E2g Phonon Mode Research Articles

Quantifying Strain in Moiré Superlattice.

In twisted van der Waals (vdW) bilayers, intrinsic strain associated with the moiré superlattice and unintentionally introduced uniaxial strain may be present simultaneously. Both strains are able to lift the degeneracy of the E2g phonon modes in Raman spectra. Because of the different rotation symmetry of the two types of strain, the corresponding Raman intensity exhibits a distinct polarization dependence. We compare a 2.5° twisted MoS2 bilayer, in which the maximal intrinsic moiré strain is anticipated, and a natural MoS2 bilayer with an intentionally introduced uniaxial strain. By analyzing the frequency shift of the E2g doublet and their polarization dependence, we can not only determine the direction of unintentional uniaxial strain in the twisted bilayer but also quantify both strain components. This simple strain characterization method based on far-field Raman spectra will facilitate the studies of electronic properties of moiré superlattices under the influence of combined intrinsic and external strains.

Raman scattering investigation of twisted WS2/MoS2 heterostructures: interlayer mechanical coupling versus charge transfer

Twisted van der Waals homo- and hetero-structures have aroused great attentions due to their unique physical properties, providing a new platform to explore the novel two-dimensional (2D) condensed matter physics. The robust dependence of phonon vibrations and electronic band structures on the twist angle has been intensively observed in transition metal dichalcogenide (TMD) homo-structures. However, the effects of twist angle on the lattice vibrational properties in the TMD heterostructures have not caused enough attention. Here, we report the distinct evolutions of Raman scattering and the underlying interlayer interactions in the twisted WS2/MoS2 heterostructures. The shifts and linewidths of E2g(Γ) and A1g(Γ) phonon modes are found to be twist angle dependent. In particular, analogous to that of the twisted TMD homostructures, the frequency separations between E2g(Γ) and A1g(Γ) modes of MoS2 and WS2 in the twisted heterostructures varying with twist angle correlate with the interlayer mechanical coupling, essentially originating from the spacing-related repulsion between sulfur atoms. Moreover, the opposite shift behaviors and broadening of A1g(Γ) modes caused by charge transfer are also observed in the twisted heterostructures. The calculated interlayer distances and band alignment of twisted WS2/MoS2 through density functional theory further evidence our interpretations on the roles of the interlayer mechanical coupling and charge transfer in variations of Raman features. Such understanding and controlling of interlayer interaction through the stacking orientation are significant for future optoelectronic device design based on the newly emerged 2D heterostructures.

Thermomechanical Response of Supported Hexagonal Boron Nitride Sheets of Various Thicknesses

Raman spectroscopy is employed to investigate the temperature dependence of the E2g phonon mode of single-layer, few-layer (FL), and bulk hexagonal boron nitride (hBN) sheets, situated over Si/SiO2 (90 nm). Depending on the sample, two temperature regimes are recorded. In the low to mid range of the examined temperatures, the monolayer and FL samples exhibit significantly higher slopes compared to the thicker ones because of the thermal expansion of the underlying substrate. In the high-temperature region, all the examined samples show almost the same temperature slope, indicating slippage of the hBN sheets relative to the substrate and providing strong evidence that the slopes of the monolayer, FL, and bulk hBN are quite similar of about −0.020 cm–1/K. This is further justified from the full width at half-maximum versus T of the studied samples and the observed similarities of the thermal expansion coefficient (TEC) of one to three layers and bulk hBN, revealing a comparable level of anharmonicity for the E2g mode from the monolayer up to bulk hBN. Moreover, using a finite element method, we have determined the TEC of the underling substrate and the strain induced by TEC mismatch between the single-layer hBN and the substrate. Consequently, the intrinsic frequency shift of the E2g mode for the monolayer hBN upon temperature change is extracted. Finally, the vibrational response of monolayer hBN upon temperature is also examined by means of computational simulations, and satisfactory agreement with the experiment is obtained.

