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  • 1. Ai-Xi, Chen
    et al.
    Hong, Wang
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hai-Ming, Zhang
    Xin, Xu
    Li-Feng, Chi
    Potential-Induced Phase Transition of N-Isobutyryl-L-cysteine Monolayers on Au(111) Surfaces2017In: Wuli huaxue xuebao, ISSN 1000-6818, Vol. 33, no 5, p. 1010-1016Article in journal (Refereed)
    Abstract [en]

    Functional solid substrates modified by self-assembled monolayers (SAMs) have potential applications in biosensors, chromatography, and biocompatible materials. The potential-induced phase transition of N-isobutyryl-L-cysteine (L-NIBC) SAMs on Au(111) surfaces was investigated by in-situ electrochemical scanning tunneling microscopy (EC-STM) in 0.1 mol.L-1 H2SO4 solution. The NIBC SAMs with two distinct structures (alpha phase and beta phase) can be prepared by immersing the Au(111) substrate in pure NIBC aqueous solution and NIBC solution controlled by phosphate buffer at pH 7, respectively. The as-prepared a phase and beta phase of NIBC SAMs show various structural changes under the control of electrochemical potentials of the Au(111) in H2SO4 solution. The a phase NIBC SAMs exhibit structural changes from ordered to disordered structures with potential changes from 0.7 V (vs saturated calomel electrode, SCE) to 0.2 V. However, the beta phase NIBC SAMs undergo structural changes from disordered structures (E < 0.3 V) to y phase (0.4 V < E < 0.5 V) and finally to the beta phase (0.5 V < E < 0.7 V). EC-STM images also indicate that the phase transition from the alpha phase NIBC SAMs to the a phase occurs at positive potential. Combined with density functional theory (DFT) calculations, the phase transition from the beta phase to the a phase is explained by the potential-induced break of bonding interactions between -COO- and the negatively charged gold surfaces.

  • 2.
    Cao, Xinrui
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ji, Yongfei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hu, Wei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Department of Chemical Physics, University of Science and Technology of ChinaHefei, China.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Department of Chemical Physics, University of Science and Technology of ChinaHefei, China .
    Feasible Catalytic Strategy for Writing Conductive Nanoribbons on a Single-Layer Graphene Fluoride2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 39, p. 22643-22648Article in journal (Refereed)
    Abstract [en]

    An accessible method for local reduction of graphene fluoride catalyzed by the Pt-coated nanotip with the assistance of a mixture of hydrogen and ethylene atmosphere is proposed and fully explored theoretically. Detailed mechanisms and roles of hydrogen and ethylene molecules in the cyclic reduction is discussed based on extensive first-principles calculations. It is demonstrated that the proposed cyclic reduction strategy is energetically favorable. This new strategy can be effectively applied in scanning probe lithography to fabricate electronic circuits at the nanoscale on graphene fluoride under mild conditions.

  • 3.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Geometrical and Electronic Structures at Molecule-Metal Interfaces from Theoretical Modeling2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, we focus on theoretical investigations on metal interfaces where many heterogeneous chemical reactions take place. Surface-enhanced Raman scattering (SERS) spectroscopy and the modern electrochemical methods are important in-situ techniques that have been widely employed for a variety of applications. Theoretical simulations have become an indispensable tool to infer the molecular details of interfacial structures that are not directly accessible from experimental measurements. In this context, we have proposed several new theoretical models for both SERS and interfacial electrochemistry, which allow us to provide molecular-level understanding of the interfacial structures under the realistic experimental conditions.

     

    The first part of the thesis has addressed the basic theory of SERS that offers the vibrational structure of the interfacial molecules. It is well known that the huge enhancement of Raman intensity in this technique can be attributed to two independent factors, namely the physical and chemical enhancements. The former is resulted from the enhanced electromagnetic field induced by the plasmonic excitations, while the latter comes from the changing of interaction between the molecule and the surface. The interplay between these two enhancement factors, which has long been an issue of debate, is revealed in this thesis. They are coupled through molecular polarizability. A practical computational approach is proposed and used to demonstrate the importance of the coupling on different molecular systems. It is found that for certain systems the coupling factor can be as large as 106. This finding is of great importance towards a comprehensive understanding of the SERS mechanisms and a quantitative prediction of the enhancement factor.

     

    The other part of the thesis is devoted to the theory of interfacial electrochemistry, in particular the effects of water solution. A novel protocol that combines classical molecular dynamics (MD) and the first principles density functional theory (DFT) calculations is proposed to address the statistical behavior of interfacial properties. Special attention has been paid to the work function of Pt(111) surface and CO adsorption energy on Pt(111) surface in aqueous solution. It has been found that in this case the work function of Pt surface illustrates a surprisingly broad distribution under the room temperature, sheds new light on the understanding of reaction activity of the surface. The proposed protocol is able to provide results in very good agreement with experiments and should be applied routinely in future studies.

