9 Since graphene was first successfully exfoliated from bulk crystals in 2004, this and other 2D materials have attracted significant attention for their intriguing properties. Despite the use of some scalable growth techniques to produce large-area graphene, exfoliation is still the primary method used to produce monolayer and few-layer graphene flakes. After over a decade of use, the exfoliation method has remained largely the same. Although the flakes it produces are high quality, this method provides only small flakes with low yield. Seeking to improve on this method, Huang et al. (DOI: 10.1021/acsnano.5b04258) developed an exfoliation technique that results in a high yield of significantly larger
flakes. The researchers achieve this feat by improving adhesion between the flakes and their SiO2/Si substrate, first by cleaning the substrate with oxygen plasma to remove ambient adsorbates. Then, after the tape loaded with graphite crystals is brought into contact with the substrates, they annealed the substrate with the tape attached for several minutes in air, promoting the removal of gas molecules between the graphite and substrate surface. When the researchers removed the tape, graphite flakes left behind had yields 20 60 times larger than those produced by conventional methods, with individual flakes hundreds of times larger. Atomic force microscopy and Raman spectroscopy showed that these monolayer and few-layer flakes were high quality, and
their use in field-effect transistors indicated excellent electrical performance. Because they were able to use the same method to produce flakes from a layered Bi2Sr2CaCu2Ox superconductor, the authors suggest that this method could be widely useful in isolating 2D materials from a range of bulk crystals.
IN NANO
Bigger, Better Graphene Flakes
Plasmonic Devices on a Roll 9 In recent years, researchers have shown the utility of arrays of metallic nanostructures, such as nanoholes, nanoparticles, and sharp tips, for applications in plasmonics, metamaterials, and near-field optics. These arrays can be patterned on solid substrates, such as silicon wafers, using advanced topdown fabrication methods. However, patterning them on stretchable substrates could make it possible to mechanically tune the optical properties of plasmonic devices and metamaterials and for applications such as strain sensors, touch panels, curved displays, and metasurfaces by wrapping them around nonplanar surfaces. In a new study, Yoo et al. (DOI: 10.1021/ acsnano.5b05279) developed a method that uses template stripping to integrate metallic nanostructures onto stretchable
and rollable substrates. The researchers first created a template on a Si wafer using nanoimprint lithography and reactive ion etching. They then deposited a Au film on the template, which was stripped and transferred onto stretchy polydimethylsiloxane (PDMS) through the use of an adhesion layer between the metal film and the PDMS, curing for several hours. Through this process, the researchers created stretchable arrays of Au nanoholes and Au pyramids. Tests showed that the optical
transmission spectra of the nanoholes and the wavelength of light resonantly scattered from the tips of the pyramids could be modulated by mechanically stretching the underlying PDMS. Using a flexible transfer layer also allowed metallic structures produced from templates, including nanoholes, nanodisks, wires, and pyramids, to be transferred onto cylindrical rollers as substrates. The authors suggest that this method could find applications in sensing, displays, plasmonics, metasurfaces, and roll-to-roll fabrication.
Viruses Let It Glow 9 Viruses represent millions of years of evolutionary fine-tuning directed toward packing, transporting, and delivering nucleic acids into hosts. All viruses package their genome inside a robust protein coat to protect it during transmission. Before it infects a new host, this capsid undergoes structural changes that ultimately lead to its complete disassembly, allowing the viral genome to separate, to become transcriptionally active, and to produce new progeny. The genetic cargo itself also often requires a maturation step to activate. Gaining a better understanding of this entire process, which is important for both natural viruses and artificial virus-inspired nanocarriers, requires an integrated approach that allows for the study of both the capsid's structural dynamics and the cargo's physical state.
