Researching on fabrication and optical properties of a single au nanohole

74 Thien Q. Duong, Khuong T. T. Pham, Nam L. Nguyen RESEARCHING ON FABRICATION AND OPTICAL PROPERTIES OF A SINGLE AU NANOHOLE NGHIÊN CỨU CHẾ TẠO VÀ KHẢO SÁT ĐẶC TÍNH QUANG CỦA LỖ NANO VÀNG Thien Q. Duong, Khuong T. T. Pham, Nam L. Nguyen University of Technology and Education - The University of Danang dqthien@ute.udn.vn, pttkhuong@ute.udn.vn, nlnam@ute.udn.vn Abstract - In this research, a detailed fabrication process of a single Au nanohole is demonstrated. An Au nanohole with various diameters is fabricated on an Si3N4 membrane through well- controlled steps of the fabrication process. The optic property of the Au nanohole is also investigated by means of COMSOL Multiphysics. The research results show that light scattering spectra from the Au nanohole exhibits a localized surface plasmon (LSP) resonance in the interaction between the light and metal nano structures. Especially, there appears on the spectrum peak a red light varying according to diameters of the nanohole. The research results also indicate the appearance of a dual extreme electric field around the nanoholes which increases along with wave force. This knowledge forms an important base for opening up possibilities to apply metal nano structures to optics and optoelectronics as well as sensory biology. Tóm tắt - Trong nghiên cứu này, chi tiết về quy trình công nghệ chế tạo lỗ nano vàng được trình bày. Lỗ nano vàng với đường kính khác nhau được chế tạo trên màng Si3N4 thông qua việc kiểm soát các bước trong quy trình công nghệ. Đặc tính quang của lỗ nano được khảo sát bằng phần mềm COMSOL. Kết quả nghiên cứu cho thấy phổ tán xạ ánh sáng của lỗ nano có sự xuất hiện của các đỉnh phổ, là tính chất đặc thù của hiệu ứng plasmon định xứ bề mặt xuất hiện trong tương tác giữa ánh sáng với các cấu trúc nano kim loại. Đặc biệt, các đỉnh phổ này xuất hiện ở vùng ánh sáng đỏ và thay đổi theo đường kính của lỗ nano. Kết quả nghiên cứu cũng cho thấy sự xuất hiện của lưỡng cực điện trường xung quanh lỗ nano và cường độ điện trường này có độ gia tăng so với cường độ sóng tới. Kết quả nghiên cứu là nền tảng quan trọng mở ra khả năng ứng dụng cấu trúc nano kim loại trong lĩnh vực quang học, quang điện tử cũng như cảm biến sinh học. Key words - Au nanohole; nano fabrication technology; Mie scaterring; localized surface plasmon; near-field enhancement. Từ khóa - lỗ nano vàng; công nghệ chế tạo nano; tán xạ Mie; plasmon định xứ bề mặt; gia tăng điện trường gần. 1. Introduction Nanoscience and nanotechnology are recent revolutionary developments of science and engineering that are evolving at a very fast pace. They are driven by the desire to fabricate materials with novel and improved properties that are likely to impact virtually all areas of physical and chemical sciences, biological sciences, health sciences, and other interdisciplinary fields of science and engineering. The physical and chemical properties of materials are mostly determined by the motion of electrons. When the motion of electrons is confined in nanometer length scale (1-100nm), which happens in nanomaterials, unusual effects are observed. Recently, nanosized metal objects have attracted more research attention and exhibited interesting optical properties with numerous applications in technology and life science. The ability of noble metal nanostructures to manipulate light at the nanoscale has resulted in an emerging research area called plasmonics. Plasmonics has become a promising new device technology that aims to exploit the unique optical properties of metallic nanostructures to enable the routing and active manipulation of light at the nanoscale [1]. Research on the metallic nanostructures has been quite intensive over the years thanks to their ability to generate surface plasmon resonances (SPRs), which are collective oscillations of conduction electrons of the metals at the interface with a dielectric media. Plasmon resonances provide the capability of the localization and manipulation of light at nanoscale [2], which makes plasmonic nanostructures attractive building blocks for various novel