Journal of Nano Research, ISSN: 1661-9897, Vol. 89, pp 1-8

doi: 10.4028/p-z5bN50

© 2025 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2025-04-23

Accepted:
2025-04-27

Online:
2025-10-13

Gold Microplates with Nano-Through-Holes for Nano-Raman Analysis of Solid Surface

The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

Oscar David Ojdany Noguez Del Aguila , Hiroki Itasaka , Takahiro Namazu , and Masayuki Nishi*1,d

Kyoto University of Advanced Science (KUAS), Faculty of Engineering, 18 Yamanouchi Gotanda-cho, Ukyo-ku, Kyoto, 615-8577, Japan

National Institute of Advanced Industrial Science and Technology (AIST), 4-205 Sakurazaka, Moriyama-ku, Nagoya, Aichi, 463-8560, Japan

ª2022md01@kuas.ac.jp, bh.itasaka@aist.go.jp, ,namazu.takahiro@kuas.ac.jp, dnishi.masayuki@kuas.ac.jp

Keywords: Raman spectroscopy, gold-microplate, nanoholes, nanoanalysis

Abstract

Raman spectroscopy is a powerful tool to analyze materials. The demand for analyzing materials in a smaller analytical region is increasing as technology advances. Tip-enhanced Raman spectroscopy (TERS) is an option. However, measuring the surface of three-dimensional bulk materials is quite challenging, since simultaneously excited micro-Raman signals hide the enhanced nano-Raman signals. In 2024, another approach of using a porous gold membrane was reported to dedicate solid surface analysis. However, this method, for example, sacrifices the lateral analytical spot size because nanopores distribute throughout the membrane. Here, thick gold microplates with nano-through-holes in the center are fabricated. The gold microplates provide a smaller analytical spot because nano-through-holes are fabricated only in a spatially limited region. The microplates and the surrounding structures are clearly visible under optical microscopes. We moved gold microplates to another location on a silicon substrate using a manipulator and successfully demonstrated nano-Raman measurements of silicon surface via nano-through-holes. The finitedifference time-domain calculations confirmed that enhanced electric fields are available by the nano-through-holes and revealed that the nano-Raman signals come from the surface of silicon within a depth of 5.4 nm .

Introduction

The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

Techniques to analyze the surface of solids play crucial roles in understanding functional materials since the surface is often the place from which chemical reactions including catalytic processes and physical phenomena including crack propagation start. Not only physical information such as roughness and morphology but also chemical information such as material, chemical bonds, atoms, valence states, and so forth, are available depending on analytical techniques. Information at a higher resolution can unveil individual structural differences than spatially averaged information. Thus, the demand for analyzing surfaces at a smaller spot size and depth is increasing as technology advances. Table 1 lists the rough typical values of common techniques for chemical information of surface. In Table 1, each technique uses either x-ray, electrons, or light for stimulating sample materials and for detecting the response. X-ray, including characteristic x-ray for identifying the atom, propagates through material well, and is difficult to have a small focal spot due to the lack of lens with high refractive index to x-ray. Electrons easily lose energy propagating through material and require vacuum for SEM, ultra-high vacuum especially for XPS and AES. Focal spots less than 10 nm are available with an electron gun and electromagnetic lenses. Raman spectroscopy uses light scattering on material and can perform in the real environment because vacuum is unnecessary. Light is easy to focus on but at most down to around its wavelength due to the diffraction limit; roughly 500 nm . This focal spot size is much bigger than that of electrons above. Those are the reasons for the differences in values in Table 1, except for TERS. TERS was firstly demonstrated in 2000 [1-4]. This relatively
new technique is attractive in terms of the small analytical spot size and depth, and has contributed to analyzing materials at nanoscales [5-13].

Table 1. Rough typical values of analytical spot size (lateral) and depth of common techniques for chemical information of surfaces.

