Solid state NMR studies of photoluminescent cadmium chalcogenide nanoparticles

Abstract

Solid state (113)Cd, (77)Se, (13)C and (31)P NMR have been used to study a number of Cd chalcogenide nanoparticles synthesized in tri-n-octyl-phosphine (TOP) with different compositions and architectures. The pure CdSe and CdTe nanoparticles show a dramatic, size-sensitive broadening of the (113)Cd NMR line, which can be explained in terms of a chemical shift distribution arising from multiple Cd environments. From (13)C NMR, it has been discovered that TOP, or its derivatives such as TOPO (trioctylphosphine oxide), is rapidly moving about the surface of the nanoparticles, indicating that it is relatively weakly bound as compared to other materials used as surface ligands, such as hexadecylamine. (31)P NMR of the nanoparticles shows at least five species arising from coordination of the ligands to different surface sites. (113)Cd NMR of CdSeTe alloy and layered nanoparticles has provided crucial information which, in conjunction with results from other techniques (especially optical characterization), has made it possible to develop a detailed picture of the composition and structure of these materials: (i) a true CdSeTe homogeneous alloy nanoparticle, (ii) a nanoparticle segregated into an alloy core region rich in Te, with a CdSeTe (close to 1 : 1 Se : Te) alloy shell and (iii) a CdSe/CdTe/CdSe layered nanoparticle in which the CdTe layer contains a small amount of Se and which forms a Quantum Dot Quantum Well (QDQW) system. The results demonstrate that solid state NMR is a vital tool in the arsenal of characterisation techniques available for nanomaterials.

Homogeneously-Alloyed CdTeSe Single-Sized Nanocrystals with Bandgap Photoluminescence

“…34 Previously we also reported solid-state NMR studies of the synthesis-structure-property relationship of ternary-alloyed regular QDs of CdTeSe with TOP capping agents and ZnCdS regular QDs with stearic acid capping agents. 28,35 The interactions of the surface ligand and surface Cd atoms were identified, together with the structural information of the ternary core, namely homogeneously alloyed vs gradiently alloyed. To the best of our knowledge, our solidstate NMR identification of such structural information of ternary QDs, including the surface environment and inner chemical composition, was the first.…”

“…20 nm of the bandgap emission is dramatically smaller than the smallest value of 35 nm claimed (but typically 50 -60 nm in the spectra shown) previously for a ternary-alloyed CdTeSe regular QD ensemble synthesized by a 300°C hot-injection approach. 27,28 It is worthy of notice that homogeneous fluorescence linewidths of different semiconductor nanocrystals are different; CdSe and PbS single dots exhibit ca. 10 nm 4 and 50 nm 24 fluorescence line widths with homogeneous broadening, respectively.…”

“…The ternary CdTeSe regular QDs were reported to have typical fwhm of 35-60 nm. 27,28 Accordingly, the combination of the sharp absorption doublet with the energy difference of 288 meV, narrow emission with 20 nm fwhm, ultrasmall NRSS, and growth kinetics with the size stability based on the constant bandgap monitored during the reaction suggests the presence of a magic-sized ensemble. The quantum yield was estimated to be ca.…”

Frozen Acrylamide Gels as Dynamic Nuclear Polarization Matrices

“…signifies Cd atoms at the surface in a broad range of different disordered coordination environments. [30] Such broad Cd spectra have previously been observed for Cd-rich surfaces in CdS NPs (0 to -750 ppm).…”

Bright Gradient-Alloyed CdSexS1–x Quantum Dots Exhibiting Cyan-Blue Emission

“…Such experimental observation is in line with our previous work on pure bulk CdSe (shift −474 ppm, halfwidth 177 Hz) and a bulk CdSeS (60:40) alloy (shift −481 ppm, halfwidth 1617 Hz) which showed that introducing S causes a shift to more negative values and a broadening of the line. 38 Further negative shifting relative to the bulk seems to be a feature of the NCs. Furthermore, a 77 Se CP signal could not be found, which might be expected if there are very few Se atoms in the surface region for 1 H to cross-polarize to.…”