattern the the realized following: following: on (1 the (1) )the the solid good good substrate reproduction reproduction (see Figure of of the the4 quasi-periodica) quasi-periodic is exactly nanostructure;reproducednanostructure; on PMP (2)(2) thethe (Figure presencepresence4b). In ofof order nanocavities;nanocavities; to investigate (3)(3) the thethe presence detailspresence of oftheof roughness copyroughness process, intointo it eachiseach useful singlesingle to nanocavity;increasenanocavity; the image(4)(4) thethe magnification, reproducibilityreproducibility as reported ofof thethe in Figuremethodmethod4c. to Weto get noticeget anyany the periodic apparentperiodic “donut”andand aperiodicaperiodic shape highlighted by the darker part in the center of each single gold element of the pattern. This result points out a refractive index difference between the edge (white) and the center (dark) of the rings that corresponds to a different gold concentration, which is higher at the edges and lower in the center. We can explain this observation taking into account that the roughness on the tip of the pillars does not allow a uniform gold exfoliation. An additional demonstration of the result of the exfoliation method is shown in Figure5 where topography (Figure5a–c) and phase (Figure5d) measurements obtained by AFM are reported for a plasmonic substrate with dodecagonal symmetry. Crystals 2020, 10, x FOR PEER REVIEW 7 of 12 nano-configuration.Crystals 2020, 10, 660 They are in good agreement with the SEM measurements reported in Figure7 of 4, 12 where the roughness in the center of each single gold nanopillar is evident.

Figure 5. Atomic Force Microscopy (AFM) topography (a–c) and phase (d) images on dodecagonal Figureplasmonic 5. Atomic crystals Force implanted Microscopy on PMP (AFM) film. topography The side of the(a–c square) and phase image ( ind) (imagesa,b) is 5 onµm; dodecagonal in (c,d), it is plasmonic2 µm. The crystals colored implanted bar on the on right PMP in (filma,c,d. )The explains side of the the square scale image in the in images. (a,b) is 5 µm; in (c,d), it is 2 µm. The colored bar on the right in (a,c,d) explains the color scale in the images. All these measurements highlight the following: (1) the good reproduction of the quasi-periodic nanostructure;The Localized (2) the Surface presence Plasmon of nanocavities; Resonance (3) (LSPR) the presence is strongly of roughness dependent into on each the singleparticular nanocavity; shape of(4) each the reproducibilitysingle nano-element; of the methodtherefore, to getwe anyexpect periodic the resonance and aperiodic peak nano-configuration.of the nanocavity structure They are to in begood located agreement in a different with the position SEM measurements with respect to reported that of the in Figure pillars’4, structure; where the moreover, roughness we in theexpected center theof eachshape single of the gold resonance nanopillar to be is affected evident. by the PMP substrate. TheThe measurement Localized Surface of the Plasmon LSPR peak Resonance has been (LSPR) performed is strongly using dependent a conventional on the set-up. particular A white shape lightof each of a single nano-element; source (HL therefore,2000-Ocean we ) expect is the coupled resonance to an peak Olympus of the nanocavitymicroscope structure by an optical to be fiberlocated and in focused a different inside position the structure with respect under to investigation that of the pillars’ by an objective structure; (M moreover, 40×, NA we0.65) expected through the a diaphragm.shape of the The resonance sample to is bepositioned affected byon the an PMPXYZ substrate.microstage that allows the careful selection of the structureThe to measurement be investigated. of the The LSPR transmitted peak has signal been performedis collected using by an a optical conventional fiber with set-up. a A whiteof 50 µmlight and of adetected halogen using source a spectrometer (HL 2000-Ocean (USB4000-Ocean Optics) is coupled Optics). to an A Olympus scheme for microscope the set-up by is anreported optical infiber a previous and focused inside [21]. the structure under investigation by an objective (M 40 , NA 0.65) through a × diaphragm.The results The of sample the LSPR is positioned measurem onents an are XYZ shown microstage in Figure that 6. allowsAt this thestage, careful we have selection measured of the thestructure LSPR peak to be shift investigated. under illumination The transmitted with a signal laser ispointer collected (λ = by 405 an nm; optical P ~25 fiber mW; with spot a corearea of~1 50cmµ2).m andThese detected data using on one a spectrometer side highlight (USB4000-Ocean the plasmonic pr Optics).operties A of scheme the realized for the nanostructure; set-up is reported on the in a otherprevious side, paper they point [21]. out the present limitations given by the weak resonance. The reason lies in the limitedThe amount results of of gold the LSPR that has measurements been exfoliated areshown and in inthe Figure homogeneity6. At this of stage, the created we have nanoholes. measured Thethe LSPRimprovement peak shift of underthese parameters illumination is with the goal a laser of pointerthe forthcoming (λ = 405 nm; investigation P ~25 mW; on spot this area subject. ~1 cm 2).

