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Two-Dimensional Materials Based Sensors

A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Biosensors".

Deadline for manuscript submissions: closed (31 May 2020) | Viewed by 41141

Special Issue Editors


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Guest Editor
1. Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, N.T.,Hong Kong SAR, China
2. Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, China
Interests: Near-field sensing; surface plasmonics; energy conversion
Special Issues, Collections and Topics in MDPI journals

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Electrical Engineering & Computer Science, Case Western Reserve University, Cleveland, OH 44106, USA
Interests: Semiconductor Devices Physics; NEMS/MEMS; Circuits & Systems; Advanced Materials
Special Issues, Collections and Topics in MDPI journals

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Guest Editor
School of Information Science and Electronic Engineering, College of Microelectronics, Zhejiang University, Hangzhou 310027, China
Interests: Graphene; Graphene-Si heterostructure; photodectors; image sensor
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

 Two-dimensional materials (graphene, MoS2, Black Phosphorus et.) with the combination of nano-fabrications can enable high performance sensors or novel sensors with new funtionalities, which can open widely applications.

Two-dimensional materials can be the building blocks for various type of sensors, such as photodectors, strain/pressure sensors, gas sensors et. The two-dimensional-based sensors can be also enlarged by creating two-dimentional heterostructures. The two-dimentional heterostructures can be produced by combining 2D-2D, 2D-3D, 2D-1D, 2D-0D structures. This special Issue aims to introduce the two-dimensional materials with the combination of nano-fabrications. Topics in general include, but are not limited, to:

  • Two-dimensional materials/heterostructures-based sensors: 2D material growth, transfer, fabrication, device prototype and demo
  • Two-dimensional materials/heterostructures as  photodectors
  • Graphene or other 2D materials-based strain/pressure sensor with high sensitivity
  • Two-dimensional materials as bio-sensors
  • Two-dimensional materials for acoustic or thermal sensing applications

Dr. He Tian
Prof. Dr. Jian-Bin Xu
Prof. Philip Feng
Prof. Dr. Yang Xu
Guest Editors

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Keywords

  • graphene
  • black Phosphorus
  • two-Dimensional Materials
  • photodetectors
  • strain/Pressure sensors
  • bio-sensors
  • acoustic or thermal sensing

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Published Papers (7 papers)

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11 pages, 1714 KiB  
Article
Two-Channel Graphene pH Sensor Using Semi-Ionic Fluorinated Graphene Reference Electrode
by Dae Hoon Kim, Woo Hwan Park, Hong Gi Oh, Dong Cheol Jeon, Joon Mook Lim and Kwang Soup Song
Sensors 2020, 20(15), 4184; https://doi.org/10.3390/s20154184 - 28 Jul 2020
Cited by 11 | Viewed by 3596
Abstract
A reference electrode is necessary for the working of ion-sensitive field-effect transistor (ISFET)-type sensors in electrolyte solutions. The Ag/AgCl electrode is normally used as a reference electrode. However, the Ag/AgCl reference electrode limits the advantages of the ISFET sensor. In this work, we [...] Read more.
A reference electrode is necessary for the working of ion-sensitive field-effect transistor (ISFET)-type sensors in electrolyte solutions. The Ag/AgCl electrode is normally used as a reference electrode. However, the Ag/AgCl reference electrode limits the advantages of the ISFET sensor. In this work, we fabricated a two-channel graphene solution gate field-effect transistor (G-SGFET) to detect pH without an Ag/AgCl reference electrode in the electrolyte solution. One channel is the sensing channel for detecting the pH and the other channel is the reference channel that serves as the reference electrode. The sensing channel was oxygenated, and the reference channel was fluorinated partially. Both the channels were directly exposed to the electrolyte solution without sensing membranes or passivation layers. The transfer characteristics of the two-channel G-SGFET showed ambipolar field-effect transistor (FET) behavior (p-channel and n-channel), which is a typical characteristic curve for the graphene ISFET, and the value of VDirac was shifted by 18.2 mV/pH in the positive direction over the range of pH values from 4 to 10. The leakage current of the reference channel was 16.48 nA. We detected the real-time pH value for the two-channel G-SGFET, which operated stably for 60 min in the buffer solution. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
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Figure 1

