Observation of Three-State Nematicity and Domain Evolution in Atomically Thin Antiferromagnetic NiPS3.

Nickel phosphorus trisulfide (NiPS3), a van der Waals 2D antiferromagnet, has received significant interest for its intriguing properties in recent years. However, despite its fundamental importance in the physics of low-dimensional magnetism and promising potential for technological applications, the study of magnetic domains in NiPS3 down to an atomically thin state is still lacking. Here, we report the layer-dependent magnetic characteristics and magnetic domains in NiPS3 by employing linear dichroism spectroscopy, polarized microscopy, spin-correlated photoluminescence, and Raman spectroscopy. Our results reveal the existence of the paramagnetic-to-antiferromagnetic phase transition in bulk to bilayer NiPS3 and provide evidence of the role of stronger spin fluctuations in thin NiPS3. Furthermore, our study identifies three distinct antiferromagnetic domains within atomically thin NiPS3 and captures the thermally activated domain evolution. Our findings provide crucial insights for the development of antiferromagnetic spintronics and related technologies.

Synthesis of 2D layered transition metal (Ni, Co) hydroxides via edge-on condensation.

Layered transition metal hydroxides (LTMHs) with transition metal centers sandwiched between layers of coordinating hydroxide anions have attracted considerable interest for their potential in developing clean energy sources and storage technologies. However, two-dimensional (2D) LTMHs remain largely understudied in terms of physical properties and applications in electronic devices. Here, for the first time we report > 20 µm α-Ni(OH)2 2D crystals, synthesized from hydrothermal reaction. And an edge-on condensation mechanism assisted with the crystal field geometry is proposed to understand the 2D intra-planar growth of the crystals, which is also testified through series of systematic comparative studies. We also report the successful synthesis of 2D Co(OH)2 crystals (> 40 µm) with more irregular shape due to the slightly distorted octahedral geometry of the crystal field. Moreover, the detailed structural characterization of synthesized α-Ni(OH)2 are performed. The optical band gap energy is extrapolated as 2.54 eV from optical absorption measurements and the electronic bandgap is measured as 2.52 eV from reflected electrons energy loss spectroscopy (REELS). We further demonstrate its potential as a wide bandgap (WBG) semiconductor for high voltage operation in 2D electronics with a high breakdown strength, 4.77 MV/cm with 4.9 nm thickness. The successful realization of the 2D LTMHs opens the door for future exploration of more fundamental physical properties and device applications.

Rapid, Multianalyte Detection of Opioid Metabolites in Wastewater.

By monitoring opioid metabolites, wastewater-based epidemiology (WBE) could be an excellent tool for real-time information on the consumption of illicit drugs. A key limitation of WBE is the reliance on costly laboratory-based techniques that require substantial infrastructure and trained personnel, resulting in long turnaround times. Here, we present an aptamer-based graphene field effect transistor (AptG-FET) platform for simultaneous detection of three different opioid metabolites. This platform provides a reliable, rapid, and inexpensive method for quantitative analysis of opioid metabolites in wastewater. The platform delivers a limit of detection 2-3 orders of magnitude lower than previous reports, but in line with the concentration range (pg/mL to ng/mL) of these opioid metabolites present in real samples. To enable multianalyte detection, we developed a facile, reproducible, and high-yield fabrication process producing 20 G-FETs with integrated side gate platinum (Pt) electrodes on a single chip. Our devices achieved the selective multianalyte detection of three different metabolites: noroxycodone (NX), 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), and norfentanyl (NF) in wastewater diluted 20× in buffer.

Electronic Raman scattering in the 2D antiferromagnet NiPS3.

Correlated-electron systems have long been an important platform for various interesting phenomena and fundamental questions in condensed matter physics. As a pivotal process in these systems, d-d transitions have been suggested as a key factor toward realizing optical spin control in two-dimensional (2D) magnets. However, it remains unclear how d-d excitations behave in quasi-2D systems with strong electronic correlation and spin-charge coupling. Here, we present a systematic electronic Raman spectroscopy investigation on d-d transitions in a 2D antiferromagnet­NiPS3, from bulk to atomically thin samples. Two electronic Raman modes originating from the scattering of incident photons with d electrons in Ni2+ ions are observed at ~1.0 eV. This electronic process persists down to trilayer flakes and exhibits insensitivity to the spin ordering of NiPS3. Our study demonstrates the utility of electronic Raman scattering in investigating the unique electronic structure and its coupling to magnetism in correlated 2D magnets.

