- General Introductions
- SERS and SHINERS
- Synthesis and Fabrication
Our group aims to develop various physicochemical methods on hard and soft surfaces and interfaces. The research interests range from surface characterization to synthesis of nanostructures and theory /modeling.
Generally, Our researches can divided into five parts, as listed in the follwing pictures.
Surface enhanced Raman scattering (SERS) is a surface plasmon resonace (SPR) based nano-photonic phenomenon. It can provide non-destructive and ultra-sensitive molecular fingerprint vibrational information down to single molecular level. However, only a few ‘free-electron-like’ metals (mainly Au, Ag and Cu) can provide a large SERS effect under the condition that the metal surface must be roughened or nanostructured. Lack of substrate (materials) and surface (morphology) generalities has severely limited the breadth of practical applications of SERS to other materials widely used in energy, life and medical sciences and industries.
Focusing on how to break SERS limitations, we have developed several pioneered methodologies step by step as below:
1. Electrodeposition of transition metals onto roughened Au or Ag surfaces.
2. Electrochemical roughening of pure transition metal surfaces.
3. Chemical deposition of ultra-thin transition metal layers onto Au nanoparticles.
4. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS)
1. SERS on Transition Metal Surfaces
Three approaches have been developed to obtain SERS-active substrates of transition metals (VIII B): 1) Electrodeposition of transition metals onto roughened Au or Ag surfaces. The obtained substrates display high SERS activities, but the ultra-thin coating layers are non-uniform with pinholes. 2) Electrochemical roughening of pure transition metal surfaces. Raman signals can be directly obtain on pure transition metal electrodes for the first time, but the enhancement is not sufficient enough to investigate some important interfacial phenomena, such as water adsorption. 3) Chemical deposition of ultra-thin (ca. 1-10 atomic layers) transition metals onto Au nanoparticles. The substrates exhibit high SERS activity with uniform morphology and pinhole-free, but it is quite difficult to apply to many other materials, and impossible to work on the smooth surface.
In general, the total surface enhancement factors of transition metals can be substantially boosted up to 104–105, for Pt, Pd, Ru, Rh, Ni and Co, respectively. The obtained good-quality SERS spectra allow us to investigate some crucial interfacial phenomena for the first time, such as the adsorption of water or hydrogen with very small Raman cross-section, on several transition metals (e.g., Pt, Pd and Rh).
In 2010, we have developed a new operation mode, Shelled-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS), where Au or Ag nanoparticles are coated with ultra-thin shells (about 1 to 5 nm) of chemically inert SiO2, Al2O3 or MnO2 shells. This mode is different from the contact mode of conventional SERS and the non-contact mode of tip-enhanced Raman spectroscopy (TERS). The Au or Ag cores provides a large enhancement to the probed molecules nearby, while the ultra-thin inert shells isolate the Au or Ag nanoparticles from the ambient environment. The main virtue of such “smart dusts” is the easy preparation and application over surfaces of any material and any morphology, which has been demonstrated with the high quality Raman spectra of probed molecules obtained at various single-crystal surfaces, semiconductors and living cells.
SHINERS method has avoided long-standing limitations (inaccessibility or difficulty) of SERS for the precise characterization of various materials, surfaces and biological samples. The concept of shell-isolated-nanoparticle enhancement may also be applicable to a wider range of spectroscopies, such as infrared spectroscopy, sum frequency generation and fluorescence, etc..
3. Applications of SERS
SERS has a broad and diverse range of chemical applications from electrochemistry and catalysis, single-molecule detection, sensing and trapping, to solid-phase synthesis, to bioanalytical applications. Very recently, the emerging of portable Raman instruments open a new window for SERS applications: on-site fast detection with high sensitivity towards food &medical safety, environmental protection, and social & national security. We mainly focus on the application of SERS to food & medical safety, such as the pesticides in fresh foods or teas, the illegal additives in processed foods and healthy products.
