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Sci Adv
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2375-2548
American Association for the Advancement of Science

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10.1126/sciadv.adq6022
Research Article
Physical and Materials Sciences
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Materials Science
Applied Sciences and Engineering
Applied Sciences and Engineering
Sub–180-nanometer-thick ultraconformable high-performance carbon nanotube–based dual-gate transistors and differential amplifiers
Sub–180-nanometer-thick high-performance electronics based on CNTs
https://orcid.org/0000-0002-3710-700X
Wang Yuru Conceptualization Data curation Formal analysis Investigation Methodology Project administration Resources Software Supervision Validation Visualization Writing - original draft Writing - review & editing 1
Wang Tingzhi Investigation Project administration Validation Writing - review & editing 1
https://orcid.org/0000-0003-2620-9788
Xiang Li Conceptualization Methodology 2
Huang Ruyi Investigation Resources Validation 3
Long Guanhua Conceptualization Methodology Software Validation 1
https://orcid.org/0009-0009-5115-7802
Wang Wanyi Investigation Methodology Resources Validation Visualization 1
Xi Meiqi Data curation Investigation Resources Software Supervision Validation Visualization 1
https://orcid.org/0000-0003-0345-1731
Tian Jiamin Investigation Resources 1
Li Wangchang Resources Visualization 1
https://orcid.org/0000-0002-1087-169X
Deng Xiaosong Investigation Methodology 1
https://orcid.org/0000-0003-4610-5353
Gong Qibei Resources 1
Bai Tianshun Data curation Investigation Project administration Software Validation 1
Chen Yufan Investigation Validation 1
Liu Hong Resources 1
https://orcid.org/0000-0002-5393-059X
Xia Yu Investigation 1
Liang Xuelei Investigation Resources 1
https://orcid.org/0000-0002-7919-5159
Chen Qing Validation 1
https://orcid.org/0000-0003-0754-074X
Peng Lian-Mao Conceptualization Funding acquisition Project administration Supervision Validation Writing - review & editing 1 3
https://orcid.org/0000-0001-9798-1631
Hu Youfan Conceptualization Data curation Formal analysis Funding acquisition Methodology Project administration Supervision Writing - review & editing 1 3 *
1 Key Laboratory for the Physics and Chemistry of Nanodevices, Center for Carbon-Based Electronics and School of Electronics, Peking University, Beijing 100871, China.
2 College of Materials Science and Engineering, Hunan University, Changsha, 410082, China.
3 Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China.
* Corresponding author. Email: youfanhu@pku.edu.cn
06 9 2024
06 9 2024
10 36 eadq602222 5 2024
01 8 2024
Copyright © 2024 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).
2024
The Authors
https://creativecommons.org/licenses/by-nc/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

There is increased interest in ultrathin flexible devices with thicknesses of <1 micrometers due to excellent conformability toward advanced laminated bioelectronics. However, because of limitations in materials, device structure, and fabrication methodology, the performance of these ultrathin devices and circuits is insufficient to support higher-level applications. Here, we report high-performance carbon nanotube–based thin-film transistors (TFTs) and differential amplifiers on ultrathin polyimide films with a total thickness of <180 nanometers. A dual-gate structure is introduced to guarantee excellent gate control efficiency and mechanical stability of the ultrathin TFTs, which exhibit high transconductance (8.96 microsiemens per micrometer), high mobility (127 square centimeters per volt per second), and steep subthreshold swing (84 millivolts per decade), and can sustain a bending radius of curvature of <10 micrometers. The differential amplifier achieves the highest gain-bandwidth product (1.83 megahertz) among flexible differential amplifiers, enabling higher-gain amplification of weak signals over an extended frequency spectrum that is demonstrated by amplification of electromyography signals in situ.

