Search

Linearly polarized microphotoluminescence ($\ensuremath{\mu}$-PL) measurements of strained ${\mathrm{WSe}}_{2}$ monolayers in out-of-plane high magnetic fields are presented. At low temperature, a splitting of the bright exciton emission into two exciton components is observed, which is attributed to an in-plane uniaxial strain based on the full polarization dependence of the photoluminescence spectrum. High magnetic field measurements directly reveal a distinct evolution of the linear polarization and allows us to extract the valley coherence time constants (${T}_{s2}^{*}$) for both exciton components. For the high-energy transition of the exciton, the valley coherence time $\ensuremath{\simeq}0.45$ ps, closely matching ${T}_{s2}^{*}$ of an unstrained monolayer ($\ensuremath{\simeq}0.34$ ps). For the low-energy exciton, however, ${T}_{s2}^{*}$ is four times larger, $\ensuremath{\simeq}1.97$ ps. This valley coherence time observed here may be explored for future valleytronic applications.

Photoluminescence and X-ray diffraction are used to investigate 2H and 3R polytypes of MoS2 layered crystals intercalated with Cl2 molecules. The spectral structure of the observed bound excitons emission is individual for each polytype, which can be used for unambiguous identification of the 2H and 3R polytypes. The DFT band structure calculations have been done for three molecular layers of 2H-MoS2 and 3R-MoS2 polytypes with and without intercalation of Cl2 molecules. The spectroscopic experimental results confirmed theoretical calculations performed for both cases and it has been demonstrated that halogen intercalation completely changes the local band structure of both 2H- and 3R-MoS2 polytypes.

The excitation spectra of the ZnAl2S4 spinel crystals codoped with chromium and vanadium are investigated in order to explain some features of the Cr3+ ions optical spectra and the inconsistency of the experimental absorption/emission bands position with the Tanabe–Sugano diagram, as well as the “missing” band in the absorption spectrum of V3+ ion although this band should be present as it is due to a spin allowed transition. The unusual high Racah parameters of the C/B ratio for Cr3+ ions have been induced to recalculate the Tanabe–Sugano diagram for the d3 electron configuration and the ratio C/B=8 in order to use it for the experimental results interpretation. The presence of Cr2+ ions in the low spin electron configurations in the octahedral coordination was confirmed; the temperature dependence of the Cr2+ emission at about 1.9 eV was studied and the Huang–Rhys factors were estimated for different temperatures. Despite the fact that the V3+ C/B ratio has a different value compared to the case of Cr3+ ions, the electron–phonon interaction is similar for both ions, with the Huang–Rhys parameter equal to 10 in both cases. Further research into optimizing the ZnAl2S4:Cr, V system to get an efficient enhancement of the vanadium emission on account on re-absorbing the chromium emission is suggested.

A systematic optical characterization of transition metal dichalcogenide layered crystals grown by chemical vapour transport method as well as of natural molybdenite were carried out by using piezoreflectance (PzR) measurements. From a detailed lineshape fit of the room-temperature PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband direct transitions were determined accurately. The possible assignments of the different origins of excitonic transitions are discussed. The near direct band edge A and B excitonic transitions detected in PzR spectra show a linear red-shift with the temperature increasing up to 525 K. The values of temperature-dependent energies of the excitonic transitions A and B are evaluated and discussed.

The experimental and theoretical studies of the optical properties of pure α-ZnAl 2 S 4 and α-ZnAl 2 S 4 :V 3+ crystals were carried out. The ab initio and crystal field calculations of the structural and optical properties of α-ZnAl 2 S 4 :V 3+ were compared with the corresponding experimental data. It was shown that the lowest vanadium 3d states are located at about 1.36 eV above the valence band's top. The complete energy level scheme of the α-ZnAl 2 S 4 :V 3+ system, which includes the host's electronic band structure and impurity ion's energy levels, was suggested on the basis of the performed calculations.

