Research

Nanoelectronics (Molecular Electronics and 2D Material Electronics)

In the next 5-20 years, 1-5 nm scale transistors (electronic devices) will emerge in the industry. At this scale, quantum, non-equilibrium, many-body, and interface effects become significantly important, and the electron transport properties of transistors cannot be simply described by the Boltzmann transport equation (diffusion or drift transport). Instead, full quantum mechanics simulations at the atomistic level are needed. However, most of the current quantum transport programs are based on a first-principle calculation, and in this framework it is challenging to simulate the full structure of a transistor (including source, drain, and gate) consisting of more than 10000 atoms. To solve this issue, our aim is to develop multiscale models combined with appropriate quantum transport techniques and apply them to investigate the next-generation electronics such as molecular electronics (e.g., molecular wires, carbon nanotubes), two dimensional electronics (e.g., graphene, molybdenum disulfide, black phosphorus), and spintronics. We have demonstrated several intriguing quantum transport phenomena in molecular electronics, including:

  1. Franck-Condon blockade and charge stability diagram caused by vibronic couplings [1]
  2. Light-driven electron transport: Photon-assisted tunneling, coherent destruction of tunneling, coherent revival of tunneling, quantum ratchet effects caused by AC driving fields [2][3][4][5]; Photo-induced anomalous Coulomb blockade [23][28]
  3. Tunneling with destructive quantum interference caused by molecular electronic structure [6][7]
  4. Conduction mechanism transition from tunneling to thermally activated hopping [8]
  5. Geometry, temperature, and length dependence of conductance in oligomer systems [9][10][11]
  6. The rule of molecular circuits in series and in parallel [12][13][14]
  7. The effects of different types of metal strings on the conductance of the smallest electric wire [15][16][22]
  8. Electric current fluctuations in a non-equilibrium open quantum system [26]


Methodology of Quantum Transport Theory at the Nanoscale

Quantum transport is a subfield of condensed matter physics and nanotechnology, which focuses on the exchange of charges (e.g., electric current), energy (e.g., heat, exciton), and angular momentum (e.g., spin) between observed and studied systems at the nanoscale. The theory of quantum transport has been extensively applied to a variety of fields including mesoscopic physics, nanoelectronics, ultracold atoms, and DNA sequencing. Our group is interested in developing a new methodology to explore novel transport phenomena at the nanoscale. We have demonstrated several intriguing quantum transport phenomena based on the following methodologies:

  1. Landauer approach [7][12][13]
  2. Quantum scattering approach [2][3][4][5][6][9][10][11]
  3. Non-equilibrium green’s function approach (Keldysh formalism) [15]
  4. Rate equation approach [1][16][23][28]
  5. Quantum master equation approach (Open quantum system techniques) [8][14][28]
  6. Molecular Dynamics-Driven Liouville von Neumann Approach (Open quantum system techniques) [26]


Light-Matter Interactions at the Nanoscale

Light-matter interaction is a fundamental issue in physics, chemistry, and engineering. Our group is especially interested in its applications such as resonance energy transfer and spectroscopy (including fluorescence [27], Raman, and nonlinear spectroscopy) near nanostructures. Resonance energy transfer plays a crucial role in the functioning of photosynthesis, biosensing, and photovoltaics. Typically, the mechanism of short-range resonance energy transfer between two molecules can be understood by Förster resonance energy transfer (FRET). However, the traditional FRET cannot describe the long-range behavior of resonance energy transfer or molecules in inhomogeneous, dispersive, or absorbing media (e.g., molecules around gold nanoparticles or surface). To address this issue, based on macroscopic quantum electrodynamics (including dielectrics), we derive a classical electrodynamics expression for RET rates and calculate them by solving Maxwell’s equations via the finite-difference time domain method [17]. Moreover, we derive an explicit Förster-type expression for the rate of plasmon-coupled resonance energy transfer (PC-RET). The proposed theory is general for energy transfer in the presence of materials with any space-dependent, frequency-dependent, or complex dielectric functions. Furthermore, the theory allows us to develop the concept of a generalized spectral overlap for understanding the wavelength dependence of PC-RET [18]. Based on our approach, we can analyze the mechanism of PC-RET around nanoparticles and their size dependence [24] and the characteristic distance of RET coupled with surface plasmon polaritons [25]. The approach is promising for applications to plasmon-coupled exciton transport in metal organic framework materials (MOFs), chromophore aggregates, and DNA-linked nanoparticle superlattices. In addition to PC-RET, we are also interested in exploring nanoscale plasmonics and other photophysical processes in the framework of macroscopic quantum electrodynamics.


