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Ultra-Fast and Ultra-Sensitive Gas Sensors Derived from Large Fermi-Level Shift in Schottky Junction with Sieve-layer Modulation Ching-Cheng Cheng, Chia-Lin Wu, Yu-Ming Liao, and Yang-Fang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03172 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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ACS Applied Materials & Interfaces

Ultra-Fast and Ultra-Sensitive Gas Sensors Derived from Large Fermi-Level Shift in Schottky

Junction

with

Sieve-layer

Modulation Ching-Cheng Cheng, Chia-Lin Wu, Yu-Ming Liao, and Yang-Fang Chen*

Department of Physics, National Taiwan University, Taipei 10617, Taiwan

KEYWORDS: Graphene, oxygen sensor, Schottky junction, fast response, sieve-layer

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ABSTRACT: Gas sensors play an important role in numerous fields, covering a wide range of application, including intelligent systems, detection of harmful and toxic gases. Even though they have attracted a great attention, the response time in the order of seconds to minutes is still very slow. To circumvent the existing problems, here, we provide a seminal attempt with the integration of graphene, semiconductor and an addition sieve layer forming a nanocomposite gas sensor with ultrahigh sensitivity and ultrafast response. The designed sieve layer has a suitable band structure that can serve as a blocking layer to prevent transfer of the charges induced by adsorbed gas molecules into the underlying semiconductor layer. We found that the sensitivity can be reduced to ppm level, and the ultra-fast response around 60 ms is unprecedented compared with published graphene based gas sensors. The achieved high performance can be interpreted well by the large change of the Fermi level of graphene due to its inherent nature of low density of states and the blocking of sieve layer to prevent charges transfer from graphene to the underlying semiconductor layer. Accordingly, our work is very useful and timely for the development of gas sensors with high performance for practical applications.

1. Introduction Gas sensors are ubiquitous in our daily life, which cover a wide range of application, including intelligent systems, greenhouse effects, detection and 2 ACS Paragon Plus Environment

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monitoring of harmful and toxic gases.1-5 A variety of materials such as optical fibers, inorganic semiconductors, conjugated polymers and carbon nanomaterials have been explored to build gas sensors.6-14 Among these materials, graphene gas sensors have recently been studied intensively, mainly due to the atom-thick two-dimensional structure and excellent properties of graphene sheets.15-26 There are several advantages in gas sensing based on graphene nanosheets. First, graphene has a large theoretical specific surface area (2630 m2g-1). All atoms of graphene can be considered as surface atoms and they can provide the largest sensing area per unit volume to adsorb gas molecules.26 Second, the interaction between graphene and adsorbates could vary from weak van der Waals interactions to strong covalent bonding. All of these interactions will affect the electronic system of graphene, which can be readily measured by electronic methods. Third, the charge carriers of graphene have zero rest mass near its Dirac point and graphene has remarkable high carrier mobility at room temperature (200000 cm2 V-1s-1) with a carrier density of ~ 1012 cm2, corresponding to a resistivity of 10-6 Ω.27-29 Furthermore, graphene exhibits inherently low electrical noise due to its high-quality crystal lattice along with its two-dimensional structure, making it capable of screening more charge fluctuation than one-dimensional carbon nanotubes counterparts. As a result, a small amount of extra electrons can cause a noticeable change in the conductance of graphene.30-33 3 ACS Paragon Plus Environment


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However, most gas sensors based on graphene in all the published reports suffer from the very slow response and recovery times, due to charges transfer from graphene to the underlying layer.5, 34-36 There are several designs of gas sensors. Even optical methods for sensing gases with high sensitivity and fast response have been developed37-40, sensing by optical measurements is relatively expensive and inconvenient compared with resistive sensors. Chemiresistor is the most widely used configuration of gas detection. In this case, the gases were detected by measuring the resistance changes of sensing layers affected by adsorbing the gas molecules. Field effect transistors (FETs) have also been used for gas sensing.41 The drain current of a FET can be effectively changed by exposure to the target gas. Among them, it is worth noting that the heterojunction Schottky diodes based on graphene/semiconductor show higher sensitivity compared with the graphene chemiresistor. This happens because the current can change exponentially with changes in Fermi level and Schottky barrier. In addition, semiconductor-based metal oxides have been used for gas sensing since 1962 with high sensitivity, when Seiyama et al. used a thin layer of zinc oxide to detect volatile organic compounds of propane and n-butane.42 However, metal oxide-based gas sensors usually rely on a high working temperature to enhance catalytic properties for gas sensing. Recently, combining metal oxides and graphene, a room temperature 4 ACS Paragon Plus Environment

