Hybrid systems combining individual optical, electronic and mechanical properties of two or more constituents have the potential of resulting in enhanced functional materials. Graphene, due to its zero bandgap, linear electronic dispersion and subsequently broadband nature, has attracted significant interest as a promising material in the optoelectronic arena. However, the ability to tune its optical properties at high fluences in a way that is compatible with large scale production without thermally induced damage has challenged adoption in industry. This work strives to overcome such issues through the development of a graphene-hybrid material. Here we combine the strong nonlinearity of graphene with the thermal stability of a DNA composite, while facilitating solution processing for large area surface coverage. The graphene-DNA composite results in a hybrid material exhibiting a tunable nonlinear response in the nanosecond and femtosecond regimes in addition to potentially possessing an extremely high damage threshold of > 170 J/cm2 when functionalized appropriately.
© 2014 Optical Society of America
Over the last decade, graphene has emerged as an ideal material in optoelectronic devices ranging from saturable absorbers for mode-locking lasers to optical modulators and photodetectors [1–7]. Recently, significant efforts have been made in the development of graphene-based materials for biotechnological applications such as biosensors, drug delivery, cell imaging and detection [8–11]. These efforts have necessitated improving graphene-compatible host materials exhibiting thermal and mechanical stability in addition to maintaining superior optical quality and response. With its unique optical and electronic properties as well as potential for chemical and physical modifications, graphene-based materials have been studied for their tunable optical properties and fast nonlinear response [12–14]. Pauli blocking has allowed the use of graphene as a saturable absorber for mode-locked lasers [6, 15–18]. In addition, functionalized graphene-based materials display broadband optical limiting due to two-photon absorption (TPA), reverse saturable absorption (RSA) and nonlinear scattering (NLS) [13, 19–29]. However graphene-composites have failed to withstand thermal effects resulting from high fluences, leading to development of a novel graphene-deoxyribonucleic acid (DNA) composite with a high damage threshold and tunable nonlinear optical platform to suit device function. The biopolymer used is formed from marine-derived DNA which has been explored as a bulk polymer material for photonic applications for the past decade . Numerous devices and applications containing the DNA biopolymer have been demonstrated; these range from purely electronic applications such as field effect transistors (FETs) to optical applications such as light emitting diodes (LEDs) and polymer electro-optic modulators [31–33]. In addition, work exhibiting the prospects of graphene-DNA hybrid materials have been discussed previously, whereby the DNA chemical structure has been shown to allow the attachment of particles, in this case graphene flakes, through either intercalation or major/minor groove binding, to the DNA double helix for enhanced material properties [34–36]. These factors have therefore led this attempt to use DNA as a host matrix for graphene. This graphene-based bio-polymer results in combining the nonlinear behavior of graphene and the thermal stability and robust nature of a DNA composite.
Here we review the fabrication of graphene-DNA composites with various concentrations, both in solution- and film-based composites. The material characterization and linear response across a broad wavelength band is shown. In addition, we demonstrate tunable nonlinear optical behavior at various wavelengths, concentrations and temporal regimes for the graphene-DNA-CTMA (g-DNA-CTMA) solution and spincoated samples. Additionally, we evaluate high-power robustness and long-term stability of these films.
The DNA used in this study is derived from salmon and purified by researchers at the Chitose Institute for Science and Technology in Japan. The protocol for developing this graphene-based biopolymer is similar to previously published papers for the DNA processing, with slight modifications to accommodate the addition of the graphene [37, 38]. To begin, 0.5 g of DNA is dissolved in deionized water at a concentration of 4 mg/mL and sonicated to a mean molecular weight of 200 kDa. 0.115 g of graphene is then added and the mixture is sonicated in timed intervals to avoid overheating the solution. Separately, 0.5 g of the cationic surfactant hexadecyltrimethyl-ammonium chloride (CTMA) is dissolved in deionized water at a concentration of 4 mg/mL.
The DNA-graphene solution is added drop-wise using a pipette into the CTMA solution forming a water insoluble compound of graphene-DNA-CTMA (g-DNA-CTMA). The solid content is then filtered and rinsed using an ethanol soxhlet-dialysis technique, and dried using similar procedures outlined in previous works . As a means of reference, the DNA-CTMA samples are developed in the same way without the addition of graphene and appear like a crystallized powder shown in Fig. 1(a). This dense substance becomes a powder-like, dark gray solute [Fig. 1(b)] with the addition of graphene. This new graphene-DNA material, therefore allows graphene to be integrated directly and effortlessly in various solvents. To create a liquid suitable for spin-coated films here, the g-DNA-CTMA is added to butanol in various amounts and sonicated until fully incorporated. The result is a stable suspension of g-DNA-CTMA as shown in Fig. 1(c) with minimal or no settling over months. The films in this work are developed using two different concentrations of the g-DNA-CTMA (18 mg/ml and 25 mg/ml) in butanol. These are spincoated at 4000 rpm for 40 s on quartz substrates and cured at 120°C for 15 min.
