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Ultrasensitive gas sensors based on vertical graphene nanowalls/SiC/Si heterostructure Pradip Kumar Roy, Golam Haider, Tsu-Chin Chou, Kuei-Hsien Chen, Li-Chyong Chen, Yang-Fang Chen, and Chi-Te Liang ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b01312 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019
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Ultrasensitive gas sensors based on vertical graphene nanowalls/SiC/Si heterostructure Pradip Kumar Roy, Golam Haider, Tsu-Chin Chou, Kuei-Hsien Chen, Li-Chyong Chen, Yang-Fang Chen*, and Chi-Te Liang* AUTHOR ADDRESS Dr. P. K. Roy, Dr. G. Haider, Prof. Y.-F. Chen, Prof. C.-T. Liang Department of Physics National Taiwan University Taipei 106, Taiwan Email:
[email protected] [email protected] Dr. T.-C. Chou, Prof. K.-H. Chen, Prof. L.-C. Chen Center for Condensed Matter Sciences National Taiwan University Taipei 106, Taiwan Prof. K.-H. Chen Institute of Atomic and Molecular Sciences Academia Sinica Taipei 106, Taiwan KEYWORDS: Gas sensors, 3D graphene nanowalls, metal/insulator/semiconductor (MIS) heterostructure, nano graphiticedges, fast electron-transfer
ABSTRACT: Gas sensors, which play an important role in the safety of human life, cover a wide range of applications including intelligent systems and detection of harmful and toxic gases. It is known that graphene is an ideal and attractive candidate for gas sensing due to its high surface area, excellent mechanical, electrical, optical and thermal properties. However, in order to fully realize its potential as a commercial gas sensor, demand for a graphene-based device of low-limit detection, high sensitivity and fast response time needs to be met. Here, we demonstrate a metal/insulator/semiconductor (MIS) based gas sensor consisting of as-grown epitaxial graphene-nanowalls (EGNWs)/silicon carbide (SiC)/silicon (Si) structure. The unique edge dominant three-dimensional (3D) EGNWs based MIS device achieved an extraordinary low limit of detection (0.5 ppm) and unprecedented sensitivity (82 µA/ppm/cm2 for H2) with a fast response of shorter than 500 ms. These unique properties of our MIS device are attributed to the abundance of vertically oriented nano graphitic-edges and structural defects that act as extra-favorable adsorption sites and exhibit fast electron-transfer kinetics through the edges. Our experimental findings can pave the way for the realization of high-performance 3D graphene-based gas sensor devices.
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It is highly desirable to detect toxic gases and measure the gas concentration for environmental monitoring, industrial chemical processing, public safety, agriculture, medicine and indoor air quality control.1, 2 To date, graphene-based gas sensors have been widely explored, due to the atom thick two-dimensional conjugated structures and excellent properties of graphene.3-5 In a single-layer graphene sheet, all atoms can be considered as surface atoms and they are capable of absorbing gas molecules, providing the largest sensing area per unit volume.6 Graphene displays a phenomenal band structure which consists of conduction and valence bands with a quasi-linear dispersion that touches at the Brillouin zone corners to form a zero-gap semiconductor. Due to these unique properties, its electronic conduction shows a strong dependence on surface adsorbates, which include gas molecules.7 The interactions between graphene sheets and adsorbates associate electronic exchange affecting the Fermi energy of graphene, which can be readily monitored by convenient electronic measurements. In addition, graphene exhibits inherently low electrical noise due to its high-quality crystal lattice, which makes it capable of screening trace amount of charge fluctuations. As a result, a small number of extra electrons can cause an evident change in the conductance of graphene.8-10 Graphene and reduced graphene oxide have been extensively used to detect different gases including NH3, NO2, H2O, Cl2 and CO.11-14 Despite the outstanding potential of graphene and carbon nanotubes as gas sensors, the path of these devices towards commercialization may be hindered by the complexity and cost of the fabrication processes, smallscale fabrication, slow response of the sensor devices and the aid of UV light or heat treatment during the recovery process to get back to the initial condition. There are a couple of reports on optical methods for sensing gases with high sensitivity and fast response,15-17 but sensing by optical measurements is relatively expensive and inconvenient compared with resistive sensors. At present, the resistive sensors are the most widely used configuration of gas detection.18 In this case, the current in a sensor device can be effectively changed by exposure to the target gas. Progress has been made towards resistive gas sensors with high sensitivity and fast response,19-21 but further efforts are required especially in the direction of low limit of detection, high sensitivity, and