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In Situ Studies of Emission Characteristics of the DC Thermal Arc Plasma Column During Synthesis of Nano-AlN Particles

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In Situ Studies of Emission Characteristics of the DC Thermal Arc Plasma Column During Synthesis of Nano-AlN Particles
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  IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 6, DECEMBER 2006 2611  In Situ  Studies of Emission Characteristics of the DCThermal Arc Plasma Column During Synthesisof Nano-AlN Particles Indrani Banerjee, N. K. Joshi, S. N. Sahasrabudhe, Soumen Karmakar, Naveen V. Kulkarni,S. Ghorui, Atul K. Tak, Shri P. S. S. Murthy, S. V. Bhoraskar, and A. K. Das  Abstract —The growth process of nanoparticles and nanowiresof AlN by thermal–plasma-assisted gas phase condensation re-action has been investigated by optical emission spectroscopy.The concentrations of the reacting precursors in the plasma havebeen correlated to the crystalline phases of nanoparticles of AlNfound from X-ray diffraction analysis. The size and morphologyof the nanoparticles have been studied by transmission electronmicroscope investigations of as-synthesized powder at a set of reactor parameters, which included arc current, reactor pressure,and standoffs of the arc column. An attempt has been made tocorrelatethegrowthofAlNtothatoftheprecursordensitypresentin the plasma reaction zone.  Index Terms —Nanoparticle synthesis, optical emission spec-troscopy (OES), plasma arc devices. I. I NTRODUCTION O NE-DIMENSIONAL (1-D) nanostructures, such as nan-otubes, nanowires, and nanocoils [1], on account of theirnumerous potential applications in medicine [2], [3], microbi-ology, biotechnology [4], [5], as well as tribology [6]–[8] have attracted considerable attention from researchers. Of these, thenanophase compounds formed out of the group III–V elementsof the periodic table are more significant. Especially, thegroup III nitrides are found to predominantly form nanoscale1-D tubular structures [9]. Among other group III nitrides,aluminum nitride, which exhibits the largest direct band gapof   ∼ 6.2 eV, has its unique importance in solid-state white-light-emitting devices [10]. Its high Curie temperature af-ter doping with transition metals makes it important in theworld of spintronics [11]. AlN possess small electron affinityfrom negative to 0.6 eV, which finds applications in field-emission vacuum microelectronic devices [12]–[14]. Further, its quantum confinement geometries [15]–[17] with superior light-emitting properties (continuously from near-infrared toultraviolet region), excellent thermal conductivity, high hard-ness, high resistance to chemicals, and high melting point have Manuscript received April 21, 2006; revised July 15, 2006. This work wassupportedbytheBoardofResearchinNuclearSciences,DepartmentofAtomicEnergy.I. Banerjee, S. Karmakar, N. V. Kulkarni, and S. V. Bhoraskar are withthe Department of Physics, University of Pune, Pune 411007, India (e-mail:svb@physics.unipune.ernet.in).N. K. Joshi, S. N. Sahasrabudhe, S. Ghorui, A. K. Tak, S. P. S. S. Murthy, andA. K. Das are with the Laser and Plasma Technology Division, Bhabha AtomicResearch Centre, Mumbai 400085, India.Digital Object Identifier 10.1109/TPS.2006.886059 attracted researchers to synthesize nanoparticles of aluminumnitride in comparison to bulk.Survey of existing literatures as well as work carried outin this paper [18]–[20] have revealed that thermal-plasma- assisted gas phase synthesis is one of the best physical routesfor the nanosynthesis of AlN. Such high-pressure plasmas areoften assumed to be under local thermodynamic equilibriumconditions [21]. Due to its chemical flexibility, high temper-ature in the reaction zone, and high efficiency of production,thermal plasma has gained special interest in large-scale syn-thesis of ultrafine particles. The extremely high quench rate(i.e.,  10 6 – 10 7 K/s) available in the plasma zone facilitates theformation of nonequilibrium phases. In addition, it is nowestablished through experiments that the presence of chargedions and radicals in the plasma reaction zone and the arcmicrofields critically influences the size and dimensionalityof the nanophase materials. Although a number of theorieshave been given about the growth mechanism, morphology, andstability