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Fabrication of P-N heterojunction based MoS2 modified CuPc nanoflowers for humidity sensing
Sensors and Actuators A 299 (2019) 111574
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fabrication of P-N heterojunction based MoS2 modified CuPc nanoflowers for humidity sensing Abir Jana a,∗ , Komal Kumari a , Anup Dey a , P.S. Sreenivas Reddy b , Bikram Biswas a , Bhaskar Gupta a , Subir Kumar Sarkar a a
Department of E.T.C.E, Jadavpur University, Kolkata, West Bengal, 700032, India Nalla Narasimha Reddy Educational Society’s Group of Institutions, Korremula X Road, Via Narapally, Ghatkesar Mandal, Chowdariguda, Telangana, 500088, India b
a r t i c l e
i n f o
Article history: Received 5 July 2019 Received in revised form 19 August 2019 Accepted 24 August 2019 Available online 12 September 2019 Keywords: CuPc nanoflowers Humidity sensing Organic-inorganic p-n heterojunction Device aging
a b s t r a c t In recent times, there has been an increased demand for cost-effective, robust and highly reliable humidity sensors. 2D material MoS2 (n-type) have demonstrated their potential application in chemical and humidity sensing. In this work, an organic-inorganic p-n heterojunction based sensor has been fabricated using the inorganic MoS2 and the organic CuPc, and its viability towards humidity sensing has been experimentally demonstrated. The thin film sensor was characterized by SEM and the results revealed the formation of CuPc nanoflowers on MoS2 surface. The XRD results also indicate excellent crystallization. The sensor shows reduced resistance with increasing RH% and the variation has been observed to be almost linear. The sensing range is from 20% RH to 98% RH. The measured sensitivity is 0.615 M/%RH. The demonstrated results are of great interest in terms of sensitivity, linear response, range of humidity monitoring and stability. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Recent advancements in the field of sensor technologies have resulted in low-power, miniaturized, high speed and cost effective sensors. Humidity is a crucial ambient parameter which needs to be monitored for a wide range of applications such as automated systems, instrumentation, climatology, agriculture, etc. Ceramic, semiconductor and organic polymer based materials have been utilized for fabrication of humidity sensors [1–9]. The sensing materials suitable for high performance humidity sensors must have-(a) high surface to volume ratio for better physisorption of water molecules, (b) the ability to interact with the water molecules repeatedly for enhanced life-cycle and faster response times. When Humidity may be given in terms of’ Relative Humidity’, ‘Parts Per Million’ by weight or by volume and ‘Absolute Humidity’. Most humidity sensors are calibrated to measure Relative humidity. Relative humidity in percentage is expressed as follows: RH
= (HA /HS )×100
∗ Corresponding author. E-mail address: [email protected] (A. Jana). https://doi.org/10.1016/j.sna.2019.111574 0924-4247/© 2019 Elsevier B.V. All rights reserved.
where, HA =Absolute Humidity; HS =Saturation Humidity Nanomaterials including nanowires, nanorods, nanofibers and pnheterojunctions have been explored widely as sensing materials owing to high surface to volume ratios that is inherent in them [10,11]. These hygrometric sensors utilize the variation in their electrical and physical properties in the presence of atmospheric humidity to provide a measure of this humidity, based on adsorption or desorption of water molecules. Ceramic materials offer superior advantages over other existing humidity sensing materials. The porous nature of ceramics (inorganic) plays a vital role in physisorption of water molecules making them suitable for humidity sensing. Intergranular and intragranular as well as pore size distribution determine the performance of these sensing materials. These inorganic sensing materials also offer the advantages of mechanical strength, thermal stability and resistance to chemical attacks over polymer based thin film humidity sensors. The disadvantage of ceramic based sensors is that they require initial heating to remove contaminants such as oil and dust. Organic polymers based sensing materials on the other hand, can operate effectively at room temperatures and are cost-effective, easy to fabricate and have good sensitivity. However, organic polymer based sensors suffer from long term drift, low water-durability, slow response times and limited operation in harsh environments. To address the aforesaid issues, hybrid inorganic-organic materials have gained
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Fig. 1. (a) CuPc nanorods (b) MoS2 deposited on silicon surface (c) CuPc deposited on MoS2.
Fig. 2. Measurement Setup.
relevance and are being viewed as a new class of sensing elements. The combined structures of inorganic-organic thin films overcome most of the disadvantages of the individual materials providing low resistance, faster and more linear response. Two dimensional materials like graphene have been widely explored for humidity sensing purposes due to their high surface to volume ratio, excellent mechanical properties and high flexibility. However, in the graphene based humidity sensors, the resistance change is not abrupt upon adsorption of water molecules. This significantly increases the response time. Graphene-like 2D monolayer MoS2 is an n-type semiconductor material that exhibits superior electrical and mechanical properties as compared to graphene [12–14]. Molybdenum disulphide (MoS2 ) is a Transition Metal Dichalcogenide (TMDC) which has recently garnered increased research interests [15–17]. TMDCs have the general formula MX2, where M = transition metal (Ti, Hf, Ta, Te, Pd, Co, Mo, Pt, Zr) and X = Chalcogens (S, Se, Te). These materials are chemically inert, have superior thermal properties and exhibit tunable bandgaps. Metal substituted phthalocyanines are organic semiconducting materials which have found wide applications in solar cells, fuel cells and thin film sensors [18–21]. CuPc (Copper Phthalacyanine) is a p-type semiconductor material that has also been used in recent works for the fabrication of solar cells, fuel cells and thin film sensors [22–26]. In this work, we have fabricated an organic-inorganic p-n heterojunction using MoS2 -CuPc nanoflower
heterostructure and verified its sensitivity towards humidity sensing.
