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One- and two-dimensional plasmonic nanostructures can be fabricated using nanoscale tensile stress. A polymer layer, coated with a thin metal film, is exposed to an interference pattern produced by ultraviolet laser beams. Crosslinking is induced between the polymeric molecules located within the bright fringes. This process not only increases the refractive index but also reduces the polymer layer thickness. Corrugations occur on the continuous thin metal film due to the nanoscale stress in the polymer layer. Thus, a periodic nanostructure of area 3 × 3 mm and depth 50 nm is created both in the polymer and metal films with excellent homogeneity and reproducibility. This method enables direct writing of a large-area plasmonic nanostructure at low cost which can be used in the design of optoelectronic devices and sensors.
© 2013 Optical Society of America
1. Introduction
Plasmonic structures have been investigated extensively both theoretically and experimentally, including design [1,2], fabrication [3,4], characterization [5,6], and application [7–9]. To date, it remains a challenge to fabricate a large-area uniform plasmonic nanostructure at low cost, and in particular implemented on organic films. Inevitably, most fabrication techniques involve either high-temperature, wet processes, or expensive processes, such as solution-processable methods [10,11], holographic fabrication [12], electron beam lithography [13,14], reactive ion-beam etching [15], and nanoimprinting [16,17]. These are not suitable in producing organic optoelectronic devices or in mass production of these devices.
Recently, we introduced a nano-patterning method using interference crosslinking to produce nanoscale periodic structures into conjugated polymers [18]. In this work, we apply nano-patterning to thin metallic films coated on conjugated polymer substrates and demonstrate a direct writing of plasmonic nanostructures with the aid of nanoscale stress. Crosslinking is known not only to increase the refractive index but also reduce the thickness of the polymer layer [19,20] forming a surface-relief structure in polymer films. It will yield nanoscale stress distribution in the polymer film, stretching the adjacent metal film to form corrugation. This one-step fabrication technique potentially enables the development of new optoelectronic devices.
2. Experimental details
2.1 Materials and the interference crosslinking technique
In our experiment, a typical light-emitting conjugated polymer, poly [(9,9- dioctylfluorenyl-2,7-diyl) -alt- co- (1,4- benzo-{2,1’,3}- thiadiazole)] (F8BT from Sigma Aldrich, USA), is employed to demonstrate the direct writing method. The polymer is spin-coated onto a 180-nm-thick indium-tin-oxide (ITO)-coated glass substrate of area 20 mm × 20 mm and thickness 1 mm at a speed of 1800 rpm. The concentration of the F8BT solution in chloroform is 20 mg/ml and the thickness of the F8BT film is about 120 nm. A thin film of metal (gold or aluminum) with a thickness of 30 nm is evaporated onto the F8BT layer, as illustrated in Fig. 1(a).
Fig. 1 Schematic of the nanoscale tensile stress induced plasmonic nanostructure. (a) ITO glass substrate coated with F8BT and metal layers. (b) The sample is exposed to a two-beam interference pattern with an included angle α1, forming a pattern with period Λ1. The nanoscale tensile stress distribution in the polymer layer is marked by the red arrows. The length of the red arrows denotes the magnitude of the nanoscale stress. (c) The plasmonic nanostructure after interference crosslinking. The insets depict uncrosslinked (left) and crosslinked (right) polymer molecule networks
During fabrication, a He-Cd laser (Kimmon Koha Co., Ltd.; model: IK3301R-G) operating at 325 nm is used as the ultraviolet (UV) light source for interference. The sample is then exposed from the glass substrate side for 15 min to an interference pattern generated by two UV beams with a diameter of 3 mm and a total power of about 30 mW. The period Λ1 of the interference pattern can be controlled by changing the included angle α1 between the two beams in Fig. 1(b). Crosslinking is initiated between polymeric molecules located within the bright fringes, whereas almost no crosslinking occurs within the dark fringes. This process not only changes the refractive index but also decreases the thickness of the polymer film because of the higher structural density.
