Field emitters comprised of aligned carbon nanotubes are shown to be promising as a primary electron source in an x-ray tube working in a nonultrahigh vacuum ambience. At a pressure of 2×10−7 Torr, the nanotube emitters continue to emit electrons for more than 1 h, and yield better resolved x-ray images than do thermionic emitters, independently of whether the sample is biological or nonbiological. The near-uniformity in energy distribution of electrons emitted from carbon nanotubes might be related to the improved image quality in the field-emission mode.

The outbreak of coronavirus disease 2019 has seriously threatened human health. Rapidly and sensitively detecting SARS-CoV-2 viruses can help control the spread of viruses. However, it is an arduous challenge to apply semiconductor-based substrates for virus SERS detection due to their poor sensitivity. Therefore, it is worthwhile to search novel semiconductor-based substrates with excellent SERS sensitivity. Herein we report, for the first time, Nb2C and Ta2C MXenes exhibit a remarkable SERS enhancement, which is synergistically enabled by the charge transfer resonance enhancement and electromagnetic enhancement. Their SERS sensitivity is optimized to 3.0 × 106 and 1.4 × 106 under the optimal resonance excitation wavelength of 532 nm. Additionally, remarkable SERS sensitivity endows Ta2C MXenes with capability to sensitively detect and accurately identify the SARS-CoV-2 spike protein. Moreover, its detection limit is as low as 5 × 10−9 M, which is beneficial to achieve real-time monitoring and early warning of novel coronavirus. This research not only provides helpful theoretical guidance for exploring other novel SERS-active semiconductor-based materials but also provides a potential candidate for the practical applications of SERS technology.

The current COVID-19 pandemic urges the extremely sensitive and prompt detection of SARS-CoV-2 virus. Here, we present a Human Angiotensin-converting-enzyme 2 (ACE2)-functionalized gold “virus traps” nanostructure as an extremely sensitive SERS biosensor, to selectively capture and rapidly detect S-protein expressed coronavirus, such as the current SARS-CoV-2 in the contaminated water, down to the single-virus level. Such a SERS sensor features extraordinary 106-fold virus enrichment originating from high-affinity of ACE2 with S protein as well as “virus-traps” composed of oblique gold nanoneedles, and 109-fold enhancement of Raman signals originating from multi-component SERS effects. Furthermore, the identification standard of virus signals is established by machine-learning and identification techniques, resulting in an especially low detection limit of 80 copies mL−1 for the simulated contaminated water by SARS-CoV-2 virus with complex circumstance as short as 5 min, which is of great significance for achieving real-time monitoring and early warning of coronavirus. Moreover, here-developed method can be used to establish the identification standard for future unknown coronavirus, and immediately enable extremely sensitive and rapid detection of novel virus.

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Novel surface-enhanced Raman scattering (SERS) substrates with high SERS-activity are ideal for novel SERS sensors, detectors to detect illicitly sold narcotics and explosives. The key to the wider application of SERS technique is to develop plasmon resonant structure with novel geometries to enhance Raman signals and to control the periodic ordering of these structures over a large area to obtain reproducible Raman enhancement. In this work, a simple Ar(+)-ion sputtering route has been developed to fabricate silver nanoneedles arrays on silicon substrates for SERS-active substrates to detect trace-level illicitly sold narcotics. These silver nanoneedles possess a very sharp apex with an apex diameter of 15 nm and an apex angle of 20°. The SERS enhancement factor of greater than 10(10) was reproducibly achieved by the well-aligned nanoneedles arrays. Furthermore, ketamine hydrochloride molecules, one kind of illicitly sold narcotics, can be detected down to 27 ppb by using our SERS substrate within 3 s, indicating the sensitivity of our SERS substrates for trace amounts of narcotics and that SERS technology can become an important analytical technique in forensic laboratories because it can provide a rapid and nondestructive method for trace detection.

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Initial stages of Al(111) oxidation by oxygen: Temperature and surface morphology effectsThe interaction of oxygen with the ordered Ni 3 Al ͑111͒ surface has been investigated in the temperature range from 300 to 1000 K using high-resolution electron-energy-loss spectroscopy ͑HREELS͒ and low-energy electron diffraction ͑LEED͒. The ''2ϫ2'' LEED pattern of the clean Ni 3 Al ͑111͒ surface indicates a bulklike termination. After oxygen adsorption at 300 K the LEED pattern is diffuse suggesting the formation of an amorphous overlayer. The HREELS spectra show evidence for oxygen interaction with both aluminum and nickel atoms. At 600 K adsorption temperature the fcc surface order is restored, however, the observed (1ϫ1) LEED pattern indicates the loss of chemical order. Again HREELS spectra suggest interaction of oxygen with both aluminum and nickel. For an adsorption temperature of 800 K LEED shows an unrotated oxygen induced superstructure with a lattice spacing of 2.93 Å in addition to the (1ϫ1) substrate spots. The HREELS spectra exhibit an intense loss at 81.9 meV, which is also known from oxygen in threefold hollow sites on Al ͑111͒. Since such sites are not available on the Ni 3 Al ͑111͒ surface, we conclude the buildup of an oxygen covered aluminum overlayer. Finally, during oxygen exposure at 1000 K we observe the growth of a ␥Ј-Al 2 O 3 structure on the reordered Ni 3 Al ͑111͒ substrate surface. This structure has been identified by means of the characteristic vibrational losses in HREELS at 54.6, 80.3, and 112.8 meV together with the emergence of overlayer spots in LEED exhibiting the lattice spacing of ␥Ј-Al 2 O 3 ͑3.02 Å͒. For oxygen exposures at 800 and 1000 K an island growth of the overlayer has been found.

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