Surface Phonon Modes Research Articles

CO2 and NO2 formation on amorphous solid water

Context. The dynamics of molecule formation, relaxation, diffusion, and desorption on amorphous solid water (ASW) is studied in a quantitative fashion. Aims. The formation probability, stabilization, energy relaxation, and diffusion dynamics of CO2 and NO2 on cold ASW following atom+diatom recombination reactions are characterized quantitatively. Methods. Accurate machine-learned energy functions combined with fluctuating charge models were used to investigate the diffusion, interactions, and recombination dynamics of atomic oxygen with CO and NO on ASW. Energy relaxation to the ASW and into water internal degrees of freedom were determined from the analysis of the vibrational density of states. The surface diffusion and desorption energetics were investigated with extended and nonequilibrium MD simulations. Results. The reaction probability is determined quantitatively and it is demonstrated that surface diffusion of the reactants on the nanosecond time scale leads to recombination for initial separations of up to 20 Å. After recombination, both CO2 and NO2 stabilize by energy transfer to water internal and surface phonon modes on the picosecond timescale. The average diffusion barriers and desorption energies agree with those reported from experiments, which validates the energy functions. After recombination, the triatomic products diffuse easily, which contrasts with the equilibrium situation, in which both CO2 and NO2 are stationary on the multinanosecond timescale.

Theoretical computation of the phononic vibrational dynamics of Au3Pd, AuPd, and AuPd3 nanostructures at the [AunPdm]/Au(110) bimetallic ordered surface alloy systems

In this work we develop a calculation for the surface phononic dynamical properties of the three ordered bimetallic surface alloy nanostructures [AunPdm]/Au(110) where [AunPdm] = Au3Pd, AuPd, and AuPd3, for the appropriate concentrations of palladium atoms in the proportions 25 %, 50 %, and 75 %, respectively, at the surface boundaries. They are ordered equilibrium structures, which have been characterized, by Atanasov and Hou (2009). Using the phase field matching theory (PFMT), and the Green's function formalism, the surface phonon dispersion curves are computed, in the harmonic approximation, along the high-symmetry directions ΓX, XS, SY and YΓ of the Brillouin zone of the corresponding surface alloy nanostructure. The local vibration densities of states (LDOS) are also calculated for these systems. The results reveal that the number and the dynamic behavior of the surface phonon and resonance modes, are very sensitive to the concentration of the palladium adsorbate atoms in the [AunPdm] nanostructures. The number and characteristics of these modes are modified at the surface and shifted to higher energies as the concentration increases. These phononic results can greatly contribute to our understanding of the dynamic effects underlying potential technological applications of these surface alloy nanostructures.

Terminal Deuterium Atoms Protect Silicon from Oxidation.

In recent years, the hybrid silicon-molecular electronics technology has been gaining significant attention for applications in sensors, photovoltaics, power generation, and molecular electronics devices. However, Si-H surfaces, which are the platforms on which these devices are formed, are prone to oxidation, compromising the mechanical and electronic stability of the devices. Here, we show that when hydrogen is replaced by deuterium, the Si-D surface becomes significantly more resistant to oxidation when either positive or negative voltages are applied to the Si surface. Si-D surfaces are more resistant to oxidation, and their current-voltage characteristics are more stable than those measured on Si-H surfaces. At positive voltages, the Si-D stability appears to be related to the flat band potential of Si-D being more positive compared to Si-H surfaces, making Si-D surfaces less attractive to oxidizing OH- ions. The limited oxidation of Si-D surfaces at negative potentials is interpreted by the frequencies of the Si-D bending modes being coupled to that of the bulk Si surface phonon modes, which would make the duration of the Si-D excited vibrational state significantly less than that of Si-H. The strong surface isotope effect has implications in the design of silicon-based sensing, molecular electronics, and power-generation devices and the interpretation of charge transfer across them.

