About

Ashwin Ramasubramaniam is a Professor in the Department of Mechanical and Industrial Engineering at the University of Massachusetts Amherst and serves as an Adjunct in Chemical and Biomolecular Engineering. He is the Director of the Materials Science & Engineering Graduate Program. His research utilizes computational modeling to study and engineer materials for optoelectronics and energy applications. His academic background includes a PhD in Engineering and multiple master's degrees in Applied Mathematics and Engineering from Brown University, as well as a BTech in Mechanical Engineering from the Indian Institute of Technology, Bombay. Ramasubramaniam has received several awards and distinctions, including a Visiting Faculty Fellowship at the Weizmann Institute of Science, the University of Massachusetts Amherst Exceptional Merit Award, and the US Department of Energy Office of Science Early Career Research Award. His professional affiliations include the American Physical Society and the Materials Research Society.

Research topics

• Materials science
• Nanotechnology
• Computational chemistry
• Chemistry
• Inorganic chemistry
• Optoelectronics
• Physical chemistry
• Chemical physics
• Metallurgy
• Crystallography
• Organic chemistry
• Composite material

Selected publications

• Efficient prediction of highly anisotropic excitonic properties in the layered antiferromagnet CrSBr via time-dependent density functional theory

  arXiv (Cornell University) · 2026-03-13

  preprintOpen access1st authorCorresponding

  CrSBr, a layered anisotropic van der Waals antiferromagnet, has recently emerged as a versatile platform where strong coupling between optical excitations and magnetic order enables magneto-optical control in low dimensions. While experiments have progressed rapidly, predictive and reliable ab initio descriptions remain limited to self-consistent, many-body perturbation theory that is computationally expensive and technically challenging. Here we present an alternative approach that accurately predicts the electronic and optical properties of CrSBr at substantially lower computational cost, while retaining quantitative accuracy in the coupling between excitons and magnetic order. Using a tuned hybrid density functional with on-site corrections, we reproduce fundamental and optical gaps and quantitatively capture the interaction between excitonic transitions and magnetic order. We then employ this functional to investigate excitonic shifts induced by spin canting, that would result from applying an external magnetic field. Our results establish an efficient framework for modeling excitonic and magneto-optical phenomena in layered magnetic semiconductors.

  PublisherDOI

• Accurate electronic and optical properties of bulk antiferromagnet CrSBr via a tuned hybrid density functional with on-site corrections

  arXiv (Cornell University) · 2026-03-13

  articleOpen access1st authorCorresponding

  CrSBr, a layered antiferromagnet, is emerging as a versatile platform for exploring strong coupling between optical and magnetic properties in low dimensions. While experimental research on this material has progressed at a rapid pace, reliable results on the ab initio front are limited to the domain of self-consistent, many-body perturbation theory, which is both computationally expensive and technically challenging. We present an alternate, less-demanding approach -- rooted in generalized Kohn-Sham density functional theory -- that can deliver accurate electronic structure and optical absorption spectra of CrSBr, as well as quantitively accurate predictions of coupling of excitons to magnetic order in CrSBr. Using a minimal two-parameter set that can be tuned to reproduce a couple of well-known experimental and/or theoretical benchmarks, such as fundamental and optical gaps, we demonstrate excellent predictive capability for the tuned functional. The approach presented here can potentially be applied broadly to other magnetic semiconductors, complementing and simplifying current approaches to modeling these materials.

  PublisherOA PDF

• Resolving Contradictory Estimates of Band Gaps of Bulk PdSe$_2$: A Wannier-Localized Optimally-Tuned Screened Range-Separated Hybrid Density Functional Theory Study

  ArXiv.org · 2025-01-25

  preprintOpen accessSenior author

  Palladium diselenide (PdSe$_2$) -- a layered van der Waals material -- is attracting significant attention for optoelectronics due to the wide tunability of its band gap from the infrared through the visible range as a function of the number of layers. However, there continues to be disagreement over the precise nature and value of the optical band gap of bulk PdSe$_2$, owing to the rather small value of this gap that complicates experimental measurements and their interpretation. Here, we design and employ a Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functional to investigate the electronic bandstructures and optical absorption spectra of bulk and monolayer PdSe$_2$. In particular, we account carefully for the finite exciton center-of-mass momentum within a time-dependent WOT-SRSH framework to calculate the \emph{indirect} optical gap and absorption onset accurately. Our results agree well with the best available photoconductivity measurements, as well as with state-of-the-art many-body perturbation theory calculations, confirming that bulk PdSe$_2$ has an optical gap in the mid-infrared (upper-bound of 0.44 eV). More generally, this work further bolsters the utility of the WOT-SRSH approach for predictive modeling of layered semiconductors.

