Publications
358 results found
Wang B, Esser BD, Bagués N, et al., 2019, Quantifying Jahn-Teller distortion at the nanoscale with picometer accuracy using position averaged convergent beam electron diffraction, Physical Review Research, Vol: 1, ISSN: 2643-1564
Using position averaged convergent beam electron diffraction (PACBED) the Jahn-Teller distortion in LaMnO3 is quantitatively measured using a straightforward pattern-matching approach. The fit between experimental patterns and PACBED patterns simulated using the quantum excitation of phonons model allows a three-dimensional measure of octahedral distortion and rotation information from the near transmitted disk region with picometer precision. The effects of plasmon and other inelastic components on quantification using this method are investigated and discussed. The results provide an avenue for accurate local studies of the crystal structure origins of emergent physics in parallel with high-resolution annular dark field scanning transmission electron microscopy imaging at interfaces and defects in quantum materials.
Mccomb DW, Lengyel J, Carter CB, 2019, Cryogenic transmission electron microscopy for materials research, MRS Bulletin, Vol: 44, Pages: 924-928, ISSN: 0883-7694
Cryogenic transmission electron microscopy is simply transmission electron microscopy conducted on specimens that are cooled in the microscope. The target temperature of the specimen might range from just below ambient temperature to less than 4 K. In general, as the temperature decreases, cost increases, especially below -77°C when liquid He is required. We have two reasons for wanting to cool the specimen - improving stability of the material or observing a material whose properties change at lower temperatures. Both types of study have a long history. The cause of excitement in this field today is that we have a perfect storm of research activity - electron microscopes are almost stable with minimal drift (we can correct what drift there is), we can prepare specimens from the bulk or build them up, we have spherical-aberration-corrected lenses and monochromated beams, we have direct-electron-detector cameras, and computers are becoming powerful enough to handle all the data we produce.
Williams REA, Mccomb DW, Subramaniam S, 2019, Cryo-electron microscopy instrumentation and techniques for life sciences and materials science, MRS Bulletin, Vol: 44, Pages: 929-934, ISSN: 0883-7694
In this article, we review some of the recent developments in instrumentation and methods that have led to the rise of cryo-electron microscopy (cryo-EM) in the life sciences community, and consider how researchers in the materials community might benefit from these advances. Transmission electron microscopy (TEM) is compared with scanning transmission electron microscopy (STEM) for cryogenic imaging in both biological and materials science applications. We discuss the developments in detector technologies that have in part powered the development of cryo-EM and anticipate exciting areas for productive overlap between life science and materials science cryo-EM applications.
Cohen L, Boldrin D, 2019, Anomalous Hall effect in noncollinear antiferromagnetic Mn 3NiN thin films, Physical Review Materials, Vol: 3, ISSN: 2475-9953
We have studied the anomalous Hall effect (AHE) in strained thin lms of the frustrated anti-ferromagnet Mn3NiN. The AHE does not follow the conventional relationships with magnetizationor longitudinal conductivity and is enhanced relative to that expected from the magnetization inthe antiferromagnetic state belowTN= 260 K. This enhancement is consistent with origins fromthe non-collinear antiferromagnetic structure, as the latter is closely related to that found in Mn3Irand Mn3Pt where a large AHE is induced by the Berry curvature. As the Berry phase inducedAHE should scale with spin-orbit coupling, yet larger AHE may be found in other members of thechemically exible Mn3AN structure.
