Three-dimensional freeform polymer microstructures containing graphene oxide (GO) nanosheets were fabricated via two-photon polymerization using Rose Bengal (RB) as the photoinitiator. To prevent photothermal damage in fabricated polymer structures and impede the reduction level in the GO nanosheets, the femtosecond laser dose was controlled as low as possible at the laser wavelength of 720 nm appropriate for superior two-photon absorption of RB. Furthermore, the GO nanosheets in the fabricated microstructure can be converted into reduced GO (rGO) by precisely increasing the laser power and controlling the desired reduction positions. In this manner, GO and rGO nanosheets contained in the designated areas of the fabricated microstructures can be achieved.
© 2015 Optical Society of America
Photopolymerization is a process which uses a combination of light with low molecular weight photoinitiators to trigger the polymerization reaction [1–3]. To fabricate three-dimensional (3D) microstructures, two-photon excited photochemistry can be utilized. Since multiphoton absorption is confined to the focal volume, photopolymerized structures with the desired 3D submicron features can be created [4–8]. This approach not only allows the creation of structures that cannot be constructed by conventional single-photon lithography, but also provides greater spatial resolution than other 3D microfabrication techniques. As a consequence, two-photon polymerization has attracted widespread interest owing to its potential use in fabricating intrinsic 3D microstructures with sub-diffraction limited spatial resolution . In such fabrication, a short laser pulse width and tight focusing are critical for inducing sufficient two-photon absorption (TPA) and for achieving high precision fabrication. For the work presented hereafter, a femtosecond laser acted as the excitation source. Previously, femtosecond laser 3D microfabrication has been demonstrated in polymer- [3,4,9,10], protein- , silica- , and metal-substrates [12,13] as well as photonic crystals .
Graphene-based materials have also attracted much interest recently due to their unique electronic, mechanical, and optical properties [15–18]. Graphene is composed of carbon atoms arranged in a two-dimensional (2D) regular hexagonal structure. Graphene oxide (GO) is low-cost and water-soluble compared to graphene, which has led to many applications [19–24]. However, GO contains oxygen functional groups that result in poor conductivity. Fortunately, GO can be reduced by light illumination and transformed into reduced graphene oxide (rGO); hence, the conductivity improves. Ultraviolet (UV) to near infrared laser have been used to reduce GO, and develop microelectronic patterns or devices on the GO film by direct laser writing or two-beam interference [25–28]. However, these studies have only formed 2D microelectronic components on the GO thin film deposited on a substrate; hence, this GO thin film is not strong enough to support the fabrication of 3D micro-electric devices. Applications of graphene-based materials in energy, environment, sensing, and biological fields often require the assembly of 2D graphene sheets into 3D architectures . Recent studies of 3D graphene architectures have produced promising outcomes, such as the integration of 2D GO nanosheets into 3D macroscopic structures (e.g., layered films and porous scaffolds) [28–31]. Further, 3D freeform polymer micro/nano structures doped with single-wall carbon nanotubes (SWCNTs) were fabricated by two-photon polymerization (TPP) . The SWCNTs improve the mechanical and electrical properties of the polymer structure.
In this study, 3D freeform GO nanosheets-doped polymer microstructures were successfully fabricated by TPP using Rose Bengal (RB) as the photoinitiator. To avoid the photothermal effect, which is enhanced by GO, destroys the fabricated polymer structure and impedes the reduction level in the GO nanosheets, the laser wavelength of 720 nm was adopted for superior TPA of RB; additionally, the laser power can be tuned to as low as possible with just the adequate dose for TPP processing. A fabrication solution consisting of hydrophilic trimethylolpropane triacrylate (WS-TMPTA) as the monomer, RB as the photoinitiator, triethylamine (TEA) as the co-initiator, and GO nanosheets as the dope was utilized. Moreover, the concentrations of the monomer, photoinitiator, and co-initiator are required to increase for enhancing the TPP efficiency. As a result, freeform 3D GO-doped polymer microstructures can be fabricated without serious photothermal damage and the reduction derivative rGO is also effectively restrained during the TPP processing. Subsequently, by precisely increasing the laser power and controlling the reduction positions, the GO nanosheets in the fabricated microstructure can be transformed into rGO. Consequently, a 3D polymer microstructure containing both GO and rGO nanosheets doped in the designated areas of the fabricated microstructures can be achieved by TPP and femtosecond laser direct writing.
