High-Energy Gain Upconversion in Monolayer Tungsten Disulfide

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Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX

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High-Energy Gain Upconversion in Monolayer Tungsten Disulfide Photodetectors Qixing Wang,† Qi Zhang,† Xiaoxu Zhao,‡ Yu Jie Zheng,§,∥ Junyong Wang,† Xin Luo,⊥ Jiadong Dan,‡ Rui Zhu,† Qijie Liang,†,○ Lei Zhang,† P. K. Johnny Wong,∇ Xiaoyue He,† Yu Li Huang,† Xinyun Wang,†,∇ Stephen J. Pennycook,‡ Goki Eda,†,#,∇ and Andrew T. S. Wee*,†,∇ †

Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore § MOE Key Laboratory of Low-grade Energy Utilization Technologies and Systems, CQU-NUS Renewable Energy Materials and Devices Joint Laboratory, Chongqing 400044, China ∥ School of Energy and Power Engineering, Chongqing University, Chongqing 400044, China ⊥ State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 12 510275, Guangdong, People’s Republic of China # Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore ∇ Centre for Advanced 2D Materials, National University of Singapore, Block S14, 6 Science Drive 2, Singapore 117546, Singapore ○ SZU-NUS Collaborative Innovation Center for Optoelectronic Science and Technology, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China

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‡

S Supporting Information *

ABSTRACT: Photodetectors usually operate in the wavelength range with photon energy above the bandgap of channel semiconductors so that incident photons can excite electrons from valence band to conduction band to generate photocurrent. Here, however, we show that monolayer WS 2 photodetectors can detect photons with energy even lying 219 meV below the bandgap of WS2 at room temperature. With the increase of excitation wavelength from 620 to 680 nm, photoresponsivity varies from 551 to 59 mA/W. This anomalous phenomenon is ascribed to energy upconversion, which is a combination effect of one-photon excitation and multiphonon absorption through an intermediate state created most likely by sulfur divacancy with oxygen adsorption. These findings will arouse research interests on other upconversion optoelectronic devices, photovoltaic devices, for example, of monolayer transition metal dichalcogenides (TMDCs). KEYWORDS: Tungsten disulfide (WS2), photodetectors, upconversion, one-photon excitation, multiphonon absorption

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Monolayer transition metal dichalcogenides (TMDCs) are an emerging group of two-dimensional (2D) semiconductors with sizable carrier mobility and direct bandgap which make them very promising for field effect transistors (FETs) and optoelectronic devices.11,19−24 To date, however, all the TMDC photodetectors are reported to work in the energy range above the bandgap of TMDCs. A systematic study in the energy range below the bandgap of TMDCs has not been wellestablished yet.

hoton energy upconversion is a phenomenon of light− matter interaction, where the energy of emitted photon is higher than that of excitation photon. Photon energy upconversion is an anti-Stokes process, the origin of which can be ascribed to an additional energy gain from multiplephoton absorption,1−3 Auger recombination,4−6 or phonon (thermal energy) absorption7−11 through virtual or real intermediate states. The investigation in energy upconversion has aroused increasing interest in recent years owing to its potential application in upconversion optoelectronic devices,12−15 optical refrigeration in solids,7,16 bioimaging,17,18 and so on. © XXXX American Chemical Society

Received: May 26, 2019 Published: June 26, 2019 A

DOI: 10.1021/acs.nanolett.9b02136 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Electrical transport properties of monolayer WS2 photodetector. (a) Schematic illustration of a monolayer WS2 photodetector. Graphene ribbons were transferred on top of monolayer WS2 as contacts, and Ti/Au were deposited on top of graphene ribbon. (b) An optical microscope image of the fabricated WS2 photodetector. The scale bar is 10 μm. (c) Ids−Vg transfer curve of a monolayer WS2 photodetector at Vds = 2 V in semilogarithmic (left, blue) and linear (right, pink) scale. (d) Ids−Vds curve of the photodetector at Vg from 0 to 50 V.

