graphene
At this point the fabrication proceeds with the deposition of the oxide layer which allows the realization of a back gate exploiting the underlying metallic metasurface for the tuning of the graphene Fermi level.
The fabrication steps mentioned in this section have not performed di- rectly by me. For this reason they are not described in great detail. Graphene has been grown and transferred by Dr. V. Misekis and Dr. N. Mishra from CNI@Nest (Istituto Italiano di Tecnologia , IIT). Sil- icon dioxide sputtering has been carried out by Dr. L. Viti (Istituto
3 Fabrication of the hybrid graphene-SRRs metasurface
Nanoscienze, CNR) and Dr. L. Baldacci (Scuola Superiore Sant’Anna, Institute of Life Sciences).
a)
b)
c)
Figure 3.4: SEM images of the hybrid graphene-SRRs metasurface. The image a), b) and c) are obtained using an magnifications of 50x, 100x and 200x. In a) and c) the contrast between the metasurface covered and uncov- ered by graphene is shown. In b) the SRRs are only visible under the graphene layer. The WD, the accelerating voltage and the beam current are 4.1 mm, 5 kV and 90 pA, respectively. The unit cell of the observed metasurface is 50 µm.
A 200 nm thick layer of SiO2 is deposited by magnetron sputtering in argon (Ar) atmosphere. The magnetron sputtering is a method of Phys- ical Vapour Deposition (PVD), in particular a plasma coating process whereby sputtering material is ejected due to bombardment of ions on the target surface. The vacuum chamber of the PVD coating machine is filled with an inert gas, in this case Ar. By applying a high voltage, a glow discharge is created, resulting in acceleration of ions to the tar- get surface and a plasma coating. The argon-ions will eject sputtering materials from the target surface (sputtering), resulting in a sputtered coating layer on the sample in front of the target. Magnetic fields keep the plasma in front of the target, intensifying the bombardment of ions. 104
Once the SiO2 layer is realized, the process for the transfer of a 4×4 mm2 graphene flake, grown on a separate host Cu foil by low-pressure chemical vapour deposition (CVD) using methane as a carbon precursor [77], has been performed. The technique used for this process is the so-called wet transfer. The graphene grown on the Cu substrate is spin coated with PMMA; then, the Cu substrate is removed using a wet etching and only the bilayer composed by PMMA and graphene remains. At this point the bilayer is put on water surface and it is directly fished from water using the desired final substrate. After the evaporation of water from the sample, the substrate with graphene is immersed in ACE to remove the layer of PMMA. The sample is then rinsed with IPA and dryed with nitrogen gas.
Examples of the final samples are shown in Figure 3.4 where three SEM images for a metasurface with the unit cell of 50 µm are reported. Each image has been obtained with different values of the magnification. The net contrast between the metasurface covered and not covered by graphene can be seen in images Figure 3.4.a and Figure 3.4.c. In the region with- out graphene the effect of charging is evident from the deformed image of the array of SRRs. In Figure 3.4.b, instead, a portion of the metasurface completely covered by graphene is shown.
of the device
This chapter is dedicated to the description of the spectroscopic mea- surements of the fabricated samples. They are all realized using Fourier Transform Infrared (FTIR) spectroscopy. For each sample, the measure- ments have been performed at three different levels of the fabrication process: after the fabrication of the metallic metasurface, after the depo- sition of the oxide layer and after the transfer of graphene. In addiction Raman spectroscopic measurements are performed in order to study the properties (in particular the presence of defects and the Fermi level) of the transferred graphene.
4.1 Experimental set up
A FTIR spectrometer [78] is composed by three fundamental part: a broadband source, an interferometer (most commonly a Michelson inter- ferometer) and a detector.
In THz range, our FTIR (Nicolet) is equipped by a globar lamp, that emits in a spectral range from 50 cm−1 to at least the near-IR. However, the emitted signal decreases strongly passing to low frequencies; below approximatly 3 THz the intensity of emitted radiation cannot be distin- guished from the background noise at room temperature.
The used interferometer is in Michelson configuration. In a Michelson interferometer (see Figure 4.1) a collimated beam of light coming from a source is split into two parts of equal intensity by a beamsplitter (BMS). One part of the beam is transmitted, continues to propagate in the same direction and then impinges onto a moving mirror M1, the other part instead is reflected in the orthogonal direction and impinges onto a fixed mirror M2. After the two beams have been reflected from the two mir- rors, they impinge onto the BMS a second time, they recombine on the other side of the BMS with respect to mirror M2 and are finally directed to the detector. Initially, the two mirrors are at the same distance from
4 Spectroscopic measurements of the device
Figure 4.1: Schematic of the Michelson interferometer with the He-Ne laser running co-axial. S = source, BMS = beamsplitter, M = mirror, D = detector, L = laser.
the BMS, implying the same optical path for both the reflected and trans- mitted beams. Moving the mirror M1 an optical path difference (OPD) is introduced to generate interferometric fringes. The used beamsplitter and detector are a silicon state substrate and a DTGS detector1, respec- tively. Due to the optical properties of these last two components, the upper limit of the spectral range for the measurements is about 600 cm−1 (about 18 THz).
The measurements are performed using the FTIR spectrometer in rapid- scan mode. In this mode, the moving mirror in the interferometer scans forward and backward in a rapid continuous fashion. Inside the FTIR spectrometer a Helium-Neon (HeNe) laser beam is used to create an in- terferogram. The HeNe fringes (generated by an OPD exactly equal to
1The deuterated-triglycine sulfate detector is a very sensitive room-temperature de-
tector for the spectral range 700 - 50 cm−1. As the temperature and hence po-
larizability of the ferroelectric crystals change (due to the absorption of infrared radiation) a charge is generated which is detected by two parallel electrodes. The deuterated crystals are used because they have a higher Curie point.
1/2-HeNe wavelength) allow to trigger the spectrometer electronics in or- der to simultaneously digitize the infrared light intensity registered at the detector. As a result of this process one obtains the digital interferogram of the whole infrared signal, i.e. a digital plot of the intensity versus the optical path difference. A digital Fourier transform applied to these data allows to obtain a spectrum of intensity versus wavenumber (cm−1). In a transmission measurement like in our case, there are actually two exper- iments that are carried out sequentially. The first is performed without putting the sample in front of the beam in order to obtain the spectrum of the source. The second instead is realized with the presence of the sample. By normalizing the last spectrum with that of the source, the desired transmission spectrum of the sample is achieved.