Aligned Epitaxial SnO₂ Nanowires on Sapphire: Growth and Device Applications

Semiconducting SnO₂ nanowires have been used to demonstrate high-quality field-effect transistors, optically transparent devices, photodetectors, and gas sensors. However, controllable assembly of rutile SnO₂ nanowires is necessary for scalable and practical device applications. Here, we demonstrate aligned, planar SnO₂ nanowires grown on A-plane, M-plane, and R-plane sapphire substrates. These parallel nanowires can reach 100 μm in length with sufficient density to be patterned photolithographically for field-effect transistors and sensor devices. As proof-of-concept, we show that transistors made this way can achieve on/off current ratios on the order of 10⁶, mobilities around 71.68 cm²/V·s, and sufficiently high currents to drive external organic light-emitting diode displays. Furthermore, the aligned SnO₂ nanowire devices are shown to be photosensitive to UV light with the capability to distinguish between 254 and 365 nm wavelengths. Their alignment is advantageous for polarized UV light detection; we have measured a polarization ratio of photoconductance (σ) of 0.3. Lastly, we show that the nanowires can detect NO₂ at a concentration of 0.2 ppb, making them a scalable, ultrasensitive gas sensing technology. Aligned SnO₂ nanowires offer a straightforward method to fabricate scalable SnO₂ nanodevices for a variety of future electronic applications

Highly Scalable, Uniform, and Sensitive Biosensors Based on Top-Down Indium Oxide Nanoribbons and Electronic Enzyme-Linked Immunosorbent Assay

Nanostructure field-effect transistor (FET) biosensors have shown great promise for ultra sensitive biomolecular detection. Top-down assembly of these sensors increases scalability and device uniformity but faces fabrication challenges in achieving the small dimensions needed for sensitivity. We report top-down fabricated indium oxide (In₂O₃) nanoribbon FET biosensors using highly scalable radio frequency (RF) sputtering to create uniform channel thicknesses ranging from 50 to 10 nm. We combine this scalable sensing platform with amplification from electronic enzyme-linked immunosorbent assay (ELISA) to achieve high sensitivity to target analytes such as streptavidin and human immunodeficiency virus type 1 (HIV-1) p24 proteins. Our approach circumvents Debye screening in ionic solutions and detects p24 protein at 20 fg/mL (about 250 viruses/mL or about 3 orders of magnitude lower than commercial ELISA) with a 35% conduction change in human serum. The In₂O₃ nanoribbon biosensors have 100% device yield and use a simple 2 mask photolithography process. The electrical properties of 50 In₂O₃ nanoribbon FETs showed good uniformity in on-state current, on/off current ratio, mobility, and threshold voltage. In addition, the sensors show excellent pH sensitivity over a broad range (pH 4 to 9) as well as over the physiological-related pH range (pH 6.8 to 8.2). With the demonstrated sensitivity, scalability, and uniformity, the In₂O₃ nanoribbon sensor platform makes great progress toward clinical testing, such as for early diagnosis of acquired immunodeficiency syndrome (AIDS)

Black Phosphorus Gas Sensors

The utilization of black phosphorus and its monolayer (phosphorene) and few-layers in field-effect transistors has attracted a lot of attention to this elemental two-dimensional material. Various studies on optimization of black phosphorus field-effect transistors, PN junctions, photodetectors, and other applications have been demonstrated. Although chemical sensing based on black phosphorus devices was theoretically predicted, there is still no experimental verification of such an important study of this material. In this article, we report on chemical sensing of nitrogen dioxide (NO₂) using field-effect transistors based on multilayer black phosphorus. Black phosphorus sensors exhibited increased conduction upon NO₂ exposure and excellent sensitivity for detection of NO₂ down to 5 ppb. Moreover, when the multilayer black phosphorus field-effect transistor was exposed to NO₂ concentrations of 5, 10, 20, and 40 ppb, its relative conduction change followed the Langmuir isotherm for molecules adsorbed on a surface. Additionally, on the basis of an exponential conductance change, the rate constants for adsorption and desorption of NO₂ on black phosphorus were extracted for different NO₂ concentrations, and they were in the range of 130–840 s. These results shed light on important electronic and sensing characteristics of black phosphorus, which can be utilized in future studies and applications

Highly Sensitive and Quick Detection of Acute Myocardial Infarction Biomarkers Using In₂O₃ Nanoribbon Biosensors Fabricated Using Shadow Masks

We demonstrate a scalable and facile lithography-free method for fabricating highly uniform and sensitive In₂O₃ nanoribbon biosensor arrays. Fabrication with shadow masks as the patterning method instead of conventional lithography provides low-cost, time-efficient, and high-throughput In₂O₃ nanoribbon biosensors without photoresist contamination. Combined with electronic enzyme-linked immunosorbent assay for signal amplification, the In₂O₃ nanoribbon biosensor arrays are optimized for early, quick, and quantitative detection of cardiac biomarkers in diagnosis of acute myocardial infarction (AMI). Cardiac troponin I (cTnI), creatine kinase MB (CK-MB), and B-type natriuretic peptide (BNP) are commonly associated with heart attack and heart failure and have been selected as the target biomarkers here. Our approach can detect label-free biomarkers for concentrations down to 1 pg/mL (cTnI), 0.1 ng/mL (CK-MB), and 10 pg/mL (BNP), all of which are much lower than clinically relevant cutoff concentrations. The sample collection to result time is only 45 min, and we have further demonstrated the reusability of the sensors. With the demonstrated sensitivity, quick turnaround time, and reusability, the In₂O₃ nanoribbon biosensors have shown great potential toward clinical tests for early and quick diagnosis of AMI