We report a chemical route to colloidal silicon (Si) nanocrystals, or quantum dots, with widely tunable average diameter, from less than 3 nm up to 90 nm and peak photoluminescence (PL) from visible wavelengths to the bulk band gap of Si at 1100 nm. The synthesis relies on the high temperature (>1100 °C) decomposition of hydrogen silsesquioxane (HSQ) to obtain Si quantum dots with good crystallinity and a narrow size distribution with tunable size embedded in SiO 2 . The oxide matrix is removed by hydrofluoric acid etching in the dark. Subsequent thermal hydrosilylation with alkenes yields free, solvent-dispersible Si nanocrystals with bright PL. The relationship between PL energy and size, exhaustively characterized by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and X-ray diffraction (XRD), is reported.

CuInSe2 (CISe) quantum dots (QDs) were synthesized with tunable size from less than 2 to 7 nm diameter. Nanocrystals were made using a secondary phosphine selenide as the Se source, which, compared to tertiary phosphine selenide precursors, was found to provide higher product yields and smaller nanocrystals that elicit quantum confinement with a size-dependent optical gap. Photovoltaic devices fabricated from spray-cast CISe QD films exhibited large, size-dependent, open-circuit voltages, up to 849 mV for absorber films with a 1.46 eV optical gap, suggesting that midgap trapping does not dominate the performance of these CISe QD solar cells.

Oxide-embedded and oxide-free alkyl-terminated silicon (Si) nanocrystals with diameters ranging from 3 nm to greater than 10 nm were studied by Raman spectroscopy. For ligand-passivated nanocrystals, the zone center Raman-active mode of diamond cubic Si shifted to lower frequency with decreasing size, accompanied by asymmetric peak broadening, as extensively reported in the literature. The size dependence of the Raman peak shifts, however, was significantly more pronounced than previously reported or predicted by the RWL (Richter, Wang, and Ley) and bond polarizability models. In contrast, Raman peak shifts for oxide-embedded nanocrystals were significantly less pronounced as a result of the stress induced by the matrix.

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Zinc sulfide-coated copper indium sulfur selenide (CuInSexS2-x/ZnS core/shell) nanocrystals were synthesized with size-tunable red to near infrared (NIR) fluorescence with high quantum yield (40%) in water. These nanocrystals were tested as an imaging agent to track a microparticle-based oral vaccine administered to mice. Poly(lactic-co-glycolic acid) (PLGA) microparticle-encapsulated CuInSexSe2-x/ZnS quantum dots were orally administered to mice and were found to provide a distinct visible fluorescent marker in the gastrointestinal tract of living mice.

Using graphene as an ultrathin support, highresolution transmission and scanning transmission electron microscopy (TEM and STEM) images of organic ligandstabilized silicon (Si) nanocrystals with unprecedented clarity were obtained. TEM images of Si nanocrystals are usually obscured on conventional amorphous carbon TEM supports because of low atomic number (Z) contrast. The atomically thin graphene supports enabled clear images of the crystalline Si cores and, for the first time, organic capping ligands. Various twin defects were observed, often accompanied by very significant lattice distortion and anisotropic strain.

Binder-free graphene-supported Ge nanowires, Si nanowires, and Si nanocrystals were studied for use as negative electrode materials in rechargeable lithium ion batteries (LIBs). Graphene obtained from reduced graphene oxide (RGO) helped stabilize electrochemical cycling of all of the nanomaterials. However, differential capacity plots revealed competition between RGO and Si/Ge lithiation. At high Si/Ge loading (>50% w/w) and low cycle rates (C/10), RGO lithiation begins to dominate. Under those conditions, only Ge nanowires exhibited significant lithiation relative to RGO, most likely due to the inherently faster lithiation of Ge compared to Si.

This paper reports our latest results using colloidal CuInSe₂ nanocrystal inks to prepare photovoltaic (PV) devices. Thus far, devices with nanocrystal layers processed under ambient conditions with no post-deposition treatment have achieved power conversion efficiencies of up to 3.1%. Device efficiency is largely limited by charge carrier trapping in the nanocrystal layer, and the highest device efficiencies are obtained with very thin layers-less than 150 nm-absorbing only a fraction of the incident light. Devices with thicker nanocrystal layers had lower power conversion efficiency, despite the increased photon absorption, because the internal quantum efficiency of the devices decreased significantly. The thin, most efficient devices exhibited internal quantum efficiencies as high as 40%, across a wide spectrum. Mott-Schottky measurements revealed that the active region thickness in the devices is approximately 50 nm.