Skip to Content Skip to Search Skip to Utility Navigation Skip to Top Navigation
Los Alamos National LaboratoryCenter for Integrated Nanotechnologies
Helping you understand, create, and characterize nanomaterials
DOE

Spring 2015

Select science highlights from Spring 2015.

Hydrogen Reduction: Converting Oxides to Alloy Films

Scientific Achievement: Epitaxial alloy films with desired magnetic properties have been prepared for the first time in the field by treating oxides in a controlled, oxygen-free environment. Hydrogen reduction of oxides for alloy films is a new route to study structure-property relationship of a range of functional materials.

Significance: Hydrogen reduction of metal-oxide compounds is simple and yet effective in producing magnetic thin film alloy with desired microstructures and physical properties. Such an approach can find applications in the fabrication of microelectromechanical systems and thin film magnetic recoding heads.h reduction

Research Details:

  • The magnetic anisotropy in epitaxial oxide films was controlled by film thickness or strain state.
  • Highly textured CoFe2 alloy films with desired magnetic properties were produced by treating CoFe2O4 in a contained hydrogen environment.
  •  The structural and magnetic properties of both films were fully characterized.

Contact:

Publications: A. P. Chen, N. Poudyal, J. Xiong, J. P. Liu, and Q. X. Jia, “Modification of Structure and Magnetic Anisotropy of Epitaxial CoFe2O4 Films by Hydrogen Reduction,” Appl. Phy. Lett., 106, 111907 (2015).

Visualizing Charge Transport in Graphene Oxide

Scientific Achievement: Unique electrostatic force microscopy (EFM) and optical spectroscopy show the development of graphene oxide’s (GO’s) visual properties with progressive reduction from GO to graphene, thus moving from insulator to conductor. We achieved the first charge transport visualization in GO using EFM.

Significance: These discoveries provide invaluable insight for advancing GO for low-cost optoelectronics applications (e.g., touch screens on cellphones) and electrical conductivity.graphene oxide

Research Details:

  • This first detailed, published study has probed the effects of tuning GO’s material properties via oxygen removal.
  • Using EFM, we achieved charge transport in progressively reduced GO.
  • GO properties can be adjusted to obtain an ideal combination of electrical conductivity and optical properties for low-cost electrical optoelectronics.

Contact:

Publications: S. E. Yalcin, C. Galande, R. Kappera, H. Yamaguchi, U. Martinez, K. A. Velizhanin, S. K. Doorn, A. M. Dattelbaum, M. Chhowalla, P. M. Ajayan, G. Gupta, and A. D. Mohite, “Direct Imaging of Charge Transport in Progressively Reduced Graphene Oxide Using Electrostatic Force Microscopy,” ACS Nano 9, 2981 (2015).

Control of Strong Light−Matter Coupling Using the Capacitance of Metamaterial Nanocavities

Scientific Achievement: We present experimental evidence that the Rabi splitting observed in metamaterials-semiconductor strongly-coupled systems  is directly proportional to the capacitance of the resonator.

Significance: Now we know how to maximize light matter interaction between metallic nanocavities and dipoles/emitters. This could enable polariton lasing & luminescence and new types of detectors or sensors.nanocavities

(a) Schematic of the system used. The metamaterial resonator is situated directly above the quantum-well stack. Rsub captures the reflection at the air-semiconductor optical interface. The strong coupling to the intersubband transitions is modeled by CIST. (b) Electron micrographs of different Au nanofabricated resonators: they span different capacitances and/or inductances. (c) Measured reflectance spectra as a function of bare cavity frequency. (d) Rabi splitting as a function of capacitance.

Research Details:

  • Fabricated metamaterial arrays on top a two-level using an intersubband transition in a semiconductor heterostructure.
  • measured Rabi-splitting from transmission curves for different resonator geometries.
  • correlated measurements with equivalent electrical circuit model (published recently by us) to describe the light−matter interaction in these systems.

Contact:

Publications: “Control of Strong Light–Matter Coupling Using the Capacitance of Metamaterial Nanocavities”, A. Benz, S. Campione, J.F. Klem, M.B. Sinclair, and I. Brener, Nano Lett. 15, 1959 (2015).

