DEVELOPMENT of a RAMAN SPECTROMETER to STUDY SURFACE-ENHANCED RAMAN SCATTERING by Nandita Biswas, Ridhima Chadha, Sudhir Kapoor, Sisir K

BARC/2011/E/003 BARC/2011/E/003

DEVELOPMENT OF A RAMAN SPECTROMETER TO STUDY SURFACE-ENHANCED RAMAN SCATTERING by Nandita Biswas, Ridhima Chadha, Sudhir Kapoor, Sisir K. Sarkar and Tulsi Mukherjee Radiation & Photochemistry Division

2011 BARC/2011/E/003

GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION BARC/2011/E/003

BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2011 BARC/2011/E/003

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01 Security classification : Unclassified

02 Distribution : External

03 Report status : New

04 Series : BARC External

05 Report type : Technical Report

06 Report No. : BARC/2011/E/003

07 Part No. or Volume No. :

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10 Title and subtitle : Development of a Raman spectrometer to study surface-enhanced Raman scattering

11 Collation : 31 p., 15 figs.

13 Project No. :

20 Personal author(s) : Nandita Biswas; Ridhima Chadha; Sudhir Kapoor; Sisir K. Sarkar; Tulsi Mukherjee

21 Affiliation of author(s) : Radiation and Photochemistry Division , Bhabha Atomic Research Centre, Mumbai

22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai - 400 085

23 Originating unit : Radiation and Photochemistry Division, BARC, Mumbai

24 Sponsor(s) Name : Department of Atomic Energy

Type : Government

Contd... BARC/2011/E/003

30 Date of submission : January 2011

31 Publication/Issue date : February 2011

40 Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai

42 Form of distribution : Hard copy

50 Language of text : English

51 Language of summary : English, Hindi

52 No. of references : 36 refs.

53 Gives data on :

60 Abstract : Raman spectroscopy is an important tool, which provides enormous information on the vibrational and structural details of materials. This understanding is not only interesting due to its fundamental importance, but also of considerable importance in optoelectronics and device applications of these materials in nanotechnology. In this report, we begin with a brief introduction on the Raman effect and various Raman scattering techniques, followed by a detailed discussion on the development of an instrument with home-built collection optics attachment. This Raman system consists of a pulsed laser excitation source, a sample compartment, collection optics to collect the scattered light, a notch filter to reject the intense laser light, a monochromator to disperse the scattered light and a detector to detect the Raman signal. After calibrating the Raman spectrometer with standard solvents, we present our results on Surface- Enhanced Raman Scattering (SERS) investigations on three different kinds of chemical systems.

70 Keywords/Descriptors : RAMAN SPECTROSCOPY; RAMAN EFFECT; MONOCHROMATORS; SPECTROMETERS; ENERGY-LEVEL TRANSITIONS; PERFORMANCE TESTING

71 INIS Subject Category : S46

99 Supplementary elements : 1

Development of a Raman Spectrometer to Study Surface-Enhanced Raman Scattering

Nandita Biswas, Ridhima Chadha, Sudhir Kapoor, Sisir K. Sarkar and Tulsi Mukherjee

Radiation & Photochemistry Division

Bhabha Atomic Research Centre, Mumbai 400085, India.

Abstract

Raman spectroscopy is an important tool, which provides enormous information on the vibrational and structural details of materials. This understanding is not only interesting due to its fundamental importance, but also of considerable importance in optoelectronics and device applications of these materials in nanotechnology. In this report, we begin with a brief introduction on the Raman effect and various Raman scattering techniques, followed by a detailed discussion on the development of an instrument with home-built collection optics attachment. This Raman system consists of a pulsed laser excitation source, a sample compartment, collection optics to collect the scattered light, a notch filter to reject the intense laser light, a monochromator to disperse the scattered light and a detector to detect the Raman signal. After calibrating the Raman spectrometer with standard solvents, we present our results on Surface-Enhanced Raman Scattering (SERS) investigations on three different kinds of chemical systems.

