Phonon Confinement Research Articles

Stress transition from compressive to tensile for silicon nanocrystals embedded in amorphous silica matrix

Silicon-rich silicon oxide films, with various Si concentrations, were deposited by plasma enhanced chemical vapor deposition and annealed at 1100°C in order to form silicon nanocrystals. For these films, it has been found that the absorption edge shifts as a function of the nanocrystal size due to the quantum confinement of exciton. This result showed that the size-related effects are present in the investigated films. Next, we examined the influence of the nanocrystal size on the vibrational modes. In this case, the Raman line related to silicon nanocrystals significantly down-shifts as a function of the nanocrystal size, which is against the predictions of the phonon confinement model. It has been shown that this effect is due to stress exerted on the nanocrystals. It has been also found that this stress changes from compressive to tensile, and the stress character depends on the film stoichiometry.

Phonon confinement effect on the polaron states in a double quantum ring structure in the presence of electric and magnetic fields

In the framework of effective mass approximation the polaron problem in double quantum ring structures in the presence of perpendicular electric and magnetic fields with taken into account the effect of phonon confinement is considered. We derived general expressions for the normalized eigenfunctions of the confined optical and interface optical phonon modes and the dispersion relation for interface optical phonons as well as electron–confined optical phonon and electron–interface optical phonon Fröhlich interaction Hamiltonians in the double quantum ring system. Results reveal that the electron–optical phonon coupling leads to the decrease of the electron ground state energy. We have shown that the polaron energy exhibits a pronounced local maximum (minimuum) in dependence on an electric (magnetic) field which corresponds to the sharp decrease (increase) of the polaronic contributions due to confined optical and interface optical phonon modes.

Anomalous Temperature-Dependent Upconversion Luminescence of Small-Sized NaYF4:Yb3+, Er3+ Nanoparticles

Size-dependent quantum confinement has important effects on the energy transfer and radiative and nonradiative transitions in nanophosphors. For lanthanide-doped nanoparticles, the confinement effect is induced mostly via electron–phonon interaction, and analysis of temperature-dependent spectroscopic properties provides an effective method for disclosing its underlying mechanism. Herein, an intriguing and unprecedented enhancement of the upconversion luminescence (UCL) at higher temperatures in hexagonal-phase NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs) is reported. Moreover, this anomalous UCL enhancement shows a strong dependence on the particle size and becomes more significant for UCNPs with a smaller size. This anomalous thermal behavior is interpreted on the basis of phonon-assisted energy transfer and phonon confinement effect. The findings are relevant to the engineering of the nanostructures of UCNPs and to the further understanding of the UCL mechanism.

Self-assembled nc-Si/a-SiNx: H quantum dots thin films: An alternative solid-state light emitting material

Nanocrystalline silicon quantum dots (QDs) of varying size from ~5.4 to 2.2nm embedded in amorphous silicon-nitride matrix (nc-Si-QDs/a-SiNx:H) were prepared via ICP-CVD (13.56MHz), using a mixture of silane (SiH4), ammonia (NH3) gases in H2-diluted plasma, by changing the low deposition pressure in a narrow range, 10–50mTorr at low substrate temperature (250°C). Strong visible photoluminescence (PL) tunable over a significantly wide range (1.67–3.02eV) has been observed. The origin of the PL is mostly attributed to band-to-band recombination due to the quantum confinement effect (QCE) in the nanocrystalline silicon QDs. A large amount of atomic hydrogen flux that originates due to the high degree dissociation of the gas molecules in high-density inductively coupled plasma (ICP) passivates well the nonradiative dangling bonds and helps in growing plenty of ultra-nanocrystallites that demonstrate intense visible photoluminescence. The red-shift of Raman peak and the corresponding line broadening have been associated to the confinement of optical phonons within the nc-Si QDs. The widening of band gap and tune-ability of visible photoluminescence over a notably wide range along with significantly high electrical conductivity of the material demonstrates enormous promise for its utilization in the fabrication of effective solid-state light emitting devices.

Computational study of in-plane phonon transport in Si thin films.

We have systematically investigated the in-plane thermal transport in Si thin films using an approach based on the first-principles calculations and lattice dynamics. The effects of phonon mode depletion induced by the phonon confinement and the corresponding variation in interphonon scattering, which may be important for the thermal conductivities of ultra-thin films but are often neglected in precedent studies, are considered in this study. The in-plane thermal conductivities of Si thin films with different thicknesses have been predicted over a temperature range from 80 K to 800 K and excellent agreements with experimental results are found. The validities of adopting the bulk phonon properties and gray approximation of surface specularity in thin film studies have been clarified. It is found that in ultra-thin films, while the phonon depletion will reduce the thermal conductivity of Si thin films, its effect is largely offset by the reduction in the interphonon scattering rate. The contributions of different phonon modes to the thermal transport and isotope effects in Si films with different thicknesses under various temperatures are also analyzed.

