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Abstract
We used ultrafast surface plasmon polariton (SPP) pulses that were focused into several tens of nm at the apex of a tapered metal tip to induce photoreduction of graphene oxide (GO), and we successfully fabricated nano graphene stripes with a minimum width of ∼200 nm. GO was reduced using about 1010 shots of SPP pulse irradiation. We evaluated the GO reduction with selective in situ coherent anti-Stokes Raman scattering measurements using spectrally focused SPP pulses.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene, which is a hexagonal two-dimensional arrangement of sp2-bonded carbon atoms, is expected to be used in the field of electronics because it has excellent electrical conductivity with a room temperature carrier mobility that is more than 100 times as large as that of silicon (Si) [1–3]. In particular, development of next-generation transistors using graphene as a channel material to replace Si is attracting attention [4].
Graphene oxide (GO) can undergo reduction by various methods to form reduced graphene oxide (rGO) [5]. The GO nanosheet is generally reduced by hydrazine, heat treatment [6] in a reducing environment, or by light irradiation using an intense light source such as laser or photocatalysts. In particular, laser pulse induced reduction is a promising technique because it permits micro GO/rGO patterning with minimal heat influence for substrates and the surrounding GO area [7–22]. In 2013, Gengler and co-workers performed transient absorption measurements using ultraviolet (uv) femtosecond laser pulses in solution and revealed that an ultrafast photoinduced chain reaction is responsible for the GO reduction. The reaction is initiated by uv femtosecond laser pulses that photoionized the solvent, liberating solvent electrons, which trigger the reduction [15]. However, such a phenomenon will only happen in a solvent that is able to produce and sustain solvated electrons.
Researchers have also achieved photoreduction of GO by femtosecond laser direct writing (FsLDW) in atmosphere without solvents nor photocatalysts. They demonstrated high resolution patterning of arbitrary shapes as compared with chemical and high-temperature thermal reduction. The main mechanism in photochemical reduction is due to the generated electron-hole pair contributing to the removal of the oxygen-containing groups [5]. The photon energy threshold for the photochemical process is known to be 3.2 eV (387-nm wavelength photon energy) [16]. Therefore, multi-photon absorption induced by focused near-infrared femtosecond laser pulses may contribute to direct electron-hole pair generation.
Table 1 shows typical examples of previous experiments of photoreduction of GO nanosheets by FsLDW without solvents. Low et al. used second-harmonic pulses of an Yb-doped fiber laser (λ0 = 515 nm). They measured rGO linewidths with different laser pulse repetition rates and pulse energies at the same scan speed of 100 mm/s. Higher repetition rates and higher pulse energies result in the gradual increase of the linewidth. The smallest linewidth they achieved was ∼1 µm [21,22]. Since accumulated laser pulse irradiation could induce temperature rise at the focused area, it is hard to conclude the major dynamics behind these photoreduction experiments of GO nanosheets by FsLDW without solvents. Zhang et al. used 80 MHz femtosecond Ti:sapphire laser pulses at a scan speed of 0.17 mm/s with a 100 nm scanning step length [9]. They observed modified patterns with sunken surfaces in the AFM images. The smallest line width was ∼500 nm and the sunken depth was ∼25 nm.
Table 1. Examples of the previous studies on graphene oxide reduction by femtosecond laser irradiation.
We demonstrated fabrication of sub-wavelength size rGO stripes with ultrafast surface plasmon polariton (SPP) pulses. Ultrafast SPP pulse nanofocusing to the sub-wavelength region by coupling femtosecond laser pulses to a tapered metal tip with a diffraction grating structure enables local spatiotemporal light excitation and electric field enhancement at the tip apex. Second harmonic generation, ultrafast nanospectroscopy measurement, and four-wave mixing generation at the tip apex were performed using ultrafast nanofocused SPP pulses [23–32]. Thus, we applied ultrafast SPP nanofocused pulses (λ0 = 800 nm), which were focused into several tens of nm at the apex of the tapered metal tip to form sub-wavelength-sized rGO stripes. Another merit of our method is that we can evaluate GO reduction by selective in situ coherent anti-Stokes Raman scattering (CARS) measurements using the same SPP nanofocus setup with a spatial resolution of ∼30 nm.
