Selective hybridization of organic dyes with CuSCN

Selective hybridization of organic dyes with CuSCN during its electrochemical growth

Yuki Tsuda, Kyota Uda, Misaki Chiba, He Sun, Lina Sun, Matthew Schuette White, Akito Masuhara & Tsukasa Yoshida Microsystem Technologies Micro- and Nanosystems Information Storage and Processing Systems ISSN 0946-7076 Microsyst Technol DOI 10.1007/s00542-017-3394-9

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Author's personal copy Microsyst Technol DOI 10.1007/s00542-017-3394-9

TECHNICAL PAPER

Selective hybridization of organic dyes with CuSCN during its electrochemical growth Yuki Tsuda1 · Kyota Uda1 · Misaki Chiba1 · He Sun1 · Lina Sun1 · Matthew Schuette White2 · Akito Masuhara1 · Tsukasa Yoshida1

Received: 14 March 2017 / Accepted: 24 March 2017 © Springer-Verlag Berlin Heidelberg 2017

Abstract Nine different organic dyes have been used for electrochemical self-assembly (ESA) of CuSCN/ dye hybrid thin films. One of these dyes was added to the bath for cathodic electrodeposition of CuSCN, containing Cu2+ and SCN− ions in ethanol. While anionic xanthene dyes having a carboxylic acid group were not loaded into the film, resulting in colorless CuSCN thin films with unchanged morphology and crystal orientation with the c-axis of β-CuSCN perpendicular to the substrate, all the other dyes were loaded to yield colorful CuSCN/dye hybrid thin films with specific hybrid nanostructures and preferential orientation to lay down the c-axis in parallel with the substrate. The hard and soft, acid and base (HSAB) principle nicely explains the successful ESA of dyes having a soft Lewis basic –NCS group for stable coordination to soft Lewis acidic Cu(I) sites of CuSCN. This further explains why the xanthene dyes, having hard Lewis basic carboxylate, did not coordinate with the CuSCN during electrodeposition, but did yield ESA with hard Lewis acidic ZnO in our previous studies. On the other hand, partial replacement of Cu+ ions of CuSCN with cationic dyes was found to be another efficient loading mechanism of ESA. These two mechanisms were confirmed by ESA in bath containing excess SCN− and Cu2+ respectively.

* Tsukasa Yoshida [email protected]‑u.ac.jp 1

Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University, Johan 4‑3‑16, Yonezawa, Yamagata 992‑8510, Japan

2

Department of Physics, Materials Science Program, University of Vermont, Burlington, VT 05405, USA

1 Introduction Hybridization of inorganic and organic materials expands the horizon of materials research. Ideally, new functionalities exploiting properties of both inorganic and organic constituents are expected (Mitzi 1999). When there is a right “chemistry” between them, they effortlessly “selfassemble” from their smallest building blocks such as inorganic ions and organic molecules. In fact, the right chemistry should in many cases be the key for such concerted functionalities to evolve. We have previously shown many examples of electrochemical self-assembly (ESA) of inorganic/organic hybrid thin films from solutions containing all the chemical ingredients (Yoshida et al. 1999, 2000, 2003, 2009; Okabe et al. 2001; Karuppuchamy et al. 2001; Nonomura et al. 2003; Pauporté et al. 2003, 2006; Oekermann et al. 2004; Sawatani et al. 2005; Sun and Yoshida 2009; Zhang et al. 2010; Ichinose et al. 2014; Iwamoto et al. 2014; Tsuda et al. 2017). For example, surface adsorptive organic dye such as eosin Y hybridizes with zinc oxide (ZnO) during its cathodic electrodeposition, simply by adding eosin Y to the bath (Yoshida et al. 2000, 2003, 2009; Okabe et al. 2001; Pauporté et al. 2003; Zhang et al. 2010; Ichinose et al. 2014). Electrochemical reduction of eosin Y makes it highly nucleophilic, strongly adsorptive to ZnO, resulting in an extreme high loading of eosin Y to occupy as high as 30% of the total film volume, and thus a deep coloration of the film (Zhang et al. 2010). Adsorption of dye molecules onto the growing surface of ZnO also impacts its crystal growth to result in unique nanostructures and alteration of the crystallographic orientation of the film. In case of ZnO/ eosin Y hybrid, a sponge-like single crystal of ZnO made as assembly of vertically aligned nanowires was obtained.

