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Highly Sensitive Gas Sensor by the LaAlO3/SrTiO3 Heterostructure with Pd Nanoparticle Surface Modulation Ngai Yui Chan, Meng Zhao, JianXing Huang, Kit Au, Man Hon Wong, Hei Man Yao, Wei Lu, Yan Chen, Chung Wo Ong, Helen Lai Wa Chan, and Jiyan Dai* Since the discovery of the two-dimensional electron gas (2DEG) at LaAlO3/SrTiO3 (LAO/STO) heterostructure interface, there has been a great attraction for scientists to study the formation mechanism of the 2DEG at oxide interface. More recently, demonstration of potential applications of the LAO/STO heterostructure in electronic devices such as field-effect transistors, sensors and memories is attracting more research interest. In this work, we demonstrate a highly-sensitive hydrogen (H2) gas sensing characteristics of LAO/STO heterostructure with palladium nanoparticle (Pd NP) surface modulation. Pd NPs act as catalysts are accommodated to the surface of LAO/STO 2DEG heterostructure to enhance its gas sensitivity and selectivity. Besides H2 gas, cross sensitivity tests indicate that the sensor shows response to ethanol, acetone and water gas vapour. The gas sensing ability has been attributed to charge coupling between the desorbed gas molecules/Pd NPs and the LAO/STO interface through the LAO layer. These results not only promise the potential interest for understanding the oxide interfacial 2DEG, but also its application in all-oxide devices, thus open a new route to complex oxide physics and ultimately for the design of devices in oxide electronics. The discovery of quasi-two dimensional electron gas at the interface between the LaAlO3 (LAO) and SrTiO3 (STO) heterostructure[1] with intriguing properties such as magnetism,[2] superconductivity,[3] quantum oscillation[4] and photoconductivity[5] have been studied extensively in recent years. The significant results discovered in this system pave a way for potential applications for multifunctional oxide based electronic devices.[6–10] Charge writing experiment by the conducting atomic force microscopy,[11–13] the polarization state of a ferroelectric thin film[14,15] and the existence of polar substance[16–18] on top of the LAO/STO heterostructure revealed that the transport properties at the interface are extremely sensitive to the surface states. The addition of metal elements on the top of
N. Y. Chan, M. Zhao, J. X. Huang, K. Au, M. H. Wong, H. M. Yao, W. Lu, Y. Chen, C. W. Ong, H. L. Wa Chan, Prof. J. Dai Department of Applied Physics The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong, P. R. China E-mail: [email protected] Prof. J. Dai The Hong Kong Polytechnic University Shenzhen Research Institute P. R. China
DOI: 10.1002/adma.201401597
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LAO layer results in significant changes in transport properties at the interface and have been studied by the recent first principle calculation and experimental work.[19,20] Pristine LAO/ STO heterostructure has also been proposed for hydrogen gas sensor based on first principle calculation,[21] but has never been experimentally realized. Motivated by these experimental results and the theoretical prediction, we have fabricated the LAO/STO 2DEG heterostructure, with surface being modulated by palladium (Pd) nanoparticles in order to trigger the LAO/STO heterostructure’s gas sensing ability. The Pd surface modulation method has been utilized to enhance the gas response of some semiconducting systems such as ZnO nanowires, AlGaN/GaN 2DEG heterostructure as well as graphene and carbon nanotube.[22–25] Herein, the surface modified LAO/STO heterostructure is demonstrated to be highly sensitive to different gases. Hydrogen, oxygen, ethanol, acetone and water vapour can be detected via changes in the transport properties of the interface. The LAO/STO heterostructures with LAO thickness of 5, 10, 20 and 40 unit cells (uc) were deposited on TiO2-terminated (001) STO substrate, and the interfacial structure and transport properties of the bare LAO/STO sample are shown in Figures S1 and S2, where typical metallic conduction can be seen (see experimental section of details). Pd nanoparticles (Pd NP) with size around 2 nm were deposited by d.c. magnetron sputtering in pure argon ambient to the surface of LAO/STO heterostructure at room temperature. Figure 1a schematically shows the device structure, and the actual picture of the sample is shown in Figure 1b, where the aluminum wires are ultrasonically bonded at the four corners for electrical characterization. It is apparent that the Pd NPs, which appear as dark spots, are distributed uniformly and densely on the surface of the LAO layer. The Pd NPs are crystallized and the density is not leading to a complete conduction percolation path. The high-resolution image shown in Figure 1c and the ring shaped selected area electron pattern (SAED) shown in Figure 1d indicate the nanocrystalline f.c.c. structure of the Pd NP. The small size and high density distribution of the nanoparticles should enhance the responsivity to the targeted gases.[26,27] Figure 2 shows the hydrogen gas sensing characteristics for the Pd NP decorated LAO/STO heterostructure with thickness of 5 uc at room-temperature and elevated (80 °C) temperature, where it is of particular interest that the conductance of the sample is extremely sensitive to H2 gas. It is noticed that our pristine LAO/STO heterostructure does not show any response to H2 gas as reflected from the current-voltage characteristics as shown in Figure S3, and there is no experimental report either
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COMMUNICATION Figure 1. (a) Schematic diagram of the Pd nanoparticle decorated LAO/STO heterostructure (the surface is a low-magnification TEM image representing the Pd NPs), (b) actual picture of the device, (c) high-resolution TEM image of a single Pd NP and (d) SAED pattern of the Pd NP.
