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Quantum Dot-Organic Semiconductor Hybrid: Using Energy Level Alignment for Efficient Carrier Transport

Multimetallic nanoframes with 3D surfaces image
(A) Schematic drawing showing the energy level alignment between two QDs and a TTFTA ligand. (B) Macroscopic structure of the hybrid film revealed by X-ray scattering patterns.

Scientific Achievement
The Inorganic/Organic Nanocomposites Program has, for the first time, demonstrated efficient charge carrier transport modulation in quantum dot (QD) films by energy level alignment between QDs and ligands.

Significance and Impact
New approach could lead to design and fabrication of ordered, coupled functional hybrid organic-inorganic nanomaterials with outstanding optoelectrical properties.

Hybrid organic-inorganic nanostructures combine the chemical versatility of organic semiconductors with the chemical stability and size-adjustable band gaps of inorganic nanomaterials. Configurations of these hybrids in an inorganic core/organic shell structure offer outstanding optoelectrical properties. Making the organic component capable of binding directly to the surface of the inorganic component can help reduce the tendency for problematic phase segregation that tends to occur during traditional methods of preparing hybrids.

One limiting factor in hybrids is how charge is transported by ligands, the organic molecule that coats the surface of quantum dots (QDs, in this case PbS) and binds them into thin films. The insulating nature of the ligands hinders in-film charge carrier transport. To improve charge transport, Xu’s team replaced native oleate ligands with a simple organic semiconductor known as TTFTA (tetrathiafulvalenetetracarboxylate) to enhance inter-particle electronic coupling.

Following experimental studies, a near-resonant alignment of PbS and TTFTA was postulated and confirmed by calculations. Considering the large width of interparticle spacing, an abnormally high field-effect hole mobility was observed and attributed to the energy level resonance. In contrast to nanoparticle devices coupled through common short-chained ligands, this system maintains a large degree of macroscopic order. This provides a different approach to the design of hybrid organic–inorganic nanomaterials, circumvents the problem of phase segregation, and holds for versatile ways to design ordered, coupled nanoparticle thin films. The study demonstrates both experimentally and theoretically that carrier transport can be effectively modulated through resonant alignment between QDs and organic semiconducting ligands.

M. Scheele, D. Hanifi, D. Zherebetskyy, S. T. Chourou, S. Axnanda, B. J. Rancatore, K. Thorkelsson, T. Xu, Z. Liu, L.-W. Wang, Y. Liu, and A. P. Alivisatos. PbS Nanoparticles Capped with Tetrathiafulvalenetetracarboxylate: Utilizing Energy Level Alignment for Efficient Carrier Transport ACS Nano 8, 2532 (2014) DOI: 10.1021/nn406127s