Use of the N,N-dialkyl-N'-benzoyl(thio)selenoureas as single source precursors for the synthesis of semiconducting quantum dots

[摘要] The successful preparation and structural characterization of a number of N,N-dialkyl-N'-benzoyl(thio)selenourealigands is described; where the intermolecular interactions are characterized by the presence of Resonance AssistedHydrogen Bonding (RAHB), π- π interactions between neighbouring benzene residues only being evident amongstthe longer alkyl chain derivatives. The first structural characterization of an asymmetrically substituted N,N-dialkyl-N'-benzoylselenourea ligand reveals an increased stability of the Z isomer in the solid state, this being reflected bythe sulfur analogue. Attempts to synthesise N,N-dicyclohexyl-N'-benzoylselenourea led to the isolation andstructural characterization of a novel 1,3,5-oxaselenazine salt and dicyclohexylaminobenzoate. The first structuralcharacterization of a 'bipodal N,N-dialkyl-N'-benzoylselenourea ligand, 3,3,3',3'-tetrabutyl-1,1'-isophthaloylbis(selenourea), reveals RAHB in the crystal lattice similar to that exhibited by the 'monopodalanalogue, N,N-dibutyl-N'-benzoylselenourea.The successful complexation of the N,N-dialkyl-N'-benzoyl(thio)selenourea ligands to a number of differenttransition metal ions is reported allowing the preparation of several potential single source precursors. Coordinationthrough the O and Se/S donor atoms to Pd(II) results in the formation of square planar metal complexes, with a cisconformation, several of which could be structurally characterized. In particular, the first structural elucidation of anasymmetrically substituted N,N-dialkyl-N'-benzoylselenourea metal complex, cis-bis(N-benzyl-N-methyl-N'-benzoylselenoureato)palladium(II) indicates the increased stability of the EZ isomer in the solid state. Structuralelucidation of the novel (N,N-diphenyl-N'-benzoylselenoureato)cadmium(II) reveals a bimetallic complex in thesolid state, where the expected 2:1 ligand : metal ratio is maintained, and the two Cd(II) centres are 5 and 6coordinated, with O and Se donor atoms. Multinuclear Nuclear Magnetic Resonance (NMR) Spectroscopy has beenemployed in the thorough characterisation of the potential single source precursors, 77Se NMR spectroscopyindicating a decreased shielding of the 77Se nucleus as the 'hardness of the central metal ion increases i.e. Pd(II) >Zn(II) > Cd(II). Use of 113Cd NMR spectroscopy indicates the preferential binding of N,N-diethyl-N'-benzoylselenourea to Cd(II) over that of its sulfur analogue, and initial studies suggest a form of chelate metathesistaking place in solution. 31P NMR spectroscopy is used to gain insight into the formation of cis-bis(N,N-diethyl-N'-benzoylselenoureato)Pt(II).Thermolysis of (N,N-diethyl-N'-benzoylselenoureato)cadmium(II) and its sulfur analogue led to the successfulsynthesis of CdSe and CdS quantum dots respectively, where thermolysis over a range of temperatures allows adegree of size control over the resulting nanoparticles. The effect of precursor alkyl chain length on nanoparticlemorphology was investigated for both the N,N-dialkyl-N'-benzoylthio- and –selenoureas. A correlation between thetwo for the (N,N-dialkyl-N'-benzoylselenoureato)Cd(II) complexes is described and possible growth mechanismsare discussed. Preliminary investigations into the use of other N,N-dialkyl-N'-benzoyl(thio)selenourea metalcomplexes as single source precursors reveal that both (N,N-diethyl-N'-benzoylselenoureato)Zn(II) and its sulfuranalogue show potential as single source precursors for the formation of ZnO and ZnS nanoparticles respectively.Initial studies into the use of N,N-dialkyl-N'-benzoyl(thio)selenourea metal complexes as single source precursorsfor the synthesis of core-shell nanoparticles is briefly described.The Aerosol Assisted Chemical Vapour Deposition (AACVD) of several N,N-dialkyl-N'-benzoyl(thio)selenoureametal complexes is reported, where both (N,N-diethyl-N'-benzoylselenoureato)Cd(II) and its sulfur analogue allowthe deposition of crystalline CdSe and CdS respectively. The AACVD of (N,N-diethyl-N'-benzoylselenoureato)Zn(II) leads to the deposition of crystalline ZnSe, ZnS being deposited by (N,N-diethyl-N'-benzoylthioureato)Zn(II). The deposition of heazelwoodite (Ni3S2) with varying morphologies results from theAACVD of cis-bis(N,N-diethyl-N'-benzoylthioureato)Ni(II). Thermal annealing of the amorphous materialdeposited by the AACVD of cis-bis(N,N-diethyl-N'-benzoylthioureato)Pd(II), allows the formation of highlycrystalline palladium. The deposition of metallic platinum using cis-bis(N,N-diethyl-N'-benzoylthioureato)Pt(II) isdescribed as well as the deposition of crystalline Pd17Se15 from cis-bis(N,N-diethyl-N'-benzoylselenoureato)Pd(II).This, to the best of our knowledge, is the first time that AACVD has been performed, using the N,N-dialkyl-N'-benzoyl(thio)selenourea metal complexes as single source precursors, in addition, we believe it to be the first timethat palladium selenide has been deposited using the AACVD technique.

[发布机构] Stellenbosch University