2. Sample preparation and microfabrication setup
2.1. Sample preparation
GO nanosheets were prepared by a modified Hummers method . A graphite powder (SP-1, Bay Carbon, USA) as the starting material was further ground to decrease the particle size. First, the ground graphite powder (5 g) and NaNO3 (2.5 g, Merck, Germany) were added to concentrated H2SO4 (18 M, 115 mL, Wako, Japan) in an ice-bath. Then, KMnO4 (15 g, J.T. Baker, USA) was gradually stirred in. During this process, the temperature of the mixture was maintained below 20 °C. The mixture was then stirred at 35 °C for 24 h to allow oxidation to occur. After oxidation, deionized (DI) water (230 mL) was slowly added to the mixture, which was subsequently heated to 98 °C for 15 min. The mixture was then diluted with DI water to 700 mL and stirred for 30 min. The reaction was terminated by adding H2O2 (12 mL, 35 wt %, Shimakyu, Japan) while stirring at room temperature, after which it was subjected to multiple washings with DI water (3 × 500 mL). Finally, GO nanosheets were obtained by drying the precipitate of the slurry at 40 °C for 24 h.
WS-TMPTA (Sartomer, USA) was utilized as the reactive monomer. The GO nanosheets in DI water was sonicated for 2 h to avoid the aggregation of GO nanosheets and then mixed into the 60% (v/v) WS-TMPTA monomer solution by vortexing. Subsequently, TEA (i.e. co-initiator) and RB (i.e. photoinitiator) were sequentially added to the solution. Ultimately, the fabrication solution consisted of 5.0 mM RB, 0.6M TEA, 3mg/ml GO nanosheets, and 60% (v/v) WS-TMPTA monomer solution. Lastly, the fabrication solution was confined between a cover slip and microscope slide by using a 100 μm-thick spacer.
2.2. Multiphoton microfabrication system
A lab-made multiphoton excited fabrication instrument/microscope combined with two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) microscopy based on a single photon counting (SPC) module, and lifetime imaging microscopy (FLIM) based on a time-correlated single photon counting (TCSPC) module has been described in our previous studies [34–36]. Key components of our instrument include a femtosecond laser (Tsunami, Spectra-Physics, USA), an inverted optical microscope (Axiovert 200, Zeiss, Germany), galvanometer x-y scanner (6215H, Cambridge, USA), a triple-axis sample positioning stage (ProScanTMII, Prior, UK), a z-axis piezoelectric nano-positioning stage (Nano-F100, Mad City Labs, USA), an acousto-optic modulator (AOM) (23080-x-1.06-LTD, Neos, USA), photomultiplier tubes (H5783P, Hamamatsu, Japan), the TCSPC module (PicoHarp 300, PicoQuant, Germany), and a data acquisition card with a field-programmable gate array (FPGA) module (PCI-7831R, National Instruments, USA). The femtosecond laser source has a pulse width of less than 100 fs and a repetition rate of 80 MHz. To overcome the group velocity dispersion of the femtosecond laser through the AOM and the objective, an SF-10 prism pair (PC-TS-KT, Newport, USA) was used for optimizing the pulse width to within the wavelength region between 700 to 840 nm . The FPGA module was designed to perform a number of simultaneous tasks, including controlling the galvanometer scanner and the z-axis piezoelectric stage for 3D focal spot positioning; modulating the AOM for rapid on/off switching of the laser and pulse selection; and processing of the SPC signals. Selected experimental parameters such as laser power, scan velocity, imaging, and sample positioning can be adjusted by the use of a customized LabVIEW program (National Instruments, USA) and several electronics interfaces. In this manner, imaging with nonlinear optical signals (TPEF/SHG) and FLIM, as well as 3D microfabrication can be achieved.
3. Experimental results and discussions
3.1. Power and wavelength selection of femtosecond laser microfabrication with GOs
To fabricate polymer microstructures with GO, GO nanosheets were added into the fabrication solution. Unfortunately, preliminary experimental results revealed that the large size (near 1 μm) of GO sheets caused a serious photothermal effect that destroyed the fabricated polymer microstructure. To decrease the thermal damage, the size of the GO sheets and the laser power must be decreased to as small as possible. Moreover, the reduction of GO nanosheets can be retarded at relatively low laser power. Hence, the graphite starting material was repeatedly ground until the size of the GO nanosheets was less than 100 nm after the oxidation process. In the fabrication process, an adequate laser dose is required for the TPP processing. Herein, two approaches were adopted for improving the TPP efficiency in order to reduce the required laser power. One is that the laser wavelength selected was for the superior TPA of RB since the polymerization efficiency is dominated by the TPA of the photoinitiator. In the TPA spectrum measurement experiment, it was found that the highest excitation wavelength corresponding to the maximum value of the relative TPA cross-section of the RB was 710 nm at the available wavelengths of the femtosecond laser ; consequently, a fabrication laser wavelength of around 720 nm was adopted. The other approach was that the concentrations of WS-TMPTA, RB and TEA were all raised according to our previous experiments . The final fabrication solution contained 0.6 M of TEA, 5 mM of RB, and 60% (v/v) WS-TMPTA to provide adequate TPP efficiency. Lastly, it should be noted that the mixing process with GO nanosheets must be performed carefully in order to avoid GO aggregation in the fabrication solution.
