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The optimal gold-coated atomic force microscopy (AFM) tip-substrate system for tip-enhanced Raman spectroscopy (TERS) was designed theoretically and demonstrated experimentally. By optimizing the tip, excitation laser, and the substrate, the TERS enhancement factor can be tuned to as high as 9 orders of magnitude, and the spatial resolution could be down to 5 nm. Preliminary experimental results for AFM tips coated with gold layer of different thicknesses reveal that the maximum enhancement can be achieved when the thickness is about 60-80 nm, which is in good agreement with the theoretical prediction. Our results not only provide a deep understanding of the underlying physical mechanism of AFM tip-based TERS, but also guide the rational construction of a working AFM-TERS system with a high efficiency.
© 2015 Optical Society of America
1. Introduction
As a rapidly developed near-field optical technique, tip-enhanced Raman spectroscopy (TERS) has been widely used for the nanoscale surface analysis, benefited from the single-molecule sensitivity and the high spatial resolution that breaks the diffraction limit [1–5]. When an SPM tip is irradiated with an external light, the Raman scattering intensity of molecules in the vicinity of the tip can be largely enhanced due to the localized surface plasmon resonance (LSPR) and the lightning-rod effect. Therefore, the tip plays an essential role in TERS.
Gold and silver have been the two best materials for fabricating TERS tips due to their high free electron density and strong LSPR effect in the visible region. Silver possesses the strongest electric field enhancement than that of gold, but its instability in air limits to some extent its application [6]. As a result, gold tips are receiving increasing application in TERS. The electrochemical etching method developed by our group has been the most important way to fabricate STM-based TERS gold tips with a high success rate [7, 8]. Thereafter, the gold tips have been used quite successfully in single molecule study and for simultaneously obtaining the electronic and chemical structure of single molecule junctions [9–11]. Some interesting works have been performed using STM-based TERS (STM-TERS) [1, 12–15]. Unfortunately, STM can only work with conductive samples, which has limited a wider application of STM-TERS. An effective way to overcome the limitation is to use an AFM-based TERS (AFM-TERS), which can be applied for a wider variety of samples than STM-TERS, including conductive and non-conductive samples.
Silicon is among the most widely used materials for fabricating AFM tips. The commercially available silicon AFM tips cannot provide sufficient TERS signal because the surface plasmon resonance (SPR) can hardly be excited in the visible region owing to a low free carrier concentration (1013~1016 cm−3) in the pure silicon [16]. Coating a metal layer over the AFM tips by physical vapor deposition method is the commonly used method to improve the sensitivity of AFM-TERS. In the past several years, some efforts have been devoted to the fabrication of metal-coated tips [17–28]. Cui et al. have numerically studied the relation between the resonance frequency, the electric field enhancement, and the optical constant of the dielectric tips [29]. The result showed that with the increase of the refractive index of the dielectric tip, the LSPR frequency of the silver-coated dielectric tip red shifts, which consequently alters the field enhancement. Taguchi et al. presented a novel technique to modify the refractive index of the silicon probe by the thermal oxidization, and successfully tuned the LSPR frequency of metal-coated silicon tips in the whole visible region without sacrificing the sharpness of the tips [30]. Poborchii et al. used aluminum-coated siliocn AFM tip for the TERS measurement, and successfully observed a weak but clear near-UV TERS signal of the strained silicon [31]. However, to our best knowledge, most of published works focused on the study of stand-alone AFM tips or the tip-dielectric substrate configuration, in which the substrate effect was ignored because of the weak near-field coupling between the tip and the substrate. In fact, recently there are some interesting reports on the use of metallic substrates to improve the detection sensitivity both theoretically and experimentally [32, 33]. In such a strong coupled tip-substrate configuration, the substrate plays a vital role in determining the overall enhancement and spatial resolution [34]. For AFM-TERS, there are still many unsolved scientific issues such as the effect of metal coating and tip configuration on the enhancement and spatial resolution of AFM-TERS, and the huge differences when an AFM tip is working above a dielectric substrate and a metallic substrate. Furthermore, most of previous calculations were performed without appropriate experimental verification.
