Towards a 2D printer: a deterministic cross contamination-free transfer method for atomically layered materials (Conference Presentation)

Photonic platforms with excellent nonlinear optical characteristics are very important to improve the devices' performance parameters such as integration, modulation speeds and working bandwidths for all-optical signal processing. The traditional processing technology of photonic platforms based on silicon, silicon nitride and silicon oxide is mature, but the nonlinear function of these optical platforms is limited due to the characteristics of materials; Although two-dimensional (2D) materials possess excellent nonlinear optical properties, their nonlinear potentials cannot be fully utilized because of their atomic layer thickness. Integrating 2D materials with mature photonic platforms can significantly improve the interaction between light and matter, give full play to the potentials of 2D materials in the field of nonlinear optics, and improve the nonlinear optical performances of the integrated platforms on the basis of fully utilizing the mature processing technology of the photonic platforms. Based on the above ideas, starting from the basic principle of nonlinear optics (Section 2), this review combs the research progress of various nonlinear photonic platforms (resonators, metasurfaces, optical fibers, on-chip waveguides, etc.) heterogeneously integrated with 2D materials, realized by traditional transfer methods (Section 3) and emerging direct-growth methods (Section 4) in recent years, and the introduction is divided into second-order and third-order nonlinearity. Comparing with the transfer methods, the advantages of using direct-growth methods to realize the heterogeneous integration of 2D materials and photonic platforms for the study of nonlinear optics are expounded, and the technical difficulties to be overcome in preparing the actual devices are also pointed. In the future, we can try to grow 2D materials directly onto the surfaces of various cavities to study the enhancement of second-order nonlinearity; we can also try to grow 2D materials directly onto the on-chip waveguides or microrings to study the enhancement of third-order nonlinearity. Generally speaking, the research on integrated nonlinearity by directly growing 2D materials onto various photonic structures has aroused great interest of researchers in this field. As time goes on, breakthrough progress will be made in this field, and technical problems such as continuous growth of high-quality 2D materials onto photonic structures and wafer-level large-scale preparation will be broken through, further improving the performance parameters of chips and laying a good foundation for optical communication, signal processing, optical sensing, all-optical computing, quantum technology and so on.

The field of two-dimensional (2D) materials has seen remarkable progress, driven by their exceptional electronic, optical, and mechanical properties. In addition to their intrinsic qualities, 2D materials are atomically thin and readily integrated with a wide range of devices, offering immense potential for next-generation on-chip technologies across various domains, beyond just electronics and optics. Central to the fabrication of 2D devices is the development of effective transfer methods, which are crucial for maintaining clean material interfaces and intact material properties. However, as some of the most fundamental techniques, transfer methods are incredibly diverse, making it challenging to navigate the various approaches. This review provides a comprehensive analysis of the state-of-the-art transfer methods, including a preliminary discussion on high-quality 2D material preparation, along with evaluation of the strengths and weaknesses of various transfer techniques. Furthermore, despite being a foundational field, there are still many significant tasks to be undertaken, with several bottlenecks awaiting breakthroughs. We also highlight emerging trends, such as reconfigurable transfer, all-transfer for chip manufacturing, and the application of transfer techniques to low-dimensional materials across broader research fields, such as chemistry, polariton, tribology, haptics, thermal transport, thermodynamic control, quantum science, neuromorphic computing, electrocatalysts, and energy, offering insights into future directions for 2D material integration.

The assembly of graphene and other two-dimensional (2D) materials into artificial crystals termed van-der-Waals stacks has great potential to produce new materials without precedence in nature and develop novel electronic devices. To reliably assemble 2D materials into such structures, however, a better understanding of the transfer process is required. Here we report a quantitative approach to examining the adhesion behavior during viscoelastic stamping of 2D materials. By measuring the adhesion of graphene to different carrier substrates and varying the peeling speed we have identified the range of adhesion of samples. The result shows that the adhesion occurs between graphene-graphene and graphene-SiO2 substrate have a higher value than the ability of polydimethylsiloxane (PDMS) stamp to pick up. The impact of surface modification and alternative substrates is investigated and our results provide guidelines to realize an effective fabrication method for two-dimensional heterostructure devices.

Two-dimensional (2D) materials are considered for numerous applications in microelectronics, although several challenges remain when integrating them into functional devices. Weak adhesion is one of them, caused by their chemical inertness. Quantifying the adhesion of 2D materials on three-dimensional surfaces is, therefore, an essential step toward reliable 2D device integration. To this end, button shear testing is proposed and demonstrated as a method for evaluating the adhesion of 2D materials with the examples of graphene, hexagonal boron nitride (hBN), molybdenum disulfide, and tungsten diselenide on silicon dioxide and silicon nitride substrates. We propose a fabrication process flow for polymer buttons on the 2D materials and establish suitable button dimensions and testing shear speeds. We show with our quantitative data that low substrate roughness and oxygen plasma treatments on the substrates before 2D material transfer result in higher shear strengths. Thermal annealing increases the adhesion of hBN on silicon dioxide and correlates with the thermal interface resistance between these materials. This establishes button shear testing as a reliable and repeatable method for quantifying the adhesion of 2D materials.

