Curvature effects on surface plasmon resonance for wearable devices

이현웅 Graduate School, Yonsei University 2023 국내박사

최근 수 십년간 플라스모닉스 응용 연구는 다양한 분야에서 활발하게 이루어져 왔다. 표면에서 강하게 집속되는 전기장을 이용하여 생체시료를 고감도로 검출, 화학 반응의 촉진, 그리고 기존에 달성할 수 없었던 새로운 광학적 특성을 구현해내는 등 다양한 학술 분야에서 중요한 역할을 하고 있다. 본 논문은 기존 평판 구조 기반의 플라스모닉스 연구에서 한 걸음 더 나아가 곡면을 갖는 플라스모닉 소자에 대해 표면 플라스몬 공명 특성을 연구하였고, 향후 플라스모닉 메타물질 등 곡면 플라스모닉 소자에 널리 활용될 수 있는 가능성을 제시하였다. 본 연구는 웨어러블 소자 개발에 대한 기술적 트렌드에 발맞추어 플라스모닉스가 웨어러블 소자에 활용될 수 있는 가능성을 보여준다. 제 1장에서는 플라스모닉 소자의 제작과 측정 등에 대한 기초적인 내용에 대해 기술하였다. 나노구조가 플라스모닉스에서 어떻게 활용되는지 살펴보았고 곡면 소자에서의 플라스모닉스 연구 동향에 대해 알아보았다. 이어 플라스모닉 소자에서 핵심적인 전자빔 리소그래피를 이용한 소자 제작에 대한 기술을 하였다. 기존 단일층 전자빔 레지스트 기반의 공정에서는 100 nm 이하의 소자제작 분해능을 달성하기 어려웠던 반면, 이중층 전자빔 레지스트 공정을 적용하여 약 70 nm 수준의 소자 제작 분해능이 달성 가능함을 보였다. 또한 곡면 구조를 제작하기 위해 기존 마이크로 렌즈 제작 등에 활용되던 열 재유동 (thermal reflow) 공정을 적용하여 플라스모닉 곡면 소자 제작하였다. 끝으로 플라스모닉 소자의 여러가지 광학적 측정 방법들에 대해 알아보고, 광시야 조명 (wide-field illumination) 광학계를 활용해 플라스모닉 소자의 특성을 측정하고 그 결과를 보였다. 제 2장에서는 곡면 플라스모닉 소자에 대해서도 평판 계산이 수행되던 기존 연구의 한계를 극복하기 위해 분할파 분석 (Segmented-wave analysis) 방법을 제안하였다. 분할파 분석법은 곡면을 분할하여 전체 곡면 소자의 특성을 근사하는 방법이다. 분할파 분석법을 이용하여 기존 광학 계산 방법으로 수행하기 어려웠던 대면적 비주기 곡면 구조를 매우 빠른 시간에 계산할 수 있음을 곡면 소자의 표면 플라스몬 공명 (Surface plasmon resonance, SPR) 계산을 통해 보였다. 또한 바이오 센서로서 성능을 평가하기 위한 센서 감도를 DNA 혼성화 모델을 이용해 계산하였다. 이후 분할파 분석법을 검증하기 위해 작은 곡면 구조에서 더 작은 단위로 곡면을 분할하였고, 유한요소법 (Finite element method, FEM)을 이용하여 분할파 분석법에 대한 검증을 수행하였다. 분할파 분석법과 유한요소법을 비교하여 분할파 분석법의 한계와 유효성에 대한 검토를 수행하였으며, 특히 수직 입사 조건 (perpendicular incidence)에서 분할파 분석법이 높은 곡률에서도 유효성을 가짐을 확인하였다. 제 3장에서는 실험적 검증을 위해 곡면 플라스모닉 소자를 직접 제작하여 SPR 특성을 측정하고, 분할파 분석법 및 유한요소법 결과와 비교하였다. 실험 결과를 통해 분할파 분석법이 실제 곡면 소자의 성능을 평가하는 데에 유의미한 결과를 제시할 수 있음을 확인하였고, 유한요소법과 상보적으로 곡면 플라스모닉 소자를 분석하는 데에 중요한 방법이 될 수 있음을 보였다. In recent decades, plasmonics and its application research have been actively conducted in various fields. It plays an important role in various research fields, such as detecting biological samples with high sensitivity using an electric field strongly confined on the surface, promoting chemical reactions, and generating unprecedented optical properties. In this paper, we went one step further from the conventional planar structure-based plasmonics research and studied the surface plasmon resonance (SPR) characteristics of curved plasmonic devices, which will be widely used in a wearable platform such as plasmonic metamaterials in the future. This study shows the possibility that plasmonics can be used in wearable devices in line with the technological trend for wearable device development. In Chapter 1, the fundamental concepts of the fabrication and measurement of curved plasmonic devices are described. How curved structures are utilized in plasmonics, and plasmonics research trends in curved surfaces are presented. Subsequently, the technology for device fabrication using electron beam lithography (EBL), which is a key element in plasmonic devices, is described. In addition, to fabricate the curved structure, a curved plasmonic device is fabricated by applying the thermal reflow process used in conventional microlens fabrication. Finally, a few types of optical measurement methods of the plasmonic device were explored, and the characteristics of the plasmonic device are measured using a wide-field illumination optical system and the results are shown. In Chapter 2, segmented-based method is proposed to overcome the limitations of previous studies in which planar-based calculation was performed for curved plasmonic devices. The segmented-wave analysis is a method of approximating the characteristics of an entire curved surface by dividing a curved surface. It is shown through SPR calculation of a curved device that the segmented-wave analysis can be used to calculate a large-area aperiodic curved structure in a very short time, which was difficult to perform with conventional optical calculation methods. In addition, the sensor sensitivity to evaluate the performance as a biosensor is calculated using a deoxyribonucleic acid (DNA) hybridization model. Furthermore, the curved surface was divided into shorter lengths to model a small, curved surface structure. Finally, segmented-wave analysis is validated using the finite element method (FEM). The evaluation of limitations and effectiveness of the segmented-wave analysis are performed by comparing it with the FEM. In Chapter 3, a curved plasmonic device is fabricated to characterize the experimental results of the SPR, and the results are compared with the calculation results. It was confirmed that the segmented-wave analysis can provide a highly efficient method to evaluate the performance of the curved plasmonic surface.