  PublisherOA PDFDOI

• Automated Workflow for Non-Empirical Wannier-Localized Optimal Tuning of Range-Separated Hybrid Functionals

  ArXiv.org · 2025-11-03

  preprintOpen access

  We introduce an automated workflow for generating non-empirical Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functionals. WOT-SRSH functionals have been shown to yield highly accurate fundamental band gaps, band structures, and optical spectra for bulk and 2D semiconductors and insulators. Our workflow automatically and efficiently determines the WOT-SRSH functional parameters for a given crystal structure and composition, approximately enforcing the correct screened long-range Coulomb interaction and an ionization potential ansatz. In contrast to previous manual tuning approaches, our tuning procedure relies on a new search algorithm that only requires a few hybrid functional calculations with minimal user input. We demonstrate our workflow on 23 previously studied semiconductors and insulators, reporting the same high level of accuracy. By automating the tuning process and improving its computational efficiency, the approach outlined here enables applications of the WOT-SRSH functional to compute spectroscopic and optoelectronic properties for a wide range of materials.

  PublisherOA PDFDOI

• Resolving contradictory estimates of band gaps of bulk PdSe2: A Wannier-localized optimally-tuned screened range-separated hybrid density functional theory study

  Applied Physics Letters · 2025-04-01 · 3 citations

  articleSenior author

  Palladium diselenide (PdSe2)—a layered van der Waals material—is attracting significant attention for optoelectronics due to the wide tunability of its band gap from the infrared through the visible range as a function of the number of layers. However, there continues to be disagreement over the precise nature and value of the optical band gap of bulk PdSe2, owing to the rather small value of this gap that complicates experimental measurements and their interpretation. Here, we design and employ a Wannier-localized optimally tuned screened range-separated hybrid (WOT-SRSH) functional to investigate the electronic band structures and optical absorption spectra of bulk and monolayer PdSe2. In particular, we account carefully for the finite exciton center-of-mass momentum within a time-dependent WOT-SRSH framework to calculate the indirect optical gap and absorption onset accurately. Our results agree well with the best available photoconductivity measurements, as well as with state-of-the-art many-body perturbation theory calculations, confirming that bulk PdSe2 has an optical gap in the mid-infrared (upper bound of 0.44 eV). More generally, this work further bolsters the utility of the WOT-SRSH approach for predictive modeling of layered semiconductors.

  PublisherDOI

• From monolayer to thin films: engineered bandgap in CVD grown Bi₂Se_{(3−<i>x</i>)}S_<i>x</i> topological insulator alloys

  Journal of Materials Chemistry C · 2024-01-01 · 5 citations

  articleOpen access

  CVD enabled scalable growth of topological insulator alloys Bi 2 Se (3− x ) S x with a composition-tunable bandgap, free from surface-state pinning.

  PublisherOA PDFDOI

• Excitations in layered materials from a non-empirical Wannier-localized optimally-tuned screened range-separated hybrid functional

  npj Computational Materials · 2024-12-19 · 12 citations

  articleOpen accessCorresponding

  Accurate prediction of electronic and optical excitations in van der Waals (vdW) materials is a long-standing challenge for density functional theory. The recent Wannier-localized optimally-tuned screened range-separated hybrid (WOT-SRSH) functional has proven successful in non-empirical determination of electronic band gaps and optical absorption spectra for covalent and ionic crystals. However, for vdW materials the tuning of the material- and structure-dependent functional parameters has only been attained semi-empirically. Here, we present a non-empirical WOT-SRSH approach applicable to vdW materials, with the optimal functional parameters transferable between monolayer and bulk. We apply this methodology to prototypical vdW materials: black phosphorus, molybdenum disulfide, and hexagonal boron nitride (in the latter case including zero-point renormalization). We show that the WOT-SRSH approach consistently achieves accuracy levels comparable to experiments and many-body perturbation theory (MBPT) calculations for band structures and optical absorption spectra, both on its own and as an optimal starting point for MBPT calculations.