  • 4.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ai, Yue-Jie
    Hu, Wei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Roles of Plasmonic Excitation and Protonation on Photoreactions of p-Aminobenzenethiol on Ag Nanoparticles2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 13, p. 6893-6902Article in journal (Refereed)
    Abstract [en]

    There is increasing evidence that surface plasmons could catalyze photochemical reactions of organic molecules on metal surfaces. However, due to the complex interactions among the substrate, the adsorbate, the environment, and the incident light, the existence and the underlying mechanism of such catalytic processes have been under intense debate. Here we present a systematic first principles study on one of the most studied and controversial systems, namely, p-aminobenzenethiol (PATP) adsorbed on silver nanoparticles. Our calculations have confirmed that the observed surface-enhanced Raman scattering (SERS) bands at 1142, 1391, and 1440 cm(-1) of PATP on silver surfaces belong to its coupling reaction product, 4,4'-dimercaptoazobenzene (DMAB). It is found that the deprotonation or protonation of N atoms is the key initial step for the transformations between PATP and DMAB. The photodecomposition reaction from DMAB to PATP can occur only under the conditions that both proton transfer and plasmonic excitations have taken place. Moreover, in addition to the widely suggested hot-electron injection mechanism of plasmon, a new photochemical channel has been revealed in the decomposition of DMAB molecules under suitable incident light. This may open up an entire new type of chemical reaction in surface chemistry that we call plasmonic photochemistry. Our theoretical calculations provide consistent interpretations for the experimentally observed pH-,wavelength-,and electrode potential dependence of the SERS spectra of PATP/DMAB adsorbed on silver surfaces. Our findings highlight the important role of theoretical investigations for better understanding of complex processes involved in photochemical reaction of surface adsorbates.

  • 5.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Fang, Ping-Ping
    Fan, Feng-Ru
    Broadwell, Ian
    Yang, Fang-Zu
    Wu, De-Yin
    Ren, Bin
    Amatore, Christian
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
    Xu, Xin
    Tian, Zhong-Qun
    A density functional theory approach to mushroom-like platinum clusters on palladium-shell over Au core nanoparticles for high electrocatalytic activity2011In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 12, p. 5441-5449Article in journal (Refereed)
    Abstract [en]

    Recently, it was found that Pt clusters deposited on Pd shell over Au core nanoparticles (Au@Pd@Pt NPs) exhibit unusually high electrocatalytic activity for the electro-oxidation of formic acid (P. P. Fang, S. Duan, et al., Chem. Sci., 2011, 2, 531-539). In an attempt to offer an explanation, we used here carbon monoxide (CO) as probed molecules, and applied density functional theory (DFT) to simulate the surface Raman spectra of CO at this core-shell-cluster NPs with a two monolayer thickness of Pd shell and various Pt cluster coverage. Our DFT results show that the calculated Pt coverage dependent spectra fit the experimental ones well only if the Pt clusters adopt a mushroom-like structure, while currently the island-like structure is the widely accepted model, which follows the Volmer-Weber growth mode. This result infers that there should be a new growth mode, i.e., the mushroom growth mode as proposed in the present work, for Au@Pd@Pt NPs. We suggest that such a mushroom-like structure may offer novel active sites, which accounts for the observed high electrocatalytic activity of Au@Pd@Pt NPs.

  • 6.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ji, Yong-Fei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Fang, Ping-Ping
    Chen, Yan-Xia
    Xu, Xin
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Zhong-Qun
    Density functional theory study on the adsorption and decomposition of the formic acid catalyzed by highly active mushroom-like Au@Pd@Pt tri-metallic nanoparticles2013In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 13, p. 4625-4633Article in journal (Refereed)
    Abstract [en]

    Local structures and adsorption energies of a formic acid molecule and its decomposed intermediates (H, O, OH, CO, HCOO, and COOH) on highly electrocatalytically active mushroom-like Au-core@Pd-shell@Pt-cluster nanoparticles with two atomic layers of the Pd shell and stoichiometric Pt coverage of around half-monolayer (Au@2 ML Pd@0.5 ML Pt) have been investigated by first principles calculations. The adsorption sites at the center (far away from the Pt cluster) and the edge (close to the Pt cluster) are considered and compared. Significant repulsive interaction between the edge sites and CO is observed. The calculated potential energy surfaces demonstrate that, with respect to the center sites, the CO2 pathway is considerably promoted in the edge area. Our results reveal that the unique edge structure of the Pt cluster is responsible for the experimentally observed high electrocatalytic activity of the Au@Pd@Pt nanoparticles toward formic acid oxidation. Such microscopic understanding should be useful for the design of new electrochemical catalysts.

  • 7.
    Duan, Sai
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Synerget Innovat Ctr Quantum Informat & Quantum P, Hefei 230026, Anhui, Peoples R China..
    Rinkevicius, Zilvinas
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. KTH, Centres, SeRC - Swedish e-Science Research Centre. Kaunas Univ Technol, Dept Phys, LT-51368 Kaunas, Lithuania..
    Tian, Guangjun
    Yanshan Univ, Sch Sci, Key Lab Microstruct Mat Phys Hebei Prov, Qinhuangdao 066004, Hebei, Peoples R China..
    Luo, Yi
    Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Synerget Innovat Ctr Quantum Informat & Quantum P, Hefei 230026, Anhui, Peoples R China..
    Optomagnetic Effect Induced by Magnetized Nanocavity Plasmon2019In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 141, no 35, p. 13795-13798Article in journal (Refereed)
    Abstract [en]

    We propose a new type of optomagnetic effect induced by a highly confined plasmonic field in a nanocavity. It is shown that a very large dynamic magnetic field can be generated as the result of the inhomogeneity of nanocavity plasmons, which can directly activate spin-forbidden transitions in molecules. The dynamic optomagnetic effects on optical transitions between states of different spin multiplicities are illustrated by first-principles calculations for C-60. Remarkably, the intensity of spin forbidden singlet-to-triplet transitions can even be stronger than that of singlet-to-singlet transitions when the spatial distribution of plasmon is comparable with the molecular size. This approach not only offers a powerful optomagnetic means to rationally fabricate molecular excited states with different multiplicities but also provides a groundbreaking concept of the light-matter interaction that could lead to the observation of new physical phenomena and the development of new techniques.

