165 research outputs found
ab initio Electronic Transport Model with Explicit Solution to the Linearized Boltzmann Transport Equation
Accurate models of carrier transport are essential for describing the
electronic properties of semiconductor materials. To the best of our knowledge,
the current models following the framework of the Boltzmann transport equation
(BTE) either rely heavily on experimental data (i.e., semi-empirical), or
utilize simplifying assumptions, such as the constant relaxation time
approximation (BTE-cRTA). While these models offer valuable physical insights
and accurate calculations of transport properties in some cases, they often
lack sufficient accuracy -- particularly in capturing the correct trends with
temperature and carrier concentration. We present here a general transport
model for calculating low-field electrical drift mobility and Seebeck
coefficient of n-type semiconductors, by explicitly considering all relevant
physical phenomena (i.e. elastic and inelastic scattering mechanisms). We first
rewrite expressions for the rates of elastic scattering mechanisms, in terms of
ab initio properties, such as the band structure, density of states, and polar
optical phonon frequency. We then solve the linear BTE to obtain the
perturbation to the electron distribution -- resulting from the dominant
scattering mechanisms -- and use this to calculate the overall mobility and
Seebeck coefficient. Using our model, we accurately calculate electrical
transport properties of the compound n-type semiconductors, GaAs and InN, over
various ranges of temperature and carrier concentration. Our fully predictive
model provides high accuracy when compared to experimental measurements on both
GaAs and InN, and vastly outperforms both semi-empirical models and the
BTE-cRTA. Therefore, we assert that this approach represents a first step
towards a fully ab initio carrier transport model that is valid in all compound
semiconductors
Few electron double quantum dot in an isotopically purified Si quantum well
We present a few electron double quantum dot (QD) device defined in an
isotopically purified Si quantum well (QW). An electron mobility of is observed in the QW which is the highest mobility
ever reported for a 2D electron system in Si. The residual concentration
of Si nuclei in the Si QW is lower than , at the
verge where the hyperfine interaction is theoretically no longer expected to
dominantly limit the spin dephasing time. We also demonstrate a
complete suppression of hysteretic gate behavior and charge noise using a
negatively biased global top gate.Comment: 4 pages, 3 figure
Electrochemical CO reduction builds solvent water into oxygenate products
Numerous studies have examined the electrochemical reduction of CO (COR) to oxygenates (e.g., ethanol). None have considered the possibility that oxygen in the product might arise from water rather than from CO. To test this assumption, C^(16)O reduction was performed in H_2^(18)O electrolyte. Surprisingly, we found that 60–70% of the ethanol contained 18O, which must have originated from the solvent. We extended our previous all-solvent density functional theory metadynamics calculations to consider the possibility of incorporating water, and indeed, we found a new mechanism involving a Grotthuss chain of six water molecules in a concerted reaction with the *C–CH intermediate to form *CH–CH(^(18)OH), subsequently leading to (^(18)O)ethanol. This competes with the formation of ethylene that also arises from *C–CH. These unforeseen results suggest that all previous studies of COR under aqueous conditions must be reexamined
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Aging and Fracture of Human Cortical Bone and Tooth Dentin
Mineralized tissues, such as bone and tooth dentin, serve as structural materials in the human body and, as such, have evolved to resist fracture. In assessing their quantitative fracture resistance or toughness, it is important to distinguish between intrinsic toughening mechanisms which function ahead of the crack tip, such as plasticity in metals, and extrinsic mechanisms which function primarily behind the tip, such as crack bridging in ceramics. Bone and dentin derive their resistance to fracture principally from extrinsic toughening mechanisms which have their origins in the hierarchical microstructure of these mineralized tissues. Experimentally, quantification of these toughening mechanisms requires a crack-growth resistance approach, which can be achieved by measuring the crack-driving force, e.g., the stress intensity, as a function of crack extension ("R-curve approach"). Here this methodology is used to study of the effect of aging on the fracture properties of human cortical bone and human dentin in order to discern the microstructural origins of toughness in these materials
Effect of Native Defects on Optical Properties of InxGa1-xN Alloys
The energy position of the optical absorption edge and the free carrier
populations in InxGa1-xN ternary alloys can be controlled using high energy
4He+ irradiation. The blue shift of the absorption edge after irradiation in
In-rich material (x > 0.34) is attributed to the band-filling effect
(Burstein-Moss shift) due to the native donors introduced by the irradiation.
