Modulation of the Anisotropic Electronic Properties in ReS 2 via Ferroelectric Film
2019; Chinese Chemical Society; Volume: 1; Issue: 3 Linguagem: Inglês
10.31635/ccschem.019.20180024
ISSN2096-5745
AutoresRenyan Wang, Fengya Zhou, Liang Lv, Shasha Zhou, Yiwei Yu, Fuwei Zhuge, Huiqiao Li, Lin Gan, Tianyou Zhai,
Tópico(s)Corrosion Behavior and Inhibition
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2019Modulation of the Anisotropic Electronic Properties in ReS2 via Ferroelectric Film Renyan Wang†, Fengya Zhou†, Liang Lv, Shasha Zhou, Yiwei Yu, Fuwei Zhuge, Huiqiao Li, Lin Gan and Tianyou Zhai Renyan Wang† State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) Shenzhen R&D Center, Huazhong University of Science and Technology, Shenzhen 518000 (China) , Fengya Zhou† State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) , Liang Lv State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) , Shasha Zhou State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) , Yiwei Yu State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) , Fuwei Zhuge State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) , Huiqiao Li State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) , Lin Gan *Corresponding authors: E-mail Address: [email protected]; E-mail Address: [email protected] State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) Shenzhen R&D Center, Huazhong University of Science and Technology, Shenzhen 518000 (China) and Tianyou Zhai *Corresponding authors: E-mail Address: [email protected]; E-mail Address: [email protected] State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074 (China) https://doi.org/10.31635/ccschem.019.20180024 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Anisotropic response in two-dimensional materials has emerged with attractive potential applications in angle-sensitive areas. However, the challenge with current studies on anisotropic response is due to the main use of the structural materials of pristine to examine the physical properties in different directions, which cause restriction of the anisotropic ratio within a narrow range, thereby, limiting their practical applications. Herein, we aimed at developing a novel strategy to effectively modulate the electronic anisotropic ratio by employing rhenium disulfide (ReS2) nanosheet in a nondestructive fashion. We integrated a thin film of ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE) on top of ReS2 nanosheet and modulated the conductivity of ReS2 within specific directions via selective polarization of the ferroelectric film. Our results demonstrated that the electronic anisotropic ratio was enhanced remarkably from the initial 1.4 to as high as ∼12.0. Thus, we concluded that this strategy is compatible with conventional processes in the semiconductor industry and may pave the way for emerging angle-sensitive applications in the near future. Download figure Download PowerPoint Introduction Anisotropic response in two-dimensional (2-D) materials is increasingly, attracting attention worldwide because of the spatial differences in the inherent physical parameters such as, carrier mobility,1–4 thermal conductivity,5–7 optical absorption coefficient,8–10 and Young's modules,11 which render these novel 2-D materials an extra degree of freedom, thus providing unprecedented opportunity for the development of applications concerning angle sensitivity, including polarized photodetector,12,13 inverter,14 and so on. In 2-D materials, the interesting anisotropic response generally stems from reduced in-plane symmetry. For example, black phosphorus (BP) is the most famous layered anisotropic material, which is an orthorhombic crystal with a puckered lattice structure. Having such reduced in-plane symmetry endows BP with a strong anisotropic response in electronic, optical, and thermal properties.15–17 Besides BP, the anisotropic properties in a series of 2-D materials have been discovered.18 Rhenium disulfide (ReS2) is one of the typical layered materials in which the quasi-one-dimensional Re4 chains (b-axis) generates distinct anisotropic response,19–22 and more importantly, its 2-D form is stable in ambient environments, in contrast to the high activity of BP ultrathin film in air.23 Moreover, the success of chemical vapor deposition (CVD) and the fabrication of high-quality ReS2 flake or film makes it more advantageous over BP for future applications;24–26 therefore, the anisotropic performance of ReS2 has become increasingly attractive. Current studies on the anisotropic response are largely based on the pristine properties of 