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. 2013 Feb 13;3:1250. doi: 10.1038/srep01250

Resonant tunneling modulation in quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure for room temperature H2S sensor application

Guangliang Cui 1, Mingzhe Zhang 1,a, Guangtian Zou 1
PMCID: PMC3570779  PMID: 23409241

Abstract

Heterostructure material that acts as resonant tunneling system is a major scientific challenge in applied physics. Herein, we report a resonant tunneling system, quasi-2D Cu2O/SnO2 p-n heterostructure multi-layer film, prepared by electrochemical deposition in a quasi-2D ultra-thin liquid layer. By applying a special half-sine deposition potential across the electrodes, Cu2O and SnO2 selectively and periodically deposited according to their reduction potentials. The as-prepared heterostructure film displays excellent sensitivity to H2S at room temperature due to the resonant tunneling modulation. Furthermore, it is found that the laser illumination could enhance the gas response, and the mechanism with laser illumination is discussed. It is the first report on gas sensing application of resonant tunneling modulation. Hence, heterostructure material act as resonant tunneling system is believed to be an ideal candidate for further improvement of room temperature gas sensing.

Recent developments of nanoscience of effectively combining two or more chemically distinct components in one single nanostructure with multifunction or new properties induced by the heterointerfaces have led to revolutionary new applications of nanomaterials in various areas, such as catalysis, photovoltaic devices, sensors, and so on1,2,3. In particular, enormous efforts have been devoted to preparing heterostructured materials, including heterojunction nanocrystals, heterojunction nanowires and heterojunction arrays4,5,6. Heterojunction nanomaterials with multi-layer structure have multi-barrier structures which lead to periodic accumulation of carriers and control of their transport properties. They play an important role in modern device physics and have many practical applications7,8. One of such applications is to improve optical detection and gas sensing.

Conventional gas-sensing materials are used based on the change of carrier concentration caused by adsorption and desorption9, which are functions of elevated operating temperature. Many of them do not have gas sensitivity at room temperature10. As a result, gas sensors often require a heating device to operate. Unfortunately, high temperature is harmful to the durability of sensors11. There have been considerable efforts to overcome the operating temperature limitation12, such as using nanostructured materials with ultra-high surface-to-volume ratios, appropriate element doping, and surface decoration with noble metals13,14,15,16. Nevertheless, production of sensors operating at room temperature with high sensitivity, high selectivity, and low power consumption at nanowatt (nW) level remains a challenging task.

P-type Cu2O17 and N-type SnO218 have been widely chosen as sensing materials in the last ten years. Their nanoscale materials, such as nanoparticle, nanowire and nanostructured film, have been investigated extensively for gas sensing applications19,20,21. With their small size, unique electrical properties, and high surface-to-volume ratios, they show excellent gas sensitivity at high temperatures. To improve the performance, we propose a Cu2O/SnO2 p-n heterostructure system. It is quasi-2D, having horizontal-multi-layer with strictly periodic structure in hundreds of microns composed of parallel one-dimensional Cu2O and SnO2 wire stripes. Different from previous reported heterojunction materials based on Cu2O/CuO and SnO222,23,24, our synthesized material can be considered as a resonant tunneling system with its application based on resonant tunneling modulation.

It is still hard to synthesize quasi-2D horizontal-multi-layer heterostructure with strictly periodic arrangement in hundreds of microns without using a template. Electrochemical deposition is an excellent method for synthesizing nanomaterials because the driving force of reaction is precisely controllable25. It is a good template-free approach to produce well-ordered nano/microstructure patterns26,27, as we will show in the following discussion.

Results

We have made a quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure using selective deposition by applying a periodic potential in quasi-2D ultra-thin liquid layer. The selective deposition was achieved by applying a half-sine wave potential to the system and utilizing different deposition potentials of Cu2O and SnO2 (Figure 1b). The frequency of the half-sine wave was 0.5 Hz and the voltage varied from 0.3 to 0.9 V. The growth velocity was closely related to the concentration of electrolyte and applied potential28. Its cyclic growth pattern was caused by the variation of ion concentration near the growth front lagging behind the variation of electrode potential29.

Figure 1. Process of electrochemical deposition.

Figure 1

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(a) Schematic diagrams showing the electrochemical deposition process. (b) Half-sine waves applied across the electrodes in the electrochemical deposition process.

