Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Carbon Nanotube Chemical Sensors

Electrically–transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high–performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical pers...

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Quantum dot-sensitized solar cells (QDSCs) have emerged as a promising candidate for next-generation solar cells due to the distinct optoelectronic features of quantum dot (QD) light-harvesting materials, such as high light, thermal, and moisture stability, facilely tunable absorption range, high absorption coefficient, multiple exciton generation possibility, and solution processability as well as their facile fabrication and low-cost availability. In recent years, we have witnessed a dramatic boost in the power conversion efficiency (PCE) of QDSCs from 5% to nearly 13%, which is comparable to other kinds of emerging solar cells. Both the exploration of new QD light-harvesting materials and interface engineering have contributed to this fantastically fast improvement. The outstanding development trend of QDSCs indicates their great potential as a promising candidate for next-generation photovoltaic cells. In this review article, we present a comprehensive overview of the development of QDSCs, including: (1) the fundamental principles, (2) a history of the brief evolution of QDSCs, (3) the key materials in QDSCs, (4) recombination control, and (5) stability issues. Finally, some directions that can further promote the development of QDSCs in the future are proposed to help readers grasp the challenges and opportunities for obtaining high-efficiency QDSCs.

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Quantum dot-sensitized solar cells

The strong and controllable chemical sensitivity of organic semiconductors (OSCs) and the amplification capability of transistors in circuits make use of OSC-based field-effect transistors compelling for chemical sensors. Analytes detected and assayed range from few-atom gas-phase molecules that may have adverse health and security implications to biomacromolecules (proteins, nucleic acids) that may be markers for physiological processes and medical conditions. This review highlights recent progress in organic field-effect transistor (OFET) chemical sensors, emphasizing advances from the past 5 years and including aspects of OSC morphology and the role of adjacent dielectrics. Design elements of the OSCs and various formats for the devices are illustrated and evaluated. Challenges associated with the present state of the art and future opportunities are also discussed.

Chemical and Biomolecule Sensing with Organic Field-Effect Transistors

Rapid improvement in photoconversion efficiency (PCE) of solution processable organometallic hybrid halide based perovskite solar cells (PSCs) have taken the photovoltaic (PV) community with a surprise and has extended their application in other electronic devices such as light emitting diodes, photo detectors and batteries. Together with efforts to push the PCE of PSCs to record values >22% – now at par with that of crystalline silicon solar cells – origin of their PV action and underlying physical processes are also deeply investigated worldwide in diverse device configurations. A typical PSC consists
of a perovskite film sandwiched between an electron and a hole selective contact thereby creating ESC/perovskite and perovskite/HSC interfaces, respectively. The selective contacts and their interfaces determine properties of perovskite layer and also control the performance, origin of PV action, open circuit voltage, device stability, and hysteresis in PSCs. Herein, we define ideal charge selective contacts, and provide an overview on how the choice of interfacing materials impacts charge accumulation, transport, transfer/
recombination, band-alignment, and electrical stability in PSCs. We then discuss device related considerations such as morphology of the selective contacts (planar or mesoporous), energetics and electrical properties (insulating and conducting), and its chemical properties (organic vs inorganic). Finally, the outlook highlights key challenges and future directions for a commercially
viable perovskite based PV technology.

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Interfaces in Perovskite Solar Cells

Breath analysis is a promising method for rapid, inexpensive, noninvasive disease diagnosis and health monitoring owing to the correlative relationship between breath biomarker concentrations and abnormal health conditions. However, current methods to identify and quantify breath components rely on large, bench-top analytical instruments. Carbon nanotube (CNT)-based gas sensors are desirable candidates to replace benchtop instruments because of their sensitive chemical-to-electrical transducer capability, high degree of chemical functionality options, and potential for miniaturization. This review seeks to give an overview of the synthetic methods used to functionalize CNT-based gas sensors, specifically those sensors that target biologically relevant breath markers. Specific examples are provided to highlight the sensing mechanisms behind different classes of CNT hybrid composites. Finally, the current challenges and prospective solutions of applying CNT-based sensors to breath analysis are discussed.