2.2 The nanoscale tensile stress induced plasmonic nanostructures
During interference crosslinking, a periodical surface-relief structure in the F8BT layer patterned on the interference pattern is created through selective reduction of the film thickness. Thus, a nanoscale tensile stress distribution appears near the interface edge between the polymer and the metal layers as shown in Fig. 1(b), which has the same period with the interference pattern. The metal film will be stretched by the nanoscale tensile stress, forming a periodic nanostructure with nonuniform thickness. The one-dimensional (1D)/two-dimensional (2D) structure can be fabricated flexibly by a single/multiexposure process [21].
3. Results and discussion
3.1 Large-area plasmonic nanostructures with homogeneity and reproducibility
Atomic force microscopic (AFM) images of the gold nanostructure in Fig. 2 show large-area homogeneity formed by interference crosslinking. The modulation depth of the structure reaches to 50 nm; that is, 42% of the total thickness of the F8BT layer. The period of the nanostructure is about 350 nm (α1 = 55°). The area of the plasmonic nanostructure based on interference crosslinking is roughly equal to that of the interference pattern.
Fig. 2 AFM images of the one- and two-dimensional gold nanostructure achieved using interference crosslinking. Λ1 = 350 nm. Scale bar in the insets, 100 nm.
3.2 Effects of the metal ductility and the adhesion of the metal/polymer interface
If metal ductility is poor or the modulation depth is too high, the thin metal film will understandably crack. For experimental comparison, a 30-nm-thick aluminium (Al) film was evaporated on the F8BT layer and the same fabrication procedure applied to the sample. The resulting structure was imaged in Fig. 3(b) and shows an Al film with breaks and jagged edges that formed because of Al’s poor ductility compared with gold [22]. Besides, it is also well known that gold can diffuse in polymers during deposition. As a consequence, adhesion properties between polymer and metal are better in the case of gold [23]. So, delamination at the Al/polymer interface can induce cracks due to a lack of adhesion. Thus, a sponge-like structure is formed in the Al film which is quite different from the gold nanostructure in Fig. 3(a). Moreover, curing also induces a change of the elastic modulus of the polymer, which has an effect on the distribution of strain in the layer. Stress profiles on different sites inside the sample can induce wrinkling (or cracks).
Fig. 3 Nanostructures formed by metals with different ductility. (a) Gold nanograting. (b) Al nanograting.
3.3. Spectroscopic characterization of the plasmonic nanostructure
The spectroscopic responses of the gold sample before and after crosslinking are measured using a spectrometer (Maya 2000 Pro, Ocean Optics, USA). Figure 4 shows that the uncrosslinked F8BT film has an extinction spectrum (solid blue line) centred at about 483 nm and a photoluminescence (PL) spectrum (solid red line) centered at about 543 nm. After a 15-min UV exposure to activate crosslinking, the PL spectrum was reduced 50% in amplitude (open red circles), implying that UV exposure of the polymer film did not degrade all polymer molecules. This conclusion is consistent with the reported results [18] that took into consideration the absorption of the glass substrate. Furthermore, electrically-driven light emission can still be observed in the crosslinked sample. Thus the structure can be integrated into optoelectronic devices. Moreover, the waveguide mode and the plasmon resonance of the gold nanogratings can be observed in the extinction spectra of the crosslinked sample. Two obvious extinction peaks can be observed in both transverse electric (TE) polarization (solid blue circles) and transverse magnetic (TM) polarization (open blue circles). One is located at about 485 nm corresponding to the absorption of F8BT, whereas the other is located at about 560 nm corresponding to the waveguide mode. It is consistent with the results reported in Ref [24]. A systematic investigation of the influence of the periodicity of the corrugation on the wavelength of the waveguide mode is reported in our previous work [25]. In the TM-polarized (polarized perpendicular to the nanowires) spectrum, a broad plasmon resonance peak can be observed around 600 nm of bandwidth 100 nm. The inset in Fig. 4 shows the measuring setup.
Fig. 4 Extinction and PL spectra of the uncrosslinked/crosslinked gold structure shown in Fig. 1(a) and (b). The inset depicts the measuring setup.