2. Experimental 2.1. Materials and methods MoS2 ultrafine powder (98% pure) was purchased from Loba Chemie Pvt. Ltd. The MoS2 sol-gel was prepared by heating 0.5 g of MoS2 in 2-propanol. The precursor materials used for the preparation of CuPc nanorods were 4 g phthalamide (0.027 mol), 1.62 g urea (0.027 mol), 1.16 g Copper (II) chloride dihydrate (0.007 mol) and 0.03 g ammonium molybdate (0.00002 mol) (catalyst). Firstly, the urea was crushed using mortar and pestle. All the other materials were then added one by one and grown until thoroughly mixed. The mixture was then place in a ceramic crucible and heated on a hot plate. When the temperature of the mixture reaches 100 ◦ C, the water gradually evaporates from the mixture. The mixture is intermittently stirred using a glass rod throughout the heating process. At approximately 200 ◦ C the powdery mixture begins to melt. At this point the mixture is covered to prevent the sublimation of the phthalamide. Finally, a dark blue glassy mixture is formed which is removed from the crucible using a glass rod. The mixture is filtered with distilled water, followed by 10 ml dil. NaOH solution, 15% HCl
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Fig. 3. XRD pattern of MoS2 after modification of CuPc.
Fig. 4. SEM image of (a) side-view of the thin film sensor (b) CuPc modified MoS2 nanoparticles (c) As-prepared CuPc nanorods (d) CuPc nanoflowers on MoS2 surface.
and finally acetone to remove the un-reacted precursor materials (Fig. 1). 2.2. Sensor fabrication P-type silicon wafer was used as substrate for deposition of the sensing film. The silvery black MoS2 sol was spray-deposited
and heated to 110 ◦ C to remove the 2-propanol. Then the substrate was subjected to heat treatment at 450 ◦ C. The silvery-black MoS2 layer turns grayish white after heat treatment. The CuPc layer was deposited onto the MoS2 layer by means of thermal evaporation at 10−5 torr vacuum and 30A current. This was followed by heat treatment to form the heterojunction. Copper contacts were made to facilitate measurement.
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Fig. 5. I–V characteristics.
Fig. 7. Resistance vs. Error.
Fig. 6. Resistance vs. Relative Humidity.
2.3. Measurement setup Fig. 8. Response Curve.
The electrical response of the sensors towards change in humidity was measured by deploying an in-house built controlled ambient chamber. The input to the controlled humidity chamber is taken from compressed air dryer (used for dehumidification up to 8% RH) and a humidifier. The amount of humidity in the controlled humidity chamber is varied using a PID controller. The fabricated humidity sensor and a reference humidity sensor are placed within the controlled humidity chamber. The reference humidity sensor used was SHT1x Humidity sensor purchased from dF Robot. The reference humidity sensor output is fed into the Intel Edison board and uploaded onto ThingsSpeak open source cloud platform. The relative humidity of the chamber can be viewed on the ThingSpeak portal. The resistance values are measured at the corresponding RH% values and graphically plotted (Fig. 2). 3. Results and discussions The as-prepared MoS2 -CuPc composite films were characterized by X-Ray diffractometer (Bruker, D2 Phaser) employing Ni-filtered Cu K␣ radiation. The XRD patterns as shown in Fig. 3 exhibits well-defined peaks. This is indicative of excellent crystallization. The diffraction peaks may be indexed to orthorhombic phase. No peaks of any other phases are detected. The lower spikes in the XRD pattern are due to background noise. The clear domi-
nant peaks of MoS2 modified CuPc is hence sufficient to justify the deposited layers. The side-view of the thin film sensor is shown in Fig. 4(a). The three layers namely, the silicon substrate, MoS2 layer and the CuPc layer, can be clearly observed. The morphology of the CuPc modified MoS2 nanoparticles were analyzed by SEM. The MoS2 CuPc heterostructure appears in the form of flakes as shown in Fig. 4(b). The morphology of CuPc nanorods is shown in Fig. 4(c). The average width of the CuPc nanorods is approximately 150 nm and average length is about 750 nm. The nanorods are not entirely straight but have uniform diameters throughout their length. CuPc modified MoS2 hybrid structures are shown in Fig. 4(d). The SEM image shows the formation of a uniformly grown network of CuPc nanoflowers lying parallel to the MoS2 surface. The average size (diameter) of the nanoflowers is approximately 750 nm. The flowers comprise of clusters of CuPc nanorods with length of around 280 nm and a few nanometers in width. The structure of such nanoflowers arises due to the interaction of CuPc molecules with MoS2 . The nanoflowers have very high surface-to-volume ratio owing to which the prepared samples exhibit high sensitivity. The I–V characteristics are shown in Fig. 5. The results conform to the expected characteristics for pn-heterojunction based sensors.