CrystalsCrystals2020 2020, 10, 10, 660, x FOR PEER REVIEW 88 of of 12 12

FigureFigure 6. 6.Localized Localized Surface Surface Plasmonic Plasmonic Resonance Resonance (LSPR) (LSPR) measurement: measurement: resonance resonance and and peak peak shift shift underunder laser laser pointer pointer illumination; illumination; black black line: line: initial initial curve; curve; red red curve: curve: under under illumination; illumination; green, green, blue: blue: λ 2 afterafter switch-o switch-off;ff; light light blue: blue: final final configuration. configuration. λ= =405 405 nm; nm; power power ~25 ~25 mW; mW; spot spot area area= =1 1cm cm2.. These data on one side highlight the plasmonic properties of the realized nanostructure; on the As already mentioned, the location (949 nm) of the resonance shows a remarkable change with other side, they point out the present limitations given by the weak resonance. The reason lies in the repect to the nanopillar structucture realized on an ITO substrate (633 nm). Additionally, under limited amount of gold that has been exfoliated and in the homogeneity of the created nanoholes. illumination, we observe a reduction of the overall transmittance, a slight change in the shape, and a The improvement of these parameters is the goal of the forthcoming investigation on this subject. small peak shift (approximately 0.5 nm), which goes back to the initial position when the laser As already mentioned, the location (949 nm) of the resonance shows a remarkable change with pointer is switched off. Actually, this shift is of the same order of magnitude of the spectral repect to the nanopillar structucture realized on an ITO substrate (633 nm). Additionally, under sensitivity of the system; therefore, no definite statement can be done on this result, even if the illumination, we observe a reduction of the overall transmittance, a slight change in the shape, and a reproducibility of this measurement is high, being observed many times. small peak shift (approximately 0.5 nm), which goes back to the initial position when the laser pointer Finally, the photomobility of the system is assessed by using a laser light to induce the motion is switched off. Actually, this shift is of the same order of magnitude of the spectral sensitivity of the of a PMP-metacrystal film having aperiodic dodecagonal symmetry. The effect is observed system; therefore, no definite statement can be done on this result, even if the reproducibility of this exploiting an optical microscope with objective 100×, N.A. 0.90, whose focal area is easily identified measurement is high, being observed many times. in the image pattern and is shown between the dashed white lines in Figure 7 (see also Video S1, Finally, the photomobility of the system is assessed by using a laser light to induce the motion of a Supporting Information). (The shape of the area in focus is determined by the non-uniform shape of PMP-metacrystal film having aperiodic dodecagonal symmetry. The effect is observed exploiting an the PMP film). optical microscope with objective 100 , N.A. 0.90, whose focal area is easily identified in the image The focus area (Figure 7a) is shifted× under illumination, due to PMP surface deformation pattern and is shown between the dashed white lines in Figure7 (see also Video S1, Supporting (Figure 7b). When the light is switched off (Figure 7c,d), the PMP film restores its initial position in a Information). (The shape of the area in focus is determined by the non-uniform shape of the PMP film). few seconds (Figure 7d). It has to be remarked that the motion starts quickly under light The focus area (Figure7a) is shifted under illumination, due to PMP surface deformation (Figure7b). illumination, while the restoring time (the time used to restore the initial position of the PMP film) When the light is switched off (Figure7c,d), the PMP film restores its initial position in a few seconds corresponds to several seconds. The onset of light-induced actuation is consistent with the (Figure7d). It has to be remarked that the motion starts quickly under light illumination, while the properties of the PMP film [16], whereas the restoring forces are probably due to the energy restoring time (the time used to restore the initial position of the PMP film) corresponds to several dissipation, depending on the thermal capacity and affecting the interfacial tension gradient of the seconds. The onset of light-induced actuation is consistent with the properties of the PMP film [16], biphasic PMP film inducing its bend, as described above. The motion at the micro-scale is also whereas the restoring forces are probably due to the energy dissipation, depending on the thermal reasonably amplified by the plasmonic effect generated by the incident light (λ = 785 nm light, capacity and affecting the interfacial tension gradient of the biphasic PMP film inducing its bend, approximately 3 mW on the sample). In fact, motion is not recorded when illuminating regions with as described above. The motion at the micro-scale is also reasonably amplified by the plasmonic effect gold on the surface but away from the nano-patterned structure, thus confirming the plasmonic role generated by the incident light (λ = 785 nm light, approximately 3 mW on the sample). In fact, motion in the observed PMP mobility. is not recorded when illuminating regions with gold on the surface but away from the nano-patterned structure, thus confirming the plasmonic role in the observed PMP mobility.