1. Introduction

1.1 The Raman Effect and Normal Raman Scattering

The Raman effect arises when a photon is incident on a molecule and interacts with the electric dipole moment of the molecule [1]. In classical terms, the interaction can be viewed as a perturbation of the electric field of the molecule. When light i.e. the incoming photons interact with the electron cloud of the molecule, it results in most photons being elastically scattered (Rayleigh scattering). The incident and scattered photons, therefore, have the same energy (frequency) and wavelength. However, a small fraction of the scattered light (approximately 1 in 10 million photons) upon interaction with the sample is scattered at optical frequencies different from the frequency of the incident photons. During this scattering, the energy of the incoming photon is either red-shifted or blue-shifted, depending on the particular process. This phenomenon is called Raman scattering and was discovered by C.V. Raman [2] in 1928. Raman scattering provides a rich variety of information on the structure and composition of matter, based on its vibrational fingerprints. The vibrational information, which usually occurs at IR frequencies, can be obtained by monitoring the frequency shifts between the excitation and the scattered light. As a scattering process, however, the Raman effect is exceedingly weak: typical Raman cross sections per molecule range between 10-30 and 10-25 cm2 with the larger values occurring during resonant conditions, when the frequency of light happens to match an electronic transition in the molecule. By comparison, fluorescence spectroscopy, based on the absorption and emission of light, exploits effective cross sections between 10-17 and 10-16 cm2. However, the advent of the lasers as an intense and monochromatic source of excitation light was a milestone in the history of Raman spectroscopy and dramatically improved the strength of scattering signals.

In quantum mechanics, the scattering phenomenon is described as an excitation to a virtual state lower in energy than a real electronic transition with instantaneous de-excitation to the final state [1]. The scattering event occurs in 10-14 second or less. The virtual-state

description of scattering is shown in figure 1. When the energy of the incident photon (hvo) is the same as that of the scattered photon, the phenomenon is termed as Rayleigh scattering, as shown in figure 1(a). When a photon of energy hvo is absorbed by the molecule in the ground 3

vibrational state, part of the photon’s energy is transferred to the Raman active mode with

frequency vm and the resulting frequency of the scattered light is reduced to vo-vm and is termed Stokes Raman scattering as shown in figure 1(b). At room temperature, the thermal population of vibrational excited states is low, although not zero. Therefore, for the majority of molecules, the initial state is the ground vibrational state, and the scattered photon will have lower energy (longer wavelength) than the exciting photon (Stokes shift). This Stokes shifted scatter is what is usually observed in Raman spectroscopy and is depicted in figure 1(b). According to the Boltzmann population of states, a small fraction of the molecules are in vibrationally excited

states. Thus, when a photon of energy hvo is absorbed by a Raman-active molecule which at the time of interaction, is already in the excited vibrational state, excessive energy of the Raman active mode is released and the molecule returns to the ground vibrational state. The resulting

frequency of scattered light goes up to vo+vm and is termed anti-Stokes Raman scattering as shown in figure 1(c). The anti-Stokes Raman scattering is always weaker than the Stokes scattering, but at room temperature it is strong enough to be useful for vibrational frequencies less than about 1500 cm-1. The Stokes and anti-Stokes spectra contain the same frequency information. The ratio of anti-Stokes to Stokes intensity at any vibrational frequency is a measure of temperature. anti-Stokes Raman scattering is useful for fluorescent molecules when the Stokes spectrum is masked by the fluorescence background.

1.2 Selection Rules and Intensities in Raman Spectroscopy

The dipole moment, P, induced in a molecule by an external electric field, E, is expressed as P= E, where,  is the polarizability of the molecule. The polarizability measures the ease with which the electron cloud around a molecule can be distorted. The induced dipole scatters light at the optical frequency of the incident light wave. The selection rule for a Raman-active vibration suggests that there will be a change in polarizability during the vibration, i.e. /Q ≠0, where, Q is the normal coordinate of the vibration. The scattering intensity is proportional to the square of the induced dipole moment, P2 i.e. (/Q)2. The observed strength of the Raman band is also proportional to the concentration of the species, as well as the intensity of the excitation laser.

