Phonon Confinement Effect Research Articles

Spurious signals identification in Brillouin light scattering spectrum

AbstractBrillouin light scattering (BLS) is the inelastic scattering of light from elementary excitations with periodic density modulation. The characteristics of non‐contact, high sensitivity, and high resolution in energy, wavevector, time, space, and phase make the BLS spectrometer widely used in investigating many intriguing physical phenomena, including the acoustic phonon confinement effects, Bose‐Einstein condensation (BEC), supercurrent, and soliton formation. Generally, the quick and correct assignment of the signals in the BLS spectra is a prerequisite for further investigations. Herein, we experimentally identify the high‐order spurious signals in the BLS spectra, which make the interpretation of the spectra difficult. The additional signals are demonstrated to originate from the laser and the temperature‐controlled laser filter used for laser filtering. Our results would contribute to the rapid assignment of the signals from the sample after excluding the spurious signals reported here. Moreover, the series of high‐order modes spaced by the free spectral range can serve as a weak broadband light source, which has great potential for investigating the optical responses of materials.

Investigating the effects of Mn content on the morphology and dielectric properties of CdS nanoparticles

CdS nanoparticles doped with Mn were synthesized by chemical precipitation using varying concentrations of Mn at Cd1-xMnxS, where (x) = 0.00, 0.03, 0.05, and 0.07. The samples were examined using scanning electron microscopy (SEM), energy dispersive X-ray techniques (EDX), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, dielectric properties, and AC conductivity measurements. SEM micrograph shows that pure CdS nanomaterial has agglomerates primarily composed of nanoparticles, whereas the sample with a concentration of 0.03 contains smaller particles. In response to the phonon confinement effect, The Raman spectra of CdS nanoparticles exhibited peaks at 303 cm−1 and 603 cm−1. In contrast, the Raman spectra of Mn:CdS nanocomposites displayed a prominent fundamental mode at 301 cm−1 and a less pronounced overtone mode at 601 cm−1. The dielectric properties and the AC conductivity of CdS have been investigated over a wide frequency range of 0.1 Hz to 10 MHz and at a variety of temperatures ranging from 298 to 423K. The real and imaginary parts of the complex dielectric constant (εʹ, εʹʹ), the electric modulus, and AC conductivity of CdS were found to depend on Mn content at different temperatures.

Magnon–phonon coupling modulation via dimensional reduction in thin antiferromagnet MnPSe3 nanoribbons

Two-dimensional (2D) magnetic materials have triggered tremendous interest in recent years due to their remarkable potential applications in magnetic storage and spintronics devices. Heat dissipation is of great significance in stability and durability of increasingly integrated magnetic devices. However, little investigation of thermal transport has been carried out in 2D magnetic materials and a comprehensive understanding of the underlying mechanism is still lacking. We experimentally demonstrate the thermal conductivity measurement of MnPSe3 nanoribbons and find a nonmonotonic thickness dependence, which is attributed to the phonon confinement effect in thin nanoribbons. The peaks of measured thermal conductivity are found to be modified with increasing thickness due to the magnon–phonon coupling. We propose that the magnon–phonon scattering rate increases with increasing thickness and causes a huge suppression in thermal conductivity. This study will deepen the understanding of the thermal properties of 2D magnetic materials and will benefit thermal management in designing magnetic devices.

Modeling Surface Vibrations and Their Role in Molecular Adsorption: A Generalized Langevin Approach.

The atomic vibrations of a solid surface can play a significant role in the reactions of surface-bound molecules, as well as their adsorption and desorption. Relevant phonon modes can involve the collective motion of atoms over a wide array of length scales. In this paper, we demonstrate how the generalized Langevin equation can be utilized to describe these collective motions weighted by their coupling to individual sites. Our approach builds upon the generalized Langevin oscillator (GLO) model originally developed by Tully. We extend the GLO by deriving parameters from atomistic simulation data. We apply this approach to study the memory kernel of a model platinum surface and demonstrate that the memory kernel has a bimodal form due to coupling to both low-energy acoustic modes and high-energy modes near the Debye frequency. The same bimodal form was observed across a wide variety of solids of different elemental compositions, surface structures, and solvation states. By studying how these dominant modes depend on the simulation size, we argue that the acoustic modes are frozen in the limit of macroscopic lattices. By simulating periodically replicated slabs of various sizes, we quantify the influence of phonon confinement effects in the memory kernel and their concomitant effect on simulated sticking coefficients.

