High-Performance Schottky Diode Gas Sensor Based on the

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High-Performance Schottky Diode Gas Sensor Based on Heterojunction of Three-Dimensional Nanohybrids of Reduced Graphene Oxide–Vertical ZnO Nanorods on 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06461 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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ACS Applied Materials & Interfaces

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

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon,

Kyunggi-do 16419, Republic of Korea ‡

Device Platform Lab., Korea Advanced Nano Fab Center, Suwon. Kyunggi-do 16229, Korea

§

Sungkyunkwan University (SKKU) Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan

University, Suwon, Kyunggi-do, 16419, Republic of Korea ∥

Samsung Advanced Institute for Health Sciences and Technology (SAIHST), Sungkyunkwan

University, Suwon, Kyunggi-do 16419, Republic of Korea KEYWORDS: gas sensor, Schottky diode, heterostructure, reduce graphene oxide, zinc oxide.

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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 sensor. 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 ultra-sensitive changes in the Schottky barrier of a van der Waals heterostructure between RGO and AlGaN layers upon the interaction with gas molecules. Advances in the design of a Schottky diode gas sensor based on the heterojunction of highmobility 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.


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INTRODUCTION The Internet of things is provoking extensive research into the variety of chemical sensor devices that enable us to collect and exchange gas sensing data and thereby open 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 more studied 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 resistor,10–22 transistor,23–25 and diode,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, 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 conventional semiconductor-based chemiresistors.33 Additionally, a main reason 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 ACS Paragon Plus Environment

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H2-sensing transistors or diodes based on AlGaN/GaN heterostructures.28,34,35,37 This sensing mechanism can be explained by the dissociation of the 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. Due to 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 temperature can decrease the sensing stability and lifetime of these materials due to thermally-induced 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 has 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.


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In this work, a new class of high electron mobility Schottky diode gas sensor is presented as a chemical sensing platform with high performance, good stability, low limit of detection down to ppb level and functionality at room temperature. The key components in our design of the device are RGOZnO 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. While 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 HCHO gases down to ppb levels, with sensitivity values of 1.88, 0.93 and 0.88 ppm1

, 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 to realize the next generation of gas sensor applications. Due to 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.


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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 Supporting Figure S1. 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 crosssectional 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 formed 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 (Supporting Figure S2). 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 m) layer. The hydrothermal method was used to grow ZnO NRs at low temperature. According to top-view and cross-sectional FE-SEM images (Supporting Figure S3a and S3b), the diameter and length of the ZnO NRs were ~0.26 and ~2.2 μm on 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 (Supporting Figure S3c and S3d). The ACS Paragon Plus Environment

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FE-SEM investigation of RGO-ZnO NR nanohybrids formed on AlGaN surface shows that neighboring ZnO NRs are connected to each other by RGO (Supporting Figure S3e and S3f). And, RGO nanosheets coated on the open area of AlGaN are connected to vertical ZnO NRs (Supporting Figure S3g). The results indicate that RGO-ZnO NRs nanohybrid can be considered as a 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 (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) (Supporting Figure S4a), which implies that occurrence of the peaks of GaN (0002) and ZnO (0002) at 34.5o 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 Supporting Figure S4b depict 3 samples in which GaN and GO bands occurred in the shifted range of 400 to 4200 cm-1, implying the existence of GO and RGO coated on an AlGaN/GaN heterostructure. In Supporting Figure S4c, two distinctive narrowed peaks at the GaN:E2 band (~563 cm-1) and the GaN:A1 band (~734 cm-1) reveal the high quality crystalline structure of the GaN epitaxial layer. ACS Paragon Plus Environment

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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 (Supporting Figure S5). 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. Since RGO nanosheets are typically p-type materials due to the removal of electrons from the conduction band with adsorption of oxygen and water 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 B, 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 the 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 ACS Paragon Plus Environment

