Gate‐controlled Multispectral Response in Graphene‐Based Heterostructure Photodetector

Multispectral photodetectors are crucial for detecting light across a wide wavelength range, serving applications requiring precise wavelength specificity and spectral imaging capabilities. However, the development of these photodetectors is hindered by several challenges, including material compatibility issues, low responsivity, the complexity of signal processing, and precise bandgap engineering. A strategy is proposed using a MoS2‐graphene photodetector to address these issues. Gate‐tunable spectral responses are achieved in a graphene photodetector by utilizing carrier transfer from MoS2 and interfacial gating effects from a SiO2/p‐doped Si substrate. Precise gate bias manipulation enables selective photocurrent capture in the range of 500–680 nm, identical to the absorption of MoS2. Furthermore, by applying a highly negative gate bias, photocurrent signals below the MoS2 bandgap, i.e., in the 680–800 nm region, are detected, significantly provoking broadband photodetection. The results highlight the versatility of gate‐tunable multispectral response, leading to an exceptional responsivity of up to 1.4 × 105 mA W−1. Moreover, through the precise modulation of gate bias and incident wavelength, it seamlessly switches between negative and positive photocurrents. This study provides important insight into carrier photogeneration in sensitized graphene‐based multifunctional optoelectronic devices, establishing a versatile platform for detecting a broad range of photocurrents with a single detector.


Introduction
Multispectral sensing is a technique to capture and analyze photon signals in multiple discrete spectral bands, which has been mechanisms include charge transport, [11][12][13] photocurrent polarity switching (negative and positive photocurrent (NPC/PPC)), [14][15][16][17] and efficient carrier generation. [12,18]Thus, comprehensive studies focusing on these aspects are both challenging and urgent to achieve high-performance multispectral detectors with improved responsivity and broadband detection capabilities, among other desirable attributes.The optical and electrical properties can be modulated by adjusting the back gate bias (V G ). [12,19] Therefore, controlling the photoresponse via gate-field is desirable for developing spectral sensing and advanced applications.
][22][23][24][25][26] However, its weak light absorption (only 2.3% in the visible spectral range) and low gain mechanism present a significant challenge for graphene-based photodetectors.Combining graphene with strong light-absorber, such as transition metal dichalcogenides, [13,21,23] silicon (Si), [27] and quantum dots, [22,28,29] enables the development of photodetectors that can achieve an ultrahigh photoconductive gain [22] and responsivity of ≈10 8 and 10 7 A W −1 , respectively.Utilizing the interfacial gating (IG) effect is an attractive method for improving the performance of graphenebased photodetectors.IG requires the absorption of incident light by the Si substrate, which generates a photovoltage that can be used to gate the graphene channel.Recent developments in this field have yielded remarkable results.[32] Moreover, the IG mechanism exploits the physics of dipole formation at the interface between Si and dielectric materials [33] .This phenomenon plays a crucial role in modulating the electrical properties of graphene in response to incident light, particularly its conductivity.These advancements have paved the way for developing advanced electric-dipole gated two-terminal phototransistors, which hold tremendous potential for applications such as chargecoupled devices in broadband image sensing [34] To unlock the potential of graphene for developing new and advanced optoelectronic devices, it is crucial to combine these mechanisms within a single device.This integration would enable a comprehensive understanding of the generated photocurrent processes and the underlying physical mechanisms driving them.However, there is currently a lack of comprehensive studies addressing these aspects.
Here, we demonstrate a novel strategy for achieving a gatecontrollable multispectral response in a graphene/MoS 2 photodetector.The design of the device incorporates both the carrier transfer (CT) from MoS 2 and the IG effect using the SiO 2 /pdoped Si substrate, with graphene as a primary channel and MoS 2 and Si as the photoactive layers.Our findings highlight that the IG effect is effectively controlled under heavily negative V G values, while the CT process is observed across a wide range of V G values.This flexibility allows V G to be utilized for tuning various types and numbers of carrier dopants for graphene channel conductance, thereby enabling control over the spectral responses of the device when exposed to light.The IG effect leads to an increase in hole density in graphene, while the CT process results in an increase in the electron population.The higher ab-sorption spectra from Si extending into the near-infrared (NIR) range have enabled a broadband photodetection solution that surpasses the limited bandgap of MoS 2 .Furthermore, through the modulation of the CT and IG processes, we demonstrate the capability to switch between NPC and PPC by adjusting the incident wavelength and V G .

