Growth Mechanism of Pine-leaf-like Nanostructure from the Backbone

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Growth Mechanism of Pine-leaf-like Nanostructure from the Backbone of SrCO Nanorods using LaMer’s Surface Diffusion: Impact of Higher Surface Energy (# = 38.9 eV/nm) {111} Plane Stacking Along #110# (# = 3.4 eV/nm) by First-Principles Calculations. 3

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Divya Arumugam, Mathavan Thangapandian, Jeshua Linu Joshua Mathavan, Archana Jayaram, Palanichamy Murugan, Selvaraj Selva Chandrasekaran, Umapathy Subramanian, Mukul Gupta, Gunadhor Singh Okram, Michael Angelo Jothirajan, and Milton Franklin Benial Amirtham Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01066 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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Crystal Growth & Design

Growth Mechanism of Pine-leaf-like Nanostructure from the Backbone of SrCO3 Nanorods using LaMer’s Surface Diffusion: Impact of Higher Surface Energy (γ = 38.9 eV/nm2) {111} Plane Stacking Along 𝟏𝟏𝟎 (γ = 3.4 eV/nm2) by First-Principles Calculations Divya Arumugam a, Mathavan Thangapandian a,*, Jeshua Linu Joshua Mathavan b, Archana Jayaram c, Murugan Palanichamy d, Selvaraj Selva Chandrasekaran d, Umapathy Subramanian e, Mukul Gupta f, Gunadhor Singh Okram f, Michael Angelo Jothirajan g, Milton Franklin Benial Amirtham a

*Author for correspondence: Dr. Mathavan Thangapandian., Ph. D., Assistant Professor, Research Department of Physics, NMSSVN College, Nagamalai Madurai-625 019, Tamilnadu, India TEL: +91-9486953567, E-mail: [email protected]. a

PG & Research Department of Physics, N. M. S. S. Vellaichamy Nadar College, Madurai-625019, Tamilnadu, India. b Department of Electrical Engineering, Karunya University, Coimbatore-641114, Tamilnadu, India. c Department of Physics and Nanotechnology, SRM University, Kattankulathur 603203, Tamilnadu, India. d Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi-630003, Tamilnadu, India. e School of physics, Department of Theoretical Physics, Madurai Kamaraj University,Madurai-625021, Tamilnadu, India. f UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore-452001, India. g Department of Physics, Arul Anandar College, Karumathur, Madurai-625514, Tamilnadu, India. Manuscript information Total Word Count Number of Text pages Number of Figures

: 10,063 : 41 : 10 with TOC

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ABSTRACT: The two-decade research on the fabrication of nanostructures has been paying attention mostly on property and application aspects, not much on their growth mechanisms by combining experimental and computational aspects. We demonstrate experimentally here the influence of surface diffusion, pH, reaction time and stress-strain on growth mechanism of pine-leaf-like SrCO3 nanostructure (PSNS) from the backbone of SrCO3 nanorods (SNRs). In order to investigate growth orientation of SNRs and PSNS, indispensable surface energy of various crystallographic planes were calculated using Vienna ab initio simulation package (VASP). Several microscopy and spectroscopic techniques were engaged to monitor surfactantless PSNS growth and structural parameters. To obtain deeper insight into this, orientation attachment-assisted single SNR has been investigated systematically by LaMer‟s surface diffusion mechanism. Analyzing further critically the transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images, occurrence of critical dimensional model of single SNR i.e. length increases as diameter decreases, was explored comprehensively according to Gibbs-Thomson effect. That would be discussed in detail with the help of modified Einstein-Stokes equation in terms of inclination angle (α) and contact angle (β). Formation of PSNS has been systematically evaluated by nanocrystal splitting mechanism using field-emission scanning electron microscopy (FESEM) images. To find out degree of orientation, the texture coefficient (TC111) value has been calculated using XRD data. The above result was correlated with surface study analysis using VASP and HRTEM results well. Furthermore, formation of PSNS with deformed twins and stacking faults (SFs) due to the emission of Shockley partial dislocation on adjacent {111} planes under compression (stress-strain) have been evaluated from stiffness and compliance coefficients using XRD data. Fine harmonized experimental and theoretical results established over the


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impact of surface energy on the growth of SNRs and PSNS. Our proposed mechanism thus leads to better understanding on growth of pine-leaf-like SrCO3 nanostructure without surfactants at room temperature. 1. INTRODUCTION Inorganic nanocrystals have received considerable attention due to their interesting novel size and shape dependent properties, which exhibited many potential innovative applications in solar cells,1 agriculture,2 biomedicine,3 catalysis,4 fuel cells,5 and magnetic data storage.6 In particular, one dimensional (1D) rod-shaped nanocrystals have drawn widespread attention in the field of nanoelectronic and nanophotonic systems due to their dimensional anisotropy. This gives rise to their unique physico-chemical properties.7,8 For example, linearly polarized light emitting CdSe and InP 1D nanomaterials have been widely used for light-emitting diodes and photodetectors. 9 Recently, substantial amount of intense research interest has been focused on the controlled synthesis of threedimensional (3D) hierarchical superarchitecture built from one-dimensional (1D) nanoscale building blocks. They are used for designing the materials and devices with advanced functionalities in areas as optics and electronics.10-12 They include dendritic CdS nanorods,13 hierarchical mesophase silicates,14 star-shaped PbS nanocrystals,15 flower-shaped Ag2O particles,16 Y-shaped Cu nanorods,17 branched MnOOH nanorods,18 and branched Cu2O crystals19 in nanometer to micrometer scales. A moment ago, Luo et al. synthesized mesoscopic flower-like ZnO nanorods.20 Compared with traditional 1D nanostructure; the hierarchical superarchitectures could enhance the device efficiency and can be used for potential applications. Owing to their prospective applications, hefty amount of hierarchy such as peanuts, flowers, split acicular crystals, frostwork-like structures and sheaf-like structures have been successfully created with controlled size and evolution of shape using various synthesis process



