Emergent functionalities enhanced by mechanical stress in SnO2-based flexible devices

Emergent functionalities created by applying mechanical stress to flexible devices using SnO2 microrods and Ga2O3/SnO2-core/shell microribbons are reviewed. Dynamic lattice defect engineering through application of mechanical stress and a voltage to the SnO2 microrod device leads to a reversible semiconductor-insulator transition through lattice defect creation and healing, providing an effective and simple solution to the persistent photoconductivity (PPC) problem that has long plagued UV semiconductor photosensors. Here, lattice defects are created near slip planes in a rutile-structured microrod by applying mechanical stress and are healed by Joule heating by applying a voltage to the microrod. Nanoscale amorphous structuring makes the Ga2O3/SnO2-core/shell microribbon with a large SnO2 surface area more sensitive to changes in temperature, while mechanical bending of the wet device improves its sensitivity to adsorbed water molecules. These results illustrate the potential for developing flexible devices with new functionalities by enhancing the intrinsic properties of materials through miniaturization, mechanical stress, and hybridization.


Introduction
Material nanoarchitectonics is a new path for the development of devices from new materials.It goes beyond the mere creation of nanomaterials to include new functionalities developed through the understanding and control of interactions between individual nanostructures and the integration of nanostructures [1].This paper provides a comprehensive review of this concept and new functionalities, such as reversible control of lattice defects, as well as enhancement of sensing functionalities through nano-and microstructured metal oxides such as SnO 2 and Ga 2 O 3 .
Lattice defects in oxides and semiconductors alter the optical, electrical, and magnetic properties of materials.In contrast to methods that control lattice defects by healing defects in crystals through thermal annealing [2][3][4] or by growing crystals from materials mixed with small amounts of impurities [5][6][7], the method used in these studies dynamically creates and heals lattice defects [8].This is a kind of lattice defect engineering by applying mechanical stress and voltage.The SnO 2 used in these studies is a wide bandgap oxide material, with a bandgap of 3.6 eV [9,10].The reason for choosing it is that SnO 2 crystals have a rutile structure.The rutile structure has inversion symmetry and does not produce a piezoelectric field effect.Thus, the changes in the properties caused by bending the crystal can be directly measured and analyzed without having to consider the effect.In the case of single crystals, it is known that their structures are elastically deformed by applying a small external mechanical stress [11].When the deformation is below the elastic limit, the crystal returns to its original shape when the stress is removed.On the other hand, bending above the elastic limit causes plastic deformation; the crystal does not return to its original state even when the stress is removed (figure 1(a)).To control the creation and healing of lattice defects, sliding of split planes in rutile-structured SnO 2 microrods can be exploited by applying mechanical stress [12,13] and through Joule heating generated by applying a voltage [8,13].In the case of amorphous and polycrystalline nano-/microstructures, deformation caused by applying mechanical stress induces gaps and expands the grain boundary, leading to an increase in surface area.This mechanical stress induced change in surface area can be exploited in order to enhance the sensing capability of Ga 2 O 3 /SnO 2 core-shell devices [14,15].
A SnO 2 -based device was fabricated on a flexible substrate (figure 1(b)) and bent by applying stress from the back of the substrate (figure 1(c)).Electrodes on both sides of the rod were used to measure the change in electrical resistance during bending and to apply a voltage to generate Joule heating for electrical healing.The strain ε of the sample was obtained from the bending of the substrate by using the following simple equation (figure 1 ( ) e = + where t polymer is the thickness of the polymer sheet, t sample is the thickness of the sample, and the denominator approximates a bend with the radius of curvature R c .Section 2 discusses the reversible control of lattice defects and the solution to the persistent photocurrent (PPC) effect by using SnO 2 microrod devices.Section 3 focuses on the sensitivity of Ga 2 O 3 /SnO 2 -core/shell microribbon devices and its enhancement by applying mechanical stress.Section 4 shows device applications of flexible SnO 2 films.
2. Emergent functionalities in SnO 2 microrod devices 2.1.Preparation and structure of SnO 2 nano/microrods SnO 2 nano/microrods were prepared by vapor phase growth in an electric furnace, and their morphology and structure were investigated.A mixture of SnO 2 powder and carbon powder in a mass ratio of 1:1 was placed in an Al 2 O 3 boat.The furnace was heated to about 473 K to evaporate water and extra gases contained in the boat and the quartz tube of the furnace for about 10 min.Then, the furnace temperature was raised to 1263 K, and the reduction reaction between SnO 2 and carbon was continued for about 60 min in high-purity argon carrier gas (9 × 10 2 Pa) mixed with 4% oxygen gas.The vapor of Sn atoms was carried by a carrier gas to grow the nano/ microrods.The SnO 2 microrods had a prismatic structure with a square cross-section of about 2 μm on one side (figure 2 The structure of a single SnO 2 microrod was investigated by making x-ray diffraction measurements (figure 2(c)) [16].Peaks corresponding to the (002), (004), and (006) orientations of the SnO 2 crystal were observed when the scattering vector q was oriented in the axial direction of the microrod, and peaks in the (200), (400), and (600) orientations of the structure were observed when q was taken diagonally across the square crosssection of the microrod.The lattice constants of this single crystal were a = 0.478 nm and c = 0.318 nm.It was confirmed that the single crystal had a rutile structure and that the (110) plane was one side of the prism [16].Transmission electron microscopy diffraction images of SnO 2 nanowires (figure 2(d)) confirmed that they also had a rutile structure with the same crystallographic orientation as the microrods (figure 2(c)) [12].

