SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Vacuum

146

To pattern electrical metal contacts, electron beam lithography or photolithography are commonly utilized, and these processes require polymer resists with solvents. During the patterning process the graphene surface is exposed to chemicals, and the residue on the graphene surface was unable to be completely removed by any method, causing the graphene layer to be contaminated. A lithography free method can overcome these residue problems. In this study, we use a micro-grid as a shadow mask to fabricate a graphene based field-effect-transistor (FET). Electrical measurements of the graphene based FET samples are carried out in air and vacuum. It is found that the Dirac peaks of the graphene devices on SiO2 or on hexagonal boron nitride (hBN) shift from a positive gate voltage region to a negative region as air pressure decreases. In particular, the Dirac peaks shift very rapidly when the pressure decreases from ~2 × 10(-3) Torr to ~5 × 10(-5) Torr within 5 minutes. These Dirac peak shifts are known as adsorption and desorption of environmental gases, but the shift amounts are considerably different depending on the fabrication process. The high gas sensitivity of the device fabricated by shadow mask is attributed to adsorption on the clean graphene surface.

Concepts: Electron, Pressure, Gas, Pascal, Photolithography, Vacuum, Electron beam lithography, Boron nitride

136

There have been several long-standing problems of cold field emission sources for electron microscopy and lithography that have prevented their widespread use, such as their inherent ultrahigh vacuum condition requirement (<10-9 torr), relatively poor current stability and rapid emission decay. This paper presents a cold field emission electron source which overcomes these problems based upon using a graphene-coated nickel point cathode. Preliminary experiments demonstrate that it provides stable emission for relatively large tip diameters (micron sizes), can operate in high vacuum conditions (>10-8 torr) and has an ultralow work function value of 1.10 ± 0.07 eV. It has an estimated reduced brightness value of 1.46 × 109 A m-2 sr-1 V-1for cathode tip-radius of 170 nm and the measured energy spread ranges from 0.246 eV to 0.420 eV for a tip radii range of 260 nm to 500 nm, which is comparable to state-of-the-art conventional cold field emission sources.

Concepts: Electron, Vacuum tube, Mathematics, Function, Quantum mechanics, Fundamental physics concepts, Vacuum, Thermionic emission

95

Thermal chemical vapour deposition techniques for graphene fabrication, while promising, are thus far limited by resource-consuming and energy-intensive principles. In particular, purified gases and extensive vacuum processing are necessary for creating a highly controlled environment, isolated from ambient air, to enable the growth of graphene films. Here we exploit the ambient-air environment to enable the growth of graphene films, without the need for compressed gases. A renewable natural precursor, soybean oil, is transformed into continuous graphene films, composed of single-to-few layers, in a single step. The enabling parameters for controlled synthesis and tailored properties of the graphene film are discussed, and a mechanism for the ambient-air growth is proposed. Furthermore, the functionality of the graphene is demonstrated through direct utilization as an electrode to realize an effective electrochemical genosensor. Our method is applicable to other types of renewable precursors and may open a new avenue for low-cost synthesis of graphene films.

Concepts: Electrochemistry, Carbon nanotube, Chemical vapor deposition, Silicon carbide, Metaphysics, Semiconductor device fabrication, Vacuum, Physical vapor deposition

64

We report the first occurrence of an icosahedral quasicrystal with composition Al62.0(8)Cu31.2(8)Fe6.8(4), outside the measured equilibrium stability field at standard pressure of the previously reported Al-Cu-Fe quasicrystal (AlxCuyFez, with x between 61 and 64, y between 24 and 26, z between 12 and 13%). The new icosahedral mineral formed naturally and was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite that experienced shock metamorphism, local melting (with conditions exceeding 5 GPa and 1,200 °C in some locations), and rapid cooling, all of which likely resulted from impact-induced shock in space. This is the first example of a quasicrystal composition discovered in nature prior to being synthesized in the laboratory. The new composition was found in a grain that has a separate metal assemblage containing icosahedrite (Al63Cu24Fe13), currently the only other known naturally occurring mineral with icosahedral symmetry (though the latter composition had already been observed in the laboratory prior to its discovery in nature). The chemistry of both the icosahedral phases was characterized by electron microprobe, and the rotational symmetry was confirmed by means of electron backscatter diffraction.

Concepts: Chondrite, Observation, Knowledge, Atmospheric pressure, Vacuum, Meteorite, Rotational symmetry, Point groups in three dimensions

39

We compared the expelled air dispersion distances during coughing from a human patient simulator (HPS) lying at 45° with and without wearing a surgical mask or N95 mask in a negative pressure isolation room.

Concepts: Vacuum, Exhalation, Ben Stein

38

We report the first meta-analysis on the impact of prophylactic use of a specific design of negative pressure wound therapy (NPWT) device on surgical site complications.

Concepts: Chronic wound, Vacuum, Negative pressure wound therapy

38

In various shocked meteorites, low-pressure silica polymorph α-cristobalite is commonly found in close spatial relation with the densest known SiO2 polymorph seifertite, which is stable above ∼80 GPa. We demonstrate that under hydrostatic pressure α-cristobalite remains untransformed up to at least 15 GPa. In quasi-hydrostatic experiments, above 11 GPa cristobalite X-I forms-a monoclinic polymorph built out of silicon octahedra; the phase is not quenchable and back-transforms to α-cristobalite on decompression. There are no other known silica polymorphs, which transform to an octahedra-based structure at such low pressures upon compression at room temperature. Further compression in non-hydrostatic conditions of cristobalite X-I eventually leads to the formation of quenchable seifertite-like phase. Our results demonstrate that the presence of α-cristobalite in shocked meteorites or rocks does not exclude that materials experienced high pressure, nor is the presence of seifertite necessarily indicative of extremely high peak shock pressures.

Concepts: Thermodynamics, Silicon, Pressure, Pascal, Quartz, Silicon dioxide, Vacuum, Cristobalite

28

Using Scanning Tunneling Microscopy, we demonstrate that the 1,3-dipolar cycloaddition between a terminal alkyne and an azide can be performed under solvent-free ultrahigh vacuum conditions with reactants adsorbed on a Cu(111) surface. X-ray Photoelectron Spectroscopy shows significant degradation of the azide upon adsorption, which is found to be the limiting factor for the reaction.

Concepts: Electron, Electron microscope, Light, Microscopy, X-ray photoelectron spectroscopy, Azide alkyne Huisgen cycloaddition, Vacuum, Ultra high vacuum

28

Xylem and phloem are essential for the exchange of solutes and signals among organs of land plants. The synergy of both enables the transport and ultimately the partitioning of water, nutrients, metabolic products and signals among the organs of plants. The collection and analysis of xylem sap allow at least qualitative assumptions about bulk transport in the transpiration stream. For quantification of element-, ion-, and compound-flow, the additional estimation of volume flow is necessary. In this chapter we describe methods for collecting xylem sap by (1) root pressure exudate, (2) Scholander-Hammel pressure vessel, (3) root pressurizing method according to Passioura, and (4) (hand/battery) vacuum pump.

Concepts: Plant, Water, Pressure, Vascular plant, Plant physiology, Xylem, Vacuum, Plant sap

28

Time of Flight secondary ion mass spectrometry (TOF-SIMS) has been used to explore the distribution of phospholipids in the plasma membrane of Tetrahymena pyriformis during cell division. The dividing cells were freeze dried prior to analysis followed by line scan and region of interest analysis at various stages of cell division. The results showed no signs of phospholipid domain formation at the junction between the dividing cells. Instead the results showed that the sample preparation technique had a great impact on one of the examined phospholipids, namely phosphatidylcholine (PC). Phosphatidylcholine and 2-aminoethylphosphonolipid (2-AEP) have therefore been evaluated in Tetrahymena cells that have been subjected to different sample preparation techniques: freeze drying ex situ, freeze fracture, and freeze fracture with partial or total freeze drying in situ. The result suggests that freeze-drying ex situ causes the celia to collapse and cover the plasma membrane.

Concepts: Cell membrane, Food preservation, Freeze drying, Vacuum, Sublimation