The main issue with Raman spectroscopy as mentioned earlier is the exceedingly weak scattering intensity. In order to get a detectable scattering intensity, it is necessary to enhance the 4

Raman scattering process by improving the Raman spectroscopy techniques. In the following sections, few enhanced Raman scattering techniques are discussed.

1.3 Stimulated Raman Scattering (SRS)

SRS is an example of “non-linear” Raman spectroscopy [1,3,4]. When the sample is irradiated with a very intense laser pulse, new “non-linear” phenomena are observed in the Raman signal. Very strong laser pulse with electric field strength > 109 V·cm-1 transforms up to 50% of all laser

pulse energy into coherent beam at Stokes frequency vo-vm as shown in figure 2. The Stokes

beam is unidirectional with the incident laser beam. Only the mode vm, which is the strongest in the normal Raman spectrum, is greatly amplified, while the weaker Raman active modes are not observed. The Stokes beam is so strong that it acts as a secondary excitation source and

generates the second Stokes line with frequency vo-2vm. The second Stokes line generates the

third one with the frequency vo-3vm, etc. SRS technique leads to an enhancement of the Raman signal by 4-5 orders of magnitude as compared to the spontaneous Raman scattering.

1.4 Coherent Anti-Stokes Raman Scattering (CARS)

CARS, is another type of “non-linear” Raman spectroscopy [1,5,6], where instead of one laser, two very strong collinear lasers irradiate a sample. Frequency of the first laser is usually constant, while the frequency of the second one can be tuned in a way that the frequency difference between the two lasers equals exactly the frequency of some Raman-active mode of interest. This particular mode will be the only strong mode in the Raman signal. With CARS, only one strong Raman peak of interest can be obtained. As shown in the CARS transition scheme (figure 3), two laser beams with frequencies v1 and v2 (v1 > v2) interact coherently, and

because of the wave mixing, produce strong scattered light of frequency 2v1-v2. If the frequency

difference between two lasers v1-v2 is equal to the frequency vm of a Raman-active vibration then

a strong light of frequency v1+vm is emitted. In other words, to obtain strong Raman signal, the

second laser frequency should be tuned in a way that v2=v1-vm. Then the frequency of the strong

scattered light will be 2v1-v2=2v1-(v1-vm) = v1+vm, which is higher than the excitation frequency

v1 and therefore considered to be anti-Stokes frequency.

1.5 Resonance Raman Scattering (RRS)

RRS takes place when the wavelength of the exciting laser is close to or coincides with the electronic absorption of a molecule [1,7]. Under resonance conditions, the intensity of the Raman-active vibrations associated with the absorbing chromophore are enhanced by a factor of 102-104. This resonance enhancement can be extremely useful, not just in significantly lowering the detection limits, but also in introducing electronic selectivity. Thus the resonance Raman technique is used for providing both structural and electronic insight [7-10] into the species of interest. For RRS, tunable lasers are the most appropriate choice.

1.6 Surface-Enhanced Raman Scattering (SERS)

In an observation, made nearly 50 years after the discovery of the Raman effect, M. Fleischmann and coworkers [11] reported an unexpectedly strong Raman signal from a monolayer of pyridine adsorbed on an electrochemically roughened silver electrode. They explained the signal strength as the result of a large number of molecules from the increased surface area on the rough electrode. Subsequently, D. L. Jeanmaire [12] and M. G. Albrecht [13] confirmed the result - a Raman enhancement of about a million times, compared to the signal from pyridine molecules in the absence of the metal. They concluded that such a strong signal cannot be explained by an increase in the surface area alone: a genuine enhancement in the Raman scattering efficiency must have been responsible. Shortly thereafter, researchers realized that the excitation of surface plasmons in the metal were largely behind the result and could explain the enhanced Raman signals. In last 35 years since its discovery and confirmation, Surface-Enhanced Raman Scattering (SERS) as shown in figure 4, has matured into a powerful spectroscopic method that exploits the interaction of light, molecules, and metal nanostructures to boost Raman signals [14,15].

