Molybdenum Disulfide Quantum Dots: Properties, Synthesis, and Applications

Abstract

Molybdenum disulfide quantum dots (MoS₂ QDs) are a unique class of zero-dimensional (0D) van der Waals nanostructures. MoS₂ QDs have attracted significant attention due to their unique optical, electronic, chemical, and biological properties due to the presence of edge states of these van der Waals QDs for various chemical functionalization. Their novel properties have enabled applications in many fields, including advanced electronics, electrocatalysis, and biomedicine. In this review, the various synthesis techniques, the novel properties, and the wide applications of MoS₂ quantum dots are discussed in detail.

1. Introduction

Since the isolation of monolayered graphene from graphite in 2004, the field of two-dimensional (2D) van der Waals materials has seen rapid development. This is due to the remarkable opportunity of using various types of 2D materials and their van der Waals heterostructures for electronic [1], optical [2], chemical [3], and biological [4,5] applications. The lack of a fundamental band gap in graphene has prevented the use of graphene in digital electronics. This has led researchers to explore van der Waals 2D materials beyond graphene, including hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDCs). In contrast to the metallic graphene, and the electrically insulating h-BN, 2D TMDCs have bandgaps close to that of silicon, which is essential for field-effect transistors (FETs). TMDCs have seen application due to their feasible electronic, optical, mechanical, chemical, and thermal properties [6]. In particular, tremendous research interest in the transition of indirect bandgaps in bulk TMDCs to direct bandgaps in monolayer TMDCs has been generated.

A further attraction of 2D TMDCs materials has recently been demonstrated when their planar dimension is reduced below 100 nm to establish the edge effects. As the planar dimension continues to reduce to several nanometers, the strong quantum confinement effect enhances the characteristic photoluminescence (PL) of TMDCs, granting them high utility in many fields. These spatially confined zero-dimensional (0D) TMDCs, known as quantum dots (QDs), have started to gain attention for applications. In particular, molybdenum disulfide (MoS₂) QDs have been used in the fields of electrocatalysis [7], solar energy production [8], energy storage [9], advanced electronics [10], chemical sensing [11], bioimaging [12], photothermal cancer therapy [13], and more. For example, the edges of MoS₂ enable hydrogen absorption similar to platinum and other rare earth metals, making it a cost-effective alternative for hydrogen evolution reactions (HERs) [7], as the increased specific surface area and active edges enhance the catalytic activity. Additionally, the direct bandgap of MoS₂ QDs along with their high biocompatibility have enabled many other applications.

Research on MoS₂ QDs is still in the infancy, but the number of publications on these QDs per year has risen each year since 2013, as shown in Figure 1. Given recent motivation to find alternatives to replace rare-earth-based materials [14] and heavy metals (such as lead and cadmium), the development of less hazardous QDs will be increasingly important. To support further development of this research area, a comprehensive review of MoS₂ QDs is provided here. While articles of this nature have been published previously, none have had the same focus; some have narrowed their view to biological applications [15], while others broadened their scope to all 0D TMDCs [16]. In this article, the properties, synthesis techniques, and applications of MoS₂ QDs will be comprehensively reviewed.

2. Properties

The physical, catalytical, and biological properties of MoS₂ QDs are summarized here. Many of these intrinsic properties are based on those of bulk and monolayer 2D MoS₂, which we have recently summarized [17,18]. Here, property differences of MoS₂ QDs as compared to those of bulk and monolayered MoS₂ are discussed.

2.1. Structure

Molybdenum disulfide is a member of the TMDC family. TMDCs take the form of MX₂,w here M is a transition metal and X is a chalcogen. TMDCs have a layered hexagonal structure with each monolayer being comprised of three stacked layers (X-M-X), and with intralayer van der Waals bonding occurring between S atoms. MoS₂ exists in three different crystal structures: a trigonal phase (1T), a hexagonal phase (2H), and a rhombohedral phase (3R). The numbers are representative of the number of layers in the unit cell while the letters correspond to the crystal symmetry. The 2H and 3R phases are semiconductive and are suitable for electronic devices. Conversely, the 1T phase is metallic, semistable, and shows high catalytic properties. The 1T phase of MoS₂ can be easily changed to the 2H phase by heating it to over 300 °C [19]. Unless otherwise denoted, our discussion will be focused on the 2H polytype.

In the 2H form, each Mo atom is covalently bonded to six S atoms, and each S atom is bonded to three Mo atoms, as seen in Figure 2. The bulk MoS₂ unit cell belongs to the P6₃/mmc space group and is defined by the hexagonal lattice constant a = 3.16 Å, the out-of-plane lattice constant c = 12.58 Å and the internal displacement parameter z = 0.12 [20]. Noncovalent interactions have been demonstrated to be essential for interlayer bonding. Figure 3 shows the binding energy of bilayer MoS₂ as a function of the out-of-plane lattice constant. The results of two ab initio density functional theory (DFT) calculations using Perdew–Burke–Ernzerhof exchange functional are plotted; one using Grimme’s dispersion corrections (filled black circles) and one without (red squares) [20]. Without the corrections for noncovalent interactions, the binding energy is found to be positive (no binding), while the results using the corrections correctly predict MoS₂ bonding. This is due to these weak interlayer bonds that mechanical exfoliation down to a single layer is possible.

2.2. Electronic Properties

The electrical, optical, and vibrational properties of TMDC nanostructures are highly dependent on interlayer coupling. Bulk, few- and monolayered MoS₂ have been studied using density functional theory and utilizing the general gradient approximation. It was shown that bulk MoS₂ is an indirect band gap semiconductor (1.2 eV), with the band gap occurring at the Γ point in the valence band and the point halfway between the Γ and K points in the conduction band. As the thickness decreased to monolayer, a direct bandgap of 1.9 eV was formed at the K point. This phenomenon has been written about extensively in the literature [21,22,23,24,25,26,27,28,29,30].

This indirect-to-direct transition can be explained by examining the bonding in bulk and monolayer MoS₂. Based on the density of states data, MoS₂ has a filled d_z² valence band that overlaps the p_z orbitals of the S atoms [24,31,32,33,34,35,36,37,38]. The conduction band is composed of the degenerate d_x²−y² and d_xy orbitals that overlap with the empty, antibonding p_z orbital of the S atoms. Near the K point, the conduction and valence bands are primarily composed of the d_x²−y² and d_xy orbitals of the Mo atoms. At the Γ point, the conduction and valence bands are primarily composed of the d orbitals of the Mo atoms and the p_z orbitals of the S atoms. The S atoms experience more interlayer coupling than the Mo atoms as the van der Waals bonds between two S atoms creates the layered structure. As MoS₂ is thinned from bulk to monolayer, the bonding attributed to the p_z orbitals of the S atoms weakens, increasing the gap near the Γ point. Conversely, since the band structure near the K point is primarily attributed to in-plane bonding, the band structure near the K point is affected only minimally. By decreasing the lateral size of MoS₂, the bonding attributed to the d_x²−y² and d_xy orbitals of the Mo atoms decreases, which would increase the band gap—this explains why smaller MoS₂ QDs have larger band gaps. The changes in electronic band structure through vertical confinement are documented in Figure 4.

MoS₂ QDs can be classified as n-type semiconductors. Vikraman et al. determined the semiconductor type by constructing FETs using thin MoS₂ QD layers [39]. The I-V characteristics of the device are shown in Figure 5 and a more in-depth discussion of the device can be found in Section 4.2.1.

2.3. Optical Properties

Raman spectroscopy measurements are common for analyzing lower-dimensional materials. MoS₂ has two characteristic peaks at 382.5 cm⁻¹ (${\mathrm{E}}_{2\mathrm{g}}^1$), due to in-plane optical vibration of the M_O -S atoms in the basal plane, and 407 cm⁻¹ (A_1g), due to out-of-plane optical vibration of S atoms along the c axis [40,41]. Previously, the separation distance between these peaks has been used to determine the thickness of a MoS₂ nanosheet; however, the distance of these peaks remains mostly invariant with QD size [42]. The intensity of the two peaks changes with particle size–in particular, the ratio of the ${\mathrm{E}}_{2\mathrm{g}}^1$ and A_1g peaks is found to decrease with a decrease in particle size [42].

The effects of quantum confinement on MoS₂ QDs are studied using optical absorption and photoluminescence spectroscopies, as seen in Figure 6 [42]. The absorption spectrum of MoS₂ QDs is defined by four peaks, labeled A–D. The most prominent peaks at ~680 and ~618 nm (labeled A and B, respectively) result from the transition of spin–orbit coupled electrons from the edges of the conduction and valence bands. The C and D peaks (at ~460 and ~395 nm, respectively) are indicative of a band nesting phenomenon commonly observed in TMDCs. This band nesting feature is facilitated by strong Van Hove singularities, which results in joint densities of states and higher transition gaps. As the QDs get smaller, the C peak experiences a blue shift, which is readily explained by the increased band gap with size reduction. The PL spectrum was found the be broad, and the smaller QDs exhibited larger intensities. The PL peak can be deconvoluted into two overlapping peaks at ~420 and ~500 nm. The peak at 420 nm is the result of transitions between quantized energy levels, and the 500 nm peak is caused by transitions mediated by defect states. With increasing size, the ratio of the intensities of the 420 and 500 nm peaks increases, which is caused by the decreased surface–volume ratio.

