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Ian Howard
Dr. Ian Howard
Advanced Materials and Optical Spectroscopy

Phone: +49 721 608-28398
ian howardRyp9∂kit edu

Staff webpage


Excited-State Dynamics in Organic Semiconductors, Nanocrystals, and Metal-organic Frameworks

Figure 1: Schematic of the processes of photon upconversion by triplet annihilation in a bilayer stack of surface-anchored metal-organic frameworks.
For more information please see the publication or press release.

We are interested in understanding and controlling the motion of excited states within novel semiconductors, be it charge-carrier hopping away from an exciton-splitting interface in an organic solar cell or the motion of triplet excitons within crystalline metal-organic frameworks. Gaining an understanding of how excited-states move in nanoscale spatial dimensions on ultrafast timescales allows us to establish the consequences of nanoscale and molecular design on the macroscopic functional performance of a material.

Our recent work on establishing the behavior of charge-transfer states at organic semiconductor interfaces can be found in the Journal of Physical Chemistry C (1,2); and our work on creating surface-anchored metal-organic framework heterojunctions that control triplet transport to allow photon upconversion can be found in Advanced Materials.

Dipolar Molecular Rotors in SURMOFs

Figure 1: Schematic of a dipolar rotor whose stator can be integrated into a surface-anchored metal-organic framework. The grey spheres indicate reactive units that bind the stator into the crystalline structure. The dipolar fragment of the molecule (indicated by the arrow) can then rotate freely around this stator.

As part of the DFG Priority Program 1928 “Coordination Networks: Building Blocks for Functional Systems”, we are investigating the dynamics of strongly interacting dipolar molecular rotors in collaboration with Prof. Herges at the Christian-Albrechts-Universität zu Kiel and Dr. Xianghui Xhang at Universität Bielefeld. We are developing materials that incorporate the dipolar molecular rotors into the building blocks of crystalline surface-anchored metal-organic frameworks; this allows the dipolar-dipolar interactions to be tuned in three dimensions and low-energy macroscopic orderings to be designed. By integrating these films into device structures, we hope to demonstrate field-induced switching of the dipolar rotor macroscopic orientation patterns and move towards realizing a new material concept for fast optical switching.

Further information can be found on the Priority Program’s website.

Figure 2: Showing a dipolar-rotor-based SURMOF light modulation device. Without applied field (left), the permanent and transition dipole moments (red arrows) lie in a collective low energy configuration in the substrate plane giving the device a high optical absorption. When bias is applied, the dipoles are rotated into a configuration with the dipoles perpendicular to the substrate plane giving the device a low optical absorption. The bottom row illustrates a zoomed view of the dipole alignment. For our prototype device we will use a (semi)-transparent gold bottom contact and graphene top contact.

Stimulated Emission and Lasing in Perovskites

Figure 1: Schematic representing stimulated emission of photons in a perovskite structure.

We collaborate closely with the Light Technology Institute at KIT to investigate the potential of perovskite semiconductors as a laser gain material.  Our first results published in Applied Physics Letters (http://scitation.aip.org/content/aip/journal/apl/109/14/10.1063/1.4963893 ) show that perovskite layers solution cast onto nanoimprinted gratings make stable distributed feedback lasers with thresholds around 120 KW/cm2.