2. Theory and experimental setup
Figure 1(a) shows the experimental setup of photoreduction of a thin GO layer and selective CARS measurements using nanofocused ultrafast SPP pulses. We used a broadband Ti:sapphire femtosecond laser source (VENTEON, Laser Technologies GmbH) operated at a repetition rate of 150 MHz with a spectrum ranging from 620 to 1000 nm. The pulse duration was ∼8 fs (full width at half maximum, FWHM). The femtosecond laser beam was passed through a 4-f pulse shaper consisting of a computer-controlled liquid-crystal spatial light modulator (SLM, Cambridge Research & Instrumentation Inc.). After reduction of GO by nanofocused SPP pulses, CARS measurement was used to evaluate the reduction of the GO. When reducing GO, the Fourier transform limited pulse compensated for optics dispersion, and the plasmon response function of the tapered Au tip was applied [31].
Fig. 1. (a) Experimental setup of photoreduction of thin GO layer using nanofocused SPP pulses and selective in situ CARS measurements. G, gratings; SLM, spatial light modulator; HWP, half wave plate; ND, neutral density filter; OAP, off-axis parabolic mirror; PMT, photomultiplier tube. Inset is the spectrum of the femtosecond laser. (b) Experimental setup of photoreduction of thin GO layer using tip-aimed direct irradiation and selective CARS measurements by nanofocused SPP pulses.
A spectral focusing technique in CARS spectroscopy was used to select the single vibration mode in the GO structure [31–35]. In single-beam CARS using a femtosecond laser with multiple combinations of difference frequency excitation, the multiple vibrational modes are simultaneously excited, resulting in a broad CARS spectrum [36]. In the spectral focusing, the spectral phase modulation is given by Eq. (1):
The laser beam was focused on the diffractive grating consisting of eight grooves with a 900-nm period, a width of ∼400 nm, and a depth of ∼300 nm fabricated on the tapered Au tip. The distance between the tip apex and gratings was ∼10 µm. The diffraction gratings were designed by Finite-difference time-domain (FDTD) numerical calculations, and were fabricated by a focused ion beam. The spot size was ∼8 µm using a parabolic mirror with an NA of 0.4 and an incident angle of 90°. The radius curvature of the tapered Au tip was ∼30 nm. The excited SPP on the tapered Au tip propagates adiabatically to the tip apex. We controlled the distance between the tapered tip and the sample less than ∼4 nm with a shear-force control system using a quartz tuning fork. The scattered light at the tip apex was captured by a microscope’s objective lens with an NA of 0.55, at 40× magnification. The CARS lights were measured by a monochromator and a photomultiplier tube.
The sample in this study was a thin GO layer. Several ml of high-concentration GO dispersed aqueous solution (6.2 mg ml-1) was dropped onto an SiO2 substrate to spin-coat it at 3000 rpm for 5 min. The spin-coated substrate was then placed on a hot plate at 80°C and baked for 10 min. Using conventional Raman spectroscopy (RENISHAW plc), we confirmed the Raman shift of the thin GO layer at a D band of 1349 cm-1 and a G band of 1598 cm-1 in Fig. 2. A D-band is caused by graphene defects, and a G-band represents the stretching vibration of the sp2 bond [37–39]. In addition to these two bands, graphene shows a Raman shift at a 2D-band, which is related to the double resonance Raman scattering process via two phonons [38,39]. The G and 2D Raman peaks change in shape, position, and relative intensity with number of graphene layers. This reflects the evolution of the electronic structure and electron-phonon interactions. Doping upshifts and sharpens the G peak for both holes and electrons [40–41]. In general, the Raman peak ratio I2D/IG has been used as an index of the number of layers of graphene sheets [42]. I2D/IG is approximately >2, 1, and <1.0 for single, few-layer and multi-layers, respectively. Since the distortion of the structure is large in GO, the 2D-band signal is quite weak as shown in Fig. 2. It was reported that a 2D-band of ∼2700 cm-1 appeared in rGO after femtosecond laser irradiation on GO [17]. The absorption spectrum of the GO shows a peak at 298 nm due to the ${n} - {{\pi }^\ast }$ transition of the C = O bond. Thus, the GO is reduced to rGO by multiphoton absorption after femtosecond laser irradiation. In this study, we evaluated the reduction of GO in terms of the change of the CARS signal at a 2D-band in situ.
