Phonon Confinement Research Articles

Phonon Confinement Effect in Two-dimensional Nanocrystallites of Monolayer MoS2 to Probe Phonon Dispersion Trends Away from Brillouin-Zone Center**Supported by the National Natural Science Foundation of China under Grant Nos 11225421, 11474277, 11434010 and 11574305, and the National Young 1000 Talent Plan.

The fundamental momentum conservation requirement q ~ 0 for the Raman process is relaxed in the nanocrystallites (NCs), and phonons away from the Brillouin-zone center will be involved in the Raman scattering, which is well-known as the phonon confinement effect in NCs. This usually gives a downshift and asymmetric broadening of the Raman peak in various NCs. Recently, the A′1 mode of 1L MoS2 NCs is found to exhibit a blue shift and asymmetric broadening toward the high-frequency side [Chem. Soc. Rev. 44 (2015) 2757 and Phys. Rev. B 91 (2015) 195411]. In this work, we carefully check this issue by studying Raman spectra of 1L MoS2 NCs prepared by the ion implantation technique in a wide range of ion-implanted dosage. The same confinement coefficient is used for both E′ and A′1 modes in 1L MoS2 NCs since the phonon uncertainty in an NC is mainly determined by its domain size. The asymmetrical broadening near the A′1 and E′ modes is attributed to the appearance of defect-activated phonons at the zone edge and the intrinsic asymmetrical broadening of the two modes, where the anisotropy of phonon dispersion curves along Γ–K and Γ–M is also considered. The photoluminescence spectra confirm the formation of small domain size of 1L MoS2 nanocrystallites in the ion-implanted 1L MoS2. This study provides not only an approach to quickly probe phonon dispersion trends of 2D materials away from Γ by the Raman scattering of the corresponding NCs, but also a reference to completely understand the confinement effect of different modes in various nanomaterials.

Optical vibrational modes of Ge nanowires: A computational approach

Although Ge nanowires (GeNWs) have been extensively studied in the last decade the information about their vibrational modes is still scarce, their correct comprehension could hasten the development of new microelectronic technologies, therefore, in this work we aimed to study the vibrational properties, Raman and IR and spectrum of GeNWs using the first principles density functional perturbation theory. The nanowires are modelled in the [001] direction and all dangling bonds are passivated with H and Cl atoms. Results show that the vibrational modes can be classified in three frequency intervals, a low frequency one (between 0 and 300cm−1) of mainly GeGe vibrations, and two of GeH bending and stretching vibrations (400–500cm−1 and 2000cm−1, respectively). There is a shift of the highest optical modes of GeGe vibrations compared to their bulk counterparts due to phonon confinement effects, however it is masked by some GeH bond bending modes as demonstrated by the IR and Raman responses. The Cl passivated case shows a larger number of modes at lower frequencies due to the higher mass of Cl compared to H, which in turn reduces the red shift of the highest optical modes frequencies. These results could be important for the characterization of GeNWs with different surface passivations.

Size effects in near-ultraviolet Raman spectra of few-nanometer-thick silicon-on-insulator nanofilms

We have fabricated Si-on-insulator (SOI) layers with a thickness h1 of a few nanometers and examined them by Raman spectroscopy with 363.8 nm excitation. We have found that phonon and electron confinement play important roles in SOI with h1 < 10 nm. We have confirmed that the first-order longitudinal optical phonon Raman band displays size-induced major homogeneous broadening due to phonon lifetime reduction as well as minor inhomogeneous broadening due to wave vector relaxation (WVR), both kinds of broadening being independent of temperature. Due to WVR, transverse acoustic (TA) phonons become Raman-active and give rise to a broad band in the range of 100–200 cm−1. Another broad band appeared at 200–400 cm−1 in the spectrum of SOI is attributed to the superposition of 1st order Raman scattering on longitudinal acoustic phonons and 2nd order scattering on TA phonons. Suppression of resonance-assisted 2-nd order Raman bands in SOI spectra is explained by the electron-confinement-induced direct band gap enlargement compared to bulk Si, which is confirmed by SOI reflection spectra.

Raman investigation of optical phonons in the ion implanted Hg1−xCdxTe

Raman scattering is studied here for Hg1−xCdxTe (x = 0.3) samples implanted with 180-keV of B11 ions with various doses up to 1 × 1015 cm−2. Considering disorder in the implanted HgCdTe material, the correlation length of Raman active optical phonons is determined as a short range order in the nanocrystals. Phonon softening and asymmetric broadening are investigated for HgTe like LO and TO phonon modes in the Raman spectrum while CdTe like modes almost disappeared for the dose greater than 5 × 1013 cm−2. Disorder is measured quantitatively for wide ranges of doses on the basis of phonon confinement model. Nanostructures of the near-surface implantation-induced damage layer are known to consist of a mixture of amorphous HgCdTe and its nanocrystals. A significant reduction of the nanocrystallites size is reported here with increasing dose i.e. L = 34–46 A0 at dose of 1 × 1015 cm−2.

