Atomic Layer Depth Resolution Research Articles

Quantitative Compositional Profiling of Conjugated Quantum Dots with Single Atomic Layer Depth Resolution via Time-of-Flight Medium-Energy Ion Scattering Spectroscopy

We report the quantitative compositional profiling of 3-5 nm CdSe/ZnS quantum dots (QDs) conjugated with a perfluorooctanethiol (PFOT) layer using the newly developed time-of-flight (TOF) medium-energy ion scattering (MEIS) spectroscopy with single atomic layer resolution. The collection efficiency of TOF-MEIS is 3 orders of magnitude higher than that of conventional MEIS, enabling the analysis of nanostructured materials with minimized ion beam damage and without ion neutralization problems. The spectra were analyzed using PowerMEIS ion scattering simulation software to allow a wide acceptance angle. Thus, the composition and core-shell structure of the CdSe cores and ZnS shells were determined with a 3% composition uncertainty and a 0.2-nm depth resolution. The number of conjugated PFOT molecules per QD was also quantified. The size and composition of the QDs were consistent with those obtained from high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. We suggest TOF-MEIS as a nanoanalysis technique to successfully elucidate the core-shell and conjugated layer structures of QDs, which is critical for the practical application of QDs in various nano- and biotechnologies.

Ion Depletion Near a Solution Surface: Is Image-Charge Repulsion Sufficient?

Grazing-incidence Rutherford backscattering and angle-resolved x-ray photoelectron spectrometry are used to determine the ion-concentration profiles near the surface of a solution consisting of a salt (TEABr) in a weakly polar organic liquid (polyethylene glycol) with atomic-layer depth resolution. The predictions of a model, in which ions in solution are repelled from the surface due to a screened Coulomb interaction with their image charge, are in good agreement with measured ion profiles. This contrasts with the behavior of salts in aqueous and highly polar organic solutions.

Grazing angle 2MeV RBS on the surface of a liquid with atomic layer depth resolution

Abstract RBS (2 MeV) has been used to study the near surface Br distribution for a salt (TEABr) in hydrocarbon (PEG200) solution. Near grazing incidence, measuring the energy of the Br edge as a function of the angle between the beam and the liquid surface indicates that Br is depleted from the surface to an average depth of 6.9 A.

Determination of deuterium adsorption site on palladium(1 0 0) using low energy ion recoil spectroscopy

Ion beam analysis has been recently applied to study the adsorption phenomena of some adsorbates on metal surfaces. In this paper, surface recoils created by low energy Ne + ions are employed to study the adsorption site of deuterium (D) atoms on Pd(1 0 0). This technique is extremely surface sensitive with the capacity for atomic layer depth resolution. From azimuthal angle observations of Pd(1 0 0) specimen, it was found that at room temperature, D was adsorbed in the fourfold hollow site of Pd(1 0 0) at a height of 0.25 ± 0.05 Å above the surface. The adsorbate remains in the hollow site at all temperatures to 383 K though the vertical height above the surface is found to depend on coverage and for the first time evidence is found of a transition to a p(2 × 2) structure for the adsorbate. There is no evidence of D sitting in the Pd(1 0 0) subsurface at room and higher temperatures.

Positron annihilation induced Auger and gamma spectroscopies of surfaces

The annihilation of positrons with core electrons results in an element specific signature in the spectra of Auger-electron and annihilation gamma rays. Because a large fraction of positrons implanted at low energies become trapped just outside the surface, annihilation induced Auger and Gamma signals probe the surfaces of solids with single atomic layer depth resolution. Recent applications of positron annihilation-induced Auger electron spectroscopy (PAES) and Auger–gamma coincidence spectroscopy (AGCS) and future applications of Auger–gamma and gamma–gamma coincidence spectroscopy are discussed.