Out-of-plane ionicity versus in-plane covalency interplay and electron–phonon coupling in MgB2 superconductor

Using the Density Functional Theory (DFT) within the Generalized Gradient Approximation (GGA) pseudopotential and plane wave basis method along with the frozen-phonon approach that starts from the ab initio evaluation of the total energy Etot of the solid with frozen-in atomic displacements, it is found that a superposition of A2u and the E2gvibrations modes is the key factor in the superconducting mechanism in MgB2 compound. Electron–Phonon coupling to these A2u and E2g phonon modes especially at the zone-boundary A point of the hexagonal Brilliouin zone leads to an interband hole charge transfer (and transfer back) between in-plane σ bond to the out-of-plane π bond along with an interatomic electron charge transfer (and transfer back) between the Magnesium s-states to the Boron out-of-plane pz-state. The direction of the electronic current is opposite to that of hole current so that it reinforces the polarization associated with these currents and may generate a large dynamical charge at a given critical temperature Tc that drives the compound into the superconducting state.

Thermal conductivity of PLD-grown thermoelectric Bi2Te2.7Se0.3 films using temperature-dependent Raman spectroscopy technique

Thermoelectric (TE) materials have increasingly entranced the attention of researchers globally, with a view to providing alternative routes to clean energy. This work reports the first thermal conductivity measurement on pulsed laser deposited Bi2Se2.7Te0.3 (BTS) thin films by a non-contact and optical-based method. Structural characterization revealed well-crystalline films whereas the scanning electron microscope images show abundant grains typical of the BTS nanostructures. A highly stoichiometric composition was observed under energy dispersive spectroscopy. The film's first-order temperature coefficient was estimated by varying the substrate temperature using advanced Raman spectrometer equipped with an inbuilt Linkam stage. The linear dependence of the E2g phonon mode on temperature and excitation laser power was harnessed to estimate the value of the suspended film's room temperature thermal conductivity. The first-order temperature coefficient was measured to be 0.0359cm−1K−1. The low thermal conductivity measured at room temperature (κ~1.12 Wm−1K−1) is comparable to previously reported values obtained using different conventional techniques. This work proved the potential of Raman spectroscopy to estimate the thermal conductivity as an excellent alternative tool to the existing expensive and laborious techniques. In addition, this work demonstrated that BTS is an excellent material for thermoelectric energy harvesting.

Optical Contrast and Raman Spectroscopy Techniques Applied to Few-Layer 2D Hexagonal Boron Nitride.

The successful integration of few-layer thick hexagonal boron nitride (hBN) into devices based on two-dimensional materials requires fast and non-destructive techniques to quantify their thickness. Optical contrast methods and Raman spectroscopy have been widely used to estimate the thickness of two-dimensional semiconductors and semi-metals. However, they have so far not been applied to two-dimensional insulators. In this work, we demonstrate the ability of optical contrast techniques to estimate the thickness of few-layer hBN on SiO2/Si substrates, which was also measured by atomic force microscopy. Optical contrast of hBN on SiO2/Si substrates exhibits a linear trend with the number of hBN monolayers in the few-layer thickness range. We also used bandpass filters (500–650 nm) to improve the effectiveness of the optical contrast methods for thickness estimations. We also investigated the thickness dependence of the high frequency in-plane E2g phonon mode of atomically thin hBN on SiO2/Si substrates by micro-Raman spectroscopy, which exhibits a weak thickness-dependence attributable to the in-plane vibration character of this mode. Ab initio calculations of the Raman active phonon modes of atomically thin free-standing crystals support these results, even if the substrate can reduce the frequency shift of the E2g phonon mode by reducing the hBN thickness. Therefore, the optical contrast method arises as the most suitable and fast technique to estimate the thickness of hBN nanosheets.

The origin of intrinsic charge transport for Dirac carbon sheet materials: roles of acetylenic linkage and electron-phonon couplings.

Within the past few years, intriguing graphene Dirac cones have attracted intense interest in novel two-dimensional (2D) Dirac materials as ultrahigh-mobility functional materials. In this work, the phonon-limited charge transport properties of α-graphyne (α-GY), α-graphdiyne (α-GDY), and β-graphyne (β-GY) were investigated using the Boltzmann transport equation within the first-principles framework while considering the electron-phonon coupling (EPC). Despite all three investigated compounds being 2D Dirac carbon materials, each demonstrated distinctly different carrier mobilities by one order of magnitude (2.2 × 104 cm2 V-1 s-1 for α-GY, 2.1 × 103 cm2 V-1 s-1 for α-GDY and 1.9 × 103 cm2 V-1 s-1 for β-GY at room-temperature and a carrier connection of n ∼ 3 × 1012 cm-2). The essential differences in the mobilities of these materials originated from the acetylenic linkage limiting the group velocity and the E2g phonon modes limiting the scattering time. For example, a few uniformly equivalent acetylenic linkages and E2g phonon modes tend to generate high mobilities. A simple mobility relationship was determined using the number of E2g photon modes, allowing for a quick estimation of the mobilities for Dirac materials. α-GY was identified as a promising alternative to graphene for next generation nanoelectronic devices.