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through different growth mechanisms, which include Ostwald Ripening or Digestive Ripening, FinkeWatzky two step mechanism, Coalescence, Oriented attachment processes and LaMer mechanism. For example, Qi et al. synthesized BaSO4 bundles and superstructures utilizing crystal modifiers such as hydrophilic block copolymers.21 Tang et al. reported the morphological evolution by splitting mechanism along the [001] direction Bi2S3 sheaflike morphology, synthesized by a simple colloidal solution method using oleic acid and elemental sulfur in 1-octadecene as crystal modifiers.22 Yang et al. reported the various morphologies of highly crystallized 3D Bi2S3 crystal structures with appropriate amount of organic molecules deposited on a anodic TiO2 nanotubes by a electrodeposition method.23 This has been assigned to the splitting of Bi2S3 into the (010) plane that grows along the [001] direction. Yang et al. reported the 3D nanostructures of self-branching anatase TiO2 nanorods with a large proportion of {010} facets synthesized by a facile one-step hydrothermal reaction utilizing tetrabutylammonium hydroxide solution (TBAH) and poly(ethylene oxide) 100-poly(propylene oxide) 65poly(-ethylene oxide)100 (F127) as capping and shape-controlling agent, respectively.24 The 3D structure was considered to be due to the oriented attachment process complexed with crystal splitting mechanism. Wang et al. suggested the formation of Y-shaped Cu nanorod branches by stacking faults from the starting point of 110 and top of the rods by {111}, {100} and {110} surfaces during the magnetron sputter deposition.17 From the above discussion, we have noted various points. First, not only various non-ecofriendly organic molecules as capping and shape-controlling agents but also difficult synthesis methods for preparation of 3D architectures have been utilized. Second, growth mechanism is not so clear. Third, possible factors on dominant influence on the formation of superarchitectures from the 1D nanoscale building blocks have not been clearly sorted out. In some reports, it is difficult to distinguish growth mechanism between shape and oriented attachment process and therefore cannot give further


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information into why such structures form. Hence, to understand the detailed growth mechanism for enhancing the functional properties of architectures from 1D building block, the influence of various factors on the growth of 1D nanostructure such as inclination angle, contact angle, diffusion coefficient, surface energy, strain and stress analysis would be essential. Carbonate materials are widely used as fillers for producing inorganic-organic composite materials. It has been proven that, strontium carbonate (SrCO3) has two traditional applications viz, an additive to fabricate the glass cathode-ray tubes for TV and computer monitors, and a constituent of magnetic ferrites for small DC motors. 25,26 Fortunately, understanding of crystallization process of SrCO3 nanostructures provide ample insight into the formation of the biomaterials. SrCO3 has additional application in chemiluminescence. 27 However, to enhance the properties of SrCO3, the investigation on the formation mechanism of nanostructures is essential. Li at al. synthesized flowerlike SrCO3 nanostructures by hydrothermal method. 28 Zhu et al. reported the synthesis of SrCO3 nanowires.29 SrCO3 nanostructures with different morphologies such as nanorods, nanowires, ellipsoidlike and sphere-like particles were fabricated by microemulsion-mediated solvothermal method.30 However, to the best of our knowledge, crucial role of surface energy on the mechanism of surface formation and growth of SrCO3 nanostructures is not so well-understood even though this is vital for various device applications. In the present study, we undertook the impact of higher surface energy on growth mechanism of pine-leaf-like SrCO3 nanostructure (PSNS) from the back bone of SNRs by a simple wet chemical method at room temperature. Remarkably, pine-leaf-like morphology of PSNS was different from others reported earlier wherein flower-like architecture. The influence of pH and reaction time on the morphology of PSNS and SNR were investigated meticulously via III stage of LaMer mechanism by means of surface diffusion. Further, to determine the growth orientation of PSNS, the critical



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dimensional model of SNR such as length increases with decreasing diameter has been evaluated from field-emission scanning electron microscopy (FESEM), TEM, HRTEM and mapping analysis. In order to predict the formation of PSNS, the detailed structural and morphological evolutions of SNR were explored by means of inclination angle (α) and contact angle (β) according with Einstein-Stokes equation. Moreover, to understand the growth orientation of SNR and PSNS quantitatively according to Gibbs-Thomson effect, surface energy calculations of orthorhombic SrCO3 were performed from the first-principles calculations. The highest surface energy twinned {111} planes may have an effect on the formation of PSNS has been discussed elaborately by splitting mechanism. In addition, to understand the probable deformation twins, the emission of Shockley partial dislocations on adjacent {111} planes under compression such as stress and strain may due to surface energy variations in crystal planes have been demonstrated. In furtherance, various spectroscopic investigations were also done to study the impact of CO2− ions on the surface of Sr2+ ions. Examine whole structural 3 investigation of PSNS and SNRs growth phenomena, impact of surface energy due to Gibbs-Thomson effect has been discussed by means of diffusion in between Sr2+ and CO2− 3 ions with the help of ab initio calculations of SrCO3 surface slabs. The proposed mechanism may open up a new prospect for fundamental transport studies. 2. EXPERIMENTAL 2.1. Materials Strontium nitrate (Sr(NO3)2, 98% Merck) and sodium hydroxide (NaOH, 98% Merck) and deionized water were used without further purification. 2.2. Synthesis of SrCO3 NRs (SNRs) and Pine-leaf-like SrCO3 nanostructures (PSNS) SNR and PSNS samples were synthesized using simple and cost-effective wet chemical procedures with slight modifications from the previous literatures.31,32 In the typical synthesis process for PSNS, 50 ml aqueous solution of 0.2 M Sr(NO3)2 was prepared. In this solution, 50 ml aqueous


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solution of 0.4 M NaOH (pH 12.6) was added drop-wise while stirring magnetically for 6 h at room temperature without any catalysts. Initially, pH of an aqueous solution is 11; it rapidly decreases and reaches the value 9 after addition of NaOH within 30 min. At pH 11, bigger diameter particles were produced, and it begun to decreases due to the acidity of the solution increases, leads to the formation of smallest nanoparticles. Hence, the implication is the diffusion rate of particles increases with increasing OH- ions adsorption on the surface of the higher surface energy faces. In order to reduce the surface free energy, nanoparticles combined to form SNR initially and then PSNS as time progresses. The obtained precipitate was separated by centrifugation, washed with deionized water and ethanol for several times to remove the alkali salt and other impurities. After this, the precipitate obtained was dried in an oven at 80 °C for 6 h. 2.3. Computational Methodology Density functional theory (DFT)-based first-principles calculations were performed to understand the surface properties of SrCO3 using Vienna ab initio simulation package (VASP-code).33 Projector augmented wave (PAW)34 formalism was used for describing the wavefunctions of the valence electrons of each atom in SrCO3. Exchange and correlation energies in many-body calculations were corrected by generalized gradient approximations (GGA) and kinetic cut-off energy of 400 eV was set for all the surface as well as bulk calculations. The Brillouin zone of unit cell was sampled by 6×4×6 kmesh for ionic optimization. Ionic and electronic optimizations were alternatively carried out until the force on each atom reaches below ±10 MeV/Å. The unit cell structure used for analyzing surface properties of SrCO3 was shown in Figure 1.



