Epitaxial Aluminum Surface-Enhanced Raman Spectroscopy Substrates for Large-Scale 2D Material Characterization

Abstract

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive technique to identify vibrational fingerprints of trace analytes. However, present SERS techniques suffer from the lack of uniform, reproducible, and stable substrates to control the plasmonic hotspots in a wide spectral range. Here, we report the promising application of epitaxial aluminum films as a scalable plasmonic platform for SERS applications. To assess the uniformity of aluminum substrates, atomically thin transition metal dichalcogenide monolayers are used as the benchmark analyte due to their inherent two-dimensional homogeneity. Besides the distinctive spectral capability of aluminum in the ultraviolet (325 nm), we demonstrate that the aluminum substrates can even perform comparably with the silver counterparts made from single-crystalline colloidal silver crystals using the same SERS substrate design in the visible range (532 nm). This is unexpected from the prediction solely based on optical dielectric functions and illustrate the superior surface and interface properties of epitaxial aluminum SERS substrates.

Epitaxial aluminum plasmonics covering full visible spectrum

“…Thus, we can precisely determine the film thickness, ranging from a few nanometers, 4.4 nm (∼19 monolayers [ML]), 10.8 nm (∼47 ML), and 20.0 nm (∼87 ML) to bulk-like (∼200 nm). In Figure 1C, we show the in-plane X-ray diffraction scan performed for the Al (220) and c-sapphire (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26) peaks, confirming the expected six-fold in-plane symmetry for epitaxial growth.…”

“…Following the discussion of fundamental structural and optical properties, we now turn to the demonstration of epitaxial film-based plasmonic applications, including aluminum SERS substrate [15] and plasmonic surface lattices [22,34,58]. For the SERS study (see Supplementary material for experimental setup), we use a vertically stacked molybdenum disulfide (MoS 2 )/tungsten diselenide (WSe 2 ) heterostructures on top of the aluminum SERS substrate (nanogroove grating) as a uniform twodimensional analyte to evaluate the SERS performance ( Figure 4A).…”

“…The Raman measurements were performed in the backscattering configuration using a 532-nm solid-state laser. Using this SERS substrate design [15], large-area chemical vapor deposition-grown MoS 2 and mechanically exfoliated WSe 2 stack on Al-SERS substrate can be clearly distinguishable ( Figure 4B, D, and F) by Raman intensity mapping because of a spatially uniform Raman hot zone and atomically smooth film surface, which prevent the formation of inhomogeneous stochastic hot-spots.…”

“…However, aluminum was not previously considered as a good candidate for alternative plasmonic material [5] before the advent of high-quality aluminum nanocrystals [10,11] and epitaxial films [9,12,13] with greatly improved material properties. During the past few years, the fast development of aluminum plasmonics has attract a great deal attention not only for the expected good performance of aluminum for UV plasmonics, such as UV surface-enhanced Raman spectroscopy (UV-SERS) [14,15], plasmonic lasers [16][17][18][19][20][21][22], and deep-UV resonances [8,23], but also for its unexpected excellent performance in the visible region, including complementary metal-oxide-semiconductor (CMOS)compatible color filters [24][25][26][27][28], photocatalysis [29], nonlinear optics [30][31][32], and SERS [15,33]. Very recently, aluminum has even been found to outperform silver in some important plasmonic applications [15,34].…”

Plasmonic metal nanostructures with extremely small features: new effects, fabrication and applications

“…Resonance peaks at wavelengths of 1.5~5 μm are shifted by hundreds of nanometres and amplitude-modulated by tens of percent through gating using relatively low voltages. Combined with a large-scale fabrication approach, the fabricated films can find applications in transparent conductors, plasmon-enhanced spectroscopy, optical biosensing and electrochromic devices.Beyond the new plasmonic effects and applications mentioned above, UTMFs imply broader future applications in the fields of optoelectronics, nonlinear optics, plasmon-enhanced detection and sensing, thermal manipulation, two photon emission and plasmonic color printing [79][80][81]. 5.…”