Nitrogen substitution reaction in a graphene sheet and carbon nanotubes of different diameter are... more Nitrogen substitution reaction in a graphene sheet and carbon nanotubes of different diameter are investigated using the generalized tight-binding molecular dynamics method. The formation of a vacancy in curved graphene sheet or a carbon nanotube is found to cause a curvature dependent local reconstruction of the surface. Our simulations and analysis show that vacancy mediated N substitution ͑rather than N chemisorption͒ is favored on the surface of nanotubes with diameter larger than 8 nm. This predicted value of the critical minimum diameter for N incorporation is confirmed by experimental results presented on nitrogen-doped multiwalled nanotubes with Ϸ5 at. % nitrogen prepared by the thermal chemical vapor deposition process.
A detailed ab initio ͑DFT/GGA and DFT/ GGA+ U͒ investigation of codoping ZnO with Co and Cu ions ... more A detailed ab initio ͑DFT/GGA and DFT/ GGA+ U͒ investigation of codoping ZnO with Co and Cu ions reveals the role of Cu and the existence of optimum structural dopant configurations which could lead to the enhancement of ferromagnetism. The role of the Cu ions in Zn͑Co,Cu͒O is to act as superexchange mediators while causing a remote delocalization through the hybridization of the Cu d 3z 2 -r 2 spin-majority states with the O p states, thereby enhancing the ferromagnetism. The ferromagnetic state is found to be more favorable than the antiferromagnetic one if the Cu-d band is spin splitted. Such a process is reminiscent of the conventional RKKY interaction among magnetic impurities embedded in a metal.
The carbon nanotubes and carbon nanobtube junctions have recently emerged as excellent candidates... more The carbon nanotubes and carbon nanobtube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale electronic devices. Single wall carbon nanotubes, as molecular wires, could be used as nanoscale interconnects, where as complex junctions of nanotubes of different chirality can be used as rectifying and transiting devices. It has been shown that pentagon-heptagon defect is responsible for the creation of simple hetero-junctions. Complex junctions, such as 3-termianl T-junctions and Y-junctions require entirely different arrangement of defects. These 3-terminal junctions form prototypes of nanoscale tunnel devices made entirely of carbon. Furthermore, either n-type or p-type doping of the semiconducting portion of these complex junctions should yield Schottky barrier type devices. We also investigate simple wires and junctions of boron and nitrogen (III-V elements) and compare with those of carbon. The structural and electronic properties on nanotubes and nanotube junctions (carbon as well as BN) are studied using a generalized tight-binding molecular dynamics (GTBMD) scheme.
Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2... more Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSb x P 1-x alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSb x P 1-x . Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSb x P 1-x nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields.
Using first principles calculations we demonstrate the existence of anisotropic ferromagnetic int... more Using first principles calculations we demonstrate the existence of anisotropic ferromagnetic interactions in Co embedded graphene nanoribbons (GNR). Spin polarization of the edge states is found to alter significantly compared to the metal free cases. Our findings can all be well justified as the output of the interplay between the development of an induced spin polarization in the neighborhood of the Co atoms and the maintaining of the polarization picture of the Co-free GNR. Based on our results we propose an efficient pathway for graphene based spintronics applications.
Nanomechanics of single-_aU C, BN and BC$_35 and B doped C nanotubes under axial compression and ... more Nanomechanics of single-_aU C, BN and BC$_35 and B doped C nanotubes under axial compression and tension are investigated through a generalized tight-binding molecular dynamics (GTBMD) and {_it ab-initio} electronic structure methods.
In this report we detail the development and application of an efficient quantum molecular dynami... more In this report we detail the development and application of an efficient quantum molecular dynamics computational algorithm and its application to the nanotube-tip assisted reactions on silicon and diamond surfaces. The calculations shed interesting insights into the microscopic picture of tip surface interactions.
