Phonon Confinement Research Articles

Hyper‐Raman spectroscopy of CeO2

AbstractCerium dioxide (CeO2) belongs to the fluorite structure group (Oh5 point group) resulting in one Raman active T2g mode and two IR active T1u TO and LO modes. The T2g Raman band in CeO2 has been used to probe the symmetric Ce–O vibrational mode of different sized nanoparticulates. Far infrared studies of the Ce–O interactions are difficult to conduct because of low optical transmission, efficiency of the detector, and water absorption bands interfering with the measurements. The selection rules of hyper‐Raman spectroscopy (HRS) allow the study of IR bands in the far infrared spectral region, such as the motion of the Ce atoms. For the first time, HRS was used to investigate the T1u TO and LO IR modes in CeO2. The position and full‐width half‐maximum (FWHM) of the TO and LO bands in the HRS spectrum were used to investigate the spectral response in relation to particle size (i.e., phonon confinement). The investigation of CeO2 with HRS provides a research platform for future work in the study of ThO2, UO2, and PuO2 IR modes.

Mo doped TiO2: impact on oxygen vacancies, anatase phase stability and photocatalytic activity

This work outlines an experimental and theoretical investigation of the effect of molybdenum (Mo) doping on the oxygen vacancy formation and photocatalytic activity of TiO2. Analytical techniques such as x-ray diffraction (XRD), Raman, x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to probe the anatase to rutile transition (ART), surface features and optical characteristics of Mo doped TiO2 (Mo–TiO2). XRD results showed that the ART was effectively impeded by 2 mol% Mo doping up to 750 °C, producing 67% anatase and 33% rutile. Moreover, the crystal growth of TiO2 was affected by Mo doping via its interaction with oxygen vacancies and the Ti–O bond. The formation of Ti–O–Mo and Mo–Ti–O bonds were confirmed by XPS results. Phonon confinement, lattice strain and non-stoichiometric defects were validated through the Raman analysis. DFT results showed that, after substitutional doping of Mo at a Ti site in anatase, the Mo oxidation state is Mo6+ and empty Mo-s states emerge at the titania conduction band minimum. The empty Mo-d states overlap the anatase conduction band in the DOS plot. A large energy cost, comparable to that computed for pristine anatase, is required to reduce Mo–TiO2 through oxygen vacancy formation. Mo5+ and Ti3+ are present after the oxygen vacancy formation and occupied states due to these reduced cations emerge in the energy gap of the titania host. PL studies revealed that the electron–hole recombination process in Mo–TiO2 was exceptionally lower than that of TiO2 anatase and rutile. This was ascribed to introduction of 5s gap states below the CB of TiO2 by the Mo dopant. Moreover, the photo-generated charge carriers could easily be trapped and localised on the TiO2 surface by Mo6+ and Mo5+ ions to improve the photocatalytic activity.

Size dependence of Raman line-shape parameters due to confined phonons in silicon nanowires

ABSTRACT A comparison of experimentally observed Raman scattering data with Raman line shapes, generated theoretically using a phonon confinement model, has been carried out to understand the sensitivity of different Raman spectral parameters towards the quantum confinement effect. Size-dependent variations of full width at half maximum, Raman peak position and asymmetry ratio have been analysed to establish the sensitivity of their corresponding physical counterparts (phonon lifetime and dispersion) in confined systems. The comparison has been done in three different confinement regimes, namely weakly, moderately and strongly. Proper reasoning has been assigned for such a variation after validation of the theoretical analysis with the experimental observations. A moderately confined system was created by preparing 6-nm-sized silicon nanostructures using metal-induced etching. An asymmetrically broadened and red-shifted Raman line shape was observed, which established that all the parameters get affected in a moderately confined system. Sensitivity of a given Raman spectral parameter has been shown to be used as a tool to understand the role of external perturbations in a material.

Quantitative study on the mechanisms underlying the phonon bottleneck effect in InN/InGaN multiple quantum wells

Herein, we quantitatively investigated the mechanisms of interfacial phonon mismatch and quantum confinement underlying the phonon bottleneck effect in InN/InxGa(1−x)N multiple quantum wells. Obvious extension of carrier thermalization with the longest relaxation time of 4.75 ns due to an enhanced phonon bottleneck effect was observed. It was found that a stronger quantum confinement could effectively reduce carrier relaxation rate via improving the elastic carrier–carrier scattering, while a larger phonon mismatch may give a higher initial carrier temperature due to the reflection of optical phonons and probably confinement of quasi-optical phonons. This study provides an essential theoretical insight into photovoltaic and other optoelectronic devices aiming to reduce the carrier relaxation rate via the phonon bottleneck effect.

