Observation of room temperature negative differential resistance in solution synthesized ZnO nanorod

We report the observation of negative differential resistance (NDR) in solution synthesized ZnO nanorod. The ZnO nanorod was fabricated as a two terminal planar device using lithographically patterned Au electrodes. The measured current–voltage response of the device has shown a negative differential resistance behavior. The peak-to-valley current ratio of the NDR is found to be greater than 4. The mechanism of this observed NDR effect has been explained based on charge trapping and de-trapping at the nanoscale contacts. It is the first observation of negative differential resistance effect in solution synthesized ZnO nanorod.     

Ni/CeO2-Al2O3催化剂上CH4-CO2转化积炭性能的研究

采用脉冲微量反应技术研究了添加n型半导体氧化物CeO₂对Ni基催化剂上CH₄积炭/CO₂消炭性能的影响,用TPR,XPS和氢吸附技术对催化剂进行了表征.结果表明,活性金属原子Ni与半导体氧化物CeO₂之间存在金属-半导体相互作用(MScI),CeO₂的添加提高了活性原子Ni⁰的d电子密度,在一定程度上抑制了CH₄分子中C-Hσ电子向d轨道的迁移,降低了CH₄裂解积炭活性;可加强Ni⁰原子d轨道向CO₂空反键π轨道的电子迁移,促进CO₂分子的活化,提高CO₂的消炭活性,使Ni/CeO₂-Al₂O₃催化剂具有较强的抗积炭性能.     

Formation of the Si/Ti interface

The formation of the Si/Ti interface during the deposition of silicon on titanium polycrystalline substrates has been studied at room temperature (RT) using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), ultraviolet photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS). The experimental results are consistent with a two-stage mechanism for Si growth: a first stage characterized by the simultaneous formation of a uniform titanium silicide layer, that reaches a limiting thickness of ∼3 monolayer (ML), and pure silicon islands 1 ML thick that grow on top of this layer up to coalescence, followed by a second stage in which pure silicon islands, with an average thickness of 9 ML, grow on top of the uniform titanium silicide layer + pure silicon ML structure formed during the first stage. As a whole, pure silicon species grows according to a Stranski-Krastanov mechanism, where the first ML is formed during the first stage and the islands during the second stage. The comparison of Ti/Si and Si/Ti interfaces shows that the structure and composition of the interface do not depend substantially on the deposition sequence, suggesting that the bulk chemistry of the compound formed at the interface dominates over the surface kinetics and the bulk substrate chemistry in determining the composition and structure of the interface.     

Adsorption of Si on Ag(0 0 1) from ab initio study

We have systematically investigated the adsorption of Si on the Ag(0 0 1) surface employing density-functional theory. Various adsorption geometries have been considered for Si coverages up to 2.0 monolayers. Our results show that the behaviors of Si at the early stages of growth on the Ag(0 0 1) surface are governed by a competition between the Si-Si and Si-Ag interactions. From the calculated results, we presented alternative models for the observed 3 × 3 and 4 × 7 structures. Our results provide a reasonable explanation for the experimental findings in a previous work.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.     

Molecular view of copper deposition chemistry: (Hexafluoroacetylacetonate)Cu(vinyltrimethylsilane) on a Si(1 0 0)-2 × 1 surface

The chemistry of a common copper deposition precursor, (hexafluoroacetylacetonate)Cu(vinyltrimethylsilane), (hfac)Cu(VTMS), on a single crystal Si(1 0 0)-2 × 1 surface is described at the molecular level using a combination of experimental surface analytical techniques under ultra-high vacuum conditions with computational analysis. At a cryogenic temperature of 100 K, (hfac)Cu(VTMS) adsorbs on this surface molecularly, without noticeable decomposition. Upon surface annealing, VTMS is easily released into the gas phase below the room temperature, while the hfac ligand is bound to the surface through the copper atom. When (hfac)Cu(VTMS) is adsorbed at room temperature, VTMS is released into the gas phase immediately, leaving surface adsorbate analogous in structure to the one formed by adsorption at cryogenic temperature and a brief annealing to room temperature. Upon surface annealing, the hfac ligand decomposes and constitutes the main source of impurities in copper deposition process.     

Submonolayer Au on Si(111) phase diagram

Based on a review of the current literature, a surface phase diagram is proposed for the submonolayer Au on Si(111) system. Kinetic considerations are reviewed and key surface phase diagram features such as the Θ < 0.4 ML metastable structure and the high temperature to Si(111)-(1 × 1)Au second order phase transition are discussed. Experiments to verify certain portions of the phase diagram are proposed.     

Buildup of the InSe/M interface (M Pd, Au) studied by X-ray photoemission and X-ray absorption spectroscopy

The electronic properties of InSe/M (M  Pd, Au) interfaces have been studied by X-ray photoemission measurements. For the InSe/Pd interface, it has been found that Pd atoms diffuse into the InSe lattice at early stages of Pd coverage, acting as acceptor centers. As the Pd coverage increases, a Pd-InSe reaction determines the electronic behaviour of the interface. However, for Pd coverages higher than 1 ML, the barrier formation tends to be controlled by an emerging bulklike Pd overlayer. Despite the atomic structure of this system is far from that expected for an ideal Schottky one, the final electronic barrier value is close to that expected for an abrupt InSe/Pd Schottky interface. On the contrary, the InSe/Au system appeared to behave as a quasi-ideal abrupt Schottky interface. Annealing processes performed at temperatures higher than 600 K alter this scheme, as revealed by X-ray absorption spectroscopy measurements, enhancing diffusion of Au atoms into InSe. In any case, the electronic barrier results to be determined by the Au overlayer formed.     

Characterization and chemical behavior of submonolayer coverages of titania on a Pt foil

The interaction between submonolayer titania coverages and Pt foil has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). The submonolayer titania can be fully oxidized to TiO₂ at 923 K under 10⁻⁸ Torr O₂, and partially oxidized to TiO_x at lower oxidation temperatures. The oxidized surface can be reduced by annealing to 1000 K or higher, or by heating in H₂ at 823 K, or by interacting with surface carbon formed from acetone decomposition. Under certain conditions (e.g., hydrogen reduction at 923 K), the surface titania can be fully reduced to metallic Ti which diffuses into bulk Pt readily. The reduced metallic Ti can resurface when the surface is oxidized at 923 K. Both XPS and HREELS data indicate the existence of subsurface oxygen, which plays an important role for the diffusion of Ti into and out of the Pt foil. Although no special interfacial active sites were revealed by HREELS studies of adsorbed acetone and CO, some TPD and XPS data suggest the presence of sites active for acetone decomposition.     

金纳米粒子的尺寸以及金-二氧化钛相互作用对表面等离激元光催化产氧的影响（英文）

表面等离激元共振效应具有独特的光-物质相互作用的性质,因而近年来被广泛的应用到太阳能转化科学领域.通过调变金属的性质(例如组分、尺寸、形貌等)可以方便地在整个太阳能光谱范围内调节其等离子共振吸收性质,从而有效地增加光催化剂的捕光效率.尽管如此,等离激元光催化目前的能量转化效率仍处于很低的水平,主要是由于光催化剂中的电荷分离以及利用效率很低.理论上可以通过控制优化金属纳米粒子的尺寸以及金属-半导体的相互作用来促进电荷分离及利用效率,从而提高光催化剂的活性.然而,关于金属纳米粒子的尺寸以及金属-半导体的相互作用在表面等离激元光催化剂中的具体作用还亟待详细研究.本文以金/二氧化钛作为典型的表面等离激元光催化剂,通过简便的焙烧后处理可同时实现金粒子尺寸和金-氧化钛界面相互作用的调控.我们发现,催化剂的可见光产氧活性随着焙烧温度呈现火山型关系,其中600 ℃处理的样品表现出最高的活性,在560 nm出的表观量子效率为0.3%,接近甚至超过文献报道的同类结果.随后通过X射线衍射、扫描电镜、光电子能谱、紫外可见光谱等表征手段系统地研究了煅烧温度对于金粒子尺寸和金-氧化钛相互作用的影响.从结果表明,通过不同温度焙烧并未改变二氧化钛自身的性质,且随着温度的升高,金尺寸在600℃之前变化较小,随后突然快速长大.在排除了不同样品的吸光性能的差异后发现,产氧活性随着Au尺寸长大而减少.从光电子能谱的结果发现,金-二氧化钛的界面相互作用随焙烧温度升高而逐步增强.进一步地,通过界面周长的归一化活性得出界面相互作用与产氧活性呈现近似线性关系.最后,通过对光沉积Au样品进行不同温度的煅烧发现大尺寸Au的粒径不随煅烧温度变化,而此时的产氧活性却随焙烧温度增加而增加,这进一步验证了界面相互作用对光催化水氧化的促进作用.综合以上结果可知,最终的等离激元诱导的可见光产氧活性是由尺寸与界面相互作用共同影响的结果,尺寸和界面相互作用的变化会共同影响热电子转移效率和表面反应效率,从而改变整体的光催化产氧效率.