In Tandem Contact-Transfer Printing for High-Performance Transient Electronics

High‐performance flexible electronics developed with resource efficient printing route will transform the way future electronics is manufactured and used to advance applications such as healthcare, Internet of Things, wearables, consumer electronics, etc. Herein, an innovative approach is presented that involves, for the first time, the in‐tandem use of contact and transfer printing methods to realize high‐quality electronic layers at selected locations on rigid (Si/SiO2), flexible (polyimide), and biodegradable (magnesium (Mg) foils). Superior grade quality of printed electronic layers is demonstrated by realizing transistors and printed UV photodetectors (PDs) employing high‐resolution electrohydrodynamic printing. The all‐printed PDs show extremely high performance for UV detection, with extraordinary high responsivity (>107 A W−1) and specific detectivity (≈1017 Jones) values at low UV intensity of 0.1 µW cm−2. Finally, the fabricated PDs on Mg foil are dissolved in deionized water at room temperature. Thus, in‐tandem contact and transfer printing has potential for ecofriendly development of transient electronics. Further, the approach allows printing of wide range of nanomaterials and heterostructures or complex superlattice structures, which can open exciting new possibilities for high‐performance electronics.


DOI: 10.1002/aelm.202200170
visible shift from integrated circuits (ICs) on conventional rigid silicon towards their ultrathin counterparts [8] and hybrid or heterogeneous integration, combining silicon technology with printed electronics. [9] In particular, printed electronics has opened interesting opportunities for flexible electronics as it offers excellent features such as design flexibility (maskless designs), efficient use of materials, low fabrication cost, low-temperature deposition, and possibility to realize devices on diverse nonconventional and degradable substrates. [10] The burgeoning printed electronics field however has few existing challenges such as modest performance with respect to conventional electronics, as a result of which its commercial applications are restricted to low-end. [10e] Innovative methods are much needed to push the performance of printed electronics and use this resource efficient route to develop faster device and circuits for high-end applications. To this end, transfer printing of 1D inorganic nanostructures has shown some potential. [11] It allows transfer of laterally aligned photolithography-defined arrays of inorganic semiconducting nanostructures from donor substrate to the target flexible substrate using elastomeric stamps (Figure 1a). To enhance the printing efficiency and registration, modified transfer printing methods involving the interface engineering, [12] thermal modulation and kinetically controlled velocity, [13] laser driven, [14] intermetallic wetting effect, [15] and direct roll transfer [11a,11b] have been proposed. These modified transfer methods have extended the printing capacities to: i) selective transfer, [16] ii) arbitrary substrate integration, [17] iii) 3D patterning of stretchable liquid metals, [15] and iv) deterministic assembly of nano to chip scale structures. [10e,12b,18] These printing approaches have led to the development of various high-performance devices on flexible and stretchable substrates, including both n-and p-channel transistors [11a,11b] needed to present a viable alternative for conventional complementary metal-oxide-semiconductor (CMOS) electronics, and functional liquid metal circuit are designed for fabricating 3D electronic devices and sensors. [15] However, transfer printing method suffers from limitations such as choice of active material limited by the availability of a bulk-wafer from where lithography-defined, laterally aligned nanostructures are High-performance flexible electronics developed with resource efficient printing route will transform the way future electronics is manufactured and used to advance applications such as healthcare, Internet of Things, wearables, consumer electronics, etc. Herein, an innovative approach is presented that involves, for the first time, the in-tandem use of contact and transfer printing methods to realize high-quality electronic layers at selected locations on rigid (Si/SiO 2 ), flexible (polyimide), and biodegradable (magnesium (Mg) foils). Superior grade quality of printed electronic layers is demonstrated by realizing transistors and printed UV photodetectors (PDs) employing highresolution electrohydrodynamic printing. The all-printed PDs show extremely high performance for UV detection, with extraordinary high responsivity (>10 7 A W −1 ) and specific detectivity (≈10 17 Jones) values at low UV intensity of 0.1 µW cm −2 . Finally, the fabricated PDs on Mg foil are dissolved in deionized water at room temperature. Thus, in-tandem contact and transfer printing has potential for ecofriendly development of transient electronics. Further, the approach allows printing of wide range of nanomaterials and heterostructures or complex superlattice structures, which can open exciting new possibilities for high-performance electronics.

