856 research outputs found
Modification of Graphene Properties due to Electron-Beam Irradiation
The authors report micro-Raman investigation of changes in the single and
bilayer graphene crystal lattice induced by the low and medium energy
electron-beam irradiation (5 and 20 keV). It was found that the radiation
exposures results in appearance of the strong disorder D band around 1345 1/cm
indicating damage to the lattice. The D and G peak evolution with the
increasing radiation dose follows the amorphization trajectory, which suggests
graphene's transformation to the nanocrystalline, and then to amorphous form.
The results have important implications for graphene characterization and
device fabrication, which rely on the electron microscopy and focused ion beam
processing.Comment: 13 pages and 4 figure
Ultraviolet Raman Spectroscopy of Single and Multi-layer Graphene
We investigated Raman spectra of single-layer and multi-layer graphene under
ultraviolet laser excitation at the wavelength of 325 nm. It was found that
while the G peak of graphene remains pronounced in UV Raman spectra, the 2D
band intensity undergoes severe quenching. The evolution of the ratio of the
intensities of the G and 2D peaks, I(G)/I(2D), as the number of graphene layers
n changes from n=1 to n=5, is different in UV Raman spectra from that in
conventional visible Raman spectra excited at the 488 nm and 633 nm
wavelengths. The 2D band under UV excitation shifts to larger wave numbers and
is found near 2825 1/cm. The observed UV Raman features of graphene were
explained by invoking the resonant scattering model. The obtained results
contribute to the Raman nanometrology of graphene by providing an additional
metric for determining the number of graphene layers and assessing its quality.Comment: 18 pages; 5 figures; submitted for publication on February 20, 200
Excitonic properties of strained wurtzite and zinc-blende GaN/Al(x)Ga(1-x)N quantum dots
We investigate exciton states theoretically in strained GaN/AlN quantum dots
with wurtzite (WZ) and zinc-blende (ZB) crystal structures, as well as strained
WZ GaN/AlGaN quantum dots. We show that the strain field significantly modifies
the conduction and valence band edges of GaN quantum dots. The piezoelectric
field is found to govern excitonic properties of WZ GaN/AlN quantum dots, while
it has a smaller effect on WZ GaN/AlGaN, and very little effect on ZB GaN/AlN
quantum dots. As a result, the exciton ground state energy in WZ GaN/AlN
quantum dots, with heights larger than 3 nm, exhibits a red shift with respect
to the bulk WZ GaN energy gap. The radiative decay time of the red-shifted
transitions is large and increases almost exponentially from 6.6 ns for quantum
dots with height 3 nm to 1100 ns for the quantum dots with height 4.5 nm. In WZ
GaN/AlGaN quantum dots, both the radiative decay time and its increase with
quantum dot height are smaller than those in WZ GaN/AlN quantum dots. On the
other hand, the radiative decay time in ZB GaN/AlN quantum dots is of the order
of 0.3 ns, and is almost independent of the quantum dot height. Our results are
in good agreement with available experimental data and can be used to optimize
GaN quantum dot parameters for proposed optoelectronic applications.Comment: 18 pages, accepted for publication in the Journal of Applied Physic
Charm production nearby threshold in pA-interactions at 70 GeV
The results of the SERP-E-184 experiment at the U-70 accelerator (IHEP,
Protvino) are presented. Interactions of the 70 GeV proton beam with C, Si and
Pb targets were studied to detect decays of charmed , ,
, mesons and baryon near their production threshold.
Measurements of lifetimes and masses are shown a good agreement with PDG data.
The inclusive cross sections of charm production and their A-dependencies were
obtained. The yields of these particles are compared with the theoretical
predictions and the data of other experiments. The measured cross section of
the total open charm production ( = 7.1
2.3(stat) 1.4(syst) b/nucleon) at the collision c.m. energy
= 11.8 GeV is well above the QCD model predictions. The
contributions of different species of charmed particles to the total cross
section of the open charm production in proton-nucleus interactions vary with
energy.Comment: 4 pages, 6 pages, 38th International Conference on High Energy
Physics 3-10 August 2016, Chicago, US
Morphometric indicators of the precentral gyrus of the large brain in boys and girls
The morphology of the precentral gyrus is of particular interest because it is the origin of the pyramidal pathway responsible for voluntary movements. The aim of the study was to determine the width of the precentral gyrus, the thickness of the cerebral cortex in it and X-ray density of neurons in young boys and girls according to X-ray computed tomographic examination. Material and methods. 35 male and 37 female with mesocephalic sculls aged 18 to 20 years old with predominance of the right hand and no pathology of the nervous system organs in the anamnesis were examined. Results. The maximal value of the precentral gyrus width was found at the point located above the superior temporal gyrus. Values were lower (p < 0.01) at the point located at the level of the middle frontal gyrus. The smallest precentral gyrus width was found at a point above the cingulate gyrus (p < 0.01). When comparing the indices in each point, there is a tendency for them to be slightly more prevalent in the left hemisphere than in the right. There is a tendency to nonsignificant prevalence in boys compared to girls (p > 0.05). Parameters of cortical thickness in the precentral gyrus in the studied points did not differ statistically significantly, but there was a tendency to their insignificant predominance in the left hemisphere compared to the right, as well as in boys compared to girls (p > 0.05). Values of the X-ray neuron density in the compared points of the precentral gyrus in both hemispheres did not differ statistically significantly for boys and girls. Conclusions. The established results will serve as the initial data as a criterion of age-specific morphological norm in young mesocephals with predominance of the right hand, which is highly demanded in the personalized approach in medical practice. These results can also be used in prospective clinical and basic research
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