Tom Linker

All Publications

• Catalysis in Extreme Field Environments: A Case Study of Strongly Ionized SiO2Nanoparticle Surfaces. Journal of the American Chemical Society Linker, T. M., Dagar, R., Feinberg, A., Sahel-Schackis, S., Nomura, K., Nakano, A., Shimojo, F., Vashishta, P., Bergmann, U., Kling, M. F., Summers, A. M. 2024

  Abstract

  High electric fields can significantly alter catalytic environments and the resultant chemical processes. Such fields arise naturally in biological systems but can also be artificially induced through localized nanoscale excitations. Recently, strong field excitation of dielectric nanoparticles has emerged as an avenue for studying catalysis in highly ionized environments, producing extreme electric fields. While the dynamics of laser-driven surface ion emission has been extensively explored, understanding the molecular dynamics leading to fragmentation has remained elusive. Here, we employ a multiscale approach performing nonadiabatic quantum molecular dynamics (NAQMD) simulations on hydrogenated silica surfaces in both bare and wetted environments under field conditions mimicking those of an ionized nanoparticle. Our findings indicate that hole localization drives fragmentation dynamics, leading to surface silanol dissociation within 50 fs and charge transfer-induced water splitting in wetted environments within 150 fs. Further insight into such ultrafast mechanisms is critical for the advancement of catalysis on the surface of charged nanosystems.

  View details for DOI 10.1021/jacs.4c08550

  View details for PubMedID 39327984

• Neutron scattering and neural-network quantum molecular dynamics investigation of the vibrations of ammonia along the solid-to-liquid transition. Nature communications Linker, T. M., Krishnamoorthy, A., Daemen, L. L., Ramirez-Cuesta, A. J., Nomura, K., Nakano, A., Cheng, Y. Q., Hicks, W. R., Kolesnikov, A. I., Vashishta, P. D. 2024; 15 (1): 3911

  Abstract

  Vibrational spectroscopy allows us to understand complex physical and chemical interactions of molecular crystals and liquids such as ammonia, which has recently emerged as a strong hydrogen fuel candidate to support a sustainable society. We report inelastic neutron scattering measurement of vibrational properties of ammonia along the solid-to-liquid phase transition with high enough resolution for direct comparisons to ab-initio simulations. Theoretical analysis reveals the essential role of nuclear quantum effects (NQEs) for correctly describing the intermolecular spectrum as well as high energy intramolecular N-H stretching modes. This is achieved by training neural network models using ab-initio path-integral molecular dynamics (PIMD) simulations, thereby encompassing large spatiotemporal trajectories required to resolve low energy dynamics while retaining NQEs. Our results not only establish the role of NQEs in ammonia but also provide general computational frameworks to study complex molecular systems with NQEs.

  View details for DOI 10.1038/s41467-024-48246-9

  View details for PubMedID 38724541

  View details for PubMedCentralID PMC11082248

• Exploring far-from-equilibrium ultrafast polarization control in ferroelectric oxides with excited-state neural network quantum molecular dynamics. Science advances Linker, T., Nomura, K. I., Aditya, A., Fukshima, S., Kalia, R. K., Krishnamoorthy, A., Nakano, A., Rajak, P., Shimmura, K., Shimojo, F., Vashishta, P. 2022; 8 (12): eabk2625

  Abstract

  Ferroelectric materials exhibit a rich range of complex polar topologies, but their study under far-from-equilibrium optical excitation has been largely unexplored because of the difficulty in modeling the multiple spatiotemporal scales involved quantum-mechanically. To study optical excitation at spatiotemporal scales where these topologies emerge, we have performed multiscale excited-state neural network quantum molecular dynamics simulations that integrate quantum-mechanical description of electronic excitation and billion-atom machine learning molecular dynamics to describe ultrafast polarization control in an archetypal ferroelectric oxide, lead titanate. Far-from-equilibrium quantum simulations reveal a marked photo-induced change in the electronic energy landscape and resulting cross-over from ferroelectric to octahedral tilting topological dynamics within picoseconds. The coupling and frustration of these dynamics, in turn, create topological defects in the form of polar strings. The demonstrated nexus of multiscale quantum simulation and machine learning will boost not only the emerging field of ferroelectric topotronics but also broader optoelectronic applications.

