Anomalous Blueshift Research Articles

Temperature-dependent Mn2+ emission in codoped CsPb2 (BrCl)5 perovskite nanocrystals

We prepared codoped CsPb2(BrCl)5 nanocrystal (NC) films by varying the Cu/Mn molar ratio using a facile two-step solution method. The Mn2+ emission peak is studied in the temperature range of 80–300 K. Strong electron–optical-phonon coupling causes an anomalous blueshift and linewidth broadening of the Mn2+ emission peak with increasing temperature. The average optical-phonon energy is approximately 50 meV. The activation energy reaches 79.32 meV, indicating that the introduction of Cu2+ in Mn-doped CsPb2(BrCl)5 NCs greatly enhances the energy transfer (ET) from the perovskite NC host to Mn2+, increasing the Mn2+ emission intensity. A clear understanding of the optical mechanism of Mn2+ emission is of great significance for the widespread application of codoped CsPb2(BrCl)5 NCs in optoelectronic devices.

Temperature dependence of photoluminescence in twisted heterobilayers of transition-metal dichalcogenides

Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDCs) are highly attractive due to their novel phenomena and potential device applications. Especially, twisted heterojunction of different 2D TMDC monolayers for various twist angles (θ) is one of the hot issues in the area of 2D TMDCs because they exhibit exotic quantum behaviors. Here, we report the effect of temperature (T) on the θ-dependent variation of photoluminescence (PL) spectra of MoS2/WS2 heterobilayers. The PL peak-vs-T variations are divided into three θ regions. Near θ = 0 and 60°, the PL peak energies show almost monotonically-decreasing behaviors with increasing T, caused by stronger electron-phonon interactions at higher T. However, in the intermediate θ range, the PL peaks show similar red shifts over almost full T range except anomalous blue shifts in a short T range near ∼150 K. We discuss this anomalous PL behaviors based on hybridization of interlayer and intralayer excitons, resulting in the formation of hybridized excitons, correlated with structural transition.

Revealing the enhanced crystalline quality mechanism of perovskites produced by microwave-assisted synthesis: toward the fabrications in a fully ambient air environment

The conventional stirring method (CSM) in a nitrogen-controlled glovebox has been widely used to synthesize the Cs/MA/FA precursor solution. However, it involves a long preparation time and limits the fabrication conditions in an ambient environment, hindering their potential for high-end applications and environmental stability. Herein, we systematically developed a fast and efficient microwave-assisted synthesis (MAS) method to produce a high-quality and stable triple cation perovskite (Cs0.1(MA0.17FA0.83)0.9Pb(Br0.17I0.83)3) solution and fabricated a device in ambient conditions. The superiority of using the MAS compared to CSM is manifested by comparing the improvements in crystal nucleation, enhanced optical properties, and its excellent stability with time, owing to the homogeneous heating of the precursor solution during the synthesis method. The MAS-prepared perovskite films showed anomalous photoluminescence blue-shift and linewidth broadening with increasing temperature, emphasizing the unique properties of the triple cation perovskite fabricated. These results were later interpreted and fitted to discuss the induced lattice thermal expansion, structural phase transition, and electron–phonon interactions that highlighted the enhanced crystal quality from the MAS-perovskite films. Considering the substantial fabrication parameters and conditions, the structural and optical stability of the MAS-perovskite films were maintained over time during exposure to air and ultraviolet light, even without encapsulation. To further elucidate the crystal quality improvements from using the MAS-perovskite solution, the fabrication of perovskite transistors in a completely ambient environment resulted in a 10-fold increase in electron carrier mobility compared to the CSM-perovskite devices. The developed synthesis method has contributed significant advantages to enhance the structural and environmental stability of perovskites, making a promising approach for larger-scale and more efficient future optoelectronic device applications.