The UV–Vis absorption spectra of MoS₂ in both bulk and quantum dot forms were compared, as shown in Figure 7. The characteristic peaks at 340, 430, 590, 650 nm in the bulk form are absent from the spectrum of the QDs. The peaks at 590 and 650 nm of the bulk form can be attributed to the K point of the Brillouin zone, and the remaining peaks are attributed to transitions from the valence band to the conduction [42,43,44,45]. The only peak shown in the QD spectrum can be explained by the excitonic features of the QDs [46], and the blue shift can be explained through quantum confinement and edge effects [47].

2.3.1. Size-Dependent Emission

The recombination of excitons in semiconductor crystals results in the emission of a photon with the same energy as the exciton. In a simple model, the energy of the photon is the sum of the band gap energy, the confinement energies of the hole and electron, and the bound energy of the exciton. As the confinement energy depends on the size of the quantum dot, the emission wavelength is directly related to the dot’s size—the larger the dot (less confined) the redder it is, and the smaller the dot (more confined), the bluer it is. Recently, there seems to evidence that the shape of the quantum dot may play a role in the emission color; however, more testing needs to be carried out to ensure this. The fluorescence lifetime is also tied to the size of the quantum dot. Since larger dots have closely spaced energy levels in which the electron-hole pair can be trapped, the fluorescence lifetime is longer than in smaller QDs [42].

2.3.2. Excitation-Dependent Emission

The photoluminescence (PL) of MoS₂ QDs has been demonstrated to be dependent on the excitation wavelength used as seen in Figure 8. There are two primary reasons for this phenomenon. The first reason comes from the issue of size polydispersity—as stated above, MoS₂ QDs exhibit size-dependent emission. The varying sizes will emit differently under varying excitation wavelengths, thus creating an excitation-dependent emission spectrum. It is theorized that monodisperse QDs would exhibit significantly less excitation-dependent emission. Another reason for this phenomenon is defect level formation after oxygen adsorbs to the edges of the QD [49].

2.4. Electrocatalytic Properties

2D materials have attracted massive interest due to their electrocatalytic activities [50]. The unsaturated Mo and S edges of MoS₂ can serve as active sites where surface reactions can be initiated. By reducing these sheets to QDs, the number of exposed edges per surface area is increased, enabling a higher amount of hydrogen atoms to bond with photocatalysts, increasing the photocatalytic activity for H₂ generation. For MoS₂, the turnover frequency of the edge sites, sulfur vacancies, and grain boundaries have been determined to be 7.5, 3.2, and 0.1 s⁻¹, and the Tafel slopes to be 66–75, 65–85, and 120–160 mV/dec, respectively [15]. 0D MoS₂ has also been demonstrated to exhibit high catalysis in oxygen reduction reactions (ORRs) [51].

2.5. Biological Properties

2.5.1. Cytotoxicity

MoS₂ QDs have been found to have excellent biocompatibility. Zhou et al. conducted methylthiazoleterazolium (MTT) assays in HeLa and RAW 264.7 cells and found high biocompatibility in micromolar concentrations [52]. The cell culture was made in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with Fetal Bovine Serum (FBS), penicillin, and streptomycin at 37 °C in a 5% CO₂ incubator. Cellular imaging was conducted in the cell culture medium, and LysoTracker green and carbon QDs (C-dots) were used for long-term cell imaging. Fluorescence imaging was performed in an inverted microscope system. It was found that it was not essential for the MoS₂ QDs to take help from any other organic reagents to penetrate the cell wall to reach the perinuclear region, implying that these QDs are compatible with aqueous biological systems. Research conducted by Xu et al. used a similar MTT assay on HeLa cell lines and determined that cell viability dropped by 1% and 12% with MoS₂ concentrations of 250 and 2000 µg/Ml [43]. Sweet et al. (2017) confirmed the high biocompatibility of MoS₂ QDs by conducting MTT assays on Lymph Node Carcinoma of the Prostate (LnCaP) and Cultured Human keratinocyte (HaCaT) cells incubated in anti-prostate-specific membrane antigen (PSMA) antibodies conjugated with MoS₂ QDs and Trypan Blue [53]. These results showed a biocompatibility of 98%. Furthermore, reactive oxygen species (ROS) assays were performed on the HaCaT cells. Cell permeable fluorogenic probes were excited at 380 nm for 7 h, and no increase in intracellular ROS was observed, again indicating high biocompatibility. Others have also performed cytotoxicity tests to prove the biocompatible nature of MoS₂ QDs [54,55,56,57,58].

2.5.2. Fluorescence Stability

When a biological cell detects a foreign substance such as nanoparticles or organic dye, it works to pump those out. Since researchers need to monitor cells for several days, retaining fluorescence markers on cells is required. Additionally, diffusion of the dyes might be responsible for the reduction in the microscopic contrast. Therefore, the reactivity of MoS₂ QDs with bio-thiols was investigated to determine the long-term fluorescence properties [29]. Phenyl isothiocyanate labeled cysteine (PITC-Cys) MoS₂ QDs were monitored using a self-assembled capillary electrophoresis system. Long-term fluorescence stability at room temperature was attributed to the covalent locking of the QDs to the thiols. The ability of MoS₂ QDs to react with intracellular thiols such as proteins, amino acids, etc., helps retain a long-term fluorescence in the live cells. The author cultured HeLa cells with MoS₂ dots, LysoTracker green, and C-dots for 48 h. Result suggests that MoS₂ dots could retain their luminescence for days, whereas LysoTracker green faded in 2 h and C-dots faded in 48 h. Supporting data with the MoS₂ labeled neuronal Pheochromocytoma (PC12) cells and the unlabeled mouse macrophage RAW 264.7 cells, also show excellent intracellular retention for 24 h, and proving the long-term fluorescence nature of MoS₂ dots.

3. Synthesis Techniques

Significant research efforts have been directed towards the development of effective strategies for the synthesis of MoS₂ QDs. As with most nanomaterials, there is the dichotomy of “bottom-up” and “top-down” synthesis methods. Top-down methods begin with bulk MoS₂ and sculpt it to the desired size; common methods include mechanical exfoliation and solvothermal techniques. In contrast, bottom-up methods begin with precursors that react to form the MoS₂ QDs, with common techniques being hydrothermal synthesis and chemical bath deposition.

3.1. Top-Down

Since the dawn of 2D materials, the mechanical exfoliation of nanomaterials has been one of the more popular top-down techniques due to its ease of execution. Micromechanical cleaving—in particular, the Scotch Tape method [59]—has proven to be effective for isolation of monolayer 2D materials; however, this method has difficulty producing the small area domains characteristic of quantum dots. Leading methods of mechanical exfoliation heavily rely on the sonication of bulk MoS₂. Other popular top-down methods include solvothermal synthesis, electrochemical synthesis, and thermal ablation.

3.1.1. Sonication-Assisted Exfoliation

MoS₂ nanosheets can be readily obtained through the sonication of MoS₂ powders. Energetic ultrasonic waves can break bonds between and within sheets of MoS₂ through an acoustic cavitation process. The sound waves introduce pressure variations within the material, forming high-temperature, high-pressure cavities that grow until they collapse. The collapse of these cavities carries enough energy to separate sheets and break interlayer bonds. In a typical experiment, MoS₂ powder is added to a solvent and sonicated for several hours to separate sheets. Then, the solution is often (but not always) refluxed to further cleave the material into smaller area domains. Finally, the solution is centrifuged and the supernatant is separated. This process is shown schematically in Figure 9.

Other modified techniques assist the sonication by carrying out thermal pretreatments of the bulk MoS₂. For example, Baby et al. heated MoS₂ powder to an optimized temperature of 340 °C before immediately quenching the powder in liquid nitrogen [67]. The temperature difference forms cracks in the bulk sample and causes the separation of exfoliated layers.

3.1.2. Ion Intercalation-Assisted Exfoliation

A common way to assist exfoliation by sonication involves the intercalation of ions (Li⁺ and Na⁺ being the most common) between sheets of layered materials (pictured in Figure 10). These methods typically result in the 1T polytype of MoS₂ being produced, which is not ideal for electronic applications due to its metallic nature; however, additional heat processing can cause a transition to the 2H phase [68].

