Atom-by-Atom Construction of a Cyclic Artificial Molecule in Silicon.

Hydrogen atoms on a silicon surface, H-Si (100), behave as a resist that can be patterned with perfect atomic precision using a scanning tunneling microscope. When a hydrogen atom is removed in this manner, the underlying silicon presents a chemically active site, commonly referred to as a dangling bond. It has been predicted that individual dangling bonds function as artificial atoms, which, if grouped together, can form designer molecules on the H-Si (100) surface. Here, we present an artificial ring structure molecule spanning three dimer rows, constructed from dangling bonds, and verified by spectroscopic measurement of its molecular orbitals. We found that removing 8 hydrogen atoms resulted in a molecular analog to 1,4-disilylene-hexasilabenzene (Si8H8). Scanning tunneling spectroscopic measurements reveal molecular π and π* orbitals that agree with those expected for the same molecule in a vacuum; this is validated by density functional theory calculations of the dangling bond system on a silicon slab that show direct links both to the experimental results and to calculations for the isolated molecule. We believe the unique electronic structure of artificial molecules constructed in this manner can be engineered to enable future molecule-based electronics, surface catalytic functionality, and templating for subsequent site-selective deposition.

Pulsed laser-deposited MoS2 thin films on W and Si: field emission and photoresponse studies.

We report field electron emission investigations on pulsed laser-deposited molybdenum disulfide (MoS2) thin films on W-tip and Si substrates. In both cases, under the chosen growth conditions, the dry process of pulsed laser deposition (PLD) is seen to render a dense nanostructured morphology of MoS2, which is important for local electric field enhancement in field emission application. In the case of the MoS2 film on silicon (Si), the turn-on field required to draw an emission current density of 10 µA/cm(2) is found to be 2.8 V/µm. Interestingly, the MoS2 film on a tungsten (W) tip emitter delivers a large emission current density of ∼30 mA/cm(2) at a relatively lower applied voltage of ∼3.8 kV. Thus, the PLD-MoS2 can be utilized for various field emission-based applications. We also report our results of photodiode-like behavior in (n- and p- type) Si/PLD-MoS2 heterostructures. Finally we show that MoS2 films deposited on flexible kapton substrate show a good photoresponse and recovery. Our investigations thus hold great promise for the development of PLD MoS2 films in application domains such as field emitters and heterostructures for novel nanoelectronic devices.

Superior field emission properties of layered WS2-RGO nanocomposites.

We report here the field emission studies of a layered WS2-RGO composite at the base pressure of ~1 × 10(-8) mbar. The turn on field required to draw a field emission current density of 1 µA/cm(2) is found to be 3.5, 2.3 and 2 V/µm for WS2, RGO and the WS2-RGO composite respectively. The enhanced field emission behavior observed for the WS2-RGO nanocomposite is attributed to a high field enhancement factor of 2978, which is associated with the surface protrusions of the single-to-few layer thick sheets of the nanocomposite. The highest current density of ~800 µA/cm(2) is drawn at an applied field of 4.1 V/µm from a few layers of the WS2-RGO nanocomposite. Furthermore, first-principles density functional calculations suggest that the enhanced field emission may also be due to an overalp of the electronic structures of WS2 and RGO, where graphene-like states are dumped in the region of the WS2 fundamental gap.

Controlled Ti seed layer assisted growth and field emission properties of Pb(Zr0.52Ti0.48)O3 nanowire arrays.

We report on the directed upright growth of ferroelectric (FE) Pb(Zr0.52Ti0.48)O3 (PZT) nanowire (NW) arrays with large aspect ratios of >60 using a Ti seed layer assisted hydrothermal process over large surface areas on ITO/glass substrates. In a two-step growth process, Ti seed layer of low surface roughness with a thickness of ~500 nm and grain size of ~100 nm was first deposited by radio frequency (RF) sputtering which was subsequently used as substrates for the growth of highly dense, single crystalline PZT NWs by controlled nucleation. The electron emission properties of the PZT NWs were investigated using the as-grown NWs as FE cathodes. A low turn-on field of ~3.4 V/µm was obtained from the NW arrays, which is impressively lower than that from other reported values. The results reported in this work give direction to the development of a facile growth technique for PZT NWs over large surfaces and also are of interest to the generation of high current electron beam from FE NW based cathodes for field emitter applications.

Enhanced field-emission behavior of layered MoS2 sheets.

Field emission studies are reported for the first time on layered MoS2 sheets at the base pressure of ∼1 × 10⁻8 mbar. The turn-on field required to draw a field emission current density of 10 µA/cm² is found to be 3.5 V/µm for MoS2 sheets. The turn-on values are found to be significantly lower than the reported MoS2 nanoflowers, graphene, and carbon nanotube-based field emitters due to the high field enhancement factor (∼1138) associated with nanometric sharp edges of MoS2 sheet emitter surface. The emission current-time plots show good stability over a period of 3 h. Owing to the low turn-on field and planar (sheetlike) structure, the MoS2 could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.