Engineering the Charge Transport of Ag Nanocrystals for Highly Accurate, Wearable Temperature Sensors through All-Solution Processes

Abstract

All-nanocrystal (NC)-based and all-solution-processed wearable resistance temperature detectors (RTDs) are introduced. The charge transport mechanisms of Ag NC thin films are engineered through various ligand treatments to design high performance RTDs. Highly conductive Ag NC thin films exhibiting metallic transport behavior with high positive temperature coefficients of resistance (TCRs) are achieved through tetrabutylammonium bromide treatment. Ag NC thin films showing hopping transport with high negative TCRs are created through organic ligand treatment. All-solution-based, one-step photolithography techniques that integrate two distinct opposite-sign TCR Ag NC thin films into an ultrathin single device are developed to decouple the mechanical effects such as human motion. The unconventional materials design and strategy enables highly accurate, sensitive, wearable and motion-free RTDs, demonstrated by experiments on moving or curved objects such as human skin, and simulation results based on charge transport analysis. This strategy provides a low cost and simple method to design wearable multifunctional sensors with high sensitivity which could be utilized in various fields such as biointegrated sensors or electronic skin.

Designing Metallic and Insulating Nanocrystal Heterostructures to Fabricate Highly Sensitive and Solution Processed Strain Gauges for Wearable Sensors

“…These applications demonstrate excellent functionality as well as high sensitivity and stability of our strain sensors. Indeed, the ability to differentiate between different stimuli such as temperature and pressure is highly important for real application of electronic skins . We believe that our solution processed heterostructure form can be used to these applications.…”

“…Since 77% CdSe NC hybrid thin films showed the highest sensitivity, they were used as building blocks for active sensing layers. Inorganic ligand‐treated Ag NC thin films were used for electrodes because they showed high conductivity and high mechanical stability as well as low gauge factor . The fabrication process of multiarray strain sensors is presented in Figure a.…”

“…As an alternative candidate for strain gauges, colloidal nanocrystal (NC) can be used as a new class of material due to following reasons: (i) NCs can be synthesized in large scales through wet chemical methods and allow solution‐based process to integrate these materials into devices with low cost . (ii) Their properties can be precisely tuned by manipulating their size, shape, surface ligand, and composition . (iii) Particularly, their unique hopping transport mechanism can be advantageously exploited to enhance their sensitivity .…”

Highly Sensitive Temperature Sensor: Ligand‐Treated Ag Nanocrystal Thin Films on PDMS with Thermal Expansion Strategy

“…In order to realize highly sensitive and reliable temperature sensors with diverse forms, various types of sensors have been developed in conjunction with investigations related to emerging materials and fabrication processes. Four types of sensors with different sensing mechanisms have been widely studied: resistive temperature sensors, capacitive temperature sensors, transistor based, and PN‐junction based temperature sensors. Among these, resistive‐type temperature sensors are one of the most promising candidates owing to the following reasons: i) their simple geometry is compatible with various materials, substrates, and fabrication processes, such as solution processed materials, flexible substrates, and environmentally friendly fabrication processes, respectively, and ii) their simple and straightforward mechanism allows facile and direct measurement of temperature with high sensitivity.…”

“…Various methods have been studied to improve sensitivity. Some examples that manipulate the TCR for sensing include the adjustment of conductivity and structural properties by using poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) with a solvent doping method, and the adjustment of the activation energy of hopping transport by using a surface treatment such as ligand exchange . Recently, highly sensitive temperature sensors have been reported utilizing geometrical change with high thermal expansion coefficient (TEC) polymer substrates .…”