Raman Investigation Research Articles

Temperature effects on the vibrational properties of the Cs2SnX6 ‘defect’ perovskites (X = I, Br, Cl)

Air-stable, vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) have been attracting particular interest due their potential applications in solar cells and thermoelectrics based on their favorable structural, optical and transport properties. In this work, a comprehensive Raman investigation of Cs2SnX6 perovskites is performed over an extended temperature range of 83–433 K in order to provide insight to their lattice dynamics. Substitution of the lighter and more electronegative Cl halogens with the heavier bromide and iodide ones resulted in the decrease of the frequencies for all Raman modes comprising the internal vibrations in the SnX6 octahedra and the νL(F2g) mode due to Cs vibrations against SnX6 in the rigid lattice. Blue shift and narrowing of the SnX6 stretching and bending bands was invariably observed for all halogen analogues with temperature decrease, evidencing the presence of lattice anharmonicity. More importantly, the elusive νL(F2g) phonon was only observed for Cs2SnCl6 for all temperatures, complying with the predictions of Hirshfeld surface analysis for the enhanced Cs–SnCl6 interactions. The latter mode emerged for Cs2SnI6 only above room temperature, indicating that combination of anharmonic lattice dynamics with the increased Cs–SnI6 oscillation energy upon increasing temperature may activate this specific “lattice mode”, which in the limit of very weak Cs–I interaction approaches the “rattling’’ Cs vibrations in the cuboctahedral cage. Strong lattice anharmonicity for the chemically stable Cs2SnX6 compounds can be accordingly inferred, which is a key aspect for their performance in future optoelectronic and thermoelectric applications.

Green synthesis of single phase hausmannite Mn3O4 nanoparticles via Aspalathus linearis natural extract

Nowadays, green synthesis of nanoparticles using plant precursors has been extensively studied. However, less attention has been given to Mn3O4. This contribution validates the synthesis of single-phase Hausmannite Mn3O4 nanoparticles by a green approach without using any standard acid/base compounds, surfactants, and organic/inorganic dissolving agents. The chemical chelation of the Mn precursor was performed via bioactive compounds of the Aspalathus Linearis’ extract, an African indigenous plant. Annealing at 400 °C for ~ 1 h was required to crystallize the small amorphous nanoparticles with an initial bimodal size distribution peaking at leftlangle {phi_{1} } rightrangle ~ 4.21 nm and leftlangle {phi_{2} } rightrangle ~ 8.51 nm respectively. Such annealing lead to increase in the diameter of the nanoparticles from 17 to 28 nm.The morphological, structural, vibrational, surface, and photoluminescence properties of the single-phase Hausmannite nanoparticles were comprehensively investigated by High Resolution Transmission Electron Microscopy(HRTEM),Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), Raman and X-rays Photoelectron Spectroscopy (XPS), spectroscopy as well as room temperature photoluminescence. Structural and morphological investigations revealed the formation of quasi-spherical nanoparticles having a single phase Hausmannite Mn3O4 crystal structure. XPS results also validated the XRD results about the formation of Hausmannite Mn3O4 nanoparticles. Raman investigations allowed a crystal-clear distinction between the Mn3O4 nature of the nanoparticles from the potential γ -Mn2O3 phase as both phases belong to the same space group and both assume tetragonally-distorted cubic lattices of nearly similar dimensions. The optical studies of the single phase Hausmannite crystalline nanoparticles exhibited a broad photoluminescence in the spectral range of 300–700 nm, which is ideal for emission devices.Graphic abstract

Efficient Yb3+ sensitized Er3+ emission of Bi2ZnOB2O6:Yb3+/Er3+ single crystal

Bi2ZnOB2O6 single crystals doped with Er3+and Yb3+ ions were grown by means of the Kyropoulos method. Bi2ZnOB2O6:Yb3+/Er3+ crystals were characterized by high values of nonlinear optical coefficients as well as efficient luminesce of Er3+ ions. The structure, vibrational and luminescent properties of Bi2ZnOB2O6:Yb3+/Er3+ crystals were investigated for the first time. The Raman investigation showed the low phonon energy of Bi2ZnOB2O6 host matrix associated to the vibrations of BO3, BO4, ZnO4 and BiO6 molecular groups, which allow effective phonon assisted energy transfer between Yb3+ and Er3+ ions. The 1532 nm emission of Bi2ZnOB2O6:Yb3+/Er3+ crystals, associated with the 4I13/2 → 4I15/2 transition of Er3+ ions, was excited at 380 (by Bi3+ ions), 488 (by Er3+ ions) and 980 nm (by Yb3+ ions) and efficient Yb3+ sensitization of Er3+ emission was observed. Moreover, the longest decay lifetime equal to 1.36 ms of Er3+ emission at 1532 nm was observed for Bi2ZnOB2O6:Yb3+/Er3+ crystals under 980 nm excitation. Additionally, using the Inokuti - Hirayama model, the kinetics of the fluorescence intensity was analyzed, besides, the energy transfer micro-parameter CDA and the energy transfer rate parameter WDA were determined for Bi2ZnOB2O6:Yb3+/Er3+ crystals. The results indicate a great potential of new multifunctional Bi2ZnOB2O6:Yb3+/Er3+ crystals for application in the integrated new-generation laser devices.

