Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing

doi:10.21203/rs.3.rs-4903114/v1

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Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing

https://doi.org/10.21203/rs.3.rs-4903114/v1

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Transparent conducting oxides (TCOs) are crucial for high-performance displays, solar cells, and wearable sensors. However, their high process temperatures and brittle nature have hindered their use in flexible electronics. We report an approach to overturn these limitations by harnessing the physics Cabrera Mott native oxidation to fabricate large-area, two-dimensional transparent electrodes via liquid metal printing. Our robotic, solvent-free and vacuum-free process deposits ultrathin (2–10 nm thick) 2D indium tin oxide (ITO) with exceptional flexibility, high transparency (> 95%) and superior conductivity (> 1300 S/cm) for wearable bioelectrodes. In a significant advance over previous work, we utilize hypoeutectic In-Sn alloys to print 2D ITO at < 140 ºC on flexible polymers. Our detailed materials characterization and microscopy reveal the efficacy of Sn-doping and high crystallinity with large, platelike grains formed by the liquid metal reaction environment. The ultrathin nature of 2D ITO yields significant enhancement to bending strain tolerance, scratch resistance exceeding durability of traditional PEDOT, and low contact impedance to skin comparable to Ag/AgCl. Finally, we utilize the conductivity and transparency of 2D ITO for synchronous, multimodal measurements via electrocardiography (ECG) and pulse plethysmography (PPG). This order-of-magnitude improvement to printed TCOs could enable new wearable biometrics and display-integrated sensors.

Physical sciences/Materials science/Materials for devices

Physical sciences/Materials science/Nanoscale materials/Two dimensional materials

Physical sciences/Engineering/Electrical and electronic engineering

Physical sciences/Nanoscience and technology/Nanoscale materials/Synthesis and processing

liquid metal printing

2D oxides

transparent electronics

wearable sensors

bioelectrodes

Transparent conducting oxides (TCOs) such as indium tin oxide (ITO) are critical materials for electronic and optoelectronic devices, providing the transparency necessary for displays, solar cells, sensors, and various user interface devices. However, state-of-the-art TCOs have two major drawbacks – they are comparatively expensive due to the need for vacuum deposition and they are traditionally considered to be brittle materials^[1] with limited suitability for flexible, lightweight systems. The minimization of film stresses via strain-tolerant serpentine structures has shown the promise of transparent metal oxides as active materials for wearable sensors, but these approaches have the drawback of requiring complex patterning and yielding low areal density circuits^[2]. Recent works have also demonstrated complementary approaches of fabricating on ultrathin substrates^[3] to minimize the bending strain of metal oxide thin films and using multilayered organic-inorganic hybrid structures to relieve bending-induced mechanical stresses^[4]. Studies of the mechanics of TCOs such as ITO have revealed that minimizing the TCO film thickness and reducing the process temperature both lead to enhanced mechanical reliability^[5].

An emerging class of ultrathin liquid metal-derived^[6] two-dimensional (2D) oxides^[7] could overturn the longstanding limitations of transparent conducting oxides, offering a platform for high-performance wearable electronics based on inorganic, wide-bandgap materials. The field of 2D oxides has demonstrated, for example, the formation of ultrathin materials with layered polymorphs (e.g. β-TeO₂) just 5–40 Å thick^[8], the potential for assembling these materials into heterostructures with unique electronic properties^[9], a tendency to favor highly crystalline phases compared with oxides printed from solution^[10], and the ability to yield conductors with extremely high transparency (> 99%) and enhanced mechanical flexibility^[11]. However, the 2D oxide field faces several ongoing challenges limiting its impact. For example, there is a demand for advancing large-area uniformity while precisely controlling the liquid metal meniscus – the key feature for modulating the printed film thickness and controlling the oxidation kinetics. Widely variable thicknesses are reported in the literature for a single material such as In₂O₃ or Ga₂O₃ due to the variable dynamics of manual methods based on blade coating^{[12, 13]}, dispensing from a syringe^[14], and squeeze printing between two flat substrates^[15]. Additionally, there is a need to introduce extrinsic dopants to control and enhance electronic properties^[7] since the precursor metal alloy constituents compete for representation in the surface oxide^[16]. If these challenges can be addressed, 2D oxides could make a significant impact on the field of flexible electronics.

