Fabrication of flexible rGO/Fe O hollow nanospheres … on-chip micro-supercapacitors for integrated...

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Fabrication of flexible rGO/Fe2O3 hollow nanospheres based on-chip micro-supercapacitors for integrated photodetecting applications Shaosong Gu1,2, Zheng Lou2, Ludong Li2, Zhaojun Chen3, Xiangdong Ma1 ( ), and Guozhen Shen2 ( ) Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-015-0923-7 http://www.thenanoresearch.com on Oct. 14, 2015 © Tsinghua University Press 2015

Just Accepted This is a “Just Accepted” manuscript, which has been examined by the peer‐review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication.

Nano Research DOI 10.1007/s12274‐015‐0923‐7

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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Fabrication of flexible rGO/Fe2O3 hollow nanospheres

based on-chip Micro-Supercapacitors for integrated

photodetecting applications

Shaosong Gu,1,2 Zheng Lou,2 Ludong Li,2 Zhaojun Chen,3

Xiangdong Ma1* and Guozhen Shen2*

1 Department of Material Science and Engineering, China

University of Mining &Technology (Beijing), Beijing,

100083, PR China. 2 State Key Laboratory for Superlattices and

Microstructures, Institute of Semiconductors, Chinese

Academy of Sciences, Beijing 100083, PR China. 3 College of Chemical Science and Engineering, Qingdao

University, Qingdao 266073, PR China

Flexible integrated on-chip Micro-supercapacitors (MSCs) were fabricated with rGO/Fe2O3 hollow nanospheres electrodes, which not only showed superior electrochemical performance, excellent mechanical flexibility, but also can drive nanowire photodetector well, demonstrating the feasibility of the flexible integrated photodetector systems.

Guozhen Shen, http://www.escience.cn/people/gzshen

1Nano Res.

Fabrication of flexible rGO/Fe2O3 hollow nanospheres based on-chip Micro-Supercapacitors for integratedphotodetecting applications Shaosong Gu,1,2 Zheng Lou,2 Ludong Li,2 Zhaojun Chen,3 Xiangdong Ma1 ( ) and Guozhen Shen2 ( ) 1 Department of Material Science and Engineering, China University of Mining &Technology (Beijing), Beijing, 100083, PR China.

2 State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, PR China 3 College of Chemical Science and Engineering, Qingdao University, Qingdao 266071, PR China

Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer‐Verlag Berlin Heidelberg 2014 KEYWORDS Fe2O3, Microsupercapacitor, Photodetector, Flexible

ABSTRACT Micro‐supercapacitors (MSCs) as important on‐chip micro‐power sources have attracted considerable attention owing to their unique and advantageous design for optimized maximum functionality within a minimized sized chipand excellent mechanical flexibility/stability on miniaturized portableelectronic devices applications. In this work, we report a novel,high‐performance flexible integrated on‐chip MSCs based on hybrid nanostructures of reduced graphene oxide (rGO)/Fe2O3 hollow Nanospheres via a microelectronic photo‐lithography technology combined with plasma etching technique. The unique structural design for on‐chip MSCs enables high performance enhancements compared to graphene‐only devices, exhibiting high specific capacitances of 11.57 F cm‐3 at a scan rate of 200 mV/s and excellent rate capability and robust cycling stability with capacitance retentionof 92.08 % after 32,000 charge/discharge cycles. Moreover, the on‐chip MSCs have superior flexibility and outstanding stability even after repetition ofcharge/discharge cycles under different bending state. As‐fabricated highly flexible on‐chip MSCs can be easily integrated with CdS nanowire‐based photodetector to form into a highly compacted photodetecting system,exhibiting comparable performance with the device driven by conventionalexternal energy storage units.

Introduction

The rapid development of miniaturized portable and wearable electronic equipment, such as micro‐electromechanical systems (MEMS),

microrobots and implantable medical devices, has greatly increased the demand for

micro/nanopower sources that can possibly be integrated on a chip with other electronic units [1,2]. It is especially important to design efficient, miniaturized energy‐storage devices that can match or exceed that of the micro‐machine being

powered and integrate the microscale energy storage units as close as possible to the electronic

Nano Research DOI (automatically inserted by the publisher)

Address correspondence to Guozhen Shen, E-mail: [email protected]; Xiangdong Ma, E-mail: [email protected].