The effect of Ag doping on the structure, optical, and electronic properties of ZnO nanostructures deposited by atmospheric pressure MOCVD on Ag/Si substrates

Atmospheric pressure metal–organic chemical vapour deposition was used to synthesize Ag-containing ZnO nanostructures of different morphology on Si substrates coated by Ag. Ag from Ag/Si substrates and Ag from silver acetylacetonate after its decomposition were used as a catalyst for ZnO nanocrystal growth for deposition of ZnO nanostructures with different morphologies. We investigated the relation of the structural parameters and chemical composition probed by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy with the photoluminescence (PL) properties and electron–phonon coupling (EPC) reflected in the multi-phonon Raman spectra. The Raman and PL spectra were studied at different powers of the exciting laser radiation (P exc). The spectral position and width of the phonon Raman peaks and the near bandgap PL band at low P exc are supposed to be determined by the structural quality of the surfaces/boundaries of the crystallites. The intensity of the near-bandgap and defect-related PL and the magnitude of the EPC are additionally affected by the dopant concentration. Because of the large crystallite size (>30 nm, determined from XRD), the effects of phonon or electron confinement are negligible in these nanostructures. The behaviour of the position and width of phonon and PL bands with increasing P exc indicates that the heat dissipation in the film, which is dependent on the nanostructure morphology and Ag content, plays an important role. In addition, the cytotoxicity of ZnO:Ag nanostructures was investigated by using monolayer cell cultures of epithelioid origin Madin-Darby bovine kidney and Madin-Darby canine kidney cells at a MTT assay revealing that the level of silver doping of ZnO nanostructures, their morphology, and geometric dimensions determine their toxic effects.

Phonon confinement and particle size effect on the low-frequency Raman mode of aurivillius phase Bi4Ti3O12 powders.

We report the systematic measurements in bismuth titanate powders of Raman frequency shift, ω and full width at half maximum (FWHM), Γ of optical phonons at q = 0 obtained between ∼300 K and 673 K in air. Both the particle size and phonon confinement effects are reasonably satisfactory to explain the Raman peak shift and asymmetric broadening observed in the ferroelectric soft phonon mode at 42 cm-1. It is shown that the lattice parameter varies as particle size x, and its contribution to size-dependent Raman shift and broadening of linewidth follows ω ∝ x -0.73 and Γ ∝ x -0.38 law, respectively. Moreover, a single phonon coupling term corresponding to a three-phonon anharmonic process is sufficient to describe the phonon coupling decay culminating from the softening of this strongly overdamped phonon mode.

The Crystalline Structure of Thin Bismuth Layers Grown on Silicon (111) Substrates.

Bismuth films with thicknesses between 6 and ∼30 nm were grown on Si (111) substrate by molecular beam epitaxy (MBE). Two main phases of bismuth — -Bi and -Bi — were identified from high-resolution X-ray diffraction (XRD) measurements. The crystal structure dependencies on the layer thicknesses of these films were analyzed. -Bi layers were epitaxial and homogenous in lateral regions that are greater than 200 nm despite the layer thickness. Further, an increase in in-plane 2 values showed the biaxial compressive strain. For comparison, -Bi layers are misoriented in six in-plane directions and have -Bi inserts in thicker layers. That leads to smaller (about 60 nm) lateral crystallites which are compressively strained in all three directions. Raman measurement confirmed the XRD results. The blue-sift of Raman signals compared with bulk Bi crystals occurs due to the phonon confinement effect, which is larger in the thinnest -Bi layers due to higher compression.

