WS2 Quantum Dot Graphene Nanocomposite Film for UV

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Cite This: ACS Appl. Nano Mater. 2019, 2, 3934−3942

WS2 Quantum Dot Graphene Nanocomposite Film for UV Photodetection Vijay K. Singh,† Sanjeev M. Yadav,‡ Himanshu Mishra,† Rahul Kumar,‡ R. S. Tiwari,† Amritanshu Pandey,*,‡ and Anchal Srivastava*,† †

Department of Physics, Institute of Science, Banaras Hindu University, Varanasi−221005, India Centre for Research in Microelectronics, Department of Electronics, IIT-BHU, Varanasi−221005, India

‡

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S Supporting Information *

ABSTRACT: The development of highly responsive, ultrathin, and cost-effective 0D-2D nanocomposite photodetectors, in which light absorption and carrier transportation may be realized separately and independently, has garnered considerable attention. In the present work, we demonstrate the fabrication of atomically thin UV photodetectors based on a hybrid structure (0D-2D) of semiconducting WS2 quantum dots (0D) with graphene (2D) on SiO2/Si substrate. Graphene and WS2 quantum dots (WS2-QDs) are synthesized through chemical vapor deposition (CVD) and hydrothermal processes, respectively. The proposed photodetector offers a remarkable response to ultraviolet (UV) light of ∼365 nm, owing to the high absorption efficiency of WS2-QDs and excellent charge mobility of graphene. The photodetector exhibits high responsivity of ∼1814 A W−1 under illumination of UV light (365 nm, power density of 50.74 μW cm−2) and a high photodetectivity of ∼7.47 × 1012 Jones (cm Hz1/2 W−1). The photodetector fabricated in this work shows a fast photoresponse time of ∼2 s (rise time) and ∼2.9 s (fall time). We have also elucidated the working principle of the proposed photodetector. Outcomes of the present work are comparable or better than other results available in the literature. Our findings suggest that this nanocomposite structure of WS2-QDs with graphene sheets is a prospective candidate for high-performance optoelectronic devices. KEYWORDS: hybrid photodetectors, TMDs, WS2, quantum dots (QDs), graphene, UV photodetector

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INTRODUCTION Ultraviolet (UV) radiation, an important constituent of electromagnetic radiations, has a profound impact in the development and survival of humankind. For instance, appropriate doses of UV radiation, which produce Vitamin D from interaction with the human stratum, are essential and required for robust bone growth.1,2 However, prolonged human exposure to UV radiation may affect the skin, eyes, immune system, and even result in skin cancer.3−5 Therefore, to identify the presence of UV radiation in a working environment, the development of an electronic device, which converts UV radiation into electrical signals (UV photodetectors), is desperately in demand. Furthermore, UV photodetectors have a wide range of commercial applications including environmental monitoring, video imaging, flame detection, missile-plume detection, space communication, and security.6−11 However, the firmly established silicon (Si) technology has a few limitations, such as it requires expensive high pass optical filters and phosphorous materials to stop low energy photons.10 On the other hand, the discovery of graphene, an atomically thin two-dimensional (2D) single sheet of carbon atoms arranged in closely packed honeycomb © 2019 American Chemical Society

lattice, has allowed us to fabricate fast next generation ultrathin photodetectors and optoelectronic devices.12−14 Owing to its excellent carrier mobility with reported theoretical values in excess of 200 000 cm2 V−1 s−1 15 at room temperature and hole mobility nearly equal as its electron mobility, it is a strong front runner for applications requiring highly conductive interconnects. Although this unique capability of graphene has sparked interest, the limitation in light absorption (only about 2.3%) is undoubtedly the bottleneck in developing useful 2D graphene based photodetectors, which show very low responsivity, typically on the order of a few mA W−1.11,16,17 However, the responsivity of graphene could be enhanced by modifying its surface with semiconducting materials having strong light absorption efficiency.13,18 In recent years, semiconducting materials of the layered transition metal dichalcogenide (LTMD) family have drawn ever-increasing attention in the field of photodetection, owing to their strong light− matter interaction and suitable band gaps Received: May 1, 2019 Accepted: June 4, 2019 Published: June 4, 2019 3934

