High Phonon Research Articles

Lattice defects and oxygen vacancies formulated ferromagnetic, luminescence, structural properties and band-gap tuning in Nd3+ substituted ZnO nanoparticles

ZnO and Zn1-xNdxO nanoparticles synthesized using the microwave assisted co-precipitation method were probed using XRD, HRTEM, UV–Vis spectroscopy, Raman spectroscopy, Photoluminescence spectroscopy and SQUID for robust and sufficient optical, structural, and magnetic profiles. The Hexagonal wurtzite structure of all the nanoparticles with a spherical shape and size in the 20-14 nm range was confirmed by XRD and HRTEM. The lower angle shift of the diffraction peaks indicated the interstitial addition of Nd3+ and collated to the lattice expansion. Urbach energy with its tail slope was used to define structural disorder in materials, and it increased with doping and decreased the optical band gap due to band-tail and tail-tail transitions. Broadening and asymmetry of the first-order E2 (high) phonon Raman mode identified structural disorder. The Spatial correlation model with changes in line center position, line width, and asymmetry of the E2 (high) phonon mode gave dopant created disorder. PL studies were used to determine the oxygen vacancy concentration, other defects, and excitation wavelength in all the nanoparticle samples that could be used as white light display devices. Magnetic studies with the BMP model were employed to confirm that Nd3+ doping leads to an increase in RTFM. Oxygen vacancies were considered to have a critical role in generating weak FM behavior in Zn1-xNdxO nanoparticles.

Temperature dependent transition of conduction mechanism from carrier injection to multistep tunneling in Fe3O4 (111)/Alq3/Co organic spin valve

We have presented experimental results addressing the origin of spin valve (SV) magneto-resistance (MR), in both injection and tunnel conduction regimes, in our fabricated Fe 3 O 4 (111)/Alq 3 /Co SV device. Experimental evidences have shown that any alternative MR process, such as ‘tunneling anisotropic MR’ is not at the origin of this SV MR. Spin resolved density of states of electrodes indicate that both the conduction mechanisms induce different spin dependent scattering which inturn modify the MR signal of the device. This modification helped in maintaining a non-monotonous quenching of MR signal with increase in temperature. We have also proposed a phenomenological model for device operation where the concept of charge gap modification at Fermi level across Verwey transition is envisaged to offer this unique scenario of tuning the conduction mode and hence MR in this ferrite based organic SV. The model is also supported by an established theoretical study which considers high temperature phonon assisted tunneling through defect states at electrode–organic interface of the device. • Conduction mechanism transition from carrier injection to multistep tunneling. • Transition due to Verwey transition of Fe 3 O 4 . • Spin flip scattering at Alq 3 spacer above Verwey transition temperature. • Non-monotonous MR quenching with temperature. • Room temperature MR signal of hybrid spin valve.

Silicate-clad Dy3+ doped multi-component phosphate glass core glass fiber for yellow laser applications

In this work, strong yellow emission was obtained in Dy3+ doped multi-component phosphate glass due to its high asymmetry and covalency of the local field and high phonon energy. The internal quantum efficiency was measured to be 39.27% and the figure of merit was calculated to be 210.53 pm2 × μs. What is more, silicate-clad Dy3+ doped multi-component phosphate core glass fiber was successfully drawn by using a rod-in-tube method, which has a strong fluorescence emission in yellow spectra band. The Dy3+ doping concentration is 2.6 wt.%, which is much larger than that of Dy3+ doped (0.2 wt.%) ZBLAN fiber. These results suggest that the silicate-clad Dy3+ doped multi-component phosphate glass fibers are potential host matrices for yellow laser.

Helicity-selective Raman scattering from in-plane anisotropic α-MoO3

Hyperbolic crystals, such as α-MoO3, can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of α-MoO3. Both helicity-preserving and helicity-reversing Raman scattering are observed. Our studies reveal that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and realizing compact photonic circuits based on helical polarized light.

