We report the rapid detection of SARS-CoV-2 in infected patients (mid-turbinate swabs and exhaled breath aerosol samples) in concentrations as low as 60 copies/mL of the virus in seconds by electrical transduction of the SARS-CoV-2 S1 spike protein antigen via SARS-CoV-2 S1 spike protein antibodies immobilized on bilayer quasi-freestanding epitaxial graphene without gate or signal amplification. The sensor demonstrates the spike protein antigen detection in a concentration as low as 1 ag/mL. The heterostructure of the SARS-CoV-2 antibody/graphene-based sensor is developed through a simple and low-cost fabrication technique. Furthermore, sensors integrated into a portable testing unit distinguished B.1.1.7 variant positive samples from infected patients (mid-turbinate swabs and saliva samples, 4000–8000 copies/mL) with a response time of as fast as 0.6 s. The sensor is reusable, allowing for reimmobilization of the crosslinker and antibodies on the biosensor after desorption of biomarkers by NaCl solution or heat treatment above 40 °C.

Point defects in solid-state materials are promising systems in the fields of quantum information, communications, and sensing. Particular to sensing, defects with coherent spin states are used for high-sensitivity room-temperature sensing under ambient conditions. Applications involving magnetic sensing with point defects are mostly dominated by the nitrogen-vacancy center in diamond, which possesses an attractive combination of spin coherence at room temperature as well as spin initialization and read-out. However, spin defects in other materials are explored as alternatives, especially in industrially mature materials, such as silicon carbide. Here, we report on the improved sensitivity of a magnetic sensor system utilizing an ensemble of silicon vacancies in silicon carbide due to isotopic purification of the host crystal. A maximum sensitivity of 4.0 nT/√Hz is reported, representing an order of magnitude improvement from the best previously reported sensitivity. Additional sensing modalities, such as angle-resolved magnetic imaging and highly broadband ac field sensing, are also demonstrated.

For solid-state spin systems, unwanted interactions with surrounding spin baths and inhomogeneity are ubiquitous challenges. In defect spin systems, part of this challenge is that the process of generating the desired defect often involves creating numerous unwanted defects that induce dephasing. Here we show that many of these issues can be alleviated with the silicon vacancy in silicon carbide ($\mathrm{SiC}$), which is a simple defect to produce as it does not require the introduction of impurity atoms into the crystal. Previous measurements of the inhomogeneous dephasing time $T_2^{\ast }$ of silicon-vacancy ensembles have thus far been limited to a few 100 ns due to the nuclear spin bath. We perform isotopic purification to minimize the influence of nuclear spins, leading to an order of magnitude improvement in the $T_2^{\ast }$ at room temperature. Further improvements emerge by suppressing the effects of strain inhomogeneity via an informed choice of basis in the spin quartet. Combining these techniques leads to about a factor 50 improvement in the $T_2^{\ast }$ of the defect ensemble even at high defect densities.

Tin(II) monosulfide (SnS) is a layered, anisotropic material that is of interest as a two-dimensional semiconductor for opto-electronic, thermoelectric, and piezoelectric applications. In this study, the effect of work function on contact behavior was investigated. Ni/Au, Pd/Au, Cr/Au, and Ti/Au contacts were fabricated onto individual, solution-synthesized, p-type SnS nanoribbons. The lower work function metals (Cr and Ti) formed Schottky contacts, whereas the higher work function metals (Ni and Pd) formed ohmic or semi-ohmic contacts. Of the ohmic contacts, Ni was found to have a lower contact resistance (∼10⁻⁴ Ω cm² or lower) than Pd (∼10⁻³ Ω cm² or lower). Both the calculated Schottky barriers (0.39 and 0.50 eV) for Cr and Ti, respectively, and the ohmic behavior for Ni and Pd agree with behavior predicted by Schottky–Mott theory. The results indicate that high work function metals should be considered to form low resistance contacts to SnS multilayers.

As part of a Special Issue in Honor of 30 years of the American Vacuum Society’s Nellie Yeoh Whetten Award, this Invited Perspective discusses results and trends from the authors’ and other published research on metal contacts to β-Ga2O3, (4H and 6H)-SiC, nanocrystalline diamond (NCD), and nanocrystalline thin films and single-crystalline nanoribbons of α-SnS. The paper is not a comprehensive review of research on contacts to each of these semiconductors; it is instead a perspective that focuses on Schottky barrier height (Φb) measurements and factors that affect Φb, such as metal work function (Φm) and crystallographic surface plane. Metals and the associated processing conditions that form ohmic or Schottky contacts to each of these semiconductors are also described. Estimates of the index of interface behavior, S, which measures the dependence of Φb on Φm, show large variations both among different semiconductors (e.g., S ∼ 0.3 for NCD and S ∼ 1.0 for SnS nanoribbons) and between different surface planes of the same semiconductor [e.g., (2¯01) vs (100) Ga2O3]. The results indicate that Φb is strongly affected by the nature of the semiconductor surface and near-surface region and suggest that the sharp distinction between covalent and ionic semiconductors as described in seminal theories can be adjustable.

The electrochemical detection of heavy metal ions is reported using an inexpensive portable in-house built potentiostat and epitaxial graphene. Monolayer, hydrogen-intercalated quasi-freestanding bilayer, and multilayer epitaxial graphene were each tested as working electrodes before and after modification with an oxygen plasma etch to introduce oxygen chemical groups to the surface. The graphene samples were characterized using X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, and van der Pauw Hall measurements. Dose–response curves in seawater were evaluated with added trace levels of four heavy metal salts (CdCl₂, CuSO₄, HgCl₂, and PbCl₂), along with detection algorithms based on machine learning and library development for each form of graphene and its oxygen plasma modification. Oxygen plasma-modified, hydrogen-intercalated quasi-freestanding bilayer epitaxial graphene was found to perform best for correctly identifying heavy metals in seawater.