Highly Enhanced Photoresponsivity of a Monolayer WSe2

†Department of Energy Science and ‡Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Highly Enhanced Photoresponsivity of a Monolayer WSe2 Photodetector with Nitrogen-Doped Graphene Quantum Dots Duc Anh Nguyen,† Hye Min Oh,*,† Ngoc Thanh Duong,† Seungho Bang,†,‡ Seok Jun Yoon,†,‡ and Mun Seok Jeong*,†,‡ †

Department of Energy Science and ‡Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Sungkyunkwan University, Suwon 16419, Republic of Korea S Supporting Information *

ABSTRACT: Hybrid structures of two-dimensional (2D) materials and quantum dots (QDs) are particularly interesting in the field of nanoscale optoelectronic devices because QDs are efficient light absorbers and can inject photocarriers into thin layers of 2D transition-metal dichalcogenides, which have high carrier mobility. In this study, we present a heterostructure that consists of a monolayer of tungsten diselenide (ML WSe2) covered by nitrogen-doped graphene QDs (NGQDs). The improved photoluminescence of ML WSe2 is attributed to the dominant neutral exciton emission caused by the n-doping effect. Owing to strong light absorption and charge transfer from N-GQDs to ML WSe2, N-GQD-covered ML WSe2 showed up to 480% higher photoresponsivity than that of a pristine ML WSe2 photodetector. The hybrid photodetector exhibits good environmental stability, with 46% performance retention after 30 days under ambient conditions. The photogating effect also plays a key role in the improvement of hybrid photodetector performance. On applying the back-gate voltage modulation, the hybrid photodetector shows a responsivity of 2578 A W−1, which is much higher than that of the ML WSe2based device. KEYWORDS: tungsten diselenide, graphene quantum dots, photoluminescence, photodetector, n-doping effect

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INTRODUCTION Layered transition-metal dichalcogenides (TMDs) have been studied widely because of their unique optoelectronic properties such as a tunable band gap, high carrier transport mobility, and an excellent ON/OFF ratio.1−4 Tungsten diselenide (WSe2) is an outstanding two-dimensional (2D) TMD and is one of the few p-type materials that can be used in optoelectronic applications.5−7 Bulk WSe2 shows an indirect band gap of 1.3 eV, and single-layer WSe2 has a direct band gap of 1.6 eV because of the quantum confinement effect.8 Consequently, strong photoluminescence (PL) can be obtained when WSe2 is scaled down from the bulk form to the monolayer (ML) form. Carrier mobilities of up to 140 cm2 V−1 s−1 can be obtained in ML WSe2 at room temperature.9 Furthermore, its internal quantum efficiency is more than 70%, which is much higher than those of other TMDs.10 These results show that ML WSe2 is an ideal material for application in optoelectronic devices such as photodetectors and lightemitting diodes.6,7,10,11 However, atomically thin material layers have limited ability to absorb and emit radiation because of their low cross-sectional area.12 Recently, much effort has been directed toward enhancing the PL and photoresponsivity of ML WSe2.12−15 Most approaches to enhance the PL involve either defect engineering or fabrication of unique structures to take advantage of the surface plasmon resonance effect.12,13 However, this either led to the suppression of carrier mobility © XXXX American Chemical Society

or required expensive and complicated processes to fabricate a unique surface. The photoresponsivity of ML WSe2 could be improved by constructing hybrid structures with layers of strong light-absorbing materials such as PbS quantum dots (QDs) or organolead halide perovskites.14,15 However, these materials are limited by their toxicity or instability because of the presence of Pb compounds or organic materials. Finding a low-cost, nontoxic, and stable material to create a hybrid structure with layers of TMDs to improve its photoelectric performance is a huge challenge. Nitrogen-doped graphene QDs (N-GQDs) have been reported as metal-free materials that exhibit strong light absorption, broad bandwidth, and tunable emission from deep-ultraviolet to near-infrared range.16,17 N-doping leads to great improvement of PL quantum yield, absorption in the visible region, and charge transfer.18,19 Moreover, N-GQDs have been recognized as nontoxic and low-cost materials that play significant roles in many applications such as photodetectors, solar cells, and bioimaging.16,20,21 N-GQDs have excellent photostability and can hold continuous excitation for 12 h.21 Hence, the approach of combining layered 2D TMDs Received: December 4, 2017 Accepted: March 6, 2018 Published: March 6, 2018 A

DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Structural and optical characteristics of pristine ML WSe2 and N-GQDs: (a) SEM image of a triangular ML WSe2 sheet. (b) Raman and (c) PL spectra of ML WSe2. (d) XRD pattern of N-GQDs. The inset shows an OM of the N-GQDs powder. (e) Raman spectrum of N-GQDs. (f) Absorption (dashed line) and PL spectra of N-GQD solutions. The insets show the OM images of the N-GQD solution under white light (yellow) and 355 nm UV light (blue).

Figure 2. Electrical characteristics of pristine ML WSe2. (a) Cartoon image of a fabricated ML WSe2 FET device. (b) Typical I−V curves of an ML WSe2 FET for different values of VG. (c) Gate voltage characteristics of an ML WSe2 FET at VDS = 1 V in atmosphere. The black line is on a linear scale, and the red line is on a logarithmic scale. The inset shows the OM image of the ML WSe2 FET device. (d) Schematic energy diagram of the ML WSe2 FET for different values of VG.

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RESULTS AND DISCUSSION ML WSe2 samples were grown directly on a SiO2/Si substrate by chemical vapor deposition (CVD) and were transferred onto a Si substrate covered with a 300 nm thick SiO2 layer by the wet-transfer method. Details of the transfer process are provided in the Experimental Section. The scanning electron microscopy (SEM) image of a triangular ML WSe2 sample presented a uniform surface (Figure 1a). The average size of the ML WSe2 flake was approximately 30 μm, and the thickness was ∼0.78 nm, which correspond to a monolayer of WSe2 (Figure S1).5,22 To characterize the optical properties of ML WSe2, Raman and PL measurements were performed under ambient conditions by employing a 532 nm solid-state laser as the excitation source. The Raman spectrum of ML WSe2 is shown in Figure 1b. It displays the out-of-plane (A1g) and inplane (E12g) phonon modes representative of layered TMDs. These two phonon modes were located at a single overlapping peak at ∼246 cm−1 for ML WSe2.23 Moreover, on applying a 532 nm excitation laser in resonance with the A exciton peak of WSe2, the second-order resonant Raman mode 2LA(M) also appeared at ∼256 cm−1 because of the LA phonons at the M point in the Brillouin zone.8,24,25 Figure 1c presents the PL

and N-GQDs is expected to enhance and stabilize the performance of optoelectronic devices effectively. In this work, we report a facile method for fabricating a hybrid structure that consists of ML WSe2 covered with a thin layer of N-GQDs (ML WSe2/N-GQDs). The PL intensity of ML WSe2 is enhanced drastically because of the reduction of the positive trion and enhancement of the neutral exciton formations through electron transfer from N-GQDs. The improved photoresponse in ML WSe2/N-GQD photodetector is attributed to strong light absorption and charge transfer from N-GQDs to ML WSe2. The photogating effect is also observed as a factor that contributes to the improvement of the hybrid device performance. The photoresponsivity of the ML WSe2/ N-GQDs enhanced and became approximately 480% higher than that of the pristine ML WSe2. By varying the back-gate voltage and illumination light power density, we obtain a responsivity of 2578 A W−1, which is much higher than that of the ML WSe2-based device. Notably, even after 30 days of exposure under ambient conditions, this hybrid photodetector exhibits good environmental stability with 46% performance retention. B

DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. Device characteristics of ML WSe2/N-GQD heterostructures: (a) OM image of ML WSe2 and ML WSe2/N-GQDs. The scale bar is 10 μm. (b) Gate voltage characteristics of ML WSe2 and ML WSe2/N-GQD heterostructure device at VDS = 1 V in atmosphere. (c) Raman spectra of ML WSe2 and ML WSe2/N-GQDs.

