Crystal-structure-dependent piezotronic and piezo-phototronic effect of

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Functional Inorganic Materials and Devices

Crystal-structure-dependent piezotronic and piezophototronic effect of ZnO/ZnS core/shell nanowires for enhanced electrical transport and photosensing performance Sehee Jeong, Min Woo Kim, Yong-Ryun Jo, Tae-Yun Kim, YoungChul Leem, Sang-Woo Kim, Bong-Joong Kim, and Seong-Ju Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06192 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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ACS Applied Materials & Interfaces

Crystal-structure-dependent piezotronic and piezophototronic effect of ZnO/ZnS core/shell nanowires for enhanced electrical transport and photosensing performance Sehee Jeong,a Min Woo Kim,b Yong-Ryun Jo, b Tae-Yun Kim, c Young-Chul Leem,b Sang-Woo Kim,c Bong-Joong Kim,b and Seong-Ju Park*,a,b

a

Department of Nanobio Materials and Electronics, Gwangju Institute of Science and

Technology, Gwangju, 61005, Republic of Korea b

School of Materials Science and Engineering, Gwangju Institute of Science and Technology,

Gwangju, 61005, Republic of Korea c

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon,

16419, Republic of Korea

*Corresponding author. E-mail: [email protected]

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ABSTRACT We report the crystal-structure-dependent piezotronic and piezo-phototronic effect of ZnO/ZnS core/shell nanowires (CS NWs) having different shell layer crystalline structures. The wurtzite (WZ) ZnO/WZ ZnS CS NWs showed higher electrical transport and photosensing properties under external strain than the WZ ZnO/zinc blende (ZB) ZnS CS NWs. The WZ ZnO/WZ ZnS CS NWs under a compressive strain of −0.24% showed 4.4 and 8.67 times larger increase in output current (1.93×10−4 A) and photoresponsivity (8.76×10−1 A/W) than those under no strain. However, the WZ ZnO/ZB ZnS CS NWs under the same strain condition showed 3.2 and 2.16 times larger increase in output current (1.13×10−4 A) and photoresponsivity (2.16×10−1 A/W) than those under no strain. This improvement is ascribed to strain-induced piezopolarization charges at both the WZ ZnO NWs and the grains of the WZ ZnS shell layer in WZ ZnO/WZ ZnS CS NWs, whereas piezopolarization charges are induced only in the ZnO core region of the WZ ZnO/ZB ZnS CS NWs. These charges can change the type-II band alignment in the ZnO and ZnS interfacial region as well as the Schottky barrier height at the junction between semiconductor and metal, thus facilitating electrical transport and reducing the recombination probability of charge carriers under ultraviolet irradiation.

KEYWORDS: ZnO/ZnS core/shell nanowire, piezotronic effect, piezo-phototronic effect, crystal structure dependence, photosensing


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1. INTRODUCTION Use of the strain-induced piezoelectric effect has drawn enormous attention because it can facilitate the high performance of a wide variety of devices such as field-effect transistors, photodetectors, light-emitting diodes, solar cells, spintronic and sensing devices.1-7 To date, most of the aforementioned devices demonstrating piezotronic or piezo-phototronic effects, which result from two- or three-way coupling of semiconductor properties, piezoelectricity, and photoexcitation, are based on the one-dimensional ZnO nanowires (NWs) with a non-symmetric wurtzite crystal structure.8-9 Numerous approaches have been proposed, including a design for an optimized configuration of the ZnO NWs or interface engineering between the ZnO NWs and metal electrode or other semiconductor layers combined with the piezoelectric effect, to improve the performance of charge carrier transport, photoresponsivity, spin precession and various sensing properties.5-6, 10-15 Recently, impressive photosensing performance via the piezo-phototronic effect has been reported in diverse composite structures based on ZnO NWs such as core/shell (CS) NWs, facilitated by the separation of the charge carriers into different conducting channels of the CS NWs.16-20 However, most research has been focused on the piezoelectric effect of only the core region in the CS NWs while that of the shell region has been disregarded, though piezoelectric materials have often been used in these shell regions. The crystal-structure-dependent piezoelectric effect of the shell layer can also effectively modify the electronic and optoelectronic properties of devices, and it is crucially important to consider this aspect for highperformance device engineering. Several studies have carried out the crystal-structure-dependent piezoelectric and piezoresistive effects of InAs NWs, and numerical simulations reporting


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comprehensive comparison of the strains and the piezoelectric potentials in two different crystalline InAs/InP CS NWs.21-22 However, the crystal-structure-dependent piezotronic and piezo-phototronic effects of a CS NW configuration have not been demonstrated experimentally. Rai et al. demonstrated a UV/visible photodetector based on the ZnO/ZnS CS NWs with piezophototronic effect, but their ZnS shell layer was only zinc blende structure which is absence of a piezoelectric effect.16 In this work, the influence of external strain by bending process on the electrical transport and photoresponsivity of ZnO/ZnS CS NWs with different ZnS shell layer crystalline structures is measured for the first time. Here, we evaluate the electrical and photosensing properties of ZnO/ZnS CS NWs with different ZnS shell layer crystalline structures on the flexible substrates to determine the crystalstructure-dependent piezotronic and piezo-phototronic effects for prospective application in multimodal functional nanoscale sensors. We demonstrate the notable influence of the crystalstructure-dependent piezoelectric effect on the charge transport and photoresponse characteristics of the wurtzite (WZ) ZnO/WZ ZnS CS NWs and the WZ ZnO/zinc blende (ZB) ZnS CS NWs under ultraviolet (UV) light irradiation.

2. RESULTS AND DISCUSSION Figure 1a shows a schematic image of ZnO NWs device structure grown by using the hydrothermal methods, where the ZnO seed layer with a silver (Ag) as a bottom electrode was deposited on the flexible polyethylene terephthalate (PET) substrate. Then, the ZnO NWs on the PET were coated using different solutions to produce the ZnO/ZnS CS NWs with different ZnS shell layer crystalline structures. To deposit a ZnS shell layer with a WZ crystalline structure, we firstly immersed the ZnO NWs on the PET substrate in a solution of sodium sulfide (Na2S) and


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then in a solution of zinc nitrate ((Zn(NO3)2) following the procedure of previous studies.23-25 The ZnS shell layer with a ZB crystalline structure was deposited by immersing the ZnO NWs in a 0.1 M thioacetamide solution at 90°C for 50 min.26-27 These synthesis methods are facile, simple and low-cost with a high yield at low process temperatures. The details of complete fabrcation process can be found in the Experimental Section, and the strategies used herein for the fabrication and synthesis of ZnO/ZnS CS NW devices with different ZnS shell layer crystalline structures are schematically illustrated in Figure S1 in the Supporting Information.

Figure 1. (a) Schematic image of ZnO NWs device before synthesizing for the ZnO/ZnS CS NWs devices with different ZnS shell layer crystalline structures. (b,c) Low-magnification bright-field TEM images of ZnO/ZnS CS NWs with (b) WZ and (c) ZB shell structures. (Insets) SAED patterns of the corresponding CS NW. (d,e) High-resolution TEM images of the corresponding regions in the low-resolution image of (d) WZ ZnO/WZ ZnS (panel (b)) and (e) WZ ZnO/ZB ZnS CS NWs (panel (c)). (Insets) FFT of the entire image of the CS NW. (d',d",e',e") FFT images of corresponding regions in the ZnO core NWs of (d') WZ ZnO/WZ ZnS and (e') WZ ZnO/ZB ZnS CS NW, as indicated by white boxes in (d) and (e); and the ZnS shell layer of the (d") WZ ZnO/WZ ZnS (green box in (d)) and (e") WZ ZnO/ZB ZnS (yellow box in (e)) CS NW.


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In order to reveal the crystal structures of the ZnO/ZnS CS NWs with different shell layer crystalline structures, transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analysis were performed. Figures 1b and 1c show a low-magnification bright-field TEM image of a WZ ZnO/WZ ZnS CS NW and a WZ ZnO/ZB ZnS CS NW, respectively. The corresponding SAED patterns are also shown in the insets of Figure 1b and 1c. These data indicate that both types of ZnO/ZnS CS NWs are composed of single-crystalline ZnO core NWs grown in the [0001] direction and polycrystalline ZnS shell layers. The SAED patterns exhibit strong diffraction spots corresponding to the (21 12), (0002), and (21 10) lattice planes of the single-crystalline ZnO core NWs. The SAED patterns also reveal weak ring patterns corresponding to the (0002) and (21 10) lattice planes of a polycrystalline WZ ZnS shell, and the (111), (200), (220) and (311) lattice planes of a polycrystalline ZB ZnS shell layer. To further investigate the crystal structure of the ZnO/ZnS CS NWs, high-resolution TEM images of two different ZnO/ZnS CS NWs with a beam direction of [01 10] as shown in Figure 1d and 1e were obtained from the interfacial regions between the core and shell layers, as shown by the boxed areas in Figure 1b and 1c, respectively. We then selected and evaluated a region in the ZnO core and one in the ZnS shell for both the WZ and ZB ZnS shell layers, as indicated by the white (core) and green/yellow (shell) dashed boxes in Figure 1d and 1e. The fast Fourier transforms (FFTs) of these image regions are shown in Figure 1d' and 1d" and Figure 1e' and 1e", respectively. The interplanar spacings for the ZnO core regions of both types of ZnO/ZnS CS NWs (indicated in the white dashed boxes in Figure 1d and 1e) are 2.6 and 1.6 Å, respectively, which correspond to the (0002), and (21 10) lattice fringes, respectively. The FFT (Figure 1d") of the WZ ZnS shell region image (green dashed box in Figure 1d) reveals lattice spacings of 1.9 and 3.1 Å, which correspond to the (21 10), and (0002) interplanar spacings,


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respectively. Finally, the FFT (Figure 1e") of the ZB ZnS shell region image (yellow dashed box in Figure 1e) shows lattice spacings of 2.7 and 3.1 Å, which correspond to the (200), and (111) interplanar spacings, respectively. These data clarify that the grains of the ZnS shell layers as marked by the green and yellow dashed boxes in Figure 1d and 1e possess single-crystalline WZ and ZB structures, respectively. Besides, TEM measurements show that the thickness of ZnS shell layer with WZ and ZB crystal structure is very uniform and their average thickness is 12±1 nm, indicating that the effect of thickness difference on the device performance is negligibly small. Figure S2 (see Supporting Information) shows a top view of the field emission scanning electron microscope (FESEM) images of the ZnO/ZnS CS NWs with WZ and ZB ZnS shell layers on the flexible PET substrates. A slightly rough surface was observed on the shell regions of both the ZnO/ZnS CS NWs, and their lengths and diameters were in the range of 600–800 nm and 60–80 nm, respectively.

Figure 2. Current-voltage (I-V) curves of (a) WZ ZnO/WZ ZnS and (b) WZ ZnO/ZB ZnS CS NWs under external compressive strain. Figure 2 shows the current–voltage (I-V) curves of the ZnO/ZnS CS NWs with WZ and ZB ZnS shell layers under compressive strain in a range of 0 to −0.24%. When no strain was applied,


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the current levels of the two different ZnO/ZnS CS NWs were not significantly different, as shown in Figure 2a and 2b. However, the current levels of both types of ZnO/ZnS CS NWs were distinctly higher than that of ZnO NWs, as shown in Figure S3a in the Supporting Information. This higher current level is because the distribution of electrons and holes arising from the typeII band structure of the ZnO/ZnS CS NWs remarkably enhances the electrical transport, whereas electron trapping at the surface states of the ZnO NWs hinders the electrical transport.28-29 Figure 2 also shows that the current flowing through both devices increases monotonically at the same forward bias with increasing compressive strain. In particular, the electrical properties of the ZnO/ZnS CS NWs with WZ and ZB ZnS shell layers are distinctively different when a compressive strain is applied. To compare the output currents of the two different ZnO/ZnS CS NWs under external strain, the relative increase of the output current is defined as ∆

=

,

(1)

where Istrain and I0 are the output current with and without a certain external strain, respectively. The results show that for WZ ZnO/WZ ZnS CS NWs, the value of ΔI/I0 is 340% under a compressive strain of −0.24% (inset of Figure 2a), while ΔI/I0 is 223% under the same strain condition for WZ ZnO/ZB ZnS CS NWs (inset of Figure 2b). The remarkably enhanced electrical transport properties of WZ ZnO/WZ ZnS CS NWs under an external compressive strain can be ascribed to the piezoelectric effects of both WZ structured ZnO and ZnS at the core and shell interface region. The single-crystalline ZnO NWs and the grains in the polycrystalline WZ ZnS shell layers can induce piezoelectric polarization charges that modulate the barrier height at the interface between the ZnO core and ZnS shell, as well as modulating the Schottky barrier height at the interface between ZnO and Ag. However, in case of WZ ZnO/ZB ZnS CS NWs, the piezopolarization charges are induced only in the ZnO core region owing to the


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absence of a piezoelectric effect in the ZB ZnS shell layer, resulting in the relatively low increment of electrical transport even under an external strain.16 These results validate that the electrical transport properties of ZnO/ZnS CS NWs can be distinctively changed by crystalstructure-dependent piezotronic effects. To verify the statistical validity, we fabricated five devices for each WZ ZnO/WZ ZnS and WZ ZnO/ZB ZnS CS NWs, and confirmed consistent electrical characteristics of devices under compressive strains, as depicted in Figure S4 of the Supporting Information.

Figure 3. The piezo-phototronic effect on the UV photosensing of the ZnO/ZnS CS NWs. (a,b) I-V curves of (a) WZ ZnO/WZ ZnS and (b) WZ ZnO/ZB ZnS CS NWs under external compressive strains and UV irradiation (0.78 mWcm2). (c,d) Photoresponses of (c) WZ ZnO/WZ ZnS and (d) WZ ZnO/ZB ZnS CS NWs at a bias of 1 V under UV irradiation.


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To confirm the crystal-structure-dependent piezo-phototronic effect of ZnO/ZnS CS NWs, the I-V curves and photosensing performance of both devices were systematically measured by applying external compressive strains under 365 nm UV irradiation with a power intensity of 0.78 mW/cm2, as presented in Figure 3. The photocurrents of both types of ZnO/ZnS CS NWs are enhanced under UV irradiation compared with those under the dark condition (Figure 2). In particular, Figure 3a and 3b clearly show that the photocurrents of both types of ZnO/ZnS CS NWs notably increase as the applied external compressive strain increases from 0 to −0.24%. The devices in the same set also exhibited consistent results under the same conditions of external compressive strain and UV irradiation. (see Figure S5 in the Supporting Information). We also estimated the relative increase of photocurrent under an external strain condition, defined as ∆ ,

=

, , ,

,

(2)

where Ip,s and Ip,0 are the photocurrent (indicated by subscript “p”) with and without external strain under UV irradiation, respectively. The results show that the value of ΔIp/Ip,0 for WZ ZnO/WZ ZnS CS NWs is 286% (inset of Figure 3a) and that for WZ ZnO/ZB ZnS CS NWs is 156% (inset of Figure 3b) under a compressive strain of −0.24%. These ΔIp/Ip,0 values are lower than those found for ΔI/I0 under no UV irradiation (340 and 223%, respectively), where the difference is owing to the piezoelectric screening effect from photo-generated charge carriers.3031

Under UV irradiation, numerous photo-generated charge carriers partially screen the

piezopolarization charges present at the heterojunction interface between the ZnO core and ZnS shell. Thus, the piezoelectric screening effect reduces the relative increase of photocurrent under external strain compared with the current under no UV irradiation. Nevertheless, we observed the significant increase in photocurrent of both types of ZnO/ZnS CS NWs, as shown in Figure 2


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and 3 owing to the type-II band structure of the ZnO/ZnS CS NWs. The photoresponsivity of the ZnO/ZnS CS NWs with WZ and ZB ZnS shell layers was evaluated by irradiating the devices with a 365 nm UV source at a power density of 0.78 mW/cm2 at a fixed bias of 1 V. Figures 3c and 3d show the time-dependent photoresponse of the ZnO/ZnS CS NWs with WZ and ZB ZnS shell layers, respectively, under compressive strain. When the UV light was switched on and off, the photocurrent response of both devices immediately changed, and the stable and repeatable switching behaviors of the photoresponses were observed under varying strains. To evaluate the performance of both types of ZnO/ZnS CS NWs as a photodetector, we calculated the photoresponsivity and external quantum efficiency (EQE) under compressive strains by using the equations previously reported elsewhere.30 We calculated

EQE

by

assuming

the

internal

photoconductive

gain

is

unity.32

The

photoresponsivities of WZ ZnO/WZ ZnS CS NWs under no strain and under −0.15 and −0.24% of compressive strain are 1.01×10−1, 2.32×10−1, and 8.76×10−1 A/W, respectively; and the EQE values under same conditions are 3.44×10−1, 7.89×10−1, and 2.98%, respectively. This result shows that the two critical parameters of photoresponsivity and EQE monotonically increase with increasing external compressive strain. Similarly, the photoresponsivities of WZ ZnO/ZB ZnS CS NWs under the same strain conditions are 1.00×10−1, 1.52×10−1, and 2.16×10−1 A/W; and the EQE values are 3.41×10−1, 5.17×10−1, and 7.34×10−1%, respectively. These results show that the photoresponsivities and EQE values of both types of ZnO/ZnS CS NWs under no strain are similar. However, when a strain of −0.24% is applied, the photoresponsivity of WZ ZnO/WZ ZnS CS NWs is increased by 8.67 times and that of WZ ZnO/ZB ZnS CS NWs is increased by 2.16 times, compared with the no strain condition. The significantly higher photoresponsivity and EQE values of the WZ ZnO/WZ ZnS CS NWs compared to those of the WZ ZnO/ZB ZnS


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CS NWs under compressive strain are attributed to the strain-induced piezopolarization charges at the heterojunction interface between the WZ ZnO core and WZ ZnS shell. These charges can effectively modulate the junction barrier and induce enhanced transport of photo-generated electrons and holes and a reduced recombination probability for the charge carriers. To evaluate the rise and decay time of the photocurrents in both types of NW devices under different compressive strains, a bi-exponential equation was employed to fit the rise and fall edges of the photoresponse curves, as shown in Figure S6 and S7 of the Supporting Information.33 As the compressive strain is varied from 0 to −0.24%, the rise time of the WZ ZnO/WZ ZnS CS NWs decreases from 7.81 to 1.24 s, whereas that of the WZ ZnO/ZB ZnS CS NWs decreases from 26.97 to 1.25 s. When a strain is applied, strain-induced piezopolarization charges in the interfacial region between the ZnO core and ZnS shell effectively modulate the barrier height. This can greatly expedite the separation of photo-generated electrons and holes into the ZnO core and ZnS shell regions, respectively; and can reduce the recombination probability of charge carriers, resulting in the faster rise times under the compressive strain compared with no strain.30 On the other hand, as the compressive strain is varied from 0 to −0.24%, the decay time of the WZ ZnO/WZ ZnS CS NWs increases from 4.31 to 4.50 s, and that of WZ ZnO/ZB ZnS CS NWs increases from 7.72 to 41.82 s. When the UV light was turned off, the spatially-separated photo-generated electrons and holes take a long time for recombination, thus inducing the slow decay times under a compressive strain of −0.24%.30 One thing to note is that the photocurrent level and response times of the WZ ZnO/WZ ZnS CS NWs are much higher and faster, respectively, than that of WZ ZnO/ZB ZnS CS NWs. Further, the photocurrent and dark current of photoresponses of WZ ZnO/ZB ZnS CS NWs are gradually increased after repeated switching of UV irradiation, implying that recover times are longer in WZ ZnO/ZB ZnS


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CS NWs than WZ ZnO/WZ ZnS CS NWs. This result shows that WZ ZnO/WZ ZnS CS NWs can achieve superior photoresponse performance with faster switching time than WZ ZnO/ZB ZnS CS NWs, suggesting that the photosensing properties can be significantly improved by the crystal-structure-dependent piezo-phototronic effect.

Figure 4. Schematics and energy band diagrams of ZnO/ZnS CS NW device with type-II band structure. (a) Schematic and the (b) corresponding energy band diagram of strain-free ZnO/ZnS CS NWs. (c) Schematic and (d,e) corresponding energy band diagrams of ZnO/ZnS CS NWs under compressive strain for (d) ZB and (e) WZ shell crystalline structures.


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To elucidate the effect of external strains on the electrical transport and photosensing properties of ZnO/ZnS CS NWs with WZ and ZB ZnS shell layers, a schematic of the energy band diagram and its realignment induced by the piezopotential change under external strains are shown in Figure 4. Figure 4a displays the schematic of the unstrained ZnO/ZnS CS NW, and Figure 4b shows its energy band diagram with a type-II band structure with a staggered alignment at the heterojunction interface.34 The potential energy gradients at the interface of the ZnO/ZnS CS NWs can separately confine the electrons in the conduction band (Ec) of the ZnO core and holes in the valence band (Ev) of the ZnS shell, respectively.29, 34 In particular, spatial separation of the photogenerated electrons and holes owing to the type-II band structure under UV irradiation minimizes the recombination probability of charge carriers, and consequently resulting in the high output photocurrent. The Schottky junctions are also formed at the interfacial regions between metals and semiconductors. The typical work function of ITO is larger than that of ZnS and the Schottky contact is formed at the interface between ZnS and ITO.35-39 However, since the typical work function of Ag is lower than that of ZnO, it is expected to form Ohmic contact at the interface between ZnO and Ag.35-36, 40-41 Nevertheless, it was often reported that the surface states induce the formation of back-to-back Schottky contact, preventing electrons from passing through the interface between Ag electrode and ZnO NW.31, 40, 42-44

When a flexible device is bent into a convex configuration, as shown schematically in Figure 4c, the NWs are subjected to a compressive strain along their axial direction. Under this compressive strain, negative piezopolarization charges are created at the top interface of the ZnO core region in the ZnO/ZnS CS NWs, which leads to upward bending of the conduction and valence bands of the ZnO NWs (Figure 4d). In addition, positive piezopolarization charges


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created at the bottom interface of the ZnO core NWs induce downward bending of the conduction and valence bands of the ZnO NWs. Thus, the electron trap state barrier (∆Ф1) at the heterojunction interface can be eliminated and the Schottky barrier height between the ZnO and the Ag electrode can be changed by external compressive strains, which are favorable to improve the electrical transport and photosensing properties. In particular, in the case of WZ ZnO/WZ ZnS CS NWs, the WZ structured ZnS shell can also generate the piezoelectric effect, as shown in Figure 4e.45 Although the ZnS shell layer of the WZ ZnO/WZ ZnS CS NWs in this study is polycrystalline, we expect that some ZnS grains in the polycrystalline WZ ZnS shell layers contribute to the generation of piezoelectric polarization charges, because the nucleation of piezoelectric material is influenced by a seed layer underneath the piezoelectric film, and the nucleation determines crystal orientation.46 Close-up HRTEM images of Figure S8 show that crystal lattices of some part of grains in WZ ZnS shell are well matched with those of ZnO NWs. These results indicate the grains of interfacial region between WZ ZnO core and WZ ZnS shell are partially aligned by preferential orientation, facilitating the generation of piezoelectric polarization charges in the thin WZ ZnS shell layer. The straininduced piezoelectric polarization charges can modulate the junction barrier height between the ZnO core and the ZnS shell, as well as the Schottky barrier height at the interface between the ZnS and ITO under external strain.47 The induced positive piezopotential in the WZ ZnS shell region contacting the top region of the ZnO NWs can eliminate the hole trap state barriers (∆Ф2) under an external strain. As a result, the spatial separation of the electrons in the ZnO core and the holes in the ZnS shell is facilitated and the recombination rate of electrons and holes is remarkably decreased. This results in enhanced electrical transport and photosensing properties. In the case of WZ ZnO/ZB ZnS CS NWs, however, the band structure can be changed only in


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the ZnO core region along the axial direction of the NWs owing to the absence of a piezoelectric effect in the ZB structured ZnS shell.16 To directly compare the piezoelectric coefficient in the ZnO/ZnS CS NWs with different ZnS shell layer crystalline structures, we measured the vertical piezoresponse of the both ZnO/ZnS CS NWs via piezoresponse force microscopy (PFM).48-50 Figure 5a shows the piezoresponse as a function of the magnitude of the external bias applied to the both ZnO/ZnS CS NWs, indicating a distinct piezoresponse from the WZ ZnO/WZ ZnS CS NW compared to that of WZ ZnO/ZB ZnS CS NW. The piezoelectric coefficient can be obtained from the slope of the solid linear fitting line, and the slope obtained for WZ ZnO/WZ ZnS and WZ ZnO/ZB ZnS CS NWs by fitting the data are 4.37×10−5, and 5.68×10−6 arbitrary units/V, respectively.49-50 Since the slope we obtained for x-quartz was 5.88×10−6 and the piezoelectric coefficient, d11, of x-quartz is known to be approximately 2.3 pm/V, the piezoelectric coefficient of both ZnO/ZnS CS NWs could be estimated as 3.8 and 0.5 pm/V, respectively by using the ratio between the CS NW and x-quartz, confirming that piezoelectricity of WZ ZnO/WZ ZnS CS NW is greater than that of WZ ZnO/ZB ZnS CS NW.49-50

Figure 5. (a) Vertical piezoresponse of WZ ZnO/WZ ZnS and WZ ZnO/ZB ZnS CS NWs measured by using vertical PFM methods. (b) Piezopotential distribution in ZnO/ZnS CS NWs with WZ and ZB crystalline structures in the ZnS shell layer under a pressure of 10 MPa, and (c) the change of piezopotential versus applied external pressure on the two types of NWs.


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We also carried out finite element analysis using the COMSOL to simulate the piezoelectric potential distribution and the piezopotential as a function of the external pressure on the ZnO/ZnS CS NWs with different ZnS shell layer crystalline structures, as presented in Figure 5b and 5c. The result clearly shows that the piezopotential of WZ ZnO/WZ ZnS CS NWs is greater than that of WZ ZnO/ZB ZnS CS NWs under the same external pressure, which is consistent with our electrical and photocurrent results. These results demonstrated that the piezopotential of ZnO/ZnS CS NWs is highly dependent on their crystal structures. In this light, we anticipate that diverse combinations of piezoelectric materials with different crystalline structures can provide alternative routes to improve the performance of a wide range of next-generation technologies such as flexible electronics, biomedical diagnostics, and smart sensors.

3. CONCLUSIONS We successfully fabricated two types of ZnO/ZnS CS NWs, having either WZ or ZB crystalline structures in the ZnS shell layer on the flexible substrates, and demonstrated the crystal-structure-dependent piezotronic and piezo-phototronic effect of both NW devices. The significantly enhanced electrical transport and photosensing properties of the WZ ZnO/WZ ZnS CS NWs under compressive strain, compared with those of WZ ZnO/ZB ZnS CS NWs, can be ascribed to the piezopotentials induced by both the WZ structured ZnO core and the ZnS shell. Under an external compressive strain of −0.24%, the output current and photoresponsivity of the WZ ZnO/WZ ZnS CS NWs were enhanced by 4.4 and 8.67 times, respectively, compared with those of NW devices under no strain. On the other hand, an enhancement of 3.2 and 2.16 times for the output current and photoresponsivity were achieved in the WZ ZnO/ZB ZnS CS NWs under the external compressive strain of −0.24%, compared with those of NW devices under no


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strain condition. These results demonstrate that the piezotronic and piezo-phototronic effects of the ZnO/ZnS CS NWs are highly dependent on their crystal structures and it can provide a feasible approach to extend new design concepts using various combinations of materials with different crystalline structures, and can broaden the scope of potential applications for the next generation of multifunctional NW sensors.

4. EXPERIMENTAL SECTION 4.1. Fabrication of ZnO/ZnS CS NW devices with different ZnS shell layer crystalline structures First, a silver (Ag) film as a bottom electrode and a nickel (Ni) thin film as an adhesion layer were deposited on the flexible polyethylene terephthalate (PET) substrate by electron-beam evaporation. Then, we deposited a ZnO seed layer using radio frequency magnetron sputtering, and grew ZnO NWs using the hydrothermal method. Detailed fabrication procedures were reported in the publication elsewhere.25 Then, we used different solutions to deposit the ZnS shell with different crystalline structures on the ZnO NWs. To deposit a ZnS shell layer with a WZ crystalline structure, we firstly immersed the ZnO NWs in a 0.16 M sodium sulfide (Na2S) solution, and then in a 0.16 M zinc nitrate ((Zn(NO3)2) solution, both at 60°C for 2h.23-25 On the other hand, the ZB ZnS shell layer was deposited by immersing the ZnO NWs into a 0.1 M thioacetamide solution at 90°C for 50 min.26-27 A thin layer of photoresist was spin-coated on the as-prepared ZnO/ZnS CS NWs, followed by oxygen plasma etching to remove the residual photoresist from the tips of the NWs. Finally, the NWs were covered with indium tin oxide (ITO)-coated PET substrates as a top electrode.16, 19, 51-52 4.2. Measurements of the effects of strain on the electrical and photosensing properties We systematically analyzed the electrical and photosensing properties of ZnO/ZnS CS NW


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devices having WZ and ZB ZnS shell layers with a semiconductor parameter analyzer (B1500A, Agilent Technologies, Inc., USA). The photoresponse characteristics of these devices were investigated using a 365 nm UV light-emitting diode at a power density of 0.78 mW/cm2.25 The compressive strain was applied to the NWs by bending the flexible PET substrates on a sample stage in various convex configurations, and the external strain in the NWs was determined using the calculation method in the literature.53-54 4.3. Measurements of piezoresponse using PFM We used an atomic force microscopy (AFM) (Park systems, XE-100)-based investigations to measure the piezoresponse and piezoelectric coefficient. The piezoelectric properties of ZnO/ZnS CS NWs with different ZnS shell layer crystalline structures were confirmed by PFM equipped with Pt coated silicon tips (Multi 75E-G, Budget Sensors, 3 N/m of force constant and 75 kHz of resonant frequency) with contact force of 10 nN. A lock-in amplifier (Stanford Research, SR830) was used for voltage sweep to ZnO/ZnS CS NWs.

APPENDIX A. SUPPORTING INFORMATION Supporting Information Available: Schematic of the fabrication process steps, field emission scanning electron microscope images, statistical results, and photoresponse properties of ZnO/ZnS CS NWs having WZ and ZB ZnS shell layers are available in the Supporting Information.

AUTHOR INFORMATION *Corresponding Author Professor Seong-Ju Park School of Materials Science and Engineering, Gwangju Institute of Science and Technology,


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Gwangju, 61005, Republic of Korea. E-mail address: [email protected]

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