of 1-D AlN, it is still far from being understood [22].Thereisanecessitytogenerateexperimentaldataonthespeciesparticipating in reaction synthesis in the plasma fireball.This paper reports results of   in situ  optical emission spec-troscopy (OES) carried out in a dc arc plasma reactor forsynthesis of nanophase structures. The plasma reactor usedearlier in the author’s laboratory [18] has been modified forachieving better process control in terms of operating parame-ters like arc current and pressure. For the first time, an attempthas been made to correlate the size of the as-synthesized AlNnanostructures with the plasma parameters.II. E XPERIMENTAL  S YSTEMS AND  M EASUREMENTS The powders of aluminum nitride were prepared by dcthermal arc plasma method. The basic experimental systemhas been detailed in our previous communication [23], andthe schematic of the setup has been presented in the inset of Fig. 1. Argon was used as the primary plasma gas. Nitrogen,being one of the reactants, was fed to the reaction zone inthree different configurations. A gas injection ring with eightports was positioned around the zone just above the substrate(anode) to inject nitrogen uniformly to the reaction zone. Inaddition, a single N 2  jet was directed at the evaporation zone onthe substrate. The next set of experiments was carried out withnitrogen being fed as the plasma gas along with argon. This hasthe advantage of creating ionized species for the reaction. 0093-3813/$20.00 © 2006 IEEE  2612 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 6, DECEMBER 2006 Fig. 1. (a) Emission spectra showing the NI lines associated with some ArIlines at  z  = 28  mm above the substrate position. (b) Emission spectra of thesame wavelength range showing the absence of NI lines at the near substrateposition  ( z  = 14  mm ) . Inset: the schematic of the plasma reactor system. OES has been coupled with the reactor chamber as the diag-nostic tool for  in situ  measurements. Experimentally observedlines as recorded through a spectrometer were identified bycomparingwithstandardspectralwavelengthsofelementsfromNIST 3.0 [24] database, which also provides other spectral data.At the end of each experiment, particles were collected bycarefully scrapping off the powder deposited on the differentportions of the chamber walls. Although a host of character-izing techniques was employed to analyze the powders, this pa-per discusses only the transmission electron microscope (TEM)micrographs to study the particle size and morphology. The arccurrent, chamber pressure, and arc length (standoffs) were theprimary variables of the experiment. The experimental systemparameters are specified in Table I.III. R ESULTS AND  D ISCUSSIONS  A. Measurements of Plasma Temperature The temperature of the plasma column during the synthe-sis was estimated by using OES techniques. Measurementof plasma temperature at different axial locations of the col-umn was necessary to estimate the axial temperature profileduring the evaporation of Al metal. Two different techniqueswere used to get the axial temperature profile. The standardBoltzmann plot [25]–[27] was used for the positions  ( z  = 28 , 56 , 65 ,  and  70  mm )  above the substrate. In this technique, aplotofthelog ( I  ul λ ul /A ul g u ) asafunctionof  E  u  givesthetem-perature of the column, where I  ul  is the spatially integrated lineintensity recorded from the spectrometer, λ ul  is the wavelength, A ul  is the transition probability between the energy levels  u and  l , and  E  u  and  g u  are the excited energy and statisticalweight of the atomic state u , respectively. Excited nitrogen (NI)lines corresponding to the wavelengths 818.48, 821.63, 824.23,856.77, 859.40, 862.92, 868.02, and 871.17 nm were used forthe purpose. Fig. 1(a) shows the typical spectrum consisting of the NI lines that have been used for Boltzmann plots.Thescenarioatthenearsubstrateposition( z  = 14 mmaboveit) was something different. Due to the high-velocity plasma jet impingement, the metal vapor front expansion takes place.The perturbation of this vapor front highly affects the spectrumcorresponding to the gaseous species, whereas the metal linesshow dominant existence. The NI and ArI lines, which havebeen used for determining the temperature of the other axialpositions, show no prominent emission at this region. This hasbeen depicted in Fig. 