Surface-enhanced Raman scattering and polarized photoluminescence from catalytically grown CdSe nanobelts and sheets

We have successfully fabricated single-crystalline CdSe nanowires, nanobelts, and sheets by a chemical vapor deposition (CVD) method assisted with laser ablation. The synthesized CdSe nanostructures have hexagonal wurtzite phase as characterized by X-ray diffraction (XRD). CdSe nanobelts can range in length from several tens to a hundred micrometers, in thickness from 40 to 70 nm, and a tapered width which is approximately 3 microm at one end and tapers off to approximately 100 nm at a catalytic gold particle. Both selected area electron diffraction (SAED) and high-resolution transmission electron microscopic (HRTEM) measurements show that the single-crystalline hexagonal belts and sheets grew along the [0.1-1.0] direction with side surface of +/-(0 0 0 1) and top surface of +/-(2 -1 -1 0). While the growth mechanism of nanobelts complies with a combination of vapor-liquid-solid (VLS) and vapor-solid (VS) processes, the formation of sheets is primarily based on the VS mechanism. For comparison, the phonon modes of CdSe nanobelts and bulk powder have been measured by surface-enhanced Raman scattering (SERS) and normal Raman scattering (NRS) spectroscopies with off- and near-resonant excitations. A blue-shift of 2.4 cm(-1) for the longitudinal optical (LO) phonon of CdSe nanobelts, relative to bulk CdSe, is attributed to a lattice contraction in the belt structure, which is confirmed by the XRD measurement. Room-temperature microphotoluminescence (PL) at approximately 1.74 eV from single CdSe nanobelts shows a 3-fold enhancement compared to that from bulk CdSe powder and displays a partial polarization dependence of emission angles.     

Reaction growth of MF2/a-C (M = Ca, Mg) core/shell nanowires at the interface of vapor and solid reactants

C(6)F(6) vapor is employed to react with CaC(2) and Mg(3)N(2) to grow CaF(2)/a-C and Mg(2)F(2)/a-C core/shell nanowires (tens of micrometers in length, tens to hundreds of nanometers in wire diameter, and tens of nanometers in core diameter), respectively, in high yields. The growth mechanism is proposed to proceed via a reaction at the interface of the vapor and solid reactants.     

High-quality luminescent tellurium nanowires of several nanometers in diameter and high aspect ratio synthesized by a poly (vinyl pyrrolidone)-assisted hydrothermal process

Large-scale selective synthesis of uniform single crystalline tellurium nanowires with a diameter of 4-9 nm, and microbelts with a width of 250-800 nm and tens of micrometers in length, can be realized by a poly (vinyl pyrrolidone) (PVP)-assisted hydrothermal process. The formation of tellurium nanowires and nanobelts in the presence of PVP is strongly dependent on the reaction conditions such as temperature, the amount of PVP, and reaction time. The results demonstrated that the keys for selective synthesis of Te nanobelts and nanowires are to modulate the growth rates of (100), (101), and (110) planes in the presence of PVP and to precisely control the reaction kinetics. High-quality luminescent ultrathin t-Te nanowires with a diameter of 4-9 nm display strong luminescent emission in the blue-violet region. This approach provides a facile route for the production of high-quality tellurium nanostructures with an interesting optical property. Furthermore, the synthesized ultrathin nanowires with deep blue color and nanobelts in gray color by this approach can be well dispersed in water or ethanol, making it possible for further engineering of their surfaces to prepare other hybrid core-shell nanostructures.     

Growth of Cu2S ultrathin nanowires in a binary surfactant solvent

We demonstrate a facile solution-phase method for the synthesis of single-crystal, high aspect ratio, and ultrathin nanowires of hexagonal-phase Cu2S by thermal decomposition of CuS2CNEt2 in a mixed surfactant solvent of dodecanethiol and oleic acid at 160 degrees C. Cu2S nanowires can be controllably synthesized with a diameter as thin as 1.7 nm and length up to tens of micrometers; they are usually aligned in the form of bundles with a thickness of hundreds of nanometers. Based on the experimental results, the formation mechanism of the ultrathin nanowires has been properly proposed. Some key synthetic parameters, which have a significant effect on the sizes and shapes of the products, have also been investigated in detail. UV-vis spectroscopy measurement reveals that the resultant ultrathin nanowires show a strong quantum size effect.     

