Fast and Low-Cost Synthesis of MoS 2 Nanostructures on Paper Substrates for Near-Infrared Photodetectors

Featured Application: This work presents an analysis of time and temperature inﬂuence of microwave-assisted hydrothermal synthesis on the direct growth of 2D-MoS2 nanostructures on cellulose paper substrates, and the production of MoS2-based low-cost photosensors with high responsivity and detectivity values. Abstract: Recent advances in the production and development of two-dimensional transition metal dichalcogenides (2D TMDs) allow applications of these materials, with a structure similar to that of graphene, in a series of devices as promising technologies for optoelectronic applications. In this work, molybdenum disulﬁde (MoS 2 ) nanostructures were grown directly on paper substrates through a microwave-assisted hydrothermal synthesis. The synthesized samples were subjected to morphological, structural, and optical analysis, using techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman. The variation of synthesis parameters, as temperature and synthesis time, allowed the manipulation of these nanostructures during the growth process, with alteration of the metallic (1T) and semiconductor (2H) phases. By using this synthesis method, two-dimensional MoS 2 nanostructures were directly grown on paper substrates. The MoS 2 nanostructures were used as the active layer, to produce low-cost near-infrared photodetectors. The set of results indicates that the interdigital MoS 2 photodetector with the best characteristics (responsivity of 290 mA/W, detectivity of 1.8 × 10 9 Jones and external quantum efﬁciency of 37%) was obtained using photoactive MoS2 nanosheets synthesized at 200 ◦ C for 120 min.


Introduction
Since its discovery in 2004, graphene has become one of the nanomaterials of great interest in the construction of devices, due to its high electronic conductivity, mechanical flexibility, and low production cost [1]. Despite the good results obtained with graphene [2,3], the absence of energy bandgap restricts its application in some devices, such as photodetectors, mostly due to low intrinsic responsivity. This led to the development of a series of other two-dimensional materials with different characteristics, such as the hexagonal boron nitride [4], silicene [5], borophene [6], black phosphourous [7], and two-dimensional transition metal dichalcogenides (2D TMDs) [8]. The latter have been thoroughly explored in recent years for several applications [8].
Among the 2D TMDs materials, composed of a transition metal (M) and a chalcogen (X) with generalized form MX 2 in which M = Mo, W, Nb, Ta, Hf, Pt, and so on, and

Materials and Methods
Molybdenum disulfide (MoS 2 ) nanosheets were grown on cellulose paper substrates by a microwave-assisted hydrothermal method optimizing the hydrothermal temperature and reaction time. Sodium molybdate dihydrate (Na 2 MoO 4 .2H 2 O) and thiourea (CS(NH 2 ) 2 ) were used as precursors of molybdenum and sulfur, respectively. Cellulose paper was used as a substrate, which was previously washed by sonication in deionized water and by acetone and isopropyl alcohol, both for 15 min.
For direct growth on the cellulose fibers, the substrates were previously immersed in a seed solution composed of sodium molybdate and thiourea (1:4) for 60 min. After that time, the substrates were dried on a hotplate at 80 • C for 60 min. Substrates with deposited seeds were placed in a hydrothermal reactor with nutrition solution also composed of sodium molybdate and thiourea (1:4). Then, they were taken to the microwave oven for synthesis that was carried out with a power of 100 W, maximum pressure of 280 psi, and with different values of temperature and time of synthesis. Three different synthesis temperatures were used: 190 • C, 200 • C, and 220 • C. These were chosen to apply the shortest possible time for the growth of the nanostructures, corresponding to each temperature used, until the maximum time of 120 min. Thus, the different temperatures and synthesis times used were: 190 • C for 30, 45 and 120 min, 200 • C for 15, 30, 45, 60 and 120 min, 220 • C for 05, 15,30,45,60, and 120 min. After the growth of the MoS 2 on the cellulose fibers, the paper with the grown nanostructures were washed with ethanol in ultrasound for 15 min to remove the nanostructures not fixed on the sample's surface. Finally, the samples were dried in a nitrogen flow.