NA,gAIVlission Need_ • Onboard computing sy;tems i_or future autonomou= intelligent vehider -powe... more NA,gAIVlission Need_ • Onboard computing sy;tems i_or future autonomou= intelligent vehider -powerful, compact, low '_;_'_.a9 power" COnsumption, radiation h_d '_ High performance computing . (Tera-and Peta-flop=) -processing _tellite data -integrated space vehicle engineering -climate modeli_ • Revolutionary computing technologies • Smart, compact senmrs, ulttarm all l_obes • "Advanced miniaturization of all systems Microspacecxaft Q "Thinking" spacecraft o Micro-, nano-rovers for planetary exploration CNT is a tubular form of carbon with diameter as small as I rim. Length: few nm to microns.
Bulletin of the American Physical Society, Mar 15, 2016
this work, we demonstrate for the first time the high pressure synthesis of α-AgGaO 2 via a solid... more this work, we demonstrate for the first time the high pressure synthesis of α-AgGaO 2 via a solid state reaction of Ag 2 O and Ga 2 O 3 . Synthesis experiments were carried out at pressures and temperatures up to 10 GPa and 600 C, respectively, using a resistively-heated diamond anvil cell. Electron diffraction confirmed the rhombohedral delafossite crystal structure of the synthesized AgGaO 2 and its corresponding lattice parameters of a = 2.99 Å and c = 18.43 Å. The vibrational modes analysis was also conducted using a combination of ab initio density functional theory (DFT) and Raman spectroscopy. This analysis yielded good agreement between the calculated Raman-active modes and experimental Raman data. Finally, the application of the GGA + U formalismbased on DFT to calculate the electronic band structure of α-AgGaO 2 provided a more realistic theoretical band gap value than those reported previously.
Bulletin of the American Physical Society, Mar 15, 2006
Submitted for the MAR06 Meeting of The American Physical Society Elastic properties of SiC nanosc... more Submitted for the MAR06 Meeting of The American Physical Society Elastic properties of SiC nanoscopic wires MAXIM MAKEEV,
A genetic algorithm procedure is developed and implemented for fitting parameters for many-body i... more A genetic algorithm procedure is developed and implemented for fitting parameters for many-body inter-atomic force field functions for simulating nanotechnology atomistic applications using portable Java on cycle-scavenged heterogeneous workstations. Given a physics based analytic functional form for the force field, correlated parameters in a multi-dimensional environment are typically chosen to fit properties given either by experiments and/or by higher accuracy quantum mechanical simulations. The implementation automates this tedious procedure using an evolutionary computing algorithm operating on hundreds of cycle-scavenged computers. As a proof of concept, we demonstrate the procedure for evaluating the Stillinger-Weber (S-W) potential by (a) reproducing the published parameters for Si using S-W energies in the fitness function, and (b) evolving a "new" set of parameters using semi-empirical tightbinding energies in the fitness function. The "new" parameters are significantly better suited for Si cluster energies and forces as compared to even the published S-W potential.
Photoinduced charge transfer from an indium tin oxide (ITO) contact into [6,6]-phenyl-C61-butyric... more Photoinduced charge transfer from an indium tin oxide (ITO) contact into [6,6]-phenyl-C61-butyric acid methyl ester (PC 60 BM) and [6,6]phenyl-C71-butyric acid methyl ester (PC 70 BM) is measured. For both, chargetransfer peaks are observed for a series of excitation energies below the absorption edge. If charge transfer is blocked using a tunnel barrier or an applied electric field, then the peaks disappear. The observed transitions are similar to those predicted by theoretical calculations of the absorption spectra for negatively charged C 60 and C 70 chains. This observation suggests that charge transfer occurs preferentially at the polaronic transition energies, providing a new means for polaronic state spectroscopy.