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Figure 1. (Color online) Unit cell of observed SrCO3 used in surface study calculations. The details concerning apparatus were given in supporting information (S1).

3. RESULTS AND DISCUSSION 3.1 Growth mechanism of PSNS from the backbone of SNRs Attain detailed information on the formation of PSNS, a possible growth mechanism of single SNR is essential that has been proposed here using classical nucleation theory. In this context, we take note of recent reviews that described the process of nucleation and growth of nanoparticles through several mechanisms.35-37 Present work discusses the growth of SNR through LaMer mechanism.38,39 The progress of nucleation and growth of SNR through LaMer mechanism may be separated into three stages (Figure 2). In stage I, concentrations of Sr2+ monomers are built up. The monomer concentration reaches the maximum concentration for nucleation (C*max) in stage II and then drops suddenly by undergoing burst nucleation. Following this, growth of nanorod starts in stage III under the control of surface diffusion of Sr2+ monomers, until the monomer concentration attains the solubility level C s. In the initial stage of pH 11, OH- ions from NaOH react with Sr2+ ions of Sr(NO3)2 to produce Sr(OH)2 molecules that act as cores. 8 ACS Paragon Plus Environment

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These lead to a relatively slow release of Sr2+ and hence lower Sr2+ concentration, expected to have a supersaturating concentration level. They have higher probability to adsorb CO 2 molecules from air to act as a shell and diffuse to Sr(OH)2 to form SrCO3 nuclei through the reaction between Sr2+ and CO2− 3 ions, with the release of OH ions, which may adsorb on the surface of former. The adsorbed OH

ions react with a nearby Sr2+ ions to produce Sr(OH)2 molecules and then react with CO2− 3 ions to form SrCO3 nuclei. As consequence, SNR could begin to grow by means of surface diffusion as decreasing pH from 11 to 9 indicates. Lower pH medium may provide the faster release of Sr 2+ and hence higher monomer concentration to form very high supersaturating level favoring 1D growth (Figure 2, III stage) according to the following reactions.40 Owing to the structural anisotropy of SrCO3 crystal, OHions would be preferentially adsorbed on the higher surface energy faces for the stable structure, which finally form 1D nanorod after periodical repeat of the previous growth process. Sr (NO3)2 + 2NaOH

Sr (OH)2 (aq) + 2Na(NO3)

(Sr2+ + 2𝑁𝑂3− + 2Na+ + 2𝑂𝐻 − ) Sr (OH)2 + CO2 (g)

SrCO3 (s) + H2O (l).

(Sr2+ + 𝐶𝑂32− + H2O)



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Figure 2. (Color online) Nucleation and growth of SNR through LaMer surface diffusion at three stages. (I) built up of monomer concentration; (II) monomer concentration reaches the C*max for nucleation; (III) growth of SNR by diffusion. The mechanism discussed so far is perhaps matched with that observed experimentally through TEM image of a single SNR (Figure 3a) that shows its growing length, but decreasing diameter as length increases (critical dimensional model). 41,42 Additional requirements to be fulfilled with a precisely defined SNR diameter and length, should be interpreted as arising from the driving force of surface diffusion due to the mass transport of the Nichols-Mullins‟ law43 between Sr2+ and CO2− 3 .

Figure 3. (Color online) (a) TEM image of single SNR growth as a function of radius and length by surface diffusion at varying pH from 11 to 9, shows decreasing SNR radius by an amount dr, as accompanied by an increase of inclination angle α and contact angle β, causes an increase in SNR length by an amount dh, such that β = dh/dr. (b) Decreasing SNR radius as a function of diffusion coefficient and contact angle β (inset: Increasing contact angle as a function of decreasing SNR radius). 10 ACS Paragon Plus Environment

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Therefore, we concentrate on the morphological changes of SNR, which is the radius as a function of length. In fact, the region close to the substrate where the nanorod exhibit larger diameter, an overgrowth of SNR by surface diffusion from the variation of pH after axial growth would be expected.40-42 Initially, under supersaturation level, the surface diffusion through Sr 2+ monomers and 38 CO2− At last pH is 3 ions form larger diameter SNR, generally corresponding to slower diffusion.

decreased to 9, solute concentration attains very high supersaturating level that gives rise to smaller radius SNRs. Consequently, SNR grows outward along the direction with decreasing diameter that corresponds to increasing surface diffusion under “orientation attachment” process. Decreasing SNR radius as a function of diffusion coefficient is shown in Figure 3b in accordance with Einstein-Stokes equation of surface diffusion D=TKB/6πηr,

(1)

where T is temperature, KB is Boltzmann constant, η is viscosity of solution and r is radius.44 Another condition is that the supersaturation in Sr 2+/ CO2− 3 is sufficiently high due to Gibbs-Thomson effect (particle size inversely proportional to solubility). For the reason that larger diameter with less solubility particles tried to produce smaller radius particles to increases the solubility level (Cs), until the SNR growth will stop. Albeit, how does the decreasing diameter increase the SNR length? By use of the Neumann quadrilateral relation and also by the modified Young‟s equation, we tend to suggest that the diameter strongly affects the SNR length.45,46 At the beginning of SNR growth, inclination angle (α) sets as zero due to the larger diameter. As the SNR continues to grow, its radius decreases by an amount dr. It increases α as accompanied by an increase of contact angle β (Figure 3b, inset). This causes an increase in SNR length (h) by an amount dh, such that β = dh/dr. Radius of SNR decreases as a function of contact angle (Figure 3b). Contact angle can be measured by θ/2 = tan-1 (h/r) method.47 It thus becomes immediately clear that as the growth starts, (i) surface diffusion increases



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due to Gibbs-Thomson effect, and (ii) decreases the SNR radius by an amount “dr” and increases the length by an amount “dh” accompanied by an increase of contact angle β. Thus the, EinsteinStokes equation can be modified as D = TKBh/6πηr,

(2)