Interdigital contacts were deposited by the screen-printing technique using commercial conductive ink CRSN2442 Suntronic, composed of silver nanoparticles dispersed on the solvent (solvent-based), acquired from Coats Screen Inks GmbH. The interdigital electrodes have a total dimension of 10.0 × 6.0 mm, 500 µm electrode wide and a 500 µm active channel between the electrodes.
Structural and optical properties of the MoS 2 nanostructures, grown on cellulose paper substrate, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and micro-Raman spectroscopy. The crystal phase was obtained by X-ray diffraction (X'Pert PRO MPD), using Cu-kα radiation (λ = 1.540598Ǻ). The XRD patterns were collected in symmetric configuration in the 2θ range of 5−70 • , using a step angle of 0.05 • , step time of 2 s, operating at 40 kV tension and 30 mA current. The morphology was studied by scanning electron microscopy (Carl Zeiss AURIGA CrossBeam workstation). Raman measurements were performed in a Renishaw inVia Reflex micro-Raman spectrometer equipped with an air-cooled CCD detector and a HeNe laser operating at 50 mW of 532 nm laser excitation. The spectral resolution of the spectroscopic system is 0.3 cm −1 . The laser beam was focused with a 50 × Leica objective lens (N Plan EPI) with a numerical aperture of 0.75. An integration time of 3 scans of 1.5 s each was used for all singlescan measurements to reduce the random background noise induced by the detector, without significantly increasing the acquisition time. The intensity of the incident laser was 2.5 mW. Triplicates were taken for all spectra. The spectrograph was calibrated between different Raman sessions using the Raman line at 521 cm −1 of an internal Si wafer for reducing possible fluctuations of the Raman system. All spectra were recorded at room temperature. Raw data were collected digitally with the Wire 5.0 software for processing. The characterization of the photodetectors was performed with the use of an AUTOLAB potentiostat (PGSTAT204) for ixV electrical measurements and chronoamperometry and a 980 nm diode laser for irradiation of the sensors, under ambient conditions. The power of the laser light was recorded using a handheld digital power meter console (PM100D) from THORLABS. Neutral density filters were used to vary the excitation power incident on the devices.

Results and Discussion
The microwave-assisted hydrothermal synthesis method was used for the direct growth of MoS 2 nanostructures on cellulose paper substrates due to the ease of the technique's application, as well as its ability to change synthesis parameters to optimize the produced structures [29][30][31]. The growth was carried out using a two-step process, where the synthesis is performed on a substrate with a thin layer of deposited seeds.
For construction of NIR photodetectors, a layer of interdigital silver electrodes was deposited on the layer of the MoS 2 nanostructures using the screen-printing technique. Further details on the growth of nanostructures and the construction of the devices can be Appl. Sci. 2021, 11, 1234 4 of 15 found in the Experimental Section. Figure 1 shows a diagram of the MoS 2 nanostructures synthesis and the photodetector production. For construction of NIR photodetectors, a layer of interdigital silver electrodes was deposited on the layer of the MoS2 nanostructures using the screen-printing technique. Further details on the growth of nanostructures and the construction of the devices can be found in the Experimental Section. Figure 1 shows a diagram of the MoS2 nanostructures synthesis and the photodetector production.    For construction of NIR photodetectors, a layer of interdigital silver electrodes was deposited on the layer of the MoS2 nanostructures using the screen-printing technique. Further details on the growth of nanostructures and the construction of the devices can be found in the Experimental Section. Figure 1 shows a diagram of the MoS2 nanostructures synthesis and the photodetector production.   Microscopy images show that the beginning of the growth process is characterized by the formation of vertically aligned MoS 2 nanosheets on the paper surface (see also Figure S1 in the Supplementary File), while the paper fibers are not fully covered. For longer synthesis times, the surface of the paper becomes completely covered by vertically aligned MoS 2 nanosheets, regardless the temperature. The effect of temperature is noticed on the time required to observe the first nanosheets on the paper surface. Such a result is expected since, for lower temperatures, the solution is exposed to lower energies during the synthesis process, requiring a longer time for nanomaterial formation. At 190 • C, it is possible to observe that, even for the longest synthesis time (120 min), the profile of the paper fibers is still perceived, indicating that it was not possible to form a thick and uniform nanostructure layer. On the other hand, at 200 • C the fibers are uniformly coated with a thick layer of MoS 2 nanosheets. It is important to note that the samples synthesized at this temperature had a more uniform surface profile than the samples produced by the conventional hydrothermal method reported in the literature [28,33,35], which generally have agglomerations of MoS 