공진 특성 조절이 가능한 Otto 구조의 표면 플라즈몬 공명 센서에 대한 연구

이연수 전북대학교 일반대학원 2021 국내박사

Surface plasmon polaritons are collective oscillation of electrons in the surface of the metals such as gold, silver and aluminum and it is caused by the incident light. When the specific conditions such as angle of incidence, wavelength and refractive index of materials, are satisfied, the incident light can be absorbed as a surface plasmon polaritons and the reflectance is rapidly decreased. This phenomenon is called as surface plasmon resonance. Characteristics of the surface plasmon resonance is changed sensitively according to a dielectric constant and refractive index change of the materials near the metal surface. Thus, if target molecules contact on the metal surface, surface plasmon resonance effect is changed and we can use this system as a sensor. The surface plasmon resonance sensor has been widely used in the chemical biology and drug design with advantages of label free detection, room temperature detection, and real time monitoring. There are two types of coupling configuration for surface plasmon resonance system using thin metal film. In the kretschmann configuration, metal film is located on the glass surface. On the other hand, metal film of the Otto configuration is located with a distance of the few micrometers from the glass. Due to the robust structure and convenience of fabrication, most of the surface plasmon resonance sensors are based on the kretschmann configuration. However, Otto configuration has a potential for tunable resonance characteristics according to the air-gap distance. If the air-gap distance between the glass substrate and the metal thin film can be stably adjusted, it will be possible to use a surface plasmon resonance sensor with the structural flexibility of the Otto configuration. In this study, the Otto configuration based sensor was fabricated through silicon on quartz bonding process. An air-gap distance of the sensor was designed to be 2.2 um and whole size of the sensor is 30*30*1 mm^3. Characteristics of the sensor was measured using a laser module with wavelength of 975.1 nm and the results was compared with FEM simulation results. It was verified that the characteristics can be sensitively changed according to the refractive index of the metal film. Performance of the sensor for gas sensing application was verified with simulation results and the results was compared with those of the other researches. The sensitivity of the fabricated sensor is 55 degree/RIU where the refractive index of the sensing medium is varied from 1 to 1.008. To verify the potential of the fabricated sensor as a pressure sensor, resonance characteristics was measured with applied pressure level from 0.23 bar to 1.3 bar. The air-gap distance of the fabricated Otto configuration was varied according to the pressure and the resonance characteristics was also changed. From the measurement results, the sensor has a sensitivity and resolution of about 0.17 degree/bar and 0.029 bar, respectively, as a pressure sensing application. Results show that the fabricated Otto configuration based sensor can be used as a hazardous gas and pressure level of the industrial facility with high sensitivity. Otto configuration with stepped air-gap (1.86 um, 2.42 um, 3.01 um, 3.43 um) was fabricated and the resonance characteristics at each air-gap was measured with 980 nm laser source. Minimum reflectance is measured to be 0.688, 0.716, 0.766, 0.86 at each section and the measurement results was analyzed with FEM simulation results. To verify the variability of characteristics of the Otto configuration and its applicability, the air-gap between the prism and the metal film was adjusted by using a commercial piezoelectric actuator. When the wavelength of the incident light was 786 nm, the reflectance was measured as 0.25 where the displacement of the piezoactuator was about 10.5 um, and it was confirmed that the maximum surface plasmon resonance occurred. Likewise, when a light source having a wavelength of 977 nm is used, the minimum reflectance becomes 0.22 where the displacement of the piezoactuator is about 9.3 um. The results shows that the wavelength of incident light for the maximum surface plasmon resonance in the Otto configuration is proportional to the air-gap in the actual measurement results. With an aim of inducing the variability of characteristics of the Otto configuration, a serpentine spring based MEMS actuator was designed and the driving characteristics were theoretically analyzed through numerical calculation and FEM simulation methods. The MEMS actuator for adjusting the air-gap of the Otto configuration was designed based on low-resistivity silicon. A 200 nm-thick gold thin film is located on the upper surface of the driving part. The initial distance between the silicon driving part and the driving electrode on the surface of the glass substrate is designed to be 18 um, and the distance between the gold film and the glass substrate is adjusted to be about 1 um from the initial value of 6 um. A resonance frequency of the driving part was calculated to be about 1.34 kHz, and it was confirmed that the driving part of the designed MEMS actuator can maintain a stable state even if an external shock is applied. FEM simulation results show that driving part of the proposed sensor can be moved vertically as much as 5.1 um when a potential difference of 59.2 V is applied between the driving electrode on the glass substrate and the driving part. It is expected that the results of this study can be used as a background data for the development of Otto configuration based surface plasmon resonance sensors with variable resonance characteristics, which has not been reported so far.

Control of Fano Resonance in Plasmonic and Optical Systems for Highly-Sensitive Spectral Responses : 플라즈모닉 및 광학 시스템에서의 스펙트럼 민감도 향상을 위한 파노 공명의 제어

박현희 서울대학교 대학원 2015 국내박사

The concept of Fano resonance from quantum physics has originally been applied to describe the configuration interaction in the rare gas excitation, which is one of the universal phenomena caused by the wave interference. The main feature of Fano resonance is the sharp, asymmetrically shaped spectrum, which is due to the interference between two resonant modes of different life-time. To exploit this intriguing spectral feature, a variety of Fano resonant nanostructures implementing optical analogue of quantum-mechanical Fano resonance have been proposed and studied, including clusters of plasmonic nanoparticles and metamaterial platforms. During my Ph.D study I have been trying to bring the concept of Fano resonance into the field of optical and plasmonic systems to provide some novel paths for the existing applications such as on-chip plasmonic devices and net optical spin excitation. In this dissertation, the characteristics of asymmetrically shaped spectral responses for the Fano resonant structures are investigated in the plasmonic stub-waveguide systems and optical chiral system. In the theoretical part of this dissertation, the theoretical descriptions about the Fano resonance phenomena are provided in several different formalism, such as the original quantum perturbation analysis, analogy with classical oscillators, and the simple and general coupled mode theory. Based on the coupled mode theory model, I also establish the Fano asymmetry parameter, which is the key physical parameter for the quantification of the Fano resonance phenomenon. Through the numerical and theoretical studies, we firstly propose a plasmonic sensor based on the control of the degree of Fano asymmetry, with the plasmonic metal-insulator-metal waveguide-stub junction structure. we also introduce an ultra-efficient plasmonic on-chip modulator based on the concept of plasmonic induced transparency. Excellent performance with 12dB transmission contrast at ~60% throughput is achieved due to the extremely sharp spectral profiles, which is caused by the Fano interference using a pair of plasmonic metal-insulator-metal stubs. A novel path for the conservative and nonmagnetic optical spin excitation is introduced utilizing the spin handedness-dependent anti-symmetric Fano resonances, with Hermitian material parameters. Utilizing the pure optical spin excitation density and its extreme spectral sensitivity in the suggested design, ‘optical spin switching’ is also introduced with experimentally accessible material parameters.

Fabrication of plasmonics platform for the chemical and environmental sensors

이수승 서울대학교 대학원 2013 국내박사

In the various field of nanotechnology and nanoscience, such as biomedical and environmental science, the design and synthesis of certain nanostructure for a desired purpose is important. Metallic nanostructures have attracted significant attention in the research area of chemical and environmental sensor due to the superior physic-chemical properties and, especially, applicability to the utilization of surface plasmon resonance phenomena. This thesis mainly deals with the application of novel metallic nanostructures to the surface plasmon resonance phenomena for the realization of effective chemical and environmental sensor. And more detailed strategies and results related with these topics, as follows; Firstly, highly selective detection system for Cu2+ ions by exploiting specific interactions between the Cu-demetallated form (E,Zn-SOD1) of Cu/Zn-superoxide dismutase (SOD1) and Cu2+ ions using surface plasmon resonance spectroscopy (SPRS) was suggested. I demonstrated that Cu2+ ions have a high affinity for vacant metal-binding sites in the E,Zn-SOD1 protein, compared to other divalent metal ions. On the basis of these measurements, it can be concluded that small amounts of Cu2+ ions can be readily detected as the result of the selective binding between the E,Zn-SOD1 protein and Cu2+ ions. It appears that metalloproteins have considerable potential for use as a novel sensing actuator, as evidenced by the selective binding of E,Zn-SOD1 proteins with Cu2+ ion. This approach can be used in conjunction with other fully or partially demetallated metalloproteins, hence it could be potentially useful in the determination of specific metal ions in aqueous media or demetallated proteins in biological fluids. Additionally, the applicability of gold nanoparticles as a ratiometric sensor was suggested. From the theoretical study using discrete dipole approximation method, it is investigated that dimer structure of gold nanoparticles has unique optical properties compared to separated single gold nanoparticle. It can be appeared new plasmon band with longer wavelength due to the electron oscillations along the longitudinal interparticle axis, and this band can be distinguished clearly with the inherent plasmon band. The intensity ratio of inherent and additional plasmon band could be utilized as the value for the ratiometric sensor. Finally, simple method to fabricate highly branched gold nanostructures with abundant petal-shaped tips by direct growth on the substrate was suggested. A lot of experiments and theoretical calculations have shown highly enhancement of the electromagnetic field in complex gold nanostructures due to the abundant ‘hot spots’ in the individual nanostructures. Among them, multi-branched nanostructure has attracted much attention because of their stronger SERS enhancement factor than other gold nanostructures. In this work, I synthesized and characterized the multi-branched gold nanoparticles, which are applicable to the surface-enhanced Raman detection. It was directly grown on the substrate with simple seed-mediated method, and the optical properties as growth procedure was investigated for the better understanding on the growth process. The multi-branched gold nanoparticles show the very high enhancement factor, thus it can be promising materials for the effective SERS substrate.

