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... It is found that FWHM(G) is not sensitive to temperature for both defect-free and defective sets, which is consistent with the recently reported T-dependent FWHM(G) results of pristine graphite [30]. It is interesting to note Fig. 3 The T-dependent Pos(G) of 1LG-4LG, 6LG, and 10LG for a defect-free and b defective sets in the temperature range of 78-318 K that T-dependent FWHM(G) in various graphene samples have been discussed [14,31,32] and have some discrepancies; for example, Lin et al. [31] observed an increase trend in unsupported graphene, Kolesov et al. [32] showed different T-dependencies in supported graphene on various substrates, and even Late et al. [14] showed slightly positive or insensitive dependencies in the case of nitrogen-doped or boron-doped graphite. However, in the low-temperature range below 350 K, FWHM(G) always kept constant in all the samples [14,31,32] probably due to weaker contribution from phonon anharmonicity and electron-phonon coupling (EPC) at low-temperature range [29,33]. ...
... It is found that FWHM(G) is not sensitive to temperature for both defect-free and defective sets, which is consistent with the recently reported T-dependent FWHM(G) results of pristine graphite [30]. It is interesting to note Fig. 3 The T-dependent Pos(G) of 1LG-4LG, 6LG, and 10LG for a defect-free and b defective sets in the temperature range of 78-318 K that T-dependent FWHM(G) in various graphene samples have been discussed [14,31,32] and have some discrepancies; for example, Lin et al. [31] observed an increase trend in unsupported graphene, Kolesov et al. [32] showed different T-dependencies in supported graphene on various substrates, and even Late et al. [14] showed slightly positive or insensitive dependencies in the case of nitrogen-doped or boron-doped graphite. However, in the low-temperature range below 350 K, FWHM(G) always kept constant in all the samples [14,31,32] probably due to weaker contribution from phonon anharmonicity and electron-phonon coupling (EPC) at low-temperature range [29,33]. ...
... It is interesting to note Fig. 3 The T-dependent Pos(G) of 1LG-4LG, 6LG, and 10LG for a defect-free and b defective sets in the temperature range of 78-318 K that T-dependent FWHM(G) in various graphene samples have been discussed [14,31,32] and have some discrepancies; for example, Lin et al. [31] observed an increase trend in unsupported graphene, Kolesov et al. [32] showed different T-dependencies in supported graphene on various substrates, and even Late et al. [14] showed slightly positive or insensitive dependencies in the case of nitrogen-doped or boron-doped graphite. However, in the low-temperature range below 350 K, FWHM(G) always kept constant in all the samples [14,31,32] probably due to weaker contribution from phonon anharmonicity and electron-phonon coupling (EPC) at low-temperature range [29,33]. In addition, FWHM(G) from 1LG to 10LG is from 9.2 to 14.6 cm −1 in the defect-free set and from 10.9 to 16.1 cm −1 in the defective set. ...
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The defects into the hexagonal network of a sp2-hybridized carbon atom have been demonstrated to have a significant influence on intrinsic properties of graphene systems. In this paper, we presented a study of temperature-dependent Raman spectra of G peak and D’ band at low temperatures from 78 to 318 K in defective monolayer to few-layer graphene induced by ion C+ bombardment under the determination of vacancy uniformity. Defects lead to the increase of the negative temperature coefficient of G peak, with a value almost identical to that of D’ band. However, the variation of frequency and linewidth of G peak with layer number is contrary to D’ band. It derives from the related electron-phonon interaction in G and D’ phonon in the disorder-induced Raman scattering process. Our results are helpful to understand the mechanism of temperature-dependent phonons in graphene-based materials and provide valuable information on thermal properties of defects for the application of graphene-based devices.
... The 2D peak position changes with the charge carrier density basically due to a dependence on graphene lattice constant [33], which is affected by doping and influences the phonon energy. Therefore, unlike the EPC-affected I 2D /I G in Fig. 6, the higher 2D peak sensitivity to substrate material in this case has a different nature and is rather related to a stronger graphene-substrate interaction for copper, which leads to a more complex phonon interactions at the interface [36,37]. For G peak position, which depends on the carrier density due to being influenced by both lattice constant and the electron-phonon interactions, the stronger graphene-copper bonding and the EPC screening by Cu seem to partially counterbalance each other, leading to mostly similar and overlapping distributions for different substrates. ...
