Ultraviolet tip‐enhanced nanoscale Raman imaging

We have constructed an ultraviolet (UV)‐apertureless near‐field scanning optical microscope‐Raman spectroscopy system by using an aluminum tip for the simultaneous measurement of topography and Raman scattering of nanomaterials with high spatial resolution. The topography, Rayleigh scattering image, and tip‐enhanced Raman scattering image of the carbon nanotube film showed that a spatial resolution of around 19 nm was achieved. This spatial resolution of UV‐Raman mapping image exceeds that of previous approaches, which have several hundred nanometers of spatial resolution. Copyright © 2012 John Wiley & Sons, Ltd.     

Nano‐imaging through tip‐enhanced Raman spectroscopy: Stepping beyond the classical limits

The spatial resolution in optical imaging is restricted by so‐called diffraction limit, which prevents it to be better than about half of the wavelength of the probing light. Tip‐enhanced Raman spectroscopy (TERS), which is based on the SPP‐induced plasmonic enhancement and confinement of light near a metallic nanostructure, can however, overcome this barrier and produce optical images far beyond the diffraction limit. Here in this article, the basic phenomenon involved in TERS is reviewed, and the high spatial resolution achieved in optical imaging through this technique is discussed. Further, it is shown that when TERS is combined with some other physical phenomena, the spatial resolution can be dramatically improved. Particularly, by including tip‐applied extremely localized pressure in TERS process, it has been demonstrated that a spatial resolution as high as 4 nm could be achieved.     

Deep‐UV tip‐enhanced Raman scattering

We report for the first time the tip‐enhancement of resonance Raman scattering using deep ultraviolet (DUV) excitation wavelength. The tip‐enhancement was successfully demonstrated with an aluminum‐coated silicon tip that acts as a plasmonic material in DUV wavelengths. Both the crystal violet and adenine molecules, which were used as test samples, show electronic resonance at the 266‐nm excitation used in the experiments. With results demonstrated here, molecular analysis and imaging with nanoscale spatial resolution in DUV resonance Raman spectroscopy can be realized using the tip‐enhancement effect. Copyright © 2009 John Wiley & Sons, Ltd.     

Exploring the origin of tip‐enhanced Raman scattering; preparation of efficient TERS probes with high yield

Tip‐enhanced Raman scattering (TERS) spectroscopy is a promising technique for nanoscale chemical analysis. However, there are several challenges preventing widespread application of this technology, including reproducible fabrication of efficient TERS probes. These problems reflect a lack of clear understanding of the origins of, and the parameters influencing TERS. It is believed that the coating characteristics at the apex of the tip have a major effect on the near‐field optical enhancement and thus the TERS activity of a metalized probe. Here we show that the aspect ratio of the tip can play a significant role in the efficiency of TERS probes. We argue that the electrostatic field arising from the lightning‐rod effect has a substantial role in the observed TERS effect. This argument is supported by ‘edge‐enhanced Raman scattering’ which is shown for a noble metal film. Furthermore, it is reported that an associated tip‐surface‐enhanced Raman scattering effect can be achieved by using a TERS‐inactive metalized probe on a surface‐enhanced Raman spectroscopy‐inactive roughened surface. This observation can be explained by an interparticle enhancement of the electromagnetic field. Copyright © 2011 John Wiley & Sons, Ltd.     

Mapping the near fields of plasmonic nanoantennas by scattering‐type scanning near‐field optical microscopy

