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Quantum chemical studies on (ZnO) n /(NiO) nheterostructured nanoclustersR. Chandiramouli a , S. Sriram a & D. Balamurugan aa School of Electrical & Electronics Engineering , SASTRA University , Tirumalaisamudram ,Thanjavur , IndiaPublished online: 14 Jun 2013.

To cite this article: R. Chandiramouli , S. Sriram & D. Balamurugan (2013): Quantum chemical studies on (ZnO) n /(NiO) nheterostructured nanoclusters, Molecular Physics: An International Journal at the Interface Between Chemistry and Physics,DOI:10.1080/00268976.2013.805846

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Molecular Physics, 2013http://dx.doi.org/10.1080/00268976.2013.805846

RESEARCH ARTICLE

Quantum chemical studies on (ZnO)n/(NiO)n heterostructured nanoclusters

R. Chandiramouli∗, S. Sriram and D. Balamurugan

School of Electrical & Electronics Engineering, SASTRA University, Tirumalaisamudram, Thanjavur, India

(Received 18 January 2013; final version received 11 May 2013)

The structural stability and electronic properties of (ZnO)n, (NiO)n, (ZnO)n/(NiO)n for n = (1 to 4) and 3D structures werestudied using density functional theory. The geometrical optimisation of clusters implies that when the atoms in the clusterincrease it leads to an increase in its stability. The stability drastically increases for the heterostructure of (ZnO)n/(NiO)n.The dipole moment of the clusters depends on the geometry of the cluster and it is found to be minimum for heterostructuresrepresenting more neutralised clusters. HOMO-LUMO energies, ionisation potential, electron affinity, chemical hardness,binding energies and vibrational analysis of different clusters are calculated and reported. The adsorption of CO on thedifferent sites of nanoclusters are studied and discussed.

Keywords: ZnO clusters; NiO clusters; DFT; heterostructure; dipole moment; HOMO-LUMO

1. Introduction

The advancement of intensive research on compound semi-conductor heterostructures leads to the discovery of manynew devices in the fabrication of semiconductor laser, light-emitting diodes and photodetectors. These new classes ofheterostructures are fabricated by contact between organicand inorganic semiconductors [1], organic and organicsemiconductors [2] and inorganic semiconductors [3]. Dif-ferent heterostructures with a thin layer of compound semi-conductor deposited over the substrate such as Si, glass [4]or a heterostructure of bulk material are made into pellets toform heterostructured junction [5]. The characteristic of thep–n junction depends on the energy gap of the semiconduc-tor and carrier concentration of the p and n type materials.The fabrication of the heterojunction may be of differenttypes, such as sputtering [6], chemical vapour deposition[7], pulsed laser deposition [8], spray deposition [9] forthin films, whereas for bulk heterojunction by preparingthe nanoparticles by different techniques, such as the liquidcoprecipitation method [10], solid state reaction [11], solgel [12] method etc. In all the cases the formation of het-erojunction depends on the size of the grains and thicknessof the layers formed.

The optimisation of the molecules during the fabrica-tion process itself will result in an effective heterojunctionthat finds potential application in optoelectronic devices.The motivation behind the work is that if the atoms in theclusters are tailored it will result in a proper heterostruc-ture. From the literature survey, it is known that much workhas not been done on the cluster studies of heterostruc-tures. Zinc oxide (ZnO) is an n-type material due to metal

∗Corresponding author. Email: rcmoulii@gmail.com

rich and nickel oxide (NiO) is a p-type material that arisesdue to metal deficit or oxygen excess. In this work, ZnOclusters are attached to NiO clusters and their minimumenergy, dipole moment, ionisation potential, electron affin-ity, chemical hardness, binding energy, vibrational analysisand energy gap between highest occupied molecular orbitalenergy (HOMO) and lowest unoccupied molecular orbitalenergy (LUMO) have been calculated using the densityfunctional theory and reported.

