Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Abstract

•Nonsupertetrahedral arrangement is unveiled in the stoichiometric CdSe clusters•Ligand-exchange-induced growth pathway of magic-sized clusters is identified•Site-specific Mn2+ doping is realized in Cd14Se13 cluster•Nanoclusters exhibit size-dependent optical and photophysical properties The size-controlled colloidal semiconductor nanocrystals promise applications in optoelectronics and catalysis. However, the atomic-level understanding of the evolution of the properties of nanocrystals is still lacking, largely due to challenges in the synthesis of atomically monodisperse nanocrystals and subsequent structural characterization. In this work, by the judicious choice of a tertiary diamine ligand, we have synthesized a single-sized Cd14Se13 cluster, the first example of a nearly stoichiometric semiconductor cluster. Its single-crystal structure reveals a nonsupertetrahedral arrangement of CdSe, which is in sharp contrast to the typical supertetrahedral structure of CdSe quantum dots. Furthermore, growth and size-conversion pathways of Cd14Se13 and Cd34Se33 clusters are mapped out. The Cd14Se13 cluster is found to be susceptible to site-specific Mn substitution, forming atomically precise dilute magnetic semiconductors for futuristic applications. Atomic-level structure characterization is central to the science and engineering of materials. However, the crystal structure determination of magic-sized nanoclusters (MSCs), typical nuclei of the semiconductor nanocrystals, is impeded by challenges in obtaining phase-pure MSCs. Herein, we report on the synthesis and X-ray crystal structure of ∼0.9-nanometer-sized 27-atom semiconductor MSC, Cd14Se13. Its structure has a central Se atom encapsulated by a Cd14Se12 cage with an adamantane-like CdSe arrangement. Two chloride ions released in situ from the dichloromethane solvent stabilize and linearly self-assemble the clusters. The formation and growth pathways of Cd14Se13 clusters provide vital insights into the size-structure-property relationships in MSCs. Furthermore, potential sites for magnetic dopants are identified in Cd14Se13 MSC, enabling research on atomically precise dilute magnetic semiconductors. Atomic-level structure characterization is central to the science and engineering of materials. However, the crystal structure determination of magic-sized nanoclusters (MSCs), typical nuclei of the semiconductor nanocrystals, is impeded by challenges in obtaining phase-pure MSCs. Herein, we report on the synthesis and X-ray crystal structure of ∼0.9-nanometer-sized 27-atom semiconductor MSC, Cd14Se13. Its structure has a central Se atom encapsulated by a Cd14Se12 cage with an adamantane-like CdSe arrangement. Two chloride ions released in situ from the dichloromethane solvent stabilize and linearly self-assemble the clusters. 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Rohrs H.W. Loomis R.A. Gross M.L. Buhro W.E. Isolation of the magic-size CdSe nanoclusters [(CdSe)13(n-octylamine)13] and [(CdSe)13(oleylamine)13].Angew. Chem. Int. Ed. 2012; 51: 6154-6157Crossref PubMed Scopus (104) Google Scholar and their Mn2+ doped counterparts (denoted as Mn2+:(CdSe)13),42Yang J. Fainblat R. Kwon S.G. Muckel F. Yu J.H. Terlinden H. Kim B.H. Iavarone D. Choi M.K. Kim I.Y. et al.Route to the smallest doped semiconductor: Mn2+-doped (CdSe)13 clusters.J. Am. Chem. Soc. 2015; 137: 12776-12779Crossref PubMed Scopus (73) Google Scholar were synthesized. The clusters were used as building blocks for self-assembly20Baek W. Bootharaju M.S. Walsh K.M. Lee S. Gamelin D.R. Hyeon T. Highly luminescent and catalytically active suprastructures of magic-sized semiconductor nanoclusters.Nat. Mater. 2021; 20: 650-657Crossref PubMed Scopus (23) Google Scholar and precursors for the nanostructured materials.30Liu Y.-H. Wang F. Wang Y. Gibbons P.C. Buhro W.E. 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Mater. 