Effects of light localization in photoluminescence and Raman scattering in silicon nanostructures

The effect of light localization in photoluminescence (PL) and Raman scattering (RS) in silicon nanowires with diameter of 100 nm was investigated. The optical excitation was done by CW radiation of a YAG:Nd laser at 1.064 μm. The PL an RS intensities were found to increase strongly for the samples with Si nanowires in comparison with corresponding values of c-Si substrate. The effect is explained by an increase of the lifetime of photons in silicon nanowire structures.     

In situ study of epitaxial growth of ZnO nanowires at the junctions of nanowall networks on zinc particles

The authors show the real-time growth of ZnO nanowalls and nanowires on zinc particles via in situ observation in an environmental scanning electron microscopy. It was observed that a ZnO polycrystalline film is first deposited on zinc particles. The nanowires started to grow when the nanowalls had just formed and they grew epitaxially on the junctions of the nanowalls. The nanowalls and the nanowires grew together until the source of zinc was exhausted. The vapor–solid mechanism is deemed to be the growth mechanism as it quantitatively accounts for the growth speed of the nanowalls and nanowires observed in the experiment. Cathodoluminescence reveals that the growth at low zinc concentration leads to blue emission from defects, which may be zinc vacancies.     

Efficient synthesis of ZnO nanoparticles,nanowalls, and nanowires by thermal decomposition of zinc acetate at a low temperature

ZnO nanoparticles, nanowires, and nanowalls were synthesized rapidly on Si via thermal decomposition of zinc acetate by a modified chemical vapor deposition at a low substrate temperature of 200–250°C for the first time. The diameters of the synthesized nanoparticles and nanowires are around 100 and 30 nm, respectively, and the thickness of nanowalls is around 20 nm. High-resolution transmission electron microscopy shows that the nanowires as well as nanowalls are single-crystalline, and the nanoparticles are highly-textured poly-crystalline structures. Room-temperature photoluminescence spectra of the nanostructures show strong ultraviolet emissions centered at 368–383 nm and weak violet emissions at around 425 nm, indicating good crystal quality. The study provides a simple and efficient route to synthesize ZnO diverse nanostructures at low temperature.     

Coherent anti-Stokes Raman scattering in silicon nanowire ensembles

In this letter, we, for the first time, report on coherent anti-Stokes Raman scattering (CARS) spectroscopy of an ensemble of silicon nanowires (SiNWs) formed by wet chemical etching of crystalline silicon with a mask of silver nanoparticles. The fabricated SiNWs have diameter ranged from 30 to 200 nm and demonstrate both visible and infrared photolumine cence (PL) and spontaneous Raman signal, with their intensities depending on presence of silver nanoparticles in SiNWs. The efficiency of CARS in SiNW ensembles is found to be significantly higher than that in crystalline silicon. The results of CARS and PL measurements are explained in terms of resonant excitation of the electron states attributed to silicon nanoparticles.     

纳米晶粒团聚GaN纳米线A1(LO) Raman峰的非对称展宽

氮化镓(GaN)是一种直接带隙Ⅲ～Ⅴ族半导体化合物,具有较宽的禁带宽度(Eg=3.4eV),较高的热稳定性,以及抗辐照等特性[1-4],是制备近紫外和蓝光光电子器件、高速微电子器件的理想材料.制备低维纳米结构,研究其物理性质既是理解低维量子现象的要求,也是未来纳米电子器件发展的需要.近年来,很多小组已经通过不同的方法成功地制备出GaN纳米棒、纳米线等一系列一维材料.这些方法包括碳纳米管辅助的方法[5]、电弧放电的方法[6]、激光刻蚀的方法[7]、升华法[8]、高温分解法以及化学气相沉积(CVD)[9-15].其中CVD方法以其制备过程简单,制备材料晶…     

Effects of gas pressure on the synthesis and photoluminescence properties of Si nanowires in VHF-PECVD method

Silicon nanowires (SiNWs) were grown on an Au-coated Si(111) substrate at various gas pressures by very high frequency plasma enhanced chemical vapor deposition via the vapor–liquid–solid mechanism. The synthesized SiNWs were characterized by field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, Raman spectroscopy and photoluminescence (PL). The SiNWs were sharp needle-shaped and possessed highly crystalline core and oxide amorphous shell. As the gas pressure increases from 70 mtorr to 85 mtorr, the average diameter of the SiNWs decreases from 250 nm to 70 nm. Furthermore, the density of the nanowires increases with the gas pressure. The PL spectra revealed a peak at about 400 nm and a broadband emission at about 700 nm.     