Results and Discussion
In the graphene hybrid photodetector, the graphene spectral response can mirror the absorption spectrum of the photosensitive layer. [22,29,31]A novel strategy was presented herein for achieving a gate-tunable photoresponse in graphene-based heterostructure devices by combining the CT effect and IG in a single device.The V G was applied to modulate the efficient photocurrent generation in both processes, as shown in Figure 1a.The top view illustrates the gate-tunable IG process, in which the Si substrate absorbs light and generates a photovoltage to gate the conductance of graphene. [30,31]The bottom view demonstrates the gate-tunable CT process, in which photogenerated carriers from MoS 2 can be transferred to graphene. [20,21]The proposed strategy has two key functionalities.First, MoS 2 and Si are light-sensitive materials to promote the photocurrent and provide different wavelengthdependent photoresponsivities in graphene.Second, E F , the carrier density, charge transport, MoS 2 -graphene band alignment, and SiO 2 /p-doped Si band bending can be gate-modulated [30] , thereby allowing the control of the photocurrent generation in both the IG and CT processes from being inefficient to efficient (following the direction of the blue and red arrows).The proposed concept was implemented by designing a device, as illustrated in Figure 1b (see details in the Experimental Section; Figure S1, Supporting Information).Figure 1c displays the optical image of the device, with the monolayer graphene represented by the hexagonal shape and the MoS 2 depicted by the triangular shape, highlighting their distinctive structural features.Raman characterization and atomic force microscopy (AFM) imaging were conducted, as demonstrated in Figure S2 (Supporting Information).The bottom hexagonal Boron Nitride (hBN) layer plays a critical role in limiting the trapping effect of the SiO 2 interface and supporting the photogenerated holes [35] .Graphene was used as the main channel to exploit the advantages of its high carrier mobility.The first absorption layer in the device was monolayer MoS 2 , selected for its advantageous optoelectronic properties, including its direct bandgap, strong light-matter interaction, and rapid CT when combined with graphene. [11,18,23,36]This layer exhibited a narrow spectral response range with a bandgap of ≈1.82 eV. [18,37]t also acted as a donor layer, facilitating the transfer of photoelectrons to graphene upon excitation. [20]The Si back gate functions as the second absorption layer, providing broad absorption spectra owing to its smaller bandgap of ≈1.1 eV. [27,31]In addition, it serves as a gate to modulate the E F of graphene.The top hBN layer protected the device from oxidation.
Figure 1d presents the transfer characteristics of the MoS 2 field-effect transistor (FET) with the graphene electrode for drain and Cr/Au electrode as the source.The device was measured under dark conditions with a V DS of 1 V.The blue curve correlates with the linear scale, while the black curve indicates the semi-logarithmic scale.Laser measurements were subsequently conducted, as illustrated in Figure S3 (Supporting Information), to further characterize the device under light illumination.The transfer characteristics exhibit the n-type behavior of the MoS 2 FET with a high on/off ratio of 10 8 .The n-type nature of MoS 2 allows electrons to be injected from MoS 2 into graphene. [20]igure 1e demonstrates the photoluminescence (PL) spectra of MoS 2 /graphene and MoS 2 with encapsulated hBN.The peaks A and B denote the neutral A-and B-excitons, respectively, with the A exciton located near 1.91 eV for both cases.In the MoS 2 /graphene heterostructure, the PL intensity of MoS 2 is significantly quenched, indicating the suppression of the A and B exciton formation owing to an efficient carrier transfer from MoS 2 to graphene. [11]The shift in the graphene charge neutrality point (Dirac point) V D further confirms the charge transfer.In Figure 1f, the transfer characteristics of a gate-tunable graphene photodetector with and without MoS 2 are presented under dark conditions, with a small V DS of 0.01 V. Graphene exhibits ambipo-lar behavior and switches between n-type and p-type at a specific gate voltage V D .The shift in V D = −3 V for bare graphene to −11 V in the presence of MoS 2 reveals electron transfer from MoS 2 to graphene when heterostructure is formed. [13,20]