Herein we report the first demonstration of high-speed automated liquid metal printing over large areas with Å-level precision control over Cabrera Mott oxidation kinetics. We apply this method to fabricate flexible, two-dimensional (2D) indium tin oxide (ITO) transparent electrodes from the surface oxides of liquid indium tin alloys at temperatures as low as 140 ºC on flexible polymer substrates. Our detailed materials characterization of these films reveals that by controlling crystallization and doping of 2D ITO it is possible to achieve superlative conductivity and high transparency at unprecedented process speeds as well as outstanding mechanical resilience, including high scratch resistance and enhanced bending strain. Finally, using these materials, we report the first demonstration of 2D oxide-based bioelectrical measurements, showing an efficient multimodal sensing approach for combining electrocardiography (ECG) and pulse plethysmography (PPG) utilizing the high transparency of liquid metal-printed ITO.

2.1 Liquid Metal Printing of 2D Indium Tin Oxide

We have developed an automated platform for liquid metal printing (Fig. 1a) that precisely controls the Cabrera-Mott oxidation kinetics of liquid metals. This is the first report of robotic, wafer-scale liquid metal printing of 2D oxides (Figure S1), replacing previous manual "touch printing" methods^[11]. Our system can vary the printing speed over a 200-fold range, from 0.1 cm/s to 20 cm/s, allowing us to print uniform films over areas greater than 100 cm². The precursors for liquid metal printing in the present study are hypoeutectic, indium-rich In-Sn alloys, though we note that the automated method can be extended to a multitude of alloys for printing various 2D oxides. Indium and tin form a eutectic mixture at 52 − 48 wt.% Sn with a melting temperature of approximately 117°C (Figure S2). The concentration of Sn in the alloy determines the melting temperature, with hypoeutectic compositions (less than 48% Sn) lowering T_m (Fig. 1b) significantly below the T_m of pure In. This allows us to print 2D ITO films at extremely low deposition temperatures, making the process compatible with thermally sensitive substrates such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), and glass (Fig. 1c).

The automated process gives us the ability to model the kinetics of liquid metal surface oxidation and explore the inherent limits to the thickness uniformity across large areas, assessing the fundamental repeatability of this synthetic method. Figure 1d shows a printed 2D ITO film covering approximately 10 x 10 cm, with a thickness of 5.8 nm and a standard deviation of just 4 Å. The thickness is measured using spectroscopic UV reflectometry (see Materials and Methods and Figure S3) and confirmed with atomic force microscopy (AFM) (Figure S4). An essential feature of our method is that it offers the ability to print films of programmed thickness by modulating the printing speed. Figure 1e shows the thickness of 2D oxide printed at 260 ºC at speeds ranging from 0.1 cm/s to 20 cm/s for both undoped In₂O₃ and ITO films (7 at. % Sn). These results show that, at high speeds, the films asymptotically approach a thickness of approximately 2.4 nm in the case of ITO and 2.5 nm in the case of In₂O₃. Reducing print speed (< 1 cm/s) allows the printing of thicker single layers approaching 5–6 nm. To understand the physics determining the 2D oxide film thickness (t_ox), we consider the Cabrera Mott oxidation kinetics model shown below (Eq. 1) where the speed of the roller (v) is treated as the inverse of time (t) for the oxide growth on the liquid metal meniscus. In this approximation, A and B are fitting constants, though we refer the reader to the literature showing their derivation from the oxygen diffusion length, Mott potential, and temperature ^{[17, 18]}:

