Chunping Hu

Preparation and Optical Properties of Fullerene/Ferrocene Hybrid Hexagonal Nanosheets and Large-Scale Production of Fullerene Hexagonal Nanosheets

The supramolecular nanoarchitectures, C(60)/ferrocene nanosheets, were prepared by a simple liquid-liquid interfacial precipitation method and fully characterized by means of SEM, STEM, HRTEM, XRD, Raman and UV-vis-NIR spectra. The highly crystallized C(60)/ferrocene hexagonal nanosheets had a size of ca. 9 microm and the formulation C(60)(ferrocene)(2). A strong charge-transfer (CT) band between ferrocene and C(60) was observed at 782 nm, indicating the presence of donor-acceptor interaction in the nanosheets. Upon heating the nanosheets to 150 degrees C, the CT band disappeared due to the sublimation of ferrocene from the C(60)/ferrocene hybrid, and C(60) nanosheets with an fcc crystal structure and the same shape and size as the C(60)/ferrocene nanosheets were obtained.

Nonadiabatic couplings from time-dependent density functional theory: Formulation in the Casida formalism and practical scheme within modified linear response

We present an efficient method to compute nonadiabatic couplings (NACs) between the electronically ground and excited states of molecules, within the framework of time-dependent density functional theory (TDDFT) in frequency domain. Based on the comparison of dynamic polarizability formulated both in the many-body wave function form and the Casida formalism, a rigorous expression is established for NACs, which is similar to the calculation of oscillator strength in the Casida formalism. The adiabatic local density approximation (ALDA) gives results in reasonable accuracy as long as the conical intersection (ci) is not approached too closely, while its performance quickly degrades near the ci point. This behavior is consistent with the real-time TDDFT calculation. Through the use of modified linear response theory together with the ground-state-component separation scheme, the performance of ALDA can be greatly improved, not only in the vicinity of ci but also for Rydberg transitions and charge-transfer excitations. Several calculation examples, including the quantization of NACs from the Jahn-Teller effect in the H3 system, have been given to show that TDDFT can efficiently give NACs with an accuracy comparable to that of wave-function-based methods.

Second-order nonadiabatic couplings from time-dependent density functional theory: Evaluation in the immediate vicinity of Jahn-Teller/Renner-Teller intersections

For a rigorous quantum simulation of nonadiabatic dynamics of electrons and nuclei, knowledge of not only the first-order but also the second-order nonadiabatic couplings (NACs) is required. Here, we propose a method to efficiently calculate the second-order NAC from time-dependent density functional theory (TDDFT), on the basis of the Casida ansatz adapted for the computation of first-order NAC, which has been justified in our previous work and can be shown to be valid for calculating second-order NAC between ground state and singly excited states within the Tamm-Dancoff approximation. Test calculations of the second-order NAC in the immediate vicinity of Jahn-Teller and Renner-Teller intersections show that calculation results from TDDFT, combined with modified linear response theory, agree well with the prediction from the Jahn-Teller/Renner-Teller models. Contrary to the diverging behavior of the first-order NAC near all types of intersection points, the Cartesian components of the second-order NAC are shown to be negligibly small near Renner-Teller glancing intersections, while they are significantly large near the Jahn-Teller conical intersections. Nevertheless, the components of the second-order NAC can cancel each other to a large extent in Jahn-Teller systems, indicating the background of neglecting the second-order NAC in practical dynamics simulations. On the other hand, it is shown that such a cancellation becomes less effective in an elliptic Jahn-Teller system and thus the role of second-order NAC needs to be evaluated in the rigorous framework. Our study shows that TDDFT is promising to provide accurate data of NAC for full quantum mechanical simulation of nonadiabatic processes.

Performance of Tamm-Dancoff approximation on nonadiabatic couplings by time-dependent density functional theory.

The Tamm-Dancoff approximation (TDA), widely used in physics to decouple excitations and de-excitations, is well known to be good for the calculation of excitation energies but not for oscillator strengths. In particular, the sum rule is violated in the latter case. The same concern arises within the TDA in the calculation of nonadiabatic couplings (NACs) by time-dependent density functional theory (TDDFT), due to the similarities in the TDDFT formulations of NACs and oscillator strengths [C. Hu, H. Hirai, and O. Sugino, J. Chem. Phys. 127, 064103 (2007)]. In this study, we present a systematic evaluation of the performance of TDDFT/TDA for the calculation of NACs. In the cases we considered, including a variety of systems possessing Jahn-Teller and Renner-Teller intersections, as well as an example with accidental conical intersections, it is found that the TDDFT/TDA performs better than the full TDDFT, contrary to the conjecture that the TDA might cause the NAC results to deteriorate and violate the sum rule. The surprisingly good performance of the TDA for NACs is probably because the TDA can partially compensate for the local-density-approximation error and give better excitation energies in the vicinity of intersections of potential energy surfaces. Our study also shows that it is important to use the TDA based on the rigorous full-TDDFT formulation of NACs, instead of using it based on an alternative approximate formulation.

