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In this division, R & D activities are carried out by several groups : Luminescent Materials and Devices;Plasma Processed Materials, Devices and Systems; Silicon and Silicon Devices; Electrochromic Materialsand Devices; Synthesis of Nanocrystalline Materials for Gas Sensors; Hybrid Organic-InorganicNanocomposites; Conducting Polymers; Surface Studies and Nanostructures; High TemperatureSuperconducting Materials & Devices; Advanced Ceramics and Optical Thin Films.

ELECTRELECTRELECTRELECTRELECTRONIC MAONIC MAONIC MAONIC MAONIC MATERIALSTERIALSTERIALSTERIALSTERIALS

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Luminescent Materials and DevicesGroup

Na+ doped ZnO quantum dots weresuccessfully prepared using Quantum ConfinementAtom (QCA) methodology that shows exceptionallyhigh PL brightness. The tuning of emission colors inthe range 480-562 nm was achieved by dispersingthem in polymeric solvents with varying index ofrefraction and studied the modifications assigned tothe surface states by the polymeric medium. This workwas immensely appreciated by Nanotech Communityof Institute of Physics (IoP), UK and published as“Technology Update” on their website http://nanotechweb.org/cws/article/tech/35724and also as a “Research News in AppliedPhysics” by VerticalNews.com (USA) ontheir website http://www.verticalnews.com/article.php?articleID=772852

High-quality field emitters are very desirablefor designing advanced optoelectronic devices suchas vacuum microwave amplifiers, parallel electronbeam microscopes, X-ray sources, field-emissiondisplays etc. The ZnO-tetrapod (TP), nanotetraneedle(NTN) and multipod (MP) with high aspect ratio andpurity were achieved via controlled pressure-assistedthermal oxidation growth technique. Severalsystematic runs have been made to tailor the optimumgrowth conditions with large production yield (>92%). We have also highlighted the synthesis mechanismto produce high-quality and homogeneouslydistributed ZnO nanostructures

Fig. 4.2: Strong green (~505 nm) emission from ZnOtetrapods at 370 nm excitation and itshomogeneous surface microstructure.

Ferromagnetism has been observed in Lidoped luminescent ZnO nanorods with Curietemperature up to 554 K, which the first reportedobservation of DMS in ZnO with only alkali doping.Dilute ferromagnetic semiconductors (DMS) withCurie temperature at or above room temperature hasimmense potential for spintronic and magneto opticdevices, which is generally been reported withtransition metal doping in ZnO. Li forms shallowacceptor states in substitutional zinc sites giving riseto p-type conductivity. Occurrence of ferromagnetismat room temperature has been established withobserved magnetic domain formation in ZnO:Lipellets in magnetic force microscopy. MagneticZnO:Li nanorods are luminescent showing strong UVemission. Li atoms can induce local moments onneighbouring oxygen atoms, which when considered

Fig. 4.1: Absorption, photoluminescence excitationand emission spectra of ZnO:Li (dotted line)and ZnO:Na (solid line) quantum dots andtuning of PL in various solvents.

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in a correlated model for oxygen orbitals couldexplain the observed ferromagnetism and high Curietemperature in ZnO:Li nanorods. This PhysicalReview B paper has been published in Virtual Journalof Nanoscale Science & Technology by the AmericanInstitute of Physics and the American Physical Societywhich covers a focused area of frontier research(http://www.vjnano.org)

Fig. 4.3: (a) B-H curve of ZnO:Li nanorods. Insetshows the TEM and MFM pictures.(b) 2-D layer of YVO4: Eu3+ nanophosphor forsolar cell applications.

Uniform 2D nanophosphor layer of red-emitting phosphor with 20% luminescence quantumefficiency and 80% transparency in the visible regionhas been developed for possible application as a solarcell phosphor towards improving efficiency of solarcell by solar spectrum modification.

(a)

In the NMITLI project with SAMTELColour Ltd., Red, Green and Blue PDP phosphorshave been developed with luminescence quantumefficiency comparable or more than commercial PDPphosphors. Degradation of PDP phosphors is aworldwide problem which reduces the lifetime of PDPTV. Degradation problem has been nearly arrestedin PDP phosphors developed at NPL, achieved bysilica coating over the phosphor grains. The PDPphosphors developed at NPL have high quantumefficienctes, shorter decay time, negligible degradationand desirable particle size for PDP panel. Green andBlue PDP phosphors have been approved bySAMTEL for PDP panel preparation with indigenousphosphors.