Gap-plasmon based fluorescence correlation spectroscopy

이홍기 Graduate School, Yonsei University 2020 국내박사

고전 광학에서 입사광은 회절 한계에 의하여 감소될 수 있는 크기에 제한을 받는다. 아베(Abbe) 회절 한계에 따르면 입사광의 크기는 주로 입사광의 파장과 사용된 광학 시스템의 개구 수(numerical aperture, NA)에 의해 결정된다. 이 회절 한계를 극복하기 위해서 다른 방법론이 광학 시스템에 적용되어야 하며 나노 스케일에서의 광 제어를 이루기 위해서 많은 종류의 연구가 수행되어왔다. 본 논문은 플라스모닉스 기술이 회절 한계를 극복한 나노 스케일 영역에서 광신호를 국소화 및 조정하여 높은 신호 대 잡음비(signal-to-noise ratio, SNR) 및 정밀도로 생물학적 정보를 획득할 수 있음을 보였다. 본 논문은 먼저 제 2 장에서 나노 스케일 물질의 플라스몬 특성을 조사하기 위해 박막의 광열 반응을 고려한 반복 계산 방법을 사용하여 금속 나노 구조의 전기장 분포와 열팽창을 분석하였다. 다양한 입사 조건에서 플라스몬 금속 박막의 전기장 향상 및 에너지 흡수의 측정이 가능한 탐침 기반의 측정법을 제시하였다. 또한, 비대칭 다층 계에서의 파동 분산 관계에 의해 설명될 수 있는 금 박막의 누설 방사(leakage radiation, LR)를 조사하였다. LR은 금속 박막의 표면 플라스몬 폴라리톤(surface plasmon polariton, SPP)의 전파를 시각화하며 이를 통하여 금속 표면의 굴절률 분포를 확인할 수 있다. 본 논문은 누설 방사 현미경(leakage radiation microscopy, LRM) 및 표면 플라스몬 공명 현미경(surface plasmon resonance microscopy, SPRM) 기술의 성능을 하드웨어 및 소프트웨어적으로 개선시킬 수 있는 방법에 대하여 논의하였다. 제 3 장은 다양한 크기 및 주기를 가지는 원형, 마름모꼴, 삼각형 모양의 나노 구조에 초단파 펄스가 가해졌을 때 인가되는 국소화된 표면 플라스몬(localized surface plasmon, LSP)에 대하여 살펴본다. 더 나아가, 플라스몬 구조에 인가되는 입사광의 특성을 조정함으로써 회절 한계로 제한된 영역 내에서 근접장 분포의 공간 제어가 가능하며, 고해상도 형광 이미징에 적용이 가능함을 보여주었다. 제 4 장에서는 18 nm 갭을 갖는 플라스몬 나노 구조체 어레이가 형광 상관 분석법 (fluorescence correlation spectroscopy, FCS)의 성능을 개선할 수 있음을 입증하였다. FCS는 형광 신호의 자기 상관 함수로부터 분자의 확산 및 결합 상호 작용과 같은 특성을 조사하는데 용이한 기술로, 특히 세포 내 및 세포막 상의 생체 분자의 운동 특성을 조사하는데 적용된다. FCS는 일반적으로 회절 한계에 의해 제한된 관찰 부피를 사용하여 수행되어 왔다. 예를 들어, 공초점 현미경을 사용한 실험 조건 하에서 FCS는 횡축으로 약 200 nm 및 종축으로 600 nm으로 제한된 관찰 부피를 갖는다. 본 논문에서는, 플라스몬을 사용하여 개선된 형광 상관 분광법(plasmonic-enhanced fluorescence correlation spectroscopy, pFCS)이 생체 분자의 운동 특성 연구에 어떻게 기여할 수 있는지 탐구하였다. p-FCS는 입사광을 나노 갭에 국소화시켜 회절 한계보다 작은 영역에서 생체분자를 관찰할 수 있도록 하였다. 나노 갭 내에 국소화된 전자기장은 근접장 주사 광학 현미경(near-field scanning optical microscopy, NSOM)을 사용하여 실험적으로 확인되었다. 이러한 나노 스케일 영역에서의 전자기장 국소화는 회절한계보다 작은 산란 단면을 가지며 LR의 세기를 증대한다. 본 논문에서는 소실파 내에서 동시에 표면 플라스몬 공명(surface plasmon resonance, SPR) 이미징을 수행할 수 있는 영상장치를 구축하여, 나노 입자를 높은 정밀도로 추적할 수 있게 하였다. 제 2 장과 3 장에서 논의한 바와 같이, 플라스몬 나노 구조체의 근접장 증폭은 형광 여기와 LR 강도 모두를 향상시킬 수 있음을 입증하였다. In classical optics, there is a theoretical limitation of the size to which a focused incident beam can be reduced. The size is mainly governed by the incident wavelength and a numerical aperture of an optical system. To break this diffraction limit, other methodologies should be employed to the optical system and many kinds of research have been performed to achieve a nanoscale light confinement. In this sense, plasmonics can be substantial to manipulate the optical signal within the nanoscale area and collect biological information from the sample with a high signal-to-noise ratio (SNR) and precision. To investigate the plasmonic properties of nanoscale materials, we first analyzed a field distribution and a thermal expansion of metallic nanostructures which were compared with an iterative calculation method considering the opto-thermal response of thin films in Chapter 2. The experimental results had a good agreement with calculation methods. The results have provided direct measurements of optical responses of plasmonic thin films by measuring field enhancements and absorption with various incident conditions. In addition, we have investigated the leakage radiation (LR) of gold thin films which can be explained by the dispersion relation in the asymmetric multi-layered system. We have discussed a good advantage of LR to visualize the surface plasmon polariton (SPP) propagation on gold thin films and how to improve the imaging performance. In Chapter 3, we have explored spatial field localization under ultrashort light pulses based on localized surface plasmon (LSP) by three-dimensional geometrical nanoapertures, which were circular, rhombic, and triangular with various combinations of size and period. Moreover, it was shown that plasmonic nanostructures enable spatial control of near-field distribution within the diffraction-limited area and also applicable to high-resolution fluorescence imaging. In Chapter 4, it was shown that the performance of fluorescence correlation spectroscopy (FCS) could be improved using plasmonic nanostructure arrays which have an 18-nm gap. FCS is a well-known technique that enables molecular detection to obtain biological information such as properties of diffusion and binding interaction by acquiring an autocorrelation of fluorescence fluctuation. FCS is typically conducted using a diffraction-limited volume in which target molecules diffuse. Under the typical experimental condition of confocal microscopy, diffraction-limited FCS has a volume of 200-nm lateral width and 600-nm axial length. We have explored how plasmon-enhanced FCS (p-FCS) was feasible and potential for biomolecular study using lysosomes in the human embryonic kidney (HEK) 293 cells. The p-FCS have provided arrays of sub-diffraction-limited light volume. The field localization within a nanodimer’s gap was confirmed experimentally using near-field scanning optical microscopy (NSOM). Those nanoscale localizations have much smaller scattering cross-section with higher SNR. We have established p-FCS imaging set-up which was able to perform surface plasmon resonance (SPR) imaging simultaneously in the evanescent field, allowing tracking nanoscale molecules with improved precision. As discussed in Chapters 2 and 3, it has been demonstrated that near-field amplification of plasmonic nanostructure had improved both fluorescence excitation and LR intensity. It should be emphasized that plasmonic nanostructures confining electromagnetic field into the nanoscale area have great potential in various biomedical engineering applications. In the dissertation, we have demonstrated near-field and far-field properties of plasmonic nanostructures and their applications for biological studies. The properties and performance of plasmonic nanostructures are expected to be investigated in more extensive areas of research as a critical component of nanotechnology and to find more applications for delivery of nanoscale biomolecular information.

