Phonon Confinement Research Articles

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.

Blue- and red-shifts of V2O5 phonons in NH3 environment by in situ Raman spectroscopy

A layer of ~30 nm V2O5/100 nm-SiO2 on Si was employed in the in situ Raman spectroscopy in the presence of NH3 effluent from a thermal decomposition of ammonium acetate salt with the salt heated at 100 °C. When the layer is placed at 25 °C, we observe a reversible red-shift of 194 cm−1 V2O5 phonon by 2 cm−1 upon NH3 gas injection to saturation, as well as a reversible blue-shift of the 996 cm−1 by 4 cm−1 upon NH3 injection. However when the sensing layer is placed at 100 °C, the 194 cm−1 remains un-shifted while the 996 cm−1 phonon is red-shifted. There is a decrease/increase in intensity of the 145 cm−1 phonon at 25 °C/100 °C when NH3 interacts with V2O5 surface. Using the traditional and quantitative gas sensor tester system, we find that the V2O5 sensor at 25 °C responds faster than at 100 °C up to 20 ppm of NH3 beyond which it responds faster at 100 °C than at 25 °C. Overall rankings of the NH3 gas sensing features between the two techniques showed that the in situ Raman spectroscopy is faster in response compared with the traditional chemi-resistive tester. Hooke’s law, phonon confinement in ~51 nm globular particles with ~20 nm pore size and physisorption/chemisorption principles have been employed in the explanation of the data presented.

Irradiation-induced defect formation and damage accumulation in single crystal CeO2

The accumulation of irradiation-induced disorder in single crystal CeO2 has been investigated over a wide range of ion fluences. Room temperature irradiations of epitaxial CeO2 thin films using 2 MeV Au2+ ions were carried out up to a total fluence of 1.3×1016 cm−2 Post-irradiation disorder was characterized using ion channeling Rutherford backscattering spectrometry (RBS/C) and confocal Raman spectroscopy. The Raman measurements were interpreted by means of a phonon confinement model, which employed rigid ion calculations to determine the phonon correlation length in the irradiated material. Comparison between the dose dependent changes in correlation length of the Raman measurements and the Ce disorder fraction from RBS/C provides complementary quantitative details on the rate of point and extended defect formation on the Ce and O sub-lattices over a broad range of ion fluences. Raman measurements, which are significantly more sensitive than RBS/C at low doses, reveal that the nucleation rate of defects is highest below 0.1 displacements per atom (dpa). Comparison between Raman and RBS/C measurements suggests that between 0.1 and 10 dpa the damage evolution is characterized by modest growth of point defects and/or small clusters, while above 10 dpa the preexisting defects rapidly grow into extended clusters and/or loops.

Robust valley polarization of helium ion modified atomically thin MoS2

Atomically thin semiconductors have dimensions that are commensurate with critical feature sizes of future optoelectronic devices defined using electron/ion beam lithography. Robustness of their emergent optical and valleytronic properties is essential for typical exposure doses used during fabrication. Here, we explore how focused helium ion bombardement affects the intrinsic vibrational, luminescence and valleytronic properties of atomically thin . By probing the disorder dependent vibrational response we deduce the interdefect distance by applying a phonon confinement model. We show that the increasing interdefect distance correlates with disorder-related luminscence arising 180 meV below the neutral exciton emission. We perform ab initio density functional theory of a variety of defect related morphologies, which yield first indications on the origin of the observed additional luminescence. Remarkably, no significant reduction of free exciton valley polarization is observed until the interdefect distance approaches a few nanometers, namely the size of the free exciton Bohr radius. Our findings pave the way for direct writing of sub-10 nm nanoscale valleytronic devices and circuits using focused helium ions.