In the deposition process, hydroxide was first formed on the front of the deposit due to its lowest deposition potential. The reaction can be written as equation (1). Hydroxide was involved in the whole deposition process of Cu2O and SnO2 as the source of oxygen. Cu2+ and Sn2+ ions were driven by the electric field to the cathode, where they were reduced and deposited on the surface as adatoms. Cu2O and SnO2 nuclei started to nucleate, grow and assemble alternatively according to the periodic half-sine wave (Figure 1a), giving rise to crystallite agglomerates. They were selectively deposited at lower and upper parts of the half-sine wave, respectively. At the lower part of the half-sine wave, only Cu2+ can be deposited, as described in equation (2). During this deposition process, Cu2+ could migrate to the growth front in time as the deposition process was relatively slow. Therefore, the formed nanocrystals were large in size and easy to accumulate. As the deposition proceeds, the Cu2+ ions near the growth front were fully consumed, leaving Sn2+ ions in the growth interface. The accumulation of Sn2+ ions in the growth interface blocked the Cu2+ ions, eventually leading to a halt in the Cu2O formation. Then, Sn2+ ions began to deposit when the voltage increased to large enough value, as described by equation (3). As the deposition process was relatively fast with high applied potential, Sn2+ could not migrate to the growth front rapidly enough to keep up with the rate of deposition, thus the formed nanocrystals were small in size with mild accumulation. Finally, the Sn2+ ions near the growth front were fully consumed, Cu2+ ions began to deposit in turn. This periodic deposition process led to the formation of ridgelike wires of Cu2O and membrane stripes of SnO2, alternating periodically.

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Scanning electron micro-scope (SEM) images show a wavelike morphology of quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure (Figure 2a and Figure 2b). This multi-layer heterostructure shows a strictly periodic arrangement in hundreds of microns. Every cycle is composed of ridgelike wire and membrane stripe, which are symmetrical and well-ordered. The ridgelike wires are narrow, only about a quarter width of a cycle. Energy-dispersive X-ray spectroscopy (EDX) equipped on the transmission electron micro-scope (TEM) was used to measure both the composition profile of Cu and Sn in one cycle across the heterojunction (Figure 2c and Figure 2d). The line-profile analysis indicates that the Cu and Sn atoms are mainly distributed respectively at ridgelike wire and membrane stripe. The fluctuation of total signal intensity of Cu and Sn also reflects the cyclical variation in thickness of the heterostructure. The selective area electron diffraction (SAED) patterns of ridgelike wire and membrane stripe show polycrystalline rings corresponding to Cu2O and SnO2, respectively (Figure 2e and Figure 2g). No other components can be found. The result is consistent with the line-profile analysis. Figure 2f and Figure 2h are high-resolution transmission electron microscopy (HRTEM) images of SnO2 and Cu2O nanocrystals. The spacings of the fringes were measured to be 0.335 nm for SnO2 and 0.246 nm for Cu2O, corresponding to the (110) plane of SnO2 and the (111) plane of Cu2O. Moreover, these HRTEM images prove that the Cu2O nanocrystal is significantly larger than SnO2 nanocrystal.

Figure 2. Morphology and structure of quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure.

Figure 2

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(a) and (b): SEM images at low and high magnifications showing the wavelike morphology of quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure. (c) and (d): Line-profile analysis across the heterojunction as well as the distribution of Cu and Sn in one cycle. (e) and (g): SAED patterns of ridgelike wire and membrane stripes, which are corresponding to Cu2O and SnO2 respectively. (f) and (h): HRTEM images of SnO2 and Cu2O nanocrystals. The spacings of the fringes were measured to be 0.335 nm for SnO2 and 0.246 nm for Cu2O.

In addition, X-ray photoelectron spectroscopy (XPS) was carried out to further confirm the presence of Cu2O and SnO2 (Figure 3a and Figure 3b). The distinct peak of 932.3 eV is assigned to Cu 2p core level, which can be used as a fingerprint to identify the presence of Cu+. The peak with binding energy of 487.2 eV is consistent with Sn 3d core level spectrum, showing that stannum is in the form of Sn4+ state. No peaks of other chemical state of Cu and Sn are observed, indicating the high purity of this product. Both Cu2O and SnO2 are gas-sensing materials19,20,21 with different behaviors to gases, and are conducive to distinction of different gases. The p-n heterojunction is the best choice because its barrier height is higher than that of other heterojunctions30,31, which means a larger range of modulation. Both Cu2O and SnO2 are in the same plane, and both of them can interact with target gases and light illumination directly.

Figure 3. Sn 3d and Cu 2p XPS spectra and temperature-dependent conductivity of quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure.

