Field emission and current noise of hydrothermally synthesized MoS2 nanosheets are investigated in ultra-high-vacuum and industrially suited high-vacuum conditions. The study reveals that the emission turn-on field is pressure dependent. Moreover, the MoS2 nano‐sheets exhibit more stable field-electron emission in high- vacuum than in ultra-high-vacuum conditions. The investigations on field-emission current fluctuations show features of 1/f-type noise in ultra-high-vacuum and high-vacuum conditions, attributed to adsorption and desorption processes. The post-field-emission results indicate the MoS2 nanosheets are a robust field emitter in high-vacuum conditions

Chandra S. Rout

Dattatray J. Late

M. Thripuranthaka

Mahendra A. More

Pracheetee D. Joag

Ranjit V. Kashid

Nanomaterials and Nanotechnology

English

 Field emission and current noise of hydrothermally synthesized MoS2 nanosheets are investigated in ultra-high-vacuum and industrially suited high-vacuum conditions. The study reveals that the emission turn-on field is pressure dependent. Moreover, the MoS2 nanosheets exhibit more stable field-electron emission in high-vacuum than in ultra-high-vacuum conditions. The investigations on field-emission current fluctuations show features of 1/f-type noise in ultra-high-vacuum and high-vacuum conditions, attributed to adsorption and desorption processes. The post-field-emission results indicate the MoS2 nanosheets are a robust field emitter in high-vacuum conditions. </jats:p

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Stable Field Emission from Layered MoS<sub>2</sub> Nanosheets in High Vacuum and Observation of 1/f Noise

