The removal of single layers from multi-layer graphene by low-energy electron stimulation

The removal of single atomic layers from multi-layer graphene using a He plasma is reported. By applying sample biases of -60 and +60 V during He plasma exposure, layer removal is found to be due to electrons instead of He ions or neutrals in the plasma. The rate of layer removal depends on exposure time, sample bias, and pre-annealing treatments. Optical contrast microscopy and atomic force microscopy studies show that the removal of C atoms occurs approximately one layer at a time across the entire multi-layer sample with no observable production of large pits or reduction in lateral dimensions. Layer removal is proposed to arise from the electron-stimulated dissociation of C atoms from the basal plane. This process differs from plasma techniques that use reactive species to etch multi-layer graphene.     

Controlled Electrodeposition of Gold on Graphene: Maximization of the Defect‐Enhanced Raman Scattering Response

A reliable method to prepare a surface‐enhanced Raman scattering (SERS) active substrate is developed herein, by electrodeposition of gold nanoparticles (Au NPs) on defect‐engineered, large area chemical vapour deposition graphene (GR). A plasma treatment strategy is used in order to engineer the structural defects on the basal plane of large area single‐layer graphene. This defect‐engineered Au functionalized GR, offers reproducible SERS signals over the large area GR surface. The Raman data, along with X‐ray photoelectron spectroscopy and analysis of the water contact angle are used to rationalize the functionalization of the graphene layer. It is found that Au NPs functionalization of the “defect‐engineered” graphene substrates permits detection of concentrations as low as 10^?16 m for the probe molecule Rhodamine B, which offers an outstanding molecular sensing ability. Interestingly, a Raman signal enhancement of up to ≈10⁸ is achieved. Moreover, it is observed that GR effectively quenches the fluorescence background from the Au NPs and molecules due to the strong resonance energy transfer between Au NPs and GR. The results presented offer significant direction for the design and fabrication of ultra‐sensitive SERS platforms, and also open up possibilities for novel applications of defect engineered graphene in biosensors, catalysis, and optoelectronic devices.     

Surface-structured gold-nanotube mats: fabrication, characterization, and application in surface-enhanced Raman scattering

The fabrication and characterization of nanostructured fibrous gold mats having high specific surface areas is reported. Freestanding porous films of 6-20-μm thickness and density 0.43 ± 0.02 g cm(3) are prepared using e-beam evaporation of gold on an electrospun nanoporous polymer template and subsequent removal of the template polymer in a suitable solvent. Structural characterization using electron microscopy techniques shows a nanofiber diameter in the range of 300-6000 nm, and the size of the nanochannels on the fiber surface is ≈200-350 nm. Such surface structuring is achieved through fast evaporation of organic solvent and phase separation of polymers during the electrospinning process. The wedge thickness varies from a few nanometers to a few tens of nanometers. The freestanding films possess good mechanical integrity and robustness. The calculated Young's modulus based on the slope in the elastic region is ≈114 MPa and gives an ultimate breaking strength of 0.7-0.8 MPa at a percentage elongation of 1.5-2.0%. X-ray diffraction and transmission electron microscopy measurements demonstrate the formation of polycrystalline gold nanostructures. Electrical characterization performed on these gold nanotubes reveals pure metallic behavior. Raman spectroscopic characterization of the fibrous membrane is performed using crystal violet (CV) adsorbed on it. Well-defined spectral peaks are obtainable at concentrations as low as 10(-7) M of CV, which did not give spectral signals at this low concentration on its own.     

Raman and optical spectroscopy characteristics of Se-doped Bi12SiO20 crystals

Crystals of BSO doped with Se in two different concentrations (BSO:Se(I) and BSO:Se(II)) were grown by the Czochralski method. It is established that doping with Se is accompanied with preferential absorption of Fe from the melt. According to the in-depth chemical analysis, it is assumed that the doping Se ions enter the tetrahedral positions by means of the substitution of 3Si⁴⁺ by (Se⁶⁺ + 2Fe³⁺) ions. The measured absorption spectrum of the low-concentration Se-doped crystal (BSO:Se(I)) confirms such a conclusion as the absorption coefficient is increased in a broad spectral interval (1.4-3.1 eV) - an effect typical for all Fe-doped Bi₁₂SiO₂₀ crystals. The polarized Raman spectra of BSO:SeI show that the doping-induced lattice distortions are small. The IR spectrum of the BSO:SeI crystal yields indications for local lowering of the symmetry of the Fe-occupied tetrahedral positions. Doping with Se at high concentration (BSO:Se(II)) leads to occasional second phase inclusions and to a downshift of practically all modes in the Raman spectrum by 2-5.5 cm⁻¹. It is concluded that the doping with Se at high concentrations follows the same mechanism as that at low concentrations but the introduced lattice distortions are more significant, and lead to an enlargement of the unit cell while preserving the overall cubic symmetry.     

