DOI:
10.1039/D1MH00538C
(Review Article)
Mater. Horiz., 2021,
8, 2123-2150
Advances in ultrasensitive piezoresistive sensors: from conventional to flexible and stretchable applications
Received
31st March 2021
, Accepted 15th June 2021
First published on 15th June 2021
Abstract
The piezoresistive effect has been a dominant mechanical sensing principle that has been widely employed in a range of sensing applications. This transducing concept still receives great attention because of the huge demand for developing small, low-cost, and high-performance sensing devices. Many researchers have extensively explored new methods to enhance the piezoresistive effect and to make sensors more and more sensitive. Many interesting phenomena and mechanisms to enhance the sensitivity have been discovered. Numerous review papers on the piezoresistive effect have been published; however, there is no comprehensive review article that thoroughly analyses methods and approaches to enhance the piezoresistive effect. This paper comprehensively reviews and presents all the advanced enhancement methods ranging from the quantum physical effect and new materials to nanoscopic and macroscopic structures, and from conventional rigid to flexible, stretchable and wearable applications. In addition, the paper summarises results recently achieved on applying the above-mentioned innovative sensing enhancement techniques in making extremely sensitive piezoresistive transducers.
Thanh Nguyen
| Thanh Nguyen is currently a postdoctoral research fellow at Griffith University, Australia. He just received his PhD degree from Griffith University, Australia in 2021. He received BE degrees in Electrical Engineering and Mechanical Engineering from Hanoi University of Science and Technology, Vietnam, in 2009 and 2012, respectively, and a MS degree from Chulalongkorn University, Thailand, in 2015. His research focuses on MEMS sensors and actuators, physics of semiconductors, soft electronics, silicon carbide MEMS/NEMS for applications in harsh environments, and new materials for sensing applications, self-powered sensors, and energy harvesting devices. |
Nam-Trung Nguyen
| Nam-Trung Nguyen received his Dip-Ing, Dr Ing and Dr Ing Habil degrees from Chemnitz University of Technology, Germany, in 1993, 1997 and 2004, respectively. Currently, he is a professor and the director of Queensland Micro and Nanotechnology Centre at Griffith University. He is a Fellow of ASME and a Member of IEEE. Nguyen's research is focused on microfluidics, nanofluidics, micro/nanomachining technologies, micro/nanoscale science, and instrumentation for biomedical applications. He has published over 400 journal papers and 3 granted US patents. Among the books he has written, the first, second and third editions of the bestseller “Fundamentals and Applications of Microfluidics” were published in 2002, 2006 and 2019, respectively. |
Dzung Viet Dao
| Dzung Viet Dao received his PhD degree from Ritsumeikan University, Japan in 2003. He then served as a Postdoctoral Research Fellow from 2003 to 2006, a lecturer from 2006 to 2007, and a Chair Professor from 2007 to 2011, all at Ritsumeikan University. From 2011 A/Prof. Dao joined Griffith University, Australia, where he has been teaching in Mechatronics and Mechanical Engineering. His current research interests include advanced manufacturing, MEMS sensors & actuators, transducers for harsh environments, and mechatronics/robotics. |
1 Introduction
The piezoresistive effect (PRE) is considered as one of the most prominent sensing effects in semiconductors besides the thermoresistive,1–6 thermoelectric,7 photovoltaic8,9 and piezoelectric effects.10 The PRE has been widely utilised in mechanical sensing technology11 and employed in a wide range of micro/nano electro-mechanical systems (MEMS/NEMS) such as strain gauges, force sensors,12–14 pressure and tactile sensors,12,15–19 accelerometers,20,21 gyroscopes,22,23 flow sensors24,25 and bio sensors.26 Because of its advantages such as low power consumption, good linearity, simple readout circuit, sensor miniaturisation, and mass production, piezoresistive sensors can be found in diverse applications such as robotics, healthcare, industry, transportation vehicles, energy harvesting,27 environmental monitoring,28 maritime security, tsunami wave detection,29 and space exploration.30,31
Even though the PRE has been discovered for more than five decades, this effect in semiconductors still extensively attracts a great deal of attention from researchers and technology developers. The need for smaller devices, lower cost, higher performance, and better abilities of operating in harsh environments (high temperature or corrosive environments) has driven advanced technologies related to the PRE. In addition, the availability of new materials (CNT, graphene, conductive composites, nanoparticles, and other low-dimensional materials) and emerging applications such as flexible, stretchable and wearable electronics require novel approaches and strategies for enhancing the PRE of these new materials and structures, hence improving the performance of piezoresistive sensors. Initially, conventional methods were introduced to enhance the intrinsic performance of the PRE in semiconductors such as aligning piezoresistors according to optimal crystal orientations,32–34 optimising doping concentrations,35 or concentrating stress at piezoresistor positions by taking advantages of novelty designs.36 Recently, novel fabrication technologies, advancements in materials science, and new applications have led to various high piezoresistive performance materials, structures, strategies, approaches and sensing mechanisms. More recently, together with the development of other physical effects, coupling methods that combine the PRE with one or more other physical effects have been introduced to modulate the original effect.
Several review papers related to the PRE have been reported in the past.26,46–50 Barlian et al.48 reviewed the PRE in semiconductors for microsystems. However, this paper only focused on fundamental aspects of the PRE such as history, notation, piezoresistive theory, piezoresistor fabrication, and some applications. Rowe49 solely reported the PRE in silicon (Si) and Si nanostructures. While Phan et al.26 only focused on silicon carbide (SiC) materials in their review paper, Chen and Yan46 and Dinh et al.50 only summarized the progress of piezoresistive flexible pressure sensors and advances in stretchable respiration sensors, respectively. Other review papers51–55 focused on stretchable, flexible, and wearable electronics where the PRE was only partially mentioned. A comprehensive overview on enhancing the PRE and the recent sensing mechanisms for high-performance PRE and their applications in high-performance piezoresistive transducers have not been reported. A review that covers a broad range of topics from rigid to flexible, stretchable and wearable applications is not available. Thus, the present paper summarizes the methods, strategies, approaches and sensing mechanisms for enhancing the performance of the PRE for high-performance piezoresistive transducers, Fig. 1. In the first section, we briefly introduce sensing mechanisms of PRE. This fundamental knowledge is important for understanding the strategies, methods, and approaches toward the enhancement of the PRE. In the second section, a detailed review is given on improving the PRE including approaches based on band energy changes, strategies of miniaturisation to the nanoscale, optimization of the macroscopic structure, and coupling the PRE with other physical effects. In addition, smart designs proposed to enhance the performance of piezoresistive sensors are also outlined. The third section summarizes the application of advanced enhancement strategies in designing high-performance piezoresistive transducers. The last section provides conclusions and perspectives for high-performance piezoresistive sensors.
|
| Fig. 1 Methods for enhancing the piezoresistive effect (PRE). Aligning piezoresistors in the orientation with maximum piezoresistive coefficient, Reproduced from ref. 34, with the permission of AIP Publishing. changing majority carriers, reproduced from ref. 37 with permission from The Royal Society of Chemistry. Optimizing doping concentration, reprinted with permission from ref. 26. Copyright 2015 IEEE. Finding alternative materials, reprinted with permission from ref. 38. Copyright (2019) American Chemical Society. Scaling down to nanofilms, reprinted with permission from ref. 39. Scaling down to nanowires (NWs), reprinted40 by permission from Springer Nature (Copyright 2006, Nature Publishing Group). Enhancing overlapping area change, reprinted41 by permission from Springer Nature (Copyright 2012, Nature Publishing Group). Enhancing crack propagation, reprinted42 by permission from Springer Nature (Copyright 2014, Nature Publishing Group). Enhancing tunnelling resistance change, reprinted with permission from ref. 43. Copyright 2016 Wiley-VCH. Coupling with the piezoelectric effect, reprinted with permission from ref. 44. Copyright (2008) American Chemical Society. Coupling with an external electric field, reprinted with permission from ref. 45. Copyright (2010) American Chemical Society. Coupling with light excitation, reprinted11 by permission from Springer Nature (Copyright 2019, Nature Publishing Group). | |
2 Piezoresistive sensing mechanisms
The PRE is defined as a change in the resistance of materials upon application of mechanical stress or strain. The performance of the PRE is determined by the magnitude of fractional change in electrical resistance versus applied strain, which is quantified by the gauge factor (GF): | | (1) |
where is the fractional change in electrical resistance, R0 is the resistance under free-strain conditions, and ε is the applied strain.
For homogeneous structures, the fractional electrical resistance change is a function of the geometric change (1 + 2ν) and the resistivity change (Δρ/ρ0) of the material:61
| | (2) |
where
ν is the Poisson's ratio of the material, and Δ
ρ and
ρ0 are absolute resistivity change and resistivity under free-strain conditions, respectively. Consequently, the GF of a homogeneous structure is:
62 | | (3) |
from
eqn (2), the fractional electrical resistance change is attributed to the resistance change due to the geometric change under strain (1 + 2
ν)
ε and the change of resistivity Δ
ρ/
ρ0. For almost all intrinsic metals, the resistivity remains constant with applied strain, so the PRE of metals results from the geometric changes. Therefore, the GFs of metals are only about 2. For some intrinsic semiconductors such as Si, germanium (Ge), SiC, and gallium arsenide (GaAs) in certain crystallographic directions, the change in resistivity is much larger than the geometric term. Thus, the changes of geometry are usually ignored.
Because the Poisson's ratio and the resistivity of inhomogeneous structures are non-uniform distributed throughout their volume, the fractional resistance change, on average, can be determined via the change in voltage or current based on the Ohm's law. For example, when the supplied current is constant, the fractional resistance change is proportional with the fractional change in the measured voltage , and hence the GF can be calculated as:
| | (4) |
For field-effect transistor based strain sensors operating in the “ON” state, the GF can also be defined as63
| | (5) |
where Δ
I is the current change due to strain and
I is the current of the unstrained sensor.
For single crystalline semiconductor materials, in addition to GF, piezoresistive coefficients (π) are utilised to evaluate the PRE. When a uniaxial stress is applied, the GF and the piezoresistive coefficient can be related by the Young's modulus of the material (E), GF = Eπ.26 Generally, the piezoresistive coefficient π is a four-rank tensor to connect a second-rank stress tensor and a second-rank resistivity tensor. For conciseness, the four-rank tensor can be collapsed to a second-rank piezoresistive coefficient tensor (e.g., π1111 to π11, π1122 to π12, π2323 to π44),64 and the PRE can be expressed as:
| | (6) |
These following sections summarise the underlying physic of the PRE in semiconductors, soft electronic sensors, and nanoscale structures. The PRE in intrinsic semiconductors mainly results from the changes in resistivity of materials, originating from changes in energy bands versus the deformation of crystals. The PRE in soft electronic sensors is usually associated with changes in macro-structures of the sensors including generation and propagation of cracks, sliding between conductive components resulting in changes in contact resistance, and the generation and disappearance of tunnelling channels. Although there are still controversial debates around the PRE in nanoscale structures (nanowires (NWs), nanobelts, and nanofilms), the physics of the PRE in nanoscale structures, which is also called size-effects, are explained by the carrier confinements and/or the formation of depletion layers on the surface of the nanostructures. Therefore, the size-effect mechanisms will be mentioned in this section. In addition, the performance of the PRE is also affected by other physical effects, and hence coupling the PRE with other physical effects can enhance the performance of piezoresistive sensors.
