Raman spectroscopy of laser‐induced oxidation of titanomagnetites

Titanomagnetites are important carriers of magnetic remanence in nature and can track redox conditions in magma. The titanium concentration in magnetite bears heavily on its magnetic properties, such as saturation moment and Curie temperature. On land and in the deep ocean, however, these minerals are prone to alteration which can mask the primary magnetic signals they once recorded. Thus, it is essential to characterize the cation composition and oxidation state of titanomagnetites that record the paleomagnetic field. Raman spectroscopy provides a unique tool for both purposes. Nonetheless, the heat generated by the excitation laser can itself induce oxidation. We show that the laser power threshold to produce oxidation decreases with increasing titanium content. With confocal Raman spectroscopy and magnetic force microscopy (MFM) on natural and synthetic titanomagnetites, a non‐destructive Raman imaging protocol was established. We applied this protocol to map out the composition and magnetization state within a single ex‐solved titanomagnetite grain in a deep‐sea basalt. Copyright © 2011 John Wiley & Sons, Ltd.     

Transition of chromium oxyhydroxide nanomaterials to chromium oxide: a hot‐stage Raman spectroscopic study

The transition of disc‐like chromium hydroxide nanomaterials to chromium oxide nanomaterials has been studied by hot‐stage Raman spectroscopy. The structure and morphology of α‐CrO(OH) synthesised using hydrothermal treatment were confirmed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). The Raman spectrum of α‐CrO(OH) is characterised by two intense bands at 823 and 630 cm⁻¹ attributed to ν₁ Cr^III O symmetric stretching mode and the band at 1179 cm⁻¹ attributed to Cr^III OH δ deformation modes. No bands are observed above 3000 cm⁻¹. The absence of characteristic OH stretching vibrations may be due to short hydrogen bonds in the α‐CrO(OH) structure. Upon thermal treatment of α‐CrO(OH), new Raman bands are observed at 599, 542, 513, 396, 344 and 304 cm⁻¹, which are attributed to Cr₂O₃. This hot‐stage Raman study shows that the transition of α‐CrO(OH) to Cr₂O₃ occurs before 350 °C. Copyright © 2010 John Wiley & Sons, Ltd.     

Study on solid structure of pentacene thin films using Raman imaging

We present a 532‐nm excited Raman imaging study of pentacene thin films (thickness, 2, 5, 10, 20, 50, 100, and 150 nm) prepared on an SiO₂ surface. The structure of the pentacene films has been investigated by images and histograms of the ratio (R) of intensity of the 1596‐cm⁻¹ band (b_3g) to that of the 1533‐cm⁻¹ band (a_g), which can be used as a marker of solid‐state phases: 1.54‐nm and 1.44‐nm phases. The Raman images showed that island‐like 1.44‐nm phase domains are grown on the 1.54‐nm phase layer from 50 nm, and all the surface of the 1.54‐nm phase layer is covered with the 1.44‐nm phase layer from 100 nm. The structural disorders have been discussed on the basis of the full width at half maximum of a band in the histogram of the R values for each film. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman micro‐spectroscopy for quantitative thickness measurement of nanometer thin polymer films

The sensitivity of far‐field Raman micro‐spectroscopy was investigated to determine quantitatively the actual thickness of organic thin films. It is shown that the thickness of organic films can be quantitatively determined down to 3 nm with an error margin of 20% and down to 1.5 nm with an error margin of 100%. Raman imaging of thin‐film surfaces with a far‐field optical microscope establishes the distribution of a polymer with a lateral resolution of ~400 nm and the homogeneity of the film. Raman images are presented for spin‐coated thin films of polysulfone (PSU) with average thicknesses between 3 and 50 nm. In films with an average thickness of 43 nm, the variation in thickness was around 5% for PSU. In films with an average thickness of 3 nm for PSU, the detected thickness variation was 100%. Raman imaging was performed in minutes for a surface area of 900 µm². The results illustrate the ability of far‐field Raman microscopy as a sensitive method to quantitatively determine the thickness of thin films down to the nanometer range. Copyright © 2015 John Wiley & Sons, Ltd.     