DOI: 10.1021/acsanm.9b00820 ACS Appl. Nano Mater. 2019, 2, 3934−3942

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ACS Applied Nano Materials

illustrated in Figure S1a. In a typical synthesis process, 0.25 g sodium tungstate (Na2WO4·2H2O) and 0.5 g L-cysteine were dissolved in 75 mL DI water with continuous stirring for 15 min. In the next step, pH of the solution was adjusted at ∼4.0 using hydrochloric acid (HCl). After stirring for 15 min, the solution was poured into a hydrothermal vessel and kept for reaction in an oven at 200 °C for 36 h. Over the completion of reaction, the oven was naturally cooled down to room temperature, and a light yellowish color solution containing WS2-QDs was obtained. Chemical Vapor Deposition Synthesis of Large Area Graphene. A graphene film was grown on 25 μm thick Cu foil by the chemical vapor deposition (CVD) technique using n-hexane as a liquid precursor.37 A schematic of the CVD setup is shown in Figure S1b. Cu foils were first loaded into a quartz tube, and it was then pumped down to 10−2 Torr using rotary pump for removing the trace gases present in it. Then, temperature of the furnace was ramped to 980 °C with a heating rate of 20 °C/min in the presence of H2 gas under a continuous flow of 50 sccm. As the desired temperature was reached, the flow of H2 gas was stopped and n-hexane vapor as a carbon precursor was injected in the quartz tube for 4 min. Further, the furnace was cooled down to room temperature under an H2 atmosphere. Fabrication of Photoconductive Type Photodetector. To fabricate the device, a commercially available SiO2/Si wafer was first cleaned following the standard cleaning procedure described in the Supporting Information. Afterward the graphene layer grown via the CVD technique was transferred from the growth substrate (Cu foil) onto the cleaned SiO2/Si wafer following the polymer based transferring process, schematically shown in Figure 1a (transferring steps are discussed in detail in the Supporting Information). Further,

in the range of 1−2 eV. Among this LTMD family, semiconducting 2D tungsten disulfide (WS2) has drawn even greater attention in the scientific community; it offers tunable band gaps as a function of layer numbers and hence gives rise to remarkable electronic and optical properties.19−22 To date, several reports are available on photodetectors based on the atomically thin or multilayered WS2 films; however, they suffer from low responsivity as well as mainly work in the visible range,23−25 whereas the emphasis of the present work is to develop an ultraviolet (UV) photodetector. Further, this issue could be addressed by using another allotrope of WS2 having dimensions down to the Bohr exciton radius ∼1.23 nm, i.e., tungsten disulfide quantum dots (WS2-QDs).26 These are among the most intensively explored nanomaterials, especially due to their optoelectronic properties, such as strong absorption of UV light, size dependent band gap tunability, efficient multiple carrier generation, and many more.27 Although QDs possess several unique properties, in layer form they show poor carrier transport in comparison to the conventional semiconductors, which is a technological barrier and limits the operating rate of devices. On the other hand, atomically thin 2D graphene, discussed earlier in this section, possesses excellent carrier mobility and is a strong contender for application as a conductive interconnector.28,29 So, bringing the two different classes of novel materials (0D WS2-QDs and 2D graphene) together as a hybrid nanostructure for their respective properties could be an excellent choice for the UV photodetector. On top of this, the hybrid system is also advantageous with respect to an individual one, where light absorption and carrier collection are realized in two different parts of the hybrid photodetector and these key functionalities may be tuned and optimized independently.30−32 In this series, to date, substantial reports are available on the photodetector devices based on the hybrid structure of colloidal QDs such as PbS, ZnO, etc., with graphene.12,33,34 Our group has also reported such a 0D-2D hybrid system (MoS2-QDs-graphene) for the visualization of room temperature Pauli blocking.35 However, to the best of our knowledge, we are the first reporting here a novel photodetector based on the hybrid structure of WS2-QDs with graphene on an SiO2/Si substrate. In this structure, WS2-QDs act as a light-harvesting material whereas the graphene layer underneath WS2-QDs provides a conducting pathway for high-speed transport of photogenerated charge carriers. The fabricated photodetector showed a responsivity of ∼1814 A W−1 under illumination of UV light (365 nm, power density of ∼50.74 μW cm−2) and a high photodetectivity of ∼7.47 × 1012 Jones (cm Hz1/2 W−1). The photodetector also showed a fast photoresponse time of ∼2 s (rise time) and ∼2.9 s (fall time).