Intragap Plasmon Resonances in Core-Gap-Shell Nanostructures

손지웅 서울대학교 대학원 2022 국내박사

Surface plasmon, the collective oscillation of electron clouds at metal/dielectric interfaces, focuses the electromagnetic energy in subwavelength region because the characteristic wavelength is much smaller, and the speed of surface plasmon is much slower than the ones at vacuum. This focusing effect dramatically increases when two metal/dielectric interfaces are adjacent. Plasmonic nanoparticles are of special interest, because their surface plasmon resonance lies in visible to near-IR region depending on their size, and the light they focus can drastically increase the response of the nearby molecules. Core-gap-shell type plasmonic nanoparticles are a promising platform because they can serve precisely controlled and robust nanogap of one to a few nm. In this thesis, the behavior of the intra-nanogap plasmons were discussed. First, fundamentals of plasmonics and properties of plasmonic nanogaps were introduced. Next, Synthetic strategies of web-above-a-ring (WAR) and web-above-a-lens (WAL) nanostructures are reported. The WAR has a controllable gap between the nanoring core and a nanoweb with nanopores for the effective confinement of electromagnetic field in the nanogap and subsequent surface-enhanced Raman scattering (SERS) of Raman dyes inside the gap with high signal reproducibility. In chapter 3, the intragap structure formed by selective etching of silver in gold core- ii gold,silver alloy shell particle. By controlling the composition of silver in the alloy shell, the gap thickness was uniform and tunable. Nanobridges formed during the raction is attributed to the discrepancy in the trends in plasmon resonance and corresponding SERS response between the synthesized and modeled structures. For proper modeling of the nanobridges, the formation mechanism was proposed via atomistic kinetic Monte Carlo simulation. In the last chapter, we define the intragap plasmon resonance, and studied how the intragap plasmon resonance change as the structure gradually, continuously transforms from cubic core-gap-cubic to spherical core-gap-sphere. The intragap plasmon resonance destructively interfered with the plasmon excited at the outer surface of the shell if they have different symmetry. If not, they did not interfere with each other. The spectral position intragap plasmon resonance did not change when two core-gap-shell particles are adjacent and the plasmonic coupling occurs. This feature is expected to be used as another degree of freedom in designing plasmonic metasurfaces. 표면 플라즈몬은 금속 절연체 계면에서 전자들이 집단으로 진동하는 현상으로서 그 파장과 진행 속도가 진공에서의 빛의 속도보다 현저하게 느리기에 전자기 에너지를 파장보다 작은 영역에 집속시킨다 이 집속 효과는 두 개의 금속 절연체 계면이 맞닿아있을 때 급격히 커진다 플라즈모닉 나노입자는 그 크기에 따라 표면 플라즈몬 공명이 가시광선 적외선 영역에 놓여 있어 빛을 집속시켜 근처에 있는 분자들의 반응을 현저히 증가시킬 수 있기에 특별한 관심을 요한다 코어 갭 쉘 형태의 플라즈모닉 나노입자들은 이러한 응용에 유망한 플랫폼인데 이는 1 nm 에서 수 nm 에 해당하는 정밀하고 견고한 나노갭을 형성하는 것이 가능하기 때문이다 이 학위논문에서는 내부 나노갭 플라즈몬의 거동이 논의된다 첫 번째로 플라즈몬 성질과 플라즈모닉 나노갭에 대한 기초 배경지식이 소개된다 제 2 장에서는 링을 덮은 거미줄 링을 덮은 렌즈 형태의 나노구조가 보고된다 링을 덮은 거미줄 입자는 나노링 코어와 구멍이 난 나노거미줄 사이에 조절 가능한 두께의 갭을 가진 입자로서 전자 기장을 이 영역에 효과적으로 집속시킬 수 있고 그에 따라 갭 안에 있는 라만 분자들의 표면증강 라만 신호를 높은 재현성을 가지고 유도할 수 있다 제 3 장에서는 금 코어 금 은 합금 쉘 구조에서 은을 선택적으로 녹여내어 만든 구조가 보고된다 . 합금 쉘에서 은의 조성을 조절함으로써 갭의 크기를 원하는 크기로 균일하게 합성할 수 있다 합성 과정에서 생기는 나노브릿지의 존재는 디자인한 구조와 실제 합성된 구조간의 플라즈모닉 공명과 그에 따른 표면증강 라만 신호의 경향성이 완전히 다르게 나타나게 만드는 역할을 하는데 이러한 나노브릿지를 적절히 모델링할 수 있도록 나노브릿지의 형성 과정을 키네틱 몬테카를로 시뮬레이션을 통해 밝혔다 마지막 제 4 장에서는 내부 갭 플라즈몬 공명을 정의하고 나노입자의 구조가 큐브 코어 갭 큐브 쉘에서 구형 코어 갭 구형 쉘로 점진적으로, 연속적으로 변할 때 어떻게 변하는지 제시한다 이 구조는최외각 플라즈몬과 같은 대칭성을 가질 땐 상쇄간섭을 하나 대칭성이 다르면 서로 간섭 을 하지 않았다 두 개의 코어 갭 쉘 구조 나노입자가 인접해서 플라즈모닉 커플링이 일어나더라도 내부갭 플라즈몬 공명의 파장은 변하지 않았다 이러한 특징은 향후 플라즈모닉 메타물질을 만들 때 또 하나의 자유도로 활용할 수 있을 것으로 기대된다

Multifunctional nanoframe architectures for advanced sensing applications : design, synthesis, and characterization

