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ABSTRACT: Using a first-principles theoretical model the adsorption of a methyl radical on different sized silver nanoparticles is compared to the adsorption of the same radical on model surfaces. Calculations of our structural, dynamical and electronic properties indicated that small changes in the local environment will lead to small changes in infrared (IR) wavenumbers, but in dramatic changes in the IR signal. Our calculations indicate the lower the adsorption site coordination, the higher is the signal strength, suggesting that small changes in the electronic charge distribution will result in bigger changes in the polarizability and hence in the spectroscopic signal intensity. This effect explains, among others, the signal magnification observed for nanoparticles in surface enhanced Raman spectroscopic (SERS) experiments.
Nanotechnology 11/2012; 23(48):485202. · 3.98 Impact Factor
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ABSTRACT: In this work a systematic study of the dependence of the structural, electronic, and vibrational properties on nanoparticle size is performed. Based on our total energy calculations we identified three characteristic regimes associated with the nanoparticle's dimensions: (i) below 1.5 nm (100 atoms) where remarkable molecular aspects are observed; (ii) between 1.5 and 2.0 nm (100 and 300 atoms) where the molecular behavior is influenced by the inner core crystal properties; and (iii) above 2.0 nm (more than 300 atoms) where the crystal properties are preponderant. In all considered regimes the nanoparticle's surface modulates its properties. This modulation decreases with the increasing of the nanoparticle's size.
Nanotechnology 07/2011; 22(27):275708. · 3.98 Impact Factor
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ABSTRACT: In this work, a 2.0 nm nanoparticle (low limit synthesized system) is compared to possible simplified models: passivated clusters, small (1.3 nm) nanoparticles and sets of plane surfaces. Our density functional theory results suggest that even when geometric aspects are properly described by the simplifications considered, electronic properties might be very different, especially when edge atoms are not properly taken into account in the nanoparticle's modeling. In addition, we propose a protocol that might help future theoretical descriptions of nanoparticles.
Journal of Physics Condensed Matter 02/2011; 23(4):045001. · 2.55 Impact Factor
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ABSTRACT: In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of furan on the silicon (001) surface. A direct comparison of different adsorption structures with x-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS), high resolution electron energy loss spectroscopy (HREELS), near edge x-ray absorption fine structure (NEXAFS), and high resolution spectroscopy experimental data allows us to identify the [4+2 ] cycloaddition reaction as the most probable adsorbate. In addition, theoretical scanning tunnelling microscopy (STM) images are presented, with a view to contributing to further experimental investigations.
Journal of Physics Condensed Matter 02/2009; 21(5):055006. · 2.55 Impact Factor
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ABSTRACT: We investigate a magnetic fluid composed of magnetite nanoparticles surfacted with dodecanoic acid molecules and stably dispersed in a hydrocarbon solvent. A comparison between Monte Carlo simulation and different experimental techniques allows us to validate our methodology and investigate the behavior of the surfactant molecules. Our analysis, based on the Langmuir model, suggests that the surfactant grafting number on isolate nanoparticles increases with the nanoparticle concentration, while the grafting on agglomerated nanoparticles presents a more complicated behavior. Our results suggests that, if properly coated and at a certain concentration range, colloids can become stable even in the presence of agglomerates. The role of the Hamaker constant, which controls the van der Waals interaction intensity, was also investigated. We have found that the ratio between grafting and Hamaker constant governs the level of nanoparticle agglomeration.
Physical Review E 01/2009; 78(6 Pt 1):061507. · 2.26 Impact Factor
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ABSTRACT: In this work, we report a systematic comparison between three-dimensional Monte Carlo simulations and magnetic resonance measurements of polydisperse Fe3O4 based nanoparticles. Magnetization curves clearly show the validity of the simple model employed by us. Our theoretical results are then compared to real ferrofluids samples that have been experimentally characterized by transmission electron microscopy and magnetic resonance measurements. Our data indicates that the number of surfactant molecules and its physical distribution (grafting) decisively affects the surface-surface particle distance within an agglomerate. Two different grafting regimes are clearly identified from our theoretical data.
Journal of Applied Physics 04/2006; 99(8):08S101-08S101-3. · 2.17 Impact Factor
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ABSTRACT: The adsorption of maleic anhydride on the Si(001) surface has been investigated using the first-principles pseudopotential formalism. Our total-energy calculations suggest that maleic anhydride (C2H2-C2O3) adsorbs preferentially through a [2+2] cycloaddition of the C=C bond ([2+2]) with an adsorption energy of around 42 kcal/mol. Besides the [2+2] configuration we have also considered other possible coverages and adsorption models, including the adsorption on inter-row and intrarow dimer sites. Based on the analysis of the relative stability of different adsorption models, we propose the formation of mixed domains, containing the [2+2] unit and an interdimer unit. The comparison of our calculated electronic band structure, vibrational modes, and scanning tunneling microscopy images for the [2+2] and the favored interdimer adsorbed structures corroborate our proposed mixed domain model.