to show the gas sensing ability of such heterostructure. We believe that the reason why H2 gas sensing in LAO/STO 2DEG has not been observed before is due to the low gas reactivity of the LAO surface. The Pd NPs on the LAO surface therefore play a crucial role for the functionalization of H2 gas sensing. It is also worth mention that the response to the gas is an interfacial effect, since the experiment with surface electrode contact without connecting to the interface does not show any measurable conductance and response upon the exposure to hydrogen. For a typical gas sensor, three physical parameters are usually studied for the sensing performance, including the sensitivity S(%), the response time τR and recovery time τS. Among them, S(%) is defined as the relative variation of the current at the interface and can be calculated as S = ΔI0I ( % ), where ΔI = I g − I 0 , I0 is the current of the interface before exposing to hydrogen and Ig is the peak current in presence of hydrogen. The parameters of τR and τS are defined respectively as the time for the 90% of the saturated current of full response and recovery. The transient response of the gas sensor to hydrogen at room temperature is shown in Figure 2a, showing the variation of current of the interface corresponding to the on (for 5 min) and off of the H2 gas flow for two cycles, with 20 ppm H2 gas concentration balanced in argon. The results show that the interfacial current quickly increases when the device is exposed to H2 gas, with τR ∼ 7.3 min, and followed by a slowly approach to saturation. While after the current of the interface saturated, the H2 gas inside the chamber was pumped out and synthetic air was flushed to the sample for the recovery process, the current of the interface decreased and essentially resumed the original state, with τS ∼ 36.7 min. The responses of the Pd NP decorated LAO/STO heterostructure was also recorded in the environment with different hydrogen gas concentrations balanced in argon (from 2 ppm to 14 ppm) at room temperature. The results shown in Figure 2b are plotted as the relationship between the sensitivity and time. A detectable change in current of ∼80% was achieved for the sample with H2 concentration as low as
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∼2 ppm. It is apparent that exposing the sensor to a higher concentration of hydrogen induces faster increase of the current to a higher peak value. These saw-tooth responses are superimposed on a monotonically increasing background; this fact can be attributed to the charge accumulation and slow recovery rate of the sensor which operated at room temperature. Nevertheless, it demonstrated that the Pd NP modulated LAO/STO heterostructure is sensitive to H2 gas. Temperature is an important factor that greatly influences the hydrogen sensing response based on the catalytic effect. Usually, higher temperature would lead to higher sensing performance due to the lowering of activation energy for gas adsorption and desorption of the sensor. Figure 2c shows the cyclic performance of the device with 20 ppm H2 balanced in argon at 80 °C, where the sensor shows both shortened τR ∼ 0.8 min and τS ∼ 0.95 min, which are much faster compared to the sensor operated at room temperature. The shorter response time can be explained by faster diffusion and dissociation of hydrogen molecules at higher temperature, where the surface has higher reactivity and more active adsorption sites for H2 molecules to Pd NP. Figure 2d shows response to H2 gas with different concentrations, with the sensitivity much higher for the device after exposed to H2 gas, a response with sensitivity up to 2400% has been recorded for the device exposed to 2 ppm H2 gas. The faster recovery is probably arose from the faster gas desorption at the Pd NPs surface due to high temperature. As shown in Figure 2e, the response time τR of the Pd NP decorated LAO/STO heterostructure at 80 °C generally tends to be shortened (for [H2 ] = 2 ppm, τ R = 3.2 min and for [H2 ] = 20 ppm, τ R = 0.8 min ). Upon exposure to dry air for the recovery process, dissolved hydrogen on Pd NP reacts with the oxygen in air and forms H2O by the reaction ( 2H2 + O2 = 2H2O). At low working temperature, the response time is longer due to slow desorption of the formed water molecules on the surface. The sensor was able to detect H2 gas with low concentration at high temperature, while it’s relatively less sensitive to
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Figure 2. (a) Electrical response for the Pd NP decorated LAO/STO heterostructure for the loading and de-loading cycle with 20ppm H2 gas balanced in argon at room temperature. (b) Real time response of sensitivity under exposure to H2 gas with various concentrations (2 – 14 ppm) at room temperature. (c) Electrical response and (d) Sensitivity response at 80 °C. (e) The relationship between response time and hydrogen concentration for the sensor operated at 80 °C. (f) Combined sensor’s sensitivity at room temperature and 80 °C.
such a low H2 concentration at room temperature. Figure 2f plots the sensitivities for the sensor to various concentration of H2 gas at room temperature and 80 °C. At low hydrogen gas concentration, the sensitivity increases with the increase of H2 concentration, while at high concentration, it begins to saturate probably due to the lack of adsorption site for the gases on the Pd NP surface. In order to assess the ability to work as a H2 gas sensor in real situation, the sensor was tested to detect H2 gas in air. This experiment using mixture of H2/O2 is very important to understand the role of H2 gas (reducing gas) and O2 gas (oxidizing gas) in the sensing characteristics of the Pd NP decorated LAO/ STO heterostructure. Figure S4 shows the time-dependent current response of the Pd NP decorated LAO/STO sensor exposed to various concentrations (10, 40, 380 and 3500 ppm) of H2 balanced in synthetic air at 80 °C, while oxygen gas was used
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for recovery after each H2/air measurement. It is apparent that with the increase of H2 gas concentration in the H2/O2 gas mixture, the current of the device increases quickly. This result indicates that, even the surface of Pd NPs are covered by oxygen species, the presence of very little H2 molecules can still result the change of interfacial carrier density and therefore result interfacial conductivity change. It is of particular interest to show the sensors’ response to the H2 gas for the samples with different LAO thicknesses, a series of samples with different LAO thicknesses (10, 20 and 40 ucs) with the same configuration were tested. Figure S5 shows the response curve of the sensors to different hydrogen concentrations at room temperature and 80 °C. All the samples exhibit response to hydrogen gas and the response tend to saturate at high hydrogen concentration. Smaller and slower gas response for thicker LAO thickness heterostructure has been observed
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COMMUNICATION Figure 3. Proposed mechanism for H2 gas sensing process: Process 1: The oxygen molecules are adsorbed on the Pd NPs and attract electrons from the 2DEG, making the Pd NPs negatively charged, which decrease the charge carrier concentration of the interface; Process 2: (i) Pd NPs changes to palladium hydride (PdHx) after the exposure to hydrogen gas and apparently decrease the work function of Pd results lower in the barrier height of the interface. (ii) The atomic hydrogen reacts with the oxygen molecules adsorbed on the Pd NPs surface, the hydrogen molecules reacts with the adsorbed oxygen species as, 2H2 + O2− ( ad) → 2H2O + e − , the released electrons thus enhance the conductivity of the interfaces.