$$\:\frac{1}{{t}_{ox}}=A-B\text{log}\left(\frac{1}{v}\right)$$

While the field of 2D oxides has postulated that the growth is governed by Cabrera Mott kinetics, this is, to our knowledge, the first study to model the kinetic dependence of 2D oxide thickness on the rate of the liquid metal meniscus deformation. This is critical because the electronic and optical properties of 2D oxides are highly thickness-dependent ^{[10, 19, 20]}, for example, due to the quantum confinement-induced bandgap widening in 2 nm thick InO_x close to the Bohr radius. To assemble thicker 2D oxide films and heterostructures, we can stack these layers to print films of 2–100 nm total thickness. Figure 1f shows a histogram of thicknesses for single, double, and triple ITO layers, indicating the ability to freely control the thickness via multilayer assembly. To put the speed of Cabrera Mott oxidation in perspective, we highlight that each of these layers is printed in less than 2 s. We note that the equivalent growth rate of these films is orders of magnitude higher than what is currently achievable with nanoscale growth methods such as Atomic Layer Deposition (ALD) ^{[21, 22]}. We also highlight the repeatability of our automated liquid metal printing process, which, over the course of ten prints executed on ten different substrates, varies by less than 6 Å, signifying a highly reproducible and reliable process for the growth of 2D ITO (Figure S5).

2.2 Materials Characterization of Liquid Metal Printed 2D ITO

Materials characterization of the liquid metal printed 2D ITO films reveals their high crystallinity and large-grained nature. AFM image in Fig. 1g shows 2D ITO exhibits grain sizes ranging from 24 to 67 nm, with an average of 40 ± 11 nm (histogram is shown in Figure S6). These large grains are notable, given that the film is just 6 nm thick. This is consistent with our previous reports of superlattices of 2D In₂O₃ that were shown by HRTEM to exhibit large platelike grains^[9]. For the 2D ITO in the present study, we have also observed that the grain size is substantially larger than that of sol-gel-derived ITO films ^{[12, 13]}. Based on other similar reports in the 2D oxide field^[7], we expect that this could be a unique feature of the liquid metal interfacial reaction. One possible explanation for the high crystallinity of our 2D ITO is the total elimination of residual organic species through our solvent-free synthesis method. Moreover, the AFM images demonstrate smooth interfaces, with an average roughness of only 0.2 nm, as shown in Fig. 1g. Such roughness number is comparable to or even smoother than some sputtered ITO films ^{[25, 26]}. HRTEM images (Fig. 1h) obtained via printing of these films onto TEM grids also show the high crystallinity of 2D ITO films. The appearance of Moiré Fringes in the 2D ITO HRTEM images indicates the overlap of stacked crystalline grains produced in a single print. This result could suggest that both the top and bottom interfaces of the liquid metal meniscus contribute to their surface oxide layers during the printing process. The ability to achieve such highly crystalline morphologies at low temperatures in such a rapid process is unique compared even with vacuum-deposited films that crystallize on longer time scales, require films above a critical thickness, and generally need post-annealing at higher temperatures^[27–29]. The enlarged view in the inset of Fig. 1h displays well-defined lattice fringes corresponding to the (400), (222), and (413) lattice planes of cubic In₂O₃. The selective area electron diffraction (SAED) data, taken from the (Fig. 1i) show a diffraction pattern typical for the cubic phase of ITO, matching reference spectra ^{[30, 31]}. We note that there is a lack of considerable amorphous signal at low angles, indicating the nearly full crystallization of the 2D ITO film printed at 260 ºC.

The potential for extrinsic Sn doping of the 2D ITO films was investigated by varying the In-Sn alloy composition and measuring the resulting ITO film via XPS. Figure 2a shows the Sn at. % extracted for films printed at 180 ºC and at 240 ºC. Sn doping concentrations in the 2D ITO match closely to the Sn concentration in the parent alloy. This corresponds well with previous literature ^{[11, 32]}. Figure 2c shows the oxygen O1s peaks and their decomposed constituent peaks, including stoichiometric M-O bonding, oxygen-deficient M-O bonding, and M-OH bonding for 2D ITO printed with varying Sn concentrations. The percentage of oxygen-deficient M-O bonding peaks at ~ 532 eV increases from 24.6% for 5 at% Sn doping to 31.6%. for 20 at. % Sn doping in the parent alloy. This matches previous literature^{[33, 34]} and could correspond to the higher free carrier concentration of these heavily doped 2D ITO films due to a higher oxygen deficiency concentration. We also observe that with high-temperature deposition, such as at 240 ºC, the XPS signal from M-OH peak bonds (~ 534 eV) weakens, while the stoichiometric M-O peak (~ 530 eV) signal increases by, compared to the film deposited at 180 ºC. (Figure S7).

