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Piezotronic Effect Enhanced Flexible Humidity Sensing of Monolayer MoS2 Junmeng Guo, Rongmei Wen, Yudong Liu, Ke Zhang, Jinzong Kou, Junyi Zhai, and Zhong Lin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17529 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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ACS Applied Materials & Interfaces

Piezotronic Effect Enhanced Flexible Humidity Sensing of Monolayer MoS2 Junmeng Guo,†,ǁ Rongmei Wen,†,ǁ Yudong Liu,†,ǁ Ke Zhang,†,ǁ Jinzong Kou,†,ǁ Junyi Zhai,*,†,‡,ǁ and Zhong Lin Wang*,†,ǁ,₸ † CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, China. ‡ Center for Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, China ǁ School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing, 100049, China ₸

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA

*Corresponding Author. E-mail: [email protected] and [email protected].

ABSTRACT: We report the piezotronic effect on the performance of humidity detection based on a back-to-back Schottky contacted monolayer MoS2 device. By introducing an upswept mechanical strain, the in-plane electrical polarization can be induced at the MoS2/metal junction region. The polarization charges can modify the Schottky barrier height at the interface of MoS2/metal junction, subsequently improved the sensitivity of the humidity sensing. An energy band diagram is proposed to explain the experiment phenomena of the humidity sensor. This work provides a simple way to enhance the sensitivity of ultrathin two dimensional materials based sensors by piezotronic effect, which has great potential applications in electronic skin, human-computer interfacing, gas sensing and environment monitoring.

Keywords: piezotronic effect, monolayer MoS2, polarization charges, humidity sensing, Schottky barrier height

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1. INTRODUCTION

The detection of humidity has attracted much attention in various fields such as textile, livestock breeding, biology, agriculture, medicine, etc.1-5 The nature of materials play a vital role in determining the performance of humidity sensing. Among numerous humiditysensitive materials, semiconductor materials has been widely studied due to unique electrical properties, such as metal oxide semiconductors with nanostructures including nanowires, nanorods and nanobelts, which showed considerable sensing performance to humidity.6-10 Generally, a high surface to volume ratio of these semiconductors materials is an important factor to realize a higher performance for humidity sensing. Compared with traditional one dimensional materials, two dimensional (2D) materials show highly attractive properties due to completely exposed atoms and stunning flexibility.11-15 Recently, monolayer MoS2 with a direct band gap of 1.9 eV, exhibits excellent performance to humidity sensing.16,17 However, monolayer MoS2 based field effect transistor (FET) sensor often need a high gate bias in order to achieve higher levels of humidity sensing, which not only increases the manufacturing complexity, but also faces many difficulties in the process of measurement, such as gate leakage and the irreversible damage to the device caused by drastic gate bias. As a result of these challenges, a more simple and stable method, such as mechanical stimulation as an energy source that is ubiquitous in our daily life, is an economic alternative to conventional hard silicon based FET sensors. The piezotronic effect is first observed at wurtzite structured semiconductor materials (such as ZnO and GaN). The strain-induced polarization charges at the interface of metal/semiconductor junction or heterojunction can attract free carriers to modify the barrier height and affect carriers transport. This tunability can be summarized as the piezotronic effect.18-21 As a non-central symmetric n-type semiconductor material, monolayer MoS2 presents excellent in-plane piezoelectric properties.22,23 Meanwhile, the 2D material 2 ACS Paragon Plus Environment

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monolayer MoS2 has an excellent adhesion with most of the flexible substrates just by van der waals force. Moreover, modulating the transport of free carriers by utilizing responding mechanical deformation to serve as portable humidity sensors is highly significant for practical application. In view of this, it is promising to enhance the humidity sensing of the single layer MoS2 by the piezotronic effect at room temperature. In this paper, we prepared an atomically thin Schottky contacted sensor using two back-toback Pd-MoS2 junctions on the flexible polyethylene terephthalate (PET) substrate for air humidity detection. It has demonstrated that the 2D monolayer MoS2 flake is a promising material for high performance humidity sensors at room temperature due to ultra-large specific surface area and high surface activity. Furthermore, we investigate the piezotronic effect on the tunability of the humidity sensor. The piezoelectric potential is produced along the zigzag edges by applying a static strain, which effectively tunes the Schottky barrier height (SBH) at the junction area. The results suggest that the strain-induced piezoelectric charges can distinctly enhance the sensitivity of single layer MoS2 humidity sensors, and the maximum relative current variation reaches 2048% with the changing of relative humidity from 63% to 5% under 0.61% tensile strain. This work proposes a simple method to enhance the detection sensitivity of single layer MoS2 based humidity sensors or 2D based flexible sensors by the piezotronic effect. 2. EXPERIMENTAL SECTION Materials Fabrications. MoS2 monolayer crystals were prepared on a 300 nm SiO2/Si substrate by chemical vapor deposition (CVD) with sulfur and MoO3 as the precursor. Before growth, the substrates were cleaned ultrasonically for 10 minutes in acetone and alcohol bath followed by a 5 min soak in deionized (DI) water and then dried with N2. MoO3 powder (18 mg) was added to a quartz boat, and SiO2/Si substrates were placed face down above the boat. Then, the boat was loaded in the center of the furnace. Solid sulfur (300 mg, Sigma Aldrich) 3 ACS Paragon Plus Environment


