Abstract
In this work, we have developed a contact-printing system to efficiently transfer the bottom-up and top-down semiconductor nanowires (NWs), preserving their as-grown features with a good control over their electronic properties. In the close-loop configuration, the printing system is controlled with parameters such as contact pressure and sliding speed/stroke. Combined with the dry pre-treatment of the receiver substrate, the system prints electronic layers with high NW density (7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs), NW transfer yield and reproducibility. We observed compactly packed (~115 nm average diameters of NWs, with NW-to-NW spacing ~165 nm) and well-aligned NWs (90% with respect to the printing direction). We have theoretically and experimentally analysed the role of contact force on NW print dynamics to investigate the heterogeneous integration of ZnO and Si NWs over pre-selected areas. Moreover, the contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with Wheatstone bridge (WB) configuration on rigid and flexible substrates. The UV PDs based on the printed ensemble of NWs demonstrate high efficiency, a high photocurrent to dark current ratio (>104) and reduced thermal variations as a result of inherent self-compensation of WB arrangement. Due to statistically lesser dimensional variations in the ensemble of NWs, the UV PDs made from them have exhibited uniform response.
Highlights
Future electronics demands new ways for integration of low-power miniaturized devices over large areas and flexible substrates such as plastic, paper, fabrics, etc
The system consists of: (1) a vertical linear position motor to control the position of the donor substrate, (2) a load cell to measure the force exerted by the donor substrate when they come in contact with the receiver substrate, (3) a three-dimensional (3D) printed platform with a spring to ensure the conformal contact between donor and receiver substrate (Fig. 1, see Supplementary Movie 1), (4) an optical microscope to analyse the alignment and conformal contact between donor and receiver substrates and (5) a horizontal linear position motor to control the sliding movement of the receiver substrate during the contactprinting process
The developed system has a close-loop configuration and allows us to carry out the transfer of both bottom-up (ZnO) and top-down (Si) NWs from growth substrate to a foreign substrate, achieving (1) high transfer-yields, i.e. preserving the asgrown NW length of 10 μm, NW crystalline structure, and NW morphology, (2) high NW densities (7 and 3 NWs/μm for ZnO and Si NWs), (3) low NW-to-NW spacings (ZnO NWs: 165 nm; Si NWs: 455 nm), (4) NW integration over areas from few mm[2] to tens of cm[2] and (5) NW integration of both Si/SiO2 rigid substrate and polyimide flexible substrate
Summary
Future electronics demands new ways for integration of low-power miniaturized devices over large areas and flexible substrates such as plastic, paper, fabrics, etc In this regard, several approaches, novel materials and structures have been investigated and devices with improved performance, higher density, energy storage and sensitivity/selectivity have been obtained[1,2,3,4,5]. The smaller dimensions of NWs increase the level of integration-related challenges especially for large-area electronics on non-conventional flexible substrates[3,9,10] Considering these issues, new methods are needed to synthesize highly crystalline semiconductor NWs with uniform aspect ratios, and to assemble aligned NWs in a way that the electronic layers made from them could lead to devices having uniform response over large areas. In contrast to single NW-based devices, the García Núñez et al Microsystems & Nanoengineering (2018)4:22 statistically dimensional variations are much lower in the case of ensemble of NWs, and multi-NW-based devices have acceptable level of response uniformity over large areas
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