Specific heat of thin phonon cavities at low temperature: Very high values revealed by zeptojoule calorimetry

Specific heat of phonon cavities is investigated in order to analyse the effect of phonon confinement on thermodynamic properties. The specific heat of free standing very thin SiN membranes in the low dimensional limit is measured down to very low temperatures (from 6~K to 50~mK). In the whole temperature range, we measured an excess of specific heat orders of magnitude bigger than the typical value observed in amorphous solids. Below 1~K, a cross-over in $c_p$ to a lower power law is seen, and the value of specific heat of thinner membranes becomes larger than that of thicker ones demonstrating a significant contribution coming from the surface. We show that this high value of the specific heat cannot be explained by the sole contribution of 2D phonon modes (Lamb waves). The excess specific heat, being thickness dependent, could come from tunneling two level systems (TLS) that form in low density regions of amorphous solids located on the surfaces. We also show that the specific heat is strongly tuned by the internal stress of the membrane by orders of magnitude, giving unprecedentedly high values, making low stress SiN very efficient for energy storage at very low temperature.

Structural and optical properties of Ce1-xGdxO2 Nanoparticles

We report on systematic study of structural and optical properties of Ce1-xGdxO2 (x = 0 to 0.2) nanoparticles (NPs). Solution combustion method was used for the synthesis of NPs. Rietveld analysis of X-ray diffraction (XRD) data confirms single-phase cubic fluorite structure of all considered compositions. Lattice expansion also observed as Gd concentration is increased. Scanning electron microscopy (SEM) confirmed the porous and spongy nature of the NPs. Raman spectroscopy shows the presence of F2g Raman mode, confirming the fluorite cubic structure of the considered NPs. The peak shifting and asymmetric broadening behaviour of Raman peaks demonstrate the phonon confinement effect. Band-edge absorption peak show the blue shifts along with direct bandgap and change in the bandgap, observed as increases the Gd content.

Non-monotonic thickness dependent and anisotropic in-plane thermal transport in layered titanium trisulphide

Due to the potential applications in many fields such as thermoelectric, transistor, and electrode materials, a novel two-dimensional layered material TiS3 has attracted tremendous attention recently. In this article, the thickness dependent and anisotropic in-plane thermal transport behavior is explored by first-principles calculation with Boltzmann transport equation. The calculation results show that when the thickness is smaller than phonon confinement size (PCS), the phonon confinement effect is dominant and the in-plane thermal conductivity has a negative dependence on thickness due to the increase of phonon–phonon scattering; when the thickness is larger than PCS, the confinement effect is weak and the dependence becomes positive due to the decrease of phonon–boundary scattering. Through further analysis, we find this non-monotonic dependence or phonon confinement phenomenon is mainly caused by the low-frequency phonons. The calculation results also show strong thermal anisotropy within the basal plane. The coupling strength, bond length, phonon group velocity, dispersiveness of optical phonon branches, iso-energy surface and iso-surface of electron density distribution are analyzed to explain the anisotropy. Based on the 1D diatomic chain model, a unit cell having similar atomic mass and strong intracell bond can enhance the dispersiveness of optical phonon branches. Those findings here can give physical insights into the thickness dependent and anisotropic in-plane thermal transport in layered materials.

The role of polymeric surfactants on morphology and volatile organic compounds sensing efficiency of tennis bat like vanadium pentoxide nanorod

Tennis Bat like V2O5 nanorods (NRs) capable of sensing volatile organic compounds (VOCs) were synthesized by co- precipitation method in presence of polymeric surfactants. The role of polymeric surfactants (PS), polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and poly(toluene sulfonic acid) (PTSA) on the formation, structural, optical and morphological properties of V2O5-NRs were studied. The efficiency of these NRs to sense acetaldehyde, toluene and xylene (ATX) vapors was also investigated. Field emission scanning electron microscope images revealed that tennis bat like NRs were obtained when PTSA was used and except PEG, all PS were rendered well defined NRs. Phonon confinement effect was highly pronounced in V2O5-NR prepared in the presence of PTSA, and also exhibited highest sensing response towards sensing ATXs, in particular, 4–16 times higher sensitivity towards toluene than other gases. Collectively, the PTSA found to be efficient for fabricating the homogenous tennis bat like V2O5-NRs capable of sensing volatile organic compounds.

