Research Projects
Revealing the role of inhomogeneities and disorder in 2D materials: Correlating transport with spatial and electronic topography:
Two-dimensional crystals form a new class of materials with robust mechanical and electrical properties, primed to change the way we build electronic devices for computers, optical detectors, and chemical or biological sensors. However, because they are only a single crystalline layer (1-3 atoms thick), their properties are more sensitive to their immediate environment than their three-dimensional counterparts. Substrates that support the 2D crystal, and impurities introduced by processing or adsorbed from the air degrade the performance of the material, and threaten the success of applications. 2D crystals also present a challenge to our fundamental understanding of electronic transport in 2D: many of these materials clearly show metallic behavior, but this is in contradiction with a very well-known theory of 2D electron transport called weak localization. The goals of this project are to understand the impact of defects and disorder on device performance and to investigate the roots of metallic behavior in 2D crystals. The research team will use a combination of electronic transport measurements and atomic-scale imaging to determine how electrons behave in the presence of disorder in 2D crystals. This project is supporting the education and training of two graduate students and a number of undergraduate students. It also supports the development of the next generation of researchers through summer workshops at University of New Hampshire for high school through graduate students. The goal of the workshops is to lower the barrier to success in research through topics that give new researchers practical information and advice, opportunities to hear about the diversity of research happening on campus, and a chance to personally connect with peers and principle investigators.
MoS2 Catalysis:
MoS2 is one of the most commonly used commercial HDS catalyst, but a microscopic understanding of the active sites and reaction mechanisms has been difficult to develop. At the Hollen Lab we plan to perform scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM) experiments on two novel experimental systems: MoS2 on SiO2 and MoS2 on anodized aluminum oxide (AAO). These studies will determine the importance of the substrate, the nature of the edge states on insulating substrates, and the role of strain and defects on the catalytic activity of MoS2.
Supported by the American Chemical Society
Petroleum Research Fund
Native Defects in Black Phosphorus:
Monolayer black phosphorus (BP) is a rising interest in 2D materials. BP has an exceptional mobility up to 1000cm^2/vs with a band gap up to 1.8eV. Here in Hollen lab we are further investigating BP by studying native p-type dopants found in commercial BP crystals. We conduct these experiments using our low temperature, ultra-high vacuum STM/STS to observe the long-range electronic signal of these native defects.
Future plans include working with mechanically exfoliated, few layer black phosphorus with our unique UHV four-point probe stystem.
Future plans include working with mechanically exfoliated, few layer black phosphorus with our unique UHV four-point probe stystem.
MoS2/Graphene Lateral Heterojuctions:
We are collaborating with Josh Robinson's group at Pennsylvania State University to characterize the interface of monolayer MoS2/graphene lateral heterojunctions using a combination of our low temperature, ultra-high vacuum STM/STS system and the 4-probe STM user tool at Penn State's 2D Crystal Consortium, an NSF Materials Innovation Platform.
Supported by NSF through the 2DCC