Raman characterization of high-pressure phase transition in AlN nanowires

High-pressure phase transition of AlN nanowires was investigated in the range of 0-33.1 GPa by in situ Raman spectrum method in diamond anvil cell (DAC). The A1 (LO) vibration mode exhibits considerably asymmetry and broadening, indicating the occurrence of wurtzite-to-rocksalt phase transition. The Raman signal of high-pressure phase can be assigned to the disorder activated Raman scattering of rocksalt AlN. After fully releasing pressure, the Raman characterization of high-pressure phase was quenched. According to the pressure dependence of phonon frequency of AlN nanowires, the difference of transiton path between AlN nanowires and bulk materials was discussed and the mode Grüneisen parameters were determined.     

Synthesis of ZnO nanowires on aluminum flake by aqueous method

ZnO nanowires are grown on aluminum flake at low temperature by using a simple aqueous solution method. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscope (TEM) are applied to determine the as-grown ZnO nanowires morphology and crystal structures. The results show that the ZnO nanowires have wurtzite structure, and the diameter and length of the nanowire are 30 nm and more than 1.5 μm, respectively. Photoluminescence spectroscopy (PL) and Raman spectrum reveal the nanowires have good optical properties with low tensile stress. Meanwhile, photoelectrochemical cell (PEC) study verifies that ZnO nanowires as photoanodes are relatively stable in the photo-oxidation process, which could be a promising technique for practical applications.     

The effect of oxygen partial pressure on the growth of ZnO nanostructure on Cu0.62Zn0.38 brass during thermal oxidation

Cu_0.62Zn_0.38 brass foils were thermally oxidized at 500 °C under various gaseous environments, including nitrogen, air and mixture of N₂–O₂ at a pressure of 1 atm for 3 h. The oxidized specimens were characterized with a scanning electron microscope, an X-ray diffractometer and a transmission electron microscope. Optical property of oxidized products was characterized by photoluminescence. It is found that the nanowires and/or nanowalls are formed from hexagon ZnO crystals under N₂–O₂ gases and air. However, nanowires and nanowalls cannot form on the specimens oxidized in N₂. When oxygen partial pressure of environment is over a critical value, increasing oxygen partial pressure is of disbenefit to the growth of nanowires and nanowalls and reduces the thickness of oxide scale on a specimen. The experimental results are explained by the defect equilibrium theory of oxide scale and the compressive growth stresses occurred during oxidation.     

Photoluminescence of GeO<Subscript>2</Subscript> films containing germanium nanocrystals

Germanium nanocrystals were formed in a GeO₂ film during the process of germanium monoxide gas-phase deposition onto a sapphire substrate and studied by photoluminescence (PL) and Raman scattering spectroscopy. A PL peak in this heterosystem was observed in the visible region at room temperature. The sizes of Ge nanocrystals were estimated from the position of a Raman peak corresponding to scattering by localized optical phonons in germanium. The PL peak position calculated with allowance for the electron and hole size quantization in Ge nanocrystals coincides well with the experimentally observed position of this peak.     

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。     

GaAs纳米线晶体结构及光学特性

采用分子束外延技术在N-型Si (111)衬底上利用自催化生长机制外延砷化镓(GaAs)纳米线,对生长的纳米线进行扫描电子显微镜测试,纳米线垂直度高,长度直径均匀度好.对纳米线进行光致发光(photoluminescence, PL)光谱测试,发现低温10 K下两个发光峰P1和P2分别位于1.493 eV和1.516 eV,推断可能是纤锌矿/闪锌矿(WZ/ZB)混相结构引起的发光以及激子复合引起的发光;随着温度升高,发现两峰出现红移,并通过Varshni公式拟合得到变温变化曲线.对纳米线进行变功率PL光谱测试,发现P1位置的峰位随功率增加而蓝移,而P2位置的峰位不变.通过拟合发现P1峰位与功率1/3次方成线性相关,判断可能是WZ/ZB混相结构引起的Ⅱ型发光;同时,对P2位置的峰位进行拟合,P2为激子复合发光.对纳米线进行拉曼光谱测试,从光谱图中发现GaAs WZ结构特有的E_2声子峰,因此证明生长出的纳米线为WZ/ZB混相结构,并通过高分辨透射电子显微镜更直观地观察到纳米线的混相结构.     

Growth,structure, and cathodoluminescence of Dy-doped ZnO nanowires

Dy-doped ZnO nanowires have been prepared using high-temperature and high-pressure pulsed-laser deposition. The morphology, structure, and composition of the as-prepared nanostructures are characterized by field emission scanning electron microscopy, X-ray diffraction, Raman scattering spectrometry, X-ray photoelectron spectrometry, transmission electron microscopy, and energy dispersive X-ray spectroscopy. The alloying droplets are located at the top of the as-prepared Dy-doped ZnO nanowires, which means that the growth of the Dy-doped ZnO nanowires is a typical vapor-liquid-solid process. The luminescence properties of Dy-doped ZnO nanowires are characterized by cathodoluminescence spectra and photoluminescence spectra at low temperature (8 K). Two peaks at 481 and 583 nm, respectively, are identified to be from the doped Dy³⁺ ions in the CL spectra of Dy-doped ZnO nanowires.     

GaN直纳米线的光学性能

对 Ga N直纳米线的拉曼光谱及光致发光光谱进行了研究。拉曼光谱表明 ,与计算值相比 ,E2 ( high)声子频率在 560 cm- 1有 -9cm- 1的移动 ,这种声子频率显示出向低能带频移及带变宽的特征 ,是由于纳米尺寸效应所引起的结果。体系的光致发光光谱在 3 44 .8nm附近的近带隙发光 ,与文献报道的 Ga N体材料的数值3 65nm相比有一蓝移 ,这是由于量子限制效应造成的     

Raman study of activated quasi-modes due to misorientation of ZnO nanowires

We present Raman scattering study of wurtzite ZnO nanowires deposited by metal–organic chemical vapor deposition (MOCVD) on (0001) sapphire. It is shown that the misorientation of the nanowires, i.e. the inclination with respect to the vertical direction, mixes A1 and E1 Raman modes, giving rise to new modes for which the propagation directions make various angles with the C-axis of ZnO nanowires. Two particular bands depending on the tilt of the nanowires are observed at 400 and 585 cm^?1. They are attributed to TO and LO quasi-phonons, and explained using Loudon's model. Morphological information of the nanowires was extracted and an average orientation of nanowires is calculated.     

Raman spectroscopic and photoluminescence study of single-crystalline SnO2 nanowires

Single-crystalline SnO₂ nanowires with sizes of 4-14 nm in diameter and 100-500 nm in length were produced in a molten salt approach by using hydrothermal synthesized precursor. Structural characters of the nanowires were examined by X-ray diffraction and high-resolution electron transmission microscopy. Raman, photoluminescence and X-ray photoelectron spectra of the samples were examined under heat treatments. Three new Raman modes at 691, 514 and 358 cm⁻¹ were recorded and assigned. The former two are attributed to activation of original Raman-forbidden A_2uLO mode and the third is attributed to defects in small-sized nanowires. A strong photoluminescence is observed at about 600 nm, the temperature effects is examined and the origin of the PL process is discussed via X-ray photoelectron spectra.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Photoluminescence and Raman spectra of SnO<Subscript>x</Subscript> nanostructures doped with Sm ions

The photoluminescence (PL) and Raman scattering of SnO_x nanoparticles deposited from vapor phase have been studied. The PL spectra are characterized by a two-band structure. The high-energy band in the range from 300 to 350 nm is due to the exciton pair annihilation and may characterize the band gap of SnO_x nanocrystals as a function of their diameter. In the red spectral region (from 600 to 700 nm), a luminescence band due to defects in nanocrystals manifests itself. The existence of defects in SnO_x nanostructures is confirmed by Raman spectroscopy. Doping of SnO_x nanoparticles with rare earth (samarium) atoms leads to the appearance of strong luminescence lines in the red region of the PL spectrum.     

Preparation,characterization, and electrochemical performances of graphene/Ni(OH)2 hybrid nanomaterials

We report a class of core–shell nanomaterials that can be used as efficient surface-enhancement Raman scattering (SERS) substrates. The core consists of silver nanowires, prepared through a chemical reduction process, that are used to capture 4-mercaptobenzoic acid (4-MBA), a model analyte. The shell was prepared through a modified Stöber method and consists of patchy or full silica coats. The formation of silica coats was monitored via transmission electron microscopy, UV–visible spectroscopy, and phase-analysis light-scattering for measuring effective surface charge. Surprisingly, the patchy silica-coated silver nanowires are better SERS substrate than silver nanowires; nanomolar concentration of 4-MBA can be detected. In addition, “nano-matryoshka” configurations were used to quantitate/explore the effect of the electromagnetic field at the tips of the nanowire (“hot spots”) in the Raman scattering experiment.     

Investigation of thermostability and phonon–phonon interactions in Mo6S3I6 nanowires by Raman scattering spectroscopy

Detailed Raman scattering measurements were performed on molybdenum–sulfur–iodine nanowires (Mo₆S₃I₆). At room temperature, 21 well‐resolved Raman modes were experimentally observed for the first time in this new compound. The phase stability and vibrational properties of the nanowires were investigated by different temperature treatments. High‐temperature Raman spectra showed that the phase separation of Mo₆S₃I₆ nanowires took place between 573 and 673 K, followed by appearance of a new mode at 819 cm⁻¹ characteristic of the MoO₃ phase. Low‐temperature Raman scattering spectra showed a significant difference in phonon–phonon interactions between internal and external Raman modes of Mo₆S₃I₆ nanowires. These interesting vibrational properties can give new insights for improved material preparation and achievement of higher conductivity and other functional properties of these otherwise interesting materials. Copyright © 2009 John Wiley & Sons, Ltd.