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In this paper, we study structural and adsorption properties of graphene irradiated with 46 MeV Ar ions and 240 keV H ions on SiO2/Si and copper substrates by micro-Raman spectroscopy. Graphene irradiated with H ions demonstrated evidence of both high and low defect density regions on a sub-micron scale. TRIM calculations showed that substrate was the dominant defect source with a contribution from about 55% for H ions in graphene on SiO2/Si to 90% for Ar in graphene on SiO2/Si. Charge carrier density analysis showed p-type adsorption doping saturating at (0.48 ± 0.08) × 10¹³ cm⁻² or (0.45 ± 0.09) × 10¹³ cm⁻² with a defect density of 1.5 × 10¹¹ cm⁻² or 1.2 × 10¹¹ cm⁻² for graphene on SiO2/Si or copper, respectively; this was analyzed in the framework of physisorption and dissociative chemisorption. This study is useful towards the development of functionalization methods, molecular sensor design, and any graphene application requiring modification of this material by controlled defect introduction.
... As a matter of fact, the Raman FWHM in unsupported 2D materials is more influenced by anharmonic phonon interactions than by thermal expansion effects. 19,20 While the incident power dependence of the FWHM is weak for all configurations, it increases for silicene on stanene-Ag(111) and shows dependence on silicene thickness. Specifically, the increase in bandwidth as a function of laser power is maximum for the single layer of silicene on stanene-Ag(111). ...
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Stabilization of silicene and preservation of its structural and electronic properties are essential for its processing and future integration into devices. The stacking of silicene on stanene, creating a Xene-based heterostructure, proves to be a viable new route in this respect. Here we demonstrate the effectiveness of a stanene layer in breaking the strong interaction between silicene and the Ag(111) substrate. The role of stanene as a 'buffer' layer is investigated by analyzing the optical response of epitaxial silicene through both power-dependent Raman spectroscopy and reflectivity measurements in the near infrared (NIR)-ultraviolet (UV) spectral range. Finally, we point out a Xene-induced shift of the silver plasma edge that paves the way for the development of a new approach to engineering the metal plasmonic response.
... Overall, the absolute values of Grüneisen parameter of stable graphene-like borophene are close to that of graphene. In physics, the anharmonicity in lattice vibrations is related to the anharmonic phonon-phonon process, thermal expansion, substrate effect and electron-phonon coupling [66]. Here we consider intrinsic borophene and the electron-phonon coupling is neglected. ...
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The last decades have seen tremendous progress in quantitative understanding of phonon transport, which is critical for the thermal management of various functional devices and the proper optimization of thermoelectric materials. In this work, using first-principles based calculation combined with non-equilibrium Green's function and phonon Boltzmann transport equation, we provide a systematic study on the phonon stability and phonon transport of monolayer boron sheet with honeycomb, graphene-like structure (graphene-like borophene) in both ballistic and diffusive regimes. For free-standing graphene-like borophene, phonon instabilities occur near the centre of Brillouin zone, implying elastic instability. Investigation of the electronic structures shows that the phonon instability is due to the deficiency of electrons. Our first-principles results show that with net charge doping and in-plane tensile strain, the graphene-like borophene is becoming thermodynamic stable in ideal plat nature, because the bonding characteristic is modified. At room temperature, the ballistic thermal conductance of graphene-like borophene 7.14 nWK-1 nm-2) is higher than that of graphene (4.1 nWK-1 nm-2), due to high phonon transmission. However, its diffusive thermal conductivity is two orders of magnitude lower than graphene, because the phonon relaxation time is dramatically reduced comparing with its carbon counterpart. Although the phonon group velocity and phonon anharmonicity are comparable with that of graphene, the suppressed phonon space results in dramatically strong phonon-phonon scattering. These thermal transport characteristics in both ballistic and diffusive regimes are of fundamental and technological relevance and provide guidance for applications of boron based nanomaterials in which their thermal conduction is major concern.
... In our work, the second factor was not taken into account since the layer of the CZTSe thin films is rather thick (about 1 μm, Fig. 1). For example, such calculations were already done for graphene (Kolesov et al., 2019), molybdenum disulfide (Najmaei et al., 2013), and germanium selenide (Deringer et al., 2014). ...
