Robust Photodetectable Paper from Chemically Exfoliated MoS2

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Robust Photodetectable Paper from Chemically Exfoliated MoS2− MoO3 Multilayers Yuefan Wei,*,† Van-Thai Tran,† Chenyang Zhao,‡ Hongfei Liu,§ Junhua Kong,*,§ and Hejun Du*,† †

School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore College of Chemistry and Environmental Engineering, Shenzhen University, 1066 Xueyuan Avenue, Nanshan District, Shenzhen 518071, PR China § Institute of Materials Research and Engineering, A*STAR, 2 Fusionopolis Way, Innovis, 138634, Singapore ‡

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S Supporting Information *

ABSTRACT: Photodetectors, which are capable of detecting light with varied wavelength, have nowadays been widely applied onto emerging fields such as security, entertainment, healthcare, environment, and so on. As the one with a two-dimensional layered structure, molybdenum disulfide (MoS2) possesses striking optical and electrical properties that can be used in photodetecting, yet the challenges remain in terms of material processing, device fabrication simplicity, and enhancement of overall photodetection performance. In this work, a photodetectable paper based on a mixture of double-phased MoS2 (1T and 2H) and MoO3 was successfully fabricated through a straightforward route, that is, chemical exfoliation and deposition of MoS2 powder on a flexible cellulose ester membrane, followed by inkjetprinted PEDOT:PSS as electrodes. The obtained device shows varied sensitivity to the light with different wavelengths. Compared with that under green and red lights, the prepared photodetector has the highest internal quantum efficiency (0.063%) and responsivity (0.134 mA W−1), while having longest response/recovery time (17.5/15.3 s) when illuminated with purple light (405 nm). The achieved responsivity is much higher than other reported liquid exfoliation- and solution-derived MoS2 photodetectors. This is ascribed to (1) the enhanced photoelectron generation caused by both MoS2 and MoO3 and (2) the good electric conductivity and efficient charge transport caused by the metallic 1T MoS2. This work demonstrates the feasibility of fabricating the MoS2-based photodetector with excellent performance through a simple exfoliation/filtration and inject printing route, and the detailed study on the response to light with different wavelengths unveils the interaction between the device and the incident light, further broadening the potential applications of such design. KEYWORDS: MoS2, inkjet printing, photoresponse, photodetector, paper dynamics for graphene11 as well as strong light absorption, electroluminescence, and tunable band gap for TMDs.12 As one of the major TMDs, molybdenum disulfide (MoS2) is an attractive functional material because of its unique mechanical, optical, and electrical properties, and has been widely used as building blocks of lubricants,13 photovoltaics,14 catalysts,15,16 supercapacitors,17−19 lithium-ion batteries,20,21 and photodetectors.10,22 MoS2 can be synthesized through various methods, including chemical vapor deposition (CVD),23 hydrothermal method,24 sol−gel method,25 physical vapor deposition,26 and mechanical/chemical exfoliation.27,28 The production of the corresponding photodetectors is normally through transferring the route from hard substrate or bulky MoS229 or direct deposition in some cases.30 The structures, morphologies, doping, and the layer number of MoS2 can be tailored into the one with different band gaps,

1. INTRODUCTION Photodetection refers to a capability process of detecting light signal via converting the light input into precise electric output (photocurrent or photovoltage) followed by measuring the produced electrical signal. Photodetection is useful in a rich variety of light-sensitive fields such as night vision, space exploration, security, entertainment, healthcare, and environment monitoring.1−4 As for the corresponding optoelectronic devices (photodetectors), their performance is critically determined by the photoelectric materials and their processing. In addition to the commercialized crystalline silicon,5,6 oxides, alloys, and inorganics from III−V semiconductors as well as some organics7 have shown appealing photodetecting properties, and the manipulation of their morphologies and structures into nanoscale and varied dimensions has proven to be an efficient strategy to further level up the performance. Among these, two-dimensional layered materials, including graphene/ graphite3,8,9 and transition-metal dichalcogenides (TMDs),10 are of particular interest of both fundamental and practical perspective for photodetecting because of the ultrafast carrier © XXXX American Chemical Society

