Near-zero-adhesion-enabled intact wafer-scale resist-transfer printing for high-fidelity nanofabrication on arbitrary substrates

There is an urgent need for novel processes that can integrate different functional nanostructures onto specific substrates,so as to meet the fast-growing need for broad applications in nanoelectronics,nanophotonics,and fexible optoelectronics.Existing direct-lithography methods are difficult to use on fexible,nonplanar,and biocompatible surfaces.Therefore,this fabrication is usually accomplished by nanotransfer printing.However,large-scale integration of multiscale nanostructures with unconventional substrates remains challenging because fabrication yields and quality are often limited by the resolution,uniformity,adhesivity,and integrity of the nanostructures formed by direct transfer.Here,we proposed a resist-based transfer strategy enabled by near-zero adhesion,which was achieved by molecular modification to attain a critical surface energy interval.This approach enabled the intact transfer of wafer-scale,ultrathin-resist nanofilms onto arbitrary substrates with mitigated cracking and wrinkling,thereby facilitating the in situ fabrication of nanostructures for functional devices.Applying this approach,fabrication of three-dimensional-stacked multilayer structures with enhanced functionalities,nanoplasmonic structures with~10 nm resolution,and MoS2-based devices with excellent performance was demonstrated on specific substrates.These results collectively demonstrated the high stability,reliability,and throughput of our strategy for optical and electronic device applications.

Advancements in transfer printing techniques for flexible electronics:adjusting interfaces and promoting versatility

The burgeoning interest in flexible electronics necessitates the creation of patterning technology specifically tailored for flexible substrates and complex surface morphologies.Among a variety of patterning techniques,transfer printing emerges as one of the most efficient,cost-effective,and scalable methods.It boasts the ability for high-throughput fabrication of 0–3D micro-and nano-structures on flexible substrates,working in tandem with traditional lithography methods.This review highlights the critical issue of transfer printing:the flawless transfer of devices during the pick-up and printing process.We encapsulate recent advancements in numerous transfer printing techniques,with a particular emphasis on strategies to control adhesion forces at the substrate/device/stamp interfaces.These strategies are employed to meet the requirements of competing fractures for successful pick-up and print processes.The mechanism,advantages,disadvantages,and typical applications of each transfer printing technique will be thoroughly discussed.The conclusion section provides design guidelines and probes potential directions for future advancements.

Switchable dry adhesive based on shape memory polymer with hemispherical indenters for transfer printing

Transfer printing based on switchable adhesive is essential for developing unconventional systems,including flexible electronics,stretchable electronics,and micro light-emitting diode(LED)displays.Here we report a design of switchable dry adhesive based on shape memory polymer(SMP)with hemispherical indenters,which offers a continuously tunable and reversible adhesion through the combination of the preloading effect and the thermal actuation of SMP.Experimental and numerical studies reveal the fundamental aspects of design,fabrication,and operation of the switchable dry adhesive.Demonstrations of this adhesive concept in transfer printing of flat objects(e.g.,silicon wafers),three-dimensional(3D)objects(e.g.,stainless steel balls),and rough objects(e.g.,frosted glasses)in two-dimensional(2D)or 3D layouts illustrate its unusual manipulation capabilities in heterogeneous material integration applications.

Properties of Novel Reactive Transfer Printing of Silk

The novel reactive transfer printing of silk was carried out through a hot-press adhesion and steaming. The special transfer paper was prepared by coating the paste mainly containing hot-melt adhesive hlgh-substituted hydroxypropyl cellulose （H-HPC） and printing thickener earboxymethyl cellulose （CMC）. The effects of each ingredient in the paste on color yield of the prints and dye penetration were investigated. The major results indicate that, color yield is chiefly governed by the adhesion extent imparted by H-HPC, the type of fixing alkaline agent, and the content of urea. Trichloroacetic acid （TCAA） as the fixing alkaline agent and adding 5% urea can enhance the color depth obviously. Dye penetration depends on the coating quantity on the transfer paper, the contents of urea and dicyandiamide. The printed silk possesses a higher color yield, color fastness of grade 3 or above, clear sharpness, and good handle when the paste contains 3 % H-HPC, 0. 7 % CMC, 3 % TCAA, 5 % urea, 3 % dicyandiamide, and 0. 5 % physical sorbent nano-silica.