Introduction
The impressive progresses made in the field of flexible/ bendable electronics has paved the path for the development of next-generation technologies such as rollable displays, [1] flexible sensors, [2] soft robotics, [2a,2b] wearable systems, [3] electronicskin, [4] epidermal electronics, [5] mobile healthcare, [6] natively flexible processing engines, [7] etc. This will also have an impact on how various current information and communication technologies (ICT) will be developed in the future. There is already a harvested. Such materials related limitations could be overcome by alternative nanostructure assembly methods such as Langmuir-Blodgett (LB) method, [19] and contact printing, [20] etc., which exploit bottom-up grown nanomaterials to realize nanoscale electronic layers on various substrates. The bottomup route for nanomaterials is attractive as it allows compositional control and thus heterostructures or complex superlattice structures can also be printed. For example, dopant materials, such as phosphine for silicon, can be introduced and removed repeatedly throughout the nanowire growth, producing superlattice structures with modulated electronic properties, [21] which can be utilized to encode unconventional electronic properties and a wide variety of shapes and structures that can be used for development of various sensing, photonics, electronics and memory devices on large area flexible substrates. This means that electronic layers with richer materials systems could be obtained with contact printing or other nanoscale assembly technique and subsequently transfer printing could be used for further processing to develop devices with unique features. In doing so, the dependence on inherently wasteful conventional fabrication processes [22] (e.g., photolithography and etching to obtain nanostructures from wafers) can also be reduced.
Herein, we use this new contact-transfer printing in tandem approach, as shown in the Figure 1b, to develop high-performance electronic devices. First, the contact printing is used to print highly laterally aligned 1D structures on an intermediate substrate to obtain uniform electronic layers. Then, conventional, or modified transfer printing is employed to selectively remove the electronic layers and print them on the final substrate. The electronic layers on final substrates are further processed to develop devices such as transistors and photodetectors (PDs). With this novel approach, we present a resource-efficient process for high-performance nanowire (NW)-based printed, transient electronics. The fabrication process steps include nanoscale material development (ZnO NWs), in-tandem use of contact and transfer printing processes, and high-resolution electrohydrodynamic (EHD) printing. All the process steps are eco-friendly, scalable, resource-efficient, and extendable for wide range of applications. First, vertically aligned ZnO NWs were grown via nontoxic Vapor Phase Transport (VPT) method using a horizontal tube furnace. Then our in-house automated contact printing system was used for uniform printing of highly dense horizontally aligned ZnO NWs on an intermediate substrate (rigid silicon). [20a,20b] Contact printing offers advantages such as: i) highly directional alignment of nanostructures, which is needed for high-device performance, ii) single-step process, iii) cost-effective, and iv) avoids contamination to NW surface as it is a dry printing method. Once NWs based electronic layer is printed, the stamp or transfer printing was employed to transfer the electronic layers from selected locations to the final device substrate (Si/SiO 2 , PI and Mg foil). The latter allowed us to fabricate many samples using a single donor. Printing at selective locations helps in reducing the sensor crosstalk and improves the heterogeneous integration of NW arrays. [23] It may be noted that contact printing could also be used to directly print the electronic layers at selected location, as has been demonstrated in past. [24] In fact, this would be efficient in terms of number of NW printed. However, the lithography and surface treatment needed each time to define the NW locations will make it cumbersome for large scale integration and contaminations by surface treatment step could influence the electrical properties of NWs. Further, contact printing cannot be used for nanostructures that are horizontally present on the donor substrate, for example, NWs and nanoribbons realized on silicon wafer using photolithography. In this regard, the in-tandem contact-transfer printing presented here is versatile and makes best use of both printing methods. Next, EHD printing was utilized to print diffusive Au contacts over the two ends of aligned NWs to define the device channel. EHD offers excellent possibilities such as drop-on-demand feature (resource-efficient), substrate independent patterning, compatibility with large area manufacturing, and the potential for lowcost operation. [25] The above approach was used to develop proof-of-concept devices such as transistor and PDs. First, n-channel transistors were developed over Si/SiO 2 substrate to extract the electrical properties of the as-gown ZnO NWs. Next, these NWs were used to develop printed PDs. The printed PDs, on both PI and Mg, showed record high responsivity (>10 7 A W −1 ) and specific detectivity (≈10 17 Jones) values at low UV intensity of 0.1 µW cm −2 . Finally, we show that the developed PDs on Mg foil can physically disintegrate and dissolve in 240 number of days in deionized (DI) water of pH ≈6 at room temperature.  