Substrate effect on phonon confinement in TiO2 nanoparticles for thermal sensing application

The great potential of anatase TiO2 nanoparticles (NPs) for high temperature applications is limited by the agglomeration, grain growth, and phase transformation into rutile. In this regard, we have decorated TiO2 NPs over the surface of Santa Barbara Amorphous-15 (SBA15) and studied the substrate effect on the thermal stability of anatase TiO2 NPs in terms of the grain growth and the phase transformation. Here, we reported the ultrafast grain growth of anatase TiO2 NPs in TiO2-SBA15 nanocomposite in response to short time thermal exposure and the resulting phonon confinement effect for high temperature thermal sensing application.

Phonon engineering in graphene and van der Waals materials

Abstract

TiO2-decorated graphite nanoplatelet nanocomposites for high-temperature sensor applications.

Temperature and/or composition mapping inside high temperature energy conversion and storage devices are challenging, yet of critical importance to improve the material design for optimum performance. Here, the great potential of TiO2 nanoparticle (NP)-decorated graphite nanoplatelet (GNP) nanocomposites as high temperature thermal senors or gas sensors is reported. Effects of the GNP substrate on phonon confinement in Raman spectrum, grain growth, and phase stability of anatase TiO2 NPs at high temperatures are systematically studied. Thermally sensitive Raman signatures, indicating the ultrafast grain growth of TiO2 NPs in response to short thermal shock treatments (0.1-25 s) at high temperatures, are exploited for high temperature thermal sensing applications. A very high accuracy of nearly 98% in temperature measurements is demonstrated for a given short-time thermal exposure. Thermal stability of anatase TiO2 NPs against transformation into the rutile phase in TiO2 -GNP nancomposites is substantially increased by controlling the surface area of the substrate, which would significantly improve the performance of TiO2 -based high temperature gas sensors.

Raman and Infrared Phonon Spectra of Ultrasmall Colloidal CdS Nanoparticles

Raman and infrared phonon spectra of ultrasmall (1.8 nm) colloidal CdS nanoparticles (usNPs) are presented. Multiphonon scattering by optical phonons up to the third order is observed in the Raman spectra at low temperature and resonant (325 nm) excitation. The first-order optical phonon peak is a superposition of several components, two of which can be assigned to surface optical (SO) and longitudinal optical (LO) modes, respectively. The LO mode, being markedly broadened compared to that of spectra of regular (>2 nm) NPs, is related to phonon confinement and bond distortion induced by a significant structural relaxation in usNPs. A shoulder observed above the LO frequency is either due to the density of phonon states induced by distorted surface bonds or due to higher-order scattering processes involving optical and acoustic phonons. The Raman peaks of usNPs do not reveal the upward shift and narrowing upon decreasing temperature from 300 K down to 85 K typical for crystalline semiconductors, even thoug...

Ge-Si based Nanowire Thermoelectric Materials in the Phonon Confinement Regime

For over a decade, nanotechnology has allowed rational synthesis of nanodot, nanowire and nanotube materials with different structure, composition, and with size control at the atomic precision. These nanoscale building blocks make it possible to explore electronic and energy harvesting devices with completely new operating principles. Similar to photonic and electronic confinement regimes where the device size approaches the wavelength of photon and electron wavefunctions, nanoscale semiconductor materials have enabled a new era of phononic engineering to turn waste heat into power and to explore the greener side of silicon. In particular, we have devised a highly sensitive differential bridge thermometry method to characterize thermal transport properties of individual Ge and heterostructure core-shell Ge/Si nanowires with diameters < 20 nm[1]. The resulting drastically suppressed thermal conductance beyond the diffusive boundary scattering limit is the first experimental evidence for the role of confinement effect on acoustic phonon dispersions in nanowires[2]. Lately we have also explored the high carrier mobility and concentration modulation in the semiconductor Ge/Si nanowires using gate modulation, and presented the first field-effect modulated thermoelectric power factor measurement on nanowires with diameters down to 11 nm[3]. The power factor was found to strongly correlate with carrier mobility and can exceed that of bulk Ge in nanowires with high hole mobility. Taken together, the well-established tunable electronic property, excellent thermoelectric power in Ge/Si nanowire materials, combined with thermal transport in the phonic confinement regime, are expected to open up a new level of thermoelectric performance beyond classical avenues.1. Wingert, M. C.; Chen, Z. C. Y.; Dechaumphai, E.; Moon, J.; Kim, J. H.; Xiang, J.; Chen, R. K. Nano Lett. 2011, 11 (12), 5507−5513.2. Zou, J.; Balandin, A. J. Appl. Phys. 2001, 89 (5), 2932−2938.3. J. Moon, J-H. Kim, Z.C.Y. Chen, J. Xiang, and R. Chen, Nano Lett. 2013, 13 (3), 1196–1202.

