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

Surface Passivation and Positive Band-Edge Shift of p-Si(111) Surfaces Functionalized with Mixed Methyl/Trifluoromethylphenylacetylene Overlayers

Chemical functionalization of semiconductor surfaces can provide high-efficiency photoelectrochemical devices through molecular-level control of the energetics, surface dipole, surface electronic defects, and chemical reactivity at semiconductor/electrolyte junctions. We describe the covalent functionalization by nucleophilic addition chemistry of p-Si(111) surfaces to produce mixed overlayers of trifluoromethylphenylacetylene (TFMPA) and methyl moieties. Functionalization of Cl-terminated Si(111) surfaces with TFMPA moieties introduced a positive surface molecular dipole that in contact with CH₃CN or Hg produced a positive band-edge shift of the semiconductor relative to junctions with CH₃-Si(111) surfaces. Methylation of the Cl/TFMPA surfaces using methylmagnesium chloride resulted in the degradation of the TFMPA moieties, whereas methylation using methylzinc chloride allowed controlled production of mixed TFMPA/methyl-terminated surfaces and permitted reversal of the order of the functionalization steps so that nucleophilic addition of TFMPA could be accomplished after methylation of Cl–Si(111) surfaces. Mixed TFMPA/methyl functionalization resulted in a Si(111) surface with surface recombination velocities of 2 × 10² cm s⁻¹ that exhibited an ∼150 mV positive band-edge shift relative to CH₃–Si(111) surfaces

Surface Passivation and Positive Band-Edge Shift of p-Si(111) Surfaces Functionalized with Mixed Methyl/Trifluoromethylphenylacetylene Overlayers

Chemical functionalization of semiconductor surfaces can provide high-efficiency photoelectrochemical devices through molecular-level control of the energetics, surface dipole, surface electronic defects, and chemical reactivity at semiconductor/electrolyte junctions. We describe the covalent functionalization by nucleophilic addition chemistry of p-Si(111) surfaces to produce mixed overlayers of trifluoromethylphenylacetylene (TFMPA) and methyl moieties. Functionalization of Cl-terminated Si(111) surfaces with TFMPA moieties introduced a positive surface molecular dipole that in contact with CH₃CN or Hg produced a positive band-edge shift of the semiconductor relative to junctions with CH₃-Si(111) surfaces. Methylation of the Cl/TFMPA surfaces using methylmagnesium chloride resulted in the degradation of the TFMPA moieties, whereas methylation using methylzinc chloride allowed controlled production of mixed TFMPA/methyl-terminated surfaces and permitted reversal of the order of the functionalization steps so that nucleophilic addition of TFMPA could be accomplished after methylation of Cl–Si(111) surfaces. Mixed TFMPA/methyl functionalization resulted in a Si(111) surface with surface recombination velocities of 2 × 10² cm s⁻¹ that exhibited an ∼150 mV positive band-edge shift relative to CH₃–Si(111) surfaces

Primary Corrosion Processes for Polymer-Embedded Free-Standing or Substrate-Supported Silicon Microwire Arrays in Aqueous Alkaline Electrolytes

Solar fuel devices have shown promise as a sustainable source of chemical fuels. However, long-term stability of light absorbing materials remains a substantial barrier to practical devices. Herein, multiple corrosion pathways in 1 M KOH(aq) have been defined for TiO₂-protected Si microwire arrays in a polymer membrane either attached to a substrate or free-standing. Top-down corrosion was observed in both morphologies through defects in the TiO₂ coating. For the substrate-based samples, bottom-up corrosion was observed through the substrate and up the adjacent wires. In the free-standing samples, uniform bottom-up corrosion was observed through the membrane with all wire material corroded within 10 days of immersion in the dark in 1 M KOH(aq)

Reductant-Activated, High-Coverage, Covalent Functionalization of 1T′-MoS₂

Recently developed covalent functionalization chemistry for MoS₂ in the 1T′ phase enables the formation of covalent chalcogenide–carbon bonds from alkyl halides and aryl diazonium salts. However, the coverage of functional groups using this method has been limited by the amount of negative charge stored in the exfoliated MoS₂ sheets to <25–30% per MoS₂ unit. We report, herein, a reductant-activated functionalization, wherein one-electron metallocene reductants, such as nickelocene, octamethylnickelocene, and cobaltocene, are introduced during functionalization with methyl and propyl halides to tune the coverage of the alkyl groups. The reductant-activated functionalization yields functional group coverages up to 70%, ∼1.5–2 times higher than the previous limit, and enables functionalization by weak electrophiles, such as 1-chloropropane, that are otherwise unreactive with chemically exfoliated MoS₂. We also explored the dependence of coverage on the strength of the leaving group and the steric hindrance of the alkyl halide in the absence of reductants and showed that functionalization was ineffective for chloride leaving groups and for secondary and tertiary alkyl iodides. These results demonstrate a substantial increase in coverage compared to functionalization without reductants, and may impact the performance of these materials in applications reliant on surface interactions. Furthermore, this method may be applicable to the covalent functionalization of similar layered materials and metal chalcogenides