2 nanosheets in microspherical profile. A surface totally covered with nanostructures is also observed in the samples synthesized at 220 • C. However, at this temperature, it is possible to observe that, for longer synthesis times, the sample surface has regions with a spherical profile very similar to those synthesized by the conventional hydrothermal method [28,32]. This indicates that there was the production of a large number of nanostructures during the synthesis process, where they tend to agglomerate in spherical shape, microflowers, formed by nanosheets. The growth kinetics of MoS 2 nanostructures on cellulose paper and other flexible substrates was recently discussed by Sahatiya et al. [28] through classical nucleation and growth theory. These spherical structures are formed when there is excessive nanosheets growth, which generally occurs for longer synthesis times or for MoS 2 synthesis without the presence of the substrate [26]. In the synthesis with direct growth on substrate, the excess of grown nanostructures ends up detaching from the paper surface and agglomerating into spherical particles to decrease the surface energy. Note the early formation of these small flowers for samples synthesized at 200 • C (Supplementary Figure S1). Figure 3 shows XRD diffractograms at different temperatures and synthesis times of the MoS 2 nanostructures, in addition to the XRD diffractogram of cellulose paper and the JCPDS n • 37-1492 card pattern of 2H-MoS 2 for comparison purposes [28,45]. Dashed vertical lines are used to facilitate the identification of phases in the diffractograms obtained at different temperatures and synthesis times. It is possible to identify regions, close do 15 • and 22 • , with the characteristic peaks of the cellulose diffractograms, the 2H-MoS 2 phases with XRD peaks at~32.8 • ,~49.8 • , and 57.8 • relative to (100), (105), and (110) planes, respectively. For 2θ > 30 • the XRD peaks fit well with JCPDS card n • 37-1492, where the presence of (100) and (110) peaks, ate~32.8 • and 57.3 • respectively, show that the samples present similar atomic arrangement along the basal planes with the bulk MoS 2 .
According to the standard 2H-MoS 2 diffractogram pattern, represented by the JCPDS card n • 37-1492 shown in Figure 3, there is a diffraction peak at 14.4 • regarding the plane (002) of the layered material. However, two other peaks appear in the diffractogram, at 9.3 • and 18.6 • , identified in Figure 3 by # symbols. MoS 2 is a representative 2D layered material and the weak van der Waals interaction between MoS 2 layers favors the intercalation of molecules and ions in this space [32,46]. The interlayer distance of the (002) plane, referring to the 14.4 • peak, of intercalation-free MoS 2 is 0.62 nm, while the corresponding interlayer distances of 9.4 • and 18.6 • peaks, present in the diffractograms of the synthesizes samples, were 0.93 nm and 0.49 nm, respectively, calculated using the Bragg equation. Several groups attribute the interlayer distances of 0.93 nm and 0.49 nm to the distance between two adjacent MoS 2 expand layers-resulting from the intercalation of different materials as CTAB [47], mesoporous carbon (MoS 2 /m-C) [48], NH 4 + [49], and NH 3 molecules [46]-and that distance between intercalated molecules and the adjacent MoS 2 layers, respectively [32,47,48,50]. Other authors have attributed the 18.6 • peak to a secondorder diffraction [49] It is important to note that, while some authors have attributed the shifted 9.4 • and 18.6 • peaks to the presence of 1T-MoS 2 phase [49,51], Lei et al. [52] have alerted about the difficult to use the expanded interlayer spacing of MoS 2 as direct evidence to confirm the existence of the 1T-MoS 2 phase. According to the standard 2H-MoS2 diffractogram pattern, represented by the JCPDS card n° 37-1492 shown in Figure 3, there is a diffraction peak at 14.4° regarding the plane (002) of the layered material. However, two other peaks appear in the diffractogram, at 9.3° and 18.6°, identified in Figure 3 by # symbols. MoS2 is a representative 2D layered material and the weak van der Waals interaction between MoS2 layers favors the intercalation of molecules and ions in this space [32,46]. The interlayer distance of the (002) plane, referring to the 14.4° peak, of intercalation-free MoS2 is 0.62 nm, while the corresponding interlayer distances of 9.4° and 18.6° peaks, present in the diffractograms of the synthesizes samples, were 0.93 nm and 0.49 nm, respectively, calculated using the Bragg equation. Several groups attribute the interlayer distances of 0.93 nm and 0.49 nm to the distance between two adjacent MoS2 expand layers-resulting from the intercalation of different materials as CTAB [47], mesoporous carbon (MoS2/m-C) [48], NH4 + [49], and NH3 molecules [46]-and that distance between intercalated molecules and the adjacent MoS2 layers, respectively [32,47,48,50]. Other authors have attributed the 18.6° peak to a secondorder diffraction [49] It is important to note that, while some authors have attributed the shifted 9.4° and 18.6° peaks to the presence of 1T-MoS2 phase [49,51], Lei et al. [52] have alerted about the difficult to use the expanded interlayer spacing of MoS2 as direct evidence to confirm the existence of the 1T-MoS2 phase.