2. Computational methods

The entire calculation in the present work is to optimisethe neutral clusters of linear ZnO structures, linear NiOstructure and the combination of ZnO cluster with NiOclusters on two layers and with different types of realisticthree-dimensional (3D) clusters. Quantum chemical DFTtechniques were used throughout the computation usingBecke’s three-parameter hybrid functional [13,14] in com-bination with the Lee—Yang–Parr correlation functionalmethod (B3LYP) utilising 6–31 basis set. The 6-31G basisset is used in this work since it can be applied to hydro-gen to krypton through (3df, 3pd) polarisation functions.The choice of B3LYP hybrid functional is that it can re-produce experimental, structural and electronic propertiesfor transition metals [15,16]. For all the optimised struc-tures the convergence energy is obtained up to 10−5eV. Thepseudopotential approximation in quantum mechanical cal-culation is to replace the complicated effects of motion ofnon-valence electrons of an atom and its nucleus with an ef-fective potential or pseudopotential where the Schrodinger

C© 2013 Taylor & Francis

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Figure 1. Optimised geometrical structures of (ZnO)n (n = 1 to 4).

equation contains a modified potential term. Since zinc andnickel have a nuclear charge of 30 and 28, respectively, acomplete basis set of 6-31G will give a better result in quan-tum calculation with pseudopotential approximation. Thisis the reason behind the selection of B3LYP/6-31G basis setthat will fully optimise the calculation of minimum energyand dipole moment. Densities of States (DOS) and HOMO-LUMO calculations are carried out with GaussSum 2.2 [17]package with the output.

2.1. Binding energy

The binding energy per atom (EBE) of nanoclusters can befound using the following relation

EBE=(n ∗ E[M]+m ∗ E[O]−E[MnOm])/(n+m), (1)

where E[M] is the energy of the metal atom (Zn, Ni), E[O]is the energy of oxygen atom, n and m are the number ofmetal atoms and oxygen atoms, respectively [18,19].

3. Results and discussion

The linear structures of (ZnO)n, (NiO)n and layers of(ZnO)n/(NiO)n clusters (n = 1 to 4) and 3D structures of

(ZnO)n/(NiO)n clusters have been simulated to study thestable energy and dipole moment among different struc-tures. Even though different stable structures are possiblewith the combination of metal oxide clusters some of thepossible geometry of structure is constructed and optimisedby DFT calculations as shown in Figures 1–3. DFT calcu-lations have been performed to study the different layers of(ZnO)n/(NiO)n clusters, the complexity of the calculationincreases with the cluster size.

3.1. Structures of (ZnO)n

The structure of (ZnO)n (n = 1 to 4) is constructed withzinc and oxygen atoms aligned in a straight line as inFigure 1(a), (b), (c) and (d). These structures are not closedstructures. As the number of atoms in the cluster increasesthe stability of the cluster also increases. For (ZnO)1 struc-ture the energy is found to be −1854.210 Hartrees, whereasfor (ZnO)2, (ZnO)3 and (ZnO)4 it is found to be −3708.537,−5562.889 and −7416.993 Hartrees, respectively. From thecalculated results of energy for various ZnO linear clusters,it is observed that, with the addition of ZnO in the clus-ter, ZnO molecular energy (i.e. (ZnO)1) is added with the

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Figure 2. Optimised geometrical structures of (NiO)n (n = 1 to 4).

clusters total energy. All the structures have the energy dif-ference of −1854 Hartrees, which is equal to the energy of(ZnO)1. The dipole moment of (ZnO)1 is calculated to be5.119 Debye. Since in this structure only one zinc atom isattached to the oxygen atom, which has 30 electrons and8 electrons, respectively, the charges are distributed asym-metrically, which gives rise to dipole moment. The (ZnO)2-,

(ZnO)3 and (ZnO)4 have dipole moments of 8.271, 10.743and 16.971 Debye, respectively. The increasing trend in thedipole moment is due to the increase in the asymmetry ofthe cluster due to more number of atoms that drasticallydeviate from the ordered structure.

