Near-field Raman Microscopy Research Articles

Formation mechanisms of sub-micron pharmaceutical composite particles derived from far- and near-field Raman microscopy

Surface enhanced Raman spectroscopy (SERS) and confocal Raman microscopy are applied to investigate the structure and the molecular arrangement of sub-micron furosemide and polyvinylpyrrolidone (furosemide/PVP) particles produced by spray flash evaporation (SFE). Morphology, size and crystallinity of furosemide/PVP particles are analyzed by scanning electron microscopy (SEM) and X-ray powder diffraction (XRPD). Far-field Raman spectra and confocal far-field Raman maps of furosemide/PVP particles are interpreted based on the far-field Raman spectra of pure furosemide and PVP precursors. Confocal far-field Raman microscopy shows that furosemide/PVP particles feature an intermixture of furosemide and PVP molecules at the sub-micron scale. SERS and surface-enhanced confocal Raman microscopy (SECoRM) are performed on furosemide, PVP and furosemide/PVP composite particles sputtered with silver (40 nm). SERS and SECoRM maps reveal that furosemide/PVP particle surfaces mainly consist of PVP molecules. The combination of surface and bulk sensitive analyses reveal that furosemide/PVP sub-micron particles are formed by the agglomeration of primary furosemide nano-crystals embedded in a thin PVP matrix. Interestingly, both far-field Raman microscopy and SECoRM provide molecular information on a statistically-relevant amount of sub-micron particles in a single microscopic map; this combination is thus an effective and time-saving tool for investigating organic sub-micron composites.

Lab-on-tip: Protruding-shaped all-fiber plasmonic microtip probe toward in-situ chem-bio detection

Lab-on-tip technology opens a new door for optical fiber surface plasmon resonance (SPR) sensors. In this paper, we first demonstrate an ultra-compact reflective SPR microtip probe on the flat fiber end in experiment, which has a length of tens of microns and a diameter of several microns. Simulations and experiments show that this new fiber SPR microtip probe coated with a nano-goldfilm has excellent refractive index (RI) sensing characteristic and great potential for biochemical detection at single cell level. Based on the high RI sensitivity, a bio-probe assembled with Chitosan/Polysodium Styrene Sulfonate (Cs/PSS) nano-polyelectrolyte film and the sheep anti-human IgG-HRP is prepared to recognize human IgG specifically. Due to the high surface sensitivity, the SPR microtip probe exhibits an ultra-low detection limit for human IgG. Due to the high bulk sensitivity and high thermo-optical coefficient (TOC) of polydimethylsiloxane (PDMS), a reference temperature probe based on SPR microtip is proposed, which is used to compensate for the temperature cross-sensitivity of the bio-probe. This new SPR microtip probe can be used not only in optical fiber sensor, but also in Tip-Enhanced Near-Field Raman Microscopy and Scanning Near-Field Optical Microscope.

Advances in Tip-Enhanced Near-Field Raman Microscopy Using Nanoantennas.

Tip-enhanced near-field Raman microscopy spectroscopy is a scanning probe technique that is capable of providing vibrational spectroscopic information on single nanoobjects and surfaces at (sub-) nanometer spatial resolution and high detection sensitivity. In this review, we first illustrate the physical principle of optical nanoantennas used in tip-enhanced near-field Raman microscopy and tip-enhanced Raman scattering (TERS) to efficiently couple light to Raman excitations on nanometer length scales. Although the antennas' electric near-field distributions are commonly understood to determine the spatial resolution, recent experiments showing subnanometer-resolved optical images put this understanding into question. This is because such images enter a regime in which classical electrodynamical descriptions might no longer be applicable and quantum plasmonic and atomistic effects could become relevant. After summarizing the current understanding of plasmonic phenomena at extremely short length scales, we discuss the different mechanisms contributing to the signal enhancement. In addition to the known contributions from electric-field and chemical enhancement, several new models have been proposed very recently that could provide important guidelines for the optimization of TERS experiments. We then review recent developments in the areas of antenna design, fabrication, and characterization. Finally, we briefly highlight recent applications to illustrate future directions of tip-enhanced near-field Raman microscopy and TERS.