Figure 1 shows the fabricated polymer microstructure without and with GO nanosheets under different fabrication laser powers at a fixed scan velocity of 250 μm/s and 1.3 NA objective lens. The designed structure has a base area of 20 × 20 μm2 and a thickness of 5 μm. As can be seen, without GO nanosheets in the fabrication solution (top row of Fig. 1), the minimum fabrication power for TPP processing is 0.67 mW. However, when the fabrication laser power exceeds 3.0 mW, the fabricated microstructure becomes blurred due to the proximity effect at strong laser powers and the diffusion of the activated co-initiator (TEA). An excess of activated TEA may lead to diffusion into non-illuminated regions and induce the polymerization of WS-TMPTA during the TEA lifetime . With GO nanosheets incorporated into the fabrication solution (bottom row of Fig. 1), the experimental results show that there are black flecks in the fabricating area, even when the fabrication laser power is low. One explanation is that the GO nanosheets absorb a portion of the fabrication laser energy, and are then transformed into rGO nanosheets that appear as black flecks. Another possibility is that the photothermal damage of WS-TMPTA resulted from some aggregative GO nanosheets. A higher laser dose may stem from the more-photon proximity effect and increased photothermal diffusion to develop undesired round areas. In the GO-doped microstructures with the fabrication laser powers of 3.0 ~10.0 mW, some dark round areas were developed. The dark may be due to the photothermal damage of WS-TMPTA and the photothermal reduction of GO nanosheets in TPP processing. Compared to the results without GO nanosheets, we can conclude that photothermal diffusion and damage occur with fabrication laser powers exceeding 3.0 mW. According to the experiments, when a 1.3 NA objective and scan velocity of 250 μm/s are used, the laser power at the wavelength of 720 nm must be controlled to within 1.0 ~3.0 mW to achieve TPP with GO nanosheets in the fabrication solution.
3.2. Reduction level control of GO nanosheets in the fabricated microstructure
In Sec. 3.1, the experimental results verified that the suitable fabrication power for this setup is between 1.0 and 3.0 mW. In order to demonstrate that the fabricated polymer microstructure contains GO nanosheets, Raman spectra were used to identify the specific spectral line of GO. In general, the characteristic Raman spectra of GO comprise two broad peaks at around 1300 cm−1 (D band) and 1600 cm−1 (G band). The G band results from the sp2 orbit attributable to the hexagonal arrangement of carbon, while the D band is the sp3 orbit of graphene due to defects of the graphene, such as oxygen functional groups. Figure 2 shows the Raman spectra of the fabricated GO nanosheets/polymer microstructures acquired by using a micro-Raman microscope (LabRAM HR, HORIBA Jobin Yvon, France). The top-left corner of Fig. 2 shows the bight-field image of the fabricated GO nanosheets/polymer microstructure with further localized laser reduction inside the red-cross pattern. The designed structure is a square with a base area of 40 × 40 μm2 and a thickness of 5 μm, for which the fabrication power was 1.0 mW at a 250 μm/s scan velocity. After the TPP process, non-polymerized fabricated solution was washed away. To further localize laser reduction in the red-cross pattern, the same scan velocity of 250 μm/s was adopted but with different laser powers. To analyze the reduction levels of GO, we calculated the ID/IG ratios from the Raman spectra. The red curve in Fig. 2 is the Raman spectrum of the fabrication solution containing GO nanosheets as an initial reference standard, i.e. without any GO reduction, for which the ID/IG ratio is 1.18. The black curve is the Raman spectrum of a fabricated GO nanosheets/polymer square with direct UV illumination for 24 h for the final reference standard, i.e. complete GO reduction, the ID/IG ratio of which lowered to 0.87. The remaining curves in Fig. 2 have the ID/IG ratios of 1.16, 1.08, 0.98, 0.91 and 0.91 for laser powers of 1.0, 2.0, 3.0, 5.0 and 10.0 mW, respectively. The ID/IG ratio decreases obviously with increasing laser power because GO defects can be removed by the laser illumination. Compared with the GO reduction at 1.0 mW, this value (1.16) was slightly smaller than that (1.18) of the fabrication solution, which demonstrates that few GO nanosheets are reduced at 1.0 mW laser power during the TPP processing. Furthermore, the ID/IG ratio (0.91) with the GO reduction at 10.0 mW is close to that (0.87) of the GO reduction with extended UV illumination. Based on the characteristic spectra of GO, Fig. 2 verifies that the fabricated squares contain both GO and rGO nanosheets. Consequently, the experimental results demonstrate that we are able to fabricate polymer microstructures containing GO nanosheets and also control the reduction level of GO in a designated area of the fabricated microstructure.