Crystals 2020, 10, 660 9 of 12 Crystals 2020, 10, x FOR PEER REVIEW 9 of 12

(a) (b)

(c) (d)

Figure 7.7.Dark Dark field field microscopy microscopy analysis analysis of the of laser-light-induced the laser-light-induced motion ofmotion a dodecagonal of a dodecagonal symmetry plasmonicsymmetry crystalplasmonic nestled crystal on thenestled PMP on film the surface. PMP film The surface. area in The focus area of thein focus microscope of the microscope objective is edgedobjective by is dashed edged whiteby dashed lines. white (a) light lines. off :(a initial) light position; off: initial (b )position; light on: (b shift) light of theon: focusshift of area the underfocus illuminationarea under illumination (λ = 785 nm; (λ power = 785 onnm; sample power approximately on sample approximately 3 mW); (c) light 3 mW); OFF: (c inertial) light OFF: motion inertial after irradiation;motion after ( dirradiation;) light OFF: (d restoring) light OFF: of the restoring initial position.of the initial position. 4. Conclusions 4. Conclusions We have discussed different approaches in PMP manufacturing focusing on a novel approach We have discussed different approaches in PMP manufacturing focusing on a novel approach based on light-induced modulation of the surface tension at the polymer interface. We have shown that based on light-induced modulation of the surface tension at the polymer interface. We have shown these materials are suitable to develop a novel method to modify the PMP properties in order to get a that these materials are suitable to develop a novel method to modify the PMP properties in order nano-patterned photomobile polymer with plasmonic properties. This is achieved by using a novel to get a nano-patterned photomobile polymer with plasmonic properties. This is achieved by using organic mixture that under photopolymerization allows obtaining sensitive PMPs and exfoliation of a novel organic mixture that under photopolymerization allows obtaining sensitive PMPs and the nanostructures. The plasmonic properties are highlighted by the LSPR whose intensity decreases exfoliation of the nanostructures. The plasmonic properties are highlighted by the LSPR whose under illumination with a moderate power light source. The plasmonic-related motion of the polymer intensity decreases under illumination with a moderate power light source. The plasmonic-related is also recorded. motion of the polymer is also recorded.

