研究背景
柔性电子器件被用于各种产品，其中包括柔性显示器、电子皮肤、可穿戴传感器和能源转换器。高效电子器件的工业化生产在该领域表现出显著的趋势。基于有机分子的方法引起了很多关注，但它们经常产生不合格的器件性能，而且不能使用标准的制造技术。目前正在研究的其他有前途的材料包括氧化锌 (ZnO) 薄膜、碳纳米管 (CNTs)、半导体纳米线、氧化锌和硒化铟 (lnSe)。然而，这些系统的高制造规模和低设备产量限制了可以用它们进行的有用应用。基于层状二维(2D)过渡金属二氯化物(TMDS)材料的范德瓦尔斯 (vdW)异质结构，由于其出色的光电、电子和半导体特性，最近吸引了很多人的兴趣。层状二维TMDs 最近变得更受欢迎，这是由于它们有可能用于柔性电子。科学界给予了它们很多关注，这是由于二维TMD 异质结构的量子效应、隧道和弱层间键。这三个特点使它们有别于传统的三维块状材料，这增加了人们对它们的兴趣。二维TMD的表面没有空键，自然很薄，这是由于这些材料内部存在弱的vdW相互作用，尽管它们的晶格不匹配，但可以用来构建功能化异质结构。二维TMD 具有拓扑有序的结构，使它们能够克服障碍，如原子扩散和位错，这是由于它们有尖锐的界面和材料结构中没有悬空键。
研究成果
具有无悬键表面和原子薄层的二维(2D)材料已被证明能够被纳入柔性电子装置。二维材料的电子和光学特性可以通过使用应变工程方法以其他方式进行调整或控制。沙特国王大学Abdullah A. Al-Kahtani教授等人在这篇综述文章中浓缩了有关创建柔性二维纳米电子器件的最新技术。这些技术有可能在近期和长期内被用于更广泛的应用。有可能使用超薄的二维材料二维材料(石黑烯、BP、WTe2、VSe2等)和二维过渡金属二氯化物 (2D TMDs)，以便能够研究设备的电气行为。一类材料是通过剥离块状材料在较小范围内生产的，而化学气相沉积(CVD)和外延生长则在较大范围内使用。该综述强调了两个不同的要求，其中包括从单一的半导体或与各种纳米材料的范德瓦尔斯异质结构。它们包括必须避免应变的地方，如为了生产应变不敏感的设备的溶液，它们包括需要应变的地方，如压力敏感的结果。弹性纳米电子学在电子皮肤中的应用，以及二维柔性电子设备的特点和功能的比较，都被认为是赋予伸展性的方法。最后，提供了关于目前在柔性电子中使用二维材料的困难和可能性的观点。相关报道以“Latest Innovations in Two-dimensional Flexible Nanoelectronics”为题发表在Advanced Materials期刊上。
图文导读
Figure 1. Our review paper's overview identifies two distinct needs, which include where the strain must be avoided, such as solutions to create strain-insensitive devices, and devices where the strain is necessary, such as pressure-sensitive results.
Figure 2. a) VdW sliding contact illustrations with and without the strain. b) A vdW sliding electrode device is used to visually represent various electrode stretching parameters. The scale bars are 10 µm. c) Schematic representation of the evaporated contact device's straining process. d) Images of the evaporated electrode device that is taken with an optical microscope display various electrode stretching values. A continuous crack can be seen under a low stretching value of 7%. The scale bars are 10 μm. e) I-V output curve with vdW sliding contact indicates intimate Au-Gr contact under strain. f) The lengthening of the Gr channel can be blamed for the change in the resistance. The electrode stretching value and the overall device resistance. g) The evaporated contacts' log-scale IV output curve. 3% electrode stretching decreases the current by more than twice, and 7% electrode stretching breaks the device. h) Bending without slipping on the test apparatus for the strain. i) PL spectrum at various tension strains. j) Eg has a modulation efficiency of 125 meV/% and a bandgap modulation of 193 meV (red line). k) Large slippage may happen under the tensile strain as a result of the weak vdW force between PVA and MoS2. l) The PL spectrum at various tension strains. m) Linear fitting (red line) shows the bandgap modulation Eg of 90 meV with a modulation efficiency of 61 meV/%. The reduced Eg and slope show that the strain transfer from the substrate to MoS2 is ineffective.
