Field emission from ZnS nanorods synthesized by radio frequency magnetron sputtering technique

The field emission property of zinc sulphides nanorods synthesized in the thin film form on Si substrates has been studied. It is seen that ZnS nanorod thin films showed good field emission properties with a low-macroscopic turn-on field (2.9-6.3 V/μm). ZnS nanorods were synthesized by using radio frequency magnetron sputtering of a polycrystalline prefabricated ZnS target at a relatively higher pressure (10⁻¹ mbar) and at a lower substrate temperature (233-273 K) without using any catalyst. Transmission electron microscopic image showed the formation of ZnS nanorods with high aspect ratio (>60). The field emission data were analysed using Fowler-Nordhiem theory and the nearly straight-line nature of the F-N plots confirmed cold field emission of electrons. It was also found that the turn-on field decreased with the decrease of nanorod's diameters. The optical properties of the ZnS nanorods were also studied. From the measurements of transmittance of the films deposited on glass substrates, the direct allowed bandgap values have been calculated and they were in the range 3.83-4.03 eV. The thickness of the films was ∼600 nm.     

Influence of Substrate on the Transportation Properties of Co/Alq3 Granular Films on a Si Wafer

A series of Co0.48 （Alq3）0.52 granular films were deposited on silicon substrates using the co-evaporating technique. A crossover of magnetoresistance （MR） from negative to positive was observed in the samples, due to conducting channel switching. The transport properties of samples are greatly influenced by hydrofluoric acid pretreatment, as a result, positive MR decreases drastically and the temperature dependence of resistance changes a lot near room temperature. The result indicates that the native oxide layer plays an important role in the transport mechanism. Moreover, different resistivities of Si substrates influence the current distribution of conducting channels, leading to different transport behaviors accordingly.     

Nanostructured ZnO thin films by chemical bath deposition in basic aqueous ammonia solutions for photovoltaic applications

This paper presents further insights and observations of the chemical bath deposition (CBD) of ZnS thin films using an aqueous medium involving Zn-salt, ammonium sulfate, aqueous ammonia, and thioure. Results on physical and chemical properties of the grown layers as a function of ammonia concentration are reported. Physical and chemical properties were analyzed using scanning electron microscopy (SEM), X-ray energy dispersive (EDX), and X-ray diffraction (XRD). Rapid growth of nanostructured ZnO films on fluorine-doped SnO₂ (FTO) glass substrates was developed. ZnO films crystallized in a wurtzite hexagonal structure and with a very small quantity of Zn(OH)₂ and ZnS phases were obtained for the ammonia concentration ranging from 0.75 to 2.0 M. Flower-like and columnar nanostrucured ZnO films were deposited in two ammonia concentration ranges, respectively: one between 0.75 and 1.0 M and the other between 1.4 and 2.0 M. ZnS films were formed with a high ammonia concentration of 3.0 M. The formation mechanisms of ZnO, Zn(OH)₂, and ZnS phases were discussed in the CBD process. The developed technique can be used to directly and rapidly grow nanostructured ZnO film photoanodes. Annealed ZnO nanoflower and columnar nanoparticle films on FTO substrates were used as electrodes to fabricate the dye sensitized solar cells (DSSCs). The DSSC based on ZnO-nanoflower film showed an energy conversion efficiency of 0.84%, which is higher compared to that (0.45%) of the cell being constructed using a photoanode of columnar nanoparticle ZnO film. The results have demonstrated the potential applications of CBD nanostructured ZnO films for photovoltaic cells.     

温度对Si衬底上低压MOCVD外延生长ZnS薄膜质量的影响

用低压MOCVD系统在(111)Si衬底上,用两步生长方法(改变/流量比)在300～400℃时外延生长了ZnS单晶薄膜。随着衬底温度的降低,ZnS薄膜结晶质量提高,并在300℃生长时获得结晶完整性较好的(111)ZnS单晶薄膜。文中讨论了衬底温度对薄膜质量的影响。     

CdS纳米带和纳米锯的热蒸发法的合成和光学性质研究

利用氢气辅助的一步热蒸发法,在镀有金膜的硅片上制备出大量的CdS纳米结构. CdS纳米结构由纤锌矿结构的CdS纳米带和纳米锯组成. 研究表明沉积温度对CdS纳米结构的形貌和尺寸具有重要影响. 用气-液-固和气-固复合生长机理解释了CdS纳米结构的形成过程.光致发光测试表明CdS纳米带和纳米锯均发射出波长为512 nm的很强的绿色发光峰,对应于CdS的带-带跃迁. 同时对CdS纳米结构的光波导性质也进行了观察和讨论.     