Confined optical-phonon-assisted cyclotron resonance in quantum wells via two-photon absorption process

The effect of confined phonons on the phonon-assisted cyclotron resonance (PACR) via both one and two photon absorption processes in a quantum well is theoretically studied. We consider cases when electrons are scattered by confined optical phonons described by the Fuchs-Kliewer slab, Ridley's guided, and Huang-Zhu models. The analytical expression of the magneto-optical absorption coefficient (MOAC) is obtained by relating it to the transition probability for the absorption of photons. It predicts resonant peaks caused by transitions between Landau levels and electric subband accompanied by confined phonons emission in the absorption spectrum. The MOAC and the full-width at half-maximum (FWHM) for the intra- and inter-subband transitions are given as functions of the magnetic field, temperature, and quantum well width. In narrow quantum wells, the phonon confinement becomes more important and should be taken into account in studying FWHM.

Tuning the Morphology of Te 1D Nanostructures By Template-Free Electrochemical Deposition in an Ionic Liquid

Tellurium and its alloys are used in many applications such as photoconductors, piezoelectric devices, thermoelectric generators and coolers [1]. For thermoelectric applications, the transport properties (thermal conductivity, electrical resistivity, Seebeck coefficient) of tellurium and its compounds can be improved by nanostructuring [2]. One-dimensional nanostructures like nanowires [3] and core-shell nanowires [4] are particularly promising due to the high phonon interface scattering and the blocking of the phonon conduction along the wire axis which lead to an increase of the thermoelectric conversion efficiency. Te nanowires can represent a base to create core-shell structures, being the core that could be further capped with a shell to get arrays of Te-based thermoelectric nanostructures like BiTe [5], AgTe [6] or LaTe [7]. Template assisted electrodeposition is a low cost technique widely used in literature for the synthesis of nanowires but implies several preparation steps and the removal of the template after the growth of the nanowires. The aim of this work is to develop a template free synthesis route that would be more convenient. For this purpose, Room-Temperature Ionic Liquids (RTILs) appear as promising solvents. Indeed, it has been highlighted that they can act as stabilizing agents for nanoparticles and nanostructured films synthesis [8]. In a previous work ([9]), our laboratory showed that Te nanowires can be obtained in an ionic liquid binary mixture: 1-ethyl-1-octyl-piperidinium bis(trifluoromethylsulfonyl)imide:1-ethyl-1-octyl-piperidinium bromide (EOPipTFSI:EOPipBr). In the present work, several morphologies were obtained by varying the synthesis parameters (applied fixed potential, Te(IV) concentration, electrolyte composition). Under charge transfer control, micrometer faceted grains are formed. Under diffusion control, the deposits are composed of nanowires, which are single crystalline as proven by SAED (Selected Area Electronic Diffraction) and TEM (Transmission Electron Microscopy) analyses. In highest tested overpotential conditions, nanowires turn progressively into crystallized hollow nanostructures (HNs) when the coulometric charge increases. This phenomenon is more accentuated at low concentration and disappears when using a soluble anode, showing that the formation of HNs is due to a limitation by species supply. The geometry of HNs, with a high surface to volume ratio, is particularly interesting to create high performances thermoelectric materials. Indeed, this geometry could further decrease the lattice part of the thermal diffusion coefficient by phonon confinement. By decreasing the bromide content of the electrolyte, hair-like single crystalline nanowires with a high aspect ratio can be obtained. HRTEM (High Resolution Transmission Electron Spectroscopy) analyses reveal a growth along the c-axis, with a smooth interface. No surface contaminants were observed by STEM (Scanning Transmission Electron Microscopy), proving that halogenide ions do not play the role of capping agents, unlike observed in other works [10].

