BITS Faculty Publications

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Ammonia activation and nitride formation pathways in transition metal clusters: insights from mass spectrometry and first-principles DFT
(ACS, 2025-08) Bandyopadhyay, Debashis
The interaction of ammonia (NH3) with laser-vaporized transition metal clusters (Ti, V, Fe, Co, and Ni) was systematically investigated using reflectron time-of-flight mass spectrometry and density functional theory. Metal-specific and size-dependent trends emerge: Ti clusters readily form (TiN)n (n = 1–7), indicating strong nitride formation. Neutral Vn and Fen clusters predominantly yield mononitrides, with the NH3 dehydrogenation efficiency varying with cluster size and charge. Con clusters show limited reactivity with mainly NH3 adsorptions and partial dehydrogenation, while Nin clusters exhibit extensive NH3 uptake, leading to stable nitride/imide species such as NiN(NH3)4 and Ni(NH)2(NH3)4, along with the formation of Ni+H2 via hydrogen release─likely resulting from the reaction of Nin+ clusters with NH3. These findings provide insights into ammonia activation, N–H bond cleavage, and transition metal nitride formation mechanisms in small clusters.
Architecture, electronic structure and stability of TM@Ge(n) (TM = Ti, Zr and Hf; n = 1-20) clusters: a density functional modeling
(Springer, 2012) Bandyopadhyay, Debashis
The present study reports the geometry, electronic structure and properties of neutral and anionic transition metal (TM = Ti, Zr and Hf)) doped germanium clusters containing 1 to 20 germanium atoms within the framework of linear combination of atomic orbitals density functional theory under spin polarized generalized gradient approximation. Different parameters, like, binding energy (BE), embedding energy (EE), energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO), ionization energy (IP), electron affinity (EA), chemical potential etc. of the energetically stable clusters (ground state cluster) in each size are calculated. From the variation of these parameters with the size of the clusters the most stable cluster within the range of calculation is identified. It is found that the clusters having 20 valence electrons turn out to be relatively more stable in both the neutral and the anionic series. The sharp drop in IP as the valence electron count increases from 20 to 21 in neutral cluster is in agreement with predictions of shell models. To study the vibrational nature of the clusters, IR and Raman spectrum of some selected TM@Gen (n = 15,16,17) clusters are also calculated and compared. In the end, relevance of calculated results to the design of Ge-based super-atoms is discussed.
Architectures, electronic structures, and stabilities of Cu-doped Ge n clusters: density functional modeling
(Springer, 2012-03) Bandyopadhyay, Debashis
The present study reports the geometries, electronic structures, growth behavior, and stabilities of neutral and ionized copper-doped germanium clusters containing 1–20 Ge atoms within the framework of linear combination of atomic orbitals density functional theory (DFT) under the spin-polarized generalized gradient approximation. It was found that Cu-capped Ge n (or Cu-substituted Ge n+1) and Cu-encapsulated Ge n clusters mostly occur in the ground state at a particular cluster size (n). In order to explain the relative stabilities of the ground-state clusters, parameters such as the average binding energy per atom (BE), the embedding energy (EE), and the fragmentation energy (FE) of the clusters were calculated, and the resulting values are discussed. To explain the chemical stabilities of the clusters, parameters such as the energy gap between the highest occupied and the lowest unoccupied molecular orbitals (the HOMO–LUMO gap), the ionization energy (IP), the electron affinity (EA), the chemical potential (μ), the chemical hardness (η), and the polarizability were calculated, and the resulting values are also discussed. Natural atomic orbital (NAO) and natural bond orbital (NBO) analyses were also used to determine the electron-counting rule that should be applied to the most stable Ge10Cu cluster. Finally, the relevance of the calculated results to the design of Ge-based superatoms is discussed.