Catalyst-assisted vapor-liquid-solid growth of single-crystal Ga2O3 nanobelts

Mass production of quasi-one-dimensional gallium oxide nanobelts is accomplished through graphite-thermal reduction of a mixture of gallium oxide powders and SnO2 nanopowders under controlled experimental conditions. Sn nanoparticles are located at or close to the tips of the nanobelts and served as the catalyst for the nanobelt growth by a vapor-liquid-solid mechanism. The morphology and microstructure of the nanobelts were characterized by scanning electron microscopy and high-resolution transmission electron microscopy. The Ga2O3 nanobelts grow along the [104] direction, the widths ranged from several tens to several hundreds of nanometers, and the lengths ranged from several tens to several hundreds of micrometers. The growth of Ga2O3 nanobelts is initiated by Sn nanoparticles via a catalyst-assisted vapor-liquid-solid process, which makes it possible to control the sizes of Ga2O3 nanobelts.     

PLD-assisted VLS growth of aligned ferrite nanorods, nanowires, and nanobelts-synthesis, and properties

We report here a systematic synthesis and characterization of aligned alpha-Fe2O3 (hematite), epsilon-Fe2O3, and Fe3O4 (magnetite) nanorods, nanobelts, and nanowires on alumina substrates using a pulsed laser deposition (PLD) method. The presence of spherical gold catalyst particles at the tips of the nanostructures indicates selective growth via the vapor-liquid-solid (VLS) mechanism. Through a series of experiments, we have produced a primitive "phase diagram" for growing these structures based on several designed pressure and temperature parameters. Transmission electron microscopy (TEM) analysis has shown that the rods, wires, and belts are single-crystalline and grow along <111>m or <110>h directions. X-ray diffraction (XRD) measurements confirm phase and structural analysis. Superconducting quantum interference device (SQUID) measurements show that the iron oxide structures exhibit interesting magnetic behavior, particularly at room temperature. This work is the first known report of magnetite 1D nanostructure growth via the vapor-liquid-solid (VLS) mechanism without using a template, as well as the first known synthesis of long epsilon-Fe2O3 nanobelts and nanowires.     

Low-temperature synthesis of single crystalline Ag2S nanowires on silver substrates

We report on the successful synthesis of silver sulfide (Ag(2)S) nanowires by a simple and mild gas-solid reaction approach. For the nanowire synthesis, a preoxidized silver substrate is exposed to an atmosphere of an O(2)/H(2)S mixture at room temperature or slightly above. The resulting Ag(2)S nanowires are phase pure with a monoclinic crystal structure and have diameters of a few tens of nanometers and lengths up to 100 mum. The influence of reaction conditions on the diameter, length, and morphology of the Ag(2)S nanowires has been studied by a number of structural and spectroscopic techniques. The nanowire growth mechanism on the Ag substrate has been discussed, which is likely characterized by continuous deposition at the tip. Additionally, we demonstrate thinning and cutting of individual Ag(2)S nanowires with electron beams and laser beams, which are potentially useful for nanowire manipulation and engineering.     

Selective synthesis of single-crystalline selenium nanobelts and nanowires in micellar solutions of nonionic surfactants

Single-crystalline nanobelts and nanowires of trigonal selenium (t-Se) have been selectively synthesized in micellar solutions of nonionic surfactants. In particular, t-Se nanobelts about 30 nm in thickness were obtained in micellar solutions of poly(oxyethylene(20)) octadecyl ether (C18EO20), whereas t-Se nanowires were obtained in micellar solutions of poly(oxyethylene(10)) dodecyl ether (C12EO10). The obtained t-Se nanobelts exhibit a low-energy absorption peak that is considerably red shifted from that for t-Se nanowires, which has been presumably attributed to the lower degree of crystal perfection for the t-Se nanobelts with rectangular cross sections.     

Study on morphology of LaFeO3 nanofibers under different voltage connections

Continuous LaFeO₃ nanowires and nanobelts were successfully synthesized using a sol–gel assisted electrospinning method followed by calcination at 500°C in air. The thermal decomposition processes of LaFeO₃ are carefully investigated and the best calcining temperature was found to be 500°C. The two nanofibers obtained were characterized using X‐ray diffraction analysis, which shows single phase and the structure of nanobelts has higher crystallinity than that of the nanowires. The scanning electron microscopy reveals that the diameter of the obtained LaFeO₃ nanowires is 139.3 nm. And the thickness and width of the nanobelts are 80 and 459 nm. Moreover, the electrospun LaFeO₃ nanobelts are endowed with a higher specific surface area compared with the nanowires, which results from the regular one dimensional morphology without any detectable agglomeration and a rough surface.     