From the preparation of SERS-active substrates, to methodology and theory, our group aims to expand the breadth and deepen the understanding of electrochemical SERS (EC-SERS) in surface science and material science.
EC-SERS on chemisorptions
For the EC-SERS characterization, our group has invented and developed diverse methods to prepare the electrochemically roughened or nanoparticles assembled film electrodes. Especially, the contributions to expand EC-SERS on transition metal (VIII B) surface of which the original SERS activity is rather limited. With the excellent SERS-active substrates, we have obtained high-quality electrochemical Raman signal of pyridine adsorbed on the coinage metal and transition metal surfaces. Concurrently, to improve our fundamental understanding of the electrode/electrolyte interface, we have successfully observed the first SERS (also the first Raman) signal of surface water on Pt-group metals.
Figure 1. a) SERS spectra of pyridine adsorbed on roughened Ag, Au, Cu and Pt electrodes at open circuit potential (left) and the peak potential (vs. SCE) of the ring breathing mode (right); b) SERS spectra of water adsorbed on Pt, Pd and Au at negative potentials in 0.1 M KClO4 (right top). The suggested models (right bottom) for the adsorbed water on different electrodes and the influence of potential on metal conduction electron are shown on the left. The suggested models (right bottom) for the adsorbed water on different electrodes and the influence of potential on metal conduction electron are presented.
EC-SERS on reaction processes and electrode kinetics
When combined with electrochemical techniques and theoretical calculation method, in-situ EC-Raman will shed the light on the in-depth understanding of reaction mechanisms in catalysis, electrochemistry, organic chemistry, etc. Take the reduction of benzyl chloride on silver electrode for instance, we carried out an in situ electrochemical surface-enhanced Raman spectroscopic study to characterize various surface species in different electrode potential regions. Corresponding DFT calculation reveals that the benzyl radical and its anionic derivate bonded on a silver electrode are the key intermediates, implying that the pathway could drastically differ from the outer sphere concerted electron reduction at inert electrodes.
Figure 2. a) CV of 5 mM PhCH2Cl in 0.1 M TEAP + CH3CN at a Ag electrode with different scan rates; b) Potential dependent SERS spectra of PhCH2Cl on a Ag electrode; DFT calculated Raman spectra of the possible solvated reaction intermediates: c) free benzyl radical, d) free benzyl anion, f) benzyl radical-Ag4 adduct, g) benzyl anion-Ag4 adduct. These are compared with e) the experimental SERS spectrum at -1.4 V vs SCE and h) a 1:5 superposition of the predicted spectra in f and g.
EC-SHINERS for in-situ monitoring reactions on single crystal surfaces
Single crystal surfaces are commonly preferred and used in surface science, because of their well-defined surface state and optic field. However, SERS is seriously limited to roughened or nanostructured surfaces. The electrooxidation processes play the crucial role in electrocatalysis investigations. Herein, electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) is utilized to in situ monitor the electrooxidation processes at Au(hkl) single crystal electrode surfaces. The experimental results are well correlated with our periodic density functional theory calculations and corroborate the long-standing speculation based on theoretical calculations in previous electrochemical studies. The presented in situ electrochemical SHINERS technique offers a unique way for a real-time investigation of an electrocatalytic reaction pathway at various well-defined noble metal surfaces.
Figure 3. a) Schematic diagram of EC-SHINERS at single crystal surface; b) The EC-SHINERS spectra of the electrooxidation processes of Au(111), Au(100), and Au(110) electrodes in 0.1 M NaClO4 (from top to bottom). The corresponding CVs are presented as well.
1. Tian, Z. Q. & Ren, B. Adsorption and reaction at electrochemical interfaces as probed by surface-enhanced Raman spectroscopy. Annu. Rev. Phys. Chem 55, 197–229 (2004).
2. Wu, D., Li, J., Ren, B. & Tian, Z. Electrochemical surface-enhanced Raman spectroscopy of nanostructures. Chem. Soc. Rev. 37, 1025-1041 (2008).