High Thermal Conductivity of Plasticine‐Based Nanocomposites Developed Using Simple Fabrication for Heat Management in Electronic Devices

Effectual thermal transport has turned out to be an imperative parameter for emerging technologies, especially in electronics. These integrated circuit technology appliances tend to undergo a surge in temperature during operation. To prevent untimely degradation and performance lag, it is important to prevent substantial overheating of the device. Polymeric thermal interface materials (TIMs) have grave issues, such as bundling of fillers, prohibitive cost, critical fabrication processes, high phonon scattering, and bad filler–matrix interaction. Herein, various carbonaceous fillers are used in the neat plasticine matrix. The results obtained at various loadings of graphite‐, xGnP‐, and graphene‐based nanocomposites are interesting. At 23 wt%, the thermal conductivities (TCs) obtained are 2.44, 4.32, and 5.22, W mK−1, respectively. The high TC is attributed to the better interaction of plasticine matrix and carbonaceous fillers, thus preventing bad dispersion issues and aggravated phonon scattering, as revealed by a scanning electron microscope (SEM). The nanocomposites exhibit good stability in TC even after prolonged heat exposure of 360 h. From this study, it is concluded that low‐cost plasticine‐based nanocomposites exhibit high TC, better dispersion of fillers, and low TC fluctuations after prolonged heat exposures, which is essential for effective heat dissipation in advanced electronic devices.

Comparison of characteristic properties of Al, Ga, and In-doped ZnO thin films formed by sol-gel method

Herein, we examine the effect of doping with Indium (In), Gallium (Ga), and Aluminum (Al) (group III elements) on the structural, optical, and vibrational properties of ZnO thin films . The characteristic properties of the ZnO films prepared by the sol-gel dip-coating method are explored by utilizing X-ray diffraction, optical spectroscopy , and Raman scattering measurements. XRD analyzes exhibit that the crystallite size reduces upon doping by Ga and Al, while it increases with In, and all films have hexagonal wurtzite structure. Additionally, Raman measurements indicate that the dominant two peaks at around 104 and 445 cm −1 are related to E 2 low and E 2 high phonon modes of ZnO, respectively. The low-frequency mode ( E 2 low ) is affected by dopant atoms, whereas the high-frequency mode ( E 2 high ) of the wurtzite phase is not influenced by the dopant. Moreover, E dop-atom phonon mode appears at low frequencies and the intensity ratio, I( E dop.atom )/I( E 2 low ), decreases as the ionic radius of dopant atoms increases. UV–Vis spectra reveal that the film transparency, optical band gap, Urbach energy, and refraction index can be effectively tuned by dopant atoms. • Al, Ga and In-doped ZnO films are prepared by sol-gel dip coating method. • XRD and Raman spectra show that films have hexagonal wurtzite structure. • The intensity of E 2 low mode rises as the ionic radius of the dopant atom increases. • The optical band gap of In-doped ZnO reduces due to expansion of crystallite size. • Raman and XRD measurements reveal that the films are under compressive stress.

Green upconversion luminescence of Er3+ and Yb3+ codoped Gd2Mo4O15 for optical temperature sensing

Er3+ and Yb3+ codoped Gd2Mo4O15 upconversion (UC) phosphors are synthesized by high temperature solid state reaction. The crystalline phase and morphology are studied in detail by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). Under 980 nm laser excitation, the phosphors show a significant green UC luminescence contributed from 2H11/2/4S3/2 → 4I15/2 transitions of Er3+, while the popular red 4F9/2 → 4I15/2 emission is quite weak. The temporal evolutions of Er3+ and Yb3+ emissions after 980 nm pulse excitation are studied as a function of Er3+ and Yb3+ concentrations. It is revealed that the weak red UC emission is due to its low emission efficiency in Gd2Mo4O15 that has a high cutoff phonon energy of 960 cm−1. The temperature sensing behaviors based on the thermally coupled 2H11/2 and 4S3/2 levels have been explored. A high absolute sensitivity of 0.00853 K−1 at 554 K is achieved and an excellent thermal durability is observed. The relative absorptance of 4I15/2 → 2H11/2 transition to the 4I15/2 → 4S3/2 transition is proposed as a good measure for evaluating the absolute sensitivity based on the comparative study of some familiar materials. Our results imply that Gd2Mo4O15 is a promising green UC phosphor for optical thermometry.

Fröhlich condensate of phonons in optomechanical systems

We propose that the Fr\"ohlich condensate of phonons can be realized in optomechanical systems. The system consists of a one-dimensional array of membranes coupled to the cavity field via a quadratic interaction, and the cavity is pumped by an external laser. Analytical and numerical results demonstrate that the high phonon occupancy of the lowest or highest mechanical mode is achievable depending on the detuning of the driving laser, the optomechanical strength, and the temperature. The feasibility of experimental implementation is discussed. Our results shed light on energy conversion and transfer, heat control, and multimode cooling using optomechanical systems.

