Farzaneh Shayeganfar

Electronic Properties of Self-Assembled Trimesic Acid Monolayer on Graphene

The adsorption of trimesic acid (TMA) on a graphene surface has been studied with density functional theory. By considering the adsorption of a single TMA molecule on different sites on graphene, we have been able to perform a detailed analysis of the equilibrium geometry, charge transfer, electronic properties in terms of density of states and band structure, and finally scanning tunneling microscopy simulations on those simple systems. The results for isolated adsorption were then compared to the behavior of the TMA unit within two different self-assembled monolayers. Our results indicate that structural deformations of TMA may significantly contribute to the magnitude of p-doping and band gap opening in graphene. The formation of a hydrogen bonding network within the assembly improves the stability of the adlayer, but its adhesion on graphene is significantly reduced. The magnitude of p-doping in graphene per TMA unit remains nearly constant from the isolated to the assembled systems, but the magnitude of the band gap opening appears to be strongly correlated with the breaking of symmetry of π-states of graphene by the TMA patterning on the surface. Our results suggest that polymorphism in self-assembled adlayers could be used to tune and control the electronic properties of graphene.

Van der Waals energy surface of a carbon nanotube sheet

Abstract The van der Walls interaction between a carbon nanotube sheet (CNTS) and a rare gas atom, is studied using both atomistic and continuum approaches. We present analytical expressions for the van der Waals energy of continuous nanotubes interacting with a rare gas atom. It is found that the continuum approach does not properly treat the effect of atomistic configurations on the energy surfaces. The energy barriers are small as compared to the thermal energy, which implies the free motion above the CNTS in heights about one nanometer. In contrast to the energy surface of a graphene sheet, the honeycomb lattice structure in the energy surface of a CNTS is imperceivable. Defects alter the energy surface which therefore influence the gas adsorption mechanism.

The level crossing and inverse statistic analysis of German stock market index (DAX) and daily oil price time series

The level crossing and inverse statistics analysis of DAX and oil price time series are given. We determine the average frequency of positive-slope crossings, να+, where Tα=1/να+ is the average waiting time for observing the level α again. We estimate the probability P(K,α), which provides us the probability of observing K times of the level α with positive slope, in time scale Tα. For analyzed time series, we found that maximum K is about ≈6. We show that by using the level crossing analysis one can estimate how the DAX and oil time series will develop. We carry out the same analysis for the increments of DAX and oil price log-returns (which is known as inverse statistics), and provide the distribution of waiting times to observe some level for the increments.

Molecular dynamics simulation of formation and growth of CdS nanoparticles

Monodispersed semiconducting nanoparticles are usually synthesised in a liquid medium using injection of an appropriate solution. A key factor in attaining a narrow particle size distribution (PSD) is the temporal separation of the nucleation and growth stages, where the former takes place during the injection. Faster injection produces a larger number of nuclei and a narrower PSD. The injection speed is expected to affect the diffusion of the ions in the solution and to create uniformly high supersaturation for a short period of time. In this paper, we study the growth of CdS nanoparticles during the injection by molecular dynamics simulation. A solution of Cd ions is injected into the simulation cell that contains sulphure ions; the variation of the PSD and its mean and variance are studied as functions of the injection velocity. Higher injection velocities produce narrower PSDs and smaller particles, hence providing a precise method for controlling both.

Electronic Properties of Adsorption of Trimesic Acid Monomer on Graphene

A large scale approach based on scanning tunneling microscopy STM simulation images and ab initio computation is used to report the adsorbed trimesic acids (TMA) monomer and monolayer behaviors on graphene. Our results unravel the interplay between the adsorption energy and band gap opening for different adsorption sites of TMA/graphene. It has been revealed that regarding to stability of different adsorption site and interaction with π electron containing surfaces (graphene), the high adsorption energy can induce maximum band gap in the system and open up possible uses of graphene in electronic device applications. A vital role on carboxyl functional groups is related to duality behaviors of combined acceptor and donor character with regard to hydrogen bonds which provides stable intermolecular self-assembly1 and monolayers (for detailed explanation and analysis see reference [1]).