Transition of synthetic chromium oxide gel to crystalline chromium oxide: a hot‐stage Raman spectroscopic study

Chromium oxide gel material was synthesised and appeared to be amorphous in X‐ray diffraction study. The changes in the structure of the synthetic chromium oxide gel were investigated using hot‐stage Raman spectroscopy based upon the results of thermogravimetric analysis. The thermally decomposed product of the synthetic chromium oxide gel in nitrogen atmosphere was confirmed to be crystalline Cr₂O₃ as determined by the hot‐stage Raman spectra. Two bands were observed at 849 and 735 cm⁻¹ in the Raman spectrum at 25 °C, which were attributed to the symmetric stretching modes of O Cr^III OH and O Cr^III O. With temperature increase, the intensity of the band at 849 cm⁻¹ decreased, while that of the band at 735 cm⁻¹ increased. These changes in intensity are attributed to the loss of OH groups and formation of O Cr^III O units in the structure. A strongly hydrogen‐bonded water H O H bending band was found at 1704 cm⁻¹ in the Raman spectrum of the chromium oxide gel; however, this band shifted to around 1590 cm⁻¹ due to destruction of the hydrogen bonds upon thermal treatment. Six new Raman bands were observed at 578, 540, 513, 390, 342 and 303 cm⁻¹ attributed to the thermal decomposed product Cr₂O₃. The use of the hot‐stage Raman spectroscopy enabled low‐temperature phase changes brought about through dehydration and dehydroxylation to be studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Two‐dimensional standoff Raman measurements of distant samples

A scanning standoff Raman spectroscopy system has been developed to measure the molecular species of distant samples in two dimensions. Computer software is used to control the scanner pan and tilt angles, and Raman measurements are collected of the distant sample area in an x–y grid pattern. The Raman spectra at each grid point are measured and processed to obtain an image of the distant surface composition. The ability to provide interpolated images of distant molecular species is illustrated. Copyright © 2011 John Wiley & Sons, Ltd.     

Micro‐Raman depth profiling of silicon amorphization induced by high‐energy ion channeling implantation

In this work, we study the silicon amorphization dependence on the crystal depth induced by 6‐MeV Al²⁺ ions implanted in the <110> and randomly oriented silicon crystal channels, which was not directly experimentally accessible in the previous similar high‐energy ion–crystal implantation cases. Accordingly, the micro‐Raman spectroscopy scanning measurements along the crystal transversal cross section of the ion implanted region were performed. The ion fluence was 10¹⁷ particles/cm². The scanning steps were 0.2 and 0.3 µm, for the channeling and random ion implantations, respectively. The obtained results are compared with the corresponding Rutherford backscattering spectra of 1.2‐MeV protons in the random and channeling orientations measured during the channeling implantation. Additionally, scanning electron microscope picture was taken on the transversal cross section of the implanted region in the channeling implantation case. We show here that the obtained silicon amorphization maxima are in excellent agreement with the corresponding estimated maxima of the aluminum concentration in silicon. This clearly indicates that the used specific micro‐Raman spectroscopy scanning technique can be successfully applied for the depth profiling of the crystal amorphization induced by high‐energy ion implantation. Copyright © 2013 John Wiley & Sons, Ltd.     

Origin of the Raman mode at 379 cm−1 observed in ZnO thin films grown on sapphire

In this work, we present a detailed Raman scattering study to clarify the origin of the mode at 379 cm⁻¹ which is observed in Raman spectra of the ZnO films grown on c‐sapphire substrates and generally attributed to the A₁‐transverse optical (A₁‐TO) mode of ZnO. The studied ZnO films were deposited by metal‐organic chemical vapor deposition on c‐sapphire and (0001) ZnO substrates. In the z(−,−)z̄ backscattering configuration, the A₁‐TO mode is forbidden, while the 379 cm⁻¹ peak is still observed in the as‐deposited film grown on sapphire substrate. However, this mode is not observed in Raman spectra of the as deposited film grown on ZnO substrate. We suggest that the peak at 379 cm⁻¹ is the E_1g mode of the sapphire substrate which is allowed in z(−,−)z̄ backscattering configuration. The effects of annealing, the substrate and the collection cross‐section on Raman active modes were analyzed. Copyright © 2011 John Wiley & Sons, Ltd.     