Figure 2c shows the XRD spectra of 2D ITO films with varying tin doping levels (1 at. %, 7 at. %, and 20 at. % Sn) as deposited. These show peaks corresponding to a mixture of (222) and (400) orientations, indicating the presence of the cubic In₂O₃ structure^[29]. Notably, at 7 at. % Sn doping, the 2D ITO film exhibits preferential growth along the [100] axis, with the (400) peak showing higher intensity than the (222) peak. With high doping, such as 20 at. %, the crystalline peaks exhibit attenuation, likely due to the heavy Sn doping impeding In₂O₃ grain growth. Figure 1d displays XRD spectra for 2D films printed with 7 at. % Sn in a Sn-In alloy at various temperatures, again revealing the presence of (222) and (400) peaks, with the latter notably more intense. Remarkably, even at a modest processing temperature of 170 ºC, a distinct crystalline (400) peak is evident. These findings suggest that the typical range of electrically optimal Sn doping in liquid metal printed films consistently favor the growth of ITO in the (400) orientation, a parallel to reports of sputtered ITO^[35].

2.3 Electrical and Electromechanical Properties of Liquid Metal Printed 2D ITO

The electrical properties of 2D ITO were extensively characterized as a function of Sn doping and print temperature, both of which have a large impact on the conductivity. Increasing Sn doping from 1 at. % to 7 at. % results in a substantial increase in the conductivity of 2D ITO. The electrical conductivity peaks at approximately 1340 S/cm at 7 at. % Sn for a deposition temperature of 260°C. For heavier Sn doping approaching 20%, the ITO films begin to exhibit slightly lower conductivity. Previous studies at lower temperatures^[11] reported that Sn concentrations above 10% resulted in the amorphization of 2D ITO. The higher print temperatures and the automated printing process of our present study could contribute to maintaining the crystalline cubic ITO phase even at high Sn concentrations, leading to high conductivity. For a film with a thickness of ~ 6 nm and without any post-annealing, the conductivity is superior to that of previous reports of printed ITO and comparable to even post-annealed vacuum-deposited ITO ^{[27–29, 36–40]}. Table 1 shows a comparison of the conductivity and post-annealing conditions for thin ITO deposited by printing (upper half of Table 1) as well as vacuum-based processing (lower half of the table). These literature results also reflect the general trend that ITO deposited by vacuum methods exhibits lower conductivity when the film thickness is scaled to allow for ultratransparency. The parity achieved here between our liquid metal printed 2D ITO and sputtered films speaks to the unique physics of Cabrera Mott oxidation and the process control facilitated by our automated methods. While a single ultrathin layer of ITO can achieve high conductivity, it is also possible to stack multiple layers to result in a lower sheet resistance, making the films suitable for various optoelectronic device applications. Figure S8 illustrates the relationship between the sheet resistance of ITO films and the number of printed layers, showing results for single, double, and triple layers. For example, stacking three layers under identical parameters—260°C, 0.5 cm/s, and 7 at. % Sn in the precursor In-Sn alloy—achieves a sheet resistance of 300 Ω/sq.

Table 1

*Summary of reported performance of printed and vacuum-deposited ultrathin indium tin oxide (ITO) as a function of thickness and annealing temperature.*
Reference	Deposition System		Conductivity, S/cm	Annealing Temperature, ºC
This Work	Liquid Metal Printing	6	1340	N/A
Pan et al.^[36]	Inkjet Printing	60	440	500
Scheideler et al.^[37]	Gravure Printing	150	500	400
Serkov et al. ^[38]	Gravure Printing	400	500	Laser Annealed
Kwok et al.^[27]	Pulsed Laser Deposition	6	830	NA
Gwamuri et al.^[39]	RF sputtering	22	62	300
Kim et al. ^[40]	RF sputtering	10	1400	240
Qiao et al.^[29]	DC Sputtering	20	12	300
Chan et al. ^[28]	DC Sputtering	20	1488	400