Confinement and surface effects on the physical properties of rhombohedral-shape hematite (α-Fe2O3) nanocrystals

Morphological, microstructural and vibrational properties of hematite (α-Fe2O3) nanocrystals with a rhombohedral shape and rounded edges, obtained by forced hydrolysis of iron(III) solutions under a fast nucleation, have been investigated in detail as a function of aging time. These studies allowed us to propose a detailed formation mechanism and revealed that these nanocrystals are composed of four {104} side facets, two {110} faces at the edges of the long diagonal of the nanocrystals and two {−441} facets as the top and bottom faces. Also, the presence of nanoscopic pores and fissures was evidenced. The vibrational bands of such nanocrystals were shifted to lower frequencies in comparison with bulk hematite ones as the nanocrystal size was reduced due to phonon confinement effects. Also, the indirect and direct transition band gaps displayed interesting dependences on the aging time arising from quantum confinement and surface effects

Short range correlation length study in a single ZnO nanowire and its impact on phonon confinement

We report the influence of the short range correlation length ζL on the optical properties of a single non-centrosymmetric ZnO nanowire. Confocal Raman scattering and one-dimension phonon confinement model were employed to investigate the phonon and geometric properties of a single ZnO nanowire. The local optical results reveal a red shift in the non-polar E2 high frequency mode and a short range correlation length of ζL = 10(2) nm is obtained due to the enhancement of the zinc diffusion during the ZnO domain nucleating.

Effect of annealing temperature on the phase transition, structural stability and photo-electrochemical performance of TiO2 multi-leg nanotubes

In this report we study the effect of annealing temperature on the structural stability, phase transformation and photo-electrochemical performance of TiO2 multi-leg nanotubes. Multi-leg nanotubes were synthesized using electrochemical anodization method, and as synthesized nanotubes were annealed in air at the temperatures ranging from 500 to 900°C.We observed that multi-leg morphology is preserved upto 900°C and a dominant rutile phase is observed at this temperature. X-ray diffraction and Raman spectra measurements were carried out to study the crystallite size, phase transformation and phonon confinement effects. The multi-leg nanotubes annealed at 900°C shows enhanced photo-electrochemical performance; this is attributed to mixed phase, increase in crystallite size and decrease in defects.

Interplay between phonon confinement and Fano effect on Raman line shape for semiconductor nanostructures: Analytical study

Theoretical Raman line shape functions have been studied to take care of quantum confinement effect and Fano effect individually and jointly. The characteristics of various Raman line shapes have been studied in terms of the broadening and asymmetry of Raman line shapes. It is shown that the asymmetry in the Raman line-shape function caused by these two effects individually does not add linearly to give asymmetry of line-shape generated by considering the combined effect. This indicates existence of interplay between the two effects. The origin of interplay lies in the fact that Fano effect itself depends on quantum confinement effect and in turn provides an asymmetry. This can not be explained by considering the two effects contribution independent of each other.

Defect structure and optical phonon confinement in ultrananocrystalline BixSn1-xO2 (x = 0, 0.03, 0.05, and 0.08) synthesized by a sonochemical method.

We report the structure of defect and the oxygen vacancy-induced optical phonon confinement in phase pure tetragonal rutile crystal structured ultrananocrystalline BixSn1-xO2 (x = 0, 0.03, 0.05, 0.08) with high surface area synthesized by sonochemical method. As the Bi ion incorporates into the SnO2 host lattice, it replaces the Sn ions marked by the lattice expansion, which leads to the formation of oxygen vacancies so as to maintain charge neutrality. The grain size reduces from 6 nm to 3 nm with increase in Bi content from 0% to 8%. The size effect and the increased oxygen vacancy concentration were found to induce phonon confinement within the grain. This has led to interesting changes in the vibrational spectra of the ultrananocrystalline BixSn1-xO2 as the size reduces below 9 nm. Absence of periodicity beyond this critical particle size relaxes the zone-centre optical phonon selection rule, causing the Raman spectrum to have contributions also from phonons away from the Brillouin-zone centre. The structure of defects, such as the in-plane, bridging and sub-bridging oxygen vacancies present, was confirmed using Raman spectroscopic analysis. The reason for enhancement in photoluminescence behaviour with increased Bi content is discussed. The energy band gap was found to be wider (∼4 eV) compared to the bulk and reveals an increasing trend as a function of Bi%. The increase in band gap with decrease in particle size marks the quantum confinement effect. The variation of band gap upon doping is due to the BM shift effect, which arises as a result of the increase in carrier concentration.