Interfacial Gating Effect
To study the impact of the IG process, a bare graphene photodetector without MoS 2 was illuminated by a 640 nm wavelength laser with a power density of 0.94 mW mm −2 , and a voltage of V DS = 0.5 V was applied during the photoresponse measurement.The photocurrent I ph was recorded as a function of time at V G ≤ −30 V, where I ph is defined as I ph = I ligh − I dark , with I light and I dark representing the drain-source current (I DS ) with and without light illumination, respectively (Figure 2a).At V G = −30 V, the IG effect was observed, and the photocurrent increased as a more negative V G was applied.In this configuration, where V G < V D , graphene exhibits p-doped behavior, and the holes are the dominant carriers.PPC is observed owing to increased hole density under light illumination.The PPC response is caused by the local gate voltage (accumulation of electrons at the SiO 2 /Si surface gated graphene channel) changing conductance and the carrier density in graphene. [30,31]Figure 2b presents the schematic of the IG mechanism, in which the band bending at the Si/SiO 2 interface, influenced by the localized interface state between SiO 2 and Si [30][31][32] (band offset), creates a built-in electric field that promotes electron-hole separation under light illumination.Consequently, the holes diffuse toward the bulk Si, while the electrons accumulate at the SiO 2 interface and act as an additional negative gate voltage, inducing an increase in the hole density in graphene.The responsivity, calculated as R = I ph /PS, where P is the light power density, and S is the area of the effective layer, achieves an exceptional value of 5 × 10 5 mA W −1 (Figure S4, Supporting Information).Moreover, the photocurrent can be adjusted by varying the applied V DS , exhibiting a linear dependence (Figure S5, Supporting Information).The incident laser power at a V G of −50 V corresponds to 4 μW mm −2 .Rapid response and decay time measurements were read out by an oscilloscope to provide insight into the sensing mechanism (Figure 2c).An ultrafast response time  d of ≈8.5 μs and decay time  r of ≈11.4 μs was observed.The rapid response time is owing to the built-in electric field that facilitates a quick separation of the electron-hole pairs, thus resulting in the rapid accumulation of electrons at the SiO 2 /Si interface.This directly impacts and modulates the conductance of the graphene channel. [31]