Received: January 23, 2019 Accepted: May 23, 2019

A

DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

cellulose ester membrane with a pore size of 220 nm was supplied by Merck Millipore. 2.2. Preparation of the MoS2 Thin Film. MoS2 powder (0.6 g) was added into 8 mL of n-butyllithium solution and stirred for 2 days under the protection of argon gas. The intercalated MoS2, LixMoS2, was washed with fresh hexane three times to remove excess lithium and organic residues. Then, exfoliation was carried out immediately by ultrasonicating LixMoS2 in deionized water (1 mg mL−1) for 1 h. The mixture was centrifuged at 2000 rpm for 5 min to remove bulky MoS2 and further diluted to 0.1 mg mL−1. The MoS2 thin film was prepared through vacuum filtering the diluted suspension on a cellulose ester membrane and then air drying. 2.3. Deposition of Electrodes. PEDOT:PSS electrodes (the advantages of PEDOT:PSS were demonstrated in S1 in the “Supporting Information”) were inkjet-printed using a Dimatix inkjet printer (DMP-2831, Fujifilm) and a cartridge with an orifice diameter of 21 μm. The PEDOT:PSS solution (0.6−1.2 wt % in water, viscosity: 5−20 cP) was filled into the cartridge. After that, droplets with a nominal volume of 10 pL were printed onto the as-prepared substrates with a fixed dot spacing of 10 μm and formed a rectangularshaped electrode with an area of 5.4 mm × 1.0 mm. The distance between two electrodes is 1.0 mm, and the overlapping length of the electrode with a MoS2 thin film is 1.0 mm, which create an active area of 1.0 mm × 1.0 mm. Jetting behavior was optimized previously by adjusting the driving voltage−time waveform and a peak voltage of 16 V for the piezoelectric print head to produce stable drops. Only 1 layer of ink was printed on each electrode. Finally, the as-printed samples were immediately annealed at 100 °C for 30 min on a hot plate to achieve good conductivity. 2.4. Characterization. The morphologies of the samples were studied using a field-emission scanning electron microscope (FESEM, JEOL 6700) and a transmission electron microscope (TEM, JEOL 2100). The optical properties of the samples were studied using a spectrophotometer (LAMBDA 950 UV/Vis/NIR, PerkinElmer) under the wavelength range of 300−800 nm. The crystalline and chemical structure of the samples was studied using an X-ray diffractometer (XRD, EMPYREAN X-ray Diffractometer, PANalytical), an X-ray photoelectron spectroscope (XPS, Theta-Probe, Thermo Fisher Scientific) with a monochromatized Al Kα X-ray source (1486.6 eV photons), and a confocal Raman microscope (WITech alpha 300) with an excitation wavelength of 532 nm. Photoresponse of the fabricated MoS2/PEDOT:PSS photodetector under dark (5 s) and light illumination (5 s, 405 ± 10 nm, 6.21 mW mm−2) after bending for different cycles with a bending radius of 8 mm was studied. The I−V tests of prepared devices were carried out in a voltage window of −6.5 to +6.5 V and a scanning rate of 0.1 V s−1 using an electrochemical workstation (CHI760C, CH Instruments, Inc.). The photoresponse tests were conducted by switching between the lighton and light-off with an interval of 30 s. The prepared devices were biased by a constant voltage of 5 V and illuminated by three commercial laser pointers, which were customized and controlled by a microcontroller. The laser pointers were powered with a dc source of around 3.1 V and produced beams at wavelengths of 405 ± 10 nm (purple), 532 ± 10 nm (green), and 650 ± 10 nm (red). The power densities and projection areas of the purple, green, and red laser sources are measured to be 6.21 and 5.09 and 0.968 mW mm−2, respectively, at a distance of 10 cm by a power meter (Field Max II, Coherent Inc.). The experimental setup for evaluating the optoelectronic properties is shown in Scheme 1.