Temperature−Dependent Raman Scattering Investigation on vdW Epitaxial PbI2/CrOCl Heterostructure

Van der Waals (vdW) epitaxial growth provides an efficient strategy to prepare heterostructures with atomically and electronically sharp interfaces. Herein, PbI2 was in situ thermally deposited onto exfoliated thin−layered CrOCl nanoflakes in high vacuum to fabricate vdW PbI2/CrOCl heterostructures. Optical microscopy, atomic force microscopy, X−ray diffraction, and temperature−dependent Raman spectroscopy were used to investigate the structural properties and phonon behaviors of the heterostructures. The morphology of PbI2 films on the CrOCl substrate obviously depended on the substrate temperature, changing from hemispherical granules to 2D nanoflakes with flat top surfaces. In addition, anomalous blueshift of the Ag1 and Au2 modes as the temperature increased in PbI2/CrOCl heterostructure was observed for the first time. Our results provide a novel material platform for the vdW heterostructure and a possible method for optimizing heterostructure growth behaviors.

Doping of charge-transfer molecules in cocrystals for the design of materials with novel piezo-activated luminescence.

A novel piezo-activated luminescent material with wide range modulation of the luminescence wavelength and a giant intensity enhancement upon compression was prepared using a strategy of molecular doping. The doping of THT molecules into TCNB-perylene cocrystals results in the formation of a weak but pressure-enhanced emission center in the material at ambient pressure. Upon compression, the emissive band from the undoped component TCNB-perylene undergoes a normal red shift and emission quenching, while the weak emission center shows an anomalous blue shift from 615 nm to 574 nm and a giant luminescence enhancement up to 16 GPa. Further theoretical calculations show that doping by THT could modify intermolecular interactions, promote molecular deformation, and importantly, inject electrons into the host TCNB-perylene upon compression, which contributes to the novel piezochromic luminescence behavior. Based on this finding, we further propose a universal approach to design and regulate the piezo-activated luminescence of materials by using other similar dopants.

Microstructure and Photoluminescence of ZrTiO4:Eu3+ Phosphors: Host-Sensitized Energy Transfer and Optical Thermometry

Orthorhombic ZrTiO4 is an attractive dielectric material; its optical properties are, however, less known. In this paper, we reported on the microstructure and luminescence studies of pristine ZrTiO4 and Eu3+-doped ZrTiO4 phosphors. The results indicated that two types of TiO6 octahedra, the isolated/ localized and coupled/delocalized, coexisted in host matrix. Eu3+ doping could induce oxygen vacancy defect states located below the bottom of the conduction band. Pristine ZrTiO4 showed bright yellow luminescence via STEs recombination at defects sites at low temperatures, but significant thermal quenching occurred due to STEs migration to quenching centers at elevated temperatures. Effective host sensitized energy transfer to Eu3+ was observed in ZrTiO4:Eu3+ phosphors and yielded the red characteristic emissions of Eu3+. Anomalous STEs luminescence enhancement and spectral blue-shift in the excitation spectra with higher Eu3 + concentration appeared and were explained by considering three factors: competitive absorption between electron transitions from the top of the valence band to the defect states and host conduction band, Eu3+ doping driving the production of more isolated TiO6 octahedra, and energy back-transfer from Eu3+ activators to other titanate groups. On the basis of the dual-emitting combination strategy involving host STEs and Eu3+ luminescence, ZrTiO4:Eu3+ phosphors were demonstrated to be ratiometric self-referencing optical thermometric materials, with a working range of 153–313 K and a maxima of relative sensitivity to ~1.1% K−1 at 243 K.

