2‐nm‐Thick Indium Oxide Featuring High Mobility

Thin film transistors (TFTs) are key components for the fabrication of electronic and optoelectronic devices, resulting in a push for the wider exploration of semiconducting materials and cost‐effective synthesis processes. In this report, a simple approach is proposed to achieve 2‐nm‐thick indium oxide nanosheets from liquid metal surfaces by employing a squeeze printing technique and thermal annealing at 250 °C in air. The resulting materials exhibit a high degree of transparency (>99 %) and an excellent electron mobility of ≈96 cm2 V−1 s−1, surpassing that of pristine printed 2D In2O3 and many other reported 2D semiconductors. UV‐detectors based on annealed 2D In2O3 also benefit from this process step, with the photoresponsivity reaching 5.2 × 104 and 9.4 × 103 A W−1 at the wavelengths of 285 and 365 nm, respectively. These values are an order of magnitude higher than for as‐synthesized 2D In2O3. Utilizing transmission electron microscopy with in situ annealing, it is demonstrated that the improvement in device performances is due to nanostructural changes within the oxide layers during annealing process. This work highlights a facile and ambient air compatible method for fabricating high‐quality semiconducting oxides, which will find application in emerging transparent electronics and optoelectronics.


Introduction
2D semiconducting materials have been intensively investigated and recognized as promising candidates for the next generation thin film electronics. [1] In particular, 2D metal oxides (2D MOXs) represent an exciting class of 2D semiconducting materials and have attracted considerable interest due to their wide bandgaps, high carrier mobility, and good environmental stability. [1c,2] However, the controlled synthesis of high-quality 2D oxide films across large areas, required to enable the development of highly integrated devices, remains a significant challenge that needs to be resolved prior to industrial and commercial applications of such emerging materials. [1b,c] To this end, liquid metal-based printing methods have recently been developed, offering an appealing approach for the facile, cost-effective, Thin film transistors (TFTs) are key components for the fabrication of electronic and optoelectronic devices, resulting in a push for the wider exploration of semiconducting materials and cost-effective synthesis processes. In this report, a simple approach is proposed to achieve 2-nm-thick indium oxide nanosheets from liquid metal surfaces by employing a squeeze printing technique and thermal annealing at 250 °C in air. The resulting materials exhibit a high degree of transparency (>99 %) and an excellent electron mobility of ≈96 cm 2 V −1 s −1 , surpassing that of pristine printed 2D In 2 O 3 and many other reported 2D semiconductors. UV-detectors based on annealed 2D In 2 O 3 also benefit from this process step, with the photoresponsivity reaching 5.2 × 10 4 and 9.4 × 10 3 A W −1 at the wavelengths of 285 and 365 nm, respectively. These values are an order of magnitude higher than for assynthesized 2D In 2 O 3 . Utilizing transmission electron microscopy with in situ annealing, it is demonstrated that the improvement in device performances is due to nanostructural changes within the oxide layers during annealing process. This work highlights a facile and ambient air compatible method for fabricating high-quality semiconducting oxides, which will find application in emerging transparent electronics and optoelectronics. and relatively low-temperature synthesis of 2D MOXs. [2b,3] These techniques rely on the Cabrera-Mott oxidation of metals, which results in the formation of an atomically thin and self-passivating oxide skin across the metal surface. [4] Due to the nature of the parent liquid metal, the interfacial oxide layer exhibits weak adhesion to the host metal and can readily be transferred to target substrates with minimal imperfections. [3a,b,4b] This simple approach can be utilized to produce 2D nanosheets of both stratified and unstratified metal oxides, which is challenging with conventional exfoliation processes. [3a-c] In comparison to vapor phase deposition techniques, liquid metal printing is straightforward and can be conducted in ambient atmosphere. [2b,3d,5] The synthesized 2D oxide films can reach wafer scale while retaining their ultrathin nature, high uniformity, and excellent electronic characteristics, demonstrating the promise for vacuum-free 2D MOXs deposition. [5a-d] Moreover, with thicknesses down to 1-2 nm, 2D MOXs deposited from liquid metals feature excellent transparency (>98%) and mechanical flexibility, making them promising candidates for transparent and flexible thin film transistors (TFTs). [5b,c,f ] Among several 2D MOXs, indium oxide is an important n-type semiconductor due to its high electron mobility (≈160 cm 2 V −1 s −1 for single crystals), tuneable electrical properties, and excellent optical transparency. [2a,6] Recent efforts to realize ultrathin In 2 O 3 from molten indium metal have also been reported. [5a,c,f,7] However, as-synthesized 2D In 2 O 3 nanosheets featured a low carrier mobility of only ≈5 cm 2 V −1 s −1 , [5c,f,7] providing plenty of room for further improvement.