Technique | Lateral spot size [nm] | Depth [nm]
X-ray photoelectron spectroscopy (XPS) | ≥ 10,000 | 1–10
Energy dispersive x-ray spectroscopy (EDS) equipped with a scanning electron microscope (SEM) | ~ 500 | ~ 500
Auger electron spectroscopy (AES) | ≥ 10 | 1–10
Micro-Raman spectroscopy | ≥ 500 | ≥ 500
Tip-enhanced Raman spectroscopy (TERS)† | ~ 10 | 2–10

†However, measurable samples are limited
TERS combines a micro-Raman spectroscope with a scanning probe microscope (SPM) [14,15]. A specially prepared metallic nanoprobe illuminated by laser light causes an enhanced electric field confined only close to the tip of nanoprobe (called near field). This enhanced electric field enables us to go beyond the diffraction limit and to detect the Raman signal from the nanoregion. Raman scattering is an inefficient phenomenon and undetectable from nanoregions without enhancement because the intensity is proportional to the active sample volume. Since the laser spot illuminating the tip of the TERS probe is much bigger than the probe tip, micro-Raman signals, called far field, simultaneously generate from the sample in the laser spot and enter the detector. To obtain a nearfield intensity high enough to ignore the far-field Raman contribution, it is necessary to have the intensity enhancement of one order of magnitude greater than the ratio of the far-field volume to the near-field one. The lower the dimension of the sample, the smaller the necessary enhancement. In fact, TERS is commonly applied to analyzing low dimensional materials: nanoparticles (0-D), nanotubes and nanowires (1-D), nanosheet (2-D). To analyze the surface of 3-D structures, the intensity enhancement of is required, considering the volume difference. To obtain this enhancement only by the TERS probe is quite challenging.

An example of TERS measurement of the surface of a 3-D structure is a work performed by Lee et al. in 2007 [16]. Silicon is a material that is often analyzed by Raman spectroscopy. It reveals crystal orientation and stress of crystalline silicon [17-19]. Lee et al. noted a need for stress mapping with nano-scale spatial resolution due to the shrinking of silicon structures to less than 100 nm . TERS point-analysis on silicon showed that optimizing the excitation beam polarization improves the contrast between near-field and far-field. They estimated as a lateral resolution via line analysis; however, the sample for the line analysis was not only silicon but "periodic lines ( 250 nm wide and 30 nm in height) on Si ," having far-field contribution deduced from the counts.

Considering the values in Table 1, prioritizing the smaller analytical depth is worth sacrificing the lateral analytical spot size. Wyss et al. developed a porous gold membrane and demonstrated surfacesensitive Raman spectroscopy in 2024 [20]. SPMs and TERS probes are not used. The nanopores randomly distributed throughout the membrane work as plasmonic hot spots to cause enhanced electric fields, while the gold membrane other than nanopores prevents the sample from being directly illuminated by laser light. In other words, it can prevent the micro-Raman signal from generating. The analytical spot size is the same as the laser spot size in this method. In other words, the analytical spot size becomes when the common high-magnification objective lens ( ) and a He-Ne 633 nm laser are used in our current system as is described later.

This study uses micro-sized gold plates with a thickness of around 70 nm and with a nano-throughhole array only in its center region, instead of the gold membrane above. These gold microplates provide smaller analytical spots because the analytical spot size is determined not by the laser spot size but by the size of a nano-through-hole array. They can also add advantages: Both microplates and the surrounding structures are clearly visible through optical microscopes. Microplates are
movable; we can move microplates to locations of interest under optical microscopes. Also, there is another degree of freedom of controlling the nano-through-hole array structure for the better electricfield enhancement. Here, we fabricated the gold microplates with nano-through-holes, moved the microplates, and demonstrated near-field Raman signal detection of silicon using the gold microplate. To make it easier to detect the near-field Raman signals as a starting point of research, we processed 4-by-4, in total 16, nano-through-holes in gold microplates, although reducing the number of nano-through-holes can further decrease the analytical spot size. This 4-by-4 still makes analytical spot size around 500 nm . Also, a silicon wafer is chosen as the starting sample to measure because it provides an intense and sharp Raman peak.

Experimental Details

The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

A flowchart of the experimental procedure is shown in Fig. 1. Single crystalline gold microplates with a thickness of around 70 nm were synthesized by a method we reported in 2007 [21]: A 3:1(v/v) mixture of a 10 mM hydrogen tetrachloroaurate aqueous solution and a 10 mM 3mercaptopropyltrimethoxysilane ethanol solution in a glass container was placed in an oven at for 48 hours. The mixture was sonicated for 10 min at 28 kHz . After gold microplates settled, the liquid above them was removed. Then, ethanol was poured into the glass container for storage.

