2D Quantum Materials Research Articles

Alice (and Bob) in Flatland.

2D quantum materials have opened infinite doors, hosting intriguing phenomena and featuring incredible engineering potential. Whether these qualities can boost the use of 2D crystals for quantum applications remains an open field with yet unexplored paths.

Development of high-reflectivity polymer/air-Bragg micromirror structures for nanophotonic applications

We report the design, nanofabrication, and characterization of high-quality polymer-based micromirror structures employing the 3D two-photon polymerization lithography technique. Compared to conventional microcavity approaches, our innovative concept provides microstructures, which allow fast prototyping. Moreover, our polymer-based mirrors are cost effective and environmentally sensitive, as well as compatible with a wide range of wavelengths from near-infrared to the telecom C-band. We demonstrate polymer/air distributed Bragg reflectors and full microcavity structures with up to 14 mirror pairs with a target wavelength of 1550 nm and a reflectivity close to 99%. Additionally, our 3D printed micromirrors are reproducible and mechanically stable, and enable hybrid nanophotonic devices based on quantum dots, molecules, or 2D quantum materials as the active medium.

Paramagnetic Singularities of the Orbital Magnetism in Graphene with a Moiré Potential.

The recent detection of the singular diamagnetism of Dirac electrons in a single graphene layer paved a new way of probing 2D quantum materials through the measurement of equilibrium orbital currents which cannot be accessed in usual transport experiments. Among the theoretical predictions is an intriguing orbital paramagnetism at saddle points of the dispersion relation. Here we present magnetization measurements in graphene monolayers aligned on hexagonal boron nitride crystals. Besides the sharp diamagnetic McClure response at the Dirac point, we detect extra diamagnetic singularities at the satellite Dirac points of the moiré lattice. Surrounding these diamagnetic satellite peaks, we also observe paramagnetic peaks located at the chemical potential of the saddle points of the graphene moiré band structure and relate them to the presence of vanHove logarithmic singularities in the density of states. These findings reveal the long ago predicted anomalous paramagnetic orbital response in 2D systems when the Fermi energy is tuned to the vicinity of saddle points.

(Invited) Controlling the Layer Stacking and Twisting of 2D Quantum Materials

Rationally and precisely controlling the structures of two-dimensional (2D) layered materials allows us to tune the electronic structures and quantum states of matter, discover new physical properties, thus enable new applications. For example, tuning the twist angles between 2D materials results in the formation of moiré patterns and the manipulation of electronic states, which have led to novel quantum phenomena, including unconventional superconductivity, moiré excitons, and tunable Mott insulators. We have shown how the phase, layer stacking and thus physical properties of 2D MX2 materials can be influenced by screw dislocations. Moreover, we achieved systematic interlayer twisting of MX2 spiral layers grown via by screw dislocations due to mismatched geometry between Euclidean crystal lattices and non-Euclidean (curved) surfaces (Science 2020, 370, 442). We experimentally demonstrated the growth of twisted spirals of WS2 and WSe2 draped over nanoparticles near their centers and microscopic structural analysis confirmed the moiré superlattices between the atomic layers. Tuning the stacking and twisting of 2D materials opens up new dimensions of 2D quantum materials (Acc. Mater. Res. 2022, 3, 369) for the study of moiré excitons, nonlinear optical properties, chiral optoelectronics, valleytronics and twistronics.

Magnetism in two-dimensional layered double hydroxides

Layered double hydroxides (LDHs) are a class of anionic clays known for a long time and properly classified in the beginning of 20th century. They are composed of positively charged metal hydroxide-based sheets endowed with interlayer anions and solvent molecules to keep the electro-neutrality. LDHs have attracted increasing attention during the last years because of their rich chemical versatility and the fact that they can be exfoliated –or directly synthesized– into two-dimensional (2D) nanosheets, impacting a wide range of potential applications. Among others, magnetism stands out as one of the most appealing properties of LDHs, this is mainly due to the possibility of modulating their magnetic interactions by judicious tuning of their composition, morphology or interlayer spacing. The combination of their modulable physical properties with good processability positions these nanosheets as excellent candidates for the development of hybrid materials and heterostructures. This review addresses from the first reports to the most recent advances in the magnetic properties of LDHs and their hybrids, showing the great potential they hold as 2D quantum materials. In addition, it is also shown how magnetic properties can be useful in energy-related applications, either evaluating the purity of materials of utmost importance such as NiFe-LDHs, or elucidating their cationic order at the atomic scale, which influences the catalytic performance.

