Upconversion Luminescence of Graphene Oxide Through Hybrid


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Article Cite This: J. Phys. Chem. C 2018, 122, 16866−16871

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Upconversion Luminescence of Graphene Oxide through Hybrid Waveguide Zongbao Li,†,‡ Jianxin Yang,† Meng Shi,# Liu Yang,∥ Yupeng Cheng,† Xiaowen Hu,† Xiaofang Jiang,† Xiaobo Xing,*,†,§ and Sailing He*,†,∥,⊥

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Centre for Optical and Electromagnetic Research, South China Academy of Advanced Optoelectronics, South China Normal University, 510006 Guangzhou, China ‡ School of Material and Chemical Engineering, Tongren University, Guizhou 554300, China § MOE Key Laboratory of Laser Life Science & Institute of Laser Life Science, College of Biophotonics, South China Normal University, Guangzhou 510631, China ∥ National Engineering Research Center of Optical Instrumentation, Centre for Optical and Electromagnetic Research, JORCEP, Zhejiang University, Hangzhou 310058, China ⊥ School of Electrical Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden # School of Provincial Key Laboratory of Laser Polarization and Information Technology, Qufu Normal University, Shandong 273165, China S Supporting Information *

ABSTRACT: Phonon-assisted upconversion is a promising way to generate short-wavelength emissions under excitation of long wavelength based on unique anti-Stokes luminescence properties. Graphene oxide nanosheets (GONs) exhibit excellent optical properties owing to quantum confinement and edge effects, which have driven research into fundamental principles and potential applications. Here, we experimentally demonstrate upconversion emission by exciting an easily fabricated GON hybrid waveguide (GHW) with enhanced photothermal effects. The results reveal different origins of short-wavelength range and long-wavelength range in the upconversion spectra, whereas the emissive surface defects of GONs and GHW structure play significant roles in the behavior of photoluminescence. Introducing other upconversion materials to promote emission efficiency, the hybrid waveguide system might readily provide the possibility for the construction of upconversion fiber lasers and remote control of the upconversion luminescence.



INTRODUCTION Upconversion, or anti-Stokes luminescence, has drawn considerable interest for the promising potential applications in laser cooling,1 frequency upconverted lasers,2 solar cells,3 photodynamic therapy,4 and biological imaging5 because of its unique ability to generate short-wavelength emissions under excitations of longer-wavelength light.6,7 First theorized in 1959,8 the photon energy upconversion from the infrared region to the visible region was demonstrated in 1960.9 For most possible upconversion mechanisms, such as triplet− triplet annihilation photon upconversion,10,11 second harmonic generation,12−14 and multiphoton ligand enhanced excitation of lanthanides,15,16 high light intensity was needed together with low conversion efficiency and narrow spectral window, which limit their applications.17 In recent years, more and more attention has been paid to the investigations of upconversion mechanism through establishing novel micro-/ nanosystems,18 designing new upconversion micro-/nanostructures,19 and tuning laser parameters20 to obtain high © 2018 American Chemical Society

upconversion efficiency and spectral modulation. Moreover, it is necessary to solve the problem of remote control of the upconversion luminescence to further extend their applications.21 Graphene oxide (GO), as an insulated and disordered analogue of graphene, is a promising photonic material for future applications.22 As an electronically hybrid material, GO has unique heterogeneous chemical and electronic structures which feature both conducting π-states from the size of sp2 clusters and a large energy gap between σ-states of sp3-bonded carbons. The appearance of the sp2 clusters in a carbon− oxygen sp3 matrix can cause the localization of electron−hole (e−h) pairs, which introduces direct band gap transition behavior and transforms the material to a semiconductor or semimetal.23 In contrast to graphene, GO is fluorescent over a Received: January 28, 2018 Revised: June 7, 2018 Published: July 2, 2018 16866

DOI: 10.1021/acs.jpcc.8b00962 J. Phys. Chem. C 2018, 122, 16866−16871

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The Journal of Physical Chemistry C

Figure 1. Experimental schematic and characterization of tapered fiber and GON hybrid waveguide (GHW). (a) Experimental schematic. Red arrows indicate light propagation. (b) Optical microscope image of fiber taper waveguide. (c,d) Optical microscopy (c) and scanning electron microscopy images (d) of GHW, integrated by GO cladding and SiO2 fiber taper waveguide. (e) Optical microscopy image of upconversion emission as GHW is excited by 1064 laser in dark field. (f) Raman spectra of GONs film on quartz without excited and GHW after excited.

