Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole–phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers ( V k center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3 d 5 / 2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron–hole recombination in KI hole traps).

The 2D nature of transition metal dichalcogenides (TMDs) makes their electronic and optical performance highly susceptible to the presence of defects. At elevated temperatures, which can be reached during growth or in operation, additional defects can be introduced and lead to further material degradation. Therefore, by studying the impact of temperature on 2D-TMDs, the formation of defects and their respective degradation pathways can be established. The electronic and geometric structure and density of thermally induced defects on 2D tungsten diselenide (WSe 2 ) layers were examined using scanning tunneling microscopy/spectroscopy (STM/STS). WSe 2 layers were grown on highly ordered pyrolytic graphite (HOPG), via molecular beam epitaxy (MBE) and annealed at 600 °C, which caused a 7-fold increase in overall defect density. A layer-dependent trend emerged whereby the defect density on the first layer was greater than the second, suggesting that the TMD−graphite and TMD−TMD van der Waals interactions influence the formation energy of thermally growth defects. The defect inventory included single-point vacancies and a collection of larger defects with complex geometric and electronic signatures. These defects were classified by matching their unique electronic structures with their respective topographical presentation via spatially resolved STS maps. Defect states at the conduction and valence band edges introduced n-or p-type character and generally lowered the local band gap around each defect site. A unique defect structure displayed an increased band gap, likely as a consequence of local delamination of the TMD due to subsurface Se− cluster formation. Density functional theory (DFT) was used to examine select defects and supported the interpretation of the STM/STS work with density of states (DOS) and local-integrated DOS calculations. The assessment of the geometric and electronic signatures and details of the local doping profile around all defect sites deepened our understanding of the thermal stability of WSe 2 .

PbSe thin films were deposited using the chemical bath deposition method and sensitized with iodine for enhanced IR photoconductivity. After sensitization, PbSe films showed a high photoresponse of 44.7% in terms of resistance change in the midinfrared wavelength range (3–5 μm). To investigate the origin of high photoresponse in sensitized PbSe films, the bandgap, work function, and valence band maximum were measured by photoluminescence (PL) and X-ray photoelectron spectroscopy secondary cutoff and valence spectra. Infrared photoluminescence spectra showed a PbSe bandgap of 0.29 eV. Visible PL spectra showed a PbI2 bandgap of 2.41 eV. Work functions of as-grown PbSe and PbI2 in sensitized PbSe were determined to be 4.30 eV and 4.50 eV, respectively. An Ag/PbSe/Ag band diagram shows a measured barrier height of 0.25 eV at the PbSe/Ag interface due to Fermi level pinning. When the Ag/PbI2/PbSe/PbI2/Ag structure is biased and exposed to midwavelength infrared illumination, the electron flow is limited due to high barriers at the interfaces. Therefore, the only hole can flow after charge separation such that the electrical resistance of PbSe film is dramatically reduced. The measured bandgap, work function, and valence band maximum along with measured barrier height for metal contacts should help in providing the understanding of the charge transport mechanism in PbSe photoconductors.

Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS2 devices; however, its effects on interface chemistry have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geological bulk single crystals of MoS2 in ultrahigh vacuum (UHV) is investigated with X-ray photoelectron spectroscopy and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS2 is observed resulting in a diffuse interface between the two materials that comprises metallic molybdenum and titanium sulfide compounds. Annealing Ti/MoS2 sequentially at 100, 300, and 600 °C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo-rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS2 interface is not thermally stable even at a transistor operating temperature of 100 °C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for electrical transport properties, highlighting the importance of interface chemistry in the metal contact design and fabrication.

Using an in-vacuo deposition/characterization tool, we study the 2D materials synthesized by molecular beam epitaxy and the interfaces formed between layered materials and in-vacuo deposited metals. The metal-2D interface is probed with x-ray photoelectron spectroscopy. Full details of sample preparation and transition metal dichalcogenide synthesis are provided. Furthermore a detailed study of in-vacuo deposited Ti on graphene, shows that while there is clear evidence of Ti carbide formation, there is no conclusive evidence of reactions with the graphene layer. Instead the carbide forms from a combination of adventitious carbon on the graphene surface, and carbon added via the deposition process.