The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Present and Future of Surface-Enhanced Raman Scattering

The wavelength corresponding to the extinction maximum, λmax, of the localized surface plasmon resonance (LSPR) of silver nanoparticle arrays fabricated by nanosphere lithography (NSL) can be systematically tuned from ∼400 nm to 6000 nm. Such spectral manipulation was achieved by using (1) precise lithographic control of nanoparticle size, height, and shape, and (2) dielectric encapsulation of the nanoparticles in SiOx. These results demonstrate an unprecedented level of wavelength agility in nanoparticle optical response throughout the visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum. It will also be shown that this level of wavelength tunability is accompanied with the preservation of narrow LSPR bandwidths (fwhm), Γ. Additionally, two other surprising LSPR optical properties were discovered:  (1) the extinction maximum shifts by 2−6 nm per 1 nm variation in nanoparticle width or height, and (2) the LSPR oscillator strength is equivalent to that of atomic silver in gas or...

Nanosphere lithography: Tunable localized surface plasmon resonance spectra of silver nanoparticles

The kinetics and microscopic mechanisms of laser melting and disintegration of thin Ni and Au films irradiated by a short, from 200 fs to 150 ps, laser pulse are investigated in a coupled atomistic-continuum computational model. The model provides a detailed atomic-level description of fast nonequilibrium processes of laser melting and film disintegration and, at the same time, ensures an adequate description of the laser light absorption by the conduction band electrons, the energy transfer to the lattice due to the electron-phonon coupling, and the fast electron heat conduction in metals. The interplay of two competing processes, the propagation of the liquid-crystal interfaces (melting fronts) from the external surfaces of the film and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for melting of metal films irradiated by laser pulses at fluences close to the melting threshold. The relative contributions of the homogeneous and heterogeneous melting mechanisms are defined by the laser fluence, pulse duration, and the strength of the electron-phonon coupling. At high laser fluences, significantly exceeding the threshold for the melting onset, a collapse of the crystal structure overheated above the limit of crystal stability takes place simultaneously in the whole overheated region within \ensuremath{\sim}2 ps, skipping the intermediate liquid-crystal coexistence stage. Under conditions of the inertial stress confinement, realized in the case of short $\ensuremath{\tau}l~10\mathrm{ps}$ laser pulses and strong electron-phonon coupling (Ni films), the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated films as well as on both homogeneous and heterogeneous melting processes. Anisotropic lattice distortions and stress gradients associated with the relaxation of the laser-induced pressure destabilize the crystal lattice, reduce the overheating required for the initiation of homogeneous melting down to $T\ensuremath{\approx}{1.05T}_{m},$ and expand the range of pulse durations for which homogeneous melting is observed in 50 nm Ni films up to \ensuremath{\sim}150 ps. High tensile stresses generated in the middle of an irradiated film can also lead to the mechanical disintegration of the film.

Combined atomistic-continuum modeling of short-pulse laser melting and disintegration of metal films

An atomic force microscope (AFM) tip is used to selectively produce surface enhanced Raman scattering (SERS) for localized Raman spectroscopy. Spectra of thin films, undetectable with a Raman microprobe spectrometer alone, are readily acquired in contact with a suitably gold-coated AFM tip. Similarly, an AFM tip is used to remove sample layers at the nanometer scale and subsequently serve as a SERS substrate for ultra-trace analysis. This demonstrates the interface of an AFM with a Raman spectrometer that provides increases sensitivity, selectivity and spatial resolution over a conventional Raman microprobe. An AFM guiding the SERS effect has the potential for targeted single molecule spectroscopy.

Locally enhanced raman spectroscopy with an atomic force microscope

Tip-enhanced Raman spectroscopy (TERS) is based on the optical excitation of localized surface plasmons in the tip-substrate cavity, which provides a large but local field enhancement near the tip apex. We report on TERS with smooth single crystalline surfaces as substrates. The adsorbates were CN- ions at Au(111) and malachite green isothiocyanate (MGITC) molecules at Au(111) and Pt(110) using either Au or Ir tips. The data analysis yields Raman enhancements of about 4 x 10(5) for CN- and up to 10(6) for MGITC at Au(111) with a Au tip, probing an area of less than 100 nm radius.

