Phonon Frequency Shifts Research Articles

Ultrafast Photo‐Response by Surface State‐Mediated Optical Transitions in Topological Insulator Bi2Te3 Nanowire

AbstractTopological insulators are a new class of materials with highly interesting optoelectronic properties such as strong light absorption, polarization‐dependent surface photocurrent, and topological phase transitions. In the present study, anomalous characteristics of the topological surface state (TSS) are discovered in a single‐crystalline Bi2Te3 nanowire (NW) through mediated optical transitions by utilizing an optical pump‐THz probe. As a result, ultra‐fast carrier recombination occurs owing to the presence of TSS, and a phonon frequency shift occurs due to enhanced electron–phonon interaction. In particular, the large optical absorption of the second TSS is closely related to high optical conductivity. The photoresponse for visible light in Bi2Te3 NW with TSS represents a remarkable improvement. The result indicates that the presence of (first and second) TSS and quantum well 2D electron gas effectively contributes to the significant improvement in photocurrent sensitivity by enhancement of the photocurrent generation. In summary, TSS‐assisted optical transitions affect the efficiency of the optoelectronic device.

Intrinsic Low Thermal Conductivity and Phonon Renormalization Due to Strong Anharmonicity of Single-Crystal Tin Selenide.

Two-dimensional (2D) van der Waals material tin selenide (SnSe) has recently attracted intensive interest due to its exceptional thermoelectric performance. However, the thermal properties and phonon transport mechanisms in its single-crystal form remain elusive. Here, we measured high-quality SnSe single crystals using nanoscale thermometry based on ultrafast optical spectroscopy and found that its intrinsic thermal conductivity is highly anisotropic in different crystallographic directions. To quantify phonon anharmonicity, we developed a new experimental approach combining picosecond ultrasonics and X-ray diffraction to enable direct measurement of temperature-dependent sound velocity, thermal expansion coefficient, and Grüneisen parameter. The measured Grüneisen parameter suggests an abnormally large temperature effect on phonon dispersion that contributes to over 90% of phonon frequency shifts. Furthermore, we performed ab initio calculations using different methods: in comparison with self-consistent phonon theory, the harmonic and quasi-harmonic models that have been widely used in current phonon calculations fail to accurately predict these important thermophysical properties at room temperature and below. Our study reveals an extremely strong intrinsic anharmonicity in SnSe that introduces phonon renormalization near room temperature. This study represents an important research benchmark in characterizing high-performance thermal energy materials and provides fundamental insight into advancing modern calculation methods for phonon transport theory.

Structural and optical study of Zn-doped As2Se3 thin films: Evidence for photoinduced formation of ZnSe nanocrystallites

Amorphous Zn-doped As2Se3 films with a nominal zinc content x up to 10 at.% were prepared by thermal evaporation. Their structure is characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy. The AFM data show a considerable increase of the film surface roughness for films with x > 5 at.%. A strong gradient of the Zn content decreasing into the film depth is confirmed by the EDX and XPS data. Heavily Zn-doped (above 7 at.%) As2Se3 films reveal photostructural changes in the course of the Raman measurements. New Raman features are attributed to TO and LO vibrations of ZnSe nanocrystallites formed in the film under laser illumination. Depending on the laser wavelength and power density, the ZnSe nanocrystallites can experience tensile strain in the film due to a non-thermal photoplastic effect in the As2Se3 film resulting in a partial removal of the material from the laser spot. The tensile strain value, estimated from the TO and LO phonon frequency shift, is shown to reach up to 2.9 GPa.

Effect of substrate polarity on the optical and vibrational properties of (311)A and (311) B oriented InAlAs/InP heterostructures

In this report, we use the photoluminescence (PL) and micro-Raman scattering techniques to systematically study the optical and vibrational properties of InAlAs/InP heterostructures grown by metal organic chemical vapor deposition (MOCVD) on either Indium (A-face) or Phosphorous (B-face) terminated (311) oriented substrates. PL measurements show that the InAlAs band gap redshifts with sample polarity switching from B to A, which is attributed to the change of the indium composition with substrate polarity. Then, micro-Raman investigations have been realized with the aim to comprehend the influence of substrate polarity on the shifts in phonon frequencies in Raman spectra. By using the modified random-element-isodisplacement (MREI) theory, we have explained the alloying effect in the InxAl1-xAs layers. A good consistency between InAs and AlAs-like phonon frequencies from the fitting of Raman spectra and calculated is achieved. AlAs-like LO phonon frequency has been employed to calculate the In composition and residual strain in our samples. In addition, the In composition of the InxAl1-xAs layer has been also determined by PL measurements. A good agreement with the indium contents obtained by both methods and those measured during growth of the epilayers has been attained. Our findings suggest that optical and crystalline quality are better for the (311)A sample.