Raman spectroscopy characterization of diamond films on steel substrates with titanium carbide arc-plated interlayer

Diamond chemical vapour deposition (CVD) on steel represents a difficult task. The major problem is represented by large diffusion of carbon into steel at CVD temperatures. This leads to very low diamond nucleation and degradation of steel microstructure and properties. Recent work [R. Polini, F. Pighetti Mantini, M. Braic, M. Amar, W. Ahmed, H. Taylor, Thin Solid Films 494 (2006) 116] demonstrated that well-adherent diamond films can be grown on high-speed steels by using a TiC interlayer deposited by the PVD-arc technique. The resulting multilayer (TiC/diamond) coating had a rough surface morphology due to the presence of droplets formed at the substrate surface during the reactive evaporation of TiC. In this work, we first present an extensive Raman investigation of 2 μm, 4 μm and 6 μm thick diamond films deposited by hot filament CVD on TiC interlayers obtained by the PVD-arc technique. The stress state of the diamond was dependent on both the films thickness and the spatial position of the coating on the substrate. In fact, on the top of TiC droplets, the stress state of the diamond was much lower than that of diamond in flatter substrate areas. These results showed that diamond films deposited on rough TiC interlayers exhibited a wide distribution of stress values and that very large compressive stress exists in the diamond film grown on flat regions of steel substrates with a TiC interlayer. Diamond films could accommodate stresses as large as 10 GPa without delamination.     

Direct measurement of the Raman enhancement factor of rhodamine 6G on graphene under resonant excitation

Graphene substrates have recently been found to generate Raman enhancement. Systematic studies using different Raman probes have been implemented, but one of the most commonly used Raman probes, rhodamine 6G （R6G）, has yielded controversial results for the enhancement effect on graphene. Indeed, the Raman enhancement factor of R6G induced by graphene has never been measured directly under resonant excitation because of the presence of intense fluorescence backgrounds. In this study, a polarization-difference technique is used to suppress the fluorescence background by subtracting two spectra collected using different excitation laser polarizations. As a result, enhancement factors are obtained ranging between 1.7 and 5.6 for the four Raman modes of R6G at 611, 1,183, 1,361, and 1,647 cm-~ under resonant excitation by a 514.5 nm laser. By comparing these results with the results obtained under non-resonant excitation （632.8 nm） and pre-resonant excitation （593 nm）, the enhancement can be attributed to static chemical enhancement （CHEM） and tuning of the molecular resonance. Density functional theory simulations reveal that the orbital energies and densities for R6G are modified bv ~raphene dots.     

Sorting of micron-sized particles using holographic optical Raman tweezers in aqueous medium

We have constructed a holographic optical tweezers system combined with Raman spectroscopy to sort trapped particles. Our software automatically moves the trapped objects to the measurement positions to obtain individual Raman signals from multiple trapped particles. We performed the sorting by comparing their spectra with the previously measured training dataset using the correlation coefficients. We used yeast cells and polystyrene beads as test particles. This study aims to show that biological particles can be separated using single cell analysis with combined holographic optical tweezers and Raman spectroscopy system.     

A SERS-active nanocrystalline pd substrate and its nanopatterning leading to biochip fabrication

Nanocrystalline metallic Pd films have been produced by thermolysis of Pd hexadecanethiolate deposited on a Si surface. The particle size was in the range of 50-60 nm based on electron and atomic force microscopy. The Pd surface produced was found to be surface-enhanced Raman scattering (SERS) active with an enhancement factor of approximately 10(5). The electron-resist behavior of Pd thiolate is exploited in conceptualizing a prototype chip with patterned Pd regions that are SERS active, called "SERS bits", which host different biomolecules.     

