Longitudinal Optical Phonon Modes Research Articles

Compatibility and optoelectronic of ZnSe nano crystalline thin film

We report the room temperature synthesis of zinc selenide (ZnSe) nano crystalline thin film on quartz by using a relatively simple and low cost closed space sublimation process (CSSP). The compatibility of the prepared thin films for optoelectronic applications was assessed by X-ray diffraction (XRD), atomic force microscope (AFM), scanning electron microscope (SEM), Raman spectroscopy, photoluminescence, and Fourier transform infrared spectroscopy (FT-IR). The XRD confirmed that the films were polycrystalline with the preferential orientation along the (111) plane corresponding to the cubic phase (2θ = 27.28°). The AFM indicated that the ZnSe film presented a smooth and compact morphology with RMS roughness 19.86 nm. The longitudinal optical phonon modes were observed at 247 cm−1 and 490 cm−1 attributed to the cubic structured ZnSe. The Zn—Se stretching band was confirmed by the FT-IR. The microstructure and compositional analysis was made with the SEM. The grain size, dislocation density, and strain calculated were co-related. All these properties manifested a good quality, high stability, finely adhesive, and closely packed structured ZnSe thin film for optoelectronic applications.

Epitaxial thin films of topological insulator Bi2Te3 with two-dimensional weak anti-localization effect grown by pulsed laser deposition

We have grown high quality epitaxial topological insulator Bi2Te3 thin films on silicon (111) substrates by pulsed laser deposition. Systematic structural characterization of the films using X-ray diffraction and transmission electron microscopy has demonstrated that a low laser pulse rate is the key to achieving epitaxial films. The films show n-type metallic behavior, consistent with Te deficiency as determined by Rutherford backscattering spectrometry measurements. The A1g longitudinal optical phonon mode of Bi2Te3 was detected by time-resolved reflectivity measurements. A 2-dimensional (2-D) weak-antilocalization effect was also observed at low temperatures, which indicates the existence of topologically protected 2-D surface states in our thin films. This growth and characterization effort paves the way to fabricate multi-layer heterostructures of topological insulators along with ferromagnetic oxides and high temperature superconductors by the same growth technique in the search for physics arising from their interfacial couplings.

Ultrafast carrier and coherent phonon dynamics in semi-insulated and n-type 4H-SiC

We investigated ultrafast carrier and phonon dynamics in semi-insulated (SI) and n-type 4H-SiC using time-resolved reflectivity measurements. In the n-type 4H-SiC, carriers are excited by an inter-conduction band transition, and thermalized by electron-electron scattering within 20 fs. We observed coherent phonons of folded phonon modes, which become Raman active as a result of the zone folding, and those of A1-symmetry longitudinal optical (A1-LO) phonon mode. In the n-type SiC, the A1-LO coherent phonon forms a coupled mode with a plasmon, resulting in an asymmetrically broadened Fourier transform spectrum. The polarization dependence and the sine-type initial phase indicate that the impulsive stimulated Raman scattering is the mechanism generating coherent phonons of E2-symmetry transverse optical and A1-LO modes in both the SI and n-type 4H-SiC.

First-order Raman scattering in cylindrical wurtzite nanowire

Abstract We have presented a theoretical study on electron resonant Raman scattering (ERRS) process associated with the bulk longitudinal optical (LO), surface optical (SO) and quasi-confined (QC) phonon modes in a free-standing wurtzite nanowire (NW). We consider the Frohlich electron–phonon interaction in the framework of the dielectric continuum model. Numerical calculations on the GaN material reveal that differential cross-section (DCS) is sensitive to the wire size. The bulk LO and high-frequency quasi-confined (QC+) phonons make main contributions to the DCS and the impact of the SO phonon can be negligible in the ERRS process. Moreover, scattering intensity of the bulk LO phonon is strongly enhanced as the incident photon energy approaches the energy band-gap of the GaN.

Characterization and photonic absorption of hierarchical tree-like CdS nanostructure synthesized by solvothermal method

Hierarchical tree-like wurtzite CdS nanostructure was successfully synthesized by the 200°C and 24h solvothermal reaction of cadmium nitrate and thiourea as starting materials in 1:1 volume ratio of diethylenetriamine and ethylenediamine as a mixed solvent. Phase and morphology of the as-synthesized CdS product were characterized by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM), including its photonic absorption by UV–visible spectroscopy. XRD spectrum and TEM image showed hexagonal wurtzite CdS structure with a 1–2μm prototypical cluster, composed of a number of limbed nanorods. Its Raman spectrum was specified as the first-order (1LO) and second-order (2LO) longitudinal optical phonon modes at 298 and 599cm−1. UV–visible spectrum shows an absorption peak centered at around 489nm, assigned as the excitonic transition, blue shifted comparing to its bulk.

