Phonon Shifts Research Articles

Temperature dependent phonon shifts in few-layer black phosphorus.

Atomically thin two-dimensional (2D) sheets of black phosphorus have attracted much attention due to their potential for future nanoelectronic and photonics device applications. Present investigations deal with the temperature dependent phonon shifts in a few-layer black phosphorus nanosheet sample prepared using micromechanical exfoliation on a 300 nm SiO2/Si substrate. The temperature dependent Raman spectroscopy experiments were carried out on a few-layer black phosphorus sample, which depicts softening of Ag(1), B2g, and Ag(2) modes as temperature increases from 77 to 673 K. The calculated temperature coefficients for Ag(1), B2g, and Ag(2) modes of the few-layer black phosphorus nanosheet sample were observed to be -0.01, -0.013, and -0.014 cm(-1) K(-1), respectively. The temperature dependent softening modes of black phosphorus results were explained on the basis of a double resonance process which is more active in an atomically thin sample. This process can also be fundamentally pertinent in other promising and emerging 2D ultrathin layer and heterostructured materials.

Suspended monolayer graphene under true uniaxial deformation.

2D crystals, such as graphene, exhibit the higher strength and stiffness of any other known man-made or natural material. So far, this assertion has been primarily based on modelling predictions and on bending experiments in combination with pertinent modelling. True uniaxial loading of suspended graphene is not easy to accomplish; however such an experiment is of paramount importance in order to assess the intrinsic properties of graphene without the influence of an underlying substrate. In this work we report on uniaxial tension of graphene up to moderate strains of ∼0.8%. This has been made possible by sandwiching the graphene flake between two polymethylmethacrylate (PMMA) layers and by suspending its central part by the removal of a section of PMMA with e-beam lithography. True uniaxial deformation is confirmed by the measured large phonon shifts with strain by Raman spectroscopy and the indication of lateral buckling (similar to what is observed for thin macroscopic membranes under tension). Finally, we also report on how the stress is transferred to the suspended specimen through the adhesive grips and determine the value of interfacial shear stress that is required for efficient axial loading in such a system.

Temperature dependent magnon-phonon coupling in bcc Fe from theory and experiment.

An ab initio based framework for quantitatively assessing the phonon contribution due to magnon-phonon interactions and lattice expansion is developed. The theoretical results for bcc Fe are in very good agreement with high-quality phonon frequency measurements. For some phonon branches, the magnon-phonon interaction is an order of magnitude larger than the phonon shift due to lattice expansion, demonstrating the strong impact of magnetic short-range order even significantly above the Curie temperature. The framework closes the previous simulation gap between the ferro- and paramagnetic limits.

Nonharmonic phonons inα-iron at high temperatures

Phonon densities of states (DOS) of bcc $\alpha$-$^{57}$Fe were measured from room temperature through the 1044K Curie transition and the 1185K fcc $\gamma$-Fe phase transition using nuclear resonant inelastic x-ray scattering. At higher temperatures all phonons shift to lower energies (soften) with thermal expansion, but the low transverse modes soften especially rapidly above 700K, showing strongly nonharmonic behavior that persists through the magnetic transition. Interatomic force constants for the bcc phase were obtained by iteratively fitting a Born-von K\'{a}rm\'{a}n model to the experimental phonon spectra using a genetic algorithm optimization. The second-nearest-neighbor fitted axial force constants weakened significantly at elevated temperatures. An unusually large nonharmonic behavior is reported, which increases the vibrational entropy and accounts for a contribution of 35 meV/atom in the free energy at high temperatures. The nonharmonic contribution to the vibrational entropy follows the thermal trend of the magnetic entropy, and may be coupled to magnetic excitations. A small change in vibrational entropy across the $\alpha$-$\gamma$ structural phase transformation is also reported.