Figure 2. Monolayer WS2 upconversion photodetector. (a) A color contour image of the photocurrent at different bottom gate voltages (Vg) and excitation wavelengths for a monolayer WS2 photodetector. There is photocurrent generation at excitation wavelength much longer than bandgap wavelength of 615 nm. (b) Ids−Vg curves of a monolayer WS2 photodetector at dark, 680, 650, 630, 610, and 530 nm laser excitations. The excitation laser power is 10 μW. There is photocurrent generation even at 680 nm laser excitation. Vds = 2 V. (c) Ids−Vg curves of a monolayer WS2 photodetector with 0, 7, 21, 35, and 49 μW 660 nm laser excitations. Vds = 2 V. (d) Ids−Vds curves of a monolayer WS2 photodetector at dark, 690, 660, 650, and 640 nm laser excitations. The excitation laser power is 10 μW. Vg = 0 V. (e) Ids−Vds curves of a monolayer WS2 photodetector at dark, 2, 4, 8, and 14 μW 690 nm laser excitations. Vg = 0 V. (f) Photoresponsivity of a monolayer WS2 photodetector at different excitation wavelengths (blue squares, left) at Vg = 50 V and absorption spectrum of a monolayer WS2 (green circles, right). The gray region represents upconversion detection range by monolayer WS2 photodetector, where excitation laser energy is smaller than monolayer WS2 bandgap energy.

Here, we demonstrate room-temperature energy upconversion in monolayer WS2 photodetectors. The WS2 photo-

detector has photoresponse at excitation wavelengths with energy below the bandgap of monolayer WS2 from 620 to 690 B

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To study the ability of the photodetector to detect photons with energy below the bandgap of monolayer WS2, Ids−Vds curves under dark, 690, 660, 650, and 640 nm excitations are compared (Figure 2d). A clear photocurrent generation under the excitation of 690 nm laser is observed. The energy gain under 690 nm excitation is 219 meV. As the excitation wavelength decreases from 690 to 640 nm, Ids increases gradually due to a higher absorption of photon at shortwavelengths and thus resulting in a larger amount of photocurrent generation. At the excitation wavelength of 690 nm, photocurrent amplifies with the excitation laser power (Figure 2e). The phenomenon of detecting photons with energy below the bandgap of semiconductors can be ascribed to energy upconversion.13−15 In addition to energy upconversion, the photocurrent generation can also possibly be generated through charge transfer from graphene contacts to WS2 by hot electron creation in graphene and photothermoelectric effect in WS2. To exclude photon absorption by graphene contacts as the possible origin of photocurrent generation, the 660 nm laser was focused in the channel center of the photodetector by a 100× objective lens with a spot diameter of 0.5 μm, which is much smaller than the channel length of 3 μm (see Methods). Besides, a photocurrent line scan was performed across the channel of WS2 photodetector (see Supporting Information, Figure S1). As is shown in Figure S1b,c, there is photocurrent generation in the channel center of WS2 photodetector, WS2/graphene interface, and WS2/ graphene stacking regions with the lowest and highest photocurrent generation in the channel center of WS2 photodetector and WS2/graphene stacking regions, respectively. A much higher photocurrent generation in WS2/ graphene interface and WS2/graphene stacking regions compared to that in WS2 channel is due to a synergic effect of photocurrent generation in WS2 and charge transfer from graphene contacts to WS2 by hot electron creation in graphene. What’s more, photocurrent generation was also observed in the center of WS2 photodetector channel using Ti/Au (5 nm/50 nm) as contacts (See Supporting Information, Figure S2a−c). The presence of photocurrent generation in the channel center of WS2 photodetector, where either graphene or Ti/Au contacts are used, can exclude the possibility that charge transfer from graphene contacts to WS2 by hot electron creation in graphene as the possible origin of detecting photons with energy below the bandgap of monolayer WS2. Next, the influence of photothermoelectric effect in WS2 will be discussed. Photothermoelectric effect is a phenomenon where a temperature gradient (ΔT) resulting from light absorption across an interface between two materials with different Seebeck coefficient (S) generates a photothermal voltage (ΔVPTE = ΔSΔT).29−31 The generated photothermal voltage (ΔVPTE) can drive current passing through the device. In this report, photocurrent measurement was performed in the condition where laser was focused in the channel center of WS2 photodetector so that laser exposed region had the same Seebeck coefficient (S) to avoid the photothermoelectric effect. Besides, the WS2 photodetector has a typical Schottky contact (Figure 1d) and the high Schottky barrier can block carriers generated from photothermoelectric effect in WS2 to diffuse into electrodes.32,33 Hence, photothermoelectric effect cannot be the dominant mechanism in photocurrent generation. Furthermore, photothermoelectric effect was evaluated in the device with Ti/Au contacts, where a photocurrent line scan

