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Abstract
Flexible, stretchable, and bendable electronics and optoelectronics have a great potential for wide applications in smart life. An environmentally friendly, cost effective and wide-angle emission laser is indispensable for the emerging technology. In this work, circumvent the challenge issue, cavity-free and stretchable white light lasers based on all carbon materials have been demonstrated by integration of fluorescent carbon quantum dots (CQDs) and crumpled graphene. The typical emission spectrum of the cavity-free laser based on all-carbon materials has a CIE chromaticity coordinate of (0.30, 0.38) exhibiting an intriguing broadband white-light emission. The unprecedented and non-toxic stretchable and white light cavity-free lasers based on all-carbon materials can serve as next-generation optoelectronic devices for a wide range application covering solid-state lighting and future wearable technologies.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
Guan-Zhang Lu, Ya-Jhu Li, Chen-Fu Hou, Rapti Ghosh, Ji-Lin Shen, Meng-Jer Wu, Tai-Yuan Lin, and Yang-Fang Chen, "All-carbon stretchable and cavity-free white lasers: publisher’s note," Opt. Express 30, 36234-36234 (2022)https://opg.optica.org/oe/abstract.cfm?uri=oe-30-20-36234
9 September 2022: A typographical correction was made to the author affiliations. A correction was made to the acknowledgments section.
1. Introduction
The developments of electronic and optoelectronic devices fabricated on flexible and lightweight substrates have resulted in vast potential for applications in unconventional electronics and optoelectronics [1–5]. Compared with established wafer-based devices, this significant development has been successfully achieved and demonstrated in impressive products, such as personal health monitors [6], e-skins [7], and stretchable and wearable computers. This vast progress has an effort to create huge demands and substantial commercial influence remains in human life. However, with the development of technology, the e-wastes generated by electronic and optoelectronic products are increasing rapidly and gradually affect the global environment. Therefore, to reduce the environmental impacts, some certain materials of the devices can be substituted with non-toxic, dissolvable, and eco-friendly materials, such as materials composed of carbon atoms.
The unique properties of carbon-based nanomaterials such as nanodiamonds, carbon nanotubes, fullerenes, graphene sheets, and carbon quantum dots (CQDs) have inspired comprehensive researches because of their great potential for the abundance of prospective applications [8–11]. CQDs, first discovered in 2004 during the purification of single-walled carbon nanotubes (SWCNTs) [12], are a novel class of carbon nanomaterials composed of discrete and quasi-spherical carbon nanoparticles. Because their excellent optical properties, they have successfully attracted plenty of interests in recent years [13–15]. Last decades, semiconductor quantum dots (QDs), the group of II-VI QDs, such as CdS, CdSe, and CdTe QDs have been extensively investigated for their strong and tunable emission properties, which enable their applications in the optoelectronic field, such as light-emitting diodes, displays, solar cells, photodetectors [16–18]. However, the group of II-VI QDs possesses certain limitations such as high toxicity owing to their ingredients including heavy metals. Due to the toxicity of cadmium elements in II-VI QDs for living beings, this prompted the creation of CQDs with cadmium-free, excellent luminescence, great biocompatibility, and covering the whole visible spectrum, which is imperatively needed.
Meanwhile, graphene, a single atomic layer of carbon atoms connected by sp2 hybridized bonds arranged like a two-dimensional (2D) honeycomb lattice structure, has attracted considerable attention in both the scientific and engineering communities since its discovery in 2004 [19]. Due to the unique lattice and electronic structures, graphene has a variety of extraordinary characteristics, such as strong ambipolar electric field effect [19], unconventional quantum Hall effect at room temperature [20–22], great thermal conductivity (5000 Wm−1K−1), high transparency (97.7%), excellent electrical conductivity (2000 Sm−1), and high mechanical flexibility [23–25]. These superior properties exhibit more promising applications and evoke huge interest in the possible implementation of graphene in a myriad of devices. These include sensors, future generations of high-speed logic devices, transparent electrodes for stretchable electronics and optoelectronics. In addition to the above characteristics, the conical band structure around the Dirac point [26] makes the behavior of carriers in graphene as massless Dirac fermions [27]. Furthermore, the nonlinear dynamics of electrons enable to fill the transient states via electron-electron collisions in the energy band and generate Pauli blocking effect. [28] According to this specific property, graphene could be applied to passively mode-locked lasers as a fast saturable absorber and play an important role in stimulated emission to generate transient population inversion phenomenon [29,30].
