Backscattering Raman Spectroscopy Research Articles

Technique for Enhancing Signal in Conventional Backscattering Fluorescence and Raman Spectroscopy of Turbid Media

We demonstrate experimentally, for the first time, the feasibility of passively enhancing fluorescence and Raman signals from diffusely scattering media in a conventional backscattering collection geometry. The method employs transmission of the collimated excitation laser beam through a "unidirectional" dielectric mirror placed directly in front of the sample. This permits laser light that escapes from the sample surface to be reflected back into the sample where it can be more usefully employed in generating Raman and fluorescence signals. This leads to improved Raman signal, higher signal-to-noise ratio, and shorter acquisition times. Feasibility studies performed on standard pharmaceutical tablets and on sheets of Teflon, using a single enhancing element, demonstrate signal enhancement factors of 6 (fluorescence) and 3 (Raman). Potential applications of this simple device include improving quality control of pharmaceutical products, disease diagnosis of biological tissue, forensics, and security screening.

Non-invasive detection of powders concealed within diffusely scattering plastic containers

We demonstrate the potential of Spatially Offset Raman Spectroscopy (SORS) for the non-invasive detection of chemicals in powder form (e.g. illicit drugs or explosives) concealed within diffusely scattering containers. The performance of the technique is compared with that of conventional backscattering Raman spectroscopy. Enhanced sensitivity is achieved by the SORS technique's inherent ability to effectively suppress fluorescence and Raman contributions originating from the packaging. The application is demonstrated on the non-invasive detection of sugar hidden in diffusely scattering plastic bottles.

Strained SiGeSn formed by Sn implant into SiGe and pulsed laser annealing

Incorporation of tin (Sn) in substitutional sites in strained Si0.75Ge0.25 was demonstrated by Sn implant and pulsed laser annealing. The surface of Si0.75Ge0.25 was amorphized by Sn implant but was recrystallized after pulsed laser annealing. The crystalline Si1−x−yGexSny layer formed was studied by Rutherford backscattering spectrometry and Raman spectroscopy. A substitutionality up to 62% Sn and 80% Ge was obtained at an optimal laser power of 400mJcm−2 for five laser pulses. A compressive strain of −1.15% was also obtained due to Sn incorporation. The presence of Sn also increased the active B dopant concentration in activating Si1−x−yGexSny to give low sheet resistance. The implantation of Sn and B followed by pulsed laser annealing could be useful for application in strain engineering of high mobility metal-oxide-semiconductor field-effect transistors.

Quantitative Transmission Raman Spectroscopy of Pharmaceutical Tablets and Capsules

Quantitative analysis of pharmaceutical formulations using the new approach of transmission Raman spectroscopy has been investigated. For comparison, measurements were also made in conventional backscatter mode. The experimental setup consisted of a Raman probe-based spectrometer with 785 nm excitation for measurements in backscatter mode. In transmission mode the same system was used to detect the Raman scattered light, while an external diode laser of the same type was used as excitation source. Quantitative partial least squares models were developed for both measurement modes. The results for tablets show that the prediction error for an independent test set was lower for the transmission measurements with a relative root mean square error of about 2.2% as compared with 2.9% for the backscatter mode. Furthermore, the models were simpler in the transmission case, for which only a single partial least squares (PLS) component was required to explain the variation. The main reason for the improvement using the transmission mode is a more representative sampling of the tablets compared with the backscatter mode. Capsules containing mixtures of pharmaceutical powders were also assessed by transmission only. The quantitative results for the capsules' contents were good, with a prediction error of 3.6% w/w for an independent test set. The advantage of transmission Raman over backscatter Raman spectroscopy has been demonstrated for quantitative analysis of pharmaceutical formulations, and the prospects for reliable, lean calibrations for pharmaceutical analysis is discussed.

Structure and mechanical properties of nickel/carbon nanocomposite films formed by microwave plasma-assisted deposition technique from argon–acetylene gas mixture

The structure and mechanical properties of nickel/hydrogenated amorphous carbon (Ni/a-C:H) films formed by microwave plasma-assisted deposition technique were investigated as a function of the carbon content using various methods: Rutherford backscattering spectroscopy (RBS), Raman spectroscopy and tribometry. The size of carbon clusters determined by Raman spectroscopy in Ni/a-C:H films deposited in gas mixtures containing 40 and 60% of C 2H 2, and in nickel free a-C:H films was 1 and 4 nm, respectively. However, the amorphous Ni/a-C:H films deposited from a gas mixture containing 60% of C 2H 2 exhibited the lowest friction coefficient (∼ 0.04), at the same time the nanohardness of these films was ∼ 7 GPa.

