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1.
J Phys Chem Lett ; : 1947-1953, 2020 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-32079400

RESUMO

Plasmonic optical antennas (POAs), often constructed from gold or silver nanostructures, can enhance the radiation efficiency of emitters coupled to POAs and are applied in surface-enhanced Raman spectroscopy (SERS) and light-emitting devices. Over the past four decades, radiation enhancement factors (REFs) of POA-emitter systems were considered to be difficult to calculate directly and have been predicted indirectly and approximately, assuming POAs are illuminated by electromagnetic plane waves without emitters. The validity of this approximation remains a significant open problem in SERS theory. Herein, we develop a method based on the rigorous optical reciprocity theorem for accurately calculating the REFs of emitters in nanoparticle-substrate nanogaps for single-molecule SERS and scanning probe-substrate nanogaps for tip-enhanced Raman spectroscopy. We show that the validity of the plane wave approximation breaks down if high-order plasmonic modes are excited. The as-developed method paves the way toward designing high-REF POA nanostructures for luminescence-related devices.

3.
ACS Nano ; 12(12): 12701-12712, 2018 Dec 26.
Artigo em Inglês | MEDLINE | ID: mdl-30543280

RESUMO

The fabrication of thin films comprising ordered nanowire assemblies with emerging, precisely defined properties and adjustable functionalities enables highly integrated technologies in the fields of microelectronics and micro system technology, as well as for efficient power generation, storage, and utilization. Shear force, theoretically, is deemed the most promising method for obtaining in-plane, uniaxial thin films comprising nanowires. The success depends largely on the assembly process, and uniform structural control throughout multiple length scales can be achieved only if a rational strategy is executed. Here, we report that in shearing processes dopants such as lyotropic cellulose nanorods can give rise to the uniaxial alignment of V2O5· nH2O nanowires. Our systematic study indicates that this finding, namely, the nanocombing effect, can be a general principle for the continuous production of uniaxial thin films comprising densely packed nanowires varying in chemical composition and aspect ratios. Conversion of the V2O5· nH2O constituents via in situ oxidative polymerization leads to in-plane, uniaxial polyaniline (PANI) thin films with anisotropic electric and optical properties, which are otherwise difficult to fabricate due to the poor processability of PANI. The uniaxial PANI thin films can be utilized to fabricate flexible gas sensors for distinguishing various analytes, including similar homologues such as methanol and ethanol. We expect the methodology to be applied to a broad spectrum of synthetic and biogenic nanowires for the integration of their collective properties in high-performance electronic devices.

4.
ACS Nano ; 12(11): 11229-11235, 2018 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-30335940

RESUMO

One of the promising approaches to meet the urgent demand for further device miniaturization is to create functional devices using single molecules. Although various single-molecule electronic devices have been demonstrated recently, single-molecule optical devices which use external stimulations to control the optical response of a single molecule have rarely been reported. Here, we propose and demonstrate a field-effect Raman scattering (FERS) device with a single molecule, an optical counterpart to field-effect transistors (a key component of modern electronics). With our devices, the gap size between electrodes can be precisely adjusted at subangstrom accuracy to form single molecular junctions as well as to reach the maximum performance of Raman scattering via plasmonic enhancement. Based on this maximum performance, we demonstrated that the intensity of Raman scattering can be further enhanced by an additional ∼40% if the orbitals of the molecules bridged two electrodes were shifted by a gating voltage. This finding not only provides a method to increase the sensitivity of Raman scattering beyond the limit of plasmonic enhancement, but also makes it feasible to realize addressable functional FERS devices with a gate electrode array.

5.
Chem Commun (Camb) ; 54(1): 10-25, 2017 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-29139483

RESUMO

In this feature article, we discuss in detail developmental bottleneck issues in Raman spectroscopy in its early stages and surface-enhanced Raman spectroscopy (SERS) in the past four decades. We divide SERS research into two different directions with different targets. Fundamental research is extending the limits of SERS to single-molecule, sub-nanometer resolution and femtosecond processes. In contrast, practical research is expanding the range of applications with the aim of providing versatile analytical tools for surface, materials, life, environmental, forensic and food sciences and also commercial instruments for use in daily life. In the second direction there have continually been many complex bottlenecks to be overcome. We attempt to enumerate the key issues in detail and also describe the achievements made to overcome the bottlenecks. In the last, but not least important part, we discuss the remaining bottlenecks and possible strategies for overcoming them to enable SERS to be an even more powerful and versatile technique.