Ultrathin, high-performance CNT-based electronics have the ability to amplify electromyography signals in situ.

http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 62101181 Natural Science Foundation of Hunan Province 2023JJ20016 National Key R & D Program of China 2021YFA1202904 National Key R & D Program of China 2022YFB4401603
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pmcINTRODUCTION

The continuous advancements in wearable and flexible electronics are expected to enable a wide range of new applications, particularly by providing advanced platforms with health assessment, physical activity tracking, and even personalized therapy capabilities (1–6). An intimate interface between electronics and organisms for high-fidelity information recording and high-performance electronics to support advanced functions are key enablers of these applications. An intimate interface with biological objects requires the electronic system to be highly conformable to adapt to complex-shaped biological surfaces. Because the conformability and adhesiveness of a thin film depend on its bending stiffness, which is proportional to the thickness of the film cubed (7), reducing the thickness of a flexible device, for example, from 2 μm to 200 nm, thus results in 1 of 1000 of its original bending stiffness, rendering the device notably more conformable and adhesive, as schematically shown in Fig. 1A. This characteristic makes the construction of ultrathin flexible devices with thicknesses of less than 1 μm very attractive for achieving ultraconformal contacts and even self-adhesive, clean, and stable interfaces with biological objects (8–12). Furthermore, a thinner device also leads to a decreased weight, which makes the flexible system more imperceptible and user-friendly (7, 8). Nevertheless, the substantial thickness reduction raises enormous challenges in device construction, including but not limited to material selection, mechanical structure design, and manufacturing (13).

Fig. 1. Ultrathin, conformal and imperceptible flexible devices.

(A) Schematic illustration of an ultrathin flexible device and a regular flexible device on the human skin. (B) Schematic illustration of the two different strategies used to realize ultrathin flexible devices: reducing substrate thickness strategy and free-standing strategy. Photograph of sub–180-nm-thick ultrathin devices: (C) warped on a glass rod, (D) placed on a soap bubble, and (E) enveloping a knife blade. Scale bars, 1 cm. (F) Photographs of an ultrathin flexible device laminated on a human finger joint with different bending angles. Scale bars, 1 cm.

In addition to achieving an ultrathin physical form, to realize the superiority of ultrathin electronics, the performance of the electronic devices is crucial to support desired advanced functions. To handle very weak biosignals, generally in the range of microvolts to millivolts for biopotentials as an example, in situ signal processing close to the source is indispensable to ensure a high signal-to-noise ratio at the start of the measurement and to minimize interference noise during transmission. Serving as the basic building blocks of flexible integrated circuits for information processing, flexible thin-film transistors (TFTs) have been the focus of many efforts to push them into the ultrathin domain (9, 14, 15). As the substrate is typically the thickest part, reducing the polymer substrate thickness is the most common method for achieving this goal (left in Fig. 1B). In addition, another approach called the “free-standing” strategy (right in Fig. 1B) has been demonstrated to push the thickness to the limit but at the cost of decreased electrostatic control of the channel. In this strategy, a polymer film functions as a self-supporting gate dielectric layer with a relatively larger thickness (several tens of nanometers) and a lower dielectric constant [2.8 for parylene C (16, 17) and 3.48 for polyimide (PI) (18)] than that of generally used high-κ dielectrics (7.5 for Al2O3, 14.9 for HfO2, and 12 for Y2O3) (19) with thicknesses of less than 10 nm. To date, almost all of these demonstrations have been carried out on the basis of organic materials. This may be attributed to the intrinsic flexibility, low processing temperature, and printability of these materials, which are compatible with ultrathin polymer films. However, the performance demonstrated thus far is still not sufficient to support the required advanced functions (13, 14).

