Nanoscale chemical characterization of materials and interfaces by tip-enhanced Raman spectroscopy

Materials and their interfaces are the core for the development of a large variety of fields, including catalysis, energy storage and conversion. In this case, tip-enhanced Raman spectroscopy (TERS), which combines scanning probe microscopy with plasmon-enhanced Raman spectroscopy, is a powerful technique that can simultaneously obtain the morphological information and chemical fingerprint of target samples at nanometer spatial resolution. It is an ideal tool for the nanoscale chemical characterization of materials and interfaces, correlating their structures with chemical performances. In this review, we begin with a brief introduction to the nanoscale characterization of materials and interfaces, followed by a detailed discussion on the recent theoretical understanding and technical improvements of TERS, including the origin of enhancement, TERS instruments, TERS tips and the application of algorithms in TERS. Subsequently, we list the key experimental issues that need to be addressed to conduct successful TERS measurements. Next, we focus on the recent progress of TERS in the study of various materials, especially the novel low-dimensional materials, and the progresses of TERS in studying different interfaces, including both solid-gas and solid-liquid interfaces. Finally, we provide an outlook on the future developments of TERS in the study of materials and interfaces.

Comparing Commercial Metal-Coated AFM Tips and Home-Made Bulk Gold Tips for Tip-Enhanced Raman Spectroscopy of Polymer Functionalized Multiwalled Carbon Nanotubes

Tip-enhanced Raman spectroscopy (TERS) combines the high specificity and sensitivity of plasmon-enhanced Raman spectroscopy with the high spatial resolution of scanning probe microscopy. TERS has gained a lot of attention from many nanoscience fields, since this technique can provide chemical and structural information of surfaces and interfaces with nanometric spatial resolution. Multiwalled carbon nanotubes (MWCNTs) are very versatile nanostructures that can be dispersed in organic solvents or polymeric matrices, giving rise to new nanocomposite materials, showing improved mechanical, electrical and thermal properties. Moreover, MWCNTs can be easily functionalized with polymers in order to be employed as specific chemical sensors. In this context, TERS is strategic, since it can provide useful information on the cooperation of the two components at the nanoscale for the optimization of the macroscopic properties of the hybrid material. Nevertheless, efficient TERS characterization relies on the geometrical features and material composition of the plasmonic tip used. In this work, after comparing the TERS performance of commercial Ag coated nanotips and home-made bulk Au tips on bare MWCNTs, we show how TERS can be exploited for characterizing MWCNTs mixed with conjugated fluorene copolymers, thus contributing to the understanding of the polymer/CNT interaction process at the local scale.

Controlling photon antibunching from 1D emitters using optical antennas

Single-photon emission is a hallmark of atom-like 0D quantum emitters, such as luminescent semiconductor nanocrystals, nitrogen vacancies in diamond and organic dye molecules. In higher dimensional nanostructures, on the other hand, multiple spatially separated electronic excitations may exist giving rise to more than one emitted photon at a time. We show that optical nanoantennas can be used to control the photon emission statistic of 1D nanostructures and to convert them into single-photon sources. Antenna-control exploits spatially confined near-field enhanced absorption and emission rates resulting in locally increased annihilation of mobile excitons and radiative recombination. As proof of concept, we experimentally demonstrate the improvement of the degree of antibunching in the photoluminescence of single carbon nanotubes using a metal tip at room temperature. Our results indicate that, in addition to improving the performance of single photon sources, optical antennas have the potential to open up a broad range of materials for quantum information technology.

Nanoscale chemical imaging using tip-enhanced Raman spectroscopy

Confocal and surface-enhanced Raman spectroscopy (SERS) are powerful techniques for molecular characterization; however, they suffer from the drawback of diffraction-limited spatial resolution. Tip-enhanced Raman spectroscopy (TERS) overcomes this limitation and provides chemical information at length scales in the tens of nanometers. In contrast to alternative approaches to nanoscale chemical analysis, TERS is label free, is non-destructive, and can be performed in both air and liquid environments, allowing its use in a diverse range of applications. Atomic force microscopy (AFM)-based TERS is especially versatile, as it can be applied to a broad range of samples on various substrates. Despite its advantages, widespread uptake of this technique for nanoscale chemical imaging has been inhibited by various experimental challenges, such as limited lifetime, and the low stability and yield of TERS probes. This protocol details procedures that will enable researchers to reliably perform TERS imaging using a transmission-mode AFM-TERS configuration on both biological and non-biological samples. The procedure consists of four stages: (i) preparation of plasmonically active TERS probes; (ii) alignment of the TERS system; (iii) experimental procedures for nanoscale imaging using TERS; and (iv) TERS data processing. We provide procedures and example data for a range of different sample types, including polymer thin films, self-assembled monolayers (SAMs) of organic molecules, photocatalyst surfaces, small molecules within biological cells, single-layer graphene and single-walled carbon nanotubes in both air and water. With this protocol, TERS probes can be prepared within ~23 h, and each subsequent TERS experimental procedure requires 3-5 h.

