Ferroelectric (Fe) materials-based devices show great promise for nonvolatile memory applications, yet few demonstrate reliable operation at elevated temperatures. In this work, we demonstrate Ni/Al0.68Sc0.32N/4H-SiC metal-ferroelectric-semiconductor capacitors for high-temperature nonvolatile memory applications. Our 30 nm thick ferroelectric Al0.68Sc0.32N film grown on SiC exhibits stable and robust ferroelectric switching up to 1000 °C. The coercive field decreases linearly from -6.4/+11.9 MV cm-1 at room temperature to -3.1/+7.8 MV cm-1 at 800 °C, further reducing to -2.5 MV cm-1 at 1000 °C. At 600 °C, the devices achieve remarkable reliability with ∼2000 endurance cycles and over at least 100 h of retention with negligible polarization loss. At 800 °C, the devices retain data for at least 10,000 s and exceed 400 write cycles. Our results further highlight the potential for ferroelectric AlScN thin films particularly when paired with SiC semiconductor substrates for high-temperature nonvolatile memory.

Silicon microelectronics, consisting of complementary metal–oxide–semiconductor technology, have changed nearly all aspects of human life from communication to transportation, entertainment and health care. Despite their widespread and mainstream use, current silicon-based devices are unreliable at temperatures exceeding 125 °C. The emergent technological frontiers of space exploration, geothermal energy harvesting, nuclear energy, unmanned avionic systems and autonomous driving will rely on control systems, sensors and communication devices that operate at temperatures as high as 500 °C and beyond. At these extreme temperatures, active (heat exchanger and phase-change cooling) or passive (fins and thermal interface materials) cooling strategies add considerable mass and complicate the systems, which is often infeasible. Thus, new material solutions beyond conventional silicon complementary metal–oxide–semiconductor devices are necessary for high-temperature, resilient electronic systems. The ultimate realization of high-temperature electronic systems requires united efforts to develop, integrate and ultimately manufacture non-silicon-based logic and memory technologies, non-traditional metals for interconnects and ceramic packaging technology. Digital electronics capable of operating at elevated temperatures are gaining importance in aerospace, space and geothermal energy as well as oil and gas exploration. This Review presents recent advances and future outlook on critical materials and devices for the same.

Covalent organic frameworks (COFs) are an attractive class of materials for sensing applications due to their inherent crystallinity, high surface area, and designable framework functionalities. While the majority of COFs have high electrical resistance, making it difficult to harness these materials for electronic sensors, real-time impedance spectroscopy can enable gas and vapor sensing of a much wider range of COFs. Herein, a set of post-synthetically modified (PSM) COFs are explored though a straightforward one-step substitution reaction by reacting phenol moieties on the pore wall, with alkyl bromides, to embed alkyl, aryl, or alcohol groups into the framework. This modular approach provides access to improved sensor properties toward the detection of volatile organics, acids, and potentially harmful gases. Sensor results indicate that post-synthetically modified COFs offer better sensitivity toward NO2 and acetic acid, with the aryl functionalized COF having an NO2 detection limit of 10 ppm. Furthermore, modified COFs also show higher selectivity toward isopropanol and toluene. This work highlights the importance of methods that facilitate post-synthetic modification of COFs so that functional groups and COF properties can be tuned. A series of covalent organic frameworks (COFs) derivatives are synthesized by the straightforward post-synthetic modification of a phenolic COF and studied as electronic sensors for harmful gases and volatile organic compounds using real-time impedance spectroscopy. This creative approach is leveraged to explore structure-property correlations and obtain COF-based gas sensor measurements of capacitance and resistance simultaneously. image

Non-volatile memory devices that can operate reliably at high temperature are required for the development of extreme environment electronics. However, creating such devices remains challenging. Here we report a non-volatile memory device that is based on an aluminium scandium nitride (Al0.68Sc0.32N) ferroelectric diode and can operate at temperatures of up to 600 degrees C. The devices are composed of metal-insulator-metal structures of nickel/AlScN/platinum grown on 4-inch silicon wafers. They exhibit clear ferroelectric switching up to 600 degrees C with distinct on and off states. At 600 degrees C, the devices exhibit one million read cycles and readable on-off ratios above 1 for over 60 h. The operating voltages of the AlScN ferrodiodes are less than 15 V at 600 degrees C and are thus compatible with silicon-carbide-based high-temperature logic technology. A non-volatile memory device that is based on an aluminium scandium nitride (Al0.68Sc0.32N) ferroelectric diode can operate at temperatures of up to 600 degrees C.

