According to theoretical studies, narrow graphene nanoribbons with atomically precise armchair edges and widths of o2 nm have a bandgap comparable to that in silicon (1.1 eV), which makes them potentially promising for logic applications. Different top-down fabrication approaches typically yield ribbons with width 410 nm and have limited control over their edge structure. Here we demonstrate a novel bottom-up approach that yields gram quantities of high-aspect-ratio graphene nanoribbons, which are only B1 nm wide and have atomically smooth armchair edges. These ribbons are shown to have a large electronic bandgap of B1.3 eV, which is significantly higher than any value reported so far in experimental studies of graphene nanoribbons prepared by top-down approaches. These synthetic ribbons could have lengths of 4100 nm and self-assemble in highly ordered few-micrometer-long 'nanobelts' that can be visualized by conventional microscopy techniques, and potentially used for the fabrication of electronic devices.

Quasi-two-dimensional (quasi-2D) Ruddlesden–Popper (RP) perovskites such as BA2Csn–1PbnBr3n+1 (BA = butylammonium, n > 1) are promising emitters, but their electroluminescence performance is limited by a severe non-radiative recombination during the energy transfer process. Here, we make use of methanesulfonate (MeS) that can interact with the spacer BA cations via strong hydrogen bonding interaction to reconstruct the quasi-2D perovskite structure, which increases the energy acceptor-to-donor ratio and enhances the energy transfer in perovskite films, thus improving the light emission efficiency. MeS additives also lower the defect density in RP perovskites, which is due to the elimination of uncoordinated Pb2+ by the electron-rich Lewis base MeS and the weakened adsorbate blocking effect. As a result, green light-emitting diodes fabricated using these quasi-2D RP perovskite films reach current efficiency of 63 cd A−1 and 20.5% external quantum efficiency, which are the best reported performance for devices based on quasi-2D perovskites so far.

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H(2)O, H(2), O(2), CO, NO(2), NO, and NH(3)), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH(2), An-CH(3), An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphene's electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

We developed a molecular interface control strategy that eliminates pinholes in perovskites by controlling the dynamics of film formation. The approach simultaneously passivates defects in perovskites by incorporating Br. As a result, the strategy prevents both shorts and non-radiative recombination, all the while providing improved charge injection and balanced charge transport. These improvements enable a 20 mm 3 20 mm perovskite LED with an external quantum efficiency of over 16%, the record efficiency for large-area perovskite electroluminescent devices.

Wave vector-resolved inverse photoelectron spectroscopy (IPES) measurements demonstrate that there is a large variation of interfacial charge transfer for graphene grown by chemical vapor deposition (CVD) on a range of dielectric or metallic substrates. Monolayer graphene grown by CVD on monolayer BN(0001)/Ru(0001) exhibits strong charge transfer from the substrate to graphene of 0.07(1) e− per carbon atom, as manifested by filling of the π* band and displacement of the Fermi level. IPES measurements of CVD single layer graphene on Ru indicate a substrate-to-graphene charge transfer from the substrate of 0.06(1) e− per carbon atom, in agreement with reported angle-resolved photoemission results. The IPES spectra of CVD single layer graphene on Ni(poly) and on Cu(poly) indicate 0.03(1) e− per carbon atom charge transfer from Ni and Cu substrates. Single layer graphene has also been grown by free radical-assisted CVD on MgO(111), resulting in a layer of graphene and an oxidized carbon interfacial layer between the graphene and the substrate. IPES measurements indicate that 0.02(1) e− per carbon atom charge is transferred from graphene to the MgO substrate. Additionally, IPES and photoemission data indicate that single layer graphene/MgO(111) exhibits a band gap. These data demonstrate that IPES is an effective method for precise measurement of substrate/graphene charge transfer and related electronic interactions, in part because of the extreme surface sensitivity of the technique, and suggest new strategies for extrinsic doping of graphene for controlled mobilities for device applications.