Photoluminescence from amorphous silica nanowires synthesized using TiN/Ni/SiO2/Si and TiN/Ni/Si substrates

Photoluminescence characteristics of amorphous silica nanowires (a-SiONWs) grown on TiN/Ni/Si and TiN/Ni/SiO₂ substrates have been studied. A-SiONWs grown on TiN/Ni/Si substrates show a Si-rich composition compared to those grown from TiN/Ni/SiO₂/Si. The emission characteristics of the nanowires were found to depend on the type of substrate. By annealing the a-SiONWs grown on TiN/Ni/Si in air, emission bands shift from blue to green bands. It is likely that silicon to oxygen ratio is an important factor in deciding the types of defects and emission bands of amorphous silica nanowires.     

Synthesis and characterization of ZnS nanobelts based on substitution reaction

ZnS nanobelts have been fabricated by ion-exchange reaction using pure Zn powder and CdS nanopowder as starting materials. The morphology and microstructure of ZnS nanobelts were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The results show that the products consist of a large quantity of ZnS nanobelts, and the nanobelts are grown as bunches randomly distributed on the substrate. The root of the bunch shows that the nanobelts are grown from a site. The ZnS nanobelts have a uniform cross-section along their lengths, and typical lengths of several tens to several hundreds of micrometers. Au nanoparticles are located at or close to the tips of the ZnS nanobelts, served as the catalyst for the growth of the vapor-liquid-solid mechanism. In addition, the nanocombs, nanosaws and nanosprings of ZnS are also found in the products.     

Simple fabrication of hexagonally well-ordered AAO template on silicon substrate in two dimensions

In this study, thin anodic aluminum oxide (AAO) templates both on silicon substrates (AAO template/SiO₂/Si) and Ti-coated silicon substrates (AAO template/Ti/SiO₂/Si) were developed for design of magnetic, electronic and optoelectronic devices, chemical sensors and chip-scale lithium-ion rechargeable microbatteries. Two types of AAO template were prepared by using a two-step anodization procedure. The templates were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The obtained thin AAO templates were approximately 50 nm in diameter and 700 nm in length with 80-nm interpore distances in a relatively large area of 6 cm². A barrier layer of the AAO templates was removed by a cathodic polarization method in KCl solution for several seconds. The current–time transient during removing the alumina barrier layer of the thin AAO template and the mechanism of electrochemical dissolution of the barrier layer are given in detail.     

Effect of substrate bias voltages on the diffusion barrier properties of Zr-N films in Cu metallization

Zr-N diffusion barriers were deposited on the Si substrates by rf reactive magnetron sputtering under various substrate bias voltages. Cu films were subsequently sputtered onto the Zr-N films by dc pulse magnetron sputtering without breaking vacuum. The Cu/Zr-N/Si specimens were then annealed up to 650 °C in N₂ ambient for an hour. The effects of deposition bias on growth rate, film resistivity, microstructure, and diffusion barrier properties of Zr-N films were investigated. An increase in negative substrate bias resulted in a decrease in deposition rate together with a decrease in resistivity. It was found that the sheet resistances of Cu/Zr-N(−200 V)/Si contact system were lower than those of Cu/Zr-N(−50 V)/Si specimens after annealing at 650 °C. Cu/Zr-N(−200 V)/Si contact systems showed better thermal stability so that the Cu₃Si phase could not be detected.     