3.2. Structures of (NiO)n

In looking at the structures of (NiO)n (n = 1 to 4) asin Figure 2(a), (b), (c) and (d) it is seen that for (NiO)1

the energy is calculated to be −1583.143 Hartrees. Allthe energies in the NiO structures are lower than that ofZnO due to less number of electrons in the NiO cluster.(NiO)2, (NiO)3 and (NiO)4 structures have the energies of−3166.428, −4749.736 and −6333.037 Hartrees, respec-tively. Energy difference is found to be −1583 Hartreesfrom that of the preceding cluster. It is evident from theobtained results that as the number of atoms in the clus-ter increases it is found to be more stable. Dipole momentpurely depends upon the distribution of charges betweenthe atoms in the cluster. For the (NiO)1 cluster the dipolemoment is 5.025 Debye in contrast to (ZnO)1, which has adipole moment of 5.119 Debye since the atomic number forzinc is 30, the dipole moment for (ZnO)1 is more as com-pared with (NiO)1. The dipole moment of (NiO)2 is 8.567Debye compared to the dipole moment of (ZnO)2, which isonly 8.271 Debye; from this observation the dipole moment

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Figure 3. Optimised geometrical structures of (ZnO)n/(NiO)n (n = 1 to 4). Bond lengths are given in A and bond angles in degrees.

not only depends on the electronic structure of the atom italso depends upon the bond length between the atoms inthe clusters. The dipole moments of (NiO)3 and (NiO)4 arecalculated to be 13.203 and 16.320 Debye, respectively. Thedipole moment varies in different ways with bond length,cluster size and the electronic configuration of the atoms inthe cluster.

3.3. Linear heterostructures of (ZnO)n/(NiO)n

In analysing the linear heterostructures of (ZnO)n/(NiO)n,interestingly it is observed that the energy of the individual

(ZnO)n/(NiO)n cluster is the sum of (ZnO)n and (NiO)n

energies. It is well inferred that instead of forming acluster of either (ZnO)n or (NiO)n, if a heterostruc-tured cluster is formed, the stability of the structureincreases that can be tailored for the fabrication of op-toelectronic devices. For (ZnO)1/(NiO)1 cluster the en-ergy is −3437.578 Hartrees, whereas the energies for(ZnO)2/(NiO)2, (ZnO)3/(NiO)3 and (ZnO)4/(NiO)4 are cal-culated to be −6875.305, −10313.059 and −13750.811Hartrees, respectively. The difference in the energy be-tween successive structures is found to be −3437 Hartrees.Moreover, the individual dipole moment of linear structures

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Molecular Physics 5

Figure 4. (ZnO)n structure (n = 1 to 4) (clusters vs. energy).

is found to be high for both (ZnO)n and (NiO)n clusters,whereas on forming the clusters with (ZnO)n and (NiO)n

the dipole moment of heterostructured clusters drasticallydecreases due to the formation of the closed structure asseen in Figure 3(a), (b), (c) and (d). Dipole moments arefound to be 0.675, 1.174, 1.950 and 2.614 Debye, respec-tively, for (ZnO)1/(NiO)1, (ZnO)2/(NiO)2, (ZnO)3/(NiO)3,(ZnO)4/(NiO)4, respectively.

Figures 4 and 5 represent cluster vs. energy and clustervs. dipole moment of the (ZnO)n cluster, the energy variesfrom −1854.210 Hartrees to −7416.993 Hartrees for n = 1to n = 4 clusters where the stability increases enormously.The dipole moment increases with the cluster size due tothe bulk zinc atoms, it increases to a value of 61.971 De-bye for n = 4 cluster. Figures 6 and 7 represent the plot ofcluster vs. energy and cluster vs. dipole moment of (NiO)n;in this cluster the energy varies from −1583.143 Hartreesto −6333.037 Hartrees, where the stability of (NiO)n iscomparatively smaller than the (ZnO)n clusters. The dipolemoment for (NiO)n varies between 5.025 to 16.320 De-bye for (NiO)n clusters. Figures 8 and 9 represent thevariation of cluster vs. energy and cluster vs. dipole mo-ment for heterostructured (ZnO)n/(NiO)n clusters. Upon

Figure 5. (ZnO)n structure (n = 1 to 4) (clusters vs. dipolemoment).

Figure 6. (NiO)n structure (n = 1 to 4) (clusters vs. energy).

Figure 7. (NiO)n structure (n = 1 to 4) (clusters vs. dipole mo-ment).

forming a heterostructured cluster there is a vast increasein the stability of the cluster that varies from −3437.578 to−13750.811 Hartrees. This is due to the formation of well-packed layers of the cluster one over the other. In contrast,the dipole moment of heterostructured clusters decreases,which varies from 0.6753 to 2.614 Debye. The decrease in

Figure 8. (ZnO)n/(NiO)n structure (n = 1 to 4) (clusters vs.energy).