Carbon nanotubes (CNTs) have been extensively explored in advanced flexible electronics due to their extraordinary intrinsic properties, including ultrathin body, high carrier mobility, excellent mechanical flexibility, and solution processability (20–24). A recent demonstration proved the favorable characteristics of CNT-TFTs for ultrathin form (25), but the achieved electrical performance remained notably inferior to that of general flexible devices (23, 26, 27) due to the adoption of a free-standing strategy utilizing a 120-nm-thick dielectric. Extending the excellent performance of CNT-TFTs to ultrathin forms by just reducing the substrate thickness is not very straightforward, as many challenges arise, including growth of high-quality sub–10-nm high-κ dielectrics on flexible substrates with increased surface roughness (28), compatible fabrication strategies to handle fragile substrates, and additional mechanical structure design considerations. In this work, in response to the above challenges in materials, device structure, and fabrication methodology, using CNT as channel materials, realizing a 125-nm-thick flexible substrate, adopting a dual-gate device structure with both electrical and mechanical designs to provide better electrostatic control and mechanical stability, and combining a robust delamination method to achieve intact performance after delamination, we demonstrate sub–180-nm-thick high-performance CNT-TFTs and differential amplifiers manufactured via conventional microfabrication processes that are compatible for large-area production. High electrical performance in terms of transconductance, mobility, and switching behavior is achieved in ultrathin CNT-TFTs, which simultaneously exhibit great mechanical stability when subjected to extreme deformation. An ultrathin differential amplifier based on these TFTs demonstrates a voltage gain of 43 dB, a common-mode rejection ratio (CMRR) of 55 dB, and a 3-dB bandwidth (f3dB) of 13 kHz, resulting in a gain-bandwidth product (GBWP) of more than 1.83 MHz, which enables the amplification of electromyography (EMG) signals from 1.5 mV to 0.8 V in situ, showing great application potential in advanced health monitoring, medical diagnostics, and more.

RESULTS

Imperceptible device based on an ultrathin PI substrate

Because of their desirable properties, such as excellent thermal resilience, chemical stability, and mechanical durability (27), PI films are used as flexible substrates in our devices. To prepare an ultrathin PI film, the solid content of the PI precursor in an N-methyl-2-pyrrolidone solution is diluted with optimization to achieve the thinnest thickness while maintaining good mechanical robustness and integrity and minimal surface roughness (fig. S1A). The silicon wafer used as the supporting substrate is subjected to oxygen plasma before spin coating of the PI solution to enhance the adhesion of the diluted PI solution and thus ensure continuity of the film at a greatly reduced thickness. After optimization, an ultrathin PI film of ~125 nm (fig. S1B) is obtained, which has good integrality over a 2-inch wafer with a root mean square value of surface roughness less than 0.17 nm (fig. S1, A and C). The ultrathin PI film with devices manufactured on it can be intactly delaminated from the silicon wafer within several minutes (fig. S2) using a capillary-assisted electrochemical delamination method (20, 27) we previously developed. As shown in Fig. 1C, the ultrathin PI film appears colorless and transparent at this greatly reduced thickness. Because of the ultrathin PI substrate, our flexible devices are very lightweight (0.90 g m−2; fig. S3) and imperceptible and can be easily placed on the surface of a soap bubble without causing it to break (Fig. 1D). As illustrated in Fig. 1E, the ultrathin film can sustain extremely small curvature radii, even on a knife cutting edge. Furthermore, the ultrathin film achieves ultraconformal contact with the wrinkled skin at finger joints and retains complete conformality throughout the finger bending process (Fig. 1F and fig. S4), which is crucial for high-fidelity signal monitoring when attached to the human body during daily activities.

Ultrathin flexible CNT dual-gate TFTs

CNT-TFTs with a dual-gate structure (Fig. 2, A and B) are directly fabricated on the ultrathin PI substrate covered with 3-nm HfO2 to enhance surface adhesion via photolithography at the wafer scale (fig. S5). Randomly oriented high-purity semiconducting CNT networks with a density of 40 to 50 tubes μm−1 are used as channel materials, as shown in the scanning electron microscopy image in Fig. 2C. Palladium (Pd) is used as source and drain contact metal (20 nm) to achieve p-type CNT-TFTs with ohmic contacts and also serves as the bottom (8 nm) and top (18 nm) gate layers. As the growth of high-quality sub–10-nm high-κ dielectrics on flexible substrates is highly challenging, a dual-gate structure is adopted to improve the gate control efficiency. Moreover, featuring a vertically symmetrical structure, this dual-gate structure provides precious mechanical design possibility in the limited space of ultrathin devices, as clarified later. The dual-gate dielectrics are yttrium oxide (Y2O3) (10 nm per layer) grown by thermal oxidization of yttrium, which has excellent wettability with CNTs that enables direct uniform growth of Y2O3 layers on the surfaces of CNTs (29–31). The dual-gate device structure can be distinguished in the cross-sectional transmission electron microscopy (TEM) image in Fig. 2D, while the spatial distribution of each composition is confirmed by the scanning TEM (STEM) image and energy-dispersive x-ray spectroscopy (EDS) results in Fig. 2E. The thickness of the dual-gate CNT-TFTs is ~46 nm, and the total thickness with the substrate is less than 180 nm. With the well-designed thickness of the different layers in the stacked structure, the analysis of the bending mechanics (32) in this material stack (note S1 and tables S1 and S2) indicates that the neutral plane, which defines a specific position through the thickness of the structure that experiences zero strain during deformation (32, 33), is situated close to the channel (located ~1 nm below the top Y2O3 dielectric layer), as shown in Fig. 2F. This situation is highly favorable for enhancing the mechanical stability and reliability of devices while maintaining ultrathin superiority without additional encapsulation or passivation, which further affirms the rationality of our dual-gate structure design.

