High-Performance Schottky Diode Gas Sensor ... - ACS Publications

Aug 21, 2017 - Advances in the design of a Schottky diode gas sensor based on the ... Electrochemical Sensors for Detecting Toxic Gases: NO2, SO2 and ...
0 downloads 0 Views 3MB Size
Download PDF
Research Article www.acsami.org

High-Performance Schottky Diode Gas Sensor Based on the Heterojunction of Three-Dimensional Nanohybrids of Reduced Graphene Oxide−Vertical ZnO Nanorods on an AlGaN/GaN Layer Nguyen Minh Triet,† Le Thai Duy,† Byeong-Ung Hwang,† Adeela Hanif,† Saqib Siddiqui,† Kyung-Ho Park,∥ Chu-Young Cho,∥ and Nae-Eung Lee*,†,‡,§ †

School of Advanced Materials Science and Engineering, ‡Sungkyunkwan University (SKKU) Advanced Institute of Nanotechnology (SAINT), and §Samsung Advanced Institute for Health Sciences and Technology (SAIHST), Sungkyunkwan University, Suwon, Kyunggi-do 16419, Republic of Korea ∥ Device Platform Laboratory, Korea Advanced Nano Fab Center, Suwon, Kyunggi-do 16229, Republic of Korea S Supporting Information *

ABSTRACT: A Schottky diode based on a heterojunction of three-dimensional (3D) nanohybrid materials, formed by hybridizing reduced graphene oxide (RGO) with epitaxial vertical zinc oxide nanorods (ZnO NRs) and Al0.27GaN0.73(∼25 nm)/GaN is presented as a new class of high-performance chemical sensors. The RGO nanosheet layer coated on the ZnO NRs enables the formation of a direct Schottky contact with the AlGaN layer. The sensing results of the Schottky diode with respect to NO2, SO2, and HCHO gases exhibit high sensitivity (0.88−1.88 ppm−1), fast response (∼2 min), and good reproducibility down to 120 ppb concentration levels at room temperature. The sensing mechanism of the Schottky diode can be explained by the effective modulation of the reverse saturation current due to the change in thermionic emission carrier transport caused by ultrasensitive changes in the Schottky barrier of a van der Waals heterostructure between RGO and AlGaN layers upon interaction with gas molecules. Advances in the design of a Schottky diode gas sensor based on the heterojunction of high-mobility two-dimensional electron gas channel and highly responsive 3D-engineered sensing nanomaterials have potential not only for the enhancement of sensitivity and selectivity but also for improving operation capability at room temperature. KEYWORDS: gas sensor, Schottky diode, heterostructure, reduce graphene oxide, zinc oxide

■

miniaturization, and outstanding sensing characteristics.29 The AlGaN/GaN materials are thermally and chemically stable, which makes them suitable for operation in chemically harsh environments, at high temperatures or under radiation fluxes, even though the cost of device fabrication can be higher than that of conventional semiconductor-based chemiresistors.33 Additionally, one of the main reasons for the great potential of AlGaN/GaN heterostructures in chemical sensing applications is the high mobility and saturation velocity of two-dimensional electron gas (2DEG). The conducting 2DEG channel is very close to the AlGaN surface and is highly sensitive to the adsorption of analytes, leading to wide utility in detecting gases, ions, proteins, and deoxyribonucleic acids.29,33 However, most recent works using AlGaN for gas detection have been limited to H2 gas detection at high operating temperatures.28,29,33−37 Electrodes of metal catalysts such as nickel, platinum, or palladium have frequently been used for H2-sensing transistors or diodes based on AlGaN/GaN heterostructures.28,34,35,37 This

INTRODUCTION The Internet of Things has provoked extensive research into the variety of chemical sensor devices that enable us to collect and exchange gas-sensing data and thereby has opened a new way to improve efficiency, accuracy, and economic benefit in smart technologies, such as smart automobiles, buildings, health, and environment.1,2 For this purpose, it is essential to develop low-powered chemical sensors for constant monitoring of environmentally hazardous gases or gases emanating from human activity. Significant efforts have been directed toward exploration and investigation into improving sensor capabilities, such as durability, detection limit, and power consumption, as well as sensitivity, selectivity, stability, and speed (response and recovery time).3−5 In particular, chemical sensors that are operational at room temperature have been studied more because they can reduce power consumption by not requiring a heater or UV/visible light to enhance performance.6−14 Among the chemical sensor structures of resistors,10−22 transistors,23−25 and diodes,26−32 Schottky diode chemical sensors based on heterostructures of epitaxial AlGaN/GaN semiconductors have shown significant potential because they can provide low power consumption, simple operation, © 2017 American Chemical Society