dimensions, and surface features, leading to significantly varied optoelectronic properties.22,31,32 Typically, the MoS2 monolayer or few-layers from mechanical exfoliation or CVD exhibit relatively high responsivity ranging from 0.1 A W−1 to a few thousand A W−1 and response time from milliseconds to seconds because of the high carrier mobility and current ON/ OFF ratio,33−40 since the pioneer study of using MoS2 as a photodetector by Yin et al.41 The variation of the responsivity and response time largely depends on the light spectrum and intensity (where normally visible light was used), the bias voltage, the contact resistance, and the positioning technique. However, the preparation of MoS2 monolayer and few-layers via the above-mentioned methods falls into several drawbacks in terms of large-scale incapability, complexity, and strict processing as well as high cost. The solution-based strategy, for instance, chemical exfoliation, is a better alternative to obtain MoS2 because of the simple processing and ease of large-scale production. The responsivity, however, is at the altitude of 0.01 mA W−1, although the produced MoS2 is also in few-layer nature,32,42,43 probably because of the changed layer quality, adsorption on the surface of the MoS2, impurities, and defects. Moreover, because of its high transparency, excellent flexibility, and mechanical durability, the MoS2 thin film from the exfoliation route can be produced on flexible/stretchable substrates, for instance, polyimide, polyethylene terephthalate, and polydimethylsiloxane, to form flexible optoelectronics,44 which has nowadays been attracting worldwide attention because of their tremendous applications in electronic skin,45 health monitoring,46−48 energy storage and conversion,49,50 and biomedical devices.51 Despite that monolayered and few-layered MoS2 is preferable to achieve high responsivity and short response time, multilayered one is more applicable in terms of the simplicity of material processing and device fabrication, the low cost, the stability as well as the robustness and could bear high practical value. So far, influences of the combined phases of semiconducting 2H and metallic 1T on photodetection properties of MoS2, as well as how it responds to certain light wavelength, remain under investigation. It has been indicated that both metallic and semiconducting phases of MoS2 influence the photodetection performance in different ways,52 and the triple-layered MoS2 shows good detection for red light, whereas the single-/double-layered one is more sensitive to green light.53,54 We believe that such concerns are important especially for some specific applications including night vision, security, and environment. Herein, in this work, a robust photodetection device based on solution-based multilayered MoS2 was designed via simple and straightforward chemical exfoliation followed by vacuum filtration and inkjetprinting using a commercially available cellulose ester membrane. The photodetection properties and the aforementioned concerns have been investigated and correlated with each other. The designed MoS2-based photodetector also shows great potential as a flexible one because of the flexibility and mechanical durability of both the membrane substrate and the MoS2, which could be the subject of the following work.

3. RESULTS AND DISCUSSION 3.1. Structures and Morphologies. The preparation procedure of the MoS 2-based photodetector paper is demonstrated in Scheme S1 in the “Supporting Information”. The typical morphologies of the as-prepared MoS2 on the cellulose membrane from chemical exfoliation are shown in Figure 1. It clearly shows that the as-prepared thin film is composed of many MoS2 thin sheets which loosely stack

2. EXPERIMENTAL DETAILS 2.1. Materials. MoS2 powder and n-butyllithium solution (1.6 M in hexane) were purchased from Sigma-Aldrich (USA) and used as received. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was supplied from Heraeus Deutschland. The B

DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

grain thickness of around 10 nm, corresponding to about 16 MoS2 atomic layers, considering that the interlayer spacing is 0.63 nm. This thickness is quite consistent with that of a single MoS2 thin sheet, as displayed in Figure 1g,h, showing the uniformity of the thickness. This indicates that each thin sheet is a single-crystal grain. The chemical exfoliation of bulky MoS2 with 2H phase into single- and few-layered MoS2 with theoretically 1T phase is a strictly controlled process with time dependence, low intercalation efficiency, and high possibility of restacking of MoS2 layers, leading to complex phase and chemical composition of the produced MoS2 thin film. This needs to be carefully evaluated because different phase and chemical compositions respond to the light in varied manner. The spectra of XPS elemental survey, Mo 3d, and S 2s core level of the prepared sample are presented in Figure 2. The survey spectrum (Figure 2a) contains Mo, S, and O species. The Mo 3d core level XPS spectrum as shown in Figure 2b can be simulated into several peaks. The two peaks located at the binding energies of 233.37 eV (Mo4+ 3d3/2) and 229.75 eV (Mo4+ 3d3/2) are assigned to 2H phase MoS2 and those located at 232.19 eV (Mo4+ 3d3/2) and 229.04 eV (Mo4+ 3d3/2) are assigned to 1T phase MoS2, which is ascribed to the partial phase transition of MoS2 from semiconducting 2H to metallic 1T. This phase transition was also consistence with the work reported.55 The peaks located at 236.38 eV (Mo6+ 3d3/2) and 232.85 eV (Mo6+ 3d3/2) reveal the existence of MoO3 (this is also confirmed by the Raman spectrum as shown in Figure S1 in the “Supporting Information”), which is due to the partial oxidation on the MoS2 surface during the process. The atomic ratio and weight ratio of 1T phase MoS2, 2H phase MoS2, and MoO3 are simulated to be around 1:4.6:2.8 and 1:4.6:2.5, respectively. It is worth noting that the ratio of the 1T and 2H phase in the final thin film can be adjusted through controlling the lithium intercalation and MoS2 exfoliation process, and the final composition of 1T MoS2, 2H MoS2, and MoO3 may affect the photodetection performance of the derived devices and require optimization. This could be an interesting subject of the following study.

