UC Merced

Surface modification is a versatile and effective route towards improving functional and structural characteristics of chemically synthesized nanomaterials. In the specific case of semiconducting nanoparticles (quantum dots) the photophysical properties are strongly tied to surface conditions. Therefore, a careful monitoring of photoluminescent (PL) behavior, both short and long term, is critical following alterations to their surface chemistry. We observe several noteworthy changes in the static and dynamic PL spectra of CdSe/ZnS core-shell quantum dots (QDs) when the as-grown native ligands are exchanged with two different mesogenic ligands – rod-like molecules attached to the particle by a flexible alkyl chain. These include reduced inter-dot energy transfer, stable recombination rates and steady emission color over more than an hour of continuous photo-excitation, all effects being more prominent in the sample with the longer attachment chain. Temperature dependence of PL and recombination rates reveals further differences. Thermally-activated PL recovery threshold is pushed to a higher temperature in the modified dots, while PL lifetime does not show the expected increase with decreasing temperature. Our results indicate that increased charge separation induced by the longer ligands is responsible for these effects, and this may be a route to fabricating quantum dot films for specific applications demanding long term emission color stability.

The design and development of multifunctional composite materials from artificial nano-constituents is one of the most compelling current research areas. This drive to improve over nature and produce ‘meta-materials’ has met with some success, but results have proven limited with regards to both the demonstration of synergistic functionalities and in the ability to manipulate the material properties post-fabrication and in situ. Here, magnetic nanoparticles (MNPs) and semiconducting QDs are co-assembled in a nematic liquid crystalline (LC) matrix, forming composite structures in which the emission intensity of the quantum dots is systematically and reversibly controlled with a small applied magnetic field (< 100 mT). This magnetic field-driven brightening, ranging between a two- to three-fold intensity increase, is a truly cooperative effect: the LC phase transition creates the co-assemblies, the clustering of the MNPs produces allows LC re-orientation at atypical low external field, and this re-arrangement produces compaction of the clusters, resulting in the detection of increased QD emission. These results demonstrate a synergistic, reversible, and an all-optical process to detect magnetic fields and additionally, as the clusters are self-assembled in a fluid medium, they offer the possibility for these sensors to be used in broad ranging fluid-based applications.

An important experimental realization resulted from the sensors. We found we could slightly modify the synthesis procedure by holding the temperature fixed approximately every 0.05 °C during the LC phase transition to generate ring-like NP structures. During the phase transition, the LC exists in a bi-phasic state which pushes the NPs to assemble in lines less than 1 μm. These rings still show potential to act as sensors, but are significantly smaller. However, to improve the sensors to be more sensitive, it became clear that we need to revise our model and mechanism behind the induced QD brightening. Namely, there is a scale mismatch between using optical microscopy to investigate, and the spatial reorganization happening on the nanoscale. By performing transmission electron microscopy (TEM), it was evident the MNPs have a tendency to remain dispersed in the LC, possibly due to stronger MNP aggregation and also stronger interactions to the sample surface, leading to the preference of “rings.” Furthermore, we investigated only MNPs in LC at the nano-scale when an external magnetic field is applied via Lorentz TEM and found that the LC matrix allows the MNPs to controllably rotate with the field, aligning their easy axis in such a way that generates a stronger net magnetization. It is this overall increase in magnetization that must be responsible for the spatial reorganization of the LC and subsequently the QDs, causing an increase in QDs per unit area and in turn the ensemble becomes brighter.

The main goal of this research combines both improving the stability of QDs and utilizing their fluorescent properties in magnetic field sensing devices. But, the mechanism for which this device works at the nanoscale is an important step toward making fluid-based processing a reality.