Deitz JI, Paul PK, Farshchi R, et al., 2019, Direct Nanoscale Characterization of Deep Levels in AgCuInGaSe<inf>2</inf> Using Electron Energy-Loss Spectroscopy in the Scanning Transmission Electron Microscope, Advanced Energy Materials, Vol: 9, ISSN: 1614-6832
A new experimental framework for the characterization of defects in semiconductors is demonstrated. Through the direct, energy-resolved correlation of three analytical techniques spanning six orders of magnitude in spatial resolution, a critical mid-bandgap electronic trap level (EV + 0.56 eV) within Ag0.2Cu0.8In1−xGaxSe2 is traced to its nanoscale physical location and chemical source. This is achieved through a stepwise, site-specific correlated characterization workflow consisting of device-scale (≈1 mm2) deep level transient spectroscopy (DLTS) to survey the traps present, scanning probe–based DLTS (scanning-DLTS) for mesoscale-resolved (hundreds of nanometers) mapping of the target trap state's spatial distribution, and scanning transmission electron microscope based electron energy-loss spectroscopy (STEM-EELS) and X-ray energy-dispersive spectroscopy for nanoscale energy-, structure, and chemical-resolved investigation of the defect source. This first demonstration of the direct observation of sub-bandgap defect levels via STEM-EELS, combined with the DLTS methods, provides strong evidence that the long-suspected CuIn/Ga substitutional defects are indeed the most likely source of the EV + 0.56 eV trap state and serves as a key example of this approach for the fundamental identification of defects within semiconductors, in general.
Ahmed AS, Lee AJ, Bagués N, et al., 2019, Spin-Hall Topological Hall Effect in Highly Tunable Pt/Ferrimagnetic-Insulator Bilayers., Nano Lett, Vol: 19, Pages: 5683-5688
Electrical detection of topological magnetic textures such as skyrmions is currently limited to conducting materials. Although magnetic insulators offer key advantages for skyrmion technologies with high speed and low loss, they have not yet been explored electrically. Here, we report a prominent topological Hall effect in Pt/Tm3Fe5O12 bilayers, where the pristine Tm3Fe5O12 epitaxial films down to 1.25 unit cell thickness allow for tuning of topological Hall stability over a broad range from 200 to 465 K through atomic-scale thickness control. Although Tm3Fe5O12 is insulating, we demonstrate the detection of topological magnetic textures through a novel phenomenon: "spin-Hall topological Hall effect" (SH-THE), where the interfacial spin-orbit torques allow spin-Hall-effect generated spins in Pt to experience the unique topology of the underlying skyrmions in Tm3Fe5O12. This novel electrical detection phenomenon paves a new path for utilizing a large family of magnetic insulators in future skyrmion technologies.
Weber D, Trout AH, McComb DW, et al., 2019, Decomposition-Induced Room-Temperature Magnetism of the Na-Intercalated Layered Ferromagnet Fe3-xGeTe2., Nano Lett, Vol: 19, Pages: 5031-5035
The creation of 2D van der Waals materials with ferromagnetism above room temperature is an essential goal toward their practical utilization in spin-based applications. Recent studies suggest that intercalating lithium in exfoliated flakes of the ferromagnet Fe3-xGeTe2 induces a nonzero magnetization at T ∼ 300 K. However, the nanoscale nature of such experiments precludes precise observations of structural and chemical changes upon intercalation. Here, we report the preparation of sodium-intercalated NaFe2.78GeTe2 as well as the investigation into its structure and magnetic properties. Sodium readily intercalates into the van der Waals gap, as revealed by synchrotron X-ray diffraction. Concurrently, the Fe2.78GeTe2 layer becomes heavily charge doped and strained via chemical pressure, yet retains its structure and ferromagnetic transition temperature of ∼140 K. However, we observe the presence of a ferromagnetic amorphous iron germanide impurity over a wide range of synthetic conditions, leading to room-temperature magnetization. This work highlights the importance of strain and electronic control for manipulating the Curie temperature in 2D ferromagnets, while emphasizing the need for careful chemical analysis when exploring phenomena in exfoliated layers.