2-D materials. However, the anisotropic ratios between various angles of these materials are usually limited in their inherent physical parameters9,14, and the magnitude of the difference is still too small to be distinguished precisely, restricting future practical applications such as flexible electronics. Recently, the use of external modulation strategies to adjust the anisotropic response in 2-D materials have been attempted in some works.27–34 For example, the strain is a widely used strategy to tune the anisotropic properties; the effect of which has been predicted by theoretical calculation29 and explored via experiments.30 Besides, many other methods, including ionic intercalation,31 sample thickness control,32 heterostructures construction,33 and chemical surface decoration34 have also been used to modulate the anisotropic response of 2-D materials. However, these current strategies still have apparent defects. For instance, deficient in controllability, in particular, have been demonstrated widely in the field of strategic strain engineering. Generally, flexible substrates or atomic force microscopy (AFM) tips are employed to achieve strain resilience or to manipulate the forces between tip and sample to change strain properties toward pliability, respectively. However, the former is difficult to apply on a specific strain site or area of the sample, and the latter is too feeble to provide uniform strain resilience on a large scale. Moreover, these approaches are likely to introduce destruction to the materials during the modulation process, causing concerns of damages caused by chemical changes encountered in the current utilized modulation strategi es. In this work, we proposed a novel highly controllable doping strategy to modulate the electronic properties of ReS2 ultrathin nanosheet in a nondestructive manner. Contrary to previous strategies, we constructed a bilayer structure of poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE)/ReS2 to control the local carrier density in ReS2 along with the specific directions, that is, parallel or perpendicular to the b axis. P(VDF-TrFE) is a kind of widely adopted ferroelectric material used to tune the electrical structure of 2-D materials, which can be fabricated into thin film via simple spin-coating technique.35–37 Under the polarization of external voltage, the polarity of P(VDF-TrFE) can be switched reversibly, and a remnant polarization is kept even after the removal of external voltage, acting as doping source either for electron or hole. Through selective polarization of the P(VDF-TrFE) film, arbitrary polarized patterns can be achieved readily. Based on rationally designed polarization patterns, the anisotropic electronic properties of ReS2 have been modulated in a large range of approximately 1.40–12.03 under the electron-doping mode of P(VDF-TrFE) film and presents the best result ever been achieved relative to previous reports. This strategy provides a simple way to uniformly modulate the anisotropic properties of 2-D materials on a large scale, and the whole modulation tends to be a nondestructive physical process. Moreover, it is compatible with modern semiconducting industry, suggesting a promising future of emerging novel angle-sensitive applications. Results and Discussion Mechanism of Anisotropic Response Modulation via Ferroelectric Film The schematic illustration of the modulation mechanism of the device is shown in Figure 1. Figure 1a depicts the basic configuration of the CVD-grown, ReS2, P(VDF-TrFE) film design, in which a simple crossed-electrodes pattern was deposited on a CVD-grown ReS2 nanosheet (∼1.6 nm, two layers) on SiO2/Si substrate. A thin layer of P(VDF-TrFE) film was then coated on the surface of the device, followed by heating to obtain a uniform and smooth film. We designed the configuration of the device as follows: One pair of electrodes was deposited along the b axis of ReS2 nanosheet, whereas the other pair was deposited with a 90° rotation to the b axis, forming a cross structure. After that, an external voltage via piezoresponse force microscopy (PFM) was employed to locally polarize the P(VDF-TrFE) film in specific directions, that is, parallel or perpendicular to the b axis (see the ). Accordingly, different types of carriers could be injected into the channel of ReS2 nanosheet under positive or negative bias (Figure 1b,c). As a good ferroelectric material, P(VDF-TrFE) film, it could maintain a considerable remnant polarization after removal of external bias, which thus, acts like a nonsource gate electrode to dope ReS2 nanosheet