Magic-Size Semiconductor Nanostructures: Where Does the Magic Come from?

Abstract

The quest for atomically precise synthesis of colloidal semiconductor nanostructures has attracted increasing attention in recent years and remains a formidable challenge. Nevertheless, atomically precise clusters of semiconductors, known as magic-size clusters (MSCs), are readily accessible. Ultrathin one-dimensional nanowires and two-dimensional nanoplatelets and nanosheets can also be categorized as magic-size nanocrystals (MSNCs). Further, the magic-size growth regime has been recently extended into the size range of colloidal QDs (up to 3.5 nm). Nevertheless, the underlying reasons for the enhanced stability of magic-size nanostructures and their formation mechanisms remain obscure. In this Perspective, we address these intriguing questions by critically analyzing the currently available knowledge on the formation and stability of both MSCs and MSNCs (0D, 1D, and 2D). We conclude that research on magic-size colloidal nanostructures is still in its infancy, and many fundamental questions remain unanswered. Nonetheless, we identify several correlations between the formation of MSCs and 0D, 1D and 2D MSNSs. From our analysis, it appears that the “magic” originates from the complexity of a dynamic and multivariate system running under reaction control. Under conditions that impose a prohibitively high energy barrier for classical nucleation and growth, the reaction proceeds through a complex and dynamic potential landscape, searching for the pathway with the lowest energy barrier, thereby sequentially forming metastable products as it jumps from one local minimum to the next until it eventually becomes trapped into a minimum that is too deep with respect to the available thermal energy. The intricacies of this complex interplay between several synergistic and antagonistic processes are, however, not yet understood and should be further investigated by carefully designed experiments combining multiple complementary in situ characterization techniques.

Room-Temperature Interconversion Between Ultrathin CdTe Magic-Size Nanowires Induced by Ligand Shell Dynamics

“…To gain insight into the role of alkylamines in the formation of CdTe MSNWs at room temperature, we followed the evolution of the reaction over time, using DDA as a representative primary alkylamine. As pointed out in ref ( 13 ) and clearly demonstrated above ( Figure 1 ), the use of absorption spectroscopy as the sole technique to study magic-size nanostructures is questionable because it cannot unambiguously identify the species responsible for the absorption transitions. Nevertheless, after the identity of the absorbing species has been established by complementary techniques, such as TEM and cryo-TEM, absorption spectroscopy becomes a powerful and convenient technique to follow the formation dynamics of the nanostructures in their native environment (i.e., in solution).…”

“…We thus propose that they can be categorized as magic-size nanowires (MSNWs). 13 For convenience, we will hereafter refer to them according to the spectral position of their lowest energy absorption transitions: NW-373, NW-418, and NW-450 for the MSNWs with the lowest energy exciton transition at 373, 418, and 450 nm, respectively.…”

“… 12 , 13 , 20 , 21 The insights provided by these studies suggest that the driving force behind the formation of MSNSs emerges from the complexity of a dynamic system running under reaction control. 13 However, the intricate framework of synergistic and antagonistic interactions that occur between multiple and dynamic variables as the reaction progresses has yet to be elucidated. In particular, the exact role of ligands and their relevance remain obscure.…”

“… 13 Semiconductor MSNSs are in the strong quantum confinement regime and can be zero-dimensional (0D, magic-size clusters and magic-size quantum dots), one-dimensional (1D, ultrathin nanowires), two-dimensional (2D, ultrathin nanosheets), or quasi-2D (weak confinement occurs also in the lateral directions, e.g., ultrathin nanoribbons and nanoplatelets). 13 The term “magic-size” is used to denote that at least one critical dimension (the confinement dimension, i.e., diameter or thickness) is atomically precise and changes only in discrete steps from one species to the next (i.e., growth is discontinuous or quantized). 13 The idea that discontinuous growth is restricted to the cluster or ultrathin size regime (≤2 nm) has been recently questioned since discrete growth of CdSe nanostructures was observed up to ∼3.5 nm, 9 , 11 , 14 which is well into the size range of colloidal quantum dots (QDs).…”

“… 13 The term “magic-size” is used to denote that at least one critical dimension (the confinement dimension, i.e., diameter or thickness) is atomically precise and changes only in discrete steps from one species to the next (i.e., growth is discontinuous or quantized). 13 The idea that discontinuous growth is restricted to the cluster or ultrathin size regime (≤2 nm) has been recently questioned since discrete growth of CdSe nanostructures was observed up to ∼3.5 nm, 9 , 11 , 14 which is well into the size range of colloidal quantum dots (QDs). Moreover, transformation and interconversion of MSNSs in response to external stimuli have been reported by several groups.…”