Molecular dynamic simulation and characterization of self-assembled monolayer under sliding friction

The frictional mechanisms of a self-assembled monolayer (SAM) during nano-scale sliding are studied using molecular dynamics (MD) simulation. The MD model consists of a gold slider and gold substrate with n-hexadecanethiol SAM chemisorbed to the substrate. The trajectory, tilt angles, normal forces, frictional forces, friction coefficients and potential energies per molecular chain of the SAM molecules are evaluated during the frictional process for various parameters including as sliding height, sliding direction (i.e. pro- or anti- the SAM tilt angle), sliding velocity and system temperature. The various parameters are discussed with regard to frictional forces, mechanisms and SAM structural transition. Results show that stick-slip occurs and is related to the sliding period and tilt angle of the SAM molecules. Amplitude of the stick-slip cycle increases with decreasing sliding height until reaching a critical sliding height, which is characterized such that sliding below the critical height causes irreversible changes in the SAM molecular organization and cumulative loss of SAM lubricating efficiency. Different SAM recovery mechanisms were found for different sliding directions relative to SAM tilt angle (pro- or anti-tilt). In both cases, minimum friction occurred during the SAM tilt-angle recovery phase. The friction force curves for these two cases also showed a regular phase shift above the critical height. For stick-slip sliding above the critical height, anti-tilt sliding had significantly lower average friction, but this trend inverted below the critical height. Sliding lower than the critical height cause progressive disorder of the SAM structure and the characteristic differences between pro- and anti-tilt sliding were progressively lost.     

Superconductivity, structure visualization, mechanical strength promotion and Raman spectra of hafnium-doped-123-YBCO synthesized via urea precursor route

The pure YBCO (YBa₂Cu₃O₇) and its variant hafnium containing superconductors with general formula: Y_1−xHf_xBa₂Cu₃O_z, where x = 0.1, 0.2, and 0.4 mole, respectively, were synthesized by solution route using urea as precursor forming agent. X-ray measurements indicated that Hf⁴⁺ ions have a negligible effect on the main crystalline structure and substitute Y-sites successfully in lattice structure of 123-YBCO at low levels of hafnium doping (x = 0.1 → 0.2 mole). From SE-microscopy mapping and EDX elemental analysis Hf⁴⁺ was detected qualitatively with good approximation to the actual molar ratio but not observed at 123-YBCO grain boundaries which confirm that hafnium (IV) has diffused regularly into material bulk of superconducting 123-YBCO-phase at low levels of concentrations. Structure visualization of Hf-doped-123-YBCO was made to confirm success of hafnium substitutions inside crystal lattice on Y-sites of 123-YBCO superconductors. Hafnium dopings affected sharply on the main vibrating modes of YBCO regime particularly on the apical oxygen (O₄) vibrational mode A₁g. Magnetic susceptibility measurements proved that hafnium dopings have strong effect on the transport properties of YBCO-composites regime. Hafnium promotes mechanical tensile coefficient recording maxima 35.7 MPa for x = 0.4 mole.     

碳纤维增强环氧树脂基复合材料湿热残余应力的微Raman光谱测试表征

采用微Raman光谱仪对碳纤维增强环氧树脂复合材料CF/EP(纤维体积分数为30%)的湿热残余应力进行了研究。实验结果表明：湿热残余应力能够使碳纤维Raman光谱发生频移,根据频移可对纤维所受湿热残余应力进行表征;选择合适的试验点是复合材料湿热残余应力Raman测试成功的关键;在湿热环境下长期吸湿,纤维所受轴向残余应力由吸湿前的热残余压应力转变成吸湿后的湿热残余拉应力;由吸湿后碳纤维所受湿热残余拉应力减去吸湿前热残余压应力获得的吸湿拉应力非常大,平均为2272 MPa,接近所用碳纤维的拉伸强度(2800 MPa);适当的加工热残余压应力有利于降低吸湿导致的应力。     

Raman and X-ray absorption near-edge structure characterization of GaN implanted with O, Ar, Xe, Te and Au

GaN samples implanted with O, Ar, Te, Xe and Au ions, at fluences high enough to cause the formation of an amorphous surface or buried layer, are studied using Raman, X-ray absorption near-edge structure (XANES) and Rutherford backscattering (RBS) spectroscopies. The amorphous character of the as-implanted layers is verified by RBS and confirmed also by the XANES spectra. The Raman spectra of the amorphous surface layers are characterized by broad bands that simulate the phonon density of states, while the contribution of the underlying material is also detected. On the contrary, the spectra of the samples containing a buried amorphous layer are more structured due to the contribution of light scattered from the partially damaged layer which exists in between the surface and the buried amorphous layer.     