Near‐field optical microscopy techniques provide information on the amplitude and phase of local fields in samples of interest in nanooptics. However, the information on the near field is typically obtained by converting it into propagating far fields where the signal is detected. This is the case, for instance, in polarization‐resolved scattering‐type scanning near‐field optical microscopy (s‐SNOM), where a sharp dielectric tip scatters the local near field off the antenna to the far field. Up to now, basic models have interpreted S‐ and P‐polarized maps obtained in s‐SNOM as directly proportional to the in‐plane ( or ) and out‐of‐plane () near‐field components of the antenna, respectively, at the position of the probing tip. Here, a novel model that includes the multiple‐scattering process of the probing tip and the nanoantenna is developed, with use of the reciprocity theorem of electromagnetism. This novel theoretical framework provides new insights into the interpretation of s‐SNOM near‐field maps: the model reveals that the fields detected by polarization‐resolved interferometric s‐SNOM do not correlate with a single component of the local near field, but rather with a complex combination of the different local near‐field components at each point (, and ). Furthermore, depending on the detection scheme (S‐ or P‐polarization), a different scaling of the scattered fields as a function of the local near‐field enhancement is obtained. The theoretical findings are corroborated by s‐SNOM experiments which map the near field of linear and gap plasmonic antennas. This new interpretation of nanoantenna s‐SNOM maps as a complex‐valued combination of vectorial local near fields is crucial to correctly understand scattering‐type near‐field microscopy measurements as well as to interpret the signals obtained in field‐enhanced spectroscopy.

     

A matter of scale: from far‐field microscopy to near‐field nanoscopy

Vibrational spectroscopy is a powerful analytical tool which provides chemical information about a sample without a priori knowledge. By combining vibrational spectroscopy with different microscopic techniques, scientists can visualize and characterize the chemical composition of a sample on length scales which cover many orders of magnitude; from far‐field radiation used in microwave astronomy and Fourier transform infrared microscopy, to near‐field scattering used in tip‐enhanced Raman spectroscopy and scanning near‐field optical or infrared microscopy. Here, various modern chemical mapping techniques are reviewed and their advantages and disadvantages are discussed. Also, a basic theoretical background is provided for each technique along with several illustrative examples.     

Plasmon‐enhanced Raman scattering by suspended carbon nanotubes

We report plasmon‐enhanced Raman scattering of the order of 10³ by a metallic carbon nanotube partially suspended inside a near‐field cavity. The tube is part of a small bundle, and is interfaced with an Au nanodisc dimer using a recently developed assembly scheme based on dielectrophoretic deposition. Spatially resolved Raman measurements with two excitation wavelengths and two orthogonal polarizations confirm that the enhancement arises from a 65 nm long suspended tube segment. We show that the orientation of the tube inside the cavity can be as effective for generating enhancement as placing the nanotube precisely in a plasmonic hotspot. Position and shape of the G‐peak show that the suspended part of the tube is free of strain and doped with a Fermi energy shift ≤40 meV. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Tip‐based plasmonics: squeezing light with metallic nanoprobes

Nanofabricated metallic tips are at the core of important research in single‐molecule imaging, near‐field scanning optical microscopy, tip‐enhanced Raman spectroscopy, as well as potential commercial applications such as heat‐assisted magnetic recording. While challenging to fabricate, much progress has been made towards the reliable production of extremely sharp (10 nm) metallic probes. In this review, the various factors that go into the design of metallic tips, their fabrication, packaging and system integration, characterization, passivation, and eventual use are discussed. Fabrication challenges, implementation issues, optical excitation schemes, and various current and emerging applications are also discussed. For the rapidly emerging fields of plasmonics and nanophotonics, the use of sharp metallic tips has generated significant scientific progress and will play an integral role well into the future.     

The Enhanced Second‐Harmonic Generation Based on Magnetic‐Lorentz‐Force Effect

Electromagnetic coupling generates a second‐harmonic signal via the magnetic component of Lorentz force of free electrons. In this work, the enhancement of artificial second‐harmonic generation (SHG) from plasmonic metamaterial with near‐field electromagnetic coupling is theoretically investigated by performing numerical simulations in both frequency and time domains. Simulation results demonstrate that a small gap between the near‐field electric dipole and magnetic resonators could increase the electromagnetic coupling and the SHG signal. In addition, the longer the near‐field electric dipole resonator, the stronger the SHG signal emits. By tuning the geometrical parameters, it has been verified that near‐field electromagnetic coupling results in enhancement of magnetic resonance, thereby improving the SHG signal by more than 3.4 times. This research paves a way toward the design of artificial nonlinearity with metamaterials.     