All‐Epitaxial Bulk Acoustic Wave Resonators

There is a growing interest in the exploration of the nitride material family for radically scaled, high frequency, ultrasonic devices by epitaxial growth techniques. Furthermore, the introduction of epitaxial growth techniques to conventional nitride‐based acoustic technology opens the door to exciting new families of structures for phonon confinement. As the need for higher frequency communications increases, both piezoelectrics and electrodes must scale to smaller dimensions. It has recently become possible to epitaxially grow single‐crystalline, wurtzite AlN/NbN piezoelectric/metal heterostructures. The epitaxial NbN films maintain high crystalline quality and electrical conductivity down to several nanometers thickness. This study demonstrates preliminary results on the feasibility of an all‐epitaxial bulk acoustic wave technology by growing and characterizing the radio frequency (RF) properties of an epitaxial AlN/NbN heterostructure.

Study the Mechanisms of Enhanced Phonon Bottleneck Effect for the Absorber of Hot Carrier Solar Cell in III-V Multiple Quantum Wells

The most vital key to realize hot carrier solar cell is reducing carrier relaxation time to nanoseconds by phonon bottleneck effect often observed in nanostructure. However, the mechanisms underlying this are still not well understood. In this paper, we systematically investigated the mechanisms of phonon interfacial mismatch and carrier quantum confinement over phonon bottleneck effect in InN/InxGa(1-x)N multiple quantum wells (MQWs). Highly promising hot carrier lifetimes due to enhanced phonon bottleneck effect were observed in these MQWs, where the longest hot carrier lifetime is 3.2±0.12 ns. It was found the quantum confinement of carriers could play more important role in the reduction of carrier cooling rate, while the optical phonon confinement is more likely to dominate the initial carrier temperature. This study clarifies two of the most important mechanisms of phonon bottleneck effect and directs a promising application of III-V MQWs on the absorber of hot carrier solar cell.

Nonlinear optical and photocatalytic dye degradation of Co doped CeO2 nanostructures synthesized through a modified combustion technique

The nanoparticles of pure and Co doped CeO2 were synthesized by means of auto ignition combustion process. Phase purity and structure of the synthesized nanostructures were examined by X-ray diffraction analysis. Crystal structure and cell parameters were refined by Rietveld analysis. The modifications in the position of Raman peaks, intensityand linewidth of F2g mode can be associated with factors such as confinement of phonons, defects and non-homogeneous distribution of particle sizes which in turn results in fluctuations in phonon relaxation. The Z-scan technique in the open and closed aperture modes reveals the self-defocusing nature and third order nonlinearity of the samples as arising due to reverse saturable absorption. Z-scan studies brought to light that the NLO properties of the Co doped CeO2 nanoparticles were significantly magnified when compared to pure CeO2, the doping of Co increased the time of electron transfer and photon transition, suppressed the electron hole recombination, and prolonged the working time of the free carrier, finally magnifying the nonlinear optical properties of the nanoparticles. Photocatalytic activity study of methylene blue and eosin yellow dye on Co doped CeO2 NPs were carried out using solar irradiation. Percentage of dye degradation and the rate at which degradation occur were calculated. High percentage of degradation shown by Co doped CeO2 NPs on EY dye. Mechanism of degradation of MB and EY dyes under solar irradiation had studied. The mechanism reveals that transition metal doping improve the trapping of e--h+ recombination during solar irradiation.

Tunable phonon nanocapacitor built by carbon schwarzite based host-guest system

The ability to control monochromatic and coherent phonons is crucial for many phonon-related applications but remains very challenging. Here, we demonstrate a promising phonon nanocapacitor for storing and emitting phonons, built by the carbon schwarzite based host-guest system (HGS). This phonon nanocapacitor takes advantage of the inherently strong phonon confinement of the hybridized modes in HGS. The monochromaticity and coherence of the stored phonons are well demonstrated via the ultralong phonon lifetime and coherent time. More interestingly, the frequency of the nanocapacitor is widely tunable from gigahertz to several terahertz by engineering the host-guest interaction. Finally, the stored phonons with different polarizations can be emitted separately with the application of uniaxial strain along a particular direction. This work may provide new opportunities for studying the coherent wave effect of phonons.