spectrum of ML WSe2, where the A exciton peak is observed at ∼767 nm. These results were in accordance with the previous reports for ML WSe2, confirming well-grown CVD samples.5,13,14 The N-GQDs were synthesized by a facile one-route hydrothermal method (Figure S2) and were characterized to determine their structures and optical properties.18 The X-ray diffraction (XRD) patterns of the N-GQD powders are shown in Figure 1d. There is a weak broad diffraction peak centered at 2θ = ∼21°, which indicates a distance d spacing of 0.421 nm; the inset image shows the N-GQDs in light yellow color. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were employed to examine the morphologies of the as-synthesized N-GQDs. The average height was ∼1.25 nm with a diameter of ∼6 nm (Figure S3). Figure 1e depicts the D band (1323 cm−1) and G band (1575 cm−1) in the Raman spectrum of N-GQDs. To explore the PL of NGQDs in detail, we carried out fluorescence measurements by using different excitation wavelengths from 340 to 420 nm (Figure 1f). The excitation-wavelength-dependent PL behavior revealed that the emission peaks shifted according to the excitation wavelength.26 The highest emission peak appeared at ∼450 nm when the N-GQDs were irradiated with a 400 nm wavelength, which is shown by the blue color of the solution under UV light (the inset in Figure 1f). The dashed line shows an absorption band near 345 nm, which is related to the electron transitions in CC, CN, and CO, from π (or n) to π*.16 Fourier transform infrared (FTIR) spectroscopy (Figure S4a) further verified the presence of nitrogen and other functional groups inside the N-GQDs, and the Mott− Schottky measurements (Figure S4b) confirmed a typical ntype semiconductor behavior. To explore the electrical properties of ML WSe2, a pristine CVD-grown sample was utilized to fabricate a field-effect transistor (FET) device, as shown in Figure 2a. Cr/Au (5/50 nm) was deposited as the source and drain electrodes, respectively, and a bottom Si substrate served as a back gate. Figure 2b depicts the typical I−V curve of the ML WSe2 FET for different values of the back-gate voltage (VG). The output current increased significantly when the gate voltage shifted to a more negative voltage. It is noted that the output I−V curves are not symmetric with opposite values of drain−source bias voltage. It is attributed to the formation of a Schottky barrier between ML WSe2 and the metal electrode (Cr/Au).27 In addition, the dependence of the drain current on the back-gate voltage modulation was shown in the linear and log scales at VDS = 1 V in atmosphere (Figure 2c). These results indicated the typical p-type FET behavior of ML WSe2. An ON/OFF ratio of up to ∼105 was obtained from the transfer curve. The field-effect mobility was calculated by utilizing the following

