Vladimir V. Plashnitsa

Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Graphenes success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in field-effect transistors, spin- and valley-tronics, thermoelectrics, and topological insulators, among many other applications.

Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H2 generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H2 generation rate of 58.06 ± 3.59 μmol h(-1) g(-1) for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H2 generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes.

Synthetic Strategy and Structural and Optical Characterization of Thin Highly Crystalline Titanium Disulfide Nanosheets

Two-dimensional (2D) nanomaterials have recently received significant attention because of their attractiveness for use in many nanostructured devices. Layered transition-metal dichalcogenides are of particular interest because reducing their dimensionality causes changes in their already anisotropic physical and chemical properties. The present study describes the first bottom-up solution-phase synthesis of thin highly crystalline titanium disulfide (TiS2) nanosheets (NSs) using abundant low-cost molecular precursors. The obtained TiS2 NSs have average dimensions of ∼500 nm × 500 nm in the basal plane and have thicknesses of ∼5 nm. They exhibit broad absorption in the visible that tails out into the near-infrared. The obtained results demonstrate new opportunities in synthesizing low-dimensional 2D nanomaterials with potential use in various photochemical energy applications.

Molybdenum Carbamate Nanosheets as a New Class of Potential Phase Change Materials

We report for the first time the synthesis of large, free-standing, Mo2O2(μ-S)2(Et2dtc)2 (MoDTC) nanosheets (NSs), which exhibit an electron-beam induced crystalline-to-amorphous phase transition. Both electron beam ionization and femtosecond (fs) optical excitation induce the phase transition, which is size-, morphology-, and composition-preserving. Resulting NSs are the largest, free-standing regularly shaped two-dimensional amorphous nanostructures made to date. More importantly, amorphization is accompanied by dramatic changes to the NS electrical and optical response wherein resulting amorphous species exhibit room-temperature conductivities 5 orders of magnitude larger than those of their crystalline counterparts. This enhancement likely stems from the amorphization-induced formation of sulfur vacancy-related defects and is supported by temperature-dependent transport measurements, which reveal efficient variable range hopping. MoDTC NSs represent one instance of a broader class of transition metal carbamates likely having applications because of their intriguing electrical properties as well as demonstrated ability to toggle metal oxidation states.