Air-Stable Humidity Sensor Using Few-Layer Black Phosphorus - ACS

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Air-Stable Humidity Sensor Using Few-Layer Black Phosphorus Jinshui Miao, Le Cai, Suoming Zhang, Junghyo Nah, Junghoon Yeom, and Chuan Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01833 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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ACS Applied Materials & Interfaces

Air-Stable Humidity Sensor Using Few-Layer Black Phosphorus Jinshui Miao1, Le Cai1, Suoming Zhang1, Junghyo Nah2, Junghoon Yeom3, Chuan Wang1,* 1

Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan

48824, United States 2

Electrical Engineering, Chungnam National University, Daejeon 34134, Korea

3

Mechanical Engineering, Michigan State University, East Lansing, Michigan 48824, United

States *Corresponding author: [email protected]

Abstract: As a new family member of two-dimensional layered materials, black phosphorus (BP) has attracted significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties. However, producing air-stable BP sensors is extremely challenging because BP atomic layers degrade rapidly in ambient conditions. In this study, we explored the humidity sensing properties of BP field-effect transistors fully encapsulated by a 6-nm-thick Al2O3 encapsulation layer deposited by atomic layer deposition. The encapsulated BP sensors exhibited superior ambient stability with no noticeable degradation in sensing response after being stored in air for more than a week. Compared with the bare BP devices, the encapsulated ones offer long-term stability with a tradeoff in slightly reduced sensitivity. Capacitance-voltage measurement results further reveal that instead of direct charge transfer, the electrostatic gating effect on BP flakes arising from the dipole moment of adsorbed water molecules is the basic mechanism governing the humidity sensing behavior of both bare and encapsulated BP sensors. This work demonstrates a viable approach for achieving air-stable BP-based humidity or chemical sensors for practical applications.

Keywords: 2D materials, black phosphorus, humidity sensor, encapsulation, long-term stability

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INTRODUCTION Two-dimensional (2D) van der Waals crystals, such as graphene and transition metal dichalcogenides (TMDs) exhibit promising prospects for electronic and sensing applications due to their unique electrical, structural, and physicochemical properties.1-7 Also, the inherent large surface-to-volume ratio of 2D materials leads to a large number of adsorption/active sites (defects, vacancies, and edges) for selective molecular adsorption.8 In the past few years, a variety of 2D materials have been extensively studied as potential chemical and gas sensing materials. For example, graphene has been demonstrated as a promising material candidate for detecting humidity,9 gases,10 and biomolecules11 owning to its ultrahigh carrier mobility, good mechanical/environmental stability, and high conductivity. Atomically thin TMDs such as MoS2 and WSe2 have also been widely studied for high-performance chemical/gas sensors12-14 and their performance can be further improved by surface functionalization.15, 16 Recently, mono- and few-layer black phosphorus (BP), the most stable allotrope of phosphorus with a puckered orthorhombic structure, has demonstrated its potential applications in the field of electronic devices and sensors.17-21 Layered BP is a p-type semiconducting material with tunable direct band gap varying from ~ 0.3 eV (bulk BP) to ~ 2.1 eV (monolayer BP).17, 22, 23 The field-effect transistors (FETs) based on few-layer BP exhibit high carrier mobility,17, 24 large current on-off ratios,24-26 high current density,27-30 and anisotropic properties.18,

24, 31

BP also holds great

promise for use in optoelectronic devices,32-35 Schottky diodes,36-39 radio-frequency transistors,40 memory devices,41, 42 and energy storage/conversion systems.43, 44 More importantly, BP shows superior chemical sensing performance compared with other 2D materials (for example, graphene and MoS2) because BP has higher molecular adsorption energy, lower out-of-plane electrical conductance, and larger surface-to-volume ratio than other 2D materials which can maximize the effect of adsorbed chemical molecules on BP thin flakes.8, 19, 20 The previous studies have demonstrated that BP exhibits high sensitivity to humidity,45 NO2,19, 20 NH3,46 and other chemical vapors.47 However, these BP sensors, using relatively thin BP flakes, will inevitably degrade rapidly in ambient conditions since O2 saturated H2O can react with BP to 2 ACS Paragon Plus Environment

