The controlled fabrication of “Tip-On-Tip” TERS probes

Abstract

Tip-enhanced near-field Raman spectroscopy has exhibited a great ability to detect in situ chemical and structural information on a sample surface with the highest lateral resolution combined with a high sensitivity at the same time. A key challenge in the TERS field is the development and improvement of the fabrication of reproducible metal tip geometries with a sharper apex. In this study, one novel apertureless “Tip-On-Tip” TERS probe with a special plasmonic nanostructure is designed, and a simple Ar⁺-ion sputtering route has been developed to fabricate silver nanoneedle arrays on the scanning probe microscopy (SPM) cantilevers for tip-enhanced Raman spectroscopy (TERS) probes. These silver nanoneedles possess a very sharp apex with an apex diameter of 15 nm and an apex angle of 20°. The novel TERS probes exhibit enhanced near-field Raman signals compared to an Ag-coated SPM probe, which are attributed to the intense electromagnetic field around the apexes of the Ag nanoneedles and the periodic structure of the Ag nanoneedles.

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1. Introduction

Tip-enhanced Raman spectroscopy (TERS) is based on the optical excitation of the localized surface plasmon in the tip-substrate cavity, which provides a large but local field enhancement near the tip apex.^1–7 At present, the TERS technique combining the SPM technique with Raman spectroscopy has been successfully applied to many fields of nano-sciences due to its unique possibility to provide in situ chemical and structural information on a sample surface with the highest lateral resolution combined with a high sensitivity at the same time. Tip-enhanced near-field Raman spectroscopy has exhibited a great ability to detect several molecules or even a single molecule Raman signal and achieve spatial resolutions as high as 14 nm.^8–11 With this technique, the very weak signal detected by Raman scattering from a small number of molecules can be significantly enhanced by a metalized apertureless tip that usually consists of a silicon or silicon nitride tip covered with a thin, smooth layer of a metal such as gold or silver to increase the scattering cross section by generating local surface plasmon.^12–14 Since the spatial resolution and sensitivity of near-field Raman experiments strongly depend on the performance of the metal coated probe, the development of a near field probe exhibiting a strong and stable enhancement, as well as a good spatial resolution, is still a central problem bothering most TERS scientists. A key challenge in the TERS field is the development and improvement in fabricating reproducible metal tip geometries and a sharper apex.¹ A metal nanoparticle-coated AFM probe, a tapered optical fiber with a nanoparticle or thin metal film at the tip,¹³ electrochemical-etched gold tips mounted on the quartz tuning fork of a shear-force AFM,⁸ and hemispherical gold droplets on top of a silicon nanowire attached on an AFM tip^15–17 have been fabricated and used in TERS measurements. Although ellipsoidal particles and plasmonic nanostructures have superior properties, they have not been used because of the difficulties in attaching them in a preferred orientation.¹⁷ Here, we present one kind of novel apertureless TERS probe with a special plasmonic nanostructure, referred to as a “Tip-On-Tip” (TOT) TERS probe as shown in Scheme 1. Based on theoretical calculations and our ion sputtering technology,^18,19 we designed a new plasmonic nanostructure that has several tens of small nanoneedles with an apex diameter of less than 15 nm and an apex angle of 20° on the apex of the Ag-coated TERS probe as a novel TOT TERS probe. The small nanoneedle array can enhance the Raman signals in the TERS measurement. Firstly, a sharper tip apex can be obtained by this kind of nanostructure, where a stronger local-field enhancement can occur; secondly, several tips can form tip arrays, where the coupling of dipole plasmon can occur and make a contribution to the Raman enhancement.

Scheme 1 (a) An SEM image of the apex of a general Ag-coated TERS probe, (b and c) a schematic front view and oblique view of the apex of a general Ag-coated TERS probe; (d–f) a schematic vertical view, front view and oblique view of the apex of a TOT TERS probe, which exhibits several tens of small nanoneedles with an apex diameter of less than 15 nm on the apex. Firstly, the sharper tip apex can be obtained by this kind of nanostructure, where a stronger local-field enhancement can occur; secondly, several tips can form tip arrays, where the coupling of dipole plasmon can occur and make a contribution to the Raman enhancement.

2. Fabrication of silver nanoneedles array on SPM tips for TERS

Recently, we have developed an ion-beam sputtering technique to prepare Au, Ag nanocone-structures as SERS-active substrates for the detection of narcotics and semiconductor nanostructures which exhibited room-temperature ultraviolet random laser action.^18,19 Our route can be used to fabricate cone-structures on all kinds of substrates such as Si, glass or plastic substrates etc. Here we demonstrated our route to fabricate an Ag nanoneedle array on commercially available scanning probe microscopy (silicon nitride) cantilevers as novel “TOT” probes.

The novel TOT TERS probes were prepared by coating a silver layer on SiN SPM cantilevers (Olympus OMCL-TR800PSA-1) by an RF helicon magnetron sputtering system and fabricating a plasmonic nanostructure by an ion-beam system. Firstly, these cantilevers were cleaned ultrasonically in ethanol for 15 min prior to deposition, and the Ag films with different thicknesses were deposited on cantilevers in an RF helicon magnetron sputtering system (ULVAC, MPS-2000-HC3). The single target of Ag (99.99%) was sputtered in high purity Ar gas. Silver was deposited at the rate of 0.1 Å s⁻¹ in order to prevent the distortion of the SPM cantilever. The thickness of the Ag film was controlled by the deposition time. The deposition system was evacuated to a background pressure of 2 × 10⁻⁶ Pa.

The Ag nanostructures on the cantilevers were fabricated by an ion-beam system with a Kaufman-type Ar⁺ ion gun (Iontech. Inc. Ltd., model MPS 3000 FC). A thin carbon film was deposited on the Ag films prior to sputtering in order to enhance the ion-induced formation of the nanostructures.²⁰ The incidence angle from the Ar⁺ ion beam to the cantilever tip axis was adjusted to be about 30°. The sputtering was done with 1 keV Ar⁺ ions at room temperature for 4 min. The basal and working pressures were 5.0 × 10⁻⁵ and 2 × 10⁻² Pa, respectively. The residue of the carbon layer was very carefully removed by sonication in ethanol several times, and restored into a glass desiccator containing fresh desiccant in an N₂ atmosphere in order to protect the Ag nanostructures of the TOT probes from oxidization before characterization.

The microstructure and morphology of the TOT TERS probes were measured with a JEOL JSM-6301 Scanning electron microscopy (SEM) operating at 15 kV. Detailed descriptions of the experimental setups for the TERS measurements have been described elsewhere.¹² Briefly, the system consists of a Horiba Jobin Yvon Labram HR-800 Raman spectrometer coupled with a Park AFM XE-100 SPM and a confocal microscope Olympus BX41. TERS measurements by different freshly-prepared tips have been estimated using measurements on an R6G layer spin coated on an aluminum mirror from an 0.1 mM R6G ethanolic solution. The tip apex is illuminated side on with a 514.5 nm laser light at an angle of roughly 60° with respect to the tip axis. For different TERS tips, the behaviors of at least three tips were analyzed, each time using three positions on a TERS sample, to determine the data reproducibility.

3. Results and discussion

Novel apertureless TERS probes with a special plasmonic nanostructure were fabricated using commercial SPM probes by our Ar⁺-ion irradiation technology. Fig. 1 shows the SEM images of the SPM tip before and after fabrication. The apex diameter of the original SPM tip is less than 20 nm, as shown in Fig. 1a. After silver coating, the apex diameter was estimated to be around 60 nm, as shown in Fig. 1b. The sharpness of the tips are similar to those reported for the Ag metal deposited directly onto AFM cantilevers with average apex diameters of 40–60 nm.¹³ The SEM image of the side face clearly shows that the SPM tip is completely coated by Ag nanoparticles and the diameter of the coated-Ag nanoparticles is around 30 nm, as shown in the inset of Fig. 1b. Fig. 1c and d show SEM images of the probe after sputtering. As we expected, the cone-structure is formed on the side-face and the apex region of the Ag-coated SPM tip by the same process as that of the Ag nanoneedles that we reported.¹⁹ As we know, the plamonic structure on the sharp tip is very important because the tip characteristics such as the shape, size and roughness under the plasmonic structure determine the spatial resolution and sensitivity of the near-field Raman experiments. Hence, an enlarged high-resolution SEM imaging of the tip apex was obtained, as shown in Fig. 1d. Very interestingly, around ten small Ag nanoneedles were grew on the apex of the SPM tip. These nanoneedles also exhibited an extremely sharp curvature with the apex diameter of less than 15 nm and apex angle of 20°, which is sharper than a typical Ag-coated TERS tip with an apex diameter of 30–60 nm,^9–11 and almost the same as the electrochemical etched best Au tips^21,22 and the single tip on the AFM probe (TOT tip) prepared by anisotropic etching in KOH.²³ The array of small Ag nanoneedles with very sharp apexes would be expected to exhibit a stronger TERS enhancement than a single nanoneedle or a single particle on the probe apex.

Fig. 1 SEM images of the SPM tip (a) before, (b) after coated by Ag nanoparticles, (c) and after sputtered by Ar⁺ ions; and (d) an enlarged SEM image of the fabricated SPM tip. The inset is the enlarged SEM image of the related part of the SPM tip.