Figure 3

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(a) and (b): Curve-fitting results of the Sn 3d and Cu 2p XPS spectra. (c) The schematic diagram of typical sensor. (d) I–V curves of the Cu2O/SnO2 p-n horizontal-multi-layer heterostructure at various temperatures.

Discussion

The conductivity of quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure was characterized by electrical measurements at various temperatures. The circuit schematic diagram and I-V curves are shown in Figure 3c and 3d, respectively. The investigation show that the conductivity increased first and then decreased as the temperature rising from 10 K to 260 K, with a maximum at 110 K. The enhanced conductivity was due to increased carrier concentration with increasing temperature. However, p-n heterojunction barrier height increased much more rapidly with increasing temperature32, and ultimately controls the conductivity of p-n multi-layer heterostructure, leading to the decrease of conductivity. Hence, the conductivity of Cu2O/SnO2 p-n horizontal-multi-layer heterostructure is mainly controlled by its barrier height near room temperature.

The special structure of the quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure determines its excellent gas sensing performance, especially for H2S at room temperature. The I–V curves of a typical sensor in air and 50 ppm H2S in air exhibit obvious nonlinear characteristics at room temperature, indicating the presence of multi-barrier (Figure 4a). Notably, the current in air is in the range of 10−8 A, which is low and enhances the sensitivity of the sensor33. The low conductivity is mainly attributed to the superimposed effect of the multi-barrier. The conductivity increased obviously when the sensor was exposed to 50 ppm H2S. Our multi-layer heterostructure shows a reversible resistance change (ΔR) relative to the original value (R) over many cycles. As shown in Figure 4b, the response of sensor to 50 ppm H2S can reach about 45% at room temperature. While the sensor responses to 50 ppm liquefied petroleum gas (LPG), NH3, NO, H2 and toluene in air are much weaker. Their I–V curves at room temperature are shown in Supplementary Figure S2. The superior sensitivity (ΔR/R) and selectivity of the sensor to H2S are evident from Figure 4b. Figure 4c shows dynamic responses of the sensor to 50 ppm H2S in air at room temperature. Upon exposure to the H2S, the sensitivity increased fast, and then reached saturation rapidly. Similarly, the sensitivity sharply reduced as soon as the gas was turned off. It can be observed in the figure 4d that the sensor has a wide detection range for H2S from 0.5 to 100 ppm at room temperature. The response increases linearly with increasing H2S concentration between 0.5 and 100 ppm. Above 100 ppm, the response has no significant change, indicating that the response becomes saturated. Notably, the sensor has reasonable response to 0.5 ppm H2S in air, so the detection limit of the sensor can reach as little as sub-ppm to H2S at room temperature. The excellent sensitivity to sub-ppm concentration H2S is due to the advanced tunneling modulation mechanism, which makes our sensor highly successful comparing with other sensors.

Figure 4. Performance of typical sensor based on the quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure at room temperature.

Figure 4

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(a) I–V curves of the sensor to 50 ppm H2S in air. (b) The sensitivities of the sensor to 50 ppm LPG, NH3, H2S, NO, H2 and toluene in air. (c) Dynamic responses of the sensor to 50 ppm H2S in air. (d) The H2S concentration dependence of the sensor.

The sensitivity of gas sensing materials depends on the extent of adsorption and desorption, while the response and recovery time depend on the speed of adsorption and desorption processes. Although the room temperature sensitivity of the sensor is greatly improved, the response and recovery behaviors are not quite optimized. Considering the above situation, we introduce laser illumination. As shown in Figure 5a, under 50 ppm H2S condition, the conductivity of the sensor with laser illumination exhibits an obvious increase. Figure 5b shows the response of sensor to 50 ppm H2S in air at room temperature is improved by almost 20% (from 45% to 65.1%) with laser illumination, while no obvious improvement are observed to other gases (Supplementary Table S1 and Figure S1). The I–V curves of the sensor to 50 ppm LPG, NH3, NO, H2 and toluene in air with laser illumination at room temperature were shown in Supplementary Figure S3. Moreover, the response and recovery time are significantly reduced from 180 to 160 s and from 500 to 240 s compared with that of no laser illumination, as shown in Figure 5c. This suggests that the speed of adsorption and desorption processes are accelerated under laser illumination. It can be found in the figure 5d that the sensor has a relative narrow detection range for H2S from 0.5 to 50 ppm with laser illumination at room temperature. The response increases linearly with increasing H2S concentration below 50 ppm. Above 50 ppm, the response has no significant change, indicating that the response becomes saturated. The response ceiling of the sensor is brought forward from 100 to 50 ppm with laser illumination (Supplementary Figure S4). This demonstrates that the adsorption is enhanced by the laser illumination. In summary, the sensor has excellent performance for sensing H2S at room temperature, and it could be further improved with laser illumination.

