Atomically thin sheets such as graphene and dichalcogenides host a rich variety of atomic structures, orbital interactions, solid-state excitations, and desirable materials properties that derive from their 2D form. The first-principles theory that our group employs runs from left to right: one (simplified) example being the inversion asymmetric structure of monolayer MoS2 → spin-split bands from Mo d frontier orbitals → valley polarization in excitons → encoding information with valley degree of freedom. But oftentimes we have a target property in mind and work our way back left for predictive design, relying on developing efficient computational screening methods, following breadcrumb trails from solid-state chemistry, and working closely with experiments to keep our feet on the ground. Below are some of our interests.
Point defects in 2D materials
When carefully engineered, defects can improve material properties or serve as key elements in quantum devices. Our interest include modeling defect as nucleation sites of 2D growth, spin-based qubits, and single-photon emitters.
Second harmonic generation (SHG) – a process converting two photons into one with doubled frequency – sits at the heart of blue/green laser technology and quantum frequency converters. We seek the upper limit of SHG responses in 2D materials, as suggested by the anomalously large nonlinear response of selected layered systems.
Modeling light scattering in 2D materials requires computational methods that consolidate the treatment of electron-phonon coupling and electronic many-body phenomena. We developed a computational framework for calculating exciton-phonon interactions from first-principles following a diagrammatic approach, with favorable scaling against structural complexity.
2D Growth control
Increasingly successful efforts are being directed to the synthesis of millimeter-scale 2D monolayers with high crystallinities. These advances motivate the nanoscale control over 2D growth by properly tailoring the topographies and the surface chemistry of the growth substrate. We employ density functional theory and reactive forcefield methods to predict control over growth rates, growth morphologies, grain boundary formation, and long-range order.
2D metals – recently realized in air-stable form – carry solid-state excitations that are distinct from ones found in 3D metals. Some are not even metallic! We investigate their electronic structure, alloying rules, superconductivity, and light-matter interaction.
Magnetic topological materials
Achieving magnetic topological phases requires a trifecta: control over magnetic elements, gap or Dirac/Weyl point stabilization, and strong spin-orbit coupling. Solids achieving this trifecta are often challenging to design and synthesize with high crystallinity. We predict and investigate the solid-state chemistry of realistic candidates that could e.g. achieve anomalous Hall effect at record temperatures or stabilize topological metals.
I have been fortunate to work with talented experimentalists across a wide gamut of disciplines. Hoping to meet more!
2D layered materials and quantum materials: Growth and advanced characterization
- Mauricio Terrones, Zhiqiang Mao, Cui-Zu Chang at Penn State Physics
- Nasim Alem, Joan M. Redwing, Joshua A. Robinson at Penn State Materials Science Engineering
- Raymond E. Schaak, Thomas E. Mallouk at Penn State Chemistry
- Daniela Radu at Florida International University
Optical properties: Raman scattering, nonlinear optics
- Bruno Carvalho at Universidade Federal do Rio Grande do Norte, Natal
- Leandro M. Malard, Marcos Pimenta at Universidade Federal de Minas Gerais, Belo Horizonte
- Venkat Gopalan, Zhiwen Liu, Shengxi Huang at Penn State MSE and EECS
- Amber McCreary at NIST
Theory and modeling
- Adri van Duin, Chaoxing Liu, Sulin Zhang at Penn State
- Richard Hennig at U Florida, Materials Science and Engineering