Chemically Resolved Electrical Measurements Research Articles

Guest Transition Metals in Host Inorganic Nanocapsules: Single Sites, Discrete Electron Transfer, and Atomic Scale Structure.

Host–guest solution chemistry with a wide range of organic hosts is an important and established research area, while the use of inorganic hosts is a more nascent area of research. In the recent past in a few cases, Keplerate-type molybdenum oxide-based porous, spherical clusters, shorthand notation {Mo132}, have been used as hosts for organic guests. Here, we demonstrate the synthetically controlled encapsulation of first-row transition metals (M = Mn, Fe, and Co) within a Keplerate cluster that was lined on the inner core with phosphate anions, {Mo132PO4}. The resulting M2+x⊂{Mo132PO4} host–guest complexes were characterized by 31P NMR and ENDOR spectroscopy that substantiated the encapsulation of the first-row transition metal guest. Magnetic susceptibility measurements showed that the encapsulation of up to 10 equiv showed little magnetic interaction between the encapsulated metals, which indicated that each guest atom occupied a single site. Visualization of the capsules and differentiation of the Mo atoms of the capsule framework and the encapsulated transition metal were possible using spherical and chromatic double aberration-corrected electron microscopy combined with energy-filtered TEM (EFTEM) elemental maps. In addition, use of visible light-induced XPS for chemically resolved electrical measurements (CREM) confirmed the successful encapsulation of M within {Mo132PO4} and furthermore showed photoinduced electron transfer from M to Mo. In the future, such targeted electron transfer between host {Mo132} and a transition metal guest could be used as photoinitiated switches using inorganic compounds and for single site photocatalytic reactions in confined space.

Submolecular Gates Self-Assemble for Hot-Electron Transfer in Proteins.

Redox reactions play key roles in fundamental biological processes. The related spatial organization of donors and acceptors is assumed to undergo evolutionary optimization facilitating charge mobilization within the relevant biological context. Experimental information from submolecular functional sites is needed to understand the organization strategies and driving forces involved in the self-development of structure-function relationships. Here we exploit chemically resolved electrical measurements (CREM) to probe the atom-specific electrostatic potentials (ESPs) in artificial arrays of bacteriochlorophyll (BChl) derivatives that provide model systems for photoexcited (hot) electron donation and withdrawal. On the basis of computations we show that native BChl's in the photosynthetic reaction center (RC) self-assemble at their ground-state as aligned gates for functional charge transfer. The combined computational and experimental results further reveal how site-specific polarizability perpendicular to the molecular plane enhances the hot-electron transport. Maximal transport efficiency is predicted for a specific, ∼5 Å, distance above the center of the metalized BChl, which is in remarkably close agreement with the distance and mutual orientation of corresponding native cofactors. These findings provide new metrics and guidelines for analysis of biological redox centers and for designing charge mobilizing machines such as artificial photosynthesis.

Contactless derivation of inner fields in gate-oxide layers: SiO2 on SiC

Probing depth electrostatic potential profiles at sub-nm resolution is a major characterization challenge. An answer is frequently proposed by chemically resolved electrical measurements (CREM); yet, CREM is limited in extracting the profile details within compositionally uniform domains. Here, we show that this principal limitation can be overcome and the CREM resolution be improved significantly. Applied to nanometric SiO2 layers on SiC, hidden impurity concentration profiles are revealed and the inner fields, before and during dielectric collapse, are quantified. With this leap improvement in resolution and sensitivity, our advanced CREM analysis promises diverse applications in device contact-free electrical studies.

Surface Oxidation as a Cause of High Open‐Circuit Voltage in CdSe ETA Solar Cells

TiO2/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open‐circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 ºC). Annealing in N2 produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N2‐annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo‐induced charge trapping, in particular at absorber grain boundaries of the air versus N2‐annealed systems, correlated with the formation of Cd–O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface‐oxidation, which both impedes electron injection from the CdSe to the TiO2, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers.

Band Alignment and Internal Field Mapping in Solar Cells

The internal fields and band offsets developing at individual interfaces, a critical aspect of device performance, are generally inaccessible by standard electrical tools. To address this problem, we propose chemically resolved electrical measurements (CREM) capable of resolving the internal details layer-by-layer. Applied to nanoporous photovoltaic cells, we thus extract a realistic band diagram for the multi-interfacial structure and, in particular, resolve the two p-n-like junction fields built spontaneously in the device. The lack of homogeneity common to many of these nanoporous cells is exploited here to “see” deep into the cell structure, beyond the typical depth limitations of the surface-sensitive technique. Further information on the cell operation under “real” working conditions is achieved by studying the charge trapping at each specific layer under optical and electrical stimuli. Our methodology overcomes a missing link in device characterization and in fundamental studies of nanoscale solid-...