Scheme 1. Experimental Setup for the Optoelectronic Properties Evaluation

together and form porous layered structures (Figure 1a). The lateral dimension of the thin sheets is within the range of a few hundred micrometers. During the exfoliation process, the Li+ ions intercalate into the MoS2 layers because of the weak van der Waals interaction between the layers, forming small fragments consisting of few-layered MoS2. The enlarged images (Figure 1b,c) further show that the sheets wrinkle up on the surface, which may be spontaneously induced by the compressive strain developed within the thin sheet with a length/thickness ratio ≫ 1 during the chemical exfoliation. The formed wrinkles may interact with incident light more efficiently through multiple reflection, facilitating the light absorption thus potentially enhancing the light detection sensitivity. Some rod-like small crystals are also found on the thin sheets (Figure 1b). As verified by the scanning electron microscopy (SEM)-energy-dispersive X-ray (EDX) spectroscope elemental mapping (Figure 1d1−d4), such small crystals are of oxygen-rich, whereas Mo/S-absence, which could be most likely identified as remained lithium oxide or hydroxide that derived from the intercalation process. It is also worth noting that oxygen is also detectable throughout the whole area as it is for Mo. This is due to the existence of MoO3 on the surface of the MoS2 thin film, which will be further elaborated later. The thickness of the MoS2 thin film on the cellulose ester membrane is about 2 μm, as indicated by the cross-sectional TEM image shown in Figure 1e. The high-resolution TEM image (Figure 1f) shows the (002) crystal lattice with a single

Figure 1. (a−c) FESEM images at different magnifications and (d1−d4) SEM-EDX elemental mapping of MoS2 thin film, (e,f) cross-sectional TEM images of MoS2 thin film, and (g,h) cross-sectional SEM images of a single MoS2 thin sheet, indicating the thickness and uniformity. C

DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a) XPS survey of the chemically exfoliated MoS2 thin film. (b) XPS spectrum of the Mo 3d and its fitting results. Contributions from 1T MoS2, 2H MoS2, and MoO3 components in the Mo 3d scanning spectrum are indicated by blue, red, and green curves, respectively.

The XRD pattern of the prepared MoS2 thin film, as shown in Figure 3a, further confirms the existence of the 2H, 1T

Figure 4a1,a2. PEDOT:PSS was printed onto the MoS2 thin film directly to form firm and complete contact and used as an electrode because of its superior electrical and optoelectronic properties, easy processing, and low cost.57−59 Light sources with different wavelengths, that is, 405 ± 10 nm (purple), 532 ± 10 nm (green), and 650 ± 10 nm (red), were applied to investigate the performance of the fabricated MoS2-based photodetector (Figure 4a3). Figure 4b shows the typical current−voltage (I−V) characteristics of the prepared flexible MoS2/PEDOT:PSS photodetector under dark condition and light illumination. It is shown that the junction between the MoS2 thin film and the PEDOT:PSS has a linear I−V behavior. This indicates that an Ohmic contact is formed between MoS2 thin film and PEDOT:PSS electrode, allowing charges to flow between the MoS2 and the PEDOT:PSS smoothly. Moreover, the resistance of the device decreases under the illumination of the light, which is due to the photoelectric effect. Figure 4c presents the time-independent photocurrent generation of the prepared photodetector under periodical light on/off with a bias voltage of 5.0 V. It is shown that the photodetector gives stable and repeatable photocurrent response under light illustration. The maximum photocurrent generated under purple, green, and red lights is around 3.56, 3.31, and 2.66 μA, respectively. An enlarged view of a single cycle is shown in Figure 4d to evaluate the response time and recovery time, where the response time is defined as the time when the photocurrent reaches to 90% of the maximum value upon switching on the light, and the recovery time was defined as the time when the photocurrent decreases to 10% of the maximum value upon switching off the light. Clearly, the response/recovery time of the photodetector upon purple light illumination, 17.5/15.3 s, respectively, is much longer than that upon green (4.3/5.6 s) and red (6.4/4.9 s) light illumination, two of which are comparable to each other. The responsivity, which presents the photocurrent generated per unit light power that projects onto a photodetector, was calculated from the following equation27

Figure 3. Optical images and XRD patterns of (a) chemically exfoliated MoS2 thin film on the cellulose membrane and (b) raw bulky MoS2 powders. The inset in (a) shows the fitting result of the selected diffraction peak.