Spectroscopic Investigation of well aligned Silicon Nano wires Fabricated by Metal Induced Etching

Well aligned Silicon nano-wires (SiNWs) were fabricated by metal induced etching (MIE). Scanning electron microscopy has been employed to study the surface morphology and cross sectional view for confirming the nano-wire like structures in our samples. Presence of electronic continuum and quantum confinement effect in the SiNWs- samples has been revealed from Raman and photoluminescence (PL) spectroscopy. Blue emission is achieved from SiNWs sample. Moreover, the effect of etching time on fabrication of SiNWs and associated PL spectra are studied here. We found with increasing etching time, pores are broader resulting in smaller size nano-wires. With further increasing in etching time, these NWs broke from the top, resulted in a blunt top surface leading to large size NWs. After a certain etching time, upper part of SiNWs is being broken, which is also reflected in PL spectra. Raman Spectroscopy shows the phonon confinement and is used to estimate the particle size. A detailed results and discussions have been presented here

Effect of crystallite size, Raman surface modes and surface basicity on the gas sensing behavior of terbium-doped SnO2 nanoparticles

In the present work, we report the structural, optical and gas sensing properties of Tb3+-doped SnO2 nanoparticles. XRD results confirmed tetragonal rutile structure of both undoped and Tb3+-doped SnO2 nanoparticles which was further confirmed from Raman results. The increase of dopant concentration resulted in decrease in crystallite size which has been confirmed from XRD and TEM results. Raman spectra exhibited bands positioned at 562 and 487cm−1 whose contribution has been found to increase with decrease in crystallite size. The shifting and broadening of Raman active bands has been explained by well-known phonon confinement model. EDS analysis inveterate presence of terbium in the doped SnO2 nanoparticles. It has been observed that 3% doped samples exhibited optimum sensor response towards ethanol vapor. The optimum operable temperature of doped samples has been reduced as compared to undoped samples. The enhanced sensor response of doped nanoparticles is attributed to the small crystallite size, high surface basicity and enhanced contribution of Raman surface vibration modes of nanoparticles.

Anomalous behavior of B1g mode in highly transparent anatase nano-crystalline Nb-doped Titanium Dioxide (NTO) thin films

The effect of Niobium doping and size of crystallites on highly transparent nano-crystalline Niobium doped Titanium Dioxide (NTO) thin films with stable anatase phase are reported. The Nb doping concentration is varied within the solubility limit in TiO2 lattice. Films were annealed in controlled environment for improving the crystallinity and size of crystallites. Elemental and thickness analysis were carried out using Rutherford backscattering spectrometry and cross sectional field emission scanning electron microscopy. Structural characteristics reveal a substitutional incorporation of Nb+5 in the TiO2 lattice which inhibits the anatase crystallites growth with increasing the doping percentage. The micro-Raman (MR) spectra of films with small size crystallites shows stiffening of about 4 cm−1 for the Eg(1) mode and is ascribed to phonon confinement and non-stoichiometry. In contrast, B1g mode exhibits a large anomalous softening of 20 cm−1 with asymmetrical broadening; which was not reported for the case of pure TiO2 crystallites. This anomalous behaviour is explained by contraction of the apical Ti-O bonds at the surface upon substitutional Nb5+ doping induced reduction of Ti4+ ions also known as hetero-coordination effect. The proposed hypotheses is manifested through studying the electronic structure and phonon dynamics by performing the near edge x-ray absorption fine structure (NEXAFS) and temperature dependent MR down to liquid nitrogen temperature on pure and 2.5 at.% doped NTO films, respectively.

Clear Experimental Demonstration of Hole Gas Accumulation in Ge/Si Core-Shell Nanowires.

Selective doping and band-offset in germanium (Ge)/silicon (Si) core-shell nanowire (NW) structures can realize a type of high electron mobility transistor structure in one-dimensional NWs by separating the carrier transport region from the impurity-doped region. Precise analysis, using Raman spectroscopy of the Ge optical phonon peak, can distinguish three effects: the phonon confinement effect, the stress effect due to the heterostructures, and the Fano effect. The Fano effect is the most important to demonstrate hole gas accumulation in Ge/Si core-shell NWs. Using these techniques, we obtained conclusive evidence of the hole gas accumulation in Ge/Si core-shell NWs. The control of hole gas concentration can be realized by changing the B-doping concentration in the Si shell.

Depth detection and fabrication of porous silicon without the stress

The stress in porous Si could affect the integration of porous Si devices with microelectronic integrated circuits based on silicon. In this paper, aligned macropore arrays without the stress were fabricated by an electrochemical etching procedure under a control voltage. The several etching stages of aligned macropore arrays were examined by microscopic technique, which indicates the variation in morphology from pore nuclei to aligned macropores with branching pores, and finally to aligned macropores without branching pores. Due to the difference of Raman peak positions between porous Si layer and substrate (unetched layer), furthermore, z-scan of Raman spectroscopy which is a nondestructive tool can be used to detect the etching depth. The difference should be ascribed to the quantum confinement of phonons in the porous Si.