Al Hammad, Hajir Hilal Khaleel Sungkyunkwan University 2023 국내박사

Metal nanoparticles (NPs) exhibit unique properties, including localized surface plasmon resonance (LSPR), which is influenced by their size, shape, and composition. This distinction sets them apart from bulk materials. Various types of plasmonic NPs have been developed and characterized to tailor their properties for specific applications. Nanoframes, a group of NPs with large cavities accessible to light and chemicals, have gained significant attention. However, previous studies on single-rim-based NFs have faced limitations in near-field focusing capabilities due to their structural simplicity, necessitating the development of a conceptually new NF architect. This thesis contributes to the comprehensive understanding of multifunctional nanoframe architectures by combining the fields of plasmonics, nanomaterial synthesis, and surface chemistry. It showcases the potential of these nanoframes in various sensing modalities, demonstrating their capabilities in different sensing applications. In Chapter 2, a brief introduction is presented on a biosensing platform that utilizes a mixture of Au nanorods and magnetically responsive Pt@Ni nanorings. The platform utilizes a rotating magnetic field to induce dynamic assays, enabling the monitoring of surface biorecognition on Au nanorods through periodic changes in extinction. This approach provides an alternative to conventional biosensors based on peak shift of localized surface plasmon resonance. In chapter 3, I address the challenges in realizing complex three-dimensional (3D) nanoframe structures for effective optical-based sensing. A novel synthesis method for complex 3D nanoframes is presented, where two-dimensional (2D) dual-rim nanostructures are engraved on each facet of octahedral nanoframes. The synthesis proceeds through multiple executables on-demand steps, involving edge-selective Pt deposition, inner Au etching, and tunable geometric patterning. The resulting plasmonic dual-rim engraved nanoframes exhibit strong light entrapping capability verified by single-particle surface-enhanced Raman scattering (SERS), highlighting their potential as nanoprobes for biosensing applications through SERS-based immunoassay. In Chapter 4, focuses on the synthesis of Au truncated octahedral dual-rim nanoframes with two functional facets. The nanoframes feature eight hot nanogaps formed by hexagonal nanoframes and six flat squares that facilitate well-ordered arrays through self-assembly. The existence of intra-nanogaps enables strong electromagnetic near-field focusing, allowing single-particle surface-enhanced Raman spectroscopy. The construction of "all-hot-spot bulk SERS substrates" using these nanoframes demonstrates highly ordered and uniform superlattices with a significantly lower limit of detection for 2-naphthalenethiol, achieved through the synergistic effect of inter- and intraparticle coupling in the superlattice. In chapter 5, presents the design and synthesis of elongated pseudo-hollow nanoframes named "Au dodecahedral-walled nanoframes" for efficient detection of gaseous analytes. The nanoframes are composed of four rectangular plates enclosing the sides and two open-frame ends with ridges for near-field focusing. The hollow interior allows for the penetration of gaseous analytes, enabling their efficient detection in combination with Raman spectroscopy. The nanoframes exhibit high homogeneity in size and shape and demonstrate significantly enhanced SERS activity compared to other nanostructures. The application of these nanoframes in detecting chemical agent simulants in the gas phase showcases their 20 times higher sensitivity compared to their solid counterpart. Overall, this thesis contributes to the advancement of multifunctional nanoframe architectures in various sensing applications, offering novel strategies for design, synthesis, and characterization in diverse sensing modalities and nanomaterial engineering. Metal nanoparticles (NPs) exhibit unique properties, including localized surface plasmon resonance (LSPR), which is influenced by their size, shape, and composition. This distinction sets them apart from bulk materials. Various types of plasmonic NPs have been developed and characterized to tailor their properties for specific applications. Nanoframes, a group of NPs with large cavities accessible to light and chemicals, have gained significant attention. However, previous studies on single-rim-based NFs have faced limitations in near-field focusing capabilities due to their structural simplicity, necessitating the development of a conceptually new NF architect. This thesis contributes to the comprehensive understanding of multifunctional nanoframe architectures by combining the fields of plasmonics, nanomaterial synthesis, and surface chemistry. It showcases the potential of these nanoframes in various sensing modalities, demonstrating their capabilities in different sensing applications. In Chapter 2, a brief introduction is presented on a biosensing platform that utilizes a mixture of Au nanorods and magnetically responsive Pt@Ni nanorings. The platform utilizes a rotating magnetic field to induce dynamic assays, enabling the monitoring of surface biorecognition on Au nanorods through periodic changes in extinction. This approach provides an alternative to conventional biosensors based on peak shift of localized surface plasmon resonance. In chapter 3, I address the challenges in realizing complex three-dimensional (3D) nanoframe structures for effective optical-based sensing. A novel synthesis method for complex 3D nanoframes is presented, where two-dimensional (2D) dual-rim nanostructures are engraved on each facet of octahedral nanoframes. The synthesis proceeds through multiple executables on-demand steps, involving edge-selective Pt deposition, inner Au etching, and tunable geometric patterning. The resulting plasmonic dual-rim engraved nanoframes exhibit strong light entrapping capability verified by single-particle surface-enhanced Raman scattering (SERS), highlighting their potential as nanoprobes for biosensing applications through SERS-based immunoassay. In Chapter 4, focuses on the synthesis of Au truncated octahedral dual-rim nanoframes with two functional facets. The nanoframes feature eight hot nanogaps formed by hexagonal nanoframes and six flat squares that facilitate well-ordered arrays through self-assembly. The existence of intra-nanogaps enables strong electromagnetic near-field focusing, allowing single-particle surface-enhanced Raman spectroscopy. The construction of "all-hot-spot bulk SERS substrates" using these nanoframes demonstrates highly ordered and uniform superlattices with a significantly lower limit of detection for 2-naphthalenethiol, achieved through the synergistic effect of inter- and intraparticle coupling in the superlattice. In chapter 5, presents the design and synthesis of elongated pseudo-hollow nanoframes named "Au dodecahedral-walled nanoframes" for efficient detection of gaseous analytes. The nanoframes are composed of four rectangular plates enclosing the sides and two open-frame ends with ridges for near-field focusing. The hollow interior allows for the penetration of gaseous analytes, enabling their efficient detection in combination with Raman spectroscopy. The nanoframes exhibit high homogeneity in size and shape and demonstrate significantly enhanced SERS activity compared to other nanostructures. The application of these nanoframes in detecting chemical agent simulants in the gas phase showcases their 20 times higher sensitivity compared to their solid counterpart. Overall, this thesis contributes to the advancement of multifunctional nanoframe architectures in various sensing applications, offering novel strategies for design, synthesis, and characterization in diverse sensing modalities and nanomaterial engineering.

Integrative bioanalysis using LSPR-driven dual sensing technologies for enhancing detection and precision