Supplementary Materials:Materials:The The following following are availableare availa onlineble online at http: //atwww.mdpi.com www.mdpi.com/xxx/s1,/2073-4352 /10Video/8/660 /S1:s1, VideoLight-induced S1: Light-induced motion of motionPlasmonic of Plasmonic PMP film. PMP film. Author Contributions: Conceptualization, R.C.; methodology, for nanostructured-PMP fabrication: R.C.; methodologyAuthor Contributions: for EBL nanopatterned Conceptualization, substrates R.C.; fabrication,methodology, M.R., for L.P., nanostructur R.C.; investigation,ed-PMP fabrication: R.C., M.R., R.C.; F.V.; datamethodology curation, for R.C., EBL M.R., nanopattern F.V.; writing—originaled substrates fabrication, draft preparation, M.R., L.P., R.C.; R.C.; writing—review investigation, andR.C., editing, M.R., F.V.; R.C. data and F.S.;curation, supervision, R.C., M.R., F.S., F.V.; L.P., writing—orig G.N., R.C., M.R.,inal F.V.; draft funding preparation, acquisition, R.C.; writing—review L.P. All authors have and readediting, and R.C. agreed and to F.S.; the publishedsupervision, version F.S., L.P., of the G.N., manuscript. R.C., M.R., F.V.; funding acquisition, L.P. All authors have read and agreed to the Funding:published Thisversion research of the was manuscript. founded by the European Union under the programme FET-OPEN within the project “PULSE-COM” grant agreement no. 863227. Funding: This research was founded by the European Union under the programme FET-OPEN within the Acknowledgments:project “PULSE-COM”Special gran thankst agreement to JordankaTasseva no. 863227. of the National Institute of Nuclear , Naples, Italy.

ConflictsAcknowledgments: of Interest: SpecialThe authors thanks declare to Dr. noJordankaTasseva conflict of interest. of the National Institute of , Naples, Italy.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Okawa, D.; Pastine, S.J.; Zettl, A.; Fréchet, J.M.J. Surface Tension Mediated Conversion of Light to Work. J. Am. Chem. Soc. 2009, 131, 5396–5398.