Liu et al. have shown that the use of the Na2MoO4.2H2O and CS(NH2)2 with high thiourea concentration induce the formation of ammonium ion intercaled into MoS2 layer [49]. In the synthesis procedure presented in this work, Na2MoO4.2H2O and thiourea were used as precursors of molybdenum and sulfur, respectively, in the proportion of (1:4). Thus, we believe that the MoS2 nanosheets grown on cellulose paper substrates by a microwave-assisted hydrothermal method present NH4 + intercaled between layers of MoS2.
It should also be noted that there is an decrease in the intensity of the cellulose diffraction peaks when subjected to heat treatments (probably associated with crystallinity of the cellulose) [53]; even so, the diffraction peaks of MoS2 stand out in the diffractogram for high synthesis times, due to the greater amount of nanomaterial grown on the paper surface, for (100), (105) and (110) planes, or the possible increase of the intercalation with the synthesis times, for the 9.4° and 18.6° peaks. In the synthesis procedure presented in this work, Na 2 MoO 4 .2H 2 O and thiourea were used as precursors of molybdenum and sulfur, respectively, in the proportion of (1:4). Thus, we believe that the MoS 2 nanosheets grown on cellulose paper substrates by a microwave-assisted hydrothermal method present NH 4 + intercaled between layers of MoS 2 . It should also be noted that there is an decrease in the intensity of the cellulose diffraction peaks when subjected to heat treatments (probably associated with crystallinity of the cellulose) [53]; even so, the diffraction peaks of MoS 2 stand out in the diffractogram for high synthesis times, due to the greater amount of nanomaterial grown on the paper surface, for (100), (105) and (110) planes, or the possible increase of the intercalation with the synthesis times, for the 9.4 • and 18.6 • peaks.
It can be noted that there is a continuous increase in peak intensity at 9.2 • up to 30 min, in samples synthesized at 220 • C. Then the intensity of the entire diffractogram, including those peaks related to cellulose paper, starts to decrease for higher times. The high temperature leads to degrading the paper structure and can affecting the growth process for longer synthesis times, leading to less production of nanostructures on the substrate surface. This result is in line with what is reported in the literature, which leads to cellulose degradation processes starting above 200 • C [53]. In summary, the growth process of the MoS 2 on paper substrate seems to be better at 200 • C.
For a complementary analysis of the presence of 1T and 2H-MoS 2 phases, Raman spectroscopy measurements were performed, looking for the presence of characteristic peaks of these phases on the samples. After confirming the presence of both phases in the synthesized samples by Raman spectroscopy, an analysis of the spatial distribution of 1T and 2H-MoS 2 was performed by the micro-Raman technique. By mapping an area of 225 µm [2] and using steps of 1 µm, the regions of the spectra between 146 to 148 cm −1 (referring to the main vibrational mode of the 1T phase), and between 378 to 385 cm −1 (referring to the E 1 2g vibrational mode of the 2H phase) [12,46], were highlighted. The Raman spectrum and the micro-Raman mapping of the samples synthesized at 220 • C, for times of 05 and 120 min, are shown in Figure 4. degradation processes starting above 200 °C [53]. In summary, the growth process of the MoS2 on paper substrate seems to be better at 200 °C.
