Structural Insights on Microwave-Synthesized Antimony-Doped Germanium Nanocrystals

Abstract

Doped and alloyed germanium nanocrystals (Ge NCs) are potential candidates for a variety of applications such as photovoltaics and near IR detectors. Recently, bismuth (Bi) as an n-type group 15 element was shown to be successfully and kinetically doped into Ge NCs through a microwave-assisted solution-based synthesis, although Bi is thermodynamically insoluble in bulk crystalline Ge. To expand the composition manipulation of Ge NCs, another more common n-type group 15 element for semiconductors, antimony (Sb), is investigated. Oleylamine (OAm)- and OAm/trioctylphosphine (TOP)-capped Sb-doped Ge NCs have been synthesized by the microwave-assisted solution reaction of GeI2 with SbI3. Passivating the Ge surface with a binary ligand system of OAm/TOP results in formation of consistently larger NCs compared to OAm alone. The TOP coordination on the Ge surface is confirmed by 31P NMR and SEM-EDS. The lattice parameter of Ge NCs increases with increasing Sb concentration (0.00–2.0 mol %), consistent with incorporation of Sb. An increase in the NC diameter with higher content of SbI3 in the reaction is shown by TEM. XPS and EDS confirm the presence of Sb before and after removal of surface ligands with hydrazine and recapping the Ge NC surface with dodecanethiol (DDT). EXAFS analysis suggests that Sb resides within the NCs on highly distorted sites next to a Ge vacancy as well as on the crystallite surface. High Urbach energies obtained from photothermal deflection spectroscopy (PDS) of the films prepared from pristine Ge NC and Sb-doped Ge NCs indicate high levels of disorder, in agreement with EXAFS data. Electrical measurements on TiO2–NC electron- and hole-only devices show an increase in hole conduction, suggesting that the Sb-vacancy defects are behaving as a p-type dopant in the Ge NCs, consistent with the vacancy model derived from the EXAFS results.

Single-Crystalline Germanium Nanocrystals via a Two-Step Microwave-Assisted Colloidal Synthesis from GeI4

“…The as-synthesized NCs show explicit particle edges in the DF-STEM image. The quality of these images is significantly improved compared with our previously published work where NCs are obtained by reducing GeI 2 in OAm. ,, Particles prepared with GeI 2 in OAm showed vague edges around the particles in the HAADF-STEM images, which were attributed to the amorphous layer on the surface of the NCs. ,, Similar results are also reported in several other studies on solution-synthesized Ge NCs, demonstrating the difficulties of making Ge NCs with high crystallinity. , The corresponding selected area electron diffraction (SAED) pattern (inset of Figure c) matches the expected pattern for the crystalline diamond cubic structure of Ge. The high intensity of the SAED rings further verifies the high crystallinity of the NCs.…”

“…Even though great progress has been made on the size control of Ge NCs, control of crystallinity is not as well-developed because of the strong covalent nature of the Ge–Ge bond. The crystallite size calculated from X-ray diffraction (XRD) is commonly reported to be significantly smaller than the particle size measured from transmission electron microscopy (TEM) images. ,− This observation may be attributed to an amorphous or reconstructed Ge surface giving rise to a larger particle size in TEM as well as the polycrystalline nature of the NCs. Ge NCs with high crystallinity should be well-capped with surface ligands providing less defects at the surface, and enhancing carrier mobility and photoluminescence and may be further beneficial to the application of various electronics.…”

Structural Characterization of Oleylamine- and Dodecanethiol-Capped Ge1–xSnx Alloy Nanocrystals

“…Small σ 2 values (σ 2 ≤ 0.010 Å 2 ) are observed for the first NN peaks for both the Ge and Sn edge data, providing support for a fairly well-ordered structure. Observation of the Ge–Ge and Sn–Ge second NN peaks also indicates the presence of long-range order. , The observed values of σ 2 for the Ge–Ge first NN peaks, 0.003–0.004 Å 2 , are similar to but slightly smaller than those of comparably sized reference cubic Ge NCs (∼0.0026–0.0029 Å 2 ) measured at 10 K . This indicates that alloying with Sn introduces more disorder than is observed in similarly sized Ge NCs.…”

“…Observation of the Ge−Ge and Sn−Ge second NN peaks also indicates the presence of long-range order. 54,55 The observed values of σ 2 for the Ge− Ge first NN peaks, 0.003−0.004 Å 2 , are similar to but slightly smaller than those of comparably sized reference cubic Ge NCs (∼0.0026−0.0029 Å 2 ) measured at 10 K. 56 This indicates that alloying with Sn introduces more disorder than is observed in similarly sized Ge NCs. The σ 2 values for the Ge−Ge second NN peaks are quite large, again indicating significant disorder beyond the first neighbor shell, but are similar to those of reference cubic Ge.…”

Anisotropic growth of ZnO nanoparticles driven by the structure of amine surfactants: the role of surface dynamics in nanocrystal growth

“…39 In this context, the use of amines with long fatty chains as surfactants, like the archetypal oleylamine, 40 is very popular to achieve good control of the morphology of NCs. The high boiling point of oleylamine, above 360 °C, allowed using it in thermolytic reduction processes whether in a "heat up" approach, [41][42][43][44][45][46][47][48][49] or through the so-called "hot-injection" [48][49][50][51] technique. Hexadecylamine, dodecylamine and octylamine are also extensively used under milder conditions for the formation of metallic, II-VI or III-V semiconductor NCs.…”