Building blocks comprised of individual nanoscale materials are commonly integrated with other materials into architectures that amplify their properties (up-scaling) or lead to new ensemble behaviors (emergent phenomena). By surveying the integrated environments of greatest potential impact, and by developing a fundamental understanding of the principles that govern the integrated properties and behaviors, we can capitalize on the greatest potential for nanomaterials to have an enduring impact on scientific and technological innovations. Nanoscale integration has the potential to revolutionize the way we live.
Innovative nanofabrication, integration, and “up-scaling” methods to incorporate quantum-size nanostructures into arbitrary 2D and 3D architectures
Semiconductor nanowires and nanotubes are important building blocks for nextgeneration energy-harvesting and energy-storage systems, optoelectronics (from single-photon sources to lowthreshold lasers), photodetectors, and even sensors for chemical or biological agents. Significant progress has been made in recent years in the precision bottom-up synthesis of single-crystalline, controllably doped, and heterostructured (both radially and axially) nanowires, as well as in the selective preparation of nanotubes with specified structure and chirality.
- Innovation in nanofabrication and integration of the growth of arbitrary, 3D, multifunctional, quantum-size structures.
- Integration and “up-scaling” methodologies for transforming 1D nanowires and nanotubes into 2D and 3D architectures and ensemble systems designed for advanced functionality.
Positional science and capabilities
CINT has state-of-the-art capabilities in molecular beam epitaxy (MBE) growth of high-mobility III-V planar heterostructures (see section 4.2). CINT’s MBE is in demand worldwide to grow high-purity, ultra-high mobility As-based III-V compound semiconductor structures with atomic monolayer precision for fundamental studies of 1-D and 2-D nanomaterials. CINT has a 9,000ft2 clean room facility with processing tools that can reach a resolution of tens of nanometers. CINT specializes in the synthesis of semiconductor nanowires by solution-phase and chemical vapor deposition (CVD) approaches to produce single crystal nanowires, radially and axially heterostructured nanowires, and complex architectures consisting of Si/Ge, III-V and other compound semiconductor materials. Hybrid 3D nanostructures are being fabricated by growing Si and Ge nanowires and their heterostructures on 2D graphene and transition metal dichalcogenides. The capability to modulate the catalyst composition in-situ in the Si-Ge CVD system enables precise control of nanomaterial characteristics. The CINT nanomanipulator is a custom two-probe device inside of a scanning electron microscope, and is used for in-situ quantitative nanostructure electrical characterization as well as the fabrication of single nanowire devices for ex-situ electrical, thermal, and optical property measurements. CINT’s Discovery Platform for in-situ TEM measurements of electrochemical processes in single nanowires has enabled many discoveries in this area. For example, silicon nanowire structures are attractive for use in next-generation energy-storage systems for their high energy density as negative electrodes for Li-ion batteries. The high surface area of the nanowire structures allows for the accommodation of the 300% volume expansion during charging. In scaling up these materials, CINT researchers are investigating the degradation mechanisms of silicon nanowire arrays in comparison to degradation within individual amorphous and single crystalline structures, and are developing design strategies to mitigate the capacity loss that occurs when active materials are disconnected from the current collector. CINT has demonstrated the application of ultrafast wide field optical microscopy for multiscale characterization, and has routinely used ultrafast pump-probe optical spectroscopies to study the carrier behavior in single nanowires.
Toward the future
Traditionally, nanomaterial system fabrication has been carried out in planar structures. More recently, however, 3D architectures have started to emerge in microand nano-electronics that will ultimately expand their utility to many technologies including consumer electronics, energy, and biomedicine (Figure2). However, this will require substantial innovation in the nanofabrication, growth, and integration of quantum-size structures.
Combinations of bottom-up and top-down growth and nanofabrication techniques have been used successfully to create “vertical” nanowires (Figure 3) in a few material systems such as group IV and certain III-V semiconductors. More complex 3D structures that contain a variety of semiconductors with sections of varying dimensional confinement (1D, 2D & 3D) are required for new applications such as low threshold lasers, quantum information and computation, new transistor architectures that go beyond the semiconductor roadmap, and novel optomechanical systems. To realize this vision, hybrid multisequential combinations of bottom-up and top-down synthesis techniques must be developed in a variety of combined semiconductor families, possibly oxides or diamond. One hypothetical example of an application where heterogeneous integration of dissimilar structures and materials could be used is single-photon sources for quantum information processing. An ideal single-photon-on-demand source could be made from an electrically injected quantum dot. One option for accomplishing this is to fabricate a nanodevice made from different direct bandgap III-V semiconductors emitting at different wavelengths. The semiconductors would have to be placed in specific locations in a 3D arrangement for subsequent coupling to passive waveguides made from group IV semiconductors for further multiplexing and routing. Developing architectures such as this will require the integration of different dimensional structures into a larger functional microstructure.
