Article Cite This: ACS Photonics XXXX, XXX, XXX-XXX
pubs.acs.org/journal/apchd5
Fully Suspended Reduced Graphene Oxide Photodetector with Annealing Temperature-Dependent Broad Spectral Binary Photoresponses Yang Cao,†,‡,§ Hua Yang,†,‡,∥ Yajing Zhao,‡,⊥ Yue Zhang,‡ Tingting Ren,‡,⊥ Binbin Jin,‡,⊥ Junhui He,*,‡ and Jia-Lin Sun*,#,○ ‡
Functional Nanomaterials Laboratory, Center for Micro/nanomaterials and Technology and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, China § Beijing Key Laboratory of Optoelectronic Measurement Technology and Beijing Engineering Research Center of Optoelectronic Information and Instruments, Beijing Information Science and Technology University, Beijing 100192, China ∥ School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China ⊥ University of Chinese Academy of Sciences, Beijing 100049, China # State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China ○ Collaborative Innovation Center of Quantum Matter, Beijing, China S Supporting Information *
ABSTRACT: A series of fully suspended reduced graphene oxide (RGO) photodetectors were fabricated based on free-standing thermally reduced graphene oxide (GO) thin films with various annealing temperatures (from 200 to 1000 °C). A new phenomenon was found that under ultraviolet (375 nm) to near-infrared (1064 nm) lasers irradiations, the photodetector consisting of fully suspended thermally reduced graphene oxide thin film and Au interdigital electrodes possess positive or zero or negative response depending on the annealing temperature of GO. This may originate from the competition between annealing temperature-dependent the positive photoresponse from RGO and the negative photoconductivity of Au interdigital electrodes. The annealing temperature-dependent wide spectral binary photoresponse offers a good thinking for designing RGO-based light triggered logic devices. Moreover, upon visible (405 nm) to near-infrared (1064 nm) irradiations, all fully suspended RGO detectors prepared at various annealing temperatures exhibit fast photoresponse, that is, rise time is only tens of milliseconds, which are 1−4 orders of magnitude faster than the reported photodetectors based on supported RGO film and are on the same order of magnitude as the supported CVD-grown and supported mechanically exfoliated graphene photodetectors. The fast photoresponse originates from the fully suspended construction of device and the thin thickness of film, which provides a new design idea for fast response RGO-based device. KEYWORDS: graphene, reduced graphene oxide, photodetectors, binary photoresponses, logic devices
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suspended graphene devices usually exhibit higher photoelectric conversion efficiency than substrate-supported graphene devices.11 And the mechanism of photoresponse of suspended graphene devices was found to be dominated by photoelectric effect. Suspended graphene-based photodetectors are more than an order of magnitude faster than substrate-supported graphene devices dominated by photothermoelectric effect.12 Moreover, compared with graphene supported by a substrate, suspended graphene has more excellent characteristics, such as high temperature coefficient of resistance,13 ultralow 1/f noise14 and lower surface plasmon losses.15 However, fabrications of
raphene is an appealing material for photoelectric conversion devices because it offers several unique advantages, including ultrawide spectrum light absorption bring high-efficiency utilization of solar energy, ultrahigh carrier mobility brings ultrafast photoelectric conversion dynamics,1,2 and ultrafast photonics,3,4 and significantly stronger surface plasmon effect compared to noble-metal bring stronger light− graphene interactions.5,6 A variety of graphene-based prototype optoelectronic devices have already been demonstrated.7−9 However, in most of these devices, graphene is supported by a substrate and interacts intimately with the substrate. Substrate surface polar phonons, which are present in common substrates such as SiO2 and h-BN, provide additional electron energy decay channels to cool photoexcited electrons.10 Thus, © XXXX American Chemical Society
Received: July 17, 2017
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DOI: 10.1021/acsphotonics.7b00768 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 1. (a) Photograph of manipulating the thin film away from the SiNW array substrate with tweezers. (b−f) Top view SEM images, (g) Raman spectra, and (h) sheet resistances of free-standing RGO thin films annealed at 200, 400, 600, 800, and 1000 °C, respectively.
