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10 nm Scale WO3/CuO Heterojunction Nanochannel for an Ultra-sensitive Chemical Sensor Soo-Yeon Cho, Doohyung Jang, Hohyung Kang, Hyeong-Jun Koh, Junghoon Choi, and Hee-Tae Jung Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b01089 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019
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Analytical Chemistry
10 nm Scale WO3/CuO Heterojunction Nanochannel for an Ultra-sensitive Chemical Sensor Soo-Yeon Cho,†,‡,〦 Doohyung Jang,†,‡,〦 Hohyung Kang,†,‡ Hyeong-Jun Koh,†,‡ Junghoon Choi,†,‡ and Hee-Tae Jung*,†,‡ †
Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ KAIST Institute for NanoCentury, Yuseong-gu, Daejeon 34141, Republic of Korea 〦
These authors contributed equally to this work
ABSTRACT: The fabrication of p–n heterostructures of a metal oxide semiconductor (MOS) showed that a large amount of heterojunction interfaces is one of the key issues in MOS gas sensor research since it could significantly enhance the sensing performance. Despite considerable progress in this area, fabrication of an ideal p–n heterojunction sensing channel has been challenging because of morphological limitations of synthetic methods in the conventional bottom-up fabrication based on precursor reductions. In this study, a 10 nm scale p–n heterojunction nanochannel was fabricated with ultra-small grained WO3/CuO nanopatterns in a large area (centimeter scale) through unique one-step top-down lithographic approaches. The fabricated p–n heterostructure nanochannel showed ultra-thinness (20 nm thickness) and high aspect ratio (>10) and consisted of highly dispersed p-type dopants and n-type channel materials. This facile heterojunction nanostructure could induce a high degree of extended depletion layer and efficient catalytic properties within its single nanochannel surfaces. Accordingly, the WO3/CuO nanochannel exhibited ultra-sensitive detection performance toward ethanol (C2H5OH) (Ra/Rg: 224 at 100 ppb), 12-times higher than that of pristine WO3 nanochannel. The limit of detection of the sensors was calculated to be below parts-per-billion levels (0.094 ppb) with significant response amplitudes (Ra/Rg = 75), which is the best ethanol-sensing performance among previously reported MOS-based sensors. Our unique lithographic approach for the p–n heterojunction nanochannel is expected to be universally applicable to various hetero-nanostructures such as the n–n junction, p–p junction, and metal–semiconductor junction without combinatorial limitations.
Recently, the development of high-performance gas sensors is one of the key issues in next-generation technology for constant extension to applied fields such as environmental monitoring, healthcare, industrial safety, militaries, and automobiles.1– 3 Many of these application fields use miniaturized solid-state gas sensors because of their portability, low operation power, as well as ability for integration as embedded devices with appropriate software,4 and mobile applications5 working within the internet of things (IoTs). Chemiresistor-type gas sensors, especially those based on metal oxide semiconductors (MOSs), have high sensitivity, are cost-effective, have a variety of material candidates, can detect a large number of gases, and have simple functionality; these are advantages that should work in their favor as new applications emerge.6–9 Until now, n-type semiconducting materials including SnO2, ZnO, TiO2, WO3, and In2O3 have been primarily used in MOS-based chemiresistors because of their high sensitivity and rapid response time.10 The sensing mechanism of the MOS-based chemiresistor gas sensor is based on electrical resistivity changes caused by analyte exposure in the adsorption/desorption reaction with chemisorbed oxygen ion species (e.g., O−, O2−) on the sensing channel surface with modulation of surface depletion layers. Thus, the enhancement of gas-sensing performance for the MOS gas sensor requires a significantly faster and larger amount of adsorption–desorption reaction of oxygen ion species, resulting in rapid and considerable change in electrical conductivity. This