Received: May 8, 2017 Accepted: August 21, 2017 Published: August 21, 2017 30722

DOI: 10.1021/acsami.7b06461 ACS Appl. Mater. Interfaces 2017, 9, 30722−30732

Research Article

ACS Applied Materials & Interfaces

Figure 1. Characterization of ZnO NRs and RGO−ZnO NRs in a Schottky diode chemical sensor. (a) The schematic structure of RGO−ZnO NR nanohybrid layers formed on an AlGaN/GaN heterostructure grown on a sapphire substrate. (b) Cross-sectional field-emission scanning electron microscopy (FE-SEM) images of ZnO NRs without a seed layer grown in a vertical direction on the AlGaN/GaN/sapphire substrate. (c) X-ray diffraction (XRD) data for pure ZnO NRs, ZnO NRs with GO, and ZnO NRs with RGO. (d) Change in intensity of Raman spectra before and after hydrazine reduction of GO. (e) I−V sweep measurement data for the samples of GO, ZnO NRs, RGO, and RGO with ZnO NRs on an AlGaN/ GaN/sapphire substrate. Schematic energy band diagrams for (f) p−n diode with RGO p-type (black-colored, Fermi levels aligned at EFp) and (g) Schottky diode with RGO−ZnO NRs n-type (red-colored, Fermi levels aligned at EFn) on AlGaN/GaN. Here, EF indicates the Fermi level and ϕB and ϕSB indicate the barrier height and Schottky barrier height (SBH), respectively, for the diode devices with RGO only and RGO−ZnO NRs. Because of oxidation annealing effects during the epitaxy of ZnO NRs on AlGaN, the energy level in the AlGaN layer in the band diagram for the RGO−ZnO NRs/AlGaN/GaN heterostructure changed from the dotted line to solid line, which led to the formation of an SBH with large ϕB. 30723

DOI: 10.1021/acsami.7b06461 ACS Appl. Mater. Interfaces 2017, 9, 30722−30732

Research Article

ACS Applied Materials & Interfaces

HCHO gases down to ppb levels, with sensitivity values of 1.88, 0.93, and 0.88 ppm−1, respectively, compared to the sensitivity values of 0.52, 0.13, and 0.59 ppm−1 of the p−n diode sensor made up of only RGO on AlGaN. In addition, the Schottky diode sensor was highly stable in response time (∼2 min) and recovery time (∼5 min). This heterostructuring of an RGO− ZnO NR active sensing layer forming a Schottky contact with a high-mobility AlGaN/GaN channel is promising for realizing the next generation of gas sensor applications. Because of its gas detection capabilities at room temperature, the Schottky diode gas sensor has great potential for integration into portable wireless electronic systems for a variety of applications.