The exact mechanism of the enhancement effect of SERS is not completely understood and is therefore still a subject of investigation and discussion.There are two primary theories and while their mechanisms are substantially different from each other, distinguishing them experimentally has not been straightforward. The electromagnetic theory relies upon the excitation of localized surface plasmons, while the chemical theory rationalizes the effect through the formation of charge-transfer complexes. The chemical theory only applies for 6

species which have formed a bond with the surface, so it cannot clearly explain the observed signal enhancement in all cases, while the electromagnetic theory can apply even in those cases where the specimen is only physisorbed to the metal surface. The enhancement factor can be as much as 1014-1015, which allows the technique to be sensitive enough to detect single molecules [16,17]. An important aspect of SERS is its potential for probing the interaction between various adsorbates and metallic surfaces. SERS has become an increasingly popular technique not only for studying the molecules at trace concentrations but also in estimating their possible orientations on the metal surfaces [18-26]. Application of the “surface selection rules” [27] helps in providing information on the sites of binding and the molecular orientation on the metal surface. Moreover, SERS has contributed to the development of plasmonics and the related field of near field optics, which are revolutionizing optics and spectroscopy.

1.7 Time-Resolved Resonance Raman Scattering (TR3S)

The RRS and SERS techniques are mostly used to study stable species in the form of solid, solution and/or mixtures. However, with the developments in laser Raman spectroscopy, it is now possible to study short-lived transient species such as electronically excited molecules, radicals, etc, with a lifetime of the order of nano (10-9) or pico (10-12) seconds by using the Time- Resolved Resonance Raman Scattering (TR3S). In TR3S technique, the short-lived transients may be generated by illuminating the sample with an excitation (pumping) source that can either be a pulsed laser or a pulsed electron accelerator to generate photo-excited species or excited radicals, respectively. The TR3S technique can provide both the structural and dynamic information of the short-lived transient species and thus, it allows one to study the time evolution of the excited molecules or radicals as well as decay of the transient species. Hence, one can obtain a detailed understanding of the changes in the chemical structure which occur within a sample after a reaction has been initiated. The energy level diagram to study the photo-excited 3 triplets of molecules by TR S is shown in figure 5. The molecules in the singlet ground state (S0)

are first excited by the pump laser to the singlet excited state (S1) where they undergo

nonradiative decay or inter-system crossing to the triplet excited state (T1). Since the pulse width of the pump laser is much narrower than the lifetime of the triplet state (normally milli-

microseconds), excitation to the S1 state by the pump laser increases the population of molecules

in T1, which may be sufficient to observe the Raman spectrum from the T1 state with a probe 7

laser. When the probe laser is in resonance with any higher Tn state, the technique is referred to as TR3S and thus can show extraordinarily strong Raman enhancement, selectivity and sensitivity (probe very low sample concentration). Using TR3 spectroscopy as a diagnostic tool, the triplet excited state of 2-methoxy-naphthalene [28] was studied. Similarly, using TR3S and p- dimethoxybenzene as a model system, G. N. R. Tripathy [29] has shown for the first time, the existence of an electron-transfer pathway in the reaction of ●OH radical.

2. Development of a Raman spectrometer

Recently, we have fabricated a Raman instrument for various Raman applications, viz. normal Raman, surface-enhanced Raman, resonance Raman and time-resolved resonance Raman scattering. The Raman instrument has been tested for normal Raman and surface-enhanced Raman scattering applications and is discussed in details in the following section.

2.1 Instrumentation

The schematic diagram of the Raman instrument as shown in figure 5 involves an excitation source viz. a laser, turning prisms (TP1, TP2 and TP3) to direct the path of the laser beam, a focusing lens to focus the laser beam onto the sample, collection optics to gather the scattered light, a notch filter to reject the laser light, a monochromator to disperse the scattered light and a detector to identify the scattered light. The detector is interfaced with the computer to record the Raman scattering signal that can be printed using a printer. The photographs of the Raman setup showing various sub systems are shown in figure 6. The details of the Raman setup are discussed below.