1(b).The temperature at the near substrate position  ( z  = 14  mm ) was calculated using the line intensity ratio of Al + (384 nm)and Al ∗ (396 nm). The temperature was estimated by therelative intensity [28] method using the ionized  ( I  1 )  and neutral ( I  2 )  species as employed for such estimation given by I  Al - I I  Al - II =  ν  ij ν  mn A ij A mn 1 N  e 2 g + i g m  2 ππ e kT  e h 2  3 / 2 exp  − E  + i  − E  m kT  e  where  v ij  and  v mn  are the transition frequencies,  A ij  and  A mn are the transition probabilities,  N  e  is the electron density,  g + i and g m  arethestatisticalweightsoftheupper levelsconsidered, m e  is the mass of the electron, and  E  + i  and  E  m  are the upperlevels of the energy considered in Al-I  ( Al ∗ )  and Al-II  ( Al + ) transitions. The temperature at the near substrate position wasdetermined to be ∼ 3500 K.Temperatures were thus determined along five axial positionsof the plasma column using the aforementioned methods. Thesewerefurtherusedforestimatinganaxialtemperatureprofile.Asa typical case, during the synthesis of sample 5, when nitrogenwas present as a plasma-forming gas along with argon, thetemperature along the plasma column was found to vary from10000 K (at the nozzle position,  z  = 70  mm) to 3500 K (at thenear substrate position,  z  = 14  mm).The nature of such a variation of temperature along theplasma column, as can be seen from the plot in Fig. 5, is thenfitted into an equation of the form given by T  ( z,L ) =  A + B, exp  − z/LC   where  A  (2834.22),  B  (12700.89), and  C   (0.35) are the con-stants for the present system,  T   is the plasma temperature,  z is the axial distance from the nozzle exit point, and  L  is thestandoff for arc length of the plasma column. The solid linein Fig. 2 depicts the axial temperature profile obtained for thepresent thermal arc plasma column.  B. Analysis of the Powder Correlation With the OES Spectra The as-synthesized powder was scrapped off from the innerwalls of the collector dome present inside the reactor [23]. Thepowder was gray in color and hygroscopic in nature. This isdue to surface reactivity as expected from nanophase materials.The samples were characterized for chemical, structural, andmorphological properties through X-ray diffraction (XRD),TEM, etc. In general, there were four variances of experimentalconditions: First, few runs were carried out with nitrogenintroduced through the injector ring distributor and the gas jet.Samples1,2,3,and4correspondtothisexperimentalcondition  BANERJEE  et al. : EMISSION CHARACTERISTICS OF PLASMA COLUMN DURING SYNTHESIS OF AlN PARTICLES 2613 TABLE IE XPERIMENTAL  O PERATING  P ARAMETERS Fig. 2. Temperature profile along the plasma jet as estimated from line inten-sities using OES during the synthesis of sample 5, with a typical Boltzmannplot shown in the inset. with other parameters as mentioned in Table I. Next, nitrogenwas introduced as the plasma-forming gas and was injectedalong with argon through the nozzle of the plasma torch. Asexpected, nitrogen gets ionized along with argon. Samples 5and 6 belong to this set.Figs. 3 and 4 present the characteristic features of the sam-ples, highlighting the effect of nitrogen in growing aluminumnitride. Fig. 3 refers to samples 1, 2, 3, and 4, whereas Fig. 4represents the results pertaining to samples 5 and 6. The struc-tural parameters of the product were obtained from the XRDanalysis done according to the Joint Committee on PowderDiffraction Standards data sheets [29]. All the XRD spectrafor samples 1–4 were identical in nature as far as crystallinityis concerned. Similarly, XRD spectra for samples 5 and 6 arealso similar.The spectrum reveals that the product is a combination of hexagonal phase of aluminum nitride with a trace of unreactedAl for samples 1–4. The crystalline phases of samples 5 and6 are presented in Fig. 4(a). In fact, samples 5 and 6 areseen to contain unreacted Al as the major part with a trace of cubic AlN. The presence of unreacted Al in samples 5 and 6indicates that the aluminum reaction has not been continuedoutside the jet. This is because the plasma column was notsurrounded by nitrogen ambience. This is a major finding thatindicates that unlike iron oxide synthesis [28], the major AlNreaction is at the boundary of the plasma fireball and notinside the plasma column where the temperature is around3500 