Flexible electronics based on one-dimensional inorganicsemiconductor nanowires and two-dimensional transition metal dichalcogenides

Kang Chen,Junan Pan,Weinan Yin,Chiyu Ma,Longlu Wang

College of Electronic and Optical Engineering & College of Flexible Electronics (Future Technology),Nanjing University of Posts & Telecommunications(NUPT),Nanjing 210023,China

Keywords:Flexible electronics One-dimensional inorganic semiconductor nanowires Two-dimensional transition metal dichalcogenides Mechanical properties Flexible device applications

ABSTRACT Flexible electronics technology is considered as a revolutionary technology to unlock the bottleneck of traditional rigid electronics that prevalent for decades,thereby fueling the next-generation electronics.In the past few decades,the research on flexible electronic devices based on organic materials has witnessed rapid development and substantial achievements,and inorganic semiconductors are also now beginning to shine in the field of flexible electronics.As validated by the latest research,some of the inorganic semiconductors,particularly those at low dimension,unexpectedly exhibited excellent mechanical flexibility on top of superior electrical properties.Herein,we bring together a comprehensive analysis on the recently burgeoning low-dimension inorganic semiconductor materials in flexible electronics,including one-dimensional (1D) inorganic semiconductor nanowires (NWs) and two-dimensional (2D) transition metal dichalcogenides (TMDs).The fundamental electrical properties,optical properties,mechanical properties and strain engineering of materials,and their performance in flexible device applications are discussed in detail.We also propose current challenges and predict future development directions including material synthesis and device fabrication and integration.

1.Introduction

In recent years,flexible electronics has drawn increasingly massive attention worldwide,since it unlocks the limitations of classical electronics arising from the rigid substrates and provides a new paradigm for energy revolution and medical technology transformation in the post-Moore era.Flexible electronic devices can be bent,folded,twisted,compressed,stretched,and even deformed into arbitrary shapes,while still maintaining superior electrical properties and high stability [1–4].At present,various flexible electronic products based on semiconductor materials have been successfully developed,such as wearable electronic products[5–8],implantable electronic products [9,10],electronic skin [11–13].Despite the success,such device-level flexibility achieved by using flexible substrates is inherently limited by the mechanical properties of the semiconductor atop.Organic semiconductors are inherently as flexible as the substrates to accommodate device deformation and are promising candidates for flexible electronics[14–16].However,the poor electrical properties and stability limit its practical application.By contrast,inorganic semiconductor materials possess high carrier mobility and long-term stability under mechanical,electrical and environmental stress,but are generally rigid in nature [17–19].Therefore,developing next-generation flexible materials is a pressing yet challenging task.

As compared with traditional silicon-based materials and organic semiconductors,low-dimensional materials with superior electrical and optical properties have attracted extensive attention in recent years [20,21].Furthermore,due to their relatively high carrier mobility and size-related physical properties,these low-dimensional materials present widely potential applications in high-performance electronic devices.

Low-dimension inorganic semiconductor materials,as novel materials being developed for flexible electronics,possess the above-mentioned attractive properties associated with inorganic semiconductor materials,while retaining the key property of mechanical flexibility [22–25].For instance,one-dimensional (1D) inorganic semiconductor nanowires (NWs) possess outstanding mechanical deformability and eminent optoelectronic properties due to the high aspect ratio of their structural characteristics [26].It is highly desirable to use them as building blocks for flexible electronics in a broad and highly interdisciplinary field.Moreover,twodimensional (2D) transition metal dichalcogenides (TMDs) are a new class of material system with unique structure and properties in the inorganic material library,featuring with a high intrinsic strain limit of ～20% and near-ideal transparency,also have shown significant potential in flexible electronics [27–37].

In light of the rapid development of low-dimensional inorganic semiconductor materials in the field of flexible electronics [38–45],we shall review 1D inorganic semiconductor NWs and 2D TMDs with emphases their physical properties and flexible device applications (Fig.1).We first introduce the electrical properties,optical properties,mechanical properties and strain engineering of 1D inorganic semiconductor NWs and 2D TMDs.Then,the application of 1D inorganic semiconductor NWs and 2D TMDs in flexible devices are discussed in detail,such as photodetectors,sensors,energy storage devices,and energy conversion devices.Finally,we comb out the general guideline that can be extracted from previous research endeavors,along with the perspective given on the future directions.We hope that this review could trigger an explosion of interest in the search of novel flexible inorganic semiconductor materials,and inject great research momentum in the development of flexible electronics.

Fig.1.Schematic illustration of the physical properties and flexible device applications of 1D inorganic semiconductor NWs and 2D TMDs typically studied in this review.Copied with permission [38].Copyright 2018,IEEE.Copied with permission [39].Copyright 2020,American Chemical Society.Copied with permission [40].Copyright 2019,American Chemical Society.Copied with permission [41].Copyright 2021,Elsevier.Copied with permission [42].Copyright 2018,Wiley-Blackwell.Copied with permission [43].Copyright 2017,Wiley-Blackwell.Copied with permission [44].Copyright 2018,Royal Society of Chemistry.Copied with permission [45].Copyright 2017,Tsinghua University Press.

2.Physical properties and strain engineering of 1D inorganic semiconductor NWs and 2D TMDs

In recent years,inorganic semiconductor NWs have been widely used in the field of flexible electronics.First,NWs have been shown to be efficient conductive channels for flexible electronics by providing intercrossed highway for directional carrier transport.Second,NWs have been proven to be highly sensitive materials for flexible sensors due to their large specific surface.Third,as the material size shrinks to the nanoscale,the inorganic NWs exhibit excellent mechanical flexibility due to the reduction of defect incorporation and the crack formation tendency [26].Currently,2D TMDs have been shown to possess many versatile physical features,such as unique and novel electronic and photonic properties,as well as exotic electronic phase transition properties,which are expected to be used in the next-generation of flexible electronic devices [46].In this subsection,we will summarize the electrical,optical,mechanical properties and strain engineering of 1D inorganic semiconductor NWs and 2D TMDs,which are emphatically needed for flexible electronic applications.

2.1.Electrical properties

The nanoscale organization of atoms in NWs leads to new phases or different atomic structures,which in turn will affect its electronic properties.The properties of electronic structure are highly dependent on the crystal phases of materials,owing to the different charge distributions from various atomic configurations.The formation and adjustment of crystal phase usually tailors the surface adsorption properties and the charge states,which can lead to better conductivity.Due to the confinement of wave functions,NWs will show quantum effects,such as quantization of the density of states and expansion of the electronic bandgap.In addition,the high chemical reactivity of the NW surface could impede its performance.For example,the electronic properties of smallsized Si NWs largely dependent on size,growth direction and surface reconstruction.In particular,the electronic bandgap width of Si NWs is size dependent,independent of the direction of the wire.Michael Nolanetal.[47] reported that the effect of surface termination on the bandgap of Si NWs (～1 nm) was investigated by density functional theory calculation.Due to quantum confinement,small-diameter Si NWs demonstrate a direct bandgap that increases with decreasing NW diameter,independent of surface termination (Figs.2A and B).

