Carrier Cooling in Colloidal Quantum Wells

It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators

Low-Threshold Stimulated Emission Using Colloidal Quantum Wells

The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 μJ/cm² and gain as high as 600 cm^–1, both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths

Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets

There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm², lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials

New Forms of CdSe: Molecular Wires, Gels, and Ordered Mesoporous Assemblies

This work investigates the structure and properties of soluble chalcogenidocadmates, a molecular form of cadmium chalcogenides with unprecedented one-dimensional bonding motifs. The single crystal X-ray structure reveals that sodium selenocadmate consists of infinite one-dimensional wires of (Cd₂Se₃)_<i>n</i>^2<i>n</i>– charge balanced by Na⁺ and stabilized by coordinating solvent molecules. Exchanging the sodium cation with tetraethylammonium or didodecyldimethylammonium expands the versatility of selenocadmate by improving its solubility in a variety of polar and nonpolar solvents without changing the anion structure and properties. The introduction of a micelle-forming cationic surfactant allows for the templating of selenocadmate, or the analogous telluride species, to create ordered organic–inorganic hybrid CdSe or CdTe mesostructures. Finally, the interaction of selenocadmate “wires” with Cd²⁺ ions creates an unprecedented gel-like form of stoichiometric CdSe. We also demonstrate that these low-dimensional CdSe species show characteristic semiconductor behavior, and can be used in photodetectors and field-effect transistors

Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te^2– Ligands Lead To Charge Carrier Diffusion Lengths Over 2 μm

We report on photoconductivity of films of CdTe nanocrystals (NCs) using time-resolved microwave photoconductivity (TRMC). Spherical and tetrapodal CdTe NCs with tunable size-dependent properties are studied as a function of surface ligand (including inorganic molecular chalcogenide species) and annealing temperature. Relatively high carrier mobility is measured for films of sintered tetrapod NCs (4 cm²/(V s)). Our TRMC findings show that Te^2– capped CdTe NCs show a marked improvement in carrier mobility (11 cm²/(V s)), indicating that NC surface termination can be altered to play a crucial role in charge-carrier mobility even after the NC solids are sintered into bulk films

Surface Functionalization of Semiconductor and Oxide Nanocrystals with Small Inorganic Oxoanions (PO₄^3–, MoO₄^2–) and Polyoxometalate Ligands

In this work, we study the functionalization of the nanocrystal (NC) surface with inorganic oxo ligands, which bring a new set of functionalities to all-inorganic colloidal nanomaterials. We show that simple inorganic oxoanions, such as PO₄^3– and MoO₄^2–, exhibit strong binding affinity to the surface of various II–VI and III–V semiconductor and metal oxide NCs. ζ-Potential titration offered a useful tool to differentiate the binding affinities of inorganic ligands toward different NCs. Direct comparison of the binding affinity of oxo and chalcogenidometallate ligands revealed that the former ligands form a stronger bond with oxide NCs (<i>e.g.</i>, Fe₂O₃, ZnO, and TiO₂), while the latter prefer binding to metal chalcogenide NCs (<i>e.g.</i>, CdSe). The binding between NCs and oxo ligands strengthens when moving from small oxoanions to polyoxometallates (POMs). We also show that small oxo ligands and POMs make it possible to tailor NC properties. For example, we observed improved stability upon Li⁺-ion intercalation into the films of Fe₂O₃ hollow NCs when capped with MoO₄^2– ligands. We also observed lower overpotential and enhanced exchange current density for water oxidation using Fe₂O₃ NCs capped with [P₂Mo₁₈O₆₂]^6– ligands and even more so for [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂] with POM as the capping ligand