Liquid exfoliation of black phosphorus nanosheets and its application as humidity sensor
Graphical abstract
Introduction
Two dimensional semiconducting layered materials such as transition metal dichalcogenides (TMDCs) have attracted great attention since the inception of search for materials with extraordinary electrical, optical and magnetic properties suitable for nanoelectronic device applications [1], [2], [3], [4], [5], [6], [7], [8], [9]. The Black phosphorus is one of the monotype layered two dimensional (2D) crystals [10], [11], [12]. Black phosphorus nanosheets have been reported to exhibit a high mobility of 1000 cm2 V−1 s−1 for a sample of thickness of 5 nm with high current ON/OFF ratio of 105 [10]. The band gap of black phosphorus was observed to be layer dependent which increases with decrease in number of layers with a value of 0.3 eV for the bulk crystal and 2.0 eV for the single layer [10], [11], [12], [13], [14]. This is exactly similar to TMDCs but with an additional advantage of a direct band gap from bulk to single layer, while the band gap changes from indirect to direct as the layer number is reduced from bulk to monolayer in the case of TMDCs [3], [4], [5], [6], [9], [17], [25]. Importantly, the bulk black phosphorus has been observed to be inherently a p-type semiconductor [15], [16]. A single layer of black phosphorus consists of phosphorus atoms covalently bonded to three nearby phosphorus atoms forming puckered structure [18]. In a bulk black phosphorus each of the single layers of P atoms are bonded to their adjacent layers through van der Waal's weak forces [11], [12], [13], [14], [17], [18].
For the development of nanoelectronics and photonic devices based on 2D materials they should have an ideal energy band gap in order to cover the wide range of electronic devices. [14], [19], [26], [39], [41], [56], The band gap of these materials can be tuned by 50% as the materials can be thinned down to single layers [18]. Thus, in the case of black phosphorus the layer dependent band gap values lay between those of graphene and semiconducting TMDCs [11]. As a result it covers the entire range of spectrum from 0 to 2.0 eV supported by other layered materials. The change in band gap of black phosphorus can be used in the devices which operate at IR optoelectronic to high mobility quantum transport devices [10].
Further, for black phosphorus the electron effective mass is observed to be several times lighter in armchair direction than in the zigzag direction [10], [14]. On integrating the possible perspectives the layered black phosphorus can have new characteristics and extraordinary properties for anisotropic electronic, optoelectronic, mechanical or magnetic properties. Recently there have been huge studies on 2D black phosphorous materials based devices such as photodetectors, field effect transistors and other optoelectronic devices [53]. Black phosphorus either in the form of single layer or few layers has so far been reported as a field effect transistor, photodetector, and p–n junction diode in heterojuction with MoS2, radio frequency transistors etc. [12], [13], [14], [15], [16], [17], [18], [40], [41].
The humidity sensors are among the most significant devices of importance in industry and instrumentation requirements. Monitoring and control of humidity in the industries, equipments, laboratories and in environment is of high importance and essential for safety precautions. Development of sensing devices with quick response and recovery times, ability to sense wide range of humidity with high sensitivity and stability is a frontier area of research in sensor. Among the materials studied for fabricating the humidity sensors includes the ceramics, metal oxides, nanowires, semiconductor nanoparticles, and composite heterostructure etc. [20], [21], [22], [23] However, these low-dimensional nano-heterostructures based sensors mostly limited to operate only at high temperature. The 2D graphene nanosheets has been utilized to sense the individual gas molecules, which lead to the ultimate limit of the sensitivity [1b]. The 2D inorganic layered materials such as MoS2 have attracted a keen interest as perspective sensor materials for next generation due to their high surface to volume ratio which can translate into extremely sensitive gas-sensor device via charge transfer on the surface and 2D planar structure favouring easy fabrication of devices.
The 2D materials have also been tested for the humidity and gas sensing performance which including graphene, reduced graphene oxide and its composites with other materials and inorganic layered materials such as MoS2, WS2, MoSe2 etc. [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [58] Though, black phosphorus has many attractive properties for optoelectronic device application so far it has not been tested for its performance towards humidity sensing. Black phosphorus is expected to possess high surface area due to its puckered structure. It is known that single to few layer thick black phosphorus nanosheets are very much sensitive to moisture [13], [38], [50], [52], [59], [60]. This property of black phosphorus could be taken advantage in the sensing of humidity. However, till date there are only a few theoretical articles describing black phosphorus as a perspective sensor material for ad-atoms, physisorption of molecules such as CO, H2, H2O, NH3, NO, and NO2 etc. [42], [43], [44], [45] Recently, there is a experimental report on the N02 gas sensor based on black phosphorus nanosheets to the best of knowledge [46]. Here, in this article the humidity sensing behaviour of black phosphorus nanosheets of three different thickness ranges and lengths synthesized by the well known liquid phase exfoliation method has been demonstrated [6], [7], [8], [47], [48], [49].
Section snippets
Materials
Bulk crystals of Black Phosphorus were purchased from smart-elements (GmbH, Ferrogasse 4/1, 1180 Vienna, AUSTRIA) and N-Methyl-2-Pyrrolidone from High Purity Laboratory Chemical Manufacturers, Mumbai, India. LiCl, MgCl2, K2CO3, NaBr, KI, NaCl, KCl and K2SO4 salts were purchased from Thomas Baker Chemicals private limited (Mumbai, India) and were used as received.
Black phosphorus nanosheets and nanoparticle synthesis
Black phosphorus nanosheets and nanoparticles were synthesized by the well known liquid phase exfoliation method through simple
Black phosphorus nanosheets and nanoparticles
The black Phosphorus nanosheets and nanoparticle were synthesized using liquid exfoliation method. Fig. S1 shows the typical optical photographs of the black phosphorus samples synthesized at 3000, 5000 and 10,000 rpm. The optical photograph clearly shows the change in colour of dispersed samples collected immediately after the synthesis at different rpm of centrifugation have varying thickness and lateral dimension. The TEM and HRTEM analysis were carried out to investigate the morphology and
Conclusion
In conclusion, the simple liquid exfoliation method for synthesis of black phosphorus nanosheets/nanoparticles with varying thickness and lengths were reported. Further, the humidity sensing performance of all the samples with varying lateral dimension and thickness were investigated. The device-3 which consists of nanosheets and nanoparticles exhibited better humidity sensing behaviour as compared to other devices. The response and recovery of the device-3 were also found to be much quicker as
Acknowledgment
The authors would like to thank Professor C. N. R. Rao, FRS (ICMS & JNCASR Bangalore), Professor Vinayak P. Dravid (Northwestern University, USA) for constant support and encouragement. One of the authors Dr. Dattatray J Late would like to thank DST (Government of India) for Ramanujan fellowship (Grant No. SR/S2/RJN-130/2012). Author would also like to thank Professor S. B. Ogale and Director, CSIR-NCL for the experimental facilities. Author would also like to thank Thripuranthaka M., Urmila
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