3D integrated electronics with layer by layer printing of NWs

Flexible electronics has drawn increasing attention recently due to the promise the field holds for various applications such as healthcare monitoring, internet of things, robotics and prosthetics [1] - [4] . These applications require high-performance electronics on flexible substrates, which is not always feasible and practical with conventional complementary-metal-oxide-semiconductor (CMOS) technology. For that, one possible solution is to use high-mobility materials based nanostructures to fabricate high-performance electronic devices and circuits [5] , [6] . During the past two decades, rapid advances have been made in this direction with quasi-1D nanowires (NWs)/nanotubes (NTs) [6] , [7] and quasi-2D materials [8] - [10] . Although many of the techniques from CMOS technology can be adopted to realize electronics with these novel materials, conflicts exist between the requirement of high-temperature process and the thermal sensitive flexible substrate [11] , [12] . In this regard, coordinating the fabrication process for flexible electronics is one topic that requires further exploration. In this paper, we adopt a printing method to avoid the use of high temperature process. The NWs synthesized on a donor substrate were printed in a layer-by-layer manner to realize the 3D stack and their scanning electron microscope (SEM) characterizations were carried out to illustrate the performance of the NW printing. A quantitative approach has been utilized in this work to quantify the NW printing result [13] . A comparison between NW printed on the 1 st and 2 nd layer have been made to understand the feasibility of using layered 3D printing to fabricate vertical devices. This strategy, although demonstrated on rigid substrate at this stage, can be potentially used for the realization of large-area flexible electronics.