Raman Signal Enhancement in Deep Spectroscopy of Turbid Media

A new, passive method for enhancing spontaneous Raman signals for the spectroscopic investigation of turbid media is presented. The main areas to benefit are transmission Raman and spatially offset Raman spectroscopy approaches for deep probing of turbid media. The enhancement, which is typically several fold, is achieved using a multilayer dielectric optical element, such as a bandpass filter, placed within the laser beam over the sample. This element prevents loss of the photons that re-emerge from the medium at the critical point where the laser beam enters the sample, the point where major photon loss occurs. This leads to a substantial increase of the coupling of laser radiation into the sample and consequently an enhanced laser photon-medium interaction process. The method utilizes the angular dependence of dielectric optical elements on impacting photon direction with its transmission spectral profile shifting to the blue with increase in the deviation of photons away from normal incidence. This feature enables it to act as a unidirectional mirror passing a semi-collimated laser beam through unhindered from one side, and at the other side, reflecting photons emerging from the sample at random directions back into it with no restrictions to the detected Raman signal. With substantial restrictions to the spectral range, the concept can also be applied to conventional backscattering Raman spectroscopy. The use of additional reflective elements around the sample to enhance the Raman signal further is also discussed. The increased signal strength yields higher signal quality, a feature important in many applications. Potential uses include sensitive noninvasive disease diagnosis in vivo, security screening, and quality control of pharmaceutical products. The concept is also applicable in an analogous manner to other types of analytical methods such as fluorescence or near-infrared (NIR) absorption spectroscopy of turbid media or it can be used to enhance the effectiveness of the coupling of laser radiation into tissue in applications such as photodynamic therapy for cancer treatment.

The influence of structural disorder on the phonon modes in zinc oxide

Abstract Undoped zinc oxide thin films and nanostructured layers were grown by pulsed laser deposition on different substrates. They were characterized by scanning electron microscopy and Raman backscattering spectroscopy. Larger substrate mismatch leads to higher structural disorder in the thin films. Simultaneously, the intensity of the phonon mode at 580 cm −1 increases. However, for the nanostructured layers it remains constant. These observations are discussed in terms of the disorder activation of forbidden Raman modes.

Stoichiometric SiO 2 thin films deposited by reactive sputtering

We report a study of experimental possibilities to produce high purity stoichiometric SiO 2 thin films by reactive ion beam sputtering. The films were deposited in a UHV chamber, 4 × 10 −9 mbar, using a high purity silicon target and 1 keV Ar + ions for sputtering. The ion beam current was varied from 1.67 to 6.85 mA, at a constant argon partial pressure of 1 × 10 −3 mbar. Different values of the oxygen partial pressure (5 × 10 −6–1 × 10 −3 mbar) were applied for reactive deposition. The substrates were held at room temperature or at 550 °C, and the films were deposited to 12.5–150 nm, at 0.0018–0.035 nm s −1. Structural characterization was performed by Rutherford backscattering spectrometry (RBS), electron microprobe, X-ray diffraction (XRD) and Raman spectroscopy. The results show that reactive ion beam sputtering can be efficient for deposition of high quality silica films at 550 °C, oxygen partial pressure of 2 × 10 −4 mbar and ion beam current on the target from 5 to 5.5 mA, or at a lower deposition rate, ion beam current of 1.67 mA and oxygen partial pressure of 6 × 10 −5 mbar. One aspect of these investigations was to study consumption of oxygen from the gas cylinder, which was found to be lower for higher deposition rates.

Noninvasive Authentication of Pharmaceutical Products through Packaging Using Spatially Offset Raman Spectroscopy

We demonstrate the use of spatially offset Raman spectroscopy (SORS) in the identification of counterfeit pharmaceutical tablets and capsules through different types of packaging. The technique offers a substantially higher sensitivity than that available from conventional backscattering Raman spectroscopy. The approach is particularly beneficial in situations where the conventional Raman backscattering method is hampered or fails because of excessive surface Raman or fluorescence signals emanating from the packaging, capsule shell, or tablet coating contaminating the much weaker subsurface Raman signals of the active pharmaceutical ingredients and excipients held in the product. It is demonstrated that such interfering signals can be effectively suppressed by SORS.

Effects of doping on structural change and hydrogen bonding in laser crystallized polycrystalline silicon films

Doped polycrystalline silicon films were produced by employing a step-by-step laser crystallization of doped hydrogenated amorphous silicon (a-Si:H). The influence of laser crystallization on structural properties and hydrogen bonding were investigated using Raman backscattering spectroscopy and hydrogen effusion measurements. Crystallization with low laser fluence, E L, results a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. In fully crystallized polycrystalline silicon the Raman lines in both P- and B-doped specimens are asymmetric, which is indicative of the Fano effect. From the hydrogen effusion spectra, the hydrogen density-of-states distribution is derived. Laser crystallization results in an increase of the hydrogen binding energy by about 0.2–0.3 eV compared to the amorphous starting material.

