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Understanding Measurement Artifacts Causing Inherent Cation Gradients in Depth Profiles of Perovskite Photovoltaics with TOF-SIMS
Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) is one of the few techniques that can obtain chemical information from the hybrid organic-inorganic perovskite solar cells (PSCs) in one dimension (standard depth profiling), two dimensions (high-resolution 100-nm imaging), as well as three dimensions (tomography combining high-resolution imaging with depth profiling). We have very recently developed thermo-mechanical methods to cleave perovskite samples at the back transparent conducting oxide/PSC interface. This allowed for depth-profiling from the back of the device to the front. The resultant profiles answered the lingering question of the apparent A-site cation gradient typically seen in TOF-SIMS depth profiles. In fact, this gradient is a measurement artifact due to beam damage that the community should be informed of as TOF-SIMS data become more commonly employed. Methods of mitigating this artifact by altering the measurement conditions are also presented.
Interpreting inorganic compositional depth profiles to understand the rate-limiting step in vapor phase infiltration processes
Vapor phase infiltration (VPI) is a post-polymerization modification technique that infuses inorganics into polymers to create organic–inorganic hybrid materials with new properties. Much is yet to be understood about the chemical kinetics underlying the VPI process. The aim of this study is to create a greater understanding of the process kinetics that govern the infiltration of trimethyl aluminum (TMA) and TiCl 4 into PMMA to form inorganic-PMMA hybrid materials. To gain insight, this paper initially examines the predicted results for the spatiotemporal concentrations of inorganics computed from a recently posited reaction–diffusion model for VPI. This model provides insight on how the Damköhler number (reaction versus diffusion rates) and non-Fickian diffusional processes (hindering) that result from the material transforming from a polymer to a hybrid can affect the evolution of inorganic concentration depth profiles with time. Subsequently, experimental XPS depth profiles are collected for TMA and TiCl 4 infiltrated PMMA films at 90 °C and 135 °C. The functional behavior of these depth profiles at varying infiltration times are qualitatively compared to various computed predictions and conclusions are drawn about the mechanisms of each of these processes. TMA infiltration into PMMA appears to transition from a diffusion-limited process at low temperatures (90 °C) to a reaction-limited process at high temperatures (135 °C) for the film thicknesses investigated here (200 nm). While TMA appears to fully infiltrate these 200 nm PMMA films within a few hours, TiCl 4 infiltration into PMMA is considerably slower, with full saturation not occurring even after 2 days of precursor exposure. Infiltration at 90 °C is so slow that no clear conclusions about mechanism can be drawn; however, at 135 °C, the TiCl 4 infiltration into PMMA is clearly a reaction-limited process, with TiCl 4 permeating the entire thickness (at low concentrations) within only a few minutes, but inorganic loading continuously increasing in a uniform manner over a course of 2 days. Near-surface deviations from the uniform-loading expected for a reaction-limited process also suggest that diffusional hindering is high for TiCl 4 infiltration into PMMA. In conclusion, these results demonstrate a new, ex situ analysis approach for investigating the rate-limiting process mechanisms for vapor phase infiltration.
Cation- and lattice-site-selective magnetic depth profiles of ultrathin Fe 3 O 4 (001) films
A detailed understanding of ultrathin film surface properties is crucial for the proper interpretation of spectroscopic, catalytic, and spin-transport data. We present x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) measurements on ultrathin Fe 3 O 4 films to obtain magnetic depth profiles for the three resonant energies corresponding to the different cation species F e o c t 2 + , F e t e t 3 + , and F e o c t 3 + located on octahedral and tetrahedral sites of the inverse spinel structure of Fe 3 O 4 . By analyzing the XMCD spectrum of Fe 3 O 4 using multiplet calculations, the resonance energy of each cation species can be isolated. Performing XRMR on these three resonant energies yields magnetic depth profiles that each correspond to one specific cation species. The depth profiles of both kinds of Fe 3+ cations reveal a (3.9 ± 1.0) -Å -thick surface layer of enhanced magnetization, which is likely due to an excess of these ions at the expense of the F e o c t 2 + species in the surface region. The magnetically enhanced F e t e t 3 + layer is additionally shifted about 2.9 ± 0.4 Å farther from the surface than the F e o c t 3 + layer.