CINT is looking to expand its growth capabilities into other semiconductor families such as large band gap semiconductors (i.e., III-Nitrides, diamond) and low band gap materials (e.g., InGaAs/InP, antimonides), and expand the MBE effort into high-mobility group IV materials. The integration of different semiconductor materials laterally and vertically will necessitate the development of new hybrid growth techniques with in situ sample handling and characterization in vacuum, all combined with new nanofabrication techniques. In addition to the expansion and combination of epitaxial capabilities, new nanofabrication techniques must be developed to create arbitrary top down nanostructures that go beyond “vertical etching.” CINT researchers envision the capability to create quantum wires that lie horizontally in several planes, and with arbitrary control of their diameters and connectivity to other topdown or bottom-up defined sections.
CINT has exceptional capabilities in synthesis and characterization of individual nanowires. Probes have been developed that make it possible to determine electrical, optical, and thermal properties at the level of a single nanowire or nanotube, and even physically manipulate single wires. However, one of the biggest challenges in nanowire science is how to integrate, up-scale, and organize structures and architectures into ensemble systems that are functionally relevant.
CINT is well positioned to answer this challenge by leveraging established capabilities in nanowire and nanotube synthesis and chemical modification, nano-manipulation for direct measurement of a nanostructure’s properties, and fabrication of single nanowire devices in order to address the fundamental issues of integration:
- manage the interface effects between individual nanostructures in interconnected networks and composite mesoscale structures;
- bridge the gap between emergent nanoscale functionality and macroscale performance;
- develop geometries that harness the exceptional axial diffusion behaviors of 1D systems;
- and determine the ion and electron transport behaviors at the junctions of nanostructures.
CINT will expand the Discovery Platforms for thermoelectric and electrical characterization and in-situ TEM measurements of transport and electrochemical processes in single nanowires. Through understanding the core degradation mechanisms that plague nanowire arrays, CINT researchers are developing design rules to implement these structures into bulk electrodes for integration into Li-ion energy-storage systems. The electrochemical TEM Discovery Platform will be redeveloped for enhanced environmental control for novel operando testing to expand the “lab-in-a-gap” capabilities. CINT will develop unprecedented capabilities in multi-scale characterization, including integrating wide field observation with nano- and mesoscale resolution in up-scaled three-dimensional architectures.
Meeting the modeling challenge
CINT’s ultimate goal in this area is to develop a fundamental understanding of the interfacial interactions between nanoscale components with 2D and 3D confinement (i.e., nanowires and quantum dots) and the host materials, and how these interfacial interactions affect the overall functionality of the hybrid structures, thus leading to optimization and control by design. CINT will pursue the following directions:
- Develop a predictive capability for the relationship between interfacial structure (microstructure and vacancies/defects) and carrier transport across interfaces. This goal will be accomplished through first-principles calculations of structural instability at interfaces and the functionality, including local electronic structure and transport, across the interfaces.
Develop a new theoretical framework to address transport properties across entire hybrid structures at the ensemble scale. This framework will be based on the first-principles calculations for both individual nanowires/QDs and the interfaces and will help establish the relation between the structure functionality and electronic, optical, and transport properties at the mesoscale.
Hybrid material interactions for generation and manipulation of light
Structured hybrid materials can be engineered to have novel photonic properties that emerge only as a result of multi-material interactions and can also include pre-designed properties for novel photon generation and manipulation. CINT is advancing the understanding and application of these revolutionary hybrid systems by addressing the most significant open questions surrounding the control, integration, and enhancement of the photonic response of two classes of materials and their associated assemblies:
- Materials and structures that control and modify electromagnetic energy (plasmonics, metamaterials); and
- Materials and assemblies that actively generate and harvest electromagnetic energy.
Positional science and capabilities
CINT’s foundations to lead in this area rest on our multidisciplinary capabilities for generation of unique photonic materials, their characterization with powerful spectroscopic tools, paired with an ability to control compositions and assembly routes to define interaction geometries across multiple length scales and degrees of complexity. We are international leaders in developing exceptional photonic materials with switchable and highly tunable photon emission properties. Examples include proprietary non-blinking quantum dots and novel microfluidic control of synthesis for axially heterostructured nanowires.