Fig. 2. Device structure and characteristics of ultrathin CNT dual-gate TFTs.

(A) Schematic illustration of the device structure of a dual-gate CNT TFT on a PI film. (B) Optical microscope image of a dual-gate CNT-TFT. Scale bar, 25 μm. (C) Scanning electron microscopy image of CNT networks in the channel. Scale bar, 400 nm. (D) TEM images of the device cross-sectional area. The Ti layer at the top is deposited to protect the device during preparing the TEM sample using the focused ion beam lift-out technique. Scale bar, 100 nm (left) and 20 nm (right). (E) STEM image and EDS analysis results for the area. Scale bars, 50 nm. (F) Schematic illustration of the stacked layers in the device cross-sectional area. The red dashed line indicates the position of the neutral plane in the device. (G) Transfer characteristics, (H) transconductance (gm) characteristics, and (I) output characteristics of a typical dual-gate CNT-TFT with a channel length of 5 μm and a channel width of 50 μm. Comparison of the electrical performance of devices with thicknesses of less than 1.5 μm in terms of the (J) mobility and (K) SS. The ultrathin region is marked in orange and blue in (J) and (K), respectively. IGZO, indium gallium zinc oxide. (L) Comparison of the W-normalized transconductance (gm/W) of flexible TFTs with different absolute Vds values.

Figure 2G shows the transfer characteristics of a typical dual-gate CNT-TFT with a channel length L = 5 μm and channel width W = 50 μm. Under a small drain-source voltage Vds = −0.1 V, the device exhibits a large current on-off ratio exceeding 106 and a subthreshold swing (SS) of 84 mV decade−1, indicating outstanding gate control efficiency. The transconductance (gm) characteristics for different Vds values are shown in Fig. 2H. At a Vds of −1 V, the maximum gm approaches 447.65 μS, equivalent to a width-normalized value gm/W = 8.96 μS μm−1, providing a high signal amplification capability that is valuable for bioinformation processing (6–9, 14, 21, 24–26, 34–44). As illustrated by the output characteristics for different gate-source voltages Vgs in Fig. 2I, the on-state current Ion at Vgs = Vds = −3 V is 1.18 mA, and, thus, a width-normalized value of 23.60 μA μm−1 is obtained. A high Ion is beneficial for achieving high-speed switches and high-bandwidth analog amplifiers. The exceptional electrical performance of the ultrathin CNT-TFTs is strongly attributed to the efficient electrostatic control of the channel provided by the dual-gate structure, as evidenced by the performance comparison with that of top-gate and bottom-gate ultrathin CNT-TFTs with similar fabrication processes, as shown in fig. S6.

The low-field mobility μ is calculated using the formula μ = gm(L/W) (1/Cox) (1/Vds), with the dielectric capacitance density Cox assigned a value of 3.68 × 10−3 F m−2 based on direct measurement of the device gate capacitance C-V curve at 1 MHz (fig. S7). The obtained μ (Vds = −0.1 V) of 127 cm2 V−1 s−1 is the highest recorded value among ultrathin flexible devices to date (Fig. 2J) (6–9, 14, 25, 34, 38, 41, 44–51). With a gate-to-source/drain overlap length of 1 μm, the transit frequency of the device is estimated to be 38.8 MHz (52). From the summarized data presented in Fig. 2K, the SS index that assesses the switching behavior of the dual-gate devices is also notably superior to that of other ultrathin devices. These results highlight the achievement of both an ultrathin thickness and high electrical performance in our devices. To realize a high-performance analog voltage amplifier with a high gain and a broad bandwidth, transistors with a larger gm are typically needed. Furthermore, given the portability of power sources and the demand for extended battery life, operation at a low voltage is highly preferable for flexible electronics. Compared to other flexible devices (Fig. 2L) (6–9, 14, 21, 24–26, 34–44, 53, 54), not limited to the ultrathin domain, our device achieves excellent gm at a low operating voltage (|Vds| = 1 V), indicating the capability to enable high-performance flexible analog circuits. The detailed data used for these comparisons are summarized in table S3.

Uniformity and mechanical flexibility of the TFTs

Device uniformity is crucial for further integration, especially for constructing analog circuits, which are more sensitive to mismatches in device characteristics. Among 110 devices fabricated in a single batch, 100 devices work normally; however, 10 devices show leakage with a gate leakage current of 1 × 10−8 (fig. S8), resulting in a device yield ~91%. To further promote the device yield, a dielectric with a higher dielectric constant and thicker thickness (e.g., 12-nm HfO2) can be adopted to reduce the probability of leakage. The transfer curves of 100 dual-gate CNT-TFTs with Vds = −0.1 V are depicted in Fig. 3A, revealing a very narrow distribution. Moreover, our devices exhibit a minimal hysteresis of only ~40 mV, which is attributed to the excellent wettability of the yttrium oxide and the CNT film, resulting in a high-quality interface. The statistical distributions of the threshold voltage (Vth), μ, and SS are −0.54 ± 0.03 V, 113 ± 5.75 cm2 V−1 s−1, and 89.9 ± 4.03 mV decade−1, respectively. This high uniformity highlights the good reproducibility and reliability of the device fabrication process, which lays a solid foundation for implementing analog circuits based on these ultrathin TFTs, particularly symmetric fully differential circuits.

Fig. 3. Uniformity and mechanical flexibility of the TFTs.

(A) Transfer characteristics of 100 dual-gate CNT-TFTs (channel length = 5 μm and channel width = 50 μm) with Vds = −0.1 V. The inset shows the hysteresis curve of a typical device. Statistical distribution of the (B) threshold voltage (Vth), (C) mobility, and (D) SS. (E) Transfer characteristics of a dual-gate CNT-TFT (channel length = 5 μm and channel width = 50 μm) in different bending states: before delamination, after delamination, and after bending at different radii = 5, 3, and 1.5 mm, five times each, and being subjected to bending at a small radius of less than 10 μm. (F) Variations in the normalized gm and SS of three representative ultrathin CNT-TFTs after different bending tests.

To assess the mechanical stability of the ultrathin flexible TFTs, the performance of a device before delamination, after delamination, and after being subjected to different bending curvatures is characterized. The device is wrapped around glass rods of three different radii (R = 5, 3, and 1.5 mm) five times each and then placed on a 10% prestretched polydimethylsiloxane elastomer that is subsequently relaxed to reach a bending curvature radius of less than 10 μm (fig. S9). The recorded transfer characteristic curves of the device following bending at various curvatures are presented in Fig. 3E. The device clearly exhibits robust mechanical stability, and the delamination process has minimal effects on the device performance. This is further confirmed by the variation of gm and SS values extracted from three representative devices, as presented in Fig. 3F. The results show that under the smallest curvature radius bending, gm only experiences a small loss less than 10%, and SS increases by only about 3%. We also characterized the variation in the device performance during a cyclic bending test (fig. S10). The results indicate that after 200 repeated bending cycles, the device experiences a 9.77% decrease in gm and a 9.09% increase in SS. Our device exhibits excellent mechanical stability due to its ultrathin physical dimensions and strategically positioned neutral plane.