applications spanning over the fields of biology, physics, chemistry, engineering, and medicine. For instance, they are widely used in sensing, surface enhanced Raman spectroscopy (SERS), plasmon-enhanced solar cells, photodetectors, drug delivery and cancer therapy as well as nanolasers, invisibility cloaks, and quantum computing [3-5]. Among noble metals, nanostructures composed of gold (Au) have attracted more research attention due to its various unique properties. Au nanostructures show intense surface plasmon resonance absorption, leading to an absorption coefficient orders of magnitude larger which make Au nanostructures to be candidate for many applications in electronics [6], solar energy [7] and especially in biosensors [8]. Au nanostructures offer, in addition to their enhanced absorption and scattering of light, good biocompatibility, facile synthesis and conjugation to a variety of biomolecular ligands, antibodies, making them suitable for use in biochemical sensing and detection, medical diagnostic and therapeutic applications. There have been several demonstrations of bio affinity sensors based on the plasmon absorption and scattering of nanoparticles and their assemblies. The optical properties of Au nanostructures are tunable throughout the visible and near-infrared region of spectrum as a function of nanoparticle size, shape, aggregation state and local environment [9]. They have recently measured the index sensitivities of Au nanospheres, nanocubes, nanobranches, nanorods, and nanobi-pyramids that exhibit different plasmon resonance wave lengths. Among these Au nanostructures, isolated nanoholes in optically thick metal films exhibit distinct optical properties [10]. There has recently been a presentation of a qualitative picture of the behavior of the electromagnetic fields around single nanohole apertures in optically thick metal films. It was argued that electric and magnetic dipoles are induced and ISSN 1859-1531 - TẠP CHÍ KHOA HỌC VÀ CÔNG NGHỆ ĐẠI HỌC ĐÀ NẴNG, SỐ 11(132).2018, QUYỂN 2 75 that these contribute significantly to the enhanced transmission [10, 11]. Such dipolar modes can be expected to result in localized surface plasmon (LSP) resonances, similar to those observed for noble metal nanoparticles. However, the presence of the metal film obviously constitutes a major difference between a hole and a particle, because a nanohole LSP resonance can be expected to couple not only to ordinary light waves but also to spatially extended surface plasmon polaritons (SPPs). In order to make a clear distinction between diffraction and plasmonic coupling effects, a deeper understanding of single nanohole optical properties is needed. In this research, the fabrication process of single Au nanohole is demonstrated. The e-beam lithography and reaction ion etching are applied to create a single Au nanohole on each Si3N4 membrane. The diameter of Au nanohole can be well-controlled during fabrication process. The interaction of light with single Au nanohole is also studied theoretically by using COMSOL Multiphysics which has been widely used in the simulation of plasmonic devices. The resulted elastic scattering spectra from the Au nanohole exhibit a localized surface plasmon (LSP) resonance in the red part of the visible spectrum. It is also found that the resonance peaks exhibit red-shift with increasing hole diameter. In addition, the electric field distribution around the resonant nanoholes, obtained from the simulations, exhibits a clear electric dipole pattern. 