The Formation Mechanism of Binary Semiconductor Nanomaterials: Shared by Single‐Source and Dual‐Source Precursor Approaches

Abstract

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.

Mechanistic Study of the Role of Primary Amines in Precursor Conversions to Semiconductor Nanocrystals at Low Temperature

“…The latter was first proposed in 2010 via one SeDPP coordination and the former in 2013 via two SeDPP coordination. [25,26] Similarly to the phosphorous-containing products, I and H can be linked by I + PPh 2 H Ð H + Se = PPh 2 H; also, G and C can be associated by G + RCOOH Ð C + PPh 2 H. [34] Scheme 1 is consistent with previous reports [5,6,14,25,26,34] and with the observations presented in Figure 1-3 and S1-S4.…”

“…Thus, these four phosphorus-containing compounds were all independently synthesized. [26,34] The chemical shift for 2 is comparable to what was reported for PPh 2 P À NHR (where R = Et, 40.4 ppm; R = n Pr, 40.9 ppm). [38] Scheme 1 addresses our proposed mechanism of the precursor conversion to NCs in Reaction (1), with R = Scheme 1 shows the reaction products, namely the CdSe NCs and the four phosphorus products identified by 31 P NMR spectroscopy.…”

“…Se=PPh 2 H reacted readily with the amine (Figure 2 d), resulting in the formation of 6 (Ph 2 PSe 2 NRH 3 , where R = C 18 H 35 , 22.4 ppm and J P-Se = 609 Hz) and HPPh 2 . [34,39] At room temperature, while 5 exhibited little reactivity toward HPPh 2 (Figure 2 b), amine-coordinated 5 displayed appreciable reactivity toward HPPh 2 , producing more 1 than 3 from the mixture with 1 equiv of HPPh 2 ( Figure 2 e) and more 3 than 1 from the mixture with 4 equiv of HPPh 2 Scheme 1. The plausible mechanism proposed for the precursor conversion in Reaction (1).…”

“…Scheme 1 is in full agreement with our previous study which suggests that, for Cd(OOCC 17 H 33 ) 2 + Se = P(C 8 H 17 ) 3 + HPPh 2 , 4 or 3 are favored by RCOOH or HPPh 2 , respectively. [34] Furthermore, Scheme 1 predicts that only 1 will be obtained from I via F + C 18 H 35 NH 2 or via E + C 18 H 35 NH 2 .…”

“…Compound 3 and 4 were reported from the syntheses of PbSe, [25] CdSe, [14,26,34,35] ZnS, [36] ZnSe, [36] and ZnSeS NCs. [36] The chemical shift for 1 is similar to that reported for Ph 2 P(Se) À NHC 12 H 25 (57 ppm, J P-Se = 775 Hz), which was monitored from a mixture of Cd(OOCPh) 2 + Se= PPh 2 H + C 12 H 25 NH 2 .…”