2.1 Carrier mobility changes due to energy band change
The energy states or energy band structures of carriers in crystals, which are calculated based on the shape and the potential magnitude of crystals, are a useful tool for understanding the physical properties of semiconductors. The PRE in intrinsic semiconductors is explained by the changes of mobilities and effective masses of the carriers, which result from the shift, warp or bend of energy bands due to the deformation of crystal lattices under applied strains and the redistribution of carriers in the shifted, warped or bended energy bands. Several methods are available for calculating the energy bands under free-strain conditions and changes of energy bands to deformation potentials. These methods are usually calculated in one-dimensional (1D) descriptions of electron and hole transport in crystalline structures, which then can be extended to three dimensions. Kimball,65 for instance, calculated the band structure of diamond, which qualitatively showed the anticipation for variations of band structure with lattice constant for crystalline materials such as silicon (Si) or germanium (Ge). Mullaney56 described structured energy bands of Si, as shown in Fig. 2a. Bardeen and Shockley66 reported the gradual change in band structure and electrostatic potential resulting from lattice distortions. Detailed information of the calculation of the energy bands and their changes due to applied strain can be found in ref. 67.
|
| Fig. 2 Piezoresistive sensing mechanisms. (a) Energy bands of Si are a function of half-internuclear distance. The curves are: I, s′ = 0; II, s/s′ + 2(p′/p + d/d) = 0; III, s = 0; IV, (doubly degenerate) p′/p + d′/d = 0; V, s/s′ + 2(p′/p + d′/d) = 0; VI, (doubly degenerate) p = 0.56 Copyright (1944) by the American Physical Society. (b) Two n-type Si valleys in k-space, aligned with the [100] axes; ΔE is the shift of band edge point under strain.57 (c) Schematic of hole energy of Si subjected to uniaxial stress.58 (d) Density of states in one band of a semiconductor as a function of dimension, reprinted with permission from ref. 59. Copyright 1996 AAAS. (e) A schematic illustration for a nanoscale crack-based mechanism. (e-1) and (e-2) Slits on leg joints of a spider for detection of external forces and vibration, reprinted42 by permission from Springer Nature (Copyright 2014, Nature Publishing Group). (f) A modeling of the overlapping mechanism in strain sensing. The initial winding angle was marked to be 7° at 0% strain (f-1), then this angle increased to 29° at 50% strain (f-2), generating gaps in-between the PE fibres (f-3), reprinted with permission from ref. 60. Copyright 2015 Wiley-VCH. (g) A schematic illustration of the tunnelling mechanism, reprinted with permission from ref. 43. Copyright 2016 Wiley-VCH. | |
The PRE theory for some n-type semiconductors such as Si, Ge and SiC has been fully explained by the many-valley model proposed by Herring.68 Originally, this model was used to explain the PRE of n-type Si, and subsequently the PRE of other semiconductors having a similar primitive cell.69,70 In his model, the lowest conduction band energies called valleys in a relaxed Si crystal are aligned along the 〈100〉 directions, as shown in Fig. 2b. Because electrons tend to occupy the lower-energy levels, the conduction electrons imaginarily occupy six equivalent energy ellipsoids near the energy minima in the conduction bands. The rotation axis of these surfaces lies along the 〈100〉 directions. The mobility of an electron in any single valley depends on the movement direction of the electron in the valley. Electrons have the lowest mobility when they move parallel to the valley directions, and the highest mobility when they move perpendicularly with the rotation axes of the ellipsoids. Because of the symmetry of the unstressed silicon crystal, the net conductivity, which is the sum of conductivity components of electrons along the three valley orientations, is independent of direction.
The gradual changes in the band structure due to lattice distortion as calculated by Bardeen and Shockley66,68,71 increase the band energies of the valley parallel to a tensile strain, while that of the valley parallel to a compressive strain reduces, Fig. 2b. The shifts of band energies in opposite ways redistribute electrons in the conduction bands, where electrons transfer from the valleys parallel to the tensile strain to the valleys parallel with compressive strain and break the symmetry of the electron distribution in the crystal. Therefore, the average mobility becomes higher in the direction of tension (longitudinal effect) and lower in the transverse directions (transverse effect).57
Although most commercial piezoresistive sensors are p-type, because of the complexity of the valence band structure, understanding of p-type PRE has not been fully achieved until recently due to advances in computation (numerical calculation).58,72,73 The valence band structure of Si includes heavy holes, light holes, and spin–orbit split-off band.58,74 Energy bands of both heavy and light holes are warped and degenerated at momentum k = 0, Fig. 2c, under stress. Because carriers tend to occupy at lower energies, the majority of holes are located at k = 0. A uniaxial stress applied to silicon results in lifts of sub-bands of heavy hole and light holes in opposite directions. Consequently, the heavy holes and light holes at k = 0 split into different energy levels, Fig. 2c, resulting in the repopulation of heavy holes and light holes under stress. In other words, uniaxial stress results in changes in the number of heavy holes and light holes. Because light holes have higher mobility than that of heavy holes, the average mobility of holes changes as uniaxial stress is applied.
2.2 Size-effect mechanisms
When the sizes of sensing elements are scaled down to the nano-meter levels, the movement of electrons is restricted due to the quantum confinement and results in significant change in electrical properties. In addition, when the sizes of sensors are scaled-down, the number of atoms near the surface becomes more considerable compared to that of atoms locating deep inside the sensor. Therefore, electromechanical properties at the surface layers will contribute more significantly in the scaled-down sensors. Fig. 2d,59 for example, shows changes in the number of densities of electronic states in a semiconductor as a sensor is scaled-down from a bulk structure. The number of densities of electronic states decreases significantly as the sample changes from a bulk structure to two-dimensional (2D), one-dimensional (1D), and nanocrystal (0D) structures. The quantum confinement squeezes electrons and holes in one dimension (2D or quantum well), two dimensions (1D-quantum wires), or three dimensions (0D-quantum dots). As such, the miniaturisation restricts electron energies being restricted to a discrete set and enlarging the band gaps which results in changes in piezoresistive properties.75 The miniaturisation of the sensor also increases the surface-area-to-volume ratio, so that the atom number at or near the surface is comparable with that in the body,76 and hence changes in surface properties of the materials caused by mechanical deformation can primarily influence the PRE.
2.3 Macro structure change mechanisms
For single crystalline semiconductors such as Si, Ge, GaAs, and SiC, the PRE results from changes in material structures at the atomic level, which is the deformation of the crystalline lattice under applied stress or strain and the changes in the energy bands of carriers. However, certain architectures, particularly ones used in stretchable, wearable, and flexible electronics, include conductive fillers such as metal NWs, carbon-based materials (CNT, graphene), or nanoparticles supported by or embedded in stretchable, wearable, and flexible nonconductive substrates. In these inhomogeneous architectures, eqn (2) cannot be used. The PRE, which is the change in the resistance of the whole device under strain/stress, primarily comes from the changes in macro structure at some positional areas with the generation of cracks, or propagation of cracks, or sliding between overlapping conductive layers, or generation or disappearance of tunnelling channels. Although the resistance changes of conductive fillers themselves under mechanical strain/stress are usually small, the changes in macro structures can result in significant changes in overall resistance of devices. Strategies, approaches or methods to enhance the PRE of these architectures are based on crack propagation, an overlapping mechanism, tunnelling mechanisms or the combination of these mechanisms.
2.3.1 Crack propagation mechanism.
For crack-based piezoresistive sensors, made of brittle nanomaterial thin films coated on top of a flexible substrate, nano/micro cracks intend to initiate at the stress concentrated areas to release the accommodated stress. By applying strains, nano/micro cracks in thin films are opened or enlarged, which critically limits the electrical conduction, hence substantially increasing the resistance of the thin films. The length, width, depth, density and position of cracks are critical factors that influence the final sensing performance and the working range of crack-based sensors.53 Practically, the crack propagation mechanism is often accompanied by the tunnelling mechanism, because some carriers still can hop through adjacent points on the cracked walls when their distances are small enough. Fig. 2e illustrates schematically the sensing mechanism based on cracks formed in Pt films on polyurethane acrylate with ideas inspired by the spider sensory system.
2.3.2 Overlapping mechanism.
Overlapping-based piezoresistive sensors consist of a number of electrical conductive components (films, sheets, wires, fibres, tubes, or bells), which are loosely connected to each other and create overlapping areas as shown in Fig. 2f. Therefore, the total electrical resistance is contributed by resistance of the conductive components themselves and the resistance of the overlapping areas or the contact resistance.52–54 Stretching the sensor results in loss in the electrical connections and decrease in the overlapping areas of adjacent components, hence increasing the contact resistance.55 In other words, the decreases in overlapping areas under stretching results from the relative slippage of the conductive components because of the weak interfacial binding between the conductive components.
2.3.3 Tunnelling effect/mechanism.
The tunnelling effect describes the crossing of electrons through quantum tunnelling junctions or nonconductive barriers as an example shown in Fig. 2g. It has been demonstrated that within a certain cut-off distance between nanomaterials, electrons can hop between closely spaced neighbouring nanomaterials through nonconductive thin layers and form quantum tunnelling junctions.55 The tunnelling resistance between two neighbouring nanomaterials can be approximately determined by Simmon's equation:54,77 | | (7) |
where V is the electrical potential difference, A is the cross-sectional area of the tunnel, J is the tunnelling current density, h is Plank's constant, d is the shortest distance between neighboring nanomaterials, e is the electron charge, m is the mass of an electron, and λ is the height of energy barrier. The cut-off tunnelling distance depends on the type of conductive materials, type of insulating media, and processing parameters. When an external strain is applied, the shortest distance, d, between neighboring nanomaterials changes and results in a change in tunnelling resistance.54,77
2.4 Coupling mechanisms
When certain physical effects are acting concurrently, they influence each other. For example, coupling of multiple physical effects in nanostructures has been employed to modulate electrical transport in logic circuits,78 enhance the sensitivity and detection resolution of bio/chemical sensors,79–81 and improve the photovoltaic performance of solar cells.82 The PRE depends on conductivity properties such as carrier mobilities, carrier concentration, and conductive channels. Thus, coupling mechanisms can be viewed as effective approaches which combine various physical effects to interfere with the conductivity property, hence modulating the PRE behaviour. The PRE can be enhanced by coupling with the piezoelectric effect,44,83 an external electric field,45 and the photovoltaic effect.11,84
3 Enhancement of the piezoresistive effect
Strategies and approaches for enhancing the PRE originate from the sensing mechanisms. With different sensing mechanisms, the strategies and approaches are often different. The PRE behaviour can also result from a combination of sensing mechanisms. This section outlines strategies and proposed solutions, achieved results, and even existing debates arising around the proposed methods to enhance the PRE.
3.1 Enhancement of the PRE based on semiconductor properties
Since the discovery of the PRE in semiconductors by Smith90 in 1954, most research on the PRE in the next five decades focused on finding out explanations of the large PRE, developing fabrication technologies, investigating the PRE in various intrinsic semiconductors (Si, Ge, SiC,…), crystalline morphology (single crystalline, polycrystalline, and amorphous), and optimizing strategies for the PRE. The PRE in intrinsic semiconductors results from the band energy changes (wrap, shift, or bend) under the applied stress/strain. Therefore, in order to enhance the PRE in a semiconductor, proposed strategies, which come to mind first, are all based on maximizing resistivity or conductivity changes with changes in energy bands.
The resistivity or conductivity of single crystalline semiconductors depend on the concentration of charge carriers and their mobilities by:94
| | (8) |
where
n,
μe and
p,
μh denote the concentrations and the mobilities of electrons and holes, respectively. From
eqn (8), there are three approaches to enhance the conductivity/resistivity change. First is enhancing the carrier mobility changes or selection of the crystallographic orientations. Second is changing the majority carriers (electron or hole) or, in other words, selection of the dopant types. Since carrier concentrations remain constant under stress, third is optimizing carrier concentration.