Research Article


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circuit on a chip, in other words, to build a self‐powered micro/nanodevice systems [3‐5]. In contrast to miniaturized micro‐batteries whose properties often suffer from fundamental problems in those redox reactions inevitably, such as relatively poor charge/discharge rates, and limited cycle life (hundreds to thousands of cycles) [6‐8], micro‐supercapacitors (MSCs) have better performance include ultrahigh power density and fast rate capability and long cycle lifetime (millions of cycles) and excellent safety which are very potential serve as miniaturized ultrahigh‐power devices that provide sufficient peak power in energy storage and harvesting applications [9‐11]. Therefore, on‐chip MSCs are used to meet the demand for micropower sources with miniaturized size and high power density, not only can fit on‐chip geometries of integrated circuits by adjustment dimension but also can be compatible with current MEMS in manufacture techniques [6,12]. To date, many kinds of MSCs have been

fabricated using different electrode materials [13‐17] and among them graphene is the most commonly used electrode because of its larger theoretical surface area [18‐20], faster electrical conductivity [21‐23]. For example, Ajayan’s group developed all‐graphene based monolithic MSCs by direct laser reduction and patterning of hydrated graphite oxide (GO) films [11]. ElKady and Kaner reported a scalable fabrication method for graphene‐based planar MSCs by direct laser writing on GO films using a standard Light Scribe DVD burner [12]. We also designed an rGO‐based planar micro‐supercapacitors with high cyclability and good flexibility by a simple micro‐manufacturing technology, and the optimized electrodes in the rGO‐based MSCs device were utilized as the source and drain electrodes of CdS photodetector [14]. However, the relatively low electrochemical capacitance and insufficient energy densities of the graphene‐based MSCs limits their applications in practical electronic utilization [15‐17]. Thus, it is necessary to overcome this restriction and improve the electrochemical performance with high capacitance and good cycling stability by developing new electrode materials.

To improve the electrochemical performance of graphene, a general and powerful method is to combine graphene with other high capacity pseudocapacitive materials to form into composite electrodes. Many kinds of graphene‐based composite electrodes have been developed including graphene/NiO [24], graphene/Co3O4 [25], and graphene/Co(OH)2 [26] etc. As expected, these composite electrodes do exhibit greatly improved electrochemical performance compared with pristine graphene electrodes. However, till now, most of the above graphene‐based composite electrodes are configured as conventional supercapacitor structures and seldom of them were used as electrodes for on‐chip MSCs using the microelectronic compatable technique. In our work, we fabricated a flexible integrated

MSCs by using the patterned reduced graphene oxide (rGO)/Fe2O3 hollow nanospheres composite as the electrodes via a conventional microelectronic photo‐lithography technology combined with plasma etching technique. Fe2O3 hollow nanospheres were selected as the electrode material to combine with graphene nanosheets because of its high theoretical specific capacitance good stability in alkaline electrolyte, low toxicity, cost effectiveness and easy synthesis[27,28]. As‐fabricated flexible on‐chip MSCs exhibited excellent volume capacitance of 11.57 F cm‐3 at a scan rate of 200 mV/s and outstanding cycling stability. About 92.08% of the initial value was remained after 32,000 charge/discharge cycles. Moreover, the MSCs have superior flexibility and stability even after repetition of charge/discharge cycles with convex and concave bending state. As‐fabricated highly flexible on‐chip MSCs can be easily integrated with CdS nanowire‐based photodetector to form into a highly compacted photodetecting system.