The diverse structural phases and chemical compositions of 2D TMDs lead to a wide range of electronic properties such as metallic,semiconducting,insulating and superconducting properties (Fig.2C) [48].Among them,the thermodynamically stable 2H phases of MoS2,WS2,MoSe2and WSe2are semiconductors that are favored by researchers as 2D materials for electronic devices due to their high stability,tunable energy band structure and lack of inversion symmetry [49].According to the density functional theory calculation,the positions of valence and conduction band edges of 2H-MoS2change with the decrease of thickness (Fig.2D).When the thickness is reduced to monolayer,the indirect bandgap semiconductor bulk material becomes a direct bandgap semiconductor.In addition,the band structure of WS2could be tuned from an indirect bandgap of 1.3 eV to a direct bandgap of 1.8 eV[50,51].Due to quantum confinement effect and quantum size effect,the edges of their valence band and conducting band may change.The effect has been obtained in most 2D TMD materials[52].The unique performance of tunable band gap makes 2D TMDs have great potential in the application of optoelectronic devices.

Fig.2.Electrical properties of 1D inorganic semiconductor NWs and 2D TMDs.(A,B) Bandgap as a function of Si NW diameter at different surface terminations.Copied with permission [47].Copyright 2007,American Chemical Society.(C) Physical properties of layered TMDs with different structural phases.(D) Band structure of 2H-MoS2 with different thicknesses.(E) Schematic diagram of the band structure at the K point of monolayer 2H-MoS2.Copied with permission [48].Copyright 2017,Nature Publishing Group.

Another important feature of monolayer 2H-TMDs is their lack of inversion symmetry,which leads to the spin splitting of electron bands driven by spin-orbit interaction [48].The band structure of the monolayer 2H-MoS2related to the actual charge-carrier concentrations is shown in Fig.2E.The conducting band minimum (CBM) and valence band maximum (VBM) in the direct semiconductor of monolayer TMDs appear at the point K in the Brillouin zone,while the CBM of the indirect bulk semiconductor at～0.55Γ-K and the VBM at the pointΓ[53].The inherent spin valley coupling of monolayer TMDs makes it possible to be used in new flexible spintronic devices without conventional magnetic materials.

2.2.Optical properties

Semiconductor NWs theoretically have different attractive physical properties due to their inherent quantum confinement nature [54,55],especially optical properties such as ultraviolet-visible absorption [56],photoluminescence [57],and Raman properties[58].Ultraviolet-visible absorption spectroscopy is commonly used to study the molecular transformations that occur within materials due to electronic excitation [59].For example,the ultravioletvisible absorption spectra of CdS NWs were investigated by Lietal.[60].The results of the absorption spectra indicated several distinct peaks centered at 406,410,411 and 425 nm.In addition,Lietal.[61] demonstrated the ultraviolet-visible properties of CdSe NWs with different diameters.When the diameters of CdSe NWs were reduced from 33 nm to 6 nm,their absorption peaks moved from 702 nm to 657 nm,respectively.Photoluminescence spectroscopy is another effective tool to study the optical characteristics of semiconductor NWs.In general,the photoluminescence properties of semiconductor NWs depend greatly on their shape,size,defect concentration and dopants [62–68],as shown in Table 1.The photoluminescence properties of the synthesized ZnO NWs with diameters of 50 and 200 nm was investigated for the existence of defects by Lupanetal.[69].For the NW samples,the emission intensity of the donor bound exciton (DOX) is higher than that of the donor-acceptor (DA) pair recombination (Fig.3A).In addition,ZnO NWs with larger diameters have higher near-band-edge-to-visible emission intensity ratio.Mousavietal.[70] also analyzed the optical properties of ZnO NWs by photoluminescence tests,and concluded that the defects caused by bound excitons contribute to the luminescence of near-band-edge.Moreover,Raman spectroscopy provides a convenient and simple route to characterize the crystallinity and overall optical quality of semiconductor NWs.Yangetal.[71] reported the Raman properties of ZnO NWs.The Raman spectra showed distinct Raman peaks at the wavenumbers of 333,380.6,438.6,583.2 and 661.5 cm-1,corresponding to 2E2,A1(TO),E2(high),E1(LO) and A1modes,respectively (Fig.3B).Furthermore,the Raman peaks of E2(high),A1(TO),E1(TO) and E1(LO) modes ex-hibit a blue shift associated with the bulk ZnO crystal.This phenomenon may be due to the lattice distortion caused by oxygen vacancies and the piezoelectric effect of the NWs themselves.

Table 1Comparison of the effects of shape,size,defect concentration and dopants on the photoluminescence properties of semiconductor NWS.

Fig.3.Optical properties of 1D inorganic semiconductor NWs and 2D TMDs.(A) Photoluminescence spectra of the ZnO NWs with diameters of 50 nm (1) and 200 nm (2).Copied with permission [69].Copyright 2010,Elsevier Ltd.(B) Raman spectrum of ZnO NWs grown on Si substrate.Copied with permission [71].Copyright 2008,Elsevier.(C)Absorption spectra of MoS2 films annealed at different temperatures.(D) Absorption spectra of MoS2 films with an average thickness of 1.3–7.6 nm.Copied with permission[72].Copyright 2011,American Chemical Society.(E) Absorption and (F) photoluminescence from 1 to 5 layers WS2.Copied with permission [73].Copyright 2013,American Chemical Society.

In general,the interlayer bandgap of 2D nanomaterials facilitates the light absorption properties over a wide spectral range[37].2D monolayer TMDs have attracted extensive attention due to their unique photoelectric characteristics.Among TMDs,such as MoS2,MoSe2,WSe2and WS2,possess periodically arranged layered structures.For instance,compared to the indirect bandgap in bulk MoS2,monolayer MoS2is a direct bandgap semiconductor material that demonstrates characteristic absorption peaks.Edaetal.[72] observed that the characteristic absorption peaks of monolayer MoS2were located at 620 and 670 nm,respectively(Figs.3C and D).Zhaoetal.[73] reported photoluminescence spectra and differential reflectance of monolayer to few-layer WS2and WSe2,indicating that the energy band structure of monolayer WS2and WSe2underwent a similar indirect to direct gap transition.The photoluminescence peak of monolayer WS2with a direct bandgap is located at 630 nm,corresponding to its energy bandgap (Figs.3E and F).

2.3.Mechanical properties

2.3.1.Mechanicalpropertiesof1DinorganicsemiconductorNWs

The mechanical properties of NWs are comprehensively described by elastic modulus,stress,strain and their relationship.Currently,a range of techniques have been developed for mechanical properties analysis of NWs,including atomic force microscope (AFM)-based testing [74–77],nanoindentation-based testing [78,79],insituelectron microscopy testing [80–83],and optical microscopy-based testing [84,85].

