About

Harley T. Johnson is a Professor in the Department of Mechanical Science and Engineering at the University of Illinois Urbana-Champaign, where he also serves as the Executive Director and CEO of the Illinois Quantum and Microelectronics Park. He holds courtesy appointments as Professor of Materials Science and Engineering and has affiliate roles in the Materials Research Lab (MRL) and the National Center for Supercomputing Applications (NCSA). Dr. Johnson is the Director and Principal Investigator of the Illinois NSF Materials Research Science and Engineering Center (I-MRSEC). His academic career includes positions as Associate Dean for Research at the Grainger College of Engineering, Associate Head for Graduate Programs in the Department of Mechanical Science and Engineering, and faculty roles at Boston University. His research focuses on the mechanics of electronic and photonic materials, nanostructures, and the optical properties of materials, with applications spanning solar energy, microelectronics, sensing, detection, and materials processing. His work employs atomistic and continuum modeling methods to simulate multiphysics phenomena, aiming to aid in the design and interpretation of experiments related to quantum dots, MEMS devices, dislocations in electronic materials, carbon nanotubes, nanoscale surface instabilities, and nanophotonic structures.

Research topics

• Computer Science
• Physics
• Condensed matter physics
• Geometry
• Mathematics
• Emergency medicine
• Quantum mechanics
• Optoelectronics
• Medicine
• Combinatorics
• Anesthesia
• Chemistry
• Composite material
• Medical emergency
• Simulation
• Optics
• Nanotechnology
• Materials science

Selected publications

• Emerging conduction pathways in semiconducting bismuth-antimony alloys

  Journal of Physics Condensed Matter · 2026-03-31

  articleOpen access

  Abstract As strong topological insulators, Bi <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi/> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> Sb x alloys with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mn>0.07</mml:mn> <mml:mo>&lt;</mml:mo> <mml:mi>x</mml:mi> <mml:mo>&lt;</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> </mml:math> are predicted to host topologically protected conduction pathways along extended defects, such as dislocations. Due to the presence of bulk conduction, these fascinating topological features have been obscured to date. We examine carrier transport and electronic states in high-purity single-crystals of Bi <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi/> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>−</mml:mo> <mml:mi>x</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> Sb x alloys with 11 at% Sb. Using high magnetic fields, we suppress bulk conduction, enabling the identification of the residual conductivity associated with extended defects. A two-band analysis of magnetotransport measurements in combination with angle-resolved photoemission spectroscopy, reveals a bandgap <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mtext>⩾</mml:mtext> </mml:mrow> </mml:math> 40 meV, larger than Arrhenius analyses of temperature-dependent resistivities. With mobility as high as 750 000 cm 2 V −1 s −1 , Bi–Sb alloys are promising candidates for topological electronics and related applications.

  PublisherDOI

• Coupling DSMC and AKMC to simulate graphite erosion due to hyperthermal oxidation at the nanoscale

  International Journal of Heat and Mass Transfer · 2026-01-07

  articleOpen accessSenior authorCorresponding

  A one-way coupling is introduced between the particle-based gas flow method, direct simulation Monte Carlo (DSMC), and the material evolution method, atomistic-kinetic Monte Carlo (AKMC), at the nanoscale. The coupled DSMC → AKMC method is used to simulate hyperthermal oxidative pitting in graphite exposed at various tilt angles, with relevance to ablative pitting in carbon fibers used for Thermal Protection System (TPS) materials during reentry. Simulated DSMC particles have a one-to-one mapping with real atoms or molecules, and the two methods are spatially coupled such that particles from DSMC can directly trigger adsorption events in AKMC. The coupled method more accurately captures gas behavior on pit geometries compared to pure uncoupled AKMC that typically uses simplified assumptions to approximate the adsorption behavior from the gas phase. Comparison between results from DSMC → AKMC and pure uncoupled AKMC reveals a discrepancy between their predicted pit geometries, especially at lower angles of graphite tilt. That is, pure uncoupled AKMC falsely overpredicts the depth and narrowness of the evolving pits due to the simplified assumption of vertical oxygen incidence rather than a realistic oblique incidence as in DSMC → AKMC. This discrepancy in pit geometry is essential to acknowledge when evaluating the influence of pit geometry on the structural stability of carbon fibers during reentry ablation. In addition, the physical insights on pit evolution under realistic gas behavior from the coupled DSMC → AKMC simulations can be used to improve uncoupled AKMC simulations by applying appropriate boundary conditions. Lastly, an exploratory analysis is performed in DSMC to determine if CO molecules emitted from the oxidizing graphite affect the number density of oxygen local to the graphite, assessing the need for a two-way coupling between DSMC and AKMC. • A one-way coupling between DSMC and AKMC is presented to simulate graphite oxidation. • Challenges in coupling the two methods at the nanoscale are discussed. • Coupling captures realistic gas-surface effects such as oblique incidence of gas particles. • Pit evolution in graphite exposed to oxygen gas at various orientations is simulated. • Realistic oblique incidence of gas particles causes defects to grow wider than deeper.

