All Publications

  • Structural Dynamics of a Novel Pseudohalide Perovskite Cs2Pb(SeCN)(2)Br-2 Investigated with Nonlinear Infrared Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY C Xing, X., Li, J., Breen, J. P., Karunadasa, H. I., Fayer, M. D. 2023
  • Long-Range Interface Effects in Room Temperature Ionic Liquids: Vibrational Lifetime Studies of Thin Films. The journal of physical chemistry. B Breen, J. P., Leibfried, L. C., Xing, X., Fayer, M. D. 2023


    Interface effects in the room temperature ionic liquids (RTILs) 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2) were investigated using ultrafast infrared polarization selective pump-probe (PSPP) spectroscopy. The CN stretch mode of SCN- dissolved in the RTILs was used as the vibrational probe. The vibrational lifetime of the SCN- was the experimental observable. Quite similar single SCN- lifetimes were observed: 59.5 ± 0.4 ps in bulk BmimBF4 and 56.4 ± 0.4 ps in bulk BmimNTf2. Thin films of both RTILs with thicknesses in the range of 15-300 nm were prepared by spin coating on functionalized substrates. PSPP experiments were performed in a small-incidence reflection geometry. In the thin films, a second, shorter lifetime was observed in addition to the bulk lifetime, with the amplitude of the shorter lifetime increasing with decreasing film thickness. By modeling the thickness dependence of the lifetime amplitudes, the correlation length of the interface effect (constant for exponential falloff of the influence of the interface) was determined to be 44.6 ± 0.6 nm for BmimBF4 and 48.3 ± 2.2 nm for BmimNTf2. The values for the shorter film lifetimes were 12.6 ± 0.1 ps for BmimBF4 and 20.2 ± 0.6 ps for BmimNTf2; the substantial differences from the bulk lifetimes showed that some of the SCN- anions near the interface experience an environment distinct from that of the bulk. It was also found that for the BmimNTf2 sample only, some of the SCN- anions reside in the surface functionalized layer with two distinct environments having distinct lifetimes.

    View details for DOI 10.1021/acs.jpcb.3c02948

    View details for PubMedID 37381928

  • Probing Lattice Dynamics in Two-Dimensional InorganicPseudohalide Perovskites with Ultrafast Infrared Spectroscopy br JOURNAL OF PHYSICAL CHEMISTRY C Xing, X., Li, J., Breen, J. P., Nishida, J., Karunadasa, H., Fayer, M. D. 2022; 126 (24): 10145-10158
  • Structured Perovskite Light Absorbers for Efficient and Stable Photovoltaics ADVANCED MATERIALS He, T., Jiang, Y., Xing, X., Yuan, M. 2020: e1903937


    Organic-inorganic hybrid lead-halide perovskite materials (ABX3 ) have attracted widespread attention in the field of photovoltaics owing to their impressive optical and electrical properties. However, obstacles still exist in the commercialization of perovskite photovoltaics, such as poor stability, hysteresis, and human toxicity. A-site cation engineering is considered to be a powerful tool to tune perovskite structures and the resulting optoelectronic properties. Based on the selection and combination of A-site cations, three types of perovskite structures, i.e., 3D perovskite, reduced-dimensional (2D/quasi-2D) perovskite, and 2D/3D hybrid perovskite can be formed. Herein, the remarkable breakthroughs resulting from these three perovskite structures are summarized, and their corresponding properties and characteristics, as well as their intrinsic disadvantages, are highlighted. By summarizing recent research progress, a new viewpoint for improving the performance and stability of perovskite photovoltaics is provided.

    View details for DOI 10.1002/adma.201903937

    View details for Web of Science ID 000533319000001

    View details for PubMedID 32419234

  • Controlling the Dynamics of Ionic Liquid Thin Films via Multilayer Surface Functionalization. Journal of the American Chemical Society Wu, B. n., Breen, J. P., Xing, X. n., Fayer, M. D. 2020


    The structural dynamics of planar thin films of an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2) as a function of surface charge density and thickness were investigated using two-dimensional infrared (2D IR) spectroscopy. The films were made by spin coating a methanol solution of the IL on silica substrates that were functionalized with alkyl chains containing head groups that mimic the IL cation. The thicknesses of the ionic liquid films ranged from ∼50 to ∼250 nm. The dynamics of the films are slower than those in the bulk IL, becoming increasingly slow as the films become thinner. Control of the dynamics of the IL films can be achieved by adjusting the charge density on substrates through multilayer network surface functionalization. The charge density of the surface (number of positively charged groups in the network bound to the surface per unit area) is controlled by the duration of the functionalization reaction. As the charge density is increased, the IL dynamics become slower. For comparison, the surface was functionalized with three different neutral groups. Dynamics of the IL films on the functionalized neutral surfaces are faster than on any of the ionic surfaces but still slower than the bulk IL, even for the thickest films. These results can have implications in applications that employ ILs that have electrodes, such as batteries, as the electrode surface charge density will influence properties like diffusion close to the surface.

    View details for DOI 10.1021/jacs.0c03044

    View details for PubMedID 32349470

  • Conjugated Alkylamine by Two-Step Surface Ligand Engineering in CsPbBr3 Perovskite Nanocrystals for Efficient Light-Emitting Diodes CHEMNANOMAT Zhang, L., Zhang, Q., Xing, X., Jiang, Y., He, T., Huang, Y., Ma, Z., Yang, J., Yuan, M. 2019; 5 (3): 318–22