Tip-enhanced Raman spectroscopy - from early developments to recent advances

An analytical technique operating at the nanoscale must be flexible regarding variable experimental conditions while ideally also being highly specific, extremely sensitive, and spatially confined. In this respect, tip-enhanced Raman scattering (TERS) has been demonstrated to be ideally suited to, e.g., elucidating chemical reaction mechanisms, determining the distribution of components and identifying and localizing specific molecular structures at the nanometre scale. TERS combines the specificity of Raman spectroscopy with the high spatial resolution of scanning probe microscopies by utilizing plasmonic nanostructures to confine the incident electromagnetic field and increase it by many orders of magnitude. Consequently, molecular structure information in the optical near field that is inaccessible to other optical microscopy methods can be obtained. In this general review, the development of this still-young technique, from early experiments to recent achievements concerning inorganic, organic, and biological materials, is addressed. Accordingly, the technical developments necessary for stable and reliable AFM- and STM-based TERS experiments, together with the specific properties of the instruments under different conditions, are reviewed. The review also highlights selected experiments illustrating the capabilities of this emerging technique, the number of users of which has steadily increased since its inception in 2000. Finally, an assessment of the frontiers and new concepts of TERS, which aim towards rendering it a general and widely applicable technique that combines the highest possible lateral resolution and extreme sensitivity, is provided.

Nanoscale chemical imaging of solid-liquid interfaces using tip-enhanced Raman spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is a powerful tool for non-destructive and label-free surface molecular mapping at the nanoscale. However, to date nanoscale resolution chemical imaging in a liquid environment has not been possible, in part due to the lack of robust TERS probes that are stable when immersed in a liquid. In this work, we have addressed this challenge by developing plasmonically-active TERS probes with a multilayer metal coating structure that can be successfully used within a liquid environment. Using these novel TERS probes, we have compared the plasmonic enhancement of TERS signals in air and water environments for both gap mode and non-gap mode configurations and show that in both cases the plasmonic enhancement decreases in water. To better understand the signal attenuation in water, we have performed numerical simulations that revealed a negative correlation between the electric field enhancement at the TERS probe-apex and the refractive index of the surrounding medium. Finally, using these robust probes we demonstrate TERS imaging with nanoscale spatial resolution in a water environment for the first time by employing single-wall carbon nanotubes as a model sample. Our findings are expected to broaden the scope of TERS to a range of scientific disciplines in which nanostructured solid-liquid interfaces play a key role.

Nano-Raman Scattering Microscopy: Resolution and Enhancement

Raman scattering microscopy is becoming one of the hot topics in analytical microscopy as a tool for analyzing advanced nanomaterials, such as biomolecules in a live cell for the study of cellular dynamics, semiconductor devices for characterizing strain distribution and contamination, and nanocarbons and nano-2D materials. In this paper, we review the recent progress in the development of Raman scattering microscopy from the viewpoint of spatial resolution and scattering efficiency. To overcome the extremely small cross section of Raman scattering, we discuss three approaches for the enhancement of scattering efficiency and show that the scattering enhancement synergistically increases the spatial resolution. We discuss the mechanisms of tip-enhanced Raman scattering, deep-UV resonant Raman scattering, and coherent nonlinear Raman scattering for micro- and nanoscope applications. The combinations of these three approaches are also shown as nanometer-resolution Raman scattering microscopy. The critical issues of the structures, materials, and reproducibility of tips and three-dimensionality for TERS; photodegradation for resonant Raman scattering; and laser availability for coherent nonlinear Raman scattering are also discussed.

Tip-Enhanced Raman Scattering Imaging of Two-Dimensional Tungsten Disulfide with Optimized Tip Fabrication Process

We successfully achieve the tip-enhanced nano Raman scattering images of a tungsten disulfide monolayer with optimizing a fabrication method of gold nanotip by controlling the concentration of etchant in an electrochemical etching process. By applying a square-wave voltage supplied from an arbitrary waveform generator to a gold wire, which is immersed in a hydrochloric acid solution diluted with ethanol at various ratios, we find that both the conical angle and radius of curvature of the tip apex can be varied by changing the ratio of hydrochloric acid and ethanol. We also suggest a model to explain the origin of these variations in the tip shape. From the systematic study, we find an optimal condition for achieving the yield of ~60% with the radius of ~34 nm and the cone angle of ~35°. Using representative tips fabricated under the optimal etching condition, we demonstrate the tip-enhanced Raman scattering experiment of tungsten disulfide monolayer grown by a chemical vapor deposition method with a spatial resolution of ~40 nm and a Raman enhancement factor of ~4,760.

