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Synthesis of 2D In2Se3: (a) Crystal structure of α-In2Se3. (b) Schematic illustration of the CVD growth process of 2D In2Se3 on to mica substrates. (c) Optical microscope image of as-grown triangular In2Se3 nanosheets on mica substrate. (d)-(f) AFM image and corresponding height profile of In2Se3 nanosheets with ~1-3 nm thickness. The as-synthesized 2D In 2 Se 3 nanosheets on transparent mica substrates are first identified by different optical contrast using optical microscopy. Figure 1c shows a typical optical image of discrete, triangular shaped In 2 Se 3 nanosheets on mica substrate with 100 μm lateral dimensions . The identical orientation observed here strongly indicates the nature of van der Waals epitaxy of In 2 Se 3 crystals on the mica substrate. The different optical contrasts for In 2 Se 3 crystals reflect different thickness. The thickness and morphology of the as synthesized atomically thin In 2 Se 3 nanosheets are measured by atomic force microscopy (AFM). Figure 1d-f show three typical AFM images of 2D In 2 Se 3 nanosheets with uniform thicknesses of 0.8, 1.9 and 3.1 nm, corresponding to mono-, bi- and trilayer , respectively. The 2D In 2 Se 3 nanosheets have flat surface, regular shape and sharp edges. The morphology  

Synthesis of 2D In2Se3: (a) Crystal structure of α-In2Se3. (b) Schematic illustration of the CVD growth process of 2D In2Se3 on to mica substrates. (c) Optical microscope image of as-grown triangular In2Se3 nanosheets on mica substrate. (d)-(f) AFM image and corresponding height profile of In2Se3 nanosheets with ~1-3 nm thickness. The as-synthesized 2D In 2 Se 3 nanosheets on transparent mica substrates are first identified by different optical contrast using optical microscopy. Figure 1c shows a typical optical image of discrete, triangular shaped In 2 Se 3 nanosheets on mica substrate with 100 μm lateral dimensions . The identical orientation observed here strongly indicates the nature of van der Waals epitaxy of In 2 Se 3 crystals on the mica substrate. The different optical contrasts for In 2 Se 3 crystals reflect different thickness. The thickness and morphology of the as synthesized atomically thin In 2 Se 3 nanosheets are measured by atomic force microscopy (AFM). Figure 1d-f show three typical AFM images of 2D In 2 Se 3 nanosheets with uniform thicknesses of 0.8, 1.9 and 3.1 nm, corresponding to mono-, bi- and trilayer , respectively. The 2D In 2 Se 3 nanosheets have flat surface, regular shape and sharp edges. The morphology  

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Two-dimensional (2D) layered semiconductors have emerged as a highly attractive class of materials for flexible and wearable strain sensor-centric devices such as electronic-skin (e-skin). This is primarily due to their dimensionality, excellent mechanical flexibility and unique electronic properties. However, the lack of effective and low-cost met...

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... selenide (In 2 Se 3 ) is a III−VI group layered chal- cogenide compound with a direct bandgap of 1.36 eV, 19 and attracts strong interest for applications in photovolta- ic and optoelectronics devices 20-21 , phase change memory 22-23 and ionic batteries 24 . Figure 1a shows the structure of one of the major polymorphs of In 2 Se 3 , name- ly α-In 2 Se 3 . It is composed of vertically stacked Se-In-Se- In-Se quintuple layers, held together by weak van der Waals forces. ...
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... strategy for CVD synthesis of 2D In 2 Se 3 nanosheets is schematically illustrated in Figure 1b. The In 2 Se 3 nanosheets are epitaxially grown on mica substrates at 660 °C by CVD using powders of Selenium (Se) and Indi- um oxide (In 2 O 3 ) as precursors and H 2 /Ar mixture as the carrier gas (see experimental section for details in sup- porting information). ...
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... as-synthesized 2D In 2 Se 3 nanosheets on transpar- ent mica substrates are first identified by different optical contrast using optical microscopy. Figure 1c shows a typi- cal optical image of discrete, triangular shaped In 2 Se 3 nanosheets on mica substrate with 100 μm lateral dimen- sions. The identical orientation observed here strongly indicates the nature of van der Waals epitaxy of In 2 Se 3 crystals on the mica substrate. ...