fast and large amount of reactions can be mainly achieved by inducing electronic sensitization, which refers to the enhancement of gas response by tuning the charge carrier concentration, which has a profound impact on the sensor performance.11 One of the methods for inducing electronic sensitization is the fabrication of a p–n heterojunction structure, which involves combining two regions of dissimilar MOSs with different band gaps and types of charge carriers.10 The p–n heterojunction can induce extended electron depletion layers on its sensing surfaces by electron–hole recombination. The catalytic effect of the higher oxidation state of the dopant acts as a strong acceptor for electrons of the host semiconductor.5 Various nanostructuring methods have been developed for high-performance gas sensors with heterojunction formation. A one-dimensional ZnO (n-type) semiconductor nanowire was decorated with discrete Mn3O4 (p) nanoclusters, and the gas response (Ra/Rg) was enhanced from 7.2 to 30.8.12 N-type ZnO nanowire (NW) film with different thicknesses of p-type CoPc and 25 nm thick CoPc coated ZnO NWs was modified and was found to exhibit a highly improved H2S response (Rg/Ra (%)) of 268% and a fast response of 26 sec.13 Despite considerable progress in this area, major challenges in induce ideal p–n heterojunction sensing channels remain. First, the p–n heterojunction nanochannel should be fabricated with ultra-small grain compositions that can form numerous p– n heterojunction interfaces within a single channel. Previously,
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Figure 1. Schematics of fabrication of the p–n heterojunction nanopattern sensor by low-energy plasma bombardment. (a) The polymer prepattern is transferred from the PDMS mold, and the target Cu and W metal is uniformly deposited on the prepattern. By low-energy plasma bombardment, Cu and W particles are physically etched at a wide angle and attached to the side surface of the prepatterns. The PS residue is removed by the RIE process, and the thermal oxidation process is followed to convert the metallic channel to a metal oxide semiconductor. (b) Schematic illustration of the sensing device and the VOC-sensing behavior of the WO3/CuO nanochannel. (c) Photograph of the sensing device. (d, e) Top-view and (f) cross-sectional view SEM images of the WO3/CuO nanochannel. The AFM profile of the nanochannel is shown as a white line.
it was hard to form large amounts of p–n heterojunction interfaces using typical precursor-based bottom-up synthesis because of the large grain size of the host channel materials and the low dispersity of dopants. Thus, only a low degree of extended depletion layer can be modulated on surfaces of the channel, leading to a small variation in the resistance of the entire sensing channel.14–16 In addition, the heterojunction nanostructure should be formed on the high-resolution channel with features below tens of nanometers to induce significantly sensitive depletion layer variations with analyte interaction. A typical MOS nanostructure including film, rod, electro-spun fiber, and particles having features above tens of nanometers cannot induce sensitive variation for analytes with heterojunction effects.17 Finally, complex bottom-up fabrication processes (e.g., electrospinning, hydrothermal synthesis) with low uniformity of heterojunction channel formation show high contact resistance within sensing channels and difficulty with integration with integrated circuits. These bottom-up approaches also have at least two steps (post deposition or precursor mixing) for
heterojunction formation and require distinct synthesis or fabrication processes for each heterojunction combination (channel dopants). Thus, it is critically required that top-down fabrication technology that can be universally applied to various heterojunction combinations in large areas with low contact resistance be developed.17–19 Herein, ideal p–n heterojunction nanochannel with ultrasmall grains and high-resolution WO3/CuO nanopattern is fabricated. On the basis of one-step unique top-down lithographic approaches, the 10 nm scale WO3/CuO nanopattern was fabricated with high uniformity in a large area (cm2 scale) and with low contact resistance because of its fully connected line structure between electrodes. The WO3/CuO nanochannel shows unique morphological characteristics: i) high-resolution (width (W) = 20 nm and height (H) = 200 nm) and high aspect ratio ((H/W) > 10) with fully exposed channel structure and ii) polycrystalline nanostructure with ultra-small grain compositions (5 nm size for p-type CuO, 10 nm size for WO3). Consequently, WO3/CuO nanopattern sensors showed ultra-sensitive ethanol response (Ra/Rg ≈ 224) at parts-per-billion (ppb) level (100 ppb)
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Analytical Chemistry
Figure 2. (a) TEM and (b) HRTEM images of the single WO3/CuO nanochannel. (c) XRD spectrum of WO3/CuO nanopattern with grain size calculations. EDS elemental mapping of the (d) WO3/CuO (50/1) and (e) WO3/CuO (15/1) single nanochannels (W: red, O: green, Cu: blue). (f) Composition ratio of the WO3/CuO nanopattern (at%) depending on the W/Cu precursor ratio.
with distinguishable response in a wide concentration range (parts per billion to percent). To the best of our knowledge, this is the best ethanol-sensing performance of MOS sensors so far. In addition, short response time (