For lithium intercalation, Qiao et al. immersed MoS₂ powder in a n-butyl lithium (n-BuLi)/hexanes solution for 2 days in an argon-filled flask [69]. Li_xMoS₂ was retrieved by filtration and washed repeatedly with hexanes. Immediately afterwards, the Li_xMoS₂ was dispersed in water and sonicated. Hydrochloric acid was added following sonication to flocculate the products, and then the mixture was washed with water until the pH was neutralized and then centrifuged several times. The flocculation was dried in a vacuum drying oven, and the whole process was repeated two or three times. Following the final cycle, the remaining lithium hydroxide was filtered out using a dialysis bag, the remaining solution was centrifuged, and the product was annealed in a bath sonicator at 90 °C. The produced QDs had a size range of 1–5 nm with an average around 3 nm, with the vast majority having a thickness of around 1.0 nm.

In the similar work carried out by An et al., MoS₂ powder was dissolved into n-hexane, and n-BuLi was added to the solution [68]. The solution was stirred for 12 h, inserting Li⁺ ions between layers, consequently resulting in the insertion of electrons into the host phase, causing a transition from the 2H to the 1T phase. Following, the solution was diluted with DI water and irradiated with an optimized femtosecond laser for 30 min to transition the QDs back to the 2H phase. The resulting QDs had a size of ~10 nm.

Similar methods utilizing Na⁺ intercalation are also effective. Zhou et al. used the ion intercalation method using Na⁺ ions by mixing MoS₂ powder with sand [52]. The mixing tube was vacuum-sealed, heated to 160 °C and maintained for 12 h to induce intercalation. The intercalated sample was exposed to ambient air before being sonicated. The yellow solution was filtered through coarse paper to collect unexfoliated MoS₂. The collected suspension was further concentrated using pressure distillation and then centrifuged. The remaining Na ions were removed through a 3-day dialysis process. The yield of MoS₂ QDs from bulk was roughly 11% and the QDs were 2.0–5.5 nm in size. Figure 10

Schematic illustration of the preparation of monolayer MoS₂ quantum dots using multiple exfoliations with Li intercalation. Reproduced with permission from [70]. Copyright Elsevier B.V., 2015.

[Figure omitted. See PDF]

3.1.3. Electrochemical Synthesis

A mechanism for electrochemical synthesis has been proposed by Gopalakrishnan et al. [44]. The generation of free radicals triggers the initial cleavage, which leads to further exfoliation of the material. This is similar to the posed mechanism for the electrochemical synthesis of graphene QDs and nanoribbons. When the highly diluted electrolytes have a DC voltage applied to them, hydroxl and oxygen free radicals are formed. The MoS₂ anode swells due to the incorporation of the anions TFSI^- (bis(trifluoromethane)sulfonimide) and Cl⁻³, and the MoS₂ begins to dissolve in the electrolyte.

Gopalakrishnan et al. demonstrated electrochemical etching to be capable of size-controlled synthesis of MoS₂ QDs. MoS₂ disks with a diameter of 1 cm were made from commercial MoS₂ powder. A constant DC potential was applied across these MoS₂ disks in a two-electrode cell in different concentrations of aqueous Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or 1-Butyl-3-methylimidazolium chloride ([BMIm]Cl). The reaction mixture was collected after 3 h and was then centrifuged. The synthesis method is schematized in Figure 11. The LiTFSI solution, at concentrations of 0.1 and 1 wt%, provided particles with sizes of 2.5 and 4.6 nm, respectively. Using the same concentrations of [BMIm]Cl yielded larger particles with sizes of 2.8 and 5.8 nm, respectively.

Li et al. developed a method for synthesizing graphene oxide and MoS₂ QDs using an electro-Fenton reaction [71]. In the electro-Fenton process, the electrochemical generation of H₂O₂ occurs at the cathode as follows—O₂ + 2e⁻ + 2H⁺ → H₂O₂. With the addition of ferrous ions, hydroxyl radicals are formed as follows—Fe²⁺ + H₂O₂ + H⁺ → Fe³⁺ + OH + H₂O, and Fe³⁺ + e⁻ → Fe²⁺.

A solution with MoS₂ nanosheets and FeSo₄ was added to the electrolytic cell. The pH of the mixture was adjusted to 3 by slowly adding single drops of sulfuric acid. A potential of −0.5 V was applied and the solution was continuously saturated with O₂ by bubbling compressed air. The mass transfer was ensured by continuously stirring the solution using a magnetic bar stirrer. The reaction was allowed to continue for one hour before the products were collected and dialyzed in sulfuric acid to remove iron ions and other impurities. An additional day of dialysis in ultrapure water was used to ensure the removal of sediments induced by the hydrolysis of iron. The as-prepared QDs ranged from 3 to 8 nm laterally and most were below 2 nm thick, indicating mostly bi- and monolayer quantum dots.

3.1.4. Thermal Ablation

Chemical vapor deposition (CVD) is a common technique used to synthesize 2D materials; however, it has had minimal success with fabricating quantum dots. Park et al. demonstrated a method to convert CVD-grown MoS₂ to MoS₂ QDs using the same CVD equipment [72]. MoS₂ nanosheets were grown on a SiO₂/Si substrate using conventional CVD techniques. Following the growth of the nanosheets, the substrates were placed in a quartz tube and heated in a tube furnace to 500 °C in ambient air and maintained for several days. The sizes of the MoS₂ domains were decreased from μm to nm through this annealing process, and the density of QDs on the substrate was determined to be 10¹¹/cm²; AFM images of these QDs can be found in Figure 12.

3.1.5. Microwave Heating

Lu et al. pioneered a cost-effective, convenient, and eco-friendly way of synthesizing MoS₂ QDs and histidine-doped MoS₂ QDs through a microwave heating process [73]. In their work, MoS₂ nanoflake solution, hydrogen peroxide, ethanol, and a stir bar were loaded into a microwave tube with the frequency set to 2445 MHz. The tube was heated to 200 °C and maintained for 30 min. The reaction solution was allowed to cool to room temperature and was left undisturbed for one day. The solution was filtered with a syringe filter (0.22 µm) and was then further purified by dialysis for 48 h. For the histidine-doped QDs, the same procedure was followed with the alteration that L-histidine powders (molar mass 155.157 g mol⁻¹) were also added to the tube before heating.

3.2. Bottom-Up

The top-down approach of synthesizing MoS₂ QDs is associated with some postprocess treatments such as centrifuging, filtration, and dialysis. The bottom-up approach is atom-by-atom manner using molybdenum and sulfur precursors giving respective ions to form MoS₂ QDs.

3.2.1. Hydrothermal Synthesis

Techniques of hydrothermal synthesis use high-temperature aqueous solutions and high vapor pressures to crystallize precursors into the desired material. These techniques typically use low-cost equipment (often only a stainless-steel autoclave and a furnace). One downside of hydrothermal methods is that they can produce by-products that can be difficult to separate from the desired product. Leading methods of hydrothermal synthesis of MoS₂ QDs use small sulfur-containing molecules to act as reduction agents to reduce molybdate (MoO₄).

For one example, Wang et al. synthesized MoS₂ QDs hydrothermally by reacting sodium molybdate (Na₂MoO₄ •2H₂O) and glutathione (C₁₀H₁₇N₃O₆S) [74]. Sodium molybdate was dissolved in water and sonicated. The pH of the solution was adjusted to 6.5 using 0.1 M HCl. Glutathione and additional water were added to the mixture and sonicated. The mixture was transferred to a Teflon-lined stainless-steel autoclave and reacted at 200 °C for 24 h. After being allowed to cool naturally, the solution was centrifuged at 9000 rpm for 5 min and the supernatant containing the MoS₂ QDs was collected.

Other researchers have used a similar process using L-cysteine [75], dibenzyldisulfides [76], thiourea [77], sodium sulfide [78], thioglycolic acid [79], and N-acetyl-l-cysteine [69], as summarized in Table 2.

3.2.2. Chemical Bath Deposition

Vikraman et al. developed a low-cost and simple chemical bath deposition (CBD) process to synthesize atomic layers of MoS₂ [39]. A precursor bath was mixed using 10–30 mM ammonium molybdate and 0.5 thiourea. HCl was used to maintain the pH of the bath at 10.0 ± 0.1. The reaction took place in the presence of 1.0 M hydrazine hydrate. The deposition time was varied between 2 and 10 min to control the thickness of the layers. The temperature of the bath was maintained at 90 °C. The solution pH, thiourea concentration, bath temperature, and hydrazine concentration were optimized and fixed at the values specified above. Following deposition, the substrates were annealed in an S environment to improve the crystallinity. The annealing took place at 450 °C for 1h with a carrier gas flow rate of 100 sccm at a constant pressure of 2 × 10⁻² Torr. The reaction proceeds as follows:

$\frac{1}{7}{\left(N{H}_4\right)}_{6}M{o}_7{O}_{24}\cdot 4H_{2}O+2C{H}_4{N}_{2}S+\frac{6}{7}HCl\stackrel{N_2{H}_4}{\to }Mo{S}_2+\frac{6}{7}N{H}_{4}Cl+2C{O}_2+2N_2+5H_2$

4. Applications

Application of MoS₂ QDs can be summarized into four major areas: (1) energy, electronics, chemical sensing, and biomedicine, as schematically illustrated in Figure 13. These applications will be discussed in the forthcoming subsections.

