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<jats:title>Abstract</jats:title> <jats:p>The fine structure of attractive Fermi polarons in van der Waals heterostructures based on monolayer transition metal dichalcogenides in the presence of elastic strain is studied theoretically. The charged excitons (trions), three particle bound states of two electrons and hole or two holes and electron, do not show any strain-induced fine structure splitting compared to neutral excitons whose radiative doublet is split by the strain into linearly polarized components. The correlation of the trions with Fermi sea holes gives rise to the attractive Fermi polarons. We show that it results in the fine structure splitting of the polaron into states polarized along the main axes of the strain tensor. The effect is related to the bosonic statistics of Fermi polarons. We develop microscopic theory of the effect and calculate the strain-induced splitting of Fermi polarons both for tungsten- and molybdenum-based monolayers identifying the role of inter- and intravalley exciton-electron interactions. The fine structure splitting of attractive Fermi polaron is proportional both to excitonic splitting and the Fermi energy. The Fermi polaron fine structure in bilayers is briefly analyzed and the role of electron and trion localization in moir'e potentials is discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>From ripples to defects, edges and grain boundaries, the 3D atomic structure of 2D materials is critical to their properties. However the damage inflicted by conventional 3D analysis precludes its use with fragile 2D materials, particularly for the analysis of local defects. Here we dramatically increase the potential for precise local 3D atomic structure analysis of 2D materials, with both greatly improved dose efficiency and sensitivity to light elements.&amp;#xD;We demonstrate light atoms can now be located in complex 2D materials with picometer precision at doses 30 times lower than previously possible. Moreover we demonstrate this using WS$_2$, in which the light atoms are practically invisible to conventional methods at low doses.&amp;#xD;The key advance is combining the concept of few tilt tomography with highly dose efficient ptychography in scanning transmission electron microscopy. We further demonstrate the method experimentally with the even more challenging and newly discovered 2D CuI, leveraging a new extremely high temporal resolution camera.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs&amp;#xD;requires in-depth understanding of their optoelectronic properties. However, how the surrounding&amp;#xD;environment of host materials, including number of layers, substrates, and strain, influences SPEs&amp;#xD;has not been fully understood. In this work, we study the dielectric screening effect due to the&amp;#xD;number of layers and substrates, and the strain effect on the optical properties of carbon dimer and&amp;#xD;nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report&amp;#xD;that the environmental screening causes lowering of the GW gap and exciton binding energy, leading&amp;#xD;to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results&amp;#xD;with an analytical model starting from the BSE Hamiltonian with Wannier basis. We also show that&amp;#xD;optical properties of quantum defects are largely tunable by strain with highly anisotropic response,&amp;#xD;in good agreement with experimental measurements. Our work clarifies the effect of environmental&amp;#xD;screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators,&amp;#xD;facilitating future applications of SPEs and spin qubits in low-dimensional systems.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Atomic doping is a widely used technique to modify the electronic properties of two-dimensional materials for various applications. In this study, we investigate the catalytic properties of single-atom doped graphene as electrocatalysts for hydrogen evolution reactions (HER) using first-principles calculations. We consider several elements, including Al, Ga, In, Si, Ge, Sn, P, As, and Sb, which were interstitially doped into single and double C vacancies in graphene. Our density functional theory calculations show that all the considered doped graphene, except for As-doped graphene, can be highly active for HER, with hydrogen adsorption free energies (∆G<jats:sub>H*</jats:sub>) close to the optimal value (∆G<jats:sub>H*</jats:sub> = 0), ranging from –0.19 to 0.11 eV. Specifically, ∆G<jats:sub>H*</jats:sub> of Al, Ga, In, and Ge are much closer to zero when doped in the single vacancy than in the double vacancy. In contrast, ∆G<jats:sub>H*</jats:sub> of Sb and Sn are much closer to zero in the double vacancy. Si and P have ∆G<jats:sub>H*</jats:sub> values close to the optimum in both vacancies. Interestingly, the vacancy numbers play a crucial role in forming orbital hybridizations, resulting in distinct electronic distributions for most dopants. As a result, a few doped graphenes show distinctive ferrimagnetic and ferromagnetic orders, which is also an important factor for determining the strength of H adsorption. These findings have important implications for designing graphene-based HER catalysis.