Solution-Processed Functionalized MoS 2 Nanosheets Composite for Photodetection Application

: Charge-transfer organic-inorganic complexes have demonstrated great potential in opto-electronic applications. Herein, a drop-casting processed photodetector based on thick composite ﬁlms made of multi-layered MoS 2 nanosheets chemically bonded to linear molecules of aromatic thiols has been developed. Composites based on multilayered nanosheets allow for facile preparation of low-cost, large-area, and ﬂexible devices. It was demonstrated that a simple functionalization of ultradispersed MoS 2 nanosheets with linear aromatic thiol results in the formation of charge and energy transfer complexes. A photodetector with functionalized MoS 2 nanosheet ﬁlm prepared by drop coating with Au electrodes demonstrated enhanced performance compared to pure materials. Our ﬁrst experiments illustrated that functionalization of MoS 2 nanosheets by a paraquaterphenyl thiol derivative leads to a signiﬁcant increase in the photoresponse speed (by a factor of 12) and decay speed (by a factor of 17.5), in addition to the enhancement of the photostability of the MoS 2 based photodetector. The photo current value has been increased by about an order of magnitude. The proposed approach offers promising prospects for further development of photodetectors.


Introduction
Photodetectors are essential for detecting and measuring light intensity using the function of converting optical signals to electrical signals.Photodetectors offer manifold various applications [1].New materials and structures for photodetectors are constantly being developed due to the evolving applications and updated requirements.Two-D materials (such as graphene, transition metal dichalcogenides, etc.) have been ubiquitously used as the most promising semiconductor materials for this purpose [2,3].Molybdenum disulfide (MoS 2 ) nanosheets are among the most studied dichalcogenides [4,5].This material has attracted more academic and applied interest due to its extraordinary properties arising from the internal structure of the nanosheets with d-electron interactions and its great degree of tunability.A single-layer MoS 2 nanosheet forms a hexagonal lattice structure and contains three sublayers of atoms, where one layer of Mo atoms is sandwiched between two layers of S atoms.Most photodetector studies [6][7][8][9][10][11][12][13][14][15][16][17] use single-layered and multilayered individual nanosheets, though positive results with multilayered nanosheet composites in the sandwich structure were shown [18].Single-layered nanosheets synthesized by CVD are widely known to be very time and energy-consuming and, therefore, expensive.Mechanically exfoliated nanosheets can be produced simply, but they have limitations in mass production.Multilayered MoS 2 nanosheets for composite formation can be easily obtained by cheap, simple, and reliable liquid phase exfoliation [4].Composites based on multilayered nanosheets can be solution-processed, which allows for facile preparation of low-cost, large-area, and flexible devices [19].Unfortunately, the characteristics of these photodetectors are insufficiently high because of a lower ordering degree than the one in independent single nanosheets, as well as the presence of contacts between the nanosheets, resulting in lower conductivity.To achieve good performance of photodetectors (high responsibility, short response time, and high detectivity), both the physical and chemical functionalization of such nanosheets are widely utilized [3,[13][14][15][16][17]20,21].Many inorganic [1,3,12] and organic [3,8,9,[13][14][15][16][17]20,21] nanomaterials have been used for functionalization.Most of the studies are devoted to physical functionalization [14][15][16][17]21].However, chemical functionalization can provide more stable materials as it is irreversible.A wide variety of organic materials with different properties gives wide possibilities for functionalization.Promising results have been obtained with aliphatic thiols [11].However, aromatic organic materials are electrically conductive and absorb light more strongly.Thus, in this study, we propose to apply the chemical functionalization of MoS 2 multilayered nanosheets with a linear aromatic thiol, which is stable and luminescent.A combination of a n-type MoS 2 nanosheet and a p-type aromatic molecule can form a charge transfer complex [22].Often, high gain can be reached owing to the photogating effect [23,24], capable of achieving higher characteristics and escaping a cooling system, lowering the detector's noise.Chemical functionalization of MoS 2 nanosheets by organic molecules can be considered as covering by surfactant, which prevents nanosheets from aggregation [25], maintains stable interaction between nanosheets, and changes the properties of these nanosheets [26,27], which can be used for the creation of stable composite films.Thus, in this work, we propose to develop solution-processed films based on the newly synthesized material and study their photodetecting properties.Our approach results in the formation of a high-quality film, the enhancement of the photoresponse value and speed, and the photostability of the MoS 2 -based photodetector.Due to the simplicity of chemical functionalization, our proposed approach is promising for the further development of solution-processable photodetectors.