Electrical properties of a bent SnO 2 microrod
The electrical properties of a single crystal SnO 2 microrod were investigated at 297 K (figure 2(e)).A SnO 2 microrod was placed on a flexible polyimide sheet with a thickness of 0.125 mm, and gold electrodes were deposited on both ends of the rod to fix it to the sheet.The resistance was measured by applying a voltage between the electrodes spaced 200 μm apart.SnO 2 becomes an n-type semiconductor due to intrinsic impurities such as oxygen defects [10], and the resistivity of the SnO 2 microrods was about 10 Ωcm at 297 K.This state is called the semiconducting state.
The resistivity change was measured at 297 K and an applied voltage of 2 V by bending the SnO 2 microrod device by applying stress from the back of the polyimide sheet (figure 1(c)) [8].The resistance of the bent microrod is plotted as a function of the strain ε in figure 2(e).In the low strain region (ε  0.12%), the resistivity changed reversibly.When the strain was removed, the resistivity returned to its original value, indicating that the microrod was elastically deformed.When the strain was further increased, the resistivity did not return to its original value even after the stress was removed.This indicates that irreversible lattice defects were created in the microrod; i.e., plastic deformation beyond the critical limit of elastic deformation occurred (figure 1(a)).When the strain was increased further, the resistance reached the limit of the measurement system (∼2 × 10 7 Ωcm).This state is called the insulating state.When the strain was reduced to zero, the device remained in the insulating state (figure 2(e)).When a voltage of 6.5 V or higher was applied to the device in the insulating state, the resistivity returned to its original value [8].Thus, the transition from the semiconductor state to the insulating state due to mechanical stress in the microrod device had nonvolatile characteristics.This means that the transition between the semiconducting and insulating states can be controlled by applying mechanical stress and voltage.

Structural analysis of a bent SnO 2 nano/microrod
The structural changes in a bent SnO 2 microrod were investigated by micro-Raman spectroscopy at 297 K to confirm that the transition from the semiconducting state to the insulating state under mechanical stress is due to lattice defects created in the microrod.Two peaks at 631 cm −1 and 733 cm −1 appeared in the Raman spectra of a SnO 2 microrod in the semiconducting state (ε = 0) before bending (figure 3(a)), corresponding to the A1 g and B2 g modes, respectively [17,18].No peaks corresponding to the E g mode appeared.On the other hand, peaks corresponding to the E g mode appeared in the spectrum of the microrod in the insulating state produced (c) XRD pattern of as-grown SnO 2 microrod: scattering vector q parallel to rod axis, scattering vector q parallel to the diagonal line of square cross-section of SnO 2 microrod [16].(d) TEM image of single SnO 2 nanorod.Upper inset: TEM diffraction pattern of nanorod near the edge of the supporting carbon sheet.Lower inset: SEM image of as-grown SnO 2 nanorods.(e) Resistivity of SnO 2 microrod device as a function of mechanical strain at V = 2.0 V.The strain is increased from 0 to 0.186% (pink hexagons) and then decreased to 0 (blue open hexagons).The resistivity returns to its original value when the applied voltage is above the threshold voltage V Th .The inset shows a schematic illustration of the bent SnO 2 device [8].Copyright 2015 Jon Wiley and Sons.by bending (ε = 0.15%) (figure 3(b)) and the A1 g peak position shifted to the lower wavelength side.The halfvalue width of the A1 g peak was broadened.The appearance of the E g mode indicates the presence of a region with a tilted crystal orientation.The shift in the peak position is due to the presence of tensor stress along the caxis [19], and the increase in the peak width is due to the reduced crystallinity of the microrod.These spectral changes remained in the insulating state of the microrod after the strain was removed (figure 3(c)), indicating that lattice defects remained in the microrod.When a voltage of more than 6.5 V was applied to the device, the spectrum returned to its original state (figure 3(d)), and the crystallinity was restored.
Since electrons accelerated by high voltage (200 kV) in a transmission electron microscope (TEM) cannot penetrate a SnO 2 microrod with a thickness of about 2 μm, a thin SnO 2 nanorod with a thickness of about 100 nm was used to study the internal structural changes by bending the nanowire [12].The bending of the nanorod on a TEM grid under the observation of a scanning electron microscope was performed using focused tungsten (W) deposition, a focused gallium (Ga) ion beam, and a sharp needle manipulator equipped with a focused ion beam (FIB) microscope.A small number of SnO 2 nanorods dispersed in organic solvent were dropped onto the TEM grid and an isolated nanorod was used in the bending procedure schematically shown in figure 4(a).One end of the nanorod was fixed to the grid by using focused W deposition (inset 1 in figure 4(a)).The needle was brought into contact with the other end and attached to it by the deposited W (inset 2).The needle was moved in order to bend the nanorod (inset 3) and was fixed to the grid by the deposited W. The fixed needle was cut off by the focused Ga ion beam irradiation (inset 4).A strain of about 3% was generated in the outer part of the bent nanorod (figure 4(b)).It is important not to irradiate the SnO 2 nanorod with the beam because the beam would easily create many defects and lose crystallinity.
The TEM images of the bent SnO 2 nanorod showed unique features resulting from plastic deformation of a rutile crystal.In the bent nanorod, black stripes appeared that were inclined at about 45°to the direction of rod growth (see the arrows in figure 4(c)).A step-like structure appeared in a magnified TEM image of the outside of the bent nanorod (figure 4(d)).On the other hand, the surface of the unbent nanorod was almost smooth (figure 4(e)).Slip planes along the 10 1 ( ̅ ) plane in a rutile crystal [20-22] were created by bending the nanorod.A step-like structure, shown schematically in figure 4(f), was caused by the slip plane.Lattice defects such as oxygen vacancies were generated near these planes and appeared as black stripes in the TEM image (figure 4(c)).In addition, a dislocation appeared in a magnified TEM image of this region (figure 4(g)).It was concluded that the plastic deformation resulting from slip planes forming could occur in a bent SnO 2 microrod with the same Copyright 2012 American Chemical Society.(e) PL spectrum of as-grown single SnO 2 microrod at 300 K. Inset: Schematic experimental setup of the PL experiment where mechanical stress is applied to the SnO 2 microrod.Repreinted from [13] with the permission of AIP Publishing.(f) PL spectra of SnO 2 microrod with one end fixed and mechanical stress applied to the other end at 300 K. Repreinted from [13] with the permission of AIP Publishing.crystal structure.In addition, charge carriers were trapped at the oxygen vacancies created near the slip plane, resulting in an increase in electrical resistance (figure 2(e)).