Plasma Processed Materials Group(A) Nano/micro crystalline silicon filmsdeposition using PECVD technique

Hydrogenated nano/micro crystalline siliconfilms were grown by SiH4+H2 +Ar gaseous mixtureusing RF (13.56 MHz) PECVD technique. Inparticular, the role of Ar, which varies from 66 - 87% in the total gaseous mixture, on the structural,electronic and optical properties of the films havebeen investigated. Raman scattering measurements(Fig 4.4 & 4.5) show that increasing Ar fractionfavorus the enhancement of crystallinity and grainsize and also in relaxing strained Si lattices. XRDstudies also reveal the crystallinity in thehydrogenated amorphous silicon films deposited atdifferent Ar fractions and the variation of grain sizefollows a similar trend to that estimated from Ramandata. Photoluminescence spectra show two radiativetransitions in the ranges 2.8-3.1 eV and 1.6-2.1 eVin the films. The high energy peaks arise from surfaceeffects of the film whereas peaks in the range of 1.6

(b)

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- 2.1 eV are due to nanocrystallinity in the filmdeposited. Optical band gap is estimated in the higherabsorption coefficient region (> 104 cm-1). Theargon dilution in the range of 70-80% has also beenfound to be favourable for crystallinity inhydrogenated amorphous silicon.

Fig 4.4: Raman spectra of nc-Si: H films deposited atdifferent argon fraction.

Fig 4.5: Crystalline, grain boundaries and amorphousphase volume fraction estimated from threepeaks fit of Raman spectra of Fig 1.

(B) Diagnostic of acetylene plasma for thegrowth of DLC films

Investigations about diagnostic of acetyleneplasma discharge using impedance probe (also calledVI probe) were carried out to identify the conditionof growth for DLC films. Plasma parameters of

gaseous mixture of argon and acetylene plasmadischarge as a function of self bias and pressure weremeasured; simultaneously DLC films were alsodeposited under same plasma conditions.

(C)Tetrahedral amorphous carbon (ta-C) filmsdeposited by filtered cathodic Vacuum arc(FCVA) technique

The effect of substrate bias and hydrogen andnitrogen incorporation on the optical constants, sp3/sp2 ratio, electrical and mechanical properties of ta-C films deposited by FCVA technique have beenstudied. Undoped and hydrogen incorporated a-Cfilms having nanoparticle inclusions at differentsubstrate bias using these modified vacuum arctechniques have also been deposited and thecharacterization of the films deposited are in progress.

Silicon and Silicon Devices GroupAntireflection Properties of Silicon NanowiresArray for Solar Cell Applications

Reflection loss of the incident light is one ofthe major factors which limit the photovoltaicconversion efficiency. An antireflection surfaceconsisting of vertically aligned silicon nanowires(SiNWs) arrays has been developed. Large areasilicon nanowires (SiNWs) arrays were made on p-Si by wet chemical etching process at roomtemperature which consisted of high density ofvertically aligned nanowires. The SiNWs in the arraywere aligned vertically as revealed by SEMinvestigations. The length of SiNWs (diameter in 50-200 nm range) was found to increase linearly withthe reaction time. The remarkable reduction inreflectivity (Rl) was observed. The value of Rl as lowas 2% was realized in the 300-600 nm wavelengthrange in the case of ~10 mm long arrays (average Rl

<3% for the entire 300-1100 nm range), a value better

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than the best Rl reported in anisotropically texturedor porous silicon surfaces. The work on solar cellfabrication using SiNW based substrates are inprogress.

Fig. 4.6: SEM image of vertically aligned siliconnanowires, (b) reflectivity vs wavelengthcurves for different etching times.

Minority Carrier Lifetime MeasurementFacility

A system for the measurement of minoritycarrier lifetime from Semi-Lab, Hungary consistingof microwave photoconductive decay (m-PCD),surface photo-voltage (SPV) and LBIC measurementtools was installed. m-PCD and SPV systems havebeen used to study process related lifetimedegradation and surface passivation.