Aperture based Raman spectroscopy on SiGe film structures with high spatial resolution

We have investigated silicon–germanium (SiGe) line structures employing metallic apertures in combination with Raman spectroscopy to obtain high‐spatial strain resolution below the diffraction limit. The apertures were cut into specifically shaped electrochemically etched tungsten tips, which were adjusted within the Raman laser beam on the sample surface by a tuning fork atomic force microscope. With this setup, line structures on patterned SiGe films with a center‐to‐center distance down to 200 nm were resolved in the Raman scans, evidently indicating a resolution clearly below the far‐field Raman resolution of about 600 nm for the used instrument. This setup allows improved local strain analysis by Raman spectroscopy and shows potential for further near‐field Raman applications. Copyright © 2008 John Wiley & Sons, Ltd.     

Tip‐enhanced Raman spectroscopy using single‐crystalline Ag nanowire as tip

We report the surface‐enhanced Raman scattering (SERS) effect from the apex of single‐crystalline Ag nanowires (NWs). We also fabricated tip‐enhanced Raman spectroscopy (TERS) tips by attaching individual Ag NWs to W wires by using the alternating current dielectrophoresis (AC‐DEP) method. The single‐crystalline Ag NW tips could overcome many of the shortcomings of conventional TERS tips. Most importantly, the results obtained from TERS using single‐crystalline metal NWs are very reproducible, and the tips are also reusable. This development represents a significant progress in making TERS a reliable optical characterization technique with nanometer spatial resolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication of NIR‐Excitable SERS‐Active Composite Particles Composed of Densely Packed Au Nanoparticles on Polymer Microparticles

A simple fabrication method is demonstrated for surface‐enhanced Raman scattering (SERS)‐active plasmonic nanoballs, which consisted of Au nanoparticles (NPs) and core–shell polystyrene and amino‐terminated poly(butadiene) particles, by heterocoagulation and Au NP diffusion. The amount of Au NPs introduced into the core–shell particles increases with the concentration of Au NPs added to the aqueous dispersion of the core–shell particles. When the amount of Au NPs increases, closely packed, three‐dimensionally arranged and close‐packed Au NPs arrays are formed in the shells. Strong SERS signals from para‐mercaptophenol adsorbed onto composite particles with multilayered Au NPs arrays are obtained by near‐infrared (NIR) light illumination.     

Two‐Dimensionally Ordered Plasmonic and Magnetic Nanostructures on Transferable Electron‐Transparent Substrates

Discovery of new plasmonic behaviors from nanostructured materials can be greatly accelerated by the ability to prepare and characterize their near‐field behaviors with high resolution in a rapid manner. Here, an efficient and cost‐effective way is reported to make 2D periodic nanostructures on electron‐transparent substrates for rapid characterization by transmission electron microscopy. By combining nanosphere lithography with a substrate float‐off technique, large areas of electron‐transparent periodic nanostructures can be achieved. For this study, the synthesis of plasmonic nanostructures of Ag, magnetic nanostructures of Co, and bimetallic nanostructures of Ag–Co are investigated. Characterization of the materials by a combination of transmission electron microscopy, far‐field optical spectroscopy, and magnetization measurements reveals that this new approach can yield useful nanostructures on transparent, flexible, and transferable substrates with desirable plasmonic and/or magnetic properties.     

Raman spectra of nitrogen‐containing organic compounds obtained using a portable instrument at −15 °C at 2860 m above sea level