Change in electronic state of a bent SnO 2 microrod
The electronic state of a bent SnO 2 microrod at 297 K was studied by using the photoluminescence (PL) method to see how the electronic state of the microrod changed by bending.One end of the microrod was fixed and mechanical stress was applied to the other end with a mechanical positioner (inset of figure 3(e)).The bent microrod was irradiated with laser light (hv = 6.3 eV), and the light emitted from the microrod was detected by using a spectrograph and a CCD camera.The PL spectrum of the unbent microrod (ε = 0) at 297 K showed no peak at 3.6 eV due to direct electron-hole recombination across the energy bandgap (figure 3(e)).This is because carriers excited to the conduction band by the irradiation decay to shallow donor levels located 0.03-0.15eV below the bottom of the conduction band, followed by an optical transition to energy levels due to oxygen defects in the band gap [17].Thus, the PL spectrum of the unbent microrod showed a broad peak centered at energies of 1.9-2.3eV (figure 3(e)).The peak intensity increased by bending the microrod (figure 3(f)).This was due to the formation of lattice defects such as oxygen vacancies near the slip planes (figure 4), which increase the density of energy levels in the band gap [17,18] and cause an increase in the optical transitions of photo-excited electrons to these levels.The intensity changed little after the stress was removed.These results demonstrate the non-volatile nature of the mechanically induced lattice defects in the microrod.

Electric healing of a lattice defect in a SnO 2 microrod
Electrical healing of lattice defects in the microrod was experimentally investigated to understand how defects are healed by applying voltage between the ends of the microrod.The creation and healing processes depended on the voltage (figure 5(a)).When more than 4 V was applied, the insulator-semiconductor transition occurred before the strain reached zero.As the voltage was increased, the threshold strain of the semiconducting-toinsulating transition increased.The threshold strains in the transition are plotted as a function of the applied voltage in figure 5(b).The increase in both strains with voltage indicates that the current flowing in the microrod caused the lattice to heal electrically, analogously to the annealing of defects by Joule heating.
The time-dependent response to an applied pulse voltage in the microrod device was used to elucidate the mechanism behind the electrical healing [13].The current change in response to the pulsed voltage was converted by a current-voltage preamplifier, and the voltage change was monitored on a storage oscilloscope (figure 5(c)).The SnO 2 microrod device was first bent to produce an insulating state and then returned to zero strain to produce an unbent microrod in the insulating state.A 7-V pulse voltage was applied to the device for 400 μs and 1 ms (figure 5(d)).A spike current signal appeared when the pulse was applied.This signal is due to accumulation of charge on the metal electrodes, similar to the accumulation of charge in a capacitor.The current began to flow after a time τ, and the current increase became slower with the passage of time, suggesting different electrical healing processes before and after the current flow.After applying the 7-V pulse voltage, the voltage was changed to 2 V.The amount of charge stored in the metal electrodes decreased and excess charge began to flow, causing a spike-like negative current to flow.After applying the pulsed voltage for 1 ms, a current of about 0.1 μA flowed through the device, indicating that the microrod had changed to the semiconducting state.
Electrical healing before the current flow was examined.The time τ from the onset of the pulse voltage to the onset of the current flow decreased as the applied voltage increased (inset in figure 5(e)).A plot of the logarithm of τ versus voltage V showed a linear relationship (figure 5(e)), indicating that carriers trapped by the potential at defect sites such as oxygen vacancies were released by the reduction of the barrier.The mechanism of trapping and release of carriers by the potential barrier is similar to the Poole-Frenkel effect [19], indicating that τ is proportional to the probability that the carriers overcome the voltage-reduced trapping barrier by thermal activation.
The electrical healing that occurs after the current flow is due to Joule heating.The transition from the insulating to the semiconducting state depends on the pulse voltage value and the pulse width (figure 5(f)) and requires a long pulse width for a small voltage pulse.Defect healing is the rearrangement of atoms into a regular order by migration of atoms and defects [2][3][4].This local recrystallization requires energy, which is provided by the Joule heating generated by the applied voltage.The gradual increase in current when the pulsed voltage is being applied (figure 5(d)) indicates that the defects are slowly healed by the Joule heating.It is important to note that in this case, lattice defects such as oxygen defects form along the slip planes.Since crystal plane migration along the planes occurs with less damage to the crystal, and since oxygen gas tends to migrate along these plane boundaries [9], the energetic barrier to defect healing in the SnO 2 microrod is low, and the lattice defects can be healed by applying a voltage for a short time.This healing process is quite different from that of an oxide nanowire with large mechanically created defects where a depressed defect region created by pressing a sharp probe on an oxide nanowire is partially healed by prolonged application of Joule heating to the nanowire [23].

