Transmission Properties of FeCl3-Intercalated Graphene and WS2 Thin Films for Terahertz Time-Domain Spectroscopy Applications
Time-resolved terahertz spectroscopy has become a common method both for fundamental and applied studies focused on improving the quality of human life. However, the issue of finding materials applicable in these systems is still relevant. One of the appropriate solution is 2D materials. Here, we demonstrate the transmission properties of unique graphene-based structures with iron trichloride FeCl3 dopant on glass, sapphire and Kapton polyimide film substrates that previously were not investigated in the framework of the above-described problems in near infrared and THz ranges. We also show properties of a thin tungsten disulfide WS2 film fabricated from liquid crystal solutions transferred to a polyimide and polyethylene terephthalate substrates. The introduction of impurities, the selection of structural dimensions and the use of an appropriate substrate for modified 2D layered materials allow to control the transmission of samples for both the terahertz and infrared ranges, which can be used for creation of effective modulators and components for THz spectroscopy systems.
KeywordsGraphene Layered materials Tungsten disulfide Liquid crystals Terahertz radiation Spectroscopy
Atomic force microscopy
Chemical vapour deposition
Few layer graphene
Intercalated few layer graphene
Intercalated multilayer graphene
Intercalated single layer graphene
Scanning electron microscopy
Single layer graphene
Terahertz time-domain spectroscopy
The field of terahertz time-domain broadband spectroscopy based on femtosecond near-infrared lasers has become an active research area due to its prospective application in non-destructive control , biomedicine , security systems, broadband communications  and others . Despite the promise for applications and observed use of the technology in both industry and scientific projects, there is still a marked lack of effective materials for generation, detection, filtering and modulation of THz radiation. Solid materials applicable for THz time-domain spectroscopy systems (THz-TDS) can be classified into several groups: nonlinear and semiconductor crystals, organic crystals and metamaterials, composites, and 2D materials. 2D materials present a promising solution due to their compact size and the additional possibility to control the properties by modifying the number and composition of layers, and the substrate type.
Layered materials that can be exfoliated to extract individual layers can be primarily grouped into three classes : graphene and its derivatives, chalcogenides and oxides. Graphene [6, 7, 8], molybdenum disulfide (MoS2) [9, 10], bismuth selenide Bi2Se3 , tungsten diselenide (WSe2) , tungsten disulfide (WS2)  and different devices based on layered heterostructures combining multiple individual 2D materials [14, 15, 16] have already been shown to demonstrate unique and exciting properties in the THz frequency ranges. It should be mentioned that, for the purposes of THz-TDS, materials which are stable at room temperature are more appropriate, as such materials minimise the additional operational requirements being placed on the overall system. Graphene has been widely proposed for different component parts of THz-TDS systems, specifically as detectors , polarizers , modulators [18, 19] and waveguides  and as a high harmonic generation medium [21, 22]. Layered WS2 has also been demonstrated as a THz generator [23, 24], as a modulator based on individual nanosheets  or liquid-exfoliated multilayer nanosheets , and furthermore as a magnetically tuned modulator [26, 27].
Typically, 2D materials are transferred to and then supported on a substrate. As laser-induced generation and detection is used in THz-TDS systems; hence, a substrate’s properties should be investigated in both the infrared and broadband THz ranges in addition to the 2D materials’ properties. Substrate materials with high transparency in the near-infrared and broad THz frequency ranges are desirable. Materials such as silicon, high-density polyethylene, polytetrafluoroethylene (Teflon), cyclic olefin copolymer (Topas), polyimide (Kapton), polyethylene terephthalate (PET) and others  are typically used in THz-TDS as they fulfil the transparency requirements. However, each substrate has a unique influence on the properties of a 2D material supported on it . The effect of the substrate and the 2D material on the overall properties of a device are intrinsically coupled. Also, the specific topography of the interface region can significantly affect the properties. Therefore, when studying new conformations of 2D materials in combination with different substrates, the overall effect should be taken into account.
In this work, we demonstrate the transmission properties of unique graphene-based structures intercalated with a FeCl3 dopant  on glass, sapphire and Kapton polyimide film substrates. This material has not previously been investigated in relation to the problems described above within the NIR and THz (0.1 – 2 THz) ranges. We also show properties of thin WS2 films fabricated from liquid crystalline (LC) solutions transferred to Kapton and PET substrates in the same electromagnetic ranges. The work shows that the introduction of dopant impurities, the selection of structural dimensions and the use of an appropriate substrate for 2D layered materials allows one to control the transmission of samples for both the terahertz and near-infrared ranges, which can then be used to create effective modulators and components for future THz spectroscopy systems.
