Low-temperature Photoluminescence Spectra Research Articles

Circumventing the polariton bottleneck via dark excitons in 2D semiconductors

Efficient scattering into the exciton polariton ground state is a key prerequisite for generating Bose–Einstein condensates and low-threshold polariton lasing. However, this can be challenging to achieve at low densities due to the polariton bottleneck effect that impedes phonon-driven scattering into low-momentum polariton states. The rich exciton landscape of transition metal dichalcogenides (TMDs) provides potential intervalley scattering pathways via dark excitons to rapidly populate these polaritons. Here, we present a theoretical and fully microscopic study exploring the time- and momentum-resolved relaxation of exciton polaritons supported by a MoSe2 monolayer integrated within a Fabry–Perot cavity. By exploiting phonon-assisted transitions between momentum-dark excitons and the lower polariton branch, we demonstrate that it is possible to circumvent the bottleneck region and efficiently populate the polariton ground state. Furthermore, this intervalley pathway is predicted to give rise to, yet unobserved, angle-resolved phonon sidebands in low-temperature photoluminescence spectra that are associated with momentum-dark excitons. This represents a distinct signature for efficient phonon-mediated polariton-dark-exciton interactions.

Identifying orientation-dependent optical properties of single-crystalline β-Ga2O3 films

We explore the effect of crystallographic anisotropy on the optical properties of high-quality β-Ga2O3 thin films by conducting experimental measurements and theoretical simulations. High resolution x-ray diffraction measurements confirm the high-quality and the single crystalline quality, ruling out the effect of defects for all films. Raman spectra reveal the presence of anisotropy, as evident from a stronger Bg(2) mode related to GaIO4 chain libration in (100) orientation. Conversely, a stronger Ag(10) mode corresponding to tetrahedral bonds is evident in the (010) orientation, while it is suppressed in the (100) orientation. Low-temperature photoluminescence spectra indicate the presence of intrinsic impurity-related emissions in the ultraviolet and blue regions for all films. An anisotropic bandgap is observed, wherein the lowest bandgap value is related to the (010) oriented sample. Spectroscopic ellipsometry measurements confirm different refractive indices and extinction coefficient values depending on crystallographic orientation, which are in good agreement with the theoretical values obtained by density functional theory, confirming the anisotropic characteristics. The theoretical calculations of charge density show that the strength of covalent bonding depends on the β-Ga2O3 orientation, while experimental findings demonstrate that as the covalent bonding character increases, the film bandgap and the refractive index decrease. The anisotropy of β-Ga2O3 with respect to the crystal orientation leads to variations in the extinction coefficient, refractive index, and bandgap energy.

The Characteristics of Luminescence from High-Temperature- and High-Pressure-Treated Diamonds

High-temperature and high-pressure (HTHP)-treated diamonds have attracted attention all over the world due to their vivid colors. In order to explore a new method for the rapid and non-destructive identification of HTHP-treated diamonds, in this paper, five IaAB-type diamonds yielded in Russia were selected as the research object and treated with HTHP. The HTHP-treated diamonds were investigated by DiamondViewTM, cathodoluminescence, micro-infrared spectrometry (micro-IR), low-temperature photoluminescence (PL) spectrometry, and three-dimensional (3D) fluorescence spectrometry. The results show that under DiamondViewTM and cathode rays, the five samples were all non-phosphorescent with different luminous patterns, such as regular annular bands, multiple groups of intersecting linear stripes, or jagged stripes. In the low-temperature PL spectra, most HTHP-treated diamonds exhibited stronger luminescence peaks at 637 nm compared to 575 nm. But there were also exceptions, such as the purple-red sample showing the opposite luminescence peak. In the 3D fluorescence spectrum of the HTHP-treated diamonds, the fluorescence peak mainly appeared in the range of 440–450 nm, accompanied by a broad band of 350–500 nm and even longer wavelengths. Meanwhile, some samples also exhibited fluorescence peaks at longer wavelengths, such as 353 nm, 676 nm, and 665 nm. These results make it possible to identify HTHP-treated diamonds by using luminescence characteristics, providing a new method for the non-destructive and rapid detection of HTHP-treated diamonds.