3.3. Freeform GO nanosheets/polymer microstructures
For 3D freeform microfabrication, sequential 2D bitmap file slices from a 3D CAD model are downloaded into the FPGA to control the laser illumination via the AOM. Since TPP is confined to the focal volume, 3D freeform polymer solid structures can be developed. Non-polymerized WS-TMPTA is washed away by DI water three times. A micro-spiral coil was fabricated on the top of a square with a base area of 50 × 50 μm2 and a height of 10 μm. The diameter of the micro-spiral coil and spiral tube are 10 μm and 1 μm, respectively. The distance between two adjacent axial fabrication layers is 0.1 μm and the scan velocity was also 250 μm/s during the fabrication processing. The TPEF images of Fig. 3 shows that the fabricated polymer micro-spiral coil containing GO nanosheets has a length of 50 μm, diameter of 10 μm, and pitch of 10 μm. In this study, 3D TPEL images were excited using a 0.13 mW, 100 fs laser at 730 nm and a scan velocity of 0.01 mm/s. Under this imaging condition, GO reduction can be avoided, while the TPEF signal is from the RB. The fabrication power is 1.5 mW in Fig. 3(a) and is 3.0 mW in Fig. 3(b). From the TPEF images, it can be clearly seen that the diameter of the spiral tube is slightly thicker with the fabrication power of 3.0 mW. Moreover, the experimental results indicate that the fabrication power must exceed 1.5 mW to fabricate 3D freeform polymer microstructures containing GO nanosheets due to the extensive scattering effect in complex fabricated structure. In Secs. 3.1 & 3.2, the fabrication power for a simple square was only 1.0 mW.
To further demonstrate microfabrication capability, a five-layer wooden-pallet structure was designed on the top of a square with a base area of 50 × 50 μm2 and a height of 10 μm. The stick pitch of the pallet is 2.4 μm and the diameter of each stick is 1.2 μm. The fabrication power was slightly increased to 2.0 mW and, as before, the scan velocity was fixed at 250 μm/s. Figures 4(a), 4(b), and 4(c) show the 2D bright-field, TPEF, and SEM images of the fabricated GO nanosheets/polymer microstructure, respectively. The zoomed-in SEM image in Fig. 4(c) shows that the fabricated microstructure is clear and intact, thereby demonstrating that 3D freeform GO nanosheets/polymer microstructures can be fabricated using a relatively low laser power to prevent the reduction of GO nanosheets in TPP processing. However, the structural quality of the GO-doped polymer microstructure is not good enough as that of pure polymer microstructure. The experimental results may be resulted from three possibilities: 1) GO nanosheets still induce photothermal damage to degrade the efficiency of TPP processing; 2) the mixture of GO nanosheets and WS-TMPTA is not uniform; and 3) the binding strength between GO nanosheets and WS-TMPTA is not strong enough. These reasons may result in lower structural quality for the fabricated GO-doped polymer microstructure.
3D freeform polymer microstructures containing GO nanosheets were fabricated by TPP processing. In order to prevent photothermal damage in the fabricated polymer structure and impede the reduction level in the GO nanosheets, the femtosecond laser power, with a scan velocity of 250 μm/s, was controlled to between 1.0 to 3.0 mW at the laser wavelength 720 nm for superior two-photon absorption of RB. Then, the laser power can be increased to 5.0 mW for effective reduction of GO in designated areas of a fabricated polymer microstructure. This approach cannot only create 3D freeform GO nanosheets/polymer microstructures without photothermal damage, but also define the GO and rGO nanosheets in designated areas of the fabricated structure by precisely increasing the laser power and controlling the reduction positions. Moreover, GO- and rGO-doped polymer microstructures might provide good conductivity and unusual optical properties that could be helpful in the design and fabrication of specific 3D micro-electronic devices and photonic crystals.
This work was supported by the Ministry of Science and Technology (MOST) in Taiwan with the grant numbers of MOST 101-2221-E-006-212-MY3, MOST 101-2221-E-006-213-MY3, MOST 103-2221-E-006-104, and Advanced Optoelectronic Technology Center of National Cheng Kung University.
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