sensing mechanism can be explained by the dissociation of H2 on the catalytic metal surface, followed by diffusion of the atomic species to the semiconductor interface, where it changes the piezoelectric charge in the channel and thus the effective barrier height at the interfaces of gate dielectrics or the Schottky contact metal.33 However, the metal catalyst electrodes have limitations in a broader scope of analyte gases. Because of the great achievements in the synthesis of nanostructured materials, which are at the heart of the evolution in high-performance chemical sensing platforms, there have been a greater number of studies on nanohybrid materials for both fundamental research and industrial applications during recent years.3 These nanohybrid materials can overcome not only the poor selectivity problem present in traditional metal oxide-based gas sensors, which is caused by the same sensing mechanism that in most cases occurs at the surface, but also the problems with operation at elevated temperature experienced by oxide semiconductor materials.3 Operation at elevated temperatures can decrease the sensing stability and lifetime of these materials due to thermallyinduced growth of metal oxide grains as well as bring the risk of ignition when the sensors detect flammable or explosive analytes, limiting the extent of their application.3 Finally, high operating temperatures result in high power consumption, which is a burden on the new generation of battery-loaded wireless sensors. Even though oxide semiconductors are limited by the heating requirement, they are generally preferred to organic semiconductors because of their durability.23 The aging effect caused by water and oxygen when the materials are exposed to an air environment can have a large influence on the degradation of the conducting channel, resulting in conductivity loss and deterioration of sensing response.3,23,38 Fortunately, it is possible to combine promising nanohybrid materials with other stable signal transducers to overcome the above problems. In this work, a new class of high electron mobility Schottky diode gas sensors is presented as a chemical sensing platform with high performance, good stability, low limit of detection down to parts per billion (ppb) level, and functionality at room temperature. The key components in our design of the device are reduced graphene oxide (RGO)−zinc oxide nanorods (ZnO NRs) as a three-dimensional (3D) nanohybrid sensing material as well as a Schottky contact material on AlGaN(∼25 nm, Al fraction of 0.27)/GaN. Gas sensors based on carbon nanostructures have shown high sensitivity at room temperature.3,4,18−22,26,39−45 Additionally, ZnO nanostructures in gas sensors have exhibited a wide range of gas detection capability.9,11,14,20,46−49 The sensing mechanism of the device in this work is based on the coupling effect between the RGO nanosheets, ZnO NRs, and the AlGaN surface. Although the RGO nanosheets showed p-type characteristics mainly caused by exposure to ambient environment and their van der Waals heterostructure with AlGaN/GaN formed a p−n diode, the RGO−ZnO NR nanohybrid materials exhibited n-type composite characteristics due to n-doping from ZnO NRs and formed a Schottky diode with AlGaN/GaN. When the diode sensors were introduced to various gases (NO2, SO2, HCHO, and NH3) for investigation of sensor responses, the two types of diodes exhibited opposite responses to the same gas under reverse bias conditions due to the different carrier transport characteristics under gas exposure. The Schottky diode sensor with the 3D nanohybrid RGO−ZnO NRs on AlGaN responded more rapidly and strongly to NO2, SO2, and

■

RESULTS AND DISCUSSION Characterization of RGO and RGO−ZnO NRs on 2DEG AlGaN/GaN. A schematic of the Schottky diode chemical sensor based on a nanohybrid of RGO−ZnO NRs on an AlGaN/GaN heterostructure is shown in Figure 1a. The structure and characteristics of the AlGaN/GaN/sapphire substrate are shown in Figure S1, Supporting Information. A key feature in the device design is that the RGO−ZnO NR nanohybrid layer was formed on an AlGaN/GaN/sapphire substrate that has high electron mobility when the sensing materials respond to environmental gases. The cross-sectional view in the schematic illustrates the sensing area (∼3.2 mm2) placed between two Au/Ti electrodes, where one Au/Ti electrode is formed directly on the AlGaN surface as the Ohmic contact, and the other Au/Ti electrode is formed on a thin Al2O3 layer deposited by atomic layer deposition (ALD). The Au/Ti electrode formed on the Al2O3 layer is connected to an RGO layer formed on ZnO NRs and an AlGaN surface, and the contact between the RGO and the AlGaN surface forms a Schottky contact. The channel length and width are 400 and 40 μm, respectively. After growing Al2O3 by ALD, the electrodes and encapsulation layers were coated on the surface. In the final step, the active layer of RGO and ZnO NRs was formed on the channel area. Full details of the fabrication process are described in the experimental section (Figure S2, Supporting Information). Figure 1b shows field-emission scanning electron microscopy (FE-SEM) images of highly vertically grown epitaxial ZnO NRs on an AlGaN(∼25 nm)/GaN (∼3 μm) layer. The hydrothermal method was used to grow ZnO NRs at low temperature. According to the top view and cross-sectional FE-SEM images (Figure S3a,b, Supporting Information), the diameter and length of the ZnO NRs were ∼0.26 and ∼2.2 μm on an average, respectively, indicating an aspect ratio (length/ diameter) of ∼8.5. The top view FE-SEM images of the RGO nanosheets coated on AlGaN/GaN sample indicate that RGO nanosheets are uniformly networked on the AlGaN surface (Figure S3c,d, Supporting Information). The FE-SEM investigation of RGO−ZnO NR nanohybrids formed on the AlGaN surface show that neighboring ZnO NRs are connected to each other by RGO (Figure S3e,f, Supporting Information), and RGO nanosheets coated on the open area of AlGaN are connected to vertical ZnO NRs (Figure S3g, Supporting Information). The results indicate that RGO−ZnO NR nanohybrid can be considered as composite material.30 Further information on the growth, fabrication, and characterization of ZnO NRs, RGO, and RGO−ZnO NRs hybrids can be found in the Experimental Section. X-ray diffraction (XRD) analysis shows that the highly vertical growth of ZnO NRs on AlGaN was contributed by the epitaxial AlGaN on the GaN/Al2O3 30724

