Novel synthesis of MoS2 nanoparticles via pulsed laser ablation in liquid for high-performance photodetection applications

TL;DR

This paper describes a new method to make MoS2 nanoparticles using laser ablation in liquid, which improves the performance of photodetectors.

Contribution

A novel synthesis method for MoS2 nanoparticles using pulsed laser ablation in liquid with SDS surfactant is introduced.

Findings

MoS2 nanoparticles synthesized with SDS have a higher energy band-gap (1.5 eV) compared to those without SDS (1.2 eV).

The responsivity of the photodetector increases from 0.9 to 1.17 A/W at 650 nm with the addition of SDS.

The detectivity and quantum efficiency of the photodetector improve significantly after adding SDS surfactant.

Abstract

In this work, MoS2 nanoparticles (NPs) are synthesized by laser ablation of molybdenum in thiourea. The effect of adding of sodium dodecyl benzene sulfonate (SDS) surfactant to thiourea on the properties of MoS2 NPs was studied. X-ray diffraction (XRD) studies reveal that the synthesized MoS2 NPs were crystalline with hexagonal structures. Field-emission scanning electron microscope (FESEM) investigations confirm the synthesized MoS2 NPs have spherical and hexagonal morphologies. The energy band-gap of MoS2 prepared in thiourea solution was about 1.2 eV and after addition of SDS is about 1.5 eV. The chemical bonds between Mo-S at peaks at 766, 894 and 1457 cm− 1 were identified by FTIR analysis. The Raman spectra of MoS2 shows formation (Mo-S) bond stretching mode. The current-voltage characteristic of n-MoS2/p-Si heterojunction prepared in thiourea and thiourea + SDS solutions were inspected during dark and illumination settings. The results reveal the responsivity of the fabricated devices increased from 0.9 to 1.17 A/W at 650 nm upon the addition of SDS surfactant to thiourea. The detectivity and quantum efficiency of the photodetector increases significantly after adding SDS surfactant. Energy band lineup of n-MoS2/p-Si photodetector under illumination was as well performed.

Figures

FESEM images of (a) MoS_2_ in thiourea and (b) MoS_2_ in thiourea + SDS, (c) TEM images of MoS_2_ prepared in thiourea, and (d) MoS_2_ prepared in thiourea + SDS. Insets of the TEM images are the particle size distribution of MoS_2_.

Elemental and EDX mapping of prepared in thiourea with free SDS at (a) mapping of MoS_2_, (b) mapping of Mo, (c) mapping of S, (d) FESEM of MoS_2_ and (e) elemental EDX spectrum of MoS_2_.

Elemental and EDX mapping of prepared in thiourea with SDS as additive at (a) mapping of MoS_2_, (b) mapping of Mo, (c) mapping of S, (d) FESEM of MoS_2_, and (e) elemental EDX spectrum of MoS_2_.

I-V characteristics of the fabricated n-MoS_2_/p-Si heterostructure; (a) dark I-V characteristics and the inset is semi-log I-V curve of MoS_2_-Si, (b,c) reverse I V curves under different illumination for MoS_2_ prepared in thiourea and SDS and thiourea solutions, respectively.

Figures-of-merit of the fabricated n-MoS_2_/p-Si heterostructure; (a) responsivity, (b) detectivity $\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{\times\:10}^{11}$$\end{document}$ , and (c) external quantum efficiency.

Time-resolved characteristics of MoS_2_-SDS (a) wavelength-dependent and (b) switching behavior and for MoS_2_-thiourea (c) wavelength-dependent and (d) switching behavior.

Band-diagram alignment of the fabricated n-MoS_2_/p-Si heterostructure under illumination.