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Table 1. Energy and dipole moment of (NiO)n, (ZnO)n, (ZnO)n/(NiO)n clusters (n = 1 to 4) and 3D clusters.

Sl no. No. of clusters (n = 1 to 4) Model Energy (Hartrees) Dipole moment (Debye)

1. 1 (ZnO)1 −1854.210 5.1192. 2 (ZnO)2 −3708.537 8.2713. 3 (ZnO)3 −5562.889 10.7434. 4 (ZnO)4 −7416.993 16.9715. 1 (NiO)1 −1583.143 5.0256. 2 (NiO)2 −3166.428 8.5677. 3 (NiO)3 −4749.736 13.2038. 4 (NiO)4 −6333.037 16.3209. 1 (ZnO)1/(NiO)1 −3437.578 0.675

10. 2 (ZnO)2/(NiO)2 −6875.305 1.17411. 3 (ZnO)3/(NiO)3 −10313.059 1.95012. 4 (ZnO)4/(NiO)4 −13750.811 2.61413. 12 3D-1 −10310.470 2.39414. 12 3D-2 −10311.235 2.18415. 12 3D-3 −10311.112 2.32616. 18 3D-4 −15325.901 1.03

Figure 9. (ZnO)n/(NiO)n structure (n = 1 to 4) (clusters vs.dipole moment).

the dipole moment of heterostructured clusters arises dueto the balancing of charges between the clusters. The ener-gies and dipole moment of different clusters are tabulatedin Table 1.

3.4. 3D structures of (ZnO)n/(NiO)n

Among the four different 3D heterostructures shown in Fig-ure 10, the energy of 3D-1 structure is −10310.47 Hartreesthat is found to be more stable than the 3D-2 and 3D-3 struc-tures; all the three structures have 12 atoms in their clus-ters. Moreover, in the 3D-1 structure the cluster resembles abuckyball-like structure, on the other hand the 3D-2 clusterresembles a hexagon-like structure and the 3D-3 resemblesa cube-like structure, because the buckyball structure is aclosed packed structure and it is more stable than the othertwo structures. In the case of the 3D-4 structure that has 18atoms in the cluster has an energy of −15325.90 Hartrees;

due to the increased number of atoms in this structure it ismore stable among all the linear clusters and 3D clusters.Looking at the dipole moments of 3D-1, 3D-2 and 3D-3structures there is no drastic variation noticed, for all thestructures the dipole moment is around 2 Debye; in thesestructures 12 atoms are packed closely, the presence of thenickel atom and zinc atom gives rise to the dipole moment.For 3D-4 structures the dipole moment is 1.03 Debye inwhich all the 18 atoms are well packed and the charge in-side the cluster is balanced giving rise to a moderate dipolemoment. The variation of energy and dipole moment withcluster type is shown in Figure 11.

3.5. HOMO-LUMO, ionisation potential,electron affinity and chemical hardness

For all the clusters, the HOMO-LUMO energy gap purelydepends on the cluster size. It is evident from Table 2that with increase in the cluster size the energy gap de-creases since an overlapping of the orbitals takes placewithin the cluster. This resembles clusters that are in zerodimension. From the simple molecule orbital theory, theHOMO energy (Ehomo) is related as the ionisation po-tential and LUMO energy (Elumo) as the electron affinity[20]. The HOMO-LUMO energy gap (Ehomo − Elumo =η) is used to calculate the chemical hardness of the clus-ter. Usually a hard molecule has a wide HOMO-LUMOgap. The chemical hardness of various clusters is tabu-lated in Table 3. Maximum hardness is found for (ZnO)1,

(NiO)1, (ZnO)1/(NiO)1 clusters, hence these clusters areharder than the other clusters [21]. The ionisation poten-tial of (ZnO)n and (NiO)n decreases when the cluster sizeincreases whereas for (ZnO)n and (NiO)n the electron affin-ity increases with increase in cluster size. However, whenthe heterostructure of (ZnO)n/(NiO)n is formed the ionisa-

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Figure 10. 3D structures of ZnO/NiO clusters.

