Ryan J. Wu

Atomic and electronic structure of exfoliated black phosphorus

Black phosphorus, a layered two-dimensional crystal with tunable electronic properties and high hole mobility, is quickly emerging as a promising candidate for future electronic and photonic devices. Although theoretical studies using ab initio calculations have tried to predict its atomic and electronic structure, uncertainty in its fundamental properties due to a lack of clear experimental evidence continues to stymie our full understanding and application of this novel material. In this work, aberration-corrected scanning transmission electron microscopy and ab initio calculations are used to study the crystal structure of few-layer black phosphorus. Directly interpretable annular dark-field images provide a three-dimensional atomic-resolution view of this layered material in which its stacking order and all three lattice parameters can be unambiguously identified. In addition, electron energy-loss spectroscopy (EELS) is used to measure the conduction band density of states of black phosphorus, which agrees well with the results of density functional theory calculations performed for the experimentally determined crystal. Furthermore, experimental EELS measurements of interband transitions and surface plasmon excitations are also consistent with simulated results. Finally, the effects of oxidation on both the atomic and electronic structure of black phosphorus are analyzed to explain observed device degradation. The transformation of black phosphorus into amorphous PO3 or H3PO3 during oxidation may ultimately be responsible for the degradation of devices exposed to atmosphere over time.

Strontium Oxide Tunnel Barriers for High Quality Spin Transport and Large Spin Accumulation in Graphene

The quality of the tunnel barrier at the ferromagnet/graphene interface plays a pivotal role in graphene spin valves by circumventing the impedance mismatch problem, decreasing interfacial spin dephasing mechanisms and decreasing spin absorption back into the ferromagnet. It is thus crucial to integrate superior tunnel barriers to enhance spin transport and spin accumulation in graphene. Here, we employ a novel tunnel barrier, strontium oxide (SrO), onto graphene to realize high quality spin transport as evidenced by room-temperature spin relaxation times exceeding a nanosecond in graphene on silicon dioxide substrates. Furthermore, the smooth and pinhole-free SrO tunnel barrier grown by molecular beam epitaxy (MBE), which can withstand large charge injection current densities, allows us to experimentally realize large spin accumulation in graphene at room temperature. This work puts graphene on the path to achieve efficient manipulation of nanomagnet magnetization using spin currents in graphene for logic and memory applications.

Enhancement of tunneling magnetoresistance by inserting a diffusion barrier in L10-FePd perpendicular magnetic tunnel junctions

We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in realizing high TMR ratios in bulk p-MTJs such as those based on FePd and Mn-based perpendicular magnetic anisotropy materials for spintronic applications.We studied the tunnel magnetoresistance (TMR) of L10-FePd perpendicular magnetic tunnel junctions (p-MTJs) with an FePd free layer and an inserted diffusion barrier. The diffusion barriers studied here (Ta and W) were shown to enhance the TMR ratio of the p-MTJs formed using high-temperature annealing, which are necessary for the formation of high quality L10-FePd films and MgO barriers. The L10-FePd p-MTJ stack was developed with an FePd free layer with a stack of FePd/X/Co20Fe60B20, where X is the diffusion barrier, and patterned into micron-sized MTJ pillars. The addition of the diffusion barrier was found to greatly enhance the magneto-transport behavior of the L10-FePd p-MTJ pillars such that those without a diffusion barrier exhibited negligible TMR ratios (<1.0%), whereas those with a Ta (W) diffusion barrier exhibited TMR ratios of 8.0% (7.0%) at room temperature and 35.0% (46.0%) at 10 K after post-annealing at 350 °C. These results indicate that diffusion barriers could play a crucial role in re...

Chemical vapor deposition of partially oxidized graphene

Herein, we report on chemical vapor deposition (CVD) of partially oxidized graphene (POG) films on electropolished polycrystalline copper foils at relatively low temperature under near-atmospheric pressure. The structural, chemical, and electronic properties of the films are studied in detail using several spectroscopic and microscopic techniques. The content of carbon and oxygen in the films is identified by chemical mapping at near-atomic scale. Electron diffraction patterns of the films possess clear diffraction spots with a six-fold pattern that is consistent with the hexagonal lattice. The fine structure of the carbon K-edge signal in STEM-EELS spectra of the films is distinguishable from that of graphene and graphite. The presence of oxygen in the films is further supported by a clear oxygen K-edge. Raman spectroscopy and XPS results provide direct evidence for a lower degree of oxidation. The work function of the films is found to be much higher than that of graphene, using UPS measurements.

