Publications

We introduce the $\ell_0\ell_2$-norm regularization and hierarchy constraints into linear regression for the construction of cluster expansion to describe configurational disorder in materials. The approach is implemented through mixed integer quadratic programming (MIQP). The $\ell_2$-norm regularization is used to suppress intrinsic data noise, while $\ell_0$-norm is used to penalize the number of non-zero elements in the solution. The hierarchy relation between clusters imposes relevant physics and is naturally included by the MIQP paradigm. As such, sparseness and cluster hierarchy can be well optimized to obtain a robust, converged, and effective cluster interactions with improved physical meaning. We demonstrate the effectiveness of $\ell_0\ell_2$-norm regularization in two high-component disordered rocksalt cathode material systems, where we compare the cross-validation and convergence speed, reproduction of phase diagrams, voltage profiles, and Li-occupancy energies with those of the conventional $\ell_1$-norm regularized cluster expansion model.

Abstract The gem ‐dimethyl groups in polyketide‐derived natural products add steric bulk and, accordingly, lend increased stability to medicinal compounds, however, our ability to rationally incorporate this functional group in modified natural products is limited. In order to characterize the mechanism of gem ‐dimethyl group formation, with a goal toward engineering of novel compounds containing this moiety, the gem ‐dimethyl group producing polyketide synthase (PKS) modules of yersiniabactin and epothilone were characterized using mass spectrometry. The work demonstrated, contrary to the canonical understanding of reaction order in PKSs, that methylation can precede condensation in gem ‐dimethyl group producing PKS modules. Experiments showed that both PKSs are able to use dimethylmalonyl acyl carrier protein (ACP) as an extender unit. Interestingly, for epothilone module 8, use of dimethylmalonyl‐ACP appeared to be the sole route to form a gem ‐dimethylated product, while the yersiniabactin PKS could methylate before or after ketosynthase condensation.

The surface structure of Pt(557) during the catalytic oxidation of hydrogen was studied with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. At 298 K, the surface Pt oxide formed after exposing Pt(557) to approximately 1 Torr of O2 can be readily removed by H2, at H2 partial pressures below 50 mTorr. Water is detected as the product in the gas phase, which also coadsorbs with hydroxyl groups on the Pt(557) surface.

No abstract is provided for this article.

One of the major challenges facing the rapidly growing field of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is the development of growth techniques to enable large-area synthesis of high-quality materials. Chemical vapor deposition (CVD) is one of the leading techniques for the synthesis of TMDCs; however, the quality of the material produced is limited by defects formed during the growth process. A very useful nondestructive technique that can be utilized to probe defects in semiconductors is the room-temperature photoluminescence (PL) quantum yield (QY). It was recently demonstrated that a PL QY near 100% can be obtained in MoS2 and WS2 monolayers prepared by micromechanical exfoliation by treating samples with an organic superacid: bis(trifluoromethane)sulfonimide (TFSI). Here we have performed a thorough exploration of this chemical treatment on CVD-grown MoS2 samples. We find that the as-grown monolayers must be transferred to a secondary substrate, which releases strain, to obtain high QY by TFSI treatment. Furthermore, we find that the sulfur precursor temperature during synthesis of the MoS2 plays a critical role in the effectiveness of the treatment. By satisfying the aforementioned conditions we show that the PL QY of CVD-grown monolayers can be improved from ∼0.1% in the as-grown case to ∼30% after treatment, with enhancement factors ranging from 100 to 1500× depending on the initial monolayer quality. We also found that after TFSI treatment the PL emission from MoS2 films was visible by eye despite the low absorption (5-10%). The discovery of an effective passivation strategy will speed the development of scalable high-performance optoelectronic and electronic devices based on MoS2.