Figure 3. a) The strain-insensitive Gr electrode is schematically depicted. b) Kirigami design schematics that were selected for use in regards to sensing applications. A picture of the actual device is shown in the inset. c) The center island and the surrounding notches were modeled using the finite element analysis at a 196% uniaxial strain. d & e) Phosphatebuffered saline-gated FET sensing and photodetection schematics. f) The fabrication procedures for vertically aligned and kirigami-patterned 2D PtSe2 layers are schematically shown on a flexible PI substrate. g) The current can vary up to 2000% in a 2D PtSe2/PI kirigami that has been optimized for it. h) A comparison of the two-terminal I-V transport properties of the same kirigami at different strain levels. i) Images captured using a camera of the kirigami in g) and h) after being manually stretched up to 2000%. j) The stretchable conductor is a 2D PtSe2/PI kirigami in this schematic for an LED continuity circuit. k) How to light an LED is demonstrated using a 2D PtSe2/PI kirigami that has been stretched to 0 and 1500%. l) Images from a finite element method simulation that shows the spatial distribution of the in-plane principal strain in a kirigami pattern at different stretch levels. m) IR images of a 2D PtTe2/PI kirigami heater and the associated Tmax vs. strain rate plots for different applied voltages. n) The same sample in m) after 1,000 cycles of stretching under a 70% strain showed variations in Tmax and R/R0.
Figure 4. a) A schematic representation of the device that was built using a VS2/MWCNTs electrode and a PVA-LiClO4 gel electrolyte. b) The capacitance retention at different bending angles. The inset shows the CV curves at different bending angles while scanning at 100 mV/s. c) An actual illustration of a device discharge is shown via a VNIT panel that is made up of 21 parallel red LEDs that are lit for 0, 30, and 60 seconds. Reproduced with permission from the publisher. d) Bilayer SnO2/WTe2resistive switching devices are schematically depicted as being positioned between metal electrodes made from Ag and Au. The electrical connections' measurement geometry for the memristive behavior is also displayed. e) The optical image of the memristive device is indicated by the yellow boxes. f) The bilayer memory devices over the flexible PET substrate demonstrate high stability for 150 consecutive cycles. g) Integrated circuits with flexible transistor arrays are shown. An epitaxy-grown 4-inch monolayer MoS2 wafer can be seen in the inset image on the left. The right inset displays the precise structure of the flexible MoS2 FETs. h) A representation of a human wrist completely covered in very large and flexible MoS2 transistor arrays with 1,518 transistors per cm2. The inset magnifies FET arrays. i) A device's on/off ratio and charge-carrier mobility were examined after it underwent 103cycles of bending and releasing tests. The data was measured at a 1% strain. Inset image shows flexible objects under a 1% strain. j) A flexible WSe2 solar cell schematic. k) The device's qualitative energy band diagram. l) A layout for bending as an example. The polyimide substrate is attached to an 8 mm metal cylinder, which results in a 4 mm radius bend. 1 cm is the scale bar. m) The JV properties of a typical flexible WSe2 solar cell that has been flattened and bent.
Figure 5. a) A schematic for a piezotronic sensor that can power itself. b) A piezotronic sensor based on α-In2Se3 is affixed to the skin in order to monitor the arterial pulse and breath. c) Tracking the signals from the arterial pulse in real-time. The measured transient arterial pulse signals are on the left. The signals zoomed are on the right. d) An illustration of the testing pulse's schematic. e) The corresponding current readings made while the pulse was being tested. f) An illustration that shows the general layout of the mechanical sensor's circuitry. g) This diagram illustrates the current output peaks and the relationship between the applied forces and the current outputs for various applied forces. h) Schematic of the fabricated MoS2 pads with an Au electrode positioned in the center and a PI substrate at the bottom. i) Images of two MoS2 pads that were fastened to the subjects' forearms in both flat and bending positions for the purpose of taking their ECGs. Double-sided tape is used in order to secure the pads to the skin. Conductive carbon oil joins a copper wire to the Au electrode for the signal transmission. The MoS2 pads' recorded ECG signals from volunteers while they are j) unwinding, k) exercising, and l) bending their arms. The variation of the cardiac electrical activity of the heart is clearly identified by examining the P, Q, R, S, and T waves in the enlarged signal curve from 5 to 5.8 seconds. m) The temperature sensor's metal micro heater is schematically shown. n) Images taken using an optical system, which match the magnified active area to the heater and sensor and the entire structure. o) The response of the sensor to a heater pulse. p) The transient heater current is increased.
Figure 6. a) An illustration that shows how the PENG device is bent and released. b) The device's output voltages were 0.36% under constant strain. c) Variations in the device's opencircuit voltages with respect to the strain. d) Visualization of the flexible device. e) An asymmetric contact InSe nanosheet-based photodetector device schematic. f) An InSe self-powered photo detector's photo switching characteristics under various tensile strains were investigated (650 nm, 0.368 mW/cm2). g) The photocurrent enhancement ratio in relation to the amount of tensile stain that was used. h) SBH changes ϕs as a result of the applied tensile strain.