2.2 Excitation source: The excitation source as shown in figures 6 and 7 is a Nd3+:YAG (Brilliant B) pumped tunable dye laser (TDL-90 system from Quantel, France). The Nd3+:YAG laser has a repetition rate of 13 Hz with pulse energy of 750 mJ/pulse at the fundamental wavelength i.e. 1064 nm, 350 mJ/pulse at the second harmonic, 532 nm and 145 mJ/pulse at the third harmonic output, i.e. 355 nm. The tunable dye laser is pumped by the second and third harmonics, i.e. 532 and 355 nm of Brilliant B. The dye laser consists of an oscillator, which generates the lasing, the pre-amplifier and the amplifier, through which the amplified output laser energy is generated. The dye laser is tunable in the range of 420-750 nm and the laser lines 8 can be separated using a grating with 2400 lines/mm. The dyes used for the lasing in the visible region are Rhodamine 590, DCM and Coumarin 500. The dye solutions prepared in methanol are circulated using a dye circulator. The output energy from the dye laser is in the range of 10-50 mJ/pulse.

2.3 Collection optics: The laser beam is steered onto the sample using turning prisms (TP1, TP2 and TP3) and is focused using a lens of focal length 25 cm. The sample is placed on the path of the laser beam in a standard 1  1 cm cuvette. Raman scattering from the sample is collected at 90 to the laser beam by a camera lens (f/1.4 from Nikon) which is then imaged on to the slit of the spectrograph. A notch filter is placed between the sample cell and the monochromator to reject the Rayleigh line from the scattered light.

2.4 Notch Filter: In the scattering process, strong elastically scattered Rayleigh background often masks the weak inelastically scattered Raman signals. The background due to Rayleigh scattering in our Raman setup was suppressed efficiently using a Notch filter with a large optical density >6.0 at the laser wavelength of 532 nm.

2.5 Monochromator: A single stage Czerny-Turner monochromator (Triax 550, Horiba Jobin Yvon, France) is used as the dispersive unit. The schematic diagram of the optics of the monochromator is shown in figure 5. This monochromator has a focal length 0.55 m and good light collection efficiency with f/6.4 aperture. It contains two concave mirrors (M1 and M2) and a 76 mm×76mm holographic grating (G with 1200 and 600 lines/mm both blazed at 500 nm) for dispersion of the spectral line. The slit is placed at the effective focus of a concave mirror (M1), so that the light reflected from the mirror is collimated (focused at infinity). The collimated light is diffracted from the grating (G) and is then collected by another concave mirror (M2) which refocuses the dispersed light on the exit slit of the monochromator. The light falling on the exit slit contains the entire image of the entrance slit. The width of the slit S can be varied from 2 µm to 2000 µm. The density of the lines (G with 1200 lines/mm) at longer wavelength provides the best resolution of the monochromator as 0.025 nm and its spectral dispersion is 1.55 nm/mm.

2.6 Detector: Multichannel detectors like charge coupled devices (CCDs) can help one to collect more scattered light signals for a wide spectral window in shorter time than what can be done by 9

a photomultiplier tube (PMT). So, to measure very weak signals CCDs are often used as a detector at the exit slit of the monochromator.

In our Raman setup (figures 6 and 7), an intensified charge coupled device (ICCD-3515, Andor iStar) is mounted at the exit port of the monochromator to detect the weak Raman scattered signal. The ICCD, comprise of a gated image intensifier and a CCD sensor. A CCD is a silicon-based semiconductor chip bearing a two-dimensional matrix of photo-sensors, or pixels.

This matrix is usually referred to as the image area. Our CCD sensor comprises of 1024×256

pixels with effective pixel size being 26×26 µm2. An image intensifier is a device that amplifies

the intensity of an image. There are three major elements in an image intensifier, the photocathode, the microchannel plate and the output phosphor screen. The size of the intensifier is 18mm. The MCP gain can be adjusted from 0-250, depending on which the ICCD signal will increase. The detector is operated in gated mode and in synchronization with the laser beam in order to avoid any spurious background signal. The quantum efficiency of the detector is ~53%. The ICCD is interfaced to the computer with an interfacing card and the Raman scattered light can be collected using a window-based program Solis (Andor’s camera and software control program) and using the same software one can collect the raw data in ASCII form for further analysis after the acquisition. The data can then be printed.