Silver-coated Si nanograss as highly sensitive surface-enhanced Raman spectroscopy substrates

We created novel surface-enhanced Raman spectroscopy (SERS) substrates by metalization (Ag) of Si nanograss prepared by a Bosch process which involves deep reactive ion etching of single crystalline silicon. No template or lithography was needed for making the Si nanograss, thus providing a simple and inexpensive method to achieve highly sensitive large-area SERS substrates. The dependence of the SERS effect on the thickness of the metal deposition and on the surface morphology and topology of the substrate prior to metal deposition was studied in order to optimize the SERS signals. We observed that the Ag-coated Si nanograss can achieve uniform SERS enhancement over large area (∼1 cm ×1 cm) with an average EF (enhancement factor) of 4.2×10⁸ for 4-mercaptophenol probe molecules. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

A low-temperature evaporation route for ZnO nanoneedles and nanosaws

A low-temperature (550 °C) catalyst-free route has been applied to fabricate ZnO nanoneedles and nanosaws via evaporation of metallic Zn powder. The nanoneedles are vertically aligned on GaN(0001)/Si(111) substrates. Based on electron-microscopy observations, we postulate that the needles grew from the closely stacked ZnO columnar grains, which were formed with 0001 texture in an early stage. The uniquely shaped ZnO nanosaws were found in a large quantity under a similar low-temperature growth condition. In addition, single-crystalline micrometer saws were also obtained under an increased Zn vapor supersaturation by changing the sample configuration. PACS 81.07.-b; 81.05.Dz; 78.67.-n; 68.70.+w; 81.16.Be     

Fabrication of ZnO nanostructures of various dimensions using patterned substrates

ZnO nanostructures were grown on silicon, porous silicon, ZnO/Si and AlN/Si substrates by low-temperature aqueous synthesis method. The shape of nanostructures greatly depends on the underlying surface. Scattered ZnO nanorods were observed on silicon substrate, whereas aligned ZnO nanowires were obtained by introducing sputtered ZnO film as a seed layer. Furthermore, both the combination of nanorods and the bunch of nanowires were found on porous silicon substrates, whereas platelet-like morphology was observed on AlN/Si substrates. XRD patterns suggest the crystalline nature of aqueous-grown ZnO nanostructures and high-resolution transmission electron microscopy images confirm the single-crystalline growth of the ZnO nanorods along [0 0 1] direction. Room-temperature photoluminescence characterization clearly shows a band-edge luminescence along with a visible luminescence in the yellow spectral range.     

Microemulsion-mediated synthesis of cadmium zinc sulfide nanocrystals with composition-modulated optical properties

Cadmium zinc sulfide nanocrystals were synthesized by a microemulsion-mediated process, which involving two steps: the preparation of CdS (or ZnS) seeds and the succedent hydrothermal growth of ZnS (or CdS) component. The XRD results show that the cadmium zinc sulfide nanocrystals with CdS seeds present a hexagonally homogeneous alloyed structure, while the ones with ZnS seeds mainly take on the characteristic of hexagonal CdS nanocrystals. The intrinsic factors influencing the crystal structures were discussed. The UV-vis and photoluminescence (PL) spectra indicate that the optical properties of the obtained nanocrystals with CdS seeds can be continuously modulated by tuning their compositions, although their sizes and size distributions are not under a strict control. The composition-modulated strategy, along with the hydrothermal microemulsion process, will be an effective route to achieve semiconductor nanocrystals with tunable optical properties under more manageable conditions.     

Quasi-Hexagonal Ordered Arrays of FePt Nanoparticles Prepared by a Micellar Method

Hexagonally ordered arrays of magnetic FePt nanoparticles on Si substrates are prepared by a self assembly of diblock copolymer PS-b-P2VP in toluene, a dip coating process and finally plasma treatment. The as-treated FePt nanoparticles are covered by an oxide layer that can be removed by a 40 s Ar＋ sputtering. The effects of the sequence of adding salts on the composition distribution are revealed by x-ray photoelectron spectroscopy measurements. No particle agglomeration is observed after 600℃annealing for the present ordered array of FePt nanoparticles, which exhibits advantages in patterning FePt nanoparticles by a micellar method. Moreover, magnetic properties of the annealed FePt nanoparticles at room temperature are investigated by a vibrating sample magnetometer.     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.     

The structure and photoluminescence properties of TiO<Subscript>2</Subscript>-coated ZnS nanowires

The structure and photoluminescence properties of TiO₂-coated ZnS nanowires were investigated. ZnS nanowires were synthesized by thermal evaporation of ZnS powder and then coated with TiO₂ by using the metal organic chemical vapor deposition (MOCVD) technique. We performed scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, and photoluminescence (PL) spectroscopy to characterize the as-synthesized and TiO₂-coated ZnS nanowires. TEM and XRD analyses revealed that the ZnS core and the TiO₂ coatings had crystalline zinc blende and crystalline anatase structures, respectively. PL measurement at room temperature showed that the as-synthesized ZnS nanowires had two emissions: a blue emission centered in the range from 430 to 440 nm and a green emission at around 515 nm. The green emission was found to be dominant in the ZnS nanowires coated with TiO₂ by MOCVD at 350°C for one or more hours, while the blue emission was dominant in the as-synthesized ZnS nanowires. Also the mechanisms of the emissions were discussed.     

Effect of sputtered films on morphology of vertical aligned ZnO nanowires

Vertical aligned ZnO nanowires were grown by MOCVD technique on silicon substrate using ZnO and AlN thin films as seed layers. The shape of nanostructures was greatly influenced by the under laying surface. Vertical nanopencils were observed on ZnO/Si, whereas the nanowires on both sapphire and AlN/Si substrate have the similar aspect ratio. XRD patterns suggest that the nanostructures have good crystallinity. High-resolution transmission electron microscopy (HRTEM) confirmed the single crystalline growth of the ZnO nanowires along [0 0 1] direction. Room-temperature photoluminescence (PL) spectra of ZnO nanowires on AlN/Si clearly show a band-edge luminescence accompanied with a visible emission. More interestingly, no visible emission for the nanopencils on ZnO/Si substrates, were observed.     

Enhanced field emission from ZnO nanopencils by using pyramidal Si(1 0 0) substrates

Zinc oxide nanopencil arrays were synthesized on pyramidal Si(1 0 0) substrates via a simple thermal evaporation method. Their field emission properties have been investigated: the turn-on electric field (at the current density of 10 μA/cm²) was about 3.8 V/μm, and the threshold electric field (at the current density of 1 mA/cm²) was 5.8 V/μm. Compared with similar structures grown on flat Si substrates, which were made as references, the pyramidal Si-based ZnO nanopencil arrays appeared to be superior in field emission performance, thus the importance of the non-flat substrates has been accentuated. The pyramidal Si substrates could not only suppress the field screening effect but also improve the field enhancement effect during the field emission process. These findings indicated that using non-flat substrates is an efficient strategy to improve the field emission properties.     

退火处理对ZnS薄膜的结构和光学性质的影响

在200 ℃下利用激光沉积技术分别在玻璃和Si(100)上沉积制备了ZnS薄膜,并在300,400,500 ℃下退火1 h。用X射线衍射(XRD)仪、紫外/可见光/近红外分光光度计、台阶仪和原子力显微镜(AFM)分别对不同衬底上样品的特征进行了观察。结果表明,玻璃上的ZnS薄膜只在28.5°附近存在着(111)方向的高度取向生长。在可见光范围内透射率为60%~90%。计算显示薄膜的光学带隙在3.46~3.53 eV之间,其小于体材料带隙的原因在于硫元素的缺失。根据光学带隙判断薄膜是单晶立方结构的β-ZnS。Si(100)上生长的是多晶ZnS薄膜:500 ℃下退火后,表面也比未退火表面更加平整致密,变化规律与ZnS/glass的类似。说明高温下退火可以有效地促进晶粒的结合并改善薄膜质量。     

Growth mechanism of scales of SiC nanowires with/without ZnS powders at varying temperatures

The growth mechanism of scales of crystalline SiC nanowires (SiC-NWs) obtained by directly evaporating solid carbon on silicon wafer with/without ZnS powders at varying temperatures is being discussed. More aligned SiC-NWs of small size and good crystalline structure were formed when ZnS was used. Random SiC-NWs of big size and poor crystalline structure were obtained at conditions free of ZnS. Furthermore, the improved crystalline structure and increased diameter of SiC-NWs were observed when the higher temperature was employed.