Figure 11. Energy and dipole moment of various 3D clusters.

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Table 2. HOMO –LUMO values of (NiO)n, (ZnO)n , (ZnO)n/(NiO)n clusters (n = 1 to 4) and 3D clusters.

Structure Cluster no. HOMO (eV) LUMO (eV) Energy gap (eV)

(NiO)n 1 −6.17 −4.12 2.052 −5.62 −4.24 1.383 −5.55 −4.39 1.164 −5.36 −4.55 0.81

(ZnO)n 1 −6.67 −4.18 2.492 −6.03 −5.05 0.983 −5.65 −5.27 0.384 −5.80 −5.49 0.31

(ZnO)n/(NiO)n 1 −5.37 −3.46 1.912 −5.10 −3.67 1.433 −5.00 −3.76 1.244 −4.92 −3.78 1.14

3D-1 12 −4.43 −3.05 1.383D-2 12 −4.07 −2.23 1.843D-3 12 −4.07 −2.8 1.273D-4 18 −2.70 −1.52 1.18

tion potential drastically decreases and the electron affinityalso decreases. The ionisation potential, electron affinityand hardness of various clusters are tabulated in Table 3.Figure 12(a), (b) and (c) represents the HOMO-LUMO en-ergy gap of (ZnO)1, (NiO)1 and (ZnO)1/(NiO)1 clusters,respectively. The obtained energy gap of ZnO is in goodagreement with earlier reported values [22]. The HOMO-LUMO diagrams of various 3D structures are shown in Fig-ure 13(a), (b), (c) and (d). The energy gap of 3D-1, 3D-2 and3D-3 are 1.38, 1.84 and 1.27 eV, respectively. Among thesethree structures, the 3D-2 structure has a higher band gap.

The higher band gap restricts the movement of electronsfrom HOMO to LUMO that will not actively take part inelectrical conductivity. The 3D-4 structure has a low valuegap of 1.18 eV that has an active participation in electri-cal conduction. Analysing the ionisation potential of 3D-1,3D-2 and 3D-3 that have a high value of IP which requiresmore energy to take electrons from these clusters. In con-trast the 3D-4 structure has an IP of 2.7 eV that implies thatelectrons can be easily removed from this cluster. The highvalue of EA denotes an active involvement in chemical re-actions. Among all the 3D structures the 3D-1 is chemically

Table 3. Ionisation potential, electron affinity and chemical hardness of (ZnO)n, (NiO)n,(ZnO)n/(NiO)n clusters (n = 1 to 4) and 3D clusters.

Ionisation potential Electron affinity Chemical hardnessCluster Size IP (eV) EA (eV) η (eV)

(ZnO)n 1 6.67 4.18 2.492 6.03 5.05 0.983 5.65 5.27 0.384 5.8 5.49 0.31

(NiO)n 1 6.17 4.12 2.052 5.62 4.24 1.383 5.55 4.39 1.164 5.36 4.55 0.81

(ZnO)n/(NiO)n 1 5.37 3.46 1.912 5.1 3.67 1.433 5 3.76 1.244 4.92 3.78 1.14

3D-1 12 4.43 3.05 1.383D-2 12 4.07 2.23 1.843D-3 12 4.07 2.8 1.273D-4 18 2.7 1.52 1.18

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Molecular Physics 9

Figure 12. (a) HOMO-LUMO energy diagram of (ZnO)1 linearstructure. (b) HOMO-LUMO energy diagram of (NiO)1 linearstructure. (c) HOMO-LUMO energy diagram of (ZnO)1/(NiO)1

structure.

highly reactive. Regarding chemical hardness, the 3D-2 hasa wide gap of 1.84 eV that is chemically harder than theother 3D structures.

