Weak Electron-phonon Coupling Research Articles

Hot carrier relaxation dynamics of an aza-covalent organic framework during photoexcitation: An insight from abinitio quantum dynamics.

In order to develop an efficient metal-free solar energy harvester, we herein performed the electronic structure calculation, followed by the hot carrier relaxation dynamics of two dimensional (2D) aza-covalent organic framework by time domain density functional calculations in conjunction with non-adiabatic molecular dynamics (NAMD) simulation. The electronic structure calculation shows that the aza-covalent organic framework (COF) is a direct bandgap semiconductor with acute charge separation and effective optical absorption in the UV-visible region. Our study of non-adiabatic molecular dynamics simulation predicts the sufficiently prolonged electron-hole recombination process (6.8 nanoseconds) and the comparatively faster electron (22.48ps) and hole relaxation (0.51ps) dynamics in this two-dimensional aza-COF. According to our theoretical analysis, strong electron-phonon coupling is responsible for the rapid charge relaxation, whereas the electron-hole recombination process is slowed down by relatively weak electron-phonon coupling, relatively lower non-adiabatic coupling, and quick decoherence time. We do hope that our results of NAMD simulation on exciton relaxation dynamics will be helpful for designing photovoltaic devices based on this two dimensional aza-COF.

Possible charge ordering and anomalous transport in graphene/graphene quantum dot heterostructure

Observations of superconductivity and charge density waves (CDW) in graphene have been elusive thus far due to weak electron–phonon coupling (EPC) interactions. Here, we report a unique observation of anomalous transport and multiple charge ordering phases at high temperatures ( T1∼213K , T2∼325K ) in a 0D−2D van der Waals (vdW) heterostructure comprising of single layer graphene (SLG) and functionalized (amine) graphene quantum dots (GQD). The presence of functionalized GQD contributed to charge transfer with shifting of the Dirac point ∼ 0.05 eV above the Fermi level (ab initio simulations) and carrier density n∼−0.3×1012 cm−2 confirming p-doping in SLG and two-fold increase in EPC interaction was achieved. Moreover, we elucidate the interplay between electron–electron and electron–phonon interactions to substantiate high temperature EPC driven charge ordering in the heterostructure through analyses of magnetotransport and weak anti-localization (WAL) framework. Our results provide impetus to investigate strongly correlated phenomena such as CDW and superconducting phase transitions in novel graphene based heterostructures.

Spinel-Based ZnAl2O4: 0.5%Cr3+ Red Phosphor Ceramics for WLED.

To address the issue of the lack of red light in traditional Ce3+: YAG-encapsulated blue LED white light systems, we utilized spark plasma sintering (SPS) to prepare spinel-based Cr3+-doped red phosphor ceramics. Through phase and spectral analysis, the SPS-sintered ZnAl2O4: 0.5%Cr3+ phosphor ceramic exhibits good density, and Cr3+ is incorporated into [AlO6] octahedra as a red emitting center. We analyzed the reasons behind the narrow-band emission and millisecond-level lifetime of ZAO: 0.5%Cr3+, attributing it to the four-quadrupole interaction mechanism as determined through concentration quenching modeling. Additionally, we evaluated the thermal conductivity and thermal quenching performance of the ceramic. The weak electron-phonon coupling (EPC) effects and emission from antisite defects at 699 nm provide positive assistance in thermal quenching. At a high temperature of 150 °C, the thermal conductivity reaches up to 14 W·m-1·K-1, and the 687 nm PL intensity is maintained at around 70% of room temperature. Furthermore, the internal quantum efficiency (IQE) of ZAO: 0.5%Cr3+ phosphor ceramic can reach 78%. When encapsulated with Ce3+: YAG for a 450 nm blue LED, it compensates for the lack of red light, adjusts the color temperature, and improves the color rendering index (R9). This provides valuable insights for the study of white light emitting diodes (WLEDs).

