Defect-laden 2D Materials for a Sustainable Future – from CO2 conversion to single photon emission

Talat S. Rahman

Department of Physics, University of Central Florida, Orlando, FL 32816, USA

In the pursuit of a sustainable future, the last decade has seen a concerted effort in accelerating the discovery of materials for energy needs, thanks to the Materials Genome Initiative. In this talk I will focus on few 2-dimensional materials which have captured our imagination. As with graphene, another common lubricant, molybdenum disulphide (MoS2) shows remarkable potential for optoelectronic applications when peeled off as a single sheet. I will show how defects and dopants in single-layer MoS2 convert it into a cheap catalyst for CO hydrogenation1. Even more interesting is the case of another 2D material, hexagonal boron nitride (h-BN), a well-known insulator.  Defects can transform h-BN to a metal-free catalyst that captures and converts CO2 to value added products such as methanol2. Interestingly, defect-laden h-BN is also being sought as a single photon emitter akin to NV centers in diamond. With a focus on electronic structural modulations of the local environment, I will draw comparisons with experimental observations made in collaborative work.  

[1] D. Le, T. B. Rawal, and T. S. Rahman, J. Phys. Chem. C 118, 5346 (2014); T.B. Rawal, et al., J. Chem. Phys. 154, 174701 (2021).

[2] K. Chagoya, et al., ACS Sustainable Chem. Eng. 9, 2447 (2021); T. Jiang, et al., Phys. Chem. Chem. Phys. 23, 7988 (2021).

To be updated...

Moiré patterns are well known in the visual arts and textile industries -- the term comes from the textured patterns seen in mohair silk fabrics.  It arises whenever two periodic structures are superimposed giving new periodicities.  Such translational symmetry breaking atomic positions is at the heart of condensed matter physics and twisting two atomically thin materials on top of each other yields designer materials where the material properties like bandwidth, electron velocity, and band topology can be controllably altered. 

Less than 15 years after the first isolation of two dimensional materials, our experimental colleagues are now able to tune the twist angle between adjacent atomic monolayers to within 0.1 degrees allowing, for example, the change in electronic bandwidth from 10,000 K in monolayer graphene to less than 10 K in twisted bilayer graphene.  Moreover, there are more than 1,000 possible “easily exfoliatable” materials to play with, giving billions of designer band structures. In the past decade, we have achieved a good handle on building non-interacting models for these materials, however, we do not have a good way to include Coulomb interactions in such moiré van der Waals systems.  I will discuss our successful work on adding Coulomb interactions to the bands of monolayer graphene [1] where we found that contrary to expectation, the interactions are strong.  However, this is masked by the effective competition between the short-range (i.e. “Hubbard-U”) and long-range (i.e. “Coulomb tail”) interaction [2]. I will speculate on how similar ideas might be applied to twisted bilayer graphene [3].     

[1] Hokin Tang, Jia Ning Leaw, Joao Rodrigues, Igor Herbut, Pinaki Sengupta, Fakher Assaad, and Shaffique Adam, " The role of electron-electron interactions in two-dimensional Dirac fermions", Science 361 570 (2018).

[2] Jia Ning Leaw, Ho-Kin Tang, Maxim Trushin, Fakher F Assaad, Shaffique Adam, “Universal Fermi-surface anisotropy renormalization for interacting Dirac fermions with long-range interactions”, Proc. Natl. Acad. Sci. (USA) 116 26431 (2019).

[2] Girish Sharma, Maxim Trushin, Oleg Sushkov, Giovanni Vignale, and Shaffique Adam

“Superconductivity from collective excitations in magic-angle twisted bilayer graphene”, Phys. Rev. Research, Rapid Comm. 2, 022040 (2020).

† This work is supported by the Singapore National Science Foundation Investigator Award (NRF-NRFI06-2020-0003).

Novel photovoltaic perovskites, the organo-metallic lead halides (e.g. CH3NH3PbI3), have revolutionized the field of solar cells by their high photon to electron conversion efficiency η of 25%.  But due to their chemical and structural tunability (one can grow crystals from nanometer size to 1000 cm3), they offer to study a wealth of exciting physical phenomena and open further possibilities for applications. To illustrate them, a selected set of measurements will be reported together with some device prototypes.