  • 8.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, G.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Univ Sci & Technol China, Peoples R China.
    Theory for Modeling of High Resolution Resonant and Nonresonant Raman Images2016In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 12, no 10, p. 4986-4995Article in journal (Refereed)
    Abstract [en]

    Tip-enhanced Raman imaging is capable of resolving the inner structure of a single molecule owing to the generation of highly localized nanocavity plasmon. Here we present a general theory and detailed computational methodology to fully describe resonant and nonresonant Raman scattering under the localized plasmonic field. We use an allylcarbinol molecule adsorbed on the gold surface as a model system to illustrate different effects on the Raman images. It is found that the ability of distinguishing an individual vibration mode is highly limited under the resonant condition due to the dominant contribution from the Franck-Condon term and the mode-independent component of the Herzberg-Teller term. The nonresonant Raman images of the single molecule are vibrationally distinguishable and present the vibrational motion of the corresponding vibrational modes in real space. Furthermore, the calculated results confirm that nonlinear optical effects can further improve the resolution of the images. The theoretical and computational methods presented here provide the basic tools to model high resolution Raman images at the single molecular level.

  • 9.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ji, Yongfei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Shao, Jiushu
    Dong, Zhenchao
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Theoretical Modeling of Plasmon-Enhanced Raman Images of a Single Molecule with Subnanometer Resolution2015In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 137, no 30, p. 9515-9518Article in journal (Refereed)
    Abstract [en]

    Under local plasmonic excitation, Raman images of single molecules can now surprisingly reach subnanometer resolution. However, its physical origin has not been fully understood. Here we report a quantum-mechanical description of the interaction between a molecule and a highly confined plasmonic field. We show that When the spatial distribution of the plasmonic field is comparable to the size of the molecule, the optical transition matrix of the molecule becomes dependent on the position and distribution of the plasmonic field, resulting in a spatially resolved high-resolution Raman image of the molecule. The resonant Raman image reflects the electronic transition density of the molecule. In combination with first-principles calculations, the simulated Raman linage of a porphyrin derivative adsorbed on a silver surface nicely reproduces its experimental counterpart. The present theory provides the basic framework for describing linear and nonlinear responses of molecules under highly confined plasmonic fields.

  • 10.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Visualization of Vibrational Modes in Real Space by Tip-Enhanced Non-Resonant Raman Spectroscopy2016In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 55, no 3, p. 1041-1045Article in journal (Refereed)
    Abstract [en]

    We present a general theory to model the spatially resolved non-resonant Raman images of molecules. It is predicted that the vibrational motions of different Raman modes can be fully visualized in real space by tip-enhanced non-resonant Raman scattering. As an example, the non-resonant Raman images of water clusters were simulated by combining the new theory and first-principles calculations. Each individual normal mode gives rise its own distinct Raman image, which resembles the expected vibrational motions of the atoms very well. The characteristics of intermolecular vibrations in supermolecules could also be identified. The effects of the spatial distribution of the plasmon as well as nonlinear scattering processes were also addressed. Our study not only suggests a feasible approach to spatially visualize vibrational modes, but also provides new insights in the field of nonlinear plasmonic spectroscopy.

  • 11.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Wu, De-Yin
    Xu, Xin
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry.
    Tian, Zhong-Qun
    Structures of Water Molecules Adsorbed on a Gold Electrode under Negative Potentials2010In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 9, p. 4051-4056Article in journal (Refereed)
    Abstract [en]

    Two stable conformations of water hexamer Clusters on gold electrode under negative potentials have been identified by density functional theory calculations. Both form a ring Structure but with different orientations of free CH bonds. Ill one of the Structures, labeled as F-Type, four free OH bonds of the water molecules point toward the gold surface and remain stable over a wide range of the negative potential. The other Structure, labeled as S-Type, starts with five Such free OH bonds pointing toward the gold surface at the low negative potential and ends LIP with six of them at higher negative potential. From the energetic point of view, the S-Type Structure is more stable than the F-Type under the same Potential. By comparing the calculated Raman spectra with the experiment, it is found that S-Type Structures are the most possible surface adsorption state of water molecules at the electrochemical interface under very negative electrode potentials. It is believed that such a novel water Structure Could also exist Oil other negative charged Surfaces.

  • 12.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Xu, Xin
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hermansson, Kersti
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Zhong-Qun
    Thermal effects on electronic properties of CO/Pt(111) in water2013In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 15, no 32, p. 13619-13627Article in journal (Refereed)
    Abstract [en]

    Structure and adsorption energy of carbon monoxide molecules adsorbed on the Pt(111) surfaces with various CO coverages in water as well as work function of the whole systems at room temperature of 298 K were studied by means of a hybrid method that combines classical molecular dynamics and density functional theory. We found that when the coverage of CO is around half monolayer, i.e. 50%, there is no obvious peak of the oxygen density profile appearing in the first water layer. This result reveals that, in this case, the external force applied to water molecules from the CO/Pt(111) surface almost vanishes as a result of the competitive adsorption between CO and water molecules on the Pt(111) surface. This coverage is also the critical point of the wetting/non-wetting conditions for the CO/Pt(111) surface. Averaged work function and adsorption energy from current simulations are consistent with those of previous studies, which show that thermal average is required for direct comparisons between theoretical predictions and experimental measurements. Meanwhile, the statistical behaviors of work function and adsorption energy at room temperature have also been calculated. The standard errors of the calculated work function for the water-CO/Pt(111) interfaces are around 0.6 eV at all CO coverages, while the standard error decreases from 1.29 to 0.05 eV as the CO coverage increases from 4% to 100% for the calculated adsorption energy. Moreover, the critical points for these electronic properties are the same as those for the wetting/non-wetting conditions. These findings provide a better understanding about the interfacial structure under specific adsorption conditions, which can have important applications on the structure of electric double layers and therefore offer a useful perspective for the design of the electrochemical catalysts.

