2025 Theses Doctoral

Dielectric Control of Exciton Transport and Correlated States in Two-dimensional Materials

Su, Haowen

The electronic and optical behavior of two-dimensional (2D) materials can be profoundly influenced by their surrounding environment, making them ideal candidates for post-synthetic tuning of their functionality. This thesis explores dielectric engineering as a versatile and scalable approach to modulate excitonic and electronic properties in 2D materials, through both local inhomogeneities and periodic structures.

In the first part, we demonstrate that nanobubbles—nanoscale dielectric inhomogeneities—can control exciton transport in the 2D semiconductor bilayer tungsten diselenide (WSe₂). Using ultrasensitive spatiotemporally resolved optical scattering microscopy, we directly visualize exciton funneling and trapping into these regions at room temperature, driven by momentum-indirect (dark) excitons whose energies are more sensitive to dielectric perturbations than bright excitons. These results establish a new toolkit for controlling exciton transport in 2D semiconductors with exceptional spatial and energetic precision using dielectric engineering of dark exciton energetic landscapes.

In the second part, we propose the use of self-assembled nanoparticles to create highly ordered dielectric superlattices interfaced directly with 2D materials. This bottom-up alternative to moiré materials enables unprecedented flexibility in the design of superlattice period, spatial scale, geometry, and gate tunability, towards the realization of correlated electronic phenomena that are not realizable in currently-used platforms. As a proof of concept, we develop complex heterostructures interfacing highly-ordered Fe₃O₄ nanoparticle superlattices with graphene. Electrostatic gating through the nanoparticles imprints a periodic potential landscape on graphene, leading to band folding and the emergence of correlated electronic interactions. Conductive-AFM and remote exciton sensing measurements confirm the presence of the electrostatic potential in graphene and of band folding, respectively, exhibiting features very similar to moiré materials. This work represents the first experimental observation of band structure modulation and correlated states in 2D materials enabled by nanoparticle self-assembly and electrostatic gate-induced dielectric imprinting on sub-20 nanometer scales.

Together, these results highlight dielectric engineering as a powerful method to control exciton transport and correlated electronic states in 2D materials, offering new pathways to design and control functional quantum materials through post-synthetic modifications.