3.6. Binding energies of (ZnO)n, (NiO)n and(ZnO)n/(NiO)n clusters

The calculated binding energies of (ZnO)n, (NiO)n and(ZnO)n/(NiO)n clusters are shown in Table 4. From the cal-culated binding energy values it is observed that the bind-ing energy value increases with cluster size for all types ofclusters. The increasing trend in the binding energy con-firms that addition of more atoms makes the cluster morestable which is also confirmed with the calculated energyvalues tabulated in Table 1. The calculated binding energyvalues of ZnO and NiO are well matched with previouslyreported values [23–26]. Looking at the binding energiesof the 3D structures as tabulated in Table 5, 3D-1 has ahigher value of BE than 3D-2 and 3D-3, which means thatamong these heterostructures, the buckyball-like structureof 3D-1 is stronger than the other two structures, which isalso in agreement with the calculated energy. There is anenormous increase in the BE of the 3D-4 structure due toits closely packed structure with 18 atoms which gives it astable structure.

3.7. Vibrational analysis of (ZnO)1, (NiO)1,(ZnO)1/(NiO)1 and 3D structures

Vibrational analysis of different isomers of (ZnO)1, (NiO)1,(ZnO)1/(NiO)1 and 3D structures were carried out and thespectrum with mode assignment is shown in Figure 14(a),(b) and (c). In case of the linear structure of (ZnO)1, max-imum intensity of 0.0318 km/mole is observed at the fre-quency of 833.03 cm−1 that is assigned to molecular stretch-ing. In contrast to (NiO)1 at the vibrational frequency of769.31 cm−1, IR intensity of 1509.89 km/mole is seenthat is mode assigned to molecular stretching. Lookingat the heterostructure of (ZnO)1/(NiO)1, three prominentIR intensities of 56.05, 51.62 and 49.88 km/mole are ob-served at frequencies of 216.43, 442.86 and 573.33 cm−1,respectively, all the vibrations are assigned to molecularstretching. It is seen that all the vibrational frequenciesare lower than the (ZnO)1 and (NiO)1 frequency that con-firms the formation of a linear heterostructure. Lookingat the vibrational spectrum of the 3D structures depictedin Figure 15(a), (b), (c) and (d), more IR intensities areobserved; among these prominent IR intensities of 40.10,87.22, 570.09 and 154.584 km/mole are seen at frequen-cies of 996.98, 1634.47. 4186.60 and 4469.24 cm−1 as-signed to molecular stretching, molecular stretching, Ni–Ostretching and Zn–O stretching, respectively. For the 3D-2 structure, even though more number of lines are seen,prominent vibrational frequencies are at 736.432, 757.236and 1086.76 cm−1 with intensities of 64.52, 68.72 and

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Figure 13. (a) HOMO-LUMO energy diagram of 3D-1 structure. (b) HOMO-LUMO energy diagram of 3D-2 structure. (c) HOMO-LUMO energy diagram of 3D-3 structure. (d) HOMO-LUMO energy diagram of 3D-4 structure.

Table 4. Binding energies of (ZnO)n, (NiO)n and(ZnO)n/(NiO)n clusters.

(ZnO)n binding (NiO)n binding (ZnO)n/(NiO)n

Cluster binding binding bindingsize (n) energy (eV) energy (eV) energy (eV)

1 2.244883 2.557806 3.9319462 3.039435 3.523786 4.4380653 3.417664 3.948273 4.6666354 2.764607 4.138747 4.778199

105.65 km/mole, respectively; all are assigned to molecularstretching. Analysing the spectrum of the 3D-3 structure,IR intensities of 70.18, 39.65 and 102.80 km/mole are ob-served at frequencies of 1581.7, 1632.82, 2102.18 cm−1

assigned to the Ni–O stretching, Zn–O stretching andNi–O stretching, respectively. Looking at the vibrationalspectrum of the 3D-4 structure, IR intensities of 59.19,80.72 and 86.48 km/mole are witnessed for frequenciesof 380.16, 400.71 and 492.11 cm−1, assigned to molecu-lar bending,molecular stretching and molecular stretching,

Table 5. Binding energies of 3D clusters.

HeterostructureCluster name Cluster size (n) binding energy (eV)

3D-1 12 1.1904983D-2 12 0.5351573D-3 12 0.2539733D-4 18 9.662825

respectively. Vibrational studies also provide informationabout the stability of clusters; a cluster with no imaginaryfrequency is more stable [27,28].