  • 13.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Xu, Xin
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Zhong-Qun
    Molecular polarization bridging physical and chemical enhancements in surface enhanced Raman scattering2011In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, Vol. 47, no 41, p. 11438-11440Article in journal (Refereed)
    Abstract [en]

    We present a practical method which demonstrates how the physical and chemical enhancements in SERS for a molecule adsorbed on metal junctions are conceptually coupled through the polarization of the molecule and its surroundings. Calculations with the state-of-the-art density functional reveal that the coupling factor considered in the present work can be as large as 10(6), such that it is indeed important for certain vibrational modes.

  • 14.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Xu, Xin
    Tian, Zhong-Qun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    A Hybrid Molecular Dynamics and First Principles Study on the Workfunction of a Pt(1 1 1) Electrode Immersed in Aqueous Solution at Room TemperatureArticle in journal (Other academic)
  • 15.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Xu, Xin
    Tian, Zhong-Qun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hybrid molecular dynamics and first-principles study on the work function of a Pt(111) electrode immersed in aqueous solution at room temperature2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 4, p. 045450-Article in journal (Refereed)
    Abstract [en]

    With a combined molecular dynamics simulation and first-principles calculations, we have investigated a metal surface immersed in aqueous solution at room temperature using a Pt(111) electrode as an example. With the inclusion of thermal average effects at room temperature, the calculated averaged work function is found to be in good agreement with the experimental measurements. The electron redistribution at the interface of the topmost Pt(111) slab layer and the first water layer plays an important role in controlling the work function. A broad distribution of calculated work functions caused by the thermal motions of the dipolar solvents is obtained from statistical sampling, which implies that the chemical reactivity of a metal electrode in aqueous solution is a dynamic property at least in the nanoscale. Such a microscopic understanding helps to understand the behavior of complex electrochemical double layers.

  • 16.
    Duan, Sai
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Xu, Xin
    Tian, Zhong-Qun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Thermal Eects on Electronic Properties of CO/Pt(111) in WaterArticle in journal (Other academic)
  • 17.
    Gong, ZuYong
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Jiang, Jun
    Xu, Xin
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Infrared spectra of small anionic water clusters from density functional theory and wavefunction theory calculations2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 19, p. 12698-12707Article in journal (Refereed)
    Abstract [en]

    We performed systematic theoretical studies on small anionic water/deuterated water clusters W/D-n=2-6(-) at both density functional theory (B3LYP) and wavefunction theory (MP2) levels. The focus of the study is to examine the convergence of calculated infrared (IR) spectra with respect to the increasing number of diffuse functions. It is found that at the MP2 level for larger clusters (n = 4-6), only one extra diffuse function is needed to obtain the converged relative IR intensities, while two or three more sets of extra diffuse functions are needed for smaller clusters. Such behaviour is strongly associated with the convergence of the electronic structure of corresponding clusters at the MP2 level. It is striking to observe that at the B3LYP level, the calculated relative IR intensities for all the clusters under investigations are diverse and show no trend of convergence upon increasing the number of diffuse functions. Moreover, the increasing contribution from the extra diffuse functions to the dynamic IR dipole moment indicates that the B3LYP electronic structure also fails to converge. These results manifest that MP2 is a preferential theoretical method, as compared to the widely used B3LYP, for the IR intensity of dipole bounded electron systems.

  • 18.
    Gong, Zu-Yong
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Zhang, Guozhen
    Jiang, Jun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Optical Excitation in Donor-Pt-Acceptor Complexes: Role of the Structure2016In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 120, no 20, p. 3547-3553Article in journal (Refereed)
    Abstract [en]

    The optical properties of the Pt complexes in the form of donor-metal-acceptor (D-M-A) were studied at the first-principles level. Calculated results show that for the frontier molecular orbitals (MOs) of a D-M-A structure the energies of unoccupied frontier MO can be mainly determined by the interaction between M and A, whereas the M-A and M-D interactions both determine the energies of occupied frontier MO. By developing a straightforward transition dipole decomposition method, we found that not only the local excitations in D but also those in A can significantly contribute to the charge-transfer (CT) excitation. Furthermore, the calculations also demonstrate that by tuning the dihedral angle between D and A the transition probability can be precisely controlled so as to broaden the spectrum region of photoabsorption. For the D-M-A molecule with a delocalized pi system in A, the CT excitation barely affects the electronic structures of metal, suggesting that the oxidation state of the metal can be kept during the excitation. These understandings for the optical properties of the D-M-A molecule would be useful for the design of dye-sensitized solar cells, photocatalysis, and luminescence systems.

  • 19.
    Gong, Zuyong
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui, China.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Department of Chemical Physics, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui, China.
    Significant Contributions of the Albrecht’s A Term to Nonresonant Raman Scattering Processes2015In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 11, no 11, p. 5385-5390Article in journal (Refereed)
    Abstract [en]

    The Raman intensity can be well described by the famous Albrecht’s Raman theory that consists of A and B terms. It is well-known that the contribution from Albrecht’s A term can be neglected without any loss of accuracy for far-off resonant Raman scattering processes. However, as demonstrated in this study, we have found that this widely accepted long-standing assumption fails drastically for totally symmetric vibration modes of molecules in general off-resonant Raman scattering. Perturbed first-principles calculations for water molecule show that strong constructive interference between the A and B terms occurs for the Raman intensity of the symmetric O-H stretching mode, which can account for ∼40% of the total intensity. Meanwhile, a minor destructive interference is found for the angle bending mode. The state-to-state mapping between Albrecht’s theory and perturbation theory allows us to verify the accuracy of the widely employed perturbation method for the dynamic/resonant Raman intensities. The model calculations rationalized from water molecule with the bending mode show that the perturbation method is a good approximation only when the absolute energy difference between the first excited state and the incident light is more than five times greater than the vibrational energy in the ground state.