Carrier Transfer Process
To investigate the mechanism of the CT process, we demonstrate the time-dependent photoresponse measurement in a graphene photodetector with MoS 2 at a fixed laser wavelength and power density (Figure 2d), while applying V G ≥ −20 V to eliminate the IG effect.The unique Dirac-cone band structure of graphene [38] facilitates distinct electrical properties, including gate-controllable carrier density and ambipolar behavior. [21,22]onsequently, the polarity of the photocurrent can be controlled by adjusting V G .The corresponding band diagram for the CT mechanism is depicted in Figure 2e.In the V G > V D regime, where the electrons are the dominant transport carriers (ndoped), the PPC is obtained, indicating an increase in the electron density under light illumination.Conversely, when V G = −20 V < V D , corresponding to p-doped graphene (hole-dominant transport), the NPC is observed due to a decrease in the hole density (opposite carrier doping compared with the IG process).The polarity of the photoconductivity is explained by the electron transfer from MoS 2 to graphene, leaving holes in the MoS 2 layer. [20,21]When V G > V D , these transferred electrons increase the electron density in graphene and enhance the net device current.However, at V = −20 V, these electrons recombine with holes in graphene, thereby reducing hole density and decreasing the current level.The switch between PPC and NPC occurs at V D .The atomically thin graphene and MoS 2 layers mean that the depletion region at their interface can be ignored.Instead, the effective field is determined by the charge polarity at the interface, which allows for the transfer of electron charges to the graphene layer when illuminated with light.The V G can control the effective electric field between the two layers. [20]The results of the photocurrent measurement indicate that, at V G = 10 V, the E F of graphene optimally aligns with the energy level of MoS 2 for the transfer of electrons, resulting in a high photocurrent.A high responsivity of R = 1.4 × 10 5 mA W −1 was achieved with a laser power of 4 μW mm −2 (Figure S6, Supporting Information).To clarify the sensing mechanism, the time-resolved photoresponse measurements of the rapid response and decay times are also presented in Figure 2f.Our results demonstrate a rapid response time  r of ≈147 μs and decay time  d of ≈211 μs.However, the response speed was notably lower than that of the IG process because the mechanisms of the two processes are distinct.The prolonged hole lifetime in MoS 2 leads to a lower decay time and persistent photoconductivity (the photocurrent is not completely removed after the light is switched off) [21] (Figure 2d; Figure S7, Supporting Information).Furthermore, the enhanced response speed observed in the IG process can be attributed to the rapid driving of the holes into the bulk Si, preventing their recombination with accumulated electrons under the influence of the builtin electric field, and the lack of charge transfer process between p-doped Si and graphene. [31]

Gate-Tunable Multispectral Response
The presence of defect states in the dielectric and MoS 2 , as well as residual impurities at the interface between the layers, can affect the photoresponse of the device.These defect states can trap charge carriers under photoexcitation, resulting in a photogating effect on the net device photocurrent.To accurately evaluate the impact of both processes on the spectral response, photocurrent spectroscopy measurements in AC mode were performed using a high-frequency chopper to minimize the effect of photogating [39] .[42] This configuration results in significant broadband absorption due to the cumulative effects of intrinsic absorption modulation within MoS 2 and p-doped Si.Various factors synergistically contribute to attaining high absorption across a broad spectrum of wavelengths while concurrently mitigating the 500-650 nm wavelength absorption observed in the SiO 2 /Si substrate (Figure S9, Supporting Information).Figure 3a presents the photocurrent spectra measured as a function of the wavelength range of 500-800 nm at V G = −50 V and 10 V.By utilizing the photonic cavity effect combined with CT and IG modulation, the results demonstrate a gate tunable response range.The results demonstrate the tunable response range.At V G = 10 V, the photocurrent response of the device is primarily generated by the CT process, which is demonstrated by the presence of peaks in the photocurrent spectra corresponding to the A exciton at ≈1.91 eV and B exciton at ≈2.05 eV, which align with the monolayer MoS 2 (Figure S9, Supporting Information) absorption and PL spectrum of MoS 2. [18] The bandgap of the monolayer MoS 2 is ≈1.82 eV [18] , thus leading to a decrease in the photocurrent for photon energies below this value.However, at V G = −50 V, we observe the photocurrent at sub-bandgap MoS 2 photon energies, which is higher than the aforementioned MoS 2 bandgap energy.This change in the spectra is related to the appearance and dominance of the IG process, where Si absorbs light, thereby resulting in the disappearance of A and B exciton peaks from MoS 2 .Opposite carrier doping simultaneously occurs in the graphene channel above the MoS 2 bandgap, thereby decreasing the photocurrent compared with the range below the MoS 2 bandgap, where no electron doping occurs from the CT process.Furthermore, through deliberate engineering of the light cavity to adjust the absorption spectrum [42] , we have effectively modulated the absorption peak, resulting in enhanced light absorption at approximately  = 750 nm (Figure S9, Supporting Information).This intricate engineering of the absorption spectrum influences the photocurrent spectra observed in the device at V G = −50 V. Figure 3b presents the color contour plot of the responsivity as a function of the wavelength with a variable V G , where the color scale indicates the intensity of the responsivity.The wavelength-dependent power density is shown in Figure S10 (Supporting Information).Regimes I and II correspond to the CT and IG processes, respectively, and regime III represents a combination of both processes.The ability to adjust the IG and CT processes through V G manipulation results in the emergence of multispectral responses.Specifically, when V G > −30 V, the device exhibits enhanced sensitivity aligned with the A and B exciton peaks within the MoS 2 layer, and the spectral response under these applied gate voltages closely mirrors the material absorption spectrum.The gate-controlled photocurrent sensitivity is consistent with the CT mechanism, as illustrated in Figure 2d.At V G ≤ −30 V, the IG process can be observed, with light sensitivity to wavelengths higher than 640 nm.The responsivity at the subbandgap energy of MoS 2 increases, and spectral behavior progressively aligns with the device absorption spectrum when a stronger negative V G is applied, which is consistent with the trend and the dominance of the IG process (Figure 2a).The use of a single detector to achieve a separate spectral response at a variable gate bias makes the device an attractive candidate for future wavelength sensors [1,3] with reconstruction algorithms.