compared to the sample with 5 uc LAO thickness. We believe that the thicker LAO film reduces the field effect and hinders the diffusion of charge carrier to the Pd NPs with increase in diffusion length, resulting in a lower sensitivity to H2 for the heterostructure. We propose the following model (as shown in Figure 3) to explain the enhanced sensitivity to gases for the Pd NPs decorated LAO/STO heterostructure. Palladium has been used in sensing applications due to its high diffusion coefficient, solubility and selectivity with respect to hydrogen.[28] As shown in the diagram, direct adsorption of the gaseous molecules or the following by-products formed by the reaction on the Pd NP surface occurs on top of the LAO layer. The presence of Pd NPs results in charge coupling and exchange with the oxygen deficient LAO film which facilitates the interfacial redox reaction of oxygen,[29–31] where the sensing mechanism can be explained in terms of oxidizing/reducing gas effect. Therefore, the surface is active and promotes further adsorption of oxygen from the atmosphere due to the presence of Pd. The surface of the Pd contains a number of oxygen related physisorption species such as O2− and O− which “spillover” on the LAO surface. On the other hand, when the heterostructure is exposed to hydrogen gas, hydrogen acts as reducing gas which decreases the concentration of oxygen species on the surface and eventually increases the concentration of charge carrier at the interface. Electrically, the work function of the Pd metal relative to the electron affinity of STO determines the transfer direction of electrons between the 2DEG to the Pd NPs. The presence of Pd metal on the surface with work function 5.6 eV is larger than the electron affinity of STO (4.0 eV), thus strong suppression of 2DEG carrier densities at the LAO/STO interface occurs due to the reduction of internal built-in electric field in the LAO layer; similar phenomenon has been observed based on the first principle calculation in the LAO/STO system.[32,33] As reported, the hydrogen sensing response of the semiconductor based gas sensors with noble metal electrodes is related to Schottky
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contact between the noble metal electrode and the heterostructure.[22,34,35] The hydrogen molecules may be adsorbed and dissociated into hydrogen atoms on Pd surface and dissolve into the Pd bulk; and consequently the Pd nanoparticles change to palladium hydride (PdHx) that apparently decrease the work function of Pd and lowers the barrier height, resulting in less suppression of the charge carrier at the interface, and therefore, fewer electrons from the interface may transfer to (PdHx) and subsequently increases the conductance of the interface.[36,37] In addition to this Pd NP electron affinity induced modulation of the interfacial conductivity, the Pd NPs catalytic electrochemical reaction with oxygen molecules and the consequence of charge exchange with the LAO film is believed to play a more important role in reducing the charge carrier density of the LAO/STO 2DEG. Pd NP’s surface is usually attached with dissociated oxygen molecules when exposed to air.[38] We have demonstrated that the electrical properties of the Pd NP decorated LAO/STO heterostructure are strongly affected by the gas environment, especially oxygen, and the conductance of the interface decreases when exposing the device in oxygen ambient.[39] The Pd NPs act as catalysis activates the dissociation of molecular oxygen,[40] and therefore, the presence of ambient oxygen has a considerable influence on the performance of Pd NP decorated LAO/STO heterostructure. When the sensor is exposed to H2 gas, the hydrogen molecules will react with the adsorbed oxygen species as, 2H2 + O2− ( ad ) → 2H2O + e − , and the released electrons will enhance the conductivity of the interfaces. Afterwards, when air is introduced to the device, oxygen molecules in air reacts with the surface adsorbed hydrogen atoms to form H2O and evaporate due to the exothermic reaction. This reaction process is exothermic and the produced H2O molecules desorbed quickly from the surface. A depletion region at the interface will be rebuilt by the adsorbed oxygen species on the Pd surface and results in O2− or O− formation. To study the importance of the presence of oxygen species to the sensing response, three carrier gases, synthetic air (∼20% oxygen
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balanced in 80% argon gas), pure oxygen and argon, were selected to study the behavior of recovery process after the sensor was exposed to hydrogen. As shown in Figure S6, the time variation of the interface’s resistance has been measured for the sensor exposed to different carrier gases for the recovery process. Compared to the case where the sample recovers in synthetic air and pure oxygen, the resistance of the interface differs from ∼21%, and there is little or no changes in resistance observed for recovery process conducted in argon gas, demonstrating the key role of oxygen adsorbed on the surface of the Pd NP decorated LAO/STO heterostructure for the sensing process. It should be noted that the type of chemisorbed oxygen species depends strongly on temperature, where a relatively higher temperature favors the redox reaction, and the interface will become more sensitive for H2 sensing. It may be argued that the hydrogen sensitivity for the Pd NP coated LAO/STO may be due to the volume expansion of the Pd NPs lattice. For bulk palladium, a lattice expansion of as high 3–4% has been reported[41] and the hydrogen sensing mechanism has been explained by the tunneling current between the Pd NPs. To rule out this effect, Pd NPs were also deposited on the TiO2 site terminated STO substrate (without LAO), and the results show that the Pd NPs are well below the conduction percolation threshold in hydrogen ambient. The working mechanism of the gas sensor lies in the conversion of electrical conductivity due to surface reactions such as oxidation or reduction that caused by different gas exposure. Oxidizing (reducing) gases can serve as electrons withdrawing (donating) groups and change the channel carrier concentration through charge coupling with the