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The temperature dependence (in range from 24 to 290 K) of Raman spectroscopy of the Cu2ZnSnSe4 (CZTSe) films with Zn-rich (series A) and Zn-poor (series B) composition obtained on a Ta foil is investigated. Analisys and approximation by the Lorentz function of the CZTSe Raman spectra suggests that the CZTSe most intense Raman peak consists of two modes (at 192/189 and 194/195 cm⁻¹), which are slightly shifted from each other. In addition, the Raman peaks around 192 and 189 cm⁻¹ lead to asymmetric broadening of dominant peaks at 194 and 195 cm⁻¹ in Raman spectra of the CZTSe films series A and B, respectively. In the case of the Sn-rich CZTSe films, we attribute of Raman peak around 189 cm⁻¹ to SnSe2 compound. However in the case of the Sn-poor CZTSe films, the observable shift is too high to assign confidently the 192 cm⁻¹ band to a SnSe2 compound, which was not detected by XRD analysis. We suppose that this mode is attributed to disordered kesterite structure. The temperature dependence Raman spectra for both series of the CZTSe films shows that a change temperature from 290 to 24 K leads to position shift and narrowing of the CZTSe Raman A-modes. The calculated temperature coefficients and anharmonic constants in Klemens model approximations for temperature dependence of shift position and FWHM of the CZTSe A-modes shown that four-phonon process has dominant contribution in damping process and as a consequence in Raman spectrum changes for two series of the CZTSe films.
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The temperature-dependent (T-dependent) linewidth (ΓG) and frequency shift (ΔωG) of the G mode provide valuable information on the phonon anharmonicity of graphene-based materials. In contrast to the negligible contribution from electron-phonon coupling (EPC) to the linewidth of a Raman mode in semiconductors, ΓG in pristine graphene is dominated by EPC contribution at room temperature due to its semimetallic characteristics. This leads to difficulty in resolving intrinsic contribution from phonon anharmonicity to ΓG. Here, we probed the intrinsic phonon anharmonicity of heavily-doped graphene by T-dependent Raman spectra based on FeCl3-based stage-1 graphite intercalation compound (GIC), in which the EPC contribution is negligible due to the large Fermi level (EF) shift. The ΔωG and ΓG exhibit a nonlinear decrease and noticeable broadening with increasing temperature, respectively, which are both dominated by phonon anharmonicity processes. The contribution of phonon anharmonicity to ΓG of heavily-doped graphene decreases as the EF approaches to the Dirac point. However, the T dependence of ΔωG is almost independent on EF and qualitatively agrees with the theoretical result of pristine graphene. These results provide a deeper understanding of the role of phonon anharmonicity on the Raman spectra of heavily doped graphene.
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Pressure dependence of the k~0 TO phonon frequencies of KCl and KBr is reported in the pressure range of 1 to 35 kbar. Using the P-V data, the volume dependence of these modes was determined. The Grüneisen parameters thus obtained compare well with those calculated from Born-Mayer-type potentials. It appears that the temperature dependence of the k~0 TO phonon frequency of KBr can be completely accounted for by its volume dependence. In the temperature range of 5 to 750°K, the "self-energy shift" due to anharmonicity thus seems to be small compared with the thermal-expansion part. The CsCl phase occurs in both KCl and KBr at appropriate pressures. The k~0 TO frequency, as expected, drops by some 10 to 12% at the transition pressure.
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Raman spectroscopy is an integral part of graphene research. It is used to determine the number and orientation of layers, the quality and types of edge, and the effects of perturbations, such as electric and magnetic fields, strain, doping, disorder and functional groups. This, in turn, provides insight into all sp(2)-bonded carbon allotropes, because graphene is their fundamental building block. Here we review the state of the art, future directions and open questions in Raman spectroscopy of graphene. We describe essential physical processes whose importance has only recently been recognized, such as the various types of resonance at play, and the role of quantum interference. We update all basic concepts and notations, and propose a terminology that is able to describe any result in literature. We finally highlight the potential of Raman spectroscopy for layered materials other than graphene.