ms). (e)
The logarithm of τ is linearly related to the applied voltage.Inset: Time delay τ of the response of SnO 2 microrod devices in the insulating state to each pulse voltage at T = 300 K. (f) Probability of transition from the insulating to the semiconducting state in the microrod device as a function of pulse width for V = 5, 7, and 10 V at T = 300 K. Repreinted from [13] with the permission of AIP Publishing.

UV photo sensitivity of a SnO 2 microrod device
SnO 2 is a wide bandgap material (E g = 3.6 eV), in which ultraviolet (UV) light excites carriers in the valence band to the conduction band across the bandgap and the measurement of the photo-induced carriers works as a monitor of the UV light, indicating that SnO 2 works as a semiconductor UV photosensor.The photo-induced current I photo was measured by changing the wavelength of the light irradiating the SnO 2 microrod (figure 6(a)) [16].The optical response R λ was calculated as R λ = I photo /SP λ , where S is the light-exposed area and P λ is the wavelength-dependent intensity of the incident light.The plot of R λ versus wavelength λ (figure 6(a)) shows that R λ increases rapidly below a wavelength close to the bandgap energy (∼345 nm) of SnO 2 .The relationship between the optical response R λ and the internal gain G in the material is as follows [24]: where q is the fundamental charge, λ is the wavelength, η is the quantum efficiency, h is Planck's constant, and c is the speed of light.R λ of the SnO 2 microrod device at 250 nm is ∼3 × 10 8 A W −1 .Assuming a quantum efficiency η of 100% and using equation (2), the internal gain G would be ∼1.5 × 10 9 .This is one to two orders of magnitude larger than G (10 6 -10 7 ) of conventional SnO 2 nanowire photosensors [25][26][27].

Solving the PPC problem by controlling lattice defects in a SnO 2 microrod device
Lattice defect engineering in SnO 2 microrod devices was used to solve the PPC problem [28] that has affected UV semiconductor sensors; that is, when these devices are irradiated with UV light (260 nm), the photo-induced current increases rapidly in a short time (figure 6(b)).Irradiation experiments were performed under three different conditions (296 K air, 331 K air, and 296 K vacuum).When the light irradiation was stopped, the photo-induced current decayed slowly.The decay at 296 K in air was faster than that in vacuum.This is because devices in a vacuum have more oxygen vacancies on the surface and inside, suggesting that carrier trapping by oxygen vacancies is the cause of the PPC problem [29].The carrier trapping energy in the PPC problem was elucidated by analyzing the decay curves of devices at different temperatures in air.The PPC decay can be described by an extended exponential function [28]: where I PPC (0) is the current immediately after the light irradiation is stopped, τ is the time constant of the PPC decay, and β is the exponential factor of the decay (0 < β < 1).τ and β were determined from the best fit of equation (3) to the data (figure 6(c)).β ranged from 0.51 to 0.54.Plotting the logarithm of τ for each temperature against the reciprocal of the temperature yielded a linear relationship (figure 6(d)), suggesting that the time constant of the decay is determined by a thermal activation process.This process can be expressed by the following equation [28]: where E C is the carrier trapping potential and τ 0 is a constant that determines the time scale.From the slope of the line obtained from the experimental data, E c is required to be 160 meV (figure 6(d)).This means that the PPC problem is caused by photo-excited carriers becoming trapped by an energy barrier of 160 meV at the oxygen vacancies.
The PPC problem was solved by exploiting the lattice defect engineering method used in the SnO 2 microrod devices.A photocurrent was induced by irradiating the devices with UV light (260 nm) in air at 296 K (figure 7(a)).After the light irradiation was stopped and the PPC decay of the current began to occur, a reset process was applied to the device (inset in figure 7(a)); the device was bent (ε = 0.15%) to bring it into the insulating state and then it was unbent (ε = 0%).A voltage of 8 V was then applied to the device for 2 s for electrical healing.After applying this reset process, the photo-induced current decreased rapidly and returned to its original value (figure 7(a)).Thus, the lattice defects created near the generated slip planes acted as recombination centers for photoexcited electron-hole pairs, and the Joule heating generated for the electrical healing released carriers trapped at the defect sites and promoted the transition to the ground state by recombination.Continuous pulsed UV light (260 nm) irradiation of the microrod device (top panel in figure 7(b)) reduced the signal-to-noise (SN) ratio of the photoinduced current response due to the PPC problem (middle panel).On the other hand, when the reset process was applied immediately after the pulsed UV irradiation, the photocurrent showed a good response with a high SN ratio (bottom panel).Thus, the reset process using lattice defect engineering proved to be an effective solution to the PPC problem of semiconductor UV photosensors.

