Thermal stability study of nitrogen functionalities in a graphene network

Catalyst-free vertically aligned graphene nanoflakes possessing a large amount of high density edge planes were functionalized using nitrogen species in a low energy N+ ion bombardment process to achieve pyridinic, cyanide and nitrogen substitution in hexagonal graphitic coordinated units. The evolution of the electronic structure of the functionalized graphene nanoflakes over the temperature range 20-800^{\circ}C was investigated in situ, using high resolution x-ray photoemission spectroscopy. We demonstrate that low energy irradiation is a useful tool for achieving nitrogen doping levels up to 9.6 at.%. Pyridinic configurations are found to be predominant at room temperature, while at 800^{\circ}C graphitic nitrogen configurations become the dominant ones. The findings have helped to provide an understanding of the thermal stability of nitrogen functionalities in graphene, and offer prospects for controllable tuning of nitrogen doping in device applications.Comment: Corresponding author: [email protected]

Sensitive Chronocoulometric Detection of miRNA at Screen printed Electrodes modified by gold decorated MoS2 Nanosheets

Here a new chronocoulometric sensor, based on semiconducting 2H MoS2 nanosheets decorated with a controlled density of monodispersed small gold nanoparticles, was fabricated via electrodeposition, for the highly sensitive detection of miRNA-21. The size and interparticle spacing of AuNPs was optimized by controlling nucleation and growth rates through tuning of deposition-potential and Au-precursor concentration and by getting simultaneous feedback from morphological and electrochemical activity studies. The sensing strategy, involved the selective immobilization of thiolated capture probe DNA (CP) at AuNPs and hybridization of CP to a part of miRNA target, whereas the remaining part of the target was complementary to a signaling non-labelled DNA sequence. Chronocoulometry provided precise quantification of nucleic acids at each step of the sensor assay by interrogating [Ru(NH3)6]3+ electrostatically bound to phosphate backbones of oligonucleotides. A detailed and systematic optimization study demonstrated that the thinnest and smallest MoS2 NSs improved the sensitivity of the AuNP@MoS2 sensor achieving an impressive detection limit of 100 aM, which is 2 orders of magnitude lower than that of bare Au electrode and also enhanced the DNA-miRNA hybridization efficiency by 25%. Such improved performance can be attributed to the controlled packing density of CPs achieved by their self -assembly on AuNPs, large interparticle density, small size and the intimate coupling between AuNPs and MoS2. Alongside the outstanding sensitivity, the sensor exhibited excellent selectivity down to femtomolar concentrations, for discriminating complementary miRNA-21 target in a complex system composed of different foreign targets including mismatched and non-complementary miRNA-155. These advantages make our sensor a promising contender in the point of care miRNA sensor family for medical diagnostics.Comment: Pagona Papakonstantinou corresponding author ([email protected]

Anomalous blueshift in emission spectra of ZnO nanorods with sizes beyond quantum confinement regime

[[abstract]]Cathodoluminescence (CL) spectroscopy has been employed to study the electronic and optical properties of well-aligned ZnO nanorods with diameters ranging from 50âtoâ180ânm. Single-nanorod CL studies reveal that the emission peak moves toward higher energy as the diameter of the ZnO nanorod decreases, despite that their sizes are far beyond the quantum confinement regime. Blueshift of several tens of meV in the CL peak of these nanorods has been observed. Moreover, this anomalous energy shift shows a linear relation with the inverse of the rod diameter. Possible existence of a surface resonance band is suggested and an empirical formula for this surface effect is proposed to explain the size dependence of the CL data.[[notice]]補正完畢[[countrycodes]]US

Unusual Structure and Magnetism in MnO Nanoclusters

We report an unusual evolution of structure and magnetism in stoichiometric MnO clusters based on an extensive and unbiased search through the potential energy surface within density functional theory. The smaller clusters, containing up to five MnO units, adopt two-dimensional structures; and regardless of the size of the cluster, magnetic coupling is found to be antiferromagnetic in contrast to previous theoretical findings. Predicted structure and magnetism are strikingly different from the magnetic core of Mn-based molecular magnets, whereas they were previously argued to be similar. Both of these features are explained through the inherent electronic structures of the clusters.Comment: 4 pages, 3 figure

Organic Solvent Based Synthesis of Gold Nanoparticle - Semiconducting 2H-MoS2 Hybrid Nanosheets

The development of simple, versatile strategies for the synthesis of gold nanoparticles (AuNPs) on semiconducting transition metal dichalcogenides (TMDC) layers is of increasing scientific and technological interest in photocatalysis, optical sensing, and optoelectronics sectors, but challenges exist on the nucleation and hybridization of AuNPs with the TMDC basal plane. At present, the widely used aqueous solution approaches suffer from poor dispersion of produced hybrids as well as from limited growth and coverage of the AuNPs on the TMDC semiconducting plane, since Au nanoclusters nucleate preferentially at the electron rich defect edges, which act as reducing agents and not on the defect free basal plane. Here, we report for the first time, the controlled synthesis of AuNPs on the basal plane of semiconducting molybdenum disulfide nanosheets (2H-MoS2 NSs) via a N,N-dimethylformamide (DMF)-based hot-injection synthesis route. This organic solvent-based synthesis route eliminates problems of poor dispersion of AuNPs@2H-MoS2 NS hybrids, whereas at the same time maintains the semiconducting crystalline quality of the pristine 2H-MoS2 NSs. In addition, the study establishes the important role of trisodium citrate, on enhancing the nucleation and improving the hybridization of AuNPs on 2H-MoS2 NSs as evidenced by the induced p-type doping. This organic solvent synthesis approach can be adopted for other hybrid systems opening the way for controlled hybridization of semiconducting layers with metal nanoparticles.Comment: corresponding author e-mail: [email protected] accepted at The Journal of Physical Chemistry