About

Thomas J. Kempa is an Associate Professor of Chemistry and of Materials Science and Engineering (by courtesy) at Johns Hopkins University. He holds a bachelor’s degree in chemistry from Boston College (2004) and spent two years as a post-graduate student at Imperial College London courtesy of a Marshall Scholarship. He pursued graduate studies in chemistry at Harvard University under the direction of Prof. Charles Lieber, focusing on the discovery and development of nanoscale materials for next-generation solar cells and photonic devices, and received his PhD in 2012. Following his doctoral work, he conducted postdoctoral studies in the laboratory of Prof. Daniel Nocera at MIT and Harvard, where he focused on harnessing electrochemical and hydrodynamic phenomena to form complex patterns of inorganic nanostructures. His research group develops new methods to prepare and study low-dimensional inorganic crystals, ranging from nanoparticles to few-atom thick sheets, which exhibit exceptional properties for applications in optoelectronic, energy conversion, and quantum science studies. His work spans physical, inorganic, and materials chemistry, with a focus on synthesizing and characterizing responsive porous molecular frameworks, 2D materials, and multi-component nanostructures to advance technologies in sensors, energy storage, electronics, and quantum devices.

Research topics

• Nanotechnology
• Materials science
• Computer Science
• Engineering
• Chemistry
• Chemical engineering
• Engineering ethics
• Optoelectronics
• Metallurgy
• Organic chemistry
• Crystallography
• Physical chemistry
• Engineering physics

Selected publications

• Plasmon Resonance Color Shifts in Nanogaps for Rapid Thickness Mapping of Two-Dimensional Materials

  ACS Applied Nano Materials · 2026-04-14

  articleSenior author
  PublisherDOI

• Manipulation of Valence Trapping through Local Order of Fe₃O Units in an Intrinsically Mixed-Valence Coordination Polymer

  Inorganic Chemistry · 2025-05-16 · 3 citations

  articleOpen accessSenior authorCorresponding

  Mixed-valence coordination polymers (CPs) can exhibit enhanced and tunable conductivity and stimulus-driven phase transitions, making them promising new electronic materials. However, the mechanisms of the electron transfer processes that underlie these properties in the solid state merit further study. Here, we report the synthesis of a new mixed-valence CP, FeIII2FeIIO(OAc)6(bpy)2 (OAc = acetate, bpy = 4,4’-bipyridine), which is obtained from the self-assembly of Fe3O(OAc)6 clusters and bpy linkers. Notably, the CP is charge-neutral in its mixed-valence form, making it one of the few established intrinsically mixed-valence CPs. Detailed characterization of the valence and spin states of the constituent Fe ions in the CP via variable-temperature Mössbauer spectroscopy, single-crystal X-ray diffraction, and magnetic susceptibility measurements reveals the occurrence of thermally activated electron transfer (i.e., valence detrapping) and antiferromagnetic exchange between FeII and FeIII ions. Intriguingly, valence-detrapping phenomena are confined to distinct regions of the CP and proceed to varying degrees of completion depending on the solid-state environment of the participating Fe ions. This study presents a convenient strategy for the direct synthesis of intrinsically mixed-valence coordination polymers and highlights the important role of solid-state structural order in manipulating electron transfer mechanisms in Class II Robin-Day mixed-valence materials.

  PublisherOA PDFDOI

• Tailoring Light–Matter Interactions in 2D Semiconductors

  Accounts of Chemical Research · 2025-06-24 · 3 citations

  articleOpen accessSenior authorCorresponding

  ConspectusTwo-dimensional (2D) crystals have had a sweeping influence on the condensed matter physics and materials science communities for over two decades. Their thinness and ability to be configured into layered and twisted heterostructures has enabled 2D crystals to become a platform material of choice to uncover many intriguing phenomena, including superconductivity, the fractional quantum anomalous Hall effect, spin textures, strain domain walls, and distinct spin-valley transitions. This versatility is on display in 2D semiconductor monolayers, which exhibit strong light-matter coupling to a rich host of excitonic states and valley selective transitions. The optical physics of 2D semiconductors is tunable through manipulation of their structure, chemical composition, mutual orientation in superlattices, and strain. Together, such adjustments give rise to a plethora of exciting applications in optics, spintronics, and quantum sensing. Our contributions to the 2D materials community have focused on developing chemical strategies for precision nanostructure synthesis, elucidating emergent optical phenomena through detailed spectroscopic analysis, and creating new 2D heterostructures that support the localization and manipulation of quasiparticle states. This Account examines selected aspects of our recent work on tailoring light-matter interactions in 2D semiconductors. We discuss how synthetic manipulation of a 2D crystal's dimensions, edge structure, strain state, and coupling to other molecular species and lattices renders specific properties. Through this article we wish to draw attention to the rich chemistry of 2D crystals and the active role chemistry should play in opening new avenues of research in 2D materials.