interface; while the adsorbents from the ambient gas molecules interact strongly with the Pd NP decorated LAO/STO heterostructure. In addition to H2 gas, other reducing gases such as ethanol, acetone and water vapour were also tested. The sensor was put in the argon ambient to obtain the base line and a fixed amount of the chemical vapour was flowed to the sensor for sensitivity test. As shown in Figures 4a and 4b, the sensitivities to acetone and ethanol gases reach ∼64% and 48%, respectively; while the response time are τ R,acetone ~ 25 min and τ R,ethanol ~ 8 min , respectively, for e and ethanol gases. We believe that this gas sensing characteristic is due to the exchange of electrons between ionosorbed species and the nanoparticles as well as the LAO film. The reaction between the ethanol and acetone with ionic oxygen species can be respectively described by CH3 CH2 OH + 60 − → 2CO2 + 3H2 O + 6e − and CH3 COCH3 + O− → CH3 CO+ CH2 + OH− + e − . The reductive gas reduces the molecular oxygen on top of the Pd NPs surface, and thus,
the charge carrier concentration at the interface increases and resumes to a stable state. The behavior of the device to water vapour is similar to acetone and ethanol, however, much higher sensitivity and shorter response time are observed. In Figure 4c, the conductance of the Pd NP decorated LAO/ STO heterostructure increases with sensitivity up to ∼8000% after exposed to H2O at room temperature. The H2O should remove adsorbed oxygen on the LAO surface. As mentioned by Xie,[17] H2O are known to alter the charge carrier density of the LAO/STO interface, and sensing of the H2O molecules with reasonable value of sensitivity, slow response and recovery have been reported. H2O molecule is proposed as the major polar gases in the air which can change the interface conductance. One probable reaction that the water vapor gets dissociated on the Pd surface give rise to H+ and OH− ions, i.e., H2O ↔ H+ + ( OH )− . First principle calculation shows that H2O binds strongly to the AlO2 outer surface[42,43] and modulate the conductivity at the interface. However, as dry air is fed in for recovery process, the recovery rate is much slower than the case in acetone and ethanol. Water molecules adsorbed on the Pd NPs surface with slower evaporation rate lead to fewer surface adsorption site for chemisorptions of oxygen species, results in a slower recovery rate. In summary, we have demonstrated the highly sensitive and room temperature workable gas sensing properties of Pd NP decorated LAO/STO heterostructure. As gas sensing device, this structure has high selectivity to oxidization and reduction gases. The outstanding gas sensing properties are due to the Pd NPs catalytic effect to different gases. With the charge coupling between the gas molecules and Pd NPs as well as the interface through either a direct charge exchange or change of electron affinity. These results enrich the physics in the LAO/ STO 2DEG system and opens up the possibility in a wide range of molecular sensing applications.
Experimental Section Sample Preparation: LAO films with 5, 10, 15 and 20 unit cells thicknesses were deposited on TiO2-terminated SrTiO3 (001) substrate with size of 5 × 5 mm2 by laser molecular beam epitaxy (Lambda Physik COMPex 205, wavelength = 248 nm). The deposition temperature was maintained at 750 °C with base vacuum lower than 2 × 10 −5 Pa and the growth was monitored by reflective high energy electron diffraction (RHEED). The sample was in-situ annealed at a reduced temperature of 550 °C at 1000 Pa and then cooled down to room temperature in the same ambient. Palladium nanoparticles were deposited on LAO/ STO surface using d.c. magnetron sputtering under a power of 15 W
Figure 4. The real time – sensitivity response of the Pd nanoparticle decorated LAO/STO heterostructure to (a) acetone, (b) ethanol and (c) water vapor in argon ambient at room temperature.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements NYC would like to thank for the support of the Hong Kong Ph.D Fellow Scheme from the Research Grants Council of Hong Kong No.RUY3. This work was supported by the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900, GRF grant No.514512 and the PolyU internal grant No. G-YJ169. Received: April 9, 2014 Revised: May 29, 2014 Published online: July 10, 2014
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with argon gas (99.995%) under a flow rate of 10 sccm at a pressure of 100 mTorr, and the time of deposition was adjusted to make sure that the Pd NPs do not form into complete conducting paths on the LAO/ STO surface. Electrical Contact: The conducting interface was contacted by ultrasonic bonding with Al wire. Hall measurement and sheet resistance measurements were carried out to illustrate the 2DEG nature of the LAO/STO interface. To test for the possibility of parallel conductivity through the Pd nanoparticles itself, contacts were made directly to the LAO surface. No significant surface conduction was detected, indicating that the Pd nanoparticles were well below the conduction percolation threshold and clearly demonstrating that the conduction is dominated by the LAO/STO interface. Synthetic Gas Measurement: The synthetic test gas mixtures were supplied from a gas mixing manifold and the sensors were enclosed in a stainless steel chamber. The sample was placed in the stainless steel chamber. For cyclic mode, the measurement chamber was first filled with synthetic air at 1 atmospheric pressure. The gas inside the measurement chamber was then evacuated using a rotary pump. The H2 gases with different concentration were balanced in argon and synthetic air were used for the recovery process. All the gases were dry with relative humidity of the testing environment effectively zero. The ethanol, acetone, and water vapour sensing performance was measured by using the static gas distribution method. Briefly, a fixed volume of liquid ∼1 mL was injected onto the heating platform in the test chamber to prepare the ethanol, acetone, water-argon gas mixture. The samples were placed in a small stainless steel chamber with several electrical feed through, gas inlet and gas outlet. All sensing measurements were performed under a dc bias voltage of 2 V.