Figure 2. (a) Upconversion emission spectra in the range of 300−800 nm with different excitation powers at a wavelength of 1064 nm. The inset shows CIE chromaticity coordinates for SWR, LWR, and entire upconversion emission of GHW. (b) Excitation power dependence of upconversion luminescence intensity for SWR and LWR. A best fit with exponent factor as 0.977 for SWR and 0.986 for LWR.

citation by inexpensive continuous-wave (CW) lasers, the phonon-assisted anti-Stokes process can obtain higher energy emission than the incident one because of the involved indirect optical transitions mediated by annihilation of one or multiple phonons.33 Here, we report an upconversion luminescence of GONs under excitation by the CW laser. To obtain remote control of the upconversion luminescence, we develop a novel fiber taper waveguide system for achieving integration of a GON hybrid waveguide (GHW) and fiber. These make a meaningful supplement to the upconversion luminescence of GO quantum dots and CQDs.

broad range of wavelengths, owing to its heterogeneous electronic structure.22 Previous research studies have demonstrated that the photoluminescence (PL) of GO and its quantum dots ranged from visible to near-infrared regions under short-wavelength light excitation, where the PL was a consequence of geminate recombination of localized e−h pairs in sp2 clusters.24−27 Recently, it is found that arising from quantum confinement effects, the nanosized carbon particles exhibit strong PL28 and upconversion luminescence properties attributed to the multiphoton active process.29 Shen and colleagues30 have developed hydrazine hydrate-reduced GO into ultrafine carbon quantum dots (CQDs) with surface passivation by PEG1500N. The CQDs with diameters of 5−19 nm were found to exhibit strong blue PL and high upconversion emission because of the anti-Stokes PL. It was revealed that the surface passivation played an important role in higher fluorescence performance and upconversion properties. On the basis of the nanosized structure, GO nanosheets (GONs) should also exhibit similar upconversion properties for the quantum confinement effects. However, there are rare reports about upconversion of GONs. Previous works have demonstrated that it could boost the temperature and intensify the thermal radiation as GO absorbed infrared photons31,32 and transferred energy to the host lattice through multiphonon relaxation.19 Under ex-



EXPERIMENTAL SECTION

Figure 1 presents the schematic illustration of the experiment. The preparation of the GONs used in this study and the properties are described in Supporting Section S1. The GHW is obtained through both thermal convection flow and optical gradient force, serving as the driving force to assemble GONs onto the fiber taper waveguide in N,N-dimethylacetamide (DMF) suspension solution of GONs (see Supporting Section S2). The end of the optical fiber is fixed and manipulated by a three-axis microstage (Kohzu Precision Co., Ltd.). To prevent burning of GONs, the GHW is introduced into a chamber filled with argon. The CW laser light with wavelengths of 1064, 980, 808, and 650 nm is separately coupled to the fiber and 16867

DOI: 10.1021/acs.jpcc.8b00962 J. Phys. Chem. C 2018, 122, 16866−16871

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Figure 3. (a−c) Upconversion emission spectra with different incident powers. (d−f) Excitation power dependence of upconverted emission.

spectra are split into several subpeaks. Because of the competition between electronic transitions, the radiation of the LWR is inhibited by SWR with weaker luminosity intensity and narrower spectral range. A key feature of the spectra is the steady increase of the luminescence emission with the increasing excitation power. Under higher excitation power, a steep boundary of SWR appears at about 400 nm with a peak localized at 407.6 nm, which is the minimum wavelength of the upconversion luminescence. Two stationary peaks of the LWR appear at 723.1 and 734.5 nm with anti-Stokes shifts of ∼341 nm (∼0.550 eV) and ∼329 nm (∼0.522 eV), respectively. In contrast, there is no detectable upconversion luminescence when GONs are substituted by graphene or carbon nanotubes on the fiber taper. Meanwhile, there is no luminescent emission of GON suspension solution irradiated by the above laser or GONs coated on the cross section of the optical fiber. It is demonstrated that both the peculiar structure of GONs and the specific waveguide structure of GHW play key roles in the upconversion process. The calculated CIE chromaticity in the inset of Figure 2a shows that the luminescence color of the spectra of SWR appears to be blue, whereas that of LWR to be red. The results reveal that the temperature should be higher than 104 K if the blue emission is obtained only by blackbody radiation. Such a high temperature is not generated because the experimental process did not destroy the original scale morphology of the fiber and GONs (Figure 1d). From the Raman spectra in Figure 1f, the positions of D and G bands remain constant and the relative density of ID/IG is similar. It reveals that there is no

