From Large-Scale Synthesis to Lighting Device Applications of Ternary I–III–VI Semiconductor Nanocrystals: Inspiring Greener Material Emitters

Quantum dots with fabulous size-dependent and color-tunable emissions remained as one of the most exciting inventories in nanomaterials for the last 3 decades. Even though a large number of such dot nanocrystals were developed, CdSe still remained as unbeatable and highly trusted lighting nanocrystals. Beyond these, the ternary I–III–VI family of nanocrystals emerged as the most widely accepted greener materials with efficient emissions tunable in visible as well as NIR spectral windows. These bring the high possibility of their implementation as lighting materials acceptable to the community and also to the environment. Keeping these in mind, in this Perspective, the latest developments of ternary I–III–VI nanocrystals from their large-scale synthesis to device applications are presented. Incorporating ZnS, tuning the composition, mixing with other nanocrystals, and doping with Mn ions, light-emitting devices of single color as well as for generating white light emissions are also discussed. In addition, the future prospects of these materials in lighting applications are also proposed

Facet Chemistry and the Impact of Surface Ligands on the Photoluminescence of Different Polyhedral-Shaped CsPbBr₃ Perovskite Nanocrystals

Controlling the surface ligand chemistry of lead halide perovskite nanocrystals remains one of the most important parameters for stabilizing different facets and maintaining high photoluminescence quantum yields (PLQYs). Successive washings or the use of antisolvents not only quenches the emission but also changes the crystal phase of these nanocrystals. However, studies to date have mostly focused on oleylammonium ion capped six-faceted hexahedron-shaped halide perovskite nanocrystals. In contrast, herein the impact of other ligands stabilizing other than facets of cube shaped nanocrystals is studied, and the physical insights of interface binding for stabilizing new facets and retaining near-unity PLQYs even with successive washings are discussed. Apart from nanocubes, 12-faceted dodecahedrons and 26-faceted rhombicuboctahedrons of CsPbBr3 having tertiary ammonium ion ligands are explored for successive dilution, precipitation, and redispersion studies with further bromide additions to investigate the change in the PLQY, crystal phase, and optical stability. After comparison, it is established that dodecahedron nanocrystals even in a larger size regime showed robust stability and retained near-unity PLQYs with four successive stages of dilutions and precipitations and hardly showed any differences in low-temperature brightness or any enhancement with extra bromide addition. These results suggest that ligands and facets remain the key features in bringing optical stability to lead halide perovskite nanocrystals

Surface Doping for Hindrance of Crystal Growth and Structural Transformation in Semiconductor Nanocrystals

Doping can strongly influence the crystal growth of semiconductor nanocrystals. It can change the surface energy and therefore the growth directions and shape of the host nanocrystals. While doping of transition metal ions in various semiconductor host nanocrystals is widely studied for obtaining new material properties, the effect of doping on crystal growth has been less explored. Herein, we study the change in the crystal growth pattern and growth rate with doping of one of the most common dopants Mn in a ZnSe host. With the help of selective surface binding ligands, hemisphere-shaped zinc blende nanostructures are designed from ZnSe seeds and dopant Mn precursors in different amounts are introduced at different stages of the synthetic process. Monitoring the sequential product and analyzing the surface or internal locations of the dopants, the possibility of shape change has been discussed. Moreover, the mutual effects of crystal growth and doping on one another are also determined considering the progress of the reactions under different conditions. We believe that the results presented here are important for understanding the doping mechanism and its effects during crystal growth in semiconductor nanocrystals, which are not clear to date

Material Diffusion and Doping of Mn in Wurtzite ZnSe Nanorods

Light-emitting transition metal ion doped 1D nanorods can be a suitable candidate for fabrication of the advanced opto-electronic-based nanodevices. Among various doped nanocrystals, Mn doped ZnSe nanocrystals are widely studied for their intense Mn d–d emission. However, this is mostly performed in the zinc blende phase of spherical ZnSe quantum dots. But, herein we study the strategy to dope Mn ions in wurtzite phase of 1D ZnSe nanorods. To achieve this, it is essential to control the 1D crystal growth of ZnSe to facilitate the adsorption of dopants. The anisotropic 1D nanostructures are designed following thermally controlled material diffusion process rather than the most widely expected kinetically driven crystal growth protocol, and the dopants are introduced at the appropriate stage of the growth for their adsorption. Using preformed magic size wurtzite ZnSe nanowires as the source material and fragmenting them at higher reaction temperature, ZnSe nanorods with variable aspect ratios are designed. These rods follow both inter- and intrarods material diffusion and retain the wurtzite phase throughout their transformation. This helps in understanding the insertion, adsorption, and retention of dopant Mn in the wurtzite phase of the 1D nanostructure

Anisotropic Zinc Blende ZnSe Nanostructures: The Interface Chemistry and the Retention of Zinc Blende Phase during Growth

Anisotropic growth in the zinc blende phase of nanomaterials in solution is normally less favorable in comparison to the wurtzite phase. Considering the case of ZnSe and using appropriate surface ligands for selective facet binding, herein we report the anisotropic growth leading to 1D rods, bullet-shaped and finally hemisphere-shaped nanostructures. A detailed study from the nucleation with magic-sized dots to all these structures has been performed, and their formation mechanism has been discussed. Interestingly, while such structures have existed to date mostly in wurtzite-type crystal phase with cell parameters <i>a</i> = <i>b</i> ≠ <i>c</i>, here for ZnSe they are formed uniquely in zinc blende phase with <i>a</i> = <i>b</i> = <i>c</i>

