Welcome to Xiang-Tian Kong's Webpage

I am a physical postdoctoral researcher studying optical and thermal properties of nanomaterials. I enjoy applying theoretical methods and simulation techniques to solving various scientific questions in nanophotonics, plasmonics, photochemistry and material sciences. These questions include

• What are the general rules of designing plasmonic components in application areas such as photocatalysis, optical and bolometric sensing, and metamaterials?
• How to manipulate resonant modes in plasmonic nanostructures and how to obtain desired near- and far-field optical properties using metallic and dielectric nanomaterials?
• How to understand the photoluminescence properties in various quantum dots?

My previous and ongoing projects are specifically listed below.

Generation of Plasmonic Hot Electrons for Boosting Photochemistry

I have theoretically and computationally studied the quantum and classical light absorption mechanisms in plasmonic nanocrystals with the hot electron effect (under supervision of Dr. Alexander Govorov at Ohio University). Metal nanocrystals with plasmonic excitation can generate large amount of hot electrons due to the quantum effect during the plasmon relaxiation. These hot electrons can be effectively injected into adjacent functional materials, such as the catalyst titanium dioxide in photo-oxidation, boosting the photochemical reactions. The processes of the generation and injection of the plasmonic hot electrons considerably rely on the size and shape of the plasmonic nanocrystals. I systematically calculated the amount of generated hot electrons and the energies carried out by the hot electrons during these processes in terms of the material and geometrical parameters of various configurations, such as isolated and coupled nanocrystals and nanogaps between a nanocrystal and a flat back-reflector. Our theory and numerical results together with the collaborative experiments (with Dr. Miguel A. Correa-Duarte at University of Vigo, Spain) show that the silver nanocrystals are overall superior to the gold ones, and multi-spiky nanocrystals and narrow nanogaps are two most efficient configurations in the hot electron generation and injection processes in the visible and near-infrared spectral ranges, respectively.

• Kong, X.-T.; Wang, Z.; Govorov, A. O., Plasmonic Nanostars with Hot Spots for Efficient Generation of Hot Electrons under Solar Illumination. Adv. Opt. Mater. 2016. Doi: 10.1002/adom.201600594
• Sousa-Castillo, A.; Comesaña-Hermo, M.; Rodríguez-González, B.; Pérez-Lorenzo, M.; Wang, Z.; Kong, X.-T.; Govorov, A. O.; Correa-Duarte, M. A., Boosting Hot Electron-Driven Photocatalysis through Anisotropic Plasmonic Nanoparticles with Hot Spots in Au–TiO2 Nanoarchitectures. J. Phys. Chem. C 2016, 120 (21), 11690-11699.
• Negrín-Montecelo, Y.; Comesaña-Hermo, M.; Kong, X.-T.; Rodríguez-González, B.; Wang, Z.; Pérez-Lorenzo, M.; Govorov, A. O.; Correa-Duarte, M. A., Traveling hot-spots in plasmonic photocatalysis: Manipulating interparticle spacing for real-time control of electron injection. ChemCatChem 2018, doi: 10.1002/cctc.201702053
• Sykes, M. E.; Stewart, J. W.; Akselrod, G. M.; Kong, X.-T.; Wang, Z.; Gosztola, D. J.; Martinson, A. B. F.; Rosenmann, D.; Mikkelsen, M. H.; Govorov, A. O.; Wiederrecht, G. P., Enhanced generation and anisotropic Coulomb scattering of hot electrons in an ultra-broadband plasmonic nanopatch metasurface. Nat. Commun. 2017, 8(1), 986.

Manipulation of Optical, Chiro-optical and Thermal Properties of Plasmonic Nanosystems

I have been working on designing of plasmonic nanosystems with strong chiro-optical responses under the supervision of Dr. Alexander Govorov. On the one hand, I focus on the mid-infrared spectral range, which contains the vibrational modes of many biomolecules and therefore is significant to bio-sensing. I use nanostructured graphene as the plasmonic system. I have found the general requirements for creating strong chiro-optical signals with graphene nanoassemblies through numerically studying of the interacting graphene nanodisks. These requirements are strong plasmonic coupling between adjacent parts in the assembly and big geometrical chirality. Now, I go on to develop practical planar graphene metamaterials that fulfill these requirements and create strong chiro-optical signals. On the other hand, I study the plasmonically induced thermal effects, especially those with chiral asymmetry. In a recent theoretical work, we show that the chiro-optical effect from a Γ-shaped plasmonic nanocrystal in a metasurface configuration can translate to a comparable chiro-thermal effect. In addition, I am always interested in finding out the potential applications of novel plasmonic nanomaterials. I have studied the plasmonic extinction properties of Cu and Al nanocups for the spectral filtering application in a recent collaborative work with Prof. Uwe Kortshagen's group at University of Minnesota.

