Optomechanical and Crystallization Phenomena Visualized with 4D Electron Microscopy: Interfacial Carbon Nanotubes on Silicon Nitride

With ultrafast electron microscopy (UEM), we report observation of the nanoscopic crystallization of amorphous silicon nitride, and the ultrashort optomechanical motion of the crystalline silicon nitride at the interface of an adhering carbon nanotube network. The in situ static crystallization of the silicon nitride occurs only in the presence of an adhering nanotube network, thus indicating their mediating role in reaching temperatures close to 1000 °C when exposed to a train of laser pulses. Under such condition, 4D visualization of the optomechanical motion of the specimen was followed by quantifying the change in diffraction contrast of crystalline silicon nitride, to which the nanotube network is bonded. The direction of the motion was established from a tilt series correlating the change in displacement with both the tilt angle and the response time. Correlation of nanoscopic motion with the picosecond atomic-scale dynamics suggests that electronic processes initiated in the nanotubes are responsible for the initial ultrafast optomechanical motion. The time scales accessible to UEM are 12 orders of magnitude shorter than those traditionally used to study the optomechanical motion of carbon nanotube networks, thus allowing for distinctions between the different electronic and thermal mechanisms to be made

Nanofriction Visualized in Space and Time by 4D Electron Microscopy

In this letter, we report a novel method of visualizing nanoscale friction in space and time using ultrafast electron microscopy (UEM). The methodology is demonstrated for a nanoscale movement of a single crystal beam on a thin amorphous membrane of silicon nitride. The movement results from the elongation of the crystal beam, which is initiated by a laser (clocking) pulse, and we examined two types of beams: those that are free of friction and the others which are fixed on the substrate. From observations of image change with time we are able to decipher the nature of microscopic friction at the solid−solid interface: smooth-sliding and periodic slip-stick friction. At the molecular and nanoscale level, and when a force parallel to the surface (expansion of the beam) is applied, the force of gravity as a (perpendicular) load cannot explain the observed friction. An additional effective load being 6 orders of magnitude larger than that due to gravity is attributed to Coulombic/van der Waals adhesion at the interface. For the case under study, metal−organic crystals, the gravitational force is on the order of piconewtons whereas the static friction force is 0.5 μN and dynamic friction is 0.4 μN; typical beam expansions are 50 nm/nJ for the free beam and 10 nm/nJ for the fixed beam. The method reported here should have applications for other materials, and for elucidating the origin of periodic and chaotic friction and their relevance to the efficacy of nano(micro)-scale devices

Irreversible Chemical Reactions Visualized in Space and Time with 4D Electron Microscopy

We report direct visualization of irreversible chemical reactions in space and time with 4D electron microscopy. Specifically, transient structures are imaged following electron transfer in copper-tetracyanoquinodimethane [Cu(TCNQ)] crystals, and the oxidation/reduction process, which is irreversible, is elucidated using the single-shot operation mode of the microscope. We observed the fast, initial structural rearrangement due to Cu^+ reduction and the slower growth of metallic Cu^0 nanocrystals (Ostwald ripening) following initiation of the reaction with a pulse of visible light. The mechanism involves electron transfer from TCNQ anion-radical to Cu^+, morphological changes, and thermally driven growth of discrete Cu^0 nanocrystals embedded in an amorphous carbon skeleton of TCNQ. This in situ visualization of structures during reactions should be extendable to other classes of reactive systems

Commentary: Energy Deregulation in Maine

Recent stories from California of power blackouts, utility bankruptcies, and skyrocketing rates have left many wondering whether Maine is going to suffer a similar fate. Like California, Maine has deregulated its electricity supply—an idea that sounded good to many, but which now has some questioning whether consumers will be made better off or worse. To address these issues, MPR asked six analysts to comment on electricity deregulation in Maine. Some address whether Maine is destined to follow in California’s footsteps. Others question whether regional decision making entities, such as the New England Power Pool and the Independent System Operator of New England, sufficiently represent the public interest. Still others address whether there is a future role for public conservation programs. Together, they suggest Maine will not befall the fate of California, but they also suggest that electricity deregulation in Maine may bring its own troubles if we’re not attentive and forward-thinking today

Photon-induced near-field electron microscopy

In materials science and biology, optical near-field microscopies enable spatial resolutions beyond the diffraction limit, but they cannot provide the atomic-scale imaging capabilities of electron microscopy. Given the nature of interactions between electrons and photons, and considering their connections through nanostructures, it should be possible to achieve imaging of evanescent electromagnetic fields with electron pulses when such fields are resolved in both space (nanometre and below) and time (femtosecond). Here we report the development of photon-induced near-field electron microscopy (PINEM), and the associated phenomena. We show that the precise spatiotemporal overlap of femtosecond single-electron packets with intense optical pulses at a nanostructure (individual carbon nanotube or silver nanowire in this instance) results in the direct absorption of integer multiples of photon quanta (nħω) by the relativistic electrons accelerated to 200 keV. By energy-filtering only those electrons resulting from this absorption, it is possible to image directly in space the near-field electric field distribution, obtain the temporal behaviour of the field on the femtosecond timescale, and map its spatial polarization dependence. We believe that the observation of the photon-induced near-field effect in ultrafast electron microscopy demonstrates the potential for many applications, including those of direct space-time imaging of localized fields at interfaces and visualization of phenomena related to photonics, plasmonics and nanostructures

A review of operations and maintenance modelling with considerations for novel wind turbine concepts

New wind turbine technologies and designs are being explored in order to reduce the cost of energy from offshore wind farms. Two potential routes to a lower cost of energy are the XRotor Concept (XRC) and Multi-Rotor System (MRS) turbines. A key cost saving for both Novel concepts included in this paper is operation and maintenance (O&M) costs savings. The major component replacement cost for conventional horizontal axis, XRC and MRS turbines are examined and the benefits of the concepts are provided in this paper. A review on existing decision support systems for offshore wind farm O&M planning is presented with a focus on how applicable these previous models are to novel turbine concepts, along with analysis of how the influential factors can be modified to effectively model XRC and MRS

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%–85% of permafrost carbon release can still be avoided if human emissions are actively reduced

Boreal forest floor greenhouse gas emissions across a Pleurozium schreberi-dominated, wildfire-disturbed chronosequence

The boreal forest is a globally critical biome for carbon cycling. Its forests are shaped by wildfire events that affect ecosystem properties and climate feedbacks including greenhouse gas (GHG) emissions. Improved understanding of boreal forest floor processes is needed to predict the impacts of anticipated increases in fire frequency, severity, and extent. In this study, we examined relationships between time since last wildfire (TSF), forest floor soil properties, and GHG emissions (CO2, CH4, N2O) along a Pleurozium schreberi-dominated chronosequence in mid- to late succession located in northern Sweden. Over three growing seasons in 2012–2014, GHG flux measurements were made in situ and samples were collected for laboratory analyses. We predicted that P. schreberi-covered forest floor GHG fluxes would be related to distinct trends in the soil properties and microbial community along the wildfire chronosequence. Although we found no overall effect of TSF on GHG emissions, there was evidence that soil C/N, one of the few properties to show a trend with time, was inversely linked to ecosystem respiration. We also found that local microclimatic conditions and site-dependent properties were better predictors of GHG fluxes than TSF. This shows that site-dependent co-variables (that is, forest floor climate and plant-soil properties) need to be considered as well as TSF to predict GHG emissions as wildfires become more frequent, extensive and severe

Persistent Pain Among Older Adults Discharged Home From the Emergency Department After Motor Vehicle Crash: A Prospective Cohort Study

Motor vehicle collisions (MVCs) are the second most common form of trauma among older adults. We sought to describe the incidence, risk factors, and consequences of persistent pain among older adults evaluated in the emergency department (ED) after an MVC