Exciting Results from our Quantum Capacitance Measurement in 2015
• Probing the electron states and metal-insulator transition mechanisms in MoS2
- See details in Nature Communications 6, Article number:6088- doi:10.1038/ncomms7088 (2015)..Click here.
The metal-insulator transition is one of
the remarkable electrical properties of atomically thin molybdenum disulphide.
Although the theory of electron–electron interactions has been used in
modelling the metal-insulator transition in molybdenum disulphide, the
underlying mechanism and detailed transition process still remain largely
unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor
heterostructures built from atomically thin molybdenum disulphide are
ideal capacitor structures for probing the electron states. The vertical
configuration offers the added advantage of eliminating the influence of large
impedance at the band tails and allows the observation of fully excited
electron states near the surface of molybdenum disulphide over a wide excitation frequency and
temperature range. By combining capacitance and transport measurements, we have
observed a percolation-type metal-insulator transition, driven by density inhomogeneities of
electron states, in monolayer and multilayer molybdenum disulphide. In
addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties
a,b) Our vertical capacitance device structures. The MoS2 flakes are fully covered by a top Ti/Au electrode. The square top electrode in a is the reference capacitor. Scale bar, 10 μm. (c) The equivalent circuit of the MoS2 capacitance devices. Total capacitance Ct measured from a 5.9-nm-thick MoS2 at 2 K at different frequencies (d) and excitation voltages (e), respectively. The measured capacitance in vertical heterostructures is almost independent of excitation frequencies, which differs greatly from that obtained in conventional FET structures. The excitation voltage used for d is 50 mV and the frequency used for e is 100 kHz.
a) Ct measured at 300 K for different excitation frequencies. (b) Ct measured at 1 kHz for different temperatures. (c) Phase information plotted as a function of excitation frequencies at 300 K for different Vg. The phase peak around 20 kHz yields a relaxation time of hole carriers at 50 μs. (d) The quantum capacitance Cq of MoS2 plotted as a function of surface potential Vs at 300 K, which yields a band gap width of around 1.14 eV. The excitation voltage used is 100 mV. The dashed line above breaks schematically shows the expected quantum capacitance at higher Fermi energies.
a–d) The schematic band diagrams of metal-BN-MoS2-metal structures at flat band (a), accumulation region (b), depletion region (c) and inversion region (d). (e–j) The schematic images showing the percolation-induced MIT under different effective thicknesses of electron states (e–g) and carrier densities (h–j). The circles denote isolated carrier puddles in MoS2. (k,l) The measured total capacitance Ct (k) and effective thickness deff (l) plotted as a function of gate voltage Vg for 2–300 K. The excitation voltage and frequency used are 50 mV and 100 kHz, respectively. (m) deff plotted as a function of temperatures at different carrier densities n.
- Our vertical cpapcitance heterostructures built from atomically thin MoS2 are ideal capacitor structures for probing the electron states and intrinsic properties of MoS2.
- The vertical heterostructures offer the added advantages of eliminating the influence of large impedance at the band tails and accessing intrinsic characteristics such as thickness-dependence dielectric constant and band gap variation in atomically thin MoS2.
- We believe that the percolationtype MIT (driven by density inhomogeneities of electron states) is the dominating mechanism of the MIT in both monolayer and multilayer MoS2.
-The present study also provides a new approach to characterizing the intrinsic properties of other atomically thin-layered materials and interface states of heterostructures built from 2D materials.