2. Fabrication and light scattering method 2.1. Fabrication of single Au nanohole Figure 1. Fabrication process of Si3N4 membrane The fabrication includes two processes: a freely suspended Si3N4 membrane is fabricated; this is followed by a set of steps to define and make a single Au nanohole on each of the membranes. In the first set of steps, as can be seen in Figure 1, a standard lithography technique and KOH etching are applied on the bottom side of Si wafer to produce the Si3N4 membrane. The Si wafer with 30nm Si3N4 layer on both sides is spinned coating with S1813 and baked at 110ºC for 1.5 minutes. The S1813 layer is exposed with UV-light for 8.5 seconds by using Mask aligner. After an exposure step, the wafer is then developed in MF319 for 15 seconds, this is followed by cleaning with DI water and N2 gas drying. The unwanted containing photoresist is cleaned by means of the reaction ion etching (RIE) with an O2 plasma for 30 seconds. This is followed by loading the wafer and etching Si3N4 layer in CF4 for 3 minutes in order to open a small window for KOH etching in the next step. The wafer is then soaked inside the mixed KOH with DI water in the ratio of 3:2 solution with temperature about 85ºC for 7 hours to produce a 30×30μm2 Si3N4 membrane. Figure 2. Fabrication process of Au nanohole on Si3N4 membrane Figure 2 shows the fabrication steps of making a Au nanohole on Si3N4 membrane. The e-beam lithography is applied to create a single nanohole pattern on each Si3N4 membrane by using PMMA 6A as photoresist. PMMA 6A is spinned coating on the Si3N4 membrane at 4000rpm for 25 seconds and baked on hot plate at 180ºC for 10 minutes. The sample is then exposed by using S-7000 e-beam system with the selected dose is 0.4μs/dot. After exposure, the sample is then developed in MIPK: IPA (1:3) solution for 60 seconds, and then immediately in IPA for 30 seconds and cleaning in DI water and N2 gas blow-drying. The unwanted containing photoresist is cleaned by using RIE with running an O2 plasma for 60 seconds. This is a highly recommended step in order to make sure that the photoresist is fully removed from the Si3N4 surface inside the designed pattern. This is followed by loading the sample and etching Si3N4 in CF4 for 8 minutes in order to open a nanohole on each Si3N4 membrane. After that, a 20nm thick of Au film is made on top of the nanohole by using e-gun evaporator with based pressure of 10-7torr and deposition rate of 1Å/s. Then, the Si3N4 on the back side of Au film is removed by CF4 etching for 8 minutes by means of RIE to form a single Au nanohole. 76 Thien Q. Duong, Khuong T. T. Pham, Nam L. Nguyen 2.2. Mie theory The historic origin of the scattering problem is connected to the diffraction by either circular cylinders or spheres, which are classical problems in both electromagnetism and acoustics. The first solution is claimed by Rayleigh to calculate the scattering for a dielectric circular cylinder, while the scattering by a dielectric sphere was first solved by Lorenz. After that, the diffraction of waves by conducting bodies, both the circular cylinder and the sphere, was presented by Thomson. However, the theoretical solution of the scattering problem by a sphere is widely known as Mie scattering, because of the well-known solution published by Mie [12]. An important concept in scattering problems is the cross section. This one indicates a quantity with the dimensions of an area related to the electromagnetic power scattered or absorbed by the object interacting with the incident wave, i.e., the scatterer. The total wave decomposes onto incident and scattered waves: 𝐸 = 𝐸𝑖𝑛𝑐 + 𝐸𝑠𝑐𝑎 (1) 𝐻 = 𝐻𝑖𝑛𝑐 + 𝐻𝑠𝑐𝑎 (2) where: 𝐸𝑖𝑛𝑐 , 𝐻𝑖𝑛𝑐: the electric and mag-netic fields of the incident wave; 𝐸𝑠𝑐𝑎 , 𝐻𝑠𝑐𝑎: the electric and magnetic fields of the scattered wave. The Time-average Poynting vector for time-harmonic fields gives the energy flux: 𝑆 = 1 2 𝑅𝑒[𝐸 × 𝐻∗] (3) It can be written as a function of the fields of the incident and scattered waves: 𝑆 = 1 2 𝑅𝑒[𝐸𝑖𝑛𝑐 × 𝐻𝑖𝑛𝑐 ∗ ] + 1 2 𝑅𝑒[𝐸𝑠𝑐𝑎 × 𝐻𝑠𝑐𝑎 ∗ ] + 1 2 𝑅𝑒[𝐸𝑖𝑛𝑐 × 𝐻𝑠𝑐𝑎 ∗ + 𝐸𝑠𝑐𝑎 × 𝐻𝑖𝑛𝑐 ∗ ] = 𝑆𝑖𝑛𝑐 + 𝑆𝑠𝑐𝑎 + 𝑆𝑒𝑥𝑡 (4) where is 𝑆𝑖𝑛𝑐 the Poynting vector of the incident wave, 𝑆𝑠𝑐𝑎 is the Poynting vector of the scattered wave, and 𝑆𝑒𝑥𝑡 is the term that arises from the interaction between the incident and the scattered fields. If we consider the scatterer’s surface 𝑆𝑠𝑐𝑎, the amount of energy absorbed by the object can be computed as: 𝑊𝑎𝑏𝑠 = −∫ �̂�. 𝑆𝑑𝑆 (5) where �̂� is the unit vector perpendicular to the surface. Moreover, we can obtain an expression of the amount of the electromagnetic energy absorbed by the scatterer as the following integral: 𝑊𝑎𝑏𝑠 = −∫ �̂�. 𝑆𝑑𝑆 = 𝑊𝑖𝑛𝑐 −𝑊𝑠𝑐𝑎 +𝑊𝑒𝑥𝑡 (6) where: 𝑊𝑖𝑛𝑐 = −∫ �̂�. 𝑆𝑖𝑛𝑐𝑑𝑆 (7) 𝑊𝑠𝑐𝑎 = −∫ �̂�. 𝑆𝑠𝑐𝑎𝑑𝑆 (8) 𝑊𝑒𝑥𝑡 = −∫ �̂�. 