2.4 Mechanical and Functional Properties of Flexible 2D ITO for Bioelectrode Applications

The utility of TCOs in user interfaces such as touch screens and displays could offer an opportunity for integrating new biosignal measurement functionality into wearable technology if the associated challenges of mechanical flexibility of TCOs can be overcome. We have characterized the mechanical properties of 2D ITO specifically to address the viability of wearable biosignal measurements. Wearable TCOs must pass rigorous mechanical testing, including abrasion, tape, and bending tests, to ensure they can withstand the physical demands of continuous use. These tests simulate conditions such as repeated friction against the skin and bending around curvilinear surfaces, ensuring the electrodes maintain functionality and integrity under mechanical stress. The clinical standard for biopotential and bioimpedance measurements involves using wet electrodes such as Ag/AgCl, though these gels can be uncomfortable, can dry out over time, and may cause irritation ^[41], rendering them unsuitable for long-term ECG signal acquisition. Previous research has demonstrated thin Au films^[42], PEDOT: PSS^[43], and Ag NW for flexible bioelectrodes^[44] due to their flexibility, but these materials have limitations to their mechanical and thermal stability.

In this work, we established 2D ITO as a potential dry electrode because it is highly flexible, transparent, and abrasion-resistant compared to traditional dry electrodes. Traditional sputtered ITO films have been extensively examined and studied for use as biosensors for various purposes^[45] but have not been shown as non-invasive epidermal bioelectrodes for measuring biopotential signals. Here we show that 2D ITO printed on polymer substrates is more strain-tolerant than sputtered flexible ITO. Figure 3a shows the normalized resistance change after 100X bending cycles at 1% tensile strain for sputtered and liquid metal-deposited ITO films. The resistance change is 5X higher in the case of sputtered ITO compared with the liquid-metal printed film. One potential factor limiting the sputtered ITO film flexibility could be residual stresses from the high-energy growth method. Another potential hypothesis is that the mixed amorphous and crystalline phases in 2D ITO could provide a better balance of mechanical flexibility and electrical performance^{[46, 47]}. This makes 2D ITO films more suitable for applications where the electrodes need to endure bending and flexing, such as wearable electronics and flexible biomedical sensors. As shown in the SEM images in Fig. 3b, the sputtered ITO films exhibit significant fractures transverse to the bending direction, whereas the 2D ITO films remain intact, demonstrating superior mechanical bendability.

The mechanical stability of 2D ITO was also characterized via abrasion and adhesion tests, which effectively simulate real-world conditions by evaluating the response to potential debonding from substrates during skin contact and the capability to repeatedly apply the films to the skin ^[48–50]. We assessed the abrasion resistance according to the ASTM standard by using pencil leads of increasing hardness to determine the hardness at which the film could be visibly scratched and to measure changes to its resistance (details in the Materials and Methods section). As shown in Fig. 3c, our 2D ITO films demonstrated abrasion resistance that is 4X higher than that of the control PEDOT: PSS film, a common dry bioelectrode material. The control PEDOT:PSS film was scratched by the softest lead (2B) available, whereas one of the hardest leads (2H, 4X stronger than 2B) did not scratch the 2D ITO film. This scratch resistance illustrates a unique advantage of ceramic bioelectrodes relative to softer alternative materials. Figure 3d illustrates the adhesion capabilities of the 2D ITO film compared to a sputtered gold film. A peel test (details in the Methods section) was conducted to observe how resistance changed with repeated tape test applications at the same locations. After the tape tests, the gold film was nearly completely delaminated, indicated by a sharp increase in normalized resistance after five applications. In contrast, the ITO film maintained its adhesion and did not de-bond, showing only a small change in normalized resistance.

