Development of positron diffraction and holography at LLNL

An error-analysis procedure that gives statistically significant error estimates for surface-structure parameters extracted from analyses of measured low-energy electron and positron diffraction (LEED and LEPD) intensities is proposed. This procedure is applied to a surface-structure analysis of Cu(100) in which experimental data are simulated by adding Gaussian-distributed random errors to the calculated intensities for relaxed surface structures. Quantitative expressions for the variances in the surface-structural parameters are given and shown to obey the expected scaling laws for Gaussian errors in the experimental data. The procedure is shown to describe rigorously parameter errors in the limit that the errors in the measured intensities are described by uncorrelated Gaussian statistics. The analysis is valid for structure determinations that are of sufficient quality to admit errors that have magnitudes within the region of convergence of a linear theory that relates perturbations of diffracted intensities to perturbations in structural parameters. It is compared with previously proposed error-estimation techniques used in LEED, LEPD, and x-ray intensity analyses.

It was not until five years after the discovery of low-energy electron diffraction (LEED) [9.1] that the positron was first observed [9.2]. By the time it became possible to form controllable, albeit very weak, low-energy positron beams in the 1970s, LEED had become an ubiquitous and powerful tool in surface structure determination. Thus, when the first observation of low energy positron diffraction (LEPD) was made in 1979 [9.3] one could well question whether it was too late for LEPD to be of any practical value in surface physics. As this review will chronicle, during the past ten years, LEPD has evolved from being a novelty to becoming a valuable technique for surface structure determination. The differences between LEPD and LEED are nontrivial; they not only raise questions of fundamental interest but also suggest that in some cases LEPD ultimately may prove superior to LEED for quantitative surface structure determination.

Theory versus experiment in low-energy positron diffraction by Cu{111}

Recently, we developed new total reflection high-energy positron diffraction (TRHEPD) apparatus [1] on a beam line of the linac-based intense positron beam of the Slow Positron Facility at KEK, Japan. The high intensity allows us to install a brightness-enhancement section, which to observation of clear positron diffraction patterns for crystal surfaces under total reflection condition. In this work, we investigated the atomic configuration of Pt-induced nanowires formed on a Ge(001) surface [2] using the apparatus. By means of the diffraction intensity analysis based on the dynamical diffraction theory, or TRHEPD rocking curve analysis, a previously proposed theoretical model [D. E. P. Vanpoucke et al., Phys. Rev. B 77, 241308(R) (2008)], composed of Ge dimers on the top layer and buried Pt arrays in the second and fourth layers, was confirmed to be the fundamental structure of the nanowire. We also investigated the atomic configuration of a rutile-TiO2(110) surface. It is well known that the structure of this surface transforms its periodicity from (1×1) to (1×2) by elevating the sample temperature above ~1100 K, whereas the detailed structure is yet to be revealed. There is a longstanding controversy between the structure models proposed by scanning tunneling microscopy, low energy electron diffraction, surface X-ray diffraction, first-principles calculation with density functional theory results, etc. To solve the problem, we have measured TRHEPD rocking curves and determined the atomic arrangements of the topmost crystal surface [3].

Recent advances in slow positron beam techniques are making it possible to study the interactions of low-energy positrons with gas molecules and solid surfaces and to measure the properties of free positronium atoms. New surface related results include the observation of surfaces with negative positron affinity and the thermionic emission of slow positronium atoms, low-energy positron diffraction measurements, and the sensitive detection of near-surface crystalline imperfections. Two recent successful experiments in atomic physics are the formation of the positronium negative ion and the optical excitation of positronium for high precisin spectroscopy. Prospects for a positron microscope and the study of exotic antimatter systems such as the two-component Fermi gas are based on the imminent possibility of enormous increases in the brightness and instantaneous intensity of positron beams.