DOI: 10.1021/acsami.7b06461 ACS Appl. Mater. Interfaces 2017, 9, 30722−30732

Research Article

ACS Applied Materials & Interfaces

Figure 2. Resistance change (ΔR/R0) of the diode gas sensors with RGO and RGO−ZnO NRs on AlGaN upon exposure to NO2, SO2, and HCHO. The data show the ΔR/R0 values of (a) a p−n diode with p-type RGO and (b) a Schottky diode with n-type RGO−ZnO NRs on AlGaN/GaN exposed to 120 ppb concentration of NO2, SO2, and HCHO gases. The changes in ΔR/R0 upon gas exposure are opposite for the two diode sensors. The schematics show the sensing mechanism for (c) the p−n diode with p-type RGO and (d) the Schottky diode with n-type RGO−ZnO NRs on AlGaN/GaN. Here, EGr and E2DEG indicate the Fermi level of RGO and AlGaN/GaN under reverse bias. The gases can diffuse and penetrate into the RGO layer, leading to formation of chemical bonds with the interaction sites and, in turn, a long and incomplete recovery. In the p−n diode under reverse bias, transport of minority holes thermally generated in AlGaN can be modulated by the change in ϕB. In the Schottky diode, under reverse bias, thermionic emission transport of minority electrons across the interface between RGO−ZnO NRs and AlGaN could be primarily modulated by the change in ϕSB.

Supporting Information, two distinctive narrowed peaks at the GaN:E2 band (∼563 cm−1) and the GaN:A1 band (∼734 cm−1) reveals the high quality crystalline structure of the GaN epitaxial layer. To observe the roles of RGO and ZnO NRs on the AlGaN/ GaN layer, I−V characteristics of proposed heterostructures with GO, RGO, ZnO NRs, and RGO−ZnO NRs on AlGaN/ GaN layers were measured for comparison (Figure 1e). The current levels of the samples with GO (black line) and ZnO NRs (green line) on an AlGaN layer under forward bias were very low, indicating high resistivity (10−11 Ω) due to the insulating nature of GO and the lack of connection between the ZnO NRs and the Au/Ti electrode on the Al2O3 layer (Figure S5, Supporting Information). Therefore, GO and ZnO NRs samples could not be used for testing purpose of gas sensing. However, it is notable that a conspicuous increase in the forward current of the diode after reduction from GO (black line) to RGO (red line) was observed. Under a sweep from −10 to 15 V, both heterojunction diodes with RGO and RGO−ZnO NRs (blue line) clearly exhibited diode characteristics, indicating that a high-level barrier exists at the contact between RGO and AlGaN. Because RGO nanosheets are typically p-type materials due to the removal of electrons from the conduction band with adsorption of oxygen and water

(1120) substrate, resulting in high crystallization of ZnO NRs (0002) on the AlGaN surface (see Figure 1c). There was no significant difference between the XRD spectra of the substrateonly sample (AlGaN/GaN on sapphire) and the samples with ZnO NRs (ZnO NRs/AlGaN/GaN on sapphire) (Figure S4a, Supporting Information), which implies that occurrence of the peaks of GaN (0002) and ZnO (0002) at 34.5° indicates hexagonal lattice matching of the GaN and ZnO structures. To verify the reduction of the GO nanosheets, which were self-assembled on the AlGaN/GaN surface by hydrazine vapors, we carried out Raman spectroscopy of GO, RGO, and RGO− ZnO NR samples, as shown in Figure 1d. The Raman spectra of all three samples displayed two unique peaks of graphene at the D band (∼1350 cm−1) and G band (∼1603 cm−1). Before performing reduction of graphene oxide using hydrazine vapor, the ID/IG ratio of the GO sample (black line) was 0.9. After completion of the reduction process, the ID/IG ratios of the RGO nanosheet sample (red line) and the RGO−ZnO NR sample (blue line) changed to 1.4 and 1.1, respectively, indicating the reduction of GO to RGO.18,42,50 The Raman spectra in Figure S4b, Supporting Information depict three samples in which GaN and GO bands occurred in the shifted range of 400−4200 cm−1, implying the existence of GO and RGO coated on an AlGaN/GaN heterostructure. In Figure S4c, 30725

DOI: 10.1021/acsami.7b06461 ACS Appl. Mater. Interfaces 2017, 9, 30722−30732

Research Article

ACS Applied Materials & Interfaces vapor from the environment,41,51,52 it is expected that a heterojunction p−n diode would be formed in a van der Waals heterostructure of RGO/AlGaN. Ideally, the RGO/AlGaN p−n diode can be described by assuming the presence of a very thin insulating layer (vacuum), which can be considered completely transparent but able to withstand a potential drop.53,54 The presence of an ultrathin vacuum layer between RGO and AlGaN is reasonable, considering the van der Waals bonding between the two surfaces.53,55 After formation of the RGO/ AlGaN heterojunction at equilibrium, the aligned Fermi level and barrier height are assumed to be EFp and ϕB, respectively (Figure 1f). In this case, the forward current of the RGO/ AlGaN heterojunction p−n diode is primarily contributed by the diffusion current of 2DEG from AlGaN/GaN toward the RGO and holes from the RGO toward the AlGaN/GaN. Interestingly, there is a decrease in electrical conductivity of the device when p-type RGO nanosheets in contact with AlGaN are hybridized with n-type ZnO NRs (Figure 1e).52 To understand the decrease in the diode current of the device with RGO−ZnO NRs compared to that of the p−n diode with only RGO under forward bias, the effect of charge carrier doping in the hybridized RGO nanosheets on the change in Schottky barrier height (SBH) at the interface between RGO and AlGaN needs to be explained (Figure 1g). Because the work functions of RGO and ZnO nanorods are known to be ∼5 and 4.2 eV, respectively,30,56,57 the Fermi level of RGO in the RGO−ZnO NRs/AlGaN/GaN heterostructure is shifted toward the conduction band by electrons transferred from n-type ZnO to RGO (i.e., n-doping). The Fermi level of ZnO is also aligned to balance at the interface, EFn, which is expected to lead to reduction of the SBH between RGO and AlGaN. However, during epitaxial processing of ZnO NRs, the AlGaN surface could have been modified by oxidation annealing, which increases the SBH at the RGO/AlGaN interface.58 Therefore, the change in SBH from ϕB to ϕSB at the RGO/AlGaN interface at equilibrium could be dominated by oxidation annealing in the ZnO NRs rather than by n-doping of RGO by the ZnO NRs (Figure 1g). In this case, a Schottky diode with an increased SBH of ϕSB is formed in a manner similar to the Schottky diode structure of n-type graphene/AlGaN/GaN.53 The forward current of the Schottky diode with an RGO−ZnO NRs/AlGaN/GaN heterostructure is dominantly contributed by the thermionic emission current of 2DEG toward the RGO, whereas the contribution of holes moving from RGO to AlGaN/GaN is negligible due to a large potential barrier for holes in n-type RGO at the RGO/AlGaN interface. In this situation, the forward current of the Schottky diode with RGO−ZnO NRs is reduced due to an overall increase in the potential barrier for 2DEG across the AlGaN layer (the band diagram under forward bias is not shown here). Gas-Sensing Mechanism of 2D RGO and 3D RGO− ZnO NR-Based AlGaN/GaN Platform. To understand the gas-sensing mechanism of the Schottky diode device with an RGO−ZnO NR nanohybrid as a chemical sensor and the principle of electrical current modulation in the device, the effects of interactions between gas molecules and the nanohybrid on the device were investigated by injecting NO2, SO2, and HCHO gases into a measurement system (Figure S6, Supporting Information). For comparison, the responses of a heterojunction device with the RGO/AlGaN structure were also measured. Sensing measurements were carried out under reverse bias for both types of devices because higher current modulation by gas adsorption was observed under reverse bias