Paintable Thermoelectric Materials from Polymer Blends of Exfoliated 2D Nanosheets

In bulk form the thermoelectric material bismuth telluride (Bi2Te3) has a high figure of merit (ZT) at room temperature. When bulk Bi2Te3 is exfoliated to smaller dimensions, such as its 2D form (nanosheets), its thermoelectric effects are improved by reduction in thermal conductivity due to phonon confinement and scattering, thus increasing the ZT. The difficulty involved in incorporating nanosheets into energy harvesting systems motivated the search for a form of Bi2Te3 that retains the high ZT apparent at the nanoscale with the capability of being deposited or painted onto device infrastructures.This work highlights a method whereby solvent exfoliation of Bi2Te3 into solution-dispersible 2D nanosheets can form a practical thin film that can be distributed across a surface. Optical transmission measurements quantify the relationship between efficient and stable exfoliation and the reduction in optical density. Optimized exfoliated suspension are also shown to form smooth, uniform blends when mixed with poly ethylene glycol and other polymers to produce a paintable Bi2Te3film that can be applied to surfaces using an innovative painting technique.Atomic force microscopy, transmission electron spectroscopy, Raman spectroscopy and scanning electron spectroscopy are used to examine the structure of the 2D nanosheets and the highly reproducible Bi2Te3 thin films. Electrical transport studies show that the films have conductive pathways over a range of surfaces and various structural formations, linking the conductivity to the percolating conduction through the nanosheet ensemble. The combination of the facile preparation method and the scope for diverse surface coating as a cohesive and conductive thin film offers a methods for integration with heat producing devices for energy harvesting applications.ReferencesK. M. F. Shahil, M. Z. Hossain, D. Teweldebrhan and A. A. Balandin, Applied Physics Letters, 96 (2010).B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang, A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen and Z. Ren, Science, 320, 634 (2008).M. Saleemi, M. S. Toprak, S. Li, M. Johnsson and M. Muhammed, Journal of Materials Chemistry, 22, 725 (2012).L. E. Bell, Science, 321, 1457 (2008).S. Li, M. S. Toprak, H. M. A. Soliman, J. Zhou, M. Muhammed, D. Platzek and E. Müller, Chemistry of Materials, 18, 3627 (2006).Y. Hernandez, N. Valeria, L. Mustafa, B. F. M., S. Zhenyu, S. De, M. T., B. Holland, M. Byrne, Y. K. Gun'Ko, J. J. Boland, P. Niraj, G. Duesberg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Ferrari and J. N. Coleman, Nature Nanotechnology, 3, 1748 (2008).V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano and J. N. Coleman, Science, 340 (2013).L. D. Hicks and M. S. Dresselhaus, Physical Review B, 47, 12727 (1993).

Unifying the low-temperature photoluminescence spectra of carbon nanotubes: the role of acoustic phonon confinement.

At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultranarrow lines in suspended nanotubes to broad and asymmetrical line shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low-energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7meV is restored. Furthermore, multipeak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

Correlation between surface phonon mode and luminescence in nanocrystalline CdS thin films: An effect of ion beam irradiation

The influence of swift heavy ion irradiation (SHII) on surface phonon mode (SPM) and green emission in nanocrystalline CdS thin films grown by chemical bath deposition is studied. The SHII of nanocrystalline CdS thin films is carried out using 70 MeV Ni ions. The micro Raman analysis shows that asymmetry and broadening in fundamental longitudinal optical (LO) phonon mode increases systematically with increasing ion fluence. To analyze the role of phonon confinement, spatial correlation model (SCM) is fitted to the experimental data. The observed deviation of SCM to the experimental data is further investigated by fitting the micro Raman spectra using two Lorentzian line shapes. It is found that two Lorentzian functions (LFs) provide better fitting than SCM fitting and facilitate to identify the contribution of SPM in the observed distortion of LO mode. The behavior of SPM as a function of ion fluence is studied to correlate the observed asymmetry (Γa/Γb) and full width at half maximum of LO phonon mode and to understand the SHII induced enhancement of SPM. The ion beam induced interstitial and surface state defects in thin films, as observed by photoluminescence (PL) spectroscopy studies, may be the underlying reason for enhancement in SPM. PL studies also show enhancement in green luminescence with increase in ion fluence. PL analysis reveals that the variation in population density of surface state defects after SHII is similar to that of SPM. The correlation between SPM and luminescence and their dependence on ion irradiation fluence is explained with the help of thermal spike model.