Figure 1 also illustrates the processing steps for conventional transfer printing. The benefits of proposed printing approach include: i) wider choice of nanomaterial to be printed, ii) resource efficient process as it avoids repeated use of conventional microfabrication steps, iii) cost-effective, and iv) more ecofriendly. To begin with, vertically aligned ZnO NWs were grown on c-plane sapphire substrates. Bottom-up approaches by vapor-liquid-solid (VLS) mechanism using VPT technique have been successful in producing environment friendly ZnO NWs. Figure S1 (Supporting Information) shows schematic representation of the NW growth VPT system, scanning electron microscopy (SEM) images, and x-ray diffraction (XRD) scan of the as-grown ZnO NWs. Figure 1b shows highdensity of highly vertically aligned ZnO NWs growth on c-plane sapphire substrates, similar to the previous reports of ZnO NW growth on (0001)-oriented hexagonal substrates. [26] It may be noted that some of the long NWs (length > 20 µm) could bend during the growth process due to their own mechanical strain. To assess the crystallographic quality of the NWs, XRD analysis was performed. As can be seen from the XRD data ( Figure S1c, Supporting Information), ZnO NWs show dominant peaks at 2θ = 34.4°, corresponding to (0002) plane orientation. This observation indicates growth of highly c-axis-oriented wurtzite ZnO NWs, as hinted from SEM image. High density of vertically aligned NWs is critical to achieve high density of horizontally aligned contact printed NWs and hence the uniform electronic layer.

In Tandem Contact-Transfer Printing Technique
Subsequently, contact printing was performed for uniform assembly of laterally aligned ZnO NWs over an intermediate receiver substrate. Figure 2 schematically shows contact printing process steps, 3D system model and the SEM images of the resulting printed NW-based electronic layer. The equipment details can be obtained from our previous work. [20a] Briefly, during the printing process, the combination of a controlled vertical and shear forces (due to sliding) results in the detaching of NWs from the donor sapphire substrate and transferring onto the intermediate receiver. This results in the NWs alignment along the direction of sliding. From the no sliding test, we see that sliding is needed not only for directional alignment but for breaking as well. The details of the printing conditions are given in methods section. To achieve uniform printing of NWs, the applied pressure at the predetermined level was maintained throughout the printing process and applied evenly along the entire area of the sample. Figure 2c-e shows the SEM images of printed NW at various magnifications. By inspecting the SEM images, we observe that the uniform printing of NWs across the entire surface area (≈1cm 2 ). This suggests that using the contact printing system with self-aligning platforms can achieve conformal contact between the donor and receiver substrates over large areas. Using SEM images and the analysis software (GTFiber2), [27] statistical analysis of the printed NWs, including the average orientation angle, mean diameter, and NW density per micrometer was obtained and is shown in Table S1 (Supporting Information). The extracted mean printed NW density is ≈1.3 NWs µm −1 . The printed high density of NWs could be correlated with the as grown NWs on donor sapphire substrate. Encouragingly, more than 93% of the printed area has shown the average NW orientation within ±5 degree of the direction of sliding (90°). This indicates a high level of NW alignment which was hinted from the SEM images. A high density of aligned NWs is indeed required for the fabrication of high-performance flexible electronics as it reduces the electron scattering at the NW-NW junction and facilitates efficient charge transport. Such a high NW alignment could also be attributed to the additional shear force applied during the printing process. To prove this hypothesis, a NW sample (silicon) was printed where no sliding motion was used (velocity = 0). The sample without sliding motion demonstrates extremely low density and large variation in alignment ( Figure S2, Supporting Information).