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EXPERIMENTAL SECTION

Chemicals. Sodium tungstate dihydrate (Na2WO4·2H2O) was obtained from Hi-media chemicals, India. L-Cysteine, hydrochloric acid (HCl), and n-hexane (C6H14) were procured from Merck, India, and Molychem chemicals India, respectively. Copper foil (25 μm thick, purity 99.8%) used as a growth substrate for graphene was purchased from Alfa Aesar, India. Deionized (DI) water was used as a solvent material in hydrothermal synthesis as well as for all the photophysical characterizations of WS2-QDs. All the chemicals used in the present work were of analytical grade and used without any further purification. Synthesis of WS2-QDs. WS2-QD was synthesized adopting our previously reported facile and ecofriendly hydrothermal method.36 Steps involved in the synthesis process have been schematically

Figure 1. Schematic illustration shows (a) polymer assisted transferring process of CVD grown graphene from Cu substrate to SiO2/Si substrate for device fabrication, (b) silver contact deposition through RF sputtering and spin coating of WS2-QDs, and (c) photodetector device. 3935


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Figure 2. (a) TEM micrographs showing highly dispersed uniform size WS2-QDs, (inset) HR-TEM image as well as particle size distribution, (b) TEM image of graphene showing single layer graphene (lighter region) having some multilayer islands (darker regions) over it, (c, d, and e) AFM images of graphene layer transferred over the SiO2/Si substrate, WS2-QDs dispersed over the SiO2/Si substrate and fabricated device (WS2-QDs dispersed over graphene layer), respectively. Height profiles recorded along the yellow arrow are shown in the insets of c, d, and e. electrical contacts (size ∼1 mm) were made through radio frequency (RF) sputtering of silver at power of 30 W for 20 min. Further, to make the photodetector device, 100 μL of colloidal WS2-QDs was spin coated over the patterned graphene−SiO2/Si substrate at 100 rpm for 2 min and then it was dried at 60 °C for 20 min (shown in Figure 1b). A photographic image of the fabricated device is shown in Figure S2. A schematic of the fabricated photoconductive type photodetector is shown in Figure 1c. Techniques Used for Characterizations. Transmission electron microscopy as well as high resolution transmission electron microscopy (HRTEM) of WS2-QDs and graphene sheets were done with a transmission electron microscope (TEM, Tecnai G2 20 TWIN, FEI, USA) operated at 200 kV. The atomic force microscopic (AFM) image was acquired using Park XE7 (Park, South Korea). Raman spectra of both WS2-QDs and graphene were recorded at

room temperature using a Raman spectrophotometer (Renishaw inVia, Germany), equipped with a solid-state laser of 532 nm wavelength. A laser beam was directed on the sample through a 50× objective at a constant laser power of 1 mW mm−2. The absorption spectrum of WS2-QDs was recorded using a UV−vis spectrophotometer (Shimadzu, Japan) having a standard quartz cuvette of path length 10 mm. I−V measurement of the fabricated photodetector was carried out at room temperature using the semiconductor parameter analyzer (Keysight, B1500A, USA), at the applied bias voltage between −5 and +5 V. A UV lamp with output optical power density 7.796 mW cm−2 (measured by PM100D, Thorlabs) was used as a source for UV light of ∼365 nm wavelength. 3936


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Figure 3. Raman spectra of (a) single/few layer graphene films and (b) WS2-QDs, respectively.

Figure 4. (a) Spectroscopic responsivity of the fabricated device for varying wavelengths of incident light, (inset) absorption spectra of WS2-QDs, (b) typical I−V curves of photodetector, under dark and illumination of UV light, (inset) ln(I)−V characteristic of the photodetector, (c) I−V characteristic of the device with temperature, (inset) ln(I)−V characteristic, and (d) time dependent response of the photodetector device measured under ambient atmosphere at a bias voltage of 5 V under the illumination of UV light (∼395 nm).