3.1.1 Enhancing the carrier mobility change or selection of the crystallographic orientations.
In single crystalline semiconductors, the charge carrier mobilities are anisotropic, and stress breaks the symmetry of the crystal, affecting the energy band structures, separates the degeneracy of valley energies, and causes transfer of charge carriers between valleys. Therefore, by applying stress/strain, the carrier mobility changes in average are anisotropic as well. Under an external electric field, free charge carriers flow parallel to the electric field direction. Conventionally, controlling charge carriers flowing in the directions, where the carrier mobility changes are maximum, is a useful approach to enhance the piezoresistive effect. In other words, the piezoresistive effect can be enhanced by positioning piezoresistors or sensing elements in the crystallographic directions (orientations) where the maximum changes in charge carrier mobility are observed. The results of investigations following this approach describe the dependence of the PRE of a semiconductor on different orientations. In early analyses of the PRE in semiconductors, Smith90 observed changes in GF following the crystal orientation. He evaluated the GF in different directions such as longitudinal [100] and [110], and transverse [100]. Extending Smiths results, Kanda85 graphically showed the dependence of the piezoresistive coefficients on the orientations in common crystal planes of Si(100), (110) and (211). These graphs provide a useful overview of how optimizing the direction of piezoresistors can improve the PRE. Henceforth, these optimal orientations are always based on aligning sensing elements for designing piezoresistive transducers. Some researchers also followed Kanda in investigating the effect of orientation in the PRE of other materials such as Shor et al.87 for n-type 3C-SiC, and Phan et al.34 for p-type 3C-SiC. Fig. 3 graphically shows the piezoresistive coefficients versus crystal orientation in common crystal planes of common semiconductors and Table 1 summarizes the crystal orientations with the largest PRE in various semiconductors. For example, in the (100) plane of single crystalline Si, the n-type has the largest PRE in the [100] direction, while the p-type has the highest GF in the [110] direction.
|
| Fig. 3 Room temperature piezoresistive coefficients in typical semiconductors. (a) (100) n-type Si. (b) (100) p-type Si. (c) (110) n-type Si. (d) (110) p-type Si, (e) (211) n-type Si. (f) (211) p-type Si.48,85 (g) (100) n-type 3C-SiC, reprinted with permission from ref. 26. Copyright 2015 IEEE. (h) (100) p-type 3C-SiC, Reproduced from ref. 34 with the permission of AIP Publishing. (i) The doping-level dependence of the PRE in 3C-SiC reprinted with permission from ref. 26. Copyright 2015 IEEE. | |
Table 1 Crystallographic orientations with the highest longitudinal piezoresistive coefficients
Materials |
Planes |
Crystal orientations |
Ref. |
Si n-type |
(100) |
〈100〉 |
85
|
Si p-type |
(100) |
〈110〉 |
85
|
Si n-type |
(110) |
〈100〉 |
85
|
Si p-type |
(110) |
〈11〉 |
85
|
Si n-type |
(211) |
〈100〉 |
85
|
Si p-type |
(211) |
〈11〉 |
85
|
Ge n-type |
(111) |
〈110〉 |
86
|
3C-SiC n-type |
(100) |
〈100〉 |
87
|
3C-SiC p-type |
(100) |
〈110〉 |
34
|
3C-SiC p-type |
(111) |
〈10〉 〈2〉 |
62
|
4H-SiC p-type |
(0001) |
Isotropic |
88
|
6H-SiC |
(0001) |
Isotropic |
89
|
3.1.2 Selection of dopant types.
Because responses of conduction bands and valence bands to the mechanical deformation are different, changes in the majority type of charge carriers significantly change the PRE. Experimental results have shown that the PRE effect of p-type silicon is larger than that of n-type. Table 2 lists the PRE of common semiconductors in both p-type and n-type. It is obvious that Si bulk materials have the highest PRE in comparison with other semiconductors.
Table 2 Comparison of the PRE in the p-type and n-type of some common single crystalline semiconductors
Type |
Doping concentration (cm−3) |
Resistivity (Ω cm) |
Π
l [×10−11 Pa−1] |
Π
t [×10−11 Pa−1] |
GF |
Ref. |
n-Si (100) |
— |
11.7 |
−102.2 |
53.4 |
— |
90
|
p-Si (100) |
— |
7.8 |
6.6 |
−1.1 |
— |
90
|
n-Si (110) |
— |
11.7 |
−31.2 |
— |
— |
90
|
p-Si (110) |
— |
7.8 |
+71.8 |
— |
— |
90
|
n-Ge (100) |
— |
16.6 |
−5.2 |
−5.5 |
— |
90
|
n-Ge (110) |
— |
16.6 |
−74.7 |
+67.9 |
— |
90
|
p-Ge |
— |
15.0 |
−10.6 |
5.0 |
— |
90
|
p-Ge |
— |
1.1 |
−3.7 |
+3.2 |
— |
90
|
n-3C-SiC (100) |
1016 to 1017 |
0.7 |
−9.6 |
5.8 |
−31.8 |
87
|
p-3C-SiC (100) |
5 × 1018 |
0.14 |
1.5 |
−1.4 |
30.3 |
91
|
n-4H-SiC |
1.5 × 1019 |
— |
— |
— |
20.8 |
92
|
p-4H-SiC (0001) |
1018 |
— |
6.43 |
−5.12 |
— |
88
|
n-6H-SiC (0001) |
3.8 × 1018 |
— |
— |
— |
−29.4 |
93
|
|
2 × 1019 |
— |
— |
— |
−22 |
89
|
p-6H-SiC (0001) |
2 × 1019 |
— |
— |
— |
27 |
89
|
3.1.3 Optimisation of carrier concentration.
As mentioned above, although the number of carriers is generally stable under strain/stress, the doping concentration also influences the PRE, and hence it is also an optimisation parameter for the improvement of PRE. The effect of doping concentration on the PRE results from the changes in the number of light holes and heavy holes of p-type and the number of electrons transferring to lower energy levels due to the shifting, wrapping or bending of energy bands under stress/strain. In many semiconductors, the GF decreases with increasing carrier concentration.87,95–97 The majority of calculations for the dependence of the PRE on doping concentration is based on the model of Kanda.85 In his model, he calculated the piezoresistive coefficient as a function of doping concentration (N) and temperature (T) via the piezoresistive factor P(N,T): | π(N,T) = P(N,T)·π(300 K) | (9) |
These calculation results agree with experimental data at low concentrations, but they substantially underestimated the values at higher doping levels.96 Similar trends are also observed in other semiconductors such as 3C-SiC87,97 as shown in Fig. 3i. These findings indicate that decreasing the doping level can enhance the PRE. However, reductions of doping levels increase the thermal vibration of the gauge factors26 and increase conductance fluctuation (1/f) noise.96,98
3.2 Enhancement of the PRE based on the size-effect
Reducing the dimensions of materials down to nanoscales, by one dimension to nanofilm (2D) or by two dimensions to nanowires (NWs), can change the electromechanical properties of piezoresistive sensors. This section will outline the achieved results in enhancing the PRE by scaling down, proposed explanations for the PRE in nanofilms and NWs, and debates surrounding reported results. Details on the fabrication technologies of NWs and nanofilms can be found in another review.112
3.2.1 One-dimensional piezoresistive effect.
In the last two decades, one-dimensional (1D) nanostructures such as nanowires have been extensively studied due to their interesting and unique electronic, optical, thermal, magnetic and mechanical properties.112 Various proposed structures and configurations indicated that miniaturisation of bulk materials to 1D nanostructures is a promising strategy to significantly enhance the performance of the PRE for mechanical sensing applications. Due to the history of use in the microelectronics industry, the massive investigations of the bulk materials and the development of Si-based fabrication technology make Si nanowires (Si NWs) ideal for fundamental research. Size dependence of the PRE was reported for the first time by Yasutada et al.,113 where the longitudinal piezoresistive coefficient of a polycrystalline nanowire piezoresistor was reported to increase with decreasing cross-sectional area. Investigation of the PRE on Si NWs received massive attentions after the report of giant PRE in bottom-up grown Si NWs by He and Yang.40Fig. 4 summarizes the results reported for enhancement of the PRE in semiconductor NWs, and details on the PRE in Si and its nanostructures are available elsewhere.49 He and Yang grew single crystalline p-type silicon nanowires of 70 nm diameter bridging a trench confined by vertical {111} faces on a 〈110〉-oriented SOI substrate, Fig. 4a. The team reported a longitudinal piezoresistive coefficient of −3550 × 10−11 Pa−1 (Fig. 4b) in 〈111〉 orientation and −660 × 10−11 Pa−1 in 〈110〉 orientation, which is an approximately 100 times increase in comparison with a bulk value.40
|
| Fig. 4 Enhancement of the PRE of Si by miniaturisation to nanowires. (a) 〈111〉-oriented Si NWs bridges on an SOI substrate. (b) First-order longitudinal piezoresistive coefficient of p-type Si NWs and its dependence on diameter and resistivity. (c) Effects of surface states on the piezoresistive coefficient, reprinted40 by permission from Springer Nature (Copyright 2006, Nature Publishing Group). (d) The conductance change Δσ/σ0versus stress of Si NWs in different diameters and depletion region widths numerically calculated. The inset shows the calculated hole concentration for 80 nm diameter Si NWs with the conducting channel represented in red, reprinted99 by permission from Springer Nature (Copyright 2008, Nature Publishing Group). (e) The IDS–VDS measurement showing the temporal changes in the zero-stress conductance manifest themselves as an apparent giant PRE. (f) The apparent and true PRE, reprinted with permission from ref. 100. Copyright (2010) by the American Physical Society. | |
At that time, it was unclear whether the enhancement of the PRE is intrinsic, caused by size reduction, or is simply due to oxidation on the NW surface as they observed a significant influence in the Si NW PRE by surface modifications.40 A few investigations on the PRE of Si NWs followed up to explain the surprisingly large PRE in Si NWs and/or investigate the PRE in Si NWs with different structures and characteristics. Several explanations were proposed for the PRE in NWs based on quantum effects in atomically thin nanowires.114–118 However, the diameter of the Si NWs should be typically on the order of 1 nm, which is much smaller than the diameters of practical NWs, which vary from 50 nm to 300 nm, so that the quantum size effect can be taken into account. Moreover, the proposed quantum size effect could not explain the change of the giant PRE in Si NWs after surface treatment, as He and Yang40 observed that both the nanowire resistance and the piezoresistive coefficient reduced after surface treatment, Fig. 4c. Fig. 4d shows the conductance change of Si NWs versus applied stress achieved by solving the Poisson–Boltzmann equation in two dimensions for diameters of 80 nm and 130 nm and the variations in the depletion region width.99 This model, proposed by Rowe,99 correlated the giant PRE in Si NWs with the partial depletion of the nanowire, and is a key realisation concerning a possible origin of the giant PRE in NWs. In NWs, where the diameter is comparable to or smaller than the surface depletion layer width, the density of free charge carriers in NWs becomes sensitive to the charge state of the surface. Particularly, for small enough diameters, the free carrier density within the conducting nanowire channel is entirely determined by the state of surface charge. An applied stress induces changes of the surface charge density, which results in a large change in carrier density, hence substantially modulating the PRE.
Although achieving breakthrough results in enhancing the PRE by miniaturisation, the reliability of the large PRE on the 1D nanostructure of conventional materials is still controversial as there were also counter-claims of a giant PRE. For example, Milne et al.100 revealed the charging effect at the sample surfaces, or dielectric relaxation, which resulted in strong variations of the resistance with time as shown in Fig. 4d, masking the true PRE of the depleted structures and mistakenly leading to claims of an apparent giant PRE. Their results were consistent with He and Yang's40 results as the GF calculated with assumption of constant free-stress resistance. However, the PRE is consistent with that of the bulk material when using the stress modulation technique, Fig. 4e.100 Although the PRE was not as high as that reported by He and Yang, an enhancement in the PRE was still observed in top-down suspended Si NWs when the variation of the resistance with time was taken into account.105Table 3 summarises achievement in enhancing the PRE by miniaturisation of piezoresistors to 1D structures.