  View details for DOI 10.1126/sciadv.abk2625

  View details for PubMedID 35319991

  View details for PubMedCentralID PMC8942355

• Simultaneous Observation of Carrier-Specific Redistribution and Coherent Lattice Dynamics in 2H-MoTe2 with Femtosecond Core-Level Spectroscopy. ACS nano Attar, A. R., Chang, H., Britz, A., Zhang, X., Lin, M., Krishnamoorthy, A., Linker, T., Fritz, D., Neumark, D. M., Kalia, R. K., Nakano, A., Ajayan, P., Vashishta, P., Bergmann, U., Leone, S. R. 2020

  Abstract

  We employ few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to reveal simultaneously the intra- and interband carrier relaxation and the light-induced structural dynamics in nanoscale thin films of layered 2H-MoTe2 semiconductor. By interrogating the valence electronic structure via localized Te 4d (39-46 eV) and Mo 4p (35-38 eV) core levels, the relaxation of the photoexcited hole distribution is directly observed in real time. We obtain hole thermalization and cooling times of 15 ± 5 fs and 380 ± 90 fs, respectively, and an electron-hole recombination time of 1.5 ± 0.1 ps. Furthermore, excitations of coherent out-of-plane A1g (5.1 THz) and in-plane E1g (3.7 THz) lattice vibrations are visualized through oscillations in the XUV absorption spectra. By comparison to Bethe-Salpeter equation simulations, the spectral changes are mapped to real-space excited-state displacements of the lattice along the dominant A1g coordinate. By directly and simultaneously probing the excited carrier distribution dynamics and accompanying femtosecond lattice displacement in 2H-MoTe2 within a single experiment, our work provides a benchmark for understanding the interplay between electronic and structural dynamics in photoexcited nanomaterials.

  View details for DOI 10.1021/acsnano.0c06988

  View details for PubMedID 33085888

• Ultrahigh energy density in high-temperature polymer dielectric reinforced by bilayer nanocoating CHEMICAL ENGINEERING JOURNAL Wang, Y., Linker, T., Lizu, K., Ortiz-Flores, L. A., Kamal, D., Zhou, J., Nguyen, H., Li, C., Gao, W., Konstantinou, A., Chen, L., Huey, B. D., Ramprasad, R., Nakano, A., Shimojo, F., Vashishta, P., Cao, Y. 2025; 507

  View details for DOI 10.1016/j.cej.2025.160613

  View details for Web of Science ID 001428402300001

• Tracking surface charge dynamics on single nanoparticles. Science advances Dagar, R., Zhang, W., Rosenberger, P., Linker, T. M., Sousa-Castillo, A., Neuhaus, M., Mitra, S., Biswas, S., Feinberg, A., Summers, A. M., Nakano, A., Vashishta, P., Shimojo, F., Wu, J., Vera, C. C., Maier, S. A., Cortés, E., Bergues, B., Kling, M. F. 2024; 10 (32): eadp1890

  Abstract

  Surface charges play a fundamental role in physics and chemistry, in particular in shaping the catalytic properties of nanomaterials. However, tracking nanoscale surface charge dynamics remains challenging due to the involved length and time scales. Here, we demonstrate time-resolved access to the nanoscale charge dynamics on dielectric nanoparticles using reaction nanoscopy. We present a four-dimensional visualization of the spatiotemporal evolution of the charge density on individual SiO2 nanoparticles under strong-field irradiation with femtosecond-nanometer resolution. The initially localized surface charges exhibit a biexponential redistribution over time. Our findings reveal the influence of surface charges on surface molecular bonding through quantum dynamical simulations. We performed semi-classical simulations to uncover the roles of diffusion and charge loss in the surface charge redistribution process. Understanding nanoscale surface charge dynamics and its influence on chemical bonding on a single-nanoparticle level unlocks an increased ability to address global needs in renewable energy and advanced health care.

  View details for DOI 10.1126/sciadv.adp1890

  View details for PubMedID 39110806

  View details for PubMedCentralID PMC11305382

• Tailoring the Angular Mismatch in MoS2 Homobilayers through Deformation Fields. Small (Weinheim an der Bergstrasse, Germany) Burns, K., Tan, A. M., Hachtel, J. A., Aditya, A., Baradwaj, N., Mishra, A., Linker, T., Nakano, A., Kalia, R., Lang, E. J., Schoell, R., Hennig, R. G., Hattar, K., Aitkaliyeva, A. 2023; 19 (29): e2300098

  Abstract

  Ultrathin MoS2 has shown remarkable characteristics at the atomic scale with an immutable disorder to weak external stimuli. Ion beam modification unlocks the potential to selectively tune the size, concentration, and morphology of defects produced at the site of impact in 2D materials. Combining experiments, first-principles calculations, atomistic simulations, and transfer learning, it is shown that irradiation-induced defects can induce a rotation-dependent moiré pattern in vertically stacked homobilayers of MoS2 by deforming the atomically thin material and exciting surface acoustic waves (SAWs). Additionally, the direct correlation between stress and lattice disorder by probing the intrinsic defects and atomic environments are demonstrated. The method introduced in this paper sheds light on how engineering defects in the lattice can be used to tailor the angular mismatch in van der Waals (vdW) solids.