  PublisherOA PDFDOI

• Control of 2D Crystal Monodispersity through a Seeded Growth Approach

  ACS Applied Nano Materials · 2025-12-22

  articleSenior authorCorresponding

  Achieving the synthesis of uniform two-dimensional (2D) transition metal dichalcogenide (TMD) crystals through chemical vapor deposition (CVD) is a sought after goal for enabling their use in future transistor, memory, and optics applications. Here, we demonstrate a seeded growth strategy that yields size monodisperse 2D TMD monolayers via CVD synthesis from pre-etched Mo oxide precursors. Controlled etching of MoO3 particles in NaOH solution produces well-defined nucleation reservoirs that take part in subsequent reactions with Se to form monodisperse 2D MoSe2 crystals. Crystal sizes are controllable by the particle etching time and the CVD reaction time down to a few hundred nanometers with size variations as low as 1.8%. Electron microscopy and Raman and photoluminescence spectroscopies verify the structural and optical quality of the as-grown MoSe2 crystals. This study reinforces the enduring need for advances in synthesis-driven control of the size, morphology, composition, and strain- and defect-states of 2D materials.

  PublisherDOI

• 33 Unresolved Questions in Nanoscience and Nanotechnology

  ACS Nano · 2025-09-04 · 22 citations

  articleOpen access

  Significant advances in science and engineering often emerge at the intersections of disciplines. Nanoscience and nanotechnology are inherently interdisciplinary, uniting researchers from chemistry, physics, biology, medicine, materials science, and engineering. This convergence has fostered novel ways of thinking and enabled the development of materials, tools, and technologies that have transformed both basic and applied research, as well as how we address critical societal challenges. In this Nano Focus, we pose and explore 33 questions whose answers could profoundly impact fields such as energy, electronics, the environment, optics, and medicine. These questions highlight the need for deeper foundational understanding, improved tools and techniques, and innovative applications─each with significant societal relevance. Together, they represent a global call-to-action for the scientific community.

  PublisherOA PDFDOI

• 33 Unresolved Questions in Nanoscience and NanotechnologyArticle link copied!

  RWTH Publications (RWTH Aachen) · 2025-01-01

  article
  Publisher

• Identification of Phase Changes in a Model 2D MOF through Polarization-Dependent Raman Spectroscopy

  ACS Applied Materials & Interfaces · 2025-10-15 · 1 citations

  articleSenior authorCorresponding

  Metal–organic frameworks (MOFs) are molecular lattices with highly tunable structures and correspondingly diverse properties. However, distinguishing MOFs that contain the same metal nodes and ligands but different connectivities is nontrivial, especially when the MOF samples approach the monolayer limit. Here we show that polarization-dependent Raman spectroscopy can be used to distinguish between two closely related structural motifs of a model MOF comprised of Mo2(isonicotinate)4 clusters. We find that certain Raman modes active under cross-polarization become more intense in the partially under-coordinated (nominally 1D) phase of the normally 2D MOF, reflecting the character of connectivity in the MOFs rather than the formal symmetry of their unit cells. This polarization-dependent change in intensity is supported by density functional theory (DFT) calculations of vibrational modes in the two phases. We anticipate polarization-resolved Raman spectroscopy will be a useful tool for identification of MOF structures, particularly for samples for which conventional structure determination is difficult or untenable.