equation: μ =

( ) dIds dVG

1 L 1 , C i W Vds

where

dIds dVG −2

is the slope of the

transfer curve, Ci = 1.15 × 10−8 F cm is the gate capacitance of the insulating layer for a 300 nm SiO2/Si substrate, L is the channel length, W is the channel width, and VDS = 1 V is the drain−source voltage. The hole mobility of the ML WSe2 FET was calculated to be approximately 2.3 cm2 V−1 s−1 at a backgate voltage of −40 V, which was consistent with the experimental data for ML WSe2 FETs without any treatment.28,29 The output current IDS could be further tuned, from the OFF to the ON working state, by lowering the back-gate voltage. This behavior is schematically illustrated by the energy band diagrams shown in Figure 2d. After applying the drain− source bias, when the gate voltage was equal to zero (OFF working state), the current was hindered by the barrier between the Au electrodes and ML WSe2. When a sufficient negative back-gate voltage was applied (ON working state), more holes could be transported owing to the lower barrier because the Fermi level shifted down to near the valence band. To investigate the effects of the N-GQDs on ML WSe2, a hybrid ML WSe 2 /N-GQD structure was prepared by depositing N-GQDs onto the surface of ML WSe2 samples (Figure S5). Then, electron beam lithography (EBL) and thermal evaporation techniques were used to fabricate the ML WSe2/N-GQD heterostructure devices for electrical measurements. The optical microscopy (OM) image shows the separation of ML WSe2, with and without the presence of NGQDs (Figure 3a). The source−drain current versus the backgate voltage transfer curves of ML WSe2 and ML WSe2/NGQDs at VDS = 1 V are presented in Figure 3b. The more negative shift of the threshold voltage (Vth) from −18 to −25 V, with a considerably lower current between ML WSe2 and ML WSe2/N-GQDs, indicated the effect of n-doping ML WSe2.30 These results demonstrated that the electron concentration of ML WSe2 with N-GQDs increased remarkably, owing to the charge transfer from N-GQDs to ML WSe2. The transfer curves of an identical ML WSe2 FET device before (Figure S6a) and after the coating of N-GQDs (Figure S6b) are presented in Figure S6c. This result further confirms the shift of Vth by N-GQDs. We also controlled the N-GQD covering concentration by spray-coating and drop-casting methods. However, we could not see n-channel turn-on for the transistor operation. Raman and PL measurements were examined to further investigate the n-doping effect of N-GQDs on ML WSe2. Figure 3c shows the Raman spectra of ML WSe2 (black line) and ML WSe2/N-GQDs (red line). Chen et al. (2D Material 2014) showed that p-doping causes a red shift of the WSe2 Raman peak (A1g and E12g).31 In this experiment, the Raman C

DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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WSe2/N-GQDs to analyze the peak contributions. Figure 4c presents the normalized PL spectrum with fitted neutral exciton X0 (yellow color) and positive trion X+ (purple color) peaks of ML WSe2 (top) and ML WSe2/N-GQDs (bottom). The distance between the neutral exciton peak X0 and the positive trion peak X+ was 7 nm (∼21 eV), which was consistent with the previously reported binding energy of the positive trion.33,34 The circles displayed the raw data points, and the red solid lines indicate the fitted PL spectra. In a typical p-type material, the positive trion X+ is predominant in ML WSe2, whereas the PL spectrum of ML WSe2/N-GQDs exhibits a rise in the neutral exciton peak X0. The intensity ratio of the neutral excitons and positive trions (I0X/IX+) of ML WSe2/N-GQDs was considerably higher than that of ML WSe2. This behavior was attributed to the change in the charge densities in ML WSe2, resulting from the n-doping effect. When the ML WSe2/NGQDs were excited by the laser source, the formation of positive trions X+ was hindered by the excess free electrons transferred from the N-GQDs to ML WSe2, resulting in a strong increase in the neutral exciton X0 emissions (Figure 4d). This phenomenon is similar to the increased PL intensity in ML WSe2, which is caused by the n-doping effect with gate modulation and is reported elsewhere.35 The formation of neutral excitons X0 rather than positive trions X+ was attributed as the main reason for the PL enhancement observed in ML WSe2/N-GQDs. To improve the performance of the ML WSe2 photodetector further, N-GQDs were introduced to hybridize with ML WSe2 and form a heterostructured device. Figure 5a depicts the

spectrum of ML WSe2/N-GQDs shows a blue shift of the A1g and E12g peaks, revealing the n-doping effect caused by NGQDs.32 The Raman peak (A1g, E12g) of the ML WSe2/NGQDs shifted with a small wavenumber because of the low doping level and large atomic mass of W and Se.26 The confocal PL mapping measurement was carried out to clarify the regions of ML WSe2 and ML WSe2/N-GQDs. Figure 4a shows the

Figure 4. (a) Confocal PL intensity mapping image. (b) PL spectra and (c) fitted PL spectra of ML WSe2 and ML WSe2/N-GQD heterostructure. (d) Schematic view of charge transfer in the ML WSe2/N-GQD heterostructure.