A thermal actuated switchable dry adhesive with high reversibility for transfer printing

Transfer printing based on switchable adhesive that heterogeneously integrates materials is essential to develop novel electronic systems,such as flexible electronics and micro LED displays.Here,we report a robust design of a thermal actuated switchable dry adhesive,which features a stiff sphere embedded in a thermally responsive shape memory polymer(SMP)substrate and encapsulated by an elastomeric membrane.This construct bypasses the unfavorable micro-and nano-fabrication processes and yields an adhesion switchability of over1000 by combining the peel-rate dependent effect of the elastomeric membrane and the thermal actuation of the sub-surface embedded stiff sphere.Experimental and numerical studies reveal the underlying thermal actuated mechanism and provide insights into the design and operation of the switchable adhesive.Demonstrations of this concept in stamps for transfer printing of fragile objects,such as silicon wafers,silicon chips,and inorganic micro-LED chips,onto challenging non-adhesive surfaces illustrate its potential in heterogeneous material integration applications,such as flexible electronics manufacturing and deterministic assembly.

Regulatable interfacial adhesion between stamp and ink for transfer printing

As an emerging processing technology,transfer printing enables the assembly of functional material arrays(called inks)on various substrates with micro/nanoscale resolution and has been widely used in the fabrication of flexible electronics and display systems.The critical steps in transfer printing are the ink pick-up and printing processes governed by the switching of adhesion states at the stamp/ink interface.In this review,we first introduce the history of transfer printing in terms of the transfer methods,transferred materials,and applications.Then,the fundamental characteristics of the transfer printing system and typical strategies for regulating the stamp/ink interfacial adhesion strength are summarized and exemplified.Finally,future challenges and opportunities for developing the novel stamps,inks,and substrates with intelligent adhesion capability are discussed,aiming to inspire the innovation in the design of transfer printing systems.

Highly durable machine-learned waterproof electronic glove based on low-cost thermal transfer printing for amphibious wearable applications

Gesture recording,modeling,and understanding based on a robust electronic glove(E-glove)are of great significance for efficient human-machine cooperation in harsh environments.However,such robust edge-intelligence interfaces remain challenging as existing E-gloves are limited in terms of integration,waterproofness,scalability,and interface stability between different components.Here,we report on the design,manufacturing,and application scenarios for a waterproof E-glove,which is of low cost,lightweight,and scalable for mass production,as well as environmental robustness,waterproofness,and washability.An improved neural network architecture is proposed to implement environment-adaptive learning and inference for hand gestures,which achieves an amphibious recognition accuracy of 100%in 26 categories by analyzing 2,600 hand gesture patterns.We demonstrate that the E-glove can be used for amphibious remote vehicle navigation via hand gestures,potentially opening the way for efficient human-human and human-machine cooperation in harsh environments.

Nanocellulose-based reusable liquid metal printed electronics fabricated by evaporation-induced transfer printing

Reusable electronics have received widespread attention and are urgently needed. Here, nanocellulosebased liquid metal(NC-LM) printed circuit has been fabricated by the evaporation-induced transfer printing technology. In this way, the liquid metal pattern is embedded into the nanocellulose membrane, which is beneficial for the stability of the circuit during use. Besides, the NC-LM circuit is ultrathin with just tens of microns. In particular, the finished product is environmentally friendly because it can be completely dissolved by water, and both the liquid metal ink and the nanocellulose membrane can be easily recollected and reused, thereby reducing waste and pollution to the environment. Several examples of flexible circuits have been designed to evaluate their performance. The mechanism of evaporation-induced transfer printing technology involves the deposition, aggregation, and coverage tightly of the nanosized cellulose fibrils as the water evaporated. This study provides an economical and environmentally friendly way for the fabrication of renewable flexible electronics.

Effect of Ink Molecular Weights and Annealing Conditions on Molecular Transfer Printing

The molecular transfer printing（MTP） technique has been invented to fabricate chemical patterns with high fidelity using homopolymer inks. In this work, we systematically studied the effects of the molecular weights of homopolymer inks and transfer conditions on the MTP process. We explored a large range of molecular weights（~3.5-56 kg·mol^（-1）） of hydroxyl-terminated polystyrene（PS-OH） and hydroxyl-terminated poly（methyl methacrylate）（PMMA-OH） in the MTP process, and found that the resulting chemical patterns on replicas from all five blends were functional and able to direct the assembly of films of the same blends. The transfer temperature and the film annealing sequences had an impact on the MTP process. MTP was sensitive to the transfer temperature and could only be performed within a certain temperature range, i.e. higher than the glass transition temperature（T_g） of copolymers and lower than the rearrangement temperature of the assembled domains. Pre-organization of the blend films was also necessary for MTP since the preferential wetting of PMMA domains at the replica surface might result in the formation of a PMMA wetting layer to prevent the presentation of underlying chemical patterns to the replica surface.