propagates along the GHW. A computer interfaced microscope (with a charge-coupled device camera) and an optical spectrograph (AvaSpec-ULS2048x64TEC) are used for observation, image capture, and real-time recording. The diameter of the fiber gradually decreases from 20 to 14 μm within a length of 84 μm and then abruptly tapers at 46 μm (Figure 1b). A quasi-ellipsoidal-shaped structure of the GHW with a width of ∼100 μm and a length of ∼220 μm is presented in Figure 1c,d. Figure 1e shows a blue luminescence image of the GHW when the GHW is excited by a 1064 nm CW laser under dark field. The fluorescence is strong enough to be observable by the naked eye (see in Supporting Video S1). Micro-Raman measurements were made by a WiTec Alpha300 system with 532 nm wavelength incident laser light and a 100× objective. The incident laser power adopted was 5 mW to avoid the sample damage. Figure 1f exhibits the Raman spectra of the GONs coated on a silicon wafer and the excited GHW. It can be seen that two samples display a D band at 1346 cm−1 and a G band at 1590 cm−1. The D peak is from the structural imperfections created by the attachment of hydroxyl and epoxide groups on the basal plane.34



RESULTS AND DISCUSSION When GONs are excited by a 1064 nm laser, Figure 2a shows the upconversion luminescence spectra as a function of excitation power. The luminescence consists of a shortwavelength range (SWR) of 400−550 nm and a longwavelength range (LWR) of 680−780 nm. As a new upconversion emission behavior of GONs, the dual-waveband 16868

DOI: 10.1021/acs.jpcc.8b00962 J. Phys. Chem. C 2018, 122, 16866−16871

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Figure 4. (a) PL spectra of GONs at different excitation wavelengths for pH = 7. (b) PLE spectra of the GONs dispersed in water. The inset shows UV−vis−NIR absorption. (c) pH-dependent PL spectra for pH = 7 and 1. (d) pH-dependent upconversion emission for PH = 7 to 4.

energy e−h pairs. The LWR appears anti-Stokes shift, whereas the SWR localized at the same position under different laser excitations. These different behaviors reveal the different combinations of the e−h pairs in the upconversion process. Because of the same spectra range,24−27 it can be inferred that the origin of SWR emission is the same with the PL spectra, whereas the origin of LWR emission is direct e−h combination. To further confirm the transition mechanism of SWR and the relationship with PL, Figure 4a,b gives out the PL spectra, PL excitation (PLE) spectra, and UV−vis−NIR absorption of GON dispersion (0.1 mg/mL). In neutral media, the weak blue excitation-wavelength-dependent PL emissions indicate a scarcity of trap states between the π* and n-state energy levels.24−27 Under excitation of 350 nm laser light, the PL spectrum shows a round peak at 407.2 nm in Figure 4a. According to the first-principles calculations,36 the PL spectrum peak at 407.2 nm (3.05 eV) is corresponding to the ribbons of ∼0.4 nm width, inducing that GONs are cut into GO quantum dots in the solution. The single PLE peak appears at 367 nm, as shown in Figure 4b, which is corresponding to an absorption band hidden in the strong background absorption.36 In the previous literature research studies,37,38 it was found that the PL was emissive surface defects of GONs because of the high concentration of free zigzag sites with triplet ground state σ1π1. In order to further confirm the similar mechanism of upconversion emission and PL, the pH-dependent PL spectra and upconversion spectra are obtained in Figure 4c,d, respectively. The PL spectrum is annihilated under 367 nm laser irradiation for pH changed from 7 to 1, which is corresponding to the reported results.37 The upconversion spectra show a similar trend excited by 1064 nm laser for pH changed from 7 to 4. The similar phenomenon reveals the similar original combinations of the e−h pairs for the SWR and PL, which is due to the emissive surface defects of GONs with a direct band gap transition behavior. On the other hand, following the excitation, thermalization takes place