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Fig. 9. Stability Curve.
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Change of resistance was measured during the process of absorption for increasing percentage Relative Humidity and then for the process of desorption for decreasing percentage Relative Humidity. Fig. 6 shows the variation of resistance with percentage Relative Humidity during absorption and desorption. The measured data shows negligible hysteresis. The resistance of the sensor gives optimum results in the range of 20%RH to 98%RH. The maximum resistance obtained is 50 M at 20%RH and the minimum resistance obtained is 2M 98% RH. The resistance decreases linearly with increasing percentage Relative Humidity. The linear response is advantageous over exponential response [18]. The sensitivity described as S=R/%RH is 0.615 M/%RH. The linear response is a superior advantage of inorganic-organic based pn heterojunction humidity sensors. The p-type CuPc nanorods present on the upper layer of the sensors have high surface to volume ratio which enhances physisorption of the water molecules. The estimated bandgap of the MoS2 -CuPc heterojunction is 1.8 eV. As RH% is increased, the water molecules distribute over the surface of the CuPc layer and enter the nanorod structures via capillary action. The water molecules dissociate into H+ and OH− . The reduced resistance and hence increased conductance is owing to the proton hopping of the H+ ions. The H+ ions in proximity with the p-type
Fig. 10. (a). Relative humidity vs. Resistance w.r.t. aging of the sensor and Fig. (b) BET analysis of the sensor.
Fig. 11. Resistivity of different samples in different conditions sample 1(annealed temperature 350◦ C) and sample 2 (annealed temperature 400◦ C).
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Table 1 Comparison performance of different humidity sensors developed in recent years with the current research work [22–26]. Materials
Device Structures
Sensing Mechanism
Detecting Ranges
Maximum Sensitivity
Response Time
Recovery Time
References
TiO2 Grapnene Oxide, Silicon LiCl, HPPMs Ce-doped ZnO Polyvinyl Alcohol MoS2 -CuPc
IDT single layer Bi-Layered Bridge Double layered cpacitor IDT Single layered Optical fiber grating Heterostructure
Resistive Resistive Impedance Based Impedance Based Optical Resistive
30% to 9o% RH 10% to 100% RH 10% to 90% RH 11% to 98% RH 30% to 95% RH 20% RH to 98% RH
3.6mv/%RH 27V/%RH Not Measured 1M/% RH 0.54 nW/%RH 0.615 M/%RH
58s 19s 2s 13s 2s 50 s
65s 10s 32s 17s Not measured 25 s
[22] [23] [24] [25] [26] Current work
CuPc makes the heterojunction forward biased and allows conduction of charge. As the RH% is further increased positive Hydronium (H3 O+ ) ions are formed which further increases the conductance. H2 O <=> H+ +OH− (atlowRH%) H2 O + H2 O <=> H3 O+ +OH− (athighRH%) Fig. 7 shows the error in measurement plot for different values of resistance. In case of absorption, it is observed that the error in measurement is upto ±3.7% for 50 Mohm resistance value. The error reduces for lower values of resistance and is a minimum of ±0.28% for 5 Mohm. Hence, the maximum error in measurement is ±3.7% which is within tolerable limits. In case of desorption, it is observed that the error in measurement is upto ±3.51% for 50 Mohm resistance value. The error reduces for lower values of resistance and is a minimum of ±0.2% for 5 Mohm. Hence, the maximum error in measurement is ±3.51% which is within tolerable limits. The response time is denoted as the time required by a sensor to reach 90% of its final value with change in the measured parameter. The response time and recovery time were calculated to be 50 s and 25 s, respectively, as shown in Fig. 8. The Fig. 9 shows the variation of resistance values over a period of ten days for three values of RH%. Fig. 10 shows the measured resistance values are almost stable over the period of observation. The graph shows variation of resistance versus Relative Humidity for three time instants, just after fabrication, one month after fabrication and 6 months after fabrication. As expected, the resistance decreases with increase in %RH. The resistance reduces with increasing time period for a given value of %RH as observed from the figure. The decrease in resistance is less than 10 Mohm beyond 40% RH value. This suggests that the sensor shows good repeatability (Fig. 11). Following bar chart shows the comparison of two sets of samples that is kept in drying agent and in open environment respectively. All the four samples were heated at 400 ◦ C after fifteen days of fabrication. There is a minor change in resistivity of samples. The change in value after heating indicates the increase of pore size due to aging of sensor in fifteen days which was proved by BET analysis by Fig. 10(b). After heating the sensor at 400 ◦ C, the hydrated wall was dried and resudual water layer inside the pore was completely destroyed. The resistance of humidity sensor is seen to decrease almost linearly with increase in RH%. The value obtained at 20% RH is 50 M which reduces to about 2 M at 98%RH. The sensitivity is measured to be 0.615 M/%RH.” (Table 1) 4. Conclusion In this work, an organic-inorganic pn-heterojunction based humidity sensor was fabricated using MoS2 and CuPc nanorods. The SEM results indicated the formation of CuPc nano flowers on the MoS2 surface. The nanoflowers result from clusters of CuPc nanorods and have an average diameter of approximately 750 nm. The nanoflowers provide the superior advantage of high surface-tovolume ratio. The XRD results suggest excellent crystallization and
MoS2 surface modification due to the formation of the nanoflowers. The sensor offers linear response as evident from Fig. 6, negligible hysteresis which is substantiated by Fig. 5 and high stability as revealed from Fig. 9. The sensitivity is measured to be 0.615 M/%RH. The response and recovery times are 50 s and 25 s, respectively as evident from Fig. 8. Thus, the hybrid organic-inorganic sensing materials overcome the inherent disadvantages of the individual material and found to be stable and highly sensitive humidity sensor which can operate at room temperature.