Well‐resolved Raman spectra of samples of nitrogen‐containing compounds were detected using a portable Raman instrument (Ahura First Defender XL) outdoors at a low ambient temperature of −15 °C and at an altitude of 2860 m (Pitztall, Austria). The portable Raman spectrometer tested here is equipped with a 785‐nm diode laser and a fixed frontal probe. Solid form of formamide, urea, 3‐methylpyridine, aniline, indene, 1‐(2‐aminoethyl)piperazine, indoline and benzofuran were detected unambiguously under high‐mountain field conditions. The main Raman features (strong, medium and partially weak bands) were observed at the correct wavenumber positions (with a spectral resolution 7–10 cm⁻¹) in the wavenumber range 200–1600 cm⁻¹. The results obtained demonstrate the possibility of employing a miniaturised Raman spectrometer as a key instrument for investigating the presence of nitrogen‐containing organic compounds and biomolecules outdoors under low temperature conditions. Within the payload designed by European Space Agency (ESA) and National Aeronautics and Space Administration (NASA) for future missions, focussing not only on Mars, Raman spectroscopy represents an important instrumentation for the detection of organic nitrogen‐containing compounds relevant to life detection on planetary surfaces or near sub‐surfaces. Copyright © 2009 John Wiley & Sons, Ltd.     

Stamping plasmonic nanoarrays on SERS‐supporting platforms

The dielectric property of a nanoparticle‐supporting film has recently garnered attention in the fabrication of plasmonic surfaces. A few studies have shown that the localized surface plasmon resonance (LSPR), and hence surface‐enhanced Raman scattering (SERS), strongly depends on the substrate refractive index. In order to create higher efficiency SERS‐active surfaces, it is therefore necessary to consider the substrate property along with nanoparticle morphology. However, due to certain limitations of conventional lithography, it is often not feasible to create well‐defined plasmonic nanoarrays on a substrate of interest. Here, an additive nanofabrication technique, i.e., nanotransfer printing (nTP), is implemented to integrate electron beam lithography (EBL) defined high‐aspect‐ratio nanofeatures on a variety of SERS‐supporting surfaces. With the aid of suitable surface chemistries, a wide range of plasmonic particles were successfully integrated on surfaces of three physically and chemically distinct dielectric materials, namely, polydimethyl siloxane (PDMS), SU‐8 photoresist, and glass surfaces, using silicon‐based relief pillars. These nTP‐created metal nanoparticles strongly amplify the Raman signal and complement the selection of suitable substrates for better SERS enhancement. Our experimental observations are also supported by theoretical calculations. The implementation of nTP to stamp out metal nanoparticles on a multitude conventional/unconventional substrates has novel applications in designing in‐built plasmonic microanalytical devices for SERS sensing and other related photonic studies. Copyright © 2011 John Wiley & Sons, Ltd.     

Immune recognition construct plasmonic dimer for SERS‐based bioassay

We have first time demonstrated the construction of a plasmonic gold dimer model for bioassays based on immune recognition with surface‐enhanced Raman scattering (SERS). To induce a strong plasmonic coupling effect, a dimer of gold nanoparticles (NPs) with a Raman label located between adjacent NPs is assembled through specific recognition in biological systems. One promising application for this model is the provision of a new type of in situ self‐calibrated and reliable SERS platform where biotinylated molecules can selectively be trapped by streptavidin and placed in the gap enhanced plasmonic field, which may enable the development of powerful, biospecific recognition‐based SERS assays. The capabilities of the dimeric constructions for analytical applications were demonstrated through the use of the SERS technique to detect biotin at very low concentrations. Additionally, the spatial SERS radiation for the gold dimer assembled on the silicon slide was simulated using the finite‐difference time‐domain method; this simulation demonstrated the distribution of the electric field as well as the utility of the proposed system, thereby introducing potential uses of bio‐specific recognition as well as opportunities for the construction of plasmonically coupled nanostructures and bioassay applications. Copyright © 2013 John Wiley & Sons, Ltd.     

Confocal full‐field X‐ray microscope for novel three‐dimensional X‐ray imaging

A confocal full‐field X‐ray microscope has been developed for use as a novel three‐dimensional X‐ray imaging method. The system consists of an X‐ray illuminating `sheet‐beam' whose beam shape is micrified only in one dimension, and an X‐ray full‐field microscope whose optical axis is normal to the illuminating sheet beam. An arbitral cross‐sectional region of the object is irradiated by the sheet‐beam, and secondary X‐ray emission such as fluorescent X‐rays from this region is imaged simultaneously using the full‐field microscope. This system enables a virtual sliced image of a specimen to be obtained as a two‐dimensional magnified image, and three‐dimensional observation is available only by a linear translation of the object along the optical axis of the full‐field microscope. A feasibility test has been carried out at beamline 37XU of SPring‐8. Observation of the three‐dimensional distribution of metallic inclusions in an artificial diamond was performed.     