Block copolymer-based LH nanocomposites: modular platform for energy transfer

Scientific Achievement: We have demonstrated the ability to generate polymer-based nanocomposites that exhibit energy transfer in non-covalent supramolecular assemblies. Results demonstrate energy transfer above 50% in 3D polymer micelles and ~95% in 2D polymer films.

Significance: nanocompositees

  • Results demonstrate potential of designing responsive photonic materials for light-harvesting/energy transfer and charge separation.
  • Polymers allow for built in functionality and exploration of incorporation of other nanoscale cofactors.

Research Details:

  • Assemblies are adaptive, forming well-defined micelles in aqueous solution and high-quality monolayer and bilayer films on solid supports with greater than 90% ETE in films.
  • Characterized via neutron scattering and physical techniques
  • ETE modeled and matches experimental results.

Contact:

Publications: “Diblock copolymer micelles and supported films with nanocovalently incorporated chromophores: A modular platform for efficient energy transfer,” P. G. Adams, A. M. Collins, T. Sahin, V. Subramanian, V. S. Urban, P. Vairaprakash, Y.  Tian, D.  G. Evans, A. P. Shreve, and G. A. Montaño, Nano Lett. 15, 1959 (2015).

Molecular Wear in Biomolecular NanoMachines

Scientific Achievement: We have shown that the mechanical activity of biomolecular motors can trigger wear at the molecular scale, which requires enhanced self-repair mechanisms in the context of biological nanosystems.

Significance: Biomolecular systems can undergo a range of active movements at the nanoscale. Our study is important to understand the limitations of biomolecular nanomachines and to improve the robustness of biomolecular-powered nanosystems through enhanced self-repair mechanisms.nanomachines

Research Details:

  • Molecular wear was proved in biomolecular nanomachines using the model system consisting of kinesin motors and microtubule shuttles.
  • Microtubules were tracked for each combination of kinesin surface density and gliding velocity to determine the factors contributing to the apparent shrinkage of the moving microtubules.
  • Magnitude of molecular wear shows non-linear dependence on nano-motor surface density, in which greater wear is observed above the mushroom-to-brush transition.

Contact:

Publications: E.L.P. Dumont, C. Do, and H. Hess, “Molecular wear of microtubules propelled by surface-adhered kinesins,” Nature Nanotechnology 10, 166 (2015).

Designing High Strength Thin Nanoparticle Membranes

Scientific Achievement: Because of their unusual strength, thin nanoparticles membranes have a number of  potential applications from sensor arrays to nanoscale filtration.  Current study resolved the fundamental mechanisms underlying their unique mechanical strength.

Significance: Simulations provided unprecedented molecular detail that cannot be obtained experimentally, and the resulting insights can be used to design nanoparticle membranes with more finely tailored properties.

nanoparticle membranes

Research Details:

  • Multi-million atom molecular dynamics simulations of alkanethiol-coated gold nanoparticle membranes were carried out to simultaneously measure nanoscale interactions while directly comparing membrane properties to experiment.
  • By replicating experimental conditions, nanoparticle membranes at a water-vapor interface were made and then water removed to form free-standing membranes.
  • Mechanical tests of the resulting membranes were then made and compared to experiment

Contact:

Publicatons: K. Michael Salerno, Dan S. Bolintineanu, J. Matthew D. Lane, and Gary Grest, "High strength, molecularly thin nanoparticle membranes," Phys. Rev. Lett. 113, 258301 (2014).

Strain Induced Room Temperature Multiferroicity 

Scientific Achievement: We have shown for the first time the coexistence of ferrimagnetism and ferroelectricity at room temperature in a complex metal oxide system by strain engineering.   

Significance: The demonstration of strain induced, high temperature multiferroism is a promising development for future spintronic and memory applications at room temperature and above.

Research Details:multiferroicity

  • Using strain to achieve the new functionality in complex metal oxide films.
  • Demonstrating the coexistence of ferromagnetism and ferroelectricity (or multiferroicity) at room temperature in a single phase BiFe0.5Mn0.5O3  films via strain.
  • Illustrating ferrimagnetic transition temperature of ∼600 K in highly strained and crystalline BiFe0.5Mn0.5O3 films which is 500 K higher than bulk BiMnO3.

