Quantum Dots Microstructural Metrology: From Time-Resolved Spectroscopy to Spatially Resolved Electron Microscopy

Colloidal quantum dots (QDs) have unique optical and electrical properties with promising applications in next-generation semiconductor technologies, including displays, lighting, solar cells, photodetectors, and image sensors. Advanced analytical tools to probe the optical, morphological, structural, compositional, and electrical properties of QDs and their ensemble solid films are of paramount importance for the understanding of their device performance. In this review, comprehensive studies on the state-of-the-art metrology approaches used in QD research are introduced, with particular focus on time-resolved (TR) and spatially resolved (SR) spectroscopy and microscopy. Through discussing these analysis techniques in different QD system, such as various compositions, sizes, and shell structures, the critical roles of these TR-spectroscopic and SR-microscopic techniques are highlighted, which provide the structural, morphological, compositional, optical, and electrical information to precisely design QDs and QD solid films. The employment of TR and SR analysis in integrated QD device systems is also discussed, which can offer detailed microstructural information for achieving high performance in specific applications. In the end, the current limitations of these analytical tools are discussed, and the future development of the possibility of interdisciplinary research in both QD fundamental and applied fields is prospected.     

Polyaniline-coated coconut fibers: Structure,properties and their use as conductive additives in matrix of polyurethane derived from castor oil

Electrically conducting fibers based on coconut fibers (CF) and polyaniline (PANI) were prepared through in situ oxidative polymerization of aniline (ANI) in the presence of CF using iron (III) chloride hexahydrate (FeCl_3.6H₂O) or ammonium persulfate (APS) as an oxidant. The PANI-coated coconut fibers (CF-PANI) displayed various morphologies, electrical conductivities and percentages of PANI on the CF surface. For both systems, a PANI conductive layer was present on the CF surface, which was responsible for an electrical conductivity of around 1.5 × 10⁻¹ and 1.9 × 10⁻² S cm⁻¹ for composites prepared with FeCl₃.6H₂O and APS, respectively; values that are similar to that of pure PANI. In order to modify the structure and properties of polyurethane derived from castor oil (PU) both CF-PANI and pure PANI were used as conductive additives. The PU/CF-PANI composites exhibited higher electrical conductivity than pure PU and PU/PANI blends. Additionally, the PU/CF-PANI composites showed a variation in electrical resistivity according to the compressive stress applied, indicating that these materials could be applied for pressure-sensitive applications.     

Edge and terrace structure of CoTPP on Au(1 1 1) investigated by ultra-high vacuum scanning tunnelling microscopy at room temperature

Edge adsorption and terrace molecular domain structures of Cobalt(II) tetraphenylporphyrin (CoTPP) on Au(1 1 1) were investigated using STM at room temperature. Two different terrace domain structures were observed. These two arrangements were found to be enantiomorphous arrangements of the molecular assemblies, where the molecular rows rotate ±16° with respect to the [1 2 1] direction of Au(1 1 1). In both arrangements, most of the CoTPP molecules were imaged as one bright dot with four legs, corresponding to a planar conformation of the macrocycle. A small proportion of the CoTPP molecules appear as two bright dots, corresponding to a saddle shape of the macrocycle. Our results show that most of the saddle-deformed CoTPP molecules are distributed in the vicinity of the bridging sites of the reconstructed gold surface. Besides terrace domains, we found that several edge adsorption structures of CoTPP are also stable enough to be imaged and analysed in detail. Furthermore, the relationship between edge structures and terrace domains was revealed.     

Catalyst patterning methods for surface-bound chemical vapor deposition of carbon nanotubes

We present three different catalyst preparation and patterning techniques for plasma-enhanced chemical vapor deposition of carbon nanostructures from acetylene and ammonia mixtures. The different merits and potential areas of application are highlighted for each technique as compared to the benchmark of e-beam-lithography patterning. Maskless, focused ion beam written Pt can nucleate aligned carbon nanofibers, thereby allowing a sub-100 nm lateral resolution on non-planar substrate geometries combined with an in-situ monitoring. Ion beam milling additionally enables the pre-shaping and marking of the substrate, which is shown for the growth of individual nanofibers on the apex of commercial scanning probe tips. Pulsed electrochemical deposition was used to form Ni and Fe catalyst islands of controlled size and density. This is also demonstrated on complex substrate geometries such as carbon cloth. Nanocontact printing was employed to deposit a highly purified Co colloid in regular patterns with feature sizes down to 100 nm onto silicon wafers for low cost patterning over large areas. We analyze the catalyst restructuring upon exposure to elevated temperatures for each technique and relate this to the nucleated nanofiber dimensions and array densities. The flexibility in catalyst and substrate material allows a transfer of our achievements to catalyst-assisted growth of nanostructures in general facilitating their hierarchical device integration and future application. PACS 81.16.Rf; 81.16.Hc; 61.46.+w     

多晶粒银纳米线形变机理对温度依赖性的分子动力学研究

基于大规模分子动力学仿真,研究了包含多个晶粒的柱状银纳米线在不同温度下沿轴向拉伸形变的行为。结果表明,当温度低于200 K时,含较大晶粒的体系中位错滑移是其形变的主要机理,最大应力随温度变化不显著。当环境温度高于200 K时,晶粒的滑动逐渐成为形变的主导因素,这一特征在含更小晶粒的体系内表现更明显。同时最大应力随温度显著降低。基于上述结果,进一步讨论了温度对Hall-Petch关系的影响。     

Excitons and multi-exciton complexes bound to a two-dimensional hole layer at a silicon surface: the Kondo effect,the Coulomb blockade and a negative photoconductivity

The recombination radiation line of surface excitons and the recombination radiation line of multi-exciton complexes bound to a two-dimensional hole layer are observed in luminescence spectra of [100] silicon metal–oxide–semiconductor structures at low two-dimensional hole density. The circular polarization of these two lines in a transverse magnetic field is defined by the average electron spin. The hole spin contribution to the circular polarization is very small due to Kondo spin correlations of holes in the excitons and complexes and holes in the two-dimensional hole layer. The Coulomb blockade excludes a direct contribution of the complexes to a surface photoconductivity. Moreover, a significant negative photoconductivity of the two-dimensional holes is observed at high excitation levels, presumably as a result of the quantum scattering of the two-dimensional holes by the complexes. A shell model of surface multi-exciton complexes is introduced.     

Recent mathematical developments in the Skyrme model

In this review we present a pedagogical introduction to recent, more mathematical developments in the Skyrme model. Our aim is to render these advances accessible to mainstream nuclear and particle physicists. We start with the static sector and elaborate on geometrical aspects of the definition of the model. Then we review the instanton method which yields an analytical approximation to the minimum energy configuration in any sector of fixed baryon number, as well as an approximation to the surfaces which join together all the low energy critical points. We present some explicit results for B=2. We then describe the work done on the multibaryon minima using rational maps, on the topology of the configuration space and the possible implications of Morse theory. Next we turn to recent work on the dynamics of Skyrmions. We focus exclusively on the low energy interaction, specifically the gradient flow method put forward by Manton. We illustrate the method with some expository toy models. We end this review with a presentation of our own work on the semi-classical quantization of nucleon states and low energy nucleon–nucleon scattering.     

Building three-dimensional nanostructures with active enzymes by surface templated layer-by-layer assembly

We show that well-defined three-dimensional nanostructures of functional enzymes can be controllably fabricated by layer-by-layer assembly of avidin and biotinylated horseradish peroxidase on micro-contact printing patterned surface templates.     

A reversible pH-driven DNA nanoswitch array

An array of surface-immobilized proton-fueled DNA nanomachines is reversibly actuated by cycling of the solution pH between 4.5 and 9, producing a conformational change between a four-stranded and a double-stranded structure, which elongates or shortens the separation distance between the 5' and 3' end of the DNA. By labeling the DNA 3' end with a fluorophore and immobilizing it onto a thin-gold surface through its 5' thiol modification, the nanoscale motion of the DNA produces mechanical work to lift up and bring down the fluorophore from the gold surface by at least 2.5 nm and transduces this motion into an optical "on-and-off" nanoswitch.     

Microstructured silicone substrate for printable and stretchable metallic films

Stretchable electronics (i.e., hybrid inorganic or organic circuits integrated on elastomeric substrates) rely on elastic wiring. We present a technique for fabricating reversibly stretchable metallic films by printing silver-based ink onto microstructured silicone substrates. The wetting and pinning of the ink on the elastomer surface is adjusted and optimized by varying the geometry of micropillar arrays patterned on the silicone substrate. The resulting films exhibit high electrical conductivity (～11?000 S/cm) and can stretch reversibly to 20% strain over 1000 times without failing electrically. The stretchability of the ≥200 nm thick metallic film relies on engineered strain relief in the printed film on patterned PDMS.