Hexagonal Boron Nitride assisted transfer and encapsulation of large area CVD graphene

Transfer of 2D Films: From Imperfection to Perfection

Atomically thin 2D films and their van der Waals heterostructures have demonstrated immense potential for breakthroughs and innovations in science and technology. Integrating 2D films into electronics and optoelectronics devices and their applications in electronics and optoelectronics can lead to improve device efficiencies and tunability. Consequently, there has been steady progress in large-area 2D films for both front- and back-end technologies, with a keen interest in optimizing different growth and synthetic techniques. Parallelly, a significant amount of attention has been directed toward efficient transfer techniques of 2D films on different substrates. Current methods for synthesizing 2D films often involve high-temperature synthesis, precursors, and growth stimulants with highly chemical reactivity. This limitation hinders the widespread applications of 2D films. As a result, reports concerning transfer strategies of 2D films from bare substrates to target substrates have proliferated, showcasing varying degrees of cleanliness, surface damage, and material uniformity. This review aims to evaluate, discuss, and provide an overview of the most advanced transfer methods to date, encompassing wet, dry, and quasi-dry transfer methods. The processes, mechanisms, and pros and cons of each transfer method are critically summarized. Furthermore, we discuss the feasibility of these 2D film transfer methods, concerning their applications in devices and various technology platforms.

Polymer transfer technique for strain-activated emission in hexagonal boron nitride

We present a hexagonal boron nitride (hBN) polymer-assisted transfer technique and discuss subtleties about the process. We then demonstrate localized emission from strained regions of the film draped over features on a prepatterned substrate. Notably, we provide insight into the brightness distribution of these emitters and show that the brightest emission is clearly localized to the underlyin-g substrate features rather than unintentional wrinkles present in the hBN film. Our results aide in the current discussion surrounding scalability of single photon emitter arrays.

Frontiers of graphene-based Hall-effect sensors

Hall sensors have become one of the most used magnetic sensors in recent decades, performing the vital function of providing a magnetic sense that is naturally absent in humans. Various electronic applications have evolved from circuit-integrated Hall sensors due to their low cost, simple linear magnetic field response, ability to operate in a large magnetic field range, high magnetic sensitivity and low electronic noise, in addition to many other advantages. Recent developments in the fabrication and performance of graphene Hall devices promise to open up the realm of Hall sensor applications by not only widening the horizon of current uses through performance improvements, but also driving Hall sensor electronics into entirely new areas. In this review paper we describe the evolution from the traditional selection of Hall device materials to graphene Hall devices, and explore the various applications enabled by them. This includes a summary of the selection of materials and architectures for contemporary micro-to nanoscale Hall sensors. We then turn our attention to introducing graphene and its remarkable physical properties and explore how this impacts the magnetic sensitivity and electronic noise of graphene-based Hall sensors. We summarise the current state-of-the art of research into graphene Hall probes, demonstrating their record-breaking performance. Building on this, we explore the various new application areas graphene Hall sensors are pioneering such as magnetic imaging and non-destructive testing. Finally, we look at recent encouraging results showing that graphene Hall sensors have plenty of room to improve, before then discussing future prospects for industry-level scalable fabrication.

Large-area integration of two-dimensional materials and their heterostructures by wafer bonding

Integrating two-dimensional (2D) materials into semiconductor manufacturing lines is essential to exploit their material properties in a wide range of application areas. However, current approaches are not compatible with high-volume manufacturing on wafer level. Here, we report a generic methodology for large-area integration of 2D materials by adhesive wafer bonding. Our approach avoids manual handling and uses equipment, processes, and materials that are readily available in large-scale semiconductor manufacturing lines. We demonstrate the transfer of CVD graphene from copper foils (100-mm diameter) and molybdenum disulfide (MoS2) from SiO2/Si chips (centimeter-sized) to silicon wafers (100-mm diameter). Furthermore, we stack graphene with CVD hexagonal boron nitride and MoS2 layers to heterostructures, and fabricate encapsulated field-effect graphene devices, with high carrier mobilities of up to [Formula: see text]. Thus, our approach is suited for backend of the line integration of 2D materials on top of integrated circuits, with potential to accelerate progress in electronics, photonics, and sensing.

Nonvolatile Rewritable Frequency Tuning of a Nanoelectromechanical Resonator Using Photoinduced Doping

Arrays of nanoelectromechanical resonators (NEMS) have shown promise for a suite of applications, from nanomechanical information processing technologies to mass spectrometry. A fundamental challenge toward broader adoption of NEMS arrays is a lack of viable frequency tuning methods, which must simultaneously allow for persistent and reversible control of single resonators while also being scalable to large arrays of devices. In this work, we demonstrate an electro-optic tuning method for graphene-based NEMS where locally photoionized charge tensions a suspended membrane and tunes its resonance frequency. The tuned frequency state persists unchanged for several days in the absence of any external power, and the state can be repeatedly written and erased with a high degree of precision. We show the scalability of this technique by aligning the frequencies of several NEMS devices on the same chip, and we discuss implications of this tuning method for both single devices and programmable NEMS networks.