High-performance differential amplifier for in situ physiological signal processing

Differential amplifiers are indispensable components of flexible circuits for physiological signal processing (5, 6, 21, 55), which are capable of amplifying differential-mode signals and suppressing common-mode noise, a crucial characteristic that is particularly important for monitoring of physiological signals with small amplitudes. To amplify physiological signals to the volt-level range and alleviate processing complexities in subsequent circuits such as analog-to-digital converters, the amplifier must have a voltage gain of 100 (40 dB) or more. In addition, certain high-frequency physiological signals can extend into the 10-kHz frequency range (2). However, previously demonstrated flexible differential amplifiers cannot simultaneously meet both the bandwidth and gain requirements due to inadequate performance.

A differential amplifier is generally formed by a pair of common-source single-stage amplifiers and a current source. As shown in Fig. 4 (A and B), two transistors (T1 in Fig. 4B) are used in parallel as the current source, and a pair of zero-Vgs amplifiers serve as the input gain stage (positive-phase input Vin+ of T2 and T4; negative-phase input Vin− of T3 and T5). Considering device uniformity, all device dimensions are standardized to the same size of W/L = 50/5 μm. By connecting two identical transistors in parallel, we effectively enhance the W/L ratio of the current source transistor, thereby expanding the output swing range of the amplifier. The zero-Vgs amplifier is a preferred choice in circuit design based on unipolar transistors because of its high gain and substantial output swing range. The single-stage zero-Vgs amplifier based on our CNT-TFTs provides an output swing close to VDD and attains a DC voltage gain of 160 at a VDD of 5 V (fig. S11). To isolate the load capacitance incurred by the measurement equipment, a source-follower buffer (T6 and T8 for the positive-phase output Vout+; T7 and T9 for the negative-phase output Vout−) is added to each of the two outputs in the fully differential configuration. A voltage divider network consisting of two linear resistors (R1 and R2; R3 and R4) made from CNT strips (W/L = 50/20 μm) delivers a VDD/2 voltage level to the bias terminal of each source-follower buffer.

Fig. 4. High-performance differential amplifier for in situ physiological signal processing.

(A) Optical microscope image of an ultrathin differential amplifier based on CNT-TFTs. Scale bar, 100 μm. GND, ground. (B) Circuit diagram of a fully differential pair amplifier based on p-type transistors. (C) Transient response of the differential amplifier for a differential-mode sine wave input signal with a Vp-p of 20 mV at a frequency of 10 kHz. (D) Bode plots of the differential amplifier for differential-mode input signals. (E) Transient response of the differential amplifier for a common-mode sine wave input signal with a Vp-p of 200 mV at a frequency of 10 kHz. (F) Bode plots of the differential amplifier for common-mode input signals. (G) Input-output characteristics of the circuit for amplifying EMG signals captured from the surface of the forearm skin of an adult male. (H) Comparison of flexible differential amplifier performance in terms of the gain versus 3-dB bandwidth. The dashed lines represent equivalent GBWP values.

To characterize the differential-mode gain of the amplifier, we apply AC sinusoidal voltage signals (peak-to-peak voltage Vp-p = 20 mV) with varied frequency to Vin+ while imposing a fixed DC bias at Vin− and simultaneously measure the output signal Vout = Vout+ − Vout−. As shown in Fig. 4C, at a supply voltage of 5 V, the ultrathin CNT differential amplifier exhibits a gain of 116 (~41 dB) for differential-mode signals at a frequency of 10 kHz. According to the Bode plots in Fig. 4D, the differential amplifier demonstrates a low-frequency voltage gain of 141 (~43 dB) and an f3dB of ~13 kHz, resulting in a maximum GBWP (GBWP = open-loop gain × f3dB) of ~1.83 MHz. Likewise, by shorting the Vin+ and Vin− terminals and simultaneously applying sinusoidal AC signals (Vp-p = 200 mV), the ability to suppress common-mode noise can be observed in the output response. The amplifier obtains an attenuation of −12 dB for common-mode signals, and the frequency response results are consistent with those of the differential-mode test. Consequently, the CMRR (the ratio of the differential-mode gain to the common-mode gain) of the amplifier reaches 55 dB. The bandwidth of our differential amplifier effectively covers the common frequency spectrum of electrophysiology signals originating from the human body, including the EMG signals with the highest frequency (2). EMG signals captured from the body surface, amplified in situ, and then subjected to a filter to eliminate the power frequency interference (50 Hz) are recorded in Fig. 4G. The test configuration is illustrated in fig. S12. The signal, initially submerged in noise at only a few millivolts, is amplified to ~0.8 V, leading to a pronounced increase in the signal-to-noise ratio. A comparison in Fig. 4H shows that our high-performance CNT differential amplifier yields the highest GBWP value among the differential amplifiers implemented on flexible substrates to date (5, 6, 21, 37, 38, 55–64). The detailed data used for the comparison are presented in table S4. The remarkable performance of our amplifier is a consequence of the high gm present in the dual-gate CNT-TFTs and the excellent uniformity of our devices.