continuously. Consequently, ReS2 nanosheet was synthesized by CVD method under 850 °C. The high growth temperature was chosen due to its benefit in the generation of high crystalline quality, and the irregular shape of sample resulted from the strong anisotropy of ReS2.25 The anisotropic (as)-synthesized ReS2 nanosheet was verified by X-ray diffraction, Raman, and photoluminescence spectrum (PL), and its intrinsic properties were thoroughly characterized via X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM; see the ). Figure 1 | The schematic representation of device structure and carrier doping of ReS2 by PFM. (a) Configuration depiction of ReS2 device with polarized P(VDF-TrFE) film. The side view of device structure with (b) forward voltage and (c) reversed voltage, corresponding to electrons and holes doping, respectively. ReS2, rhenium disulfide; PFM, piezoresponse force microscopy; P(VDF-TrFE), ferroelectric poly(vinylidene fluoride-co-trifluoroethylene). Download figure Download PowerPoint Identification of the Orientation of ReS2 and Verification of Ferroelectric Film Angular resolution-polarized Raman spectra (ARPRS) is an effective technique used to assess the anisotropy of 2-D materials.38–40 For anisotropic ReS2, it has been demonstrated that the maximal-polarized Raman intensity of Eg-like Raman mode at 213 cm−1 is along the b-axis direction under parallel Raman scattering, which is generally defined as the Re4-chain lattice orientation.41 Therefore, we precisely deposited electrodes along specific directions in ReS2 nanosheet. Figure 2a shows the angular resolution Raman spectrum of the ReS2 sample with an interval of 60° rotation under 532 nm laser excitation at room temperature. It can be observed that peak intensities of most Raman modes exhibit a period of 180° with unchanged peak positions, in particular, for Eg-like modes at 151 and 213 cm−1. Considering the Ci point group and P-1 space group of ReS2, the Raman tensor R for given Eg-like in-plane scattering process is defined as follows19: R = ( u v v w ) (1) Figure 2 | Identification of the orientation of ReS2 and verification of the polarization effect of P(VDF-TrFE) film. (a) Angular resolution Raman spectrum of the ReS2 sample under 532 nm laser excitation. (b) Polar plots of parallel angle-resolved Raman scattering intensities of Eg-like mode (213 cm−1) at 532 nm, corresponding ReS2 sample are shown. (c) Raman mapping of the parallel-polarized collection with different rotation angles under 532 nm. (d) PFM phase hysteresis loop of thin P(VDF-TrFE) film at −10 V to 10 V polarization voltages. (e) SHG signal of ferroelectric P(VDF-TrFE) film under 10 V, −10 V, and no polarization situation. ReS2, rhenium disulfide; P(VDF-TrFE), ferroelectric poly(vinylidene fluoride-co-trifluoroethylene); PFM, piezoresponse force microscopy; SHG, second harmonic generation. Download figure Download PowerPoint For parallel detection and excitation polarizations, the intensity I (θ) of a given Eg-like Raman mode varies with θ as below: I ( θ ) ∝ ( u cos 2 θ + 2 v sin θ cos θ + w sin 2 θ ) 2 (2) Polar plots of the peak intensity of Raman mode at 213 cm−1 are shown in Figure 2b. Most Raman plots fitted well, using equation (2), hence, indicating the anisotropic lattice structure and phonon properties. This dependence is more intuitively described using Raman mapping with the polar plots of peak intensity as a function of the polarization angle (θ) of the laser (Figure 2c). According to the angle-resolved spectrum information, the direction of the b axis in ReS2 nanosheet could be precisely identified. To verify the effect of polarized P(VDF-TrFE) film, we first conducted a polarization test in the bias range of −10 to 10 V on the P(VDF-TrFE) film. The PFM phase hysteresis loop of thin P(VDF-TrFE) film (Figure 2d) indicates significant ferroelectric properties according to the sharply modulated phase between relatively small polarized voltage windows and the rectangle-like hysteresis loop, implying a well-maintained polarization condition.42 In addition, the difference between the two conditions is nearly 180°, which is an ideal polarized transformation, thereby, further confirming its excellent ferroelectric polarization property. Figure 2e shows the intensity of second harmonic generation (SHG) of P(VDF-TrFE) film under different polarization voltage conditions. The appearance of the SHG signal represents the breakdown of the centrosymmetric structure along the laser incident direction.43 No peaks are observed at the absence of external electric field, whereas small peaks appear