Materials
The MoS 2 ultrafine powder with hexagonal lamellar structure was purchased from Graphene Supermarket (~90 nm average particle size, purity 99%, product number EM092222) and used as supplied.Functionalized MoS 2 (MoS 2 -M) has been synthesized using this powder.Chlorobenzene (99.5%, CB) and isopropanol (99.8%) were used as received without further purification.MoS 2 nanosheets were exfoliated in isopropanol based on a widely used exfoliation technique [4] with an ultrasonic tip for 12 h, resulting in the formation of a dispersion of MoS 2 nanosheets with a wide range of sizes and thicknesses.After sonication, the dispersion has been used for optical measurements and film preparation.

MoS 2 Nanosheets Functionalization
For the functionalization of the MoS 2 nanosheets, 4-Hexyl-3 -methyl-4 -mercaptop-quaterphenyl (MQP) has been chosen.The chemical structure of MQP is presented in Figure 1.The process of the chemical functionalization of MoS2 nanosheets with thiol molecules is simple and reliable, as presented in detail herein.Ultrasonic (US) disaggregation of the MoS2 ultrafine powder suspension in an organic solvent with the addition of MQP to maximize the MoS2 specific area and the MQP chemisorption has been applied.For this purpose, the following materials and apparatus were used: MoS2 ultrafine powder, DMF and n-hexane from Merck (Germany); MQP (synthesized); ultrasonic cleaner GT Sonic 1860QT (150 W, 40 kHz); centrifuge MDCEN-302-SD; diaphragm pump Wiggens C610; UV-lamp with 264 and 365 nm sources; glass round-bottom flasks.
A 50 mg MoS2 (MW 160 g/mol; 0.3 mmol) suspension in DMF (10 mL) was treated by US for 15 min in a round-bottom flask.After dispersion and continuing US treatment, a solution of 140 mg MQP (MW 436 g/mol; 0.3 mmol) in DMF (5 mL) was added with a rate of about one drop/s.Then, the reaction mixture was sonicated again for 1 h.The obtained suspension was evaporated and dried under vacuum (15 mm Hg, 50 °C, 30 min).The residue was dispersed in n-hexane (20 mL) under US to remove the excess MQP.This mixture was next separated in a centrifuge (6000 RPM, 5100 g, 30 min); the residue was rinsed with hexane (five times) until the solvent after centrifugation showed no fluorescence due to the traces of MQP when excited with a 365 nm UV light.After the purification, a wet material was dried on air at 50 °C.
MQP was synthesized specifically for this work, as it is not commercially available.The synthesis of MQP was carried out by the interaction of 4-hexyl-3′-methyl-4‴-Br-p-quaterphenyl (II) with n-butyllithium at a temperature ranging from -70 to -78 °C, followed by the addition of elemental sulfur.Next, 4-hexyl-3′-methyl-4‴-Br-p-quaterphenyl was obtained by the Suzuki reaction of 4-hexyl-3′-methyl-4‴-B(OH)2-p-terphenyl (I) with 4-bromoiodobenzene (see Scheme 1).The 4-Hexyl-3′-methyl-4‴-bromo-p-quaterphenyl (II) (4,83 g, 10 mmol) was dissolved in 50 mL of dry THF, and the solution was cooled in an argon atmosphere to −80-70 °C with continuous stirring.At this temperature, an n-butyllithium solution (5 mL of 2.5M solution in hexane) was added by syringe, and the reaction mixture was stirred for 1 h at the same temperature.The sulfur solution (0.384 g, 12 mmol) in 30 mL THF was added to this mixture by syringe.The mixture was gradually allowed to warm up to 20 °C, followed by the addition of 50 mL of 2% hydrochloric acid.The organic layer was separated, and The process of the chemical functionalization of MoS 2 nanosheets with thiol molecules is simple and reliable, as presented in detail herein.Ultrasonic (US) disaggregation of the MoS 2 ultrafine powder suspension in an organic solvent with the addition of MQP to maximize the MoS 2 specific area and the MQP chemisorption has been applied.For this purpose, the following materials and apparatus were used: MoS 2 ultrafine powder, DMF and n-hexane from Merck (Germany); MQP (synthesized); ultrasonic cleaner GT Sonic 1860QT (150 W, 40 kHz); centrifuge MDCEN-302-SD; diaphragm pump Wiggens C610; UV-lamp with 264 and 365 nm sources; glass round-bottom flasks.