  • 20.
    Hu, Wei
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Theoretical modeling of surface and tip-enhanced Raman spectroscopies2017In: Wiley Interdisciplinary Reviews. Computational Molecular Science, ISSN 1759-0876, E-ISSN 1759-0884, Vol. 7, no 2, article id UNSP e1293Article, review/survey (Refereed)
    Abstract [en]

    Raman spectroscopy is a powerful technique in molecular science because of the ability of providing vibrational 'finger-print'. The developments of the surfaceenhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) have significantly improved the detection sensitivity and efficiency. However, they also introduce complications for the spectral assignments, for which advanced theoretical modeling has played an important role. Here we summarize some of our recent progresses for SERS and TERS, which generally combine both solid-state physics and quantum chemistry methods with two different schemes, namely the cluster model and the periodic boundary condition (PBC) model. In the cluster model, direct Raman spectra calculations are performed for the cluster taken from the accurate PBC structure. For PBC model, we have developed a quasianalytical approach that enables us to calculate the Raman spectra of entire system. Under the TERS condition, the non-uniformity of plasmonic field in real space can drastically alter the interaction between the molecule and the light. By taking into account the local distributions of the plasmonic field, a new interaction Hamiltonian is constructed and applied to model the super-high-resolution Raman images of a single molecule. It shows that the resonant Raman images reflect the transition density between ground and excited states, which are generally vibrational insensitive. The nonresonant Raman images, on the other hand, allow to visualize the atomic movement of individual vibrational modes in real space. The inclusion of non-uniformity of plasmonic field provides ample opportunities to discover new physics and new applications in the future. 

  • 21.
    Hu, Wei
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Zhang, Guangping
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ma, Yong
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Hefei National Laboratory for Physical Sciences, Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui, China.
    Quasi-Analytical Approach for Modeling of Surface-Enhanced Raman Scattering2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 52, p. 28992-28998Article in journal (Refereed)
    Abstract [en]

    Surface-enhanced Raman scattering has become a powerful analytical tool for the characterization of molecules adsorbed on metal surfaces. The lack of reliable computational methods to accurately assign the complicated Raman spectra has hampered its practical applications. We propose here a quasi-analytical method that allows for the effective evaluation of Raman tensors in periodic systems based on density functional perturbation theory and the finite-difference approach. Its applicability has been validated by simulating Raman spectra of 4,4’-bipyridine (4,4’-bpy) in various conditions. The calculated Raman spectra of isolated 4,4’-bpy as well as its adsorption on flat gold surfaces nicely reproduce their experimental counterparts. The same method has also been successfully applied to a more complicated system, namely 4,4’-bpy inside gold nano junctions. By comparing with the in situ experimental spectra, four interfacial configurations are identified, which are further verified by the good agreement between the simulated charge transfer properties and the experimental measurements. These results indicate that the proposed low-cost quasi-analytical method can provide accurate interpretation for the experimentally measured surface-enhanced Raman spectra and unambiguously determine the structures of the molecules on metal surfaces.

  • 22. Hu, Wei
    et al.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Zhang, Yujin
    Ren, Hao
    Jiang, Jun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Identifying the structure of 4-chlorophenyl isocyanide adsorbed on Au(111) and Pt(111) surfaces by first-principles simulations of Raman spectra2017In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 19, no 48, p. 32389-32397Article in journal (Refereed)
    Abstract [en]

    Surface Raman spectroscopy has become one of the most powerful analytical tools for interfacial structures. However, theoretical modeling for the Raman spectra of molecular adsorbate on metallic surfaces is a long-standing challenge because accurate descriptions of the electronic structure for both the metallic substrates and adsorbates are required. Here we present a quasi-analytical method for high-precision surface Raman spectra at the first principle level. Using this method, we correlate both geometrical and electronic structures of a single 4-chlorophenyl isocyanide (CPI) molecule adsorbed on a Au(111) or Pt(111) surface with its Raman spectra. The "finger-print'' frequency shift of the CN stretching mode reveals the in situ configuration of CPI is vertical adsorption on the top site of the Au(111) surface, but a bent configuration when it adsorbs on the hollow site of the Pt(111) surface. Electronic structure calculations reveal that a pi-back donation mechanism often causes a red shift to the Raman response of CN stretching mode. In contrast, sigma donation as well as a wall effect introduces a blue shift to the CN stretching mode. A clear relationship for the dependence of Raman spectra on the surface electronic and geometrical information is built up, which largely benefits the understanding of chemical and physical changes during the adsorption. Our results highlight that high-precision theoretical simulations are essential for identifying in situ geometrical and electronic surface structures.

  • 23.
    Hu, Wei
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Tian, Guangjun
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Lin, L. -L
    Ma, Yong
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. Shandong Normal University, China.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    Vibrational identification for conformations of trans-1,2-bis (4-pyridyl) ethylene in gold molecular junctions2015In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 453-454, p. 20-25Article in journal (Refereed)
    Abstract [en]

    The surface-enhanced Raman scattering (SERS) spectroscopy and inelastic electron tunneling spectroscopy (IETS) are employed to study trans-1,2-bis (4-pyridyl) ethylene (BPE)/gold system. Both junction and complex forms are considered for the SERS simulations. It is predicted that the peak at 1581 cm-1 is more intense in the junction forms than that in the complex forms. Time dependent density functional theory calculations show that the relative intensity is mainly controlled by the excitation energy derivative respect to the normal modes, and the total intensity is governed by the excitation energy of the excited states. The CH bending modes dominate the IET spectra when BPE adsorbed on the flat gold surfaces. While, the pyridyl ring deformation modes are more active when BPE adsorbed on the edge of the gold clusters. For BPE adsorbed on the tip of gold clusters, the pyridyl ring and CC stretching modes show significant contribution to the IET spectra.