𝑆𝑒𝑥𝑡𝑑𝑆 (9) In fact, all the incident power is both incoming and outgoing from the surface; then 𝑊𝑖𝑛𝑐vanishes identically for a non-absorbing medium (for simplicity). Hence: 𝑊𝑒𝑥𝑡 = 𝑊𝑠𝑐𝑎 +𝑊𝑎𝑏𝑠 (10) At this point, we can give a physical interpretation to the term 𝑊𝑒𝑥𝑡 . It is the sum of the energy absorbed by the scatterer and of the scattered energy, i.e., it is the amount of energy subtracted from the incident wave because of the inter-action with the scatterer. For this reason, 𝑊𝑒𝑥𝑡 is called extinction power. The scattering, absorption, and extinction cross sections are defined as follows: 𝜎𝑒𝑥𝑡 = 𝑊𝑒𝑥𝑡 𝑆𝑖𝑛𝑐 , 𝜎𝑠𝑐𝑎 = 𝑊𝑠𝑐𝑎 𝑆𝑖𝑛𝑐 , 𝜎𝑎𝑏𝑠 = 𝑊𝑎𝑏𝑠 𝑆𝑖𝑛𝑐 (11) The relation between the cross sections is the same as the one between the powers: 𝜎𝑒𝑥𝑡 = 𝜎𝑠𝑐𝑎 + 𝜎𝑎𝑏𝑠 (12) The scattering cross section represents the amount of power scattered by the object over the amount of power per unit area carried by the incident wave. Similarly, the absorption cross section represents the amount of power absorbed by the scatterer over the amount of power per unit area carried by the incident wave. Finally, the extinction cross section represents the amount of overall power subtracted from the incident wave over the amount of power per unit area carried by the incident wave. 2.3. Localized surface plasmon Figure 3. Schematics of the stimulated plasmonic polarization in the nanosphere and the corresponding electric field modcation in its surrounding [13] A localized surface plasmon (LSP) is the result of the confinement of a surface plasmon in a nano object of size comparable to or smaller than the wavelength of light used to excite the plasmon. The LSP has two important effects: electric fields near the nano object’s surface are greatly enhanced and the object’s optical absorption has a maximum at the plasmon resonant frequency. The enhancement falls off quickly with distance from the surface and, for noble metal nanostructures, the resonance occurs at visible wavelengths. Gold nanostructures have been extensively used for applications both in biology (e.g. bio-imaging) and technology (e.g. photonics) due their unique optical properties. These properties are conferred by the interaction of light with electrons on the gold nanostructure surface. At a specific wavelength (frequency) of light, collective oscillation of electrons on the gold nanostructure surface causes a phenomenon called surface plasmon resonance resulting in strong extinction of light (absorption and scattering). Localized surface plasmon resonance (LSPR) associated with noble metal nanostructures, create sharp spectral absorption and scattering peaks as well as strong electromagnetic near-field enhancements at the surface as illustrated in Figure 3 [13]. ISSN 1859-1531 - TẠP CHÍ KHOA HỌC VÀ CÔNG NGHỆ ĐẠI HỌC ĐÀ NẴNG, SỐ 11(132).2018, QUYỂN 2 77 3. Results and discussion 3.1. Single Au nanohole The Au nanohole is fabricated at Quantum physics lab, Institute of physics, Academia Sinica, Taiwan. Figure 4 shows the SEM images of the Au nanoholes with different diameters based on Si3N4 membrane, the thickness of Au film is evaporated of 20 nm. As illustrated in Figure 2, the diameter of the nanohole on the Si3N4 membrane is strongly determined by e-beam lithographic parameters, the smaller in the original pattern designed for the smaller in the final nanohole is fabricated. It can be seen that the size of the nanohole can be well-controlled. Figure 4. SEM images of single Au nanohole with different diameters: (a)65 nm, (b)81 nm 3.2. Surface plasmon resonance of single Au nanohole Modeling and simulation of the optical properties of single Au nanoholes are of great importance to achieve an insight into the surface plasmon phenomena. In this study, an analytical model based on an approximate Mie theory has been designed using COMSOL Multiphysics software. The simulation domain comprises of an Au nanohole with an air domain truncated by a perfectly matched layer (PML). The Maxwell’s equations for the scattering of electromagnetic radiation by nanohole have been solved with the model. In this model, the wavelength dependent optical properties such as