setting up of in-situ x ray absorption spectroscopy...

N E W S L E T T E RFounder’s Day Special Issue October 2016

Setting up of In-situ X ray Absorption Spectroscopy measurement facility at Indus-2

SRS & Indigenous development of thin film multilayer neutron supermirrors

D.Bhattacharyya, C.Nayak, A. Biswas, S.N. Jha and N.K. Sahoo

Atomic & Molecular Physics Division

Dr. D. Bhattacharyya is the recipient of the DAE Homi Bhabha Science and

Technology Award for the year 2014

In-situ X-ray Absorption Measurement Facility at Indus-2 SRS

X-ray absorption spectroscopy (XAS) generally deals with measurement of absorption coefficient as a function of X-ray photon energy around an X-ray absorption edge of an element in a material. X-ray absorption spectrum consists of two parts: (i) The spectrum near the absorption edge (viz., the X-ray near edge structure or the XANES part) gives information about the external perturbations in the valence states to which electrons make transitions from core levels upon absorption of X-ray photon energy and hence can yield information regarding hybridization of orbitals in case of molecule or long range order existing in a crystalline sample apart from the oxidation states of the absorbing atom in the material. (ii) The second part of the spectrum which extends from 50 eV to ~700 eV above the absorption edge is generally called the Extended X-ray absorption fine structure (EXAFS) part which is generally characterized by the presence of fine structure oscillations and can give precise information regarding the short range order and local structure around the particular atomic species in the material. This determination is confined to a distance given by the mean free path of the photoelectron in the condensed matter, which is between 5-7 Å radius from the element. The above characteristic along with the fact that EXAFS is an element specific tool, makes it a powerful structural local probe. With the advent of modern bright Synchrotron radiation sources, XAS has emerged out to be the most powerful local structure determination technique which can be applied to any type of material viz. amorphous, polycrystalline, polymers, surfaces, solutions. Furthermore, XAS does not require any

particular experimental conditions, such as high vacuum and hence samples of various physical forms can be adapted for measurements in the experimental stations [1].

Over the last few years, a comprehensive XAS measurement facility has been developed at INDUS-2 SRS at RRCAT, Indore which consists of two working beamlines viz., Energy Dispersive EXAFS beamline (BL-08) and Energy Scanning EXAFS beamline (BL-09) [2,3]. A large number of users from R&D institutions, universities and industries across the country are using the above facility and more than 85 papers have so far been published in reputed international journals in last 5-6 years where the data measured in the above beamlines have been used. The energy dispersive EXAFS beamline (BL-08) covers the photon energy range of 5-20 keV and in this beamline, the entire EXAFS spectrum of the samples can be recorded in a single shot within a time scale of ∼300 msec. Hence this is best suited for studying in-situ fast and time-resolved processes. One of the major applications of this beamline is in-situ studies on growth of nanoparticles.

Since its invention, nanoparticles find wide varieties of applications in the field of medicine, catalysis, biotechnology, fuel cells, solar cells, sensors and environmental science etc. [4]. This is well established that the properties of these nanoparticles can be tuned over a wide range by controlling their size and shape and this seeks the need to understand the mechanism of nucleation and growth of these nanoparticles. This envisages a new era of “in-situ” studies on the growth of nanoparticles. However, the scarcity of suitable fast techniques, which is one of the pre-requisites of in-situ studies, that can actually throw some light into the

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growth and nucleation process, made these studies unpopular for a long time. UV-Vis spectroscopy is one of the most popular and oldest techniques to study the kinetics of growth. However, this technique is limited to systems which give absorption peak in the UV-Vis wavelength regime. Transmission Electron Microscopy (TEM) is also another popular technique that can be used in-situ to study the changes in particle size during nucleation and growth. However, in-situ TEM has the limitation that the electron beam can interfere with the reacting species and thus can change the whole redox reaction process to be studied. Also, it is very difficult to be employed in-situ in solution phase due to vacuum requirement. Small angle X-ray scattering (SAXS) is an important X-ray based technique which can act as a substitute for TEM measurements revealing the changes in particle size during growth and nucleation. However, all the above techniques mostly yield information on real

time changes in particle sizes only and cannot throw light on the evolution of the reduction process that is mainly responsible for the nucleation and growth of nanoparticles in such systems. Also these techniques cannot find out the coordination of atoms in clusters which are formed in the initial phase of the growth and act as seeds for further growth of the nanoparticles. In this regard XAS has played a crucial role in giving insight into the nucleation and formation process of nanoparticles from a very different aspect as compared to the other popular in-situ techniques [5].