Raman micro‐spectroscopy for quantitative thickness measurement of nanometer thin polymer films

The sensitivity of far‐field Raman micro‐spectroscopy was investigated to determine quantitatively the actual thickness of organic thin films. It is shown that the thickness of organic films can be quantitatively determined down to 3 nm with an error margin of 20% and down to 1.5 nm with an error margin of 100%. Raman imaging of thin‐film surfaces with a far‐field optical microscope establishes the distribution of a polymer with a lateral resolution of ~400 nm and the homogeneity of the film. Raman images are presented for spin‐coated thin films of polysulfone (PSU) with average thicknesses between 3 and 50 nm. In films with an average thickness of 43 nm, the variation in thickness was around 5% for PSU. In films with an average thickness of 3 nm for PSU, the detected thickness variation was 100%. Raman imaging was performed in minutes for a surface area of 900 µm². The results illustrate the ability of far‐field Raman microscopy as a sensitive method to quantitatively determine the thickness of thin films down to the nanometer range. Copyright © 2015 John Wiley & Sons, Ltd.     

Micro‐Raman analysis and finite‐element modeling of 3 C‐SiC microstructures

Micro‐ and nano‐electromechanical systems (MEMS and NEMS) fabricated in 3 C‐SiC are receiving particular attention thanks to the material physical properties: its wide band gap (2.3 eV), its ability to operate at high temperatures, its mechanical strength and its inertness to the exposure in corrosive environments. However, high residual stress (which is normally generated during the hetero‐epitaxial growth process) makes the use of 3 C‐SiC in Si‐based MEMS fabrication techniques very limited leading to a failure of micro‐machined/sensor structures. In this paper, micro‐Raman characterizations and finite‐element modeling (FEM) of microstructures realized on poly and single‐crystal (100) 3 C‐SiC/Si films are performed. Transverse optical (TO) Raman mode analysis reveals the stress relaxation on the free standing structure (796.5 cm⁻¹) respect to the stressed unreleased region (795.7 cm⁻¹). Also, microstructures as cantilever, bridge and planar rotating probe show an intense stress field located around the undercut region. Here, the TO Raman mode undergoes an intense shift, up to 2 cm⁻¹, ascribed to the modification of the Raman stress tensor. Indeed, the generalized axial regime, described by diagonal components of the Raman stress tensor, cannot be applied in this region. Raman maps analysis and FEM simulations show the ‘activation’ of the shear stress, i.e. non‐diagonal components of the stress tensor. The stress‐Raman modes shift correlation, in the case of fully non‐diagonal stress tensors, has been investigated. The aim of future works will be to minimize the stress field generation and the defects density within the epitaxial layer. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and attachment of silver nanowires on atomic force microscopy cantilevers for tip‐enhanced Raman spectroscopy

Surface‐enhanced Raman spectroscopy is based on the absorption of light by nanometer‐sized metal particles, resulting in large enhancement of the Raman signal. By replacing the metal particles by a metallic nanotip, the enhancement can be localized. The resulting tip‐enhanced Raman spectroscopy is capable of measuring Raman spectra with high spatial resolution, effectively overcoming the diffraction limit. A successful tip‐enhanced Raman spectroscopy experiment depends heavily on the ability to fabricate tips of a definite metal with the appropriate shape and size, which is still a challenging process. We have prepared silver nanowires with a diameter of 200–300 nm by templated electrochemical deposition and attached them onto atomic force microscope cantilevers by focused electron beam induced deposition. We found that they produce a reproducible enhancement of the Raman signal intensity. Other metals and smaller nanostructures might also be produced, suggesting an interesting development potential for these novel nanoprobes. Copyright © 2011 John Wiley & Sons, Ltd.