The field of valleytronics, which exploits the valley degree of freedom in two-dimensional (2D) materials, has emerged as a promising frontier in next-generation electronic and photonic devices. In this context, a valley refers to an energy extremum in the electronic band structure of a material, which can serve as a discrete quantum number — akin to charge or spin — for information encoding. Because many 2D materials possess two inequivalent valleys in momentum space, they can naturally represent binary logic states, making them attractive for low-power electronics and quantum computing. This topical review explores the valley-dependent Berry phase effects and their significant role in various valleytronic applications. We begin by introducing the fundamentals of topology in solid-state physics, emphasizing the Berry phase — a geometric phase with profound quantum mechanical implications — and its associated Berry curvature, which acts as an effective magnetic field in momentum space. Next, we review key 2D materials, including monolayer and bilayer graphene as well as transition metal dichalcogenides, to illustrate how valley-dependent Berry curvature gives rise to orbital magnetic moments and the valley Hall effect. Advanced concepts, such as Chern numbers and valley Chern numbers, are discussed to quantify the topological order in these systems. The review also highlights the emergence of topological kink states and domain wall states in graphene, which are linked to robust edge conduction. Furthermore, we examine the valley Zeeman effect and the mechanisms underlying valley polarization (VP), wherein an imbalance of electrons between distinct momentum valleys enables novel device functionalities. Recent experimental breakthroughs — such as observations of the exciton Hall effect — demonstrate the technological potential of Berry phase effects in controlling VP and current. Finally, we underscore the critical role of these geometric and topological phenomena in driving future valleytronic innovations in 2D materials.

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) allows species-level microbial identification quickly and cost-effectively. The aim of this study was to validate MALDI-TOF MS for the identification of Legionella anisa, Legionella longbeachae and Legionella pneumophila and to examine whether incubation time or sample preparation method affect identification results. Environmental Legionella strains and ATCC reference strains were used in the study. The strains underwent 16S rRNA sequencing and latex agglutination and fluorescence tests to which MALDI-TOF MS results were compared. The strains, incubated for 24 and 48h, were analysed with MALDI-TOF MS using direct transfer and extraction methods. Following performance characteristics were calculated from the results: sensitivity, specificity, relative accuracy, predictive values, repeatability and reproducibility. Species-level identification was achieved for 63 out of the 69 environmental and reference strains regardless of incubation time or preparation method. MALDI-TOF MS proved to be very specific (100%) and sensitive (94.6-100%) to the Legionella species analysed. With the extraction method, incubation time had a statistically significant, but species-dependent effect on the interpretative criteria, i.e. score values, in all species studied. No significant difference in any of the species' score values was found between the direct transfer and extraction methods with 24 h incubation. MALDI-TOF MS score values showed high repeatability (std 0.01-0.23) and reproducibility (std 0.02-0.18) for all Legionella species analysed. Based on the results, MALDI-TOF MS suits well for the analysis of environmental Legionella strains already after 24h of incubation and using direct transfer method.

Transistors with exfoliated two-dimensional (2D) materials on a SiO2/Si substrate have been applied and have been proven effective in a wide range of applications, such as circuits, memory, photodetectors, gas sensors, optical modulators, valleytronics, and spintronics. However, these devices usually suffer from limited gate control because of the thick SiO2 gate dielectric and the lack of reliable transfer method. We introduce a new back-gate transistor scheme fabricated on a novel Al2O3/ITO (indium tin oxide)/SiO2/Si "stack" substrate, which was engineered with distinguishable optical identification of exfoliated 2D materials. High-quality exfoliated 2D materials could be easily obtained and recognized on this stack. Two typical 2D materials, MoS2 and ReS2, were implemented to demonstrate the enhancement of gate controllability. Both transistors show excellent electrical characteristics, including steep subthreshold swing (62 mV dec-1 for MoS2 and 83 mV dec-1 for ReS2), high mobility (61.79 cm2 V-1 s-1 for MoS2 and 7.32 cm2 V-1 s-1 for ReS2), large on/off ratio (~107), and reasonable working gate bias (below 3 V). Moreover, MoS2 and ReS2 photodetectors fabricated on the basis of the scheme have impressively leading photoresponsivities of 4000 and 760 A W-1 in the depletion area, respectively, and both have exceeded 106 A W-1 in the accumulation area, which is the best ever obtained. This opens up a suite of applications of this novel platform in 2D materials research with increasing needs of enhanced gate control.

Solvothermal-assisted liquid-phase exfoliation of large size and high quality black phosphorus

A comprehensive review of the fabrication process, fundamental properties and functionalities and device applications of heterogeneously integrated two-dimensional (2D) materials is provided. An extensive library of atomic 2D materials with selectable material properties exists and it is rapidly expanding, with which it is possible to construct hybrid or heterostructures that display novel properties with unique functionalities. Such heterostructures adding a degree of freedom to carriers in the third direction provide interesting possibilities for the design of functional novel devices. The subject of 2D heterostructures has now become so vast that no single article can cover the entire topic in any reasonable manner. This review is intended to bridge the gap between the major developments already discovered and current trends in 2D heterostructures.

