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The waveguiding of surface enhanced Raman scattering (SERS) signals was demonstrated by using organic semiconducting microrods (MRs) hybridized with functionalized gold nanoparticles (Au-NPs). Organic semiconducting 1,4-bis(3,5-bis(trifluoromethyl) styryl)-2,5-dibromobenzene (TSDB) crystalline MRs were fabricated as active optical waveguiding system using a self-assembly method. The static SERS effect and the enhancement of photoluminescence were simultaneously observed for the TSDB MRs hybridized with Au-NPs. The waveguiding characteristics of the SERS signals through the hybrid MR of TSDB/Au-NPs were investigated using a high-resolution laser confocal microscope (LCM) system. The enhanced output Raman characteristic modes of TSDB molecules were clearly observed along the hybrid MR of TSDB/Au-NPs, which is attributed to stronger scattering of the light and the increased coupling efficiency of waveguiding due to the presence of Au-NPs. The waveguiding of the SERS signals exhibited different decay constants for the corresponding characteristic Raman modes, such as -C = C- aromatic, -CF3, and C-Br stretching modes. The observed waveguiding characteristics of various SERS modes enable multi-modal waveguiding with relatively narrow spectral resolution for nanophotonic information.
© 2017 Optical Society of America
1. Introduction
Two fundamental optical excitations, the propagation of surface plasmon polaritons (SPPs) and localized SPPs, have tremendous application prospects [1,2]. The excessively large enhanced electromagnetic (EM) field caused by the localized SPPs has been applied to a surface-enhanced optical spectrum [3–5], a surface plasmon resonance sensor [6,7], ultra-transmission [8,9], a plasmonic-enhanced emission [10,11], and quantum communication [12,13]. One of the most promising applications of SPPs is surface-enhanced Raman scattering (SERS), which has been intensively studied for bio- and chemical-sensing and structural characterization, including molecular vibration and rotation [14–20].
As an optical fingerprint, Raman spectrum has been measured for the structural analysis of materials, because Raman characteristics represent various phonon modes corresponding to the chemical constituent of molecules. However, due to the small number of molecules, the relatively small Raman scattering cross-section causes weak signals, even with high laser power. Hybridization with metal nanostructures and the use of a rough metal surface have been adopted to realize the SERS effect through enhancement of the EM field by localized SPPs [21,22], resulting in the detection of magnified Raman signal at a single molecular level [23,24].
With the rapid development of nanoscale fabrication for light-emitting organic semiconductors, organic-based optoelectronics, plasmonics, and photonics have been intensively studied [25]. Organic luminescent nano- and micro-crystals have been used as active optical waveguiding systems. The waveguided photoluminescence (PL) spectra and the decay characteristics in these systems have been intensively investigated for their application to photonics and chemical-/bio-sensing [26,27]. For efficient PL waveguiding, the input excitation wavelength must be selected within the absorption region of such materials. In contrast, the waveguiding of Raman signal can uniquely provide direct signal transport by means of specific molecular interaction modes, such as vibrational and rotational modes, in crystalline organic semiconducting nano- and micro-structures [28,29]. Raman waveguiding, when the excited wavelength is outside the optical absorption range, has been shown to be capable of transporting multi-signals corresponding to various different molecular orientations and vibrations of materials [30,31]. Also, the spectral widths of the waveguided multi-modal Raman signals are usually much narrower than that of the PL signal, suggesting that Raman signal propagation can transport a significantly greater amount of information. Therefore, the waveguiding of SERS signals using organic luminescent nano- or micro-crystals contributes to accessing new research fields and applications in nanophotonics and chemical-/bio-sensing, with the merits of multi-modal waveguiding with narrow bandwidth.