Figure 1
<p>XPS and Raman spectra of graphene. (<b>a</b>) Survey spectra of XPS on pristine, oxygenated and fluorinated graphene; (<b>b</b>) deconvoluted C 1s peaks on partially oxygenated graphene; (<b>c</b>) deconvoluted F 1s peaks on partially fluorinated graphene; (<b>d</b>) Raman spectra of pristine, oxygenated and fluorinated graphene.</p>
Full article ">Figure 2
<p>Typical three-dimensional pH sensor using graphene field-effect transistor. (<b>a</b>) Schematic illustration of G-ISFET with Ag/AgCl reference electrode. For oxygenated G-ISFET: (<b>b</b>) <span class="html-italic">I<sub>DS</sub></span>–<span class="html-italic">V<sub>DS</sub></span> transfer characteristic with respect to <span class="html-italic">V<sub>GS</sub></span> and (<b>c</b>) <span class="html-italic">I<sub>DS</sub></span>–<span class="html-italic">V<sub>GS</sub></span> transfer characteristic with respect to pH value.</p>
Full article ">Figure 3
<p>Characteristic graphs of fluorinated G-ISFET: (<b>a</b>) <span class="html-italic">I<sub>DS</sub></span>–<span class="html-italic">V<sub>DS</sub></span> transfer characteristic with respect to <span class="html-italic">V<sub>GS</sub></span> and (<b>b</b>) <span class="html-italic">I<sub>DS</sub></span>–<span class="html-italic">V<sub>GS</sub></span> transfer characteristic with respect to pH value.</p>
Full article ">Figure 4
<p>Real-time detection of pH in electrolyte solution using (<b>a</b>) oxygenated G-ISFET and (<b>b</b>) fluorinated G-ISFET. (<b>c</b>) Long-term stability of oxygenated G-ISFET in a buffer solution of pH 8.</p>
Full article ">Figure 5
<p>Two-dimensional pH sensor using graphene field-effect transistor. (<b>a</b>) Schematic illustration of two-channel graphene solution gate field-effect transistor (G-SGFET) and sensor image. For two-channel G-ISFET: (<b>b</b>) <span class="html-italic">I<sub>DS</sub></span>–<span class="html-italic">V<sub>DS</sub></span> transfer characteristic with respect to <span class="html-italic">V<sub>GS</sub></span> and (<b>c</b>) <span class="html-italic">I<sub>DS</sub></span>–<span class="html-italic">V<sub>GS</sub></span> transfer characteristic with respect to pH value (<b>d</b>) <span class="html-italic">I<sub>DS</sub>–V<sub>DS</sub></span> transfer characteristic with respect to pH value. (<b>e</b>) The Dirac point of the two-channel G-STGFET.</p>
Full article ">Figure 6
<p>(<b>a</b>) Real-time detection of pH in electrolyte solution using two-channel G-SGFET. (<b>b</b>) The long-term stability of two-channel G-SGFET in a buffer solution of pH 8.</p>
Full article ">
11 pages, 5044 KiB  
Article
Temperature Characteristics of a Pressure Sensor Based on BN/Graphene/BN Heterostructure
by Mengwei Li, Teng Zhang, Pengcheng Wang, Minghao Li, Junqiang Wang and Zewen Liu
Sensors 2019, 19(10), 2223; https://doi.org/10.3390/s19102223 - 14 May 2019
Cited by 19 | Viewed by 4179
Abstract
Temperature is a significant factor in the application of graphene-based pressure sensors. The influence of temperature on graphene pressure sensors is twofold: an increase in temperature causes the substrates of graphene pressure sensors to thermally expand, and thus, the graphene membrane is stretched, [...] Read more.
Temperature is a significant factor in the application of graphene-based pressure sensors. The influence of temperature on graphene pressure sensors is twofold: an increase in temperature causes the substrates of graphene pressure sensors to thermally expand, and thus, the graphene membrane is stretched, leading to an increase in the device resistance; an increase in temperature also causes a change in the graphene electrophonon coupling, resulting in a decrease in device resistance. To investigate which effect dominates the influence of temperature on the pressure sensor based on the graphene–boron nitride (BN) heterostructure proposed in our previous work, the temperature characteristics of two BN/graphene/BN heterostructures with and without a microcavity beneath them were analyzed in the temperature range 30–150 °C. Experimental results showed that the resistance of the BN/graphene/BN heterostructure with a microcavity increased with the increase in temperature, and the temperature coefficient was up to 0.25%°C−1, indicating the considerable influence of thermal expansion in such devices. In contrast, with an increase in temperature, the resistance of the BN/graphene/BN heterostructure without a microcavity decreased with a temperature coefficient of −0.16%°C−1. The linearity of the resistance change rate (ΔR/R)–temperature curve of the BN/graphene/BN heterostructure without a microcavity was better than that of the BN/graphene/BN heterostructure with a microcavity. These results indicate that the influence of temperature on the pressure sensors based on BN/graphene/BN heterostructures should be considered, especially for devices with pressure microcavities. BN/graphene/BN heterostructures without microcavities can be used as high-performance temperature sensors. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
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Figure 1