Facile fabrication of 2D material multilayers and vdW heterostructures with multimodal microscopy and AFM characterization

Reliable transfer processes that enable manipulation of two-dimensional (2D) materials, e.g., transition metal dichalcogenides (TMDCs) and MXenes, from one substrate to another has been a necessity for successful device fabrication. With both mechanical exfoliation and chemical vapor deposition (CVD) widely used, a versatile, clean, deterministic, and yet simple transfer technique is highly needed. To address such need, we developed a transfer method that takes advantage of wettability contrast between interfaces without the use of sacrificial layers or chemical processes. More importantly, a setup was developed to carry out this transfer method with high sample selectivity and fine control of the position and orientation of transferred TMDC crystals, a feature required for fabrication of the devices based on vertical 2D heterostructures. Using both exfoliated and CVD grown materials and subsequent atomic force microscopy (AFM), photoluminescence (PL), confocal Raman and tip enhanced Raman spectroscopy (TERS) characterization, we ascertained the quality of interfaces resulting from the transfer process while preserving excellent 2D material integrity. PL and TERS maps revealed nanometer-scale heterogeneities in the interfaces of fabricated heterostructures, which should enable further perfection of the transfer technique. TERS/TEPL information were employed to identify areas suitable for nanodevice fabrication, making the reported transfer and characterization methods ideal for making high quality assembly of 2D heterostructure more accessible, which should facilitate exploration of vertical 2D heterostructures for applications in electronics, batteries, solar cells, and twistronics.     

纳米TiO2粉末的结构表征:X射线衍射、透射电镜及高温Raman光谱分析

采用沉淀-胶溶-絮凝法，以偏钛酸为反应物．制备出锐钛矿型纳米TiO2粉末．用X射线衍射(XRD)、透射电镜(TEM)和高温Raman光谱对其结构进行表征。并采用高温Raman光谱仪对所制备的纳米TiO2从25～1200℃进行了原住Raman光谱研究。温度较低时，锐钛矿相的Raman谱峰明显可见．由于高温下Raman信号减弱，随着温度升高，Raman特征谱峰逐渐减弱以至于全部消失，仅显示出几个宽宽的凸起。同时，高温Raman光谱存在明显的温度效应，因而导致了随温度升高144cm^-1和196cm^-1谱峰的蓝移,640cm^-1谱峰的红移以及谱峰宽化。     

拉曼光谱的应用及进展

本文介绍了拉曼光谱与红外光谱的应用区别,重点综述了拉曼光谱在高温、高压、共振、表面增强技术上的应用,以及拉曼光谱的快速分析检测,提出了拉曼光谱技术存在的主要问题与发展前景.     

Raman characterization of boron doped tetrahedral amorphous carbon films

Boron doped tetrahedral amorphous carbon films, having boron content from 0.59 to 6.04 at.%, have been prepared by a filtered cathodic vacuum arc system using boron mixed graphite targets. The influence of boron on the surface morphologies and microstructures of the films was studied by atomic force microscopy and Raman spectroscopy. The surface images showed that the irregular tops on the surface of the films tended to form larger clusters as boron content increased. The Raman spectra of the films were, respectively, deconvoluted using Gaussian and Breit-Wigner-Fano line shapes. The Raman parameters, including the intensity ratios, peak positions, peak widths and coupling coefficients, obtained from both line shapes were described and compared. It was found that both line shapes could produce consistent results except the peak widths of G bands. The major effect of boron introduction was to increase the clustering of the sp² phase in the films.     

Composition dependent structural,Raman and nonlinear optical properties of PVA capped Zn1-x-yCdxCuyS quantum dots

Composition dependent structural, optical nonlinear and limiting properties of PVA capped Zn_1-x-yCd_xCu_yS quantum dots at different Cu:Zn ratio synthesized by insitu technique is subjected to detailed investigation. Cubic phase of the quantum dots were identified from XRD with particle size in the range 2.5 nm–3.5 nm find excellent correlation with the particle size measured from TEM. With increase in Cu concentration: systematic increment in lattice parameter, red shift in absorption edges and luminescence quenching is observed. Raman scattering reveals good photoactivity evidenced by intensity variation and shifting of LO and TO phonon modes. The intensity dependent third order nonlinearity is studied using Q switched Nd: YAG laser with 532 nm irradiation. Progressive increase in 3 PA coefficient indicated that prepared samples exhibit good nonlinear and optical limiting properties.