4.1. Energy

4.1.1. Electrocatalysis

Hydrogen is a promising candidate to replace fossil fuels due to its high calorific value (~143 kJ/g) and as the only combustion by-product is water. Of the various methods to produce H₂, the photocatalytic splitting of water is widely regarded as the greenest as the process can be carried out at room temperature and atmospheric pressure under light radiation. The redox reaction begins when electrons in the valence band of the photocatalysts are excited to the conduction band via photoexcitation. These electrons are subsequently consumed to produce H₂. Thus, it is essential to develop efficient photocatalysts with suitable redox potentials, broad absorption, and high photostability. Traditionally, noble metals such as Pt, Au, and Ag have been used [84,85,86,87,88]; however, these materials are scarce and expensive. It is thus essential to develop efficient photocatalytic systems free of costly noble metals. An ideal replacement would possess a high exchange current density, low Tafel slope, low overpotential, and catalytic stability.

The free energy of atomic hydrogen bonding to MoS₂ edges was found to be close to that of conventional platinum. The electrocatalytic performance of MoS₂ was found to be tied to the number of exposed active sites, and thus the exposure of active sites (i.e., the increase in surface–volume ratio) results in improved performance. A summary of the use of MoS₂ QDs for electrocatalysis can be found in Table 3.

A promising low-cost and scalable approach to hydrogen evolution using MoS₂ QDs and graphene sheets was proposed by Najafi et al. [90]. The study used both the 1T and 2H polytypes of MoS₂. The HER electrocatalytic activity of these graphene/MoS₂ QD structures were tested in 0.5 M H₂SO₄ on a glassy carbon electrode. The overpotential at 10 mA/cm² cathodic current density of the graphene/2H MoS₂ QD structures was lower than that of the 2H MoS₂ flake structures (~60 mV), the 1T MoS₂ structures (~235 mV) and the graphene/1T MoS₂ flake structures (~151 mV). The Tafel slopes of the 2H MoS₂ flakes, 2H MoS₂ QDs, and 1T MoS₂ flakes were reported to be ~145, ~98, and ~78 mV/dec, respectively. This further demonstrates that, for the 2H MoS₂ flakes, the limited number of edge sites of flakes compared to quantum dots lowers electrocatalytic performance. The Tafel slope of the 1T MoS₂ flake structures further facilitates the posed Volmer–Heyrovsky mechanism. The Tafel slopes (shown in Figure 14) of the graphene 2H MoS₂ flake, graphene 2H MoS₂ QD, and graphene 1T MoS₂ flake structures were reported as ~163, ~141, and ~82 mV/dec, respectively—demonstrating increases of 0.13%, 0.44%, and 0.05%, respectively.

4.1.2. Solar Cells

Two-dimensional MoS₂ films have previously found application in solar cells as electron and hole transport layers (ETL and HTL, respectively) [92]. With this in mind and with the increasing popularity of QD-based photovoltaic technology, it is without great surprise that MoS₂ QDs have begun seeing applications in solar cells. A brief summary of the use of MoS₂ QDs in solar cells can be found in Table 4.

Organic–inorganic halide perovskite solar cells (PSCs) have gained a lot of attention from the photovoltaic community as high power conversion efficiency (PCE) can be obtained through low-cost, low-temperature processing with scalable potential [96,97,98,99,100,101,102]. The theoretical PCE limit of PSCs has been determined to be ~31%, with a practical value of 29.5% [103,104]. Two-dimensional materials are emerging in interface engineering to boost the PV performance. 2D materials offer useful electronic and optoelectronic properties that are tunable via morphological modification. Najafi et al. reported a CH₃NH₃PbI₃ PSC as shown in Figure 15, with a PCE exceeding 20% that used MoS₂ QDs with functionalized reduced graphene oxide (f-RGO) hybrids as the HTL and active buffer layer (ABL) [82]. The MoS₂ QDs exhibited both hole-extracting and electron-blocking properties, making them ideal for the HTL and ABL. The effects of quantum confinement increase the optical band gap of MoS₂ from 1.2 eV in bulk to >3.2 eV for QDs, and this raises the minimum energy of the conduction band of MoS₂ above the lowest unoccupied molecular orbital (LUMO) of methylammonium lead iodide (MAPbI₃), which in turn blocks electron injection in the HTL. The creation of MoS₂: f-RGO hybrids is necessary for a homogenous HTL. Reduced graphene oxide (RGO) was chosen to “plug the pinholes” in the MoS₂ QD films for two reasons: the first is that the (3-mercaptopropyl)trimethoxysilane (MPTS) molecules effectively anchor onto RGO, and the second being that exposed thiol moieties enable f-RGO to participate in S-S van der Waals physisorption and/or S-vacancy passivation/filling. As shown in Figure 16, the tested device exhibited a PCE of up to 20.12% with an average PCE of 18.8%. Najafi et al. speculate that “graphene interface engineering” will play a significant role in the future of efficient and stable perovskite solar cells [82].

The organic cations used in PSCs are hygroscopic and have led to device instability. In addition to PSCs, bulk heterojunction (BHJ) organic solar cells (OSCs) have also been at the forefront of photovoltaic research due to their low-cost production, flexibility, and light weight. A typical BHJ cell is composed of electrodes to control the charge flow, an active layer to absorb light, and interfacial layers for the extraction of charge carriers. Traditionally, Poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) has been used as the HTL or hole extraction layer (HEL) in OSCs. However, PEDOT:PSS’s acidity [105] and hygroscopicity [106] limit device stability and practical application. Recently, 2D materials have seen application as HEL in BHJ due to their lamellar and electrical structures [107,108,109,110,111,112,113,114,115,116,117,118,119]. In the work carried out by Xing et al., UV–ozone treated MoS₂ (O-MoS₂) with a tunable work function was used as an efficient HEL material for high-performance BHJ OSCs [95]. The reported OSCs used different donor materials, Poly(3-hexylthiophene-2,5-diyl) (P3HT) and Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b] thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th), and the PCE for the PTB7-Th cell was 8.7%, which is slightly higher than the PCE for OSCs with PEDOT:PSS as the HEL.

Hybrid structures-based phototransistors have been used to achieve high-performance optoelectronic devices. Ulganathan et al. fabricated a phototransistor with InSe nanosheets decorated with MoS₂ QDs (see Section 4.2.2) and used the device as the active layer in a solar cell [94]. A schematic representation of the device can be found in Figure 17. In this device, TiO₂ was used at the ETL, and 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (Spiro-OMeTAD) was used as the HTL. The PCE of this hybrid solar cell was determined to be 3.03%.

4.1.3. Energy Storage

Conventional lithium-ion battery (LIB) technology uses graphite-based materials as the anode, but the low capacity of graphite (~372 mA h g⁻¹) prevents current LIBs from seeing application in electric vehicles and load leveling of power grids. To address this, the search for novel anode materials broadened, revealing that metal sulfides are capable of achieving high reversible capacity and long cycle life [120,121]. MoS₂ is readily intercalated by Li⁺, enabling a lithium storage capacity that is 80% higher than graphite (~670 mA g⁻¹). Due to its low electrical conductivity, high volume change during lithiation, and slow lithium storage kinetics, MoS₂ has not been readily applied as a LIB anode material. However, when MoS₂ is combined with graphene nanosheets, the nanostructure results in a shortened charge transport path and a high strain tolerance, which shows promise for LIB applications. Wang et al. studied MoS₂ nanoparticle-bonded graphene sheets as LIB anode materials [9]. The researchers began with synthesizing MoO₂ QDs onto graphene sheets, and then used a sulfide treatment to convert the MoO₂ to MoS₂. These MoS₂/graphene composites were found to have a higher capacity of 1497 mA h g⁻¹ at 100 mA g⁻¹ with remarkable rate retention and cycling stability.

In addition to LIB, supercapacitors are a promising candidate for flexible energy storage devices due to their high durability, quick charge–discharge ability, long cycling stability, and high power density [122,123,124,125,126,127,128]. Supercapacitors using TMDCs (specifically MoS₂) and conducting polymer polyaniline (PANI) have recently been used [129,130,131,132,133]. MoS₂ sheets easily aggregate due to the interlayer van der Waals interactions, so it is necessary to use other materials to improve dispersion. PANI hydrogels exhibit high electrical conductivity, high porosity, and high specific surface area. In a recent work, Das et al. synthesized a 3D nanostructured conductive PANI MoS₂ hydrogel for the fabrication of a high energy density supercapacitor device [134]. The prepared gel had a high capacitance value of 791 F/g at 1.0 A/g in a three-electrode system. Using a similarly prepared gel in a two-electrode all-solid-state device, the capacitance remained relatively high at 331.2 F/g at a current density of 1.0 A/g. This hybrid xerogel was demonstrated to have a high energy density of 29.4 Wh/kg at a power density of 398 W/kg and high cyclic stability of 84.2% retention after ten thousand cycles. The all-solid-state device’s practical utility was demonstrated by powering various LEDs.