fully suspended graphene devices using mechanically exfoliated graphene and chemical vapor deposited (CVD) graphene are quite challenging due to difficulties in accurately transferring graphene on prefabricated trenches11 or dry-etching or wetetching underlying substrate on the premise of nondamaging graphene.12,14 Alternatively, free-standing RGO films were developed to fabricate suspended graphene devices.13 Owing to preparation of the RGO by reducing GO, regulating reduction degree can regulate the types and densities of oxygenous functional groups in the RGO, and thereby can control optical and electrical properties of the RGO even changing its conduction mechanism.16 This deep tunability can be very easily implemented for the RGO. The RGO can respond to very wide spectral ranges from ultraviolet to terahertz,17 which is incomparable by other emerging 2D materials such as black phosphorus and molybdenum disulfide.18 Nevertheless, in terms of responsivity and current ON/OFF ratio, the RGO are significantly inferior to those 2D materials as typical semiconductors.19 Very recently, we reported a new method, that is, layer-by-layer drop-casting GO/ethanol suspension on pretextured substrate followed by heat treatment, to produce free-standing RGO films with outstanding electrical conductivity.20 On this basis, a series of fully suspended RGO photodetectors were facilely fabricated based on as-prepared free-standing RGO thin films prepared at various annealing temperatures in this work. The photodetectors reveal remarkable annealing temperature-dependent photoresponse, that is, photocurrent changes from positive to zero and to negative with increase of annealing temperature. Furthermore, the highest responsivity of the photodetectors was up to 51.46 mA/W at a small bias voltage of 0.5 V, which is 1 order of magnitude higher than those of supported graphene photodetectors. All of the fully suspended photodetectors exhibit fast (tens of milliseconds) photoresponses over a wide spectral
regime from visible (405 nm) to near-infrared (1064 nm), which are comparable to those of photodetectors based on supported CVD grown-graphene and supported mechanically exfoliated-graphene.
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EXPERIMENTAL SECTION Preparation and Characterization of Free-Standing RGO Thin Films. GO was synthesized from natural graphite by a modified Hummers method.21 The as-prepared GO sheets were dispersed in ethanol to give a concentration of 0.7 mg/mL GO suspension by sonication. Silicon nanowire (SiNW) arrays were prepared by chemical etching silicon wafers.22 Here, SiNW arrays were used as substrates, and the GO suspensions were respectively drop-casted (75 μL/∼1 cm2) on the top surfaces of several SiNW arrays. The obtained SiNW arrays were then dried at 60 °C in an oven for only several minutes prior to the next drop-casting. After six cycles of drop-casting and drying overnight in air, the SiNW arrays were annealed at 200, 400, 600, 800, and 1000 °C, respectively, under Ar (95%)−H2 (5%) flow with 50 mL/min for 3 h to produce freestanding RGO thin films with various annealing temperatures. Details of preparation please refer to our previous work.20 The morphologies of RGO films with various annealing temperatures were characterized using field emission scanning electron microscopy (SEM, Hitachi S-4800) operating at 5 kV. Raman spectra of the RGO films with various annealing temperatures were obtained on a Raman spectrometer (inVia-Reflex, Renishaw, U.K.) with the incident laser light of 532 nm. The sheet resistances of the free-standing RGO films with various annealing temperatures were measured using the four-point probe method (Guangzhou Four Probes Tech Co.). X-ray photoelectron spectra (XPS) of RGO obtained at different annealing temperatures were collected by a ESCALAB 250Xi X-ray photoelectron spectrometer. Their sheet resistances were B
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Figure 2. (a) Survey XPS spectra of thermally reduced GO at different annealing temperatures. C 1s XPS spectra of thermally reduced GO thin films annealed at (b) 200, (c) 400, (d) 600, (e) 800, and (f) 1000 °C, respectively.
were achieved using a 785 nm laser with spot diameter of ∼2 mm (MDL-III-785−500 mW) and a 1064 nm laser with spot diameter of ∼3 mm (MIL-III-1064−500 mW). The abovementioned lasers were obtained from Changchun New Industries Optoelectronics Tech. Co. The position of laser spot on sample could be accurately controlled by installing the samples on a platform with two micrometer caliper which can be adjusted in both horizontal and vertical directions. For all the measurements under illumination, the laser spot was positioned in the middle of suspended RGO thin film. Turn-on and turn-off of the illumination were achieved by electromagnetic shutter (SSH-C2B, OptoSigma). Incident optical power on the sample was adjusted using neutral density filters. Current−voltage (I−V) characteristics were measured in dark and upon illumination, respectively, using a Keithley 2400 Source. Time-resolved photoresponses were recorded using a Keithley 2400 Source Meter.