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form oxidized phosphorus.48 Relatively stable BP sensors can be achieved by using very thick and stacked BP flakes that were liquid exfoliated.8,

45

However, such devices will lose the

transistor behavior (no gate dependence) and can be very hard to scale down. Considering the above mentioned issues, achieving air-stable sensing performance is extremely critical to few-layer BP sensors for practical applications. To the best of our knowledge, few-layer BP-based ChemFET sensors with long-term ambient stability have not been reported. In fact, it was speculated that passivated BP FETs would not be applicable for chemical sensing applications due to lack of direct exposure of the device active materials to chemicals.19 In this study, we have systematically investigated the humidity sensing characteristics of few-layer BP FETs with and without the presence of passivation. Because the few-layer BP flakes are known to undergo degradation rapidly in ambient conditions, a thin Al2O3 dielectric layer (6 nm in thickness) was deposited on top using atomic layer deposition (ALD) to serve as the encapsulation layer, which is thick enough to preserve BP devices from degradation as confirmed by atomic force microscopy (AFM), Raman spectroscopy, and electrical measurements. We exposed the BP FETs to various controlled humidity levels at room temperature and monitored changes in their electrical characteristics. The BP sensor prior to Al2O3 encapsulation showed p-type behavior and exhibited a monotonic increase in conductance upon exposure to higher humidity levels. The transfer characteristics of the same BP device became ambipolar after Al2O3 encapsulation, and exhibited an increased p-branch current and decreased n-branch current upon exposure to water vapor. More importantly, the encapsulated BP sensors were found to exhibit drastically improved long-term stability in air with almost no change in sensing response after being stored in air for over a week. Finally, the capacitance-voltage (C-V) measurements revealed that the electrostatic interaction between dipole moment of water molecules and BP flakes is a probable mechanism governing the humidity sensing behavior of BP sensors. This work demonstrates the feasibility of achieving air-stable BP-based humidity or chemical sensors for practical applications.



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EXPERIMENTAL METHODS Device fabrication. The few-layer BP flakes were mechanically exfoliated from bulk BP crystals (99.998%, Smart Elements), using a Scotch tape, onto heavily doped Si wafers with a 300-nm-thick SiO2 on the top. After transfer, the samples were dipped into warm acetone for ~ 1 hr to remove the tape residues. The few-layer BP flakes were identified and characterized by the Olympus BX51 optical microscope, HORIBA Raman spectroscopy, and Dimension 3100 atomic force microscopy. For the BP device fabrication, PMMA resist 495 A6 and 950 C2 were spin coated on the SiO2/Si wafers followed by baking on a hot plate at 175 degree Celsius for 5 min, respectively. The device S/D patterns were defined by e-beam lithography using Hitachi SU5000 with Nanometer Pattern Generation System (NPGS). To make electrical contacts to the BP flake, a 50-nm-thick Au film was deposited onto the BP flake using thermal evaporation (Edward Auto306) and lift-off. Finally, a 6-nm-thick Al2O3 dielectric was deposited onto the entire BP devices at 250 degree Celsius using atomic layer deposition (Cambridge Nanotech Savannah S100). Humidity sensing measurements. The humidity sensing measurements were performed by exposing BP FETs to water vapor carried by argon (99.99%). The humidity levels were adjusted by changing the argon flow rate and the hygrometer was used to monitor the relative humidity in real-time. The electrical conductance change of BP sensors was characterized by Agilent B1500A semiconductor parameter analyzer and Signatone S-1160 probe station and recorded as the sensing signal. All the measurements were conducted in the ambient conditions at room temperature.