4.2. Electronic and Optoelectronic Devices

4.2.1. FETs

Transistors fabricated with MoS₂ monolayers demonstrate excellent on/off ratios and high mobilities, making them suitable candidates for next-generation transistors [135,136,137,138,139,140]. Additionally, atomically thin MoS₂ crystals have been shown to exhibit mechanical and optical properties that make them useful for backplane thin-film transistors for flexible, transparent displays [141].

Vikraman et al. fabricated bottom-gated 2D FETs from MoS₂ QDs [39]. The device structure and contacts were accounted for by patterning contact points using photolithography and reactive ion SF₆/O₂ plasma etching. Source and drain electrode patterns with Ti/Au (10/50 nm) were deposited on top of the MoS₂ film via e-beam evaporation. After forming the electrode contacts, the device was annealed at 200 °C for 2 h in a vacuum tube under 100 sccm Ar flow to remove residual photoresist and to reduce the contact resistance. The electrical properties were measured using a two-probe method at room temperature in a vacuum chamber. The MoS₂ QD devices exhibit typical n-type behavior. The linear I_DS-V_DS characteristics can be attributed to the reduced charge injection Schottky barriers. At V_DS = 1 V, the on/off ratio was measured at ~10⁵, the transconductance was measured at ~2.49 × 10⁻⁸ S, and the field-effect mobility was measured at 0.21 cm²V⁻¹s⁻¹. These values are comparable to monolayer MoS₂ devices made by more conventional and expensive methods [142,143,144].

4.2.2. Photodetectors and Phototransistors

Phototransistors based on hybrids of 2D materials have recently been demonstrated to have high detection sensitivity and wide wavelength response, providing potential for frontier photodetector technology. As shown in Figure 18, Ulganathan et al. combined MoS₂ QDs with indium selenide nanosheets (henceforth denoted as MoS₂/InSe) and used the hybrid nanostructure as the conducting channel of a high-performance broadband photodetector [94]. In an earlier study [145], Ulaganathan et al. demonstrated the photoresponsitivity of an InSe FET to be 12.3 A W⁻¹. The MoS₂/InSe device exhibited photoresponsivity almost one thousand times as high, recorded at 9304 A W⁻¹. This photoresponsivity enhancement was studied in the NIR and visible regions. Under this radiation, the junction interfaces and trap states of the MoS₂/InSe FET capture only one type of charge carrier to delay the recombination of electron-hole pairs in the wake of photoabsorption. As only one type of carrier can be captured, the other charge carriers have prolonged lifetimes and can make several transits in the InSe channel, leading to a high photoconductive gain. Additionally, the photoinduced electrons in the MoS₂ QDs can transit to the InSe channel as additional charge carriers.

π-Conjugated polymers have gained interest in semiconductor devices due to their tunable conductivities and engineerable band gaps [146,147,148,149,150,151]. P3HT is one such polymer that is a p-type semiconductor and hole-transport material for optoelectronic applications [143,144,145,146,147,148,149,150,151]. A major setback with P3HT is that its photoconductivity has proven difficult to tune. In their 2018 work, Nair et al. demonstrated a method of controllably confining monodisperse MoS₂ QDs in P3HT fibers, leading to the enhancement and tunability of the photoconductivity of the QD-P3HT hybrid [152]. The encapsulation of MoS₂ QDs in the P3HT fibers assists with defect healing of the fibers, and the charge transfer process introduced by the QDs results in a tunable photoconductivity. The increased lifetime of charge carriers leads to an increased hole concentration in the P3HT fibers, consequently leading to a higher excited state population in the QDs. The combination of these effects provides enhanced fluorescence in the QDs, and enhanced photoconductivity in the QD-P3HT hybrid. The maximum photoconductivity of this hybrid occurred under an excitation wavelength of 360 nm and corresponds to an 82% enhancement of electric conductivity in comparison to pristine P3HT, as shown in Figure 19. This photoconductivity enhancement is of high interest in organic photovoltaics and optoelectronics as it paves the way for higher efficiency P3HT solar cells, and in a broader view, this work provides new opportunities for hybrid low-dimensional material design [152].

4.2.3. LEDs

QD-LEDs have been an attractive area of research due to their potential uses as low-cost backlighting for LCD displays with good color saturation and white lighting with a high color rendering index. QD-LEDs have high color purity (narrow full-width, half-maximum), easily processable solutions, and good stability. As the wavelength of emitted light is controlled by the size of the QDs, it is possible to synthesize QDs that cover the whole visible light spectrum. White LEDs (WLEDs) consist of RGB QDs and they are attractive due to their small size, light weight, high efficiency, low power consumption, and long working lifetime [153,154,155]. Conventional QD-WLEDs use cadmium-based molecules, and due to the toxicity of cadmium, new QD emitters are needed for mass adoption of QD-LED technology. Recently, researchers have made progress fabricating QD-LEDs with MoS₂ QDs [73,156].

Histidine-doped MoS₂ QDs exhibit a broadband emission property with emission intensities much higher than undoped pristine MoS₂ QDs. In the work by Lu et al., the authors fabricated a proof-of-concept histidine-doped MoS₂ QD-WLED [73]. PEDOT:PSS solutions were spin-coated onto ITO-coated glass at 2000 rpm for 30 s and baked at 140 °C for 30 min before depositing the next layer. The histidine-doped MoS₂ QDs were similarly spin-coated at 2000 rpm for 30 s and baked at 80 °C for 30 min. The TPBi (2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) and Ag electrode was deposited using a thermal evaporation system through a shadow mask under vacuum at a pressure of 1.5 × 10⁻⁵ torr. The final device had an area of 0.05 cm² defined by the overlapping area of the ITO and Ag electrode. The device exhibited strong broadband white-light emission, with Commission Internationale de l’Eclairge chromaticity coordinates of (0.30, 0.36).

Biopolymer-based materials have recently been used in organic LEDs (OLEDs) [156,157,158]. When embedded in DNA hydrogels, QDs can act as pseudo-cross-linkers that enhance the gel strength and the melting temperature. As shown in Figure 20, Pandey et al. embedded MoS₂ QDs in a DNA hydrogel matrix to fabricate OLEDs [156]. The formed nanocomposite hydrogels exhibited enhanced gel strength and higher melting temperatures. Furthermore, the gel exhibited blue fluorescence. As shown in Figure 21, the device achieved a maximum luminance of 37,420 cd/m² and exhibited a current efficiency of 19 cd/A. The systematic enhancement of the properties of polyanionic hydrogels caused by colloidal particles of the same polarity is poorly understood; however, these hybrid materials have great potential due to the unusual properties demonstrated here.

Silicon-based optoelectronics are limited due to silicon’s indirect band gap [159], and so III-V group materials are predominantly used in these devices. However, III-V technologies have difficulty coupling to silicon platforms and they suffer from heat dissipation [160]. It would prove useful to develop LEDs that could be readily integrated into existing silicon electronics. Shrivastava et al. incorporated MoS₂ QDs with porous silicon (PS) to enhance the electroluminescence properties of PS nanostructures [160]. The PS/MoS₂ QD nanostructures exhibit excitation-dependent bands due to the partial oxidation of PS. The incorporation of MoS₂ greatly enhanced the F-band due to the charge transfer of photo-generated carriers of MoS₂ QDs to PS nanocrystals, resulting in increased radiative recombination. The incorporation of the MoS₂ QDs also resulted in the complete suppression of the S-band due to the blocking of the irradiation centers on the PS surface. The MoS₂ QD embedded PS exhibited concentration-dependent electroluminescence in the blue region—this is due to the increase in the number of injected charge carriers for radiative recombination. Remarkable electroluminescence enhancement was observed.

4.2.4. Resistive Switching/Memory Devices

As discussed earlier, MoS₂ exists in many allotropes; the 2H form being semiconducting with a thickness-dependent bandgap between 1.2 and 1.9 eV, and a metallic 1T phase [30,161,162]. The 1T phase can form when alkali ions or metal nanoparticles are intercalated between MoS₂ nanosheets [163,164,165,166,167,168]. This is because the electron transfer from the intercalated atoms to the MoS₂ destabilizes the 2H structure and favors the 1T coordinated Mo atoms [162,163,164]. It has previously been reported that the 2D crystals can be transformed from metallic to semiconducting phases via photoinduction with sub-picosecond dynamics [169]. Fu et al. demonstrated a reversible 2H-1T phase transition of a MoS₂ nanosheet/QD (2D/0D) structure controlled by photoexcitation of the QDs, and used the transition as a dynamic photoresistive memory device [170]. The QD and nanosheet dispersion was deposition between two graphene electrodes on a SiO₂/Si chip. As shown in Figure 22, when the structure is illuminated, photoresistive switching can be observed at lower voltages due to the additional transfer of excited electrons from the QDs to the nanosheets. These memristive states are maintained when voltage is applied across the electrodes. This resistive switching allows for similar devices to be implemented in artificial neural networks configured with a wide optical range.