In this study, by integration of crumpled graphene and CQDs, the first stretchable and cavity-free white laser based on all-carbon materials has been successfully designed and demonstrated. The emitted lights from the CQDs can go through multiple scattering in between the hills and valleys of the crumpled structure of graphene generating population inversions and inducing the laser actions arising from the rapid carriers transfer at the interface between CQDs and the graphene [31–33]. Furthermore, our device based on the crumpled structure of graphene/CQDs, compared with the conventional lasers, in which inflexible lens are required to serve as a resonant cavity, will pave the way to develop the stretchable functionality of the unconventional electronics and optoelectronics because the laser action can be modulated by simply applying external strain.
2. Results and discussion
A transmission electron microscope (TEM) image has been carried out to investigate the sizes of the CQDs as shown in Fig. 1(a). The histogram of the size distribution of CQDs was shown in Fig. S1 in Supporting Information, and the CQDs have sizes range from 8.0 to 13.0 nm with an average size of 10.1 nm. As illustrated in Fig. 1(b), the property of tunable emissions of the CQDs in solution was clearly visible, where the light at different wavelengths was emitted upon light excitation with different wavelengths. This property has also been observed in lots of published researches, which portrayed multicolor emissions when excited at different wavelengths [34–36]. The absorbance of the CQDs in solution was shown in Fig. 1(c), where the shoulder at 297 nm is attributed to the π–π* transition of C = C bonds in sp2 basal plane. [37,38] In the (αE)2 versus (E) plot derived from the UV−vis spectrum, where α is the normalized absorption coefficient and E is photon energy, the bandgap of the CQDs can be determined by the horizontal intercept of the extrapolation from the linear region shown in Fig. 1(d). The bandgap of CQDs is around 3.4 eV.
Fig. 1. (a) The TEM image of a carbon quantum dot. (b) The excitation-dependent PL of the CQDs in solution. (c) UV-vis absorbance spectra of the CQDs in solution. (d) of (αE)2 versus photon energy (E) of CQDs.
The X-ray photoelectron spectroscope (XPS) measurements were performed to investigate the chemical state and composition of the materials. The C 1s, N 1s, and O 1s peaks were respectively identified at 284.8, 399.8, and 531.8 eV shown in Fig. 2(a). The results of XPS showed that all the CQDs mainly consisted of carbon, nitrogen, and oxygen. The high-resolution spectrum of C 1s was deconvoluted into three peaks at 284.9, 286.0, and 288.0 eV, which originated from C−C, C−O, and C = O, respectively shown in Fig. 2(b) [39–41]. The high-resolution spectrum of N 1s was shown in Fig. 2(c). The N peak was deconvoluted into two subpeaks related to different carbon to nitrogen bond configurations: pyridinic N (398.9 eV) and pyrrolic N (400.1 eV) [40,42]. The Raman spectrum of the CQDs was shown in Fig. 2(d). In the Raman spectrum, the two characteristic D band and G band can be obviously seen at 1345.0 and 1574.0 cm−1, respectively, where the D band is attributed to defects and disordered structure in crystalline sp2 clusters or the presence of sp3 defects [43,44]. The crystalline G band corresponds to the in-plane stretching vibration of sp2 carbon atoms. The intensity ratio of D to G bands can be used to estimate the defects of carbon-based samples. The intensity ratio ID/IG of the prepared CQDs wasround 0.9, indicating that the CQDs have surface defects [44]. The CQDs did have oxygen defects according to the C 1s spectrum including the C-O and C = O. And this result is consistent with the Raman spectrum which has a higher intensity of the D band.
