Tip-enhanced Raman spectroscopy: tip-related issues
- 1.3k Downloads
After over 15 years of development, tip-enhanced Raman spectroscopy (TERS) is now facing a very important stage in its history. TERS offers high detection sensitivity down to single molecules and a high spatial resolution down to sub-nanometers, which make it an unprecedented nanoscale analytical technique offering molecular fingerprint information. The tip is the core element in TERS, as it is the only source through which to support the enhancement effect and provide the high spatial resolution. However, TERS suffers and will continue to suffer from the limited availability of TERS tips with a high enhancement, good stability, and high reproducibility. This review focuses on the tip-related issues in TERS. We first discuss the parameters that influence the enhancement and spatial resolution of TERS and the possibility to optimize the performance of a TERS system via an in-depth understanding of the enhancement mechanism. We then analyze the methods that have been developed for producing TERS tips, including vacuum-based deposition, electrochemical etching, electrodeposition, electroless deposition, and microfabrication, with discussion on the advantages and weaknesses of some important methods. We also tackle the issue of lifetime and protection protocols of TERS tips which are very important for the stability of a tip. Last, some fundamental problems and challenges are proposed, which should be addressed before this promising nanoscale characterization tool can exert its full potential.
KeywordsTERS Tip Electrochemical etching Coating Protection
Raman spectroscopy is a vibrational spectroscopic technique capable of providing molecular fingerprint information for chemical identification. As an analytical method, it has been widely used in various fields such as chemical analysis , material sciences , and biomedical applications . However, the sensitivity of Raman spectroscopy is low because of the inherent low efficiency of the inelastic scattering  compared with other optical techniques. Fortunately, the discovery and development of surface-enhanced Raman spectroscopy (SERS) [5,6] have enormously improved the sensitivity of Raman spectroscopy. By exploiting the giant enhancement effect of Raman scattering from molecules in close proximity to plasmonic metal nanostructures, SERS has achieved sensitivity down to monolayer molecules or even single molecules [7, 8, 9, 10]. Nevertheless, the ideal spatial resolution of normal Raman spectroscopy and SERS achieved on a confocal system is in the range of half a wavelength of the excitation laser, determined by the optical diffraction limit. In 2000, tip-enhanced Raman spectroscopy (TERS) was first demonstrated [11, 12, 13, 14]; in the following 15 years, TERS has been developed into a promising and powerful tool for chemical identification at the nanometer scale. TERS combines scanning probe microscopy (SPM) with Raman spectroscopy and is able to provide not only a high detection sensitivity down to single-molecule level but also a high spatial resolution down to sub-nanometers, offering chemical fingerprint information and morphologic information about the surface simultaneously. TERS has found increasing applications in areas including catalysis [15, 16, 17], materials [18, 19, 20, 21, 22, 23, 24, 25, 26], biology [27, 28, 29], molecular electronics , and surface science [31, 32, 33, 34, 35, 36, 37, 38, 39].
In TERS, a sharp metallic or metallized tip (gold or silver) is brought in close proximity to the surface of a sample controlled by SPM. Three SPM techniques, i.e., scanning tunneling microscopy (STM), atomic force microscopy (AFM), and shear force microscopy (SFM), have been adopted in TERS setups. Then, a beam of laser light with an appropriate wavelength and polarization is focused onto the tip apex to excite the TERS signal. In TERS, the tip is the dominant enhancing source of Raman signals: it determines, to a large extent, not only the enhancement of the signal but also the spatial resolution of TERS and SPM imaging. In addition, the reliability of TERS results also relies on the stability of the TERS tip. As a decisive component of the TERS technique, the development of reliable and highly enhanced TERS tips is the basis for further development and application of TERS. However, how to reproducibly fabricate the TERS tip with a high enhancement is still one of the bottleneck problems in TERS. The low reproducibility is mainly due to the lack of control over the morphology and geometry of the tip, especially the apex, which in turn prevents the comprehensive understanding and thus optimization of TERS measurements. Therefore, the first key issue related to the TERS tip is to figure out the influence of various tip parameters, including materials, radius, cone angle, and surface roughness, on the enhancement and spatial resolution, which will be very helpful for the design and selection of suitable tips for TERS studies. The second issue is to develop methods to fabricate TERS tips (including that to be used with STM, AFM, and SFM), with optimal enhancement and high reproducibility on the basis of the above understanding, which will improve the performance and expand the applications of TERS. Last but not least is the issue of the lifetime of the TERS tip, i.e., how long can a tip retain the enhancement? It requires methods to prevent the tip from contamination and improve the stability of the tip.