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Highly Sensitive Gas Sensor by the LaAlO3/SrTiO3 Heterostructure with Pd Nanoparticle Surface Modulation Ngai Yui Chan, Meng Zhao, JianXing Huang, Kit Au, Man Hon Wong, Hei Man Yao, Wei Lu, Yan Chen, Chung Wo Ong, Helen Lai Wa Chan, and Jiyan Dai* Since the discovery of the two-dimensional electron gas (2DEG) at LaAlO3/SrTiO3 (LAO/STO) heterostructure interface, there has been a great attraction for scientists to study the formation mechanism of the 2DEG at oxide interface. More recently, demonstration of potential applications of the LAO/STO heterostructure in electronic devices such as field-effect transistors, sensors and memories is attracting more research interest. In this work, we demonstrate a highly-sensitive hydrogen (H2) gas sensing characteristics of LAO/STO heterostructure with palladium nanoparticle (Pd NP) surface modulation. Pd NPs act as catalysts are accommodated to the surface of LAO/STO 2DEG heterostructure to enhance its gas sensitivity and selectivity. Besides H2 gas, cross sensitivity tests indicate that the sensor shows response to ethanol, acetone and water gas vapour. The gas sensing ability has been attributed to charge coupling between the desorbed gas molecules/Pd NPs and the LAO/STO interface through the LAO layer. These results not only promise the potential interest for understanding the oxide interfacial 2DEG, but also its application in all-oxide devices, thus open a new route to complex oxide physics and ultimately for the design of devices in oxide electronics. The discovery of quasi-two dimensional electron gas at the interface between the LaAlO3 (LAO) and SrTiO3 (STO) heterostructure[1] with intriguing properties such as magnetism,[2] superconductivity,[3] quantum oscillation[4] and photoconductivity[5] have been studied extensively in recent years. The significant results discovered in this system pave a way for potential applications for multifunctional oxide based electronic devices.[6–10] Charge writing experiment by the conducting atomic force microscopy,[11–13] the polarization state of a ferroelectric thin film[14,15] and the existence of polar substance[16–18] on top of the LAO/STO heterostructure revealed that the transport properties at the interface are extremely sensitive to the surface states. The addition of metal elements on the top of
N. Y. Chan, M. Zhao, J. X. Huang, K. Au, M. H. Wong, H. M. Yao, W. Lu, Y. Chen, C. W. Ong, H. L. Wa Chan, Prof. J. Dai Department of Applied Physics The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong, P. R. China E-mail: [email protected] Prof. J. Dai The Hong Kong Polytechnic University Shenzhen Research Institute P. R. China
DOI: 10.1002/adma.201401597
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LAO layer results in significant changes in transport properties at the interface and have been studied by the recent first principle calculation and experimental work.[19,20] Pristine LAO/ STO heterostructure has also been proposed for hydrogen gas sensor based on first principle calculation,[21] but has never been experimentally realized. Motivated by these experimental results and the theoretical prediction, we have fabricated the LAO/STO 2DEG heterostructure, with surface being modulated by palladium (Pd) nanoparticles in order to trigger the LAO/STO heterostructure’s gas sensing ability. The Pd surface modulation method has been utilized to enhance the gas response of some semiconducting systems such as ZnO nanowires, AlGaN/GaN 2DEG heterostructure as well as graphene and carbon nanotube.[22–25] Herein, the surface modified LAO/STO heterostructure is demonstrated to be highly sensitive to different gases. Hydrogen, oxygen, ethanol, acetone and water vapour can be detected via changes in the transport properties of the interface. The LAO/STO heterostructures with LAO thickness of 5, 10, 20 and 40 unit cells (uc) were deposited on TiO2-terminated (001) STO substrate, and the interfacial structure and transport properties of the bare LAO/STO sample are shown in Figures S1 and S2, where typical metallic conduction can be seen (see experimental section of details). Pd nanoparticles (Pd NP) with size around 2 nm were deposited by d.c. magnetron sputtering in pure argon ambient to the surface of LAO/STO heterostructure at room temperature. Figure 1a schematically shows the device structure, and the actual picture of the sample is shown in Figure 1b, where the aluminum wires are ultrasonically bonded at the four corners for electrical characterization. It is apparent that the Pd NPs, which appear as dark spots, are distributed uniformly and densely on the surface of the LAO layer. The Pd NPs are crystallized and the density is not leading to a complete conduction percolation path. The high-resolution image shown in Figure 1c and the ring shaped selected area electron pattern (SAED) shown in Figure 1d indicate the nanocrystalline f.c.c. structure of the Pd NP. The small size and high density distribution of the nanoparticles should enhance the responsivity to the targeted gases.[26,27] Figure 2 shows the hydrogen gas sensing characteristics for the Pd NP decorated LAO/STO heterostructure with thickness of 5 uc at room-temperature and elevated (80 °C) temperature, where it is of particular interest that the conductance of the sample is extremely sensitive to H2 gas. It is noticed that our pristine LAO/STO heterostructure does not show any response to H2 gas as reflected from the current-voltage characteristics as shown in Figure S3, and there is no experimental report either
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COMMUNICATION Figure 1. (a) Schematic diagram of the Pd nanoparticle decorated LAO/STO heterostructure (the surface is a low-magnification TEM image representing the Pd NPs), (b) actual picture of the device, (c) high-resolution TEM image of a single Pd NP and (d) SAED pattern of the Pd NP.