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oxygen sensor was developed with an enhanced sensing performance by UV light.43 However, the reported sensors still possess the drawback of slow response time. We have summarized many parameters of outstanding graphene hybrid based gas sensors in Table 1.5, 43-47 It is clear that the response time of all reported sensors is quite slow. To circumvent the difficulties described above, here, we provide a seminal attempt with the integration of graphene, semiconductor and an addition sieve layer forming a nanocomposite gas sensor with ultrahigh sensitivity and ultrafast time response. The designed sieve layer has a suitable band structure that can serve as a blocking layer to prevent transfer of the charges induced by adsorbed gas molecules into the underlying semiconductor layer. Besides, the thickness of the sieve layer has to be thin enough to allow the current flow in the graphene/semiconductor Schottky junction. Because graphene is a two-dimensional sheet with low density of states, a small amount of charge carriers can generate a large variation of Fermi energy. As a consequence, the exponential change of the current through the barrier height of the Schottky diode can be greatly enhanced. Furthermore, since there is no charge transfer between graphene and the underlying semiconductor layer during the gas detection, the response and recovery times therefore can be extremely fast.

2. Experimental Section



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To illustrate our working principle, the gas sensor consisting of graphene, ZnO sieve layer, heavily-doped p-type silicon substrate has been fabricated and demonstrated. The Graphene/ZnO/Silicon heterojunction devices were fabricated on commercially purchased highly p-doped (resistivity of 0.1 Ω-cm) Si wafers with 300 nm SiO2 layer. First, these wafers were patterned by photolithography and etching in buffered oxide etch solution (6:1 volume ratio of 40% NH4F in water to 49% HF in water) to create a window to expose the p-doped silicon. Second, a thermal evaporation deposition technique was used to deposit Ti/Au (5nm/100nm) film contact pads on top of 300 nm SiO2 layer as well as on the corner of the exposed surface of Si wafer. Third, a RF magnetron sputtering deposition technique was used to deposit ZnO film on the exposed silicon window. The graphene films were grown at 1000 ° C by using a mixed gas of CH4 (40 sccm) and H2 (20 sccm) via a CVD method in which 25 µm thick Cu foils were employed as the catalytic substrates. Forth, to transfer graphene films onto the window, the as-grown graphene films (on metal foils) were spin-coated with PMMA, and the metal foils were dissolved in a dilute FeCl3 solution. The PMMA-coated graphene films were then transferred onto the top of the exposed window of ZnO/Si. After that, the devices were thoroughly rinsed with acetone and isopropanol to remove the PMMA, and dried.


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Two measurement systems (Keithley 236 and Agilent 4155C) were utilized to supply the dc voltage and to record the current curves, such as the current-voltage (I-V) and current-time (I-T) and FET characteristics. In order to reveal the electrical characteristics under different gas concentrations, a home build gas measurement system was utilized. This system contained two chambers: a mixing chamber and a testing chamber. The mixing chamber was used to mixing gases, while testing chamber was used to record electrical characteristics. All these chambers were connected to a mechanical vacuum pump and could be evacuated. A check valve was connected to the testing chamber for discharging excess pressure above 1 atm in the testing chamber. The gas concentration was precisely controlled and mixed by two mass flow controllers from Brooks Instrument. During measurements, all the chambers were evacuated in the beginning. Then, the gases were mixed in the mixing chamber with specified pressure (a little higher than 1 atm) and gas concentration. The mixing gas was then sent to the testing chamber to record electrical characteristics. The testing chamber was evacuated by the mechanical vacuum pump to extract the injected gas molecules. The Raman spectrum of graphene was measured by Horiba LabRAM 800 and shown in the Supporting Information Figure S1. The single layer graphene is confirmed by the intensity ratio of G to 2D band. The absence of D band intensity at 7 ACS Paragon Plus Environment


