Stephen P. Stagon

Functionalized aligned silver nanorod arrays for glucose sensing through surface enhanced Raman scattering

Glucose detection through surface enhanced Raman scattering (SERS) has recently attracted a lot of interest due to its potential as a minimally-invasive, in vivo sensing technology. However, the application of SERS to glucose detection is greatly limited because of its small Raman scattering cross-section and low affinity with bare metal surfaces. In this work, an active SERS substrate composed of nearly aligned silver nanorods with uniform distribution was fabricated using high vacuum electron beam physical vapor deposition. A monolayer of 4-mercaptophenylboronic acid (MPBA) was self-assembled on the Ag nanorod surfaces, through the covalent interaction between its thio group and the Ag surface, thus resulting in a functional SERS substrate for glucose detection. The results from X-ray photoelectron spectroscopy and Raman spectroscopy clearly indicate that MPBA was successfully functionalized on the Ag nanorod surfaces. The specific binding of glucose with the boronic acid motif in MPBA significantly affects the SERS signal of MPBA on Ag nanorods, which can be measured and correlated to glucose concentrations. Quantitative detection of glucose in a clinically relevant (0–20 mM) concentration range was successfully demonstrated. The fundamental mechanism behind this approach was also discussed, and both electromagnetic and chemical enhancement mechanisms are attributed to the enhanced SERS signals. These results provide new insights into the development of SERS-based glucose sensors using Ag nanorod arrays.

Anomaly of film porosity dependence on deposition rate

This letter reports an anomaly of film porosity dependence on deposition rate during physical vapor deposition – the porosity increases as deposition rate decreases. Using glancing angle deposition of Cu on SiO2 substrate, the authors show that the Cu film consists of well separated nanorods when the deposition rate is 1 nm/s, and that the Cu films consists of a more uniform film when the deposition rate is 6 nm/s; all other deposition conditions remain the same. This anomaly is the result of interplay among substrate non-wetting, density of Cu nuclei on the substrate, and the minimum diameter of nanorods.

Enhanced thermal stability of Ag nanorods through capping

Ag nanorods may serve as sensors in the detection of trace amounts of chemical agents, even single molecules, through surface enhanced Raman spectroscopy (SERS). However, thermal coarsening of Ag nanorods near room temperature limits their applications. This letter proposes the use of a thin oxide capping layer to enhance the thermal stability of Ag nanorods beyond 100 °C. Using electron microscopy characterization and SERS tests, the authors show that the proposed method is effective in stabilizing both morphology and sensitivity of Ag nanorods. The results of this work extend the applicability of Ag nanorods as chemical sensors to higher temperatures.

Airtight metallic sealing at room temperature under small mechanical pressure

Metallic seals can be resistant to air leakage, resistant to degradation under heat, and capable of carrying mechanical loads. Various technologies – such as organic solar cells and organic light emitting diodes – need, at least benefit from, such metallic seals. However, these technologies involve polymeric materials and can tolerate neither the high-temperature nor the high-pressure processes of conventional metallic sealing. Recent progress in nanorod growth opens the door to metallic sealing for these technologies. Here, we report a process of metallic sealing using small well-separated Ag nanorods; the process is at room temperature, under a small mechanical pressure of 9.0 MPa, and also in ambient. The metallic seals have an air leak rate of 1.1 × 10−3 cm3atm/m2/day, and a mechanical shear strength higher than 8.9 MPa. This leak rate meets the requirements of organic solar cells and organic light emitting diodes.

Combined Hydrophobicity and Mechanical Durability through Surface Nanoengineering

This paper reports combined hydrophobicity and mechanical durability through the nanoscale engineering of surfaces in the form of nanorod-polymer composites. Specifically, the hydrophobicity derives from nanoscale features of mechanically hard ZnO nanorods and the mechanical durability derives from the composite structure of a hard ZnO nanorod core and soft polymer shell. Experimental characterization correlates the morphology of the nanoengineered surfaces with the combined hydrophobicity and mechanical durability, and reveals the responsible mechanisms. Such surfaces may find use in applications, such as boat hulls, that benefit from hydrophobicity and require mechanical durability.

Control of Separation and Diameter of Ag Nanorods through Self-organized Seeds

This paper proposes a mechanism of controlling the diameter and separation of metallic nanorods from physical vapor deposition through self-organized seeds and experimentally demonstrates the feasibility using Ag as the prototype metal, In as the seed, and Si the substrate. Being non-wetting on Si substrates, deposited In atoms self-organize into islands. Subsequently deposited Ag atoms attach to In islands, rather than to Si substrates, due to preferential bonding and geometrical shadowing. The experimental results show that self-organized In seeds of 5 nm nominal thickness give rise to the best separation and the smallest diameter of Ag nanorods.

Degradation Mechanism of Ag Nanorods for Surface Enhanced Raman Spectroscopy

This paper reports a degradation mechanism of silver (Ag) nanorods that are used as substrates for surface enhanced Raman spectroscopy (SERS). The attachment of sulfur and hydrocarbons to the surfaces of Ag nanorods is observed when they are stored in ambient over four months. This attachment is observed to correlate with ~20% decrease in SERS signal. The attachment, and thereby the signal degradation, takes three weeks to complete, and remains stable after the initial decay over the rest of the four month test period. While this degradation mechanism is a limitation to the gross enhancement, the ensuing stability beyond three weeks is encouraging.

Effects of adhesion layer on Ag nanorod growth mode and morphology using glancing angle physical vapor deposition

This letter reports on the transition from a non-wetting to an effectively wetting growth mode of silver (Ag) nanorods when an adhesion layer is used during glancing angle physical vapor deposition growth. When deposited onto a silicon substrate without an adhesion layer, Ag nanorods grow from partially interconnected non-wetting islands with diameters of ∼100 nm, although many connect with their neighbors due to small rod-to-rod spacing. When a 1 nm thick Cr adhesion layer is used, which is shown not to completely coat the substrate, the growth mode becomes effectively wetting through the coalescence of closely spaced nuclei, and both Ag nanorod diameter and spacing increase. Alternatively, when a thicker 10 nm Cr adhesion layer is used, the growth mode becomes mixed, as both small effective wetting regions and film gaps exist. For the cases of no adhesion layer and 1 nm Cr adhesion layer, the nanorods are oriented at ∼23° from the substrate but lay down onto the substrate when a 10 nm thick Cr adhesion ...