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Microsyst Technol

Eosin Y solid is totally phase-separated, fills the nanopores in the ZnO crystal, and thus can be completely extracted by soaking the film in a mild alkaline. As the remaining porous ZnO crystal achieves both high crystallinity and surface area, it was ideally suited as a photoelectrode material in dye-sensitized solar cells (DSSCs) (Yoshida et al. 2009). Taking other dyes such as coumarin343 (Yoshida et al. 2009) or simply citric acid (Ichinose et al. 2014) as a structure-directing agent (SDA) added to the electrolytic bath, the crystal orientation of the electrodeposited film was changed by 90° to lay down the c-axis of ZnO to be in parallel with the substrate plane. A common feature among the organic molecules adsorptive to ZnO is the presence of relatively hard Lewis basic groups such as carboxylate (Yoshida et al. 2000, 2003, 2009; Okabe et al. 2001; Nonomura et al. 2003; Pauporté et al. 2003, 2006; Sun and Yoshida 2009; Zhang et al. 2010; Ichinose et al. 2014), sulfonate (Yoshida et al. 1999; Oekermann et al. 2004; Zhang et al. 2010) and phosphonate (Karuppuchamy et al. 2001) to bind to the relatively hard Lewis acidic Zn(II) sites of ZnO. Differences in the molecular structure bring about different anisotropies in the crystal growth, resulting in a variety of hybrid nanostructures. Thus, ESA indeed is a good test bed to check the matched or unmatched chemistry between inorganic and organic constituents. While ZnO is an n-type wide bandgap semiconductor, copper(I) thiocyanate (CuSCN) is a wide bandgap p-type compound (Pattanasattayavong et al. 2013; Treat et al. 2015) and its well crystallized thin film can directly be CN− electrodeposited from solutions containing C u2+ and S ions (Sun and Yoshida 2009; Sun et al. 2011). Diffusion limited cathodic reduction of 1:1 complex, [Cu(SCN)]+, simply results in a formation of β-CuSCN (Sun et al. 2011).

[Cu(SCN)]+ + e− → CuSCN

(1)

Thus, CuSCN can be a very good counterpart to the ZnO/dye system to study ESA with organic dye molecules. Certainly, the chemistry has to be right between CuSCN and the organic dyes to be used for a successful ESA of CuSCN/dye hybrid thin films. We have recently reported several examples of ESA of CuSCN/dye hybrid thin films employing dyes such as zwitter ionic rhodamine B (RB) (Iwamoto et al. 2014) and cationic stilbazolium chromophore from 4-N,N-dimethylamino-4′-N′methylstilbazolium tosylate (DAST) (Tsuda et al. 2017). While RB has both anionic carboxylate and cationic ammonium, DAS+ is cationic from its methylpyridinium moiety. These dyes can be loaded into CuSCN at high concentrations and exhibit action as SDAs to create unique hybrid nanostructures and alter crystallographic orientation of CuSCN. They can also be completely extracted by soaking the film in N-dimethylacetamide (DMA) without dissolving CuSCN. The remaining porous crystal CuSCN performed

