Solubility of Monolayer MoS2 and Expected Bioabsorbable LED

Transient electronic technology is a new technology, which is characterized by the ability to dissolve, decompose in a controlled way. Expected adhibitions include bioabsorbable or biodegradable medical implants, hardware-safe storage devices and biodegradable sensors.2D materials may play a vital role in these systems due to their unique electrical, thermal, mechanical and optical properties. Monolayer MoS2 is a recently discovered biosoluble two-dimensional material. Monolayer MoS2 has unique semiconductor characteristics and biological absorption properties. Single-layer MoS2 is a direct band gap semiconductor material, and it has a high electron mobility, good current switching ratio, very low power consumption. These characteristics of single-layer MoS2 make it have great potential in electronic/optoelectronic devices, sensors, photothermal therapeutics, and biomedical applications. In this aspect, the ability to dissolve single-layer MoS2 in the biological liquid can be used to regulate the performance and life of the relevant bio-absorbable devices and systems. Based on monolayer MoS2, we designed a bio-absorbable LED. Furthermore, the wet transfer process of single layer MoS2 was optimized to some extent.


Introduction
Molybdenum disulfide is a pivotal member of the layered transition metal dihalide class (TMDC) of materials.It has attracted great interest in the past few time on account of its applications in electronic, photoelectric, physicochemical and biomedical sensors [1][2][3][4][5].
Single-layer MoS 2 can gradually oxidize in air or dissolve in a water solution after a few days, causing its environmental degradation [6][7][8]. Because of these characteristics, single-layer MoS 2 is a well suited material for bioabsorbable electronic devices (for instance biosensors and short time biomedical sensors), they can be completely absorbed after implantation in the human body, and the implant can dissolve in the body without a second operation. The long-term cytotoxicity and immunobiocompatibility of monolayer MoS 2 grown by chemical vapour deposited (CVD) have been reported in biological fluids and tissues of live animal models [9,10]. Temporary monitoring of various intracranial physiological activities in animals of single-layer MoS 2 based on bioabsorption and multi-function sensors has also proven that such technology is in the short-term diagnosis/treatment function during the recovery process of traumatic brain injury has a specific clinically relevant role [10].

Dissolve Model
The biodegradation process of single-layer MoS 2 crystals in PBS solution is caused by intrinsic defects in the granules or along the grain boundaries (GBs) [14,15].In phosphate buffer saline (PBS) solution, the probable reaction during the biodegradation process of MoS 2 was studied. Because the dissolution test of single-layer MoS 2 involved NaCI, KCI, Na 2 HPO 4 , KH 2 PO 4 , O 2 and H 2 O in PBS, the possible reactions are as follows [9,10,14]: where the molybdate ion (MoO 4 2− ) is the main Mo-containing byproduct, which has been confirmed by inductively coupled plasma-mass spectrometry (ICP-MS) measurements [9,14]. Note that Na + and K + ions in the PBS solution may lead to lattice distortions of MoS 2 and the formation of Na 2 S (from 2H-MoS 2 to 1T-NaMoS 2 and then to soluble Na 2 S). Therefore, the increase of Na + and K + concentration will speed up the degradation process. In addition, MoS 2 will be oxidized by oxygen in the PBS solution to generate MoO 4 2-, and MoO 2 2itself is easily dissolved (Eq. 3). Besides, based on Reaction Eq. 3, the increase in pH caused by increasing the OH -1 concentration can further accelerate the reaction rate. The following empirical kinetic equations regarding pH dependence in room temperature are given [9,14]: Then, Eq.4 can be calculated by ordinary differential transformation: (5) where y is the dissolution percentage, the units of the t and OH -1 concentration are days and mol/L, respectively. According to Eq.5, the dissolution kinetics simulation shown in figure 1 (a) can be obtained.  From the previous research report [16], the temperature dependence of the single-layer MoS 2 can be derived, the rates can be written as: (6) K B means Boltzmann constant, E a means activation energy.
According to the literature, the activation energy at pH 7.4 is ~105 mev and the activation energy at pH 12 is ~147 mev [9].We can simulate the temperature dependence curve of single layer MoS 2 at pH 7.4 and pH 12 in 37 °C PBS solution by Eq.6, as shown in figure 1(b).