Compositional changes in the surface layers of the Cu 3Au(1 0 0) below the bulk transition temperature

The surface compositional changes in the surface layers of Cu 3Au(1 0 0) have been studied by medium-energy ion-scattering spectroscopy (MEIS). Using 100 keV N + ions in a MEIS experiment, single atomic layer depth resolution was obtained. At a temperature starting at about 500 K, the Au atoms in the surface layer of Cu 3Au(1 0 0) apparently began to move to the second layer with increasing temperature. The atomic concentration of Au in the first layer as measured by MEIS then decreased from 50% at room temperature (RT) to 32% at 720 K. This decrease in the atomic concentration of Au atoms in the first layer was continuous, rather than abrupt at the critical temperature. Moreover, it was found that the compositional change between the first and the second layers was reversible at temperatures up to 720 K. The atomic concentration of Au atoms in the third layer remained constant over the temperature range from RT to 720 K. The difference between this result, where the first layer Au concentration decreases, and the previous low-energy ion-scattering spectroscopy (LEIS) measurements which show a relatively constant Au concentration, lies in the production of surface vacancies and adatoms at elevated temperatures. These defects have no significant effect on yield in LEIS experiments but are seen using MEIS.

Medium-energy ion scattering spectroscopy for quantitative surface and near-surface analysis of ultrathin films

Medium-energy Ion Scattering Spectroscopy (MEISS) can provide non-destructive quantitative surface and near-surface composition with atomic layer depth resolution. The special features of MEISS can be utilized to improve quantification of conventional surface analysis techniques by studying preferential sputtering, which is one of the major problems in quantitative surface analysis using sputtering techniques. This phenomenon was studied in detail with MEISS, in situ and ex situ XPS analysis, RBS, and (inductively coupled plasma optical emission spectrometry) for WSi 2 .

Elastic recoil detection with single atomic layer depth resolution

The necessary conditions for single atomic layer depth resolution in Elastic Recoil Detection (ERD) analysis will be summarized in this paper: First there are rigorous requirements for the ion beam conditions and for the energy resolution of the recoil ion detector in order to obtain an energy resolution in the order of 5 × 10 −4. Second, the most limiting factors in depth resolution are due to the physical limits imposed by small angle scattering effects and due to the energy loss spread of the incident and recoil ions. Last but not least, the third point deals with irradiation damage which has to be carefully controlled in order to measure the original depth profile before it is altered by the ion beam. As studied by energy loss measurements in transmission geometry through thin carbon foils the energy spread of heavy ions strongly depends on the charge state of the incident ions. In order to obtain the smallest energy spread the data show that the charge state of the incident ions should be near equilibrium. Using a 60 MeV 127I 23+ ion beam the resolution of single atomic carbon layers of a highly oriented pyrolythic graphite (HOPG) sample could be demonstrated analysing the 12C 5+ recoils with the Munich Q3D magnetic spectrograph. As far as we know it is the first time that separated signals of neighbouring atomic layers could be resolved using ERD.

Glancing incident MeV ion beams for total reflection PIXE (TPIXE) and RBS surface analysis

In a total reflection geometry at small incident angles ϕ (0–35 mrad), MeV proton and α beams were used to analyse Au and Cu layers on flat quartz substrates by detection of X-rays and backscattered protons. The reflected proton beams were detected as a function of incident angle ϕ. An improved model for theoretical X-ray yield calculations is developed. These calculations showed that a proton beam of 2.5 MeV will be totally reflected from a Au atomic surface layer at an incident angle smaller than 8 mrad. At these small angles the backscattered protons and α particles show a pattern which indicates atomic layer depth resolution.

Surface analysis with a position-sensitive atom probe

A position-sensitive detector has been combined with time-of-flight mass spectrometry in a short-flight-path atom probe to yield an instrument in which both the chemical identity and surface position are obtained for individual atoms removed by field evaporation from the surface of a field-ion specimen. This enables thin films and surface layers to be analysed, not only with atomic-layer depth resolution, but also with sub-nanometre lateral resolution. The new instrument is described, and examples are presented showing the application of the technique to the study of the oxidation of metal surfaces, and surface segregation in an yttrium barium cuprate ceramic-oxide superconductor.