Surface phonons limit heat conduction in thin films

Understanding microscopic heat conduction in thin films is important for nano/micro heat transfer and thermal management for advanced electronics. As the thickness of thin films is comparable to or shorter than a phonon wavelength, phonon dispersion relations and transport properties are significantly modulated, which should be taken into account for heat conduction in thin films. Although phonon confinement and depletion effects have been considered, it should be emphasized that surface-localized phonons (surface phonons) arise whose influence on heat conduction may not be negligible due to the high surface-to-volume ratio. However, the role of surface phonons in heat conduction has received little attention thus far. In the present work, we performed anharmonic lattice dynamics calculations to investigate the thickness and temperature dependence of in-plane thermal conductivity of silicon thin films with sub-10-nm thickness in terms of surface phonons. Through systematic analysis of the influences of surface phonons, we found that anharmonic coupling between surface and internal phonons localized in thin films significantly suppresses overall in-plane heat conduction in thin films. We also discovered that specific low-frequency surface phonons significantly contribute to surface--internal phonon scattering and heat conduction suppression. Our findings are beneficial for the thermal management of electronics and phononic devices and may lead to surface phonon engineering for thermal conductivity control.

Raman characterization of phonon confinement and strain effects from latent ion tracks in α‐quartz

AbstractThis study reports phonon confinement and strain effects in the Raman spectrum of ion‐irradiated and subsequently etched α‐quartz. Y‐ and Z‐cut α‐quartz single crystals were irradiated at room temperature with 20‐MeV Ni6+ and 40‐MeV I7+ ions. Latent ion tracks were produced with areal densities ranging from the isolated track regime to the overlapping track regime (nominal fluences of 1 × 109, 1 × 1010, and 1 × 1011 ions cm−2). Nanowell structures were revealed after vapor etching with hydrofluoric acid (HF) aqueous solutions. A phonon confinement model was invoked to explain the observed changes in the shape of the strong Raman peak located around 463 cm−1. Phonon coherence lengths of the irradiated samples were determined by fitting the confinement model to the experimental data. It was found that the phonon coherence lengths (L) decreased with increasing fluence (L ~ 66 nm for samples irradiated with 1 × 109 ions cm−2, L ~ 58 nm at 1 × 1010 ions cm−2, and L ~ 45 nm at 1 × 1011 ions cm−2), suggesting that ion tracks may confine optical phonons in quartz. An additional contribution to the upshift was observed at the highest fluence. This was attributed to a contribution from lattice strain when track separations are small and the overlapping strain fields around ion tracks extend through a large volume fraction of the crystal. Through an empirical correlation, the induced stress was estimated to be on the order of 100 MPa.

The influence of confined acoustic phonon on the quantum Peltier effect in doped semiconductor superlattice in the presence of electromagnetic wave

Based on the kinetic equation method, the quantum Peltier effect has been theoretically studied under the influence of confined acoustic phonon in doped semiconductor superlattice in the presence of the electromagnetic wave (laser radiation). There were complicated dependences of the analytical expression of the Peltier coefficient (PC) on quantities such as doped concentration of the superlattice, amplitude of the laser radiation, the cyclotron frequency of electrons and temperature of the system. In detailed consideration, the quantum number m was changed in order to characterize the influence of confined acoustic phonon. When setting the m to zero, we obtained the results that corresponded to the case of unconfined phonon. The theoretical results have been numerically evaluated and discussed for the GaAs:Si/GaAS:Be doped semiconductor superlattice (DSS). We found the oscillation of the PC according to enhancement of cyclotron frequency of electrons. Moreover, position of resonant peaks has shifted under the influence of phonon confinement. In the case of low doped concentration (n D), the PC decreased in non-linear way; then it reached a negative constant in high n D value. The non-linear change of the PC also has been detected when investigating its dependence on the laser amplitude. All numerical results have shown that the magnitude of the PC decreased due to the increase of phonon confinement effect. In short, the confinement of acoustic phonon caused the change of the quantum Peltier effect in DSS of GaAs:Si/GaAS:Be in quantitatively as well as qualitatively.