Characterization of the electrode-skin impedance for 2D ITO was performed to assess its suitability for various biopotential measurements. Impedance testing ensures that dry bioelectrodes make consistent and adequate contact with the skin, which is crucial for accurately measuring bioelectric signals with a high signal-to-noise ratio ^[51]. We conducted electrode-skin impedance tests on a 2D ITO film and compared it to clinical standard Ag/AgCl gel control electrodes using a three-electrode setup (details in the Material and Methods section). As shown in Fig. 3e, the 2D ITO electrode exhibits comparable skin-contact impedance to Ag/AgCl per unit area from 1 Hz to 100 kHz. For comparison, the 2D ITO electrodes also show better skin-contact impedance (∼95 kΩ/cm²) compared to PEDOT: PSS values reported in the literature (169–194 kΩ/cm² )^{[52, 53]}, as well as gold electrodes (305 kΩ/cm²)^[53]. This low contact impedance makes the 2D ITO electrodes promising candidates for dry bioelectrodes in biopotential measurements. Figure 3f shows a spider chart of various dry bioelectrodes’ performance in terms of flexibility, transparency, conductivity, adhesion, bio-compatibility, and wear resistance. As highlighted in this chart, 2D ITO excels in its transparency and its mechanical resilience to abrasion while offering sufficient conductivity for accurate biopotential measurements.

2.5 Multimodal Biosignal Acquisition

ECG measurements using flexible 2D ITO electrodes were performed by contacting the index fingers on both hands (as described in Materials and Methods). The left inset of Fig. 4a shows a zoomed-in view of the P wave, QRS complex, and T wave of a single period of the electrocardiogram signal. We performed a Pearson correlation analysis comparing the measured heart rates from both the standard gel electrodes and the 2D ITO electrode. As shown in Fig. 4b, the analysis revealed a high positive correlation of 0.98 between measurements from both the 2D ITO and gel control electrodes under variable conditions (resting and light exercise). This strong correlation signifies the reliability and practical potential of the 2D ITO dry electrodes. Moreover, we employed our highly conductive 2D ITO films to perform electromyography (EMG) measurements for tracking two different hand gestures (open and closing a hand). Figure 4c shows the placement of the electrodes on the forearm and the measured EMG signal overlaid with the corresponding time divisions for open and closed hand gestures.

To leverage the multifunctionality of these transparent flexible bioelectrodes, we next demonstrated a set of multimodal heart rate measurements using both biopotential and optical methods. This is possible because 2D ITO is highly transmissive to the source wavelengths utilized in PPG (e.g. 530 nm and 940 nm), as shown in Fig. 4d, with an average transmittance above 95%. This allows these electrodes to be co-located with light-based PPG sensors without hindering light transmission from the PPG LED or the collection of reflected light by the PPG photodetectors. The high transmittance of 2D ITO would also allow vertical integration of these bioelectrodes with displays in future wearable systems. Figure 4e shows a schematic of the dual measurement of both ECG and PPG signals. Such integration enables simultaneous ECG and PPG measurements from the same site, combining the detailed electrical activity captured by ECG with the vascular blood volume changes detected by PPG. The advantage of performing these measurements simultaneously for cardiovascular monitoring is that ECG can be utilized to validate and correct motion artifacts commonly seen in PPG signals ^{[54, 55]}. As shown in Fig. 4f, heart rate measured using PPG (78–83 BPM) during light activity matches the precise measurements from electrical monitoring with the 2D ITO electrode (80–82 BPM). Additionally, as seen in Fig. 4f, the peak of the PPG coinciding with the T peak of the ECG indicates temporally synchronized cardiac cycle phases. An additional advantage of these synchronous measurements could be the ability to detect anomalies, such as arrhythmias, that are impossible to perceive via PPG measurements alone. This dual measurement capability, facilitated by the transparency of ITO electrodes, provides a more robust and holistic approach to cardiovascular health monitoring while also minimizing the area needed for multimodal measurements in a miniaturized device form factor.

