Metal and semiconductor nanowires (NWs) become the core components of electronics. Integration of NWs involves the use of electrically driven Joule heating, which is not site-selective. In addition, Joule heating increases the complexity during the fabrication of nanoelectronics. Researchers at Seoul National University, Korea, University of Michigan, USA, University of California, Berkeley, USA, Ajou University, Korea, Hanyang University, Korea and Kyungpook National University, Korea have demonstrated a selective laser-induced hydrothermal growth (LIHG) process for integrating hierarchical heterogeneous nanowire-on-nanowire structure which can be used on-demand without the need for conventional photolithography or vacuum deposition. They have also fabricated an all-nanowire UV sensor using this methodology.
Single-crystalline Ag NW (length: ∼300 μm; diameter: >200 nm) was prepared by modified polyol synthesis. The Ag NW was deposited on a clean glass substrate with a help of a fluidic channel and post-treated at 150 °C for 30 min to completely remove the polyol on the surface of Ag NW. Subsequently the Ag NW coated glass was wetted with ZnO quantum dot (QD) seed solution (prepared by mixing 10 mM Zn(OAc)2 in 60 mL of ethanol with 30 mM NaOH in 30 mL of ethanol and heating the mixture at 60 °C for 2 h). Then, the ZnO seeded Ag NW coated glass was immersed in ZnO precursor solution (prepared by mixing 25 mM Zn(NO3)2·6H2O, 25 mM hexamethylenetetramine, 5−7 mM polyethylenimine and 100 mL of deionized water and, heating the mixture at 95 °C for 1 h) and subjected to LIGH process (Fig. 1).
Fig. 1 Schematic of the LIHG process with hybrid background heating; Nd:YAG laser (532 nm) is focused at a specific spot on the ZnO seeded Ag NW coated glass immersed in ZnO precursor solution. As the temperature within a confined region (at the center of the laser focused area) rises above the threshold temperature, growth of ZnO NW is initiated and the growth continues only within the laser heated spot.
Scanning electron micrograph of the hierarchical ZnO NW branched on a Ag NW backbone that is suspended on an etched Si substrate (Fig. 2(a)) confirms selective growth of ZnO NW on Ag NW by the LIHG process. The micrograph of ZnO NW grown after 10 min of laser irradiation possesses a crystalline structure with a hexagonal cross section (Fig. 2(b)), which is further confirmed by transmission electron microscopy (inset of Fig.2(b)).
Fig. 2 (a) SEM image of selective growth of ZnO NW branches on Ag NW suspended on an etched Si substrate by LIGH process; (b) Magnified view of ZnO NW array with a hexagonal cross section (Inset: TEM image)
Scanning electron micrographs of ZnO NW grown at different polarization angles of 45°, 60°, and 90° on Ag NW at 1 W laser power for 6 min (Fig. 3) indicate that size of the secondary ZnO NW branch array is increased as the laser polarization becomes perpendicular to the Ag NW and the lateral size can be elongated up to 2.5 μm.
Fig. 3 SEM images of ZnO NW arrays grown on a single Ag NW at various polarization angles: (i) 45°; (ii) 60°; and (iii) 90°, after 6 min of laser irradiation
An all-nanowire UV sensor is fabricated by placing two Ag NWs adjacent to each other so that the ZnO NW grown from them can be connected as a photoconductive channel network. Electrical pads are attached at each end by laser sintering (Fig. 4(a)). The photocurrent measured by switching UV illumination under 0.1 V bias indicates that the current is around 0.7 nA, which is much higher than the dark current of <0.3 nA (Fig. 4(b)). In spite of a small on/off ratio, the rise in photocurrent and decay time are relatively fast when compared to other UV sensors with a similar configuration.
Fig. 4 (a) Schematic illustration of the fabrication of all-nanowire UV sensor; and (b) Photocurrent measurement with switching UV illumination (0.1 V bias).
The LIGH process seems promising for the bottom-up fabrication of next-generation all-nanowire electronics and multifunctional environmental sensors.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Habeom Lee et al., Nanowire-on-Nanowire: All-Nanowire Electronics by On-Demand Selective Integration of Hierarchical Heterogeneous Nanowires, ACS Nano, Article ASAP DOI: 10.1021/acsnano.7b06098
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