Acknowledgements Abir Jana, Komal Kumari and Anup dey thankfully acknowledge the financial support obtained from RUSA2.0 in the form of Project Assistant; Reference No.: R-11/370/19 and CSIR in the form of SRFs; ACK No.: 143705/2K18/1; File No.: 09/096(0964)/2019-EMR-I and (file no: 09/096(0883)/2017-EMR-I, Ack. No. - 143170/2K15/1) respectively.
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Biographies
Abir Jana (S’18) was born in Hooghly-Chinsurah, West Bengal in 1993 and obtained his B.Tech degree from WBUT, 2014. He completed his M.Tech degree from Jadavpur University (JU), Kolkata in 2018. He is currently working towards his Ph.D from Dept. of Electronics and Telecommunication Engineering, Jadavpur University. He has published 2 journals. His research interests includes Simulation and Fabrication of Non-conventional Solar Cell.
Komal Kumari (S’18) was born in Bhagalpur, Bihar in 1991 and obtained her B.Tech degree from WBUT, 2014. She completed her M.Tech degree from Jadavpur University (JU), Kolkata in 2018. She is currently working towards her Ph.D from Dept. of Electronics and Telecommunication Engineering, Jadavpur University. She has published 2 journals. Her research interests includes Simulation and Fabrication of Perovskite, Dye-sensitized and Tandem Solar Cell.
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Anup Dey received his B.Tech. degree from the St. Thomas College of Engineering and Technology in 2012. He completed his M.Tech in the Dept. of Electronics and Telecommunication Engineering, Jadavpur University in 2015. Presently he is continuing research work leading to PhD (CSIR, SRF, India) in Jadavpur University in the department of Electronics & Telecommunication Engineering. His current research focuses on Metal oxides based thinfilm gas sensors and its applications.
Mr P. S. Sreenivas Reddy is currently an Associate Professor in the dept. of electronics and telecommunication engineering of Nalla Narasimha Reddy Group of Institutions (NNRG).
Bikram Biswas received his B.Tech. Degree in Electronics and Communication Engineering from Ideal Institute of Engineering, West Bengal, India in 2016, and is currently pursuing M.Tech. in VLSI Design and Microelectronics Technology in the department of Electronics and Telecommunication Engineering, at Jadavpur University, West Bengal, India. His research interests include metal oxides based thin-film gas sensors and its applications.
Bhaskar Gupta (S’83–A’89–M’90–S–’99) was born in Kolkata, India, in 1960. He received the B.E.Tel.E., M.E.Tel.E., and Ph.D. degrees from Jadavpur University, Kolkata, India, in 1982, 1984, and 1996, respectively. He is currently a Professor in the Department of Electronics and Telecommunications Department, Jadavpur University, where he has been teaching since 1985.He has published about 360 research articles in refereed journals and conferences and coauthored three books on advanced research topics. His present area of interest is planar antennas, dielectric resonator antennas, wearable antennas, photonic band gap materials, computational electromagnetic, MEMS design and application of soft computing techniques in microwave engineering and antennas. Dr. Gupta is a Fellow of IETE, a Fellow of the Institution of Engineers (India) and a Life Member of SEMCE(I). He was the Chairman of AP-MTT Joint Chapter, IEEE Calcutta Section and Chairman, Students’ Activities, IEEE Calcutta Section. He has supervised twenty five doctoral theses and is presently guiding ten more. He served as Referee in different internationally acclaimed journals and as Guest Editor to the Asian Journal of Physics. Further, he successfully completed nineteen research projects sponsored by various agencies and is currently working on two more, including national and international collaborative projects. He has been named in the 2009 edition of Marquis’ Who’s Who of the World. Subir Kumar Sarkar is currently a Professor of Electronics and Telecommunication Engineering Department, Jadavpur University, India. He has completed 18 R&D projects sponsored by different Government of India funding agencies and published more than 590 technical research papers in international/national journals and peer reviewed conferences. His research interest includes nano devices and low power VLSI circuits, computer networks, digital watermarking and RFID. He is also a senior member of IEEE, IEEE EDS distinguished lecturer, life fellow of IE (India) and IETE, life member of ISTE and life member of IACS.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fabrication of P-N heterojunction based MoS2 modified CuPc nanoflowers for humidity sensing Abir Jana a,∗ , Komal Kumari a , Anup Dey a , P.S. Sreenivas Reddy b , Bikram Biswas a , Bhaskar Gupta a , Subir Kumar Sarkar a a
Department of E.T.C.E, Jadavpur University, Kolkata, West Bengal, 700032, India Nalla Narasimha Reddy Educational Society’s Group of Institutions, Korremula X Road, Via Narapally, Ghatkesar Mandal, Chowdariguda, Telangana, 500088, India b
a r t i c l e
i n f o
Article history: Received 5 July 2019 Received in revised form 19 August 2019 Accepted 24 August 2019 Available online 12 September 2019 Keywords: CuPc nanoflowers Humidity sensing Organic-inorganic p-n heterojunction Device aging
