Hassana Samassekou

Optical evidence for blue shift in topological insulator bismuth selenide in the few-layer limit

Optical bandgap properties of high-quality few-layer topological insulator Bi2Se3 thin films are investigated using broadband absorption spectroscopy. We provide direct optical evidence for blue shift in the bulk bandgap of Bi2Se3 as it approaches the two-dimensional limit. The blue shift is robust and observed in both protected (capped) and exposed (uncapped) thin films. The behavior is strongest below six quintuple layers (QLs), particularly at the 2 and 3 QL level, where finite-size effects are known to be most significant in Bi2Se3. A further bandgap increase is observed in all films that we attribute to the Burstein-Moss effect. Our result provides additional insights into the scaling behavior of topological materials. The bandgap increase has a significant impact on the electronic and optoelectronic applications of topological insulators.

A simple approach to analyze layer-dependent optical properties of few-layer transition metal dichalcogenide thin films

Intense interest in transition metal dichalcogenides (TMDs) is driven, among other things, by their intriguing layer-dependent optical properties down to the monolayer and strong excitonic effects. Experimental determination of optical constants such as the complex refractive index and dielectric function of large-area TMDs thin films is prone to large uncertainties at the few-layer level using a single measurement technique such as spectroscopic ellipsometry (SE). In this work, we demonstrate an approach to accurately determine the optical constants by combining transmission spectroscopy with SE. Using MoS2 as an example, the prototypical TMD material, we demonstrate our approach on transparent (Quartz, Al2O3) and absorbing (e.g. Si/SiO2) substrates. We find that pre-characterized optical properties from transmission spectroscopy significantly improve the quantitative accuracy of optical constants obtained using SE on both transparent and absorbing substrates. We show that the extracted optical constants from samples deposited on transparent substrates are highly reliable in obtaining layer-dependent optical properties of TMDs while data on absorbing Si/SiO2 substrate suffer from strong substrate contribution. Overall, we strongly conclude that SE must be combined with transmission spectroscopy to obtain optical constants with high quantitative accuracy. Our approach, though demonstrated on few-layer films, will be applicable to monolayer and bilayer TMDs.