Experiments

Synthesis of rGO /Fe2O3 nanocomposites: In a typical procedure, graphene oxide (GO) was first synthesized via a modified Hummer method using natural graphite powder [29]. rGO/Fe2O3 hollow Nanospheres was synthesized via an easy one‐step hydrothermal process. Briefly, ~40 mg of graphene oxide (GO) was dispersed in 35ml deionized water under ultrasonication for 1 h to form homogeneous solution. Then, 0.26g FeCl3 were added into the


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above solution under ultrasonication for 0.5 h. Finally, the mixture was sealed in a Teflon lined stainless steel autoclave for hydrothermal reaction at 180 ºC for 12 h. The product was filtered, washed with deionized water for several times resulted in the formation of rGO nanosheets/Fe2O3 nanospheres hybrid composites. The PVA/KOH polymer electrolyte was prepared as follows: in a typical process, 4 g PVA was dissolved in 35 mL DI water with stirring at 100 ºC for 2 h. Then, 1.63 g KOH was dissolved in 5 mL DI water. Finally, the above two solutions were mixed together at 60 ºC under vigorous stirring until the solution became clear. Synthesis and characterizations of CdS nanowire: CdS nanowires were synthesized via a simple vapour transport method In a typical process, 0.01 g CdS powder (99.999%) was placed on an alumina boat at the centre of a tube furnace as the source material. A silicon wafer coated with ~10 nm thick Au layer as the catalysts was placed on the other alumina boat located downstream, at a distance of 16‐17 cm from the tube centre. After the tube was purged with pure Ar at 110 ºC for 2 h, the system was heated to 1000 ºC in 35 min and maintained at that temperature for 2 h. The flow rate of the carrier gas (Ar) was kept at 300 sccm (standard cubic centimeters per minute). After the reaction ended, the furnace was cooled down to room temperature naturally. Fabrication of CdS nanowire photodetector on the rGO/Fe2O3 MSCs: After the CdS nanowires being removed from the Si slice, 10 μL of CdS solution (20 mg/mL in isopropanol) was spread in one of the gaps between the square electrodes. Nanowires formed a dense film connecting the middle electrode and one of the electrodes of MSCs. After drying, the device was available for testing. Materials characterization: The synthesized products were characterized with an X‐ray diffractometer (XRD; X’Pert PRO, PANalytical B.V., the Netherlands) with radiation from a Cu target (Kα, λ = 0.15406 nm). The morphologies of the samples were characterized using electron microscopy (SEM; NANOSEM 650‐6700F, 15 kV) and transmission electron microscopy (TEM; JEOL JEM‐2010HT). X‐ray photoelectron spectra of the products were measured by an X‐ray photoelectron spectrometer (GENESIS, Americaʹs dax co., LTD). Raman spectra were obtained on a Renishaw RM2000 Raman spectrometer with 457.9 nm

wavelength incident laser light. The electrochemical performance of the fabricated on‐chip MSCs were measured by the CHI 760D electrochemical work stations. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 0.01 Hz to 100 kHz at open circuit potential with an ac perturbation of 5 mV. Calculations: All the on‐chip MSCs were tested in a symmetrical two electrode configuration. The specific capacitance of each micro‐device obtained through cyclic voltammograms was based on the following equation:

∫=V

0IdV

SVX1

XC

(1) Where Cx is the specific capacitance (F cm‐2, F cm‐3; X stands for area (cm2) and volume (cm3) of the electrode, respectively), S is the scan rate in cyclic voltammograms (V s‐1), V represents the potential window (V), and I stands for current (A). The energy density (E, in Wh cm‐3) and power density (P, in W cm‐3) of the micro‐device were calculated as follows:

36002V×C

2

V

×=E

(2)

t3600EΔ×

=P

(3) where Cv is the volumetric capacitance obtained from Equation (1) (in F cm‐3), V is the operating voltage window (in V), and ∆t is the discharge time of the micro‐device (in second).