Unlocking the Synergy: Simultaneous Occurrence of IG and CT Processes
Figure 4a presents a schematic of the physical mechanism underlying the spectral response for a combination of the IG and CT processes (Regime III in Figure 3b).The applied V G can control the photoresponse and the superior process, which determines the type of carrier doping in the graphene channel.When hole doping in graphene is stronger than electron doping (IG dominant), the hole density increases in graphene (PPC), as indicated by the left side in the red-dashed ellipse.Conversely, with a stronger electron doping in graphene owing to the dominance of the CT process, the hole density decreases (NPC), as illustrated by the right side in the red-dashed ellipse.The effects of the gate-tunable IG and CT dominance on the photocurrent spectroscopy are shown in Figure 4b.At V G = −30 V, the CT process is dominant, and the A and B exciton peaks from MoS 2 were still observed.At V G = −40 V, the IG mechanism was superior (photocurrent induced by hole doping); however, electron transfer from MoS 2 also influenced the carrier densities in graphene.This reduces the number of holes in the IG process, thereby leading to the disappearance of the A and B exciton photocurrent peaks of MoS 2 .In addition, the highest A and B exciton photocurrent peaks at V G = −30 V were reversed to become the lowest photocurrent points at V G = −40 V. Figure 4c presents an intriguing observation of the photocurrent as a function of V G , ranging from −50 to −30 V.This photocurrent polarity observation is not owing to the gate-controlled graphene polarity, but rather the simultaneous occurrence of the IG and CT processes, which results in opposite carrier doping in the graphene channel.As the G varies from −50 V, the IG process becomes dominant, leading to PPC.When the V G varies from −38 to −30 V, electron doping by the CT process becomes more prominent, resulting in NPC.Switching between these two processes also enables the modulation of the photocurrent polarity by wavelength below and above the MoS 2 bandgap.Figure 4d presents the photoresponse of the device when illuminated with lasers at 640 and 811 nm at a V G of −30 V.The inset depicts a strategy for selectively detecting NIR wavelengths and visible light without bandpass filters.The NPC response is achieved by a laser at 640 nm, whereas the PPC response is achieved by a laser at 811 nm.The gate-and wavelength-controlled photocurrent polarity can be utilized for broadband photoelectric sensing, multifunctional logic gates, and multispectral detectors. [16,17]