Color Tunable Single-phase Cyclosilicate KBaScSi3O9: Eu2+/Tb3+/Sm3+ for n-UV pumped wLEDs

The exploration of stable/efficient phosphor for n-UV chip pumped white light-emitting diodes (wLEDs) remains an arduous task. Herein, a novel series of color tunable Eu2+/Tb3+/Sm3+ tridoped Sc-based cyclosilicate phosphors (KBaScSi3O9: Eu2+/Tb3+/Sm3+) have successfully designed and obtained via facile solid-state reaction based on comprehensive consideration of rigidly structural network of host, weak electron-phonon coupling strength and spectral adjustability of codopants via energy transfer (ET). The crystallographic occupancies of Eu, Tb and Sm primary in Ba/K, Ba and Ba sites in the monoclinic host, respectively were determined on the basis of the Rietveld refinements and crystal chemistry rules. The photoluminescence (PL) properties were systematically investigated in conjunction with structural analyses, implying that the ‘warm’ white light with excellent thermal stability is stemmed from combination of optimized blue (Eu2+), green (Tb3+) and red (Sm3+) emissions via Eu → Tb → Sm ET strategy. Interestingly, Tb ions play both roles of green light luminescent center and ET bridge to achieve cascade ET for connecting Eu/Sm ion pairs because of the adverse metal-metal charge transfer (MMCT) effect (Eu2+ + Sm3+ → Eu3+ + Sm2+). Furthermore, the phosphor-converted wLEDs (pc-wLEDs), by encapsulating the optimal KBS: 4%Eu/4%Tb/6%Sm with commercial n-UV LED chip, show high thermal stability with satisfactory electroluminescence (EL) performances. These results indicate the KBS: Eu/Tb/Sm phosphors are potential candidates for application in high power n-UV pumped pc-wLEDs.

Van Hove singularity driven enhancement of superconductivity in two-dimensional tungsten monofluoride (WF)

Superconductivity in two-dimensional materials has gained significant attention in the last few years. In this work, we report phonon-mediated superconductivity investigations in monolayer Tungsten monofluoride (WF) by solving anisotropic Migdal Eliashberg equations as implemented in EPW. By employing first-principles calculations, our examination of phonon dispersion spectra suggests that WF is dynamically stable. Our results show that WF has weak electron–phonon coupling (EPC) strength (λ) of 0.49 with superconducting transition temperature (T c ) of 2.6 K. A saddle point is observed at 0.11 eV below the Fermi level (E F ) of WF, which corresponds to the Van Hove singularity (VHS). On shifting the Fermi level to the VHS by hole doping (3.7 × 1014 cm−2), the EPC strength increases to 0.93, which leads to an increase in the T c to 11 K. However, the superconducting transition temperature of both pristine and doped WF increases to approximately 7.2 K and 17.2 K, respectively, by applying the Full Bandwidth (FBW) anisotropic Migdal–Eliashberg equations. Our results provide a platform for the experimental realization of superconductivity in WF and enhancement of the superconducting transition temperature by adjusting the position of E F to the VHS.

Electron-phonon coupling dictates electron mean free paths and negative thermal diffusion in metals

The spatiotemporal dynamics of the fundamental energy carriers in metals underpin the efficiency of a wide array of technologies. Although much progress in our understanding of the temporal response of electronic relaxation processes following an external field perturbation has been achieved, the spatial relaxation dynamics of electrons after excitation remains unclear. Here, we employ a scanning ultrafast microscopy with high spatial resolution to directly measure the mean free paths of electrons in different metals. We show that the strength of electron-phonon coupling not only dictates the mean free paths, but also controls a peculiar spatial shrinking of the electronic temperature profile immediately after the electrons have thermalized with the ‘colder’ lattice. More specifically, from our experimental tracking of the spatial relaxation profiles, a clear observation of an effective negative thermal diffusion is apparent for metals with weaker electron-phonon coupling, while for metals with stronger electron-phonon coupling, the negative thermal diffusion effect is not observable. Our spatiotemporal modeling based on the two-temperature approach provides evidence that although negative diffusion occurs universally for materials with two different species that have varying diffusivities and are thermodynamically coupled, effective negative diffusion is masked for cases with stronger coupling between the species.