Chemically resolved electrical measurements in organic self-assembled molecular layers

The spectroscopy of charged particles, e.g. X-ray photoelectron spectroscopy (XPS), proposes unique capabilities for electrical studies of nanometric heterostructures. By turning the problem of surface charging into a consistent and general method for chemically resolved electrical measurements (CREM), selected surface and sub-surface regions can be probed in a non-contact manner, with useful advantages over standard electrical tools. When applied to organic systems, CREM can read electrical information out of atomic sites within molecular assemblies, and hence, map the electrostatic potential variations under external stimuli, approaching atomic resolution. This paper reviews CREM studies of self-assembled monolayers (SAMs), with an emphasis on questions regarding the surface charge distribution. Surprising observations in work function changes under self-assembly of dipolar molecules are presented, as well as the direct detection of sub-molecular polarization effects, and photo-induced charge redistribution across SAMs that are situated between semiconducting media. The review concludes by suggesting future applications of CREM and further challenges that may arise.

Hot-electron characteristics in chemically resolved electrical measurements of thin silica and SiON layers

We use the recently developed chemically resolved electrical measurements (CREM) to sensitively measure hot-electron transport characteristics in thin dielectric layers. By comparing bare gate-oxide layers, SiO2 and SiON, pronounced differences are revealed that are absent from standard contact measurements and from CREM conducted on top metallic pads. The “on pad” and standard measurements obey a similar defect-assisted “Poole–Frenkel” transport, whereas I∼Vα characterizes the hot-electron transport through the bare overlayer, with a clear thickness dependence of α. These unique CREM features offer useful advantages in gate-oxide characterization.

X‐ray photoelectron spectroscopy and tribology studies of annealed fullerene‐like WS2 nanoparticles

AbstractThe time dependent chemical changes occurring at the surface of inorganic fullerene‐like (IF) nanoparticles of WS2 were investigated using X‐ray photoelectron spectroscopy (XPS) and compared to those of bulk powder, 2H‐WS2. It was possible to follow the long term surface oxidation and carbonization occurring at defects on the outermost surface (0001) molecular layers of the inorganic fullerene‐like nanoparticles. Vacuum annealing was shown to remove most of these contaminants and bring the surface close to its pristine stoichiometric composition. In accordance with previous measurements, further evidence was obtained for the existence of water molecules, which were entrapped in the hollow core and interstitial defects of the fullerene‐like nanoparticles during the synthesis. These water molecules were also shown to be removable by the vacuum annealing process. Chemically resolved electrical measurements (CREM) in the XPS showed that the IF samples had become less p‐type after the vacuum annealing. Finally, tribological measurements showed that the vacuum annealed IF samples performed better as an oil additive than the non‐annealed IF samples and the 2H‐WS2 powder. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Secondary electron emission control in X-ray photoelectron spectroscopy

Secondary electron emission (SEE) is a major player in surface charging during X-ray photoelectron spectroscopy (XPS); its characteristics and applicability as a current source for electrical measurements are studied. We employ sample biasing and a top retarding grid to control the photoelectron current, and further compare their I– V characteristics with direct spectroscopy of the secondary electrons. Using silica-coated gold substrates, the effect of sample work function on the emitted secondary electrons is shown and fine control over the surface potential gradients, in the range of 10–100 meV, is achieved. XPS-based chemically resolved electrical measurements (CREM) can thus be extended to the positive current regime.

Chemical Site Capacitance: Submolecular Measurements and a Model

The study of molecular electric properties is an intriguing, rapidly developing field in which technological and basic scientific challenges and developments are evolving. Nevertheless, understanding of the interplay of intermolecular interactions, substrate effects, and electrode contacts remains challenging. Here, we present noncontact chemically resolved electrical measurements (CREM) of halide-terminated molecular layers and a straightforward model for quantitative analysis of submolecular chemical site capacitance. We demonstrate that under low current densities, the main electronic effects can be accounted for by considering the (sub)molecular properties of the monolayers, whereas the excess potential due to charge injection can be described as site capacitance corresponding to chemically identifiable molecular sites.

Chemically Resolved Photovoltage Measurements in CdSe Nanoparticle Films

Light-induced chemically resolved electrical measurements (CREM) under controlled electrical conditions are used to study photovoltaic effects at selected regions in nanocrystalline CdSe-based films. The method, based on X-ray photoelectron spectroscopy (XPS), possesses unique capabilities for exploring charge trapping and charge transport mechanisms, combining spectrally filtered input signals with photocurrent detection and with a powerful, site-selective, photovoltage probe.

Transient Charge Accumulation in a Capacitive Self-Assembled Monolayer

Charge accumulation in an organosilane monolayer self-assembled on silicon is studied using electron-spectroscopy-based chemically resolved electrical measurements (CREM). By resolving the net electrical response of the organic layer, a significant capability of holding extra charge is indicated. Quantum size effects at a molecularly thin layer and the role of competing discharge mechanisms, including defect-assisted leakage currents, are discussed.