Green Materials for Electronics by unknow

Author:unknow
Language: eng
Format: epub
ISBN: 9783527692965
Publisher: John Wiley & Sons, Inc.
Published: 2017-08-30T00:00:00+00:00

5.4 Tunable/Programmable Transience

The properties of the encapsulation layers and/or the substrates for transient electronics provide a means to tune/program the lifespan of a device, with good engineering control. Material options for the encapsulation range from inorganic materials such as MgO, SiO2, and SiNx to degradable polymers, such as PLGA, PCL, silk films, and others, or a combined use of multiple layers of these materials. Figure 5.3a shows measured changes in electrical resistance of a meander trace of Mg, encapsulated with layers of MgO with different thicknesses, and combinations of MgO with crystallized silk films, each due to immersion in DI water at room temperature. Here, as-deposited Mg traces without any encapsulation lose their electrical continuity within an hour. A single layer of MgO with different thicknesses allows adjustment of the lifetime of the device up to 12 h, while a combination of MgO and silk extends the life to over 4 days [1]. Figure 5.3b provides an example of an encapsulation strategy at the device level using metal-oxide-semiconductor field-effect transistors (MOSFETs) with Si NMs for the semiconductor, Mg for the conductor, and MgO/SiO2 for the gate dielectric. When partially submerged in DI water, the device exhibits stable operation for up to 90 h, that is, no degradation in measured transfer curves and drain currents. Rapid transience occurs in a short time interval that follows this period of stable operation. Device characteristics, therefore, exhibit two-step kinetics in function: (i) The encapsulation layers determine the first timescale (stable operation); (ii) the Mg electrodes define the second (rapid transience) owing to a relatively higher rate for dissolution compared to other device materials [1]. Tuning and programming of transience behavior using this type of strategy and more complex variants of it provide important capabilities. Since the solubility of silk fibroin films in water depends on the degree of crystallinity, adjusting this parameter allows the period of device operation to be selected across a wide range, from seconds to years [24, 25].

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