Our doped carbon nanotubes provide new multifunctionality and boosted quantum yields and highlight CINT ability to isolate specific tube structures and control their surface chemistries. Together, our emitters provide multi-photon to single photon behaviors across classical to quantum regimes (see Figure 4). Pioneering efforts in nanomaterials assembly include soft templating approaches for creation of hybrid functional systems with hierarchical structures that are reconfigurable and responsive. Innovative dip pen nanolithography is providing unprecedented control over placement of emitters on photonic, plasmonic, and metamaterials structures, which provide significant opportunity for manipulation of light.
CINT has been a world leader in the area of metamaterials (THz to near-infrared) for nearly a decade. We now lead the world in all-dielectric metamaterials as well. CINT's position in this field is enabled by access to nanofabrication and expitaxial growth facilities. Advances in these materials are driven by world-class spectroscopic characterization, including ultrafast tools providing fs resolution across THz to soft X-ray energies unavailable anywhere else. Paired with the broadest continuous excitation range available for Raman spectroscopy (near-IR to UV) and state-ofthe art tools for microscopic imaging, spectroscopy, and dynamics measurements of single nanoelements, CINT abilities for optical characterization of nanomaterials are unmatched. Our first-principles DFT simulation capability encompasses nearly all flavors of electronic structure codes for understanding electronic, optical, and vibrational properties of complex materials. In particular, LANL-owned nonadiabatic excitedstate molecular dynamics capability excels at modeling the largest systems accessible for nonlinear and time-dependent spectroscopy. Theory efforts are further founded in DFT and classical electromagnetic theory simulation for metamaterials modeling and design.
Toward the future
Drawing on these strengths, CINT will approach the following opportunities in hybrid photonic materials as we move forward:
Generation and active manipulation of novel emitting states and photon correlation statistics
Accessing new emission regimes of expanded wavelengths, enhanced quantum yields, and tunable or selectable photon statistics and dynamics requires defining interactions in terms of the relative placement and orientation of materials within the hybrid structure, while also controlling the hybrid composition over multiple dimensionalities. CINT will expand the hybrid materials community’s ability to generate and actively manipulate novel emitting states and photon correlation statistics by addressing the following challenges:
- Identifying and realizing candidate materials that are likely to generate targeted optical behaviors from hybrid interactions, such as tailoring of plasmonic interactions aimed at enhancing biexciton emission.
- Synthesizing and/or integrating multi-component systems with the appropriate interaction geometries to create a desired functionality, such as harnessing metamaterial interactions with dopant states of emitters for directional emission or enhanced coupling to photonic waveguides.
- How can emergent electronic structures be manipulated and probed across multiple length-scales within interfacial environments to generate desired optical responses?
- Generating desired optical responses in emergent electronic structures by manipulating interactions across multiple length-scales within interfacial environments. Examples include use of soft responsive systems to modulate coupling between embedded optical emitters for ondemand behaviors.
CINT will employ a variety of materials processing techniques, including direct synthesis, self-assembly, nanofabrication and directed placement, to further our understanding of these issues. CINT will also move beyond traditional synthetic and processing approaches by tapping soft-materials assembly methods with the potential to harness the responsive and highly tunable nature of bio-inspired systems.
Active, multifunctional plasmonic and metamaterial interactions
Hybrid materials interactions have significant potential for establishing new functionality and enhanced manipulation of the medium in which light is generated, harvested, or propagated. Hybrids enable a move from passive to active plasmonic and metamaterials, and form a basis for new concepts including metamolecules (in which the collective interactions of individual metamaterial elements or atoms create new function) and “plasmonics on demand” (where localized materials interactions automatically generate desired resonances in optimized locations). Unprecedented multifunctionality will result, giving simultaneous control of polarization states, beam steering, and focusing; integrating perfect absorption of light directly into optoelectronic architectures; or ultimately integrating hybrid metatmaterials directly with emerging concepts in emitting materials. To realize this extraordinary multifunctionality, CINT will utilize our integrated efforts in synthesis, characterization, and modeling to address the following questions:
- Devising non-traditional plasmonic systems (e.g., graphene hybrids or emerging epitaxial oxides) that cannot be accessed with more traditional noble metal approaches.
- Designing multifunctional metamaterial behaviors through metamolecule concepts (shown in Figure 5).
- Generating hybrid interactions coupled to metamaterials architectures to provide active/dynamic control and tuning of enhanced metamaterial response and their optical nonlinearities.