Measuring the Atomic and Electronic Structure of Black Phosphorus with STEM

Black phosphorus, a layered two-dimensional crystal with tunable electronic properties and high hole mobility, is quickly emerging as a promising candidate for future electronic and photonic devices [1]. Although theoretical studies using ab initio calculations have tried to predict its atomic and electronic structure [2, 3], uncertainty in its fundamental structural properties due to a lack of clear experimental evidence continues to stymie our full understanding and application of this novel material.

Cross-sectional STEM Imaging and Spectroscopy of Devices with Embedded 2D Materials

Two-dimensional (2D) systems have been demonstrated to be excellent materials for charge [1] and spin transport [2] in devices. Their exceptional performance in these devices is afforded by their unique electronic structure in their singleor few-layer states. As such, it would be advantageous to characterize how their atomic and electronic structures change while embedded in actual devices, as compared to in their free-standing states. Although simulations have modeled 2D materials embedded within a solar cell or field effect transistor in order to predict the material’s performance [3], experimental results to corroborate these theoretical predictions that show the structure of the 2D material or the interface it shares with the substrate or contacts in the device remain scarce. This region, together with its interface, is as thick as the embedded 2D material in the device, which is often only a few atomic layers, making access inherently difficult.

Probing Two-dimensional (Bi,Sb)2Te3/h-BN Heterostructures Using Complementary S/TEM and Simulation Techniques

To revolutionize electronics, materials must be developed, characterized, and engineered that can outperform conventional, silicon-based technology. Because of their momentum-locked surface states, topological insulators (TIs) are emerging as materials that could provide important advances for magnetoelectronic technologies. Already, spin-transfer torque [1], current-induced spin polarization [2,3], and room-temperature spin injection [4] have been demonstrated as phenomena of interest in TI-based devices.

Simplifying Electron Beam Channeling in STEM

The channeling behavior for electron probes in conventional transmission electron microscopy (TEM) and scanning TEM (STEM) remains an active area of research because it influences the quantitative interpretation of high-resolution images and spectroscopies. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the number of electrons incident at different depths [1]. Understanding these short-range oscillations and the parameters that influence them is critical due to the short depths of focus of probes in modern aberration-corrected STEMs. Although sophisticated mathematical descriptions have modeled beam channeling accurately [2, 3], a less rigorous approach can provide a more accessible understanding of how this complex phenomenon affects STEM results acquired from experiments.

Simplifying Electron Beam Channeling in Scanning Transmission Electron Microscopy (STEM)

Sub-angstrom scanning transmission electron microscopy (STEM) allows quantitative column-by-column analysis of crystalline specimens via annular dark-field images. The intensity of electrons scattered from a particular location in an atomic column depends on the intensity of the electron probe at that location. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the beam intensity incident at different depths. Understanding the parameters that control this complex behavior is critical for interpreting experimental STEM results. In this work, theoretical analysis of the STEM probe intensity reveals that intensity oscillations during specimen propagation are regulated by changes in the beams angular distribution. Three distinct regimes of channeling behavior are observed: the high-atomic-number (Z) regime, in which atomic scattering leads to significant angular redistribution of the beam; the low-Z regime, in which the probes initial angular distribution controls intensity oscillations; and the intermediate-Z regime, in which the behavior is mixed. These contrasting regimes are shown to exist for a wide range of probe parameters. These results provide a new understanding of the occurrence and consequences of channeling phenomena and conditions under which their influence is strengthened or weakened by characteristics of the electron probe and sample.

Challenges of Oversimplifying Z-contrast in Atomic Resolution ADF-STEM

Using thickness or atomic number (Z) to interpret variations in the intensity of atomic columns has been a trademark of annular dark-field scanning transmission electron microscopy (ADF-STEM) images. As widely accepted theory, higher Z elements, in general, show increase scattering compared to lower Z elements [1]. For experiments involving a predetermined binary compound (BN, GaAS, AlN, etc), Z contrast, alone, is often times enough to conclude the chemical identity of an atomic column. This interpretation is accurate under ideal electron beam behavior where the STEM probe propagates with the majority, if not all, of its intensity along a single atomic column. However, Odlyzko [2], among others, established electron beam channeling can spatially spread the intensity of a propagating electron beam beyond a single atomic column. Under these conditions, even when a STEM probe is localized to an atomic column, a non-negligible amount of probe intensity could transfer to neighboring columns. This behavior makes image interpretation more complex, and oversimplifying conventional Z-contrast can potentially lead to wrong conclusions from ADF-STEM images.