Organometal halide (OMH) perovskites are highly promising for photovoltaic (PV) and other applications. However, their instability toward environmental factors such as humidity presents a major challenge in their potential commercial use. In this study, we developed a method to modify the surface of CH 3 NH 3 PbI 3 perovskite films by spin coating oleic acid (OA) to create a water resistant layer that results in enhanced stability and PV performance. The OA-surface passivated perovskites were studied using FT-IR spectroscopy, UV−vis absorption spectroscopy, and X-ray diffraction (XRD). The samples were aged in dark humid air at ∼76% relative humidity (RH) for 4 weeks. The surface passivated films showed minimal signs of decomposition, and the PV devices showed better performance than the unpassivated devices. A possible explanation is the carboxyl group (−COO − ) of OA binds to surface Pb 2+ and/or CH 3 NH 3 + to both passivate these surface defect sites, resulting in the formation of a thin layer of OA with their hydrophobic tail away from the perovskite film surface that effectively prevents water molecules from reaching the perovskite.

The numerous electronic and optoelectronic applications that rely on semiconductors require tuning their properties through doping. Germanium quantum dots (Ge QDs) were successfully doped with bismuth up to 1.5 mol %, which is not achievable in the bulk Ge system. The structures of oleylamine-and dodecanethiol-capped Ge QDs were probed with EXAFS, and the results are consistent with Bi dopants occupying surface lattice sites. Increasing the amount of Bi dopant from 0.50 to 1.5 mol % results in increasing disorder. In particular, the nearestneighbor Bi−Ge bond length is much longer than the Ge−Ge bond length in Ge QDs. Oleylamine to dodecanethiol ligand exchange was shown to partially restore order in doped QDs. Transport measurements of the Bidoped Ge QD thin films revealed that Bi doping leads to a significant increase in dark current and photocurrent. These results indicate that doping can provide a pathway for improving the performance of group IV quantum dots for energy conversion applications including photodiodes and photovoltaic cells.

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.

Microwave-assisted heating methods have been used to synthesize oleylamine-capped Ge1–x Sn x nanocrystals. By varying the reaction temperature while keeping the Ge and Sn precursor concentrations constant, Ge1–x Sn x nanocrystals with Sn compositions of 13.9 ± 2.1% over the 3.8–9.3 nm size range have been achieved. The oleylamine-capping ligand is replaced with dodecanethiol without affecting the Sn composition of the nanocrystals. The Sn composition is obtained from employing Vegard’s law on the lattice parameters obtained from the Rietveld refinement of powder X-ray diffraction patterns. Scanning transmission electron microscopy provides particle size and morphology. Extended X-ray absorption fine structure at the Sn and Ge edges of the dodecanethiol-capped nanocrystals confirms the solid solution, which is further supported by Raman spectroscopy. 1H NMR and FTIR are used to characterize the surface ligand capping before and after ligand exchange. Tauc plot analysis of UV–vis–NIR spectra to determine the indirect optical band gap for oleylamine- and dodecanethiol-capped Ge1–x Sn x nanocrystals is reported. This work demonstrates the effects of synthesis temperature on the particle size, composition, and structure of Ge1–x Sn x nanocrystals.