Nitrogen and phosphorus co-doped graphene quantum dots: synthesis from adenosine triphosphate, optical properties, and cellular imaging.

Graphene quantum dots (GQDs) are emerging zero-dimensional materials promising a wide spectrum of applications, particularly, as superior fluorescent reporters for bio-imaging and optical sensing. Heteroatom doping can endow GQDs with new or improved photoluminescence properties. Here, we demonstrate a simple strategy for the synthesis of nitrogen and phosphorus co-doped GQDs from a single biomolecule precursor (adenosine triphosphate - ATP). Such ATP-GQDs exhibit high fluorescence quantum yield, strong two-photon upconversion, small molecular weight, high photostability, and good biocompatibility. Furthermore, transferrin conjugated ATP-GQDs have been used for imaging and real-time tracking of transferrin receptors in live cells.

Band-gap manipulations of monolayer graphene by phenyl radical adsorptions: a density functional theory study.

Phenyl radical (Ph˙) adsorption on monolayer graphene sheets is used to investigate the band-gap manipulation of graphene through density functional theory. Adsorption of a single Ph˙ on graphene breaks the aromatic π-bond and generates an unpaired electron, which is delocalized to the ortho or para position. Adsorption of a second radical at the ortho or para position saturates the radical by electron pairing and results in semiconducting graphene. Adsorption of a second radical at the ortho position (ortho-ortho pairing) is found to be more favorable than adsorption at the para position (ortho-para pairing), and the ortho-ortho pairing has stronger effects on band-gap opening compared with ortho-para pairing. Adsorption of even numbers of Ph˙ on graphene by ortho-ortho and ortho-para pairings, in general, increases the band gap. Our study shows promise of band-gap manipulation in monolayer graphene by Ph˙ adsorption, leading to potential wider applications of graphene.

Scalable and effective enrichment of semiconducting single-walled carbon nanotubes by a dual selective naphthalene-based azo dispersant.

Semiconducting single-walled carbon nanotubes (s-SWNTs) have emerged as a promising class of electronic materials, but the metallic (m)-SWNTs present in all as-synthesized nanotube samples must be removed for many applications. A high selectivity and high yield separation method has remained elusive. A separation process based on selective chemistry appears to be an attractive route since it is usually relatively simple, but more effective chemicals are needed. Here we demonstrate the first example of a new class of dual selective compounds based on polycyclic aromatic azo compounds, specifically Direct Blue 71 (I), for high-purity separation of s-SWNTs at high yield. Highly enriched (~93% purity) s-SWNTs are produced through the simple process of standing arc-discharge SWNTs with I followed by centrifugation. The s-SWNTs total yield is up to 41%, the highest yet reported for a solution-based separation technique that demonstrates applicability in actual transistors. 91% of transistor devices fabricated with these s-SWNTs exhibited on/off ratios of 10(3) to 10(5) with the best devices showing mobility as high as 21.8 cm(2)/V s with on/off ratio of 10(4). Raman and X-ray photoelectron spectroscopic shifts and ultraviolet-visible-near-infrared (UV-vis-NIR) show that I preferentially complexes with s-SWNTs and preferentially suspends them. Preferential reaction of naphthyl radicals (generated from I with ultrasonication) with m-SWNTs is confirmed by changes in the D-band in the Raman spectroscopy, matrix-assisted desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and molecular simulation results. The high selectivity of I stems from its unique dual action as both a selective dispersion agent and the generator of radicals which preferentially attack unwanted metallic species.

Modulating the electronic properties of germanium nanowires via applied strain and surface passivation.

We report a systematic study on the surface passivation and strain effects on the electronic properties of hydrogenated germanium nanowires (H-GeNWs) with different growth orientations and diameters using density functional theory calculations. We show that increasing the coverage percentage of halogen passivations--fluorine (F) and chlorine (Cl) in particular--reduces the band gap of the GeNWs drastically but not linearly, depending on the chemical environment of the passivation sites. Moreover, we find that in general, applying strain--either compression or tensile--can only induce a decreased band gap in GeNWs but exception is found in <110> GeNWs: an increased band gap can be induced which is determined to be related to their surface structures. The current work reveals that electronic response upon structural changes caused by external factors is more sensitive in <110> GeNWs than in <100> GeNWs, suggesting that GeNWs with selected growth orientation can be applied in specialized applications that require different degrees of sensitivity or robustness.

Stress induced half-metallicity in surface defected germanium nanowires.

Germanium nanowires (GeNWs) with single, double, quadruple and octuple surface dangling bonds (SDBs) are investigated using density-functional-theory calculations. We show that single SDB defected GeNWs remain semiconducting as their non-defected form while double or multiple SDB defects result in either semiconducting or metallic GeNWs, depending on the defect's locations on the surface. More importantly, we show that the electronic properties of surface defected GeNWs can also be fine-tuned by applying tensile and compressive strains. Upon the right loading, the surface defected GeNWs become half-metallic. In addition, we determine that the surface defected GeNWs can be classified into three classes: (1) GeNWs with zero magnetic moment, which are either metallic or semiconducting; (2) GeNWs with net magnetic moments equal to the number of SDBs, which are semiconducting with distinct spin-up and spin-down configurations; and (3) GeNWs with net magnetic moments significantly lower than the number of SDBs. We also find that only the defected GeNWs that fall under (3) are potentially half-metallic. Our results predict that half-metallic GeNWs can be obtained via engineering of the surface defects and the structures without the presence of impurity dopants.

Water induced electrical hysteresis in germanium nanowires: a theoretical study.

We apply DFT calculations to evaluate the electronic properties of germanium nanowires (GeNWs) upon adsorption of water molecules and reveal the possible causes of the experimentally observed electrical hysteresis in GeNWs based electronic devices. We show that the absorption of water molecules on the GeNW surface would lead to the formation of hydroxyl passivated GeNWs (OH-GeNWs). The first step of the formation mechanism is physisorption of water molecules toward a Ge atom then followed by dissociation of water molecules to form OH-GeNWs, consistent with experimental observation of reversible and irreversible electrical hystereses. More importantly, we also predict that the effective masses of OH-GeNWs depend strongly on their growth orientation and depend nonlinearly on the OH coverage percentage. We propose that the electrical hysteresis phenomenon observed in experiment can be attributed to the formation of OH-GeNWs with different OH coverage percentages, along with different alignments of the OH groups on the GeNW surface, and also the presence of surface trap state defects, during the different stages of I-V measurement.