Room-Temperature Magnetic-Induced Circularly Polarized Photoluminescence in Two-Dimensional Er2O2S.

Two-dimensional (2D) materials with spin polarization have great potential for achieving next-generation spintronic applications. However, spin polarization of 2D materials is usually produced at a cryogenic temperature because of thermal fluctuations, which severely hinder their further applications. Here, we report room-temperature intrinsic magnetic-induced circularly polarized photoluminescence (PL) in 2D Er2O2S flakes. The geff factor of 2D Er2O2S stays at around -6.3 from the liquid He temperature limit to room temperature, which is independent of temperature. This anomalous phenomenon in Er2O2S is totally different from previous materials, which all have a decreasing Zeeman splitting with increasing temperature resulting from thermal fluctuations. The anomalous temperature-dependent magnetic-induced circularly polarized PL originates from the weak electron-phonon coupling in 2D Er2O2S, which has been proven by both the temperature-dependent Raman and theoretical calculations. This work sheds light on the understanding and manipulation of 2D materials for practical spintronic applications.

Effect of the rigidity of an organic interlayer on the nonradiative recombination and exciton dissociation in hybrid Dion–Jacobson 2D lead iodide perovskites

3AMP-based 2D perovskites produce higher rigidity octahedral structures through more symmetric hydrogen bonding, ultimately resulting in weaker electron–phonon coupling and lower defect density.

Weak electron-phonon coupling contributing to enhanced thermoelectric performance in n-type TiCoSb half-Heusler alloys

n-type TiCoSb half-Heusler (HH) alloys show lower thermoelectric performance may be notably owing to the low valley degeneracy of the conduction band. Here, we show that this drawback can be counterbalanced by a decrease in the deformation potential coefficient and hence, weak electron-phonon coupling strength driven by the substitution of Nb for Ti in the n-type alloys Ti1-xNbxCoSb0.96Bi0.04. The combined substitutions of Nb and Bi yield a reduction of ∼86% in the deformation potential of the conduction band minimum, with the lowest value of ∼5 eV at 300 K achieved in Ti0.85Nb0.15CoSb0.96Bi0.04. This effect leads to enhanced power factor from ∼0.018 mW m−1 K−2 for x = 0 to ∼1.69 mW m−1 K−2 for x = 0.15 at RT due to the resulting strong increase in electron mobility by one order of magnitude. Both the isovalent Bi and aliovalent Nb substitutions further contribute to decrease the lattice thermal conductivity owing to enhanced mass and strain field fluctuations. The beneficial combined effects of weaker electron-phonon coupling and enhanced point-defect phonon scattering results in a higher dimensionless thermoelectric figure of merit ZT, with a peak value ∼ 0.37 at 870 K in Ti0.85Nb0.15CoSb0.96Bi0.04, representing a ∼ 375% improvement with respect to pristine TiCoSb.

Realization of Broadband Near-Infrared Emission with High Thermal Stability in YGa3(BO3)4: Cr3+ Borate Phosphor.

As a key material for phosphor-converted light-emitting diodes (pc-LEDs) applications, broadband near-infrared (NIR) phosphors currently face poor thermal stability issues. In this work, we synthesized a broadband near-infrared phosphor YGa3(BO3)4: Cr3+ (YGBO: Cr3+) with a high thermal stability. The YGBO: Cr3+ sample exhibits a broadband near-infrared emission centered at 770 nm with a full width at half-maximum (fwhm) of 2130 cm-1 under blue light excitation. Benefiting from the borate host crystal's strong structural rigidity, wide optical band gap, and weak electron-phonon coupling strength, YGBO: Cr3+ demonstrates strong luminescence thermal stability, and the corresponding luminescence intensity can maintain 80% at 150 °C compared to room temperature. Furthermore, we fabricated a pc-LED device using a blue light chip and YGBO: Cr3+ phosphor, and confirmed its application potential as a near-infrared light source in the spectral analysis of fruit freshness.

