Mesostructured Materials with Controllable Long-Range Orientational Ordering and Anisotropic Properties.

Inorganic-organic mesophase materials provide a wide range of tunable properties, which are often highly dependent on their nano-, micro-, or meso-scale compositions and structures. Among these are macroscopic orientational order and corresponding anisotropic material properties, the adjustability of which are difficult to achieve. This is due to the complicated transient and coupled transport, chemical reaction, and surface processes that occur during material syntheses. By understanding such processes, general criteria are established and used to prepare diverse mesostructured materials with highly aligned channels with uniform nanometer dimensions and controllable directionalities over macroscopic dimensions and thicknesses. This is achieved by using a micropatterned semipermeable poly(dimethylsiloxane) stamp to manage the rates, directions, and surfaces at which self-assembling phases nucleate and the directions that they grow. This enables mesostructured surfactant-directed silica and titania composites, including with functional guest species, and mesoporous carbons to be prepared with high degrees of hexagonal order, as well as controllable orthogonal macroscopic orientational order. The resulting materials exhibit novel anisotropic properties, as demonstrated by the example of direction-dependent photocurrent generation, and are promising for enhancing the functionality of inorganic-organic nanocomposite materials in separations, catalysis, and energy conversion applications.

Fast Electron and Slow Hole Relaxation in InP-Based Colloidal Quantum Dots.

Colloidal InP-based quantum dots are a promising material for light-emitting applications as an environment friendly alternative to their Cd-containing counterparts. Especially for their use in optoelectronic devices, it is essential to understand how charge carriers relax to the emitting state after injection with excess energy and if all of them arrive at this desired state. Herein, we report time-resolved differential transmission measurements on colloidal InP/ZnS and InP/ZnSe core/shell quantum dots. By optically exciting and probing individual transitions, we are able to distinguish between electron and hole relaxation. This, in turn, allows us to determine how the initial excess energy of the charge carriers affects the relaxation processes. According to the electronic level scheme, one expects a strong phonon bottleneck for electrons, whereas holes should relax easier as their energy levels are more closely spaced. On the contrary, we find that electrons relax faster than holes. The fast electron relaxation occurs via an efficient Auger-like electron-hole scattering mechanism. On the other hand, a small wave function overlap between core and shell states slows the hole relaxation. Additionally, holes can be trapped at the core/shell interface, leading to either slow detrapping or nonradiative recombination. Overall, these results demonstrate that it is crucial to construct devices enabling the injection of charge carriers energetically close to their emitting states in order to maximize the radiative efficiency of the system.

Semiconductor Seeded Nanorods with Graded Composition Exhibiting High Quantum-Yield, High Polarization, and Minimal Blinking.

Seeded semiconductor nanorods represent a unique family of quantum confined materials that manifest characteristics of mixed dimensionality. They show polarized emission with high quantum yield and fluorescence switching under an electric field, features that are desirable for use in display technologies and other optical applications. So far, their robust synthesis has been limited mainly to CdSe/CdS heterostructures, thereby constraining the spectral tunability to the red region of the visible spectrum. Herein we present a novel synthesis of CdSe/Cd1-xZnxS seeded nanorods with a radially graded composition that show bright and highly polarized green emission with minimal intermittency, as confirmed by ensemble and single nanorods optical measurements. Atomistic pseudopotential simulations elucidate the importance of the Zn atoms within the nanorod structure, in particular the effect of the graded composition. Thus, the controlled addition of Zn influences and improves the nanorods' optoelectronic performance by providing an additional handle to manipulate the degree confinement beyond the common size control approach. These nanorods may be utilized in applications that require the generation of a full, rich spectrum such as energy-efficient displays and lighting.

Electronically coupled hybrid structures by graphene oxide directed self-assembly of Cu(2-x)S nanocrystals.

Here, we describe an electronically coupled hybrid material consisting of graphene oxide (GO) flakes and inorganic Cu(2-x)S nanocrystals (NCs) formed via a self-assembly route. As a result of the amphiphilic nature of the water-dispersible GO flakes, the hydrophobic Cu(2-x)S NCs self-assemble in between the GO flakes, resulting in a large-interface hybrid structure with ordered close-packed NCs. We demonstrate that the optical properties of the hybrid GO/Cu(2-x)S structures are governed by the injection of electrons from the GO flakes to the valence band of the vacancy-doped plasmonic Cu(2-x)S NCs. This leads to a suppression of the plasmon band of the Cu(2-x)S NCs and to a softening of the Raman G-band of the GO flakes. Our results indicate that graphene derivatives can act not only as a self-assembly directing template, but also as a tool to affect the optical properties of self-assembled NCs in a chemical process, enhanced by the high interface area of the composite.