nm with highest energy gain of 219 meV. At the excitation wavelength of 680 nm, the highest photoresponsivity, photoconductive gain, and specific detectivity is 59 mA/W, 0.1, and 7.5 × 105 Jones, respectively. Photoluminescence (PL) studies at varying excitation power, excitation wavelength, and temperatures, together with scanning transmission electron microscopy−annular dark field (STEM-ADF) imaging and density functional theory (DFT) calculations were performed to reveal the underlying upconversion mechanism. The observed anomalous energy upconversion is attributed to energy gained by absorbing multiphonons through intermediate states created by defects, most likely sulfur divacancy (VS2) with oxygen adsorption. The photodetector is made up of a monolayer WS2 as energy gain conducting channel, few layer graphene ribbons on top of WS2 as contact electrodes, and Ti/Au (5 nm/50 nm) on top of graphene ribbons as conducting metal (Figure 1a). Graphene ribbons were chosen as contact electrodes in order to improve the device performance. The detailed fabrication process is described in Methods. Figure 1b shows an optical microscope image of the fabricated WS2 photodetector. Figure 1c depicts the drain-to-source current (Ids) versus bottom gate voltage (Vg) (Ids−Vg) transfer curve of a monolayer WS2 transistor at Vds of 2 V. This transistor exhibits a typical n-type transport with an electron mobility of 3 cm2 V−1 s−1 and current on/off ratio of 105, which is comparable to results of previous reports.25,26 The n-type transport together with a threshold voltage of −17 V manifests that monolayer WS2 is intrinsically electron doped. Figure 1d displays the Ids−Vds curve of the transistor at Vg varying from 0 to 50 V. The nonlinear Ids−Vds curve indicates a Schottky contact between monolayer WS2 and few layer graphene. Figure 2a shows a photocurrent intensity image of a monolayer WS2 photodetector as a function of Vg and excitation wavelength. When the excitation wavelength increases from 490 to 680 nm, two resonant peaks are observed at 517 and 615 nm, which correspond to positions of B and A excitons of monolayer WS2, respectively. At 517 and 615 nm excitations, a small Vg can generate a large photocurrent owing to enhanced resonant absorption. When the excitation laser wavelength (energy) is longer (smaller) than the bandgap wavelength (energy) of 615 nm (2.016 eV), photocurrent generation can also be observed. Figure 2b compares Ids−Vg transfer curve under dark, 680, 650, 630, 610, and 530 nm laser excitations. With the decrease of excitation wavelength from 680 to 530 nm, the threshold voltage shifts to negative side gradually. The shift in threshold voltage is attributed to photogating effect in monolayer WS2 photodetector.27,28 Under light illumination, electron−hole pairs are generated, and subsequently photogenerated holes are trapped by trap states in SiO2 substrate, giving rise to positively charged traps, which leads to a strong photogating effect and electron doping in monolayer WS2. Therefore, the threshold voltage shifts to the negative direction under light illumination. With the increase of excitation wavelength, the shift reduces owing to the decrease in absorption. A similar photocurrent generation and threshold voltage shift is also shown in Ids−Vg transfer curve at varying powers under the excitation of 660 nm laser (Figure 2c). As the laser power intensifies, the amount of generated photocurrent increases. At the same time, the threshold voltage shifts to negative direction with the increase of laser power. C

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Figure 3. Photoresponsivity, photoconductive gain, and specific detectivity of a monolayer WS2 upconversion photodetector. (a−c) The photoresponsivity (a), photoconductive gain (b), and specific detectivity (c) of a monolayer WS2 upconversion photodetector as a function of excitation wavelength from 620 to 680 nm at Vg = 0 V (black squares), 10 V (blue circles), 30 V (green up-triangles), and 50 V (red downtriangles). The excitation power is 10 μW. Vds = 2 V. (d−f) The photoresponsivity (d), photoconductive gain (e), and specific detectivity (f) of a monolayer WS2 upconversion photodetector as a function of 660 nm excitation power at Vg = 0 V (black squares), 10 V (blue circles), 30 V (green up-triangles), and 50 V (red down-triangles). Vds = 2 V.