Confined Selenium in N-Doped Mesoporous Carbon Nanospheres for Sodium-Ion Batteries.

In this paper, we have adopted a simple and etching-free method to prepare mesoporous carbon spheres in one step. Selenium can be deposited in the internal cavity, which can avoid pulverization due to the combined effect of volume expansion and a solid-electrolyte interphase (SEI) film while charging and discharging. Therefore, the as-prepared selenium and nitrogen codoped mesoporous carbon nanosphere (Se@NMCS) composites can deliver an outstanding sodium-storage performance of 336.6 mAh g-1 at a present density of 200 mA g-1 and great long-cycling performance. For a further understanding of the Na+ storage mechanism of the Se@NMCS anode in sodium-ion batteries (SIBs), the phase evolution of the Se@NMCS anode has been explored during the charge/discharge process by conducting in situ Raman investigation.

Raman investigation of corrosion products on Roman copper-based artefacts

<p class="Abstract">This paper illustrates a case study related to the characterisation of corrosion products present on recently excavated artefacts. The archaeological findings, from the Rakafot 54 site (Beer-Sheva, Israel), consist of 23 coins and a pendant, all dating back to the Roman period. Raman spectroscopy was used to identify the corrosion products that compose the patina covering the objects. To facilitate and support their identification, spectra were then processed using principal components analysis. This chemometric technique allowed the identification of two main compounds, classified as atacamite and clinoatacamite, which formed the main components of the patinas. The results of this investigation can help in assessing the conservation state of artefacts and defining the correct restoration strategy.</p>

The split-up of G band and 2D band in temperature-dependent Raman spectra of suspended graphene

Dedicated Raman spectroscopy investigations in the temperature range from 78 K to 298 K were performed for the graphene suspended above round holes on SiO2/Si substrate (the suspended graphene, SUG), in comparison with the surrounding graphene adhered to solid SiO2/Si substrate (the substrate supported graphene, SSG). The results confirmed that one-layer SUG (1L-SUG) was subjected to strains originated from the surrounding 1L-SSG due to the different area and shape around the hole along different directions. In further, the split-up of G band and 2D band were observed at low temperature because of the enhanced anisotropic strains in 1L-SUG due to the mismatch of thermal expansion coefficient (TEC) between the 1L-SSG and SiO2/Si substrates. The temperature-dependent (T-dependent) frequency shifts for the G band and 2D band in 1L-SUG and 1L-SSG were consistent with previous Raman investigations. Therefore, the micro-hole boundary effect in suspended graphene play a significant role in determination of the properties for graphene materials and graphene-based flexible devices.

Raman investigation of hydration structure of iodide and iodate* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11374123 and 12004132) and Science and Technology Planning Project and Talent Project of Jilin Province, China (Grant Nos. 20170204076GX, 20180101006JC, 20180101238JC, 20190201260JC, 20200201179JC, 2019C0355-5, JJKH20200935KJ, and JJKH20200936KJ)

In the troposphere, the destruction of ozone and the formation of new particles are closely related to the iodine content, which mainly comes from iodide (I−) and iodate () in the seawater. Therefore, understanding the interactions between I−, and water molecules plays a certain role in alleviating the destruction of the ozone layer. Raman spectroscopy is commonly used to obtain the information of the interaction between I−, and water molecules quickly and accurately. Herein, the effect of I− and on the change in Raman band characteristics of water is investigated to reflect the associated intermolecular interactions change. With the addition of the two ions, the Raman band corresponding to OH stretching vibration moves towards the high wavenumber, indicating the formation of hydration structure. The narrowing of the Raman band from OH stretching vibration under weak hydrogen bond agrees well with the hydrogen bond variation, while the abnormal broadening of the Raman band from OH stretching vibration under strong hydrogen bond indicates the formation of H-down structure. With the increase of ions concentration, the frequency shift of the Raman band from OH stretching vibration under both weak and strong hydrogen bonds becomes more apparent. Meanwhile, the frequency shift of I− is more obvious than that of , which indicates that I− is more likely to form the hydration structure with water than . These results contribute to analyzing the different interactions between I−–water and –water, then helping to prevent ozone depletion.