Optoplasmonic probe to realize scanning near-field Raman microscopy.

Tip-enhanced Raman spectroscopy (TERS) is a powerful scanning probe technique for Raman detections in nanotechnology to date. However, limited by the physical principles of a nanosize tapered metal (or metal-coated) probe used in a TERS device, only far-field without near-field Raman signal can be collected by the TERS with the metal probe. This makes conventional TERS lower in efficiency and cannot be a real near-field Raman microscopy. In this paper, we propose a simple and realizable optoplasmonic probe model, which is composed of a dielectric microsphere and a metal nanobowtie, to realize an ideal scanning near-field Raman microscopy (SNRM). Using finite-difference time-domain (FDTD) method, calculation results of electric field distributions of the proposed probe demonstrate that the probe provides three outstanding characteristics, including strong enhancement of local electric field, nanoscale distributions of the produced electric filed, and collection enhancement of emitted energy with wide wavelength range in near field. These characteristics of the probe resolve the detecting restrictions of metal probes and provide a real near-field scanning method. Therefore, a potentially novel SNRM can be expected to extend Raman application range greatly.

Plasmonic optical antenna design for performing tip-enhanced Raman spectroscopy and microscopy

This paper highlights optical plasmonic antennas designed with dc-pulsed low-voltage electrochemical etching of a gold wire for implementing tip-enhanced Raman scattering (TERS) measurements. We demonstrate a versatile electrochemical system that allows one to engineer TERS-active metallic gold tips with diverse shapes and sizes in a highly reproducible fashion. The underlying etching mechanism at a voltage-driven meniscus around a gold wire immersed into an electrolyte is discussed in detail. We show that the developed method is suitable to produce not only the simplest geometries such as cones and spheroids, but more complex designs. Attempts have been made to design plasmonic tapered antennas with quasi-uniformly spaced nano-sized bumps on the mesoscopic zone for the extra surface plasmon-light coupling. The capability of the patterned antenna to enhance and localize optical fields is demonstrated with near-field Raman microscopy and spectroscopy of single-walled carbon nanotubes bundles.

Near-Field Raman Microscopy for Nanometric Observation

Near-Field Raman Microscopy for Nanometric Observation

Highly sensitive detection of near-field Raman scattered light from strained Si∕SiGe heterostructures by scanning near-field optical Raman microscope using ultraviolet resonant Raman scattering

We have developed a scanning near-field Raman microscope (SNORM) with a hollow pyramidal probe that uses ultraviolet resonant Raman scattering and measured changes in the near-field and far-field Raman intensities of strained Si∕SiGe heterostructures as a function of a probe-sample distance R. We observed that the near-field Raman intensity of a strained Si film dramatically decreased with an increase in the probe-sample distance. The decay curve of the near-field Raman intensity was approximately expressed by the R−3 plot calculated from a simple model based on dipole-dipole interaction. This confirms that our SNORM detects the near-field light from the sample.

Characterization of Si nanostructures using a noncontact mode scanning near-field optical Raman microscope, with 100nm spatial resolution and 5nm depth resolution, using ultraviolet resonant Raman scattering

The authors developed a noncontact mode scanning near-field Raman microscope (NC-SNORM) with a hollow pyramidal probe, using ultraviolet resonant Raman scattering, measured the stress distribution in very-large-scale integration standards made of silicon dioxide (SiO2) film and Si, and characterized Si nanodot structures. The authors found that the areas covered by SiO2 are under tensile stress and the areas not covered by SiO2 are under compressive stress. Compressive stress concentrates on the interface, about 100nm wide, between the covered and noncovered areas. They measured near-field Raman spectra of the Si nanodot structures and found that the Raman intensities change periodically approximately every 100nm.