was placed upstream from the SiO2/Si substrate warming up by heating tape. The furnace temperature was increased to 850 ℃ in 15 ℃/min with 10 sccm argon gas and maintained for 30 min. Meanwhile, the S-zone temperature was ramped up to about 210 °C by the heating tape. After growth, the sample was cooled down to room temperature naturally. The whole process was carried out under atmospheric pressure. The poly (methyl methacrylate) (PMMA) was spun onto the samples and was dried through warm table. The samples were then placed in diluted hydrofluoric acid (10:1) to etch the SiO2 layer. After cleaned by deionized water, the PMMA/MoS2 film was transferred onto a PET substrate. Finally, the sample was put into acetone to remove the PMMA. Material Characterization. The layer number of grown MoS2 by CVD was identified using Raman spectroscopy (Renishaw Raman with a 532 nm excitation laser and a 100x objective with laser spot size of ∼1 µm). Plan view of crystalline structures of monolayer MoS2 and selected area electron diffraction patterns were analyzed by using a transmission electron microscopy (TEM, TecnaiTM G2 F30, FEI, USA). The method of preparation for TEM samples was described in previous study. Device Fabrication and Measurements. E-beam lithography was used to fabricate the electrodes of MoS2 devices. Cr/Pd/Au (1 nm/20 nm/50 nm) was deposited by an ebeam/thermal evaporator as source and drain contacts as the metal–MoS2 interface runs parallel to the zigzag direction. Electrical characterization of MoS2 sensors was performed by using a Keithley 4200 semiconductor characterization system to record the I–V characteristics under different strains with various relative humidity in a sealed chamber. In this work, we used silica gel to control the relative humidity. Humid air was first introduced into the chamber through an air inlet and then a quantity of silica gel was placed in the chamber. Through adjusting the quantity of silica gel, we can get a precise relative humidity in the sealed chamber. During the whole process of test, the MoS2 sensor device was placed in a dark environment to avoid light interference. 4 ACS Paragon Plus Environment

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3. RESULT AND DISCUSSION

MoS2 triangle flakes were synthesized by chemical vapor deposition (CVD). The insert graph of Figure 1a shows Raman spectrum of a grown MoS2 flake. The prominent peaks are at 385.4 and 405.4 cm−1, which are precisely corresponding to in-plane mode E2g1 and out-ofplane mode A1g. The gap between E2g1 and A1g peaks is 20 cm−1, indicating that the MoS2 flake is monolayer.24 We also observed that the general morphology of MoS2 monolayers maintain triangle after transfer from Si/SiO2 substrate to PET substrate (Figure 1a). The suspended MoS2 triangles on the top of holey carbon TEM grid were used to analyze the crystal structures and lattice orientation by the transmission electron microscopy (TEM) and electron diffraction techniques in details. The associated Fast Fourier transforms (inset of Figure S1) reveals the hexagonal lattice structure with the lattice spacing of 2.7 and 1.6 Å assigned to the (100) and (110) planes. In this paper, the x-axis is taken to be along the “armchair” direction, and the y-axis along the “zigzag” direction. Figure 1b (The sharper and straighter edges indicate the molybdenum zigzag triangles) and Figure 1c show that the zigzag is the dominant morphologies of the CVD grown MoS2 triangles, which is consistent with the previous reports.25 The relevance to this morphology allowed us to easily identify the armchair ‘X’ and zigzag ‘Y’ orientation with the triangle edge terminations of CVD grown MoS2 by optical microscopy, which is very important and convenient to identify the direction with the maximum in-plane piezoelectric constant for monolayer MoS2. To fabricate MoS2 based piezoelectric device, triangles MoS2 flakes were first transferred to a flexible PET substrate. Two metal electrodes were subsequently painted parallel to the zigzag direction. Figure 1d shows the channel length between two electrodes of this device, which is about 10 µm. When the flexible PET substrate is bended upward, the uniaxial strain is applied to the device, thus generating the in-plane polarization charges along ‘X’ direction (Figure 1e).26 In 5 ACS Paragon Plus Environment


