Surface Optical Phonon Research Articles

Invoking Obscured Surface Optical, Acoustic, and Anharmonic Phonon Modes and Their Modulation by Synthesis Conditions and Arrangement of the Surface in 0D ZnS

For low-dimensional systems, symmetry breaking and surface reconstruction are critical for the observation of special vibrational properties of technical significance such as surface optical (SO), ...

Effects of Density and Diameter on Surface Optical Phonon Modes in GaAs Nanowire Bundles.

We report the systematic investigation of the surface optical phonon modes in Au-catalyzed GaAs nanowires grown on an Au pre-patterned GaAs(111)B substrate using μ-Raman spectroscopy. We employed electron-beam dose rate as a control parameter during the substrate patterning step for adjusting the nanowire base diameter and coverage, which are independent from the nanowire growth conditions. We have experimentally studied the effect of the fill factor and average diameter on the surface optical phonon modes and explained the red-shift and broadening of the surface optical phonon frequencies by employing the dielectric continuum model. The surface optical phonon mode shift is exhibited to be sensitive to fill factor, rather than base diameter. The decrease in the average diameter from 280 nm to 180 nm results in the asymmetric broadening and red-shift of the surface optical phonon frequency (~1.83 cm-1) but the theoretical calculation from the isolated single nanowire-based dielectric continuum model cannot solely explain the behaviors of the surface optical phonon mode. In contrast, the change in the fill factor from 0.01 to 0.83 results in a shift of the surface optical phonon frequency (~6.5 cm-1) from the GaAs bulk value. The red-shift and asymmetric broadening of the surface optical phonons, in an agreement with the Maxwell-Garnett approximation, are consequences of dipolar interaction of randomly aligned neighboring nanowires and the polar nature of GaAs nanowire bundles. This work suggests the pre-patterning parameter dependent surface optical phonon characteristics of GaAs nanowire bundles which are of great importance in the nondestructive characterization of low-dimensional opto-electronic materials and devices.

Resolution of polaron spin states by the infrared absorption in monolayer transition metal dichalcogenides

Abstract We theoretically investigate the infrared absorption of polaron with spin-orbit coupling in monolayer transition metal dichalcogenides, in which the polaron states are formed arising from the electron coupled with the intrinsic longitudinal optical phonon and the induced surface optical phonon modes. We find that the magnitude of optical absorption are different for the polaron with spin-up and spin-down states in the same valley. Moreover, the absorption peaks of spin-up and spin-down states show the opposite changing trends with increasing the strength of Rashba spin-orbit coupling. These results indicate that the polaron spin state could be used for an effective valley information carrier in two-dimensional valleytronic materials by the infrared optical absorption, which offers new opportunities for potential applications in infrared and valleytronic devices.

Raman Spectral Probe on Size-Dependent Surface Optical Phonon Modes and Magnon Properties of NiO Nanoparticles

The particle size-dependent surface optical (SO) phonon and magnon modes of NiO nanoparticles were extensively studied using room-temperature confocal Raman spectroscopy. The required nanoparticles...

Enhancement of effective masses of the surface polaron in FeSe thin film on SrTiO3 substrate

Abstract We study the correction of the effective mass of the surface polaron in monolayer FeSe on SrTiO 3 substrate using a variational approach, in which surface polaron states are formed arising from the coupling between electrons in FeSe film and two kinds of surface optical (SO) phonon modes in SrTiO 3 substrate. We give a concise formula for the relations of the effective masses with the electron–SO phonon coupling matrixes, and find that the effective masses are enhanced significantly due to these coupling, which are very good agreement with the recent experimental measurements. Moreover, the effective masses depend strongly on the cutoff wave vectors of SO phonon modes and the interlayer distance between FeSe and SrTiO 3 substrate. Our theoretical results provide a practical route to analyze the electron–SO phonon coupling by measuring the variation of effective masses in experiments.