2004; 3: 99-102Crossref PubMed Scopus (433) Google Scholar Although the intriguing stoichiometry has triggered computational studies that have proposed several structures for (CdSe)13 clusters,15Kasuya A. Sivamohan R. Barnakov Y.A. Dmitruk I.M. Nirasawa T. Romanyuk V.R. Kumar V. Mamykin S.V. Tohji K. Jeyadevan B. et al.Ultra-stable nanoparticles of CdSe revealed from mass spectrometry.Nat. Mater. 2004; 3: 99-102Crossref PubMed Scopus (433) Google Scholar,28Bootharaju M.S. Baek W. Lee S. Chang H. Kim J. Hyeon T. Magic-sized stoichiometric II–VI nanoclusters.Small. 2021; 17: e2002067Crossref PubMed Scopus (10) Google Scholar experimental validation of such structures remains challenging. In this work, we report the synthesis and crystal structure of a highly desirable and elusive, Cd14Se13 MSC, which narrowly meets the QD-like composition as well as the 1:1 metal:chalcogenide stoichiometry. The crystal structure of the Cd14Se13 cluster provides valuable insights into the structure, MS results, and optical properties of (CdSe)13 MSCs. The experimental work is in agreement with density functional theory (DFT) calculations and provides clues to the formation and size-growth pathways of Cd14Se13 clusters. Furthermore, Mn2+ doping studies are carried out, revealing the number of dopants along with their positions in the Cd14Se13 clusters. In the previous work,20Baek W. Bootharaju M.S. Walsh K.M. Lee S. Gamelin D.R. Hyeon T. Highly luminescent and catalytically active suprastructures of magic-sized semiconductor nanoclusters.Nat. Mater. 2021; 20: 650-657Crossref PubMed Scopus (23) Google Scholar,41Wang Y. Liu Y.-H. Zhang Y. Wang F. Kowalski P.J. Rohrs H.W. Loomis R.A. Gross M.L. Buhro W.E. Isolation of the magic-size CdSe nanoclusters [(CdSe)13(n-octylamine)13] and [(CdSe)13(oleylamine)13].Angew. Chem. Int. Ed. 2012; 51: 6154-6157Crossref PubMed Scopus (104) Google Scholar,42Yang J. Fainblat R. Kwon S.G. Muckel F. Yu J.H. Terlinden H. Kim B.H. Iavarone D. Choi M.K. Kim I.Y. et al.Route to the smallest doped semiconductor: Mn2+-doped (CdSe)13 clusters.J. Am. Chem. Soc. 2015; 137: 12776-12779Crossref PubMed Scopus (73) Google Scholar,44Hsieh T.-E. Yang T.-W. Hsieh C.-Y. Huang S.-J. Yeh Y.-Q. Chen C.-H. Li E.Y. Liu Y.-H. Unraveling the structure of magic-size (CdSe)13 cluster pairs.Chem. Mater. 2018; 30: 5468-5477Crossref Scopus (32) Google Scholar for the synthesis of (CdSe)13 clusters, n-alkylamines or n-alkyldiamines were employed as both ligands and solvents. Unfortunately, hydrophobic ligand-ligand interactions among the as-synthesized (CdSe)13(n-alkylamine/n-alkyldiamine)13 clusters led to the formation of insoluble lamellar or sheet-like structures, which makes atomic-level structural characterization of MSCs impossible (Scheme 1). In this work, by using a tertiary diamine as the ligand and halocarbon as the solvent, we made a significant advancement (Scheme 1) in the synthesis and atomic-level structural characterization of under-explored stoichiometric CdSe MSCs. Particularly, the Cd14Se13 MSCs were synthesized through reaction between cadmium acetate and bis(trimethylsilyl)selenide in the presence of N,N,N′,N′-tetramethylethylenediamine (TMEDA) ligands in dichloromethane (DCM) (see experimental procedures for details). TMEDA ligands not only provide rigid binding with appropriate steric constraints but also disable the intercluster interactions due to short carbon chain, leading to the formation of soluble Cd14Se13 clusters instead of lamellar (CdSe)13 assemblies.20Baek W. Bootharaju M.S. Walsh K.M. Lee S. Gamelin D.R. Hyeon T. Highly luminescent and catalytically active suprastructures of magic-sized semiconductor nanoclusters.Nat. Mater. 2021; 20: 650-657Crossref PubMed Scopus (23) Google Scholar,41Wang Y. Liu Y.-H. Zhang Y. Wang F. Kowalski P.J. Rohrs H.W. Loomis R.A. Gross M.L. Buhro W.E. Isolation of the magic-size CdSe nanoclusters [(CdSe)13(n-octylamine)13] and [(CdSe)13(oleylamine)13].Angew. Chem. Int. Ed. 2012; 51: 6154-6157Crossref PubMed Scopus (104) Google Scholar,42Yang J. Fainblat R. Kwon S.G. Muckel F. Yu J.H. Terlinden H. Kim B.H. Iavarone D. Choi M.K. Kim I.Y. et al.Route to the smallest doped semiconductor: Mn2+-doped (CdSe)13 clusters.J. Am. Chem. Soc. 2015; 137: 12776-12779Crossref PubMed Scopus (73) Google Scholar,44Hsieh T.