Efficient Carrier Multiplication in Colloidal CuInSe₂ Nanocrystals

Transient absorption spectroscopy (TAS) was used to study carrier multiplication (CM) (also called multiexciton generation (MEG)) in solvent-dispersed colloidal CuInSe₂ nanocrystals with diameters as small as 4.5 nm. Size-dependent carrier cooling rates, absorption cross sections, and Auger lifetimes were also determined. The energy threshold for CM in the CuInSe₂ nanocrystals was found to be 2.4 ± 0.2 times the nanocrystal energy gap (<i>E</i>g) and the CM efficiency was 36 ± 6% per unit <i>E</i>g. This is similar to other types of nanocrystal quantum dot materials

Germanium Nanorod Extinction Spectra: Discrete Dipole Approximation Calculations and Experiment

Optical extinction spectra were measured for dispersions of germanium (Ge) nanorods produced by arrested solution–liquid–solid (SLS) growth using bismuth (Bi) seeds. Peaks in the real (<i>n</i>) and imaginary (<i>k</i>) parts of the complex index of refraction of Ge give rise to an absorbance peak at ∼600 nm, which shifts to slightly longer wavelengths with increased aspect ratio. Discrete dipole approximation calculations of absorption and scattering cross sections reveal that the length-dependent optical properties result from enhanced light trapping and absorption

Chains, Sheets, and Droplets: Assemblies of Hydrophobic Gold Nanocrystals with Saturated Phosphatidylcholine Lipid and Squalene

Assemblies of saturated 1,2-diacylphosphatidylcholine lipid and hydrophobic dodecanethiol-capped 1.8 nm diameter gold nanocrystals were studied as a function of lipid chain length and the addition of the naturally occurring oil, squalene. The gold nanocrystals formed various lipid-stabilized agglomerates, sometimes fusing with lipid vesicle bilayers. The nanocrystal assembly structure depended on the hydrocarbon chain length of the lipid fatty acids. The lipid with the shortest fatty acid length studied, dilauroylphosphatidylcholine, created extended chains of gold nanocrystals. The lipid with slightly longer fatty acid chains created planar sheets of nanocrystals. Further increases of the fatty acid chain length led to spherical agglomerates. The inclusion of squalene led to lipid- and nanocrystal-coated oil droplets

Low Temperature Colloidal Synthesis of Silicon Nanorods from Isotetrasilane, Neopentasilane, and Cyclohexasilane

Isotetrasilane, neopentasilane, and cyclohexasilane were studied as reactants for silicon (Si) nanorod growth in solution. These polysilane hydrides were found to enable lower growth temperatures in solution than any other silane reactants used to date. Cyclohexasilane enabled the lowest growth temperature of 200 °C, using a single-step solution–liquid–solid (SLS) reaction from tin (Sn) seeds. The potential for nanorod growth is determined by the reactivity of the silane. Cyclohexasilane is the most reactive of the polysilane hydrides studied here, requiring the lowest energy for dehydrogenation. The formation of silylene by cycolhexasilane also facilitates chemisorption onto the Sn surface during nanorod growth. Relatively bright photoluminescence (emission quantum yields of 2%) could still be achieved from Si nanorods grown at these low temperatures

Self-Assembly and Thermal Stability of Binary Superlattices of Gold and Silicon Nanocrystals

Simple hexagonal (sh) AB₂ binary superlattices (BSLs) of organic ligand-capped silicon (A; 5.40(±9.8%) nm diameter) and gold (B; 1.88(±10.1%) nm diameter) nanocrystals were assembled by evaporation of colloidal dispersions and characterized using transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). When deposited on tilted substrates by slow evaporation, the sh-AB₂ superlattice contracted slightly toward the substrate with centered orthorhombic structure. Heating the BSL to 200 °C in air led to gold coalescence and segregation to the surface of the assembly without disrupting the Si nanocrystal sublattice, thus creating a sh superlattice of Si nanocrystals

Chloroform-Enhanced Incorporation of Hydrophobic Gold Nanocrystals into Dioleoylphosphatidylcholine (DOPC) Vesicle Membranes