  View details for DOI 10.1002/smll.202300098

  View details for PubMedID 37026674

• Allegro-Legato: Scalable, Fast, and Robust Neural-Network Quantum Molecular Dynamics via Sharpness-Aware Minimization Ibayashi, H., Razakh, T., Yang, L., Linker, T., Olguin, M., Hattori, S., Luo, Y., Kalia, R. K., Nakano, A., Nomura, K., Vashishta, P. edited by Bhatele, A., Hammond, J., Baboulin, M., Kruse, C. SPRINGER INTERNATIONAL PUBLISHING AG. 2023: 223-239

  View details for DOI 10.1007/978-3-031-32041-5_12

  View details for Web of Science ID 001463178800012

• Squishing Skyrmions: Symmetry-Guided Dynamic Transformation of Polar Topologies Under Compression. The journal of physical chemistry letters Linker, T., Nomura, K. I., Fukushima, S., Kalia, R. K., Krishnamoorthy, A., Nakano, A., Shimamura, K., Shimojo, F., Vashishta, P. 2022; 13 (48): 11335-11345

  Abstract

  Mechanical controllability of recently discovered topological defects (e.g., skyrmions) in ferroelectric materials is of interest for the development of ultralow-power mechano-electronics that are protected against thermal noise. However, fundamental understanding is hindered by the "multiscale quantum challenge" to describe topological switching encompassing large spatiotemporal scales with quantum mechanical accuracy. Here, we overcome this challenge by developing a machine-learning-based multiscale simulation framework─a hybrid neural network quantum molecular dynamics (NNQMD) and molecular mechanics (MM) method. For nanostructures composed of SrTiO3 and PbTiO3, we find how the symmetry of mechanical loading essentially controls polar topological switching. We find under symmetry-breaking uniaxial compression a squishing-to-annihilation pathway versus formation of a topological composite named skyrmionium under symmetry-preserving isotropic compression. The distinct pathways are explained in terms of the underlying materials' elasticity and symmetry, as well as the Landau-Lifshitz-Kittel scaling law. Such rational control of ferroelectric topologies will likely facilitate exploration of the rich ferroelectric "topotronics" design space.

  View details for DOI 10.1021/acs.jpclett.2c03029

  View details for PubMedID 36454058

• Probing the presence and absence of metal-fullerene electron transfer reactions in helium nanodroplets by deflection measurements. Physical chemistry chemical physics : PCCP Niman, J. W., Kamerin, B. S., Villers, T. H., Linker, T. M., Nakano, A., Kresin, V. V. 2022; 24 (17): 10378-10383

  Abstract

  Metal-fullerene compounds are characterized by significant electron transfer to the fullerene cage, giving rise to an electric dipole moment. We use the method of electrostatic beam deflection to verify whether such reactions take place within superfluid helium nanodroplets between an embedded C60 molecule and either alkali (heliophobic) or rare-earth (heliophilic) atoms. The two cases lead to distinctly different outcomes: C60Nan (n = 1-4) display no discernable dipole moment, while C60Yb is strongly polar. This suggests that the fullerene and small alkali clusters fail to form a charge-transfer bond in the helium matrix despite their strong van der Waals attraction. The C60Yb dipole moment, on the other hand, is in agreement with the value expected for an ionic complex.

  View details for DOI 10.1039/d2cp00751g

  View details for PubMedID 35438706

• Deep Well Trapping of Hot Carriers in a Hexagonal Boron Nitride Coating of Polymer Dielectrics. ACS applied materials & interfaces Linker, T., Wang, Y., Mishra, A., Kamal, D., Cao, Y., Kalia, R. K., Nakano, A., Ramprasad, R., Shimojo, F., Sotzing, G., Vashishta, P. 2021; 13 (50): 60393-60400

  Abstract

  Polymer dielectrics can be cost-effective alternatives to conventional inorganic dielectric materials, but their practical application is critically hindered by their breakdown under high electric fields driven by excited hot charge carriers. Using a joint experiment-simulation approach, we show that a 2D nanocoating of hexagonal boron nitride (hBN) mitigates the damage done by hot carriers, thereby increasing the breakdown strength. Surface potential decay and dielectric breakdown measurements of hBN-coated Kapton show the carrier-trapping effect in the hBN nanocoating, which leads to an increased breakdown strength. Nonadiabatic quantum molecular dynamics simulations demonstrate that hBN layers at the polymer-electrode interfaces can trap hot carriers, elucidating the observed increase in the breakdown field. The trapping of hot carriers is due to a deep potential well formed in the hBN layers at the polymer-electrode interface. Searching for materials with similar deep well potential profiles could lead to a computationally efficient way to design good polymer coatings that can mitigate breakdown.

