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Topic Review
Gallium Nitride Epitaxy
Gallium nitride (GaN) is a wide-bandgap semiconductor material with excellent electrical and optical properties, making it a promising candidate for various electronic and optoelectronic devices. In particular, the unique characteristics of GaN make it a popular choice for high-power and high-frequency applications, such as power electronics, RF amplifiers, and light-emitting diodes (LEDs).
  • 715
  • 25 Apr 2023
Topic Review
Gabor-Domain Optical Coherence Microscopy
Gabor-domain optical coherence microscopy (GDOCM) is a high transverse resolution variant of spectral domain optical coherence tomography (SD-OCT). It was proposed to break the cellular resolution limit of optical coherence tomography (OCT). GDOCM achieves invariant transverse and axial resolutions of 2 micron in 3D by fusing together multiple volumetric images that are acquired employing a liquid lens to dynamically refocus at different depths inside the sample with no moving parts.
  • 2.5K
  • 29 Oct 2020
Topic Review
G-Factor
A g-factor (also called g value or dimensionless magnetic moment) is a dimensionless quantity that characterizes the magnetic moment and angular momentum of an atom, a particle or the nucleus. It is essentially a proportionality constant that relates the different observed magnetic moments μ of a particle to their angular momentum quantum numbers and a unit of magnetic moment (to make it dimensionless), usually the Bohr magneton or nuclear magneton.
  • 3.5K
  • 28 Oct 2022
Topic Review
Fundamentals and Mechanisms of Light-Activated Gas Sensors
The light-activated gas sensors show promising results, particularly using visible light as an external trigger that lowers the power consumption as well as improves the stability, sensitivity and safety of the sensors. It effectively eliminates the possible damage to sensing material caused by high operating temperature or high energy light. 
  • 119
  • 07 Mar 2024
Topic Review
Fundamental Interaction
In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist: the gravitational and electromagnetic interactions, which produce significant long-range forces whose effects can be seen directly in everyday life, and the strong and weak interactions, which produce forces at minuscule, subatomic distances and govern nuclear interactions. Some scientists hypothesize that a fifth force might exist, but these hypotheses remain speculative. Each of the known fundamental interactions can be described mathematically as a field. The gravitational force is attributed to the curvature of spacetime, described by Einstein's general theory of relativity. The other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics. Within the Standard Model, the strong interaction is carried by a particle called the gluon, and is responsible for quarks binding together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction is carried by particles called W and Z bosons, and also acts on the nucleus of atoms, mediating radioactive decay. The electromagnetic force, carried by the photon, creates electric and magnetic fields, which are responsible for the attraction between orbital electrons and atomic nuclei which holds atoms together, as well as chemical bonding and electromagnetic waves, including visible light, and forms the basis for electrical technology. Although the electromagnetic force is far stronger than gravity, it tends to cancel itself out within large objects, so over large (astronomical) distances gravity tends to be the dominant force, and is responsible for holding together the large scale structures in the universe, such as planets, stars, and galaxies. Many theoretical physicists believe these fundamental forces to be related and to become unified into a single force at very high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this. Devising a common theoretical framework that would explain the relation between the forces in a single theory is perhaps the greatest goal of today's theoretical physicists. The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg for which they received the 1979 Nobel Prize in physics. Some physicists seek to unite the electroweak and strong fields within what is called a Grand Unified Theory (GUT). An even bigger challenge is to find a way to quantize the gravitational field, resulting in a theory of quantum gravity (QG) which would unite gravity in a common theoretical framework with the other three forces. Some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything (ToE).