Using the MoS₂ QDs synthesized in an earlier section, An et al. fabricated a resistive switching device based on the oxygen movements in oxidized QDs [68]. The operating mechanism of the device can be explained in terms of ion drift, conducting filament formation and rupture, and the trapping and detrapping of charge carriers. As pristine 2H MoS₂ does not have intrinsic resistive switching behavior, it is necessary to functionalize the QDs to use them in such a device. Oxidation is one of the simplest forms of functionalization, and conveniently the QDs in this study underwent spontaneous oxidation during the laser exfoliation process described in Section 3.1.2. A Au/Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA)/MoS₂ QD/TiO₂/FTO resistive device was fabricated and its electrical properties characterized as shown in Figure 23. Following continuous cycling of bias voltages ranging from ±2.2 V, constant hysteresis loops were observed. From 0 to ~1.9 V, the current was fairly low, corresponding to the OFF state, and at around 1.9 V the current suddenly and substantially increased to the ON state. Under the opposite bias, the current remained high until ~1.8 V (ON), until it dramatically decreased (OFF), which is characteristic of unipolar resistive switching. The characteristics were identical but inverted for negative biases. The retention time of the ON and OFF states was approximately 10 s, which is short compared with 2D MoS₂ devices. The resistances of the states were calculated using Ohm’s law using the average current, and their values were 9.72 ± 0.14 Ω and 354.55 ± 11.04 Ω for the ON/OFF states, respectively. Similar devices without the inclusion of the MoS₂ QDs did not exhibit this behavior. Past studies suggest that the resistive switching behavior of most MoS₂-based devices is mostly caused by ion drift across oxygen, and many MoS₂-based devices exhibit bipolar characteristics [171,172,173,174]. The unipolar behavior of this device can be similarly explained.

4.3. Chemical Sensors

2D materials have seen use as chemical sensors due to their high surface areas and reactivity. MoS₂ QDs have the added utility of tunable photoluminescence. Chemical detection using MoS₂ QDs is often carried out using fluorescence quenching techniques via photoexcited electron transfer (PET) [175] and is occasionally assisted by exploiting the Förster resonance energy transfer (FRET) [78,176] mechanism. Using FRET to detect requires that absorption spectra of the desired material overlap with the PL spectra of the sensor [177,178,179]. By reducing the size of the MoS₂ QDs, the photoemission wavelength can be tuned, making these QDs ideal for the chemical sensors. The use of MoS₂ QDs as chemical sensors is summarized in Table 5.

Global security concerns and environmental protection have made the detection of nitro aromatic explosives—particularly 2,4,6,-trinitrotoluene (TNT) and 2,4,6-trinitrophenol (TNP)—incredibly important [180,181,182,183,184]. Wang et al. demonstrated the capability of MoS₂ QDs to detect TNP; however, their sensor was tested in the presence of metal ions and not in the presence of other nitro explosives save TNT [75]. A few years after that publication, Haldar et al. fabricated sensors capable of sensing without the presence of metal ions and of sensing TNP in the presence of other nitro explosives [78]. The experiments were carried out by adding 4.5 mL of 1,4-Diaminobutane capped MoS₂ QDs in a 5 mL quartz tube at neutral pH. Varied concentrations (0.1–0.8 µm) of the explosive analytes were gradually added to the MoS₂ QD solution, and the PL quenching spectra were measured with a 340 nm excitation wavelength. As shown in Figure 24, the highest PL quenching was shown with only 50 µL of TNP, and the PL intensity of the MoS₂ QDs was reduced by roughly 50%. When other aromatic nitro compounds were added to the MoS₂ QD and TNP solution, the PL quenching remained high, as shown in Figure 24, demonstrating the high selectivity of these functionalized QDs towards TNP.

In addition to nitro explosives, MoS₂ QDs were also used for the detection of other organic molecules. For example, Huang et al. reported on the utilization of MoS₂ QDs as a fluorescent detector of tetracycline hydrochloride (Tc) [79]. Using a time-correlated single-photon counting technique, the excitation behavior of MoS₂ QDs in the presence of Tc was studied. As shown in Figure 25, fluorescence quenching was observed and might be attributed to a nonradiative energy transfer from excited states in the QDs to the Tc structure. The fluorescence quenching of other antibiotics was tested; however, minimal quenching was observed.

As another example, MoS₂ QDs were used for glucose sensing. Glucose has a vital role in living systems—particularly, blood glucose levels—and thus being able to accurately measure blood glucose levels is of high importance. Conventional blood glucose monitoring technology is hindered by high cost, complicated testing equipment, and complex sample preparation techniques [185,186]. Contrastingly, fluorescence assays have been demonstrated to be inexpensive, sensitive sensors. The glucose sensing capabilities of water-soluble MoS₂ QDs were demonstrated by Wang et al. [74]. As with other MoS₂ QD chemical sensors, the primary sensing mechanism was fluorescence quenching. In their sensor, the PL spectra of the MoS₂ QDs were quenched in the presence of H₂O₂, a compound formed through the oxidation of glucose by dissolved oxygen in the presence of glucose oxidase. The concentration of glucose can be determined indirectly by examining the intensity of the fluorescence quenching caused by the amount of H₂O₂ formed (see Figure 26). To further demonstrate the viability of this sensor, the researchers tested it in fetal bovine serum samples and the results agreed with their controlled study.

Functionalized MoS₂ QDs were also applicable for selective sensing of metallic ions and heavy metals. Heavy metal pollution poses a threat to environmental health and is known to lead to many biological and physiological conditions in humans. Cysteine-functionalized MoS₂ QDs have been demonstrated to be highly selective dual-model sensing capabilities of Fe³⁺ and Al³⁺ [83]. In the presence of Al³⁺, the PL intensity of the QDs rose tremendously, while the intensity roughly halved in the presence of Fe³⁺. The PL intensity was also measured in the presence of other common metals, and no significant quenching or enhancement was observed, demonstrating the high selectivity of the cysteine-functionalized MoS₂ QDs. The capabilities of the sensor were tested with tap water, lake water, and living cells to great success.

Label-free sensing of heavy metal pollution such as Pb was also demonstrated by the use of small-area MoS₂ nanosheets [77]. In the presence of Pb²⁺, MoS₂ nanosheets experienced a significant enhancement of fluorescence due to the formation of PbSO₄ on the surface of the nanosheets. The selectivity of the sensor was assessed by measuring the fluorescence response of 17 common metal ions at a concentration of 5.0 μm, and Pb²⁺ had a significantly greater enhancement than others (Figure 27). The feasibility of the sensor was tested using water from Longten Lake (Nanchang, China) and the performance was satisfactory. This study reinforces the heavy metal sensing capabilities of MoS₂.

4.4. Biological Applications

Given their high biocompatibility and photostability, MoS₂ QDs are great candidates for biological applications.

4.4.1. Bioimaging

Xu et al. use MoS₂ QDs to perform bioimaging tests on HeLa Cells [43]. For this, HeLa cells were incubated in DMEM containing MoS₂ QDs for 7 h to determine the cell permeability. As shown in Figure 28, intracellular fluorescence was observed in a confocal laser scanning microscope (CLSM). The overlapping of the fluorescence of MoS₂ probes and LysoTracker in cells indicates that MoS₂ QDs entered the cells by endocytosis. Additionally, the QDs were found in the cytosol of the perinuclear region, which proves MoS₂ QDs can penetrate the cell wall but cannot penetrate the nucleus. This reduces the risk of aberrant genetic mutation. Lin et al. also confirm that the fluorescent signal comes from the cytoplasmic region, indicating the usefulness of MoS₂ QD probes in cellular imaging [81]. Furthermore, Dai et al. used intracellular microribonucleic acid (miRNA) in HeLa and HaCaT cells as a model, to demonstrate the feasibility of MoS₂ QDs for intracellular molecular imaging analysis [60]. In this case, a molecular beacon (MB) probe of miRNA-21 was loaded in the MoS₂ QDs via the van der Waals forces between the nucleobases and the basal plane of MoS₂ QDs. The intrinsic blue PL from MoS₂ QDs and the red PL from the MB hybridization with the miRNA-21 can be differentiated, which suggested specific recognition of the miRNA-21 target.

MoS₂ QDs were also used for tumor cell imaging. For example, Shi et al. did fluorescent imaging in vivo and in vitro of SW480 tumor cells [187]. Gluthathione (GSH)–MoS₂ QD probes were injected into mice with colon cancer via their tail veins. Fluorescence imaging was performed on several organs before and after the injection. The fluorescence intensity and therefore the concentration of the QDs in the liver, heart, and spleen was increased for 24 h after the injection, whereas the concentration of QDs was increased for an hour and decreased after 24 hrs in the lungs and kidney. The possible clearance pathway of QDs in the latter case could be from liver to bile and feces. The clearance and the biodistribution of the QDs are organ-specific. Additionally, it depends on several other factors such as hydrodynamic size, surface charge, and surface modifications of the QDs [187].