DISCUSSION

Devices and circuitry embody a combination of ultrathin design, and high performance can provide new capability for health monitoring and medical diagnostics. Here, we have developed sub–180-nm-thick ultrathin dual-gate CNT-TFTs and differential amplifiers that show exceptional flexibility and establish good conformal contact with complex skin surfaces. The high-performance CNT-TFTs based on a dual-gate design demonstrate the highest mobility among ultrathin devices to date, ~127 cm2 V−1 s−1. The rational design of the neutral plane position in the device ensures that the device performance is maintained even at a radius curvature that is less than 10 μm. On the basis of the substantial high transconductance (8.96 μS μm−1) and the superior uniformity of the devices, we have effectively implemented a high-performance differential amplifier. The amplifier provides a high voltage gain of 43 dB, a large bandwidth of 13 kHz, and a CMRR of 55 dB. Among the differential amplifiers based on flexible TFTs, our amplifier achieves the highest GBWP of 1.83 MHz. With the high-performance amplifier, skin surface–collected EMG signals are amplified in situ. The processed signals with enhanced quality can be used in the next stage for health assessment, medical diagnostics, etc. Other key functional modules, such as analog-to-digital converters, filters, etc., are highly needed to be developed to improve the information processing capabilities and achieve more comprehensive ultrathin integrated systems. Our research extends the performance capabilities of ultrathin flexible electronics, establishing a pathway for the creation of future ultraconformable, lightweight, and multifunctional ultrathin electronic systems for diverse medical and health care applications.

MATERIALS AND METHODS

Fabrication of ultrathin PI substrates

The surface of the phosphorus heavily doped silicon wafers was treated by subjecting it to an oxygen plasma operating at 250 W for 60 s. Subsequently, a PI solution (3022L, POME Sci-tech Company) diluted in N-methyl-2-pyrrolidone solution, to realize a solid content of 8% by weight, was dispensed onto the surface of the silicon wafer. Next, the silicon wafer was spun at a speed of 4000 rpm for a duration of 30 s. The silicon wafer coated with PI solution was placed in an oven and heated at a rate of 2°C min−1 until it reached 300°C, where it was maintained for 1 hour. Last, on the cured PI surface, a 3-nm-thick HfO2 layer was grown through atomic layer deposition at 90°C to improve the adhesive strength between the metal layer and the flexible substrate.

Preparation of CNT thin films

Two hundred milligrams of arc-discharged CNTs sourced from Carbon Solutions Inc. were combined with 400 mg of 9-(1-octylnonyl)-9H-carbazole-2,7-diyl in 200 ml of chloroform. Dispersion was achieved through ultrasonication utilizing a top-tip dispergator (VC500, Sonics) for 30 min at 300 W. Subsequently, the solution was subjected to high-speed centrifugation at 20,000g for 1 hour (Allegra X-22R, Beckman Coulter) to facilitate the removal of bundles. The resultant upper supernatant was then subjected to an additional centrifugation at 50,000g for 2 hours to further eliminate residual metallic nanotubes. Last, the upper 90% of the supernatant was collected as the CNT solution for subsequent applications. The CNT films were fabricated using a dip-coating process. Initially, a PI film substrate on a silicon wafer was securely affixed to the dip-coating machine, oriented vertically above the surface of the CNT solution. The substrate was methodically immersed and withdrawn from the CNT solution at a controlled rate. As the solvent within the CNT solution rapidly evaporated during the withdrawal process, CNTs were deposited on and adhered to the PI film surface. This iterative process was repeated 30 times, yielding a thin film characterized by a randomly oriented CNT network exhibiting a density of ~40 to 50 tubes μm−1.