Figure 1
<p>Structure and fabrication process of the sensors: the process flow of the non-cavity-type graphene sensor: (<b>a</b>). The silicon substrate is cleaned; (<b>b</b>). SiO<sub>2</sub> insulating layer is formed on the surface of the silicon substrate through Plasma Enhanced Chemical Vapor Deposition (PECVD); (<b>c</b>). Sputtering of metal, forming a heterogeneous junction bottom electrode; (<b>d</b>). The underlying BN is transferred and the graphics of BN are completed; (<b>e</b>). Graphene is transferred and its graphics completed; (<b>f</b>). The top-level BN is transferred and the graphics of BN are completed; (<b>g</b>). The top metal electrode is vaporized; (<b>h</b>). Vertical view of the device structure.</p>
Full article ">Figure 2
<p>SEM images of the sensors: (<b>a</b>) Local diagram of the cavity-type graphene sensor chip; (<b>b</b>) Enlarged image of the bottom devices shown in (<b>a</b>); (<b>c</b>) Local diagram of a non-cavity-type graphene sensor chip; (<b>d</b>) Enlarged image of the bottom devices shown in (<b>c</b>).</p>
Full article ">Figure 3
<p>Raman test results of the BN/graphene/BN heterostructure: The intensity of peak G <span class="html-italic">I<sub>G</sub></span> of graphene is 270, the peak <span class="html-italic">I<sub>2D</sub></span> of graphene is 1050, and the peak strength ratio <span class="html-italic">I<sub>2D</sub></span>/<span class="html-italic">I<sub>G</sub></span> ≈ 3.89. The intensity of peak G <span class="html-italic">I<sub>G</sub></span> of BN/graphene/BN is 296, the peak <span class="html-italic">I<sub>2D</sub></span> of BN/graphene/BN is 958, and the peak strength ratio <span class="html-italic">I<sub>2D</sub></span>/<span class="html-italic">I<sub>G</sub></span> ≈ 3.24.</p>
Full article ">Figure 4
<p>Electrical characterization of the BN/graphene/BN pressure sensor: The resistance of the BN/graphene/BN pressure sensor with a microcavity is approximately 1092 ± 4.6 Ωsq<sup>−1</sup>, and that of the BN/graphene/BN pressure sensor without a microcavity is approximately 1320 ± 6.5 Ωsq<sup>−1</sup>.</p>
Full article ">Figure 5
<p>Experimental characterizations of the BN/graphene/BN pressure sensors: (<b>a</b>) test structure diagram of the pressure sensor with a cavity; (<b>b</b>) the fractional change in the electrical resistivity variation of the BN/graphene/BN pressure sensor with a microcavity over the range 30–150 °C; (<b>c</b>) the fitting curve of the BN/graphene/BN pressure sensor with a microcavity over the range 30–150 °C; (<b>d</b>) test structure diagram of the pressure sensor without a cavity; (<b>e</b>) the fractional change in the electrical resistivity variation of the BN/graphene/BN pressure sensor without a microcavity over the range 30–150 °C; (<b>f</b>) the fitting curve of the BN/graphene/BN pressure sensor without a microcavity over the range 30–150 °C.</p>
Full article ">
11 pages, 6909 KiB  
Article
Effects of Acetone Vapor on the Exciton Band Photoluminescence Emission from Single- and Few-Layer WS2 on Template-Stripped Gold
by Samantha Matthews, Chuan Zhao, Hao Zeng and Frank V. Bright
Sensors 2019, 19(8), 1913; https://doi.org/10.3390/s19081913 - 23 Apr 2019
Cited by 4 | Viewed by 4121
Abstract
Two-dimensional (2D) materials are being used widely for chemical sensing applications due to their large surface-to-volume ratio and photoluminescence (PL) emission and emission exciton band tunability. To better understand how the analyte affects the PL response for a model 2D platform, we used [...] Read more.
Two-dimensional (2D) materials are being used widely for chemical sensing applications due to their large surface-to-volume ratio and photoluminescence (PL) emission and emission exciton band tunability. To better understand how the analyte affects the PL response for a model 2D platform, we used atomic force microscopy (AFM) and co-localized photoluminescence (PL) and Raman mapping to characterize tungsten disulfide (WS2) flakes on template-stripped gold (TSG) under acetone challenge. We determined the PL-based response from single- and few-layer WS2 arises from three excitons (neutral, A0; biexciton, AA; and the trion, A). The A0 exciton PL emission is the most strongly quenched by acetone whereas the A PL emission exhibits an enhancement. We find the PL behavior is also WS2 layer number dependent. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
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Graphical abstract