Additionally, Chen et al. performed intracellular fluorescence imaging of GSH in HeLa cells [54]. The study showed 6-mercaptopurine functionalized monolayer MoS₂ QDs are better probed than other QD probes for GSH detection in terms of sensitivity, selectivity, practicality, and quantification [54]. In addition to these studies, many other researchers have also used MoS2 QD probes for bioimaging [61,67,188,189,190,191].

On the other hand, blue PL can be observed under the excitation at 700 nm, suggesting that MoS₂ QDs could serve for multiphoton imaging labeling. Since the excitation light is in the near IR region, it can prevent damages on living cells or biosystems that are often caused by UV or blue excitations. Dai et al. performed multiphoton imaging in the HeLa and HaCaT cells incubated with MoS₂ QDs [60]. As the PL property of MoS₂ QDs is not sensitive to the NIR excitation, no change was observed in the PL brightness even after continuous excitation over 30 min. Additionally, a negligible change was observed in cell morphology when excited at 700 nm, in contrast to the case when cells are exposed to UV irradiation for 30 min where apoptotic vesicles started to emerge. This result suggests that near IR excitation is less harmful than UV excitation for living cells (multiphoton is less harmful than single-photon excitation) [60].

Since MoS₂ QDs showed extremely high two-photon brightness, good biocompatibility, and high photostability, Sweet et al. used anti-PSMA (prostate-specific membrane antigen) attached MoS₂ QDs for multiphoton imaging of live prostate cancer cell [53]. In a series of selectivity tests, i.e., enzyme-linked immunosorbent assay was used in different cell lines, it was inferred that MoS₂-based two-photon imaging in biological II window is capable of distinguishing targeted LnCaP cells from nontargeted cells.

4.4.2. Photothermal Therapy (PTT) and Photodynamic Therapy (PDT) and Radiation Therapy (RT)

Current PDT agents/photosensitizers are suffering from the issues of photobleaching and low oxygen production rate. Since MoS₂ QDs are capable in producing highly reactive oxygen, singlet oxygen (¹O₂), [55] and are relatively photostable, they are proposed to be an excellent candidate as a PDT agent [55]. Additionally, MoS₂ QDs exhibit an enhanced antibacterial activity than MoS₂ nanosheets. As demonstrated by Tian et al., MoS₂ QDs can produce more electron-hole pairs than MoS₂ nanosheets when illuminated with simulated solar light [57]. These electron-hole pairs can produce reactive oxygen species (ROS) in the presence of water and oxygen to destruct the antioxidant defense system for enhancing the antibacterial activity. A feasibility study was performed on mice using Gram-negative, Escherichia coli (E. coli). Mice were found to be bacteria-free for 7 days after the treatment with MoS₂ QDs. Several mice organs were inspected and find no organ damage after being treated with MoS₂ QDs and the light illumination, thereby proving their effective photodynamic antibacterial activity [57].

Li et al. have explored the use of MoS₂ QDs for combined PDT and PTT effect [58]. Nanohybrids of MoS₂ and disulfide SiO2 (MoS₂@ss-SiO2) are conjugated with hydraulic acid (HA) and chlorin e6 (Ce6) for the combined PTT and PDT treatment of tumor cell. These integrated MoS₂@ss-SiO2-Ce6/HA nanohybrids were used for multimodel imaging, which includes fluorescence/CT/MSOT imaging (CT: computed tomography, MSOT: multispectral optoacoustic tomography). Since all the components in this nanohybrid are biocompatible/biodegradable, these nanoparticles could later be cleared through the renal route, which was later proved by CT imaging. The increased Hounsfield unit (HU) from 119.3 ± 24 to 225.2 ± 41.2 suggested predominant CT imaging capability and tumor-targeting capability of the MoS₂@ss-SiO2-Ce6/HA nanohybrid. Additionally, the MSOT signal intensity is also depicted linearly with the formulation concentration (1 to 8 mg/mL) of this nanohybrid, which suggests that this nanohybrid is a suitable MSOT contrast agent. This means the nanohybrid can be used to track tumors efficiently. Moreover, the performance of the Dichloro-dihydro-fluorescein diacetate (DCFH-DA) assay test on these nanohybrids showed the production of ROS generation, a requirement for PDT. The efficacy of the photothermal conversion efficiency was found to be 22.34%. When the tumor-bearing mice were irradiated with NIR radiation (808 nm, 1.5 W/cm²), the temperature at the tumor site in the MoS₂@ss-SiO2-Ce6/HA treatment group rapidly increased from 25 to 55.2 °C within 5 min, along with the suppression of the tumor size, proving the efficiency of the PTT effect. Additionally, the insignificant weight loss of the mice after 21 days shows the tumor inhibition activity. Additionally, the hematoxylin and eosin (H&E) staining of the organ tissue slices, depicted no significant necrotic cells in major organs in the mice treated by MoS₂@ss-SiO2-Ce6/HA nanoparticle, whereas tumor tissues had more necrosis and apoptosis in the combined therapy (PTT/PDT) for the control group [58].

Wang et al. studied the effect of MoS₂ QDs@polyaniline (MoS₂@PANI) nanohybrids as photothermal adsorbing agents and radiosensitizer [191]. There had been studies for TMDC QDs, but not for this kind of organic–inorganic hybrid. MoS₂ was chosen as a radiosensitizer, as it offers a high atomic number, whereas PANI offers great photothermal conversion efficiency, photostability, and biocompatibility. As a result, this nanohybrid exhibits a strong X-ray attenuation capability and a high NIR absorption efficiency. Therefore, these nanohybrids can be used for a combined PTT and Radiation Therapy (RT) on 4T1 tumor-bearing mice. For this study, PTT was carried out with laser irradiation at 808 nm (1.5 W cm⁻², 5 min), and RT was performed by X-ray radiation at a dose of 6 Gy. As shown in Figure 29, different control groups were used to inspect the enhanced efficacy of the treatment with MoS₂@PANI with the combined PTT/RT technique. H&E staining was studied to further confirm the antitumor efficiency. Severe destruction of tumor cells was detected with no noticeable sign of toxicity with the combined PTT and RT treatment. Additionally, effective reduction in tumor hypoxia was observed in different parts of a mice body due to enhanced oxygenation under hyperthermia immediately after mild PTT treatment. This combination therapy showed remarkably enhanced oxygenation in tumors immediately after PTT treatment [191].

4.4.3. Biosensing

One of the biosensing mechanisms is switching of fluorescence quenching of the bioreceptor-transducer after the detection of the analyte. Based on the same mechanism, MoS₂ QDs were used to detect dopamine and GSH in erythrocytes and live cells and nitro groups, respectively [37,77].

Fluorescence quenching can occur by various pathways, including electron transfer (ET) [77], FRET [192], and PET [78]. Fluorescence quenching that happened between PANI and MoS₂ QDs was due to FRET. This was used to detect bovine serum albumin (BSA) [192], where fluorescence will be recovered upon success detection. Several other organic and nonorganic molecules were used to inspect its selectivity [192]. Additionally, 1,4-di-aminobutane functionalized MoS₂ QDs were used as the fluorophores to detect the presence of the nitro group in an aqueous medium with >90% selectivity [78]. This high selectivity is due to the proton exchange between the electron-deficient nitro group and the functionalized MoS₂ QDs.

Swaminathan et al. suggested that spectrum overlap of the electron donor and acceptor molecules is needed for FRET to occur [192]. Ha et al. reported that MoS₂ can play the roles of both acceptor and donor [193]. The authors suggested that MoS₂ is a FRET donor with 11.73% of efficiency at a distance of 4.42 nm and can also act as a fluorescence quencher acceptor. For this dual function, an Alexa Flour 430 dye molecule was labeled with a double-stranded deoxyribonucleic acid (DNA) with a Guanine (G) base at the end and conjugated with MoS₂ QDs. The donor–acceptor distance in the nanoconjugate Alexa Flour 430-DNA-MoS₂ QDs was used to study the dual nature of MoS₂ QDs. The G base was used to tune the distance between the MoS₂ and the dye molecule. When there was no space between the molecules, MoS₂ acted as a fluorescence quencher acceptor whereas, in the increased distance up to 13 bases (of G) in between, it acted as a donor [193].

In addition to molecular sensing, MoS₂ QDs were exploited for use in DNA sequencing. Faramarzi et al. used time-dependent density functional theory and quasi-static finite and time-domain approach to show that Graphene oxide (GO) QDs and MoS₂ QDs can be used to sequence DNA [12]. It was proposed that the wavelength of the emitted light is the function of incident light and each type of DNA nucleobase can shift the emission wavelength by 1 to 130 nm. Additionally, this concept helps to figure out the unknown DNA nucleobase. The sensitivity and selectivity of this wavelength shifting in the presence of the DNA nucleobase method are higher than the ones with ionic, plasmonic, tunneling, and Raman methods.