Fig. 2. Raman spectrum of a GO sample measure by a conventional Raman spectroscopy. In addition to D and G bands, a weak 2D-band is visible around 2700 cm-1.
3. Results and discussion
Before employing nanofocused SPP pulses, we have done photoreduction of a thin GO layer by simply focusing the broadband Ti:sapphire femtosecond laser pulses on the thin GO surface. The focused spot size diameter was ∼20 µm with an irradiation fluence of 0.20 mJ/cm2. The scanning speed was 10 µm/s. Clear D-, G- and 2D-band Raman peaks were observed by conventional Raman spectroscopy excited by cw 532-nm laser light. The peak intensity ratios between D- and G-bands and 2D- and G-bands were ID/IG = 0.84, I2D/IG = 0.041, respectively. Since the I2D/IG measured for a GO layer before laser irradiation was ∼0.018, accumulated pulse laser irradiation even at such a low laser pulse fluence can induce reduction of GO thin layers.
Figure 3 shows the selective CARS results for a rGO sheet measured after photoreduction by nanofocused ultrafast SPP pulses. We measured 2D-band (2700 cm-1) CARS spectra appearing around 590 nm (2700 cm-1) at 30 sec ( = 4.5 × 109 shots) and 60 sec ( = 9 × 109 shots). Change in 2D-band CARS signal intensity during photoreduction is shown in Fig. 3(b). The CARS signal intensity increased until the irradiation time of 50 sec, and it was saturated at the subsequent irradiation time. The origin of the 2D-band is the double resonance effect associated with the matching of the phonon vectors connecting the electronic states. Therefore, electrostatic force in layer interaction will easily fluctuates the Raman signal during reduction process since the layer overlap changes the electronic state.
Fig. 3. (a) 2D-band (2700 cm-1) CARS spectra of rGO after photoreduction by nanofocused SPP pulses with selective excitation at 2D-band at d > 5 µm (black) and at d < 4 nm (red and blue). (b) Change in 2D-band CARS signal intensity during photoreduction. The repetition rate of laser pulse irradiation was 150 MHz.
The I2D/IG ratio reached 0.091, which is higher than the ratio obtained by direct focused irradiation of the femtosecond laser pulse by more than a factor of two.
Figure 4 shows the experimental results of two-dimensional GO reduction after 50 sec of reduction irradiation ( = 7.5 × 109 shots) at each fixed spatial point. We scanned the tapered Au tip only in the horizontal direction of the figure every 50 nm. During the CARS measurement, we scanned the probe in both vertical and horizontal directions every 10 and 100 nm, respectively. Figure 4(a) shows the results of the 2D-band CARS measurements, and Fig. 4(b) shows the results of the topography measurements. The scale bars shown in Figs. 4(a) and 4(b) correspond to 200 nm. Figure 4(c) shows the 2D-band CARS signal (blue) measured along the line AB shown in Fig. 4(a). Figure 4(d) shows the topography (blue) measured along the line AB shown in Fig. 4(b). The red lines in Figs. 4(c) and 4(d) represent the results after 30 sec of plasmon pulse irradiation ( = 4.5 × 109 shots). The CARS signal spatial width was ∼200 nm (FWHM) and the topography width was ∼180 nm (FWHM) at an irradiation time of 30 sec. At an irradiation time of 50 sec, the CARS signal width increased to ∼210 nm (FWHM), and the topography width was ∼190 nm (FWHM). Since the radius of curvature of the tapered Au tip used in this experiment was ∼30 nm, the reduced area was considerably wider than the radius of curvature of the tip. If the photoreduction was caused through the photochemical process, the reduced GO should exhibit a similar size to the tip apex.
Fig. 4. (a) CARS image of rGO sheet for 2D-band (2700 cm-1) after 50 sec plasmon pulse irradiation. (b) Topography image of rGO of (a). Scale bar is 200 nm. (c) CARS signal (blue) measured along line AB as shown in (a). (d) Topography height (blue) measured along line AB as shown in (b).