Ali, Mohamed Ragab Elsayed Sungkyunkwan University 2025 국내박사

본 논문은 고도화된 색 변화 기반 및 이중 모드 바이오센서 개발을 위한 핵심 메커니즘으로서 국소 표면 플라스몬 공명(Localized Surface Plasmon Resonance, LSPR)의 응용에 대한 심층적 연구를 제시한다. LSPR은 금속 나노구조와 빛의 상호작용에서 발생하는 현상으로, 분석 물질(analyte) 결합에 따른 가시적 색 변화 반응을 유도하는 다양한 광학 센서의 기반 원리로 활용된다. 본 연구는 나노입자의 조성, 형태, 그리고 주변 유전체 환경이 LSPR의 스펙트럼 특성과 감도에 미치는 영향을 중심으로 LSPR의 기초 이론을 포괄적으로 고찰하는 것으로 시작된다. 이러한 이론적 기반을 바탕으로, 본 논문은 LSPR을 색 변화(colorimetric) 및 형광(fluorometric) 센서 플랫폼에 통합하는 다양한 전략을 탐색하며, 특히 색 변화와 형광 신호를 동시에 활용하는 이중 모드 시스템에 주목한다. 이와 같은 이중 신호 플랫폼은 검출 정확도 향상, 위양성 감소, 자가보정 기능을 가능하게 하며, 실제 생체 진단에 적용되어 요소(urea) 및 티로시나아제(tyrosinase)와 같은 임상 관련 바이오마커를 효과적으로 검출하였다. 첫 번째 바이오센서는 고감도 형광-색 변화 하이브리드 탄소 점 기반 나노센서로, 요소를 이중 모드로 모니터링한다. 요소가 요소분해효소(urease)에 의해 분해되어 생성되는 이산화탄소와 암모니아는 용액의 pH를 증가시키며, 이는 탄닌산의 환원력을 활성화시켜 플라스몬 은 나노입자(AgNPs)를 in situ로 생성하게 한다. 생성된 AgNPs의 흡수 스펙트럼은 탄소 점(CDs)의 형광 스펙트럼과 중첩되어, 요소 존재 시 형광이 효과적으로 소광된다. 해당 시스템은 형광 기반으로는 18 nM, 색 변화 기반으로는 1.05 μM의 낮은 검출한계를 보였으며, 각각 100 nM–1 mM 및 50 µM–1 mM 범위에서 우수한 직선성을 나타내어 현재까지 보고된 센서 중 가장 낮은 검출한계를 기록하였다. 두 번째 바이오센서는 LSPR 파장 이동 기반의 개폐형 형광 신호를 이용하는 새로운 비율형 형광 분석법을 제시하며, 흑색종 진단에 활용되었다. 이 시스템은 다중 색 변화(multicolorimetric) 및 비율형 형광 검출(ratiometric fluorometric detection)을 통합한 초고감도 티로시나아제 정량 플랫폼이다. 금 나노바이피라미드(AuNBPs)의 조절 가능한 LSPR과 상향변환 나노입자(UCNPs)를 기반으로 한 FRET 기전을 활용하며, 티로시나아제에 의해 카테콜이 산화되어 퀴논을 형성하면서 Ag의 침착이 억제되어 농도에 따른 색 변화가 유도된다. 이 방식은 색 변화 및 형광 감지에서 각각 4.13 × 10⁻⁵ U/mL 및 2.71 × 10⁻⁵ U/mL의 우수한 검출한계를 보이며, 복잡한 생물학적 샘플에서도 높은 선택성과 안정성을 입증하였다. 마지막으로, 별도의 라벨링 및 희석 없이 원뇨(raw urine) 내 요소를 직접 검출할 수 있는 플라스몬-양자점 하이브리드 분석 시스템이 개발되었다. 이 시스템은 urease 효소에 의한 요소의 가수분해로 생성된 암모니아가 국소적인 pH 변화를 유도하며, 이는 카테콜의 환원 활성화 및 금 나노바이피라미드 표면에서의 은 침착 효율을 조절한다. 변화된 LSPR은 AuNBPs와 양자점(QDs) 간의 FRET를 조절하여 형광 채널 중 하나를 선택적으로 소광시킨다. 해당 이중 모드 플랫폼은 QDs 기반 비율형 형광과 LSPR 변조를 결합하여 복잡한 샘플 전처리 없이도 민감하고 정확한 요소 검출이 가능함을 보여준다. 이처럼, 본 논문에서 제시된 LSPR 기반 이중 감지 바이오센서는 고감도, 고선택성, 높은 실용성을 동시에 갖추고 있으며, 향후 현장 진단(point-of-care) 및 환경 모니터링 등의 분야에 폭넓게 응용될 수 있는 가능성을 보여준다. Abstract This thesis presents an in-depth investigation into the application of Localized Surface Plasmon Resonance (LSPR) as a core mechanism for the development of advanced dual-mode biosensors. The work explores the dual-mode systems that combine LSPR-based color change with complementary fluorescence signals to enhance detection accuracy, reduce false positives, and enable self-calibrating responses. These dual-signal platforms were applied to real-world biomedical diagnostics, highlighting their promise for future utilization in point-of-care diagnostics and environmental monitoring. The first biosensor is a highly-sensitive fluorometric and colorimetric hybrid carbon-dot nanosensor for dual monitoring of urea. The research wherein urease enzyme can specifically hydrolyze urea to generate carbon dioxide and ammonia, causing an increase in the pH, which activates the reduction affinity of tannic acid to generate plasmonic AgNPs in situ. The absorption spectra of the AgNPs overlapped closely with the fluorescence spectrum of the CDs, enabling effective quenching of the CD fluorescence upon urea exposure. This fluorogenic and chromogenic dual-signal is used to quantify the accurate urea concentrations, showing limit-of-detection (LOD) of 18 nM and 1.05 μM for fluorometric and colorimetric sensing, respectively. The second biosensor introduces a novel ratiometric fluorometric assay, employing LSPR wavelength shift-driven on-off signals for melanoma diagnostics. This method leverages a dual-mode biosensing platform integrating multicolorimetric and ratiometric fluorometric detection for ultra-sensitive tyrosinase quantification. This platform leverages LSPR modulation of gold nanobipyramids (AuNBPs) alongside a fluorescence resonance energy transfer (FRET)-based mechanism using upconversion nanoparticles (UCNPs). The tunable LSPR of AuNBPs provided a bidirectional ratiometric multicolor emission improving sensitivity and selectivity. Tyrosinase-mediated oxidation of catechol to quinone inhibits silver deposition on AuNBPs, inducing a distinct concentration-dependent color shift for precise visual quantification. This dual-modality assay achieves an exceptional LOD of 4.13 × 10-5 and 2.71 × 10-5 U/mL for colorimetric and fluorometric sensors, respectively, demonstrating high selectivity, robustness in complex biological samples. Lastly, a plasmonic-quantum dot hybrid assay is introduced for label-free, dilution-free urea detection in raw urine. This system utilizes the catalytic properties of urease to hydrolyze urea into ammonia and carbon dioxide, creating a localized pH shift that induces the reduction ability of catechol indirectly governs silver deposition efficiency on the AuNBP surface. In undiluted urine, this reaction dynamically alters the LSPR behavior of AuNBPs, which modulates the FRET between the QDs and the AuNBPs. The proximity-induced energy transfer leads to selective quenching of one fluorescence emission channel, enabling highly sensitive urea detection without the need for dilution or complex sample preparation. This dual-mode platform, combining LSPR modulation and ratiometric fluorescence using QDs, exhibits high sensitivity and reliability for direct urea detection in undiluted urine. Ultimately, these innovative biosensors, leveraging LSPR principles, exhibit exceptional sensitivity, selectivity, and applicability across diverse fields, promising rapid and accurate analyte detection for various diagnostic and monitoring applications. Conclusion This dissertation presents a comprehensive exploration of localized surface plasmon resonance (LSPR)-driven dual-mode biosensing strategies for high-sensitivity bioanalysis. By integrating colorimetric and fluorometric signal transduction mechanisms, we developed multifunctional nanohybrid platforms tailored for real-time, label-free detection of clinically and environmentally significant analytes. The research journey begins by the first platform that features nitrogen-doped carbon dots (N-CDs) and plasmonic silver nanoparticles (AgNPs) for dual-signal urea detection, enabling ratiometric fluorescence quenching via FRET and LSPR-based colorimetric modulation. The system exhibited exceptional sensitivity (LOD: 18 nM fluorometric, 1.05 µM colorimetric) and selectivity in complex matrices, including human urine. The second system employs gold nanobipyramids (AuNBPs) and upconversion nanoparticles (UCNPs) for tyrosinase-based melanoma diagnostics. Tyrosinase-catalyzed catechol oxidation governs silver deposition on AuNBPs, modulating LSPR shifts and dual-peak UCNP fluorescence via inverse ratiometric switching. This sensor achieved a remarkably low detection limit (LOD: 4.13 × 10-5 U/mL) and demonstrated high precision in human serum. The third platform overcomes the chloride interference barrier in raw urine sensing by employing a stabilized [Ag(NH3)2]+ complex and crown ether ligands to facilitate silver shell growth on AuNBPs for dilution-free urea quantification. Combined with dual-emissive quantum dot nanohybrids, this system achieved robust multicolorimetric and ratiometric responses with minimal background interference. All sensing platforms were successfully immobilized into hydrogel matrices, enabling portable, point-of-care (POC) biosensing with excellent stability and reproducibility. Collectively, these LSPR-driven dual-mode biosensors address key challenges in sensitivity, matrix compatibility, and signal reliability, offering powerful solutions for clinical diagnostics, food safety, and environmental monitoring.