In this review, we will mainly focus on these tip-related issues. We will first discuss the principle of enhancement mechanism of TERS, especially the role of the tip in the TERS enhancement. Then we will introduce various methods for fabrication of tips together with our opinions on their advantages and challenges. Finally we will deal with the methods to extend the lifetime of TERS tips. The review aims to provide a guideline to choose the right tips for those who are interested in or working on TERS.
Principles of TERS tip enhancement
It is generally accepted that when the wavelength of the incident laser matches the LSPR peak of the TERS system, a strong EM field can be produced at the tip apex [42,43]. Since only the electric field component along the tip axis plays a dominant role in the TERS enhancement, a p-polarized incident beam can induce a stronger near-field enhancement than that with an s-polarized incident beam [42,44, 45, 46]. Recently, the radially polarized beam has been increasingly used in TERS [47,48], because it has a large component of the longitudinal field that can efficiently excite the LSPR at the tip apex to create a strong EM enhancement. It can increase the TERS signals by about eightfold compared to that excited by the linear polarized laser. In addition, the incidence angle-dependent TERS enhancement has been studied theoretically, which reveals the maximum field enhancement at an incidence angle in the range of ca. 40–60° [49,50].
In a practical TERS measurement, the tip apex has to approach the substrate sufficiently close, because the EM field significantly increases with the decrease of the distance between the tip and the substrate [49,51, 52, 53, 54, 55]. The typical working distance is ca. 1–2 nm. In addition, both experimental results and theoretical simulations have demonstrated that the properties of substrate can dramatically influence the field enhancement [49,56, 57, 58]. According to the substrate materials, the TERS working mode can be categorized into two types: uncoupled and coupled. The uncoupled-type TERS usually involves the use of substrates of dielectric materials (SiO2, Al2O3, etc.), which gives a weak enhancement [49,56]. It will not lead to an obvious shift of the LSPR peak of the tip compared with that of an isolated tip . The coupled-type TERS, also called gap-mode TERS, employs a substrate of noble metals (Au, Ag, and Cu), which will induce a strong electromagnetic coupling between the tip and substrate and result in a huge field enhancement and an obvious red shift (by tens of nanometer) of the LSPR peak . It has been found that the TERS enhancement of the coupled type is hundreds of times larger than that of the uncoupled type [49,56]. Therefore, most of the works with single-molecule sensitivity were achieved by using the coupled-type TERS. In comparison with noble metals, the use of transition metals (Pd, Pt, Ni, etc.) as substrates gives a weaker TERS enhancement due to the lower density of the surface conduction electrons [33,49,56]. Therefore it is challenging to obtain TERS signals from the transition metals that are important in surface science and heterogeneous catalysis. Anyhow, the enhancement from transition metals is still about 100 times higher than the uncoupled type, pointing to the promising future of this direction. Furthermore, it was found that the nanoscale roughness on the noble metal substrates can increase TERS enhancement by an order of magnitude .
It is well known that the dielectric permittivity of the surrounding medium can affect the LSPR position of metal nanostructures. We would expect a sizable red shift of the LSPR peaks when the tip is in water compared with that in air  without much change in the enhancement . However, more delicate calculation is still necessary to obtain the exact peak position. For the sake of comparison, there is about a 30-nm red shift of the LSPR peak when a Ag nanoparticle is immersed in water compared with that in air .
In addition to the aforementioned three variables (laser, substrate, and surrounding medium), the TERS enhancement also depends strongly on the tip itself. In the following section, we focus on the influence of the tip parameters on the TERS enhancement.
The conclusion of this part is that a higher spatial resolution can be achieved at a smaller tip radius both for the uncoupled and coupled-type TERS. However, the enhancement follows different trends depending on the substrate properties. For the uncoupled type, the trend is that the smaller the tip, the higher the enhancement. However, for the coupled type, the maximum TERS enhancement can be achieved when the tip radius is in the range of 50 to 80 nm.
