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Ultra-sensitive and Fully Reversible NO2 Gas Sensing based on p-type MoTe2 Under ultra-violet Illumination Enxiu Wu, Yuan Xie, Bo Yuan, Hao Zhang, Xiaodong Hu, Jing Liu, and Daihua Zhang ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00461 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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Ultra-sensitive and Fully Reversible NO2 Gas Sensing based on p-type MoTe2 Under ultra-violet Illumination Enxiu Wu1, Yuan Xie1, Bo Yuan2, Hao Zhang1, Xiaodong Hu1, Jing Liu1, *, Daihua Zhang1, * 1

State Key Laboratory of Precision Measuring Technology and Instruments, School of Precision Instruments and Opto-electronics Engineering, Tianjin University, No. 92 Weijin Road, Tianjin 300072, China

2

Qiushi Honors College, Tianjin University, No. 92 Weijin Road, Tianjin, 300072, China

E-mail: [email protected], [email protected] Abstract The unique properties of two-dimensional (2D) materials make them promising candidates for chemical and biological sensing applications. However, most 2D material sensors suffer from extremely long recovery time due to the slow molecular desorption at room temperature. Here, we report an ultra-sensitive p-type Molybdenum ditelluride (MoTe2) gas sensor for NO2 detection with greatly enhanced sensitivity and recovery rate under ultraviolet (UV) illumination. Specifically, the sensitivity of the sensor to NO2 is dramatically enhanced by one order of magnitude under 254 nm UV illumination as compared to that in the dark condition, leading to a remarkable low detection limit of 123 ppt. More importantly, the p-type MoTe2 sensor can achieve full recovery after each sensing cycle well within 5 min at room temperature. Finally, the p-type MoTe2 sensor also exhibits excellent sensing performance to NO2 in ambient air and negligible response to H2O, indicating its great potential in practical applications, such as breath analysis and ambient NO2 detection. Such impressive features originate from the activated interface interaction between the gas molecules and p-type MoTe2 surface under UV illumination. This work provides a promising and easily applicable strategy to improve the performance of the gas sensors based on 2D materials.

KEYWORDS: p-type MoTe2, gas sensor, NO2, UV illumination, interface interaction

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Nitrogen dioxide (NO2) is one of the most common gas exhaustions in numerous industry applications, which can be particularly dangerous to human beings at the concentration level greater than 1 ppm, as it can cause severe and permanent damages to the respiration system.1,2 Furthermore, the sensing of nitrogen oxides (NOx, a group mainly consisting of NO2 and NO) can be potentially implemented in diagnostic processes. For instance, detection of NOx in exhaled breath (at parts per billion (ppb) levels) is helpful for identifying infections of lung tissues.3 In addition, the NOx can possibly be used as a biomarker for some of the gastrointestinal disorder symptoms such as irritable bowel disease.4 Conventional metal oxide semiconductor sensors have delivered satisfactory sensitivity to NO2 in several publications,5-7 however, they require very high temperature (generally above 200 ℃) to activate the chemisorption of atmospheric oxygen on the metal oxide surface, which raises several thermal-related issues and restricts their applications in oxygen-free environments or hazardous environments containing flammable gas species. Low-dimensional materials, including carbon nanotubes, graphene and black phosphorus, have also been investigated as potential candidates.8-14 Recently, semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely studied and considered as promising gas-sensing materials due to their high surface-to-volume ratio and favorable surface energy level for gas adsorption.15-17 However, those sensors exhibit similar slow recovery kinetics as other low-dimensional materials, which restricts them from multi-time tests. Therefore, it is critically important to find a reversible approach to sensitively and selectively detect trace level NO2. In the past decade, researchers have developed various approaches to detect NO2 with fast adsorption/desorption kinetics, for instance, such as surface functionalization18 and applying external electronic field through gate bias19. Nevertheless, these methods usually exist a trade-off between the sensitivity and recovery rate. In contrast, light illumination is another effective and widely used way to dramatically enhance the kinetic performance of low-dimensional semiconductor chemical gas sensors that does not suffer from the trade-off problem.20-22 However, few reports have systematically investigated the influence of light illumination on the performance of 2D TMDCs based gas sensors.