Conclusion
We have demonstrated the ability to modulate and integrate IG and CT processes in a graphene-based hybrid photodetector.This allows us to obtain a gate-tunable multispectral response.The dominance of IG in graphene results in hole doping, whereas the dominance of CT from MoS 2 leads to electron doping.The photoresponse of our photodetector is broader than the MoS 2 absorption spectrum, enhancing spectral resolution.Through applying V G , we have demonstrated the wavelength sensitivity within the range of 500-680 nm originating from MoS 2 , and above 680 nm originating from Si, making it suitable for wavelength detection.Our device obtains an extraordinary responsivity of up to 1.4 × 10 [5] mA W −1 by utilizing the high carrier mobility of graphene.Furthermore, we have produced opposite photocurrent responses (NPC and PPC) by manipulating V G and wavelength.Moreover, we have developed a strategy for detecting NIR wavelengths selectively without the need for a bandpass filter, making the device useful for broadband photoelectric sensing and multispectral detectors.Our findings not only advance the understanding of photocurrent processes in sensitized graphene, but also provide insights for the design of advanced optoelectronic devices based on graphene materials.

Experimental Section
Device Fabrication: First, multilayer bottom hBN (HQ Graphene) flakes were mechanically exfoliated on a heavily p-doped Si substrate with 300 nm SiO 2 .The monolayer graphene flakes synthesized on a copper foil using CVD were spin-coated with poly (methyl methacrylate) (PMMA) C4 at 3000 rpm for 30 s; subsequently, the monolayer graphene was separated from the Cu foil by the bubbling method, rinsed six times with deionized water, and then turned over and transferred onto the hBN target via dry transfer.The PMMA-C4 spin-coated CVD-grown monolayer MoS 2 flakes on the silicon wafer were detached from the substrate via etching using sodium hydroxide 0.1 mol L −1 solutions, and then flipped and transferred to the Gr/hBN target similar to the transfer of the monolayer graphene.PMMA-C4 was immediately removed using an acetone solution in each case after the transfer process.Subsequently, the sample was annealed at 200 °C for 3 h under a flow of argon (200 sccm) gas to enhance the interfacial contact between the layers and remove the polymer residues.Subsequently, the target electrical electrodes were patterned via electron-beam (e-beam) lithography, and Cr/Au (5/50 nm) metal contacts were deposited via e-beam deposition.Finally, the top hBN flakes were exfoliated onto the PDMS substrate and transferred onto the top of the sample using the dry transfer method.
Photocurrent Spectroscopy Measurement: The monochromatic light was extracted from the white light source and focused on the sample using an objective lens; the beam size was ≈630 μm in length and 100 μm in width after focusing.The photocurrent response was measured by combining a current preamplifier and lock-in amplifier, whereas a Keithley SMU was utilized to apply a gate and drain-source bias.An oscilloscope was used to monitor the photocurrent, following the current preamplifier stage.The power density was measured using a silicon detector PD300-BB and Nova II Power Meter, with a 50 μm pinhole.The monochromator adjusts the wavelength of white light, controls the photocurrent spectroscopy measurement process, and reads out the data monitored by a computer. [39]haracterization of the Sample (PL, Raman, Electrical Measurement, and Photocurrent Spectroscopy measurement): All Raman and PL spectra were obtained using commercial equipment (NT-MDT) with a 532 nm (2.33 eV) laser under ambient conditions.The AFM topographic image was measured by E-sweep/NanoNavi Station SP.The transfer curve and photoresponse measurements were conducted using an SMU Keithley 4200-SCS system.Laser excitations at wavelengths of 640 nm (≈1.94 eV) and 811 nm (≈1.53 eV) were employed, with respective beam diameters of 1.06 × 1.02 and 1.35 × 1.12 mm.The power density was determined using a silicon detector PD300-BB and Nova II Power Meter, employing a 35 μm pinhole for accurate measurements.The photocurrent spectroscopy was measured with a home-built system [39] using an EQ-99X LDLS white-light source, monochromator SP2150 (Princeton), Femto DLPCA-200 current preamplifier, a Stanford Research SR 830 lock-in amplifier, an SMU Keithley 4200-SCS system, and 2 CH 70 MHz oscilloscope.The response speed was measured with the Femto DLPCA-200 current preamplifier and read out by a 2 CH 70 MHz oscilloscope.