In fact, there are still some open questions concerning the influence of the tip radius. First, it is still necessary to perform comprehensive theoretical calculations so that the differences in tip materials (also the dielectric layer), substrate materials, and the coating properties can all be included [68,70,71]. Second, the lightning-rod effect needs to be carefully considered, especially when the tip radius is below 10 nm . Lastly, the effect of the tip radius for all different configurations should be carefully verified by elegantly designed experiments.
Morphology of the tip apex
The NP tip can be treated as a single metal nanoparticle. Therefore, it will have the same optical response as a metal nanoparticle. However, it is time consuming to attach an individual nanoparticle to the tip. In comparison, NPs tips and NPs-layer tips are more widely used. For the NPs tip, the distance between the neighboring nanoparticles is too large to significantly affect the optical response of the nanoparticle at the apex. Therefore, the NPs tip can be also treated as a single nanoparticle. However, different from the NP tip, the nanoparticles other than the one at the apex of the NPs tip will also be excited by the laser; this gives a continuum background to the TERS spectra and reduces the signal to noise ratio. It is still very challenging to control the surface morphology, including the size and shape of evaporated nanoparticles, during the fabrication of the NPs tip and NPs-layer tip. It was recognized recently that the dipole direction of the tip significantly influences the optical response and the imaging quality of TERS. Therefore, a defocused imaging method has been developed to evaluate the direction of the tip dipole of NPs-layer tips and to correlate the morphology and optical response of the NPs-layer tip .
The field enhancement of the full metal tip arises from the lightning-rod effect and SPR effect. When the gold/silver tip is excited by a light polarized along the tip axis, the surface charge will oscillate at the same frequency as the exciting field. The surface charges form an oscillating standing wave and support the surface plasmon. Therefore, the induced surface charges have a large chance to accumulate at the apex of the tip and give a strongly enhanced electric field [81,82]. The NPs-layer tip shares a similar optical process with the full metal tip, but the scattering from the rough surface of the NPs-layer tip will reduce the efficiency of surface plasmon and leads to a dramatic decrease of the field enhancement. Benefitting from the lighting-rod effect and the effective accumulation of surface charges, the full metal tip shows much higher enhancement than that of the NP tip, NPs tip, and NPs-layer tip . This may partially explain why STM-based TERS (using full metal tip) provides stronger signals and better spatial resolution than those of AFM-based TERS (especially using NPs-layer tip) .
Methods for fabricating TERS tips
STM, AFM, and SFM are commonly employed to build TERS systems. Accordingly, there are three types of tips: STM TERS tips, AFM TERS tips, and SFM TERS tips. In the STM TERS, a tip with a high conductivity is required for the flow of the tunneling current. In AFM TERS, the feedback relies on either the changed reflection of a laser beam or a change in the resonance frequency of the tip, and it requires a cantilever-based tip with different resonance frequencies in the range of ca. 10–400 kHz. In SFM TERS, almost all kinds of tips can be attached to a quartz crystal tuning fork to work in the SFM feedback. Up to now, considerable efforts have been devoted to the fabrication of TERS tips with an improved enhancement and reproducibility. In this section, we provide an overview of the existing methods for fabrication of TERS tips.
STM TERS tips
The quality of the tip fabricated by the EC etching depends on the composition of the etchant, the etching voltage, and the method used to cut off the circuit, etc. However, the detailed etching mechanism and determining parameters are still too complicated to discuss, because it was found that the etching temperature and the concentration of the original chemicals also play key roles in the reproducibility.
Fabrication of gold STM TERS tips
The depletion of Cl− ions during the dissolution of gold leads to the formation of gold oxides which may act as a passivation layer to inhibit the electrochemical reaction and thus result in a decrease of the reaction current. The Cl− ion is then replenished by diffusion from solution to dissolve the surface oxide and the fresh gold surface is exposed again resulting in an increase of the reaction current. The process is repeated during the etching process, leading to the oscillation of current [87,89,90]. On the gold ring cathode, some reduction reactions take place, such as the hydrogen evolution and Au deposition, resulting in rigorous bubbling. It was found that the generated bubbles are driven to the gold tip on etching. If the bubbles are stuck on the Au wire surface, the surface will be rough. A cell with a jacket structure [91,92] has been developed to avoid the bubbling effect and stabilize the etching process. To overcome the volatile and corrosive nature of fuming HCl, some groups have used less toxic chloride salts like CaCl2 [93,94] and NaCl  instead of HCl as etchants. In fact, if Cl− ion is used, it is unavoidable to produce Cl2, which may be an irritant to eyes and lungs. Since the mixture of fuming HCl and anhydrous ethanol is still the best recipe for fabrication of gold tips concerning the high reproducibility and sharp apex, it is currently the most widely used recipe in TERS laboratories and companies.