3. Wang, A. et al. In Situ Identification of Intermediates of Benzyl Chloride Reduction at a Silver Electrode by SERS Coupled with DFT Calculations. J. Am. Chem. Soc. 132, 9534–9536 (2010).
4. Li, J. et al. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature 464, 392-395 (2010).
5. Li, C. Y. et al. In Situ Monitoring of Electrooxidation Processes at Gold Single Crystal Surfaces Using Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. J. Am. Chem. Soc. 137, 7648-7651 (2015).
A concept to increase the efficiceny and selectivity of assembly processes
One important objective of molecular assembly research is to create highly complex functional chemical systems capable of responding, adapting, and evolving. Compared with living systems, the synthetic systems are still rather primitive and are far away from realizing those features. Nature is by far the most important source of inspiration for designing and creating such systems. Here, we summerize an alternative approach, inspired by catalysis, to examine and describe some molecular assembly processes.
A new term, “catassembly,” is suggested to refer to the increase in rate and control of a molecular assembly process. This term combines the words “catalysis” and “assembly,” and identifiably retains the Greek root of “cat” of catalysis. The corresponding verb is “catassemble”and the noun is “catassembler”, referring to the “helper” species. Catassembly in molecular assembly is a concept that is analogous to catalysis in chemical synthesis. (Figure 1)
We used some primitive examples of catassembly to present their distinct characteristics and their difference from template-assisted self-assembly (table 1).[1,2] These highly efficient assembly processes involve small molecules, large molecules and molecule-modified NPs and demonstrate the characteristics and superiority of catassembly. In some of these examples, the strategy of catassembly has apparently been applied unknowingly.
More importantly, because most efforts have focused on how to rationally design and synthesize molecular building blocks, and the creation of functional chemical systems relying on the self-assembly of these components, we wish to emphasize the seemingly missing yet critical consideration in this field: the design and utilization of molecular catassemblers for the construction of new functional chemical systems with significantly high efficiency and selectivity.
Epitaxy induced assembly: a new strategy to fabricate multi-scale composite materials
Nanoparticles (NPs) usually need further self-assembly or co-assembly with other materials to form composite functional materials for advanced uses. Moreover, the performance of the composite material is related not only to its own atomic or nanoscale structures but also to the mesoscale structures in which defects, interfaces, and non-equilibrium processes are more important and complex. Constructing composite materials with well-defined structures at the mesoscale provides rich opportunities to obtain novel functional materials by controllable hierarchical assembly. It is highly desirable to develop new methods to construct 2D NA composites by directly assembling NPs on the functional substrates, which will allow better control of the interface structures and reduction of mesoscale defects.
Inspired by atomic epitaxial growth, we propose an “epitaxy induced assembly” method to form two-dimensional nanoparticle arrays (2D NAs) directly onto desired materials. For layered materials (substrate), by functionalizing the NPs with one component of those materials while using the other components to tune the interaction between surface molecules on NPs and the substrate, NP “epitaxy induced assembly” can be achieved as an analogue of the atomic epitaxial growth. (Fig. 2)
As an illustration, we employ a series of surfactant-capped nanoparticles as the “artificial atoms” and layered hybrid perovskite (LHP) materials as the substrates and obtain 2D NAs in a large area with few defects. (Fig. 3) This method is universal for nanoparticles with different shapes, sizes, and compositions and for LHP substrates with different metallic cores. Raman spectroscopic and X-ray diffraction data support our hypothesis of epitaxial assembly. The novel method offers new insights into the controllable assembly of complex functional materials and may push the development of materials science at the mesoscale.
1. Wang, Y; Lin, HX; Chen, L; Ding, SY; Lei, ZC; Liu, DY; Cao, XY; Liang, HJ; Jiang, YB; Tian, ZQ, Chemical Society Reviews,43(1):399-411, 2014
2. Wang, Y; Lin, HX; Ding, SY; Liu, DY; Chen, L; Lei, ZC; Fan FR; Tian, ZQ, Science China Chemistry, 42(4): 525-547, 2012
3. Lin, H. X.; Chen, L.; Liu, D. Y.; Lei, Z. C.; Wang, Y.; Zheng, X. S.; Ren, B.; Xie, Z. X.; Stucky, G. D.; Tian, Z. Q. J. Am. Chem. Soc. 2015, 137, 2828.
