Spring 2018

When atomically thin two-dimensional (2D) materials are layered, they often form incommensurable noncrystalline structures that exhibit long-period moiré patterns when examined by scanning probes. In this presentation we will use graphene and hexagonal boron nitride as examples of gapless and gapped Dirac materials to illustrate how the moire superlattices due to interlayer coupling can alter the materials' intrinsic electronic properties. The derivation of the effective models for these van der Waals materials heterojunctions for arbitrary twist angles can benefit from input obtained from ab initio calculations carried out for commensurate short period crystalline structures. We will discuss how the moire pattern modified electronic structures give rise to a variety of experimentally measurable features including enhanced density of states through van Hove singularities, and flat bands, or to their suppression due to formation of band gaps.

The incidence rates of cancers and other chronic diseases have been increasing in many regions and populations.  There are more than 70,000 new cases of inflammatory bowel diseases (IBD) such as ulcerative colitis, diagnosed every year. Established diagnostic techniques for cancers and ulcerative colitis are invasive, cause discomfort, and are not cost-effective. The compliance rate for the screening of such diseases is very small due to this discomfort, expense, and the risk of complications. Thus, it is important to develop minimally invasive or noninvasive and cost-effective prescreening strategies.   

Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy accompanied with different data analysis frameworks can provide an excellent spectroscopic technology to extract biochemical information from bio-fluids which can lead to the identification of diseases. Results show that dried serum samples can be used to detect the biochemical changes induced by cancers and IBDs. This potential technology can be further developed into a noninvasive, personalized diagnostic tool in which patient-to-patient differences in molecular signatures would allow the assessment of disease status and personalized drug management.  

We investigate the energetic relaxation and spatial localization of photoexcited states in conformationally disordered p-conjugated models, choosing poly(para-phenylene vinylene) as a model system. Assuming vertical excitations, the initial photoexcited eigenstates are obtained via the disordered Frenkel model. The subsequent relaxation and localization of the excited statesis determined via the disordered Frenkel-Holstein model coupled to a dissipative environment. In particular, we solve the Lindblad master equation via the time-evolving block decimation (TEBD) and quantum jump trajectory methods.

The values of the model parameters physically relevant to polymer systems naturally lead to a separation of time scales, with the ultra-fast dynamics corresponding to energy transfer from the exciton to the internal phonon modes (i.e., the C-C bond oscillations), while the longer time dynamics correspond to damping of these phonon modes by the external dissipation. Associated with these time scales, we investigate the following processes that are indicative of the system relaxing onto the emissive chromophores of the polymer: 1) Exciton-polaron formation occurs on an ultra-fast time scale, with the associated exciton-phonon correlations present within half a vibrational time period of the C-C bond oscillations. 2) Exciton decoherence is driven by the decay in the vibrational overlaps associated with exciton-polaron formation, occurring on the same time scale. 3) Exciton density localization is driven by the external dissipation, arising from ‘wavefunction collapse’ occurring as a result of the system-environment interactions. Finally, we show how fluorescence anisotropy measurements can be used to investigate the exciton decoherence process during the relaxation dynamics.