figure of merit to evaluate the performance of the photodetector. Assuming external quantum efficiency η = 100%, the photoconductive gain is calculated as G = Rhυ/ηe, where R is photoresponsivity, h is the Planck constant, ν is the frequency of the excitation laser, and e is electron charge.21,32 Similar to photoresponsivity, photoconductive gain at varying Vg also decreases with the increase of excitation wavelength (Figure 3b). At the excitation wavelength of 620 and 680 nm, the highest photoconductive gain is 1.1 and 0.1 at Vg = 30 V, respectively. In terms of photoresponsivity and photoconductive gain, the upconversion performance of the monolayer WS2 photodetector is comparable to or even better than traditional upconversion photodetectors.13−15 To assess the upconversion sensitivity of the WS2 photodetector against the background noise, specific detectivity is calculated. Assuming the shot noise from the dark current contributes most to the total noise, the specific detectivity is given by D* = RA1/2/(2eId)1/2, where A is the effective device area and Id is the dark current.32 The detectivity declines with the increase of laser wavelength from 620 to 680 nm (Figure 3c). In contrast to photoresponsivity and photoconductive gain, detectivity is higher at a lower gate voltage because at lower Vg the dark current is smaller. At Vg = 0 V, detectivities are 1.6 × 107 and 7.5 × 105 Jones at 620 and 680 nm excitation, respectively, which are much smaller than values in previous reports of photodetectors in detecting photons with energy above the bandgap of monolayer TMDCs.21,32 At the excitation wavelength of 660 nm, the overall trends of photoresponsivity, photoconductive gain, and specific detectivity decline with the increase of laser power owing to pronounced recombination of photoexcited electron−hole pairs (Figure 3d−f).32

was performed across the photodetector under the excitation of 660 nm laser at Vds = 0 V (see Supporting Information, Figure S2d). Figure S2d shows that photocurrent arising from photothermoelectric effect is below system noise level. On the basis of the discussions above, it is assured that the ability of the device to detect photons with energy below the bandgap of WS2 is due to energy upconversion. In order to quantitatively evaluate upconversion performance of the photodetector, photoresponsivity (R = Iph/P) is calculated at varying excitation wavelengths from 490 to 680 nm at Vg = 50 V and Vds = 2 V (blue squares, Figure 2f), where Iph is photocurrent and P is absorbed laser power. An absorption spectrum of monolayer WS2 is plotted as well for comparison (green circles, Figure 2f). Two peaks at 517 and 615 nm of the photoresponsivity match well with the absorption lines of B and A excitons in the absorption spectrum. The gray region in Figure 2f represents upconversion detection range by the photodetector, where excitation laser energy is below the bandgap of monolayer WS2. With the increase of excitation wavelength from 620 to 680 nm, the photoresponsivity declines from 512 to 27 mA/W which is much larger than reported values of a few mA/W for traditional upconversion photodetectors.13−15 In the upconversion detection range from 620 to 680 nm, the photoresponsivity at Vg = 0, 10, 30, and 50 V decreases gradually with the increase of excitation wavelength owing to reduced absorption at long wavelengths (Figure 3a). Photoresponsivity is higher at larger Vg because the Fermi level is closer to the conduction band minimum, resulting in smaller Schottky barrier. At Vg = 30 V, the highest and lowest responsivity are 551 and 59 mA/W at 620 and 680 nm laser excitations, respectively. Photoconductive gain is another D

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Figure 4. Photocurrent response of a monolayer WS2 upconversion photodetector. (a) Time-resolved photoresponse curve of the device at 640 nm (green), 660 nm (blue), and 680 nm (black) laser excitation. All the excitation powers are 10 μW. (b) Time-resolved photoresponse curve of the device at 10 μW (black), 20 μW (blue), 30 μW (green) 660 nm laser excitation. (c) An enlarged figure in (b) at 10 μW 660 nm laser excitation showing the rise time an decay time of 2.2 and 6.1 s, respectively. All the measurement were performed at Vds = 2 V and Vg = 0 V.