Molybdenum ditelluride (MoTe2) is a new addition to the class of 2D-TMDCs, which has a narrower band gap of ~1.0 eV than other TMDCs. It extends the photo detection range of TMDCs from visible to near-infrared range.23 Recent experimental investigations show that MoTe2 photodetector has a high photo-responsivity and a fast photo-response in a broad spectral range.24,25 In view of growing attention on MoTe2 for various optoelectronic applications, it is also important to investigate the influence of light illumination on the sensing performance of MoTe2 based chemical sensors.


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In this work, we report ultra-sensitive detection of NO2 with the detection limit as low as 123 ppt, using p-type MoTe2 gas sensor. At the same time, we demonstrate excellent reversibility of the sensor and systematically investigated the inﬂuence of UV illumination on the recovery kinetics. We confirmed that the performance improvement originated from activated interface interaction between gas molecules and MoTe2 surface under UV illumination. Thus, our results prove a significant improvement for NO2 sensing and demonstrate the potential of 2D MoTe2 as a promising candidate for ultra-sensitive chemical sensing applications. The recent advancement of micro UV LEDs and chip packaging techniques will eventually enable on-chip integration of photo-assistant chemical sensors that deliver minimum power consumption, form factor and manufacturing cost.

Result and Discussion

Figure 1. (a) Schematic diagram of MoTe2 transistor. (b) Optical image of the device. Scale

bar is 2um. The sensing channel is highlighted by the green dashed rectangle. (c) AFM image of the sensing channel. The white curve is an AFM height profile. (d) Raman spectra of 2-H MoTe2 crystal.

Few-layered MoTe2 flakes were exfoliated and transferred onto a SiO2/n+ doped Si substrate by scotch tape, on which contact patterns were defined by electron-beam lithography and 10/30 nm Ti/Au were subsequently deposited as metal contacts. The fabricated device was


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finally annealed at 250 °C for 5 min in N2/H2 mixtures to acquire p-type MoTe2 transistor.26 Selecting p-type MoTe2 as sensing unit is vital to achieve high-performance sensing of NO2, which will be discussed later in detail. Figure 1a and b present the schematic and optical image of the device. The thickness of MoTe2 flake was 3.7 nm, as confirmed by atomic force microscopy (AFM) measurements in Figure 1c. In Figure 1d, characteristic Raman modes of A1g (171 cm-1), E1g (232 cm-1), and A21g (288 cm-1) are clearly observed using a confocal Raman spectroscope, indicating the exfoliated MoTe2 is 2H-MoTe2 crystals.

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We then carried out a series of electrical measurements to characterize the transport behavior of the MoTe2 transistor. The transfer characteristics of MoTe2 transistor are shown in both linear and logarithmic scales in Figure 2a, when it was placed under dark and UV light illumination with different wavelengths. We kept the drain-source bias at 500 mV while sweeping the gate voltage (Vgs) from -60 to 60 V. The device shows unipolar p-type transfer properties with a current on/off ratio over 104. It is noteworthy that there is an evident leftward shift of the transfer characteristic curve in UV illumination as compared with that in darkness. It can be attributed to UV-induced desorption of atmospheric oxygen (O2), since O2 molecules adsorbed on the surface of MoTe2 flake may act as electron acceptors which induced p-type doping.27 Besides, the photo-induced O2 desorption strongly depends on the wavelength of light,28 and thus, 254 nm UV with higher energy removes O2 molecules more effectively than 365 nm UV light. We notice that there is an obvious Schottky barrier in p-type MoTe2 conduction channel, as evidenced by the non-linear behavior of the output characteristics as shown in Figure 2b. The observation is also consistent with previous studies,29 which concludes that the Schottky barrier-type sensor generally delivers higher sensitivity to gas adsorption compared to homogeneous materials.15