Emergent sensitivity in hybrid SnO 2 -based sensors
SnO 2 materials are widely used as commercial gas sensors and exhibit good sensitivity to relative humidity (RH) at low temperatures [9,10].However, the described SnO 2 microrod sensor did not function as a gas sensor (lower inset in figure 9(a)) because its electrical properties did not change when water molecules adsorbed on the surface under high RH conditions.The current was dominated by the flow inside of the microrod due to the small surface-to-volume ratio.This section describes functionalized SnO 2 -based sensors as an example of using material nanoarchitectonics to overcome this weakness.In particular, to show the potential of material nanoarchitectonics, Ga 2 O 3 , which has a large bandgap (4.7 eV) and a polymorphic nature, was hybridized with SnO 2 .In addition, mechanical stress was applied to the hybridized sensor as a way to activate its functionality.

Preparation and characterization of microribbons composed of SnO 2 and Ga 2 O 3
The hybridized structure was formed in a one-step fabrication using vapor phase growth of SnO 2 and Ga 2 O 3 [14].Since Ga 2 O 3 has a higher melting point than SnO 2 , there is a difference in the temperature at which the respective atomic gases aggregate and crystallize.This difference was exploited to create a hybridized structure.A mixture of SnO 2 , Ga 2 O 3 , and carbon powder in a mass ratio of 1:1:2 was placed in an Al 2 O 3 boat, and vapor phase growth was performed in an electric furnace at 990 °C by using argon gas (9 × 10 2 Pa) mixed with 4% oxygen as a carrier gas.A Ga-based material was first grown as the core of the structure.As the temperature decreased, Sn atomic gas was adsorbed on the surface and formed microribbons with scaly-like shell structures (figures 8(a), (b)).The microribbon had a rectangular cross-section of about 10 μm × 1.5 μm (figure 8(b)).The shell was covered with scaly structures with a lateral size of about 200 nm.Local EDX measurements of the microribbon showed that the shell contained Sn atoms with a thickness of about 300 nm (figure 8(c)).Since the XPS spectrum in the energy range of 0-1200 eV shows peaks ascribed to Sn, O, C (figure 8(d)), the ribbon is covered by Sn-based layers.Figure 8(e) shows double spectral lines of Sn 3d at binding energies of 485.9 eV (Sn 3d 5/2) and 494.3 eV (Sn 3d 3/2) with a spin-orbit splitting of 8.4 eV, which is consistent with Sn 4+ ion bound to oxygen in the SnO 2 matrix [14].The O 1s peaks are composed of a peak (531.1 eV) of O 2-from the rutile structure of SnO 2 and a peak (532.7 eV) of thin hydroxide on the surface of amorphous SnO 2 (figure 8(f)) [14].There were no peaks corresponding to SnO 2 in the x-ray diffraction pattern of the single microribbon (figure 8(g)), suggesting that the shell was composed of amorphous SnO 2 .Analysis of the x-ray diffraction pattern corresponding to the Ga-based core (figure 8(g)) showed that the lattice constant was the same as that of β-Ga 2 O 3 among the polymorphic oxide gallium, indicating that the core was composed of a β-Ga 2 O 3 single crystal.From these analyses, it was concluded that the microribbon had a β-Ga 2 O 3 /amorphous SnO 2 core/shell structure (figure 8(h)).3.2.Ultra-sensitive humidity sensing using Ga 2 O 3 /SnO 2 -core/shell microribbon sensor A core/shell microribbon sensor was fabricated by attaching gold electrodes to both ends of a core/shell microribbon placed on a polyimide sheet [14].The resistance of the device was 3.5 × 10 12 Ω at 296 K, and the current changed significantly in response to the adsorption of water molecules (figure 9(a)).On the other hand, a SnO 2 microrod sensor showed no change when water molecules adsorbed on the microrod (lower inset).The resistances of the Ga 2 O 3 microrod sensors with and without water adsorption were below the measurement limit (upper inset).Their hybridization of SnO 2 and Ga 2 O 3 through material nanoarchitectonics enabled us to create a new functionality as follows.This section describes humidity sensing using these core-shell devices and elucidates the mechanism behind their high sensitivity and high response speed.
When the Ga 2 O 3 /SnO 2 -core/shell sensor was placed in an atmosphere in which the relative humidity (RH) was repeatedly changed between 75% and 5% at 296 K, the current in the device responded reproducibly and rapidly to the humidity change (figure 9(b)).The response time, which is defined here as the time required to reach 90% RH of the saturation value of the current change, was about 28 s at high humidity and about 7 s at low humidity (inset in figure 9(b)).These response times are shorter than those of conventional humidity sensors [30][31][32][33].The mechanism behind the fast response time of the core-shell device is as follows.The shell in dry air is covered with molecular oxygen ions (figure 9(c)).Most of the carriers in the