Black Phosphorus Photodetector for Multispectral, High-Resolution Imaging

Black phosphorus is a layered semiconductor that is intensely researched in view of applications in optoelectronics. In this letter, we investigate a multilayer black phosphorus photodetector that is capable of acquiring high-contrast (<i>V</i> > 0.9) images both in the visible (λ_VIS = 532 nm) as well as in the infrared (λ_IR = 1550 nm) spectral regime. In a first step, by using photocurrent microscopy, we map the active area of the device and we characterize responsivity and gain. In a second step, by deploying the black phosphorus device as a point-like detector in a confocal microsope setup, we acquire diffraction-limited optical images with submicron resolution. The results demonstrate the usefulness of black phosphorus as an optoelectronic material for hyperspectral imaging applications

Symmetry Considerations for the Targeted Assembly of Entropically Stabilized Colloidal Crystals <i>via</i> Voronoi Particles

The relationship between colloidal building blocks and their assemblies is an active field of research. As a strategy for targeting novel crystal structures, we examine the use of Voronoi particles, which are hard, space-filling particles in the shape of Voronoi cells of a target structure. Although Voronoi particles stabilize their target structure in the limit of high pressure by construction, the thermodynamic assembly of the same structure at moderate pressure, close to the onset of crystallization, is not guaranteed. Indeed, we find that a more symmetric crystal is often preferred due to additional entropic contributions arising from configurational or occupational degeneracy. We characterize the assembly behavior of the Voronoi particles in terms of the symmetries of the building blocks as well as the symmetries of crystal structures and demonstrate how controlling the degeneracies through a modification of particle shape and field-directed assembly can significantly improve the assembly propensity

Hot Spot Dynamics in Carbon Nanotube Array Devices

We report on the dynamics of spatial temperature distributions in aligned semiconducting carbon nanotube array devices with submicrometer channel lengths. By using high-resolution optical microscopy in combination with electrical transport measurements, we observe under steady state bias conditions the emergence of time-variable, local temperature maxima with dimensions below 300 nm, and temperatures above 400 K. On the basis of time domain cross-correlation analysis, we investigate how the intensity fluctuations of the thermal radiation patterns are correlated with the overall device current. The analysis reveals the interdependence of electrical current fluctuations and time-variable hot spot formation that limits the overall device performance and, ultimately, may cause device degradation. The findings have implications for the future development of carbon nanotube-based technologies

Antenna-Enhanced Photocurrent Microscopy on Single-Walled Carbon Nanotubes at 30 nm Resolution

We present the first photocurrent measurements along single carbon nanotube (CNT) devices with 30 nm resolution. Our technique is based on tip-enhanced near-field optical microscopy, exploiting the plasmonically enhanced absorption controlled by an optical nanoantenna. This allows for imaging of the zero-bias photocurrent caused by charge separation in local built-in electric fields at the contacts and close to charged particles that cannot be resolved using confocal microscopy. Simultaneously recorded Raman scattering images reveal the structural properties and the defect densities of the CNTs. Antenna-enhanced scanning photocurrent microscopy extends the available set of scanning-probe techniques by combining high-resolution photovoltaic and optical probing and could become a valuable tool for the characterization of nanoelectronic devices

Entropically Patchy Particles: Engineering Valence through Shape Entropy

Patchy particles are a popular paradigm for the design and synthesis of nanoparticles and colloids for self-assembly. In “traditional” patchy particles, anisotropic interactions arising from patterned coatings, functionalized molecules, DNA, and other enthalpic means create the possibility for directional binding of particles into higher-ordered structures. Although the anisotropic geometry of nonspherical particles contributes to the interaction patchiness through van der Waals, electrostatic, and other interactions, how particle shape contributes entropically to self-assembly is only now beginning to be understood. The directional nature of entropic forces has recently been elucidated. A recently proposed theoretical framework that defines and quantifies directional entropic forces demonstrates the anisotropicthat is, patchynature of these emergent, attractive forces. Here we introduce the notion of entropically patchy particles as the entropic counterpart to enthalpically patchy particles. Using three example “families” of shapes, we show how to modify entropic patchiness by introducing geometric features to the particles <i>via</i> shape operations so as to target specific crystal structures assembled here with Monte Carlo simulations. We quantify the emergent entropic valence <i>via</i> a potential of mean force and torque. We show that these forces are on the order of a few <i>k</i>_B<i>T</i> at intermediate densities below the onset of crystallization. We generalize these shape operations to shape anisotropy dimensions, in analogy with the anisotropy dimensions introduced for enthalpically patchy particles. Our findings demonstrate that entropic patchiness and emergent valence provide a way of engineering directional bonding into nanoparticle systems, whether in the presence or absence of additional, non-entropic forces

High-Performance p‑Type Black Phosphorus Transistor with Scandium Contact

A record high current density of 580 μA/μm is achieved for long-channel, few-layer black phosphorus transistors with scandium contacts after 400 K vacuum annealing. The annealing effectively improves the on-state current and <i>I</i>_on/<i>I</i>_off ratio by 1 order of magnitude and the subthreshold swing by ∼2.5×, whereas Al₂O₃ capping significantly degrades transistor performances, resulting in 5× lower on-state current and 3× lower <i>I</i>_on/<i>I</i>_off ratio. The influences of moisture on black phosphorus metal contacts are elucidated by analyzing the hysteresis of 3–20 nm thick black phosphorus transistors with scandium and gold contacts under different conditions: as-fabricated, after vacuum annealing, and after Al₂O₃ capping. The optimal black phosphorus film thickness for transistors with scandium contacts is found to be ∼10 nm. Moreover, p-type performance is shown in all transistors with scandium contacts, suggesting that the Fermi level is pinned closer to the valence band regardless of the flake thickness

Photocurrent Spectroscopy of (<i>n</i>, <i>m</i>) Sorted Solution-Processed Single-Walled Carbon Nanotubes

Variable-wavelength photocurrent microscopy and photocurrent spectroscopy are used to study the photoresponse of (<i>n</i>, <i>m</i>) sorted single-walled carbon nanotube (SWNT) devices. The measurements of (<i>n</i>, <i>m</i>) pure SWCNT devices demonstrate the ability to study the wavelength-dependent photoresponse <i>in situ</i> in a device configuration and deliver photocurrent spectra that reflect the population of the source material. Furthermore, we show that it is possible to map and determine the chirality population within a working optoelectronic SWCNT device

Virial Coefficients and Equations of State for Hard Polyhedron Fluids

Hard polyhedra are a natural extension of the hard sphere model for simple fluids, but there is no general scheme for predicting the effect of shape on thermodynamic properties, even in moderate-density fluids. Only the second virial coefficient is known analytically for general convex shapes, so higher-order equations of state have been elusive. Here we investigate high-precision state functions in the fluid phase of 14 representative polyhedra with different assembly behaviors. We discuss historic efforts in analytically approximating virial coefficients up to <i>B</i>₄ and numerically evaluating them to <i>B</i>₈. Using virial coefficients as inputs, we show the convergence properties for four equations of state for hard convex bodies. In particular, the exponential approximant of Barlow et al. (<i>J. Chem. Phys</i>. <b>2012</b>, <i>137</i>, 204102) is found to be useful up to the first ordering transition for most polyhedra. The convergence behavior we explore can guide choices in expending additional resources for improved estimates. Fluids of arbitrary hard convex bodies are too complicated to be described in a general way at high densities, so the high-precision state data we provide can serve as a reference for future work in calculating state data or as a basis for thermodynamic integration

Long-Term Once-Daily Tiotropium Respimat® Is Well Tolerated and Maintains Efficacy over 52 Weeks in Patients with Symptomatic Asthma in Japan: A Randomised, Placebo-Controlled Study

<div><p>Background</p><p>This study assessed the long-term safety and efficacy of tiotropium Respimat, a long-acting inhaled anticholinergic bronchodilator, in asthma, added on to inhaled corticosteroids (ICS) with or without long-acting β₂-agonist (LABA).</p><p>Methods</p><p>285 patients with symptomatic asthma, despite treatment with ICS±LABA, were randomised 2:2:1 to once-daily tiotropium 5 μg, tiotropium 2.5 μg or placebo for 52 weeks (via the Respimat SoftMist inhaler) added on to ICS±LABA, in a double-blind, placebo-controlled, parallel-group study (NCT01340209). Primary objective: to describe the long-term safety profile of tiotropium. Secondary end points included: trough forced expiratory volume in 1 second (FEV₁) response; peak expiratory flow rate (PEFR) response; seven-question Asthma Control Questionnaire (ACQ-7) score.</p><p>Results</p><p>At Week 52, adverse-event (AE) rates with tiotropium 5 μg, 2.5 μg and placebo were 88.6%, 86.8% and 89.5%, respectively. Commonly reported AEs with tiotropium 5 μg, 2.5 μg and placebo were nasopharyngitis (48.2%, 44.7%, 42.1%), asthma (28.9%, 29.8%, 38.6%), decreased PEFR (15.8%, 7.9%, 21.1%), bronchitis (9.6%, 13.2%, 7.0%), pharyngitis (7.9%, 13.2%, 3.5%) and gastroenteritis (10.5%, 3.5%, 5.3%). In the tiotropium 5 μg, 2.5 μg and placebo groups, 8.8%, 5.3% and 5.3% of patients reported drug-related AEs; 3.5%, 3.5% and 15.8% reported serious AEs. Asthma worsening was the only serious AE reported in more than one patient. At Week 52, adjusted mean trough FEV₁ and trough PEFR responses were significantly higher with tiotropium 5 μg (but not 2.5 μg) versus placebo. ACQ-7 responder rates were higher with tiotropium 5 μg and 2.5 μg versus placebo at Week 24.</p><p>Conclusions</p><p>The long-term tiotropium Respimat safety profile was comparable with that of placebo Respimat, and associated with mild to moderate, non-serious AEs in patients with symptomatic asthma despite ICS±LABA therapy. Compared with placebo, tiotropium 5 μg, but not 2.5 μg, significantly improved lung function and symptoms, supporting the long-term efficacy of the 5 μg dose.</p><p>Trial Registration</p><p>ClinicalTrials.gov <a href="https://www.clinicaltrials.gov/ct2/show/NCT01340209?term=NCT01340209&rank=1" target="_blank">NCT01340209</a></p></div

Overall summary of adverse events (treated set).

<p>^aPatients were randomised 2:2:1 to the tiotropium Respimat 5 μg, tiotropium Respimat 2.5 μg and placebo Respimat groups, respectively.</p><p>^bAs determined by the investigator.</p><p>^cElevation of aspartate aminotransferase and/or alanine aminotransferase ≥3 × upper limit of normal combined with elevated total bilirubin ≥2 × upper limit of normal at the same visit.</p><p>^dAsthma worsening.</p><p>AE, adverse event.</p><p>Overall summary of adverse events (treated set).</p