Raman scattering spectroscopy for solid-phase and metal-induced crystallization of extremely thin germanium films on glass

Raman scattering spectroscopy was applied to characterize polycrystalline germanium (Ge) on noncrystalline substrates fabricated by solid-phase crystallization and metal-induced crystallization (MIC) using copper (Cu) and tin (Sn) as the catalyzer. The subject was focused on the material for thin-film transistors in which the precursors, with the thickness of only 15 nm, were deposited by sputtering. The optical phonon mode exhibited considerable asymmetry and was separated into the following three components: crystal, low-dimensional nanosized crystallite (NC), and amorphous. The width and frequency shift of the crystal component were analyzed using the phonon confinement effect, thermal stress, and Sn composition. The transition from amorphous to the NC state was associated with the disorder in the precursor. The addition of a restrained amount of Cu led to the evident progress of crystallization and simultaneous suppression of the NC transition. The Sn composition of 3 at% was insufficient to exhibit a noticeable MIC effect.

Role of the phonon confinement effect and boundary scattering in reducing the thermal conductivity of argon nanowire.

The thermal conductivity of model argon nanowires over a wide range of temperatures from 20 K to 70 K has been calculated using the formula obtained by solving the Boltzmann equation and independently by molecular dynamic (MD) simulations. The theoretical predictions for thermal conductivity take into account the effect of phonon confinement and boundary scattering. Two known theoretical approaches were used. The first approach is based on the solution of the Boltzmann equation with given boundary conditions and uses bulk acoustic phonon dispersion and neglects the phonon confinement effect. The second approach includes also the modification of acoustic phonon dispersion due to spatial confinement. In simulations, the square and circular shapes of wire with the transverse size of nanowires from 4.3 nm to 42.9 nm have been considered. It was found that MD simulation results match the theoretical predictions reasonably well. The obtained results showed that the phonon confinement effect influences the thermal conductivity of nanowires, but the dominant factor decreasing the thermal conductivity with the thickness of nanowires is boundary scattering. Moreover, the values of the interface specular parameter indicate that the specular phonon-boundary scattering prevails over diffuse phonon-boundary scattering.

Doping-Induced Combined Fano and Phonon Confinement Effect in La-Doped CeO2: Raman Spectroscopy Analysis

The effect of La-doping on the crystallographic structure and the Raman line-shape of CeO2 has been investigated. X-ray diffraction analysis suggests that the lattice parameter and the Ce/La–O bond...

Phonon confinement and size effect in Raman spectra of TiO2 nanocrystal towards Photocatalysis Application

Phonon confinement and size effect in Raman spectra of TiO2 nanocrystal towards Photocatalysis Application

Infrared spectroscopy of CeO2 nanoparticles using Bergman's spectral representation: effects of phonon confinement and lattice strain.

The dielectric function of a cerium oxide nanopowder has been investigated by infrared spectroscopy. The use of Bergman's spectral representation and a semi-quantum dielectric function model allows an accurate retrieval of the main features of the lattice dynamics of this nanocompound. Due to the absence of significant lattice strain or vacancy concentration, the observed differences between the dielectric functions of the nanopowder and a single crystal can be explained mainly by the phenomenon of phonon confinement. The results are validated by comparison to literature data and additional spectroscopic techniques, as well as by direct measurements of powder filling factors.