Etching voltage is an important parameter to produce sharp gold tips with a smooth surface and an appropriate aspect ratio. For different etchants the optimal voltages are different. Both DC [87, 88, 89,91,96] and AC (including square DC-pulsed voltage) [92,94, 95, 96, 97] voltage have been used for etching. Generally, DC etching is simple and the etching process generally finishes in minutes; whereas, the AC method can further accelerate the mass transfer process and thus the supplement of the etchants, yielding a faster etching rate . For the AC method etching parameters such as high limit voltage, low limit voltage, period time, and duty cycle can also be controlled to fabricate gold tips with designed geometries .
In addition to the etchant and voltage, one should also pay attention to the crystallinity of the gold wire to be etched, which may also affect the shape and roughness of a tip . Usually a soft gold wire (Premion type) has less defects and grain boundaries than the hard wires, which may have a better reproducibility and yield of sharp and smooth gold tips. Annealing the wire prior to etching may be a good practice to reduce the defects and grain boundaries .
Fabrication of silver STM TERS tips
Although various methods have been developed to fabricate silver tips, the EC etched silver tips are still relatively blunt and rough and the reproducibility is still low compared with gold tips. Therefore, there is still plenty of room to improve the reproducibility and sharpness, and more statistical data to show the reproducibility are still needed [103,109].
Other fabrication methods and specialized STM TERS tips
AFM TERS tips
Different from the STM case, most AFM studies use the commercial tips for measurement. Therefore, it is also common practice to fabricate AFM TERS tips by deposition of metal onto a commercially available silicon cantilever. In addition, different kinds of specialized tips have been developed and used in AFM TERS studies.
The most prevalent method of preparing the AFM TERS tip is to use vacuum evaporation to deposit metal onto the surface of a commercial silicon tip. The vacuum evaporation method allows one to controllably transfer atoms from a heated source to a substrate located at a distance away from the source to grow a film in the vacuum environment. Many factors may affect the quality of the as-deposited film [114,115]. The morphology of the metal film could be controlled by the evaporation rate [114,116]. The metal film is rough with distinct particles at a slow evaporation rate and becomes a smooth continuous structure with the increase of the evaporation rate. The shapes and sizes of the metal film depend on the migration rate of the metal atoms on the substrate surface. The metal atoms migrate over the substrate surface until they are adsorbed at the active sites to form a strong chemical bond with the substrate, or these migrated metal atoms may collide with each other to form an agglomerate. The active sites and agglomerates act as the nuclei for the growth of the metal film. The arrival of new atoms will disrupt the migration of the atoms on the surface to increase the probability of nucleation. Consequently, high nucleation density is obtained at a higher deposition rate and leads to relatively smooth metal film. Temperature is another important factor that interplays with the deposition rate [117,118]. In general, the temperature of the substrate is slightly higher than the room temperature, considering the long separation distance between the source and the substrate (e.g., 12 cm). When the distance becomes shorter, e.g., 5 cm, a large variation in temperature across the substrate may affect the uniformity of the surface morphology of the metal film. Furthermore, vapor pressure also affects the rate and moving direction of the vapor atom and thus changes the crystalline texture of the film. In practice, the dependence of the vapor pressure at a given temperature for varied materials is different. In addition, other factors, such as the placement of the heater, source, and substrate may also affect the structure of the deposited films.
Compared with the vacuum deposition processes, chemical deposition has emerged as a more simple, environmentally friendly, and cost-effective micro/nanofabrication method. The quality of the coating layer is mainly controlled by the reductant (or deposition potential or current) and deposition time. In addition, as a benchtop method, chemical deposition is convenient for the mass production of TERS tips in the lab. Two kinds of chemical deposition methods have been used to fabricate the metal-coated AFM tip: electrodeposition method and electroless deposition method.
Electrodeposition is widely employed in applications ranging from surface finishing, corrosion inhibition, to nanoscale feature fabrication for the ultra-large-scale integration (ULSI) [124,125]. The deposition thickness can range from a few angstroms to several millimeters by controlling the deposition current, potential, or time. The surface roughness depends on the nucleation density on the substrate. Similar to the vacumm evaporation methods, the surface roughness is high at low overpotentials with a low deposition rate and becomes low at high overpotentials with a high deposition rate. In addition, the stronger the complexing ability of the ligand used in the electrodeposition bath is, the more negative the overpotential that is required, which is beneficial for the formation of a smooth metal layer. Several methods have been developed to obtain a uniform coating, such as direct-current, direct-potential, and pulsed eletrodeposition. Recently, we developed a new method to conveniently fabricate Au-coated AFM TERS tips with a high reproducibility by pulsed electrodeposition .
Unlike the electrodeposition method, electroless deposition allows metals to be deposited on the surface without controlling the tip potential. The latter is an important method to coat a metal film on a nonconductive surface, such as SiO2. In particular, the uniform coating on substrates of complicated shapes can be obtained with the electroless deposition . Some efforts have been devoted to the fabrication of metal-coated tips by electroless deposition [126,127,129,130]. The mirror reaction is one of the most commonly used electroless deposition techniques to produce silver on Si AFM tips . Figure 12c shows a tip that was decorated with isolated Ag nano islands owing to the sparse nucleation. The low density of nucleation sites may be a result of the smooth Si surface with few defects for the formation of the nuclei and the low concentration of the reactants. To achieve a continuous film, the nucleation density may be improved by increasing the concentration of the reaction species and self-assembly of a layer of Au or Ag nanoparticles on the AFM tip surface to act as the nucleation sites. Galvanic reaction of Ag(I) ions with elemental silicon was developed by Brejna and Griffiths  to electrolessly deposit silver film on silicon. As a result of the nature of the galvanic reaction, part of the silicon acted as the cathode so that the silver can be deposited on it; the other part of the silicon was dissolved. As a result, the tip morphology is rough. Surprisingly, the particles do not form nuclei on the apex (Fig. 13d), and the silver coating is an extension of the coating from the shaft resulting in a large tip radius.
Fabrication of specialized AFM TERS tips
In addition to the metal-coated tips, considerable efforts have been devoted to produce non-standard AFM TERS tips. In general, non-standard tips have special metal nanostructures at the tip apex to confine the laser energy and produce a strong field enhancement. The tip can be obtained by the attachment of nanoparticles to an AFM tip, or by the microfabrication method, or bending of the electrochemical etched metal tip in the lab.
Attachment of nanoparticles
Selective deposition of nanoparticles
Owing to the precise control of the size and shape of the nanomaterials, micromanufacturing technology is a promising tool for fabricating special tips to study the tip effect on the near-field enhancement. By using the FIB milling, metal-coated AFM tips with different lengths have been fabricated [137,144]. It was found that the resonant wavelength red shifts with increasing tip length and a high TERS enhancement could be obtained when the laser wavelengths were near the plasmonic resonance of the tip. Benefitting from the precise control of FIB milling, the plasmon resonance of the tip can be tuned to obtain better TERS enhancement and imaging resolution. More recently, a “tip-on-tip” TERS tip has been fabricated by combining a simple Ar+ ion sputtering route to obtain silver nanoneedle arrays on the tip apex . It should be noted that this specialized TERS tip exhibits a higher enhancement than that of a Ag-coated AFM tip because of the intense electromagnetic field around the apexes of the Ag nanoneedles.
Full metal AFM TERS tip
SFM TERS tips
There are several advantages of the tuning fork-based SFM . First, it is not necessary to use a feedback laser to control the tip–sample distance. Therefore, the structure of the SFM head can be significantly simplified compared with AFM. Second, SFM can be applied to all different types of samples as in AFM. Third, almost all kinds of tips can be attached to the quartz tuning forks for later use.
The most widely used tip for SFM-based TERS was made by gluing a silver or gold tip prepared by EC etching to the quartz tuning fork [51,148, 149, 150]. The mass of the glued tip and the amount of glue have a great effect on the quality factor of the quartz tuning fork. They should be as small as possible to maintain a high quality factor to achieve sufficiently high force sensitivity. SFM TERS tips can also be made by coating a metal layer or attaching nanoparticles [151, 152, 153] to the sharp end fiber apex, which appears to have a low reproducibility and efficiency. In 2012, Johnson et al.  proposed a promising method to fabricate over one million gold pyramids in one wafer by using a template-stripping fabrication method. These pyramids are highly reproducible and can be stored for a long time. However, the pyramid should be glued to a thin wire and then glued to a tuning fork for TERS studies. This method is very promising for commercialization if the procedure can be further simplified.
Cleaning, protection, and lifetime
A clean tip with a high stability guarantees the reliability and reproducibility of the TERS measurement. Generally, some organic contaminants adsorbed on the tip surface may interfere with the TERS measurement when the tip is exposed to air. Similarly, when TERS is performed in liquids, analyte molecules and contaminants in the aqueous solution may be adsorbed on the tip and thus interfere with TERS signals . Thus, the contamination may prevent the re-use of a TERS tip or even shorten the lifetime of the tip. A simple way to inhibit the adsorption of contaminants including carbon and analyte molecules is to coat a SAM (self-assembly monolayer) of ethanethiolate (EtS) on the tip . The SAM may protect the tip from contaminants in both the aqueous solution and air. Furthermore, it is possible to remove the contaminants when the gold tip is contaminated. Our group  proposed a method to clean gold tips by dipping them in concentrated sulfuric acid. The tip is then soaked in ultrapure water to remove sulfuric acid and in ethanol for quick drying. With this method, the stored gold tip can be regenerated with a high TERS activity.
On the other hand, the stability of TERS tips, including chemical stability and mechanical stability, is a critical issue for their use. As is known, silver shows low chemical stability compared with that of gold. As a result of the oxidation and tarnish of silver, the lifetimes of silver tips reported in the literature are different, ranging from several hours  to several days . Recently, we found that metal silver tips kept a high activity without obvious degradation of enhancement after storing in air (T approx. 22 °C, RH approx. 45 %) for more than 2 months (Huang et al. 2015, in review). The main reason is the formation of a compact oxide layer (only about 1.7 nm thick) when silver is exposed to air, which prevents the further oxidation of silver. More interestingly, such an ultrathin layer of silver oxide protects the silver tip from contamination.
Conclusion and perspectives
The tip plays dual roles in TERS. It is the most important part in TERS to provide not only the highly localized enhanced electric field to boost Raman signal of the sample so that the molecular information can be achieved with a nanometer spatial resolution but also simultaneously the morphological and other information that SPM can provide. The further development of TERS highly depends on the development of reliable methods for fabricating TERS tips and the in-depth understanding of the role of the tip in the TERS enhancement.
In this review, we discussed the role of the tip including the materials, radius, cone angle, and morphology in the TERS enhancement. It is clear that gold and silver are still the best materials to achieve a high enhancement in the visible light region. The tip apex with a smaller radius gives a higher spatial resolution. The choice of the tip radius depends on the application because there is a compromise between the enhancement and spatial resolution as a result of their different dependencies on the radius. An optimized TERS experiment not only involves the optimization of the tip parameters but also requires a good match between the incident laser and the LSPR of TERS systems.
We also introduced methods to fabricate TERS tips, including physical deposition (vacuum evaporation), chemical methods (EC etching, electrodeposition, electroless deposition, etc.), and microfabrication techniques (FIB, Ar+ ion sputtering, etc.). Vacuum evaporation is the most prevalent method of fabricating AFM TERS tips. Further optimization is still required to improve the reproducibility of the obtained TERS tip. Chemical methods are suitable for the fabrication of all kinds of TERS tips. Among them, electrochemical etching is particularly useful for preparing STM TERS tips. Microfabrication methods are a powerful but expensive tool, which can be used for preparing specialized TERS tips. In addition to all the above methods, the tip can be further refined by sharpening with FIB or modifying with high-voltage pulses to increase the TERS enhancement.
We further tackled the issues of lifetime and protection protocols of TERS tip, which have seldom been discussed in the TERS literature. Cleaning with concentrated sulfuric acid is an effective way to regenerate the Au tip with retained TERS activity. An ultrathin film of silver oxide naturally grown on the silver tip can provide good protection from contamination and further oxidation. The Al2O3 or SiOx can be used to improve the mechanical stability of metal-coated AFM TERS tips and extend the lifetime of TERS tips.
