Ultrafast Exfoliation of 2D Materials by Solvent Activation and One-Step Fabrication of All-2D-Material Photodetectors by Electrohydrodynamic Printing

Printed Two-Dimensional Materials for Flexible Photodetectors: Materials, Processes, and Applications

With the rapid development of micro-nano technology and wearable devices, flexible photodetectors (PDs) have drawn widespread interest in areas such as healthcare, consumer electronics, and intelligence interfaces. Two-dimensional (2D) materials with layered structures have excellent optoelectronic properties and mechanical flexibility, which attract a great deal of attention in flexible applications. Although photodetectors based on mechanically exfoliated 2D materials have demonstrated superior performance compared to traditional Si-based PDs, large-scale manufacturing and flexible integration remain significant challenges for achieving industrial production. The emerging various printing technology provides a low-cost and highly effective method for integrated manufacturing. In this review, we comprehensively introduce the most recent progress on printed flexible 2D material PDs. We first reviewed the most recent research on flexible photodetectors, in which the discussion is focused on substrate materials, functional materials, and performance figures of merits. Furthermore, the solution processing for 2D materials coupled with printing functional film strategies to produce PDs are summarized. Subsequently, the various applications of flexible PDs, such as image sensors, healthcare, and wearable electronics, are also summarized. Finally, we point out the potential challenges of the printed flexible 2D material PDs and expect this work to inspire the development of flexible PDs and promote the mass manufacturing process.

Direct or Indirect Sonication in Ecofriendly MoS2 Dispersion for NO2 and NH3 Gas-Sensing Applications

Unlike the most used, this study explores the effects of direct and indirect sonication methods on the dispersion and gas sensing performance of MoS2 nanoflakes. The obtained dispersions are characterized using various techniques, such as field emission scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy, dynamic light scattering, and Raman and X-ray diffraction, to evaluate their morphological and structural properties. Gas sensing measurements are conducted using exfoliated MoS2 on interdigitated electrode structures, and the response to multiple gases is recorded. The sensitivity and selectivity of the sensors are analyzed and compared between the direct and indirect sonication methods. The results demonstrate that both direct and indirect methods lead to the formation of well-dispersed MoS2 multilayer nanosheets, whereas the indirect approach exhibits a uniform and bigger flake size. Gas sensing experiments reveal that the MoS2 nanoflakes prepared via indirect sonication have enhanced sensitivity by 17 and 46% toward NO2 and NH3 gases, respectively, compared to the ones achieved by the direct sonication method. Both methods demonstrated its selectivity for NO2 and NH3 and the preferential temperature to detect NO2 and NH3 gas are 50 and 100 °C, respectively. This research contributes to the development of eco-friendly MoS2-based gas sensors by providing insights into the influence of direct (probe) and indirect (bath) sonication methods on dispersion quality and gas sensing performance. The findings highlight the potential of indirect sonication as a reliable technique for fabricating high-performance MoS2 gas sensors, opening venues for the design and optimization of eco-friendly sensing platforms for environmental monitoring and industrial applications.

Printed Electronics Based on 2D Material Inks: Preparation, Properties, and Applications toward Memristors

Printed electronics, which fabricate electrical components and circuits on various substrates by leveraging functional inks and advanced printing technologies, have recently attracted tremendous attention due to their capability of large-scale, high-speed, and cost-effective manufacturing and also their great potential in flexible and wearable devices. To further achieve multifunctional, practical, and commercial applications, various printing technologies toward smarter pattern-design, higher resolution, greater production flexibility, and novel ink formulations toward multi-functionalities and high quality have been insensitively investigated. 2D materials, possessing atomically thin thickness, unique properties and excellent solution-processable ability, hold great potential for high-quality inks. Besides, the great variety of 2D materials ranging from metals, semiconductors to insulators offers great freedom to formulate versatile inks to construct various printed electronics. Here, a detailed review of the progress on 2D material inks formulation and its printed applications has been provided, specifically with an emphasis on emerging printed memristors. Finally, the challenges facing the field and prospects of 2D material inks and printed electronics are discussed.

One-Step Exfoliation Method for Plasmonic Activation of Large-Area 2D Crystals

Advanced exfoliation techniques are crucial for exploring the intrinsic properties and applications of 2D materials. Though the recently discovered Au-enhanced exfoliation technique provides an effective strategy for the preparation of large-scale 2D crystals, the high cost of gold hinders this method from being widely adopted in industrial applications. In addition, direct Au contact could significantly quench photoluminescence (PL) emission in 2D semiconductors. It is therefore crucial to find alternative metals that can replace gold to achieve efficient exfoliation of 2D materials. Here, the authors present a one-step Ag-assisted method that can efficiently exfoliate many large-area 2D monolayers, where the yield ratio is comparable to Au-enhanced exfoliation method. Differing from Au film, however, the surface roughness of as-prepared Ag films on SiO₂ /Si substrate is much higher, which facilitates the generation of surface plasmons resulting from the nanostructures formed on the rough Ag surface. More interestingly, the strong coupling between 2D semiconductor crystals (e.g., MoS₂ , MoSe₂ ) and Ag film leads to a unique PL enhancement that has not been observed in other mechanical exfoliation techniques, which can be mainly attributed to enhanced light-matter interaction as a result of extended propagation of surface plasmonic polariton (SPP). This work provides a lower-cost and universal Ag-assisted exfoliation method, while at the same time offering enhanced SPP-matter interactions.