Full text

Introduction

Molybdenum disulfide (MoS_2_) is a layered structured semiconductor material along a great antifriction performance^1,2^. The MoS_2_ with a direct band gab have important applications such as photocatalysts^3^, photodetectors^4^, and field effect transistors^5^. The application of MoS_2_ has protracted within the future electronic circuits field which requires relatively low stand-by power, solar energy funnels, electrocatalysis, optoelectronic devices, catalysis, solid lubrication, electrochemical interaction^1,6^, flexible devices^7^, biomedicine, energy storage, gas sensing^8^, tumor therapy by means of the photothermal effects with endorsing apoptosis of cancer-cells^9^. It is significant noteworthy to mention that that the maximum operative method to reduce the wear and friction at interfaces’ contact which is the utilization of lubricants mixed along MoS_2_ NPs^10^. MoS_2_ semiconductor along nanometer-scale exhibit a well-oriented catalytic activity, a small friction coefficient, and exclusive physical features as compared to their micro scale. Additionally, this type of material possesses a relatively large active surface area which allows an augmented high reactivity, outstanding electrical conductivity, adsorption capacity as well as a wide-ranging resistance against oxidation within a moist air environment^11^. MoS_2_ has been a widely used solid lubricant for many years because it is readily available, with a small friction coefficient, and it is somewhat oxidation-resistant semiconductor up to about 355 °C^12–19^. Next, several physical and chemical methods have been developed to prepare these NPs, with a particular and controlled morphology, including thermal decomposition^20^, sonochemical reaction^21^, chemical vapor deposition^22^, etc. MoS_2_ is an extensively considered van der Waals materials, revealing diversity of features including direct band gap, outstanding up conversion photoluminescence, down conversion photoluminescence, and optical limiting, when exfoliated in to quantum dots and mono/few layers^23–28^. The optical recombination shaped a stable exciton because of high Coulomb interaction which in turn allows a considerable optical characteristic. Emergence of strong photoluminescence in MoS_2_ shows the onset of direct band gap, as the optical properties reflect the electronic band structure and the band structure in turn gets modified with confinement effects in nanostructures^29^.

The MoS_2_ has been investigated and its shows that properties exhibited can be potential candidates for planar device technologies and to improve the performance of transistors, LED, solar cells and so on. In addition, few reports suggested methods to produce nanostructure of MoS_2_ as well as NPs of another material with good efficiency, which can be used practical application for the fabrication of technological various devices^30–32^. Among these various ways to produce nanostructure, pulsed laser ablation in liquid (PLAL) is considered as one of useful, simple and easy ways to synthesize nanostructure. As compared to other experimental methods, PLAL can obtain stable suspensions of various nanostructures in a wide range of liquids with a one-step, simple and economic procedure^33,34^. PLAL has many advantages such as catalyst free, cost-effectiveness simplicity, and good control on morphology and size of the NPs^35–37^. Key Laser parameters that effect properties of the product include laser fluence, wavelength, and pulse width. The type of liquid used during synthesis significantly influences the characteristics of the resulting NPs^38^.

Recent reports showed that PLAL is widely-established for 2D dichalcogenides preparation including MoSe_2_, WS_2_, MoS_2_, and WSe_2_. Nanosheets and/or quantum dots were produced from bulk MoS_2_ using PLAL^24^. Such framework was further developed for WS_2_ nanorods and MoSe_2_ nano-scrolls synthesis^39^. Overall, the reported studies highlighted that the proposed method, PLAL, is well-considered as an effective framework for surfactant-free colloidal preparation method of 2D dichalcogenides. However, none of the addressed methods demonstrated a combination of both SDS and thiourea as surfactant throughout PLAL for syntheses of MoS_2_ nanoparticles. The objective of this work is to synthesize MoS_2_ NPs using a green and novel technique based on laser ablation of a molybdenum target in thiourea (Tu) or sodium dodecyl sulfate (SDS) solutions, which act as sulfur sources. Using this approach, high-performance n-MoS_2_/p-Si heterojunction photodetectors were fabricated. The performance of the MoS_2_/Si devices was compared with those prepared by other synthesis methods.

Experimental procedure

Colloidal MoS_2_ NPs were prepared using pulsed laser ablation of high purity (99.9%) molybdenum pellet in two solutions: 0.5 M of thiourea (Tu) (:CH_4_N_2_S) and SDS (NaC_12_H_25_SO_4_). The laser used was second harmonic Q-switched Nd: YAG laser operating at wavelength of λ = 532 nm (second harmonic), pulse duration of 7 ns, pulse repetition frequency of 4 Hz, laser fluence of 4.7 J cm^− 2^, beam radius of 1 mm, and laser energy of 36.9 mJ. The molybdenum pellet was with diameter of 1 cm in diameter and 2 mm in thickness. It was formed by compressing molybdenum powder using a hydraulic compressor at 5 tons. The Mo pellet was placed in the bottom of glass vessel filled with 0.5 M thiourea aqueous solution (3.81 g dissolved in 100 ml). In the second laser ablation process, the pellet was located in the bottom of glass vessel contain 0.5 M of thiourea + SDS solution (3.81 g of thiourea mixed with 14.84 g of SDS dissolved in 100 ml water). The height of the solutions was 2 mm above the Mo pellet. Moreover, the laser was focused on the Mo pellet by using a positive lens with 10 cm focal length. The ablation time was adjusted to be 15 min for all experiments. Mirror-like single crystal of p-type 1cm^2^ silicon substrate with an orientation of (111) and electrical resistivity of 3–5Ω cm was used as substrate for photodetector fabrication. The substrate was cleaned with distilled water and treated with diluted HF. The MoS_2_ layer was deposited on silicon substrate by spin coating method. Figure 1a, shows the schematic diagram of experimental steps used for deposition process of MoS_2_ layer and fabrication photodetectors. Ohmic contacts were performed by evaporation of In film on the MoS_2_ layer and Al film on the back side of the silicon substrate using thermal evaporation system through thin metal mask. As shown in the inset of Fig. 1a, the cross-sectional FE-SEM image of MoS_2_ nanostructure film-Si heterostructure prepared in thiourea + SDS solution. The MoS_2_ nanostructure film is covered the entire silicon substrate with clear boundary, the thickness of the MoS_2_ film was around 2.2 μm.