Synthesis and Fabrication of Nanostructures
Monodisperse Single-crystal Metal Nanocubes
Metal nanocrystals with special shapes have attracted steadily growing interest due to unique properties and intriguing applications. In most of their potential applications, the quality and the structure of the surface of nanocrystals will undoubtedly play a very important role in determining their functions. Surface-enhanced Raman scattering (SERS) with its high surface sensitivity becomes a powerful technique in studying the optical property of metal nanocystals. Usually, the SERS activity strongly depends on the styles and shapes of metal nanocystals. In the past years, SERS substrate usually is the rough surface, which is difficult to develop the theoretical model based on the experimental data obtained from the ill-defined surfaces because of the heterogeneity in the surface structure. To cope with this difficulty, it is worthwhile to construct the atomic-scale well-defined nanostructure surface which has well-controlled shape, size and space between each other.
Recently, Our group have developed some simple methods in liquid-phase for synthesizing monodisperse single-crystalline Pd, Au, Ag and Au@Pdnanocubes. Noble metal nanocubes exhibit high SERS activity, which are explained by a theoretical calculation using three-dimensional finite difference time domain (FDTD) method. The size- and shape-controlled nanocrystals may be a promising material to bridge the gap between highly rough and single crystal surfaces, and hence allows us to obtain a deep insight into the SERS mechanism and make a mighty advance on understanding and application of nanocrystals.
Growth Mechanisms of Nanocrystals
To extend the shape-controlled growth of nanocrystals, it is highly desirable to develop new synthesis methods and get the deep insight of the growth mechanism. We propose that the exposed crystal faces can be simply tuned by controlling the supersaturation, and higher supersaturation will result in the formation of crystallites with higher surface-energy faces. We have successfully applied it for the growth of ionic (NaCl), molecular (TBPe), and metallic (Au, Pd) micro/nanocrystals with high-surface-energy faces. The above proposed strategy can be rationally designed to synthesize micro/nanocrystals with specific crystal faces and functionality toward specific applications.
Based on the systematic study on the heterogeneous growth mechanism, we have preliminarily proposed a general rule that the atomic radius, bond dissociation energy, and electronegativity of the core and shell metals play key roles in determining the conformal epitaxial layered growth mode. This rule would be help for designing and fabricating more complex nanostructures, such as multiple-shell nanostructures and metal-semiconductor nanocomposites.
The growth of nanoparticles will be affected by the external environment, we found that without centrifugation the Au nanooctahedron seeds grew into truncated octahedrons cuboctahedrons, nanocubes truncated along the  and  directions, and slightly truncated nanocubes. The mass transfer of Au monomers can be accelerated and the surface reaction step then plays a critical role in shape evolution.
Fabrication of Nanostructures by MEMS method
Nanostructures with gaps in tunneling regime exhibit great research interests because of their widely application in nano-electronics and molecular electronics, and their unique properties in plamonics and catalysis. Mechanically Controllable Break Junction (MCBJ) is an excellent technology to create gap and control the gap distance in tunneling regime precisely and in-situ.
Our group has developed two approaches for the MCBJ method. By using the Microelectromechanical Systems (MEMS) technologies, we firstly fabricate microelectrodes pairs on silicon chips. One of our approach is combining nanoelectrochemistry to electrodeposit interested metals to make the microelectrodes pair form suspended nano-restriction contact. The other one is using Electron-Beam Lithography to fabricate suspended nano-bridge on microelectrodes pair. By bending the chips using a pushing screw underneath, the nanostructures will be broken into nano-electrodes pairs with gaps. With a large reduction ratio, the screw’s sub-micrometer pushing makes gap separation sub-angstrom changes, which means the gap can be controlled precisely in tunneling regime.