A 50 mg MoS 2 (MW 160 g/mol; 0.3 mmol) suspension in DMF (10 mL) was treated by US for 15 min in a round-bottom flask.After dispersion and continuing US treatment, a solution of 140 mg MQP (MW 436 g/mol; 0.3 mmol) in DMF (5 mL) was added with a rate of about one drop/s.Then, the reaction mixture was sonicated again for 1 h.The obtained suspension was evaporated and dried under vacuum (15 mm Hg, 50 • C, 30 min).The residue was dispersed in n-hexane (20 mL) under US to remove the excess MQP.This mixture was next separated in a centrifuge (6000 RPM, 5100 g, 30 min); the residue was rinsed with hexane (five times) until the solvent after centrifugation showed no fluorescence due to the traces of MQP when excited with a 365 nm UV light.After the purification, a wet material was dried on air at 50 • C.
MQP was synthesized specifically for this work, as it is not commercially available.The synthesis of MQP was carried out by the interaction of 4-hexyl-3 -methyl-4 -Br-pquaterphenyl (II) with n-butyllithium at a temperature ranging from −70 to −78 • C, followed by the addition of elemental sulfur.Next, 4-hexyl-3 -methyl-4 -Br-p-quaterphenyl was obtained by the Suzuki reaction of 4-hexyl-3 -methyl-4 -B(OH) 2 -p-terphenyl (I) with 4-bromoiodobenzene (see Scheme 1).The process of the chemical functionalization of MoS2 nanosheets with thiol molecules is simple and reliable, as presented in detail herein.Ultrasonic (US) disaggregation of the MoS2 ultrafine powder suspension in an organic solvent with the addition of MQP to maximize the MoS2 specific area and the MQP chemisorption has been applied.For this purpose, the following materials and apparatus were used: MoS2 ultrafine powder, DMF and n-hexane from Merck (Germany); MQP (synthesized); ultrasonic cleaner GT Sonic 1860QT (150 W, 40 kHz); centrifuge MDCEN-302-SD; diaphragm pump Wiggens C610; UV-lamp with 264 and 365 nm sources; glass round-bottom flasks.
A 50 mg MoS2 (MW 160 g/mol; 0.3 mmol) suspension in DMF (10 mL) was treated by US for 15 min in a round-bottom flask.After dispersion and continuing US treatment, a solution of 140 mg MQP (MW 436 g/mol; 0.3 mmol) in DMF (5 mL) was added with a rate of about one drop/s.Then, the reaction mixture was sonicated again for 1 h.The obtained suspension was evaporated and dried under vacuum (15 mm Hg, 50 °C, 30 min).The residue was dispersed in n-hexane (20 mL) under US to remove the excess MQP.This mixture was next separated in a centrifuge (6000 RPM, 5100 g, 30 min); the residue was rinsed with hexane (five times) until the solvent after centrifugation showed no fluorescence due to the traces of MQP when excited with a 365 nm UV light.After the purification, a wet material was dried on air at 50 °C.
MoS 2 nanosheets can be presented by several polymorphs, depending on the interlayer stacking arrangement and intralayer coordination between the central Mo atom and the surrounding S atoms [3][4][5].The exact nature of organic thiol and exfoliated MoS 2 interactions remains unclear [29].Thus, it is not clear where the thiol group of the molecule can be bonded to the MoS 2 nanosheet.Typically, the defect-free area of a MoS 2 basal plane is inert.However, it was observed that an S atom of thiol can be attached to the Mo [30,31] or S [30,32,33] atoms at vacancy defects.Mo atoms can be reached at the edges of the nanosheet, while S atoms can be accessible at the basal planes.

Thin Film Preparation
Thin films were obtained by drop-casting the obtained suspension on the substrate or by spin-coating.For thin film deposition, we used either 0.5 mm thick quartz substrates transparent for wavelengths above 200 nm or silicon surface for device preparation.All substrates were carefully cleaned with acetone and ethanol and were dried before thinfilm deposition.