In summary, we present flexible 2D TCOs fabricated at ultralow temperatures using vacuum-free Cabrera Mott oxidation of liquid hypoeutectic In-Sn alloys, demonstrating wafer-scale films with angstrom-level thickness control via an automated, kinetically driven approach. Our synthesis rapidly (at up to 20 cm/s) produces smooth, ultrathin 2D ITO with unprecedented conductivity (> 1300 S/cm) comparable to vacuum-deposited films. Surface morphology and structural characterization confirm the effectiveness of Sn-doping, revealing the high crystallinity of these 2D oxides and the large, plate-like grains formed in the liquid metal reaction environment. We observe that a significant result of the ultrathin nature of 2D ITO and the liquid metal printing process is enhanced bending strain tolerance, superior scratch resistance, and low contact impedance for 2D ITO when used as wearable bioelectrodes. Finally, leveraging the conductivity and transparency of 2D ITO, we enable simultaneous, multimodal measurements via electrocardiography (ECG) and photoplethysmography (PPG). These findings represent a significant improvement in the performance of printed metal oxides and introduce a promising new material for multimodal biometrics.

Alloy Preparation

The In-Sn alloys were prepared by melting In (Luciteria, 99.995%) and Sn (Luciteria, 99.995%) pellets in a graphite crucible at 300°C for 2 hours in an inert nitrogen atmosphere glovebox (< 10 ppm O₂, < 10 ppm H₂O) to minimize surface oxidation. Alloys containing 1, 2.5, 5, 7, 10, 20, and 30 at. % Sn was prepared by mixing the respective amounts of Sn with In.

Automated 2D TCO Synthesis and Deposition

2D indium tin oxides (ITOs) were deposited by rolling molten In-Sn alloy droplets along a substrate (Si/SiO₂, glass, PET, or PEN) on a hotplate using a silicone roller controlled by a 3-axis inline gantry robot (Fisnar 5300N). The deposition process was conducted at speeds from 0.1 to 20 cm/s and temperatures from 140 to 290°C (printing at 140 ºC was specifically enabled by high Sn-concentrations, as detailed in Fig. 1b). Prior to deposition, the target substrates were treated with approximately 10 seconds of atmospheric plasma using a Plasma-Etch 1000W system supplied with 30 LPM compressed dry air to promote the 2D ITO film adhesion. Two dummy substrates placed before and after the target substrate were used to allow the deposition process to reach equilibrium and produce a uniform and continuous metal oxide film deposition. After deposition, the residual liquid metal on the surface of the oxide film was removed with a squeegee while still on the hotplate and again once the sample had cooled to room temperature.

Thin Film Fabrication

For the abrasion tests, the control PEDOT: PSS films were prepared by spin-coating a 1 wt. % PEDOT: PSS solution in H₂O onto a SiO₂ (300nm)/Si substrate at 6000 RPM for 1 minute, followed by drying on a hotplate at 100°C. An Anatech LTD Hummer 6.2 sputtering system was utilized to deposit a 15 nm-thick gold film on a soda lime glass substrate for the peeling test.

Materials Characterization:

A Differential Scanning Calorimeter (DSC) (Discovery DSC 250, TA Instruments) was used to measure the melting temperature of the Sn-In alloys with various Sn compositions with a ramp rate of 10°C /min under N₂ flow. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Supra XPS at approximately 10⁻⁹ Torr on three layers of 2D ITO films printed on 100 nm SiO₂ substrates. Elemental analysis of the 2D ITO films was performed by comparing the Sn 3d, In 3d, and O1s peaks. Optical microscope images were captured with a Keyence VHX-7100 microscope. UV-Vis spectroscopy was conducted using a DeNovix DS-11 FX + spectrophotometer to measure the printed ITO films' absorbance spectra (270–800 nm) on glass substrates. AFM was performed using an AIST-NT instrument in tapping mode to measure film thickness and grain morphology. High-resolution transmission electron microscopy (HRTEM) was carried out with a Thermo Scientific Talos F200i instrument. Samples for HRTEM imaging were prepared by liquid metal printing of 2D ITO (7 at. % Sn) directly onto TEM grids (Carbon Square Mesh, Cu, 300 Mesh, UL, EMS) at 260 ºC, with excess liquid metal removed using a silicone squeegee. X-ray diffraction (XRD) was performed using a Rigaku UltraX Cu-anode diffractometer (Cu Kα radiation at 40 kV, 300 mA, λ = 0.154 nm) with a scanning rate of 0.5° per minute on single printed layers of 2D ITO on 300 nm SiO₂ substrates. Grain size analysis was conducted via AFM phase imaging and confirmed through HRTEM images. Scanning electron microscopy (SEM) was performed using a Thermo Scientific Helios 5 CX tool.