Stress Characterization of Si by Near-Field Raman Microscope Using Resonant Scattering

We have developed a tapping-mode scanning near-field optical Raman microscope (SNORM) with a caved and pyramidical probe, using resonant Raman scattering, and have measured the stress distribution of Si. The peak frequency shifts to a lower frequency by 0-0.5 cm(-1) in the area covered by silicon dioxide, whereas it shifts to a higher frequency by 0-0.3 cm(-1) in the area uncovered by silicon dioxide, showing that the areas covered and uncovered by silicon dioxide are under tensile and compressive stresses, respectively. It has been found that compressive stresses of about 0.69 GPa/cm2 are concentrated on the corner of the area uncovered by silicon dioxide. The comparison of stress distributions measured with and without the cantilever shows that the SNORM we developed has a spatial resolution of at least less than 250 nm.

Near-field Raman microscopy

Near-field microscopy offers the power of optical characterization with nanometer spatial resolution (∼15 nm). Combining tip-enhanced microscopy with Raman scattering spectroscopy results in the ability to localize distinct spectral features, providing a unique opportunity to characterize materials on length scales of a few nanometers using visible light.

Near‐field scanning Raman microscopy using apertureless probes

AbstractWe describe an apertureless near‐field scanning Raman microscope in reflection geometry developed by integrating a near‐field scanning optical microscope and a Raman spectrometer. This set‐up offers some unique features that make it applicable to more samples. The fabrication of the metal tips is also explained in detail. Near‐field Raman mappings have been realized on real silicon devices for the first time. The results illustrate the capability of our near‐field Raman microscope in 2D Raman imaging. The apertureless configuration is a breakthrough to the limitation set by the low optical throughput of metal‐coated optical fiber tips, reducing drastically the integration time for Raman spectra. The reflection scattering geometry makes the system applicable to any samples without preparation. Copyright © 2003 John Wiley & Sons, Ltd.

Tip‐enhanced Raman microscopy: practicalities and limitations

AbstractThe feasibility of apertureless scanning near‐field Raman microscopy, exploiting the local enhancement in Raman scattering in the vicinity of a silver or gold tip, was investigated. Using the finite difference time domain method we calculated the enhancement of electric field strength, and hence Raman scattering, achieved through the resonant excitation of local modes in the tip. By modelling the frequency‐dependent dielectric response of the metal tip we were able to highlight the resonant nature of the tip‐ enhancement and determine the excitation wavelength required for the strongest electric field enhancement, and hence Raman scattering intensity, which occurs for the excitation of modes localized at the tip apex. It is demonstrated that a peak Raman enhancement of 107‐fold should be achievable with <5 nm spatial resolution. We show that surface‐enhanced Raman scattering from carbon contamination on a silver or gold tip can be significant. However, we find for a tip of radius of curvature 20 nm that the Raman enhancement should decay totally within 20 nm from the tip. Hence withdrawal of the tip by this distance should lead to the disappearance of the tip‐enhanced signal, leaving only that from carbon contamination on the tip itself and the intrinsic signal from the sample. Copyright © 2003 John Wiley & Sons, Ltd.

Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy

A tip-enhanced near-field Raman microscope has been applied to the detection of an individual single-wall carbon nanotube (SWNT). The detected information is a color (Raman-shift) image of molecular distribution without having to resort to staining non-fluorescent molecules of interest. In addition to nanometric-sensing and -imaging capability, local field-enhancement of the metallic tip has been utilized to detect a weak Raman scattering from nanometer region, which cannot be observed by conventional micro-Raman configuration (far-field Raman). The experimental results are shown with analysis of distinct vibration modes of both radial breathing mode and G-band.

Optical near-field Raman microscopy

Optical near-field Raman microscopy

High-resolution near-field Raman microscopy of single-walled carbon nanotubes.

We present near-field Raman spectroscopy and imaging of single isolated single-walled carbon nanotubes with a spatial resolution of approximately 25 nm. The near-field origin of the image contrast is confirmed by the measured dependence of the Raman scattering signal on tip-sample distance and the unique polarization properties. The method is used to study local variations in the Raman spectrum along a single single-walled carbon nanotube.