3. Material analysis
Images of these films are shown in Fig. 1(d) and (e) whereby it can be seen that the graphene flakes disperse over the surface with the 25 mg/ml sample exhibiting more graphene flake aggregation. A typical tapping mode atomic force microscopy (AFM) figure and height scans are also shown in Fig. 1(f) and (g) for the higher concentration sample. Before processing and sonication the commerically bought graphene flakes have laterally sizes of upto 50 μm. With sufficient sonication and processing these flakes get reduced in size. As shown, the lateral size of the resulting graphene flakes vary between 1 μm for the single flakes to around 6 μm for the clustered graphene flakes. The height of these flakes above the DNA-CTMA matrix ranges from 0.2–0.5 nm which would indicate fairly uniform films with minimal surface roughness.
Figure 2 shows the linear transmission and absorption coefficient of 25 mg/ml g-DNACTMA suspension in a 100 μm cuvette. The prominent features are due to the butanol which can be eliminated via background subtraction. Although the butanol exhibits absorption peaks at wavelengths > 2000 nm, these do not interfere in our range of interest. The films are unaffected by the butanol, which evaporates during the annealing process. As seen with the graphene film the transmittance for the solution decreases towards shorter wavelengths due to the graphene absorption peak. The corresponding absorption coefficient (α○) for these solutions are calculated using the transmittance and thickness of the cuvette and is shown in Fig. 2. It should be noted that no shift in the linear transmittance is seen when the graphene is added to the DNA-CTMA composite, indicating no chemical bonding between the graphene and the helical structure of the DNA. This confirms that the graphene flakes do not alter the chemical composition of the composite but “place” themselves in the strategic positions along the structure resulting in a stable solution whereby the graphene flakes refrain from settling or falling out of the solvent.
Linear measurements (both transmittance and reflectance) are then carried out on these spincoated samples using a “VW” configuration in a CARY spectrophotometer as shown in Fig. 3(a). The samples show no prominent absorption features over a wide range demonstrating the broadband nature of graphene. The thickness of the films measured using a surface profilometer varies from 140 – 160 nm, showing no remaining contributions from butanol. The calculated α○ for these films is shown in Fig. 3(b), where the graphene density is responsible for the dramatic increase in the absorption coefficient. The decrease in transmittance towards shorter wavelengths results from the high absorption of graphene at 230 nm due to π-π* transitions of the C=C bonds in the aromatic ring of graphene (as described above) . The abrupt features at approximately 800 nm are due to a detector change in the spectrophotometer, and are not indicative of film characteristics.
To investigate the quality and electronic structure of these samples, Raman measurements of the higher concentration film and the commercially purchased graphene flakes are performed using a Renishaw InVia Raman system at 514.5 nm with a 1800 l/mm grating and 50× objective. The Raman spectra of the 25 mg/ml graphene film is shown for regions with and without clustering in Fig. 4(a). The inset shows the Raman data for the graphene flake before its integration into the DNA composite. Carbon allotropes exhibit the three characteristic peaks as shown. The D peak located at ∼1357 cm−1 results from defects while the G peak (at 1581 cm−1) is the doubly degenerate phonon mode at the Brillouin zone center corresponding to the first order scattering of the E2g mode. The 2D peak at 2712 cm−1 is a two-phonon band which exists in the second order of zone boundary phonons in the Raman spectra of graphene. The D peak is prominent in both regions which could result from edge effects or ripples on the graphene flakes since the Raman beam spot size is approximately on par with the lateral sizes of the smaller graphene flakes. In regions of high flake aggregation the 2D peak is fully suppressed and broadened. Because the 2D peak is sensitive to effects such as stacking order along the c-axis and number of layers, such a spectra could be indicative of random, multistacking of graphene flakes in such regions. The Raman data carried out on a graphene flake (before embedding it in the composite) however shows all three peaks clearly with a low intensity D. The 2D although has a wider full-width half max (FWHM), its symmetric shape and relative intensity to the G peak indicates single to few layered stacked graphene flakes electronically decoupled from each other. The change in the spectra post-integration into the composite is primarilty due to the aggressive sonication of the graphene flakes in the composite which may be the cause of wrinkles across their surface. For the g-DNA-CTMA spincoated sample, the Raman spectra in the region consisting of non-aggregated, well dispersed graphene flakes exhibits a low intensity 2D peak due to the thickness of the graphene-composite layer. A closer look at the 2D peak in Fig. 4(b) reveals it is a wider, blue-shifted peak with respect to monolayer graphene (and the graphene flake) and can be fit with four Lorentzians each with a FWHM of 26 cm−1, characteristic of bilayer graphene [39–41].