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Figure 3. (a) Dynamic sensing performance toward NO2 in N2 environment at concentration from 20 ppb to 2 ppm under different light illumination. Inset: dynamic sensing behaviors to NO2 at concentration from 20 ppb to 200 ppb. The light blue bars denote the gas concentration of each exposure. (b) Linear fitting of sensitivity versus concentration from 20 ppb to 800 ppb. Inset: sensitivity to NO2 at concentration from 20 ppb to 2000 ppb. (c) Relative recovery rate of the sensor toward NO2 at concentration from 20 ppb to 2 ppm under different light illumination. (d) Three cycles of sensing 20 ppb NO2 in dark and UV illumination.

Figure 3a shows the real time response of the p-type MoTe2 sensor under the exposure of NO2 with concentration ranging from 20 ppb to 2 ppm under different light illumination. We define the sensor response as (G − G0 ) / G0 ×100% , where G0 and G are the channel conductance before and 5 min after NO2 exposure, respectively. Under all light illumination conditions, the conductance of the sensor increases monotonically with NO2 concentration. This can be explained by the following two effects: 1) NO2 is a strong oxidizer so it withdraws electrons from MoTe2 and boosts its hole concentration, which downshifts the Fermi level of MoTe2 towards the valence band, 2) at the same time, the downshifted Fermi level of MoTe2 narrows the Schottky barrier at the MoTe2/metal interface. Moreover, we note that the response of sensor is highly dependent on light illumination. Specifically, in dark condition, the MoTe2 sensor exhibits ~6.3% to ~270% conductance change to NO2 at 20 ppb to 300 ppb level; while under constant UV illumination, the response of the MoTe2 sensor is significantly


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enhanced by 58% (44%) to ~1744% (~1060%) conductance change to NO2 at 20 ppb to 300 ppb level under 254 (365) nm UV illumination. The wavelength dependent sensing response is consistent with the changes of transfer and output characteristic curves under different light illuminations. Figure 3b plots the sensor response as a function of NO2 concentration. The response is linear at low concentrations and shows a quasi-saturation behavior as the concentration increases. The linear portion of the curve is used to estimate the sensitivity and detection limit, which is typically defined as the concentration level corresponding to a signal-to-noise ratio (SNR) of 3.13 The calculated sensing detection limits of NO2 under various conditions are shown in Supporting Information (SI) Figure S1. Under 254 nm UV light illumination, the detection limit

is estimated to be ~123 ppt by comparing the noise level (0.18%) to the sensitivity (3200% ppm-1) of the sensor at low concentration levels. This number is superior than the detection limits obtained by those sensors based on other 2D materials and ~8 times better than those measured in the dark (~1200 ppt) [14-19]. So far, the lowest detection limit for NO2 is ~2 ppt, which is achieved by the pristine carbon nanotube under UV light illumination.13 However, the conductance of the device gradually drops under continuous UV light illumination due to structural degradation, resulting in unstable sensing. In our case, there is no structural damage of MoTe2 after UV illumination and the Raman peak position change (SI Figure S2), which indicates that the MoTe2 sensor possesses stable sensing performance in constant UV illumination. Figure 3c shows the relative recovery rate of sensor to NO2 exposure within five minutes. We defined relative recovery rate as: (Gexp osure − Gre cov ery ) / (Gexp osure − Ginitial ) ×100% , where Ginitial ,

Gexp osure , Gre cov ery represent the initial conductance , the conductance after 5 min adsorption of NO2 and conductance after 5 min desorption of NO2 in pure N2 flow. The sensor achieves complete reversibility within 5 min under 254 nm UV, in contrast to the maximum relative recovery rate of ~45% in dark condition. In the 365 nm UV illumination, the recovery rate is between the values under the two sensing conditions. Reversibility of NO2 sensing is of great importance for practical applications, which is a main challenge for 2D materials based gas sensors.14-19