Effects of Mg doping rate on physical properties of Mg and Al co-doped Zn1−x−0.02MgxAl0.02O transparent conducting oxide films prepared by rf magnetron sputtering

The transparent and conductive Mg–Al co-doped ZnMgAlO films with different Mg contents have been sputter grown on quartz substrates at room temperature. Modulation of the band-gap and conductivity was tried by varying the Mg doping rate from 5at.% to 20at.% at a fixed Al content of 2at.%. Optical characterization of the deposited ZnMgAlO films confirmed that the average optical transmittance in the visible region was ∼85% and the transparency of the near infra-red (NIR) region expanded to further IR region with increasing the Mg doping level, which can be profitable for the application where higher NIR transmittance is required. At a low Mg doping level of 5at.%, the best quality of electrical properties were achieved with the resistivity of 3.7×10−4Ωcm, the electron concentration of 7×1020/cm3, and the Hall mobility of 25.66cm2/Vs, whereas with further Mg doping up to 20at.%, the resistivity increased to 1.9×10−2Ωcm, which is accompanied with a degradation of crystal quality. Optical band-gap reached to 3.87eV at a high Mg doping level of 20at.%. Additional phonon modes at 275cm−1 and 648cm−1 found in the Raman spectra of the ZnMgAlO films were assigned to the host lattice defect vibrational complexes related with Al dopants and it was suggested that the broadening, weakening, and shift of phonon peaks with increasing the Mg doping level may be related with the enhanced lattice distortion in ZnO crystal and the reduced mass change induced by substitution of Mg in Zn sites.

(Invited) Anisotropoic Strain Evaluation in the Finite Si Area by Surface Plasmon Enhanced Raman Spectroscopy

Introduction The strain in the channel of a transistor can enhance carrier mobility. The strain state is complicated because the channel is affected by the various components. Raman spectroscopy has been performed for the strain evaluation in Si owing to high-strain sensitivity and high-spatial resolution [1]. However, it is difficult to evaluate the complicated strain state using conventional Raman. In this study, Raman spectroscopy using z-polar light excitation was performed in order to evaluate anisotropic biaxial strain states in the finite Si area. Experiment Two types of strained Si substrates, strained Si on insulator (SSOI) as shown by the TEM image in the inset of Fig. 1, and strained-Si followed by SiGe on insulator (SGOI) as shown in the inset of Fig. 2, were used as the samples. The Si substrate with the line-and-space pattered SiN film was also used for the strain evaluation in the finite Si area. The space widths were varied from 5.0 to 0.6 mm.The excitation light component perpendicular to the surface, i.e., z-polarization, were achieved by oil immersion Raman with high-NA objective lens or surface enhanced Raman spectroscopy (SERS) with Ag particle on the sample surface. The z-polar light excitation allows the excitation of the transverse optical (TO) phonon as well as longitudinal optical (LO) phonon [2]. The relationship between the Raman wavenumber shifts Dw LO and Dw TO and biaxial stresses were expressed by the following equation (1) (The coordinate is defined: x: [110], y: [1(_)10], and z: [001]) [3].In the experiment, the 532 nm laser and the oil-immersion lens were used. The numerical aperture and refractive index of oil were 1.4 and 1.5, respectively. Results and Discussion Figure 1 shows the oil-immersion Raman spectra from SSOI in the LO and TO active conditions. Although the peak positions originating in the Si substrate are the same, the peak positions originating in the strained Si layer are different each other. This result shows the excitation of the TO phonon. Figure 2 shows the Raman spectra from the strained Si on SGOI with SERS (w/ Ag) and the oil-imersion (w/o Ag) conditions. It should be noticed that the intensity ratio of the strained Si peak around 515 cm- 1 to the Si substrate peak at 520 cm- 1 increased in the SERS condition. Furthermore, the line width of the strained Si peak in the SERS conditions was broadened. This was caused by the strong excitation of the TO phonon owing to the depolarization. The electrical field component ratios were calculated by finite-difference time-domain method [4]. From the calculation, the value of the z-polarization was the highest for the SERS conditions among the SERS, oil-immersion, and dry conditions, as shown in Table 1. Figure 3(a) shows the wavenumber shifts of TO and LO phonons from the Si substrate with the L&S pattered SiN film as a function of the space width obtained by SERS. Using the above equation, the biaxial stresses s xx and s yy were calculated, as shown in Fig. 3(b). s xx perpendicular to the pattern increased with the decrease in the space width, while s yy parallel to the pattern remained in the low value. The biaxial stress states were able to be evaluated efficiently by using the SERS technique. Acknowledgement This study was partially supported by the Semiconductor Technology Academic Research Center (STARC), the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research B (No. 24360125), and the Japan Science and Technology Agency through the Adaptable and Seamless Technology transfer Program (A-STEP) through target-driven R&D.