Non-volatile memory (NVM) devices that reliably operate at temperatures above 300 $^\circ$C are currently non-existent and remains a critically unmet challenge in the development of high-temperature (T) resilient electronics, necessary for many emerging, complex computing and sensing in harsh environments. Ferroelectric Al$_x$Sc$_{1-x}$N exhibits strong potential for utilization in NVM devices operating at very high temperatures (> 500 $^\circ$C) given its stable and high remnant polarization (PR) above 100 $\mu$C/cm$^2$ with demonstrated ferroelectric transition temperature (TC) > 1000 $^\circ$C. Here, we demonstrate an Al$_{0.68}$Sc$_{0.32}$N ferroelectric diode based NVM device that can reliably operate with clear ferroelectric switching up to 600 $^\circ$C with distinguishable On and Off states. The coercive field (EC) from the Pulsed I-V measurements is found to be -5.84 (EC-) and +5.98 (EC+) (+/- 0.1) MV/cm at room temperature (RT) and found to decrease with increasing temperature up to 600 $^\circ$C. The devices exhibit high remnant polarizations (> 100 $\mu$C/cm$^2$) which are stable at high temperatures. At 500 $^\circ$C, our devices show 1 million read cycles and stable On-Off ratio above 1 for > 6 hours. Finally, the operating voltages of our AlScN ferrodiodes are < 15 V at 600 $^\circ$C which is well matched and compatible with Silicon Carbide (SiC) based high temperature logic technology, thereby making our demonstration a major step towards commercialization of NVM integrated high-T computers.

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS2, MoSe2, MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe2, with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

Piezoelectricity in low-dimensional materials and metal-semiconductor junctions has attracted recent attention. Herein, a 2D in-plane metal-semiconductor junction made of multilayer 2H and 1T' phases of molybdenum(IV) telluride (MoTe2 ) is investigated. Strong piezoelectric response is observed using piezoresponse force microscopy at the 2H-1T' junction, despite that the multilayers of each individual phase are weakly piezoelectric. The experimental results and density functional theory calculations suggest that the amplified piezoelectric response observed at the junction is due to the charge transfer across the semiconducting and metallic junctions resulting in the formation of dipoles and excess charge density, allowing the engineering of piezoelectric response in atomically thin materials.

Effective chemical sensor devices must facilitate both the detection of analytes at ultralow concentrations and the ability to distinguish one analyte from another. Sensors built using two-dimensional nanomaterials have demonstrated record-level sensitivity toward certain chemical vapor species, but the specificity of chemical analyte detection remains lacking. To address this deficiency, this work pioneers the use of a broadband fiber-optic sensor coated with thin-film MoS2 where selectivity is achieved through observing changes in the visible spectrum transmission during exposure to different aliphatic and aromatic vapors. A significant loss in transmission across the fiber was observed near peaks in the refractive index associated with the C, B, and A excitons as well as at peaks associated with defect states. Several mechanisms for achieving selectivity are investigated, including deciphering donor/acceptor molecules, aromatic compounds, analytes with high refractive index, and intercalants such as aniline-based compounds. Moreover, the sensor device is entirely reusable and demonstrates reversible, empirical, and selective detection of aniline down to 6 ppm.

Ultrasensitive Molecular Sensors In article number 2106830, Nicholas R. Glavin and co-workers describe a real-time impedance spectroscopy approach, which enables ultrasensitive molecular sensors in solution processed 2D nanomaterials. Through bypassing traditionally dominant interflake interactions and selectively extracting intraflake doping effects, detection of NO2 vapor down to 1 ppb is readily achievable with an ultimate limit of detection approaching 63 ppt. Image by Dr. Jo Richers (www.jorichers.com).

Multiplex electronic antigen sensors for detection of SARS-Cov-2 spike glycoproteins and hemagglutinin from influenza A are fabricated using scalable processes for straightforward transition to economical mass-production. The sensors utilize the sensitivity and surface chemistry of a 2D MoS2 transducer for attachment of antibody fragments in a conformation favorable for antigen binding with no need for additional linker molecules. To make the devices, ultra-thin layers (3 nm) of amorphous MoS2 are sputtered over pre-patterned metal electrical contacts on a glass chip at room temperature. The amorphous MoS2 is then laser annealed to create an array of semiconducting 2H-MoS2 transducer regions between metal contacts. The semiconducting crystalline MoS2 region is functionalized with monoclonal antibody fragments complementary to either SARS-CoV-2 S1 spike protein or influenza A hemagglutinin. Quartz crystal microbalance experiments indicate strong binding and maintenance of antigen avidity for antibody fragments bound to MoS2. Electrical resistance measurements of sensors exposed to antigen concentrations ranging from 2?20 000 pg mL?1 reveal selective responses. Sensor architecture is adjusted to produce an array of sensors on a single chip suited for detection of analyte concentrations spanning six orders of magnitude from pg mL?1 to µg mL?1.