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Temperature-dependent Raman scattering is performed on unsupported vertical graphene sheets, which are approximate to free graphene without supporting the substrate. Here the observed G peak line shift with temperature is completely consistent with the theoretical prediction based on the first-principles calculation on free graphene, and our result is helpful to understand intrinsic anharmonic phonon characteristics of free graphene and the divergence on the G peak line shift with temperature. However, the observed linewidth variation is different from the prediction. To reveal the origins, a simplified Klemens model is used, and the dominating anharmonic phonon scattering mechanism is explored. In addition, line shift and linewidth variations of D and 2D peaks of the graphene sheets with temperature are revealed, and the possible mechanisms dominating the results are discussed.
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Direct measurement of the adhesion energy of monolayer graphene as-grown on metal substrates is important to better understand its bonding mechanism and control the mechanical release of the graphene from the substrates, but it has not been reported yet. We report the adhesion energy of large-area monolayer graphene synthesized on copper measured by double cantilever beam fracture mechanics testing. The adhesion energy of 0.72 ± 0.07 J m(-2) was found. Knowing the directly measured value, we further demonstrate the etching-free renewable transfer process of monolayer graphene that utilizes the repetition of the mechanical delamination followed by the regrowth of monolayer graphene on a copper substrate.
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We present the results of a thorough study of wet chemical methods for transferring chemical vapor deposition grown graphene from the metal growth substrate to a device-compatible substrate. On the basis of these results, we have developed a "modified RCA clean" transfer method that has much better control of both contamination and crack formation and does not degrade the quality of the transferred graphene. Using this transfer method, high device yields, up to 97%, with a narrow device performance metrics distribution were achieved. This demonstration addresses an important step toward large-scale graphene-based electronic device applications.
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The thermal expansion coefficient (TEC) of single-layer graphene is estimated with temperature-dependent Raman spectroscopy in the temperature range between 200 and 400 K. It is found to be strongly dependent on temperature but remains negative in the whole temperature range with a room temperature value of (-8.0 ± 0.7) × 10(-6) K(-1). The strain caused by the TEC mismatch between graphene and the substrate plays a crucial role in determining the physical properties of graphene, and hence its effect must be accounted for in the interpretation of experimental data taken at cryogenic or elevated temperatures.
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The phonon dispersion of graphene is known to display two strong Kohn Anomalies (kinks) in the highest optical branch (HOB) at the high-symmetry points Γ and K [ Piscanec , S. ; et al. Phys. Rev. Lett. 2004 , 93 , 185503 ]. The phonon slope around the Kohn anomalies is related to the electron-phonon-coupling (EPC) with the graphene π bands. We show that this EPC, which has strong impact, for example, on Raman scattering and electron transport, can be strongly modified due to interaction with a metallic substrate. For graphene grown on a Ni(111) surface, a total suppression of the Kohn anomaly occurs; the HOB around Γ and K becomes completely flat. This is due to the strong hybridization of the graphene π-bands with the nickel d bands that lifts the linear crossing of the π bands at K. In addition, the out-of-plane modes are also found to be strongly affected by the binding to the substrate. For other metallic substrates, where the distance between the graphene sheet and the substrate is larger, hybridization is much less pronounced and the Kohn anomaly is only weakly perturbed. From experimental phonon dispersions, one can therefore draw conclusions about the interaction strength between graphene and its different substrates.
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Results are presented from an experimental and theoretical study of the electronic properties of back-gated graphene field effect transistors (FETs) on Si/SiO(2) substrates. The excess charge on the graphene was observed by sweeping the gate voltage to determine the charge neutrality point in the graphene. Devices exposed to laboratory environment for several days were always found to be initially p-type. After approximately 20 h at 200 degrees C in approximately 5 x 10(-7) Torr vacuum, the FET slowly evolved to n-type behavior with a final excess electron density on the graphene of approximately 4 x 10(12) e/cm(2). This value is in excellent agreement with our theoretical calculations on SiO(2), where we have used molecular dynamics to build the SiO(2) structure and then density functional theory to compute the electronic structure. The essential theoretical result is that the SiO(2) has a significant surface state density just below the conduction band edge that donates electrons to the graphene to balance the chemical potential at the interface. An electrostatic model for the FET is also presented that produces an expression for the gate bias dependence of the carrier density.
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Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.