Jamison JS, May BJ, Deitz JI, et al., 2019, Ferromagnetic Epitaxial μ-Fe<inf>2</inf>O<inf>3</inf> on β-Ga<inf>2</inf>O<inf>3</inf>: A New Monoclinic Form of Fe<inf>2</inf>O<inf>3</inf>, Crystal Growth and Design, Vol: 19, Pages: 4205-4211, ISSN: 1528-7483
Here we demonstrate a new monoclinic iron oxide phase (μ-Fe2O3), epitaxially stabilized by growth on (010) β-Ga2O3. Density functional theory (DFT) calculations find that the lattice parameters of freestanding μ-Fe2O3 are within ∼1% of those of β-Ga2O3, and that its energy of formation is comparable to that of naturally abundant Fe2O3 polytypes. A superlattice of μ-Fe2O3/β-Ga2O3 is grown by plasma assisted molecular beam epitaxy, with resulting high-resolution X-ray diffraction (XRD) measurements indicating that the μ-Fe2O3 layers are lattice-matched to the substrate. The measured out-of-plane (b) lattice parameter of 3.12 ± 0.4 Å is in agreement with the predicted lattice constants and atomic-resolution scanning transmission electron microscopy (STEM) images confirm complete registry of the μ-Fe2O3 layers with β-Ga2O3. Finally, DFT modeling predicts that bulk μ-Fe2O3 is antiferromagnetic, while the interface region between μ-Fe2O3 and β-Ga2O3 leads to ferromagnetic coupling between interface Fe3+ cations selectively occupying tetrahedral positions. Magnetic hysteresis persisting to room temperature is observed via SQUID measurements, consistent with the computationally predicted interface magnetism. ©
Smith TM, Good BS, Gabb TP, et al., 2019, Effect of stacking fault segregation and local phase transformations on creep strength in Ni-base superalloys, Acta Materialia, Vol: 172, Pages: 55-65, ISSN: 1359-6454
In this study, two similar, commercial polycrystalline Ni-based disk superalloys (LSHR and ME3) were creep tested at 760 °C and 552 MPa to approximately 0.3% plastic strain. LSHR consistently displayed superior creep properties at this stress/temperature regime even though the microstructural characteristics between the two alloys were comparable. High resolution structural and chemical analysis, however, revealed significant differences between the two alloys among active γ′ shearing modes involving superlattice intrinsic and extrinsic stacking faults. In ME3, Co and Cr segregation and Ni and Al depletion were observed along the intrinsic faults - revealing a γ′ to γ phase transformation. Conversely in LSHR, an alloy with a higher W content, Co and W segregation was observed along the intrinsic faults. This observation combined with scanning transmission electron microscopy (STEM) simulations confirm a γ′-to-D019 χ phase transformation along the intrinsic faults in LSHR. Using experimental observations and density functional theory calculations, a novel local phase transformation strengthening mechanism is proposed that could be further utilized to improve the high temperature creep capabilities of Ni-base disk alloys.
Lee AJ, Ahmed AS, Guo S, et al., 2019, Epitaxial Co<inf>50</inf>Fe<inf>50</inf>(110)/Pt(111) films on MgAl<inf>2</inf>O<inf>4</inf>(001) and its enhancement of perpendicular magnetic anisotropy, Journal of Applied Physics, Vol: 125, ISSN: 0021-8979
Perpendicular magnetic anisotropy (PMA) in magnetic thin films with low coercivity is desirable for magnetic memory devices. It has been found that a (111)-oriented or textured Pt seed layer can enhance PMA and is, therefore, commonly utilized in spintronic structures. We grow (111)-oriented Pt epitaxial films via off-axis sputtering on various substrates and investigate the optimal substrate and orientation for high quality, epitaxial growth. Our results show that Pt(111) epitaxial films grow remarkably well on MgAl2O4(001) with an exceptionally narrow X-ray diffraction rocking curve. This high-quality seed layer is found to promote epitaxial growth of Pt/Co50Fe50/Pt trilayers with strong PMA comparable to many repeats of the magnetic multilayers reported previously. In addition, the Pt seed layer enhances the maximum thicknesses of Co50Fe50 that can still maintain PMA up to 1.07 nm.
Meng K-Y, Ahmed AS, Baćani M, et al., 2019, Observation of Nanoscale Skyrmions in SrIrO3/SrRuO3 Bilayers., Nano Lett, Vol: 19, Pages: 3169-3175
Skyrmion imaging and electrical detection via topological Hall (TH) effect are two primary techniques for probing magnetic skyrmions, which hold promise for next-generation magnetic storage. However, these two kinds of complementary techniques have rarely been employed to investigate the same samples. We report the observation of nanoscale skyrmions in SrIrO3/SrRuO3 (SIO/SRO) bilayers in a wide temperature range from 10 to 100 K. The SIO/SRO bilayers exhibit a remarkable TH effect, which is up to 200% larger than the anomalous Hall (AH) effect at 5 K, and zero-field TH effect at 90 K. Using variable-temperature, high-field magnetic force microscopy (MFM), we imaged skyrmions as small as 10 nm, which emerge in the same field ranges as the TH effect. These results reveal a rich space for skyrmion exploration and tunability in oxide heterostructures.
