Covalent bonds between 2D materials unlock enhanced optoelectronic capabilities

Researchers have chemically linked 2D materials using a molecular “velcro,” resulting in a device with improved optoelectronic properties. The device, made of palladium nanosheets covalently bonded with MoS₂, shows an enhanced optoelectronic response in the infrared thanks to the chemically bonded interface between the two materials, in comparison to its van der Waals counterpart. This next generation of 2D-2D heterostructures goes beyond van der Waals thanks to the strong covalent bonds between its 2D materials.

Combining the best of different crystals to obtain the ultimate material is the motto that drives two-dimensional (2D) materials research. 2D structures are typically built by atomic deposition and weakly bonded to each other by van der Waals interactions. In the last few years, an alternative approach for creating robust 2D structures has been introduced, involving the chemical linkage of nanosheets of distinct materials. Now, researchers are leveraging this technique to create improved devices with a richer optoelectronic response.

In a recent collaboration between IMDEA Nanociencia, ICMM (Madrid), INMA and ARAID Foundation (Zaragoza), researchers have synthesized and characterized a 2D structure composed of palladium nanosheets and molybdenum disulfide (MoS₂). The study is published in the journal Small.

MoS₂ is one of the most popular 2D materials because of its facile exfoliation and excellent optoelectronic properties. It features a well-defined bandgap in its 2H type and good absorbance in the visible range of the spectrum. However, a notable limitation of MoS₂ is its poor absorbance in the infrared. The broadband optical detection ability, especially from ultraviolet to the near infrared range, is critical for applications including medical monitoring, video imaging or optical communications.

Researchers have combined MoS₂ with palladium nanosheets to create 2D structures with broadband detection that provide absorbance in the infrared. The prototype device, consisting of a single layer of MoS₂ covalently functionalized with palladium nanosheets, showed an enhanced optoelectronic response, both in terms of width and intensity, in comparison with a van der Walls structure with the same components.

Researchers proved that the enhancement stemmed from the chemically bonded interface between the two materials. The spectroscopic analysis of the palladium-MoS₂ device revealed an electronic interaction between the two materials that evidenced the effectiveness of the chemical connection.

The device reported here presents three key features. First, a MoS₂ large lateral size in the micrometer range combined with an ultrathin thickness of less than 5 nanometers. Second, the palladium nanosheets 2D morphology, which enables a strong absorbance in the infrared region. Last, the chemical connection between the two nanomaterials is facilitated via a bifunctional molecule.

The work highlights the advantages of the covalent connection. First, the device is robust against solvents or thermal processes. Further, the covalent connection between its 2D components improves the device’s optoelectronic response in comparison to its van der Waals counterpart. These findings demonstrate that covalent linked 2D materials hold promise for their application in broad-band photodetection.