scattering cross section have been simulated where the far field calculations are done on the inner boundary of the PML. In addition, the near-field distribution around the nanohole also can be calculated. Figure 5 presents a comparison of scattering spectra of three Au nanoholes with diameters of 60 nm, 78 nm and 90 nm. As can be seen from the result, the elastic scattering spectra from the nanoholes exhibit a broad resonance in the red part of the visible spectrum, which is qualitatively similar to localized surface plasmon (LSP) resonances in gold nanodisks [14]. The averaged peak positions extracted from the scattering spectra are 729, 770 and 800 nm for 60, 78 and 90 nm holes, respectively. The nanohole resonance peaks red-shift with an increasing hole diameter, similar to gold nanodisks LSP resonances. As can be seen in Figure 5, it is also noted that an increase in nanohole diameter not only leads to a red-shift but also causes spectral broadening, which can be attributed to the shortened lifetime of the hole resonance. This is in contrast to gold nanodisks, for which the LSP lifetime is not significantly affected by a change in a disk diameter [14]. Figure 5. Normalized scattering cross section as a function of wavelength for different Au nanohole diameters Figure 6. Calculated E-field distribution around 60 nm, 78 nm and 90 nm holes in a 20 nm thick Au film. The calculations were produced using the COMSOL multiphysics, and the normally incident monochromatic light was polarized in the x direction In order to further our understanding of the nature of the resonant hole excitations, the near-field distribution around single Au nanohole excited at resonance wavelength are calculated. Figure 6 shows calculated E- field distribution around 60 nm, 78 nm and 90 nm holes in a 20 nm thick Au film and the normally incident monochromatic light was polarized in the x direction. As can be seen from the results, the E-field distribution exhibits clear electric dipole patterns for three nanoholes which arise from the localized surface plasmon. The maximum E-field enhancement is observed close to the 78 Thien Q. Duong, Khuong T. T. Pham, Nam L. Nguyen nanohole edges because it is at these positions that the charge accumulation associated with the dipolar excitation is the largest. As can be seen from the result, E-field enhancements due to surface plasmon, which is defined as E/E0 where E0 is the incident field, are 5.25, 6.63 and 7.87 nm for 60, 78 and 90 nm holes, respectively. A maximum value of this field increases with the diameter of the hole which agrees with reported results for nanohole plasmon field enhancement [14]. The calculations are thus fully consistent with the idea that the nanohole resonance can be viewed as a localized electric dipole plasmon. 4. Conclusion In conclution, the fabrication process of single Au nanohole is demonstrated. The e-beam lithography and reaction ion etching are applied to create a single Au nanohole on each Si3N4 membrane. The diameter of nanohole can be well-controlled during fabrication process. The optical properties of a single Au nanohole is also studied by using COMSOL Multiphysics. The interaction of light with single Au nanohole exhibits a localized surface plasmon (LSP) resonance in the red part of the visible spectrum. It is also found that the resonance peaks exhibit red-shift and broadening with an increasing hole diameter. In addition, the electric field distribution around the resonant nanoholes exhibits a clear electric dipole pattern which can be assigned for localized surface plasmon. Furthermore, we find that the size of the nanohole affects plasmonic field enhancement in the edges. This knowledge is opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical sensing and biophysics. This research is funded by Fund for Science and Technology Development of the University of Danang under project number B2018-ĐN06-11. REFERENCE [1] M. L. Brongersma, M. V. Shalaev, “The Case for Plasmonics”, Science, 328, 440 (2010). [2] J. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White, M. L. 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