Aiming at a highly bright emitter, hybridized thin films consisting of organic dye TDBC J-aggregates (JA) and titanium oxide (TO) nanoparticles (NPs) have been fabricated successfully. The fluorescence intensity and the corresponding fluorescence quantum yield multiplied ca. 10 times and ca. twice, respectively. TO NPs have a high refractive index, and have no absorption loss like metal NPs. On the other hand, extinction (absorption) and fluorescence spectra are in general overlapped in organic dye JA, that is, so a small Stokes’ shift. Namely, the present phenomenon could be qualitatively explained by the simultaneously optical processes of both “excitation enhancement” induced directly by near-field effect from TO NPs and “emission enhancement” as a radiation of scattering field from TO NPs, which were polarized by the excitation energy from TDBC JA to TO NPs. In other words, the definite scattering peak in the extinction spectrum of TO NPs should be tuned efficiently with extinction (absorption) and fluorescence peak bands of TDBC JA.
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Fluorescence and/or emission behaviors from organic dyes have extensively been investigated so far in both fields of photophysics and photochemistry for many years . In addition, the fluorescence and/or emission spectra for organic nanocrystals (NCs) of dyes are interestingly shifted, depending on crystal size . This is due to thermally softened crystal lattice structure in organic NCs . Recently, highly luminescent point light source  has been much attractive from the viewpoints of fundamental photophysics such as SPASER (surface plasmon amplification by stimulated emission of radiation) [5,6] and some applications in the fields of nonlinear photonics device [7–10], quantum computing systems , bio-sensing , etc. Especially, the organic dye-metal hybridized nanoparticles (NPs) are one of the most promising nanomaterials as an intensively luminous body [13–16]. Fluorescence intensity (FI) emitted from organic dye is often enhanced considerably by utilizing localized surface plasmon resonance (LSPR) effect [17–19], when the hybridized nanostructure, e.g., a distance between organic dye and noble metal NPs like gold (Au) and silver (Ag), could be controlled and optimized properly [20–22]. There are the two optical processes in enhancement of FI in hybridized NPs [23,24]: so-called “Excitation Enhancement”  and “Emission Enhancement” . The increase in probability of photoexcitation is induced directly by LSPR effect in the former case. On the other hand, excitation energy is resonantly transferred to noble metal NPs before causing non-radiative transition from photoexcited organic dye in the latter case. As a result, LSPR is induced in noble metal NPs, and then scattering light (or scattering field) is irradiated efficiently. Many literatures concerning Emission Enhancement have been reported so far [26–28]. In fact, Emission Enhancement and gap mode effect based upon M-I-M (metal-insulator-metal) type hybridized nanostructure were responsible to much intensive FI in the core (Au)-dual shells (inner shell: organic dye in SiO2 matrix, and outer shell: Au) type hybridized NPs by considering increase in fluorescence quantum yield (QY) and reduction of fluorescence life time . On the other hand, FI became high in the core (Ag)-dual shell (inner shell: SiO2, and outer shell: QDs (quantum dots)) hybridized NPs, owing to Excitation Enhancement . There are, however, some problems in noble metal NPs as follows. Excitation energy is partially loosed by Drude absorption loss in noble metal NPs . Thus, FI is rather reduced, when non-radiative inactivation in noble metal NPs becomes more than that of organic dye having high QY [23,24]. FI is also quenched, due to dipole-dipole interaction, when organic dye is located (or directly contact) with extremely neighbored noble metal NPs [20–22,30].