Graphical abstract
Full article ">Figure 1
<p>Typical AFM results for a WS<sub>2</sub> flake on TSG. (<b>a</b>) Typical AFM height (<b>left</b>) and phase (<b>right</b>) images. The dotted curve is used to outline the WS<sub>2</sub> flake. Two vectors (V1 and V2) are shown in the height image. (<b>b</b>) Typical height profiles along the V1 (<b>left</b>) and V2 (<b>right</b>) vectors shown in panel a. The V1 and V2 vectors traverse largely single- and two-layer regions, respectively.</p>
Full article ">Figure 2
<p>Typical single point Raman spectra and band assignments for different areas on a WS<sub>2</sub> flake on TSG. (<b>a</b>) Single-layer region. (<b>b</b>) Few-layer region.</p>
Full article ">Figure 3
<p>Typical WS<sub>2</sub> layer count maps for a single WS<sub>2</sub> flake on TSG. (<b>a</b>) <span class="html-italic">I<sub>2LA</sub></span>/<span class="html-italic">I<sub>A1g</sub></span> intensity ratio map. (<b>b</b>) <span class="html-italic">Δ</span> (E<sub>2LA</sub> − E<sub>A1g</sub>) band energy difference map. (<b>c</b>) Combined a and b WS<sub>2</sub> layer count map.</p>
Full article ">Figure 4
<p>Typical total PL emission intensity maps (1.8–2.1 eV) for a single WS<sub>2</sub> flake on TSG. (<b>a</b>) Air atmosphere. (<b>b</b>) Acetone atmosphere. (<b>c</b>) <span class="html-italic">PL<sub>Air</sub></span>/<span class="html-italic">PL<sub>Acetone</sub></span> intensity response map. The black and white dots indicate single- and few-layer WS<sub>2</sub> regions determined from <a href="#sensors-19-01913-f003" class="html-fig">Figure 3</a>c.</p>
Full article ">Figure 5
<p>Typical single point PL emission spectra and curve fits from specific locations on the WS<sub>2</sub> flake shown in <a href="#sensors-19-01913-f004" class="html-fig">Figure 4</a>. (<b>a</b>) Spectra from a single-layer WS<sub>2</sub> region. (<b>b</b>) Spectra from a few-layer WS<sub>2</sub> region. (1) Spectra recorded in air. (2) Spectra recorded under acetone vapor. (3) Energy-dependent <span class="html-italic">PL<sub>Air</sub></span>/<span class="html-italic">PL<sub>Acetone</sub></span> response spectra. The dashed line in spectra 3 located at 1.0 denotes no quenching. Spectral regions with <span class="html-italic">PL<sub>Air</sub></span>/<span class="html-italic">PL<sub>Acetone</sub></span> &lt; 1 exhibit a PL enhancement under acetone vapor. Spectral regions with <span class="html-italic">PL<sub>Air</sub></span>/<span class="html-italic">PL<sub>Acetone</sub></span> &gt; 1 exhibit a PL quench under acetone vapor.</p>
Full article ">Figure 6
<p>Typical PL emission A<sup>0</sup>, AA, and A<sup>−</sup> exciton band amplitude maps for a single WS<sub>2</sub> flake on TSG under air and acetone vapors. Maps are generated by curve fitting PL emission spectra at each pixel across the entire flake. (<b>a</b>,<b>b</b>,<b>c</b>) Maps in air. (<b>d</b>,<b>e</b>,<b>f</b>) Maps under acetone vapor.</p>
Full article ">Figure 7
<p>Typical PL emission A<sup>0</sup>, AA, and A<sup>−</sup> exciton band maxima (eV) maps for a single WS<sub>2</sub> flake on TSG under air and acetone vapors. Maps are generated by curve fitting PL emission spectra at each pixel across the entire flake. (<b>a</b>,<b>b</b>,<b>c</b>) Maps in air. (<b>d</b>,<b>e</b>,<b>f</b>) Maps under acetone vapor.</p>
Full article ">Figure 8
<p>Typical PL emission A<sup>0</sup>, AA, and A<sup>−</sup> exciton band FWHM (eV) maps for a WS<sub>2</sub> flake on TSG under air and acetone vapors. Maps are generated by curve fitting PL emission spectra at each pixel across the entire flake. (<b>a</b>,<b>b</b>,<b>c</b>) Maps in air. (<b>d</b>,<b>e</b>,<b>f</b>) Maps under acetone vapor.</p>
Full article ">Figure 9
<p>Typical acetone vapor-dependent PL emission A<sup>0</sup>, AA, and A<sup>−</sup> exciton band intensity ratio (<span class="html-italic">PL<sub>Air</sub></span>/<span class="html-italic">PL<sub>Acetone</sub></span>), band energy maxima difference (<span class="html-italic">E<sub>max,Air</sub></span> − <span class="html-italic">E<sub>max,Acetone</sub></span>), and band FWHM energy difference (<span class="html-italic">FWHM<sub>Air</sub></span> − <span class="html-italic">FWHM<sub>Acetone</sub></span>) maps for a single WS<sub>2</sub> flake on TSG.</p>
Full article ">Figure 10
<p>Model of the overall effect of acetone on exciton bands. A WS<sub>2</sub> flake in an air environment denoting single- and few-layer areas. The red arrows indicate a quench in PL and the blue arrow indicates an enhancement. The arrow length represents the effect magnitude.</p>
Full article ">
11 pages, 2589 KiB  
Article
Controlled Growth of an Mo2C—Graphene Hybrid Film as an Electrode in Self-Powered Two-Sided Mo2C—Graphene/Sb2S0.42Se2.58/TiO2 Photodetectors
by Zhe Kang, Zhi Zheng, Helin Wei, Zhi Zhang, Xinyu Tan, Lun Xiong, Tianyou Zhai and Yihua Gao
Sensors 2019, 19(5), 1099; https://doi.org/10.3390/s19051099 - 4 Mar 2019
Cited by 31 | Viewed by 5341
Abstract
The monotonic work function of graphene makes it difficult to meet the electrode requirements of every device with different band structures. Two-dimensional (2D) transition metal carbides (TMCs), such as carbides in MXene, are considered good candidates for electrodes as a complement to graphene. [...] Read more.
The monotonic work function of graphene makes it difficult to meet the electrode requirements of every device with different band structures. Two-dimensional (2D) transition metal carbides (TMCs), such as carbides in MXene, are considered good candidates for electrodes as a complement to graphene. Carbides in MXene have been used to make electrodes for use in devices such as lithium batteries. However, the small lateral size and thermal instability of carbides in MXene, synthesized by the chemically etching method, limit its application in optoelectronic devices. The chemical vapor deposition (CVD) method provides a new way to obtain high-quality ultrathin TMCs without functional groups. However, the TMCs film prepared by the CVD method tends to grow vertically during the growth process, which is disadvantageous for its application in the transparent electrode. Herein, we prepared an ultrathin Mo2C—graphene (Mo2C—Gr) hybrid film by CVD to solve the above problem. The work function of Mo2C—Gr is between that of graphene and a pure Mo2C film. The Mo2C—Gr hybrid film was selected as a transparent hole-transporting layer to fabricate novel Mo2C—Gr/Sb2S0.42Se2.58/TiO2 two-sided photodetectors. The Mo2C—Gr/Sb2S0.42Se2.58/TiO2/fluorine-doped tin oxide (FTO) device could detect light from both the FTO side and the Mo2C—Gr side. The device could realize a short response time (0.084 ms) and recovery time (0.100 ms). This work is believed to provide a powerful method for preparing Mo2C—graphene hybrid films and reveals its potential applications in optoelectronic devices. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
Show Figures