A novel sensing platform based on self-doped TiO2 nanotubes for methylene blue dye electrochemical monitoring during its electro-Fenton degradation

The synthesis of a self-doped TiO2 nanotubes (SD-TNT) electrode and its potential applicability as a sensing platform electrode for voltammetric determination of methylene blue (MB) dye is reported. Highly ordered TiO2 nanotubes vertically oriented to the titanium surface were grown and the electrochemical doping did not change the morphology of the TNT. Raman spectroscopy demonstrated that the electrochemical doping of TNT promoted a blue shift of the Eg1 mode (from 152 to 158 cm−1) which results in an increased conductivity due to the phonon confinement resultant from the formation of non-stoichiometry Ti3+ donor states. The structural changes allowed the application of SD-TNT as a voltammetric sensor of MB without any UV irradiation. The SD-TNT electrode allowed an efficient quantification of MB, with a limit of detection of 0.48 and 0.90 μmol L−1 when considered the oxidation and reduction peak, respectively. Then, SD-TNT electrode was applied to the monitoring of MB concentration during electro-Fenton oxidation. MB removal fitted well with pseudo-first-order kinetics, suggesting a constant formation of •OH radicals and oxidation of MB. These results revealed that the SD-TNT is a versatile, simple, and promising material for the voltammetric determination of synthetic dyes.

Temperature-dependence calculation of lattice thermal conductivity and related parameters for the zinc blende and wurtzite structures of InAs nanowires

Theoretical calculations are performed on lattice thermal conductivity (LTC) and related parameters for the zinc blende and wurtzite structure of InAs nanowires (NWs) with diameters of 50, 63, 66, 100 and 148 nm through the Morelli–Callaway model. For the model to be efficiently applicable, the longitudinal and transverse modes are considered. The melting point of the various-sized NWs is considered to estimate the Debye and phonon group velocities. The impacts of Gruneisen parameter, dislocations and surface roughness are also successfully utilized to address the calculated and measured LTC of the semiconductor under investigation. Results show that the Gruneisen parameter increases with decreasing NW diameter and that phonon confinement leads to an observable deviation of the calculated LTC curve from that of the experimental one in the case of bulk InAs. We assume that NW boundaries, dislocations and imperfections are responsible for the scattering of phonons along with electrons and other phonons because of normal and Umklapp processes. Therefore, at a specified temperature, LTC depends on the size and crystal structure of the semiconductor. As such, the thermal and mechanical parameters of InAs can be greatly modified by decreasing the size and dimension of the semiconductor as a result of the quantum-confinement effect.

High-pressure synthesis and optical properties of nanodiamonds obtained from halogenated adamantanes

Following our previous brief communication the carbonization of halogenated adamantanes C10H14Br2 and C10H15Cl under high pressure with the aim to produce nanodiamonds have been studied in details. A fixed pressure of 8 GPa and temperatures up to 2000 K were applied. Carbonization behavior of both precursors is very similar and starts in range of 900–1000 K with formation of about 1–2 nm diamond nanocrystals. With increasing process temperature the size of obtained nanodiamonds slowly grows up to tens of nanometers at 2000 K. The observed behavior provides favorable conditions for the synthesis of the nanodiamonds of the precisely controlled crystal size. Raman spectroscopy, X-ray diffraction and high resolution transmission microscopy were used to monitor the phase composition of the samples and the crystal size of nanodiamonds. Lattice parameter of nanodiamonds, calculated by using the Rietveld analysis of the X-ray diffraction patterns, increases by up to 2–3% as an average crystallite size decreases to the minimal value of 1–2 nm. Based on the experimental results and density functional theory calculations, extremely high increase in the lattice parameter of nanodiamonds is interpreted in terms of the strain exerted on the nanocrystals due to diamond surface reconstruction, particularly on the (110) face. Lowering of the crystal size is accompanied by the red-shift and broadening of the diamond Raman line. The effects are explained in terms of the crystal lattice expansion and the phonon confinement model.