플라즈몬 공명을 활용한 광학기반 물리적 복제 방지 기능

이주한 중앙대학교 대학원 2025 국내석사

정보 통신의 발달로 정보 보안에 대한 요구가 커졌다. 이뿐만 아니라 의약품, 전자제품 등의 불법 복제가 늘어나면서 식별된 사용자와 정품 인증에 대한 암호화 시스템이 고도화되었다. 이에 맞추어 아예 물리적인 복제가 불가능하여 안정성을 높인 보안 시스템의 구축에 이목이 집중되었고 이것이 물리적 복제 방지 기능(Physically unclonable function, PUF)이다. PUF는 무질서한 물리적 배치나 자체적으로 내재한 무작위한 방식을 통해 사실상 복제가 불가능한 고유 식별자를 생성하는 보안 기능이다. 이로 인해 동일한 공정으로 제작하더라도 통제 불가한 공정상의 오류가 발생해 결과물에 차이가 발생하고 여기서 차별화되는 물리량을 암호화에 사용한다. 그 결과 같은 방법으로 복제를 시도하더라도 다른 결과물이 나와 복제가 불가하다는 특징을 가진다. 이뿐만 아니라 체계 내에 내장된 고유의 물리적 방식에 의해 암호화 키를 만들기 때문에 알고리즘에 의존하지 않은 비결정론적인 방식을 사용해 머신 러닝 공격에도 강한 저항성이 있다. 최근 연구에서는 광학의 파장, 편광, 투과율을 비롯한 다양한 변수를 활용해 복잡도를 높일 수 있다는 점과 비접촉 방식으로 샘플 손상 없이 빠른 측정이 가능하다는 점에 주목해 광학 기반의 PUF에 대한 연구가 늘어나고 있다. 본 연구에서는 이에 대한 방안으로 플라즈몬 기반의 PUF를 제안하였다. 무작위로 배치된 플라즈몬 구조물에서 플라즈몬 간의 결합이 일어나며 예측 불가능 형태의 빛의 산란과 간섭이 발생한다. 이에 의한 이미지를 암호화에 사용하여 무작위성이 높은 암호화 키를 생성하였다. 첫 번째로 금 나노 입자 기반 PUF에서는 기판 위에 무작위로 배치된 금 나노 입자들이 국소적 표면 플라즈몬 공명에 의해 전기 쌍극자 모드가 발생하고 강한 산란에 의한 이미지를 얻는다. 이미지로 얻은 암호화 키는 0.4981의 평균 해밍 거리 비율을 보여주어 높은 무작위성을 보였다. 두 번째로 연구한 알루미늄 필름 구멍 구조 기반 PUF는 석영 기판 위에 겹치지 않게 무작위로 구멍이 뚫린 알루미늄 필름이 있는 구조이다. 금속 표면과 구멍의 경계 끝부분에서 각각 표면 플라즈몬 공명과 국소적 표면 플라즈몬 공명이 발생하여 예측하기 어려운 이미지를 형성하였다. 성능을 확인한 결과, 평균 해밍 거리 비율과 표준편차가 각각 0.4987, 0.03113로 높은 무작위성과 좁은 분포를 가졌다. 자유도는 258bits로 물리적 비트 수인 256 bits와 유사한 값을 가져 각 비트가 독립적으로 동작하고 있음 또한 입증하였다. 추가로 실제 공정 상황을 가정해 구멍에 편차를 적용해 확인하였고 기존과 유사한 성능을 보여 실제 제작 시에도 준수한 성능을 보임을 확인했다. 마지막 연구에서는 두번째 구조의 구멍을 타원형으로 변경해 편광에 대한 반응성을 극대화해 하나의 PUF에서 편광에 따라 여러 개의 키를 얻을 수 있음을 보였다. 이 연구는 유한 미분 시간 영역 전산모사로 시행하였다. 절대적인 작은 크기 40 μm로 인해 소형 칩으로 만들기에 적합할 것으로 생각하고 공정 방식상 대량 생산이 쉽고 대형화에 유리해 크기를 키워 많은 양의 암호화 데이터를 담을 수 있다. 구조물은 이로 인해 ID 키나 정품 인증에 활용할 수 있을 것으로 기대한다. With the development of information and communication technologies, the demand for enhanced information security has grown. In addition, the rise in illegal reproduction of pharmaceuticals and electronic devices has led to the sophistication of encryption systems for user identification and product authentication. This has driven the desire to build security systems that are physically unclonable, ensuring heightened stability and security. Such systems are referred to as Physically Unclonable Functions (PUFs). PUFs generate unique identifiers that are virtually impossible to replicate by leveraging disordered physical arrangements or inherently random mechanisms. Even when produced under identical processes, uncontrollable manufacturing defects lead to differences in output. These unique physical properties are used in encryption. As a result, attempts to replicate the same process yield different outputs, making duplication impossible. Furthermore, because PUFs create encryption keys through the intrinsic physical mechanisms embedded within the system, they adopt a non-deterministic approach that does not rely on algorithms. As the results, those exhibit strong resistance to machine learning attacks. Recent studies have highlighted the advantages of photonic PUFs. For example, they exhibit high complexity using a lot of variables such as wavelength, polarization, and transmittance, which enhance their functionality. Also, they can conduct fast, non-destructive measurements. In this study, a plasmon-based PUF is proposed as a solution. Randomly arranged plasmonic structures create unpredictable scattering and interference patterns through plasmonic coupling. These patterns are used for encryption, generating highly random cryptographic keys. At first, a gold nanoparticle-based structure PUFs was studied. They generate electric dipole mode via localized surface plasmon resonance, resulting in strong forward scattering and producing complex images. Those images are converted into encryption keys through binning and digitizing. This method achieved an average fractional Hamming distance of 0.4981, demonstrating high randomness. In the second study, aluminum films with randomly distributed non-overlapping holes on a substrate were studied. Localized surface plasmon resonance at the hole edges and surface plasmon resonance on the metallic surface produced highly unpredictable images. Performances analysis revealed an average Hamming distance ratio and standard deviation of 0.4987 and 0.03113, which indicates high randomness and a narrow distribution. The degrees of freedom reached 258 bits. Considering the physical bit count of 256 bits, each bit operates independently. Additionally, to account for real manufacturing conditions, simulations were conducted incorporating deviations in hole diameters. The performance remained almost unchanged Hence, I expect the performance to be reliable even when producing real products. In the final study, the hole shapes were modified to ellipses at the second structure to enhance polarization sensitivity. This adjustment allowed multiple keys to be generated from a single PUF depending on the polarization state. This research was conducted using finite-difference time-domain (FDTD) simulations. Due to its compact size of 40 μm, the proposed structure is well-suited for miniaturized chips. Also, when considering the manufacturing process, it supports mass production and enables larger designs capable of storing substantial encryption data. These attributes make the plasmon-based PUFs promising candidates for use in ID keys and product authentication systems.