Graphene has a high thermal conductivity (∼5000 Wm-1K-1). In the numerical calculation of the heat transfer of graphene using COMSOL multiphysics software, Subrina and Kotchetkovit reported that the maximum temperature reached ∼524 K at the focal point of 488-nm CW light (2 mW at a focal spot size of 50 nm in dia.) and heat propagated by ∼300 nm from the spot until the temperature dropped to 400 K [43]. In fact, the estimated averaged plasmon power at 800 nm in our experiment was less than 0.05 mW. Therefore, at such high thermal conductivity, the local graphene temperature cannot be elevated to 400 K. However, the thermal conductivity of GO is much lower than that of graphene, depending on the coverage of oxygen atoms. At 20% coverage of oxygen, the thermal conductivity of GO lowers to ∼8.8 W m-1K-1 [44]. Therefore, we speculate that in our experiment the SPP-induced reduction of GO was caused by a thermal process and that the rGO stripe width of ∼200 nm corresponds to the temperature distribution width while keeping temperature necessary for thermal reduction. Several researchers reported that the reduction begins at ∼100-130°C [45–47].
To confirm our hypothesis, we increased the local plasmon pulse intensity in a tip-aimed direct irradiation method since we could not increase the laser irradiation power. Figure 1(b) shows the experimental setup for the photoreduction of a thin GO layer using tip-aimed direct irradiation with an objective lens having NA of 0.2 and laser incident angle of 45°. We used the nanofocused SPP pulses for selective CARS measurements after the reduction process. Further reduction of a GO layer during CARS measurement was negligible.
Even when using the tip-aimed direct irradiation method, the local plasmon was significantly enhanced (>103) at the tip apex. Therefore, stray excitation of a sample around the tip apex by focused femtosecond laser pulse was much lower. We estimate that the plasmon intensity at the tip direct irradiation with a focused laser peak intensity of ∼8.3 GW/cm2 is ∼2 times as large as that in the SPP nanofocusing method with an incident laser peak intensity of ∼5.2 GW/cm2 on the grating structure.
Figure 5 shows the results of CARS measurements when reducing GO by tip-aimed direct irradiation. The direct irradiation time was 60 sec ( = 9 × 109 shots). The CARS signal width was ∼300 nm (FWHM), the topography width was ∼280 nm (FWHM), and the maximum depth of topography was ∼30 nm. The rGO stripe width is larger than that in Fig. 4 because the local temperature under the tip apex was higher than that in the experiment for Fig. 4, and the temperature distribution width while keeping temperature >100-130°C became wider.
Fig. 5. (a) CARS spectrum of rGO after tip-aimed direct irradiation with selective excitation at 2D-band at d > 5 µm (black) and at d < 4 nm (red and blue). (b) CARS signal (blue). (c) topography after 60 sec of reduction irradiation.
In our GO reduction experiments with ultrafast plasmon pulses, the major photoreduction dynamics would be multi-photon absorption at 800 nm and following thermal dissociation process induced by 10-fs plasmon pulses irradiated at 150 MHz. The minimum reduction size was limited to ∼200 nm by the heat conduction. Therefore, nanofocused SPP pulses act as a delta-like heat source for GO sheets when the thermal reduction process is more dominant than direct photochemical reduction. The rGO stripe size is determined by the temperature distribution formed by heat conduction in GO/rGO. Therefore, if the radius of curvature of the tapered tip, that is, the plasmon size is much smaller than the region size above the GO reduction critical temperature (100-130°C), the radius of curvature does not affect the rGO stripe size. On the other hand, if we operate GO reduction at lower plasmon pulse powers so that the elevated temperature reaches just the threshold of reduction only just under the tip apex, we could achieve much smaller reduction sizes with the present scheme. Under these well-controlled GO reduction conditions, the radius of curvature will determine the rGO stripe size.
Currently, Hada and co-workers determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations [48]. If one can find a way to enhance the photoexcitation and suppress the thermal reduction, the optimum nano GO structures for the intended applications can be prepared via FsLDW.