Mn掺杂SnO_2一维纳米结构的制备、形貌及光学性质

用化学气相沉积(CVD)法制备了Mn掺杂的SnO2一维纳米结构(纳米线及纳米带),X射线衍射(XRD)显示样品为金红石型SnO2晶体,其生长机理可分别归结为气-液-固(VLS)和气-固(VS)机理,生长温度和气态原料浓度的差别是造成样品形貌及生长机理不同的主要原因.样品的拉曼谱出现了500、543、694和720cm-1四个新拉曼谱峰,分别是由活性的红外模和表面模引起的.纳米线及纳米带发光峰位于520nm处,发光强度随样品中氧空位的增减出现由强到弱的变化.     

氧化铟八面体、纳米带、锯齿状纳米线和纳米链的可控合成

在N₂/H₂O混合气流中将硅片上金覆盖的金属铟颗粒加热到800 ℃制备出了不同形貌的In₂O₃纳米结构, 在距铟源不同距离处依次得到In₂O₃的八面体、纳米带、锯齿状纳米线和纳米链. 采用拉曼光谱、扫描电镜、X射线衍射和透射电镜对产物进行了表征分析. 结果表明, 八面体、纳米带、锯齿状纳米线和纳米链均为立方相单晶结构的In₂O₃. 基于气-固和气-液-固生长机理详细分析了八面体、纳米带、锯齿状纳米线和纳米链的生长过程, 提出了不同形貌In₂O₃纳米结构的生长模式.     

Mass production of very thin single-crystal silicon nitride nanobelts

We report the large-scale synthesis of very thin single-crystalline Si₃N₄ nanobelts with high yield via catalyst-assisted pyrolysis of polymeric precursors. The obtained nanobelts, which show a perfect crystal structure and smooth surface, are up to several millimeters in length with typical width and thickness of ∼800 nm and tens of nanometers, respectively. It is believed that the nanobelts were grown via a vapor-solid process, in which Al catalyst played a key role. This result provides a possibility for mass producing high quality, very thin Si₃N₄ nanobelts.     

Resistivity and Field Electron Emission of Nanowires Formed by Electron Beam Induced Chemical Vapor Deposition

"Self-standing iron nanowires were fabricated at the apex of a tungsten needle tip by electron beam induced deposition. This sharp needle tip which adhered to the nanowire can be moved with a stepping motor and piezo-driving device, and was attached inside a specially designed transmission electron microscope pecimen holder. A copper conductor substrate, with which the approaching nanowires will build up a closed electric circuit, was set on the holder. The tungsten needle tip accompanied with the EBICVD nanowires made contact with the substrate and then a voltage was applied between the two electrodes. Resistivity values of the examined nanowires, by a devised Lock-in-Amplifier circuit, range from 0.1 -m to 10-3 -m. Our investigation might have implications in the fabrication and characterization of nano-electronics device. Precursor with phenanthrene (C4H10) was used and the deposition experiment was done using a scanning electron microscope at room temperature. It was found that the surface structure at the top of the nanorod, such as a small protrusion within only several nanometers scale, has significant influence on the field emission property. An emission current of several tens of nano-ampere flowing through this nanorod could induce resistance heating. In several minutes, this thermal energy could transform the original amorphous carbon into a graphite-like structure embedded with fullerenes. The turn-on voltage of the graphite-like nanorod was about 11 V less than that of the original amorphous case."     

Size-controllable one-dimensional SnO2 nanocrystals: synthesis, growth mechanism, and gas sensing property

Single crystalline one-dimensional (1-D) SnO(2) nanocrystals with controllable sizes, including the diameter and the aspect ratio, were synthesized by modulating the precursor concentration, reaction time and temperature via a solution method. By regulating the growth in a kinetic regime, a higher temperature range (220-240 degrees C) was beneficial to the growth of SnO(2) nanowires, while reactions below 220 degrees C only resulted in nanorods or even nanoparticles. The aggregates of SnO(2) nanocrystals in the forms of hollow spheres and dendrites were observed as the intermediates for the nanowires. Based on the TEM and SEM observations, the growth mechanism is discussed from the viewpoints of the nature of the reverse micelles and the crystal habit of rutile SnO(2). CO gas sensing measurements were also carried out for SnO(2) nanocrystals with different assembly styles. The results indicate that the sensitivity had close correlation to the specific surface area of the nanocrystals.     

Strongly birefringent pb3o2cl2 nanobelts

Orthorhombic Pb3O2Cl2 (mendipite) nanobelts micrometers in length and tens of nanometers wide were synthesized by a solventless thermolysis of a single-source precursor in the presence of capping ligands. The nanobelts are single crystals elongated preferentially in the [010] direction. Pb3O2Cl2 is a birefringent material due to its anisotropic crystal structure. The nanobelts exhibit birefringence enhanced by 1 order of magnitude as a result of their small size and belt geometry exceeding the birefringence of naturally occurring minerals, including CaCO3 and TiO2. The preferential elongation of the nanobelts in the [010] direction contributes to this enhancement.     

Boron nanowires synthesized by laser ablation at high temperature

Boron nanowires have been synthesized by laser ablation at high temperature. The as-synthesized boron nanowires were characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and electron energy loss spectroscopy (EELS). The boron nanowires have lengths of several tens of micrometers long and diameters of 30–60 nm. The effects of the synthesis conditions on the formation of the boron nanowires were investigated and possible growth mechanisms of the boron nanowires are discussed.     

Temperature-controlled growth of ZnO nanowires and nanoplates in the temperature range 250-300 degrees C

Starting from a mixture of Zn and BiI3, we grew nanowires and nanoplates on an oxidized Si substrate at relatively low temperatures of 250 and 300 degrees C, respectively. The ZnO nanowires had diameters of approximately 40 nm and grew along the [110] direction rather than the conventional [0001] direction. The nanoplates had thicknesses of approximately 40 nm and lateral dimensions of 3-4 microm. The growth of both the nanowires and nanoplates is dominated by the synergy of vapor-liquid-solid (VLS) and direction conducting. Analysis of photoluminescence spectra suggested that the nanoplates contain more oxygen vacancies and have higher surface-to-volume ratios than the nanowires. The present results clearly demonstrate that the shapes of ZnO nanostructures formed by using BiI3 can be controlled by varying the temperature in the range 250-300 degrees C.     

Hierarchical saw-like ZnO nanobelt/ZnS nanowire heterostructures induced by polar surfaces

Saw-like nanostructures composed of single-crystalline ZnO nanobelts and single-crystalline ZnS nanowires have been successfully synthesized by a vapor-solid process. Several techniques, including scanning electron microscope, transmission electron microscopy, and photoluminescence spectroscopy, were used to investigate the structures, morphology, and photoluminescence properties of the products. Due to the similar crystal habits of wurtzite ZnO and ZnS with chemically active Zn-terminated (0001) and chemically inactive O-terminated (or S-terminated) (000) polar surfaces, hierarchical saw-like nanostructures were considered to be formed by the initiation of a chemically active Zn-terminated ZnO (0001) polar surface. Photoluminescence properties of the heterostructures, different from pure ZnO nanobelts or ZnS nanowires, were also studied at room temperature.     

Electron microscopy study of exotic nanostructures of cadmium sulfide.

In this article, two simple methods, evaporation-condensation and catalytic thermal evaporation, were used to investigate the synthesis of CdS nanostructures for nanoscale optoelectronic applications. To understand their growth mechanisms, various electron microscopy and microanalysis techniques were utilized in characterizing their morphologies, internal structures, growth directions and elemental compositions. The electron microscopy study reveals that when using the evaporation-condensation method, branched CdS nanorods and self-assembled arrays of CdS nanorods were synthesized at 800 degrees C and 1000 degrees C, respectively. Instead of morphological differences, both types of CdS nanorods grew along the [0001] direction. However, when using the catalytic thermal evaporation method (Au as the catalyst), patterned CdS nanowires and nanobelts were formed at the temperature region of 500-600 degrees C and 600-750 degrees C, respectively. Their growth direction was along the direction [1010] instead of [0001]. Based on the microscopy and microanalysis results, we propose some growth mechanisms in relation to the growth processes of those exotic CdS nanostructures.     

Catalyst-nanostructure interfacial lattice mismatch in determining the shape of VLS grown nanowires and nanobelts: a case of Sn/ZnO

Vapor-liquid-solid (VLS) is a well-established process in catalyst-guided growth of nanowires. The catalyst particle is generally believed to be in liquid state during growth, and it is the site for adsorbing incoming molecules; the crystalline structure of the catalyst may not have any influence on the structure of the grown one-dimensional nanostructures. In this paper, using tin particle guided growth of ZnO nanostructures as a model system, we show that the interfacial region of the tin particle with the ZnO nanowire/nanobelt could be ordered (or partially crystalline) during the VLS growth, although the local growth temperature is much higher than the melting point of tin, and the crystallographic lattice structure at the interface is important in defining the structural characteristics of the grown nanowires and nanobelts. The interface prefers to take the least lattice mismatch; thus, the crystalline orientation of the tin particle may determine the growth direction and the side surfaces of the nanowires and nanobelts. This result may have important impact on the understanding of the physical chemical process in the VLS growth.