4. Nonlinear measurements
Nonlinear transmission measurements are carried out using intensity-scan (I-scan) and z-scan. The I-scan setup is similar to the one described in  at λ = 1 μm with a pulsewidth of 3.63 ns, a pulse repetition frequency (PRF) of 10 Hz, a Rayleigh length (zr) of approximately 250 μm and beam radius (at 1/e2 of the maximum irradiance) of 15.7 μm. The z-scan setup consists of a Ti:sapphire laser source with a 125 fs pulse width, a 1 kHz repetition rate a beam radius of approximately 25 μm and Rayleigh length, zR, calculated to be 0.25 cm. In both experiments, the field will interact primarily with regions of well dispersed graphene, although some interactions with aggregated flakes will occur. The I-scan data is fit by numerically propagating a paraxial electric field through the material with thickness L using a finite-difference beam propagation method where time dependence is neglected and continuous-wave (CW) propagation is assumed. This is done since the Rayleigh length for the I-scan is smaller than the thickness L of the cuvette. The nonlinear coefficients are then obtained by calculating the transmittance using Isat and βeff as the fitting parameters and performing a nonlinear fit to the data obtained . The z-scan measurements however, are fit using a standard thin-film model with an intensity dependent absorption function of the form: [43–45]Fig. 2 and Fig. 3(b), ξ is the optical confinement at the graphene-DNA film (which ranges from 0.585 – 0.62 for the 18 mg/ml and 25 mg/ml film respectively and is calculated using transfer matrix-based calculations), Isat is the saturation intensity and βeff is the nonlinear absorption coefficient.
We first characterize the 25 mg/ml solution in a 100 μm cuvette in the nanosecond regime, and results are shown in Fig. 5(a). Initially a rise in transmission is observed at lower fluences, however with increased pumping nonlinear absorption effects dominate, resulting in decreasing transmittance with 3 dB point occurring at ≈ 55 J/cm2 (15 GW/cm2). Accounting for effects such as NLS, RSA and TPA, the effective nonlinear absorption coefficient βeff = 21.7 cm/GW with an Isat < 0.1 GW/cm2.
To understand the wavelength and pulsewidth dependent nonlinear absorption mechanisms the 25 mg/ml solutions in a 100 μm cuvette are now analyzed using femtosecond z-scan measurements at 640 nm and 1000 nm and results are shown in Fig. 5(b). For both the wavelengths, as the samples move through focus, a dip in transmittance occurs due to optical limiting effects. Although a slight dip is seen for the DNA-CTMA solution it is significant when compared after the addition of graphene. The dip is much “wider” at λ =640 nm than at λ =1000 nm, indicating a lower value of Isat. At 640 nm the proximity to the π-π* absorption peak of graphene results in a lower limiting threshold driven by this change in Isat. Since RSA requires a higher contribution from the linear absorption to keep the samples in an excited state the change from SA to RSA takes place at a lower fluence at 640 nm [25, 27]. At longer wavelengths however, the limiting threshold is offset by the slight increase in transmission due to stronger saturable absorption effects. This therefore leads to a modestly lower βeff of 11 cm/GW and significantly higher Isat of 13.5 GW/cm2 at 1000 nm when compared to βeff = 28 cm/GW and Isat = 1.7 GW/cm2 at 640 nm.
Further, as expected, the nonlinear absorption (NLA) coefficient obtained from Fig. 5(a) for nanosecond pulsewidths βeff = 21 cm/GW is higher than those obtained in the femtosecond temporal regime (βeff =11 cm/GW). In the nanosecond regime effects such as nonlinear scattering dominate through the creation of microbubbles due to solvent heating, therefore leading to a larger βeff.