4.4.4. Other Theranostic Applications

MoS₂ QDs offer an excellent platform for theranostic applications. In 2020, Yang et al. performed numerous analyses including cell cytotoxicity, cell survival analysis, enzyme-linked immunosorbent assay, immunoblotting assay ROS assessment, lysosomal contents and stability assessment, flow cytometry assay, and immunofluorescence [56]. From these studies, it was determined that MoS₂ QDs caused cell death of microglia cells, as MoS₂ QDs induce caspase-1-dependent pyroptosis of microglia cells. Results showed that the nucleotide-binding domain-like receptor protein 3 (NLRP3) inflammasomes were activated in microglia, as MoS₂ QDs trigger it. This can harm the stability of the central nervous system. Additionally, the mitochondria-derived reactive oxygen species (mtROS) triggers autophagy. This autophagy was found to diminish the activation of MLRP3 activation. It is suggested to use MoS₂ QDs rationally for theranostic use in neuroscience as it was found that autophagy prevented MoS₂ QD treated microglia from pyroptotic cell death.

Additionally, due to its biocompatibility and excellent performance as a biosensor, MoS₂ has been a promising candidate for drug delivery. Chen et al. performed a feasibility study by conjugating thiol-related anticancer drug and thiolated doxorubicin (DOX-SH) on monolayer MoS₂ QDs. The authors demonstrated in vitro and in vivo tests on the release of DOX-SH in the presence of GSH [54]. Additionally, Liang et al. [194] theoretically studied antituberculosis drug delivery, whereas Liu (2020) [195] and Dong (2018) [190] figured out the feasibility of chemotherapeutic drug DOX, using MoS₂ QDs.

5. Conclusions

Since the successful synthesis in 2014, MoS₂ QDs have gained a lot of attention for their photochemical properties, photostability, and biocompatibility. All the publications so far have shown that the synthetic route is simple and cost-efficient; however, more creativity is needed to scale-up the synthesis techniques to a much larger scale for real-world application of MoS₂ QDs. Thus far, the method of thermal ablation seems the most promising as it uses the same equipment as a large-area synthesis method of MoS₂ nanosheets [72,196]. Regarding potential applications, we are expecting to see more advances in electronic and photovoltaic applications of MoS₂ QDs. For example, one may attempt to use composite materials, i.e., combining 2D van der Waals materials with MoS₂ QDs, or QDs of various TMDCs in tandem, to broaden the absorption range for next-generation photovoltaics. Many of the current chemical sensing applications rely on fluorescence quenching (i.e., a turn-off method), and novel techniques using a fluorescence turn-on method could improve the accuracy of chemical detection. For the further development of biomedical applications, researchers should look for synthetic routes with higher quantum yields and longer fluorescence emission wavelengths, as many of the current QDs have low quantum yields and high-energy, blue emissions.

There is no doubt that MoS₂ QDs have already shown immense potential in sensors, bioimaging, phototherapy, photocatalysis, and biomedicine. We are expected to see creativity in the use of MoS₂ QDs for many other applications, for example, based on edge functionalization of these van der Waals nanostructures, and solar energy harvesting based on the broad light absorption nature of these environmentally friendly QDs. All these will rely on the high-yield synthesis of MoS₂ QDs with controllable sizes. Breakthroughs in large-scale synthesis and real-world applications will promote the future science, engineering, and real-world application of MoS₂ QDs.

Author Contributions

J.K., S.S. and A.A. have performed a literature study and discuss the findings with D.Z. and Y.K.Y. J.K. and S.S. wrote the manuscript. A.A., D.Z. and Y.K.Y. further discussed, reviewed, and edited the contents of the manuscript with J.K. and S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the Elizabeth and Richard Henes Center for Quantum Phenomena.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All data presented here are adopted from the published work cited in the references.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Table 1

Methods of sonication-assisted synthesis of MoS₂ QDs and experimental parameters.

Solvents	Notes	Emission (Excitation)	Size	Reference
Ethylene Glycol	Natural molybdenite suspended in the solvent and sonicated in a pressurized ultrasound reactor for 20 min, and then refluxed at 193 °C for 24 h. Dispersion filtered using Pragopor membrane.	440 nm (350 nm)	4–70 nm, 1 nm thick	[65]
Sulfuric Acid	MoS2 powder suspended in sulfuric acid and sonicated at 65 °C for 20 h. The solution was centrifuged for 30 min, dialyzed, and filtered.	425 nm (300 nm)	3.5 nm, 1–1.5 thick	[60]
DMF, NMP, DMEU, DI water, ethanol, acetone	Six solvents were tested. MoS2 powder suspended in a solvent and sonicated for 3 h. The top two-thirds were decanted and refluxed for 6 h below the solvent’s boiling point. The solution was allowed to sit for several hours, then centrifuged for 5 min. The supernatant was evaporated under vacuum, and the QDs were resuspended in DI water.	DMF: 465 nm (390 nm) NMP: 455 nm (380 nm)	DMF: 3.3 nm (avg.), 1.2 nm thickNMP: 3.4 nmDMEU: 3–4 nm	[43]
NMP	MoS2 powder dispersed in the solvent and sonicated it continuously in an ice bath for 3.5 h before tip sonicating it for another 3.5 h. The dispersion was left undisturbed overnight and then centrifuged for 90 min.	575 nm (400 nm)	0.5–4.0 nm (2.5 nm avg.)	[66]
Liquid Nitrogen + IPA	Heated MoS2 powder in a quartz boat to 340 °C under ambient air and maintained it for 3 min. Immediately after, it was quenched in a Dewar flask of liquid nitrogen. Once the liquid nitrogen was fully evaporated, quenched MoS2 was dispersed in IPA and sonicated for 30 min using a cycle of 7 s “on” and 3 s “off”. The solution was centrifuged for 30 min.	440 nm (360 nm)	1.41 nm (avg.), 1.5 nm thick	[67]

Table 2

Hydrothermal synthesis of MoS₂ quantum dots with various precursors.

Precursors	Emission (Excitation)	Size	Reference
Sodium molybdate + Glutathione	425 nm (340 nm)	2.7 ± 0.3 nm (avg.) spherical	[74]
Sodium molybdate + L-cysteine	402 nm (308 nm)	3.5 nm, 1–1.5 thick	[75]
Sodium molybdate + Dibenzyl-disulfides	280 nm (205 nm)	DMF: 3.3 nm (avg.), 1.2 nm thickNMP: 3.4 nmDMEU: 3–4 nm	[75]
Sodium molybdate + Thiourea	406 nm (250 nm)	0.5–4.0 nm (2.5 nm avg.)	[77]
[(NH4)6Mo7O24 • 4H2O] + Sodium Sulfide		1.41 nm (avg.), 1.5 nm thick	[78]
Molybdenyl acetylacetonate + ThioglycolicAcid + Sodium Sulfide			[79]
(NH4)6Mo7O24 + Thiourea + N-acetyl-l-cysteine	480 nm (380 nm)	2.1 nm (avg.) < 0.9 nm thick	[69]
(NH4)2MoS + N2H4	400 nm (330 nm)	2.8 nm (avg.) 1.4–2.5 nm thick	[80]
(NH4)2MoS + Oleylamine	575 nm (500 nm)	4.5 ± 0.5 nm (avg.) 3 nm thick	[81]

Table 3

Use of MoS₂ QDs in HER.

Material	Overpotential (mV)	Tafel Slope (mV/dec)	Reference
MoS2 QDs in Aerogel	53	41	[89]
MoS2 QDs between MoS2 Nanosheets	190	74	[66]
MoS2 QDs on Glassy Carbon	210	60	[44]
MoS2 QDs	140	66	[48]
MoS2 QDs on Graphene Flakes	136	141	[90]
MoS2 QDs	160	59	[76]
Fe-doped MoS2 QDs		121	[91]
Li-doped MoS2 QDs		109	[91]
Mg-doped MoS2 QDs		91	[91]
MoS2 QDs on Au	130	94	[39]
MoS2 QDs	120	115	[44]

Table 4

Uses of MoS₂ QDs in solar cell technology and reported power conversion efficiencies.

Design	PCE (%)	Reference
MoS2 on F-doped SnO2 as a Counter Electrode	3.69	[93]
MoS2 QDs/InSe–FET	3.03	[94]
MoS2 QDs with UV–ozone on MoS2 Nanosheets	8.66	[95]
MoS2 QDs on Graphene Flakes	20.12	[82]

Table 5

Chemical Sensing Capabilities of MoS₂ QDs.

Chemical	Selectivity	Concentration	Reference
2,4,6–trinitrophenol	F0/F = 1.42	10.0 μm	[75]
2,4,6–trinitrophenol	F/F0 = 0.95	1.0 mM	[78]
Pb	(F- F0)/F0 > 0.9	5.0 μm	[77]
S	(F- F0)/F0 > 0.3	10.0 μm	[77]
Tetracycline hydrochloride	(F- F0)/F0 > 0.4	0.05 mM	[79]
Glucose	(F- F0)/F0 = 0.17	0.5 mM	[74]
Al	(F- F0)/F0 > 2.7	1.0 mM	[83]
Fe	(F- F0)/F0 > 0.5	1.0 mM	[83]