Ultraviolet Raman spectrometry of oil mixtures, biological samples and catalysts; a new fibre-based optical method for characterization of fluorescing materials.

Fig.1 Deep UV Argon-ion laser (Lexel 95-SHG™ )

Fig.2 Deep UV Raman instrument (Renishaw plc, UK)

In Raman spectroscopy fluorescence (that is emission of unwanted light) is a persistent problem. Surprisingly, it has been discovered that a way to avoid this fluorescence is to excite the Raman spectra with deep ultraviolet (DUV) light. “Fluorescence does not exist if the exciting light has a wavelength shorter than 260 nm” [1]. The DTU project has accordingly been started to get experience with the use of DUV-laser light in Raman spectroscopy to characterize samples that with visible light would be strongly fluorescing. This method has only - over the many years since 1984 - been very sparsely investigated. We foresee many applications of results of the DUV Raman method. A main focus is on characterization of food and oil products. The DUV Raman technique - when implemented – will help oil characterization and identification of waste products found at sea and shores. Small amounts of polycyclic aromatic hydrocarbons (PAHs) and sulfur-containing compounds contained in oil products should be detectable (on a ppm to ppb scale or less).
A major aim in the project has dealt with the implementation of this not so easy kind of deep UV spectroscopy in practice. With regard to DUV light sources, our laboratory is p.t. equipped with two continuous wave (CW) lasers. The first one is based on the second harmonic generation (SHG) from an Ar-ion laser. The second one is a 4th harmonic line generated from a Nd:YVO4 laser.
 

1. A DUV Ar-ion laser, (Lexel 95-SHG™ laser, see Fig. 1).

Table 1. Argon-ion laser lines, wavelengths and power levels.

2. A solid-state 266 nm laser (CRYLAS FQCW 266-50 laser, see Fig. 3)

Fig. 3: The 266 nm solid-state laser, CRYLAS FQCW 266-50.
 
We have several Raman systems installed in the Lab. The UV Raman system is a Renishaw™ instrument (Fig. 2), equipped with UV-enhanced Suprasil™ quartz optics, a microscope, a highly sensitive CCD detector, and the necessary accessories. The setup of our system is shown in Fig. 4.
 

Fig. 4: The UV Raman Setup in our laboratory.
 
An important future aspect of the DUV-Raman method is application of optical fibres to totally encapsulate the UV radiation, thus avoiding any risk of damage to persons. Optical hollow micro-structured “photonic crystal” quartz fibres with minute air-holes may be used, guiding light to pipettes with sample chemicals and solutions (e.g. series of oil/gasoline/methanol mixtures) that one wants to analyze.
 

Applications

Gasoline analysis

Fig. 5: Raman spectra of Statoil 95 Gasoline and Naphthalene, excited with the 229 nm laser.

Fig. 6: Raman spectra of Statoil 95 Gasoline, Toluene and Naphthalene, respectively. Excited with 244 nm laser

Fig. 7: (left) The monocyclic ring breathing mode in Toluene at 1004 cm^-1; (right) the bicyclic ring breathing mode in naphthalene at 1381 cm^-1. The calculated vibrational modes and Raman active bands were obtained by use of the Gaussian 03W software package. The optimisation of the molecular structure were performed. The structure of the Melamine molecule and its vibrations were obtained iteratively, in three subsequent calculations. Initially the geometry was roughly optimized using the semi-empirical PM3 type procedure, followed next by the ab initio Hartree-Fock/DFT method with 3rd order restricted Becke-Lee-Yang-Parr procedure (RB3LYP), at first with the 3-21G basis sets. Then the energy minimum structure and the vibrational modes were calculated by using the 6-31+G basis sets.

Food additives: Sudan I

 
The red dye Sudan I is considered to be toxic and carcinogenic. In recent years, several European control authorities have discovered the presence of Sudan I in series of imported food items, such as red canned chili fruits [6]. The recently applied methods for determination of Sudan I give detection limits on the order of around 0.5 ppm [7]. No known threshold limit seems to exist with respect to long-term health risks, and therefore there is a need for further development of the detection methods.
a) b)

Scheme 1. The tautomeric inversion of Sudan I: (a) the azo form and (b) the hydrazo form. The equilibrium is shifted far to the right. The structure of azo dyes has been intensely discussed in the chemical literature. The azo dyes formally contain the azo group –N=N–, but are in most cases partly or completely transformed into the tautomeric hydrazo form (Scheme 1, right). According to our studies with the Gaussian 03W software package, the azo forms gave poor resemblance to the experiments. In contrast to this, the calculated spectrum for the hydrazo form showed good resemblance to the experimental spectrum [8].

Fig. 8: Raman spectra of solid Sudan I obtained with (a) 244 nm UV light and (b) 1064 nm IR light. Useful spectra could not be obtained with green or blue light. The calculated spectrum is shown as (c). The band at 2331 cm^-1 in (a) is due to N₂ gas.