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ARTICLENanomaterials and NanotechnologyStable Field Emissionfrom Layered MoS2 Nanosheetsin High Vacuum andObservation of 1/f NoiseRegular PaperRanjit V. Kashid1, Pracheetee D. Joag2, M. Thripuranthaka4*, Chandra S. Rout3,Dattatray J. Late4 and Mahendra A. More11 Centre for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune, India2 Maharashtra Institute of Technology College of Engineering, Pune, India3 School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar, India4 Physical and Materials Chemistry Division, National Chemical Laboratory (CSIR), Dr Homi Bhabha Road, Pune, India* Corresponding author(s) E-mail: mam@physics.unipune.ac.inReceived 02 July 2014; Accepted 23 December 2014DOI: 10.5772/60071© 2015 The Author(s). Licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided theoriginal work is properly cited.AbstractField  emission  and  current  noise  of  hydrothermallysynthesized  MoS2  nanosheets  are  investigated  in  ultra-high-vacuum  and  industrially  suited  high-vacuumconditions.  The study reveals that the emission turn-onfield  is  pressure  dependent.  Moreover,  the  MoS2  nano‐sheets exhibit more stable field-electron emission in high-vacuum  than  in  ultra-high-vacuum  conditions.  Theinvestigations on field-emission current fluctuations showfeatures of 1/f-type noise in ultra-high-vacuum and high-vacuum conditions, attributed to adsorption and desorp‐tion processes. The post-field-emission results indicate theMoS2  nanosheets  are  a  robust  field  emitter  in  high-vacuum conditions.Keywords Layered materials, MoS2, Electron emission,Noise measurement, Raman spectroscopy1. IntroductionOf late, advances in the area of two-dimensional (2D)materials like graphene and its inorganic analogues haveattracted a great deal of research attention because of theirpromising applications in various electronic devices owingto their exotic physicochemical properties. One of thesematerials, molybdenum disulphide (MoS2), has beenexplored for a variety of applications including field-effecttransistors [1-3], phototransistors [4], sensors [5-7], inte‐grated circuits [8, 9], and field emission [10-12]. We recentlyobserved that MoS2 with several layers exhibits excellentfield-emission properties in an ultra-high vacuum (UHV)[10]. However, to ensure practical applicability of a fieldemitter it is necessary to understand its current stability andnoise/fluctuations in industry-suited high-vacuum condi‐tions (~10-6 mbar). The noise provides the informationregarding its source. Especially the 1/f type of noise reflectsmany processes at the electronic and atomic level, and thusis an evaluating parameter for reliability of the device [13].1Nanomater Nanotechnol, 2015, 5:10 | doi: 10.5772/60071Therefore, we have investigated field-emission propertiesand current noise from hydrothermally synthesized MoS2nanosheets in UHV (P~3x10-8 mbar) and high-vacuum (HV:P~1x10-6 mbar) conditions.2. Experimental procedureMoS2 sheets were synthesized by a one-step hydrothermalreaction. In a typical experiment, 3 mM Na2MO4(Sigma-Aldrich, 99.98%) and 15 mM thioacetamide (C2H5NS,Sigma-Aldrich, ≥ 99%) were dissolved in 40 mL DI waterand stirred for one hour at room temperature using amagnetic stirrer. The solution was transferred to a 50 mLteflon-lined stainless-steel autoclave, heated up to 200°Cand left for 24 hours. After cooling naturally, the productwas filtered, washed with DI water and dried in a vacuumat 60°C for six hours. The as-synthesized product was thencharacterized by Raman spectroscopy, TransmissionElectron Microscopy (TEM) and Field Emission ScanningElectron Microscopy (FESEM). The Raman spectrum wasrecorded with high-resolution Raman spectrometer(LabRAM HR Horiba Jobin Yvon) using 514 nm as excita‐tion wavelength at room temperature.For field-emission measurements, as-synthesized MoS2nanosheets were ultra-sonicated in ethanol for 10 minutes.The nanosheets were solution cast on pre- cleaned Sisubstrate (~1 cm2). The field-emission investigation ofMoS2 nanosheets on Si substrate was carried out in a diodeconfiguration in both UHV and HV conditions. In the diodeconfiguration, the specimen (MoS2/Si, cathode) wasmounted parallel to a semi-transparent conducting phos‐phor screen (Cu-doped ZnS, the anode), at an inter-electrode separation of ~500 μm. The field-emission current(I) versus applied voltage (V) measurements were carriedout using an electrometer (Keithely 6514) and a high-voltage dc power supply (Spellman, 40 kV). The field-emission current stability was recorded at the pre-setcurrent value of 1 μA using a computer-controlled data-acquisition system, with a sampling time of 10 s. The field-emission current-time traces and the corresponding powerspectra were recorded using a Fast Fourier Transform (FFT)analyser (IWASTU SM-2701). The results were reproduci‐bly repeated for two different specimens prepared underidentical conditions.3. Results and discussionFigure 1 shows the Raman spectrum of