Flexible Inorganic Electronics Research Articles

Laser-driven noncontact bubble transfer printing via a hydrogel composite stamp

Transfer printing that enables heterogeneous integration of materials into spatially organized, functional arrangements is essential for developing unconventional electronic systems. Here, we report a laser-driven noncontact bubble transfer printing via a hydrogel composite stamp, which features a circular reservoir filled with hydrogel inside a stamp body and encapsulated by a laser absorption layer and an adhesion layer. This composite structure of stamp provides a reversible thermal controlled adhesion in a rapid manner through the liquid-gas phase transition of water in the hydrogel. The ultrasoft nature of hydrogel minimizes the influence of preload on the pick-up performance, which offers a strong interfacial adhesion under a small preload for a reliable damage-free pick-up. The strong light-matter interaction at the interface induces a liquid-gas phase transition to form a bulge on the stamp surface, which eliminates the interfacial adhesion for a successful noncontact printing. Demonstrations of noncontact transfer printing of microscale Si platelets onto various challenging nonadhesive surfaces (e.g., glass, key, wrench, steel sphere, dry petal, droplet) in two-dimensional or three-dimensional layouts illustrate the unusual capabilities for deterministic assembly to develop unconventional electronic systems such as flexible inorganic electronics, curved electronics, and micro-LED display.

Nonlinear stretching mechanics of planar Archimedean-spiral interconnects for flexible electronics

Stretchable island-bridge structures are widely used in flexible inorganic electronics. The recently proposed Archimedean-spiral interconnects are good candidates for achieving a high level of stretchability. The optimal design of the Archimedean spiral depends on systematic exploration of the geometric variables, which is still lacking in theoretical fundamentals. In this work, a theoretical model for the mechanical responses of Archimedean-spiral interconnects under the in-plane stretching is developed based on the finite deformation plane-strain beam theory. The key parameters of the mechanical responses including the effective tensile stress, maximum strain and deformed configurations are analytically derived and validated by finite element analysis for a wide range of geometric variables. The theoretical and numerical results demonstrate that the stress–strain responses and elastic stretchability can be well tailored by three nondimensionless variables. Quantitative comparison shows that the Archimedean-spiral interconnects may have certain advantage over the widely-adopted serpentine interconnects. The results obtained in this work are beneficial for facilitating ultra-stretchable spiral interconnects for future stretchable electronics.

Continuous Liquid Metal Printed 2D Transparent Conductive Oxide Superlattices

Abstract2D conducting metal oxides offer unprecedented control of thin film electrostatics at the nanoscale. A scalable, rapid, and low‐cost approach is presented to printing transparent conductive oxides (TCOs) via spontaneous low‐temperature Cabrera‐Mott oxidation of compliant liquid metals. Repeating heterostructures of these 2D oxide layers are exploited to produce an exceptional, 100× increase in conductivity while simultaneously raising the visible range optical transmittance. This innovative approach employs defect modulation doping at the type I heterojunction between InOx/GaOx, exceeding the achievable performance with ITO, which is otherwise limited by poor dopant activation at low temperatures. The exceptional performance of these multilayer 2D TCO superlattices exceeds that of competing TCOs printed from sol‐gels and nanoparticles, establishing a 100× faster process to fabricate flexible inorganic electronics.

Review on property regulation of semiconducting materials in flexible electronics

Flexible electronics technology plays an important role in regulating the properties of semiconducting materials, leading to the breakthrough in traditional strain engineering that is limited by the rigid and brittle inorganic materials and the fixed strain values. Thereby, the relevant research not only provides a new clue for strain regulation of semiconductor materials or other functional materials, but also lays a theoretical foundation for the performance evaluation of stretchable and flexible electronic devices based on inorganic functional materials in large-deformation environments. In this paper, the research progress of flexible inorganic electronics and strain effects on band structures, especially the property regulation of semiconducting materials in flexible electronics, is reviewed. Firstly, the nano-diamond particles based thinning process and the transfer printing are emphatically expounded with their influence on the properties of semiconducting electronics explored. In addition, the development and application of strain effect on band structure in recent years are introduced. In particular, the strain control based on buckling GaAs nanoribbon and buckling quantum well structure are studied to demonstrate the superior advantage of flexible electronics technology in the property regulation of semiconducting materials. The application and developing trend of strain engineering in the future are prospected finally.

Programmable and scalable transfer printing with high reliability and efficiency for flexible inorganic electronics.