Recent Advances in Semiconductor Nanowire Heterostructures

We review a variety of recent optical experiments which provide unique characterizations of the energy landscape in semiconductor nanowires, both simple nanowires and more complex semiconductor heterostructures. The optical experiments include the techniques of photoluminescence, time-resolved photoluminescence, photoluminescence excitation spectroscopy, photocurrent spectroscopy, and Raman scattering brought to bear on structures from simple core-shell structures, to axial heterostructures, to core-multiple-shell heterostructures. The physics that is illuminated ranges from localization of excitons in GaAs quantum well tubes, to strain in GaAs/GaP axial heterostructures, to carrier relaxation dynamics in both ZB and WZ InP probed by the new technique of transient Rayleigh scattering. We briefly explore these several examples in what follows.First we discuss a core-multi-shell structure consisting of a GaAs quantum well bounded by two AlGaAs shells all wrapped around a [111] oriented GaAs nanowire. Recently our group (Fickenscher et al. Nano Letters 2013, 1016-1022) has obtained photoluminescence (PL) and photoluminescence excitation (PLE) data which indicate these materials are strong emitters and that the quantum well exhibits strong quantum confinement. We have modeled these strain-free quantum well tubes (QWT) and find that the electron and hole ground states are strongly confined to the corners of the hexagonally symmetric quantum well. Thus this confinement defines truly quantum wires which run along the length of the nanowires along its corners. Single nanowire PL and PLE measurements show that for quantum well tube widths larger than 5 nm excited states are observed, but for smaller widths only the ground state is observed. For a 4 nm QWT sample, we observe narrow emission lines on the high energy side of the luminescence band which suggest highly localized states similar to that observed for quantum dots (see Figs. below). The use of semiconductor nanowires for both basic and applied studies takes advantage of the unique opportunities they provide for growing heterostructures even with materials that have quite different lattice constants. Both radial heterostructures and axial heterostructures have been explored. We have used Raman scattering to study the spatially-resolved strain and stress in a GaAs/GaP axial heterostructure nanowire. These two materials have widely different (3.6%) lattice constants. To characterize the stress or strain of this nanowire heterostructure, we have used high spatial resolution Raman scans along the nanowires which show the GaAs/GaP interface is clearly identifiable. We interpret the phonon energy shifts in each material as one approaches the interface. Near the interface we observe both a GaAs TO phonon as well as a GaP TO phonon. The GaAs phonon is clearly shifted to higher energies due to compression from the GaP which has a smaller lattice constant. A strain gradient exists across the interface so the GaP phonon energies shift to lower and GaAs phonon shift to higher energies as one approaches the interface from the GaP section of the nanowire. We observe that the strain does not return to the unstrained value even microns away from the interface. We note that the stresses reflected in both the GaP and GaAs Raman spectra are of a magnitude which implies that the electronic energy landscape of such structures are significantly modified.As a third example we briefly describe a promising new technique (Montazeri et al. Nano Letters 2012, 5389-5395), transient Rayleigh scattering spectroscopy [TRS], which enables us to probe the carrier dynamics of single semiconductor nanowires on the picosecond scale. A 200 fs pump photoexcites a high density of carriers in the nanowire and a delayed probe monitors the change in reflectivity. The photoexcited carriers modify the real and imaginary parts of the complex index of refraction through band-filling, band gap renormalization, and plasma screening – and thus the measured reflectivity response. A model calculation enables one to extract the density of electrons and holes and the temperature of the electron-hole plasma as a function of time. We first explored carrier dynamics in a simple core-shell GaAs/AlGaAs structure. Very recently we have used TRS spectroscopy to investigate both zincblende and wurtzite InP nanowires. We see distinctively different behaviors including the A, B, and C excitons in the wurtzite phase, as well as distinctively different time-dependent responses.

Failure processes in embedded monolayer graphene under axial compression.

Exfoliated monolayer graphene flakes were embedded in a polymer matrix and loaded under axial compression. By monitoring the shifts of the 2D Raman phonons of rectangular flakes of various sizes under load, the critical strain to failure was determined. Prior to loading care was taken for the examined area of the flake to be free of residual stresses. The critical strain values for first failure were found to be independent of flake size at a mean value of –0.60% corresponding to a yield stress up to -6 GPa. By combining Euler mechanics with a Winkler approach, we show that unlike buckling in air, the presence of the polymer constraint results in graphene buckling at a fixed value of strain with an estimated wrinkle wavelength of the order of 1–2 nm. These results were compared with DFT computations performed on analogue coronene/PMMA oligomers and a reasonable agreement was obtained.

Temperature-dependent nonlinear phonon shifts in a supported MoS2 monolayer.

