Growth Schemes of Tunable Ultrathin CdS_xSe_1–x Alloyed Nanostructures at Low Temperatures

Abstract

The new 2D colloidal semiconductor ultrathin nanosheets provide an appealing combination of properties. Controlling both their morphology and composition offers another path to control their physical properties. Homogeneously alloyed structures with tunable properties were obtained using NaBH4 which controls the precursor reactivity. The effect of NaBH4 on the degree of alloying, shape control, and optical properties of the alloyed colloidal nanosheets is presented here. The alloyed structures are a monolayer thicker than the pure CdS or CdSe. In relatively low Se contents, the addition of NaBH4 produced high quality alloyed nanosheets that are uniform, in the wurtzite phase and with small thickness distribution as evident from transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical characterization. Atomic resolution phase images provide evidence to both stacking faults formation across the entire width of the sheet as well as local disorder, suggesting a combined mechanism of oriented attachment of patches that are fused and extended by unidirectional growth.

Tuning the surface properties of alloyed CdSxSe1−x 2D nanosheets

“…the absorption and emission. 28,29 We have used aberration corrected TEM to investigate the structure of the nanosheets on the atomic scale in order to understand their formation mechanism, concluding that a mechanism of oriented attachment of patches exists alongside a unidirectional growth of the anion-rich facet. The surface defects produced by this growth mechanism serve for trapping of excitons by the sites located at the CdSe nanoplatelet surface instead of the commonly resolved multi-particle Auger recombination mechanism, 30 making the surface sensitive to photo etching in contrast to edge degradation, typical of the zinc blende structures.…”

“…15 The 2D alloyed CdS x Se 1Àx nanosheets were synthesized using a mixture of oleylamine and octylamine as the solvent. 28,29,32 Following the addition of the S-Se-NaBH 4 precursor, the reaction growth temperature was set to 65 C, 75 C, 85 C and 95 C, respectively, and kept for 10 h (see ESI † for detailed protocols). TEM images of all the synthetic batches showed uniform contrast of the platelets, as seen in Fig.…”

“…An addition of NaBH 4 is required to balance the different reactivities of the Se and S precursors. 28,29 Se is the more reactive species and it existed in excess in the nal product of all the samples, all of which were synthesized with a feed ratio of 1 : 1 Se : S. EDS analysis (Table 1 and Fig. S1 †) presents the nal composition of the structures.…”

“…Increasing the temperature enhanced the incorporation of Se within the structures such that the nal Se : S ratio was 4 : 1 for the synthesis at 95 C compared with Se : S ratio of 1.4 : 1 for the synthesis at 65 C. Our previous work has shown that in contrast to other synthetic protocols, the alloyed 2D nanosheets grow via oriented attachment under so-templating conditions, resulting in a homogenous composition without S-rich and Se-rich areas. 28,29 The homogenous composition promotes sharp emission peaks, whereas a composition gradient produces emission in a variety of wavelengths, thus degrading the optical quality of the samples.…”

“…The 1st excitonic transition for the lower Se-content (65 C) nanosheets was at 444 nm and at 517 nm for the highest Se-content (95 C), in accordance with the literature. Additional Se is known to red shi the absorbance and photoluminescence of CdS x Se 1Àx nanosheets, 20,28,29 CdS x Se 1Àx QD 33 and CdS x Se 1Àx nanowires. 34 The normalized PL spectra of all the samples showed bandedge emission and narrow emission peak, with full width at half maximum (FWHM) as small as 19 nm at 444 nm for the synthesis temperature of 65 C and 16 nm at 517 nm for 95 C, indicating an extremely uniform thickness.…”