Vertically-oriented MoS2 nanosheets for nonlinear optical devices.

Transition metal dichalcogenides such as MoS2 represent promising candidates for building blocks of ultra-thin nanophotonic devices. For such applications, vertically-oriented MoS2 (v-MoS2) nanosheets could be advantageous as compared to conventional horizontal MoS2 (h-MoS2) given that their inherent broken symmetry would favor an enhanced nonlinear response. However, the current lack of a controllable and reproducible fabrication strategy for v-MoS2 limits the exploration of this potential. Here we present a systematic study of the growth of v-MoS2 nanosheets based on the sulfurization of a pre-deposited Mo-metal seed layer. We demonstrate that the sulfurization process at high temperatures is driven by the diffusion of sulfur from the vapor-solid interface to the Mo seed layer. Furthermore, we verify an enhanced nonlinear response in the resulting v-MoS2 nanostructures as compared to their horizontal counterparts. Our results represent a stepping stone towards the fabrication of low-dimensional TMD-based nanostructures for versatile nonlinear nanophotonic devices.

Boosting Hole Mobility in Coherently Strained [110]-Oriented Ge-Si Core-Shell Nanowires.

The ability of core-shell nanowires to overcome existing limitations of heterostructures is one of the key ingredients for the design of next generation devices. This requires a detailed understanding of the mechanism for strain relaxation in these systems in order to eliminate strain-induced defect formation and thus to boost important electronic properties such as carrier mobility. Here we demonstrate how the hole mobility of [110]-oriented Ge-Si core-shell nanowires can be substantially enhanced thanks to the realization of large band offset and coherent strain in the system, reaching values as high as 4200 cm2/(Vs) at 4 K and 1600 cm2/(Vs) at room temperature for high hole densities of 1019 cm-3. We present a direct correlation of (i) mobility, (ii) crystal direction, (iii) diameter, and (iv) coherent strain, all of which are extracted in our work for individual nanowires. Our results imply [110]-oriented Ge-Si core-shell nanowires as a promising candidate for future electronic and quantum transport devices.

Cracking the Si Shell Growth in Hexagonal GaP-Si Core-Shell Nanowires.

Semiconductor nanowires have increased the palette of possible heterostructures thanks to their more effective strain relaxation. Among these, core-shell heterostructures are much more sensitive to strain than axial ones. It is now accepted that the formation of misfit dislocations depends both on the lattice mismatch and relative dimensions of the core and the shell. Here, we show for the first time the existence of a new kind of defect in core-shell nanowires: cracks. These defects do not originate from a lattice mismatch (we demonstrate their appearance in an essentially zero-mismatch system) but from the thermal history during the growth of the nanowires. Crack defects lead to the development of secondary defects, such as type-I1 stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.

III-V nanowire arrays: growth and light interaction.

Semiconductor nanowire arrays are reproducible and rational platforms for the realization of high performing designs of light emitting diodes and photovoltaic devices. In this paper we present an overview of the growth challenges of III-V nanowire arrays obtained by molecular beam epitaxy and the design of III-V nanowire arrays on silicon for solar cells. While InAs tends to grow in a relatively straightforward manner on patterned (111)Si substrates, GaAs nanowires remain more challenging; success depends on the cleaning steps, annealing procedure, pattern design and mask thickness. Nanowire arrays might also be used for next generation solar cells. We discuss the photonic effects derived from the vertical configuration of nanowires standing on a substrate and how these are beneficial for photovoltaics. Finally, due to the special interaction of light with standing nanowires we also show that the Raman scattering properties of standing nanowires are modified. This result is important for fundamental studies on the structural and functional properties of nanowires.

Growth mechanisms and process window for InAs V-shaped nanoscale membranes on Si[001].

Organized growth of high aspect-ratio nanostructures such as membranes is interesting for opto-electronic and energy harvesting applications. Recently, we reported a new form of InAs nano-membranes grown on Si substrates with enhanced light scattering properties. In this paper we study how to tune the morphology of the membranes by changing the growth conditions. We examine the role of the V/III ratio, substrate temperature, mask opening size and inter-hole distances in determining the size and shape of the structures. Our results show that the nano-membranes form by a combination of the growth mechanisms of nanowires and the Stranski-Krastanov type of quantum dots: in analogy with nanowires, the length of the membranes strongly depends on the growth temperature and the V/III ratio; the inter-hole distance of the sample determines two different growth regimes: competitive growth for small distances and an independent regime for larger distances. Conversely, and similarly to quantum dots, the width of the nano-membranes increases with the growth temperature and does not exhibit dependence on the V/III ratio. These results constitute an important step towards achieving rational design of high aspect-ratio nanostructures.

Self-assembled quantum dots in a nanowire system for quantum photonics.

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.

Gallium assisted plasma enhanced chemical vapor deposition of silicon nanowires.

Silicon nanowires have been grown with gallium as catalyst by plasma enhanced chemical vapor deposition. The morphology and crystalline structure has been studied by electron microscopy and Raman spectroscopy as a function of growth temperature and catalyst thickness. We observe that the crystalline quality of the wires increases with the temperature at which they have been synthesized. The crystalline growth direction has been found to vary between <111> and <112>, depending on both the growth temperature and catalyst thickness. Gallium has been found at the end of the nanowires, as expected from the vapor-liquid-solid growth mechanism. These results represent good progress towards finding alternative catalysts to gold for the synthesis of nanowires.

Plasma-enhanced low temperature growth of silicon nanowires and hierarchical structures by using tin and indium catalysts.

Plasma-enhanced low temperature growth (<300 degrees C) of silicon nanowires (SiNWs) and hierarchical structures via a vapor-liquid-solid (VLS) mechanism are investigated. The SiNWs were grown using tin and indium as catalysts prepared by in situ H(2) plasma reduction of SnO(2) and ITO substrates, respectively. Effective growth of SiNWs at temperatures as low as 240 degrees C have been achieved, while tin is found to be more ideal than indium in achieving a better size and density control of the SiNWs. Ultra-thin (4-8 nm) silica nanowires, sprouting from the dendritic nucleation patterns on the catalyst's surface, were also observed to form during the cooling process. A kinetic growth model has been proposed to account for their formation mechanism. This hierarchical structure combines the advantages of the size and position controllability from the catalyst-on-top VLS-SiNWs and the ultra-thin size from the catalyst-on-bottom VLS-ScNWs.

A transcription factor regulatory circuit in differentiated pancreatic cells.

Mutations in the human genes encoding hepatocyte nuclear factors (HNF) 1alpha, 1beta, 4alpha, and IPF1(PDX1/IDX1/STF1) result in pancreatic beta cell dysfunction and diabetes mellitus. In hepatocytes, hnf4alpha controls the transcription of hnf1alpha, suggesting that this same interaction may operate in beta cells and thus account for the common diabetic phenotype. We show that, in pancreatic islet and exocrine cells, hnf4alpha expression unexpectedly depends on hnf1alpha. This effect is tissue-specific and mediated through direct occupation by hnf1alpha of an alternate promoter located 45.6 kb from the previously characterized hnf4alpha promoter. Hnf1alpha also exerts direct control of pancreatic-specific expression of hnf4gamma and hnf3gamma. Hnf1alpha dependence of hnf4alpha, hnf4gamma, hnf3gamma, and two previously characterized distal targets (glut2 and pklr) is established only after differentiated cells arise during pancreatic embryonic development. These studies define an unexpected hierarchical regulatory relationship between two genes involved in human monogenic diabetes in the cells, which are relevant to its pathophysiology. Furthermore, they indicate that hnf1alpha is an essential component of a transcription factor circuit whose role may be to maintain differentiated functions of pancreatic cells.

Hepatic nuclear factor 1-alpha directs nucleosomal hyperacetylation to its tissue-specific transcriptional targets.

Mutations in the gene encoding hepatic nuclear factor 1-alpha (HNF1-alpha) cause a subtype of human diabetes resulting from selective pancreatic beta-cell dysfunction. We have analyzed mice lacking HNF1-alpha to study how this protein controls beta-cell-specific transcription in vivo. We show that HNF1-alpha is essential for the expression of glut2 glucose transporter and L-type pyruvate kinase (pklr) genes in pancreatic insulin-producing cells, whereas in liver, kidney, or duodenum tissue, glut2 and pklr expression is maintained in the absence of HNF1-alpha. HNF1-alpha nevertheless occupies the endogenous glut2 and pklr promoters in both pancreatic islet and liver cells. However, it is indispensable for hyperacetylation of histones in glut2 and pklr promoter nucleosomes in pancreatic islets but not in liver cells, where glut2 and pklr chromatin remains hyperacetylated in the absence of HNF1-alpha. In contrast, the phenylalanine hydroxylase promoter requires HNF1-alpha for transcriptional activity and localized histone hyperacetylation only in liver tissue. Thus, different HNF1-alpha target genes have distinct requirements for HNF1-alpha in either pancreatic beta-cells or liver cells. The results indicate that HNF1-alpha occupies target gene promoters in diverse tissues but plays an obligate role in transcriptional activation only in cellular- and promoter-specific contexts in which it is required to recruit histone acetylase activity. These findings provide genetic evidence based on a live mammalian system to establish that a single activator can be essential to direct nucleosomal hyperacetylation to transcriptional targets.

Design of a photowritten cavity neodymium-doped fiber laser tunable around an absorption molecular line.

Experiments have shown that it is possible to realize laser cavities photowritten within rare-earth-doped silicate fibers. Experimental results on a particular application, namely, the detection of gas traces by identification of one of the gas's absorption lines, are presented. Experiments on an absorption line of atmospheric water have been carried out to illustrate this purpose. Two lasers have been designed with particular characteristics (spectral linewidths, slope efficiencies, thresholds). Preliminary experimental results on the detection of an absorption line of atmospheric water have been obtained with fiber lasers and optoacoustic detection.

Ultraviolet-induced permanent Bragg gratings in cerium-doped ZBLAN glasses or optical fibers.

A holographic method was used to write refractive-index gratings in ZBLAN fluoroziroconate glasses or fibers doped by a Ce concentration of 5000 or 10,000 parts-in-10(6) weight. Direct pumping of the (2)F(5/2)-5d transitions of Ce(3+) ions near 245 nm resulted in a change in the refractive index. The photoinduced change partly recovered on a time scale of several hours at room temperature. The remaining change in the refractive index looked stable on a time scale of a month. This permanent change reaches 2 x 10(-5) at 1560 nm.

Dynamic holographic memory showing readout, refreshing, and updating capabilities.

We propose a new refreshing procedure for reading out a dynamic holographic memory without loss of information. The retrieved images are fed back to the memory after being thresholded and amplified. Experimental demonstration is performed with a LiNbO(3) photorefractive crystal.