High-temperature oxidation behaviors of CVD diamond films

In this study, high-temperature oxidation of single-crystal diamond and diamond films prepared by hot filament chemical vapor deposition (HF-CVD), were characterized using thermal analysis and high-temperature in-situ Raman analysis. The measurements were performed in various temperatures up to 1300 °C in air and N₂ atmospheres. The results indicate that the initial oxidization temperature of diamond film deposited at 700 °C (D700 film) is ≈629 °C, lower than those of diamond film deposited at 900 °C (D900 film, ≈650 °C) and single-crystalline diamond (≈674 °C) in air. Oxidation rate of D700 film at high temperatures appeared to be the highest among the samples studied. A likely cause lies in the fact that, compared to their D900 sample, D700 diamond film contains a larger amount of non-diamond carbon and grain boundaries. However, D900 and D700 diamond films as well as single-crystalline diamond showed no detectable weight loss and oxidization when they were heated up to 1300 °C in N₂ atmosphere.     

Micro‐Raman investigation of transition‐metal‐doped ZnO nanoparticles

We measured the Raman spectra of ZnO nanoparticles (ZnO‐NPs), as well as transition‐metal‐doped (5% Mn(II), Fe(II) or Co(II)) ZnO nanoparticles, with an average size of 9 nm. A typical Raman peak at 436 cm⁻¹ is observed in the ZnO‐NPs, whereas Zn_1−xMn_xO, Zn_1−xFe_xO and Zn_1−xCo_xO presented characteristic peaks at 661, 665 and 675 cm⁻¹, respectively. These peaks can be related to the formation of Mn₃O₄, Fe₃O₄ and Co₃O₄ species in the doped ZnO‐NPs. Moreover, these samples were analyzed at various laser powers. Here, we observed new vibrational modes (512, 571 and 528 cm⁻¹), which are specific to Mn, Fe and Co dopants, respectively, and ZnO‐NPs did not reveal any additional modes. The new peaks were interpreted either as disorder activated phonon modes or as local vibrations of Mn‐, Fe‐ and Co‐related complexes in ZnO. Copyright © 2007 John Wiley & Sons, Ltd.     

Nanostructured thin films as surface‐enhanced Raman scattering substrates

Nanoporous thin films with silver nanoparticles were synthesized with a bottom–up approach, and its potential as effective surface‐enhanced Raman scattering (SERS) substrates was demonstrated. The use of mesoporous titania films as substrates allowed to control the growth of nanoparticles on the film surface. Atomic force microscopy measurements, Ultraviolet‐visible and X‐ray diffraction analysis confirmed the photoreduction of Ag⁺ to Ag⁰ with the formation of nanoparticles with crystallite dimensions of 32 to 36 nm. The new substrates allowed the detection of two analytes (rhodamine B isothiocyanate and cytochrome c), present in solutions at very low concentrations, highlighting their potential in SERS sensing. Reproducibility, homogeneity, enhancement factor of the substrate, consistency of results and detection limits were also assessed. Copyright © 2012 John Wiley & Sons, Ltd.     

Raman investigation of silicon nanocrystals: quantum confinement and laser‐induced thermal effects

We present a detailed experimental and theoretical Raman investigation of quantum confinement and laser‐induced local thermal effects on hydrogenated nanocrystalline silicon with different nanocrystal sizes (3.6–6.2 nm). The local temperature was monitored by measuring the Stokes/anti‐Stokes peak ratio with the laser power density range from ~120 to 960 kW/cm². In combination with the three‐dimensional phonon confinement model and the anharmonic effect, which incorporates the three‐phonon and four‐phonon decay processes, we revealed an asymmetrical decay process with wavenumbers ~170 and 350 cm^–1, an increasing anharmonic effect with nanocrystal sizes, and a shortening lifetime with enhanced temperature and decreasing nanocrystal dimension. Furthermore, we demonstrated experimentally that for Si nanocrystals smaller than 6 nm, the quantum confinement effect is dominant for the peak shift and line broadening. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman study of single wall carbon nanotube thin films treated by laser irradiation and dynamic and isothermal oxidation