than forward bias. The resistance change of the diode devices under a reverse bias of −10 V was defined as ΔR/R0, where ΔR = RG − R0, RG is the resistance of the p−n and Schottky diodes in the environment of the gas exposure (NO2, SO2, or HCHO) and R0 is the resistance of the sensor in dry air. The resistance changes of the nanohybrid device and the RGO/AlGaN device were monitored and plotted in Figure 2a,b, respectively. The resistance change of the device under reverse bias is dominantly caused by modulation of SBH upon gas exposure, as discussed below in detail. Figure 2a describes one response and recovery cycle of the p−n diode with only networked RGO film on AlGaN upon exposure to gases at a concentration of 120 ppb. The response and recovery times are described as the times required to achieve ∼90% of response and recovery in the sensing signal, respectively, when the introduced gases are in and out. The RGO device exhibited an obvious decrease in ΔR/R0 upon exposure to NO2 and SO2 but an increase upon exposure to HCHO gas, confirming that the carriers in RGO are holes.4,19,43,59 In the RGO sensor, the p−n heterojunction plays a role in the sensing mechanism of RGO/AlGaN. Once the NO2 or SO2 molecules are exposed to the p−n diode under reverse bias, the electrons in RGO are withdrawn upon adsorption on it and the barrier height (ϕB) increases. In this case, thermally generated minority electrons in RGO and minority holes in GaN cannot easily drift toward AlGaN and RGO, respectively, due to very large potential barriers at the RGO/AlGaN and AlGaN/GaN junctions. However, the drift of thermally generated minority holes within the AlGaN toward RGO and GaN can be enhanced and subsequently modulated, possibly due to the reduced barrier height for hole transport (Figure 2c). Consequently, the reverse saturation current of the RGO/AlGaN heterojunction p−n diode increased and, in turn, the ΔR/R0 of the device decreased. More evidently, the RGO/ AlGaN p−n diode showed the opposite current direction under HCHO gas, implying that there was a transfer of electrons from HCHO to RGO, which reduced the barrier height (ϕB). As a consequence, the saturation diode current decreased due to the reduced drift of thermally generated minority holes within AlGaN toward RGO, which increased ΔR/R0 upon exposure to HCHO. Those observations concur with the p-type behavior of RGO nanosheets, which leads to the formation of a p−n diode with n-type AlGaN and allows the reverse saturation current to be modulated by the change in barrier height (ϕB) upon gas exposure. The same gases were also injected to the Schottky diode sensor with RGO−ZnO NRs to provide a clear explanation of the sensing mechanism. When RGO nanosheets and ZnO NRs are hybridized together, the RGO−ZnO NR nanohybrid acts as an n-type sensing layer, which can form a Schottky diode with AlGaN. As observed from the FE-SEM images of RGO nanosheets coated on the ZnO NR/AlGaN structure, RGO nanosheets connect the ZnO NRs and also connect between the AlGaN surface covered by RGO and the ZnO NRs (Figure S3e−g, Supporting Information). Therefore, RGO is not only the main component in the nanohybrid in contact with the AlGaN surface to form a Schottky junction but also the sensing layer is connected to the metal electrode in the RGO−ZnO NR device, whereas ZnO NRs play a role in increasing the surface area and doping electrons to RGO. In selectivity, the RGO− ZnO NR/AlGaN/GaN Schottky diode sensor presented contrasting responses compared to those of the p−n diode device with RGO only, revealing that the nanohybrid formed as 30726

DOI: 10.1021/acsami.7b06461 ACS Appl. Mater. Interfaces 2017, 9, 30722−30732

Research Article

ACS Applied Materials & Interfaces

Figure 3. Sensor responses of the diodes with RGO−ZnO NRs on AlGaN for NO2, SO2, and HCHO gases. Resistance changes (ΔR/R0) of the diode sensor with the RGO−ZnO NRs on AlGaN were measured under exposure to (a) NO2, (b) SO2, and (c) HCHO gases of varying concentrations. The maximum values of the resistance change (ΔR/R0) from the response data were plotted as a function of concentration of (d) NO2, (e) SO2, and (f) HCHO gas.