Figure 7. a) A monolayer WSe2 structure with a dielectric rod and a nanogap single-photon source is shown in the schematic. b) The schematic X-axis energy bandgap diagram is shown in (A). The WSe2 bandgap strain is designed in order to nanogap the potential well that is created at the nanogap site. Excitons that are generated by the optics flow into the potential well and combine there in order to create a localized emission. c) Schematics of the nanogapinduced deformation of the monolayer of WSe2. There is a saddle-shaped deformation for the narrow (wide) nanogap along the x- (y-) axis. The elongation direction coincides with the exciton oscillation. d) The PL spectra of the strained monolayer WSe2 were calculated as a function of the temperature and wavelength. The temperature ranged from 4 to 20 K, and the pump's power was 12.1 µW. e) The highest peak in (A) was measured for the peak intensity, which is shown in black, and spectral line width, which is shown in blue, as a function of temperature At a wavelength of 758 nm. The intensity suppression and line width broadening were observed above 20 K. f) A cross-section of a WSe2 monolayer that is placed on top of a grid of SiOx pillars is schematically shown on the left, and a quasi-1D localized exciton that was photographed using the strain-induced potential at the edge of the SiOx pillar while being illuminated with a laser is shown on the right. The arrow indicates the direction of the oscillating exciton dipole, and the pillar edge's one-dimensional strain-induced potential is parallel to it. g) An elastomeric substrate with a vertical vdW heterobilayer is schematically shown. h) The mechanically pre-stretched elastomeric substrate is released, which results in a wrinkled heterostructure with a heterogeneous strain profile of the alternate tension, which is at the crest, and the compression, which is at the valley. i) MoS2/WSe2 heterobilayer image that was captured using optical microscopy with an estimated twist angle of 50.7°. The scale bar is 10 µm. j) An MoS2/WSe2 heterobilayer with wrinkles, which is seen using optical microscopy. The scale bar is 10 µm. k) The flat monolayers of MoS2, WSe2, and MoS2/WSe2 have different PL spectra.
Figure 8. a) Schematic of a p-n device. b) WS2 monolayer induced compressive in-plane strain calculation and the measurement setup schematic illustration. c) EL spectra. d) Position of the EL peak and energy shift in relation to the amount of the applied force. e) Two-point bending schematic, the applied strain calculation method R is the bending radius, l is the substrate half-thickness, SP is the screw posts, and TS is the transition stages. f) Monolayer MoS2Raman spectra with strains that range from 0% to 1.2% g) Raman spectra of WSe2monolayers with an increasing strain. h) Development of the E2g -1and A1g peaks in the monolayers of MoS2 and WSe2, and the E2g -1peaks in the monolayers of those two materials, which is in response to the applied tensile strains.
总结与展望
柔性二维及其异质结构电子器件有可能成为一个新的技术平台，只要不断努力解决晶圆规模和低成本生产以及多功能和可靠设备运行方面的困难。为了实现这一目标，除了客户定制的电气和机械性能外，还必须寻求薄、透明、灵活和抗裂设备的商业应用。最有前途的应用领域涉及人类集成的可穿戴技术，它可以无线通信数据和监测日常生理数据。这一类别还包括灵活的移动设备显示器和高速晶体管。这些应用包括集成逻辑晶体管和无线电路、多用途传感器以及能源转换和存储设备，取决于材料合成和制造技术的不断发展。因此，如果柔性设备要在日常生活中得到更成功的应用，必须进一步改进集成技术。
一个具有高灵敏度和直接制造工艺的传感器，生产起来很简单。然而，正在进行的研究是使用各种工艺方法，以改善这些传感器的可重复性，这是由于传感器的可重复性差。目前正在研究在压力和触觉传感器中使用纳米和微结构阵列，目的是在不需要改变材料本身的情况下提高传感器在灵敏度和滞后性方面的性能。然而，当两个或更多的刺激同时作用时，对刺激的辨别的研究还不够。此外，由于耐用性差和大规模生产的挑战，也有限制。因此，人们正在研究具有高耐久性的建筑材料，这些材料可以利用不同形状的微观结构进行大量生产。由导电材料制成的可穿戴设备打算使用纺织品结构来生产。纤维和织物产品有各种形状，尽管有人体运动或形状变化带来的压力，它们仍有很高的伸展性和强度。它们确实有一个缺陷，那就是它们容易受到外部污染和潮湿的影响。因此，目前正在进行一项调查，开发一种即使放在极端环境中也能保持高度耐久性的结构。
另一方面，为了利用这些纳米管所具有的各向异性的特性，排列的 CNT 已经在生产智能和灵活的传感器方面得到了广泛的应用。这些灵活的传感器无法像人类皮肤那样伸展，这对持续和实时的健康监测没有用。因此，创造可负担得起、使用简单、对人体舒适的可穿戴设备对于持续监测心电图信号以进行安全测量至关重要。由于新的制造技术和新型材料的使用，灵活和可弯曲的生物医学电子装置现在已经成为可能。通过整合贴片式传感器，将有可能以较低的成本收集临床上准确的病人数据，然后将其发送到现有的医疗监测系统或基于云的系统进行即时分析。患者对他们将获得基于数据的个性化治疗的信心将增加，因为医疗专业人士可以更快做出治疗决定。预计电子皮肤、可穿戴技术和医疗监测在未来都将利用由改进的研究产生的人机界面。
文献链接
Latest Innovations in Two-dimensional Flexible Nanoelectronics
https://doi.org/10.1002/adma.202301280
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