Rapid replication and facile modulation of subwavelength antireflective polymer film using injection nanomolding and optical property of multilayer coatings
© Fuh et al.; licensee Springer. 2013
Received: 9 July 2013
Accepted: 20 September 2013
Published: 2 October 2013
A rapid, cost-effective and high-throughput process for nanotexturing subwavelength structures with high uniformity using the polycarbonate (PC) is realized via injection nanomolding. The process enables the precise control of nanohole array (NHA) surface topography (nanohole depth, diameter, and periodicity) over large areas thereby presenting a highly versatile platform for fabricating substrates with user-defined, functional performance. Specifically, the optical property of the PC substrates were systematically characterized and tuned through the modulation of the depths of NHA. The aspect ratio submicron holes can be easily modulated and experimentally proven by simply adjusting the molding temperature. The nanotextured depths were reliably fabricated in the range of 200 to 400 nm with a period of approximately 700 nm. The fabricated PC films can reduce the reflectivity from an original bare film of 10.2% and 8.9% to 1.4% and 2.1% with 400-nm depth of nanoholes at the wavelength of 400 and 550 nm, respectively. Compared with conventional moth-like nanostructures with nanopillar arrays with heights adjustable only by an etching process, this paper proposes a facile route with submicron holes to achieve a similar antireflective function, with a significantly reduced time and facile height modulation capability. Furthermore, the effects of multilayer coatings of dielectric and metallic layers on the nanomolded NHA have been performed and potential sensing application is explored.
KeywordsSubwavelength Polycarbonate (PC) Injection nanomolding Nanohole array (NHA) Reflectivity
Nanotechnology is a prioritized research topic and triggers great interest among scientists, engineers and energy researchers around the world [1, 2]. Among them, surface nanotexturing has been extensively utilized in the recent years for enabling new functionalities and tailoring excellent physical and chemical properties. A wide range of examples explored recently include antireflective coatings [3, 4], superhydrophobic surfaces [5, 6], bio-engineered thin film , anti-stiction surfaces  and bio-mimic gecko adhesives . Experimentally, artificially fabricated inverted surface patterns of NHA and high fidelity nanopillar arrays have been proposed for substrates with structural antireflective and enhanced light management properties and practical applications include high-efficiency solar cells and synthetic gecko adhesives. In particular, antireflective coating technology using thin-film stacks are mainstream tools maturely used in many optical systems [10, 11]; however, critical limitations in the coating materials such as adhesion, thermal mismatch, and instability are the main drawbacks [12, 13]. Therefore, nanotexturing antireflective surfaces and associated fabrication technology is booming and in great demand.
Fabrication method for the antireflective coatings
Applications (other than antireflective coatings)
Micro-replication process (MRP)
Capable of creating nano/micro features on substrates of slicon or plastics. By combining three major steps of micro/nanostructure masters, metallic mold electroplating and replication into plastics.
Backlight guide plate, grating, micro-mirror arrays, photonic crystals and other micro/nano features
Roll-to-roll (R2R) printing
Capable of creating electronic devices on flexible substrates (plastics or metal foil) Typically includes steps of coatings, printing, laminating, re-reeling, and rewinding processes
Organic light emitting diodes (OLED), thin film solar cells, optical brightness enhancement films or organic thin film transistors (OFET)
Anodic aluminum oxide (AAO)
By anodizing high-purity aluminum to generate a porous alumina membrane as templates such that a closed-packed hexagonal array of columnar cells can be obtained. Typically, can be categorized as a self-ordering synthesis of nanopores
Molecular separation, energy generation and storage, electronics, photonics, sensors (biosensors), drug delivery, and template synthesis
In this paper, we present a facile and fast fabrication route for high-throughput, low-cost nanotexturing of surfaces with tunable NHA depths. The optical properties of the textured films were systematically characterized as a demonstration to validate the proposed technique for enabling substrates with functional performance of tunable reflectivities. In addition, this NHA can be integrated with multilayer coatings of both metallic and dielectric layers to further tailor the optical properties such as ultra-sensitive sensing applications.
Preparation of the PC film via precision injection nanomolding
Characterization of the replication process and operating parameters
To characterize the nanotextured surfaces, both SEM (LEO 1530 Gemini, Zeiss, Oberkochen, Germany) and AFM (Digital Instruments nanoscope, Tonawanda, NY, USA) were utilized. For the optical reflectivity measurements, spectrophotometer STEAG ETA-Optic (Heinsberg, Germany) and n&k analyzer 1280 (n&k Technology, Inc., San Jose, CA, USA) were used at the angle of 90°.
Results and discussion
The precision injection nanomolding process has been widely accepted as one of the rapid replication methods to transfer nanostructures and is considered a major mass production technique for a wide range of commercial products . In particular, the major processing parameters can be classified into the following: injection and mold temperatures, packing time and pressure, injection speed, etc. The diameter of the injection nanomolded film is a disk shape which geometric dimension is 120 mm in diameter and 0.6-mm thick. For a typical injection nanomolding operation, the following parameters apply: mold temperature is intentionally controlled in the range of 115 to 130°C, respectively, while the following parameters are fixed: 0.5-s packing time and 130-MPa packing pressure, injection speed 120 cm/s while the PC viscous flow was maintained at 320°C, total clamping force is fixed at 350 KN. Total cycle time for one shot of process including automatic transfer can be as low as 4 s while maintaining a high-fidelity replication. An automatic monitoring system is included in the injection process and deviation for the molding temperature is within ±0.5°C. In previous studies, the molding and PC flow temperature play a significant role on the replicated structure, both in terms of precise fidelity of depth and pitch. Other experimental work can be briefly explained as following: a stock PC pellets is fed into the system and used as the supply material. The mold holds a temperature controlled water circulation system for the purpose of heating and cooling function that facilitates the continuous operation and to ensure uniformity of viscous flow. The NHA stamp is held in the machine firmly and symmetrically about the mold geometric center while the transfer mechanism is concurrently applied. Upon finishing the molding process, the molded part is transferred to a conveyer for later rinsing deionized (DI) water bath. The system allows the user to control all the above parameter settings, and in particular, both the material and the molding temperatures are the most crucial ones.
In summary, a versatile and rapid process is presented based on the well-established injection nanomolding of PC polymer for the controlled nanotexturing of NHA surfaces over large areas with tunable depth topography. In addition, with the change of master Ni stamp, feature size diameter and density/periodicity can also be adjusted accordingly. The NHA-engineered surfaces exhibit a functional optical property that can be optimized for anti-reflection coatings. The proposed technology of rapidly replicated NHA surfaces may be used for efficient and cost-effective solar cells, highly light extracted light-emitting diodes (LED) and self-cleaning surfaces. The scalability of the process can be sufficiently addressed due to the reduced cycle time of 4 s and is fully compatible with the well-established mass production of DVD/BD industries. This work presents an important advance in the rapidly growing field of nanomanufacturing. Furthermore, we have also experimentally demonstrated an approach to quantitatively control transmission of light through NHA and multilayer coating of both dielectric and metallic layers with the potential use of sensing applications. The future work can be extended to the transmission of light through current NHA/multilayer structures and geometry-dependent selectivity in terms of both frequency and resonant width.
This work was supported by the Taiwan National Science Council under contract no. NSC 101-2221-E-008-014 and NSC 102-2221-E-008 -067.
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