Phonon thermodynamics and elastic behavior of GaN at high temperatures and pressures

The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorter-wavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20 GPa, respectively. The Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zinc blende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes.

Mode Grüneisen parameters of an efficient thermoelectric half-Heusler

Half-Heusler compounds have recently been identified as promising thermoelectric materials, but their relatively high thermal conductivities impede the further improvement of thermoelectric performance. The knowledge of phonon vibrational properties provides a fundamental understanding of the thermal transport behavior of solids and thus could serve as a guidance on further suppressing the thermal conductivity. Herein, a highly efficient p-type half-Heusler thermoelectric alloy FeNb0.8Ti0.2Sb is taken as an example to explore its phonon vibrational properties. Phonon spectrum with the frequencies down to 10 cm−1 and its evolution with pressure for the studied material are provided by Raman scattering. It is found that two vibrational modes with the frequency >200 cm−1 display a common mode frequency increase with increasing pressure. Based on the bulk modulus from synchrotron X-ray powder diffraction and phonon frequency shifts, the mode Grüneisen parameters are obtained. Our results establish characteristic phonon vibrational properties of this high-performance half-Heusler thermoelectric alloy.

Phonon anharmonic frequency shift induced by four-phonon scattering calculated from first principles

Phonon energies at finite temperatures shift away from their harmonic values due to anharmonicity. In this paper, we have realized the rigorous calculation of phonon energy shifts of silicon by three- and four-phonon scattering from first principles. The anharmonic fourth-order force constants are calculated by considering up to the fifth nearest neighbors. The results agree reasonably well with available data from inelastic neutron scattering throughout the Brillouin zone. Surprisingly, the frequency shifts of optical phonon modes near the Γ point are sensitive to the cutoff radius of the fourth-order force constants, in contrast to the four-phonon scattering rates, which nearly saturate when considering the second nearest neighbors. We have also compared the results with ab initio molecular dynamics simulations and found that the higher order of anharmonicity is important for optical phonons. Our work provides critical insight into the anharmonic phonon frequency shift and will have a significant impact on the thermal and optical applications.

Optical properties of highly luminescent, monodisperse, and ultrastable CdSe/V2O5 core/shell quantum dots for in-vitro imaging

Herein, a first report on the formation of high-quality CdSe/V2O5 core/shell quantum dots (QDs). A single-source cluster approach has been adopted to synthesize highly luminescent CdSe and CdSe/V2O5 core/shell QDs. X-ray diffraction pattern depicts a crystal structural phase transformation from Zinc blende to Wurzite for core/shell QDs. Steady-state absorption and emission studies indicate a significant red-shift in both absorption and emission peak after shell growth. Formation of core/shell structures have been confirmed by absorption spectra of CdSe (1–12 h) along with TEM images analysis. Raman studies of core/shell QDs shows a lower wave number shift in phonon frequency which is correlated with lattice contraction and indicates the intensive electron interaction between CdSe and V2O5. Biocompatibility test (with A549 cell line) of CdSe and CdSe/V2O5 core/shell QDs have been carried out to know about toxicity of these QDs and results exhibited a noticeable increase in viability of cell lines for core/shell QDs. These highly luminescent, ultrastable core/shell QDs can be used in many applications such as catalysis, solar cell, and cellular imaging.

