Probing interfacial electronic structure in quantum materials using stem cathodoluminescence spectral imaging

Hou, Hanyu

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https://hdl.handle.net/2142/132742 Copy

Description

Title

Probing interfacial electronic structure in quantum materials using stem cathodoluminescence spectral imaging

Author(s)

Hou, Hanyu

Issue Date

2025-09-30

Director of Research (if dissertation) or Advisor (if thesis)

• Zuo, Jian-Min
• Wen, Jianguo

Doctoral Committee Chair(s)

Huang, Pinshane

Committee Member(s)

• Goldschmidt, Elizabeth
• Cao, Qing

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Quantum Materials, Quantum Emitters, hBN

Abstract

Quantum information science (QIS) critically relies on materials that can generate, process, and sense quantum states with high fidelity at scales. The development of quantum material requires measurements that can connect atomic registry, symmetry, and local electromagnetic fields to optical and electronic responses at precise locations where quantum entanglement resides. The demands for such measurements range from the detection of defect centers and excitons supplying single photons for quantum networking to the characterization of superconducting structures that enable fast, low-loss circuits. Quantum emitters (QEs) are especially important because of their capability to provide controlled photon emissions with two-level models. Within this landscape, hexagonal boron nitride (hBN) is a promising host for QEs at room temperature. For superconductors, the two-dimensional electron gas formed between europium oxide (EuO) and potassium tantalate oxide (KTaO3, KTO) illustrates how interfacial chemistry and symmetry breaking drive the superconductivity. These two systems motivated the structure-property relation study here that is simultaneously structure aware and optically sensitive at atomic to nanoscopic scale. This thesis starts with the development of scanning transmission electron microscopy (STEM) with cathodoluminescence (CL) detection for the structure and optical property measurements. The STEM/CL instrumentation is coupled to a spectrometer in a time-resolved measurement system for hyperspectral mapping in position and optical energy, as well as time resolution. Combined with high-resolution imaging, four-dimensional electron microscopy for imaging lattice distortion and electron energy-loss spectroscopy (EELS), the STEM/CL setup enables the study of where and why emission is enhanced in a material down to precise nanoscale. Specifically, STEM/CL is co-registered for structure, composition, optical response, and orbital texture characterization to gain insights about connections between subtle structural motifs with quantitative optical observables. On hBN, the introduction of twisted interface between two layers of hBN brightens CL emissions. This enhancement enables the location and identification of a blue emitter with a zero-phonon line at 440 nm and correlation to a vertically aligned carbon dimer in thin regions of twisted hBN at approximately ten nanometer spatial resolution. Guided by this critical insight, we demonstrate deterministic, site-selective creation of blue centers by depositing a carbon overlayer and by applying targeted electron-beam doses. In addition to the emission of single defects, the twisting of hBN layers tunes their electronic structure and optical responses. The Moire lattice formed by the twisted layers modulate photon emissions, which can be mapped using STEM/CL. Results show twist-angle dependent CL emission between domain and domain boundaries by different stacking order. In addition, domain centers with AB/BA stacking orders in near 0° twisted hBN are dependent on is brighter than the domain walls with AA and saddle parallel (SP) stacking order. At near 60° twisted hBN, the AA’ domain centers have weaker emissions than the AB1’/AB2’ domain corners. To determine the origin of the optical modulation, we further investigate the structural modulation and polarity in twisted hBN. 4D-STEM based differential phase contrast (DPC) imaging reveals chiral CoM displacement vortices within the Moire lattice cell. Examination of diffraction patterns show an chiral orientation of atomic mismatch at the interface. In complementary angle-resolved EELS also reveals a chiral π-bonding manifold at the domain. Together, these results establish a close link between periodic lattice distortions and in-plane polarization, to orbital texture, and insights into the CL enhancement mechanism of twisted hBN and polar topological texture in low angle twisted hBN. The same STEM/CL platform is applied to the study of the EuO/KTO superconducting interface. The Eu2+ ion diffusions from EuO side to KTO contribute additional electron and spin texture for the two-dimensional electron gas (2DEG), facilitating electron-phonon coupling at the interface thus enable superconducting Cooper pairs formation below the critical temperature. To examine the Eu ion diffusion into KTO, we combine atomic resolution imaging with energy dispersive X-ray (EDS) mapping and core-loss electron energy-loss spectroscopy (EELS). Results indicate Eu ion interdiffusion with mixed valence of 2+ and 3+ states, and Eu3+ can diffuse further than Eu2+. Furthermore, CL at the interface reveals Eu3+ emission that tracks the absence of inversion symmetry which suggests local disordering of Eu dopant environment These results unveil the EuO/KTO interfacial chemical distribution and electronic states, providing solid evidence supporting the existence of Eu2+ for superconductivity, new possible mechanisms towards Eu3+, and their local environment. Overall, this thesis advances STEM/CL as a quantitative, co-registered probe that bridges atomic structure and device-relevant light emission in quantum materials. The STEM/CL investigation of carbon-doped hBN outlines a practical path to engineer bright, stable quantum emitters and tunable twisted interface for QIS.

Graduation Semester

2025-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/132742

Copyright and License Information

Copyright 2025 Hanyu Hou

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