Next, the transfer printing was employed for selective assembly of ZnO NWs on rigid Si/SiO 2 , and flexible polyimide (PI) and biodegradable magnesium (Mg) foil from contact printed uniform electronic layers. It is to note that optimization studies for transfer printing were performed on PI substrate.  Figure S3 (Supporting Information) shows the optical microscopy images of etched silicon master template and PDMS mold with square pillar structures. From the Figure S3 (Supporting Information) we can see that the length and width of square structures are 100 and 100 µm, respectively. Using the fabricated microstructure PDMS stamp, we transferred the contact printed ZnO NWs selectively over target device substrate. To have reasonably high transfer efficiency (during the picking-up step), we used previously reported capillary-force-assisted stamp technique that uses a thin layer of evaporated water as an instant glue to increase the adhesion energy between nanomaterials and PDMS during the pick-up step. [28] To have a thin layer of water, we hold the fabricated PDMS mold over a pre-heated water at 80 °C for 10 s. Next, immediately the stamp was placed over the contact printed NWs substrate (pillar side facing NWs) and constant force of 10N was applied for 10 s. After the liquid evaporates, the NWs were transferred onto PDMS stamp. The stamp with NWs was immediately placed over the target device substrate and a constant force was applied. The yield and registration factor of printed nanomaterials are low due to the viscoelastic nature of the PDMS stamp, but this is inherent to the conventional transfer printing. To optimize the transfer yield, the magnitude of the applied force was varied from 0.5 to 10 N. The number of NWs transferred were dependent on the magnitude of the force applied ( Figure S4, Supporting Information). Increasing the applied force enhances the number of NW transferred. It is to note that due to system limitation we could not apply higher forces to further increase the transfer yield. The use of water as an adhesion promoter ensures the low contamination level on the NW materials and largely preserves their electrical properties. This is needed for the high performance offered by inorganic NWs.

N-channel Transistors and Electrical Characterizations
To confirm the material's electrical quality, ZnO NW based bottom-gated field-effect transistors (NW-FETs) were fabricated and characterized. The ZnO NW-FETs (based on printed NWs) were fabricated on Si/SiO 2 substrates using standard microfabrication processes. The resulting electrical characterization results, output, and transfer curves along with the ZnO NW-FET schematic are shown in Figure 4. It is to note that the transfer of NWs was performed using the exact procedure    −13 V. The extracted peak field effect mobility (μ FE ) was found to be ≈50 cm 2 V -1 s -1 , using the standard cylinder-plate capacitor model. [29] The extracted mobility compares well with the state of the art for ZnO NWs which confirm the high quality of our nanomaterial and in tandem printing methods. The output characteristics (V DS -I DS ) was obtained by varying V GS from −20 to 40 V with step of 10 V (Figure 4d). Notably, the low-V DS region shows a linear dependence of I DS with increasing V DS without any inflection point (confirming formation of ohmic contacts with Ti metal). This is needed to extract the near correct values of nanomaterials. The high ohmicity of the metal-semiconductor (MS) contacts further confirms the clean transfer of NWs with negligible contamination on the NW surface.
It is to note that the fabricated transistors showed some device-to-device performance variations including V T , mobility etc. To improve the performance reproducibility of devices from single batch or from different batches, many important aspects need attention. First, uniform ZnO NW synthesis which includes NWs having controlled diameter, doping, surface states, etc., on the donor substrate. To-date, it is highly challenging to reproduce the NW growth process with controlled electrical properties. Second, highly controlled NW printing approach to have uniform electronic layer. This will ensure similar number of NWs in the device channel. The use of an ensemble of NWs to form a device (instead of single NW) should also lead to statistically lower dimensional variations and hence improve the uniformity among devices. Lastly, NW-FETs which work in depletion/accumulation mode are heavily influenced by the quality of metal−semiconductor contact interface, dielectric/semiconductor interface quality, and adsorbed species on NWs. [30] Because of the low dimensionality of NWs, it is very challenging to realize high quality contacts primarily due to the electrostatics involved at nanocontact interfaces. [31] Currently, we are working on some of these aspects to further reduce the batch-to-batch performance variations of devices and will be presented in our future works.