confocal PL mapping intensity of the A exciton peak of ML WSe2, with and without N-GQDs. The purple color presents the PL intensities of the SiO2/Si substrate and electrodes without any observed PL. The violet and blue colors show the low PL intensity, which can be observed in the ML WSe2 or NGQDs regions. The green and yellow colors present the high PL intensities observed in the ML WSe2/N-GQD region. From the PL mapping image, we could obtain the local PL spectra of ML WSe2, N-GQDs, and ML WSe2/N-GQDs (Figure S7), which were consistent with the PL mapping intensity images. From this, we could clearly see that the PL intensity of ML WSe2/N-GQDs enhanced remarkably, compared to that of the pristine ML WSe2. To evaluate the PL enhancement, we compared the PL intensities of ML WSe2 and ML WSe2/NGQDs. Figure 4b shows the PL spectra of the ML WSe2 (black line) and ML WSe2/N-GQDs (red line) samples. The PL intensity observed in ML WSe2/N-GQDs was enhanced dramatically (by approximately 800%) compared to that of ML WSe2. In addition, a blue shift of approximately 4 nm in the PL peak of ML WSe2/N-GQDs confirmed the n-doping effect by the N-GQDs.14 Furthermore, the X-ray photoelectron spectroscopy (XPS) data of ML WSe2 before and after the coating of N-GQDs are shown in Figure S8. The upshifts of W 4f (∼0.1 eV) and Se 3d (∼0.3 eV) peaks before coating (black scatters) to a higher binding energy after coating (red scatters) provide evidence of the n-doing effect from N-GQDs to ML WSe2.32 The C 1s and N 1s peaks of N-GQDs were also observed in the XPS data of ML WSe2/N-GQD heterostructure, indicating the well coating of N-GQDs on ML WSe2. To elucidate the major reason for the enhanced PL intensity of ML WSe2 caused by the deposited N-GQDs, we applied Lorentzian fitting for the PL spectra of ML WSe2 and ML

Figure 5. Photoresponse of the hybrid ML WSe2/N-GQD photodetector. (a) Schematic illustration of an ML WSe2/N-GQD photodetector; highly n-doped Si serves as a back gate, and Cr/Au (5/50 nm) is used as drain (D) and source (S) contacts, respectively. (b) Time-dependent photocurrent of ML WSe2 (black line) and ML WSe2/N-GQD devices after fabrication (red line) and after 30 days under ambient (blue line) at VDS = 1 V and VG = 0 V. (c) Energy band diagram of N-GQDs and ML WSe2 before contact and after formation of heterostructure under 405 nm light illumination.

schematic diagram of an ML WSe2/N-GQD photodetector under 405 nm laser illumination, which is the most effective wavelength for generating electrons and holes in N-GQDs (Figure 1f). Figure 5b shows the time-dependent photocurrents of different photodetector samples at VDS = 1 V, VG = 0 V, and a power density of 170 μW cm−2. It can be clearly seen that the output photocurrent (Iph = Il − Id) of the ML WSe2/N-GQD photodetector (red line) is greatly enhanced for the currents D

DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) Transfer curves of ML WSe2 and ML WSe2/N-GQD photodetector in dark and under light illumination. (b) Transfer curves of the ML WSe2/N-GQD photodetector under different illumination power densities. (c) Dependence of the ML WSe2/N-GQD photodetector responsivity on the back-gate voltage with different power densities. (d) Dependence of the ML WSe2/N-GQD photodetector responsivity on the power density with different values of back-gate voltage. (e) Time-dependent photocurrent of the ML WSe2/N-GQD photodetector at VG = −20 V and a power density of 170 μW cm−2 under different values of bias voltage. (f) ML WSe2/N-GQD photodetector responsivity as a function of both back-gate voltage and bias voltage. The illumination light wavelength is 405 nm.