Results and discussion

Fig. 1a shows typical X‐ray diffraction (XRD) patterns of the as‐prepared rGO/Fe2O3 composites. Obvious evidence indicates that all of the observed peaks position is corresponding well with the pure crystal phase of α‐Fe2O3 (JCPDS No:33‐0664) [30], demonstrating that Fe2O3 with high purity can be well synthesized on the graphene sheets. Fig. 1b shows the Raman spectra of reduced graphene oxide (rGO) and the rGO/Fe2O3 composites. Both samples display similar two bands at 1348 and 1591 cm‐1, which correspond to the disordered (D) band


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and graphitic (G) band of carbon materials respectively. The ratios between the intensity of the D band and the G band (ID/IG) were usually taken as a measure of defect concentration. The values of ID/IG increased from 0.96 to 1.0, indicating the increase of surface disorders for the rGO/Fe2O3 composites. This enhancement of the disordered carbon content should be ascribed to the partial insertion of Fe2O3 nanospheres into graphene layer, which is in accordance with previous reports about graphene‐based nanocomposites [31]. In order to further confirm the surface information of the obtained samples, X‐ray photoelectron spectroscopy (XPS) was also carried out, as shown in Fig. 1c. It can be clearly seen that the survey spectrum of the rGO/Fe2O3 composites mainly

shows carbon, oxygen, and iron species. The peak located at 284.89 eV is assigned to the characteristic peak of C1s, which can be divided into several peaks (Fig. S1a). The intensities of all C1s peaks of the carbon binding to oxygen are very weak indicating that the GO is reduced [32]. The Fe 2p XPS spectra of the sample exhibit two main peaks at 711.6 and 724.8 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 spin‐orbit peaks of Fe2O3 (Fig. S1b). Moreover, a satellite peak at 718.9 eV was also observed, which is in accordance with the characteristic of α‐Fe2O3. This result is also consistent with the XRD analysis as mentioned above and further identify the existence of Fe2O3 in the composites.

Figure 1 (a) XRD patterns, (b) Raman spectra, (c) XPS spectrum, and (d-f) SEM, TEM and HRTEM images of the as-obtained

rGO/Fe2O3 composites.

The morphology and detailed microstructures of the as‐synthesized rGO/Fe2O3 composites were then measured by SEM and TEM, as shown in Fig. 1d‐f. Fig. 1d and 1e are typical SEM image and TEM image of the rGO/Fe2O3 hollow Nanospheres, where both Fe2O3 nanospheres and rGO nanosheets can be clearly seen, with the Fe2O3 hollow nanospheres well dispersed on the surface of rGO nanosheets. As clearly shown in Fig. 1f, the HRTEM image of Fe2O3 hollow nanospheres shows the fringe spacing of ~0.27 nm, which is well consistent with the (104) crystal planes of α‐Fe2O3

[30]. On‐chip MSCs were then fabricated by using the

rGO/Fe2O3 composites as the electrodes. Fig. 2a shows the schematic illustration of a typical fabrication process of the flexible on‐chip MSCs on PET substrate. To enhance the hydrophility and wettability of the PET substrate, its top side was first treated with air plasma (750V, 45min) in a plasma cleaner with 0.6 L/min of air flow. Then, the as‐synthesized rGO/Fe2O3 hybrid composites dispersed in ethanol were spin‐coated on the treated PET substrate at 500 revolutions per minute


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(rpm) for 15 s and 4000 rpm for 30s to produce an uniform rGO/Fe2O3 film as electrodes material. After spin coating, the substrate was heated at 100 ºC for 4 min to evaporate the water. Subsequently, a thin Ni film (~40 nm) was deposited on the rGO/Fe2O3 film to enhance the conductivity of the electrodes by thermal evaporation, followed with the spin coating of a thin layer of photoresist. Photolithograph was then performed to pattern the electrodes with desired device structures. Un‐wanted Ni film and rGO film was removed by

2 M HNO3 and air plasma, respectively. PVA/KOH gel electrolyte was spread on the MSCs, resulted in the formation of flexible all‐solid‐state devices. Fig. 2b illustrated the three‐dimensional (3‐D) structure of the as‐fabricated symmetric interdigital MSCs with the rGO/Fe2O3 electrodes. The conventional photolithography process is readily scalable for the efficient fabrication of the MSCs and multiple integrated on‐chip MSCs can be easily produced on a flexible PET substrate, as shown in Fig. 2c.