A focused ion beam (FIB) system (SMI 3050, SII NanoTechnology) was used to process nano-through-holes in gold microplates. Gold microplates were deposited on a silicon substrate by dropping the solution onto it, and then the substrate was placed in an oven at for 60 min for drying. A gold microplate is irradiated with a FIB at , and ions with an overlap scan mode. A FIB-processed gold microplate was moved onto a different location on the silicon substrate using a manipulator (SS-305, MicroSupport). SEM (S-4800, Hitachi) observation of the FIB-processed area of silicon, on which the FIB-processed gold microplate had been, revealed the formation of nano-through-holes: Nanoholes formed at silicon which was underneath the gold microplate during FIB irradiation. Atomic force microscopy (Smart SPM-1000, AIST-NT) was also performed to check whether the region of nano-through-holes of the gold microplate is in contact with the surface of silicon after the gold microplate was moved.

Raman spectra were measured using a Raman microscope (LabRAM HR Evolution, HORIBA) with a He-Ne laser (Wavelength: 633 nm ) focused by an objective lens ( , NA=0.9). To confirm that electric-field enhancement is available by the nano-through-hole structure fabricated and to know the profile of the enhanced electric field, the finite-difference time-domain (FDTD) method was performed using a commercial software (Poynting for Optics, Fujitsu). In FDTD calculations, only a single nano-through-hole is considered. The calculation was performed in a three-dimensional region with . The gold microplate was modeled with dimensions of . A plane wave source was placed so that the plane wave at a wavelength of 633 nm was incident perpendicular to the surface of the gold microplate. The plane wave was linearly polarized in the parallel direction to the surface of the microplate. The Lorenz-Drude model was employed for all mediums in the calculation. The calculation region was discretized by a nonuniform mesh with the cell size ranged from 2 to 16 nm .

[picture]

Fig. 1. Flowchart of the experimental procedure.

Fig. 1. Flowchart of the experimental procedure.

Results and Discussion

The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

Figure 2(a) is an SEM image of nanoholes of a gold microplate processed by a FIB, and (b) is an image of silicon at the same FIB-processed area after the gold microplate was moved. Nanoholes in Fig. 2(b) were similar to those in Fig. 2 (a). This reveals that the nanoholes of the gold microplate are through-holes. The diameter of the nanoholes of gold was 100 nm and that of silicon was 80 nm . This shows that nano-through-holes are truncated in diameter from top to bottom, where "bottom" means the silicon side. This truncated structure of 100 nm to 80 nm in diameter is used as a model of the nano-through-hole to perform FDTD calculations of electric fields.

[picture]

Fig. 2. a) an SEM image of nanoholes of a gold microplate processed by a FIB, and b) that of silicon at the same FIB-processed area after the gold microplate was moved.

Fig. 2. a) an SEM image of nanoholes of a gold microplate processed by a FIB, and b) that of silicon at the same FIB-processed area after the gold microplate was moved.

Figure 3(a) shows an optical microscope image of a gold microplate which was moved from its FIB-processed location. At the tail end of the arrow, a darker gray area with a shape similar to the gold microplate at the arrowhead was seen in the figure. This dark gray area is where the gold had been in FIB processing. The reason for the dark gray is as follows. The dark gray area is inside the rectangle of a lighter gray area. This rectangle is where a scanning ion image was taken to see and confirm that a FIB focus was on the gold microplate before nanohole processing. Since silicon which was underneath the gold microplate was hidden from FIB illumination throughout the process, it shows the same dark gray color as the outside the rectangle. The gold microplate showed the location of nanoholes by the orange spot, a little different color spot from the other part. Similarly, the former gold-microplate area, that is, the dark gray area in the rectangle showed a lighter gray spot at its center. Thus, this lighter gray color also proves that the nanoholes are through-holes. This is another way to confirm the through-hole formation without SEM observation. Figures 3(b) and (c) show the result of AFM measurement of the gold microplate in Fig. 3 (a). Figure 3 (b) is a line profile of the height along the dashed arrow in Fig. 3 (c), where one step of the measurement was . Nanoholes are on the dashed arrow. Figure 3(b) shows that the height of the gold microplate from the silicon surface is around 70 nm . This height is equivalent to the thickness of the gold microplate. This implies that the nano-through-holes are probably in contact with silicon after the gold microplate was moved.