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as a dye-sensitized photocathode (Iwamoto et al. 2014). Undoubtedly, there are ample opportunities for the ESA of CuSCN/dye hybrid thin films and their use in devices such as hybrid solar cells. However, still not much is known about the underlying chemistry to govern the ESA of CuSCN/dye system. Specifically, requirements about the molecular properties for the ESA need to be clarified. In this study, we have examined nine selected organic dyes for ESA with CuSCN (Fig. 1). Ethanol was chosen as the reaction medium as it is the standard solvent for the electrodeposition of CuSCN and all these dyes dissolve relatively well to ethanol. Fluorescein (FL), 3,4,5,6-tetrachlorofluorescein (TCFL) and 2′,4′,5′,7′-tetrabromofluorescein (TBFL) are in the family of classical anionic xanthene dyes. TBFL is also called eosin Y and is a sort of a reference dye for the ESA with ZnO. Fluoresceinisothiocyanate (FLNCS) is also a member of anionic xanthene dye but its unique feature is the presence of –NCS group. Rhodamine B (RB) and Rhodamine 6G (R6G) are rhodamine dyes having cationic ammonium groups. While RB can be zwitter ionic due to the presence of free acid form of carboxylic acid group, R6G always stays as a cation as its carboxylic acid group is esterified. Nile blue (NB) is a cationic naphthoxazinium dye, highly fluorescent, used for DNA staining and photodynamic cancer therapy. Methylene blue (MB) is also a cationic classical phenothiazine dye and its facile and reversible redox reaction makes it useful as a redox indicator. Basic violet 1 (BV1, also called Methyl violet 2B) is a member of triphenyl methane dye. BV1 is mono-cationic in its purple form but acts as a pH indicator due to the presence of multiple amino groups which become protonated under highly acidic (pH 90%) of dye samples, FL (TCI), TCFL (TCI), TBFL (Aldrich), FLNCS (Aldrich), RB (Aldrich), R6G (Aldrich), NB (Aldrich), MB (Fluka) and BV1 (TCI) were purchased and used without further purification. Ethanolic solution containing 2.5 mM Cu(ClO4)2, 2.5 mM LiSCN and 0.1 M LiClO4 served as the electrolytic bath for the electrodeposition of CuSCN, to which one of the dye samples was added at 0.25 mM. Degreased F-doped S nO2 (FTO) coated conductive glass (AGC, Asahi-DU, 10 Ω/sq.) furnished into a rotating disk electrode (RDE) was cathodized at +0.2 V versus Ag/AgCl

Author's personal copy Microsyst Technol

Fig. 1 Structures of the organic dyes used in this study

for up to 600 s to obtain thin films. The deposition was carried out at room temperature (298 K), under air and under controlled mass transport by setting the rotation speed at ω = 500 rpm. The deposited film samples were rinsed with ethanol and dried under air. UV–Vis absorption spectra of the films were measured in transmission with air as the reference, on a SHIMADZU Solid Spec-3700 spectrophotometer. X-ray diffraction patterns were measured on a Rigaku RINT-Ultima III using Cu Kα radiation. Surface morphology of the films were observed by field emission scanning electron microscope (FE-SEM, JEOL JSM-6700F).

3 Results and discussion Pictures of the product thin films electrodeposited for 180 s are shown in Fig. 2. CuSCN (no dye) has a wide bandgap (3.6–3.9 eV) and thus is not colored. These pictures

already clearly reveal “bad chemistry” with the anionic xanthene dyes, FL, TCFL and TBFL, “good chemistry” for the rest, since colorful CuSCN/dye hybrid thin films are obtained. Loading of the dye chromophore is obvious from the absorption spectra of the films, showing characteristic absorption peaks in the visible for the successful ones, originating from the respective dye (Fig. 3). The dye loading seems to occur most efficiently for BV1, followed by NB for their high absorbance. The films electrodeposited in the presence of FL, TCFL and TBFL do not show such clear absorption peaks expected from these dyes, and thus remain white. It is interesting to note that many of the successful products of CuSCN/dye ESA are more transparent than the others as recognized from the decreased absorbance at 800 nm, caused by light scattering. Such changes are caused by the act of the dye molecules as SDAs to alter the film morphologies. FE-SEM images of all the product films are shown in Fig. 4. CuSCN thin film (a) electrodeposited without dye

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Fig. 2 Pictures of the CuSCN (no dye) and CuSCN/dye hybrid thin films electrodeposited for 180 s

Fig.  3 a Absorption spectra of the dyes used in this study for their ethanol solutions, normalized for their absorption maxima. Dye concentration was adjusted to approximately 10 μM without taking the purity of the purchased chemicals into account. b Transmission UV–Vis absorption spectra of the CuSCN (no dye) and CuSCN/dye hybrid thin films electrodeposited on FTO glass substrates for 180 s, measured with air as the reference

consists of rugged particles. The close-up image shows they are poly-crystalline, each one of the crystal grains having a smooth surface. All these characteristics are preserved for the films with FL (b), TCFL (c) and TBFL (d), the colorless films not incorporating these dyes, although the crystal grains grow larger than those of (a) and they are elongated for (c) and (d). They also show smooth crystal facets. On the contrary, clear change of morphology is recognized for the colorful films incorporating the dyes (e–j).