Size-effect of germanium (Ge) nanocrystals

Fig. 9: TEM cross-sectional images of samples (from left to right): 0126, 0269 and 0194.

Samples were prepared by standard clean-room techniques on 4 inch (100) oriented Si substrates. First, one layer of Ge doped SiO₂ was deposited. Then a SiGe alloy layer was deposited on top of thts SiO₂ layer. The ratios of the Si to Ge were tuned from 1:1 to 1:0.5 and 1:0.25 for samples 0194, 0269 and 0126 respectively. Thirdly, one identical layer of Ge doped SiO₂ was deposited on the SiGe alloy layer. By repeating the deposition of alternate layers of SiGe alloy and Ge doped SiO₂, multilayered structures were readily made. Samples were annealed at 1100°C in N₂ for 4 hours after deposition. More details of the fabrication parameters are found in [9]. Samples 0126, 0269 and 0194 had 3-layer structures. The Ge nanocrystals were found to give Raman peaks at about 295-300 cm^-1, shifting linearly with the excitation power density. Sample 0351 had a 5-layer structure with 1:1 Si/Ge ratio, made in order to increase the interaction with the light. A size effect was demonstrated within these Ge nanocystals (black dots in Fig. 9) after excluding any thermal effect as having effect on the properties, see Fig. 10 and [10,11].
 
 

Fig.10: Raman spectra of samples 0126, 0269, 0194, 0351 and a reference bulk Ge wafer. The inset is a zoom-in on the main peak of the Germanium lattice [10,11].

Solid state technology

Sample	B concentration [cm-3]	N concentration [cm-3]
a	8.0 x 1018	4.0 x 1016
b	6.9 x 1018	3.2 x 1018
c	6.9 x 1018	6.0 x 1018
d	4.4 x 1018	9.0 x 1018
e	5.2 x 1018	9.2 x 1018

 

Fig. 11: Raman backscattering spectra of 6H-SiC samples with different dopant concentrations showing longitudinal optical phonon-plasmon coupled (LOPC) modes (inset: positions of the LOPC modes of doped SiC samples).
 
The mechanisms of Raman shifts in p- and n-type samples are different. The LOPC mode would broaden, lower its intensity, and shift toward higher wavenumbers with increasing free carrier concentrations. Usually, it is more sensitive to the amount of free electrons than to free holes. In p-type samples, very few acceptor states are ionized due to the large ionization energy, and the Raman shift is mainly contributed to the atomic size effect [14]. B atoms usually occupy Si lattice positions in SiC. The inter-atomic distance of a Si-C bond is longer than that of a B-C bond due to the smaller atomic radius of B. The biaxial tensile stress will be released which results in a decrease of the phonon oscillation frequency. So the LOPC mode shifts toward smaller wavenumbers with higher B concentrations. In n-type samples, the predominant mechanism causing the Raman shift of the LOPC mode is the free carrier (electron) concentration. Although no obvious peak shift has been observed between sample d and e due to a relatively small concentration difference, the peak intensity of the LOPC mode decreases as expected when the free electron concentration increases from sample e to d. Furthermore, one can see from Fig. 11 that the LOPC modes of n-type samples occur at significantly higher wavenumbers than the p-type ones [14].

Melamine

Fig. 12: Raman spectra of milk, 0.1 % melamine in water and Milk by use of 229 nm (10 mW) excitation.

Fig. 13: C3 Triangle Breathing mode coupled to NH2-scissoring (667 cm-1 on measurement, 682.0 cm-1 on calculation). Calculation methods were as given in Fig.7.

UV Raman spectroscopy resolution techniques

Fig. 14: Raman spectral resolution modeling. The wide green points are the measurements of Raman bandwidth ΔωM of diamond for various slit widths. The narrow red points, connected by red curves, are the experimental spectral resolution data Δω obtained by means of the new model equation [20, 21]. The black dotted curve is the theoretical spectral resolution results (spectral resolution) based on our first principle equations [20, 21]. a) Green light source. b) Ultraviolet light source.

Fig. 15: The observed dependence of the spectral resolution along the Raman shift axis [20, 21]. The green and violet lines show the theoretical results calculated by use of our equation for only the central part of the CCD (in extended scanning mode using only few pixels). The black lines show the spectral resolution increments in four spectrograms and three spectrograms, for 514.5 nm and 257.3 nm laser excitation, respectively, obtained in static mode recordings, using many pixels and settings of the central pixel of the CCD at Raman shift wavenumbers of ω0 = 500, 1500, 2500 and 3500 cm^-1 for 514.5 nm excitation, and ωo = 750, 2250 and 3750 cm^-1 for 257.3 nm excitation, respectively.

General on UV

The UV Raman project was sponsored by The Danish Research Council for Technology and Production Sciences through the grant no. 274-08-0168. We thank Jakob Thyr from K-Analys, Sweden for help during the project.

REFERENCES:

For more information please contact
Associate Professor, Ph.D. Rolf W. Berg (rwb@kemi.dtu.dk )
Phone +45 4525 2512 or +45 4050 4191

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