Molybdenum Disulfide Atomic Layers Research Articles

Enabling Ultrasensitive Photo-detection Through Control of Interface Properties in Molybdenum Disulfide Atomic Layers.

The interfaces in devices made of two-dimensional materials such as MoS2 can effectively control their optoelectronic performance. However, the extent and nature of these deterministic interactions are not fully understood. Here, we investigate the role of substrate interfaces on the photodetector properties of MoS2 devices by studying its photocurrent properties on both SiO2 and self-assembled monolayer-modified substrates. Results indicate that while the photoresponsivity of the devices can be enhanced through control of device interfaces, response times are moderately compromised. We attribute this trade-off to the changes in the electrical contact resistance at the device metal-semiconductor interface. We demonstrate that the formation of charge carrier traps at the interface can dominate the device photoresponse properties. The capture and emission rates of deeply trapped charge carriers in the substrate-semiconductor-metal regions are strongly influenced by exposure to light and can dynamically dope the contact regions and thus perturb the photodetector properties. As a result, interface-modified photodetectors have significantly lower dark-currents and higher on-currents. Through appropriate interfacial design, a record high device responsivity of 4.5 × 103 A/W at 7 V is achieved, indicative of the large signal gain in the devices and exemplifying an important design strategy that enables highly responsive two-dimensional photodetectors.

Chemical vapor deposition growth and transport properties of MoS2–2H thin layers using molybdenum and sulfur as precursors

This paper introduces a feasible process to achieve the molybdenum disulfide atomic layers using chemical vapor deposition (CVD) method, with molybdenum thin film and solid sulfur as precursors. And some improvements were made to reduce the amount of metastable MoS2–3R. The morphology of the acquired MoS2 layers, existing as triangular flakes or large-area continuous films, can be controlled by adjusting the synthesis time and reacting temperature. The characterization results show that the monolayer MoS2 flakes reveal a (002)-oriented growth on SiO2/Si substrates, and its crystalline domain size is approximately 30 μm, and the thickness is 0.65 nm. Since the synthesis of MoS2–3R is restrained, the electronic transport properties of MoS2 with different layers were investigated, revealing that those properties equal with those of MoS2 samples prepared by exfoliation methods.

Controlled synthesis and optical properties of polycrystalline molybdenum disulfide atomic layers grown by chemical vapor deposition

We report a scalable growth of mono to few-layer molybdenum disulfide (MoS2) atomic layers on different substrates by chemical vapor deposition (CVD). The effects of the source material (sulfur and MoO3 powder) and the growth temperature were systematically optimized for the growth of both highly crystalline and large area MoS2. The deposited film thickness could be precisely controlled by varying the growth temperatures, and this was confirmed by Raman and AFM results. The monolayer, bilayer, and multilayer MoS2 could be obtained at 650 °C, 700 °C, and 750–800 °C, respectively. The mobility value of ∼0.89 cm2/V s and current on/off ratio in the order of ∼104 was estimated for monolayer MoS2. The mobility value increased to ∼7.6 cm2/V s for the bilayer MoS2. Our results pave the way for the controlled synthesis of high-quality transition metal dichalcogenide materials, which are an attractive option for applications in electronic and optoelectronic devices.

Synthesis and defect investigation of two-dimensional molybdenum disulfide atomic layers.