Fig. 2. (a) Binding energy obtained from XPS spectra for the CQDs; high-resolution XPS shows the binding energy of (b) C 1s and, (c) N 1s electrons for the CQDs. (d) Raman spectrum of the CQDs.
The fabrication process of the crumpled structure is schematically illustrated in Fig. 3(a). The crumpled structures of graphene formed by releasing the strained PDMS films are characterized by the deformation ratio according to the following equation:
where α is the length of the strained PDMS films and β is the length of the released PDMS films sustaining the crumpled structure. The Raman spectrum of the graphene was shown in Fig. 3(b). And the intensity ratio of G to 2D bands is around 0.7, indicating that the CVD-grown graphene may contain a mixture of monolayer and bilayer [45,46]. Figure 3(c) shows that the surface morphology of the crumpled structure of graphene confirmed by an atomic force microscope (AFM) image. The AFM image clearly revealed the distinct hills and valleys structures of the crumpled graphene, where the crumple height and width are about tens of nanometers and several micrometers, respectively. It is worth noting that in Fig. 3(c) there exists a hierarchical crumpled structure. This intriguing hierarchical crumpled structure may be caused by the movement of the graphene layer during the pre-strain PDMS releasing process arising from different Young’s modulus coefficient between graphene and PDMS, because the stacking of graphene on the top of PDMS is bonded by a weak van der Waals force. Consequently, the size of the crumpled structure is sufficient enough to trap the emitted light and can serve as an excellent matrix for light amplification due to the multiple reflections of photons at the valley of crumpled graphene [47,48].Fig. 3. (a) Schematic illustration of the device fabrication process. (b) Raman spectrum of the graphene on a Si substrate under 532 nm laser. (c) An AFM image of the surface morphology of the crumpled structure of graphene.
The light emission phenomenon of the device was studied under the 374 nm pulsed laser of frequency 1 MHz and pulse width of 55 ps through an optical microscope. The excitation power-dependent emission spectra of the CQDs/PDMS device at the releasing condition were shown in Fig. S2(a), exhibiting a broadband emission spectrum without any shrouding sharp peaks. And the relation between the integrated intensity and pumping power density is linear without a threshold as shown in Fig. S2(b), indicating that the crumpled CQDs/PDMS structure without graphene is unable to achieve population inversion to generate the laser action [33]. In contrast, Fig. 4(a) shows the excitation power-dependent emission spectra from the crumpled CQDs/graphene/PDMS structure with a deformation ratio of −30%. At the excitation power up to and above 74 Wcm−2, the narrow spikes at random frequencies emerged in the broadband emission spectra. The multiple spikes in the emission spectra demonstrated the coherent random lasing phenomena. In contrast to the crumpled CQDs/PDMS structure without graphene, the integrated intensity as a function of the pumping power density of the crumpled CQDs/graphene/PDMS structure shown in Fig. 4(b) exhibits a threshold behavior, and the value of threshold is about 40 Wcm−2. Above the pumping threshold, the emission intensity increased rapidly with the increasing pumping power density. Accordingly, the crumpled CQDs/graphene structure forms an excellent matrix for light scatterings and amplification. Subsequently, the stimulated cavity-free laser emission from the crumpled structure of CQDs/graphene thus was achieved exhibiting broadband white light with many ominently sharp peaks. The corresponding CIE chromaticity diagram of the device under different pumping power densities is shown in Fig. 4(c). The device corresponds to the best CIE color coordinates of (0.30, 0.38).