For extended reaction time, it is interesting to found the formation of PSNS from the backbone of SNRs (Figure 4 a, b). In earlier works, morphology of the obtained product depends on the structure of raw material, concentration of alkaline solution, temperature and reaction time. Interestingly, in the present work, morphology of SrCO3 particles strongly depends on the reaction duration while keeping other parameters fixed. Based on our careful observation on morphology formation, we believe that nanocrystal splitting is the most likely answer for the formation of PSNS. It has been known fact that different crystal structures tend to form different morphology in nature by nanocrystal splitting during their growth. For example, a mineral of the zeolite group stiblite displays “desmine” sheaflike structure, wavellite a mineral of aluminum phosphate displays spherulite structure, and aragonite (CaCO3) tends to form split acicular or frostwork-like structure. Intuitively, why nanocrystal splitting occurs? What is the reason for shape evolution during growth of NRs? These questions are not answered clearly. Generally, crystal splitting is associated with the fast crystal growth at solution oversaturation as discussed above. Grigor‟ev et al. reported that there were mechanical splitting caused by extra molecules appeared in the crystallographic network.48 Punin et al. demonstrated that, if the oversaturation attains, “critical level” crystal splitting was possible. 49 We believe that high surface energy among the exposed surfaces was responsible for the nanocrystal splitting, leading to the formation of PSNS. According to the previous results, energetically higher-energy facets are unfavorable for the thermodynamically stable crystal structure. That‟s why by means of minimizing the surface energy (Gibbs-Thomson


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effect), the reactivity of high-energy surface was typically greater than that of low-energy surface during crystal growth,50 and thus due to the higher-energy facets, orthorhombic phase of SrCO3 has higher splitting ability than the other crystal phases. Therefore, we demonstrate that when the reaction time is extended, the fast crystal growth by the highest surface energy of SrCO 3 crystals takes place, leading to the nanocrystal splitting to form PSNS by secondary nucleation. During the reaction with concentrated NaOH, bonds of SrCO3 crystals are broken.

Figure 4. (Color online) (a) FESEM image of PSNS, thin SNRs with curved structure (red arrow). (b) TEM image of meticulous PSNS from Figure (a). (c) TEM image of straight thick SNRs with long lengths due to their rigidity. (d) Schematic representation of PSNS formation by nanocrystal splitting when reaction time extended, (inset) zooming of PSNS morphology. (e-g) Elemental mapping images of PSNS.



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As consequence CO2 molecules and OH- ions were partially diffused into the broken Sr2+ layer to form SrCO3 nanoseeds due to secondary nucleation under certain supersaturation levels and SNRs have grown directly from the SrCO3 particles through orientation attachment mechanism. This process continues to form the PSNS (Figure 4d). Meanwhile, when some other smaller particles did not attain certain supersaturation levels, they diffused further to form SNRs. In addition, as shown in Figure 4 a, c, thin SNRs with curved structure (Red arrow) and straight thick SNRs (black arrow) with long lengths are formed. According to Wu et al. it can be predicted that, when SNRs begin to further split, thin SNRs were flexible to form curved structure and thick SNRs were straight due to their rigidity.51 This depends upon the stress and strain fields of various crystal planes; at which we shall come back. Elements present in PSNS were confirmed by elemental mapping (Figures 4e-g) and EDX (energydispersive X-ray spectrometry) analysis (Figure S2), which confirmed that no elements other than Sr, C and O were present. Obviously, the modified critical dimensional model of SNR is relative to the reduction of Gibbs free energy of the system (surface energy of various crystallographic planes) that thus satisfies the Gibbs-Thomson effect. This is the major reason for the single crystalline nanorod formation, providing information on the growth orientation of deposited crystals. Oriented attachment process is thus proposed for SNR growth. Hence, these synthesized 1D nanostructures may be oriented in certain directions. As already mentioned in the above discussion, nanocrystal splitting is responsible for PSNS formation. Nevertheless, why these single SNR and PSNS grew out along 𝟏𝟏𝟎 ? This cannot be easily understood without further analysis. To understand the growth mechanism of single SNR and PSNS fully, the evaluation of surface energy of various crystallographic planes are indispensable. In order to investigate how surface energy relates to the surface structure, surface energy values of various crystallographic planes of SrCO3 were computed using VASP code. SrCO3 unit cell is


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crystallized in orthorhombic structure with Pmcn space group and unit-cell contains four formula units. It is optimized using first-principles calculations and the obtained lattice parameters are a = 5.102 Å, b = 8.401 Å, and c = 5.968 Å. Using optimized lattice parameters, surface slabs are modeled along (001), 010), (100), (110), (111), and (130) planes.

Figure 5. (Color online) Structures of the slabs used for SrCO3 surface energy calculations. (a) (001) surface, (b) (010) surface, (c) (100) surface, (d) (110) surface, (e) (111) surface and (f) (130) surface.



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The optimized surface models of the selected crystal planes have been given in Figure 5. The surface energy (γ) of these planes were calculated using

𝜸(

𝒆𝑽 𝒏𝒎𝟐

)=

𝑬𝒔𝒖𝒓𝒇𝒂𝒄𝒆 −𝒏𝑬𝒃𝒖𝒍𝒌 𝟐 × 𝒂𝒓𝒆𝒂 𝒐𝒇 𝒔𝒖𝒓𝒇𝒂𝒄𝒆 𝒖𝒏𝒊𝒕 𝒄𝒆𝒍𝒍

(3)

where, Esurface, Ebulk, and n are total energies of surface slabs, bulk and number of unit cells on the surface, respectively. Calculated surface energy values of (001), (010), (100), (110), (111) and (130) planes were 2.8 eV/nm2, 2.2 eV/nm2, 5.2 eV/nm2, 3.4 eV/nm2, 38.9 eV/nm2 and 5.4 eV/nm2, respectively. This demonstrated that (010) surface of SrCO3 has lowest γ of 2.2 eV/nm2 and (111) surface possesses the highest γ of 38.9 eV/nm2, which understood be due to the more dangling bonds of (111) surface. Shirota et al. reported that the order of stabilities of SrCO3 crystal in dilute aqueous acetic acid has following the sequence (100), (130), (111) ≫ (112), (110), (010), (011), (021).52 In this work, based on the structural property analysis using VASP code, the growth rate of SrCO 3 crystal structure follows the sequence (111) > (130) > (100) > (110) > (001) > (010) and found to be agreed well with the previous results. In order to compare the calculated growth sequence of SrCO 3 to the experimental results, structural and morphological analyses have been performed by using XRD and HRTEM. XRD pattern of PSNS has been present in Figure 6a. It has been evident that all the distinct Bragg reflections match well with the standard orthorhombic crystal structure of SrCO 3 (JCPDS card no. 05-0418, space group Pmcn and lattice constants of a = 5.107 Ǻ, b = 8.414 Ǻ and c = 6.026 Ǻ) and no other phases can be detected indicating the single phase nature of prepared sample. The peaks located at 20.36°, 25.20°, 25.76°, 29.68°, 31.48°, 35.20°, 36.10°, 41.40°, 44.12°, 45.60°, 47.72° and 50.04° match well with (110), (111), (021), (002), (012), (200), (130), (220), (221), (041), (132) and (113) diffraction planes, consistent with earlier report.32 16 ACS Paragon Plus Environment