MoS2, and MoO3, as well as the crystal structure. The diffraction peaks at 2θ = 14.1°, 32.8°, 39.4°, 49.6°, 58.5°, and 60.1° are assigned to planes (002), (100), (103), (105), (110), and (008), respectively, of hexagonal 2H phase MoS2 (JCPDS no. 65-1951), whereas the peak at about 13°, 24°, 33°, and 38.5° can be exclusively indexed to plane (020), (110), (101), and (060) of MoO3 (JCPDS no. 05-0508). The asymmetric peak at around 14° can be further deconvoluted into three Gaussian peaks at about 12.8°, 14.1°, and 14.8°, corresponding to the MoO3, 2H MoS2, and 1T MoS2, respectively.56 The composition ratio of 2H phase MoS2 and 1T phase MoS2 is calculated as around 4.6:1, which is in a good agreement with the XPS results. Compared with those of bulky MoS2 (Figure 3b), the diffraction peaks of the exfoliated MoS2 are more broaden, indicating smaller grain size, that is, fewer MoS2 layers in the crystal. On the basis of the Scherrer equation, the average crystallite size of exfoliated 2H and 1T MoS2 along the c axis of plane (002) is calculated to be about 7.3 nm (12 layers in each grain) and 10.0 nm (16 layers in each grain), respectively, whereas that of bulky MoS2 is calculated to be at around 42.2, that is, around 66 layers. This is quite close to the TEM observation for the obtained MoS2 thin film as stated above. 3.2. Photoelectrical Property Evaluation and the Underlying Mechanism. The MoS2 thin film is firmly attached onto the cellulose ester membrane, and the obtained paper-based device is very flexible and robust, as shown in

R=

Iph Po

× G* =

|Ilight − Idark| Po

×

AL APD

(1)

where R is the responsivity, Iph is the absolute photocurrent generated by the photodetector, Po is the incident optical power, G* is the geometry factor, and Ilight and Idark are the photocurrents generated under illumination and dark, respectively. APD is the exposure area of the photodetector, that is, active area; AL is the projected area of the laser. In this work, because the distance of two PEDOT:PSS electrodes is smaller than the diameter of the laser light, that is, the active D

DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a1) Schematic and (a2) optical image of a cellulose ester membrane deposited with MoS2 film and PEDOT:PSS electrodes. (a3) Schematic of a prepared photodetector under illumination. (b) Current−voltage (I−V) characteristics and (c) time-independent photoresponse of the fabricated MoS2/PEDOT:PSS photodetector under dark condition or light illumination with different wavelengths, i.e., purple (405 ± 10 nm, 6.21 mW mm−2), green (532 ± 10 nm, 5.09 mW mm−2), and red (650 ± 10 nm, 0.968 mW mm−2). (d) Enlarged view of (c), presenting a single cycle of the photoresponse. (e) External/internal quantum efficiency and responsivity of the prepared photodetector under different wavelengths.

Table 1. Optoelectronic Properties of Paper-Based MoS2/PEDOT:PSS Photodetector and the Summary of Some Typical MoS2-Based Photodetectors Reported in the Literature Recently light

wavelength (nm)

maximum/absolute photocurrent at bias = 5 V (μA)

purple green red

405 ± 10 532 ± 10 650 ± 10

3.56/0.835 3.31/0.636 2.66/0.07

response time (s)

recovery time (s)

17.5 4.3 6.4 reported results

liquid-exfoliated MoS2 under UV−visible−NIR (ref 34) inkjet-printed MoS2 under visible (ref 32) solution-derived MoS2 under UV−visible (ref 42) solution-processed MoS2 films under broadband light (ref 43)

area which was illuminated by the laser is smaller than the projected area of the laser, the geometry effect was taken into consideration. It is worth noting that there is no photocurrent generated when the light illuminates onto the PEDOT:PSS electrode; thus, the photocurrent is fully generated from the active area of the MoS2 thin film. The responsivity is calculated to be 0.134, 0.125, and 0.072 mA W−1, corresponding to purple, green, and red light, respectively, as shown in Figure 4e. It reveals that the responsivity of the prepared photodetector under purple light is 1.1 times and 1.9 times as high as that under green and red light, respectively, suggesting that the prepared flexible photodetector is more sensitive to the purple and green light.