The unique mechanical properties of 1D inorganic semiconductor NWs have been extensively explored [86–91],as shown in Table 2.Si NWs are one of the most crucial and versatile building blocks in nanoelectronics,and their mechanical properties have been extensively studied by various techniques.Kimetal.[74] tested the mechanical properties of Si NWs with radii ranging from 15 nm to 70 nm by AFM bending and nanoindentation(Figs.4A and B).The force and deformation (f-d) curves obtained from the AFM bending experiments are shown in Fig.4C,where the sharp drop in force corresponds to the brittle failure of the Si NWs.Thef-drelationship given by the line-tension-only model analysis model,i.e.,

Table 2Summary of mechanical properties of 1D inorganic semiconductor NWs and 2D TMDs.

Fig.4.Mechanical properties of 1D inorganic semiconductor NWs.(A) Schematic diagram of the AFM bending test structure of Si NW.(B) Schematic diagram of the nanoindentation test structure of Si NW.(C) Representative f-d curves obtained from the AFM bending test.The inset shows the AFM images taken before and after the bending test.(D) Representative P-h curves obtained from nanoindentation with Berkovich and cube-corner tips.The inset shows the AFM images taken before and after the nanoindentation test.(E) Variation in the elastic modulus of Si NWs as a function of NW radius.Copied with permission [74].Copyright 2011,WILEY-VCH Verlag.(F)Elastic modulus of the cylindrical and hexagonal ZnO NWs as a function of NW effective diameter.Copied with permission [97].Copyright 2015,American Chemical Society.(G) The structural model of the single-crystal GaN NW.(H) The structural model of the obtuse-angle twin GaN NW.(I) Young’s modulus with diameter of GaN NWs.Copied with permission [83].Copyright 2015,American Chemical Society.

whereE,randLare the elastic modulus,radius,and span length of NW,respectively [92].Finally,the elastic modulus of Si NWs with radii of 15–70 nm was derived to be about 185 GPa.The loaddisplacement (P-h) curves obtained for nanoindentation using a typical Berkovich indenter and cube-corner indenter are shown in Fig.4D.The results showed that the modulus values obtained by Berkovich indenter and cube-corner indenter are 37.9±13.3 GPa and 59.5±13.3 GPa,respectively,which are significantly lower than the modulus values obtained by AFM bending.It is possible that the surface effect of the circular surface of the NWs on the predicted contact area leads to an underestimation of the elastic modulus measurements.In addition,there is no significant change in the elastic modulus as the Si NW radius decreases from 70 nm to 15 nm (black squares),as shown in Fig.4E.However,Zhuetal.[93] reported a different trend,that is,the elastic modulus of Si NWs decreased with decreasing radius.

A number of experimental studies have been devoted to analyzing the effect of NW size on elastic modulus.The most likely mechanisms for the size-dependent elastic modulus of Si NWs are surface reconstruction and the presence of surface oxide/amorphous layers [94,95].Stanetal.[96] observed that the elastic modulus of ZnO NWs increases with decreasing size,which is different from the variation trend of Si NWs.This discrepancy is mainly attributed to the uncertainty caused by the inherent limitations of the various testing methods and the microstructure and internal defects of the NW samples used.For example,the indentation test is difficult to obtain accurate absolute modulus of nanostructured materials due to its complex stress/strain conditions and empirical formulas based on bulk materials.For bending loading tests,the measured modulus will be inversely proportional to the fourth power of the measured diameter,which can lead to large deviations due to small measurement errors.In addition,smallvariations in microstructure caused by the growth conditions may lead to obvious differences in the results of the actual tests [97],as shown in Fig.4F.Note that defects in crystalline materials alter the distribution of electrons within the atomic structure,thereby changing the elastic modulus of the material [98].Generally,point defects can increase the average bond length,resulting in a decrease in elastic modulus [99].Planar defects can change the modulus values of NWs,which may depend on the microstructure of the NWs under test [100,101].Daietal.[83] confirmed the effect of planar defects on the elastic modulus at nanoscale byinsituTEM vibration and tensile tests.They modeled the GaN NW with planar defects based on the characterization results,with the red area representing (001) stacking fault (SF) and the blue area representing twin boundary (TB),as shown in Figs.4G and H.The average elastic modulus of single-crystal GaN NWs with diameters greater than 92 nm is 338±16 GPa.Stacking faults (SFs) parallel to the wire axis could be regarded as parallel fibers in the Voigt rule of the mixture,and a limited number of SFs have no significant effect on the NW modulus.While for obtuse-angle twin GaN NWs with diameters ranging from 98 nm to 171 nm,the elastic modulus is only about 66±5 GPa (Fig.4I).

2.3.2.Mechanicalpropertiesof2DTMDs

2D TMDs are a new class of material systems with unique structure and properties in the inorganic semiconductor materials library,and are considered as one of the candidates with high potential for applications in flexible electronics [48,102].

The unique mechanical properties of 2D semiconductor materials represented by TMDs at the nanoscale have been extensively explored [103–105],as shown in Table 2.The mechanical properties of the monolayer MoS2were characterized by AFM,and the elastic modulus and fracture strength were measured to be 270±100 GPa and 15±3 N/m (23 GPa) [106].Subsequently,Castellanos-Gomezetal.[107] investigated the mechanical properties of MoS2with several layers (5–25 layers)by AFM.Thef-dcurves demonstrated the elastic properties of MoS2free-suspension nanosheets with 5 to 20 layer thicknesses,with nonlinear behaviors for 5–8 layers and linear behavior for thicknesses above 10 layers (Fig.5A).Elastic modulus can be calculated from the quantitative relationship between force and deformation [108,109].The mean value of elastic modulus for MoS2nanosheets with 8-layer thickness was calculated to be 0.33±0.07 TPa (Fig.5B).Compared with Mo-based TMDs (MoX2),W-based TMDs (WX2) have larger elastic modulus and tensile strength.The W-based TMDs reveals a greater maximum strength than the Mo-based TMDs because the hybridization leads to a charge redistribution in the region shared by the transition metal and chalcogen atoms [110].Falinetal.[111] explored the mechanical properties of monolayer (1 L),bilayer (2 L) and trilayer(3 L) WS2,WSe2and WTe2through experiments and theoretical calculations.The elastic modulus and fracture strength of 1–3 L tungsten dichalcogenides are shown in Figs.5C and D,respectively.

Fig.5.Mechanical properties of 2D TMDs.(A) f-d traces in the center of suspended sections of 5-,10-and 20-layer MoS2 nanosheets.Inset: Schematic diagram of the nanoscale bending test experiments performed on freely suspended MoS2 nanosheets.(B) The mean value of Young’s modulus for MoS2 nanosheets with 8-layer thickness.Copied with permission [107].Copyright 2012,Wiley-Blackwell.(C) The Young’s modulus and (D) fracture strengths of 1–3 L WS2,WSe2,and WTe2.Copied with permission[111].Copyright 2021,American Chemical Society.(E) Young’s modulus of MoS2 at different types of defect densities.(F) Yield stress and corresponding yield strain of monolayer MoS2 under different vacancy defects.Copied with permission [113].Copyright 2019,Elsevier.(G) Interaction coefficient α for different types of bilayers.Copied with permission [114].Copyright 2014,American Chemical Society.

The mechanical properties of 2D materials depend sensitively on the presence of defects (e.g.,vacancies,dislocations,and grain boundaries) and their evolution during deformation [112].Mahataetal.[113] reported the effect of intrinsic defects on the mechanical properties of monolayer MoS2.All six different defects reduce the elastic modulus of monolayer MoS2,and this effect increases with increasing defect density (Fig.5E).In addition,the yield stress of the monolayers with different types of defects are all lower than those of the original monolayer MoS2(Fig.5F).Currently,there are few studies on the effects of defects on the mechanical properties of TMDs,and there is an urgent need to gain insight into the effects caused by different types of defects.In addition,the mechanical properties of 2D heterostructures and their interlayer interactions are also a worthy research direction.Liuetal.[114] studied different bilayer heterostructures and indicated that the 2D modulus of the heterostructure is not equal to the sum of the modulus of the individual layers.This is caused by the weak interaction between layers and the sliding effect.The interaction coefficientαvalue of graphene/graphene bilayer is almost 1,while the interaction coefficientαvalue of MoS2/graphene heterostructure is only 0.69 as well as 0.8 for MoS2/WS2heterostructure (Fig.5G).This is due to the strong interaction between graphene homogeneous layers preventing interlayer sliding,while the interlayer coupling of MoS2/graphene and MoS2/WS2is weak.

2.4.Strain engineering

It is known from the above that the mechanical properties of 1D inorganic semiconductor NWs and 2D TMDs have been extensively studied.A multitude of research groups have further investigated the effect of strain on the optoelectronic properties of 1D inorganic semiconductor NWs and 2D TMDs and realized new strain engineering applications.The so-called strain engineering refers to a simple and effective method to change the physical properties of materials by stretching or compressing [37].For example,Niquetetal.[115] demonstrated that a moderate strain in the 2% range enhances or reduces the carrier mobility in Si NWs by a factor of>2 (up to 5).Mechanical strain not only changes the electron mobility and transport properties of Si NWs,but also has a considerable impact on the bandgap and work function.Lietal.[116] observed that the band structure evolution of Si NWs demonstrated anisotropy in the three directions with increasing strain within the strain from 0 to 20.The Si NWs maintain the indirect bandgap under increasing strain along the (100) direction,while the indirect to direct bandgap transition is achieved at 4% and 14% loading strain along the (110) and (111) directions,respectively.Furthermore,the semiconductor-to-metal transition is achieved at 14% and 20% loading strain along the (110) and (111) directions,respectively.Fuetal.[117] revealed the effect of bending strain on the optical and optoelectronic properties of GaN NWs.The bent GaN NW under scanning cathodoluminescence test,the near-band-edge peak split into two peaks.The low-energy peak was linearly red-shifted with increasing bending strain (Fig.6A),while the high-energy peak was blue-shifted and decreased slightly with increasing strain (Fig.6B).In addition,the photo current of GaN NW increased almost linearly with the bending strain in ultraviolet response measurements,indicating that bending strain significantly enhanced the photoconductivity (Figs.6C and D).

Compared with 1D inorganic semiconductor NWs,the strain engineering of 2D TMDs is more abundant,including folds,bubbles,tents,and conformal patterns [37].Strain modulation is an effective method to modulate the material properties of 2D TMDs,such as large strains to modulate their energy band structure [107,118].For example,monolayer MoS2completes the direct to indirect bandgap transition at about 2% uniaxial tensile strain,while it transforms from semiconductor to metal at 10%-15% biaxial tensile strain [119].McCrearyetal.[120] found that the Raman inplane modes and photoluminescence energies of few-layer MoS2changed significantly at ～1.5% uniaxial tensile strain.However,continuous and uniform bandgap tuning is difficult to achieve with chemical composition tuning during synthesis and external tuning including applied stress after synthesis [121–123].Zengetal.[124] developed a sphere diameter engineering (SDE) technique to accurately and reproducibly manipulate the bandgap of 2D materials and achieved a completely uniform bandgap tuning for MoS2.The technique achieves bandgap reduction and increase by constructing a sphere with positive or negative curvature.The basic process of the SDE technique is shown in Fig.6E.The correspondence is established by the bandgap and the curvature (2/diameter) of the spheres,such as the curve relationship extracted from 83 glass spheres grown with MoS2(Fig.6F).In addition,the low room temperature carrier mobility of 2D semiconductors due to strong phonon scattering is also a challenge.Studies have shown that strain engineering can reduce the effective mass of carriers and thus improve the carrier mobility of 2D semiconductors[125,126].Ngetal.[127] realized the improvement of charge carrier mobility by introducing local strain to suppress phonon scattering.They obtained rippled MoS2(r-MoS2) samples by transferring the MoS2flakes to a substrate with a bulge morphology(Fig.6G).Since the dielectric constant (ε) serves as an important parameter for identifying high carrier mobility,the effect of different curvatures on theεof r-MoS2was investigated.The results revealed that both in-plane (ε||) and out-of-plane (ε⊥)εincrease with increasing curvature (Fig.6H).Furthermore,the room temperature mobility in r-MoS2is enhanced by two orders of magnitude,reaching ～900 cm2V-1s-1(Fig.6I).

From the above,strain engineering holds great potential for tuning the electrical and optoelectronic properties of lowdimensional semiconductors.Due to the high flexibility of lowdimensional semiconductors,the strain effect is greatly manifested,which makes strain engineering promising for the design of high-performance micro/nanoscale functional devices.Furthermore,more precise and reproducible strain modulation methods need to be developed for various performance engineering aspects of low-dimensional semiconductors.

3.Flexible device applications

Ideally,all components of flexible electronic devices including the functional component should conform to deformation to ensure structural and functional integrity.Simultaneously,flexible electronic devices must maintain their performance under harsh environment such as various mechanical stresses (bending,folding,rolling and stretching),humidity,and heat [37,128,129].

3.1.Flexible photodetectors

Photodetectors are optoelectronic devices that could convert light signals into electrical signals.Compared to traditional rigid photodetectors,the flexible ones are able to adapt to different surfaces,thus enabling new functions.For example,flexible photodetectors attached to the human body can more precisely monitor the ultraviolet radiation that people receive.Flexible photodetectors mounted on eye lenses could be implanted to restore vision to the blind [130,131].1D inorganic semiconductor NWs and 2D TMDs possess great potential for optoelectronic applications due to their unique structures and optoelectronic properties.In this section,we present the recent advances in flexible photodetectors based on 1D inorganic semiconductor NWs and 2D TMDs [132–138],as shown in Table 3.

1D inorganic NWs possess a large number of trap states at the surface due to their large surface area and volume ratio,which can prolong the lifetime of photocarriers.In addition,its lowdimensional structure and single-crystal properties in turn ensure high carrier mobility of free carriers [130,139].To date,numerous high-performance flexible photodetectors made of inorganic semiconductor NWs have been realized.For example,the NW form of Si is widely used for flexible photodetector applications.Kimetal.[140] used the metal catalyst-assisted etching method to prepare Si NW with a diameter of 120 nm,which achieved highperformance multi-wavelength photodetection.The photodetector can withstand bending radii as low as 5 mm and survive the repeated bending and unbending more than 10,000 cycles.Hossainetal.[133] integrated the a single-crystalline percolative Si NW to a transparent and flexible photodetector.With a wide direct bandgap and high carrier mobility,ZnO NWs are excellent materials for fabricating photodetectors [141,142].Nunezetal.[38] fabricated a flexible photodetector based on ZnO NWs for wearable ultraviolet dosimetry.The SEM characterization of the ZnO NWs assembly is shown in Fig.7A,and microfiber formation on a pair of Au electrodes was observed.The photodetector showed little dark current (Idark) variation under compressive and tensile bending with bending radius ranging from 5 nm to 27 mm (Fig.7B),and exhibited the oppositeIdarkvariation behavior.In addition,II-VI semiconductor NWs,such as CdS and ZnTe have also been investigated as functional components in flexible photodetectors[143,144].

Fig.7.Flexible photodetectors based on 1D inorganic semiconductor NWs and 2D TMDs.(A) SEM image of ZnO NWs.The inset is the ZnO NWs-based microfibers bridging the Au electrodes.(B) Dark current test of a flexible ZnO NWs photodetector under compressive and tensile stress.The inset shows the snapshot during dynamic bending.Copied with permission [38].Copyright 2018,IEEE.(C) I-V characteristics of photodetector based on ZnO-SnO2 heterostructure NWs arrays in three different bending states.Copied with permission [147].Copyright 2018,IOP Publishing.(D) Schematic diagram of the flexible photodetector based on few-layer MoS2.(E) Photocurrent and dark current of the photodetector before and after 100 bending cycles.Copied with permission [39].Copyright 2020,American Chemical Society.(F) Schematic diagram of the fabrication process of the photodetector based on the MoS2/Si heterojunction.(G) Sense performance of MoS2/Si photodetector varies with the bending radius of plane,5,8,10,12 and 14 mm.(H) Stability of the MoS2/Si heterojunction with 1000 bending cycles.Copied with permission [132].Copyright 2021,Royal Society of Chemistry.

Moreover,the device performance can be improved by constructing 1D heterostructures,such as higher sensitivity,wider spectral response range,and faster response speed [145].For example,Zhengetal.[146] reported the fabrication of a photodetector based on ZnO–CdO heterostructure NWs arrays with a broad spectral response range by electrospinning method.The performance test results showed that the hybrid arrays have a wider spectral response range from ultraviolet to visible light compared to pure ZnO or CdO NW arrays.Similarly,Louetal.[147] synthesized ZnO-SnO2NWs arrays by an electrospinning technique,and developed a high-performance ultraviolet photodetector based on the hybrid arrays.The ZnO-SnO2heterostructure NWs arrays have the advantages of high stability,large on/off current ratio and fast response speed in ultraviolet photodetector applications.The I-V curves of the photodetector based on the ZnO-SnO2heterojunc-tion NWs array under three different bending test regimes did not change significantly (Fig.7C),demonstrating its excellent mechanical flexibility and stability.Electrospinning technology represents a facile way to achieve multiple compositions by using predetermined precursors to fabricate multicolor responsive photodetectors,which may open new shortcuts for efficient photodetector applications.

Correspondingly,2D materials represented by graphene have been reported to be important channel materials for photodetectors.The use of graphene as a channel material for photodetectors can provide broad spectral detection and ultrafast sensing[148].However,graphene has a low switching ratio due to its zero bandgap,which limits its further development in optoelectronic applications such as photodetectors.Therefore,it is particularly important to search other 2D materials as alternative channel materials.With numerous prominent electrical and optical properties,such as continuously tunable bandgap with the number of layers,strong light-matter interaction,etc.,TMDs are one of the most promising candidates for the study of new high-performance photodetectors [149–151].It has been found that photodetectors based on 2D TMDs exhibit the advantages of broad spectral detection,low dark current,high response and short response time.For example,Limetal.[152] synthesized a wafer-uniform MoS2layer at low temperature,and fabricated a flexible visible-light photodetector array based on the MoS2layer.Recently,Schneideretal.[39] used several layers of MoS2nanosheets as photoactivated channels to fabricate a flexible photodetector with a responsivity of 150 A/W and a detection rate of up to 1012Jones at drain voltage(Vds) of 2 V and gate voltage (Vgs) of -2 V.The schematic crosssection of the device is shown in Fig.7D.The photocurrent and dark current of the photodetector hardly changed after 100 bending cycles,manifesting splendid mechanical stability (Fig.7E).

2D van der Waals heterostructures to improve the photoresponsivity by simply stacking heterogeneous layers are being widely studied.Choietal.[132] fabricated an ultra-flexible 2D MoS2/Si heterojunction photodetector with a responsivity of 10.07 mA/W to near-infrared light (λ=850 nm) and a specific detectivity of 4.53×1010Jones.The fabrication process of this photodetector is shown in Fig.7F.The photocurrent in the on/off state of this photodetector did not have a significant inhibitory effect on the optical response characteristics at bending radii of 5,8,10,12 and 14 mm(Fig.7G).In addition,the on and off-state stabilities of the heterojunction device do not change drastically even over 1000 consecutive bending cycles (Fig.7H).The p-n junction photodetectors can be fabricated by stacking TMD layered semiconductors with different bandgaps and carrier types,which affords excellent on/off current ratio and high near infrared detection rate,such as MoS2/WS2,MoTe2/MoS2and WSe2/SnS2[153–156].Pataniyaetal.[157] recently showed a flexible photodetector based on MoS2/WSe2heterostructure with an on/off ratio of the order of 103and a photoresponsivity of 124 mA/W.In addition to building heterostructures to achieve more functionality or improve performance,doping can also greatly improve device optoelectronic performance.For example,Sunetal.[158] developed a p-n homojunction photodetector with rectification ratios as high as 103using cetyltrimethylammonium bromide for local electron doping of WSe2channel materials.Compared with the intrinsic photodetector,the optical switching ratio of the p-n homojunction has increased 1000-fold,and the response speed is also improved by a factor of 3.

Despite great progress has been made in the field of 1D inorganic semiconductor NWs and 2D TMDs based flexible photodetectors,there still exist challenges for practical applications.Developing sensing materials with both high carrier transport and mechanical durability,exploring largescale production technologies that enable precise control of morphology,crystallinity,directional assembly,and even physical and chemical properties,as well as expanding the multifunctional integration of photodetectors with different devices are the directions toward the future development of flexible photodetectors.

3.2.Flexible gas sensors

Air pollution is one of the global problems that need to be solved,and the development of efficient gas sensors is a matter of urgency.Due to the immense demand for wearable electronic devices in the future,researchers have focused on developing reliable and efficient flexible gas sensors.Especially,the development of flexible gas sensors capable of detecting toxic gasses (NOx,H2S,CO,etc.) or biomarkers (NH3,CH4S,etc.) from human breath has gradually become a research hotspot [159].The sensing material is of paramount importance for a flexible gas sensor,which is responsible for interacting with the target gas and converting the chemical signal into an electrical signal [160].

Inorganic metal oxide semiconductors,as traditional gas sensing materials,are capable of detecting toxic gasses at low concentrations,and have become a hot research topic in the field [161].In general,the sensing mechanism of conventional metal oxide gas sensors depends on the surface reaction that takes place between the target gas molecules and the oxygen ions pre-adsorbed on the metal oxide surface.The sensing performance of inorganic metal oxide semiconductors improves significantly as the crystal size decreases,which further facilitates the development of gas sensors.1D metal oxide semiconductor nanomaterials such as NWs have been widely investigated for flexible gas sensors due to their high surface-to-volume ratio and the extension of the surface charge region [162–164].For example,Tonezzeretal.[165] developed a flexible gas sensor based on ZnO NWs,which reacted at an optimal temperature condition of 280°C with response values for oxygen and hydrogen are 10 and 11,respectively.Subsequently,the researchers found that combining NWs and metal nanoparticles can boost the reaction between the target gas and the sensing material,thereby enabling higher performance gas sensors [166,167].In this case,Fuetal.[168] designed a Cu-ZnO NW-based flexible gas sensor with high response and selectivity,which is capable of detecting the toxic gas H2S at room temperature.Yangetal.[40] proposed a strategy to construct flexible gas sensors by decorating ZnO NWs with palladium (Pd) nanoparticles.Specifically,the surface of ZnO NWs was decorated with Pd nanoparticles,and then the surface was sulfided by H2S gas to form a ZnS layer,which eventually formed a core-shell structure for monitoring H2S gas (Fig.8A).Pd nanoparticles significantly enhanced the sensitivity of the ZnO/ZnS core-shell NW sensor (Fig.8B).Furthermore,Pd-modified ZnO/ZnS core-shell NWs exhibited shorter response and recovery time than ZnO/ZnS core-shell NWs due to the surface catalytic effect of Pd nanoparticles.Therefore,the Pdmodified ZnO/ZnS core-shell NW sensor manifested fast response and recovery.There was no major difference in the response of the sensor to H2S gas under flat and bent conditions (Fig.8C).These confirmed that metal nanoparticle decoration is an effective way to improve the performance of gas sensors,and also inspired the design of new flexible gas sensors to detect more gas species.In addition,the introduction of heterostructures has been shown to improve the performance of metal oxides in sensing a wide range of gasses by increasing or decreasing the resistance of heterostructures nanomaterials [169].

Fig.8.Flexible gas sensors based on 1D inorganic semiconductor NWs and 2D TMDs.(A) Schematic diagram of the fabrication process of the flexible gas sensor based on Pd-decorated ZnO NWs.(B) Comparison of response,response time and recovery time between ZnO/ZnS core-shell NWs and Pd nanoparticles decorated ZnO/ZnS core-shell NWs.(C) Sensing performance of Pd-decorated ZnO NWs-based gas sensor under flat and bent surfaces.Copied with permission [40].Copyright 2019,American Chemical Society.(D) Flexible gas sensor scheme based on WSe2 nanosheets.(E) The sensing curve of the flexible sensor based on WSe2 nanosheets with and without ultraviolet light irradiation for different concentrations of NO2.Inset: Function of sensor response at low NO2 concentration (0.05–0.5 ppm).(F) The histogram response of WSe2 sensor to 1 ppm NO2 under different bending conditions with ultraviolet illumination.Copied with permission [41].Copyright 2021,Elsevier.

Recently,2D TMDs have been used to fabricate flexible gas sensors due to their excellent mechanical strength,high carrier mobility,high conformality,and tunable semiconducting properties[170].The sensing mechanism of flexible gas sensors based on 2D TMDs mainly depends on the charge transfer process between target gas molecules and TMDs materials [171].Among 2D TMDs,MoS2possesses the advantages of large surface area and simple synthesis process,which is considered to be one of the biggest concerns in gas sensing applications [172].For example,Burmanetal.[173] synthesized large MoS2films using an ultrasoundassisted liquid exfoliation method,and developed a flexible gas sensor based on the films.As a p-type inorganic 2D layered material with extraordinary physical and chemical properties,WSe2has been developed for flexible gas sensors.For example,Yangetal.[41] reported a highly selective and flexible gas sensor based on exfoliated WSe2nanosheets for detecting trace concentrations of NO2under ultraviolet activation at room temperature (25°C)(Fig.8D).The response recovery transient of the sensor increasedwith the stepwise increase of NO2concentration (Fig.8E).What is more,the sensor displayed high response to each concentration of NO2under ultraviolet irradiation.Even if the bending radius of the gas sensor is reduced to 1 mm,it still maintained an excellent sensing response to 1 ppm NO2,proving its outstanding mechanical stability (Fig.8F).Other 2D layered materials such as MoSe2,WS2,VS2,SnS2and SnSe2,have been investigated and used to detect gasses on flexible sensing platforms [174–178].Moreover,the sensing performance of the sensor can be improved by adding another material to the TMDs sensing material to form nanocomposites or van der Waals heterostructures [179].For instance,a NO2gas sensor based on WOx/WSe2heterostructure was fabricated by Medinaetal.[180].Hanetal.[181] proposed a sensing mechanism based on modulation of the heterojunction barrier potential formed at the ZnO/MoS2interface upon exposure to gas.Recently,Yietal.[182] realized stretchable NO2sensors based on MoS2@rGO nanocomposites,which deliver encouraging stability before and after applying 20% tensile strain.

Finally,we summarize the performance of flexible gas sensors based on 1D inorganic semiconductor NWs and 2D TMDs[183–185],as shown in Table 4.To facilitate the further development and broader applications of flexible gas sensors,various challenges such as selectivity to specific gasses in practical applications,stability in atmospheric environment,self-healing capability after mechanical damage,rapid response in the part-per-trillion range (mainly for medical applications),and biocompatibility need to be addressed.

Table 4Summary of the performance of 1D inorganic semiconductor NWs and 2D TMDs based flexible gas sensors.

3.3.Flexible energy storage devices

With the rapid development of lightweight,flexible,and even wearable electronic devices,the energy storage devices that provide energy for them also need to develop towards flexibility.Rechargeable batteries,represented by lithium-ion batteries,are considered as a suitable choice for storing energy by converting chemical energy into electrical energy,and have received extensive attention due to their high energy density.However,properties such as short cycle life and slower charge/discharge rates limit their further application.As an effective alternative to batteries,supercapacitors offer excellent cycle life,ultra-fast charge/discharge rates,high power density,etc.Flexible supercapacitor is an emerging branch of supercapacitor,which is suitable for backup power supply of flexible electronic products due to its controllable size,large mechanical compliance and excellent safety [186–188].

A flexible supercapacitor consists of two electrodes with current collectors,an electrolyte and a separator [189].The material properties of these components and the connections between them greatly affect the performance of supercapacitors.For example,the instability of the interface between the electrode,diaphragm and electrolyte hinders the successful fabrication of flexible supercapacitors [42].Conventional stretchable supercapacitors mainly coat the electrodes with gel electrolytes and use them as separators,but they cannot effectively prevent electrode dislocation and short circuit problems [190,191].Therefore,mechanically enhanced and separator-integrated electrodes are vital for fabricating editable supercapacitors.Lvetal.[42] fabricated editable supercapacitors with honeycomb-like structures based on mechanically strengthened MnO2NW composite (Fig.9A).The stretchability of this supercapacitor is highly programmable,with a maximum stretchable strain of more than 780% (Fig.9B).The honeycomb-like supercapacitor still maintained nearly 98% of the initial capacitance after 10,000 stretching and releasing cycling tests at 400% tensile strain (Fig.9C),demonstrating extremely high cycling stability.Furthermore,when an integrated device consisting of the editable supercapacitor and a strain sensor was worn on the arm,the supercapacitor successfully powered the strain sensor(Fig.9D).Furthermore,other flexible supercapacitors based on inorganic semiconductor NWs have been developed [192–194],as shown in Table 5.In the future research,it is expected to design flexible/stretchable integrated energy storage devices with more functions for various portable and wearable electronics.

Table 5Summary of the performance of 1D inorganic semiconductor NWs and 2D TMDs based flexible supercapacitor.

Improving the performance of supercapacitors by optimizing the properties of electrode materials has been a hot topic of extensive research.Rigid electrode materials greatly hinder the feasibility of flexible applications of supercapacitors.2D layered materials with high specific surface area,high mechanical strength,and high electrochemical activity are widely used as electrodes for supercapacitors [195].The ultrathin thickness and high conductivity of the electrodes enhance electron transfer and increase electrode electrolyte contact for high-performance energy storage applications [30].TMDs are highly sought due to their large surface area,active edge sites and variable oxidation states that allow for large energy storage capacity as well as the presence of transition metals that provide the required redox reactions for enhanced capacitance [196,197].Among TMDs,MoS2has received increasing attention in flexible supercapacitor applications due to its unique structure and high capacitance.Ghoraietal.[198] synthesized few-layer MoS2nanosheets by liquid phase exfoliation and constructed a high-performance flexible supercapacitor.Later studies found that the 1T phase of MoS2has higher conductivity than the 2H phase,and is more suitable as an electrode material for supercapacitors.Wangetal.[199] used multilayer 1T-MoS2nanosheets as electrode material to fabricate a flexible supercapacitor with a high areal capacitance of 310 mF/cm2at 1 mA/cm2.However,the low electrical conductivity of MoS2as an electrode limits the development of supercapacitors.Even though the presence of 1T-MoS2would alleviate the problem,the 1T phase orientation is difficult to maintain [200].The introduction of other materials to form a hybrid structure with MoS2to create a synergistic effect could enhance the electrochemical properties.Graphene,with its large aspect ratio and excellent mechanical properties,completes a well match with 2D MoS2nanosheets.For example,flexible supercapacitors constructed from graphene/MoS2composites are able to withstand high tensile strains of 60% without obvious degradation of electrochemical performance [201].Carbon nanotubes have also been used to synthesis MoS2-based composites due to their high mechanical properties and superior electrical conductivity.The supercapacitor developed based on MoS2nanosheets and carbon nanotube showed a specific capacitance of 13.16 F/cm3and a capacitance retention of 98% after 10,000 charge-discharge cycles [191].Furthermore,Wangetal.[202] reported a flexible supercapacitor by using a composite of MoS2,reduced graphene oxide,and carbon nanotubes,which has a high specific capacitance of 129 mF/cm2at 0.1 mA/cm2.Changetal.[203] recently demonstrated a MoS2-single-wall carbon nanotube/cellulose nanofibril flexible supercapacitor (Figs.9E and F).The specific capacitance of the supercapacitor remained at 91.01% of its original value after 10,000 galvanostatic charge/discharge (GCD) cycling tests (Fig.9G).This indicated the excellent cycling stability of the supercapacitor.The cyclic voltammetry (CV) test results of this supercapacitor under different bending angles are shown in Fig.9H.

In addition to MoS2,other flexible supercapacitors based on TMDs have been developed [204–206],as shown in Table 5.Compared with 2D S-based and Se-based TMDs,Te-based TMDs possess better electrical conductivity,which is more beneficial to improve the capacitance of supercapacitors.Yuetal.[43] first reported the flexible supercapacitor based on Te-based TMDs(Fig.9I).The device has nearly identical CV behavior at bending angles of 0° and 90°,demonstrating excellent flexibility,high mechanical strength,and structural integrity (Figs.9J and K).Moreover,the supercapacitor has excellent long-term stability,with a capacitance retention close to 0.92 and an efficiency of 95% after 5500 cycles (Fig.9L).

From the above,the electrode material is particularly important for the comprehensive performance of flexible supercapacitors.Various nanocomposites bring opportunities for fabricating high-performance flexible supercapacitors due to the synergistic effect of their different materials,but the interaction,electron transportation between the composite constituents,and the effect of the compositions on the overall performance need to be further explored.Meanwhile,the development of novel flexible electrode materials may open new avenues for improving the performance of flexible supercapacitors.In addition,the electrolyte is regarded as a vital component of flexible supercapacitors so that their parameters influencing the final performance of the flexible supercapacitors and the working mechanism are necessary to be carefully studied.

3.4.Flexible energy conversion devices

With the rapid growth of the world population and the intensification of human activities,the development of sustainable and renewable energy sources has become an important issue for maintaining a clean environment in the modern world [207].Electrocatalysis plays a central role in clean energy conversion,providing a host of sustainable processes for future technologies [208].Electrocatalytic water splitting for sustainable hydrogen production is widely regarded as one of the most promising approaches to obtain renewable energy due to its high energy conversion efficiency and no carbon emissions.In electrocatalytic processes,especially at high current densities,the tension and vibrational forces generated during bubble escape and collapse are generally considered to be important factors leading to poor stability [209].The effect of bubble overflow and collapse on the catalyst is prevented under high current conditions to obtain long-term stability of the catalyst [210].Recently,the prominent applications of flexible devices such as flexible sensors and flexible supercapacitors have inspired the design of novel flexible electrodes for hydrogen evolution reaction (HER).The designed flexible electrode cushions the shock of electrolyte convection and hydrogen bubble rupture through stress release,thereby ensuring long-cycle stability.

Studies have identified that molybdenum sulfide (MoSx) is a promising catalyst for highly active HER in acidic solutions [211].For example,MoS2is a typical cheap substitute for Pt,and its high activity mainly comes from the sulfur atoms at the edge [212].In addition,amorphous MoSxhas also been shown to be an efficient HER catalyst [213].The electrochemical performance of MoSxcatalysts can be improved by employing suitable conductive substrates.For example,Zhangetal.[207] reported a free-standing and binder-free hybrid film composed of graphene mesoporous structure (GMS) and single-walled carbon nanotubes (SWCNTs),and then deposited amorphous MoSxon the film to obtain a novel catalyst with excellent HER catalytic activity.The MoSx@GMS/SWCNT hybrid electrode exhibited splendid flexibility and undergoes various deformations without causing significant structural damage(Figs.10A-D).More importantly,the flexibility of the electrode facilitates its ability to maintain stable catalytic performance under different deformations,as shown in Fig.10E.For MoS2,the poor electrical conductivity between both the dispersed nanostructured MoS2catalyst and the S-Mo-S layer of MoS2limits its activity[214].Electron transport can be accelerated by coupling MoS2with a conductive substrate.In particular,graphene conductive substrates have attracted extensive attention due to their excellent electron transport properties and chemical stability [215].Zhangetal.[44] fabricated a flexible hybrid MoS2/graphene-independent electrocatalytic electrode.The current of the developed hybrid film is almost unchanged under various bending and recovery states,indicating the stable conductivity of the MoS2/graphene catalyst(Fig.10F).In addition,the effect of MoS2/graphene electrode on the catalytic activity under four different bending states was also investigated,which was measured in 0.5 mol/L H2SO4acidic solution at 10 mV/s (Figs.10G-J).The results showed that the polarization curves remain almost constant under various bending states(Fig.10K).

Fig.10.Flexible energy conversion devices based on 1D inorganic semiconductor NWs and 2D TMDs.(A-D) MoSx@GMS/SWCNT hybrid electrode in different deformation states.(E) Polarization curves of MoSx@GMS/SWCNT under different deformation states.Copied with permission [207].Copyright 2016,Wiley-VCH Verlag.(F) I-t curves of MoS2/graphene films at different bending curvatures.(G-J) Optical photos of MoS2/graphene electrode in four different bending states.(K) Polarization curves of MoS2/graphene electrode in different bending states.Copied with permission [44].Copyright 2018,Royal Society of Chemistry.(L) Digital camera photographs of CoP NW/Hb electrode after 0,100,200 and 300 bending/extending cycles during linear-sweep voltammetry.(M) Polarization curves of CoP NW/Hb after 0,100,200 and 300 bends extending cycles.Copied with permission [45].Copyright 2017,Tsinghua University Press.

Moreover,the NW structure possesses rather efficient catalytic properties for HER [216].For example,compared with common high surface area nanoparticle and nanostructured film morphologies,catalytic NW arrays directly integrated with conducting electrodes could facilitate charge carrier transport to further enhance performance and improve stability [217].Huangetal.[45] developed a cobalt phosphide (CoP) nanonetwork catalytic electrode based on CoP NWs,which has excellent flexibility and maintains remarkable catalytic stability.The flexibility and structural integrity of the CoP network electrodes were evaluated under different bending times,as shown in Fig.10L.The polarization curve of the CoP network electrode remained almost unchanged after repeated bending in 1 mol/L phosphate-buffer solution for 300 times,which indicates the prominent catalytic stability of the electrode(Fig.10M).

To date,flexible electrodes for HER have been rarely reported but highly desired.In future research,the design of the novel flexible electrode should pay more attention to its mechanical properties.The excellent mechanical properties can make the catalyst resist the stress caused by the stretching and rupture of bubbles in the high-current catalytic reaction,thereby improving the mechanical stability of the catalytic system.

4.Summary and outlook

With the deepening of research,1D inorganic semiconductor NWs and 2D TMDs have made great progress in various aspects,which have been widely used in practical applications such as electronics and optoelectronics.In this review,we summarize the latest advances of 1D inorganic semiconductor NWs and 2D TMDs based flexible electronic devices,involving the physical properties and strain engineering of 1D inorganic semiconductor NWs and 2D TMDs and their applications in several typical flexible electronic devices.These flexible devices include photodetectors,sensors,energy storage devices,and energy conversion devices [218–222].

Despite the impressive progress has been made in 1D inorganic semiconductor NWs and 2D TMDs based flexible electronics,there are still challenges coupled with opportunities that require more dedicated investigations.In the following,we highlight some of the most important from the perspective of material synthesis,device fabrication and integration.

(1) Material synthesis.Low-dimensional materials play an important role in flexible applications due to their excellent mechanical and optoelectronic properties.However,the synthesis of high quality low-dimensional materials remains a challenge.For example,the size heterogeneity of 1D NWs results in significant differences in performance,requiring the development of efficient synthesis methods with precisely controllable morphology and size.Furthermore,although the different methods are available to construct NWs,the yield of NWs for scalable fabrication for the practical applications is low.Therefore,it is necessary to explore feasible methods for the large-scale fabrication of NWs with controllable structures and desirable properties.For 2D materials,highquality and large-scale synthesis methods need to be further optimized.For instance,most of the 2D layered materials can be obtained by the mechanical exfoliation method,but the lateral dimensions of the prepared samples are small and the thickness is not easy to control.Liquid phase exfoliation or chemical vapor deposition methods are difficult to control the number of layers,edge morphology,defect density,doping concentration and other parameters of the sample.Additionally,the synthesis of novel low-dimensional nanostructured materials is also one of the important research directions.For example,compared with 1D or 2D nanostructures of single materials,1D or 2D nanostructures with complex morphology composed of heterogeneous materials can realize functions that monomers cannot.In addition,1D or 2D heterogeneous nanostructures have a broad application prospect in various flexible devices and multifunctional systems.

(2) Device fabrication and integration.Currently,photolithography and printing processes are used for the fabrication of flexible devices.However,traditional photolithography processes are difficult to achieve low-cost and high-throughput nanofabrication.Printing processes enable low-cost and large-scale fabrication of flexible electronics,but it is difficult to reduce line resolution and alignment accuracy down to sub-micron,which limits the manufacturing of highly integrated flexible devices.In recent years,new device manufacturing technologies have emerged,such as interference lithography,laser direct writing,scanning probe lithography,directed self-assembly,nanoimprint lithography and 3D printing.However,all these technologies have shortcomings and need to be further optimized.Moreover,the integration of flexible devices with different functions would be the most exploratory and challenging topic for future research.The development of integrated systems with multiple functions might be a viable path towards commercial and industrial applications.

We believe that the breathtaking applications of 1D inorganic semiconductor NWs and 2D TMDs based flexible electronics in ubiquitous wearable devices are possible in the near future.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work,there is no professional or other personal interest of any nature or kind in any product,service and/or company that could be construed as influencing the position presented in,or the review of,the manuscript entitled.

Acknowledgments

This work was financially supported by the Natural Science Foundation of China (No.51902101),Natural Science Foundation of Jiangsu Province (No.BK20201381),Science Foundation of Nanjing University of Posts and Telecommunications (No.NY219144),and Postgraduate Research &Practice Innovation Program of Jiangsu Province (No.SJCX22_0254).