when −10 V bias or 10 V bias is set, indicating that the polarization slightly alters the symmetry of electronic states in P(VDF-TrFE) film. Initial Anisotropic Ratio of Device We first collected the pristine electrical transport properties of ReS2 nanosheet demonstrated in Figure 3a. I–V curves display the notable difference between directions parallel and perpendicular to b axis, manifesting strong lattice direction dependence. Moreover, the straight I–V lines also suggest that the contacts between the electrode and ReS2 nanosheet are in Ohmic-like mode. In view of the identical channel lengths of the two directions and uniform thickness of ReS2 nanosheet, the conductance value of different directions could be utilized precisely to evaluate the electrical anisotropy. Plot curves of the electronic conductance as a function of bias voltage fitted into two straight lines (Figure 3b), showing that the electronic conductance of the b axis and cross direction are within the average value of 26.74 and 18.77 nS, respectively. In other words, the intrinsic conductance ratio (Gcross/Gparallel) between the two directions is ∼ 0.70. Figure 3 | Electronic transport properties of ReS2 device. (a) I–V curves of original ReS2 at parallel and perpendicular to b-axis directions; inset showed parallel and crossed b-axis orientation of ReS2 atomic structure. (b) Electronic conductance data and their corresponding fit lines at two different directions. ReS2, rhenium disulfide. Download figure Download PowerPoint Polarization of Ferroelectric Film Perpendicular to b Axis Subsequently, we checked the influence of polarized P(VDF-TrFE) film on the electronic performance of beneath ReS2 nanosheet. Herein, we took a selective polarization across the b axis of ReS2 nanosheet as an example to demonstrate the effect first. Tip bias of the conductive probe was set as 0 V, and voltage modulation was accomplished through an external control circuit. High-scan resolution mode was applied during the whole experimental process to ensure accuracy and uniformity of polarization. Output electrical curves showed that the current increased monotonously with an increase in the polarization voltage, and the Ohmic-like contact mode was maintained during the whole test process. Interestingly, it is easy to observe that the increase in current is nonlinear along with the polarization voltage (Figure 4a), suggesting that the remnant polarization in P(VDF-TrFE) film is also nonlinear to external bias, consistent with previous reports.44 Compared with initial ReS2, the current under a bias of 0.5 V increased to 3.5 times when −10 V polarization voltage was applied on P(VDF-TrFE) film, demonstrating effective and successful electron injection. The phase change of ferroelectric P(VDF-TrFE) film is a more direct reflection of the polarization process. As shown in Figure 4b, with the increase of polarization voltage, the phase of P(VDF-TrFE) film readily changed from ∼−50° to 130°, demonstrating the evolution of polarization states. In order to visualize the electrical evolution of ReS2 under varied polarized situations, the histogram of the electronic conductance along different directions is summarized in Figure 4c. For the electrical performance of the device along the b axis and the corresponding average conductance analysis at different directions, detailed information has been included in . For the unpolarized b-axis direction, the conductance was almost constant, showing just a slight fluctuation, probably due to the influence of the partially overlapped area within the polarization pattern (inset of Figure 4a). In contrast, for the polarized direction across the b axis, the electronic conductance increased notably as polarization voltage rises, and reached a maximum value at −10 V. To evaluate the modulation more clearly, the electronic conductivity ratio between two different directions was calculated, as shown in Figure 4d. As the polarization voltage increases from 0 to −10 V, the ratio changes significantly from ∼0.70 to ∼4.01, which is strong evidence of the remarkable ability of ferroelectric P(VDF-TrFE) film on the modulation anisotropic electronic properties of 2-D materials. According to reported studies45, strain (that might be induced by ferroelectric coating film) can also contribute to the change of electric conductivity. To verify the dominant role of ferroelectric polarization in our case, we made a rough estimation on this issue as explained below (also see ). Figure 4 | Electrical properties of ReS2 device under polarization of P(VDF-TrFE) film across the b-axis