Enhancement of superconducting parameters of MgB2 by low energy carbon ion implantation

The aim of work is to study the effects of carbon ion implantation on enhancement of superconducting properties of MgB2. Pure MgB2 bulk samples are irradiated by carbon ion beam of energy 40 keV (beam current 1 µA) at different fluences. After irradiation, there is enhancement in superconducting properties viz., critical current density (Jc), critical magnetic field (Hc2), comparing to pristine sample. Estimated with Ginzburg-Landau theory method at zero kelvin, the critical magnetic field Hc2(0) for irradiated sample shows an increase of 47 T than that of 38 T reported for pristine sample. There is also a four times increase in Jc after implantation. Improvement in grain connectivity with ion fluence is found in surface morphological studies. Raman spectroscopic studies show a shift in resonance peak related with E2g phonon modes after irradiation. This shifting is caused due to improvement in scattering and alteration of Fermi surface in implanted samples. These results are explained in correlation with the lattice disorder.

Electron-phonon coupling and superconductivity in LiB1+xC1−x

By means of the first-principles density-functional theory calculation and Wannier interpolation, electron-phonon coupling and superconductivity are systematically explored for boron-doped LiBC (i.e., ), with x between 0.1 and 0.9. Hole doping introduced by boron atoms is treated through virtual-crystal approximation. For the investigated doping concentrations, our calculations show that the optimal doping concentration corresponds to 0.8. By solving the anisotropic Eliashberg equations, we find that LiB1.8C0.2 is a two-gap superconductor, whose superconducting transition temperature, Tc, may exceed the experimentally observed value of MgB2. Similar to MgB2, the two-dimensional bond-stretching E2g phonon modes along the line have the largest contribution to electron-phonon coupling. More importantly, we find that the first two acoustic phonon modes B1 and A1 around the midpoint of the line play a vital role for the rise of Tc in LiB1.8C0.2. The origin of strong couplings in B1 and A1 modes can be attributed to enhanced electron-phonon coupling matrix elements and softened phonons. It is revealed that all these phonon modes couple strongly with σ-bonding electronic states.

Giant Mechano-Optoelectronic Effect in an Atomically Thin Semiconductor

Transition metal dichalcogenides (TMDs) are particularly sensitive to mechanical strain because they are capable of experiencing high atomic displacements without nucleating defects to release excess energy. Being promising for photonic applications, it has been shown that as certain phases of layered TMDs MX2 (M = Mo or W; X = S, Se, or Te) are scaled to a thickness of one monolayer, the photoluminescence response is dramatically enhanced due to the emergence of a direct electronic band gap compared with their multilayer or bulk counterparts, which typically exhibit indirect band gaps. Recently, mechanical strain has also been predicted to enable direct excitonic recombination in these materials, in which large changes in the photoluminescence response will occur during an indirect-to-direct band gap transition brought on by elastic tensile strain. Here, we demonstrate an enhancement of 2 orders of magnitude in the photoluminescence emission intensity in uniaxially strained single crystalline WSe2 bilayers. Through a theoretical model that includes experimentally relevant system conditions, we determine this amplification to arise from a significant increase in direct excitonic recombination. Adding confidence to the high levels of elastic strain achieved in this report, we observe strain-independent, mode-dependent Grüneisen parameters over the entire range of tensile strain (1-3.59%), which were obtained as 1.149 ± 0.027, 0.307 ± 0.061, and 0.357 ± 0.103 for the E2g, A1g, and A21g optical phonon modes, respectively. These results can inform the predictive strain-engineered design of other atomically thin indirect semiconductors, in which a decrease in out-of-plane bonding strength may lead to an increase in the strength of strain-coupled optoelectronic effects.