In this paper, we devote ourselves to not only optimization of the tip structural parameters, but also at the same time obtaining an optimized configuration to guide the design and construction of an AFM-TERS system with a high efficiency. The three-dimensional finite-difference time-domain (3D-FDTD) method was introduced to theoretically investigate the LSPR properties of silicon tips and gold coated silicon tips. The electric field enhancement and spatial resolution were investigated by varying the thickness of the gold coating, and further to be verified experimentally. We believe the calculation results will be a good reference for the fabrication of gold-coated AFM-TERS tip and for the experimental design of the TERS configuration. TERS experiments were in agreement with the calculation results well.
2. Simulation and experimental methods
The 3D-FDTD method, which numerically solves the Maxwell’s differential equations [35], was employed to obtain the LSPR-related optical properties of the silicon AFM tip with or without substrates and gold-coated silicon AFM tip (Si@Au) with an gold substrate. The simulations were performed using FDTD Solutions 7.5 (Lumerical Solutions, Vancouver). A p-polarized plane wave is incident from the side at an angle θ on the nanocavity formed by the tip and substrate, and its electric field amplitude is chosen to be 1.0 V/m. The cone angle of the silicon tip is set as 30°. A truncated tip with a tip length of 500 nm is used in the calculation to save the computational resources without sacrificing the accuracy [32, 36]. Perfectly matched layers (PML) boundary conditions were used on all boundaries for all simulations. To accurately simulate the 1-2 nm tip–substrate distance, the Yee cell size was set to be 0.5 nm. Nonuniform FDTD mesh method was used to save the computation resources and simulation time. Time-domain monitor was added to ensure the convergence of our calculations. Optical constants for gold and silicon were taken from ref. 37 and ref. 38, respectively.
To verify our simulation results, TERS experiments were performed. The enhancement can be derived by varying the thickness of gold coating for the tip-enhanced Raman response. We chose malachite green isothiocyanate (MGITC) as the probe molecules adsorbed on a well-defined gold (111) single-crystal surface that was prepared by flame-annealing following the Clavilier method [39]. Monolayer of MGITC was accomplished by immersing freshly flame-annealed gold (111) single-crystal in a 6 μM MGITC ethanolic solution for 3 h and then the gold (111) was rinsed with copious amounts of ethanol to remove physisorbed multilayer species. The commercially available silicon AFM tips (VIT_P/IR, NT-MDT) with radius down to ~5 nm were used. Electroplating method was used to deposit gold coating onto silicon tips. For the experiment Si@Au tips were used in the semicontact mode to measure the tip-enhanced Raman spectrum. The 632.8 nm He-Ne laser was focused into the nanogap between the tip and substrate using a × 100 long working length objective (Numerical Aperture NA = 0.7). An upright Raman microscope (NTEGRA Spectra, NT-MDT, Russia) was employed for TERS measurement.
3. Results and discussion
This work deals with silicon AFM tip, so it is essential to understand the optical properties of silicon tips themselves. The electric field enhancement (defined as |M|2 = |Eloc/Ein|2, where Eloc and Ein are local and incident electric field, respectively) located 0.5 nm below the tip was considered for all calculations in this study. Figure 1 shows the calculation model and the related results for tips with different radii. In this model, the tip is simulated as a conical taper terminated by a hemisphere of various radii, r. Previous results from our group and other groups revealed that the maximum electric field enhancement for the side-illumination TERS is in the same order for an incident angle in the range from 30 o to 60 o for a metallic gold tip [34, 36]. We chose an angle of 40 o for incident in considering the practical accessible angle in a real TERS setup.
Fig. 1 (a) Schematic diagram of a silicon AFM tip without a substrate; (b) The tip radius dependent electric field enhancement (|M|2).
Figure 1(b) shows the electric field enhancement provided by silicon AFM tips. In all tip radii, three peaks can be found in the range from 400 nm to 800 nm. These peaks blue shift and the electric field enhancement increases sharply with the decrease of the tip radius. The nonresonant effect, namely the lightning rod effect, which results from the increase of the surface charge density at the tip apex, becomes more and more apparent for sharper tips and thus leads to a remarkable electric field enhancement. It is also found that the maximum enhancement appears in a wavelength that is far shorter than the commonly used laser wavelength (e.g., 632.8 nm) in most TERS instruments. Although there are peaks in the red region, the electric field enhancement is very weak even for the sharpest tip, which is only about 2 orders of magnitudes. The strongest electric field enhancement occurs at the smallest tip radius of 1 nm, which is similar to a previous study of the gold tip [40]. The differences between these two kinds of tips lie in the maximum intensity and position of the peak. The maximum TERS enhancement factor, that is proportional to the fourth power of the local E-field enhancement (|M|4 = |Eloc/Ein|4), for the gold tip is as high as 107~108 in the range of 750-1000 nm, whereas that of the silicon tip is only 104~105 in the range of 400-700 nm (for simplicity, we neglect the shift in Raman frequency relative to the excitation frequency). The free electron density in silicon (1013~1016 cm−3) is too low to support the surface plasmon resonance (SPR) in the visible range [16]. Instead, these peaks mainly arise from the lightning rod effect.