Atomlike interaction and optically tunable giant band-gap renormalization in large-area atomically thin MoS2

Coulomb interactions in atomically thin transition metal dichalcogenides can be dynamically engineered by exploiting the dielectric environment to control the optical and electronic properties. Here we demonstrate an optically tunable giant band-gap renormalization (BGR) $\ensuremath{\sim}1200$ and 850 meV from the edge of the conduction band and complete suppression of the exciton absorption in large-area single-layer (1L) and three-layer (3L) ${\mathrm{MoS}}_{2}$, respectively. The observed giant BGR is two orders of magnitude larger than that in the conventional semiconductors, and it persists for tens of ps. Strikingly, our results demonstrate photoinduced transparency at the electronic band gap using an intense optical field at room temperature. Exciton bleach recovery in 1L and 3L show a contrasting fluence-dependent response, demonstrating the layer-dependent optical tuning of exciton lifetime in a way that would be both reversible and real time. We find that the optical band gap (exciton resonance peak) shows a transient redshift followed by an anomalous blueshift from the lowest energy point as a function of the photo-generated carrier density. The observed exciton energy shift is analogous to atom-atom interactions, and it varies as a Lennard-Jones like potential as a function of the interexciton separation.

Anomalous Stark shift of excitonic complexes in monolayer WS2

Monolayer transition metal dichalcogenide semiconductors host strongly bound two-dimensional excitonic complexes, and form an excellent platform for probing many-body physics through manipulation of Coulomb interaction. Quantum confined Stark effect is one of the routes to dynamically tune the emission line of these excitonic complexes. In this work, using a high quality graphene/hBN/WS$_2$/hBN/Au vertical heterojunction, we demonstrate for the first time, an out-of-plane electric field driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. Such universal non-monotonic Stark shift with electric field arises from a competition between the conventional quantum confined Stark effect driven red shift and a suppressed binding energy driven anomalous blue shift of the emission lines, with the latter dominating in the low field regime. We also find that the encapsulating environment of the monolayer TMDC plays an important role in wave function spreading, and hence in determining the magnitude of the blue Stark shift. The results for neutral and charged excitonic species are in excellent agreement with calculations from Bethe-Salpeter Equation that uses seven-band per spin tight binding Hamiltonian. The findings have important implications in probing many-body interaction in the two dimension as well as in developing layered semiconductor based tunable optoelectronic devices.

Anomalous blueshift of aperture resonance enabled by the loss of a thin film

The substrate effects on aperture resonance have been widely studied because the resonance peak position is key for sensing, communications, and field enhancement applications. So far, the theoretical works have focused on the lossless infinite substrate case, which only explains the resonance peak redshift by the substrate index. The loss effect has not been investigated yet because lossy infinite substrate significantly reduces the aperture transmission. Therefore, this work studied the loss effect on aperture resonance via an analytic model of the transmission though slot antennas on a finite substrate. When the substrate thickness was very thin compared to the wavelength, the transmittance remained high even for a large imaginary part of the refractive index; furthermore, the developed model predicted a strong blueshift when this imaginary part exceeds the real part. Besides, the zero of the imaginary part of the coupling strength was crucial in determining the resonance frequency for both infinite and finite substrates. Thus, this study can contribute to the material characterization, label-free detection, and efficient design of active metamaterials.

Intermediate scale bandgap fluctuations in ultrathin Cu(In,Ga)Se2 absorber layers

Ultrathin single- and three-stage Cu(In,Ga)Se2 absorber layers were analyzed with room temperature photoluminescence (PL) spectra. An anomalous blueshift was observed upon increasing carrier injection for both samples. This blueshift was attributed to the presence of bandgap fluctuations that are of the same order as the minority carrier diffusion length. From time resolved measurements, a diffusion length of a few 100 nms was deduced. The single-stage spectrum consists of two peaks, and the sample was, therefore, also analyzed by hyperspectral imaging, providing lateral PL and reflectance data with 1 μm resolution. Marginal variations were observed in the PL yield and spectra. This homogeneity could again be attributed to an intermediate scale of the bandgap fluctuation with an upper limit of 1 μm for the scale of the lateral bandgap fluctuations. The two peaks in the PL spectra of the single-stage sample could be attributed to interference, and correction methods were applied. The bandgap fluctuations were extracted for the three-stage and single-stage sample and were 45 meV and 72 ± 3 meV, respectively. It is suggested that this difference is attributed to the smaller grains and larger amount of grain boundaries in the single-stage sample.