Carbon Nanotube‐based Gas Sensors toward Breath Analysis

Titanium dioxide (TiO2) nanostructures and TiO2/graphene nanocomposites are intensively studied materials for energy conversion, energy storage, and organic contaminant photodegradation. However, for TiO2/graphene composites, impermeability across the graphitic basal plane for electrolytes, metal ions, and gas molecules hinders their practical applications. Herein we report a simple, environmentally friendly synthetic route for mesoporous anatase TiO2 nanospindles, and successfully apply this method to obtain in situ grown TiO2 nanospindles/graphene oxide composite. After a thermal reduction at 400 °C, holes are created in the reduced graphene oxide (RGO) sheets through a photocatalytic oxidation mechanism. The formation of holes in RGO is promoted by photogenerated hydroxyl radicals that oxidize and subsequently decarboxylate the graphitic surface of RGO. The proposed mechanism was supported by photocatalytic electrochemical properties of the nanomaterials. The resulting TiO2/holey RGO composites may ove...

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In Situ Grown TiO2 Nanospindles Facilitate the Formation of Holey Reduced Graphene Oxide by Photodegradation

The design and synthesis of materials that can be used for sensing gases and vapors at ambient temperature is of much significance to environmental monitoring, public health and safety. Herein, single-walled carbon nanotubes (SWCNTs) templated CuO networks (SWCNT/CuO) were synthesized via a facile sol–gel method. Raman and XPS characterization proved that CuO was covalently linked to SWCNTs via Cu–O–C bonds. After deposition of the obtained SWCNT/CuO onto the interdigitated gold electrodes to assemble gas sensors, both ethanol and water vapors were successfully detected by the sensors at ambient temperature. Electron donation from both analytes to the p-type sensing material results in a conductance decrease effect on the SWCNT/CuO sensor. The ambient operating temperature overcomes the high operating temperature shortcoming of most reported CuO-based sensors. Meanwhile, the high sensitivity (2 ppm for ethanol) indicates promising sensing applications for the as-prepared SWCNT/CuO material described in this study.

Single-walled carbon nanotubes templated CuO networks for gas sensing

Utilizing a sol-gel synthesis, indium oxide is grown on the surface of oxidized single-walled carbon nanotubes (SWCNT) to form a hybrid material with high conductivity and sensitivity toward certain organic vapors. The room-temperature sensing of dilute ethanol and acetone vapors on the surface of indium oxide/SWCNT hybrid material is studied using electrical conductance experiments in a nonoxidizing environment. Through testing of variously calcinated materials, it was observed that the degree of annealing greatly affects the material's response to acetone and ethanol, such that the intermediate calcination condition yields the best sensitivity. DFT simulations are used to study the interface between defective SWCNT and indium oxide, as well as the interaction between ethanol and acetone molecules with the indium oxide/SWCNT hybrid material.

Indium Oxide—Single-Walled Carbon Nanotube Composite for Ethanol Sensing at Room Temperature

Compared to the fast electron transport in perovskite and rapid electron injection from perovskite to TiO2 nanoparticle scaffold, the slower electron transport rate in mesoporous TiO2 is reported to be a hindrance factor for power conversion efficiency. One-dimensional nanomaterials are believed to show faster carrier transport rate. In this paper, vertically aligned one-dimensional TiO2 nanocones on transparent conducting oxide were synthesized and utilized as a promising scaffold for perovskite solar cells. A simple concentration-dependent CH3NH3PbI3 seeding spin-coating method was developed to effectively improve the surface coverage at low cost. The resultant perovskite solar cells realized an average power conversion efficiency up to 11%, which is higher than that of rectangular TiO2 nanorods-based device. Besides the faster electron transport rate and slower recombination in such one-dimensional nanostructures, we attribute the superior performance of the nanocone-based device to the fact TiO2 nanocone arrays allow more CH3NH3PbI3 to deposit. By scaling up the nanocone-based devices to 1.2 cm2, they yielded a decent average power conversion efficiency of ∼6%. The combination of TiO2 nanocones with perovskite paves a way to take full advantage of one-dimensional nanomaterials and organic–inorganic perovskites for photovoltaic applications.

James E. Ellis

Papers

Carbon Nanotube‐based Gas Sensors toward Breath Analysis

In Situ Grown TiO2 Nanospindles Facilitate the Formation of Holey Reduced Graphene Oxide by Photodegradation

Single-walled carbon nanotubes templated CuO networks for gas sensing

Indium Oxide—Single-Walled Carbon Nanotube Composite for Ethanol Sensing at Room Temperature

Perovskite solar cells based on bottom-fused TiO2 nanocones