In the present study, nanoscale variations in the work function values and the resulting changes in junction properties of chemical vapour deposited two-dimensional (2D) MoS 2 domains has been investigated as a function of number of layers using Kelvin Probe Force Microscopy (KPFM) and Conductive Atomic Force Microscopy(CAFM) techniques. Raman spectroscopy has been employed to obtain the magnitude of difference between E 2g and A 1g peaks which has been used as a signature of the number of layers. Surface potential of MoS 2 monolayer sample exhibits a value of -427 mV (~7.2 mV for bulk) along with a large spread of about 29mV (~3mV for bulk). The present study shows that the optical and electronic properties of MoS 2 1-2 layer samples exhibit a large difference from its bulk counterpart. These characteristic features remain intact even in the presence of adsorbates and defects which result in spread in surface potential values and corresponding changes in junction characteristics. These results are important for the application of Chemical Vapor Deposition (CVD) grown MoS 2 monolayers for semiconductor devices.
In the present study, we report a controlled growth of tin oxide and tin oxide: carbon nanoparticles by an integrated method comprising of the gas phase agglomeration, electrical mobility based size selection, and in–flight sintering steps. The effect of in-flight sintering temperature and variation in growth environment (N2, H2 and O2) during nanoparticle formation, morphology and composition has been investigated by carrying out High Resolution Transmission Electron microscopy and X-Ray diffraction studies. The results highlight the novelty of the present technique to grow alloy and core-shell nanoparticles in which the stoichiometery (x) of SnOx and the mode of incorporation of carbon into the tin oxide lattice (alloy or core-shell structure), along with well-defined size can be controlled independently. Detailed Photoluminescence (PL) studies of well sintered monocrystalline SnO, SnOx and SnO2 nanoparticles along with SnOx:C and SnO2:C alloy and C@SnO core-shell nanoparticle has been carried out. The shift in the position and nature of PL peaks due to band edge, Sn interstitials and oxygen vacancy defect level energy states has been understood as a function of stoichiometery and nanoparticle structure (alloy and core-shell). These results suggest the possibility of tailoring the position of these levels by controlling the size, composition and alloying which is potentially important for gas sensing, photoconductivity and photo-electrochemical applications.
The present study reports the fabrication of MoS2 based optical sensors for tunable, broadband and wavelength selective light detection with single layer and multilayer MoS2 samples. The I–V measurements are performed in a two-terminal configuration with bias voltage from −1 to +1 V and I–t at +1 V for light wavelengths ranging from UV (300–450 nm), visible (500–670 nm), to near infrared (700–1100 nm). A sigmoidal I–V behavior is observed in dark and for optically generated current in single layer and multilayer MoS2 samples. The photoconductivity is studied as a function of different number of layers of MoS2, namely, single layer (1L) and multilayer (seven layers: 7L) on SiO2/Si as the growth substrates. The high value of photoresponse (403 µA mW−1) and responsivity (0.92 A W−1) at 1000 nm exhibited by the 7L MoS2 sample makes it a suitable candidate for highly selective photodetector applications. The quantum efficiency also shows a high value of 120% for 7L sample at 900 nm. UV–visible absorption spectra collected for few layer MoS2 samples grown on sapphire and quartz substrates gives an insight into the presence of van Hove singularities, thereby, leading to high photocurrent in the UV range. An interesting phenomenon of broadband selection of 1L and specific wavelength response of 7L MoS2 samples is observed. Various photocurrent generation mechanisms are seen to be prevalent, namely, due to the inherent van Hove singularities in the band structure, band edge excitation of MoS2, trap assisted and IR generated photocurrent. Hence, based on the selection of number of layers of the active material (2D MoS2), a tunable response ranging from broadband in 1L to near infrared (upto 1000 nm) wavelength selective in 7L is possible, paving the way towards a multifunctional optical sensor.
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