May BJ, Hettiaratchy EC, Selcu C, et al., 2019, Enhanced uniformity of III-nitride nanowire arrays on bulk metallic glass and nanocrystalline substrates, Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics, Vol: 37, ISSN: 2166-2746
Nanowires possess unique strain relieving properties making them compatible with a wide variety of substrates ranging from single crystalline semiconductors, amorphous ceramics, and polycrystalline metals. Flexible metallic foils are particularly interesting substrates for nanowires for both flexible optoelectronics and high throughput manufacturing techniques. However, nanowires grown on polycrystalline metals exhibit grain-dependent morphologies. As an alternative route, the authors demonstrate the growth of highly uniform III-Nitride nanowires on bulk metallic glass (amorphous metal) and nanocrystalline Pt metal films using molecular beam epitaxy. Nanowire arrays on metallic glass substrates show uniformity over length scales >100 μm. The quality of these nanowires is explored by photoluminescence spectroscopy. The electrical characteristics of individual nanowires are measured via conductive atomic force microscopy, and mesoscale light-emitting diodes (LEDs) are fabricated. Nanowires grown on nanocrystalline Pt films showed an increase in output power by a factor of up to 32, and an increase in the overall LED efficiency by up to 13× compared with simultaneously grown nanowire LEDs on bare Si.
Zhang C, Zhang X, Zhao W, et al., 2019, Chemotherapy drugs derived nanoparticles encapsulating mRNA encoding tumor suppressor proteins to treat triple-negative breast cancer, Nano Research, Vol: 12, Pages: 855-861, ISSN: 1998-0124
Triple-negative breast cancer (TNBC) is one type of the most aggressive breast cancers with poor prognosis. It is of great urgency to develop new therapeutics for treating TNBC. Based on current treatment guideline and genetic information of TNBC, a combinational therapy platform integrating chemotherapy drugs and mRNA encoding tumor suppressor proteins may become an efficacious strategy. In this study, we developed paclitaxel amino lipid (PAL) derived nanoparticles (NPs) to incorporate both chemotherapy drugs and P53 mRNA. The PAL P53 mRNA NPs showed superior properties compared to Abraxane® and Lipusu® used in the clinic including high paclitaxel loading capacity (24 wt.%, calculated by paclitaxel in PAL), PAL encapsulation efficiency (94.7% ± 6.8%) and mRNA encapsulation efficiency (88.7% ± 0.7%). Meanwhile, these NPs displayed synergetic cytotoxicity of paclitaxel and P53 mRNA in cultured TNBC cells. More importantly, we demonstrated in vivo anti-tumor efficacy of PAL P53 mRNA NPs in an orthotopic TNBC mouse model. Overall, these chemotherapy drugs derived mRNA NPs provide a new platform to integrate chemotherapy and personalized medicine using tumor genetic information, and therefore represent a promising approach for TNBC treatment. [Figure not available: see fulltext.].
Zhang C, Zhao W, Bian C, et al., 2019, Antibiotic-Derived Lipid Nanoparticles to Treat Intracellular Staphylococcus aureus., ACS Appl Bio Mater, Vol: 2, Pages: 1270-1277
Intracellular survival of pathogenic bacteria leads to high chances of bacterial persistence and relapse in the bacteria-infected host. However, many antibiotics fail to clear the intracellular bacteria due to their low internalization by cells. In order to increase delivery of antibiotics in cells and eliminate intracellular bacteria, we developed antibiotic-derived lipid nanoparticles. First, we synthesized antibiotic-derived lipid conjugates using two widely used antibiotics including penicillin G (PenG) and levofloxacin (Levo). Then, we formulated them into antibiotic-derived lipid nanoparticles and evaluated their antibacterial effects. We found that penicillin G derived phospholipid nanoparticles (PenG-PL NPs) were able to enhance cellular uptake of penicillin G as compared with free penicillin G and eliminate up to 99.9998% of ~108.5 intracellular methicillin sensitive Staphylococcus aureus (S. aureus) in infected A549 cells, a lung epithelial cell line. The PenG-PL NPs showed the potential for inhibiting intracellular S. aureus and are promising to be further studied for in vivo antibacterial applications.