We report Raman spectra measurements on a MoS(2) monolayer supported on SiO(2) as a function of temperature. Unlike in previous studies, the positions of the two main Raman modes, E(2g)(1) and A(1g) exhibited nonlinear temperature dependence. Temperature dependence of phonon shifts and widths is explained by optical phonon decay process into two acoustic phonons. On the basis of Raman measurements, local temperature change under laser heating power at different global temperatures is derived. Obtained results contribute to our understanding of the thermal properties of two-dimensional atomic crystals and can help to solve the problem of heat dissipation, which is crucial for use in the next generation of nanoelectronic devices.

Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3

Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO3 over a temperature range of 20–300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a Néel transition at TN ∼ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below TN. Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near TN. An additional phonon anomaly was identified at T* ∼ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman frequencies between TN and T* suggest competing WFM and AFM interactions. A close match between the phonon frequency of B3g (3)-octahedral rotation mode with the square of sublattice magnetization between TN and T* is indicative of the presence of spin-phonon coupling in multiferroic YCrO3.

Stress relaxation of GaN microstructures on a graphene‐buffered Al2O3 substrate

AbstractGaN microstructures were grown on c‐Al2O3 with a multi‐stacked graphene buffered layer using metal metal‐organic chemical‐vapor deposition. Under the same growth conditions, the nucleation of GaN was suppressed by the low surface energy of graphene, resulting in a much lower density of microstructures relative to those grown on c‐Al2O3. Residual stress in the GaN microstructures was estimated from the peak shift of the E2 phonon using micro‐Raman spectroscopy. The results showed that the compressive stress of approximately 0.36 GPa in GaN on c‐Al2O3 caused by lattice mismatch and the difference in the thermal expansion coefficient was relaxed by introducing the graphene layer. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

From isotope labeled CH3CN to N2inside single-walled carbon nanotubes

The observation of one-dimensional N₂ inside single-walled carbon nanotubes raises the questions, how are the N₂ molecules formed and how do they manage to make their way to this peculiar place? We have used N(15) and C(13) isotope labeled acetonitrile during the synthesis of single-walled carbon nanotubes to investigate this process. The isotope shifts of phonons and vibrons are observed by Raman spectroscopy and X-ray absorption. We identify the catalytic decomposition of acetonitrile as the initial step in the reaction pathway to single-walled carbon nanotubes containing encapsulated N₂.

Temperature-dependent phonon shifts in monolayer MoS2

We present a combined experimental and computational study of two-dimensional molybdenum disulfide and the effect of temperature on the frequency shifts of the Raman-active E2g and A1g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A1g mode than for the E2g mode. This is in contrast to previously reported bulk behavior, in which the E2g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system.

Stress State of GaN Epilayer Grown on Sapphire and 6H-SiC Substrates

Abstract The stress states and influence of two opposite stress types under similar stress intensities on the structural and optical properties of GaN films grown on sapphire and 6H-SiC substrates via metalorganic chemical vapor deposition were investigated. The E2 (high) phonon shifts of Raman spectra show that tensile stresses exist in the GaN epilayer grown on 6H-SiC, whereas compressive stresses appear in the film grown on sapphire, indicating that the thermal mismatch between the epilayers and the substrates plays a major role in determining the residual stresses in GaN films. Narrower full widths at half-maximum of E2 (high) phonon and double crystal X-ray diffraction peaks and the higher E2 (high) phonon intensity are visible for the GaN film grown on sapphire, illustrating that under almost equivalent stress intensities, tensile stresses have a much more negative influence on the crystalline quality of GaN epilayers. Finally, a numerical relationship between the luminescent band gap and the biaxial stresses of the GaN films is obtained at 10 K.

On the Anomalous Magnetic Behavior and the Multiferroic Properties in BiMn2O5

Sealed-tube synthesis of BiMn2O5 materials and their physical properties have rationally been reinvestigated depending on the reactants. The aim of the study was to characterize its potential multiferroic properties and to investigate the anomalous magnetic properties in relation to the expected ferroelectric properties. Rietveld refinement of the room temperature X-ray diffraction data shows the stability of the crystallographic structure with a Mn(3+)/Mn(4+) ratio far from 1 because of bismuth and oxygen deficiencies despite the sealed-tube synthesis. Our detailed magnetic susceptibility and specific heat data analysis unambiguously support an intrinsic anomalous magnetic behavior in relation to the establishment of a magnetic short-range ordering far from the Néel temperature. Around room temperature, oxygen vacancies are responsible for supporting the dielectric loss peak measured, and, interestingly, the so-called T*, which was underlined in relation to an anomalous phonon shift (García-Flores, A. F.; et al. Phys. Rev. B 2006, 73, 104411), is not a characteristic temperature in relation to the multiferroic properties because no ferroelectric transition was detected.

