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Posted: Mar 09, 2017

Tunable and flexible 2D organic-inorganic hybrid photodetectors

(Nanowerk Spotlight) Photodetectors with a spectral response from the ultraviolet (UV) to visible light have significant importance in modern industrial and scientific applications such as imaging, communication, environmental monitoring and day and nighttime surveillance.
Compared to other materials, the photo-current conversions of two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 nanosheets are impressive, making them great candidates for next-generation visible light photodetectors.
However, the large dark current and limited spectral selectivity in the visible region have limited this material's potential practical application in broadband photodetection, especially in the ultraviolet region.
This has led researchers to develop approaches such as hybridization of MoS2 with quantum dots and heterostructures with other 2D materials to suppress the dark current and enhance the broad spectral range in the NIR region were attempted. While convenient, these approaches are not scalable for large-scale implementation due to the complicated procedures as well as expensive material and instrument-related costs.
The generation of hybrids with other 2D materials by simple solution mixing will resolve the aforementioned issues and provide improved performance. In addition, the arrays of hybrid photodetectors fabricated on flexible substrates, such as plastic and paper, would be beneficial for future wearable applications.
In new work, researchers at King Abdullah University of Science & Technology (KAUST) have developed a facile and low-cost solution processing strategy to fabricate mechanically flexible 2D organic–inorganic hybrid thin-film photodetectors on a conventional filter paper consisting of inorganic MoS2 and organic g-C3N4 nanosheets for broadband UV-Visible light photodetection.
process for preparation of MoS2 and g-C3N4 hybrid dispersions
a) Schematic representation of the crystal structure of MoS2 and g-C3N4 and the process for preparation of MoS2 and g-C3N4 hybrid dispersions. b) Schematic illustration of the two-terminal, planar photodetector consisting of exfoliated MoS2 and g-C3N4 nanosheets. Photographs of c) the 5:5 hybrid film deposited on nylon membrane filter paper by a vacuum filtration method and d) photodetector. e) Energy dispersive X-ray spectroscopy (EDS) mapping of 5:5 hybrid films. Elemental mapping of the individual elements in the 5:5 hybrid film are also shown (Mo, blue; S, yellow; N, green; C, red). The Mo, S, C, and N atoms are uniformly distributed in the hybrid film, which implies that MoS2 and g-C3N4 are mixed homogeneously. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
"The unique electrical and optical properties of g-C3N4 with wide bandgap (∼ 2.7 eV) and low dark current extends the hybrid films photo response to the UV region and suppresses the dark current of MoS2," Husam N. Alshareef, Professor of Materials Science & Engineering at KAUST, tells Nanowerk. "Simple but robust solution mixing of 2D MoS2 and g-C3N4 offers an extremely convenient route to controlling their composition in the hybrid films without affecting the structural integrity of the individual layers and thus allows for tuning the optoelectronic properties."
As Alshareef's team demonstrated in a paper in Advanced Functional Materials ("2D Organic–Inorganic Hybrid Thin Films for Flexible UV–Visible Photodetectors"), hybrid thin films with 5:5 ratio of MoS2 and g-C3N4 exhibited excellent photodetection performance in terms of ON/OFF photocurrent ratio, specific detectivity, responsivity, and response time upon both 365 and 532 nm illumination.
"This performance was upheld even when the films were severely deformed at a bending radius of approximately 2 mm," adds Dr. Dhinesh Velusamy, first author of the paper. "The detailed charge transfer and separation processes at the interface and tunable photodetection of the MoS2 and g-C3N4 hybrid films were confirmed by thorough investigation of photo-induced, carrier-relaxation dynamics elucidated with the femtosecond transient absorption spectroscopy with broadband capability."
The team found that photodetectors based on pure MoS2 showed a photo response only in the visible region, while pure g-C3N4 photodetector operate only in the UV region.
"The ratios of the photocurrent to the dark current of the hybrid photodetectors clearly show that the addition of g-C3N4 not only improves the photodetector performance of MoS2 in the visible region, but also extends its photo response to the UV region due to the larger band gap of g-C3N4," Velusamy points out. "In addition, and importantly, the wavelength at the maximum photocurrent was controlled by the composition ratio."
He adds that, "in light of previous works reporting photodetectors based on liquid exfoliation of TMDs, the performance of our flexible hybrid films considering the Ion/Ioff ratio, detectivity, and response time is high and even comparable to the results of single or few-layer of other TMD photodetectors."
The researchers confirmed the detailed charge transfer and separation processes at the interface and tunable photodetection of the hybrid films by thorough investigation of photo-induced, carrier-relaxation dynamics elucidated with the femtosecond Transient Absorption (fs-TA) with broadband capability.
In the MoS2/g-C3N4 hybrid both time constants decrease, suggesting the presence of ultrafast charge transfer and charge recombination channels in the hybrid films.
More importantly, more efficient charge transfer is observed in the 5:5 hybrid, indicating that there is more active area in contact at the interface and thus giving rise to the high photocurrent.
"The observed hole transfer from MoS2 to g-C3N4 indicates that there is more active area in contact at the interface and thus giving rise to the high photocurrent," notes Alshareef.
Another benefit of these hybrid thin-films is their mechanical flexibility. The arrays of photo-detectors that the KAUST team fabricated are readily bendable and photocurrent arising from applied light is detected in situ under various bending states.
"We found that the initial Ion/Ioff ratio values were barely altered as a function of bending radius," says Velusamy. "The values were still maintained even at a bending radius of approximately 2 mm. Our mechanically flexible TMD composite photo-detectors are also resistant to multiple and repeated deformation. After 400 bending cycles at the bending radius of 2 mm, the devices worked properly without significant deterioration of the performance."
"The demonstration of our new method for fabricating micron-thick, flexible films consisting of a variety of highly separated TMD nanosheets for high performance band tunable photo-detection will not only be of interest in researchers working in the field of 2D materials and photodetectors but also instructive for chemists, physicists and materials scientists," concludes Alshareef. "Our findings will open up new potential usage of 2D materials in various opto-electronic, photovoltaic and flexible thin film electronics for system on-panel and system on-film applications. In addition, our photodetectors fabricated on flexible substrates would be beneficial for future wearable and patchable applications."
There are numerous 2D materials with intriguing photoelectronic properties corresponding to their material-dependent energy band gaps. Going forward, the team will utilize their universal strategy to fabricate hybrid 2D materials photodetectors from deep UV to far NIR wavelengths.
Systematically controlling 2D materials composition in hybrid films without affecting the structural integrity of the individual layers allows for tuning their optoelectronic properties.
2D materials dispersed in solvent are not only suitable for fabricating hybrids with different 2D materials with controlled composition but also for various solution-based film processes such as spin-coating, dip-coating and layer-by-layer assembly.
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