Figure 5. PL studies of energy upconversion in monolayer WS2. (a) A comparison between upconversion PL (pink) and normal PL spectra of a monolayer WS2. The excitation laser wavelengths for upconversion PL and normal PL are 658 and 532 nm, respectively. (b) A comparison of upconverion PL spectra of monolayer WS2 at powers from 5 to 49 μW. The excitation laser is 658 nm. (c) Integrated upconversion PL intensity as a function of excitation power. The red line is a linear fitting. Error bars correspond to standard deviations of Lorentz fitting of the upconversion PL spectra at different excitation powers. (d) A comparison of upconverion PL spectra at excitation wavelengths from 650 to 690 nm. The spectrum is vertically shifted for comparison. (e) Upconverion PL intensity as a function of excitation wavelength from 650 to 690 nm. (f) A comparison of upconverion PL spectra of monolayer WS2 from 260 to 340 K excited by 658 nm laser. (g) Arrhenius plot of IUCPL/IPL as a function of 1/T. IUCPL/ IPL represents upconversion PL intensity (IUCPL) normalized by normal PL intensity (IPL) at respective temperatures. The red line is a linear fitting. From the slope of the fitting, the activation energy (Eact) is calculated as 127 ± 4 meV. (h) A schematic energy diagram of multiphonon-assisted energy upconversion processes. State |0⟩, |1⟩, and |2⟩ represent ground state, intermediate state, and free exciton state, respectively.

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As the temperature lowers down, the upconversion PL intensity weakens with the peak position blueshift (see also Supporting Information, Figure S4), just as the case in normal PL (see Supporting Information, Figure S5). To exclude the effect of dark exciton state on upconversion PL intensity at varying temperatures, upconversion PL intensity (IUCPL) is normalized through being divided by the normal PL intensity (IPL) at respective temperatures. After normalization, upconversion PL intensity (IUCPL/IPL) still increases with temperature (Figure 5g), indicating that the energy upconversion is a thermally assisted (or phonon-mediated) process. At elevated temperatures, there are an increased number of phonons available for phonon-mediated energy upconverison.8,9 The upconversion PL intensity strengthens with phonon population, which follows Bose−Einstein distribution (I(T) = I0/(exp(Eact/kBT) − 1)), where kB, T, and Eact are Boltzmann constant, temperature, and activation energy, respectively. At an excitation wavelength of 658 nm, the energy gain of the upconversion is about 133 meV, which is much larger than room-temperature (300 K) thermal energy kBT (26 meV). Hence, a simplified distribution formula I(T) = I0 exp(−Eact/ kBT) is adopted to fit IUCPL/IPL at varying temperatures (red line, Figure 5g). The fitted Eact of 127 ± 4 meV coincides well with the upconversion energy gain of 133 meV at an excitation wavelength of 658 nm, further corroborating the phononmediated energy upconversion mechanism. In order to figure out the phonon modes involved in the energy upconversion process, a Raman spectrum study was conducted. The first-order phonon modes are LA (M), E12g(Γ), and A1g (Γ) with Raman shifts (energy) of 175 cm−1 (21.7 meV), 356.5 cm−1 (44.2 meV), and 417.4 cm−1 (51.8 meV) (see Supporting Information, Figure S6). The fourth-order resonant 4LA (M) mode is at 700.2 cm−1 (86.9 meV). The energies of single Raman modes are too small compared to the energy gain of 219 meV, suggesting that the energy upconversion is a multiphonon absorption process. The multiphonon can be a combination of the same or different phonon modes as has been reported in WS2 PL upconversion.10 Intermediate states usually participate in the energy upconversion process. To verify the presence of intermediate states, low-temperature PL was studied. At 120 K, a broad emission peak from defects appears at 698 nm, which is 290 (250) meV below the exciton (trion) peak (see Supporting Information, Figure S7). The peak position (698 nm) of the defect state is close to 690 nm under the excitation of which both upconversion photodetection and PL can be observed. Hence, this defect state most probably participates in the energy upconversion process. Gas adsorption was previously found to passivate defects in TMDCs and thus probably affects photon energy upconversion.38 To study the effect of gas adsorption on energy upconversion, few layer hexagonal boron nitride (h-BN) was used to encapsulate monolayer WS2. After encapsulating with h-BN, PL intensity of monolayer WS2 is enhanced, indicating an increased PL quantum efficiency (see Supporting Information, Figure S8a). In contrast, the upconversion PL intensity of WS2 with h-BN encapsulation is reduced compared to the naked WS2 (see Supporting Information, Figure S8b). Besides, the upconversion PL intensity of WS2 in vacuum is reduced compared to the WS2 in ambient (see Supporting Information, Figure S8d). These observations can be understood if the intermediate states for energy upconversion