Table 3 Piezoresistive effect in nanowires
Material |
Type, orientation |
Size (nm) |
Fabrication |
Π
l [ × 10−11 Pa−1] |
Comments |
Ref. |
Si NW |
p, 〈110〉 |
100 nm × 53 nm |
Top-down |
+38.7 |
54.8% larger than the values obtained from p+ diffused piezoresistors |
101
|
Si NW |
p, 〈110〉 |
53 nm × 53 nm |
Top-down |
+48 |
Increased up to 60% with a decrease in the cross-sectional area |
102
|
Si NW |
p, 〈110〉 |
35 nm × 35 nm |
Top-down |
130 |
Increased up to 60% when the width of SiNWs down from 490 nm to 35 nm |
103
|
|
|
35 nm × 480 nm |
Top-down |
82 |
|
|
Si NW |
p, 〈110〉 |
40 nm × 53 nm |
Top-down |
130 |
Increase 60% when the width of SiNWs down from 480 nm to 35 nm |
104
|
|
|
40 nm × 480 nm |
|
82 |
|
|
Si NW |
p, 〈111〉 |
70 nm |
Bottom-up |
−3550 |
— |
40
|
|
p, 〈110〉 |
75 nm thick |
|
−660 |
|
|
Si NW |
p, 〈110〉 |
40 nm ×40 nm |
Top-down |
+147 |
Released nanowires |
105
|
|
|
|
|
29.4 |
Non-released structures |
|
Si NW |
p, 〈110〉 |
140 nm × 200 nm |
Top-down |
455 |
6.3 times increase compared to bulk silicon |
106
|
|
|
480 nm × 340 nm |
|
70 |
|
|
Si NW |
p, 〈111〉 |
100 nm diameter |
Bottom-up |
−131 |
High strain level |
107
|
|
|
|
|
+25 |
Low strain level |
|
Si NWs |
p, 〈110〉 |
13 nm × 45 nm |
Top-down |
290 |
— |
108
|
Si NWs |
p, 〈110〉 |
300 nm × 300 nm |
Top-down |
3000 |
Stress concentration is the principal source of giant PZR effect |
109
|
Si Nanoribbon |
p, 〈110〉 |
200 nm × 2000 nm |
Top-down |
205 |
Separated the time-varying of resistance |
100
|
Si beam |
p, 〈110〉 |
100 nm thick |
Top-down |
29.2 |
Dopant concentration of 1 × 1019 cm−3 |
110
|
|
|
|
|
177.8 |
Dopant concentration of 5 × 1017 cm−3 |
|
Si film |
p, 〈110〉 |
9 nm thickness |
Top-down |
−400 |
Stress-enhanced Si/SiO2 interface electron-trapping effect |
76
|
3C-SiC NW |
p, 〈110〉 |
300 nm × 300 nm |
Top-down |
10.6 |
— |
111
|
3.2.2 Two-dimensional piezoresistive effect.
Although the PRE properties of polycrystalline and single-crystalline Si films initially reported were not better than single crystalline bulk Si,119–123 structures with sufficiently small film thickness can provide a significant improvement in piezoresistive properties induced by quantum confinement effect or interface electron-trapping effect. At a low temperature, the GF in the surface n-type inversion layer of bulk silicon is found to be much larger than that of bulk silicon (GF ≈ 1500).124 The change of effective masses of conduction electrons on the surface due to the quantization of the carrier wave function in the surface channel is determined as the main reason for this improvement. The quantisation of carrier motion perpendicular to the semiconductor surface caused a splitting of bands in electric sub-bands and a valley anisotropy.124–126 The improvement of the PRE by miniaturisation of bulk sensors to nanofilms was supported by theoretical studies,127 which investigated the effect of quantum size to the PRE in ultrathin piezoresistors. When the holes are confined in a space with the size comparable with the de Broglie wavelength, the quantum size effect plays a significant role in PRE.127 Together with the reduction in thickness of the Si layer, Yang and Li76 proposed a structure with a 9 nm-thick Si film covered by two SiO2 layers, which created two interfaces to trap holes. This stress-enhanced interface-trapping, which influences the carrier-concentration change, became more remarkable as the Si thickness reduced. The interface-trapping results in a giant PRE with piezoresistive coefficients of πl ≈ πt ≈ − 400 × 10−11 Pa−1 in a 9 nm-thick Si film, which was at least one-order-of-magnitude improvement in comparison with the bulk structure.76 The improvement in the PRE by reducing the thicknesses to quantum size also proved the effectiveness in other materials such as AlAs75 and GaAs.128 Shkolnikov et al.75 reported a PRE in n-type AlAs quantum wells with a GF exceeding 10000 and even up to 56000 in the presence of a moderate magnetic field perpendicular to the planes of the 2D system. In a (311) GaAs 2D hole system, distortion of the heavy hole valence band with strain results in a large GF of approximately 3600.128
3.3 Enhancing changes in macro-structure of piezoresistive sensors
3.3.1 Enhancement of the PRE based on the crack propagation mechanism.
Cracks in conductive materials are considered as defects to avoid in general, but if we consider this problem from another perspective, the cracks in conductive materials may provide an approach to realise ultra-high PRE.134 Crack-based piezoresistive sensors are typically made from stretchable nanocomposites, which includes electrically conductive fillers embedded on flexible supporting materials. Different conductive fillers can be used for creating a crack-based piezoresistive sensor, such as metal nanowires (e.g. AuNWs, AgNWs, CuNWs), nanoparticles, carbon-based materials (e.g. graphene, CNT), or conductive polymers.
Inspired by spiders’ ability of detecting extremely small vibrations using a crack-shaped slit organ near theirs leg joints, Fig. 2e, Kang et al. introduced the first nanoscale crack-based sensors which can attain ultrahigh sensitivity with a GF of over 2000 in the 0–2% strain range.42 The team mimicked the geometry of the slit organ by depositing a stiff, 20 nm-thick platinum (Pt) layer on top of viscoelastic polyurethane acrylate. Cracks in the Pt film were then formed by bending the Pt film on PUA in a controlled manner in terms of crack density and direction. The effectiveness of enhancing the PRE by a crack-based-mechanism can be seen obviously in Fig. 5a. Large variation in resistance was obtained with high repeatability for a cracked sample, in sharp contrast to the case with a nearly flat bare Pt film with no crack (yellow curve). When loading was switched between ON and OFF, the crack sample exhibits a 450-fold-higher resistance variation at 0.5% strain compared with the case with no crack (Fig. 5b).42 The high strain sensitivity originated from the rare yet large gap-bridging steps on opposite edges of a zigzag crack and the disconnection–reconnection events of the crack edges. When stretched, a cracked film could be extended in the axial direction, which disconnects the crack edges, while being compressed in the transverse direction, which reconnects them.42 Similar structures with metallic nanofilms deposited on elastic substrates and cracks generated via a controlled pre-stretching process were reported with the highest GF of 5000.134,135 Hybrid conductive fillers such as AgNWs/graphene hybrid particles136 and CNT/KH550†137 can be used instead of one conductive filler, but the achieved sensitivity was lower. Moreover, the crack-based PRE can be further enhanced by generating parallel microcracks.138 The GF of a graphite thin film/elastic substrate-based strain sensor, for example, was significantly improved to as high as 11300 as parallel cracks in graphite thin films were formed and controlled.138 In addition, the sensitivity of a nanoscale crack-based sensor can be enhanced remarkably by modulating the crack geometry, particularly depth.129 The crack depths were controlled by applying additional tensile forces after generating initial cracks (Fig. 5c), without changing other geometrical factors such as crack density and asperity. The crack depth-propagated sensor exhibited ultrasensitivity with GF of approximately 16000 at 2% strain, Fig. 5d, which is a dramatic enhancement compared to bare platinum (non-cracked) and bending platinum (40 nm-depth cracks).129
|
| Fig. 5 Crack mechanisms for enhancement of the PZR effect. (a) and (b) The effectiveness of enhancing the piezoresistive effect by a crack-based-mechanism. (a) Reversible loading–unloading behaviour of a crack-based sensor for various final strains. (b) Resitance variation of the crack sample and no crack sample as the load was ON and OFF, reprinted42 by permission from Springer Nature (Copyright 2014, Nature Publishing Group). (c) Controling the crack depth by applying a tensile force after initial cracks generated by mechanically bending the curved surface. (d) Performance of the sensors with various crack depths. (d-1) Crack depth fabricated by different tensile forces. (d-2) Normalised resistance change measured at a 2% strained obtained by the sensors. The inset in (d-1) and (d-2) show the results for various moduli of PUA, reprinted with permission from ref. 129. Copyright 2016 Wiley-VCH. (e) and (f) Crack-based piezoresistive structures with a network of conductive channels. (e) Gold microwire network embedded in PDMS, reprinted with permission from ref. 130. Copyright (2018) American Chemical Society. (f) Effect of strain on the structure and morphology of the percolating network microstructures, reproduced from ref. 131 with permission from The Royal Society of Chemistry. (g) Stress concentration for enhancing the PRE of crack based fibres, reprinted with permission from ref. 132. Copyright 2017 Wiley-VCH. (h) Multifilament-structured fibres, reprinted with permission from ref. 133. Copyright (2018) American Chemical Society. | |
On conductive film/elastic substrate-based structures, the conductive fillers fully cover the elastic substrate and the cracks are predefined by a pre-stretching process. Arranging conductive fillers as a network of conducting channels can be another approach. Nanometric break junctions are formed throughout the wire network under strain. The strain itself also increases the number of such junctions, which leads to a large change in the sheet resistance of the mesh.130 Gupta et al. fabricated a strain sensor made of a gold micromesh partially embedded in the PDMS substrate (Fig. 5e) with GF of over 108.130 Wang et al. prepared strain sensors with AgNW conductive networks supported by PDMS films with GF of 846 at 150% strain.139 By introducing and patterning percolating network microstructures, the sensitivities of the strain sensor proposed by Liao et al. are greatly enhanced by 136 times that of the sensors without surface microstructures.131 In addition, in another report, the ultrahigh GF of approximately 15000 within 60% strain range results in changes of microstructure and the conductive path due to applied strain, Fig. 5f.131
Redistribution of strain has been proposed as an effective method to enhance the sensitivity of piezoresistive strain sensors.52,132,143 For example, Fig. 5g shows the redistribution of strain along the fibre by adding microbeads, resulting in strain/stress concentration areas where longer and wider microcracks were induced.132 The sensitivity increases with increasing size of the micro beads and is significantly improved compared to fibres without microbeads.
For crack-based piezoresistive sensors, besides enhancing the sensitivity, expanding the sensing range is also a critical need. However, it is difficult to simultaneously meet both expectations.133 Lee et al. proposed an effective approach for fabricating highly sensitive and stretchable fibre strain sensors that simultaneously exhibit outstanding sensitivity and excellent sensing range, by incorporating only silver nanoparticles into multifilament-structured stretchable fibres, as shown in Fig. 5h.133 The multifilament structure and silver-rich shells of the fibre strain sensor enable the sensor to achieve outstanding sensitivity (9.3 × 105 and 659 in the first stretching and subsequent stretching, respectively) and wide sensing range (450 and 200% for the first and subsequent stretching, respectively) at the same time.133 Achievement results of the PRE based on the crack-based mechanism are summarized in Table 4.
