Surface Phonon Modes Research Articles

Sea Urchin Like Shaped CdSe Nanoparticles Grown in Aqueous Solutions via Electron Beam Irradiation

Cadmium selenide (CdSe) nanoparticles have been grown from an aqueous solutions containing equimolar ammoniated cadmium sulphate and sodium selenosulphate as precursors in presence of citric acid as a capping agent, via electron beam irradiation. The radiolytic processes occurring in the medium result in the formation of CdSe nanoparticles through the reactions mediated by hydrated electrons (e(aq)-). The dynamics of the formation of these nanoparticles was investigated by pulse radiolysis studies. The size of the primary nanoparticles as estimated from the absorption spectra recorded immediately was less than 3 nm. These nanoparticles exhibited strong excitonic absorption pattern and broad photoluminescence at room temperature, which has been attributed to the presence of surface states/defects. This has been confirmed by Raman spectral studies, where CdSe nanoparticles exhibited characteristic surface phonon modes at around 250 cm(-1). The photoluminescence lifetime decay measurements further supported the existence of surface defects on the as-grown CdSe nanoparticles. These nanoparticles were found to exist in the agglomerated form of sea urchin like shapes of uniform size of about 500 nm as revealed from TEM and SEM images. These sea urchin like shaped CdSe nanoparticles grown in this route were found to be very stable under the ambient conditions. We infer that citric acid influences the growth as well as stability of these nanoparticles. It is expected that these nanomaterials could find potential applications in the field of sensors, catalysis and photovoltaics.

Thickness evolution of phonon properties in ultrathin Bi (111) films

Ultrathin Bi films have been theoretically predicted to be an elemental two-dimensional topological insulator recently. Investigation on phonon properties in topological insulators, especially down to a few-layer regime, is an interesting topic. In this paper, the evolution of structures and phonon properties of ultrathin Bi (111) films with thickness is studied from first-principles calculations by including spin–orbit coupling. The calculations of lattice dynamics show that both two Raman modes A1g and Eg in ultrathin Bi (111) films harden compared with those in bulk, which is contrary to other nanomaterials. The possible origin of this anomalous phonon behavior is discussed by considering the peculiar semimetallicity of Bi. With increasing thickness, the frequencies of these two Raman modes decrease gradually toward the values in the bulk. Our calculations also show that the softening and hardening of surface phonon modes coexist in ultrathin Bi films.

Splitting of the Surface Phonon Modes in Wurtzite Nanowires

We analyze the surface optical modes of GaN nanowires (NW) and perform a comparative study with the characteristics expected for other polar NWs. The theoretical analysis of the modes is performed within the context of the effective medium theory that takes into account the dipolar interaction between neighboring NWs (Maxwell-Garnett approximation). It is shown that deviations of the exciting light from the NWs axis, which coincides with the wurtzite c-axis, result in the anticrossing of two distinct surface phonon branches, leading to their splitting in axial and planar components and the appearance of two peaks in the Raman spectra. Additional calculations are performed that determine the relevant filling factor at which this anticrossing takes place for NWs based on AlN, InN, ZnO, as well as other material systems that may crystallize in the wurtzite structure when grown in the form of NWs, such as GaP and InAs. We analyze the splitting of the surface modes as a function of the material's ionicity.

Surface lattice dynamics and electron–phonon interaction in ultrathin Bi(111) film

The surface phonon and electron–phonon interactions in the two-dimensional topological insulator Bi(111) film are calculated, including the spin–orbit coupling from density-functional perturbation theory. By analyzing the zone-center phonons, an anomalous phonon hardening of two Raman modes, Eg and A1g, in ultrathin films is found and is explained by considering the redistribution of the charge density on the surface of the semimetallic Bi. Surface phonon band structures and the surface phonon density of states are given, and we find that softening and hardening of surface phonon modes occur simultaneously in ultrathin Bi film, but the softening may dominate over the hardening. The calculated electron–phonon coupling constant λ for Bi(111) film is much larger than that for the bulk, which might induce surface-localized superconductivity in this two-dimensional topological insulator.

Excitation of Coherent Phonons in the One-Dimensional Bi(114) Surface

We present time-resolved photoemission experiments from a peculiar bismuth surface, Bi(114). The strong one-dimensional character of this surface is reflected in the Fermi surface, which consists of spin-polarized straight lines. Our results show that the depletion of the surface state and the population of the bulk conduction band after the initial optical excitation persist for very long times. The disequilibrium within the hot electron gas along with strong electron-phonon coupling cause a displacive excitation of coherent phonons, which in turn are reflected in coherent modulations of the electronic states. Beside the well-known A(1g) bulk phonon mode at 2.76 THz, the time-resolved photoelectron spectra reveal a second mode at 0.72 THz which can be attributed to an optical surface phonon mode along the atomic rows of the Bi(114) surface.