direction. (a) I–V curves of perpendicular to the b-axis direction at different polarization voltages; inset shows the schematic polarization of the ReS2 device with P(VDF-TrFE) film. (b) Phase image of P(VDF-TrFE) film at different polarization voltages. (c) Electronic conductance histogram of ReS2 at two directions under different polarization voltages. (d) Electronic conductivity ratio (σcross/σparallel) curve at different polarization voltages. P(VDF-TrFE), ferroelectric poly(vinylidene fluoride-co-trifluoroethylene). Download figure Download PowerPoint Polarization of Ferroelectric Film Parallel to b Axis We conducted a polarization along the b axis of ReS2 on the same device. Figure 5a showed the I–V curves measured at direction parallel to b axis of ReS2 nanosheet. The inset shows a schematic image of a simple polarization operation. Unlike the Ohmic contact after polarization, shown in Figure 4a, almost all I–V curves in Figure 5a showed distinct deviation from a straight line, suggesting Schottky contact between the electrode and ReS2 nanosheet (see also shown in ), in which the device was polarized along the direction crossing the b axis. We attributed such an Ohmic-like to Schottky contact mode changing to the difference of electron concertation between overlap area and other channel area. Taking the I–V curve in the , for example, the polarization direction was set to cross with the b axis. Therefore, for the I–V measured along the b axis, the channel must have a partial area overlapping with the polarization pattern. As a result, distinct carrier concentration difference existed both at interfaces between the polarized area and the unpolarized area, resulting in Schottky barriers. A similar case is displayed in Figure 5a, except that the polarization direction was changed to parallel the b axis. The I–V curves were measured along the direction across the b axis, and subsequently, a corresponding average conductance analysis was performed at the different directions (see details included in ). Similarly, the electronic conductance of the two directions under different polarization voltages was also extracted, as shown in Figure 5b. The blue rectangular histogram representing the electronic conductance of polarized b-axis direction showed a gradual increasing trend with an increase in the polarization voltage, confirming the steady enhancement of the carrier doping from the P(VDF-TrFE) film. By contrast, the orange rectangular histogram referred to the electronic conductance in the cross b-axis direction (without extra polarization treatment since last time), which demonstrated an initial sharp reduction, followed by a gradual increase with voltage enhancement. The reason for such a distinct reduction in conductance is not clear right now, but we suggested that previous polarization (across b axis) was subdued at the overlapped area. A possible reason for such degradation is the self-depolarization effect, resulting from the induced charges in ReS2 nanoflake, likely to be suppressed via deposition of a layer of conductor on the other side of P(VDF-TrFE) film to counteract the influence of charges in ReS2 nanosheet.46 As for the progressive enhancement of conductivity with the increase of polarized voltage, it is plausible to have resulted from the newly induced carriers that compensated gradually for the loss of self-depolarization. The electronic conductivity ratio of the two directions is displayed as a line graph in Figure 5c, which shows that the increase in electron concentration in the b-axis direction conduces to the decrease of conductivity ratio from 4.01 to 0.96, manifesting the significance in the modulation of ReS2 electrical anisotropy. Figure 5 | Electrical properties of ReS2 device under polarization of P(VDF-TrFE) film along parallel to b-axis direction after polarization along the cross direction above. (a) I–V curves of parallel b-axis at different polarization voltages; inset shows the schematic polarization of ReS2 device with P(VDF-TrFE) film. (b) Electronic conductance histogram of ReS2 device at parallel and cross b-axis directions. (c) Electronic conductivity ratio (σcross /σparallel) curve at different polarization voltages. (d) I–V curves of ReS2 under polarization of P(VDF-TrFE) film at 10 V along b-axis direction after −10 V along cross b-axis orientation; inset shows electronic conductance histogram of ReS2 at two directions. P(VDF-TrFE), ferroelectric poly(vinylidene fluoride-co-trifluoroethylene); ReS2, rhenium disulfide. Download figure Download PowerPoint Polarization of Ferroelectric Film in