An in-situ XAS measurement facility has been developed at Indus-2 SRS to study the growth and nucleation of nanoparticles [6]. The experimental facility has been improvised for simultaneous measurement of EXAFS and UV-Vis spectroscopy. Fig. 1(a) shows the schematic diagram of the setup while fig. 1(b) shows the photograph of the same.

(a) (b) Fig. 1: (a) Schematic diagram and (b) Photograph of the in-situ XAS measurement set-up.

In the above set-up, synthesis of nanoparticles is carried out in a specially designed teflon reaction cell having paths for both X-rays and UV-Vis radiation in mutually perpendicular directions. X-ray is transmitted through kapton windows while the optical light is passed using optical fibres which are capped with teflon ferrules and are directly immersed into the solution. The cell has been designed and fabricated in such a way that the optical paths can vary from 5 mm to 20 mm for X-rays and from 2 mm to 20 mm for UV-Vis radiation. The volume of the cell varies according to the adjustment of the kapton windows and optical fibers. The sample mount of the beamline has been modified so that the in-situ reaction cell can be placed on a magnetic stirrer-cum-heater for mixing and heating the reaction solution. The precursor is taken in the teflon reaction cell and the reducer is

injected into it through teflon tube using a computer-controlled motor-driven syringe pump. Using the above set-up the growth of Au and Pt nanoparticles from their respective chloride precursors using block copolymer based reduce-cum-stabilizer have been studied by simultaneous in-situ measurement of XAS and UV-Vis spectroscopy [6]. Figs. 2(a) shows the time-resolved XANES spectra of the precursors showing reduction of Au+3 and Pt+4 ions to metallic states, whereas fig. 2(b) shows the evolution of Au and Pt coordination with time as obtained from time resolved in-situ EXAFS spectra.

The growth kinetics of both gold and platinum nanoparticles are found to be almost similar and are found to follow the three stages viz., (i) reduction of metal ions by block copolymer and formation of small

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cluster of typical 5 coordinations, (ii) absorption of block copolymer and reduction of metal ions on the surface of the cluster and increase in cluster size and (iii) growth of gold nanoparticles stabilized by block copolymer. It can be observed from fig. 2(b) that the time scale involved in the 2nd and 3rd stages of the nanoparticle formation are similar for both gold and platinum nanoparticles. This may be due to the fact that these stages are mainly governed by the block copolymer which is same in both the cases. However, the 1st stage of nucleation takes place earlier in case of Au than in case of Pt due to the difference in the reduction potential of the respective precursors. The cluster sizes at the various stages of growth of the nanoparticles have been estimated theoretically from the metal-metal coordination numbers obtained from the time-resolved EXAFS spectra shown in fig. 2(b). It has been found that in both the cases, 12-atom clusters are formed which act as seeds for further growth. However, for gold, these 12-atom cluster first grows into the magic number 13-atom cluster and then 147-atom clusters which further grows into gold nanoparticles. For platinum, the 12-atom cluster grows into the magic number 55-atom cluster which further grows into platinum nanoparticles. The first two stages of the growth of Au and Pt nanoparticles as obtained by in-situ XAS measurements are also corroborated by simultaneous in-situ measurement of UV-Vis spectroscopy.

(a)

(b)

Fig. 2: (a) Time-resolved XANES spectra of the precursors showing reduction of Au+3 and Pt+4 ions to metallic states, (b) Evolution of Au and Pt coordination with time as obtained from time resolved in-situ EXAFS spectra.

References:

1. D.C. Konigsberger, R. Prince, X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES (Wiley, New York, 1988).

2. D. Bhattacharyya, A.K. Poswal, S.N. Jha, Sangeeta and S.C. Sabharwal, Nuclear Instruments Method. in Phys. Res., A 609 (2009) 286.

3. S. Basu, C. Nayak, A. K. Yadav, A. Agrawal, A. K. Poswal, D. Bhattacharyya, S. N. Jha, and N. K. Sahoo, J. Phys.: Conf. Ser., 493 (2014) 012032.

4. Gunter Schmid, ed. Nanoparticles: From Theory to Application. 2004, Wiley-VCH, Weinheim.

5. J. Polte, T. T. Ahner, F. Delissen, S. Sokolov, F. Emmerling, A. F. Thünemann and R. Kraehnert, J. Am. Chem. Soc., 132 (2010) 1296.

6. C. Nayak, D. Bhattacharyya, S. N. Jha and N.K. Sahoo, J. Synch. Rad., 23 (2016) 293.

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Indigenous development of thin film multilayer neutron supermirrors

Worldwide availability of high flux research reactor boosted the application of neutron as a probe in the research of condensed matter physics. Due to comparable de Broglie wavelength (λ=h/mv) of neutron with the inter-atomic spacing in many physical systems and comparable energy to many atomic and electronic processes, neutron scattering has become an established technique in material characterisation. Since neutron scattering cross-section of the nucleus of an atom does not depend on its atomic number, neutron can very effectively used in probing low Z atoms which cannot be probed by X-rays. Also since neutrons are charge-less particles and are not easily absorbed by materials, they can be used in “in-situ” experiments where samples are kept inside specialised sample environment equipments, such as furnaces, cryostats or pressure cells. Moreover, magnetic properties of neutron can be used in characterising magnetic structure of materials. Other than scattering, neutrons are excessively used in radiography, tomography and imaging applications also. During all these experiments collimating and focusing of the neutron beam emerging from a reactor source is always a challenging task.

In practice neutrons are generally guided by using total external reflection from a metallic layer (viz., Ni film) at a very narrow grazing angle of incidence. In nature, no material is available which can efficiently reflect neutrons in large wide angle, though artificial periodic and non-periodic thin film multilayer structure can solve this problem to some extent [1]. For a periodic multilayer with uniform bi-layer thickness due to the constructive interference of the neutrons reflected from each interface, a high reflectivity Bragg peak appears at some angular position. Such periodic multilayers with alternate layers of high and low neutron scattering lengths, work as high reflecting neutron mirror and monochromator. In case of non-periodic multilayer where the bilayer thickness varies along the depth, several Bragg peaks due to different portions of the multilayers arise, which subsequently merge and a continuous high reflectivity profile is achieved up to a large value of grazing angle of incidence. Such a structure is known as a supermirror and in case of neutron it is characterised by its ‘m-value’ which signifies the ratio of critical angle of the supermirror compared to natural Ni.

Moreover, if these supermirrors are made up with alternate layers of magnetic and non-magnetic elements such as Co/Ti, Fe/Si, FeCoV/Ti etc., so that contrast in neutron scattering lengths increases under the presence of magnetic field then they reflect neutrons of one spin state while neutrons with other spin states are transmitted and in that case these supermirrors are also used as neutron polarizers [2]. These are not only used in polarized neutron reflectivity (PNR) set-ups for measuring magnetization in thin film and multilayer, but also for the separation of nuclear, magnetic and nuclear spin-incoherent scattering on a multi-detector neutron spectrometer and in neutron spin-echo (NSE) spectrometers.

Mezei et. al. [3] first proposed the multilayer design structure of neutron supermirrors for increasing the critical angle of total external reflection as well as spin polarization of neutrons. Their design is based on the idea of continuously depth graded multilayer such that successive bilayers vary in thickness. The thickness is chosen by a method analogous to that used in the design of broad band filter for visble optics. However it failed to yield the desired reflectivity. Since then different design structures have been proposed for achieving high reflectivity in the case of high-m value supermirrors, among which the most frequently used is the Hayter and Mook model [4].

Electron beam evaporation method was used first time by Mezei [5] for realisation of Fe/Ag neutron supermirror polarizer. However, as have been seen, compared to evaporation method in sputtering techniques, the energy of the adatoms are generally high enough (∼10 eV) so that adatoms can re-organise themselves on the surface of the growing films leading to smoother two-dimensional growth. Hence, presently different variants of sputtering technology are generally used to realize these multilayers. Some of the researchers

have reported deposition of very large m -value supermirrors by Ion Beam Sputtering (IBS) technique [6]. In spite of several advantages of IBS technique like low air pressure plasma, higher stability of process and non-contact of plasma with deposited film, IBS technique is not suitable for large area and high throughput deposition due to the enormous cost involved in procurement of large size ion guns and low

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deposition rate of the ion beam sputtering process. DC magnetron sputtering which can yield higher sputtering rate and which has easy scalability and provisions for reactive sputtering in various gaseous ambient is thus a preferred technique for depositing these multilayer neutron supermirrors for actual applications [7,8]. In our laboratory a 9 m long fully computer controlled in-

line dc/rf magnetron sputtering system has been developed recently indigenously where several neutron mirrors consisting of a large number of bilayers and each having 150 mm x 250 mm dimension can be deposited simultaneously. The photograph of the system is shown in fig.3, while the full description of it has been reported elsewhere [9,10].