K. This is consistent with the temperature values asestimated at the near substrate position where the centerlinetemperature was determined to be ∼ 3500 K, and the peripheraltemperature of the fireball was approximately  ∼ 2300 K, withan expected temperature gradient of 50 K/mm. However, theeffect of temperature variation along the plasma column in thepresence of different types of ambience will also be different,and this will change the temperature gradient in each case.This might also be responsible to the observed variation inthe crystallography and morphology. This might result in thecrystallization of h_AlN under this atmosphere. Moreover, thepresence of c_AlN in the latter case (with N 2  as plasma gas)along with major part of unreacted Al indicates the presenceof insufficient nitrogen existing in the periphery and outsidethe plasma for the formation of stable AlN powders. This isalso well in agreement with the increase in enthalpy of plasma-forming gas when nitrogen is introduced along with Ar, therebyincreasing the rate of evaporation of metal Al in this case.An interesting feature that has been observed during theexperiment is the molecular band spectrum of nitrogen speciesas presented in Fig. 3(b). It refers to the nitrogen band spec-trum [30] obtained when the ambient gas was nitrogen andthe plasma-forming gas was argon. Here, the peak obtainedat 738.6 nm corresponds to the first positive nitrogen bandcorrespondingtothetransitionB 3 Π − A 3  .Theemissionbandis broad and is well defined. Whereas, in Fig. 4(b), the samemolecular band appearing at 738.6 nm is observed, which isassociated with the line spectrum of excited atomic nitrogenspecies along with it. This indicates that the nitrogen enteringthrough the plasma jet dissociates into atomic species and isexcited to higher energy levels.InFigs. 3(b) and 4(b),the morphology and particle size of thenanoaluminum nitride synthesized in these experiments werestudied through TEM micrographs. All the samples invariablyexhibited the 1-D feature that indicates the formation of nano-tubes or nanowires along with nanoparticles. In most of thecases, the nanowires with diameters varying from 20 to 80 nmare seen to be attached at the top with the spherical nanoballs of size approximately 20 nm in diameter. For structure and processcorrelations, two of the significant parameters, i.e., the ratio of number density of nanowires to that of nanoparticles  ( n w /n  p ) and the aspect ratio of the length of nanowire to the diameter of the nanowire ( L/D ), have been studied.A glance at these ratios, as determined from TEM mi-crographs for the two sets of samples, shows that  L/D  isremarkably high for samples 5 and 6  ( L/D  = 25  and  26)  ascompared to 18, 14.4, 12, and 7 for samples 1–4, respectively.The second ratio, i.e.,  n w /n  p  has almost remained identi-cal. It therefore appears that the growth of nanostructures interms of the crystalline phase and the aspect ratio is definitely  2614 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 6, DECEMBER 2006 Fig. 3. (a) Typical spectrum showing the molecular band of nitrogen during the synthesis of samples 1, 2, 3, and 4. (b) Typical TEM micrograph for samples 1,2, 3, and 4. (c) Typical XRD patterns for samples 1, 2, 3, and 4.Fig. 4. (a) Typical spectrum showing the excited atomic nitrogen lines associated with the molecular band during the synthesis of samples 5 and 6. (b) TEMmicrograph for samples 5 and 6. (c) Typical XRD patterns for samples 5 and 6. influenced by the stoichiometry of nitrogen in the system andthe temperature at the reaction zone. In the case of plasma, thetemperature is high enough to have aluminum and nitrogen inthe atomic state. As aluminum vapor diffuses out, the reactionatthetemperaturearound2500K(edgeofthefireball)produceshexagonal AlN, whereas the absence of nitrogen envelop nearthe edge of fireball allows the Al vapor to escape to the lowertemperature zone where the cubic phase of AlN is likely tobe formed.The behavior of particle morphology with the relative abun-dance of Al + with respect to nitrogen was interesting. Fig. 5shows a typical spectrum with the nitrogen ring allowing anitrogen ambience surrounding the plasma. It highlights theemission lines of Al + and N 2 . The intensities of the emissionlines of Al + ( I  Al +  at  λ  = 384 . 