The time-resolved photoswitching behavior of the photodetector was studied under the illumination of 10 μW 640, 660, and 680 nm laser for six cycles at Vds = 2 V and Vg = 0 V (Figure 4a). The photodetector exhibited a repeatable photocurrent response under 640, 660, and 680 nm laser excitations even though the energy of excitation laser is much smaller than the bandgap of WS2. The photocurrent at 640 nm excitation (8 × 10−9A) is about 20 times higher than that at 680 nm excitation (4 × 10−10A) due to higher absorption at shorter wavelength. At the excitation wavelength of 660 nm, photocurrent increases almost linearly as the laser power increases from 10 to 30 μW (Figure 4b). Figure 4c shows one cycle of the time-resolved photoresponse curve at 10 μW 660 nm excitation. The rise (decay) time of the time-resolved photocurrent curve is calculated to be 2.2 (6.1) s estimated by the time required for the photocurrent to rise (decay) from 10% (90%) to 90% (10%) of the final (initial) value, which is similar to the values of traditional upconversion photodetectors.13,14 To unravel the underlying mechanism of photon energy upconversion, photoluminescence (PL) study was performed. Figure 5a compares PL spectra of a monolayer WS2 at 658 nm (pink) and 532 nm (blue) laser excitations. At the excitation of 532 nm, the PL spectrum of WS2 shows a prominent A exciton emission centered at 615 nm and a trion A− emission centered at 628 nm (see also Supporting Information, Figure S3), which agrees well with previous reports.34,35 In contrast, at the excitation of 658 nm, the energy of which is 133 meV below the bandgap of monolayer WS2, PL spectrum can still be observed, which is consistent with the above observation of photocurrent generation at the excitation of 660 nm. The phenomenon of energy of emitted photons is higher than that of excitation photons is called PL upconversion.8,9 The PL spectrum of energy upconversion (pink) almost totally overlaps the normal PL spectrum (blue). Hence, the upconversion PL peak at 614.7 nm is attributed to WS2 A exciton emission, which is also observed in a previous report.10 At the excitation of 658 nm, upconversion PL spectra are compared at different excitation powers. With the increase of excitation power, the intensity of upconversion PL increases gradually without distinct changes in peak shape and peak position (Figure 5b). Figure 5c presents integrated intensity of upconversion PL as a function of excitation power. The intensity of upconversion PL increases almost linearly with the excitation power. The linear dependence of the upconversion PL intensity with the excitation power manifests that the energy upconversion is a one-photon excitation process instead of Auger recombination or multiple-photon excitation process. Because Auger recombination or multiple-photon excitationinduced energy upconversion usually shows a superlinear (square or more) excitation power dependence.8,9,36 Figure 5d depicts upconverion PL spectrum of monolayer WS2 at excitation wavelengths from 650 to 690 nm under the same excitation power with energy gain ranging from 109 to 219 meV. The intensity of upconversion PL attenuates with the increase of excitation wavelength, whereas the peak position and spectral shape remain almost unchanged at different excitation wavelengths (Figure 5e). From these observations, anti-Stokes Raman scattering can be ruled out as the origin of energy upconversion because peak position shifts with excitation wavelength in Raman process.37 Figure 5g compares the upconversion PL spectra at temperatures varying from 260 to 340 K at 658 nm excitation. F