Ultrahigh thermal conductivity in hexagonal BC6N- An efficient material for nanoscale thermal management- A first principles study

Engineering materials with high thermal conductivity are of fundamental interest for efficiently dissipating heat in micro/nanoelectronics. Using first principles computations we report an ultra-high thermal conductivity of 2090 Wm−1K−1 (1395 Wm−1K−1) for hexagonal pure (natural) BC6N(h-BC6N). This value is among the highest thermal conductivities known after diamond and cubic boron arsenide. This ultra-high lattice thermal conductivity (k) is mainly attributed with high phonon group velocities of both acoustic and optical phonons arising from strong CC and BN bonds as well as the light atomic mass of the constituent elements such as boron (B), carbon (C) and nitrogen (N). We also report size dependent thermal conductivity of h-BC6N nanostructures by including boundary scattering. At room temperature (300 K) and at nanoscale length (L) of 100 nm, a high k value of 175 Wm−1K−1 is observed (higher than the bulk k value of silicon). Optical phonons with large group velocities are mainly responsible for this high thermal conductivity in h-BC6N nanostructures. High thermal conductivity of h-BC6N makes it a candidate material for heat dissipation in micro/nano thermal management applications.

Ballistic Heat Transport in Nanocomposite: The Role of the Shape and Interconnection of Nanoinclusions.

In this article, the effect on the vibrational and thermal properties of gradually interconnected nanoinclusions embedded in an amorphous silicon matrix is studied using molecular dynamics simulations. The nanoinclusion arrangement ranges from an aligned sphere array to an interconnected mesh of nanowires. Wave-packet simulations scanning different polarizations and frequencies reveal that the interconnection of the nanoinclusions at constant volume fraction induces a strong increase of the mean free path of high frequency phonons, but does not affect the energy diffusivity. The mean free path and energy diffusivity are then used to estimate the thermal conductivity, showing an enhancement of the effective thermal conductivity due to the existence of crystalline structural interconnections. This enhancement is dominated by the ballistic transport of phonons. Equilibrium molecular dynamics simulations confirm the tendency, although less markedly. This leads to the observation that coherent energy propagation with a moderate increase of the thermal conductivity is possible. These findings could be useful for energy harvesting applications, thermal management or for mechanical information processing.

Heat transport through propagon-phonon interaction in epitaxial amorphous-crystalline multilayers

Managing heat dissipation is a necessity for nanoscale electronic devices with high-density interfaces, but despite considerable effort, it has been difficult to establish the phonon transport physics at the interface due to a “complex” interface layer. In contrast, the amorphous/epitaxial interface is expected to have almost no “complex” interface layer due to the lack of lattice mismatch strain and less associated defects. Here, we experimentally observe the extremely-small interface thermal resistance per unit area at the interface of the amorphous-germanium sulfide/epitaxial-lead telluride superlattice (~0.8 ± 4.0 × 10‒9 m2KW−1). Ab initio lattice dynamics calculations demonstrate that high phonon transmission through this interface can be predicted, like electron transport physics, from large vibron-phonon density-of-states overlapping and phonon group velocity similarity between propagon in amorphous layer and “conventional” phonon in crystal. This indicates that controlling phonon (or vibron) density-of-states and phonon group velocity similarity can be a comprehensive guideline to manage heat conduction in nanoscale systems.

Investigation of the thermal conductivity of tetrabenzo[8]circulene (TB8C) by molecular dynamics simulation

ABSTRACT In this work we investigated the thermal conductivity of tetrabenzo[8]circulene (TB8C) through the Equilibrium Molecular Dynamics (EMD) simulations. Through this investigation we propose an estimated value for the thermal conductivity of TB8C not yet reported in the literature. The TB8C thermal conductivity was simulated for different temperatures. We compared the average thermal conductivity value, (216 ± 2) W m−1 K−1 with the simulated thermal conductivity of graphene, carbon nanotube and C80H30 molecule, reported in the literature. This comparison suggests that the size and structure containing defects of TB8C, in relation to the intrinsic graphene plane hexagonal structure model, are responsible for its good thermal conductivity. To clarify the mechanism of the thermal behaviour of TB8C, we calculated the Phonon Density of States (PHDOS) and compared with C80H30 and graphene. When compared to graphene, a higher phonon density of state is observed for the TB8C in the frequency range from 0 to 10 THz, which may be an explanation for the high thermal conductivity of the TB8C. By calculating the group velocity, we proposed the hypothesis that the lifetime of phonons, for TB8C, should be greater than that of graphene and proposed that thermal transport should occur, predominantly, in ballistic form.