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this work, the maximum applied strain is limited to 0.7% to avoid sample slippage. The strain applied to the sample can be estimated using the formula27

ε = h/2R

(1)

where ε is the strain of monolayer MoS2, h is the thickness of the flexible PET and R is the radius of the bending PET. We first investigated the response of the monolayer MoS2 based humidity sensor under various relative humidity (RH) without tensile strain at room temperature, as shown in Figure 2a. In this device, the metal semiconductor metal (MSM) contacts can be equivalent to two back-to-back Schottky barrier structures, and the reversely bias Pd-MoS2 Schottky barrier primarily control the carriers transport performance when bias voltage is applied. The measured I-V curves in Figure 2a display nonlinear and rectification behavior, and such asymmetric electrical feature can be clearly related to the formation of two distinctly different Schottky junctions due to the interface/surface state at the contacts.28 Here, the SBH of source side is higher than that of drain side (Figure S2). When the RH discretely changes from 63% to 5%, the current of the monolayer MoS2 sensor increases stepwise from 22.3 pA to 125 pA at the bias voltage of 10 V, showing a good linearity to RH. For this sensor, there are two reasons to explain the decrease of current with increasing the RH. On the one hand, under high RH, lots of water molecules are absorbed on the surface of the monolayer MoS2, which trap electrons and subsequently reduce the current of the device under a positive bias condition. On the other hand, the adsorption in close to the Pd-MoS2 junction can also increase the SBH and reduce the current. Then a fixed external strain (0.61%) is applied to the same humidity sensor along zigzag direction to check whether the strain can change the humidity response of monolayer MoS2 based sensor. As shown in Figure 2b, the current of the sensor increases from 38.7 to 478 pA 6 ACS Paragon Plus Environment

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(by 1235%) with decreasing the RH from 63% to 5% when a bias voltage of 10 V is applied. It is clear that the output current monotonically increases as the RH decreases, and similar trends have also been observed at various RH under other certain strain conditions for this device (Figure S3). Obviously, when a 0.61% strain is applied, the sensor has larger output current compared with strain-free condition at the same RH environment, which demonstrates that the flow of the free electrons through the conduction band increases. Meanwhile, the I-V curve also maintains the nonlinear and asymmetric behavior, suggesting that the reversely bias Pd-MoS2 Schottky barriers become lower. Figure 2c and 2d show I-V performance of the device at 5% and 63% RH under various tensile strain conditions, respectively. At 5% RH condition, the current increases from 125 pA to 478 pA (by 383%) with changing the tensile strain from 0 to 0.61% at the bias voltage of 10 V. When the RH increases to 63%, the current increases from 22.3 pA to 38.7 pA (by 174%) as the tensile strain changes from 0 to 0.61% at the bias voltage of 10 V. Combining with the result of I-V characteristics for other RH (Figure S4), it can be noted that the device has similar trends when it is subjected to different tensile strains under 20%, 32%, 43% and 51% RH conditions, respectively. All above results demonstrate that the piezoelectric charges induced by the applied strain can significantly enhance the output current of MoS2 based sensor at certain RH condition. Moreover, the current of this device are capable of being maintained at a fixed RH or strain over several days (Figure S5a-b), indicating excellent stability and flexibility of our humidity sensor. Figure S5c shows the current variation with radius of curvature. The current increase with the increasing of the bending. The maximum current is observed for 4.1 mm radius of curvature coresponding to a strain of 0.61%. The results obtained in this study suggest that the single layer MoS2 device can be suitable for flexible humidity sensing. 29 A 3D scatter diagram, which includes of all output current under different relative humidity and applied strain conditions, is plotted in Figure 3a under the bias of 10 V, and the whole 7 ACS Paragon Plus Environment