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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). Since 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 the SBH fromB toSB 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 SB is formed in a similar manner 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, while 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 NRs based AlGaN/GaN platform In order to understand the gas sensing mechanism of the Schottky diode device with an RGOZnO 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


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investigated by injecting NO2, SO2, and HCHO gases into a measurement system (Supporting Figure S6). 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 R/R0, where R = 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 and 2b, respectively. The resistance change of the device under reverse bias is dominantly caused by modulation of the 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 R/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, p-n heterojunction play a role in 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 (B) 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 R/R0 of the device ACS Paragon Plus Environment

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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 (B). As a consequence, the saturation diode current decreased due to the reduced drift of thermally generated minority holes within AlGaN toward RGO, which increased R/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 (B) 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 ZnO NRs/AlGaN structure, RGO nanosheets connect the ZnO NRs and also connect between the AlGaN surface covered by RGO and the ZnO NRs (Supporting Figure S3e-g). Therefore, RGO is not only the main component in the nanohybrid contacting with AlGaN surface to form a Schottky junction but also the sensing layer connected to metal electrode in the RGO-ZnO NR device while ZnO NRs play a role in increasing the surface area and doping electrons to RGO. In selectivity, the RGO-ZnO NRs/AlGaN/GaN Schottky diode sensor presented contrasting responses compared to those of the pn diode device with RGO only, revealing that the nanohybrid formed as an n-type sensing material on AlGaN/GaN (Supporting Figure S7). The Schottky diode sensor under reverse bias responded to NO2 and SO2 gases with a R/R0 increase but to HCHO gas with a R/R0 decrease, as shown in Figure 2b. For RGO-ZnO NRs sensor, thermionic emission transport is a main part in sensing mechanism of RGO-ZnO NRs/AlGaN. The opposite tendencies in the response to the different gases were likely caused by the modulation of the SBH (SB), which causes the thermionic emission transport of the electrons in RGO to dominate the reverse saturation current. The electrons withdrawn ACS Paragon Plus Environment

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from the n-type RGO nanosheet layer to the adsorbed NO2 or SO2 gas raised the SBH (SB) 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 R/R0 (Figure 2d). Similarly, the HCHO molecules adsorbed onto the nanohybrid lowered the SBH (SB) 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 R/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 (Supporting Figure S6), the performance of the nanohybrid Schottky diode showed a larger modulation of R/R0 than that of the 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 n-doping of RGO by ZnO NRs, which causes different charge transport behaviors under reverse bias, and increase in the sensing area of the devices with ZnO NRs. Furthermore, 3D RGO-ZnO NRs nanohybrid with 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 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 NRs 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 ACS Paragon Plus Environment

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in Figure 3. In the case of detecting NO2 and SO2 (Figure 3a and 3b, respectively), the R/R0 ratio increased with gradually increasing gas concentration in the diode with RGO-ZnO NRs due to the electron-withdrawing 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 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 R/R0 decreased with gradually increasing gas concentration due to the electron-donating nature of reducing gas.41,42 Comparing the sensing gases of the diode with RGO only (Supporting Figure S8) with 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 the RGO-only device’s exposure to NO2, SO2, and HCHO gases are presented in Supporting Figure S9a, S9b, and S9c, respectively. The results indicate that ZnO NRs grown on AlGaN are a crucial element in the enhancement of sensing signal not only by increasing the surface area, but also by doping electrons to RGO, as aforementioned. Because of the baseline drift while sensing the gases, a new baseline was created and re-plotted after each cycle to determine a reliable sensitivity value. The edited plots of the responses to various concentrations of NO2, SO2, and HCHO are illustrated in Figure 3d, 3e, and 3f, respectively. There is a clear enhancement in the sensor signal with increasing concentration of each gas. However, the plots reveal clearly that the response of the diode with RGO-ZnO NRs was mostly non- linear, and a similar result was found for the diode with RGO under NO2, SO2, and HCHO gas (Supporting Figure S9d, S9e, and S9f). This is possibly related to incomplete recovery caused by the reaction of the gases with the interaction sites available for the subsequent response and the slow out-diffusion of gas molecules in the multilayer stacked structure of the RGO networked layer.12,22 The response of the diode with RGO-ZnO NRs plotted against gas concentration showed a linear regime at high ACS Paragon Plus Environment

