4.2 Tunable Color Filters as Electrochromic Windows
4.2.8 Prospective Fabrication Method and Characterization Technique 133
A typical fabrication flow for the realization of our theoretically proposed design is rather straightforward. MDM based devices, composed of multilayer thin films of Ag–
DAST–Ag can be fabricated by successive electron beam (e-beam) evaporation tech- nique [169–171]. For the realization of an absorption-mode color filter, first, a 125 nm thick Ag layer needs to be coated on a smooth silicon (Si) wafer [170]. Thereafter, 80 nm thick DAST layer needs to be deposited on top of Ag–Si layer. Finally, 45 nm thick Ag layer can be deposited on top of DAST–Ag–Si layer. For the case of a transmission- mode color filter, top and bottom Ag layers, each 32 nm thick, need to be coated on top and bottom surfaces of a two-sided polished DAST layer of 77 nm thickness [169, 171].
For optical characterization, Fourier Transform infrared (FT-IR) microscope or a micro- scope integrated with a spectrometer and an electron multiplication charge-coupled device (CCD) camera of prescribed specifications given in [169, 170] may be used.
4.2.9 Summary
We presented a theoretical design of electrotunable absorption- and transmission- mode nanophotonic windows as specific color filter based on metal–dielectric–metal (MDM) structurewith significantly reduced voltage requirements. We illustrated the design of three color filters (blue, green, and red) to show that only±10 V power sup- ply should be sufficient to realize any intermittent color filter in the visible regime.
Our theoretical results obtained using transfer matrix method and transmission line method agree well with those of finite-element-method based full-wave simulations, which validates our results. For practical realization, our design is power-efficient, large-area compatible, lithography-free, angle-insensitive, polarization-independent, and has extremely narrow-bandwidth.
5
Static and Electrotunable Windows Based on Insulator–Metal–Insulator Multilayer Thin-films
Contents
5.1 Insulator–Metal–Insulator Thin-films Based Static Windows 142 5.2 Insulator–Metal–Insulator Thin-films Based Electrotunable
Windows . . . 145 5.3 Static versus Electrotunable Windows . . . 151
In chapters 3 and 4, we mainly focused on the designs of passive and electrotun- able windows, respectively, based on metal–insulator–metal multilayer thin-films. We showed that the optimal performances of those windows were obtained using metallic layers which were 5 nm in thickness. Today, with the existing nanofabrication tech- nology, it is still very challenging to deposit uniform 5 nm thin top and bottom metal- lic layers in a metal–insulator–metal structure. However, with rapid advancement in nanofabrication technology we believe that soon it may be possible in the near future.
So, keeping in mind the current state-of-the-art nanofabrication technology, we ex- plored alternative designs of passive and electrotunable windows based on insulator–
metal–insulator (IMI) multilayer thin-films, which would allow the use of a thicker metal film that would be easier to fabricate.
This chapter presents the designs of static and electrotunable ‘smart’ windows, fol- lowed by a comparative study between those.A unique approach is adoptedto design insulator–metal–insulator thin-films, deposited over a silica glass substrate to filter vis- ible and infrared solar radiation selectively. For static windows, we optimize our de- sign to operate in diverse climatic conditions by choosing different combinations and thicknesses of metal and insulator layers. Whereas for electrotunable windows, we use an electro–optic polymer as the insulator layers to dynamically control portions of transmitted solar radiation over a voltage range of−12 V to +12 V.
5.0.1 Background
In literature, many visible or infrared filters based on metal–insulator–metal (MIM) thin-films have been reported [197–199]. However, there is a lack of work on insulator–
metal–insulator (IMI) thin-films based filters. Moreover, such thin-films have not been explored in the context of smart windows that simultaneously require filtering of visi- ble and infrared radiations. Here, it is important to highlight that the MIM structures
use of two metallic layers at the top and bottom. Since smart window applications demand transmission-mode filters with high visible transmittance, the metallic layer thickness should be minimal in MIM structures to achieve desired visible light trans- mission. Unfortunately, such ultra-thin layers of metals are very challenging to realize even with the existing state-of-the-art nanofabrication facilities. In this context, IMI thin-films could be a more practical choice due to the use of a single metallic layer (that could be comparatively thicker than those used in MIM structures), sandwiched between two insulator layers.
Here we present a comparative study on insulator–metal–insulator (IMI) thin-films based static and electrotunable windows. We use noble metals [gold (Au), silver (Ag), and copper (Cu)] as well as their relatively inexpensive alternatives [lithium (Li), ti- tanium nitride (TiN), and aluminium oxynitride (ALON)] as a choice for the metallic layer. For static windows, we make use of different dielectrics [such as, silica (SiO2), titanium oxide(TiO2), and silicon (Si)] as a choice for both the top and bottom insula- tor layers. For electrotunable windows, we use an electro–optic polymer: 4-dimethyl- amino-N-methyl-4-stilbazoliumtosylate (DAST), as the insulator layers, which would allow to dynamically control the portions of transmitted solar radiation over a voltage range of−12 V to +12 V [132].