  • 666
  • 15 Nov 2022
Topic Review
Fully Integrated Photoacoustic NO2 Sensor
A fully integrated photoacoustic nitrogen dioxide (NO2) sensor is developed and demonstrated. In this sensor, an embedded photoacoustic cell was manufactured by using an up-to-date 3D printing technique. A blue laser diode was used as a light source for excitation of photoacoustic wave in the photoacoustic cell. The photoacoustic wave is detected by a sensitive microelectromechanical system (MEMS) microphone. Homemade circuits are integrated into the sensor for laser diode driving and signal processing. The sensor was calibrated by using a chemiluminescence NO–NO2–NOX gas analyzer. And the performance of this sensor was evaluated. The linear relationship between photoacoustic signals and NO2 concentrations was verified in a range of below 202 ppb. The limit of detection was determined to 0.86 ppb with an integration time of 1 s. The corresponding normalized noise equivalent absorption was 2.0 × 10−8 cm−1∙W∙Hz−1/2. The stability and the optimal integration time were evaluated with an Allan deviation analysis, from which a detection limit of 0.25 ppb at the optimal integration time of 240 s was obtained. The sensor was used to measure outdoor air and the results agree with that obtained from the NO–NO2–NOX gas analyzer. The low-cost and portable photoacoustic NO2 sensor has a potential application for atmospheric NO2 monitoring.
  • 995
  • 01 Nov 2020
Topic Review
FTIR for Vehicle Exhaust Emissions
In a Fourier Transform InfraRed (FTIR) spectrometer, some of the infrared (IR) radiation is absorbed by the sample, and some of it is passed through (transmitted). The resulting molecular absorption and transmission response can be used to identify the components of the sample and their concentration.
  • 1.7K
  • 20 Aug 2021
Topic Review
Frost Diagram
A Frost diagram or Frost–Ebsworth diagram is a type of graph used by inorganic chemists in electrochemistry to illustrate the relative stability of a number of different oxidation states of a particular substance. The graph illustrates the free energy vs oxidation state of a chemical species. This effect is dependent on pH, so this parameter also must be included. The free energy is determined by the oxidation–reduction half-reactions. The Frost diagram allows easier comprehension of these reduction potentials than the earlier-designed Latimer diagram, because the “lack of additivity of potentials” was confusing. The free energy ΔG° is related to reduction potential E in the graph by given formula: ΔG° = −nFE° or nE° = −ΔG°/F, where n is the number of transferred electrons, and F is Faraday constant (F = 96,485 J/(V·mol)). The Frost diagram is named after Arthur Atwater Frost (de), who originally created it as a way to "show both free energy and oxidation potential data conveniently" in a 1951 paper.
  • 3.4K
  • 01 Dec 2022
Biography
Friedrich Hund
Friedrich Hermann Hund (4 February 1896 – 31 March 1997) was a Germany physicist from Karlsruhe known for his work on atoms and molecules.[1] Hund worked at the Universities of Rostock, Leipzig, Jena, Frankfurt am Main, and Göttingen. Hund worked with such prestigious physicists as Schrödinger, Dirac, Heisenberg, Max Born, and Walter Bothe. At that time, he was Born's assistant, working
  • 1.1K
  • 18 Nov 2022
Topic Review
Friction Force Microscope
In materials science, chemical force microscopy (CFM) is a variation of atomic force microscopy (AFM) which has become a versatile tool for characterization of materials surfaces. With AFM, structural morphology is probed using simple tapping or contact modes that utilize van der Waals interactions between tip and sample to maintain a constant probe deflection amplitude (constant force mode) or maintain height while measuring tip deflection (constant height mode). CFM, on the other hand, uses chemical interactions between functionalized probe tip and sample. Choice chemistry is typically gold-coated tip and surface with R–SH thiols attached, R being the functional groups of interest. CFM enables the ability to determine the chemical nature of surfaces, irrespective of their specific morphology, and facilitates studies of basic chemical bonding enthalpy and surface energy. Typically, CFM is limited by thermal vibrations within the cantilever holding the probe. This limits force measurement resolution to ~1 pN which is still very suitable considering weak COOH/CH3 interactions are ~20 pN per pair. Hydrophobicity is used as the primary example throughout this consideration of CFM, but certainly any type of bonding can be probed with this method.
  • 271
  • 17 Nov 2022
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