Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications.Here we review recent results on the Raman spectroscopy and imaging of graphene.We show ...Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications.Here we review recent results on the Raman spectroscopy and imaging of graphene.We show that Raman spectroscopy and imaging can be used as a quick and unambiguous method to determine the number of graphene layers.The strong Raman signal of single layer graphene compared to graphite is explained by an interference enhancement model.We have also studied the effect of substrates,the top layer deposition,the annealing process,as well as folding(stacking order)on the physical and electronic properties of graphene.Finally,Raman spectroscopy of epitaxial graphene grown on a SiC substrate is presented and strong compressive strain on epitaxial graphene is observed.The results presented here are highly relevant to the application of graphene in nano-electronic devices and help in developing a better understanding of the physical and electronic properties of graphene.展开更多
There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful ...There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar＊ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy （AFM）. Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to Alg modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.展开更多
We quantitatively study the Raman and photoluminescence （PL） emission from single-layer molybdenum disulfide （MoS2） on dielectric （SiO2, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film） and c...We quantitatively study the Raman and photoluminescence （PL） emission from single-layer molybdenum disulfide （MoS2） on dielectric （SiO2, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film） and conducting substrates （Au and few-layer graphene）. We find that the substrate can affect the Raman and PL emission in a twofold manner. First, the absorption and emission intensities are strongly modulated by the constructive/destructive interference within the different substrates. Second, the position of the Alg Raman mode peak and the spectral weight between neutral and charged excitons in the PL spectra are modified by the substrate. We attribute this effect to substrate-induced changes in the doping level and in the decay rates of the excitonic transitions. Our results provide a method to quantitatively study the Raman and PL emission from MoSa-based vertical heterostructures and represent the first step in ad hoc tuning the PL emission of 1L MoS2 by selecting the proper substrate.展开更多
The spherical CaCO3 composite was synthesized in the presence of the amphiphilic block copolymer, i.e. polystyrene-b-poly(acrylic acid). It was revealed by XRD analysis that the grains in the composite was calcite wit...The spherical CaCO3 composite was synthesized in the presence of the amphiphilic block copolymer, i.e. polystyrene-b-poly(acrylic acid). It was revealed by XRD analysis that the grains in the composite was calcite with great distortion. Infared and Raman analysis showed that the composite was composed of PS-b-PAA calcium salt and calcite, and the calcite was nano-scaled.展开更多
We report the thermal conductivities of monolayer （1L） and bilayer （2L） WS2 grown by chemical vapor deposition （CVD）, which are determined by use of temperature and excitation dependences of E2g^1 and A1g Raman ...We report the thermal conductivities of monolayer （1L） and bilayer （2L） WS2 grown by chemical vapor deposition （CVD）, which are determined by use of temperature and excitation dependences of E2g^1 and A1g Raman modes. The first-order temperature coefficients of E2g^1 and Alg modes in both supported and suspended WS2 layers were extracted. The frequency shift of the A3g mode with temperature is larger than that of the E1 mode for 1L-WS2, which is 2g attributed to stronger electron-phonon coupling for the A1g mode than that for the E12g mode. Moreover, by use of the shift of the phonon mode induced by laser heating, the thermal conductivities at room temperature were estimated to be 32 and 53 W/（m.K） for 1L- and 2L-WS2, respectively. Our results provide fundamental information about the thermal properties of WS2 layers, which is crucial for developing applications of atomically-thin WS2 devices.展开更多
We report the preparation of nanocomposites of reduced graphene oxide with embedded Fe3O4/Fe nanorings （FeNR@rGO） by chemical hydrothermal growth. We illustrate the use of these nanocomposites as novel electromagnet...We report the preparation of nanocomposites of reduced graphene oxide with embedded Fe3O4/Fe nanorings （FeNR@rGO） by chemical hydrothermal growth. We illustrate the use of these nanocomposites as novel electromagnetic wave absorbing materials. The electromagnetic wave absorption properties of the nanocomposites with different compositions were investigated. The preparation procedure and nanocomposite composition were optimized to achieve the best electromagnetic wave absorption properties. Nanocomposites with a GO：cx-Fe203 mass ratio of 1：1 prepared by annealing in HdAr for 3 h exhibited the best properties. This nanocomposite sample （thickness = 4.0 mm） showed a minimum reflectivity of -23.09 dB at 9.16 GHz. The band range was 7.4-11.3 GHz when the reflectivity was less than -10 dB and the spectrum width was up to 3.9 GHz. These figures of merit are typically of the same order of magnitude when compared to the values shown by traditional ferric oxide materials. However, FeNR@rGO can be readily applied as a microwave absorbing material because the production method we propose is highly compatible with mass production standards.展开更多
文摘Graphene has many unique properties that make it an ideal material for fundamental studies as well as for potential applications.Here we review recent results on the Raman spectroscopy and imaging of graphene.We show that Raman spectroscopy and imaging can be used as a quick and unambiguous method to determine the number of graphene layers.The strong Raman signal of single layer graphene compared to graphite is explained by an interference enhancement model.We have also studied the effect of substrates,the top layer deposition,the annealing process,as well as folding(stacking order)on the physical and electronic properties of graphene.Finally,Raman spectroscopy of epitaxial graphene grown on a SiC substrate is presented and strong compressive strain on epitaxial graphene is observed.The results presented here are highly relevant to the application of graphene in nano-electronic devices and help in developing a better understanding of the physical and electronic properties of graphene.
