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A route towards ultra-sensitive layered carbon based piezoresistive sensors through hierarchical contact design Xiaoshuang Duan, Jiangjiang Luo, Yanbo Yao, and Tao Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14495 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 26, 2017
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ACS Applied Materials & Interfaces
A route towards ultra-sensitive layered carbon based piezoresistive sensors through hierarchical contact design ‡Xiaoshuang Duan, ‡ Jiangjiang Luo, Yanbo Yao, and Tao Liu* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Soochow, No. 199 Renai Road, Soochow City 215000, P.R. China. KEYWORDS piezoresistive sensor, gauge factor, direct laser writing, layered carbon, hierarchical contact.
ABSTRACT
Ultrahigh sensitive piezoresistive sensors at small deformation are highly desired in many applications. Here we propose a hierarchical contact design concept and implement it through direct laser writing technique for fabricating the layered carbon piezoresistive sensors with ultrahigh sensitivity. The sensors with unprecedented gauge factors (~ 5000 – 10,000) at small deformation (ε < 0.1 %) were successfully fabricated and demonstrated for their use in sensing both static and high-frequency (20 – 30 kHz) dynamic mechanical loads. A simple basic structure unit (BSU) contact network model was developed for understanding the importance of the BSU/BSU contact strength and network fractal dimension in dictating the piezoresistive
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sensitivity of the layered carbon piezoresistive sensors with designed hierarchical contact structures. The hierarchical contact design concept and the contact network model proposed in our work would open a general route for developing ultrasensitive piezoresistive sensors based on granular matters and composite materials.
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1. Introduction The change of material electrical resistance in response to mechanical stress or strain has been recognized long time ago since Lord Kevin1. Such phenomenon is now known as piezoresistive effect and it has been widely utilized in developing various piezoresistive sensors for sensing human motions, displacement and level, velocity and acceleration, force and strain, pressure, etc.2 One of the key characteristics for a piezoresistive sensor is its sensitivity, which is typically measured through gauge factor (GF), defined as the relative resistance change per unit strain ε GF = (∆R/R)/ε. Significant progresses have been made in the past several decades in enhancing the sensitivity of various piezoresistive materials. With bulk silicon - the working horse in the current sensing technology as a comparison baseline, we highlight in Figure 1 the works reported in the literature with GFs ≥ 100 at small deformation (ε < 0.5%). According to these previous findings, one can identify two different approaches being taken in the past for achieving ultrahigh GFs. The first approach is through discovering new materials that have inherent electronic band structures that is very sensitive to stress/strain variations. This includes the most prominent piezoresistive material – bulk silicon that has GFs in a range of 40 – 2002 and other types of semiconductive carbon materials3,4. With down-sizing the material to microscale and even nanoscale, the same approach has led to the discovery of some novel piezoresistive nanomaterials with extremely high GFs, such as thin films of single crystalline CdS (GF ~2970) 5 and p-GaN (GF ~260)6, individual silicon nanowire (GF ~6000)7 and carbon nanotube (GF ~200 – 2900) 8-11 . Another approach to achieve high GFs is based upon creating a heterogeneous network structure formed by physical contacts of the basic structural unit (BSU) as in granular matters or composite materials. This approach relies on the stress/strain induced breakage/formation of the physical contacts between BSUs and/or the related electron tunneling
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resistance variation to achieve high GFs. The BSUs being explored include different types of metallic nanoparticles12-19, nanographenes20-25, and the metal-carbon nanotube hybrid particles2628
, which respectively impart the GFs in a range of 100 – 460, 110 – 507, and 155 – 220. Due to
the availability of a wide range of BSUs in terms of composition, size and shape, the approach of creating heterogeneously structured granular matter and composite materials is more versatile and potentially has general applicability for developing ultrasensitive piezoresistive sensors at small deformations. To fully exploit the potentials of this approach, the key is to have a generally applicable method for rationally designing the physical contacts between BSUs and an efficient method to implement such a design. Herein, we report our work on developing ultrasensitive layered carbon based piezoresistive sensors guided by a hierarchical contact design concept. With this new design concept and a direct laser writing implementation technique, we are able to achieve the piezoresistive sensor with a record high GF of ~5700 – 10, 000 at small deformation (ε < 0. 1%). Prototypical sensors with extremely high sensitivity and fast dynamic response were fabricated and demonstrated for their use in pressure sensing and ultrasound sonication process monitoring. Moreover, with extension of a phenomenological model on the breakage of BSU/BSU contact network previously developed by Krauss29, we arrived at a simple formula that can be used for explaining the unique piezoresistive behavior of the layered carbon sensor with designed hierarchical contacts. Detailed fitting and analysis based on this formula reveal that the sensor sensitivity is closely related to the fractal dimension of the network formed by the layered carbon BSUs and the characteristic strain for BSU contact breakage.
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≡ ∂GF/∂ ∂ε > 0 10
≡
≡ ∂GF/∂ ∂ε = 0
≡ ∂ GF/∂ ∂ε < 0
4
Nano-Semiconductor
Carbon
Metal
nSC1
CCAS1 S1
nSC2
CCAS2 10
LLAS1
3
S2
CCAS3 C1 M1
M4 M3 M5 102
G1
M2
M6 M8
M7
LLAS2 C2
S3
S4 LLAS3 G5
G2 G3 G4
nSC3
SM1 SM2 Silicon
Gauge Factor (GF)
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G6
DDLS1 10-5
10-3
10-1
10-5
10-3
10-1
10-5
10-1
10-3
Strain ε
Figure 1. Highlights of the ultra-sensitive piezoresistive sensors reported in the literature (open symbols) and this work (filled symbols) categorized according to material composition. M1 – M8
12-19
: aggregation/assembly of metallic nanoparticles; S1 – S4
nanotube sensors; G1-G6
20-25
8-11
: individual carbon
: aggregation/assembly of nanographene; SM1 – SM2
aggregation/assembly of metal-carbon nanotube hybrid particles; C1 – C2
3,4
26-27
:
: carbon material
sensor other than carbon nanotube and graphene; nSC1 – nSC3: nano-sized various semiconductor sensors5-7;
CCAS1-3, LLAS1-2 and DDLS1: layered carbon sensors with
designed hierarchical contact structures respectively representing circle-circle contact area sensor (CCAS), line-line contact area sensor (LLAS), and dot-dot contact line sensor (DDLS). The symbols of ⊿ , (filled and unfilled), □ respectively represent ∂GF/∂ε>0, ∂GF/∂ε
