Alpha 300r raman microscope

Raman measurements were carried out using a WiTec alpha 300R Raman microscope connected to a 532 nm laser. A 600 g/mm grating was used, which provided a spectral resolution of around 2.3 cm⁻¹. An EMCCD camera (1600 × 200 pixels, 16 μm pixel size, Andor Newton) was used for detection of the scattered photons. The EMCCD gain was set at a numerical value of 250. The pre-amplifier gain value was set to 1. For high spatial as well as depth resolution, a 100 × objective (Zeiss EC ‘Epiplan-Neofluar’ DIC, numerical aperture (NA) = 0.9) was chosen. The laser power at the sample was measured using an optical power meter (ThorLabs). Raman maps are made by integrating the area under the band of interest for every pixel: from 1550 to 1750 cm⁻¹ (G band) for GO and graphene, 2600–2800 cm⁻¹ for the G′ peak of graphene, 253–400 cm⁻¹ for WS₂, 1330–1390 cm⁻¹ for BN, 350–427 cm⁻¹ for MoS₂ and 2800–3000 cm⁻¹ for PAA.

CRM analysis was performed using an Alpha300R Raman microscope (WiTec, Ulm, Germany) equipped with 532 nm laser source. To avoid photo damage, the power was set to 10 mW at the samples, corresponding to a laser density of 7.54 mW µm⁻². A 600 lines/mm grating was selected, and the back scattered light was collected on a back illuminated deep depletion CCD detector over the spectral range 0–3600 cm⁻¹ with a spectral resolution ~5 cm⁻¹. Spectra were collected using a 20 × objective (Zeiss EC-Epiplan-NEOFLUAR, NA = 0.5, spot size ~1.3 µm) and the acquisition time was set to 30 s × 2 accumulations. For each disc, 20 maps were performed across the SC, to account for spatial heterogeneity. For each map, 9 spectra (3-by-3) were collected using a 4 µm step size, resulting in 180 spectra recorded per sample and a total of 1440 spectra for the study. The instrument is calibrated daily using a 2-step procedure. Firstly, the True Cal function of Project 5 (WITec, Ulm, Germany) is used. It is an automatic multipoint calibration routine performed with a mercury–argon (HgAr) light source integrated in the Raman microscope. Secondly, prior to data acquisition, a verification was done using the peak at 520.7 cm⁻¹ from a silicon substrate.

For Raman measurements, the samples were placed in the gas-tight optical cell. The Raman spectra was measured using a WITec alpha300 R—Raman microscope with a 514 nm laser. In the experiments, the position of the G-band of graphene is traced as a function of applied bias (see Supplementary Fig. 5). The shift in the G band (Δω_G) reveals the Fermi energy of electrons in graphene and the charge carrier concentration (n) via the following relations^{38 (link)}. For electrons, E_F [meV] = 21 Δω_G + 75[cm⁻¹], for holes, E_F [meV] = −18 Δω_G − 83[cm⁻¹]. Both fittings are consistent with our data within the experimental scatter. For both type of carriers, their density is given by n[cm⁻²] = (E_F/11.65)² x 10¹⁰. We fit the data with the formula E_F = ħv_F $\sqrt{\mathrm{\pi }n\left(V\right)}$ , which $n\left(V\right)$ is gate tuneable carrier density ( $n$ ) as a function of bias ( $V$ ). The relationship between bias ( $V$ ) and charge carrier density ( $n$ )^{18 (link)} is given by $∣V-NP∣=\frac{\hslash {\nu }_F\sqrt{\pi n}}{e}+\frac{ne}{C}$ with the fitting parameters C ≈ 3.6 μF cm⁻², the gate capacitance and NP = -0.18 V, the neutrality point.