a b s t r a c t In recent times, there has been an increased demand for cost-effective, robust and highly reliable humidity sensors. 2D material MoS2 (n-type) have demonstrated their potential application in chemical and humidity sensing. In this work, an organic-inorganic p-n heterojunction based sensor has been fabricated using the inorganic MoS2 and the organic CuPc, and its viability towards humidity sensing has been experimentally demonstrated. The thin film sensor was characterized by SEM and the results revealed the formation of CuPc nanoflowers on MoS2 surface. The XRD results also indicate excellent crystallization. The sensor shows reduced resistance with increasing RH% and the variation has been observed to be almost linear. The sensing range is from 20% RH to 98% RH. The measured sensitivity is 0.615 M/%RH. The demonstrated results are of great interest in terms of sensitivity, linear response, range of humidity monitoring and stability. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Recent advancements in the field of sensor technologies have resulted in low-power, miniaturized, high speed and cost effective sensors. Humidity is a crucial ambient parameter which needs to be monitored for a wide range of applications such as automated systems, instrumentation, climatology, agriculture, etc. Ceramic, semiconductor and organic polymer based materials have been utilized for fabrication of humidity sensors [1–9]. The sensing materials suitable for high performance humidity sensors must have-(a) high surface to volume ratio for better physisorption of water molecules, (b) the ability to interact with the water molecules repeatedly for enhanced life-cycle and faster response times. When Humidity may be given in terms of’ Relative Humidity’, ‘Parts Per Million’ by weight or by volume and ‘Absolute Humidity’. Most humidity sensors are calibrated to measure Relative humidity. Relative humidity in percentage is expressed as follows: RH
= (HA /HS )×100
∗ Corresponding author. E-mail address: [email protected] (A. Jana). https://doi.org/10.1016/j.sna.2019.111574 0924-4247/© 2019 Elsevier B.V. All rights reserved.
where, HA =Absolute Humidity; HS =Saturation Humidity Nanomaterials including nanowires, nanorods, nanofibers and pnheterojunctions have been explored widely as sensing materials owing to high surface to volume ratios that is inherent in them [10,11]. These hygrometric sensors utilize the variation in their electrical and physical properties in the presence of atmospheric humidity to provide a measure of this humidity, based on adsorption or desorption of water molecules. Ceramic materials offer superior advantages over other existing humidity sensing materials. The porous nature of ceramics (inorganic) plays a vital role in physisorption of water molecules making them suitable for humidity sensing. Intergranular and intragranular as well as pore size distribution determine the performance of these sensing materials. These inorganic sensing materials also offer the advantages of mechanical strength, thermal stability and resistance to chemical attacks over polymer based thin film humidity sensors. The disadvantage of ceramic based sensors is that they require initial heating to remove contaminants such as oil and dust. Organic polymers based sensing materials on the other hand, can operate effectively at room temperatures and are cost-effective, easy to fabricate and have good sensitivity. However, organic polymer based sensors suffer from long term drift, low water-durability, slow response times and limited operation in harsh environments. To address the aforesaid issues, hybrid inorganic-organic materials have gained
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Fig. 1. (a) CuPc nanorods (b) MoS2 deposited on silicon surface (c) CuPc deposited on MoS2.
Fig. 2. Measurement Setup.
relevance and are being viewed as a new class of sensing elements. The combined structures of inorganic-organic thin films overcome most of the disadvantages of the individual materials providing low resistance, faster and more linear response. Two dimensional materials like graphene have been widely explored for humidity sensing purposes due to their high surface to volume ratio, excellent mechanical properties and high flexibility. However, in the graphene based humidity sensors, the resistance change is not abrupt upon adsorption of water molecules. This significantly increases the response time. Graphene-like 2D monolayer MoS2 is an n-type semiconductor material that exhibits superior electrical and mechanical properties as compared to graphene [12–14]. Molybdenum disulphide (MoS2 ) is a Transition Metal Dichalcogenide (TMDC) which has recently garnered increased research interests [15–17]. TMDCs have the general formula MX2, where M = transition metal (Ti, Hf, Ta, Te, Pd, Co, Mo, Pt, Zr) and X = Chalcogens (S, Se, Te). These materials are chemically inert, have superior thermal properties and exhibit tunable bandgaps. Metal substituted phthalocyanines are organic semiconducting materials which have found wide applications in solar cells, fuel cells and thin film sensors [18–21]. CuPc (Copper Phthalacyanine) is a p-type semiconductor material that has also been used in recent works for the fabrication of solar cells, fuel cells and thin film sensors [22–26]. In this work, we have fabricated an organic-inorganic p-n heterojunction using MoS2 -CuPc nanoflower
heterostructure and verified its sensitivity towards humidity sensing.