RESULTS AND DISCUSSION Figure 1a shows the schematics of two types of BP FET-based humidity sensors (with and without Al2O3 encapsulation layer) compared in this study. Firstly, few-layer BP flakes were mechanically exfoliated from bulk BP crystals, using a scotch tape, onto a p+Si substrate with 300-nm-thick SiO2. Gold source/drain (S/D) electrodes with a thickness of 50 nm were then 4 ACS Paragon Plus Environment

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defined by standard e-beam lithography (EBL), metal evaporation, and lift-off processes. Finally, a 6-nm-thick Al2O3 dielectric layer was deposited by ALD to fully encapsulate the BP device, which can effectively prevent the underlying BP flake from reacting with water molecules and oxygen in the ambient conditions. Figure 1b shows the experimental setup used for BP humidity sensing measurements. The as-fabricated BP sensors were tested with different humidity levels at room temperature. An argon gas passes through a bubbler filled with water and delivers moisture to the surface of the BP sensor. Relative humidity (RH) level was monitored in real-time by a hygrometer inside a sealed container along the gas flow path. The RH level can be effectively controlled by varying the argon flow rate. Figure 1c illustrates the efficacy of using Al2O3 as a passivation layer. Four days after it was fabricated, the bare BP device degraded completely under the ambient condition, as shown in Figure 1c(ii) and Figure S1b. In contrast, the BP device passivated by a 6-nm-thick Al2O3 layer did not show any noticeable change after being stored in the ambient condition for over 5 days (Figure 1c(iv) and Figure S1d). More details regarding the BP stability study are presented in Supporting Information Figure S2 and further demonstrate that ALD Al2O3 can effectively suppress the BP degradation. The Raman spectra of BP flakes with and without Al2O3 passivation are also studied and compared as shown in Figure 1d and e. The three Raman peaks located at ~ 362, ~ 438, and ~ 466 cm-1 correspond to the Ag1, B2g, and Ag2 phonon modes of typical few-layer BP flake.18 For the bare BP flake, no Raman peaks were observed after 7 days of ambient exposure (red line, Figure 1d). In contrast, all of the three Raman peaks remain at the identical positions for the encapsulated BP flake and show very similar peak intensities before and after being exposed to the ambient condition for over 7 days (Figure 1e), further demonstrating the effectiveness of ALD Al2O3. The influence of ALD Al2O3 passivation on the electrical characteristics of BP FETs is presented in Figure 2. The transfer curves (IDS-VGS) of the same BP transistor before (blue line) and after (red line) depositing a 6-nm-thick Al2O3 dielectric show a transition from p-type to ambipolar transistor behavior with a dramatically enhanced n-branch current (before ALD ~ 2.3 nA/µm, after ALD ~ 260.6 nA/µm) and transconductance (before ALD ~ 10-5 µS/µm, after ALD 5 ACS Paragon Plus Environment


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~ 10-2 µS/µm), as shown in Figure 2a and b. Statistical study on eight BP devices before and after Al2O3 encapsulation (Supporting Information Figure S3) indicates that all devices exhibit similar transition to ambipolar behavior. It is also worth noting that the device hole mobilities (~ 100 cm2V-1s-1) and current on-off ratios (~ 104) show little change after Al2O3 passivation (Supporting Information Figure S3c, d). The enhanced n-branch current can be attributed to the doping effect of ALD Al2O3 and the subsequent reduction of Schottky barrier thickness for electrons at the BP-Au contact edge (Figure 2c). For the BP FETs with Au contact, Schottky barriers exist for both holes and electrons at the interface. The positive fixed charges in the Al2O3 dielectric induced by the deficiency of oxygen atoms will bend the energy band downward (solid line, Figure 2c), leading to a reduced Schottky barrier thickness for electrons,49, 50 which makes it much easier for electrons to inject from metal into conduction band through a tunneling process. Figure 2d and e show the device output characteristics (IDS-VDS) before and after Al2O3 encapsulation under various gate voltages. It can be clearly seen again that the p-branch current (Vg = -70, -50 and -30 V) exhibits little change after depositing Al2O3 (Figure 2d) while the n-branch current (Vg = 70, 50 and 30 V) is significantly enhanced (Figure 2e). The humidity sensing experiment results of both unencapsulated and encapsulated BP FET sensors are shown in Figure 3. Under the ambient condition (RH ~ 21% and room temperature), the BP FET without Al2O3 passivation exhibited p-type transistor behavior with a current on-off ratio of ~ 3.4 × 103 and on-current of ~ 20 µA/µm at a VDS = 0.5 V (black line, Figure 3a). An up-shift in the transfer curves was observed as the BP sensor was exposed to increasing RH levels (blue line RH = 59%, orange line RH = 71%, and red line RH = 83%). The increased electrical conductivity in Figure 3a can be attributed to the increased hole concentration induced by water molecules adsorbed on the BP flake. In addition, the subthreshold slope of the transfer curves decreased with the increasing RH level, indicating a reduced carrier mobility, which can be ascribed to the surface scattering caused by the adsorbed water molecules on top of the BP flake. Here it is important to recognize that the sensing mechanism of the unencapsulated BP FET sensors may be quite different depending on how the sensors are fabricated. In case of the 6 ACS Paragon Plus Environment