the as-synthesizedMoS2 nanosheets, depicting the fingerprints of in-planevibrational mode as E12g at ~ 378 cm-1 and out-of-planevibrational mode as A1g at ~404 cm-1 of MoS2. Carefulobservation of the spectrum reveals appearance of ashoulder at ~ 384 cm-1 near the E12g peak, which may beattributed to defects present in MoS2.Figures 2 (a-b) show low- and high-magnification TEMimages of the MoS2 nanosheets, respectively. The averagedimension of the nanosheets is found to be 1-3 μm. Figures2 (c-d) show the FESEM images of solution-cast MoS2nanosheets on Si substrate. The images clearly reveal thepresence of a number of protruding edges, which act asemission sites.Figure 1. Raman spectrum of as-synthesized MoS2 nanosheetsThe field-emission current-density (J) versus applied-electric-field (E) plots of MoS2 nanosheets in HV and UHVare shown in Figure 3(a). The values of the turn-on fieldrequired to draw emission current density of 1 μA/cm2 inHV and UHV conditions are found to be 10.8 and 11.6V/μm, respectively. Figure 3 (b) shows the correspondingFowler-Nordheim (F-N) plots (a plot of ln (J/E2) versus 1/E)derived from the J-E plots of MoS2 nanosheets in HV andUHV conditions. Both the F-N plots show a linear natureover the entire range of the applied field. Figures 4(a-b)depict the field-emission current stability of MoS2 nano‐sheets recorded in HV and UHV, respectively. The emis‐sion current in UHV exhibits an initial rise and furtherstabilization at a 2 μA current level. The observed initialrise in the current is attributed to the preconditioning of thespecimen due to residual gas ion bombardment [14]. Themagnitude of the standard deviation around the meanvalue is found to be 0.15 and 0.24 μA for the emissioncurrent in HV and UHV, respectively, indicating thatMoS2 nanosheets exhibit relatively stable behaviour in HVcompared to UHV. In UHV and HV conditions, the typicalnumber density of molecules is of the order of ~108 and1010 molecules/cm3, respectively. The time to form amonolayer is ~73 s in UHV and ~2.2 s in HV [15]. In UHVconditions, the coverage of adsorbing gas molecules on thesurface of MoS2 nanosheets is much less in comparison withHV conditions. The continuous ion bombardment duringthe operation of the emitter desorbs the adsorbed gasmolecules from the surface of the emitter in UHV condi‐tions. In HV conditions, the coverage of the adsorbingmolecule is high enough that it is not altered due to ionbombardment. Therefore, in UHV, the temporal changes inwork function due to adsorption/desorption events lead tomore deviations from the average value of the current(Figure 4a) than re observed in the HV conditions (Figure2 Nanomater Nanotechnol, 2015, 5:10 | doi: 10.5772/600714b). This is also reflected in the values of standard devia‐tion, as discussed above. The field-emission patterns wererecorded at ~50 μA and are shown in Figures 4 (c-d). Thefield-emission pattern observed in the HV conditionsexhibits brighter emission spots than in the UHV. Theobserved spots indicate that the emission comes from thesharp protrusions on the edges of the MoS2 nanosheets (asseen in FESEM images, Figures 2 (c-d)).Figure 2. (a-b) Low- and high-magnification TEM images, and (c-d) FESEM images of MoS2 nanosheetsFigure 3. (a) J-E plots and (b) F-N plots of MoS2 nanosheets on Si substrate in HV and UHV, respectivelyTo gain an understanding of the origin of the observedfluctuations in the emission current (noise), the FFTanalyser was used. Figures 5 (a-b) depict the current timetraces and Figures 5 (c-d) show the corresponding powerspectra of MoS2 field emitters operated in UHV and HV,respectively. Figures 5 (a-b) show that the MoS2 fieldemitter exhibits step-like fluctuation, indicating ‘flip-flop’behaviour between two states in the UHV environment,and between multiple states in the HV environment. Thepower spectra corresponding to the current time tracesreveal 1/fα-type behaviour, with α ~0.86 and 0.98 in UHVand HV, respectively. In HV, the emitter is covered withresidual gases, and in such a case α ~1 [15]. α is less than 1for UHV, as found in the present case. These fluctuations3Ranjit V. Kashid, Pracheetee D. Joag, M. Thripuranthaka, Chandra S. Rout, Dattatray J. Late and Mahendra A. More:Stable Field Emission from Layered MoS2 Nanosheets in High Vacuum and Observation of 1/f Noisemay be related to surface diffusion. The observed ‘flip-flop’can be treated as switching between two or more adsorp‐tion states mediated by surface diffusion, as modelled byKlient [16].Figure 4. Field-emission current stability at the pressure of (a) P~3x10-8 mbar, (b) P~1x10-6 mbar. Field-emission patterns recorded at ~50 μA of MoS2 nanosheetsat the pressure of (c) P~3x10-8 mbar, (d) P~1x10-6 mbarFigure 5. Field-emission current-time traces recorded at (a) P ~ 3x10-8 mbar, (b) P ~ 1x10-6 mbar, at the preset value of 1 μA, and the corresponding powerspectra recorded at (c) P ~ 3x10-8 mbar, (d) P ~ 1x10-6 mbar4 Nanomater Nanotechnol, 2015, 5:10 | doi: 10.5772/60071Figure 6. FESEM image of MoS2 nanosheets, (a) pre field emission, (b) postfield emission measurementsAfter taking the field-emission measurements, we furtherinvestigated the surface morphology of the MoS2 nano‐sheets by FESEM (shown in Figures 6(a-b)). It is interestingto note that the surface morphology shows no significantdeterioration after long-term operation, even in HVconditions. The results indicate that the MoS2 nanosheetsconstitute a robust field emitter.4. ConclusionsThe investigation of field-emission current noise showsfeatures of 1/fα noise in UHV and HV conditions, with α ~0.86 and 0.98, respectively. The temporal variations in thelocal work function are due to the adsorption-mediatedsurface diffusion. The MoS2-nanosheets emitter exhibitssignificant stable emission current in HV conditions. TheMoS2 nanosheets prove to be robust in the industry-suitedvacuum environment.5. AcknowledgementsRVK would like to thank the Council of Scientific andIndustrial Research (CSIR), New Delhi, India, for the SRFScheme. DJL would like to thank the Department of Scienceand Technology (DST), Govt. of India, for the award of aRamanujan Fellowship. Thanks are also due to Prof. D. S.Joag, Emeritus Scientist-CSIR, Department of Physics,University of Pune, for useful discussions. This field-emissions work has been carried out as part of the CNQS-UPE-UGC programme.6. References[1] Alam K, Lake R K (2012) Monolayer MoS2 Transis‐tors beyond the Technology Road Map. ElectronDevices. IEEE Transactions on Nanotechnolgy59:3250-3254.[2] Late D J, Liu B, Matte H S, Dravid V P, Rao C N(2012) Hysteresis in single-layer MoS2 field effecttransistors. ACS Nano 6:5635-5641.[3] Radisavljevic B, Radenovic A, Brivio J, GiacomettiV, Kis A (2011) Single-layer MoS2 transistors. Nat.Nanotechnol. 6:147-150.[4] Yin Z, Li H, Li H, Jiang L, Shi Y, Sun Y, Lu G, ZhangQ, Chen X, Zhang H (2011) Single-Layer MoS2Phototransistors. ACS Nano 6:74-80.[5] He Q, Zeng Z, Yin Z, Li H, Wu S, Huang X, ZhangH (2012) Fabrication of Flexible MoS2 Thin-FilmTransistor Arrays for Practical Gas-Sensing Appli‐cations. Small 8:2994-2999.[6] Late D J, Huang Y-K, Liu B, Acharya J, Shirodkar SN, Luo J, Yan A, Charles D, Waghmare U V, DravidV P, Rao C N R (2013) Sensing Behavior of Atomi‐cally Thin-Layered MoS2 Transistors. ACS Nano7:4879-4891.[7] Wu S, Zeng Z, He Q, Wang Z, Wang S J, Du Y, YinZ, Sun X, Chen W, Zhang H (2012) Electrochemi‐cally reduced single-layer MoS2 nanosheets:charac‐terization, properties, and sensing applications.Small 8:2264-2270.[8] Radisavljevic B, Whitwick M B, Kis A (2011)Integrated Circuits and Logic Operations Based onSingle-Layer MoS2. ACS Nano 5:9934-9938.[9] Wang H, Yu L, Lee Y-H, Shi Y, Hsu A, Chin M L, LiL-J, Dubey M, Kong J, Palacios T (2012) IntegratedCircuits Based on Bilayer MoS2 Transistors. NanoLett. 12:4674-4680.[10] Kashid R V, J. Late D, Chou S S, Huang Y-K, De M,Joag D S, More M A, Dravid V P (2013) EnhancedField-Emission Behavior of Layered MoS2 Sheets.Small 9:2730-2734.[11] Li Y B, Bando Y, Golberg D (2003) MoS2 nanoflowersand their field-emission properties. AppliedPhysics Letters 82:1962-1964.[12] Zhang Q, Yu K, Zhao B, Wang Y, Song C, Li S, YinH, Zhang Z, Zhu Z. (2013) Synthesis of a MoS2-MWNT nanostructure with enhanced field emis‐sion and electrochemical properties. RSC Advances3:10994-11000.[13] Jones B K (1993) Electrical Noise as a Measure ofQuality and Reliability in Electronic Devices.Advances in Electronics and Electron Physics87:201-257.[14] Late D J, More M A, Joag D S, Misra P, Singh B N,Kukreja L M (2006) Field emission studies on well5Ranjit V. Kashid, Pracheetee D. Joag, M. Thripuranthaka, Chandra S. Rout, Dattatray J. Late and Mahendra A. More:Stable Field Emission from Layered MoS2 Nanosheets in High Vacuum and Observation of 1/f Noiseadhered pulsed laser deposited LaB6 on W tip.Applied Physics Letters. 89:123510.[15] Roth A (1976) Vacuum Technology. North Holland.The Netherlands.[16] Kleint C (1971) Surface diffusion model of adsorp‐tion-induced field emission flicker noise: I. Theory.Surface Science 25:394-410.6 Nanomater Nanotechnol, 2015, 5:10 | doi: 10.5772/60071

Stable Field Emission from Layered MoS2 Nanosheets in High Vacuum and Observation of 1/f Noise

Field emission and current noise of hydrothermally synthesized MoS2 nanosheets are investigated in ultra- high-vacuum and industrially suited high-vacuum conditions. The study reveals that the emission turn-on field is pressure dependent. Moreover, the MoS2 nano‐ sheets exhibit more stable field-electron emission in high- vacuum than in ultra-high-vacuum conditions. The investigations on field-emission current fluctuations show features of 1/f-type noise in ultra-high-vacuum and high- vacuum conditions, attributed to adsorption and desorp‐ tion processes. The post-field-emission results indicate the MoS2 nanosheets are a robust field emitter in high- vacuum conditions

Stable Field Emission from Layered MoS2 Nanosheets in High Vacuum and Observation of 1/f Noise

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