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This paper reports on the emission characteristics of amino-functionalized graphene quantum dots (af-GQDs). We employed the variable stripe length method to measure the net optical gain of af-GQDs. Photoluminescence emission was enhanced through the efficient confinement of photons using an optical resonator. The two-dimensional resonator is made up of a cholesteric liquid crystal (CLC) reflector to enable the redistribution of spontaneous emission from the af-GQDs. The proposed method was shown to increase the intensity of peak emission to more than three times that of the reference sample without a CLC reflector. The peak emission intensity of af-GQDs in the optical resonator grows exponentially with an increase in excitation energy. These results demonstrate the feasibility of two-dimensional optical amplifiers based on CLC reflectors.
© 2017 Optical Society of America
1. Introduction
Fluorescent materials are crucial to the further development of opto-electronic devices. Quantum dots (QDs) have risen as fascinating materials because of high emission performance originating from the quantum confinement effect [1–5]. Newly emerging luminescent graphene quantum dots (GQDs) have been shown to possess many extraordinary advantages. Compared to conventional semiconductor QDs containing heavy metals, GQDs are superior in terms of low cytotoxicity, chemical stability, biocompatibility, hydrophilicity, and low cost [6–10]. GQDs also have unique photoluminescence properties including high photostability and tunable emission. The emission spectrum spreads over a broad range, from the visible to the near-infrared region, depending on their excitation wavelength and structure [11–17].
GQDs comprise a two-dimensional graphene sheet of nano sp2 carbon clusters decorated with characteristic functional groups on the basal plane and lateral edge, which makes GQDs soluble in most hydrophilic solvents. Functional groups play the role of potential well, providing quantum confinement that can lead to discrete quantum states [18,19]. The radiative recombination of localized electron-hole pairs in sp2 domains causes photoluminescence emission in GQDs [19–21]. The energy band gap depends on the size and shape of the sp2 domains [18,19], the degree of oxidation in carbonaceous structures, and the arrangement of functional groups [22–25]. The band structure can be modulated by adjusting the size of sp2 clusters or through surface chemical functionalization to enhance the tunability of optical properties. Amino-functionalized graphene quantum dots (af-GQDs) have been proposed as excellent light emitters with high luminance quantum yield and wide photoluminescence tunability [26]. The graphene nanostructure is edge-terminated by a primary amine, which drastically alters the electron cloud distribution of af-GQDs due to its strong electron-donating ability. The strong orbital resonance of amino groups with a graphene core is the basis of the high quantum efficiency and optical tunability.
In addition to chemical synthesis techniques, the inclusion of cavities is another powerful strategy by which to enhance the photoluminescence of af-GQDs by increasing the rate of spontaneous emission [27]. According to Fermi’s golden rule, the rate of spontaneous emission depends on the inherent properties of the luminescent materials and the density of electromagnetic modes in a given environment [28]. An optical resonator can be used to modify the density of optical modes by sustaining radiation at particular frequencies while inhibiting radiation at others, thereby enhancing emission of resonant modes. The ability to boost the spontaneous emission rate would promote the development of high-efficiency light emitters.
The most common type of optical resonator is the Fabry-Perot cavity, consisting of two facing flat or spherical reflecting surfaces, which can be metal mirrors or dielectric distributed Bragg reflectors. In this work, we investigated the photoluminescence properties of af-GQDs in a capillary tube surrounded by cholesteric liquid crystal (CLC), which acts as the reflector in a two-dimensional optical resonator. CLCs comprising nematic liquid crystal (LC) and chiral dopants present an ordered helical structure with periodically modulated index of refraction, producing a selective reflection band for circularly polarized light with the same handedness as a cholesteric helix [29]. The central wavelength of the reflection band is proportional to the intrinsic helical pitch length. CLCs possess a number of advantages over traditional reflectors. The LC directors are self-organized within a one-dimensional periodic photonic structure and the reflection band is easily tuned according to the concentration of chiral dopant and external stimuli. Through self-assembly and flexibility, CLCs can be used to create resonators of various shapes and dimensions. In this study, we injected CLC into the gap between two coaxial capillary tubes to encircle the aqueous dispersion of af-GQDs in the inner tube. This increased by a factor of 3 the intensity of radial emission in the resonant mode, compared to its non-cavity equivalent.