Phonon Confinement Induced Non-Concomitant Near-Infrared Emission along a Single ZnO Nanowire: Spatial Evolution Study of Phononic and Photonic Properties

The impact of mixed defects on ZnO phononic and photonic properties at the nanoscale is only now being investigated. Here we report an effective strategy to study the distribution of defects along the growth direction of a single ZnO nanowire (NW), performed qualitatively as well as quantitatively using energy dispersive spectroscopy (EDS), confocal Raman-, and photoluminescence (PL)-mapping technique. A non-concomitant near-infrared (NIR) emission of 1.53 ± 0.01 eV was observed near the bottom region of 2.05 ± 0.05 μm along a single ZnO NW and could be successfully explained by the radiative recombination of shallowly trapped electrons with deeply trapped holes at . A linear chain model modified from a phonon confinement model was used to describe the growth of short-range correlations between the mean distance of defects and its evolution with spatial position along the axial growth direction by fitting the E2H mode. Our results are expected to provide new insights into improving the study of the photonic and photonic properties of a single nanowire.

Investigation of ethanol gas sensing properties of Dy-doped SnO2 nanostructures

In this paper we report doping induced enhanced sensor response of SnO2 based sensor towards ethanol at a working temperature of 200 °C. Undoped and dysprosium-doped (Dy-doped) SnO2 nanoparticles were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD and Raman results verified tetragonal rutile structure of the prepared samples. It has been observed that crystallite size reduced with increase in dopant concentration. In addition, the particle size has been calculated from Raman spectroscopy using phonon confinement model and the values match very well with results obtained from TEM and X-ray diffraction investigations. Dy-doped SnO2 sensors exhibited significantly enhanced response towards ethanol as compared to undoped sensor. The optimum operating temperature of doped sensor reduced to 200 °C as compared to 320 °C for that of undoped sensor. Moreover, sensor fabricated from Dy-doped SnO2 nanostructures was highly selective toward ethanol which signifies its potential use for commercial applications. The gas sensing mechanism of SnO2 and possible origin of enhanced sensor response has been discussed.

Influence of confined optical phonons and laser radiation on the hall effect in a compositional superlattice

The effect of confined optical phonons and laser radiation are taken into account to study the Hall effect in a compositional semiconductor superlattice (CSSL). The new theoretical expressions of the Hall conductivity, the Hall coefficient and the magnetoresistance are obtained as functions of the external fields (the magnetic field B, the laser frequency Ω and amplitude E), the CSSL parameters (the well length dI, the barrier length dII and height U) and the quantum number m characterizing the phonon confinement effect. The magneto-confined phonon resonance condition in a CSSL is also carried out to explain the numerical evaluation results for the GaAs/AlxGa1−xAs CSSL. When optical phonons are confined, the Hall conductivity has more resonance peaks in comparison with that for bulk phonons. In addition, these sub-peaks are more separate from the main peaks as the CSSL layers thickness decreases. The tighter CSSL period is, the more forcefully the phonon confinement makes. When the CSSL period d is smaller than 30nm, the contribution of confined optical phonons is significant and could not be neglected. The magnetophonon resonance for bulk semiconductor could be achieved in the limit the CSSL period goes to infinity. Furthermore, the Hall conductivity increases and the Hall coefficient, the magnetoresistance decrease due to the laser radiation and the effect of confined optical phonons. Especially, the near-linear dependence of the magnetoresistance on temperature is in good agreement with the experimental result.

Optomechanical properties of GaAs/AlAs micropillar resonators operating in the 18 GHz range.

Recent experiments demonstrated that GaAs/AlAs based micropillar cavities are promising systems for quantum optomechanics, allowing the simultaneous three-dimensional confinement of near-infrared photons and acoustic phonons in the 18-100 GHz range. Here, we investigate through numerical simulations the optomechanical properties of this new platform. We evidence how the Poisson's ratio and semiconductor/vacuum boundary conditions lead to very distinct features in the mechanical and optical three-dimensional confinement. We find a strong dependence of the mechanical quality factor and strain distribution on the micropillar radius, in great contrast to what is predicted and observed in the optical domain. The derived optomechanical coupling constants g0 reach ultra-large values in the 106 rad/s range.

