Large portfolio of 2D semiconductor materials benefits next-generation flexible electronics

(Nanowerk Spotlight) Inspired by the unique optical and electronic property of graphene, two-dimensional (2D) layered materials – as well as their hybrids – have been intensively investigated in recent years, driven by their potential applications for nanoelectronics. The broad spectrum of atomic layered crystals includes transition metal dichalcogenides (TMDs), semiconducting dichalcogenides, monoatomic buckled crystals, such as black phosphorous (BP or phosphorene), and diatomic hexagonal boron nitride (h-BN), etc.
This class of materials can be obtained by exfoliation of bulk materials to small scales, or by epitaxial growth and chemical vapor deposition (CVD) for large areas.
Such atomically thin, single- or few-layer crystals are featured with strong intralayer covalent bonding and weak interlayer van der Waals bonding, resulting in superior electrical, optical and mechanical properties.
They form a complete set of conductors, semiconductors and insulators that are transparent and mechanically compliant on flexible polymer substrates, making them attractive and promising for next generation flexible electronics.
A recent Review article in Small ("Flexible Device Applications of 2D Semiconductors") the author, Dr. Li Gao from Nanjing University of Science and Technology, examines the recent advancement of flexible 2D electronic devices based on TMDs and BP.
Flexible 2D transistor applications
Flexible 2D transistor applications. a) Photograph of the bent flexible MoS2 transistor array. The inset shows the schematic layer-by-layer structure of the flexible device. b) Another schematic image of a bendable MoS2 transistor. c) Illustration of a flexible phosphorene transistor. (© Wiley-VCH Verlag) (click on image to enlarge)
She discusses the mechanical properties and strain-tunability of typical 2D semiconductors first, followed by novel growth and fabrication techniques that are compatible for 2D flexible devices.
Subsequently, she presents detailed application examples of field-effect transistors, photodetectors, strain and chemical sensors, and supercapacitors, with a final discussion of potential approaches and challenges to achieve 2D stretchable devices.
The device applications covered in detail are flexible transistors; flexible optoelectronics; flexible sensors; and flexible supercapacitors. From flexible to stretchable: Flexible 2D devices that can undertake bending and mild straining deformations have been a fundamental step in realizing high performance, low-cost, transparent, and wearable electronics. However, in order to obtain truly wearable and human-body-integrated electronic device functions, it is critical to further investigate and explore novel device structures that can enable stretchable and conformal 2D semiconductors.
Strategies of achieving stretchable 2D semiconductor devices
Strategies of achieving stretchable 2D semiconductor devices. a) SEM images of patterns developed on the graphene sheet delaminated buckles as the substrate is uniaxially relaxed. b) Rippled structure in the released state. c) An image of the transparent graphene electrotactile stimulator attached on pig skin. (© Wiley-VCH Verlag) (click on image to enlarge)
Since the first demonstration of flexible TMDs and phosphorene electronics a couple of years ago, device functionality and performance have already outperformed organic and amorphous thin-film electronics, and even some graphene-based devices.
"With continuous efforts addressing challenges in multifunctional and reliable device operation, and wafer-scale and low-cost production, flexible 2D and their heterostructure electronics would have the potential to enable a novel technology platform, by seeking commercial applications in fast, ultrathin, transparent, flexible, and crack-resistant devices, with customer tailored electrical and mechanical performances," Gao concludes her review. "The most promising application areas range from high speed transistors and flexible displays for mobile devices, ultrasensitive and label-free sensors, and more efficient energy storage devices, to human-integrated wearable devices that can monitor daily physiological information and communicate data wirelessly. These applications rely on the continuous development of materials synthesis and fabrication techniques, to form integrated logical transistors and wireless circuits, multifunctional sensors, energy conversion, and storage devices, etc."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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