The absorption and luminescent properties of α-ZnAI2S4:V spinel type crystals in the temperature range 10–300 K are investigated. The spectra are assigned to the electronic transitions of trivalent vanadium ions located in octahedral sites. It is shown that at low temperatures the three main components of the revealed IR luminescence spectra are caused by the 1A1g(1G)→1Eg(1D), 1T2g(1D), 3T2g(3F)→3T1g(3F), and 1Eg(1D)→3T1g(3F) transitions. The observed dependencies of the emission components intensities on temperature are explained assuming that there is a phonon assisted tunnelling between 3T2g(3F) and 1Eg(1D) states. On temperature rise, the 3T2g(3F)→3T1g(3F) vibronic transitions suppress other emission channels, which leads to the enhancement of the integral luminescence intensity and to the broadening of the spectrum centred at λ=1.4 μm.

The absorption and luminescent properties of α-ZnAl 2 S 4 :Co spinel type crystals grown by chemical transport reactions method are investigated. The spectra are assigned to the electronic transitions of Co 2+ ions located in tetrahedral sites. It is shown, that the revealed four radiative spectral components are caused by the 2 A( 2 G)→ 4 T 1 ( 4 F), 4 T 1 ( 4 P)→ 4 T 2 ( 4 F), 2 E( 2 G)→ 4 T 2 ( 4 F) and 2 E( 2 G)→ 4 T 1 ( 4 F) transitions.

The photoexcitation and relaxation of carriers is studied in WSe2 nanolayers on Si/SiO2 substrate by optical pump–probe technique. The excitation wavelength falls in the vicinity of the band edge. The thickness variation of WSe2 layers acts as tuning of band gap of the layers. In this way, a band edge spectroscopy is carried out using the constant excitation wavelength. It is shown that the relaxation time constants increase with the thickness increase, while the amplitude of transient reflectivity changes nonmonotonically. A model is suggested describing the amplitude behavior.

Optical spectroscopy in high magnetic fields $B\leq65$ T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe$_{2}$, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe$_{2}$ where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model., just accepted in Nano Letters http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00626 (equations corrected)

The results of studying the optical properties of nanoscale single crystals of MoS2:Cl2 and WS2:Br2 semiconductor compounds are presented. In microscopic images obtained at the wavelength of the second (400 nm), edge effects are detected, which consist in enhancement or reduction in the second-harmonic signal intensity. Unlike previously proposed interference mechanisms of edge effects, non-interference mechanisms are considered. The occurrence of edge effects is associated with either an increased Cl2 and Br2 halogen molecule concentration or with an electrically induced second harmonic caused by band bending at the edges of individual crystal layers.

Zinc oxide thin nanostructured film and single crystal were studied by photoluminescence (PL) and optical second harmonic generation (SHG). The single crystal reveals a strong blue excitonic luminescence, while for thin films “yellow”-defect PL is dominant, pointing to a strong contribution from the surface of film nanograins. In SHG the bulk contribution is dominant for both film and single crystal, although SHG enhancement was found in the film. Nonlinear parameters were calculated from the model and preferable orientation of the film grains was found.

The comparative study of the excitonic radiative recombination in 2H-MoS2:Cl2 and 2H-WS2:Br2 layered crystals is presented. The steady-state emission spectra of both the investigated materials include two distinct regions. The short-wavelength region consisting of sharp lines is produced by bound excitons, related to the halogen molecules intercalated in the layered semiconductors during the growth process. The second spectral region—a vibronic broad band identical for both types of crystals, is attributed to the deep radiative centres, caused by the intrinsic defects of the host lattices. The atypical temperature increase of the excitonic radiative decay time observed for 2H-WS2:Br2 in the range T = 2 – 7 K is related to the presence of a long lifetime sublevel, placed 0.3 meV above the lowest excitonic state.

Resonant Raman spectra of single layer WS$_{2}$ flakes are presented. A second order Raman peak (2LA) appears under resonant excitation with a separation from the E$^{1}_{2g}$ mode of only $4$cm$^{-1}$. Depending on the intensity ratio and the respective line widths of these two peaks, any analysis which neglects the presence of the 2LA mode can lead to an inaccurate estimation of the position of the E$^{1}_{2g}$ mode, leading to a potentially incorrect assignment for the number of layers. Our results show that the intensity of the 2LA mode strongly depends on the angle between the linear polarization of the excitation and detection, a parameter which is neglected in many Raman studies., Comment: 6 pages, 4 figures

We studied the photoluminescence (PL) spectra of thiophene oligomers in a solution, and in a solid state: aggregated film and self-assembled monolayer (SAM). We showed a strong influence of packing conditions on the PL spectra. In the solid state a new electronic state was observed, which does not reveal itself in the solution.