shell are trapped by these ions adsorbed on the SnO 2 surface and at the boundaries of the nanostructured shell, while some of the carriers in the SnO 2 shell move into the Ga 2 O 3 core due to the difference between the Fermi surfaces (figure 8(i)).This causes carrier depletion in the SnO 2 layer and low current flow.In the case of the SnO 2 shell at 75% RH, on the other hand, the oxygen molecular ions on the surface are replaced by water molecules.The charge carriers trapped by the oxygen molecular ions are released and flow through the SnO 2 layers and H + ions also follow along the surface water molecular layer (figure 9(d)) [30][31][32], resulting in high current.Because trapping and release of charge carriers occurs primarily within the shell as it switches between low and high humidity, the sensor can respond quickly to changes in humidity.
Next, the current of the core-shell sensor was measured as RH was varied from 15% to 85% at temperatures of 285, 293, 303, and 311 K.The sensitivity S = I humid air /I dry air , where I humid air and I dry air represent the current of the sample in dry air (5% RH) and humid air, respectively, is plotted against RH in figure 9(e).As shown, the sensitivity changed by about five orders of magnitude at 285 K under high RH conditions.When the relative humidity was set at 75% RH and the temperature was varied, the current changed by about four orders of magnitude with no hysteresis between the heating and cooling times (figure 9(f)).At 5% RH, however, there was almost no change in current over the temperature range (inset in figure 9(f)).Since I dry air below 5% RH is on the order of 10 -4 nA, the humidity sensitivity of core-shell sensors is determined by the change in I humid air due to adsorbed water molecules replacing oxygen ions on the SnO 2 shell.Water adsorption induces the release of electrons from the oxygen ion sites, and the electrons inside the shell and the H + ions along the water layers act as a current that is nearly proportional to the number of the water molecules on the shell.When the SnO 2 shell was wet, the device current changed significantly with temperature (figure 9(f)).This temperature-dependent sensitivity is due to the intrinsic nature of SnO 2 materials, namely the physical adsorption property of water molecules on the SnO 2 surface [34].The molecular partial pressure required for water molecules to adsorb on the SnO 2 surface at a given temperature indicates that water molecules adsorb very easily on the SnO 2 surface at temperatures below 293 K, while adsorption becomes more difficult at temperatures above 293 K.This water adsorptive nature of the microribbon sensors originates from the intrinsic nature of the SnO 2 forming the shell.The large surface area of the amorphous nanostructured shell makes it possible to access the nature of the SnO 2 surface through current measurements as a function of temperature.
3.3.Sensitivity enhanced by bending wet Ga 2 O 3 /SnO 2 -core/shell microribbon sensor Bending a core/shell sensor by applying mechanical stress causes an increase in the humidity sensitivity (figure 10(a)) [15].The linear current-voltage relationship at 297 K and 75% RH under different strains (figure 10(b)) indicates that the ohmic contact properties between the gold electrode and the device were unchanged by the application of strain.The relative resistance ΔR/R 0 , where R 0 is the resistance at zero strain and R strain is the resistance under strain, varied linearly with strain ε (figure 10(c)).The gauge factor (GF) of the strain is expressed by the following equation: This equation gives a GF of −41 for the core-shell device at 45%-75% RH (inset of figure 10(c)).The large and nearly constant GF is due to the linear increase in the surface area of the amorphous SnO 2 shell as the device is stretched.The response times of the sensor to the absorption and desorption of water molecules were almost the same regardless of the degree of strain (ε = 0 or 0.77%) (figure 10(a)), indicating that the chemical properties of the SnO 2 forming the shell were constant with or without strain.When the device was bent, the interstices and grain boundaries in the scaly structured shell expanded.Water molecules adsorbed on this surface and allowed current to flow through the wet nanostructured SnO 2 , leading to a large change in the current and a large gauge factor when bending the sensor (figure 10(c)).The wet flexible strain sensor showed reproducible response to alternating strain changes and no degradation due to aging at least for several months of use.The GF of −41 for the wet sensor was better than that of commercial strain sensors (GF ≈ 2) and non-piezoresistive strain sensors composed of a graphene ribbon (GF = 1.9) [35], and worse than that of piezoresistive strain sensors composed of a carbon nanowire (GF ≈ 600-1000) [36] and a ZnO nanowire (GF = 1250) [37].