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>MXene, an ultra-thin two-dimensional conductive material, has attracted considerable interest in various fields due to its exceptional material properties. In particular, Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub>x</jats:sub> MXene exhibits distinct optical properties, enabling it to support surface plasmons in the shortwave infrared (SWIR) region. However, it is challenging to enhance the field confinement of MXene surface plasmons in a single-interface structure due to the substantial intrinsic absorption of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub>x</jats:sub> MXene. Herein, we explore various multilayer structures capable of supporting high field confinement of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub>x</jats:sub> MXene plasmons, including insulator-MXene-insulator (IMI), MXene-insulator-MXene (MIM), and insulator-MXene-insulator-MXene (IMIM) configurations. We observe that the field confinement of MXene plasmons improves as the thickness of either the MXene or insulator layers decreases, which is attributed to the strong coupling between plasmons at the multilayer interfaces. Furthermore, the IMIM structure demonstrates the most substantial enhancement in field confinement. In an IMIM structure with a 1.3 nm-thick MXene monolayer and a 1.0 nm-thick SiO<jats:sub>2</jats:sub> layer, the wavelength and effective field size of the plasmon at a frequency of 150 THz (λ<jats:sub>0</jats:sub> = 2.0 μm) are calculated to be 24.61 nm and 1.50 nm, respectively. These values demonstrate a reduction by factors of 55 and 596, respectively, compared to those obtained in a single SiO<jats:sub>2</jats:sub>-MXene interface structure. Multilayer-based MXene plasmons provide a solution for enhancing the field confinement of MXene plasmons in the SWIR region, and we expect them to play a crucial role in a variety of 2D material-based SWIR plasmonic applications.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub> <jats:italic>x</jats:italic> </jats:sub> MXene is emerging as the enabling material in a broad range of wearable and implantable medical technologies, thanks to its outstanding electrical, electrochemical, and optoelectronic properties, and its compatibility with high-throughput solution-based processing. While the prevalence of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub> <jats:italic>x</jats:italic> </jats:sub> MXene in biomedical research, and in particular bioelectronics, has steadily increased, the long-term stability and degradation of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub> <jats:italic>x</jats:italic> </jats:sub> MXene films have not yet been thoroughly investigated, limiting its use for chronic applications. Here, we investigate the stability of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub> <jats:italic>x</jats:italic> </jats:sub> films and electrodes under environmental conditions that are relevant to medical and bioelectronic technologies: storage in ambient atmosphere (shelf-life), submersion in saline (akin to the in vivo environment), and storage in a desiccator (low-humidity). Furthermore, to evaluate the effect of the MXene deposition method and thickness on the film stability in the different conditions, we compare thin (25 nm), and thick (1.0 µm) films and electrodes fabricated via spray-coating and blade-coating. Our findings indicate that film processing method and thickness play a significant role in determining the long-term performance of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub> <jats:italic>x</jats:italic> </jats:sub> films and electrodes, with highly aligned, thick films from blade coating remarkably retaining their conductivity, electrochemical impedance, and morphological integrity even after 30 days in saline. Our extensive spectroscopic analysis reveals that the degradation of Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:sub> <jats:italic>x</jats:italic> </jats:sub> films in high-humidity environments is primarily driven by moisture intercalation, ingress, and film delamination, with evidence of only minimal to moderate oxidation.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We revisit and extend the standard bosonic interpretation of interlayer excitons (ILX) in the moiré potential of twisted heterostructures of transition-metal dichalcogenides. In our experiments, we probe a high quality MoSe<jats:sub>2</jats:sub>/WSe<jats:sub>2</jats:sub> van der Waals bilayer heterostructure via density-dependent photoluminescence spectroscopy and reveal strongly developed, unconventional spectral shifts of the emergent moiré exciton resonances. The observation of saturating blueshifts of successive exciton resonances allow us to explain their physics in terms of a model utilizing fermionic saturable absorbers. This approach is strongly inspired by established quantum-dot models, which underlines the close analogy of ILX trapped in pockets of the moiré potential, and quantum emitters with discrete eigenstates.