《Nature Nanotechnology (IF:40.523)》EPFL Andras Kis & Aleksandra Radenovic Explore the Optoelectronic Potential of Monolayer Molybdenum Disulfide

2D materials are a class of emerging materials with wide-ranging electrical properties and potential practical applications. While graphene has been extensively studied as a 2D material, monolayers of other materials such as insulating boron nitride (BN) and semiconducting MoS2 or WSe2 have garnered increasing attention for their application in field-effect transistors. Professor Andras Kis, from the Department of Electrical Engineering at EPFL (école Polytechnique Fédérale de Lausanne), and Professor Aleksandra Radenovic, from the Department of Bioengineering, have been at the forefront of this research. As early as 2013, they published an article titled "Ultrasensitive photodetectors based on monolayer MoS2" in the journal Nature Nanotechnology, focusing on single-layer MoS2. Due to quantum mechanical constraints, it is a semiconductor with a direct bandgap and explores its potential applications in optoelectronic devices, particularly photovoltaic cells.

Single-layer MoS2 has a hexagonal structure with a molybdenum atom sandwiched between two sulfur atoms. The bandgap of single-layer MoS2 is about 1.8 eV at the K point in the Brillouin zone, which is suitable for absorbing visible light. A single monolayer of MoS2 can absorb 10% of incident light with energy above the bandgap. Moreover, single-layer MoS2 exhibits strong photoluminescence due to the direct bandgap transition, which can be enhanced by 1000 times compared to the bulk crystal. However, the photoluminescence quantum yield of single-layer MoS2 is still low, about 0.4%, which limits its efficiency as a light emitter.

On the other hand, single-layer MoS2 has shown great promise as a light absorber and photocatalyst. Recently, researchers have demonstrated an ultrasensitive single-layer MoS2 phototransistor with improved device mobility and on-current. The device exhibits a maximum external light responsivity of 880 A W at a wavelength of 561 nm, which is comparable to the best reported values for other 2D materials. The high responsivity is attributed to the efficient generation and separation of electron-hole pairs under photoexcitation in single-layer MoS2.

This study investigates the effect of different pre-deposition surface treatments and contact materials on the attenuation of the photoresponse. Different surface cleaning treatments were found to reduce the decay time. This can be explained by the difference in the hydrophilicity and hydrophobicity of the functionalized SiO2 surface. Using different growth techniques such as wet oxidation and dry oxidation to deposit SiO2 can further reduce the characteristic decay time τdecay. We use the following SiO2 surface treatment methods for pre-micromechanical exfoliation: The KOH-SiO2 substrate was soaked in 30% KOH solution for 30 minutes at room temperature. Then O2 plasma treatment was performed with 270W RF power for 20 minutes, and finally lift-off was performed. Piranha - SiO2 substrates were soaked in piranha cleaning solution (H2SO4:H2O2 3:1) for 45 minutes. Then O2 plasma treatment was performed with 270W RF power for 20 minutes, and finally lift-off was performed. The HF-SiO2 substrate was soaked in 2 ml of 50% formic acid and 70 ml of deionized water for 30 seconds. Then O2 plasma treatment was performed with 270W RF power for 20 minutes, and finally lift-off was performed. Using different contact metals, such as Ti/Au (10/50nm) or Cr/Au (10/50nm), can further reduce the decay time, but at the expense of photoresponse sensitivity, the lowest decay time τdecay is 320 ms, SiO2 devices grown using Cr/Au contacts and wet oxidation.

Another exciting application of single-layer MoS2 is in photovoltaic cells, where it can form type-II heterojunctions with other organic or inorganic materials. A type-II heterojunction is a junction between two materials with different band alignments, such that the conduction band minimum of one material is higher than that of the other, and the valence band maximum of one material is lower than that of the other. This creates a staggered band alignment that facilitates charge separation and transport across the junction. For example, single-layer MoS2 can form a type-II heterojunction with the organic polymer PTB7, which is a widely used donor material in organic solar cells. The heterojunction exhibits mutual photoluminescence quenching and photovoltaic effect, indicating efficient charge transfer between the two materials. The internal quantum efficiency of the heterojunction exceeds 40% for an overall cell thickness of less than 20 nm, resulting in exceptional current density per absorbing thickness in comparison to other organic and inorganic solar cells.

Single-layer MoS2 can also be doped to modify its electronic structure and enhance its photocatalytic activity. For instance, p-type doping can introduce acceptor levels in the bandgap of single-layer MoS2, which can act as electron traps and increase the lifetime of photo-generated holes. This can improve the hydrogen evolution reaction on single-layer MoS2 under visible light irradiation. Moreover, doping can also tune the bandgap and energy levels of single-layer MoS2 to match those of other materials in heterojunctions.

In summary, single-layer MoS2 is a versatile 2D material that has many potential applications in optoelectronic devices, especially photovoltaic cells. It has a direct bandgap that allows high absorption coefficients and efficient electron-hole pairs under photoexcitation. It can also form type-II heterojunctions with other materials to achieve charge separation and transport across the junction. Furthermore, it can be doped to modify its electronic structure and enhance its photocatalytic activity. Recent developments in large-scale production techniques, such as liquid exfoliation and chemical vapor deposition-like growth, show significant application potential in MoS2-based integrated optoelectronic circuits, light sensing, biomedical imaging, video recording, and spectroscopy.

source:https://www.nature.com/articles/nnano.2013.100

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