Two Directions Simultaneously So far, we have verified the ability to modulate the electronic anisotropic of ReS2 nanosheet via electrons doping. We were then prompted to test whether the hole doping could modulate the anisotropic ratio effectively. To enhance the contrast between the different directions of ReS2 nanosheet, we took negative and positive polarization along different directions simultaneously. Typically, 10 V polarization was applied down the concentration of electrons along b-axis direction by just following the operation of −10 V polarization along the cross direction. I–V curves showed that the currents parallel to and in the cross directions of the b-axis were quite close and the value of both reduced vastly, compared with the case of exclusive electron injection (Figure 5d). However, the anisotropic ratio still stayed at a relatively low level (∼1.19), in relation to their electronic conductance (inset of Figure 5d). This result indicated that depletion of carriers in ReS2 nanosheet greatly suppressed the current and even the electron-doping channel (along the direction across the b axis) and the final current dominated an overlapped area with the hole-doping channel. Therefore, modulation of the electronic anisotropy of 2-D materials via depletion is probably not a good choice for our device configuration. Largest Anisotropic Ratio Can be Achieved Finally, to explore the utmost degree of modulation on the electronic anisotropy of ReS2 nanosheet, we conducted a polarization experiment with a high voltage up to −16 V along b axis alone on a new device (to avoid the possible polarization residue in a previous device, Figure 6). As already known, ReS2 has the best electronic performance along the b axis; therefore, we envisioned that improving the performance of the b axis could further enhance the anisotropic ratio of the device. We determined the initial electronic conductivity ratio of the b axis and cross direction to be ∼1.40. As expected, the conductivity rapidly increased with augmentation of the polarization voltage. Nonetheless, saturation was reached as the voltage was set at ∼−8 V. Accordingly, the ultimate anisotropic ratio of conductivity was recorded at ∼12, which is the highest noted among several reports to the best of our knowledge. Furthermore, we would like to point out that the modulation effect of this method is dependent on sample thickness and the essence of charge-doping in ferroelectric polarization. Generally, the remnant polarization of ferroelectric film of a certain value induces a corresponding value of charge doping. If sample thickness is increased, the doping effect sharply receded, suggesting that a thin sample has a more pronounced modulation effect than a thick one (see ). Figure 6 | Utmost anisotropic ratio achieved by our study ReS2-P(VDF-TrFE) film integrated device strategy. (a) The schematic of ReS2 device under polarization of P(VDF-TrFE) film. (b) Electronic conductance histogram of a new ReS2 device at parallel and cross directions to b axis under polarization of P(VDF-TrFE) film along the b-axis direction. (c) Electronic conductivity ratio (σparallel /σcross) of ReS2 devices at different polarization voltages. ReS2, rhenium disulfide; P(VDF-TrFE), ferroelectric poly(vinylidene fluoride-co-trifluoroethylene). Download figure Download PowerPoint Methods Device Fabrication The ReS2 devices on a silicon substrate (300 nm SiO2) with marks were constructed by the wet transfer method (…),47 using a spin-coating poly(methyl methacrylate) as the supporting membrane. To fabricate the device for measurements, the electrode patterns were defined by an electron-beam lithography (EBL) system (FEI Quanta 650 SEM and Raith Elphy Plus), and Cr/Au (10/80 nm) metals were deposited by the thermal evaporation (Nexdap, Angstrom Engineering). The constructed device was then annealed at 200 °C for 1 h in Ar/H2 (100/5 sccm) environment to remove the residual organic pollution, which could result in a high-quality interface. Ferroelectric film on ReS2 device was obtained by spin coating P(VDF-TrFE) and then baked at 135 °C for 15 min. Device Characterization The as-synthesized ReS2 flakes were characterized by an optical microscope (BX51; Olympus). The crystalline structure and composition investigations were determined, using X-ray diffraction (Empyrean; PANalytical B.V.) and transmission electron microscope (FEI Tecnai G2 F30). The electronic structure was measured by X-ray photoelectron spectroscopy (Kratos-AXIS ULTRA DLD-600W). A confocal Raman sy
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