Formation of Stable Nitrene Surface Species by the Reaction of Adsorbed Phenyl Isocyanate at the Ge(100)‑2 × 1 Surface

The reaction of phenyl isocyanate (PIC) following adsorption at the Ge(100)-2 × 1 surface has been investigated both experimentally and theoretically by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, quantum chemical calculations, and molecular dynamics simulations. PIC initially adsorbs by [2 + 2] cycloaddition across the CN bond of the isocyanate, as previously reported, but this initial product converts to a second product on the time scale of minutes at room temperature. The experimental and theoretical results show that the second product formed is phenylnitrene (C₆H₅N) covalently bonded to the germanium surface via a single Ge–N bond. This conclusion is further supported by FTIR spectroscopy experiments and density functional theory calculations using phenyl isocyanate-¹⁵N and phenyl-<i>d</i>₅ isocyanate

Formation of Stable Nitrene Surface Species by the Reaction of Adsorbed Phenyl Isocyanate at the Ge(100)‑2 × 1 Surface

The reaction of phenyl isocyanate (PIC) following adsorption at the Ge(100)-2 × 1 surface has been investigated both experimentally and theoretically by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, quantum chemical calculations, and molecular dynamics simulations. PIC initially adsorbs by [2 + 2] cycloaddition across the CN bond of the isocyanate, as previously reported, but this initial product converts to a second product on the time scale of minutes at room temperature. The experimental and theoretical results show that the second product formed is phenylnitrene (C₆H₅N) covalently bonded to the germanium surface via a single Ge–N bond. This conclusion is further supported by FTIR spectroscopy experiments and density functional theory calculations using phenyl isocyanate-¹⁵N and phenyl-<i>d</i>₅ isocyanate

Formation of Stable Nitrene Surface Species by the Reaction of Adsorbed Phenyl Isocyanate at the Ge(100)‑2 × 1 Surface

The reaction of phenyl isocyanate (PIC) following adsorption at the Ge(100)-2 × 1 surface has been investigated both experimentally and theoretically by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, quantum chemical calculations, and molecular dynamics simulations. PIC initially adsorbs by [2 + 2] cycloaddition across the CN bond of the isocyanate, as previously reported, but this initial product converts to a second product on the time scale of minutes at room temperature. The experimental and theoretical results show that the second product formed is phenylnitrene (C₆H₅N) covalently bonded to the germanium surface via a single Ge–N bond. This conclusion is further supported by FTIR spectroscopy experiments and density functional theory calculations using phenyl isocyanate-¹⁵N and phenyl-<i>d</i>₅ isocyanate

Highly Textured Tin(II) Sulfide Thin Films Formed from Sheetlike Nanocrystal Inks

Highly textured tin­(II) sulfide thin films are prepared from a nanocrystal ink comprised of high-aspect-ratio nanosheets. The orthorhombic nanosheets are synthesized colloidally to isolate lateral growth and minimize the presence of alternate crystal phases. The tin sulfide films deposited from the nanosheets exhibit pure elemental composition, micrometer-sized grains, and a remarkable degree of texturing. The films consist of lamellar stacking of nanosheets with some intercalation, and the average sheet thickness is ∼30 nm. The SnS films have an indirect band gap of 1.23 eV, and density functional theory calculations indicate minimal quantum confinement contributions. The anisotropic electronic properties of tin sulfide are greatly intensified in films formed by this process, yielding an in-plane mobility of 5.7 cm²/(V s) but an out-of-plane resistivity as high as 30 kΩ cm. This work represents a new strategy for nanocrystal inks in which the nanocrystal morphology is tailored to direct film orientation, grain size, and transport properties. The method provides a route for the deposition of high-quality, layered semiconductor thin films with applications in photovoltaics and two-dimensional (2-D) electronics

Improving Performance in Colloidal Quantum Dot Solar Cells by Tuning Band Alignment through Surface Dipole Moments