We present optical and infrared (IR) observations of the Type II SN 2002hh from 3 to 397 d after explosion. The optical spectroscopic (4–397 d) and photometric (3–278 d) data are complemented by spectroscopic (137–381 d) and photometric (137–314 d) data acquired at IR wavelengths. This is the first time L-band spectra have ever been successfully obtained for a supernova (SN) at a distance beyond the Local Group. The VRI light curves in the first 40 d reveal SN 2002hh to be an SN IIP (plateau) – the most common of all core-collapse SNe. SN 2002hh is one of the most highly extinguished SNe ever investigated. To provide a match between its early-time spectrum and a coeval spectrum of the Type IIP SN 1999em, as well as maintaining consistency with K I interstellar absorption, we invoke a two-component extinction model. One component is due to the combined effect of the interstellar medium (ISM) of our Milky Way Galaxy and the SN host galaxy, while the other component is due to a ‘dust pocket’ where the grains have a mean size smaller than in the ISM. The early-time optical light curves of SNe 1999em and 2002hh are generally well matched, as are the radioactive tails of these two SNe and SN 1987A. The late-time similarity of the SN 2002hh optical light curves to those of SN 1987A, together with measurements of the optical/IR luminosity and [Fe II] 1.257 μm emission indicate that 0.07 ± 0.02 M⊙ of 56Ni was ejected by SN 2002hh. However, during the nebular phase the HKL′ luminosities of SN 2002hh exhibit a growing excess with respect to those of SN 1987A. We attribute much of this excess to an IR-echo from a pre-existing, dusty circumstellar medium. Based on an IR-echo interpretation of the near-IR (NIR) excess, we deduce that the progenitor of SN 2002hh underwent recent mass-loss of ∼0.3 M⊙. A detailed comparison of the late-time optical and NIR spectra of SNe 1987A and 2002hh is presented. While the overall impression is one of similarity between the spectra of the two events, there are notable differences. The Mg I 1.503 μm luminosity of SN 2002hh is a factor of 2.5 greater than in SN 1987A at similar epochs, yet coeval silicon and calcium lines in SN 2002hh are fainter. Interpreting these differences as being due to abundance variations, the overall abundance trend between SN 1987A and 2002hh is not consistent with explosion model predictions. It appears that during the burning to intermediate-mass elements, the nucleosynthesis did not progress as far as might have been expected given the mass of iron ejected. Evidence for mixing in the ejecta is presented. Pronounced blueshifts seen in the more isolated lines are attributed to asymmetry in the ejecta. However, during the time-span of these observations (∼1-yr post-explosion) we find no evidence of dust condensation in the ejecta such as might have been revealed by an increasing blueshift and/or attenuation of the red wings of the emission lines. Nevertheless, the clear detection of first overtone CO emission by 200 d and the reddening trend in (K−L′)0 suggest that dust formation in the ejecta may occur at later epochs. From the [O I]λλ6300, 6364 Å doublet luminosity we infer a 16–18 M⊙ main-sequence progenitor star. The progenitor of SN 2002hh was probably a red supergiant with a substantial, dusty wind.

Restoring degraded peat soils presents an attractive, but largely untested, climate change mitigation approach. Drained peat soils used for agriculture can be large greenhouse gas sources. By restoring subsided peat soils to managed, impounded wetlands, significant agricultural emissions are avoided, and soil carbon can be sequestered and protected. Here, we synthesize 36 site-years of continuous carbon dioxide and methane flux data from a mesonetwork of eddy covariance towers in the Sacramento-San Joaquin Delta in California, USA to compute carbon and greenhouse gas budgets for drained agricultural land uses and compare these to restored deltaic wetlands. We found that restored wetlands effectively sequestered carbon and halted soil carbon loss associated with drained agricultural land uses. Depending on the age and disturbance regime of the restored wetland, many land use conversions from agriculture to restored wetland resulted in emission reductions over a 100-year timescale. With a simple model of radiative forcing and atmospheric lifetimes, we showed that restored wetlands do not begin to accrue greenhouse gas benefits until nearly a half century, and become net sinks from the atmosphere after a century. Due to substantial interannual variability and uncertainty about the multi-decadal successional trajectory of managed, restored wetlands, ongoing ecosystem flux measurements are critical for understanding the long-term impacts of wetland restoration for climate change mitigation.

The widely accepted active galactic nucleus (AGN) paradigm has been recently challenged by the discovery of the so-called ``changing-look'' (CL) phenomenon characterized by spectral-type transitions. By comparing the SDSS-V and SDSS DR16 spectroscopic datasets, here we report the identification of 14 new CL-AGNs (redshift $z<0.5$) exhibiting spectral-type changes on a timescale of $\sim 10$yr. Follow-up spectroscopy was conducted with the Lick Shane 3m and Keck 10m telescopes for three of the objects. Detailed analysis of these spectra enables us to arrive at the following two main results. (1) By compiling a sample of 65 CL-AGNs with good measurements, we reinforce the previous claim that CL-AGNs tend to be biased against both a high Eddington ratio ($\lesssim 0.1$) and a high bolometric luminosity ($\lesssim 10^{46}\,\mathrm{erg\,s^{-1}}$). This bias suggests that the disk-wind broad-line-region model is a plausible explanation of the CL phenomenon. (2) The host galaxies of CL-AGNs tend to be dominated by intermediate stellar populations, which motivates us to propose that CL-AGNs are probably particular AGNs at a special evolutionary stage, such as a transition stage from ``feast'' to ``famine'' fueling of the supermassive black hole. In addition, with our spectra, we identify SDSS J025951.22+003744.2 as a new repeat CL narrow-line Seyfert 1 galaxy with a rapid ``turn-on'' timescale of $\sim 1$yr.