The present work investigates the effect of the anionic co-doping on the magnetic properties of t... more The present work investigates the effect of the anionic co-doping on the magnetic properties of the DMSs doped with transition metal (TM) ions, namely that of Zn(Mn,X)O, X=C,N,P and S. The present findings are in complete analogy with the corresponding results for the cationic co-doping of Zn(TM1,TM2)O as studied in our earlier works. For both, the cationic and anionic co-dopings, we find that the magnetic impurities induce magnetic moments on their nearest neighbors with the largest induced magnetic moment observed on the cation or the anion which mediates two magnetic impurities. The magnetic interaction in the case of the anionic co-doping can be seen as a double exchange or a superexchange type of mechanism. On the other hand, in the cationic co-doping, the magnetic interactions take the form of a double-exchange or a superexchange between polarized tetrahedra each centered at one of the co-dopants.
In the past, molecular-dynamics computer simulations have employed model potentials of the intera... more In the past, molecular-dynamics computer simulations have employed model potentials of the interaction between atoms. Here we introduce a new molecular-dynamics technique in which atomic forces are computed from the total electronic energy of the system of interacting atoms. The method is illustrated by trajectories for Al and As atoms scattering off the relaxed (110) surface of GaAs.
Abstract The microscopic aspects of interfacial growth are only crudely accessible to experiment.... more Abstract The microscopic aspects of interfacial growth are only crudely accessible to experiment. We have therefore developed a technique for computer-experimental studies of atoms and molecules deposited on solid surfaces, in which forces are determined from the electronic structure rather than atomic potentials. Here we report simulations of Cl, O, Se, N, As, Si, Ge, Al, Ga, Zn, Hg, Cu, and Au on the (1 1 0) surface of GaAs. The results exhibit non-trivial variety in both the dynamics and final bonding sites.
As the papers of this conference make clear, there are many approaches to the calculation of tota... more As the papers of this conference make clear, there are many approaches to the calculation of total energies and atomic forces. During the past four years, we have developed a technique that is particularly useful for computer simulations of atomic motion and chemical reactions at solid surfaces [1–18]. The long-range goal of such simulations is microscopic understanding of complex processes like interfacial growth and catalysis. A feasible short-range goal is to study the more specific atomic processes that occur when an atom, molecule, or cluster interacts with a solid surface.
Nitrogen substitution reaction in a graphene sheet and carbon nanotubes of different diameter are... more Nitrogen substitution reaction in a graphene sheet and carbon nanotubes of different diameter are investigated using the generalized tight-binding molecular dynamics method. The formation of a vacancy in curved graphene sheet or a carbon nanotube is found to cause a curvature dependent local reconstruction of the surface. Our simulations and analysis show that vacancy mediated N substitution ͑rather than N chemisorption͒ is favored on the surface of nanotubes with diameter larger than 8 nm. This predicted value of the critical minimum diameter for N incorporation is confirmed by experimental results presented on nitrogen-doped multiwalled nanotubes with Ϸ5 at. % nitrogen prepared by the thermal chemical vapor deposition process.
A detailed ab initio ͑DFT/GGA and DFT/ GGA+ U͒ investigation of codoping ZnO with Co and Cu ions ... more A detailed ab initio ͑DFT/GGA and DFT/ GGA+ U͒ investigation of codoping ZnO with Co and Cu ions reveals the role of Cu and the existence of optimum structural dopant configurations which could lead to the enhancement of ferromagnetism. The role of the Cu ions in Zn͑Co,Cu͒O is to act as superexchange mediators while causing a remote delocalization through the hybridization of the Cu d 3z 2 -r 2 spin-majority states with the O p states, thereby enhancing the ferromagnetism. The ferromagnetic state is found to be more favorable than the antiferromagnetic one if the Cu-d band is spin splitted. Such a process is reminiscent of the conventional RKKY interaction among magnetic impurities embedded in a metal.