Figure 2. (a) Schematic illustration of the fabrication process of the flexible on-chip MSCs on PET substrate. (b) 3-D Schematic illustration of the fabricated on-chip MSCs. (c) Digital photograph of integrated on-chip MSCs on a single PET substrate.

Electrochemical performance of the rGO/Fe2O3 on‐chip MSCs was firstly evaluated by two‐electrode cyclic voltammetry (CV) and galvanostatic charge‐discharge measurements. CV curves at various scan rates varying from 0.2 to 3.0 V s‐1 were shown in Fig. 3a. The quasi‐rectangular shapes of CV loops obviously indicate fast charge transfer reaction and efficient electrical double layer (EDL) formation for ideal capacitive behavior [33,34]. Note that there are no apparent redox peaks observed at the voltage range of 0 to 1 V, which reveals that this double‐layer capacitor‐like CV behavior actually originates from the fast, reversible successive surface redox reactions of Fe2O3 by means of surface electro‐adsorption of ion, as well as proton incorporation [6,18]. Moreover, the galvanostatic charge‐discharge measurements under different current density (from 60 to 200 μA/cm2) were shown in Fig. 3b‐c. Both the charge/discharge curves display linear and relative symmetric triangular shapes, indicating excellent double‐layer capacitor performance, in accordance with the CV curves. Fig. 3d shows the voltage drop of the MSCs at various current densities. The linear

increase of voltage drop with current densities conformed to the overall internal resistance, signifying the lower overall internal resistance of the MSCs. Fig. 3e shows the change of the capacitance

retention as the number of cycles increase at a charge/discharge current density of 100 μA/cm2. The device retained about 92.08% of the initial value after 32,000 cycles, which is comparable to that of other active materials including laser‐scribed graphene electrochemical capacitors (96.5% retention after 10000 cycles) [35], rGO‐CNT micro‐supercapacitors (95% retention after 1000 cycles) [36], and rGO/PANI composite (90% retention after 1700 cycles) [37]. This result suggested an excellent long‐term electrochemical stability, which can also be certificated by the CV and Galvanostatic charge‐discharge curves of the 1st and 32,000th cycles measured as shown in Fig. S2a‐b. The areal capacitance and volume capacitance values of the rGO/Fe2O3 electrode can be calculated and plotted in Fig. 3f, respectively. Clearly, the on‐chip MSCs achieved a volume capacitance of 11.57 F/cm3 and an areal capacitance


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of 347 μF/cm2 even when applied with a scan rate as high as 200 mV/s. To our best knowledge, the volume capacitance of our rGO/Fe2O3 based device is much higher than the reported values of graphene‐based device (1.3 F cm‐3 for onion‐like carbon/graphene based MSCs, 3.1 F cm‐3 for laser‐written reduced graphene oxide MSCs, and 3.05 F cm‐3 for laser‐scribed graphene MSCs) [10‐12].The great enhancement in the volume capacitance of the current on‐chip MSCs can be

contributed to the synergistic effect of composites, namely, Fe2O3 nanospheres in this case as a good pseudocapacitive candidate material has superior capacity, and the large surface area of rGO nanosheets in this case leads to much higher loading of Fe2O3 nanospheres. Moreover, the capacitance is reduced with the increase of the scan rate in Fig. 3f, owing to the efficient infiltration of ions in the electrolyte is step by step reduced to micro‐electrodes, according to previous report [22].

Figure 3. Electrochemical properties of the on-chip MSCs. (a) CV curves at different scan rates varying from 0.2 to 3.0 V s-1. (b-d) Galvanostatic charge-discharge curves at different current density from 60 to 200 µA/cm2 and corresponding voltage drop. (e) Variation of capacitance retention of the MSCs with 32000 cycle number. The inset shows the 10 last cycles of the charge/discharge process. (f) Areal capacitance and volume capacitance of the MSCs at different scan rates.