Herein, we utilize the liquid metal printing technique to prepare ultrathin and large-area In 2 O 3 nanosheets. The effects of low-temperature annealing on the crystal structure and electronic properties of these materials are then investigated. We demonstrate that a simple postsynthesis annealing process at 250 °C results in improved uniformity and larger grain sizes, while also reducing disorder at the grain boundaries, boosting the performance of field effect transistors (FETs) and solarblind UV detectors. The observed carrier mobility approaches that of single crystal In 2 O 3 , highlighting an exciting pathway towards the next generation of miniaturized electronic and optoelectronic circuits.

Results and Discussion
A schematic of 2D In 2 O 3 nanosheets synthesized by employing the liquid metal printing technique is shown in Figure 1a,b.
The synthesis was carried out in an ambient atmosphere. The ultrathin oxide layers grow spontaneously on the surface of molten indium metal and are then transferred to the desired substrate by squeeze printing (see the Experimental Section for the detailed synthesis process). Continuous 2D nanosheets deposited on SiO 2 /Si substrates approaching millimeter scale are routinely achieved and can be seen in Figure 1c and Figure S1 in the Supporting Information. It should be noted that larger amounts of molten metal can be used to readily increase the lateral dimension of 2D In 2 O 3 nanosheets. [5c,f ] Furthermore, a continuous printing process has recently been developed, demonstrating that wafer scale deposition via liquid metal processing is quite feasible. [5d] To explore the electron transport properties of the produced 2D oxide nanosheets, back-gated field-effect transistors (FETs) were fabricated using 2D In 2 O 3 as the semiconducting channels (Figure 2a, inset). The performance of a non-annealed 2D In 2 O 3 FET has been shown in Figure S2, Supporting Information, featuring a mobility of 7 cm 2 V −1 s −1 and an on/off ratio of 10 2 . Upon annealing at 250 °C for 2 h in ambient air, the FET performances were remarkably improved. Figure 2a,b presents the output curves (I DS − V DS ) and transfer curves (I DS − V GS ) of the produced transistors based on annealed 2D In 2 O 3 nanosheets. This indicates n-channel semiconducting behavior with an excellent carrier mobility of ≈96 cm 2 V −1 s −1 , which is comparable to the reported value for polycrystallinesilicon TFTs (50-100 cm 2 V −1 s −1 ). [8] In addition, the transistors based on annealed 2D In 2 O 3 exhibited a considerable on/off current ratio of ≈10 4 with a clear depletion mode observed at V GS = −40 V ( Figure 2c). It is important to note that the on/off ratio can be further improved by optimizing the dielectric layer or engineering the device architecture. [3d,9] Figure 2d illustrates the mobility statistics, demonstrating the highly consistent performance of these FET devices. The optimized annealing condition was identified to be 250 °C for a duration of 2 h, which led to the FET mobility of 95.7 ± 4.8 cm 2 V −1 s −1 . Herein, the maximum temperature of the whole annealing process was limited to 250 °C, which enables device fabrication on flexible
Taking advantage of the wide bandgap of 2D In 2 O 3 (E g opt = ≈3.7 eV), we have recently demonstrated the use of few-nm-thick In 2 O 3 for optoelectronic applications in the UV region. [5c] Several works have reported that a photodetector's performance can be enhanced by annealing, [11] and here we investigate annealed 2D In 2 O 3 nanosheet based photodetectors. The optoelectronic properties of annealed 2D In 2 O 3 nanosheets were investigated by irradiating two-terminal planar devices (Figure 2a,inset) with light sources of 285 and 365 nm. The UV detection performances were then compared with published data for UV-detectors based on as-exfoliated 2D In 2 O 3 nanosheets (Table S1, Supporting Information).
As shown in Figure 2e,f, and Table S1, Supporting Information, annealing 2D-In 2 O 3 nanosheets at 250 °C in air for 2 h likely reduced the recombination rate of electron-hole pairs, which ultimately increased the photocurrent multifold. [12] This finding suggests that the annealing process reduced defects in the In 2 O 3 layer which can act as recombination sites, and improved charge carrier extraction due to improved mobility resulting in larger carrier diffusion length. [13] Besides the improved photocurrent, the annealing also facilitated in enhancing the figures of merit (FoM) for all three parameters of responsivity (R), detectivity (D*), and external quantum efficiency (EQE), which were an order of magnitude higher after annealing when compared with pristine In 2 O 3 , thereby showing superior performance (Table S1, Supporting Information).