Contact:

Publications: E. M. Choi, T. Fix, A. Kursumovic, C. J. Kinane, D. Arena, S. L. Sahonta, Z. Bi, J. Xiong, L. Yan, J. S. Lee, H. Wang, S. Langridge, Y. M. Kim, A. Y. Borisevich, I. MacLaren, Q. M. Ramasse, M. G. Blamire, Q. X. Jia, and J. L. MacManus-Driscoll, “Room temperature ferrimagnetism and ferroelectricity in strained, thin films of BiFe0.5Mn0.5O3,”Adv. Funct. Mater. 24, 7478 (2014).

Using Light to Couple Electricity and Magnetism within a Few Trillionths of a Second

Scientific Achievement: Ultrashort optical pulses were used to explore and optically manipulate the coupling between ferroelectric (FE) and ferromagnetic (FM) order in an oxide heterostructure, revealing the microscopic mechanisms that limit the speed of the response.

Significance: Materials in which electric and magnetic order coexist, known as multiferroics, have great potential for applications in areas including data storage and magnetic sensing. Many of these applications require a high-speed response, making it critical to understand and control the timescales governing the coupling between magnetism and electricity in multiferroic devices.electricity and magnetism

Time-and-polarization dependent response of the FE layer after optically perturbing magnetic order in the FM layer. The timescale taken for the response to reach its peak, as well as the detailed polarization dependence, reveals the microscopic mechanisms underlying ME coupling.

Research Details:

  • Using femtosecond optical pulses to manipulate the coupling between FE and FM order in a complex oxide heterostructure
  • Optically perturbing the magnetization in FM layer and selectively probing the associated change in the FE properties of FE layer
  • Coupling between FE and FM order induced within tens of picoseconds, mediated through elastic coupling between the FE and FM layers

Contact:

Publications: Y. M. Sheu, S. A. Trugman, L. Yan, Q. X. Jia, A. J. Taylor, and R. P. Prasankumar, “Ultrafast optical manipulation of magnetoelectric coupling at a multiferroic interface,” Nat. Commun. 5, 5832 (2014).

High-efficiency solution-processed perovskite solar cells with millimeter-scale grains

superconductivity

Hot casting techniques produce large crystalline grain perovskites in highly efficient and hysteresis-free solar cells.

The Science: A solution-based hot-casting technique was usedto grow continuous, pinhole-free thin films of organometallic perovskiteswith millimeter-scale crystalline grains. The fabricated planar solar cells have efficiencies approaching 18%, with little cell-to-cell variability with no photocurrent-voltage hysteresis.

Significance: The recent discovery of organic-inorganic perovskites offers promising routes for the development of low-cost solar based clean global energy solutions for the future.

Summary:

graph near a Mott transition

Solution-processed organic-inorganic hybrid perovskite solar cells have achieved high average power conversion values (PCE), making them an excellent candidate for next generation solar cells. The high PCE values in perovskite solar cells have been attributed to strong light absorption, weakly bound excitons and highly mobile charge carriers. However, the average efficiencies greatly vary compared to the most efficient device, indicating persistent challenges of stability and reproducibility. Moreover, hysteresis during device operation, possibly due to defect-assisted trapping, has been identified as a critical roadblock to the commercial viability of perovskite photovoltaic technology. Researchers at Los Alamos National Laboratory have focused on improving film surface coverage by increasing the crystal size and improving the crystalline quality of the grains, which is expected to reduce the overall bulk defect density and mitigate hysteresis by suppressing charge trapping during solar cell operation. LANL researchers have found by using a hot-casting technique for perovskite formation there is prolonged growth of the perovskite crystals, yielding grain sizes as large as 1 to 2 mm, in comparison with grains sizes of 1 to 2m that are typical of conventional post-annealing processes. Large grain crystals have less interfacial area and therefore suppress charge trapping and eliminate photocurrent-voltage hysteresis, while still yielding PCEs of 18%.