[picture]

Fig. 3. a) An optical microscope image of a gold microplate with nano-through-holes which was moved from its FIB-processed location. b) and c) are the result of AFM measurement of the gold microplate in (a). b) is a line profile of the height along the dashed arrow in c).

Fig. 3. a) An optical microscope image of a gold microplate with nano-through-holes which was moved from its FIB-processed location. b) and c) are the result of AFM measurement of the gold microplate in (a). b) is a line profile of the height along the dashed arrow in c).

Figure 4(a) shows a Raman map showing the intensity of silicon peak at [22] inside and around the gold microplate in Fig. 3(a). In this map, lighter gray is more intense than black. Figure 4(b) shows Raman spectra measured on silicon, at nanoholes, and on gold. In the spectra at nanoholes and on gold, broad photoluminescent background across the measurement window was removed by Raman software. At nanoholes, which was confirmed as nano-through-holes, the silicon peak was successfully detected. Some weak broad peaks were noticed, too. The intensity of the peak at at nanoholes was much lower than that of silicon outside the gold. No specific peak was observed on gold. This shows that the gold microplate except nano-through-holes worked as a shield to prevent the silicon underneath gold from being illuminated.

Figure 5(a) shows the result of the FDTD calculations. Two types of mediums were employed in the calculation region on the two sides of the gold microplate as a border: vacuum on the left-hand side where the plane wave was incident and silicon on the right-hand side. A vacuum media with a circular truncated cone shape was placed at the center of the gold microplate to act as a hole through the microplate. The hole diameters were set as 100 and 80 nm on the vacuum and silicon sides, respectively. These diameters were determined by the result shown in Fig. 2. FDTD calculations show that the electric field that is greater than that of the incident light is available at the nano-throughhole. Also, this enhanced electric field enters silicon. Figure 5(b) is the depth profile of normalized intensity at specific points from the gold edge along the "depth" arrow in Fig. 5(a). An exponential decay curve was in good agreement with the depth profile and provided 2.4 nm as the analytical depth defined as of the initial intensity. If we define the analytical depth as of the initial intensity, we obtain 5.4 nm . Thus, the silicon peak observed at the nano-through-holes comes from the silicon surface at least within 5.4 nm .

[picture]

Fig. 4. (a) a Raman map showing the intensity of silicon peak at inside and around the gold microplate in Fig. 3(a). (b) Raman spectra measured on silicon, at nanoholes, and on gold.

Fig. 4. (a) a Raman map showing the intensity of silicon peak at inside and around the gold microplate in Fig. 3(a). (b) Raman spectra measured on silicon, at nanoholes, and on gold.

[picture]

Fig. 5. (a) A result of FDTD calculations. (b) Depth profile of normalized intensity of the timeaveraged electric field from the gold edge along the "depth" arrow in (a).

Fig. 5. (a) A result of FDTD calculations. (b) Depth profile of normalized intensity of the timeaveraged electric field from the gold edge along the "depth" arrow in (a).

To clarify that the electric field enhancement enabled us to detect the spectrum at nano-throughholes experimentally, we calculated the enhancement factor :