Vesicles of dioleoylphosphatidylcholine (DOPC) formed by extrusion (liposomes) with hydrophobic alkanethiol-capped Au nanocrystals were studied. Dodecanethiol-capped 1.8-nm-diameter Au nanocrystals accumulate in the lipid bilayer, but only when dried lipid–nanocrystal films were annealed with chloroform prior to hydration. Without chloroform annealing, the Au nanocrystals phase separate from DOPC and do not load into the liposomes. Au nanocrystals with slightly longer capping ligands of hexadecanethiol or with a larger diameter of 4.1 nm disrupted vesicle formation and created lipid assemblies with many internal lamellar attachments

Synthesis and Ligand Exchange of Thiol-Capped Silicon Nanocrystals

Hydride-terminated silicon (Si) nanocrystals were capped with dodecanethiol by a thermally promoted thiolation reaction. Under an inert atmosphere, the thiol-capped nanocrystals exhibit photoluminescence (PL) properties similar to those of alkene-capped Si nanocrystals, including size-tunable emission wavelength, relatively high quantum yields (>10%), and long radiative lifetimes (26–280 μs). X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy confirmed that the ligands attach to the nanocrystal surface via covalent Si–S bonds. The thiol-capping layer, however, readily undergoes hydrolysis and severe degradation in the presence of moisture. Dodecanethiol could be exchanged with dodecene by hydrosilylation for enhanced stability

The Role of Ligand Packing Frustration in Body-Centered Cubic (bcc) Superlattices of Colloidal Nanocrystals

This paper addresses the assembly of body centered-cubic (bcc) superlattices of organic ligand-coated nanocrystals. First, examples of bcc superlattices of dodecanethiol-capped Au nanocrystals and oleic acid-capped PbS and PbSe nanocrystals are presented and examined by transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). These superlattices tend to orient on their densest (110) superlattice planes and exhibit a significant amount of {112} twinning. The same nanocrystals deposit as monolayers with hexagonal packing, and these thin films can coexist with thicker bcc superlattice layers, even though there is no hexagonal plane in a bcc lattice. Both the preference of bcc in bulk films over the denser face-centered cubic (fcc) superlattice structure and the transition to hexagonal monolayers can be rationalized in terms of packing frustration of the ligands. A model is presented to calculate the difference in entropy associated with capping ligand packing frustration in bcc and fcc superlattices

The Role of Ligand Packing Frustration in Body-Centered Cubic (bcc) Superlattices of Colloidal Nanocrystals

This paper addresses the assembly of body centered-cubic (bcc) superlattices of organic ligand-coated nanocrystals. First, examples of bcc superlattices of dodecanethiol-capped Au nanocrystals and oleic acid-capped PbS and PbSe nanocrystals are presented and examined by transmission electron microscopy (TEM) and grazing incidence small-angle X-ray scattering (GISAXS). These superlattices tend to orient on their densest (110) superlattice planes and exhibit a significant amount of {112} twinning. The same nanocrystals deposit as monolayers with hexagonal packing, and these thin films can coexist with thicker bcc superlattice layers, even though there is no hexagonal plane in a bcc lattice. Both the preference of bcc in bulk films over the denser face-centered cubic (fcc) superlattice structure and the transition to hexagonal monolayers can be rationalized in terms of packing frustration of the ligands. A model is presented to calculate the difference in entropy associated with capping ligand packing frustration in bcc and fcc superlattices

Colloidal Luminescent Silicon Nanorods

Silicon nanorods are grown by trisilane decomposition in hot squalane in the presence of tin (Sn) nanocrystals and dodecylamine. Sn induces solution–liquid–solid nanorod growth with dodecylamine serving as a stabilizing ligand. As-prepared nanorods do not luminesce, but etching with hydrofluoric acid to remove residual surface oxide followed by thermal hydrosilylation with 1-octadecene induces bright photoluminescence with quantum yields of 4–5%. X-ray photoelectron spectroscopy shows that the ligands prevent surface oxidation for months when stored in air