3D Raman mapping of uniaxially loaded 6H‐SiC crystals

Raman spectroscopy is used to investigate the three‐dimensional stress distribution in 6H‐silicon carbide (SiC) specimens subjected to stresses up to 3.7 GPa along the c‐axis. Specifically, the relative Raman shift of the longitudinal optic phonon of 6H‐SiC is used to evaluate the local stress across the bulk crystal. For this purpose, an anvil device with opposed 6H‐SiC and sapphire specimens was used. After subjecting the anvils to uniaxial load, several series of two‐dimensional Raman maps were registered at different depths in the 6H‐SiC anvil. The analysis of the Raman spectra reveals an exponential decay of the stress as a function of the depth. A novel phenomenological Grüneisen‐like model is introduced here to account for such observation. On the contrary, the in‐plane stress analysis shows a radial Gaussian‐like distribution regardless the depth, a distinct behavior that is attributed to the appearance of shear stress components. The suitability of both models and their applicability to other materials are discussed, along with some future directions. Copyright © 2013 John Wiley & Sons, Ltd.

Renormalization effects and phonon density of states in high temperature superconductors

Using the versatile double time thermodynamic Green's function approach based on many body theory the renormalized frequencies, phonon energy line widths, shifts and phonon density of states (PDOS) are investigated via a newly formulated Hamiltonian (does not include BCS type Hamiltonian) that includes the effects of electron-phonon, anharmonicities and that of isotopic impurities. The automatic appearance of pairons, temperature, impurity and electron-phonon coupling of renormalized frequencies, widths, shifts and PDOS emerges as a characteristic feature of present theory. The numerical investigations on PDOS for the YBa2Cu3O7 − δ crystal predicts several new feature of high temperature superconductors (HTS) and agreements with experimental observations.

Implicit phonon shifts and thermodynamical properties of rigid carbon nanotube bunches

We calculate phonon shifts of external modes of a bunch of carbon nanotubes. The bunches form a 2-dimensional hexagonal arrangement of lattice with long molecules of carbon nanotubes. A simple model based on atom-atom potential has been used to calculate the implicit anharmonicity in the phonons of carbon nanotube bundles (also called ropes or bunches) having rigid tubes, with the assumption that under hydrostatic pressure only the inter-tube distance in the bunch varies. Various bulk and thermodynamic properties like thermal expansion, bulk modulus and the Gruneisen constants and external phonon shifts which naturally enter into the calculation are also described and compared with the available data. The specific heat capacity has also been calculated.

Electronic heat capacity of an isotopically disordered semiconductor crystals

An interaction of an electron with phonon in a crystal shows the dependence of electronic heat capacity on temperature in low temperature limit. It is also found from theory that electronic heat capacity is a linear as well as non-linear function of electron–phonon coupling constant, linear function of force constant change parameter and polaron state upto second order. An electron Green’s function has been evaluated using equation of motion technique of quantum dynamics. The response function has been obtained from electron Green’s function to give electron–phonon linewidth and shift. This electron–phonon linewidth has been responsible for electron density of states.

Charge transfer between carbon nanotubes and sulfuric acid as determined by Raman spectroscopy

The spontaneous interaction between sulfuric acid and carbon nanotubes is studied using Raman spectroscopy. We are able to determine the charge transfer without any additional parameter using the spectral signature of inner and outer walls of double-wall carbon nanotubes. While for the outer wall both the lattice contraction and the nonadiabatic effects contribute to the phonon shift, only the lattice contraction contributes for the inner nanotube. For the outer nanotube, we are able to separate these two contributions of the Raman G-band shift as a function of the charge transfer. We have carried out density functional theory calculations on graphene to see how different chemical species (HSO4 − ,H 2SO4 ,H + ) affect the electronic band structure and electron-phonon coupling. The Raman G-band shift for the outer nanotube, �ω , as a function of hole charge transfer per carbon atom, fC, is found to be �ω (cm −1 ) = (350 ± 20)fC + (101 ± 8) √ fC.