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BP humidity sensors made of very thick films (tens of µm; synthesized by depositing liquid-exfoliated BP flakes), the absorbed water molecules would form conducting ions within the highly resistive random network of BP flakes, significantly modulating the overall electric transport.45 Meanwhile, working principles of humidity or gas sensors consisting of an individual few-layer BP flake is generally believed to rely on direct charge transfer from water or gas molecules to the BP surface.19, 46, 51 For example, in case of NO2 and NH3 sensing, upon gas adsorption on the BP surface, the electron withdrawing molecule NO2 (electron donating molecule NH3) would lead to an increased (decreased) hole concentration and consequently higher (lower) device conductance in p-type BP FET device. The above is in good agreement with results found in the literature. However, the same charge transfer argument cannot be used to explain the increased conductance upon moisture injection as seen in our work and others.51 This is because water molecules are electron donating, which should lead to decreased conductance in p-type BP FET upon exposure (similar to NH3) if direct charge transfer was the main mechanism. Therefore, an alternative explanation is required to explain the observed response (increased conductance) of the bare BP sensor to humidity. As will be discussed later in the paper, we attribute the sensing mechanism to the electrostatic gating effect induced by the dipole moment of adsorbed water molecules. Moreover, to address a concern that the bare BP devices may degrade rapidly during the humidity sensing experiments, we conducted a short-term stability study on one of the bare BP FET devices by exposing it to humidity (Supporting Information Figure S4). After initial injection of water vapor (RH ~ 69%) on the BP device, both transfer and output curves show an abrupt up-shift (red lines in Figure S4a and b). After the water vapor injection was turned off, the characteristics of the BP sensor gradually recovered to their original states. After around 1 hour, both transfer and output curves almost overlap with the original curves at 21% RH. From the results, one can safely conclude that although the bare BP sensor will inevitably degrade in ambient condition due the chemical reaction with moisture, the devices can remain stable for a short period of time, at least within the time frame of the experiments. Therefore, it would be 7 ACS Paragon Plus Environment


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valid to compare the sensing response of the same BP device before and after encapsulation with the ALD Al2O3 layer. Next, the sensing behavior of the same BP device in Figure 3a was studied after deposition of a 6-nm-thick Al2O3 dielectric layer on top. After encapsulation, the transfer curves (black line, Figure 3b) converted to ambipolar behavior, which is in agreement with the discussion above in Figure 2. Upon exposure to different RH levels, the encapsulated BP sensor exhibited increased p-branch current and decreased n-branch current, as shown in Figure 3b (blue line RH = 60%, orange line RH = 71%, and red line RH = 83%). Similarly, the behavior could also be attributed to the electrostatic dipole moment of the adsorbed water molecules on Al2O3 inducing an downward bending of the energy band, thus changing the Schottky barrier for both electrons and holes at the BP-Au contacts.52, 53 More specifically, the electrostatic gating effect from the water molecules causes a positive shift of the threshold voltage, which leads to the increased p-branch current (due to the reduced Schottky barrier thickness for holes) and the decreased n-branch current (due to the increased Schottky barrier thickness for electrons). The output characteristics of the same BP sensor before (Figure 3c) and after (Figure 3d) Al2O3 encapsulation measured at a VGS of -70 V both exhibited monotonic current increase as the RH level increased. On the other hand, it is clear that the response (current increase) for the unencapsulated BP device is larger than the encapsulated one upon exposure to the same RH level. The slightly reduced response/sensitivity is caused by the existence of the 6-nm-thick Al2O3 dielectric layer, which weakens the electrical field induced by the electrostatic dipole moment of water molecules. The dynamic sensing response (VDS = 1 V, VGS = -60 V) for the BP sensor without (red) and with (blue) Al2O3 encapsulation are compared in Figure 3e. Figure 3e clearly shows the increase in the drain current upon injection of water vapor (without Al2O3 dielectric: ~ 3-fold enhancement in S/D current at RH = 82%; with Al2O3 dielectric: ~ 1.7-fold enhancement in S/D current at RH = 81%), and the BP sensor can completely recover to its original state as the water vapor injection is switched off. As discussed previously, the Al2O3-encapsulated BP sensor exhibits ambipolar behavior whose n-branch current decreases 8 ACS Paragon Plus Environment