The structural properties of MoS_2_ were examined using X–ray diffractometer (XRD-6000, Shimadzu). The optical properties of colloidal MoS_2_ samples were measured by UV-Vis absorption spectra (Shimadzu UV-1800 spectrophotometer). The infrared absorption of MoS_2_ as measured via using Fourier transformed infrared spectrophotometer FTIR (8400 S, Shimadzu/ Japan). The vibration modes of the product ere determined using Raman spectroscopy (Malvern HORIBA XploRA PLUS-UK). The morphology of the synthesized MoS_2_ NPs as investigated using Field emission scanning electron microscope (FE-SEM, TESCAN, MIRA3) and transmission electron microscope (TEM, Philips-EM-208 S, Iran). The chemical composition of the MoS2 was identified using energy dispersive X-ray EDX equipped with FESEM. The current- voltage properties of MoS_2_/Si heterojunction were measured under dark and white illumination conditions. The responsivity of MoS_2_/Si photodetector was measured in the spectral range 400–900 nm using Jobin Yvon monochromator, beam splitter, DC digital power supply, electrometer, and halogen lamp with variac.

Results and discussion

The formation of MoS₂ NPs by laser ablation of a Mo target immersed in a thiourea solution can be explained as follows: when the laser irradiates the Mo target, it generates a hot plasma plume containing Mo atoms and ions, which rapidly expands into the surrounding solution. As the laser pulses continue, the intense local temperature and photon energy cause thermal and photochemical decomposition of thiourea, releasing S²⁻ ions. These sulfur ions subsequently react with the Mo species in the plasma to form MoS_2_ colloids according to the following chemical reaction:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:Mo+2SC{\left({NH}_{2}\right)}_{2}+2{H}_{2}O\left(laser\:ablation\right)\to\:{MoS}_{2}+2CO(N{H}_{2}{)}_{2}+2{H}_{2}$$\end{document}

Since CO (NH_2_)2 (dissolved urea) is by-product remains in solution. Figure 1b shows the XRD patterns of Mo pellet, MoS_2_ NPs prepared in thiourea, and MoS_2_ NPs prepared in thiourea mixed with SDS. The XRD pattern of Mo pellet shows the presence of three peaks located at 2θ = 41.8°, 59.7°, and 74.6° corresponding to (110), (200), and (211) planes, respectively. These peaks are indexed to body centered cubic BCC Mo according to JCPDS # 42-1120. The XRD pattern of MoS_2_ NPs prepared by laser ablation of Mo in thiourea illustrates four peaks observed at 2θ = 32.64°, 40.48°, 58.6°, and 73.68°, corresponding to planes (100), (101), (110) and (203) planes, respectively. These peaks are belonged to hexagonal 2-H MoS_2_ which is well matched with JCPDS #37-14920. The absence of plane (002) at 2θ = 14° can be attributed to the fact that the basal planes (which are parallel to the c-axis) lie either parallel to the substrate or parallel to the beam path. Furthermore, when the MoS_2_ is poorly crystalline along the c-axis or has only a few layers, as is typical in laser ablation in liquid route, the (002) reflection becomes very weak or even nonexistent. It can be noticed from Fig. 1b, the intensity of the XRD peaks of the MoS_2_ prepared in thiourea only is higher than those for the sample prepared in mixture of thiourea and SDS. This can be ascribed to the reason that the sample synthesized in thiourea only solution results in growing of more freely crystalline MoS_2_, which in turn leading to form MoS_2_ NPs with higher degree of crystallinity. While in the case of the MoS_2_ NPs prepared in thiourea + SDS solution, SDS can lead to better dispersion of Mo and S precursors in the solution, possibly leading to more nucleation sites and formation of finer particles with lower crystalline quality. In fact, SDS added to the thiourea solution improves the morphology of the resulting MoS_2_ NPs, since it represents an anionic surfactant and its molecules adsorb onto the surface of newly formed MoS_2_ NPs, with their hydrophilic heads interacting with the particle surface and hydrophobic tails extending into the solution. This produces both steric and electrostatic barriers that prevent particle agglomeration, leading to a more uniform and narrower MoS_2_ particle distribution. The lattice constants of synthesized MoS_2_ NPs were calculated from XRD analysis, as shown in Table 1. Where a = b = 3.14 Å and c = 13.3 Å which in consistent with those for bulk MoS_2_. The crystallite size of MoS_2_ NPs was calculated using Scherrer equation and listed in Table 1. No phases concerning to other phases of molybdenum sulfide like MoS_3_, Mo_2_S_3_, etc. were detected in XRD pattern.