• Kong, X.-T.; Zhao, R.; Wang, Z.; Govorov, A. O., Mid-infrared Plasmonic Circular Dichroism Generated by Graphene Nanodisk Assemblies. Nano Lett. 2017, 17 (8), 5099-5105.
• ​Kong, X.-T.​; Khorashad, L. K.; Wang, Z.; Govorov, A. O., ​Photothermal Circular Dichroism Induced by Plasmon Resonances in Chiral Metamaterial Absorbers and Bolometers​. Nano Lett. ​2018​​, DOI: 10.1021/acs.nanolett.7b05446
• Qin, Y.; Kong, X.-T.; Wang, Z.; Govorov, A. O.; Kortshagen, U. R., Near-Infrared Plasmonic Copper Nanocups Fabricated by Template-Assisted Magnetron Sputtering. ACS Photonics 2017, 4(11), 2881-2890.
• Vazquez Besteiro, L.; ​Kong, X.-T.​; Wang, Z.; Rosei, F.; Govorov, A. O., ​Plasmonic Glasses and Films Based on Alternative Inexpensive Materials for Blocking Infrared Radiation​. Nano Lett. 2018​​, 18(5), 3147–3156.
• Review: Urban, M. J.; Shen, C.; Kong, X.-T.; Zhu, C.; Govorov, A. O.; Wang, Q.; Hentschel, M.; Liu, N., Chiral Plasmonic Nanostructures Enabled by Bottom-Up Approaches. Annu. Rev. Phys. Chem. 2019, 70(1), DOI: 10.1146/annurev-physchem-050317-021332
• Review: Kong, X.-T.; Vazquez Besteiro, L.; Wang, Z.; Govorov, A. O., Plasmonic Chirality and Circular Dichroism in Bio-assembled and Non-biological Systems: Theoretical Background and Recent Progress. Adv. Mater. 2018, DOI: 10.1002/adma.201801790 (First published: 09 September 2018)

Also, I am interested in surface plasmon polaritons guided by metal‒dielectric interfaces. These tightly confined guided waves hold the promise of the next generation miniaturized photonic devices. I analytically studied the guided modes in the metal‒multi-insulator‒metal plasmonic waveguides, and numerically designed the mode converters using transformation optics to change the guided wave properties in metal‒insulator‒metal waveguides (my PhD project supervised by Dr. Jian-Guo Tian and Dr. Zu-Bin Li at Nankai University, China). I also proposed to use corrugated substrates to increase the propagation length of the mid-infrared graphene plasmons (my independent work at NCNST, Beijing, China).

• Kong, X.-T.; Li, Z.-B.; Tian, J.-G., Mode converter in metal-insulator-metal plasmonic waveguide designed by transformation optics. Opt. Express 2013, 21 (8), 9437-9446.
• Kong, X.-T.; Yan, W.-G.; Li, Z.-B.; Tian, J.-G., Optical properties of metal-multi-insulator-metal plasmonic waveguides. Opt. Express 2012, 20 (11), 12133-12146.
• Kong, X.-T.; Bai, B.; Dai, Q., Graphene plasmon propagation on corrugated silicon substrates. Opt. Lett. 2015, 40 (1), 1-4.
• Kong, X.-T.; Yang, X.; Li, Z.; Dai, Q.; Qiu, X., Plasmonic extinction of gated graphene nanoribbon array analyzed by a scaled uniform Fermi level. Opt. Lett. 2014, 39 (6), 1345-1348.

Electronic Properties and Photoemission of Quantum Dots

In a collaborative project with Prof. Federico Rosei's group at INRS, Canada, I have modeled the carrier recombination process and the wavefunctions of the electrons and holes in the giant core/shell semiconductor QDs with various shapes. Core/shell QDs are frequently used as photosensitizers in photochemical reactions. While the shell layer separates the core from reacting with the ambient chemicals, it changes the degree of quantum confinement of the electrons in the core, thus shifting the photoluminescence spectrum of the QDs and prolonging the photoluminescence lifetime. My calculations show that the lifetime monotonically increases with increasing shell layer thickness for spherical QDs, and also significantly depends on the shape of the shell layer for anisotropic QDs. These properties are important for optimizing QDs as photosensitizers.