The effect of visible and infrared irradiation on the structure of single wall carbon nanotubes (SWCNTs) is still an insufficiently resolved subject. In this paper, we report a detailed study of two types of SWCNT thin films treated by pulsed laser irradiation and dynamic and isothermal oxidation, respectively. Raman features of treated films were investigated by 532, 633 and 780 nm excitation lines, respectively. It was established that the 532 nm excitation laser line probes the surface of SWCNT films, while the two others probe the bulk of SWCNT films. It was found that during Raman measurements, one type of SWCNTs warms up significantly while the other type does not change temperature. Raman analysis of SWCNTs probed with 532 nm showed that effects of high power laser irradiation on two types of SWCNTs are completely different. Major effects are decomposition, debundlation and photooxidation. It was found that debundlation and removal of defective carbon are much more pronounced in one type of SWCNTs treated with dynamic oxidation. Dynamic oxidation affects mostly thinner SWCNTs that easily burn in air. The only significant effects of isothermal oxidation are the increase of defective amorphous carbon in the first type of SWCNTs and the decrease of D band and debundlation in the second type. Copyright © 2012 John Wiley & Sons, Ltd.     

Nanomechanical characterization of amorphous hydrogenated carbon thin films

Amorphous hydrogenated carbon (a-C:H) thin films deposited on a silicon substrate under various mixtures of methane-hydrogen gas by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD) was investigated. Microstructure, surface morphology and mechanical characterizations of the a-C:H films were analyzed using Raman spectroscopy, atomic force microscopy (AFM) and nanoindentation technique, respectively. The results indicated there was an increase of the hydrogen content, the ratio of the D-peak to the G-peak (I_D/I_G) increased but the surface roughness of the films was reduced. Both hardness and Young's modulus increased as the hydrogen content was increased. In addition, the contact stress-strain analysis is reported. The results confirmed that the mechanical properties of the amorphous hydrogenated carbon thin films improved using a higher H₂ content in the source gas.     

The characteristics of laser‐driven shock wave investigated by time‐resolved Raman spectroscopy

An accurate and simple method, Raman peak‐shift simulation, is proposed to determine the characteristics of a laser‐driven shock wave. Using the principle of the Raman peaks shifting at high pressure and the pressure distribution in the gauge layer, the profile of the Raman peak can be numerically simulated. Combined with time‐resolved Raman spectroscopy, some main characteristics of the shock wave were determined. In the experiment, polycrystalline anthracene was used as the pressure gauge. The pump–probe technique was used to obtain the time‐resolved Raman spectra of anthracene under shock loading. The velocity of the shock wave, the peak pressure and the rise time of the shock front were determined by simulating the experimental spectra numerically. The result shows that the method of Raman peak‐shift simulation is effective in obtaining the characteristics of a laser‐driven shock wave. Copyright © 2010 John Wiley & Sons, Ltd.     

分子束外延PbTe单晶薄膜的反常拉曼光谱研究

采用分子束外延(MBE)方法在BaF₂(111)衬底上生长了高质量的PbTe单晶薄膜， 拉曼光谱测量观察到了表面氧化物的振动模、布里渊区中心(q≈0)纵光学(LO)声子振动模以 及声子-等离子激元耦合模振动.随着显微拉曼光谱仪激光光斑聚焦深度的改变，各拉曼散射 峰的峰位、积分强度、半高宽等都表现出不同的变化趋势. 随着激光光斑聚焦位置从样品表 面上方3μm处变化到表面下方3μm处，PbTe外延薄膜的LO声子频率从119cm^-1移 动到124cm^{-1关键词： PbTe外延薄膜 拉曼散射 纵光学声子}     

Identification of Mn‐related Raman modes in Mn‐doped ZnO thin films

This article aims to investigate the Raman modes present in Mn‐doped ZnO thin films that are deposited using the magnetron co‐sputtering method. A broad band ranging from 500 to 590 cm⁻¹ is present in the Raman spectra of heavily Mn‐doped ZnO films. The multi‐peak‐fitting results show that this broad band may be composed of six peaks, and the peak at 528 cm⁻¹ could be a characteristic mode of Mn₂O₃. The results of this study suggest that the origin of the Raman peaks in Mn‐doped ZnO films may be due to three major types: structural disorder and morphological changes caused by the Mn dopant, Mn‐related oxides and intrinsic host‐lattice defects. Copyright © 2010 John Wiley & Sons, Ltd.     