基金Acknowledgements The authors would like to thank Prof. Wei Ji from Renmin University for his kindness in sharing with us the unpublished results on the electronic structure calculations of black phosphorus, Prof. Pingheng Tan for his guidance on early Raman characterization, and Dr. Shuo Ding on her assistance with obtaining the optical image used in TOC. This work is financially supported by the National Natural Science Foundation of China （Nos. 51222202, 11104026, and 61376104）, the National Basic Research Program of China （No. 2014CB932500） and the Program for Innovative Research Teams in Universities of the Ministry of Education of China （No. IRT13037） and the Fundamental Research Funds for the Central Universities （No. 2014XZZX003-07）.
文摘There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar＊ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy （AFM）. Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A2g mode stiffens, and the intensity ratio of A2g to Alg modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.
文摘We quantitatively study the Raman and photoluminescence （PL） emission from single-layer molybdenum disulfide （MoS2） on dielectric （SiO2, hexagonal boron nitride, mica and the polymeric dielectric Gel-Film） and conducting substrates （Au and few-layer graphene）. We find that the substrate can affect the Raman and PL emission in a twofold manner. First, the absorption and emission intensities are strongly modulated by the constructive/destructive interference within the different substrates. Second, the position of the Alg Raman mode peak and the spectral weight between neutral and charged excitons in the PL spectra are modified by the substrate. We attribute this effect to substrate-induced changes in the doping level and in the decay rates of the excitonic transitions. Our results provide a method to quantitatively study the Raman and PL emission from MoSa-based vertical heterostructures and represent the first step in ad hoc tuning the PL emission of 1L MoS2 by selecting the proper substrate.
文摘The spherical CaCO3 composite was synthesized in the presence of the amphiphilic block copolymer, i.e. polystyrene-b-poly(acrylic acid). It was revealed by XRD analysis that the grains in the composite was calcite with great distortion. Infared and Raman analysis showed that the composite was composed of PS-b-PAA calcium salt and calcite, and the calcite was nano-scaled.
文摘We report the thermal conductivities of monolayer （1L） and bilayer （2L） WS2 grown by chemical vapor deposition （CVD）, which are determined by use of temperature and excitation dependences of E2g^1 and A1g Raman modes. The first-order temperature coefficients of E2g^1 and Alg modes in both supported and suspended WS2 layers were extracted. The frequency shift of the A3g mode with temperature is larger than that of the E1 mode for 1L-WS2, which is 2g attributed to stronger electron-phonon coupling for the A1g mode than that for the E12g mode. Moreover, by use of the shift of the phonon mode induced by laser heating, the thermal conductivities at room temperature were estimated to be 32 and 53 W/（m.K） for 1L- and 2L-WS2, respectively. Our results provide fundamental information about the thermal properties of WS2 layers, which is crucial for developing applications of atomically-thin WS2 devices.
基金This work was supported by the National Basic Research Program of China （No. 2013CB932602）, the Program of Introducing Talents of Discipline to Universities （No. B14003）, National Natural Science Foundation of China （No. 51527802, 51372020 and 51232001）, Beijing Municipal Science ＆ Technology Commission, Beijing Higher Education Young Elite Teacher Project （No. YETP0354）, Program for New Century Excellent Talents in University （No. NCET- 12-0777）.
文摘We report the preparation of nanocomposites of reduced graphene oxide with embedded Fe3O4/Fe nanorings （FeNR@rGO） by chemical hydrothermal growth. We illustrate the use of these nanocomposites as novel electromagnetic wave absorbing materials. The electromagnetic wave absorption properties of the nanocomposites with different compositions were investigated. The preparation procedure and nanocomposite composition were optimized to achieve the best electromagnetic wave absorption properties. Nanocomposites with a GO：cx-Fe203 mass ratio of 1：1 prepared by annealing in HdAr for 3 h exhibited the best properties. This nanocomposite sample （thickness = 4.0 mm） showed a minimum reflectivity of -23.09 dB at 9.16 GHz. The band range was 7.4-11.3 GHz when the reflectivity was less than -10 dB and the spectrum width was up to 3.9 GHz. These figures of merit are typically of the same order of magnitude when compared to the values shown by traditional ferric oxide materials. However, FeNR@rGO can be readily applied as a microwave absorbing material because the production method we propose is highly compatible with mass production standards.
文摘利用低压金属有机化学气相淀积(LP MOCVD)在Si基片上外延生长ZnO薄膜,制备了两类样品一类是在Si上直接外延ZnO,另一类是在Si上通过SiC过渡层来外延ZnO.根据两类样品的拉曼光谱、x射线衍射、原子力显微图和光致发光的结果,表明ZnO外延薄膜中的张应力对薄膜的结晶状况有着重要的影响,使用SiC过渡层能够有效缓解ZnO薄膜中的张应力,减小缺陷浓度,提高ZnO外延层的质量;然后根据缺陷的形成机制进一步提出,对于ZnO Si,其中较大的张应力导致了高浓度的非辐射复合缺陷的形成,使得样品的紫外和绿峰的发射强度均大大降低;对于ZnO SiC Si,其中较小的张应力导致ZnO薄膜中主要形成氧替位缺陷OZn,从而使发光中的绿峰增强.
