Talks and presentations

Nanoscale Optoelectronic Materials through Theoretical Modeling

March 17, 2025

Conference Posters, APS March Meeting 2025 in HK, APS March Meeting 2025 in HK

Light-matter interaction, as the origin of optoelectronics and further nanoscale materials and devices, has earned huge research interests. Given the difficulties in producing perfect defect-free samples in nanoscale, theoretical methods exhibit superiorities in predicting and explaining physical phenomena in this field. Here, we picked two examples to demonstrate how theoretical methods such as density function theory (DFT) benefit the analysis and promotion of optoelectronics development. In the optoelectronic device aspect, 4H-SiC devices demonstrate substantial promise in high power electronics with the wide bandgap and excellent thermal conduction performance of 4H-SiC. Experimental results on ALD ZnO decorated 4H-SiC devices showed optimized I-V performance, but experiments could not provide explanations in electronic scale. Our DFT modeling and theoretical models presented predictions as reliable evidence for experimental guesses on surface work function values, Fermi level pinning effect, and the presence of polarity as well as its compensation by surface decoration. In addition, the close looks at electron distribution, band structure, and structural properties consolidated our guesses and provided more information to explore the capabilities of this structure for advanced applications of devices. Alongside the device aspect, the material aspect of light response was also considered. Twisted bilayer graphene showed exceptional potential in converting traditional circuits to a new regime of 2D materials-based electronics and 3D circuits. This mainly comes from the remarkable electrical properties of graphene and the introduced symmetry by the additional atomic layer. Theoretical computations were done on revisiting the structural parameters of twisting bilayer graphene with large twisting angles, following the careful check of van der Waals interaction correction in DFT. The dielectric response of twisted bilayer graphene was visited to provide a more general picture of how it interacts with incident light. Results showed that the additional atomic layer and the varying twisting angle posed inevitable influences on van der Waals interlayer distance, and then phase differences of reflected light. The two examples of nanoscale optoelectronic materials demonstrate how theoretical modeling boosts the advance of optoelectronics performance and efficiency.

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Metal-Semiconductor Contact Fermi Level Tunning Engineering

April 30, 2024

Talks, MECH7001 ME Department Student Seminar, The University of Hong Kong, HK

The ubiquitous metal-semiconductor contacts in semiconductor devices play the dominant role in device performance. They are as influential as semiconductor materials themselves. Two key factors affected by metal-semiconductor contact are the contact resistance and charge carrier barrier at the interface. These factors determine the electrical characteristics and efficiency of devices directly. The energy balance process after contact results in either facilitating or blocking effects on charge carrier transport across the interface, depending on the energy difference of the Fermi levels between two sides. This implies that adjusting the Fermi level before contact offers the potential to customize contact characteristics, allowing for larger flexibility in device fabrication and performance enhancement. In this talk, I will introduce the well-established theoretical models for evaluating metal-semiconductor contact, providing guidance for modifying and manipulating the material properties to design and improve device performance. Within this framework, I will discuss several selected works that demonstrated the engineering of Fermi level tunning through theoretical predictions and experimental validations. The successful manipulation of metal-semiconductor contacts propounds an effective strategy for designing ideal electronic devices with a better understanding of per se of the contact phenomenon.

Multiscale Multiphysics Simulation Model of Laser-Induced Ultrasonic Energy Conversion

