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High Resolution Tip-Enhanced Raman Images of Single Molecules from First Principles Simulations

Time: Fri 2019-12-06 14.00

Location: FP 41, Roslagstullsbacken 33, Stockholm (English)

Subject area: Theoretical Chemistry and Biology

Doctoral student: Zhen Xie , Teoretisk kemi och biologi, Research group of Prof. Yi Luo

Opponent: professor Filippo De Angelis, Department of chemistry, biology and biotechnology, university of Perugia

Supervisor: Professor Yi Luo, Teoretisk kemi och biologi, Bioteknologi, Kemi

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With the precise control of spatially confined plasmon (SCP), tip-enhanced Raman spectroscopy (TERS) has achieved sub-nanometer resolution, leading to the chemical and physical characterization of the single molecule by optical Raman images. In the high resolution TERS measurements, the SCP spatial distribution generates the position-dependent Raman images. The position dependence challenges the conventional response theory, because the assumption of interactions between the molecule and the uniform electromagnetic field does not hold anymore. Moreover, as an emerging technology, potential applications of high resolution TERS are required to be fully explored. In this thesis, the developed theory for modeling high resolution Raman images is presented. By taking a series of typical molecular systems as examples, we theoretically predict some fine applications of single-molecule TERS.

The first part of the thesis introduces the development of Raman spectroscopy and images. To achieve the final target of single molecule characterization, high spatial resolution single-molecule TERS is established and improved. As a nondestructive measuring tool, Raman imaging technology offers the means to study single molecules with unprecedented spatial resolution.

The high resolution Raman images theory with detailed derivations is given in the second part of the thesis. The key factor is to take the inhomogeneous spatial distribution of SCP field into account, when we construct the interaction Hamiltonian between the localized light field and the molecule. This makes the numerical simulations of Raman images feasible.

Other parts of the thesis give some theoretical predictions for potential applications of the emerging Raman imaging technology. Specifically, resonance Raman images can visualize the geometric changes of a single molecule switch and the intramolecular structure in real space. Since the localized plasmonic field can affect the electron transition, the excited quantum states can thus be effectively manipulated. This breaks down the intrinsic spatial selection rule imposed in conventional spectra. In addition, an effective linear response algorithm is used to simulate nonresonance Raman images. The unique superiority of spatial vibration resolution from non-resonance cases provides rich information about the single molecule. By constructing images from different vibrational modes, the spatial chemical distribution within a single molecule can be visualized. All these findings will facilitate fine applications of the emerging TERS technology in the coming years.