to show the gas sensing ability of such heterostructure. We believe that the reason why H2 gas sensing in LAO/STO 2DEG has not been observed before is due to the low gas reactivity of the LAO surface. The Pd NPs on the LAO surface therefore play a crucial role for the functionalization of H2 gas sensing. It is also worth mention that the response to the gas is an interfacial effect, since the experiment with surface electrode contact without connecting to the interface does not show any measurable conductance and response upon the exposure to hydrogen. For a typical gas sensor, three physical parameters are usually studied for the sensing performance, including the sensitivity S(%), the response time τR and recovery time τS. Among them, S(%) is defined as the relative variation of the current at the interface and can be calculated as S = ΔI0I ( % ), where ΔI = I g − I 0 , I0 is the current of the interface before exposing to hydrogen and Ig is the peak current in presence of hydrogen. The parameters of τR and τS are defined respectively as the time for the 90% of the saturated current of full response and recovery. The transient response of the gas sensor to hydrogen at room temperature is shown in Figure 2a, showing the variation of current of the interface corresponding to the on (for 5 min) and off of the H2 gas flow for two cycles, with 20 ppm H2 gas concentration balanced in argon. The results show that the interfacial current quickly increases when the device is exposed to H2 gas, with τR ∼ 7.3 min, and followed by a slowly approach to saturation. While after the current of the interface saturated, the H2 gas inside the chamber was pumped out and synthetic air was flushed to the sample for the recovery process, the current of the interface decreased and essentially resumed the original state, with τS ∼ 36.7 min. The responses of the Pd NP decorated LAO/STO heterostructure was also recorded in the environment with different hydrogen gas concentrations balanced in argon (from 2 ppm to 14 ppm) at room temperature. The results shown in Figure 2b are plotted as the relationship between the sensitivity and time. A detectable change in current of ∼80% was achieved for the sample with H2 concentration as low as
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∼2 ppm. It is apparent that exposing the sensor to a higher concentration of hydrogen induces faster increase of the current to a higher peak value. These saw-tooth responses are superimposed on a monotonically increasing background; this fact can be attributed to the charge accumulation and slow recovery rate of the sensor which operated at room temperature. Nevertheless, it demonstrated that the Pd NP modulated LAO/STO heterostructure is sensitive to H2 gas. Temperature is an important factor that greatly influences the hydrogen sensing response based on the catalytic effect. Usually, higher temperature would lead to higher sensing performance due to the lowering of activation energy for gas adsorption and desorption of the sensor. Figure 2c shows the cyclic performance of the device with 20 ppm H2 balanced in argon at 80 °C, where the sensor shows both shortened τR ∼ 0.8 min and τS ∼ 0.95 min, which are much faster compared to the sensor operated at room temperature. The shorter response time can be explained by faster diffusion and dissociation of hydrogen molecules at higher temperature, where the surface has higher reactivity and more active adsorption sites for H2 molecules to Pd NP. Figure 2d shows response to H2 gas with different concentrations, with the sensitivity much higher for the device after exposed to H2 gas, a response with sensitivity up to 2400% has been recorded for the device exposed to 2 ppm H2 gas. The faster recovery is probably arose from the faster gas desorption at the Pd NPs surface due to high temperature. As shown in Figure 2e, the response time τR of the Pd NP decorated LAO/STO heterostructure at 80 °C generally tends to be shortened (for [H2 ] = 2 ppm, τ R = 3.2 min and for [H2 ] = 20 ppm, τ R = 0.8 min ). Upon exposure to dry air for the recovery process, dissolved hydrogen on Pd NP reacts with the oxygen in air and forms H2O by the reaction ( 2H2 + O2 = 2H2O). At low working temperature, the response time is longer due to slow desorption of the formed water molecules on the surface. The sensor was able to detect H2 gas with low concentration at high temperature, while it’s relatively less sensitive to
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Figure 2. (a) Electrical response for the Pd NP decorated LAO/STO heterostructure for the loading and de-loading cycle with 20ppm H2 gas balanced in argon at room temperature. (b) Real time response of sensitivity under exposure to H2 gas with various concentrations (2 – 14 ppm) at room temperature. (c) Electrical response and (d) Sensitivity response at 80 °C. (e) The relationship between response time and hydrogen concentration for the sensor operated at 80 °C. (f) Combined sensor’s sensitivity at room temperature and 80 °C.
such a low H2 concentration at room temperature. Figure 2f plots the sensitivities for the sensor to various concentration of H2 gas at room temperature and 80 °C. At low hydrogen gas concentration, the sensitivity increases with the increase of H2 concentration, while at high concentration, it begins to saturate probably due to the lack of adsorption site for the gases on the Pd NP surface. In order to assess the ability to work as a H2 gas sensor in real situation, the sensor was tested to detect H2 gas in air. This experiment using mixture of H2/O2 is very important to understand the role of H2 gas (reducing gas) and O2 gas (oxidizing gas) in the sensing characteristics of the Pd NP decorated LAO/ STO heterostructure. Figure S4 shows the time-dependent current response of the Pd NP decorated LAO/STO sensor exposed to various concentrations (10, 40, 380 and 3500 ppm) of H2 balanced in synthetic air at 80 °C, while oxygen gas was used
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for recovery after each H2/air measurement. It is apparent that with the increase of H2 gas concentration in the H2/O2 gas mixture, the current of the device increases quickly. This result indicates that, even the surface of Pd NPs are covered by oxygen species, the presence of very little H2 molecules can still result the change of interfacial carrier density and therefore result interfacial conductivity change. It is of particular interest to show the sensors’ response to the H2 gas for the samples with different LAO thicknesses, a series of samples with different LAO thicknesses (10, 20 and 40 ucs) with the same configuration were tested. Figure S5 shows the response curve of the sensors to different hydrogen concentrations at room temperature and 80 °C. All the samples exhibit response to hydrogen gas and the response tend to saturate at high hydrogen concentration. Smaller and slower gas response for thicker LAO thickness heterostructure has been observed
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COMMUNICATION Figure 3. Proposed mechanism for H2 gas sensing process: Process 1: The oxygen molecules are adsorbed on the Pd NPs and attract electrons from the 2DEG, making the Pd NPs negatively charged, which decrease the charge carrier concentration of the interface; Process 2: (i) Pd NPs changes to palladium hydride (PdHx) after the exposure to hydrogen gas and apparently decrease the work function of Pd results lower in the barrier height of the interface. (ii) The atomic hydrogen reacts with the oxygen molecules adsorbed on the Pd NPs surface, the hydrogen molecules reacts with the adsorbed oxygen species as, 2H2 + O2− ( ad) → 2H2O + e − , the released electrons thus enhance the conductivity of the interfaces.