approximately 3.5 eV larger than that of N-GQDs.27 The energy band diagrams of the N-GQDs and ML WSe2 before contact are shown at the top, whereas the bottom figure presents the energy band diagram of the ML WSe2/N-GQD heterostructure after formation. When the interface of N-GQDs was in contact with ML WSe2, band bending occurred between ML WSe2 and N-GQDs. In the dark, the excess free electrons from N-GQDs were transferred to ML WSe2, improving the current of ML WSe 2 /N-GQDs. Under 405 nm laser illumination, photoexcited carriers were generated in ML WSe2 and more efficiently in N-GQDs because of their good light absorption property. The photogenerated holes in the valence band of N-GQDs were transferred to the ML WSe2 by the built-in potential between N-GQDs and ML WSe2, while the conduction band spike at the heterojunction blocks the flow of the photogenerated electrons from N-GQDs to ML WSe2.14 The trapped electrons in the N-GQD layer contribute the photogating effect to the ML WSe2 device, which will be discussed in the next part. Furthermore, we suggest that the photoexcited charge carriers (holes) of N-GQDs were transferred effectively to ML WSe2 owing to the longer lifetimes of N-GQDs (nanoseconds) compared to those of ML WSe2 (picoseconds).18,38 Consequently, a much higher photocurrent and photoresponsivity were obtained for ML WSe2/NGQDs. To understand the working mechanism of the ML WSe2/NGQD photodetector more clearly, we measure the transfer curves of ML WSe2 and ML WSe2/N-GQD photodetector in dark and under light illumination (Figure 6a). The difference between the transfer curves of the ML WSe2 photodetector in dark (black line) and light conditions (yellow line) is negligible. On the contrary, the photocurrent Iph of the ML WSe2/NGQD photodetector was enhanced significantly. In addition, Vth of the ML WSe2/N-GQD transfer curve in dark (red line) is shifted toward a more positive gate voltage under light illumination (blue line), which reveals the photogating effect.39 It is originated from the accumulation of photogenerated electrons in the N-GQD layer under light illumination, which

under both dark (Id) and light (Il) conditions, compared to that of the ML WSe2 photodetector (black line). Notably, after 30 days under ambient conditions, the output photocurrent of the ML WSe2/N-GQD photodetector (blue line) was approximately 46% of that of a fresh device (red line). On the contrary, the photoresponsivity of ML WSe2 almost disappeared after 30 days (Figure S9). The environmental stability of this photodetector is expected to be useful for applications in devices that operate for long periods of time. The photoresponsivity, one of the key parameters of a photodetector, can be calculated using the equation R = Iph/Peff, where Peff is the effective light power illuminated onto the device working area.15 Accordingly, the photoresponsivity of the ML WSe2/NGQD photodetector was 240 mA W−1, which was much higher than that of the pristine ML WSe2 photodetector, which was 5 mA W−1. Additionally, the photoresponsivity of the ML WSe2/ N-GQD photodetector kept under ambient conditions after 30 days was 110 mA W−1. This value was still 22-fold higher than the responsivity of the pristine ML WSe2 photodetector. The AFM images in Figure S10 show the surface morphologies of the samples. The AFM image of pristine ML WSe2 (Figure S10a) presented a uniform surface of the transferred ML WSe2 sample. After 30 days under ambient conditions, the damage to the surface of ML WSe2 (Figure S10b) caused by oxidation by moisture or oxygen molecules could be observed clearly.36 Notably, there was no damage to the surface of the ML WSe2/ N-GQD heterostructure, even after 30 days under ambient conditions (Figure S10c). In addition, Raman and PL spectra of a fresh photodetector and a photodetector exposed to ambient conditions for 30 days are shown in Figure S11. These confirm the stability of the ML WSe2/N-GQD heterostructure. To explain the photoresponsivity enhancement mechanism and the influence of N-GQDs on the ML WSe2 photodetector, the schematic energy band diagram is presented in Figure 5c. According to the previous results, the electron affinity and energy gap of N-GQDs were approximately 2.9 and 2.6 eV, respectively.17,37 In addition, ML WSe2 was reported to have a band gap of 1.6 eV with an electron affinity that was E

DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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days). These findings are expected to open a novel avenue for the fabrication of efficient and stable optoelectronic devices.