Towards a correlative approach for characterising single virus particles by transmission electron microscopy and nanoscale Raman spectroscopy

The morphology and structure of biological nanoparticles, such as viruses, can be efficiently analysed by transmission electron microscopy (TEM).

Tip-enhanced Raman scattering--Targeting structure-specific surface characterization for biomedical samples

Tip-enhanced Raman scattering (TERS) has become a powerful tool for nanoscale structural analysis for several branches of organic, inorganic, and biological chemistry. This highly sensitive technique enables molecular characterization with a lateral resolution far beyond Abbe's diffraction limit and correlates structural and topographic information on a nanometer scale. In this review, the current experimental concepts with respect to their strengths and obstacles are introduced and discussed. A further focus was set to biochemistry comprising applications like nucleic acids, proteins, and microorganisms, thus demonstrating the potential use towards the pharmaceutically relevant challenges where nanometer resolution is required.

Tip enhancement of upconversion photoluminescence from rare earth ion doped nanocrystals

We present tip-enhanced upconversion photoluminescence (PL) images of Er(3+)- and Yb(3+)-doped NaYF4 nanocrystals on glass substrates with subdiffraction spatial resolution. Tip-sample distance dependent measurements clearly demonstrate the near-field origin of the image contrast. Time-resolved PL measurements show that the tip increases the spontaneous emission rate of the two emission channels of Er(3+) in the visible region. Very efficient enhancement of upconversion PL is discussed in the context of the two-photon nature of the excitation process and homoenergy transfer between the ions within the nanocrystals. Comparison between different nanocrystals and tips shows a strong influence of the tip shape on the image contrast that becomes particularly relevant for the larger dimensions of the investigated nanocrystals.

Shape evolution and single particle luminescence of organometal halide perovskite nanocrystals

Organometallic halide perovskites CH3NH3PbX3 (X = I, Br, Cl) have quickly become one of the most promising semiconductors for solar cells, with photovoltaics made of these materials reaching power conversion efficiencies of near 20%. Improving our ability to harness the full potential of organometal halide perovskites will require more controllable syntheses that permit a detailed understanding of their fundamental chemistry and photophysics. In this manuscript, we systematically synthesize CH3NH3PbX3 (X = I, Br) nanocrystals with different morphologies (dots, rods, plates or sheets) by using different solvents and capping ligands. CH3NH3PbX3 nanowires and nanorods capped with octylammonium halides show relatively higher photoluminescence (PL) quantum yields and long PL lifetimes. CH3NH3PbI3 nanowires monitored at the single particle level show shape-correlated PL emission across whole particles, with little photobleaching observed and very few off periods. This work highlights the potential of low-dimensional organometal halide perovskite semiconductors in constructing new porous and nanostructured solar cell architectures, as well as in applying these materials to other fields such as light-emitting devices and single particle imaging and tracking.

Surface- and tip-enhanced resonant Raman scattering from CdSe nanocrystals

Surface- and tip-enhanced resonant Raman scattering (resonant SERS and TERS) by optical phonons in a monolayer of CdSe quantum dots (QDs) is demonstrated. The SERS enhancement was achieved by employing plasmonically active substrates consisting of gold arrays with varying nanocluster diameters prepared by electron-beam lithography. The magnitude of the SERS enhancement depends on the localized surface plasmon resonance (LSPR) energy, which is determined by the structural parameters. The LSPR positions as a function of nanocluster diameter were experimentally determined from spectroscopic micro-ellipsometry, and compared to numerical simulations showing good qualitative agreement. The monolayer of CdSe QDs was deposited by the Langmuir-Blodgett-based technique on the SERS substrates. By tuning the excitation energy close to the band gap of the CdSe QDs and to the LSPR energy, resonant SERS by longitudinal optical (LO) phonons of CdSe QDs was realized. A SERS enhancement factor of 2 × 10(3) was achieved. This allowed the detection of higher order LO modes of CdSe QDs, evidencing the high crystalline quality of QDs. The dependence of LO phonon mode intensity on the size of Au nanoclusters reveals a resonant character, suggesting that the electromagnetic mechanism of the SERS enhancement is dominant. Finally, the resonant TERS spectrum from CdSe QDs was obtained using electrochemically etched gold tips providing an enhancement on the order of 10(4). This is an important step towards the detection of the phonon spectrum from a single QD.