In a new study, Ortega-Esteban et al. (DOI: 10.1021/acsnano.5b03020) developed a method to track viral genome release using a combination of atomic force microscopy (AFM) and single-molecule fluorescence microscopy. Using human adenovirus as a model, the researchers triggered genome unpacking by using the AFM probe to repeatedly indent the capsid, ultimately causing it to rupture. They were then able to collect fluorescent signals from the released genome through the use of a DNA-specific intercalating fluorescent dye, YOYO-1, present in the buffer solution. Their experiments showed that the genomes of mature wild-type viruses expand after release, leaving behind larger fluorescent spots after uncoating compared to those of immature viruses. This expansion, the authors suggest, likely plays an important role in infectivity. VOL. 9
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The method used here to study the uncoating process, they add, can be used to study uncoating in other viruses.
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9 Nanopores have attracted increasing attention as a promising method for sequencing DNA, with potential for quickly and accurately reading long sequences with characteristic changes in ionic current as DNA strands pass through the pore. However, sequencing methods based on biological pores have a number of limitations, including fragility of the processing enzymes and lipid bilayers used to control DNA transport at high salt concentrations and deletion and insertion errors caused by enzymes skipping and backstepping along the DNA strand in a stochastic manner. Though these problems can be controlled by using synthetic solid-state nanopores, the high speed of DNA transit through these pores often makes its residence time too short to identify individual nucleotides.
Seeking a better solution, Belkin et al. (DOI: 10.1021/acsnano.5b04173) demonstrate the theoretical potential for plasmonic nanopores to sequence DNA. Using allatom molecular dynamics simulations, the researchers constructed nanopores in solidstate membranes topped with two gold triangular prisms forming a bowtie structure. This structure is capable of concentrating the electromagnetic field from an incident laser beam onto nanometer-sized hotspots near the nanopore. Taking advantage of the plasmonic field this bowtie structure produces, the researchers show that the resulting optical field of the hotspots could control the flow of DNA through the nanopore, restraining the flow when the laser beam was on and allowing DNA to slip through when it was off. In conjunction with
this process, surfaced-enhanced Raman scattering provided a readout of the DNA's nucleotide composition as fragments are confined in the hotspots. The authors suggest that this approach could offer a solution to the challenges of sequencing DNA in nanopores.
IN NANO
Sequencing DNA with Plasmonic Pores
Localized Surface Plasmon Resonance Sensing of a Different Color 9 Localized surface plasmon resonance (LSPR) sensing enables the detection of various analytes through monitoring the shifts of the plasmon resonances of finite nanostructures as the dielectric environment is changed. Larger screening charges induced in the surrounding dielectric of a nanostructure lead to larger red shifts in its plasmon resonance. Although this red shift should, in principle, provide a direct report of the dielectric permittivity, those of typical analytes are too similar to induce distinguishable red shifts. Thus, the plasmonic nanostructure is usually functionalized with molecules that can only bind specific analytes. For gold structures, often used for plasmonic applications, the highest LSPR sensitivity is for plasmon resonances in the
infrared, requiring instrumentation such as spectrophotometers. This setup is bulky and requires stable, ambient conditions and electrical power, limiting its applications. Seeking an alternative, King et al. (DOI: 10.1021/acsnano.5b04864) developed a colorimetric LSPR sensor that relies on aluminum nanoclusters with plasmonic Fano resonances. Each cluster was composed of a core disk surrounded by several satellite disks. By varying the size of the nanocluster and the number of satellite particles, the researchers were able to tune the nanoclusters' resonances from the near-UV into the visible region of the spectrum. By designing the clusters with specific chromaticities in the blue-green region,
where the eye is most sensitive to color changes, the researchers created sensors that visibly changed color in the presence of different analytes, obviating the need for extra detection equipment. The authors suggest that this platform could be useful for a variety of sensing applications.
nanocomposites. The ability to mimic biological designs could lead to new highperformance materials. However, finding facile preparation pathways that produce the complex hierarchical ordering of these natural materials has remained challenging. In a new study, Wang and Walther (DOI: 10.1021/acsnano.5b05074) demonstrate a single-step self-assembly process for preparing materials that mimic crustacean cuticles, such as the shells of crabs and lobsters. The researchers added a dilute suspension of cellulose nanocrystals (CNCs), renewable materials derived from trees and farm waste, into a solution of poly(vinyl alcohol) (PVA), allowing the PVA to coat the CNCs and the mixture to evaporate into a
solid film. The resulting films were highly iridescent, a strong indicator of ordered cholesteric structures. By varying the ratio of CNCs to PVA, the researchers created materials that ranged from strong and stiff to more ductile, with differing helical pitches and photonic band gaps. By casting different dispersions of these materials on top of one another, they created materials with stacked periodicities and varying properties. The authors suggest that this strategy opens avenues for combining photonic properties, multilayer structures, and mechanical robustness in materials with a facile synthesis.