Recently, transparent inorganic materials having high refractive index (RI) and without absorption loss in the visible (Vis) region , such as GaP (gallium phosphide)  and Si (silicon) , have been now noted in order to avoid and overcome above-mentioned problems in noble metal NPs. Total reflection of excitation light is repeated inside these inorganic NPs, and then only excitation light with a particular wavelength is selectively enhanced resonantly. As a result, intense near-field could be generated on the surface of inorganic NPs, depending on wavelength dispersion of complex RI and the size of inorganic NPs . In brief, it is much important to tune and overlap scattering peak in extinction spectrum of inorganic NPs with absorption and/or fluorescence peak bands of organic dye so as to remarkably enhance FI [31–35]. Especially, one can expect the simultaneous appearance of Excitation Enhancement and Emission Enhancement effects caused by inorganic NPs, instead of LSPR effect of noble metal NPs, if organic dye J-aggregates (JA) would be chosen properly [35–39], because the so-called Stokes’ shift is much small, that is, extinction (absorption) and fluorescence spectra are in common almost overlapped in the case of dye JA.
In the present article, the hybridized thin films have been fabricated finely by composing organic dye JA and titanium oxide (TO) NPs with high RI (n = 2.72) [40,41], and the hybridized nanostructural correlations of FI and enhancement mechanism will be discussed in details. Only a few articles about FI enhanced by using TO NPs have been published so far [31,40,41].
2. Experimental section
TDBC (Fig. S1) was purchased from Hayashibara Co., Ltd. Tetraethoxysilane (TEOS), ammonium aqueous solution (NH3 aq.: 28%), and ethanol were commercially available (FUJIFILM Wako Pure Chemical Corp.). All chemical reagents and ultra-pure water (18.2 MΩ cm) was used without further purification.
2.2 Fabrications of silica layer-coated TO NPs
TO NPs (10 mg) were dispersed into ultra-pure water medium (10 mL). Subsequently, TEOS (100 μL), ethanol (10 mL), and NH3 aq. (5 mL) were added into an aqueous dispersion liquid of TO NPs, and then kept at room temperature for 6 hours under the condition of stirring .
2.3 Preparations of hybridized thin films consisting of TDBC JA and TO NPs
An aqueous dispersion liquid (10 mL) of TO NPs (0 to 2 mg) were added dropwise on a glass substrate, and then air-dried at room temperature. Next, aqueous dispersion liquids of TDBC JA (0.1 to 1.0 mM, 200 μL) were further dropped, and then dried up again enough so as to prepare hybridized thin films. This fabrication processes were performed in a same manner in both cases of TO NPs with 100 nm and 200 nm in different sizes. On the other hand, the similar hybridized thin films have been prepared in the same manner by employing an aqueous dispersion liquid (10 mL) of silica layer-coated TO NPs (0.1 mg) and aqueous dispersion liquid of TDBC JA (0.1 mM, 200 μL).
2.4 Characterization methods
The obtained hybridized thin films were all characterized with scanning electron microscope (SEM: JEOL, JSM-6700F, 15 kV) as well as dynamic light scattering (DLS) and zeta-potential measurement instrument (Malvern, Zetasizer Nano-ZS). The linear optical properties were measured with UV (ultra-violet) - Vis absorption spectrophotometer (JASCO, V-570) and fluorescence spectrophotometer (Hitachi, F-2500, excitation wavelength: λex = 560 nm). In addition, QY was evaluated carefully by the measurement apparatus (Bunkoukeiki Co., Ltd., IZ-CT-25TP, λex = 400 nm).
3. Results and discussion
The shape of TO NPs was not necessarily spherical but cuboidal-like, and the size was about 200 nm [Fig. S2(a)]. The extinction spectrum of aqueous dispersion liquids of TO NPs was much broad in the wide Vis region [Fig. S2(b)], due to mainly size distribution of TO NPs. (The extinction spectrum of TO NPs obtained by Mie scattering simulation [43,44] will be discussed later in Fig. S6.)