Characterization of nanowire light-emitting diodes with InP/InAsP heterostructures emitting in telecom band

We report the growth and characterization of InP/InAsP/InP nanowires (NWs) and NW LEDs (NW-LEDs), which emit light at telecom wavelengths. InP-based NWs were grown by selective-area metal-organic vapor-phase epitaxy, and a thin InAsP layer was embedded in the NWs. The NW exhibited emission lines in their low-temperature photoluminescence spectra, suggesting the formation of quantum dots (QDs) in the NW. NW-LED operation was demonstrated at both room and low temperatures in the telecom band, but it was found that the emission wavelength range and blueshift behavior induced by current injection differed considerably between room and low temperatures. Our results suggest that an efficient path for carrier injection into the active InAsP layer should be explored for NW-QD-based single-photon sources operating via current-injection.

Luminescence spectrum of cadmium chalcogenide photovoltaic film structures and their power enhancement

A technology has been developed for producing photovoltaic film structures of CdTe, CdTe:In, CdTe/CdS by thermal vacuum deposition, which allows increasing their operating power. An analysis of their low- temperature photoluminescence spectra shows a significant increase in their spectral sensitivity. It has been experimentally shown that there is a clear correlation between the anomalous photovoltaic (APV) properties and the shape of the intrinsic photoluminescence band of obliquely deposited CdTe films. The photoluminescence spectrum of a polycrystalline CdTe film with APV property is qualitatively different from the spectra of a single crystal, large-block polycrystal and single microcrystal. The main contribution to the photoluminescence of the film comes from radiative recombination of free carriers (A-line with half-width ΔEA≈14.2±0.1 meV) and edge luminescence with a wide doublet structure (B- and C - lines with half- widths 18.5±0.1 meV and 32.2±0.1 meV). When doped with In impurities and as a result of heat treatment, the emission spectrum is greatly transformed in accordance with the change in the photoelectric properties of the film. In the photoluminescence spectrum of a CdTe layer in a CdTe/CdS heterostructure grown under identical technological conditions as a CdTe monolayer, an additional broad spectral line appears due to the presence of a heterointerface, the superhot region disappears, and the A-emission line narrows somewhat (ΔEA≈11.2±0.1 meV), which significantly differ the studied film structures from the known CdS/CdTe heterosystems of other authors. The proposed method for analyzing low-temperature photoluminescence spectra makes it possible to purposefully control the technology for manufacturing photovoltaic film structures. The results obtained are of interest for film optoelectronics and solar cell manufacturing.

Effect of Strain and Surface Proximity on the Acceptor Grouping in ZnO.

According to the present knowledge, the level of zinc oxide conductivity is determined by donor and acceptor complexes involving native defects and hydrogen. In turn, recently published low-temperature cathodoluminescence images and scanning photoelectron microscopy results on ZnO and ZnO/N films indicate grouping of acceptor and donor complexes in different crystallites, but the origin of this phenomenon remains unclear. The density functional theory calculations on undoped ZnO presented here show that strain and surface proximity noticeably influence the formation energy of acceptor complexes, and therefore, these complexes can be more easily formed in crystallites providing appropriate strain. This effect may be responsible for the clustering of acceptor centers only in certain crystallites or near the surface. Low-temperature photoluminescence spectra confirm the strong dependence of acceptor luminescence on the structure of the ZnO film.

Effect of Neutron Irradiation on the Electronic and Optical Properties of AlGaAs/InGaAs-Based Quantum Well Structures.