Micro‐Raman spectroscopic characterization of a tunable electrochromic device for application in smart windows

An asymmetric electrochromic (EC) device based on an active EC tungsten oxide–titanium oxide (WO₃–TiO₂) layer was constructed. The EC active layer consisted predominantly of monoclinic WO₃ nanocrystallites with a minor additional component of hexagonal WO₃ and amorphous TiO₂. Detailed micro‐Raman spectroscopic studies of the intercalation and deintercalation of lithium in the EC active layer of the EC device as a function of the applied voltage were performed. Three significant structural stages occur upon intercalating Li into the WO₃–TiO₂ layer when coloration potentials of 1.0, 1.5, 2.0, and 3.0 V are applied to the EC device. In the first stage (applied potential of 1.0 V), the m‐Li_x WO₃ phase is retained. In the second stage, (applied potential of 1.5 and 2.0 V) the m‐Li_x WO₃ transforms to a tetragonal phase. In the third stage, (applied potential of 3.0 V) the Raman spectrum exhibits no spectral bands, showing that Li_x WO₃ has attained the highest‐symmetry cubic phase. This phase sequence is confirmed by the X‐ray diffraction (XRD) measurement. These phase transitions can be reversed and, upon complete deintercalation, m‐WO₃ with traces of h‐WO₃ is recovered. Optical transmission studies were performed in conjunction with Raman and XRD studies. A shift of the optical transmittance peak position from 639 to 466 nm and reduction in the width of the transmittance curve with increasing applied potential opens up the possibility of smart window applications for the nanocrystalline WO₃‐based EC device. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and Raman investigation of sol–gel‐derived relaxor ferroelectric thin films

Pb(Fe_2/3W_1/3)O₃ (PFW) thin films were deposited on platinized silicon substrate by a chemical solution deposition technique. Room‐temperature X‐ray diffraction (XRD) revealed a pure cubic crystal structure of the investigated material. The microstructure indicated good homogeneity and density of the thin films. A Raman spectroscopic study was carried out on PFW to study the polar nano‐regions in the temperature range 85–300 K. The Raman spectra showed a change in the peak intensity and a shift towards the lower wavenumber side with temperature. The Raman spectra also revealed the transition from the relaxor to the paraelectric state of PFW. There was no evidence of a soft mode in the low‐temperature region, in contrast to the normal ferroelectric behavior. The polar nano‐regions tend to grow and join at low temperatures (∼85 K), which become smaller with increase in temperature. The presence of strong Raman spectra in the cubic phase of the material is due to the presence of distributed Fm3m(Z = 2) symmetry nano‐ordered regions in the Pm3m(Z = 1) cubic phase. Copyright © 2007 John Wiley & Sons, Ltd.     

ZnO：Sb薄膜的光致发光及拉曼特性研究

利用射频磁控溅射法（MS．RF）在玻璃基片上制备了不同掺杂浓度的ZnO：Sb薄膜．借助X射线衍射仪（XRD）、透射光谱、光致发光谱（PL）和拉曼散射光谱（Raman）等手段研究了Sb掺杂浓度对ZnO薄膜的微结构、光致发光和拉曼特性的影响．结果表明：所有样品均呈现ZnO六角纤锌矿结构且具有高度C轴择优取向;在Sb掺杂ZnO薄膜的拉曼光谱中观察到位于532cm^-1的振动模式,结合XRD分析认为此峰归因于Sb替代Zn位且与0成键的局域振动模式（LVMSb--O）;光致发光谱测试发现,仅在ZnO：Sb薄膜中观察到位于3．11eV附近的紫光发射峰,结合拉曼光谱分析认为此峰与XbZn-O复合体缺陷相关．