Evaluation of Thermal Conductivity Characteristics in Polycrystalline Silicon - The Effect of Nanostructure in the Grains

1.Introduction Polycrystalline silicon (poly-Si) is a promising candidate for channel materials such as thin film transistors (TFTs). Moreover, poly-Si is also expected for the next-generation thermoelectric device materials. It has been reported that poly-Si has low thermal conductivity due to their grain boundaries and defects, and so on [1,2]. We have previously confirmed that nanostructures exist in poly-Si grains, and these nanostructures become the phonon scatters [3]. However, the detailed effect of annealing on the nanostructure size has not been sufficiently. In this study, we focused on only the nanostructure size by changing the additional annealing conditions during poly-Si thin film fabrication with keeping the same grain size, and evaluated the thermal conductivity characteristics of the poly-Si thin film. 2.Experiments The 100 nm thick SiO2 films were thermally grown on Si substrates. The 150 nm thick Amorphous Si (a-Si) thin films were deposited at 510 °C by low pressure chemical vapor deposition (LPCVD) using the mono-silane (SiH4) gases under the pressure of 0.4 Torr. The a-Si thin films on SiO2/Si substrate were then annealed at 700 °C 2 hrs (Sample A), 700 °C 2hrs + 900 oC 2hrs (Sample B) and 700 °C 2 hrs +1100 °C 2 hrs (Sample C), respectively. The grain size is determined by the first annealing. Therefore, the grain size of each sample is almost the same at 600 nm.The fabricated samples were measured by UV Raman spectroscopy for the investigation of thermal conductivity characteristics. The excitation source was the UV laser (λ=355 nm), and the focal length of the spectrometer was 2,000 mm. The laser power was controlled by a variable neutral density (ND) filter in the incident light path from 1 to 10 mW in 1 mW steps. Five-point Raman spectra measurements were performed on each sample to evaluate the thermal conductivity characteristics. 3.Results and Discussion Figure 1 shows the laser-power-dependent Raman spectra of Sample A. It was confirmed that the higher laser power resulted in the higher Raman spectral intensity. In addition, it was confirmed the Raman spectrum tends to shift toward lower wavenumber as the laser power increases. It has been reported that the Raman spectrum for Si-Si mode shift toward lower wavenumber due to the thermal effect as temperature increases. Therefore, it is considered that the change in the Raman shift is due to the temperature change caused by local laser heating. Table 1 shows the average nanostructure size of each sample. The nanostructure size is calculated by the full width at half maximum (FWHM) of the UV Raman spectroscopy [4]. Figure 2 shows that the laser-power-dependent Raman peak shift from the poly-Si thin films. It was confirmed that there is a difference in zero power intercept due to phonon confinement effect. It is considered that the Raman spectrum for sample A shifted due to the phonon confinement effect possibly caused by the smaller nanostructures inside the grain. In addition, it was also confirmed that the discrepancy in the Raman shifts between each sample becomes larger as the laser power increases, and it indicates that there is a difference in the temperature change for each sample. The relation between Raman shift and temperature are described by following Eq. (1) [5].dω/dT = -0.024 cm-1/K (1)Figure 3 shows that temperature in poly-Si thin film measured by Si Raman peak shift. It was revealed that Sample B has lower thermal conductivity despite the nanostructure size is larger. It may need to consider not only size but also density of the nanostructure to understand these phenomena. In conclusion, we clearly indicated that not only the average grain size but also the nanostructures inside grains should be carefully considered to control thermal conductivities. REFERENCE Y. Nakamura, Sci. Technol. Adv. Mater. 19, 31 (2018).K. Valalaki, N. Vouroutzis, and A. G. Nassiopoulou, J. Phys. D: Appl. Phys. 49, 315104 (2016).H. Takeuchi, R. Yokogawa, K. Takahashi, K. Komori, T. Morimoto, N. Sawamoto and A. Ogura, 236th ECS Meeting. G04-1210 (2019).H. Yamazaki, M. Koike, M. Saitoh, M. Tomita, R. Yokogawa, N. Sawamoto, M. Tomita, D. Kosemura, and A. Ogura, Sci. Rep. 7,16549 (2017).D. Fan, H. Sigg, R. Spolenak, and Y. Ekinci, Phys. Rev. B 96, 115307 (2017). Figure 1