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![Improved resolutions of STEM to correlate emergent phenomena to atomistic structure of quantum materials. a) Emergent vibrational modes in oxide superlattices. Dark field profiles show sharp boundaries between layers, and off‐axis EELS line profiles show atomistic variations of vibrational modes that are fundamentally different between i,ii) 27 unit cell superlattices and iii,iv) 2 unit cell superlattices (reprinted under the terms of the CC BY 4.0 license.[⁵⁶] Copyright 2022, The Authors. Published by Springer Nature). b) High spatial/spectral‐resolution EELS imaging of planar defects. i) Stacking fault in SiC (Reproduced with permission.[⁵⁷] Copyright 2021, Springer Nature). ii) Interface mode in Si/Ge heterostructure (Reproduced under the terms of the CC BY 4.0 license.[⁵⁸] Copyright 2021, The Authors. Published by Springer Nature). c) EELS imaging of defect modes from a single Si atom in graphene. i) DFT calculation, ii) schematic of experiment, iii) experimental EELS measurements (Reproduced with permission.[⁵⁹] Copyright 2020, American Association for the Advancement of Science). d) Efficient high spatial/spectral/momentum resolved EELS dispersion measurements of phonons in hBN. i) Brillouin zone schematic of parallel EELS dispersion measurement. ii) 16 minute dispersion acquisition at ≈5 nm spatial resolution (Reproduced with permission.[⁶⁰] Copyright 2020, Microscopy Society of America). e) 4D‐EELS measurements on hBN nanotubes. Images show real‐space imaging of spectral features in momentum‐energy space (Reproduced under the terms of the CC BY 4.0 license.[⁶¹] Copyright 2021, The Authors. Published by Springer Nature).](publication/358640008/figure/fig4/AS:11431281172671920@1688630549037/Improved-resolutions-of-STEM-to-correlate-emergent-phenomena-to-atomistic-structure-of.png)
Improved resolutions of STEM to correlate emergent phenomena to atomistic structure of quantum materials. a) Emergent vibrational modes in oxide superlattices. Dark field profiles show sharp boundaries between layers, and off‐axis EELS line profiles show atomistic variations of vibrational modes that are fundamentally different between i,ii) 27 unit cell superlattices and iii,iv) 2 unit cell superlattices (reprinted under the terms of the CC BY 4.0 license.[⁵⁶] Copyright 2022, The Authors. Published by Springer Nature). b) High spatial/spectral‐resolution EELS imaging of planar defects. i) Stacking fault in SiC (Reproduced with permission.[⁵⁷] Copyright 2021, Springer Nature). ii) Interface mode in Si/Ge heterostructure (Reproduced under the terms of the CC BY 4.0 license.[⁵⁸] Copyright 2021, The Authors. Published by Springer Nature). c) EELS imaging of defect modes from a single Si atom in graphene. i) DFT calculation, ii) schematic of experiment, iii) experimental EELS measurements (Reproduced with permission.[⁵⁹] Copyright 2020, American Association for the Advancement of Science). d) Efficient high spatial/spectral/momentum resolved EELS dispersion measurements of phonons in hBN. i) Brillouin zone schematic of parallel EELS dispersion measurement. ii) 16 minute dispersion acquisition at ≈5 nm spatial resolution (Reproduced with permission.[⁶⁰] Copyright 2020, Microscopy Society of America). e) 4D‐EELS measurements on hBN nanotubes. Images show real‐space imaging of spectral features in momentum‐energy space (Reproduced under the terms of the CC BY 4.0 license.[⁶¹] Copyright 2021, The Authors. Published by Springer Nature).
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Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entangleme...
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Optically active defects in 2D materials, such as hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs), are an attractive class of single-photon emitters with high brightness, operation up to room temperature, site-specific engineering of emitter arrays with strain and irradiation techniques, and tunability with external electr...