A Controlled Growth Process To Design Relatively Larger Size Semiconductor Nanocrystals

The growth of semiconductor nanocrystals in solution is mostly governed by the kinetic and thermal modes of control of the reaction process. In most of the cases, the size of the particles is limited within 5–6 nm, and further annealing mostly defocuses the particles size distribution. But, herein, we report a self-driven growth protocol which supplies the monomer continuously to significant extent and delays the thermal diffusion-controlled ripening process. This has been achieved by choosing appropriate sulfur precursor in the synthesis of metal sulfide nanocrystals which controls the sulfide ion concentration in the reaction medium via establishing an appropriate chemical equilibrium. As a consequence, the monomer concentration retains above their critical limit and it delays the ripening process. Finally, the nanocrystals can grow even larger than 10 nm, which are difficult to obtain from different established synthetic approaches. This has been observed for several semiconductor nanocrystals such as ZnS, CdS, CdZnS, and also in ZnSe nanocrystals. Further, this growth process has been adopted to dope Mn in larger sized ZnS and CdZnS nanocrystals, and efficient dopant emission has been obtained

Correlation of Dopant States and Host Bandgap in Dual-Doped Semiconductor Nanocrystals

Excitation of a semiconductor nanocrystal generates an electron–hole pair, which on recombination results in band edge excitonic emission. Insertion of impurity or dopant state capable of donating and/or accepting electrons can change the recombination process and leads to new emission called dopant emission. However, the presence of more than one impurity state generates multiple recombination possibilities, and the allowed transition might follow selective, additive, or a new path to get a new emission. To understand this, herein we report the correlation of host bandgap with dopant states in different dual-doped semiconductor nanocrystals. This has been achieved by doping two optically active (Mn and Cu) dopants in one semiconducting nanocrystal and observing the dopant emission changes with continuous variation of host bandgap. It has been observed that Mn d-state emission is predominated in dual-doped ZnS and Cu impurity state emission for ZnSe in spite of presence of both Mn and Cu in each semiconductor nanocrystal. However, further tuning their bandgap by appropriate alloying again reversed the recombination process where Cu became predominant for alloyed ZnS and Mn for alloyed ZnSe. From these emission changes the dopants states are correlated with the host bandgap and allowed recombination processes have been established

Hybrid Dot–Disk Au-CuInS₂ Nanostructures as Active Photocathode for Efficient Evolution of Hydrogen from Water

The synthesis of hybrid 0D-2D dot–disk Au-CIS heterostructures is enabled through nucleating wurtzite ternary I–III–VI CuInS₂ (CIS) semiconductor nanostructures on cubic Au particles via thiol-activated interface reactions. Chemistry of formation of these unique hybrid metal–semiconductor nanostructures is established by correlating successive X-ray diffraction patterns and microscopic images. Furthermore, these nanostructures are explored as an efficient photocathode material for photoelectrochemical (PEC) production of hydrogen from water. Although CIS nanostructures are extensively used as PEC active materials for solar-to-hydrogen conversion, the coupled structures with Au for their exciton–plasmon coupling is observed in producing a higher photocurrent with efficient evolution of hydrogen. In the comparison of materials properties, it is observed that the cathodic photocurrent, onset potential, and the half-cell solar-to-hydrogen efficiency (HC-STH) are recorded to be superior to all CIS-based photocathodes reported up to the current time. These results suggest that designing proper heterostructured functional materials can enhance the hydrogen production in the PEC cell and would be helpful for the ongoing technological needs for a greener way of generating and storing hydrogen energy

Mn-Doped Multinary CIZS and AIZS Nanocrystals

Multinary nanocrystals (CuInS₂, CIS, and AgInS₂, AIS) are widely known for their strong defect state emission. On alloying with Zn (CIZS and AIZS), stable and intense emission tunable in visible and NIR windows has already been achieved. In these nanocrystals, the photogenerated hole efficiently moves to the defect-induced state and recombines with the electron in the conduction band. As a result, the defect state emission is predominantly observed without any band edge excitonic emission. Herein, we report the doping of the transition-metal ion Mn in these nanocrystals, which in certain compositions of the host nanocrystals quenches this strong defect state emission and predominantly shows the spin–flip Mn emission. Though several Mn-doped semiconductor nanocrystals are reported in the literature, these nanocrystals are of its first kind that can be excited in the visible window, do not contain the toxic element Cd, and provide efficient emission. Hence, when Mn emission is required, these multinary nanocrystals can be the ideal versatile materials for widespread technological applications

Zinc Blende 0D Quantum Dots to Wurtzite 1D Quantum Wires: The Oriented Attachment and Phase Change in ZnSe Nanostructures

Oriented attachment of nanocrystals has been recently studied as one of the important tools to organize the nanocrystals in a regular array to design new nanostructures. This is mostly a thermodynamically driven process where the nanocrystals align in a certain crystallographic direction and merge, minimizing the interfacial energy of the system during the course of reaction. While this has been widely studied for several group II–VI semiconductor nanocrystals, we explore herein ZnSe 0D quantum dots which on merging change to 1D quantum nanowires. Importantly, the phase of the nanocrystals is found to be transformed from zinc blende to wurtzite after the fusion. To understand this, we have analyzed the intermediate samples and studied the high-resolution transmission electron microscopy (HRTEM) of single, twin, and triple connected dots as well as the final nanowires and address the phase change during the shape conversion. Additionally, we have provided density functional theory (DFT) calculation to support our experimental observations