Optimal carrier concentration for FeSb2 colossal thermopower

Crystals of FeSb2 correlated narrow-gap semiconductor host colossal thermopower values. By tuning the impurity level here, we show that electron–phonon scattering that transfers phonon momentum to electrons is efficient only for certain optimal carrier concentration in the low-mobility band. Phonon drag acting on such states in crystals with high phonon mean free path enhances thermopower to colossal values, whereas for different carrier concentration, dominant thermal transport mechanism is electronic diffusion. This highlights the dual nature of correlated in-gap states that take part in the phonon drag but also reduce phonon mean free path.

The investigations of Dy:YAG and Dy,Tb:YAG as potentially efficient GaN blue LD pumped solid state yellow laser crystals

With the fast development of GaN blue laser diode, Dy3+ doped laser crystals are receiving renewed interests due to their potential yellow laser emission under GaN laser diode pumping. In this study, Dy:YAG and Dy,Tb:YAG with high quality have been grown successfully with Czochralski method. Their spectroscopic properties as well as the energy transfer mechanism between Dy3+ and Tb3+ were discussed in details. The 583 nm emission cross-section of Dy:YAG crystal is improved by co-doping of Tb3+, which indicates the efficient quasi-resonant energy transfer between the Dy3+-6H13/2 state and Tb3+-7F4 state is realized. The thermal properties of Dy:YAG, including specific heat and thermal conductivity, were investigated. The high thermal conductivity suggests the Dy:YAG crystal should be very suitable for high-power laser operation. In addition, considering the high phonon energy of Dy:YAG and Dy,Tb:YAG crystals, the depletion of population on the lower laser level (6H13/2) of Dy3+ could be accelerated. The results suggesting that both Dy:YAG and Dy,Tb:YAG could be highly appraised as excellent competitors for all solid state yellow lasers.

Mechanical response, thermal conductivity and phononic properties of group III-V 2D hexagonal compounds

Two-dimensional hexagonal compounds have gained a lot of attention due to their potential application in energy, photovoltaics, mechanics and optoelectronics. In this work, we investigate the mechanical, thermal and phononic properties of group III-V binary compounds using ab-initio calculations. Pressure and strain dependency are also explored. The four compounds, BN, BP, AlN and AlP, are found to exhibit hyperelastic softening behavior and high interatomic rigidity that is highly desirable in the automotive and aeronautical industries. Besides an anisotropic elastic response that appears in the nonlinear regime and at non-zero pressure, the compounds contract when the temperature increases. The results related to thermal conductivity reveal the potential application of BN in thermal management. However, the high Grüneisen parameter and the high phonon sccatering rate render BP and AlN compounds good candidates for thermoelectric devices.

Thermally conductive, self-healing, and elastic Polyimide@Vertically aligned carbon nanotubes composite as smart thermal interface material

Smart thermal interface materials must exhibit self-healability and high thermal conductivity (k) and elastic deformation as they can experience repeated compression and undergo sudden damage. However, it remains challenging to ensure high phonon mobility/efficient phonon transfer, self-healing, and good elasticity by optimizing its molecular orientation or cross-links. Herein, a self-healing and elastic polyimide copolymer (EMPI) cross-linked by flexible and rigid segments is uniformly filled into the gaps of a forest of vertically aligned carbon nanotubes (VACNTs). The EMPI@VACNTs composite exhibits a high k value at 10.83 ± 0.22 W m−1 K−1, low interfacial thermal resistance at 6.83 ± 0.15 K mm2 W−1, high elastic compressive deformation (30% at 2.5 MPa), and strong surface adhesion (0.3 MPa). Further, it could recover 90.8% of its elastic modulus and 92% of its thermal resistance after self-healing at 80 °C for 80 h. An EMPI@VACNTs-Cu device exhibits efficient heat conduction not only by recovering after gradient compressive strains of up to 30% but also by self-healing its damage or reforming the interface with Cu. Thus, the thermally conductive, self-healing, and elastic EMPI@VACNTs composite opens new avenues for smart thermal management in various high-power/intelligent devices.

