Simulated and Experimental Studies of Size Effects on Plasmonic Gas Sensing with Nanohole Arrays
Authors: Sun Libin , Zhao Yangyang , Dong Chaobo , Benkstein Kurt D. , Semancik Steve , Zaghloul Mona E. 
Published: May 2020

Introduction

In the last decade, plasmonic sensors have drawn enormous attention. They have been widely applied in the biochemical and chemical gas sensing field due to their attractive properties including low cost, room-temperature operation, high sensitivity, and near real-time response [1]-[4]. A nanohole array (NHA) plasmonic sensor platform coated with copper benzenetricarboxylate (Cu-BTC) metal-organic framework (MOF), has been successfully developed in our previous work [4], which demonstrated capabilities for detecting and quantifying volatile organic compounds (VOCs), such as acetone and ethanol, at low concentration in the 500 nmol/mol (parts-per-billion (ppb)) range. Sensitivity is one of the critical factors for sensing in both research and industrial settings. Here, we investigated the effect of reduced feature size in NHA sensors for enhancing gas sensing performance, especially sensitivity. During experimental measurements, peak shifts of the localized surface plasmon resonance (LSPR), as measured by a spectrometer, are attributed to adsorption and desorption processes of the target gases molecules in the MOF layer. These processes cause changes at the interface of the metal and dielectric layers proximal to the NHA area. As the refractive index of the MOF layer is changed by the adsorption of the targeted gas molecules, a stronger E-field surrounding NHA areas can lead to larger wavelength and intensity shifts at the peak position of LSPR, which would improve sensitivity.

Simulations

The strategy we implement for higher sensitivity is to enhance the electromagnetic field surrounding nanohole areas by reducing the diameter of the nanoholes. To verify the feasibility of this approach, we performed simulations using finite difference time domain solutions. Figure 1 shows the NHA configuration in top/cross-sectional views. In the simulation, we chose Si3N4 as the substrate and included a NHA of 50 nm gold holes above it. A 15 nm adsorption layer simulates the MOF layer. To simulate the adsorption and desorption processes, we swept the refractive index (1.0 to 1.2) in the gas-containing MOF layer. Next, we set the incident light wavelength from 400 nm to 800 nm. A frequency-domain-field monitor was set at the Au/Si3N4 interface layer to measure the E-field. Another monitor was placed above the adsorption layer to detect the wavelength and intensity shifts.

Fabrication and Measurements

Advances with nanofabrication technology tools, such as E-Beam Lithography (EBL) and Ion Milling (in place of Stepper or Focused Ion Beam Lithography) gave us opportunities to fabricate and explore a large scale NHA (40000 µm2) for our sensor, with the nanohole diameter smaller than 200 nm, at a reasonable process cost. First, a 50 nm thick Au layer was deposited over the Si/Si3N4 substrate. Second, the sample was coated with 280 nm of photoresist, to pattern the NHA structure with EBL. Ion Milling was used to etch through the Au layer to form nanoholes. A 15 nm Cu-BTC MOF layer was added as an adsorption layer. Figure 1 (c) shows an SEM image of the NHA with the diameter/period of 160 nm/400 nm. The chemical sensing experiments were performed by exposing NHA sensors with different hole diameters to varied concentrations of methanol, from (2 to 20) µmol/mol (ppm), at room temperature. The sensing system is composed of our NHA sensors and an optical microscope integrated with a spectrometer, as previously reported [4].

Results and Conclusions

Figure 2 (a) shows the simulated distribution of the E-field surrounding the nanohole area at the peak wavelength. The simulation results for the E-field intensity of the NHA with different diameters from 10 nm to 300 nm and fixed period equal to 400 nm is shown in Figure 2 (b). As can be seen, the E-field reaches the strongest point with the nanohole diameter at 160 nm, and is about twice as strong as the previous NHA design [4] with a diameter of 200 nm. As shown in Figure 3 (a), we swept the refractive index of the MOF layer from 1.0 to 1.2 (step size of 0.05) to simulate the gas adsorption process, and the simulated monitor can detect shifts of both peak wavelength and intensity. For comparison, we fixed the refractive index change of the MOF layer and varied the NHA diameters, and the responses are shown in Figure 3 (b). The shifts of wavelength and normalized intensity both show the highest value when the nanohole diameter is 160 nm. Figure 4 (a) shows the response of the NHA with diameter of 160 nm to methanol. To examine the sensitivity of the NHA sensors with diameters of 160 nm and 200 nm, we plot the normalized intensity shift versus varied concentrations ranging from 2 µmol/mol to 20 µmol/mol of methanol, as presented in Figure 4 (b). The slope and intensity shifts of the sensor with diameter of 160 nm are larger than for a diameter of 200 nm. These simulations and experiments show that the NHA sensor with a diameter of 160 nm provides higher sensitivity.

References

[1] C. Genet and T. W. Ebbesen, "Light in tiny holes," Nature, vol. 445, (7123), pp. 39-46, 2007. Available:http://dx.doi.org/10.1038/nature05350. DOI: 10.1038/nature05350.
[2] J. R. Mejía-Salazar and J. Oliveira Osvaldo N, "Plasmonic Biosensing," Chemical Reviews, vol. 118, (20), pp. 10617-10625, 2018. Available: https://www.ncbi.nlm.nih.gov/pubmed/30247025. DOI: 10.1021/acs.chemrev.8b00359.
[3]. A. G. Brolo, "Plasmonics for future biosensors," Nature Photonics, vol. 6, (11), pp. 709-713, 2012. Available: https://doi.org/10.1038/nphoton.2012.266. DOI: 10.1038/nphoton.2012.266.
[4] Y. Zhao et al, "Miniaturized Nanohole Array Based Plasmonic Sensor for the Detection of Acetone and Ethanol with Insights into the Kinetics of Adsorptive Plasmonic Sensing," Nanoscale, 2019.

Figure 1