Zero thermal quenching and suppressed concentration quenching in a highly efficient structure‐confined fluoride near-infrared phosphor

The development of thermally stable and efficient broadband near-infrared (NIR) phosphors suffers from severe hindrance. Herein, a unique anti-fluorescence quenching is realized in a novel Cs2NaGaF6: Cr3+ NIR fluoride phosphor with a strategy of integrating the structural confinement effect with weak electron-phonon coupling effect. Upon blue light excitation, this phosphor exhibits a wide emission band with a peak wavelength of ∼789 nm and a full width at half maximum (FWHM) of 117 nm. Thanks to the structural confinement effect, a large quenching content of 20 mol% and a high internal quantum efficiency (IQE) of 87.4% are achieved, showing a suppressed concentration quenching behavior. Significantly, Cs2NaGaF6: 0.20Cr3+ phosphor possesses an abnormal zero thermal quenching, which could be related with the weaker electron-phonon coupling effect. The emission intensity of Cs2NaGaF6: 0.20Cr3+ at 423 K can maintain up to 101.96% of that at 298 K. A fabricated NIR phosphor-converted light-emitting diode (pc-LED) exhibits an output power of 57.71 mW and a photoelectric conversion efficiency as high as 19.24% at 100 mA. Finally, utilizing a high-power NIR LED as the illuminant for traffic surveillance, significant information can be captured even in low-visibility environments. As a result, Cs2NaGaF6: Cr3+ possesses tremendous prospects in high-power NIR LEDs.

How the Stacking Pattern Influences the Charge Transfer Dynamics of van der Waals Heterostructures: An Answer from a Time-Domain Ab Initio Study.

Here, we perform a time domain density functional study in conjunction with a non-adiabatic molecular dynamics (NAMD) simulation to investigate the charge carrier dynamics in a series of van der Waals heterostructures made of two-dimensional (2D) SnX2 (X = S or Se)-supported ZrS2, ZrSe2, and ZrSSe monolayers. Results from NAMD simulation reveal delayed electron-hole recombination (in the range of 0.53-2.13 ns) and ultrafast electron/hole transfer processes (electron transfer within 108.3-321.5 fs and hole transfer between 107.6 and 258.8 fs). The most interesting finding of our study is that switching from AB to AA stacking in the heterostructures extends the carrier lifespan by a significant amount. The delayed electron-hole recombination because of the switching stacking pattern can be rationalized by weak electron-phonon coupling, lower non-adiabatic coupling (NAC), and fast decoherence time. Thus, these insightful NAMD studies of excited charge carriers reveal that the stacking pattern variation is an effective tool to develop efficient photovoltaic devices based on 2D van der Waals heterostructures.

Thermally stable near-infrared emission from a double perovskite K2LiScF6:Cr3+ phosphor for light-emitting diodes

The exploration of Cr3+-doped near-infrared (NIR) phosphors for light-emitting diodes has attracted widespread attention. In this work, we present a fluoride double perovskite K2LiScF6:Cr3+ NIR-emitting phosphor through a co-precipitation method. It was found that the Cr3+ activators undergo a weak crystal field and present a NIR emission band peaking at 768 nm under blue light excitation (430 nm). Meanwhile, the K2LiScF6:Cr3+ exhibits a high activation energy (0.537 eV), a weak electron-phonon coupling (EPC) effect and a remarkable photoluminescence thermal stability at 423 K, with an intensity of 95.1% of that at room temperature. Profiting from the luminescent properties, clear night vision images and human palm vein photographs were realized by coating the K2LiScF6:Cr3+ phosphor on a blue InGaN chip and using it as a lighting source, suggesting its applicability in light-emitting diodes for NIR spectroscopy applications.