2. Experimental 2.1. Materials and methods MoS2 ultrafine powder (98% pure) was purchased from Loba Chemie Pvt. Ltd. The MoS2 sol-gel was prepared by heating 0.5 g of MoS2 in 2-propanol. The precursor materials used for the preparation of CuPc nanorods were 4 g phthalamide (0.027 mol), 1.62 g urea (0.027 mol), 1.16 g Copper (II) chloride dihydrate (0.007 mol) and 0.03 g ammonium molybdate (0.00002 mol) (catalyst). Firstly, the urea was crushed using mortar and pestle. All the other materials were then added one by one and grown until thoroughly mixed. The mixture was then place in a ceramic crucible and heated on a hot plate. When the temperature of the mixture reaches 100 ◦ C, the water gradually evaporates from the mixture. The mixture is intermittently stirred using a glass rod throughout the heating process. At approximately 200 ◦ C the powdery mixture begins to melt. At this point the mixture is covered to prevent the sublimation of the phthalamide. Finally, a dark blue glassy mixture is formed which is removed from the crucible using a glass rod. The mixture is filtered with distilled water, followed by 10 ml dil. NaOH solution, 15% HCl
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Fig. 3. XRD pattern of MoS2 after modification of CuPc.
Fig. 4. SEM image of (a) side-view of the thin film sensor (b) CuPc modified MoS2 nanoparticles (c) As-prepared CuPc nanorods (d) CuPc nanoflowers on MoS2 surface.
and finally acetone to remove the un-reacted precursor materials (Fig. 1). 2.2. Sensor fabrication P-type silicon wafer was used as substrate for deposition of the sensing film. The silvery black MoS2 sol was spray-deposited
and heated to 110 ◦ C to remove the 2-propanol. Then the substrate was subjected to heat treatment at 450 ◦ C. The silvery-black MoS2 layer turns grayish white after heat treatment. The CuPc layer was deposited onto the MoS2 layer by means of thermal evaporation at 10−5 torr vacuum and 30A current. This was followed by heat treatment to form the heterojunction. Copper contacts were made to facilitate measurement.
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Fig. 5. I–V characteristics.
Fig. 7. Resistance vs. Error.
Fig. 6. Resistance vs. Relative Humidity.
2.3. Measurement setup Fig. 8. Response Curve.
The electrical response of the sensors towards change in humidity was measured by deploying an in-house built controlled ambient chamber. The input to the controlled humidity chamber is taken from compressed air dryer (used for dehumidification up to 8% RH) and a humidifier. The amount of humidity in the controlled humidity chamber is varied using a PID controller. The fabricated humidity sensor and a reference humidity sensor are placed within the controlled humidity chamber. The reference humidity sensor used was SHT1x Humidity sensor purchased from dF Robot. The reference humidity sensor output is fed into the Intel Edison board and uploaded onto ThingsSpeak open source cloud platform. The relative humidity of the chamber can be viewed on the ThingSpeak portal. The resistance values are measured at the corresponding RH% values and graphically plotted (Fig. 2). 3. Results and discussions The as-prepared MoS2 -CuPc composite films were characterized by X-Ray diffractometer (Bruker, D2 Phaser) employing Ni-filtered Cu K␣ radiation. The XRD patterns as shown in Fig. 3 exhibits well-defined peaks. This is indicative of excellent crystallization. The diffraction peaks may be indexed to orthorhombic phase. No peaks of any other phases are detected. The lower spikes in the XRD pattern are due to background noise. The clear domi-
nant peaks of MoS2 modified CuPc is hence sufficient to justify the deposited layers. The side-view of the thin film sensor is shown in Fig. 4(a). The three layers namely, the silicon substrate, MoS2 layer and the CuPc layer, can be clearly observed. The morphology of the CuPc modified MoS2 nanoparticles were analyzed by SEM. The MoS2 CuPc heterostructure appears in the form of flakes as shown in Fig. 4(b). The morphology of CuPc nanorods is shown in Fig. 4(c). The average width of the CuPc nanorods is approximately 150 nm and average length is about 750 nm. The nanorods are not entirely straight but have uniform diameters throughout their length. CuPc modified MoS2 hybrid structures are shown in Fig. 4(d). The SEM image shows the formation of a uniformly grown network of CuPc nanoflowers lying parallel to the MoS2 surface. The average size (diameter) of the nanoflowers is approximately 750 nm. The flowers comprise of clusters of CuPc nanorods with length of around 280 nm and a few nanometers in width. The structure of such nanoflowers arises due to the interaction of CuPc molecules with MoS2 . The nanoflowers have very high surface-to-volume ratio owing to which the prepared samples exhibit high sensitivity. The I–V characteristics are shown in Fig. 5. The results conform to the expected characteristics for pn-heterojunction based sensors.
