Silver Nanorods Wrapped with Ultrathin Al2O3 Layers Exhibiting Excellent SERS Sensitivity and Outstanding SERS Stability

Abstract

Silver nanostructures have been considered as promising substrates for surface-enhanced Raman scattering (SERS) with extremely high sensitivity. The applications, however, are hindered by the facts that their morphology can be easily destroyed due to the low melting points (~100 °C) and their surfaces are readily oxidized/sulfured in air, thus losing the SERS activity. It was found that wrapping Ag nanorods with an ultrathin (~1.5 nm) but dense and amorphous Al2O3 layer by low-temperature atomic layer deposition (ALD) could make the nanorods robust in morphology up to 400 °C, and passivate completely their surfaces to stabilize the SERS activity in air, without decreasing much the SERS sensitivity. This simple strategy holds great potentials to generate highly robust and stable SERS substrates for real applications.

Site-Selective Controlled Dealloying Process of Gold-Silver Nanowire Array: a Simple Approach towards Long-Term Stability and Sensitivity Improvement of SERS Substrate

“…However, the previously reported techniques such as the use of metal oxide or inert metal coating layers only offer stability improvement at the sacrifice of SERS sensitivity1122232427282930313233. In order to truly examine the value of the proposed technique towards SERS substrate fabrication, the stability of both p-AuAg-NWs array (Fig.…”

“…We have considered the other techniques commonly applied for stability improvement of SERS substrate. For examples, atomic layer deposition (ALD) was used to coat Ag NWs array with an ultrathin TiO 2 22 or Al 2 O 3 11 coating layer, a highly stable signal can be achieved for approximately 50 days in the air for both materials. Nevertheless, it is important to note that by coating Ag NWs array with TiO 2 or Al 2 O 3 using even single ALD cycle (<1 nm thick), normalized Raman intensity of the coated substrate was calculated to be only 60% of the uncoated structure and slowly decrease to approximately 40% of its original signal after 50 days.…”

“…Even though, the use of Ag NWs in SERS applications offers great electromagnetic field enhancement especially hierarchical Ag structures, the major drawback of metallic Ag is the rapid oxidation at the surface of nanostructures. This results in a dramatic decrease of SERS activity over time1112.…”

“…Several designs of SERS structures were demonstrated using a thin oxide layer (i.e. TiO 2 , Al 2 O 3 )1122, Au shell23, and graphene coatings24. Unfortunately, a long-term stability cannot be achieved without compromising SERS sensitivity.…”

Pinhole-Containing, Subnanometer-Thick Al2O3 Shell-Coated Ag Nanorods as Practical Substrates for Quantitative Surface-Enhanced Raman Scattering

“…From our previous report, 19 we have demonstrated that the SERS enhancement dropped along with the distance between the Ag surface and analytes, so Ag NRs coated with this subnanometer-thick Al 2 O 3 layer ensured better Raman amplification compared with that of the Ag NRs with thicker Al 2 O 3 coatings. Meanwhile, although the oxide shell was ultrathin and with a few pinholes, it was robust enough to guarantee the long-term chemical stability of SERS substrates against adverse environments.…”