Poly(3,4-ethylenedioxythiophene) (PEDOT)films with enhanced performance characteristicsfor electro-chromic smart windows

PEDOT films have been functionalized afterpolymerization via a photochemical nitrene reactionusing the reactive 1-fluoro-2-nitro-4-azidobenzene(FNAB), typically used for bio-chemical applications.The unusual surface morphology of the functionalizedPEDOT film with micron sized outgrowths with apartially crystalline structure (Figure 4.7) isresponsible for its extremely high coloration efficiency(340 cm2 C-1 at 550 nm), large charge storagecapacity (0.7 mC cm-2 at 5 mV s-1 and goodelectrochemical cycling stability (as high opticalcontrast is retained even after 3000 clear to dark anddark to clear cycles). These results demonstrate theunparalleled potential this functionalizing methodologyhas for creating electro-active polymer films withunique morphologies and electrochemical propertiesfor electro-chromic applications.

Journal of Materials Chemistry 19 (2009) 2236– 2348

Electrochimica Acta 54 (2009) 1292 – 1303

Fig. 4.7: Functionalization of doped PEDOT films byFNAB shown through an equation; SEM onleft hand side corresponds to PEDOT beforefunctionalization and SEM on right hand sideis for PEDOT after functionalization.

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Nanostructured Materials andDevices and Surface Studies GroupOne-Pot Synthesis of Composition-TunableCdSe-ZnSe (core-shell) and ZnxCd1-xSe(Ternary-Alloy) Nanocrystals with highluminescence and stability

In this work, a simple single-pot synthesisprocedure has been adopted to prepare both CdSe-ZnSe core-shell as well as ZnxCd1-xSe ternary alloynanocrystals systematic alloy varying the amountsof Zn-precursor. Few samples are also beingprepared with the conventional double-pot approachand the results are compared with the correspondingsamples obtained from single-pot synthesisapproach.In this work, a simple, effective andreproducible synthetic route (single-pot approach)for the preparation of high quality core-shell CdSe-ZnSe quantum dots without the use of anypyrophoric organometallic precursors is presented.Effective surface passivation of stoichiometric,monodispersed, small-sized (~ 5 nm) CdSenanocrystallites is achieved by overcoating them witha ZnSe shell using zinc acetate as a zinc source bysingle-pot approach. The resulting core-shellnanocrystallites exhibit high quantum yield values ~11.33%, narrow line-width of the PL band, stablesurface-bonds configuration and superior structuralproperties at lower Zn content (~ 10 at%). Withincreasing Zn content (e” 20 at%), a composition-tunable emission across the visible spectrum hasbeen demonstrated by a systematic blue-shift inemission wavelength due to the formation of ternaryZnxCd1-xSe quantum dots with acceptableluminescence properties. Here, contribution toemission process from surface states ofnanocrystallites increases with zinc content. Thecore-shell and ternary QD’s formed by different

routes are modeled, based on the observations ofseveral complimentary techniques (XPS depth-profiling, PL, UV-VIS absorbance, TEM/TED).

Formation Of Water-Soluble And BiocompatibleTOPO-Capped CdSe Quantum Dots WithEfficient Photoluminescence

In this work, polysorbate surfactants withsame functional groups but with varying molecularmasses (Tween-80, Tween-40 & Tween-20) indifferent concentrations (0.1% to 20% w/w) wereused to study the effect of the length of the surfactantchain on the luminescence of the entrapped TOPO-capped CdSe nanocrystals. Various phospholipidswith different functional headgroups such as ethyleneglycol (-PEG) and amine (-NH2) were used toimprove biocompatibility and provide sites forbioconjugation respectively. It is understood that thatthe hydrophobic ends of the surfactant binds with thewater repelling groups of the cap layer, thus modifyingthe CdSe cap layer and making it soluble in aqueousmedia. It was observed that the PL emission intensityof CdSe increases with increase in concentration ofTween-series surfactant unlike in the case of thiol-coated CdSe nanoparticles. However, higher PLintensity was obtained in the case of stoichiometricCdSe with Tween-40 corresponding to 20% w/w.The efficient PL sustainability of water-soluble CdSeQD’s can be attributed to the simpler chain structureof Tween-40 surfactant resulting in better passivationof the micelle.