The effect of liquid type on the Raman spectra of synthesized MoS_2_ NPs was investigated, as illustrated in Fig. 1c. The band in the low Raman-shift region at 480 cm^− 1^ is ascribed to the molybdenum-sulfur (Mo-S) stretching mode^10,40^. This mode is assigned to second order 2xLA(M) mode^41^. Besides, the band of symmetric stretching of C-O vibration mode is located at 735 cm^− 1^ as shown in Fig. 2b^42–45^. For the MoS_2_ NPs synthesized in mixture of thiourea and SDS solution, the peak at 481 cm^− 1^ is belonging to Mo-S bond stretching mode. The peaks located at 570 cm^− 1^ and 640 cm^− 1^, were related to the second-order processes of 2E_1g_(Γ) and A_1g_(M) + LA(M), respectively, and the peak at 932 cm^− 1^ is due to E^1^2g(M) + LA(M)^41^. The MoS2’s vibration mode (A_1_g) is attained through a relatively weak signal at around 403 cm^− 1^. Such low strength of mode is believed to be usual due to both robust surface effect and decreased crystallinity which in turn suppresses the Raman response characteristic. Additionally, the presence of Mo-S bonding is confirmed via the A_1_g shoulder; this was found to be in up-right alignment with XRD analysis. It is believed that despite the advantages effect in NPs dispersion due to the presence of SDS, the lower Raman intensity in MoS_2_ produced in thiourea and SDS solution is probably caused by a combination of decreased crystallinity, smaller particle size, higher structural disorder, and thinner layer formation.

The FT-IR transmission spectra of the MoS_2_ NPs are shown in Fig. 1d. Most of the observed peaks are assigned to Mo-S and S-S vibrations in addition to other FTIR peaks indexe4d to hydroxyl groups. The peaks at 766 cm^− 1^ belonged to deformation vibration of Mo-S. The peak observed at 894 cm^− 1^ indexed to Mo-O stretching and the origin of the peak found at 1457 cm^− 1^ is C-H bending comes from the presence of SDS and thiourea^46,47^. The band at 1648 cm^− 1^ is the related to H-O-H bending from adsorbed water. The characteristic vibrational bands at 2304 cm^− 1^ can be assigned to the stretching of the C–H alkyl stretching band in Thiourea (Tu)^48^. The spectrum of MoS_2_ NPs shows the broad bands between 3750 cm^− 1^ and 3649 cm^− 1^ which is attributed to the characteristic’s bands of O-H stretching of the intermolecular and intramolecular hydrogen bonds^49,50^. We can see that the presence of SDS results in more intense FTIR peaks due to the dispersion improvement. We may attribute the presence of additional FTIR bands corresponding to C–H (2900–2950 cm^− 1^) stretching to the successful adsorption of SDS molecules onto the surface of MoS_2_ NPs.