From Elastic Spin to Phonon Spin: Symmetry and Fundamental Relations

This work is mainly based on postgraduate lectures at Tongji University since 2020 spring. We firstly revisit the elastic spin and orbital angular momentum [Proc. Natl. Acad. Sci. USA 115, 9951 (2018)] but more general for anisotropic systems by applying Noether’s theorem to the elastic Lagrangian and by applying the symmetry argument in the field theory. Then, fundamental relations between elastic energy flux and elastic spin are uncovered. In particular cases, the wave spin is closely related to the vorticity of energy flux and momentum. Secondly, we move forward from the elastic spin to revisit the phonon spin [Fizika Tverdogo Tela 3, 2160 (1961)] by applying the second quantization to elastic fields. We show that the uncovered phonon spin, a polarized elastic-vibration quanta, is generally not restricted to transverse phonon modes, but applying to general phonon modes, such as the longitudinal phonon modes, surface phonon modes, and hybridized phonon modes, regarded as a consequence of mode interferences. The elastic spin and phonon spin originate from the local rotating of the field polarization in time domain, not the local circulation (vorticity) of displacement or velocity in space domain. It is hopeful that the present results could advance the fundamental understanding of phonon spin and elastic spin, and promote the spin phononics for hybrid quantum sensing and technology with multiple degrees of freedom.

Visible to near-infrared broadband photodetector employing thin film topological insulator heterojunction (p-TlBiSe2/n-Si) diode

The work presents a study of quasi-2D thin topological insulator film of TlBiSe2 and its heterojunction TlBiSe2/Glass and TlBiSe2/Si by thermal coating. The vibrational modes were identified in both the heterojunctions using Raman spectroscopy with the identification of the surface phonon mode (SPM) in the TlBiSe2/Si heterojunction. Ultrafast dynamics were studied for the relaxation dynamics of the charge carriers in these heterojunctions. The study identified the interface phenomenon in the form of charge states (CT) in TlBiSe2/Si p-n heterojunction. The p-TlBiSe2/n-Si heterojunction showed good p-n diode characteristics with a rectification ratio under the dark. Its optical response was studied by varying optical power (2.37–3.78 µW) and wavelength (500–1900 nm). It showed excellent photoresponse in the visible to near-infrared (NIR) range which peaks at 900 nm wavelength in both forward as well reverse bias. The explanation of its optical and electrical performance was modeled in terms of band structure, surface states, and interface phenomenon. The study is important in terms of the implementation of next-generation topological insulator-based photodetectors.

Near-field radiative heat transfer between irregularly shaped dielectric particles modeled with the discrete system Green's function method

Near-field radiative heat transfer (NFRHT) between irregularly shaped dielectric particles made of SiO2 and morphology characterized by Gaussian random spheres is studied. Particles are modeled using the discrete system Green's function (DSGF) approach, which is a volume integral numerical method based on fluctuational electrodynamics. This method is applicable to finite, three-dimensional objects, and all system interactions are defined independent of thermal excitation by a generalized system Green's function. The DSGF method is deemed suitable to model NFRHT between irregularly shaped particles after verification against the analytical solution for chains of two and three SiO2 spheres. The NFRHT results reveal that geometric irregularity in particles leads to a reduction of the total conductance from that of comparable perfect spheres at vacuum separation distances smaller than the particle size, a regime in which NFRHT is a surface phenomenon. At vacuum separation distances larger than the particle size, NFRHT becomes a volumetric process, and the total conductance between irregularly shaped particles converges to that of comparable perfect spheres. Spectral analysis reveals, however, that particle irregularity leads to damping and broadening of resonances at all separation distances, thereby highlighting the importance of the DSGF method for spectral engineering in the near field. The reduced spectral coherence when particle size is larger than the vacuum separation distance is attributed to coupling of surface phonon-polaritons within the randomly generated, distorted particle features. For particle size smaller than the vacuum separation distance, resonance broadening and damping is linked with the multiple localized surface phonon modes supported by the composite spherical harmonic morphologies of the Gaussian random spheres.

Degenerate line modes in the surface and bulk phonon spectra of orthorhombic NaMgF3 perovskite

Degenerate bulk-line phonon modes have been widely reported in various crystal system types; however, degenerate surface-line phonon modes have only been reported in monoclinic crystal systems, such as SnIP with space group P2/c (No. 13). Herein, we propose that degenerate surface-line phonon modes can also emerge in solids with orthorhombic structures. Based on first-principle calculations and symmetry analysis, we propose that orthorhombic NaMgF3 fluoroperovskite with space group Pnma (No. 62) is a material candidate with degenerate line states in both the bulk phonon mode and the (010) surface phonon mode. We discovered four closed nodal loops (two type-I and two hybrid-type) on the ky = 0 plane in the bulk phonon mode, all of which coexisted with Dirac points on the Z–U and X–U paths. Moreover, we discovered symmetry-projected doubly degenerate nodal lines along the X¯–U¯ surface path in the (010) surface phonon mode. The proposed degenerate surface-line phonons in NaMgF3 is quite clean and protected by symmetries, which will aid future experimental detection.