April 22, 2024

Conference Posters, PIERS2024 at Chengdu, PIERS2024 at Chengdu

With rapid advancement on pushing the resolution and strong optical contrast at desired imaging depth of laser-induced ultrasound, compact and miniature photoacoustic imaging and diagnosis systems are emerging as a promising functional imaging technology for modern biomedical applications. In this context, a structured absorber medium exposed by laser excitation increases their temperature and launches a pressure wave that propagates as ultrasound emission through the subject under inspection. In contrast to direct laser excitation of the testing objects, this structured absorber allows generation of ultrasound emission at higher central frequency and minimizing the variance of ultrasound intensity injected in biological tissue, therefore enabling more accurate and reliable quantitative measurements. However, due to the complex interaction of photothermal conversion and photoacoustic waves involved, the signal pathway of laser-induced ultrasound emission is not fully established. There is a critical need to elaborate the energy conversion process spanning from electronic scale, atomistic scale, to microscale, in order to properly control the laser dose and design structured absorbers. To unravel the hidden physical pictures, we propose a multiscale multiphysics simulation model for capturing the interconnected process of laser-induced ultrasonic energy conversion. As the first and foremost task, we focus on the optical, plasmonic, and thermal responses of metallic and semiconductor particles to laser irradiation. Understanding of this process unveils the energy conversion and heat transport during and after photoexcitation of these materials. The developed simulation model is used to illustrate and evaluate the advantages of the widespread carbon materials-based laser-induced ultrasonic transducers, while also aiding in the comprehension of the process of light-matter interaction. As a result, our findings of this research can lead to quantifiable metrics and design guidelines of novel high-performance laser-induced ultrasonic transducers, ultimately benefiting modern biomedical diagnosis.

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Ultrafast Electronic Dynamics Observation in Energy Storage Materials

August 14, 2023

Talks, Prof. Fang's Research Group Weekly Meeting, The University of Hong Kong, HK

Thin film electrodes are able to enhance energy storage performance by their microscale thickness while ensuring mechanical flexibility. Transition metal oxides are typical electrode materials with unique properties on charge carriers, among them tricobalt tetraoxide possesses a band gap of near-infrared excitation, widely applied in photocatalysis, optoelectronics, and so forth. However, the lack of research on charge carrier dynamics and photo-accelerated charging of tricobalt tetraoxide restricts the application of this class of materials for thin film electrode energy storage devices. This work summarized the basic concept of absorption spectroscopy and raised one scheme for building a transient absorption spectrum experimental setup. Afterward, steady-state and transient absorption spectroscopy were employed to identify the optical transition in tricobalt tetraoxide, then photoexcited free charge carriers and their lifetime was analyzed. Based on transient spectroscopy, all free charge carriers observed had a long lifetime of >10 ns, surpassing the general value. It is inferred that the abnormally long charge carrier lifetime is due to small polaron formation by the observed phenomenon and literature research. In addition to the experimental findings, first principles calculation was implemented to validate the result by steady-state absorption spectrum, and deviations of the calculated absorption spectra from the experiment caused by low calculation accuracy and the implementation of Hubbard correction were found, the calculation result still plays a role of guidance to the experiments. Based on the small polaron formation observed in the experiments of tricobalt tetraoxide thin film, this work proposes the application potential of tricobalt tetraoxide thin film electrode for photo-accelerated charging-energy storage devices and prospects the future research directions.

Ab Initio calculation of Electron Temperature Dependent Heat Capacity and Electron Phonon Coupling Factor of Noble Metals

June 24, 2022

Conference Presentation, IEEE-MTNM 2022, Virtual Conference

Noble metal nanoparticles show fantastic catalytic property in both scientific research and industrial applications. To generate nanoparticles via femtosecond laser ablation is gaining increasing interest in the past decades. In this presentation, a serials of ab initio calculations for four typical noble metals (gold, palladium, iridium, and rhodium) are to be delivered, focusing on three pivotal parameters in the two-temperature model (electron heat capacity, phonon heat capacity, and electron-phonon coupling factor), which describes the thermal repsonse of the electron subsystem to the femtosecond laser irradiation and the electron-phonon coupled energy transfer between the electron subsystem and the lattice subsystem. In respect of the electron subsystem, by increasing electron temperature as the deposition of laser energy in the electron subsystem, the electron transitions from d orbitals to sp orbitals and from sp orbitals to d orbitals are observed for metals with fully and partially filled d block. It is highlighted that the change of electron density of states and Fermi-Dirac distribution function with the increasing electron temperature are the major factors affecting the electron heat capacity and electron-phonon coupling factor. In respect of the lattice subsystem, electron temperature shows little contribution to phonon heat capacity in spite of conspicuous impact on phonon density of states. Debye model and Debye temperature are tools to investigate thermodynamic properties straightforwardly, the calculation results reveal that Debye temperature as a disparate function of electron temperature for the investigated metals. In summary, this presentation conducts an insight attempt to explain the influence of femtosecond laser irradiation on noble metals, providing a ground of fabricating noble metal nanoparticles with femtosecond laser theoretically.