The PULSTAR is a 1-MWth nuclear research reactor located at North Carolina State University. It is fueled by uranium dioxide assemblies enriched to 4% or 6% in U-235 and is currently under licensing for operation at 2-MWth power. The PULSTAR is a center for irradiation testing and pre/post irradiation examination of materials. Among its unique capabilities are positron annihilation spectroscopy (PAS) and neutron powder diffraction (NPD) facilities. The PAS facility provides an intense positron beam reaching 6 × 108 e+/s, which drives two spectrometers; the e+-PAS and the Ps-PAS, used for studies of defects in thin film materials. A Na-22 bulk PAS system is also operational, which is used for studying millimeter scale materials. All spectrometers are capable of performing Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). To date, the PAS systems have been used to characterize various materials (unirradiated and irradiated) that include graphite, soft matter, and metal-organic frameworks (MOF). The NPD facility uses a double focusing single crystal silicon rotating monochromator producing neutron beams with different energies. A position sensitive detection bank covers a scattering angle of 5°-125°. The facility is used in the examination of unirradiated and irradiated materials including graphitic materials, magnetic materials without rare-earth elements, and anode materials used in lithium batteries. The collected diffraction patterns can be processed to produce atomic pair distribution functions. The PAS and NPD facilities are available through user programs including the US DOE’s Nuclear Science User Facilities (NSUF) and the US NSF’s Research Triangle Nanotechnology Network (RTNN).

Two-dimensional (2D) materials have attracted interest because of their excellent properties and potential applications. A key step in realizing industrial applications is to synthesize wafer-scale single-crystal samples. Until now, single-crystal samples, such as graphene domains up to the centimeter scale, have been synthesized. However, a new challenge is to efficiently characterize large-area samples. Currently, the crystalline characterization of these samples still relies on selected-area electron diffraction (SAED) or low-energy electron diffraction (LEED), which is more suitable for characterizing very small local regions. This paper presents a highly efficient characterization technique that adopts a low-energy electrostatically focused electron gun and a super-aligned carbon nanotube (SACNT) film sample support. It allows rapid crystalline characterization of large-area graphene through a single photograph of a transmission-diffracted image at a large beam size. Additionally, the low-energy electron beam enables the observation of a unique diffraction pattern of adsorbates on the suspended graphene at room temperature. This work presents a simple and convenient method for characterizing the macroscopic structures of 2D materials, and the instrument we constructed allows the study of the weak interaction with 2D materials.

Recent developments of the Slow Positron Facility at the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) are reported. We have modified the converter/moderator assembly for slow positron beam production, yielding an increase of an order of magnitude in the intensity of the beam. The first observation of the photodetachment of the positronium negative ion (Ps−), and the installation of a reflection high-energy positron diffraction (RHEPD) station and the initial data obtained are reported.

Among lithium battery cathode materials, Li1.2Ni0.13Mn0.54Co0.13O2 (LR-NMC) has a high theoretical capacity, but suffers from voltage and capacity fade during cycling. This is partially ascribed to transition metal cation migration, which involves the local transformation of the honeycomb layered structure to spinel-like nano-domains. Determination of the honeycomb layered/spinel phase ratio from powder X-ray diffraction data is hindered by the nanoscale of the functional material and the domains, diverse types of twinning, stacking faults, and the possible presence of the rock salt phase. Determining the phase ratio from transmission electron microscopy imaging can only be done for thin regions near the surfaces of the crystals, and the intense beam that is needed for imaging induces the same transformation to spinel as cycling does. In this article, it is demonstrated that the low electron dose sufficient for electron diffraction allows the collection of data without inducing a phase transformation. Using calculated electron diffraction patterns, we demonstrate that it is possible to determine the volume ratio of the different phases in the particles using a pair-wise comparison of the intensities of the reflections. Using this method, the volume ratio of spinel structure to honeycomb layered structure is determined for a submicron sized crystal from experimental three-dimensional electron diffraction (3D ED) and precession electron diffraction (PED) data. Both twinning and the possible presence of the rock salt phase are taken into account. After 150 charge–discharge cycles, 4% of the volume in LR-NMC particles was transformed irreversibly from the honeycomb layered structure to the spinel structure. The proposed method would be applicable to other multi-phase materials as well.