In the above simulation, no substrate is present and the electric field enhancement is very weak in the visible range even for the sharpest tip, which is insufficient for a TERS experiment. However, in TERS measurements, the sample should be placed on a certain substrate and the presence of a substrate provides a way to introduce the near field electromagnetic coupling between the tip and substrate. Such a coupling may lead to an enhanced electric field and provide an effective way to improve the TERS detection sensitivity [34, 41]. Aiming at understanding the coupling effect between the tip and substrate, we calculated the spectral characteristics for a silicon AFM tip on three different types of substrates (e.g., silica, silicon, and gold substrates). Figure 2(a) shows the calculation model, where the incident angle, tip-substrate distance and substrate thickness are θ = 40°, d = 2 nm, and t = 100 nm, respectively. The tip radius is set as r = 50 nm where the electric field enhancement reaches its maximum for a gold substrate. Figure 2(b) shows the spectral responses of the electric field enhancement on the three types of substrates. The most pronounced but very weak peak for silicon AFM tip without a substrate is in the green-yellow region (about 567 nm). After a substrate was placed at a distance of 2 nm below the silicon-AFM tip, the field enhancement could be significantly improved. The maximum TERS enhancement factor is 2 × 107 for a gold substrate, 6.4 × 105 for a silicon substrate, and 5.6 × 103 for a silica substrate. It is clear that the maximum enhancement can be obtained for the gold substrate and is three orders of magnitude larger than that for a substrate. It means that the electric field enhancement can be improved greatly while introducing a plamon-active substrate even for a non-plasmon silicon tip. The coupling system we calculated in this work is a side-illumination geometry, so the vertical component of the incident field plays a dominant role for the electric field enhancement, which means that the coupling effect between the tip and the substrate is the key contribution to the enhancement. For the tip alone case, the electric field enhancement is weak. Once a plasmon-active substrate is placed below the tip, a huge enhancement will be generated because of the strong near-field coupling between the tip and substrate.
Fig. 2 (a) Schematic diagram of a silicon AFM tip with a substrate; (b) The substrate dependent electric field enhancement. The radius of the silicon tip is 50 nm.
Such an enhancement is already comparable with most reported TERS studies, which may have implication for conducting TERS study over a gold substrate to achieve a good TERS signal using a silicon tip. The overall near-field coupling effect follows a same trend for the three substrates over the whole spectral range between 400 and 1000 nm. The surface plasmon resonance wavelength red shifts when the substrate is changed from silica to silicon and gold. It is interesting and meaningful to find that the greatest enhancement appears in the red region for a gold substrate.
From the above study, in the strong near-field coupled AFM-TERS configuration, the substrate plays a key role in determining the position and intensity of the LSPR peaks. The LSPR response of the coupled system is dominantly contributed by the coupling effect between the substrate and the tip. However, we found that the tip size also has a great impact on both the resonant wavelength and the intensity. As it can be seen in Fig. 3, for a gold substrate, when we change the tip radius from 1 to 70 nm, the resonant wavelength can be tuned from 500 to 700 nm. The electric field enhancement increases sharply with the increase of the tip radius from 1 nm to 50 nm. The strongest enhancement can be achieved with a tip radius of 50 nm, which will give a resonant wavelength of 616 nm and electric field enhancement of 4000. Further increase of the tip radius will then lead to a decrease in the enhancement. This tendency is different from that of the silicon AFM tip without the gold substrate. In the tip alone system, the tip is modeled as a conical taper terminated by a hemisphere, and the maximum enhancement appears at the smallest tip size as a result of the lightning rod effect. Whereas, in the case of tip-substrate coupling system, these sharp SPR peaks arise from the dipole-dipole coupling effect between the silicon tip and the gold substrate. In this case, we should consider the competing effects of the radiation damping and the surface scattering of the tip [42, 43]. As a result, there will be an optimal tip size for TERS. When the size of the tip is small, the surface scattering is very weak leading to a low electric field enhancement. The enhancement value increases with the increased tip size until 50 nm. Then, the radiation damping effect becomes strong and results in the decrease of the electric field enhancement. The enhancement is at the same order of magnitude (which is 103) for the tip radius in the range of 10 nm to 70 nm. Therefore, in order to achieve a higher spatial resolution, one may choose a sharper tip (for example 10 nm) without sacrificing much the TERS enhancement.