Electron Density Response to Phonon Dynamics in MgB<sub>2</sub>: An Indicator of Superconducting Properties

Electron density differences resulting from atom displacement patterns aligned with phonon modes in MgB2 have been calculated using density functional theory (DFT). The extent of phonon anomalies, identified as indicators of the superconducting transition temperature, Tc, under a range of conditions in AlB2-type structures, reduce as boron atoms are displaced from their equilibrium positions along E2g mode directions. The Fermi energy for displacements along the directions of the E2g phonon mode accounts for changes in the covalent B-B bond electronic charge density. We applied differential atom displacements to show that the shifted σ band structure associated with the light effective mass became tangential to the Fermi level and that the Fermi surface undergoes a topological transition at a critical relative displacement of ~0.6% of the boron atoms from equilibrium. The difference in Fermi energies at this critical displacement and at the equilibrium position correspond to the superconducting energy gap. The net volume between tubular σ surfaces in reciprocal space correlated with the depth of the phonon anomaly and, by inference, it is a key to an understanding of superconductivity. This ab initioapproach offers a phenomenological understanding of the factors that determine Tc based on knowledge of the crystal structure.

Mechanical pressure induced chemical cutting of boron nitride sheets into boron nitride quantum dots and optical properties

The ability to exfoliate 2D hexagonal boron nitride nanosheets (h-BNNSs) into nano-sized layered particles to form the boron nitride quantum dots (BNQDs) has been discussed by mechanical pressure through high intensity sonication. The top-down, simple and efficient synthesis approach has been applied for synthesis of BNQDs directly from bulk h-BN powder by high intensity sonication processes. The time dependent high intensity sonication shows the thinning of BNNSs or delaminated BNNSs are broken into nano-size in form of BNQDs. Higher ultrasonication time duration reduce the number of h-BN layers from h-BNNSs and produce the physical defects on the surface and finally transform into BNQDs. To evaluate the structural analysis and properties of BNQDs were employed by XRD, HRTEM, EDX, XPS, AFM and PL techniques. The average and uniform size of BNQDs observed from TEM analysis were 3–6 nm and containing an average thickness of 3–12 atomic BN layers. Raman spectroscopy confirms the formation of few layered BNQDs nanostructure from bulk h-BN powder by indicating the weak and small E2g phonon mode (1365 cm−1). The photoluminescence (PL) measurements carried out of BNQDs show typical luminescence spectra and strong peak emission at 448 nm.

Isotope effect on electron-phonon interaction in the multiband superconductorMgB2

We investigate the effect of isotope substitution on the electron-phonon interaction in the multi-band superconductor MgB2 using tunable laser based Angle Resolved Photoemission Spectroscopy. The kink structure around 70 meV in the $\sigma$ band, which is caused by electron coupling to the E2g phonon mode, is shifted to higher binding energy by ~3.5 meV in Mg(10)B2 and the shift is not affected by superconducting transition. These results serve as the benchmark for investigations of isotope effects in known, unconventional superconductors and newly discovered superconductors where the origin of pairing is unknown.

Direct observation of the generation of coherent optical phonons in thin antimony films by the femtosecond electron diffraction method

The generation of coherent optical phonons in an antimony film has been directly observed by the femtosecond electron diffraction method. The sample has been excited by a femtosecond laser pulse (λ = 800 nm) and probed with a pulsed photoelectron beam. Oscillations of the intensity corresponding to vibration frequencies of optical phonons excited by the laser have been observed in the obtained diffraction patterns: totally symmetric (A1g) and twofold degenerate (E2g) phonon modes of antimony and their combinations.

Strong interaction between electrons and collective excitations in the multiband superconductorMgB2

We use a tunable laser ARPES to study the electronic properties of the prototypical multiband BCS superconductor MgB2. Our data reveal a strong renormalization of the dispersion (kink) at ~65 meV, which is caused by coupling of electrons to the E2g phonon mode. In contrast to cuprates, the 65 meV kink in MgB2 does not change significantly across Tc. More interestingly, we observe strong coupling to a second, lower energy collective mode at binding energy of 10 meV. This excitation vanishes above Tc and is likely a signature of the elusive Leggett mode.

Pristine Basal- and Edge-Plane-Oriented Molybdenite MoS2 Exhibiting Highly Anisotropic Properties.