Boron nitride nanotubes as efficient surface absorbers for air pollutant gas molecules: insights from density functional theory
(2025-11) Bandyopadhyay, Debashis
This study investigates into the adsorption sensing capabilities of single-walled (5,5) boron nitride nanotubes (BNNTs) towards environmental pollutant gas molecules, including CH2, SO2, NH3, H2Se, CO2 and CS2. Employing a linear combination of atomic orbital density functional theory (DFT) and spin-polarized generalized gradient approximation (GGA), the investigation reveals the nanotube's robust adsorption behavior without compromising its structural integrity. Thermodynamic and chemical parameters, such as adsorption energy, HOMO-LUMO gap, vertical ionization energy, and vertical electron affinity, highlight the (5,5) BNNTs' potential as efficient absorbents for pollutant molecules. Infrared spectroscopy confirms the formation of distinct BNNT-gas complexes. These findings underscore the promising application of BN nanotubes as absorbents for common gaseous pollutants, essential for developing sensors to enhance indoor air quality.
The C-Hf-Ti System (Carbon-Hafnium-Titanium)
(JOURNAL OF PHASE EQUILIBRIA, 2000) Bandyopadhyay, Debashis
The assessed phase diagram of the Ti-C system shown in[1990Oka] conducted a detailed assessment of the C-Hf Fig. 1 is taken from [1998Oka].[1995Alb],[1996Jon], and system, where only one intermediate phase exists. The [1996Sei] reported other assessments of this system, which assessed phase diagram shown in Fig. 3 is taken from consists of two terminal solids α-Ti and β-Ti and a refractory[1990Oka]. Three terminal solids, C (graphite), α-Hf, and monocarbide TiC. The other phases present are liquid and β-Hf, are present in this system.[Massalski1] has reported graphite (C). Two eutectic and one peritectoid reaction appear the temperature of β-Hf⇔ α-Hf allotropic transformation as in this system at 1646, 2776, and 920 C, respectively. There 1743 C. There are several conflicting reports in the literature seems to be a tendency of carbon ordering at compositions([1954Cot],[1961Por],[1962Kat], and [1965Rud]) regarding below stoichiometry; as a result, the Ti2C phase does not the eutectic reaction temperature between C and Hf and its appear in the binary phase diagram. The crystal structure corresponding composition. The crystal structure data of the data shown Table 1 are taken from [Massalski2]. C-Hf system shown in Table 3 are taken from [1990Oka].
The C-Nb-Ti system (Carbon - Niobium - Titanium)
(Springer, 2000-01) Bandyopadhyay, Debashis
The assessed phase diagram of the Ti-C system in Fig. 1 is taken from [1998Oka]. Other recent assessments on this were done by [1996Jon] and [1996Sei]. The system consists of two terminal solids aTi and bTi and a refractory monocarbide TiC. Other phases present are liquid and graphite C. Two eutectic and one peritectoid reaction appear in this system at 1646, 2776, and 920 °C, respectively.
The C-Ti-Zr system (carbon-titanium-zirconium)
(Springer, 2001-01) Bandyopadhyay, Debashis
The assessed phase diagram of the Ti-C system in Fig. 1 is taken from [1998Oka],[1995ALB],[1996Jon], and [1996Sei] have reported other assessments of this system; all show two terminal solids alpha-Ti and beta-Ti and a refractory monocarbide TiC with other phases being liquid and graphite (C). Two eutectic reactions and one peritectoid reaction take place in this system at 1646 deg C, 2776 deg C, and 920 deg C, respectively. There seems to be a tendency of carbon ordering at compositions below stoichiometry. As a result, the Ti^ sub 2^ C phase does not appear in the binary phase diagram. Crystal structure data of the Ti-C system
The C-Ti-Zr system (carbon-titanium-zirconium) (vol 22, pg 64, 1998)
(JOURNAL OF PHASE EQUILIBRIA, 2001-04) Bandyopadhyay, Debashis
Calculation of the Debye Temperature and Study of the Lattice Dynamics of Fe80–xNixCr20 by 57Fe Mössbauer Spectroscopy
(De Gruyter, 2022) Bandyopadhyay, Debashis
This paper reports a Mössbauer spectroscopic study of Fe80–xNixCr20 [19 ≤ x (at.%) ≤ 30] alloys in the temperature range 10–295 K. From the temperature variation of the center shift, the Debye temperatures of different alloys have been calculated and the lattice dynamics of the system has been discussed. From the temperature dependence of the relative broadening of the full-width-at-half-maximum, the relaxation nature of the system has also been discussed.