The whisker of polyoxymethylene produced in the cationic polymerization process of trioxane is a single crystal of extended chain crystal morphology. Since it has a quite small size of several microns in radius and several tens of microns in length, the crystal structure analysis of POM whisker had been tried by using an electron diffraction technique, but it was easily damaged by an electron beam. We have succeeded in taking the electron diffraction patterns of the POM whisker from the different directions by reducing the electron beam intensity as much as possible and by using a highly sensitive imaging plate detector. Application of the direct method, which is useful for solving the so-called phase problem in structure analysis, allowed us to obtain the helical conformation of POM chain packed in the trigonal unit cell. However, the refinement of the thus-obtained initial structure was difficult since the electron diffraction intensities were modified seriously from the original values by the multiple reflection effect in the single crystal. Rather, the refinement was made successfully by combining the initial structure obtained by the electron diffraction data with the X-ray diffraction data taken for the γ-ray-polymerized POM multiple crystalline sample, suggesting a usefulness of an organized combination of electron and X-ray diffraction techniques in the structure analysis of polymer crystals.

Total reflection high-energy positron diffraction (TRHEPD) promises to be an ideal technique for determining the atomic arrangement of solid surfaces. TRHEPD makes full use of the total reflection of the positron beam from a surface in reflection high-energy positron diffraction (RHEPD), the positron counterpart of reflection high-energy electron diffraction (RHEED). Owing to the development and use of a brightness-enhanced intense positron beam at KEK, it is now possible to obtain clear TRHEPD patterns. It is shown that the TRHEPD pattern from a Si(111)-7×7 reconstructed surface for the total reflection condition does not contain contributions from atoms in the bulk.

Positron annihilation is a well established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. Pulsed and continuous beams of moderated positrons have been designed and developed to study very efficiently surface and near surface regions in metals and semiconductors. Especially a pulsed positron beam with narrow beam pulses (∼ 150 ps) enables positron lifetime experiments as a function of the positron energy. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. Developments at several institutes in Europe are under way. The aim is to obtain a beam intensity in the order of ∼ 10 10 positrons/sec. Parallel to the installment of intense positron beams, the development of positron microscopes is pursued. Two types of positron microscopes are being set up: a scanning positron microscope with a pulsed beam of 100 ps and a beam diameter of 1 μm and a positron reemission microscope with about the same beam diameter

The chemisorption of CO on the (111) surface of Ir at room temperature and below has been investigated using both low-energy electron diffraction (LEED) and thermal desorption mass spectrometry. The CO is adsorbed rapidly onto the (111) Ir forming a well-ordered (√3×√3) R30° overlayer structure after an exposure of approximately 2 to 3 Langmuirs. For larger exposures the overlayer compresses continuously as additional CO is adsorbed until a (2√3×2√3) R30° structure is formed. The surface coverage of CO corresponding to the (√3×√3) R30° structure is one-third monolayer, and mass spectrometry has shown that the surface coverage corresponding to the (2√3×2√3) R30° structure is 7/12 monolayer, where monolayer coverage is based on the number of surface Ir atoms. The isosteric heat of adsorption at one-third monolayer coverage was measured by monitoring the intensity of the overlayer LEED beams as a function of CO pressure and surface temperature, and it was found to be 35±1 kcal/mole. Thermal desorption revealed the existence of two apparent binding states of the CO at saturation coverage. At low coverage (ϑ≲0.4), the CO desorbs in a single peak, whereas the second state builds in at a higher coverage. The appearance of the second state in the thermal desorption spectra coincides with the transformation between the two overlayer structures. The probability of adsorption of the CO is nearly constant for ϑ≲0.2 and has a value of essentially unity, whereas the adsorption probability at larger surface coverages is much smaller, e.g., the value has dropped by more than an order of magnitude by the time ϑ=0.4 has been reached. Models are suggested for the observed surface structures, and the results are compared with other data of CO chemisorption on the close-packed surface of other Group VIII transition metals.

It is shown that a relativistic electron-positron plasma formed by the coalescence of two counterstreaming intense relativistic electron and positron beams can collapse into a very-small-diameter filament. The collapse is accompanied by the emission of intense coherent $\ensuremath{\gamma}$ radiation. The radius of the collapsed state is determined by the quantum-mechanical uncertainty principle. At its maximum contraction the electron-positron plasma can approach nuclear densities. The predicted effect may have many interesting applications, some of which are briefly mentioned. The generation of the intense beams seems to be possible with existing techniques.

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC