Temperature dependent phonon confinement in silicon nanostructures

Temperature dependent variation in Raman line-shape from silicon (Si) nanostructures (NSs) is studied here. Asymmetry and red-shift in room temperature Raman spectrum is attributed to phonon confinement effect. Raman spectra recorded at higher temperatures show increase in FWHM and decrease in asymmetry ratio with respect to its room temperature counterpart. Theoretical Raman line-shape analyses of temperature dependence of phonon confinement is done by incorporating the temperature dependence of phonon dispersion relation. Experimental and theoretical temperature dependent Raman spectra are in good agreement.     

Multiple Fano resonances in nanorod and nanoring hybrid nanostructures

Multiple Fano resonances of plasmonic nanostructures have attracted much attention due to their potential applications in multicomponent biosensing. In this paper, we propose a series of hybridized nanostructures consisting of a single nanoring and multiple nanorods to generate multiple Fano resonances. One to three Fano resonances are achieved through tuning the number of nanorods. The interaction coupling process between different components of the nanostructures is recognized as the mechanism of multiple Fano resonances. We also theoretically investigate the applications of the produced multiple Fano resonances in refractive index sensing. The specific properties of multiple Fano resonances will make our proposed nanostructures beneficial to high-sensitivity biosensors.     

Surface enhanced Raman scattering and photoluminescence properties of catalytic grown ZnO nanostructures

Sword-like (diameter ranging from 40 nm to 300 nm) and needle-like zinc oxide (ZnO) nanostructures (average tip diameter ∼40 nm) were synthesized on annealed silver template over silicon substrate and directly on silicon wafer, respectively via thermal evaporation of metallic zinc followed by a thermal annealing in air. The surface morphology, microstructure, chemical analysis and optical properties of the grown samples were investigated by field emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray analysis, room temperature photoluminescence and Raman spectroscopy. The sword-like ZnO nanostructures grown on annealed silver template are of high optical quality compared to needle-like ZnO nanorods for UV emission and show enhanced Raman scattering.     

Porous silicon as efficient surface enhanced Raman scattering (SERS) substrate

Silver nanocrystallites are obtained through immersion of porous silicon samples in AgNO₃ solutions and a successive thermal annealing. The efficiency of nanostructures as surface enhanced Raman scattering (SERS) substrates is checked on cyanine-based dyes and horseradish peroxidase, evidencing detectable concentrations as low as 10⁻⁷ to 10⁻⁸ M. The substrate efficiency is strictly related to the Ag particle morphology, which could yield to either local surface plasmons (LSP) coupled to individual particles or to inter-particle short-range interaction.     

Electron-phonon interaction in boron-doped silicon nanocrystals: Effect of Fano interference on the Raman spectrum

Raman spectroscopy is employed for studying silicon nanocrystal arrays in boron-doped amorphous silicon films. The nanocrystals were formed in the initial amorphous films by the pulsed impact of an excimer laser. The electron-phonon interaction effects are observed experimentally in the heterostructure formed by a silicon nanocrystal and an amorphous matrix. These effects can be described in the framework of the familiar Fano interference model.     

Raman investigation of the electron-phonon interaction in n-type silicon nanocrystals

Silicon nanocrystals prepared in phosphorus-doped (at a concentration of 3.3 × 10²⁰ cm^?3) amorphous silicon films under pulsed irradiation with an excimer laser are studied using Raman spectroscopy and electron microscopy. The experimental data can be interpreted in terms of the Fano interference as a manifestation of the electron-phonon interaction effects in n-type silicon nanocrystals. It is assumed that a strong electron-phonon interaction (as compared to similar interactions in n-type bulk silicon) is due to the weakening of the momentum selection rules in nanocrystals.     

硅纳米线拉曼光谱的研究

声子限制效应会引起本征硅纳米线拉曼光谱红移及不对称宽化,但研究发现其并非引起硅纳米线拉曼光谱改变的主要因素。研究表明,由于在拉曼光谱测量中,通常使用的入射激光功率都在5 mW以上,激光加热会导致很高的局部温度,从而引起拉曼光谱大幅度红移并对称宽化,这是硅纳米线拉曼光谱红移的主要影响因素。另外,激光功率很高时,由激光激发的载流子会与声子发生Fano型干涉,从而使硅纳米线拉曼光谱发生Fano型红移和不对称宽化。除此之外,对小直径本征硅纳米线,声子限制效应导致波矢选择定弛则弛豫,使不在布里渊区中心的声子也可以参与拉曼散射,因而其拉曼光谱中除常见的几个拉曼峰外还会出现新拉曼峰。     

Entanglement via tunable Fano-type interference in asymmetric semiconductor quantum wells

Entanglement is realized in asymmetric coupled double quantum wells (DQWs) trapped in a doubly resonant cavity by means of Fano-type interference through a tunneling barrier, which is different from the previous studies on entanglement induced by strong external driven fields in atomic media. We investigate the generation and evolution of entanglement and show that the strength of Fano interference can influence effectively the degree of the entanglement between two cavity modes and the enhanced entanglement can be generated in this DQW system. The present investigation may provide research opportunities in quantum entangled experiments in the DQW solid-state nanostructures and may result in a substantial impact on the technology for entanglement engineering in quantum information processing.     