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Fig. 9. Stability Curve.
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Change of resistance was measured during the process of absorption for increasing percentage Relative Humidity and then for the process of desorption for decreasing percentage Relative Humidity. Fig. 6 shows the variation of resistance with percentage Relative Humidity during absorption and desorption. The measured data shows negligible hysteresis. The resistance of the sensor gives optimum results in the range of 20%RH to 98%RH. The maximum resistance obtained is 50 M at 20%RH and the minimum resistance obtained is 2M 98% RH. The resistance decreases linearly with increasing percentage Relative Humidity. The linear response is advantageous over exponential response [18]. The sensitivity described as S=R/%RH is 0.615 M/%RH. The linear response is a superior advantage of inorganic-organic based pn heterojunction humidity sensors. The p-type CuPc nanorods present on the upper layer of the sensors have high surface to volume ratio which enhances physisorption of the water molecules. The estimated bandgap of the MoS2 -CuPc heterojunction is 1.8 eV. As RH% is increased, the water molecules distribute over the surface of the CuPc layer and enter the nanorod structures via capillary action. The water molecules dissociate into H+ and OH− . The reduced resistance and hence increased conductance is owing to the proton hopping of the H+ ions. The H+ ions in proximity with the p-type
Fig. 10. (a). Relative humidity vs. Resistance w.r.t. aging of the sensor and Fig. (b) BET analysis of the sensor.
Fig. 11. Resistivity of different samples in different conditions sample 1(annealed temperature 350◦ C) and sample 2 (annealed temperature 400◦ C).
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Table 1 Comparison performance of different humidity sensors developed in recent years with the current research work [22–26]. Materials
Device Structures
Sensing Mechanism
Detecting Ranges
Maximum Sensitivity
Response Time
Recovery Time
References
TiO2 Grapnene Oxide, Silicon LiCl, HPPMs Ce-doped ZnO Polyvinyl Alcohol MoS2 -CuPc
IDT single layer Bi-Layered Bridge Double layered cpacitor IDT Single layered Optical fiber grating Heterostructure
Resistive Resistive Impedance Based Impedance Based Optical Resistive
30% to 9o% RH 10% to 100% RH 10% to 90% RH 11% to 98% RH 30% to 95% RH 20% RH to 98% RH
3.6mv/%RH 27V/%RH Not Measured 1M/% RH 0.54 nW/%RH 0.615 M/%RH
58s 19s 2s 13s 2s 50 s
65s 10s 32s 17s Not measured 25 s
[22] [23] [24] [25] [26] Current work
CuPc makes the heterojunction forward biased and allows conduction of charge. As the RH% is further increased positive Hydronium (H3 O+ ) ions are formed which further increases the conductance. H2 O <=> H+ +OH− (atlowRH%) H2 O + H2 O <=> H3 O+ +OH− (athighRH%) Fig. 7 shows the error in measurement plot for different values of resistance. In case of absorption, it is observed that the error in measurement is upto ±3.7% for 50 Mohm resistance value. The error reduces for lower values of resistance and is a minimum of ±0.28% for 5 Mohm. Hence, the maximum error in measurement is ±3.7% which is within tolerable limits. In case of desorption, it is observed that the error in measurement is upto ±3.51% for 50 Mohm resistance value. The error reduces for lower values of resistance and is a minimum of ±0.2% for 5 Mohm. Hence, the maximum error in measurement is ±3.51% which is within tolerable limits. The response time is denoted as the time required by a sensor to reach 90% of its final value with change in the measured parameter. The response time and recovery time were calculated to be 50 s and 25 s, respectively, as shown in Fig. 8. The Fig. 9 shows the variation of resistance values over a period of ten days for three values of RH%. Fig. 10 shows the measured resistance values are almost stable over the period of observation. The graph shows variation of resistance versus Relative Humidity for three time instants, just after fabrication, one month after fabrication and 6 months after fabrication. As expected, the resistance decreases with increase in %RH. The resistance reduces with increasing time period for a given value of %RH as observed from the figure. The decrease in resistance is less than 10 Mohm beyond 40% RH value. This suggests that the sensor shows good repeatability (Fig. 11). Following bar chart shows the comparison of two sets of samples that is kept in drying agent and in open environment respectively. All the four samples were heated at 400 ◦ C after fifteen days of fabrication. There is a minor change in resistivity of samples. The change in value after heating indicates the increase of pore size due to aging of sensor in fifteen days which was proved by BET analysis by Fig. 10(b). After heating the sensor at 400 ◦ C, the hydrated wall was dried and resudual water layer inside the pore was completely destroyed. The resistance of humidity sensor is seen to decrease almost linearly with increase in RH%. The value obtained at 20% RH is 50 M which reduces to about 2 M at 98%RH. The sensitivity is measured to be 0.615 M/%RH.” (Table 1) 4. Conclusion In this work, an organic-inorganic pn-heterojunction based humidity sensor was fabricated using MoS2 and CuPc nanorods. The SEM results indicated the formation of CuPc nano flowers on the MoS2 surface. The nanoflowers result from clusters of CuPc nanorods and have an average diameter of approximately 750 nm. The nanoflowers provide the superior advantage of high surface-tovolume ratio. The XRD results suggest excellent crystallization and
MoS2 surface modification due to the formation of the nanoflowers. The sensor offers linear response as evident from Fig. 6, negligible hysteresis which is substantiated by Fig. 5 and high stability as revealed from Fig. 9. The sensitivity is measured to be 0.615 M/%RH. The response and recovery times are 50 s and 25 s, respectively as evident from Fig. 8. Thus, the hybrid organic-inorganic sensing materials overcome the inherent disadvantages of the individual material and found to be stable and highly sensitive humidity sensor which can operate at room temperature.