Influence of surface optical phonons on exciton binding energies of a quasi-one-dimensional wurtzite GaN-based nanowire: Quantum size effect

Based on the surface optical phonon states and their electron-phonon coupling functions obtained recently, the surface polaronic exciton states in a quasi-one-dimensional (Q1D) wurtzite nanowire (NW) are investigated by using the variational method and Lee-Low-Pines transform. In order to reflect the different confined features of Q1D wurtzite NWs in axial and radial directions and anisotropy of wurtzite nitride crystals, a two-parameter variational approach is proposed and applied to describe the polaronic exciton states in the NWs. Numerical calculations are performed for GaN NWs. The quantum size effects on the binding energies of polaronic exciton and the contributions of surface phonon modes are analyzed in detail. Our results show that the binding energy of polaronic exciton and surface phonon contribution in the wurtzite GaN NWs reaches 190 and 95 meV, respectively, which are one or two orders of magnitude larger than those in cubic GaAs-based quantum wells and NWs with the same radius. This is mainly ascribed to the strong electron-phonon interaction, the large effective masses of carriers and relatively small dielectric constants in GaN material. The numerical results also show that the two-parameter variational approach is reasonable and necessary for the description of polaronic exciton states in Q1D wurtzite GaN NWs. Moreover, the behaviors of the two variational parameters for the polaronic exciton states are quite different from those for the bound polaron states in Q1D wurtzite NWs.

Electrical modulation of emissivity

We demonstrate that it is possible to modulate the thermal emission through an electrical modulation of the emissivity. The basic idea is to design a device where absorption is due to a resonant phenomenon. If the resonance can be electrically controlled, then absorption and, therefore, thermal emission can be controlled. We demonstrate this general concept using THz resonant absorption by surface phonon polaritons coupled through a gold grating. In our device, absorption is mostly due to a surface phonon mode confined in a single quantum well (QW). The THz emissivity is electrically controlled by controlling the quantum well refractive index using a gate voltage to modulate the electron density. This paves the way to electrically modulated incandescent sources.

Surface enhanced Raman scattering effect of CdSe/ZnS quantum dots hybridized with Au nanowire

Functionalized CdSe/ZnS quantum dots (QDs) were attached to the surface of Au nanowire (NW). Analysis of optical absorption spectra disclosed surface plasmon bands of Au NWs at 562 and 627 nm and showing an overlap with the QD absorption band. Micro Raman spectra (λex = 514 nm) of the QDs/Au single NW exhibited surface enhanced Raman scattering (SERS) peaks at 180, 205, and 277 cm−1, corresponding to surface, longitudinal, and transverse optical phonon modes, respectively. From time-resolved fluorescence spectra, the exciton lifetime of QDs decreased after hybridization with Au NW due to the energy transfer, supporting the SERS effect.

Size Effects on Properties of NiO Nanoparticles Grown in Alkalisalts

NiO nanoparticles with sizes of 3.5–12.4 nm were grown by thermal decomposing of nickel acetate at different temperatures in NaCl and Li2CO3 alkalisalts. The properties of the nanoparticles were characterized by X-ray diffraction spectrometer, transmission electron microscope, absorption spectrometer, micro-Raman microscope, and superconducting quantum interference device. The effects of the nanoparticle sizes on the crystal structure, exciton ground state energy, vibration modes, and magnetic properties were studied. Lattice parameter of NiO increases with a decrease in nanoparticle sizes. The band gap of NiO nanoparticles increases with a decrease in the nanoparticle size. LO modes of NiO nanoparticles shift red, and the intensity increases with a decrease in the nanoparticle sizes. Surface phonon modes are observed. Bifurcation temperature and blocking temperature of NiO nanoparticles shift to lower temperature with a decrease in nanoparticle sizes. Two peaks are present in all nanoparticles’ zero-field-cooled magnetization curves, and the saturation magnetization, remanet magnetization, and coercivity increase with a decrease in the nanoparticle sizes. The nanoparticles exhibit size-dependent anomalous magnetic properties that make the remagnetization curve surpass the initial magnetization curve in the M–H hysteresis curves taken at 5 K.

