Integration of Functional Thin Films Structure by Capillary Transfer

Films with a low flexural rigidity, referred here to as “soft” films that could be made of a wide variety of materials, not only intrinsically soft materials with a low modulus such as elastomeric/polymeric materials of polydimethylsiloxane (PDMS), but also stiff/brittle materials with micro/nanoscale thickness such as ultrathin silicon membrane and metal foils, are the basis for an entire class of wearable technologies in flexible electronics/optoelectronics, biomedical devices, energy storage and conversion systems, and micro/nanoelectromechanical systems. Unfortunately, existing technologies that enable a mechanical transfer of these soft films onto receiver substrates for practical applications are based on their as-fabricated, growth or intermediate solid substrates such as glass slides, semiconductor wafers, and native metals. Besides, these processes usually require assistance of external stimuli such as heating or/and chemical etching with sophisticated structural designs and fabrications that help reduce the interfacial energy and facilitate the physical separation of films, and rely largely on trial-and-error methods, which results often in a low yield and inevitable contamination, degradation and/or damage to films. Compared with solid native substrates, the liquid phase, an intrinsically deformation-free substrate due to fluidity, provides a unique and tactful platform that helps release residual stress or/and avoid deformation mismatch with surrounding solid constraints during growth, self-assembly and fabrication of materials and structures, and is emerging as a powerful host medium in the preparation of a wide variety of functional films from two-dimensional materials, to Janus films, and to biofilms. Moreover, the fluidity of liquid would allow films to move upwards or downside the liquid substrate, which enabled a selective contact of the two film surfaces with the receiver substrates. However, the conventional transfer approaches and fundamentals of films that are deeply established in the framework of native solid substrates are not applicable for developing a scalable, fast and defect-free transfer technology toward a reliable transferring of film from liquid surface. We will report a capillary approach that enables a fast transfer of soft films from a versatile set of non-corrosive liquid environment in a defect-free manner. This capillary transfer is underpinned by a transfer front that is a dynamic interface of contact among solid receiver substrate, liquid native substrate and transfer film with a small capillary force, and can be well controlled by the moving direction (push-down or pull-up) of receiver substrates in a high speed, thereby leading to a damage and defect free film delivery with a desirable surface in contact with the receiver substrate. We demonstrate in extensive experiments, together with theoretical models and computational analysis, the capillary transfer of a versatile set of soft films with a broad material diversity of both film and liquid, thickness, surface wetting properties, and geometric patterns of soft films onto various solid substrates in a well-defined order. With a combination of the push-down and pull-up transfers, we further demonstrate the application capability of capillary transfer in the assembly of multiple-layer structures with a desirable assembly order. Our approach offers a novel, scalable route for transferring soft films of complex patterns and on-demand surface functions onto substrates, potentially useful for fabrication, assembly and patterning of film-based devices, structures and systems.