Synthesis and Optical Properties of PbSe Nanorods with Controlled Diameter and Length

The synthesis of PbSe nanorods with low branching (<1%), high aspect ratios (up to ∼16), and controlled lengths and diameters was demonstrated via the removal of water and oleic acid from the synthesis precursors. It was determined that the proper combination of reaction time and temperature allows for the control of PbSe nanorod length and diameter and therefore control over their electronic states, as probed through absorbance and photoluminescence measurements. Similar to PbSe nanowires, nanorods display higher Stokes shifts than for spherical nanocrystals due to intrananorod diameter fluctuations

Anisotropic Absorption in PbSe Nanorods

We present absorption anisotropy measurements in PbSe nanostructures. This is accomplished <i>via</i> a new means of measuring absorption anisotropy in randomly oriented solution ensembles of nanostructures <i>via</i> pump–probe spectroscopy, which exploits the polarization memory effect. We observe isotropic absorption in nanocrystals and anisotropic absorption in nanorods, which increases upon elongation from aspect ratio 1 to 4 and is constant for longer nanorods. The measured volume-normalized absorption cross section is 1.8 ± 0.3 times larger for parallel pump and probe polarizations in randomly oriented nanorods as compared to nanocrystals. We show that this enhancement would be larger than an order of magnitude for aligned nanorods. Despite being in the strong quantum confinement regime, the aspect ratio dependence of the absorption anisotropy in PbSe nanorods is described classically by the effects of dielectric contrast on an anisotropic nanostructure. These results imply that the dielectric constant of the surrounding medium can be used to influence the optoelectronic properties of nanorods, including polarized absorption and emission, phonon modes, multiple exciton generation efficiency, and Auger recombination rate

Control of PbSe Nanorod Aspect Ratio by Limiting Phosphine Hydrolysis

The aspect ratio and yield of PbSe nanorods synthesized by the reaction of Pb-oleate with tris­(diethylamino)­phosphine selenide are highly sensitive to the presence of water, making it critical to control the amount of water present in the reaction. By carefully drying the reaction precursors and then intentionally adding water back into the reaction, the nanorod aspect ratio can be controlled from 1.1 to 10 and the yield from 1 to 14% by varying the water concentration from 0 to 204 mM. ³¹P­{¹H} and ¹H NMR show that water reacts with tris­(diethylamino)­phosphine to create bis­(diethylamido)­phosphorous acid. It was determined that bis­(diethylamido)­phosphorous acid is responsible for the observed aspect ratio and yield changes. Finally, it was found that excess oleic acid in the reaction can also react with tris­(diethylamino)­phosphine to create bis­(diethylamido)­phosphorous acid, and upon the removal of both excess oleic acid and water, highly uniform, nonbranching nanorods were formed

Size and Temperature Dependence of Band-Edge Excitons in PbSe Nanowires

We report the attenuance and temperature-dependent photoluminescence spectra of PbSe nanowires with diameters between 5.6 and 26.4 nm (12−23% relative standard deviation) and lengths greater than 1 μm. The nanowire first exciton energy varies between 0.3 and 0.6 eV as the diameter decreases from 26.4 to 5.6 nm, respectively. Compared to spherical PbSe nanocrystals, PbSe nanowires show less quantum confinement and larger Stokes shifts. The band gap temperature coefficient (d<i>E</i>_g/d<i>T</i>) decreases as the nanowire diameter decreases, consistent with previous results for PbSe spherical nanocrystals

Sulfur-Capped Germanium Nanocrystals: Facile Inorganic Ligand Exchange

The development of applications for germanium nanocrystals has been hindered by the limited availability of synthetic methods coupled with poorly understood ligand-exchange chemistry. Herein we describe the synthesis of germanium nanocrystals and ligand exchange experiments designed to establish facile routes toward ligand replacement and, consequently, layers that are amenable to charge-transfer. After assessing thiols, carboxylates, and dithiocarbamates, sulfur dissolved in 1-ocatadecene was determined to be the most amenable to ligand exchange, with over 95% of the initial alkylamine ligand replaced as determined by Fourier transform infrared spectroscopy (FTIR). These results were in good agreement with density functional theory calculations showing a strong preference for Ge–S bonding. The materials were fully characterized via powder X-ray diffraction, FTIR, transmission electron microscopy, and X-ray and UV photoelectron spectroscopy. This new ligand exchange procedure provides a possible route toward the fabrication of thin films that may be employed in such applications as photovoltaic devices

Synthesis and Characterization of PbS/ZnS Core/Shell Nanocrystals

We demonstrate a synthetic method to add a ZnS shell, with controlled thickness, to PbS nanocrystals using Zn oleate and thioacetamide as Zn and S precursors. The ZnS shell reaction is self-limiting and deposits approximately a monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. The reaction is self-limiting because the sulfur precursor, thioacetamide, is less reactive toward the PbS/ZnS core/shell nanocrystal surface as compared to the Zn oleate precursor present in the reaction solution. To increase the ZnS shell thickness beyond a monolayer, subsequent ZnS shell reactions are modified by adding the thioacetamide 10 minutes before the Zn oleate. This gives the thioacetamide time to react at the PbS/ZnS core/shell nanocrystal surface before the Zn oleate is added. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) shows most ZnS shells lack crystalline order. However, select core/shell nanocrystals have epitaxial crystalline (zinc-blende) ZnS shells or crystalline (zinc-blende) shells with no obvious epitaxial relationship to the PbS core. The PbS core 1S_h–1S_e absorbance and photoluminescence peak energies redshift upon shell addition due to relief of a ligand-induced tensile strain and wave function leakage into the shell. The photoluminescence quantum yield decreases after ZnS shell addition likely due to nonradiative defect states at the core/shell interface