A Guide to and Review of the Use of Multiwavelength Raman Spectroscopy for Characterizing Defective Aromatic Carbon Solids: from Graphene to Amorphous Carbons

sp2 hybridized carbons constitute a broad class of solid phases composed primarily of elemental carbon and can be either synthetic or naturally occurring. Some examples are graphite, chars, soot, graphene, carbon nanotubes, pyrolytic carbon, and diamond-like carbon. They vary from highly ordered to completely disordered solids and detailed knowledge of their internal structure and composition is of utmost importance for the scientific and engineering communities working with these materials. Multiwavelength Raman spectroscopy has proven to be a very powerful and non-destructive tool for the characterization of carbons containing both aromatic domains and defects and has been widely used since the 1980s. Depending on the material studied, some specific spectroscopic parameters (e.g., band position, full width at half maximum, relative intensity ratio between two bands) are used to characterize defects. This paper is addressed first to (but not limited to) the newcomer in the field, who needs to be guided due to the vast literature on the subject, in order to understand the physics at play when dealing with Raman spectroscopy of graphene-based solids. We also give historical aspects on the development of the Raman spectroscopy technique and on its application to sp2 hybridized carbons, which are generally not presented in the literature. We review the way Raman spectroscopy is used for sp2 based carbon samples containing defects. As graphene is the building block for all these materials, we try to bridge these two worlds by also reviewing the use of Raman spectroscopy in the characterization of graphene and nanographenes (e.g., nanotubes, nanoribbons, nanocones, bombarded graphene). Counterintuitively, because of the Dirac cones in the electronic structure of graphene, Raman spectra are driven by electronic properties: Phonons and electrons being coupled by the double resonance mechanism. This justifies the use of multiwavelength Raman spectroscopy to better characterize these materials. We conclude with the possible influence of both phonon confinement and curvature of aromatic planes on the shape of Raman spectra, and discuss samples to be studied in the future with some complementary technique (e.g., high resolution transmission electron microscopy) in order to disentangle the influence of structure and defects.

Influence of Confined Optical Phonons and Laser Radiation on the Radioelectric Effect in a Cylindrical Semiconductor Quantum Wire with Parabolic Potential

Abstract. Based on the quantum kinetic equation method, the influence of confined optical phonons and laser radiation on the Radioelectric effect in a cylindrical semiconductor quantum wire with parabolic potential subjected to a dc electric field and a linearly polarized electromagnetic wave has been theoretically studied. The obtained analytical expression of the Radioelectric field (REF) depends on frequencies and amplitudes of the external electromagnetic waves, the quantum wire parameters, the temperature of the system, and especially the quantum numbers (n and m) which characterize the phonon confinement. Numerical calculations for the GaAs/GaAsAl quantum wire show the strongly impact of the confined optical phonons as well as the laser radiation on the REF magnitude and posture. The REF also has more resonance peaks in comparison with that in case of bulk phonon.

Lattice diffusion and vapor solid growths forming nanoarchitectures on ZnO nanowires

We report hierarchical nanoarchitectures formed on the tips and sidewalls of ZnO nanowires which is formed on the top of microtubes. The whole growth process of these micro/nanostructures during thermal oxidation combines lattice/grain/surface ionic diffusion along with vapor solid mechanism. All the process takes place along with the presence of an electric current, which plays an important role forming the ZnO molecules due to Zn metal evaporation and attracting them to condense into nanostructures of several morphologies. The observation of a very long needle-like nanowire reveals the stack nature of the growth. These nanoarchitectures are rarely observed experimentally. Raman scattering confirms phonon confinement in the nanostructures. Photoluminescence measurements indicate a route for engineering defects on the surface of ZnO microtubes after the complete coalescence of the nanostructures through heat treatment. This experiment would be useful for improving nanostructure organization which could provide an impact in the manufacturability of nanostructure-based systems.