Printed Photodetectors on Flexible Substrates and Electrical Characterizations
Further, we fabricated flexible PDs on PI substrate using EHD printing and performed detailed characterization (Figure 5). The EHD printed contact electrodes over the aligned ZnO NWs were made using the commercial Au ink (Figure 5a). Electrical measurements were performed on PDs under ambient conditions to monitor their performance. First, the current-voltage (I-V) response of a single device under dark and different UV illumination intensities is measured and results are shown in Figure 5b. For I-V measurements, the bias voltage is swept from −1 to 1 V. A typical symmetrical nonlinear behavior was observed for the fabricated PD device. This is because Au (work function −5.1 eV) is expected to form a Schottky contact on n-ZnO NWs (electron affinity ≈4.5 eV). Further, the figure shows that photocurrent increases from ≈0.5 µA (dark current) to ≈5 µA (2 µW cm −2 ) in 365 nm UV light (one order change in photocurrent) at V D = 1 V. The working mechanism for ZnO NW based PDs is well documented. [32] Briefly, in the dark condition, the absorbed oxygen molecules on the NW surface results in the depletion of electrons. The electron depletion leads to upward band bending and an electric barrier across the MS interface. When illuminated with UV light, the separation of photogenerated electron-hole pairs leads to the desorption of oxygen molecules and thus, increase in the concentration of free electrons. This will also result in the decrease of MS barrier. Both phenomenon results in an enhancement of drain current.
The I-V measurements shown in Figure 5b confirm that the device can be operated at low drain voltages. Accordingly, low bias voltage (V D = 1) is applied to measure time-resolved photoresponse with varying UV intensity from 0.1 to 2.0 µW cm −2 (Figure 5c). The PD shows a reasonably high response to the UV light intensity, like Figure 5b. Furthermore, repeated dark-UV response cycles were performed at intensity of 0.5 µW cm −2 . The obtained set of data showed good repeatability (Figure 5d). Using this data, rise and recovery time is calculated. A response (15 s) and recovery (30 s) time are exhibited by the printed UV sensor. Further, other important parameters including responsivity (R), specific detectivity (D*), external quantum efficiency (EQE), and I Light /I Dark ratio are extracted from the time-resolved photoresponse curves at different UV intensity to further evaluate the performances of printed PDs. First, the R of printed PDs was calculated using equation (1) [33] ( ) where I Light represents the current under UV light illumination; I Dark is the current under dark condition; P in is the incident light power per unit area and S is the effective sensing area of the device. It can be seen from this set of data ( Figure 5e); responsivity monotonically decreases with increasing UV intensity. At low UV intensity (0.1 µW cm −2 ), a maximum responsivity of 8 × 10 7 was achieved (for S = 1.8 × 10 -7 cm 2 , considering NWs as cylindrical structure with diameter 100 nm). At higher UV intensities, the R values decreases but remained more than 10 7 . In addition, the D* is another important PD parameter which is given by [33] = * where e is the elementary charge and J D is dark current density. The detectivity indicates the capability of PDs to detect small signals. Like responsivity, the D* dependent of applied UV intensity curve (Figure 5f) exhibits the same trend. The D* reaches maximum value of 8 × 10 16 Jones at the low UV illumination of 0.1 µW cm −2 . Finally, EQE, i.e., the ratio of the number of converting absorbed photons to electrons and the total number of excitation photons, was extracted from the equation below: where h is Planck's constant, c is velocity of light, and λ is the wavelength of incident light. The EQE decreases gradually with increasing UV light intensity (Figure 5g). The largest EQE reaches 2.5 × 10 10 % under 0.1 µW cm −2 of UV intensity. These results indicate that the obtained printed PDs have outstanding UV detectivity. Finally, I Light /I Dark ratio was extracted and shown in Figure 5h. The extracted I Light /I Dark ratio showed opposite increasing trend compared to R, D* and EQE with an increase of UV intensity. At higher UV intensity (2 µW cm −2 ), the ratio increases to ≈9 from ≈2 under 0.1 µW cm −2 . The PD device performance is compared with the state-ofthe-art ZnO nanomaterials-based UV detectors ( Table 1 and Figure 5i). [32,34] The systematic studies shown above have confirmed the high performance of the all-printed PDs based on ZnO NWs which is comparable to the reported record for ZnO NWs. Particularly, they have shown the detectivity values comparable to the best reported printed PDs based on ZnO NWs. [34o] Furthermore, the obtained R value is among the best performance in reported ZnO-based printed and nonprinted PDs (Figure 5i). However, the I Light /I Dark ratio is significantly lower than the state-of-the-art reports. This could be enhanced by increasing the NWs in the channel and decreasing the doping concentration in ZnO NWs to have higher Schottky barrier energy. [34o] Higher potential barrier at MS contact will help to decrease the level of dark current which also helps in terms of enhancing the detectivity value of the PD. Despite low I Light /I Dark ratio, such a high PD performance could be attributed to the following: i) low-device area (thanks to highresolution EHD printing), ii) efficient charge transport because of single crystallinity and excellent alignment of printed NWs, and iii) negligible impurities inclusion during contact/transfer printing (dry process), as confirmed from the transistor device electrical characterization results. Furthermore, we have compared our printed PD's performance with a similar PD device fabricated using conventional lithography ( Figure S5, Supporting Information). Compared with the conventionally fabricated device ( Table 2), printed sensor showed higher I Light /I Dark ratio and fast response/recovery times. But the conventionally fabricated device demonstrated approximately one order higher responsivity and EQE values than printed ones whereas comparable detectivity values are obtained for both devices. This is  expected as deposited Ti/Au metal with conventional fabrication route forms nearly ideal ohmic MS contact with ZnO. [30,35] It is important to highlight here that compared with conventional fabricated devices, the present adopted printed manufacturing route is highly resource-efficient (for both active and electrode material usage).

Printed Photodetectors on Biodegradable Substrates, Electrical Characterizations, and Transience Study
Finally, we demonstrate the fabrication steps can be applied over biodegradable substrates to realize transient devices.
Towards this, PDs were printed on biodegradable Magnesium (Mg) foils (≈20 µm thick) as device substrate and the electrical characterization results are shown in Figure 6. It is to note that before starting the fabrication process on Mg foil, ≈1 µm thick biodegradable SiO 2 was deposited to passivate the rough surface (rms ≈800 nm) using the plasma-enhanced chemical vapor deposition (PECVD) technique. Figure 6a shows an exploded view of various biodegradable and nonbiodegradable layers of the fabricated PDs. Magnesium is chosen as device substrate due to its inherent advantages including high thermal stability (required to anneal at 250°C to cure the printed Au electrodes), biodegradability, and biocompatibility. Following this, the previously described fabrication steps were followed to develop all printed PD. The fabricated devices were characterized under similar conditions. Time resolved photoresponse was measured at the constant UV intensity 0. Previous studies to explore the dissolution behavior of biodegradable Mg metal foil has been performed in phosphate buffer solution (PBS, pH 7.4). [39] In the present study, the PDs were placed inside DI water (pH ≈ 6) at room temperature (RT) to study the transience behavior of the device and the process is schematically shown in Figure 6j,k. Figure S6 (Supporting Information) presents a series of images during the transience by hydrolysis of PDs on a Mg foil (≈20 µm) at various times. Day 0 ( Figure S6i, Supporting Information) image represents first day when the PD was placed inside the DI water. After 10 days inside the DI water, the PD components started to disintegrate (Au electrodes observed floating in DI, see inset in Figure  S6iii, Supporting Information). The disintegration starts at the interfaces between the different layers (SiO 2 /Mg and SiO 2 / Au). After 60 days, the PD was lifted out of the DI water and thickness monitoring was performed using an optical profilometer and SEM. The extracted thickness using the SEM images of PD device was ≈15 µm ( Figure S7, Supporting Information). This shows the etching rate of the device (mainly Mg) is ≈83 nm day −1 . With this rate, the complete dissolution of the Mg foil would occur after ≈240 d under these conditions. To enhance the etching rate of the PD device, solution with higher basicity or temperature (36-37 °C) could be used. [10b] It is important to note that in the present device, the metal contacts are printed using nonbiodegradable Au electrodes. In the future work, novel biodegradable printing inks of materials such as Mg, Fe, etc. will be explored to realize all printed and fully biodegradable electronics.