modulates the current in the ML WSe2 channel. To investigate the factors that affect the responsivity behavior of photodetectors, the transfer curves of the ML WSe2/N-GQD hybrid device at VDS = 1 V with different illumination power densities are shown in Figure 6b. As one can see from the figure, the stronger illumination power leads to a higher photocurrent and a shift of Vth to a more positive gate voltage, resulting from the photogating effect.39 The dependence of the ML WSe2/NGQD photodetector responsivity on the back-gate voltage with different power densities is presented in Figure 6c. The responsivity values increase significantly when the back-gate voltage shifts to more negative values, demonstrating the switching of the working state from OFF to ON stage. In addition, the photoresponsivity could also be improved by decreasing the power density. Figure 6d shows the dependence of the hybrid device responsivity on the illumination light power density under different values of back-gate voltage modulation. Higher responsivities can be achieved at a lower power density. This is due to the low density of occupied trap states in ML WSe2 or at the interfaces between ML WSe2 and SiO2/Si substrate at a low power illumination.40,41 Moreover, the lower power density also leads to a decreased recombination rate of photoexcited carriers, which enhances the photoresponsivity.42 As a result, a photoresponsivity of 2578 A W−1 is achieved from the ML WSe2/N-GQD photodetector at VG = −60 V and a power density of 1.9 μW cm−2. This value is much higher than that of ML WSe2 devices. The periodic photoresponse behavior of ML WSe2/N-GQDs at VG = −20 V and VDS = 1 V under different power densities is shown in Figure S12a. By extracting a single period of timedependent photoresponse, the response time of this photodetector was observed to be smaller than 0.5 s for both rise time and decay time (Figure S12b). Furthermore, we characterize the photoresponsivity as a function of bias voltage. Figure 6e presents the time-dependent photocurrent of the ML WSe2/N-GQD photodetector at VG = −20 V and a power density of 170 μW cm−2 under different values of bias voltage. The magnitude of Iph at the same value of the back-gate voltage and power density is increased as the bias voltage increases from 1 to 4 V. From the above results, the profile of responsivity as a function of both back-gate voltage and bias voltage is shown in Figure 6f. The responsivity is increased by either increasing the bias voltage or decreasing the back-gate voltage.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Materials. All chemicals used in our work were commercially available and were used without further purification. Citric acid monohydrate and urea were purchased from Sigma-Aldrich. Synthesis of N-GQDs. Citric acid (0.21 g, 1 mmol) and 0.18 g (3 mmol) of urea were dissolved in 5 mL of deionized (DI) water. After stirring for 15 min to form a clear solution, the mixture was transferred into a Teflon-lined stainless autoclave (20 mL). The sealed autoclave was heated in an electric oven at 160 °C for 3 h. After cooling down to room temperature, the N-GQD sample was obtained by adding ethanol into the solution and centrifuging at 8000 rpm for 10 min. The final product was dialyzed using a membrane with a molecular weight cutoff of 2000 Da in DI water for 3 days. Preparation of ML WSe2. ML WSe2 samples were grown directly on a SiO2/Si substrate by CVD and transferred onto a Si substrate covered with a 300 nm thick SiO2 layer by the wet-transfer method. Through a typical process, a layer of poly(methyl methacrylate) (PMMA) was coated onto the surface of the as-grown ML WSe2 to serve as the supporting layer for the transfer process. After dipping in hydrogen fluoride solution, PMMA/ML WSe2 was detached from the substrate and remained on the solution, followed by washing with DI to remove any residual etchant. The washed samples were transferred to a clean SiO2/Si substrate. After the transfer process was complete, PMMA was removed using acetone. Fabrication of an ML WSe2/N-GQD Photodetector. After transferring ML WSe2 onto the SiO2/Si substrate with highly doped ntype Si that served as the back gate, the N-GQD solution was spraycasted onto the ML WSe2 at 80 °C on a hot plate. Subsequently, the electrode pattern was first created by EBL. Cr/Au (5/50 nm) was thermally deposited as the drain and source electrodes, respectively, followed by the lift-off process with acetone. Characterizations. The morphologies and structures of the samples were obtained through SEM (JSM7000F, JEOL) and X-ray diffractometry (Rigaku, SmartLab). The height profile was measured using an atomic force microscope (SPA400, SEIKO). The UV−vis absorption, PL spectra, and FTIR spectra of the N-GQD samples were measured using a UV−vis absorption spectrophotometer (V-670, JASCO), a Cary Eclipse fluorescence spectrophotometer (Agilent Technologies), and an FT/IR-4700 spectrometer (JASCO), respectively. The Mott−Schottky measurements were performed using a three-electrode system with a Pt rod as the counter electrode, glassy carbon as the working electrode, and calomel (3 M KCl) as the reference electrode (VMP3 Potentiostat Bio-Logic Science). Raman, PL, and XPS measurements of ML WSe2 and ML WSe2/N-GQDs were examined by using a multifunctional optical microscopy system (NTEGRA Spectra, NT-MDT) and a Multichamber XPS UHV system (PREVAC). Electrical measurements were conducted at room temperature with a probe station and source/measure units (Keithley 4200 and Agilent B2900A) under ambient conditions. A monochromatic 405 nm laser was used as the illumination source.