Comprehensive materials characterization has been conducted to further clarify the role of thermal annealing in boosting carrier mobility. It is well established that the electronic and optical properties of 2D materials are typically thickness dependent, [1c,14] and inconsistencies in the performance of electronic devices are often caused by nonuniform thicknesses and defects created during the deposition process. [1c] Therefore, atomic force microscopy (AFM) has been used to assess the uniformity and surface morphology of isolated 2D In 2 O 3 nanosheets. From AFM data, no obvious changes in film thickness were observed after annealing. Both freshly deposited and annealed samples featured atomically flat and uniform 2D sheets covering large areas with minimal cracks, folds, and pinholes (Figure 3a; Figure S4a, Supporting Information). Analysis of 15 edge locations indicated that liquid metal printing is highly reproducible with an average thickness of 1.98 ± 0.08 nm for annealed 2D In 2 O 3 (Figure 3a, lower right inset and Figure S4b in the Supporting Information), which is similar to the thickness of as-exfoliated 2D In 2 O 3 reported in our recent work (2.08 ± 0.07 nm) [5c] and matches well with two unit cells of cubic In 2 O 3 (a = 10.12 Å). [15] Due to their atomically thin nature, 2D In 2 O 3 nanosheets offer an ultra-high transparency across the visible and near-infrared regions ( Figure S5, Supporting Information). The as-synthesized and annealed 2D In 2 O 3 nanosheets exhibited a transmittance of 98.5% and 99.6% at the wavelength 550 nm, respectively, clearly demonstrating that annealing improves the optical properties of the 2D In 2 O 3 nanosheets. The optical absorption of pristine In 2 O 3 is comparable, while that of annealed samples is superior to the 97.4% optical transmittance of monolayer graphene. [16] The chemical composition of the as-synthesized and annealed 2D In 2 O 3 nanosheets was determined by X-ray photoelectron spectroscopy (XPS), with the spectra of In 3d and O 1s presented in Figure 3b,c. The characteristic doublets of In 3d 5/2 and In 3d 3/2 can be observed at ≈444.6 and ≈452.2 eV, respectively, identifying the +3 oxidation state of indium in the 2D oxide sheets (Figure 3b). [5b,17] Figure 3c shows O 1s spectra

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which contain peaks associated with various oxygen species. The peak at ≈529.9 eV pertains to oxygen bound to indium, whilst the remainder are related to either the oxide layer on the Si/SiO 2 substrate (≈532.4 eV) or to residual organic contaminants (≈531.3 and ≈532.8 eV for CO and CO, respectively). No change to the oxidation state was observed after annealing. The only variation in the O 1s spectrum observed after annealing was a small reduction in the contaminant peaks. This was accompanied by a 10% reduction in the total C 1s signal relative to In 3d.
Optical characterization of the nonannealed and annealed 2D In 2 O 3 nanosheets was used to investigate possible changes in the band structure of 2D oxide nanosheets due to thermal annealing. Tauc plot analysis revealed an optical bandgap of ≈3.6 eV for the annealed 2D In 2 O 3 , which is slightly lower than the ≈3.7 eV measured for as-exfoliated samples ( Figure S6, Supporting Information), indicating that if any changes to the electronic band structure occurred, they were likely minimal.
Repeated printing resulting in the deposition of thicker films enabled the collection of X-ray diffraction (XRD) patterns which are displayed in Figure 3d. The XRD patterns reveal the crystal structure of 2D In 2 O 3 nanosheets synthesized from liquid metal. The peak at ≈30.6 o can be attributed to the (222) plane of cubic indium oxide [15] with the intensity increasing significantly upon annealing the sample. As such, annealing was found to not change the crystallization habit of cubic In 2 O 3 , but instead to increase the overall crystallinity of the deposited material, hence improving the electronic properties of the 2D In 2 O 3 nanosheets. Nevertheless, XRD in of itself does not provide sufficient evidence to identify the precise nanostructural changes that occur within the 2D materials upon thermal treatment. Therefore, transmission electron microscopy (TEM) with in situ heating was performed at 250 °C to study the microscopic changes in detail and gain further insight into modifications to the crystal structure and transformation of 2D In 2 O 3 nanosheets during annealing (Figure 4). This technique allows real-time observation of structural evolution at the atomic scale during the thermal annealing process, and Video S1 in the Supporting Information showcases a time-lapse of the twohour-annealing process.