Influence of Sm doping on the microstructural properties of CdS nanocrystals

Incorporation of rare earth ions of samarium into CdS nanocrystals has been successfully prepared by a simple co-precipitation method. X-ray diffraction (XRD) analysis of Sm3+:CdS nanocrystals exhibits the diffraction peaks corresponding to the cubic structure. A powder composition by energy dispersive analysis of X-rays (EDX) and by the X-ray photoelectron spectroscopy (XPS), the binding energy of Sm 3d5/2 (1084.37eV), reveals the presence of samarium into the CdS. Micro Raman spectroscopy showed peaks at 294 and 590cm−1 matches with the first and second order scatterings of the longitudinal optical phonon modes. The 1LO and 2LO mode, respectively from 299cm−1 and 599cm−1 shifted towards a lower frequency due to the optical phonon confinement and the structural disorder induced by the RE ions incorporation. A larger specific surface area of ~140m2g−1 was measured for Sm doped CdS nanocrystals using N2 adsorption–desorption isotherm.

Significant reduction of lattice thermal conductivity due to phonon confinement in graphene nanoribbons

Lattice thermal conductivity, ${\ensuremath{\kappa}}_{p}$, of suspended and supported graphene nanoribbons (GNRs) is studied over a wide temperature range, taking into account the dispersive nature of confined acoustic phonon modes. Employing a modified Callaway model, an expression for ${\ensuremath{\kappa}}_{p}$ is developed, considering the explicit contributions from in-plane longitudinal, transverse, and torsional acoustic, and out-of-plane flexural acoustic phonon modes. Numerical calculations of ${\ensuremath{\kappa}}_{p}$($T$) are presented assuming the confined acoustic phonons to be scattered by sample boundaries, impurities, and other phonons via both normal and umklapp processes. The effect of phonon confinement is to modify the phonon group velocities and the temperature dependence of ${\ensuremath{\kappa}}_{p}$. In a suspended 5-nm-wide GNR at room temperature, a decrease in ${\ensuremath{\kappa}}_{p}$ by \ensuremath{\sim}70% is predicted. Our study brings out the relative importance of the contributing phonon modes and reveals the influence of flexural phonons on ${\ensuremath{\kappa}}_{p}$ as a marked shoulder at low temperatures. The role of the various sample-dependent scattering mechanisms is examined. The substrate, in supported GNRs, is shown to curtail the phonon mean free path and suppress the low-temperature ${\ensuremath{\kappa}}_{p}$. Our results are in good agreement with recent experimental data of Bae et al. [M. H. Bae, Z. Li, Z. Aksamija, P. N. Martin, F. Xiong, Z. Y. Ong, I. Knezevic, and E. Pop, Nat. Commun. 4, 1734 (2013)] for supported GNRs.

Probing high‐energy ion‐implanted silicon by micro‐Raman spectroscopy

The effect of ion implantation (4 MeV12C2+, 5 MeV16O2+, and 8 MeV28Si2+) on [110] silicon wafers in channeling and random orientation is investigated by micro‐Raman spectroscopy. The profiles were measured using Scanning Electron Microscope (SEM) showing that the ions were penetrating deeper inside the wafer in the channeling case creating a 1–2 µm wide strongly modified region and agreeing with the d‐nuclear reaction analysis measurements. Micro‐Raman spectroscopy was employed for the assessment of the lattice damage, probing the side surface of the cleaved wafers at submicron step. The phonon modifications show strong lattice distortions in zones parallel to the front surface of the wafers and at depths, which agree with the results of the characterization techniques. In these strongly damaged zones, there is a substantial reduction in the phonon intensity, a small shift in wavenumber position, and a large increase in the phonon width. On the basis of a modification of the phonon confinement model that takes under consideration the laser beam profile, the reduction in intensity of scattered light, and the nanocrystallite size distribution from the simulation of the lattice displacements, the main characteristics of the Raman spectra could be reproduced for the random C and O implantations. The results indicate that at a critical doping level, the induced defects and lattice distortions relax by breaking the silicon single crystal into nanocrystallites, thus creating the observed zones of strongly distorted lattice. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Determining the crystalline degree of silicon nanoclusters/SiO2 multilayers by Raman scattering