6.
Faraday Discuss ; 205: 457-468, 2017 12 04.
Artigo em Inglês | MEDLINE | ID: mdl-28885636

RESUMO

After surface-enhanced Raman spectroscopy (SERS) was initiated over four decades ago, its practical application seems to be far behind the fundamental research that has made tremendous progress. SERS as a highly sensitive technique has not been widely adopted by the materials science and surface science communities or in the market of analytical instruments. In this discussion, we first classify the previous approaches along this direction over the past four decades and divide them into three strategies. Based on our recent theoretical and experimental approaches, we discuss in more detail the third strategy related to shell-isolated nanostructures. It can significantly expand the SERS study on nontraditional SERS-active (i.e. weakly SERS-active) materials (e.g. Pt, Ni, Fe, etc.) and even SERS-inactive materials (e.g. Si and Al2O3). We then focus on a single shell-isolated nanoparticle and how to controllably locate the strong electromagnetic field just at the probe surface of various materials. The use of side illumination at a high incident angle and/or nanocubes can further enhance the Raman signal by one to two orders of magnitude, which could be helpful for quantitative studies for surface science, heterogeneous catalysis, and soft matter science.

7.
Chem Soc Rev ; 46(13): 4042-4076, 2017 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-28660954

RESUMO

Surface-enhanced Raman spectroscopy (SERS) and related spectroscopies are powered primarily by the concentration of the electromagnetic (EM) fields associated with light in or near appropriately nanostructured electrically-conducting materials, most prominently, but not exclusively high-conductivity metals such as silver and gold. This field concentration takes place on account of the excitation of surface-plasmon (SP) resonances in the nanostructured conductor. Optimizing nanostructures for SERS, therefore, implies optimizing the ability of plasmonic nanostructures to concentrate EM optical fields at locations where molecules of interest reside, and to enhance the radiation efficiency of the oscillating dipoles associated with these molecules and nanostructures. This review summarizes the development of theories over the past four decades pertinent to SERS, especially those contributing to our current understanding of SP-related SERS. Special emphasis is given to the salient strategies and theoretical approaches for optimizing nanostructures with hotspots as efficient EM near-field concentrating and far-field radiating substrates for SERS. A simple model is described in terms of which the upper limit of the SERS enhancement can be estimated. Several experimental strategies that may allow one to approach, or possibly exceed this limit, such as cascading the enhancement of the local and radiated EM field by the multiscale EM coupling of hierarchical structures, and generating hotspots by hybridizing an antenna mode with a plasmonic waveguide cavity mode, which would result in an increased local field enhancement, are discussed. Aiming to significantly broaden the application of SERS to other fields, and especially to material science, we consider hybrid structures of plasmonic nanostructures and other material phases and strategies for producing strong local EM fields at desired locations in such hybrid structures. In this vein, we consider some of the numerical strategies for simulating the optical properties and consequential SERS performance of particle-on-substrate systems that might guide the design of SERS-active systems. Finally, some current theoretical attempts are briefly discussed for unifying EM and non-EM contribution to SERS.

8.
Chem Rev ; 117(7): 5002-5069, 2017 Apr 12.
Artigo em Inglês | MEDLINE | ID: mdl-28271881

RESUMO

Core-shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core-shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surface-enhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core-shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core-shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core-shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core-shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core-shell nanomaterials with different functionalities for SERS and other surface-enhanced spectroscopies.

9.
J Am Chem Soc ; 137(18): 5923-9, 2015 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-25894840

RESUMO

We report an electrochemically assisted jump-to-contact scanning tunneling microscopy (STM) break junction approach to create reproducible and well-defined single-molecule spintronic junctions. The STM break junction is equipped with an external magnetic field either parallel or perpendicular to the electron transport direction. The conductance of Fe-terephthalic acid (TPA)-Fe single-molecule junctions is measured and a giant single-molecule tunneling anisotropic magnetoresistance (T-AMR) up to 53% is observed at room temperature. Theoretical calculations based on first-principles quantum simulations show that the observed AMR of Fe-TPA-Fe junctions originates from electronic coupling at the TPA-Fe interfaces modified by the magnetic orientation of the Fe electrodes with respect to the direction of current flow. The present study highlights new opportunities for obtaining detailed understanding of mechanisms of charge and spin transport in molecular junctions and the role of interfaces in determining the MR of single-molecule junctions.

10.
Chem Soc Rev ; 43(1): 399-411, 2014 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-24061231

RESUMO

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 from realizing those features. Nature is by far the most important source of inspiration for designing and creating such systems. In this critical review, we summarize 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 the rate and control of a molecular assembly process. This term combines the words "catalysis" and "assembly," and identifiably retains the Greek root "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. After using several examples to illustrate the characteristics of catassembly, we discuss future methodological and theoretical developments. We also emphasize the significance of the synergy between chemical synthesis and molecular assembly, especially for hierarchical assembly systems. Because most efforts in the field of molecular assembly have been devoted to the design and synthesis of molecular building blocks, we wish to stress the apparently missing yet critical link to complex chemical systems, i.e., the design and utilization of molecular catassemblers to facilitate the formation of functional molecular assemblies from building blocks with high efficiency and selectivity. This rational control and accelerated method will promote the systems chemistry approach, and may expand the spectrum of molecular assembly from basic science to applications.