Figure 1

Figure 1
<p>The schematic diagrams of Mo<sub>2</sub>C crystal growth and the novel Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub>/FTO vertical heterostructure photodetector. (<b>a</b>) Schematic diagram of the chemical vapor deposition (CVD) method to grown Mo<sub>2</sub>C—graphene. (<b>b</b>) The schematic diagram of the transfer of the Mo<sub>2</sub>C—Gr layer. (<b>c</b>) The schematic diagram of the self-driven two-sided photodetector.</p>
Full article ">Figure 2
<p>The schematic diagrams (<b>upper</b>) and SEM images (<b>bottom</b>) of Mo<sub>2</sub>C crystal and Mo<sub>2</sub>C—Gr growth under the various ratios of methane to hydrogen. (<b>a</b>) 0, (<b>b</b>) 1:600, (<b>c</b>) 1:100.</p>
Full article ">Figure 3
<p>The schematic diagram and optical image showing the growth of Mo<sub>2</sub>C and Mo<sub>2</sub>C—Gr under various ratios of methane to hydrogen and various growth times. (<b>a</b>) The schematic diagram of the distribution of Mo<sub>2</sub>C crystals on the Cu/Mo substrate at the ratio of 1:600. (<b>b</b>–<b>e</b>) The distribution of Mo<sub>2</sub>C crystals on the Cu/Mo substrate with a growth time of 10, 30, 60 and 120 min, respectively. (<b>f</b>) The schematic diagram of the distribution of the grown Mo<sub>2</sub>C—Gr on the Cu/Mo substrate at the ratio of 1:100. (<b>g</b>–<b>j</b>) The growth of Mo<sub>2</sub>C—Gr and distribution of Mo<sub>2</sub>C crystals on the Cu/Mo substrate with a growth time of 10, 30, 60 and 120 min, respectively.</p>
Full article ">Figure 4
<p>The characterization analysis of Mo<sub>2</sub>C. (<b>a</b>) The element distribution of Mo<sub>2</sub>C on Cu. (<b>b</b>) The transmission electron microscope (TEM) image. (<b>c</b>) The high-resolution TEM image of Mo<sub>2</sub>C. The inset image is a magnified image of a selected region. (<b>d</b>) The selected area electron diffraction (SAED) pattern along the [100] zone axis.</p>
Full article ">Figure 5
<p>The spectral analysis of Mo2C and Mo<sub>2</sub>C—Gr. (<b>a</b>) The Raman spectrum of Mo<sub>2</sub>C. The inset is the optical image of Mo<sub>2</sub>C. (<b>b</b>) The X-ray diffraction patterns of Mo<sub>2</sub>C. (<b>c</b>) The UPS spectra of graphene (<b>black</b>) and Mo<sub>2</sub>C—Gr (<b>red</b>) on n-type silicon. The inset of the left panel shows the magnified region from 18 eV to 15 eV.</p>
Full article ">Figure 6
<p>The optoelectronic characteristics of the Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub> photodetector. (<b>a</b>) The schematic diagram of photodetectors. (<b>b</b>) The energy band diagram of Mo<sub>2</sub>C—Gr, Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>, TiO<sub>2</sub> and FTO in the photodetector. (<b>c</b>) Dark current-voltage of Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub> photodetectors. (<b>d</b>) Current-voltage curves of Mo<sub>2</sub>C/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub>, Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub> and Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub> photodetectors, respectively, under 1.5 G illumination (100 mW/cm<sup>−2</sup>).</p>
Full article ">Figure 7
<p>The detection performance of the Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub>/FTO photodetector under illumination from the Mo<sub>2</sub>C—Gr side. (<b>a</b>) The current-time curves of the photodetector under a 2 mW/cm<sup>2</sup> illumination with a wavelength of 400 nm to 1000 nm at 0 bias. (<b>b</b>) Self-powered photoresponse under an illumination of 650 nm light. (<b>c</b>) At various bias voltages, the photoresponse under a 2.5 mW/cm<sup>2</sup> illumination with 650 nm wavelength. (<b>d</b>) The current-time curves of the photodetector in atmosphere and vacuum under a 2 mW/cm<sup>2</sup> illumination of 650 nm wavelength light at 0 bias. (<b>e</b>) Photoresponse at various temperatures. (<b>f</b>) The voltage response and recovery time of the photodetector.</p>
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<p>The impedance analysis of the photodetector. (<b>a</b>) The schematic equivalent circuit diagram of the Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub>/FTO photodetector. (<b>b</b>,<b>c</b>) Nyquist diagram and frequency-dependent relationships of the Mo<sub>2</sub>C—Gr/Sb<sub>2</sub>S<sub>0.42</sub>Se<sub>2.58</sub>/TiO<sub>2</sub>/FTO photodetector in the dark (<b>black</b>) and under illumination (<b>red</b>), respectively. CPE: constant-phase element.</p>
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11 pages, 2619 KiB  
Article
A Sprayed Graphene Pattern-Based Flexible Strain Sensor with High Sensitivity and Fast Response
by Wei Xu, Tingting Yang, Feng Qin, Dongdong Gong, Yijia Du and Gang Dai
Sensors 2019, 19(5), 1077; https://doi.org/10.3390/s19051077 - 3 Mar 2019
Cited by 31 | Viewed by 6208
Abstract
Flexible strain sensors have a wide range of applications in biomedical science, aerospace industry, portable devices, precise manufacturing, etc. However, the manufacturing processes of most flexible strain sensors previously reported have usually required high manufacturing costs and harsh experimental conditions. Besides, research interests [...] Read more.
Flexible strain sensors have a wide range of applications in biomedical science, aerospace industry, portable devices, precise manufacturing, etc. However, the manufacturing processes of most flexible strain sensors previously reported have usually required high manufacturing costs and harsh experimental conditions. Besides, research interests are often focused on improving a single attribute parameter while ignoring others. This work aims to propose a simple method of manufacturing flexible graphene-based strain sensors with high sensitivity and fast response. Firstly, oxygen plasma treats the substrate to improve the interfacial interaction between graphene and the substrate, thereby improving device performance. The graphene solution is then sprayed using a soft PET mask to define a pattern for making the sensitive layer. This flexible strain sensor exhibits high sensitivity (gauge factor ~100 at 1% strain), fast response (response time: 400–700 μs), good stability (1000 cycles), and low overshoot (<5%) as well. Those processes used are compatible with a variety of complexly curved substrates and is expected to broaden the application of flexible strain sensors. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
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Graphical abstract

