Improving gas sensing properties of graphene by introducing dopants and defects: a first-principles study

The interactions between four different graphenes (including pristine, B- or N-doped and defective graphenes) and small gas molecules (CO, NO, NO2 and NH3) were investigated by using density functional computations to exploit their potential applications as gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the graphene structure and the molecular adsorption configuration. All four gas molecules show much stronger adsorption on the doped or defective graphenes than that on the pristine graphene. The defective graphene shows the highest adsorption energy with CO, NO and NO2 molecules, while the B- doped graphene gives the tightest binding with NH3. Meanwhile, the strong interactions between the adsorbed molecules and the modified graphenes induce dramatic changes to graphene's electronic properties. The transport behavior of a gas sensor using B- doped graphene shows a sensitivity two orders of magnitude higher than that of pristine graphene. This work reveals that the sensitivity of graphene-based chemical gas sensors could be drastically improved by introducing the appropriate dopant or defect

Long-Run Underperformance And The Offering Price Clustering Phenomenon

The study proposes a new informational role for the offering price of an equity IPO.  Offering prices are quoted either in whole prices (e.g., $2,$11, $19, etc) or fractional prices (e.g.,$2.35, $11.15,$15.75, etc).  Using Jay R. Ritter’s sample of 1,526 IPOs issued during the period 1975 to 1984, the study examines the relation between the presence of whole price clusters and long-run underperformance.  The results indicate that fractional offering prices are associated with better long-run performance. &nbsp

Investor Sentiment And Close-End Country Funds?

An innovative method to estimate the duration of investor sentiment is applied to closed-end country fund returns and it finds that U.S. investor sentiment has a short life.  The effects of sentiment on closed-end country fund returns are largely consistent with existing literature however, it is only apparent in daily time-series regressions.  Sentiment rapidly fades at a weekly frequency and virtually disappears using monthly return observations.  These results suggest that the kind of investor sentiment for country fund prices does not have a persistent component

Avalanche breakdown characteristics of Al1-xGaxAs0.56Sb0.44 quaternary alloys

Avalanche breakdown characteristics are essential for designing avalanche photodiodes. In this work, we investigated the effects of adding Ga to Al1-xGaxAs0.56Sb0.44 quaternary alloys. Using p-i-n diodes with a 100 nm i –region and alloy composition ranging from x = 0 to 0.15, we found that the bandgap energy of Al1-xGaxAs0.56Sb0.44 reduces from 1.64 to 1.56 eV. The corresponding avalanche breakdown voltage decreases from 13.02 to 12.05 V, giving a reduction of 64.7 mV for every percent addition of Ga. The surface leakage current was also found to be significantly lower in the diodes with x = 0.10 and 0.15 suggesting that Ga can be added to reduce the surface leakage current while still preserving the lattice match to InP substrate

Hamiltonicity of 3-arc graphs

An arc of a graph is an oriented edge and a 3-arc is a 4-tuple $(v,u,x,y)$ of vertices such that both $(v,u,x)$ and $(u,x,y)$ are paths of length two. The 3-arc graph of a graph $G$ is defined to have vertices the arcs of $G$ such that two arcs $uv, xy$ are adjacent if and only if $(v,u,x,y)$ is a 3-arc of $G$. In this paper we prove that any connected 3-arc graph is Hamiltonian, and all iterative 3-arc graphs of any connected graph of minimum degree at least three are Hamiltonian. As a consequence we obtain that if a vertex-transitive graph is isomorphic to the 3-arc graph of a connected arc-transitive graph of degree at least three, then it is Hamiltonian. This confirms the well known conjecture, that all vertex-transitive graphs with finitely many exceptions are Hamiltonian, for a large family of vertex-transitive graphs. We also prove that if a graph with at least four vertices is Hamilton-connected, then so are its iterative 3-arc graphs.Comment: in press Graphs and Combinatorics, 201

Space-filling design for nonlinear models

Performing a computer experiment can be viewed as observing a mapping between the model parameters and the corresponding model outputs predicted by the computer model. In view of this, experimental design for computer experiments can be thought of as devising a reliable procedure for finding configurations of design points in the parameter space so that their images represent the manifold parametrized by such a mapping (i.e., computer experiments). Traditional space-filling designs aim to achieve this goal by filling the parameter space with design points that are as "uniform" as possible in the parameter space. However, the resulting design points may be non-uniform in the model output space and hence fail to provide a reliable representation of the manifold, becoming highly inefficient or even misleading in case the computer experiments are non-linear. In this paper, we propose an iterative algorithm that fills in the model output manifold uniformly---rather than the parameter space uniformly---so that one could obtain a reliable understanding of the model behaviors with the minimal number of design points