The structural phases and vibrational properties of Mo1−xWxTe2 alloys

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T′ semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the Td Weyl semimetal phase. The WxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk WxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T′, and Td structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2–WTe2 system, including single-phase 2H, 1T′, and Td regions, as well as a two-phase 1T′ + Td region. Disorder arising from compositional fluctuations in WxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T′-MoTe2 mode and the enhancement of a double-resonance Raman process in WxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in WxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

Optical anisotropy in vertically oriented TiO2 nanotube arrays

Nanofabricated optically anisotropic uniaxial thin films with deep submicron feature sizes are emerging as potential platforms for low-loss all-dielectric metamaterials, and for Dyakonov surface wave-based subwavelength optical confinement and guiding at interfaces with isotropic media. In this context, we investigate the optical properties of one such uniaxial platform, namely self-organized titania nanotube arrays (TNTAs) grown by the bottom-up nanofabrication process of electrochemical anodization on silicon wafer substrates, and subsequently annealed at different temperatures, i.e. 500 °C and 750 °C. We performed detailed quantitative analysis of the structure of the TNTAs using x-ray diffraction and Raman spectroscopy, which revealed a measurable phonon confinement in TNTAs annealed at 500 °C. Variable angle spectroscopic ellipsometry was used to investigate the optical anisotropy in two kinds of TNTAs—those constituted by anatase-phase and those containing a mixture of anatase and rutile phases. Both kinds of TNTAs were found to have positive birefringence (Δn) exceeding 0.06 in the spectral region of interest while mixed phase TNTAs exhibited Δn as high as 0.15. The experimentally measured anisotropy in the refractive index of the TNTAs was compared with the predictions of two different effective medium approximations incorporating the uniaxial geometry. The measured value of Δn for TNTAs exceeded that of bulk anatase single crystals, indicating the potential of nanostructured dielectrics to outperform dielectric crystals of the same material with respect to the magnitude of the achievable directional refractive index contrast.

New explanation of Raman peak redshift in nanoparticles

In this letter, we propose a new model that explains the Raman peak downshift observed in nanoparticles with respect to bulk materials. The proposed model takes into account discreteness of the vibrational spectra of nanoparticles. For crystals with a cubic lattice (Diamond, Silicon, Germanium) we give a relation between the displacement of Raman peak position and the size of nanoparticles. The proposed model does not include any uncertain parameters, unlike the conventionally used phonon confinement model (PCM), and can be employed for unambiguous nanoparticles size estimation.

Finite-size effects on the lattice dynamics in spin crossover nanomaterials. II. Molecular dynamics simulations

In the first part of this work, an experimental study of the lattice dynamics of spin crossover nanoparticles was performed using the nuclear inelastic scattering (NIS). A size dependence of low energy phonon modes appears under 10 nm, but its origin is not well understood. In this paper, we investigate the phonon confinement effects in the framework of molecular dynamics simulations by modeling three-dimensional nanoparticles considering a cubic lattice with an octahedral pattern. The vibrational density of states is computed and compared to the experiment. The simulations allow one to highlight both the role of the phonon quantification and the role of the size and shape distributions of particles on the extracted parameters leading to a better understanding of the experimental results.

Size dependent Raman and absorption studies of single walled carbon nanotubes synthesized by pulse laser deposition at room temperature

In this article, size dependent correlation of acoustic states is established for radial breathing mode (RBM). Single walled carbon nanotubes (SWCNTs) are synthesized along with carbon encapsulated iron nanoparticles by pulse laser deposition at room temperature. Ferrocene is used as a catalyst for growth of SWCNTs. Various studies such as HR-TEM, X-Ray Diffraction (XRD), Raman spectroscopy and NIR-Absorption spectroscopy are utilized to confirm the presence of SWCNTs in the as-synthesized and purified samples. RBM of SWCNTs can be differentiated here from Raman modes of carbon encapsulated iron nanoparticles by comparing their line shape asymmetry as well as oscillator strength. Furthermore, a quantum confinement model is proposed for RBM. It is invoked here that RBM is manifestation of quantum confinement of acoustic phonons. Well reported analytical relation of RBM is utilized to explore the nature of phonons responsible for RBM on the basis of quantum confinement model. Diameters of SWCNTs estimated by Raman studies are found to be in reasonably good agreement with that of NIR-absorption studies.