where is the Raman spectral intensity of silicon measured outside the gold microplate. is the one measured at nano-through-holes. and are the analytical volume of silicon outside the gold microplate and at nano-through-holes, respectively. Both volumes were assumed as those of cylinders. For calculating , the lateral analytical spot size is equivalent to the laser spot size. The laser spot size was determined by the knife edge method [23]: A line profile of intensity of Raman peak of silicon at was measured from silicon to across a nanohole-free gold microplate. The nanohole-free gold microplate was chosen to have one side perpendicular to the line and to be large enough to maintain this perpendicularity. The distance until the peak intensity decreases from to was in average, in other words, the radius of the laser spot is 750 nm . The analytical depth of was calculated by [24-26], where is the wavelength of the laser: 633 nm , and , resulting in 781 nm . For calculating , the diameter of the 16 nano-through-holes on the silicon side, was used as the lateral analytical size of each nano-through-hole (in other words, the radius is 40 nm ), although the strongly enhanced region is not homogeneously distributed as shown in Fig. 5(a). The analytical depth of 5.4 nm was chosen by the depth profile in Fig. 5(b). These values and the assumption of the cylindrical volume were determined not to overestimate the EF . The resulting EF was 82 . Since the average intensity of the Raman peak of silicon was 650 counts at the nanoholes in our measurement, the enhancementfree counts would be around 8, which is within errors in the measurement. Therefore, the Raman peak of silicon was detected owing to the electric-field enhancement and that is considered as Raman signals from the surface within 5.4 nm .

Experiments have revealed that SEM observation of nanoholes contaminates them, and SEM observation to confirm the detailed structure of nanoholes eventually became the last procedure in the series of experiments. The gold microplate shown in Fig. 3(a) was observed with the SEM after all of Raman and AFM measurements are performed. In fact, another Raman measurement after SEM observation showed carbon-contaminated Raman spectra at the nanoholes. Figures 6(a) and (b) show SEM images of the gold microplate, shown in Fig. 3(a), with different magnifications. Nanoholes are focused in (b). Figure 6(c) shows an SEM image of silicon that was underneath the gold microplate in nanohole-processing. Figure 6(a) shows that a part of gold, on the left-hand side, was deformed by manipulation. This happens especially when microplates are moved from the original location, where microplates were settled from the solution and dried, due to the stronger interaction between microplates and silicon. Figures 6(b) and (c) revealed that unfortunately or fortunately, the nanoholes are distorted probably due to a deviation from proper alignment of the ion beam or desired condition. Also, cracks are observed. When those cracks appeared is unclear. The total area of 16 nanoholes in silicon was calculated by an image analysis of Fig. 6(c). This provided us with , because the total area was smaller than that of 16 circular nanoholes with a diameter of 80 nm . How this distortion and cracks of nano-through-holes contributed to the experimentally obtained nano-Raman signal are unclear yet. However, this provides another degree of freedom to have more enhancement in the future.

[picture]

Fig. 6. (a) An SEM image of the gold microplate shown in Fig. 3(a); (b) a magnified image of (a) with a focus on nanoholes; (c) an SEM image of silicon in the same area as (b), and was underneath the gold microplate in the FIB nanohole-processing.

Fig. 6. (a) An SEM image of the gold microplate shown in Fig. 3(a); (b) a magnified image of (a) with a focus on nanoholes; (c) an SEM image of silicon in the same area as (b), and was underneath the gold microplate in the FIB nanohole-processing.

As we discussed above, silicon peak at nano-through-holes is from the surface within a depth of 5.4 nm and is comparable with TERS. The lateral analytical spot size is determined by the size of the nano-hole array: as is seen in Fig. 6. This lateral spot size was one third of that of the laser spot size with a conventional micro-Raman set-up: wavelength: 633 nm , objective lens:
, NA=0.9. Further small lateral spot sizes are one of our targets in future work. Just placing the gold microplates onto a desired location on the sample surface allows the users to have chemical information of a surface using a conventional micro-Raman. This is an advantage of our method. However, our method is currently limited to point analysis. Also, future issues are methods to move gold microplates smoothly under precise control and methods to more easily check the contact between the microplate and sample surface and enable contact if not.

Conclusions

The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

A nano-through-hole array was processed at around the center of thick gold microplates using a FIB. This gold microplate with nano-through-holes enabled us to measure nano-Raman spectra of the surface of silicon within a depth of 5.4 nm using a He-Ne laser. The enhancement factor was greater than 82 . Future work will find more appropriate nano-through-hole structures for measurement and better understanding of a variety of solid surfaces.

Acknowledgements

The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

This research was partially supported by JSPS KAKENHI Grant Number 25 K 08795.

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The original version of this paper is available on https://www.scientific.net/JNanoR.89.1.pdf

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