Due to their layered structure, 2H layered transition metal dichalcogenide indirect bandgap compounds MoS2, WS2 can be intercalated with foreign atoms and molecules. The 2H-MoS2, 2H-WS2 and 2H-WSe2 crystals intercalated with halogen molecules at low temperatures exhibit a strong luminescence due to the bound excitons recombination. In this work, the results of comparative study of the excitonic luminescence kinetics for 2H-WS2:Br2 and 2H-MoS2:Cl2 single crystals are presented. A simple kinetic model describing the temperature dependencies of the steady-state luminescence intensity and of the radiative decay time is proposed. The lifetimes of the excitonic levels and the activation energy of the excitonic luminescence quenching caused by an extrinsic self-trapping process for both studied compounds were determined.

Photoluminescence measurements have been performed on chromium doped α-ZnAl 2 S 4 spinel-type crystals in the temperature range 2–540 K. a single configuration coordinate diagram for the Cr 3+ ions in the α-ZnAl 2 S 4 host is constructed to interpret the absorption and emission spectra and to allow a determination of the energy gaps between the 2 E g − 2 T 1g and 2 E g − 4 T 2g states. A fit of the measured temperature dependence of the luminescence decay, using a simplified four level model gave the lifetimes of the 2 E g , 2 T 1 g and 4 T 2 g excited levels and confirmed the gap values obtained from steady state measurements.

The increase of the resistance up to two orders of magnitude under laser illumination (λ=760 nm) was observed in La0.7Ca0.3MnO3 (LCMO) epitaxial thin films in ferromagnetic state. Optical absorption also increases by 10–15 % and the magnetic second-harmonic generation signal decreases down to zero under the irradiation. The light induced changes are reversible with characteristic relaxation times τ∼1–30 s. Magnetic field, B=4 T, suppresses the photoconductivity and decreases its relaxation time. Photoinduced effects are caused by the injection of a large number of extra carriers, which change the (antiferromagnetic) AFM/FM phase balance in LCMO, favoring the insulating AFM state.

The transfer matrix method has been widely used to calculate wave propagation through the layered structures consisting entirely of either linear or nonlinear optical materials. In the present work, we develop the transfer matrix method for structures consisting of alternating layers of linear and nonlinear optical materials. The result is presented in a form that allows one to directly substitute the values of material constants, refractive index and absorption coefficient, into the expressions describing the second harmonic generation (SHG) field. The model is applied to the calculation of second harmonic (SH) field generated in nano-thin layers of transition metal dichalcogenides exfoliated on top of silicon oxide/silicon Fabry-Perot cavity. These structures are intensively studied both in view of their unique properties and perspective applications. A good agreement between experimental and numerical results can be achieved by small modification of optical constants, which may arise in an experiment due to a strong electric field of an incident focused pump laser beam. By considering the SHG effect, this paper completes the series of works describing the role of Fabry-Perot cavity in different optical effects (optical reflection, photoluminescence and Raman scattering) in 2D semiconductors that is extremely important for characterization of these unique materials.

The mapping of two-photon excited luminescence in aggregations of free-standing zinc oxide microrods has been carried out at room temperature. Two-photon luminescence spectra in the excitonic region for individual microrods have been recorded. The luminescence intensity exhibits a power-law dependence on the optical pump power with the exponent n > 2. This fact, along with the existence of a threshold power above which the dependence in the exciton region deviates from a quadratic one, indicates the onset of light amplification in individual ZnO microrods and the conditions preceding laser oscillation.

Photoluminescence (PL) of chromium-doped spinel-type semiconductor α-ZnAl 2 S 4 has been studied in the temperature range 2–540 K. The transitions between ground state 4 A 2g and 2 E g , 2 T 1g , 2 T 2g , 4 T 2g , 4 T 1g excited levels of Cr 3+ ions in absorption and emission spectra have been observed. A fit of the measured temperature dependence of the PL decay, using a four-level model, gave the lifetimes of the 2 E g , 2 T 1g and 4 T 2g levels. The energy gaps between 2 E g – 2 T 1g and 2 E g – 4 T 2g states have been determined and they are very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr 3+ ions in the α-ZnAl 2 S 4 :Cr spinel host has been constructed.