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Figure 6. (Color online) (a) XRD pattern of PSNS with standard JCPDS of SrCO3. (b) HRTEM image of single SNR faceted with SrCO3 {111} faces and preferred orientation attachment in the c-axis 110 direction with a low surface energy (inset: Fourier filtered image). (c, e) TEM and HRTEM images of hexagonal cross section single SNR with two high-surface energy (111) planes corresponding to well-developed low-surface energy 110 growth direction. (d, f) Schematic illustrations of figures c and e. (g) FFT image of single SNR. (h) Schematic illustration for SrCO3 bond breaking in non-acidic growth condition. (i) Formation of stable polar faces of SrCO 3. (j) Fourier filtered image of single SNR showing rugged arrangement of C-O bonds of carbonate ions (inset: zoomed view of filtered image). It may be noted that the higher intensity (111) plane reflection is stronger than the other reflections that indicate nanorod is highly oriented along (111) surface. The (111) and (021) peaks were



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used to estimate the growth direction of PSNS. The relative texture coefficient (TC111) of (111) over (021) diffraction peak has been used here to find out the degree of orientation53 given by,

TC111 =

𝑰𝟏𝟏𝟏 /𝑰°𝟏𝟏𝟏 𝑰𝟏𝟏𝟏 /𝑰°𝟏𝟏𝟏 + 𝑰𝟎𝟐𝟏 / 𝑰°𝟎𝟐𝟏

(4)

where I111 and I021 are the measured diffraction intensities due to (111) and (021) planes, respectively. I°111 and I°021 are the corresponding values of standard JCPDF obtained from randomly oriented powder samples. The texture coefficient value for a preferential orientation along the c-axis (power sample) is 0.5. The TC111 value of our sample is 0.624, which indicates that the preferred crystalstacking domain is along c-axis. To shed light on the formation of single SNR and the splitting structure of PSNS, we studied their temporal morphological evolution by HRTEM analysis. A HRTEM image of single SNR (Figure 6b) shows that lattice fringes with spacing of 0.344 nm, could be indexed to the (111) plane (γ = 38.9 eV/nm2) with higher surface energy. The stepped surface of the single SNR faceted with SrCO3 {111} faces as shown by red lines (Figure 6b), showing the preferred orientation attachment was occurring in the c-axis 110 direction with a low surface energy (γ of 3.4 eV/nm2) indicating the single-crystal nature of SNR (Fourier filtered image, inset of Figure 6b). A hexagonal cross section of single SNR with two high-surface energy (111) planes corresponding to welldeveloped low-surface energy 110 growth direction is shown by TEM and HRTEM images (Figure 6c,e) and their schematic illustrations are given in Figures 6 d and f. Also, interesting orientation of the single SNR has been discovered from the fast Fourier transformation (FFT) image (Figure 6g). This shows the stacking of single SNR consists two (111) planes. It is also noteworthy that the orthorhombic crystal structure has four faces of two polar [(110), (010)] and two neutral [(100), (130)] in nature. They are responsible for the formation of stable structures parallel to c-axis. This might belongs to acidic and alkaline media.54 Schematic illustration of the crystal structure by using atomic model and Fourier filtered image of single SNR are given in Figures 6h to j. Each carbonate ion formulates nine 18 ACS Paragon Plus Environment

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Sr-O bonds with six Sr ions. According to Shindo et al. in the non-acidic growth condition, one or two Sr-O bonds of each CO2− 3 ions need to be broken to form stable faces (Figure 6h), while O atoms of CO2− ions pointing upward (encircled). This would give polarity normal to the surface by the 3 cancellation of positive charges make polar faces stable and destabilize the neutral faces (Figure 6i). The corresponding rugged arrangement of C-O bonds of carbonate ions pointing upward is shown by Fourier filtered image of single SNR (Figure 6j, inset: zoomed view of filtered image).55 In our present study, the experiment was carried out in the alkaline media. So, it could be precise for the polar {110} face with low surface energy. One may argue that since {010} is another polar face that has less surface energy compared with {110} face, why should the growth not in this direction? We believe that it may depend upon the time duration of reaction. As we discussed above, when the reaction time increases, formation mechanism of PSNS is proven reasonably using HRTEM result analysis as due to nanocrystal splitting, and hence PSNS growth shows good analogy to the growth direction of single SNR (Figure 7a). A closer examination of single SNR and PSNS suggests that each tip of an SNR consist of two (111) planes combined to yield a growth axis of 110 (Figure 6c, d and Figure 7b marked by red line). A cross sectional HRTEM image of single SNR with well-developed facets is given in Figure 6e. This reveals that SNR has a hexagonal cross section with two (111) planes called the twin planes.56,57 Nanotwin structure is also supported by the selected area electron diffraction pattern (SAED) and fast Fourier transformation (FFT) image (Figure 7c, d).



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Figure 7. (Color online) HRTEM image of PSNS due to nanocrystal splitting shows 110 growth direction (inset: Fourier filtered images of PSNS shows (111) plane stacking along 110 direction. (b) Twinned structure of PSNS using HRTEM (inset: One of the SNR having twinned plane. (c) SAED pattern of PSNS, having (111) mirror planes. (d) FFT image of PSNS clearly proves that nano-twin structure (inset: axial elongation of PSNS mirror planes (179.81° and 0.00°) matched well with SAED patterns given in Figure 7).