2. Experiment
Three types of graphene quantum dots with different surface functional groups were purchased from Conjutek Co., Ltd to enable a comparison of emission efficiency. Under the same excitation conditions, the af-GQDs exhibited the highest photoluminescence intensity. Thus, we used af-GQDs to study the enhancement of photoluminescence in optical resonators. A bottom-up method involving the pyrolysis of glutamic acid was adopted for the synthesis of the af-GQDs. The concentration of af-GQDs in water was 20 mg/ml with an appropriate surfactant. Native solution (i.e., without dilution) was used throughout the study. The morphology of the af-GQDs was investigated using a JOEL JEM-2100F high-resolution transmission electron microscope. Functional groups attached to af-GQDs were studied using a Nicolet 460 Fourier transform infrared spectroscopy in attenuated total reflectance mode. The UV-Vis absorption spectrum was characterized using a Hitachi U4100 spectroscopy.
As shown in Fig. 1, we employed the variable-stripe-length (VSL) method to measure the net optical gain in order to quantify the emission efficiency of af-GQDs [30]. The aqueous dispersion of af-GQDs was poured into a rectangular glass cell with an inner width of 5 mm and inner length of 10 mm. The sample was optically excited by a frequency-tripled Nd:YAG pulsed laser beam with wavelength of 355 nm, pulse duration of 8 ns, and repetition rate of 10 Hz. A half-wave plate and polarizer were used to alter the excitation energy, which was measured by a powermeter via a beam splitter. The excitation light beam was focused into a 0.5 mm wide stripe using a cylindrical lens atop the sample parallel to the short side of the glass cell. The excitation length (l) was varied by shading part of the stripe using a movable shield. The emitted light was collected by an optical fiber connected to a spectrometer.
The two-dimensional resonator was composed of two coaxial capillary tubes with CLC injected into the gap. Figure 2 presents a schematic illustration of the fabrication procedure. The resonator was fabricated using glass capillaries of two sizes: a) outer diameter (1.44 mm), inner diameter (1.05 mm), length (0.8 cm), and b) outer diameter (0.97 mm), inner diameter (0.70 mm), length (1.5cm). The long segments (b) were immersed in a fluid of 3-glycidyloxy propyl trimethoxy silane (GPTMS). The short segments (a) were filled with GPTMS under capillary force. The segments were then placed in a dish and baked in an oven at 120°C for 24 h so that the surfaces of the tubes were coated with a thin film of GPTMS. A long segment was affixed vertically on a microscope stage. A short segment was attached to a ruler mounted on an XYZ manipulator. Under a microscope, the short segment was adjusted by the manipulator so that the two segments were coaxial . The bottom of the short segment was adhered to the long segment using AB glue. This created a set of coaxial capillary tubes separated by a 40-μm gap. A single-mode fiber was moistened with CLC to fill the 40-μm gap. The homogeneous alignment of GPTMS caused the CLC to spontaneously self-assemble into a planar helical structure. The CLC was prepared by mixing a nematic LC host HTW114200-100 (ne = 1.799, no = 1.513) and a left-handed chiral agent S811 at a ratio of 68.3:31.7. The 40 μm gap between the two coaxial capillary tubes was filled with the mixture under capillary action. We selected the long wavelength edge of the reflection band of the CLC to match the peak wavelength of the emission from af-GQDs. The inner tube was filled with the aqueous dispersion of af-GQDs under capillary action.
Photoluminescence measurements were performed using the setup in Fig. 3. The excitation beam from a frequency-tripled Nd:YAG pulsed laser (wavelength of 355 nm) was focused by a lens in the center of the front facet of the coaxial capillary tubes.
Fig. 3 Experiment setup used to measure photoluminescence of af-GQDs in two-dimensional optical resonator. The inset shows an enlarged schematic view of the sample with the laser beam focused at the center of the cross-section.
3. Results and discussion
Figure 4 presents the ultraviolet-visible (UV-Vis) absorption spectrum and Fourier transform infrared (FT-IR) spectrum of the af-GQDs. The inset presents a schematic structure model of af-GQDs containing characteristic functional groups in the basal plane and at the lateral edge [26]. The UV-Vis absorption spectrum presents an absorption peak at 335 nm, which is associated with the n-π* transition of C = O bonds in the oxygen-containing functional groups [31]. The characteristic absorption bands at 1650 and 3410 cm−1 in the FT-IR spectrum were respectively assigned to amide-carbonyl (-NH-CO-) stretching vibrations and N-H in-plane stretching of the amine groups [26].
Fig. 4 (a) UV-Vis absorption spectrum and (b) FT-IR spectrum of af-GQDs. The inset in (a) presents a schematic representation of the af-GQDs.