Chatterjee S, Ricciardi L, Deitz JI, et al., 2019, Manipulating acoustic and plasmonic modes in gold nanostars, Nanoscale Advances, Vol: 1, Pages: 2690-2698
In this contribution experimental evidence of plasmonic edge modes and acoustic breathing modes in gold nanostars (AuNSs) is reported. AuNSs are synthesized by a surfactant-free, one-step wet-chemistry method. Optical extinction measurements of AuNSs confirm the presence of localized surface plasmon resonances (LSPRs), while electron energy-loss spectroscopy (EELS) using a scanning transmission electron microscope (STEM) shows the spatial distribution of LSPRs and reveals the presence of acoustic breathing modes. Plasmonic hot-spots generated at the pinnacle of the sharp spikes, due to the optically active dipolar edge mode, allow significant intensity enhancement of local fields and hot-electron injection, and are thus useful for size detection of small protein molecules. The breathing modes observed away from the apices of the nanostars are identified as stimulated dark modes-they have an acoustic nature-and likely originate from the confinement of the surface plasmon by the geometrical boundaries of a nanostructure. The presence of both types of modes is verified by numerical simulations. Both these modes offer the possibility of designing nanoplasmonic antennas based on AuNSs, which can provide information on both mass and polarizability of biomolecules using a two-step molecular detection process.
Trout AH, Wang Y, Esser BD, et al., 2019, Identification of turbostratic twisting in germanane, Journal of Materials Chemistry C, Vol: 7, Pages: 10092-10097, ISSN: 2050-7534
Germanane, a van der Waals layered material predicted to have a direct band gap and high carrier mobility, is a promising two-dimensional material with applications in optoelectronics. The electronic properties of germanane have been well studied; however, experimentally measured properties are orders of magnitude lower than predicted values, potentially limiting future device applications. The structure of germanane contains an inherent disorder along the c-axis, resulting in a diffuse halo with hexagonal symmetry in the electron diffraction pattern. The origin of this disorder is not well understood, further limiting the device application of germanane. Here, we have used experimental and simulated electron diffraction patterns to show that this diffuse scattering arises from turbostratic disorder present in the germanane structure with rotational disorder as the main contribution. The maximum rotation angle in the examined germanane crystal is limited to three degrees. For larger angles, germanane would become unstable as DFT calculations show. DFT calculations also indicate that small angle rotations cause a change in charge distribution in Ge and H atoms, and thus should affect the electronic properties of germanane considerably. This study explains for the first time the origin of the c-axis disorder in this van der Waals structure and establishes computationally analyzed diffraction patterns as a tool to quantify turbostratic disorder.
Chatterjee S, Ricciardi L, Deitz JI, et al., 2018, Heterodimeric plasmonic nanogaps for biosensing, Micromachines, Vol: 9
We report the study of heterodimeric plasmonic nanogaps created between gold nanostar (AuNS) tips and gold nanospheres. The selective binding is realized by properly functionalizing the two nanostructures; in particular, the hot electrons injected at the nanostar tips trigger a regio-specific chemical link with the functionalized nanospheres. AuNSs were synthesized in a simple, one-step, surfactant-free, high-yield wet-chemistry method. The high aspect ratio of the sharp nanostar tip collects and concentrates intense electromagnetic fields in ultrasmall surfaces with small curvature radius. The extremities of these surface tips become plasmonic hot spots, allowing significant intensity enhancement of local fields and hot-electron injection. Electron energy-loss spectroscopy (EELS) was performed to spatially map local plasmonic modes of the nanostar. The presence of different kinds of modes at different position of these nanostars makes them one of the most efficient, unique, and smart plasmonic antennas. These modes are harnessed to mediate the formation of heterodimers (nanostar-nanosphere) through hot-electron-induced chemical modification of the tip. For an AuNS-nanosphere heterodimeric gap, the intensity enhancement factor in the hot-spot region was determined to be 10 6 , which is an order of magnitude greater than the single nanostar tip. The intense local electric field within the nanogap results in ultra-high sensitivity for the presence of bioanalytes captured in that region. In case of a single BSA molecule (66.5 KDa), the sensitivity was evaluated to be about 1940 nm/RIU for a single AuNS, but was 5800 nm/RIU for the AuNS-nanosphere heterodimer. This indicates that this heterodimeric nanostructure can be used as an ultrasensitive plasmonic biosensor to detect single protein molecules or nucleic acid fragments of lower molecular weight with high specificity.