The electrodes pair created by MCBJ can be stable, and the break and junction process can be repeated over ten thousands times, whilst the gap can be easily tuned to fit the target molecule length. Thus it is useful to fabricate and measure the electronic properties of Metal Quantum Point Contacts (MQPCs) and Metal-Molecule-Metal (MMM) junctions
As nanostructures show Surface Plasmonic Resonance (SPR) under visible laser illumination. There exists great enhanced electromagnetic field between the gap of nanostructures. By combining Surface-Enhanced Raman Scattering (SERS) technology, we are able to characterize the Raman and electronic properties of molecular junction simultaneously. Besides, as the laser provide a forceful stimuli, the molecular junction in the tunneling regime might also exhibit extraordinary phenomena. By the MCBJ method, the gap distance can be precisely controlled, which can be helpful to measure the nanostructure scattering in-situ, and investigate the relationship between SPR and SERS, even by applying the bias. On the same time, in order to stabilize the molecular junction and eliminate the influence of impurities, and do some low-temperature or temperature-adjustable experiments, we have designed and fabricate a Low-Temperature Ultra-High-Vacuum MCBJ-SERS combined system (LT-UHV-MCBJ-SERS).
Conduction electrons in metal or metal-like nanomaterials can be coherently excited by incident light to collectively oscillate at metal/dielectric interfaces. The collective oscillating mode of electrons and the nanomaterials that support are referred to as surface plasmons and plasmonic materials, respectively. There are two types of surface plasmons: (i) localized surface plasmons (LSP), in which coherent electrons oscillate around the NP surfaces or nanoscale crevices, and (ii) propagating surface plasmons (that is, surface plasmon polaritons (SPPs)), in which the coherent electrons oscillate as a longitudinal wave at extended metal surfaces. Nano-optics, especially the sub-field plasmonics is the study of light at nanoscale and the study of the interaction between light and nano-scale objects supporting surface plasmons.
Surface plasmon supports the characteristic extinction energy and strong electromagnetic field in the vicinity of the nanostructures, which has great importance for a wide variety of applications ranging from biosensors, spectroscopies and microscopy, optoelectronics and solar cells.
Our research interests in nano-optics and plasmonics are to rational design novel nano-devices for nano-spectroscopies, nano-sensor and nano-imaging based on the comprehensive understanding of the mechanism of plasmon-enhanced spectroscopies and its correlation to the plasmonic properties of nanostructures, by means of new theories, concepts and simulation methods. The relation of our plasmonic researches to other researches in our group could be described in Figure 1.
Plasmonic properties and SERS activity from transition metal nanostructures
1. SERS from transition-metal cauliflower-like structures prepared by electrochemical roughening methods
Figure 2. STM (a) and AFM (b) images of electrochemically roughened Pt and Rh electrodes, respectively. FDTD simulated electric field distribution for spherical (c) and cauliflower-like (d) Rh nanoparticles. The incident beam illuminates along the y direction with x-polarization.
2. SERS from gold@transition-metal core@shell nanostructures
Figure 3. Schematic illustration of the electromagnetic field distribution around Au@Pt core–shell nanoparticles under laser irradiation (left). The dependence of the electromagnetic field strength (normalized with the strength on the Pt surface) on the distance from the Pt shell is shown in the right plot, indicating a substantially strong field enhancement can still be obtained on the surface.
Plasmonic properties of a hybrid structures including plasmonic nanoparticles on probing material surfaces
1. The first and second generations of hotspots
The first generation of hotspots were generated from single nanostructures such as nanospheres and nanocubes (FIG. 4a,b), or nanorods freely suspended in a homogeneous medium. These hotspots exhibit moderate SERS-activity; however, some rationally designed single nanoparticles with sharp corners and/or with intra-particle gaps, such as Au and Ag nanostars, nanoflowers and mesocages exhibit much higher SERS-activity.
