Atomically Thin Resonant Tunnel Diodes built from Synthetic van der Waals Heterostructures

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant tunneling in an atomically thin stack with spectrally narrow room temperature negative differential resistance characteristics

J Regularization Improves Imbalanced Multiclass Segmentation

We propose a new loss formulation to further advance the multiclass segmentation of cluttered cells under weakly supervised conditions. When adding a Youden's J statistic regularization term to the cross entropy loss we improve the separation of touching and immediate cells, obtaining sharp segmentation boundaries with high adequacy. This regularization intrinsically supports class imbalance thus eliminating the necessity of explicitly using weights to balance training. Simulations demonstrate this capability and show how the regularization leads to correct results by helping advancing the optimization when cross entropy stagnates. We build upon our previous work on multiclass segmentation by adding yet another training class representing gaps between adjacent cells. This addition helps the classifier identify narrow gaps as background and no longer as touching regions. We present results of our methods for 2D and 3D images, from bright field images to confocal stacks containing different types of cells, and we show that they accurately segment individual cells after training with a limited number of images, some of which are poorly annotated

Signature of a three-dimensional photonic band gap observed on silicon inverse woodpile photonic crystals

We have studied the reflectivity of CMOS-compatible three-dimensional silicon inverse woodpile photonic crystals at near-infrared frequencies. Polarization-resolved reflectivity spectra were obtained from two orthogonal crystal surfaces corresponding to 1.88 pi sr solid angle. The spectra reveal broad peaks with high reflectivity up to 67 % that are independent of the spatial position on the crystals. The spectrally overlapping reflectivity peaks for all directions and polarizations form the signature of a broad photonic band gap with a relative bandwidth up to 16 %. This signature is supported with stopgaps in plane wave bandstructure calculations and with the frequency region of the expected band gap.Comment: 9 pages, 5 figure

Directional wetting in anisotropic inverse opals

Porous materials display interesting transport phenomena due to the restricted motion of fluids within the nano- to micro-scale voids. Here, we investigate how liquid wetting in highly ordered inverse opals is affected by anisotropy in pore geometry. We compare samples with different degrees of pore asphericity and find different wetting patterns depending on the pore shape. Highly anisotropic structures are infiltrated more easily than their isotropic counterparts. Further, the wetting of anisotropic inverse opals is directional, with liquids filling from the side more easily. This effect is supported by percolation simulations as well as direct observations of wetting using time-resolved optical microscopy

Strong modulation of optical properties in black phosphorus through strain-engineered rippling

Controlling the bandgap through local-strain engineering is an exciting avenue for tailoring optoelectronic materials. Two-dimensional crystals are particularly suited for this purpose because they can withstand unprecedented non-homogeneous deformations before rupture: one can literally bend them and fold them up almost like a piece of paper. Here, we study multi-layer black phosphorus sheets subjected to periodic stress to modulate their optoelectronic properties. We find a remarkable shift of the optical absorption band-edge of up to ~0.7 eV between the regions under tensile and compressive stress, greatly exceeding the strain tunability reported for transition metal dichalcogenides. This observation is supported by theoretical models which also predict that this periodic stress modulation can yield to quantum confinement of carriers at low temperatures. The possibility of generating large strain-induced variations in the local density of charge carriers opens the door for a variety of applications including photovoltaics, quantum optics and two-dimensional optoelectronic devices.Comment: 16 pages main text + 13 pages S

Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics

We review our recent efforts in building atom-scale quantum-dot cellular automata circuits on a silicon surface. Our building block consists of silicon dangling bond on a H-Si(001) surface, which has been shown to act as a quantum dot. First the fabrication, experimental imaging, and charging character of the dangling bond are discussed. We then show how precise assemblies of such dots can be created to form artificial molecules. Such complex structures can be used as systems with custom optical properties, circuit elements for quantum-dot cellular automata, and quantum computing. Considerations on macro-to-atom connections are discussed.Comment: 28 pages, 19 figure

Exciton Mobility and Localized Defects in Single Carbon Nanotubes Studied with Tip-Enhanced Near-Field Optical Microscopy

In this work, single-walled carbon nanotubes (SWNTs) have been studied using tip-enhanced near-field optical microscopy (TENOM). This technique provides a sub-diffraction spatial resolution of 15 nm on the basis of strong local signal enhancement, which allows for nanoscale imaging of the photoluminescence (PL) intensity and energy along single semiconducting SWNTs. Thereby, the mobility of excitons and their interaction with defects and spatial exciton energy variations can be directly visualized. Similarly, the local Raman scattering properties of metallic SWNTs have been investigated, revealing the microscopic relation of localized defects and the resulting Raman D-band intensity. The first part of the thesis presents a newly developed numerical description of exciton mobility and local quenching at defect sites, accounting also for the TENOM imaging process. This highly flexible model is used to quantitatively evaluate experimental observations such as photo-induced PL blinking and strong spatial PL intensity variations of single semiconducting SWNTs. The main finding is that exciton propagation can be described as ne-dimensional diffusion with a diffusion length of 100 nm for the studied nanotubes, determined independently from both the PL blinking characteristics and the direct visualization using high-resolution TENOM. The temporal and spatial PL variations result from efficient exciton quenching at localized defects and the nanotube ends. The second part reports on the first observation of exciton localization in SWNTs at room temperature, leading to strongly confined and bright PL emission. Localization results from narrow exciton energy minima with depths of more than 15 meV, evidenced by energy-resolved near-field PL imaging. Complementary simulations using a modified numerical model accounting for energy gradients are in good agreement, predicting a significant directed diffusion towards energy minima yielding locally enhanced exciton densities. The energy variations are attributed to inhomogeneous DNA-wrapping of the nanotubes, used for their separation during sample preparation. In the last part, the microscopic relation between the defect-induced Raman D-band and the defect density has been investigated for metallic SWNTs. The length scale of the D-band scattering process in the vicinity of defects was imaged with TENOM for the first time and found to be about 2 nm. Furthermore, localized defects have been photo-generated intentionally by the strong fields at the tip while recording the evolution of the local Raman spectrum. Based on this data, a quantitative relation could be determined, that is highly relevant for the characterization of carbon nanotubes via Raman spectroscopy

Coupling of emitters to surface plasmons investigated by back focal plane microscopy

Current efforts in the field of plasmonics towards device integration and miniaturization require detailed knowledge about the coupling between surface plasmons and emitters. In this work coupling between surface plasmon polaritons and different emitter systems has been investigated by the technique of back focal plane imaging. To develop a deeper understanding of the interaction phenomena the studies focused on single emitters in elementary plasmonic configurations that allow for an analytical description. The first part of the thesis reports on the successful demonstration of surface plasmon polaritons launched by a single dipolar carbon nanotube emitter on a metal thin film after local optical excitation. Leakage radiation microscopy images, recorded in the back focal plane of a microscope objective, could be modeled successfully and contained the propagation length and direction of surface plasmon polaritons. Corresponding real-space images revealed plasmon propagation away from the single dipolar plasmon source. The polarization behavior of surface plasmon polaritons launched by single carbon nanotubes was found to be radial as predicted by theoretical calculations. Remote excitation of single walled carbon nanotube excitons via propagating surface plasmons is demonstrated in the second part. A scanning aperture probe was used as source for propagating surface plasmons with fine controllability over excitation position and propagation direction. It was raster scanned in close proximity over a single carbon nanotube located on a metal film while recording the emission response from the nanotube. The carbon nanotube showed an emission response while the aperture plasmon source was still far away from the nanotube position. Theoretical modeling of the excited surface plasmon fields confirmed that the nanotube maps the surface plasmons locally with sub-diffraction resolution. In the last part, radiation channels in the vicinity of a plasmonic nanowire were investigated. Radiation patterns of a coupled system of rare earth nanocrystals and silver nanowires in the back focal plane revealed that the emission in the vicinity of a nanowire can be approximately described by two emission channels that can be calculated analytically: Dipolar emission, also observed in the absence of the nanowire, and leakage radiation from the nanowire. The latter can be calculated using an antenna-resonator model that considers the air-dielectric interface on which the nanowire is deposited and the position of excitation along the nanowire. Fitting of the experimentally observed patterns provides estimates for the branching ratio between the two emission channels and further enable the determination of the plasmon wave-vector supported by the nanowires

Electrochemical growth of three-dimensionally ordered macroporous metals as photonic crystals

Over the last two decades three dimensionally ordered macroporous (3-DOM) materials have turned out to be very promising in many applications ranging from optics, plasmonics, to catalyst scaffolds. The thesis presents a systematic study on formation and characterisation of 3-DOM metals as photonic crystals. Metals are nearly perfect reflectors with low adsorption at microwave or millimetre wavelengths. Meanwhile they generally absorb visible light because of their negative imaginary part of the dielectric constant that could destroy the band gap in the visible though they. Howevers, for noble metals such as gold, silver and copper, considering the Drude-like behaviour, the adsorption will be small enough to achieve a complete photonic band gap for optical or even shorter wavelengths, with silver performing the best. In order to fabricate the 3-DOM metallic nanostructures, template-directed electrochemical deposition has been employed in which, initially a highly ordered film of submircon sized colloidal spheres is deposited on to electronically conducting substrates, for instance, indium-tin oxide (ITO) coated glass substrate, through evaporation-induced self-assembly; and subsequently it is infiltrated with metallic elements electrochemically reduced from corresponding electrolytes; fiannly removal of the colloidal templating film reveals a metallic film comprised of periodically arranged spherical voids. Field Emission Gun Scanning Electron Microscopy (FEGSEM) was used to examine the surface morphology and periodicity of the 3-DOM metallic films. It revealed that highly ordered structures are homogenous and uniform over a large scale for both the original colloidal templates and metallic inverse structures. However for silver electroplated from either silver thiosulfate or silver chlorate bath, voids in the template are fully infiltrated, including both the interstitial spaces between the colloidal spheres and any cracks between film domains, forming a complete solid network over large length scales; for copper the filling factors are strongly dependent on the bath chemistry and in copper sulfate bath isolated macroporous domains can be formed due to those in the cracks will be dissolved back to the solution while those reduced from copper glycerol bath resulted in fully infiltrated structures. Moreover, angle-resolved reflectance spectroscopy has further confirmed the three-dimensional periodicity and indicated the inverse structures have stop band properties in the visible wavelength region, consistent with variation in the effective refractive index of the films. In addition, surface enhanced Raman scattering (SERS) spectroscopy has been used to evaluate applications of the inverse metals as SERS-active substrates. SERS has nearly exclusively been associated with three noble metals copper, silver (by far the most important) and gold. The 3-DOM metallic thin films possess excellent features for SERS detection arising from their long range periodical void geometry, which gives significant enhancement to Raman intensity. Preliminary measurements have demonstrated the 3-DOM metallic structures are well suited for SERS enhancement. Series spectra from different points of each specimen have given reproducible intensities. Variables associated with Raman intensity such as pore size, dye concentration, and film thickness, have been tuned to achieve maximal enhancement for visible and near-IR wavelengths