D retention and depth profile behavior for single crystal tungsten with high temperature neutron irradiation
Single crystalline W (tungsten) samples irradiated at 633, 963 and 1073 K by neutrons to a damage level of 0.1 dpa were exposed to a high-flux D (deuterium) plasma at 673, 873 and 973 K, respectively, in TPE (Tritium Plasma Experiment) at INL (Idaho National Laboratory). Deuterium desorption was analyzed by TDS (Thermal Desorption Spectroscopy), and D depth profiles were determined by NRA (Nuclear Reaction Analysis) at SNL (Sandia National Laboratories). HIDT (Hydrogen Isotope Diffusion and Trapping) simulation code was applied to evaluate D behavior for neutron-damaged W at higher temperature. The D retention at depths up to 3 f.tm for the neutron-damaged sample at 673 K was two orders of magnitude larger than that for undamaged tungsten, and its D desorption spectrum had a single broad stage at around 900 K. As the neutron irradiation/plasma exposure temperature increased, D retention was largely reduced, and the desorption temperature was increased to higher temperatures above 1100 K. The D depth profiles by NRA also showed D migration toward bulk by higher temperature irradiation, compared to undamaged W. The HIDT simulation indicated that the major binding energy of D was changed from 1.43 eV to 2.07 eV at higher neutron irradiation and plasma exposure temperatures, suggesting that some vacancies and small vacancy clusters would aggregate to form larger voids, or depopulation of weak traps at high D plasma exposure temperatures. It can be said that more stable trapping sites played dominant roles in the D retention at higher neutron irradiation and plasma exposure temperature. The binding energy by HIDT simulation was almost consistent with the reported value by TMAP, but the consideration of not only total D retention measured by TDS but also D depth profile by NRA led to the more accurate D behavior in neutron-damaged W.
Depth profiling and standardization from the back side of a sample for accurate analyses: Emphasis on quantifying low‐fluence, shallow implants in diamond‐like carbon
Rationale Back‐side thinning of wafers is used to eliminate issues with transient sputtering when analyzing near‐surface element distributions. Precise and accurate calibrated implants are created by including a standard reference material during the implantation. Combining these methods allows accurate analysis of low‐fluence, shallow features even if matrix effects are a concern. Methods Implanted Na (<2.0 × 10 11 ions/cm 2 , peaking <50 nm) in diamond‐like carbon (DLC) film on silicon (solar wind returned by NASA's Genesis mission) was prepared for measurement as follows. Implanted surfaces of samples were epoxied to wafers and back‐side‐thinned using physical or chemical methods. Thinned samples were then implanted with reference ions for accurate quantification of the solar wind implant. Analyses used a CAMECA IMS 7f‐GEO SIMS in depth‐profiling mode. Results Back‐side‐implanted reference ions reduced the need to change sample mounts or stage position and could be spatially separated from the solar wind implant even when measuring monoisotopic ions. Matrix effects in DLC were mitigated and the need to find an identical piece of DLC for a reference implant was eliminated. Accuracy was only limited by the back‐side technique itself. Conclusions Combining back‐side depth profiling with back‐side‐implanted internal standards aides quantification of shallow mono‐ and polyisotopic implants. This technique helps mitigate matrix effects and keeps measurement conditions consistent. Depth profile acquisition times are longer, but if sample matrices are homogeneous, procedural changes can decrease measurement times.
Chemical depth profiles of the GaAs/native oxide interface
The final-state oxidation products and their distribution in thin native oxides (30-40 A) on GaAs have been studied using X-ray photoelectron spectroscopy in conjunction with chemical depth profiling. Extended room-temperature-oxidation conditions have been chosen to allow the native oxide to attain its equilibrium composition and structure. The work emphasizes the use of chemical depth-profiling methods which make it possible to examine the variation in chemical reactivity of the oxide structure. A minimum of two distinct regions of Ga2O3 with differing chemical reactivity is observed. Chemical shift data indicate the presence of As2O3 in the oxide together with an elemental As overlayer at the interface. A change in relative charge transfer between oxygen and both arsenic and gallium-oxide species is observed in the region of the interface.