Fabrication of dual-gate CNT-TFTs

Patterning of the devices was achieved through ultraviolet photolithography (EVG610, EV Group). Metal deposition was carried out using electron-beam evaporation and lift-off processes. The substrate surface was first coated with a stacked layer of 0.3-nm titanium (Ti) and 8-nm Pd, serving as the bottom gate. To enhance adhesion between the metal and the substrate, preliminary deposition of 0.3 nm of Ti preceded the palladium plating. Subsequently, 8 nm of Y was deposited, followed by heating in an oven to 260°C for 30 min to induce the formation of 10-nm-thick Y2O3 as a bottom gate dielectric. CNT films were subsequently prepared on the substrate using a dip-coating method. Next, an inductively coupled plasma (Minilock, Trion) process was used to selectively remove unwanted CNTs except for those in the channel area through oxygen plasma etching, with the process parameters set at 100 W for 30 s. Afterward, 20 nm of Pd was deposited onto the CNT film to serve as the source and drain contacts. The top gate dielectric layer of Y2O3 was prepared using the same method as described earlier. Last, device fabrication process was completed by depositing 18-nm-thick Pd as the top gate.

Characterization of CNT-TFTs and amplifiers

The DC electrical characteristics of the CNT-TFTs and amplifiers were obtained through testing using an interconnected testing system with a semiconductor parameter analyzer (Keithley 4200A-SCS, Tektronix) and a probe station (Summit 1100, Cascade Microtech). For AC testing, the input signal was provided by a signal generator (33220A, Agilent), and the output signal was measured using an oscilloscope (MSOS254A, Keysight). The experiments with human individuals were performed under the protocol (approval no. IRB00001052-21156) that was approved by the Institutional Review Board of Peking University. We confirm that these experiments with human individuals were carried out with the full, informed consent of the individuals.

Capillary-assisted electrochemical delamination

First, a NaCl electrolyte solution (1 M) was prepared. After trimming off the fringes of the PI layer and exposing the surface of the heavily doped silicon wafer (with a resistivity of 0.009 ohm·cm), the silicon wafer was fixed at the angle of 45° by clamping equipment, while the electronic devices were still facing upward. Then, the lower edge of the silicon wafer was inserted into the NaCl solution. Following the positive (20 V) and ground (0 V) potential applied to the silicon wafer and NaCl solution, respectively, the anodic reaction of Si − 8e− + 8OH− → H2SiO3↓ + 3H2O + 2O2↑ was induced, generating gaps between the PI layer and silicon wafer. A capillary force was then triggered to allow the NaCl solution to climb upward and delaminate the PI along the inclined silicon wafer. After delamination, the ultrathin substrate was laid flat on a supporting silicon wafer during electrical characterization.

Acknowledgments

Funding: This work was supported by the National Key R&D Program of China (grant nos. 2021YFA1202904 and 2022YFB4401603), National Natural Science Foundation of China (62101181), and Natural Science Foundation of Hunan Province (2023JJ20016).

Author contributions: Y.W., X.L., and Y.H. conceived the experiments. Y.W., T.W., R.H., G.L., W.W., W.L., Q.G., T.B., and Y.C. fabricated the device, performed the electrical measurements, and analyzed and interpreted the data with input from Y.H., X.D., and L.-M.P. M.X. and X.L. prepared the CNT solution and thin films. J.T. and Q.C. performed the TEM, STEM, and EDS characterization. H.L. and Y.X. performed the atomic force microscopy characterization. The manuscript was written with contributions from all authors, and all authors approved the final version of the manuscript.

Competing interests: The authors declare that they have no competing interests.

Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

Supplementary Materials

This PDF file includes:

Figs. S1 to S12

Supplementary Note S1

Tables S1 to S4

References
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