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... Besides their pristine form, heterogeneities such as impurities, dopants, structural defects, disorders, and interfaces may be controlled through substitutional doping and defect engineering to engineer electronic structures, induce magnetization, and trigger quantum phenomena in host materials. 10 As a result, 2D materials are a rich platform for a broad spectrum of research from theory to nextgeneration (opto)electronics, fundamental physics, sensors, energy, and artificial intelligence and machine learning. The substantial interest in this field is reflected in the publication record: The number of publications relevant to 2D materials has increased from merely ≈1900 in 2004, to ≈9000 in 2013, and over 49,000 in 2021. ...
... Recently, controllable substitutional doping of TMDs with transition metal elements during MOCVD for modulating TMDs-based transistors has been demonstrated. Kozhakhmetov et al. 164,165 introduced Re 2 (CO) 10 and V 2 (C 5 H 5 ) 2 during WSe 2 growth to substitute W with Re and V for n-and p-type doping, respectively. Furthermore, by controlling carrier gas flow going through the bubblers of precursors that provide dopants, dopant concentrations can be tuned the parts per million to the percentage level in WSe 2 ( Figure 10e). ...
... 2D heterostructures were shown to operate in a multimodal regime� producing several signals in response to a single analyte, allowing for reporting the analyte in a complex media. 10 Multiplexed sensing of doxorubicin, a common cancer drug, was demonstrated by Ignatova et al. 329 using a 2D material vertical heterostructure. A biosensor composed of monolayer MoS 2 coated with graphene utilized GERS, MoS 2 photoluminescence (PL), and graphene Raman shift signals to report drug molecules with a 60 nM threshold. ...
Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.
... These transitions are particularly complex in quantum, spin, and magnetic materials, where competing interactions between multiple coupled variables often lead to nanoscale heterogeneity, [3][4][5] both spatial and temporal, which profoundly influence phase coherence and correlation, as well as transitions between phases. [6] For example, in high-temperature superconductivity where multiple nearly degenerate phases exist, thermal fluctuations store entropy, and play a key role in phase stability and metastability, and in transition paths between phases. [7][8][9][10] This interplay between fluctuations, phases, and macroscopic properties lies at the heart of a very active and rich area of materials science research An important but less studied question is, how temporal heterogeneities influence correlation properties and fluctuations that ultimately guides between phases at the nanoscale in the temperature regime where the theory of critical phenomena is not very relevant. ...
Equilibrium phase transitions are influenced by fluctuations and often discussed within the framework of the Gibbs free energy, wherein the exchange of energy between system and thermal bath is stationary and all regions of the sample exhibit the same phase. Presence of spatial heterogeneity in the magnetic structures such as pinning centers, domain walls, topological defects, etc. may cause temporal heterogeneity that modifies the nature of the magnetic phase transition. This study reports that interplay of nanoscale thermodynamics with spatio‐temporal heterogeneity gives rise to complex phase transition pathways in amorphous FexGe1‐x thin films with temperature and Fe‐concentration (x). Coherent resonant soft X‐ray scattering experiments that have simultaneous spatial, temporal, and spectral sensitivity show that the origin of helical to paramagnetic phase transition in amorphous Fe‐Ge thin films lies in the appearance of enhanced‐fluctuation spots deep inside the ordered state. The fluctuations are heterogeneous, starting over a small fraction of the domains that increases and becomes isotropic over the entire film as the temperature increases or the Fe‐concentration decreases. The fluctuating‐fraction, when normalized to magnetization for different Fe‐concentrations, follows a single power law behavior, suggesting that the nature of the transition can be described in terms of the underlying spatio‐temporal fluctuations.
... However, most such studies are inherently limited to macroscopically averaged properties. Alternatively, atomic scale imaging and spectroscopy by scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) have been key to deciphering chemical disorder and electronic heterogeneity in quantum materials [7][8][9] . Nevertheless, traditional methods of visual inspection of these images can be challenging for multi-component alloys, particularly beyond the dilute limit due to chemical disorders and electronic inhomogeneities. ...