Device Fabrication
First, the sample powder was added to the CB solvent and sonicated for 12 h to form a dispersion.After that, uniform films were cast on a clean SiO 2 /Si substrate by drop/spin coating, followed by annealing at a specific temperature for 1 h.To ensure the cleanliness of the substrate, it is necessary to perform ultrasonic cleaning treatment on the substrates before usage.Specifically, the SiO 2 /Si substrates are cleaned with acetone and isopropyl alcohol and deionized water for 30 min.Acetone and alcohol can remove the majority of organic impurities from the substrates' surface, and deionized water can wash away the remaining particle contaminants.To improve film homogeneity, the samples need to be annealed.However, because regular annealing temperatures for inorganic materials [34] are too high for organic materials, we lowered the annealing temperature and shortened the annealing time for composite samples to 80 • C for 1 h.Unfortunately, such a low temperature cannot remove all defects.The thickness of the active layer film for both devices is about 1.3 µm (1.32 µm for MoS 2 and 1.28 µm for MoS 2 -M).After optimizing synthesis parameters and obtaining uniform, high-quality films, Au electrodes (70 nm thick) were plated on the film by thermal vacuum evaporation.As a result, two kinds of photodetectors (PDs) based on pure MoS 2 and doped MoS 2 were formed.

Characterization
Systematic characterizations were carried out to determine the properties of functionalized MoS 2 nanosheets and their individual counterparts.
Absorption spectra were recorded with a Cary 500 (Varian, Palo Alto, CA, USA) spectrophotometer.Photoluminescence and excitation spectra were measured with a multifunctional spectral fluorimeter, Fluorolog-3 (Horiba Scientific, Palaiseau, France), which provides highly sensitive and stable measurements in the ultra-violet and visible range.All photoluminescence measurements for solutions were made perpendicularly to the excitation beam, and thin films were oriented at 30 • .Raman spectra have been obtained at room temperature with a micro-Raman spectrometer Nanofinder High End (Lotis TII, Minsk, Belarus-Tokio Instruments, Tokio, Japan; 1800 lines/mm grating; 50× objective; 532 nm excitation wavelength).The morphology of the dispersed phase was examined using a field emission scanning electron microscope (FESEM), Zeiss Sigma, and scanning electron microscope MIRA3 (Tescan, Brno, Chech Republic).The thickness of the film is determined by a step profilometer (KLA-Tencor, Milpitas, CA, USA).All measurements were carried out at room temperature and ambient conditions.The molecular structure of the synthesized organic compounds was verified using 1 H NMR spectra acquired on a Bruker AVANCE 500 spectrometer at room temperature and mass spectra acquired on an Agilent 6410 Triple Quadrupole LC/MS System.
Optoelectronic properties were collected using the semiconductor characterization system Keithley 4200-SCS (Tektronix, Beaverton, OR, USA) connected to a vacuum probe station (Lake Shore).A 75 W Xe lamp equipped with a monochromator was used as a light source.Light density was measured by a NOVA II power meter (OPHIR photonics, Jerusalem, Israel).All the measurements were performed at ambient environment, i.e., room temperature and atmospheric pressure.

Microscopy
Electron microscopy provides insights into the morphology of the films.Figure 2 represents an SEM picture of the morphology of MoS 2 and MoS 2 −M films.It can be seen that this sample consists of large nanosheets (of several micrometers) covered with small particles, hinting that US treatment was not complete, especially for pure MoS 2 .Dispersion size and concentration are known to depend on sonication time and power [35].The plane of the large nanosheets is located mainly parallel to the plane of the substrate surface, and the small nanosheets are positioned similarly on the surface of the large nanosheets (see Figure 2, right).Additionally, no fundamental difference between MoS 2 and MoS 2 -M film morphology has been observed.Hence, only a small part of the nanosheet surface has been covered with molecules that are typically oriented perpendicularly to its surface.The combination of large and small particles can result in higher conductivity owing to the denser packing of the particles.Note that the low-resolution microscopy reveals a much lower quality of the pure MoS 2 nanosheet film than the film made of MoS 2 -M nanosheets.MoS 2 film samples contain numerous inhomogeneities and hollow cavities.Functionalized MoS 2 nanosheet films are much more homogeneous than nonfunctionalized material because of better interaction between the nanosheets caused by thiol molecules.