To examine the performance of the probe, we measured the tip-enhanced Raman spectra of the R6G molecules adsorbed on an aluminum mirror. With the Ag-coated SPM probe and TOT probe in contact with the aluminum coated with R6G, Raman spectra were acquired using 0.2 mW laser power and an integration time of 1 s. Those probes were retracted and the third spectrum was acquired with the laser spot at the same position on the aluminum mirror. The same laser power and same integration time was used. Fig. 2 shows the tip-enhanced Raman scattering spectra of R6G on aluminum acquired with a TOT tip, an Ag-coated tip in contact with the sample and without a tip. The peaks around 1317, 1512 and 1655 cm⁻¹ could be attributed to the Raman signal of R6G, and the peaks around 1370 and 1590 cm⁻¹ may originate from the Raman signal of R6G or the photodecomposition products of R6G (this probability is excluded below).^24,25 While the far-field Raman spectra display the related peaks around 1190, 1365, 1510, 1575 and 1650 cm⁻¹.¹⁹ The near-field Raman spectra exhibit different spectral features including the spectral position fluctuations of the characteristic bands and the variations of the relative peak amplitudes with their far-field Raman spectra. This behavior has been observed by R. Zenobi,^26,27 Hayazawa,²⁸ B. Pettinger et al.^24,25 and Neacsu et al.^21,22 When these tips were retracted from the sample, no distinguishable peak was observed. When the Ag-coated TERS tip and TOT tip was in contact with the surface of the samples, clear enhancements in the Raman signal were observed. As we expected, the TOT probe produce a stronger tip-enhanced Raman signal of R6G than the Ag-coated SPM probe, due to the huge electric field enhancement of the Ag nanoneedle array on the SPM probe.^21,22,29

Fig. 2 The tip-enhanced Raman spectra of R6G adsorbed on an aluminum mirror detected by a TOT tip, an Ag-coated tip and without a tip. The top is the far-field Raman spectra of R6G from ref. 19.

The giant electromagnetic field around the tip apex can very easily lead to a chemical alteration and/or decomposition of the investigated species in the TERS experiments. And the conventional dyes or laser dyes (like rhodamine) are very susceptible to photo-bleaching. Recently the problem whether the spectral features really originate from the target molecules or have to be assigned to carbonaceous decomposition products give rise to many TERS-scientists' doubt and even causes arguments. In the present work, it is very important and necessary to distinguish between the target R6G molecules and the carbonaceous species from our TERS spectra. Here the time evolution of the TERS spectra of the R6G monolayer at the Al mirror is shown in Fig. 3 in order to illustrate the problem mentioned above. In the beginning, the spectrum exhibited the slightly-shifted characteristic R6G fingerprint Raman bands located at 1312, 1370, 1512, 1590 and 1655 cm⁻¹ compared with their far-field Raman bands. With the continuing illumination of the sample, the characteristic R6G Raman band intensities decreased. With the further illumination of the sample for 30 s, a new Raman spectrum with clearly different bands located at 1365 and 1585 cm⁻¹ appeared. Obviously, this spectrum originated from the typical carbon broad-band Raman structure with peaks at around 1350 and 1580 cm⁻¹,^30,31 suggesting that photobleaching or decomposition of the investigated R6G molecules has taken place. The photobleaching, photodesorption, photoinduced surface diffusion, substrate heating, and possibly substrate morphology changes (through photooxidation for example), or even laser forces, may cause the SERS signal decay.³² From the decay process of the R6G molecules mentioned above, the Raman spectrum before illumination for 30 s can be attributed to the targeted R6G molecules. Therefore, we can confirm that these peaks around 1312, 1365, 1512, 1590 and 1655 cm⁻¹ can be attributed to the Raman signal of R6G, not the photodecomposition products of R6G.

Fig. 3 The time evolution of the TERS spectra of the R6G monolayer at Al with 0.2 mW of laser power at the sample and 10 s of integration time per spectrum.

4. Conclusions

In summary, one novel “Tip-On-Tip” apertureless TERS probe was designed and prepared by fabricating Ag nanoneedle arrays on commercial SPM cantilevers by using a simple Ar-ions sputtering techniques. The novel TERS probes exhibit enhanced Raman signals compared to the normally-used Ag-coated TERS tip in TERS measurements, and have the potential to improve the TERS enhancement and related in situ topography resolution.

Acknowledgements

Y. Yang thanks the Century Program (One-Hundred-Talent Program) of the Chinese Academy of Sciences for special funding support. This study was also supported in part by a fund from the National Natural Science Foundation of China (NSFC, Contract no. 51071167, 51102266). Y. Yang is also thankful for the support by Fund from Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (no. 12CS01). Z. Y. Li thanks the financial support from the National Natural Science Foundation of China under Grant no. 10525419, 60736041 and 10874238.

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