The second generation of SERS hotspots are generated from coupled nanostructures with controllable inter-particle nanogaps (for example, the NP dimers shown in FIG. 4c,d or oligomers and NP arrays shown in Fig. 4e,f) or inter-unit nanogaps in nano-patterned surfaces (FIG. 4g,h). Such hotspots exhibit excellent SERS activity. Because the average SERS intensities from coupled plasmonic nanostructures are typically 2–4 orders of magnitude greater than that from single nanostructures, they are more commonly used for trace-molecule detection. SERS hotspots from coupled nanostructures are extremely small (1–5 nm), but the Raman signals of probe molecules at the hotspots contribute significantly to the total Raman signal.
Figure 4. First- and second-generation SERS hotspots for trace-molecule detection.
2. The third-generation hotspots
In general, first- and second-generation hotspots are not well suited for surface analysis of many materials. For example, widely used materials such as silicon wafer or ceramics cannot be squeezed into the extremely tiny and narrow regions of the hotspots formed by coupled nanostructures, such as those shown in FIG. 4c-h. Therefore, it is highly desirable to design plasmonic nanostructures that can have hotspots right on the surface of the materials to be probed. The goal can be realized by taking into account of the effect of the materials to be probed in the design of the plasmonic nanostructures. Because, the localized SPR response and local EM field distribution not only depends on the shape, size of plasmonic nanostructures, but also depends sensitively on the dielectric properties of probe materials close to the plasmonic nanostructures. The hotspot generated from the hybrid structures consisting of plasmonic nanostructures and the probe materials can be considered third-generation hotspots. Representative examples of such hybrid structures are shown in FIG. 5.
Figure 5. Third-generation SERS hotspots for surface analysis of materials.
3. The plasmonic nature of SHINERS from general materials
SHINERS is a representative technique supporting third-generation hotspots. The working principle of SHINERS for gold, silver, and more general metal single-crystal surfaces have been extensively studied in our group[4-8].
Figure 6. Fano Resonance nature of a Au@SiO2 nanoparticle dimer on a flat gold surface. Coupled dipole of Nanoparticles induce the imaginary dipole on the metal or dielectric surfaces, to form a magnetic-dipole-like mode. It is the plasmonic dark mode with less irradiative efficiency, but with strong near field.
1. Tian, Z.-Q. et al. Surface-enhanced raman scattering from transition metals with special surface morphology and nanoparticle shape. Faraday Discuss. 132, 159-170 (2006).
2. Tian, Z.-Q., Ren, B., Li, J.-F. & Yang, Z.-L. Expanding generality of surface-enhanced raman spectroscopy with borrowing sers activity strategy. Chem. Commun., 3514-3534 (2007).
3. Ding, S.-Y. et al. Nanostructure-based plasmon-enhanced raman spectroscopy for surface analysis of materials. Nature Reviews Materials 1, 16021 (2016).
4. Li, J.-F. et al. Extraordinary enhancement of raman scattering from pyridine on single crystal au and pt electrodes by shell-isolated au nanoparticles. J. Am. Chem. Soc. 133, 15922-15925 (2011).
5. Li, J.-F. et al. Shell-isolated nanoparticle-enhanced raman spectroscopy. Nature 464, 392-395 (2010).
6. Li, C.-Y. et al. “Smart” ag nanostructures for plasmon-enhanced spectroscopies. J. Am. Chem. Soc. 137, 13784-13787 (2015).
7. Li, J.-F., Anema, J.R., Wandlowski, T. & Tian, Z.-Q. Dielectric shell isolated and graphene shell isolated nanoparticle enhanced raman spectroscopies and their applications. Chem. Soc. Rev. (2015).
8. Ding, S.-Y., Yi, J., Li, J.-F. & Tian, Z.-Q. A theoretical and experimental approach to shell-isolated nanoparticle-enhanced raman spectroscopy of single-crystal electrodes. Surf. Sci. 631, 73-80 (2015).
9. Chen, S. How to Light Special Hot Spots in Multiparticle-Film Configurations. ACS Nano 10, 581-587 (2016).