High Selectivity and Sensitivity in Chemiresistive Sensing of Co(II) Ions with Liquid-Phase Exfoliated Functionalized MoS2 : A Supramolecular Approach.

Chemical sensing of water contamination by heavy metal ions is key as it represents a most severe environmental problem. Liquid-phase exfoliated two-dimensional (2D) transition metal dichalcogenides (TMDs) are suitable candidates for chemical sensing thanks to their high surface-to-volume ratio, sensitivity, unique electrical characteristics, and scalability. However, TMDs lack selectivity due to nonspecific analyte-nanosheet interactions. To overcome this drawback, defect engineering enables controlled functionalization of 2D TMDs. Here, ultrasensitive and selective sensors of cobalt(II) ions via the covalent functionalization of defect-rich MoS2 flakes with a specific receptor, 2,2':6',2″-terpyridine-4'-thiol is developed. A continuous network is assembled by healing of MoS2 sulfur vacancies in a tailored microfluidic approach, enabling high control over the assembly of thin and large hybrid films. The Co2+ cations complexation represents a powerful gauge for low concentrations of cationic species which can be best monitored in a chemiresisitive ion sensor, featuring a 1 pm limit of detection, sensing in a broad concentration range (1 pm - 1 µm) and sensitivity as high as 0.308 ± 0.010 lg([Co2+ ])-1 combined with a high selectivity towards Co2+ over K+ , Ca2+ , Mn2+ , Cu2+ , Cr3+ , and Fe3+ cations. This supramolecular approach based on highly specific recognition can be adapted for sensing other analytes through specific ad-hoc receptors.

Two-Dimensional Violet Phosphorus: A p-Type Semiconductor for (Opto)electronics.

The synthesis of novel two-dimensional (2D) materials displaying an unprecedented composition and structure via the exfoliation of layered systems provides access to uncharted properties. For application in optoelectronics, a vast majority of exfoliated 2D semiconductors possess n-type or more seldom ambipolar characteristics. The shortage of p-type 2D semiconductors enormously hinders the extensive engineering of 2D devices for complementary metal oxide semiconductors (CMOSs) and beyond CMOS applications. However, despite the recent progress in the development of 2D materials endowed with p-type behaviors by direct synthesis or p-doping strategies, finding new structures is still of primary importance. Here, we report the sonication-assisted liquid-phase exfoliation of violet phosphorus (VP) crystals into few-layer-thick flakes and the first exploration of their electrical and optical properties. Field-effect transistors based on exfoliated VP thin films exhibit a p-type transport feature with an Ion/Ioff ratio of 104 and a hole mobility of 2.25 cm2 V-1 s-1 at room temperature. In addition, the VP film-based photodetectors display a photoresponsivity (R) of 10 mA W-1 and a response time down to 0.16 s. Finally, VP embedded into CMOS inverter arrays displays a voltage gain of ∼17. This scalable production method and high quality of the exfoliated material combined with the excellent optoelectronic performances make VP an enticing and versatile p-type candidate for next-generation more-than-Moore (opto)electronics.

Mechanism of ultrafast energy transfer between the organic-inorganic layers in multiple-ring aromatic spacers for 2D perovskites.

Lead halide based perovskite semiconductors self-assemble with distinct organic cations in natural multi-quantum-well structures. The emerging electronic properties of these two-dimensional (2D) materials can be controlled by the combination of the halide content and choice of chromophore in the organic layer. Understanding the photophysics of the perovskite semiconductor materials is critical for the optimization of stable and efficient optoelectronic devices. We use femtosecond transient absorption spectroscopy (fs-TAS) to study the mechanism of energy transfer between the organic and inorganic layers in a series of three lead-based mixed-halide perovskites such as benzylammonium (BA), 1-naphthylmethylammonium (NMA), and 1-pyrenemethylammonium (PMA) cations in 2D-lead-based perovskite thin films under similar experimental conditions. After optical excitation of the 2D-confined exciton in the lead halide layer, ultrafast energy transfer is observed to organic singlet and triplet states of the incorporated chromophores. This is explained by an effective Dexter energy transfer, which operates via a correlated electron exchange between the donating 2D-confined exciton and the accepting chromophore under spin conservation.

Emerging perovskite monolayers.

The library of two-dimensional (2D) materials has been enriched over recent years with novel crystal architectures endowed with diverse exciting functionalities. Bulk perovskites, including metal-halide and oxide systems, provide access to a myriad of properties through molecular engineering. Their tunable electronic structure offers remarkable features from long carrier-diffusion lengths and high absorption coefficients in metal-halide perovskites to high-temperature superconductivity, magnetoresistance and ferroelectricity in oxide perovskites. Emboldened by the 2D materials research, perovskites down to the monolayer limit have recently emerged. Like other 2D species, perovskites with reduced dimensionality are expected to exhibit new physics and to herald next-generation multifunctional devices. In this Review, we critically assess the preliminary studies on the synthetic routes and inherent properties of monolayer perovskite materials. We also discuss how to exploit them for widespread applications and provide an outlook on the challenges and opportunities that lie ahead for this enticing class of 2D materials.

Embedded Nickel-Mesh Transparent Electrodes for Highly Efficient and Mechanically Stable Flexible Perovskite Photovoltaics: Toward a Portable Mobile Energy Source.

