Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit, in discrete steps, from 460 to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shell structures. Here, we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl, resulting in a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm with a line width (fwhm) of 9-13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a short emission lifetime of 5-11 ns. Finally, due to the increased red shift of the NPL band edge photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into both the NPL synthesis as well as their optoelectronic properties.

Interface engineering of organic-inorganic halide perovskite solar cells (PSCs) plays a pivotal role in achieving high power conversion efficiency (PCE). In fact, perovskite photoactive layer needs to work synergistically with the other functional components of the cell, such as charge transporting/active buffer layers and electrodes. In this context, graphene and related twodimensional materials (GRMs) are promising candidates to tune "on demand" the interface properties of PSCs. In this work, we fully exploit the potential of GRMs by controlling the optoelectronic properties of hybrids between molybdenum disulfide (MoS2) and reduced graphene oxide (RGO) as hole transport layer (HTL) and active buffer layer (ABL) in mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite (MAPbI3)-based PSC. We show that zero-dimensional MoS2 quantum dots (MoS2 QDs), derived by liquid phase exfoliated MoS2 flakes, provide both holeextraction and electron-blocking properties. In fact, on the one hand, intrinsic n-type doping-induced intra-band gap states effectively extract the holes through an electron injection mechanism. On the other hand, quantum confinement effects increase the optical band gap of MoS2 (from 1.4 eV for the flakes to > 3.2 for QDs), raising the minimum energy of its conduction band (from -4.3 eV for the flakes to -2.2 eV for QDs) above the one of conduction band of MAPbI3 (between -3.7 and -4 eV) and hindering electron collection. The van der Waals hybridization of MoS2 QDs with functionalized reduced graphene oxide (f-RGO), obtained by chemical silanization-induced linkage between RGO and (3-mercaptopropyl)trimethoxysilane, is effective to homogenize the deposition of HTLs or ABLs onto the perovskite film, since the two-dimensional (2D) nature of RGO effectively plug the pinholes of the MoS2 QDs films. Our "graphene interface engineering" (GIE) strategy based on van der Waals MoS2 QD/graphene hybrids enable MAPbI3-based PSCs to achieve PCE up to 20.12% (average PCE of 18.8%). The possibility to combine quantum and chemical effects into GIE, coupled with the recent success of graphene and GRMs as interfacial layer, represents a promising approach for the development of next-generation PSCs. Figure 1. (a) Sketch of mesoscopic MAPbI3-based PSC exploiting MoS2 QDs:f-RGO hybrids as both HTL and ABL. (b) Scheme of the energy band edge positions of the materials used in the different components of the assembled mesoscopic MAPbI3-based PSC. The energy band edge positions of MoS2 flakes and MoS2 QDs were determined from OAS and UPS measurements detailed along the text, while those of the other materials were taken from literature: FTO, 52 TiO2, 52 MAPbI3, 134-139 spiro-OMeTAD 52 and Au 52 . (c) State-of-the-art and predicted PCE evolution for PSCs, highlighting the synergistic potential of GIE and the formulation of advanced perovskite chemistries. The RGO flakes are effective to plug the pinholes MoS2 QDs films, thus to homogenize the HTL. The choice of the functionalization for RGO relies on the bifunctional r...

In the past few years, several protocols have been reported on the synthesis of CdSe nanoplatelets with narrow photoluminescence (PL) spectrum, high PL quantum efficiency, and short exciton lifetime. The corresponding core/shell nanoplatelets are however still mostly based on CdSe/CdS, which possess an extended lifetime and a strong red shift of the band-edge absorption and emission, in accordance with a quasi-type-II band alignment. Here we report on a robust synthesis procedure to grow a ZnS shell around CdSe nanoplatelets at moderate temperatures of 100–150 °C, to improve the optical properties of CdSe nanoplatelets via a type-I core/shell heterostructure. The shell growth is performed under ambient atmosphere, in either toluene or 1,2-dichlorobenzene. The variation of the shell thickness induces a continuous red shift of the PL peak, eventually reaching 611 nm. The PL quantum efficiency is increased compared to the original CdSe cores, with values up to 60% depending on the shell thickness. High-resolution transmission electron microscopy reveals a bending of the nanoplatelets caused by strain due to 12% lattice mismatch between CdSe and ZnS. The present procedure can easily be translated to other core/shell nanocrystals, such as CdSe/CdS and CdSe/CdZnS nanoplatelets.

The miniaturization of energy storage units is pivotal for the development of next-generation portable electronic devices. Micro-supercapacitors (MSCs) hold great potential to work as on-chip micro-power sources and energy storage units complementing batteries and energy harvester systems. Scalable production of supercapacitor materials with costeffective and high-throughput processing methods is crucial for the widespread application of MSCs. Here, wet-jet milling exfoliation of graphite is reported to scale up the production of graphene as a supercapacitor material. The formulation of aqueous/alcohol-based graphene inks allows metal-free, flexible MSCs to be screen-printed. These MSCs exhibit areal capacitance (C areal ) values up to 1.324 mF cm −2 (5.296 mF cm −2 for a single electrode), corresponding to an outstanding volumetric capacitance (C vol ) of 0. 490 F cm −3 (1.961 F cm −3 for a single electrode). The screen-printed MSCs can operate up to a power density above 20 mW cm −2 at an energy density of 0.064 µWh cm −2 . The devices exhibit excellent cycling stability over chargedischarge cycling (10 000 cycles), bending cycling (100 cycles at a bending radius of 1 cm) and folding (up to angles of 180°). Moreover, ethylene vinyl acetate-encapsulated MSCs retain their electrochemical properties after a home-laundry cycle, providing waterproof and washable properties for prospective application in wearable electronics.