Structural and optical properties of the a-Si:H thin films irradiated by high energetic electron beams

Hydrogenated amorphous silicon (a-Si:H) films deposited on quartz substrate by radio-frequency plasma enhanced chemical vapour deposition were irradiated by an high energetic electron beam source from an electronic gun device. The effects of the high energetic electron beam current density (0.32–0.81 mA/mm2) irradiations on the structural and optical properties of the nc-Si:H thin films have been investigated by atomic force microscopy (AFM), Fourier transform infrared spectroscopy, Raman scattering spectroscopy, X-ray diffraction (XRD) spectroscopy, ultraviolet and visible spectroscopy. The AFM analysis showed increase in the root-mean-square roughness and increase in average grain size. Raman and XRD measurements revealed increasing electron beam current favored enhancement of crystallinity and enlargement of crystallites. The optical band gap of the films is broaden, which can be explained in terms of the reduced hydrogen content and phonon confinement effect. The increasing trend of the B with the variation of electron beam current reveals a more ordered film structure disorder. From the results, it was interesting to found that the optical constants showed a good correlation with the crystallinity within the variation of electron beam current density. The variation of the optical energy gap based on structure disorder and the quantum confinement effect is also discussed.

Thermal Conductivity Suppression in Nanostructured Silicon and Germanium Nanowires

The inherent low lattice thermal conductivity (TC) of semiconductor nanowires (s-NW) due to one-dimensional phonon confinement might provide a solution for the long-lasting figure-of-merit problem for highly efficient thermoelectric (TE) applications. Standalone diameter modulation or alloying of s-NW serve as a toolkit for TC control, but realizing the full potential of nanowires requires new atomic-scale designs, growth, characterization, and understanding of the physical mechanisms behind the structure–property (TC) relationship. Before undertaking time-consuming and expensive experimental work, molecular dynamics (MD) simulations serve as an excellent probe to investigate new designs and understand how nanostructures affect thermal transport properties through their capability to capture various phenomena such as phonon boundary scattering, phonon coherence resonance, and phonon backscattering. On the other hand, because different research groups use different structural and MD parameters in their simulations, it is rather difficult to make comparisons between different nanostructures and select appropriate ones for potential TE applications. Therefore, in this work, we systematically investigated pristine, core–shell (C–S), holey (H-N), superlattice (SL), sawtooth (ST), and superlattice sawtooth (SL-ST) nanowires with identical structural parameters. Specifically, we aim to compare the relative TC reduction achieved by these nanostructures with respect to pristine nanowires in order to propose the best structural design with the lowest lattice TC, using Green–Kubo method-based equilibrium molecular dynamics simulations at 300 K. Our results show that the TC can be minimized by changing specific parameters such as the core diameter and monolayer separation for C–S, H-N, and ST structures. In the case of SL structures, the TC is found to be independent of these parameters. However, surface roughness in the form of a ST morphology provides a TC value below 2 W/m-K, approaching the amorphous limit.

Raman spectroscopy for study of interplay between phonon confinement and Fano effect in silicon nanowires

A combined effect of doping (type and species) and size on Raman scattering from silicon (Si) nanowires (NWs) has been presented here to study interplay between quantum confinement and Fano effects. The SiNWs prepared from low doping Si wafers show only confinement effect, as evident from the asymmetry in the Raman line-shape, irrespective of the doping type. On the other hand SiNWs prepared from wafer with high doping shows the presence of electron–phonon interaction in addition to the phonon confinement effect as revealed from the presence of asymmetry and antiresonence in the corresponding Raman spectra. This combined effect induces an extra asymmetry in the lower energy side of Raman peak for n-type SiNWs whereas the asymmetry flips from lower energy side to the higher energy side of the Raman peak in p-type SiNWs. Such an interplay can be represented by considering a general Fano-Raman line-shape equation to take care of the combined effect in SiNWs. Copyright © 2015 John Wiley & Sons, Ltd.

Influence of spin–orbit interactions on the polaron properties in wurtzite semiconductor quantum well

We have study the simultaneous effect of Rashba and Dresselhaus spin–orbit interactions on the polaron properties in wurtzite semiconductor quantum wells. The linear and cubic contributions of the bulk Dresselhaus spin–orbit coupling and the effects of phonon confinement on electron–optical-phonon interaction Hamiltonians are taken into account. We have found analytical solutions for the polaron energies as well as polaron effective mass within the range of validity of perturbation theory. It is shown that the polaron energy and effective mass correction are both significantly enhanced by the spin–orbit coupling. Wave number dependent phonon contribution on the electron energy has minima and varies differently of the spin-up and spin-down states. Polaron self-energy due to interface optical phonon modes has larger values than of the confined optical phonon modes ones. The polaron effective mass exhibits anisotropy and the contribution of the Dresselhaus spin–orbit coupling term on the polaron effective mass is dominated by Rashba one.