Physics Colloquium - Spring 2021

Atomically thin layers are known as two-dimensional (2D) materials and have attracted a growing attention due to their great potential as building blocks for a future generation of low-power and flexible 2D optoelectronic devices. Similar to the well-established 3D electronics, the development of functional 2D devices will depend on our ability to fabricate heterostructures and junctions where the optical and electronic properties of different compounds are brought together to create new functionalities. Vertical heterostructures can be produced by selective van der Waals stacking of different monolayers with distinct chemical composition. However, in-plane lateral heterostructures, where different materials are combined within a single 2D layer, have proven to be more challenging. During the formation of the hetero-junction, it is important to minimize the incorporation of undesired impurities and the formation of crystal defects at the junction that will impact the functionality of the 2D device. When fabricating periodic structures, it is equally important to develop the ability of controlling the domain size of each material.
In this talk, we will review different techniques that have been used to create 2D lateral heterostructures of transition metal dichalcogenide compounds. Emphasis will be made in two synthesis approaches developed in our group. The first, a one-pot modified CVD, utilizes a single heterogeneous solid source, for the continuous fabrication of lateral multi-junction heterostructures of TMD monolayers. In this method, the heterojunctions are sequentially created by only changing the composition of the reactive gas environment in the presence of water vapor. This allows to selectively control the water-induced oxidation and volatilization of each transition metal precursors, as well as its nucleation on the substrate, leading to sequential edge-epitaxy of distinct TMDs. This simple method have proven to be effective for continuous growth of TMD-based multi-junction lateral heterostructures, including selenides, sulfides and ternary alloys. Basic devices with field effect transistor configuration were fabricated to study the electrical behavior of these heterojunctions, their diode-like response, photo-response as a function of laser power as well as photovoltaic behavior of the heterojunctions will be discussed.
The second approach consists in a laser-assisted chemical modification of ultra-thin TMDs, locally replacing selenium by sulfur atoms that allows the local tuning of the physical properties. The photo-conversion process takes place in a controlled reactive gas environment and the heterogeneous reaction rates are monitored via in situ real-time Raman and photoluminescence spectroscopies. The spatially localized photo-conversion resulted in a heterogeneous TMD structure, with chemically distinct domains, where the initial high crystalline quality of the film is not affected during the process. This was confirmed via transmission electron microscopy as well as Raman and Photoluminescence spatial maps. Additionally, we also applied this method for post-growth local electronic doping, where small amounts of chalcogen atoms are replaced by nitrogen increasing the hole concentration and hence the p-type doping.

Have you ever wondered what goes on inside the black box after a paper is submitted to one of the Nature journals? There are many urban myths. Some of them may be true! Find out what actually happens, how to improve your own papers for submission anywhere and how to become part of the process.

A majority of disk galaxies host stellar bars that are the prime driver of internal galaxy evolution. Bars also trigger the flow of angular momentum between the disks and their parent dark matter (DM) halos. Using high-resolution N-body stellar and DM numerical simulations, I model and analyze the dynamical and secular evolution of stellar bars in disk galaxies and their DM counterparts, including the induced DM bars in spinning halos with a range of cosmological spin parameter. My main results emphasize a new effect: the DM halo spin has a profound effect on the evolution of stellar and DM bars. Broadly, I find that secular torques between stellar bars and DM halo orbits lead to a coupling that lasts for the lifetime of the bar. Most importantly, the stellar bars dissolve when residing in halos of large prograde spin. These disks can represent the unbarred branch of galaxies on the Hubble Fork Diagram. The dark matter over-densities produced by the halo-bar coupling provide a novel space where baryons are evacuated in which to look for DM annihilation or decay signals. I will describe the mechanisms that allows low inclination dark matter halo orbits to couple to the stellar bar and make predictions on the mass and location of previously unreported dark matter over-densities in our own Milky Way.

