Modulating the Electronic Properties of Au-MoS2 Interfaces Using Functionalized Self-Assembled Monolayers.

Molybdenum disulfide (MoS2) is a transition-metal dichalcogenide with many applications including in electronic devices and sensors. A critical issue in the development of these devices is the high resistance between the metal contact and the molybdenum disulfide layer. In this study, we employ Raman spectroscopy and X-ray photoelectron spectroscopy to investigate the modification of Au-MoS2 contact properties using functionalized alkanethiolate self-assembled monolayers (SAMs). We demonstrate that both 2H and 1T MoS2 strongly interact with the underlying Au substrate. The electronic properties of the interface are mediated by the dipole moment of the alkanethiolate SAM, which have a -CH3, -CO2C6F5, -OH, or -COOH terminal group. Finally, we demonstrate the site-selective deposition of 2H and 1T MoS2 on micropatterned SAMs to form conducting-semiconducting patterned MoS2 films.

Chemical Bath Deposition of Copper Sulfide on Functionalized SAMs: An Unusual Selectivity Mechanism.

We have investigated the chemical bath deposition (CBD) of CuS using thioacetamide on functionalized self-assembled monolayers (SAMs) using scanning electron and optical microscopies, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. For all SAMs studied, the amount of CuS deposited is strongly dependent on the bath pH and can be attributed to the interaction of the SAM terminal groups with the chalcogenide ions present in solution. For -CH3-terminated SAMs, there is a steady increase in the amount of CuS deposited with an increase in the bath pH because there is an increase in the concentration of chalcogenide ion. However, for -OH- and -COOH-terminated SAMs, we observe that the maximum amount of CuS is deposited at pH 10. We attribute this behavior to a competition between the repulsion of the chalcogenide ions by the negatively charged SAM terminal groups and an increase in the chalcogenide ion concentration with an increase in the bath pH. Using the interaction of the chalcogenide ions with the different SAM terminal functional groups, we demonstrate that CuS can be selectively deposited on the -CH3-terminated areas of patterned -OH/-CH3- and -COOH/-CH3-terminated SAMs.

Polytype control of MoS2 using chemical bath deposition.

Molybdenum disulfide (MoS2) has a wide range of applications from electronics to catalysis. While the properties of single-layer and multilayer MoS2 films are well understood, controlling the deposited MoS2 polytype remains a significant challenge. In this work, we employ chemical bath deposition, an aqueous deposition technique, to deposit large area MoS2 thin films at room temperature. Using Raman spectroscopy and x-ray photoelectron spectroscopy, we show that the deposited MoS2 polytype can be changed from semiconducting 2H MoS2 on hydrophobic -CH3 and -CO2C6F5 terminated self-assembled monolayers (SAMs) to semimetallic 1T MoS2 on hydrophilic -OH and -COOH terminated SAMs. The data suggest that the deposition of MoS2 polytypes is controlled by the substrate surface energy. High surface energy substrates stabilize 1T MoS2 films, while 2H MoS2 is deposited on lower surface energy substrates. This effect appears to be general enabling the deposition of different MoS2 polytypes on a wide range of substrates.

Emergence of Magnetic States in Pr2Fe(4-x)Co(x)Sb5 (1 < x < 2.5).

Single crystals of Pr2Fe(4-x)Co(x)Sb5 (1 < x < 2.5) were grown from a Bi flux and characterized by X-ray diffraction. The compounds adopt the La2Fe4Sb5 structure type (I4/mmm). The structure of Pr2Fe(4-x)Co(x)Sb5 (1 < x < 2.5) contains a network of transition metals forming isosceles triangles. The x ∼ 1 analogue is metallic and exhibits a magnetic transition at T1 ≈ 25 K. The magnetic moment obtained from the Curie-Weiss fit is 11.49(4) µ(B), which is larger than the spin-only Pr(3+) moment. The x ∼ 2 analogue orders magnetically at T1 ≈ 80 and T2 ≈ 45 K. This is the first case of the substitution of Co into the La2Fe4Sb5 structure type, evidenced by the increased concentration of dopant with decreased lattice parameters coupled with a change in the transition temperature with a change in the cobalt concentration. The added complexity in the magnetic behavior of the x ∼ 1 and 2 analogues indicates that the increased concentration of Co invokes an additional magnetic contribution of the transition metal in the sublattice. Furthermore, X-ray photoelectron spectroscopy measurements support the change in the oxidation states of transition metals with increasing cobalt concentration.

Application of visible-light photosensitization to form alkyl-radical-derived thin films on gold.

Visible-light irradiation of phthalimide esters in the presence of the photosensitizer [Ru(bpy)3]2+ and the stoichiometric reducing agent benzyl nicotinamide results in the formation of alkyl radicals under mild conditions. This approach to radical generation has proven useful for the synthesis of small organic molecules. Herein, we demonstrate for the first time the visible-light photosensitized deposition of robust alkyl thin films on Au surfaces using phthalimide esters as the alkyl radical precursors. In particular, we combine visible-light photosensitization with particle lithography to produce nanostructured thin films, the thickness of which can be measured easily using AFM cursor profiles. Analysis with AFM demonstrated that the films are robust and resistant to mechanical force while contact angle goniometry suggests a multilayered and disordered film structure. Analysis with IRRAS, XPS, and TOF SIMS provides further insights.