X. Y. Cui

Atom probe microscopy investigation of Mg site occupancy within δ′ precipitates in an Al–Mg–Li alloy

The composition and site occupancy of Mg within ordered δ′ precipitates in a model Al–Mg–Li alloy have been characterized by atom probe microscopy and first-principles simulations. The concentration in the precipitates is found to be almost the same as that of the matrix; however, we show evidence that Mg partitions to the sites normally occupied by Li in the L12 structure. Density functional calculations demonstrate that this partitioning is energetically favorable, in agreement with experimental results.

Embedded clustering in Cr-doped AlN: Evidence for general behavior in dilute magnetic III-nitride semiconductors

We report a systematic density-functional theory investigation of the “structure-property relationship” of Cr:AlN by doping up to 5 Cr atoms in large supercells, for which exhaustive structural and magnetic configurations have been calculated—including full atomic relaxation. Our results demonstrate that the Cr atoms tend to segregate to form Cr-N-Cr bonded clusters, which are embedded in the AlN host wurtzite structure. Significantly, while the ferromagnetic state with a spin moment close to 3 μB∕Cr is the ground state for both isolated “single” and “pair” doping configurations, for larger cluster configurations states containing antiferromagnetic or ferrimagnetic coupling with net spin in the range of 0−1.53 μB∕Cr are found to be energetically more favorable. Electrical conductivity (half-metallic or insulating) is predicted to be sensitively dependent on the dopant concentration. We propose a picture that various sized Cr-N-Cr bonded clusters coexist and the statistical distribution and associated magn...

Built-in electric fields and valence band offsets in InN/GaN(0001) superlattices: First-principles investigations

Based on all-electron density functional theory calculations, we systematically investigate the built-in electric fields and valence band offsets in wurtzite InN/GaN(0001) superlattices, where their correlations with biaxial strain, as well as the superlattice geometry, are determined. Both the built-in electric fields (several MV/cm) and the valence band offsets (0.16 –1.1 eV) are found to be strongly dependent on the superlattice geometry and strain growth conditions. Spontaneous polarization and strain-induced piezoelectric polarization are comparable in contribution to the macroscopic electric field. Relative to the fully relaxed superlattices, tensile (compressive) strain significantly weakens (strengthens) the magnitude of the electric field, and decreases (increases) the value of the valence band offset. The results will be valuable in relation to practical heterojunction-based device optimization and design.

Atomically resolved tomography to directly inform simulations for structure–property relationships

Microscopy encompasses a wide variety of forms and scales. So too does the array of simulation techniques developed that correlate to and build upon microstructural information. Nevertheless, a true nexus between microscopy and atomistic simulations is lacking. Atom probe has emerged as a potential means of achieving this goal. Atom probe generates three-dimensional atomistic images in a format almost identical to many atomistic simulations. However, this data is imperfect, preventing input into computational algorithms to predict material properties. Here we describe a methodology to overcome these limitations, based on a hybrid data format, blending atom probe and predictive Monte Carlo simulations. We create atomically complete and lattice-bound models of material specimens. This hybrid data can then be used as direct input into density functional theory simulations to calculate local energetics and elastic properties. This research demonstrates the role that atom probe combined with theoretical approaches can play in modern materials engineering.

Performance modulation of α-MnO2 nanowires by crystal facet engineering

Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO2 nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

Direct observation of local potassium variation and its correlation to electronic inhomogeneity in (Ba1-xKx)Fe2As 2 pnictide

Local fluctuations in the distribution of dopant atoms are thought to cause the nanoscale electronic disorder or phase separation in pnictide superconductors. Atom probe tomography has enabled the first direct observations of dopant species clustering in a K-doped 122-phase pnictide. First-principles calculations suggest the coexistence of static magnetism and superconductivity on a lattice parameter length scale over a wide range of dopant concentrations. Our results provide evidence for a mixed scenario of phase coexistence and phase separation, depending on local dopant atom distributions.

Quantitative dopant distributions in GaAs nanowires using atom probe tomography

Controllable doping of semiconductor nanowires is critical to realize their proposed applications, however precise and reliable characterization of dopant distributions remains challenging. In this article, we demonstrate an atomic-resolution three-dimensional elemental mapping of pristine semiconductor nanowires on growth substrates by using atom probe tomography to tackle this major challenge. This highly transferrable method is able to analyze the full diameter of a nanowire, with a depth resolution better than 0.17 nm thanks to an advanced reconstruction method exploiting the specimens crystallography, and an enhanced chemical sensitivity of better than 8-fold increase in the signal-to-noise ratio.

A computational study of photoisomerization in Al3O3- ­clusters

Ab initio calculations are employed to understand the photoisomerization process in small Al3O3− clusters. This process is the first example of a photoinduced isomerization observed in an anion cluster gas-phase system. Potential energy surfaces for the ground state and the excited state (S1 and T1) are explored by means of B3LYP, MP2, CI-singles, and CASSCF methods. We demonstrate that the isomerization process occurs between the global minimum singlet state Book structure (C2v,1A1) and the triplet state Ring structure (C2v,3B2). The calculated vertical excitation energy is 3.62 eV at the CASSCF level of approximation, in good agreement with the experimental value (3.49 eV). A nonplanar conical intersection, which hosts the intersystem crossing between the S1 and T1 surfaces is identified at the region of around R(1,6)=2.4 A. Beyond the experimental results, we predict, that this isomerization is reversible upon absorption of a phonon with energy of 1.92 eV. Our results describe a unique system, whose st...

Band gap engineering of wurtzite and zinc-blende GaN/AlN superlattices from first principles

Based on all-electron density functional theory calculations, we systematically investigate the electronic structure of (0001)-oriented wurtzite (wz) and (111)-, (100)-, and (110)-oriented zinc-blende (zb) GaN/AlN superlattices, where the band gap, strength of the electric field and their correlation with biaxial stain as a function of the superlattice thickness are calculated. For the polar wz-(0001) and zb-(111) systems, the band gap values are found to continuously decrease with increasing thickness of the superlattice period due to the built-in electric field. By mapping the core-level shift, we demonstrate the presence of spontaneous polarization in both wz-(0001) and zb-(111) superlattices. The built-in electric field is calculated to be about 5.1±0.3 and 1.4±0.4 MV/cm in the “free-standing” (fully relaxed) wz-(0001) and zb-(111) superlattices, respectively. Strain-induced piezoelectric polarizations are estimated to contribute only about 5% for the wz-(0001) superlattice, and about 30% for the zb-(...

Full tip imaging in atom probe tomography

Atom probe tomography (APT) is capable of simultaneously revealing the chemical identities and three dimensional positions of individual atoms within a needle-shaped specimen, but suffers from a limited field-of-view (FOV), i.e., only the core of the specimen is effectively detected. Therefore, the capacity to analyze the full tip is crucial and much desired in cases that the shell of the specimen is also the region of interest. In this paper, we demonstrate that, in the analysis of III-V nanowires epitaxially grown from a substrate, the presence of the flat substrate positioned only micrometers away from the analyzed tip apex alters the field distribution and ion trajectories, which provides extra image compression that allows for the analysis of the entire specimen. An array of experimental results, including field desorption maps, elemental distributions, and crystallographic features clearly demonstrate the fact that the whole tip has been imaged, which is confirmed by electrostatic simulations.