Size Effects in Single- and Few-Layer MoS2 Nanoflakes: Impact on Raman Phonons and Photoluminescence.

The high optical absorption and emission of bidimensional MoS2 are fundamental properties for optoelectronic and biodetection applications and the opportunity to retain these properties in high quality nano-sized flakes would bring further possibilities. Here, a large set of single-layer and few-layer (2-3 layers) MoS2 flakes with size in the range from 10 nm to 20 µm are obtained on sapphire by vapor deposition techniques and evaluated combining the information from the Raman phonons with photoluminescence (PL) and absorption bands. The flakes have triangular shape and are found to be progressively relaxed from the tensile strain imposed by the sapphire substrate as their size is reduced. An increasing hole doping as size decreases is deduced from the blue shift of the A1g phonon, related to charge transfer from adsorbed oxygen. No clear correlation is observed between defects density and size, therefore, doping would be favored by the preferential adsorption of oxygen at the edges of the flakes, being progressively more important as the edge/surface ratio is incremented. This hole doping also produces a shift of the PL band to higher energies, up to 60 meV. The PL intensity is not found to be correlated to the size but to the presence of defects. The trends with size for single-layer and for 2-3 layer samples are found to be similar and the synthesis method does not influence PL efficiency which remains high down to 40 nm being thus promising for nanoscale photonics.

Raman and Fluorescence Enhancement Approaches in Graphene-Based Platforms for Optical Sensing and Imaging.

The search for novel platforms and metamaterials for the enhancement of optical and particularly Raman signals is still an objective since optical techniques offer affordable, noninvasive methods with high spatial resolution and penetration depth adequate to detect and image a large variety of systems, from 2D materials to molecules in complex media and tissues. Definitely, plasmonic materials produce the most efficient enhancement through the surface-enhanced Raman scattering (SERS) process, allowing single-molecule detection, and are the most studied ones. Here we focus on less explored aspects of SERS such as the role of the inter-nanoparticle (NP) distance and the ultra-small NP size limit (down to a few nm) and on novel approaches involving graphene and graphene-related materials. The issues on reproducibility and homogeneity for the quantification of the probe molecules will also be discussed. Other light enhancement mechanisms, in particular resonant and interference Raman scatterings, as well as the platforms that allow combining several of them, are presented in this review with a special focus on the possibilities that graphene offers for the design and fabrication of novel architectures. Recent fluorescence enhancement platforms and strategies, so important for bio-detection and imaging, are reviewed as well as the relevance of graphene oxide and graphene/carbon nanodots in the field.

Raman amplification in the ultra-small limit of Ag nanoparticles on SiO2 and graphene: Size and inter-particle distance effects.

Size, shape and hot spots are crucial to optimize Raman amplification from metallic nanoparticle (NPs). The amplification from radius = 1.8 ± 0.4 nm ultra-small silver NPs was explored. Increasing NP density redshifts and widens their plasmon that, according to simulations for NPs arrays, is originated by the reduction of the interparticle distance, d, becoming remarkable for d ≤ R. Inter-particle interaction red-shifts (N130 nm) and widens (N90 nm) the standard plasmon of non-interacting spherical particles. Graphene partly delocalizes the carriers enhancing the NIR spectral weight. Raman amplification of graphene phonons is moderate and depends smoothly on d while that of Rhodamine 6G (R6G) varies almost exponentially due to their location at hotspots that depend strongly on d. The experimental correlation between amplification and plasmon position is well reproduced by simulations. The amplification originated by the ultra-small NPs is compared to that of larger particles, granular silver films with 7 < R < 15 nm grains, with similar extinction values. The amplification is found to be larger for the 1.8nm NPs due to the higher surface/volume ration that allows higher density of hot spots. It is demonstrated that Raman amplification can be efficiently increased by depositing low density layers of ultra-small NPs on top of granular films.

Breast cancer biomarker detection through the photoluminescence of epitaxial monolayer MoS2 flakes.

In this work we report on the characterization and biological functionalization of 2D MoS2 flakes, epitaxially grown on sapphire, to develop an optical biosensor for the breast cancer biomarker miRNA21. The MoS2 flakes were modified with a thiolated DNA probe complementary to the target biomarker. Based on the photoluminescence of MoS2, the hybridization events were analyzed for the target (miRNA21c) and the control non-complementary sequence (miRNA21nc). A specific redshift was observed for the hybridization with miRNA21c, but not for the control, demonstrating the biomarker recognition via PL. The homogeneity of these MoS2 platforms was verified with microscopic maps. The detailed spectroscopic analysis of the spectra reveals changes in the trion to excitation ratio, being the redshift after the hybridization ascribed to both peaks. The results demonstrate the benefits of optical biosensors based on MoS2 monolayer for future commercial devices.

Supported Ultra-Thin Alumina Membranes with Graphene as Efficient Interference Enhanced Raman Scattering Platforms for Sensing.

The detection of Raman signals from diluted molecules or biomaterials in complex media is still a challenge. Besides the widely studied Raman enhancement by nanoparticle plasmons, interference mechanisms provide an interesting option. A novel approach for amplification platforms based on supported thin alumina membranes was designed and fabricated to optimize the interference processes. The dielectric layer is the extremely thin alumina membrane itself and, its metallic aluminum support, the reflecting medium. A CVD (chemical vapor deposition) single-layer graphene is transferred on the membrane to serve as substrate to deposit the analyte. Experimental results and simulations of the interference processes were employed to determine the relevant parameters of the structure to optimize the Raman enhancement factor (E.F.). Highly homogeneous E.F. over the platform surface are obtained, typically 370 ± (5%), for membranes with ~100 nm pore depth, ~18 nm pore diameter and the complete elimination of the Al2O3 bottom barrier layer. The combined surface enhanced Raman scattering (SERS) and interference amplification is also demonstrated by depositing ultra-small silver nanoparticles. This new approach to amplify the Raman signal of analytes is easily obtained, low-cost and robust with useful enhancement factors (~400) and allows only interference or combined enhancement mechanisms, depending on the analyte requirements.