We review our developed visualization method of charge transfer (CT) for chemical enhancement mechanism on surface‐enhanced Raman scattering (SERS) and tip‐enhanced Raman spectroscopy (TERS). Firstly, we describe our visualization method of charge difference density, which provides direct visual evidence for photoinduced CT. And then, using the visualization method of CT, we interpreted the mechanism of SERS and TERS. Photoinduced charge transfer in the processes of SERS and TERS can be clearly seen. Our visualization method provides a visual and easy understanding way for the mechanism of SERS and TERS. Copyright © 2014 John Wiley & Sons, Ltd.

Ferroelectricity in calcium doped hafnium oxide (Ca:HfO2) thin films has been experimentally proved for the first time in this work. All films prepared by chemical solution deposition exhibited smooth and crack-free surfaces, which were observed using an atomic force microscope. After 104 field cycling, a maximum remanent polarization of 10.5 μC/cm2 was achieved in HfO2 films with 4.8 mol. % Ca content. Meanwhile, the breakdown of the film occurred after 7 × 106 electric cycles. A phase transition from the monoclinic phase to cubic/orthorhombic phases was observed with increasing Ca concentration. We suggest the change in oxygen vacancy concentration as the origin of phase evolution, which was confirmed by X-ray photoelectron spectroscopy analysis. These results open a new pathway for realizing ferroelectricity in HfO2-based films.

The influence of a static external electric field on surface-enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine-Au 20 complex with the density functional theory and time-dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm À1 ) but suppresses the trigonal ring breathing mode (~1051 cm À1 ). On the contrary, the mode at 1051 cm À1 is more enhanced than the mode at 1009 cm À1 when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field.

Para-aryl-dithiols (PADTs, HS-(C 6 H 4 ) n -SH, n = 1, 2, and 3) have been used extensively in molecular electronics, surface-enhanced Raman spectroscopy (SERS), and quantum electron tunneling between two gold or silver nanoparticles (AuNPs and AgNPs). One popular belief is that these dithiols cross-link noble metal nanoparticles (NPs) as monolayer dithiolate spacers. Reported herein is our finding that PADTs predominantly exist as monothiolate forms on AuNPs or AgNPs. No PADT-induced NP cross-linking was observed regardless of the NP/PADT concentration ratios. Moreover, only one PADT thiol can be deprotonated even when PADTs are treated with concentrated NaOH or AgNO 3 solutions. In contrast, 1,4-benzenedimethanethiol (HS-CH 2 -(C 6 H 4 ) 1 -CH 2 -SH) and alkyl dithiol 1,2-ethanedithiol can be dithiolated on AuNPs and AgNPs, and in excess NaOH and AgNO 3 solutions. This study should be of broad importance for plasmonic NP research given the popularity of PADTs in molecular electronics and SERS applications.

Density functional theory (DFT) and time-dependent DFT calculations have been performed to investigate the Raman scattering spectra of metal-molecule complex and metal-molecule-metal junction architectures interconnected with 4-aminothiophenol (PATP) molecule. The simulated profiles of normal Raman scattering (NRS) spectra for the two complexes (Ag(2)-PATP and PATP-Au(2)) and the two junctions (Ag(2)-PATP-Au(2) and Au(2)-PATP-Ag(2)) are similar to each other, but exhibit obviously different Raman intensities. Due to the lager static polarizabilities of the two junctions, which directly influence the ground state chemical enhancement in NRS spectra, the calculated normal Raman intensities of them are stronger than those of two complexes by the factor of 10(2). We calculate preresonance Raman scattering (RRS) spectra with incident light at 1064 nm, which is much lower than the S(1) electronic transition energy of complexes and junctions. Ag(2)-PATP-Au(2) and Au(2)-PATP-Ag(2) junctions yield higher Raman intensities than those of Ag(2)-PATP and PATP-Au(2) complexes, especially for b(2) modes. This effect is mainly attributed to charge transfer (CT) between the metal gap and the PAPT molecule which results in the occurrence of CT resonance enhancement. The calculated pre-RRS spectra strongly depend on the electronic transition state produced by new structures. With excitation at 514.5 nm, the calculated pre-RRS spectra of two complexes and two junctions are stronger than those of with excitation at 1064 nm. A charge difference densities methodology has been used to visually describe chemical enhancement mechanism of RRS spectrum. This methodology aims at visualizing intermolecular CT which provides direct evidence of the Herzberg-Teller mechanism.

The absorption spectra of dianionic tetrocyanoethylene and dicationic tetrathiafulvalene dimers have been studied theoretically with the time-dependent density functional theory and the recently proposed Coulomb-attenuated model. The nature of the excited states was further explored by means of the two-dimensional (2D) site (transition density matrix) and three-dimensional (3D) cube (transition density and charge difference density) representations. By use of the 3D transition density and charge difference density, we visualized the orientation of transition dipole moment and also explained charge-transfer characteristics occurring in the dianionic/dicationic pi-dimers system. It is found that for the dianionic/dicationic pi-dimers system there exist two kinds of charge-transfer patterns for the mainly excited states, the intermolecular charge transfer and the mixture of intramolecular charge transfer coupled with intermolecular charge transfer. Meanwhile, the coupling effect of excitation and the oscillation of electron-hole pairs between the monomers have been revealed with 2D site representation of transition density matrix, which also indicates the electron-hole coherence upon photon excitation.

The Raman and absorption spectra of a Ag 2 -PATP-Au 2 junction adsorbed on graphene and boron-doped graphene were investigated by using density functional theory (DFT) and time-dependent DFT methods.The interactions between the graphene and junction result in charge transfer (CT) from the graphene to the junction due to their different work functions. This CT leads to charge redistribution on the junction, and then the changes of static polarizabilities, which directly influence the enhancement of normal Raman spectra. The absorption spectra show that the graphene and boron-doped graphene induce some CT excited states in the visible and infrared regions. When the energy of incident light is close to the energy of these CT excited states, these electronic transitions will be excited, which leads to the enhancement of pre-resonant Raman scattering (pre-RRS) spectra. In pre-RRS spectra, the B-doped model has stronger Raman intensities, since it produces more CT excited states with intense oscillator strength near the incident light than the graphene model. The non-totally symmetric modes (b 2 ) are strongly enhanced as well as the totally symmetric modes (a 1 ), indicating the contribution of Herzberg-Teller (HT) scattering. The charge difference densities (CDDs) method was employed to directly visualize the CT from the graphene sheet to the molecule.

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