Smith–Purcell Radiation from Highly Mobile Carriers in 2D Quantum Materials

AbstractTerahertz (THz) radiation has broad applications ranging from medical imaging to spectroscopy. One viable source of high‐intensity THz radiation is the Smith–Purcell (SP) effect, which involves charge carriers moving over a periodic surface. Conventional SP emitters use electron beams to generate charge carriers, necessitating bulky electron acceleration stages. Here, a compact design for generating THz SP radiation using mobile charge carriers within 2D materials is proposed. This circumvents the beam alignment and beam divergence challenge, allowing for a reduction in the electron‐grating separation from tens of nm to 5 nm or less, leading to more efficient near‐field excitation and a potentially chip‐level THz source. In such a configuration, it is shown that the optimal electron velocity and the corresponding maximum radiation intensity can be predicted from the electron‐grating separation. The numerical demonstration shows that hot electrons can excite SP radiation in graphene on a silicon grating, and the radiation intensity can be increased by graphene surface plasmons. This study can be extended to a broad variety of charge carriers in 2D materials, thus allowing for compact, tunable, and low‐cost THz sources.

Artificial Graphene Spin Polarized Electrode for Magnetic Tunnel Junctions.

2D materials offer the ability to expose their electronic structure to manipulations by a proximity effect. This could be harnessed to craft properties of 2D interfaces and van der Waals heterostructures in devices and quantum materials. We explore the possibility to create an artificial spin polarized electrode from graphene through proximity interaction with a ferromagnetic insulator to be used in a magnetic tunnel junction (MTJ). Ferromagnetic insulator/graphene artificial electrodes were fabricated and integrated in MTJs based on spin analyzers. Evidence of the emergence of spin polarization in proximitized graphene layers was observed through the occurrence of tunnel magnetoresistance. We deduced a spin dependent splitting of graphene's Dirac band structure (∼15 meV) induced by the proximity effect, potentially leading to full spin polarization and opening the way to gating. The extracted spin signals illustrate the potential of 2D quantum materials based on proximity effects to craft spintronics functionalities, from vertical MTJs memory cells to logic circuits.

Steep-slope transistors enabled with 2D quantum coupling stacks

As down scaling of transistors continues, there is a growing interest in developing steep-slope transistors with reduced subthreshold slope (SS) below the Boltzmann limit. In this work, we successfully fabricated steep-slope MoS2 transistors by incorporating a graphene layer, inserted in the gate stack. For our comprehensive study, we have applied density functional theory to simulate and calculate the change of SS effected by different 2D quantum materials, including graphene, germanene and 2D topological insulators, inserted within the gate dielectric. This theoretical study showed that graphene/MoS2 devices had steep SS (27.2 mV/decade), validating our experimental approach (49.2 mV/decade). Furthermore, the simulations demonstrated very steep SS (8.6 mV/decade) in WTe2/MoS2 devices. We conclude that appropriate combination of various 2D quantum materials for the gate-channel stacks, leads to steep SS and is an effective method to extend the scaling of transistors with exceptional performance.

Nanodiamond islands confined between two graphene sheets as perspective 2D quantum materials

Based on direct space DFT PBE0/6-31G* electronic structure calculations, the structure and properties of nanodiamond islands confined between two finite graphene fragments (NDI-c2G) was proposed and theoretically explored. DFT simulations revealed that fusion of planar aromatic molecules with two parent graphene fragments may form either cubic or hexagonal allotropes of NDI-c2Gs lattices accompanied with formation of local corrugated sp3 sites with substituted dangling bonds embedded into graphene sublattices. It was shown that at the DFT level of theory, low distortion energies of NDI-c2Gs lattices are comparable or smaller than the energy of van-der-Waals interactions, which allows the NDI-c2G lattices to be stabilized by a support and finally synthesized. The Nuclear Independent Chemical Shift calculations, shows that in the vicinity of NDI regions, graphene lattices are estimated to be either low-, or anti-aromatic. Furthermore, the formation of NDI-c2Gs leads to localization of HOMO and LUMO states at different sites of the NDI-c2Gs lattices. The NDI-c2G regions with confined frontier orbitals can be considered as arrays of quantum dots isolated from each other by NDI scattering centres of 3.82 Å dimension. The results demonstrates that NDI-c2G should be considered as strongly-correlated entangled hybrid quantum dots which may form extended quantum ensembles with great potential for advanced quantum applications.

Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization.

Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface-mediated growth process for the controlled synthesis of high-quality monolayer hBN is proposed and further demonstrated. It is discovered that the in-plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single-domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep-ultraviolet emission at 6.12eV stems from the 1s-exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer-scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

Electrically tunable Goos-Hänchen shift in two-dimensional quantum materials

We theoretically investigate the tunable Goos-Hänchen (GH) shifts in silicene subjected to an external electric field and circularly polarized light. The prominent feature of these 2D quantum materials is the tunable bandgap that can be tuned by an external electric field or by irradiating circular polarized light beam. Using angular spectrum analysis, we obtain the analytical expressions for the spin and valley polarized spatial and angular GH shifts. We find that tuneable giant spatial and angular GH shifts exhibit extreme values near Brewster’s angles and away from the optical transition frequencies in the silicene. We demonstrate that both positive and negative giant GH shifts can be achieved in the graphene family by tuning the electric field and circularly polarized light in distinct topological regimes. Due to the topological properties of these materials, the GH shift is sensitive to the coupled spin and valley indices of the Dirac fermions as well as to the number of closed gaps. We further demonstrated that topology and spin-orbit interactions play a crucial role in beam shifts and topological quantum phase transitions of the silicene can be comprehensively and efficiently probed through GH shift at the nanoscale.

Photoexfoliation Synthesis of 2D Materials

Quantum two-dimensional (2D) materials discovered in the early 21st century have outsmarted existing nanomaterials in various frontiers of applications. Among the preparation of 2D materials, molecular beam epitaxy, atomic layer deposition, and chemical vapor deposition are nonscalable costly methods, whereas sonochemical and Hummer’s exfoliation methods provide functionalized sheets. Conversely, ultrafast liquid-phase laser processing promises quick delivery of defect-free 2D quantum materials. We report photoexfoliation synthesis of atomic graphene layers, boron nitride (BN), and molybdenum disulfide (MoS2) by the intense KrF laser irradiation into aqueous dispersions of parent material powders in DMF taken in quartz beakers. The number of atomic layers and the lateral size of the sheets gradually decrease with an increase in the laser irradiation duration. Also, the laser fluence becomes the critical control parameter of the lateral size and the number of layers. The average lateral size shrinks from ∼400 nm at 1.5 J/cm2 to 20–30 nm at 4 J/cm2, which accompanies a surge in the ratio of sheets with fewer layers. We correlate the laser processing parameters with the sample size and analyze the molecule-atom-scale interactions. Simulation and DFT calculations suggest the mild out-of-plane thermal expansion of atomic layers followed by solvent intercalation stretches interlayer distance to ∼6.68 Å and thereby lowers the activation energy of exfoliation. The optimum photon fluence at the solvent-assisted condition reduces the activation barrier, enabling us to synthesize 2D crystals in the solution phase. Photoexfoliation synthesis of pure crystals of 2D materials can be promising for next-generation electronic devices.

Visualization of Charge-Density-Wave Reconstruction and Electronic Superstructure at the Edge of Correlated Insulator 1T-NbSe2.

Edges of low-dimensional quantum systems have profound effects on both fundamental research and device functionality. Real-space investigation of the microscopic edge structures and understanding the edge-modulated electronic properties are of great essence. Here we report the nanoscale structural reconstruction at the atomically sharp edge of a charge-density-wave (CDW) correlated insulator 1T-NbSe2 and the induced electronic properties. We find the CDW unit cells at the edge of single layer (SL) 1T-NbSe2 evolve from the well-defined CDW order in bulk and spontaneously reconstruct into the quartet along the edge. Moreover, we capture an anomalous electronic superstructure along the edge, the periodicity of which is four times that of ordinary CDW lattice. Our findings provide a way to design the one-dimensional electronic superstructure in 2D quantum materials.

Probing topological quantum phase transitions via photonic spin Hall effects in spin-orbit coupled 2D quantum materials

Topological photonics is an emerging field in photonics in which various topological and geometrical ideas are used to manipulate and control the behavior of light photons. The interplay between topological matter and the spin degree of freedom of photons provides new opportunities for achieving spin-based photonics applications. In this paper, the photonic spin Hall effect (PSHE) of reflected light from the surface of the topological silicene quantum systems subjected to external electric and radiation fields in the terahertz regime is theoretically investigated. By tuning the external electric and the applied laser fields, we can drive the silicenic system through different topological quantum phase transitions. We demonstrate that the in-plane and transverse spatial spin dependent shifts exhibit extreme values near Brewster’s angles and away from the optical transition frequencies. We reveal that the photonic spin Hall shifts are sensitive to the spin and valley indices as well as to the number of closed gaps. We believe that the spin and valley-resolved PSHE will greatly impact the research in spinoptics, spintronics, and valleytronics.