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under higher humidity levels. As a results, when the dynamic sensing response of the encapsulated BP device is measured with a positive gate bias of VGS = 70 V, the n-branch current decreases upon multiple pulses of water vapor injection (Supporting Information Figure S5). In Figure 3f, the device relative conductance change (∆G/G0) is plotted as a function of the RH level, where ∆G = G – G0, G is the instantaneous conductance exposure to various RH levels and G0 is device conductance in ambient conditions (RH ~ 21%). The ∆G/G0 increases linearly with the increasing RH level for both sensors, suggesting the sensor’s linear dynamic behavior. With regard to sensitivity, defined as the slope in Figure 3f, the bare BP sensor (red cycles) shows higher performance than the encapsulated one (blue cycles). Although the unencapsulated BP sensor is slightly more sensitive, it is challenging to make them air-stable for practical applications because atomically-thin BP flakes are known to undergo degradation upon exposure to moisture.17, 48 Therefore, adding a 6-nm-thick Al2O3 layer on top is a good tradeoff between sensitivity and long-term stability. Figure 4a, b, and c show the sensing response of a representative unencapsulated BP sensor, when it was fresh and after exposure to ambient conditions for 3 days. Comparing Figure 4a and b, one can clearly see that the fresh BP sensor without Al2O3 encapsulation is completely degraded and exhibits no response to water vapor after being stored in ambient for 3 days. The dynamic sensing response in Figure 4c also demonstrates the irreversible degradation. In contrast, a fresh BP sensor, when properly encapsulated by depositing a 6-nm-thick Al2O3 layer on top, could still preserve a good sensing response even when stored in ambient conditions for over 7 days (Figure 4d and e). The dynamic sensing response of the encapsulated device before and after 7 days of ambient exposure presented in Figure 4f also shows no noticeable change. On the basis of above experiments, one can conclude that the ALD Al2O3 dielectric layer provides a sufficient barrier for moisture diffusion and effectively suppress degradation of BP sensors while still maintaining good sensing sensitivity. To further shed light on the sensing mechanism of the BP humidity sensors with and without Al2O3 encapsulation, capacitance-voltage (C-V) measurements were further performed as 9 ACS Paragon Plus Environment