The UV-Vis absorption spectrum of synthesized MoS_2_ recorded in the wavelength range of 250–1100 nm as shown in Fig. 1e. The absorption decreases sharply after 300 nm and 350 nm for MoS_2_ synthesized in mixture of thiourea and thiourea solutions, respectively, and then they trend to saturate. The origin of these peaks is the quantum size effect and the blue shift of the sample prepared in mixture of thiourea and SDS solution is due to the obtaining of monodispersed NPs. SDS surfactant plays a significant role in prevention the agglomeration of the NPs via lowering the surface tension and contributes in improvement of the stability of the NPs^51^. The optical energy gap of colloidal MoS_2_ NPs can be determined from the optical absorbance using Tauc plot^52^. By plotting (αhυ)^2^ vs. photon energy (hυ), the extrapolation of the linear part to the photo energy axis yields the energy gap according to Tauc equation: (αhυ)^1/n^ = (hυ-E_g_), the value of n is found to be 0.5, indicating the MoS_2_ has direct transition. The calculated direct band gap value for the MoS_2_ in thiourea solution and mixture of thiourea and SDS was 1.2 and 1.75 eV, respectively, as shown in the inset image in Fig. 1e. The attained optical bandgap alteration (1.2–1.75 eV) as a function of SDS addition indicated quantum sized particles. Therefore, the utilized PLAL-based thiourea along SDS is considered as a novelty key role that allows in situ surfactant and sulfurization-assisted MoS_2_ growth. Increase of the optical energy gap of MoS_2_ NPs after adding of SDS can be explained as follows: serves as a capping agent to stop the NPs from aggregating together when it is added to the thiourea solution during laser ablation. Consequently, the MoS_2_ NPs continue to be smaller and more consistent in size. The energy gap of MoS_2_ NPs rises with decreasing particle size, according to quantum confinement effects. This happens because more energy is needed for electron-hole excitation as the electronic states become separated and the effective distance over which charge carriers are confined is decreased.

Fig. 1(a) Deposition process of MoS_2_ layer, inset is layer thickness, (b) XRD patterns, (c) Raman spectroscopy, (d) FT-IR, and (e) UV-Vis and inset is the calculated E_g_.

Table 1. Lattice parameters of MoS_2_ prepared in thiourea (Tu) and mixture of thiourea and SDS solutions.Lattice constant for hexagonalSample2θ (degree)hklCrystallite size (nm)StrainDislocation density (nm^− 2^)a = b (Å)c (Å)c/a3.14813.344.23Thiourea40.48(101)50.973.84 × 10^− 3^0.4663.14813.354.24Thiourea + SDS40.48(101)453.92 × 10^− 3^0.467

Table 2 demonstrates other reported outcomes for MoS_2_ synthesis through PALA framework. Different morphologies depend directly on the solution/liquid utilized throughout the synthesis process.

Table 2. Similar reported approach for MoS_2_ synthesis.Study/referenceTarget materialLiquid medium/sulfur sourceMorphology/structureKey findingsPLAL of MoS_2_ liquid N_2_^53^MoS_2_Liquid N_2_MoS_2_ nanosheetsDoping MoS_2_ with N_2_PLAL of MoS_2_ in methanol solution^11^MoS_2_ metalmethanolLayered MoS_2_Layered structure with micrometer sizesPLAL of MoS_2_ in distilled water^54^MoS_2_ metalDistilled waterQuantum dotsDependence of particle size on laser energy and ablation timePresent workMo metalThiourea & Thiourea + SDSThiourea: semi-spherical, agglomerated NPsPresent work

The morphology of synthesized MoS_2_ NPs was investigated using FESEM as shown in Fig. 2 (a and b). It can be seen that the morphology of the MoS_2_ NPs prepared in the thiourea solution consists mainly of rough and irregular particles with sizes ranging from 32 to 140 nm. These particles exhibit spherical to ellipsoidal shapes, and their agglomeration leads to the formation of coral-like or cauliflower-like structures, as shown in Fig. 2a. In contrast, the MoS_2_ NPs synthesized in the thiourea + SDS solution (Fig. 2b) display a well-defined hexagonal morphology with uniformly dispersed particles. These structures may be due to the formation of MoS_2_ with various crystal orientations which are consistent with XRD results.

TEM images of MoS_2_ synthesized in thiourea and in thiourea + SDS solutions are presented in Fig. 2c and d, respectively. The sample prepared in thiourea shows the formation of semi-spherical, agglomerated MoS_2_ NPs, whereas the thiourea + SDS solution leads to a higher concentration of well-defined spherical MoS_2_ NPs. The reason beyond this result can be ascribed to the negatively hydrophobic charged sulfate that generate a repulsive electrostatic force between MoS_2_ NPs and in turn prevents the agglomeration of the MoS_2_ NPs^55^. The particle size distribution of MoS_2_ NPs synthesized in thiourea and thiourea + SDS solution are depicted in insets of Fig. 2c, d, respectively. The average particle size of MoS_2_ NPs synthesized in thiourea and mixture of thiourea and SDS solutions was 31 nm and 133 nm, respectively, since the agglomeration of the MoS_2_ NPs prepared in thiourea resulted in formation of larger particles.