Deitz JI, Paul PK, Karki S, et al., 2018, Nanoscale Electronic Structure Characterization in CIGS with Electron Energy-Loss Spectroscopy, Pages: 3914-3917
Electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) was used to provide electronic structure characterization of CuIn 1-x Ga x Se 2 (CIGS) solar cells. Two applications within this context were explored: Spatially-resolved bandgap profiling using a newly-developed and simplified analysis approach, and detection of sub-gap defect states with high energy and spatial resolution. The simplified STEM-EELS bandgap profiling method was found to closely track nominal bandgap trends calculated from compositional data, with only a small, fixed offset (0.35 eV). Sub-gap energy levels in two different CIGS samples were identified at EV + 0.43 eV and EV + 0.56 eV, 0.56 eV, exactly consistent (i.e.< 0.01 eV discrepancy) with trap states measured via conventional deep level transient spectroscopy (DLTS) in both cases. Furthermore, direct correlation between STEM-EELS and scanning DLTS (S-DLTS) measurements made at the same sample site providing strong confirmation that the energy levels detected via STEM-EELS are indeed the same trap levels detected via (S-)DLTS.
Marsh JM, Mamak M, Wireko F, et al., 2018, Multimodal Evidence of Mesostructured Calcium Fatty Acid Deposits in Human Hair and Their Role on Hair Properties., ACS Appl Bio Mater, Vol: 1, Pages: 1174-1183
We provide the first conclusive evidence for the presence of exogenous calcium fatty acid deposits, which not only form in-between the cuticle layers in the lipid-rich cell membrane complex, but also grow to dimensions large enough to cause the structure to bulge, thereby impacting the optical and mechanical properties of the hair fiber. The composition and phase of these deposits were probed using a multimodal analytical approach with spatially resolved techniques including synchrotron micro X-ray fluorescence coupled with X-ray scattering, focused ion beam (FIB)-scanning electron microscopy (SEM), scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, and Fourier transform infrared and Raman imaging where the collective analysis is consistent with a meso-phase composed of calcium C16/C18 saturated fatty acids from natural sources such as sebum. X-ray microtomography and serial "slice and view" FIB/SEM both reveal the location and volumetric shape of the deposits.
Luo X, Zhao W, Li B, et al., 2018, Co-delivery of mRNA and SPIONs through amino-ester nanomaterials, Nano Research, Vol: 11, Pages: 5596-5603, ISSN: 1998-0124
Nanoparticles have been widely explored for combined therapeutic and diagnostic applications. For example, lipid-based nanoparticles have been used to encapsulate multiple types of agents and achieve multi-functions. Herein, we enabled a co-delivery of mRNA molecules and superparamagnetic iron oxide nanoparticles (SPIONs) by using an amino-ester lipid-like nanomaterial. An orthogonal experimental design was used to identify the optimal formulation. The optimal formulation, MPA-Ab-8 LLNs, not only showed high encapsulation of both mRNA and SPIONs, but also increased the r2 relaxivity of SPIONs by more than 1.5-fold in vitro. MPA-Ab-8 LLNs effectively delivered mRNA and SPIONs into cells, and consequently induced high protein expression as well as strong MRI contrast. Consistent herewith, we observed both mRNA-mediated protein expression and an evident negative contrast enhancement of MRI signal in mice. In conclusion, amino-ester nanomaterials demonstrate great potential as delivery vehicles for theranostic applications.
