Marco Cirillo

Thermal Charging of Colloidal Quantum Dots in Apolar Solvents: A Current Transient Analysis

We analyze thermal charging in additive-free colloidal CdSe quantum dot (QD) dispersions by means of the transient electric current resulting from a voltage step applied across the QD dispersion. On the basis of the initial current and the total charge separated, we find that the CdSe dispersion behaves as a 1:1 electrolyte where equal fractions of the QDs carry a single positive or a single negative charge. This conclusion is confirmed by a more detailed fitting of the current transient using the Nernst-Planck-Poisson equations. Using equilibrium thermodynamics, we relate the fraction of charged QDs to the QD charging energy. The magnitude of the charging energy corresponds to values found using known models for the charging energy of either a spherical surface in a dielectric or a charge within a dielectric sphere. However, the experimental dependence of the charging energy on the dielectric constant of the solvent is far less pronounced than predicted by these models. A better correspondence is found based on the charging energy of a spherical surface embedded in a compound medium consisting of the ligand shell and the solvent.

Carbon nanotube growth from Langmuir–Blodgett deposited Fe3O4 nanocrystals

We investigate colloidal Fe(3)O(4) nanocrystals as a catalyst system for carbon nanotube (CNT) growth that allows for decoupling the CNT growth step from the catalyst shaping and activation step. The system consists of 6.4 nm Fe(3)O(4) nanocrystals synthesized using a solution-based thermal decomposition reaction and, subsequently, transferred as hexagonally ordered Langmuir-Blodgett (LB) monolayers on TiN substrates. We demonstrate for the first time aligned CNT growth from LB deposited nanocrystals on a metallic underlayer. The hexagonally ordered monolayers of catalyst particles show promising stability up to the CNT growth temperature. In situ TEM heating experiments were performed to find this onset of particle deformation and showed stability of the nanoparticles up to 600 °C. The particle coalescence at high temperatures was also evidenced by the increasing CNT diameter, from 9.5 nm at 580 °C to 16 nm at 630 °C. By choosing to work at temperatures below the onset particle coalescence temperature, equivalent CNT diameters were obtained under different catalyst activation and growth conditions. The high stability of the catalyst on the metallic underlayer enables us to study CNT growth kinetics independently of the catalyst shaping step. This work opens a route towards combining growth studies with an electrical evaluation of the CNT growth as the TiN can be used as the bottom contact.

The micropatterning of layers of colloidal quantum dots with inorganic ligands using selective wet etching

The micropatterning of layers of colloidal quantum dots (QDs) stabilized by inorganic ligands is demonstrated using PbS core and CdSe/CdS core/shell QDs. A layer-by-layer approach is used to assemble the QD films, where each cycle involves the deposition of a QD layer by dip-coating, and the replacement of the native organic ligands by inorganic moieties, such as OH(-) and S(2-), followed by a thorough cleaning of the resulting film. This results in a smooth and crack-free QD film on which a photoresist can be spun. The micropatterns are defined by a positive photoresist, followed by the removal of uncovered QDs by selective wet etching with an HCl/H3PO4 mixture. The resulting patterns can have submicron feature dimensions, limited by the resolution of the lithographic process, and can be formed on planar and 3D substrates. It is shown that the photolithography and wet etching steps have little effect on the photoluminescence quantum yield of CdSe/CdS QDs. Compared with the unpatterned CdSe/CdS QD film, only a 10% degradation in the quantum yield is observed. These results demonstrate the feasibility of the proposed micropatterning method to implement the large-scale device integration of colloidal quantum dots.