Conclusions
We have presented a novel resource-efficient, printed electronics route to fabricate high-performance inorganic nanowire (NW)-based flexible and transient electronics. The presented approach exploits the best of both contact and transfer printing methods. Starting from the synthesis of NWs (Vapor Phase Transport process) to their integration process (in-tandem contact and transfer printing), and finally to define metal electrodes (electrohydrodynamic printing), greener pathways were followed. The major advantages of the proposed NW-printing approach, as compared to the conventional transfer printing, are two folds: i) no need of printing material to be available in bulk (wafer) form which will increase the range of nanomaterials that can be used for the fabrication of high-performance nanoscale devices, and ii) reduced-dependence on conventional microfabrication steps including lithography and etching which will reduce the generation of toxic chemical waste. The efficacy of the developed manufacturing route was demonstrated by fabricating transistors and high performance all-printed UV

Experimental Section
Vapor Phase Transport (VPT) Method: The VPT growth of ZnO NWs by vapor liquid solid (VLS) mechanism was performed inside a horizontal quartz tube furnace by carbothermal reduction of ZnO nanopowder on (0001) c-plane sapphire substrates (Figure 1a). [21,34c,40] Prior to NW synthesis, the cleaned substrates were coated with Au film (3 ± 1 nm) using an electron-beam evaporator. Coated substrates and the mixture of source material (ZnO and carbon powder at 1:1 weight ratio) are placed in an alumina "boat," which is inserted inside a high temperature furnace. The samples were heated in an Ar ambient at 950 °C (growth on sapphire) for 120 min for the growth of ZnO NWs in the sub-100 diameter range.
Morphological and Structural Characterizations: The morphological analysis of as grown and contact printed ZnO NWs is performed using scanning electron microscopy (SEM-Hitachi SU8240). ZnO NWs crystallinity was studied using X-ray diffraction (XRD) with Cu Kα 1 radiation on the Bruker D8 Venture kappa geometry diffractometer equipped with a Photon-II CPAD detector. The scans were performed in the 2θ range from 30° to 40° at a scanning rate of 0.01° s −1 .
Contact Printing: To carry out the NW printing, a well-controlled custom-built contact printing system was used (Figure 2a,b). The details of the developed system are reported elsewhere. [20a] In the contact printing process, a flat rigid donor substrate with vertically grown ZnO NWs was brought into physical contact with a receiver substrate. Then, a controlled uniform pressure is exerted between the donor and receiver and subsequently the donor substrate is slide over the receiver while maintaining the applied pressure. In the present study, the donor sample (NWs on sapphire) was used to print NWs on a Si receiver substrate. The printing pressure and sliding velocity were set to be 33 kPa and 1 mm s −1 , respectively.
Transfer Printing: The selective printing of the array of aligned ZnO NWs was carried out using controlled transfer printing (Figure 3). To prepare a master template, periodic square hollow structures were fabricated on a silicon wafer. To realize the structures, a positive-type photoresist was patterned using photolithography. Following, the silicon was etched by employing deep reactive ion etching (DRIE) technique. The PDMS base and curing agent were mixed at a weight ratio of 10:1. After degassing in a vacuum chamber, the mixture was casted using the master Si mold and degassed again. Next, the master silicon substrate with PDMS was placed inside to pre-heated convection oven at 80 °C for 2 h. Once cured, PDMS stamp with microstructures was carefully peeled off. The details of the process steps and mechanism are discussed in results section.
ZnO Nanowire-Based Field Effect Transistor (NW-FET): The fabrication of back-gated ZnO NW-FETs was performed following conventional microfabrication processes including photolithography and metal deposition (Ti (10 nm)/Au (90 nm)) for source and drain contacts using e-beam evaporation method and lift-off. Highly doped silicon with 200 nm thick SiO 2 was used as the gate and gate dielectric, respectively.
EHD Printing: To fabricate printed PDs, the contact electrodes were realized over the aligned NWs using an electrohydrodynamic (EHD) printer (super inkjet (SIJ R )). The printed ink is commercial gold ink (CAu-2000). A thermal treatment is applied to the substrate at 100 °C for 10 minutes prior to printing. The printing is performed using a 75% sine wave, 300 V amplitude, 0 DC bias, 80 Hz frequency, and the stage speed of 0.5 mm s −1 . To increase the conductivity of the printed lines, the same pattern is repeated over five layers. The curing step is performed at 250 °C for 2 h.
Electrical and Optoelectrical Characterizations: The electrical characterization of ZnO NW-FETs and printed PDs such as time resolved photoresponse and IV characteristics were performed in the ambient environment using semiautomated summit 12k Auto prober and semiconductor device parameter analyzer (B1500A, Agilent). A UV light emitting diode (LED) with the wavelength of 365nm was used for evaluating photodetection capabilities of the fabricated devices.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.