For a complementary analysis of the presence of 1T and 2H-MoS2 phases, Raman spectroscopy measurements were performed, looking for the presence of characteristic peaks of these phases on the samples. After confirming the presence of both phases in the synthesized samples by Raman spectroscopy, an analysis of the spatial distribution of 1T and 2H-MoS2 was performed by the micro-Raman technique. By mapping an area of 225 μm [2] and using steps of 1 μm, the regions of the spectra between 146 to 148 cm −1 (referring to the main vibrational mode of the 1T phase), and between 378 to 385 cm −1 (referring to the E 1 2g vibrational mode of the 2H phase) [12,46], were highlighted. The Raman spectrum and the micro-Raman mapping of the samples synthesized at 220 °C, for times of 05 and 120 min, are shown in Figure 4. As can be seen in Figure 4a,b, it was possible to identify characteristic Raman peaks of the 1T (vibrational modes J1, J2 and J3) and 2H (vibrational modes E 1 2g and A1g) phases in the sample spectrum [46], thus confirming the presence of both phases in the synthesized nanostructure. Figure 4c,d shows the micro-Raman mapping, confirming the presence of both phases, the 1T (blue) and the 2H (green), in both samples and in the same space region. These phases are not occupying well-defined regions, but they are dispersed As can be seen in Figure 4a,b, it was possible to identify characteristic Raman peaks of the 1T (vibrational modes J 1 , J 2 and J 3 ) and 2H (vibrational modes E 1 2g and A 1g ) phases in the sample spectrum [46], thus confirming the presence of both phases in the synthesized nanostructure. Figure 4c,d shows the micro-Raman mapping, confirming the presence of both phases, the 1T (blue) and the 2H (green), in both samples and in the same space region. These phases are not occupying well-defined regions, but they are dispersed throughout the mapping area. This behavior is observed for all samples (regardless synthesis time and temperature), as shown in Figure 4 and Figure S2 of the supplementary information. The dispersion of the metallic and semiconductor phases of MoS 2 in the samples may be associated with the continuous production of the metallic phase during synthesis. Over time, the 1T phase is converted to a 2H phase, due to the low stability of the metallic phase and presence of microwave radiation during the synthesis process [12]. Thus, the 1T and 2H phases are present in the samples, as can be seen in the diffractogram of the sample synthesized at 200 • C for 120 min. The greater dispersion of phases on the surface of the substrate can be good for certain applications, since it increases the metal/semiconductor interface (1T/2H) which leads to greater carrier mobility due to the presence of the 1T phase.
As shown in Figure 1, low-cost interdigital photodetectors were built on cellulose paper substrate with a MoS 2 active layer synthesized by the microwave-assisted hydrothermal method at different temperatures and times. Figure 5a-c show the current x voltage curves under laser illumination (980 nm) of the photodetectors with a MoS 2 photosensitive layer synthesized at 190, 200, and 220 • C, respectively. The curves are linear and symmetric for the small bias voltage, indicating a ohmic-like contact as observed for the single-layer MoS 2 phototransistor synthesized using the CVD technique [54], or by photodetector built with a multilayer MoS 2 synthesized by the conventional hydrotermal method, using in this case Ag NPs (nanoparticles) as contact [28]. The small bandgap energy of the multilayer MoS 2 may generate a very small Schottky barrier between Ag NPs electrodes and MoS 2 . After illumination, carriers are injected into the small conduction band of the multilayer MoS 2 generating photocurrent. The photocurrent increase can be associated with a bias voltage through to the reduction of the carrier transit time (τ transit = l 2 /µV), where l is the length of the channel, µ is the carrier mobility, and V the bias potential [22]. The curves in Figure 5b,c, for longer synthesis times, present a strong increase in the current x bias voltage (with a non-linear behavior) for bias voltages higher than ~2V. This non-linearity in the IxV curve is less intense in the samples synthesized at lower temperature, 190 °C- Figure 5a, and in the samples with shorter synthesis times for the synthesis temperatures of 200 and 220 °C, possibly due to the smaller amount of MoS2 material. Figure 5d-f show that there is an increase in both the dark current and the photocurrent, The curves in Figure 5b,c, for longer synthesis times, present a strong increase in the current x bias voltage (with a non-linear behavior) for bias voltages higher than~2V. This non-linearity in the IxV curve is less intense in the samples synthesized at lower temperature, 190 • C- Figure 5a, and in the samples with shorter synthesis times for the synthesis temperatures of 200 and 220 • C, possibly due to the smaller amount of MoS 2 material. Figure 5d-f show that there is an increase in both the dark current and the photocurrent, a result that goes according to the discussion held for Figure 5a-c. The increase in the dark current with synthesis time can be associated with the presence of the metallic phase and trap states.