3.8. CO adsorption on heterostructured(ZnO)n/(NiO)n nanoclusters

The adsorption of carbon monoxide (CO) on different sitesof (ZnO)2/(NiO)2 and 3D-2 are studied to observe thepossibility of CO to get adsorbed on the clusters are shownin Figures 16 and 17. Table 6 shows the energies obtained for

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Figure 14. (a) Simulated IR spectrum of (ZnO)1 cluster (frequency in cm−1 and IR intensity in km/mole). (b) Simulated IR spectrumof (NiO)1 cluster (frequency in cm−1 and IR intensity in km/mole). (c) Simulated IR spectrum of (NiO/ZnO)1 cluster (frequency in cm−1

and IR intensity in km/mole).

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Figure 15. (a) Simulated IR spectrum of 3D-1 cluster (frequency in cm−1 and IR intensity in km/mole). (b) Simulated IR spectrum of3D-2 cluster (frequency in cm−1 and IR intensity in km/mole). (c) Simulated IR spectrum of 3D-3 cluster (frequency in cm−1 and IRintensity in km/mole). (d) Simulated IR spectrum of 3D-4 cluster (frequency in cm−1 and IR intensity in km/mole).

normal and CO-adsorbed (ZnO)2/(NiO)2 and 3D-2 clusters.In analysing the (ZnO)2/(NiO)2 cluster, when CO gets ad-sorbed on the zinc atom the energy is found to be −6988.525Hartrees whereas at the nickel atom the energy is found to be−6988.478 Hartrees. Likewise in the 3D-2 structure at thezinc atom the calculated energy is −10424.260 Hartrees

and for the nickel atom site it is −10423.909 Hartrees.From these results it is inferred that the adsorption of COin the heterostructured nanocluster is more in favour of thezinc atom site compared to nickel atom site [29,30]. Theheterostructured nanocluster can be used for gas sensingapplications.

Figure 16. CO adsorption on (ZnO)2/(NiO)2 clusters: (a) on Zn atom site; (b) on Ni atom site.

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Molecular Physics 13

Figure 17. CO adsorption on 3D-2 clusters: (a) on Zn atom site; (b) on Ni atom site.

Table 6. Energies of CO adsorbed on (ZnO)2/(NiO)2 and 3D-2clusters.

Cluster Energy (Hartrees) CO adsorbed position

(ZnO)2/(NiO)2 −6875.305 –−6988.525 Zn−6988.478 Ni

3D-2 −10311.236 –−10424.260 Zn−10423.909 Ni

4. Conclusion

Visualisation of the clusters gives a clear picture aboutits own characterisation that provides a clear idea for thesynthesis of material. The geometrical structures, dipolemoment, stability, HOMO-LUMO energies, ionisation po-tential, electron affinity, binding energies and vibrationalanalysis of (ZnO)n, (NiO)n,(ZnO)n/(NiO)n (n = 1 to 4) andheterostructured 3D structures have been studied using DFTwith B3LYP exchange correlation function using 6-31G asthe basis set. Different linear structures are completely opti-mised with DFT techniques. An important observation thatneeds to be highlighted is that the stability of the clusterincreases with increase in the cluster size. Instead of be-ing either (ZnO)n or (NiO)n if a cluster is formed with aheterostructure its stability increases due to its structure.In 3D structures, 3D-4 is found to be more stable. Dipolemoment is significant for linear structures of (ZnO)n and(NiO)n, in contrast if a heterostructure is formed, the dipolemoment decreases due to its closed structure. With increasein cluster size the energy gap decreases since more numberof atoms are accommodated in the cluster. The ionisationpotential is found to decrease with cluster size and electron

affinity increases. Maximum chemical hardness is observedfor single clusters of (ZnO)1, (NiO)1, (ZnO)1/(NiO)1. Thechemical hardness for 3D-2 is more than the other 3D struc-tures. When the cluster size increases the binding energiesof linear structures also increase, which also confirms thatstability increases with cluster size. Binding energy is largefor the 3D-4 cluster, which confirms that it is more stable.Vibrational analysis confirms the formation of stable het-erostructured (ZnO)n/(NiO)n clusters. More IR intensitiesare seen for the 3D structures, which confirm that the 3Dstructures are more stable. From the adsorption studies ofCO to nanoclusters, it is inferred that the adsorption of COto the zinc atom site is more favourable than the nickelatom site. The information given in this article provides aplethora for the experimentalists to tailor new materials thathave industrial importance.

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