Transfer printing that enables heterogeneous integration of materials in desired layouts offers unprecedented opportunities for developing high-performance unconventional electronic systems. However, large-area integration of ultrathin and delicate functional micro-objects with high yields in a programmable fashion still remains as a great challenge. Here, we present a simple, cost-effective, yet robust transfer printing technique via a shape-conformal stamp with actively actuated surface microstructures for programmable and scalable transfer printing with high reliability and efficiency. The shape-conformal stamp features the polymeric backing and commercially available adhesive layer with embedded expandable microspheres. Upon external thermal stimuli, the embedded microspheres expand to form surface microstructures and yield weak adhesion for reliable release. Systematic experimental and computational studies reveal the fundamental aspects of the extraordinary adhesion switchability of stamp. Demonstrations of this protocol in deterministic assemblies of diverse challenging inorganic micro-objects illustrate its extraordinary capabilities in transfer printing for developing high-performance flexible inorganic electronics.

Recent Advances on Thermal Management of Flexible Inorganic Electronics.

Flexible inorganic electronic devices (FIEDs) consisting of functional inorganic components on a soft polymer substrate have enabled many novel applications such as epidermal electronics and wearable electronics, which cannot be realized through conventional rigid electronics. The low thermal dissipation capacity of the soft polymer substrate of FIEDs demands proper thermal management to reduce the undesired thermal influences. The biointegrated applications of FIEDs pose even more stringent requirements on thermal management due to the sensitive nature of biological tissues to temperature. In this review, we take microscale inorganic light-emitting diodes (μ-ILEDs) as an example of functional components to summarize the recent advances on thermal management of FIEDs including thermal analysis, thermo-mechanical analysis and thermal designs of FIEDs with and without biological tissues. These results are very helpful to understand the underlying heat transfer mechanism and provide design guidelines to optimize FIEDs in practical applications.

Low-temperature growth of Three dimensional ReS2/ReO2 metal-semiconductor heterojunctions on Graphene/polyimide film for enhanced hydrogen evolution reaction

Flexible inorganic electronics (FIE) have shown unique advantages in energy conversion, aerospace and wearable devices due to excellent electronic properties and thermostability of inorganic materials. The industrialization of high-performance flexible inorganic electronics (FIE) devices requires universal approaches to fabricate inorganic crystal on polymer substrates at acceptable temperature. Herein, we firstly developed the vortex flow chemical vapor deposition (VFCVD) for low temperature synthesis of high-quality vertical ReO2 arrays at 450 °C on flexible graphene-polyimide (G-PI) conductive film. The Euler equations suggest that the vapor pressure of ReO2 is almost 100-times higher than that of free space with VFCVD at identical conditions. The derived ReS2/ReO2 metal-semiconductor heterojunction arrays are performed outstanding hydrogen evolution reaction (HER) activity with high long-term stability as an energy conversion device. This work opens up an opportunity for low temperature growth of inorganic nanomaterials on polymer substrates by VFCVD for the industrialization of high-performance flexible energy devices.

Self-powered flexible electronics beyond thermal limits

Self-powered flexible electronics using high-performance inorganic materials have been studied and developed for the essence of future electronics due to the thing, lightweight, self-sustainable, and biocompatible characteristics, which can be applied to body sensor network and next generation Internet of Things (IoT). However, most of inorganic materials should be processed in the high-temperature processes such as the semiconductor fabrication, which is not compatible flexible plastic substrates. Therefore, the new approaches must be demonstrated to overcome the thermal limits of previous methodology and achieve the flexible inorganic electronics on various flexible plastic substrates. In this review paper, we introduce the recent progress of technologies to realize flexible and high-performance inorganic electronics on plastic substrates over the thermal limits, i.e., laser-assisted procedure, chemical or mechanical exfoliation approaches. They are compatible not only to flexible plastic substrates but also to conventional device processes. We also explain the novel application devices such as flexible optoelectronics, flexible large-scale integration (LSI) devices, flexible energy harvesters, and flexible sensors using the recent-developed technologies beyond the previous thermal limit. This paper highlights the proper direction to complete future flexible inorganic electronics for high-performance self-powered systems.

Review on flexible photonics/electronics integrated devices and fabrication strategy

In recent years, to meet the greater demand for next generation electronic devices that are transplantable, lightweight and portable, flexible and large-scale integrated electronics attract much more attention have been of interest in both industry and academia. Organic electronics and stretchable inorganic electronics are the two major branches of flexible electronics. With the semiconductive and flexible properties of the organic semiconductor materials, flexible organic electronics have become a mainstay of our technology. Compared to organic electronics, stretchable and flexible inorganic electronics are fabricated via mechanical design with inorganic electronic components on flexible substrates, which have stretchability and flexibility to enable very large deformations without degradation of performance. This review summarizes the recent progress on fabrication strategies, such as hydrodynamic organic nanowire printing and inkjet-assisted nanotransfer printing of flexible organic electronics, and screen printing, soft lithography and transfer printing of flexible inorganic electronics. In addition, this review considers large-scale organic and inorganic flexible electronic systems and the future applications of flexible and stretchable electronics.

Vertical ZnO Nanotube Transistor on a Graphene Film for Flexible Inorganic Electronics.