To evaluate the infrared photosensors, figures of merit Responsivity (R) and Specific Detectivity (D * ) were used, which can be calculated using the equations R = (I on -I 0ff )/P and D* = R/ √ ((2eI 0ff )⁄A). Here, I on is the generated photocurrent in the device under illumination, I off is the dark current, P is the incident light power on the effective area of the device, e is the elementary charge and A is the effective surface area. Figure 6 shows that R and D* values increase with increasing of the synthesis time for three synthesis temperatures and for a bias voltage of 4V. The device built with a sample synthesized at 200 • C for 120 min has the highest value for both D * and R. There is also a big increase of these values for synthesis time longer than 60 min (see yellow curve). The decrease in the R value obtained in the sample synthesized at 220 • C for 120 min, when compared to the sample synthesized at 200 • C for 120 min, may be attributed to the degradation of the cellulose paper.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 15 the elementary charge and A is the effective surface area. Figure 6 shows that R and D* values increase with increasing of the synthesis time for three synthesis temperatures and for a bias voltage of 4V. The device built with a sample synthesized at 200 °C for 120 min has the highest value for both D * and R. There is also a big increase of these values for synthesis time longer than 60 min (see yellow curve). The decrease in the R value obtained in the sample synthesized at 220 °C for 120 min, when compared to the sample synthesized at 200 °C for 120 min, may be attributed to the degradation of the cellulose paper. Therefore, the experimental data show that the temperature of 200 °C and time of 120 min were the best conditions for the production of near infrared photodetectors based on MoS2 active layer grown directly on cellulose paper substrates by the microwave-assisted hydrothermal method. This R value is above the value reported in the literature for photodetectors with active MoS2 layer produced by the hydrothermal method with the same structure. Nahid Chaudhary et al. [55] for example, obtained the maximum R value of 23.8 μA/W for excitation at 635 nm, and Parikshit Sahatiya et al. [28] obtained the R value of 60 μA/W for excitation at 554 nm.
It is important to note that temperatures above 200 °C have generally been used in the literature for synthesis of MoS2, or heat treatments have been made after the synthesis to increase the fraction of phase 2H over phase 1T. At 200 °C (as in this work), we still have a large proportion of the 1T phase when compared with samples synthesized by the conventional method followed by heat treatment [13,28]. Therefore, we believe that the increase in the 1T/2H interface, caused by the simultaneous and continuous formation of the metallic phase and its conversion to the semiconductor phase, and the consequent increase in the mobility of carriers in the sample, is of great importance to obtain high values of responsivity in MoS2 photodetectors. The interface between the paper substrate Therefore, the experimental data show that the temperature of 200 • C and time of 120 min were the best conditions for the production of near infrared photodetectors based on MoS 2 active layer grown directly on cellulose paper substrates by the microwaveassisted hydrothermal method. This R value is above the value reported in the literature for photodetectors with active MoS 2 layer produced by the hydrothermal method with the same structure. Nahid Chaudhary et al. [55] for example, obtained the maximum R value of 23.8 µA/W for excitation at 635 nm, and Parikshit Sahatiya et al. [28] obtained the R value of 60 µA/W for excitation at 554 nm.