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illustrated in Figure 5. All C-V data presented are measured at a frequency of 1 MHz. Figure 5a shows the schematic of C-V measurement setup where the gate of the BP FET is connected to the HIGH terminal while the S/D electrodes are both connected to the LOW terminal of the C-V analyzer. Unlike the back-gated BP devices for the I-V measurements, it is crucial for the devices under C-V characterization to have a localized back-gate (Figure 5b) in order to minimize the parasitic pad capacitance. Otherwise, if a conventional back-gated BP FET was used for the C-V measurement, the device capacitance versus the gate voltage curve would just show a straight line due to the large parasitic capacitance of S/D electrodes and pads (Supporting Information Figure S6). To achieve the locally-gated BP FET, we first fabricated periodic Ti/Au (1 nm/40 nm) disks on the SiO2/Si substrate and then deposited a 10-nm-thick Al2O3 layer by ALD on the entire substrate as the gate dielectric layer. The few-layer BP flakes were transferred onto the sample using standard mechanical exfoliation, and an optical microscope was used to locate the BP flakes that were near the edge of the metal disks (Figure 5b(i)). Finally, the BP devices with localized gate were fabricated using EBL, metal evaporation, and lift-off as described previously. Figure 5c presents the C-V characteristics of a representative BP FET before and after a 6-nm-thick Al2O3 layer was deposited on the top. Without encapsulation (blue line, Figure 5c), the BP FET can be fully depleted with VBIAS greater than ~ 1 V, which is expected for conventional p-type FET behavior. However, the same BP device after Al2O3 encapsulation (red line, Figure 5c) exhibits ambipolar transistor behavior and enters accumulation region for both negative and positive gate voltages. The channel is depleted only when the VBIAS is in between ~ -1.5 V and ~ 0 V. Figure 5d shows the C-V characteristics of the BP device without Al2O3 upon exposure to a high RH level of ~ 86%. It is clear that the entire C-V curve underwent a positive shift of around 1 V upon exposure to high moisture levels. A similar positive shift of the C-V curve was also observed in the same BP device after Al2O3 encapsulation (Figure 5e). The underlying sensing mechanism of BP FET responding to humidity can be attributed to the electrostatic interaction between water molecules and BP flakes for both unencapsulated and encapsulated devices and 10 ACS Paragon Plus Environment

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explained using the schematics in Figure 5f. Water molecule is the only one having a dipole moment among all the gasses in ambient air. The surface defects of substrate (Al2O3 or SiO2) will give rise to charge transfer and dipole moment will be formed in the substrate layer.53,

54

Therefore, substrate surface defects can lead to an impurity band, and the electrostatic dipole of water molecules will change the impurity band resulting in increased hole concentration in the BP flake.52,

53, 55

For the encapsulated BP sensor (Figure 5f(iii)) with a 6-nm-thick Al2O3

dielectric between BP flake and water molecules, the electrostatic field induced by water molecules will be weakened leading to a slightly smaller sensitivity to humidity.

CONCLUSIONS In conclusion, we have studied the humidity sensing behavior of few-layer BP FETs under ambient conditions. Because the BP flakes are known to degrade rapidly in ambient conditions, a 6-nm-thick Al2O3 dielectric layer was used to passivate the BP sensors in order to make them air-stable. Thorough characterizations reveal that the Al2O3 encapsulation can effectively suppress BP degradation, which lead to devices with drastically improved long-term stability in air with a tradeoff in slightly reduced sensitivity. Lastly, C-V measurements were used to elucidate the humidity sensing mechanism of BP devices. The electrostatic interaction between water molecules and BP flakes would result in increased hole concentration in BP, thus enhancing the p-branch current. Our results demonstrate a viable approach for achieving air-stable BP-based chemical sensors for practical applications.



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Conflict of Interest: The authors declare no competing financial interest.

Acknowledgement: This work was funded by Michigan State University. The BP device fabrication was done in the Keck Microfabrication Facility (KMF) of Michigan State University. The authors would like to thank Dr. Baokang Bi for helpful discussions regarding the fabrication process.

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: Optical microscope and AFM images of BP flakes and FETs (S1); Long-term stability of few-layer BP flake with Al2O3 encapsulation (S2); Statistical study of the effect of ALD Al2O3 encapsulation on the electrical characteristics of eight BP devices (S3); Short-term stability of an unencapsulated BP sensor during humidity sensing experiments (S4); Sensing response of an encapsulated BP sensor biased with a positive gated voltage (S5); C-V measurement of back-gated BP FETs (S6).