  PublisherDOI

• Excitons at the interface of 2D TMDs and molecular semiconductors

  The Journal of Chemical Physics · 2024-05-28 · 6 citations

  articleOpen accessSenior author

  Van der Waals heterostructures (vdWHs) of vertically stacked two-dimensional (2D) atomic crystals have been used to elicit intriguing phenomena stemming from strong electronic correlations, magnetic textures, and interlayer excitons spawned at the heterointerface. However, vdWHs comprised of heterointerfaces between these 2D atomic crystal lattices and molecular assemblies are emerging as equally intriguing platforms supporting properties to be harnessed for photovoltaic energy conversion, photodetection, spin-selective charge injection, and quantum emission. In this perspective, we summarize recent research examining exciton dynamics in heterostructures between semiconducting 2D transition metal dichalcogenides and molecular organic semiconductors. We discuss methods for assembly of these heterostructures, the nature of interlayer or charge-transfer excitons at transition-metal dichalcogenide (TMD)-molecule interfaces, explicit exciton transfer between organics and TMDs, and other interfacial phenomena driven by the merger of these two material classes. We also suggest key new research directions extending the remit of these 2D atomic-molecular lattice heterointerfaces into the domains of condensed matter physics, quantum sensing, and energy conversion.

  PublisherOA PDFDOI

• Exfoliation and Optical Properties of S=1 Triangular Lattice Antiferromagnet NiGa$_2$S$_4$

  arXiv (Cornell University) · 2024-06-05

  preprintOpen access

  Two-dimensional (2D) van der Waals (vdW) materials have been an exciting area of research ever since scientists first isolated a single layer of graphene. Single layer magnetic materials can provide a pathway for vdW heterostructures with magnetic properties. While most of the magnetic vdW materials exhibit ordering transitions in the bulk, here we report a successful exfoliation of a triangular lattice S=1 antiferromagnet NiGa$_2$S$_4$, which already demonstrates exotic magnetism in the bulk material. We establish the number of layers of the material by atomic force microscopy (AFM) and detail a careful characterization using Raman and optical spectroscopy to demonstrate how the optical, electronic, and structural properties of NiGa$_2$S$_4$ change as a function of sample thickness. Optical measurements and electronic structure calculations of bulk versus monolayer NiGa$_2$S$_4$ confirm the material to be a Mott insulator with an electronic gap of about 1.5 eV, which slightly increases for layers below 10 L. We conclude with a theoretical analysis of the possibility of doping monolayer NiGa$_2$S$_4$ by proximity to a metal.

  PublisherOA PDFDOI

• Exfoliation and optical properties of S = 1 triangular lattice antiferromagnet NiGa2S4

  Scientific Reports · 2024-11-14 · 3 citations

  articleOpen access

  Two-dimensional (2D) van der Waals (vdW) materials have been an exciting area of research ever since scientists first isolated a single layer of graphene. Single layer magnetic materials can provide a pathway for vdW heterostructures with magnetic properties. While most of the magnetic vdW materials exhibit ordering transitions in the bulk, here we report a successful exfoliation of a triangular lattice S = 1 antiferromagnet NiGa $$_2$$ S $$_4$$ , which already demonstrates exotic magnetism in the bulk material. We establish the number of layers of the material by atomic force microscopy (AFM) and detail a careful characterization using Raman and optical spectroscopy to demonstrate how the optical, electronic, and structural properties of NiGa $$_2$$ S $$_4$$ change as a function of sample thickness. Optical measurements and electronic structure calculations of bulk versus monolayer NiGa $$_2$$ S $$_4$$ confirm the material to be a Mott insulator with an electronic gap of about 1.5 eV, which slightly increases for layers below 10 L. We conclude with a theoretical analysis of the possibility of doping monolayer NiGa $$_2$$ S $$_4$$ by proximity to a metal.

  PublisherOA PDFDOI

Recent grants

• CAREER: Coordination Polymer Superlattices with Tailored Properties through Chemical Vapor Deposition Synthesis

  NSF · $756k · 2019–2024

Frequent coauthors

• Charles M. Lieber

  20 shared

• F. James Claire

  14 shared

• Minyuan M. Li

  Pacific Northwest National Laboratory

  12 shared

• Marina A. Solomos

  Merck & Co., Inc., Rahway, NJ, USA (United States)

  11 shared

• Sun‐Kyung Kim

  11 shared

• Hong‐Gyu Park

  Seoul National University

  10 shared

• Maxime A. Siegler

  10 shared

• Bozhi Tian

  University of Chicago

  9 shared

Labs

• Kempa GroupPI

Education

• Ph. D., Chemistry

  Harvard University

  2012

• B. S., Chemistry

  Boston College

  2004

Awards & honors

• MRS Graduate Student Award
• Dudley Herschbach Teaching Award
• 2013 IUPAC Young Chemist Prize
• DARPA Young Faculty Award
• NSF CAREER Award