It is important to note that temperatures above 200 • C have generally been used in the literature for synthesis of MoS 2 , or heat treatments have been made after the synthesis to increase the fraction of phase 2H over phase 1T. At 200 • C (as in this work), we still have a large proportion of the 1T phase when compared with samples synthesized by the conventional method followed by heat treatment [13,28]. Therefore, we believe that the increase in the 1T/2H interface, caused by the simultaneous and continuous formation of the metallic phase and its conversion to the semiconductor phase, and the consequent increase in the mobility of carriers in the sample, is of great importance to obtain high values of responsivity in MoS 2 photodetectors. The interface between the paper substrate and the MoS 2 layer may be another important characteristic for devices, once it can be a source of trap states [54] and temperature and time synthesis had a greater effect on the photoconductivity. Thus, the quick increase of the photocurrent to bias voltage (>2 V), as observed in Figure 5, can be associated with the detrapping charge from trap states. It is known that photogain in photoconductors increases as carrier lifetime and decreases as transit time G = τ life /τ transit [22]. Figure 6 shows the external quantum efficiency (EQE) of the devices. EQE was obtained by using EQE(%) = Rhc/(λe)x100. Here, R is the responsivity, h is the Planck's constant, c is the speed of light in vacuum, e is the elementary charge, and λ is the wavelength of the excitation light. As can be seen, the EQE increases as the time synthesis increases, similar to R and D*. The device built from MoS 2 , synthesized at 200 • C for 120 min, presents an EQE value of 26%.
The MoS 2 photodetector has a relatively high R value, compared to other similar devices based on MoS 2 synthesized by the hydrothermal method. The rise and the decay response time has high values (τ rise = 3.7 s e τ decay = 4.7 s) which may be associated with the high concentration of the metallic phase in MoS 2 and trap states in the MoS 2 and/or paper/MoS 2 interface. In comparison, the HfO 2 -gated single-layer MoS 2 phototransistors, with high responsivity due to the photogate effect, present a slow response of 0.6-9.0 s. In this case, the slow response time and the high responsivity was induced by the trap states in the MoS 2 or MoS 2 /SiO 2 interface [54] Jin et al. [56] deposited Ag NPs on monolayer MoS 2 phototransistor and obtained a very high R value and slow response time of 18.7 s, showing other strategy to induce photoconductive effect. The improvement was attributed to the localized surface plasmon resonance of Ag NPs with the increase of light absorption and carrier injection from Ag NPs to MoS 2 under illumination. Thus, we believe that, in our device, the surface plasmon resonance in the Ag NPs electrode is light excited and generates hot electrons via nonradiative decay plasmonic resonance, since the Ag NPs show a plasmonic resonance signal in the near-infrared region, as shown in Figure S3 in the supplementary file. The energy of the hot electrons may be greater than the small Schottky barrier between Ag NPs and MoS 2 , and it can be injected into the MoS 2 multilayer generating a photocurrent [57][58][59]. Additionally, the effect of the MoS 2 phases (1T and 2H) on the electrical performances of hybrid PDs was compared by Wang et al. [60] and results indicated that the metallic 1T phase exhibited a high R value. However, these devices showed low on/off ratios (<2) and slow response times (0.75 s) due to their metallic conducting nature similar to that of the graphene. On the other hand, the semiconducting 2H phase demonstrated lower R value and fast response time (<25 ms). Thus, we believe that our PDs built with MoS 2 synthesized at 200 • C for 120 min presented higher metallic phases than other devices built with materials synthesized for other temperatures and times, and that the hot electrons from Ag NPs could be contributing to the photocurrent improvement. Therefore, the contact between Ag NPs and the MoS 2 multilayer must convert from a small Schottky junction to an Ohmic-like contact when the device is illuminated, as observed in Au nanorods/Perovskite photoconductor [56,61]. The device works as a light illuminated photoconductor, explaining the slow response time and high R compared to a similar device based on a MoS 2 synthesized by the conventional hydrothermal method [28,55]. Figure 7 shows the dependence of the photocurrent (I on -I off ) as a function of the light power on the photosensor synthesized at 200 • C for 120 min.