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Figure 1. Air-stable humidity sensor using few-layer BP. (a) Schematic of BP FET based humidity sensors (i) without and (ii) with Al2O3 passivation layer. (b) Schematic illustrating the experimental setup used for humidity sensing measurements. (c) AFM images of BP humidity sensor without Al2O3 encapsulation (i) right after fabrication and (ii) after 4 days in ambient condition; (iii) with a 6-nm-thick Al2O3 encapsulation layer right after fabrication and (iv) after 5 days in ambient condition. The white dashed lines outline the device S/D electrodes. The scale bar is 2 µm in (i) and (ii) and 2.5 µm in (iii) and (iv). (d) Raman spectra of an unencapsulated BP flake right after exfoliation (blue line) and after 7 days in ambient condition (red line). (e) Raman spectra of an encapsulated BP flake with a 6-nm-thick Al2O3 encapsulation right after exfoliation (blue line) and after 7 days in ambient condition (red line).


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Figure 2. Electrical characteristics of the BP FET before and after ALD Al2O3 encapsulation. (a) Transfer characteristics of the same BP FET before (blue line) and after (red line) depositing a 6-nm-thick Al2O3 dielectric layer on top. (b) Device transconductance before (blue) and after (red) Al2O3 encapsulation deduced from panel (a). (c) Illustrations of the energy band diagram at BP-Au contact before (dashed line) and after (solid line) Al2O3 encapsulation under various gate voltages (Vg < 0 V, Vg = 0 V, and Vg > 0 V). (d, e) Semi-logarithmic scale plot of the output characteristics for the same BP device (d) under negative gate voltages (Vg = -70, -50 and -30 V) and (e) under positive gate voltages (Vg = 70, 50 and 30 V). Dashed line: without Al2O3 layer. Solid line: with Al2O3 layer.



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Figure 3. Sensing response of the BP humidity sensors. (a, b) Transfer characteristics of a representative BP sensor (a) without and (b) with a 6-nm-thick Al2O3 encapsulation layer upon exposure to different RH levels. (c, d) Output characteristics of the same BP device (c) before and (d) after depositing Al2O3 upon exposure to various RH levels. (e) Dynamic sensing response for the same BP sensor without (red) and with (blue) Al2O3 capping layer upon exposure to different RH levels. (f) Relative conductance change (∆G/G0) plotted as a function of RH levels showing the sensitivity of the device. 20 ACS Paragon Plus Environment

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Figure 4. Long-term stability of the BP humidity sensors. (a, b) Transfer characteristics of a BP sensor without Al2O3 encapsulation exposed to various levels of RH when (a) it was freshly made and (b) after being stored in ambient condition for 3 days (black line RH ~ 21%, orange line RH ~ 65%). (c) Dynamic sensing response for the unencapsulated BP sensor right after fabrication (orange) and after being stored in ambient for 3 days (black). (d, e) Transfer characteristics of a BP sensor encapsulated by Al2O3 exposed to various levels of RH when (d) it was freshly made and (e) after being stored in ambient condition for 7 days (blue line RH ~ 21%, red line RH ~ 69%). (f) Dynamic sensing response for the encapsulated BP sensor right after fabrication (red) and after being stored in ambient for 7 days (blue).



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Figure 5. Capacitance-voltage characteristics of the BP humidity sensors. (a) Schematic illustrating the C-V measurement setup used in this study. (b) Optical microscope images of (i) the BP flake, (ii) the BP FET with localized gate, and (iii) zoomed-in view of the device channel marked by the dashed box in ii. The scale bars in i, ii, iii are 6 µm, 30 µm, and 8 µm, respectively. (c) C-V characteristics of the BP FET in panel b before (blue) and after (red) deposition of a 6-nm-thick Al2O3 capping layer. (d) C-V characteristics of the BP sensor without Al2O3 encapsulation upon exposure to different RH levels. (e) C-V characteristics of the same BP device with ALD Al2O3 encapsulation upon exposure to different RH levels. (f) Schematics of (i) a fresh unencapsulated BP FET, (ii) unencapsulated BP FET exposed to water vapor, and (iii) encapsulated BP FET exposed to water vapor, illustrating the humidity sensing mechanism.


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Figure for TOC


Air-Stable Humidity Sensor Using Few-Layer Black Phosphorus - ACS

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