Surface Chemistry Exchange of Alloyed Germanium Nanocrystals: A Pathway Toward Conductive Group IV Nanocrystal Films

We present an expansion of the mixed-valence iodide reduction method for the synthesis of Ge nanocrystals (NCs) to incorporate low levels (∼1 mol %) of groups III, IV, and V elements to yield main-group element-alloyed Ge NCs (Ge_1–<i>x</i>E_<i>x</i> NCs). Nearly every main-group element (E) that surrounds Ge on the periodic table (Al, P, Ga, As, In, Sn, and Sb) may be incorporated into Ge_1–<i>x</i>E_<i>x</i> NCs with remarkably high E incorporation into the product (>45% of E added to the reaction). Importantly, surface chemistry modification via ligand exchange allowed conductive films of Ge_1–<i>x</i>E_<i>x</i> NCs to be prepared, which exhibit conductivities over large distances (25 μm) relevant to optoelectronic device development of group IV NC thin films

Perturbation of the Electron Transport Mechanism by Proton Intercalation in Nanoporous TiO₂ Films

This study addresses a long-standing controversy about the electron-transport mechanism in porous metal oxide semiconductor films that are commonly used in dye-sensitized solar cells and related systems. We investigated, by temperature-dependent time-of-flight measurements, the influence of proton intercalation on the electron-transport properties of nanoporous TiO₂ films exposed to an ethanol electrolyte containing different percentages of water (0–10%). These measurements revealed that increasing the water content in the electrolyte led to increased proton intercalation into the TiO₂ films, slower transport, and a dramatic change in the dependence of the thermal activation energy (<i>E</i>_a) of the electron diffusion coefficient on the photogenerated electron density in the films. Random walk simulations based on a microscopic model incorporating exponential conduction band tail (CBT) trap states combined with a proton-induced shallow trap level with a long residence time accounted for the observed effects of proton intercalation on <i>E</i>_a. Application of this model to the experimental results explains the conditions under which <i>E</i>_a dependence on the photoelectron density is consistent with multiple trapping in exponential CBT states and under which it appears at variance with this model

Tuning Electrical, Optical, and Thermal Properties through Cation Disorder in Cu₂ZnSnS₄

Chemical disorder in semiconductors is important to characterize reliably because it affects materials performance, for instance by introducing potential fluctuations and recombination sites. It also represents a means to control material properties, to far exceed the limits of equilibrium thermodynamics. We present a study of highly disordered Cu–Zn–Sn–S (d-CZTS) films along the Cu2SnS3–Cu2ZnSnS4–ZnS binary line, deposited by physical vapor deposition. Deposition at low temperature kinetically stabilizes compositions that are well outside of the narrow, equilibrium solid solution of kesterite (Cu2ZnSnS4). Here we study d-CZTS and its thermal treatment using complementary characterization techniques: X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). We find that cations in d-CZTS are highly disordered while the sulfur anions remain in a well-defined, cubic close-packed lattice. On the atomic scale, composition fluctuations are accommodated preferentially by stacking faults. Kinetically-stabilized cation disorder can produce nonequilibrium semiconductor alloys with a wide range of band gap, electronic conductivity, and thermal conductivity. d-CZTS therefore represents a processing route to optimizing materials for optoelectronic device elements such as light absorbers, window layers, and thermal barriers