Flexible SnO 2 film sensors
SnO 2 is a well-known gas sensing material that has been widely used as a 2D film sensor [38,39] rather than the 1D wire sensor mentioned in this review.This is because SnO 2 films have a larger surface area, can adsorb more gases, and are easier to fabricate.The films can be fabricated on both solid substrates and flexible films by atomic layer deposition, electrochemical deposition, sputtering, pulsed laser deposition [40], ink jet printing methods [41], and spin-coating of SnO 2 nanoparticles.In addition, flexible SnO 2 films have been used as the n-type electron transparent layer (ETL) [42] of perovskite solar cells with n-i-p device architecture [43], because the SnO 2 material itself has excellent properties as an ETL material, including high light transmission with wide bandgap, high conductivity, high chemical stability, and the proper band energy alignment with the perovskite layer.Moreover, instead of SnO 2 films in the solar cells, SnO 2 and TiO 2 nanowire aggregated layer have been applied as other candidates for the ETL of perovskite solar cells [44].
Interfacial chemical engineering and modification between the perovskite and SnO 2 layers in the n-i-p structure of perovskite solar cells are known to play an important role in forming efficient and stable interfacial energy alignment and producing high electron mobility of the SnO 2 layers [45,46].In order to solve this interfacial problem, surface modification of the SnO 2 layer with alkaline halide salts [47] and with ammonium halide salts [48], and passivation of the SnO 2 surface with functional organic compounds and self-assembled molecules (SAMs) [45,49,50] have been applied to the perovskite solar cells.These procedures improved the stability and reliability of the cells and produced above 25% certified efficiency [51].Note that the structural electronic changes caused by mechanical bending of the flexible solar cells with SnO 2 layers are not required in their practical applications.

Summary
The present study demonstrates that novel functionalities can be created in rutile-structured SnO 2 microrods by dynamic lattice defect engineering through the application of mechanical stress and voltage and that β-Ga 2 O 3 /amorphous SnO 2 core/shell microribbons exhibit novel humidity and temperature sensitivities that are enhanced by increasing the surface area of the SnO 2 through the application of mechanical stress.The design and fabrication of these hybridized oxide structures by nanoarchitectonics and the enhancement of the functionality by applying external stimuli pave the way for new sensor applications.Nano/microrods with novel functionalities can be fabricated by appropriate material selection and their unique hybridized growth, namely 'material nanoarchitectonics', then the combination of these hybridized oxide wire aggregated layers with functional 2D materials can produce new functionalities by electronic linkage between them and open a new avenue for the development of new 2D materials and sensors.
(a)) and a length of 3-5 mm[8].The microrods grew from tiny SnO 2 crystal nuclei which were formed by the high-density atomic Sn gas on the surface of the Al 2 O 3 boat (figure2(b)).The SnO 2 nanorods were formed by the catalytic reaction of gold particles on the sapphire (001) substrate[12].The Transmission electron microscope (TEM) image and diffraction pattern of the SnO 2 nanorod show the growth in the [001] direction (figure2(d)).

Figure 1 .
Figure 1.(a) Schematic states of bent single-crystal rod with rutile-structure studied in this review.Slip planes form when the rod is bent beyond its elastic deformation limit.(b) Photograph of wire device bent by mechanical force.(c) Strain (ε) of the rod as determined from the polar radius R c , thickness of the polymer substrate (t polymer ) and sample (t sample ), as in equation (1).

Figure 2 .
Figure 2. (a) Scanning electron microscope (SEM) image of as-grown single-crystal SnO 2 microrod.(b) Optical microscope image of as-grown SnO 2 microrods grown on the edge of an Al 2 O 3 boat.(c) XRD pattern of as-grown SnO 2 microrod: scattering vector q parallel to rod axis, scattering vector q parallel to the diagonal line of square cross-section of SnO 2 microrod [16].(d) TEM image of single SnO 2 nanorod.Upper inset: TEM diffraction pattern of nanorod near the edge of the supporting carbon sheet.Lower inset: SEM image of as-grown SnO 2 nanorods.(e) Resistivity of SnO 2 microrod device as a function of mechanical strain at V = 2.0 V.The strain is increased from 0 to 0.186% (pink hexagons) and then decreased to 0 (blue open hexagons).The resistivity returns to its original value when the applied voltage is above the threshold voltage V Th .The inset shows a schematic illustration of the bent SnO 2 device [8].Copyright 2015 Jon Wiley and Sons.

Figure 3 .
Figure 3. Micro-Raman spectra of SnO 2 microrod at 300 K under different strain conditions.The vertical dotted line shows the peak of the A 1g mode at ε = 0. (a) Original straight SnO 2 microrod in the semiconducting state (strain ε = 0).(b) Bent SnO 2 microrod in the insulating state (ε = 0.15 %).(c) Microrod in the insulating state after straightening (ε = 0).(d) Microrod in the semiconducting state by applying a voltage across the microrod (ε = 0) [8].Copyright 2012 American Chemical Society.(e) PL spectrum of as-grown single SnO 2 microrod at 300 K. Inset: Schematic experimental setup of the PL experiment where mechanical stress is applied to the SnO 2 microrod.Repreinted from[13] with the permission of AIP Publishing.(f) PL spectra of SnO 2 microrod with one end fixed and mechanical stress applied to the other end at 300 K. Repreinted from[13] with the permission of AIP Publishing.