2D Quantum materials: Magnetism and superconductivity

First Page

Superradiant Emission from Coherent Excitons in van Der Waals Heterostructures

AbstractRecent advancements in isolation and stacking of layered van der Waals materials have created an unprecedented paradigm for demonstrating varieties of 2D quantum materials. Rationally designed van der Waals heterostructures composed of monolayer transition‐metal dichalcogenides (TMDs) and few‐layer hBN show several unique optoelectronic features driven by correlations. However, entangled superradiant excitonic species in such systems have not been observed before. In this report, it is demonstrated that strong suppression of phonon population at low temperature results in a formation of a coherent excitonic‐dipoles ensemble in the heterostructure, and the collective oscillation of those dipoles stimulates a robust phase synchronized ultra‐narrow band superradiant emission even at extremely low pumping intensity. Such emitters are in high demand for a multitude of applications, including fundamental research on many‐body correlations and other state‐of‐the‐art technologies. This timely demonstration paves the way for further exploration of ultralow‐threshold quantum‐emitting devices with unmatched design freedom and spectral tunability.

Electron Microscopy to Probe Flat Band Topological Systems of 2D and Pseudo 2D Quantum Materials

Electron Microscopy to Probe Flat Band Topological Systems of 2D and Pseudo 2D Quantum Materials

Monolayer 2D quantum materials subjected to gamma irradiation in high-vacuum for nuclear and space applications

The stability and reliability of emerging two-dimensional (2D) quantum materials subjected to harsh environments, such as high-energy radiation, are of high importance, particularly in the fields of space, defense, and energy applications. In this work, we explored the effects of gamma radiation on the structural and optical properties of monolayer WSe2 and WS2 crystals. Raman and photoluminescence spectroscopies were employed to study and probe radiation-induced changes to the samples after exposure to intense gamma radiation (from a 60Co source) in a high-vacuum environment (∼1 × 10−6 Torr) and with various exposure times to vary the total accumulated dosage (up to ∼56 Mrad). In general, very small changes in optical or vibrational properties were observed compared to pristine samples, suggesting noteworthy stability even for high dosages of gamma radiation. Moreover, we found that WSe2 monolayer samples exhibited higher tolerance to gamma radiation compared to WS2 samples. These findings highlight the inherent stability of these 2D quantum materials in harsh radioactive environments, which motivates further investigation of their optical, electrical, and structural properties and exploration for use in future space, energy, and defense applications.

Sparse sampling for fast quasiparticle-interference mapping

Scanning tunneling microscopy (STM) is a notoriously slow technique; Data-recording is serial which renders complex measurement tasks, such as quasiparticle interference (QPI) mapping, impractical. However, QPI would provide insight into band-structure details of quantum materials which can be inaccessible to angle-resolved photoemission spectroscopy. Here we use compressed sensing (CS) to fundamentally speed-up QPI mapping. We reliably recover the QPI information from a fraction of the usual local density of state measurements. The requirement of CS is naturally fulfilled for QPI, since CS relies on sparsity in a vector domain, here given by few nonzero coefficients in Fourier space. We exemplify CS on a simulated Cu(111) surface using random sampling of constant and varying probability density. We further simplify the motion of the STM tip through an open traveling salesman's problem for greater efficiency. We expect that the implications of our CS approach will be transformative for the exploration of 2D quantum materials.

Experimental Realization of Two-Dimensional Buckled Lieb Lattice.

Two-dimensional (2D) materials with a Lieb lattice host exotic electronic band structures. Such a system does not exist in nature, and it is also difficult to obtain in the laboratory due to its structural instability. Here, we experimentally realized a 2D system composed of a tin overlayer on an aluminum substrate by molecular beam epitaxy. The specific arrangement of Sn atoms on the Al(100) surface, which benefits from favorable interface interactions, forms a stabilized buckled Lieb lattice. Theoretical calculations indicate a partially broken nodal line loop and a topologically nontrivial insulating state with a spin-orbital coupling effect in the band structure of this Lieb lattice. The electronic structure of this system is experimentally characterized by angle-resolved photoemission spectroscopy, in which the hybridized states between topmost Al atoms and Sn atoms are revealed. Our work provides an appealing method for constructing 2D quantum materials based on the Lieb lattice.