Graphical abstract
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<p>Schematic illustration of graphene flexible strain sensor fabrication process.</p>
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<p>(<b>a</b>–<b>b</b>) Photograph and schematic images of the strain sensor using polydimethylsiloxane (PDMS) substrate; (<b>c</b>) optical image of PDMS substrate treated by oxygen plasma after 10 pre-stretch cycles (0–10% strain range); (<b>d</b>) SEM image of the graphene film on PDMS after 500 loading and unloading (0–1% strain range) cycles.</p>
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<p>The flexible strain sensor shows high performances. (<b>a</b>) Resistance changes of the strain sensor under 0–1% strain at the 1000th cycle of loading/unloading test; (<b>b</b>) the response time is within the range of 400 to 700 μs under the impact acceleration of 500 g; (<b>c</b>) the measurement of the creep characteristic, blue curve represents the strain measurement curve of sensor with time and the red one is the actual ΔR/R0 measurement curve; (<b>d</b>) ΔR/R0 value of the sensor at strain 1% under 1000 cyclic measurement of 0–1% strain, sampling every 50 cycles.</p>
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<p>The finite element method (FEM) simulation analysis of the slip of graphene sheets when the substrate is under uniaxial stretching. (<b>a</b>) The strain distribution of the sensor (<b>b</b>) and (<b>c</b>) is the strain simulation of graphene sheet and PDMS substrate respectively; (<b>d</b>) the relative movement of the graphene sheet with substrate before stretching and (<b>e</b>) after stretching; (<b>f</b>) the schematic microscopic stacking of graphene sheets before stretching and (<b>g</b>) after stretching.</p>
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<p>The sensors are applicable to detect complex curved surfaces. (<b>a</b>) Illustration of micro-strain detection on the complex model surface under light illumination; (<b>b</b>) The change of ΔR/R0 of sensor with illumination time.</p>
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8 pages, 2329 KiB  
Article
Sensitivity Enhancement of a Surface Plasmon Resonance with Tin Selenide (SnSe) Allotropes
by Xiaoyu Dai, Yanzhao Liang, Yuting Zhao, Shuaiwen Gan, Yue Jia and Yuanjiang Xiang
Sensors 2019, 19(1), 173; https://doi.org/10.3390/s19010173 - 5 Jan 2019
Cited by 58 | Viewed by 5359
Abstract
Single layers of tin selenide (SnSe), which have a similar structure as graphene and phosphorene, also show excellent optoelectronic properties, and have received much attention as a two-dimensional (2D) material beyond other 2D material family members. Surface plasmon resonance (SPR) sensors based on [...] Read more.
Single layers of tin selenide (SnSe), which have a similar structure as graphene and phosphorene, also show excellent optoelectronic properties, and have received much attention as a two-dimensional (2D) material beyond other 2D material family members. Surface plasmon resonance (SPR) sensors based on three monolayer SnSe allotropes are investigated with the transfer matrix method. The simulated results have indicated that the proposed SnSe-containing biochemical sensors are suitable to detect different types of analytes. Compared with the conventional Ag-only film biochemical sensor whose sensitivity is 116°/RIU, the sensitivities of these SnSe-based biochemical sensors containing α-SnSe, δ-SnSe, ε-SnSe, were obviously increased to 178°/RIU, 156°/RIU and 154°/RIU, respectively. The diverse biosensor sensitivities achieved with these three SnSe allotropes suggest that these 2D materials can adjust SPR sensor properties. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
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<p>Sketch diagram of the SPR biochemical sensor made using three types of SnSe allotropes to enhance the sensitivity.</p>
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<p>Measured refractive indices for the three SnSe allotropes [<a href="#B22-sensors-19-00173" class="html-bibr">22</a>].</p>
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<p>Variation of reflectivity with incidence angle for (<b>a</b>) the conventional biochemical sensor based on a simplex Ag film, (<b>b</b>–<b>d</b>) are the proposed biochemical sensors with α-SnSe, δ-SnSe, ε-SnSe, respectively.</p>
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<p>(<b>a</b>) Change of reflectivity relative to incidence angles, (<b>b</b>) the electric field distributions of the biochemical sensors proposed by SnSe from <span class="html-italic">n<sub>s</sub></span> = 1.33 to <span class="html-italic">n<sub>s</sub></span> = 1.37, (<b>c</b>) incidence angles of three SnSe allotropes from <span class="html-italic">n<sub>s</sub></span> = 1.33 to <span class="html-italic">n<sub>s</sub></span> = 1.37, (<b>d</b>) the variation of the sensitivities of the proposed biochemical sensors with different number of SnSe layers with the refractive indices of the sensing medium.</p>
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<p>(<b>a</b>) Change of the reflectance of α-SnSe biosensor, (<b>b</b>) variation of sensitivity with respect to different numbers of SnSe allotrope layers, (<b>c</b>) change of resonance angle with respect to different layer numbers of SnSe allotropes, (<b>d</b>) the electric field distribution for the proposed SPR sensor.</p>
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Review