CONSPECTUS: The unique physical properties of two-dimensional (2D) molybdenum disulfide (MoS2) and its promising applications in future optoelectronics have motivated an extensive study of its physical properties. However, a major limiting factor in investigation of 2D MoS2 is its large area and high quality preparation. The existence of various types of defects in MoS2 also makes the characterization of defect types and the understanding of their roles in the physical properties of this material of critical importance. In this Account, we review the progress in the development of synthetic approaches for preparation of 2D MoS2 and the understanding of the role of defects in its electronic and optical properties. We first examine our research efforts in understanding exfoliation, direct sulfurization, and chemical vapor deposition (CVD) of MoS2 monolayers as main approaches for preparation of such atomic layers. Recognizing that a natural consequence of the synthetic approaches is the addition of sources of defects, we initially focus on identifying these imperfections with intrinsic and extrinsic origins in CVD MoS2. We reveal the predominant types of point and grain boundary defects in the crystal structure of polycrystalline MoS2 using transmission electron microscopy (TEM) and understand how they modify the electronic band structure of this material using first-principles-calculations. Our observations and calculations reveal the main types of vacancy defects, substitutional defects, and dislocation cores at the grain boundaries (GBs) of MoS2. Since the sources of defects in two-dimensional atomic layers can, in principle, be controlled and studied with more precision compared with their bulk counterparts, understanding their roles in the physical properties of this material may provide opportunities for changing their properties. Therefore, we next examine the general electronic properties of single-crystalline 2D MoS2 and study the role of GBs in the electrical transport and photoluminescence properties of its polycrystalline counterparts. These results reveal the important role played by point defects and GBs in affecting charge carrier mobility and excitonic properties of these atomic layers. In addition to the intrinsic defects, growth process induced substrate impurities and strain induced band structure perturbations are revealed as major sources of disorder in CVD grown 2D MoS2. We further explore substrate defects for modification and control of electronic and optical properties of 2D MoS2 through interface engineering. Self-assembled monolayer based interface modification, as a versatile technique adaptable to different conventional and flexible substrates, is used to promote significant tunability in the key MoS2 field-effect device parameters. This approach provides a powerful tool for modification of native substrate defect characteristics and allows for a wide range of property modulations. Our results signify the role of intrinsic and extrinsic defects in the physical properties of MoS2 and unveil strategies that can utilize these characteristics.

Exploring the Flatlands: Synthesis, Characterization and Engineering of Two-Dimensional Materials

In this talk, we report the controlled vapor phase synthesis of MoS2 atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. The atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulfide atomic layers are examined and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are examined and the role of grain boundaries is evaluated. More importantly, if precise two-dimensional domains of graphene, h-BN and MoS2 atomic layers can be seamlessly stitched together, in-plane heterostructures with interesting electronic applications could potentially be created. Here, we show that planar graphene/h-BN heterostructures can be formed by growing graphene in lithographically-patterned h-BN atomic layers. Our approach can create periodic arrangements of domains with size that ranging from tens of nanometers to millimeters. The resulting graphene/h-BN atomic layers can be peeled off from their growth substrate and transferred to various platforms including flexible substrate. Finally, we demonstrate how self-assembled monolayers with a variety of end termination chemistries can be utilized to tailor the physical properties of single-crystalline MoS2 atomic-layers. Our data suggests that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS2, illustrating an engineering approach for local and universal property modulations in two-dimensional atomic-layers.

Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers

Single-layered molybdenum disulphide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for the next generation of nanoelectronics. Here, we report the controlled vapour phase synthesis of molybdenum disulphide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area, single- and few-layered films. Using high-resolution electron microscopy imaging, the atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulphide atomic layers are examined, and the primary mechanisms for grain boundary formation are evaluated. Grain boundaries consisting of 5- and 7- member rings are directly observed with atomic resolution, and their energy landscape is investigated via first-principles calculations. The uniformity in thickness, large grain sizes, and excellent electrical performance signify the high quality and scalable synthesis of the molybdenum disulphide atomic layers.

Quantitative analysis of the temperature dependency in Raman active vibrational modes of molybdenum disulfide atomic layers

Raman spectroscopy is utilized to quantify the temperature dependency of the vibrational modes in molybdenum disulfide (MoS2) atomic layers. These analyses are essential for understanding the structural properties and phononic behaviors of this two-dimensional (2D) material. We quantitatively analyze the temperature dependent shifts of the Raman peak positions in the temperature range from 300 to 550 K, and find that both planar and out-of-plane characteristic modes are highly sensitive to temperature variations. This temperature dependency is linear and can be fully explained by the first-order temperature coefficient. Using a semi-quantitative model, we evaluate the contributions of the material's thermal expansion and intrinsic temperature effects to this dependency. We reveal that the dominating source of shift in the peak position of planar mode E2g(1) for samples of all thicknesses investigated is the four-phonon process. In addition to the four-phonon process, thermal expansion plays a significant role in the temperature dependency of the out-of-plane mode, A1g. The thickness dependency of the temperature coefficient for MoS2 and a drastic change in behaviors of samples from bi- to single-layered are also demonstrated. We further explore the role of defects in the thermal properties of MoS2 by examining the temperature dependency of Raman modes in CVD-grown samples.