Figure 3d demonstrates the repeatability and recoverability of the sensor to NO2 gas at 20 ppb level under 254nm UV illumination. Under 254nm UV illumination, the MoTe2 sensor exhibits 40% response and 90% reversibility within 5 min to 20 ppb NO2. In comparison, the sensor exhibits negligible recovery when it is in darkness, which directly hinders multi-time


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Figure 4. (a) Real-time response of MoTe2 sensor upon exposure 1 ppm NO2 under 254 nm UV light illumination with different light intensities. (b) Response of the MoTe2 sensor as a function of light intensity when NO2 concentration is 1 ppm. (c) Linear fitting of fully recovery time versus UV light intensity (d) Dynamic response of the MoTe2 sensor to NO2 with concentration from 20 ppb to 800 ppb under UV illumination with optimized UV intensity of 3.0 mW/cm2.

Figure 4a shows the dynamic response of MoTe2 sensors to 1 ppm NO2 under UV light illumination (254 nm) with intensity ranging from 1.5 to 3.5 mW/cm2. We note that the sensor response and fully recovery time are strongly influenced by the light intensity. Figure 4b and c present the dependence of response and fully recovery time on the UV light intensity, respectively. Both the sensing response and fully recovery time decrease linearly when the light intensity increases from 1.5 to 3.5 mW/cm2. These trends can be explained that the UV illumination can promotes the desorption of both O2 and NO2. When the light intensity is lower than a certain threshold (the threshold intensity is lower than 1.5 mW/cm2), UV light illumination accelerates the desorption of residual O2 and provides more active adsorption sites to NO2 molecules, which increases the sensing response to NO2. However, as the light intensity further increases beyond the threshold, the adsorption of NO2 molecules is limited. Thus, high intensity UV illumination lowers sensor response but accelerates recovery. In order to achieve optimum detection limit (at sub ppb level for NO2 detection in exhaled


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breath3) and recovery speed simultaneously, we choose UV light intensity of 3 mW/cm2 as the optimal value, under which the dynamic response of MoTe2 sensor to NO2 is shown in Figure 4d. The MoTe2 sensor exhibits high sensitivity as well as excellent recovery characteristic. The calculated detection limit for NO2 sensing under 3 mW/cm2 UV illumination is 252 ppt. Although detection limit value increases by ~2 times, the fully recovery time reduces ~ 3 times. The dependence of response and recovery time on NO2 concentration are shown in SI Figure S3a and b. In Table 1, we compared the NO2 gas sensor based on MoTe2 with the sensors based on other 2D material in terms of testing condition, sensitivity, detection limit and recovery time. Our sensor showed ultrahigh sensitivity with complete recovery at room temperature under UV illumination. Detailed sensing mechanism will be discussed in the following section.

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Figure 5. (a) Dynamic response to NO2 in air condition at concentration with NO2 concentration from 80 ppb to 1 ppm under dark (blue line) and 254nm UV light illumination (red line), respectively. (b) Linear fitting of sensor response versus NO2 concentration from 80 ppb to 200 ppb. (c) Relative recovery rate of the sensor when NO2 concentration ranges from 80 ppb to 1000 ppb under dark and UV illumination, respectively. (d) Dynamic response of the MoTe2 sensor to H2O in N2 environment with concentration from 2800 ppm to 14000 ppm. The intensity of UV light is 3 mW/cm2.

We then demonstrate the sensing performance of the MoTe2 sensor to NO2 in air condition in