Optical and structural investigation of ZnS:Cu thin films synthesized by spray pyrolysis

We have investigated the effect of Cu dopant and substrate temperature on the structural and optical properties of ZnS thin films deposited on glass substrate by spray pyrolysis technique. The glass substrate temperature varied from 300 to 450 °C. Different characterization methods such as X-ray diffractometry, EDS, SEM, and Raman scattering analyses for studying the structural, morphological and optical properties of the prepared ZnS and ZnS:Cu samples were carried out. Improvement in crystallization, increase in crystallite size and also better growth orientation along the (111) direction are clearly evident due to the increase in the substrate temperature. Cu doping effects on the displacement of diffraction peaks towards larger angles and decrease in crystallite size which caused by this impurity are the main results of this studies. The absence of any other impurity in samples, smooth surface, homogenous structure and spherical shape of crystals have approved by EDS analysis and SEM imagining, respectively. Raman scattering spectra of ZnS and ZnS:Cu in ambient condition not only indicate E and A Raman first order scattering of ZnS, but also indicate an intense peak about 334 cm−1 related to surface optical phonon mode in this nanostructure. Optical properties of thin films determined by UV–Vis spectroscopy and the energy band gap was calculated with ZnS doping rates. Photoluminescence analysis and origin of green emission peak at 533 nm and red peak at 745 nm were discussed.

Manipulation of surface phonon polaritons in SiC nanorods

Surface phonon polaritons (SPhPs) are potentially very attractive for subwavelength control and manipulation of light at the infrared to terahertz wavelengths. Probing their propagation behavior in nanostructures is crucial to guide rational device design. Here, aided by monochromatic scanning transmission electron microscopy-electron energy loss spectroscopy technique, we measure the dispersion relation of SPhPs in individual SiC nanorods and reveal the effects of size and shape. We find that the SPhPs can be modulated by the geometric shape and size of SiC nanorods. The energy of SPhPs shows red-shift with decreasing radius and the surface optical phonon is mainly concentrated on the surface with large radius. Therefore, the fields can be precisely confined in specific positions by varying the size of the nanorod, allowing effective tuning at nanometer scale. The findings of this work are in agreement with dielectric response theory and numerical simulation, and provide novel strategies for manipulating light in polar dielectrics through shape and size control, enabling the design of novel nanoscale phonon-photonic devices.

Micro-Raman investigation on the size effects of surface optical phonon modes in single cone-shape InGaN/GaN microrods

As one of the third generation semiconductor representative materials, nitride has been commonly used in the field of semiconductor optoelectronics. Low dimensional structures of GaN have become research hotspot. In this paper, the size effects of single InGaN/GaN core-shell microrods on the surface optical phonon mode (SO mode) are studied based on micro-Raman technology. The experimental results show that the size of the microrod would lead to the SO mode. Furthermore, the frequency of SO mode is changed with different radius of microrod. The larger the radius is, the higher the frequency of SO mode. Combined with theoretical calculation, it is found that the experimental results are in agreement with calculation. In addition, the experimental results show that the coupling of the free carrier and the longitudinal optical phonon caused the Raman shift of E1 (LO) mode. Based on this, we obtain the distribution of carrier concentration along the [0 0 0 1] growth direction.

Power loss of hot Dirac fermions in silicene and its near equivalence with graphene

The power loss P of hot Dirac fermions through the coupling to the intrinsic intravalley and intervalley acoustic and optical phonons is analytically investigated in silicene as a function of electron temperature T e and density n s . At very low T e , the power dissipation is found to follow the Bloch–Grüneisen power-law and , as in graphene, and for 20–30 K, the power loss is predominantly due to the intravalley acoustic phonon scattering. On the other hand, dispersionless low energy intervalley acoustic phonons begin to dominate the power transfer at temperatures as low as ∼30 K, and optical phonons dominate at K, unlike the graphene. The total power loss increases with T e with a value of ∼1010 eV s−1 at 300 K, which is the same order of magnitude as in graphene. The power loss due to intravalley acoustic phonons increases with n s at higher T e , whereas due to the intervalley acoustic and optical phonons is found to be independent of n s . Interestingly, the energy relaxation time in silicene is about four times higher than that in graphene. For this reason, silicene may be superior over graphene for its applications in bolometers and calorimeters. Power transfer to the surface optical phonons is also studied as a function of T e and n s for silicene on Al2O3 substrate and it is found to be greater than the intrinsic phonon contribution at higher T e . Substrate engineering is discussed to reduce .

