972 research outputs found
Dark states of single NV centers in diamond unraveled by single shot NMR
The nitrogen-vacancy (NV) center in diamond is supposed to be a building
block for quantum computing and nanometer scale metrology at ambient
conditions. Therefore, precise knowledge of its quantum states is crucial.
Here, we experimentally show that under usual operating conditions the NV
exists in an equilibrium of two charge states (70% in the expected negative
(NV-) and 30% in the neutral one (NV0)). Projective quantum non-demolition
measurement of the nitrogen nuclear spin enables the detection even of the
additional, optically inactive state. The nuclear spin can be coherently driven
also in NV0 (T1 ~ 90 ms and T2 ~ 6 micro-s).Comment: 4 pages, 3 figure
Work function of few layer graphene covered nickel thin films measured with Kelvin probe force microscopy
Few layer graphene and graphite are simultaneously grown on a similar to 100 nm thick polycrystalline nickel film. The work function of few layer graphene/Ni is found to be 4.15 eV with a variation of 50 meV by local measurements with Kelvin probe force microscopy. This value is lower than the work function of free standing graphene due to peculiar electronic structure resulting from metal 3d-carbon 2p(pi) hybridization. (C) 2016 AIP Publishing LLC
Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications
Based on the arrangement of two-dimensional 'melon', we construct a unit cell
for polymeric carbon nitride (PCN) synthesized via thermal polycondensation,
whose theoretical diffraction powder pattern includes all major features
measured in x-ray diffraction. With the help of this unit cell, we describe
the process-temperature-induced crystallographic changes in PCN that occur
within a temperature interval between 510 and 610â°C. We also discuss further
potential modifications of the unit cell for PCN. It is found that both
triazine- and heptazine-based g-C3N4 can only account for minor phases within
the investigated synthesis products
Charge Generation and Selective Separation at PbS Quantum Dot Metal Oxide Interfaces
Charge separation and transfer at the interface between layers of oleic acid capped PbS quantum dots QDs and Titanium and Indium Tin oxide TiO2 and ITO films were investigated by surface photovoltage SPV measurements. Photoluminescence PL measurements were performed in order to check for excitonic transitions and determine the QD band gaps. The QDs diameter of 4.2 nm and 5.0 nm were estimated by using the PL band gaps and the theoretical equation derived by Wang et al. [J. Chem. Phys. 87 1987 7315]. The SPV spectra of the PbS QDs TiO2 system reveal a positive charge on the PbS film surface and show three distinguished regions which demonstrate i the charge separation across QDs, ii the electron injection from QDs into TiO2 and iii the fundamental absorption in TiO2. The on set of the electron injection depends on the QD size QD band gap it shifts to lower photon energies for lower QD dimensions for higher QD band gaps . Thus, a better conduction band alignment is achieved in the latter case. In contrast to PbS QDs TiO2, the SPV spectra of the PbS QDs ITO structure reveal the negative charge on PbS surface. Moreover, the charge transfer at this interface is not observed. Instead, the SPV peculiarities in the photon energy range 1.4 3.0 eV point out to trapped holes on the ITO surface state
An optical quasimonomer
A comprehensive investigation of the luminescent properties of carbon nitride
polymers, based on tri-s-triazine units, has been conducted. Steady-state
temperature- and excitation-power-dependent as well as time-resolved
measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong
photoluminescence, covering the visible spectrum. The spectral, thermal, and
temporal features of the photoluminescence can be satisfactorily described by
the excitation and radiative recombination of molecular excitons, localized at
single tri-s-triazine units. The discussed model is in accordance with the
recently reported absorption features of carbon nitride polymers. Thus, from
the point of view of optical spectroscopy, the material effectively behaves as
a monomer
Formation of the physical vapor deposited CdS Cu In,Ga Se2 interface in highly efficient thin film solar cells
We report on the buffer absorber interface formation in highly efficient 14.5 , AM1.5 ZnO CdS Cu In,Ga Se2 solar cells with a physical vapor deposited CdS buffer. For Se decapped Cu In,Ga Se2 CIGSe absorbers we observe sulfur passivation of the CIGSe grain boundaries during CdS growth and at the interface a thermally stimulated formation of a region with a higher band gap than that of the absorber bulk, determining the height of the potential barrier at the CdS CIGSe interface. For air exposed CIGSe samples the grain boundary passivation is impeded by a native oxide adsorbate layer at the CIGSe surface determining the thermal stability of the potential barrier heigh
On mesogranulation, network formation and supergranulation
We present arguments which show that in all likelihood mesogranulation is not
a true scale of solar convection but the combination of the effects of both
highly energetic granules, which give birth to strong positive divergences
(SPDs) among which we find exploders, and averaging effects of data processing.
The important role played by SPDs in horizontal velocity fields appears in the
spectra of these fields where the scale 4 Mm is most energetic; we
illustrate the effect of averaging with a one-dimensional toy model which shows
how two independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are degraded.
The role of SPDs in the formation of the photospheric network is shown by
computing the advection of floating corks by the granular flow. The coincidence
of the network bright points distribution and that of the corks is remarkable.
We conclude with the possibility that supergranulation is not a proper scale of
convection but the result of a large-scale instability of the granular flow,
which manifests itself through a correlation of the flows generated by SPDs.Comment: 10 pages, 11 figures, to appear in Astronomy and Astrophysic
Regularly arranged indium islands on glass/molybdenum substrates upon femtosecond laser and physical vapor deposition processing
A bottom-up approach is presented for the production of arrays of indium
islands on a molybdenum layer on glass, which can serve as micro-sized
precursors for indium compounds such as copper-indium-gallium-diselenide used
in photovoltaics. Femtosecond laser ablation of glass and a subsequent
deposition of a molybdenumfilm or direct laser processing of the
molybdenumfilm both allow the preferential nucleation and growth of indium
islands at the predefined locations in a following indium-based physical vapor
deposition(PVD) process. A proper choice of laser and deposition parameters
ensures the controlled growth of indium islands exclusively at the laser
ablated spots. Based on a statistical analysis, these results are compared to
the non-structured molybdenumsurface, leading to randomly grown indium islands
after PVD
A complementary neutron and anomalous x-ray diffraction study
Distinguishing the scattering contributions of isoelectronic atomic species by
means of conventional x-ray- and/or electron diffraction techniques is a
difficult task. Such a problem occurs when determining the crystal structure
of compounds containing different types of atoms with equal number of
electrons. We propose a new structural model of Cu(InxGa1âx)3Se5 which is
valid for the entire compositional range of the CuIn3Se5âCuGa3Se5 solid
solution. Our model is based on neutron and anomalous x-ray diffraction
experiments. These complementary techniques allow the separation of scattering
contributions of the isoelectronic species Cu+ and Ga3+, contributing nearly
identically in monoenergetic x-ray diffraction experiments. We have found that
CuIII3Se5 (III=In,Ga) in its room temperature near-equilibrium modification
exhibits a modified stannite structure (space group I4ÂŻ2m). Different
occupation factors of the species involved, Cu+, In3+, Ga3+, and vacancies
have been found at three different cationic positions of the structure
(Wyckoff sites 2a, 2b, and 4d) depending on the composition of the compound.
Significantly, Cu+ does not occupy the 2b site for the In-free compound, but
does for the In-containing case. Structural parameters, including lattice
constants, tetragonal distortions, and occupation factors are given for
samples covering the entire range of the CuIn3Se5âCuGa3Se5 solid solution. At
the light of the result, the denotation of Cu-poor 1:3:5 compounds as
chalcopyrite-related materials is only valid in reference to their
composition
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