The effect of neutron irradiation on the structural, optical, and electronic properties of doped strained heterostructures with AlGaAs/InGaAs/GaAs and AlGaAs/InGaAs/AlGaAs quantum wells was experimentally studied. Heterostructures with a two-dimensional electron gas of different layer constructions were subjected to neutron irradiation in the reactor channel with the fluence range of 2 × 1014 cm-2 ÷ 1.2 × 1016 cm-2. The low-temperature photoluminescence spectra, electron concentration and mobility, and high-resolution X-ray diffraction curves were measured after the deactivation. The paper discusses the effect of neutron dose on the conductivity and optical spectra of structures based on InGaAs quantum wells depending on the doping level. The limiting dose of neutron irradiation was also estimated for the successful utilization of AlGaAs/InGaAs/GaAs and AlGaAs/InGaAs/AlGaAs heterostructures in electronic applications.

Understanding the Optical Properties of Doped and Undoped 9-Armchair Graphene Nanoribbons in Dispersion.

Graphene nanoribbons are one-dimensional stripes of graphene with width- and edge-structure-dependent electronic properties. They can be synthesized bottom-up in solution to obtain precise ribbon geometries. Here we investigate the optical properties of solution-synthesized 9-armchair graphene nanoribbons (9-aGNRs) that are stabilized as dispersions in organic solvents and further fractionated by liquid cascade centrifugation (LCC). Absorption and photoluminescence spectroscopy reveal two near-infrared absorption and emission peaks whose ratios depend on the LCC fraction. Low-temperature single-nanoribbon photoluminescence spectra suggest the presence of two different nanoribbon species. Based on density functional theory (DFT) and time-dependent DFT calculations, the lowest energy transition can be assigned to pristine 9-aGNRs, while 9-aGNRs with edge-defects, caused by incomplete graphitization, result in more blue-shifted transitions and higher Raman D/G-mode ratios. Hole doping of 9-aGNR dispersions with the electron acceptor F4TCNQ leads to concentration dependent bleaching and quenching of the main absorption and emission bands and the appearance of red-shifted, charge-induced absorption features but no additional emission peaks, thus indicating the formation of polarons instead of the predicted trions (charged excitons) in doped 9-aGNRs.

A family of mono-nuclear lanthanide-radical complexes: Synthesis, structures, magnetic and fluorescent properties

Two new rare earth complexes based on TEMPO derivative namely [Ln(hfac)3(H2O)hmb-TEMPO] (Ln = Gd 1, Tb 2; hmb-TEMPO = 4-((2-hydroxy-3-methoxybenzylidene) amino)-2,2,6,6-tetramethyl; hfac = hexafluoroacety lacetonate) were successfully synthesized through reacting hmb-TEMPO radical and Ln(hfac)3. Single-crystal X-ray diffraction reveals that 1 and 2 are isostructural complexes in which the LnIII ion is surrounded nine oxygen atoms and displayed distorted tricapped trigonal prism geometry. Magnetic studies show that antiferromagnetic interactions dominate in complex 1. Nonzero out-of-phase signals were observed for complex 2, indicating single-molecule magnet behavior. The low-temperature photoluminescence spectrum of complex 2 were investigated and discussed in detail.

Quantum-dimensional near-surface recombination of photocarriers in CdTe microcrystals

The low-temperature (T = 2 K) photoluminescence spectra of a p-CdTe/n-CdS film heterostructure containing CdTe microcrystals have been studied. In the spectral region above the intrinsic absorption edge of bulk CdTe, a dominant “superhot” radiation band was found. The band arises because of optical transitions of electrons from near-surface levels of the spatial quantization of a microcrystal to the valence band states.

Growth of non-polar m-plane GaN pseudo-substrates by Molecular beam epitaxy

Non-polar m-plane GaN films were grown by Plasma Assisted Molecular Beam Epitaxy on γ-LiAlO2 (100) substrates by a controlled coalescence of GaN nanocolumns obtained by a two-step process including a top-down nanopillars etching from a GaN buffer and a subsequent bottom-up overgrowth. Transmission electron microscopy data show a significant reduction of extended defects density in the coalesced film as compared to the initial GaN buffer, most likely due to a filter effect by the regrowth process on the nanopillars inclined walls. Low temperature photoluminescence spectra back this reduction by a strong intensity decrease of the stacking faults fingerprint emission peaks, while a very intense donor-bound excitonic emission at 3.472 eV, 2.8 meV wide, becomes dominant.