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RESULTS AND DISCUSSION

well with a Gaussian distribution, revealing that the average lateral size of WS2-QDs is ∼5 nm. Further, the high resolution TEM image of WS2-QDs is shown in the inset of Figure 2a, showing the crystal lattice d-spacing of 0.27 nm, which corresponds to the (100) plane of the WS2 crystal.38 The TEM image of graphene shown in Figure 2b depicts that the CVD grown graphene is a highly continuous single layer (lighter region) in nature having few multilayer islands (darker region)

Structural Characterization of WS2-QDs and Graphene. The surface morphology of the as synthesized WS2QDs, graphene, and photodetector device has been analyzed through TEM and AFM techniques. It is evident from the TEM image (Figure 2a) that the as synthesized WS2-QDs are uniform in size and are highly dispersed. The variation of particle size is shown in the inset of Figure 2a, which is fitted 3937


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ACS Applied Nano Materials over it. The AFM images of graphene, WS2-QDs, and the fabricated device (WS2-QDs/graphene layer) were recorded in noncontact mode with a scanning rate of 0.5 Hz and 512 pixels (shown in Figures 2c, d, and e). The AFM image in Figure 2c shows the continuous layer of graphene. A large number of wrinkles and few defects are visible, which occurs due to polymer assisted transferring of graphene from Cu substrate to SiO2/Si substrate. The height profile (shown in inset of Figure 2c) recorded along the yellow arrow shows a height difference of ∼1 nm, which suggests the presence of monolayer graphene in the particular region. Figure 2d shows the AFM image of WS2-QDs dispersed over the SiO2/Si substrate. From here, we can clearly see the highly dispersed quantum dots over the substrate. The height profile of quantum dots (in the inset of Figure 2d) recorded along the yellow arrow shows an average height of ∼6 nm, which corresponds to the multilayer WS2QDs. Further, we have also recorded the AFM image of the fabricated device having WS2-QDs dispersed over a graphene layer, which is shown in Figure 2e. Bright spots (enclosed in white circles) in the AFM image correspond to the WS2-QDs and the darker region below it corresponds to the graphene layer. This shows that WS2-QDs are uniformly dispersed over the graphene layer. The height profile (inset of Figure 2e) recorded along the yellow arrow clearly shows the dispersion of QDs (vertical size ∼6 nm) over the graphene layer. The as-grown graphene layer, as well as WS2-QDs, was also analyzed through the well-known nondestructive Raman spectroscopy technique. Raman spectra were recorded using a Raman spectrophotometer (Renishaw in Via, Germany) with a laser excitation of 532 nm. Raman spectra of single and few layer graphene are shown in Figure 3a. In both cases, three peaks are observed at ∼1350, ∼1580, and ∼2700 cm−1, which corresponded to the D, G, and 2D bands of graphene, respectively. In some regions of graphene film, it has been observed that intensity ratio of 2D and G bands were higher than 2, which corresponds to the single layer graphene. In some regions the intensity ratio was less than 1 also, which corresponds to the multilayer graphene. So, Raman analysis also reveals the presence of single layer of graphene film having some multilayer islands over it. Figure 3b shows the Raman spectrum of WS2-QDs. In this case, two characteristic peaks of WS2 were observed at ∼350 and ∼418 cm−1, which correspond to the in-plane (E12g) and out of plane (A1g) vibrations of S−W−S atoms, respectively. Device Characterization. The qualitative performance of the photodetector has been characterized in terms of responsivity, detectivity, and time response. The experimental setup to measure the responsivity of our WS2-QDs/graphene based photodetector consists of a monochromator (SP2150i, Princeton Instruments), digital multimeter (Agilent, 34410A) and power meter (PM100D, Thorlabs). Responsivity and detectivity characteristics (Figure 4a) have been determined at +5 V by varying wavelength of incident light on the photodetector. The responsivity of the hybrid structure of graphene with WS2-QDs is highly sensitive only for UV light. The strong sensitivity of WS2-QDs/graphene based photodetectors toward UV regions is attributed to the high absorption of WS2-QDs for this region, which is confirmed by the absorption spectra as shown in the inset of Figure 4a. In the absorption spectrum two intense absorption bands at ∼330 and ∼245 nm are observed. The band at ∼330 nm corresponds to the excitonic absorption band of WS2, and the band at ∼245 nm corresponds to the surface defects.36,39

