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The rise of 2D non-layered materials in optoelectronics

(Nanowerk Spotlight) For the first time, liquid-phase exfoliation (LPE) – a widely explored technique to obtain two-dimensional (2D) layered nano-architectures – has been successfully used to exfoliate a non-layered material. This intriguing finding could help in the search for other 2D materials for innovative applications.
In layered materials, the atoms within the layers are held together by strong covalent forces, while van der Waals interaction enables stacking of the layers. It is relatively easy to obtain 2D structures from bulk layered materials through exfoliation. Chemical vapor deposition (CVD) growth has also been employed to synthesize 2D materials. Among these physical and chemical methods, LPE has been widely used owing to its facile and efficient process.
Distinct from the layered materials, non-layered materials have chemical covalent bonds in all three dimensions. It seems to be difficult or even impossible to obtain layered structures from these materials. However, if this could be achieved, we can not only increase the family of 2D materials but also dream up a number of innovative applications. Scientists have hypothesized that the LPE method may be used for exfoliating non-layered materials via ultrasound.
New research, led by Professor Han Zhang at Shenzhen University in China, has achieved 2D nanosheets from non-layered material by utilizing the LPE method (Advanced Functional Materials, "Ultrathin 2D Nonlayered Tellurium Nanosheets: Facile Liquid-Phase Exfoliation, Characterization, and Photoresponse with High Performance and Enhanced Stability"). These findings make the LPE method of universal significance in the field of 2D materials.
In this work, the authors realized ultrathin 2D non-layered tellurium (Te) nanosheets via the LPE method. The as-prepared 2D Te nanosheets possess a typical 2D structure with lateral dimension ranging from 41.5 nm to 177.5 nm and a thickness ranging from 3 nm to 6 nm.
Liquid-phase exfoliation can be used to exfoliate non-layered materials, such as tellurium
Liquid-phase exfoliation (LPE) can be used to exfoliate non-layered materials, such as tellurium (Te). The newly exfoliated non-layered 2D nanomaterial possesses great potential for instance in the area of photo-responsive devices. (Image: Dr. Zhongjian Xie, Dr. Chenyang Xing, Shenzhen University)
The strong anisotropy in bulk crystalline Te, with a typical chain-like structure that originates from the strong Te-Te covalent bonds in intra-chains and weak Van der Waals forces in inter-chains, may be responsible for the successful synthesis of 2D Te nanosheets.
More interestingly, due to the narrow band gap of Te, the as-prepared 2D Te nanosheets can be used to fabricate high-performance photodetectors.
The 2D Te nanosheets exhibit excellent photoresponse behavior from the UV to the visible regime in association with strong time and cycle stability for on/off switching behavior.
A higher bias potential, higher light power intensity as well as higher concentration of KOH electrolyte can lead to higher photocurrent intensity under the illumination of simulated light.
The authors observed similar trends in samples irradiated by light with wavelengths of 350 nm, 365 nm, 380 nm, 400 nm, and 475 nm. Compared with other kinds of illumination, simulated light can generate a larger photocurrent intensity, while low-wavelength light produces a higher photoresponsivity.
By employing Raman characterization, the team observed no significant change in vibration peak of fresh and 2-week-old 2D Te nanosheets, indicating not only the unchangeable crystalline characteristics of 2D Te nanosheets but also a good stability in ambient conditions.
The stability of the photoresponse performance of 2D Te nanosheets is crucial for practical applications. To that end the cyclic and temporal stability of the photoresponse performance of 2D Te nanosheets was measured.
A long measurement time of 10,000 seconds with intervals of 5 seconds covering 1,000 on/off cycles was chosen to investigate the cyclic stability of the photoresponse performance. After 1,000 on/off cycles, the photocurrent intensity of 2D Te nanosheets only decreased by 16.6%, indicating a quite good cyclic stability.
In addition, after storage in 0.1 M KOH for 1 month, the photo-response behavior of the same sample was detected again. Results showed that the 1-month-old 2D Te nanosheets exhibited a reduced photocurrent intensity of 24%, which again indicates the good temporal and chemical stability of 2D Te nanosheets in KOH electrolyte.
In terms of these findings, the authors anticipate that the LPE-produced 2D Te nanosheets have promising potential in UV-visible photodetection as well as liquid optoelectronics.
This work may open up new avenues for obtaining high-quality 2D nanostructures from bulk materials, which could considerably expand the family of 2D materials.
Provided by Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics, Shenzhen University, as a Nanowerk exclusive.

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