Since its first application as light absorbing materials in photovoltaic technology, perovskite solar cells (PSCs) have achieved a remarkable certified record power conversion efficiency (PCE) of over 25% in just over a decade. However, hybrid perovskite absorbers still face the issue of chemical instability as they degrade under continued exposure to moisture, light illumination, and UV light and are unstable at high temperatures. These instabilities are related to the deposition method used and the intrinsic properties of the material. We have demonstrated the deposition of pure, polycrystalline, smooth, and compact MAPbI3 perovskite films, using a sequential low-pressure chemical vapor deposition (LPCVD) method in a single reactor. This material was incorporated into a planar single-junction PSC (with no additives or additional interfacial engineering) that was fabricated, stored and tested under open-air conditions, yielding a best PCE of 11.7%. The solar cell maintains 85% of its performance up to 13 days in the open air with a relative humidity up to 80%.

This LPCVD method was further developed to produce mixed-halide and Sn-Pb perovskite thin films. We will report on the deposition procedure of these thin films and its resultant structural, compositional and optical properties. Furthermore, the impact of Cl-doping on the PSC performance will be discussed.

Chemically doped semiconducting polymers exhibit electronic transport characteristics that range from localized (or hopping-like) transport to delocalized (or metal-like) transport.  While a multitude of electronic transport models have been proposed, none of them capture the full spectrum from localized to delocalized transport.[1]  Additionally, existing models do not quantitatively capture the dependency on charge carrier density (or carrier concentration or carrier ratio) that manifests through the measured temperature-dependent electrical conductivity and Seebeck coefficient.  Recently, we developed a semi-localized transport (SLoT) model, building upon past insight,[2] that can describe the full spectrum from localized to delocalized transport.[3]  This new model provides quantitative insight into charge carrier localization that is capable of more accurately describing electronic transport in a broad spectrum of organic electronic and thermoelectric semiconducting polymers.  This invited talk will discuss our recent publication[3] where I will first present motivation showing the previous short comings of our collective understanding with existing models.  Next, I will briefly discuss the development of the SLoT model in the context of the organic thermoelectric field.  I will then discuss the utility and prospects of deeper insight that the SLoT model affords.  Then I will validate the model using the prototypical P3HT polymer doped with FeCl3 and show its broad applicability in accurately describing other polymers/organic materials (namely, PBTTT, PA, PEDOT, SWCNT, and N2200) that were not previously well described by other models.  I will then extend this process to new polymers (namely PE2) and describe the deeper insight gained from this model.  I will then conclude my talk describing the relevant experimental measurements that research groups should undertake in characterization of their polymers to be able to use the SLoT model, in hopes of encouraging uniform material characterization internationally.  The future implications of the SLoT model in developing semiconducting polymers could be profound.  When coupled with chemical and structural characterization, the SLoT model connects the chemistry and structure to the macroscopic transport properties.  Once the SLoT model parameters are calculated, we can quantify fundamental limits to a polymer’s potential (e.g., ability to achieve high electrical conductivity or high Seebeck coefficient).  Ultimately, this allows us to accelerate the rational development of chemically doped organic electronics affording new functionality (e.g., thermal or electronic switching, thermoelectric cooling or power generation, etc.).

TBA

Magnetic systems on the range of nanometers are of great interest for application as well as fundamental physics. When macroscopic magnetic materials are scaled down to the nanoscale the energy balance changes and single-domain and super-paramagnetic states arise. When such systems are in close proximity to each other the dipole-dipole interaction becomes important and complex new magnetic correlations can arise. In addition, the contact to other materials at interfaces introduces new interactions that can lead to emergent phaenomena like the giant magneto resistance effect that is present in our everyday life in the read-heads of magnetic HDDs.

While there was and still is a large interest from the scientific community in these systems the magnetic order on these lengths scales is hard to study. Only few techniques exist that have the required spatial resolution and sensitivity to magnetism. While there have been great advances in various microscopy techniques with magnetic sensitivity they have their limitations. The resolution is often in the 25-100nm range, they sometimes rely on resonances of certain elements and they only provide local information from the surface of the sample. For buried structures and to access the global ensemble of a statistically distributed state neutron scattering is still the method of choice, as the magnetic moment of the neutron can directly interact with the sample magnetization.

I will present neutron methods for the study of surface near magnetic nanostructures. Starting from the one dimensional case of polarized neutron reflectometry (PNR) to measure layered magnetic structures over the "bulk" small angle neutron scattering (SANS) technique to the surface sensitive grazing incidence neutron scattering (GISANS), all relevant techniques will be covered. In addition to the relevant techniques, I will discuss some general background relevant to the scattering physics as well as examples of scientific systems.

The microscopic structure and dynamics of molten salts is a fascinating subject. It is a very diverse family of materials; while molten NaCl is a simple mix of single-valence ions with a minimal structural organization, transition-metal ions form chains of octahedra, a structural motif present in the solid phases. The interest in molten salts is motivated by both scientific curiosity and practical applications. For example, molten salts are used as coolants in solar power. Molten-salt nuclear reactor (MSR) concepts are candidates for next-generation nuclear power reactors, which promise to be safer and more efficient than existing water-based ones. In these applications, Cr is the principal corrosion product, and NaCl is a common constituent. Therefore, we studied the atomic structure of molten NaCl−CrCl3. We found networks of [CrCl6]3− octahedra and intermediate-range order in remarkable agreement with ab initio simulations. Such studies were enabled and benefited immensely from developments in neutron and X-ray diffraction methods. The availability of Cr isotopes with different neutron scattering properties makes Cr an ideal model multivalent ion for experimental validation of atomistic simulations of molten salts.

Dr. Boris Khaykovich is a Research Scientist at the Nuclear Reactor Laboratory at MIT. Boris received MSc in Chemical Physics and a Ph.D. in Physics (1999) at Weizmann Institute of Science in Israel, followed by postdoctoral training at MIT. Boris is a physicist who has extensive experience in X-ray and neutron scattering methods and instrumentation for materials science. He is known for the development of neutron focusing optics for neutron imaging and small-angle scattering. Boris has been leading projects on the determination of the molecular structure of molten salts and, in the past, conducted crystallographic studies of magnetic and biological materials. Boris has been a Guest Editor of Journal Imaging, Special Issue on Neutron Imaging, and has been the Chair of the SNS/HFIR User Group Executive Committee, representing users of neutron-scattering facilities at Oak Ridge National Lab. 

Ben Krewson: Defect Dynamics and Selection for Quenched Striped Patterns

We study transverse modulational dynamics of striped pattern formation in the wake of a directional quenching mechanism. Such mechanisms have been proposed to control pattern-forming systems and suppress defect formation in many different physical settings, such as light-sensing reaction-diffusion equations, solidification of alloys, and eutectic lamellar crystal growth. Furthermore, they are a simple model of a growth process in a patterned biological system. In the context of the complex Ginzburg-Landau and Swift-Hohenberg equations, two prototypical models of pattern formation, we show that long-wavelength and slowly varying modulations of striped patterns are governed by a one-dimensional viscous Burgers equation, with viscous and nonlinear coefficients determined by the quenched stripe selection mechanism. This reduced model allows for accurate description of defect dynamics as shock/rarefaction dynamics.

Troy Schneider: Structured Glass-Ceramic Scintillators  

We studied compounds with a desirable x-ray mass attenuation coefficient, such as Terbium Oxide and Gadolinium Fluoride, which are very good at absorbing x-rays and turning them into visible light. This makes them quite useful in medical imaging devices such as x-rays. We varied the amount of these chemicals and observed the changes in the glass-ceramic. We also looked into alternative chemicals with better mass attenuation coefficients. We found that the Terbium Oxide and the Gadolinium Fluoride were very useful in attenuating x-rays, and that altering the percentages of a few other materials in the glass, such as Calcium Fluoride, produced a more stable glass at high energies. 

Lauryn Williams: Identifying Dynamical Masses of Long Period Companions using Orbit Fitting Code with HGCA Astrometry

Sweeping advances in space telescopes paired with the innovative methods of computational astronomy give us a unique opportunity to detect and measure the dynamical masses of substellar companions orbiting stars outside our solar system. ESA’s space based HIPPARCOS and Gaia observatories have yielded precise astrometric measurements that -- when combined-- open a window on the detailed astrometric behavior of stars. Brandt et al. 2021 recently created the cross-calibration Hipparcos-Gaia Catalog of Accelerations (HGCA). In this work, we concentrate on 46 accelerating stars within 20pc of our sun focusing on those objects with long-term precise radial velocity curves from Keck’s HIgh Resolution Echelle Spectrometer (HIRES). Combining the acceleration information with the radial velocity curve, we use the complex orbit-fitting code called ORVARA (Orbits from Radial Velocity, Absolute, and/or Relative Astrometry), to obtain explicit constraints on the dynamical masses of the companions. Preliminary results have yielded the dynamical masses for companions around two well studied stars: 14 Her and 15 Sge. There remain 44 sources within 20pc that can be further analyzed. Studies of these accelerating stars are a pathway to understanding the hidden multiplicity fraction of our solar neighborhood. This research was possible through the RGGS-AMNH REU Program, funded by the National Science Foundation.