Fig. 2FESEM images of (a) MoS_2_ in thiourea and (b) MoS_2_ in thiourea + SDS, (c) TEM images of MoS_2_ prepared in thiourea, and (d) MoS_2_ prepared in thiourea + SDS. Insets of the TEM images are the particle size distribution of MoS_2_.

Figure 3a–e shows the EDX spectra and elemental mapping analysis of MoS_2_ NPs synthesized in thiourea aqueous solution. The EDX confirmed the presence of peaks of Mo and S which are related to main elements of the MoS_2_ NPs and other peaks are come from the glass substrate. The weight% of Mo and S elements was found to be 7.7% and 92.3%, respectively, indicating formation of incomplete stoichiometric MoS_2_. These weight percentages mean that the product is severe sulfur deficiency due to low concentration of thiourea which means that there are no enough sulfur atoms to convert Mo to MoS_2_ as shown in elemental mapping Fig. 3e. On the contrary, the MoS_2_ prepared in thiourea + SDS solution having percentage ratios of 73.9% S and 26.1% Mo, indicating the formation of sulfur rich MoS_2_ product due to the presence of much content of sulfur in the solution as shown in Fig. 4a–e.

Fig. 3. Elemental and EDX mapping of prepared in thiourea with free SDS at (a) mapping of MoS_2_, (b) mapping of Mo, (c) mapping of S, (d) FESEM of MoS_2_ and (e) elemental EDX spectrum of MoS_2_.

Fig. 4. Elemental and EDX mapping of prepared in thiourea with SDS as additive at (a) mapping of MoS_2_, (b) mapping of Mo, (c) mapping of S, (d) FESEM of MoS_2_, and (e) elemental EDX spectrum of MoS_2_.

Hall measurement was carried out and they showed that the synthesized MoS_2_ exhibited a negative Hall coefficient, indicating its n-type conductivity. The origin of n-type conductivity in MoS_2_ is due to the excess free electrons come from the sulfur vacancies which create donor states near the conduction band. Figure 5a, shows the dark current-voltage (I-V) characteristics of n-MoS_2_/p-Si heterojunctions fabricated in thiourea and thiourea + SDS solutions in the forward and under reverse directions in the voltage span of -8 to 8 V, inset into the figure is semi-log I-V curve scale. It is evident from Fig. 5a that the forward current increases exponentially with forward bias voltage, while the reverse bias current increases very slowly with voltage. The Figure shows that the heterojunctions display a rectification characteristic and the heterojunction fabricated in thiourea + SDS exhibits higher rectification. The forward current was found to be consisted of two regions, since the first region, which occurs at low voltage bias which is known as the recombination current. At such low biases, the applied voltage is insufficient to significantly reduce the barrier height, so the depletion region remains wide. As a result, only a limited number of electrons and holes can cross the junction, leading to a small current. Whereas the second region of the forward current is known as diffusion current, where the current increases exponentially with high voltage bias as a result of the bias voltage potential exceeding the potential barriers. The forward bias voltage provides enough energy to the electron to overcome the barrier height and move to silicon. The ideality factor (n) of the heterojunction as determined from the diode equation \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\left[I={I}_{o}\:\left({e}^{qV/nKT}-1\right)\right]\:$$\end{document} where I_o_ is the saturation current, K is the Boltzmann constant, q is the electron charge, T is the operating temperature, The value of I_o_ was estimated from plot of semilogarithmic forward current versus bias voltage. After substituting the values in Eq. 1, the ideality factor of MoS_2_/Si heterojunctions fabricated in thiourea and thiourea +SDS solution was 4.1 and 3.3, respectively. The large ideality factor values are attributed to the large mismatch in lattice constants between MoS_2_ and silicon substrate which is calculated and found to be around 42%. The value of mismatch may induce structural defects that affect the junction properties. Based on the obtained results, the junction quality of MoS_2_/Si fabricated in thiourea + SDS is better than that of MoS_2_/Si fabricated in thiourea solution only. The reason beyond this finding can be attributed to the agglomeration of MoS_2_ NPs for the heterojunction prepared in thiourea only.

Figure 5b, c illustrates the illuminated I-V characteristic of the n-MoS_2_/p-Si heterojunction photodetectors fabricated in thiourea and thiourea + SDS solutions, respectively, at various white light intensities. Increases in current after illumination the photodetector with light is due to the generation of (e-h) pairs in the depletion and diffusion length regions of the heterojunction.