-E. Yang T.-W. Hsieh C.-Y. Huang S.-J. Yeh Y.-Q. Chen C.-H. Li E.Y. Liu Y.-H. Unraveling the structure of magic-size (CdSe)13 cluster pairs.Chem. Mater. 2018; 30: 5468-5477Crossref Scopus (32) Google Scholar The single-sized MSCs in the synthesized material was confirmed via matrix-assisted laser desorption ionization-time of flight-MS (MALDI-TOF-MS) analysis (Figure S1). The high-resolution mass spectra identified the species, [Cd14Se13Cl(TMEDA)]+ and [Cd14Se13Cl]+ (Figures 1A, 1B, and S2), and [Cd13Se13Cl]− (Figure S3), in their respective ionization modes (Note S1). Note that two Cl− ions are generated from DCM to charge-balance the 14th Cd2+ ion of the MSCs, since 13 Cd2+ ions are neutralized by 13 Se2− ions, establishing the chemical formula of MSCs Cd14Se13Cl2. The weaker binding of amine ligands with the MSCs results in the detection of only a single TMEDA-attached species, although the remaining TMEDA molecules are desorbed in the ionization process. Nevertheless, the total molecular formula Cd14Se13Cl2(TMEDA)6 of the MSCs (Cd14Se13 in short) is established by the combined X-ray crystallography and DFT results. Encouraged by the MS results of the single-sized MSCs, the surface of DCM solution of clusters is covered with n-pentane to grow single crystals to determine the molecular structure (see experimental procedures for details). The single-crystal X-ray diffraction (SCXRD) data analysis (Table S1) reveals a nearly spherical inorganic fraction of the Cd14Se13Cl2 cluster with a diameter of ∼0.9 nm, which is stabilized by six organic TMEDA ligands (Figure 1C), confirming the crystallization of the as-synthesized clusters. The Cd14Se13 cluster contains a Se atom at the center surrounded by a Cd14Se12 cage of six-membered (CdSe)3 rings. The Cd and Se atoms are arranged in an alternating zig-zag fashion (Figure 1D) similar to the adamantane-like structure. The central Se gains a tetrahedral configuration by connecting with four Cd atoms, whereas other 12 Se atoms exhibit a tri-coordination mode. Notably, 13 Cd atoms exhibit tetrahedral coordination, whereas the remaining one Cd shows only a tri-coordination. The four inner Cd atoms, connected to the central Se atom, each achieve tetrahedral geometry by bonding with three surface Se atoms. Of ten surface Cd atoms, six gain tetrahedral arrangement by bonding with two Se atoms and two nitrogen atoms of one TMEDA ligand (Figure 1C). Three of the remaining four surface Cd atoms attain tetrahedral geometry by each binding with three Se atoms and one Cl atom (Figure 1E). The weak binding of TMEDA with Cd14Se13, evidenced by the MS data, seems to be compensated by strengthening of the Cd–Se bonds. This is reflected in the shortening of Cd–Se bonds (average: 2.550 Å), which are part of the Cd-TMEDA surface motifs. On the other hand, other Cd–Se bonds are relatively longer with an average Cd–Se distance of 2.639 Å. The Cl− ions play critical role not only in providing fourth coordination to the surface Cd atoms, which renders the clusters neutral, but also in stabilizing them through self-assembly (Figure 1E). The cluster-assembly is driven by strong Cd–Cl bonds (∼2.580 Å), for those involved in the intercluster interactions (red arrows in Figure 1E), compared with the terminal Cd–Cl bonds (2.608 Å) (blue arrows in Figure 1E). As the obtained crystal structure is representative of a solid-state cluster, the binding of TMEDA ligands with the clusters in the solution state was confirmed by proton nuclear magnetic resonance spectroscopy (1H NMR). Compared with the pristine TMEDA, the proton signals of methyl groups connected to nitrogen atom shift and split due to TMEDA-cluster binding in different chemical environment (Figure 1F). The coordination of TMEDA with the cluster surface is further supported by a slight blue-shift of infrared peaks (Figure S4). The energy