Scanning tunneling microscopy (STM) is a powerful technique for imaging atomic structure and inferring information on local elemental composition, chemical bonding, and electronic excitations. However, traditional methods of visual inspection can be challenging for such determination in multi-component alloys, particularly beyond the dilute limit due to chemical disorder and electronic inhomogeneity. One viable solution is to use machine learning to analyze STM data and identify patterns and correlations that may not be immediately apparent through visual inspection alone. Here, we apply this approach to determine the Se/S concentration in superconducting single-layer FeSe1-xSx alloy epitaxially grown on SrTiO3(100) substrate by molecular beam epitaxy. First, defect-related dI/dV tunneling spectra are identified by the K-means clustering method, followed by singular value decomposition to distinguish between those from S and Se. Such analysis provides an efficient and reliable determination of local elemental composition, and further reveals correlations of nanoscale chemical inhomogeneity to superconductivity in single-layer iron chalcogenide films.
... DFXM has been used to study structure-property relationships in a variety of materials (Refs. [2,3] and the references therein), but its effective use beyond room-temperature studies remains a challenge, particularly at the cryogenic temperatures frequently encountered in investigations of quantum materials [4]. This is due to competing requirements of low vibrations and mechanical stability, versus the operational demands and physical connections of a cryostat mounted on an X-ray diffractometer. ...
Dark field X-ray microscopy (DFXM) is an experimental technique employed to investigate material properties by probing their 'mesoscale,' or microscale structures, in a bulk-sensitive manner using hard X-rays at synchrotron radiation sources. However, challenges remain when it comes to applications of this technique to examine low-temperature phenomena in quantum materials, which exhibit complex phase transitions at cryogenic temperatures. One such material is NaMnO2, which hosts an antiferromagnetic transition at 45 K that is suspected to coincide with local structural transitions from its majority monoclinic phase to nanoscale triclinic domains. Direct observation of local heterogeneities and this effect at low temperatures in NaMnO2 is an important step in understanding this material, and serves as an ideal candidate study for expanding the DFXM experimental design space. This paper details a foundational high-resolution DFXM study, down to liquid-helium temperature and below, conducted to explore phase transitions in NaMnO2. The outlined experiment ushers in the evaluation of other functional materials at low temperatures using this technique.
... Consequently, tuning the carrier density has been of critical importance for the design and fabrication of semiconductor devices, as well as for exploring novel electronic states in complex oxides such as superconductivity [2][3][4] and topological effects [5]. Chemical doping is often exploited to tune the carrier density in solids, although it may introduce chemical and electronic inhomogeneities at the nanoscale [6][7][8]. These inhomogeneities are often detrimental to carrier mobility and device performance [9][10][11]. ...
Doping inhomogeneities in solids are not uncommon, but their microscopic observation and understanding are limited due to the lack of bulk-sensitive experimental techniques with high-enough spatial and spectral resolution. Here, we demonstrate nanoscale imaging of both dopants and free charge carriers in La-doped BaSnO3 (BLSO) using high-resolution electron energy-loss spectroscopy (EELS). By analyzing both high- and low-energy excitations in EELS, we reveal chemical and electronic inhomogeneities within a single BLSO nanocrystal. The inhomogeneous doping leads to distinctive localized infrared surface plasmons, including a novel plasmon mode that is highly confined between high- and low-doping regions. We further quantify the carrier density, effective mass, and dopant activation percentage from EELS data and transport measurements on the bulk single crystals of BLSO. These results represent a unique way of studying heterogeneities in solids, understanding structure-property relationships at the nanoscale, and opening the way to leveraging nanoscale doping texture in the design of nanophotonic devices.
Materials containing Dirac fermions (DFs) have unique electronic properties, and have been extensively studied. Electron spin resonance revealed that α-ET2I3 (ET = bis(ethylenedithio)-tetrathiafulvalene) at 1 bar contained a nearly three-dimensional DFs above ∼100 K coexisting with standard fermions. The close charge-transfer ET–I3 interactions account for temperature-sensitive three-dimensional (3D) band structures and temperature-independent resistivity behaviour. As 3D band structures cannot be depicted in a four-dimensional space, the analysis method proposed herein provides a general way to present important and easy-to-understand information of such band structures that cannot be obtained otherwise.
