Optimized Colorimetric Photonic‐Crystal Humidity Sensor Fabricated Using Glancing Angle Deposition

Abstract

Colorimetric sensing, where environmental changes are transduced into visual color changes, provides an intuitively simple yet powerful detection mechanism that is well‐suited to the realization of low‐cost and low‐power sensors. A new approach in colorimetric sensing exploits the structural colour of photonic crystals (PCs) to create new color‐changing materials, however much work is still required to simultaneously achieve optimized sensor response and low‐cost, scalable nanofabrication. This work responds to these challenges by designing, fabricating and evaluating a mesoporous PC sensor optimized to exhibit as large as possible color‐shift in response to small changes in relative humidity (RH). A novel design optimization is achieved by employing a colorimetric framework that translates simulated/measured spectral quantities into numeric color values directly related to color perception. The sensor design is then realized using a mesoporous TiO2 PC, fabricated using glancing angle deposition (GLAD). The GLAD technique is a bottom‐up, single‐step nanofabrication method providing the nanoscale precision required to successfully realize the optimized PC design. The PC sensor is shown to be highly sensitive and stable: the PC structural‐color changes visibly due to RH changes smaller than 1%, and the response is stable over hundreds of hours of sensor operation. Additionally, measurements and simulations are used to reveal the important link between the PC optical modes, pore geometry, and sensor response which will be useful in future PC sensor experiments. The combination of bottom‐up nanofabrication with visible color‐based sensing, coupled with the useful design methodology, will lead to further developments in low‐cost, widely deployable optical sensors.

Perspectives on oblique angle deposition of thin films: From fundamentals to devices

“…The dependence of the film's refractive index and the overall optical response of these systems on the conditions of the medium have been one of the main motivations for using nanocolumnar films or nanostructures as optical sensors. Some examples of optical sensors based on changes in the optical properties of OAD structures are the use of helical nanocolumns [490] and Bragg stacks [285,502] for infiltrated liquid sensing, or the use of photonic crystals for high-speed water vapor detection [514,515] and colorimetric detection of humidity [516] (see Section 5.4 for a more complete review of humidity sensors).…”

Humidity sensing properties of MoO3-NiO nanocomposite materials

“…Particularly, the development of an excellent humidity sensing material is an important aspect in the creation of a high-powered humidity sensor. In the past decades, many kinds of materials including polymer [1][2][3][4], electrolytes [5][6][7], organic-inorganic hybrid composites [8][9][10][11], photonic crystal [12][13][14] and metal oxides or ceramic [15][16][17][18][19][20] have been explored to fabricate humidity sensing detectors. But each kind of material cannot completely meet all requirements desired for an excellent sensor such as high sensitivity, good linearity, quick response-recovery and long-term stability.…”

Touchless Optical Finger Motion Tracking Based on 2D Nanosheets with Giant Moisture Responsiveness

“…[ 21 ] In addition, several humidity sensors were reported with smaller stopband shifts in the range of tens of nanometers and response times on the order of a few seconds. [ 22,23 ] Recently, Steele et al [ 24 ] and Hawkeye and Brett [ 25 ] presented mesoporous TiO 2 gratings used for humidity applications, which show an ultrafast response time in the subsecond range, but rather small stop band shifts (and therefore resolution) on the order of 30 nm. Here, we replace the dimensionally fl exible organic component in BS by phosphatoantimonic acid H 3 Sb 3 P 2 O 14 nanosheets [ 26,27 ] with giant moisture responsiveness.…”