Fig. 3 The curvature radius dependent electric field enhancement for a silicon AFM tip with a gold substrate.
From the above calculation, it is found that the electric field enhancement can be improved drastically after introducing a plasmon-active noble metal substrate. If we further coat the AFM tip with a plasmon-active material, we would expect a higher electric field enhancement. For this purpose, we further simulate the cases for silicon tip with a gold layer. The model used in the calculations is shown in Fig. 4(a). A p-polarized plane wave is incident from the side at an angle of θ. The thickness of the gold coating, the tip-substrate distance and the radius of the silicon AFM tip in the calculation model are h, d, and r, respectively. As shown in Fig. 4(a), the maximum of the electric field enhancement is located at the center right below the tip. Figure 4(b) shows the dependence of the electric field enhancement on the thickness of the gold coating, where θ = 40°, d = 2 nm, and r = 5 nm. We can find that the electric field enhancement, resulting from the dipole-dipole coupling effect between the gold coating and gold substrate, will increase sharply with the increase of the thickness of gold coating from 5 nm (peak at 528 nm) to 80 nm (peak at 613 nm) as a result of the increased size effect of the tip. Further increase of the thickness of the gold coating will no longer bring additional electric field enhancement. In addition, a new LSPR peak around 596 nm, resulting from the quadrupole-quadrupole coupling effect, appears as the thickness of gold coating reaches 50 nm. The intensities of these quadrupole peaks increase while the thickness of gold coating increases from 50 nm (peak at 596 nm) to 80 nm (peak at 613 nm). Further increase of the thickness of the gold coating leads to the decrease in the peak intensity. We also observe that both dipole and quadrupole peaks red shift with the increase of the thickness of the gold coating. Furthermore, it can be found from the above result that in a tip-gold substrate coupling system, the maximum TERS enhancement as high as 9 orders of magnitude can be obtained when a silicon tip is coated with a gold layer of 80 nm thickness. This value is roughly 2 orders of magnitude higher than that for a silicon AFM tip with a tip radius of 50 nm (see Fig. 3). Most importantly, the maximum enhancement is of the same order of magnitude for the thickness of the gold coating in the range between 50 and 100 nm and the peak positions are located in the region from 580 to 720 nm. It indicates that 632.8 nm from He-Ne laser would be an ideal choice as the excitation source for the coupled gold-coated silicon tip and gold-substrate TERS system. It is no wonder that this wavelength is the most widely used excitation in the majority of TERS experimental studies using gold tips [44, 45]. Figure 4(c) shows the electric field distribution of Si@Au AFM tip on a gold substrate excited at 615 nm, where θ = 40°, d = 2 nm, h = 80 nm, r = 5 nm. We can find that the highest TERS enhancement is located in the nanogap between the tip and substrate.
Fig. 4 (a) Calculation model for a Si@Au AFM tip over a gold substrate; (b) The dependence of the electric field enhancement on the thickness (h) of the gold coating; (c) The near field distribution of Si@Au AFM tip on a gold substrate excited at 615 nm, where θ = 40°, d = 2 nm, h = 80 nm, r = 5 nm; (d) The dependence of the spatial resolution (SR) (using 632.8 nm as the excitation wavelength) on the thickness (h) of the gold coating.