compared to the sample with 5 uc LAO thickness. We believe that the thicker LAO film reduces the field effect and hinders the diffusion of charge carrier to the Pd NPs with increase in diffusion length, resulting in a lower sensitivity to H2 for the heterostructure. We propose the following model (as shown in Figure 3) to explain the enhanced sensitivity to gases for the Pd NPs decorated LAO/STO heterostructure. Palladium has been used in sensing applications due to its high diffusion coefficient, solubility and selectivity with respect to hydrogen.[28] As shown in the diagram, direct adsorption of the gaseous molecules or the following by-products formed by the reaction on the Pd NP surface occurs on top of the LAO layer. The presence of Pd NPs results in charge coupling and exchange with the oxygen deficient LAO film which facilitates the interfacial redox reaction of oxygen,[29–31] where the sensing mechanism can be explained in terms of oxidizing/reducing gas effect. Therefore, the surface is active and promotes further adsorption of oxygen from the atmosphere due to the presence of Pd. The surface of the Pd contains a number of oxygen related physisorption species such as O2− and O− which “spillover” on the LAO surface. On the other hand, when the heterostructure is exposed to hydrogen gas, hydrogen acts as reducing gas which decreases the concentration of oxygen species on the surface and eventually increases the concentration of charge carrier at the interface. Electrically, the work function of the Pd metal relative to the electron affinity of STO determines the transfer direction of electrons between the 2DEG to the Pd NPs. The presence of Pd metal on the surface with work function 5.6 eV is larger than the electron affinity of STO (4.0 eV), thus strong suppression of 2DEG carrier densities at the LAO/STO interface occurs due to the reduction of internal built-in electric field in the LAO layer; similar phenomenon has been observed based on the first principle calculation in the LAO/STO system.[32,33] As reported, the hydrogen sensing response of the semiconductor based gas sensors with noble metal electrodes is related to Schottky
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contact between the noble metal electrode and the heterostructure.[22,34,35] The hydrogen molecules may be adsorbed and dissociated into hydrogen atoms on Pd surface and dissolve into the Pd bulk; and consequently the Pd nanoparticles change to palladium hydride (PdHx) that apparently decrease the work function of Pd and lowers the barrier height, resulting in less suppression of the charge carrier at the interface, and therefore, fewer electrons from the interface may transfer to (PdHx) and subsequently increases the conductance of the interface.[36,37] In addition to this Pd NP electron affinity induced modulation of the interfacial conductivity, the Pd NPs catalytic electrochemical reaction with oxygen molecules and the consequence of charge exchange with the LAO film is believed to play a more important role in reducing the charge carrier density of the LAO/STO 2DEG. Pd NP’s surface is usually attached with dissociated oxygen molecules when exposed to air.[38] We have demonstrated that the electrical properties of the Pd NP decorated LAO/STO heterostructure are strongly affected by the gas environment, especially oxygen, and the conductance of the interface decreases when exposing the device in oxygen ambient.[39] The Pd NPs act as catalysis activates the dissociation of molecular oxygen,[40] and therefore, the presence of ambient oxygen has a considerable influence on the performance of Pd NP decorated LAO/STO heterostructure. When the sensor is exposed to H2 gas, the hydrogen molecules will react with the adsorbed oxygen species as, 2H2 + O2− ( ad ) → 2H2O + e − , and the released electrons will enhance the conductivity of the interfaces. Afterwards, when air is introduced to the device, oxygen molecules in air reacts with the surface adsorbed hydrogen atoms to form H2O and evaporate due to the exothermic reaction. This reaction process is exothermic and the produced H2O molecules desorbed quickly from the surface. A depletion region at the interface will be rebuilt by the adsorbed oxygen species on the Pd surface and results in O2− or O− formation. To study the importance of the presence of oxygen species to the sensing response, three carrier gases, synthetic air (∼20% oxygen
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balanced in 80% argon gas), pure oxygen and argon, were selected to study the behavior of recovery process after the sensor was exposed to hydrogen. As shown in Figure S6, the time variation of the interface’s resistance has been measured for the sensor exposed to different carrier gases for the recovery process. Compared to the case where the sample recovers in synthetic air and pure oxygen, the resistance of the interface differs from ∼21%, and there is little or no changes in resistance observed for recovery process conducted in argon gas, demonstrating the key role of oxygen adsorbed on the surface of the Pd NP decorated LAO/STO heterostructure for the sensing process. It should be noted that the type of chemisorbed oxygen species depends strongly on temperature, where a relatively higher temperature favors the redox reaction, and the interface will become more sensitive for H2 sensing. It may be argued that the hydrogen sensitivity for the Pd NP coated LAO/STO may be due to the volume expansion of the Pd NPs lattice. For bulk palladium, a lattice expansion of as high 3–4% has been reported[41] and the hydrogen sensing mechanism has been explained by the tunneling current between the Pd NPs. To rule out this effect, Pd NPs were also deposited on the TiO2 site terminated STO substrate (without LAO), and the results show that the Pd NPs are well below the conduction percolation threshold in hydrogen ambient. The working mechanism of the gas sensor lies in the conversion of electrical conductivity due to surface reactions such as oxidation or reduction that caused by different gas exposure. Oxidizing (reducing) gases can serve as electrons withdrawing (donating) groups and change the channel carrier concentration through charge coupling with the interface; while the adsorbents from the