The band gaps of synthesized WS2-QDs are estimated through the Tauc plot (Figure S3). The WS2-QDs used in the device are variable in size; therefore, multiple band gaps of the WS2QDs are observed, but all correspond to the UV region only. This is why a band of UV spectrum is absorbed by our fabricated photodetector and not just a wavelength of UV radiation. Therefore, upon illumination by UV light on WS2QDs, large numbers of photocarriers are generated accounting for every WS2-QDs’s photoactivity and then swept through the field created via external voltage. This mechanism results in a large photocurrent and hence a high responsivity photodetector is obtained with fast response speed. It is also observed that the responsivity increases gradually with decreasing wavelength (increasing the photon energy). However, the responsivity was very low for the light with wavelength more than 430 nm. So, from the obtained result, we can say that the fabricated photodetector is intrinsically “visible-blind”. Further, responsivity (R) of the device is calculated using the following equation: R=

IP Popt

(1)

Where, IP is the photocurrent at applied bias voltage and Popt is the optical power density of illuminated light. The sources of noise in the photodetector are mainly due to the contributions from dark current and thermal fluctuations. The shot noise occurs mainly due to the dark current while Johnson and flicker noises occur due to thermal fluctuations. The flicker and Johnson noise play significant roles at very low and high frequency. So, here assuming the major contribution from shot noise only, the detectivity (D)33,40,41 of the photodetector is calculated using the following equation: D=

qηλ ji RA zy1/2 jj zz hc k 4kT {

(2)

Where, the ratio of qηλ denotes the responsivity (R) of the hc device in term of external quantum efficiency (η), illuminated light wavelength (λ), electronics charge (q = 1.6× 10−19 C), Planck’s constant (h = 6.63 × 10−34 m2 kg s−1) and c = 3 × 108 m s−1. In addition in eq 2, k (= 1.38 × 10−23 m2 kg s−2 K−1) is the Boltzmann constant, T (300 K) is room temperature, and RA is the resistance and area product. The resistance and area product (RA) of the photodetector at an applied bias under the consideration of shot noise only is defined as follows: i dJ y RA = jjj zzz k dV {

−1

=

kT qJ

(3)

Where, J and V are optical current density and applied voltage of photodetector, respectively. The responsivity and detectivity of our photodetector at applied voltage (+5 V) and central wavelength ∼365 nm are found to be ∼1814 A W−1 and ∼7.47 × 1012 Jones, respectively. The responsivity and detectivity of our photodetector are showing large improvement with respect to graphene based photodetectors.42,43 Its higher responsivity, simpler structure (due to the absence of any supplementary layer for absorption tuning), and lower fabrication cost could be of great interest to many researchers. Further, when the photodetector device was exposed to a UV light of wavelength 365 nm, it showed a high photo3938


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ACS Applied Nano Materials Table 1. Optical Characteristics of WS2-QDs/Graphene Based UV Photodetector applied bias (volt)

wavelength (nm)

responsivity (R) (A W−1)

2.5

∼320 ∼365 ∼320 ∼365

∼1486.4 ∼697.4 ∼3873.3 ∼1814.3

5

detectivity (D) (Jones) ∼9.9 ∼4.6 ∼1.6 ∼7.5

× × × ×

time response (s) at ∼24.32 mW cm−2, at ∼395 nm

contrast ratio (Ilight/Idark) (at ∼7.796 mW cm−2, at 365 nm) ∼22

1012 1012 1013 1012

Trise = ∼2.04 Tfall = ∼2.89

∼21

Figure 5. Schematic diagram for the photoresponse mechanism of the WS2-QDs/graphene hybrid photodetector. (a) WS2-QDs dispersed on the graphene layer and (b) band structure of graphene and n-WS2 junction under thermal equilibrium before and after contact with graphene.