We use Raman scattering to investigate the size distribution, built-in strains and the crystalline degree of Si-nanoclusters (Si-nc) in high-quality Si-rich oxynitride/SiO2 multilayered samples obtained by plasma enhanced chemical vapor deposition and subsequent annealing at 1150 °C. An initial structural characterization of the samples was performed by means of energy-filtered transmission electron microscopy (EFTEM) and X-ray diffraction (XRD) to obtain information about the cluster size and the presence of significant amounts of crystalline phase. The contributions to the Raman spectra from crystalline and amorphous Si were analyzed by using a phonon confinement model that includes the Si-nc size distribution, the influence of the matrix compressive stress on the clusters, and the presence of amorphous Si domains. Our lineshape analysis confirms the existence of silicon precipitates in crystalline state, in good agreement with XRD results, and provides also information about the presence of a large compressive stress over the Si-nc induced by the SiO2 matrix. By using the Raman spectra from low temperature annealed samples (i.e., before the crystallization of the Si-nc), the relative scattering cross-section between crystalline and amorphous Si was evaluated as a function of the crystalline Si size. Taking into account this parameter and the integrated intensities for each phase as extracted from the Raman spectra, we were able to evaluate the degree of crystallization of the precipitated Si-nc. Our data suggest that all samples exhibit high crystalline fractions, with values up to 89% for the biggest Si-nc. The Raman study, supported by the EFTEM characterization, indicates that this system undergoes a practically abrupt phase separation, in which the precipitated Si-nanoclusters are formed by a crystalline inner part surrounded by a thin amorphous shell of approximately 1–2 atomic layers.

Modeling of optomechanical coupling in a phoxonic crystal cavity in diamond.

A photonic and phononic crystal (phoxonic crystal PxC) is a periodically patterned material that can at the same time localize optical and mechanical modes. Here we theoretically model one-dimensional PxC in diamond and find high quality mechanical resonances with very high frequencies > 10 GHz and optical properties comparable to those of PxC in other materials. The simultaneous confinement of photons and phonons leads to an optomechanical interaction that we calculate in a perturbation approach. The optomechanical coupling strengths reach values in the MHz range. We identify design rules to simultaneously achieve high optical and mechanical quality factors along with strong optomechanical coupling.

Theoretical approach to the phonon modes and specific heat of germanium nanowires

The phonon modes and specific heat of Ge nanowires were computed using a first principles density functional theory scheme with a generalized gradient approximation and finite-displacement supercell algorithms. The nanowires were modeled in three different directions: [001], [111], and [110], using the supercell technique. All surface dangling bonds were saturated with Hydrogen atoms. The results show that the specific heat of the GeNWs at room temperature increases as the nanowire diameter decreases, regardless the orientation due to the phonon confinement and surface passivation. Also the phonon confinement effects could be observed since the highest optical phonon modes in the Ge vibration interval shifted to a lower frequency compared to their bulk counterparts.

Band gap narrowing in BaTiO3 nanoparticles facilitated by multiple mechanisms

In the present work, BaTiO3 nanoparticles of four different size ranges were prepared by sol-gel method. The optical band gap of these particles at some size ranges has come down to 2.53 eV from 3.2 eV, resulting in substantial increase in optical absorption by these ferroelectric nanoparticles making them potential candidates for light energy harvesting. XRD results show the presence of higher compressive strain in 23 nm and 54 nm size particles, they exhibit a higher band gap narrowing, whereas tensile strain is observed in 31 nm and 34 nm particles, and they do not show the marginal band gap narrowing. The 23 nm and 54 nm particles also show a coupling of free carriers to phonons by increasing the intensity of LO phonon mode at 715 cm−1. The higher surface charge density is expected in case of enhanced surface optical Raman modes (638 cm−1) contained in 31 and 34 nm size particles. In addition to this, the red shift in an LO mode Raman spectral line at 305 cm−1 with decrease in particle size depicts the presence of phonon confinement in it. The enhanced optical absorption in 23 nm and 54 nm size particles with a narrowed band gap of 3 eV and 2.53 eV is due to exchange correlation interactions between the carriers present in these particles. In 31 nm and 34 nm range particles, the absorption got bleached exhibiting increased band gaps of 3.08 eV and 3.2 eV, respectively. It is due to filling up of conduction band resulting from weakening of exchange correlation interactions between the charge carriers. Hence, it is concluded that the band gap narrowing in the nanoparticles of average size 23 nm/54 nm is a consequence of multiple effects like strain, electron-phonon interaction, and exchange correlation interactions between the carriers which is subdued in some other size ranges like 31 nm/34 nm.