In this study, we simultaneously observed the SERS effect and the enhanced PL for the crystalline organic π-conjugated 1,4-bis(3,5-bis(trifluoromethyl) styryl)-2,5-dibromobenzene (TSDB) microrods (MRs) after hybridization with functionalized Au-NPs. The waveguiding of SERS spectra along the hybrid MR of TSDB/Au-NPs was clearly observed using a high-resolution laser confocal microscope (LCM) system. The enhanced Raman characteristics (i.e. SERS signals) in the TSDB molecule, such as -C = C- aromatic, -CF3, and C-Br stretching modes, were waveguided through the organic TSDB MRs with different decay constants. The energy propagation through the intermolecular π-π interaction between π-conjugated molecules and the stronger light scattering due to surface plasmon (SP) excitation by Au-NPs assist efficient waveguiding of SERS signals.
2. Experimental setup
2.1 Materials
All starting chemical materials were purchased from commercial suppliers (Sigma Aldrich and Fisher Scientific). The 1,1'-[(2,5-dibromo-1,4-phenylene)bis(methylene)]bis[1,1,1-triphenylphosphonium dibromide (1 g, 1.057 mmol) and 3,5-bis(trifluoromethyl) benzaldehyde (0.51 g, 2.114 mmol) were dissolved in anhydrous tetrahydrofuran [32]. Potassium tert-butoxide (0.3 g, 2.5 eq) was proportionally added over the duration of 10 min. The mixture was then warmed to room temperature, and stirred overnight. After pouring the mixture into water, the precipitate was collected, and washed with methanol. Finally, the TSDB powder was obtained through recrystallization from a chloroform (CHCl3) solution, after flash column chromatography [32]. The TSDB powder dissolved in CHCl3 was filtered using a syringe filter. The TSDB molecules in the super-saturated solution then self-assembled in the form of MRs within a few minutes. The dodecanethiol (C12H25SH) functionalized Au-NPs were fabricated using the Brust method [33]. For the hybridization of TSDB MR with the functionalized Au-NPs, the Au-NPs were homogeneously spin coated on the glass substrate. The TSDB MR was then physically placed on the surface of Au-NPs, in order to realize the energy transfer of the SP.
2.2 Instruments and measurements
The formation and surface morphology of the TSDB and its hybrid MRs were investigated by scanning electron microscopy (SEM, Hitachi S-4300) and high-resolution scanning transmission electronic microscope (HR-S/TEM, Tecnai G2 F30ST). The optical microscope and CCD images were measured using the OLYMPUS BXIS system (BX51M-N53MF2&DP73-SET). The UV-Vis absorption spectra of the samples were measured by using a HP8453 spectrometer at room temperature. The nanoscale PL, Raman, and SERS spectra of the MRs, including their output spectra (i.e. waveguiding characteristics), were measured using a high-resolution LCM system built around an inverted optical microscope (Axiovert 200, Zeiss GmbH). For the LCM PL mapping experiment, an unpolarized diode laser (λex = 405 nm) was used for PL excitation. For the LCM Raman experiment, a He-Ar laser (λex = 488 nm and 514 nm) and He-Ne gas laser (λex = 633 nm) were used for the Raman excitation. The spot size of the focused laser beam on the sample in the LCM system was estimated to be approximately 500 nm. The incident laser power on the sample and the exposed time for each Raman spectrum varied with the use of excitation lasers. The detailed methods of the LCM experiments have previously been reported [31,32].
3. Results and discussion
3.1 Steady-state SERS and PL characteristics
The steady-state and dynamic (i.e. waveguiding) characteristics of optical signals, such as PL and Raman, are closely related to the crystalline structures and the refractive index (n) of the active materials. For efficient waveguiding of PL and Raman signals, highly crystalline luminescent nano- or micro-structures are preferred, because high crystallinity enables better energy propagation. In this study, the crystalline TSDB with the form of MR was synthesized through a self-assembly method in organic CHCl3 solvent. From SEM and HR-TEM images as shown in Fig. 1, the width and thickness of the TSDB MRs were typically about 1 ~2 μm and 500 nm, respectively. The length of the MRs was about 50 ~55 μm. The high crystallinity of the TSDB MRs obtained from XRD pattern has earlier been reported [32]. For the SERS effect, the functionalized Au-NPs were spin-coated on the glass substrate, and the TSDB MRs were then drop-cast on the Au-NPs. The inset of Fig. 1(a) shows a schematic chemical structure of TSDB molecule. Figure 1(b) shows HR-TEM image of the hybrid MR of TSDB attached with functionalized Au-NPs. The number density of Au-NPs on the surface of the MR was about 395 ea / 100 × 100 nm2. From the HR-TEM image of Fig. 1(b) and HR-S/TEM images of Fig. 1(c,d), the relatively homogeneous attachment of Au-NPs on the surface of a TSDB MR was observed. Figure 1(e) shows UV-Vis absorption spectrum (blue curve) in ethanol solvent and LCM PL spectrum (red curve) of TSDB MRs. The TSDB MR showed blue light-emission with PL peak at 490 nm. The main optical absorption peak of the TSDB MRs was observed at 338 nm due to the π–π* transition. The UV-Vis absorption peak of the Au-NPs (black curve) corresponding to SP absorption was observed at 510 nm, which is closely matched with the PL peak of the TSDB MR, implying the possible energy transfer of SP.
Fig. 1 (a) SEM image of TSDB MR. The inset shows a schematic chemical structure of TSDB molecule. (b) HR-TEM image of hybrid MR of TSDB attached with Au-NPs. The small black dots represent the functionalized Au-NPs. The inset shows a schematic structure of the functionalized (dodecanethiol, C12H25SH) Au-NP. (c) HR-S/TEM image of hybrid MR of TSDB/Au-NPs. (d) Magnification of the HR-S/TEM image of the hybrid MR corresponding to yellow box region in Fig. 1(c). (e) UV-Vis absorption spectrum of TSDB MR (blue curve) in ethanol, and Au-NPs (black curve) in hexane. LCM PL spectrum of TSDB MR (red curve; λex = 405 nm, laser power = 3 μW, exposed time = 47.5 ms).
Figures 2(a) and 2(b) show the optical microscopy and CCD images of the pristine TSDB MR, respectively, in which blue light-emission was observed. Figures 2(c) and 2(d) show the LCM PL mapping images of the corresponding systems. The red color in the CCD counts represents the higher intensity of the PL signals. The brighter image of Fig. 2(d) compared to Fig. 2(c) was observed for the hybrid MR of TSDB/Au-NPs, indicating the enhancement of PL intensity. Figure 2(e) shows the LCM PL spectra of the pristine TSDB MR and the hybrid MR of TSDB/Au-NPs. The main LCM PL peak was observed at 490 nm. Considerable enhancement (about 100%) of the PL intensity of the TSDB MR was detected after the hybridization of Au-NPs. The UV-Vis absorption peak for the Au-NPs was observed at 510 nm (Fig. 1(e)), representing the SP absorption energy [14–17,34]. A laser beam incident on the TSDB/Au-NPs resulted in the SP resonance coupling between the Au-NPs and the TSDB MR because of the close match between SP energy of the Au-NPs and photon energy (490 nm) of the TSDB MR, as shown in Fig. 1(e). This contributed to the energy transfer effect in SP resonance coupling, resulting in the enhancement of PL intensity of the hybrid MR, as shown in Fig. 2(f) [25,35–38].
Fig. 2 (a) Optical microscopy, and (b) CCD images of the pristine TSDB MR. LCM PL mapping images of (c) the pristine TSDB MR, and (d) the hybrid MR of TSDB/Au-NPs (λex = 405 nm). The color scale bar in the middle represents the LCM PL intensity. (e) LCM PL spectra of the pristine TSDB MR (black curve) and the hybrid MR of TSDB/Au-NPs (red curve) (λex = 405 nm, laser power = 3 μW, exposed time = 47.5 ms). (f) Schematic illustration of energy transfer mechanism of SP of the hybrid MR of TSDB/Au-NPs.