In the above calculations, we have discussed the dependence of the electric field enhancement on the thickness of the gold coating over the silicon AFM tips. Actually, the detection sensitivity and spatial resolution are the two most important factors for TERS experiments. Although, a spatial resolution of 0.5 nm has been demonstrated under a very special condition [1], improving the spatial resolution is still one of the key goals and the effort of most TERS studies. The electric field in TERS is highly localized in the nanogap between the tip and the substrate, which consequently generates huge Raman signal for the molecules adsorbed on the substrate and provides a high spatial resolution close to or smaller than the tip size. To this end, the spatial resolution of Si@Au TERS was theoretically investigated to find out the optimal geometric parameters of the tip and the gold substrate. Figure 4(d) gives a plot of the spatial resolution of TERS with the thickness of gold coating, where r = 5 nm, d = 2 nm, and θ = 40°. It can be seen in the figure that the spatial resolution of TERS can be improved with the decreased thickness of gold coating (corresponding to a decrease in the tip radius). With the decrease of the thickness from 50 to 5 nm, the spatial resolution will increase from 11 to 5 nm. This tendency is similar to that of an isolated gold tip [46]. The only difference is that the spatial resolution for this coupled system is higher than that for the gold tip alone when the tip radius is of the same value. The main reason is that the strong coupling effect of the gold substrate with the gold tip will help improve the spatial resolution. However, we should point out that the highest spatial resolution of 5 nm is achieved with a gold coating thickness of 5 nm. At this thickness, the electric field enhancement is the lowest. It means that increasing the thickness of gold coating will lead to a larger TERS enhancement but a smaller spatial resolution in a specific coating thickness range. Therefore, a suitable gold coating thickness should be selected to balance the electric field enhancement and the spatial resolution depending on the experiment purpose.
To verify the theoretical calculation, we prepared silicon AFM tips with gold-coating of three thicknesses, that is, 115 nm (h1), 65 nm (h2), and 30 nm (h3), and used them for TERS measurements of the MGITC coated gold (111) surface. These three thicknesses are close to the optimal thickness of the gold coating for achieving the highest TERS enhancement in the range from 50 nm to 100 nm. Figure 5(a) shows the side-view SEM images of Si@Au AFM tips with gold coatings of the three thicknesses. The radius of the original silicon tip is 5 nm. The corresponding TERS spectra of MGITC monolayer on a freshly flame-annealed gold (111) surface are shown in Fig. 5(b). The experimental results demonstrate that the tip with 65 nm thickness is superior to the other two tips, no matter the signal strength or signal-to-noise ratio, which qualitatively matches our simulation results well. It should be noted that the experimentally obtained amplitude of variation of signal strength for the three tips seems, to some extent, to be deviated from the quantitative simulations. The possible reason is that the experimentally prepared tips (see Fig. 5(a)) have some roughness, which is different from the theoretical model, in which an ideal cone tip was used.
Fig. 5 (a) SEM images of Si@Au AFM tips with various gold coating, where h1 = 115 nm, h2 = 65 nm and h3 = 30 nm. The radius of the silicon tip is 5 nm. (b) The corresponding TERS spectra of MGITC monolayer on a freshly flame-annealed gold (111) surface in (a). h1 was obtained at a laser power of 0.1 mW and an acquisition time of 3 s. h2 was obtained at 10 μW and an acquisition time of 1s. h3 was obtained at a laser power of 0.2 mW and an acquisition time of 3 s.
4. Conclusions
In conclusion, both theoretical simulations and experimental works have been done to investigate the gold-coated atomic force microscopy (AFM) tip-substrate system for tip-enhanced Raman spectroscopy. The resonance frequency of TERS tip could be tuned by changing the geometric parameters of the calculation TERS system. The maximum TERS enhancement factor for a silicon AFM tip on a gold substrate is 1.5 × 107, which is over two orders of magnitude larger than that of silicon AFM tip without a substrate. The gold coating on a silicon AFM tip will further help improve the TERS enhancement roughly by two orders of magnitude, to reach nine orders of magnitude. Increasing the thickness of the gold coating will bring a larger TERS enhancement but reduce the spatial resolution. The spatial resolution for a 5-nm-radius silicon tip with gold coating layer of 5 nm placed over a gold substrate could reach as high as 5 nm. The calculation results agree well with the TERS experiments using silicon-AFM tips with different thicknesses of the gold coating.
Acknowledgments
This work was supported by the National Instrumentation Program (2011YQ03012406), the NSF of China (21173171, 11474239, and 21321062), and Ministry of Education of China (IRT13036).
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