The rate performance is another important characteristic for high‐performance capacitors in addition to large reversible capacities and good cyclabilities. Fig. 4a shows the rate performance of the on‐chip MSCs at varied current density. After 2500 times of continuous cycling from 100 to 60 μA/cm2, respectively, almost without noticeable decrease for the initial capacitance further revealing the excellent electrochemical stability of the on‐chip MSCs. Typical Nyquist plots in frequencies ranging from 100 kHz to 0.01 Hz are presented in Figure 4b. The equivalent series resistance (ESR) of the MSCs was estimated to be 200 Ω, evidencing that our electrodes have very small resistance values with good ion response.

The Nyquist curve displayed a vertical line to the x‐axis in the low frequency region, which indicates that the MSCs acts like an ideal capacitive behavior. Fig. 4c shows the Ragone plot in which the volume energy and power densities of the rGO/Fe2O3 MSCs are compared with other energy storage devices reported in the literatures. The on‐chip MSCs exhibit a maximum volumetric energy density of 1.61 mW h cm‐3 and a maximum volumetric power density of 9.82 W cm‐3. Remarkably, these values are higher than those obtained in other all‐solid‐state supercapacitors [38,39] and graphene‐based MSCs [14] as well as a commercial lithium thin‐film battery (4 V/500 μA h) [10] and an Al electrolytic capacitor (3 V/300 μF) [12]. This


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result also confirms the excellent performance of our rGO/Fe2O3 MSCs.

Figure 4. (a) Cycle performance at progressively varying current densities. (b) Nyquist plots of the on-chip MSCs. The inset shows the high-frequency region. (c) Ragone plots (energy density vs power density) providing a comparison of the current on-chip MSCs (red squares) with other similar devices reported in the literature.

The flexibility of on‐chip MSCs under different bending states was further investigated in this work. Fig. 5a demonstrates the optical image of a device under bending state, revealing the excellent flexibility of the on‐chip MSCs. Fig. 5b and 5c present the CV curves (at a scan rate of 1V/s) and galvanostatic charge/discharge curves (at 100 μA/cm2) of the on‐chip MSCs under different bending states from the flat state to convex bending (bending radius = 3.2 mm), concave bending, and then returning to the flat state again. The total area of curves in both cases remained the same shape and did not show obvious variations, further confirming the outstanding flexibility and mechanical stability of the fabricated device. Moreover, the stable features of the on‐chip MSCs can also be displayed through long‐term charge/discharge measurements under different bending states, as shown in Fig 5d. Even after 2000 bending cycles, the capacitance almost kept its original value (~96.6%), revealing that the on‐chip MSCs device can be strong enough to bear the

generated tensile force, which is very useful in practical applications as flexible energy storage units.

Figure 5. (a) Photograph of the fabricated rGO/Fe2O3 MSCs under the bending states and its (b) CV curves and (c) galvanostatic charge/discharge curves under flat, concavx bending and convex bending states, respectively. (d) Capacitance retention as a function of the cycle number under different bending states.

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Figure 6. (a)SEM images, (b) XRD pattern of the as-synthesized CdS nanowires. (c) The schematic illustration of the electric circuit.(d) The self-discharge curve, (e) I-V test result of the device under different power intensities of 470 nm light illumination and in dark. (f) Photocurrent versus time plots of the device under illumination at a bias of 0.24 V supplied by an external power source and driven by the MSCs, respectively. (g) Photoresponse charateristics with different light intensities driven by the MSCs device. (h) The dependence of photocurrent on light intensity. The fitting result is I∼P0.62.

In order to achieve a flexible integrated on‐chip photodetecting applications, CdS nanowires which were produced by conventional chemical vapor deposition method were used as the photoresponsing materials. Fig 6a shows the typical scanning electron microscopy (SEM) image of the as‐grown CdS nanowires, which clearly have the diameter in the range 80‐�200 nm and length of several hundreds of micrometers, possessing high aspect ratio. Fig. 6b is the X‐ray diffraction pattern of CdS nanowires. Obvious evidence indicates that all of the observed peaks position is corresponding well with a hexagonal structure (JCPCD No. 65‐3414). The sharp diffraction peaks in each crystal facets demonstrates the good crystallinity, which is fundamental for an excellent photodetector device. As for the flexible integrated photodetecting system, a complete integration should contain energy storage device and the powering of photodetectors, so we printed CdS nanowires on the electrodes of the MSCs to get a integrated on‐chip photodetecting applications, as shown in Fig 6c. The current collector of MSCs acts as one electrode of the CdS photodetector while the middle square electrode between the MSCs acts as another electrode. Due to the excellent electrochemical performance of our device, the self‐discharge properties of the MSCs