The LCM Raman spectra of the pristine TSDB MR and the hybrid MR of TSDB/Au-NPs were compared to investigate the SERS effect. Figures 3(a) and 3(b) show LCM Raman mapping images of the pristine TSDB MR and the hybrid MR of TSDB/Au-NPs, respectively. The red color in the CCD counts represents the higher intensity of Raman signals. Increased Raman intensity for the hybrid MR of TSDB/Au-NPs was observed from the LCM mapping images, which qualitatively agrees with the increase of LCM PL intensities. Figures 3(c) and 3(d) show the LCM Raman spectra of the pristine TSDB MR and the hybrid MR of TSDB/Au-NPs, respectively, measured in the same experimental conditions of excitation laser = 488 nm, laser power = 500 μW, and exposed time = 100 ms. The representative Raman characteristics corresponding to the -CF3 and -C = C- aromatic modes from the TSDB molecules were detected near 1,196 (1,320) and 1,562 (1,618) cm−1, respectively, as shown in Figs. 3(c) and 3(d). The maximum intensity of the -C = C- aromatic stretching mode at 1,562 cm−1 for the pristine TSDB MR was 479 as units of photon counts. However, that for the hybrid MR of TSDB/Au-NPs (at 1,562 cm−1) was drastically enhanced up to 754 as units of photon counts. The other Raman characteristic peak corresponding to the -CF3 stretching modes near 1,196 cm−1 was also considerably enhanced for the hybrid MR of TSDB/Au-NPs, as shown in Fig. 3(e). A similar SERS effect was also observed for the identical systems when using the 514 nm excitation laser (laser power = 1.5 mW and exposed time = 250 ms), as shown in Fig. 3(f). Therefore, the static SERS effect was clearly observed for the hybrid MR of TSDB/Au-NPs, originated from the energy transfer of SP.
Fig. 3 LCM Raman mapping images (λex = 488 nm) of (a) pristine TSDB MR, and (b) hybrid MR of TSDB/Au-NPs. The color scale bar in the middle represents the Raman intensity. LCM Raman spectra (λex = 488 nm) of (c) pristine TSDB MR, and (d) hybrid MR of TSDB/Au-NPs. The numbers of 01, 02, 03, and 04 indicate the positions of the measurement of the spectrum for Figs. 3 (a) and (b). Simultaneous comparison of LCM Raman spectra of the pristine TSDB MR (black curve) and the hybrid MR of TSDB/Au-NPs (red curve) by using the (e) 488 nm, and (f) 514 nm excitation lasers.
The enhancement factor (EF) of the SERS signals is calculated by the following equation:
where ISERS is the SERS intensity, IRS is the Raman intensity under non-SERS condition, Nvol is the average number of molecules in the scattering volume for Raman measurement, and NSurrf is the average number of adsorbed molecules in the scattering volume for the SERS experiments [39]. The SERS EF of the -C = C- aromatic stretching mode at 1,562 cm−1 was estimated to be about 73,000 with λex = 488 nm, and 155,000 with λex = 514 nm. Table 1 lists the EFs of the other Raman modes with the different mapping conditions. Note that the static SERS effect in the hybrid MR of TSDB/Au-NPs was also observed for the use of the 633 nm excitation laser, as listed in Table 1. In Eq. (1) for the enhancement factor (EF) of the SERS spectrum, Nvol and NSurrf are calculated using the following equations: The chemical structure of the TSDB molecule was very similar to that of the 1,4-bis(3,5-bis(trifluoromethyl) cyanostyryl)-2,5-dibromobenzene (CN-TSDB) molecule.28 Therefore, we assumed that the number of molecules was estimated using the X-ray diffraction (XRD) data of CN-TSDB microplate. In the estimation, the molecular volume was 652.48 Å3, the molecular area was 94.08 Å2, the scattered area was 1.9635 × 107 Å2 (laser spot area), and the thickness of TSDB MR was 5,000 Å. The number density of the Au-NPs was about 395 ea / 100 × 100 nm2, and the number density of the TSDB molecules was 12,134 ea / 100 × 100 nm2. Table 1 shows that the SERS modes were clearly observed when the 488 nm excitation laser was used. Therefore, the waveguiding experiments of SERS were performed by using the 488 nm excitation laser.