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27 pages, 7173 KiB  
Review
Structure-Property Relationships in Graphene-Based Strain and Pressure Sensors for Potential Artificial Intelligence Applications
by Zewei Luo, Xiaotong Hu, Xiyue Tian, Chen Luo, Hejun Xu, Quanling Li, Qianhao Li, Jian Zhang, Fei Qiao, Xing Wu, V. E. Borisenko and Junhao Chu
Sensors 2019, 19(5), 1250; https://doi.org/10.3390/s19051250 - 12 Mar 2019
Cited by 77 | Viewed by 10472
Abstract
Wearable electronic sensing devices are deemed to be a crucial technology of smart personal electronics. Strain and pressure sensors, one of the most popular research directions in recent years, are the key components of smart and flexible electronics. Graphene, as an advanced nanomaterial, [...] Read more.
Wearable electronic sensing devices are deemed to be a crucial technology of smart personal electronics. Strain and pressure sensors, one of the most popular research directions in recent years, are the key components of smart and flexible electronics. Graphene, as an advanced nanomaterial, exerts pre-eminent characteristics including high electrical conductivity, excellent mechanical properties, and flexibility. The above advantages of graphene provide great potential for applications in mechatronics, robotics, automation, human-machine interaction, etc.: graphene with diverse structures and leverages, strain and pressure sensors with new functionalities. Herein, the recent progress in graphene-based strain and pressure sensors is presented. The sensing materials are classified into four structures including 0D fullerene, 1D fiber, 2D film, and 3D porous structures. Different structures of graphene-based strain and pressure sensors provide various properties and multifunctions in crucial parameters such as sensitivity, linearity, and hysteresis. The recent and potential applications for graphene-based sensors are also discussed, especially in the field of human motion detection. Finally, the perspectives of graphene-based strain and pressure sensors used in human motion detection combined with artificial intelligence are surveyed. Challenges such as the biocompatibility, integration, and additivity of the sensors are discussed as well. Full article
(This article belongs to the Special Issue Two-Dimensional Materials Based Sensors)
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<p>Trends in the development of graphene-based sensors and graphene-based strain and pressure sensors. The data is from the Web of Science. Hundreds of papers have been published, and a selected subset is represented since 2008 [<a href="#B36-sensors-19-01250" class="html-bibr">36</a>,<a href="#B37-sensors-19-01250" class="html-bibr">37</a>,<a href="#B38-sensors-19-01250" class="html-bibr">38</a>,<a href="#B39-sensors-19-01250" class="html-bibr">39</a>,<a href="#B40-sensors-19-01250" class="html-bibr">40</a>,<a href="#B41-sensors-19-01250" class="html-bibr">41</a>,<a href="#B42-sensors-19-01250" class="html-bibr">42</a>,<a href="#B43-sensors-19-01250" class="html-bibr">43</a>,<a href="#B44-sensors-19-01250" class="html-bibr">44</a>,<a href="#B45-sensors-19-01250" class="html-bibr">45</a>,<a href="#B46-sensors-19-01250" class="html-bibr">46</a>,<a href="#B47-sensors-19-01250" class="html-bibr">47</a>,<a href="#B48-sensors-19-01250" class="html-bibr">48</a>,<a href="#B49-sensors-19-01250" class="html-bibr">49</a>,<a href="#B50-sensors-19-01250" class="html-bibr">50</a>,<a href="#B51-sensors-19-01250" class="html-bibr">51</a>,<a href="#B52-sensors-19-01250" class="html-bibr">52</a>]. Copyright 2009, American Association for the Advancement of Science; Copyright 2011, American Association for the Advancement of Science; Copyright 2008, Wiley; Copyright 2012, Nature Publishing Group; Copyright 2013, Nature Publishing Group; Copyright 2013, Nature Publishing Group; Copyright 2013, Wiley; Copyright 2013, Nature Publishing Group; Copyright 2014, Wiley; Copyright 2015, Nature Publishing Group; Copyright 2015, Wiley; Copyright 2016, American Association for the Advancement of Science; Copyright 2016, American Association for the Advancement of Science; Copyright 2016, American Association for the Advancement of Science; Copyright 2017, Wiley; Copyright 2017, Wiley.</p>
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<p>Overview of the graphene-based strain and pressure sensors in the various fabrications of the structures and wide range of potential applications [<a href="#B76-sensors-19-01250" class="html-bibr">76</a>,<a href="#B77-sensors-19-01250" class="html-bibr">77</a>,<a href="#B78-sensors-19-01250" class="html-bibr">78</a>,<a href="#B79-sensors-19-01250" class="html-bibr">79</a>]. Copyright 2015, Wiley; Copyright 2014, Nature Publishing Group; Copyright 2016, Nature Publishing Group; Copyright 2017, Nature Publishing Group; Copyright Google Images.</p>
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<p>Schematic illustrations of the transduction methods: (<b>a</b>) resistive, (<b>b</b>) capacitive, and (<b>c</b>) piezoelectric [<a href="#B79-sensors-19-01250" class="html-bibr">79</a>,<a href="#B96-sensors-19-01250" class="html-bibr">96</a>,<a href="#B105-sensors-19-01250" class="html-bibr">105</a>,<a href="#B106-sensors-19-01250" class="html-bibr">106</a>]. Copyright 2015, American Association for the Advancement of Science.</p>
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<p>Fabrication processes and structural characterization of 0D structure strain sensors [<a href="#B136-sensors-19-01250" class="html-bibr">136</a>]. Copyright 2018, Wiley. (<b>a</b>) The structure diagram for 0D strain sensors based on 0D fullerene structure before stretch. (<b>b</b>) The structure diagram of the 0D strain based on 0D fullerene structure after stretch. (<b>c</b>) The preparation process of this 0D structure strain sensor. (<b>d</b>) Schematic illustration of a sensing mechanism for films under stretching. (<b>e</b>) Surface SEM images for sensing films at 0% applied strains. (<b>f</b>) Surface SEM images for sensing films at 60% applied strains. (<b>g</b>) The Gauge factor (GF) and linear behavior of the strain sensor. (<b>h</b>) The hysteresis of the strain sensor.</p>
