A semiconductor is a material whose conductivity lies somewhere between that of a conductor and an insulator. This property allows semiconductors to serve as the base material for modern electronics and transistors. It is no understatement that the technological advances in the latter part of the 20th centuryTh the speech was largely led by the semiconductor industry.
Today, technological advances in semiconductor nanocrystals are currently underway. For example, quantum dots and lines from semiconductor materials are of great interest for displays, photocatalytic and other electronic devices. However, many aspects of the colloidal nanocrystals still remain to be understood at the fundamental level. An important one of them is the elucidation of the mechanisms at the molecular level for the formation and growth of the nanocrystals.
These semiconducting nanocrystals are grown starting from small individual precursors made of a small number of atoms. These precursors are called “nanoclusters”. Isolation and molecular structure determination of such nanoclusters (or simply clusters) have been the subject of enormous interest in recent decades. The structural details of clusters, typical cores of the nanocrystals, are expected to provide critical insights into the evolution of the properties of the nanocrystals.
Different “seed” nanoclusters result in the growth of different nanocrystals. As such, it is important to have a homogeneous mixture of identical nanoclusters if one wants to grow identical nanocrystals. However, the synthesis of nanoclusters often results in the production of clusters of all possible different sizes and configurations, and purifying the mixture to obtain only the desired particles is very challenging.
Therefore, it is important to produce nanoclusters with homogeneous sizes. “Magic nanoclusters, MSCs”, which preferentially form over random sizes in a uniform manner, range in size from 0.5 to 3.0 nm. Among these, MSCs consisting of non-stoichiometric cadmium and chalcogenide ratio (not 1:1) are the most studied. A new class of MSCs with a 1:1 stoichiometric ratio of the metal-chalcogenide ratio has been in the limelight due to the prediction of intriguing structures. For example, Cd13See13CD33See33 and Cd34See34consisting of equal numbers of cadmium and selenium atoms have been synthesized and characterized.
Recently, researchers at the Center for Nanoparticle Research (led by Professor HYEON Taeghwan) within the Institute of Basic Sciences (IBS) in collaboration with the teams at Xiamen University (led by Professor Nanfeng ZHENG) and at the University of Toronto (led by Professor Oleksandr VOZNYY) reported the colloidal the synthesis and atomic-level structure of stoichiometric semiconductor cadmium selenide (CdSe) clusters. This is the smallest nanocluster synthesized today.
Synthesis of Cd14See13 was achieved after many previous failures with Cd13See13, which always ended up in unwanted assemblies, making them impossible to characterize. Director Hyeon said: “We found that the tertiary diamine and halocarbon solvent play a critical role in achieving nearly single stoichiometric clusters. The tertiary diamine ligands (N,N,N’,N’-tetramethylethylenediamine) not only provide rigid bonding. with suitable steric constraints but also disables the intercluster interactions due to the short carbon chain, leading to the formation of soluble Cd14See13 cluster, instead of unwanted insoluble lamellar Cd13See13 congregations.”
The dichloromethane solvent provides chloride ions in place to simultaneously achieve charge balancing of the 14Th cadmium ion, which enables the self-assembly of the clusters to form (Cd14See13Cl2)n. As a result, single crystals of adequate quality could be obtained for the researchers to determine the structure of the clusters. The composition of the clusters obtained from the single crystal X-ray diffraction data analysis agreed very well with the mass spectrometry and nuclear magnetic resonance data. The overall shape of the cluster was spherical with a size of about 0.9 nm.
While most other MSCs with non-1:1 metal-chalcogenide ratios tend to have supertetrahedral geometry, the new Cd14See13 was found to have a core-cage arrangement of constituent atoms. Specifically, the cluster comprised a central Se atom encapsulated by a Cd14See12 cage with an adamantane-like CdSe arrangement. Such a unique arrangement of atoms opens up the possibility of growing nanocrystals with unusual structures, which needs to be further explored in the future.
The optical properties of the cluster showed the presence of quantum confinement effects with band-edge photoluminescence. However, the photoluminescence properties related to defect states were prominent due to the extremely small size of the clusters. The structure and absorption peaks observed in the experiments were well supported by the density functional theory calculations.
The researchers created the CD14See13 cluster through an intermediate Cd34See33 cluster, which is the next known large-sized stoichiometric cluster. Interestingly, both of these two clusters could be doped via substitution with a maximum of two Mn atoms, illustrating the potential to realize dilute magnetic semiconductors with tailored photoluminescence properties. The calculation results showed that the Cd sites bound to halides were more susceptible to Mn substitution.
The implications of this study may go far beyond the synthesis of simple semiconductor clusters, as the tertiary diamines of different chemical structures may be extended to other clusters. Synthesis and determination of the atomic-level structure of other clusters may eventually help to understand the molecular-level growth mechanism of the semiconductor nanocrystals.
It was shown that Cd34See33 clusters could be kinetically stabilized by a ligand-exchange-induced size conversion process developed in this work. However, more efforts and new strategies are needed to improve the solution state stability for the structure determination of the next large cluster CD34See33, which are the critical nuclei for the cadmium selenide-based nanocrystal growth. It is hoped that further studies of size, structure and dopant dependence in the optoelectronic, photocatalytic and spintronic applications can open new directions for scientific research on the semiconductor clusters.
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