Graphene-Based Environmental Sensors: Electrical and Optical Devices.

In this review paper, we summarized the recent progress of using graphene as a sensing platform for environmental applications. Especially, we highlight the electrical and optical sensing devices developed based on graphene and its derivatives. We discussed the role of graphene in these devices, the sensing mechanisms, and the advantages and disadvantages of specific devices. The approaches to improve the sensitivity and selectivity are also discussed.

Spin-induced linear polarization of photoluminescence in antiferromagnetic van der Waals crystals.

Antiferromagnets are promising components for spintronics due to their terahertz resonance, multilevel states and absence of stray fields. However, the zero net magnetic moment of antiferromagnets makes the detection of the antiferromagnetic order and the investigation of fundamental spin properties notoriously difficult. Here, we report an optical detection of Néel vector orientation through an ultra-sharp photoluminescence in the van der Waals antiferromagnet NiPS3 from bulk to atomically thin flakes. The strong correlation between spin flipping and electric dipole oscillator results in a linear polarization of the sharp emission, which aligns perpendicular to the spin orientation in the crystal. By applying an in-plane magnetic field, we achieve manipulation of the photoluminescence polarization. This correlation between emitted photons and spins in layered magnets provides routes for investigating magneto-optics in two-dimensional materials, and hence opens a path for developing opto-spintronic devices and antiferromagnet-based quantum information technologies.

Modulation Doping via a Two-Dimensional Atomic Crystalline Acceptor.

Two-dimensional nanoelectronics, plasmonics, and emergent phases require clean and local charge control, calling for layered, crystalline acceptors or donors. Our Raman, photovoltage, and electrical conductance measurements combined with ab initio calculations establish the large work function and narrow bands of α-RuCl3 enable modulation doping of exfoliated single and bilayer graphene, chemical vapor deposition grown graphene and WSe2, and molecular beam epitaxy grown EuS. We further demonstrate proof of principle photovoltage devices, control via twist angle, and charge transfer through hexagonal boron nitride. Short-ranged lateral doping (≤65 nm) and high homogeneity are achieved in proximate materials with a single layer of α-RuCl3. This leads to the best-reported monolayer graphene mobilities (4900 cm2/(V s)) at these high hole densities (3 × 1013 cm-2) and yields larger charge transfer to bilayer graphene (6 × 1013 cm-2).

A cleanroom in a glovebox.

The exploration of new materials, novel quantum phases, and devices requires ways to prepare cleaner samples with smaller feature sizes. Initially, this meant the use of a cleanroom that limits the amount and size of dust particles. However, many materials are highly sensitive to oxygen and water in the air. Furthermore, the ever-increasing demand for a quantum workforce, trained and able to use the equipment for creating and characterizing materials, calls for a dramatic reduction in the cost to create and operate such facilities. To this end, we present our cleanroom-in-a-glovebox, a system that allows for the fabrication and characterization of devices in an inert argon atmosphere. We demonstrate the ability to perform a wide range of characterization as well as fabrication steps, without the need for a dedicated room, all in an argon environment. Finally, we discuss the custom-built antechamber attached to the back of the glovebox. This antechamber allows the glovebox to interface with ultra-high vacuum equipment such as molecular-beam epitaxy and scanning tunneling microscopy.

Deep-Learning-Enabled Fast Optical Identification and Characterization of 2D Materials.

Advanced microscopy and/or spectroscopy tools play indispensable roles in nanoscience and nanotechnology research, as they provide rich information about material processes and properties. However, the interpretation of imaging data heavily relies on the "intuition" of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, the optical characterization of 2D materials is used as a case study, and a neural-network-based algorithm is demonstrated for the material and thickness identification of 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, flake sizes, and their distributions, based on which an ensemble approach is developed to predict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other optical identification applications. This artificial-intelligence-based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials, and potentially accelerate new material discoveries.