TDBC JA is formed with increasing concentration of an aqueous solution of TDBC . Figure 1 shows the extinction (absorption) spectra and fluorescence spectrum of TDBC JA dispersed in an aqueous medium. The weak and broad absorption peak (M) was located around 450 to 550 nm in wavelength at low concentration of TDBC monomer (0.005 mM). The strong and sharp absorption peak (J) appeared at 585 nm in the longer wavelength region [45,46], when TDBC monomer was converted into TDBC JA in highly concentrated aqueous medium (0.01 mM) . TDBC JA also provided the narrow fluorescence peak in the almost same wavelength region as that of absorption peak (J) of TDBC JA (0.01 mM). That is to say, the Stokes’ shift is much small in the extinction (absorption) and fluorescence spectra of TDBC JA [32,42]. Interestingly, the whole shape of TDBC JA was fiber-like from SEM observation as shown in Fig. S3(a). The diameter was approximately uniform and ca. 150 nm. The primary NPs of TDBC JA (10 to 20 nm in size) were condensed inside the fiber-like structure in the inset of Fig. S3(a).
It was experimentally speculated from SEM observations in Figs. S3(b) and S3(c) that the surface of hybridized thin films was nearly covered with TDBC JA at low loading amount of TO NPs (0.01 mg). On the other hand, the bare surface of TO NPs partially appeared and exposed with relatively increasing the loading amount of TO NPs (0.1 mg) in the hybridized thin films. There were no extinction (absorption) peaks from TDBC monomers in TDBC JA thin film (without TO NPs) and the hybridized thin film (Fig. S4). Probably, TDBC monomers were all incorporated into TDBC JA during condensation and drying-up in the preparation process of the (hybridized) thin films in the solid states, which were different from an aqueous dispersion liquid of TDBC JA in Fig. 1 . The vertical shift of baseline for the hybridized thin film in Fig. S4 was mainly due to scattering effect from TO NPs .
Based on the above-mentioned experimental results, Fig. 2 exhibits the difference in FI of by Excitation Enhancement as well as Emission Enhancement in TDBC JA thin film and the hybridized thin films. The fluorescence peak positions were scarcely shifted within the experimental errors in any case [45,46]. The FI in the hybridized thin films was remarkably dependent on loading amount of TO NPs, and was at most 10 times approximately as much as that of TDBC JA thin film. Namely, the FI at monitored λ = 592 nm has provided the maximum value as shown in Fig. 3, and the QY was also multiplied by a factor of about two.
Incidentally, neither FI nor QY was increased in the hybridized thin film consisting of TDBC JA and polystyrene NPs (monodispersed 200 nm in size) with low RI (n = 1.59), instead of TO NPs (not shown). In Fig. 3, there was interestingly the difference in loading amounts of TO NPs at the maximum values of FI and QY. As described in Introduction, FI is usually influenced by Excitation Enhancement as well as Emission Enhancement [23,24], whereas QY is closely related to only Emission Enhancement .
Let us further discuss on fluorescence properties of the hybridized thin films by considering near-filed effect from TO NPs [31,34]. In Fig. 4, the fluorescence spectra have been evaluated by employing the hybridized thin films, in which TO NPs were replaced by silica layer-coated TO NPs or by TO NPs having different sizes, to investigate the influences of near-field and size effects of TO NPs. The FI was reduced remarkably and became less than that of TDBC JA thin film, when silica layer-coated TO NPs were used [Fig. 4(a)]. This fact suggests that the silica layer effectively obstructed near-field effect from TO NPs . It is likely that the silica layer takes ca. 20 nm in thickness (Fig. S5), which roughly corresponds to the region of near-field around the surrounding TO NPs (ca. 200 nm in size) . Similarly, FI was much low and below that of TDBC JA thin film in the case of TO NPs with 100 nm in size [Fig. 4(b)]. As a result of Mie scattering simulation under the assumptions of monodispersed size and spherical shape in TO NPs (Fig. S6) [43,44], there is no effective scattering peak in the case of TO NPs with 100 nm in size. On the contrary, the definite scattering peak appears in TO NPs with 200 nm in size, and is overlapped enough with absorption and fluorescence peak bands of TDBC JA .