measured using a HL 5500PC Hall Effect Measurement System by heating films from 27 to 69 °C. Fabrication and Characterization of Fully Suspended RGO Photodetectors. The free-standing RGO films prepared at various annealing temperatures were respectively fixed on insulating glass grooves to ensure the films keeping suspended state. Then the Au interdigital electrodes were deposited on the surface of suspended RGO films through shadow masks by thermal evaporation. The interdigitated electrode has a spacing of 200 μm and a width of 200 μm. The two terminals of the interdigitated electrodes were then bonded using Au paint with Cu wire for electrical measurements. All the measurements were performed in air at room temperature. UV irradiation was achieved using a 375 nm laser with spot diameter of ∼2 mm (MDL-III-375−100 mW). VIS irradiations were achieved using a 405 nm laser with spot diameter of ∼2 mm (MDL-III-405− 100 mW) and a 532 nm semiconductor laser with spot diameter of ∼2 mm (MGL-III-532−200 mW). NIR irradiations C
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Figure 3. Digital photographs of fully suspended RGO photodetectors based on free-standing RGO thin films produced by heat-treatment at (a) 200, (b) 400, (c) 600, (d) 800, and (e) 1000 °C, respectively. (f) Schematic illustration of fully suspended RGO photodetector for I−V characterizations and photoresponse measurements.
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between Ilight and Idark, that is, Iph = Ilight − Idark. The Iph obtained from the current−voltage (I−V) characteristics of the photodetectors reveal a regular change. With the increase of annealing temperature, the Iph value changes from positive (200 and 400 °C) to approximate 0 (600 °C) and to negative (800 and 1000 °C), as shown in Figure 4. The change of Iph with increasing annealing temperature also clearly appears in photoresponses under illuminations of UV (375 nm) up to NIR (1064 nm; Figure 5). The following phenomena were observed: (1) for photodetectors based on the RGO films obtained at 200 and 400 °C, currents of the photodetectors markedly increase when light is ON; (2) for photodetector based on the RGO film obtained at 600 °C, only a slight change (under 532 and 1064 nm light illuminations) in current or hardly observed change in current (under other wavelengths illuminations) appears when light is ON; (3) for photodetectors based on the RGO films obtained at 800 and 1000 °C, the currents of the photodetectors decrease when light is ON. The good reproducibility of photoresponses upon multiple on and off cycles of switching lasers with different wavelengths testifies that the Iph changes are not accident. Whatever the illumination wavelength (energy of photon) is, the Iph always changes from positive to zero (or approximates to zero) and then to negative with rising annealing temperature. This may be the result of competition between the positive photoresponse from RGO and the negative photoconductivity from Au interdigital electrodes. On one hand, the Au interdigital electrodes as a noble metal film with nanoscale thickness exhibit significant negative photoconductivity caused by photoinduced surface plasmon polaritons offering another scattering channel for transportation of electrons.28,29 On the other hand, the RGO possesses positive photoresponse, which was produced by the incident photons and heating associated with the incident photons.30 The intensity of the positive photoresponse depends on the reduction level of RGO, because the content of oxygenous functional groups in RGO greatly affect lifetime of photogenerated carriers, carrier mobility31 and temperature coefficient of resistance of RGO.16 For example, in the presence of oxygenous defects, some photoexcited electrons are trapped and holes can circulate