Anomalous Linear Layer-dependent Blue Shift of Interband Transition in Two-Dimensional Materials

Anomalous Linear Layer-dependent Blue Shift of Interband Transition in Two-Dimensional Materials

Squeezing of the Super-Elastic Double-Helix Photon in the Gravitational Field (Hidden in Very-Well Known Old Formulae). Pound - Rebka - Snider Effect Studied by the Advanced LIGO Instrument, Pioneer Anomaly and CMB (Cosmic Microwave Background) (23.11.201

In our approach we have combined knowledge of Old Masters (working in this field before the year 1905), New Masters (working in this field after the year 1905) and Dissidents under the guidance of Louis de Broglie and David Bohm. Based on the great experimental work of Robert Pound, Glen A. Rebka and J.L. Snider we have proposed a squeezing of the super-elastic double-helix photon in the gravitational field. We have newly defined the squeeze rate of that photon particle on the helical path. We have inserted this squeeze rate into the very-well old formulae of Newton, Soldner, Gerber and Einstein and might glimpse traces of the quantum gravity. The squeeze rate of photons can be studied in details using the Great instrument - the Advanced LIGO - located on the surface of the Earth (USA, Italy, Japan). The observed strains on the level 5*10-19 should be caused by the gravitational field of our Earth. The observed strains on the level 5*10-22 should be caused by the gravitational fields of the Moon and the Sun. We estimate that the experimental value of the gravitational constant G studied by the LIGO instrument can achieve the accuracy to the level of ppb (parts per billion) after the removal of those strains from the measured signal and the removal of the gravitational influences of the Earth, the Moon, the Sun, Venus and Jupiter. To study the squeeze effect on a bigger scale we propose to analyze the Pioneer anomaly where Pioneer´s photons have been flying around the planets in our Solar system causing the squeeze effect - the anomalous blueshift. Similarly, we can study cosmic microwave photons flying around the objects in our Solar system that might create “the axis of evil” - temperature fluctuations in the CMB map (Wien displacement law). Can we prepare in our Solar system “tired” light by frequent blueshift - redshift transitions? Can it be that Nature cleverly inserted the squeeze rate into our very-well known Old Formulae? We want to pass this concept into the hands of Readers of this Journal better educated in the Mathematics and Physics.

The High-resolution Transmission Spectrum of HD 189733b Interpreted with Atmospheric Doppler Shifts from Three-dimensional General Circulation Models

Abstract The signature of wind patterns caused by the interplay of rotation and energy redistribution in hot Jupiters is detectable at high spectral resolution, yet no direct comparison has been attempted between predictions from general circulation models (GCMs) and observed high-resolution spectra. We present the first such comparison on near-infrared transmission spectra of the hot Jupiter HD 189733b. Exploring 12 rotation rates and two chemical regimes, we have created model spectra from 3D GCMs and cross-correlated them with the observed spectra. Comparing our models against those of HD 189733b, we obtain three key results: (1) we confirm CO and H2O in the planet’s atmosphere at a detection significance of 8.2σ; (2) we recover the signature of day-to-night winds with speeds of several km s−1 at pressures of several millibars; and (3) we constrain the rotation period of the planet to between 1.2 and 4.69 days (synchronous rotation (2.2 days) remains consistent with existing observations). Our results do not suffer from the shortcomings of 1D models as cross-correlation templates—these models mainly tend to overconstrain the slower rotation rates and show evidence for anomalous blueshifts. Our 3D models instead match the observed line-of-sight velocity of this planet by self-consistently including the effects of high-altitude day-to-night winds. Overall, we find a high degree of consistency between observations of HD 189733b and our GCM-based spectra, implying that the physics and chemistry are adequately described in current 3D forward models for the purpose of interpreting observations at high spectral resolution.