was also measured and the corresponding result was shown in Fig 6d. It can be seen that, after being charged to 1.0 V, the MSCs maintained at 0.24 V for more than 21,000 s, demonstrated the possibility of integrating the MSCs with a photodetector device. Similarly, to confirm CdS photoresponsing properties, the I‐V curves of the CdS nanowires assembled film were measured in different power intensities (0.569 to 1.281 cmW/cm2) of the 470 nm light irradiation and in the dark, respectively. We can see that with the light

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intensities increased, the light current between source electrode and drain electrode significantly increases at the same voltage, demonstrates that the CdS nanowires photodetector exhibited a high sensitivity, as can be seen in Fig 6e. Then, we studied the photoresponse charateristics of the CdS nanowire‐based photodetector in the integrated photodetecting system. The time dependent photoresponse of the flexible photodetecting system, driven by an external power source with the integrated device, was measured by periodically turning the white light on and off, as shown in Fig 6f. From the curve measured by the MSCs and compared with an external bias of 0.24 V, we found that, current on/off ratios of 10.53 and 9.81 were obtained for the two systems, respectively, which indicated the feasibility and stability of the use of MSCs to substitute the conventional energy unit as driven source for a photodetector. In addition, we also found that the photoelectric response time and recover time are 1.21 s and 1.40 s for the self‐powered photodetector, which is even more sensitive than external power source driven one (1.22 s and 1.41 s). In order to further study the photoresponse characteristics driven by the MSCs, we also measured two cycles of the I‐T curve under different power intensities of 470 nm light illumination as shown in Fig. 6g. It can be seen that the saturated photocurrent increases as the raising of light intensity, and then recovers much the initial value as the recovery of light intensity, indicating the good reproducibility of the photodetector driven by the MSCs. The photocurrent versus light intensity can be described as a power law (Fig. 6h), I=APθ [40], which can be interpreted as follows: the photocurrent I, a constant A, the light intensity P and the empirical value θ. As shown in Fig. 6h, the photocurrent and light intensity driven by the MSCs, have a strong linearly dependence, that the value of the θ is 0.62 (I~P0.62) which proved the CdS nanowire‐based photodetector in the integrated photodetecting system possessing an amazing photocurrent capability. From our results, we deduced that, compared with the conventional external energy unit, the MSCs as the driven source not only ensures stable and comparable performance, but also provide new features such as excellent flexibility, microminiaturization, and highly compact integration of the full systems, which are believed to be integratable to the update microelectronic technology.

Conclusions In conclusion, a simple and scalable microelectronic photo‐lithography technology combined with

plasma etching technique to fabricate flexible rGO/Fe2O3 hollow nanospheres based on‐chip Micro‐Supercapacitors for integrated photodetecting applications has been reported. Such fabricated on‐chip MSCs on the flexible PET substrate worked in the potential range of 0 to 1 V, and the volume capacitances can reach 11.57 F cm‐3 at a scan rate of 200 mV/s, and about 92.08% of the initial capacitance was remained even after 32,000 cycles. Moreover, the on‐chip MSCs have superior flexibility and outstanding stability even after repetition of charge/discharge cycles with convex and concave bending states. Furthermore, the MSCs were utilized as the source and drain electrodes of the CdS photodetector, demonstrating the feasibility of the flexible integrated photodetector systems which are promising for further large‐scale and integrated applications. Acknowledgements This work was supported by the National Natural Science Foundation (61377033). References [1] S, C, Yao.; X, D, Tang.; C, C, Hsieh.; Y, Alyousef and C, H, Amon. Micro-electro-mechanical systems (MEMS)-based

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Fabrication of flexible rGO/Fe O hollow nanospheres … on-chip micro-supercapacitors for integrated...

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