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References

1. Okawa, D.; Pastine, S.J.; Zettl, A.; Fréchet, J.M.J. Surface Tension Mediated Conversion of Light to Work. J. Am. Chem. Soc. 2009, 131, 5396–5398. [CrossRef][PubMed] 2. Yu, Y.; Nakano, M.; Ikeda, T. Directed bending of a polymer film by light. Nature 2003, 425, 145. [CrossRef] [PubMed] 3. Briand, D.; Yeatman, E.; Roundy, S. Micro Energy Harvesting; Wiley-VCH Verlag GmbH & Co. KGaA: Wheinheim, Germany, 2015. 4. Harrop, P.; Das, R. Energy Harvesting and Storage for Electronic Devices 2009–2019; IDTechEx Ltd.: Cambridge, UK, 2009. 5. Ikeda, T.; Ube, T. Photomobile polymer materials: From nano to macro. Mater. Today 2011, 14, 480–487. [CrossRef] 6. Marangoni, C. Sul principio della viscosita’ superficiale dei liquidi stabilito dal Sig. J. Plateau. Nuovo Cimento 1871, 5, 239–273. [CrossRef] 7. Lucchetta, D.E.; Simoni, F.; Nucara, L.; Castagna, R. Controlled-motion of floating macro-objects induced by light. AIP Adv. 2015, 5, 77147. [CrossRef] 8. Lucchetta, D.E.; Simoni, F.; Nucara, L.; Castagna, R. Light Moves Macro-objects. Prog. Electr. Res. Symp. 2015, 1486, 113716. 9. Lucchetta, D.E.; Castagna, R.; Simoni, F. Light-actuated contactless macro motors exploiting Bénard–Marangoni convection. Opt. Express 2019, 26, 13574–13580. [CrossRef] 10. Maggi, C.; Saglimbeni, F.; Dipalo, M.; De Angelis, F.; Di Leonardo, R. Micromotors with asymmetric shape that efficiently convert light into work by thermocapillary effects. Nat. Comm. 2015, 6, 7855. [CrossRef] 11. Mallea, R.T.; Bolopion, A.; Beugnot, J.-C.; Lambert, P.; Gauthier, M. Laser-induced thermocapillary convective flows: A new approach for non-contact actuation at microscale at the fluid/ interface. IEEE/ASME Trans. Mechatron. 2017, 22, 693–704. [CrossRef] 12. Hendarto, E.; Gianchandani, Y.B. Thermocapillary actuation of millimeter-scale rotary structures. J. Microelectromech. Syst. 2014, 23, 494–499. [CrossRef] 13. Angeloni, A.S.; Caretti, D.; Carlini, C.; Chiellini, E.; Galli, G.; Altomare, A.; Solaro, R.; Laus, M. Photochromic liquid-crystalline polymers. Main chain and polymers containing azobenzene mesogens. Liq. Cryst. 1989, 4, 513–527. [CrossRef] 14. White, T.J.; Tabiryan, N.V.; Serak, S.V.; Hrozhyk, U.A.; Tondiglia, V.P.; Koerner, H.; Vaia, R.A.; Bunning, T.J. A high frequency photodriven polymer oscillator. 2008, 4, 1796–1798. [CrossRef] 15. Kondo, M.; Takemoto, M.; Fukae, R.; Kawatsuki, N. Photomobile Polymers from Commercially Available Compounds: Photoinduced Deformation of Side-Chain Polymers Containing Hydrogen-Bonded Photoreactive Compounds. Polym. J. 2012, 44, 410–414. [CrossRef] 16. Castagna, R.; Nucara, L.; Simoni, F.; Greci, L.; Rippa, M.; Petti, L.; Lucchetta, D.E. An Unconvetional Approach to Photomobile Composite Polymer Films. Adv. Mater. 2017, 29, 1604800. [CrossRef][PubMed] 17. Pirani, F.; Angelini, A.; Frascella, F.; Rizzo, R.; Ricciardi, S.; Descrovi, E. Light-Driven Reversible Shaping of Individual Azopolymeric Micro-Pillars. Sci. Rep. 2016, 6, 31702. [CrossRef][PubMed] 18. Pirani, F.; Angelini, A.; Frascella, F.; Descrovi, E. Reversible Shaping of Microwells by Polarized Light Irradiation. Int. J. Polym. Sci. 2017, 6812619, 1–5. [CrossRef] 19. Unser, S.; Bruzas, I.; He, J.; Sagle, L. Localized Resonance Biosensing: Current Challenges and Approaches. Sensors 2015, 15, 684–688. [CrossRef][PubMed] 20. Bingham, J.M.; Hall, W.P.; Van Duyne, R.P. Exploring the Unique Characteristics of LSPR Biosensing. Nanoplasmonic Sens. 2012, 29–58. [CrossRef] 21. Rippa, M.; Castagna, R.; Pannico, M.; Musto, P.; Tkachenko, V.; Zhou, J.; Petti, L. Engineered Plasmonic Thue-Morse Nanostructures for LSPR Detection of the Pesticide Thiram. Nanophotonics 2017, 6, 1083–1088. [CrossRef] 22. Zheng, J.; He, L. Surface-Enhanced Raman for the Chemical Analysis of Food. CRFSFS 2014, 13, 317–321. [CrossRef] 23. Hughes, J.; Izake, E.L.; Lott, W.B.; Ayoko, G.A.; Sillence, M. Ultra Sensitive Label Free Surface Enhanced Method for the Detection of . Talanta 2014, 130, 20–25. [CrossRef] [PubMed] Crystals 2020, 10, 660 11 of 12

