2D II–VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

Abstract

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II−VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II−VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II−VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron−hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

Study of the Mechanism and Increasing Crystallinity in the Self-Templated Growth of Ultrathin PbS Nanosheets

“…For colloidal 2D nanocrystals, the often suggested reaction mechanism is soft templated growth; the long organic ligands that are present are supposed to form close-packed lamellar mesophases, which could guide 2D growth. 5 , 31 , 46 − 50 …”

“…To understand both the origin of the deformed orthorhombic structure and the stepwise thickness increase by 2 MLs of the NSs, we studied the reaction in more detail. For colloidal 2D nanocrystals, the often suggested reaction mechanism is soft templated growth; the long organic ligands that are present are supposed to form close-packed lamellar mesophases, which could guide 2D growth. ,,− …”

“… 1 , 2 Colloidal synthesis can thus result in molar quantities of solution-processable quantum objects with spectrally pure and tunable optical transitions, which is of large interest in material science and physics. 3 − 5 …”

“…Hence, these systems share characteristics with epitaxial quantum wells. Moreover, in some systems, such as CdX (X = Se, S or Te) nanoplatelets, the thickness is atomically defined and these show well-defined spectral features without inhomogeneous broadening. , Colloidal synthesis can thus result in molar quantities of solution-processable quantum objects with spectrally pure and tunable optical transitions, which is of large interest in material science and physics. − …”

Spontaneous Strain in Quasi-Two-Dimensional Janus CdSe Nanoplatelets and Its Microscopic Mechanisms

“…Colloidal semiconductor nanoplatelets have been a topic of intense research in recent years. − Among them, nanoplatelets of cadmium chalcogenides have attracted much attention due to their outstanding optical properties. They are direct band gap semiconductors.…”