Hairong Zheng

In-situ plasmon-driven chemical reactions revealed by high vacuum tip-enhanced Raman spectroscopy

With strong surface plasmons excited at the metallic tip, tip-enhanced Raman spectroscopy (TERS) has both high spectroscopic sensitivity and high spatial resolution, and is becoming an essential tool for chemical analysis. It is a great challenge to combine TERS with a high vacuum system due to the poor optical collection efficiency. We used our innovatively designed home-built high vacuum TERS (HV-TERS) to investigate the plasmon-driven in-situ chemical reaction of 4-nitrobenzenethiol dimerizing to dimercaptoazobenzene. The chemical reactions can be controlled by the plasmon intensity, which in turn can be controlled by the incident laser intensity, tunneling current and bias voltage. The temperature of such a chemical reaction can also be obtained by the clearly observed Stokes and Anti-Stokes HV-TERS peaks. Our findings offer a new way to design a highly efficient HV-TERS system and its applications to chemical catalysis and synthesis of molecules, and significantly extend the studies of chemical reactions.

Plasmonic Scissors for Molecular Design

Heterogeneous catalysts play an important role in surface catalytic reactions, but selective bond breaking and control of reaction products in catalytic processes remain significant challenges. High-vacuum tip-enhanced Raman spectroscopy (HV-TERS) is one of the best candidates to realize surface catalytic reactions. Herein, HV-TERS was employed in a new method to control dissociation by using hot electrons, generated from plasmon decay, as plasmonic scissors. In this method, the N=N bond in 4,4-dimercaptoazobenzene was selectively dissociated by plasmonic scissors, and the reaction products formed from the radical fragment (SC6H5N) were controlled by varying the pH value. Under acidic conditions, p-aminothiophenol was produced from the radical fragment by attachment of hydrogen ions, whereas under alkaline conditions, 4-nitrobenzenethiol was obtained by attachment of oxygen ions to the substrate.

Insights into the nature of plasmon-driven catalytic reactions revealed by HV-TERS

The nature of plasmon-driven chemical reactions is experimentally investigated using high vacuum tip-enhanced Raman spectroscopy (HV-TERS). It is revealed that the coupling between the tip and the substrate can produce intense plasmon resonance, which then decays to produce sufficient hot electrons and thus catalyses the chemical reaction. The photoelectron emission from the laser illuminated silver substrate alone cannot drive the reaction.

Surface enhanced fluorescence and Raman scattering by gold nanoparticle dimers and trimers

Dimers and trimers of gold nanoparticles were synthesized using wet chemistry method for surface enhanced fluorescence and Raman scattering. The dimers and trimers provide hot spots for enhancing the fluorescence and Raman signals, and significantly obvious enhancement is obtained from Raman signals in solution. Using finite element method, we calculate the enhancement of fluorescence and Raman signals in the experimental system. Both experimental and theoretical results show that the dimers and trimers solution can be used in micro-quantitative detection from fluorescence and Raman signals.

Enhanced red upconversion luminescence by codoping Ce3+ in β-NaY(Gd0.4)F4:Yb3+/Ho3+ nanocrystals

In this work, hexagonal phase NaY(Gd0.4)F4:Yb3+/Ho3+ nanocrystals were obtained by solvothermal method. The upconversion emission tuning from green to red in NaY(Gd0.4)F4:Yb3+/Ho3+ nanocrystals was successfully achieved by replacing Y3+ ions in the nanocrystal structure with Ce3+ ions under 980 nm excitation. The red upconversion emission intensity was enhanced with Ce3+ concentration increasing. The output colors for the samples can be clearly observed in a confocal microscopy setup. It was found that two efficient cross-relaxation processes between Ho3+ and Ce3+ ions had been employed to enhance red emission and suppress green emission. The possible upconversion mechanisms and conversion efficiency between Ho3+ and Ce3+ ions were investigated in detail. In addition, the influence of the matrix and surface properties on the upconversion emission of the samples was also discussed. The red upconversion emission of Yb3+, Ho3+ and Gd3+ codoped nanocrystals in this work will have great potential applications in biological imaging, magnetic resonance imaging agents, and display and anti-counterfeiting applications.