24. Luo, S.-C.; Sivashanmugan, K.; Liao, J.-D.; Yao, C.-K.; Peng, H.-C. Nanofabricated SERS-Active Substrates for Single to Virus Detection in Vitro: A Review. Biosens. Bioelectron. 2014, 61, 232–235. [CrossRef] [PubMed] 25. Rippa, M.; Castagna, R.; Pannico, M.; Musto, P.; Bobeico, E.; Zhou, J.; Petti, L. Plasmonic Nanocavities-Based Aperiodic Crystal for -Protein Recognition SERS Sensors. Opt. Data Process. Storage 2017, 3, 54–60. [CrossRef] 26. Rippa, M.; Castagna, R.; Pannico, M.; Musto, P.; Bobeico, E.; Zhou, J.; Petti, L. High-Performance Nanocavities-Based Meta-Crystals for Enhanced Plasmonic Sensing. Opt. Data Process. Storage 2016, 2, 22–29. [CrossRef] 27. Salomon, A.; Prior, Y.; Fedoruk, M.; Feldmann, J.; Kolkowski, R.; Zyss, J. Plasmonic coupling between metallic nanocavities. J. Opt. 2014, 16, 114012. [CrossRef] 28. Lee, K.-L.; Wu, T.-Y.; Hsu, H.-Y.; Yang, S.-Y.; Wei, P.-K. Low-Cost and Rapid Fabrication of Metallic Nanostructures for Sensitive Biosensors Using Hot-Embossing and -Heating Nanoimprint Methods. Sensors 2017, 17, 1548. [CrossRef] 29. Aksu, S.; Huang, M.; Artar, A.; Yanik, A.A.; Selvarasah, S.; Dokmeci, M.R.; Altug, H. Flexible Plasmonics on Unconventional and Nonplanar Substrates. Adv. Mater. 2011, 23, 4422–4432. [CrossRef] 30. Kahraman, M.; Daggumati, P.; Kurtulus, O.; Seker, E.; Wachsmann-Hogiu, S. Fabrication and Characterization of Flexible and Tunable Plasmonic Nanostructures. Sci. Rep. 2013, 3, 3396. [CrossRef] 31. Childs, W.R.; Nuzzo, R.G. Decal Transfer Microlithography: A New Soft-Lithographic Patterning Method. J. Am. Chem. Soc. 2002, 124, 13583–13596. [CrossRef][PubMed] 32. Henzie, J.; Lee, M.H.; Pryce, I.M.; Aydin, K.; Kelaita, Y.A.; Briggs, R.M.; Atwater, H.A. Highly Strained Compliant Optical Metamaterials with Large Frequency Tunability. Nano Lett. 2010, 10, 4222–4235. 33. Lee, M.H.; Huntington, M.D.; Zhou, W.; Yang, J.-C.; Odom, T.W. Programmable Soft Lithography: Solvent-Assisted Nanoscale Embossing. Nano Lett. 2010, 11, 311–320. [CrossRef][PubMed] 34. Rippa, M.; Capasso, R.; Petti, L.; Nenna, G.; De Girolamo Del Mauro, A.; Maglione, M.G.; Minarini, C. Nanostructured PEDOT:PSS Film with Two-Dimensional Photonic Quasi Crystals for Efficient White OLED Devices. J. Mater. Chem. C 2015, 3, 147–152. [CrossRef] 35. Criante, L.; Vita, F.; Castagna, R.; Lucchetta, D.E.; Simoni, F. Characterization of Blue Sensitive Holographic Polymer Dispersed Liquid Crystal for Microholographic Data Storage. Mol. Cryst. Liq. Cryst. 2007, 465, 203–207. [CrossRef] 36. Criante, L.; Lucchetta, D.E.; Vita, F.; Castagna, R.; Simoni, F. Distributed feedback all-organic microlaser based on holographic polymer dispersed liquid crystals. Appl. Phys. Lett. 2009, 94, 111114. [CrossRef] 37. Vita, F.; Lucchetta, D.E.; Castagna, R.; Francescangeli, O.; Criante, L.; Simoni, F. Detailed investigation of high-resolution reflection gratings through angular-selectivity measurements. J. Opt. Soc. Am. B 2007, 24, 471–476. [CrossRef] 38. Venhoven, B.A.; De Gee, A.J.; Davidson, C.L. Polymerization contraction and conversion of light- BisGMA-based methacrylate . 1993, 14, 871–875. [CrossRef] 39. Peutzfeld, A. Composites in Dentistry: The Systems. Eur. J. Oral Sci. 1997, 105, 97–116. [CrossRef] 40. Lucchetta, D.E.; Spegni, P.; Di Donato, A.; Simoni, F.; Castagna, R. Hybrid Surface-Relief/Volume One Dimensional Holographic Gratings. Opt. Mater. 2015, 42, 366–369. [CrossRef] 41. Qi, J.; Sousa, M.E.; Fontecchio, A.K.; Crawford, G.P. Temporally Multiplexed Holographic Polymer-Dispersed Liquid Crystals. Appl. Phys. Lett. 2003, 82, 1652–1654. [CrossRef] 42. Qi, J.; De Sarkar, M.; Warren, G.T.; Crawford, G.P. In situ shrinkage measurement of holographic polymer dispersed liquid crystals. J. Appl. Phys. 2002, 91, 4795–4800. [CrossRef] 43. Castagna, R.; Vita, F.; Lucchetta, D.E.; Criante, L.; Simoni, F. Superior Performance Polymeric Composite Materials for High Density Optical Data storage. Adv. Mater. 2009, 21, 589–592. [CrossRef] 44. Watts, D.C.; Hindi, A. Intrinsic ‘Soft-Start’ Polymerisation Shrinkage-Kinetics in an Acrylate-Based Resin-Composite. Dent Mater. 1999, 15, 39–45. [CrossRef] 45. Castagna, R.; Vita, F.; Lucchetta, D.E.; Criante, L.; Greci, L.; Ferraris, P.; Simoni, F. Nitroxide Radicals Reduce Shrinkage in Acrylate-Based Holographic Gratings. Opt. Mater. 2007, 30, 539–544. [CrossRef] Crystals 2020, 10, 660 12 of 12

