AVS2017 Session 2D-WeA: Properties and Characterization of 2D Materials

The mechanical behavior of graphene oxide is length scale dependent: orders of magnitude different between the bulk forms and monolayer counterparts. Understanding the underlying mechanisms plays a significant role in their versatile application. A systematic multiscale mechanical study from monolayer to multilayer, including the interactions between layers of GO, can provide fundamental support for material engineering. In this talk, an experimental coupled with simulation approach will be introduced studying the multiscale mechanics of graphene oxide (GO).

GO is a layered nanomaterial comprised of hierarchical units whose characteristic dimension lies between monolayer GO (0.7 nm 1.2 nm) and bulk GO papers (≥ 1 µm). Mechanical behaviors of monolayer GO and GO nanosheets (10 nm- 100 nm) were comprehensively studied by our group. Monolayer GO was measured to have an average strength of 24.7 GPa, orders of magnitude higher than previously reported values for GO paper and approximately 50% of the 2D intrinsic strength of pristine graphene. The huge discrepancy between the strength of monolayer GO and that of bulk GO paper motivated the study of GO at the intermediate length scale (GO nanosheets). Experimental results showed that GO nanosheets possess high strength in the gigapascal range. Molecular Dynamic simulations showed that the transition in the failure behavior from interplanar fracture to intraplanar fracture was responsible for the huge strength discrepancy between nanometer scale GO and bulk GO papers. Additionally, the interfacial shear strength between GO layers was found to be a key contributing factor to the distinct mechanical behavior among hierarchical units of GO. The understanding of the multiscale mechanics of GO is transferrable in the study of other 2D materials.

Raman spectroscopy has proved to be an indispensable tool for characterization of both carbon-based 2D materials, such as graphene or graphene oxide, and TMDCs, providing information on: number of layers, distribution of defects, type of structure and (in case of TMDCs) excitonic behavior. Despite the wealth of the information Raman spectroscopy can provide about the structure of 2D materials, its spatial resolution is limited to approximately 300-500nm, depending on the wavelength of the excitation laser; in many cases this is insufficient for precise characterization of spatial distribution of the variations of structural and the related electronic and optoelectronic properties. Hyperspectral imaging using tip enhanced optical spectroscopy (TEOS), which includes tip enhanced Raman spectroscopy (TERS) and tip enhanced photoluminescence spectroscopy (TEPL), can dramatically improve the spatial resolution of both Raman and PL optical microscopy, providing this much-needed sensitivity.

TERS characterization of graphene and graphene oxide, both pristine and functionalized, reveals a dramatic increase in Raman response at folds and wrinkles in the flakes, as well as over the patterns imprinted in the flakes using an ultrasharp single crystal diamond probe. Interestingly, the wrinkles in graphene oxide flakes exhibit increased conductivity compared to flat areas of the flake, a rather unexpected discovery; as graphene oxide in its normal state is a dielectric, increased thickness of the material might have been expected to result in worse conductivity.

TERS and TEPL analysis of as-grown WS₂ demonstrated significant heterogeneity of the flakes. In an approximately 100nm-wide band adjacent to the flake edges we see both decreased, blue-shifted intensity of the photoluminescence and also increased intensity of the TERS signal; this can be attributed to decreased density of the charge carriers in the vicinity of the edges resulting in increased efficiency of Raman scattering and decreased concentration of trions, which have red-shifted PL relative to regular un-charged exitons.

TERS and correlated surface potential imaging of the WS₂ flakes exfoliated to gold revealed significant nanoscale (from tens to low hundreds of nanometer) variations in intensity of the major Raman bands across the flake. A high resolution map of the photoluminescence distribution extracted from the TERS map of the WS₂ flake clearly delineates that portion of the flake that is single layer. Correlation of the TERS hyperspectral image Raman band intensity with the distribution of surface potential may be diagnostic of TMDC doping level and of the exact sign of the charge carriers.

Low dimensional semiconductor materials with different sized band-gaps such as 2D atomic crystals, for example WS2 or MoS2 layered van der Waals materials, or 1D nanowires (NWs) and nanoribbons such as ZnO, or semiconducting carbon nanotubes (CNTs), have drawn significant attention due to their unique physical, chemical, mechanical properties, and other just to name a few. Low dimensional materials when assembled in vertical or lateral arrangements often lead to the largely enhanced properties, and new functionalities. While the preparation of layered architectures usually involves multi-step fabrication processes it also relies on mask assisted lithographic processes.

Here we present controlled selective preparation of 1D and 2D nanostructures of MoS2, WS2 and ZnO in the variety of geometric assemblies by employing parallel direct write patterning (DWP) of aqueous ink precursors on substrates at predefined locations. In a two-step process (1^st patterning and 2^nd growth) our unconventional fabrication approach enables simple and flexible production of hetero-structures and other architectures based on “mix and match” principle in precisely controlled fashion. Location specific synthesis of materials also provides access to as-grown interfaces and rapid testing of materials quality, crystallinity and chemical composition which was confirmed by various characterization methods (Raman Spectroscopy, PL, AFM, XRD etc)

Acknowledgement

Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02 06CH11357

Transition metal dichalcogenides (TMDs) have been extensively studied because of their unique electronic and optical properties. In addition, having the non-zero band gap, they can be directly integrated in logic devices. However, it is of crucial importance to tune the band gap and Fermi level positions of TMDs effectively to enhance the ON/OFF current ratio. In this study, the band gap of WSe₂ was engineered using a (NH₄)₂S solution and a gaseous O₃ treatment. Prior to the chemical treatments, the surface of MBE (molecular beam epitaxy) grown WSe₂ was studied using STM (scanning tunneling ,microscopy). Large grains of WSe₂ about 100 nm were observed. The band gap of monolayer (ML) WSe₂ was about 2.1 eV in STS (scanning tunneling spectroscopy), consistent with previous STM studies. A bare WSe₂ sample was emerged in ammonium sulfide solution (40% (NH₄)₂S(aq)) at 300K. Afterward, the sulfur treated (ST) WSe₂ sample was transferred to a UHV scanning tunneling microscopy (STM) chamber to observe the effect of the treatment on the band gap. The STM imaging revealed that new electronic states were generated by ST across the entire WSe₂ surface, with asymmetric bias dependence. In STS, band gap of the ST WSe₂ was reduced to 1.2 eV, while Fermi level was pinned near the valence band. A WSe₂ field effect transistor (FET) was fabricated to show the effect of ST on electrical properties. Electrical measurement showed that ST of monolayer WSe₂ transistors increased ON states current by two order of magnitude in the p-branch, while ON state current in n -branch was increased by an order of magnitude which is consistent with the (NH₄)₂S decreasing the band gap and inducing p-type doping. The band gap was also tuned by ozone (O₃) treatment at room temperature. A MBE grown WSe₂ was inserted in a vacuum chamber. O₃ gas was prepared by UV lamp excitation of a continuous O₂ flow into the chamber at 300 K for 2 min. After dosing O₃ gas for 2 min, the band gap of the WSe₂ was about 1.3 eV, while Fermi level was near conduction band indication (n-type). As exposure time of O₃ increased to 6 min, the band gap of ML WSe₂ was decreased to 1.1eV and the formation of WOx was observed on the ML terraces. Therefore, O₃ had the opposite behavior of (NH₄)₂S since O₃ treatment maintained n-type doping while shrinking the band gap.