Raman shifts and photoluminescence of the InSb nanocrystals ion beam-synthesized in buried SiO2 layers

The InSb nanocrystals embedded in buried SiO2 layers were obtained by the In+ and Sb+ ion implantation into the SiO2 layers thermally grown on Si wafers followed by the hydrogen transfer of a Si film and high-temperature annealing at 800–1100 °C. Transmission electron microscopy, Raman spectroscopy and photoluminescence were used to study the sample properties. The spherical-shaped InSb nanocrystals distributed close to the implanted atom profiles were obtained in buried SiO2. The InSb TO- and LO-like phonon modes at 187 cm−1 and 195 cm−1 were observed in the Raman spectra. The effect of both phonon quantum confinement and stresses on the phonon frequency shift was calculated. The TO-LO splitting obtained from the ion-beam synthesized InSb nanocrystals was 3 cm−1 less than that from the unstressed bulk monocrystalline InSb. The obtained effect is discussed in the frames of decreasing the transverse effective charge, as well as that of the surface phonon influence. The photoluminescence peak at 1524 nm (0.81 eV) was seen in the low-temperature PL spectra from the samples annealed at 900–1000 °C. Its energy position corresponds to the localized charge carrier energy in the InSb nanocrystals.

Temperature Measurement by a Nanoscale Electron Probe using Energy Gain and Loss Spectroscopy

Heat dissipation in integrated nanoscale devices is a major issue that requires the development of nanoscale temperature probes. Here, we report the implementation of a method that combines electron energy gain and loss spectroscopy to provide a direct measurement of the local temperature in the nanoenvironment. Loss and gain peaks corresponding to an optical-phonon mode in boron nitride were measured from room temperature to $\ensuremath{\sim}1600\text{ }\text{ }\mathrm{K}$. Both loss and gain peaks exhibit a shift towards lower energies as the sample is heated up. First-principles calculations of the temperature-induced phonon frequency shifts provide insights into the origin of this effect and confirm the experimental data. The experiments and theory presented here open the doors to the study of anharmonic effects in materials by directly probing phonons in the electron microscope.

Optical properties of radio-frequency magnetron sputtered α-(Cr1-xAlx)2O3 thin films grown on α-Al2O3 substrates at different temperatures

We report the optical properties of the (Cr1-xAlx)2O3 thin films (0.08 ≤ x ≤ 0.16) with a thickness of ~300 nm grown on c-plane (0001) α-Al2O3 substrates at different temperatures (i.e. room temperature, 320 °C and 400 °C) by non-reactive radio-frequency magnetron sputtering. (0001) oriented single-phase corundum structured crystalline thin films were grown at temperatures of 320 °C and 400 °C, while the films deposited at room temperature are amorphous. The stoichiometric composition of the films was analyzed and verified by electron probe micro-analyses. The phase composition and lattice parameter c were characterized by X-ray diffraction in Bragg-Brentano geometry. The crystal quality increases with increasing the deposition temperature independent of the film composition, and the lattice constant c increases with decreasing the Al concentration which is independent of the deposition temperature. Raman spectra supported the interpretation of phase composition analysis and showed a significant shift of phonon frequency with Al concentration and the substrate temperature. Infrared reflection absorption spectroscopy of the films was carried out to determine the absorption band shifts in relation to the lattice parameter c which the smaller lattice constant c the larger frequency of the Eu mode. Further, the band gap of the thin films was calculated from visible light absorption spectra.

Pressure effect on the superconducting and the normal state of β−Bi2Pd

The pressure effect up to 24.0 kbar on superconducting and normal-state properties of $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ single crystal (${T}_{\mathrm{c}}\ensuremath{\approx}4.98\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure) has been investigated by measurements of the electrical resistivity. In addition, we have performed the heat capacity measurements in the temperature range 0.7--300 K at ambient pressure. The recent calculations of electronic density of states, electron-phonon interaction spectral function, and phonon density of states of $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$ [Zheng and Margine, Phys. Rev. B 95, 014512 (2017)], are used to fit the resistivity and the heat capacity data. In the superconducting state we have focused on the influence of pressure on the superconducting transition temperature ${T}_{\mathrm{c}}$ and upper critical field ${H}_{\mathrm{c}2}$ and a negative effect with $d{T}_{\mathrm{c}}/dp=--0.025\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{kbar}$ and $d{H}_{\mathrm{c}2}/dp=--8\phantom{\rule{0.16em}{0ex}}\mathrm{mT}/\mathrm{kbar}$ is found. A simplified Bloch-Gr\uneisen model was used to analyze the pressure effect on the temperature dependence of the normal-state resistivity. The obtained results point to a decrease of the electron-phonon coupling parameter \ensuremath{\lambda} and to a shift of phonon frequencies to higher values with pressure. Moreover, the temperature dependence of the normal-state resistivity follows a ${T}^{2}$ dependence above ${T}_{\mathrm{c}}$ up to about 25 K. Together with the enhanced value of Sommerfeld coefficient $\ensuremath{\gamma}=13.23\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ these results point to a certain role of the electron-electron interaction in the superconducting pairing mechanism in $\ensuremath{\beta}\ensuremath{-}\mathrm{B}{\mathrm{i}}_{2}\mathrm{Pd}$.