The bottom-up integration of a 1D-2D hybrid semiconductor nanostructure into a vertical field-effect transistor (VFET) for use in flexible inorganic electronics is reported. Zinc oxide (ZnO) nanotubes on graphene film is used as an example. The VFET is fabricated by growing position- and dimension-controlled single crystal ZnO nanotubes vertically on a large graphene film. The graphene film, which acts as the substrate, provides a bottom electrical contact to the nanotubes. Due to the high quality of the single crystal ZnO nanotubes and the unique 1D device structure, the fabricated VFET exhibits excellent electrical characteristics. For example, it has a small subthreshold swing of 110 mV dec-1 , a high Imax /Imin ratio of 106 , and a transconductance of 170 nS µm-1 . The electrical characteristics of the nanotube VFETs are validated using 3D transport simulations. Furthermore, the nanotube VFETs fabricated on graphene films can be easily transferred onto flexible plastic substrates. The resulting components are reliable, exhibit high performance, and do not degrade significantly during testing.

Self-powered flexible inorganic electronic system

Self-powered flexible inorganic electronic systems have been demonstrated to be the core of next-generation electronics due to their lightweight, thin, self-sustainable and biocompatible properties, which are applicable to cramped or corrugated surfaces. Many researchers have studied myriad approaches for high performance flexible electronics, e.g. energy harvesters, batteries, high-density memories, large-scale integration (LSI), light-emitting diodes (LEDs), and sensors. Moreover, innovative devices for in vivo biomedical applications have been demonstrated on curvilinear and isolated regions of human bodies for detecting or even treating diseases. This paper reviews recent advances in self-powered flexible inorganic electronics, covering material selection, mechanical design, fabrication method and their all-in-one integration on a single plastic substrate.

Room to High Temperature Measurements of Flexible SOI FinFETs With Sub-20-nm Fins

We report the temperature dependence of the core electrical parameters and transport characteristics of a flexible version of fin field-effect transistor (FinFET) on silicon-on-insulator (SOI) with sub-20-nm wide fins and high-k/metal gate-stacks. For the first time, we characterize them from room to high temperature (150 °C) to show the impact of temperature variation on drain current, gate leakage current, and transconductance. Variation of extracted parameters, such as low-field mobility, subthreshold swing, threshold voltage, and ON-OFF current characteristics, is reported too. Direct comparison is made to a rigid version of the SOI FinFETs. The mobility degradation with temperature is mainly caused by phonon scattering mechanism. The overall excellent devices performance at high temperature after release is outlined proving the suitability of truly high-performance flexible inorganic electronics with such advanced architecture.

Slip zone model for interfacial failures of stiff film/soft substrate composite system in flexible electronics

Delicate designs of flexible inorganic electronics are based on the composite structure of the stiff film/soft substrate system, which substantially relieves stresses in the stiff inorganic functional units and utilizes the soft substrate to realize flexibility and stretchability. Resulted from the large mismatch of the mechanical properties of each layer in such system, the interface between the stiff electronic film and the soft substrate suffers serious stress concentration so that interfacial failures become more dangerous than material cracks during operating. The existing approaches for interfacial failures do not work well to explain the diverse failure modes, nor give the detailed stress evolution as the increasing of applied loading. In this paper, we employ the slip zone model in composite structure and strength theory to acquire the stress evolution, and then fundamentally and quantitatively explain the interfacial failure behaviors of such flexible electronic devices. The consistence between our theoretical prediction and the experimental results gives adequate supports to our cohesive constitutive relations in the slip zone. The criteria to determine the failure modes have also been established, which are important to guide the design and evaluation of flexible electronic devices.

Review on stretchable and flexible inorganic electronics

In order to meet the further demand of the next-generation electronic devices in the transplantable, lightweight and portable performances, flexible and stretchable inorganic electronics attract much more attention in both industry and academia in recent years. Compared to organic electronics, stretchable and flexible inorganic electronics are fabricated with the integrated structures of inorganic components on complaint substrates, which own the stretchability and flexibility via mechanical design. Thus stretchable and flexible inorganic electronics have the high electron mobility and excellent conformability to non-planar environment subjected to large deformation. This paper reviews the recent progress on principle, design based on mechanics, integration based on transfer printing and the reliability analysis of stretchable and flexible inorganic electronics. Finally, the prospective is also described for future application in bioengineering and medicine.

Interfacial Failure in Flexible Electronic Devices

Flexible inorganic electronics with the delicate integration of stiff electronic films on soft substrate have attracted much attention recently. For the large mismatch of mechanical properties between the soft substrate and stiff electronic films in such devices, interfacial failures are always extremely dangerous. In this letter, we present theoretical analysis for the interfacial failures based on fracture mechanics and establish the criteria for the failure modes. It is found that the thin films always slip first and then transit to delamination from the substrate as the increasing of applied loading. The theoretical prediction is consistent with the experiment, which can guide the design and evaluate the reliability of flexible electronics.