Two-dimensional (2D) materials and their heterostructures offer a rich platform for studying the fundamentals of condensed matter physics and engineering new functional devices. Scanning tunneling microscopy (STM) has revealed various atomic structures and electronic properties of 2D materials. However, STM measurements require atomically clean surfaces, which are difficult to maintain, as many 2D materials readily degrade upon air exposure. Moreover, the small size of exfoliated flakes, typically around 10 μm, presents a challenge for accurate and repeatable tip positioning in conventional STM systems. To overcome these limitations, we designed a home-built low-temperature STM integrated with a glove box and a specially designed sample stage. The glove box, directly connected to a load-lock chamber of STM system, enables the transfer and measurement of samples without exposure to ambient air, minimizing contamination and oxidation. The sample stage, designed with dual pockets and micron-level positioning capability, facilitate in-situ tip conditioning and precise navigation to small 2D flakes. The synergistic integration of these components ensures reliable, high-resolution STM/STS measurements, enabling comprehensive investigations into the electronic properties of a wide range of 2D materials.

The mechanical properties of two-dimensional (2D) materials are critical for their applications in functional devices as well as for strain engineering. Here, we report the Young’s modulus and breaking strength of multilayered InSe, an emerging 2D semiconductor of the layered group III chalcogenide. Few-layer InSe flaks were exfoliated from bulk InSe crystal onto Si/SiO2 substrate with micro-fabricated holes, and indentation tests were carried out using an atomic force microscopy probe. In combination with both continuum analysis and finite element simulation, we measured the Young’s modulus of multilayer 2D InSe (>5 L) to be 101.37 ± 17.93 GPa, much higher than its bulk counterpart, while its breaking strength is determined to be 8.68 GPa, approaching the theoretical limit of 10.1 GPa. Density functional theory calculations were also carried out to explain the insensitivity of Young’s modulus to the layer count. It is found that 2D InSe is softer than most 2D materials, and exhibits breaking strength higher than that of carbon fiber, yet remaining more compliant, making it ideal for flexible electronics applications. The reliability of our method is also validated by measurement of graphene.

Recently, 2D materials have been intensively studied as emerging materials for future electronics, including flexible electronics, photonics, and electrochemical energy storage devices. Among representative 2D materials (such as graphene, boron nitride, and transition metal dichalcogenides) that exhibit extraordinary properties, graphene stands out in the flexible electronics field due to its combination of high electron mobility, high thermal conductivity, high specific surface area, high optical transparency, excellent mechanical flexibility, and environmental stability. This review covers the synthesis, transfer, and characterization methods of graphene and 2D materials and graphene's application to flexible devices as well as comparison with other competing materials.

Near-ultraviolet (NUV) detectors based on two-dimensional (2D) materials have garnered considerable attention owing to their atomic-scale thickness, varied bandgap, and strong interaction with incident light. In this work, homostructures composed of tin disulfide/chromium–doped tin disulfide (SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Cr–SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) were proposed for 405 nm near-ultraviolet detectors. Afterward, superlative detectors were fabricated through the employment of micro-mechanical exfoliation and polydimethylsiloxane-assisted dry-transfer methods, anchored by the SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cr–SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> homostructures. The detectors demonstrated exceptional detection performance for 405 nm NUV radiation with a significant responsivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.04\times 10^{{3}}\,\,\text {A}\cdot \text {W}^{-{1}}$ </tex-math></inline-formula> and a short response time of 8 ms, exceeding that of detectors based on most 2D materials and even commercial light detectors (Thorlabs FD11A–Si). The work presents an effective homostructure for high-performance SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based detectors, thereby opening an opportunity for the utilization of SnS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in NUV detectors.

The clean and reliable transfer of two-dimensional (2D) materials is critical for preserving their intrinsic properties and enabling high-performance device applications. This study presents a method utilising UV-ozone treated polydimethylsiloxane (UV-PDMS) as a transfer medium to achieve residue-free 2D materials. The UV-PDMS transfer method increases surface rigidity, reduces surface polymeric residues, and ensures intimate contact between 2D materials and target substrates. This is translated into cleaner samples as confirmed by atomic force microscopy, photoluminescence spectroscopy, and x-ray photoelectron spectroscopy. Field-effect transistors based on monolayer MoS2 fabricated with UV-PDMS transferred method exhibit lower subthreshold swing, reduced hysteresis, and higher carrier mobility compared to devices fabricated with PDMS transfer. Additionally, vertical solar cells based on multilayer WSe2 prepared with UV-PDMS method demonstrate enhanced fill factor and power conversion efficiency.

Plasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high‐quality fabrication, all‐dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light‐matter interactions and contribute to the revolutions on nanophotonic devices.