Raman Spectra
Raman spectroscopy is known to be a reliable diagnostic tool for studying the thickness of MoS2 nanosheets, as well as various interactions [36].Raman spectra of unsonicated MoS2 and functionalized MoS2−M samples (Figure 3) excited with a 532 nm laser were obtained.Both spectra contain two main modes of out-of-plane vibrations (A1g) of S atoms and in-plane vibrations (E 1 2g) of Mo atoms and S atoms.The original MoS2 nanosheets show these peaks at about 381 and 407 cm −1, with the difference between them of ~26 cm −1 implying that this sample contains nanosheets with thicknesses ranging from monolayer to, preferably, several layers.Functionalized MoS2 nanosheets show a negligible (about 1 cm −1 ) redshift of both peaks with the same peak intensity ratio but a noticeable broadening of both peaks.These data confirm the presence of both S−S and S−Mo bonds owing to the successful MoS2 functionalization.Note that the measurements were made at five points with different nanosheet dimensions, distribution, and morphology for both samples.It was shown that the only observed difference was the signal intensity.

Absorption and Luminescence
Optical absorption and luminescence spectra can give information about the position of energy levels and energy transformation in the considered system.Figure 4 illustrates the normalized absorption, excitation, and luminescence spectra of MoS2−M and the corresponding components (MoS2 and MQP).The absorption spectrum of MoS2−M is approximately a superposition of the absorption of both individual components.However, a longwave shoulder at about 350 nm can be considered a charge transfer band.Indeed, the relative intensity of this shoulder is much higher than that of the MoS2 spectrum, though the deposition of MQP is negligible.Partial overlapping of MoS2 absorption and MQP

Raman Spectra
Raman spectroscopy is known to be a reliable diagnostic tool for studying the thickness of MoS 2 nanosheets, as well as various interactions [36].Raman spectra of unsonicated MoS 2 and functionalized MoS 2 −M samples (Figure 3) excited with a 532 nm laser were obtained.Both spectra contain two main modes of out-of-plane vibrations (A 1g ) of S atoms and in-plane vibrations (E 1 2g ) of Mo atoms and S atoms.The original MoS 2 nanosheets show these peaks at about 381 and 407 cm −1, with the difference between them of ~26 cm −1 implying that this sample contains nanosheets with thicknesses ranging from monolayer to, preferably, several layers.Functionalized MoS 2 nanosheets show a negligible (about 1 cm −1 ) redshift of both peaks with the same peak intensity ratio but a noticeable broadening of both peaks.These data confirm the presence of both S−S and S−Mo bonds owing to the successful MoS 2 functionalization.Note that the measurements were made at five points with different nanosheet dimensions, distribution, and morphology for both samples.It was shown that the only observed difference was the signal intensity.

Raman Spectra
Raman spectroscopy is known to be a reliable diagnostic tool for studying the thickness of MoS2 nanosheets, as well as various interactions [36].Raman spectra of unsonicated MoS2 and functionalized MoS2−M samples (Figure 3) excited with a 532 nm laser were obtained.Both spectra contain two main modes of out-of-plane vibrations (A1g) of S atoms and in-plane vibrations (E 1 2g) of Mo atoms and S atoms.The original MoS2 nanosheets show these peaks at about 381 and 407 cm −1, with the difference between them of ~26 cm −1 implying that this sample contains nanosheets with thicknesses ranging from monolayer to, preferably, several layers.Functionalized MoS2 nanosheets show a negligible (about 1 cm −1 ) redshift of both peaks with the same peak intensity ratio but a noticeable broadening of both peaks.These data confirm the presence of both S−S and S−Mo bonds owing to the successful MoS2 functionalization.Note that the measurements were made at five points with different nanosheet dimensions, distribution, and morphology for both samples.It was shown that the only observed difference was the signal intensity.