In situ Raman investigation of resting thermal effects on gas emission in charged commercial 18650 lithium ion batteries

The inherent risk of side reactions in lithium ion batteries (LIBs) is a crucial issue for large battery packs. To mitigate this problem, there have been previous studies that have attempted to reveal the possible chemical reactions from decomposition of electrolytes and electrode materials’ reactions during cycling. In particular, the gases expelled during decomposition have garnered research attention as they are explosive, flammable, toxic, and raise the inner pressure of a cell drastically. In addition, it is important to understand thermal effects on gas evolution or degradation of the electrode layer because batteries are normally exposed to a warm operation condition. Herein, the gases expelled within commercial 18650 cylindrical type LIB cells were investigated in moderate thermal conditions in a charged state. The investigation was conducted using a lab-made in situ Raman spectroscopic analysis system and performing comprehensive transient electrochemical analyses. Our Raman spectroscopy system could explore the battery reactions non-disruptively, which is critical for the interior of a cell. Through this in situ technique, the gas expelling trends upon the thermal conditions were elucidated; furthermore, detrimental effects of gas generation reactions to a cell integrity were proposed through comparison with the electrochemical results.

Raman spectroscopic detection of polyene-length distribution for high-sensitivity monitoring of photo- and thermal degradation of polyvinylchloride

The degradation of the ubiquitous polyvinylchloride (PVC) material under the influence of various factors is known to result commonly in polyene formation. Such polyene defects occur in the form of conjugated aliphatic chains with different lengths and contents, and their sensitive and length-specific monitoring is important for the assessment of PVC degradation. Here, we report on the resonance-enhanced Raman signatures of polyene sequences of varying lengths in photo- and thermally degraded PVC films. The lengths of polyene segments have been estimated based on their selectively enhanced and spectrally resolved contributions to the Raman bands assigned to the stretching vibrations of conjugated double carbon bonds. Using deconvolution analysis of a characteristic Raman band of polyenes, we especially demonstrate that the spectral signatures of polyene segments corresponding to other various electronic resonances contribute to the Raman spectral envelope observed at a given resonant excitation. In most cases, we observe an asymmetric band profile, indicating an asymmetric length distribution of polyene defects formed in PVC films upon a mild degradation extent less than 1% mass loss. We also demonstrate that the wavenumber (ν1) of the stretching vibrations of single carbon bonds in the polyene sequences of degraded PVC is inversely related to the number (n) of double carbon bonds by an empirical equation n=476·cm-1/ν1-1082·cm-1. To the best of our knowledge, while considering different laser excitations spanning the range of possible electronic resonances from blue to near-infrared for Raman investigations, the present work includes (i) the first Raman spectral deconvolution study for the 532.0 nm excitation wavelength used in most portable Raman probes nowadays and (ii) the screening of polyene defects also beyond the red edge of the visible spectrum and the evidence of a resonance-enhanced polyene signal at 647.1 nm. Important new information has been obtained about polyene lengths and spectral distribution for PVC, whose critical physical properties ranging from flexibility to electrical resistance are severely affected by polyene formation.

Operando Raman and UV-Vis spectroscopic investigation of the coloring and bleaching mechanism of self-powered photochromic devices for smart windows

Abstract Automatically controlling the optical transmittance of solar light provides an energy-saving strategy for designing energy-efficient buildings. Ideally, the smart windows should be able to show homogeneous and large-scale color-switching performance while modulating the intensity of daylight. Among the suitable candidates for smart windows, the photochromic device has a great potential for homogenous and large-scale color-switching material. In a photochromic device, the intercalation/deintercalation of Li-ions is a key step determining the kinetics and efficiencies of the coloring/bleaching process. Herein, we exploit operando Raman spectroscopy to investigate the photochromic mechanism, which reveals the movement of Li ions into specific sites within the tunnels that exist in hexagonal tungsten oxide (WO3). These spectroscopic results demonstrate that the intercalation/deintercalation of the Li ions is related to specific sites in the WO3 lattice which stabilizes the energy of the intermediate state during the coloring/self-bleaching process. Additionally, theoretical simulation supports that electrons, as well as Li ions, participate in the color-switching performance. This study improves our understanding of the photochromic performance of this special device and opens up a research field toward the development of smart windows applications.