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around 1350 cm-1 confirms the high quality of the transferred graphene. The x-ray diffraction spectrum of ZnO on silicon is shown in the Supporting Information Figure S2, which indicates that the obtained ZnO layer is hexagonal-wurtzite structure with a c-axis preferred orientation.48 As shown in the Supporting Information Figure S3, the SEM image of graphene on ZnO/Si was obtained using a JEOLJSM-6700F FEG microscope operating at 20.0 kV for secondary-electron imaging. It shows that the graphene layer can be transferred well on top of ZnO film surface. 3. Results and Discussion

Figure 1a is the scheme illustration of the graphene/ZnO/p-Si device configuration and the inset is a digital photograph of the device. A ﬁlm consisting of single layer graphene was coated conformably onto a patterned ZnO/SiO2/p-Si substrate with a ZnO/p-Si half-opened window and pre-deposited Ti/Au contacts on top of the SiO2 area and on the corner of half-opened silicon area as contacts. The thickness of ZnO is optimized to 34 nm to achieve the highest performance of the device. The electrical properties were measured by the top Ti/Au contact on SiO2 and the Ti/Au contact on the corner. Figure 1b is the current-voltage (I-V) relationship, which shows the characteristic of the p-n junction formed by n-type ZnO film and p-type Si substrate for the external bias between -1.5V to 4V. When the reverse bias exceeds -2V, the 8 ACS Paragon Plus Environment

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current increases drastically due to electrons transport through the Schottky barrier between graphene and ZnO sieve layer. This I-V behavior can be understood by the band alignment as shown in Figure 1c. Because of the large conduction band offset between n-type ZnO and p-type Si, most of the voltage drop occurs at the p-n junction, which will dominate the current flow initially. Afterwards, when the reverse bias is large enough (exceeds -2V) to let electron pass through p-n junction, the charge transport is then determined by the Schottky barrier between graphene and ZnO layer. Figure 1d shows the I-V curves under the exposure of different concentrations of oxygen ranging from 100 ppm to 10% of atmospheric pressure (1atm). Under reverse bias, with a small amount of oxygen concentration, such as 100 ppm, the change of the reversed current flow can be clearly observed. By increasing the concentration of oxygen, the reversed current drastically decreases. With 10% of oxygen, the reversed current disappears and the curves exhibit a well-behaved rectifying characteristics. Therefore, with negative bias on the device, we could take advantage of this sensitive change of reversed current flow to monitor the oxygen concentration. Figure 2a shows the real time percentage of current response to different concentrations of oxygen ranging from 100 ppm to 10% in atmospheric pressure. All the curves were measured under applied -4V bias on top electrode (graphene), and the introduced gases were under normal temperature and pressure. At the beginning, the 9 ACS Paragon Plus Environment


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device was under a vacuum condition with a pressure of 2.6x10-5 atm. Then, the mixed gas was introduced at 100 s and evacuated to vacuum again at 250 s. First, with N2 introduced, there shows no obvious current change. With 100 ppm oxygen introduced, there is a distinct current change near 20 %, which shows a highly sensitive detection of low oxygen concentration. By extrapolation, for the injection of 1 ppm oxygen concentration, the percentage of current change is expected to be around 1%, which is still in the measurable range. With higher oxygen concentrations such as 1000 ppm, the rising of current response became saturated. Notably, all the current change curves react and reach equilibrium very fast, and they also recover instantly. Quite interesting, after evacuating, all the current changes go back to the original value, which shows a good repeatability and implies that all the adsorbed oxygen molecules go away. This important feature demonstrates that other additional processes to remove residual gas are unnecessary. As shown in Figure 2b, the percentage of current response as a function of oxygen concentration is plotted in linear–log scale. The percentage of current response changes obviously below 1000 ppm and gets saturated at higher concentration. In Figure 2a, we can clearly see that the response of the newly design sensor is really fast. In order to further explore the fast time response, we have estimated the percentage of current response qualitatively. As shown in Figure 3a and 3b, the rising 10 ACS Paragon Plus Environment