Mass transfer techniques for large-scaleand high-density microLED arrays

Inorganic-based micro light-emitting diodes (microLEDs) offer more fascinating properties and unique demands in next-generation displays. However, the small size of the microLED chip (1–100 µm) makes it extremely challenging for high efficiency and low cost to accurately, selectively, integrate millions of microLED chips. Recent impressive technological advances have overcome the drawbacks of traditional pick-and-place techniques when they were utilized in the assembly of microLED display, including the most broadly recognized laser lift-off technique, contact micro-transfer printing (µTP) technique, laser non-contact µTP technique, and self-assembly technique. Herein, we firstly review the key developments in mass transfer technique and highlight their potential value, covering both the state-of-the-art devices and requirements for mass transfer in the assembly of the ultra-large-area display and virtual reality glasses. We begin with the significant challenges and the brief history of mass transfer technique, and expand that mass transfer technique is composed of two major techniques, namely, the epitaxial Lift-off technique and the pick-and-place technique. The basic concept and transfer effects for each representative epitaxial Lift-off and pick-and-place technique in mass transfer are then overviewed separately. Finally, the potential challenges and future research directions of mass transfer are discussed.

A single micro-LED manipulation system based on micro-gripper

Micro-LEDs(μLEDs)have advantages in terms of brightness,power consumption,and response speed.In addition,they can also be used as micro-sensors implanted in the body via flexible electronic skin.One of the key techniques involved in the fabrication ofμLED-based devices is transfer printing.Although numerous methods have been proposed for transfer printing,improving the yield ofμLED arrays is still a formidable task.In this paper,we propose a novel method for improving the yield ofμLED arrays transferred by the stamping method,using an innovative design of piezoelectrically driven asymmetric micro-gripper.Traditional grippers are too large to manipulateμLEDs,and therefore two micro-sized cantilevers are added at the gripper tips.AμLED manipulation system is constructed based on the micro-gripper together with a three-dimensional positioning system.Experimental results using this system show that it can be used successfully to manipulateμLED arrays.

Conformal manufacturing of soft deformable sensors on the curved surface

Health monitoring of structures and people requires the integration of sensors and devices on various 3D curvilinear,hierarchically structured,and even dynamically changing surfaces.Therefore,it is highly desirable to explore conformal manufacturing techniques to fabricate and integrate soft deformable devices on complex 3D curvilinear surfaces.Although planar fabrication methods are not directly suitable to manufacture conformal devices on 3D curvilinear surfaces,they can be combined with stretchable structures and the use of transfer printing or assembly methods to enable the device integration on 3D surfaces.Combined with functional nanomaterials,various direct printing and writing methods have also been developed to fabricate conformal electronics on curved surfaces with intimate contact even over a large area.After a brief summary of the recent advancement of the recent conformal manufacturing techniques,we also discuss the challenges and potential opportunities for future development in this burgeoning field of conformal electronics on complex 3D surfaces.

High-precision transfer-printing and integration of vertically oriented semiconductor arrays for flexible device fabrication

Flexible electronics utilizing single crystalline semiconductors typically require post-growth processes to assemble and incorporate the crystalline materials onto flexible substrates. Here we present a high-precision transfer-printing method for vertical arrays of single crystalline semiconductor materials with widely varying aspect ratios and densities enabling the assembly of arrays on flexible substrates in a vertical fashion. Complementary fabrication processes for integrating transferred arrays into flexible devices are also presented and characterized. Robust contacts to transferred silicon wire arrays are demonstrated and shown to be stable under flexing stress down to bending radii of 20 mm. The fabricated devices exhibit a reversible tactile response enabling silicon based, nonpiezoelectric, and flexible tactile sensors. The presented system leads the way towards high-throughput, manufacturable, and scalable fabrication of flexible devices.