Chemisorptions effect of oxygen on the geometries, electronic and magnetic properties of small size Nin (n = 1-6) clusters
(Springer, 2011-05) Bandyopadhyay, Debashis
The present study reports the effect of oxygen addition on small size Nin (n = 1-6) clusters in different spin states within the framework of linear combination of atomic orbital (LCAO) density functional theory (DFT) under spin polarized generalized gradient approximation (GGA) functional. Relative stabilities of the optimized clusters are discussed on the basis of the calculated parameters, such as, binding energy (BE), embedding energy (EE) and fragmentation energy (FE). Other parameters, like ionization potential (IP), electron affinity (EA), etc. show that though the additions of oxygen can affect the chemical properties of Nin clusters with an additional stability to NinO. In most of the cases the magnetic moment of the stable isomers are geometry dependent for a particular size both in pure and oxidized clusters. Calculated magnetic moments of NinO (n = 1-6) clusters reveal that the magnetic moment of ground state Ni4O isomers in different geometries is same as in pure Ni4 isomers. Present study also explains the cause of stable magnetic moment in Ni4O cluster through the distribution of electrons in different orbitals.
Density Functional Investigation of Structure and Stability of Gen and GenNi (n = 1−20) Clusters: Validity of the Electron Counting Rule
(ACS, 2010-01) Bandyopadhyay, Debashis
Structure and electronic properties of neutral and cationic pure and Ni-doped Ge clusters containing 1−20 Ge atoms are calculated within the framework of linear combination of atomic orbitals density functional theory. It is found that in clusters containing more than 8 Ge atoms the Ni atom is absorbed endohedrally in the Ge cage. Relative stability of Ni-doped clusters at different sizes is studied by calculating their binding energy, embedding energy of a Ni atom in a Ge cluster, highest-occupied molecular orbital to lowest-unoccupied molecular orbital gap, and the second-order energy difference. Clusters having 20 valence electrons turn out to be relatively more stable in both the neutral and the cationic series. There is, infact, a sharp drop in IP as the valence electron count increases from 20 to 21, in agreement with predictions of shell models. Relevance of these results to the designing of Ge-based superatoms is discussed.
A density functional theory–based study of the electronic structures and properties of cage like metal doped silicon clusters
(AIP, 2008-10) Bandyopadhyay, Debashis
Ab initio electronic-structure calculations were performed by using density functional theory with polarized basis set (LanL2DZ) within the spin polarized generalized gradient approximation for metal doped clusters where varies from 9 to 20. In the first step of the calculation, geometrical optimizations of the nanoclusters have been done. In the next step, these optimized geometries have been used to calculate the binding energy (BE) and HOMO-LUMO gap of the clusters. In order to check the stability of the clusters, the second order energy differences of the optimized geometries have been calculated. To study the optical behavior of the clusters, IR and Raman spectra calculation have been done. Further calculations on cation and anion clusters have been done to obtain their ionization potential (IP), electron affinity (EA), and chemical potential.
Effect of Transition Metal Doping on Hydrogenated Germanium Nanocages: A Density Functional Investigation
(American Scientific Publishers, 2010-01) Bandyopadhyay, Debashis
In this report we present an ab initio electronic-structure calculations of hydrogenated germanium cages GenHnTM (TM = Cu and Zn, n = 12 to 24) using density functional theory with polarized basis set (SDD) nanoclusters. In the first step of the calculation, geometrical optimizations of the nanoclusters have been done. In the next step only the ground state optimized geometries are used to calculate the binding energy (BE), HOMO-LUMO gap and embedding energy (EE) of the clusters. Based on these values a comparative study on different doping and also with respect to the pure cages are done. It is found that though the doping with Cu can be taken favorable in the cages, but Zn is not.