3. Calibration

The first step after the installation of the spectrometer is to calibrate using a few standard samples. The entrance slit of the monochromator was kept at 50 µm for the calibration. At this slit width the resolution of the instrument is 0.03 nm (1 cm−1). The room light response was obtained at 546.07 and 578.7 nm to check the accuracy of the instrument. Subsequent to the installation and the calibration of the monochromator, a few standard samples, viz. benzene and carbon tetrachloride, were studied.

3.1 Benzene: The Raman spectrum of benzene in the range 500-1400 cm-1 recorded using 532 nm laser line is shown in figures 8(a) and (b) with 600 and 1200 lines/mm gratings, respectively. This spectrum in the range of 970-1020 cm-1 is expanded and is shown in figures 9(a) and (b) with 600 and 1200 lines/mm gratings, respectively. The Raman peak is fitted with a Lorentzian 10

and the peak position is obtained at 992.47 and 992.69 cm-1, respectively. The corresponding -1 peak due to C-C vibration is expected at 992.36 cm [30]. The full width at half maxima (FWHM) of the peak with 600 and 1200 lines/mm is 6.63 and 3.81 cm-1, respectively. Thus, our measurement is in good agreement with the data available in the literature.

3.2 Carbon tetrachloride: The Raman spectrum of carbon tetrachloride in the range 100-1000 cm-1 recorded using 532 nm laser excitation with 600 and 1200 lines/mm is shown in figures 10 (a) and (b), respectively. The Raman bands of carbon tetrachloride are observed at 218, 314, 459, 762 and 790 cm-1 which is in agreement with the data available in literature. The Raman spectrum of carbon tetrachloride were recorded as a function of various parameters like entrance slit width (figure 11) and MCP gain (figure 12) in order to get the optimum signal. The slit was opened to 20, 50, 100, 150 and 200 µms and the spectrum recorded in the 150-550 cm-1 region with a 600 lines/mm grating. The Raman signal increases considerably with the increase in the slit width from 20 to 150 µm, but does not change much beyond 200 µm. The MCP gain was increased from 0 to 150. From figure 12, it is seen that the change in the gain from 0 to 20 does not lead to any significant change in the Raman signal, which, however, increases substantially with the increase in the gain from 50 to 150. Thus, equipped with a calibrated Raman spectrometer that was tested with standard samples like benzene and carbon tetrachloride, we have demonstrated its use to study nanomaterials for SERS applications.

4. Surface-Enhanced Raman Scattering (SERS) Studies

SERS studies were carried out for chlorogenic acid, benzotriazole and cadmium sulphide (CdS)

on TiO2. SERS spectra were recorded at room temperature using the 532 nm laser line, from the Nd3+:YAG laser (Brilliant B). The laser power used to record the Raman spectra was 30 mW. The Raman scattered light as mentioned earlier was collected at the 90 geometry and detected using a Triax 550 monochromator and an ICCD together with a notch filter, covering a spectral range of 150-1700 cm-1. The entrance slit width of the monochromator and the MCP gain was kept at 200 µm and 150, respectively for getting the optimum signal. The observed results for

chlorogenic acid, benzotriazole and CdS on TiO2 are presented below. 11

4.1 Chlorogenic acid (CGA): CGA is an ester of caffeic acid and quinic acid and is known to be an antioxidant. It is an important plant metabolite with anti-viral and anti-bacterial properties and thus, it is useful to study its surface adsorption characteristics. The structure of CGA is shown in the inset of figure 13. To study the SERS of CGA, colloidal silver was prepared by the reduction of silver nitrate with sodium citrate using the method of Lee and Meisel [31]. The SERS spectra of CGA were recorded at different concentrations as shown in figure 13. Strong peaks are -1 observed at 1212, 1363, 1489 and 1583 cm and are assigned to the phenyl ring CH bend, CO2 symmetric stretch, ring CO stretch and the phenyl ring stretch, respectively. From the enhanced bands observed in SERS spectra it has been inferred that CGA is chemisorbed to the silver surface through the oxygen atoms of the carboxylate group.