Polarons and bipolarons in one-dimensional systems (arbitrary coupling)

The properties of a one-dimensional (1D) polaron and a bipolaron in the region of strong and intermediate coupling are investigated. The Buimistrov–Pekar (BP) method was chosen for the calculation of the energy of self-trapped states. Calculations were carried out by the variational method using wave functions of various types. The lowest polaron and bipolaron energies were obtained using the Gaussian system of functions. In the limit of weak electron-phonon coupling, both the BP and the Feynman methods lead to a linear dependence of the polaron energy where is the Fröhlich constant of electron-phonon coupling. Such dependence can be interpreted as a transition from an autolocalized (in the strong coupling region) to a delocalized polaron state. At where, and are the high-frequency and static dielectric constants, the critical value of the electron-phonon coupling constant, at which the bipolaron states become unstable, is in the range where the upper limit was calculated with respect to the double energy of the Feynman polaron, and the lower bound was obtained using the Buimistrov–Pekar method. The triplet states of a bipolaron do not form a bound state. The Heisenberg exchange interaction of polarons has an antiferromagnetic character.

Enhanced Layer-Breathing Modes in van der Waals Heterostructures Based on Twisted Bilayer Graphene.

The characterization of interlayer coupling in two-dimensional van der Waals heterostructures (vdWHs) is essential to understand their quantum behaviors and structural functionalities. Interlayer shear and layer-breathing (LB) phonons carry rich information on interlayer interaction, but they are usually too weak to be detected via standard Raman spectroscopy due to the weak electron-phonon coupling (EPC). Here, we report a universal strategy to enhance LB modes of vdWHs based on twisted bilayer graphene (tBLG). In both tBLG/hBN and tBLG/MoS2 vdWHs, the resonantly excited electrons in tBLG can strongly couple to LB phonons extended over the entire layers in the vdWHs, whose resonance condition is tunable by the twist angle of tBLG. In vdWHs containing twisted graphene layers with multiple twisted interfaces, the EPC of LB phonons coming from the collective LB vibrations of entire heterostructure layers can be tuned by resonant excitation of programmable van Hove singularities according to each twisted interface. The universality and tunability of enhanced LB phonons by tBLG make it a promising method to investigate EPC and interlayer interaction in related vdWHs.

Stable Polyhedron-Cluster-Based Rigid Fluoride Framework toward Zero-Thermal-Quenching Near-Infrared Emission for Prolonged LED Applications.

Development of highly thermally stable broadband near-infrared (NIR) luminescence materials is crucial for advancing the prolonged stable application of smart NIR light sources. In this study, a zero-thermal-quenching and reversible temperature-dependent broadband NIR-emitting Cs2NaAl3F12:Cr3+ phosphor is demonstrated, benefiting from its stable polyhedron-cluster-building rigid structure. The excellent thermal stability of Cs2NaAl3F12:Cr3+ is rooted in its stable [Al6Na4F45] cluster building unit, which provides a rigid structure with a weak electron-phonon coupling effect and a wide band gap with a huge thermal activated barrier. Such characteristics are well revealed by multiple studies on crystal structure, electronic structure, Huang-Rhys factor S, configuration coordinate model, and Debye temperature. The incorporation of Li or K instead of Na weakens the luminescence thermal stability, directly proving the importance of the stable [Al6Na4F45] cluster for stable Cr3+ substitution and rigid structure construction. Furthermore, Cs2NaAl3F12:Cr3+ presents much superior thermal stability compared to traditional rigid garnet-type fluorides Na3X2Li3F12:Cr3+ (X = Al, Ga, In). A high-power NIR LED is presented, utilizing the high quantum efficiency (∼71%) and extremely thermally stable broadband NIR emission around 750 nm of Cs2NaAl3F12:Cr3+. It realizes clear vein and cartilage imaging in the human hand, demonstrating its potential in medical diagnosis applications. This result provides important insights for designing new-type rigid crystal structures using stable polyhedron clusters as basic units, advancing the development of highly thermally stable NIR-emitting phosphors.