  • 24.
    Hu, Wei
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China.
    Zhang, Guang-Ping
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Fu, Qiang
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China.
    Molecular Design to Enhance the Thermal Stability of a Photo Switchable Molecular Junction Based on Dimethyldihydropyrene and Cyclophanediene Isomerization2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 21, p. 11468-11474Article in journal (Refereed)
    Abstract [en]

    Photoswitchable molecular devices based on dimethyldihydropyrene (DHP) and cyclophanediene (CPD) isomers in gold junctions have been systematically studied by using first-principles calculations. The reaction pathways for the forth- and back-isomerization between DHP and CPD have been explored. It is found that along the ground state, the calculated barrier for the back-isomerization from CPD to DHP is as high as 23.2 kcal/mol. The forth- and back-isomerization on excited state was found to be much easier compared to that on the ground state. Our calculations have shown that the same conclusions about the reaction pathways can be drawn for the DHP/CPD derivatives that were experimentally studied. It is revealed that the thermal stability of the molecular switch can be significantly enhanced when certain substitutions are employed. A desirable substitution that gives a larger ON/OFF ratio and higher thermal stability is proposed for these isomeric systems. We have also found that the electrode distance has a huge impact on the electron transport properties, as well as the switching performance, of these junctions, which nicely explains some puzzling experimental observations.

  • 25.
    Huang, Bingru
    et al.
    South China Normal Univ, South China Acad Adv Optoelect, Ctr Opt & Electromagnet Res, Guangdong Prov Key Lab Opt Informat Mat & Technol, Guangzhou 510006, Guangdong, Peoples R China.
    Bergstrand, Jan
    KTH, School of Engineering Sciences (SCI), Applied Physics, Experimental Biomolecular Physics.
    Duan, Sai
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Zhan, Qiuqiang
    South China Normal Univ, South China Acad Adv Optoelect, Ctr Opt & Electromagnet Res, Guangdong Prov Key Lab Opt Informat Mat & Technol, Guangzhou 510006, Guangdong, Peoples R China.
    Widengren, Jerker
    KTH, School of Engineering Sciences (SCI), Applied Physics, Experimental Biomolecular Physics.
    Ågren, Hans
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Liu, Haichun
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Overtone Vibrational Transition-Induced Lanthanide Excited-State Quenching in Yb3+/Er3+-Doped Upconversion Nanocrystals2018In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 12, p. 10572-10575Article in journal (Refereed)
  • 26.
    Li, Xin
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Zhu, Liangliang
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Zhao, Yanli
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Aggregation-induced chiral symmetry breaking of a naphthalimide-cyanostilbene dyad2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 43, p. 23854-23860Article in journal (Refereed)
    Abstract [en]

    Spontaneously emerged supramolecular chirality and chiral symmetry breaking from achiral/racemic constituents remain poorly understood. We here report that supramolecular chirality may emerge from the structural flexibility of achiral aryl nitrogen centres which provide instantaneous chirality. Employing a naphthalimide-cyanostilbene dyad as a model, we explored the underlying mechanism of aggregation-induced chiral symmetry breaking and found that the conformations of the N-naphthylpiperazine and the N, N-dimethylaniline units facilitate the formation of ordered supramolecular structures and offer opposite handedness. Furthermore, chiral symmetry breaking of the monomers was amplified by the formation of dimers. The microscopic and the macroscopic observations from the theoretical simulations and experimental measurements are thus rationalized by connecting the population of the dihedral angles of the aryl nitrogen centres, the morphology of the self-assemblies, and the observed circular dichroism spectra.

  • 27.
    Li, Xiyu
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Liu, Haichun
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Chen, Guanying
    Harbin Inst Technol, Sch Chem Engn & Technol, Harbin 150001, Heilongjiang, Peoples R China.
    Luo, Yi
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China ; Federal Siberian Research Clinical Centre under FMBA of Russia, 26 Kolomenskaya, Krasnoyarsk 660037, Russia.
    On the Mechanism for the Extremely Efficient Sensitization of Yb3+ Luminescence in CsPbCl3 Nanocrystals2019In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 10, no 3, p. 487-492Article in journal (Refereed)
    Abstract [en]

    Rare earth ion (RE3+)-doped inorganic CsPbX3 (X = Cl or Cl/Br) nanocrystals have been presented as promising materials for applications in solar-energy conversion technology. An extremely efficient sensitization of Yb3+ luminescence in CsPbCl3 nanoparticles (NCs) was very recently demonstrated where quantum cutting is responsible for the performance of photoluminescence quantum yields over 100% (T. J. Milstein, et al. Nano Letters 2018, 18, 3792). In the present work, based on the cubic phase of inorganic perovskite, we seek to obtain atom-level insight into the basic mechanisms behind these observations in order to boost the further development of RE3+-doped CsPbX3 NCs for optoelectronics. In our calculations of cubic crystal structure, we do not find any energy level formed in the middle of the band gap, which disfavors a mechanism of stepwise energy transfer from the perovskite host to two Yb3+ ions. Our work indicates that the configuration with "right-angle" Yb3+-V-Pb-Yb3+ couple is most likely to form in Yb3+-doped CsPbCl3. Associated with this "right-angle" couple, the "right-angle" Pb atom with trapped excited states would localize the photogenerated electrons and act as the energy donor in a quantum cutting process, which achieves simultaneous sensitization of two neighboring Yb3+ ions.