High-Mobility, Wet-Transferred Graphene Grown by Chemical Vapor Deposition

We report high room-temperature mobility in single-layer graphene grown by chemical vapor deposition (CVD) after wet transfer on SiO₂ and hexagonal boron nitride (hBN) encapsulation. By removing contaminations, trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to ∼70000 cm² V^-1 s^-1 at room temperature and ∼120 000 cm² V^-1 s^-1 at 9K. These are more than twice those of previous wet-transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room-temperature mobilities of ∼30 000 cm² V^-1 s^-1. These results show that, with appropriate encapsulation and cleaning, room-temperature mobilities well above 10 000 cm² V^-1 s^-1 can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

The Potential Application of BAs for a Gas Sensor for Detecting SO2 Gas Molecule: a DFT Study

Different atmospheric gas molecules (e.g., N2, O2, CO2, H2O, CO, NO, NO2, NH3, and SO2) are absorbed on the pristine hexagonal boron arsenide (BAs) through density functional theory calculations. For each gas molecules, various adsorption positions were considered. The most stable adsorption depended on position, adsorption energy, charge transfer, and work function. SO2 gas molecules had the best adsorption energy, the shortest distance for BAs surface in the atmospheric gas molecule, and a certain amount of charge transfer. The calculation of work function was important for exploring the possibilities of adjusting the electronic and optical properties. Our results presented BAs materials can be the potential gas sensor of SO2 with high sensitivity and selectivity.

Plasmon induced thermoelectric effect in graphene

Graphene has emerged as a promising material for optoelectronics due to its potential for ultrafast and broad-band photodetection. The photoresponse of graphene junctions is characterized by two competing photocurrent generation mechanisms: a conventional photovoltaic effect and a more dominant hot-carrier-assisted photothermoelectric (PTE) effect. The PTE effect is understood to rely on variations in the Seebeck coefficient through the graphene doping profile. A second PTE effect can occur across a homogeneous graphene channel in the presence of an electronic temperature gradient. Here, we study the latter effect facilitated by strongly localised plasmonic heating of graphene carriers in the presence of nanostructured electrical contacts resulting in electronic temperatures of the order of 2000 K. At certain conditions, the plasmon-induced PTE photocurrent contribution can be isolated. In this regime, the device effectively operates as a sensitive electronic thermometer and as such represents an enabling technology for development of hot carrier based plasmonic devices.

The photoresponse of graphene-based photodetectors is dominated by photovoltaic and photothermoelectric effects. Here, the authors leverage strongly localised plasmonic heating of graphene carriers to detect a second photothermoelectric effect occurring across a homogeneous channel in the presence of an electronic temperature gradient.

Graphene gas sensing using a non-contact microwave method

We report a non-contact CVD graphene gas sensing method that utilises a high Q microwave dielectric resonator perturbation technique. A graphene sample is coupled to the evanescent field of a dielectric resonator whereupon nitrogen dioxide (NO₂), a p-doping gas, is detected by monitoring the change in the linewidth and frequency of the resonant mode. The resonant peak shape is dependent on the number of carriers in the graphene sheet. Therefore, the linewidth perturbation can be converted to a measurement of the graphene sheet resistance. To demonstrate the strength of this technique, sensor response curves for NO₂ at different concentrations and temperatures are measured showing sub ppm sensitivity. This technique eliminates interactions between the trace gas and metal contacts that otherwise effect the sensor response of the graphene device.

Formation mechanisms of Fe3-xSnxO4 by a chemical vapor transport (CVT) process

Our former study reported that Fe-Sn spinel (Fe_3-xSn_xO₄) was easily formed when SnO₂ and Fe₃O₄ were roasted under CO-CO₂ atmosphere at 900-1100 °C. However, the formation procedure is still unclear and there is a lack of theoretical research on the formation mechanism of the Fe-Sn spinel. In this work, the reaction mechanisms between SnO₂ and Fe₃O₄ under CO-CO₂ atmosphere were determined using XRD, VSM, SEM-EDS, XPS, etc. The results indicated that the formation of Fe_3-xSn_xO₄ could be divided into four steps: reduction of SnO₂ to solid phase SnO, volatilization of gaseous SnO, adsorption of gaseous SnO on the surface of Fe₃O₄, and redox reaction between SnO and Fe₃O₄. During the roasting process, part of Fe³⁺ in Fe₃O₄ was reduced to Fe²⁺ by gaseous SnO, and meanwhile Sn²⁺ was oxidized to Sn⁴⁺ and entered into Fe_3-xSn_xO₄. The reaction between SnO₂ and Fe₃O₄ could be summarized as Fe₃O₄ + xSnO_(g) → Fe_3-xSn_xO₄ (x = 0-1.0).