The carbon nanotubes and carbon nanobtube junctions have recently emerged as excellent candidates... more The carbon nanotubes and carbon nanobtube junctions have recently emerged as excellent candidates for use as the building blocks in the formation of nanoscale electronic devices. Single wall carbon nanotubes, as molecular wires, could be used as nanoscale interconnects, where as complex junctions of nanotubes of different chirality can be used as rectifying and transiting devices. It has been shown that pentagon-heptagon defect is responsible for the creation of simple hetero-junctions. Complex junctions, such as 3-termianl T-junctions and Y-junctions require entirely different arrangement of defects. These 3-terminal junctions form prototypes of nanoscale tunnel devices made entirely of carbon. Furthermore, either n-type or p-type doping of the semiconducting portion of these complex junctions should yield Schottky barrier type devices. We also investigate simple wires and junctions of boron and nitrogen (III-V elements) and compare with those of carbon. The structural and electronic properties on nanotubes and nanotube junctions (carbon as well as BN) are studied using a generalized tight-binding molecular dynamics (GTBMD) scheme.
Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2... more Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSb x P 1-x alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSb x P 1-x . Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSb x P 1-x nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields.
Using first principles calculations we demonstrate the existence of anisotropic ferromagnetic int... more Using first principles calculations we demonstrate the existence of anisotropic ferromagnetic interactions in Co embedded graphene nanoribbons (GNR). Spin polarization of the edge states is found to alter significantly compared to the metal free cases. Our findings can all be well justified as the output of the interplay between the development of an induced spin polarization in the neighborhood of the Co atoms and the maintaining of the polarization picture of the Co-free GNR. Based on our results we propose an efficient pathway for graphene based spintronics applications.
Nanomechanics of single-_aU C, BN and BC$_35 and B doped C nanotubes under axial compression and ... more Nanomechanics of single-_aU C, BN and BC$_35 and B doped C nanotubes under axial compression and tension are investigated through a generalized tight-binding molecular dynamics (GTBMD) and {_it ab-initio} electronic structure methods.
In this report we detail the development and application of an efficient quantum molecular dynami... more In this report we detail the development and application of an efficient quantum molecular dynamics computational algorithm and its application to the nanotube-tip assisted reactions on silicon and diamond surfaces. The calculations shed interesting insights into the microscopic picture of tip surface interactions.
NA,gAIVlission Need_ • Onboard computing sy;tems i_or future autonomou= intelligent vehider -powe... more NA,gAIVlission Need_ • Onboard computing sy;tems i_or future autonomou= intelligent vehider -powerful, compact, low '_;_'_.a9 power" COnsumption, radiation h_d '_ High performance computing . (Tera-and Peta-flop=) -processing _tellite data -integrated space vehicle engineering -climate modeli_ • Revolutionary computing technologies • Smart, compact senmrs, ulttarm all l_obes • "Advanced miniaturization of all systems Microspacecxaft Q "Thinking" spacecraft o Micro-, nano-rovers for planetary exploration CNT is a tubular form of carbon with diameter as small as I rim. Length: few nm to microns.
Bulletin of the American Physical Society, Mar 15, 2016
this work, we demonstrate for the first time the high pressure synthesis of α-AgGaO 2 via a solid... more this work, we demonstrate for the first time the high pressure synthesis of α-AgGaO 2 via a solid state reaction of Ag 2 O and Ga 2 O 3 . Synthesis experiments were carried out at pressures and temperatures up to 10 GPa and 600 C, respectively, using a resistively-heated diamond anvil cell. Electron diffraction confirmed the rhombohedral delafossite crystal structure of the synthesized AgGaO 2 and its corresponding lattice parameters of a = 2.99 Å and c = 18.43 Å. The vibrational modes analysis was also conducted using a combination of ab initio density functional theory (DFT) and Raman spectroscopy. This analysis yielded good agreement between the calculated Raman-active modes and experimental Raman data. Finally, the application of the GGA + U formalismbased on DFT to calculate the electronic band structure of α-AgGaO 2 provided a more realistic theoretical band gap value than those reported previously.
