Acoustic properties of buried graphitized layers in diamond formed by ion implantation followed by annealing were studied using the picosecond ultrasonic technique with spatial resolution. Two ...methods of elastic pulse generation were used: heating an aluminum film deposited on a diamond sample by femtosecond laser pulses and direct illumination of the graphitized layers by these pulses. We applied a multilayered model of the acousto-optical response to fit experimental results and estimate the distribution of the acoustical parameters (wave resistance, viscoelastic damping, and longitudinal sound speed) of the structures under study in depth. It was found that unique sets of spectral lines are present in the Fourier spectra of measured responses in regions with different internal structures. Mapping of the Fourier spectra made it possible to visualize regions with different internal structures. The combined use of depth profiling and mapping can serve as a tool for hypersound tomography.
The effect of pulsed laser exposure on electronic states formed by partial dislocation cores in cadmium telluride is demonstrated using in situ measurements of low-temperature (5 K) ...microphotoluminescence. It is shown that laser pulses 5 ns long with a wavelength of 1053 nm allows local transformation of dislocation cores, not affecting the surrounding unperturbed CdTe lattice.
Based on the measurements of spectra and polarization of low-temperature (5K) microphotoluminescence excited by stabilized continuous-wave laser pumping, we find a new type of isolated emitters ...formed due to the dislocations in crystalline CdZnTe. For an emitter of this type, zero-phonon luminescence is located 30–50 meV above the usual dislocation luminescence and is subjected to giant fluctuations of spectral position (~10 meV) and intensity. A noticeable degree of linear polarization in the plane containing directions indicates the strongly anisotropic nature of the corresponding electronic states and confirms their connection with dislocations.
Using low-temperature (5 K) micro-photoluminescence measurements of a ZnSe/ZnMgSSe quantum well, isolated (quantum) emitters produced by interacting acceptor and hydrogen-like donor have been found ...and studied. These emitters enable optical probing of a single acceptor (donor) center if the nature of the donor (acceptor) constituting the distant donor-acceptor pair is known. Here we demonstrate the simplest realization of proposed optical probing: frequencies of optically active phonons are successfully determined for different types of single residual acceptors in a ZnSe-based quantum well.
We show that the low-temperature (5 K) emission spectrum of InSe nanoflakes, obtained by mechanical exfoliation of β-InSe, is characterized by unusual luminescence bands that are not present in the ...source material. These bands were observed both in the center and at the edges of the flakes, and their location at 30–70 meV below the expected position of the excitonic band bottom rules out intrinsic emission. In addition, the most intense luminescence band exhibits line narrowing and a pronounced blue shift of ~40 meV with the increase of excitation power from 40 to 1000 W/cm2. The shift is not observed in the low-temperature emission spectra of ultra thin (four monolayers) InSe flakes. Our data suggest that producing InSe flakes using mechanical exfoliation leads to the generation of optically active structural defects, presumably associated with a violation of the layer sequence and/or layer spacing inherent to β-InSe. The simplest type of these defects are bubbles and/or folds, the formation of which is accompanied by an increase in the distance between InSe layers and, as a consequence, produces repulsive (attractive) potential for electrons (holes). At low temperatures, these defects govern the luminescent properties of the flakes up to 6 monolayers by suppressing the intrinsic (exciton) emission. Some qualitative changes were observed only in ultrathin flakes with a thickness of ~4 monolayers.
•Low-temperature luminescence of β-InSe nanoflakes has been studied.•Emission spectra of ultrathin InSe flakes have been obtained at 5K.•The impact of InSe exfoliation process on the intrinsic and defect-related emission has been demonstrated.
We employ here a picosecond ultrasonic technique to study Van der Waals heterostructures. Temporal variation of the reflection coefficient of the Al film that covers Van der Waals hBN/WSe\(_2\)/hBN ...heterostructures on a sapphire substrate after the femtosecond laser pulse excitation is carefully measured using an interferometric technique with spatial resolution. The laser pulse generates a broadband sound wave packet in aluminum film propagating perpendicular to the plane direction and partially reflecting from the heterostructural interfaces. The demonstrated technique has enough sensitivity to resolve a WSe\(_2\) monolayer embedded in hBN. We apply a multilayered model of the optical and acoustical response that allows to evaluate the mechanical parameters, in particular, rigidity of interfaces, inaccessible from the other measurements. Mapping of the Fourier spectra of the response clearly visualizes different composition regions and can therefore serve as an acoustic tomography tool. Our findings demonstrate almost zero acoustic phonon dissipation below 150 GHz at the interfaces and in the layers that makes Van der Waals heterostructures perspective for nano-acoustical applications.
The results of E-190 experiment (project Thermalization) with 50 GeV proton beam irradiation of SVD-2 setup are presented. MC simulation has shown the linear dependence of number of photons detected ...in electromagnetic calorimeter and the average number of neutral pions. Multiplicity distribution of neutral pion, N0, for total number of particles in the event, Ntot = Nch + N0, are obtained with corrections on the setup acceptance, triggering and efficiency of the event reconstruction. The scaled variance of neutral pion fluctuations, ω = D/ < N0 >, versus total multiplicity is measured. The fluctuations increase at Ntot > 18. According to quantum statistics models this behavior can indicate a pion condensate formation in the high pion multiplicity system. This effect has been observed for the first time.
The mechanical properties of the layered crystals in the few layer limit are largely unexplored. We employ a picosecond ultrasonic technique to access the corresponding mechanical parameters. ...Temporal variation of the reflection coefficient of the Al film that covers hBN/WSe2/hBN (where hBN is hexagonal boron nitride) heterostructures on a sapphire substrate after the femtosecond laser pulse excitation is carefully measured using an interferometric technique with spatial resolution. The laser pulse generates a broadband sound wave packet propagating perpendicularly to the Al plane and partially reflecting from the heterostructural interfaces. The demonstrated technique allows one to resolve a WSe2 monolayer embedded in hBN. We apply a multilayered model of the optoacoustical response to evaluate the mechanical parameters, in particular, the rigidity of the interfaces. Mapping of the Fourier spectra of the response visualizes different composition regions and may serve as an acoustic tomography tool. Almost zero phonon dissipation below 150 GHz demonstrates the van der Waals heterostructures’ potential for nanoacoustical applications.
Film samples produced by codeposition of salts Na(form), Ni(form)
2
, Co(Аcac)
3
, and Pb(Аc)
2
from graphene oxide dispersions are studied. The thus-prepared materials are characterized by spectral ...and X-ray analytical methods. The structure of graphene oxide containing salt crystals is studied by scanning electron microscopy (SEM). Crystallization occurs both on the surface and in the interlayer spaces of graphene oxide. No side processes occur during crystallization; the precursor salts retain their phase composition, except for lead acetate, which is partially hydrolyzed in aqueous solution. The incorporation of salt crystals increases film imperfection and enhances the resistivity of the material severalfold. The results of this study can appear useful in the development of graphene oxide based membranes.
PDF