4.2 Benzotriazole: Benzotriazole and its derivatives have found several applications in the field of corrosion of metals and have been widely used in the surface treatment of materials as corrosion inhibitors. Here, we have reported the results of our observation of benzotriazole adsorbed on silver colloid in an alkaline medium. The focus of this work is to study the orientation of benzotriazole on the silver surface and to study the effect of concentration on the surface orientation. The structure of benzotriazole is shown in the inset of figure 14. To study SERS of benzotriazole, colloidal silver was prepared by the reduction of silver nitrate with sodium borohydride using the method of Creighton et. al. [32]. The size of silver particles in the silver colloid was determined by TEM studies and was found to be 15-20 nm [33]. SERS spectra of benzotriazole were recorded at different concentrations as shown in figure 14. The spectra show selective enhancement of the Raman bands at 788, 1010, 1282, 1388 and 1574 cm-1. The bands at 788 and 1010 cm-1 are assigned to the phenyl ring breathing. The bands at 1282, 1388 and 1574 cm-1 are assigned to CH in plane bend in combination with CC stretch, triazole ring stretch and phenyl ring stretch, respectively. The interpretation of the data based on the surface selection rules suggest a near end-on configuration for the benzotriazole molecule with the N- atoms of the triazole ring mainly interacting with the silver surface.

4.3 CdS/TiO2: 20 mol% doped CdS in TiO2 semiconductor composites were prepared and

characterized by SERS. The SERS spectra of the CdS/TiO2 nano-composites annealed at different temperatures were recorded and shown in figure 15. The room temperature SERS spectrum shows weak Raman features at 398, 518 and 643 cm-1 with the fluorescence 12 background due to CdS. With increase in annealing temperatures to 400 C and 600 C, the peaks became more intense and resolved.

4.4 Future Strategies: Scanning-Probe Tips and Two-Photon Methods

We plan to couple the existing tunable laser Raman system with an excitation (pumping) source that can either be a pulsed laser or a pulsed electron accelerator in order to generate photo- excited species or excited radicals, respectively. These short-lived (10-9 s) photo-excited species or excited radicals that are usually formed in very low concentration will be studied using TR3S technique. This will allow us to obtain a detailed understanding of the structural changes which occur within a sample after the initiation of a chemical reaction.

The strong field enhancement and local confinement of the SERS effect open up exciting new capabilities that are pushing spectroscopic methods to new limits. One of those methods, Tip-Enhanced Raman Spectroscopy (TERS) [34], combines highly confined probe volumes with scanning-probe microscopy. A fine metal tip, brought within a few nanometers of the surface of a molecular film, strongly enhances the Raman scattered signal from the film (as shown in figure 4). In addition to the enhanced Raman signal at nanoscale resolution, the scanning tip also provides complementary topographic information of the sample. Other methods that are gaining interest among researchers include two-photon techniques. By using two lower energy photons during the scattering process, light can be shifted to longer wavelengths than is possible during the one-photon scattering. The lower energy incident photons penetrate many materials more deeply and simultaneously reduce the degradation of light sensitive samples. So far, most two-photon applications are based on fluorescence. Hyper-Raman scattering (HRS) [35], is a potential tool for probing the chemistry of materials using two-photon scattering. So far, the extremely small hyper-Raman cross sections (10-65 cm4 s/photon) have precluded applications of HRS as a practical spectroscopic tool. The extremely strong electromagnetic enhancement that accompanies HRS suggests that researchers can now utilize versatile surface-enhanced hyper Raman scattering (SEHRS) [36] process as well.

5. Summary We have developed a Raman spectrometer to study the vibrational and structural details of materials. The excitation source was 532 nm from the Nd3+:YAG laser. The Raman scattered light was collected at 90 to the laser beam by a camera lens (f/1.4 from Nikon) which is then imaged on to the slit of the monochromator (Triax 550, Jobin Yvon, France). The Rayleigh scattering was rejected using a notch filter placed in front of the monochromator. An intensified charge coupled device (ICCD, Andor) is mounted at the exit port of the monochromator to detect the Raman scattered light. The Raman system was calibrated and tested with known solvent standards. SERS and Raman studies were carried out for chlorogenic acid, benzotriazole and

CdS/TiO2 semiconductor nano-composites, respectively.