Figure 5 presents optical images of af-GQD dispersion illuminated by white and ultraviolet (UV) light, photoluminescence spectrum, the multi-photon excitation microscopy images, and high-resolution transmission electron microscopy (HRTEM) images of af-GQDs. The aqueous dispersion of af-GQDs appears slightly yellow under white light illumination, due to weak absorption in the blue spectral region. When illuminated by UV light, the af-GQDs produced strong emission across a wide spectral range of 400 to 600 nm. The multi-photon excitation microscopy images illustrate the uniform distribution of af-GQDs in the water. The diameter and thickness of af-GQDs were estimated from HRTEM images at 15 and 5 nm, respectively. The lattice spacing of 0.33 nm in the side view image is consistent with the interlayer spacing in crystalline graphite [32].
Fig. 5 (a) Optical photographs of af-GQDs dispersion illuminated under white (left; daylight lamp) and UV light (right; 365 nm). (b) Photoluminescence spectrum excited at 355 nm. (c) Multiphoton excitation microscopic images of aqueous dispersion of af-GQDs excited at 840 nm before (left) and after (right) evaporation of water. (d) Top (left) and side (right) views of the HRTEM images of af-GQDs.
Figure 6(a) presents the emission spectra of af-GQDs recorded using the VSL method at various excitation lengths with excitation energy of 3 mJ/pulse. Figure 6(b) plots the peak emission intensity (I) at a wavelength of 486 nm, as a function of the excitation length l under various excitation energies. By assuming a one-dimensional amplifier model, I is related to net optical gain g (i.e., the difference between gain and loss coefficients), as follows [33]:
where J is the power density of spontaneous emission. By fitting Eq. (1) with the measured data, g can be deduced at every excitation energy level and wavelength. The excited stripe of the sample was equivalent to a single-pass amplifier for luminescence photons traveling within the stripe using intense excitation energy to reach a population inversion. At higher excitation energies, a more intense signal from spontaneous emission would induce greater gain through stimulated emission.Fig. 6 (a) Emission spectra of af-GQDs under various excitation lengths at an excitation energy of 3 mJ/pulse. (b) Stripe-length dependence of photoluminescence of af-GQDs at a wavelength of 486 nm under various pump energies. Marks indicate the measured intensities, and curves are fits by Eq. (1).
Figure 7 presents a cross-sectional image of the two-dimensional resonator and reflection spectrum of the CLC. Figure 8 presents the photoluminescence spectra of a sample and a reference without CLC reflector. The periodic photonic structure of CLCs makes it possible to confine photons in the cavity under the effects of Bragg reflection, and to enhance emission by coupling af-GQD emission to the mode of the cavity at the edge of the reflection band. The intensity of peak emission in the optical resonator was approximately three times stronger than that of the reference sample. The CLC reflector also improved the integrated intensity of the emission.
Fig. 7 (a) Microscopic images showing the two-dimensional resonator in cross section: before (left) and after (right) being filled with CLC mixture. (b) Reflection spectrum of CLC in the two-dimensional resonator.
Fig. 8 Photoluminescence of af-GQDs in the two-dimensional resonator and in a reference sample without CLC reflector, at an excitation energy of 3 mJ/pulse.
Figure 9(a) presents the emission spectra of af-GQDs in the optical resonator excited at various excitation energy levels. The emission intensity increased with an increase in excitation energy. Figure 9(b) summarizes the dependences of the peak emission intensity on the excitation energy. The peak emission intensity reveals an exponential growth with excitation energy. At lower excitation energies, the dependence is linear because only spontaneous emission is present. Above a excitation energy threshold, spontaneous emission was amplified by stimulated emission resulting in incoherent optical feedback.
Fig. 9 (a) Emission spectra of af-GQDs at various excitation energies. (b) Peak intensity as a function of excitation energy.
4. Conclusion
The paper reports on the emission characteristics of af-GQDs when using a two-dimensional optical resonator based on a CLC reflector for the enhancement of spontaneous emission. The graphene nanostructure edge-terminated by a primary amine produces strong photoluminescence through the strong orbital resonance of amino groups with a graphene core. The net optical gain of the af-GQDs was measured using the VSL method. The presence of the CLC reflector enhanced the mode density due to the redistribution of spontaneous emission. The peak emission intensity in the optical resonator exceeded that of the reference sample by a factor of 3. An increase in excitation energy resulted in a further enhancement. This investigation opens new prospects for the use of two-dimensional confinement to enhance photoluminescence emission.
Funding
Ministry of Science and Technology of Taiwan (MOST) (103-2221-E-327-016).
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