Colloidal quantum dots (CQDs) have received recent attention for low cost, solution processable, high efficiency solid-state photovoltaic devices due to the possibility of tailoring their optoelectronic properties by tuning size, composition, and surface chemistry. However, the device performance is limited by the diffusion length of charge carriers due to recombination. In this work, we show that band engineering of PbS QDs is achievable by changing the dipole moment of the passivating ligand molecules surrounding the QD. The valence band maximum and conduction band minimum of PbS QDs passivated with three different thiophenol ligands (4-nitrothiophenol, 4-fluorothiophenol, and 4-methylthiophenol) are determined by UV–visible absorption spectroscopy and photoelectron spectroscopy in air (PESA), and the experimental results are compared with DFT calculations. These band-engineered QDs have been used to fabricate heterojunction solar cells in both <i>unidirectional</i> and <i>bidirectional</i> configurations. The results show that proper band alignment can improve the directionality of charge carrier collection to benefit the photovoltaic performance

Dynamical Orientation of Large Molecules on Oxide Surfaces and its Implications for Dye-Sensitized Solar Cells

A dual experimental-computational approach utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory-molecular dynamics (DFT-MD) is presented for determining the orientation of a large adsorbate on an oxide substrate. A system of interest in the field of dye-sensitized solar cells is studied: an organic cyanoacrylic acid-based donor-π-acceptor dye (WN1) bound to anatase TiO₂. Assessment of nitrogen K-edge NEXAFS spectra is supported by calculations of the electronic structure that indicate energetically discrete transitions associated with the two π systems of the C–N triple bond in the cyanoacrylic acid portion of the dye. Angle-resolved NEXAFS spectra are fitted to determine the orientation of these two orbital systems, and the results indicate an upright orientation of the adsorbed dye, 63° from the TiO₂ surface plane. These experimental results are then compared to computational studies of the WN1 dye on an anatase (101) TiO₂ slab. The ground state structure obtained from standard DFT optimization is less upright (45° from the surface) than the NEXAFS results. However, DFT-MD simulations, which provide a more realistic depiction of the dye at room temperature, exhibit excellent agreementwithin 2° on averagewith the angles determined via NEXAFS, demonstrating the importance of accounting for the dynamic nature of adsorbate–substrate interactions and DFT-MD’s powerful predictive abilities

TiO₂ Conduction Band Modulation with In₂O₃ Recombination Barrier Layers in Solid-State Dye-Sensitized Solar Cells

Atomic layer deposition (ALD) was used to grow subnanometer indium oxide recombination barriers in a solid-state dye-sensitized solar cell (DSSC) based on the spiro-OMeTAD hole-transport material (HTM) and the WN1 donor-π-acceptor organic dye. While optimal device performance was achieved after 3–10 ALD cycles, 15 ALD cycles (∼2 Å of In₂O₃) was observed to be optimal for increasing open-circuit voltage (<i>V</i>_OC) with an average improvement of over 100 mV, including one device with an extremely high <i>V</i>_OC of 1.00 V. An unexpected phenomenon was observed after 15 ALD cycles: the increasing <i>V</i>_OC trend reversed, and after 30 ALD cycles <i>V</i>_OC dropped by over 100 mV relative to control devices without any In₂O₃. To explore possible causes of the nonmonotonic behavior resulting from In₂O₃ barrier layers, we conducted several device measurements, including transient photovoltage experiments and capacitance measurements, as well as density functional theory (DFT) studies. Our results suggest that the <i>V</i>_OC gains observed in the first 20 ALD cycles are due to both a surface dipole that pulls up the TiO₂ conduction band and recombination suppression. After 30 ALD cycles, however, both effects are reversed: the surface dipole of the In₂O₃ layer reverses direction, lowering the TiO₂ conduction band, and mid-bandgap states introduced by In₂O₃ accelerate recombination, leading to a reduced <i>V</i>_OC