In Ga-rich material, optical absorption measurements show that the
irradiation-introduced native defects are inside the bandgap, where they are
incorporated as acceptors. The observed irradiation-produced changes in the
optical absorption edge and the carrier populations in InxGa1-xN are in
excellent agreement with the predictions of the amphoteric defect model
Electrochemical CO reduction builds solvent water into oxygenate products
Numerous studies have examined the electrochemical reduction of CO (COR) to oxygenates (e.g., ethanol). None have considered the possibility that oxygen in the product might arise from water rather than from CO. To test this assumption, C^(16)O reduction was performed in H_2^(18)O electrolyte. Surprisingly, we found that 60–70% of the ethanol contained 18O, which must have originated from the solvent. We extended our previous all-solvent density functional theory metadynamics calculations to consider the possibility of incorporating water, and indeed, we found a new mechanism involving a Grotthuss chain of six water molecules in a concerted reaction with the *C–CH intermediate to form *CH–CH(^(18)OH), subsequently leading to (^(18)O)ethanol. This competes with the formation of ethylene that also arises from *C–CH. These unforeseen results suggest that all previous studies of COR under aqueous conditions must be reexamined
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Fatigue threshold R-curves predict small crack fatigue behavior of bridging toughened materials
Small crack fatigue is a widely recognized problem in the fatigue of materials; however, there has been limited progress in developing methods for predicting small crack fatigue behavior. In this paper, small crack effects due to crack bridging are addressed. A fatigue threshold R-curve was measured for a 99.5% pure polycrystalline alumina using standard compact tension specimens and it was used to 1) determine the bridging stress profile for the material and 2) make fatigue endurance strength predictions for realistic semi-elliptical surface cracks. Furthermore, is has been shown that the fatigue threshold R-curve can equivalently be determined by measuring the bridging stress distribution, in this case using fluorescence spectroscopy, using only a long crack compact tension specimen without the need for difficult small crack experiments. It is expected that this method will be applicable to a wide range of bridging toughened materials, including composites, toughened ceramics, intermetallics, and multi-phase materials.Keywords: fracture, toughness, fatigue, crack bridgingKeywords: fracture, toughness, fatigue, crack bridgin
A nanoporous capacitive electrochemical ratchet for continuous ion separations
Directed ion transport in liquid electrolyte solutions underlies numerous
phenomena in nature and industry including neuronal signaling, photosynthesis
and respiration, electrodialysis for desalination, and recovery of critical
materials. Here, we report the first demonstration of an ion pump that drives
ions in aqueous electrolytes against a force using a capacitive ratchet
mechanism. Our ratchet-based ion pumps utilize the non-linear capacitive nature
of electric double layers for symmetry breaking which drives a net
time-averaged ion flux in response to a time varying input signal. Since the
devices are driven by a non-linear charging and discharging of double layers,
they do not require redox reactions for continual operation. Ratchet-based ion
pumps were fabricated by depositing thin gold layers on the two surfaces of
anodized alumina wafers, forming nanoporous capacitor-like structures. Pumping
occurs when a wafer is placed between two compartments of aqueous electrolyte
and the electric potential across it is modulated. In response to various input
signals, persistent ionic voltages and sustained currents were observed,
consistent with net unidirectional ion transport, even though conduction
through the membrane was non-rectifying. The generated ionic power was used in
conjunction with an additional shunt pathway to demonstrate electrolyte
demixing
Band offset determination of the GaAs/GaAsN interface using the DFT method
The GaAs/GaAsN interface band offset is calculated from first principles. The
electrostatic potential at the core regions of the atoms is used to estimate
the interface potential and align the band structures obtained from respective
bulk calculations. First, it is shown that the present method performs well on
the well-known conventional/conventional AlAs/GaAs (001) superlattice system.
Then the method is applied to a more challenging nonconventional/conventional
GaAsN/GaAs (001) system, and consequently type I band lineup and valence-band
offset of about 35 meV is obtained for nitrogen concentration of about 3 %, in
agreement with the recent experiments. We also investigate the effect of strain
on the band lineup. For the GaAsN layer longitudinally strained to the GaAs
lattice constant, the type II lineup with a nearly vanishing band offset is
found, suggesting that the anisotropic strain along the interface is the
principal cause for the often observed type I lineup
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