Brandon Lee: Stellarator Coil Optimization Supporting Multiple Magnetic Configurations

Effective and efficient stellarator optimization is of great interest to the nuclear fusion community. Our goal in this work is to develop techniques that can be used to design devices capable of supporting multiple magnetic configurations. Such devices would presumably be much more economical than those optimized for a single magnetic configuration. We utilize PyPlasmaOpt, a Python package that couples coil and vacuum field optimization, as our base software. Our primary methods for adding flexibility to the coil design are adjusting the currents in the modular coils and adding control coils to the device. We find that changing the currents in the modular coils without the addition of control coils leads to flexible stellarators with very irregular coil shapes and minute plasma volumes. By adding control coils, we introduce enough degrees of freedom to achieve multiple magnetic configurations with one coil configuration while maintaining reasonable coil shapes and low quasisymmetry error and creating larger plasma volumes. We add quadratic flux minimizing surfaces to the optimizations to increase the plasma volumes further. Overall, we achieve greater flexibility by accounting for multiple magnetic configurations during the coil design stage.

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Our planet is called Earth, but based on the surface composition, perhaps it should have been called Water. Despite the ubiquitous presence on our planet, water's solid form, ice does not readily enter the ground state upon cooling, owing to residual disorder of the hydrogen atoms in the lattice. In the 1990s, a class of magnetic ceramic materials was discovered, with similar behavior of its magnetic moments as the protons in ice, and these materials came to be known as Spin Ice. In a cleanroom facility, researchers today can design nanoscale magnetic latticework structures that mimic the behavior of spin ices and water ices, with the advantage that the interactions and behavior can be controlled by design. Such materials are known as Artificial Spin Ice, and I will present work on the behavior of these materials when intentional defects are programmed into the latticework. These artificial defects mimic the real defects that are known to occur in a wide range of materials, including water ice and the ceramics phases that play host to the spin ices. The implications for other ground state phases, such as high temperature superconductors, will be discussed.

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Abstract TBD

The chromosomes of eukaryotic cells are based on tremendously long DNA molecules that must be replicated and then physically separated to allow successful cell division.  I will discuss what we have learned about chromosome structure from our group's biophysical experiments and mathematical modeling of chromosome structure. A key emerging feature of chromosome organization is the role of active chromatin loop formation, or "loop extrusion" as a mechanism leading to chromosome compaction, individualization, and segregation.  I will discuss a number of aspects of the SMC complexes thought to be the loop-extruding elements. I will also discuss our group's studies of the role of chromosomal epigenetic marks in control of the structure and integrity of the cell nucleus.

The world and the Universe we live in are composed of fermions and bosons. The quantum statistics of these particles overwhelmingly governs what we see around us. But  one could wonder, can other kinds of quantum particles exist? I will begin this colloquium with an introduction of quantum statistics and the fascinating possible existence of anyons, particles which obey 'fractional' statistics.  

The quantum Hall system forms a marvelous two-dimensional realm for hosting many rich phenomena, including fractional statistics. I will describe how anyons can emerge in this setting, how they could be detected borrowing from beam-splitter and other principles used to detect bosons and fermions, and how landmark experiments of last year did perform such detection. I will also illustrate how the same setting can probe dynamics akin to that found in the astrophysical realm of black holes. Specifically, point-contact geometries can exhibit phenomena parallel to Hawking-Unruh radiation and black hole quasinormal modes associated with ringdowns in gravitational wave detection.  

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Quantum systems with strong interaction exist in many branches of physics and present a challenge for theory. We will discuss some recent methods to solve the problem, focusing on one  particular example: the fractional quantum Hall fluid. Many phenomena occurring in this fluid can be explained by postulating a new type of quasiparticle - the composite fermion. I will describe theories of this composite fermion, in particular the recently proposed "Dirac composite fermion theory", and how research in condensed matter physics has stimulated the discovery of a large number of new dualities between quantum field theories, previously unknown to high-energy physics.