Smith TM, Esser BD, Good B, et al., 2018, Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, Vol: 49, Pages: 4186-4198, ISSN: 1073-5623
In this study, local chemical and structural changes along superlattice intrinsic stacking faults combine to represent an atomic-scale phase transformation. In order to elicit stacking fault shear, creep tests of two different single crystal Ni-based superalloys, ME501 and CMSX-4, were performed near 750 °C using stresses of 552 and 750 MPa, respectively. Through high-resolution scanning transmission electron microscopy (STEM) and state-of-the-art energy dispersive X-ray spectroscopy, ordered compositional changes were measured along SISFs in both alloys. For both instances, the elemental segregation and local crystal structure present along the SISFs are consistent with a nanoscale γ′ to D019 phase transformation. Other notable observations are prominent γ-rich Cottrell atmospheres and new evidence of more complex reordering processes responsible for the formation of these faults. These findings are further supported using density functional theory calculations and high-angle annular dark-field (HAADF)-STEM image simulations.
Asel TJ, Yanchenko E, Yang X, et al., 2018, Identification of Ge vacancies as electronic defects in methyl- and hydrogen-terminated germanane, Applied Physics Letters, Vol: 113, ISSN: 0003-6951
We use a combination of optical and electrostatic surface science techniques to measure electronically active native defects in multilayer GeCH3 and GeH, two-dimensional (2D) functionalized materials. Chemical processing techniques coupled with density functional theory enable us to identify the specific physical nature of both native point defects and synthesis-related impurities which can limit the optical and charge transport properties of these materials. Direct comparison of optical measurements with calculated electronic levels provides identification of these localized, deep level gap states and confirms partial H-passivation of dangling bonds, revealing synthesis and processing methods needed to control specific defects and optimize these 2D materials for emergent solid state-electronics.
Liu Y, McComb DW, Lee YU, et al., 2018, Workflow for correlatively imaging mouse pulmonary valve extracellular matrix, Microscopy and Microanalysis, Vol: 24, Pages: 1436-1437, ISSN: 1431-9276
O'Hara DJ, Zhu T, Trout AH, et al., 2018, Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit., Nano Lett, Vol: 18, Pages: 3125-3131
Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe x) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that, in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe2) monolayer, while for thicker films it could originate from a combination of vdW MnSe2 and/or interfacial magnetism of α-MnSe(111). Magnetization measurements of monolayer MnSe x films on GaSe and SnSe2 epilayers show ferromagnetic ordering with a large saturation magnetization of ∼4 Bohr magnetons per Mn, which is consistent with the density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe2. Growing MnSe x films on GaSe up to a high thickness (∼40 nm) produces α-MnSe(111) and an enhanced magnetic moment (∼2×) compared to the monolayer MnSe x samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveals an abrupt and clean interface between GaSe(0001) and α-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe2 monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.
Boldrin D, Mihai A, Zou B, et al., 2018, Giant Piezomagnetism in Mn3NiN., ACS Appl Mater Interfaces, Vol: 10, Pages: 18863-18868
Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn3NiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of Mn3NiN with intrinsic biaxial strains of ±0.25% result in Néel transition shifts up to 60K and magnetisation changes consistent with theory. Films grown on BaTiO3 display a striking magnetisation jump in response to uniaxial strain from the intrinsic BaTiO3 structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient α (where α=dB/dE) of 0.018 G cm/V. The latter agrees with the 1000-fold increase over Cr2O3 predicted by theory. Overall our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature.
Ahmed AS, Rowland J, Esser BD, et al., 2018, Chiral bobbers and skyrmions in epitaxial FeGe/Si(111) films, Physical Review Materials, Vol: 2
We report experimental and theoretical evidence for the formation of chiral bobbers - an interfacial topological spin texture - in FeGe films grown by molecular beam epitaxy. After establishing the presence of skyrmions in FeGe/Si(111) thin-film samples through Lorentz transmission electron microscopy and the topological Hall effect, we perform magnetization measurements that reveal an inverse relationship between the film thickness and the slope of the susceptibility (dχ/dH). We present evidence for the evolution as a function of film thickness L from a skyrmion phase for L<LD/2 to a cone phase with chiral bobbers at the interface for L>LD/2, where LD∼70nm is the FeGe pitch length. We show using micromagnetic simulations that chiral bobbers, earlier predicted to be metastable, are in fact the stable ground state in the presence of an additional interfacial Rashba Dzyaloshinskii-Moriya interaction.
