Control of Halogen Atom in Inorganic Metal-Halide Perovskites Enables Large Piezoelectricity for Electromechanical Energy Generation.

Regulating the strain of inorganic perovskites has emerged as a critical approach to control their electronic and optical properties. Here, an alternative strategy to further control the piezoelectric properties by substituting the halogen atom (I/Br) in the CsPbX3 perovskite (X = Cl, Br) structure is adopted. A series of piezoelectric materials with excellent piezoelectric coefficients (d33 ) are unveiled. Iodine-incorporated CsPbBr2 I demonstrates the record intrinsic piezoelectric response (d33 ≈47 pC N-1 ) among all inorganic metal halide perovskites. This leads to an excellent electrical output power of ≈ 0.375 mW (24.8 µW cm-2 N-1 ) in the piezoelectric energy generator (PEG) which is higher than those of the pristine/mixed perovskite references with CsPbX3 (X = I, Br, Cl). With its structural phase remaining unchanged, the strained CsPbBr2 I retains its superior piezoelectricity in both thin film and nanocrystal powder forms, further demonstrating its repeatability and versatility of applications. The origin of high piezoelectricity is found to be due to halogen-induced anisotropic lattice strain in the unit-cell along the c-axis, and octahedral distortion. This study reveals an avenue to design new piezoelectric materials by modifying their halide constituents and paves the way to design efficient PEGs for improved electromechanical energy conversion.

Electronic field effect detection of SARS-CoV-2 N-protein before the onset of symptoms.

As part of the efforts to contain the pandemic, researchers around the world have raced to develop testing platforms to detect the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes the Coronavirus disease 2019 (COVID-19). Within the different detection platforms studied, the field effect transistor (FET) is a promising device due to its high sensitivity and fast detection capabilities. In this work, a graphene-based FET which uses a boron and nitrogen co-doped graphene oxide gel (BN-GO gel) transducer functionalized with nucleoprotein antibodies, has been investigated for the detection of SARS-CoV-2 nucleocapsid (N)-protein in buffer. This biosensor was able to detect the viral protein in less than 4 min, with a limit of detection (LOD) as low as 10 ag/mL and a wide linear detection range stretching over 11 orders of magnitude from 10 ag/mL-1 µg/mL. This represents the lowest LOD and widest detection range of any COVID-19 sensor and thus can potentially enable the detection of infected individuals before they become contagious. In addition to its potential use in the COVID-19 pandemic, our device serves as a proof-of-concept of the ability of functionalized BN-GO gel FETs to be used for ultrasensitive yet robust biosensors.

An ultrasensitive heart-failure BNP biosensor using B/N co-doped graphene oxide gel FET.

Heart failure (HF) is the number one cause of death in the world. B-type natriuretic peptide (BNP) is a recognized biomarker for HF and can be used for early detection. Field effect transistor (FET) biosensors have the ability to sense BNP in much shorter times than conventional clinical studies. The lowest limit of detection (LOD) of state-of-the-art HF FET biosensors is 100 fM and detection ranges are short, being less than 4 orders of magnitude. In this work, a B/N co-doped graphene oxide (GO) gel (BN-GO) was used as the channel material in an FET biosensor targeting BNP. The sensor was able to sense BNP in as little as 2 min, with an LOD as low as 10 aM and a wide linear detection range of 10 aM-1 µM, stretching over 11 orders of magnitude. The biosensor showed great selectivity and minimal response towards K+ and OH- ions and the human epidermal growth factor receptor (HER2) protein. This biosensor serves as a proof-of-concept of the ability of BN-GO gel FETs to be used for ultrasensitive biosensors.

Near-infrared properties of silver nanowire networks.

Due to their ease of fabrication and mechanical flexibility, silver nanowire transparent electrodes have been touted as a promising replacement for metal oxides such as indium tin oxide (ITO). Here we study an additional advantage: their high transparency in the near-infrared region (NIR) which is highly desirable for some applications. For electrodes that are 96% transparent in the visible, ones made from ITO are only 35% transparent at a wavelength of 2500 nm, but those made from silver nanowires maintain a transparency as high as 94%. Experiments and modelling show that to minimize the transparency drop from the visible to the NIR, the nanowires should be sparse and larger in diameter. This is found to be attributed to both the larger average spacing between nanowires in such networks and the lower absoprtion losses of larger diameter nanowires in the NIR.

Rapid synthesis of ultra-long silver nanowires for high performance transparent electrodes.

By using 1,2-propanediol instead of the classic polyol solvent, ethylene glycol, ultra-long silver nanowires are obtained in only 1 h. These nanowires lead to transparent electrodes with a sheet resistance of 5 Ohms per sq at a transparency of 94%, one of the highest figures of merit for nanowire electrodes ever reported.

The Joule heating problem in silver nanowire transparent electrodes.

Silver nanowire transparent electrodes have shown considerable potential to replace conventional transparent conductive materials. However, in this report we show that Joule heating is a unique and serious problem with these electrodes. When conducting current densities encountered in organic solar cells, the average surface temperature of indium tin oxide (ITO) and silver nanowire electrodes, both with sheet resistances of 60 ohms/square, remains below 35 °C. However, in contrast to ITO, the temperature in the nanowire electrode is very non-uniform, with some localized points reaching temperatures above 250 °C. These hotspots accelerate nanowire degradation, leading to electrode failure after 5 days of continuous current flow. We show that graphene, a commonly used passivation layer for these electrodes, slows nanowire degradation and creates a more uniform surface temperature under current flow. However, the graphene does not prevent Joule heating in the nanowires and local points of high temperature ultimately shift the failure mechanism from nanowire degradation to melting of the underlying plastic substrate. In this paper, surface temperature mapping, lifetime testing under current flow, post-mortem analysis, and modelling illuminate the behaviour and failure mechanisms of nanowires under extended current flow and provide guidelines for managing Joule heating.