Table 4 Enhancing the PRE based on the crack mechanism and tunnelling mechanism
Enhancing mechanism |
Materials |
GF |
At strain (%) |
Comments |
Ref. |
Conductive fillers (active materials) |
Substrates (supportive layers) |
Crack |
Pt |
PUA |
2000 |
0–2 |
Two-part crack-based structure |
42
|
Crack |
Au |
PDMS |
200 |
<0.5 |
Two-part crack-based structure |
135
|
|
|
|
1000 |
0.5–0.7 |
|
|
|
|
|
5000 |
0.7–1 |
|
|
Crack |
Au/Ti |
PDMS |
5000 |
0–1 |
Two-part crack-based structure |
134
|
Crack |
AgNWs/graphene oxides |
PU |
20 |
<0.3 |
Two-part crack-based structure |
136
|
|
|
|
1000 |
0.3–0.5 |
|
|
|
|
|
4000 |
0.8 –1 |
|
|
Crack |
CNT/KH550 |
PDMS |
5–1000 |
2–250 |
— |
137
|
Crack |
Graphite |
Eco-flex |
11300 |
50 |
Two-part crack-based structure-parallel cracks |
138
|
Crack |
Pt |
PUA |
16000 |
2 |
Two-part crack-based structure-crack depth-propagated sensor |
129
|
Crack |
Au microwire |
PDMS |
108 |
0.02–4.5 |
Au microwire network |
130
|
Crack |
AgNWs |
PDMS |
846 |
150 |
AgNWs conductive network |
139
|
Crack |
AgNWs |
Patterned-PDMS |
150000 |
60 |
AgNWs conductive network |
131
|
Crack |
Graphene |
PDMS |
282 |
20 |
Elastomer-felled graphene woven fabric network |
140
|
Crack |
Au or CNT |
PDMS fibres |
100 |
— |
Strain distribution |
132
|
Crack |
Ag |
PU fibres |
9.2 × 105 |
450 |
Multifilament structure |
133
|
Tunnelling |
ZnO particle |
PDMS |
>104 |
0.1–0.9 |
Sea urchin-shaped ZnO microparticles |
141
|
Tunnelling |
Metal particle |
PU elastomer |
— |
|
Sea-urchin shaped metal microparticles |
43
|
Tunnelling |
CNT |
Polymer |
22.4 |
— |
A higher sensor GF corresponds to higher resistance in the composites |
142
|
Tunnelling |
AgNWs |
PDMS |
14 |
— |
A strong piezoresistivity with tunable GFs depending the density of AgNWs |
77
|
3.3.2 Enhancement of the PRE based on the overlapping mechanism.
Although many structures have been proposed for enhancing PRE by the overlapping mechanism, it can be classified into four main groups consisting of fibre-based,60,144,147,148 thin film-based,145,149–151,155 nanotube-based,146,153 and interlocking-based structures.41,154
In fibre-based structures, conductive fillers (e.g., graphene,60,147 silver NWs (Ag-NWs)144) are coated onto elastic fibres (e.g., polyester fibres,60 polyurethane fibre,144 rubber fibre) creating conductive elastic fibres. Subsequently, the as-prepared conductive fibres are twisted around each other (Fig. 6a), or around another elastic core fibre (Fig. 2f) to construct a bunch of stretchable fibres. Consequently, piezoresistive sensors consist of a bunch of stretchable fibres60,144,147 or a network of bunches.148 As shown in Fig. 2f, Cheng et al.60 developed a graphene-based composite fibre with “compression spring” architecture, which consists of graphene-coated polyester fibres winding around a highly elastic polyurethane core fibre.60 The elongation caused by stretching was accompanied by the increase of the winding angle and gaps between the conductive fibres, leading to the decrease of the contact area between the inner and outer layers of conductive fibres.60 The graphene-based fibre reached an averaged GF of 10 within 1% strain, 3.7 within 50% strain, and 35 at 0.2% strain endowing the sensor with a high sensitivity at small strain and broad sensing range. Another example, conductive core–shell fibres with multiscale wrinkled microstructures on the surface (Fig. 6a) prepared through coating AgNW ink on prestrained polyurethane fibres, were twisted to construct flexible piezoresistive fibres. The investigations to the sensing mechanism indicated that increasing the contact points in the flexible piezoresistive fibres has significantly improved the sensitivity.144
|
| Fig. 6 Changing overlaping areas under stretching. (a) A fibre-based structure. Schematic illustration of the twisting fabrication of flexible piezoresistive fibres, reprinted with permission from ref. 144. Copyright 2016 Wiley-VCH. (b) A structure with thin films overlapped each other. Changes in contact areas between rGO films when applying strain and schematic illustration of sensing mechanism of FSG strain sensor upon stretching, reprinted with permission from ref. 145. Copyright (2016) American Chemical Society. (c) A nanotube overlapping structure. Schematics of the response of the CNT connections to strain, reprinted with permission from ref. 146. Copyright 2019 Wiley-VCH. (d) An interlocking structure. Schematic illustrations of the pressure, shear and torsion loads and their possible geometric distortions of the paired hairs, reprinted41 by permission from Springer Nature (Copyright 2012, Nature Publishing Group). | |
Various conductive films overlapping each other are another architecture to effectively enhance the PRE based on the overlapping mechanism. The shear forces parallel with the conductive films result in the relative sliding between adjacent films which changes the contact areas and hence the contact resistance.150,151 In addition, pressure perpendicular to the conductive films increases the contact pressure between adjacent films, which reduces the contact resistance between the adjacent films.149 Although, for example, the PRE of the graphene sheet under a mechanical strain possessing changes in the barrier height is small (GF of 1.9),156 by fabricating two layers of graphene oxide (GO) film overlapped each other on an elastic tape (Fig. 6b), the GO-based strain sensors had the GF of 16.2 within 60% strain and about 150 at strains >60%.145
Another very effective way to enhance the PRE using the overlapping mechanism was proposed by Lee et al.,146Fig. 6c. Vertically aligned carbon nanotube (VACNT) bundles were transferred to ecoflex film so that VACNT bundles were laid down, overlapped with adjacent bundles and carbon nanotubes (CNTs) lay parallel to the strain axis. The sliding and separation of the overlapped CNTs under stretch created an ultrahigh sensitive strain sensor with GF of 42300 at a strain of 125% to 145%.146 A similar idea also was given by Yamada et al.,153 but CNTs were aligned perpendicular to the strain axis and hence the observed GF was lower. In another research, SWCNTs were aligned onto a flat elastomer to make long films of arbitrary length and packed into a highly densely packed solid form using a droplet of isopropyl alcohol. When stretched, the nanotube films fracture into gaps and islands, and bundles bridging the gaps, which change the contact resistance between adjacent CNTs.153
Interlocking-based strain-gauge sensors have a simple architecture that enables the detection of pressure, shear and torsion,41,154 including two interlocked arrays supported on thin elastic layers, Fig. 6d. When different sensing stimuli are applied, the degree of interconnection and the electric resistance of the sensors change in a reversible, directional manner with specific discernible strain-gauge factors.41 Interlocking structures were demonstrated to be an effective approach to substantially improve the sensitivity of piezoresistive sensors. For example, the Pt-coated interlocking-based strain-gauge sensor presented by Pang et al.41 had a GF of 11.5 (pressure), 0.75 (shear), and 8.35 (torsion) with a strain range of ≤ 5% which are fairly comparable or superior to those of existing graphene-based films and metal alloys. Park et al.154 proposed interlocked micro-dome arrays, which were based on carbon nanotube composite elastomer films with surface microstructures of hexagonal microdome arrays, with a GF as high as 9617 at strain of 90–120%, which is significantly (10–395 times) higher than the values for planar films. Suen et al.157 developed a highly sensitive tactile sensor, in which the top electrode and bottom electrode layers were interlocked by ZnO nanorods vertically grown on the PDMS surface. Also using ZnO nanorods, but combined with ZnO nanosheet, Pu et al.158 have successfully fabricated a hierarchical-structure tactile sensor with an exceptional sensitivity in the low-pressure range. Table 5 lists the achievement of enhancing the PRE based on the overlapping mechanism.
Table 5 Enhancement of the PRE based on an overlapping mechanism
Structure |
Materials |
Performance |
Stretchability |
Comments |
Ref. |
Conductive materials |
Core fibre |
GF |
At strain (%) |
Fibres |
Graphene |
Polyurethane Polyester |
3.7 |
50 |
200 |
Double-covered yarn |
60
|
|
|
|
10 |
1 |
|
|
|
|
|
|
35 |
0.2 |
|
|
|
Fibres |
AgNWs |
Polyurethane |
Sensitivity of 0.12 kPa−1 |
400 |
Multiscale wrinkled microstructure |
144
|
Fibres |
Graphene |
Rubble Wool |
2800 |
150 |
150 |
The rubber structure is much better than nylon covered rubber and wool yarns structures |
147
|
Fibres |
Carbonized silk fibres |
5.8 |
0–1 |
500 |
Plain-weave structure |
148
|
|
|
|
9.6 |
<250 |
|
|
|
|
|
|
37.5 |
250–500 |
|
|
|
Thin films |
Graphene oxide (GO) |
Elastic tape |
16.2 |
<60 |
82 |
Ultralow limit of detection (<0.1% strain) |
145
|
|
|
|
150 |
>60 |
|
|
|
Thin films |
Graphene platelets |
Silicon rubber |
164.5 |
<12 |
12 |
The GF is higher for thicker samples |
149
|
Thin films |
Reduced graphene oxide |
PDMS |
34.4 |
<2.5 |
7.5 |
Self-locked overlapping structure |
150
|
|
|
|
99.9 |
<5 |
|
|
|
|
|
|
402.3 |
<7.5 |
|
|
|
Thin films |
GO–AgNWs–C60 |
25 |
0–3 |
62 |
C60 lowers the friction between graphene oxide-based layers |
151
|
|
|
|
466.2 |
3–35 |
|
|
|
|
|
|
1000.2 |
35–52 |
|
|
|
|
|
|
2392.9 |
52–62 |
|
|
|
Thin films |
Ti3C2Tx MXene/CNT |
|
64.6 |
0–30 |
130 |
Ultralow limit of detection (< 0.1% strain), tunable sensing range |
152
|
|
|
|
772.60 |
40–70 |
|
|
|
Nanotubes |
SWCNT |
PDMS |
0.82 |
0–40 |
200 |
SWCNT vertically aligned perpendicular to strain axis |
153
|
Nanotubes |
CNT |
Ecoflex |
256 |
0–80 |
>145 |
CNTs aligned parallel with strain axis |
146
|
|
|
|
3250 |
80–125 |
|
|
|
|
|
|
42300 |
125–145 |
|
|
|
Interlocking structure |
Pt-Coasted PU nanofibres |
PDMS |
11.5 |
0–2 |
2 |
Enable detect pressure, shear and torsion |
41
|
Interlocking structure |
CNT |
PDMS |
27.8 |
0–40 |
120 |
GF is 10 to 395 times higher than that of the planner films. Detection capability of various mechanical stimuli |
154
|
|
|
|
1084 |
40–90 |
|
|
|
|
|
|
9617 |
90–120 |
|
|
|
3.3.3 Enhancement of the PRE based on the tunnelling mechanism.
Enhancing the tunnelling mechanism is another approach for improving the PRE. The resistance of sensors based on the tunnelling mechanism is attributed to the resistance of conductive fillers themselves, which negligibly change with applied strain, and tunnelling resistance between adjacent conductive filler elements (wire fragments, or particles). For example, as shown in Fig. 7a,142 the resistance of a CNT/polymer composite based sensor includes resistance of CNTs themselves (RCNT) and tunnelling resistance between adjacent CNTs (Rtunnel). Under strain, the number of tunnelling channels and tunnelling distances changes, which significantly affects the resistance of tunnelling based sensors under an applied strain. In order to create tunnelling channels, conductive fillers are structured as fragments of nanowires such as CNTs142,159–161 and AgNWs,77 or sea-urchin shapes.43,141 Tunnelling resistance of carbon nanotube-based composites has been numerically and experimentally investigated, providing insights into the tunnelling resistance and proposing strategies to enhance the resistance change under strain. Ning et al.160 concluded that the tunnelling effect is considered to be the principal mechanism of a polymer/CNT nanocomposite strain sensor under small strains, which is consistent with results reported by Li et al.162 A much higher sensitivity or larger PRE can be obtained if the volume fraction of CNT is close to the percolation threshold.161,162 Ning et al. also142 numerically and experimentally evaluated the effect of processing parameters and material properties on the PRE of the polymer/CNT composite. Both numerical and experimental results indicated that higher tunnelling resistance or a higher ratio of tunnelling resistance to the total resistance lead to a higher PRE, which is indicated by the increase of GF with lower curing temperatures and higher stirring rate.142
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| Fig. 7 Tunnelling mechanism for enhancement of the PRE. (a) Schemetic view of the CNT conductive network including tunnelling effect, reprinted with permission from ref. 142. Copyright 2009 Elsevier. (b and c) The tunnelling piezoresistive sensor with interlocked microdome arrays. (d) Resistance change versus applied pressure for different structures: planar (black), microdome (red), and interlocked microdome (blue). (e) Tunnelling resistances of interlocked arrays for different CNT concentrations, reprinted with permission from ref. 159. Copyright (2014) American Chemical Society. (f) Sea urchin-shaped synthetic zinc oxide microparticles (SUSMs) with a forest of nanostructured spines. (g) A sensor made from a SUSM thin film sandwiched between two electrodes. (h) Local spine–spine connections, reprinted141 by permission from Springer Nature (Copyright 2018, Nature Publishing Group). | |
Tunnelling based PRE can be enhanced by changing the configuration of sensors according to the strategy proposed by Park et al.159 The team developed flexible electronic skins based on composite elastomer films (composed of CNT and PDMS) that contain interlocked micro-dome arrays, Fig. 7b and c. The team compared the performance of three different configurations (a single planar arrays, a single micro-dome array, and an interlocked micro-dome array). Composite films with an interlocked micro-dome array exhibit an abrupt switching behaviour with ROFF/RON ratios on the order of 105 in comparison with 100 and 2 for single micro-dome arrays and planar structures, respectively, when the applied pressure was increased from 0 to 10 kPa, Fig. 7d.159
In addition, the tunnelling resistance strongly depends on the conductive filler concentrations. Therefore, optimizing the conductive filler concentration suitable with sensor structure is another approach to enhance the tunnelling PRE. For instance, Park et al.159 observed that the variation in relative resistance with pressure increases with CNT concentration (Fig. 7e) in the interlocked micro-dome array composite films, while this behaviour differs distinctly from that of a planar structure. In another example, the GFs of a AgNW network sandwiched between two layers of PDMS ranged from 2 to 14 depending on the density of the AgNWs, which was controlled by the AgNW solution in a fabrication process.77 The GF was large (GF ≈ 14) when the AgNW solution was small (6 mg mL−1), and hence the resistance of the strain sensor was relatively large (R0 ≈ 246 Ω).