The Journal of Chemical Physics 09/2005; 123(7):074708. · 3.33 Impact Factor
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ABSTRACT: A possible model for the ZnSe growth on GaAs(110) is proposed based on a first-principles pseudopotential method. Our calculations suggest that ZnSe growth on GaAs(110) could be understood in a two-step process: (i) Zn atoms will be adsorbed over Ga and As sites of the GaAs(110) surface, and (ii) the Zn atom over the Ga site will be replaced by a Se atom, followed by layer-by-layer ZnSe growth. We have also investigated Zn-induced features at the GaAs(110) surface, during the initial Zn interaction with the surface. Zn was found to adsorb preferentially at Ga substitutional sites at the subsurface layer and over Ga and As surface atoms. Theoretical STM images show the presence of bright features related to the Zn at Ga substitutional sites in the subsurface layers in agreement with recent experimental works. © 2002 American Institute of Physics.
Applied Physics Letters 07/2002; 81(3):481-483. · 3.84 Impact Factor
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ABSTRACT: Using a first-principles pseudopotential method we have compared the adsorption and dissociation of the common n-type dopant molecule PH3 on the Si(001)-(21) and Ge(001){(21) surfaces. We find that the dissociated state is energetically more favourable than the molecular state by 1.70(0.81) eV, whereas the latter is 0.58(0.25) eV more stable than the system composed of the free silicon(germanium) surface and PH3(g). The chemisorbed system is characterised by elongated Si{Si(Ge{Ge) dimers that are symmetric in the dissociative case and asymmetric in the molecular case and by the fact that the Si(Ge){PH2 as well as the PH3(ads) groups retain the pyramidal geometry of the phosphine molecule. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental works.
Brazilian Journal of Physics. 01/2002;
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ABSTRACT: Using a first-principles pseudopotential method we have studied the adsorption and dissociation of NH3, PH3, and AsH3 on the Si(001)–(2×1) surface. Apart from the existence of a barrier for the adsorption of the precursor state for arsine, we observe that the global behavior for the chemisorption of the XH3 molecules considered in this work is as follows: the gas phase XH3 adsorbs molecularly to the electrophilic surface Si atom and then dissociates into XH2 and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms, respectively. The energy barrier, corresponding to a thermal activation, is much smaller than the usual growth temperature, indicating that all three molecules will be observed in their dissociated states at room temperature. All adsorbed systems are characterized by elongated Si–Si dimers that are (almost) symmetric in the dissociative case but asymmetric in the molecular case. According to our first-principles calculations, all XH3 and XH2 systems retain the pyramidal geometry observed for the gas molecules. Our calculated vibrational spectra further support the dissociative model for the XH3 molecules considered here. © 2001 American Institute of Physics.
The Journal of Chemical Physics 05/2001; 114(21):9549-9556. · 3.33 Impact Factor
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R. Miotto
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ABSTRACT: Using a first-principles pseudopotential technique, we have investigated four possible models for the adsorption of Zn on the GaAs(001)-β2(2×4) surface. In agreement with the experimental studies of Ohtake and co-workers, our total energy calculations suggest that Zn atoms adsorb preferentially in trench (third-layer) sites. All structures are characterized by the breaking of the As dimer and the subsequent formation of two mixed dimers. Despite the formation of mixed dimers, all other structural features for the four models studied (i.e., minimum interplanar distance and remaining As dimer bond lengths) retain the characteristics of the free-surface structure. We observed that all four studied models present a very similar band structure, but while the binding energies and dispersion of the surface states are very similar to that observed for the free surface, their orbital characters depend on the model considered. From a theoretical scanning-tunneling-microscopy (STM) image simulation, we have shown that Zn adsorption in the top layer leads to a clearly identifiable change in the brightness. On the other hand, Zn adsorption in the trench results in small changes in the brightness in the trench region, which is not easily detectable in a common STM experiment.
Phys. Rev. B. 11/2000; 62(20).
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ABSTRACT: The atomic structures of four possible models for the adsorption of Zn on GaAs(001)–(2×4) are investigated by means of a first-principles pseudopotential technique. Our calculations suggest that Zn atoms adsorb preferentially in trench (third layer) sites. All structures are characterized by the breaking of the arsenic dimer and the formation of two mixed dimers in the vertical plane containing the original As–As dimer. The Zn atom lies 0.16 Å higher than the As atoms, and the mixed dimer has a bond length of 2.31 Å. All other structural features for the four models studied (e.g., minimum interplanar distance, and remaining As dimer bond lengths) retain the characteristics of the free surface. © 2000 American Institute of Physics.
Applied Physics Letters 06/2000; 76(25):3735-3737. · 3.84 Impact Factor
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ABSTRACT: Using the first-principles pseudopotential approach, we have performed calculations of the total energy, atomic structure, and chemical bonding for the bulk and surfaces of GaAs and cubic GaN to assess the role of the generalized gradient approximation (GGA) for the exchange-correlation energy. Our calculations show that the combined effect of nonlinear core correction and GGA is responsible for significant changes in the structural parameters and electronic band structure of the (001) surface, regardless of the ionicity of the compound studied, but not for bulk or the nonpolar (110) surface.
Phys. Rev. B. 01/1999; 59(4).