Elastic and strain--tunable electronic and optical properties of la2algao6 hybrid perovskite: a first-principles study
(2025-11) Bandyopadhyay, Debashis
Perovskite materials, known for their structural versatility and multifunctional properties, continue to draw interest for advanced electronic and optoelectronic applications. In this study, we investigate the elastic and strain--engineered mechanical, electronic properties and optical properties of the orthorhombic La2AlGaO6 (LAGO) hybrid perovskite using first--principles quantum mechanical calculations based on density functional theory (DFT). Structural optimizations were performed using both the local density approximation (LDA) and the generalized gradient approximation (GGA). The mechanical stability of LAGO was confirmed through the Born--Huang criteria, and key elastic constants (C11, C12, C33, C44, and C66) were evaluated. These constants were further used to derive mechanical parameters such as Young's modulus, bulk modulus, shear modulus, Poisson's ratio, Cauchy's pressure, and anisotropic factor, offering insights into the material's ductility, hardness, and elastic anisotropy. Crucially, we explored the influence of biaxial strain on the electronic band structure, DOS/PDOS, and Fermi energy, revealing significant band gap modulation under compressive and tensile strain, and hence, varying the optical properties. The coupling between elastic response and electronic structure highlights LAGO's potential for tunable device applications, where mechanical stimuli can be employed to tailor its electronic functionality.
Electronic structure and stabilities of Ni-doped germanium nanoclusters: a density functional modeling study
(Springer, 2012-12) Bandyopadhyay, Debashis
The present study reports the geometry, electronic structure, growth behavior and stability of neutral and ionized nickel encapsulated germanium clusters containing 1–20 germanium atoms within the framework of a linear combination of atomic orbital density functional theory (DFT) under a spin polarized generalized gradient approximation. In the growth pattern, Ni-capped Gen and Ni-encapsulated Gen clusters appear mostly as theoretical ground state at a particular size. To explain the relative stability of the ground state clusters, variation of different parameters, such as average binding energy per atom (BE), embedding energy (EE) and fragmentation energy (FE) of the clusters, were studied together with the size of the cluster. To explain the chemical stability of the clusters, different parameters, e.g., energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO gap), ionization energy (IP), electron affinity (EA), chemical potential (μ), chemical hardness (η), and polarizability etc. were calculated and are discussed. Finally, natural bond orbital (NBO) analysis was applied to understand the electron counting rule applied in the most stable Ge10Ni cluster. The importance of the calculated results in the design of Ge-based superatoms is discussed.
Electronic structure and stability of anionic AuGen (n = 1–20) clusters and assemblies: a density functional modeling
(Springer, 2018-12) Bandyopadhyay, Debashis
In the present study electronic structure and stabilities of cationic gold-doped germanium clusters, AuGen (n = 1 to 20), and their assemblies have been investigated by density functional theory (DFT) modeling. Computational results show a good relationship between the thermodynamic parameters, average binding energy, embedding energy, fragmentation energy, etc., with the percentage hybridization between different Ge 4s, Ge 4p, and Au 5d atomic orbitals, which plays a dominating role in the stabilization of anionic AuGe7, AuGe10, Au(Ge7)2, Au(Ge9)2, and Au(Ge10)2 clusters. Other thermodynamic and chemical parameters are also found consistent with the observed thermodynamic stabilities of the nanoclusters. In smaller size range (n < 11), Au atom always absorbs on the surface or vertex of pure Ge cluster. From n = 11, endohedral doping starts. In the assembled clusters, Au atom play the role as a bridging atom in Au(Ge7)2, Au(Ge9)2, and Au(Ge10)2 clusters. Stability of the AuGe7, AuGe10, Au(Ge7)2, Au(Ge9)2, and Au(Ge10)2 are explained using magic number in shell-filled model and mixed (π-σ) aromatic rule. As per the symmetry and structure of AuGe12 cluster, it is comparable to a nido-cluster, and hence, its stability is explained using Wade-Mingos rule. Calculated VDE, ADE, HOMO-LUMO gap, and VIP have very close agreement with the experimental results. IR and Raman frequencies show that the vibration nature of the clusters could produce electromagnetic radiation in the far infrared region which is useful for medical applications.
