PHYSICAL REVIEW B 89, 125422 (2014) Universal ac conduction in large area atomic layers of CVD-grown MoS 2 S. Ghosh, 1 S. Najmaei, 2 S. Kar, 3 R. Vajtai, 2 J. Lou, 2 N. R. Pradhan, 4 L. Balicas, 4 P. M. Ajayan, 2 and S. Talapatra 1 1 Department of Physics, Southern Illinois University, Carbondale, Illinois 62901, USA 2 Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005, USA 3 Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA 4 National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA (Received 20 August 2013; published 19 March 2014) Here, we report on the ac conductivity [σ ′ (ω); 10 mHz <ω< 0.1 MHz] measurements performed on atomically thin, two-dimensional layers of MoS 2 grown by chemical vapor deposition (CVD). σ ′ (ω) is observed to display a “universal” power law, i.e., σ ′ (ω) ∼ ω s measured within a broad range of temperatures, 10 K <T< 340 K. The temperature dependence of ‘‘s ” indicates that the dominant ac transport conduction mechanism in CVD-grown MoS 2 is due to electron hopping through a quantum mechanical tunneling process. The ac conductivity also displays scaling behavior, which leads to the collapse of the ac conductivity curves obtained at various temperatures into a single master curve. These findings establish a basis for our understanding of the transport mechanism in atomically thin, CVD-grown MoS 2 layers. DOI: 10.1103/PhysRevB.89.125422 PACS number(s): 72.20.−i, 63.22.Np, 71.23.−k, 72.80.Ng Van der Waals bonded layered solids such as MoS 2 , WS 2 , MoSe 2 , h-BN, etc. have emerged as the materials of choice for obtaining atomically thin, two-dimensional (2D) systems [1–4] with fascinating electrical as well as optical properties [5–10]. Field-effect transistors composed of a single, or few layers of MoS 2 were found to display high electron mobilities, making them potentially useful as active elements in thin-film transistors [1,8–10]. These observations, coupled with the fact that single-layer MoS 2 is a direct-band-gap material (∼1.8 eV), in contrast to its bulk counterpart which is an n-type semiconductor with an indirect band gap of ∼1.3 eV, stimu- lated intensive research on the electrical and optoelectronic properties of single-layer MoS 2 transistors [3–7]. Such observations, mostly on mechanically exfoliated layers of MoS 2 from single crystals, provided enough impetus to explore innovative methods for large-scale synthesis of atomically thin MoS 2 layers [3,11–15]. Among these, liquid phase exfoliation [3,11], laser thinning [12], as well as the chemical vapor deposition (CVD) method [13–15] are now being utilized to synthesize large-scale area MoS 2 layers. However, the materials produced using these techniques are, in general, susceptible to structural disorder [16,17]. Such disorder is known to affect the properties of the material; for example, in semiconductors, atomic defects and bonding disor- der influence their band structure, which in turn influences their charge transport [18,19] properties. Therefore, understanding the correlation between the structure and physical properties of these materials is of fundamental interest. In particular, studying the electrical conduction mechanisms [20–28] of 2D layered solids is of major importance, since it would play a relevant role in many of the envisioned optoelectronic applications based on these materials. In recent times a large body of research has been exploring the exciting electronic properties of CVD-grown MoS 2 layers, revealing rich new science and its technological potential. A question of fundamental importance then arises with regard to how the electrical performance of these materials will compare with their crystalline counterparts. In this paper, we present a study on the electrical conduction mechanisms of large area, atomically thin CVD-grown MoS 2 layers by critically investigating dc transport and more importantly, ac transport measurements, and show that the electrical performance of large area CVD-grown MoS 2 layers are extremely similar to mechanically exfoliated samples from naturally occurring crystals. Our observations indicate that atomically thin CVD MoS 2 layers show “universal” ac features, with the real part of the ac conductivity [σ ′ (ω)] constant at low frequencies but following an approximate power law σ ′ (ω) ∼ ω s at high frequencies. The exponent “s ” has a weak temperature dependence and is close to unity within the studied range of 10 K <T< 340 K. The weak temperature dependence of “s ” indicates that the ac conduction occurs via quantum- mechanical tunneling (QMT) processes of electrons and is typically observed in highly crystalline and commercially available MoS 2 [20]. Finally, we show that these samples follow the “time-temperature superposition principle” (TTSP), as indicated by the collapse of the ac conductivity data onto a single master curve through proper scaling. Large area MoS 2 layers were synthesized through the CVD technique on SiO 2 substrates. The process involves a direct chemical reaction between Mo and S and is described elsewhere [13] in detail. The topographical homogeneity of the samples was measured using atomic force microscopy (AFM, Agilent PicoScan 5500). Raman spectroscopy (Renishaw inVia), with a 514.5-nm laser excitation wavelength and a power of 2 mW, was used to characterize the structure and the number of layers of the films. These large area flakes were electrically contacted on top using patterns of Au with an underlying Ti layer through standard photo lithography techniques. The ac transport properties were measured (under high vacuum; pressure <10 −5 Torr) from 10 mHz to 0.1 MHz within a temperature range of 10–340 K using a temperature- controlled helium cryostat (Janis, closed cycle) coupled to a potentiostat/galvanostat with a built-in frequency response analyzer at 10 μHz to 2 MHz (PARSTAT 2263, Princeton Applied Research). An ac amplitude of 1 V was applied between the contacts, and the response was recorded in the form of a standard Bode plot and a Nyquist plot. The dc transport measurements were performed using a Keithley source meter model no. 2410. 1098-0121/2014/89(12)/125422(5) 125422-1 ©2014 American Physical Society