In this work, we demonstrate the successful application of two-dimensional (2D) materials, i.e., graphene and functionalized MoS 2 , in perovskite solar cells (PSCs) by interface engineering the standard mesoscopic n−i−p structure. The use of 2D materials has the dual role to improve both the stability and the overall power conversion efficiency (PCE) of the PSCs compared to standard devices. The application of 2D materials is successfully extended to large-area perovskite solar modules (PSMs), achieving PCEs of 13.4% and 15.3% on active areas of 108 cm 2 and 82 cm 2 , respectively. This performance results in record-high active area-indexed aperture PCE (AIAPCE) of 1266.5% cm 2. In addition, the 2D materials-based PSMs show a stability under a prolonged (>1000 h) thermal stress test at 65°C (ISOS-D2), representing a crucial advancement in the exploitation of perovskite photovoltaic technology. I n recent years, lead-halide perovskite solar cells (PSCs) have catalyzed the attention of the scientific community, with power conversion efficiency (PCE) exceeding 20%, by using cost-effective and potentially scalable solution processing approaches. 1,2 In particular, the global research effort boosted the PCE of PSCs up to 24.2% for singlejunction 3 and 27.3% for tandem perovskite/silicon 4 solar cells. Despite these important achievements, long-term stability 5 and scalability 6 are still the major constraints for the market entry of the perovskite photovoltaic technology. 7,8 In fact, the photoactive lead-halide perovskites typically lack stability due 34 to their hygroscopicity and propensity to back-convert into 35 their precursors during exposure to moisture, 9 oxygen, 10,11 and 36 light illumination. 10,12 Moreover, they experience a tetragonal-37 to-cubic phase transition at the temperature reached during 38 typical solar cell operation (>80°C), 13 resulting in making 39 them unfit for standard solar module certifications. 14,15 In 40 addition, the deposition of both pin-hole-free homogeneous

Colloidal lead sulfide nanosheets have attracted broad interest for a wide variety of device applications, including field-effect transistors, solar cells, and spintronic devices. Whereas confinement effects in PbS quantum dots are well studied, they are still unclear in 2-dimensional ultrathin PbS nanosheets, especially in the 1 nm thickness range. In this work, we report a synthesis of monodisperse, rectangular-shaped PbS nanosheets with a thickness of 1.2 nm, using Pb­(thiocyanate)2 as a single source precursor. These nanosheets have an orthorhombic crystal structure, a direct bandgap, and weak optical absorption properties. This is evident from the lack of both excitonic absorption features and photoluminescence, and was corroborated by density functional theory calculations. Although these properties make the PbS nanosheets unsuitable for emission based applications, the nanosheets are highly photoconductive in films, with a responsivity up to 0.1 A W–1 and a detectivity of 1.3 × 109 Jones. We detected higher photoconductivity of these films under bending stress compared to that of films of PbS quantum dots.

Two dimensional (2D) colloidal PbS nanoplatelets (NPLs) with a thickness of 1.8–2.8 nm have been synthesized using a single-molecule precursor approach with lead octadecylxanthate. The lateral dimensions were tuned by varying the reaction temperature, growth time, and capping ligands. Transmission electron microscopy and X-ray diffraction reveal that the NPLs have an orthorhombic crystal structure rather than the rocksalt phase usually reported for bulk and nanostructured PbS. The 1.8 nm thickness, in combination with the tunable lateral dimensions, results in a blue-shifted absorption peak at 715–730 nm and a 48–68 nm narrow emission spectrum with a surprisingly small, 18 nm Stokes shift at room temperature. The fluorescence lifetime of these PbS NPLs is 2 orders of magnitude shorter than the typical lifetime in 0D colloidal PbS quantum dots, highlighting the advantageous properties of colloidal 2D nanostructures that combine strong transversal with weak lateral confinement.

Gallium selenide (GaSe) is a layered compound, which has been exploited in nonlinear optical applications and photodetectors due to its anisotropic structure and pseudodirect optical gap. Theoretical studies predict that its 2D form is a potential photocatalyst for water splitting reactions. Herein, the photoelectrochemical (PEC) characterization of GaSe nanoflakes (single‐/few‐layer flakes), produced via liquid phase exfoliation, for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both acidic and alkaline media is reported. In 0.5 m H2SO4, the GaSe photoelectrodes display the best PEC performance, corresponding to a ratiometric power‐saved metric for HER (Φsaved,HER) of 0.09% and a ratiometric power‐saved metric for OER (Φsaved,OER) of 0.25%. When used as PEC‐type photodetectors, GaSe photoelectrodes show a responsivity of ≈0.16 A W−1 upon 455 nm illumination at a light intensity of 63.5 µW cm−2 and applied potential of −0.3 V versus reversible hydrogen electrode (RHE). Stability tests of GaSe photodetectors demonstrated a durable operation over tens of cathodic linear sweep voltammetry scans in 0.5 m H2SO4 for HER. In contrast, degradation of photoelectrodes occurred in both alkaline and anodic operation due to the highly oxidizing environment and O2‐induced (photo)oxidation effects. The results provide new insight into the PEC properties of GaSe nanoflakes for their exploitation in photoelectrocatalysis, PEC‐type photodetectors, and (bio)sensors.