Emergent Moiré Phonons Due to Zone Folding in WSe2-WS2 Van der Waals Heterostructures.

Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240-260 cm-1, which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments, we show that these moiré Raman modes are a consequence of the large moiré length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moiré Raman modes and the WSe2 A1g Raman mode (∼250 cm-1) is observed when exciting with a 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects.

Room-Temperature Oxygen Transport in Nanothin BixOySez Enables Precision Modulation of 2D Materials.

Oxygen conductors and transporters are important to several consequential renewable energy technologies, including fuel cells and syngas production. Separately, monolayer transition-metal dichalcogenides (TMDs) have demonstrated significant promise for a range of applications, including quantum computing, advanced sensors, valleytronics, and next-generation optoelectronics. Here, we synthesize a few-nanometer-thick BixOySez compound that strongly resembles a rare R3m bismuth oxide (Bi2O3) phase and combine it with monolayer TMDs, which are highly sensitive to their environment. We use the resulting 2D heterostructure to study oxygen transport through BixOySez into the interlayer region, whereby the 2D material properties are modulated, finding extraordinarily fast diffusion near room temperature under laser exposure. The oxygen diffusion enables reversible and precise modification of the 2D material properties by controllably intercalating and deintercalating oxygen. Changes are spatially confined, enabling sub-micrometer features (e.g., pixels), and are long-term stable for more than 221 days. Our work suggests few-nanometer-thick BixOySez is a promising unexplored room-temperature oxygen transporter. Additionally, our findings suggest that the mechanism can be applied to other 2D materials as a generalized method to manipulate their properties with high precision and sub-micrometer spatial resolution.

Stacking-dependent optical properties in bilayer WSe2.

The twist angle between the monolayers in van der Waals heterostructures provides a new degree of freedom in tuning material properties. We compare the optical properties of WSe2 homobilayers with 2H and 3R stacking using photoluminescence, Raman spectroscopy, and reflectance contrast measurements under ambient and cryogenic temperatures. Clear stacking-dependent differences are evident for all temperatures, with both photoluminescence and reflectance contrast spectra exhibiting a blue shift in spectral features in 2H compared to 3R bilayers. Density functional theory (DFT) calculations elucidate the source of the variations and the fundamental differences between 2H and 3R stackings. DFT finds larger energies for both A and B excitonic features in 2H than in 3R, consistent with experimental results. In both stacking geometries, the intensity of the dominant A1g Raman mode exhibits significant changes as a function of laser excitation wavelength. These variations in intensity are intimately linked to the stacking- and temperature-dependent optical absorption through resonant enhancement effects. The strongest enhancement is achieved when the laser excitation coincides with the C excitonic feature, leading to the largest Raman intensity under 514 nm excitation in 2H stacking and at 520 nm in 3R stacked WSe2 bilayers.

Twist Angle-Dependent Atomic Reconstruction and Moiré Patterns in Transition Metal Dichalcogenide Heterostructures.

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moiré patterns. No atomic reconstruction, that is, any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here, we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles ≤1° undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moiré pattern as has been assumed in prior experimental work. Using conductive atomic force microscopy (CAFM), we show that TMD vdWH at small twist angles exhibit large domains of constant conductivity. The domains in samples with R-type stacking are triangular, whereas the domains in samples with H-type stacking are hexagonal. Transmission electron microscopy provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moiré pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provide fundamental insight into the behavior of this important class of heterostructures.

Manipulating Topological Domain Boundaries in the Single-Layer Quantum Spin Hall Insulator 1T'-WSe2.

We report the creation and manipulation of structural phase boundaries in the single-layer quantum spin Hall insulator 1T'-WSe2 by means of scanning tunneling microscope tip pulses. We observe the formation of one-dimensional interfaces between topologically nontrivial 1T' domains having different rotational orientations, as well as induced interfaces between topologically nontrivial 1T' and topologically trivial 1H phases. Scanning tunneling spectroscopy measurements show that 1T'/1T' interface states are localized at domain boundaries, consistent with theoretically predicted unprotected interface modes that form dispersive bands in and around the energy gap of this quantum spin Hall insulator. We observe a qualitative difference in the experimental spectral line shape between topologically "unprotected" states at 1T'/1T' domain boundaries and protected states at 1T'/1H and 1T'/vacuum boundaries in single-layer WSe2.

Rapid significant weight loss and regional lipid deposition: Implications for insulin sensitivity.