Gas sensor array using metal oxidesemiconductors

With the aim to integrate the sensor deviceon microheaters, sol-gel process was investigated tocoat the alumina substrate. Preliminary investigationswere carried out to drop deposition of the sol on thesilicon microheater substrates. Indium, tungsten and

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niobium doping were found very effective in reducingthe resistance and imparting good sensing capacityover these substrates. The gas sensing properties wereinvestigated after doping with Nb, Co and Ni, Sr,Sb. The gas response was measured for various gasessuch as ethanol, acetone, TMA, DMA, ammonia,NOx, CO, LPG and CNG, DMMP, acetonitrile etc.Efforts were made to fabricate sensor arrays forselective gas detection. For this separate sensors wereprepared and integrated to make a sensor array. Toimpart selective determination other metal oxideswere also investigated such as ZnO nanorods, WO3,TiO2 etc. Various chemical routes were investigatedto prepare nanocrystalline powders. Materials werecharacterized and gas sensitivities of these materialswere investigated for bulk as well as thin films.Mesoporous oxide materials were synthesized andtested using BET surface area technique.

Synthesis of metal oxide / MWCNT composite

Recently, nanostructures such as nanotubes,nanowires etc have been studied for high gas sensitivityas they would provide more surface sites for moreoxygen to be adsorbed and contacted with the gases.Composite films based on tin or tungsten oxide andcarbon nanotubes have been investigated as new gassensitive materials with improved sensitivity.Composites containing carbon nanotubes exhibitcooperative or synergetic effects between the metaloxide and carbon nanotube, and useful for manyapplications. Metal oxides SnO2, W doped SnO2 andWO3 were used to prepare composites sensors. Asol-gel processing route was used to preparecomposite films. Films were studied by X-raydiffraction, scanning electron microscopy (SEM).Sensing properties of the prepared composites havebeen tested with respect to NO2 gas. The addition ofcarbon nanotubes during synthesis resulted in a highly

porous structure. A high surface area and smallcrystallites present in the sol-gel synthesizedcomposite films were attributed to this high sensitivity.NO2 is a common air pollutant; presence of a lowconcentration even at the ppb level may causebreathing difficulties, particularly in children and theelderly. Low concentrations of NO2 could be detectedeven at room temperature using these composite filmswhich was not possible without CNT addition. ACimpedance spectroscopy was used to study theelectrical behaviour of these hybrid sensors.

The photo-catalytic oxidation of organiccompound in aqueous solution containing a suspensionof titanium (IV) oxide has been used for the removalof organic impurities from water. However, afterdegradation of these impurities, the suspendedcatalyst water has been removed to obtain freshstream of water. In order to enhance photo catalyticactivity of these compounds, metal ions having atomicradii near to Ti(IV) ion. These metal ion substitutes incrystalline network of quantum size. This quantumsize effect anchor on solid support like glass bids,fibre glass, ceramic fibres, glaze tiles etc. whichremoves the pollutant from air and water.

Optical characterization of a large number ofsamples from NPL and outside NPL (JNU, NewDelhi) were carried out using UV-Vis-NIR Shimadzumake Spectrophotometer for the development ofcoatings and materials for various applications. Someof the significant spectrophotometric studies pursuedduring the period are : Variation of reflectivity aftereach major process step of a single crystal Si solarcell ; reflectivity studies for various types of ARcoatings on silicon, viz., silicon nanowires (SiNWs)made on porous silicon wafers, texturedmonocrystalline silicon with and without ZnO layeron it , ZnO:Al coatings on silicon with different Al

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content, etc. for use in Si solar cells ; absorptionstudies of nanocrystals of PbSe, CdSe, nano-composites of polymer-CdSe for solar cellapplications; changes in optical properties withvariation in doping level and annealing temperaturefor silver doped sodium phosphate glasses for possiblephotonic applications . Holmium filters were calibratedusing Shimadzu spectrophotometer for M/s TextilesCommittee, Mumbai and M/s AVT McCormickIngredients Pvt.Ltd., Ernakulam.

Surface Physics & Nanostructures

The change in the surface morphology of theoxygen induced faceted Re )1312(

− surface has beenstudied due to the growth of gold nanoclusters. Thenano trenches (two sided ridges) faceted rheniumsurface was formed by oxygen interaction followedby annealing to elevated temperature which consistsof facet along )1211(

− and )0101(− orientation is used

as a template to grow the gold nanoclusters. Thegrowth modes and the influence of gold on the facetedrhenium surface were characterized by AES, LEEDand STM. It is observed that the gold grows in theform of 2D & 3D islands on top of the ridgesdepending upon the gold coverages. On annealingthe gold covered faceted surface undergoes drasticmorphological change and form three sided nanopyramids. The facets of the nano pyramids weredetermined as, and. The STM analyses revealed thatthe facet of the nano pyramid covered with zigzaggold nano chains.