Absorption and Luminescence
Optical absorption and luminescence spectra can give information about the position of energy levels and energy transformation in the considered system.Figure 4 illustrates the normalized absorption, excitation, and luminescence spectra of MoS2−M and the corresponding components (MoS2 and MQP).The absorption spectrum of MoS2−M is approximately a superposition of the absorption of both individual components.However, a longwave shoulder at about 350 nm can be considered a charge transfer band.Indeed, the relative intensity of this shoulder is much higher than that of the MoS2 spectrum, though the deposition of MQP is negligible.Partial overlapping of MoS2 absorption and MQP

Absorption and Luminescence
Optical absorption and luminescence spectra can give information about the position of energy levels and energy transformation in the considered system.Figure 4 illustrates the normalized absorption, excitation, and luminescence spectra of MoS 2 −M and the corresponding components (MoS 2 and MQP).The absorption spectrum of MoS 2 −M is approximately a superposition of the absorption of both individual components.However, a longwave shoulder at about 350 nm can be considered a charge transfer band.Indeed, the relative intensity of this shoulder is much higher than that of the MoS 2 spectrum, though the deposition of MQP is negligible.Partial overlapping of MoS 2 absorption and MQP luminescence spectra can result in the partial energy transfer from MQP to MoS 2 .Blue luminescence of MoS 2 occurs, probably, from higher energy levels as described in [37][38][39][40].It is weak compared to the luminescence of MQP and the composite.No red luminescence is observed in the system of few-layered MoS 2 nanosheets.The luminescence spectrum of the hybrid sample is red-shifted by about 19 nm and broadened compared to the MQP spectrum because of the partial superposition of the luminescence spectra of both counterparts.The intensity of its luminescence is much weaker than that of MQP.This finding indicates that a charge transfer from MQP to MoS 2 occurs, which is confirmed by the almost fully coinciding excitation spectra of MQP and MoS 2 -M.Thus, in this system, both energy and charge transfer can be observed.
luminescence spectra can result in the partial energy transfer from MQP to MoS2.Blue luminescence of MoS2 occurs, probably, from higher energy levels as described in [37][38][39][40].It is weak compared to the luminescence of MQP and the composite.No red luminescence is observed in the system of few-layered MoS2 nanosheets.The luminescence spectrum of the hybrid sample is red-shifted by about 19 nm and broadened compared to the MQP spectrum because of the partial superposition of the luminescence spectra of both counterparts.The intensity of its luminescence is much weaker than that of MQP.This finding indicates that a charge transfer from MQP to MoS2 occurs, which is confirmed by the almost fully coinciding excitation spectra of MQP and MoS2-M.Thus, in this system, both energy and charge transfer can be observed.

Current-Voltage Characteristics of Photodetectors
Subsequently, the optoelectronic performance of the constructed devices was tested.The length of the devices' channel is 100 µm, and the width of the gold electrodes is 500 µm.The thickness of SiO2 is 300 nm, which is sufficient to eliminate the interference of the bottom substrate (silicon) signal.As seen from Figure 5a,b, the structures of the devices based on pure MoS2 and MoS2−M are similar, with the only difference in the composition of the photosensitive layer with the same thickness of about 1.3 µm.As a result, as shown in Figure 5c,d representing I-V test results, there are two obvious improvements in optoelectronic performance after the doping of organic molecules.On the one hand, both dark and photocurrent were increased by about one order of magnitude.On the other hand, the current signals are much more stable than that of pure MoS2.Compared to the pure MoS2 device, the organic molecules−doped MoS2 device shows significant positive effects.Before doping, the fabricated device shows obvious current instability, both dark current and photocurrent, indicating that the charge carrier transmission is very inefficient.This may be caused by the high defect density of materials, low carrier mobility, small diffusion length, etc.More importantly, the film quality of pure MoS2 is limited without the MoS2 doping, which will affect the generation, separation, transportation, and recombination of photogenerated carriers to a great degree, thus leading to poor performance with jittering intense current signals and slow response behavior under light illumination.It should be noted that the dark current at negative bias is higher than the photocurrent at different wavelengths of laser irradiation, indicating that the photodetectors do not have any photo gain at negative bias.In contrast, positive bias does not have this problem.The reason for this phenomenon can be the formation of Schottky contacts between metal and semiconductors.In the case of Schottky contact, the band of semiconductor at the interface is curved, thus forming a Schottky barrier and showing a nonlinear I-V behavior under