In contrast to a recent study reporting an amorphouscrystalline heterophase of 2D In 2 O 3 printed from liquid metal at a lower temperature of 165 °C, [5a] our work demonstrates that printing at a higher temperature (i.e., 250 °C) directly results in polycrystalline 2D In 2 O 3 nanosheets (Figure 4a). Interestingly, many nanocrystals within the as-printed nanosheet were found to be surrounded by an approximate 0.5 nm thick distorted boundary region (Figure 4b). After 10 min of annealing (Figure 4i), several additional nucleation sites appeared and grew throughout the annealing process, resulting in the partial recrystallization of the 2D oxide film (Figure 4i-l).
As shown in Figure 4a-h, the crystallographic structure of 2D In 2 O 3 nanosheets significantly changed after annealing. It is clear from the grain size distributions shown in Figure 4d, h that the crystal domain size increased from 11.5 ± 5.9 to 16.1 ± 3.7 nm after annealing at 250 °C for 2 h. This finding explains the origin of increased carrier mobility of annealed 2D In 2 O 3 in part, as the increase in crystal domain sizes results in fewer grain boundaries, and hence fewer charge transport barriers. [1c,18] More importantly, the intergranular distorted regions, which may present significant potential barriers to charge transport, were evident in the as-made 2D In 2 O 3

Figure 4. a-h) HRTEM images with corresponding zoom-in areas, SAED patterns, and grain size distributions of a-d) pristine and e-h) annealed 2D
In 2 O 3 nanosheets. Color-coded areas in (b) and (f) represent the crystal grains with red arrows indicating the grain boundaries. i-l) In situ TEM of 2D In 2 O 3 nanosheets at 250 °C indicating the recrystallization of 2D In 2 O 3 . The nucleation sites were indicated in (i) and the growth of crystal grains can be observed during the thermal annealing. All TEM images were taken from the same 2D In 2 O 3 nanosheet.
www.advmatinterfaces.de ( Figure 4b). As grains ripened during annealing, these regions were absorbed by the growing crystalline domains (Figure 4f), contributing to the observed increase in carrier mobility. Additional TEM images of 2D In 2 O 3 can be seen in Figure S7 in the Supporting Information.
Interestingly, the selected area electron diffraction (SAED) pattern shown in Figure 4c reveals a partially aligned crystal structure for the pristine 2D In 2 O 3 , in which the (222) diffraction spots appear in clusters and are thus preferentially oriented. In contrast, the annealed nanosheets exhibited a random crystal orientation with diffraction spots forming a ring without significant clustering (Figure 4g). Here, it should be noted that from the viewpoints of TFT device fabrication, randomly oriented nanocrystalline 2D semiconductors are preferred over highly oriented ones, since it minimizes the effect of anisotropic carrier mobilities along a material's crystal axes. This finding highlights the potential of implementing a thermal annealing process for practical applications of liquid metal derived 2D In 2 O 3 nanosheets.

Conclusions
In this work, we have demonstrated a scalable method for synthesizing large-area and ultrathin In 2 O 3 nanosheets. The obtained 2D sheets exhibit a polycrystalline nature with a thickness of ≈2 nm. TEM with in situ heating reveals that upon thermal annealing at 250 °C for 2 h, the 2D oxide nanosheets showed slightly increased domain sizes, while the intergranular distorted regions were effectively removed. FETs were fabricated using annealed 2D In 2 O 3 as the semiconducting channels, leading to a remarkable improvement in electron mobility when compared with un-annealed samples, and significantly surpassing the performance of devices based on many state-ofthe-art 2D semiconductors. UV-detectors based on annealed 2D In 2 O 3 also exhibited figures of merit that are an order of magnitude higher than those of devices made from freshly printed 2D In 2 O 3 . Overall, the 2D In 2 O 3 nanosheets featured an excellent transparency -exceeding 99% -with a promising combination of electronic and optoelectronic features. These results confirm 2D In 2 O 3 nanosheets derived from liquid metal are potential candidate for emerging transparent electronics applications such as invisible circuits, digital displays, smart contact lenses, and neural interfaces.

Experimental Section
Materials: Indium metal (In, 99.99%) was provided by Indium Corporation. Unless stated otherwise, all solvents were products from Sigma-Aldrich and used as received. 300 nm SiO 2 / p + Si wafers were purchased from D&X Co., Ltd.
