One thing in common: The formation of binary colloidal semiconductor nanocrystals from single- (M(EEPPh2 )n ) and dual-source precursors (metal carboxylates M(OOCR)n and phosphine chalcogenides such as E=PHPh2 ) is found to proceed through a common mechanism. For CdSe as a model system (31) P NMR spectroscopy and DFT calculations support a reaction mechanism which includes numerous metathesis equilibriums and Se exchange reactions.
Both singlesource and dual-source precursor approaches (SSPA and DSPA) to binary semiconductors have been documented. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Such materials consist of elements from two groups such as IIB and VIA. The single-source precursors (SSPs) consist of the metallic and nonmetallic elements of the semiconductor constituents in a single molecule. [9][10][11][12][13][14] DSPA uses separated metallic-element and nonmetallic-element precursors, which commonly involve metal carboxylates (M(OOCR) n such as M = Zn, Cd, Pb, Cu, In) and phosphine chalcogenides (such as E = PHR 2 where E = S, Se, Te), [15][16][17][18][19][20][21][22][23][24][25][26][27] respectively. Despite the large number of recipes developed for the various colloidal semiconductor nanocrystals (NCs) in the past 20 years, there is still little understanding of their formation mechanisms. To realize the full potential of semiconductor materials, there is an urgent need to advance our mechanistic understanding. Filling this gap in our knowledge should have practical implications such as lowering the high temperature currently employed for syntheses and offering new avenues to optimize the design of low-temperature approaches to novel semiconductor nanomaterials.Recent evidence suggests that the formation of various binary semiconductor NCs by SSPAs and DSPAs may share analogous mechanisms. A DSPA to CdS quantumm dots (QDs) at 160 8C in tetradecane (CH 3 (CH 2 ) 12 CH 3 ) was reported from the reaction of cadmium stearate (Cd(OOCC 17 H 35 ) 2 ) and diphenylphosphine sulfide (S= PHPh 2 ). [25] DSAPs to E-based semiconductor QDs have become popular with metal carboxylates as cation precursors and diphenylphosphine chalcogenides E = PHPh 2 as anion precursors. [15][16][17][18][19][20][21][22][23][24][25][26][27] Astonishingly, the lack of a common formation mechanism is actually accompanied by the same 31 P NMR identification of RCOO-PPh 2 (R = C 17 H 33 99 ppm (3 in Scheme 1) or C 6 H 5 102 ppm) and Ph 2 P-PPh 2 (À14 ppm, 4) for the various DSPAs to PbSe, [18] CdSe, [19][20][21][22] ZnSe, [23,24] ZnS, [24] and ZnSeS, [24] together with C 17 H 33 COO-P(Se)Ph 2 (77 ppm, 5) for the Se-based NCs. [18,[21][22][23][24] Furthermore, the conversion of Se = PHPh 2 to diphenyldiselenophosphinate derivatives (ÀSeSePPh 2 ) has been documented. [18,19,22] For instance, [22] the formation of RCOOCdSeSePPh 2 (c) was proposed from a Cd(OA) 2 + Se=PHPh 2 reaction after the release of oleic acid (C 17 H 33 COOH or RCOOH, R = C 17 H 33 ) from (RCOO) 2 Cd(Se = PHPh 2 ) 2 (b) followed by diphenylphosphine (HPPh 2 or DPP) from RCOOCd(Se À PPh 2 )-(Se = PHPh 2 ) (d) through cleavage of the Se = P bond of the Se=PHPh 2 coordination arm.
Primary alkyl amines (RNH2) have been empirically used to engineer various colloidal semiconductor nanocrystals (NCs). Here, we present a general mechanism in which the amine acts as a hydrogen/proton donor in the precursor conversion to nanocrystals at low temperature, which was assisted by the presence of a secondary phosphine. Our findings introduce the strategy of using a secondary phosphine together with a primary amine as new routes to prepare high-quality NCs at low reaction temperatures but with high particle yields and reproducibility and thus, potentially, low production costs.
Understanding the size-dependent structures and properties of ligand-capped nanoclusters in solvent is of particular interest for the design, synthesis and application of II-VI colloidal QDs. Using DFT and TDDFT calculations, we studied the structure and optical property evolution of the cysteine-capped (CdSe)N clusters of N = 1-10, 13, 16 and 19 in gas, toluene, water and alkaline aqueous solution, and made a comparison with their corresponding bare clusters. The cysteine binds with (CdSe)Nvia several patterns depending on the medium they exist in, affecting the cluster structures and in consequence their optical absorption. In general, the absorption bands of (CdSe)N blueshift when cysteine is added, and the shift varies with the interaction strength between the cluster and the ligand, and the dielectric constant of the solvent. However, bare clusters retain their size sensitivity, in particular the redshift trend with increasing cluster size, and some similarity was noted for the optical absorption of the bare and ligated clusters regardless of the gas or solvent media. Population analysis reveals that the excitations are mainly from orbitals distributing on the (CdSe)N part, while the ligand is negligibly involved in the excitations. This is an important feature for the II-VI QDs as biosensors with which the information of biomolecules is detected from the size dependent optical absorption or emission of the QDs other than the biomolecules.
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