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concentrations. The responses of this diode were 50%, 24%, and -20% for NO2, SO2, and HCHO gas at 1 ppm, respectively. At the same concentration, the diode with RGO only showed lower R/R0 of -11.5%, -9.5%, and 15.5% for NO2, SO2, and HCHO gas, respectively, confirming the importance of ZnO NRs in the nanohybrid for performance improvement. The sensitivity in gas sensing is defined by the slope of the response curve between resistance response (R/R0) and gas concentration (n): S = (R/R0)/∆n.39 We summarized our sensitivity data and compared them with those of other studies in Table 1. When NO2 gas is injected at the same ppb level, the sensitivity of 1.875 ppm-1 for the diode with RGO-ZnO NRs was three times higher than that of the diode with RGO only, 0.517 ppm-1. Similarly, the diode with RGO-ZnO NRs had higher sensitivities of 0.933 and 0.875 ppm-1 for SO2 and HCHO gases, respectively, compared to sensitivity values of 0.133 and 0.591 ppm-1 from the diode with RGO only. It is possible that the high surface area of the 3D RGO-ZnO NR nanohybrid also affected the sensitivity because the density of the molecules adsorbed on the surface of the nanohybrid is higher than that of 2D RGO.41 In order to evaluate the reproducibility of the device, we examined five cycles of repetitive gas sensing to observe the changes after each gas exposure. The NO2, SO2, and HCHO gases were introduced into the measurement chamber at a concentration of 120 ppb. As shown in Figure 4, the devices with RGO-ZnO NRs exposed to NO2, SO2, and HCHO for five cycles showed baseline drift during each cycle. As shown in Supporting Figure S10, after re-plotting using a normalized baseline value, it is evident that the sensor response was not significantly changed after sensing. A slight change in response might be caused by the slow desorption of molecules, which would be visible for each gas. However, based on the test curve, it was confirmed that the RGO-ZnO NR sensor is relatively stable, and its response is consistently detectable under low concentrations (ppb level) of NO2, SO2, and HCHO at room temperature.


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A summary of the response of each device to each gas is depicted in Figure 5a. Our devices showed a gas detection range of 120 ppb to 1 ppm at room temperature. At the smallest gas concentration of 120 ppb, the device with RGO only could detect SO2 with the lowest responsivity of -1.6%. The highest responsivity was 49.5% from the diode with RGO-ZnO NRs under NO2. The responses of RGO and RGO-ZnO NR devices to toxic gases clearly show different response directions as well as different modulation, which can provide a way of selectively differentiating between oxidizing and reducing gases. Furthermore, as discussed in the sensing mechanism, combining ZnO NRs with RGO nanosheets in the fabrication of hybrid sensing materials as part of a Schottky diode improved sensing responsivity. There have been many reports on gas sensors based on ZnO nanostructured materials working at high temperatures with high sensitivity.5,14,46,47 The results of this work suggest that the advance of combining RGO with ZnO NRs not only increases the conductivity of the active layer, but also reduces the working temperature of the device. There are various parameters for evaluating the performance of gas sensors. Two critical factors, sensitivity and detection limit, are involved. The sensitivity represents the responsiveness of the device to gas, while the detection limit represents the device’s capability for detecting the gas. The detection limit vs. sensitivity of several recent gas sensor devices, including our diode devices, is summarized in Figure 5b. Here, the devices located on the right side of the x-axis have higher sensitivity, while the devices located on the lower end of the y-axis have a lower detection limit. Our diode devices with RGO-ZnO NRs not only exhibited high sensitivity, but also had a low detection limit. This very low detection limit is attributed to the design structure in which a heterojunction Schottky diode based on 3D nanohybrid sensing materials is combined with 2D RGO and vertical 1D ZnO NRs on an AlGaN/GaN heterostructure, forming a 2DEG with electron mobility of ~2000 cm2/V.s between the AlGaN and GaN.66 The thermionic electron current of the Schottky diode under reverse bias abruptly changes when gas molecules are adsorbed on the active layer of the diode. As ACS Paragon Plus Environment