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optoelectronic devices, photovoltaic devices, for example, of monolayer TMDCs. Methods. Sample Preparation and Characterization. Monolayer WS2 was exfoliated by polydimethylsiloxane (PDMS) from bulk crystals (HQ graphene) and then transferred onto a 300 nm SiO2/Si substrate.21 Layer thicknesses of the exfoliated flakes were determined by their optical contrast under an optical microscope first and then confirmed by PL and Raman spectroscopy. Few-layer graphene were exfoliated onto a 300 nm SiO2/Si substrate directly, which was followed by electron beam lithography (EBL) (FEI) patterning and deep RIE (Oxford Plasma Pro Cobra 100) etching into graphene ribbons. Poly(methyl methacrylate) (PMMA) A5 950 was spin-coated onto graphene ribbons and baked at 180 °C on a hot plate for 1 min. PMMA-covered samples were etched by KOH solution (3 mol/L). After floating on KOH solution, graphene ribbons attached on PMMA film were put onto deionized (DI) water for 2 h to remove KOH residue. Then PMMA-covered graphene ribbons were precisely transferred onto exfoliated monolayer WS2 with a transfer station. After that, the samples were immersed in acetone for 2 h and IPA for 5 min to remove PMMA and residues. After transfer graphene ribbons onto WS2, Ti/Au (5 nm/50 nm) were patterned by EBL and thermoevaporated onto graphene ribbons as conducting metal followed by lift-off in acetone (2 h) and IPA (5 min). Then the fabricated devices were annealed at 200 °C in ultrahigh vacuum (∼10−6 mbar) for 2 h to improve device performance. Optoelectronic Measurement. The electrical transport properties of WS2 photodetectors were measured by Agilent B2902A in ultrahigh vacuum (∼10−5 mbar). The photocurrent measurement of the upconversion WS2 photodetectors were excited by monochromator-coupled supercontinuum light source. Lasers were focused by a 100× objective lens with spot diameter of 0.5 μm. PL and Raman Spectroscopy. Room-temperature (300 K) PL and Raman spectra were captured by a commercial WITec Alpha 300 R Raman system. The continuous wave (CW) laser wavelength is 532 nm and the spot diameter is 0.5 μm using a 100× objective lens. The laser power of 3 μW, integration time of 1 s, and accumulation times of 2 were used for PL spectrum measurement. The laser power of 0.65 mW, integration time of 1 s, and accumulation times of 2 were used for Raman spectrum measurement. Low-Temperature PL Spectroscopy. Low-temperature PL spectra were captured by a laser confocal microscope (NTMDT, NTEGR Spectra) using a 100× objective lens. The excitation laser wavelength is 532 nm (CW). Low-temperature PL measurements were performed in a vacuum chamber coupled with a liquid nitrogen (78 K) container (Janis). PLE Spectroscopy. Photoluminescence excitation (PLE) spectra were captured by a confocal microscope (NT-MDT, NTEGR Spectra) with monochromator-coupled supercontinuum light as excitation source. The PLE experiment was conducted in ultrahigh vacuum (∼10−5 mbar) using a 100× objective lens. To obtain upconversion PL spectra, 625 and 650 nm short pass filters were used together in the collection optical path to filter out long wavelength laser. STEM-ADF Imaging. Atomic resolution STEM-ADF images were taken from JEOL ARM200F equipped with an ASCOR aberration corrector operating at 60 kV. The collection angle is 30 to 110 mrad in order to enhance the S contrast. The convergence angle is set at 30 mrad.