Temperature-dependent photoluminescence characterization of compressively strained AlGaInAs quantum wells

A series of compressively strained AlGaInAs quantum well (QW) structures with different annealing treatment durations at 170 °C were investigated by temperature-dependent photoluminescence (PL). An abnormal S-shaped behavior in emission energy and a non-monotone evolution in spectral band width were observed at low temperature. A significant negative linear correlation between full width at half maximum (FWHM) and emission energy at different test temperature was exhibited in samples with different heat treatment durations. The highly linear relation demonstrated that the dependencies and relationships between PL peak energy and FWHM vs temperature are consistent. The anomalous blue shift and concomitant narrowing of FWHM in AlGaInAs QW were affected by lattice strain fluctuations due to the difference of thermal expansion coefficients between adjacent layers and strong carriers’ localization at low temperature.

Anomalous blue-shift of terahertz whispering-gallery modes via dielectric and metallic tuning.

The vast majority of resonant systems show a red-shift for the resonance frequency when a perturbation, e.g.,losses, is introduced to the system. In contrast, here we report for the first time, to the best of our knowledge, the experimental demonstration of both red- and anomalous blue-shifting of whispering-gallery modes (WGMs) using dielectric and metallic substrates. The maximum blue-shift is more than three times as large as the expected red-shift, proving that the anomalous blue-shift is more than a peculiar curiosity. The experiments are performed in the terahertz frequency range with coherent continuous-wave spectroscopy. The results establish dielectric and metallic tuning as a novel and viable approach to tune high-quality WGMs and provide valuable insights into the anomalous blue-shift of WGM cavity systems. The tuning capabilities for these compact monolithic resonators are of significant interest for fundamental science and technological applications alike.

Bright Mid-Infrared Photoluminescence from Thin-Film Black Phosphorus.

Recently rediscovered layered black phosphorus (BP) provides rich opportunities for investigations of device physics and applications. The band gap of BP is widely tunable by its layer number and a vertical electric field, covering a wide electromagnetic spectral range from visible to mid-infrared. Despite much progress in BP optoelectronics, the fundamental photoluminescence (PL) properties of thin-film BP in mid-infrared have rarely been investigated. Here, we report bright PL emission from thin-film BP (with thickness of 4.5 to 46 nm) from 80 to 300 K. The PL measurements indicate a band gap of 0.308 ± 0.003 eV in 46 nm thick BP at 80 K, and it increases monotonically to 0.334 ± 0.003 eV at 300 K. Such an anomalous blueshift agrees with the previous theoretical and photoconductivity spectroscopy results. However, the observed blueshift of 26 meV from 80 to 300 K is about 60% of the previously reported value. Most importantly, we show that the PL emission intensity from thin-film BP is only a few times weaker than that of an indium arsenide (InAs) multiple quantum well (MQW) structure grown by molecular beam epitaxy. Finally, we report the thickness-dependent PL spectra in thin-film BP in mid-infrared regime. Our work reveals the mid-infrared light emission properties of thin-film BP, suggesting its promising future in tunablemid-infraredlight emitting and lasing applications.

Theoretical Prediction and Analysis of the UV/Visible Absorption and Emission Spectra of Chiral Carbon Nanorings.