Raman enhancement (SERS) of the surface phonon modes of TiO2

We report the first observation of the enhancement of Raman forbidden surface phonon modes (SO) of A2u symmetry in TiO2 nanoparticles on the adsorption of 2,2′-bipyridine and N-719 dye molecules. We identify the observed surface modes in conjunction with HREELS studies and attribute their enhancements to (Herzberg-Teller) vibronic coupling between the charge-transfer and the molecular transitions. We conclude that the vibronic coupling is responsible for the enhancements of the surface modes through intensity borrowing. Furthermore, using SERS selection rules, we examine possible orientations of the molecules relative to the surface confirming the Raman forbidden A2u symmetry of the surface modes.

Structure and Reactivity of α-Al2O3(0001) Surfaces: How Do Al–I and Gibbsite-like Terminations Interconvert?

The α-Al2O3(0001) surface has been extensively studied because of its significance in both fundamental research and application. Prior work suggests that in ultra-high-vacuum (UHV), in the absence of water, the so-called Al–I termination is thermodynamically favored, while in ambient, in contact with liquid water, a Gibbsite-like layer is created. While the view of the α-Al2O3(0001)/H2O(l) interface appears relatively clear in theory, experimental characterization of this system has resulted in estimates of surface acidity, i.e., isoelectric points, that differ by 4 pH units and surface structure that in some reports has non-hydrogen-bonded surface aluminol (Al–OH) groups and in others does not. In this study, we employed vibrational sum frequency spectroscopy (VSFS) and density functional theory (DFT) simulation to study the surface phonon modes of the differently terminated α-Al2O3(0001) surfaces in both UHV and ambient. We find that, on either water dosing of the Al–I in UHV or heat-induced dehydroxylation of the Gibbsite-like in ambient, the surfaces do not interconvert. This observation offers a new explanation for disagreements in prior work on the α-Al2O3(0001)/liquid water interface─different preparation methods may create surfaces that do not interconvert─and shows that the surface phonon spectral response offers a novel probe of interfacial hydrogen bonding structure.

Symmetry-enforced nodal chain phonons

Topological phonons in crystalline materials have been attracting great interest. Most cases studied so far are direct generalizations of the topological states from electronic systems. Here, we reveal a class of topological phonons - the symmetry-enforced nodal-chain phonons, which manifest the characteristic of phononic systems. We show that in five space groups with D2d little co-group at a non-time-reversal-invariant-momentum point, the phononic nodal chain is guaranteed to exist owing to the vector basis symmetry of phonons, which is a character distinct from electronic and other systems. In other words, this symmetry enforcement feature of the proposed nodal chain is limited to phononic systems. Interestingly, the chains in these five space groups exhibit two different patterns: for tetragonal systems, they are one-dimensional along the fourfold axis; for cubic systems, they form a three-dimensional network structure. Based on first-principles calculations, we identify K2O as a realistic material hosting the proposed nodal-chain phonons. We show that the effect of LO-TO splitting helps to expose the nodal-chain phonons in a large frequency window. In addition, the nodal chains may lead to drumhead surface phonon modes on multiple surfaces of a sample.

Charge transfer induced symmetry breaking in GaN/Bi2Se3 topological heterostructure device

In topological insulators (TI) for surface electron transport, dissipationless surface states are required and are activated by symmetry breaking usually by reducing thickness of the film. Substrates play an important role in modulating the surface properties by modifying the surface electronic and mechanical properties. In the present work, we have studied the n-GaN/p-Bi2Se3 topological heterojunction for the topological surface states and analyzed by Raman and ultrafast transient absorption (TA) spectroscopy probed in visible and NIR regions. Raman spectrum clearly shows the electron-phonon interaction at the surface by appearance of surface phonon modes (SPM) in heterojunction. TA spectroscopy is performed on Glass/Bi2Se3 and n-GaN/Bi2Se3 heterojunction to identify surface states, energy levels, charge transfer and carrier relaxation processes. Electrical measurements under dark and illuminated conditions were performed for deeper understanding of the interface states and their effect on electrical and optical performance. The study provides complete understanding of n-GaN/TI-based interfaces by spectroscopic and electrical measurements for their application in next-generation electronic and optical devices.