Characterization of semiconductor materials using synchrotron radiation-based near-field infrared microscopy and nano-FTIR spectroscopy

We describe the application of scattering-type near-field optical microscopy to characterize various semiconducting materials using the electron storage ring Metrology Light Source (MLS) as a broadband synchrotron radiation source. For verifying high-resolution imaging and nano-FTIR spectroscopy we performed scans across nanoscale Si-based surface structures. The obtained results demonstrate that a spatial resolution below 40 nm can be achieved, despite the use of a radiation source with an extremely broad emission spectrum. This approach allows not only for the collection of optical information but also enables the acquisition of near-field spectral data in the mid-infrared range. The high sensitivity for spectroscopic material discrimination using synchrotron radiation is presented by recording near-field spectra from thin films composed of different materials used in semiconductor technology, such as SiO2, SiC, SixNy, and TiO2.

Antenna-enhanced optoelectronic probing of carbon nanotubes

We report on the first antenna-enhanced optoelectronic microscopy studies on nanoscale devices. By coupling the emission and excitation to a scanning optical antenna, we are able to locally enhance the electroluminescence and photocurrent along a carbon nanotube device. We show that the emission source of the electroluminescence can be pointlike with a spatial extension below 20 nm. Topographic and antenna-enhanced photocurrent measurements reveal that the emission takes place at the location of highest local electric field indicating that the mechanism behind the emission is the radiative decay of excitons created via impact excitation.

Tip-enhanced near-field optical microscopy

Tip-enhanced near-field optical microscopy (TENOM) is a scanning probe technique capable of providing a broad range of spectroscopic information on single objects and structured surfaces at nanometer spatial resolution and with highest detection sensitivity. In this review, we first illustrate the physical principle of TENOM that utilizes the antenna function of a sharp probe to efficiently couple light to excitations on nanometer length scales. We then discuss the antenna-induced enhancement of different optical sample responses including Raman scattering, fluorescence, generation of photocurrent and electroluminescence. Different experimental realizations are presented and several recent examples that demonstrate the capabilities of the technique are reviewed.

Nanoscale imaging of InN segregation and polymorphism in single vertically aligned InGaN/GaN multi quantum well nanorods by tip-enhanced Raman scattering

Vertically aligned GaN nanorod arrays with nonpolar InGaN/GaN multi quantum wells (MQW) were grown by MOVPE on c-plane GaN-on-sapphire templates. The chemical and structural properties of single nanorods are optically investigated with a spatial resolution beyond the diffraction limit using tip-enhanced Raman spectroscopy (TERS). This enables the local mapping of variations in the chemical composition, charge distribution, and strain in the MQW region of the nanorods. Nanoscale fluctuations of the In content in the InGaN layer of a few percent can be identified and visualized with a lateral resolution below 35 nm. We obtain evidence for the presence of indium clustering and the formation of cubic inclusions in the wurtzite matrix near the QW layers. These results are directly confirmed by high-resolution TEM images, revealing the presence of stacking faults and different polymorphs close to the surface near the MQW region. The combination of TERS and HRTEM demonstrates the potential of this nanoscale near-field imaging technique, establishing TERS as a very potent, comprehensive, and nondestructive tool for the characterization and optimization of technologically relevant semiconductor nanostructures.

Ultrafast optical wide field microscopy

We have developed a new imaging method, ultrafast optical wide field microscopy, capable of rapidly acquiring wide field images of nearly any sample in a non-contact manner with high spatial and temporal resolution. Time-resolved images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon nanowire after optical excitation are captured using a two-dimensional smart pixel array detector. These images represent the time-dependent carrier dynamics with high sensitivity, femtosecond time resolution and sub-micrometer spatial resolution.

Bound 1D Excitons in Single CdSe Quantum Wires

Photogenerated electron-hole pairs are observed to be bound as 1D excitons in CdSe quantum wires (QWs) at room temperature. Microscopy experiments performed on dilute samples of CdSe QWs prepared on coverslips with patterned electrodes reveal that there is no change in either the overall photoluminescence (PL) intensity or the distribution of the PL intensity with the application of an external electric field. Changes in the PL intensity, and thus evidence for separate charge carriers within the QWs, are observed only for concentrated samples. In these concentrated samples, a thin film of other compounds, including trioctylphosphine oxide and a bismuth salt formed in the synthesis, is observed to encompass the QWs. The separate charge carriers that influence the PL intensity are attributed to the other compounds in the sample.