Magneto-optical transport properties of monolayer transition metal dichalcogenides

We study the optical transport properties of the monolayer transition metal dichalcogenides (TMDCs) such as ${\mathrm{MoS}}_{2}$, ${\mathrm{WS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WSe}}_{2}$ in the presence of a magnetic field. The TMDCs band structures are obtained and discussed by using the effective massive Dirac model, in which the spin and valley Zeeman effects as well as an external electric field are included. The magneto-optical absorption coefficient (MOAC) is derived as a function of absorbed photon energy when the carriers are scattered by random impurities combined with the intrinsic acoustic and optical phonons in TMDCs and the surface optical (SO) phonons of substrates. Our result shows that the spin-splitting feature appeared in all four TMDC materials. The combination of strong spin-orbit coupling (SOC) and Zeeman fields has doubled the Landau levels but has not changed the energy gap of the TMDCs monolayer, which can be controlled by the electric field. Because of their strong SOC effect, the absorption spectrum in monolayer TMDCs is separated into two separate peaks caused by spin up and down. At the low temperature, the MOAC intensity via impurity scattering is the biggest followed by that of the SO phonons while the intrinsic acoustic and optical phonon scatterings display the smallest. For the monolayer TMDCs on substrates, ${\mathrm{SiO}}_{2}$ always shows its superiority in comparison with the others. Among the four TMDC materials, ${\mathrm{MoSe}}_{2}$ shows the biggest MOAC intensity, while ${\mathrm{WS}}_{2}$ has the biggest value of the absorbed photon energy. The full-width at half-maximum (FWHM) via impurity scattering achieves its highest value in ${\mathrm{WS}}_{2}$, while this occurs in ${\mathrm{MoSe}}_{2}$ and ${\mathrm{MoS}}_{2}$ via intrinsic acoustic and optical phonon scatterings, respectively. Our estimation of mobility from FWHM gives good agreement with the experimental results in ${\mathrm{WS}}_{2}$.

Optimizing Al-doped ZrO2 as the gate dielectric for MoS2 field-effect transistors

In this work, we investigate the effects on the electrical properties of few-layered MoS2 field-effect transistors (FETs) following Al incorporation into ZrO2 as the gate dielectrics of the devices. A large improvement in device performance is achieved with the Al-doped ZrO2 gate dielectric when Zr:Al = 1:1. The relevant MoS2 transistor exhibits the best electrical characteristics: high carrier mobility of 40.6 cm2 V−1 s−1 (41% higher than that of the control sample, and an intrinsic mobility of 68.0 cm2 V−1 s−1), a small subthreshold swing of 143 mV dec−1, high on/off current ratio of 6 × 106 and small threshold voltage of 0.71 V. These are attributed to the facts that (i) Al incorporation into ZrO2 can decrease its oxygen vacancies; densify the dielectric film; and smooth the gate dielectric surface, thus reducing the traps at/near the Zr0.5Al0.5Oy/MoS2 interface and the gate leakage current; (ii) adjusting the dielectric constant of the gate dielectric to an appropriate value, which achieves a reasonable trade-off between the gate screening effect on the Coulomb-impurity scattering and the surface optical phonon scattering. These results demonstrate that optimized Zr0.5Al0.5Oy is a potential gate dielectric material for MoS2 FET applications.

Plasmon induced enhancement of surface optical phonon modes and magnon properties of NiO nanoparticles: Raman spectral probe.

In the present work, the influence of Ag-induced plasmons on the surface optical (SO) phonon modes of NiO nanoparticles was extensively studied using room temperature Raman spectroscopy. Remarkable intensity enhancements were observed for the rarely reported SO phonon modes compared to the other first-order phonon modes of NiO nanoparticles. The occurrence of SO modes was further studied using an approximate dielectric continuum (DC) model and a difference between the calculated and experimental SO frequencies was observed, which can be attributed to the presence of one magnon background over the first order phonon modes. The experimental and theoretical SO frequencies became closer at higher Ag concentration and the second-order magnon (2M) and phonon bands disappeared in the NiO:Ag samples. The absence of magnon and higher order phonon modes in the NiO:Ag samples indicates changes in the magnetic properties of the nanomaterials, which has been further supported by the vibrating-sample magnetometer (VSM) measurements.