First - principle studies of surface phonon modes in GeSe Multi‐Layers

The surface phonon dispersion spectra of GeSe layered crystals were investigated by first‐principles calculations within the density functional perturbation theory (DFPT). We model the GeSe surface as an arrangement of periodically placed slabs with a varying number of layers. To minimize the artificial interaction between the periodic images of the slabs, large enough vacuum space is taken in the perpendicular direction, and the slabs extend to infinity in the other directions. Detailed first‐principles analysis of modes revealed surface phonons outside the bulk frequency zones in addition to vibrations in the bulk frequency range. To obtain the phonon states, the derivative of the forces with respect to the atomic coordinates were calculated at equilibrium positions in DFPT. In the phonon spectra, vibrational modes localized at the surface appear above the bulk phonon bands and inside the bulk phonon gap. The lowest frequency modes are reminiscent of Rayleigh surface waves. The thickness dependence of the phonon spectra was explored by comparing the results of the slabs with 1–8 layers. A few suggestions are provided to verify our findings in future experiments.

ZnSe nanoparticles growth by aqueous colloidal solution at room temperature

ZnSe nanoparticles (ZnSe-NPs) were obtained by aqueous colloidal solution at room temperature (25 °C) at different pH. A physicochemical study of the aqueous solution was proposed. The chemical reagents of the colloidal solution were zinc chloride, selenium elementary powder, Extran® as a surfactant, and sodium borohydride as a reducing agent. The species distribution diagrams (SDDs) and solubility curves (SCs) as pH functions were plotted as a result of the physicochemical analysis. The SDDs and SCs predicted the ZnSe-NPs formation and their growing process, therefore, a series of experiments according to these diagrams were performed at various pH of the colloidal solution through different Extran® content. The ZnSe-NPs were obtained without further purification. The ZnSe-NPs were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) techniques. The XRD results revealed that the ZnSe-NPs have mixed cubic and hexagonal crystalline structures, with crystal size at the nanoscale. The Raman spectra exhibited two peaks at 253 cm−1 of the longitudinal optical (LO) phonon mode, and 235 cm−1 of the surface phonon mode corresponding to the ZnSe bulk. The SEM images showed a variety of morphologies with different Zn/Se atomic ratios estimated from the EDS analysis.

Correction to “Core and Surface Electronic States and Phonon Modes in SiC Quantum Dots Studied by Optical Spectroscopy and Hybrid TDDFT”

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to “Core and Surface Electronic States and Phonon Modes in SiC Quantum Dots Studied by Optical Spectroscopy and Hybrid TDDFT”Yuanyuan LiYuanyuan LiMore by Yuanyuan Li, Xiaoyu LiuXiaoyu LiuMore by Xiaoyu Liu, Tianyuan LiangTianyuan LiangMore by Tianyuan Liang, and Jiyang Fan*Jiyang FanMore by Jiyang Fanhttp://orcid.org/0000-0003-1894-036XCite this: J. Phys. Chem. C 2021, 125, 17, 9590Publication Date (Web):April 26, 2021Publication History Published online26 April 2021Published inissue 6 May 2021https://doi.org/10.1021/acs.jpcc.1c03357Copyright © 2021 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views311Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (426 KB) Get e-Alerts Get e-Alerts

Electrochemical and magnetic properties of perovskite type RMnO3 (R = La, Nd, Sm, Eu) nanofibers

Perovskite type rare earth manganates RMnO3 (LaMnO3, NdMnO3, SmMnO3, and EuMnO3) nanofibers were fabricated by a simple electrospinning process. LaMnO3 nanofibers showed rhombohedral structure, and NdMnO3, SmMnO3, and EuMnO3 nanofibers exhibited orthorhombic structure. The surface structure, phonon modes, optical property, morphology, and valence states of RMnO3 nanofibers were examined. The electrochemical behaviors of RMnO3 nanofibers as supercapacitor electrode materials were explicitly illustrated. It confirms the feasible applications of RMnO3 nanofibers as supercapacitor electrodes. In addition, the magnetic properties of RMnO3 nanofibers were investigated. At room temperature, RMnO3 nanofibers exhibited paramagnetic behaviors. At low temperature, LaMnO3 and NdMnO3 nanofibers presented magnetic transition to ferromagnetic state, while SmMnO3 and EuMnO3 nanofibers showed magnetic transition to antiferromagnetic state.

