Stijn Flamée

Giant and Broad-Band Absorption Enhancement in Colloidal Quantum Dot Monolayers through Dipolar Coupling

The absorption cross section of colloidal quantum dots in close-packed monolayers shows a 4 (CdSe) to 5-fold (PbS) enhancement compared to quantum dots in a dilute dispersion. Quantitative agreement is demonstrated between the value and the size dependence of the enhancement and theoretical model predictions based on dipolar coupling between neighboring quantum dots. This collective optical behavior offers a new degree of freedom in the custom design of optical properties for electro-optical devices.

Annealing of sulfide stabilized colloidal semiconductor nanocrystals

The effect of thermal annealing on layers of CuInS2 nanocrystals (NCs) stabilized with (NH4)2S was investigated using in situ transmission electron microscopy (TEM), in situ X-ray diffraction (XRD), thermogravimetric analysis combined with mass spectrometry (TGA-MS) and X-ray photoelectron spectroscopy (XPS). It is shown that these inorganic, chalcogen containing ligands inhibit NC sintering up to 450 °C in an inert atmosphere. On the other hand, sintering can be promoted by annealing in hydrogen gas. A similar behavior is found with Cu2ZnSnSe4 and CdSe NCs. We attribute the inhibited sintering to the oxidation of the S2− originally stabilizing the NCs to sulfite or sulfate moieties, where oxidation is possible either by exposure of the films to air or by thermal decomposition of residual solvent molecules present in the film under inert conditions.

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.

Study of hole mobility in poly(N-vinylcarbazole) films doped with CdSe/ZnS quantum dots encapsulated by 11-(N-carbazolyl) undecanoic acid (C11)

We report the experimental study of hole transport in poly(vinylcarbazole) (PVK) films doped with colloidal CdSe/ZnS core-shell quantum dots (QDs) determined using the Time-of-Flight (TOF) method. The miscibility between PVK and the QDs is improved by capping the QDs with a novel 11-(N-carbazolyl) undecanoic acid (C11) ligand instead of commonly used organic ligands, such as oleic acid. The study of the hole mobility of the pristine and doped PVK films with a doping concentration of the C11 capped QDs ranging from 1.61 × 1017 to 7.10 × 1018 dots/cm3 was performed as a function of electric field and temperature in the range of 105–106 V/cm and 298–338 K, respectively. Upon increasing the QD concentration, a decrease of hole mobility was observed by up to nearly 2 orders in magnitude at a doping concentration of 3.87 × 1018 dots/cm3 at T = 298 K. This suggests that the QDs induce shallow hole traps. The field and temperature dependence of the hole mobility was characterized using the Bassler disorder model and showed an increase of the energetic disorder (σ) from 124 to 204 meV as well as of the spatial disorder (Σ) from 0.95 to 5 when the concentration of the QDs was increased to 3.87 × 1018 dots/cm3. At higher concentration of the QDs (7.10 × 1018 dots/cm3), an increase of the hole mobility was observed suggesting hopping of the holes through the QD clusters. In addition, we also found that for this high doping concentration, the field dependence of the hole mobility was no longer in agreement with the Bassler disorder model. One should consider that at this doping concentration, the volume occupied by the inorganic (CdSe + ZnS) and organic (C11) components of the QDs in the doped film was estimated to be 14.6 and 15.8 volume %, respectively. This implies that the volume fraction of the inorganic material is very close to the percolation threshold, which amounts to 17 volume % for small spherical particles embedded in a three dimensional matrix. Furthermore, the conductivity data suggest a qualitative change in film properties between the samples with 3.87 × 1018 and 7.10 × 1018 dots/cm3. The study of film morphology by atomic force microscopy (AFM) experiment shows that while for the film with 3.87 × 1018 dots/cm3 the surface of the film has still the same features as that of a pristine PVK film, this is no longer the case for the film with 7.10 × 1018 dots/cm3, where shallow holes with a diameter of 100 to 200 nm become visible. These holes with the size much larger than the diameter of an individual QD likely correspond to clusters of the QDs. Upon further increasing the QD concentration to 9.68 × 1018 dots/cm3, the density of these holes is also increased. A correlation between the conductivity data and the film morphologies indicates that the presence of these QD clusters in the sample with 7.10 × 1018 dots/cm3 does not only change the homogeneity and roughness of the film but also leads to a significant change in the shape of the density of states of the energy sites for hopping holes resulting in a field and temperature dependence of the hole mobility that is no longer compatible with the Gaussian disorder model for this sample. Furthermore, the presence of these “hole” structures observed with AFM might imply a formation of large QD clusters in the polymer film, which form continuous pathways for charge carrier hopping between the opposite electrodes.