RESULTS AND DISCUSSION As-prepared RGO thin film was easily taken off the top surface of SiNW array using tweezers to get free-standing film (Figure 1a) for further characterization. SEM observations (Figure 1b− f) show that the free-standing RGO thin films annealed at different temperatures have similar morphology, that is, continuous thin film with a few folds. Raman spectra (Figure 1g) of RGO films annealed at different temperatures indicate that RGO films have two strong characteristic peaks at 1358 and 1597 cm−1 corresponding to the D and G bands, respectively. The intensity ratio of two peaks (ID/IG) increases from 0.796 to 1.160 with increase in heat-treatment temperature from 200 to 1000 °C. This implies that the increase of annealing temperature decreases the size of each sp2 region in graphene, but increases their overall presence in the samples, resulting in conductivity enhancement of the film.23,24 As show in Figure 1h, the sheet resistance of RGO film markedly decreases when heat treatment temperature increases. Particularly, when the sample was annealed at 1000 °C, the sheet resistance of RGO film could be reduced to 39.7 Ω sq−1, which is among the lowest values of sheet resistances of RGO films reported so far.25−27 XPS survey spectra (Figure 2a) show that the amounts of oxygenous functional groups in RGO gradually decrease with increase of annealing temperature. The amounts of oxygenous functional groups in RGO decrease over 50% when annealing temperature up to 800 °C. Multipeak fitting of C 1s spectra (Figure 2b−f) show that the C−C bond (∼284.1 eV) still largely exists in all the samples. The C−O bond (∼285.6 eV) is the main oxygenous species in RGO with fewer CO (∼287.5 eV) and OC−O (∼288.8 eV). It is noted that even if annealing temperature is as high as 1000 °C, RGO can not be completely reduced because the C−O bond still exists as shown in Figure 2f. A series of fully suspended RGO photodetectors were fabricated based on the free-standing RGO thin films produced at different annealing temperatures (Figure 3). The light current (Ilight) and dark current (Idark) are defined as the current of photodetector under illumination and in the dark, respectively. The photocurrent (Iph) is defined as the difference D
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Figure 4. (a, c, e, g, and i) I−V characterizations measured in dark (black curve) and upon 1064 nm laser illumination (red curve), respectively, of fully suspended RGO detectors based on RGO thin films produced at varied annealing temperatures. (b, d, f, h, and j) Enlarged partial views of the I−V characteristics in the bias voltage range of 0.8−1.0 V.
temperature dependence, with a large negative temperature coefficient of resistance. The absolute value of negative temperature coefficient of resistance depends on the reduction level of RGO, that is, decreases with increasing the reduction level.16 The as-prepared RGO thin films with different
in the circuits for many times, resulting in a high external quantum efficiency.31 Furthermore, the presence of oxygenous functional groups leads to the transport mechanism of variable range hopping (VRH) for the conduction of electrons.16 This VRH behavior causes the RGO resistance to exhibit a strong E
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Figure 5. Time-resolved photoresponses of fully suspended RGO detectors based on free-standing RGO thin films produced at varied annealing temperatures under laser illumination of (a) 375 nm (UV) of 25 mW, (b) 405 nm (VIS) of 83 mW, (c) 532 nm (VIS) of 258 mW, (d) 785 nm (NIR) of 463 mW, and (e)1064 nm (NIR) of 547 mW at a bias voltage of 0.5 V.
positive photoresponse from RGO dominates the observed photoresponse of device, resulting in the Iph > 0. With increase of annealing temperature, the reduction degree of RGO
reduction levels exhibit similar properties discussed above (see Supporting Information). At lower annealing temperatures such as 200 and 400 °C, RGO possesses a lower reduction level, the F
DOI: 10.1021/acsphotonics.7b00768 ACS Photonics XXXX, XXX, XXX−XXX