15.3 5.6 4.9 (below)

external quantum efficiency (%)

internal quantum efficiency (%)

responsivity (mA W−1)

0.041 0.029 0.014

0.063 0.042 0.020

0.134 0.125 0.072

0.036 ∼0.10 0.063 0.10 (at 15 V)

The external and internal quantum efficiency of the prepared MoS2 thin-film photodetector under different light sources was calculated and is shown in Figure 4e. It is shown that the highest external/internal quantum efficiency, that is, 0.041%/ 0.063%, is achieved under purple light illumination, approximately 1.4/1.5 times as high as that under green light illumination and 3.0/3.2 times as high as that under red light illumination. It reveals that under purple light illumination, the prepared photodetector is capable of converting absorbed photons into photocurrent more efficiently. The photodetecting properties of the prepared MoS2 thin film under different light illuminations are compared and summarized in Table 1. It is suggested that the MoS2 thin-film photodetector exhibits the best overall performance under purple light E

DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Because the energy of all of the three light sources in this work is larger than the direct binding energy of the 2H phase in the prepared MoS2 thin film, electrons and holes can be generated and transport in the effective zone upon the projection of the incident light. In this specific design, the metallic 1T phase in the obtained MoS2 thin film facilitates the conduction of the generated electron out to the PEDOT:PSS electrode, whereas the MoO3 that formed on the surface of the thin film acts as hole extraction contacts because of its high work function.61−63 This reduces the chance of recombination of the generated electrons and holes. Moreover, the depth of effective zone theoretically depends on the light penetration depth in the film at certain wavelength, that is, the smaller the penetration depth, the shallower the light can penetrate through the material, resulting in a shallower effective area that can generate electron−hole pairs. According to the Lambert− Beer Law, the light penetration depth is proportional to the inverse of the square root of the complex part of the dielectric constant. For the prepared MoS2 thin film, which has an overall thickness of around 2.5 μm and average grain size of 7.3 nm (2H phase), the theoretical light penetration depth is calculated to be about 16.0, 50.3, and 56.2 nm at wavelengths of 405 nm (purple), 510 nm (green), and 650 nm (red), respectively.64 This indicates that the incident light can penetrate several MoS2 grains because of the smaller grain size in out-of-plane direction than penetration depth and the porous layered structure of the thin film as aforementioned. The purple light has a shallowest effective area among all, leading to that most electrons and holes accumulation within the shallow surface area under purple light illumination, that is, highest carrier density, therefore highest responsivity. The excessive carriers, which cannot efficiently contribute to the photocurrent, interact with the surface adsorbates, oxygen (O2)/water (H2O), and slow down the response/recovery (17.5/15.3 s) compared with the other two. Despite the similar optical penetration depth, that is, similar effective area/volume, as aforementioned for that under green and red light, the much higher responsivity under green light than red light can be explained by the saturation of the photocarrier generation.65 The excess energy will not contribute to the photoelectron conversion process, but interact with the atomic lattice structure and cause lattice vibrations, releasing in the form of thermal energy and lower responsivity. Upon light illumination, a quick followed by slow response is observed in all cases and the similar phenomenon occurs during the recovery period. Taking the photoresponse under green light illumination as a typical example, the photocurrent increases dramatically in the first second but slowly thereafter. After switching off the light, the photocurrent dropped quickly in the first second, but takes much longer to reach stable status. This can be explained by the generation and transportation of photoinduced electron−hole pairs and their interaction with O2 and H2O that exists in the surrounding or on the surface of the active MoS2 thin film. When photons interact with the MoS2 crystal, the photoexcited carriers quickly recombine via Auger process.66 The generation and the recombination could equilibrate within nanoseconds time scale which causes the fast rise curve of photocurrent. Meanwhile, some of the generated holes react with absorbed species via electron−hole recombination at trap states on the surface of the MoS2, which causes the release of these species and resulting in the slow photoresponse.38