vert from a small Schottky junction to an Ohmic-like contact when the device is illuminated, as observed in Au nanorods/Perovskite photoconductor [56,61]. The device works as a light illuminated photoconductor, explaining the slow response time and high R compared to a similar device based on a MoS2 synthesized by the conventional hydrothermal method [28,55]. Figure 7 shows the dependence of the photocurrent (Ion-Ioff) as a function of the light power on the photosensor synthesized at 200 °C for 120 min.   Figure 7a shows a decrease in photocurrent behavior with decreasing light power intensity. This is an expected result, since with the decrease in the incident light power, there is a decrease in the number of photons reaching the sensor and, consequently, there is a decrease in the number of carriers promoted from the valence band to the conducting band of the MoS 2 nanostructures. With the decrease in the number of carriers in the conduction band, there is a decrease in the generated photocurrent (I on -I off ). An analysis of the relationship between the generated photocurrent and the incident light power on the photodetector is showed in the inset of Figure 7a, where a fitting with power law is used (I = AP θ ). Here A is a proportionality constant and θ is the exponent that determines the relationship between the photogenerated current and the light source power. From the adjustment, a linear relationship is observed (θ = 1.1 ± 0.1). Similar behavior of the photocurrent with light intensity was observed for the single-layer MoS 2 phototransitor [54]. Figure 7b shows the relationship between responsivity/detectivity and the light source power used. Note that the highest responsivity and detectivity values (290 mA/W and 1.8 × 10 9 Jones) occur for the power of 2 mW. In the range of 6 to 2 mW, the responsivity and detectivity values increase rapidly with decreasing power (these results indicate that for lower radiation intensity, in the µW range, the photodetector can present very high responsivity values-these results are not showed here). This behavior has been associated with the increase in the recombination probability as carriers photoexcitation increases [59]. EQE measurements (inset of Figure 7b) obtained at various excitation powers show that the EQE gradually increases as the power decreases, similar to the results for the graphene-MoS 2 -graphene devices (responsivity of ≈ 0.2 A W −1 and EQE = 55%) [62].

Conclusions
MoS 2 nanostructures were grown directly on cellulose paper substrates using the microwave-assisted hydrothermal synthesis method with different temperatures and synthesis times. Variations were observed in the amount of MoS 2 grown on top of cellulose fibers as a function of the synthesis parameters. The analysis also showed the distribution of the metallic and semiconductor phases of the MoS 2 nanostructures throughout the sample, generating metal/semiconductor interfaces. Near-infrared sensors were produced from the synthesized samples, where the screen-printing technique was used to deposit interdigital Ag NPs contacts in the samples. The results of the electrical characterizations showed an improvement in the performance of the sensor with the increase in nanostructures synthesis time, for the three temperatures of synthesis used. Such results can be explained by the increase in the amount of MoS 2 , according to the model created for the sensors operation. Based on the data analysis, the photodetector produced from the sample synthesized at 200 • C and 120 min presented a high responsivity value (290 mA/W), a specific detectivity value (1.8 × 10 9 Jones) and an external quantum efficiency of 37%, with response times τ rise = 3.7 s and τ decay = 4.7 s. The sensors produced from the samples synthesized at a temperature of 220 • C also showed high values of responsivity, however, the high temperature causes substrate degradation, making the device fragile and the application difficult. The improvement in the responsivity value of the photodetector can be associated with the photoconductive effect due to three probably factors: (i) the high concentration of the metallic 1T phase in MoS 2 layers; (ii) trap states in the MoS 2 and/or paper/MoS 2 interface; and (iii) hot electron injection from surface plasmon resonance in the Ag NPs electrodes to MoS 2 nanomaterial.
An analysis of the photocurrent dependence as a function of the incident radiation power on the sensor showed a linear dependence of the device's photocurrent in relation to the power of the light source. The responsivity showed a great increase with the reduction of the irradiation power, at the value of 290 mA/W for the power of 2 mW. This result must be associated with the increase in the probability of recombination with the increase in the photoexcited carriers. This work provides a functional example, as well as a promising strategy, to improve the performance of 1T/2H MoS 2 -based photodetectors. Further studies on the surface state passivation for device optimization, for example, will be conducted to improve photocurrent in MoS 2 photodetector synthesized by the microwave-assisted hydrothermal synthesis method.
Supplementary Materials: The following are available online at https://www.mdpi.com/2076-341 7/11/3/1234/s1, Figure S1: SEM images of the MoS2 nanostructures grown directly on cellulose paper substrates with different temperature and time of synthesis, Figure S2: Micro-Raman mapping and detachment of phases 1T (blue) and 2H (green) in samples of MoS2 nanostructures grown directly on paper substrates with different temperature and time of synthesis, Figure S3: Absorbance spectrum of silver nanoparticles ink in an isopropyl alcohol solution.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.