The layered structure of molybdenum disulfide (MoS2 ) is structurally similar to that of graphite, with individual sheets strongly covalently bonded within but held together through weak van der Waals interactions. This results in two distinct surfaces of MoS2 : basal and edge planes. The edge plane was theoretically predicted to be more electroactive than the basal plane, but evidence from direct experimental comparison is elusive. Herein, the first study comparing the two surfaces of MoS2 by using macroscopic crystals is presented. A careful investigation of the electrochemical properties of macroscopic MoS2 pristine crystals with precise control over the exposure of one plane surface, that is, basal plane or edge plane, was performed. These crystals were characterized thoroughly by AFM, Raman spectroscopy, X-ray photoelectron spectroscopy, voltammetry, digital simulation, and DFT calculations. In the Raman spectra, the basal and edge planes show anisotropy in the preferred excitation of E2g and A1g phonon modes, respectively. The edge plane exhibits a much larger heterogeneous electron transfer rate constant k(0) of 4.96×10(-5) and 1.1×10(-3) cm s(-1) for [Fe(CN)6 ](3-/4-) and [Ru(NH3 )6 ](3+/2+) redox probes, respectively, compared to the basal plane, which yielded k(0) tending towards zero for [Fe(CN)6 ](3-/4-) and about 9.3×10(-4) cm s(-1) for [Ru(NH3 )6 ](3+/2+) . The industrially important hydrogen evolution reaction follows the trend observed for [Fe(CN)6 ](3-/4-) in that the basal plane is basically inactive. The experimental comparison of the edge and basal planes of MoS2 crystals is supported by DFT calculations.

Pressure effects on the electronic structure and superconducting critical temperature of Li2B2

We present the structural, electronic and superconducting properties of Li2B2 under pressure within the framework of the density functional theory. The structural parameters, electronic band structure, phonon frequency of the E2g phonon mode and superconducting critical temperature Tc were calculated for pressures up to 20 GPa. We predicted that the superconducting critical temperature of Li2B2 is about 11 K and this decreases as pressure increases. We found that even though the lattice dynamics of the E2g phonon mode is similar to MgB2, the reduction of the σ-band density of states at Fermi level and the raising of the E2g phonon frequency with pressure were determinant to decrease λ and consequently Tc.

Effect of boron and nitrogen doping with native point defects on the vibrational properties of graphene

Boron and nitrogen doping in graphene has important implications in graphene-based devices. We investigate systematically the vibrational properties of B- and N-doped graphene with vacancies using forced vibrational method. We have calculated the phonon density of states (PDOSs), typical mode patterns and phonon localization length for different concentration of B, N and vacancies. We find that the interference between native point defects and B or N dopant break down the phonon degeneracy at the Г point of the LO and TO modes, distort and shift down the PDOSs significantly. We observe a broadening and softening of the Raman active E2g phonon mode with an increase of B and N atoms. The PDOS peaks for the mixture of vacancies and B or N atoms show the remarkable increase in the low-frequency region induced by their defect formations. Our computer experiments demonstrate that the disordered graphene show the spatially localized vibrations due to the resonant vibrations of the impurity atoms relative to the main C atoms. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the phonon is localized strongly within a region of several nanometers in the random disordered graphene structures. In particular, a typical localization length is on the order of ≈9.5nm for B- and N-doping, ≈9nm for mixture of B-doping and vacancy, and ≈8.5nm for mixture of N-doping and vacancy concentrations of 20%. This study provides a useful basis for the understanding of a wide variety of physical properties such as thermal conductivity, specific heat capacity, and electron–phonon interaction, as well as in the experiments of infrared, Raman, and neutron-diffraction spectra of doped-graphene.

Alpha to omega martensitic phase transformation pathways in pure Zr

Abstract This first principles study attempts to find the lowest enthalpy barrier transformation pathway (TP) for pressure induced α → ω transformation in pure Zr by systematically studying TPs suggested experimentally along with TPs generated from symmetry analysis. We find that symmetry generated pathways and model pathways given from experimental inputs essentially outline the same subset of possible pathways. The most favorable TP, comprising of smaller strains and atomic shuffles compared to others, has a C2/c as a common space group along the TP with enthalpy barrier of 22 meV/atom at 0 K. It is evident from the study that the softening of shear elastic constant C44 and Γ point transverse optical phonon frequency as a function of pressure in α-Zr are associated with change in effective potential landscape for the E2g phonon at Γ point of the Brillouin zone. The calculated electronic band structure as a function of pressure and E2g phonon mode shows a similar topological change in the band structure which drives this transformation.

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb2AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb2AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E2g (ω1) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E2g (ω2) and A1g (ω4) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E2g (ω2) and A1g (ω4) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.