Design of near-field optical probes with optimal field enhancement by finite difference time domain electromagnetic simulation

We report the three-dimensional electromagnetic simulation of gold nanoparticles with specific geometries as a means to the rational design of apertureless near-field scanning optical microscopy (NSOM) probes. Analytical solutions for field enhancement by spheroidal particles are used to provide physical insight for probe design. These solutions indicate that probes need to be not only sharp, but also finite in length in order to generate the highest field enhancement. Finite difference time domain (FDTD) simulations of gold particles illuminated by near infrared radiation are performed. Field enhancements for right trigonal pyramids are found to be size and wavelength dependent. Furthermore, the enhancements for these pyramidal particles are higher than for similar length conical particles, which in turn perform better than quasi-infinite conical probes. The particles we design with FDTD can be made using current nanofabrication techniques, and therefore hold great promise as apertureless NSOM probes. These right trigonal pyramids are particularly well suited for use in tip enhanced nonlinear optical microscopy or near-field Raman microscopy.

The role of tip plasmons in near-field Raman microscopy.

The enhancement in electric field strength in the vicinity of a metal tip, through the excitation of plasma modes in the tip, is investigated using the finite difference time domain method; such tip enhancement has significant potential for application in scanning near-field Raman microscopy. To represent an experimentally realistic geometry the near-field probe is described by a conical metal tip with a spherical apex, with radii 20 nm and 200 nm considered, in close proximity to a glass substrate. Illumination through the substrate is considered, both at normal incidence and close to the critical angle, with the polarization in the plane of incidence. By modelling the frequency dependent dielectric response of the metal tip we are able to highlight the dependence on the scattering geometry of the nature of the electromagnetic excitations in the tip. In particular, the strongest electric field enhancement with the greatest confinement occurs for the excitation of modes localized at the tip apex, excited only for off-normal incidence. Bulk modes excited in the tip also produce enhancement, although over a larger area and with significantly less enhancement than that of the localized modes; however, the excitation of bulk modes is independent of the angle of incidence.

Near-Field Surface-Enhanced Raman Spectroscopy on Single Silver Nanoparticles

We report the development of a near-field Raman microscope for nanometer-scale vibrational spectroscopy at room temperature. With the use of surface-enhanced Raman scattering and tapered single-mode fiber probes, near-field Raman spectra have been obtained from single Ag colloidal nanoparticles with excitation intensities as low as 10 nW. Far-field confocal studies have also been carried out on spatially isolated, single Ag nanoparticles. A direct comparison of the near-field and confocal results shows that near-field optical excitation does not lead to significantly different selection rules in Raman spectroscopy. This work further demonstrates that single metallic nanoparticles can be attached to the tip of a near-field probe for potential application in surface-enhanced optical microscopy and nanofabrication.

Nano-Raman Spectroscopy and Imaging with a Near-Field Scanning Optical Microscope

Raman spectroscopy was performed using a near-field scanning optical microscope. This avoids the diffraction limit inherent in conventional optical microscopy techniques involving far-field optical components, and allows volumes significantly smaller than the cube of the wavelength to be investigated. The small sample volume coupled with the light-starved nature of the Raman effect itself makes such nano-Raman studies difficult. A near-field Raman microscope is described and results showing near-field effects in an investigation of Rb-doped KTP are presented. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability, and that several unique aspects of the near-field data recommend this approach.

Nano-Raman Spectroscopy and Imaging with a NearField Scanning Optical Microscope

Raman spectroscopy was performed using a near-field scanning optical microscope. This avoids the diffraction limit inherent in conventional optical microscopy techniques involving far-field optical components, and allows volumes significantly smaller than the cube of the wavelength to be investigated. The small sample volume coupled with the light-starved nature of the Raman effect itself makes such nano-Raman studies difficult. A near-field Raman microscope is described and results showing near-field effects in an investigation of Rb-doped KTP are presented. An image taken within a Raman feature demonstrates that nano-Raman imaging is indeed possible if the near-field instrument has considerable long-term stability, and that several unique aspects of the near-field data recommend this approach.