Identifying Charge Transfer Mechanisms across Semiconductor Heterostructures via Surface Dipole Modulation and Multiscale Modeling.

The design and fabrication of stable and efficient photoelectrochemical devices requires the use of multifunctional structures with complex heterojunctions composed of semiconducting, protecting, and catalytic layers. Understanding charge transport across such devices is challenging due to the interplay of bulk and interfacial properties. In this work, we analyze hole transfer across n-Si(111)- R|TiO2 photoanodes where - R is a series of mixed aryl/methyl monolayers containing an increasing number of methoxy units (mono, di, and tri). In the dimethoxy case, triethylene glycol units were also appended to substantially enhance the dipolar character of the surface. We find that hole transport is limited at the n-Si(111)- R|TiO2 interface and occurs by two processes- thermionic emission and/or intraband tunneling-where the interplay between them is regulated by the interfacial molecular dipole. This was determined by characterizing the photoanode experimentally (X-ray photoelectron spectroscopy, voltammetry, impedance) with increasingly thick TiO2 films and complementing the characterization with a multiscale computational approach (first-principles density functional theory (DFT) and finite-element device modeling). The tested theoretical model that successfully distinguished thermionic emission and intraband tunneling was then used to predict the effect of solution potential on charge transport. This prediction was then experimentally validated using a series of nonaqueous redox couples (ferrocence derivatives spanning 800 mV). As a result, this work provides a fundamental understanding of charge transport across TiO2-protected electrodes, a widely used semiconductor passivation scheme, and demonstrates the predictive capability of the combined DFT/device-modeling approach.

Oxygen-Induced Ordering in Bulk Polycrystalline Cu2ZnSnS4 by Sn Removal.

Solid-state nuclear magnetic resonance spectroscopy, X-ray diffraction, and Raman spectroscopy were used to show that Cu2ZnSnS4 (CZTS) bulk solids grown in the presence of oxygen had improved cation ordering compared to bulk solids grown without oxygen. Oxygen was shown to have negligible solubility in the CZTS phase. The addition of oxygen resulted in the formation of SnO2, leading to Sn-deficient CZTS. At the highest oxygen levels, other phases such as Cu9S5 and ZnS were observed. Beneficial ordering was only observed in samples produced with more than 2 at. % oxygen in the precursor materials but did not occur in samples designed with excess Sn and O. Thus, it is the removal of Sn and formation of Sn-deficient CZTS that improves ordering rather than the presence of SnO2 or O alone. These results indicate that using oxygen or air annealing to tailor the Sn content of CZTS followed by an etching step to remove SnO2 may significantly improve the properties of CZTS.

CdCl2 Treatment-Induced Enhanced Conductivity in CdTe Solar Cells Observed Using Conductive Atomic Force Microscopy.

The rear surfaces of CdTe photovoltaic devices without back contacts, grown by close-spaced sublimation (CSS), were analyzed using conductive atomic force microscopy (C-AFM). As-deposited and CdCl2-treated CdTe samples were compared to clarify the effect of the treatment on charge flow through grains and grain boundaries. The CdCl2-treated samples exhibit a more homogeneous and enhanced current flow across the grains as compared to the as-deposited samples. The grain boundaries show variable current. Under high bias, grain boundaries dominate current flow when the main junction is reverse biased and with the conducting current in reverse breakdown. Under the opposite bias conditions, where the contact of the conductive tip to the surface is reverse biased and under breakdown conditions, the current flow is uniform with little contrast between grains and grain boundaries. The results are interpreted as resulting from the improved crystallinity of the CdTe with reduced p-type doping along the grain boundaries.

Towards Direct Synthesis of Alane: A Predicted Defect-Mediated Pathway Confirmed Experimentally.

Alane (AlH3 ) is a unique energetic material that has not found a broad practical use for over 70 years because it is difficult to synthesize directly from its elements. Using density functional theory, we examine the defect-mediated formation of alane monomers on Al(111) in a two-step process: (1) dissociative adsorption of H2 and (2) alane formation, which are both endothermic on a clean surface. Only with Ti dopant to facilitate H2 dissociation and vacancies to provide Al adatoms, both processes become exothermic. In agreement, in situ scanning tunneling microscopy showed that during H2 exposure, alane monomers and clusters form primarily in the vicinity of Al vacancies and Ti atoms. Moreover, ball milling of the Al samples with Ti (providing necessary defects) showed a 10 % conversion of Al into AlH3 or closely related species at 344 bar H2 , indicating that the predicted pathway may lead to the direct synthesis of alane from elements at pressures much lower than the 10(4)  bar expected from bulk thermodynamics.

XPS investigation of a CdS-based photoresistor under working conditions: operando-XPS.

A noncontact chemical and electrical measurement X-ray photoelectron spectroscopy (XPS) technique is performed to investigate a CdS-based photoresistor during its operation. The main objective of the technique is to trace chemical- and location-specified surface potential variations as shifts of the XPS Cd 3d(5/2) peak position without and under photoillumination with four different lasers. The system is also modeled to extract electrical information. By analyzing the measured potential variations with this model, location-dependent resistance values are represented (i) two dimensionally for line scans and (ii) three dimensionally for areal measurements. In both cases, one of the dimensions is the binding energy. The main advantage of the technique is its ability to assess an element-specific surface electrical potential of a device under operation based on the energy deviation of core level peaks in surface domains. Detection of the variations in electrical potentials and especially their responses to the energy of the illuminating source in operando, is also shown to be capable of detecting, locating, and identifying the chemical nature of structural and other types of defects.

Direct writing of sub-5 nm hafnium diboride metallic nanostructures.

Sub-5 nm metallic hafnium diboride (HfB(2)) nanostructures were directly written onto Si(100)-2 × 1:H surfaces using ultrahigh vacuum scanning tunneling microscope (UHV-STM) electron beam induced deposition (EBID) of a carbon-free precursor molecule, tetrakis(tetrahydroborato)hafnium, Hf(BH(4))(4). Scanning tunneling spectroscopy data confirm the metallic nature of the HfB(2) nanostructures, which have been written down to lateral dimensions of ∼2.5 nm. To our knowledge, this is the first demonstration of sub-5 nm metallic nanostructures in an STM-EBID experiment.

Origin of bulklike structure and bond length disorder of Pt37 and Pt6Ru31 clusters on carbon: comparison of theory and experiment.

We describe a theoretical analysis of the structures of self-organizing nanoparticles formed by Pt and Ru-Pt on carbon support. The calculations provide insights into the nature of these metal particle systems-ones of current interest for use as the electrocatalytic materials of direct oxidation fuel cells-and clarify complex behaviors noted in earlier experimental studies. With clusters deposited via metallo-organic Pt or PtRu(5) complexes, previous experiments [Nashner et al. J. Am. Chem. Soc. 1997, 119, 7760; Nashner et al. J. Am. Chem. Soc. 1998, 120, 8093; Frenkel et al. J. Phys. Chem. B 2001, 105, 12689] showed that the Pt and Pt-Ru based clusters are formed with fcc(111)-stacked cuboctahedral geometry and essentially bulklike metal-metal bond lengths, even for the smallest (few atom) nanoparticles for which the average coordination number is much smaller than that in the bulk, and that Pt in bimetallic [PtRu(5)] clusters segregates to the ambient surface of the supported nanoparticles. We explain these observations and characterize the cluster structures and bond length distributions using density functional theory calculations with graphite as a model for the support. The present study reveals the origin of the observed metal-metal bond length disorder, distinctively different for each system, and demonstrates the profound consequences that result from the cluster/carbon-support interactions and their key role in the structure and electronic properties of supported metallic nanoparticles.