Phonon confinement and transport in ultrathin films

Thermal transport by phonons in films with thicknesses of less than 10 nm is investigated in a soft system (Lennard-Jones argon) and a stiff system (Tersoff silicon) using two-dimensional lattice dynamics calculations and the Boltzmann transport equation. This approach uses a unit cell that spans the film thickness, which removes approximations related to the finite cross-plane dimension required in typical three-dimensional-based approaches. Molecular dynamics simulations are performed to obtain finite-temperature structures for the lattice dynamics calculations and to predict thermal conductivity benchmarks. Thermal conductivity decreases with decreasing film thickness until the thickness reaches four unit cells (2.1 nm) for argon and three unit cells (1.6 nm) for silicon. With a further decrease in film thickness, thermal conductivity plateaus in argon while it increases in silicon. This unexpected behavior, which we identify as a signature of phonon confinement, is a result of an increased contribution from low-frequency phonons, whose density of states increases as the film thickness decreases. Phonon mode-level analysis suggests that confinement effects emerge below thicknesses of ten unit cells (5.3 nm) for argon and six unit cells (3.2 nm) for silicon. Thermal conductivity predictions based on the bulk phonon properties combined with a boundary scattering model do not capture the low thickness behavior. To match the two-dimensional lattice dynamics and molecular dynamics predictions for larger thicknesses, the three-dimensional lattice dynamics calculations require a finite specularity parameter that in some cases approaches unity. These findings point to the challenges associated with interpreting experimental thermal conductivity measurements of ultrathin silicon films, where surface roughness and a native oxide layer impact phonon transport.

Localized phonons in Raman spectra of nanoparticles and disordered media

AbstractSeveral approaches may be used to link the Raman spectra to the characteristic size scale of nanomaterials. To treat the vibrations of the nanocrystal as localized phonons is a basic idea of the phonon confinement model (PCM). This review describes different approximations proposed since the pioneering work of Richter et al. in 1981. Early implementations of PCM received criticism for the arbitrariness of the introduced parameters and physically unsound assumptions. Later, the physical consistent version was proposed in which the model was coupled with quantum‐chemical calculations. Representative cases of applying the PCM are discussed: model benchmark system (nanodiamond), localized acoustic phonons in nanoparticles (ZnO), nanowires (Ge), and phonon‐like modes in the hydrogen‐bonding network of liquid water.

Simultaneous characterization of cross- and in-plane thermal transport in insulator patterned by directionally aligned nano-channels

Anisotropic thermal transport behavior was investigated in a single crystal sapphire patterned by vertically aligned few-nanometer diameter and several micrometer long cylindrical ion tracks. These ion tracks were introduced by exposing the sapphire to energetic ions of xenon accelerated to 167 MeV with fluences ranging from 1012 to 1014 ions/cm2. It was found that, in the low ion-track density regime, cross-plane thermal conductivity is larger, whereas in the high track density regime, the trend reverses and in-plane conductivity becomes larger. The crossover between these regimes is attributed to the interplay between phonon scattering with ion track boundaries and phonon confinement effects. In the low track density regime, the material is described by bulk phonon dispersion and anisotropy in thermal transport is attributed to the aligned nature of tracks that effectively reduce the mean free path of phonons traveling in the in-plane direction more than in the cross-plane direction. In the high-density regime, larger conductivity reduction in the cross-plane direction is consistent with previous observations, where the anisotropic reduction in thermal conductivity is owed to the anisotropic reduction of acoustic velocity caused by phonon confinement. Our results are further supported by an analytical model describing phonon mediated thermal transport.

Polaronic effects on the impurity-related linear and nonlinear optical properties in a nanowire with magnetic field

Abstract Using the Landau–Pekar variational method, the binding energies of a hydrogen-like impurity-bound magnetopolaron as well as related optical properties in a cylindrical nanowire are theoretically investigated. The influence of polar optical phonon confinement in a nanowire on the electron–phonon interaction is considered. The predictions of the impurity-related polaronic effects on the linear and nonlinear optical properties due to resonant transitions between hydrogen-like states are presented. A notable influence of the polaronic effect on optical characteristics and binding energies is observed. Particularly it is shown that the electron–phonon interaction leads to blue shifts of the extrema of all absorption coefficients and refractive index changes. The polaronic effect on the optical characteristics is noticeably manifested when the electron interacts mainly with either confined optical or interface optical phonons. It is found that, regardless of the magnetic field, the extrema of the refractive index change, without taking into account the electron–phonon interaction, are observed at the same photon energy, and taking into account this interaction, the extrema are somewhat distant from each other.