Fig. 3: Photographs of the 9 meter long in-line sputtering system.

Co/Ti neutron supermirror polarizers of m =2.0 (100 layers), m =2.25(204 layers) and m =2.5 (312 layers) have been developed by using the above system. Prior to the development of the supermirrors, several Co, Ti single layer films and Co/Ti tri-layers have been deposited under various depositions conditions and were characterised by specular and diffused grazing incidence X-ray reflectivity measurements with laboratory X-ray source. Optimum deposition conditions have thereby been achieved to obtain films with very low roughness and sharp interfaces and the supermirrors have been deposited under this optimised deposition condition. Subsequent to the deposition, the supermirror polarizers have been characterized by measuring Polarized Neutron Reflectivity (PNR) with neutron beam of 2.5 Å wavelength at DHRUVA reactor, BARC, Trombay [11]. It has been found that though the spectral characteristics of the m=2.0 supermirror polarizer closely matches with the theoretically designed spectra, the PNR spectra of the m=2.25 and m=2.5 supermirrors contain undesirable

oscillations and the measured reflectivity values of up-spin neutrons are relatively low. On further

Fig. 4: Polarized neutron reflectivities of m=2.25 and m=2.5 Co/Ti supermirrors where all Co layers have been deposited under a mixed ambient of argon and air.

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investigation, it has been observed that the presence of magnetic roughness or magnetic dead layer at the Co/Ti interface is the reason for such degraded performance of these supermirrors. The above problems have subsequently been rectified by depositing all the Co layers under a mixed ambient of argon and air. In fig. 4, improved neutron reflectivities of m=2.25 and 2.5 Co/Ti supermirrors where all Co layers have been deposited under mixed ambient of argon and air are shown. High reflectivity (~80%) up to a reasonably large critical wavevector transfer ( q ) of ~0.06 Å-1 have been observed in the above multilayer structures. The neutron mirror and supermirrors (SM) fabrication facility which has been developed in our laboratory will give a boost to the neutron-based research in the country. The above development has significant commercial implications also since such neutron supermirrors are available from very few commercial sources internationally at a very high cost. References

1. J. Daillant and A. Gibaud, X-ray and Neutron Reflectivity Principles and Applications, Springer, Berlin Heidelberg, 2009

2. O. Schärpf, AIP Conference Proceedings, 89 (1982) 182.

3. F. Mezei and P.A. Dagleish, Commun. Phys., 2 (1977) 41.

4. J.B. Hayter and H.A. Mook, J. Appl. Cryst., 22 (1989) 35.

5. F. Mezei, Commun. Phys., 1 (1976) 81. 6. D. Yamazaki, R. Maruyama, K. Soyama, H. Takai,

M. Nagano and K. Yamamura, Journal of Physics: Conference Series, 251 (2010) 012076.

7. P. Høghøj, I. Anderson, R. Siebrecht, W. Graf and K. Ben-Saidane, Physica B: Condensed Matter, 355 (199) 267.

8. S. Maidul Haque, A. Biswas, Debarati Bhattacharya, R.B. Tokas, D. Bhattacharyya and N.K. Sahoo, Journal of Applied Physics, 114 (2013) 103508.

9. A. Biswas, R. Sampathkumar, A. Kumar, D. Bhattacharyya, N.K. Sahoo, K.D. Lagoo, R.D. Veerapur, M. Padmanabhan, R.K. Puri, D. Bhattacharya, S. Singh and S. Basu, Rev. Sci. Instrum., 85 (2014) 123103.

10. A. Biswas, S.M. Haque, J. Misal, K.D. Lagoo, R.D. Veerapur, M. Padmanabhan, R.K. Puri, R. Sampathkumar, Ajaykumar, D. Bhattacharya, D. Bhattacharyya and N.K. Sahoo, AIP Conference Proceedings, 1591 (2014) 985.

11. S. Basu and S. Singh, Journal of Neutron Research, 14 (2006) 109.

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