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absolute value of responsivity appears always at 1000 °C, which indicates the photodetector based on the RGO film annealed at 1000 °C is most sensitive. This may be because the RGO film annealed at 1000 °C possesses the best conductivity (Figure 1g), which is very favorable for transport and collection of photoexcited carriers. The highest responsivity can be up to 51.46 mA/W at a small bias voltage of 0.5 V, which is 1 order of magnitude higher than those of reported photodetectors only using graphene as light active material.2 The response speed is another important parameter of a photodetector. The rise time and decay time of the photodetector are times gaps between 10% and 90% of the peak values of the Iph at rise and fall edges, respectively.32 The time interval between two collected data points was set as 18 ms when the current−time profile was recorded using Keithley 2400 Source Meter. Using single on/off cycle under 1064 nm NIR light illumination as an example, almost all the fully suspended RGO detectors prepared at various annealing temperatures possess fast photoresponse time (both the rise time and the decay time) less than 100 ms as shown in Figure 6. The fast response speed of fully suspended RGO detectors prepared at various annealing temperatures is appropriate not only for NIR laser irradiation, but also for UV and VIS laser irradiations. As shown in Table 2, upon VIS to NIR laser
increases, causing decrease of the positive photoresponse from RGO. At an annealing temperature of 600 °C, the positive photoresponse from RGO reduces nearly to the absolute value of negative photoconductivity of Au interdigital electrodes, and thus almost no photoresponse (Iph ≈ 0) was observed. Further increasing the annealing temperature to 800 and 1000 °C would cause the negative photoconductivity of Au interdigital electrodes to become dominant in the observed photoresponse of device, and thus the Iph < 0. The responsivity (Rλ) is one important parameter of a photodetector, which is defined as the photocurrent generated per unit power of the incident light on the effective area of a photoconductor. Here, we define responsivity is negative when Iph is negative. Because only 40.26% of effective area of laser spot is RGO and the other is Au interdigital electrodes, the laser power received by RGO thin film was estimated to be 40.26% of incident power accordingly. The responsivities of the detectors based on the RGO films produced at varied annealing temperatures under laser illuminations of different wavelengths were summarized in Table 1. It can be found that the maximum Table 1. Responsivities at a Bias Voltage of 0.5 V under Laser Illumination of Different Wavelengths of the Detectors Based on RGO Films Produced at Varied Annealing Temperaturesa
Table 2. Response Speed of the RGO Photodetectors Based on RGO Films Produced at Various Annealing Temperature under Laser Illumination of Different Wavelengths
responsivity (mA/W) wavelength (nm)
200 °C
400 °C
600 °C
800 °C
1000 °C
375 (25 mW) 405 (83 mW) 532 (258 mW) 785 (463 mW) 1064 (547 mW)
20.57 8.88 2.27 1.31 1.09
1.09 0.45 0.25 0.17 0.30
0 0 0.06 0 0.01
−1.09 −0.60 −0.35 −0.26 −0.49
−51.46 −48.48 −24.07 −16.15 −14.17
response speed (ms): rise time and decay time
a
Due to covering of Au interdigital electrodes, laser power actually received by RGO thin film should be calculated as 40.26% of incident power.
wavelength (nm)
200 °C
400 °C
800 °C
1000 °C
375 405 532 785 1064
102, 102 55, 109 64, 103 51, 161 60, 115
108, 127 75, 139 72, 118 60, 118 40, 79
146, 86 97, 139 55, 104 60, 95 44, 66
101, 103 76, 90 65, 93 65, 88 71, 81
Figure 6. Enlarged views of single on/off cycle of fully suspended RGO detectors produced at varied annealing temperatures under 1064 nm NIR light illumination at a bias voltage of 0.5 V. G
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Figure 7. Photocurrents of detector prepared at (a) 200 °C and (b) 1000 °C as a function of illumination power under laser illumination of 375 nm (UV) at a bias voltage of 0.5 V. The solid lines are linear fitting curves and dashed are extensions of the linear fitting curves. Insets: their respective curves in detail in the power range below 8 mW, corresponding fitting equations and linear correlation coefficients (R2).