Temperature Measurement by a Nanoscale Electron Probe Using Energy Gain and Loss Spectroscopy.

Heat dissipation in integrated nanoscale devices is a major issue that requires the development of nanoscale temperature probes. Here, we report the implementation of a method that combines electron energy gain and loss spectroscopy to provide a direct measurement of the local temperature in the nanoenvironment. Loss and gain peaks corresponding to an optical-phonon mode in boron nitride were measured from room temperature to ∼1600 K. Both loss and gain peaks exhibit a shift towards lower energies as the sample is heated up. First-principles calculations of the temperature-induced phonon frequency shifts provide insights into the origin of this effect and confirm the experimental data. The experiments and theory presented here open the doors to the study of anharmonic effects in materials by directly probing phonons in the electron microscope.

Generalisation of phonon confinement model for interpretation of Raman line-shape from nano-silicon

A comparative analysis of two Raman line-shape functions has been carried out to validate the true representation of experimentally observed Raman scattering data for semiconducting nanomaterials. A modified form of already existing phonon confinement model (PCM) incorporates two basic considerations, phonon momentum conservation and shift in zone centre phonon frequency. After incorporation of the above mentioned two factors, a rather symmetric Raman line-shape is generated which is in contrary to the usual asymmetric Raman line-shapes obtained from nanostructured semiconductor. By fitting an experimentally observed Raman scattering data from silicon nanostructures, prepared by metal induced etching, it can be established that the Raman line-shape obtained within the framework of PCM is a true representative Raman line-shape of sufficiently low dimensions semiconductors.

Transverse acoustic phonon anomalies at intermediate wave vectors in MgV2O4

Magnetic spinels (with chemical formula $AX_{2}$O$_{4}$, with $X$ a 3$d$ transition metal ion) that also have an orbital degeneracy are Jahn-Teller active and hence possess a coupling between spin and lattice degrees of freedom. At high temperatures, MgV$_{2}$O$_{4}$ is a cubic spinel based on V$^{3+}$ ions with a spin $S$=1 and a triply degenerate orbital ground state. A structural transition occurs at T$_{OO}$=63 K to an orbitally ordered phase with a tetragonal unit cell followed by an antiferromagnetic transition of T$_{N}$=42 K on cooling. We apply neutron spectroscopy in single crystals of MgV$_{2}$O$_{4}$ to show an anomaly for intermediate wavevectors at T$_{OO}$ associated with the acoustic phonon sensitive to the shear elastic modulus $\left(C_{11}-C_{12}\right)/2$. On warming, the shear mode softens for momentum transfers near close to half the Brillouin zone boundary, but recovers near the zone centre. High resolution spin-echo measurements further illustrate a temporal broadening with increased temperature over this intermediate range of wavevectors, indicative of a reduction in phonon lifetime. A subtle shift in phonon frequencies over the same range of momentum transfers is observed with magnetic fields. We discuss this acoustic anomaly in context of coupling to orbital and charge fluctuations.

Strain-phonon coupling in (111)-oriented perovskite oxides

Strain-phonon coupling, in terms of the shift in phonon frequencies under biaxial strain, is studied by density functional theory calculations for 20 perovskite oxides strained in their (111) and (001) planes. While the strain-phonon coupling under (001) strain follows the established, intuitive trends, the response to (111) strain is more complex. Here we show that strain-phonon coupling under (111) strain can be rationalized in terms of the Goldschmidt tolerance factor and the formal cation oxidation states. The established trends for coupling between (111) strain and in-phase and out-of-phase octahedral rotational modes as well as polar modes provide guidelines for rational design of (111)-oriented perovskite thin films.