Current-Voltage Characteristics of Photodetectors
Subsequently, the optoelectronic performance of the constructed devices was tested.The length of the devices' channel is 100 µm, and the width of the gold electrodes is 500 µm.The thickness of SiO 2 is 300 nm, which is sufficient to eliminate the interference of the bottom substrate (silicon) signal.As seen from Figure 5a,b, the structures of the devices based on pure MoS 2 and MoS 2 −M are similar, with the only difference in the composition of the photosensitive layer with the same thickness of about 1.3 µm.As a result, as shown in Figure 5c,d representing I-V test results, there are two obvious improvements in optoelectronic performance after the doping of organic molecules.On the one hand, both dark and photocurrent were increased by about one order of magnitude.On the other hand, the current signals are much more stable than that of pure MoS 2 .Compared to the pure MoS 2 device, the organic molecules−doped MoS 2 device shows significant positive effects.Before doping, the fabricated device shows obvious current instability, both dark current and photocurrent, indicating that the charge carrier transmission is very inefficient.This may be caused by the high defect density of materials, low carrier mobility, small diffusion length, etc.More importantly, the film quality of pure MoS 2 is limited without the MoS 2 doping, which will affect the generation, separation, transportation, and recombination of photogenerated carriers to a great degree, thus leading to poor performance with jittering intense current signals and slow response behavior under light illumination.It should be noted that the dark current at negative bias is higher than the photocurrent at different wavelengths of laser irradiation, indicating that the photodetectors do not have any photo gain at negative bias.In contrast, positive bias does not have this problem.The reason for this phenomenon can be the formation of Schottky contacts between metal and semiconductors.In the case of Schottky contact, the band of semiconductor at the interface is curved, thus forming a Schottky barrier and showing a nonlinear I-V behavior under positive/negative bias.It is, therefore, rational that the photocurrent is higher than the dark current at positive bias and lower at negative bias because of the existence of a built-in electric field.This property can be changed with Ohmic contacts in the case of corresponding energy levels of electrode and semiconductor.On the other side, gold electrodes can be replaced by polymer electrodes.
positive/negative bias.It is, therefore, rational that the photocurrent is higher than the dark current at positive bias and lower at negative bias because of the existence of a builtin electric field.This property can be changed with Ohmic contacts in the case of corresponding energy levels of electrode and semiconductor.On the other side, gold electrodes can be replaced by polymer electrodes.

Photoresponse Kinetics
To further study the enhancement of the optoelectronic performance of doped−MoS2based photodetectors, we performed time-dependent I-t tests of the devices.The incident laser power over the fabricated device is 228 µW cm −2 (at 350 nm).As seen in Figure 6b, compared to pure MoS2, the MoS2−M−based PD exhibits better properties with higher currents and more stable photocurrents, especially with faster response speed.As specifically shown in Figures 6c and 6d, the rise τrise and decay τdecay time of pure MoS2 are 5.2 s and 14.8 s, respectively, and the τrise and τdecay time of MoS2−M are 0.4 s and 0.8 s, respectively, which means the response speed is increased by 12 times, and the decay speed is increased by 17.5 times.These rise and decay times originate because of the presence of nanosheet edges acting as traps [17] retarding the movement of charges.Note that there are fast and slow decay components (Figure 6).This tail of the slow decay component can be attributed to the adsorption of molecules of oxygen and water from air [15] and other long-lived traps.
Interestingly, the response speed and photocurrent stability were greatly improved.We attribute these improvements to the good electrical conductivity of organic molecules, which creates better contacts and connections between the nanosheets.This result improves film quality with better morphology, such as better particle distribution, improved uniformity, more substantial filling, etc., that is in consistent with the microscopy data.This factor facilitates the transport of photo-generated carriers and improves charge carrier dynamics and photocurrent stability.Indeed, better contacts between nanosheets in functionalized samples increase the flow of charges as occurs in graphene nanosheets with metal nanoparticles fully covered by aromatic thiol molecules [41], which can be noticed as the increase in response speed and value.Also, light absorption is increased after the MoS2 functionalization due to the additional absorption by the organic molecules.It can be concluded from the above results that doping by organic molecules enhances the photoresponse speed and the photostability of MoS2.Although the device performance is not outstanding compared to the device based on the mechanically exfoliated MoS2, this novel