Electronic-temperature estimation of Joule-heated graphene via Raman investigations

Temperature-dependent Raman scattering provides valuable information on electron–phonon coupling and phonon anharmonicity of graphene. In this study, we show an enhancement of Joule heating in graphene by confining the current flow in a narrow channel. In addition, we performed a detailed analysis of the anharmonic effect in the hBN/monolayer graphene/hBN heterostructure based on the behaviour of the full-width at half maximum of the G mode with increasing electric power: a non-monotonic trend, leading to the key of approximation of the electronic temperature in graphene. We believe our results could offer a convenient analysis tool to study electron–phonon coupling and anharmonic phonon-decay processes in a high-temperature regime.

In situ electrochemical Raman investigation of charge storage in rGO and N-doped rGO.

In this study, in situ electrochemical Raman spectroscopy was applied to clarify the charge storage mechanism in three types of anodes, synthetic graphite, reduced graphene oxide (rGO), and nitrogen-doped reduced graphene oxide (N-rGO). The Li+ intercalation phenomenon was measured in LiPF6 electrolyte solution using a modified coin cell setup. The synthetic graphite anode showed the splitting of the G peak at the potential E < 0.2 V vs. Li/Li+, corresponding to the formation of a graphite intercalation compound (GIC) and its second-order 2D peak was found to be red-shifted due to charge transfer and induced strain in the potential region of 0.5 to 0.15 V vs. Li/Li+. In the case of rGO, the lattice defects assisted in large and early intercalation of electrolyte ions, which is confirmed by the red-shift in the G peak (∼36 cm-1) and its early disappearance below 0.3 V vs. Li/Li+, respectively. Unlike rGO, nitrogen vacancies in N-rGO provide active sites for Li+ intercalation, resulting in enhanced charge transfer, displayed by the large red-shift in the G peak (∼55 cm-1) and blue-shift in the D peak. In addition, a new Raman peak at 1850 cm-1 was observed in N-rGO for the first time, corresponding to the formation of a reversible intermediate species from the interaction between Li+ and nitrogen vacancies. This work demonstrates the use of a simple in situ technique to get insight into the nano-carbon electrodes during device operation and to reveal the role of doped nitrogen atoms for Li+ intercalation.

In situ Raman investigation of the phase transition of NaVO2F2 under variable temperature conditions.

In this study, the phase transition of NaVO2F2 was measured at different temperatures via in situ Raman spectroscopy. The NaVO2F2 compounds were synthesized by a hydrothermal method and were identified to be monoclinic with the P21/c space group at room temperature by XRD. Accordingly, the variations of Raman shifts and intensities of the characteristic peaks for NaVO2F2 associated with temperature were obtained and investigated. It was confirmed that NaVO2F2 had three types of phase transitions, which occurred in the temperature region from 78 K to 573 K. Further, the results indicate that transition from a low-temperature phase (I) to another low-temperature phase (II), low-temperature phase (II) to P21/c phase and P21/c phase to P21/m phase occurred near the three temperature points of 93 K, 233 K, and 453 K, respectively, during the heating process. Therefore, a novel characterization method was provided for further research on the phase transition theory and performance of vanadate compounds.

LA-ICP-MS U-Pb dating of metamorphic monazite and zircon of the Qinling metamorphic complex, Xixia and Neixiang areas, Henan Province