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ABSTRACT: Using the first-principles pseudopotential–local-density-approximation approach, we have studied the dissociative molecular adsorption of NH3 on Si(001)-(2×1). We find that upon adsorption the Si dimer becomes symmetric and somewhat elongated. We also find that the Si-N bond is inclined at 10° with respect to the surface normal, the N-Si bond length is 1.75 Å, and the N-H bond length is 1.05 Å. Finally, our electronic structure calculations suggest that the adsorption of ammonia almost completely passivates the silicon surface.
Phys. Rev. B. 09/1998; 58(12).
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ABSTRACT: In this work, we report three-dimensional Monte Carlo simulations of a polydisperse magnetic fluid sample based on magnetite nanoparticles surface coated with dodecanoic acid dispersed in hydrocarbon. Monodisperse simulations are also performed and indicate that polydispersity is a key issue. Our simulations are consistent with both static magnetic birefringence and magnetic resonance measurements for a particle volume fraction smaller than 4%.
Journal of Magnetism and Magnetic Materials 293(1):553-558. · 1.78 Impact Factor
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ABSTRACT: The adsorption of ethylene oxide (oxirane) and diethylene dioxide (1,4-dioxane) on the Si(0 0 1) surface was investigated using the first-principles pseudopotential method within a generalised gradient approximation to the density functional theory. Our results indicate that oxirane adsorption on the silicon surface probably occurs via C–C bond. This interaction induces the breaking of the C–C bond and the formation of a Si–C–O–C–Si ring. The 1,4-dioxane interaction with the silicon (2 × 2) surface, on the other hand, results in the decomposition of the considered molecule in different radicals, depending on the original adsorbed structure. In order to disregard the possible influence of the slab considered, we suggest that high order reconstruction surfaces, i.e. (4 × 2) or (4 × 4), should be investigated.
Surface Science. 601(13):2576-2579.
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ABSTRACT: Based on a first-principles pseudopotential calculation, within the local density approximation, we have studied a series of different structural models for the In-rich InAs(0 0 1) surface, including a very recent model proposed by Kumpf et al. based in X-ray diffraction experiments. Our data suggests that the ζ(4 × 2) model corresponds to the most probable structure, from the energetic point of view. The stability of the ζ(4 × 2) structure is mainly attributed to electrostatic and structural characteristics of the surface. The atomic and electronic structure of this system is analysed in detail. Experimental scanning tunnelling microscopy images are reinterpreted in the lights of our theoretical simulated images for the proposed structure, with a view to explain contradicting experimental and theoretical results.
Surface Science. 542:101-111.
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ABSTRACT: In this work we have performed an ab initio theoretical study of the In-covered GaAs(001) surface. Our total-energy calculations indicate that for an In “rich” environment, with a coverage of 0.75 monolayer, the ζ(4×2) model represents the energetically most stable structure. The β2(4×2) model is dismissed based upon energetic analysis and theoretical scanning tunneling microscopy (STM) images. It is argued that the α model is not expected to be stabilized, in contrast to the interpretation of recent STM images.
Phys. Rev. B. 67(4).
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R. Miotto
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ABSTRACT: In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density-functional theory, to investigate the adsorption process of acetonitrile on the silicon surface. Our first-principles calculations indicate that CH3CN adsorb via a [2+2] cycloaddition reaction through the CN group (di-σCN model) with an adsorption energy around 37 kcal∕mol. Although thedi-σCN model is found to be the most probable adsorbed structure from the energetic point of view, we have also found that the adsorption via the Si-N dative bond (Si-Ndat model) is also possible. Based in our energetic analysis, and in the spectroscopy data by Bournel and co-workers, we suggest the existence of a mixed domain surface composed by both the di-σCN and Si-Ndat adsorbates, although the existence of the latter structure is disregarded by the core level shift analysis by Tao and co-workers. Possible reasons for the contrast between available data are addressed. We present theoretical scanning tunneling microscopy images of the possible adsorbed systems and suggest that this experimental procedure could possibly identify the existence of the proposed mixed domain structure. In addition, the electronic structure and surface states for both the di-σCN and Si-Ndat models are discussed in detail.
Phys. Rev. B. 69(23).
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R. Miotto
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ABSTRACT: Using a first-principles pseudopotential method we have studied the interaction processes involved in the adsorption of ammonia on the germanium surfaces. We have found that ammonia adsorbs molecularly on the germanium surface and, in contrast to the interaction of the same species with the silicon surface, no dissociation takes place. The adsorption process is barrierless and the adsorption energy for the first NH3 molecule is 19.6 kcal/mol. A second ammonia molecule will be adsorbed in a (2×2) periodicity, i.e. following the flip of the dimer tilt direction of every second dimer row, with an adsorption energy of 12 kcal/mol. This suggests that there is most certainly a critical coverage around 1/4 ML (monolayer) after which the adsorption rate decreases, but nothing forbids the completion of 1/2 ML. The most probable structure is characterized by asymmetric and elongated Ge-Ge dimers, and by an adsorbed NH3 specie with structural characteristics very close to the free molecule. Our calculated vibrational spectra further support the molecular adsorption model for the NH3 interaction with the germanium surface.
Phys. Rev. B. 68(11).