Experimental indications of a BCS behaviour in superconducting diamond

AbstractWe present a scanning tunneling spectroscopy study of a superconducting single‐crystalline heavily boron‐doped diamond which provides an experimental evidence for BCS behaviour. The superconducting gap, its temperature dependence, its relationship to the bulk critical temperature as well as the shape of the tunneling density of states are found to be fully compatible with a conventional BCS‐type weak‐coupling mechanism. A Raman backscattering spectroscopy study of diamond epilayers with different doping levels indicates by a slight phonon softening a coupling between carriers and optical phonons. In addition, in the mixed state, the disordered Abrikosov lattice of vortices were imaged using STM spectroscopy. Localized states at energies strikingly much higher than expected for low‐lying quasiparticles excitations appear in the tunneling density of states of the vortex core. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Friction properties of ta-C and a-C:H coatings under high vacuum

The tribological behaviour of ta-C and a-C:H carbon films was analysed by pin-disc tests under high vacuum (< 10 − 3 Pa). The tests were executed on a commercial High Vacuum Tribometer (CSM-Instruments) and compared with results obtained under controlled atmospheric pressure conditions. In this work, we present the comparison of the friction coefficients under high vacuum and under controlled atmosphere conditions of two sets of carbon films, one prepared by Filtered Cathodic Vacuum Arc (FCVA; giving rise to hydrogen-free carbon), the other by Plasma Enhanced Chemical Vapour Deposition (PECVD; leading to hydrogenated carbon). The structure of these films was analysed by Raman and Rutherford BackScattering (RBS) spectroscopy and Elastic Recoil Detection (ERD) as well as by X-rays reflectometry (XRR). We will demonstrate that under vacuum, FCVA carbon layers do not slide ( μ = 0.5–0.7) but PECVD carbon films can have a good sliding behaviour ( μ = 0.1–0.3). An inverse behaviour was observed under controlled atmospheric conditions. These first results indicate a predominant role of the structure of the films, as analysed by Raman and RBS-ERD spectroscopy as well as by XRR, on the tribological characteristics of the carbon films. In particular, the influence of hydrogen concentration seems to be directly linked to the vacuum tribological behaviour of the coatings.

Raman and photoluminescence spectroscopy from n- and p-type 6H-SIC alpha-particle irradiated

We report optical properties of n-nitrogen and p-boron-type 6H-SiC alpha-particle irradiated at doses 10 12–10 14 cm −2. The back scattering Raman Spectroscopy (RS) from the reference samples reveals all the four first-order Raman active modes (1-RAMs) as well as all the 12th folded vibrational modes (FVMs) previously observed and tabulated. The irradiation at low doses induces several Raman optically active bands in the n- and p-type 6H-SiC. In order to identify the new modes we have derived some group-theoretical Raman selection rules (RSRs). Using the RSRs we have distinguished treatment-induced modes from the first-order RAM belonging to 6H-SiC. Moreover, by means of the RSRs we have made an attempt to identify the origins of some of the second-order phonons traditionally labelled as: a, b, ….., h. Photoluminescence spectroscopy (PL) also monitors some new bands treatment induced. Some of them are temperature and laser power dependent.

Formation and characterization of silicon films on flexible polymer substrates

Polysilicon thin film transistors on flexible substrates are of considerable interest for applications in flexible displays. This paper investigates the formation of nanocrystalline silicon on flexible, transparent polymer substrates. An 800-nm layer of amorphous silicon was deposited on a polyimide substrate followed by a 20-nm layer of aluminum. Samples were rapid thermal annealed at 900 °C for 20 s, forming silicon nanocrystallites in a porous amorphous silicon film. The films were analyzed using Rutherford backscattering spectrometry, Raman Spectroscopy and cross-section transmission electron microscopy. A mechanism is proposed for the formation of silicon nanocrystallites and pores in the a-Si layer.

Three phonon processes in GaN

In the harmonic approximation the Hamiltonian of the vibrational states gives the picture of non-interacting phonons in a crystal. At finite temperatures all crystals contain anharmonic lattice forces. The effect of anharmonicity introduces the interactions of independent phonons in crystals. The number of independent vibrational modes originating from the high symmetry points and lines in the Brillouin zone is determined by lattice mode representation (LMR). The three phonon processes such as overtones and combinations are subjected to group theoretical selection rules. The overtones result from the Symmetrized Cubes of active species (irreducible representations (irrps)) and combinations from the complete reduction of the triple Kronecker product of different irrps according to which the vibrational states of crystals are classified. We have determined the symmetry allowed overtones, simple and general combinations of three phonon processes in GaN. The back-scattering Raman spectroscopy from GaN in the region 900–2200 cm −1 yields several optically active transitions; these are analyzed in terms of the derived three phonon Raman selection rules.