4. Conclusion
We processed thin nanographene stripes by reducing GO with nanofocused SPP pulses using a tapered Au tip and evaluated the GO reduction with selective in situ coherent anti-Stokes Raman scattering measurements using spectrally focused SPP pulses. The GO reduction was evaluated by the amplitude of the selective CARS signal at a 2D-band in situ. At a laser wavelength of 800 nm, it is difficult to induce GO reduction through a pure photochemical process. However, due to the low thermal conductivity of GO, we confined the photoinduced thermal energy in a small area and created a sub-wavelength rGO structure with multi-photon absorption induced by ultrafast plasmon pulses. When we carefully optimize the irradiation laser intensity to target a local temperature just above the critical temperature necessary for GO reduction, we will be able to create a smaller rGO stripe size which is determined by the radius of curvature of a tapered Au tip with our processing scheme.
Appendix A : Selective CARS measurement with a spectral focusing scheme
To realize selective CARS measurement with a spectral focusing scheme, we programmed the spectral phase of the femtosecond laser pulses by a 4f pulse shaper consisting of 600-line gratings, Fourier lenses, and a liquid crystal spatial light modulator (SLM) (Cambridge Research & Instrumentation Inc.). This SLM was composed of two liquid crystal panels. Each panel has 128 pixels of which voltage was independently controlled by a computer-controlled external circuit. Then, we can arbitrarily shape the femtosecond laser pulse both in amplitude and phase.
Figure 6 shows the conceptions of spectral focusing that indicate instantaneous frequency of excitation pulse and CARS using broadband pump pulse, Stokes pulse and probe pulse. We applied a linear frequency chirp in the spectral range and a delay at a lower frequency than the branched frequency ${\omega _b}$. When applying the proper delay based on the target Raman mode ${\mathrm{\Omega }_\textrm{R}}$, selective excitation of an arbitrary vibration mode is possible [31–35]. Figure 7 shows a spectral amplitude and phase when using the spectral phase function described in Eq. (1) for selective CARS measurement of the graphene 2D-vibrational mode. We did not shape the spectral amplitude of the laser pulse except for filtering the both sides of the broad band spectrum. The spectral phase function corresponds to that in Eq. (1) with ${\omega _b} = 2.59 \times {10^{15}}\;\textrm{rad}/\textrm{s}$ (725 nm), ${\omega _0} = 2.41 \times {10^{15}}\; \textrm{rad}/\textrm{s}$ (780 nm), ${\omega _{max}} = 2.73 \times {10^{15}}\; \textrm{rad}/\textrm{s}$ (690 nm), ${\omega _{min}} = 2.07 \times {10^{15}}\; \textrm{rad}/$s (910 nm), $\varphi^{\prime\prime} = 750\; \textrm{f}{\textrm{s}^2}$, ${\mathrm{\Omega }_\textrm{R}} = 2700\; \textrm{c}{\textrm{m}^{ - 1}}$. The shaped laser pulse shape with this phase modulation can be obtained by Fourier transform as shown in Fig. 8. The frequency beat induced by the two chirped pulse components designed in the spectral focusing appears as a fast temporal modulation during the early part of the laser pulse.
Fig. 6. The conceptions of spectral focusing that indicate instantaneous frequency of the laser pulse and CARS using broadband pump pulse, Stokes pulse and probe pulse.
Fig. 7. Spectral amplitude and phase which we intended to use to achieve the spectral focusing. The spectral phase corresponds to the spectral phase function described in Eq. (1) for selective CARS measurement of the graphene 2D-vibrational mode. We did not shape the spectral amplitude of the laser pulse except for filtering the both sides of the broad band spectrum.
The spectral phase shown in Fig. 7 must be wrapped to the phase ranging from $- \pi $ to $+ \pi $ when we establish the phase mask with the actual SLM. Figure 9 shows the pixelized spectral phase values applied on the SLM.
Fig. 9. Actual spectral phase applied to 128-pixel-SLM to achieve the spectral phase shown in Fig. 7.
Funding
Ministry of Education, Culture, Sports, Science and Technology Quantum Leap Flagship Program (MEXT Q-LEAP), Grant Number JPMXS0118067246.
Disclosures
The authors declare no conflicts of interest.
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