Core/shell structured metal halide perovskite nanocrystals (NCs) are emerging as a type of material with remarkable optical and electronic properties. Research into this field has been developing and expanding rapidly in recent years, with significant advances in the studies of the shell growth mechanism and in understanding of properties of these materials. Significant enhancement of both the stability and the optical performance of core/shell perovskite NCs are of particular importance for their applications in optoelectronic technologies. In this review, the recent advances in core/shell structured perovskite NCs are summarized. The band structures and configurations of core/shell perovskite NCs are elaborated, the shell classification and shell engineering approaches, such as perovskites and their derivative shells, semiconductor shell, oxide shell, polymer shell, etc. are reviewed, and the shell growth mechanisms are discussed. The prospective of these NCs in lighting and displays, solar cells, photodetectors, and other devices is discussed in the light of current knowledge, remaining challenges, and future opportunities.

light with the emission wavelength of 620-640 nm is an essential part for highdefinition display; however, compared with the green-counterpart, the electroluminescence (EL) performance of the colorsaturated red and blue perovskite emitters is much poorer. Ever since the first red-PeLED with an EL peak at 630 nm was demonstrated by Friend and coworkers, [1] efforts have been paid to improve the emitter photoluminescence quantum yield (PL QY) toward high-performing devices. Tan and coworkers applied a trimethylaluminum vapor-based crosslinking method to increase the PL QY of CsPb(Br/I) 3 nanocrystals (NCs) and gave rise to an EQE of 1.4%. [28] Brighter emitters leading to more efficient LEDs have been also observed in perovskite NCs of other colors and components, and strategies including doping, [29][30][31][32][33][34] surface capping, [35][36][37][38][39][40][41] core-shell structures, [42,43] and anion-exchange [25,[44][45][46][47] have been employed. For lead-halide perovskites, most charge recombination centers are localized on the crystal surfaces, and lead atoms who may cause strong exciton quenching are facile to form. [35,[38][39][40] Thus, the PL QY of red CsPb(Br/I) 3 NCs can increase through reducing the number of surface lead atoms. Aside from the excellent optical properties, the electric conductivity of perovskite NCs is another crucial factor for the performance of perovskite electroluminescent devices. Metal doping has been considered as a promising avenue to control over the electronic and optical performance of perovskite NCs. However, the limited improvement of metal doped CsPb(Br/I) 3 PeLEDs indicates that new methods are necessary. [32] Furthermore, doping perovskite with transition metal ions (Cu 2+ , Ni 2+ , Zn 2+ ) broadened the emission bands, and incorporation of Mn 2+ or lanthanide ions introduced new emission centers in perovskite NCs, leading to poor color purity of PeLEDs. [31,33,34] Here, benzyl iodide (BI) was chosen to passivate the CsPb(Br/I) 3 NC surface where the iodide can bond to the surface Pb atoms leading to reduced non-recombination centers. Interestingly, we found that the electrons tend to transfer from CsPb(Br/I) 3 NC to the surface electron acceptor-aromatic rings, leading to p-doping of the CsPb(Br/I) 3 NCs, thus allowing us to control over the energy levels and electrical conductivity. With the help of BI, the NC PL QY is greatly increased Color-saturated red light-emitting diodes (LEDs) with emission wavelengths at around 620-640 nm are an essential part of high-definition displays. Metal halide perovskites with very narrow emission linewidth are promising emitters, and rapid progress has been made in perovskite-based LEDs (PeLEDs); however, the efficiency of the current color-pure red PeLEDs-still far lags behind those of other-colored ones. Here, a simple but efficient strategy is reported to gradually down-shift the Fermi level of perovskite nanocrystals (NCs) by controlling the interaction between NCs and their surface molecular electron acceptor-benzyl iodide ...

Metal halide perovskites combine excellent electronic and optical properties, such as defect tolerance and high photoluminescence efficiency, with the benefits of low‐cost, large‐area, solution‐based processing. Composition‐ and dimension‐tunable properties of perovskites have already been utilized in bright and efficient light‐emitting diodes (LEDs). At the same time, there are still great challenges ahead to achieving operational and spectral stability of these devices. In this review, the origins of instability of perovskite materials, and reasons for their degradation in LEDs are considered. Then, strategies for improving the stability of perovskite materials are reviewed, such as compositional engineering, dimensionality control, defect passivation, suitable encapsulation matrices, and fabrication of core/shell perovskite nanocrystals. For improvement of the operational stability of perovskite LEDs, the use of inorganic charge‐transport layers, optimization of charge balance, and proper thermal management are considered. The review is concluded with a detailed account of the current challenges and a perspective on the key approaches and opportunities on how to reach the goal of stable, bright, and efficient perovskite LEDs.