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This might be due to the axial elongation of higher surface energy (111) plane drive the nucleation of a second (111) plane to minimizing the surface energy along the axis of 110 direction. Further investigations on morphology of PSNS have shown that surfaces are not as smooth as those found in the FESEM images. They indicate that the tips of the SNRs are not oriented in a usual manner. Notably, they are composed of non-uniform twin boundaries. This further reveals that this anisotropic growth geometry consists of high degree of stacking faults (SFs) i.e. high degree of splitting do occur in the formation of PSNS. Based on above analysis, complex splitting growth with twinning has been observed in PSNS. Although, this is not common in all type of crystals, in some cases of quartz, it has been observed.58 These twinned nanostructures and their resultant performances could be understood by the fundamental lattice symmetry of nanocrystal. As we mentioned in the above atomic model of SrCO3, each carbonate ion makes nine Sr-O bonds. To form a stable polar surface or otherwise in the period of nanocrystal splitting some bonds need to be broken and the regular lattice sequence of bilayer, for example AaBbCcAaBbCc can be misplaced by the another bilayer (AaBbCcAaCcBbAc) called the mirror plane or twin plane. 59 We have statistically demonstrated that to minimize the surface energy of the (111) plane, the oriented attachment process held in SNR grew with decreasing diameter with increasing length. Hence, to form a stable structure, Sr2+ and CO2− 3 ions in SNR diffuse along the c-axis due to the spontaneous 2+ polarization produced by the oppositely charged CO2− according with Gibbs-Thomson 3 ions to Sr

effect. This leads to stacking of ions alternatively along the 110 direction with a lowest surface energy. Indeed, with the extended reaction time, these ions have largest divergence in the highest surface energy (111) plane and tend to induce nanocrystal splitting making fastest growth along the 110 direction to form PSNS. Therefore, it should be stressed that the formation of PSNS appears to be motivated by the minimization of surface energy leading to perhaps fusion of several particles into a



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single PSNS and its subindividuals grow from a single nucleus without separation from each other for a stable structure through oriented attachment. It can also be found that the intergrowth of SNR has twinned tips at the initial stage of the reaction and is complexed with splitting growth when the reaction time is increased. It should be noted that the morphological features of resultant product have an effect strongly on the concentration of alkaline materials such as KOH and NaOH and the architecture could be controlled by the concentration of OH - ions.60 We suggest that not only by the diffusion of OH- ions 2+ but also by the spontaneous polarization of oppositely charged CO2− induces the fastest 3 ions to Sr

growth towards the formation of SNR and hence PSNS. Furthermore, to analyze the bond length and growth orientation of SrCO3 molecules in PSNS, XRD pattern has been evaluated by using Rietveld technique with the help of Fullprof Suite program. XRD pattern along with Rietveld refinement data is shown in Figure 8a. It is clear that all the measured profiles match well with the calculated Bragg 2θ positions for Pmcn space group. Refined fractional atomic positions of SrCO3 in PSNS sample and its isothermal parameters are presented in Table 1. The fitting quality of the experimental data validated by the reliability factors (R factors) such as profile factor (Rp), weighted profile factor (Rwp), expected weighted profile factor (Rexp), Bragg factor (RB), crystallographic factor (RF), goodness of fit „χ2’ and lattice parameters are listed in Table 2. Lattice constant value of „c‟ obtained from Rietveld refinement was (c = 6.0342 Ǻ) greater than lattice constant c = 5.968 Ǻ obtained from surface study analysis using VASP. This is explained by the particle size dependence of lattice parameters. It may be point out that, initially the larger lattice parameter value of a and b were less sensitive to attract the CO2− 3 ions while the smaller lattice parameter along c axis was more sensitive to attract CO2− 3 ions execute strong influence on the particle size and surface charge, correlated well with the above FESEM and TEM analysis.


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Figure 8. (Color online) (a) Rietveld refined XRD pattern of PSNS. Black color represents measured data points and the solid red color represents calculated refined data. Bottom purple color line shows the allowed Bragg positions of PSNS sample and blue color line resembles the difference between measured and refined data points. (b) Stress-strain plot for PSNS clearly shown that particularly (110) plane has high strain and stress.



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Table 1. Refined fractional atomic positions and isothermal parameters of PSNS.

Parameters X

Y

Z

Biso

Atoms Sr

0.25000 0.41522

0.75879

0.0129

C

0.25000 0.76674 -0.05057 0.1200

O

0.25000 0.91609 -0.10452 0.0350

O

0.46408 0.67879 -0.09684 0.0650

Table 2. Reliability factors of PSNS by Rietveld fitting.

Parameters

Value

Rp (%) Rwp (%) Rexp (%) RB (%) RF (%) χ2 a (Å) b (Å) c (Å)

8.98 11.33 6.85 2.84 2.18 2.74 5.1007 8.4329 6.0342

The value of u parameter from the output of Rietveld refinement data was 0.478942, which has been defined as the relative shifting of anionic sublattice corresponding to cationic sublattice in c axis obtained from refinement data. The molecular geometry of PSNS would be obtained by “uc” for c


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direction,

𝒂𝟐 𝟑

+

𝟏 𝟐

𝟐

𝟏/𝟐 𝟐

−𝒖 𝒄

for other direction.61 The calculated bond distances 2.890 Å along c axis

was corresponds to Sr-O, whereas in other direction, it was found as 2.947 Å, which may be ascribed to Sr-C were in good agreement with previous results [O‟Day et al. Sr-O (2.61 Å) and Sr-C (3.04 Å)].62 The calculated Sr-O bond distance in c axis higher than previous report can be taken to mean that internal pressure or higher strain in the c direction as [110]. Expanded unit cell volume of SrCO 3 = 259.55 Å3 (for bulk SrCO3 = 258. 99 Å3), further proved particle size was reduced in PSNS. From HRTEM analysis (Figure 7b), it is seen that formation of PSNS has SFs and twins, predict that emission of Shockley partial dislocations

𝟏 𝟔

𝟏𝟐𝟏

(𝟏𝟏𝟏)

+

𝟏 𝟔

𝟐𝟏𝟏

(𝟏𝟏𝟏)

from grain boundaries

promote deformation twins and SFs.63 High nucleation stress-strain, low temperature, diffusion of atoms and nanometer size grains are the foremost contributions for the deformation. We have already discussed at length about the formation of PSNS via diffusion by the impact of room temperature and diameter. We discussed further about dislocation with stress and strain. Firstly, dislocation is a crystallographic defect which increases with decreasing grain size (δ = 1/D2) while increasing stressstrain implies material hardness. Second, the emission of partial dislocation by shear stress increases with decreasing grain size. The plot of strain versus stress is given in Figure 8b. The obtained value of strain, stress and young‟s modulus for corresponding crystallographic planes are given in Table 3. Strain and stress can be calculated by using ε = β

hkl/4

tanθ and σ = Yhkl ε. Young‟s modulus (E) is a

measure of elasticity which can be obtained by, Yhkl = (h2+k2+l2)2/(S11h4+S22k4+S33l4+(2S12+S66)h2k2+(2S13+S55)h2l2+(2S23+S44)k2l2)