response. A family of I−V curves of the device under dark and illumination of UV light is shown in Figure 4b. The I−V measurement of the fabricated photodetector was carried out at room temperature using the semiconductor parameter analyzer. The applied bias voltage was kept between −5 and +5 V to perform the I−V analysis. A UV lamp with an output optical power density of 7.796 mW cm−2 at ∼365 nm wavelength was used as an optical source. The photodetector decorated with WS2-QDs (WS2-QDs/graphene/SiO2/Si structure) shows a huge contrast ratio (IUV/Idark ∼ 22) much higher than the other reported WS2-based UV photodetectors.25,44 The large illumination current of our device with respect to the dark current clearly shows the potential of the fabricated device for UV photodetection applications. The temperature-dependent I−V characteristics of the hybrid photodetector have also been obtained using the semiconductor parameter analyzer in the voltage range of −5 to +5 V with varying temperature ranging from 300 to 403 K as shown in Figure 4c. I−V characterization of our device shows the stability of our device at high temperature with very small variation in their current values with respect to the current at room temperature (27 °C). As temperature increases, some thermal charge carriers are generated and under the influence of bias reach to the electrode; hence, a small change in current is observed as shown in Figure 4c. Furthermore, the small increment in current with respect to the large variation of temperature potentially proves the thermal stability of our device unlike the other thermally unstable UV photodetector reported in the literature.45 As shown in Figure 4c, the variation in current of our photodetector at an applied voltage of +5 V is found to be in the range 70−85 μA as temperature was raised almost double from 70 to 130 °C. The small

variation in the current of our device with respect to temperature makes it potentially suitable for UV detection at high temperature with high accuracy as well as stability. The time response of the WS2-QDs/graphene photodetector is shown in Figure 4d. The time response characteristic of the device is recorded using a UV LED source with an optical power density of ∼24.32 mW cm−2 with the center frequency at 395 nm, under +5 V bias conditions. The rise time (the time required for a pulse to rise from 10% to 90% of steady value of current) and fall time (time from 90% to 10% of steady value of current) of photodetector are calculated to be ∼2.04 and ∼2.89 s, while the rise and fall times in the case of graphene/ SiO2/Si are found to be ∼2.44 and ∼8.22 s (shown in the Supporting Information). The higher mobility of free electrons generated in WS2-QDs is responsible for the fast response time of the WS2-QDs/graphene hybrid structure.46 This is because the generated fast electrons (in WS2-QDs) will cross the junction to contribute the resultant current, whereas the generated holes will be trapped on the WS2-QD side.44 Since the time response of the device collectively depends on hole and electron mobility and photogenerated electrons have higher mobility, hence an improvement in time response is observed. Therefore, we attribute the fast response speed of hybrid detectors to the combination of WS2-QDs acting as photoactive materials and graphene with a high mobility providing a fast channel. So, it is clearly evident that the addition of WS2-QDs with monolayer graphene is improving the time response of our device. The optical characteristics of our fabricated photodetectors for UV ranges are summarized in Table 1. Principle of Operation. The schematic of WS2-QDs dispersed over graphene is shown in Figure 5a. The band 3939


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ACS Applied Nano Materials Table 2. Comparison of the Characteristic Parameters of the Present Photodetector with the Other Reported UV Photodetectors device structure TiO2/graphene ZnO nanorod/CVD graphene/ Al2O3/glass ZnO-QDs/CVD graphene/SAM/ SiO2/Si ZnO-QDs/graphene graphene/ZnO/SiO2/Si WS2 films WS2/graphene/Al2O3/SiO2/Si graphene-WS2 QDs based

spectra range 250−400 nm 250−400 nm

operating bias (V) 3

200−400 nm 10.0 300−400 nm 370−1064 nm 340−680 nm

−3

5

responsivity (A/W) 0.482 (at 330 nm) 3 × 105 (at 365 nm) 2.4 × 107 (at 335 nm) 9.9 × 108 (at 335 nm) 3 × 104 (at 365 nm) 0.51 950 (at 405 nm) ∼1814.0 (at 365 nm)

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structure of the n-WS2 and graphene (having a lower work function than n-WS2) junction under thermal equilibrium before and after contact is presented in Figure 5b. In Figure 5b, ΦG and ΦWS2 are the work function of graphene and WS2QDs, respectively. EC, EV, and EF,WS2 are the conduction energy level, valence energy level, and Fermi level of WS2-QDs, respectively. EF and χWS2 are the Fermi energy level of graphene and electron affinity of WS2-QDs, respectively. Under light illumination, a large numbers of electron−hole pairs are generated in WS2-QDs. The photogenerated holes remain in the WS2-QDs side while the electrons move toward graphene as suggested by the band diagram (Figure 5b). These electrons reach the electrode due to the application of external biasing and produce a large photocurrent.47−51 Finally, we have summarized the performance of our fabricated device in Table 2 along with comparison to the other reported photodetectors in terms of their responsivity, detectivity, and photoresponse time, etc.