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CONCLUSIONS Photoresponsivity of the ML WSe2 photodetector was enhanced by the incorporation of N-GQDs. An increase in the PL intensity and a shift in the Vth position of the ML WSe2 were also observed, which were attributed to the effect of ndoping in the N-GQDs. By analyzing the PL spectra, we found that the improved PL intensity in ML WSe2 could be the result of the reduction of positive trions and the increase in the number of neutral excitons. In addition, an enhanced photoresponsivity was observed from the ML WSe2/N-GQD photodetector, which was approximately 480% higher than that of the ML WSe2-based photodetector, because of the charge transfer from the N-GQDs to ML WSe2 and the photogating effect by photogenerated electrons in the N-GQD layer. A highest photoresponsivity of 2578 A W−1 was obtained at VG = −60 V and a power density of 1.9 μW cm −2 . The photoresponsivity of the ML WSe2/N-GQD photodetector was stable under ambient conditions for a long time (up to 30

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18419. Optical and AFM images of ML WSe2 CVD samples; schematic procedure of the synthesis of N-GQDs; AFM, TEM, FTIR, and Mott−Schottky spectra of N-GQDs; schematic procedure to fabricate ML WSe2/N-GQDs; OM images and transfer curves of an identical ML WSe2 device; local PL spectra of ML WSe2, N-GQDs, and ML WSe2/N-GQD heterostructure; XPS survey scan of ML WSe2 before and after coating of N-GQDs; photoresponse of ML WSe2 photodetector after fabrication F

DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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and after 30 days in ambient; AFM images of ML WSe2 and ML WSe2/N-GQDs after 30 days at ambient conditions; Raman and PL spectra of a fresh ML WSe2/N-GQD device and after 30 days under ambient; and photoresponse behavior of the ML WSe2/N-GQD photodetector (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-31-299-4053 (H.M.O.). *E-mail: [email protected] (M.S.J.). ORCID

Mun Seok Jeong: 0000-0002-7019-8089 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the IBS-R011-D1 of Korea and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2016R1A2B2015581) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1A6A3A11936024).

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ABBREVIATIONS AFM, atomic force microscopy; CVD, chemical vapor deposition; 2D, two-dimensional; DI, deionized; EBL, electron beam lithography; FET, field-effect transistor; FTIR, Fourier transform infrared; IQE, internal quantum efficiency; ML, monolayer; MWCO, molecular weight cutoff; N-GQD, nitrogen-doped graphene QD; PL, photoluminescence; PMMA, poly(methyl methacrylate); QD, quantum dot; SEM, scanning electron microscopy; TMD, transition-metal dichalcogenide; TEM, transmission electron microscopy; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy.

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DOI: 10.1021/acsami.7b18419 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX