Atomic Positional Versus Electronic Order in Semiconducting ZnSe Nanoparticles

Size-controlled ZnSe nanoparticles with high extents of atomic positional order are shown to exhibit large size-dependent variations in their local electronic environments. Solid-state ${}^{77}\mathrm{Se}$ and ${}^{67}\mathrm{Zn}$ NMR spectra reveal increasingly broad distributions of ${}^{77}\mathrm{Se}$ and ${}^{67}\mathrm{Zn}$ environments with decreasing nanoparticle sizes, in contrast with high degrees of atomic positional order established by transmission electron microscopy and x-ray diffraction. First-principles calculations of NMR parameters distinguish between atomic positional and electronic disorder that propagate from the nanoparticle surfaces and yield insights on the order and disorder present.

Figure 1

(a)–(c) Representative high-resolution TEM images of ZnSe nanoparticles with different mean sizes: (a) 9, (b) 5, and (c) 1.3 nm, and (d) the corresponding powder XRD patterns of the same samples. The * symbols indicate reflections assigned to surfactant species.

Figure 2

Solid-state (a)–(d) single-pulse ${}^{77}\mathrm{Se}$ (11.7 T) and (e)–(h) spin-echo ${}^{67}\mathrm{Zn}$ (19.6 T) MAS NMR spectra of (a),(e) bulk ZnSe, and the same ZnSe nanoparticle samples in Fig. 1 with mean diameters: (b),(f) 9, (c),(g) 5, and (d),(h) 1.3 nm.

Figure 3

Calculated isotropic ${}^{77}\mathrm{Se}$ chemical shifts versus the distances of ${}^{77}\mathrm{Se}$ atoms from the nearest surface of a 9-layer ZnSe ZB supercell (right) with two exposed (110) surfaces with 50% coverage of ${\mathrm{H}}_3\mathrm{PO}$ molecules. In the upper layers, there are two types of ${}^{77}\mathrm{Se}$ atoms (•); below the fifth layer ($>1\mathrm{nm}$), separate calculations on a 17-layer supercell yield a single ${}^{77}\mathrm{Se}$ environment (▴) with a bulklike ${}^{77}\mathrm{Se}$ chemical shift [22]. The ${}^{77}\mathrm{Se}$ MAS NMR spectrum of 5-nm ZnSe nanoparticles along the upper axis is the same as in Fig. 2c.