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... thickness and morphology of the as synthesized atomical- ly thin In 2 Se 3 nanosheets are measured by atomic force microscopy (AFM). Figure 1d-f show three typical AFM images of 2D In 2 Se 3 nanosheets with uniform thicknesses of 0.8, 1.9 and 3.1 nm, corresponding to mono-, bi- and tri- layer, respectively. The 2D In 2 Se 3 nanosheets have flat surface, regular shape and sharp edges. ...
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... 2D In 2 Se 3 nanosheets have flat surface, regular shape and sharp edges. The morphology of 2D In 2 Se 3 nanosheets on mica substrates is notably dif- ferent from that of the irregular In 2 Se 3 nanocrystals that we synthesize on SiO 2 substrate (see Figure S1). We at- tribute this variation to surface electronic structure of these two substrates (see additional discussions in sup- porting information and Figure S1). ...
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... morphology of 2D In 2 Se 3 nanosheets on mica substrates is notably dif- ferent from that of the irregular In 2 Se 3 nanocrystals that we synthesize on SiO 2 substrate (see Figure S1). We at- tribute this variation to surface electronic structure of these two substrates (see additional discussions in sup- porting information and Figure S1). The crystal structure of the as-grown In 2 Se 3 is charac- terized by Raman spectroscopy. ...

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... The peaks at 2θ of 9.3°, 18.7°, 28.3°, 47.9°, and 58.3° correspond to the (003), (006), (009), (0015), and (1016) planes of the β-In 2 Se 3 crystal (PDF#35-1056), respectively [30]. Besides, the strong peak at 14.3°i n the XRD patterns is the (002) lattice plane of MoS 2 crystal (PDF#37-1492) [31]. The vertical heterostructure inherits all the peaks from both In 2 Se 3 and MoS 2 , indicating the successful coupling of In 2 Se 3 and MoS 2 (basic characterizations of fewlayered In 2 Se 3 grown on mica sheets are shown in Fig. S2 in the ESM). ...
... PL quenching in nL (n ≥ 3) In 2 Se 3 /MoS 2 heterostructures can be attributed to the type-II band alignment between In 2 Se 3 and monolayer MoS 2 ( Fig. 2(d)). As previously reported, both the conduction band minimum and the valance band maximum of In 2 Se 3 are lower than those of MoS 2 [31]. Under the laser irradiation, many electron-hole pairs are generated in In 2 Se 3 and MoS 2 layer. ...
... The excited electrons in MoS 2 layer transfer from the conduction band of MoS 2 to that of In 2 Se 3 . Oppositely, holes transfer from the valence band of In 2 Se 3 to that of MoS 2 [31]. As a result, excitons tend to separate and electron-hole recombination rarely occurs in MoS 2 , leading to the significant PL quenching. ...
Article
The layer-dependent properties are still unclarified in two-dimensional (2D) vertical heterostructures. In this study, we layer-by-layer deposited semimetal β-In2Se3 on monolayer MoS2 to form vertical β-In2Se3/MoS2 heterostructures by chemical vapor deposition. The defect-mediated nucleation mechanism induces β-In2Se3 nanosheets to grow on monolayer MoS2, and the layer number of stacked β-In2Se3 can be precisely regulated from 1 layer (L) to 13 L by prolonging the growth time. The β-In2Se3/MoS2 heterostructures reveal tunable type-II band alignment arrangement by altering the layer number of β-In2Se3, which optimizes the internal electron transfer. Meanwhile, the edge atomic structure of β-In2Se3 stacking on monolayer MoS2 shows the reconstruction derived from large lattice mismatch (∼ 29%), and the presence of β-In2Se3 also further increases the electrical conductivity of β-In2Se3/MoS2 heterostructures. Attributed to abundant layer-dependent edge active sites, edge reconstruction, improved hydrophilicity, and high electrical conductivity of β-In2Se3/MoS2 heterostructures, the edge of β-In2Se3/MoS2 heterostructures exhibits excellent electrocatalytic hydrogen evolution performance. Lower onset potential and smaller Tafel slope can be observed at the edge of monolayer MoS2 coupled with 13-L β-In2Se3. Hence, the outstanding conductive layers coupled with edge reconstruction in 2D vertical heterostructures play decisive roles in the optimization of electron energy levels and improvement of layer-dependent catalytic performance.