By increasing the reverse bias voltage, the depletion region expanded towards the MoS_2_ layer, and in turns increases the photocurrent and enhancing the photosensitivity of the photodetector. Increasing the light intensity leads to significantly increase in the photocurrent due to the increases in the number of e-h pairs, indicating the good linearity characteristics of the photodetector. Moreover, Fig. 5b, c indicates that there was no noticeable saturation of photocurrent at high light intensity, demonstrating that the MoS_2_/Si photodetector exhibits good linearity properties. Continuously, the photosensitivity of the photodetector ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:S={I}_{ph}/{I}_{d}$$\end{document} ) was calculated and found to be 70 and 200 photodetectors fabricated in thiourea and thiourea + SDS solutions, respectively.

Fig. 5I-V characteristics of the fabricated n-MoS_2_/p-Si heterostructure; (a) dark I-V characteristics and the inset is semi-log I-V curve of MoS_2_-Si, (b,c) reverse I V curves under different illumination for MoS_2_ prepared in thiourea and SDS and thiourea solutions, respectively.

Figure 6a shows the spectral responsivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:[{R}_{\lambda\:}={I}_{ph}-{I}_{d}/{P}_{in}]$$\end{document} ^56^ plot of MoS_2_ fabricated in thiourea and thiourea + SDS. For the n-MoS_2_/p-Si photodetector prepared with thiourea (Tu), where two peaks could be perceived. The first peak, located at 650 nm, has a responsivity of approximately 0.863 A/W. The second peak, at 850 nm, corresponds to the silicon substrate with a responsivity of approximately 0.69 A/W. For the n-MoS_2_/p-Si synthesized with thiourea + SDS, the responsivity of the first peak was 1.017 A/W at 650 nm, and the second peak, with a responsivity of approximately 0.9 A/W, corresponds to the silicon substrate at 850 nm. The red shift in the photodetector’s responsivity after adding SDS to Tu can be attributed to the reduction in the optical energy gap of MoS_2_ NPs, as indicated by the red line. Beyond 600 nm, the responsivity showed a semi-flat response, and a significant peak was observed at 760 nm. These findings demonstrate that the addition of a surfactant like cationic SDS to the Tu solution decreases the number of recombination centers, thereby reducing the density of charge carrier recombination. In comparison to silicon-based heterojunction photodetectors, the photodetector incorporating SDS exhibited a higher responsivity over a wider range of visible light. Beyond 650 nm, the incoming light is no longer absorbed by MoS_2_ nanostructured film and transmitted to the silicon substrate. As for the second increase around 850 nm, the main reason is the absorption edge of silicon. Silicon has strong absorption near its energy gap (~ 1.12 eV), which corresponds to wavelengths around 850–900 nm. At this wavelength, photons still have enough energy to excite carriers across the Si energy gap.

As shown in Fig. 6b, the specific detectivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:[{D}^{*}={R}_{\lambda\:}(A{)}^{1/2}/\sqrt{2q{I}_{D}}]$$\end{document} of n-MoS_2_/p-Si photodetector prepared with thiourea was 4.319 × 10^11^ Jones at a wavelength of 650 nm, whereas the detectivity of photodetector fabricated in Tu + SDS was around 5.082 × 10^11^ Jones at 650 nm. The significant increase in detectivity of the photodetector after adding the SDS surfactant can be attributed to the widening of the depletion layer width and the increase in minority carrier diffusion length due to reduction in the concentration of agglomerated NPs. The external quantum efficiency \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:[EQE={1240\:\times\:\:\mathrm{R}}_{{\uplambda\:}}/{\uplambda\:}]$$\end{document} can be defined as the number of electron-hole pairs generated for each incident photon. Figure 6c displays the EQE as a function of the wavelength for the n-MoS_2_/p-Si HJs photodetector synthesized MoS_2_ in thiourea and thiourea + SDS solution was 1.64 × 10^2^% and 1.9 × 10^2^%, respectively. These findings can be ascribed to the reduction in agglomeration of MoS_2_ NPs and decreases in surface states and recombination centers as well as due high surface area of MoS_2_. Table 3 displays the performance of fabricated n-MoS_2_/p-Si photodetector compared to other heterojunction photodetectors reported in previous studies.

Fig. 6. Figures-of-merit of the fabricated n-MoS_2_/p-Si heterostructure; (a) responsivity, (b) detectivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{\times\:10}^{11}$$\end{document} , and (c) external quantum efficiency.