A depth profile of C-14 in the lunar rock 12002.
The procedure involved separating the rock sample into depth fractions, designated OPI-OP6, grinding and dissolving each sample in a teflon still. The results obtained for the C-14 depth profile are shown in a table. The value for OP4 is unusually high. This phenomenon can be explained by leakage of atmospheric C-14 during the test. The profile shows a definite decrease in C-14 activity with increasing depth in the rock.
Auger depth profiling of Au-Al/x/Ga/1-x/As interfaces and LPE Al/x/Ga/1-x/As-GaAs heterojunctions
An attempt is made to report Auger depth profiles produced in a UHF ion-milling Auger system for intentionally 'graded' Al(x)Ga(1-x)As-GaAs heterojunctions, 'abrupt' growth heterojunctions of the same material, and Au-Al(x)Ga(1-x)As interfaces. It is found that the interface widths of the 'abrupt' growth heterojunctions did not fluctuate greatly from an average value of 110 A and that 'graded' growth samples had interface widths of about 525 to 1500 A. The results for the Au-containing samples indicate that an oxide layer was present between the Au and the Al(x)Ga(1-x)As.
Depth Profiling Analysis of Aluminum Oxidation During Film Deposition in a Conventional High Vacuum System
The oxidation of aluminum thin films deposited in a conventional high vacuum chamber has been investigated using x-ray photoelectron spectroscopy (XPS) and depth profiling. The state of the Al layer was preserved by coating it with a protective MgF2 layer in the deposition chamber. Oxygen concentrations in the film layers were determined as a function of sputter time (depth into the film). The results show that an oxidized layer is formed at the start of Al deposition and that a less extensively oxidized Al layer is deposited if the deposition rate is fast. The top surface of the Al layer oxidizes very quickly. This top oxidized layer may be thicker than has been previously reported by optical methods. Maximum oxygen concentrations measured by XPS at each Al interface are related to pressure to rate ratios determined during the Al layer deposition.
Aluminum 26, Be-10 and Cl-36 depth profiles in the Canyon Diablo iron meteorite
We have measured activities of the long-lived cosmogenic radionuclides Al-26, Be-10, and Cl-36 in 12 fragments of the iron meteorite Canyon Diablo and have constructed production rate-versus-depth profiles of those radionuclides. Profiles determined using differential particle fluxes calculated with the LAHET code system are in good agreement with Al-26, Be-10, and Cl-36 experimental data, but the agreement for Cl-36 was obtained only after neutron-induced cross sections were modified. Profiles calculated with lunar particle fluxes are much lower than experimental Canyon Diablo profiles. The cosmic ray exposure ages of most samples are near 540 m.y.
The X-ray photoelectron spectroscopy depth profiling and tribological characterization of ion-plated gold on various metals
For the case of ion-plated gold, the graded interface between gold and a nickel substrate and a nickel substrate, such tribological properties as friction and microhardness are examined by means of X-ray photoelectron spectroscopy analysis and depth profiling. Sliding was conducted against SiC pins in both the adhesive process, where friction arises from adhesion between sliding surfaces, and abrasion, in which friction is due to pin indentation and groove-plowing. Both types of friction are influenced by coating depth, but with opposite trends: the graded interface exhibited the highest adhesion, but the lowest abrasion. The coefficient of friction due to abrasion is inversely related to hardness. Graded interface microhardness values are found to be the highest, due to an alloying effect. There is almost no interface gradation between the vapor-deposited gold film and the substrate.
Mars Sample Return: The Value of Depth Profiles
Sample return from Mars offers the promise of data from Martian materials that have previously only been available from meteorites. Return of carefully selected samples may yield more information about the history of water and possible habitability through Martian history. Here we propose that samples collected from Mars should include depth profiles of material across the interface between weathered material on the surface of Mars into unweathered parent rock material. Such profiles have the potential to yield chemical kinetic data that can be used to estimate the duration of water and information about potential habitats on Mars.