It has become apparent evidently that the enhancement of FI and the changes of QY are closely correlated not only to near-field effect from TO NPs, spectral tuning and overlapping but also to hybridized nanostructure, e.g., size of TO NPs, distance between TDBC JA and TO NPs, loading amount of TO NPs, and so on. Figure 5 illustrates the proposed scheme of enhancement mechanism of FI. From the viewpoints of hybridized nanostructure, the near-field effect is insufficient at the low loading amount of TO NPs [Fig. 5(a)], and then FI becomes weak. When the loading amount of TO NPs increases and is adequate [Fig. 5(b)], the near-field effect becomes effectively enough. In addition, localized photoelectric field (so-called hot- spots) is formed between the adjacent TO NPs , which could further contribute to enhancement of FI . As a result, strong fluorescence is emitted intensively. On the contrary, FI becomes weak again under the condition of excess loading amount of TO NPs [Fig. 5(c)], owing to the opposite influences such as confinement effect of emitted fluorescence by TO NPs with high RI inside the hybridized thin films , and to self-absorption effect by TDBC JA having much small Stokes’ shift .
Figure 5(d) displays the near-field effect from TO NPs and the optoelectronic interaction with TDBC JA. When excitation light is irradiated and its intensity is multiplied inside TO NPs with high RI, due to total reflection effect, near-field as well as hot-spots would appear efficiently at the surrounding TO NPs as already-mentioned in Fig. 5(b) [31,34]. These optical processes would surely induce Excitation Enhancement. Although excitation energy in photoexcited organic dyes and/or JA is usually transferred resonantly to noble metal NPs with Drude absorption loss , polarization would be rather induced in contrast in TO NPs by the excitation energy in photoexcited organic dyes and/or JA . As a result, scattering light (or scattering field) is irradiated from TO NPs without scattering loss, since TO NPs have no absorbance and transparent in Vis region. This optical process seems to be a kind of Emission Enhancement in the case of transparent inorganic NPs having high RI like TO NPs. Thus, both Excitation and Emission Enhancements would occur simultaneously in the hybridized thin films consisting of TDBC JA and TO NPs. Actually, the following factors would extensively affect the maximal FI and QY, mainly depending on loading amount of TO NPs and hybridized nanostructure, i.e., number density of hot-spots as well as near-field effect, macroscopically diffusive reflection (or multiple scattering) and optical path length of excitation light on the surface of and inside the hybridized thin films, confinement and self-absorption effects of emitted fluorescence inside JA, and so on.
FI (ca. 10 times) and QY (ca. twice) were multiplied in the hybridized thin films consisting of TDBC JA and TO NPs. The present phenomenon could be qualitatively explained by the simultaneously two optical processes. One is Excitation Enhancement induced directly by near-field effect from TO NPs, and the other is a kind of Emission Enhancement as a radiation of scattering field from TO NPs polarized by the excitation energy from TDBC JA. In other words, the definite scattering peak in the extinction spectrum of TO NPs should be overlapped and tuned efficiently with absorption and fluorescence peak bands of TDBC JA. In contrast, the FI was enhanced about three times in the similar hybridized thin films consisting of TDBC JA and Au NPs, due to not only Drude absorption loss but also imperfect tuning (or overlapping) of LSPR peak of Au NPs with absorption and fluorescence peak bands of TDBC JA. Unexpectedly, any scattering peaks of GaP, ZrO2 (zirconia oxide), and Nb2O5 (niobium pentoxide) are not located in the range of absorption and fluorescence peak bands of TDBC JA. Anyway, it is important to overlap and tune scattering peak of inorganic NPs having high RI with absorption and/or fluorescence peak bands of organic dyes and/or JA in a hybridized material system so as to highly obtain FI toward optoelectronics and photonics applications, for examples, large-area emitter for LED, light-amplifier for optical guide plate, optically pre-amplifier for solar cell, and so on in the near future.
Ministry of Education, Culture, Sports, Science and Technology (20191104).
The present research was performed by the financial support from Management Expenses Grant in Tohoku University (National University Corporation), Cooperative Research Program of “Network Joint Research Center of Materials and Devices” (No. 20191104), and “NIMS Joint Research Hub Program”. These budgets were all delivered from Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan
The authors declare that there are no conflicts of interest related to this article.
See Supplement 1 for supporting content.
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