  • 28.
    Lian, Ke-Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Li, Xiao-Fei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    Jin, Ming-Xing
    Ding, Da-Jun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    The Choice of the Supercell Controlling the Properties of Armchair/Zigzag Hybrid Graphene NanoribbonsArticle in journal (Other academic)
  • 29.
    Lian, Ke-Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Li, Xiao-Fei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Jin, Ming-Xing
    Ding, Da-Jun
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tuning electronic and magnetic properties of armchair vertical bar zigzag hybrid graphene nanoribbons by the choice of supercell model of grain boundaries2014In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 10, p. 104303-Article in journal (Refereed)
    Abstract [en]

    Grain boundaries (GBs) attract much interest for its ability to tune the property of hybrid materials. Theoretically predicting the properties of hybrid graphene with GBs, even a linear GB remains challenging due to its inhomogeneous structure, which makes supercell model tough to choose in theoretic studies. For the first time, the applicability of supercells with different GBs and lattice-mismatches for describing armchair-zigzag hybrid graphene nanoribbons was validated by ab initio molecular dynamic simulations and first principles electronic structure calculations. And to what extent the electronic properties can be tuned by the strain effects resulting from the lattice-mismatch and the GBs distortion in supercells was demonstrated. This work showed that the intrinsic strain in such system plays a decisive role in determining the band structure and spin polarization properties. Hybrid graphene nanoribbon was found to be ferromagnetic in the ground state, especially for the case of using the supercell with nearly-perfect lattice match. Its high Curie temperature suggests the potential applications of this material in spintronics.

  • 30.
    Tian, Guangjun
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Zhang, Guang-Ping
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hu, Wei
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology. University of Science and Technology of China, China.
    The effect of Duschinsky rotation on charge transport properties of molecular junctions in the sequential tunneling regime2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 35, p. 23007-23016Article in journal (Refereed)
    Abstract [en]

    We present here a systematic theoretical study on the effect of Duschinsky rotation on charge transport properties of molecular junctions in the sequential tunneling regime. In the simulations we assume that only two electronic charging states each coupled to a two dimensional vibrational potential energy surface (PES) are involved in the transport process. The Duschinsky rotation effect is accounted by varying the rotational angle between the two sets of displaced PESs. Both harmonic potential and anharmonic Morse potential have been considered for the cases of the intermediate to strong electron-vibration couplings. Our calculations show that the inclusion of the Duschinsky rotation effect can significantly change the charge transport properties of a molecular junction. Such an effect makes the otherwise symmetric Coulomb diamond become asymmetric in harmonic potentials. Depending on the angle of the rotation, the low bias current could be significantly suppressed or enhanced. This effect is particularly prominent in the Franck-Condon (FC) blockade regime where the electron-vibration coupling is strong. These changes are caused by the variation of the FC factors which are closely related to the rotational angle between the two sets of PESs involved in the charge transport process. For a molecular junction with Morse potentials, the changes caused by Duschinsky rotation are much more complicated. Both the amplitude and shape of the Coulomb diamond are closely dependent on the rotational angle in the whole range from 0 to 2 pi. One interesting result is that with a rotation angle of pi (and also pi/2 for certain cases) symmetric Coulomb diamonds can even be formed from the intrinsically asymmetric Morse potential. These results could be important for the interpretation of experimental observations.

  • 31.
    Wei, Yong
    et al.
    Yanshan Univ, Sch Sci, Key Lab Microstruct Mat Phys Hebei Prov, Qinhuangdao 066004, Hebei, Peoples R China..
    Pei, Huan
    Yanshan Univ, Coll Informat Sci & Engn, Qinhuangdao 066004, Hebei, Peoples R China..
    Sun, Dexian
    Yanshan Univ, Sch Sci, Key Lab Microstruct Mat Phys Hebei Prov, Qinhuangdao 066004, Hebei, Peoples R China..
    Duan, Sai
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Tian, Guangjun
    Yanshan Univ, Sch Sci, Key Lab Microstruct Mat Phys Hebei Prov, Qinhuangdao 066004, Hebei, Peoples R China..
    Numerical investigations on the electromagnetic enhancement effect to tip-enhanced Raman scattering and fluorescence processes2019In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 23, article id 235301Article in journal (Refereed)
    Abstract [en]

    In the present work, we theoretically study the electromagnetic (EM) enhancement of the Raman and fluorescence signals for a molecule placed in a nanocavity formed by a metallic tip and substrate that mimics a tip-enhanced Raman scattering (TERS) setup using three-dimensional finite element method calculations. The influence of tip size and tip-molecule distance on the EM enhancements of the incident field as well as the radiative and non-radiative decay rates of the molecule are systematically investigated. Simulation results show that the maximum EM enhancement to the incident light as provided by the localized surface plasmon resonance in the nanocavity can reach similar to 285 for the configuration considered in the present work. Meanwhile, it was found that, at the classical limit, decreasing the apex radius or the tip-molecule distance can both reduce the spatial distribution (as characterized by the full width at half maximum) of the Raman enhancement in a linear fashion. Moreover, simulation results show that the nonlocal dielectric response of the tip and the substrate plays a key role to the fluorescence quantum yield of the molecule. However, it was found that the strong EM excitation enhancement is the dominating factor for the tip enhanced fluorescence (TEF) effect and stronger fluorescence enhancement has been found when increasing the apex radius or reducing the tip-molecule distance with an incident wavelength of 532 nm. The best TERS and TEF enhancements were found to be similar to 6 x 10(9) and similar to 1.7 x 10(4), respectively, with the tip-molecule distance around 1 nm.

