Abstract
This paper reports the design of Ag-Al2O3-Ag heterojunctions based on Ag nanorods (AgNRs) and their applications as thermally stable and ultrasensitive substrates of surface-enhanced Raman scattering (SERS). Specifically, an ultrathin Al2O3 capping layer of 10 nm on top of AgNRs serves to slow down the surface diffusion of Ag at high temperatures. Then, an additional Ag layer on top of the capping layer creates AgNRs-Al2O3-Ag heterojunctions, which lead to giant enhancement of electromagnetic fields within the Al2O3 gap regions that could boost the SERS enhancement. As a result of this design, the SERS substrates are thermally stable up to 200 °C, which has been increased by more than 100 °C compared with bare AgNRs, and their sensitivity is about 400% that of pure AgNRs. This easy yet effective capping approach offers a pathway to fabricate ultrasensitive, thermally stable and easily prepared SERS sensors, and to extend SERS applications for high-temperature detections, such as monitoring in situ the molecule reorientation process upon annealing. Such simultaneous achievement of thermal stability and SERS sensitivity represents a great advance in the design of SERS sensors and will inspire the fabrication of novel hetero-nanostructures.
Highlights
Surface-enhanced Raman scattering (SERS) is the foundation of a powerful spectroscopic technique for rapid and non-destructive determination of chemical [1], environmental [2,3] and biological [4] analytes at trace levels, even at the level of a single molecule [5]
While the nanoscale dimension gives rise to high sensitivity, it leads to thermal instability because nanostructures coarsen at elevated temperatures [8,9]
Ag nanorods (AgNRs) array and Ag colloids coarsen so much that they fuse at a temperature as low as 50 ◦C [10,11], limiting their practical surface-enhanced Raman scattering (SERS) applications, such as monitoring in situ the thermal crystallization and the catalysis process
Summary
Surface-enhanced Raman scattering (SERS) is the foundation of a powerful spectroscopic technique for rapid and non-destructive determination of chemical [1], environmental [2,3] and biological [4] analytes at trace levels, even at the level of a single molecule [5]. Aiming to preserve the high sensitivity and yet to achieve thermal stability of SERS substrates, we have recently proposed and demonstrated the capping of AgNRs using high-melting temperature Al2O3 [12]. The SERS efficiency of AgNRs-Al2O3 substrates with 6 nm to 12 nm capping declined moderately with Al2O3 deposition. By depositing 10 nm or thicker Al2O3 onto AgNRs, the substrates exhibited no discernible morphology variation at 200 ◦C. The total nominal deposition thickness of Al2O3 we used in this work is 10 nm [12]. The evaporation target is switched to Ag again in the chamber, the incident angle is kept at 86◦, and the deposition rate is decreased to 0.3 nm/s for better coverage of the Al2O3. Characterizations of AgNRs-Al2O3-Ag Substrates – Morphology, Structure, and SERS. The data collection time for each spectrum is set to be five seconds and each SERS spectrum is obtained by measuring and averaging the signals collected from five different spots on a substrate
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