Fig. 4. (a) The excitation power-dependent emission spectra from the crumpled CQDs/graphene/PDMS structure with deformation ratio −30%; inset is the optical picture of the device. (b) The integrated intensity as a function of the pumping power density of the crumpled CQDs/graphene/PDMS structure with deformation ratio −30%. (c) The corresponding CIE chromaticity diagram of the device under different pumping power density. (d) Schematic band structure and electronic transitions of the crumpled CQDs/graphene structure under optical excitations.
An all-carbon composed white light laser has been successfully fabricated and demonstrated. It is of great potentialities that optoelectronic devices can be produced by composing a single carbon element only. To further illustrate the stimulated cavity-free laser behavior from the crumpled CQDs/graphene structure, the schematic band structure and electronic transitions were shown in Fig. 4(d) to explain the carrier dynamics of CQDs/graphene under 374 nm pulsed laser. The different parabolic lines represent the quantum confined energy levels in the conduction and valence bands corresponding to the quantum confinement size effect (QCSE) of CQDs [49]. According to the Raman spectrum of CQDs, there are many surface defects states that exist in the CQDs. The different horizontal lines represent the surface defect states of CQDs of different sizes. Accordingly, the transitions of the carriers in CQDs/graphene can be divided into three processes. First, when the light of 374 nm pulsed laser illuminates the crumpled CQDs/graphene hybrid structure, CQDs and graphene both absorb the incident photons, and electron-hole (e-h) pairs are thus generated. For the CQDs, the valence band electrons can transit to higher energy levels in the conduction band, following the relaxation to the surface defect states near the conduction band edge. For the graphene, with the ultra-high saturation absorption strength readily under strong excitation due to Pauli blocking effect [29], the high-intensity laser pulses are able to induce a population inverted transient state [30]. Second, due to the close proximity between the CQDs and graphene, it is highly tenable that the excited electrons of the graphene can tunnel to the proximate energy levels in CQDs. Since the e-h recombination in CQDs is the subsequent process after the filling of energy states by the resonant tunneling of the carriers from graphene. Consequently, many additional carriers from graphene finally can be injected into surface defect levels in CQDs, which enables to generate population inversion and induce stimulated emissions in CQDs by excellent matrix for light amplification of crumpled graphene.
Comparing with the conventional lasers with fixed modes and strong directionality, the cavity-free lasers can be recorded at different angles and the modes of lasing are not fixed. The angular dependence of the emission spectra of the crumpled structure of CQDs/graphene/PDMS at a pumping power density of 1.2 kW cm−2 are shown in Fig. S3(a). The emission peak intensity recorded at different measuring angles is shown in Fig. S3(b), indicating a close lasing strength in each detected angle. To further elucidate the influences of crumpled graphene on the role of multiple scattering for light amplification, the emission spectra of the CQDs/graphene/PDMS devices with different deformation ratios are shown in Fig. 5(a)-(c). Note, there is no discernibly sharp peak spreading in the spectrum for the sample with zero deformation ratio which is free of external strains. On the other hand, for the crumpled samples with non-zero deformation ratios a variety of obviously sharp peaks are dominant in the spectra. For the CQDs/graphene sample with more crumpled graphene structure, the more lasing behaviors arise characterized by the densely sharp peaks in the emission spectra.
Fig. 5. (a)-(c) The emission spectra of the CQDs/graphene/PDMS devices with different deformation ratios, range from −30% to 0% at a pumping power density of ∼ 45 W cm−2. (d) Absorbance spectra of the crumpled graphene with different deformation ratios, ranged from 0% to −40%. (e) The TRPL spectra of the deformed and undeformed CQDs/graphene devices at a pumping power density of ∼ 235.4 W cm−2. (f) Carrier decay time of the crumpled CQDs/graphene devices with deformation ratios of 0% and −30%, respectively, as a function of pumping power density, monitored at 470.0 nm.