Ultrafast Dynamics and Signal Processing

Electronic dynamics (attosecond science) in molecules and solid states is a fundamental topic in chemistry and physics. Traditionally, dynamics of a quantum system such as lifetime and energy difference can be revealed by spectral lines in the frequency domain by using the Fourier analysis, but the Fourier transform presents limited chronological information of a dynamical system. To explore chronological information in a quantum dynamical system, advanced signal processing techniques are needed. By using the new time-frequency methods developed by Prof. Hau-Tieng Wu and Dr. Yae-Lin Shue and the first-principle method (time-dependent generalized pseudospectral method) by Prof. Shih-I Chu, we can understand several fascinating phenomena in a hydrogen atom within a strong laser field [19] [20] [21]. In the future, our goal is to apply modern signal processing techniques to explore the electronic dynamics of molecules, monolayers, and 2d materials.



Reference (Further Information can be found in Publications):

[1] J. Chem. Phys., 2010, 133, 144705.
[2] Phys. Rev. Lett., 2012, 109, 186801. 
[3] J. Chem. Phys., 2014, 141,124703.
[4] Phys. Rev. B, 2015, 92, 035410.
[5] Phys. Chem. Chem. Phys., 2015, 17, 20617-20629.
[6] Chem. Phys. Lett. 2008, 457, 279-283. 
[7] Chem. Phys., 2009, 355, 177-182.
[8] J. Phys. Chem. Lett., 2014, 5, 1831-1836.
[9] Nano Lett., 2013, 13, 5020-5025.
[10] J. Phys. Chem. C, 2015, 119, 4753-4759.
[11] J. Chem. Phys., 2016, 145, 234702.
[12] J. Am. Chem. Soc., 2014, 136, 1832-1841.
[13] J. Am. Chem. Soc., 2015, 137, 5948-5954.
[14] Phys. Chem. Chem. Phys., 2016, 18, 32087-32095.
[15] J. Phys. Chem. C, 2008, 112, 10538-10541.
[16] Angew. Chem. Int. Ed., 2015, 54, 15734-15738. (Chosen as a very important article) 
[17] J. Chem. Phys., 2017, 146, 064109.
[18] J. Phys. Chem. Lett., 2017, 8, 2357-2367. (Chosen as a Cover article)
[19] AIP Advances, 2014, 4, 117138.
[20] Opt. Express, 2015, 23, 30459-30482.
[21] Int. J. Data Sci. Anal. (JDSA), 2017, 3, 231-245.
[22] Chem (Cell), 2017, 3, 373–379.
[23] Nano Lett., 2018, 18, 5015-5023.
[24] J. Phys. Chem. C, 2018, 122, 22650-22659.
[25] J. Phys. Chem. Lett, 2018, 9, 7032-7039. 

[26] J. Phys. Chem. C, 2019, 123, 10746-10755. (Invited Article, Special Issue -- Abraham Nitzan Festschrift)

[27] J. Chem. Phys., 2019, 151, 014105.  (Editor’s Pick, Special Issue: “Dynamics of Open Quantum Systems”)

[28] J. Chem. Phys., 2019, 151, 054704.

[29] J. Phys. Chem. C, 2019,123, 29298-29305.

[30] J. Phys. Chem. Lett., 2020, 11, 5948-5855.

[31] J. Chem. Phys., 2020, 153, 044103.

[32] J. Phys. Chem. Lett., 2020, 11, 6796-6804.

[33] J. Chem. Phys., 2020, 153, 184102.