(5)

Where, S11 = 0.00697, S12 = -0.00237, S13 = 0.00043, S22 = 0.01320, S23 = -0.00304, S33 = 0.01220, S44 = 0.02430, S55 = 0.03900 and S66 = 0.02350 1/Gpa, are compliance coefficients.64 In Table 3, it has been shown that particularly E = 102.7 Gpa for (110) plane has high tensile stress (2.95 Gpa) which



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further indicates (111) plane stacking is along 110 direction, while compressive and shear stress can be obtained by Bulk modulus (K = 1/9 (C11+C22+C33) + 2/9 (C12+C13+ C23)) and Shear modulus (G = 1/15 (C11+C22+C33-C12-C13-C23) + 1/5 (C44+C55+C66)). K = 66.11 Gpa and G = 33.27 Gpa, have been calculated from the reported stiffness coefficients of SrCO3 (C11 = 152, C12 = 54, C13 = 33, C22 = 109, C23 = 43, C33 = 74, C44 = 34, C55 = 26, and C66 = 38 Gpa),65 and found to be agreed well with reported values. Via „K‟ and „G‟, compressive and shear stress values of 110 were 1.90 Gpa and 0.96 Gpa respectively. Even though, this tensile, compressive and shear stress values were quite less for SFs and twins. Table 3. Calculated values of strain, stress and young‟s modulus for corresponding crystallographic planes of PSNS. S. No

Plane

Strain (ε) 0.02875

Yhkl Gpa 102.7

Stress (ζ) Gpa 2.95

1

(110)

2

(111)

0.00384

247.2

0.95

3

(021)

0.00375

84.4

0.32

4

(002)

0.00487

81.9

0.39

5

(012)

0.00457

88.9

0.41

6

(200)

0.00338

143.5

0.49

7

(130)

0.00263

80.3

0.21

8

(220)

0.00284

102.7

0.29

9

(221)

0.00306

93.4

0.29

10

(041)

0.00357

78.5

0.28

11

(132)

0.00194

86.8

0.17

12

(113)

0.00230

78.1

0.18


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3.2 Various spectroscopic investigations of PSNS The impact of CO2− 3 ions on the morphology of PSNS was examined by vibrational analysis such as FTIR and Raman spectrum was given as supporting information (S3). To gain further information on the structure of PSNS, we collected X-ray photoelectron spectroscopy (XPS) data. The high resolution XPS spectrum of strontium (Sr) 3p, Oxygen (O) 1s and carbon (C) 1s states are given in Figure 9. According to previous literature from Young et al.69 they have concentrated on Sr 3p1/2 + C 1s and Sr 3d levels and concluded that SrCO3 was easily identified by C 1s peak position (289.5-290.0 eV) rather than O 1s binding energies. However, they have not considered the effect of carbonate on C 1s and O 1s electronic structure of SrCO3. Under convenient condition carbonation occurs and can formulate significant results in the oxides of rare earth or alkali metals. By this way, we were paying attention on how C 1s and O 1s electronic structure of PSNS can be affected by carbonate contribution. Figure 9a, shows the high resolution XPS spectrum of Sr 3p level, situated at 276 eV can be assigned to strong spin-orbit splitting with Sr 3p1/2 state.69 The major peak located at 284.5 eV is designated to carbon atoms with sp2 hybridized orbitals, like highly oriented pyrolytic graphite (HOPG) structure.70 Figure 9b, shows high resolution XPS results of O 1s region. For quantitative analysis of O 1s XPS spectrum, curve fitting were carried out to determine the effect of carbonate contribution. At the binding energy of about 529.97 eV can be assigned to lattice oxide ions (O2− ) and the binding energy at 531.83 eV obtained for carbonate ions (CO2− 3 ), which confirms the effect of carbonate on O 1s and C 1s band structure of PSNS.71 Figure 9c, shows the high resolution XPS spectra of Sr 3d state with two symmetric peaks centered at 134 eV and 134.74 eV corresponding to Sr 3d3/2 and Sr 3d5/2 respectively.72 The first order X-band differential EPR spectrum of PSNS was presented in Figure 10. It can be pragmatic that PSNS exhibits EPR signals. By using Lorentzian fit, the g-factor value of free radical is



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found to be nearly equal to 2.174 for PSNS, which has been assigned to the non-spin paired (s = ½) polaron. Previous studies reported by Stoyanova et al. who found that the EPR signal was mainly induced by the CO2• radicals.73 This indicates the formation of paramagnetic defects in PSNS. This signal should correspond to carbon particles, due to the formation of CO− 2 radicals as intermediate products by the reaction of e− with CO2 , which are subsequently transformed into CO2− 3 . In particular, three types of defects are believed to be direct consequences with the interaction of host CO2− 3 ions. 3− − − These are CO− 3 , CO3 and CO2 . However, herewith we studied the paramagnetic defect centre of CO2

radicals on the surface of PSNS done by EPR absorption spectroscopy whose absorption signals are of greater magnitude than signals due to other paramagnetic species. 74


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Figure 9. XPS spectra of PSNS sample showing (a) high resolution Sr 3p1/2 and C 1s state. (b) O 1s high resolution spectra and fitting curves. (c) High resolution XPS spectra of fitting Sr 3d3/2 and Sr 3d5/2 states respectively. The EPR spectrum of PSNS represented a superposition of one singlet signal with g = 2.174 at the power of the 10 mW microwave field in the spectrometer cavity. The EPR absorption spectral data were found to be best fit for the Lorentzian function shown in Figure 10. The obtained correlation coefficient (R2), spin concentration and full width half maximum (FWHM) values are 0.89205, 29 ACS Paragon Plus Environment