  • 32.
    Xie, Zhen
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Shandong Normal Univ, Sch Phys & Elect, Shandong Prov Key Lab Med Phys & Image Proc Techo, Jinan 250014, Shandong, Peoples R China.
    Duan, Sai
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Tian, Guangjun
    Yanshan Univ, Coll Sci, Qinhuangdao 066004, Peoples R China..
    Wang, Chuan-Kui
    Shandong Normal Univ, Sch Phys & Elect, Shandong Prov Key Lab Med Phys & Image Proc Techo, Jinan 250014, Shandong, Peoples R China..
    Luo, Yi
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Univ Sci & Technol China, Hefei Natl Lab Phys Sci Microscale, Hefei 230026, Anhui, Peoples R China..
    Theoretical modeling of tip-enhanced resonance Raman images of switchable azobenzene molecules on Au(111)2018In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 10, no 25, p. 11850-11860Article in journal (Refereed)
    Abstract [en]

    With a highly localized plasmonic field, tip-enhanced Raman spectroscopy (TERS) images have reached atomic-scale resolution, providing an optical means to explore the structure of a single molecule. We have applied the recently developed theoretical method to simulate the TERS images of trans and cis azobenzene as well as its derivatives on Au(111). Our theoretical results reveal that when the first excited state is resonantly excited, TERS images from a highly confined plasmonic field can effectively distinguish the isomer configurations of the adsorbates. The decay of the plasmonic field along the surface normal can be further used to distinguish different nonplanar cis configurations. Moreover, subtle characteristics of different molecular configurations can also be identified from the TERS images of other resonant excited states with a super-high confined plasmonic field. These findings serve as good references for future TERS experiments on molecular isomers.

  • 33.
    Xie, Zhen
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Duan, Sai
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Wang, Chuan-Kui
    Shandong Normal Univ, Shandong Prov Key Lab Med Phys & Image Proc Techn, Sch Phys & Elect, Jinan 250014, Shandong, Peoples R China..
    Luo, Yi
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Monitoring Hydrogen/Deuterium Tautomerization in Transient Isomers of Single Porphine by Highly Localized Plasmonic Field2019In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 123, no 17, p. 11081-11093Article in journal (Refereed)
    Abstract [en]

    Inner proton transfer between two trans isomers (tautomerization) in porphyrins plays a crucial role in many biological systems as well as molecular nanotechnology. Although the stepwise mechanism of tautomerization is well accepted, the involved intermediate cis-isomer has not been directly detected owing to its short lifetime and the extremely low intensities of corresponding hydrogen vibrations. Here, taking a single porphine as the prototype, we theoretically demonstrate that Raman intensities of the hydrogen vibrations become accessible under the highly localized plasmonic field because of the symmetry breaking effect. In addition, with the ultrafast incident excitations, we find that Raman signals of cis-porphine could be distinguished from the stable trans isomer, suggesting a general protocol for the direct characterization of transient isomers. Moreover, calculated results reveal that the position of inner hydrogen/deuterium can be unambiguously visualized from Raman images of the corresponding stretching modes, providing a unique optical means for the chemical monitoring of tautomerization in porphine and its derivatives.

  • 34. Zhang, Qun
    et al.
    Zheng, Hongjun
    Geng, Zhigang
    Jiang, Shenlong
    Ge, Jing
    Fan, Kaili
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Chen, Yang
    Wang, Xiaoping
    Luo, Yi
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    The Realistic Domain Structure of As-Synthesized Graphene Oxide from Ultrafast Spectroscopy2013In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 135, no 33, p. 12468-12474Article in journal (Refereed)
    Abstract [en]

    Graphene oxide (GO) is an attractive alternative for large-scale production of graphene, but its general structure is still under debate due to its complicated nonstoichiometric nature. Here we perform a set of femto-second pump-probe experiments on as-synthesized GO to extrapolate structural information in situ. Remarkably, it is observed that, in these highly oxidized GO samples, the ultrafast graphene-like dynamics intrinsic to pristine graphene is completely dominant over a wide energy region and can be modified by the localized impurity states and the electron-phonon coupling under certain conditions. These observations, combined with the X-ray photoelectron spectroscopy analysis and control experiments, lead to an important conclusion that GO consists of two types of domain, namely the carbon-rich graphene-like domain and the oxygen-rich domain. This study creates a new understanding of the realistic domain structure and properties of as-synthesized GO, offering useful guidance for future applications based on chemically modified/functionalized graphenes.

  • 35. Zhang, Yue-Jiao
    et al.
    Li, Song-Bo
    Duan, Sai
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Lu, Bang-An
    Yang, Ji
    Panneerselvam, Rajapandiyan
    Li, Chao-Yu
    Fang, Ping-Ping
    Zhou, Zhi-You
    Phillips, David Lee
    Li, Jian-Feng
    Tian, Zhong-Qun
    Probing the Electronic Structure of Heterogeneous Metal Interfaces by Transition Metal Shelled Gold Nanoparticle-Enhanced Raman Spectroscopy2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 37, p. 20684-20691Article in journal (Refereed)
    Abstract [en]

    In heterogeneous catalysis, characterization of heterogeneous metal interfaces of bimetallic catalysts is a crucial step to elucidate the catalytic performance and is a key to develop advanced catalysts. However, analytical techniques such as X-ray photoelectron spectroscopy can only work in vacuum conditions and are difficult to use for in situ analysis. Here, we present efficient and convenient core-shell nano particle-enhanced Raman spectroscopy to explore the in situ electronic structures of heterogeneous interfaces (Au@Pd and Au@Pt core-shell NPs) by varying the shell thickness. The experimental observations reported here clearly show that Pd donates electrons to Au, while Pt accepts electrons from Au at the heterogeneous interfaces. This conclusion gains further support from ex situ X-ray photoelectron spectroscopy results. The Au core greatly affects the electronic structures of both the Pd and Pt shells as well as catalytic behaviors. Finally, the as prepared core-shell nanoparticles were used to demonstrate their improved catalytic properties in real electrocatalytic systems such as methanol oxidation and oxygen reduction reactions.

1 - 35 of 35
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