ambient gas molecules interact strongly with the Pd NP decorated LAO/STO heterostructure. In addition to H2 gas, other reducing gases such as ethanol, acetone and water vapour were also tested. The sensor was put in the argon ambient to obtain the base line and a fixed amount of the chemical vapour was flowed to the sensor for sensitivity test. As shown in Figures 4a and 4b, the sensitivities to acetone and ethanol gases reach ∼64% and 48%, respectively; while the response time are τ R,acetone ~ 25 min and τ R,ethanol ~ 8 min , respectively, for e and ethanol gases. We believe that this gas sensing characteristic is due to the exchange of electrons between ionosorbed species and the nanoparticles as well as the LAO film. The reaction between the ethanol and acetone with ionic oxygen species can be respectively described by CH3 CH2 OH + 60 − → 2CO2 + 3H2 O + 6e − and CH3 COCH3 + O− → CH3 CO+ CH2 + OH− + e − . The reductive gas reduces the molecular oxygen on top of the Pd NPs surface, and thus,
the charge carrier concentration at the interface increases and resumes to a stable state. The behavior of the device to water vapour is similar to acetone and ethanol, however, much higher sensitivity and shorter response time are observed. In Figure 4c, the conductance of the Pd NP decorated LAO/ STO heterostructure increases with sensitivity up to ∼8000% after exposed to H2O at room temperature. The H2O should remove adsorbed oxygen on the LAO surface. As mentioned by Xie,[17] H2O are known to alter the charge carrier density of the LAO/STO interface, and sensing of the H2O molecules with reasonable value of sensitivity, slow response and recovery have been reported. H2O molecule is proposed as the major polar gases in the air which can change the interface conductance. One probable reaction that the water vapor gets dissociated on the Pd surface give rise to H+ and OH− ions, i.e., H2O ↔ H+ + ( OH )− . First principle calculation shows that H2O binds strongly to the AlO2 outer surface[42,43] and modulate the conductivity at the interface. However, as dry air is fed in for recovery process, the recovery rate is much slower than the case in acetone and ethanol. Water molecules adsorbed on the Pd NPs surface with slower evaporation rate lead to fewer surface adsorption site for chemisorptions of oxygen species, results in a slower recovery rate. In summary, we have demonstrated the highly sensitive and room temperature workable gas sensing properties of Pd NP decorated LAO/STO heterostructure. As gas sensing device, this structure has high selectivity to oxidization and reduction gases. The outstanding gas sensing properties are due to the Pd NPs catalytic effect to different gases. With the charge coupling between the gas molecules and Pd NPs as well as the interface through either a direct charge exchange or change of electron affinity. These results enrich the physics in the LAO/ STO 2DEG system and opens up the possibility in a wide range of molecular sensing applications.
Experimental Section Sample Preparation: LAO films with 5, 10, 15 and 20 unit cells thicknesses were deposited on TiO2-terminated SrTiO3 (001) substrate with size of 5 × 5 mm2 by laser molecular beam epitaxy (Lambda Physik COMPex 205, wavelength = 248 nm). The deposition temperature was maintained at 750 °C with base vacuum lower than 2 × 10 −5 Pa and the growth was monitored by reflective high energy electron diffraction (RHEED). The sample was in-situ annealed at a reduced temperature of 550 °C at 1000 Pa and then cooled down to room temperature in the same ambient. Palladium nanoparticles were deposited on LAO/ STO surface using d.c. magnetron sputtering under a power of 15 W
Figure 4. The real time – sensitivity response of the Pd nanoparticle decorated LAO/STO heterostructure to (a) acetone, (b) ethanol and (c) water vapor in argon ambient at room temperature.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements NYC would like to thank for the support of the Hong Kong Ph.D Fellow Scheme from the Research Grants Council of Hong Kong No.RUY3. This work was supported by the National key Basic Research Program of China (973 Program) under Grant No. 2013CB632900, GRF grant No.514512 and the PolyU internal grant No. G-YJ169. Received: April 9, 2014 Revised: May 29, 2014 Published online: July 10, 2014
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with argon gas (99.995%) under a flow rate of 10 sccm at a pressure of 100 mTorr, and the time of deposition was adjusted to make sure that the Pd NPs do not form into complete conducting paths on the LAO/ STO surface. Electrical Contact: The conducting interface was contacted by ultrasonic bonding with Al wire. Hall measurement and sheet resistance measurements were carried out to illustrate the 2DEG nature of the LAO/STO interface. To test for the possibility of parallel conductivity through the Pd nanoparticles itself, contacts were made directly to the LAO surface. No significant surface conduction was detected, indicating that the Pd nanoparticles were well below the conduction percolation threshold and clearly demonstrating that the conduction is dominated by the LAO/STO interface. Synthetic Gas Measurement: The synthetic test gas mixtures were supplied from a gas mixing manifold and the sensors were enclosed in a stainless steel chamber. The sample was placed in the stainless steel chamber. For cyclic mode, the measurement chamber was first filled with synthetic air at 1 atmospheric pressure. The gas inside the measurement chamber was then evacuated using a rotary pump. The H2 gases with different concentration were balanced in argon and synthetic air were used for the recovery process. All the gases were dry with relative humidity of the testing environment effectively zero. The ethanol, acetone, and water vapour sensing performance was measured by using the static gas distribution method. Briefly, a fixed volume of liquid ∼1 mL was injected onto the heating platform in the test chamber to prepare the ethanol, acetone, water-argon gas mixture. The samples were placed in a small stainless steel chamber with several electrical feed through, gas inlet and gas outlet. All sensing measurements were performed under a dc bias voltage of 2 V.
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