Angelo Bongiorno

Oligonucleotide-stabilized Ag nanocluster fluorophores.

Single-stranded oligonucleotides stabilize highly fluorescent Ag nanoclusters, with emission colors tunable via DNA sequence. We utilized DNA microarrays to optimize these scaffold sequences for creating nearly spectrally pure Ag nanocluster fluorophores that are highly photostable and exhibit great buffer stability. Five different nanocluster emitters have been created with tunable emission from the blue to the near-IR and excellent photophysical properties. Ensemble and single molecule fluorescence studies show that oligonucleotide encapsulated Ag nanoclusters exhibit significantly greater photostability and higher emission rates than commonly used cyanine dyes.

Room-temperature metastability of multilayer graphene oxide films.

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.

Origin of the Chemical and Kinetic Stability of Graphene Oxide

At moderate temperatures (≤ 70°C), thermal reduction of graphene oxide is inefficient and after its synthesis the material enters in a metastable state. Here, first-principles and statistical calculations are used to investigate both the low-temperature processes leading to decomposition of graphene oxide and the role of ageing on the structure and stability of this material. Our study shows that the key factor underlying the stability of graphene oxide is the tendency of the oxygen functionalities to agglomerate and form highly oxidized domains surrounded by areas of pristine graphene. Within the agglomerates of functional groups, the primary decomposition reactions are hindered by both geometrical and energetic factors. The number of reacting sites is reduced by the occurrence of local order in the oxidized domains, and due to the close packing of the oxygen functionalities, the decomposition reactions become – on average – endothermic by more than 0.6 eV.

Atomistic models of the Si(100)–SiO2 interface: structural, electronic and dielectric properties

We review the structural, electronic and dielectric properties of atomistic models of the Si(100)-SiO2 interface, which have been purposely designed in order to match a large variety of atomic-scale experimental data. After describing the generation procedure and the structural properties of two specific interface models, we study the corresponding electronic structure and dielectric response within the framework of density-functional theory. Particular emphasis is given to a systematic comparison between the atormstic properties of our model interfaces and experiment. Besides. synthesizing the present status of our experimental knowledge on the Si(100)-SiO2 interface, these models provide a solid and necessary basis for future investigations in the area of gate stacks for Si-based microelectronics.

Equivalent oxide thickness of a thin oxide interlayer in gate insulator stacks on silicon

We investigate the equivalent oxide thickness of a thin oxide interlayer in gate insulator stacks on silicon. Through the use of a first-principles approach, we map the profile of the local permittivity across two interface models showing different suboxide structures. These models incorporate the available atomic-scale experimental data and account for the amorphous nature of the oxide. The equivalent oxide thickness of the interfacial oxide layer is found to be smaller than the corresponding physical thickness by 0.2-0.3 nm. We discuss implications of these results for future device scaling. (c) 2005 American Institute of Physics.

Thermodynamic stability limits of simple monoatomic materials.

This computational study addresses the thermodynamical stability of superheated crystals. Molecular dynamics simulations are employed to derive the caloric curves of the solid and liquid phases of a material. Caloric curves are used to derive thermodynamic state functions, the parameters of the equilibrium melting phase transition, and the regions of thermodynamical stability of the liquid and solid phases. Molecular dynamics trajectories are also analyzed to gain insight on the mechanisms leading to the instability of the homogeneous superheated solid phase. This study shows that in simple and homogeneous solids the configurational entropy is not zero and that its excitations can occur without disrupting the crystallinity of the lattice. The superheating and supercooling limits of the solid and liquid phases are found to correspond to states of equal entropy and enthalpy.

Film Structure of Epitaxial Graphene Oxide on SiC: Insight on the Relationship between Interlayer Spacing, Water Content, and Intralayer Structure

Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter-layer structure of these ultra-thin oxide films. X-ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter-layer registry, little amount (<10%) of intercalated water, and unexpectedly large interlayer separations of about 9.35 A. Density functional theory calculations show that the apparent contradiction of “little water but large interlayer spacing in the graphene oxide films” can be explained by considering a multilayer film formed by carbon layers presenting, at the nanoscale, a non-homogenous oxidation, where non-oxidized and highly oxidized nano-domains coexist and where a few water molecules trapped between oxidized regions of the stacked layers are sufficient to account for the observed large inter-layer separations. This work sheds light on both the vertical and intra-layer structure of graphene oxide films grown on silicon carbide, and more in general, it provides novel insight on the relationship between inter-layer spacing, water content, and structure of graphene/graphite oxide materials.

Atomic-scale modelling of kinetic processes occurring during silicon oxidation

We model the fundamental kinetic processes occurring during silicon oxidation at the atomic scale. We first focus on the diffusion of the, neutral O-2 molecule through the oxide layer. By combining ab initio and classical simulations, we derive a statistical description for the O-2 potential energy landscape in the oxide. Statistical distributions are then mapped onto lattice models to investigate the O-2 diffusive process in the bulk oxide and across an oxide layer at the Si(l 00)-SiO2 interface. We find that the diffusion of O-2 is a percolative process, critically influenced by both energetical and geometrical features of the potential energy landscape. At the interface, the occurrence of a thin densified oxide layer in contact with the substrate limits percolative phenomena and causes the O-2 diffusion rate to drop below its value for ordinary amorphous SiO2. Then, we use first-principles calculations to address the kinetic processes occurring in the proximity of the Si(100)-SiO2 interface. We first focus on the energetics of negatively charged oxygen species in the oxide, and on the diffusive and dissociative properties of the charged molecular species. We find that negatively charged oxygen species incorporate in the oxide at Si sites, giving rise to additional Si-O bonds and important network distortions. Finally, we focus on the oxidation reaction at the Si(100)-SiO2 interface. We find that the O-2 oxidation reaction occurs by crossing small energy barriers, regardless of the spin or charge state of the molecular species. Our findings are consistent with kinetics pictures of the silicon oxidation process entirely based on diffusive phenomena.

First-principles study of hydrogen permeation in palladium-gold alloys

Density functional theory and lattice model calculations are combined to study the permeability of hydrogen in Pd lightly alloyed with Au. This study shows that small amounts of Au substitutions in Pd leads to, respectively, an increase and decrease of the diffusivity and solubility of hydrogen in the alloy. The competition between these two phenomena depends on temperature and can yield dilute PdAu membranes with a hydrogen permeability higher than pure Pd.

Electronic Structure at Realistic Si(100)-SiO2 Interfaces

We study the interfacial electronic properties of a model Si-SiO2-Si structure which is intended to simulate the substrate-oxide-polysilicon stack in metal-oxide-semiconductor devices. The structural properties of this model are shown to match closely those of the real system, as determined by a number of atomic-scale experimental probes. In particular, the model introduced here takes into account the disordered reconstruction at the Si-SiO2 interface, and the amorphous nature of the 2 nm thick oxide. For this model, we calculate the local valence and conduction band edges within the framework of density functional theory, and show that the width of the electronic structure transition between the silicon substrate and the oxide is about 8-9 Angstrom. Finally, we calculate the typical decay length of silicon-induced gap states and find a value of 1.2 Angstrom for states lying close to the Si band edges. This result is in agreement with experimental measurements of the leakage current through ultrathin gate oxides.