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trend of output signals can be observed visually. The current increases as the RH decreases and the tensile strain increases. In order to discriminate the current changes between different RH and strains, two 2D graphs have been plotted from the 3D graph, as shown in Figure S6a and Figure S6b. First, we fixed five strains (0%, 0.3%, 0.43%, 0.54%, 0.61%, respectively) and changed the RH from 5% to 63% in Figure S6a. The output current value of the RH between 5% and 63% stepwise increases as the strain increases from 0% to 0.61%. It is obvious that the piezotronic effect enhances the current output of the humidity sensor remarkably. So, a good tuning behavior can be obtained by the piezotronic effect, which is helpful to enhance the sensitivity to humidity of the monolayer MoS2 based sensor. In a word, a larger applied tensile strain indicates a larger current and a better selectivity for the humidity sensor. Second, we fixed six relative humidity (5%, 20%, 32%, 43%, 51%, 63%, respectively) and changed the tensile strain from 0 to 0.61% in Figure S6b. The modulation effect of this sensor by piezotronic effect will become poor as the relative humidity increases from 5% to 63%, which indicates that the polarization charges induced by strain is more effective at low RH condition, and this can be understood that the big relative humidity can partially screen the piezoelectric polarization charges due to the adsorption of water molecule and the increase of SBH at the junction area as reported before.30 All in all, the monolayer MoS2 sensor performance is determined by the coupling of the adsorption of water molecules and the piezotronic effect. The relative current variation of the sensor to humidity by piezotronic effect can be calculated by the following equation

dI/I = (Ihumidity, ε – I0)/I0

(2)

where I0 is the current in 63% RH under strain-free condition, and Ihumidity, ε is the current under certain RH and strain condition. In this paper, the response sensitivity is defined as the 8 ACS Paragon Plus Environment

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relative current variation. As shown in Figure 3b, the relative current variation of the sensor under a certain RH is significantly enhanced with the strain increasing. This result suggests that the piezotronic effect can effectively improve the sensitivity of humidity detection of single layer MoS2 sensor. The free electrical transport property of the MoS2 humidity sensor is determined by the Schottky barrier at the metal/semiconductor interface region. In order to understand the nature of the free carriers transport behavior across the MSM junction, it is very important to deepen the analysis of the change of the SBH. The corresponding band diagram of our device with a monolayer MoS2 sandwiched between two opposite Schottky barriers is presented in Figure S2. The I-V curves of this sensor is asymmetrical, and the barrier height of the source side is dramatically higher than the drain side, meanwhile the output current is determined by the reversely bias Schottky barrier at the drain side (φd) when there is a positive bias applied. The change of the SBH is quantitatively extracted by the classic thermionic emission-diffusion theory (note S1).31 Figure 3c shows that the changes of the SBH (△ △SBH) at drain side with different RH and strains when a positive 10 V bias is applied. It is obvious that the △φd increases with the increasing of the strain from 0 to 0.61% under three different RH (20% RH, 43% RH and 63% RH), which means that the SBH at drain side decreases at the same condition. Therefore, the Schottky contact of the humidity sensor has high tunability owing to the piezotronic effect. The sensing mechanism of 2D monolayer MoS2 nanomaterial is based on a charge transfer process (Figure S7).17 When the sensors are exposed to a humidity environment, the water molecules are adsorbed on the surfaces of sensing channels, leading to the variation of sensor current. The band diagram is plotted in Figure 4 to elucidate the piezotronic effect on the performance of the humidity sensor. When no strain is applied to this MSM structure, as shown in Figure S2, the two back-to-back Schottky junctions between the single layer MoS2 9 ACS Paragon Plus Environment


and the Pd electrodes have distinctly different barrier heights (φs > φd). When low RH condition is introduced into the chamber (Figure 4a), the adsorption amount of water molecules on the surface of the device is very few, and thus the ability of withdraw electrons from the MoS2 surface is very weak. In this case, the impact of water molecules adsorbed at the junction area on the Schottky barrier is also very little. The electronic transfer properties in the conduction band of the MoS2 are mainly dominated by the Pd-MoS2 contact and the applied external field. When tensile strain is introduced by bending the PET substrate upward in this condition, the piezoelectric potential is created along the zigzag and Pd electrode edges due to the non-central symmetric crystal structure (Figure 4b). The positive polarization charges at the drain side decreases the conduction band minimum (CBM) throughout the entire semiconductor, and therefore decreases the Schottky barrier height (SBH) for electrons at the junction. Therfore, more free electrons get through the conduction band under an external positive bias condition. As a result, the observed output current increases by applying tensile strains under positive bias voltage. The sensitivity of humidity is also enhanced when the tensile strain is applied. Figure 4c shows the energy-band change of the humidity sensor under a high RH and without strain. The adsorption amount of water molecules on the surface of the single layer MoS2 is huger as compared to the case at low RH, which can effectively reduce the electronic transport of conduction band because the large number of electrons is trapped by water molecules. This process forms an electron depletion region at the 2D materials surface and reduces the MoS2 conductance. Especially, massive water molecules around the vicinity of Schottky contacted Pd-MoS2 interface can attract negative electrons to accumulate at the junction, and increase the SBH at the both side of the electrode, which lead to the decrease of the output current of the sensor at high RH in the chamber. When tensile strain is applied, piezoelectric charges are created at the two sides of the MSM structure, and the positive piezoelectric charges at the drain side reduce the SBH of the junction (Figure 4d). It is also 10 ACS Paragon Plus Environment