Table 1. Enhancement factors (EFs) of Raman characteristic modes of TSDB with different excitation conditions.
3.2 Waveguiding characteristics of Raman and SERS modes
Figure 4(a) shows a schematic illustration of the SERS waveguiding experiment for the hybrid MR of TSDB/Au-NPs using the LCM system. The functionalized Au-NPs were homogeneously spin-coated on the glass substrate, and the TSDB MR was then attached to the surface of the Au-NPs. The detecting position was fixed at the end of the TSDB MR. The input focused excitation laser position was movable along the TSDB MR (indicated by the arrow direction), as shown in Fig. 4(a). The excitation laser beam, which was focused using the objective lens, was perpendicularly irradiated on the sample. Figure 4(b) clearly shows that the Raman characteristic signals corresponding to the C-Br, -CF3, and -C = C- aromatic modes were waveguided through the TSDB MR. The intensities of the output Raman spectra decreased with increasing propagation distance. The waveguiding of Raman signals implies the energy propagation through π-π intermolecular interaction [31]. We note that Raman signals are significantly stronger for the hybrid MR of TSDB/Au-NPs than TSDB MR, on the initial coupling location as well as along the MR. We attribute such superior performance of SERS waveguiding to the increased light coupling efficiency and the stronger scattering of the light due to the presence of Au-NPs comprising the hybrid MR.
Fig. 4 (a) Schematic illustration of the SERS waveguiding experiment for the hybrid MR of TSDB/Au-NPs using the LCM system. Output (i.e. waveguided) LCM Raman spectra through (b) pristine TSDB MR, and (c) hybrid MR of the TSDB/Au-NPs, with various propagation distances. The numerical values in the inset represent the propagation distance.
For reference, we performed the same waveguiding experiment of Raman modes by using polystyrene (PS) nanowire, as shown in Fig. 5. Figure 5(a) shows a microscopic image of PS NW with a schematic chemical structure of PS (inset). For the waveguiding experiment of Raman modes in PS NW, the excitation wavelength was 633 nm, the excitation power was ~5 mW, and the excitation time was 10 min. Figures 5(b) and 5(c) show the output LCM Raman spectra along the PS NW. The Raman characteristic modes showed almost no waveguiding effect along the PS NW, because of the amorphous characteristics of the PS sample. In this study, however, the waveguiding of the SERS signals was clearly observed through the hybrid MR of the TSDB/Au-NPs, as shown in Fig. 4(c). The SERS signals corresponding to the C-Br mode at 650 cm−1, the -CF3 mode at 1,196 (1,320) cm−1, and the -C = C- aromatic mode at 1,562 (1,618) cm−1 were successfully waveguided through the hybrid MR.
Fig. 5 (a) Microscopic image of polystyrene (PS) NW. Inset: Schematic chemical structure of PS. (b) Output (i.e., waveguided) LCM Raman spectra along PS NW. (c) Magnified output LCM Raman spectra along PS NW. Inset: Propagation distance.
Figures 6(a) and 6(b) show the magnification of the output LCM Raman spectra of the -C = C- aromatic mode at 1,562 cm−1 with various propagation distances (i.e. waveguiding characteristics of Raman modes) for the pristine TSDB MR and the hybrid MR of TSDB/Au-NPs, respectively. The intensities of the Raman characteristic modes of the hybrid MR of TSDB/Au-NPs were much higher than those of the pristine TSDB MR, because of the SERS effect. Table 2 lists the SERS EFs of the -C = C- aromatic and -CF3 stretching modes for the hybrid MR of the TSDB/Au-NPs. These SERS EFs were obtained from the experimental conditions with the power of 200 μW and the exposed time of 10 s, which differed from those for Table 1. The results of Figs. 6(a) and 6(b) indicate that the Raman and SERS signals can be waveguided through the crystalline TSDB MR.