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and falling edges of current response to 500 ppm are estimated to be 33 ms and 60 ms, respectively, which are unprecedented fast comparing with the published results for graphene based resistive gas sensors.5 In order to stand out the importance of the additional sieve layer, we have fabricated a gas sensor based on graphene/p-Si device without the sieve layer for comparison. As shown in Figure 4, the response time of the sensor without the sieve layer is in the order of 10 minutes, which is much slower than that of the sensor with the sieve layer and comparable with published reports.5 Note that the sensitivity as shown in Figure 4 is also much less than that of the sensor with the sieve layer. For example, under the exposure of 10% O2, the percentage of amount change of the sensor with sieve layer is about 90%, while that of the sensor without the sieve layer is less than 10%. The underlying mechanism of this ultra-fast and sensitive oxygen sensor derived from graphene/ZnO/silicon device can be understood as follows. As shown in Figure 5a, when the device is under reverse bias condition, the device exhibits a reversed current flow as shown in Figure 1b. With a reverse bias of -4V, electrons can pass through the small Schottky barrier and form a measurable reversed current flow. When different oxygen concentrations were introduced, oxygen molecules will be adsorbed on top of graphene, which results in hole donation from oxygen to graphene. 11 ACS Paragon Plus Environment


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That is, oxygen molecule acts as a p type dopant.49 The Fermi level of graphene is shifted downward by the adsorption of oxygen, which causes the increase of the Schottky barrier height between graphene and ZnO. The Schottky type I-V characteristics can be described using the thermionic emission model as

  eV    eVa  −eΦ B   I = I s exp  a  − 1 = AA*T 2 exp   exp   nkT    nkT    nkT 

   − 1  

(1)

where A is the contact area, A∗ is the effective Richardson constant [≈46.32 A cm-2K−2 for p-type silicon], T is the absolute temperature, e is electronic charge, ΦB is the barrier height, k is the Boltzmann constant, n is the diode ideality factor. Is is the reversed saturated current. In the thermionic emission model, the reversed current change is exponentially proportional to the change of Schottky barrier height (ΦB). The higher the oxygen concentration we introduce, the smaller the reversed current will be. Note that the donated holes from adsorbed oxygen molecules will remain in the graphene layer without transferring into the Si substrate due to the blocking of the ZnO sieve layer, which prevents the excess charges trapped by the defects in the underlying layer. With the lack of the charge transfer process, the response and recovery times therefore can be drastically reduced. In addition, the remaining excess charges in the graphene can easily modulate the Fermi level of the graphene layer due to its small density of states, which enables to greatly change the current across the heterojunction. As a result, a 12 ACS Paragon Plus Environment

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highly sensitive and fast response oxygen sensor by the optimized graphene/ZnO/p-Si configuration can be obtained. The sensor shows near 20% current response to oxygen in a very low oxygen concentration region (100 ppm), which is an unprecedented feature compared with all published reports in terms of graphene based resistive gas sensors.5 To further support our proposed mechanism, the working mechanism of the sensor without the sieve layer is shown in Figure S4. Because there is no sieve layer to block the transfer of holes from graphene into Si layer, the change of the Fermi level and the Schottky barrier is therefore greatly reduced due to the large density of states of bulk silicon. Therefore, the sensitivity is much less than the sensor with the sieve layer. Furthermore, due to the addition process of charge transfer between graphene and underlying layer, the response time becomes slow. In order to verify that the Fermi level of graphene is shifted downward by the adsorption of oxygen, a graphene FET with 80 nm SiO2 was fabricated. As shown in Figure 5b, the electrical measurements for a typical FET were performed under different oxygen concentrations. The Dirac point of graphene shifts toward positive direction under higher oxygen concentration, which is a clear evidence showing that the Fermi level of graphene is shifted downward by the adsorption of oxygen.