... In particular, novel skininterface devices such as E-skins, textile sensors, and hydrogel sensors have attracted widespread attention due to their potential for applications in human body motion monitoring 12 , disease treatment 13 , drug delivery 14 , blood glucose monitoring 15 , and blood pressure analysis 16 . For instance, in the past years, various attempts have been made to design signal collectors in skininterface devices for long-term body motion monitoring during natural daily activities, which would combine lightweight, flexibility, high permeability and imperceptibility [17][18][19][20][21][22][23][24][25][26][27][28][29] . The recent emergence of wearable motion sensors based on liquid metals 30 , hydrogels 31 , graphene aerogels 32 , functional fibers 33 , and piezoelectric materials 34,35 have enabled flexible devices to be applied in body motion monitoring and short-term healthcare. ...
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New-generation human body motion sensors for wearable electronics and intelligent medicine are required to comply with stringent requirements in terms of ultralight weight, flexibility, stability, biocompatibility, and extreme precision. However, conventional sensors are hard to fulfill all these criteria due to their rigid structure, high-density sensing materials used as the constituents, as well as hermetical and compact assembly strategy. Here, we report an ultralight sensing material based on radial anisotropic porous silver fiber (RAPSF), which has been manufactured by phase separation and temperature-controlled grain growth strategy on a modified blow-spinning system. The resistance of RAPSF could be dynamically adjusted depending on the deflected shape. Furthermore, an all-fiber motion sensor (AFMS) with an ultra-low density of 68.70 mg cm−3 and an overall weigh of 7.95 mg was fabricated via layer-by-layer assembly. The sensor exhibited outstanding flexibility, breathability, biocompatibility, and remarkable body motion recognition ability. Moreover, the AFMS was shown to have great potential as an artificial intelligence throat sensor for throat state identification at the accuracy above 85%, allowing one to spot the early onset of the viral throat illness.
... Indium selenide (In 2 Se 3 ) is a member of Group III-VI layered compounds. A single cell of In 2 Se 3 comprises three quintuple layers (QLs) connected by weak van der Waals force, and each QL is composed of five-atomic-layer (Se-In-Se-In-Se) stacked along the c-axis by a strong covalent bond [16], as shown in Fig. 1a. According to earlier reports, In 2 Se 3 has several crystal phases, a [17][18][19][20], b [21], c [22], d [23], j [24], a 0 [23]and b 0 [23], and the crystal structures as well as physical and chemical properties of these phases are significantly different. ...
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Indium selenide (In2Se3) is an attractive layered semiconductor material with promising optoelectronic and piezoelectric applications. In this work, single-crystalline α-In2Se3 nanobelts are synthesized using a catalyst-free chemical vapor deposition method through the direct selenization of In2O3 powders. The as-grown nanobelts are single crystals with high crystallinity and regular outer shapes. The piezoelectric response d33 of a 50 nm-thick α-In2Se3 nanobelt is, for the first time, determined to be approximately 1.6 pm·V⁻¹. The photodetector using the In2Se3 nanobelts exhibits high photoresponsivity of 66.6 A·W⁻¹ at 442 nm and high stability under an atmospheric environment. This work provides a new route for the development of α-In2Se3 nanobelts as promising candidates for low-cost, nanoscale actuators, sensors, and detectors.
... In these reports, gauge factors as high as ∼600 42 were obtained for CVD-grown nanographene films. Similarly, strain sensors based on CVD-grown films 44,45 and nanocomposites 46 of other 2D materials have also been demonstrated. ...
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Printed strain sensors will be important in applications such as wearable devices, which monitor breathing and heart function. Such sensors need to combine high sensitivity and low resistance with other factors such as cyclability, low hysteresis, and minimal frequency/strain-rate dependence. Although nanocomposite sensors can display a high gauge factor (G), they often perform poorly in the other areas. Recently, evidence has been growing that printed, polymer-free networks of nanoparticles, such as graphene nanosheets, display very good all-round sensing performance, although the details of the sensing mechanism are poorly understood. Here, we perform a detailed characterization of the thickness dependence of piezoresistive sensors based on printed networks of graphene nanosheets. We find both conductivity and gauge factor to display percolative behavior at low network thickness but bulk-like behavior for networks above ∼100 nm thick. We use percolation theory to derive an equation for gauge factor as a function of network thickness, which well-describes the observed thickness dependence, including the divergence in gauge factor as the percolation threshold is approached. Our analysis shows that the dominant contributor to the sensor performance is not the effect of strain on internanosheet junctions but the strain-induced modification of the network structure. Finally, we find these networks display excellent cyclability, hysteresis, and frequency/strain-rate dependence as well as gauge factors as high as 350.
... The same TEM investigations were also carried out on α-In 2 Se 3 flakes, as shown in Figures 5D-F. The lattice constant is measured to be about 0.35 nm, which corresponds to d-spacing (100) lattice planes of α-In 2 Se 3 (Ho et al., 2013;Zhou et al., 2015;Feng et al., 2016;Zhou et al., 2017;Tang et al., 2019). Figure 6 shows the PL spectra of the γ-InSe flakes exfoliated from the as-grown crystals. ...
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The controlled growth of indium selenides has attracted considerable research interests in condensed matter physics and materials science yet remains a challenge due to the complexity of the indium–selenium phase diagram. Here, we demonstrate the successful growth of indium selenides in a controllable manner using the high-pressure and high-temperature growth technique. The γ-InSe and α-In 2 Se 3 crystals with completely different stoichiometries and stacking manner of atomic layers have been controlled grown by subtle tuning growth temperature, duration time, and growth pressure. The as-grown γ-InSe crystal features a semiconducting property with a prominent photoluminescence peak of ∼1.23 eV, while the α-In 2 Se 3 crystal is ferroelectric. Our findings could lead to a surge of interest in the development of the controlled growth of high-quality van der Waal crystals using the high-pressure and high-temperature growth technique and will open perspectives for further investigation of fascinating properties and potential practical application of van der Waal crystals.
... However, the size and the thickness of In 2 Se 3 nanosheets obtained by the ME method are uncontrollable, and this leads to further limiting the optical response. Additionally, the chemical vapor deposition (CVD) and the physical vapor deposition (PVD) methods were also successfully employed for fabricating In 2 Se 3 nanoflakes [43,44]. Compared with other methods, the PVD method could accurately control the thickness of In 2 Se 3 nanoflakes. ...
... The Raman result is displayed in Figure 1C. Apparently, three peaks at 107, 172, and 205 cm −1 are considered to be done by A 1 (LO + TO), A 1 (TO), and A 1 (LO) phonon modes of In 2 Se 3 [42], which evidently prove that we successfully synthesized the In 2 Se 3 nanoflakes [43,44]. Diffraction peaks including (004), (006), (106), (0010), (1115), and (1118) were detected, as shown in Figure 1D. ...
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The output power in ultrafast fiber lasers is usually limited due to the lack of a versatile saturable absorber with high damage threshold and large modulation depth. Here we proposed a more efficient strategy to improve the output energy of erbium-doped fiber laser based on indium selenide (In 2 Se 3 ) prepared by using the physical vapor deposition (PVD) method. Finally, stable mode-locked bright pulses and triple-wavelength dark–bright pulse pair generation were obtained successfully by adjusting the polarization state. The average output power and pulse energy were 172.4 mW/101 nJ and 171.3 mW/100 nJ, which are significantly improved compared with the previous work. These data demonstrate that the PVD-In 2 Se 3 can be a feasible nonlinear photonic material for high-power fiber lasers, which will pave a fresh avenue for the high-power fiber laser.
... A multitude of studies have been conducted to develop e cient strain sensors, and materials that respond to structural changes and exhibit such change have been mainly explored [1][2][3] . Strain sensors require great exibility and play an important role in a variety of applications such as electronic skin 4,5 and soft robots 6,7 . ...
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A strain sensor characterized by elasticity has recently been studied in various ways to be applied to monitoring humans or robots. Here, 4 types of 3D-printed auxetic lattice structures using thermoplastic polyurethane (TPU) as raw material were characterized: truss and honeycomb with positive Poisson's ratio and chiral truss and re-entrant with negative Poisson's ratio. Each structure was fabricated as a flexible and stable strain sensor by coating graphene through a dip-coating process. The fabricated auxetic structures have excellent strength, flexibility, and electrical conductivity desirable for a strain sensor and detect a constant change in resistance at a given strain. The 3D-printed auxetic lattice 4 type structures coated with CWPU/Graphene suggest potential applications of multifunctional strain sensors under deformation.
... 314 In another work, strain sensors produced from large-scale CVD-grown In 2 Se 3 exhibited two orders of magnitude higher sensitivity (gauge factor % 237) than conventional metal-based (gauge factor %1-5) and graphene-based strain (gauge factor % 2-4) sensors under similar uniaxial strain. 322 Additionally, the integrated strain sensor array, fabricated from the template-grown 2D In 2 Se 3 films, displayed a high spatial resolution of %500 lm in strain distribution, making this material platform highly attractive as e-skins for robotics and human body motion monitoring. ...
Article
The interest in two-dimensional and layered materials continues to expand, driven by the compelling properties of individual atomic layers that can be stacked and/or twisted into synthetic heterostructures. The plethora of electronic properties as well as the emergence of many different quasiparticles, including plasmons, polaritons, trions, and excitons with large, tunable binding energies that all can be controlled and modulated through electrical means, has given rise to many device applications. In addition, these materials exhibit both room-temperature spin and valley polarization, magnetism, superconductivity, piezoelectricity that are intricately dependent on the composition, crystal structure, stacking, twist angle, layer number, and phases of these materials. Initial results on graphene exfoliated from single bulk crystals motivated the development of wide-area, high purity synthesis and heterojunctions with atomically clean interfaces. Now by opening this design space to new synthetic two-dimensional materials “beyond graphene,” it is possible to explore uncharted opportunities in designing novel heterostructures for electrically tunable devices. To fully reveal the emerging functionalities and opportunities of these atomically thin materials in practical applications, this review highlights several representative and noteworthy research directions in the use of electrical means to tune these aforementioned physical and structural properties, with an emphasis on discussing major applications of beyond graphene 2D materials in tunable devices in recent years and an outlook of what is to come in the next decade.
... Various conductive nanomaterials (e.g., graphene, multi-walled carbon nanotubes (MWCNTs) and carbon black (CB) [4][5][6]) and supporting materials (e.g., polydimethy-lsiloxane (PDMS), PU foam, and Ecoflex [7]) have been utilized for flexible sensors. Flexible tensile sensors which convert the mechanical deformation into electrical signals are researched for high stretchability and flexibility and easily accessible systems [8][9][10]. Because of their fast response time, easy accessible mechanism and simple structure, the resistive pressure sensors are investigated intensively which convert the applied pressure to the changes in resistance [11,12]. ...
Article
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A flexible strain sensor which integrates both pressure sensing and tension sensing functions is demonstrated with an active layer comprising of polydimethy-lsiloxane (PDMS) elastomer, liquid crystal (LC), and multi-walled carbon nanotubes (MWCNTs). The introduction of LC improves the agglomeration of MWCNTs in PDMS and decreases Young’s modulus of flexible resistive sensors. The tension/pressure integrated resistive sensor not only shows a broad tensile sensing range of 140% strain but also shows a good sensitivity of the gauge factor, 40, with tensile force. Besides, the tension/pressure integrated resistive sensor also shows good linearity and sensitivity under pressure. The resistance of the pressure sensor increases as the applied pressure increases because of the decrease in the cross-sectional area of the path. The sensor also shows good hydrophobic properties which may help it to work under complex environment. The tension/pressure integrated sensor shows great promising applications in electronic skins and wearable devices.
... Piezoresistive crystals usually include silicon or other semiconductors. Various semiconductors with inherent piezoresistive effects, such as Si, CNT, graphene, α-In2Se3, MoS2, VO2, and PtSe2, are introduced into piezoresistive tactile sensors to realize tactile sensing via band structure changes under external strain [50][51][52][53][54]. Although the piezoresistive crystal possesses brittle and rigid characteristics, it can be integrated on a flexible substrate, such as polyimide materials, so as to realize flexible tactile sensing, miniaturization, and high-density integration [55]. ...
... Piezoresistive crystals usually include silicon or other semiconductors. Various semiconductors with inherent piezoresistive effects, such as Si, CNT, graphene, α-In 2 Se 3 , MoS 2 , VO 2 , and PtSe 2 , are introduced into piezoresistive tactile sensors to realize tactile sensing via band structure changes under external strain [50][51][52][53][54]. Although the piezoresistive crystal possesses brittle and rigid characteristics, it can be integrated on a flexible substrate, such as polyimide materials, so as to realize flexible tactile sensing, miniaturization, and high-density integration [55]. ...
Article
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Tactile sensors are an important medium for artificial intelligence systems to perceive their external environment. With the rapid development of smart robots, wearable devices, and human-computer interaction interfaces, flexible tactile sensing has attracted extensive attention. An overview of the recent development in high-performance tactile sensors used for smart systems is introduced. The main transduction mechanisms of flexible tactile sensors including piezoresistive, capacitive, piezoelectric, and triboelectric sensors are discussed in detail. The development status of flexible tactile sensors with high resolution, high sensitive, self-powered, and visual capabilities are focused on. Then, for intelligent systems, the wide application prospects of flexible tactile sensors in the fields of wearable electronics, intelligent robots, human-computer interaction interfaces, and implantable electronics are systematically discussed. Finally, the future prospects of flexible tactile sensors for intelligent systems are proposed.