西峡-内乡地区位于北秦岭造山带东部， 出露黑云斜长片麻岩、变质砂岩、石英片岩、石墨大理岩、石榴角闪岩等变质岩石。激光拉曼测试表明，变质岩锆石与独居石包裹体中， 未发现高压-超高压变质特征矿物。值得注意的是，显微岩相研究也未见到多期变质作用的证据。对变质独居石和变质锆石进行的LA-ICP-MS U-Pb定年表明， 副变质岩记录的变质高峰期时代为414~405Ma， 石榴角闪岩记录的变质高峰期时代为401~379Ma， 属晚志留世-中泥盆世。区内岩石普遍经历了角闪岩相变质作用， 变质高峰期P-T条件为660~710℃/0.53~1.02GPa， 属于中压变质相系。不同变质岩的变质高峰期压力条件差异明显， 说明这些岩石俯冲深度不同， 在折返阶段才以构造岩片的形式混杂在一起。各种变质岩中退变质矿物组合普遍不发育， 暗示抬升速率较快。;Metapelite, metasandstone, quartzite, marble and garnet amphibolite expose in the Xixia and Neixiang areas, eastern North Qinling Orogenic Belt. No (ultra-) high pressure mineral inclusions were found within the metamorphic monazite or zircon through laser Raman investigation, and no petrographic evidences suggest they experienced polymetamorphism. LA-ICP-MS U-Pb dating of metamorphic monazite and zircon suggest that the metamorphic peak occurred at 414~405Ma recorded by metapelite or 401~379Ma by amphibolite, i.e., Late Silurian to Middle Devonian. The rocks experienced amphibolite facies metamorphism with peak P-T conditions estimated to be 660~710℃/0.53~1.02GPa, which is ascribed to the medium P/T facies series. However, significant diverse pressure conditions of these rocks imply that they reached different depths and later they were amalgamated in the tectonic exhumation. Ubiqutous absence of retrograde mineral assemblages imply that the exhumation was relatively fast.

Influence of Al3+ doping for V5+ on the structural, optical, thermal and electrical properties of V2-xAlxO5-δ (x=0–0.20) ceramics

This paper reports an investigation on the structure-properties correlation of trivalent metal oxide (Al2O3)-doped V2O5 ceramics synthesized by the melt-quench technique. XRD patterns confirmed a single orthorhombic V2O5 phase formation with increasing strain on the doping of Al2O3 in place of V2O5 in the samples estimated by Williamson-Hall analysis. FTIR and Raman investigations revealed a structural change as [VO5] polyhedra converts into [VO4] polyhedra on the doping of Al2O3 into V2O5. The optical band gap was found in a wide semiconductor range as confirmed by UV–visible spectroscopy analysis. The thermal and conductivity behavior of the prepared samples were studied using thermal gravimetric analysis (TGA) and impedance analyzer, respectively. All the prepared ceramics exhibit good DC conductivity (0.22–0.36 Sm-1) at 400 ᵒC. These materials can be considered for intermediate temperature solid oxide fuel cell (IT-SOFC)/battery applications due to their good conductivity and good thermal stability.

MoS2 nanorods anchored reduced graphene oxide nanohybrids for electrochemical energy conversion applications

Herein, the hydrothermal route has been explored to design a novel network of molybdenum sulfide (MoS 2 ) nanorods/reduced graphene oxide (rGO) by varying wt% of MoS 2 (10–30 wt%) for the highly stable and efficient electrochemical energy conversion applications involving dye sensitized solar cells (DSSCs); and direct methanol fuel cells (DMFCs). The MoS 2 /rGO nanohybrids network has been systematically characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). XRD, HRTEM, Raman and XPS investigations confirmed the uniform and homogenous anchoring of MoS 2 nanorods on the rGO sheets in the MoS 2 /rGO nanohybrids. It has been revealed that the wt% of MoS 2 in MoS 2 /rGO nanohybrids strongly affects the anchoring of MoS 2 nanorods on rGO sheets which in turn affects their electrocatalytic behavior. The optimized MoS 2 /rGO nanohybrids with 20 wt% of MoS 2 nanorods exhibited long term stability and highly efficient electrocatalytic behavior. DSSCs assembled with MoS 2 /rGO nanohybrids as CE exhibited comparable power conversion efficiency (PCE) relative to standard DSSC. The optimized anchoring of MoS 2 on rGO sheets resulted in high current density as compared to rGO based electrocatalyst in MORs. Moreover, reproducibility of CV curves revealed high stability of MoS 2 /rGO nanohybrids under harsh electrolyte medium. • Synthesis of novel network of molybdenum sulfide nanorods (MoS 2 )/ reduced graphene oxide (rGO) using hydrothermal method. • Optimized MoS 2 /rGO nanohybrids (20 wt% of MoS 2 ) exhibits long term stability and high electrocatalytic behavior. • MoS 2 /rGO nanohybrids act as effective replacement of Pt electrocatalyst in DSSCs and methanol oxidation reactions in DMFCs.