Representative longitudinal optical phonon modes in polar semiconductor quantum dots

Existence of representative longitudinal optical (LO) phonon modes is theoretically discussed for the case of polar semiconductor cylindrical quantum dots embedded in a semiconductor matrix. The approach is developed within the dielectric continuum model considering the Fröhlich interaction between electrons and the confined LO phonons. The theory is applied to cylindrical GaAs/AlAs quantum dots within an adiabatic treatment.

CHARACTERIZATION OFCdSexS1-xQUANTUM DOTS USING XRD, UV-VIS ABSORPTION AND RAMAN SPECTROSCOPY MEASUREMENTS

CdSexS1-xquantum dots (QDs) have been grown in borosilicate glass matrix using two step annealing technique and are characterized by AFM, XRD, UV-Vis absorption, HRTEM and Raman spectroscopy measurements. AFM studies show that nearly spherical clusters of about 5–10 nanocrystals were grown. From XRD measurements, it is found that the nanocrystals are grown in hexagonal phase. From HRTEM measurements, it is observed that the size of QDs increases from 5.1 to 9.5 nm with increasing annealing duration from 3 to 10 hrs. Blue shift in absorption spectrum indicates that quantum confinement increases with decreasing annealing durations. Raman spectrum of CdSexS1-xQDs shows two mode type of behavior i.e., CdSe like longitudinal optical phonon mode (LO1) and CdS like longitudinal optical phonon mode (LO2). It has been observed that with increasing the size of QDs from 5.1 to 9.5 nm, LO1shifts from 205 to 210 cm-1while LO2shifts from 278 to 291 cm-1. The shift in the LO1and LO2Raman peaks towards high frequencies has been interpreted due to increase in the lattice compressive strain with the increase in QDs size. The effect of phonon negative dispersion on the shift of LO1and LO2is found to be comparatively insignificant. It is also found that the ratio of intensity of overtones to the fundamental mode increases from 0.34 to 0.48 with decreasing QD size. This indicates that the electron–phonon coupling increases with the decrease in size of QD due to increasing overlap of electron and hole wave function in small size QDs. It is further observed that the frequency of the surface optical phonon mode remains unchanged as the size of QD changes but the intensity decreases with the increase in the size of QDs finally merging in LO mode Raman spectrum.

Synthesis of ZnTe Nanoparticles by Microwave Irradiation Technique, and Their Characterization

The solid state syntheses of cubic Zinc Telluride nanoparticles were successesfully achieved for the first time by microwave irradiation technique within ∼25 min as per the author's knowledge. The crystalline phase was confirmed by powder X-ray diffraction. The crystallite size was calculated and found to be ∼40 nm. Scanning electron microscope was also used to determine the size and shape of the nanoparticles, and the average particle size was found to be ∼60 nm with spherical shapes. The particle size distribution was in range of ∼50 to 70 nm. One transverse optic (TO) and two longitudinal optic (1LO and 2LO) phonon modes were observed at 167 and 205, 408 cm–1 in the FT-Raman spectrum. The optical band gap was calculated which has found to be 2.51 eV higher than the bulk ZnTe (E g = 2.26 eV), it reveals a clear blue shift with an energy difference 0.25 eV, this may be due to the crystallite size, exciton or charge carrier confinement.

How Quickly Does a Hole Relax into an Engineered Defect State in CdSe Quantum Dots

Intraband hole relaxation of colloidal Te-doped CdSe quantum dots is studied using state-selective transient absorption spectroscopy. The dots are excited at the band edge, and the defect band bleach caused by state filling of the hole is probed. Close to the defect energy, the hole relaxation is substantially slowed down, indicating a gap separating the defect state from the CdSe band edge. A clear dependence of the relaxation time with the QD's size is presented, implying that the hole relaxation is mediated by longitudinal optical (LO) phonon modes of the CdSe host. In addition, we find that overcoating the quantum dots by two monolayers of a ZnS shell extends the hole relaxation time by a factor of 2, suggesting a combined effect of LO phonons and surface effects governing intraband hole relaxation.

