Publications

Excitons, the bosonic quasiparticle emerging from Coulomb interaction between electrons and holes, will undergo a Bose-Einstein condensation(BEC) and transition into a superfluid state with global phase coherence at low temperatures. An important platform to study such excitonic physics is built on double-layer quantum wells or recent two-dimensional material heterostructures, where two parallel planes of electrons and holes are separated by a thin insulating layer. Lowering this separation distance ($d$) enhances the interlayer Coulomb interaction thereby strengthens the exciton binding energy. However, an exceedingly small $d$ will lead to the undesired interlayer tunneling, which results the annihilation of excitons. Here, we report the observation of a sequences of robust exciton condensates(ECs) in double bilayer graphenes twisted to $\sim 10^\circ$ with no insulating mid-layer. The large momentum mismatch between the two graphene layers well suppress the interlayer tunneling, allowing us to reach the separation lower limit $\sim$ 0.334 nm and investigate ECs in the extreme coupling regime. Carrying out transport measurements on the bulk and edge of the devices, we find incompressible states corresponding to ECs when both layers are half-filled in the $N=0$ and $N=1$ Landau levels (LLs). The comparison between these ECs and theoretical calculations suggest that the low-energy charged excitation of ECs can be meron-antimeron or particle-hole pair, which relies on both LL index and carrier type. Our results establish large-angle twisted bilayers as an experimental platform with extreme coupling strength for studying quantum bosonic phase and its low-energy excitations.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Two-dimensional (2D) tungsten disulfide (WS2), tungsten diselenide (WSe2), and tungsten ditelluride (WTe2) draw increasing attention due to their attractive properties deriving from the heavy tungsten and chalcogenide atoms, but their mechanical properties are still mostly unknown. Here, we determine the intrinsic and air-aged mechanical properties of mono-, bi-, and trilayer (1–3L) WS2, WSe2, and WTe2 using a complementary suite of experiments and theoretical calculations. High-quality 1L WS2 has the highest Young's modulus (302.4 ± 24.1 GPa) and strength (47.0 ± 8.6 GPa) of the entire family, overpassing those of 1L WSe2 (258.6 ± 38.3 and 38.0 ± 6.0 GPa, respectively) and WTe2 (149.1 ± 9.4 and 6.4 ± 3.3 GPa, respectively). However, the elasticity and strength of WS2 decrease most dramatically with increased thickness among the three materials. We interpret the phenomenon by the different tendencies for interlayer sliding in an equilibrium state and under in-plane strain and out-of-plane compression conditions in the indentation process, revealed by the finite element method and density functional theory calculations including van der Waals interactions. We also demonstrate that the mechanical properties of the high-quality 1–3L WS2 and WSe2 are largely stable in air for up to 20 weeks. Intriguingly, the 1–3L WSe2 shows increased modulus and strength values with aging in the air. This is ascribed to oxygen doping, which reinforces the structure. The present study will facilitate the design and use of 2D tungsten dichalcogenides in applications such as strain engineering and flexible field-effect transistors.

The use of collaborative robots (Cobots) for materials development in chemical laboratories is currently of high priority. Herein, the Cobot is used for autonomous continued analysis and synthesis of graphene oxide–polyethyleneimine‐based membrane to unify a method and prospects for big data collection are shown. Membranes have already demonstrated a selective affinity to potassium cations and promised to adjust permeability for other cations by changing pH. The Cobot allows a variation of membrane properties by its composition modification. The present strategy combines a novel perspective of material production by Cobots and the application of machine learning. Moreover, the current approach can be adapted for different modern chemical laboratories for various scientific research, and the proper workflow is provided.

