Direct Imaging of Long-Range Exciton Transport in Quantum Dot Superlattices by Ultrafast Microscopy.

Long-range charge and exciton transport in quantum dot (QD) solids is a crucial challenge in utilizing QDs for optoelectronic applications. Here, we present a direct visualization of exciton diffusion in highly ordered CdSe QDs superlattices by mapping exciton population using ultrafast transient absorption microscopy. A temporal resolution of ∼200 fs and a spatial precision of ∼50 nm of this technique provide a direct assessment of the upper limit for exciton transport in QD solids. An exciton diffusion length of ∼125 nm has been visualized in the 3 ns experimental time window and an exciton diffusion coefficient of (2.5 ± 0.2) × 10(-2) cm(2) s(-1) has been measured for superlattices constructed from 3.6 nm CdSe QDs with center-to-center distance of 6.7 nm. The measured exciton diffusion constant is in good agreement with Förster resonance energy transfer theory. We have found that exciton diffusion is greatly enhanced in the superlattices over the disordered films with an order of magnitude higher diffusion coefficient, pointing toward the role of disorder in limiting transport. This study provides important understandings on energy transport mechanisms in both the spatial and temporal domains in QD solids.

In situ optical and structural studies on photoluminesence quenching in CdSe/CdS/Au heterostructures.

We report here detailed in situ studies of nucleation and growth of Au on CdSe/CdS nanorods using synchrotron SAXS technique and time-resolved spectroscopy. We examine structural and optical properties of CdSe/CdS/Au heterostructures formed under UV illumination. We compare the results for CdSe/CdS/Au heterostructures with the results of control experiments on CdSe/CdS nanorods exposed to gold precursor under conditions when no such heterostructures are formed (no UV illumination). Our data indicate similar photoluminescence (PL) quenching and PL decay profiles in both types of samples. Via transient absorption and PL, we show that such behavior is consistent with rapid (faster than 3 ps) hole trapping by gold-sulfur sites at the surface of semiconductor nanoparticles. This dominant process was overlooked in previous end-point studies on semiconductor/metal heterostructures.

Submicron trenches reduce the Pseudomonas fluorescens colonization rate on solid surfaces.

Bacterial adhesion and spreading on biomaterials are considered key features of pathogenicity. Roughness and topography of the substrate have been reported to affect bacterial adhesion, but little is known about their effect on spreading. Submicron row and channel tuning with bacterial diameter (S2) were designed to test bacterial motility on these surfaces. Random nanometer-sized structures (S1) were used as controls. Optical microscopy and AFM were employed to detect biological and surface pattern details in the micro- and nanoscale, respectively. Results showed that motility strategies (flagella orientation, elongation, aggregation in rafts, formation of network structures, and development of a bacterial frontier) were affected by the presence of submicropatterns. Importantly, the rate of bacterial spreading on S2 was significantly reduced and influenced by the orientation of the submicropatterns. Consequently, submicroengineered substrates could be employed as a tool to downgrade bacterial colonization. Such patterns could impact on the design of proper engineered structures to control biofilm spreading on solid surfaces.

Electrochemical deposition onto self-assembled monolayers: new insights into micro- and nanofabrication.

Pattern transfer with high resolution is a frontier topic in the emerging field of nanotechnologies. Electrochemical molding is a possible route for nanopatterning metal, alloys and oxide surfaces with high resolution in a simple and inexpensive way. This method involves electrodeposition onto a conducting master covered by a self-assembled alkanethiolate monolayer (SAMs). This molecular film enables direct surface-relief pattern transfer from the conducting master to the inner face of the electrodeposit, and also allows an easy release of the electrodeposited film due their excellent anti-adherent properties. Replicas of the original conductive master can be also obtained by a simple two-step procedure. SAM quality and stability under electrodeposition conditions combined with the formation of smooth electrodeposits are crucial to obtain high-quality pattern transfer with sub-50 nm resolution.