AIM: To compare regional lipid deposition and insulin sensitivity after differing weight loss strategies: very low calorie diet (VLCD) and laparoscopic adjustable gastric banding (LAGB). METHOD: Thirty-nine obese women underwent anthropometry, proton magnetic resonance (MR) spectroscopy for assessment of liver fat (LFAT) and MR imaging for visceral (VAT) and subcutaneous abdominal fat volume (SAT) determination. Fasting blood was taken for insulin, glucose and free fatty acid (FFA) analysis. Measurements were repeated after 6-weeks Optifast VLCD (n = 14) or 3 months after LAGB (n = 25). RESULTS: Similar, significant (p < 0.001) weight loss occurred after VLCD (8%) and LAGB (9%). Both interventions induced significant (p < 0.001) and similar reductions in body mass index (BMI) and waist circumference, and in SAT and VAT (VLCD p < 0.05, LAGB p < 0.001). LFAT fell only after VLCD (p < 0.05). Plasma FFA only fell after LAGB (p < 0.05). Homeostasis model assessment (HOMA-R) improved only following VLCD (p < 0.05). No relations were detected between the changes in LFAT, VAT and SAT. The change in LFAT related to the change in HOMA-R in both interventions combined (r = 0.410, p = 0.013) and in the VLCD group (r = 0.660, p = 0.020). There was no change in relative dietary fat intake after LAGB (p = 0.11). CONCLUSION: Caloric and fat restriction for 6 weeks (VLCD) reduces weight, SAT, VAT, LFAT and HOMA-R. Less severe caloric restriction for 12 weeks (LAGB) causes significant loss of weight, VAT and SAT but no detectable change in LFAT and HOMA-R. Following weight loss, a change in LFAT is related more to changes in insulin sensitivity or dietary fat than to abdominal adiposity loss.

Change in liver size and fat content after treatment with Optifast very low calorie diet.

BACKGROUND: Laparoscopic adjustable gastric banding (LAGB) requires surgical access to the gastroesophageal junction, which may be compromised by the enlarged, fatty liver that is frequently encountered in the obese. Liver size appears reduced and surgical access improved following preoperative weight loss with Optifast Very Low Calorie Diet (VLCD). The aim of this study was to assess the effects of 6 weeks Optifast VLCD on liver volume and fat content. METHODS: 18 morbidly obese subjects underwent magnetic resonance imaging and spectroscopy to measure liver size and fat content before and after intensive treatment with Optifast VLCD for 6 weeks. RESULTS: All subjects completing 6 weeks Optifast VLCD lost weight. Body weight and BMI (median [interquartile range]) reduced from 119.7 [111.9-131.3] kg and 44 [40-51] kg/m(2) respectively, to 110.6 [98.0124.5] kg and 40 [36-47] kg/m(2), P<0.001. Median excess weight loss (EWL) was 15.1 [9.6-21.1]%. Baseline liver volume and fat content were related (r=0.633, P=0.005). After 6 weeks Optifast VLCD, there was a 14.7% reduction in mean liver volume (P<0.001) and a 43% reduction in mean liver fat (P=0.016). The change in liver volume was predicted by the change in the liver fat (r = 0.610, P=0.012). CONCLUSION: This study has demonstrated that a 6 week diet with Optifast VLCD results in significant related reductions in liver size and liver fat content. This suggests that the reduction in liver volume is due to loss of fat. The reduction in liver fat and volume likely accounts for the perceived improved operability in patients undergoing LAGB.

The early effects of weight loss surgery on regional adiposity.

BACKGROUND: Weight loss beyond 6 months following laparoscopic adjustable gastric banding (LAGB) is associated with a preferential mobilization of visceral adipose tissue and an improvement in insulin sensitivity in insulin resistant subjects. Because the rate of weight loss is greatest in the first 3 months after LAGB, we investigated the impact of LAGB on changes in regional lipid deposition and insulin sensitivity over this period. METHODS: 10 female obese non-diabetic subjects underwent magnetic resonance (MR) imaging and spectroscopy before and 12 weeks after LAGB (using the Swedish band), for the quantification of abdominal subcutaneous and visceral adipose tissue areas and intrahepatic lipid. Fasting blood free fatty acids were analyzed. Insulin sensitivity was monitored by fasting insulin and homeostasis model assessment (HOMA). RESULTS: Median weight loss 12 weeks after gastric banding was 9.5 kg [interquartile range (IQR): -16.5 to -6]. There were significant reductions in median abdominal subcutaneous (-20% [IQR: -24 to -13]) and visceral (-15% [IQR: -49 to -8]) adipose tissue depots as well as plasma free fatty acids (-34% [IQR: -79 to -8]). The amount of weight lost was directly proportional to the initial BMI (r=0.778; P=0.008). Visceral fat loss was proportional to initial visceral adiposity (r=0.80, P=0.01). There was no significant improvement in insulin sensitivity. CONCLUSION: Significant fat loss occurs 3 months after LAGB. The absence of a concurrent improvement in insulin sensitivity may reflect the relatively small reduction in visceral adipose tissue at this stage. Improvement in insulin sensitivity beyond 3 months after LAGB may be due to the continued loss of visceral adipose tissue.