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The grains are larger than the pure CuSCN and their surface is not smooth, exhibiting textures in nano scale. These nanostructures are obviously caused by the loading of the dye molecules into the grains. Some regularity is recognized in the nanostructures, especially visible for (h) and (i), made by oriented attachment of thin platelets. The dyes successfully achieving ESA indeed act as SDAs to modify the crystal growth of CuSCN. Crystallographic orientation of the films has also been greatly changed by the dyes. All of the electrodeposited thin films show XRD peaks of rhombohedral β-CuSCN else than those arising from the FTO substrate (Fig. 5). However, the relative peak intensities from the different crystal planes clearly differ from film to film, especially so between those not incorporating dyes and those hybridized with dyes. The powder sample should exhibit diffraction from (003) planes with the highest intensity, followed by the slightly weaker one from (101). The films grown without dye, with TCFL and TBFL exhibit very strong (003) peaks. That with FL show (003) and (101) with almost the same intensities, while all the others, thus those hybridized with dyes, show (101) peak stronger than (003). β-CuSCN has a hexagonal unit cell elongated for the c-axis (a = b = 3.857 Å, c = 16.449 Å), so that the (101) planes occur almost in parallel with the c-axis (crossing at 11.5° with each other). Thus, comparison of (101)/(003) peak ratio tells how the c-axis of β-CuSCN crystallites is oriented with respect to the substrate. The (101)/(003) ratio measured in XRD was divided by that of JCPDS ((101)/ (003) = 0.75) to evaluate the orientation index. The values for the films are summarized as a bar graph in Fig. 6. Those with the index smaller than 1 prefer to orient the c-axis in perpendicular to the substrate, whereas those larger than 1 lay down the c-axis to be in parallel with the substrate. Pure CuSCN electrodeposited without dye is strongly oriented with its c-axis perpendicular to the substrate. This tendency is somewhat weakened for the films with TCFL and TBFL, but they also show the same preference. That with FL should be seen as almost the same as that of the powder sample with its index close to 1. Then, all the dye loaded CuSCN show the opposite trend to be oriented with


Fig. 4 FE-SEM images of the CuSCN and CuSCN/dye hybrid thin films electrodeposited for 600 s: a no dye, b FL, c TCFL, d TBFL, e FLNCS, f RB, g R6G, h NB, i MB and j BV1

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Microsyst Technol 10

Orientation index for (101) / (003)

BV1 MB NB R6G RB FLNCS TBFL TCFL FL No Dye FTO

9 8 7 6 5 4 3 2 1 0

Intensity / a.u.

dye

20

30

(110) (107)

ș-CuSCNࠉJCPDS ࠉ㸦#29-0581 㸧

(015)

(006) (104)

(012)

(003)

(101)

Fig. 6 Orientation index calculated from the XRD peak intensity ratio of (101)/(003) of the electrodeposited CuSCN (no dye) and CuSCN/dye hybrid thin films, with respect to the (101)/(003) ratio of the powder diffraction standard. Those higher than 1 indicates a tendency to orient the c-axis of β-CuSCN in parallel with the substrate plane, whereas those lower than 1 in vertical

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2ȟ / degree (Cu KȘ㸧 Fig. 5 XRD patterns of the CuSCN and CuSCN/dye hybrid thin films electrodeposited for 600 s, compared with that of the FTO glass substrate and the powder diffraction standard of β-CuSCN (JCPDS #29-0581)

the c-axis parallel with the substrate. Such preference is most prominent for RB, followed by BV1. These changes in the crystal orientation of the films can be understood as a consequence of the anisotropy in the crystal growth. The crystal seeds deposited at the beginning of the electrolysis should be randomly oriented on the randomly structured FTO substrate. Dye adsorption results in a selection of the crystallites to grow further, i.e., those oriented with their preferential growth direction vertical to the substrate can continue to grow, whereas the others cannot and buried underneath as the film grows thick. Such a mechanism of the orientation control was reasonably concluded from our previous study on switching SDA from citric acid to eosin Y in the middle of the electrodeposition of ZnO (Ichinose et al. 2014). ESA with citric acid results