Enhanced Raman Scattering on Nine 2D van der Waals Materials.

Since the discovery of graphene-enhanced Raman scattering in 2010, other 2D materials have been reported to show a Raman enhancement effect on molecules adsorbed on their surfaces. The mechanism for this phenomenon, however, still remains elusive. Here we performed a comparative investigation of the Raman enhancement effect on nine 2D materials with an identical number of copper phthalocyanine (CuPc) as probe molecules. Furthermore, the degree of charge transfer for different CuPc/2D material combinations was calculated, and a positive correlation with enhancement factors was observed, providing evidence to support the charge-transfer-dominated chemical mechanism for this amplification. This study also suggests that Raman enhancement spectroscopy can be used as a nondestructive and rapid probe for the interface interaction between molecules and 2D materials, crucial for organic molecule/2D material-based electronic and optoelectronic devices.

Chemical and Bio Sensing Using Graphene-Enhanced Raman Spectroscopy.

Graphene is a two-dimensional (2D) material consisting of a single sheet of sp² hybridized carbon atoms laced in a hexagonal lattice, with potentially wide usage as a Raman enhancement substrate, also termed graphene-enhanced Raman scattering (GERS), making it ideal for sensing applications. GERS improves upon traditional surface-enhanced Raman scattering (SERS), combining its single-molecule sensitivity and spectral fingerprinting of molecules, and graphene's simple processing and superior uniformity. This enables fast and highly sensitive detection of a wide variety of analytes. Accordingly, GERS has been investigated for a wide variety of sensing applications, including chemical- and bio-sensing. As a derivative of GERS, the use of two-dimensional materials other than graphene for Raman enhancement has emerged, which possess remarkably interesting properties and potential wider applications in combination with GERS. In this review, we first introduce various types of 2D materials, including graphene, MoS2, doped graphene, their properties, and synthesis. Then, we describe the principles of GERS and comprehensively explain how the GERS enhancement factors are influenced by molecular and 2D material properties. In the last section, we discuss the application of GERS in chemical- and bio-sensing, and the prospects of such a novel sensing method.

Probing the Domain Architecture in 2D α-Mo2 C via Polarized Raman Spectroscopy.

MXenes are emerging 2D materials with intriguing properties such as excellent stability and high conductivity. Here, a systematic study on the Raman spectra of 2D α-Mo2 C (molybdenum carbide), a promising member in MXene family, is conducted. Six experimentally observed Raman modes from ultrathin α-Mo2 C crystal are first assigned with the assistance of phonon dispersion calculated from density functional theory. Angle-resolved polarized Raman spectroscopy indicates the anisotropy of α-Mo2 C in the b-c plane. Raman spectroscopy is further used to study the unique domain structures of 2D α-Mo2 C crystals grown by chemical vapor deposition. A Raman mapping investigation suggests that most of the α-Mo2 C flakes contain multiple domains and the c-axes of neighboring domains tend to form a 60° or 120° angle, due to the weak MoC bonds in this interstitial carbide and the low formation energy of the carbon chains along three equivalent directions. This study demonstrates that polarized Raman spectroscopy is a powerful and effective way to characterize the domain structures in α-Mo2 C, which will facilitate the further exploration of the domain-structure-related properties and potential applications of α-Mo2 C.

Anomalous Phonon Modes in Black Phosphorus Revealed by Resonant Raman Scattering.

Black phosphorus (BP), a layered material with puckered crystalline structure in each layer, has drawn intense interest due to its unique optical and electronic properties. In particular, the intricate Raman scattering effect in BP is intriguing and provides a platform for researchers to probe the physical properties of BP in depth. Here we report the first observation of anomalous modes with the frequency in the range of 100-900 cm-1 in BP due to the resonant Raman effect. The origin and assignment of the anomalous modes are discussed based on the excitation energy- and angle-dependent Raman measurements. Density functional theory (DFT) calculated electronic band structure is used to support our understanding. The newly observed phonon modes could serve as a unique probe for the fine electronic structures and the exciton-phonon couplings, which promote a better understanding of BP for potential nanoelectronic and nanophotonic applications in the future.