A global trend towards miniaturization and multiwavelength performance of nanophotonic devices drives research on novel phenomena, such as bound states in the continuum and Mietronics, as well as surveys for high-refractive index and strongly anisotropic materials and metasurfaces. Hexagonal boron nitride (hBN) is one of the promising materials for future nanophotonics owing to its inherent anisotropy and prospects of high-quality monocrystal growth with an atomically flat surface. Here, we present highly accurate optical constants of hBN in the broad wavelength range of 250-1700 nm combining imaging ellipsometry measurements, scanning near-field optical microscopy and first-principles quantum mechanical computations. hBN's high refractive index, up to 2.75 in the ultraviolet (UV) and visible range, broadband birefringence of ∼0.7, and negligible optical losses make it an outstanding material for UV and visible range photonics. Based on our measurement results, we propose and design novel optical elements: handedness-preserving mirrors and subwavelength waveguides with dimensions of 40 nm operating in the visible and UV ranges, respectively. Remarkably, our results offer a unique opportunity to bridge the size gap between photonics and electronics.

Memristors‐based integrated circuits for emerging bio‐inspired computing paradigms require an integrated approach utilizing both volatile and nonvolatile memristive devices. Here, an innovative architecture comprising of 1D CVD‐grown core–shell heterostructures (CSHSs) of MoO 2 ‐MoS 2 is employed as memristors manifesting both volatile switching (with high selectivity of 10 7 and steep slope of 0.6 mV decade −1 ) and nonvolatile switching phenomena (with I on / I off ≈10 3 and switching speed of 60 ns). In these CSHSs, the metallic core MoO 2 with high current carrying capacity provides a conformal and immaculate interface with semiconducting MoS 2 shells and therefore it acts as a bottom electrode for the memristors. The power consumption in volatile devices is as low as 50 pW per set transition and 0.1 fW in standby mode. Voltage‐driven current spikes are observed for volatile devices while with nonvolatile memristors, key features of a biological synapse such as short/long‐term plasticity and paired pulse facilitation are emulated suggesting their potential for the development of neuromorphic circuits. These CSHSs offer an unprecedented solution for the interfacial issues between metallic electrodes and the layered materials‐based switching element with the prospects of developing smaller footprint memristive devices for future integrated circuits.

We demonstrate that optical transparency of any two-dimensional system with a symmetric electronic spectrum is governed by the fine structure constant and suggest a simple formula that relates a quasiparticle spectrum to an optical absorption of such a system. These results are applied to graphene deposited on a surface of oxidized silicon for which we measure ellipsometric spectra, extract optical constants of a graphene layer and reconstruct the electronic dispersion relation near the $K$ point using optical transmission spectra. We also present spectroscopic ellipsometry analysis of graphene placed on amorphous quartz substrates and report a pronounced peak in ultraviolet absorption at 4.6 eV because of a van Hove singularity in graphene's density of states. The peak is asymmetric and downshifted by 0.5 eV probably due to excitonic effects.

Tunneling and magnetic tunneling are investigated in graphene/ h -BN/graphene van der Waals heterosystems. Two new types of systems are found, in which negative differential conductivity regions are implemented due to resonant tunneling through defect levels in the h -BN barrier, and current caused by their presence is generated.

<title>Abstract</title> Nanophotonics relies on precise nanoscale structuring, yet conventional fabrication techniques remain complex and costly. Layered van der Waals (vdW) materials, with their intrinsic anisotropy and high refractive indices, offer a promising route toward simplified nanostructuring and tunable optical functionality. However, no vdW material has previously been shown to exhibit a strong photorefractive effect—a key requirement for light-based modulation. Here we report a giant photorefractive response (Δ<italic>n</italic> ≈ 0.1) in crystalline arsenic trisulfide (As₂S₃), observed even under low-intensity illumination. In addition to refractive index modulation, light exposure enables controlled thickness tuning of As₂S₃. The material exhibits a giant photoexpansion of up to 5%, depending on the illumination intensity. Building on this photoexpansion effect, we introduce a maskless, low-cost nanopatterning technique based on continuous-wave laser writing, achieving resolutions up to 50,000 dots per inch without the need for ultrafast lasers. The combination of high photosensitivity, optical anisotropy, and transparency positions As₂S₃ as a versatile platform for integrated photonics, adaptive optics, data storage, biomedical imaging, and nanoscale sensing.