Core and Surface Electronic States and Phonon Modes in SiC Quantum Dots Studied by Optical Spectroscopy and Hybrid TDDFT

The surface of the SiC quantum dot (QD) is a complex two-dimensional system that comprises fruitful surface reconstruction and passivation which affect its chemistry and photophysics. Here, we repo...

Phonon spectra of clean and Ni-terminated diamond (111) surfaces: An ab-initio study

The phonon densities of states (ph-DOSs) of clean and Ni-terminated C(111) surfaces with 1 × 1 and 2 × 1 surface structures were investigated using ab-initio density functional perturbation theory. The ph-DOSs showed vibrational spectra associated with the surface structures of C(111) and Ni/C(111). Further analyses of various surface phonon modes were performed to identify vibrational features involving the surface atoms of C(111) and Ni/C(111). These features provide important information for experimentally verifying the formation of a diamond bulk-like structure at Ni/C(111), as suggested in a previous study.

Phonon density of states in lanthanide-based nanocrystals

We report a combined inelastic neutron and X-ray scattering study of the phonon density of states of the nano- and microcrystalline lanthanide-based materials NaY$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ and NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$. While large (20 nm) nanocrystals display the same vibrational spectra as their microcrystalline counterparts, we find an enhanced phonon density of states at low energies, $E \leq 15\,\rm{meV}$, in ultra-small (5 nm) NaGd$_{0.8}$Yb$_{0.18}$Er$_{0.02}$F$_4$ nanocrystals which we assign to an increased relative spectral weight of surface phonon modes. Based on our observations for ultra-small nanocrystals, we rationalize that an increase of the phonon density of states in large nanocrystals due to surface phonons is too small to be observed in the current measurements. The experimental approach described in this report constitutes the first step toward the rationalization of size effects on the modification of the absolute upconversion quantum yield of upconverting nanocrystals.

Screen printing coating of (ZnO)0.8(CdO)0.2 material for optoelectronic applications

Here we report X-ray response and optical properties of composite (ZnO)0.8(CdO)0.2 thick film deposited by simple, easy and low cost screen printing technique on glass substrates followed by sintering at 500 °C. X-ray diffraction pattern confirms the polycrystalline structure of the film having hexagonal and cubic structures with preferred orientations of grains along (101) plane for ZnO and (111) plane for CdO. While Raman spectra exhibited strong peaks of E2 (high) phonon and overtone of surface phonon modes at 431 cm−1 and 1145 cm−1 respectively. The absorption coefficient and band gap which are essential for the optoelectronic applications were determined by using UV–visible absorbance data. Photoluminescence spectroscopy of the (ZnO)0.8(CdO)0.2 films showed a strong emission peak at 407 nm near the band edge along with a weak green–yellow emission peak spanning the wavelength range from 450 to 500 nm. The Arrhenius plot of DC conductivity shows semiconductor nature with existing activation energy of about 0.33 eV. The film thickness was measured by profilometry, obtaining a thickness of around 3 µm.

Spectrally selective mid-infrared absorber/emitter by controlling optical Tamm states with thin phononic film

Abstract We demonstrate the near-unity absorption of multilayer structures in the mid-infrared (MIR). The first configuration consists of a distributed Bragg reflector and a subwavelength-thin phononic film on top. Optical waves are confined within the cavity and the interface of the top layer, resulting in narrowband selective perfect absorption at the wavelength of 12.56 μm, which is of particular interest for many technological applications. Thus, highly spectral selective thermal emission can emerge from the engineered phononic-photonic structure. Tamm absorber can also be realized when the light from the phononic film side is perfectly absorbed. Reflectance of the fabricated sample is measured using a Fourier transform infrared spectrometer (FTIR) and matches the simulations well. Our findings will pave a new avenue for harness surface phonon (SPh) modes in the MIR, which could benefit applications such as radiative cooling, selective thermal emission and MIR sensing.