Figure 5. Performance of typical sensor based on the quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure with laser illumination (405 nm, 20 m·cm−2) at room temperature.

Figure 5

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(a) I–V curves of the sensor to 50 ppm H2S in air with laser illumination. (b) The sensitivities of the sensor to 50 ppm LPG, NH3, H2S, NO, H2 and toluene in air with laser illumination. (c) Dynamic responses of the sensor to 50 ppm H2S in air with laser illumination. (d) The H2S concentration dependence of the sensor with laser illumination.

The quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure is a multi-barrier system, where carriers transport by resonant tunneling. When it is exposed to H2S, part of the absorbed oxygen atoms on the surface of Cu2O and SnO2 are removed, leaving oxygen-bound electrons in the surface. As a result, the free electron concentration of SnO2 increased, while the hole concentration of Cu2O decreased. Because of the reduction of hole concentration, the carrier diffusion between SnO2 and Cu2O reduced, leading to narrower depletion layer of the p-n heterojunction. Finally, the heterojunction barrier decreases for the changed Fermi level. Therefore, electrons have a relatively large probability of crossing the multi-barrier by resonant tunneling. Because the adsorption and desorption are functions of operating temperature9, the change of carrier concentration is limited at room temperature. However, the conductivity of Cu2O/SnO2 p-n horizontal-multi-layer heterostructure depends on heterojunction barrier, which is sensitive to the carrier concentration34. Hence, the resonant tunneling modulation requires a low level change of carrier concentration. The resonant tunneling modulation process is very rapid and easy to restore, even at room temperature. Hence, room temperature sensitivity to H2S can be obtained by the application of Cu2O/SnO2 p-n horizontal-multi-layer heterostructure. Further, if the application of resonant tunneling modulation in gas sensing can be implemented, it will make significant advances in room temperature gas sensing. The nano size of Cu2O and SnO2 crystals and wavelike morphology with large surface areas also contribute to the superior room temperature sensitivity.

When the laser illumination is applied, it is necessary to determine whether the sensitivity changes are caused by thermal or nonthermal mechanisms. If it is from the heating of laser illumination, its rate should be a function of energy deposited on the sensor. As shown in Figure 6a, the response of sensor to 50 ppm H2S in air is 45% without laser illumination, and 60.1% under laser illumination with the power of 5 mW·cm−2. However, the response is not significantly increased with the increase of the power. Therefore, the improved gas sensitivity with laser illumination is not caused by the heating effect of laser. Only laser illumination cannot lead to such a substantial increase of conductivity. Conductivity increased slightly only under laser illumination at room temperature (Figure 6b). We believe that the increased sensitivity is due to a synergistic effect of laser illumination and H2S.

Figure 6. Expression of Laser illumination effect to sensor.

Figure 6

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(a) The sensitivities of sensor to 50 ppm H2S in air with various laser (405 nm) intensities at room temperature. (b) I–V curves of sensor with and without laser illumination (405 nm, 20 mW·cm−2).

The laser illumination excites the electron-hole pairs on the surface of Cu2O and SnO2, which increases the conductivity of materials, and also modifies the electric field due to charge accumulation near the heterojunction barriers35. The conductivity of Cu2O/SnO2 p-n horizontal-multi-layer heterostructure is improved with laser illumination, which makes the change of conductivity caused by H2S more obvious. More important, laser illumination reduces the energy barrier of desorption36, causing further reduction of heterojunction barrier. Accordingly, resonant tunneling modulation is greatly improved with laser illumination for the enhanced interaction between material and gas. At the same time, the improvement of desorption speed leads to the acceleration of response behavior. All these contribute to the increase of sensitivity. On the other hand, the sensitivity of sensor due to laser illumination doesn't seem to apply to other gases, so the selectivity is retained and improved with laser illumination.

In summary, we fabricate a quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure with strictly periodic arrangement in hundreds of microns which can be considered as a resonant tunneling system. Our work makes a breakthrough in gas-sensing application of resonant tunneling modulation. The resonant tunneling modulation in this system can be achieved at room temperature by H2S irritation. The detection limit of sensor based on this heterostructure system can reach as little as sub-ppm to H2S at room temperature. Compared with the existing room temperature H2S sensors37,38,39,40, our sensors are based on the advanced tunneling modulation mechanism, and have higher sensitivity and selectivity to low concentration (sub-ppm) H2S at room temperature. Moreover, the response and recovery time are much shorter for the agility of resonant tunneling modulation at room temperature. Notably, Laser illumination also improves the sensor performance. Therefore, this kind of p-n multi-layer heterostructure provides a new direction of designing materials for room temperature gas sensing.