Laser-Driven Field Emission from Graphene Nanoribbons : Time-Dependent Density-Functional Theory Simulations

First-principles simulations were carried out to explore the excited electron dynamics of graphene nanoribbons (GNRs) under femtosecond laser pulses and an electrostatic field. Electron emission was found to be governed by both the electric dipole transition probability and the characters of the excited states, which are dependent on the edge termination of the GNR. For the laser parameters used in the simulations, the emission mechanism was demonstrated to be one-photon photoemission. The present approach enables a physical interpretation of highly nonequilibrium electron emission processes.

Calculation of atomic excitation energies by time-dependent density functional theory within a modified linear response.

Time-dependent density functional theory (TDDFT) has become a standard tool for investigation of electronic excited states. However, for certain types of electronic excitations, TDDFT is known to give systematically inaccurate results, which has been attributed to the insufficiency of conventional exchange-correlation functionals, such as the local density approximation (LDA). To improve TDDFT performance within LDA, a modified linear response (MLR) scheme was recently proposed, in which the responses from not only the ground state, but also the intermediate excited states are taken into account. This scheme was shown to greatly improve TDDFT performance on the prediction of Rydberg and charge-transfer excitation energies of molecules. Yet, for a validation of this TDDFT-MLR scheme for excitation energies, there remain issues to be resolved regarding Rydberg transitions of single atoms before going to larger systems. In the present work, we show an adapted algorithm to construct the intermediate excited states for rare-gas atoms. With the technique, Rydberg transition energies can be well decoded from LDA, as will also be shown in the application of the TDDFT-MLR scheme to other types of atoms.

Nanoplasmon dynamics and field enhancement of graphene flakes by first-principles simulations

We investigated nanoplasmon dynamics in graphene flake (GF) monomers and dimers using real-time and real-space time-dependent density functional theory. By showing the characteristic features of dynamical polarizability, we verified that the two distinct peaks in the optical absorption spectra can be attributed to π and π + σ plasmons. We clearly show a significant difference between the on- and off-resonance responses of the π plasmon by demonstrating the spatial distribution of the induced charge and the electric field. We thus reveal the mechanism of plasmon-induced field enhancement near GF edges, which shows great importance of first-principles studies on molecular nanoplasmonics.

Patterns of Field Electron Emission from Carbon Nanotubes: Ab Initio Simulations by Time-Dependent Density Functional Theory

We present the theoretical field-emission (FE) patterns from pristine and H2-adsorbed carbon nanotubes (CNTs), using ab initio time-dependent density functional theory. The field-emitted electrons are treated in the equal footing with the electrons in the nanotubes and the spatial distributions of FE current densities are calculated directly by the time-propagated Kohn-Sham wave functions. The simulated results of pristine CNTs clearly show either five-fold or six-fold symmetries, corresponding to the symmetries of the features of pentagons on the CNT caps. Further simulations on the H2 molecule adsorbed CNTs verify that the bright spots in the FE pattern are of signatures of atom adsorption onto CNTs, and the adsorption site should be close to the bright spots.

Optical Properties of Boron Nitride and Graphene Nanoribbons: A Time Dependent Density Functional Theory Simulation

The boron nitride (BN) sheet [1], just the analogue of graphene, has attracted wide attention due to its useful properties such as a complementary 2D dielectric substrate for graphene electronics. The BN nanoribbons (BNNRs), which were recently produced under unwrapping multiwalled BN nanotubes, show semiconducting properties due to edge states and imperfections, and have width-dependent energy-band gaps [2] that can be tuned by transverse electric fields. Thus, the BN nanostructures can be promising materials in opto-electronics as functional semiconductors. In the present study we report the results of dielectric properties of the BN nanostructures with the comparison of their carbon analogues by the real-time time-dependent density functional theory in the linear response regime to calculate the dielectric function [3]. Our results have reproduced plasmon peaks in electron energy loss spectra (EELS) of h-BN and the BN sheet observed in a very recent experiment [4]. The profiles of EELS spectra become more similar to those of absorption spectra for BN structures with lower dimension, especially for BNNRs, indicating that the electronic excitations of low dimensional BN structures occur mostly through particle-hole excitations. The detailed comparisons of the results with their carbon analogues and also with the results calculated by the independentparticle approximation will be given in the presentation. [1] H. Zeng, C. Zhi, Z. Zhang, X. Wei, X. Wang, W. Guo, Y. Bando, and D. Golberg, Nano Lett. 10, 5049 (2010). [2] C. Hu, R. Ogura, N. Onoda, S. Konabe, and K. Watanabe, Phys. Rev. B 85, 245420 (2012). [3] G. F. Bertsch, J.-I. Iwata, A. Rubio, and K. Yabana, Phys. Rev. B 62, 7998 (2000). [4] C. T. Pan, R. R. Nair, U. Bangert, Q. Ramasse, R. Jalil, R. Zan, C. R. Seabourne, and A. J. Scott, Phys. Rev. B 85 , 045440 (2012). APPC12 The 12th Asia Pacific Physics Conference

Connecting single conductive polymers to a single functional molecule

We have developed a method using a scanning tunneling microscope (STM) probe tip to control the chain polymerization of diacetylene compounds in a self-ordered layer, thereby creating conjugated polydiacetylene nanowires. When a small amount of phthalocyanine was deposited on a molecular layer of diacetylene compound, we found adsorbed and stabilized phthalocyanine single molecules. The initiation of chain polymerization on the diacetylene molecular row to which the single phthalocyanine molecule was adsorbed was also demonstrated.