A method of rapidly obtaining concentration-depth profiles from X-ray diffraction
A broadened diffraction peak, or intensity band, is observed in the case diffraction from a nonhomogeneous phase in which the variations in compositions result in a range of lattice spacings. An intriguing aspect regarding the relationship between the X-ray diffraction band and the composition-depth profile is the hypersensitivity of the intensity band to the shape of the profile. A number of investigators have sought to use this sensitivity to construct high-precision profiles. Difficulties encountered are related to complications due to intensity broadening, and prohibitive computational requirements. Simulation techniques have provided the most accurate interpretation of the intensity band. However, the involved calculations have been prohibitively long. The present study discusses a technique which has simple computational requirements and is as accurate and flexible as the simulation techniques.
Nonaqueous chemical depth profiling of YBa2Cu3O(7-x)
A nonaqueous solution of Br in absolute ethanol (EtOH) has recently been reported to be effective at removing nonsuperconducting surface species from YBa2Cu3O(7-x) films, leaving the surface close to the ideal stoichiometry. This same etchant is shown here to be an effective bulk etchant in chemical depth profiling through 1-micron-thick films. The Cu remains in the 2 + oxidation state and the stoichiometry, as determined by X-ray photoelectron spectroscopy, is close to ideal and nearly constant throughout the profile, indicating the absence of any large preferential etching effects. The reaction of YBa2Cu3O(7-x) films with HF/EtOH, HCl/EtOH, and I/EtOH solutions is also reported.
Scanning Electron Microscopy Investigation of a Sample Depth Profile Through the Martian Meteorite Nakhla
The ongoing scientific debate as to whether or not the Martian meteorite ALH84001 contained evidence of possible biogenic activities showed the need to establish consistent methods to ascertain the origin of such evidence. To distinguish between terrestrial organic material/microbial contaminants and possible indigenous microbiota within meteorites is therefore crucial. With this in mind a depth profile consisting of four samples from a new sample allocation of Martian meteorite Nakhla was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis. SEM imaging of freshly broken fractured chips revealed structures strongly recent terrestrial microorganisms, in some cases showing evidence of active growth. This conclusion was supported by EDX analysis, which showed the presence of carbon associated with these structures, we concluded that these structures represent recent terrestrial contaminants rather than structures indigenous to the meteorite. Page
Auger electron spectroscopy and depth profile study of oxidation of modified 440C steel
Auger electron spectroscopy (AES) and sputtering were used to study selective oxidation of modified 440C steel. The sample was polycrystalline. Oxidation was performed on initially clean surfaces for pressures ranging from 1 x 10 to the minus 7th power to 1 x 10 to the minus 5th power torr and temperatures ranging from room temperature to 800 C. AES traces were taken during oxidation. In situ sputtering depth profiles are also obtained. A transition temperature is observed in the range 600 to 700 C for which the composition of the outer surface oxide changed from iron oxide to chromium oxide. Heating in vacuum about 5 x 10 to the minus 10 power torr to 700 C causes conversion of the iron oxide surface to chromium oxide.
Controlling Magnetization Vector Depth Profiles of La 0.7 Sr 0.3 CoO 3 /La 0.7 Sr 0.3 MnO 3 Exchange Spring Bilayers via Interface Reconstruction
The La 0.7 Sr 0.3 CoO 3–δ /La 0.7 Sr 0.3 MnO 3–δ (LSCO/LSMO) bilayer system is an ideal perovskite oxide platform for investigating interface reconstruction and its effect on their magnetic properties. Previous studies have shown that LSCO can separate into magnetic sublayers, which possess distinct trends as the total LSCO thickness increases. Here, we used polarized neutron reflectometry to quantify changes in the magnetic and chemical depth profiles, and it confirms the formation of ~12 Å-thick interfacial LSCO and LSMO layers, characterized by a decreased nuclear scattering length density compared to the bulk of the layers. This decrease is attributed to the combined effects of oxygen vacancy formation and interfacial charge transfer, which lead to magnetically active Co 2+ ions with ionic radii larger than the Co 3+ /Co 4+ ions typically found in bulk LSCO or single-layer films. The interfacial magnetization values, as well as Co 2+ ion and oxygen vacancy concentrations, depend strongly on the LSCO layer thickness. These results highlight the sensitive interplay of the cation valence states, oxygen vacancy concentration, and magnetization at interfaces in perovskite oxide multilayers, demonstrating the potential to tune their functional properties via careful design of their structure.