  PublisherDOI

• Coupling DSMC and AKMC to Simulate Graphite Erosion due to Hyperthermal Oxidation at the Nanoscale

  SSRN Electronic Journal · 2025-01-01

  preprintOpen accessSenior author
  PublisherDOI

• Quantifying superlubricity of bilayer graphene from the mobility of interface dislocations

  arXiv (Cornell University) · 2025-01-09

  preprintOpen access

  Van der Waals (vdW) heterostructures subjected to interlayer twists or heterostrains demonstrate structural superlubricity, leading to their potential use as superlubricants in micro- and nano-electro-mechanical devices. However, quantifying superlubricity across the vast four-dimensional heterodeformation space using experiments or atomic-scale simulations is a challenging task. In this work, we develop an atomically informed dynamic Frenkel--Kontorova (DFK) model for predicting the interface friction drag coefficient of an arbitrarily heterodeformed bilayer graphene (BG) system. The model is motivated by MD simulations of friction in heterodeformed BG. In particular, we note that interface dislocations formed during structural relaxation translate in unison when a heterodeformed BG is subjected to shear traction, leading us to the hypothesis that the kinetic properties of interface dislocations determine the friction drag coefficient of the interface. The constitutive law of the DFK model comprises the generalized stacking fault energy of the AB stacking, a scalar displacement drag coefficient, and the elastic properties of graphene, which are all obtained from atomistic simulations. Simulations of the DFK model confirm our hypothesis since a single choice of the displacement drag coefficient, fit to the kinetic property of an individual dislocation in an atomistic simulation, predicts interface friction in any heterodeformed BG. By bridging the gap between dislocation kinetics at the microscale to interface friction at the macroscale, the DFK model enables a high-throughput investigation of strain-engineered vdW heterostructures.

  PublisherOA PDFDOI

• Modeling the effect of pitting on the tensile behavior of amorphous carbon and carbon fiber

  Carbon Trends · 2025-08-01

  articleOpen access

  FiberForm, the substrate of the Phenolic Impregnated Carbon Ablator (PICA), contains various fundamental forms of carbon, including vitreous or highly ordered graphitic regions in the carbon fiber core and amorphous or turbostratic carbon in the binder material, which provides stiffness to the material. In this study, we use the AIREBO potential to investigate the effect of pitting on the elastic properties of carbon fiber (CF) and amorphous carbon (AC). The generation, structural characterization, and loading of pristine AC and CF are compared with their pitted counterparts over a range of densities (1.27 g/cm 3 to 2.93 g/cm 3 ) and porosities. Results show a reduction of up to 18.7% in elastic modulus for AC and a reduction of 13.7% in modulus for CF. We also observe AC to have weaker tensile behavior for both oxidized and pristine states, supporting the hypothesis that FiberForm is likely to fail at the binder. The present work advances fundamental understanding of the coupling between oxidation and mechanical behavior of carbon-based TPS materials and serves as a basis for larger-scale simulations.

  PublisherDOI

• Fundamental microscopic properties as predictors of large-scale quantities of interest: Validation through grain boundary energy trends

  Acta Materialia · 2025-01-13 · 5 citations

  articleOpen access
  PublisherOA PDFDOI

• Probing Topological Surface States and Conduction via Extended Defects in (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ Films

  ArXiv.org · 2025-12-14

  preprintOpen access

  (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ alloys are non-degenerate topological insulators (TIs) whose Dirac point (DP) can be tuned within the bulk bandgap by varying the composition, effectively reducing bulk conduction while allowing surface carrier conduction. Magnetotransport measurements of a series of (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films indicate electron-dominated conduction, with weak anti-localization attributed to topological surface states (TSSs). Due to the similarity of phase coherence lengths and twin boundary spacings ($\sim$100 nm), we consider the role of twin boundaries as additional conducting paths. Density functional theory calculations reveal an enhanced density of states near the Fermi level at $60^\circ$ twin boundaries, with 2D carrier concentration in excess of $3 \times 10^{13}$ cm$^{-2}$. Furthermore, an analysis of the longitudinal magnetoconductivity yields an upper bound of $7.3 \times 10^{-4}$ S for twin boundary conductivity, resulting in a carrier mobility as high as $142$ cm$^2$/(V$\cdot$s). We discuss the role of twin boundaries in facilitating a transition from a massive Dirac cone dispersion to gapless, topologically protected surface states. Understanding the role of twin boundaries on carrier conduction in non-degenerate TIs is critical for the development of novel TI-based electronic devices.