Acknowledgements:

The authors gratefully acknowledge Mr. S. A. Nadkarni, Ms. M. Toley and Mr. S. J. Shinde of Radiation and Photochemistry Division for their help and support in setting up of the Raman spectrometer.

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Figure 1: Energy level diagram for Raman scattering; (a) Rayleigh scattering (b) Stokes Raman scattering (c) anti-Stokes Raman scattering.

Figure 2: Stimulated Raman Scattering transition scheme.

Figure 3: Coherent Anti-Stokes Raman Scattering transition scheme.

Figure 4: Surface-Enhanced Raman Scattering (SERS). Molecules (blue) are adsorbed onto metal nanoparticles (orange) either in suspension or on a surface. SERS spectrum reveals molecular vibration energies based on the frequency shift between the incident (green) and the

scattered (red) laser light. The power PSERS of the Raman signal depends on the number N of

molecules involved in the process, the laser intensity IL, the effective Raman cross section of the R 2 2 adsorbed molecule  ads, and enhancement factors A(L) and A(S) , which quantify the magnification of laser-excitation and scattered fields, respectively.

Figure 5: Energy level diagram for Time-Resolved Resonance Raman Scattering (TR3S).

Figure 6: Schematic of the surface-enhanced Raman scattering (SERS) setup.

Figure 7: Photographs of the Raman setup showing various sub-systems.

Figure 8: Raman spectra of benzene in the region 500-1400 cm-1 with (a) 600 and (b) 1200 lines/mm grating.

(a)

(b)

Intensity (arb. units)

600 800 1000 1200 1400 -1 Raman shift / cm

Figure 9: Raman spectra of benzene in the region 970-1020 cm-1 with (a) 600 and (b) 1200 lines/mm grating.

(a)

6.63 cm-1

(b)

Intensity (arb. units)

3.81 cm-1

970 980 990 1000 1010 1020

Raman shift / cm-1

Figure 10: Raman spectra of carbon tetrachloride in the region 100-1000 cm-1 with (a) 600 and (b) 1200 lines/mm grating.

(a)

(b)

Intensity (arb. units)

200 400 600 800 1000 Raman shift / cm-1

Figure11: Raman spectra of carbon tetrachloride in the region 150-550 cm-1 with a grating of 600 lines/mm as a function of slit width, 20 (open stars), 50 (dashed line), 100 (dotted line), 150 (open circles) and 200 (solid line) µm.

Intensity (arb. units)

200 300 400 500

-1 Raman shift / cm

Figure 12: Raman spectra of carbon tetrachloride in the region 150-550 cm-1 with a grating of 600 lines/mm as a function of MCP gain, 0 (solid line), 20 (open stars), 50 (dashed line), 100 (dotted line) and 150 (open circles).

Intensity (arb. units)

200 300 400 500

Raman shift / cm-1

Figure 13: SERS spectra at (a) 510-5 M, (b) 110-4 M and (c) 210-4 M concentrations of CGA. In the inset is shown the structure of CGA.

O OH

O OH HO C 2

HO OH OH

(c)

(b)

Intensity (arb. units)

(a)

200 400 600 800 1000 1200 1400 1600

Raman shift / cm-1

Figure 14: SERS spectra at (a) 210-4 M, (b) 1.510-4 M and (c) 110-4 M concentrations of benzotriazole. In the inset is shown the structure of benzotriazole.

H N N N

(c)

(b)

Intensity (abb. units)

(a)

200 400 600 800 1000 1200 1400 1600 -1 Raman shift / cm

Figure 15: Raman spectra of 20% doped CdS in TiO2 annealed at different temperatures (a) 27C (b) 400C and (c) 600C.

(c)

Intensity (arb. units) (b)

(a)

200 400 600 800 1000 1200 1400 1600 Raman shift / cm-1