Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and ...integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, single-crystal 2D gallium, indium and tin that are stabilized at the interface of epitaxial graphene and silicon carbide. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to the graphene overlayer; that is, they are 'half van der Waals' metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals offer compelling opportunities for superconducting devices, topological phenomena and advanced optoelectronic properties. For example, the reported 2D Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly free-electron Fermi surface that closely approaches the Dirac points of the graphene overlayer.
The functionality of atomic quantum emitters is intrinsically linked to their host lattice coordination. Structural distortions that spontaneously break the lattice symmetry strongly impact their ...optical emission properties and spin-photon interface. Here we report on the direct imaging of charge state-dependent symmetry breaking of two prototypical atomic quantum emitters in mono- and bilayer MoS
by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). By changing the built-in substrate chemical potential, different charge states of sulfur vacancies (Vac
) and substitutional rhenium dopants (Re
) can be stabilized. Vac
as well as Re
and Re
exhibit local lattice distortions and symmetry-broken defect orbitals attributed to a Jahn-Teller effect (JTE) and pseudo-JTE, respectively. By mapping the electronic and geometric structure of single point defects, we disentangle the effects of spatial averaging, charge multistability, configurational dynamics, and external perturbations that often mask the presence of local symmetry breaking.
The study on breakdown characteristics at microscale is very vital to the plasma physics community and micro/nanoelectronic industries. However, the dynamic development processes including the ...electron avalanche and discharge channel during the breakdown event are still unclear experimentally so far due to the restriction of the in-situ diagnostic technique. In this paper, an optical diagnostic technique is put forward to achieve an in-situ observation of the dynamic breakdown process across microgaps at atmospheric pressure. By using the nanosecond temporally and micron spatially resolved optical measurement method, the light emission appearances of microgap breakdown under nanosecond pulse have been captured and imaged directly. The initial and propagation mechanisms for electrical breakdown across the very small air gap are discussed based on the light emission properties. Results show that the light emission initiates from the cathode as the initial electron avalanche, then the avalanche transits to the streamer due to the space charge effect and photoionization, and propagates towards the anode with a velocity of 4.40×10 3 m/s, demonstrating a cathode-directed streamer in the pulsed breakdown. The space charge induced field enhancement and photoionization are proved to play a significant role in the pulsed breakdown across microgaps. In addition, the time to breakdown is estimated to be about 20ns while the formative time lag is about 9ns through the temporal sequence of the optical images. The in-situ optical measurement technique presented in this paper would be of a great help for better understanding the physical mechanism of microgap breakdown.
Abstract
Contacting two-dimensional (2D) semiconductors with van der Waals semimetals significantly reduces the contact resistance and Fermi level pinning due to defect-free interfaces. However, ...depending on the band alignment, a Schottky barrier remains. Here we study the evolution of the valence and conduction band edges in pristine and heavily vanadium (0.44%), i.e.,
p
-type, doped epitaxial WSe
2
on quasi-freestanding graphene (QFEG) on silicon carbide as a function of thickness. We find that with increasing number of layers the Fermi level of the doped WSe
2
gets pinned at the highest dopant level for three or more monolayers. This implies a charge depletion region of about 1.6 nm. Consequently, V dopants in the first and second WSe
2
layer on QFEG/SiC are ionized (negatively charged) whereas they are charge neutral beyond the second layer.
Two-dimensional (2D) materials are popular for fundamental physics study and technological applications in next-generation electronics, spintronics, and optoelectronic devices due to a wide range of ...intriguing physical and chemical properties. Recently, the family of 2D metals and 2D semiconductors has been expanding rapidly because they offer properties once unknown to us. One of the challenges to fully access their properties is poor stability in ambient conditions. In the first half of this Review, we briefly summarize common methods of preparing 2D metals and highlight some recent approaches for making air-stable 2D metals. Additionally, we introduce the physicochemical properties of some air-stable 2D metals recently explored. The second half discusses the air stability and oxidation mechanisms of 2D transition metal dichalcogenides and some elemental 2D semiconductors. Their air stability can be enhanced by optimizing growth temperature, substrates, and precursors during 2D material growth to improve material quality, which will be discussed. Other methods, including doping, postgrowth annealing, and encapsulation of insulators that can suppress defects and isolate the encapsulated samples from the ambient environment, will be reviewed.
Since the isolation of graphene in 2004, there has been an exponentially growing number of reports on layered two-dimensional (2D) materials for applications ranging from protective coatings to ...biochemical sensing. Due to the exceptional, and often tunable, electrical, optical, electrochemical, and physical properties of these materials, they can serve as the active sensing element or a supporting substrate for diverse healthcare applications. In this review, we provide a survey of the recent reports on the applications of 2D materials in biosensing and other emerging healthcare areas, ranging from wearable technologies to optogenetics to neural interfacing. Specifically, this review provides (i) a holistic evaluation of relevant material properties across a wide range of 2D systems, (ii) a comparison of 2D material-based biosensors to the state-of-the-art, (iii) relevant material synthesis approaches specifically reported for healthcare applications, and (iv) the technological considerations to facilitate mass production and commercialization.
•CVD growth parameters are systematically manipulated to optimize the crystal quality of graphene.•High-quality wafer-scale monolayer graphene on both c-plane and r-plane sapphire is achieved.•C-O ...bonding between graphene and sapphire is the site for graphene domain formation on sapphire.•Compres to r-plane sapphire, oxygen-rich surface of c-plane sapphire leads to higher nucleation density and smaller domains.
The direct growth of graphene on insulating substrates presents significant promise for the advancement of next-generation semiconductor technologies. Here, the synthesis of graphene directly on sapphire substrates via chemical vapor deposition (CVD) is reported. The CVD growth parameters are systematically manipulated to optimize the crystal quality of graphene. Furthermore, the dependency of graphene growth on the crystallographic orientation of the sapphire substrate is explored to elucidate the underlying mechanisms. It is found that robust C-O chemical bonds are established between the oxygen atoms on the sapphire surface and the carbon atoms in the graphene, predominantly influencing the nucleation process. The oxygen-rich nature of the c-plane sapphire surface contributes to a higher graphene nucleation density and the formation of smaller grains comparing to r-plane sapphire. Accordingly, high quality and uniform monolayer graphene is successfully obtained on both c-plane and r-plane sapphire.
Hybrid rice row detection at the pollination stage is critical for the automation of in-field pollination agricultural vehicles. The parental crops of hybrid rice are planted at intervals in seed ...production fields with narrow inter-row spacing. During the advance of the pollination vehicle, in addition to the centerline of the crop row, information on the crop region boundaries is required to guide the vehicle and prevent it from running over the crop. For complete crop row detection, a novel machine vision-based method was presented to identify each of the individual regions of the crop rows, more than the centerlines, by line-shaped mask scanning combined with the vanishing point of the crop rows. The approach consisted of grayscale transformation, vanishing point detection, crop region identification, boundary position fine-tuning and crop region segmentation. Its region detection performance outperformed the convolutional neural network-based (CNN-based) methods with an intersection over union (IoU) of 0.832, an accuracy of 90.48%, a recall of 86.36%, a precision of 98.96% and an f1-Score of 92.23%. Its centerline extraction ability was compared with Hough Transform-based and SegNet-based methods on the basis of average lateral distance (ALD) between the ground truth line and the detected line. The proposed method resulted in an ALD of 1.943 pixels in a 640*360 resolution image, which was superior to the Hough Transform-based (5.704) and the SegNet-based (3.555) methods.
Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical ...properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.
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