Figure 4 .
Figure 4. (a) Schematic diagram showing the fabrication of a bent nanowire using a sharp metal probe and focused tungsten deposition in a FIB machine.The insets are scanning electron microscope (SEM) images of a SnO 2 nanowire bending by the FIB method.The red arrow indicates the SnO 2 nanowire.The number in the inset corresponds to the area numbered in the schematic diagram.(b) TEM image of bent single-crystal SnO 2 nanowire on a thin carbon support layer of a TEM grid.(c) TEM image of bent single-crystal SnO 2 nanowire under 3% strain.Black stripes with blue arrows indicate the formation of slip planes in the rutile structure.(d) High-magnification TEM image of the outer surface under tensile strain.Arrows indicate step-like surface structures.(e) High-magnification TEM image of the outer surface of an unbent SnO 2 nanowire.(f) Schematic diagram showing slip planes formed by tensile strain.The inset shows the geometric relationship between the [001] growth direction and the 10 1 ( ̅ ) plane in the rutile structure.(g) High-magnification TEM image of a SnO 2 nanowire with dislocations under tensile strain.The inset schematically shows the dislocation at the slip plane [12].Copyright 2014 IOP Publishing.

Figure 5 .
Figure 5. (a) Strain versus resistance curves for the SnO2 microrod device at V b = 2, 4, 5, and 7 V and T = 300 K. (b) Threshold strain at semiconducting-to-insulating transitions (closed circles) and insulating-to-semiconducting transitions (open circles) versus voltage applied between the electrodes.(c) Schematic illustration of dynamic measurement of SnO 2 microrod devices under application of mechanical stress and pulse voltage.The distance between the Au electrodes is 120 μm for a single-crystal SnO 2 microrod.(d) Change in current of the device in the insulating state after a time delay τ in response to pulse voltage (V = 7 V and pulse width of 400 μs and 1 ms).(e) The logarithm of τ is linearly related to the applied voltage.Inset: Time delay τ of the response of SnO 2 microrod devices in the insulating state to each pulse voltage at T = 300 K. (f) Probability of transition from the insulating to the semiconducting state in the microrod device as a function of pulse width for V = 5, 7, and 10 V at T = 300 K. Repreinted from [13] with the permission of AIP Publishing.

Figure 6 .
Figure 6.(a) Photoresponse spectrum of SnO 2 single microrod photosensor at 296 K for an applied voltage of 2 V.The inset shows the schematic experimental setup.(b) Typical PPC behavior in SnO 2 single microrod irradiated by 260 nm UV light at 2 V applied voltage under different conditions: 296 K in vacuum, 296 K in air and 331 K in air.The excitation intensity is about 0.05 μW cm −2 .(c) Normalized photocurrent decay in air at 296 K (open squares) and 331 K (open circles).The PPC decay time constants are determined by fitting the decay curves with the stretched-exponential equation.(d) Arrhenius plot of PPC decay time constant as a function of inverse temperature [16].Copyright 2015 Jon Wiley and Sons.

Figure 7 .
Figure 7. (a) Photoresponse spectrum of SnO 2 single microrod photosensor at 296 K for an applied voltage of 2 V. (b) Time-dependent photocurrent response measured in dry air at a bias voltage of 2 V under 260 nm illuminations with different intensities.Good reproducibility is obtained after the reset process [16].Copyright 2015 Jon Wiley and Sons.

Figure 8 .
Figure 8. SEM images of as-grown β-Ga 2 O 3 /amorphous-SnO 2 core/shell microribbons at (a) low and (b) high magnifications.(c) SEM image of microribbon and EDX line scanning profiles of Ga, O, Sn components in core/shell microribbon.XPS spectra showing binding energy of (d) all, (e) Sn, and (f) O atoms.The Sn 3d 5/2 peak shows a symmetric component without shoulders (Red curve: Gausian fitting).(g) XRD patterns of as-grown single microribbon with the scattering vector q parallel to the rod axis.(h) Schematic structure of a Ga 2 O 3 / SnO 2 core/shell microribbon.(i) Schematic energy band diagram of Ga 2 O 3 and SnO 2 [14].Copyright 2012 Royal Society of Chemistry.

Figure 9 .
Figure 9. (a) I-V curves of core/shell microribbon-based humidity sensor in dry air (5% RH) and 75% RH air at 298 K.The upper and lower insets show the I-V curves of Ga 2 O 3 microribbon and SnO 2 microwire control samples, respectively.Both control samples have negligible humidity sensing at 298 K. (b) Dynamic response of Ga 2 O 3 /SnO 2 -core/shell microribbon sensor for detection of 75% RH air.The inset shows that the response time and recovery time were ∼28 and ∼7 s, respectively.Conductivity models of β-Ga 2 O 3 / amorphous-SnO 2 core/shell microribbon in (c) dry air and (d) humid air.(e) Sensitivity as a function of RH at different operating temperatures: 285 K (red solid square line), 293 K (blue open square line), 303 K (pink solid circle line) and 311 K (green open circle line).(f) Temperature dependence of sensor current in 75% RH air at V = 1 V.The inset shows the temperature dependence of the sensor current in dry air (5% RH) [14].Copyright 2012 Royal Society of Chemistry.

Figure 10 .
Figure 10.(a) Dynamic response of flexible sensor to different levels of RH under 0 (blue line) and 0.77% strain (red line).The device was operated at 297 K with an applied voltage of 10 V. (b) Linear voltage-current relationship for different strains.The inset shows the time-dependent response at 0% and 0.77 %, respectively.(c) The relative change in resistance (ΔR/R 0 ) as a function of the tensile strain at 75% RH.The slope corresponds to a GF of −41.The inset shows GF of the flexible sensor at RH of 45, 55, 65 and 75% [15].Copyright 2012 Jon Wiley and Sons.