Electrical Characterization

Sheet resistance was measured using a four-point probe at room temperature in air.

Thickness Characterization:

Films for measuring thickness were printed on SiO₂/Si substrates (300 nm SiO₂). The exact SiO₂ thickness was measured to facilitate the modeling of the reflectance spectrum of the 2D ITO films on SiO₂ spectroscopic reflectometry (F3-sX, Filmetrics) from 380 nm – 1050 nm to extract the thickness of the films. These thicknesses measured by reflectometry were confirmed via AFM line scans of films patterned by wet etching. Multilayer films > 20 nm thick were also measured via stylus profilometry (KLA Tencor D-500) to confirm the reflectometry measured thickness.

Mechanical Characterization of Flexible 2D ITO Films

Bending resilience measurements were performed on 2D ITO films deposited onto 60 µm thick polyimide substrates at 260°C and sputtered ITO onto 175 µm PET substrate. The films were measured after the substrate was bent to 1%, 1.25%, and 1.5% of tensile strain until 100 bending cycles were achieved. The film hardness test was conducted using the ASTM D3363 standard, which entails scratching thin films, such as 2D ITO (printed with 7 at. % Sn doped at 260°C) on silicon and spin-coated PEDOT, using pencil leads of varying hardness. The films were imaged with optical microscopy, and changes in electrical resistance were observed after each iteration of abrasion with the specified pencil leads. Abrasion was applied with a force of 7 N, an angle of around 45°, and a speed of approximately 0.5 cm/s. An adhesion test for the 2D ITO film (printed with 7 at. % Sn doped at 260°C) on polyimide and sputtered Au on glass was performed using Kapton tape (Uline). The tape was removed at a speed of approximately 1 cm/s and at an angle of ~ 90° to the tested film.

2D ITO Bioelectrode Characterization

ECG measurements were conducted by wrapping printed ITO electrodes that are printed with 7 at% Sn doped at 260°C on polyimide in a single lead setup (Fig. 4a). As a control, a gel electrode (3M-2238 Electrode) was placed on the forearm. The heart rate was calculated using the Eq. 2, shown below:

Heart rate (beats per minute) = 60/RR

(2)

Where RR is the time interval in seconds between two consecutive R peaks of the measured ECG signal. Both measurements were performed using a Vernier EKG Sensor (Go Direct). EMG measurements were performed by placing these ITO electrodes on polyimide on the forearm (Fig. 4c) and using the Vernier Go Direct system for data collection. A Polar Verity sensor was used to measure the PPG from the wrist. The transparent ITO electrode on polyimide, which measures the ECG signal, was placed between the PPG LED and the wrist. This setup allows simultaneous measurements of both PPG and ECG.

Supporting Information

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

Conflict of Interest

The authors declare no conflict of interest.

Data Availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

Saifur Rahman was supported by the Dartmouth PhD Innovation Fellowship. This research was supported by the National Science Foundation Electronic and Photonic Materials Program (Award #2202501) as well as the National Science Foundation Electronics, Photonics, and Magnetic Devices program (Award #2219991). We acknowledge Paul Defino at Dartmouth College for assistance in AFM scans as well as John Wilderman at the University of New Hampshire for performing XPS measurements.

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Kinetic Liquid Metal Synthesis of Flexible 2D Conductive Oxides for Multimodal Wearable Sensing

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Abstract

Figures

1. Introduction

2. Results and Discussion

2.1 Liquid Metal Printing of 2D Indium Tin Oxide

2.2 Materials Characterization of Liquid Metal Printed 2D ITO

2.3 Electrical and Electromechanical Properties of Liquid Metal Printed 2D ITO

2.4 Mechanical and Functional Properties of Flexible 2D ITO for Bioelectrode Applications

2.5 Multimodal Biosignal Acquisition

3. Conclusion

4. Experimental Methods

2D ITO Bioelectrode Characterization

Declarations

References

Additional Declarations

Supplementary Files

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