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shown, by combining ZnO NRs with the RGO layer, the sensitivity of the devices was improved, shifting the data to the right on the graph. By using 3D ZnO NRs, the enlarged catchment area induced more interactions between the gas and the sensing surface in a given time. Even though the device is operated at a relatively large reverse bias voltage, the power consumption of the device is a few nanowatts becasue the reverse saturation current level is low and the nanohybrid sensing materials allow the device to be operated at room temperature without thermal heating. Besides, humidity effect on the sensor performance cannot be neglected because adsorption of water and oxygen molecules in ambient envronment influences the sensor performances due to the withdrawal of electrons from the n-type RGO surface and, in turn, change the current output. The resistance of the nanohybrid Schottky diode can be increased when the water molecules are adsorbed on the RGO surface, leading to the withdrawal of electrons and in turn, an increase in the R/R0 responses. In contrast, adsorption of water or oxygen molecules can increase the conductivity of a 2D RGO p-n diode due to the increase of holes, resulting in a decrease in the R/R0 responses.

CONCLUSION In summary, a new class of chemical sensors based on a combination of 3D nanostructured hybrid materials, RGO-ZnO NRs, and a high electron mobility channel, AlGaN/GaN, was developed and designed for environmental monitoring applications. RGO nanosheets were coated directly onto 3D ZnO NRs to create a 3D sensing material for chemical sensors that operates at room temperature, thereby overcoming the heating requirement of metal oxide sensing materials. Furthermore, by incorporating 3D ZnO NRs, the p-type RGO active layer was changed to n-type RGO, forming a Schottky diode of RGO-ZnO NRs and AlGaN/GaN. The resistance of the Schottky diode gas sensor under reverse bias was effectively modulated upon adsorption of oxidizing and reducing gases due to ACS Paragon Plus Environment

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sensitive changes in the Schottky barrier height, which governs the thermionic transport of electrons across the heterojunction between RGO-ZnO NRs and AlGaN. Also, the sensitivity of the Schottky diode with RGO-ZnO NRs was higher that of the RGO-AlGaN p-n diode due to the increased surface area in the active layer produced by the incorporation of 3D ZnO NRs. Therefore, our devices also provide a way to control the selectivity for different kinds of gases at various concentration levels. Additionally, the diode with 3D nanohybrids of RGO-ZnO NRs showed good stability and reproducibility in detecting toxic gases of NO2, SO2, and HCHO. The combination of the AlGaN/GaN heterostructure with high electron mobility and stability and the nano-engineered sensing materials with high responsivity to hazardous gases may provide a new and generalizable approach to developing a chemical sensor device that has high sensitivity, low detection limit, low power consumption, and functionality at room temperature.

EXPERIMENTAL SECTION Fabrication of Schottky Diode Gas Sensors Schottky diode devices were fabricated on a heterostructure of AlGaN(~25 nm, Al fraction = 0.25)/low-temperature GaN(~3 μm) epitaxially grown on a sapphire substrate by metal organic chemical vapor deposition (Supporting Figure S1a). The AlGaN layer was undoped, and the uniformity of resistivity and thickness of the sample is shown in Supporting Figures S1b and S1c, respectively. Supporting Figure S2 shows a schematic of the fabrication process of the nanohybrid structure of RGO-ZnO NRs on the epitaxial AlGaN/GaN layer to create a Schottky diode. During device fabrication, the samples were cleaned in a sonication bath with acetone, ethanol, iso-propanol, and DI water for 5 mins at each step. To preclude a leakage current flowing from the 2DEG to the electrodes, a 150-nm-thick Al2O3 dielectric layer was grown by atomic layer deposition (ALD) at ACS Paragon Plus Environment