The GF reduced to 2 with initial resistance of R0 ≈ 7.5 Ω, when a much denser network of AgNWs was formed.77 In particular, a GF of as higher than as 104 of the tunnelling PRE was achieved with structures in which particles were shaped with nanostructured spines (sea urchins shape).141Fig. 7f141 shows the SEM image of one sea urchin-shaped synthetic zinc oxide microparticle (SUSM) which included a forest of nanostructured spines. The electrical carriers (electrons) can tunnel from one particle to another through contacting or adjacent spines, Fig. 7f. The resistance of a thin film made from these SUSM sandwiched between two flexible electrodes (Fig. 7g) was contributed from the resistance of particle and inter-spine contact resistance. Applying a strain results in changes in distances and contact between spines and bends the spines themselves, leading to significantly improved PRE.141
3.4 Enhancement of the PRE based on multi-physic coupling
More recently, coupling of the PRE with other physical effects, such as piezoelectricity, optoelectronics, or electrical modulation, has emerged as an advanced and promising approach to boost the PRE.11Table 6 shows recent achievements in enhancing the PRE by multi-physic coupling.
Table 6 Enhancing the PRE by coupling with other physical effects
Coupled effect |
Materials |
Structures |
GF |
Comments |
Ref. |
Piezoelectric |
ZnO fine wires |
Ag–ZnO–Ag |
1250 |
ZnO fine-wires. The SBHs as function of strain |
44
|
Piezoelectric |
ZnO NWs arrays |
Au–ZnO NWs arrays-ZnO seed |
1813 |
Vertical ZnO nanowire arrays. Conductivity of the device was significantly tuned by the change of ZnO/Au Schottky barrier |
83
|
Piezoelectric |
Indium-doped ZnO nanobelt |
Polar surface controlled In-doped ZnO nanobelt |
4036 |
The SBHs at the interfaces with the source and drain electrodes were influenced by the induced piezopotential. The top surface was the monopolar surface |
165
|
Piezoelectric |
ZnSnO3 NWs/microwires |
Ag–ZnSnO3–Ag |
3740 |
The GF is 19 times and three time higher than that of Si and ZnO nanowires |
166
|
Piezoelectric |
MoS2/graphene |
FET |
575294 |
Variable Schottky barrier |
163
|
Piezoelectric |
ZnO/SiC |
ZnO/SiC heterojunction NWs |
50.93 |
The piezoelectric effect of ZnO nanolayers improved the PRE of SiC nanowire |
164
|
Electric field |
SiNWs |
MOSFET |
5000 |
p-Type 〈100〉 Si NW at VGS of 3.75 V |
45
|
Electric field |
SiNWs |
FET |
1861 |
n-Type Si NW |
167
|
Electric field |
SiNWs |
MOSFET |
1320 |
n-Type Si NW |
168
|
Electric field |
SiNWs |
FET |
350 |
At gate bias of 0.6 V |
169
|
Electric field |
MoS2 |
FET |
–40 |
At Vbg = 20 V |
63
|
Illumination |
3C-SiC |
3C-SiC/Si |
–450 |
Visible light |
84
|
Illumination and tuning current |
3C-SiC |
3C-SiC/Si |
58000 |
Visible light |
11
|
Illumination |
3C-SiC NWs |
Pt/Ir–SiC NW-graphite |
— |
Ultraviolet (UV) light |
170
|
3.4.1 Enhancement of the PRE by coupling the piezoelectric effect.
The strain-modulated electric potential in piezoelectric materials can be used to control or tune the transport of charge carriers, hence possibly enhancing the PRE, known as piezotronics. Jun et al. fabricated a fully packaged strain sensing device based on a single zinc oxide (ZnO) piezoelectric nano or micro wire bonded laterally on a polystyrene substrate, Fig. 8a.44 Utilising the crucial role of the Schottky barriers at the metal/semiconductor interfaces in determining the electrical transport properties of the metal–semiconductor–metal structure, the single ZnO wire was sandwiched between two opposite Schottky barriers with distinctly different barrier heights, Fig. 8b. The highest GF demonstrated for their sensor was 1250 (Fig. 8c), which is much higher than the state-of-the-art doped Si strain sensor (200). The underlying mechanism of the enhancement of the GF was attributed to the change of Schottky barrier height (SBH) (Fig. 8d), resulting from the combined effects from strain induced band structure change and the piezoelectric effect.44 As such, in piezotronic strain sensors, the strain induced piezopolarization at the interface changed the barrier height through modulating the carrier distribution and transportation. In other words, the resistance of the piezotronic strain sensors consisting of resistance of the functional material (e.g. ZnO) and resistance of the Schottky contacts. Under strain, the resistance of the functional material changes due to induced band structures change while the resistance of Schottky contacts in total significantly changes thanks to generation of piezoelectric voltage. As a result, the resistance of the piezotronic strain sensors substantially changes under strain/stress. Combining with the PRE, the I–V performance of the sensor can be modified by strain.165,171,172
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| Fig. 8 (a–g) Coupling of the PRE with piezoelectric effect. (a) A single zinc oxide (ZnO) piezoelectric fine wire (PFW) laterally bonded on a polystyrene substrate with two Schottky contacts with metal electrodes. (b) The asymmetric Schottky barrier heights (SBHs) at the source and drain contacts of the PFW. (c) GF of the PFW as a function of strain. (d) The derived change in SBH as a function of strain, reprinted with permission from ref. 44. Copyright (2008) American Chemical Society. (e) Strain sensor based on a variable Schottky barrier in MoS2/graphene heterostructure field effect transistor. (f) The relative change in drain current and the GF of the MoS2/graphene heterostructure field effect transistor, reprinted with permission from ref. 163. Copyright (2019) American Chemical Society. (g) Enhancing the PRE of SiC nanowire by coupling with piezoelectric effect, reprinted with permission from ref. 164. Copyright (2020) American Chemical Society. (h–j) Electrically controlling the PRE. (h) and (i) Si NW embedded within the cantilever above an electrically conductive substrate, in which the electric field around the NW was modulated by an external electrical bias. (j) Extracted GF as a function of gate voltage, reprinted with permission from ref. 45. Copyright (2010) American Chemical Society. | |
In another example, the conductivity of the sensor based on vertical ZnO nanowires grown on polyethylene terephthalate was significantly tuned by the change of the ZnO/Au Schottky barrier so that a GF up to 1813 was achieved.83 This value is higher than that of sensors based on a lateral ZnO microwire.44 With two electrodes connecting to the two ends of nanowires, an opposite polarization was generated at the two ends of the nanowire.44,83 However, Zheng et al. [161] presented a piezotronic strain sensor with the same polar surface on top of nanobelts at both the source and drain electrodes. Indium was doped to effectively change the polar direction parallel to the growth direction.165 As a result of utilising the monopolar surface, the sensor achieved a large GF of 4036 at a bias of +3 V. Beside ZnO piezoelectric materials, the performance of piezotronic strain sensors has been investigated in other materials such as ZnSnO3,166,173 and monolayer MoS2.174–176 Jyn et al.166 demonstrated a flexible strain sensor based on ZnSnO3 nanowires/microwires for the first time with a GF of 3740, which is 19 times higher than that of Si and three times higher than those of ZnO nanowires. The reason for the significant enhancement of the sensitivity is also from the change of the Schottky barrier height under small variation of compressive and tensile strain.166
The performance of piezotronic strain sensors can be enhanced by using electrode materials whose work function can be modulated as demonstrated by Ilmen et al.163 The team introduced a strain sensor based on a variable Schottky barrier in a MoS2/graphene heterostructure field effect transistor, Fig. 8e. The Schottky barrier in a MoS2/graphene junction was significantly changed by strain-induced polarized charges when the Fermi level (EF) of graphene was located near the Dirac point, where the density of states was relatively low, by controlling the gate voltage. In other words, the low density of states near the Dirac point in graphene allowed a large modulation of the graphene Fermi level and corresponding Schottky barrier in a MoS2/graphene junction by strain-induced polarized charges of MoS2.163 The authors demonstrated that near the Dirac point in graphene (Vgate − ΔVth = −0.15) the Schottky barrier changed dramatically (ΔΦSB = 118 meV), corresponding to a GF of 575294. Fig. 8f shows the dependence of the GF on the gate voltage. The GF dramatically increased with decreasing gate voltage and reached its maximum of 575294 at Vgate − ΔVth = −0.15. In addition to coupling with the piezoelectric effect to modify the Schottky barrier height, resulting in significantly enhancing the piezoresistive effect, the induced-strain polarization of the piezoelectric material can be coupled with non-piezoelectric materials to modulate the PRE of the non-piezoelectric materials. For example, the PRE of a SiC nanowire was improved by coupling with the piezoelectric effect of a ZnO nanolayer.164 As a result, the GF of the ZnO/SiC heterojunction nanowires could be up to 50.93 which was profoundly higher than those of SiC counterparts. The improvement in the PRE mainly resulted from the reduction in resistance of the ZnO/SiC structure in comparison with the SiC counterpart (Fig. 8g), which resulted in an increase in the current of the as-constructed ZnO/SiC heterojunction. The lower resistances in ZnO/SiC nanowires than SiC nanowires can be mainly assigned to facilitating the separation of interface charges and increasing the electron density within the conduction band due to formation of a ZnO/SiC heterojunction and ZnO nanolayer limiting the recombination of the carriers.164
3.4.2 Enhancement of the PRE by coupling with external electric field.
Another approach to enhance the PRE is coupling the piezoresistive with an external electric field, which is commonly seen in strain sensors based on field effect transistor (FET) sensors. In these sensors, the piezoresistive performance are electrically controlled by inducing an electric field with an external electrical bias. For instance, Pavel et al.45 fabricated Si NWs oriented in the 〈110〉 direction buried on SiO2 layers. Two terminals at the two ends of the NWs play the roles of source and drain of the metal–oxide–semiconductor field effect transistor (MOSFET) while the substrate (Si) serves as the gate, Fig. 8h and i. The authors confirmed that the substrate voltage strongly influences the device performance or the GF of the device. As shown in Fig. 8j, the GF is a function of gate voltage (VGS). The device operated as a conventional piezoresistor with GF just around 50 for negatively large gate voltage (VGS from −10 to −3 V). Then the device was changed to an intermediate state, where a combination of the PRE and carrier depletion increased the GF to almost 300 as the VGS increased to 1 V. Further increasing VGS resulted in a GF of up to 5000 at 3.75 V when the pinch-off region had the strongest influence on the device behaviour. Optimising the biasing conditions (gate voltage) could lead to the formation of a pinch-off region within the NW, which dominates the overall NW resistance. Mechanical stress applied on the NW caused an increase in the charge carrier concentration. By electrically modulating the formation of the pinch-off region, a modulation of the GF by 2 orders of magnitude from 50 to 5000 was achieved.45 In other words, by controlling the gate voltage, a pinch-off region can be created within the nanowire. Therefore, the NW structure can be then represented as two serial resistors, the pinch-off and bulk resistors, in which the pinch-off region has a dominant influence on the total resistance of the device. Modulating the carrier concentration by stress in the pinch-off region has significantly modulated the total resistance changes of the device. The anomalous PRE is a combined effect of electric field and mechanical stress on a constricted current channel.169 A giant PRE was also reported in a similar structure but with n-type Si NWs.167,168 The Si NWs depleted by a back-gate bias resulted in substantial increase in the subthreshold drain current to applied strain, leading to a single order of magnitude increase in GF to as high as 1861 at the back-gate voltage of VGS = −2.6 V.167
A similar approach has been reported but with a gate-all-around nanowire FET structure to provide easier carrier depletion with the use of low gate bias.169 With narrow gate bias span of 0.6 V near threshold region, the piezoresistive coefficient was enhanced up to seven times from 29 × 10−11 Pa−1 for an inversion region to 207 × 10−11 Pa−1, corresponding to a GF of 350.169 For two-dimensional (2D) semiconductors such as MoS2, the Fermi level can be adjusted by gate bias which significantly tuned the PRE.63 Similarly, the GF of transistor-based sensors reached the highest values in subthreshold regime, where exponential dependence of drain current on back-gate voltage and the change of ΔI was large.63 The GF was close to 0 as the transistor was switched off (Vbg < 10 V) and reached a maximum of approximately −40 in the subthreshold regime (around Vbg = 20 V) before decreasing again in the transition region to the linear regime (Vbg > 20 V).63 Even with piezoresistive sensors based on piezoelectric materials, their performance also strongly depended on the electrical field created by the gate voltage. For example, although the Schottky barrier decreased with the increase of the mechanical strain because of strain-induced polarization, the GF strongly depended on the gate voltage, Fig. 8f.163