Transfer of 1L-MoS 2 onto Different Substrates
Generally, the necessary step to transfer a single layer of MoS 2 grown by CVD using a wet method is to use a polymer layer (such as PMMA) as a support layer [17].PMMA has many outstanding characteristics, such as relatively low viscosity, excellent wetting ability, flexibility and good solubility in several organic solvents. After a series of experiments, we optimized the single-layer MoS 2 wet transfer as follows [18]: First, clean the silicon wafer: place the silicon wafer in acetone solution and sonicate for 10 minutes, then in ethanol solution for 10 minutes, then rinse with deionized water, and finally dry with nitrogen and plasma cleaner (Enhance the hydrophilicity of the target substrate) for 10 minutes; coat a thin layer of PMMA (950 A4, 40 g/L) to single-layer MoS 2 which growing on SiO 2 with a dropper. Place the silicon wafer on a heating table (100°C) for 30 minutes to cure PMMA; after cooling to room temperature, float the chip in 30% KOH (aq) for 20 minutes, where the Si substrate sinks due to the etching of SiO 2 .The PMMA/ MoS 2 layer floats on the KOH solution. Next, wash PMMA / MoS 2 for 4 times in DI water, use the target substrate remove the molybdenum disulfide film from bottom to top, and dry it naturally for more than 4 hours. Then put it in a drying oven at 100°C for 30 minutes to evaporate the remaining water and improve the combination of MoS 2 and substrate. Remove PMMA with warm acetone, soak in ethanol/isopropanol for 30 minutes. In the end, blow dry with nitrogen and store. Figure 2

LED Device Design
The wide application of light-emitting diodes (LEDs) in displays and lighting sources, the increase in production speed and the shortened lifespan have led to a corresponding increase in concerns about environmental pollution caused by related electronic waste generated [19]. Many components in commercial LEDs, such as organic emitters, metal electrodes, and connecting wires, have high chemical stability, and they have the risk of releasing toxic heavy metals after use [20]. The cost of waste management and potential environmental hazards have prompted research into bio-soluble LEDs that can degrade into non-toxic substances under natural conditions. In addition, biomedical implants, including LEDs, have the potential to provide unprecedented opportunities in diagnostic and therapeutic functions, and biodegradable LEDs can provide useful functions that cannot be provided by existing equipment combinations in this field [21].
The transient effect achieved by bioabsorbable eliminates the traces of the equipment, thereby avoiding the cost, complexity and risk associated with the secondary operation of equipment retrieval.Such a system usually requires a full set of bioabsorbable electronic materials (including semiconductors, dielectrics and conductors) as the basic modules to achieve functions [2,22]. Moreover, this system must not only consider its basic electrical properties, but also consider the degradation chemistry and biocompatibility of electronic materials and their products after reacting with biological fluids.
Although a lot of research has been done on optical fibers, photovoltaic cells and photodetector cells in the body [1], the development of bioabsorbable optical components has been neglected compared to similar photovoltaic products, and fully bioabsorbable LEDs were not reported until autumn 2019.According to the previously reported bioabsorbable LED, a bioabsorbable LED structure based on a monolayer MoS 2 was designed.as shown in figure 4.   4 ] to biological systems [9,19,22].Therefore, they are also harmless to the environment.

Conclusion
The dissolution mechanism of single-layer MoS 2 is analyzed. According to the dissolution kinetics equation, the dependence of single-layer MoS 2 on temperature and pH dependence curve is simulated. The dissolution rate depends on the temperature, pH value and the type and concentration of ions in the biological fluid. Considering the dissolution kinetics of single layer MoS 2 in PBS solution, high pH and high temperature will result in higher dissolution rate. And according to the previously reported bioabsorbable LED, designed a bioabsorbable LED based on a single-layer MoS 2 . The final degradation products either have low toxicity to biological systems or are necessary for life. Therefore, they are also harmless to the environment. And the wetting transfer process of single-layer MoS 2 during device manufacturing was optimized, the substrate transfer film processed by the plasma cleaning machine is better.