Strain-Based Chemiresistive Polymer-Coated Graphene Vapor Sensors.

Suspended chemiresistive graphene sensors have been fabricated using well-established nanofabrication techniques to generate sensors that are highly sensitive to pyridine and with excellent discrimination between polar and nonpolar analytes. When coated with a polymer surface layer and suspended on 3-D patterned glass electrodes, a hybrid combination of polymer and graphene yields chemiresistive vapor sensors. Expansion and contraction of the polymer layer produces strain on the suspended graphene (Gr). Hence, when organic vapors permeate into the polymer layer, the high gauge factor of the graphene induces substantial electrical resistive changes as folds and creases are induced in the graphene. The hybrid suspended polymer/Gr sensor exhibits substantial responses to polar organic vapors, especially pyridine, while also exhibiting reversibility and increased discrimination between polar and nonpolar analytes compared to previous approaches. This sensor design also allows for potential tunability in the types of polymers used for the reactive surface layer, allowing for use in a variety of potential applications.

Selective-Area, Water-Free Atomic Layer Deposition of Metal Oxides on Graphene Defects.

Passivating defective regions on monolayer graphene with metal oxides remains an active area of research for graphene device integration. To effectively passivate these regions, a water-free atomic layer deposition (ALD) recipe was developed and yielded selective-area ALD (sa-ALD) of mixed-metal oxides onto line defects in monolayer graphene. The anisotropically deposited film targeted high-energy defect sites that were formed during synthesis or transfer of the graphene layer. The passivating layer exceeded 10 nm thickness with minimal deposition onto the basal plane of graphene. The mixed-metal oxide film was of comparable quality to films deposited using nonselective water-based ALD methods, as shown by X-ray photoelectron spectroscopy. The development of sa-ALD techniques to target defect regions on the graphene sheet, while keeping the basal plane intact, will provide a new mechanism to passivate graphene defects and modify the electronic and physical properties of graphene.

Defect-Seeded Atomic Layer Deposition of Metal Oxides on the Basal Plane of 2D Layered Materials.

Atomic layer deposition (ALD) on mechanically exfoliated 2D layered materials spontaneously produces network patterns of metal oxide nanoparticles in triangular and linear deposits on the basal surface. The network patterns formed under a range of ALD conditions and were independent of the orientation of the substrate in the ALD reactor. The patterns were produced on MoS2 or HOPG when either tetrakis(dimethylamido)titanium or bis(ethylcyclopentadienyl)manganese were used as precursors, suggesting that the phenomenon is general for 2D materials. Transmission electron microscopy revealed the presence, prior to deposition, of dislocation networks along the basal plane of mechanically exfoliated 2D flakes, indicating that periodical basal plane defects related to disruptions in the van der Waals stacking of layers, such as perfect line dislocations and triangular extended stacking faults networks, introduce a surface reactivity landscape that leads to the emergence of patterned deposition.

Macroscale and Nanoscale Photoelectrochemical Behavior of p-Type Si(111) Covered by a Single Layer of Graphene or Hexagonal Boron Nitride.

Two-dimensional (2D) materials may enable a general approach to the introduction of a dipole at a semiconductor surface as well as control over other properties of the double layer at a semiconductor/liquid interface. Vastly different properties can be found in the 2D materials currently studied due in part to the range of the distribution of density-of-states. In this work, the open-circuit voltage (Voc) of p-Si-H, p-Si/Gr (graphene), and p-Si/h-BN (hexagonal boron nitride) in contact with a series of one-electron outer-sphere redox couples was investigated by macroscale measurements as well as by scanning electrochemical cell microscopy (SECCM). The band gaps of Gr and h-BN (0-5.97 eV) encompass the wide range of band gaps for 2D materials, so these interfaces (p-Si/Gr and p-Si/h-BN) serve as useful references to understand the behavior of 2D materials more generally. The value of Voc shifted with respect to the effective potential of the contacting solution, with slopes (ΔVoc/ΔEEff) of -0.27 and -0.38 for p-Si/Gr and p-Si/h-BN, respectively, indicating that band bending at the p-Si/h-BN and p-Si/Gr interfaces responds at least partially to changes in the electrochemical potential of the contacting liquid electrolyte. Additionally, SECCM is shown to be an effective method to interrogate the nanoscale photoelectrochemical behavior of an interface, showing little spatial variance over scales exceeding the grain size of the CVD-grown 2D materials in this work. The measurements demonstrated that the polycrystalline nature of the 2D materials had little effect on the results and confirmed that the macroscale measurements reflected the junction behavior at the nanoscale.

Lightly Fluorinated Graphene as a Protective Layer for n-Type Si(111) Photoanodes in Aqueous Electrolytes.

The behavior of n-Si(111) photoanodes covered by monolayer sheets of fluorinated graphene (F-Gr) was investigated under a range of chemical and electrochemical conditions. The electrochemical behavior of n-Si/F-Gr and np(+)-Si/F-Gr photoanodes was compared to hydride-terminated n-Si (n-Si-H) and np(+)-Si-H electrodes in contact with aqueous Fe(CN)6(3-/4-) and Br2/HBr electrolytes as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. Illuminated n-Si/F-Gr and np(+)-Si/F-Gr electrodes in contact with an aqueous K3(Fe(CN)6/K4(Fe(CN)6 solutions exhibited stable short-circuit photocurrent densities of ∼10 mA cm(-2) for 100,000 s (>24 h), in comparison to bare Si electrodes, which yielded nearly a complete photocurrent decay over ∼100 s. X-ray photoelectron spectra collected before and after exposure to aqueous anodic conditions showed that oxide formation at the Si surface was significantly inhibited for Si electrodes coated with F-Gr relative to bare Si electrodes exposed to the same conditions. The variation of the open-circuit potential for n-Si/F-Gr in contact with a series of nonaqueous electrolytes of varying reduction potential indicated that the n-Si/F-Gr did not form a buried junction with respect to the solution contact. Further, illuminated n-Si/F-Gr electrodes in contact with Br2/HBr(aq) were significantly more electrochemically stable than n-Si-H electrodes, and n-Si/F-Gr electrodes coupled to a Pt catalyst exhibited ideal regenerative cell efficiencies of up to 5% for the oxidation of Br(-) to Br2.