Structural and optical characterization of mechanochemically synthesized copper doped CdS nanopowders

Abstract Incorporation of copper into CdS crystals has been successfully prepared by mechanical alloying using a planetary ball mill. The powders are prepared with different milling times at 300 rpm with various Cu/Cd ratios from 0.1 to 25 at%. X-ray diffraction (XRD) analysis of milled powders showed peaks corresponding to hexagonal structure with a detection of phase transition to a cubic structure with increasing milling time. Grain sizes varied from 21 to 30 nm corresponding to different Cu/Cd ratios. Field emission scanning electron microscopy (FESEM) images reveal agglomerated materials with particle size of approximately 28 nm (5 Cu at%) and layered structures caused due to the milling process. Powder composition by energy dispersive analysis of X-rays (EDAX) reveals the incorporation of copper into the CdS. Micro Raman spectroscopy showed peaks approximately at 301 and 585 cm −1 corresponding to first and second order scatterings of longitudinal optical phonon mode. The LO mode at 301 cm −1 shifted towards lower wave number due to decrease of grain size by increase in milling time. From high resolution transmission electron microscope (HRTEM), the dominant phase of individual CdS nanocrystals was found to be hexagonal structure along with cubic structure.

Infrared absorption and Raman spectroscopy studies of InSbBi layers grown by liquid phase epitaxy

We report on the results of optical absorption and Raman spectroscopy measurements on InSbBi layers grown by liquid phase technique. A maximum Bi content of 0.4at.%, as measured by energy dispersive X-ray (EDX) technique, is used in the experiments. Optical absorption measurements made on the samples indicate a room temperature energy band gap reduction up to about 6meV with respect to undoped InSb layers grown by the same technique. Bi content calculated from this band gap reduction agrees with that obtained from EDX. A weak peak obtained at 152cm−1 in the Raman spectrum of the material is identified with the longitudinal optical phonon mode of InBi. Further a mode at 140cm−1 is observed due to isolated Bi atoms at the interstitial sites.

Raman spectroscopy of ZnS nanostructures

We report Raman scattering results of wurtzite ZnS nanowires, nanocombs, and nanobelts. The Raman spectrum obtained from ZnS nanowires exhibits first‐order phonon modes at 272, 284, and 350 cm−1, corresponding to A1/E1 transverse optical, E2 transverse optical, and A1/E1 longitudinal optical phonons, respectively. Several multiphonon modes are also observed. The longitudinal optical phonon mode varies in wavenumber for nanocombs and nanobelts, indicating that the residual strain varies during the morphological change from ZnS nanowires to nanocombs and ultimately to nanobelts. Interestingly, a surface optical (SO) phonon mode varies in wavenumber depending on the shape and surface roughness of the ZnS nanostructures. The surface modulation wavelengths of the ZnS nanowires, nanocombs, and nanobelts are estimated using the SO phonon dispersion relations and the observed SO phonon wavenumbers. Copyright © 2012 John Wiley & Sons, Ltd.

Optical phonon modes in ordered core-shell CdSe/CdS nanorod arrays

We investigate the optical phonons in ``dot-in-a-rod'' core--shell CdSe/CdS nanorods at wavelengths resonant with the optical transitions either in the core or in the shell. At a wavelength above the CdS band gap, only CdS phonon modes were detected. In contrast, at excitation in resonance with the core transitions, we observed phonon modes of both the CdSe core and the CdS shell. In laterally ordered nanorod assemblies, the CdS longitudinal-optical phonon mode manifested a low-energy shoulder that could be related to higher-order longitudinal-optical phonon modes. Furthermore, we report on surface-optical modes that originate from the tracklike superstructure of the nanorod assemblies.

The infrared fingerprint signals of silica nanoparticles and its application in immunoassay

Infrared absorption properties of silica nanoparticles were studied. The transverse optical and the longitudinal optical phonon modes from the silica were proved to be the characteristic spectroscopic fingerprint signals. Based on this, a sandwich-structured immunoassay was performed, and the detection of the analyte (human IgG) was achieved by using biofunctional silica nanoparticles as infrared probes. The immunoassay based on Fourier transform infrared reflection absorption spectroscopy of silica nanoparticles shows significant value for potential applications in many areas, such as biomedicine, food safety, and waste treatment.

Structural, optical and luminescence studies of ZnSe nanowires

Abstract ZnSe nanowires have been successfully synthesized through chemical vapor deposition assisted by laser ablation in a tube furnace on a silicon substrate coated with a gold film of 2 nm thickness. X-ray powder diffraction measurements reveal that the synthesized products had pure hexagonal wurtzite structure. The microstructures and chemical composition of the as-grown nanowires have been investigated by means of electron microscopy, energy dispersive spectroscopy, photoluminescence and Raman spectroscopy. The results reveal that the as-grown material consists of ZnSe nanowires with diameters ranging from 60 – 100 nm and with lengths up to several tens of micrometers. High resolution transmission electron microscopy and selected area electron diffraction indicated that as-synthesized nanowires were single crystalline in nature. Micro-photoluminescence studies on ZnSe nanowire reveal strong emission at 460 nm. The Raman peak at 251 cm−1 is attributed to the longitudinal optic phonon mode of ZnSe.

