Flexible Photodetectors Based on 1D Inorganic Nanostructures

Flexible photodetectors with excellent flexibility, high mechanical stability and good detectivity, have attracted great research interest in recent years. 1D inorganic nanostructures provide a number of opportunities and capabilities for use in flexible photodetectors as they have unique geometry, good transparency, outstanding mechanical flexibility, and excellent electronic/optoelectronic properties. This article offers a comprehensive review of several types of flexible photodetectors based on 1D nanostructures from the past ten years, including flexible ultraviolet, visible, and infrared photodetectors. High‐performance organic‐inorganic hybrid photodetectors, as well as devices with 1D nanowire (NW) arrays, are also reviewed. Finally, new concepts of flexible photodetectors including piezophototronic, stretchable and self‐powered photodetectors are examined to showcase the future research in this exciting field.


1D Inorganic-Nanostructures-Based Flexible PDs
The photoresponse of 1D inorganic nanostructures to light irradiation with different wavelengths is mainly depended on their band gap. For example, metal oxides such as ZnO, [ 48 ] SnO 2 [ 49 ] and In 2 O 3 , [ 50 ] have large band gaps, thus their 1D nanostructures are widely used as the sensing candidates for high-performance UV PDs. For visible light PDs, some semiconductors with moderate band gaps such as CdS, [ 51 ] Si [ 52 ] and In 2 Se 3 [ 53 ] are chosen as the active materials. In 2 Te 3 [ 54 ] and InAsSb, [ 55 ] and semiconductors, with narrow band-gaps may be used to fabricate infrared PDs. Here, recent progress of fl exible, 1D, inorganic-nanostructure-based PDs are summarized, focusing on the above mentioned three types devices: UV PDs, visible PDs and infrared PDs.

Flexible Ultraviolet PDs
As an important type of optoelectronic device, UV PDs show wide ranging applications in fi re monitoring, biological, environmental sensors, space exploration, and UV irradiation detection. [56][57][58] High performance UV PDs have been widely developed based on 1D metal oxides or multicomponent metal oxides. In 2002, Yang et al. fi rst reported single-crystalline ZnO and SnO 2 NWs based UV PD. [ 59 ] The results revealed that the conductance of ZnO/SnO 2 NWs changed greatly when the devices were exposed to ultraviolet light irradiation. Hence, great attention has been paid on developing high performance UV PDs with 1D metal oxide nanostructures, such as binary metal oxides, multicomponent metal oxides, and complex metal oxides as well.

Flexible Binary-Oxide-Based PDs
1D binary metal oxide nanostructures are good candidates for fl exible UV PDs because of their excellent electronic properties, mechanical fl exibility, and stability. Up to now, fl exible UV PDs have been built on many kinds of 1D binary metal oxide nanostructures, including ZnO NWs/nanorods/nanobelts, SnO 2 NWs/nanobelts, TiO 2 nanorods, In 2 O 3 NWs, etc. [60][61][62][63][64][65] An example is the fl exible UV PDs developed by Albiss  transparency and fl exibility. [ 66 ] Figure 1 c shows the currentvoltage ( I-V ) properties of the device with and without bending when the device was exposed to 365 nm UV light and in the dark, respectively, while Figure 1 d shows the photocurrent versus light intensities curves of the device under bending for different cycles. These results indicated that the fl exible device based on ZnO nanorods has a good electronic stability and mechanical robustness.
Flexible UV PDs built on other 1D binary metal oxides nanostructures also exhibited attractive performances. For instance, a single SnO 2 microrod UV photoconductor on a fl exible substrate demonstrated excellent UV light selectivity and ultrahigh internal gain (1.5 × 10 9 ), much higher than other SnO 2 UV PDs. [ 67 ] The fi ndings in this research results in a novel and feasible method for realizing SnO 2 microrod UV PD with both fast response speed and ultrahigh gain. In addition, a fl exible UV PDs of ZnO NWs with 5-20 nm in diameter have also been fabricated using a UV decomposition process. [ 68 ] The signifi cant I on / I off ratio of fl exible device is about 11300% under 365 nm UV light. These studies indicate that 1D binary oxides have a great potential in fl exible UV PDs. [69][70][71]

Flexible Multicomponent-Oxide-Based PDs
Compared to simple binary oxides, multicomponent oxides exhibit superior performances, and functionalities of REVIEW ( [ 72,73 ] Furthermore, most of the multicomponent oxides have wide-bandgaps which make them essential components of a UV PD. 1D ternary oxides used in PDs, such as Zn 2 SnO 4 , [ 74 ] In 2 Ge 2 O 7 , [ 75 ] Zn 2 GeO 4 , [ 76,77 ] etc. have been investigated as functional materials for UV PDs. Flexible multicomponent-oxide-based PDs were fi rst fabricated with Zn 2 GeO 4 and In 2 Ge 2 O 7 NW networks, as shown in Figure 2 . [ 78 ] Zn 2 GeO 4 and In 2 Ge 2 O 7 NW networks were successfully prepared via a standard chemical vapor deposition (CVD) technology. After transferring the NWs to the fl exible PET substrate, the silver paste were then printed as parallel lines. As-fabricated fl exible PDs exhibited outstanding photoresponse performance to UV light irradiation.
Individual single-crystalline ZnGa 2 O 4 NW was also investigated as the UV sensing material. [ 79 ] Via a standard microfabrication process, the single-NW fl exible PD was fabricated: ZnGa 2 O 4 NWs were stripped and dissolved in isopropyl alcohol solution from the substrate, after that the NWs were dispersed on a PET substrate. The interdigitated electrodes (Cr/Au) were patterned at both ends of the NWs by photolithography techniques and lift-off process. The fl exible PD revealed a high response to 350 nm UV light illumination with an excellent photoresponse performance. The current on/off ratio is about 43 when the applied voltage is 5 V. The response and recovery times of the fl exible ZnGa 2 O 4 PDs are about 13 s and 9 s, respectively. The conductance of the device is hardly infl uenced by the bending stress, revealing the outstanding folding tolerance of the fl exible ZnGa 2 O 4 PD.

Flexible Complex-Structure-based PDs
Some complex structures, such as heterostructures, and superlattice structures, have unique electronic and photonic properties with promising applications in optoelectronics. The determination of the characteristics of the materials with these structures are not only the collection of the contributions by each component, but also the reactions occurring between the various components of the interface. [ 80 ] Recently, fi eld-effect transistors and fl exible UV PDs were successfully fabricated with InGaO 3 (ZnO) superlattice NWs. [ 81 ] The SEM image of the fl exible device based on InGaO 3 (ZnO) superlattice NWs is shown in the inset of Figure 3 a. Figure 3 a exhibited the I-V measurement of the fl exible PD under a monochromatic UV light or in the dark. It can be seen that the device has a good Ohmic contact between the Cr/Au electrodes on the PET substrate and the superlattice NW. Figure 3 b showed a good dependence between the photocurrent and the light intensity, which further proved a good photocapture in the superlattice NW. As shown in Figure 3 c is the response and recovery properties of the device. During four testing cycles, the PD still keeps its original photocurrent, which further confi rm that the as-prepared fl exible devices have good reproducibility. The infl uence of superlattice structure on the optoelectronic properties was proposed that recombination of the electron-hole pairs can be greatly decreased because of the spatial separation of the photogenerated carriers at the interface within the superlattice NWs. This work would be a reference for the superlattice NWs used in the fi eld of PDs. Undoubtedly, more research works will focus on this fi eld. Wei and his groups also reported a transparent UV PD based on the ZnO/SnO 2 heterojunction nanofi bers. Compared to pure SnO 2 or ZnO nanostructures the ZnO/SnO 2 device has a great enhancement of UV sensitivity due to the spatial of the photogenerated carriers. [ 82 ] Similar results were also found in other research works, such as ZnS/ZnO, [ 83 ] ZnO/graphene, [ 84 ] etc.

Flexible Silicon-NW-based PDs
Being the major material of the semiconductor industry, Si NWs have become one of the most emergent 1D materials which have been widely used in many application, such as solar cells, [ 93 ] fi eld effect transistors (FETs), [ 94 ] thermoelectric applications, [ 95 ] lithium batteries, [ 96 ] and PDs. [ 97 ] The use of Si NWs in PDs can enhance light harvesting and high quantum effi ciencies, but the rigidity of the structure prevents fl exible applications. Recently, the fabrication of highly fl exible, Si NWs (via a metal-assisted-chemical etching method) network based PDs was studied by Unalan et al. [ 98 ] They showed that the device provided both fl exibility and transparency, because both the active part and the electrodes are composed by NWs networks.  (Figure 4 c) reveal that the active area of the device was 3 mm. Photoresponsivity of the Si NW network PDs with different NW densities were also investigated. From the light on/off measurements, PDs were found to exhibit fully reversible switching behavior. As shown in Figure 4 e, the typical rise time was found to be 0.43 ms, while the fall time was 0.58 ms. In order to measure the fl exibility of the MSM PDs, the author fabricated the devices on PET substrate underwent a bending test. The photoconductor performance was recorded as a function of number of bending cycles for a fi xed bending curvature of 1 cm (Figure 4 f). The dark and photo current of the device measured as function of cycles up to a maximum bending cycles of 500 are shown in Figure 4 g. Bending resulted in a decrease in both the light and dark current of the device during the fi rst 200 cycles; however, further increase in the number of bending cycles did not lead to any change in the current. The main reason may be due to the loss of mechanical contacts between Si NW junctions and also Ag and Si NWs. This work provides the basis for the fabrication of cost-effective and fl exible PDs using Si NWs, which offer fl exibility to the Si world and could certainly be helpful for the fabrication of other optoelectronic and sensing devices.  Reproduced with permission. [ 78 ] Copyright 2012, Optical Society of America.
Si NWs can also be used in the fl exible avalanche PDs (APDs). Kim et al. reported the fabrication of APDs composed by p + -i-n + Si NWs on a fl exible plastic substrate. [ 99 ] The maximum responsivity and avalanche gain of the APDs are estimated to be 1.4 × 10 −3 A/W and 5.9 × 10 4 , respectively. The superior photoresponse properties of the fl exible APD are associated with the length of the intrinsic region of the NWs and the doping concentration. Furthermore, the APDs on the fl exible plastic substrates showed excellent recovery properties as the device was bent and returned to the fl at condition.

Flexible II-V-compound-based PDs
II-V group semiconductors holds appeal for a diverse range of applications. Examples range from the highly earth-abundant Zn 3 P 2 , which is an attractive material for photovoltaics, [ 100 ] to materials of lesser abundance such as Zn 4 Sb 3 , [ 101 ] which is among the most effi cient of thermoelectric materials, and Cd 3 As 2 , [ 102 ] which was recently identifi ed as one of the few known examples of a three-dimensional topological Dirac semimetal. Less well studied, Zn 3 As 2 is an earth abundant semiconductor with 1.0 eV band gap and the potential to realize high hole mobilities. [ 103 ] Recently, single-crystalline Zn 3 As 2 NWs have been synthesized which are 100-240 nm in diameter, and hundreds of micrometers in length by a simple CVD method ( Figure 5 a). [ 104 ] Then FETs and visible-light PDs are also fabricated and studied. The results showed that Zn 3 As 2 NW revealed a typical p-type characteristic with an effect hole mobility of 305.5 cm 2 V −1 s −1 . The photoresponse of the fl exible device has also been investigated. As shown in Figure 5 b is the current-time ( I-T ) curves of the fl exible PD under a white light irradiation with different bias voltage of 2, 4, and 8 V. At the bias of 8 V, the dark current was 1.20 µA, then the photocurrent increased to 7.4µA as the white light illuminated. Compared with the rigid device, the fl exible PD has a much lower photocurrent and dark current. It may be due to the worse contact between PET substrate and the NWs.   [ 81 ] that there is no signifi cant change of the conductance even after the fl exible PD is bent by 30, 60, 90, and 120 cycles.
Besides Zn 3 As 2 NWs, a series of other II-V compound-NWbased fl exible visible PDs have also been fabricated including, Zn 3 P 2 , [ 105 ] Cd 3 P 2 , [ 90 ] and Cd 3 As 2 . [ 106 ] These II-V NWs exhibited great photoresponse to the visible light due to their long minority carrier diffusion length, large optical absorption coeffi cient, and high carrier mobility. Meanwhile, fl exible devices based on these materials also showed excellent fl exibility and electrical stability. Through these researches we can see that II-V NWs have a potential application as next-generation photoconductive materials to building blocks the fl exible nanooptoelectronic devices.

Flexible Sulfi de-Based PDs
β-In 2 S 3 NW with a band gap of 2.0-2.3 eV has a potential application for high-performance fl exible visible PDs. as reported by Shen et al. [ 107 ] The as-prepared fl exible PD showed an ultrahigh I on / I dark ratio of up to 10 6 , which is six orders of magnitude higher than the best I on / I dark values reported till now, and an excellent response to visible incident light with quantum effi ciency and responsivity as high as 2.28 × 10 7 % and 7.35 ×  process, which were used to fabricate fl exible visible PDs with excellent photoresponse properties. [ 108 ] Very recently, Amos et al. introduced a universal and inexpensive way to fabricate CdS nanobelts onto a fl exible substrate by utilizing the UV photooxidation patterned to directly induce the nucleation and growth from aqueous solutions (shown in Figure 6 ). [ 109 ] This technology is highly compatible with polymer substrates due to benign solvents utilized in the preparation of semiconductor and the absence of high temperature processing. As the fl exible device was exposed under 514 nm light irradiated, it possessed an excellent photoresponse with the detectivity of 3 × 10 11 cm Hz 1/2 W −1 at a light frequency of 90 Hz.

Flexible Infrared PDs
Compared with the other types of PDs, infrared PDs have great importance in numerous civilian and military applications, including heat capacity mapping, thermal remote sensing, target tracking and environment monitoring. [110][111][112][113] Common materials for infrared PDs include GaSb, [ 114 ] GaAs, [ 115 ] InAs, [ 116 ] and HgCdTe. [ 117 ] Recent years, 1D nanostructures are also used in infrared PDs. For example, Lu et al. reported InAs NW PDs with a detection wavelength about ≈1.5 µm at room temperature with a photoresponsivity of 5.3 × 10 3 AW −1 . [ 118 ] In order to avoid the negative effects of surface defect states and atmospheric molecules, the author introduced a half-wrapped topgate by using 10 nm HfO 2 as the top-gate dielectric.
Till now, there are only a few examples could be found on fl exible infrared PDs with 1D inorganic nanostructures as the sensing materials. Recently, single GaSb NW based PDs were fabricated on PET substrates, which exhibited high responsivity, fast-response, and long-term stability in photoswitching over a broad spectral range from ultraviolet to near infrared. Due to the much lower dark current on PET, the device has lower noise equivalent power of 2.0 × 10 −12 W Hz −1/2 . [ 119 ] Although this fl exible device showed great photoresponse properties and good fl exibilities, the detective range was still less than 800 nm which limited its use as an infrared PD.
Carbon nanotubes (CNTs) have been extensively studied as electronic, photoelectronic, and piezoresistance materials. [ 120 ] With a wide absorption light wavelengths of 354-2480 nm, PDs based on CNTs can be used to detect near-infrared light. However, it still remains a challenge to achieve fl exible CNT-based infrared PD. Recently, Paltiel presented a simple wet chemistry technology for inkjet printing fl exible PDs based on CNTs and CdTe nanocrystals (shown in Figure 7 ). [ 121 ] Figure 7 a is a   [ 104 ] schematic drawing of the fabrication process for multi-walled CNTs (MWCNTs)-nanocrystals assembly. In this process, the MWCNTs lines are printed between a net of metal contacts and the CdTe NCs are adsorbed on top of the CNT lines. A typical high resolution SEM view of the printed CNT line with the adsorbed NCs is shown in Figure 7 b. The as-fabricated device has an obvious response to 980 nm infrared light at room temperature, with a fast response and recovery speed.

Flexible Organic-Inorganic-Hybrid-Based PDs
Organic-inorganic hybrid PDs have not only the advantages of inorganic based devices such as the broad-band absorption and the excellent intrinsic carrier mobilities, but also the features of organic based devices, including tunable functionality and easyformation properties. [122][123][124] In addition, the reaction at the interface between the organic polymers and inorganic semiconductors is good for the photoresponse properties. [125][126][127][128] In recent years, 1D organic-inorganic hybrid nanostructures have been widely investigated, which usually exhibited superior rectifi cation, lightemitting, and photovoltaic behaviors. [129][130][131][132][133] However, the inorganic components in these hybrids are either quantum dots or nanoparticles. And devices with 1D inorganic semiconductor nanostructures as the inorganic component are rarely studied.
We developed the fi rst fl exible hybrid PDs with P3HT and CdSe NWs as the components. [ 134 ] P3HT was chosen as the organic component because it has great absorption in the visible range and high hole transport rate. [ 135 ] CdSe NWs are utilized as the inorganic part because of its excellent electrical conductivity and caontrollable surface charge. [ 136 ] Moreover, both the CdSe and P3HT can adsorb the spectra in the visible spectrum. Figure 8 a illustrates the mechanical fl exibility of the hybrid PD under different bending conditions. The P3HT:CdSe NW hybrid device shows fast response properties with the rise and decay times measured on the millisecond timescale at high-frequency light signals of 50 Hz. The probable enhancing mechanism for the hybrid PD has also been investigated. The interface of the hybrid fi lm is a key factor in charge dissociation and transportation. When the photo-carriers are generated, the electrons transfer to the material with higher electron affi nity; in contrast, the holes transfer to the lower ionization potential. As shown in Figure 8 b, in such a hybrid system, CdSe NWs could have a large interfacial area by combining with conjugated P3HT. The schematic energy-level diagram of the hydrid fi lms is shown in Figure 8 c, we can see that P3HT are utilized as the hole-transport material, while CdSe NWs is an effective electronic-transport material, indicating the superlative photocurrent revealed in hybrid PDs. Furthermore, a 3D interconnected network in the hybrid fi lm can be formed as CdSe NWs dispersed in the P3HT matrix, resulting in a large interface area for charge separation. Hence, good carrier transport and   [ 109 ] long-lived charge separation could be obtained in the hybrid PD. These effects led to an impressive improvement of photoresponse of the hybrid PD compared with the single-component devices. Similar results were also reported in other hybrid systems, such as the n-type phenyl-C61-butyric acid methyl ester (PCBM)-p-type Cd 3 P 2 NWs system, [ 90 ] and the n-type PCBM-ptype GaP NW-based, hybrid fl exible PDs. [ 137 ] The above studies exhibited good performance of the hybrid PDs, but the photoresponse properties can still be improved by changing the kind of organic and inorganic materials. Curry and his team reported that the photoresponse of the C60-nanorod-based PD could be improved 400 times due to an ultralow photodoping mechanism with a detectivity of >10 9 Jones, a rise time 60 µs, and a linear dynamic range of 80 dB, which can measure 250 kHz AC signals. [ 138 ] Figure 9 a shows a typical SEM image of the C60 nanorod fi lm of more than a dozen micrometers in length and hundreds of nanometers in diameter. Figure 9 b,c shows a schematic illustration of the PD and a photograph of the C60-nanorod-based fl exible device. It can be seen that the device has a good fl exibility. The organic TCNQ, R6G, P3HT and inorganic semiconductor PbS and CdSe NCs have been investigated to study the effect of photodoping on C60 nanorods. Figure 9 d,e are the optical absorption spectrum of the photodopants. From Figure 9 d, it is revealed that the use of P3HT and CdSe NCs as the photodopant provides 400-and 50-fold improvement, respectively. Figure 9 e shows the spectral responsivity of the PD based on PbS NCs photodoped at different wavelength and applied electric fi eld. The devices also show a great enhancement in sensitivity, meanwhile they also display broadband UV-vis-NIR photosensitivity (350 to 1150 nm). These enhancements are achieved due to the ultralow photodoping mechanism. Some other works focused on the organic-inorganic hybrid PDs have also been reported, but most of them have not applied them in fl exible devices.

1D-Nanostructure-Array-Based Flexible PDs
1D nanostructure arrays have become an important subject for electronic and optoelectronic applications. [ 139 ] To achieve the expected performance, 1D nanostructures with good alignment and precise patterning have been widely needed by most electronic applications. In consequence, more and more has been devoted to developing the alignment/patterning of inorganic 1D nanostructures. Recently, several techniques such as the bubble blown method, [ 140 ] contact printing, [ 141 ] Languir-Blodgett (LB) technique, [ 142 ] electric/magnetic fi eld alignment, [ 143 ] and microfl uidic-assisted NW alignment, [ 144 ] have been investigated. Compared with the single 1D nanostructure based PD, the aligned 1D nanostructure array is an effi cient and simple remedy for low-level photocurrents. Furthermore, compared with the random NW network, there is no contactpotential barrier among the 1D nanostructure arrays which is benefi cial for electronic transmission. The capability of assembling/printing different 1D nanostructures with tunable atomic composition on fl exible substrates is very important in the fi eld of fl exible electronic with a broad spectrum.  In the following, two types of NW array based fl exible PDs are introduced: direct growth of aligned NWs and contact printing aligned NWs.

Direct Growth of NW Arrays on Flexible Substrates
Selecting a suitable substrate is an important issue for development of both the method of direct growth of 1D nanostructure arrays and the structure of device design. For soft/fl exible substrates, this is a diffi cult task, because high-quality and aligned 1D nanostructures typically require specifi cally oriented, smooth substrates, as well as treatment with extreme temperatures. Among fl exible substrates, paper with extreme surface roughness and thermal lability displays signifi cant engineering challenges for the realization of 1D nanostructure growth. Chen et al. reported the controlled growth of aligned single-crystal ZnO NWs on an economic and green paper substrate with a nonhazardous chemical solution and low temperature process. [ 145 ] The innovation of this work is to regulate the paper's surface properties to control its conducting and semiconducting. The schematic of the paper PD is shown inset of Figure 10 a. The typical I-V properties of the PDs under continuous UV light illumination and in the dark were also shown. Figure 10 b shows the photoresponse switching behavior of the PD. By modulating UV exposure, the photocurrent can be reproducibly switched with sensitivity of 60 at a low bias voltage of 5 V. Stable electrical performance at various twisting or bending conditions is an important factor for fl exible substrates. Systematic analysis of the mechanical stability was carried out by controlling the PD to a specifi c bending angle and width. The results revealed that the fl exible paper PD has a tiny change in the turn-on voltage with increasing of bending angle because there are small traps formed between the NWs and the electrodes during bending.
Aside from paper, NW arrays can also be grown on other fl exible substrates. For instance, ZnO NW array is grown on Kevlar fi ber via the CVD method. [ 146 ] Fiber-based fl exible UV PDs are then fabricated with these NW arrays. To speed up the UV response, P-type conductive polymer poly (3,4-  fl exibility of the fi ber-based UV PD, the device was bent into a "U" shape. Even under such bending conditions, it still has a photosensitivity to UV light which means that the device has a very good fl exibility. Such NW arrays grown on fl exible polymer fi ber used for PDs has also been reported by Bayindir. [ 147 ] Some other methods to grow 1D nanostructure arrays on fl exible substrates were also studied. Electrospinning is a typical method to synthesize inorganic NWs and nanofi bers which can spin the NWs directly onto the substrate. However, most of the electrospun NWs are disorderly and have rarely been employed for PDs. Recently, Fan and his group introduce an all-printable fabrication technique to uniquely produce parallel ZnO NWs arrays onto a fl exible substrate via the near-fi eld electrospinning technique, as shown in Figure 11 . [ 148 ] Electrodes with a spacing of 2 µm were ink-jet printed onto the NWs to investigate the optoelectronic properties. The device showed the highest detectivity and responsivity among the previous reports for NW-based devices. The exciting results in this research can be used as a guideline utilizing other system materials to design high performance PDs.

Contact-Printing of Aligned NWs on Flexible Substrates
The contact-printing method has proved to be an simple, and effi cient way to assemble high-density 1D nanostructure arrays on a large scale. [149][150][151] This method can transfer the NWs to the receiver substrates to align the disorderly NWs into parallel arrays with a wide range of applications in electronic fi elds. [151][152][153][154] Flexible PDs based on NW array fabricated by the contact printing method were reported by several groups. Bai et al. reported an integrated ZnO NW UV fl exible PDs at the macroscopic scale. [ 155 ] Figure 12 a,b show a schematic diagram and SEM image of the device with Ag electrodes pressed onto the aligned NWs. As shown in Figure 12 c, the I-V measurement confi rmed the good Ohmic contact between the ZnO NWs and the Ag electrodes. The results show that as the device is exposed under UV radiation, the photocurrent has a great increased compared with the dark current. The I-T property of the device at a 4.5 mW cm −2 UV irradiance further confi rm the ultrahigh on/off ratio which indicates the capacity to detect weak UV light. Compared to the individual NW devices, NW arrays devices have a great enhancement of photoresponse performance.
Similar improvement of photoresponse was also observed in InP NW [ 156 ] and Zn 3 P 2 NW [ 157 ] arrays, as well as in ternary oxide NW arrays. [ 158 ] All the fabricated flexible devices exhibited good flexibility, and electrical stability. These merits demonstrate that the contact printing is a good method for future fabrication of electronic and optoelectronic nanodevice.   b) The plot demonstrates the photoresponse at 5 V bias voltage. c) I-V characteristics of different p-n junction diode device structures. d) Electrical properties measured at different bending angles. Reproduced with permission. [ 145 ]

New Concepts in Flexible PD Design with 1D Inorganic Nanostructures
Next-generation electronic devices designed with a compact integration and small scale have been of widespread interest for scientists from different research areas. Nanoscale PDs, as one of the most widely used electronic devices, have shown great improvement from the fast development of nanotechnologies and nano-science. Among the efforts devoted to this enhancement, some new concepts for fl exible PDs based on 1D nanostructure are discussed in this section.

Piezo-Phototronic PDs
The piezo-phototronic effect, which exists in wurtzite structure materials such as CdS, GaN and ZnO, has great potential in optoelectronic devices. Using strain, the properties of charge transport and recombination process can be regulated at the metal-semiconductor interface or the p-n junction. Previous reports have discussed more fundamental principles of this effect in detail. 1D nanostructures could be very useful in the preparation of strain controlled piezoelectronic devices where the deformation can be controlled by implementing the device on a fl exible substrate. The piezo-phototronic effect combined with fl exible optoelectronics technology is used applied in solar cells, light-emitting diodes, PDs etc. for enhancing the performance of the device. [159][160][161] Recently, Wang et al. [ 162 ] prepared a piezo-potential enhanced PD based on the ZnO-CdS core-shell NW, and then proposed a theoretical model. Strain induced piezo-potential can further enhance the performance of the PD by modulation of the Schottky barrier heights (SBHs) at the source and drain contacts. They illustrated how the SBHs and photocurrent of the PD under illumination have been affects by the piezoelectric polarization. Moreover, when the PD is set up to a -0.31% compressive strain the photoresponse of the ZnO-CdS device can be further improved for 10 times due to the piezo-phototronic effect ( Figure 13 ). In addition, Liu et al. [ 163 ] found that, at a low light intensity, the piezo-phototronic effect greatly enhances the sensitivity for weak light detection. But as the light intensity increased, the SBHs got lower which gave the I-V curve a similar form to that of the Ohmic contact, so that the contribution of piezo charges had a weak infl uence compared with that of in low-power illumination.

Stretchable PDs
Stretchable electronic devices have great advances for implantable devices in the human body. [164][165][166] Stretchable devices, such as pressure sensors, gas sensors and PDs, could also be used in industry, bio-organs, and under harsh environments. [167][168][169][170] Among these applications, stretchable PDs could be used to convert light as optical signal into an electrical signal. Stretchable PDs can be integrated with biological systems including electronic eye cameras, wearable monitoring devices as well as many other applications. However, very few reports have been focused on 1D nanostructure based stretchable PDs.  [ 155 ] www.MaterialsViews.com www.advancedscience.com Adv. Sci. 2016, 3,1500287 Poly dimethylsiloxane (PDMS) with high stretchability, excellent optical transparency and good biocompatibility is one of the most applied substrate for stretchable devices. Lee and his group fi rst introduced a lithographic fi ltration technology to manufacture embedded NWs based stretchable PDs. [ 171 ] The PDs keep their functionalities even as they are stretched up to 100% condition. However, the I on / I off ratio decreased when the device stretched with a slower response and recover rate, due to the NW-polymer chain interactions considering the low oxygen content and slow gas diffusion rate. Then, Je et al. employed high-performance, stretchable UV-vis-NIR NWPDs by a direct-writing, meniscusguided method. [ 172 ] They employed an effective method to grow NW arches between two electrodes on PDMS substrates as schematically illustrated in Figure 14 a. The NWPD array exhibited superior stretch ability (up to 100%) and fl exibility, as obviously investigated by a photograph (Figure 14 b). The photoresponse of the NWPDs was almost unchanged even under substantial or repeated stretching. In addition, under stretching of up to 100%, the on/off ratio and response time of the device were almost constant. These outstanding properties clearly display the great photoelectrical stability under ultra-stretching states, because the NW arches with the stretchable architecture can relax the external strain. The direct writing method of fabrication of NW arches presents potential application for next-generation stretchable and high-performance photoelectronic devices.

Integrated Self-Powered PDs
Integrated self-powered nanosystem with power supplies and functional nanodevices can be used in various applications, such as medical therapy, environmental monitoring and optoelectronics. [173][174][175] Integrated PDs received extensive attention since they may play an important role in the particular applications, including chemical and biosensing and wireless sensor networks. [ 176 ] Due to the production of devices that do not require an external power supply, the extra weight of the system can be avoided. In general, this type of self-powered PD is made up of a light sensor, an electrical measurement system, and a power unit. For instance, Wang et al. power a fl exible integrated ZnO NW based UV PD by a fl exible transparent nanogenerator (FTNG). [ 177 ] Figure 15 a shows the schematic of a self-powered nanosystem. The corresponding voltage drop is about 0.2 V on the UV PD with the large resistance, when the UV light is off. Since irradiated under the UV light, the carriers increase leading to the decreasing of the resistance. More carriers can be generated with the UV light intensity increased. Meanwhile the voltage drop decreases further. By monitoring the changes of the voltage on the device, UV light can be quantitatively detected. Therefore, a FTNG can be successfully used to drive the device to form a fl exible selfpowered UV PD. Our group also fabricated several kinds of self-powered PD systems with different device structures. For example, CdSe NW PDs were integrated with a fl exible all-solidstate GeSe 2 supercapacitor to form a self-PD nanosystem without any external bias voltage. [ 178 ] A self-powered visible PD system driven by graphene microsupercapacitors was also recently designed. [ 179 ] The optimized electrodes in the microsupercapacitor device were utilized as the source and drain electrodes of the CdS PD as well, confi guring an integrated, self-powered, visible light PD without an external power source. Compared to the time-dependent photoresponse of the PD driven by an external power source, the integrated PD system showed similar performance, indicating the potential applications of the integrated self-powered systems for next generation highly compacted, highly fl exible and extremely light electronics.
Although integrated self-powered PDs are generally planar, fi ber-based integrated PDs have also been studied recently. However, compared with the planar integrated self-powered PDs which may have a restriction for the actual applications, a fi ber-based integrated PD may have its special advantages. Recently, a fi ber-based integrated Reproduced with permission. [ 177 ] Copyright 2012, American Chemical Society. Reprinted with permission from ref. [ 177 ] ©2012, American Chemical Society. self-powered PD has been successfully fabricated. [ 180 ] In this system, a microscale fl exible asymmetric supercapacitor (SC) using Co 3 O 4 NWs on titanium wire and graphene on carbon fi bers electrodes was used as energy-storage device. Meanwhile, a two-dimensional graphene layer was used as the light-sensitive material. The device shows an excellent photoresponse to white light. The photoresponse mechanism of the integrated system can be explained that when the device exposed to the light, the electron-hole pairs became separated under an external fi eld supplied by the fully charged SC, leading to the increased leakage current of the SC. Hence, by monitoring the changes of the leakage current, photodetection can be accomplished. As a fl exible integrated system, the performances of both the SC and PD changed little when the device was bent into different states, demonstrating the excellent fl exibility of the integrated self-powered PDs which combined a SC and a PD on a single fi ber.

Summary and Outlook
In this review, the most recent developments on 1D, inorganicnanostructure-based, fl exible PDs have been presented. We have given a summary of the applications of inorganic 1D nanostructures and the performance of advanced fl exible PDs fabricated using such nanomaterials as reported in the latest literature. The fascinating achievements to date should encourage more researchers to make additional efforts to tackle new challenges in the future.
To further improve the performance of nanostructured PDs, the properties of 1D nanostructures and the design of the fl exible devices should be paid more attention in the future. Although the research on various 1D nanostructures has made remarkable progress and signifi cant achievements, some problems still exist. Firstly, the effect of size heterogeneity on the performance is very obvious. So more precise control of morphology, hierarchical, crystallization, and orientation assembly is urgently needed. However, 1D nanostructures grown by bottom-up processes still have challenges because the small dimensions have a tendency to aggregate. To address this problem, two effective approaches can be used. One is a novel 3D printing method used to print quality NWs or uniform NW arrays directly. The other is the top-down approaches improving which has the advantage for large scale integration. Secondly, uniform 1D nanostructures still do not ensure the uniform physical and chemical properties, electronic transport properties etc. It is very important but still a challenge to minimize the variability NW-to-NW. Thirdly, for fabricating and integrating the scalable fl exible PDs, the yield of 1D nanostructures is still signifi cantly insuffi cient. Nowadays, there are more or less advantages or weaknesses on the 1D nanostructures assembly techniques. This situation will create some issues in printing 1D nanostructures accurately on fl exible substrates. Hence, more suitable 1D nanostructure assembly methods need to be prepared to provide better quality of 1D nanostructures for fl exible PDs.
Although signifi cant progress about the design of fl exible PDs has been made, there are still challenges and future directions to study. Firstly, compared with the classical PDs, fl exible PDs based on 1D nanostructures are still at a primary stage. Because 1D-nanostructure-based fl exible PDs have much lower properties in bandwidth and effi ciency and the performance of between devices is variable; their progress and applications have been greatly confi ned. To solve these problems, we should develop new measurement and evaluation technologies. If an effective method can be employed to guarantee both outstanding fl exibility and high performance for PDs, more commercial products based on 1D nanostructures will be developed. Secondly, the biggest challenging to put the nanoscale fl exible PDs into commercial applications is their high cost. It is well known that most of the assembly of 1D nanostructures into fl exible PDs still needs complex technology such as lithography processes with expensive instruments and time-consuming. Hence, more attention should be focused on how to achieve low-cost preparation of fl exible PDs on a large scale. Printed electronics have the advantages of large scale, fl exibility and low-cost which may be good for fabricating large scale fl exible PDs and needs more studies from researchers. At last, for future directions on fl exible PDs based on 1D nanostructures, integrated systems with multi-functional nanodevices will become a hot topic for next generation fl exible nanodevices. If one can assemble other functional nanodevices with a PD into a nanosystem, a large variety of functions can be incorporated, and then it can be introduced to the market with great potential of commercial and industrial applications.