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Nanoscale Probing of Adsorbed Species by Tip-Enhanced Raman Spectroscopy

The scanning tunneling microscope (STM) has been combined with laser excitation and was used for modification of metal surfaces in air. This technique enables processing of structures with a lateral resolution of approximately 10 nm. The form of the created features ranges from craters and ditches to hillocks. The process has been demonstrated on gold and gold/palladium substrates by utilization of tungsten, silver, and platin/iridium tips. Using pure silver tips or silver‐coated tungsten tips, a transfer of tip atoms to the substrate occurred. In the case of uncoated tungsten tips, we observed substrate evaporation and surface grain reorganization at low laser intensities, respectively. No distortion of the employed tips during the structuring process was observed. Several future oriented applications are conceivable, such as, for example, high density data storage and fresnel optics for x rays.

Nanostructure fabrication using laser field enhancement in the near field of a scanning tunneling microscope tip

A brief outline of the methods and approaches in the theory for field enhancement of laser radiation in the nearfield of a scanning probe microscope tips is given. The simple analytical small spheroid model (mainly used in Surface Enhanced Raman Spectroscopy) as well as a more realistic numerical model based on the boundary element method are used for field enhancement calculations on the tip apex. The small spheroid model gives a rough estimation from below for tips with spheroid geometry, whereas the numerical model can take into account arbitrary tip geometry and include the effect of substrate surface. Tremendous field concentration with nanometer localization for certain tip and substrate materials is used for applications in surface nano modification with laser radiation. Experimental data supporting the surface modification due to the enhanced field mechanism are presented.

Field enhancement of optical radiation in the nearfield of scanning probe microscope tips

Shape and composition of electrochemically etched tungsten tips for use in scanning tunneling microscopy (STM) were investigated in a transmission electron microscope (TEM) with a Gathan imaging filter (GIP). The tips are prepared by a lamella drop-off technique. We observe typical tip radii of less than 10 nm. After a storage of some days under ambient conditions, an amorphous oxide film is detectable. The electron energy-loss spectroscopy confirms that the surface is contaminated by compounds that contain carbon, too.

Characterization of electrochemically etched tungsten tips for scanning tunneling microscopy

We report preliminary results of using an SFM(Scanning Force Microscope)/laser combination in FOLANT configuration (FOcusing of LAserradiation in Nearfield of a Tip) to perform nanostructuring on insulator and metal surfaces in air This technique is based on local intensity enhancement of laser radiation in the nearfield of a tip and enables processing of structures with a lateral resolution of approximately 10 nm The structuring has been carried out on polycarbonate and metal (gold) surfaces - appropriate materials for data storage applications - as well as on organic media to demonstrate possible applications in biotechnology Some details of nanostructuring mechanisms are discussed

Nanostructuring with laser radiation in the nearfield of a tip from a scanning force microscope

Due to its physical and chemical features silver is a promising material for scanning tunneling microscopy (STM) tips in the field of nanostructuring by a laser/STM combination. This nanostructuring is archived by field enhancement of optical radiation by a factor of up to 103 in the near field between a STM tip and a substrate. The magnitude of the field enchantment depends on the geometry and material of the utilized tip. State of the art procedures of processing silver tips cannot be applied to this application due to insufficient quality and reproducibility, respectively. We have developed a new simple single‐step etching procedure for silver tips with a high reproducibility and a tip radius of less than 100 nm. This was realized by electrochemical etching in an ammonia solution and subsequent electronic controlled movement of the tip out of the electrolyte.

K. Dickmann

Papers

Nanostructure fabrication using laser field enhancement in the near field of a scanning tunneling microscope tip

Field enhancement of optical radiation in the nearfield of scanning probe microscope tips

Characterization of electrochemically etched tungsten tips for scanning tunneling microscopy

Nanostructuring with laser radiation in the nearfield of a tip from a scanning force microscope

New etching procedure for silver scanning tunneling microscopy tips