Spectroscopy of phonon modes of a single-wall armchair carbon nanotube using measurements of nonlinear conductance: Theory

We present a theoretical study of the differential conductance of a finite-length armchair single-wall carbon nanotube in the presence of electron–phonon interaction by employing a perturbative approach. It is shown that the nonlinear conductance measurement can be regarded as a novel method for detecting longitudinal optical phonon modes, which have non-negligible electron–phonon coupling in a proper applied bias range. Furthermore, it is found that at high temperatures, differential conductance depends on temperature, while this dependence is smeared out as temperature decreases.

Exciton–phonon interaction in cylindrical semiconductor quantum dots

The exciton–longitudinal optical phonon interaction is theoretically investigated for the case of polar semiconductor cylindrical quantum dots embedded in semiconductor matrix. The theory is developed within the dielectric continuum model considering the Fröhlich interaction between electrons and confined bulk longitudinal optical phonons for a configurational interaction model of quantum dot. Representative longitudinal optical phonon mode for the exciton–phonon interaction is predicted for cylindrical InAs/GaAs quantum dots.

Structural and dielectrical studies on mechano-chemically synthesized indium doped CdS nanopowders

Incorporation of indium (dopant) into CdS crystals have been successfully achieved by a mechanical alloying process. Powders are prepared with various In/Cd ratio from 1 to 10 at% and milled at 300 revolution per minute (rpm) for 60 min. X-ray diffraction (XRD) analysis of milled In doped CdS compound showed that the major phase of the product was wurtzite with grain sizes varying from 37 to 42 nm corresponding to change in In/Cd compositions. High resolution transmission electron microscopy (HRTEM) images as well as Fourier transformation in reciprocal space provide a good pathway to identify the structure of individual CdS nanocrystals, whose dominant phase was determined to be wurtzite structure along with zinc blende structure. Field emission scanning electron microscopy (FESEM) images reveal that CdS crystal prefers to grow along the (001) direction rather than (110) due to its high surface energy. The Raman spectra of CdS:In particles present well-resolved lines at approximately 303 and 600 cm−1, corresponding to the first and second-order scatterings, respectively, of the longitudinal optical (LO) phonon mode. Dielectrical studies showed that dielectrical constant (e′) decreased with increase in frequency, whereas AC conductivity (σAC) in In doped CdS increases with increase in frequency and also both the values increased with increase in doping concentration.

Synthesis, Structural and Optical Properties of PVP Encapsulated CdS Nanoparticles

Highly monodispersed CdS nanoparticles using polyvinyl pyrrolidone (PVP) as the capping agent were synthesised by chemical coprecipitation method. The surface-modified cadmium sulfide nanoparticles were found to be remarkably stable. In the presence of PVP, cubic phase with small grain size of CdS were observed in XRD. The peaks were identified to originate from (111), (220) and (311) planes of CdS, respectively. The crystallite size of the synthesised CdS nanoparticles was about 3 nm calculated from the (111) plane of XRD pattern and it was also confirmed through HRTEM. Morphology and elemental mapping of the synthesised nanoparticles were studied by SEM and EDX analyses. Increase in the band gap with decrease in the particle size was observed from the reflectance mode UV spectrum, which confirms the quantum confinement effect. From the photoluminescence studies, enhanced near-band-edge blue light emission and significantly reduced defect-related green emission were observed. Longitudinal optical (LO) phonon modes, corresponds to pure CdS were observed in Raman spectrum.

Origin and tuning of surface optic and long wavelength phonons in biomimetic GaAs nanotip arrays

Nano-texturization provides sensitive routes for selection of preferred phonon modes. Biomimetic gallium arsenide (GaAs) nano-tips, with a pencil-like structure, prepared by an electron cyclotron resonance plasma etching of planar GaAs wafer demonstrates tunable strength of the surface optic (SO), and long wavelength transverse optic and longitudinal optic phonon modes. These modes can be tuned as a function of the length (L) of the nano-tips enabling phonon engineering. Invalidation of symmetry rules due to nano-texturization results in the excitation of a SO mode that can also be tuned, in strength and position, with L. Red shift of this mode with a change in the dielectric constant of the medium (air to aniline) confirms the SO nature. The theoretically estimated length scales indicate that the diameter modulated apexes of the nano-tips, whose length (L’) increases consistently with L, could be responsible in transferring the required momentum to the SO phonons.