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observed that the tunable performance of the sensor by the piezotronic effect in high RH is weaker than that in low RH case. With the increasing of the RH, the more water molecules are absorbed on the surface of the sensing channels. The accumulation electrons at the Schottky contact area have also resulted in partial screening the strain induced polarization charges and could have reduced its piezoelectric performance. On the contrary, with the decreasing of the RH, the less water molecules are absorbed on the surface of the sensing channels. The screening effect of the less accumulation electrons to the strain induced polarization charges is little. In this case, the piezotronic effect gets a very good presentation. Therefore the modulation of the ultrathin sensor by applied strain at high humidity is less effective compared with the modulation at low RH. In this work, the modulation of the sensitivity of the humidity sensor is due to the competition among piezoelectricity and water molecules adsorption on the single layer MoS2 device. Anyhow, the free carriers transport property can be efficiently modulated by piezotronic effect, resulting in enhanced current output and sensitivity of the single layer MoS2 sensor. Considering the significant effect of strain at low RH, the strained device might perform significantly better than a relaxed device at low RH. For instance, the strained device might be able to work at much lower RH than a relaxed device and have a much smaller low detection limit.

4. CONCLUSIONS

In summary, we fabricated ultrathin flexible Schottky contacted 2D monolayer MoS2 sensor for air humidity detections at room temperature. The results suggest that the sensitivity and output signals can be dramatically enhanced by the piezotronic effect, and the maximum relative current variation reaches 2048% with the changing of relative humidity from 63% to 5% under 0.61% tensile strain. The band structure diagram is proposed to explain the enhance performance of the sensor, which shows that the SBH is reduced as the positive polarization 11 ACS Paragon Plus Environment


charges is produced at one side of the Schottky junctions induced by the piezotronic effect, thus the performance of the humidity sensor can be effectively tuned by the piezotronic effect in details. This study demonstrates the coupling of the adsorption of water molecules and the piezoelectric properties with ultrathin 2D devices, which will have a significant scientific and research value to develop excellently sensitive sensors for other flexible electric device system by coupling the electrons, piezoelectric, gas molecules and bioactive macromolecules in the future.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. More details images and equations about the crystallographic plane, band diagram, I-V curves and change in Schottky Barrier Height (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]. Author Contributions §

J.M. Guo and R.M. Wen contributed equal

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by National Key R & D Project from Minister of Science and Technology, China (2016YFA0202703, 2016YFA0202704), NSFC 51472056, the “thousands talents” program for pioneer researcher and his innovation team, China, the Recruitment Program of Global Youth Experts, China.

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Towards Intrinsic MoS2 Devices for High Performance

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Figure 1. Single-layer MoS2 humidity sensor and operation scheme. a) Optical image of the single atomic layer MoS2 flake transferred onto PET flexible substrate. Inset: Raman spectrum of single-layer MoS2 flake prepared by CVD. b, c) High resolution TEM images of single-layer MoS2 flake. d) A flexible two-terminal single atomic layer MoS2 humidity sensor device. e) The homecustomized setup for characterizing piezotronic process in single-layer MoS2.



Figure 2. I – V curves of the humidity sensor under different strains and relative humidity conditions. a) Left: 0% strain and different RH from 5% to 63%. Right: The corresponding current of this sensor under 10 V bias. b) Left: 0.61% strain and different RH from 5% to 63%. Right: The corresponding current of this sensor under 10 V bias. c) Left: 5% relative humidity and different strains from 0% to 0.61%. Right: The corresponding current of this sensor under 10 V bias. d) Left: 63% RH and different strains from 0% to 0.61%. Right: The corresponding current of this sensor under 10 V drain bias.


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Figure 3. Piezotronic effect on the performance of single layer MoS2 based humidity sensor. a) 3D graph depicting the current response of the humidity sensor under different strains and RH at a bias voltage of 10 V. b) The relative current variation of the humidity sensor to tensile strain at a fixed RH of 5%, 20%, 32%, 43%, 51%, and 63%, respectively. c) The derived change in SBH based on the thermionic emission-diffusion model, as a function of strain at a bias of 10V and a fixed RH of 20%, 43%, and 63%, respectively.



Figure 4. Schematic energy band diagram illustrating the piezotronic effect of a single layer MoS2 based humidity sensor under different conditions: a) low RH with no strain, b) low RH with tensile strain, c) high RH with no strain, d) high RH with tensile strain.


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Piezotronic Effect Enhanced Flexible Humidity ... - ACS Publications

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