Table 3A comparison between the performance of our fabricated n-MoS_2_/p-Si photodetector and several other heterojunction photodetectors reported in previous studies.PhotodetectorKey notesWavelength (nm)Rλ (A/W)D* (Jones)Ref.MoS_2_ QD/SiColloidal MoS_2_ QDs600–6500.85~ 8 × 10^11^^24^MoS_2_/SiPerpendicular MoS_2_ sheets6253.6 × 10^− 4^~ 1 × 10^9^^57^MoS_2_/Si + BTO layerMoS_2_ monolayer5500.603~ 1 × 10^13^^58^MoS_2_/SiLarge-area CVD MoS_2_5808.5~ 1 × 10^12^^59^MoS_2_/SiPLAL in thiourea + SDS6501.017~ 5.1 × 10^11^This work

The time-resolved characteristics of the fabricated photodetectors are elucidated in Fig. 7, with respect to the medium utilized during preparation. This particular investigation was conducted using narrow band-pass filters ranging from 400 nm to 900 nm in conjunction with source measure unit where the response/recovery periods were considered from 10% to 90% of the total attained photocurrent. The fabricated n-MoS_2_/p-Si heterojunction devices exhibited rather wavelength-dependency behavior as a function of the applied wavelengths where both photodetectors exhibited the highest photocurrent under incident wavelength of 650 nm (Fig. 7a,c) for MoS_2_ fabricated in thiourea and thiourea + SDS, respectively. Herein, the response/recovery time perceived during this investigation was found to be 81/86 and 83/86 ms. for the addressed heterostructures, respectively. The attained results indicate that MoS_2_ prepared in thiourea + SDS demonstrated rather higher response time as compared to that prepared in thiourea alone. Additionally, the response time was found to be relatively higher than recovery which suggests faster electron/hole pairs generation in comparison to that of recombination. Continuously, as demonstrated in Fig. 7b,d, the proposed geometries clearly revealed repeatable performance over three multiple cycles with pulse width of ~ 10 s.

Fig. 7. Time-resolved characteristics of MoS_2_-SDS (a) wavelength-dependent and (b) switching behavior and for MoS_2_-thiourea (c) wavelength-dependent and (d) switching behavior.

The related energy band lineup (Fig. 8) of the proposed heterojunction was estimated as pre-described by Anderson’s model. The attained band diagram revealed staggered (Type-II) band alignment at which the valence band VB and conduction band CB of MoS_2_ are positioned at lower energy levels as compared to that of p-Si substrate. In particular, the demonstrated band alignment exhibited ΔE_C_ of 0.55 eV along ΔE_V_ of 1.32 eV; such bands discontinuities allow the generation of potential barriers which in turn enable electron allocation from p-Si wafer to n-MoS_2_. The latter may promote a well-oriented separation of charges upon illumination. Subsequently, the internal field boosts the photo-generated pairs (electron/hole) separation and therefore enhances the rectifying behavior and photo-responsive features of the fabricated heterojunction^60^. Thus, carrier dynamics around the proposed heterojunction allow the combination of photovoltaic diffusion as well as separation (at the depletion region).

Fig. 8. Band-diagram alignment of the fabricated n-MoS_2_/p-Si heterostructure under illumination.

Conclusion

This work demonstrates a simple and novel PLAL approach for the synthesis of crystalline hexagonal MoS_2_ NPs using thiourea and thiourea + SDS solutions as sulfur sources, followed by their direct integration into n-MoS_2_/p-Si heterojunction photodetectors. The structural, vibrational), and morphology analysis confirm the formation of phase-pure MoS_2_, with morphology and dispersion strongly influenced by the liquid medium. The optical measurements confirmed that energy gap increased from 1.2 to 1.75 eV after adding SDS surfactant due to decreasing the agglomeration of the NPs. Elemental analysis confirms Mo–S bond formation and indicates that surfactant-assisted ablation promotes more favorable surface chemistry for device fabrication. Electrical and optoelectronic properties of MoS_2_/Si heterojunction demonstrate robust diode behavior with pronounced rectification. The SDS surfactant plays a vital role in improving the junction quality, characterized by a lower ideality factor and reduced recombination, resulting in a linear and bias-tunable photoresponse across the visible to near-infrared region. The spectral responsivity shows a distinct peak between 650 and 850 nm, with a maximum responsivity of ~ 1 A W⁻¹ and a specific detectivity in the range of 10¹¹–10¹² Jones, comparable to state-of-the-art Si-based heterojunction photodetectors. The observed photoresponse is attributed to a staggered (type-II) MoS_2_/Si band alignment that facilitates efficient carrier separation. Overall, these findings validate PLAL as a controllable and green synthesis route for MoS₂ nanostructures and highlight the crucial role of SDS in tuning NPs dispersion, interfacial quality, and device performance.