Recent Progress on Plasmon-Enhanced Fluorescence

Abstract The optically generated collective electron density waves on metal–dielectric boundaries known as surface plasmons have been of great scientific interest since their discovery. Being electromagnetic waves on gold or silver nanoparticle’s surface, localised surface plasmons (LSP) can strongly enhance the electromagnetic field. These strong electromagnetic fields near the metal surfaces have been used in various applications like surface enhanced spectroscopy (SES), plasmonic lithography, plasmonic trapping of particles, and plasmonic catalysis. Resonant coupling of LSPs to fluorophore can strongly enhance the emission intensity, the angular distribution, and the polarisation of the emitted radiation and even the speed of radiative decay, which is so-called plasmon enhanced fluorescence (PEF). As a result, more and more reports on surface-enhanced fluorescence have appeared, such as SPASER-s, plasmon assisted lasing, single molecule fluorescence measurements, surface plasmoncoupled emission (SPCE) in biological sensing, optical orbit designs etc. In this review, we focus on recent advanced reports on plasmon-enhanced fluorescence (PEF). First, the mechanism of PEF and early results of enhanced fluorescence observed by metal nanostructure will be introduced. Then, the enhanced substrates, including periodical and nonperiodical nanostructure, will be discussed and the most important factor of the spacer between molecule and surface and wavelength dependence on PEF is demonstrated. Finally, the recent progress of tipenhanced fluorescence and PEF from the rare-earth doped up-conversion (UC) and down-conversion (DC) nanoparticles (NPs) are also commented upon. This review provides an introduction to fundamentals of PEF, illustrates the current progress in the design of metallic nanostructures for efficient fluorescence signal amplification that utilises propagating and localised surface plasmons.

Higher Order Fano Resonances and Electric Field Enhancements in Disk-Ring Plasmonic Nanostructures with Double Symmetry Breaking

Optical properties of disk-ring plasmonic nanostructures with double symmetry breaking are investigated theoretically. Tunable higher order Fano resonance is achieved, and it is sensitive to the degree of asymmetry of the nanoring, the offset and the dimension of the nanodisk. It is demonstrated that such higher order Fano resonances originate from the destructive interference between the bright mode of the displaced nanodisk and the dark mode of the asymmetric nanoring. By tunning the asymmetry degree of the nanoring, the offset, and the dimension of the nanodisk, certain higher order Fano resonances can be suppressed or enhanced. Double asymmetry breaking also allows the realization of the stronger electric field enhancement, resulting from the stronger interaction between the displaced nanodisk and the asymmetric nanoring.

Surface enhanced fluorescence on three dimensional silver nanostructure substrate

Multi-dimensional metallic nanostructures were fabricated by self-assembling silver nanoparticles on 3-aminopropyltrimethoxysilane-modified glass substrate and using p-aminothiophenol molecule as a linker. Surface enhanced fluorescence was investigated for Rhodamine 6G fluorophore molecules on the prepared 2D and 3D substrates. The experimental observation showed that the 3D nanostructured substrate presented stronger fluorescence enhancement, comparing with what was observed on the 2D nanoparticle arrays. Higher intensity of local electric field and stronger coupling of surface plasmon resonance in 3D silver nanostructure enhanced the excitation and emission of fluorophore molecules more effectively, leading to a stronger fluorescence enhancement on 3D nanostructured substrate. The result suggests that a metallic substrate with 3D nanostructures can produce better fluorescence enhancement, which is important for studying the mechanism and expanding the potential applications of enhanced fluorescence effect.

Tip-Enhanced Ultrasensitive Stokes and Anti-Stokes Raman Spectroscopy in High Vacuum

We report ultrasensitive Stokes and anti-Stokes Raman spectra of 1,2-benzenedithiol monolayer on Ag film with home-made high-vacuum tip-enhanced Raman spectroscopy (HV-TERS) system. Raman peaks that were orginally very weak were observed experimentally and assigned theoretically. The local temperature was obtained based on the observed Stokes and anti-Stokes HV-TERS spectra.

Plasmon resonances and strong electric field enhancements in side-by-side tangent nanospheroid homodimers.

The plasmon resonance and electric field enhancement in a side-by-side tangent nanospheroid homodimer (TNSHD) have been investigated theoretically by using DDA and FDTD methods, respectively. The simulation results indicate that this side-by-side TNSHD has its novel optical properties. We find that the plasmon resonance with a distinct Fano lineshape can be achieved and the electric field intensity can be enhanced strongly. The tunability of the Fano resonance could provide important applications in biosensing. The obtained electric field enhancement might open a promising pathway for surface-enhanced Raman scattering (SERS) and light trapping in solar cells.