irradiations, the rise time of all the fully suspended RGO detectors prepared at various annealing temperatures are tens of milliseconds. These response speeds are one to 4 orders of magnitude faster than those of the other reported supported RGO photodetectors,31,33−36 and is on the same order of magnitude as that of the supported CVD-grown37 and supported mechanically exfoliated38 graphene photodetectors. These response speeds are also faster than that of our previous work.20 More importantly, the response speed results (see Table 2) of fully suspended RGO detectors prepared at various annealing temperatures reveal that when the annealing temperature reaches or exceeds 200 °C, the detector is able to possess fast response speed, which means fast response RGO-based device could be fabricated at low annealing
temperature instead of previously reported high temperature of 1000 °C.20 The effects of film thickness on photoresponse was preliminarily studied. The photocurrent and response time both increase with increasing the film thickness (see Supporting Information). This is because a thicker film led to a higher light absorption to produce more photocurrent, and simultaneously suffered from longer time to reach to the steady state of current upon on and off cycles of switching lasers. Based on the results of comparison with substrate supported photodetector (see Supporting Information) and results of photoresponses from fully suspended photodetector with different film thickness (see Supporting Information), there may be two main reasons responsible for the fast response. One H
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is the fully suspended construction of device, which avoids the slow heat transfer between the RGO thin film and the substrate, and thus can quickly reach to steady state of current. The other is the thickness of film, for fully suspended photodetector, the thinner the film, the shorter the time to achieve steady state of current (see Supporting Information). Because detectors prepared at 200 and 1000 °C possess high responsivity (Table 1), the relationships between the Iph and incident laser power for the two detectors were further studied. Figure 7a,b show that both photocurrents of detector prepared at 200 and 1000 °C were commendably proportional to incident laser power ranging from 128 μW (or 400 μW) to 8 mW. Nevertheless, with further increase of incident laser power, the photocurrents gradually become lower than the linear fitting, that is, the photocurrents appear saturation. Due to the limited power (maximum power of 25 mW) of 375 nm laser, the saturation of photocurrent is not remarkable. More significant phenomenon of saturation of photocurrent was observed in the detector based on a thicker RGO thin film under higher powers illumination of 532 nm laser (see Supporting Information). Long-term stability of the fully suspended RGO photodetector was investigated. After the photodetector was placed for 15 days at room temperature in air, the performance of device does not decay (see Supporting Information).
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00768. Figure S1: The saturation of the photocurrent under high power illumination; Figure S2: Long-term stability of fully suspended RGO photodetector; Figure S3: Sheet resistance−temperature plots of RGO thin films prepared at different annealing temperatures; Figure S4: Effects of film thickness on photoresponse; Figure S5: Comparison of photoresponses of fully suspended photodetector with supported photodetector (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Junhui He: 0000-0002-3309-9049 Jia-Lin Sun: 0000-0002-1849-5916 Author Contributions †
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These authors contributed equally to this work.
Notes
CONCLUSIONS A series of free-standing thermally reduced graphene oxide thin films were prepared at various annealing temperatures from 200 to 1000 °C. Based on the as-prepared RGO thin films, fully suspended RGO photodetectors were fabricated. Effects of the annealing temperature on I−V characteristic and photoresponse of fully suspended RGO photodetectors were systematically studied. A new physical phenomenon has been found that the photocurrent of photodetectors changes from positive to zero to negative with the increase of the annealing temperature over a wide spectral regime from ultraviolet (375 nm) to near-infrared (1064 nm). That may be due to the existence of the competition between the positive photoresponse from RGO depending on the degree of reduction of GO and the significant negative photoconductivity of Au interdigital electrodes caused by photoexcited surface plasmon polaritons scattering transportation of electrons. The maximum of responsivities of photodetectors appears at the highest annealing temperature of 1000 °C. The highest responsivity can be up to 51.46 mA/W at a small bias voltage of 0.5 V, which is 1 order of magnitude higher than those of supported photodetectors using only graphene as light active elements. Upon visible (405 nm) to near-infrared (1064 nm) irradiations, all the fully suspended RGO detectors prepared at various annealing temperature exhibit fast rise time with tens of milliseconds, which is one to 4 orders of magnitude faster than previously reported supported RGO photodetectors and is on the same order of magnitude as the photodetectors based on supported CVD-grown and supported mechanically exfoliated graphene. The fast photoresponse originates from the fully suspended construction of device and the thin thickness of film, which provides a new design idea for fast response RGO-based device. The finding that the photoresponse of RGO photodetector can be accurately controlled to be positive or negative by tuning the degree of reduction of GO has great significance in developing and designing RGO-based light triggered logic devices applied to integrated optoelectronic systems.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Technology Commission of Beijing Municipality (Grant No. Z151100003315018), Foundation of Technical Institute of Physics and Chemistry (Grant No. 2014-JP-01), National Natural Science Foundation of China (Grants No. 11104283, 21571182, 21271177, 11174172, 11374172), and Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT_16R07).
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ACS Photonics
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DOI: 10.1021/acsphotonics.7b00768 ACS Photonics XXXX, XXX, XXX−XXX