Meeting the modeling challenge
CINT’s ultimate goal in this area is to create hybrid materials by design. This will entail significant advances in predictive modeling. In particular, the state-of-the-art must be dramatically advanced in such areas as electronic structure, dynamics, environment, and interfacial interactions at length-scales between the molecular and macroscopic. To meet the modeling challenge, CINT will pursue the following opportunities:
- Develop a predictive capability for designing new optical functionality arising from materials interactions such as between plasmons in metallic systems (e.g. metal nnanoparticles, Dirac metals) and excitons in semiconductor nanoemitters.
- Understand materials coupling mechanisms and identify the most interesting and promising materials interactions, both in terms of composition and interaction geometries, to pursue as routes to novel optical behaviors.
- Develop new theoretical concepts capable of optimizing electronically active networked structures by accessing the middle-length scales of significance for understanding integrated hybrid behaviors and obtaining targeted optical responses.
CINT’s current expertise and capabilities in hybrid materials provide a strong foundation for pursuing the above questions However, the full range of effort in this area will require CINT to expand its materials generation capability to include new techniques capable of placement of optical nanoparticles with nm precision. Additionally, our strengths in single-nanoparticle spectroscopic characterization could be significantly enhanced by adding capability for single nano-element Raman and magneto-optical spectroscopy while expanding ultrafast capabilities to include single-photon counting techniques at wavelengths longer than the near-IR. CINT will also bring the full strength of our integrated efforts in synthesis, characterization, and modeling (Figure 1) to bear on these issues. Model development will work hand-in-hand with experiment in this rapidly expanding field, to allow us to establish the most relevant test systems for validation of predictive models. There is significant new opportunity for developing robust approaches to model exciton-plasmon coupling and pioneer the area of “phononics” (manipulation of phonons and phonon coupling phenomena)..
Multi-scale structure and dynamics in soft matter
A grand challenge in nanoscience integration is the ability to propagate the intrinsically unique behaviors of nanoscale materials into functional materials and systems at the macroscale. Nature provides a vast array of blueprints by which this challenge can be achieved in soft materials. For example, cephalopods (e.g., squids) are able to rapidly change their color at the organismal level based on changes in structure (and associated function) across multiple length scales, beginning with the active reorganization of pigment granules at the sub-cellular level. Our goal is to develop strategies (principles and soft materials) that will enable the hierarchical assembly of individual nanoconstituents so as to harness their collective or emergent behaviors. It is anticipated that the multiscale and multidimensional assembly of nanoscale building blocks will lead to next generation photonic (e.g., solid-state lighting, lasing, color tuning), electronic (i.e., beyond silicon electronics), and energy storage technologies.
Positional science and capabilities
Current CINT capabilities include the synthesis of a wide range of both natural and engineered functional nano-constituents, structured soft materials that can serve as platforms for the spatial/orientational organization of the individual nanocomponents, and multiscale modeling and visualization tools, all of which serve to position CINT for realizing this scientific opportunity. Specifically, CINT has established expertise in the large-scale production of naturallyderived, functional biomolecules including transport nanomotors, (kinesin and dynein motor proteins), light-driven proton pumps (bacteriorhodopsin), light-gated ion transporters (channel rhodopsin), and rotary actuators (F1-ATP synthase). This effort is complimented by unique capabilities in the synthesis of engineered nanoparticles, including fluorescent metal nanoclusters, non-blinking quantum dots, high explosive detonation nanocarbons, plasmonic nanoparticles, magnetic nanoparticles, and chemically tailored carbon nanotubes and graphene.
In the area of hierarchically-structured soft materials for the assembly of nano-constituents into functional complexes / composites our capabilities include the development of bio-derived (lipid) matrices, artificial biomembranes, genetically-engineered responsive peptides / polymers, wholly synthetic block copolymer constructs (vesicles), structured dual conducting poly(ionic liquids), and stimuliresponsive blends of lipids and polymers.
Advanced characterization techniques are crucial for understanding the structure and functional dynamics of assemblies/complexes across a broad range of time and length scales. CINT has worldleading capabilities for the visualization of both the spatial distribution of nano-constituents within complex environments and their dynamics. Three-dimensional single molecule / particle tracking microscope, developed at CINT, images the fine details of complex dynamic systems, For example, the precise step sizes of motor proteins propagating within a biomembrane have been imaged (Figure 6). Super resolution fluorescence microscopy provides images of fluorescently labeled samples to a resolution of 10 ~ 20 nm (a factor of 10 below the diffraction limit of 250 nm), approaching the resolution of electron microscopy. In concert with these imaging techniques, correlated AFM and fluorescence imaging affords unique capabilities combining single-molecule sensitivity with time-correlated singe-photon counting. These world-class visualization capabilities are augmented with an environmental scanning force microscope that yields information on the adhesion and binding characteristics of nanocomponents; and an in-house X-ray scattering (SAXS/WAXS) instrument which provides greater structural details (from 100’s of nm to Å).