To reduce such scattering effects, g-DNA-CTMA films, rather than solutions, are measured, also at 1 μm. At lower peak intensities of 226 GW/cm2, both samples undergo optically induced transparency (saturable absorption) when moved through focus as seen in Fig. 6(a) and (b). As the peak intensity is increased the saturable absorption effect in the lower concentration 18 mg/ml g-DNA-CTMA film increases giving us nonlinear coefficients βeff = 0.5 cm/GW and Isat = 59 GW/cm2. Unlike lower concentrations, an increase in graphene results in the appearance of a dip in transmittance at comparable input intensities. For the 25 mg/ml concentration at I○ = 589 GW/cm2, optical limiting starts to overtake saturable absorption effects resulting in a higher NLA coefficient of βeff = 12 cm/GW and subsequently higher Isat of 150 GW/cm2 seen in Fig. 6(b). For reference, the inset in Fig. 6(a) shows the z-scan measurement of the DNACTMA in the absence of graphene, exhibiting no change in nonlinear transmittance, demonstrating the g-DNA-CTMA nonlinear response is indeed due to the embedded graphene flakes.
Because nonlinear scattering effects are negligible in the femtosecond regime and at 1000 nm the graphene absorption peak does not contribute significantly to excited state absorption, the nonlinearity in these samples are considered to be primarily due to two photon absorption effects. This is apparent when compared to the solution counterpart in the nanosecond regime, whereby the larger βeff comprises of contributions from nonlinear scattering effects due to the longer pulsewidths. The higher contribution from NLS is obvious in the graphene-based solution since the absorption coefficient for the solution although much lower than the films gives a higher nonlinear absorption coefficient. Hence the addition of the solvent and the formation of microbubbles due to the long pulsewidths leads to the higher NLA coefficient in solution. Nonlinear coefficients for these graphene-DNA composites are summarized in Table 1.
Outside the tunability of nonlinear response, perhaps the most important feature of the graphene-DNA hybrid is how it is able to respond to thermal effects and withstand high optical fluences without damage. Previous studies testing laser damage thresholds in graphene materials (graphene-hybrids, single layer graphene, etc.) have been carried out [23, 47, 48]. In most of these works damage thresholds occurred in the femtosecond and picosecond regimes and were considerably low — although it was seen that the addition of graphene to composite-based samples did increase its resistance to damage. A DNA-CTMA composite has been shown to be resilient with a thermal conductivity of 0.6 W/mK compared to standard polymer composites such as PMMA which has a thermal conductivity of 0.12 W/mK . To test the damage thresholds of these films, we irradiate the 25 mg/ml film with 3.63 ns at a pulse repetition rate of 1 Hz at λ = 1000 nm. Here, the g-DNA-CTMA films are irradiated with 30 single shots from 0.02 J/cm2 to 178 J/cm2.
Initially no evidence of damage upto a fluence of 178 J/cm2 was indicated. The sample was then kept for a period of weeks and tested again. The damage threshold dropped dramatically which we attribute to humidity exposure as reported for DNA-CTMA previously . However, once the samples are heated above 100°C to remove the absorbed water and the measurements are repeated at λ = 1000 nm (again up to 178 J/cm2) the samples as before, do not exhibit any evidence of damage. Repeated measurements on these samples (without heating) over a period of weeks resulted in no further damage. The reason for this extended stability is currently not clearly understood and will require further analysis. Further work is also underway to understand the nature of the chemical interaction between the graphene and the DNA-CTMA structure.
In conclusion, a series of nonlinear measurements were conducted on graphene-DNA composite films and solutions to exhibit their nonlinearity in the nanosecond and femtosecond regime. The films are shown to exhibit strong optical transparency at lower fluences but result in optical limiting when irradiated at higher fluences. Further, it is shown that optical limiting effects were dominant in the nanosecond regime, which we attribute to thermally induced nonlinear scattering. However, optical limiting was also exhibited in the femtosecond regime when the samples are irradiated at higher fluences resulting in dominant optical limiting effects. Finally, nanosecond pulse testing showed the potential of a thermally stable composite with damage thresholds >170 J/cm2. Such materials demonstrating tunable and strong nonlinear optical properties imply great potential for optical limiting in systems vulnerable to laser damage as well as saturable absorbers to mode-lock without damaging at high fluences.
Part of this work was conducted while S.H. held a National Research Council Postdoctoral Fellowship. We would like to thank Dr. Matthew Sfier at the Center of Functional Nanomaterials at Brookhaven National Labs for assistance with the z-scan measurements. We would also like to acknowledge Jonathan Slagle at the Materials Directorate at Wright Patterson Air Force Base for his help in I-scan measurements. Special thanks to Xiaoze Liu for carrying out initial z-scan measurements on the solution samples. This project was partially supported by AFOSR LRIR 14RY09COR.
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