2  nm ) , corresponding to thetransition  3 s  ·  4 d– 3 s  ·  10 f to that of N ∗ 2 ( I  N ∗ 2 at  λ  = 405 . 94  nm ) ,correspondingtothetransitionC 3 Π  →  B 3 Π observedat14mmabove the substrate position as recorded from OES, are shownin this figure.Fig. 6(a) highlights the comparative variation of the  n w /n  p factor with the relative abundance of aluminum with respect toreactive nitrogen molecules.This variation depicts the fact that higher abundance of Al + inside the plasma zone enhances the growth of 1-D structurerelative to that of the particles. Similarly, the variation of the coefficient, i.e.,  L/D  has been compared with the rela-tive abundance of aluminum in terms of the intensity ratio Fig. 5. Typical spectrum showing the lines for the Al, Al + , and N 2  bands. I Ar ∗ (384 . 2) / I N +2  (405 . 94)  observed at 14 mm above the substrateposition as recorded from OES.Interestingly, in contrast to the variation of  n w /n  p , the aspectratio  L/D  decreases with an increase in the relative abundanceof Al + . This has been highlighted in Fig. 6(b).This inverse relation of the abundance of ionized Al withrespect to N 2  helps in understanding the growth mechanism.The coefficient  n w /n  p  is influenced by nucleation, whereasthe coefficient  L/D  is related to the growth phenomenon. Itseems that nucleation is favored by higher concentration of   BANERJEE  et al. : EMISSION CHARACTERISTICS OF PLASMA COLUMN DURING SYNTHESIS OF AlN PARTICLES 2615 Fig. 6. (a) Correlation of the relative abundance of Al + with  n w /n p .(b) Correlation of the relative abundance of Al + with L/D . aluminum relative to N 2 , whereas growth rate of 1-D struc-ture is restricted. High aluminum concentration leads into theformation of nanowires of AlN. It is possible that a largerconcentration of Al bonds together and grows in a highlypreferred direction, which is   111   for Al. Consequently, it getsnitrided, and the sequence continues, which might be the reasonwhy the 1-D structure is obtained. On the other hand, whenthe concentration of aluminum is lower as compared to that of nitrogen, the nucleation site is AlN, and subsequent depositionof AlN into the embryo results into the formation of minimumenergy configuration, which is thermodynamically probable,resulting into the particles. In addition, the rate of nucleation n w /n  p  and the rate of growth  L/D  are also controlled by therelative abundance of aluminum in the reaction zone.Apart from the parameters that have been observed to influ-ence the growth morphology and the structure, there are variousother influencing features in gas phase condensation that havenot been discussed. For example, a major parameter includesthe differences of the properties of the gases, such as specificheat and heat conductivity [21], which will affect not only thegas temperature but also the evaporation rate of aluminum.IV. C ONCLUSION In conclusion, the presence of reactive nitrogen specieswithin the plasma reaction zone has high impact on the crys-tallinity and aspect ratio of the synthesized powders. Theresults imply that nitrogen present at higher stoichiometry atthe edge of the fireball seems to be essential for the growthof cubic aluminum nitride, inhibiting the hexagonal phase. Themetastable cubic phase seems to stabilize under the high-energystate, which is thermodynamically not achievable under normalconditions. High values of the abundance of Al + relative to re-active nitrogen species favor the 1-D growth of AlN irrespectiveof other experimental operating parameters.A CKNOWLEDGMENT The authors would like to thank the Abdus Salam Interna-tional Centre for Theoretical Physics for supporting the entireexperimental program, B. Stewart for organizing it, the par-ticipating members for taking interest, H. J. Kunze for beingthe mentor for spectroscopy, and M. Hravobsky for being thementor for the total program. The authors would also liketo thank the National Chemical Laboratory for providing thecharacterization facility.R EFERENCES[1] C. N. R. Rao, A. Govindaraj, G. Gundiah, and S. R. C. Vivekchand,“Nanotubes and nanowires,”  Chem. Eng. Sci. , vol. 59, no. 22/23,pp. 4665–4671, Nov./Dec. 2004.[2]  NanowiresCanDetectMolecularSignsofCancer,ScientistsFind  .(2006).[Online]. Available: http://www.biologynews.net/link.entry.php[3] M.-W. Shao, H. Yao, M.-L. Zhang, N.-B. Wong, Y.-Y. Shan, andS.-T. 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