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<p>Fabrication processes and structural characterization of 1D structure strain and pressure sensors [<a href="#B34-sensors-19-01250" class="html-bibr">34</a>,<a href="#B163-sensors-19-01250" class="html-bibr">163</a>]. Copyright 2017, American Chemical Society; Copyright 2015, American Chemical Society. (<b>a</b>) The structure diagram of the 1D strain and pressure sensors based on 1D graphene structure before the strain and pressure. (<b>b</b>) The structure diagram of the 1D strain and pressure sensors based on 1D graphene structure after the strain and pressure. (<b>c</b>) The fabrication process of a 1D structure pressure sensor and its representations. (<b>d</b>) The sensitivity and linearity of this pressure sensor. (<b>e</b>) The hysteresis of this pressure sensor. (<b>f</b>) The fabrication process of a 1D structure strain sensor and its representations. (<b>g</b>) The sensitivity and linearity of three types of strain sensors. (<b>h</b>) The function of the Polydimethylsiloxane (PDMS) coating.</p>
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<p>Fabrication processes and structural characterization of 2D structure strain and pressure sensors [<a href="#B111-sensors-19-01250" class="html-bibr">111</a>,<a href="#B155-sensors-19-01250" class="html-bibr">155</a>]. Copyright 2017, Elsevier; Copyright 2017, American Chemical Society. (<b>a</b>) The structure diagram of the 2D strain and pressure sensors based on 2D graphene layers structure before the strain and pressure. (<b>b</b>) The structure diagram of the 2D strain and pressure sensors based on 2D graphene layers structure after the strain and pressure. (<b>c</b>) The fabrication process of a 2D structure pressure sensor and its representations. (<b>d</b>) The sensitivity and linearity of this pressure sensor. (<b>e</b>) The response time and hysteresis of this pressure sensor. (<b>f</b>) The fabrication process of a 2D structure pressure sensor and its representations. (<b>g</b>) The sensitivity and linearity of this pressure sensor (<b>h</b>) The response time and hysteresis of this pressure sensor.</p>
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<p>Fabrication processes and structural characterization of 3D structure strain and pressure sensors [<a href="#B142-sensors-19-01250" class="html-bibr">142</a>,<a href="#B153-sensors-19-01250" class="html-bibr">153</a>]. Copyright 2017, Wiley; Copyright 2018, Elsevier. (<b>a</b>) The structure diagram of the 3D strain and pressure sensors based on 3D graphene sponge structure before the strain and pressure. (<b>b</b>) The structure diagram of the 3D strain and pressure sensors based on 3D graphene sponge structure after the strain and pressure. (<b>c</b>) The fabrication process of a 3D structure strain and pressure sensor and its representations. (<b>d</b>) The sensitivity and linearity of this strain and pressure sensor. (<b>e</b>) The repeatability and hysteresis of this strain and pressure sensor. (<b>f</b>) The fabrication process of a 3D structure strain and pressure sensor and its representations. (<b>g</b>) The linearity and hysteresis of this strain and pressure sensor. (<b>h</b>) The sensitivity and hysteresis of this pressure sensor.</p>
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<p>Monitoring in real-time of the graphene-based strain and pressure sensor for human motion detection [<a href="#B179-sensors-19-01250" class="html-bibr">179</a>,<a href="#B180-sensors-19-01250" class="html-bibr">180</a>,<a href="#B181-sensors-19-01250" class="html-bibr">181</a>]. The insets figures source from [<a href="#B185-sensors-19-01250" class="html-bibr">185</a>,<a href="#B186-sensors-19-01250" class="html-bibr">186</a>,<a href="#B187-sensors-19-01250" class="html-bibr">187</a>,<a href="#B188-sensors-19-01250" class="html-bibr">188</a>,<a href="#B189-sensors-19-01250" class="html-bibr">189</a>,<a href="#B190-sensors-19-01250" class="html-bibr">190</a>,<a href="#B191-sensors-19-01250" class="html-bibr">191</a>,<a href="#B192-sensors-19-01250" class="html-bibr">192</a>]. Copyright 2015, Wiley; Copyright 2017, Nature Publishing Group; Copyright 2018, American Chemical Society. (<b>a</b>) A graphene-based strain sensor applied to detect throat movement on human skin. (<b>b</b>) A graphene-based strain sensor applied to detect cheek movement on human skin. (<b>c</b>) A graphene-based strain sensor applied to detect wrist pulse on human skin. (<b>d</b>) A graphene-based pressure sensor applied to detect chest pulse on human skin. (<b>e</b>) A graphene-based pressure sensor applied to detect foot pressure on human skin. (<b>f</b>) A graphene-based strain sensor applied to distinguish six different hand positions.</p>
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<p>(<b>a</b>) A smart electronic skin on a hand, showing robotics [<a href="#B195-sensors-19-01250" class="html-bibr">195</a>]. Copyright 2018, Nature Publishing Group. (<b>b</b>) Mechatronics in artificial intelligence [<a href="#B196-sensors-19-01250" class="html-bibr">196</a>]. Copyright 2018, American Chemical Society. (<b>c</b>) Interaction demonstration of smart recognition of the Braille diagram. The Braille diagram is shown on the left corner. Pressure distribution of braille ‘E’ ‘C’ ‘N’ ‘U’ of the Gr-GO heterostructure film pressure sensor array is shown on the right. The pressure array wirelessly communicates with external devices via Bluetooth. (<b>d</b>) A smart electronic prosthetic hand, which can be used in human-machine interaction [<a href="#B197-sensors-19-01250" class="html-bibr">197</a>]. Copyright 2014, Nature Publishing Group.</p>
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