rise time (s)

decay time (s)

0.7 3.2

0.5 66.3

ref 52 12

5.1 × 1013 (at 368 nm) 7.5 × 1014

2.3 5.0

85.1

54

4.33 × 1014 2.7 × 109

1.0 4.1 7.85 2.04

22.0 4.4 5.61 2.89

55 24 44 this work

7.47 × 1012

53

AUTHOR INFORMATION

Corresponding Authors

*Email: anchalbhuatgmail.com. Phone: +91- 9453203122 (A.S.). *Email: apandey.eceatiitbhu.ac.in. Phone: +91-9454749047 (A.P.). ORCID

Vijay K. Singh: 0000-0001-5675-8281 Sanjeev M. Yadav: 0000-0003-0806-284X Himanshu Mishra: 0000-0002-5804-6840 R. S. Tiwari: 0000-0002-0966-4028 Anchal Srivastava: 0000-0002-6573-5345 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.K.S. acknowledges the UGC for providing a Senior Research Fellowship (Fellowship Grant No: F.25-1/2014-15/(BSR)/5127/2007/(BSR)). H.M. is thankful to CSIR, New Delhi, for providing an SRF fellowship (Fellowship Grant No: 09/ 013(0752)/2018-EMR-I). A.S. acknowledges DST-PURSE, SERB, India (Project Code: EMR/2016/007720), and CASBHU for providing financial support. The authors acknowledge the Bio-Physics laboratory of the Physics Department, Institute of Science, BHU, for access to facilities like UV−vis and PL. R.K., S.M.Y., and A.P. would like to thank Central Instrument Facility (CIF), IIT (BHU), Varanasi, for extending the characterization facilities.

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CONCLUSION In summary, we have fabricated an atomically thin UV photodetector based on hybrid structure (0D-2D) of semiconducting WS2 quantum dots (0D) with graphene (2D) on SiO2/Si substrate, which exhibits a remarkable photoresponse to UV light. The photodetector has shown high responsivity of ∼1814 A W−1 and high photodetectivity of ∼7.47 × 1012 Jones. The photodetector exhibits a fast photoresponse time of ∼2 s (rise time) and ∼2.9 s (fall time). The results obtained in the present work, exploring for the first time the novel hybrid system WS2-QDs/graphene, are comparable or better than the other available literature works on graphene and WS2. Our findings suggest that the hybrid structure of WS2-QDs with graphene sheets is a prospective candidate for high-performance optoelectronic devices.

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detectivity (cm Hz1/2W−1)

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REFERENCES

(1) Holick, M. F. Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am. J. Clin. Nutr. 2004, 80 (6), 1678S−1688S. (2) Reichrath, J.; Saternus, R.; Vogt, T. Challenge and perspective: the relevance of ultraviolet (UV) radiation and the vitamin D endocrine system (VDES) for psoriasis and other inflammatory skin diseases. Photochemical & Photobiological Sciences 2017, 16 (3), 433− 444. (3) De Gruijl, F. Skin cancer and solar UV radiation. Eur. J. Cancer 1999, 35 (14), 2003−2009. (4) Diepgen, T. L.; Mahler, V. The epidemiology of skin cancer. Br. J. Dermatol. 2002, 146, 1−6. (5) Armstrong, B. K.; Kricker, A. The epidemiology of UV induced skin cancer. J. Photochem. Photobiol., B 2001, 63 (1−3), 8−18. (6) Munoz, E.; Monroy, E.; Pau, J.; Calle, F.; Omnes, F.; Gibart, P. III nitrides and UV detection. J. Phys.: Condens. Matter 2001, 13 (32), 7115.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00820. Exciton Bohr radius of WS2-QDs, schematic for hydrothermal and CVD synthesis of WS2-QDs and graphene, substrate cleaning procedure, details of graphene transfer from Cu to SiO2/Si substrate, photographic image of photodetector device, Tauc plot for band gap calculation of WS2-QDs, time response of graphene (PDF) 3940


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WS2 Quantum Dot Graphene Nanocomposite Film for UV

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