Current artificial magnetic conductor (AMC) designs use metallic patterns on rigid substrates and focus on shapes and sizes of AMC structures, rather than on material performance, which has hindered operation bandwidth and design flexibility. Here, we introduce printed graphene AMC-based broadband and flexible microwave radar absorbers, which not only redirect but also absorb the incident wave so to broaden the operation bandwidth. Contrasting to other reported works, the phase characteristics of the AMCs are realized through the control of the surface resistance provided by printed graphene laminates. We produced a variety of AMC structures, composed of printed graphene circular ring arrays with exactly the same shape and size, but different sheet resistances. By carefully designing the sheet resistance of printed graphene laminates, the optimized anti-phase reflection cancellation between AMCs can be achieved. With printed graphene AMCs and flexible dielectric substrate, the absorber presented in this work has a broadband effective absorption (above 90% absorptivity) from 7.58 GHz to 18, is polarization insensitive under normal incident, and can work at relatively wide incident angles. Furthermore, this absorber is capable of bending easily with notable performance, which makes it ideal for applications with irregular and uneven shapes.

Direct conversion of CO₂ into valuable organic products is probably the most important but challenging issue for global sustainability efforts. Metal carbides are promising as vital catalytic components in achieving this goal. Understanding the evolution of chemical orbitals and the corresponding energy levels on their interfaces are essential for targeted product synthesis. In this study, we discover that a highly active FeCo alloy carbide has a distinctive oxygen-bonding ability to regulate the evolution of oxygen-containing reaction intermediates. Combining with the copper/zinc/aluminum catalytic component, the designed tandem catalyst allows for the extremely high C₂₊ alcohols selectivity (49.1 percent) and space-time yield (245.7 milligram per gram catalyst per hour) at a CO₂ conversion of 51.1 percent. The excellent catalyst stability (>1000 hours) and potential economic viability make this process promising in eliminating carbon emissions at industrial application scale.

No abstract is provided for this article.

1- and 2-iodoheptafluoropropane were characterized extensively as potential replacement etchants for perfluorocompounds used in an oxide etch application. In the present study, via holes of critical dimension down to 0.35 μm were etched in an inductively coupled high density plasma tool. Oxide etch rate, mask and stop layer selectivities, and feature profile were among the principal metrics used to evaluate the performance of these compounds. A conventional (C3F8-based) etch process was used as a reference. Process behavior as a function of a number of variables—namely source power, bias power, etch gas flow, additive gas CH3F) flow, roof, wall, and chiller temperatures, and pressure was studied. While good etch rates and feature profiles were obtained, mask and stop layer selectivity was found to be limited. As a supplement to the experimental work, a set of ab initio quantum chemical calculations was undertaken to obtain enthalpies of dissociation for each of the bonds in the iodoheptafluoropropane molecules in order to better understand their dissociation pathways in plasma environments. Part I focuses on trends in process behavior as a function of the variables explored and discusses key mechanisms responsible for the observed effects. Parts II and III will focus on deposited film characterization and global warming emissions, respectively.

No abstract is provided for this article.

Higher-order exchange and quantum effects are widely known to play an important role in the properties of nanomagnets. In article number 2004138, Elton J. G. Santos and co-workers show that recently discovered two-dimensional CrI3 magnets require biquadratic exchange interactions and quantum-rescaling to describe their long-range ordering. Metastability of the magnetic domains induces domain walls with different chirality, which is important for the new generation of quantum-materials-based technologies.

We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For a few layers (<6), the monochromatic contrast increases linearly with thickness. The data can be adequately understood by considering the samples behaving as a superposition of single sheets that act as independent two-dimensional electron gases. Thus, Rayleigh imaging is a general, simple, and quick tool to identify graphene layers, which is readily combined with Raman scattering, that provides structural identification.