We use optical pump-probe spectroscopies and time- and angle-resolved photoemission spectroscopy  to study excitons and carriers in two-dimensional semiconductors, such as transition metal dichalchogenides (TMDCs). Of particular interest are excitonic phase transitions in TMDC heterobilayers.

Nano Lett. 2021, 21, 23, 9903–9908

Nat. Mater. 2021, 20, 1657–1662

Nano Lett. 2021, 21(8), 3511-3517

Science 2020, 367, 903-906

Nat. Mat. 2020, 19, 1068-107  



Superatom solids are a new class of materials assembled with nanoscale building blocks of superatomic motifs. This synthetic approach allows us to manipulate material structures and their optical, electronic, and magnetic properties. We can tune optical gaps broadly from mid-IR to the visible region, and control the structural dimensions between 0D, 1D, 2D, and 3D. We are studying emergent properties of the materials with optical spectroscopies and scanning-tunneling microscopy.

Adv. Mat. 2019, 30, 1903209

Adv. Funct. Mater. 2019, 29, 1902951

Nano Lett. 2018, 18, 1483-1488


We study the fundamental interaction of light with matter, such as lasing and polariton condensates in low-dimensional cavities. In particular, we aim to understand the fundamental scattering processes responsible for polariton condensation and coherent light emission.

Sci. Adv. 2021, 7, eabj7667

Nature Commun. 2019, 10, 265

Acct. Chem. Res. 2019, 52, 2950-2959

Nat. Mater. 2015, 14, 636-642.



Singlet fission is the photo-chemical process where one singlet exciton splits into two triplet excitons. The exciton multiplication process can potentially boost the performance of a single-junction solar cell, a photo detector, and perhaps a multi-electron photocatalyst, but the microscopic understanding of singlet fission is necessary. We use multiple ultrafast spectroscopic techniques to understand the singlet fission rates and to explore the harvesting of triplets from the singlet fission process.

Nature Chem. 2017, 9, 341-346.

Science Adv. 2017, 3, e1700241.

J. Am. Chem. Soc. 2019, 141, 13143-13147


cropped-cropped-perovskite_final-21-e1503590282154.jpgPOLARON DYNAMICS IN LEAD HALIDE PEROVSKITES

We seek to understand the exceptional carrier properties in lead halide perovskites from the viewpoint of polarons, the quasi particles of carriers associated with surrounding lattice distortion. Specifically, we propose the ferroelectric polaron concept to explain the efficient screening of charge carriers. We extend the idea to other ferroelectric and paraelectric semiconductors as basis for defect tolerant semiconductors.

Nat. Rev. Chem. 2021, 5, 838–852

Nature Mat. 2018, 17, 379-381

Science Adv. 2017, 3, e1701217

Science, 2016, 353, 1409-1413.