Synthesis of local epitaxial α-(Cr1 − xAlx)2O3 thin films (0.08 ≤ x ≤ 0.16) on α-Al2O3 substrates by r.f. magnetron sputtering

(0001) oriented nanocrystalline α-(Cr1-xAlx)2O3 (0.08≤x≤0.16) thin films with an average thickness of ~270nm were grown on c-plane α-Al2O3 (0001) single crystal substrates at 400°C by non-reactive r.f. magnetron sputtering by a combinational approach using a segmented ceramic target. The stoichiometric composition of the films was determined by electron probe micro-analysis (EPMA). The composition dependent crystallization behavior of the α-(Cr1−xAlx)2O3 thin films was characterized by detailed X-ray diffraction (XRD) analyses (i.e. XRD in Bragg-Brentano geometry, Rocking curve, Pole figure, Reciprocal space mapping (RSM)). Transmission electron microscopy (TEM) was carried out to study the microstructure and describe the orientation and epitaxial relationship between the films and the substrates. Further, Raman spectra show a significant shift of phonon frequency with Al concentration.

Anharmonic interatomic force constants and thermal conductivity from Grüneisen parameters: An application to graphene

Phonon-mediated thermal conductivity, which is of great technological relevance, fundamentally arises due to anharmonic scattering from interatomic potentials. Despite its prevalence, accurate first-principles calculations of thermal conductivity remain challenging, primarily due to the high computational cost of anharmonic interatomic force constant (IFCs) calculations. Meanwhile, the related anharmonic phenomenon of thermal expansion is much more tractable, being computable from the Gr\"uneisen parameters associated with phonon frequency shifts due to crystal deformations. In this work, we propose a novel approach for computing the largest cubic IFCs from the Gr\"uneisen parameter data. This allows an approximate determination of the thermal conductivity via a much less expensive route. The key insight is that although the Gr\"uneisen parameters cannot possibly contain all the information on the cubic IFCs, being derivable from spatially uniform deformations, they can still unambiguously and accurately determine the largest and most physically relevant ones. By fitting the anisotropic Gr\"uneisen parameter data along judiciously designed deformations, we can deduce (i.e., reverse engineer) the dominant cubic IFCs and estimate three-phonon scattering amplitudes. We illustrate our approach by explicitly computing the largest cubic IFCs and thermal conductivity of graphene, especially for its out-of-plane (flexural) modes that exhibit anomalously large anharmonic shifts and thermal conductivity contributions. Our calculations on graphene not only exhibits reasonable agreement with established DFT results, but also presents a pedagogical opportunity for introducing an elegant analytic treatment of the Gr\"uneisen parameters of generic two-band models. Our approach can be readily extended to more complicated crystalline materials with nontrivial anharmonic lattice effects.

Formation of CdSe nanocrystals in Cd-doped thin arsenic selenide films under laser irradiation

Amorphous Cd-doped As2Se3 films with nominal Cd contents up to 12at.% were prepared by thermal evaporation. Atomic force microscopy studies confirm the uniform film structure with surface roughnesses below 1nm, independent of the Cd content. As shown by energy-dispersive X-ray spectroscopy, the Cd content in the film reveals a strong gradient and decreases with the film depth. For heavily Cd-doped (above 7at.%) As2Se3 films, Raman features attributed to CdSe longitudinal optical (LO) phonon and its overtone (2LO) are revealed in the Raman spectra as an evidence for the formation of CdSe nanocrystallites in the As2Se3:Cd film under above-bandgap or below-bandgap laser illumination. The CdSe nanocrystallites undergo tensile strain due to a photoplastic effect in the As2Se3 film, i. e. partial removal of the material from the laser spot. The tensile strain value, estimated from the LO phonon frequency shift, is shown to reach nearly 2GPa.

Superconductivity-induced shift of phonon frequency in the system with itinerant and strongly correlated electrons

We investigate a system consisting of strongly correlated localized and itinerant electron states mixed by optical vibration. The linear vibronic interaction causes the softening of active optical mode and it may induce a structural instability. The modification of renormalized phonon frequency by the gap in the energy of itinerant electrons in the superconducting phase has been established. The disposition of the electron spectrum and chemical potential influences substantially the corresponding effect. It is demonstrated that the presence of van Hove singularity in the center of itinerant electron band introduces qualitative changes into the superconductivity-induced shifts of phonon frequency.