Lead-free double perovskite materials, A 2 M(I)M′(III)X 6 , have recently attracted attention as environment-friendly alternatives to lead-based perovskites, APbX 3 , because of both rich fundamental science and potential applications. We report band gap tuning via alloying of Cs 2 AgBiCl 6 nanocrystals (NCs) with nontoxic, abundant Na. It results in a series of Cs 2 Na x Ag 1−x BiCl 6 (x = 0, 0.25, 0.5, 0.75, and 1) double perovskite NCs, leading to increase in optical band gap from 3.39 eV (x = 0) to 3.82 eV (x = 1) and 30-fold increment in weak photoluminescence. The tuning of band gap has been further explored by electronic structure calculation under the framework of density functional theory (DFT). The latter confirms that the increase in band gap is due to reduction of Ag contribution near valence band maxima (VBM) on incorporation of Na ion in place of Ag. These alloyed double perovskites can have useful potential applications in optoelectronic devices.

Mixed-dimensional van der Waals nanohybrids (MvNHs) of two-dimensional transition-metal dichalcogenides (TMDs) and zero-dimensional perovskites are highly promising candidates for high-performance photonic device applications. However, the growth of perovskites over the surface of TMDs has been a challenging task due to the distinguishable surface chemistry of these two different classes of materials. Here, we demonstrate a synthetic route for the design of MoSe2–CsPbBr3 MvNHs using a bifunctional ligand, i.e., 4-aminothiophenol. Close contact between these two materials is established via a bridge that leads to the formation of a donor–bridge–acceptor system. The presence of the small conjugated ligand facilitates faster charge diffusion across MoSe2–CsPbBr3 interfaces. Density functional theory calculations confirm the type-II band alignment of the constituents within the MvNHs. The MoSe2–CsPbBr3 nanohybrids show much higher photocurrent (∼2 × 104-fold photo-to-dark current ratio) as compared to both pure CsPbBr3 nanocrystals and pristine MoSe2 nanosheets owing to the synergistic effect of pronounced light–matter interactions followed by efficient charge separation and transportation. This study suggests the use of a bifunctional ligand to construct a nanohybrid system to tune the optoelectronic properties for potential applications in photovoltaic devices.

Lead-free halide double perovskites (DPs) have emerged as a stable and greener alternative to very toxic lead-based perovskites owing to their outstanding photophysical properties. Here, a series of Cs 2 (Na x Ag 1−x )BiBr 6 (x = 0, 0.25, 0.5, 0.75, and 1) double-perovskite nanocrystals (NCs) are synthesized, which have not been much explored yet. The formation of highly miscible Cs 2 (Na x Ag 1−x )BiBr 6 solid solutions is favored because of very low lattice mismatch between Cs 2 NaBiBr 6 and Cs 2 AgBiBr 6 . These DP NCs have tunable band gaps, which successively increase with increasing Na/Ag ratio, enabling us to tune their band gap by more than 0.5 eV. The band gap tuning is further confirmed by electronic structure calculations using density functional theory (DFT), which is in line with the band gap estimated from the absorption data. The Cs 2 (Na x Ag 1−x )BiBr 6 (x = 0, 0.25, 0.5) DP NCs are found to be more stable in the ambient conditions than the Cs 2 (Na x Ag 1−x )BiBr 6 (x = 0.75, 1.00) DP NCs.

We propose a computational methodology that highlights the intimate connection between surface and interface dipoles and work functions or valence band alignments.We apply the methodology to inspect the energy level alignments of halide perovskites and explore various situations relevant to perovskite-based heterostructures: i) the effect of surface termination, the ability to fine-tune and interpret the shift in energy alignments via ii) surface coating and iii) surface functionalization and / or passivation with molecules. We highlight the importance of local strain relaxation at the surfaces or interfaces, and revisit classical approaches based on capacitor models. Finally, we show that surface dipoles are additive in heterostructures and illustrate it through a 2D/3D perovskite interface. This provides a handy tool to interpret band alignments in complex perovskite-based heterostructures and buried interfaces. The scope of our work goes far beyond halide perovskites and can be useful to scrutinize the surfacial and interfacial behaviors of other semiconductors and heterojunctions. It allows bridging results from atomistic ab initio calculations and classical simulation approaches for multilayered thin film devices.

The electrocatalytic water splitting activity of layered transition metal dichalcogenides (TMDs) is limited by inert basal planes and slower reaction kinetics. Here, we demonstrate the use of MoSe2–Cu2S nanoheterostructures (NHSs)...