  View details for DOI 10.1021/acsami.1c14587

  View details for PubMedID 34890506

• MISTIQS: An open-source software for performing quantum dynamics simulations on quantum computers SOFTWAREX Powers, C., Bassman, L., Linker, T. M., Nomura, K., Gulania, S., Kalia, R. K., Nakano, A., Vashishta, P. 2021; 14

  View details for DOI 10.1016/j.softx.2021.100696

  View details for Web of Science ID 000663433800024

• Ex-NNQMD: Extreme-Scale Neural Network Quantum Molecular Dynamics Rajak, P., Aditya, A., Fukushima, S., Kalia, R. K., Linker, T., Liu, K., Luo, Y., Nakano, A., Nomura, K., Shimamura, K., Shimojo, F., Vashishta, P., IEEE IEEE COMPUTER SOC. 2021: 943-946

  View details for DOI 10.1109/IPDPSW52791.2021.00145

  View details for Web of Science ID 000689576200117

• Optically Induced Three-Stage Picosecond Amorphization in Low-Temperature SrTiO3. The journal of physical chemistry letters Linker, T., Tiwari, S., Fukushima, S., Kalia, R. K., Krishnamoorthy, A., Nakano, A., Nomura, K. I., Shimamura, K., Shimojo, F., Vashishta, P. 2020; 11 (22): 9605-9612

  Abstract

  Photoexcitation can drastically modify potential energy surfaces of materials, allowing access to hidden phases. SrTiO3 (STO) is an ideal material for photoexcitation studies due to its prevalent use in nanostructured devices and its rich range of functionality-changing lattice motions. Recently, a hidden ferroelectric phase in STO was accessed through weak terahertz excitation of polarization-inducing phonon modes. In contrast, whereas strong laser excitation was shown to induce nanostructures on STO surfaces and control nanopolarization patterns in STO-based heterostructures, the dynamic pathways underlying these optically induced structural changes remain unknown. Here nonadiabatic quantum molecular dynamics reveals picosecond amorphization in photoexcited STO at temperatures as low as 10 K. The three-stage pathway involves photoinduced charge transfer and optical phonon activation followed by nonlinear charge and lattice dynamics that ultimately lead to amorphization. This atomistic understanding could guide not only rational laser nanostructuring of STO but also broader "quantum materials on demand" technologies.

  View details for DOI 10.1021/acs.jpclett.0c02873

  View details for PubMedID 33124829

• Material considerations for thermoelectric enhancement via modulation doping APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING Beekman, M., Heaton, G., Linker, T. M., Johnson, D. C. 2020; 126 (7)

  View details for DOI 10.1007/s00339-020-03673-5

  View details for Web of Science ID 000542224400001

• Towards simulation of the dynamics of materials on quantum computers PHYSICAL REVIEW B Bassman, L., Liu, K., Krishnamoorthy, A., Linker, T., Geng, Y., Shebib, D., Fukushima, S., Shimojo, F., Kalia, R. K., Nakano, A., Vashishta, P. 2020; 101 (18)

  View details for DOI 10.1103/PhysRevB.101.184305

  View details for Web of Science ID 000534165800002

• Field-Induced Carrier Localization Transition in Dielectric Polymers. The journal of physical chemistry letters Linker, T. M., Tiwari, S., Kumazoe, H., Fukushima, S., Kalia, R. K., Nakano, A., Ramprasad, R., Shimojo, F., Vashishta, P. 2020; 11 (2): 352-358

  Abstract

  Organic polymers offer many advantages as dielectric materials over their inorganic counterparts because of high flexibility and cost-effective processing, but their application is severely limited by breakdown in the presence of high electric fields. Dielectric breakdown is commonly understood as the result of avalanche processes such as carrier multiplication and defect generation that are triggered by field-accelerated hot carriers (electrons or holes). In stark contrast to inorganic dielectric materials, however, there remains no mechanistic understanding to enable quantitative prediction of the breakdown field in polymers. Here, we perform systematic study of different electric fields on hot carrier dynamics and resulting chemical damage in a slab of archetypal polymer, polyethylene, using nonadiabatic quantum molecular dynamics simulations. We found that high electric fields induce localized electronic states at the slab surface, with a critical transition occurring near the experimentally reported intrinsic breakdown field. This transition in turn facilitates strong polaronic coupling between charge carriers and atoms, which is manifested by severe damping of the time evolution of localized states and the presence of C-H vibrational resonance in the hot-carrier motion leading to rapid carbon-carbon bond breaking on the surface. Such polaronic localization transition may provide a critically missing prediction method for computationally screening dielectric polymers with high breakdown fields.

  View details for DOI 10.1021/acs.jpclett.9b03147

  View details for PubMedID 31867972

• Comparison of Predicted Thermoelectric Energy Conversion Efficiency by Cumulative Properties and Reduced Variables Approaches Linker, T. M., Lee, G. S., Beekman, M. SPRINGER. 2018: 3085-3090

  View details for DOI 10.1007/s11664-017-5870-5

  View details for Web of Science ID 000431920400004