Phonon propagation scale and nanoscale order in vitreous silica from Raman spectroscopy

For nanoscale systems such as nanoparticles and 3D-bonded networks, the range of spatial coherence is well reflected in the Raman spectral pattern. For confined, or localized, phonons, the range of q-points contributing to the spectrum depends on the phonon confinement length, which makes it possible to derive size information from the spectra. In this work, the Raman spectrum of vitreous silica is described as localized phonons of an SiO2 network. The convergence of the spectral pattern with the confinement size is studied. It is shown that the phonon propagation scale in vitreous silica is within the 0.5–2 nm range.

Study of Transition Metal Ion Doped CdS Nanoparticles for Removal of Dye from Textile Wastewater

Simple and cost-effective co-precipitation route was employed to prepare pure and transition metal ions doped CdS nanoparticles using 2-mercaptoethanol as surfactant. Doping of Ni, Co and Fe ions to CdS nanoparticles have been characterized adopting various techniques. X-ray diffraction confirmed CdS has single cubic phase with crystal size varying from 1 to 2 nm. UV–Vis spectroscopy analysis reveals an increase in absorption intensity upon doping. From Raman spectra, small shift in wave number is observed with inclusion of dopants which may be attributed to optical phonon confinement. Scanning electron microscopy confirmed agglomerated spherical morphology and Cd–S linkage along with other related functional groups was recorded by Fourier transform infrared spectroscopy. Photocatalytic measurements of undoped and doped CdS were studied by irradiating methylene blue solution under visible light. It was observed that Co doped CdS effectively bleaches out methylene blue than Ni and Fe doped CdS upon exposure of visible light. This report also highlights effect of prepared nanoparticles in degradation of methylene blue by catalytic agent NaBH4.

Investigation of Irradiation Damage of Sic Single Crystal

A SiC single crystal was irradiated with 1.248 MeV Ar6+ ions at a fluence of 5×1014 cm−2. X-ray diffractometry and Raman spectroscopy were employed to investigate the irradiation damage. The simulation results suggested that the damage peak was at the depth of about 700 nm, and the corresponding value was about 0.35 dpa. The XRD peak at about 35.503° was detected. It should originate from the irradiation-induced amorphous SiC. For Raman measurement, the 199, 265, 435, 540, and 659 cm−1 peaks were recorded and assigned to Si-Si vibrations. The 1415 cm−1 peak was attributed to C-C vibration. The asymmetric broadening of 786 and 963 cm−1 peaks are observed. The tails at the frequency ranges of 690-777 and 816-925 cm−1 were caused by the phonon confinement effect resulting from the irradiation-induced damage.

Influence of Confined Phonons on the Hall Coefficient in a Cylindrycal Quantum wire with an infinite potential (for electron – confined optical phonon scattering)

The influence of confined optical phonons on the Hall Coefficient (HC) in a Cylindrycal Quantum Wire (CQW) with an infinite potential (for electron – confined optical phonons scattering). Consider a case where CQW is placed in a perpendicular magnetic field , a constant - electric field and an intense electromagnetic wave . By using the quantum kinetic equation for electrons interacting with Confined Optical Phonon (COP), we obtain analytical expressions for (HC), which are different from in comparison to those obtained for the HC in the case of normal bulk semiconductor and in the case of cylindrycal quantum wire with electron – unconfined phonons scattering mechanism. Numerical calculations are also applied for AlGaAs/GaAs/AlGaAs cylindrycal quantum wire, we see the HC depends on magnetic field B, temperature T, frequency Ω and amplitude E0 of laser radiation and especially quantum index m1 and m2 characterizing the phonon confinement. This influence is due to the quantum index m1 and m2, which makes an increase of Hall coefficient by 2,3 times in comparition with the case of unconfined phonons. When the quantum number m1 and m2 goes to zero, the result is the same as in the case of unconfined phonons.

Determination of crystallite size of nanodiamond by Raman spectroscopy

Although the phonon confinement model (PCM) was claimed to be successfully used to accurately calculate the size of larger Si nanocrystals, quantitative size characterization by Raman spectra still remains a challenge in the case of nanodiamonds due to its complexity. Here, we find that a local-mode model of Raman spectra developed recently can be employed to determine the bond number of the ordered diamond core in nanodiamonds, and then furtherly determine the size of nanodiamonds. The Raman lines of nanodiamonds of 3.0 nm, 2.0 nm, 2.2 nm, 3.3 nm, 3.7 nm 4.42 nm and 6.3 nm are calculated. Results are in good agreement with the measured Raman spectra. It not only provides a new approach to predict the size of nanodiamonds accurately by Raman spectra, but also helps to clarify issues in Raman spectra of nanodiamond and other carbon nanomaterials.