Ultra-sharp-Line Emission in Isotopic c-10BP

Cubic boride crystals with pure boron isotopes have attracted great attention in recent years. Here, high-quality cubic boron phosphide single crystals with isotopic 10B and a maximum width reaching 1.5 mm are grown via the high-temperature molten salt method. An interesting phenomenon that there are many discrete sharp lines in the low-temperature (4–80 K) photoluminescence spectra is observed. Besides, as temperature increases, the luminescence intensity of these sharp lines shows a nonmonotonic decay with two different trends exhibited. Based on analyses, the sharp-line emission originates from phonon-assisted donor–acceptor pair radiative recombination. The nonmonotonic decay is affected by the number of assisting phonons and the degree of thermal ionization under the effect of temperature, and the different decay trends are attributed to the effect of thermal ionization dependent on the distance between the donor and acceptor. Results obtained in this research are expected to enrich the knowledge concerning cubic boride crystals in the aspect of physical properties.

Impact of the organic cation on the band-edge emission of two-dimensional lead-bromide perovskites.

Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in k-space (where k is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.

Thickness-dependent excitonic properties of WSe2/FePS3 van der Waals heterostructures.

van der Waals heterostructures (vdWHs), with their flexible combination of various two-dimensional (2D) materials, are continuously revealing new physics and functionalities. 2D magnetic materials have recently become a focus due to their fascinating electronic and spintronic properties. However, there has rarely been any investigation of the optical properties of 2D magnetic materials-based heterostructures. Herein, we construct a new WSe2/FePS3 heterostructure, in which WSe2 works as a "sensor" to visualize the thickness-dependent properties of FePS3. As characterized by photoluminescence (PL) spectra, whether under or on top of the FePS3, the PL intensity of the monolayer WSe2 is strongly quenched. The quenching effect becomes more obvious as the FePS3 thickness increases. This is because of the efficient charge transfer process occurring at the WSe2/FePS3 interface with type II band alignment, which is faster for thicker FePS3, as is evident from transient absorption measurements. The thickness-dependent charge transfer process and corresponding excitonic properties are further revealed in low-temperature photoluminescence spectra of WSe2/FePS3 heterostructures. Our results show that the thickness of 2D magnetic materials can work as an experimental tuning knob to manipulate the optical performance of conventional 2D semiconductors, endowing van der Waals heterostructures with more unexpected properties and functionalities.

Диполярные биэкситоны в латеральных ловушках в гетероструктурах Si/SiGe/Si

Si/SiGe/Si heterostructures with long-range lateral potential fluctuations that appear in the capping Si layer near the SiGe/Si heterointerface due to the presence of relaxed areas in the SiGe layer have been studied. Analysis of the low-temperature photoluminescence spectra indicates that, upon the photoexcitation of the structure, the accumulation of nonequilibrium charge carriers, formation of dipolar excitons, and their recombination take place in large-scale lateral traps formed by these fluctuations. It is found that, at temperatures T < 10 K, a new narrow line appears with an increase in the excitation level at the blue side of the broad photoluminescence band of dipolar excitons localized by short-range potential fluctuations. At temperatures T ~ 2 K, this line is dominant in the spectra even at the lowest excitation levels. It is shown that, at moderate excitation levels, this line is caused by the recombination of free dipolar biexitons in large-scale traps. At high excitation levels, the width of the new line increases by more than a factor of 2 compared to that at lower excitation levels, and under these conditions this line is associated with the recombination of dipolar electron–hole plasma in large-scale traps.

Wurtzite Gallium Phosphide via Chemical Beam Epitaxy: Impurity-Related Luminescence vs Growth Conditions.