Although ultrahigh sub-nanometer resolution has been achieved, the mechanism is still under hot debate [37,157,158]. The challenges lie in how to characterize the atomic structure of the very end of the tip and how to reliably simulate the electromagnetic response at atomic resolution precision without sacrificing the role of the rest of the bulky part of the tip .
Furthermore, when the tip is coupled with a substrate, the challenges are how to precisely simulate the distribution of the electromagnetic field in the narrow gap, how the molecular species (including the dipole orientation  and conductivity) will influence the enhancement effect, and how the far-field and near-field strengths contribute to the obtained TERS signal.
Most experimentalists are now doing TERS experiments by choosing tips from a batch of tips and directly measuring the TERS signal. This is a time-consuming approach. If certain physical properties of a tip can be related to the TERS enhancement by normal optical methods, this will accelerate the quality control of TERS and the TERS measurement. This requires a good understanding of the optical response of the tips and TERS system.
Concerning the fabrication of STM tips, it is still a great challenge to obtain a highly enhanced silver tip with a nice tip shape in a reproducible way. Furthermore, if the tip end of both silver and gold can be engineered to a specific shape and/or LSPR wavelength in a convenient way, it will also be very important to TERS.
Concerning the commercialization of TERS tips, there are some reports on the batch production of AFM-based TERS tips using special protocols and recently Bruker is even commercializing TERS tips . However, the real commercialization of TERS tip requires the tip to have a good reproducibility of the signal enhancement to resist them wearing off during measurement. Most importantly, if the price of the commercial TERS tips can be lowered to that close to AFM tips (instead of 10 times higher), then true wide application of TERS may eventually come as the operator need not worry about breaking the tip.
We acknowledge the support from the Ministry of Science and Technology (MOST, 2011YQ03012406 and 2013CB933703), National Natural Science Foundation of China (NSFC; 21227004, 21321062, and J1310024), and Ministry of Education of the People’s Republic of China (MOE, IRT13036).
Conflict of interest
The authors declare no competing financial interest.
- 2.Weber WH, Merlin R (2013) Raman scattering in materials science. Springer Science & Business Media, BerlinGoogle Scholar
- 14.Pettinger B, Picardi G, Schuster R, Ertl G (2000) Electrochem Jpn 68:942–949Google Scholar
- 24.Zhu L, Atesang J, Dudek P, Hecker M (2007) Mater Sci-Pol 25(1):19–31Google Scholar
- 44.Demming AL, Festy F, Huang F, Richards D (2005) J Korean Phys Soc 47:S1–S4Google Scholar
- 64.Palik ED (1998) Handbook of optical constants of solids III. Academic, San DiegoGoogle Scholar
- 68.Demming AL, Festy F, Richards D (2005) J Chem Phys 122(18)Google Scholar
- 73.Le Ru EC, Etchegoin PG (2009) Principles of surface-enhanced Raman spectroscopy and related plasmonic effects. Elsevier, AmsterdamGoogle Scholar
- 89.Wang X, Cui Y, Ren B (2007) Chem J Chinese Univ 28(3):522–525Google Scholar
- 123.Kawata S, Shalaev VM (2011) Tip enhancement. Elsevier, AmsterdamGoogle Scholar
- 131.Vakarelski IU, Higashitani K (2006) Langmuir 22(7):2931–2934Google Scholar
- 132.Sqalli O, Bernal MP, Hoffmann P, Marquis-Weible F (2000) Appl Phys Lett 76(15):2134–2136Google Scholar
- 133.Umakoshi T, Yano TA, Saito Y, Verma P (2012) Appl Phys Express 5(5):052001–052003Google Scholar
- 134.Okamoto T, Yamaguchi I (2001) J Microsc-oxford 202(1):100–103Google Scholar
- 153.Le NV, Mevellec JY, Minea T, Louarn G (2012) Int J Opt 2012:591083–591088Google Scholar
- 159.(2015) IRIS TERS probes. http://www.brukerafmprobes.com/t-IRIS-TERS-Probes.aspx. Accessed 14 Jan 2015