Development of a Surface-Plasmon Resonance Sensor Testbed for Bimetallic Sensors

Babu, Roshni S.; Colenso, Hamish R.; Gouws, Gideon; Auguié, Baptiste; Moore, Ciaran P.

doi:10.1109/i2mtc.2019.8827029

Cited by 4 publications

(11 citation statements)

References 26 publications

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“…However, wide adoption of Ag sensors has been prevented because Ag is unstable, and therefore an inert capping layer must be placed on top of the Ag thin film to protect it from contamination. However, the bilayer combination of these two materials is interesting as the sensing performance of Ag could be combined with the inert reliability of Au [52][53][54][55][56].…”

Section: Spr Sensorsmentioning

confidence: 99%

“…The creation of SPR sensors using Ag has been explored with dielectric capping layers and thin capping layers made of inert plasmonic Au [52][53][54][55][56]. The capping of Ag has also been explored with materials other than Au, such as graphene oxide (GO) [57], barium titanate (BaTiO 3 ) [58], zinc oxide (ZnO) [59], and titanium dioxide (TiO 2 ) [60].…”

Section: Spr Sensorsmentioning

confidence: 99%

“…However, the prism-coupled Kretschmann configuration is used for the modelled sensor performance calculated in this work [55,56].…”

Section: Monochromaticmentioning

confidence: 99%

“…For a sodium chloride (NaCl) analyte, it has been shown that a 20 nm Ag film capped with a 10 nm Au film can protect the Ag from degradation and improve the sensitivity when compared to a sensor consisting of a single Au film [55]. It has also been shown that the incidence angle chosen for sensing can influence the results [78].…”

Section: Monochromaticmentioning

confidence: 99%

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Silver Surfaces and Nanostructures for Imaging and Sensing Applications.

Colenso

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Silver is an exciting material in nanofabrication. It exhibits the capability to couple surface plasmons and enhance evanescent modes used in imaging and sensing. However, it is also extremely difficult to fabricate into the desired device structures. Silver initially deposits not as a uniform thin film but as individual islands that alter its plasmonic performance. Even when uniform silver thin films are deposited, they corrode readily in air and dewet from the substrates they are deposited on. Techniques typically used to create silver nanostructures also use halogenated plasmas. These halogenated plasmas generate halide ions, which are undesirable for people or the environment. In this thesis, the performance of thin silver films protected by inert plasmonic capping layers was modelled. The models indicated that 40 nm silver films capped by 8 nm gold films and 20 nm silver films capped by 10 nm platinum films were appealing for use as sensors. The models indicated that a 10 nm silver film capped by an 11 nm gold film and a 10 nm silver film capped by a 15 nm platinum film was appealing for use as lenses. The models indicated that silver layers approaching 10 nm thickness were desirable for these plasmonic bilayer devices and informed the fabrication work that followed.Silver thin films, of approximately 10 nm thickness, were deposited on amorphous borofloat substrates. The as-deposited films had an RMS roughness of 2.0 nm and a peak roughness of 26.1 nm. However, the silver was deposited in Volmer-Weber islands. To form a continuous film, the as-deposited films were subjected to thermal anneals between 200 °C and 350 °C. The anneal increased both the surface roughness and grain size of the thin films. The anneal also changed the optical transmission spectrum of the thin films. It was found the use of a seed layer is preferred for silver thin films of approximately 10 nm thickness. The risk of contaminating the silver during transfer between process steps outweighs any benefit obtained from an anneal.Silver nanostructures were fabricated using a combination of laser interference lithography and reactive ion etching. A dual exposure interference lithography process was used to create a periodic hole array in a photoresist film. A dual etch reactive ion etch process was developed using helium, argon, and oxygen gases for etching back anti-reflective coating. A plasma containing a halogenated gas mixture typically removes the anti-reflective coating. Removing the halogenated plasma from the effluent gas stream demonstrates an exciting proof-of-concept of an environmentally friendly anti-reflective coating etch.It was found for 1000 nm period nanostructures that the duty cycle ratio of the pattern changed by 0.1 as a result of the etch. A 0.1 duty cycle ratio change indicates approximately 50 nm of material was removed in all horizontal directions from the photoresist pattern, enlarging the photoresist hole. At the same time, the etch removes 110 nm of the back anti-reflective coating, revealing the substrate. Arrays of 700 nm period, 0.50 duty cycle ratio silver nanostructures were reliably created using this process. These analytical and experimental results contribute to our understanding of silver nanostructures and their behaviour in optical devices.