Ultrafast demagnetization dynamics in the epitaxial FeGe(111) film chiral magnet

The exploring of ultrafast demagnetization is a significant subject for understanding the versatility of spin dynamics. However, the laser-induced ultrafast demagnetization process in chiral magnet FeGe has remained elusive. Here, we report the ultrafast demagnetization processes of FeGe film with various magnetic structures using time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Our work reveals a transition from one-step ultrafast demagnetization (type I) to two-step demagnetization (type II) process by changing ambient temperature, laser fluence, and magnetic field. We confirm that the transition from one-step to two-step ultrafast demagnetization mainly results from the weak electron-phonon (e-p) coupling. In the skyrmion phase region, TRMOKE curves demonstrate a two-step ultrafast demagnetization process. The two-step ultrafast demagnetization will further enhance the magnetization loss, and consequently facilitate the magnetization switching and pave a pathway to engineer chiral magnetic devices on picosecond timescales.

Broadband near-infrared emitting Cr3+-activated InGaO3(ZnO)4 phosphor and its application in pc-LEDs

In broadband near-infrared (NIR) phosphor converted to light-emitting diodes (pc-LEDs), Cr3+-activated phosphors have received a lot of attention. Herein, we developed a novel InGaO3(ZnO)4: xCr3+ (x = 0.001–0.02) with NIR broadband emission through a solid-state reaction at high temperature. Its excitation spectrum covers the blue light region, with the peak at 467 nm, which is suitable for blue light chip excitation. The optimum Cr3+ ion doping content is x = 0.01 and the emission spectrum ranges from 650 nm to 950 nm with a large full width at half maximum (FWHM) of 119 nm. Moreover, the internal quantum efficiency (IQE) of InGaO3(ZnO)4: 0.01Cr3+ phosphor is about 79%. Meanwhile, the phosphor exhibits good thermal stability due to the relatively weak electron-phonon coupling effect, and the emission intensity at 373 K can retain 57.7% of that at 303 K. The packaged pc-LED shows good NIR light penetration ability in spectral transmission tests, which indicates that the pc-LED has great application potential in portable devices as a light source.

Revealing the promising near-room-temperature thermoelectric performance in Ag2Se single crystals

With high carrier mobility and intrinsic low lattice thermal conductivity, Ag2Se compounds have attracted increasing attention for thermoelectric application near room temperature. Due to its phase transition at ∼406 K and resulting thermal volume expansion, the growth and thermoelectric properties of large-sized Ag2Se single crystals have seldom been reported so far. In this work, the vertical Bridgeman method was used for growing bulk Ag2Se single crystal, with an orientation preference along low-symmetric (201) plane. The Hall mobility as high as 2 000 cm2/(V·s) and weak electron-phonon coupling contributes to a high electronic quality BE of ∼7.0 in near-room-temperature β-Ag2Se single crystals, which is superior to the high-temperature phase α-Ag2Se. The observed low lattice thermal conductivity of 0.8 W/(m·K) at 300 K is due to the low group speeds and strong anharmonicity. A promising peak zT of 0.66 at 375 K and an average zT of 0.65 at 300–375 K were realized in β-Ag2Se crystals. The low Vickers hardness and good ductile properties were confirmed by experiment and theoretical analysis. This work not only synthesized large-sized and highly-orientated Ag2Se crystals, but also revealed its great potential of thermoelectric performance and mechanical properties for various applications near room temperature.

Multi-sites energy transfer in Fe3+-doped KAl11O17 phosphor toward zero thermal quenching near-infrared luminescence.

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have demonstrated great potential for optoelectronic and biomedical applications, while the exploration of NIR phosphors with high thermal stability remains a challenge. Herein, we report an NIR phosphor KAl11O17:Fe3+ with zero thermal quenching (TQ) behavior up to 200°C. The asymmetrical broadband NIR emission with three sub-bands centered at 700, 770, and 800 nm is related to the superposition of different Fe3+ emission centers located in Al2O4, Al3O6, and Al4O6 sites of the KAl11O17 host, respectively. Temperature- and Fe3+ concentration-dependent emission spectra verify that the energy transfer (ET) between multiple Fe3+ emitters and the weak electron-phonon coupling (EPC) effect contribute to the thermally stable broadband NIR emission. The fabricated NIR pc-LED using optimized KAl11O17:Fe3+ phosphor exhibits great potential in information encryption applications.