Toward the future
Synthesis and fabrication of soft materials
Drawing on the above mentioned strengths of CINT, we will push the state-of-the-nanoscience beyond the synthesis and fabrication of simple homogeneous building blocks (nano-constituents) and composites and seek to create integrated systems which combine multiple functional components and exhibit emergent properties (Figure 7). In addition, the responsive soft matrices will promote dynamic and/or programmable interfaces that drive structural reorganization and associated changes in macroscopically observed composite/system function. Through optimization of our “toolbox” of stimuliresponsive, structured soft materials we will achieve multi-scale assembly of disparate nanoobjects, serving to fulfill, for example, our vision of hybrid materials for the controlled manipulation of light (see section 2.2). Critical to achieving these goals will be the nano- to mesoscale assembly of nanoparticles. By controlling the spatial arrangement of the individual nanocomponents within a responsive matrix will allow for dynamic tuning of their spatial proximity and therefore active regulation of the macroscopic properties of the material. For example, the spatial organization of nanoscale emitters (gQDs / nanocarbons) and plasmonic (metal) nanoparticles doped or in-situ synthesized within a hierarchically-structured soft matrix will offer a means for achieving super-radiance, plasmon assisted lasing and dynamic control over photon emission or light interaction (“color tuning”), all component materials that could ultimately be integrated to form next generation nanophotonic devices. Our current collection of structured soft materials, however, will require synthetic modification so as to possess the following materials attributes:
- Distinct regions for spatial organization of the functional nano-constituents,
- Environmental-responsivity for active reconfiguration of the nanoparticle arrangement and hence composite properties,
- Processablility for extension of structure into macroscopic dimensions,
- Compliant interfacial chemistry for coupling to traditional device materials (e.g., metals and ceramics) / architectures,
- Requisite balance of mechanical durability without sacrificing dynamics.
Another area requiring significant investment will be infrastructure (instrumentation, tools, clean room space, and expertise) to expand our efforts in top-down patterning / lithography and processing of selfassembled (bottom-up) soft materials. That is, full realization of nanocomposites with complex functionality will require the ability to generate 2D and 3D patterned materials that will allow for the integration of nano-constituents over a full range of length scales (spanning nano → macro), this aspect of our work will require the addition of a Soft Fabrication Laboratory at CINT. The facility will contain a suite of 2D and 3D patterning tools, including capabilities for micro/nano-contact printing, moulding, inkjet printing, optical lithography, and scanning-probe based direct write techniques.
Strategic plan and investment
Concurrent with advancing the fabrication of soft materials, substantial improvements in modeling tools applicable for understanding multi-length scale and temporal phenomena will also be pursued, aiding in the a priori design of the materials and analysis of multimodal characterization data. For example, LAMPS codes development at Sandia will be used to improve our image analysis capabilities.
In recent years, considerable progress in optical microscopy has enabled characterization of soft and biological materials with spatial resolution well below optical diffraction limits, and with high temporal resolution. CINT has led the effort in developing imaging and visualization tools for single biomacromolecule / particle tracking. While these techniques are well suited for evaluating the dynamics of individual nanoscale components, they often lack the ability to capture details of the molecular structure of complex assemblies. A full understanding of complex composite materials requires imaging structure and structural dynamics over a full range of length and time scales. Future efforts within CINT will focus on achieving this goal by improving optical tracking capabilities to achieve single molecule resolution and by combining other characterization tools, such as force microscopy, X-ray scattering and electron microscopy into a single platform. A specific opportunity for advancing multimodal characterization will be the coupling of X-ray scattering/ diffraction which provides atomic resolution with the images and trajectory information provided by 3D optical tracking. It is anticipated that integration of these techniques onto a single platform will yield structure and structural dynamics over the Angstrom to micron length scale.
Meeting the modeling challenge
CINT’s ultimate goal in this area is to develop a fundamental understanding of the interactions among nanoscale components, and how these interactions affect the overall functionality of the heirarchical structures, thus leading to optimization and control by design. The world leading LAMMPS molecular dynamics code is our base for performing these simulations. CINT will pursue the following directions:
- Develop exascale code capability to treat the heirarchical systems studied by the experimentaliststhrough code development to use accelerators (e.g. GPUs) of exascale computers.
- Develop coarse-grained models from the underlying atomistic models to expand the spatial and temporal ranges significantly, while maintaining key (molecular) chemistry.