Studies of solid surfaces at atomic resolution: Atom-probe and field ion microscopy

The structure and composition are basic information of a surface. Using a field ion microscope the atomic structure of a surface can be observed, whereas using a time-of-flight atom-probe the composition of a surface can be analyzed, often with true single atomic layer depth resolution. We present here a brief review of studies with this technique, emphasizing recent developments. These include: (1) Observation of surface atomic reconstructions. (2) Successful imaging of silicon surfaces of well ordered atomic structures. (3) Using ns pulsed-laser heating, atomic processes in atom transport and surface reconstructions can be studied in 5 ns steps with true atomic resolution. (4) By using ps pulsed-laser assisted field desorption for the atom-probe operation, dissociation of compound ions in high electric field by atomic tunneling can be studied with a time resolution of 20 fs. These and other new theoretical and experimental developments in atom-probe and field ion microscopy and its application to surface science will be briefly described, focusing especially on the atomic resolution and the quantitative aspects of these studies.

Compositional variations in the near surface layers, an atom-probe study of cosegregation of sulfur in Pt–Rh and Pt–Ir alloys

Equilibrium composition depth profiles with true single atomic layer depth resolution have been obtained for the (001) plane of a Pt–Ir and five Pt–Rh alloys using the atom-probe field ion microscope. Pt segregates to the top two layers in Pt–Ir and Rh segregates to the top layer in Pt–Rh alloys after annealing at 700 °C for 5 min. In addition, an overlayer of sulfur is found to cosegregate to the surface of these alloys even though the bulk impurity content is less than 100 ppm. The coverage of sulfur overlayer for Pt–Rh alloys varies between 0.22 to 0.52 of a monolayer, and it increases with increasing surface Rh concentration. The depth dependence of the composition depth profiles has been found to be monotonic for the Pt–Ir alloy and nonmonotonic, or possibly oscillatory, for the Pt–Rh alloys. Pending the uncertainty of the effect of the sulfur overlayer, Pt segregation in Pt–Ir is consistent with most theoretical models, and Rh segregation in Pt–Rh is consistent with the bond-breaking model and the atomic model, but is inconsistent with a surface energy model and an electronic model. Rh segregation may also be an effect of chemisorption enhanced surface segregation. While the experimental result on the top surface layer can be explained by various theoretical models, the compositional variation in the near surface layers may shed some light on the atomic interactions in the surface region. For the Pt–Ir alloy, we have also observed a diffusion kinetic effect in a composition depth profile.

A nonmonotonic concentration depth profile of Pt–Rh alloys: A surface segregation study using the atom-probe field ion microscope

Equilibrium concentration depth profiles with true single atomic layer depth resolution have been obtained for the surface segregation of Pt–Rh alloys using a time-of-flight atom-probe field ion microscope. The depth profiles obtained for the (001) plane of Pt–Rh alloys of five different bulk composition all show an enrichment of Rh to the top surface layer. It is also observed that there is a significant Rh depletion in the second layer. An additional feature of cosegregated sulfur impurity and overlayer formation is observed. The question of the possible nonequilibrium depth profiles has been resolved by a careful experimental testing. The surface segregation of Rh is consistent with several theoretical models, whereas the nonmonotonic concentration profile may indicate an oscillatory structure in the potential of single ions in the near surface layers.

Surface segregation and diffusion kinetics study of a Pt-Ir alloy using the time-of-flight atom-probe field ion microscope

Two absolute composition depth profiles of single atomic layer depth resolution have been obtained for the (001) surface of a Pt-20% Ir alloy. Pt segregates only to the top two surface layers. A feature of a diffusion kinetic effect has also been observed from which diffusion parameters of alloy species in the near surface layers are estimated.

Surface segregation of a Pt–Au alloy: An atom-probe field ion microscope investigation

An absolute composition depth profile of a dilute gold alloy of platinum is obtained with a true atomic layer depth resolution. At 600±20 °C, the top {001} layer Au concentration is found to be as high as 99±20% for a sample with a bulk gold concentration of 4.1±0.7%. The Au concentration decreases monotonically into the bulk, with a characteristic depth of about three atomic layers.