Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-layer MoS2 (1407.2478v1)

Abstract: Ultrathin layers of semiconducting molybdenum disulfide (MoS2) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS2, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS2. Micro-fabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ~30 mV/K at room temperature are observed, which are much larger than those observed in other two dimensional crystals and bulk MoS2. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows S~T^1/3 suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS2 and the high thermopower value is of interest to future thermoelectronic research and application.

• The paper demonstrates enhanced thermopower (~30 mV/K) in single-layer MoS₂ through a variable range hopping conduction mechanism.
• It employs micro-fabricated heaters and thermometers to measure temperature-dependent electrical conductivity from 20 K to 300 K.
• The study reveals gate voltage-tunable thermoelectric properties, highlighting potential for efficient heat-to-electricity conversion.

Analysis of Large Thermoelectricity via Variable Range Hopping in CVD-Grown Single-layer MoS<sub\>2</sub>

The paper of thermoelectric properties of two-dimensional materials has gathered significant attention, particularly the thermoelectric power, or thermopower, of molybdenum disulfide (MoS<sub\>2</sub>). This paper focuses on a systematic investigation of the thermoelectric properties of single-layer MoS<sub\>2</sub> grown using chemical vapor deposition (CVD), which is a pivotal advance in the field due to its relevance in scalable application technologies. The research provides insights into the band structure, carrier density, and conduction mechanisms of CVD-grown MoS<sub\>2</sub> through a combination of electrical and thermoelectric measurements.

Key Findings and Methodology

Through the utilization of micro-fabricated heaters and thermometers, the paper meticulously measures the thermopower and electrical conductivity across an extensive temperature range (20 K - 300 K). The remarkable observation is the measurement of a thermopower value of approximately 30 mV/K at room temperature, which significantly surpasses that of other two-dimensional materials such as graphene and is notably higher than bulk MoS<sub\>2</sub>.

The research attributes the large thermopower values observed to a two-dimensional variable range hopping (VRH) transport mechanism. This conclusion is supported by the Seebeck coefficient's dependence on temperature, which follows a S ~ T<sup\>1/3</sup> relationship. This finding aligns consistently with the electrical transport behavior of the material, thus reinforcing the association between the electrical conductivity and thermopower.

Implications and Theoretical Considerations

The implications of these findings are extensive. From a practical standpoint, the high thermopower of CVD-grown MoS<sub\>2</sub> presents a promising avenue for the development of thermoelectric materials, which could potentially enhance energy conversion systems through efficient heat-to-electricity conversion. The dependence of the thermopower on gate voltage demonstrates the capability for tunable thermoelectric properties through electrostatic gating, making such materials highly adaptable for various electronic and optoelectronic applications.

Theoretically, the VRH mechanism described herein provides a framework for understanding electron transport in highly disordered two-dimensional systems. This is particularly pertinent when the charge carrier density is low, and conduction is dominated by phonon-assisted hopping between localized states as opposed to band conduction. This nuanced understanding of charge transport offers insights into the potential manipulation of electronic properties through structural engineering at the nanoscale.

Future Perspectives

Looking towards future developments in this field, the ability to further optimize the growth and structural properties of CVD-grown MoS<sub\>2</sub> could potentially lead to even higher efficiency thermoelectric materials. Investigating the effects of different substrates, annealing processes, and compositional tuning could lead to enhancements in both thermopower and overall material stability.

In conclusion, this paper significantly contributes to the understanding of two-dimensional thermoelectric materials by linking the mechanical synthesis of MoS<sub\>2</sub> with its intrinsic electronic and thermal transport properties. It opens up pathways for subsequent material studies aimed at leveraging the unique properties of transition metal dichalcogenides for advanced electronic applications.