Goran Tripun Karapetrov

We report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS2 films. We use scanning tunneling microscopy and Kelvin probe force microscopy in order to obtain measurements of the local density of states, work function and nature of defects in MoS2 films. We track the evolution of defects that are formed under annealing in ultra-high vacuum conditions. We observe formation of metastable domains with different work function values after annealing the material in ultra-high vacuum to moderate temperatures. We attribute these metastable values of the work function to evolution of crystal defects forming during the annealing. The experiments show that sulfur vacancies formed after exposure to elevated temperatures diffuse, coalesce, and migrate bringing the system from a metastable to equilibrium ground state. The process could be thermally activated with estimated energy barrier for sulfur vacancy migration of 0.6 eV in single unit cell MoS2. The results provide estimates of the thermal budgets available for reliable fabrication of MoS2-based integrated circuit electronics and indicate the importance of defect control and layer passivation.

In this work, we report on the vapor-based synthesis of molybdenum disulphide (MoS 2 ), which is one of the most widely studied two-dimensional (2D) layered materials since the large body of work conducted on the quintessential layered material, graphene. In addition, we have fabricated and characterized a MoS 2 photodetector which was formed using a visco-elastic stamping technique. This device exhibits a low Schottky barrier height of ~27 meV and a photoresponsivity of 1,314 A/W at room temperature using a broad band light source with irradiance power of 0.134 nW at 5 V drain-source voltage. Our results show the promise of MoS 2 for photodetectors and a range of other applications.

  Conference Title: 2016 Lester Eastman Conference on High Performance Devices (LEC) Conference Start Date: 2016, Aug. 2 Conference End Date: 2016, Aug. 4 Conference Location: Bethlehem, PA, USA In this work, we report on the vapor-based synthesis of molybdenum disulphide (MoS 2 ), which is one of the most widely studied two-dimensional (2D) layered materials since the large body of work conducted on the quintessential layered material, graphene. In addition, we have fabricated and characterized a MoS 2 photodetector which was formed using a visco-elastic stamping technique. This device exhibits a low Schottky barrier height of ∼27 meV and a photoresponsivity of 1,314 A/W at room temperature using a broad band light source with irradiance power of 0.134 nW at 5 V drain-source voltage. Our results show the promise of MoS 2 for photodetectors and a range of other applications.

Cobalt and Manganese intercalated NbSe2 single crystals have been synthesized and characterized by DC magnetization and scanning tunnelling microscopy (STM) at low temperatures. We observed a pronounced peak effect in magnetization for both Co and Mn intercalated samples that we further investigated by low temperature STM. A structural phase transition of the vortex lattice (VL) has been observed for applied magnetic fields corresponding to the peak in magnetization.

The ability to tune the electronic and structural properties of nanocatalysts can potentially lead towards their superior catalytic activity. This can be achieved by numerous ways such as controlling the size and shape of nanoparticles, the introduction of another metal into the bulk of the catalyst and tuning of the nanoscale surface morphology. The later has been in focus of this report and can be controlled by either preparation procedure or by in-situ electrochemical treatment.

We studied the magneto-transport properties of magnetically coupled superconductor-ferromagnet MoGe/Permalloy bilayers. The rotatable anisotropy Permalloy ferromagnet with stripe domain structure induces in-plane anisotropy in superconducting order parameter. Superconducting phase diagram shows that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels just above the magnetic domain walls. By changing the in-plane direction of magnetic stripe domains it is possible to re-configure the direction of the superconducting channels and controllably rotate the direction of the in-plane anisotropy axis in the superconductor.

We fabricated and characterized one- and two- dimensional nanoscale arrays of platinum for study of model catalysts. One-dimensional arrays of nanoscale facets were fabricated by annealing a high-index plan of platinum single crystals. The high-index plane forms rows of alternating two low-index facets, (111) and (100), widths of which are similar to 10 nanorneters. Two-dimensional arrays were fabricated lithographically from the epitaxial films of platinum grown on SrTiO3 substrates. Electron beam lithography was used to create precisely registered square arrays of millions of identical platinum nanocrystals with similar to 30 nm in diameter.

A laser beam, precisely focused on the patterned superconducting structure, was used to nucleate a resistive area that is sensitive to external thermal effects. The electron beam lithography and wet chemical etching were applied as pattern transfer processes in epitaxial Y-Ba-Cu-O films. Two different sensor designs were tested: (i) 3 millimeters long and 40 micrometers wide stripe and (ii) 1.25 millimeters long, and 50 micron wide meander-like structure. It is shown experimentally that scanning the laser beam along the stripe leads to physical displacement of the sensitive area and, therefore may be used as a basis for imaging over a broad spectral range. For example, patterning the superconducting film into a meander structure is equivalent to a two-dimensional detector array. In additional to the simplicity of the detector fabrication sequence (one step mask transfer), a clear advantage of this approach is the simplicity of the read-out process: an image is formed by registering the signal with only two electrical terminals. The proposed approach can be extended for imaging over a wide spectral range.