Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen
University zur Erlangung des akademischen Grades eine/s(r)
Doktor/s(in) der Naturwissenschaften oder eine/s(r) Doktor/s(in) der Ingenieurwissenschaften genehmigte Dissertation
Nanoparticle based Strain and Chemical Sensors


Nanoparticles (NPs) as individual building blocks can be assembled to form socalled “artificial materials”. These NP materials possess unique electrical and mechanical characteristics and have been widely used in various applications, such as strain sensing, chemical detection, photo detectors, memory devices and so on. Specially, owing to their unique electronic properties, NP sensors based on electrical transduction mechanism have attracted considerable attention. In this thesis, NP materials were utilized to fabricate strain sensors and chemical sensors. Meanwhile, the influence of humidity on electronic properties of the NP materials was investigated.

In the first part of this thesis, new types of binary NP material were synthesized and utilized to fabricate strain sensors. These binary NP materials were fabricated by self-assembly with either homogenous or heterogeneous arrangement of NPs. Variable electrical and electro-mechanical properties were observed in both materials. It is shown that the electrical and electro-mechanical properties of these binary NP materials are highly tunable and strongly affected by the NP species and their corresponding volume fraction ratio. The conductivity and the gauge factor of these binary NP materials can be manipulated by about five and two orders of magnitude, respectively.

These binary NP materials with different arrangements of NPs also demonstrate different volume fraction dependent electro-mechanical properties. The binary NP materials with heterogeneous arrangement of NPs exhibit a peaking of the sensitivity at medium mixing ratios, which arise from the aggregation induced local strain enhancement. Studies on electron transport regimes and micro-morphologies of these binary NP materials revealed the different mechanisms accounting for the variable electrical and electro-mechanical properties. A model based on effective medium  theory is used to describe the electrical and electro-mechanical properties of such binary nanomaterials and shows an excellent match with experiment data. These binary NP materials possess great potential applications in high performance strain sensing technology due to their variable electrical and electro-mechanical properties.

In the second part of this thesis, NP based chemical sensors were fabricated and investigated. Molecules containing carboxylic groups but with different lengths were used to modify the NPs. NPs were then assembled on the quartz substrates to form NP stripes as the sensing area. It is shown that molecules with different lengths provide different tunneling barrier for the charge transport in the NP stripes. We studied the conductivity change of these chemical sensors upon the exposure to various ions. It was observed that these NP based sensors exhibited high sensitivity and specificity to Hg2+ ions. Specifically, the sensitivity of these chemical sensors was found to be dependent on the length of surface modification molecules of the NPs. Chemical sensors utilizing NPs modified with 11-mercaptoundecanoic acid (MUA) exhibited the highest sensitivity. By studying the charge transport regime and morphology change of the NP stripes, the mechanism accounting for the molecule length dependent sensitivity and the specificity was explained.

Since most NP based sensors are operated under ambient condition or even in aqueous solution, the humidity is always problematic, influencing their electrical properties via the interaction between the water molecules and NP materials. In the third part of this thesis, we investigated the influence of humidity on the electronic response of the NP materials. Multilayer and monolayer AuNP stripes with different dominated charge transport regimes were fabricated. It is shown that the humidity dependent electronic response is strongly correlated with the morphology and electron transport regimes in the AuNP stripes. Due to the differences in AuNP arrangements and the resulting dominant charge transport regime, the multilayer and mono layer AuNP stripes response differently to the humidity. This work revealed the possible mechanism accounting for their different responses and could help the development of high performance nanoparticle based devices.



Convective self-assembly setup configuration

A homemade convective self-assembly (CSA) setup was established including a
temperature controlled sample holder, fixtures, lifting platform, glass deposition slide, angle adjustable rotator, and a precise computer controlled step motor, Figure 3-1. The temperature controlled sample holder possesses an accuracy of ±1ºC. The lifting  platform can be moved up and down from micron meter scale to centimeter. The rotator
can be adjusted from 30º to 60º with 0.5º accuracy. The computer controlled step motor  is based on the modification of a dip coater machine (model: ND-DC 11/175, Nadetech
Innovations S.L.). The step motor can be controlled at a minimum moving speed of
0.17 μm·s-1.

Electro-mechanical measuring setup configuration

… Cyclic strain loading and bending tests of the strain sensor were performed in  a homemade computer controlled bending setup, Figure 3-3(b). The fixtures are connected with dip coater machine (model: ND-DC 11/175, Nadetech Innovations S.L.), which can be controlled and programmed on the computer. In this way the frequency and bending times of the step motor can be well controlled