UV/vis absorption and emission spectra of recently synthesized chiral carbon nanorings were simulated using first-principles-based molecular dynamics and time-dependent density functional theory (TD-DFT). The chiral carbon nanorings are derivatives of the [ n]cycloparaphenylene ([ n]CPP) macrocycles, containing an acene unit such as naphthalene, ([ n]CPPN), anthracene ([ n]CPPA), and tetracene ([ n]CPPT), in addition to n paraphenylene units. In order to study the effect of increasing molecular size on absorption and emission spectra, we investigated the cases where n = 6 and 8. Frontier molecular orbital analysis was carried out to give insight into the degree of excitation delocalization and its relationship to the predicted absorption spectra. The lowest excited singlet state S1 corresponds to a HOMO-LUMO π-π* transition, which is allowed in all chiral carbon nanorings due to lack of molecular symmetry, in contrast to the forbidden HOMO-LUMO transition in the symmetric [ n]CPP molecules. The S1 absorption peak exhibits a blue-shift with increasing number of paraphenylene units in particular for [ n]CPPN and [ n]CPPA and less so in the case of [ n]CPPT. In the case of CPPN and CPPA, the transition density is mainly localized over a semicircle of the macrocycle with the acene unit in its center but is strongly localized on the tetracene unit in the case of CPPT. Molecular dynamics simulations performed on the excited state potential energy surfaces reveal red-shifted emission of these chiral carbon nanorings when the size of the π-conjugated acene units is increased, although the characteristic [ n]CPP blue-shift with increasing paraphenylene unit number n remains apparent. The anomalous emission blue-shift is caused by the excited state bending and torsional motions that stabilize the π HOMO and destabilize the π* LUMO, resulting in an increasing HOMO-LUMO gap.

Anomalous Raman and photoluminescence blue shift in mono- and a few layered pulsed laser deposited MoS2 thin films

We report on the Raman blue shift of layered MoS2 films as well as an anomalous blue shift of the photoluminescence spectra of nanocrystallite MoS2 films grown by pulsed laser deposition. Monolayer, bilayer and trilayer MoS2 films were fabricated by ablating MoS2 target for a deposition time of around 14, 24 and 30 s at a laser fluence of 2.2 Jcm−2. The two Raman active modes showed lattice strain-induced blue shift of 1–3 cm−1. The layered films showed an out-of-plane compressive strain as well as an in-plane tensile strain confirmed by Riet-Veld refinement of the X-ray diffraction pattern. TEM image of the MoS2 film for 14 s demonstrated the presence of nanocrystallites of an average size of 13 nm along with the layered film of MoS2. An anomalous photoluminescence peak was observed corresponding to the MoS2 nanocrystallites at 2.18 eV, attributed to quantum confinement effect.

Anomalous anisotropic exciton temperature dependence in rutile TiO2

Elucidating the details of the electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile TiO$_2$, revealing a strong anisotropic polarization dependence with increasing temperature, namely an anomalous blueshift for light polarized along the a-axis and a conventional redshift for light polarized along the c-axis. By employing many-body perturbation theory, we identify two terms in the electron-phonon interaction Hamiltonian that contribute to the anomalous blueshift of the a-axis exciton. Our approach paves the way to a complete ab initio treatment of the electron-phonon interaction and of its influence on the optical spectra of polar materials.

Synthesis and photoluminescence characteristics of blue‐green luminescent aluminum oxynitride

AbstractAluminum oxynitride (AlON), which can be regarded a nitrogen‐stabilized cubic γ‐Al2O3, has attracted attention in terms of its good mechanical, chemical, and optical stability. Because of its optical inertness, however, photoluminescence (PL) emission from nominally pure AlON has not been carefully investigated and evaluated. In this work, we prepared visibly luminescent AlON by nitridation of γ‐Al2O3 under N2 atmosphere without adding aluminum nitride (AlN) using a high‐frequency induction heating unit. The resulting AlON exhibits a broad PL emission in the blue/green spectral region under excitation with light of ~260 nm. In the luminescent AlON sample, the excitation and emission events will occur at different sites; the electron transfer from the excitation site to the emission site is preceded by the radiative recombination process. It has also been found that the PL peak wavelength shows an anomalous blue shift by ~50 nm with increasing temperature from 78 to 500 K. The observed temperature dependent PL characteristics are governed by thermalization among multiple emitting levels. Aluminum vacancies and oxygen vacancies, both of which are introduced into the crystalline lattice during nitridation without the presence of AlN, are very likely candidates for the excitation and emission centers, respectively. Hence, the present direct nitridation method provides a simple and effective way to add an additional optical functionality to otherwise optically inactive AlON.