illumination, and the achieved responsivity is comparable to other recently reported liquid exfoliation- and solution-based MoS2 photodetectors as presented in Table 1, although the incident light wavelength and power may be different.32,42,43 The photoresponse of the fabricated photodetector after bending was further investigated to verify the stability (Supporting Information, Figure S2). It is shown that no obvious degradation can be observed on the photocurrent under the bending cycles of up to 600. The photocurrent reduces slightly after that. Moreover, the response time of the photodetector upon purple light illumination increases with the increasing of the bending cycle, whereas the recovery time remains same. This verifies the good bending stability of the designed flexible photodetector paper. In order to unveil the reason behind the varied respond to different light sources, the optical property of prepared MoS2 thin film was studied. The light absorption is obtained by subtracting the light reflection and transmission from the total incident light, as shown in Figure 5. The strong light

Figure 5. Light absorption of chemically exfoliated MoS2 thin film on cellulose paper. The inset shows the enlarged image of the rectangular area.

absorption peak located at around 350 nm (3.5 eV) can be attributed to the formed MoO3 on the surface, which has a direct band gap at around 2.8−3.5 eV.60 Two characteristic absorption peaks A and B (inset of Figure 5), which are located around 586 nm (2.1 eV) and 659 nm (1.9 eV), respectively, can be assigned to the direct band gap transition of MoS2, indicating the splitting that arises from the combined effect of interlayer coupling and spin−orbit coupling.31 This further confirms the existence of MoS2 and MoO3 in the thin film. Noting that the light transmission of the prepared sample is nearly zero and the light absorption of the cellulose paper occurs mainly under the wavelength of less than 300 nm, the major energy loss of the incident light therefore comes from the light reflection by the MoS2 thin film. The rough surface of the MoS2 thin film, that is, the wrinkles as presented above, facilitates the light absorption through multiple reflections within the thin film. Figure 5 shows that the light absorption of the prepared MoS2 thin film at 395−415 nm (purple), 522− 542 nm (green), and 640−660 nm (red) is around 65, 69, and 70%, respectively, representing photon energy utilization of the specific incident light for the MoS2 thin film. The trend is on the order of purple < green < red, whereas the difference is minor. The purple light illumination thus leads to most intensive energy excitation onto the MoS2 thin film because of the highest power density thus highest possibility of the generation of the electron−hole pairs. F

DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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4. CONCLUSIONS In summary, a flexible paper photodetector based on the MoS2 thin film was successfully fabricated via a two-step process. The MoS2 thin film was first synthesized via chemical exfoliation from bulk MoS2 powder and deposited onto a flexible cellulose ester membrane. PEDOT:PSS was then printed onto the MoS2 thin film as an electrode via inkjet printing. It was shown that the prepared MoS2 thin film composes of stacking wrinkled MoS2 thin nanosheets with 1T and 2H phase as well as trace of MoO3 on the surface. The thin nanosheets possess tens of MoS2 layers. Compared with that illuminated with green and red light, the prepared photodetector shows the highest quantum efficiency and responsivity, while having longest response/recovery time when illuminated with purple light. This work has successfully demonstrated the effectiveness of the designed cellulose membrane-supported MoS2 thin film to detect specific light with high sensitivity. Although there are still some factors awaiting optimization in order to further improve its overall performance, including number of MoS2 layers, thin film thickness and quality, chemical and phase composition, as well as device fabrication, the MoS2-based photodetectable paper can be potentially applied to wearable electronic devices because of its flexibility and robustness. The optimization and the potential application could be the subject of the following work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b01515.

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Selection of electrodes, fabrication schematics, Raman spectrum, and bending test results (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.W.). *E-mail: [email protected] (J.K.). *E-mail: [email protected] (H.D.). ORCID

Yuefan Wei: 0000-0003-3050-1589 Hongfei Liu: 0000-0002-9905-7028 Junhua Kong: 0000-0002-5296-6603 Hejun Du: 0000-0002-6204-7520 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Ministry of Education Academic Research Fund and Agency for Science Technology and Research (A*STAR), Singapore, Science and Engineering Research Council (SERC), Additive Manufacturing Programme. We appreciate the help in using equipment from Singapore Centre for 3D Printing.

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REFERENCES

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DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.9b01515 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX