Layered Ferroelectric NbOI2 Flakes Toward In‐Plane Anisotropic Self‐Powered Sensing

2D ferroelectric materials have attracted much interest due to their potential for developing flexible self‐powered nanogenerators. Niobium oxide diiodide (NbOI2) has in‐plane anisotropy of electrical properties and large lateral piezoelectric coefficient, which makes it possess high‐performance and unique behavior in flexible sensing. In this work, multidirectional piezoelectric nanogenerator (PENG) devices using NbOI2 flake are fabricated and excellent energy harvesting and sensing capabilities are found. Specifically, the NbOI2‐based PENG can exhibit a long‐time stable voltage output of 215 mV at a large strain of 1.1%. More importantly, the periodic output signal pattern of the twelve‐electrode NbOI2‐based PENG in six directions is investigated, and this anisotropy provides the possibility of achieving simultaneous signal harvesting in multiple directions. This work broadens the scope of applications of 2D materials in nano‐energy and provides new ideas and insights for further exploration of nano‐energy and smart wearable nano‐electronic devices.

material into devices can be substantially simplified.In addition, the strong anisotropy of NbOI 2 offers the possibility of measuring multiangle signals.Therefore, the combination of NbOI 2layered materials into flexible and wearable PENGs will have great potential for applications.
Here, we obtained large-area NbOI 2 flakes by mechanical exfoliation method.Several NbOI 2 -based PENGs device containing up to multiple electrodes were prepared by photolithography, and the output signals of NbOI 2 at multiple angles in the in-plane were investigated for the first time.The NbOI 2 -based PENG exhibited stable voltage signal responses of 215 and 30 mV in the directions of the optimal and worst signals, respectively, with the two directions perpendicular to each other.Further, we measured up to six directions spaced 30°apart on the same NbOI 2 film, and this anisotropic piezoelectric signal response was symmetrically distributed with angle.This signal variability can be attributed to the relationship between O-Nb-O chains and I-Nb-I chains presenting %90°angles within the NbOI 2 face.In addition, the NbOI 2 -based PENGs can be attached to the human body, demonstrating excellent biosensing performance and enabling more accurate recognition of organisms in motion and speech.1c. [37]We prepare many NbOI 2 samples by mechanical exfoliation and transfer them to flexible PET substrates for subsequent PENG preparation.Contact mode AFM measurements indicate that the thickness of a mechanically exfoliated NbOI 2 flake is about 70 nm (Figure S1, Supporting Information).Figure 1d shows an STEM of the exfoliated NbOI 2 sample with a rectangular network-like arrangement that corresponds to the lattice of Figure 1b. Figure 1e shows an X-ray diffraction (XRD) pattern of the exfoliated NbOI 2 flake, where four diffraction peaks reveal the selective orientation of the flake as (100).Figure 1f shows the Raman spectrum of NbOI 2 with the presence of five peaks in the detection range (P 1 -P 5 from low to high frequencies).It is worth mentioning that the exfoliated NbOI 2 flakes always show a rectangular or elongated shape with nearly vertical edges, which is consistent with the STEM and the growth orientation.This rectangular morphology helps us to determine the lattice orientation directly from the optical microscopy images.

Piezoelectricity of NbOI 2 -Based PENG Under Multiple Directions
To initially evaluate the electrical performance of the device, a four-electrode NbOI 2 -based PENG was prepared as shown in Figure 2b.The thickness of NbOI 2 is about 70 nm (Figure S1, Supporting Information).Figure 2a shows the bending schematic of the four-electrodes PENG.When a tensile strain is applied, a piezoelectric polarization charge with opposite polarity is generated at two ends of the NbOI 2 sample to form electrical potential difference.Once the squeezing force is released, the strain disappears and the piezoelectric potential on the electrodes decreases at the same time.Afterward, the accumulated charges on the electrodes move in the opposite direction, resulting in an electrical signal opposite to the previous process.
Figure 2c,d shows the detected I SC and V OC signals for the four-electrodes device in both 90°and 0°directions, and the experiments were performed with one tensile strain and one compressive strain.These periodically varying signals also show that the 90°direction is a good signal direction in the fourelectrodes device: its V OC and I SC signals reach 400 mV and 450 pA.Respectively, the V OC and I SC signals in the 0°direction are only 60 mV and 85 pA, which are significantly weaker than those in the 90°direction.We did not observe significant voltage outputs on the bare PET substrate without transferring NbOI 2 , establishing that these signals were indeed from NbOI 2 (Figure S2a, Supporting Information).Meanwhile, we performed unidirectional bending and stretching tests on the device to further check whether the electrical signals were generated by piezoelectric phenomena.When tensile strain was performed, a negative voltage signal was detected first, and a positive voltage signal was generated during rebound (Figure S2b, Supporting Information).In contrast, the output signals detected in compressive strain have the opposite shape (Figure S2c, Supporting Information).These output signals are reversible, proving that the variability of the electrical signals in the vertical direction indeed comes from the anisotropic piezoelectric effect of the NbOI 2 material and not from the interference of external signals.
To demonstrate the durability of the NbOI 2 -based PENG, and to further illustrate the difference in the same signals of the PENG in both directions, tensile cycling tests were performed at 1.1% of the same high strain in both directions.The method of strain calculation is described in the detail in Figure S3 (Supporting Information).As Figure 2g and Figure S4, Supporting Information show, after 1000 cycles, there are stable and V OC signals in both vertical directions.The signals in the 90°and 0°directions are stable at 215 and 30 mV.The I SC at 180 pA is also stable for 1000 cycles under the same conditions (Figure S5, Supporting Information).In Table 1, we compare the performance of NbOI 2 -based PENG with other reported 2D PENG, and multilayer NbOI 2 shows very good voltage signals.
It is exciting to exhibit anisotropic piezoelectric signal differences on the same sample, which represents the possibility of collecting information in multiple directions simultaneously using only one PENG.To illustrate the signal difference of NbOI 2 -based PENG in the vertical direction, we simulate the polarization intensity when the NbOI 2 crystal is stressed in different directions (Figure 3a).Where the b-axis direction is the inplane polarization axis direction where the O-Nb-O chain is  [37] This work located, and the vertical c-axis direction is the nonpolarization axis direction where I-Nb-I chain is located (Figure 1b).The change in polarization intensity ΔP = 0.7 pC m À1 when a 2% strain occurs in the axial direction along the b-axis, while the axial strain along the c-axis in the same case leads to a change of only %0.045 pC m À1 .Although in NbOI 2 crystals the c-axis direction has a smaller resistance than the b-axis direction due to the weaker electronegativity of the I atom, R b /R c = 1.7. [36]he difference in electrical signal due to the resistance is much smaller than the piezoelectric signal due to the polarization.The results show that the strongest direction of the electrical signal is always along the b-axis.
To expand the range of orientations that can be detected by the NbOI 2 -based PENG and to check the simulation results, we selected NbOI 2 crystals with stripped dimensions larger than 40 um and fabricated a PENG with twelve equally spaced electrodes on it.Figure 3b shows an optical microscopy image of a twelve-electrode NbOI 2 -based PENG with a 30°separation maintained between each adjacent electrode.To ensure comparability of the multiangle signal, the tips of the twelve electrodes were controlled to all be on the same circle with a radius of 34 μm.Pairs of electrodes were connected for each test and bent in the direction perpendicular to the electrodes.Figure 3c shows the open-circuit voltages at multiple tensile strains in a typical 90°direction, and at a single direction, the open-circuit voltage increases with increasing strain, which is consistent with previous experimental observations. [36]The V OC in this direction is 94 mV at 0.97% strain.We further examine the V OC in the remaining five directions for multiple tensile strains, and the relevant data are summarized in Figure 3d.To focus on comparing the differences in signals at different angles, we use linear fit over the zero point for V OC and strain.The slope of the fit can represent the V OC signal due to tensile strain in that direction at 1% strain.The piezoelectric signals at different angles are comparable only if the same strain is applied.
The slope information with angle is presented in Figure 3e.For the twelve-electrode NbOI 2 -based PENG, the V OC in the 0°, 30°, 60°, 90°, 120°, and 150°directions are 43, 59, 78, 97, 89, and 66 mV.Because the electrodes are measured in pairs, the curves exhibit centrosymmetry, and the images show a distribution pattern similar to the number "8".In this twelveelectrode NbOI 2 -based PENG, the 90°and 0°directions are the directions of the strongest and weakest piezoelectric signals, and both of their electrode directions are also parallel to the edges of the rectangular NbOI 2 sheet,corresponding to the b-axis and caxis in the lattice.The signals of the remaining four angles are approximately symmetrically distributed along the vertical line.

Energy Harvesting from the Human Body
We explored the energy collection and flexible sensing of NbOI 2based PENGs on the human body.Due to their small size and flexibility, the PENGs can be made into a variety of monitoring devices that are attached to the human body for sensing and collecting energy (Figure 4a).No formal ethical approval was required for the experiments demonstrated herein.Written informed consent was obtained from all the participants prior to the publication of this study.Figure 4b shows the energy harvesting and motion feedback detection when the NbOI 2 -based PENG is attached to the back of the neck.A voltage response of %40 mV can be generated when the tester tilts his head and lowers his head, and the electrical signal is directional, allowing discrimination between opposing forms of motion.In addition, as shown in Figure 4c,d, the PENG can respond to the bending curvature of the wrist and fingers, with the output age increasing with the bending angle.We then tried to attach the device to the throat to detect the vibrations caused by the vocal cords during speech.As shown in Figure 4e, the same tester pronounced "A", "B", "C", "D", "E", and "F", The PENG of NbOI 2 effectively collects the vibration energy and converts it into a voltage signal.When pronouncing different letters, different voltage waveforms are obtained, and when pronouncing the same letter, the waveforms have similarity (Figure S6

Conclusions
We have successfully prepared a variety of NbOI 2 -based PENGs with the help of photolithography.The NbOI 2 -based PENGs have excellent stability, generating a stable signal output of about 215 mV and 180 pA for a long time at a large strain of 1.1%.In contrast, only 30 mV of voltage signal is available in the vertical direction for the same test conditions.Through theoretical simulations, we clarify that the signal difference in these two directions corresponds to the polarization difference between the mutually perpendicular O-Nb-O and I-Nb-I chain directions in the NbOI 2 lattice.Combined with measurements of the PENG of the twelve-electrode NbOI 2 in six directions, it is verified that the electrical signals in the remaining directions are the sum of the components generated by the electrical signals in the directions of these two crystallographic axes.This anisotropy provides the possibility to achieve simultaneous signal harvesting in multiple directions.The NbOI 2 -based PENGs can not only collect biomechanical energy from each joint on the human body but also effectively collect acoustic signals.This study broadens the scope of applications of 2D materials in the nano-energy and provides new ideas and insights for further exploration of nano-energy and smart wearable nano-electronic devices.

Experimental Section
Material Synthesis: NbOI 2 samples were synthesized by the chemical vapor transport reaction using stoichiometric elemental precursors of high-purity Nb, Nb 2 O 5, and I 2 with additional small amount of iodine as the transport agent.The precursors were sealed inside a quartz tube under high vacuum and were subjected to a two-zone horizontal tube furnace for a reaction time of 7 days under 500-600 °C.The shiny rectangle crystals were collected from the cold end of the tube.
Device Fabrication: 2D NbOI 2 flakes were mechanically exfoliated by using Scotch tape from bulk NbOI 2 crystals and then transferred onto an oxygen plasma dry etching pretreated flexible poly(ethylene terephthalate) (PET) substrate.The PET substrate was heated to 100 °C for 10 min during the transferring process to enhance adhesion force between it and NbOI 2 flakes.A suitable size of NbOI 2 flake is found on the substrate by the photolithography machine, and the expected electrode structure is drawn and photoengraved.Sequentially, Cr (10 nm) and Au (70 nm) thin layers were deposited on the PET substrate by sputtering, followed by a metal lift-off process.
Material Characterization: An optical microscope (Moticam Pro 205A) equipped was used for optical imaging.The XRD characterization was carried out on a Bruker D8 Advance diffractometer operating with a Cu Kα radiation (λ = 1.5406Å).Raman spectroscopy was conducted on a Jobin-Yvon LabRAM HR-800 spectrometer with a laser excitation wavelength of 532 nm.The thicknesses were accurately determined by using a contact mode AFM (AIST-NT).STEM operating at 300 kV was used to examine crystal structures, of which the data were analyzed using Digital Micrograph software.

2. 1 .
Figure 1a,b shows the crystal structure of NbOI 2 , which consists of distorted [NbO 2 I 4 ] octahedra connected by edge sharing of I-I edges along the c-axis and by corner sharing of O atoms along the b-axis.NbOI 2 layer has a rectangular network-like lattice, made of

Figure 1 .
Figure 1.Characterization of NbOI 2 .a) Side view along the b-axis and b) top view to the crystal structure of NbOI 2 .Green balls denote Nb atoms; purple balls denote I atoms; red balls denote O atoms.c) Schematic, viewed along the c-axis, shows that Nb atoms are far from the equilibrium position along the b-axis.The yellow circles denote the position of the equilibrium center of the [NbO 2 I 4 ] octahedron; the arrows denote the direction of spontaneous polarization.d) STEM image of the 2D NbOI 2 crystal.e) Powder X-ray pattern and f ) Raman spectrum of the NbOI 2 crystals.

Figure 2 .
Figure 2. Performance of the four-electrode NbOI 2 -based PENG.a) Bending schematic of PENG.b) Photograph and c) optical microscopy image of the four-electrode NbOI 2 -based PENG.d) Open-circuit voltage (V OC ) and e) short-circuit current (I SC ) in two perpendicular directions.f ) 1000 cycles of V OC under 1.1% strain in 90°direction.

Figure 3 .
Figure 3. Performance of twelve-electrode NbOI 2 -based PENG.a) Theoretical calculation of polarization strength changes when stress is applied to polarized or unpolarized axes.b) Optical microscopy images of the twelve-electrode NbOI 2 -based PENG.c) Relationship between strain and output voltage at 90 degrees direction.d) Strain-output voltage image and its linear fit at different directions.e) Polar coordinate images of output voltage in each direction at 1% strain after fitting.

Figure 4 .
Figure 4. Application of NbOI 2 -based PENG as a wearable device.a) NbOI 2 -based PENGs can be used for a variety of human monitoring.b) The neckmounted device can recognize postural changes and respond to conditioning signals.c,d) Devices that collect energy from the wrist (c) and fingers (d) can recognize the degree of flexion.e) Devices attached to the throat can respond differentially to different letter sounds.No formal ethical approval was required for the experiments demonstrated herein.Written informed consent was obtained from all the participants prior to the publication of this study.
, Supporting Information).Based on the above tests, these NbOI 2 -based PENGs can effectively collect the mechanical energy generated by human joints and convert it into electrical signals.Therefore, NbOI 2 -based PENGs have great potential as wearable devices and self-powered sensors in biomechanical energy collection and human activity monitoring.

Table 1 .
Performance of the NbOI 2 -based PENG in this work compared with other 2D PENGs reported.