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200oC. An electrode of Au/Ti was directly deposited by thermal evaporation through a shadow mask for an Ohmic contact. Before the growth of ZnO NRs, a hydrophobic SiO2 layer was deposited by ebeam deposition to prevent the growth of ZnO NRs on the surface of the Ohmic electrode. A plasma treatment for cleaning was also implemented to eliminate contamination that could prevent the uniform growth of ZnO NRs on the AlGaN surface. A hydrophobic layer of tetratetracontane (TTC) was capped on the Ohmic electrode before initiating the coating process of the GO solution. The alignment of the shadow masks for the Au/Ti, SiO2, and TTC layers on the substrates was manually performed by matching the alignment channel. The thicknesses of the Au, SiO2, and TTC layers were 60, 200, and 60 nm, respectively. The channel dimensions of the characterized devices were less than 400 m in length and 1 cm in width. After depositing the Ohmic contact on the AlGaN layer, ZnO NRs were grown on the patterned Schottky diode devices without using a seed layer. A nutrient solution was prepared with equal molar concentrations of 30 mM zinc nitrate hexahydrate (Zn(NO3)2.6H2O) and hexamethylenetetramine (C6H12N4, HTMA) in 200 mL of DI water. After depositing the hydrophobic SiO2 layer, the patterned samples were placed in the nutrient solution at a temperature of 90oC for 4 hrs. Then, the patterned samples of ZnO NRs were cleaned by DI water and dried at 100oC for half an hour. Graphene oxide (GO) nanosheets were first synthesized from exfoliated graphene oxide nanosheets prepared from graphite flakes using a modified Hummers’s method.67,68 GO nanosheets have the average thickness of ~3 nm and sheet size of ~ 200 nm regarding to our RGO, as reported in the previous works.18,69,70 The patterned sample surface with ZnO NRs was modified with a 20 wt% solution of poly(diallydimethylammonium chloride) (PDDA) in water to form self-assembled monolayers (SAMs) for enhanced adsorption of the GO nanosheets. The fabrication process is described as follows. First, GO networks were adsorbed to the PDDA-modified samples by a drop casting method for 1 hr. Because the rapid response of RGO nanosheets increases during gas exposure, the steep slope ACS Paragon Plus Environment

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region increases with increasing GO reduction time. For this reason, the networked GO film was reduced to RGO by exposure to a hydrazine hydrate vapor (hydrazine monohydrate 99+%, Alfa Aesar) at 40 oC for 24 hrs. Then, the channels were annealed at 120 oC for 3 hrs in a nitrogen environment to eliminate any adsorption interactions between the hybrid channel and oxygen or water, which could cause electrical instability. The RGO nanosheets also formed the Schottky contact with AlGaN/GaN layer. Characterization of gas sensing performance Before execution of the gas sensing measurement, the current-voltage (I-V) characteristics of our devices were measured with a semiconductor parameter analyzer (HP 4145B, Agilent) to explore the Schottky behavior of our structures. I-V measurements were performed with different channels, including GO only, RGO only, ZnO NRs only, and RGO-ZnO NR nanohybrid materials, in order to clearly understand the electrical conduction mechanism in the devices. The voltage was swept from 10 to 10 V in ambient conditions. All measurements were performed in a gas sensing system with gas supply and control units, as illustrated in Supporting Figure S6. The device was loaded into a glass tube chamber placed in the center of a thermostatic furnace. Gas flow between the gas bottles and the glass tube was allowed and was controlled by a mass flow controller (MFC). The measurement was stabilized at room temperature (300 ± 1 K), while the mass flow was constantly maintained at 500 sccm. The gases used for testing were NO2, SO2 and HCHO. The environment for sensing gases was dry synthetic air (21% v/v O2/N2), which is a carrier gas suitable for diluting gases and characterizing the mechanism of gas sensors. The gases were introduced for a two-minute response time and then exposed to dry air gas for five minutes as a recovery time. The measurements were conducted by a Keithley 2400 meter. The