To unravel the above results, the absorbance spectra of the samples with crumpled graphene with different deformation ratios, ranged from 0% to −40% were shown in Fig. 5(d). Obviously, the absorbance of the crumpled graphene sample increases extensively with the increasing deformation ratio, indicating the increase in the deformation ratio negatively will increase the density of the crumpled graphene matrix which will lead to the effective light trapping and more strong light scatterings, and thus the higher absorption of the incident excitation laser light. The time-resolved photoluminescence (TRPL) spectra of the deformed and undeformed CQDs/graphene devices at a pumping power density of ∼ 235.4 W cm−2 were shown in Fig. 5(e). The carrier lifetime of the crumpled CQDs/graphene device (deformation ratio of −30%) is much faster than that of the undeformed sample (i.e. deformation ratio of 0%). Figure 5(f) demonstrated the comparison of the carrier decay time of the devices with deformation ratios of 0% and −30%, respectively, as a function of pumping power density. The monitored emission wavelength is 470 nm. In contrast to the lifetime of the device with flat graphene (0% deformation ratio), the carrier decay time of the crumpled CQDs/graphene device showed a drastic reduction beginning at 50.0 W cm−2 representing the increase in the number of the carrier transitions per unit time, which indicated the change of emission process from spontaneous emission to stimulated emission. The prominent threshold value for the change of the carrier decay time is a strong indication of laser action. Accordingly, the crumpled graphene structure can form a virtual cavity for trapping the emitted photons from the CQDs. The stronger photon confinement by the densely crumpled CQDs/graphene structure easily leads to the formation of coherent loops, which enhances the radiative transition rate to produce stimulated emission with a shorter carrier decay time. Thus, the crumpled CQDs/graphene matrix can offer effectively optical feedbacks by multiple scatterings of photons for light amplification which achieves the cavity-free white light lasing.
Finally, in order to prove the structure-induced tunability for a long operation period, the reproducibility of the device was assessed by repeatedly stretching and releasing strain to the device. One stretching and releasing cycle is defined as the device with the initial deformation ratio −30%, then stretching the device with the deformation ratio from −30% to 0%. The deformation ratio of the device returns to −30% after the device is released. Emission spectra of the device after each cycle of stretching and releasing strain were recorded under a pumping power density of 150 W cm−2, and the normalized emission intensity under repeated stretching and releasing strain is shown in Fig. S4.
3. Experimental section
3.1 Stretchable substrate
Polydimethylsiloxane (PDMS) was used as the stretchable substrates to fabricate the crumpled structure of the devices. The PDMS was purchased from Dow Corning Sylgard 184A and 184B. The mixture with the weight ratio of 184A to 184B is about 12 and baked at 80 °C for one hour to solidify and produce the PDMS.
3.2 Graphene
The graphene was fabricated from a standard chemical vapor deposition (CVD) method [50]. First, the 99.98% copper foil was polished by the electropolishing method with 85% H3PO4 under 1.5 V for 15 min to reduce the surface roughness of the copper foil. Then, the prepared copper foil was put into the furnace with 60 sccm hydrogen (H2) flow and baked at 1000 °C for 1 hr. Immediately, methane (CH4) was flown into the furnace with 3.5 sccm for 30 min and kept the temperature (1000 °C) after the H2 treatment. Then, kept the same flow in the system (with 60 sccm H2 and 3.5 sccm CH4) to cool down the temperature of the furnace to room temperature. After finishing the above procedure, the graphene was deposited on the copper foil.
3.3 Synthesis of carbon quantum dots
In a typical experiment, 0.5 mL hydrogen peroxide (H2O2) solution, 6 mL alcohol, 0.1 g L-histidine powders, and stir bar were loaded into a microwave tube and heated to the desired temperature (180 °C) at 800 r.p.m. for 30 min. After the described reaction, the mixture was cooled down to room temperature and must be allowed standing at room temperature for one day. Finally, the carbon quantum dots solution was filtered with filter paper.