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Figure 5. Figure 5a plots the real-time response to NO2 in dark and UV light illumination. The MoTe2 sensors shows no obvious difference in response magnitude in air as compared to in N2: the detection limit is estimated to be ~0.7 ppb and ~1.4 ppb under dark and UV light illumination, respectively. The noise level and the slope of response of MoTe2 sensor are 0.35% (0.19%) and 0.64% ppb-1 (0.8% ppb-1) in UV illumination (dark) in Figure 5b. However, significant enhancement in fully recovery characteristic of MoTe2 sensor is achieved under UV illumination, as shown in Figure 5c. We further confirm the response of the sensor to H2O under UV illumination in Figure 5d. The sensor shows response to H2O as well, but the sensitivity is significantly lower than that to NO2, which ranges from 5.3% to 9.4% in response to 2800-14000 ppm H2O in N2. In contrast, the response to 20 ppb NO2 has already reached 18.2%. The high sensing sensitivity to NO2 in air condition with excellent recovery properties and poor selectivity to H2O, indicating MoTe2 sensor great potential in practical applications, such as breath analysis and ambient NO2 detection.

Table 1. Comparison between Literature on 2D Material Based Gas Sensors and our Work (p-type MoTe2 Based Gas Sensor) Materials Mechanically

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Figure 6. Transfer characteristics of ambipolar MoTe2 (a) and p-type MoTe2 (b) before and

after exposure to air for one hour, respectively. (c) Schematic band alignment diagrams at the Ti/MoTe2 interface after adsorption of acceptor molecules. (d) Optical absorbance

spectrum of MoTe2 flake on a SiO2 substrate. Inset: diagram of UV-induced electron-hole pairs of MoTe2.

Mechanisms The enhanced sensitivity of MoTe2 sensor under 254 nm UV illumination can be attributed to the following reasons:

First, we choose p-type MoTe2 as the sensing channel, which is more sensitive to oxidizing


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gas such as NO2. Since pristine MoTe2 exhibits ambipolar (n-type dominated) charge transport behavior with electrons as the major carrier at 0 V gate bias (as shown in Figure 6a), we anneal pristine MoTe2 in N2/H2 mixtures to tune it to unipolar p-type as evidenced in the transfer curve in Figure 6b. The potential mechanism of tuning the carrier type of MoTe2 through annealing has been investigated in our previous work.26 In order to demonstrate that the unipolar p-type MoTe2 is more sensitive to oxidizing gases, we placed both ambipolar and unipolar p-type MoTe2 in air atmosphere for an hour. The transfer characteristic curves of which are shown in Figure 6a and Figure 6b, respectively. We find that the shift of the threshold voltage is negligible for the ambipolar device after air exposure. On the contrary, the source-drain current of the unipolar p-type MoTe2 increases remarkably, which indicates higher sensitivity to air adsorption. This phenomenon is attributed to modulation of the Schottky barrier at the MoTe2/metal interface, which results from the adsorption of oxidizing molecules (O2, H2O) on MoTe2,15,27 as shown in Figure 6c. When oxidizing gas is adsorbed on the MoTe2 channel surface, large amounts of electrons are transferred from MoTe2 channel to oxidizing gas molecules, leading to the downshift of Fermi level of MoTe2 closer to the valence band and decrease of the Schottky barrier width. As a result, it becomes easier for the hole carriers to tunnel through the barrier.

Second, the introduction of UV light illumination can efficiently clean the MoTe2 channel and promote desorption of O2 molecules. The defects in MoTe2 are expected to act as active sensing sites under light illumination, which enhances the sensitivity to target gases. However, the defects such as Te vacancies can be easily occupied by O2 molecules in the air, which yields much larger binding energy with MoTe2 (~166 meV) compared to the case of defect-free MoTe2 (~36–64 meV).36,37 Therefore, removal of O2 molecules can provide more active sites and significantly enhance the sensitivity. However, the thermal energy at room temperature cannot desorb O2 molecules from MoTe2.38,39 In this respect, UV illumination is necessary to provide enough energy for O2 desorption.28,40 The study in Reference [28] suggests that photo induced desorption is strongly dependent on the wavelength of light and the light with shorter wavelength provides higher desorption efficiency. Therefore, 254nm UV has higher efficiency than 365 nm UV to refresh the nanomaterials surface, leading to larger sensitivity improvement.