Fabrication of Samples
WS2 films were fabricated from liquid crystalline tungsten disulfide dispersions. Films from LC phase solutions show higher homogeneity than those fabricated from non-LC dispersions [43, 44, 45]. To obtain a LC phase dispersion, an initial 500 mL solution was prepared in a sealed beaker. IPA was used as the solvent and bulk WS2 particles (Sigma-Aldrich 243639), with dimensions around a few microns on average as the solute at a concentration of 5 mg mL −1. To break down the material, a process of ultrasonication in an ultrasonic bath (James Products 120 W High Power 2790 mL Ultrasonic Cleaner) filled with deionised water was used. Five hour-long periods, separated by 30 min each to prevent excessive heating of the solvent, were used to ensure sufficient exfoliation of the sample. The resultant dispersions were then put through a process of centrifugation for 10 min at 2000 rpm to remove residual bulk material and narrow the distribution of particle sizes present in the solution. After centrifugation, the solution was fractioned, with only the supernatant extracted, to ensure only suitably sized particles remained. The resultant solution was then dried under vacuum (∼ 0.1 atm) in a Schlenk line to fully remove the solvent, before being re-dispersed in IPA again at concentration of 1, 5 and 100 mg mL −1. After re-dispersion, the solutions were again ultrasonicated (for a few minutes) to prevent any aggregated exfoliated particles remaining in the solutions. As the concentration is changed significantly following the centrifugation step, it is necessary to re-establish the concentration following that step. Re-dispersing allows for accurate knowledge of the concentrations of the solutions without affecting the properties of the dispersed 2D material particles. The tungsten disulfide dispersions of all concentrations showed a separation of phases as the volume fraction of the liquid crystal phase was less than 100%.
This solution was then transferred to Kapton and PET substrates with 0.125 and 1 mm thickness, respectively. These substrates were chosen due to their low absorption in the terahertz region from 0.1 to 2.0 THz. For transfer to Kapton, a drop casting method was used with the 100 mg mL −1 dispersion. For the first sample (denoted WS2 S), 50 μL of solution from the upper, lower concentration, non-LC phase fraction was drop cast directly onto the Kapton substrate and allowed to dry. For the second sample (WS2 L), 50 μL of solution from the lower, higher concentration, LC phase fraction was used. Drop cast samples were dried on a hot plate at 70 circC for 5 min. In both cases, individual particle sizes were measured by atomic force microscopy and scanning electron microscopy, with average sizes determined as 2.5 μm2 laterally and thickness of 3.9 nm. The difference was the significantly greater overall film thickness for the L sample versus the S sample, owing to the greater concentration of tungsten disulfide in the liquid crystal phase fraction. For transfer to PET, a thin film transfer method was used. First 20 mL of the liquid crystalline solution was filtered using a Büchner flaskunder vacuum—under vacuum—onto a nano-porous polytetrafluoroethylene membrane. The film on the membrane was then transferred to the substrate using a heat- and IPA-assisted method. The substrate was wetted slightly with IPA while heating to 70 circC on a hot plate. The membrane was quickly transferred onto the substrate, and as the IPA evaporated through the membrane, the thin film of tungsten disulfide was released from the membrane and hence transferred to the substrate after removal of the membrane. Two samples were produced—one from the 1 mg mL −1 dispersion (WS2_LC) and the other from the 5 mg mL −1 dispersion (WS2_HC). Again, average individual tungsten disulfide particle sizes were determined as 2.5 μm2 laterally and thickness of 3.9 nm. The overall film thicknesses were determined to be approximately 1 and 10 μm respectively. Figure 3 shows SEM and optical images of the WS2 samples. In both cases, the uniformity of the coverage is noticeable. From SEM analysis, it can be seen that the majority of the particles are well aligned with the substrate, although some (typically smaller) particles are aligned perpendicular to the substrate. This general alignment is expected when depositing thin films from LC dispersions [43, 44, 45, 46].
Raman spectroscopy measurements were conducted using a Raman spectrometer (Renishaw) with linearly polarised incident light at a wavelength of 532 nm and approximate power of 0.1 mW. Spectra were gathered with an accumulation time of 10 s.