Superheating of silicon nanocrystals observed by Raman spectroscopy

In the Raman spectra of silicon nanocrystals a new anomalous component was detected. Close to the usual first order Raman peak situated for a bulk crystal at 521 cm⁻¹ at room temperature, two peaks arise shifting towards lower energy and demonstrating a huge temperature increase, as measured by the ratio of the Stokes/anti-Stokes peak intensities. This behavior is dependent on the laser power and on the morphology of the nanocrystals. We can exclude, however, confinement effects, although surface enhanced phonon modes could be responsible of such superheating. Alternative explanations are also suggested and discussed.     

Probing disorder and charged impurities in graphene by Raman spectroscopy

Disorder and doping can strongly affect the properties of graphene. Here we analyze these effects on several samples by Raman spectroscopy. In particular, we show that pristine and unprocessed graphene samples deposited on silicon, covered with a thin silicon oxide layer, show strong variations in their Raman spectra, even in absence of disorder. The variation in the Raman parameters is assigned to charged impurities. This shows that as‐deposited graphene is unintentionally doped, reaching charge concentrations up to 10¹³ cm^–2 under ambient conditions. The doping varies from sample to sample and the charges are inhomogeneously distributed on a submicron scale. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two-state double-continuum Fano resonance at the Si(100) surface

Fano resonances are well-known manifestations of the interference between a direct and an indirect ionization process. Here we report on a more complicated interference pattern observed in two-photon photoemission at the Si(100) surface. This two-dimensional Fano profile involves two discrete surface resonances which couple as initial and intermediate states to the silicon valence and conduction band, respectively. Tuning the photon energy across the surface resonance reveals asymmetric line profiles with pronounced destructive interference in the two-photon photoelectron intensities of both initial and intermediate states. The interference pattern is explained by an analytic extension of Fano's model to describe the coupling of two discrete states with two continua. This coupling strongly modifies the photoabsorption and is of general importance for light conversion in nanostructures and light-harvesting devices.     

The influence of boron concentrations on structural properties in disorder silicon films

In this work we present a detailed structural of a series of B-doped hydrogenated microcrystalline silicon (μc-Si:H) films deposited by plasma-enhanced chemical vapor deposition (PECVD) and B-doped polycrystalline silicon (poly-Si) films produced by step-by-step laser crystallization process from amorphous silicon. The influence of doping on the structural properties and structural changes during the sequential crystallization processes were monitored by Raman spectroscopy. Unlike μc-Si:H films, that consist of a two-phase mixture of amorphous and ordered Si, partially crystallized sample shows a stratified structure with polycrystalline silicon layer at the top of an amorphous layer. With increasing doping concentration the LO-TO phonon line in poly-Si shift to smaller wave numbers and broadens asymmetrically. The results are discussed in terms of resonant interaction between optical phonons and direct intraband transitions known as a Fano resonance. In μc-Si:H films, on the other hand, the Fano effect is not observed. The increase of doping in μc-Si:H films suppressed the crystalline volume fraction, which leads to an amorphization in the film structure. The structural variation in both μc-Si:H and poly-Si films leads to a change in hydrogen bonding configuration.     

Cancellation of Raman pulse walk-off by slow light

We theoretically demonstrate in a nonlinear optical fiber system with a narrowband Raman gain that pulse walk-off between the pump and the Raman Stokes waves can be fully compensated for by Raman slow light, leading to group-velocity matching between the interacting waves, greater useful interaction length, and thereby enhanced Raman amplification efficiency. Limitations due to Kerr effect are further discussed.     

Raman spectroscopy for study of interplay between phonon confinement and Fano effect in silicon nanowires

A combined effect of doping (type and species) and size on Raman scattering from silicon (Si) nanowires (NWs) has been presented here to study interplay between quantum confinement and Fano effects. The SiNWs prepared from low doping Si wafers show only confinement effect, as evident from the asymmetry in the Raman line‐shape, irrespective of the doping type. On the other hand SiNWs prepared from wafer with high doping shows the presence of electron–phonon interaction in addition to the phonon confinement effect as revealed from the presence of asymmetry and antiresonence in the corresponding Raman spectra. This combined effect induces an extra asymmetry in the lower energy side of Raman peak for n‐type SiNWs whereas the asymmetry flips from lower energy side to the higher energy side of the Raman peak in p‐type SiNWs. Such an interplay can be represented by considering a general Fano‐Raman line‐shape equation to take care of the combined effect in SiNWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Silver-Doping Induced Lattice Distortion in TiO2 Nanoparticles

The Ag-doping effects on Ti02 nanoparticles are investigated by means of x-ray diffraction （XRD） and Raman scattering spectroscopy. XRD and Raman results indicate that Ag-doping stabilizes the rutile phase in TiO2. We find an Ag-doping induced lattice expansion in both anatase and rutile phases. The Ag-doping has different influences on the lattice distortion for anatase and rutile phases, that is, the e/a-value for the anatase phase decreases with 0.5% Ag-doping and then increases with 1~ Ag-doping while that for the rutile phase shows a gradual increase with increasing Ag-doping. We have ascribed the different variations of lattice distortion due to Ag-doping to the change of interracial interaction between the anatase and rutile phases induced by different Ag concentrations.     

Methods for increasing the efficiency of nonlinear optical interactions in nanostructured semiconductors

Methods for increasing the efficiency of the optical second-and third-harmonic generation in gallium phosphide and silicon nanostructures formed by electrochemical etching of crystalline semiconductors are discussed. The efficiency of nonlinear optical interactions can be increased by using phase matching in anisotropic nanostructured semiconductors that exhibit form birefringence or by increasing the local field, as in scattering in macroporous semiconductors. The efficiencies of third-harmonic generation in porous silicon and of second-harmonic generation in porous gallium phosphide are found to increase by more than an order of magnitude.     

Electronic transport for armchair graphene nanoribbons with a potential barrier

We theoretically investigate the electronic transport properties through a rectangular potential barrier embedded in armchair-edge graphene nanoribbons (AGNRs) of various widths. Using the Landauer formula and Dirac equation with the continuity conditions for all segments of wave functions at the interfaces between regions inside and outside the barrier, we calculate analytically the conductance and Fano factor for the both metallic and semiconducting AGNRs, respectively. It is shown that, by some numerical examples, at Dirac point the both types of AGNRs own a minimum conductance associated with the maximum Fano factor. The results are discussed and compared with the previous relevant works.     

Fabrication and Characterization of Si Nanocrystals Synthesized by Electron Beam Evaporation of Si and SiO2 Mixture

Silicon nanocrystals synthesized by electron beam （e-beam） evaporation of Si and SiO2 mixture are studied. Rutherford backscattering spectrometry of the as-deposited Si-rich silicon dioxide or oxide （SRO） thin film shows that after evaporation, the Si and SiO2 concentration is well kept, indicating that the e-beam evaporation is suitable for evaporating mixtures of Si and SiO2. The SRO thin films are annealed at different temperatures for two hours to synthesize silicon nanoerystals. For the sample annealed at 1050℃, silicon nanoerystals with different sizes and the mean diameter of 4.5 nm are evidently observed by high resolution transmission electron microscopy （HRTEM）. Then the Raman scattering and photoluminescence spectra arising from silicon nanocrystals are further confirmed the above results.     

One-phonon-assisted resonant electron Raman scattering of GaAs quantum dots in an AlAs matrix

We have presented a theoretical calculation of the differential cross section (DCS) for the electron Raman scattering (ERS) process associated with the bulk-like longitudinal optical (LO) and interface optical (IO) phonon modes in semiconductor quantum dots (QDs). Electron states are considered to be confined within the QDs. We consider the Fröhlich electron-phonon interaction in the framework of the dielectric continuum approach. We study selection rules for the processes. Some singularities in the Raman spectra are found and interpreted. A discussion of the phonon behavior for QDs with large and small size is presented. The numerical results are also compared with that of experiments.     

Fano resonances in heterogeneous dimers of silicon and gold nanospheres

We theoretically investigate the optical properties of dimers consisting of a gold nanosphere and a silicon nanosphere. The absorption spectrum of the gold sphere in the dimer can be significantly altered and exhibits a pronounced Fano profile. Analytical Mie theory and numerical simulations show that the Fano profile is induced by constructive and destructive interference between the incident electric field and the electric field of the magnetic dipole mode of the silicon sphere in a narrow wavelength range. The effects of the silicon sphere size, distance between the two spheres, and excitation configuration on the optical responses of the dimers are studied. Our study reveals the coherent feature of the electric fields of magnetic dipole modes in dielectric nanostructures and the strong interactions of the coherent fields with other nanophotonic structures.