Figure 1 .
Figure 1.Gate-tunable spectral response in graphene-based heterostructure devices via CT and IG processes and device characterization.a) Mechanism for gate-controllable IG and CT processes for manipulating the spectral response in graphene-based devices.The different absorption spectra from the Si and MoS 2 photosensitive layers lead to distinct photocurrent spectroscopy, and the wavelength sensitivity is modulated by V G , resulting from inefficient to efficient photocurrents (blue and red arrows).b) Schematic of the MoS 2 /graphene heterostructure photodetector.c) The optical images of the device, respectively, with the white scale bar corresponding to 10 nm.The numeric labels correspond to the electrodes in the schematic.d) Transfer curve of MoS 2 FET with V DS = 1 V. e) PL spectra of the MoS 2 /graphene heterostructure and MoS 2 with hBN encapsulation; A and B correspond to A-neutral exciton and B-exciton peaks.f) Transfer curve of the graphene with and without MoS 2 on top with V DS = 0.01 V under dark conditions.The black arrow indicates the shift of the graphene Dirac point in the MoS 2 /graphene heterostructure.

Figure 2 .
Figure 2. Gate-tunable photocurrent generation mechanism of the CT and IG processes.a) Photoresponse as a function of time in the bare graphene device at V G ≤ −30 V under a 640 nm wavelength laser with a power density of 0.94 mW mm −2 .Without MoS 2 , the photocurrent originates from the Si back gate (IG process).b) Schematic of the IG process at V G < −30 V.The photogenerated electrons (circled minus symbols) accumulate at SiO 2 , thus inducing a decrease in the graphene Fermi level (red arrow).c) Fast time-resolved photoresponse at V G = −50 V; the rise and decay times are  r = 11.4 μs and  d = 8.5 μs, respectively.d) Photoresponse as a function of time in the MoS 2 /graphene photodetector with the fixed parameters.The photocurrent is induced by the CT from MoS 2 .e) Schematic of the CT process for V G < V D (right panel) and V G > V D (left panel).The green arrow illustrates the electron transfer from the MoS 2 conduction band to graphene after photoexcitation, thus leading to changes in the graphene Fermi level.f) Fast time-resolved photoresponse at V G = 10 V; the rise and decay times are  r = 147 μs and  d = 211 μs, respectively.

Figure 3 .
Figure 3. Gate-tunable photocurrent spectroscopy and multispectral response.a) Photocurrent as a function of wavelength ranging from 500 nm to 800 nm at V G of −50 V and 30 V. The green and purple color strips indicate the wavelength corresponding to photon energies above and below the MoS 2 bandgap, respectively.b) Color contour plot of the responsivity plotted as a function of the wavelength at various gate biases.Regimes I, II, and III indicate three different photocurrent generation mechanisms: CT, IG, and a combination of the two, respectively.

Figure 4 .
Figure 4. Photocurrent generation with the recombination of IG and CT processes (Regime III in Figure 3b).a) Schematic of the combination of the IG and CT processes at V G ≤ −30 V.The dominant process between the two determines the type of carrier doping, which influences the graphene Fermi level.In the dashed red circle, the left side indicates hole doping in graphene owing to the IG process; conversely, the electron doping by the dominant CT process is shown on the right side.b) Photocurrent spectra at different gate biases (−40 and −30 V) demonstrating the reversed A and B exciton photocurrent peak.c) Gate dependence of the photocurrent at fixed parameters; the red and blue arrows indicate the gate-controlled PPC and NPC, respectively.d) Photoresponse as a function of time under lasers of different wavelengths (811 nm and 640 nm) at a V G = −30 V. PPC is detected by an 811 nm laser, whereas NPC is observed by a 640 nm laser.