3.4.3 Enhancement of the PRE by coupling with the photovoltaic effect.
The photovoltaic effect, the generation of voltage when a device is exposed to light, has been studied intensively for scientific interest and as a sustainable energy source since its first observation in the nineteenth century.177,178 The most common application of the photovoltaic effect is solar cells, in which the photoexcited carriers were separated by a built-in electrical field developed at a p–n junction. Interestingly and more recently, the photovoltaic effect can be coupled to enormously enhance the PRE. Abu et al.84 reported a GF of 28 under dark conditions for a p-3C-SiC/p-Si heterostructure; however, it significantly increased to about −455 under illumination of 635 nm wavelength at 3.0 mW cm−2, which is over 200 times higher than that of commercial metal strain gauges, and 16 times higher than that of 3C-SiC thin films.84 In particular, by coupling the photoexcitation of the charge carriers, the strain modification of the carrier mobility and the electric field modulation of the carrier energy, a giant PRE was demonstrated in a semiconductor heterostructure.11 Visible light was utilised to non-uniformly illuminate the sensing element on the top layer of the heterojunction structure and a tuning current was supplied flowing through the sensing element as shown in Fig. 9a. Under visible light illumination and optimal tuning current, a stable GF value of the 3C-SiC/Si heterojunction as high as 58000 was achieved (Fig. 9b and c), while the value under dark conditions was approximately 20, which was a modulation of approximately 2950 times from the dark condition. We note that this GF is approximately 30000 time greater than that of commercial metal strain gauges and more than 2000 times greater than that of 3C-SiC.11
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| Fig. 9 Piezoresistive effect enhanced by optoelectronic coupling in a heterojunction. (a) Enhancing mechanism by optoelectronic coupling. (b) & (c) Giant PRE achieving by coupling with non-uniform illumination and tuning current. (d) The GF as a function of tuning current. (e) Formation mechanism of the lateral photovoltage. (f) A relatively small voltage V0 at optimal tuning current, reprinted11 by permission from Springer Nature (Copyright 2019, Nature Publishing Group). | |
The performance of the PRE was enhanced by optoelectronic coupling in a 3C-SiC/Si heterojunction resulting from a combination of two key elements: light illumination and tuning current. Fig. 9d shows the role of the tuning current in the enhancement of the PRE. The GF strongly changed corresponding with value of the tuning current, in which the GF increased from approximately −16 then reached a maximum value of 95500 and then decreased to about 50 as the tuning current was swept from 15 to 45 μA. The non-uniform illumination formed a gradient of charge carriers from electrode R to electrode L, Fig. 9e, which resulted in a difference in the electric potential or Fermi levels described as eVph = EF,SiC@R − EF,SiC@L and determined by a generated lateral photovoltage Vph.11 The tuning current was introduced to reduce the difference in the Fermi energy levels of 3C-SiC at the two electrodes, resulting a relatively small voltage V0 between the two electrodes at optimal tuning current, Fig. 9f. In other words, by controlling light illumination and tuning current, the total resistance of the device is ultrasmall under free-strain conditions. Hence, change in total resistance of the device under strain/stress has been significantly modulated. Although this strategy was demonstrated on p+-3C-SiC/p-Si, this method can be extended to enhance the sensitivity of other materials and smart structures that have simultaneous photovoltaic and piezoresistive properties.11 Coupling with illumination to enhance the PRE of 3C-SiC nanowires also has been reported with using ultraviolet (UV) light.170 Instead of using a vertical heterojunction as in the above reports,11,84 two Schottky junctions with different barrier heights were created at two ends of the 3C-SiC nanowires with Pt/Ir on the AFM tip and graphite substrate, inducing formation of built-in electric fields, hence promoting the separation of photogenerated electron–hole pairs. The UV light illumination increased the conductivity of SiC nanowires and hence contributed to enhancing the PRE. Thanks to 405 nm UV light illumination, the PRE has been enhanced approximately 3 times.170
3.5 Enhancing the performance of the piezoresistive sensors with optimal designs
Enhancing the PRE results in improvement in performance of piezoresistive sensors. Therefore, using the above strategies can significantly improve the performance of the piezoresistive sensors. However, the performance of piezoresistive sensors still can be substantially improved by optimal designs such as designs with a stress-amplifier or strain concentration structures although the PRE is not improved. Conventionally, for improving the sensitivity of piezoresistive sensors, the piezoresistors are located at the maximum strain/stress location. For example, the piezoresistors are located at the middle of the edges of the square diaphragm in pressure sensors, where the generated strain/stress are maximum. Furthermore, square diaphragms are used instead of circular diaphragms because the maximum strain generated in square diaphragms is 1.64 times higher than that of circular diaphragms with equivalent dimensions.15,17,179 In addition, the stress can be amplified at the location of piezoresistors thanks to smart structures. For example, inspired by the dog-bone structure, Phan et al.36 proposed a nanowire-based strain amplifying structure, in which strain can be magnified in a desired area. Using this structure, the sensitivity of their device increased approximately by 6 fold compared to that of conventional micro and nano structures.151 Xu et al.180 developed a bossed diaphragm combined with a peninsula-island structure, which introduced a stiffness mutation at the gap region between the peninsula and island and generated a stress concentration region at the gap region with low strain energy dissipation to enhance the sensor sensitivity.
4 Application of advanced enhancement strategies in developing highly sensitive mechanical sensors
On the basis of the above-mentioned strategies, significant enhancements in performance (sensitivity, response time, signal to noise ratio, resolution, detectable range, and so on) of piezoresistive sensing devices have been achieved. This section briefly introduces the application of advanced enhancement strategies proposed to enhance the PRE in mechanical piezoresistive sensors, which have been demonstrated in a laboratory scale. In addition, this section will analyse challenges of transferring these advanced enhancement methods into commercial piezoresistive sensors. Finally, the differences in requirements in material and device structures between conventional application and flexible/stretchable ones are discussed as well. This section does not discuss the performance, structures, and designs in details, which can be found elsewhere.
4.1 Conventional rigid piezoresistive sensors
Pressure sensors are considered as one of the most prominent MEMS devices, so there have been substantial advances in development of piezoresistive pressure sensors. Due to the large PRE in the miniaturised structure of piezoresistors, pressure sensors with sensing elements based on nanowire structures have been investigated such as SiNWs,187 SiC NWs,16 and Si nanofilm.188 There is controversial debate around the effectiveness of the miniaturisation approaches, and while there have been reports showing pressure sensors either showing the enhancement in the performance of pressure sensors with nano-piezoresistor, others do not show significant improvement. Together with proposing the electrically coupling approach, this approach has been successfully applied on pressure sensors. Ultrasensitive pressure sensors achieved through the effective tuning of the transverse electric field have been reported189,190 with the sensitivity as high as 13 Pa−1. The high sensitivity was achieved through the effective tuning of the transverse electric field across a p-type 100 nm cross-section Si NW.189 Piezotronic pressure sensors, based on the coupling between the polarization under stress of piezoelectric materials and the PRE, have been induced as well.191,192 The combination of the piezoelectric effect of the ZnO nanoarrays and the electron-tunnelling modulation of the MgO nanolayer has resulted in a high sensitivity of 7.1 × 10−4 gf−1 and a fast response time of 128 ms of the piezotronic pressure sensor.192 The discovery of a giant PRE in the SiC/Si heterojunction by opto-electronic coupling11 which likely creates a new direction for development of mechanical sensors, has been realised by an application on pressure sensors.17 By coupling between light illumination and tuning current, the sensitivity of the pressure sensor (0.87 kPa−1) was enhanced up to 185000 times compared to the unilluminated condition, which was more than 1700 times higher than the best result of the recently reported micromachined semiconductor pressure sensors.17
While the GF of the metal strain sensor is around 2, the GF of bulk intrinsic semiconductors with conventional methods are up to 200. By introducing novel strategies, the performance of the strain sensors has been significantly enhanced. The GF of rigid semiconductor strain sensors can be as high as 6570 by miniaturising piezoresistors, or 1861 by coupling with electrical control,167 58000 by coupling with light illumination and tuning current, or 575294 by coupling with the piezoelectric effect.163 Nanowire piezoresistors such as SiNWs,193–195 and ZnO NWs196 have been utilised as sensing elements in accelerometers. Vertically aligned piezoelectric nanowire arrays of barium titanate were grown to fabricate a NEMS accelerometer with high sensitivity and unity coherence and wide operating bandwidth.197 Nguyen et al.27 demonstrated the combination of the piezoresistive effect and light illumination to tune the sensitivity of the accelerometer in a monolithic structure.
4.2 Flexible and stretchable piezoresistive sensors
For enhancing and optimizing the sensitivity and stability of high-performance flexible piezoresistive sensors, substates and conductive components are the two most important factors that must be comprehensively considered in the device design and fabrication processes. In particular, the substrates are referred to the backbone of the sensors where various sensing elements can be integrated for diverse sensing purposes. In general, polymeric materials are usually chosen for the substrates used in flexible piezoresistive sensors due to their high elasticity and chemical stability as well as their low-cost compared to other materials. Some common polymers employed to build-up flexible piezoresistive sensors are polydimethylsiloxane (PDMS), polyethylene naphthalate (PEN) or polyethylene terephthalate (PET). Among them, PDMS is the most widely used material for the fabrication of such piezoresistive sensors, because this material affords many advantages such as easy fabrication, low-cost, optical transparency, and biocompatibility. On the other hand, conductive components are sensing elements of devices which play a crucial role in determining the device performance. Conventional conductive materials are usually semiconductor, metal, or carbon-based materials which possess an intrinsic brittle and rigid nature and thus, cannot be used for flexible sensing devices in conventional structures. However, several innovative approaches have been recently developed to produce many novel stretchable devices which could be capable of sustaining a large level of strain. In recent years, the use of substrates with microstructure surfaces has been emerging as an effective way to fabricate highly sensitive piezoresistive sensing devices.183,198,199 A pioneering example was reported by Choong et al.198 The team designed and constructed micro-pyramid PDMS arrays to enhance the pressure sensitivity of the sensor. In this work, a PDMS substrate with micro-pyramids was replicated from a silicon mould and then grafted with a sub-micrometre-thick PEDOT:PSS/PUD composite polymer. This structure then served as a piezoresistive electrode in which the electrical resistance changes with the pressure. As a result, the authors unambiguously demonstrated the enhanced pressure sensitivity of the pyramid-structured sensor relative to unstructured films.198 In another interesting example, Su et al.199 reported PDMS thin films with an irregular pattern of microdomains using mimosa leaves. The authors found that the bio-inspired flexible pressure sensor was generated with high sensitivity, durable stability and quick response time.199 Wang et al.199 fabricated a micro-patterned PDMS substrate using a piece of delicate silk scarf. In this work, a free standing single walled carbon nanotub (SWCNT) ultrathin film was transferred onto the micro-patterned surface to form double layers of SWCTs/PDMS films in a face-to-face configuration. As a result, the as-fabricated sensor exhibited a high sensitivity, fast response time, and ultralow detection limit.199
Although operating similar conventional rigid sensors, the flexible and stretchable sensors exhibit mechanical properties resembling those of human tissues, and thus they can be used to detect human motions. Using fibre structures, Wei et al.144 constructed flexible piezoresistive fibres, which presented high sensitivity to pressure and bending deformations (0.12 kPa−1 and 0.012 rad−1) with potential applications as wearable devices and smart fabrics. As shown in Fig. 10a1, the piezoresistive fibre was integrated into the finger of a glove to detect the finger motion.144 An overlap-based carbon nanotube ultrasensitive strain sensor with a gauge factor of 42300 was introduced recently by Lee et al.146 The sensor consisted of overlapped carbon nanotube bundles, which can slide and disconnect under stretching. The wearable applications of the sensor were demonstrated with abilities of distinguishing the amount of bending of the finger and arm and detecting subtle strains from movements of neck muscles during saliva swallowing and speaking.146 Applying the tunnelling mechanism, Yin et al.141 mimicked tapering the spine of biological bristles using synthetic zinc oxide microparticles to create a strain sensor with a strain gauge factor >104. The prototype sensors were demonstrated for capturing the bending movement in the wrist, detecting swallowing motion and monitoring physiological pulses for health diagnostics.141Fig. 10a2 and a3 show applications of CNT-based strain sensors for human-motion detection, in which the strain sensors were integrated in a glove to detect fine finger motions181 or fixed to a stocking over the knee joint to monitor movement of the knee joint.153 Though achieving significant progress in developing flexible and stretchable for human motion detection, many of human activities are still not addressed for monitoring purposes and currently sensors are mostly attached on the skin rather than embedded in wearable devices.
