Photoluminescence and resonant Raman scattering in N-doped ZnO thin films

A combination of studies on photoluminescence and resonant Raman scattering in N-doped ZnO thin films were carried out at room temperature. In the photoluminescence spectra, a transformation of radiative recombination mechanism from free-exciton to donor-acceptor-pair transition was observed. An enhancement of resonant Raman scattering processes as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed in the Raman spectra. Also, the nature of the 1LO phonon underwent a transformation from a pure A₁(LO) mode to a quasimode with mixed A₁ and E₁ symmetry. The underlying mechanisms accounting for the influences of N doping on the optical properties of ZnO were related to the incorporation of extrinsic defects in the crystal lattice.     

Finding electromagnetic and chemical enhancement factors of surface-enhanced Raman scattering

The authors report two methods to determine electromagnetic and chemical enhancement factors in surface-enhanced Raman scattering (SERS), which are based on saturation property and decay dynamics of photoluminescence and concurrent measurements of photoluminescence and resonance Raman scattering intensities. Considerations for experimental implementation are discussed. This study is expected to facilitate the understanding of SERS mechanisms and the advancement of the usage of SERS in chemical and biological sensor applications.     

From anti‐Stokes photoluminescence to resonant Raman scattering in GaN single crystals and GaN‐based heterostructures

The progress on anti‐Stokes photoluminescence and Stokes and anti‐Stokes Raman scattering in GaN single crystals and GaN/AlN heterostructures is reviewed. Anti‐Stokes photoluminescence investigated in the past was primarily attributed to two‐photon absorption, three‐photon absorption, and phonon‐assisted absorption. On the other hand, anti‐Stokes Raman scattering was used to determine electron‐phonon scattering time and decay time constant for longitudinal‐optical phonons. In a typical high electron mobility transistor based on GaN/AlN heterostructures, strong resonances were reached for first‐order and second‐order Raman scattering processes. Therefore, both Stokes and anti‐Stokes Raman intensities were dramatically enhanced. The feasibility of laser cooling of a nitride structure has been demonstrated. Anti‐Stokes photoluminescence and Raman scattering have potential applications in upconversion lasers and laser cooling of nitride ultrafast electronic and optoelectronic devices.     

Annealing-induced structural transformation of gelatin-capped Se nanoparticles

Selenium nanoparticles were obtained by a new facile synthetic route from gelatin-stabilized aqueous colloidal solutions. The dependence of structural and optical properties of the nanoparticles upon duration of post-synthesis thermal treatment of Se sols was studied by optical absorption, photoluminescence and Raman scattering. We have shown that ageing of colloidal Se solutions at elevated temperatures results in an enlargement and crystallization of selenium nanoparticles, accompanied with noticeable changes in their optical spectra: an enhancement of the photoluminescence of the NPs, incorporated in gelatin films; a “red” shift of both the absorption threshold and emission band maximum; appearance of new peaks in Raman spectra, attributed to the formation of the crystalline phase in the course of the ageing.     

基于硫蒸气处理转移的单层二硫化钼的荧光增强

采用化学气相沉积法在SiO2/Si衬底上制备了单层MoS2,再通过300℃硫蒸气处理用聚甲基丙烯酸甲酯(Polymethyl Methacrylate,PMMA)转移下的单层MoS2.使用原子力显微镜、真空荧光检测和拉曼光谱等手段表征了样品的形貌和光致发光性能.结果表明:经过硫蒸气处理转移后的单层MoS2的光致发光强度比由化学气相沉积法制备的未处理的单层MoS2的光致发光强度增强了约5倍.光致发光强度增强是由于在硫蒸气处理过程中,单层MoS2的部分硫空位被硫原子纳米团簇所填补,从而提高了光致发光效率.此外,分别将单层MoS2转移到SiO2/Si衬底、石英、三氧化铝及氟化镁衬底再经过硫处理后,也观察到了类似的荧光增强现象.     

发射可见光多孔硅的拉曼光谱

本文报导了多孔硅的拉曼散射和光致发光的研究。给出了多孔硅的拉曼和光致发光谱之间的对应关系,根据拉曼峰的移动,估算了多孔硅量子线横截面的平均尺度为2.1～4.2nm。     

Plasmon resonance modulated photoluminescence and Raman spectroscopy of diindenoperylene organic semiconductor thin film

In a comparison between a bare diindenoperylene (DIP) film and a DIP film spin-coated with a layer of gold nanoparticles, we have investigated the influence of plasmon resonances in the gold particles on spectroscopic properties of the molecular film. Under off-resonant excitation with a laser at 633 nm, the bare DIP film showed only weak photoluminescence (PL) and Raman signals, but after spin-coating gold nanoparticles on such a DIP film, we found an enhancement of both the PL and Raman signals by a factor of about 3, whereas no enhancement could be observed when the same sample was excited with laser light of 488 nm. This difference reveals that at 633 nm, plasmon resonances in the gold nanoparticles are excited, leading in turn to an enhancement of PL and Raman signals of the weakly absorbing DIP film via coupling between plasmons in the gold particles and exciton-polaritons in the molecular film. For the laser at 488 nm, due to a much larger absorption coefficient of DIP, excitons in the molecular film are directly excited, out-weighing the influence of an off-resonant coupling to the plasmon resonances in the gold particles occurring at much lower energy.     

Synthesis and optical spectroscopy of ZnO nanowires

Single crystal ZnO nanowires with lengths and diameters ranging from 2 to 30 μm and 100 to 300 nm, respectively, have been grown by the vapor transport method on SiO₂/Si substrates using Au as catalyst. Their Raman and emission properties under different excitation wavelengths have been studied at the nanoscale. Whereas Raman measurements on nanowires corroborate the well-known ZnO phonon characteristics, their photoluminescence spectra exhibit a very broad emission band, mainly in the visible region from 450 to 800 nm, which corresponds to different defect-related recombination processes. Spectrally resolved scanning near-field optical microscopy, SNOM, of single ZnO nanowires have also been performed for a direct imaging of the photoluminescence emission with high spatial resolution below 100 nm, establishing a relationship with the simultaneously acquired topography.     

Size effects on phonon localization and Raman enhancement in silicon nanotips

Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic properties associated with their dimensionality. We here investigate the effect of size‐induced changes on phonon localization and explain the enhanced Raman response. The occurrence of normally forbidden transitions in the photoluminescence spectra provides evidence for the predicted localization effect. Spatially resolved Raman spectroscopy reveals a continuous change of the silicon Raman peak position and peak width along the nanotip that is attributed to a smooth change between bulk properties at the base to size‐induced phonon confinement in the apex of the nanotip. This approach allows to exclude heating effects that normally overwhelm the phonon confinement signature. The Raman spectra are in excellent agreement with the spatial correlation model and the extracted correlation length is comparable to the tip dimensions. The observed phonon confinement coincides with an enhancement of the Raman scattering efficiency at the tip apex and results in a 40‐fold increase of the sample's Raman intensity compared with bulk silicon. These results provide a step toward the integration of Si based optoelectronic devices. Copyright © 2012 John Wiley & Sons, Ltd.     

Visible photoluminescence from silicon nanopowders produced by silicon evaporation in a high-power electron beam

Silicon nanopowders produced by electron-beam-induced evaporation of a bulk silicon sample in an argon atmosphere are studied by the photoluminescence technique and Raman scattering spectroscopy. A photoluminescence peak in the visible region of the spectrum has been detected at room temperature in powders consisting of silicon nanocrystals. The strong short-wavelength shift of the photoluminescence peak can be attributed to the quantum size effect of electrons and holes in small silicon nanocrystals (about 2 nm). The size of silicon nanocrystals is determined by analyzing Raman spectra, and it is consistent with estimates obtained from photoluminescence data.     

入射激光功率对硅纳米线拉曼光谱及荧光光谱的影响

一维纳米材料硅纳米线是目前重要的光电材料之一,采用化学气相沉积法制备了硅纳米线,实验研究了不同功率532 nm激光激发下的拉曼光谱和荧光光谱,随着入射激光功率的增加,一阶拉曼光谱出现红移和非对称加宽,而且红移同入射激光功率成正比,光致荧光光谱出现蓝移和双峰结构。使用声子限域效应、应变效应和激光非均匀加热效应对实验结果进行了分析,并采用matlab模拟了入射激光功率同拉曼频移的理论关系曲线,结果表明激光非均匀加热效应是引起拉曼光谱和光致荧光光谱变化的主要原因。     

InGaN/GaN多量子阱热退火的拉曼光谱和荧光光谱

通过拉曼光谱及荧光光谱测量研究了采用低压金属有机化学沉积(MOCVD)方法生长的InGaN／GaN多量子阱高温快速热退火处理对量子阱光学性质的影响。观测到退火后InGaN／GaN量子阱的拉曼光谱E2,A1(LO)模式的峰位置出现了红移,而且该振动峰的半高宽也有微小变化。温度升高退火效果更明显。退火使量子阱内应力部分消除,同时In,Ga原子扩散出现相分离使拉曼谱表现出变化。在常温和低温下的光荧光谱表明,退火处理的量子阱发光主峰都出现了红移;而且低温退火出现红移,退火温度升高相对低温退火出现蓝移;同时在低温荧光光谱里看到经过退火处理后原发光峰中主峰旁边弱的峰消失了。讨论了退火对多量子阱光学性质的影响。     

a-C:N:H纳米尖端荧光产生的机理

用CH4,H2和NH3为反应气体,利用等离子体增强热丝化学气相沉积在沉积有碳膜的Si衬底上制备了a-C:N:H纳米尖端,并用扫描电子显微镜和微区Raman光谱仪对碳膜和纳米尖端进行了表征。结果表明:Raman谱中含有与碳和氮相关的峰,且纳米尖端的Raman谱比碳膜的Raman谱有很强的荧光背景。Raman谱中的峰说明沉积的碳膜和纳米尖端是a-C:N:H薄膜和a-C:N:H尖端。a-C:N:H纳米尖端的Raman谱中强荧光背景的产生表明其在激发光源照射的过程中发射了强荧光,对a-C:N:H纳米尖端产生强荧光的机理进行了探讨。     

Raman scattering of Zn doped CuGaS2 layers grown by vapor phase epitaxy

Raman spectra for non-site-selectively and site-selectively Zn-doped CuGaS₂ layers grown by vapor phase epitaxy (VPE) were investigated. Although an appearance of characteristic Raman line(s) related with the doped Zn atom was not seen, an enhancement of the Raman intensity ratio of the highest LO mode to the A₁ mode (I_LO/I_A1) was observed. The site-selectively Zn-doped layers with p-type conductivity exhibited larger I_LO/I_A1 ratio compared to those with n-type conductivity. The observed correlation between the I_LO/I_A1 ratio and the peak energy of the photoluminescence characteristic for Zn-doped p-type samples (L emission) suggests that the enhancement of I_LO/I_A1 is due to the increase of Zn atom substituting Ga site (Zn_Ga) which is acting as an acceptor.     

Electron-polariton scattering in semiconductor microcavities

In semiconductor microcavities, electron-polariton scattering has been proposed as an efficient process that can drive polaritons from the bottleneck region to the ground state, achieving Bose amplification of the optical emission. We present clear experimental observation of this process in a structure that allows control of the electron density and we report substantial enhancement of photoluminescence. We show that this enhancement is more effective at higher temperatures due to the different way that electron scattering processes either broaden or relax polaritons.     

Formation and Raman scattering of seed‐like ZnO nanostructure

This work demonstrates seed‐like ZnO nanostructure aggregated with nano‐entities using the colloid chemical method. The formation mechanism and optical properties of the ZnO nanostructure are studied. The X‐ray diffraction spectrum indicates that the ZnO nanostructures are polycrystalline pure wurtzite phase. Multiphonon processes in the nanostructure are identified from the Raman scattering spectra in the spectral range of 300–2000 cm⁻¹. A strong violet photoluminescence band emission was visible in the room temperature photoluminescence spectra. This work determines the quality of such crystals and extends the optical applications of ZnO nanostructures. Copyright © 2009 John Wiley & Sons, Ltd.     

Interplay between phonon confinement effect and anharmonicity in silicon nanowires

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role.     

Limiting efficiencies of nonlinear-optical processes in microstructure fibers

The ways of achieving limiting waveguide enhancement of nonlinear-optical processes in microstructure and photonic-crystal fibers are studied. The waveguide enhancement of nonlinear-optical processes is shown to be physically limited because of the competition of diffraction and refractive-index-step radiation confinement. In the case of the limiting refractive-index step values for fused silica fibers, the maximum waveguide enhancement of nonlinear-optical processes is achieved with submicron fiber core diameters. The maximum waveguide enhancement of coherent anti-Stokes Raman scattering in a hollow microstructure fiber relative to the regime of tight focusing is shown to scale as λ ²/α^a4 with radiation wavelength λ, the inner fiber radius a, and the magnitude of radiation losses α.     

PHOTOLUMINESCENCE AND RAMAN SCATTERING OF PURE GERMANIUM/SILICON SHORT PERIOD SUPERLATTICE

Pure Ge/Si short period superlattice (SPS) samples grown by gas source molecular beam epitaxy (GS-MBE) were studied by photoluminescence and Raman scattering. For SPS samples with Germanium layer thickness (L_Ge) of 1.5 monolayer (ML), a new band of photoluminescence is observed for silicon layer thickness (L_Si) in an intermediate range of 1.9-2.9 nm. In contrast to pure Ge/Si quantum wells, the energy of the new band shows a red-shift with the increase of L_Si. Raman scattering results show that when the intensity of the photoluminescence of the new band reaches a maximum, the Raman shift relating the vibration of Si-Si reaches a minimum. It is therefore considered that the new band of the pure Ge/Si SPS is related with some kind of strain relaxation process.