Enhanced electron-phonon coupling at the Au/Mo(112) surface

A detailed investigation of the electronic structure and electron-phonon coupling for a Au monolayer on the Mo(112) surface is presented. The electronic states of bulk Mo and the (112) surface-derived states are seen to strongly hybridize with those of the Au overlayer, resulting in the formation of surface resonance states localized near the surface and the interface of Au/Mo(112). The experimentally extracted self-energy due to the electron-phonon coupling on one of the surface resonance bands gives a good quantitative agreement with the calculations. The strength of electron-phonon coupling for Au/Mo(112) is discussed in terms of the mass enhancement factor and is considerably larger than for the Mo(112) surface. Such an increase in the mass enhancement factor in the vicinity of the Fermi level likely derives from the soft surface phonon modes created upon Au adsorption.

Surface lattice vibration and electron-phonon interaction in topological insulator Bi2Te3 (111) films from first principles

Surface phonon and electron-phonon (EP) interaction in Bi2Te3 (111) films are calculated by including spin-orbit coupling from the density-functional perturbation theory. We present a detailed analysis of the zone-center phonons by considering the interlayer relaxation of the films. The phonon density of states and the localized surface phonon modes along the and directions in the surface Brillouin zone are given. Our results clearly show that softening and stiffening of the surface phonons coexist in Bi2Te3 ultrathin films. Furthermore, we report on a first-principles study of the EP coupling constant for Bi2Te3 films. Our calculated EP interaction in Bi2Te3 films is very weak, which is in favor of application for room temperature electronic devices.

Preparation of clean and well-ordered hydrogen-terminated Si(1 1 0)-(1 × 1) surfaces and the measurements of vibrational modes

We propose electrochemical and wet chemical processes to prepare the clean and well-ordered hydrogen-terminated Si(110)-(1×1) surfaces, with our evaluation by low-energy electron-diffraction (LEED), high-resolution electron-energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). Clear (1×1) LEED patterns indicated the two-dimensional space group of p2mg. The intrinsic SiH bending and stretching modes and surface phonon modes, as well as hydrocarbon impurities were observed by HREELS. Among the procedures tested, the electrochemical etching in a NH4F solution yielded the best ordered surface. The wet chemical etching produced the clean samples suitable for HREELS measurements.

Surface phonons of Ge(001) and their correlation with thep(2×1) andc(4×2) reconstruction as shown by Raman spectroscopy

The $p$(2$\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$1)/$c$(4$\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$2) and the $c$(4$\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$2) reconstruction of the Ge(001) surface have been studied by polarized Raman spectroscopy at 300 and 40 K, respectively. Raman spectra show several well-defined surface phonon modes related to the atomic structure of the Ge surface. Four modes are detected in the range between 5.70 and 28.15 meV. Their eigenenergies and polarization dependence agree with reported calculation results from the adiabatic bond charge model and from density-functional perturbation theory. The temperature-induced phase transition between both reconstructions is reflected in the symmetry selection rules. Moreover, our results reveal that in Raman scattering the impact of the well-known flipping of the buckled dimers in the $p$(2$\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$1) reconstruction is fundamentally different from the time averaging, which occurs for low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Since the phonon time scale is several orders of magnitude faster than the dimer flipping, the phonon oscillations and their Raman scattering are described consistently within the framework of quasistatic buckled dimers with a short-range antiferromagnetic in-row buckling order.

Structural, optical and magnetic properties of Mn doped ZnO thin films prepared by pulsed laser deposition

Abstract Zn1−xMnxO thin films were grown by pulsed laser deposition. The phase purity and the structure were confirmed by X-ray diffraction studies. The films have a transmittance more than 80% in the visible region. The refractive index of Zn0.90Mn0.10O films is found to be 1.77 at 550 nm. The presence of non-polar E 2 high and E 2 low Raman modes in thin films indicates that ‘Mn’ doping does not change the wurtzite structure of ZnO. Apart from the normal modes of ZnO the Zn1−xMnxO ceramic targets show two additional modes at 332 cm−1 (I1) and 524 cm−1 (I2). The broad Raman peaks (340–600 cm−1) observed Zn0.90Mn0.10O thin films can be deconvoluted into five peaks, denoted as P1–P5. The possible origins of Raman peaks in Zn1−xMnxO films are the structural disorder and morphological change caused by the Mn dopant. The B 1 low , B 1 low 2 , B 1 high and A 1 LO modes as well as the surface phonon mode have been observed in heavily Mn-doped ZnO films. Zn0.98Mn0.02O thin film shows room temperature ferromagnetism. The saturation magnetic moment of the Zn0.98Mn0.02O thin film is 0.42μB/Mn atom. The undoped ZnO film prepared under the same condition shows diamagnetic nature. At higher doping concentrations the formation of Mn clusters suppress the room temperature ferromagnetism in Zn1−xMnxO thin films and shows paramagnetism. XPS confirms the incorporation of Mn2+ into the ZnO lattice.

Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots

The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E2 Raman active optical phonon at 436 cm–1 is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260–340 cm–1, attributed to surface vibrations. The E1-LO mode at 585 cm–1 is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 Å) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode.

Ultrafast dynamics of occupied quantum well states in Pb/Si(111)

We investigate the ultrafast electron dynamics of occupied quantum well states (QWSs) in Pb/Si(111) with time-resolved photoemission spectroscopy. We find an ultrafast increase in binding energy of the QWSs driven by the optical excitation, while the electronic system is in a non-equilibrium state. We explain this transient energetic stabilization in the photoexcited state by an ultrafast modification of the Fermi level pinning, triggered by charge transfer across the Pb/Si interface. In addition, we observe the excitation of a coherent surface phonon mode at a frequency of ∼2 THz, which modulates the QWS binding energy.

Role of Fe doping on structural and vibrational properties of ZnO nanostructures

In this report, Raman and Fourier Transform Infrared (FTIR) measurements were carried out to study the phonon modes of pure and Fe doped ZnO nanoparticles. The nanoparticles were prepared by sol–gel technique at room temperature. The X-ray diffraction measurements reveal that the nanoparticles are in hexagonal wurtzite structure and doping makes the shrinkage of the lattice parameters, whereas there is no alteration in the unit cell. Raman measurements show both \(E_{2}^{\mathrm{low}}\) and \(E_{2}^{\mathrm{High}}\) optical phonon mode is shifted towards lower wave number with Fe incorporation and explained on the basis of force constant variation, stress measurements, respectively. In addition, Fe related local vibrational modes (LVM) were observed for higher concentration of Fe doping. FTIR spectra reveal a band at 444 cm−1 which is specific to E1 (TO) mode; a red-shift of this mode in Fe doped samples and some surface phonon modes were observed. Furthermore, the observation of additional IR modes, which is considered to have an origin related to Fe dopant in the ZnO nanostructures, is also reported. These additional mode features can be regarded as an indicator for the incorporation of Fe ions into the lattice position of the ZnO nanostructures.

Femtosecond Laser‐Induced Formation of Wurtzite Phase ZnSe Nanoparticles in Air

We demonstrate an effective method to prepare wurtzite phase ZnSe nanoparticles from zincblende ZnSe single crystal using femtosecond pulse laser ablation. The fabricated ZnSe nanoparticles are in spherical shape and uncontaminated while synthesized under ambient environment. By controlling the laser fluences, the average size of ZnSe nanoparticles can be varied from ~16 nm to ~22 nm in diameter. In Raman spectra, the surface phonon mode becomes dominant in the smaller average particle size with uniform size distribution. The interesting phase transition from the zinc blende structure of ZnSe single crystal to wurtzite structure of ZnSe nanoparticles may have been induced by the ultrahigh ablation pressure at the local area due to the sudden injection of high energy leading to solid‐solid transition.

Growth and lattice dynamics of single-crystalline SnO 2 nanowires prepared by annealing a gel precursor

A large amount of ultra-long single-crystalline SnO 2 nanowires were successfully synthesized through a polymeric sol–gel approach followed by a post-annealing in a crucible covered with a lid. The experimental results indicate that the product is composed of tetragonal SnO 2 nanowires, and moreover, their growth mechanism should arise from the self-catalyzed Sn-terminated polar surfaces due to the enrichment of Sn at the growth front of nanowires. In Raman scattering spectrum, in addition to three fundamental vibrational modes, some infrared (IR) active and surface phonon modes (SPM) are also observed due to size and shape confinement effects and surface disorder. Similarly, low frequency non-active IR and surface phonon mode absorptions are also observed in IR spectra.

A density functional theory study of the electron–phonon coupling at the Mo(112)surface

The electron–phonon (e–p) coupling for the surface bands of Mo(112) has been studied bydensity functional theory (DFT). There are significant contributions from the surfacephonon modes to the low frequency part of the phonon density of states and Eliashbergfunction, which results in enhanced e–p coupling at the surface and, consequently, anincrease of the mass enhancement factor and the imaginary part of the self-energy.We suggest that the enhanced e–p coupling, together with anharmonic effects,scattering off impurities, as well as decay channels through adjacent bulk bands,leads to the increase of the linewidth of the electronic surface bands crossing theFermi level observed in high resolution photoemission spectra of the Mo(112)surface.