Hydrogen local vibrational modes in zinc oxide.

Using Raman backscattering spectroscopy we have observed six local vibrational modes in as-grown state-of-the-art nominally undoped zinc oxide single crystals. The local vibrational modes are located at nu=2854, 2890, 2918, 2948, 2988, and 3096 cm(-1). Some specimens were annealed up to 950 degrees C to remove hydrogen. A subsequent Raman backscattering measurement revealed that the local vibrational modes disappeared. This establishes that the observed local vibrational modes are caused by the presence of hydrogen in the ZnO crystals.

Optical properties of nitrogenated tetrahedral amorphous carbon films

The chemical composition, structural, and optical properties of nitrogenated tetrahedral amorphous carbon (ta-C:N) films deposited by a pulsed filtered vacuum arc deposition system were characterized by non-Rutherford backscattering spectroscopy, Raman spectroscopy, and ultraviolet-visible spectroscopy. It was observed that the amount of nitrogen atoms incorporated into the ta-C:N films, as well as the sp2 fraction of the films, increased with increasing nitrogen pressure PN during deposition. As a result, the optical band gap of the ta-C:N films also decreased with increasing PN. At a fixed nitrogen partial pressure of 4×10−3 Pa, the nitrogen content was found to first increase with increasing substrate negative bias voltage (−Us), up to a maximum of about 14.5 at. % at −Us of 100 and 150 V, then decreases with further increase of −Us. The sp2 fraction however increased monotonically with increasing −Us. The optical band gap of the ta-C:N films initially increased with increasing −Us, up to a maximum at a certain −Us, and then decreased with further increase in −Us. The variation of the optical band gap with the negative substrate bias voltage was discussed in terms of the different sp2-bonded carbon configurations existing in the films and the graphitization of the ta-C:N films, as indicated by the Raman and density measurement results.

Synthesis and characterisation of dual-phase Y-TZP and RuO2 nanopowders: dense electrode precursors

The synthesis and characterisation of nanopowders in the dual-phase system tetragonal-Y2O3-doped ZrO2 (Y-TZP) and RuO2 are described. Five powders were prepared from a co-precipitation (CP) method with stoichiometric variation in the RuO2 content (5–46 mol%) and two powders were prepared from solid-phase mechanical mixing of the above oxides prepared separately. In the CP method, an aqueous chlorometal solution containing appropriate precursor ions was co-precipitated in a concentrated aqueous ammonia (pH y14) solution. Following filtration, washing and drying (100 uC), the CP synthesis route yielded black coloured amorphous powders. Crystalline dual-phase powders were obtained after calcination in stagnant air at 600 uC for 2 h. The average Y-TZP and RuO2 crystallite sizes were, respectively, 10 and 20 nm, i.e. a nano/nano powder. The powders were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), backscatter Raman spectroscopy (ZrO2 phase determination) and quantitative X-ray fluorescence (XRF). Time-dependent calcination experiments for samples of similar composition revealed that under the synthesis conditions employed, a composite was formed where a fraction of the crystalline RuO2 phase was initially dissolved in the ZrO2 phase and which gradually transforms to a more stable, distinctly dual-phase system upon prolonged (¢20 h) calcination at 600 uC.

Silver incorporation in Ge–Se glasses used in programmable metallization cell devices

We investigate the nature of thin films formed by the photodissolution of Ag into Se-rich Ge–Se glasses for use in programmable metallization cell devices. These devices rely on ion transport in the film so produced to create electrically programmable resistance states. The way in which Ag incorporates into the chalcogenide film during photodiffusion is examined using Rutherford backscattering spectroscopy analysis and Raman spectroscopy. The results suggest that an Ag-rich phase separates due to the reaction of Ag with free Se from the chalcogenide glass leaving a Ge-rich chalcogenide matrix.

Influence of in situ ultrasound treatment during ion implantation on amorphization and junction formation in silicon

We report the first study of the effect of in situ ultrasound treatment during ion implantation on amorphization and transient enhanced diffusion (TED) in silicon. Rutherford backscattering spectroscopy, ion channeling, and Raman spectroscopy measurements show that amorphization of Si during Ar ion implantation is increased by ultrasound treatment, especially at ultrasound frequencies around 2 MHz. By secondary ion mass spectroscopy we investigate the influence of ultrasound treatment on TED of B atoms. Our data show that TED is reduced due to the influence of ultrasound treatment. The results are discussed in terms of the interaction of ultrasound waves with point defects and the ultrasound stimulated enhanced diffusion of interstitials.