Nicholas Kirkwood

Ga for Zn Cation Exchange Allows for Highly Luminescent and Photostable InZnP-Based Quantum Dots

In this work, we demonstrate that a preferential Ga-for-Zn cation exchange is responsible for the increase in photoluminescence that is observed when gallium oleate is added to InZnP alloy QDs. By exposing InZnP QDs with varying Zn/In ratios to gallium oleate and monitoring their optical properties, composition, and size, we conclude that Ga3+ preferentially replaces Zn2+, leading to the formation of InZnP/InGaP core/graded-shell QDs. This cation exchange reaction results in a large increase of the QD photoluminescence, but only for InZnP QDs with Zn/In ≥ 0.5. For InP QDs that do not contain zinc, Ga is most likely incorporated only on the quantum dot surface, and a PL enhancement is not observed. After further growth of a GaP shell and a lattice-matched ZnSeS outer shell, the cation-exchanged InZnP/InGaP QDs continue to exhibit superior PL QY (over 70%) and stability under long-term illumination (840 h, 5 weeks) compared to InZnP cores with the same shells. These results provide important mechanistic insights into recent improvements in InP-based QDs for luminescent applications.

Two Mechanisms Determine Quantum Dot Blinking

Many potential applications of quantum dots (QDs) can only be realized once the luminescence from single nanocrystals (NCs) is understood. These applications include the development of quantum logic devices, single-photon sources, long-life LEDs, and single-molecule biolabels. At the single-nanocrystal level, random fluctuations in the QD photoluminescence occur, a phenomenon termed blinking. There are two competing models to explain this blinking: Auger recombination and surface trap induced recombination. Here we use lifetime scaling on core-shell chalcogenide NCs to demonstrate that both types of blinking occur in the same QDs. We prove that Auger-blinking can yield single-exponential on/off times in contrast to earlier work. The surface passivation strategy determines which blinking mechanism dominates. This study summarizes earlier studies on blinking mechanisms and provides some clues that stable single QDs can be engineered for optoelectronic applications.

Protic additives determine the pathway of CdSe nanocrystal growth

The formation of semiconductor nanocrystals by hot-injection synthesis follows complex reaction mechanisms that are not yet fully understood. In particular the occurrence of intermediate species indicated by sharp, stationary spectral lines poses an important deviation from the predictions of classical nucleation theory. We show that trace amounts of water and other protic additives strongly impact the structure of these reaction intermediates, forming either coordination polymers under dry conditions or small clusters in the presence of moisture. These intermediates bind monomer during the initial nucleation phase. The structure of the intermediate determines the monomer release rate, either continuously or in a rapid dissolution event, and hence controls the reaction kinetics. From this we propose a kinetic model that allows us to predict secondary nucleation events. By directing the type of intermediate formed, protic additives provide a lever to manipulate this equilibrium and control nanocrystal synthesis in a rational fashion.

Tuning Single Quantum Dot Emission with a Micromirror

The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.

Colloidal Stability of Apolar Nanoparticles: Role of Ligand Length

Inorganic nanoparticle cores are often coated with organic ligands to render them dispersible in apolar solvents. However, the effect of the ligand shell on the colloidal stability of the overall hybrid particle is not fully understood. In particular, it is not known how the length of an apolar alkyl ligand chain affects the stability of a nanoparticle dispersion against agglomeration. Here, small-angle X-ray scattering and molecular dynamics simulations have been used to study the interactions between gold nanoparticles and between cadmium selenide nanoparticles passivated by alkanethiol ligands with 12-18 carbons in the solvent decane. We find that increasing the ligand length increases colloidal stability in the core-dominated regime but decreases it in the ligand-dominated regime. This unexpected inversion is connected to the transition from ligand-dominated to core-dominated agglomeration when the core diameter increases at constant ligand length. Our results provide a microscopic picture of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.

Spectroelectrochemical Signatures of Surface Trap Passivation on CdTe Nanocrystals

The photoluminescence (PL) quantum yield of semiconductor nanocrystals (NCs) is hampered by in-gap trap states due to dangling orbitals on the surface of the nanocrystals. While crucial for the rational design of nanocrystals, the understanding of the exact origin of trap states remains limited. Here, we treat CdTe nanocrystal films with different metal chloride salts and we study the effect on their optical properties with in situ spectroelectrochemistry, recording both changes in absorption and photoluminescence. For untreated CdTe NC films we observe a strong increase in the PL intensity as the Fermi-level is raised electrochemically and trap states in the bandgap become occupied with electrons. Upon passivation of these in-gap states we observe an increase in the steady state PL and, for the best treatments, we observe that the PL no longer depends on the position of the Fermi level in the band gap, demonstrating the effective removal of trap states. The most effective treatment is obtained for Z-type passivation with CdCl2, for which the steady state PL increased by a factor 40 and the PL intensity became nearly unaffected by the applied potential. X-ray Photoelectron Spectroscopy measurements show that treatment with ZnCl2 mainly leads to X-type passivation with chloride ions, which increased the PL intensity by a factor four and made the PL less susceptible to modulation by applying a potential with respect to unpassivated nanocrystal films. We elucidate the spectroelectrochemical signatures of trap states within the bandgap and conclude that undercoordinated Te at the surface constitutes the largest contribution to in-gap trap states, but that other surface states that likely originate on Cd atoms should also be considered.

Highly-photoconductive InP quantum dots films and solar cells

InP and InZnP colloidal quantum dots (QDs) are promising materials for application in light-emitting devices, transistors, photovoltaics, and photocatalytic cells. In addition to possessing an appropriate bandgap, high absorption coefficient, and high bulk carrier mobilities, the intrinsic toxicity of InP and InZnP is much lower than for competing QDs that contain Cd or Pb–providing a potentially safer commercial product. However, compared to other colloidal QDs, InP QDs remain sparsely used in devices and their electronic transport properties are largely unexplored. Here, we use time-resolved microwave conductivity measurements to study charge transport in films of InP and InZnP colloidal quantum dots capped with a variety of short ligands. We find that transport in InP QDs is dominated by trapping effects, which are mitigated in InZnP QDs. We improve charge carrier mobilities with a range of ligand-exchange treatments and for the best treatments reach mobilities and lifetimes on par with those of PbS QD films used in efficient solar cells. To demonstrate the device-grade quality of these films, we construct solar cells based on InP & InZnP QDs with power conversion efficiencies of 0.65 and 1.2%, respectively. This represents a large step forward in developing Cd- and Pb-free next-generation optoelectronic devices.