High pressure lattice dynamics and stress-strain responses of LaIn3 and LaTl3

A detailed study of the ground state structure, non-zero elastic constants, high pressure lattice dynamics as well as theoretical tensile and shear strengths of the cubic intermetallic compounds LaIn3 and LaTl3 with AuCu3 (L12)-type has been made through first-principles. The optimized equilibrium crystal constants, cell volumes, bulk modulus and its pressure derivative, and single-crystalline elastic constants are in fair agreement with available experimental data and other calculations. More importantly, high pressure phonon spectrum, total and partial phonon densities of states in LaIn3 and LaTl3 up to 10 GPa are reported by employing Density Functional Perturbation Theory (DFPT), and the present results have demonstrated that both systems are dynamically stability up to 10 GPa. The differences between the dynamical properties of LaIn3 and LaTl3 have also been discussed systematically. Additionally, the tensile and shear stress-strain responses and the theoretical strengths of LaIn3 and LaTl3 are further calculated, we have found that LaIn3 exhibits larger tensile and shear strengths than LaTl3. These investigations are expected to provide beneficial references for further exploring the physical properties of rare earth compounds.

Density Functional Theory Investigation of Thermal Conductivity in α-CN and α-CP Monolayers: Implications for Thermal Management of Electronic Devices

This work presents the density functional theory based investigations of two-dimensional carbon pnictides α-CX (X = N and P). Despite being from the group of the pnictide family, there are significant differences in the trends of the electronic and vibrational properties of both systems. The calculated electronic band gaps are 3.77 eV and 1.79 eV for α-CN and α-CP, respectively. The direct nature and moderate gap magnitude of α-CP indicate its applicability in the field of optoelectronic and photovoltaic device applications, which is confirmed by the calculated absorption spectra of the system. Distinguished phonon dispersion curves of both systems show a highly anisotropic trend. The diverse and steep nature of the acoustic phonon modes together with high phonon group velocity, low phonon anharmonicity, and high phonon mean free path (MFP) leads to a pronounced magnitude of thermal conductivity in the case of α-CN. In contrast to α-CN, α-CP owing to the lower gap between the optoacoustic phonon modes, lower MFP, and high anharmonicity possesses a remarkably lower thermal conductivity of 292 W m–1 K–1. The thermoelectric performance of the system is assessed by calculating the figure-of-merit ZT in the case of α-CP, which is 0.36 at 300 K that further attains > 0.7 at higher temperatures; whereas, for α-CN, the ZT value remains < 0.1 due to its ultra-high thermal conductivity. The present study paves the pathway toward exploration of carbon-based low-dimensional materials for high mobility and high thermal conductivity-based nanoelectronic device applications.

Prediction and observation of intermodulation sidebands from anharmonic phonons in NaBr

A quantum Langevin model, similar to models used recently for optomechanics, was used to predict intermodulation phonon sidebands (IPS). Ab initio calculations of anharmonic phonons in rocksalt NaBr showed these spectral features as "many-body effects". Modern inelastic neutron scattering measurements on a crystal of NaBr at 300K revealed diffuse intensity at high phonon energy from a predicted upper IPS. The transverse optical (TO) part of the new features originates from phonon intermodulation between the transverse acoustic (TA) and TO phonons. The longitudinal optical (LO) spectral features originate from three-phonon coupling between the TA modes and the TO lattice modes. The partner lower IPS proves to be an "intrinsic localized mode". Interactions with the thermal bath broaden and redistribute the spectral weight of the IPS pair. These sidebands are a probe of anharmonicity and quantum noise, which originate from interactions between the phonons in NaBr.

Enhancement of thermal conductivity in presence of macroscopic polarization fields: Role of dispersive transverse phonon modes in nanoscale GaN

The macroscopic polarization effects on lattice thermal conductivity of GaN is theoretically investigated. The explicit contributions from dispersive low and high frequency transverse and longitudinal phonons modes to thermal conductivity are investigated in the presence of polarization fields. Our numerical calculations of the thermal conductivity including polarization fields for the different dimension samples obtain good fit with experimental reports. The significant increase in thermal conductivity is observed at room temperature in presence of macroscopic polarization fields and in the low temperature region effect is found to be negligible. Contribution from high frequency transverse mode phonons to the total thermal conductivity is dominant in nanoscale dimension GaN samples in presence of polarization fields.