dispersive X-ray spectroscopy (EDS) confirms the presence of Cd, Se, and Cl elements in the cluster samples (Figures S5 and S6). The thermogravimetric analysis (TGA) shows that the cluster has a TMEDA fraction of ∼20.3%, which is close to that (20.7%) of Cd14Se13Cl2(TMEDA)6 (Figure S7). The Cd14Se13 clusters show a characteristic absorption profile with two clear peaks at 354 and 329 nm (Figure 2A). Similarity in the solution- and solid-state absorption profiles suggests the crystallization of the as-synthesized clusters (Figure 2B) and the determined solid-state structure most likely corresponds to solution state. The (CdSe)13 MSCs also have a similar absorption pattern, however with the significant light scattering and spectral shifts (350 and 335 nm) in the absorption spectrum due to a different self-assembly driving force through alkyl groups of amine ligands (e.g., n-octylamine).28Bootharaju M.S. Baek W. Lee S. Chang H. Kim J. Hyeon T. Magic-sized stoichiometric II–VI nanoclusters.Small. 2021; 17: e2002067Crossref PubMed Scopus (10) Google Scholar41Wang Y. Liu Y.-H. Zhang Y. Wang F. Kowalski P.J. Rohrs H.W. Loomis R.A. Gross M.L. Buhro W.E. Isolation of the magic-size CdSe nanoclusters [(CdSe)13(n-octylamine)13] and [(CdSe)13(oleylamine)13].Angew. Chem. Int. Ed. 2012; 51: 6154-6157Crossref PubMed Scopus (104) Google Scholar42Yang J. Fainblat R. Kwon S.G. Muckel F. Yu J.H. Terlinden H. Kim B.H. Iavarone D. Choi M.K. Kim I.Y. et al.Route to the smallest doped semiconductor: Mn2+-doped (CdSe)13 clusters.J. Am. Chem. Soc. 2015; 137: 12776-12779Crossref PubMed Scopus (73) Google Scholar The Cd14Se13 clusters show photoluminescence (PL) profiles in the 360–600 nm region, which are found to be independent of the excitations at 354 and 329 nm. The emission at ∼380 nm is attributed to band-edge PL.42Yang J. Fainblat R. Kwon S.G. Muckel F. Yu J.H. Terlinden H. Kim B.H. Iavarone D. Choi M.K. Kim I.Y. et al.Route to the smallest doped semiconductor: Mn2+-doped (CdSe)13 clusters.J. Am. Chem. Soc. 2015; 137: 12776-12779Crossref PubMed Scopus (73) Google Scholar The qualitative match of the optical properties of Cd14Se13 MSCs with those of the compositionally similar, semiconductor-like (CdSe)13 MSCs42Yang J. Fainblat R. Kwon S.G. Muckel F. Yu J.H. Terlinden H. Kim B.H. Iavarone D. Choi M.K. Kim I.Y. et al.Route to the smallest doped semiconductor: Mn2+-doped (CdSe)13 clusters.J. Am. Chem. Soc. 2015; 137: 12776-12779Crossref PubMed Scopus (73) Google Scholar suggests that the Cd14Se13 clusters may also possess semiconductor-like behavior. Since the cluster is in the molecular size regime, the emission related to surface trap states is significant.45Bowers M.J. McBride J.R. Rosenthal S.J. White-light emission from magic-sized cadmium selenide nanocrystals.J. Am. Chem. Soc. 2005; 127: 15378-15379Crossref PubMed Scopus (615) Google Scholar A qualitative agreement between the absorption and PL excitation (PLE) profiles (Figure S8), however with slight red-shifts, suggests that the surface states indeed belong to the clusters. The red-shift of PLE features may be attributed to the structural distortion in the photo-excited state since the absorption spectrum is unchanged after the PLE measurement (Figure S8). DFT calculations were performed to corroborate the experimental results. The calculations established the complete ligand-shell (Figure 2C) as well as confirmed the molecular structure of Cd14Se13 clusters determined by SCXRD. The absorption spectrum generated via DFT calculations was found to be in good agreement with the experimental results (Figure 2D). The minor differences in the simulated and experimental spectra can be attributed to the spectral broadening in the high energy range of the measured absorption that is often observed for CdSe MSCs.46Singh V. Priyanka X. More P.V. Hemmer E. Mishra Y.K. Khanna P.K. Magic-sized CdSe nanoclusters: a review on synthesis, properties and white light potential.Mater. Adv. 2021; 2: 1204-1228Crossref Google Scholar DFT is not susceptible to such distortio