19


implemented software allowed the device to be fixed at -10 V, while the current was limited to 0.01 A to record the output signal of the current and resistance during measurement. FIGURE CAPTIONS

a

c

1 µm

Sapphire (0006)

(3) (2) (1)

30

d

(1) GO sample (2) RGO sample (3) RGO-ZnO NRs sample

G

(1)

2D

(3) (2)

1500

40

45

50

GO RGO ZnO NRs RGO-ZnO NRs

6.0x10-6

Current, I (A)

D

35

2 theta (degree)

e

1000

(1) Reference sample (2) ZnO NRs sample (3) RGO-ZnO NRs sample

GaN  ZnO (002)

Intensity (a.u)

b

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

4.5x10-6 3.0x10-6 1.5x10-6 0.0

2000

2500

-10

Raman shift (cm-1)


-5

0

5

10

15

Voltage, V (V)

20

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f

g

Figure 1. Characterization of ZnO NRs and RGO-ZnO NRs in a Schottky diode chemical sensor. (a) Schematic structure of RGO-ZnO NR nanohybrid layers formed on an AlGaN/GaN heterostructure grown on a sapphire substrate. (b) Cross-sectional FE-SEM images of ZnO NRs without a seed layer grown in a vertical direction on AlGaN/GaN/sapphire substrate. (c) X-ray diffraction 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 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 B and SB indicate the barrier height and Schottky barrier height (SBH), respectively, for the diode devices with RGO only and RGO-ZnO NRs. Due to 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 B.


21


b

12

Response, R/R0 (%)

a Response, R/R0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HCHO

8 4 NO2

SO2

0 -4 GAS ON

GAS OFF

0

400

DRY AIR 800

1200

1600

30

NO2

20

SO2

10

HCHO

0 -10

GAS ON

GAS OFF

0

400

DRY AIR 800

1200

1600

Time (s)

Time (s)

c

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d

Figure 2. Resistance change (R/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 R/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 R/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 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, the 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 B. In the Schottky


22

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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 SB.

a

b 500

40

250 120

20 0 0

400

800

1200

d

500

20

120

250

10

0

1600

Time (s)

1000 ppb 750

SO2

0

500

1000

1500

2000

Time (s)

e


60

HCHO 0 -7

120

-14

250 500

-21

750 1000 ppb 0

500

1000

1500

2000

Time (s)

f

40 30 20 0

250

500

750

Concentration (ppb)

1000


NO2

50

SO2

24 20 16 12 8 0

250

500

750

1000

Concentration (ppb)


28

60

10

c 30

1000 ppb 750

NO2



80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-8

HCHO

-12 -16 -20 -24

0

250

500

750

1000

Concentration (ppb)

Figure 3. Sensor responses of the diodes with RGO-ZnO NRs on AlGaN for NO2, SO2, and HCHO gases. Resistance changes (R/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 (R/R0) from the response data were plotted as a function of concentration of (d) NO2, (e) SO2, and (f) HCHO gas.


23


30

NO2

20 10 0

120 ppb 0

500

1000

1500

Time (s)

2000

2500


b

c 16

SO2

12 8 4 0

120 ppb 0

500

1000

1500

2000

2500

Time (s)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

HCHO 0 -5 -10

120 ppb

-15 0

500

1000

1500

2000

2500

Time (s)

Figure 4. Sensor responses (R/R0) of the diode sensor with 3D RGO-ZnO NRs under gas exposure. The data show the responses of the devices exposed to (a) NO2, (b) SO2, and (c) HCHO gases with the gas concentration of 120 ppb for all gases.