The metastable wurtzite crystal phase in gallium phosphide (WZ GaP) is a relatively new structure with little available information about its emission properties compared to the most stable zinc-blend phase. Here, the effect of growth conditions of WZ GaP nano- and microstructures obtained via chemical beam epitaxy on the optical properties was studied using power- and temperature-dependent photoluminescence (PL). We showed that the PL spectra are dominated by two strong broad emission bands at 1.68 and 1.88 eV and two relatively narrow peaks at 2.04 and 2.09 eV. The broad emissions are associated with the presence of carbon and a small number of extended crystal defects, respectively. For the sharp emissions, two main radiative recombination channels were observed with ionization energies estimated in the range of 50-80 meV and lower than 10 meV. No variation of the low-temperature PL spectra was observed for samples grown at different P precursor flows, while increasing Ga content enhanced the dominant broad emission at around 1.68 eV, suggesting that the group III organometallic precursor is the main source of impurities. Finally, Be-doped samples were grown, and their characteristic optical emission at 2.03 eV was identified. These results contribute to the understanding of impurity-related luminescence in hexagonal GaP, being useful for further crystal growth optimization required for the fabrication of optoelectronic devices.

Synthesis and Properties of Stable Amino Metal Halide Molecular Clusters in the Solid State.

Nanosized molecular clusters (MCs) composed of PbBr2 and neutral ligand butylamine (BTYA) with unique optical properties in solution and solid states have been synthesized using ligand-assisted reprecipitation and spin-coating, separately. The studies of their optical properties using ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) show the first electronic absorption and PL band of the MCs at 401 and 411 nm, respectively, for the solution and solid state samples that exhibit good stability under ambient conditions. Low-temperature PL spectra below 30 K show vibronic peaks indicative of a single size or a very narrow size distribution of the MCs. On the basis of Raman, X-ray diffraction, and transmission electron microscopy measurements, a layered structural model is proposed for the MCs with a BTYA ligand capping on the surface of the corner-shared tilted [PbBr6]4- octahedral framework. The stable and retained structure of MCs in the solid state is promising for photonics applications.

On-demand generation of optically active defects in monolayer WS2 by a focused helium ion beam

We demonstrate that optically active emitters can be locally generated by focusing a He-ion beam onto monolayer WS2 encapsulated in hBN. The emitters show a low-temperature photoluminescence spectrum, which is well described by an independent Boson model for localized emitters. Consistently, the photoluminescence intensity of the emitters saturates at low excitation intensities, which is distinct to the photoluminescence of excitonic transitions in the investigated WS2 monolayers. The demonstrated method allows us to position defect emitters in WS2 monolayers on demand. A statistical analysis suggests the generation yield of individual emitters to be as high as 11% at the highest investigated He-ion doses.

In(Ga)N 3D Growth on GaN-Buffered On-Axis and Off-Axis (0001) Sapphire Substrates by MOCVD.

In(Ga)N epitaxial layers were grown on on-axis and off-axis (0001) sapphire substrates with an about 1100 nm-thick GaN buffer layer stack using organometallic chemical vapor deposition at 600 °C. The In(Ga)N layers consisted of a thin (~10–25 nm) continuous layer of small conical pyramids in which large conical pyramids with an approximate height of 50–80 nm were randomly distributed. The large pyramids were grown above the edge-type dislocations which originated in the GaN buffer; the dislocations did not penetrate the large, isolated pyramids. The large pyramids were well crystallized and relaxed with a small quantity of defects, such as dislocations, preferentially located at the contact zones of adjacent pyramids. The low temperature (6.5 K) photoluminescence spectra showed one clear maximum at 853 meV with a full width at half maximum (FWHM) of 75 meV and 859 meV with a FWHM of 80 meV for the off-axis and on-axis samples, respectively.

Disorder and wave-function intermixing effects in the luminescence spectra of LiYF4:Ho in external magnetic fields

On the basis of the high-resolution low-temperature photoluminescence spectra of LiYF4:Ho crystals we discuss spectral manifestations of disorder caused by random distribution of either the lithium isotopes or defects in the crystal lattice. A peculiar discrete spectral structure of a different nature was found in the magnetic-field-dependent spectra. We show that it arises due to mixing of the wave functions at the anticrossings of hyperfine levels in a magnetic field.