3.4 Fabrication process for a CQD/graphene/PDMS structure
To fabricate the device, a pre-strained PDMS film was formed by uniaxially stretching the PDMS thin film fixed on a glass substrate with tapes. Then, the graphene was transferred on the pre-strained PDMS film. The CQDs are deposited onto the surface of the graphene uniformly by drop coating process and baked at 50 °C for 30 min. Finally, after removing the fixed tapes on pre-strained PDMS, the pre-strained CQDs/graphene/PDMS device is released with about 1 cm s−1 to produce the hierarchical crumpled CQDs/graphene structure.
3.5 Characterization
Transmission electron microscope (TEM) images were measured by Philips Tecnai F30 Field Emission Gun Transmission Microscope (Instrumentation Center, National Taiwan University) to characterize the morphology of the CQDs. Photoluminescence (PL) spectra of the CQDs in solution were measured by a FluoroMax-4 (HORIBA) fluorescence spectrometer. Absorbance spectrum was recorded by an ultraviolet-visible spectrophotometer (PerkinElmer, LAMBDA 750). X-ray photoelectron spectroscope (XPS) spectra were conducted by using the Electron Spectroscopy for chemical Analysis System (Thermo Scientific ESCALAB 250, Instrumentation Center, National Taiwan University), equipped with XR5 monochromated X-ray aluminum Gun, maximum energy 15 kV, 200 W, range of beam size from 120 µm to 650 µm. Raman spectrum was conducted at room temperature with a system combining a synapse thermoelectric cooled charge-coupled device (CCD) guaranteed to −75°C connected to the spectroscopy software SynerJYTM, optical microscope, 532 nm laser. Surface morphology image of crumpled graphene was obtained by using an atomic force microscope (AFM) system (Solver P47). The emission spectra and Time-resolved photoluminescence (TRPL) spectra were performed by a pulse diode laser (Picoquant, PDL 800-B, center wavelength of 374 nm, 55 ps, 1 MHz), and a Horiba-Jobin-Yvon TRIAX 320 spectrometer to collect the signals.
4. Conclusion
In summary, all-carbon stretchable and cavity-free white light lasers based on the crumpled structure of CQDs/graphene hybrid devices have been fabricated and demonstrated. The various radiative states of the carbon quantum dots were utilized to generate an intrinsic broadband white light under optical injection through the device structure of CQDs/graphene. The emitted lights from the CQDs can go through multiple scattering in between the hills and valleys of the crumpled structure of graphene generating population inversions and inducing the laser actions arising from the rapid carriers transfer at the interface between carbon quantum dots and graphene. A typical emission spectrum of the novel carbon-based laser device consisting of crumpled graphene/CQDs has a CIE chromaticity coordinate of (0.30, 0.38) exhibiting an intrinsic broadband white-light emission. Accordingly, we have successfully used the crumpled structure of non-toxic CQDs/graphene to provide a proof of concept of stretchable and cavity-free white light lasers based on all-carbon materials, which should be very useful and timely to advance the potential application of carbon-based materials for future stretchable optoelectronics, such as the electrically driven QDs random lasers developed recently. [51]
Funding
Ministry of Science and Technology, Taiwan (MOST 110-2112-M-002-044, MOST 110-2112-M-019-003, MOST 110-2524-F-002-043); Advanced Research Center for Green Materials Science and Technology, National Taiwan University (110L9006).
Acknowledgments
Y.-F.C. conceived the idea of this project. T.-Y.L. provided CQDs. G.-Z.L. and Y.-J.L. performed the main experiments. Y.-F.C. and T.-Y.L. supervised the study. J.-L.S. provided the fruitful suggestions in the experimental sections. C.-F.H., R.G., and M.-J.W. participated in sample characterizations and analyses. Y.-F.C., T.-Y.L., and G.-Z.L. wrote the manuscript. Y.-F.C.*, T.-Y.L.** are the corresponding author and co-corresponding author, respectively.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data are available from corresponding author upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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