originate from defects with oxygen adsorption. Defect states are passivated by h-BN encapsulation or reduced by oxygen desorption, resulting in reduced upconversion PL due to the lack of intermediate states for upconversion. To reveal the origin of this defect, a scanning transmission electron microscopy−annular dark field (STEM-ADF) imaging study of mechanically exfoliated monolayer WS2 was performed (see Supporting Information, Figure S9). From this study, it is found that sulfur vacancy (VS) and sulfur divacancy (VS2) dominate the defects and other defects or chemical elements except tungsten and sulfur can hardly be observed. The concentration of VS and VS2 in the mechanically exfoliated monolayer WS2 is 0.44 and 0.066 nm−2, respectively. Oxygen atoms cannot be distinguished due to the low knockon damage threshold and much smaller atomic number of oxygen compared to sulfur and tungsten. In order to figure out which one of VS and VS2 is involved in the energy upconversion and the influence of oxygen adsorption, electronic band structures of VS and VS2 without and with oxygen absorption are calculated based on density functional theory (DFT) method. Many-electron GW-BSE calculations are not performed here due to the computation being prohibitive for our structures. But the PBE bandgap (∼1.8 eV) is closed to the theoretical exciton peak computed with GW-BSE (1.95 eV)39 and our experimental exciton peak (∼2 eV). Hence, we could evaluate the possibility of the defects by comparing the energy difference between the conduction band minimum (CBM) and defect levels in DFT to the energy difference between the defect peak and the A exciton peak in experiment. From the calculation results (see Supporting Information, Figure S10), it is found that the energy differences between the defect states for VS, VS2, and VS2 with oxygen adsorption and CBM are around 460, 500, and 440 meV, respectively. All of them are close to the experimental value of 290 meV of the energy difference between defect peak and A exciton peak in the PL spectrum of monolayer WS2 (see Supporting Information, Figure S7) which suggests that all of them could contribute to the upconversion. However, as discussed above, the upconversion PL intensity of WS2 with h-BN encapsulation or in vacuum is reduced compared to the naked WS2 or WS2 in ambient, which indicates that VS2 with oxygen adsorption is most likely the origin of the intermediate state involving in the energy upconversion. A schematic illustration of the PL energy upconversion process in this report is shown in Figure 5h. Electrons from ground state |0⟩ are excited by photons with energy below the bandgap of WS2 into an intermediate state |1⟩ (red arrow), followed by absorbing energy from multiphonons and being upconverted into a higher energy free exciton state |2⟩ (brown arrow). Then, free excitons recombine to ground state |0⟩ and emit photons with energy higher than excitation photon energy (green arrow). In conclusion, we have reported monolayer WS2 upconversion photodetectors with a highest energy gain of 219 meV. Under the illumination of laser with energy below the bandgap of monolayer WS2, the photoresponse is attributed to energy upconversion by absorbing multiphonons through an intermediate state originating most likely from sulfur divacancy (VS2) with oxygen adsorption. Further study of optimizing photodetector device structure to improve its performance at energy upconversion regions can be performed. Our study will also inspire research interests on other upconversion G

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DFT Calculations. The density functional theory (DFT) calculations were performed with the VASP code40,41 using the projector-augmented plane wave (PAW) approach and a kinetic energy cutoff of 400 eV. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional42 was employed to perform all calculations and Grimme’s D343 correction was used to consider the van der Waals (vdW) interactions. Single layer WS2 with defects was modeled using a 5 × 5 supercell with more than 15 Å of vacuum. Geometry optimization was performed with a force convergence criteria of 0.01 eV/Å. Monkhorst Pack k-grid sampling of 10 × 10 × 3 in the bulk WS2 unit cell and 2 × 2 × 1 in the 5 × 5 supercells were used for geometry optimization.

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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/acs.nanolett.9b02136. Additional figures (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +65 66013757.. ORCID

Qixing Wang: 0000-0003-0623-1910 Qi Zhang: 0000-0003-4201-7196 Xiaoxu Zhao: 0000-0001-9746-3770 Qijie Liang: 0000-0003-4252-7997 Yu Li Huang: 0000-0003-3699-4708 Goki Eda: 0000-0002-1575-8020 Andrew T. S. Wee: 0000-0002-5828-4312 Author Contributions

Q.W., Q.Z., and X.Z. contributed equally to this work. Q.W. conceived and designed the experiment. Q.W. prepared the sample, fabricated devices, and performed PL, Raman characterizations. Q.W. and Q.Z. performed PLE, temperature-dependence, power-dependence, and optoelectronic device characterizations together. X.Z. performed STEM study. Y.J.Z. performed DFT calculations. J.W. helped to set up equipment for optoelectronic device characterization. Q.W. analyzed the data and wrote the manuscript. All the authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support by NUS research scholarship (WQX), A-STAR 2D Pharos Grant (SERC 1527000012), Singapore Ministry of Education Tier 2 grant MOE2016-T2-2110, and MOE grant R-144-000-389-114. G.E. also acknowledges funding support from the Ministry of Education (MOE), Singapore, under AcRF Tier 2 (MOE2015-T2-2-123, MOE2017-T2-1-134). Computations were performed on High-Performance Computing platform of Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences. H

DOI: 10.1021/acs.nanolett.9b02136 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.9b02136 Nano Lett. XXXX, XXX, XXX−XXX