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS<jats:sub>2</jats:sub>, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS<jats:sub>2</jats:sub>/graphene stacks employing first-principles calculations and find that the lower conduction band of WS<jats:sub>2</jats:sub> at the <jats:inline-formula> <jats:tex-math><?CDATA $K/K^{^{\prime}}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>K</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msup> <mml:mi>K</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">′</mml:mi> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmacd5dfieqn1.gif" xlink:type="simple" /> </jats:inline-formula> valleys (the <jats:inline-formula> <jats:tex-math><?CDATA $\mathrm{CB}^-$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">B</mml:mi> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmacd5dfieqn2.gif" xlink:type="simple" /> </jats:inline-formula> band) is strongly influenced by the graphene layer on the orbital level. Specifically, our detailed microscopic analysis reveals that the conduction band at the <jats:italic>Q</jats:italic> point of WS<jats:sub>2</jats:sub> mediates the coupling between <jats:inline-formula> <jats:tex-math><?CDATA $\mathrm{CB}^-$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">C</mml:mi> <mml:mi mathvariant="normal">B</mml:mi> </mml:mrow> <mml:mo>−</mml:mo> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmacd5dfieqn3.gif" xlink:type="simple" /> </jats:inline-formula> and graphene due to resonant energy conditions and strong coupling to the Dirac cone. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step energetically resonant process, with different bands mediating the interlayer interactions. Such effects can be further exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Semiconductor transition metal dichalcogenides (TMDs) have equivalent dynamics for their two spin/valley species. This arises from their energy-degenerated spin states, connected via time-reversal symmetry. When an out-of-plane magnetic field is applied, time-reversal symmetry is broken and the energies of the spin-polarized bands shift, resulting in different bandgaps and dynamics in the K<jats:inline-formula> <jats:tex-math><?CDATA $_+$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi /> <mml:mo>+</mml:mo> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmacd12aieqn1.gif" xlink:type="simple" /> </jats:inline-formula> and K<jats:inline-formula> <jats:tex-math><?CDATA $_-$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi /> <mml:mo>−</mml:mo> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tdmacd12aieqn2.gif" xlink:type="simple" /> </jats:inline-formula> valleys. Here, we use time-resolved Kerr rotation to study the magnetic field dependence of the spin dynamics in monolayer MoSe<jats:sub>2</jats:sub>. We show that the magnetic field can control the light-induced spin accumulation of the two valley states, with a small effect on the recombination lifetimes. We unveil that the magnetic field-dependent spin accumulation is in agreement with hole spin dynamics at the longer timescales, indicating that the electron spins have faster relaxation rates. We propose a rate equation model that suggests that lifting the energy-degeneracy of the valleys induces an ultrafast spin-flip toward the stabilization of the valley with the higher valence band energy. Our results provide an experimental insight into the ultrafast charge and spin dynamics in TMDs and a way to control it, which will be useful for the development of new spintronic and valleytronic applications.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>With the rapid development of electronic communication technology, electromagnetic pollution has become increasingly severe, leading to a growing need for absorption materials with excellent absorption performance. Herein, nano-sized oxide-intercalated Ti<jats:sub>3</jats:sub>C<jats:sub>2</jats:sub>T<jats:italic> <jats:sub>x</jats:sub> </jats:italic> MXene composites were synthesized via <jats:italic>γ</jats:italic>-radiation in an aqueous system. The synthesis was conducted in a reducing environment at room temperature, which effectively prevented the oxidation of MXene. Nano-oxides (Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> and Cu<jats:sub>2</jats:sub>O) were formed <jats:italic>in situ</jats:italic> between the MXene layers, forming a homogeneous oxide-intercalated structure. Compared with the original MXene, the intercalated structure significantly enhanced the electromagnetic wave absorption (EMA) performance of the MXene/Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> composite in the frequency range of 2–18 GHz, with smaller reflection loss (RL) (−58.7 dB), thinner thickness (2.35 mm). The RL<jats:sub>min</jats:sub> value of the MXene/Cu<jats:sub>2</jats:sub>O composite was −50.9 dB (2.55 mm) and the effective frequency width was 4.88 GHz at a thickness of 1.3 mm. Compared with ordinary chemical methods, the <jats:italic>γ</jats:italic>-radiation is a facile and ‘green’ method that is suitable for large-scale preparation. The absorption mechanism was attributed to conductive loss, polarization loss, and magnetic loss, which contributed to efficient EMA. This work demonstrates that <jats:italic>γ</jats:italic>-radiation is a promising method for preparing nano metal oxide-intercalated MXene with enhanced EMA performance.</jats:p>