Methods

Preparation

The Quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure was synthesized by an electronchemical deposition system in a quasi-2D ultra-thin electrolyte liquid layer. This system consisted of a growth chamber, a low-temperature cycle water bath (Polystat, Cole-parmer, USA), a dc power supply (DF1731SB5A, Zhongce, China), an arbitrary function generator (AFG310, Tektronix, USA), and a CCD camera (A311f, Leica, Germany). The electrolyte was prepared using analytical reagent Cu(NO3)2, SnCl2 and Millipore water. The ion concentrations of Cu2+ and Sn2+ in electrolyte were 0.08 M and 0.02 M respectively. The pH value was adjusted to 2.4 by nitricacid. The Si substrate was first oxidized to form a layer of SiO2 on the surface, and the SiO2 layer was used to protect the electron transport from the substrate. Two parallel Cu foil electrodes (30 μm-thick, 99.9%) were put on the Si substrate, which was placed on the Peltier element in the growth chamber. Then the prepared solution was added dropwise. After that, a cover glass was carefully put on the two electrodes, and ensures the space between the coverglass and the SiO2/Si substrate was filled with electrolyte. The Peltier element and low-temperature cycle water bath were used to control the temperature and solidify the electrolyte. An ultra-thin ice layer could be formed between the silicon substrate and the cover glass by adjusting the temperature of the Peltier element and low-temperature cycle water bath accurately. The solute in the electrolyte was partially expelled from the solid in the solidification process due to the partitioning effect. Eventually two ultra-thin liquid layers of concentrated electrolyte were formed between the ice layer and the lower silicon substrate as well as the ice layer and the higher cover glass separately. The thickness of the ultra-thin electrolyte layers are about 300 nm, and the electrodeposition process was carried at −4.5°C by applying a half-sine wave potential (0.3–0.9 V) with a frequency of 0.5 Hz across the ultra-thin liquid layers. The growth time is about 1 h, during which the real-time growth process can be observed by the optical microscope (Leica Dmlm). When the growth process finished, the coverglass and silicon wafer were taken out, and cleaned by the Millipore water. Finally, Cu2O/SnO2 p-n heterostructure left on the surface of coverglass and Si substrate. The as-prepared quasi-2D Cu2O/SnO2 horizontal-multi-layer heterostructure was examined by field-emission SEM (JSM-6700F, JEOL, Japan), TEM (Tecnai G2, FEI, USA), and XPS (ESCALAB MKII, VG, UK) for morphology, composition, crystallography, and structure characterization, respectively.

Gas sensitivity test

The samples on the cover glass were chosen as the test object. First, the sample was covered by a linear mask with a width of 100 μm and parallel to the heterointerface. Two copper wires were fixed on both sides of the mask separately. Then the Au film was deposited on the glass by vacuum ion sputtering for 300 s. After that, the mask was taken away, and the heterostructure system was connected into the circuit. The target gases with desired concentrations were prepared by static volumetric method. The gas response measurements were carried out in a vacuum test system, including a test chamber, optical platform, laser source and an air pump. The sensor was kept in the test chamber equipped with appropriate optical window, inlets and outlets for laser and target gases flow, and the air and target gases were alternately introduced into the test chamber. During the measurement, the electrical signals were recorded every 30 seconds by a Keithley 2400 sourcemeter (2400, Keithley, USA) and a Digital Phosphor Oscilloscope (TDS5034B, Tektronix, USA) under laser (405 nm) illumination with a low power density of 20 mW·cm−2.

Author Contributions

M.Z.Z. conceived and designed the experiments. G.L.C. performed the experiments. M.Z.Z., G.T.Z. and G.L.C. interpreted the results. All authors co-wrote the manuscript.

Supplementary Material

Supplementary Information

Resonant tunneling modulation in quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure for room temperature H2S sensor application

srep01250-s1.pdf (394.3KB, pdf)

Acknowledgments

This work was funded by the National Science Foundation of China, Nos. 90923032, 11174103 and 20873052.

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Supplementary Materials

Supplementary Information

Resonant tunneling modulation in quasi-2D Cu2O/SnO2 p-n horizontal-multi-layer heterostructure for room temperature H2S sensor application

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