Synthetic strategies and optical properties of anisotropic plasmonic nanostructures

이정훈 서울대학교 대학원 2014 국내박사

Designing and synthesizing plasmonic nanostructures allow us to manipulate and mix the optical, magnetic and electrical characteristics of materials. Optical response of individual nanoparticles mainly dominated by localized surface plasmon resonance (LSPR) strongly depends on their size, shape, composition. This structure- and environment-dependent LSPR has been used to enhance optical signals such as Raman scattering or fluorescence in a wide range of fields from optics and spectroscopy to biomedical applications. Important reason that enhances such an optical signal is the extraordinarily amplified electromagnetic fields (EM fields) at the junction area positioned between a pair of nanoparticles or at internal crevice of the single nanoparticle. Among many structures, dimeric nanostructures with inter-particle spacing have been the most heavily studied because it is simple and they generate a very strong EM field in their junctions. In a homodimer system, redshift of longitudinal bonding plasmon modes is typically observed while decreasing inter-particle distance (hybridization of individual particle plasmons) but antibonding mode is dark. However, recent results show that diverse plasmon modes can be shown and excitable in the heterodimers with different size of nanoparticles or composition. This phenomenon is due to the symmetry breaking of plasmon modes and resulted in strong enhancement of various plasmon coupling. In both cases, it is particularly important to control ~1 nm gap because effective plasmon coupling can be generated in ~1 nm gap or less. Therefore, it is still highly challenging to precisely and reproducibly synthesize and assemble them into well-defined structures at nanometer scale for making them ideal for a range of fundamental studies and applications. In this thesis, we introduce DNA-based synthetic strategies to study optical properties and design plasmonic nanostructures with high structural controllability. These strategies allow us to manipulate the shape and assembly of nanostructures, such as dimer/trimmers with inter-particle spacing, single particles with intra-nanogap or different compositions. We investigate the relationships between plasmon response, signal amplification and structural changes of those plasmonic nanostructures such as inter-/intra-particle distance and particle size/composition. In the assembled system with inter-particle spacing, we performed single-molecule correlation studies on single-DNA-tethered Au-Ag core-shell dimeric nanostructures for surface-enhanced Raman scattering (SERS). We showed strategies to design the assembly of plasmonic nanoparticles and control the distance between particles for amplifying SERS signal. With DNA-modified AuNPs, We developed a new salt-tuned synthetic strategy for anisotropic growth of secondary nanoparticles on primary nanoparticles to form Au-Ag bimetallic heterodimers. We further focused in controlling the extent and sharpness of overlapped region on merged Au-Ag heterodimers and investigated the plasmon response dependent on structural changes and also size of nanoparticles. Understanding and utilizing the relationships between structural changes and plasmon coupling in nanostructures herein could greatly increase our knowledge in plasmonics, give insights in designing and synthesizing the plasmonic nanostructures. The chapter 1 provides an overview and perspective of recent advances in the use of DNA-tailored plasmonic nanostructures in biosensing applications. The plasmonic properties of metallic nanoparticles (NPs) such as Au and Ag NPs and the plasmonic coupling between them are of enormous interest for their strong and controllable optical signal enhancement and manipulation capabilities. The strong optical properties of these plasmonic structures are promising for various biosensing applications, but the widespread use of these structures is limited largely due to the absence of high-yield synthetic method for targeted nanoprobes with nanometer precision and the poor understanding of the plasmonics of these structures. DNA is a promising material that can be used as both specific biorecognition and versatile synthetic template in forming and controlling plasmonic nanostructures and their aggregations. In chapter 2, two different single-DNA-tethered Au-Ag core-shell nanodumbbell (GSND) designs with an engineerable nanogap were used in this study: the GSND-I with various inter-particle nanogaps from ∼4.8 nm to <1 nm or with no gap and the GSND-II with the fixed inter-particle gap size and varying particle size from a 23-30 nm pair to a 50-60 nm pair. With two probe design, we extensively study the relationships between single-molecule surface-enhanced Raman scattering (SMSERS) intensity, enhancement factor (EF) distribution over many particles, inter-particle distance, particle size/shape/composition and excitation laser wavelength using the single-particle AFM-correlated Raman measurement method and theoretical calculations. From the GSND-I, we learned that synthesizing a <1 nm gap is a key to obtain strong SMSERS signals with a narrow EF value distribution. Importantly, in the case of the GSND-I with <1 nm inter-particle gap, an EF value of as high as 5.9 × 1013 (average value = 1.8 × 1013) was obtained and the EF values of analyzed particles were narrowly distributed between 1.9 × 1012 and 5.9 × 1013. In the case of the GSND-II probes, a combination of >50 nm Au cores and 514.5 nm laser wavelength that matches well with Ag shell generated stronger SMSERS signals with a more narrow EF distribution than <50 nm Au cores with 514.5 nm laser or the GSND-II structures with 632.8 nm laser. Our results show the usefulness and flexibility of these GSND structures in studying and obtaining SMSERS structures with a narrow distribution of high EF values and that the GSNDs with<1nm are promising SERS probes with highly sensitive and quantitative detection capability when optimally designed. In chapter 3, we performed single-molecule correlation studies on DNA-tethered Au-Ag core-shell heterodimers to find out the relationship between SERS and LSPR. Using a multistep AFM tip-matching strategy that enables us to gain the optical spectra with the optimal signal-to-noise ratio as well as high reliability in correlation measurement between LSPR and SERS, the coupled longitudinal dipolar and high-order multipolar LSPs were detected within a dimeric structure, where a single Raman dye is located via a single-DNA hybridization between two differently sized Au-Ag core-shell particles. On the basis of the characterization of each LSP component, the distinct phase differences, attributed to different quantities of the excited quadrupolar LSPs, between the transverse and longitudinal regimes were observed for the first time. By assessing the relative ratio of dipolar and quadrupolar LSPs, we found that these LSPs of the dimer with ∼1 nm gap were simultaneously excited, and large longitudinal bonding dipolar LSP/longitudinal bonding quadrupolar LSP value is required to generate high SERS signal intensity. Interestingly, a minor population of the examined dimers exhibited strong SERS intensities along not only the dimer axis but also the direction that arises from the interaction between the coupled transverse dipolar and longitudinal bonding quadrupolar LSPs. Overall, our high-precision correlation measurement strategy with a plasmonic heterodimer with ∼1 nm gap allows for the observation of the characteristic spectral features with the optimal signal-to-noise ratio and the subpopulation of plasmonic dimers with a distinct SERS behavior, hidden by a majority of dimer population, and the method and results can be useful in understanding the whole distribution of SERS enhancement factor values and designing plasmonic nanoantenna structures. In chapter 4, we report a salt-tuned synthetic strategy using DNA-modified Au nanoparticles (DNA-AuNPs) to form Au-Ag head-body nanosnowman structures in >95% yield. We propose a mechanism for the formation of asymmetric Au-Ag nanosnowmen from DNA-AuNPs, salts, and Ag-precursor-loaded polymers. Importantly, we show that oriented assemblies of various nanostructures are readily obtained using nanosnowmen with asymmetrically modified DNA as building blocks. Synthesizing and assembling nanoscale building blocks to form anisotropic nanostructures with the desired composition and property are of paramount importance for the understanding and use of nanostructured materials. In chapter 5, we further developed the DNA-based synthetic strategies for asymmetric Au-Ag bimetallic heterodimers with structural controllability especially within overlapped region. These strategies allow us to manipulate the shape and composition of nanostructures within single particle level. We investigate the relationships between plasmon response and structural changes. We have shown that the extent of overlap and sharpness of anti-wedge region in merged conductive area along with symmetry breaking by different composition and size are responsible for various plasmon modes. Especially, the charge transfer plasmon and capacitive plasmon modes at low frequency showed most sensitive response on those changes. We have further shown that reproducible SERS signal can be generated from this structures that show linear dependence on particle concentration (5 fM). SERS enhancement factor was confirmed to 1.3 × 106 ~ 1.9 × 106. Besides, Tunable and wide range of LSPR from visible to near IR makes this structure attractive for many plasmon-based applications.