• Tong, X.; Kong, X.-T.; Zhou, Y.; Navarro‐Pardo, F; Selopal, G. S.; Sun, S.; Govorov, A. O.; Zhao, H; Wang, Z. M.; Rosei, F., Near‐Infrared, Heavy Metal‐Free Colloidal “Giant” Core/Shell Quantum Dots. Adv. Energy Mater. 2018, 8(2), 1701432.
• Tong, X.; Kong, X.-T.; Wang, C.; Zhou, Y.; Navarro‐Pardo, F; Barba, D.; Ma, D.; Sun, S.; Govorov, A. O.; Zhao, H; Wang, Z. M.; Rosei, F., Optoelectronic Properties in Near-infrared Colloidal Heterostructured Pyramidal "Giant" Core/shell Quantum Dots. Adv. Sci. 2018, DOI: 10.1002/advs.201800656
• Madathumpady Abubaker, H. M.; Lamers, M.; Baumann, V.; Dey, P.; Blanch, A. J.; Polishchuk, I.; Kong, X.-T.; Levy, D.; Urban, A. S.; Govorov, A. O.; Pokroy, B.; Rodríguez-Fernández, J.; Feldmann, J., ​Strong Quantum Confinement Effects and Chiral Excitons in Bio-Inspired ZnO−Amino Acid Cocrystals​. J. Phys. Chem. C ​2018​​, DOI: 10.1021/acs.jpcc.8b01567

Manipulation of Light-Matter Interaction Using Graphene

I developed a computational tool for solving the optical properties of the multilayer systems according to the transfer matrix method, and applied it to a series of layered nanostructures containing graphene in a collaborative work with Dr. Xiao-Qing Yan and Dr. Zhi-Bo Liu at Nankai University, China (my PhD project). I phenomenologically explained the linear and nonlinear polarization-dependent optical responses of graphene with various illumination conditions.

• Xing, F.; Liu, Z.-B.; Deng, Z.-C.; Kong, X.-T.; Yan, X.-Q.; Chen, X.-D.; Ye, Q.; Zhang, C.-P.; Chen, Y.-S.; Tian, J.-G., Sensitive real-time monitoring of refractive indexes using a novel graphene-based optical sensor. Sci. Rep. 2012, 2, 908.
• Ye, Q.; Wang, J.; Liu, Z.; Deng, Z.-C.; Kong, X.-T.; Xing, F.; Chen, X.-D.; Zhou, W.-Y.; Zhang, C.-P.; Tian, J.-G., Polarization-dependent optical absorption of graphene under total internal reflection. Appl. Phys. Lett. 2013, 102 (2), 021912-4.
• Gao, C.; Zhao, X.; Yao, J.; Yan, X.-Q.; Kong, X.-T.; Chen, Y.; Liu, Z.-B.; Tian, J.-G., Sign of differential reflection and transmission in pump-probe spectroscopy of graphene on dielectric substrate. Photon. Res. 2015, 3 (2), A1-A9.
• Yao, J.; Zhao, X.; Yan, X.-Q.; Kong, X.-T.; Gao, C.; Chen, X.-D.; Chen, Y.; Liu, Z.-B.; Tian, J.-G., Making transient optical reflection of graphene polarization dependent. Opt. Express 2015, 23 (19), 24177-24188.
• Yan, X.-Q.; Liu, F.; Kong, X.-T.; Yao, J.; Zhao, X.; Liu, Z.-B.; Tian, J.-G., Polarization dependence of graphene transient optical response: interplay between incident direction and anisotropic distribution of nonequilibrium carriers. J. Opt. Soc. Am. B 2017, 34 (1), 218-226.

In a collaboration with Dr. Haider Butt at University of Birmingham, UK, I co-designed the thinnest lenses in the world using multilayer graphene. I did the computational part of the work and predicted the lensing effect of the graphene Fresnel plates.

• Kong, X.-T.; Khan, A. A.; Kidambi, P. R.; Deng, S.; Yetisen, A. K.; Dlubak, B.; Hiralal, P.; Montelongo, Y.; Bowen, J.; Xavier, S.; Jiang, K.; A. J. Amaratunga, G.; Hofmann, S.; Wilkinson, T. D.; Dai, Q.; Butt, H., Graphene-based ultrathin flat lenses. ACS Photonics 2015, 2 (2), 200-207.

I am/was supported by