Third, we also note that the sensor recovers faster under UV light illumination than in darkness, which can be explained by the following reasons. Figure 6d depicts the optical absorption spectrum of MoTe2 flake on SiO2 substrate. Since the strong optical absorption of MoTe2 around the broad 254 nm peak is due to π-electron plasmon excitation,41 it is plausible that photoexcited plasmons in MoTe2 are responsible for induced molecular desorption. The


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collective electron oscillation in MoTe2 de-excites into single-particle hot-electron excitation.42 The excited hot electrons (or holes) may attach to the adsorbed molecules and promote desorption. Part of the plasmon excitation energy is dissipated through breaking molecule-MoTe2 binding. The same photoinduced molecular desorption has been studied on the surface of single-walled carbon nanotubes (SWCNs).28 Another reason is that UV radiation can efficiently generate electron-hole pairs in MoTe2 channel because of its small bandgap (~1 eV), as shown in inset of Figure 4d. The electron-hole pairs are then separated by the surface built-in potential, and the holes are transferred to MoTe2 surface to recombine with the trapped electrons in adsorbed NO2 molecules,40,42 as shown in the following reaction: h + + NO2− ( ad ) → NO2 ( g )

resulting in the fast desorption of NO2.

Conclusion In summary, ultrahigh sensitive and fully reversible NO2 sensing are simultaneously achieved using a p-type MoTe2 gas sensor under UV light illumination. Compared to dark condition, the sensitivity is enhanced by one order of magnitude under continuous UV illumination, which results in a detection limit of 123 ppt. Furthermore, the recovery rate is significantly improved from 10-40% in dark condition to 100% under UV illumination within 300 s at different NO2 concentration ranges. It is also found that the MoTe2 sensors achieves excellent sensing performance to NO2 in air atmosphere and negligible response to H2O, making it promising candidate for practical applications, such as breath analysis and ambient NO2

detection. On-chip integration of 2D nanomaterials with micro UV LEDs would enable high performance gas sensors with ultra-low power consumption and small form factors.

Supporting Information Supporting Information Available: The following files are available free of charge. The calculated sensing detection limits of NO2 under various conditions (S1). Raman spectrum of MoTe2 materials before and after UV illumination (S2). The dependence of response and recovery time on concentration under UV illumination with an appropriate intensity of 3 mW/cm2 (S3).

Methods Device fabrication and measurements Few-layer MoTe2 sheet was mechanically exfoliated from the bulk MoTe2 crystal onto


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pre-cleaned 285 nm SiO2/Si at room temperature in ambient conditions. Electrical contacts were patterned on the surface of exfoliated MoTe2 flake using standard e-beam lithography techniques, followed by sequentially depositing 10 nm Ti and 30 nm Au via e-beam evaporation. After gold deposition, the devices were annealed at 250 ° C for 5 min in N2/H2 mixtures. The device measurements were performed using an Agilent B1500A semiconductor parameter analyzer in ambient conditions.

Sensing experiments The device was wire-bonded on a chip carrier and then mounted into a sealed chamber. Sensing measurements were carried out by exposing the MoTe2 FET device to NO2 gas with various concentrations diluted by ultrahigh purity nitrogen. Gas concentrations were adjusted by modulating the flow rates of both the target and carrier gases. The gas chamber was flushed with nitrogen to recover the device after each measurement.

Characterizations We used commercial Raman spectrometer (Renishaw, Inc.) to obtain the Raman spectroscopy with a 532 nm laser source. The AFM images were taken with Bruker Dimension Icon. The electrical properties were measured with Agilent B1500A semiconductor parameter analyzer in ambient air.

Corresponding Author [email protected], [email protected] Author Contributions Enxiu Wu and Yuan Xie contributed equally. / The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / The authors declare no competing financial interest. Acknowledgment This work is supported by National Science Foundation of China (No. 21405109) and Seed Foundation of State Key Laboratory of Precision Measurement Technology and Instruments, China (No. Pilt1710).

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Ultra-sensitive and Fully Reversible NO2 Gas ... - ACS Publications

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