Chemical sensors based on solution‐processed 2D nanomaterials represent an extremely attractive approach toward scalable and low‐cost devices. Through the implementation of real‐time impedance spectroscopy and development of a three‐element circuit model, redox exfoliated MoS 2 nanoflakes demonstrate an ultrasensitive empirical detection limit of NO 2 gas at 1 ppb, with an extrapolated ultimate detection limit approaching 63 ppt. This sensor construct reveals a more than three orders of magnitude improvement from conventional direct current sensing approaches as the traditionally dominant interflake interactions are bypassed in favor of selectively extracting intraflake doping effects. This same approach allows for an all solution‐processed, flexible 2D sensor to be fabricated on a polyimide substrate using a combination of graphene contacts and drop‐casted MoS 2 nanoflakes, exhibiting similar sensitivity limits. Finally, a thermal annealing strategy is used to explore the tunability of the nanoflake interactions and subsequent circuit model fit, with a demonstrated sensitivity improvement of 2× with thermal annealing at 200 °C.

Since graphene, a variety of 2D materials have been fabricated in a quest for a tantalizing combination of properties and desired physiochemical behavior. 2D materials that are piezoelectric, i.e., that allow for a facile conversion of electrical energy into mechanical and vice versa, offer applications for sensors, actuators, energy harvesting, stretchable and flexible electronics, and energy storage, among others. Unfortunately, materials must satisfy stringent symmetry requirements to be classified as piezoelectric. Here, 2D ultrathin single‐crystal molybdenum oxide (MoO 2 ) flakes that exhibit unexpected piezoelectric‐like response are fabricated, as MoO 2 is centrosymmetric and should not exhibit intrinsic piezoelectricity. However, it is demonstrated that the apparent piezoelectricity in 2D MoO 2 emerges from an electret‐like behavior induced by the trapping and stabilization of charges around defects in the material. Arguably, the material represents the first 2D electret material and suggests a route to artificially engineer piezoelectricity in 2D crystals. Specifically, it is found that the maximum out‐of‐plane piezoresponse is 0.56 pm V −1 , which is as strong as that observed in conventional 2D piezoelectric materials. The charges are found to be highly stable at room temperature with a trapping energy barrier of ≈2 eV.

Multiplex electronic antigen sensors for detection of SARS-Cov-2 spike glycoproteins or hemagglutinin from Influenza A in liquid samples with characteristics resembling extracted saliva were fabricated using scalable processes with potential for economical mass-production. The sensors utilize the sensitivity and surface chemistry of a two-dimensional MoS 2 transducer for attachment of antibody fragments in a conformation favorable for antigen binding. Ultra-thin layers (3 nm) of amorphous MoS 2 were directly sputtered over the entire sensor chip at room temperature and laser annealed to create an array of semiconducting 2H-MoS 2 active sensor regions between metal contacts. The semiconducting region was functionalized with monoclonal antibody Fab (fragment antigen binding) fragments derived from whole antibodies complementary to either SARS-CoV-2 S1 spike protein or Influenza A hemagglutinin using a papain digestion to cleave the antibodies at the disulfide hinges. The high affinity for the MoS 2 transducer surface with some density of sulfur vacancies for the antibody fragment base promoted chemisorption with antigen binding regions oriented for interaction with the sample. The angiostatin converting enzyme 2 (ACE2) receptor protein for the SARS-CoV-2 spike glycoprotein, was tethered to a hexa-histidine (his 6 ) tag at its c-terminus both for purification purposes, as well as a motif for binding to MoS 2 . This modified protein was also investigated as a bio-recognition element. Electrical resistance measurements of sensors functionalized with antibody fragments and exposed to antigen concentrations ranging from 2-20,000 picograms per milliliter revealed selective responses in the presence of complementary antigens with sensitivity to SARS-CoV-2 or influenza A on the order of pg/mL and comparable to gold-standard diagnostics such as Polymerase Chain Reaction (PCR) analysis. Lack of antigen sensitivity for the larger ACE2 BRE further demonstrates the utility of the engineered antibody fragment/transducer interface in bringing the target antigen closer to the transducer surface for sensitivity required for early detection viral diagnostics.

A seedless solution process is developed for controllable growth of crystalline ZnO micro/nanowire arrays directly on single-layer graphene sheets made in chemical vapor deposition (CVD). In particular, the alignment of the ZnO micro/nanowires correlates well with the density of the wires, which is determined by both the sample configuration in solution and the graphene surface cleaning. With increasing wire density, the ZnO micro/nanowire array alignment may be varied from horizontal to vertical by increasing the physical confinement. Ultraviolet photodetectors based on the vertically aligned ZnO micro/nanowires on graphene show high responsivity of 1.62 A W-1 per volt, a 500% improvement over epitxial ZnO sensors, a 300% improvement over ZnO nanoparticle sensors, and a 40% improvement over the previous best results for nanowire/graphene hybrid sensors. This seedless, floating growth process could be scaled up for large scale growth of oriented ZnO micro/nanowires on graphene at low costs.