Investigation of temperature and temporal stability of AlGaAsSb avalanche photodiodes

Since avalanche gain and breakdown voltage in most semiconductor materials change with temperature, instruments utilizing Avalanche Photodiodes (APDs) for their avalanche gains need to incorporate either temperature stabilization or voltage adjustment in the APD operation circuits. In this work we evaluated the temperature and temporal stability of avalanche gain in Al 0.85 Ga 0.15 As 0.56 Sb 0.44 , a wide bandgap semiconductor lattice-matched to InP substrates. We investigated the temperature and temporal stability of the gain and breakdown voltage at temperatures of 24 °C (room temperature) to 80 °C. The breakdown voltage varies linearly with temperature with a temperature coefficient of 1.60 mV/K. The avalanche gain reduces from 10 to 8.5, a reduction of 15%, when the temperature increases from 24 to 80°C. The temporal stability of gain was recorded when the APD was biased to achieve an avalanche gain of 10. Fluctuations are within ± 0.7% at 24°C, increasing to ± 1.33% at 80°C. The temperature and temporal stability of avalanche gain indicates the potential of using Al 0.85 Ga 0.15 As 0.56 Sb 0.44 APDs grown on InP substrates to achieve high tolerance to temperature fluctuation

InAs Photodiodes for 3.43 mu(text)m Radiation Thermometry

We report an evaluation of an epitaxially grown uncooled InAs photodiode for the use in radiation thermometry. Radiation thermometry measurements was taken using the photodiode covered blackbody temperatures of 50 °C-300 °C. By determining the photocurrent and signal-to-noise ratio, the temperature error of the measurements was deduced. It was found that an uncooled InAs photodiode, with the peak and cutoff wavelengths of 3.35 and 3.55 μm, respectively, measured a temperature of 50 °C, with an error of 0.17 °C. Many plastics have C -H molecular bond absorptions at 3.43 μm and hence radiate thermally at this wavelength. Our results suggest that InAs photodiodes are well suited to measure the temperature of plastics above 50 °C. When tested with a narrow bandpass filter at 3.43 μm and blackbody temperatures from 50 °C-300 °C, the InAs photodiode was also found to produce a higher output photocurrent, compared with a commercial PbSe detectors

Avalanche Breakdown Timing Statistics for Silicon Single Photon Avalanche Diodes

CCBY Silicon-based Single Photon Avalanche Diodes (SPADs) are widely used as single photon detectors of visible and near infrared photons. There has however been a lack of models accurately interpreting the physics of impact ionization (the mechanism behind avalanche breakdown) for these devices. In this work, we present a statistical simulation model for silicon SPADs that is capable of predicting breakdown probability, mean time to breakdown and timing jitter. Our model inherently incorporates carriers & #x0027; dead space due to phonon scattering and allows for non-uniform electric fields. Model validation included avalanche gain, excess noise factor, breakdown voltage, breakdown probability, and timing statistics. Simulating an n on-p and a p-on-n SPAD design using our model, we found that the n-on-p design offers significantly improved mean time to breakdown and timing jitter characteristics. For a breakdown probability of 0.5, mean time to breakdown and timing jitter from the n-on-p design were 3 and 4 times smaller compared to those from the p on n design. The data reported in this paper is available from the ORDA digital repository (DOI: 10.15131/shef.data.4823248)

An InGaAlAs-InGaAs two-color photodetector for ratio thermometry

We report the evaluation of a molecular-beam epitaxy grown two-color photodetector for radiation thermometry. This two-color photodetector consists of two p+in+ diodes, an In0.53Ga0.25Al0.22As (hereafter InGaAlAs) p+in+ diode, which has a cutoff wavelength of 1180 nm, and an In0.53Ga0.47As (hereafter InGaAs) p+in+ diode with a cutoff wavelength of 1700 nm. Our simple monolithic integrated two-color photodetector achieved comparable output signal and signal-to-noise (SNR) ratio to that of a commercial two-color Si-InGaAs photodetector. The InGaAlAs and InGaAs diodes detect blackbody temperature as low as 275°C and 125°C, respectively, with an SNR above 10. The temperature errors extracted from our data are 4°C at 275°C for the InGaAlAs diode and 2.3°C at 125°C for the InGaAs diode. As a ratio thermometer, our two-color photodetector achieves a temperature error of 12.8°C at 275°C, but this improves with temperature to 0.1°C at 450°C. These results demonstrated the potential of InGaAlAs-InGaAs two-color photodetector for the development of high performance two-color array detectors for radiation thermometry and thermal imaging of hot objects