46. Ackam, N.; Crisp, J.; Holman, R.; Kakkar, S.; Kennedy, R. Shrinkage: Its Measurement and Consequences. Radtech Eur. 1995, 71. Available online: https://www.radtech-europe.com/knowledge-center/articles/other/ shrinkage-its-measurement-and-consequences (accessed on 19 July 2020). 47. Heilmann, S.M.; Moon, J.D. Multiacrylate Cross-Linking Agents in Pressure-Sensitive Photoadhesives. U.S. Patent 4,379,201 A, 5 April 1983. 48. Lazear, N.R.; Stackman, R.W. Ultraviolet Initiator Systems for Pressure-Sensitive . U.S. Patent 4,150,170 A, 17 April 1979. 49. Kaczmarek, H.; Decker, C. Interpenetrating Polymer Networks. I. Photopolymerization of Multiacrylate Systems. J. Appl. Polym. Sci. 1994, 54, 2147–2156. [CrossRef] 50. Tobing, S.D.; Klein, A. Molecular Parameters and their relation to the Performance of Acrylic Pressure-Sensitive Adhesives. J. Appl. Polym. Sci. 2001, 79, 2230–2244. [CrossRef] 51. Pradeep, K.; Deshpande, D.A.; Babu, G.N. Pressure Sensitive Adhesives of Acrylic Polymers Containing Functional Monomers. Polymer 1982, 23, 937–939. 52. Dewaele, M.; Truffier-Boutry, D.; Leloup, G.; Devaux, J. Volume Contraction in Photocured Dental Resins: The Shrinkage-Conversion Relationship Revisited. Dent. Mater. 2006, 22, 359–365. [CrossRef][PubMed] 53. Pappas, S.P. Radiation Curing Science and Technology; Plenum Press: New York, NY, USA, 1992. 54. Loshaeck, S.; Fox, T.G. Cross-linked Polymers. I. Factors Influencing the Efficiency of Cross-linking in of Methyl Methacrylate and Glycol Dimethacrylates. J. Am. Chem. Soc. 1953, 75, 3544–3550. [CrossRef]

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