Synthesis of 2D Indium Oxide Nanosheets: A set of 300 nm thick SiO 2 /p + Si substrates were cleaned with acetone, propanol, and Mili-Q water, followed by blow-drying with compressed air. A squeeze printing technique was utilized to synthesize ultrathin In 2 O 3 sheets by adapting previous reports. [5b,c] Prior to the synthesis, 300 nm SiO 2 /Si substrates were preheated on a hot plate at 250 °C. An indium shot (0.3-0.5 g) was then placed and melted on a glass slide. As can be seen in Figure  S8, Supporting Information, the molten indium metal appeared dullgray due to the presence of a thick oxide layer with possible air-borne contaminants that formed during the storage of the precursor metal. Prior to the squeeze printing, this pre-existing oxide layer was removed by squeezing the molten metal with another glass slide. A small piece of liquid indium was then collected utilizing a glass pipette and transferred onto a desired substrate to perform the squeeze printing. The same method can also be applied to synthesize the material on glass and quartz substrates. A further cleaning process was conducted to remove metallic residues using Kapton sticky tape and solvent assisted cleaning described in previous studies. [5c] Prior to device fabrication, the obtained materials were annealed on a hotplate at a range of temperatures and durations. The synthesis and annealing processes were carried out in ambient air. As for the TEM samples, 2D In 2 O 3 was directly printed onto the copper TEM grids via touch-printing. [19] Characterizations of 2D In 2 O 3 Nanosheets: All optical images were obtained using a Leica DM2500 optical microscope. The thickness of the 2D nanosheet and surface morphology were acquired with the aid of a Bruker Dimension Icon AFM operating under ScanAsyst-air mode. The collected data was then processed and analyzed with Gwyddion 2.55. TEM images were taken from a JEOL JEM-2100F TEM operating at an acceleration voltage of 200 kV equipped with a Gatan OneView camera. A Gatan 652 in situ heating holder was used to perform in-situ annealing. Gatan Microscopy suite software was utilized to analyze the data. XRD patterns were obtained using a Bruker D4 diffractometer with Cu Kα radiation (1.5418 Å). XPS spectra were obtained from a Thermo Scientific K-alpha XPS spectrometer with 1486.7 eV (Al Kα) X-ray source and concentric hemispherical electron analyzer. The XPS data were processed with CasaXPS software. UV-vis spectra and optical transmittance measurements were acquired from 2D In 2 O 3 nanosheets prepared on quartz with a Cary 60 UV-vis spectrophotometer.
Device Fabrication and Measurement: The ultrathin In 2 O 3 -based field-effect transistors were fabricated via a photolithography process. First, the AZ 5214E photoresist was spin-coated on 2D In 2 O 3 sheets deposited on 300 nm thick SiO 2 /p + Si substrates prior to the electrode patterning using a Maskless Aligner -Heidelberg MLA150. An e-beam evaporator (PVD75 -Kurt J. Lesker) was then employed to deposit Cr/Au (10/100 nm) electrodes, followed by a lift-off process utilizing acetone to remove the photoresist residue. The FET measurements were performed using a probe station equipped with a Keysight B2902A Precision Source/Measure Unit (SMU). Variable-temperature measurements were conducted using a Linkam PE95 temperature controller equipped with a liquid nitrogen-cooled chamber. It is important to note that the electrode dimensions were assumed to be the length and width of the active channel and fringing currents might lead to a slightly overestimated carrier mobility. However, the determined values are sufficient to observe the effect of annealing on the overall electric properties of ultrathin indium oxide nanosheets. Optoelectronic measurements of two terminal devices were performed on a Linkam stage using a keysight B2912A source meter. The devices were illuminated using monochromatic LEDs from Thorlab (285 and 365 nm) at 0.4 mW cm −2 incident power and a bias voltage of 0.5 V. For cyclic repeatability measurements, an optical stimulus of 2000 s was used each cycle with a 4000 s recovery time before the next incoming pulse. All device performances were carried out at ambient conditions.
FET mobility was subsequently calculated by using the following equation [3d,5c,f] where µ e refers to the electron mobility (cm 2 V −1 s −1 ), L is the channel length (20 µm) and W is the channel width (40 µm). I DS , V DS , V GS , and C ox represent the drain-source current and bias, the gate bias, and the gate insulator capacitance per unit area, respectively. Photoresponsivity R (A W −1 ), photodetectivity D* (Jones), and external quantum efficiency EQE (%) were extracted from the photodetector performance by using the equations below [5c,20]  where ΔI ph , P, S, and I dark are the change in photocurrent at incident illumination, power density, effective area subjected to illumination, and dark current. h, c, e, and λ represent Planck's constant, light speed, electron charge, and the wavelength of the incident light, respectively.

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