evident chemical change occurring when GONs are illuminated. Further theoretical calculations confirm that the experimental temperature is about 1800 K at the incident power of 200 mW (see Figure S5). By combining with the localized peaks in Figure 2a, it is inferred that the emission is not blackbody radiation. The emission peak intensity (If) of the anti-Stokes as a function of incident laser power (P) is analyzed in a logarithmic coordinate, as shown in Figure 2b. For an upconversion process, the integrated If is proportional to Pn, where n is the number of photons required in populating the upper emitting state. From the slopes of the fitting lines, the n values are 0.986 and 0.977 for LWR and SWR, respectively. Such an upconversion process is similar to those occurring in monolayers MoSe2 and WSe2.33 To get a clear picture of the upconversion emission, the PLs excited by higher-energy incident lights are investigated. The upconversion emissions of three incident lights (with wavelengths of 980, 808, and 650 nm) at different powers are presented in Figure 3a−c. Using a 980 nm laser to substitute the 1064 nm one, the LWR has a ∼190 nm blue shift (0.59 eV, from ∼735 to ∼545 nm) with an anti-Stokes shift of ∼435 nm (∼1.01 eV, from 980 to 545 nm). The SWR remains the original position and has a small overlap with the LWR. Reducing the incident light wavelength to 808 nm, Figure 3b shows that the spectrum has a localized SWR narrow to ∼100 nm, whereas LWR is absent. When the GHW is excited by a 650 nm laser, the spectrum only exhibits signal emission peak at 407.6 nm with full width at half-maximum of ∼10 nm, as shown in Figure 3c. With the same slopes ∼1 of the cases (Figure 3d−f), the upconversion mechanism can be inferred that under laser excitation, one electron in the Urbach tail of valance band can transport to the tail of the conduction band by absorbing a photon, and a new hole is produced in the valence band.35 Then, the electrons and holes in the tails can transport to higher energy levels assisted with multiphonon absorption for the momentum conservation. Finally, upconversion emission occurs as the combination of the higher 16869

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Technology Research and Development Program (863 Program) of China (no. 2012AA012201), and Natural Science Foundation of Guizhou Province ([2016]1150 and [2015]67).

and emits a small amount of higher energy LO-phonon (∼kT).35 For the LWR spectra, the anti-Stokes shifts of ∼0.5 eV (excited by 1064 nm) and ∼1.01 eV (excited by 980 nm) reveal that the energy of the absorbed LO-phonons may be ∼0.5 eV (see in Supporting Section S3). Therefore, the mechanism of upconversion emission of LWR should be due to the direct combinations of the e−h pairs in high energy level by absorbing photon-assisted different numbers of phonons in a high-temperature field.



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CONCLUSIONS In this paper, we demonstrate a phonon-assisted upconversion of GONs in a hybrid waveguide system. The results show that both the peculiar structure of GONs and the specific waveguide structure of GHW play key roles in the upconversion process. Because of the high absorption of GO in the near-infrared region, higher temperature produces a large amount of higher energy phonons. By absorbing a photon with assistance of multiphonons, radiative recombinations of e−h pairs take place and lead to an upconversion process of LWR for direct band transition, whereas that of SWR is similar to PL owing to emissive surface defects of GONs. However, the other similar carbon-based materials, such as graphene, C60, and carbon nanotubes, do not exhibit a similar upconversion phenomenon. As an accessible and widely used material, GONs will change the upconversion emissions from an academic and exotic phenomenon into a realistic and viable tool, such as for analysis and imaging techniques in medicine and diagnostics.



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00962. Characterization of GO; fabrication of GO hybrid waveguide; and optical and thermal characteristics of GHW (DOCX) Luminescence of GHW excited by a 1064 nm laser under 40× microscope in bright and dark field (MPG)



REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.X.). *E-mail: [email protected] (S.H.). ORCID

Xiaobo Xing: 0000-0002-8720-8175 Sailing He: 0000-0002-3401-1125 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors highly appreciate Prof. Shen Lan from South China Normal University for the help at the mechanism of upconversion. This work is partially supported by Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (2017B030301007), Natural Science Foundation of Guangdong Province (2016A020221030 and 2016B090906004), Swedish VR grant (621-2011-4620), National Natural Science Foundation of China (91233208, 81371877, and 81772246), Special Support Program of Guangdong Province (2016TQ03R749), the National High 16870

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