|
| Fig. 10 Some applications of flexible and stretchable piezoresistive sensors. (a) Human motion detection. (a-1) Finger motion detections based on the resistance change in piezoresistive fibre integrated into a glove, reprinted with permission from ref. 144. Copyright 2016 Wiley-VCH. (a-2) Data glove with capability of detecting fine finger motions and collecting electric motion data, reprinted with permission from ref. 181. Copyright (2016) American Chemical Society. (a-3) A stocking assembled with SWCNT film over the knee joint to detect and distinguish every movement of the knee, reprinted153 by permission from Springer Nature (Copyright 2011, Nature Publishing Group). (b) Healthcare monitoring. (b-1) An adhered sensor mounted onto a human throat for the real-time sensing of the physiological motion of swallowing. (b-2) A sensor attached to the human wrist to real-timely sense the blood pulse, reprinted with permission from ref. 182. Copyright 2018 Wiley-VCH. (b-3) E-skin device for monitoring wrist pulses of a healthy person and a pregnant womant, reprinted with permission from ref. 183. Copyright 2014 Wiley-VCH. (c) Electronic skin. Ultrathin silicon nanoribbon sensor arrays conformally integrated on human skin, reprinted184 by permission from Springer Nature (Copyright 2014, Nature Publishing Group). (d) Human machine interface. Schematic illustration of a wearable wireless music instrument built by GWF/PDMS sensors into an open source hardware with wireless communication capabilities and userfriendly interfaces, reproduced from ref. 185 with permission from The Royal Society of Chemistry. (e) Robotic application. A robotic hand was remotely controlled by four flexion sensors sewed on a textile glove, reprinted with permission from ref. 186. Copyright (2015) American Chemical Society. | |
Healthcare monitoring and disease diagnostics are some of the main applications of flexible and stretchable piezoresistive sensors, which are high sensitivity, low cost, large stretchability, and comfortability. The sensors are usually attached onto the human throat, wrist, arm, finger, neck, and chest to measure and quantify electrical signals generated by human activities. For example, Guo et al.182 developed a flexible wearable pressure sensor with combining microcrack and interlocking mechanisms. Their sensors were mounted onto a human throat (as shown in Fig. 10b1) or to the wrist (as shown in Fig. 10b2), to sense real-time swallowing actions or the blood pulse, respectively. Their sensor exhibited wide full-range healthcare monitoring with high strain deformation, fast response/recovery time, high sensitivity, which potentially predicts early-stage Parkinson's disease and can be used as smart artificial electronic or for wirelessly monitoring human-motion interactivities [182]. A flexible, ultra-sensitive and highly stable e-skin made of PDMS film covered single-walled carbon nanotube ultrathin film can distinguish blood pulses of a healthy person and pregnant woman as shown in Fig. 10b3.183 Flexible and stretchable sensors for monitoring heart beats have also achieved critical progress,53 but there are various challenges for practical usage such as distinguishing body movements with heart-beat signals and/or extracting other physiological information form collected data. In addition, the sensitivity, accuracy, long-term stability, and biocompatibility of sensors need to be improved to reach the standard requirements.
The sensors can be distributed as an array in a wide area, which serve as electronic skin. An ultra-sensitive pressure sensor based on overlapping mechanism have been synthesised for electronic skin and health monitoring applications.200 Another example of applying the advanced enhancement methods in developing electronic skins, which can use in robotic hands or healthcare monitoring devices, was introduced by Ha et al.201 Arrays of hierarchical micro- and nanostructured ZnO nanowire in an interlocked geometry, which were sensitive to both static and dynamic tactile stimuli, were designed. The flexible skins exhibited a high pressure sensitivity (−6.8 kPa−1) and an ultrafast response time.201 As shown in Fig. 10c, Kim et al.184 demonstrated a smart prosthetic skin instrumented with ultrathin, single crystalline silicon nanoribbon strain sensor arrays. The sensor arrays, which serve as prosthetic skins, mimic real skin and transmit the stimuli-responsive electrical signals for nerve stimulation, hence allowing patients to comfortably use their devices. However, it is critical to utilise materials with low elastic moduli and good stretchability. Based on the crack propagation mechanism, Lee et al.133 fabricated highly stretchable and sensitive fibre strain sensors with Ag nanoparticles embedded into stretchable fibres. Their strain sensor showed a gauge factor as high as approximately 9.2 × 105. They also demonstrated potential application of the fibre strain sensor for electronic textiles, wearable electronics and biomedical engineering.133
For human machine interface application, flexible and stretchable sensors, which can be attached on curved surfaces, detect contact force and location, physical activities and surrounding environment, and sent the detected data to the integrated smart systems. A good example, as shown in Fig. 10d, was reported by Liu et al.185 The team fabricated highly sensitive, flexible graphene woven fabric/PDMS sensors integrated with wireless communication and wearable interface, which allows their users to rearrange melodies to play music by moving their fingers. For robotic application, flexible and stretchable sensors can be utilised to real-timely monitor sophisticated movements of robots, to remotely control and actuate the smart robots, and to interact with surrounding objects. Fig. 10e186 shows a demonstration of a remote controlled robotics arm using a textile glove sewed with four flexible sensors.
4.3 Common challenges of applying advanced enhancement strategies in commercial products
Although advanced strategies for enhancing the piezoresistive effect as mentioned above have been successfully demonstrated in the laboratory, there are still many challenges for applying these methods in commercial sensors. For size-effect-based enhancement of the PRE, the controversial debate around reliability of the giant PRE of nanowires still exists. The performance of nanowire piezoresistive sensors is degraded by the surface conditions of nanowires, which are easily affected by surrounding environment, and play a critical role in their piezoresistive properties. In addition, the difficult mass production, high cost and poor reproducibility of nanowire piezoresistive devices are also obstacles for using nanowire piezoresistive sensors in everyday situations.
Although flexible/stretchable piezoresistive devices have been significantly progressing over the last decade, most devices are still demonstrated only at a laboratory level. There are two main obstacles in transferring these laboratory-demonstrated sensors into commercial sensors. First, the reproducibility of flexible/stretchable piezoresistive devices is low. For example, it is very difficult to control the crack density and crack depth of crack-based piezoresistive sensors so that all the sensors have the same crack properties. Second, degradation in properties of materials and/or electronic properties of devices over time is another obstacle. Although the piezoresistive effect in conventional materials such as Si, have been investigated a long time ago and have been successfully used in commercial sensors, it is difficult to use these materials for flexible and stretchable applications. The requirements for high sensitivity, stability and durability, flexibility and/or stretchability properties are critical for flexible/stretchable sensors, and remain the main obstacles in using conventional materials in flexible/stretchable applications. The conventional materials (e.g. Si or SiC) with large Young's modulus have low stretchability and flexibility, which limits the ability of using them in flexible/stretchable applications. Coupling with photovoltaic effects has been successfully demonstrated. However, with using an external light source, it still needs to overcome this bottle neck for applying in practical applications. In addition, most studies were just proof of concept, and several practical challenges still remain to be addressed including integration of sensors with power, data analysis, data processing, and packaging.
5 Conclusions and perspectives
We presented a comprehensive review on methods, strategies and approaches to enhance the PRE, in substrates ranging from rigid to flexible/wearable/stretchable materials, from conventional applications to soft electronics. Sensing mechanisms of the PRE including carrier mobility changes resulting from energy band change, macro structure change, size-effect and multi-physic couplings are first reviewed, to understand proposed methods, strategies and approaches that are then discussed in detail. Conventional strategies such as aligning piezoresistors along the orientations in which the stress-induced mobility changes of carriers are maximum in those directions, optimizing doping concentration, alternating doping atoms, or finding alternative materials which have a higher PRE, are limited by the nature of materials and utilised mainly on enhancing the PRE of semiconductors. Thanks to the recent advancement in fabrication technologies, the sensing elements can be scaled down to the nanoscale, which was reported as an effective approach to improve the PRE of nano-size sensing elements. However, this approach also received controversial debates on both claims and counterclaims of giant PRE in nanostructures. Coupling of the PRE and other physical effects is an advanced strategy to modulate the performance of the PRE which achieved remarkable results. Coupling of the PRE with controlling the electrical field surrounding the sensing elements opened a new direction of development of piezoresistive sensors with field effect transistor (FET) structures. The performance of the FET piezoresistive sensors strongly depends on the voltage between the gate and source. Coupling the PRE with piezoelectric potential created in the piezoelectric material by applying strain leads to piezotronic sensors. The performance of the piezotronic sensors was significantly modulated by the changes of Schottky barrier height resulting from piezoelectric potential under applied strain. Optoelectronic coupling is another approach which enormously affects the PRE in the heterostructure. This approach is promising for extending to enhance the sensitivity of various materials and smart structures that have simultaneous photovoltaic and piezoresistive properties. With the needs of high flexibility and stretchability in soft/wearable electronic applications, the strategies, proposed to enhance the PRE of rigid materials, seem to be unsuitable because of the stiffness of conventional materials or the low natural PRE of new materials. Enhancing crack propagation, encouraging changes in overlapping areas or contact pressure between adjacent layers, or boosting the change in tunnelling resistance have demonstrated the effectiveness in improving the performance of the piezoresistive flexible/stretchable/wearable sensors.
Research studies on improving the PRE have significantly enhanced the performance (sensitivity, signal to noise ratio, resolution, detectable ranges, etc.,) of piezoresistive sensors. Together with the development of new materials and new applications, the new sensing mechanisms and strategies for improving the PRE will be proposed. These mechanisms and strategies can be developed from the proposed ones or are totally new directions. In addition, further investigations to push the limits of the PRE to higher levels based on the proposed methods, but applied to new materials, novel structures and new applications continue to be carried out. The next generation of electromechanical sensors is integrating multiple electromechanical sensing functionalities, power supplies, and wireless connectivity into a single technology platform. Coupling the PRE with other physical effects will be investigated further to modulate the PRE and/or the physical effects.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The work is supported by the Foundation for Australia-Japan Studies under the Rio Tinto Australia-Japan Collaboration Project and Aus4Innovation Partnership Grants. This work has been partially supported by Australian Research Council grants LP160101553 and DE210100852. T. D. is grateful for the support from USQ Capacity Building Grants 2020. T. N. is grateful for the support from the Publication Assistance Scholarship, Griffith University.
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Footnote |
† 3-Aminopropyltriethoxysilane. |
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