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in a strong orientation in c-axis parallel with the substrate, whereas eosin Y orients c-axis vertical. When the primarily electrodeposited ZnO is strongly oriented with citric acid, eosin Y is not able to change the orientation and ZnO/ eosin Y hybrid layer grows to inherit the same orientation, because structure control by homoepitaxy of ZnO strongly operates and prevails to the effect of eosin Y as the SDA. Such a situation is reasonably expected for a system with instantaneous nucleation followed by atomic layer-bylayer growth, namely the systems in which the film growth is achieved only by crystal growth, without forming new nuclei in the course of electrodeposition. Both ZnO and CuSCN obey such a rule and thus result in highly crystallized thin films. It is therefore understood that β-CuSCN naturally likes to grow along its c-axis, in the absence of dye and in the presence of TCFL, TBFL and perhaps also FL, which do not hybridize with CuSCN, resulting in the films to exhibit a strong (003) XRD peak. On the other hand, the dyes loaded into the film hinder the growth along the c-axis, probably due to their strong adsorption onto the (003) facet, so that the growth of the crystallites oriented with their c-axis parallel with the substrate prevail to achieve the observed orientation. The differences of the dyes in the ESA with CuSCN must be considered with respect to their molecular structures. The anionic xanthene dyes, FL, TCFL and TBFL did not show any affinity to CuSCN, thus were not loaded into the film, and did not change the morphology and crystal orientation as well. Irrespective of the presence, absence and positions of halogen substituents, these dyes with a carboxylic acid group cannot bind to CuSCN, although they are nicely loaded into ZnO, especially well for TBFL


(eosin Y) (Yoshida et al. 2000, 2003, 2009; Okabe et al. 2001; Pauporté et al. 2003; Zhang et al. 2010; Ichinose et al. 2014). According to the hard and soft, acid and base (HSAB) principle, relatively hard Lewis basic carboxylate should find a favorable coordination to the relatively hard Lewis acidic sites of Zn(II) of ZnO, whereas Cu(I) of CuSCN is a typical soft Lewis acid and thus does not find good affinity with the carboxylated dyes. The same experience applies to the stable binding of multiply carboxylated photosensitizer dyes to titanium dioxide (TiO2) used in dye-sensitized solar cells (DSSCs) (Nazeeruddin et al. 2005). Hard Lewis acidic Ti(IV) of TiO2 even permits stable coordination of hydroxylated alizarin dye having no carboxylic acid groups (Akin et al. 2016). Hard Lewis basic deprotonated oxide anionic moiety binds to Ti(IV) in this case. On the contrary, hardness of Zn(II) is relatively modest, so that hard Lewis basic sulfonated eosin Y showed reduced affinity to ZnO, whereas doubly carboxylated eosin Y increased its affinity, as compared to eosin Y (Yoshida et al. 2009). Thus, a balanced hardness/softness for acid/base pairs is needed for stable bond formation. Successful ESA of CuSCN/FLNCS has to be discussed, because FLNCS is also a carboxylated anionic xanthene dye, but acted differently from FL, TCFL and TBFL. In this case, soft Lewis basic S atom of –NCS group should be the anchor to Cu(I). In fact, the famous Ru complex used in DSSCs, Ru(dcbpy)2(NCS)2 (dcbpy = 2,2′-bipyridine-4,4′dicarboxylic acid) also could be hybridized with CuSCN during its electrochemical growth, due to the presence of –NCS groups (Yoshida et al. 2009). Also, physiologically active tripeptide glutathione having a soft Lewis basic –SH group resulted in a successful ESA with CuSCN (Sun and Yoshida 2009). All these experiences tell us that the HSAB principle is able to nicely predict success or failure of ESA, with either CuSCN or ZnO. All the other dyes tested here, RB, R6G, NB, MB and BV1, were also successful for the ESA with CuSCN. The structure of the chromophore of these dyes differ but what is common among them is the presence of a cationic group, making them mono-cationic except for RB which can be zwitter ionic (neutral) due to the presence of carboxylic acid group. The positive charge resides on N atom of the ammonium groups, except for the phenothiazine MB for which the positive charge is located in the S atom. However, these cationic moieties are not expected to act as a soft Lewis base to Cu(I). All these dyes also have lone pairs from 2sp3 hybridization of N atom which could be used for coordination to Cu(I), although their electron donation ability must be weakened due to conjugation with the π-electronic system. Another possibility is partial substitution of C u+ ions of CuSCN with these cationic dyes, instead of substituting SCN− of CuSCN with dyes. In fact, loading of RB was