fast response sensors in accompany with UV light or heat treatment free recovery process towards its practical application. Three-dimensional (3D) graphene has a unique porous structure combined with the structural interconnectivity and the outstanding properties of graphene. It offers several fascinating features, such as low density, high porosity, large surface area, stable mechanical properties, fast mass and electron transport. The unique 3D few-layer epitaxial graphene nanowalls (EGNWs), unlike the 2D planar graphene, provides an abundance of vertically oriented nano graphitic edges that exhibit fast electron-transfer kinetics and high electroactive surface area to geometrical area.22, 23 In addition, 3D graphene provides a higher surface area and much more space for the transportation or storage of catalyst nanoparticles, gas molecules, and electrons/ions.24 These exceptional features are advantageous for designing novel gas sensing devices. In this work, we have synthesized large-scale few-layer EGNWs arrays by microwave plasma enhanced
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chemical vapor deposition. The 3D EGNWs show an asgrown inherent metal/insulator/semiconductor (MIS) structure consisting of EGNWs/SiC/p-doped Si. It is worth noting that the MIS-based gas sensors might provide higher sensitivity compared with the graphene planner devices, because the current can change exponentially with changes in the Fermi level due to adsorption of the gas molecule on graphene surface. In this paper, as a proof-of-concept study, gas sensing experiments have been performed in edge oriented 3D EGNWs/SiC/Si based MIS device using trace amounts of O2 and H2 at room temperature and atmospheric pressure. EXPERIMENTAL: Synthesis of EGNWs via MPECVD: The growth process of vertical graphene nanowalls carried out by using the MPECVD technique, as reported earlier. 25 The graphene-sheathed SiC nanowalls are directly grown in an As Tex 5 kW microwave chamber using silicon (111) wafer as a substrate. Synthesis of such hybrid nanostructure is achieved through the following stages. At first H2 (99.999%) was introduced into the chamber to remove native oxide from the Si surface by the generation of H2 plasma at 800 °C, at a pressure of 40 Torr for few minutes while keeping the microwave power fixed at 1500 W. Subsequently, a thin buffer layer (10–20 nm) of 3C-SiC (1 1 1) was epitaxially grown on Si (1 1 1) surface by feeding a gas mixture of H2, CH4, and SiH4 into the chamber at 1100 °C. The chamber pressure is kept fixed at few tens of Torr with a microwave power of 1500–2000 W during the buffer layer formation. Afterward this SiC buffer layer serves as a seeding layer to provide nucleation sites and facilitates the anisotropic growth of SiC nanowalls. At the final stage, graphene sheathed SiC nanowalls were achieved as a result of H2 plasma etching of the SiC nanowalls surface for last few minutes. Throughout the synthesis process, the substrate temperature was supervised by an optical pyrometer (CHINO IR-H) via a quartz window. Measurements: The current−voltage (I−V) and current−time (I−t) measurement were carried out by two measurement systems (Keithley 236 and Agilent 4155C multimeters) to supply the dc voltage and record the current curves. In order to unfold the electrical characteristics under different gas concentrations, a home-built gas measurement system was used (Figure S1). This home-built system composed of two chambers: a mixing chamber and a testing chamber. The mixing chamber was used for mixing gases. On the other hand, the testing chamber was utilized to record electrical characteristics. All these chambers were connected to a mechanical vacuum pump with an evacuation facility. In addition, a check valve was connected to the testing chamber for discharging excess pressure above 1 atm in the testing chamber. Two mass flow controllers from Brooks Instrument were used to precisely control and mixed the gas concentration. All chambers were evacuated at first before starting the measurements. After that, the gases were mixed in the mixing chamber at a specified pressure (a little higher than 1 atm) and gas concentration. Finally, mixed gas
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was inserted to the testing chamber to record electrical characteristics. After measurements, the testing chamber was evacuated by the mechanical vacuum pump to extract the injected gas molecules. The surface morphology and structural characterization of EGNWs were acquired by using fieldemission scanning electron microscopy (FESEM, 6700F, JEOL, Japan), high-resolution transmission electron microscopy (HRTEM, JEOL-2100) and Raman spectroscopy (Jobin Yvon HR800, 532 nm laser). Preparation of Liner and MIS EGNWs Device: At first commercially purchased mask design was placed on the 3D graphene networks to cover up the whole graphene part except two windows to make contact. A thermal evaporation deposition technique was used to deposit Ag film contact pads on top of the sample under high vacuum condition (