  PublisherOA PDFDOI

• Graphene-hBN interlayer interactions from quantum Monte Carlo

  Physical review. B./Physical review. B · 2025-08-26 · 1 citations

  articleSenior author

  The authors use here large-scale quantum Monte Carlo calculations to simulate van der Waals (vdW) correlations in boron nitride and graphene to a new level of accuracy and detail. This interaction determines the corrugation and phonon behavior of encapsulated graphene layers, which is critical to the behavior of these materials. The authors find that standard approximations to the vdW interactions fail to describe both graphene and boron nitride accurately. An interatomic potential is provided to enable high-accuracy calculations of the mechanical properties of these 2D materials.

  PublisherDOI

• Mechanics of out-of-plane screw dislocation in a 2D material

  Extreme Mechanics Letters · 2025-04-08 · 1 citations

  articleOpen accessSenior authorCorresponding

  We study the mechanics of out-of-plane screw dislocations in two-dimensional (2D) materials using elastic membrane theory and atomistic simulations . Through elastic membrane theory, we derive a closed-form equation for the excess energy of the out-of-plane screw dislocation, revealing that the strain associated with the dislocation in a 2D material diminishes more rapidly with distance compared to that in a bulk material. We utilize this equation to compute energy profiles of out-of-plane screw dislocations in graphene. Various core radii across Burgers vectors (i.e., number of layers) under conditions with and without hydrogen termination on the dislocation core are considered, and computed energies are validated by atomistic simulations. Our results show that the screw dislocation core has a finite core radius which increases as the Burgers vector increases to avoid a high stress concentration at the dislocation core, thereby minimizing the total energy. Furthermore, we extend our theory to include the interaction between screw dislocations in a dipole configuration. Simulation results indicate that the relaxation narrows a high-strain concentration region near the dislocation core, where the interaction energy becomes negligible. Additionally, we examine the influence of out-of-plane screw dislocations on alternating twisted multilayer graphene. Our results reveal that the screw dislocation induces additional in-plane strain on the structure near the dislocation core, but that this additional strain is confined to within 2 nm of the dislocation core. • Mechanics of out-of-plane screw dislocations in 2D materials is examined. • The core energy is impacted by hydrogen termination, radius, and Burgers vector. • Interactions between screw dislocations are also thoroughly examined.

  PublisherDOI

• Interatomic potential development for topological insulator Bi1-xSbx and its dislocation by force-following active learning

  ArXiv.org · 2025-10-21

  preprintOpen accessSenior author

  We introduce a force following active learning algorithm that integrates density functional theory DFT with the Gaussian Approximation Potential GAP framework to develop a robust interatomic potential IP for a dislocation in a topological insulator Bi1xSbx. Starting from an initial potential IP0 trained on unit cell data from strained Bi Sb binaries our active learning approach iteratively refines the IP during a structural relaxation. In each cycle if the force error uncertainty of any atom near the dislocation core exceeds a threshold value the IPi is efficiently retrained IPi to IPi1 by incorporating DFT computed forces and energies of atoms near the high uncertainty atom. This strategy ensures that the relaxation process maintains a low force error until full convergence is achieved. Consequently the final IP here IP5 has two capabilities 1 it reproduces the relaxation pathway observed during the active learning process unlike the initial IP0 which lacks prior dislocation core knowledge and 2 it captures the lattice and elastic properties of Bi Sb binaries across a range of Sb concentrations. We also evaluate dislocation properties Peierls stresses and dislocation generation by compression to assess the performance of the trained potential IP5.

  PublisherOA PDFDOI

Recent grants

• Probing and optimizing quantum dot confined states for next generation intermediate band solar cells

  NSF · $382k · 2009–2013

• GOALI: Polarized Infrared Imaging for the Mechanics of Photovoltaic Wafers

  NSF · $398k · 2013–2017

• Material Removal Mechanisms in Focused Ion Beam Nanopore Drilling

  NSF · $300k · 2015–2019

• Atomistic Origins of Ion Bombardment Nanoscale Surface Instability

  NSF · $280k · 2005–2009

• NER: Optimized Photonic Bandgap Devices with Nanoscale Disorder

  NSF · $100k · 2005–2007

Frequent coauthors

• Pascal Pochet

  Commissariat à l'Énergie Atomique et aux Énergies Alternatives

  90 shared

• Jonathan B. Freund

  25 shared

• L. B. Freund

  22 shared

• Sameh Tawfick

  19 shared

• Mitisha Surana

  University of Illinois Urbana-Champaign

  18 shared

• Kaihao Zhang

  University of Illinois Urbana-Champaign

  18 shared

• Jad Yaacoub

  University of Illinois Urbana-Champaign

  18 shared

• Ganesh Ananthakrishnan

  University of Illinois Urbana-Champaign

  18 shared

Awards & honors

• NSF Materials Research Science and Engineering Center (I-MRS…