Optical absorption were investigated in CdGa/sub 2/S/sub 4/ in the temperature range 85-300 K. The energy gap (3.17 eV at 85 K) and its temperature dependence were determined. The latter was studied by means of a three-parameter thermodynamic model and the Einstein model. Band tail states were found to give rise to the absorption below the fundamental edge.

The spectroscopic characteristics of the α-ZnAl(2)S(4) wide bandgap semiconductor doped with Ti ions are investigated. It is shown, that the ZnAl(2)S(4):Ti spinel-type crystals exhibit luminescence in the IR spectral range 0.8-1.4 µm. The observed spectroscopic characteristics are assigned to the emission bands arising from the ligand -Ti(4+) charge transfer for octahedral sites of titanium that is in agreement with the experimental evidence for the absence of the EPR signal from Ti ions. A qualitative explanation of the experimental data is given.

Structural characterization of sample was carried out through the Laue showing a very perfect ternary symmetry and a high crystallinity. Optical absorption was investigated in CuIn3Se5 in the temperature range 85-300K. The energy gap (1.42 eV at 85K) and its temperature dependence were determined. The latter was studied by means of a three-parameter thermodynamic model and the Einstein model. The values of the band gap at zero temperature, average phonon energy, and a dimentionless constant related to the electron-phonon coupling were estimated. Band tail states were found to give rise to the absorption below the fundamental edge.

Recombination radiation in Cd3(AsxP1−x)2 solid solutions near cadmium phosphide (x ⪅ 0.1) is studied. A complex structure of the stimulated emission spectra in Cd3(AsxP1−x)2 single crystals is detected. It is shown that for Cd3P2 the width of emission spectrum and its position relative to the transmission edge may be adequately described in terms of a direct interband-transition model. The same model is used to calculate the luminescence Icv1(ω) and the optical transmission Tcv1(ω) in the presence of a small (i.e. up to 5% of the Brillouin zone size) K1 shift between the extrema of the heavy hole valence band V1 and conduction band C. It is shown that at k1 ≠ 0 the dependence Icv1(ω) may have two maxima. The structure of stimulated emission spectra and the reference data on luminescence in Cd3(AsxP1−x)2 single crystals are interpreted taking into account the calculations carried out in this work. Rekombinationsstrahlung in Cd3(AsxP1−x)2-Mischkristallen in der Nahe von Kadmiumphosphid (x ⪅ 0,1) wird untersucht und eine komplexe Struktur der stimulierten Emissionsspektren in Cd3(AsxP1-x)2-Einkristallen entdeckt. Es wird gezeigt, das fur Cd3P2 die Breite des Emissionsspektrums und seine Lage bezuglich der Transmissionskante adaquat mit einem Modell direkter Interbandubergange beschrieben werden kann. Dasselbe Modell wird fur die Berechnung der Lumineszenz Icv1(ω) und optischen Transmission Tcv1(ω) beim Vorliegen einer kleinen (d. h. bis zu 5% der Grose der Brillouinzone) K1-Shift zwischen den Extremwerten des Valenzbandes schwerer Locher V1 und des Leitungsbandes C benutzt. Es wird gezeigt, das bei k1 ≠ 0 die Icv1(ω)-Abhangigkeit zwei Maxima besitzen kann. Die Struktur der stimulierten Emissionsspektren und die Bezugswerte der Lumineszenz in Cd3(AsxP1−x)2-Einkristallen werden unter Berucksichtigung der durchgefuhrten Berechnungen interpretiert.

Optical absorption, photoacoustic spectroscopy and photoconductivity were investigated in ReS2 single crystals in the temperature range 50–300 K. The energy gap (1.55 eV at 80 K) and its temperature dependence, the value of the average phonon energy (17 meV) and the electron phonon coupling parameter (S=2.40) as well as the electron‐hole mobility due to lattice scattering were determined.