Figure 5 demonstrates the extinction spectra of the waveguide mode with changing the incidence angle of a white light beam from a tungsten halogen lamp (Ocean Optics; model: HL-2000). The experimental setup of the measurement has been detailed in our early work [25]. The waveguide mode is observed at 558 nm and 567 nm at normal incidence for TM and TE polarization, respectively, indicating that the effective refractive index of the nanostructure for the TM wave is slightly smaller than that for the TE wave. The waveguide mode is split into two branches as incidence angle is increased. The angle-resolved tuning rate is about 4.5 and 4.3 nm/degree for TM and TE waves, respectively. The bandwidth of the waveguide mode and the plasmon resonance measures to be about 10 nm and 100 nm, respectively. These characteristics indicate the excellent quality of the nanostructure based on the direct nanofabrication technique.
Fig. 5 Angle-resolved tuning properties of the waveguide mode of the gold nanogratings based on interference crosslinking. (a) TM polarization. (b) TE polarization. The angle changes from 0 to 24° with a step of 2°. The insets show the enlarged view of the dip/peak in the spectra.
It should be noted that the peaks in the extinction spectra change into dips when the incidence angle is larger than 20° for the TM polarization. In brief, the anti-crossing behavior between the waveguide mode and the plasmon resonance can be observed for large incidence angle. The anti-crossing effect due to the Fano resonance as a result of the coupling between localized surface plasmon of the gold nanostructures and waveguide modes has been discussed in detail [26, 27]. The most important feature of this kind of behaviour is a dip observed in the extinction spectrum, implying enhanced transmission. A transition is thus generally observed from an extinction peak to a dip for the emergence of the localized surface plasmon, indicating the happening of the coupling process. Therefore, the change for the waveguide-mode feature from a peak to a dip in the extinction spectra in Fig. 5(a) is a convincing verification of the excitation of the localized surface palsmon resonance in the nanostructured gold films. Furthermore, for larger incidence angle, the directly transmitted light wave becomes weaker due to reflection and plasmon resonance scattering. In this case, the directly transmitted light wave is comparable with waveguide mode. Therefore, they counteract each other completely, changing the spectra from peaks to dips [28].
4. Discussions
Figure 6 shows a scanning electron microscopic (SEM) image of the cross-section of the sample, where clear gold/polymer/ITO interfaces can be observed, validating the fabrication technique. Moreover, the gold film is almost continuous, only some cracks exist in the gold film as shown in the left inset of Fig. 6. Thus, the broad plasmon resonance is mainly caused by the inhomogeneous thickness of the gold film. The thickness of gold film is larger where the polymer layer is thicker and, one the other hand, gold film is quite thin inside the grooves as shown in the right inset of Fig. 6. If the modulation depth or period of the grating is very large, the subsequent tensile stress will become large enough to crack the gold film forming gold lines. So, using thicker polymer film or larger period grating, the response of the plasmon resonance may be improved. Then the plamonic grating may be used for sensing applications as our previous work [29].
Fig. 6 Cross-sectional SEM image of the sample shown in Fig. 2(a). The left inset presents the top view of the sample. The red circle denotes the cracks. Scale bar = 1 μm. The right inset is the enlarged view of the sample. White lines denote grating ridges. The thickness of the grating ridge/valley is roughly indicated by red/blue arrows. Scale bar = 400 nm.
The proposed fabrication technique might be used in the design of metal electrode of electrically pumped polymer lasers, which is one of the motivations of this fabrication work. It is a challenge to fabricate metal electrodes on the nanograting without disturbing the function (or mode) of the nanograting. Inhomogeneous thickness of the metal film may induce spatial modulation of charge injection and recombination in the active layer, which might be useful for exploiting organic light-emitting devices (OLEDs), which is difficult to achieve by conventional evaporation technique. Besides, this kind of structures can be used in enhancing the output coupling efficiency, for example, when a polymer layer is coated onto the gold nano-grating structures to produce OLEDs.
5. Conclusions
We introduced a direct writing technique for fabricating plasmonic structure on conjugated polymer film using interference. Different patterns generated by interference patterns can be “recorded” conveniently on the thin metal film. This direct nanofabrication technique provides an alternative means to pattern plasmonic devices.
Acknowledgments
The authors acknowledge the 973 Program (2013CB922404), the National Natural Science Foundation of China (11104007, and 11274031), Beijing Natural Science Foundation (1132004, 4133082), the Beijing Educational Commission (KM201210005034), and Beijing Nova Program (2012009) for the financial support.
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