Structural and spectroscopic comparison between polycrystalline, nanocrystalline and quantum dot visible light photo-catalyst Bi2WO6

The structural and spectroscopic features of the visible light photocatalyst Bi2WO6 have been studied. Polycrystalline (PC), nanocrystalline (NC) and quantum dot (QD) sized samples were produced using solid state reaction, hydrothermal and flame spray pyrolysis methods, respectively. While the crystal structures of PC and NC Bi2WO6 are well characterized using X-ray powder diffraction data Rietveld refinements, the structural information of the QD are obtained from the complementary pair distribution function analysis and high-resolution transmission electron microscopy. The Raman spectra of the samples are compared with the phonon density of states calculated by DFT. A continuous phenomenological model describes selective optical phonon confinement into the QDs. The type of the electronic bandgaps obtained from the UV-VIS absorbance-spectra have been analyzed using two different methods, and compared with those calculated from the electronic band structures.

Correlation of TEM data with confined phonons to determine strain and size of Ge nanocrystals embedded in SixNy matrix

Models that use phonon confinement fail to provide consistent results for nanocrystal sizes in differing dielectric matrices due to varying stress experienced by nanocrystals in different dielectric environments. In cases where direct measurement of stress is difficult, the possibility of stress saturation as a function of size opens up a window for the use of phonon confinement to determine size. We report on a test of this possibility in Ge: SixNy system. Ge nanocrystals (NCs) embedded in silicon nitride matrix have been fabricated using plasma enhanced chemical vapor deposition (PECVD) followed by post annealing in Ar ambient. Nanocrystal size dependence of Raman spectra was studied taking into account associated stress and an improved phonon confinement approach. Our analysis show same stress for NCs which have sizes below 7.0 nm allowing the use of phonon confinement to determine the nanocrystal size. The results are compared with TEM data and good agreement is observed.

Phonon thermal transport in silicene-germanene superlattice: a molecular dynamics study

Two-dimensional (2D) hybrid materials have drawn enormous attention in thermoelectric applications. In this work, we apply a molecular dynamics (MD) simulation to investigate the phonon thermal transport in silicene-germanene superlattice. A non-monotonic thermal conductivity of silicene-germanene superlattice with period length is revealed, which is due to the coherent–incoherent phonon conversion and phonon confinement mechanisms. We also calculate the thermal conductivity of a Si-Ge random mixing monolayer, showing a U-shaped trend. Because of the phonon mode localizations at Ge concentration of <20% and >80%, thermal conductivity varies dramatically at low doping regions. By changing the total length (Ltotal), the infinite-length thermal conductivities of pure silicene, pure germanene, silicene-germanene superlattice, and Si-Ge random mixing monolayer are extracted as 16.08, 15.95, 5.60 and 4.47 W/m-K, respectively. The thermal boundary conductance (TBC) of the silicene-germanene is also evaluated, showing a small thermal rectification. At Ltotal = 274.7 nm, the TBC of silicene to germanene is 620.49 MW/m2-K, while that of germanene to silicene is 528.76 MW/m2-K.

Influence of phonon confinement on the optically detected magneto-phonon resonance line-width in parabolic quantum wells

We investigate the influence of phonon confinement described by the slab mode model on the optically detected magneto-phonon resonance (ODMPR) effect and ODMPR line-width in parabolic quantum wells by using the operator projection. The ODMPR conditions as functions of the well’s parameters and the photon energy are also obtained. The shifts of ODMPR peaks caused by the confined phonon are discussed. The numerical result for a specific quantum well shows that in the two cases of confined and bulk phonons, the line-width (LW) is sensitive to the change of the confinement frequency, and temperature. Furthermore, the influence of slab mode phonon plays an important role and cannot be neglected in reaching the ODMPR line-width for all range of the well’s confinement frequency.