24

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RGO NO2

60 40

RGO SO2 RGO HCHO RGO/ZnO NRs NO2 RGO/ZnO NRs SO2 RGO/ZnO NRs HCHO

20 0 -20 120ppb 250ppb 500ppb 750ppb

1 ppm

Concentration

b Detection Limit (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

10

[18] [64] [66]

[67]

1 [22] [42]

0.1

RGO/ZnO-AlGaN/GaN

RGO-AlGaN/GaN THIS WORK

[36]

0.01

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Sensitivity (ppm-1)

Figure 5. Comparison of sensor responses and performance. Summary of (a) the responses of the diodes with RGO and RGO-ZnO NRs to gases of various concentrations and (b) a performance ACS Paragon Plus Environment

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comparison of the diode sensors in this work with other gas sensors with respect to the detection of NO2 at room temperature.

Table 1. Room temperature gas-sensing properties of materials exposed to NO2, SO2, and HCHO gases. Material

Gas

Detection limit

Sensitivity

Response/Recovery time

Ref.

PAN-WO3

SO2

5 ppm

0.009 ppm-1

~ 200/180 s

61

Ppy/WO3

NO2

5 ppm

0.52 ppm-1

~ 500/5000 s

62

Ag-LaFeO3

HCHO

500 ppb

0.15 ppm-1

67/104 s

63

SWCNTs

NO2

44 ppb

0.034 ppm-1

Order of 10 h

39

CNT/RGO

NO2

500 ppb

0.06 ppm-1

~ 3600 s

22

RGO-EDA

NO2

1 ppm

0.25 ppm-1

~ 600/900 s

64

RGO/PEDOT

SO2

1 ppm

0.05 ppm-1

< 180/70 s

43

RGO/Cu2O

NO2

400 ppb

0.4 ppm-1

~ 300/500 s

45

RGO/SnO2

NO2

1 ppm

2.87 ppm-1

65/N.A. s

65

SnO-Sn/Graphene

HCHO

200 ppb

0.25 ppm-1

300/1200 s

19

ZnO/Graphene

HCHO

25 ppm

0.01 ppm-1

30/40 s

20

RGO pillar

NO2

5 ppm

0.054 ppm-1

900/5400 s

18

RGO-ZnO NRs on 2DEG AlGaN/GaN (this work)

NO2, SO2, HCHO

120 ppb

1.875 ppm-1 0.933 ppm-1 0.875 ppm-1

120/320 s

This work

RGO networks on 2DEG AlGaN/GaN (this work)

NO2, SO2, HCHO

120 ppb

0.517 ppm-1 0.133 ppm-1 0.591 ppm-1

120/300 s

This work

N.A. : not available


26

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ASSOCIATED CONTENT Supporting Information. Additional fabrication process, FE-SEM, XRD, Raman spectra, sensing gas data and detailing supplementary text and Supporting Figure S1 to S10. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(Nae-Eung Lee) [email protected].

ACKNOWLEDGMENT This

research

was

supported

by the

Basic

Science

Research

Program

(Grant

No.

2016R1A2A1A05005423) and the NanoMaterial Technology Development Program (Grant No. 2015M3A7B7044548) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning.


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(2)

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Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Adv. Mater. 2016, 28 (5), 795–831.

(4)

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Wei, A.; Pan, L.; Huang, W. Recent Progress in the ZnO Nanostructure-Based Sensors. Mater. Sci. Eng. B 2011, 176 (18), 1409–1421.

(6)

Zhang, Y.; Xu, J.; Xu, P.; Zhu, Y.; Chen, X.; Yu, W. Decoration of ZnO Nanowires with Pt Nanoparticles and Their Improved Gas Sensing and Photocatalytic Performance. Nanotechnology 2010, 21 (28), 285501.

(7)

Comini, E.; Faglia, G.; Sberveglieri, G. UV Light Activation of Tin Oxide Thin Films for NO2 Sensing at Low Temperatures. Sens. Actuators B Chem. 2001, 78 (1–3), 73–77.


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(8)


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High-Performance Schottky Diode Gas Sensor Based on the

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