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1.7262 E11 and 2.9 mT. In the simplest case, g-factor of the orthorhombic CO− 2 center here can be observed by an isotropic g tensor, such that all three principal axes of the paramagnetic centre are identical. Accordingly, the motion about all three possible axes would lead to one isotropic broad absorption spectrum at the mean g factor in the absence of any hyperfine interaction. These results are well correlated by Marshall et al. 74 These orientations may be understood as the planes of all the molecule-ion were perpendicular to the strontianite [111] direction. The corresponding EPR spectrum of SrCO3 obtained by thermal decomposition method reported by S. Angelov et al. consists of a single EPR line at g = 2.0033 with the microwave power of 30 mW.75 Nevertheless, in our case the broad singlet EPR spectrum can be obtained by 10 mW power. This is attributed to the nanostructure of the present SrCO3, wherein signals are only from the surface and the surface defects. Furthermore, it proves that Sr 2+ also plays an important role for the formation of carbonates. Lunsford et al. reported − the g-tensor of the new radical species such as CO−, CO− 2 and O2 spontaneous formation due to the

interaction of gas-phase molecules with electrons trapped in paramagnetic surface defects, which depend upon the normal valence states of the species. 76


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Figure 10. (Color online) EPR spectral data of PSNS (inset bottom: best fit for the Lorentzian function, Up: paramagnetic defect centre of CO− 2 radicals on the surface of PSNS). 2+ In strontianite, the lattice host ions are divalent, that is CO2− 3 and Sr . Once the irradiation

occurs, the CO2 interacts with the electrons trapped at specific sites of the SrCO 3 surface like Sr 2+ cations. These are usually classified as shallow traps that correspond to weakly binding sites for electrons. At high electrostatic potential generated by a Sr 2+ cation, an excess electron in one of their sites stabilizes and the presence of molecule accepting species such as CO 2, the electron can be transferred to the adsorbate with the formation of CO− 2 radical. Furthermore, due to the formation of pine-leaf-like SrCO3 nanostructures at electron-rich surface presupposes the formation of large amount of CO− 2 radicals that stabilize the broad EPR singlet spectrum. Our hypothesis is that the paramagnetic centre associated with singlet at g = 2.174 were due to probably rapidly rotating CO− 2 radical and or surface defects. 31 ACS Paragon Plus Environment


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4. CONCLUSIONS The growth of PSNS from the backbone of SNRs has been proposed comprehensively based on surface diffusion, pH, reaction time, surface energy and stress-strain. Firstly, in order to predict the formation of PSNS, the critical dimensional model of SNR such as decreasing particle size with increasing length has been evaluated by III stage of LaMer‟s surface diffusion mechanism. It was found that the decrease in pH from 11 to 9 increases surface diffusion by following the Gibbs-Thomson effect. This leads to decreasing SNR radius (dr) with an amount of increasing SNR length (dh) according to increase of inclination angle (α) and contact angle (β) leading to proposing a modified surface diffusion formula D = TKBh/6πηr, from Einstein-Stokes equation. Consequently, formation of PSNS due to extending reaction time was proposed based on splitting mechanism. Secondly, to predict the growth orientation of PSNS and SNR according to surface energy, the surface study analysis of various crystallographic planes was carried out by using VASP. They provide the clear growth sequence (111) > (130) > (100) > (110) > (001) > (010) proving vividly that SNR and PSNS are oriented with higher surface energy γ = 38.9 eV/nm2 {111} plane stacking along lower surface energy γ = 3.4 eV/nm2 110 direction, correlating well with calculated TC111 (0.624) and d-spacing values from XRD and HRTEM analysis respectively. The growth orientation of SrCO3 molecules in PSNS could also have been proved by increased bond length value of 2.890 Å corresponds to Sr-O along c-axis. Thirdly, to explore the probable responsibility for deformation twinning and SFs, the emission of Shockley partial dislocations has been evaluated by tensile stress (2.95Gpa), compressive stress (1.90 Gpa) and shear stress (0.96 Gpa) using E, K and G = 102.7, 66.11 and 33.27 Gpa. Furthermore, impact 2 of CO2− 3 ions on the structure of PSNS was confirmed by (i) the sp hybridized orbitals on C 1s and

− CO2− 3 ions on O 1s high resolution XPS spectra and (ii) with the paramagnetic defect centre of CO2


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radicals with g factor (2.174) by EPR measurement. Finally, we conclude that by following GibbsThomson effect, the LaMer‟s surface diffusion with decreasing pH led to the formation of bigger to smaller particle size nanorod formation. Consequently, higher surface energy {111} plane stacking along lower surface energy 110 direction promote PSNS with deformation twins and SFs when extending reaction time. Supporting Information Available Supporting Information S1 The details concerning apparatus and performing experiments such as XRD, FESEM, TEM, HRTEM, vibrational analysis, XPS and EPR analysis were used for validate the growth mechanism of SNR and PSNS formation. Supporting Information S2 EDX spectrum of PSNS for can be used to find out the elemental composition of SrCO3. Supporting Information S3 FTIR spectral investigation of PSNS for analyzing the impact of CO2− 3 ions on the morphology of PSNS and vibrational modes of PSNS investigated using μ-Raman data. This information is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements One of the author T.M thank the centre director Dr. V. Ganesan, UGC-DAE Consortium for Scientific Research, Indore, India for allowing us to utilize freely available national facilities to university, college/ research institute user scientists, students/ professors of India as sponsored by University Grants Commission, New Delhi. This work was supported by UGC-DAE CSR, CRS, Indore (CSR-ICBL-71/CRS-188/2016-17/852). We also thank the Management of NMSSVN College for encouragement and permission to carry out this work and Department of chemistry, NMSSVN College to provide chemicals for synthesis process.



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TOC Graphic for the manuscript “Growth mechanism of Pine-leaf-like Nanostructure from the Backbone of SrCO3 Nanorods using LaMer’s Surface Diffusion: Impact of Higher Surface Energy (γ = 38.9 eV/nm2) {111} Plane Stacking Along 𝟏𝟏𝟎 (γ = 3.4 eV/nm2) by First-Principles Calculations” Divya Arumugam a, Mathavan Thangapandian a,*, Jeshua Linu Joshua Mathavan b, Archana Jayaram c, Murugan Palanichamy d , Selvaraj Selva Chandrasekaran d, Umapathy Subramanian e, Mukul Gupta f, Gunadar Singh Okram f, Michael Angelo Jothirajan g, Milton Franklin Benial Amirtham a

*Author for correspondence: Dr. Mathavan Thangapandian., Ph. D., Assistant Professor, Research Department of Physics, NMSSVN College, Nagamalai Madurai-625 019, Tamilnadu, India TEL: +91-9486953567, E-mail: [email protected].


Growth Mechanism of Pine-leaf-like Nanostructure from the Backbone

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