Synthesis of Nanomaterials

Quantum dots (QDs) were discovered in the early 1980s by two scientists independently, and their methods later revolutionized the field, enabling the production of extremely high-quality nanocrystals. Research on quantum dots has focused largely on 0D nanomaterials, particularly CdSe QDs. With advancements in synthesis techniques, not only the size but also the shape, morphology, and crystal structure of QDs have become highly tunable. Thermal-decomposition hot-injection methods, in particular, yield quantum dots with highly crystalline lattices and narrow size distributions. Heterostructuring has also been extensively applied, leading to the development of core/shell quantum dots. Depending on the band alignment between the core and shell materials, the spatial distribution of electron and hole wavefunctions can be engineered. Recently, advanced Type-I heterostructures featuring gradient energy barriers have been introduced. These structures funnel excitons toward the core center, maximizing radiative recombination while suppressing Auger recombination.(1) Introducing asymmetric strain into the core lattice during epitaxial shell growth causes further splitting between heavy-hole and light-hole energy states. This reduction in degeneracy mitigates spectral diffusion and linewidth broadening.

1. Nature 617, 79-85 (2023).

Recently, perovskite quantum dots (QDs) have emerged as a new class of nanomaterials with distinct crystal structures and compositional flexibility, attracting significant attention for their transformative potential in nanotechnology. In particular, CsPbX₃ (X = Cl, Br, I) have demonstrated exceptional promise for photovoltaic and light-emitting applications due to their outstanding optoelectronic properties, including high absorption coefficients, widely tunable bandgaps, and near-unity photoluminescence (PL) quantum yields (~100%). In contrast to conventional CdSe QDs, CsPbX₃ QDs possess a soft, ionic lattice, which fundamentally alters their excitonic and carrier dynamics. This lattice softness leads to enhanced defect tolerance, modified carrier–phonon coupling, and reduced sensitivity to surface trap states. Perovskite QDs exhibit small exciton fine-structure splitting and fast radiative recombination, arising from strong electronic coupling. Moreover, through appropriate surface engineering, CsPbX₃ QDs can display narrow ensemble PL linewidths and suppressed spectral diffusion properties. These characteristics make perovskite QDs increasingly attractive platforms for generating indistinguishable single photons and achieving coherent emission, a regime in which CdSe QDs face intrinsic limitations.

In recent years, two-dimensional (2D) transition-metal chalcogenides (TMCs) have emerged as an important class of materials, where the new material properties are obtained by reducing the dimensionality. For instance, the band gap of various semiconducting TMCs changes from an indirect to a direct gap with a concomitant appearance of strong exciton photoluminescence (PL) when they become single-layer thick. In particluar, the valley-selective excitation, providing a new degree of freedom to manipulate the material properties, also becomes possible in single-layer TMCs. Recently, we discovered that the reduction of the lateral size to the nanoscale can potentially combine the unique property of the 2D exciton with additional lateral confinement.(1) It creats the ‘single-layer quantum dot (SQD)’ that is distinctly different from more common QDs derived from 3D crystals. Building on these discoveries, current research has expanded to heterostructured 2D QDs, in which vertically stacked or laterally coupled architectures enable emergent phenomena such as ultrafast interlayer tunneling, the formation of new excitonic states, and tunable magnetic properties.

1. J. Am Chem. Soc. 138, 13253-13259 (2016). 

The incorporation of paramagnetic, spin-5/2 manganese (Mn) impurities into II–VI colloidal quantum dots (QDs) results in considerable modifications of their optical and magneto-optical properties.(1) In particular, strong sp-d exchange interactions between a semiconductor host and magnetic impurities lead to fast excitation transfer from the QD to the Mn ions, which enables highly efficient emission in the intragap region due to radiative relaxation of the excited Mn state. Recent studies of Mn-doped CdSe QDs demonstrate that energy transfer from an excited Mn ion (Mn*) to carriers residing in QD intrinsic states is extremely fast (~100 fs timescale), which allows for the ejection of a hot electron outside the dot prior to its cooling to the band edge.(2) The estimated energy gain rate reaches very high values of more than 10 eV ps–1, and as a result, it overshoots the energy loss rate by a factor of approximately seven. This is a dramatic departure from the standard situation, which is expected to lead to the spin-exchange driven new phenomena. 

1. J. Am. Chem. Soc. 142, 18160-18173 (2020).

2. Nat. Photon. 16, 433-440 (2022).