The electronic structures and properties of transition metal-doped silicon nanoclusters: A density functional investigation
(Elsevier, 2008-11) Bandyopadhyay, Debashis
We report an ab initio all electron molecular-orbital electronic-structure calculation by using density functional theory (DFT) and with polarized basis set (LanL2DZ) within the spin polarized generalized gradient approximation for metal-doped silicon clusters, SinM (n = 14–20 and M = Ti, Zr, Hf). As the first step of calculation, geometrical optimizations of the nanoclusters have been done. In the next step, these optimized geometries are used to calculate the binding energy and HOMO–LUMO gap (band gap) of the clusters. In order to check the dynamical stability of the clusters, IR and Raman spectra have been calculated. Further calculations have been done on cation and anion clusters to obtain ionization potential (IP), electron affinity (EA), chemical potential and chemical hardness of the optimized clusters.
Evolution of electronic and vibrational properties of M@Xn (M = Ag, Au, X = Ge, Si, n = 10, 12, 14) clusters: a density functional modeling
(Springer, 2018-03) Bandyopadhyay, Debashis
Evolution of electronic and vibrational properties of M@Xn (M = Ag, Au, X = Ge, Si, n = 10, 12, 14) nanoclusters is investigated by using first-principle density functional theory (DFT)-based calculations with effective core potentials. To explain the thermodynamic and chemical stability of the ground state cluster in each size, variation of different thermodynamic and chemical parameters, like, binding energy (BE), HOMO–LUMO gap (ΔE), vertical ionization potential (VIP) and vertical electron affinity (VEA) was studied with the variation of the size of the clusters for emphasizing the differences and similarities in the clusters. It is found that Au doping in Ge and Si cages prefers endohedral position, whereas Ag prefers to take the position at the surface of the cages. In addition, IR and Raman spectra of the clusters are also studied to understand the vibrational nature of the stable clusters. At the end, present theoretical results are compared with existing experimental data. Theoretical knowledge of the thermodynamic, chemical and vibrational properties of these specific ground state structures is important for understanding its potential application in the field of optoelectronic science.
First principle study on the electronic and optical properties of ktao3 for optoelectronic properties
(IEEE, 2023) Bandyopadhyay, Debashis
In this theoretical study, the structural, optical and electronic properties of KTaO3 have been investigated. The Density of States clearly depicts the strong contribution of the p orbitals of Oxygen in the bonding, indicating the structure's stability. With a band gap of 2.65 eV coupled with negligibly low energy loss and significantly high absorptivity in the visible region, the material implies strong application in Optoelectronic and solar cell purposes.
First-Principal Study of the Effect of Ti Doping on Hydrogenation of Mgn (N=2-8) Nanoclusters
(SSRN, 2023-02) Bandyopadhyay, Debashis
We report the effect of single Ti atom doping Mgn (n=2-8) clusters on hydrogenation kinetics using density functional theory (DFT). During growth process, variations of different energetic parameters, such as binding energy, stability, etc., with the size of the clusters, Mg4 and TiMg5, are found as thermodynamically stable. From chemisorption energies in hydrogenated Mgn and TiMgn clusters, we have estimated the catalytic effect of Ti doping in Mgn clusters to improve the dose of hydrogen. It is found that the activation barrier reduces by about 32.8% after Ti doping, as evident from the IRC study. ELF mapping indicates that the Ti doping increase the reactive sites in the Mg5 cluster, and the TiMg5 can absorb extra hydrogen molecule to attain the average binding energy of Mg5-2H. So, Ti doping in the Mg5 cluster improves the hydrogenation process as well as the dose concentration because of its catalytic activities