Plasmonic Films Based On Colloidal Lithography And Corresponding Properties

Metals have been widely used in daily life and science research. Besides mechanical and conducting properties, some noble metals can have interaction with light in some specific cases, resulting in surface plasmon resonance(SPR). The electric field around the metal surface can be redistributed and enhanced. This offers great potential in optical devices, sensors, photonic circuit and medical diagnose. In general, metal should be structured for the excitation of SPR. Fabrication of various metal micro-nanostructures has been a hot spot in the field of physic and material research. The great progress in the field of plasmonic films is, in part, due to the advances in nanofabrication methodologies. Scanning beam lithography techniques, such as electron beam lithography and focused ion beam lithography, are the main choices to fabricate metal structured films. These conventional lithography techniques are capable of precise control over the size, shape, and spacing of metallic nanostructures, which are appropriate for lab fabrication and theory analysis. However, there are limitations in fabricating structures for these techniques, especially in three-dimension(3D). Besides, the fabrication processes are high-cost and inefficiency. More recent research has focused on unconventional lithographic techniques that are capable of facilely fabricating 3D structures and patterning large areas in parallel at low cost. In particular, the colloidal lithography technique, possessing the advantages of a low-cost and flexible fabrication process, has been more frequently used in preparing plasmonic films. In this paper, various 3D plasmonic films are designed and fabricated through colloidal lithography technique. The plasmonic performances are enhanced by introducing the features of cavities, nanotips and nanogaps in the structured metal films. The plasmonic films are investigated for applications of plasmonic sensors, color displays, biological detection and asymmetric optics.1. In this paper, a novel nanohole array with three-dimensional structure was generated by colloidal lithography existing of a free-standing nanohole array that experiences the same environment on both sides in 3D space. In a practicable way, the nanoholes were lifted up from the substrates, resulting in undulate morphology. Therefore, we name this a 3D nanohole array(3D-NHA), and the performance in optical applications was much improved compared to conventional 2D nanohole arrays(2D-NHAs) due to a nearly perfect surface plasmon energy matching on both surfaces of the arrays. There was a 1.5-fold improvement in extraordinary optical transmission and 1.7-fold enhancement in response sensitivity towards a change of refractive index compared to 2D-NHAs. Furthermore, a(1, 1) Au/air transmission peak appeared in the transmission spectra and showed high relative sensitivity and an excellent linear response over a wide spectral range. Besides improved optical properties, good transferability of nanohole arrays without substrate dependence was designed and realized experimentally for the first time due to the unique protuberant holes of 3D-NHAs, which can be flexibly used in plasmonic sensors. 2. In this paper, a novel nanohole array with volcano shaped holes is fabricated on glass substrates with excellent stability by a low-cost, large-area and well-controlled colloidal lithography(CL) method. By controlling the hole size and height, Ag nanovolcano arrays can be designed to provide optimal transmission spectra to yield structural colors possessing only one single sharp transmission peak with effective suppression of undesired wavelength light, while which doesn’t need any index-matching layers. Three primary red-green-blue(RGB) monochromatic colors are obtained by adjusting the period. Moreover, just by changing the surrounding environments, the colors can be tuned facilely and inexpensively retaining a single sharp transmission peak across the whole visible range, showing an efficient responsive color display. Based on the model of nanovolcano, disk-in-volcano arrays are fabricated by carrying out another deposition process. By tuning structural parameters, the disk-in-volcano arrays show greatly enhanced resonances in the nanogaps formed by the disks and the inner wall of the volcanos. Therefore, they respond to the surrounding environment with a sensitivity as high as 977 nm per refractive index unit(RIU) and with excellent linear dependence on the refraction index. Moreover, through mastering the fabrication process, biological sensing can be easily confined to the cavities of the nanovolcanos. This novel sensor concentrates the surface plasmon energy density, reduces the sensing background and saves expensive reagents. The nanovolcano arrays and the disk-in-volcano arrays present great potential in applications of color displays and plasmonic sensors.3. We have developed a method to prepare a structured silver film with a hollow nanocone array. The topologically continuous film is demonstrated to show greatly enhanced optical transmission compared to a flat silver film with the same thickness. The hollow nanocones possess sharp top tips and bottom nanoholes, which are both prominent places where strong SPRs can be excited. Due to the contributions of SP energy from these two types of structures, the SP field of hollow nanocones is not only greatly enhanced but also redistributed inside the hollow core(air) outside the dielectric substrate, resulting in a new kind of resonant optical transmission. Furthermore, the effect of structural parameters on the spectra and sensing performances is investigated, showing precise control over the spectra and great potential for plasmonic sensors. These hollow nanocone arrays are further inverted to make the tips contact the substrates through a transferring process. The inverse hollow nanocone arrays show different optical performances and resonant modes, indicating the significant role of surrounding environment. By changing the material, Au hollow nanocone array films are fabricated. Through tuning the height and thickness of the nanocones, the wavelength of the resonance peak can be controlled so as to overlap with the bulk plasmonpeak at around 500 nm. This causes a strong overlap of the spectrum of the structured films with that of planar Au films in air, which leads to difficulty in distinguishing the colors of the two different films. However, the plasmonic peaks can shift away from the bulk plasmon peak by changing the surrounding environment to show different colors, showing the property of smart color display. Moreover, patterns of hollow nanocone array films are fabricated by photolithography or direct writing, which can also be displayed in a smart way. Furthermore, based on the model of hollow nanocone array and combining the shadow deposition technique, structured films with periodic arrays of nanoholes covered by half-cone shells are fabricated. The asymmetric half-cone/nanohole array films are capable of reshaping the optical transmission for differently polarized light, structural parameters and incidence angle. The 3D asymmetric arrays show an advanced property of asymmetric angle-dependent transmission with the intensity increasing with changing the direction of the incident light from the shelter(half-cone shells) side to the empty side. The nanostructures thus have one more degree of asymmetry and hence of freedom for manipulation compared to the previous ones. The nanostructured films in this chapter will be useful for metallic nanophotonic elements in many applications, including surface plasmon enhanced optical sensing and optical devices.In summary, three types of plasmonic films are fabricated based on colloidal lithography. The extensive structures of the three models are also explored. In terms of each unique structure, the plasmonic property and the performances in sensors, color display, biology detection and asymmetric optics are investigated. The work in this paper will play a positive role in the research of plasmonic theory, developing the plasmonic performances and exploring novel applications.