New Trends in Nanoarchitectured SERS Substrates: Nanospaces, 2D Materials, and Organic Heterostructures

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

Spontaneous formation of gold nanoparticles on MoS2 nanosheets and its impact on solution-processed optoelectronic devices

Understanding size-dependent properties of 2D materials is crucial for their optimized performance when incorporated through solution routes. In this work, the chemical nature of MoS₂ as a function of nanosheet size is investigated through the spontaneous reduction of chloroauric acid. Microscopy studies suggest higher gold nanoparticle decoration density in smaller nanosheet sizes, resulting from higher extent of reduction. Further corroboration through surface-enhanced Raman scattering using the gold-decorated MoS₂ nanosheets as substrates exhibited an enhancement factor of 1.55 × 10⁶ for smaller nanosheets which is 7-fold higher as compared to larger nanosheets. These plasmonic-semiconductor hybrids are utilized for photodetection, where decoration is found to impact the photoresponse of smaller nanosheets the most, and is optimized to achieve responsivity of 367.5 mAW^-1 and response times of ∼17 ms. The simplistic modification via solution routes and its impact on optoelectronic properties provides an enabling platform for 2D materials-based applications.

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Reducing agent-free Au nanoparticle decoration on aqueously dispersed 2H-MoS₂.

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Control on Au nanoparticle decoration density through nanosheet size-selection.

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SERS as a probe for determining the decoration density along with microscopy.

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Enhanced photodetection by spontaneous modification with Au on MoS₂ films.

Engineering sensitivity and spectral range of photodetection in van der Waals materials and hybrids

Exploration of van der Waals heterostructures in the field of optoelectronics has produced photodetectors with very high bandwidth as well as ultra-high sensitivity. Appropriate engineering of these heterostructures allows us to exploit multiple light-to-electricity conversion mechanisms, ranging from photovoltaic, photoconductive to photogating processes. These mechanisms manifest in different sensitivity and speed of photoresponse. In addition, integrating graphene-based hybrid structures with photonic platforms provides a high gain-bandwidth product, with bandwidths ≫1 GHz. In this review, we discuss the progression in the field of photodetection in 2D hybrids. We emphasize the physical mechanisms at play in diverse architectures and discuss the origin of enhanced photoresponse in hybrids. Recent developments in 2D photodetectors based on room temperature detection, photon-counting ability, integration with Si and other pressing issues, that need to be addressed for these materials to be integrated with industrial standards have been discussed.

Cationic complex directed thiostannate layers with excellent proton conduction and photocatalysis properties

Three isostructural thiostannates SnS-M (M = Fe, Mn and Zn) have been fabricated using metal-amine complex cations as structure-directing agents. These thiostannates are composed of typical two-dimensional lamellar [Sn3S7]n2n- anionic...

Liquid-Exfoliated 2D Materials for Optoelectronic Applications

Two-dimensional (2D) materials have attracted tremendous research attention in recent days due to their extraordinary and unique properties upon exfoliation from the bulk form, which are useful for many applications such as electronics, optoelectronics, catalysis, etc. Liquid exfoliation method of 2D materials offers a facile and low-cost route to produce large quantities of mono- and few-layer 2D nanosheets in a commercially viable way. Optoelectronic devices such as photodetectors fabricated from percolating networks of liquid-exfoliated 2D materials offer advantages compared to conventional devices, including low cost, less complicated process, and higher flexibility, making them more suitable for the next generation wearable devices. This review summarizes the recent progress on metal-semiconductor-metal (MSM) photodetectors fabricated from percolating network of 2D nanosheets obtained from liquid exfoliation methods. In addition, hybrids and mixtures with other photosensitive materials, such as quantum dots, nanowires, nanorods, etc. are also discussed. First, the various methods of liquid exfoliation of 2D materials, size selection methods, and photodetection mechanisms that are responsible for light detection in networks of 2D nanosheets are briefly reviewed. At the end, some potential strategies to further improve the performance the devices are proposed.

Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO₂) in monolayer molybdenum disulfide (MoS₂) when interfacing with a VO₂ film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS₂ directly synthesized on a SiO₂/Si substrate by chemical vapor deposition was first transferred onto a VO₂/c-Al₂O₃ substrate in which the VO₂ film was prepared by a sputtering method. We compared the MoS₂ interfaced with the VO₂ film with the as-synthesized MoS₂ by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E_2g¹ and A_1g modes in the monolayer MoS₂. Specifically, for the as-synthesized MoS₂, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E_2g¹ and A_1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E_2g¹ and A_1g modes for the MoS₂ transferred onto VO₂ were clearly observed across the phase transition of VO₂, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS₂/VO₂ interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.