The rapid development of Internet of Things mobile terminals has accelerated the market's demand for portable mobile power supplies and flexible wearable devices. Here, an embedded metal-mesh transparent conductive electrode (TCE) is prepared on poly(ethylene terephthalate) (PET) using a novel selective electrodeposition process combined with inverted film-processing methods. This embedded nickel (Ni)-mesh flexible TCE shows excellent photoelectric performance (sheet resistance of ≈0.2-0.5 Ω sq-1 at high transmittance of ≈85-87%) and mechanical durability. The PET/Ni-mesh/polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS PH1000) hybrid electrode is used as a transparent electrode for perovskite solar cells (PSCs), which exhibit excellent electric properties and remarkable environmental and mechanical stability. A power conversion efficiency of 17.3% is obtained, which is the highest efficiency for a PSC based on flexible transparent metal electrodes to date. For perovskite crystals that require harsh growth conditions, their mechanical stability and environmental stability on flexible transparent embedded metal substrates are studied and improved. The resulting flexible device retains 76% of the original efficiency after 2000 bending cycles. The results of this work provide a step improvement in flexible PSCs.

Topochemical Synthesis of Two-Dimensional Transition-Metal Phosphides Using Phosphorene Templates.

Transition-metal phosphides (TMPs) have emerged as a fascinating class of narrow-gap semiconductors and electrocatalysts. However, they are intrinsic nonlayered materials that cannot be delaminated into two-dimensional (2D) sheets. Here, we demonstrate a general bottom-up topochemical strategy to synthesize a series of 2D TMPs (e.g. Co2 P, Ni12 P5 , and Cox Fe2-x P) by using phosphorene sheets as the phosphorus precursors and 2D templates. Notably, 2D Co2 P is a p-type semiconductor, with a hole mobility of 20.8 cm2 V-1 s-1 at 300 K in field-effect transistors. It also behaves as a promising electrocatalyst for the oxygen evolution reaction (OER), thanks to the charge-transport modulation and improved surface exposure. In particular, iron-doped Co2 P (i.e. Co1.5 Fe0.5 P) delivers a low overpotential of only 278 mV at a current density of 10 mA cm-2 that outperforms the commercial Ir/C benchmark (304 mV).

Fluoride-Free Synthesis of Two-Dimensional Titanium Carbide (MXene) Using A Binary Aqueous System.

Two-dimensional (2D) titanium carbide (Ti3 C2 ) is emerging as an important member of the MXene family. However, fluoride-based synthetic procedures remain an impediment to the practical applications of this promising class of materials. Here we demonstrate an efficient fluoride-free etching method based on the anodic corrosion of titanium aluminium carbide (Ti3 AlC2 ) in a binary aqueous electrolyte. The dissolution of aluminium followed by in situ intercalation of ammonium hydroxide results in the extraction of carbide flakes (Ti3 C2 Tx , T=O, OH) with sizes up to 18.6 µm and high yield (over 90 %) of mono- and bilayers. All-solid-state supercapacitor based on exfoliated sheets exhibits high areal and volumetric capacitances of 220 mF cm-2 and 439 F cm-3 , respectively, at a scan rate of 10 mV s-1 , superior to those of LiF/HCl-etched MXenes. Our strategy paves a safe way to the scalable synthesis and application of MXene materials.

A Delamination Strategy for Thinly Layered Defect-Free High-Mobility Black Phosphorus Flakes.

Extraordinary electronic and photonic features render black phosphorus (BP) an important material for the development of novel electronics and optoelectronics. Despite recent progress in the preparation of thinly layered BP flakes, scalable synthesis of large-size, pristine BP flakes remains a major challenge. An electrochemical delamination strategy is demonstrated that involves intercalation of diverse cations in non-aqueous electrolytes, thereby peeling off bulk BP crystals into defect-free flakes comprising only a few layers. The interplay between tetra-n-butylammonium cations and bisulfate anions promotes a high exfoliation yield up to 78 % and large BP flakes up to 20.6 µm. Bottom-gate and bottom-contact field-effect transistors, comprising single BP flakes only a few layers thick, exhibit a high hole mobility of 252±18 cm2 V-1 s-1 and a remarkable on/off ratio of (1.2±0.15)×105 at 143 K under vacuum. This efficient and scalable delamination method holds great promise for development of BP-based composites and optoelectronic devices.

Solution-Processable High-Quality Graphene for Organic Solar Cells.

The unique optical and electronic properties of graphene open up new opportunities for optoelectronics. This work reports the use of solution-processed high-quality graphene as transparent conductive electrode in an organic solar cell using an electrochemical approach. The fabricated thieno[3,4-b]thiophene/benzodithiophene:phenyl-C71-butyric acid methyl ester (PTB7:PCB71M) bulk heterojunction organic solar cell based on the exfoliated graphene (EG) anode exhibits a power conversion efficiency of 4.23%, making EG promising for next-generation flexible optoelectronic devices.

Ultrafast Delamination of Graphite into High-Quality Graphene Using Alternating Currents.

To bridge the gap between laboratory-scale studies and commercial applications, mass production of high quality graphene is essential. A scalable exfoliation strategy towards the production of graphene sheets is presented that has excellent yield (ca. 75 %, 1-3 layers), low defect density (a C/O ratio of 21.2), great solution-processability, and outstanding electronic properties (a hole mobility of 430 cm2 V-1 s-1 ). By applying alternating currents, dual exfoliation at both graphite electrodes enables a high production rate exceeding 20 g h-1 in laboratory tests. As a cathode material for lithium storage, graphene-wrapped LiFePO4 particles deliver a high capacity of 167 mAh g-1 at 1 C rate after 500 cycles.