John J. Steele

Nanostructured Metal Oxide Thin Films for Humidity Sensors

Capacitive humidity sensors were fabricated using countersunk interdigitated electrodes coated with amorphous nanostructured TiO2, SiO2, and Al2O3 thin films grown by glancing angle deposition. The capacitive response and response times for each sensor were measured. The sensor utilizing TiO2 exhibited the largest change in capacitance, increasing exponentially from ~ 1 nF to ~ 1muF for an increase in relative humidity from 2% to 92%. Adsorption and desorption response times were measured using flow rates of 2.5 l/min and were between 90 ms and 300 ms for the sensors studied here. A simple model of the capacitive response of the devices has been developed and used to calculate the dielectric constant of the combined system of our films and adsorbed water. The obtained dielectric constants are found to be much higher than bulk or literature values for similar systems.

Capacitive Humidity Sensors With High Sensitivity and Subsecond Response Times

Capacitive-based humidity sensors were fabricated using coplanar interdigitated electrodes coated with nanostructured TiO2 thin films produced by glancing angle deposition. In this letter, we show that increased sensitivity (nF/%RH) is obtained by decreasing the electrode periodicity or by increasing the planar area of the electrodes, or both. The devices were sensitive over a wide range of relative humidity levels (<1% to >92%) and exhibited extremely fast, subsecond response times. Typical adsorption and desorption response times were measured to be <220 and >400 ms, respectively

Impact of morphology on high-speed humidity sensor performance

Capacitive-based humidity sensors were fabricated using unique nanostructured aluminum-oxide thin films. These sensors exhibited extremely fast desorption response times as short as 42 ms. In this paper, we present the effects of varying the thin-film porosity on sensor performance. Specifically, we look at the capacitive response and the desorption response time of the sensors. It was found that increased porosity tends to decrease the desorption response time and increase the relative humidity where the devices become sensitive.

Ion-Beam Assisted Glancing Angle Deposition for Relative Humidity Sensors

Ion-beam assisted glancing-angle deposition is used to fabricate relative humidity sensors. Ion currents of 3,5, and 7 mA were tested. The morphology and capacitance of the sensors are found to be dependent on the ion current density. The capacitance increases with increasing ion current, especially for the 7 mA case which exhibits a capacitance approximately one order of magnitude greater than a standard glancing-angle deposited film over much of the sensor range.

Performance and Modeling of a Nanostructured Relative Humidity Sensor

Capacitive humidity sensors were fabricated using interdigitated electrodes coated with amorphous nanostructured TiO2 thin films grown by glancing angle deposition. The sensor exhibited a large change in capacitance, increasing exponentially from ∼ 1 nF to ∼ 1 μF for an increase in relative humidity from 2 % to 92 %. A simple model of the capacitive response and dielectric constant of the devices has been developed and compared to the experimental results. From this comparison, it is clear that the magnitude of the device response observed cannot be explained with bulk dielectric constants or literature values.

Microstructured humidity sensors fabricated by glancing angle deposition: characterization and performance evaluation

We have used the glancing angle deposition technique to fabricate highly porous nanostructured optical thin films that act as humidity sensors. The responsiveness and repeatability of these sensors has been investigated for samples stored under different environmental conditions. It has been found that samples stored in air have a more stable performance than those stored in a dry nitrogen environment. It has also been found that annealing impacts the responsiveness of the optical thin film sensors.

Sub-Second Humidity Sensing based on Nanostructured Narrow-Bandpass Optical Filters

An optical-based humidity sensor with a sub-second response time was fabricated from a nanostructured titanium dioxide thin film. A refractive index profile designed to yield a narrow-bandpass optical interference filter was obtained through nanoscale porosity variations produced by glancing angle deposition (GLAD). Under varying humidity conditions the transmittance spectrum of the filter shifts due to effective index changes of the porous structure resulting from adsorption/desorption of water vapor. In the following we will show that this device is highly sensitive, exhibits minimal hysteresis, and is extremely fast. The adsorption and desorption response times were measured to be 270 ms and 160 ms, respectively.

Surface functionalization and porosity studies of high-speed humidity sensors

Presented here are the results of a study on the effects of varying the porosity of unique nanostructured thin films on their humidity sensing properties. Specifically, we look at the capacitive response and the desorption response time of the sensors. It was found that increased porosity tends to decrease the desorption response time and increase the relative humidity where the devices become sensitive. Also presented are the results of surface functionalization effects on device sensitivity. It was found that the dynamic range of the sensors could be attenuated by as much as three orders of magnitude by covalently bonding hydrophobic organic molecules to the surface of the films.