Contact:

Sergei Tretiak, Los Alamos National Laboratory

serg@lanl.gov

Publications: W. Nie, H Tsai, R. Asadpour, J.C. Blancon, A.J. Neukirch, Gautum Gupta, J. Crochet, M. Chhowalla, S. Tretiak, M.A. Alam, H.L. Wang, A.D. Mohite. High efficiency solution processed perovskite solar cells with millimeter-scale grains; Science, 347, 522 (2015).

Molecules Know When Size Matters in Crystal Growths

Discovery of size-dependent crystal growth at small size enables radial nanowire solar cell fabrications

graph near a Mott transition

A) Silicon crystal growth rates by chemical vapor deposition is reduced as facet size decreases due to edge desorption and undergoes; a transition to defected growth at a critical thickness with phosphorous additions. B) Model for enhanced SiH4 desorption at facet edge. C) Three-dimensional radial wire solar cell array. D) Light absorption profile of three-dimensional radial wire for solar cell

The Science: Epitaxial crystal growth studies reveal a new, unexplored size-dependent growth rate and crystallinity transition at mesoscale dimensions. The control of molecular incorporation on small crystal facets is shown to enable growth of defect free, radial electrical junctions on nanowires.

Significance: Controlling the crystal growth of three-dimensional (3D) nano and mesoscale architectures is essential for designing and manufacturing next-generation semiconductor devices. Recent results demonstrate such three-dimensional crystal growth can boost the efficiency of 3D microwire solar cells by 30%.

Summary: Single crystalline materials growth rate is governed by size-dependent precursor adsorption rate which has not been considered previously in chemical vapor deposition crystal growth for three-dimensional structures. The observation is revealed by a combination of epitaxy measurement and nano/microfabrication techniques. Silicon micro/nanostructures as the world-best studied materials for crystal growth are the platforms to study size-dependent growth behavior. Single crystalline growth of radial shell and planar thin films on silicon nanowires and nanoridges with well-defined dimensions shows size-dependent growth rate which cannot be explained by conventional explanation of epitaxy. A newly developed size-dependent precursor adsorption model is fit in the experimental observation. The size-dependent crystal growth on nanowire leads to three-dimensional crystal growth in micro/nanoscale. The three-dimensional crystalline architecture enhances solar light absorption by 30 to 40% due to light focusing, which can be useful for photovoltaics. The size-dependent growth behavior is also affected by impurity incorporation. The first observation of transition of crystallinity during phosphorus doped silicon radial shell on nanowire is a key result for designing next-generation semiconductor devices which industry is developing.

Contact:

Jinkyoung Yoo, Los Alamos National Laboratory

jyoo@lanl.gov

Publications: Jinkyoung Yoo, Shadi A. Dayeh, Norman C. Bartelt, Wei Tang, Alp T. Findikoglu, S. Tom Picraux, Size-dependent silicon epitaxy at mesoscale dimensions, Nano Letters 14, 6121 (2014).

Novel Electroforming-Free Nanoscaffold Memristor with Very High Uniformity, Tunability, and Density

Scientific Achievement: We demonstrate that vertical interface can act as an active device. Emergent behavior (resistive switching) is obtained right across the vertical interface by interfacing SrTiO3 with Sm2O3 at nanoscales.

Significance: Our results illustrate that resistive memories with a density of 40 Tb/in2 could be fabricated based on a nanoscaffold metal-oxide composite structure.

tamm couplinggraph tamm states

Fig. 1 (left) High-angle annular dark-field scanning transmission electron microscopy image of vertical interface of SrTiO3 and Sm2O3 in cross sectional view.

Research Details:

  • CINTs expertise in multifunctional materials and close collaborations between users and CINT scientist enabled the innovation in the design and development of nanoscaffold memristors or resistive switching devices (Fig. 1).
  • Switching is detected only across the vertical interface, while both nanocolumn and matrix are insulating (Fig. 2). The resistance variations exceed two orders of magnitude with very high uniformity and tunability.
  • A large concentration of oxygen deficiency right across the vertical interface (Fig. 3) is attributed to the resistive switching across the interface between SrTiO3 and Sm2O3

Publications: S. Lee, A. Sangle, P. Lu, A. Chen, W. Zhang, J. S. Lee, H. Wang, Q. X. Jia, and J. L. MacManus-Driscoll, Adv. Mater. 26, 6284 (2014).