Periodic field-dependent photocurrent from a tungsten field emitter

We report the observation of a periodic electric-field-dependent component in the photo-current from a tungsten field emitter illuminated by the focused beam of a krypton ion laser tuned to a photon energy of ~3.51 eV. Over the field range we have investigated, the phase φ of the oscillations in the photocurrent can be related to the electric field strength F by the power law φ ∝ F^α, where $\text{α ≈ ?}\text{1}\text{2}$. In this preliminary study, we describe the frequency, amplitude, and harmonic structure of oscillations detected with the probe hole centred over the (510) region of the field emission pattern. A Fowler-Nordheim plot of the photocurrent is presented, and the nonoscillatory component is analysed on the basis of a simple theory of barrier penetration. The origin of the oscillatory component of the photocurrent is discussed.     

Sensitive detection of hydroxymethylcytosine levels in normal and neoplastic cells and tissues

A new sensitive analytical method using capillary electrophoresis with laser induced fluorescence (CE‐LIF) was applied for the simultaneous detection of DNA methylation and hydroxymethylation levels in human cancers of different origin. DNA hydroxymethylation, measured as 5‐hydroxymethylcytosine (5hmC) levels, was decreased in gliomas with mutation in the isocitrate dehydrogenase 1 (IDH1) gene when compared to IDH1‐wildtype gliomas. Independent from IDH1 mutation, 5hmC levels were decreased in lung carcinomas when compared to normal lung tissue. Reduced DNA hydroxymethylation was also observed upon dedifferentiation in cultured murine embryonic fibroblasts. Our data show that reduced DNA hydroxymethylation is related to cellular dedifferentiation and can be detected in various types of cancers, independent from the IDH1 mutation status. Quantitative determination of altered 5hmC levels may therefore have potential as a biomarker linked to cellular differentiation and tumorigenesis.     

UHV STM I(V) and XPS studies of aryl diazonium molecules assembled on Si(111)

Molecular layers formed from 4-trifluoromethylbenzenediazonium tetrafluoroborate and 4-Methylbenzenediazonium tetrafluoroborate have been assembled on H-passivated Si(111) and studied by UHV STM and XPS. STM imaging shows well-developed Si(111) step edges and terraces both on Si(111):H and Si(111) substrates covered with a molecular layer. STM I(V) data acquired at different tip-substrate separations reveals a factor of approximately 10 enhancement in current for positive bias voltage when current flows through the 4-trifluoromethyl molecule when compared to the 4-methyl variant. The observed current enhancement in I(V) can be understood by comparing the projected density of states of the two molecule-Si systems calculated using a density functional theory local density approximation after geometry optimization was performed via the conjugate gradient method. XPS data independently confirm that H-passivated Si(111) remains oxygen free for short exposures to ambient conditions and provide evidence that the molecules chemically react with the silicon surface.     

Quantum interference study of electron lifetimes in magnesium

The results of an experiment at 4.2 K using the magnetoresistance oscillations caused by the interferometer trajectories in magnesium are reported. The magnesium crystal used in this experiment is believed to have an impurity concentration of ～1 ppm. The quantum state lifetime of electron states traversing the interferometer was determined to be .01±.002 nsec. In addition, the values of the three magnetic breakdown parameters needed to describe the size of the spin orbit energy gaps occurring near the AHL zone plane in magnesium were found to be:H ₁=2.90±.1KG,H ₂=12.5±.5KG,H ₃=4.25±.25KG. The presence of higher harmonics was also observed in the magnetoresistance oscillations and the higher harmonics were used to allow an experimentally meaningful value ofH ₃ to be determined. It is presently believed that the sensitivity of the interferometer to quantum state lifetimes can be extended by using the higher harmonics and hence information about electron-phonon and possibly electron-electron interactions will be obtained.     

Label-free detection of a bacterial pathogen using an immobilized siderophore, deferoxamine

Pathogenic bacteria obtain the iron necessary for survival by releasing an iron chelator, termed a siderophore, and retrieving the iron-siderophore complex via a cell surface siderophore receptor. We have exploited the high affinity of Yersinia enterocolitica for its siderophore, deferoxamine, to develop a rapid method for capture and identification of Yersinia. In this methodology, a deferoxamine-bovine serum albumin conjugate is printed onto a gold-plated chip in a parallel line pattern. After flowing a suspension of Yersinia across the siderophore-derivatized chip, any Yersinia that binds to the chip is detected by dark-field microscopy analysis of the scattered light, followed by Fourier transform analysis of the scattering pattern. Since peak intensities are found to correlate with pathogen concentration, pathogen titers as low as 10(3) cfu/ml can be readily detected. Moreover, immobilized deferoxamine can distinguish Y. enterocolitica, which binds ferrioxamine (deferoxamine-Fe), from Staphylococcus aureus, Mycobacterium smegmatis and Pseudomonas aeruginosa, which don't. Because human pathogens cannot easily mutate their iron retrieval systems without loss of viability, we suggest that few if any mutant Yersinia will emerge that can avoid detection. Together with previous results demonstrating selective capture of Pseudomonas aeruginosa by its immobilized siderophore (pyoverdin), these data suggest that pathogen-specific siderophores may constitute effective and immutable capture ligands for rapid detection and identification of their cognate pathogens.     

Folding and cracking of graphene oxide sheets upon deposition

Graphene Oxide (GO) sheets, suspended in an aqueous solution, were deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG) and studied using Raman spectroscopy, atomic force microscopy (AFM) and scanning tunneling microscopy (STM). AFM phase imaging shows a distinct contrast between GO and the underlying HOPG substrate. Raman spectroscopy clearly showed the presence of GO sheets on the top of HOPG substrate. The AFM and STM images also reveal wrinkling, folding, and tearing of individual GO sheets after depositing onto an HOPG substrate. We have also observed a distinct cracking of a GO sheet after folding. We attribute this new cracking phenomenon to a weakening of C–C bonds during the oxidation of a graphene sheet.     

Numerical transmission probabilities and the oscillatory photo-induced field emission current: Static and dynamic image charge effects

Numerical calculations are reported for the transmission probability of electrons incident upon a model potential barrier typically used in discussing electron emission from metal surfaces. These calculations are utilized in an attempt to explain the unexpected oscillatory photocurrent which was observed in a previous study of the electric field dependence of the photoinduced field emission current. Transmission resonances are observed in these calculations and a simple theory is described which adequately accounts for the electric field strengths at which these resonances occur. An important result of these calculations is that the model potential barrier given by $\text{V(x) = ?eFx ? (}\text{1}\text{4π?}{}_0\text{)(}\text{e}{}^2\text{4x}\text{)}$ cannot explain the oscillatory behavior of the photo-field current. Arguments are presented which question the static nature of the image charge potential employed in this simple model and a modification is introduced which incorporates in a very approximate fashion the time dependence involved in the formation of the image charge potential. Calculations based on this model correctly predict the periodicity of the observed oscillations and give a frequency which is in reasonable agreement with the measured value.