11.
Chemphyschem ; 14(10): 2217-24, 2013 Jul 22.
Artigo em Inglês | MEDLINE | ID: mdl-23824871

RESUMO

Herein, we employ Ag@TiO2 core-shell nanoparticles for surface-enhanced Raman scattering (SERS) investigations of TiO2-N719 dye interfaces. In situ electrochemical SERS investigations of the Ag@TiO2-N719 interaction are systematically carried out under a series of electrode-potential controls. By comparing the potential dependence of resonant and pre-resonant SERS spectra recorded with different laser excitations, bidentate carboxylate linkage is considered to be involved in N719 adsorption on TiO2. Meanwhile, SCN ligand shows obvious interactions with TiO2, and their role in the adsorption and orientation of N719 on TiO2 should not be underestimated. The in situ SERS spectra of Ag@TiO2 show a clear bell-shaped intensity-potential relation for the major bands of N719. A molecule-to-TiO2 charge-transfer resonance is tentatively attributed to account for such a phenomenon. Under the influence of such a charge-transfer resonance, valuable information about the N719-TiO2 interaction as well as the intramolecular deformation of N719 is obtained.


Assuntos
Corantes/química , Nanopartículas/química , Compostos Organometálicos/química , Prata/química , Tiocianatos/química , Titânio/química , Ligantes , Tamanho da Partícula , Análise Espectral Raman , Propriedades de Superfície
12.
Chem Commun (Camb) ; 48(59): 7353-5, 2012 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-22713905

RESUMO

A comparative study of gold nanoparticles (Au NPs) growth employing cetyltrimethylammonium bromide (CTAB) adsorbent was performed. Au nanooctahedrons transformed into slightly truncated nanocubes without centrifugation, whereas they transformed into nanocubes with centrifugation. Our results indicate that the mass transfer of Au monomers can influence the shape evolution of NPs.

13.
Chem Commun (Camb) ; 48(41): 4962-4, 2012 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-22506270

RESUMO

Aiming to solve the problem of simulation of the potential dependent surface Raman spectra of anion containing surface complexes on electrodes, we developed a new simulation model by adding different cations (Li(+), Na(+), K(+), Rb(+) or Cs(+)) attached to the bottom layer of a large metallic cluster while the surface complex sits on the top layer.

14.
J Am Chem Soc ; 133(40): 15922-5, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21899270

RESUMO

We used shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to systematically study the adsorption of pyridine on low-index Au(hkl) and Pt(hkl) single crystal electrodes. Our gold-core silica-shell nanoparticles (Au@SiO(2) NPs) boost the intensity of Raman scattering from molecules adsorbed on atomically flat surfaces. The average enhancement factor reaches 10(6) for Au(110) and 10(5) for Pt(110), which is comparable to or even greater than that obtained for bare gold NPs (a widely adopted SERS substrate). 3D-FDTD simulations reveal that this large enhancement is due to the transfer of the "hotspots" from NP-NP gaps to NP-surface gaps. We also found that the SHINERS intensity strongly depends on the surface crystallographic orientation, with differences up to a factor of 30. Periodic DFT calculations and theoretical analysis of dielectric functions indicate that this facet-dependence is predominantly governed by the dielectric property of the surface. The results presented in this work may open up new approaches for the characterization of adsorbates and reaction pathways on a wide range of smooth surfaces.

15.
Nat Commun ; 2: 305, 2011.
Artigo em Inglês | MEDLINE | ID: mdl-21556059

RESUMO

The conductance of single-molecule junctions may be governed by the structure of the molecule in the gap or by the way it bonds with the leads, and the information contained in a Raman spectrum is ideal for examining both. Here we demonstrate that molecule-to-surface bonding may be characterized during electron transport by 'fishing-mode' tip-enhanced Raman spectroscopy (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature. Density functional theory calculations reveal that the most significant spectral change seen for a gold-4,4'-bipyridine-gold junction results from the deformation of the pyridine ring in contact with the drain electrode at high voltage, and these calculations suggest that a stronger bonding interaction between the molecule and the drain may account for the nonlinear dependence of conductance on bias voltage. FM-TERS will lead to a better understanding of electron-transport processes in molecular junctions.


Assuntos
Compostos Organoáuricos/análise , Análise Espectral Raman/métodos , Condutividade Elétrica , Eletrodos , Campos Eletromagnéticos , Fenômenos Eletromagnéticos , Transporte de Elétrons , Ouro/química , Microscopia de Tunelamento , Estrutura Molecular , Compostos Organoáuricos/química , Piridinas , Propriedades de Superfície
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