Deitz JI, Sarwar ATMG, Carnevale SD, et al., 2018, Nano-Cathodoluminescence Measurement of Asymmetric Carrier Trapping and Radiative Recombination in GaN and InGaN Quantum Disks., Microsc Microanal, Vol: 24, Pages: 93-98
The ability to characterize recombination and carrier trapping processes in group-III nitride-based nanowires is vital to further improvements in their overall efficiencies. While advances in scanning transmission electron microscope (STEM)-based cathodoluminescence (CL) have offered some insight into nanowire behavior, inconsistencies in nanowire emission along with CL detector limitations have resulted in the incomplete understanding in nanowire emission processes. Here, two nanowire heterostructures were explored with STEM-CL: a polarization-graded AlGaN nanowire light-emitting diode (LED) with a GaN quantum disk and a polarization-graded AlGaN nanowire with three different InGaN quantum disks. Most nanowires explored in this study did not emit. For the wires that did emit in both structures, they exhibited asymmetrical emission consistent with the polarization-induced electric fields in the barrier regions of the nano-LEDs. In the AlGaN/InGaN sample, two of the quantum disks exhibited no emission potentially due to the three-dimensional landscape of the sample or due to limitations in the CL detection.
Deitz JI, Karki S, Marsillac SX, et al., 2018, Bandgap profiling in CIGS solar cells via valence electron energy-loss spectroscopy, Journal of Applied Physics, Vol: 123, ISSN: 0021-8979
A robust, reproducible method for the extraction of relative bandgap trends from scanning transmission electron microscopy (STEM) based electron energy-loss spectroscopy (EELS) is described. The effectiveness of the approach is demonstrated by profiling the bandgap through a CuIn1-xGaxSe2 solar cell that possesses intentional Ga/(In + Ga) composition variation. The EELS-determined bandgap profile is compared to the nominal profile calculated from compositional data collected via STEM-based energy dispersive X-ray spectroscopy. The EELS based profile is found to closely track the calculated bandgap trends, with only a small, fixed offset difference. This method, which is particularly advantageous for relatively narrow bandgap materials and/or STEM systems with modest resolution capabilities (i.e., >100 meV), compromises absolute accuracy to provide a straightforward route for the correlation of local electronic structure trends with nanoscale chemical and physical structure/microstructure within semiconductor materials and devices.
Heczko M, Esser BD, Smith TM, et al., 2018, Atomic resolution characterization of strengthening nanoparticles in a new high-temperature-capable 43Fe-25Ni-22.5Cr austenitic stainless steel, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, Vol: 719, Pages: 49-60, ISSN: 0921-5093
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Blissett AR, Deng B, Wei P, et al., 2018, Sub-cellular In-situ Characterization of Ferritin(iron) in a Rodent Model of Spinal Cord Injury., Sci Rep, Vol: 8
Iron (Fe) is an essential metal involved in a wide spectrum of physiological functions. Sub-cellular characterization of the size, composition, and distribution of ferritin(iron) can provide valuable information on iron storage and transport in health and disease. In this study we employ magnetic force microscopy (MFM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS) to characterize differences in ferritin(iron) distribution and composition across injured and non-injured tissues by employing a rodent model of spinal cord injury (SCI). Our biophysical and ultrastructural analyses provide novel insights into iron distribution which are not obtained by routine biochemical stains. In particular, ferritin(iron) rich lysosomes revealed increased heterogeneity in MFM signal from tissues of SCI animals. Ultrastructural analysis using TEM elucidated that both cytosolic and lysosomal ferritin(iron) density was increased in the injured (spinal cord) and non-injured (spleen) tissues of SCI as compared to naïve animals. In-situ EELs analysis revealed that ferritin(iron) was primarily in Fe3+ oxidation state in both naïve and SCI animal tissues. The insights provided by this study and the approaches utilized here can be applied broadly to other systemic problems involving iron regulation or to understand the fate of exogenously delivered iron-oxide nanoparticles.
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