The as high as 8.9 × 1014 cm?2

The advent of two-dimensional (2-D) graphene, single atomic thick layer of graphite, has stimulated the research interest in other 2-D materials. Transition-metal dichalcogenides (TMDs) are among them. Recently, 2D materials have demonstrated a promising material from the perspective of both fundamentals as well as applications. They exhibit unique optoelectronic properties, which can be largely tuned by varying the thickness. More recently, thermal chemical vapor deposition (CVD) or HFCVD have been developed for the growth of these 2D materials. Microwave measurements have been performed in microstrip line based test fixture where first real and imaginary parts of complex relative permittivity have been obtained, then the loss tangent and absorption coefficients of bilayer and single layer graphene are extracted. Lower absorption in bilayer graphene is 0.6 dB/cm at 26.5 GHz, this led to slightly lower optical effective mass of photons than in single layer graphene. We envision that reported results can help to understand the dielectric and absorption properties of as-grown graphene layer in microwave frequency range. These studies aim to utilize the 2D materials as transparent conducting electrode in Si heterojunction solar cell.


 1         Introduction   


Recently, few layer graphene (FLG) is reported to exhibit carrier mobility as high as 105 cm2/V.s at room temperature 1, controllable foil resistance from 100 to 8.8 ?/sq (ohms per square), foil carrier density as high as 8.9 × 1014 cm?2 and carrier mean free path as large as ~ 0.6 ?m 2, along with impressive flexibility and controllable conductivity. In addition, graphene is considered as a massless and gapless Dirac quasi-particle system with rough linear energy dispersion 3. These outstanding properties of graphene have made it a potential candidate for application in sensors to terahertz (THz) devices. In addition, the characteristic of graphene has been explored for the microwave absorption and electromagnetic shielding abilities. Different materials like polymer microcellular foams, polyaniline nanorod arrays, nanoplatelet-epoxy etc. with graphene particles have been studied 4–7 and demonstrated interesting electromagnetic absorption. Such composite improves the electromagnetic wave absorption through additional relaxation processes, namely dielectric, dipole, and polarization relaxations. However, the absorption property of graphene layer is unexplored and so the present paper deals with determining the graphene absorption property in 10 MHz – 26.5 GHz frequency range. Such information could be useful for the microwave isolation of components developed using the multilayer graphene and related microwave absorption applications.


2         Graphene Samples


In the present study, single layer graphene transferred on both glass and quartz of thickness 0.7 mm and 1.0 mm, respectively of area 1²×1² were used which was procured from Graphene Industries, USA. Bilayer graphene was grown by using Hot Filament Chemical Vapor Deposition (HFCVD) System. Commercially available Cu foil with a thickness of 25 mm were cut into 1/2²×1/4² pieces to be used as substrate. Initially, Cu foil was ultrasonicated in acetone for 10 min, acetic acid for 10 min, and then washed with DI water. After drying the foil, it was placed in the chamber. Initially, bilayer graphene sample on Cu foil was developed by heating upto 850°C from the room temperature (RT) 27°C along with flow of H2 at 50 sccm and pressure of 35 torr. After 10 minutes, CH4 at a flow rate of 10 sccm and pressure of 35 torr was supplied to the processing chamber for next 10 minutes along with H2. Then temperature is cooled down to the RT with flow of H2. These bilayer graphene samples on Cu foil are designated as BLG/Cu, whereas the procured graphene samples are designated as SLG/glass and SLG/quartz, respectively. 1         Characterization of Graphene samples 3.1 Raman Spectroscopy   In the Raman spectra shown in Fig.1 for single layer graphene transferred on glass and quartz (a), three sharp peaks were indexed as G, 2D satellite and 2D bands. The peak position of various bands are found at wD ~1341.83 cm-1, wG ~1587.12 cm-1, w2Dsatelite ~2452.092 cm-1, and w2D  ~2688.67 cm-1 for graphene on glass substrate, whereas for graphene on quartz substrate, these values are at wD ~1341.83 cm-1, wG ~1587.16 cm-1, w2Dsatelite ~ 2452.73  cm-1, and w2D ~2688.82 cm-1. The insignificant D-band indicating that graphene layers are free from defects. The I2D/IG ratio is calculated as ~3.69 (on glass) and ~ 4.1 (on quartz), which has confirmed that a single layer graphene was present on both substrates. Raman spectra for bilayer graphene on Cu is shown after curve fitting in Fig. 1(b), three sharp peaks were indexed as D, G and 2D bands for as-grown graphene samples. The peak positions for BLG/Cu are found at wD ~1342 cm-1, wG ~1588 cm-1, and w2D ~ 2713 cm-1. The I2D/IG ratio is evaluated as ~0.55 for BLG/Cu, which verified that a bilayer graphene is present.3.2 Microwave characterization of graphene samples


A 5.08 cm long microstrip line was fabricated on FR-4 substrate (er = 4.4 and height= 1.5 mm). As shown in Fig. 2(a), the microstrip line with sample holder was connected between ports (A and B) of calibrated vector network analyzer model N5227A of Keysight Technologies Inc. and S-parameters were measured for unloaded microstrip line from 10 MHz to 26.5 GHz. Then microstrip line was loaded by SLG/glass, SLG/quartz and BLG/Cu one by one and then S-parameters were measured individually. Figs. 2(b) and 2(c) show the arrangements of loading on the microstrip line for single layer graphene and bilayer graphene, respectively. For the middle part of the microstrip line i.e. loaded by the sample, the S-parameters were obtained by using standard transmission line (ABCD) parameters based de-embedded method. A MATLAB program was made to obtain de-embedded S-parameters 8. For unloaded microstrip line and the ones loaded with SLG/glass and BLG/Cu, the measured complex reflection (S11) and transmission (S21) parameters have been shown in Fig. 3(a-d). As shown in Fig. 4a, input reflection coefficient (S11) is found to be high for SLG/glass and BLG/Cu loading in the lower frequency range up to 12 GHz, which may be due to confinement of electromagnetic fields into FR4 substrate. As the bilayer graphene is covered by Cu, its loading changes the effective impedance of microstrip line at lower frequency 9. At higher frequency, fields are more concentrated in FR4 substrate, so S11 for both loadings is similar to the unloaded microstrip line after 12 GHz. The phase of S11 also has random shifts at lower frequencies, whereas loading shifted the peaks of phase towards lower frequencies as observed in Fig. 4(b). Differences in the phase shifts are indicating that both graphene samples have different material properties at the microwave frequencies. In Fig. 3(c-d), the magnitudes of forward (S21) transmission parameters are found to be decreased as frequency increased, due to the lossy nature of microstrip line. Compared to unloaded microstrip line, lower values of S21 for SLG/glass and BLG/Cu, indicate loss of transmission power, which is more with BLG/Cu (> 12 dB) upto 20 GHz. The peak values of S21 phase is found to be shifted towards lower frequencies with loadings in Fig. 4 (d), which is in result of dielectric kind of loading. In addition, high values > -15 dB of S11 suggest that unloaded microstrip line turned to be more reflecting at frequencies above 10 GHz.