Fig. 6 Magnification of output LCM Raman spectra of the -C = C- aromatic mode with various propagation distances along (a) pristine TSDB MR, and (b) hybrid MR of TSDB/Au-NPs. Output LCM Raman intensities of the C-Br (black markers), -CF3 (red markers), and -C = C- aromatic (blue markers) modes as a function of propagation distance along the (c) pristine TSDB MR, and (d) hybrid MR of TSDB/Au-NPs.
Table 2. Enhancement factor (EF) of Raman modes of -C = C- aromatic and –CF3 stretching modes using Eq. (1). The SERS EFs were obtained from the conditions of λex = 488 nm with the power of 200 μW and exposed time of 10 s.
Figures 6(c) and 6(d) show the output LCM Raman intensities corresponding to the C-Br, -CF3, and -C = C- aromatic modes as a function of propagation distance for the pristine TSDB MR and the hybrid MR of TSDB/Au-NPs, respectively. The LCM Raman intensities decreased with the increasing propagation distance between the input laser position and the detection position. The decay characteristics of waveguided Raman signals are fitted with the following equation:
where IR is the output LCM Raman intensity, IR0 is the proportional constant of the intensity, αR is the decay constant in units of μm−1, and x is the propagation distance of the incident Raman signal. The values of αR of the C-Br (at 650 cm−1), -CF3 (at 1320 cm−1), and -C = C- aromatic (at 1618 cm−1) stretching modes along the pristine TSDB MR were estimated to be about 0.152, 0.175, and 0.178 μm−1, respectively. The αR values of the C-Br, -CF3, and -C = C- aromatic stretching modes along the hybrid MR of TSDB/Au-NPs were estimated to be about 0.142, 0.162, and 0.163 μm−1, respectively, which are lower than those of the pristine MR. These results suggest that due to the SERS effect, the hybridization of Au-NPs has improved the decay characteristics corresponding to the Raman modes of the TSDB molecules. The diffusion length (l ≡ 1/αR) of the Raman signals corresponding to the -C = C- aromatic stretching mode was calculated to be about 5.62 and 6.14 μm for the pristine and hybrids MRs, respectively. The results suggest the enhancement of Raman signal waveguiding efficiency due to the SERS effect. It has elsewhere been noted that the diffusion length of exciton in the organic semiconductors is about 20 nm, which is much shorter than our diffusion length of Raman signals [40]. The diffusion length of the polariton for inorganic Al(Ga)N nanowire has been reported to be about 60 μm [41]. Therefore, it can be suggested that the propagation of polariton in the TSDB MR plays an important role in the waveguiding of SERS signal. The results in this study show the steady-state Raman intensity and its waveguiding efficiency of the TSDB MR were clearly enhanced by the hybridization with Au-NPs, because of the energy transfer effect of SP.4. Conclusion
Organic crystalline TSDB MRs were fabricated as active optical waveguiding system using a self-assembly method. The functionalized Au-NPs were synthesized using the Brust method, and hybridized with the TSDB MRs to realize the SERS effect. The nanoscale PL spectra, Raman spectra, SERS spectra, and their waveguiding characteristics of organic crystalline TSDB MR and hybrid MRs, were measured using a high-resolution LCM system. We observed the enhancement of the Raman and PL intensity of the TSDB MR after the hybridization of Au-NPs, because of the energy transfer of SP. Characteristic Raman signals of the TSDB molecule, such as the C-Br, -CF3, and -C = C- aromatic stretching modes, were observed at the output position, which demonstrates the successful performance of the waveguiding of Raman signals. The waveguiding of SERS signals through the hybrid MR of TSDB/Au-NPs was also observed using the LCM system. The waveguiding efficiency of the SERS signals was improved in comparison with that of the pristine signal, benefited from the stronger light scattering by Au-NPs. The waveguiding of multi-modal SERS signals with relatively narrower bandwidth can be used in the fields of nanophotonics, optical communications, and bio-sensing.
Funding
Center for Advanced Meta-Materials (CAMM), funded by the Ministry of Science, ICT and Future Planning as the Global Frontier Project (2014M3A6B3063710).
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