Coupled plasmon–phonon modes in monolayer MoS2

We present a theoretical study of the plasmon–phonon coupling in a suspended monolayer MoS2 and a MoS2 substrate system using a diagrammatic self-consistent field theory. The four coupled plasmon–phonon modes and the four plasmon-surface phonon modes are observed due to the spin–orbit and electron-optic phonon interactions. The two of coupled plasmon–phonon and plasmon-surface phonon modes are optic-like and the other two are acoustic-like. The plasmon are strongly coupled with the optic-phonon in MoS2 and the surface optic-phonon in the substrates as the electron density or wave-vector increases. The strong plasmon–phonon coupling shows that the optoelectronic properties of monolayer MoS2 are evidently modulated by electron–phonon interactions. The hybrid plasmon–phonon polaritons can be achieved by strong light-mater interactions. This study is relevant to the application of MoS2 as novel plasmonic and nanophotonic devices.

Substrate-limited helical edge states

We derived analytical results for the gapless edge states of two-dimensional topological insulators in the presence of electron--surface optical (SO) phonon interaction due to substrates. We followed an analytical algorithm, called the Lee-Low-Pines variational approximation in the conventional polaron theory, to examine the substrate-induced effects on both bulk and edge states of a two-dimensional topological insulator within the frame work of the Bernevig-Hughes-Zhang (BHZ) model. By implementing this algorithm, we propose a phonon-dressed BHZ Hamiltonian which allows one to investigate the effects of various substrates not only on bulk states but also on the associated gapless helical edge states (HESs). We found that both the bulk and HESs are significantly renormalized in the momentum space due to the substrate-related polaronic effects. The model we developed here clarifies which substrates favor the HESs of the quantum spin Hall system and which do not. Correspondingly, our work demonstrates that the substrate-related polaronic effects have a significant role in the emergence of HESs. In other words, we show that SO phonons due to substrates modify the electronic band topology of topological insulators together with the associated HESs, and therefore, they can be used to tune quantum phase transitions between topological insulators and nontopological ones.

Influence of Gate Doping Concentration on the Characteristics of Amorphous InGaZnO Thin-Film Transistors With HfLaO Gate Dielectric

Amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) with HfLaO as high-k gate dielectric have been fabricated, and p-type silicon wafers with resistivities of 0.001 ~ 0.002, 0.005, 0.02 ~ 0.021 and $5\sim 10\Omega \cdot $ cm are used as their gate electrodes. The carrier mobility of the samples show an increase with increasing gate acceptor concentration, indicating that the TFT performances are affected by the hole concentration in the gate electrode. The HfLaO gate dielectric is highly polarizable and so produces surface-optical phonons, which can scatter the electrons in the IGZO channel to reduce their mobility. Therefore, this work directly demonstrates that the holes in the gate electrode can have a screening effect to suppress this remote phonon scattering of the gate dielectric on the channel carriers, just like the electrons in the metal gate, ITO gate and n-silicon gate of field-effect transistors.

Possible Fröhlich surface polaron formation in graphene on a polar substrate

Abstract We theoretically study the formation of Frohlich polaron in monolayer graphene on a substrate made of polar materials. In this calculation, the Rayleigh-Schrodinger perturbation theory is used to obtain analytical expression of polaron energy. The decrease (increase) in the electron (hole) energy due to interaction with surface optical phonons indicates that the formation of a surface polaron is preferred. The charge carrier-phonon interaction in monolayer graphene on a polar substrate does not lead to the formation of a gap between the valence and the conduction bands, as there exist the same energy shifts for both electron and hole at k = 0 . The expression for the renormalized Fermi velocity due to carrier-phonon coupling for small values of k is obtained. Our results show that the influence of the screening effect on polaron energies is significant for k ≤ k F .