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Section: Spr Sensorsmentioning

confidence: 99%

Section: Spr Sensorsmentioning

confidence: 99%

“…However, the prism-coupled Kretschmann configuration is used for the modelled sensor performance calculated in this work [55,56].…”

Section: Monochromaticmentioning

confidence: 99%

Section: Monochromaticmentioning

confidence: 99%

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Silver Surfaces and Nanostructures for Imaging and Sensing Applications.

Colenso

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“…Figure 6.7 (a) Schematic of Ag-Au bimetallic SPR sensor, and (b) colour mapped reflectivity spectral data of bimetallic Ag-Au SPR sensors obtained at respective resonant angles[200]. The colour map shows that a near to zero reflectivity is obtained at an overall thickness of around 50 nm.…”

mentioning

confidence: 99%

Bimetallic structures to enhance the performance of surface plasmon resonance sensors

Babu¹

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Surface plasmon resonance (SPR) sensing is a label−free and rapid detection method and has extensive applications in the field of medical diagnostics, food control, and environmental monitoring. However, the lack of sensitivity to detect small molecules is a continuing concern in the application of this technique. Past research has explored different plasmonic structures such as metal nanoparticles, metallic nanoslits, nanoholes, colloidal Au nanoparticles, 2D nanomaterials, and multilayer structures as the sensing layer to improve the sensitivity of these sensors. However, the sensitivity improvement could be realised only with the cost of the increased complexity of optical configuration and sensor chip fabrication. Silver (Ag) is a very good candidate as the metallic layer for the sensor chip due to its higher electrical conductivity as compared to gold (Au). Besides cost−effectiveness, Ag thin film based sensors have better sensitivity with a sharp resonance dip and a high signal−to−noise ratio. However, the poor chemical stability of Ag thin films prevents their use in practical applications. Noble metals such as Au and platinum (Pt) offer greatly enhanced chemical stability. This work investigated the development of SPR sensors composed of a silver−noble metal bilayer structure to utilize both the sensitivity of silver and the chemical stability of the noble metal. To enable this research, an automated experimental SPR testbed for sensor characterisation was designed and constructed. This testbed is based on the Kretschmann configuration, using a He−Ne laser source at 632.8 nm. SPR sensor consisting of multilayer metal structures was fabricated using standard microelectronic fabrication techniques. The influence of the relative thickness of a noble metal capping layer on the SPR response and sensitivity from the Ag layer was systematically optimised, using both theoretical and experimental approaches. A theoretical analysis of the performance of the bimetallic SPR sensors was done using the transfer matrix method (TMM) by assuming a five−layer configuration. In the case of an Au capping layer, these simulations indicate an optimised thickness of 45 nm for Ag and 5 nm for Au. The observation from experimental analysis of different thickness combinations of Ag and Au matched the simulated results. However, the results of the stability studies exclude the practical use of 45 nm Ag/5 nm Au structures, as long−term degradation of the Ag layer occurs. A structure of 40 nm Ag/10nm Au was thus selected as the best composition for sensor applications. It is showed that sensors fabricated with this structure showed enhanced sensitivity compared to single−layer Au sensors, with a sensitivity 50% higher than that of the single−layer Au sensor. In the case of Ag/Pt structures, simulations indicated enhanced sensitivity from a 10 nm Ag/16 nm Pt structure. However, experimental measurements did not show any evidence for SPP excitation of Pt at the measured wavelength of 632.8 nm, making it unsuitable as a capping layer in our studies. The application of 40 nm Ag/10 nm Au bimetal layers as biosensors was done by the immobilization of thiol−terminated vitamin B12 aptamers on the Au sensor surface. However, the results were not reproducible, and more work on the binding kinetics of this aptamer will need to be performed to use this in a biosensor structure.

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