Photoresponse Kinetics
To further study the enhancement of the optoelectronic performance of doped−MoS 2based photodetectors, we performed time-dependent I-t tests of the devices.The incident laser power over the fabricated device is 228 µW cm −2 (at 350 nm).As seen in Figure 6b, compared to pure MoS 2 , the MoS 2 −M−based PD exhibits better properties with higher currents and more stable photocurrents, especially with faster response speed.As specifically shown in Figure 6c,d, the rise τ rise and decay τ decay time of pure MoS 2 are 5.2 s and 14.8 s, respectively, and the τ rise and τ decay time of MoS 2 −M are 0.4 s and 0.8 s, respectively, which means the response speed is increased by 12 times, and the decay speed is increased by 17.5 times.These rise and decay times originate because of the presence of nanosheet edges acting as traps [17] retarding the movement of charges.Note that there are fast and slow decay components (Figure 6).This tail of the slow decay component can be attributed to the adsorption of molecules of oxygen and water from air [15] and other long-lived traps.
Interestingly, the response speed and photocurrent stability were greatly improved.We attribute these improvements to the good electrical conductivity of organic molecules, which creates better contacts and connections between the nanosheets.This result improves film quality with better morphology, such as better particle distribution, improved uniformity, more substantial filling, etc., that is in consistent with the microscopy data.This factor facilitates the transport of photo-generated carriers and improves charge carrier dynamics and photocurrent stability.Indeed, better contacts between nanosheets in functionalized samples increase the flow of charges as occurs in graphene nanosheets with metal nanoparticles fully covered by aromatic thiol molecules [41], which can be noticed as the increase in response speed and value.Also, light absorption is increased after the MoS 2 functionalization due to the additional absorption by the organic molecules.It can be concluded from the above results that doping by organic molecules enhances the photoresponse speed and the photostability of MoS 2 .Although the device performance is not outstanding compared to the device based on the mechanically exfoliated MoS 2 , this novel doping strategy of combining organic molecules with transition metal dichalcogenides can result in the realization of stable and accurate control of photo-generated carriers in a composite film.It has great significance for expanding the application potential of two-dimensional materials in semiconductor optoelectronic devices.
doping strategy of combining organic molecules with transition metal dichalcogenides can result in the realization of stable and accurate control of photo-generated carriers in a composite film.It has great significance for expanding the application potential of twodimensional materials in semiconductor optoelectronic devices.

Conclusions
The photodetector based on thick composite films made of multi-layered MoS2 nanosheets chemically bonded with linear molecules of aromatic thiols has been developed.Composites based on multilayered nanosheets can be solution-processed, which allows for the simple fabrication of low-cost, large-area, and flexible devices.The sample consists of large nanosheets (of several micrometers) covered with small MoS2 particles.Raman line-broadening and red-shift to lower-wavenumbers of the MoS2 peaks confirm the formation of the functionalized composite.Both absorption and luminescence spectra are the superposition of the corresponding components.However, the long-wavelength shoulder at about 350 nm in the absorption spectrum of the composite can be considered as a charge transfer band.It was found that the functionalization of MoS2 nanosheets with a paraquaterphenyl thiol derivative greatly enhances the photoresponse value, speed, and photostability of MoS2-based photodetector.This study represents our first steps in the proposed direction and requires further experiments.The proposed approach is very promising for further development, for example, the use of higher dispersity, choice of a functionalizing molecule, and degree and type of functionalization.

Figure 5 .
Figure 5. (a,b) The device structures of pure MoS2−based and MoS2−M−based PDs; (c) I-V curves of pure MoS2−based PD; and (d) I-V curves of doped−MoS2−based PD.

Figure 5 .
Figure 5. (a,b) The device structures of pure MoS 2 −based and MoS 2 −M−based PDs; (c) I-V curves of pure MoS 2 −based PD; and (d) I-V curves of doped−MoS 2 −based PD.

Figure 6 .
Figure 6.(a) Schematic illustration of the device architecture (i) and an optical microscope image of the real device (ii); (b) photoresponse comparison of pure MoS2−based and MoS2−M−based PDs; (c) semilogarithmic I-t curves of pure MoS2−based PD under 350 nm illumination on/off switching at 3 V; (d) semilogarithmic I-t curves of MoS2−M−based PD under 350 nm illumination on/off switching at 3 V.