Influence of spatial dispersion on spectral tuning of phonon-polaritons

The field of nanophotonics has long sought to identify mechanisms to realize dynamical control of optical modes. In most approaches, the magnitude of tuning is dependent upon the degree to which the optical permittivity is malleable upon some material change, such as carrier concentration. Here, through a multiwavelength Raman spectroscopic examination of 4H-SiC nanopillars, momentum is identified as an alternative means to enhance spectral tunability of nanophotonic modes, owing to the spatial dispersion implicit in the infrared (IR) optical permittivity of polar semiconductors. Experimentally, this is deduced through the observation of a ``forbidden'' Raman mode at $\ensuremath{\approx}780\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and the emergence of the surface-optical phonon polariton at $\ensuremath{\approx}950 {\mathrm{cm}}^{\ensuremath{-}1}$, which evolved with intensities dependent upon the nanopillar diameter and the wavelength of the incident light. The evolution of these modes is accompanied by a redshift and spectral narrowing of the longitudinal-optical plasmon coupled (LOPC) mode exhibiting a similar wavelength and diameter dependence. Mie resonances, identified using ultraviolet-visible spectroscopy and excited by the visible laser excitation of the Raman experiment, acted to vary the momentum sampled during the Raman experiment leading to these spectral dependencies. This was deduced by fitting the Raman response accounting for both the presence of the surface phonon and the overdamped LOPC mode under the Lindhard-Mermin approximation. This fitting not only explains the Raman response, but also clearly exhibits the spatially disperse permittivity of the SiC, which is shown to have a momentum-dependent sensitivity to carrier concentration. Such sensitivity, in turn, highlights the potential of spatial dispersion as a means to accentuate the performance of active IR nanophotonic approaches employing phonon polaritons.

A comparative study of substrates disorder on mobility in the Graphene nanoribbon: Charged impurity, surface optical phonon, surface roughness

The effects of substrate on the electronic properties of Graphene remains unclear. Many theoretical and experimental efforts have been done to clarify this discrepancy. In this work, we studied the electronic transport in armchair Graphene nanoribbons (AGNR) in the presence of substrate's disorder. The three main substrate's disorders -- surface roughness, charged impurity and surface optical phonon -- are investigated. Non-Equilibrium Green's function along with the tight-binding model is employed to investigate the electronic properties of Graphene Nanoribbons. The effects of these disorders are investigated individually, finally, the effects of them are compared to determine the dominant source of scattering.

Gate-tunabe gap of heated monolayer graphene on substrates

Temperature effect of energy gap is researched in the presence of the homogeneous magnetic field based on the electrons/holes - surface optical phonons coupling induced by different polar substrates under monolayer graphene. The quantum statistical theory and LLP variational method are used to calculate the energy gap of monolayer graphene on the polar substrate. We found that substrate-induced gap can be tunable in the landau energy level by controlling the temperature T, confinement strength parameter g, polarization strength η of substrate. The energy gap matches with the recent experimental data. These conclusions obtained highlight the vital role of polaron effect in deciding substrate-induced gap and provide a promising way to increase the substrate-induced gap of monolayer graphene.

Multiphonon Raman scattering mediated by the exciton states in monolayer transition metal chalcogenides

In the frame of Huang-Rhys's model, we study multiple orders of Raman scattering in monolayer transition metal chalcogenides (TMDC) based on the intrinsic excitons coupled with three types of phonon modes, including longitudinal optical (LO) phonon, surface optical (SO) phonon, and longitudinal acoustic (LA) phonon. We find that multiphonon structures display multiple order overtones at equal interval energies of $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}$ and $\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{SO}}$, respectively, and intensities of overtones depend on the obtained values of Huang-Rhys factor, which can be modulated by the strength of exciton-optical phonons coupling, cutoff wave vector of optical phonon modes, and large exciton Bohr radius. Moreover, we propose the combined multiphonon processes of $p\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}+m\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{SO}}$ and $p\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LO}}(m\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{SO}})+\ensuremath{\hbar}{\ensuremath{\omega}}_{\text{LA}}$. These combined processes with weaker intensity could be used to analyze the fine structure of multiphonon Raman spectra in experiments. Our theoretical results indicate that the strong exciton effect in monolayer TMDC provides an excellent platform for exploring multiphonon Raman scattering.