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One additional feature worth mentioning is that when the device was under forward bias, the I-V curve exhibits no clear difference under different oxygen concentrations, unlike the data shown in Figure 1b. This interesting behavior indicates that the polarity of the bias on graphene will affect the absorption of oxygen molecules on our device. When applying negative voltage on graphene, it is easier for oxygen molecules to absorb electrons and be adsorbed on graphene and donate holes. On the other hand, when a positive voltage is applied on graphene, oxygen molecules will be repelled and cannot be adsorbed on graphene because it is difficult to absorb electrons from a much deeper Fermi level. We envisage that this unique feature of our device could be utilized as a selective gas sensor, in which under reverse bias the sensor can detect adsorbed molecules that donate holes, while under forward bias, it can detect adsorbed molecules that donate electrons. A proof-of-concept demonstration for the detection of hydrogen gas under forward bias is shown in the Supporting Information Figure S5. Similar to the measurement for the detection of oxygen gas, the primary result also shows that our device can detect hydrogen gas with high sensitivity and fast response. 4. Conclusion

In summary, we have demonstrated that graphene Schottky junction with the mediation of a sieve layer can serve as an excellent sensitive gas sensor with ultra-fast 14 ACS Paragon Plus Environment

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response time. The sensitivity can be reduced to ppm level, and the ultra-fast response time around 60 ms is unprecedented compared with published graphene based gas sensors. The achieved high performance can be interpreted well by the large change of the Fermi level of graphene due to its inherent nature of low density of states and the blocking capability of the sieve layer to prevent charge carriers transfer from graphene to the underlying semiconductor layer. It is expected that with surface decoration on top of graphene, our device can be further improved as a good template for chemical sensing, such as biosensors and many other chemical molecules. Our work shown here therefore is very useful and timely for the development of sensing devices with high performance for practical applications.

Table 1. Summary of gas sensor parameters of graphene hybrid based gas sensors (see also recent review article reference 5).



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Figure 1. (a) Schematic illustration and photograph of the device configuration. The thickness of ZnO is optimized to be 34 nm. (b) The diode characteristic of diode behavior. (c) Band diagrams of the graphene, ZnO and p-Si before contact and under equilibrium conditions. EF is the Fermi level. (d) Current-voltage (I–V) characteristics to different concentrations of oxygen ranging from 100 ppm to 10% in atmospheric.


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Figure 2. (a) Real time percentage change of current response to different concentrations of oxygen ranging from 100 ppm to 10% of atmospheric pressure under applied -4V bias on graphene. (b) The percentage of current response as a function of oxygen concentration plotted in linear–log scale. The percentage current change saturates around 1000 ppm.

Figure 3. (a) The rising real time percentage of current response to 500 ppm oxygen in a short time. (b) The falling real time percentage of current response to 500 ppm oxygen. The rising and falling response times are 33 ms and 60 ms, respectively.



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Figure 4. The response time of the graphene/p-Si device without sieve layer is in the order of 10 mins, which is much slower than that of the sensor with sieve layer.


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Figure 5. (a) The energy diagram shows the charge carriers flow under O2 exposure and under reverse bias, where e- represents electron and h+ represents hole donated by oxygen molecule. (b) Electrical properties of a graphene field effect transistor under different oxygen concentration.

ASSOCIATED CONTENT Supporting Information. Experimental results about the Raman characterization of graphene; the x-ray diffraction (XRD) spectrum of ZnO film; the SEM image of graphene on ZnO/Si; the working mechanism of a gas sensor based on graphene/p-Si device without sieve layer; real time percentage change of current response to H2 exposure. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Author Contributions C.C.C. planned the project, supervised the overall project and designed the experiments, set up the measurement equipment. C.L.W fabricated the device, and 19 ACS Paragon Plus Environment


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performed electrical and optical measurements. Y.F.C. and C.C.C. and Y.M.L analyzed the data and wrote the manuscript. All authors discussed and commented on the manuscript. ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology and the Ministry of Education of the Republic of China. REFERENCES (1) Li, C.; Bai, H.; Shi, G. Conducting Polymer Nanomaterials: Electrosynthesis and Applications. Chem. Soc. Rev. 2009, 38 (8), 2397-2409. (2) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Metal–Organic Framework Materials as Chemical Sensors. Chem. Rev. 2011,

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