Reductant-Activated, High-Coverage, Covalent Functionalization of 1T′-MoS₂

Recently developed covalent functionalization chemistry for MoS₂ in the 1T′ phase enables the formation of covalent chalcogenide–carbon bonds from alkyl halides and aryl diazonium salts. However, the coverage of functional groups using this method has been limited by the amount of negative charge stored in the exfoliated MoS₂ sheets to <25–30% per MoS₂ unit. We report, herein, a reductant-activated functionalization, wherein one-electron metallocene reductants, such as nickelocene, octamethylnickelocene, and cobaltocene, are introduced during functionalization with methyl and propyl halides to tune the coverage of the alkyl groups. The reductant-activated functionalization yields functional group coverages up to 70%, ∼1.5–2 times higher than the previous limit, and enables functionalization by weak electrophiles, such as 1-chloropropane, that are otherwise unreactive with chemically exfoliated MoS₂. We also explored the dependence of coverage on the strength of the leaving group and the steric hindrance of the alkyl halide in the absence of reductants and showed that functionalization was ineffective for chloride leaving groups and for secondary and tertiary alkyl iodides. These results demonstrate a substantial increase in coverage compared to functionalization without reductants, and may impact the performance of these materials in applications reliant on surface interactions. Furthermore, this method may be applicable to the covalent functionalization of similar layered materials and metal chalcogenides

Characterization of Electronic Transport through Amorphous TiO_2 Produced by Atomic-Layer Deposition

Electrical transport in amorphous titanium dioxide (a-TiO_2) thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO_2|metal substrates that had various top metal contacts has been characterized by ac conductivity, temperature-dependent dc conductivity, space-charge-limited current spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy, and current density versus voltage (J–V) characteristics. Amorphous TiO_2 films were fabricated using either tetrakis(dimethylamido)-titanium with a substrate temperature of 150 °C or TiCl_4 with a substrate temperature of 50, 100, or 150 °C. EPR spectroscopy of the films showed that the Ti^(3+) concentration varied with the deposition conditions and increases in the concentration of Ti^(3+) in the films correlated with increases in film conductivity. Valence band spectra for the a-TiO_2 films exhibited a defect-state peak below the conduction band minimum (CBM) and increases in the intensity of this peak correlated with increases in the Ti^(3+) concentration measured by EPR as well as with increases in film conductivity. The temperature-dependent conduction data showed Arrhenius behavior at room temperature with an activation energy that decreased with decreasing temperature, suggesting that conduction did not occur primarily through either the valence or conduction bands. The data from all of the measurements are consistent with a Ti^(3+) defect-mediated transport mode involving a hopping mechanism with a defect density of 10^(19) cm^(–3), a 0.83 wide defect band centered 1.47 eV below the CBM, and a free-electron concentration of 10^(16) cm^(–3). The data are consistent with substantial room-temperature anodic conductivity resulting from the introduction of defect states during the ALD fabrication process as opposed to charge transport intrinsically associated with the conduction band of TiO_2

Characterization of Electronic Transport through Amorphous TiO_2 Produced by Atomic-Layer Deposition

Electrical transport in amorphous titanium dioxide (a-TiO_2) thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO_2|metal substrates that had various top metal contacts has been characterized by ac conductivity, temperature-dependent dc conductivity, space-charge-limited current spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy, and current density versus voltage (J–V) characteristics. Amorphous TiO_2 films were fabricated using either tetrakis(dimethylamido)-titanium with a substrate temperature of 150 °C or TiCl_4 with a substrate temperature of 50, 100, or 150 °C. EPR spectroscopy of the films showed that the Ti^(3+) concentration varied with the deposition conditions and increases in the concentration of Ti^(3+) in the films correlated with increases in film conductivity. Valence band spectra for the a-TiO_2 films exhibited a defect-state peak below the conduction band minimum (CBM) and increases in the intensity of this peak correlated with increases in the Ti^(3+) concentration measured by EPR as well as with increases in film conductivity. The temperature-dependent conduction data showed Arrhenius behavior at room temperature with an activation energy that decreased with decreasing temperature, suggesting that conduction did not occur primarily through either the valence or conduction bands. The data from all of the measurements are consistent with a Ti^(3+) defect-mediated transport mode involving a hopping mechanism with a defect density of 10^(19) cm^(–3), a 0.83 wide defect band centered 1.47 eV below the CBM, and a free-electron concentration of 10^(16) cm^(–3). The data are consistent with substantial room-temperature anodic conductivity resulting from the introduction of defect states during the ALD fabrication process as opposed to charge transport intrinsically associated with the conduction band of TiO_2