sensing area of the devices with ZnO NRs. Furthermore, 3D RGO−ZnO NR nanohybrid with a larger surface area than that of a 2D RGO active layer can contain more defects, edges, and other high-energy binding sites, which can increase the adsorbed amount of gas molecules and, in turn, increase the sensing signal as well as the recovery time.18,60 Furthermore, the Schottky diode under reverse bias provided higher responsivity to gases of the same concentration, most likely due to higher modulation of thermionic charge transport across the SBH, which was controlled by electron donation or withdrawal during the adsorption of gas molecules. Comparison of Gas-Sensing Performance of 2D RGO and 3D RGO−ZnO NR Sensor. For further investigation into the concentration-dependent responses of the diode sensor, the device with RGO−ZnO NRs was used to detect reactive gases of various concentrations, as depicted in Figure 3. In the case of detecting NO2 and SO2 (Figure 3a,b, respectively), the ΔR/R0 ratio increased with gradually increasing gas concentration in the diode with RGO−ZnO NRs due to the electronwithdrawing nature of oxidizing gases. The diode’s response to NO2 was greater than its response to SO2. This can be explained by the larger electronegativity of NO2 than that of SO2, resulting in greater electron attraction behavior after they are adsorbed onto the nanohybrid channel. For HCHO gas (Figure 3c), the response of the diode with RGO−ZnO NRs showed that ΔR/R0 decreased with gradually increasing gas concentration due to the electron-donating nature of the reducing gas.41,42 Comparing the sensing gases of the diode with RGO with (Figure S8, Supporting Information) that of the diode with RGO−ZnO NRs indicates that the sensing performance of the diode with RGO−ZnO NRs was significantly higher, as discussed previously. More details of

an n-type sensing material on AlGaN/GaN (Figure S7, Supporting Information). The Schottky diode sensor under reverse bias responded to NO2 and SO2 gases with a ΔR/R0 increase but to HCHO gas with a ΔR/R0 decrease, as shown in Figure 2b. For the RGO−ZnO NR sensor, thermionic emission transport is the main part in the sensing mechanism of RGO− ZnO NRs/AlGaN. The opposite tendencies in the response to the different gases were likely caused by the modulation of SBH (ϕSB), which causes the thermionic emission transport of the electrons in RGO to dominate the reverse saturation current. The electrons withdrawn from the n-type RGO nanosheet layer to the adsorbed NO2 or SO2 gas raised the SBH (ϕSB) at the RGO/AlGaN heterojunction, resulting in a reduction in the thermionic transport of electrons from the n-type sensing layer toward the AlGaN layer under reverse bias and, in turn, an increase in ΔR/R0 (Figure 2d). Similarly, the HCHO molecules adsorbed onto the nanohybrid lowered the SBH (ϕSB) by donating electrons to the nanohybrid, leading to an increase in the thermionic transport of electrons from RGO−ZnO NRs to AlGaN/GaN under reverse bias. As a result, the saturation diode current of the Schottky diode gas sensor under reverse bias increased, and the ΔR/R0 value decreased. Importantly, when comparing the sensing behaviors of the p−n diode with RGO only and of the Schottky diode with a nanohybrid of RGO−ZnO NRs (Figure S6, Supporting Information), the performance of the nanohybrid Schottky diode showed a larger modulation of ΔR/R0 than that of the performance of p−n diode with RGO. The higher performance of the nanohybrid Schottky diode under the presence of adsorbed gas molecules can be presumably attributed to the ndoping of RGO by ZnO NRs, which causes different charge transport behaviors under reverse bias and increase in the 30727

DOI: 10.1021/acsami.7b06461 ACS Appl. Mater. Interfaces 2017, 9, 30722−30732

Research Article

ACS Applied Materials & Interfaces Table 1. Room-Temperature Gas-Sensing Properties of Materials Exposed to NO2, SO2, and HCHO Gasesa material

gas

detection limit

sensitivity (ppm−1)

response/recovery time

ref

polyaniline−WO3 polypyrrole/WO3 Ag−LaFeO3 single-walled carbon nanotubes carbon nanotube/RGO RGO−ethylenediamine RGO/poly(3,4-ethylenedioxythiophene) RGO/Cu2O RGO/SnO2 SnO−Sn/graphene ZnO/graphene RGO pillar RGO−ZnO NRs on 2DEG AlGaN/GaN (this work)

SO2 NO2 HCHO NO2 NO2 NO2 SO2 NO2 NO2 HCHO HCHO NO2 NO2 SO2 HCHO NO2 SO2 HCHO

5 ppm 5 ppm 500 ppb 44 ppb 500 ppb 1 ppm 1 ppm 400 ppb 1 ppm 200 ppb 25 ppm 5 ppm 120 ppb

0.009 0.52 0.15 0.034 0.06 0.25 0.05 0.4 2.87 0.25 0.01 0.054 1.875 0.933 0.875 0.517 0.133 0.591

∼200/180 s ∼500/5000 s 67/104 s order of 10 h ∼3600 s ∼600/900 s