Bulletin of the American Physical Society, Mar 15, 2006
Submitted for the MAR06 Meeting of The American Physical Society Elastic properties of SiC nanosc... more Submitted for the MAR06 Meeting of The American Physical Society Elastic properties of SiC nanoscopic wires MAXIM MAKEEV,
A genetic algorithm procedure is developed and implemented for fitting parameters for many-body i... more A genetic algorithm procedure is developed and implemented for fitting parameters for many-body inter-atomic force field functions for simulating nanotechnology atomistic applications using portable Java on cycle-scavenged heterogeneous workstations. Given a physics based analytic functional form for the force field, correlated parameters in a multi-dimensional environment are typically chosen to fit properties given either by experiments and/or by higher accuracy quantum mechanical simulations. The implementation automates this tedious procedure using an evolutionary computing algorithm operating on hundreds of cycle-scavenged computers. As a proof of concept, we demonstrate the procedure for evaluating the Stillinger-Weber (S-W) potential by (a) reproducing the published parameters for Si using S-W energies in the fitness function, and (b) evolving a "new" set of parameters using semi-empirical tightbinding energies in the fitness function. The "new" parameters are significantly better suited for Si cluster energies and forces as compared to even the published S-W potential.
Photoinduced charge transfer from an indium tin oxide (ITO) contact into [6,6]-phenyl-C61-butyric... more Photoinduced charge transfer from an indium tin oxide (ITO) contact into [6,6]-phenyl-C61-butyric acid methyl ester (PC 60 BM) and [6,6]phenyl-C71-butyric acid methyl ester (PC 70 BM) is measured. For both, chargetransfer peaks are observed for a series of excitation energies below the absorption edge. If charge transfer is blocked using a tunnel barrier or an applied electric field, then the peaks disappear. The observed transitions are similar to those predicted by theoretical calculations of the absorption spectra for negatively charged C 60 and C 70 chains. This observation suggests that charge transfer occurs preferentially at the polaronic transition energies, providing a new means for polaronic state spectroscopy.
The present work investigates the effect of the anionic co-doping on the magnetic properties of t... more The present work investigates the effect of the anionic co-doping on the magnetic properties of the DMSs doped with transition metal (TM) ions, namely that of Zn(Mn,X)O, X=C,N,P and S. The present findings are in complete analogy with the corresponding results for the cationic co-doping of Zn(TM1,TM2)O as studied in our earlier works. For both, the cationic and anionic co-dopings, we find that the magnetic impurities induce magnetic moments on their nearest neighbors with the largest induced magnetic moment observed on the cation or the anion which mediates two magnetic impurities. The magnetic interaction in the case of the anionic co-doping can be seen as a double exchange or a superexchange type of mechanism. On the other hand, in the cationic co-doping, the magnetic interactions take the form of a double-exchange or a superexchange between polarized tetrahedra each centered at one of the co-dopants.
In the past, molecular-dynamics computer simulations have employed model potentials of the intera... more In the past, molecular-dynamics computer simulations have employed model potentials of the interaction between atoms. Here we introduce a new molecular-dynamics technique in which atomic forces are computed from the total electronic energy of the system of interacting atoms. The method is illustrated by trajectories for Al and As atoms scattering off the relaxed (110) surface of GaAs.
Abstract The microscopic aspects of interfacial growth are only crudely accessible to experiment.... more Abstract The microscopic aspects of interfacial growth are only crudely accessible to experiment. We have therefore developed a technique for computer-experimental studies of atoms and molecules deposited on solid surfaces, in which forces are determined from the electronic structure rather than atomic potentials. Here we report simulations of Cl, O, Se, N, As, Si, Ge, Al, Ga, Zn, Hg, Cu, and Au on the (1 1 0) surface of GaAs. The results exhibit non-trivial variety in both the dynamics and final bonding sites.
As the papers of this conference make clear, there are many approaches to the calculation of tota... more As the papers of this conference make clear, there are many approaches to the calculation of total energies and atomic forces. During the past four years, we have developed a technique that is particularly useful for computer simulations of atomic motion and chemical reactions at solid surfaces [1–18]. The long-range goal of such simulations is microscopic understanding of complex processes like interfacial growth and catalysis. A feasible short-range goal is to study the more specific atomic processes that occur when an atom, molecule, or cluster interacts with a solid surface.
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Papers by Madhu Menon