Stimulated Raman scattering and defect-based photoconductivity in mixtures of CdS/TiO2

Stimulated Raman scattering (SRS) is studied in polycrystalline CdS/TiO2 mixtures based on laser power and temperature dependences in Raman investigation. Our Raman analysis reveals several features of nonlinear behavior, such as nonlinear laser power dependencies or high enhancement of Raman lines at a low temperature due to the intrinsic nonlinear nature of CdS, a high light scattering inside the matter, and a trap-based diffusive medium in a grain-boundary system. In particular, building up experimental evidence of conductivity at various temperatures, it is shown that the samples exhibit thermally activated conductivity due to potential local barriers revealed by a considerable fraction of the grain-boundaries in a diffusive medium. Taking into account the defect-based conductivity, a double Poole–Frenkel barrier is established within the grain-boundaries of CdS/TiO2 mixtures, and their barrier energies are investigated considering the temperature-dependent dark current of the grain-boundary model. We thus demonstrate that diffusive medium-based defects in a grain-boundary model makes CdS/TiO2 mixtures behave efficiently in nonlinear processes such as SRS. These features are of particular interest for optical communication technology, and the fundamental application of the SRS effect is semiconductor optical amplifiers.

High-pressure Raman investigation of high index facets bounded α-Fe2O3 pseudocubic crystals

High index facet bounded α-Fe2O3 pseudocubic crystals has gained the attention of the scientific community due to its promising electrochemical sensing response towards aqueous ammonia. The structural stability of α-Fe2O3 pseudocubic crystals is investigated through high-pressure Raman spectroscopy up to 22.2 GPa, and those results are compared with our ab initio theoretical calculations. The symmetry of the experimental Raman-active modes has been assigned by comparison with theoretical data. In addition to the Raman-active modes, two additional Raman features are also detected, whose intensity increases with compression. The origin of these two additional peaks addressed in this study, reveals a strong dependence on the geometry and the low dimensionality as the most plausible explanation.

Local ferroelectric polarization in antiferroelectric chalcogenide perovskite BaZrS3 thin films

Bulk chalcogenide perovskite BaZrS3 (BZS), with a direct band gap in visible region, is an important photovoltaic material, albeit with limited applicability owing to its antiferroelectric (AF) nature. Presently, ferroelectric (FE) perovskite-based photovoltaics are attracting enormous attention for environmental stability and better energy conversion efficiency through enhanced charge separation, owing to loss of center of inversion symmetry. We report on antiferroelectric-ferroelectric (AF-FE) phases of BZS thin film, grown with chemical vapor deposition (CVD), using temperature-dependent Raman investigations and first-principles calculations. The origin of FE phases is established from anomalous behavior of A7g ~ 300 cm-1 and B1g5 ~ 420 cm-1 modes, which involves the vibration of atoms at apical site of ZrS6 octahedra. Additionally, below 60 K, B1g1 and B2g2 ( ~ 85 cm-1) modes appear whereas B12g (~ 60 cm-1) disappears to stabilize the Pnma structure against ferroelectricity by local distortion. Here, B2g2 and B1g2 involve vibrations of Ba atoms in AF manner while B1g1 involves, in addition, the rotation of octahedra as well. Our first-principles calculations confirm that FE appears as a result of loss of center of inversion symmetry in ZrS6 octahedra due to existence of oxygen (O) impurities placed locally at apical sites of sulfur (S) atom.

Temperature-dependent Raman investigation and photoluminescence of graphene quantum dots with and without nitrogen-doping

The study of the temperature dependence of Raman spectrum of graphene quantum dots is important for further understanding of the fine structure and properties of the material such as atomic bonds, thermal expansion, and thermal conductivity. In this work, we present new results on the temperature dependence of the frequency of G peak in the Raman spectra of bare GQDs and nitrogen-doped graphene quantum dots (N-GQDs), respectively. The nature of these GQDs is investigated using high-resolution transmission electron microscopy together with Raman, absorption, and photoluminescence spectra. We monitored a shift in G peak frequency toward lower frequencies with the temperature ranging from 81 to 663 K. In addition, a slight decrease in G peak intensity and an increase in D peak intensity have been observed, which is especially clear for Raman spectra of N-GQD sample. From the linear relationship of G peak frequency with temperature, we determine for the first time the value of the temperature coefficient χ of the G mode. Values of − 0.0222 ± 0.001 cm−1 K−1 for GQDs, and − 0.0243 ± 0.0013 cm−1 K−1 for N-GQDs were deduced. These results show a clear heat expansion in the graphene lattice, although small in size, when sample temperature is increased from 81 to 663 K. Our result is important to further understand physical effects induced by anharmonic phonons in QGDs and should be taken to account when designing efficient nanoelectronic devices employing GQDs.