VI 반도체 나노 결정 표면에 형성되는 금속(카드뮴, 아연)이온–
리간드(트리옥틸포스핀 칼코겐 화합물, 친수성 싸이올레이트) 착물의 기체상 상대적인 안정도와 반응 연
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- The metal (M = Cd2+ and Zn2+) complexes with chalcogen precursors and hydrophilic thiols with reference molecules (chalcogen precursors: trioctylphosphine oxide, sulfide, and selenide (TOPE
E = O, S, and Se)
hydrophilic thiols: 3-mercaptopropionic acid (MPA), 3-mercapto-propanol (MPO), dithiothreitol (DTT), 2-mercapto-ethanol (BET), and aminoethanethiol (AET)) are prepared by electrospray ionization and their relative stabilities and intramolecular reactions are studied by collision-induced dissociation (CID) with Xe under single collision conditions.
TOPE complexes are considered as molecular precursors for the colloidal synthesis of II–
VI compound semiconductor nanocrystals employing TOPO as a metal-coordinating solvent and TOPS or TOPSe as a chalcogen precursor. Of the various [M + nTOPE]2+ (n = 2–
7) ions generated by electrospray ionization (ESI), the n = 2–
4 complexes are characterized by CID as a function of collision energy. The collision energy at 50% dissociation (E50%) is determined from the cracking curve and the relative stabilities of the complexes are established. Between the two metal ions, the zinc–
TOPE complexes are more stable than the cadmium-TOPE complexes when n = 2–
3, whereas their relative stabilities are reversed when n = 4. Of the TOPE, TOPO binds most strongly to the metal ion, while TOPSe does most weakly. Upon CID, loss of TOPE occurs exclusively from the tetra-TOPE complexes, while extensive fragmentation of TOPE takes place from the di-TOPE complexes, showing the signature of the metal chalcogenide formation. The nucleation of nanocrystals appears to begin with the cracking of [M + 2TOPE]2+ (E = S and Se).
Hydrophilic thiols are widely used as surface ligands to make semiconductor nanocrystals soluble in water. The metal–
hydrophilic thiol complexes are considered to be formed at the surface on the water soluble II–
VI semiconductor nanocrystals and determine the stability of nanocrystals in water as well as the solubility of nanocrystals. To understand the stability and the binding structure of metal–
thiolate complexes, we investigated collision induced dissociations of [M + L −
H]+ (M = Cd, Zn) in the gas phase where L = MPA, MPO, DTT, BET, AET, propionic acid (PA), methyl 3-mercaptopropionate (MMP). [M + L −
H]+ is prepared by electrospray ionization and CID was performed. The major reactions of [M + L −
H]+ by CID are a ligand loss and a neutral loss (HX, CO, C2H4
X = OH, NH2, OCH3, SH). Upon CID of [Cd + L −
H]+, a ligand loss reaction occurs as the first reaction except for DTT. For [Zn + L −
H]+, neutral loss (HX and CO) occurs as the first reaction except for AET. AET undergoes a ligand loss first. Of the ligands, BET binds most strongly to the cadmium ion and AET does so to the zinc ion. MMP binds most weakly to the cadmium ion, and MPO does so to the zinc ion. In the CID spectra, fragments including trace amounts of MSH+, MOH+ or MNH2+ reveal that the structure of complex is formed by bidentate structure. When [M + L −
H]+ dissociates to M with [L −
H], a neutral M with [L −
H]+ is formed by two electron transfer from thiolate to the metal ion for L = MPA, MPO, BET, DTT, AET. On the other hand, in the PA complexes that does not have a mercapto group, the only one or no electron is transferred to the metal ion. These two electron transfers from thiolate to the metal ion in the hydrophilic thiolate complexes are originated from thiolate group. It suggests that the thiolates on the nanocrystal surface can be hole traps on the nanocrystal surface, lower the quantum yield of nanocrystals.
To give insights to the most stable structures and a ligand loss path of [M + L −
H]+ for M = Cd, Zn
L = MPA, MPO, BET, DTT, AET, PA, and MMP, density functional theory (DFT) calculations were performed. We calculated the zero-point energy of the most stable structures of [M + L −
H]+ and several charge states of Mα with [L −
H]β where α = 0, 1, 2, β = −
1, 0, 1, and α + β = 1. The most stable structures of parent are bidentate structures. Comparing the DFT calculation results and the daughter ion by CID, [M + L −
H]+ dissociates directly to M with [L −
H]+ without the change of structures when it undergoes the ligand loss reaction.
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