Lithographic printing inspired in-situ transfer of MXene-based films with localized topo-electro tunability for high-performance flexible pressure sensors

MXene-based films have been intensively explored for construction of piezoresistive flexible pressure sensors owing to their excellent mechanical and electrical properties.High pressure sensitivity relies on pre-molding a flexible substrate,or regulating the micromorphology of MXene sheets,to obtain a micro-structured surface.However,the two avenues usually require complicated and time-consuming microfabrication or wet chemical processing,and are limited to non-adjustable topographicelectrical(topo-electro)properties.Herein,we propose a lithographic printing inspired in-situ transfer(LIPIT)strategy to fabricate MXene-ink films(MIFs).In LIPIT,MIFs not only inherit ridge-and-valley microstructure from paper substrate,but also achieve localized topo-electro tunability by programming ink-writing patterns and cycles.The MIF-based flexible pressure sensor with periodical topo-electro gradient exhibits remarkably boosted sensitivity in a wide sensing range(low detection limit of 0.29 Pa and working range of 100 kPa).The MIF sensor demonstrates versatile applicability in both subtle and vigorous pressuresensing fields,ranging from pulse wave extraction and machine learning-assisted surface texture recognition to piano-training glove(PT-glove)for piano learning.The LIPIT is quick,low-cost,and compatible with free ink/substrate combinations,which promises a versatile toolbox for designing functional MXene films with tailored morphological-mechanical-electrical properties for extended application scenarios.

Cost-effective fabrication of liquid metals via a laser induced graphene stamp for highly-stretchable integrated optoelectronics

Stretchable electronics have found widespread applications in various fields such as wearable electronics,soft robots,and bioelectronics.As an important promising alternative of traditional rigid conductors,liquid metals have demonstrated immense potential to provide high conductivity and stretchability for the stretchable electronic systems.However,limited by their fluidity and high surface tension,challenges remain in achieving liquid metal patterns with low-cost,high-precision,large-scale,and complex geometry.Here,a fabrication technique was proposed based on laser-induced graphene(LIG)stamps to enable liquid metal self-selectively adhere to substrates.Liquid metal patterns could thus be achieved in different designed geometries and could be transferred onto stretchable substrates.The liquid metal patterns exhibit exceptional electrical conductivity(3.24×10^(6)S/m even under 1000%strain),high stretchability(1000%strain,maximum of 2500%),small resistance changes under significant deformations(with a quality factor of 62.5 under 1000%strain),and high resolution(down to 50μm).Utilizing the patterned liquid metals,a stretchable integrated multifunctional optoelectronic system was demonstrated,encompassing a stretchable display matrix,a pressure sensor array,a wireless powering coil,and cardiovascular sensors,which further highlight the remarkable application potential of liquid metals in optoelectronic user-interaction and advanced physiological monitoring.

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude,polarization,wave vector and frequency of light.Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density.Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials.We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies.Importantly,InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering,resulting in a reduced carrier mobility thereby damping the plasmonic response.We demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers,including flexible nonlinear absorbers achieved by transferring the disks to polyimide films.Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz(THz)optics and for passive protection of sensitive electromagnetic devices.

Stick-and-play system based on interfacial adhesion control enhanced by micro/nanostructures

The evolution of membrane-type electronics has facilitated the development of stick-and-play systems,which confer diverse electrical functions to various planar or arbitrary curvilinear surfaces.The stick-and-play concept is based on the development of thin electronic devices in a printable format and their subsequent transfer to target surfaces.The development of this technology requires control of the interfacial adhesion of the electronic prints for retrieval from a carrier and transfer to the target surface.First,we discuss the transfer printing for membrane-type electronics,starting from an overview of materials available for flexible substrates,transfer printing of electronic prints for retrieval,and assembly for further integration.Second,we explain the stick-and-play concept based on fabricated membrane-type electronics;"stick" and “play"refer to the transfer of electronic devices and the performance of their electronic functions,respectively.In particular,we broadly survey various methods based on micro/nanostructures,including gecko-inspired,interlocking,cephalopod-sucker-inspired,and cilia structures,which can be employed to stick-and-play systems for enhancing interfacial adhesion with complex target surfaces under dynamic and wet conditions.Finally,we highlight the stick-and-play system application of micro/nanostructures for skin-attachable biomedical electronics,e-textiles,and environmental monitoring electronics.