Further, we investigate morphologicalinstability of a catalytically important hcp Rusurface.The Ru undergoes various surface reconstructions atlow oxygen coverage followed by high temperatureannealing. The study revealed the possibility of theformation oxygen-induced facets on morphologicallystable Ru surface. The LEED observations and tilt

angle calculation indicates two stable orientationsformed on Rusurface specified as (10-10) and (01-10) surface. We also report the sub-monolayeradsorption of Indium metal on reconstructed highindex Si (5512) surfaces under controlledthermodynamic & kinetic growth. The growth modeof the Indium adsorption is determined by AES whilethe formations of various superstructural phases areidentified by LEED.

High Temperature SuperconductingDevices, Advanced Ceramics andOptical Thin Films

The High Temperature SuperconductingMaterials and Devices Group has been engaged inthe development of (Bi,Pb)2Sr2Ca2Cu3O10+x [Bi-2223] bulk tube/rod and current leads and longlength tapes for high current transport. Jointassemblies of 1 – 1.5 m long three to five bulk tubes(L = 300 mm, OD = 12 mm, ID = 10 mm) carryingtransport critical current not less than 80% of thatof the individual tubes carrying 500 – 1000A and

Fig. 4.8: Variation of Jc and Tc as a function of Smconcentration

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contact resistivity of the order of 10-7 – 10-8 ÙPo(‘¨– cm2 at 77K in self field has been developed.Optimization of preparation parameters to improvethe transport current though the joint is in progress.Multifilamentary Bi-2223 tapes (> 35 m long) having7 filaments carrying Jc of the order of 103 A/cm2 at77 K in self-field have been developed. Tounderstand the nature of pairing and in search ofeffective pinning centers, XRD, SEM, ACsusceptibility, resistivity and EPR studies ofBi1.84Pb0.35Sr1.91Ca2.05SmxO10+y (x = 0 – 0.12M%)bar shaped (35 x 12 x 2 mm3) samples preparedunder optimized conditions have been carried out.XRD studies show that almost all peaks belong tothe Bi-2223 and Bi-2212 phases. Transitiontemperature Tc and Jc have been found to vary withSm concentration (x), as shown in Fig.4.8. SEMstudies show that addition of Sm leads to a changesin grain size and porosity, depending on the value ofx. EPR studies of these samples are in progress.

In the Advanced Ceramics Group, the betaalumina ionic conductor project was successfullycompleted. The microwave furnace designed underthe above project is being used for microwavesintering of CCTO dielectric materials. One weightpercent Chromium doped calcium-copper-titanate(Cr2O3-doped CCTO) ceramics were prepared bysolid-state reaction method. These samples weresintered at temperatures 1100%C, 1150%C, and1200%C. Samples sintered at 1200%C shows welldefined helical grain growth. The average grain sizeof Cr2O3-doped CCTO increased with increase insintering temperature. XRD patterns of thesesamples show diffraction profiles similar to the bccperovskite structure. Dielectric properties of Cr2O3-doped CCTO materials were improved withincreasing sintering temperature.

Fig. 4.9: Microstructure of Cr2O3-doped CCTO sample

Under the Optical Thin Films activity, thecharacterization of a large number of transparent orweakly absorbing thin films from different users todetermine the optical constants and thicknesses hasbeen carried out using manual ellipsometry and white-light spectrophotometry. A PC controlled fibre-opticbased mini spectrophotometer has been set up tomeasure the transmittance and reflectance of samplesover the wavelength range 200 to 1700 nm. Using

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the in-house designed, fabricated and assembledplasma polymerization deposition system, with aargon-oxygen RF plasma at 13.56 MHz and non-toxic liquid organic precursors, processes have beensuccessfully developed for the dpreparation ofagglomerates of pure anatase-phase titaniananocrystals of large surface area, by applying

Fig. 4.10: Anatase phase titania agglomerates, nanotubes, nanoparticles andnanorods prepared by PECVD technique

different substrate bias voltages, at room temperature.Along with these, nanorods, nanotubes andnanoparticles of anatase phase titania have also beenprepared (Fig. 4.10). A thin film deposition systemwhich can execute simultaneous PECVD and ionbeam sputtering processes for the deposition ofnanocomposite films is in the process of fabrication.