  PublisherOA PDFDOI

• Spatial and Bidirectional Work Function Modulation of Monolayer Graphene with Patterned Polymer “Fluorozwitterists”

  ACS Central Science · 2024-08-06 · 3 citations

  articleOpen access

  diode configuration, with local increase or decrease of work function. Overall, this polymeric fluorozwitterist design is suitable for enabling simple, solution-based surface patterning and is anticipated to be useful for spatial work function modulation of 2D materials integrated into electronic devices.

  PublisherDOI

• Thermodynamics and electronic structure of edges in monolayer MoSi2N4

  Journal of Applied Physics · 2024-07-16 · 3 citations

  articleOpen accessSenior author

  MoSi2N4 is a two-dimensional ternary nitride semiconductor that has attracted attention for its excellent mechanical and thermal properties. Theoretical studies predict that zigzag edges of this material can host magnetic edge states and Dirac fermions, but the stability of such edges has not been examined. Here, we present a density functional theory study of the electronic and thermodynamic properties of MoSi2N4 edges. We develop a (partial) ternary phase diagram that identifies a region of chemical potentials within which MoSi2N4 is stable over competing elemental or binary phases. Based on this phase diagram, we determine the thermodynamic stability of several armchair and zigzag edges and elucidate their electronic structures. Bare zigzag edges, predicted to host exotic electronic states, are found to be substantially higher in energy than armchair edges and, thus, unlikely to occur in practice. However, with hydrogen passivation, these zigzag edges can be stabilized relative to their armchair counterparts while retaining metallicity and magnetic order. Our analysis provides a solid thermodynamic basis for further exploration of MoSi2N4 in nanoscale electronics and spintronics.

  PublisherOA PDFDOI

• Ultrafast Shift Current in SnS₂ Single Crystals: Structure Considerations, Modeling, and THz Emission Spectroscopy

  Advanced Optical Materials · 2024-04-13 · 9 citations

  articleOpen access

  Abstract Above‐band gap optical excitation of non‐centrosymmetric semiconductors can lead to the spatial shift of the center of electron charge in a process known as shift current. Shift current is investigated in single‐crystal SnS 2 , a layered semiconductor with the band gap of ≈2.3 eV, by THz emission spectroscopy and first principles density functional theory (DFT). It is observed that normal incidence excitation with above gap (400 nm; 3.1 eV) pulses results in THz emission from 2H SnS 2 () polytype, where such emission is nominally forbidden by symmetry. It is argued that the underlying symmetry breaking arises due to the presence of stacking faults that are known to be ubiquitous in SnS 2 single crystals and construct a possible structural model of a stacking fault with symmetry properties consistent with the experimental observations. In addition to shift current, it is observed THz emission by optical rectification excited by below band gap (800 nm; 1.55 eV) pulses but it requires excitation fluence more than two orders of magnitude higher to produce same signal amplitude. These results suggest that ultrafast shift current in which can be excited with visible light in blue–green portion of the spectrum makes SnS 2 a promising source material for THz photonics.

  PublisherOA PDFDOI

Recent grants

• NSF-BSF: The Hard-Soft Interface -- Integrating 2D Semiconductors with Functional Polymers for Nanoscale Optoelectronics

  NSF · $450k · 2018–2022

• Collaborative Research: NSF-BSF: On-Chip High-Resolution Mid-Infrared Spectroscopy with a Single Tunable van der Waals Heterostructure Photodetector

  NSF · $192k · 2022–2025

• NSF-BSF: Controlling Phase Selectivity and Electrocatalytic Activity of Transition-Metal Dichalcogenide Overlayers in Core-Shell Nanoparticles for CO2 Reduction

  NSF · $341k · 2018–2022

Frequent coauthors

• Vivek B. Shenoy

  University of Pennsylvania

  41 shared

• Doron Naveh

  25 shared

• Dimitrios Maroudas

  22 shared

• Todd Emrick

  University of Massachusetts Amherst

  14 shared

• Michael D. Barnes

  University of Massachusetts Amherst

  13 shared

• Raúl Arenal

  Universidad de Zaragoza

  12 shared

• Ioanna Fampiou

  Harvard University Press

  12 shared

• Ryan Selhorst

  United States Air Force Research Laboratory

  11 shared

Awards & honors

• Visiting Faculty Fellowship, Weizmann Institute of Science,…
• University of Massachusetts Amherst Exceptional Merit Award…
• US Department of Energy Office of Science Early Career Resea…
• Young Leader Professional Development Award, The Minerals, M…
• Materials Research Society Graduate Student Silver Award (20…