Visible and IR Range Spectroscopy
Measurements of intercalated graphene samples’ and tungsten disulfide films’ transmission in the visible and near infrared ranges were carried out using a research-class spectrophotometer (Evolution-300). This spectrometer allows measurement of the transmittance in 190–1100 nm range with standard deviation of 10 measurements < 0.05 nm and photometric accuracy of 1%.
The transmission in the THz range was investigated by a laboratory THz time-domain spectroscopy system [47, 48] which is systematised in Fig. 1b. In this system, the generation of THz radiation is based on the optical rectification of femtosecond pulses in an InAs crystal located in a magnetic field . Femtosecond laser radiation from a Yb-doped solid-state fs oscillator (central wavelength 1050 nm, duration 100 fs, pulse energy 70 nJ, repetition rate 70 MHz) is divided by a beamsplitter (BS) to the pump and probe beams. The pump beam—modulated by an optical chopper—passes through a delay line and is focused on the THz generator InAs crystal placed in the magnet (M) with 2.4 T field. A Teflon filter (F1) is used to cut off the IR pump beam. The THz radiation (estimated average power 30 μW, FWHM ∼1.8 ps) is focused at normal incidence on the sample (S). The transmitted THz pulse is collimated by a -oriented CdTe electro-optical crystal (EOC) for EO detection by an off-axis parabolic mirror (PM). The probe beam polarization is fixed by a Glan prism (G) to be 45circ relative to the THz polarization. The probe beam is also focused onto the same spot of the CdTe crystal. The birefringence in the CdTe crystal induced by the electric field of the THz pulse changes the polarization of the probe beam. The polarization change is measured using a quarter wave plate (λ/4), a Wollaston prism (W) and a balanced photo detector (BPD). A lock-in amplification (LA) technique is used to raise the signal-to-noise ratio. The amplified signal is then transferred to the computer via an analogue-to-digital converter.
The THz-TDS measurements were performed several times at different points of the samples and the averaged values were taken. The beam size in this setup is around 3 mm. The integral transmittance of the sample surface was measured. The obtained time dependencies of the THz pulse electric field (wave forms) without samples presence, when passed though substrates, and when passed through films on substrates were used to calculate THz frequency-domain spectra by means of Fourier analysis. The transmitted amplitudes were then compared for different samples.
Results and Discussions
Figure 3a illustrates the Raman spectrum for tungsten disulfide film transferred from LC state to a silicon-on-insulator substrate. The typical peaks specific to crystalline WS2 E2g and A1g can be seen in the spectrum. Using Raman mapping for the thin films, high homogeneity of the Raman signal was observed over large areas.
Varying the structure dimensions, specifically the film thickness, from 1 to 10 μm for WS2 LC based thin films on polyethylene terephthalate (PET) causes a change of the transmission in the range from 400–1100 nm of up to 35%. This is expected due to the greater overall optical density of the thicker film produced from the higher concentration solution.
In summary, the transmission properties of 2D layered materials based on graphene and tungsten disulfide in near infrared and terahertz ranges are demonstrated. Unique graphene-based structures intercalated with a FeCl3 dopant on glass, sapphire and Kapton polyimide substrates as well as thin WS2 film fabricated from liquid crystal solutions transferred to a Kapton and PET substrates were observed. The introduction of impurities, the intercalation, the selection of structural dimensions and the use of an appropriate substrate for modified 2D layered materials allow one to control the transmission of samples for both the terahertz and infrared ranges, which can be used for creation of effective modulators and components for THz spectroscopy systems. This work represents application-oriented results for future studies, which will concentrate on new devices for terahertz time-domain spectroscopy systems.
MOZ, AB, MFC and ANT initiated the research. BTH, EK and KKW worked on the sample fabrication. MOZ, YVG, ENO and PSS designed and executed the experiments. MOZ, YVG, BTH, EK, MFC, AB and ANT discussed the experimental results. MOZ, EK and BTH wrote the manuscript. All authors read and approved the final manuscript.
This work has been supported by the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom via the EPSRC Centre for Doctoral Training in Electromagnetic Metamaterials (Grant No. EP/L015331/1) and via Grant Nos. EP/N035569/1, EP/G036101/1 and EP/M002438/1; Government of the Russian Federation (08-08).
The authors declare that they have no competing interests.
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