As reionization is inherently coupled with galaxies —the dominant sources of ionizing photons— understanding galaxy evolution in the early universe relies on understanding the process of reionization. Lyman-alpha emission has played a pivotal role in constraining the neutral hydrogen (HI) fraction of the intergalactic medium (IGM) during the epoch of reionization (EoR) thanks to the resonant nature of Lyman-alpha scattering with HI. A spectroscopic dataset of galaxies in the early universe, obtained by the DEIMOS (optical) and MOSFIRE (near-infrared; NIR) spectrographs on the Keck telescopes, is utilized for investigating the evolution of the IGM during EoR by measuring the Lyman-alpha equivalent width (EW) distribution. I will present the results of the spectroscopic analysis, including our IGM HI fraction measurement and the discovery of a highly ionized region at z ~ 7.6, which highlights the inhomogeneity of reionization. I will also discuss future directions for improving the use of Lyman-alpha as a probe of reionization.

Type Ia supernovae (SNe Ia) are powerful distance indicators for cosmology because of their extreme and consistent luminosities, however the exact mechanism behind the explosion remains an open question. In order to better understand SNe Ia and improve our cosmological parameters, we need a method to distinguish between progenitor types and to characterize the local environment of individual SNe Ia. A powerful way to achieve both is through light echoes. Three-dimensional maps of the dust distribution around SNe Ia, as revealed by light echoes, can provide insight into the progenitor of the system: compact, disk-like echoes are likely caused by circumstellar material. Multi-color imaging of light echoes can also reveal properties of the dust in the local environment (e.g. dust grain size, distribution, and chemical composition). We can infer the existence of a light echo from late-time photometry of a supernova, but in order to map out the dust distribution, the light echo must be resolved. Here I present a newly identified light echo around SN 2009ig, first inferred from the late-time light curve form the Large Binocular Telescope and now resolved in archival Hubble Space Telescope images, and what we can learn about the local environment of this SN Ia.

Charlotte M. Wood is a fifth-year PhD candidate at the University of Notre Dame. She works with Dr. Peter Garnavich studying type Ia supernova cosmology, light echoes, and compact binary systems. Her thesis focuses on determining type Ia progenitor types through light echoes and how the relative rate of progenitor types affects Hubble constant measurements. Previously she worked with Dr. Justin Crepp characterizing directly imaged brown dwarfs and their stellar companions. Charlotte received her Bachelor of Science in Physics from Hofstra University in 2016, where she worked with Dr. Stephen Lawrence studying light echoes and recurrent novae. She will be on the job market next year looking for postdoctoral positions.

The Universe has expanded by a factor of over one billion between the present-day and the early thermal epoch known as the neutrino decoupling. We observe markers that track the dynamics of this expansion in many forms: the recession of galaxies (Hubble Expansion), the dim afterglow of the hot plasma epoch (Cosmic Microwave Background) and the abundances of light elements (Big Bang Nucleosynthesis). The epoch of neutrino decoupling produced a fourth pillar of confirmation – the Cosmic Neutrino Background (CNB). In our current understanding, the CNB was created in the first second after elementary particles spontaneously filled the void of the early Universe. These early universe relics have cooled under the expansion of the Universe and are sensed indirectly through the action of their diminishing thermal velocities on large-scale structure formation. Recent experimental advances open up new opportunities to directly detect the CNB through the process of neutrino capture on tritium, an achievement which would profoundly confront and extend the sensitivity of precision cosmology data. PTOLEMY, an experiment at the Gran Sasso National Laboratory in Italy, is a novel method of 2D target surfaces, fabricated from graphene, that forms a basis for a large-scale relic neutrino detector. Recent PTOLEMY publications [1,2] describe the underlying technique for achieving CNB sensitivity and redefines the future direction of neutrino mass measurements. The discussion of PTOLEMY focusses on experimental challenges, recent developments and the path forward to discovery sensitivity.

References

[1] M.G. Betti et al., "A Design for an Electromagnetic Filter for Precision Energy Measurements at the Tritium Endpoint”, Progress in Particle and Nuclear Physics, 106, (2019) 120-131, https://doi.org/10.1016/j.ppnp.2019.02.004

[2] M.G. Betti et al., "Neutrino physics with the PTOLEMY project”, Journal of Cosmology and Astroparticle Physics, 07, (2019) 047, https://doi.org/10.1088/1475-7516/2019/07/047