Acknowledgements Abir Jana, Komal Kumari and Anup dey thankfully acknowledge the financial support obtained from RUSA2.0 in the form of Project Assistant; Reference No.: R-11/370/19 and CSIR in the form of SRFs; ACK No.: 143705/2K18/1; File No.: 09/096(0964)/2019-EMR-I and (file no: 09/096(0883)/2017-EMR-I, Ack. No. - 143170/2K15/1) respectively.
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Biographies
Abir Jana (S’18) was born in Hooghly-Chinsurah, West Bengal in 1993 and obtained his B.Tech degree from WBUT, 2014. He completed his M.Tech degree from Jadavpur University (JU), Kolkata in 2018. He is currently working towards his Ph.D from Dept. of Electronics and Telecommunication Engineering, Jadavpur University. He has published 2 journals. His research interests includes Simulation and Fabrication of Non-conventional Solar Cell.
Komal Kumari (S’18) was born in Bhagalpur, Bihar in 1991 and obtained her B.Tech degree from WBUT, 2014. She completed her M.Tech degree from Jadavpur University (JU), Kolkata in 2018. She is currently working towards her Ph.D from Dept. of Electronics and Telecommunication Engineering, Jadavpur University. She has published 2 journals. Her research interests includes Simulation and Fabrication of Perovskite, Dye-sensitized and Tandem Solar Cell.
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Anup Dey received his B.Tech. degree from the St. Thomas College of Engineering and Technology in 2012. He completed his M.Tech in the Dept. of Electronics and Telecommunication Engineering, Jadavpur University in 2015. Presently he is continuing research work leading to PhD (CSIR, SRF, India) in Jadavpur University in the department of Electronics & Telecommunication Engineering. His current research focuses on Metal oxides based thinfilm gas sensors and its applications.
Mr P. S. Sreenivas Reddy is currently an Associate Professor in the dept. of electronics and telecommunication engineering of Nalla Narasimha Reddy Group of Institutions (NNRG).
Bikram Biswas received his B.Tech. Degree in Electronics and Communication Engineering from Ideal Institute of Engineering, West Bengal, India in 2016, and is currently pursuing M.Tech. in VLSI Design and Microelectronics Technology in the department of Electronics and Telecommunication Engineering, at Jadavpur University, West Bengal, India. His research interests include metal oxides based thin-film gas sensors and its applications.
Bhaskar Gupta (S’83–A’89–M’90–S–’99) was born in Kolkata, India, in 1960. He received the B.E.Tel.E., M.E.Tel.E., and Ph.D. degrees from Jadavpur University, Kolkata, India, in 1982, 1984, and 1996, respectively. He is currently a Professor in the Department of Electronics and Telecommunications Department, Jadavpur University, where he has been teaching since 1985.He has published about 360 research articles in refereed journals and conferences and coauthored three books on advanced research topics. His present area of interest is planar antennas, dielectric resonator antennas, wearable antennas, photonic band gap materials, computational electromagnetic, MEMS design and application of soft computing techniques in microwave engineering and antennas. Dr. Gupta is a Fellow of IETE, a Fellow of the Institution of Engineers (India) and a Life Member of SEMCE(I). He was the Chairman of AP-MTT Joint Chapter, IEEE Calcutta Section and Chairman, Students’ Activities, IEEE Calcutta Section. He has supervised twenty five doctoral theses and is presently guiding ten more. He served as Referee in different internationally acclaimed journals and as Guest Editor to the Asian Journal of Physics. Further, he successfully completed nineteen research projects sponsored by various agencies and is currently working on two more, including national and international collaborative projects. He has been named in the 2009 edition of Marquis’ Who’s Who of the World. Subir Kumar Sarkar is currently a Professor of Electronics and Telecommunication Engineering Department, Jadavpur University, India. He has completed 18 R&D projects sponsored by different Government of India funding agencies and published more than 590 technical research papers in international/national journals and peer reviewed conferences. His research interest includes nano devices and low power VLSI circuits, computer networks, digital watermarking and RFID. He is also a senior member of IEEE, IEEE EDS distinguished lecturer, life fellow of IE (India) and IETE, life member of ISTE and life member of IACS.