enhanced in the presence of excess SCN− ions in the bath as compared to the situation with an excess of Cu2+ ions, when the concentration of the electrochemically active species [Cu(SCN)]+ was fixed, thus the rate of CuSCN growth being constant (Iwamoto et al. 2014). Such a result implies that the cationic character of RB is important to form a salt with S CN− ions. Dyes with cationic character are loaded into CuSCN by substituting Cu+, while anionic FLNCS substituting SCN−, as the different mechanisms of ESA. In order to verify such a hypothesis, we ran a series of experiments to change Cu2+:SCN− ratio for the ESA with FLNCS and BV1 (Fig. 7). Although the experimental conditions were allowed to differ for the absolute concentrations of [Cu2+], [SCN−], [dye] and deposition time for each set of electrodeposition, the concentration of the active species, [Cu(SCN)]+, determining the rate of CuSCN growth, and that for respective dye, FLNCS or BV1, were fixed to see the change of dye loading efficiency by the Cu2+:SCN− ratio. Opposite trends were found for FLNCS and BV1. FLNCS increases its loading when the electrolyte contains an excess of C u2+, whereas BV1

Fig. 7 Transmission absorption spectra of a CuSCN/FLNCS and b CuSCN/BV1 hybrid thin films electrodeposited from the baths containing Cu2+ and SCN− at various ratios, while dye concentrations in the bath and deposition time were fixed as [FLNCS] = 50 μM, [BV1] = 250 μM and 20 and 3 min, respectively. The spectra are offset to align for the absorbance at 800 nm to be zero for the sake of clear comparison of the dye absorption peak heights

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does so when there is an excess of S CN−, as recognized from the change of dye absorption peak heights in the respective sets of film spectra. The trend found for BV1 is the same as that for RB (Iwamoto et al. 2014), indicating partial substitution of Cu+ cation with cationic BV1. When the bath contains an excess of C u2+, it impedes the loading of BV1 to CuSCN to reduce the peak height. Just the opposite is the case for FLNCS. Increase of the excessive Cu2+ leads to stabilization of electrochemically generated Cu+ and thus stabilizes coordination of FLNCS. However, the excess of S CN− competes with + FLNCS for its coordination to Cu and thus drastically reduces its loading. Although the absolute efficiency of loading of BV1 appears to be higher than that of FLNCS as recognized from the difference of the absorbance, their opposite behavior in response to the Cu2+:SCN− ratio nicely confirms their difference in the mechanism of ESA with CuSCN. Although further experiments on the other dyes are advised to confirm the idea, it is reasonable to consider that the cationic dyes, including RB, are able to replace a part of C u+ of CuSCN, as the mechanism of their loading. On the other hand, that of FLNCS occurs as a replacement of SCN− ions according to the HSAB principle.

4 Conclusion Nine different kinds of organic dyes were employed in the ESA with CuSCN to test their chemistry for hybridization. Clear differences were observed in their behavior. Anionic xanthene dyes, FL, TCFL and TBFL, do not hybridize with CuSCN, indicating that the carboxylate is not the right anchor to CuSCN. Soft Lewis basic –NCS in FLNCS, however, found the right chemistry to coordinate to the soft Lewis acidic Cu(I) to replace S CN− ions of CuSCN for the successful ESA. All the other dyes, RB, R6G, NB, MB and BV1, behave as cations to replace Cu+ for their ESA with CuSCN. Details of the mechanisms of the ESA to account for the differences in the composition, structure and the efficiency of dye loading are yet to be analyzed. However, the method of ESA already nicely serves as a test bed to check the chemistry between organic molecules and the inorganic compounds. For example, very good “chemistry” is already implied for dyes like RB, NB, MB and BV1 because of their high loading into CuSCN. These dyes have unique properties in photoluminescence, redox, and pH sensing, so that concerted functionalities are anticipated from these hybrid thin films. Further explorations to expand the variety of hybrid materials for better understanding of the underlying chemistry as well as their properties in device applications are under way.

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Microsyst Technol Acknowledgements The present work was financially supported by Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers, “Advanced Next Generation Energy Leadership (R2601, FY2014-2016)” and Grants-in-Aid for Scientific Research B (15H03854) of Japan Society for the Promotion of Science (JSPS).

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