Nanocomposites Get Crabby 9 Nacre, wood, bone, and crustacean cuticles are natural, lightweight materials, combining high stiffness, strength, and toughness. These biological nanocomposites combine high fractions of hard reinforcements arranged in orderly fashions within energy-dissipating soft matrices. This setup is the opposite of most presentday engineered nanocomposites, which use small amounts of reinforcements within commodity polymers, and whose performance often falls short of those of biological
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9 Engineering strain into materials provides a means to tune electrical, optical, and magnetic properties in nondestructive ways. A typical example is the strained-layer superlattice, in which periodic stacking of two-dimensional semiconductor layers with different lattice constants generates dislocation-free strain up to several percent. This concept can also be transferred to onedimensional systems known as a strainedwire superlattice (SWS). Though this type of structure has attracted growing interest from both fundamental and applied perspectives, achieving this design has been challenging, particularly as wire dimensions approach the atomic scale. In a new study, Song et al. (DOI: 10.1021/ acsnano.5b04377) demonstrate a SWS
composed of atomic-scale wires fabricated through self-assembly on Si surfaces. The
researchers prompted Au Si atomic wire formation by incorporating submonolayer Au atoms into the surface layer of vicinally cut and regularly stepped Si(111) surfaces. Each of these wires consists of two major atomic chains, a double Au chain on the nanoterrace and a Si chain with a graphene-
like reconstruction on the step edge. Tests show that the pristine Si nanoterraces impose a strain of several percent on the neighboring Au Si wires. Calculations suggest that this strain modifies both the band structure of metallic chains and the magnetic properties of spin chains. The authors suggest that this SWS represents the ultimate 1D version of a strained-layer superlattice. Better understanding of structures of this type, they add, could help direct precise engineering of self-assembled atomic scale wires.
IN NANO
Atomic Wire Superlattices Feel the Strain
Helping Photons and Electrons Go with the Flow 9 A variety of nanostructured surfaces, such as nanopillars, nanocones, nanowires, inverted pyramids, and nanospheres, have been developed to enable strong, broadband absorption in semiconductors for optoelectronics. However, engineered optoelectronic surfaces must also control the flow of electrons in addition to light. Typically, electrical conductivity at this interface has been provided through a transparent electrode deposited over the photon management structure. Thus, despite the need for both strong absorption and high conductivity at optoelectronic interfaces, nanostructures for photon and electron management have usually been studied and optimized separately. In a new study, Narasimhan et al. (DOI: 10.1021/acsnano.5b04034) created a hybrid metal semiconductor surface that
has both strong light absorption and high electrical conductivity. The design of this new surface is based on nanopillars with small aspect ratios that protrude from a substrate through holes in a metal film. Using metal-assisted chemical etching, the researchers used a patterned gold film as a catalyst to etch a silicon substrate, causing the film to sink into the substrate and an array of silicon nanopillars to protrude through the film's holes. When this surface was coated with a silicon nitride anti-reflection layer, tests showed broadband absorption of up to 97%, even though metal covers about 60% of the top surface. Simulations showed that Mielike resonances in the nanopillars were responsible for this effect, funneling light around the metal layer and into the substrate. In addition, this surface displayed
low sheet resistance. Together, the authors say, these results suggest a new paradigm for jointly managing light and electrons for high-performance optoelectronic interfaces.
Bottom-Up and Top-Down Meet in the Middle 9 The III V semiconductors' direct band gaps have made these materials promising components for optoelectronic devices, including solar cells, light-emitting diodes (LEDs), and single-photon emitters. However, scaling up the production of III V semiconductor nanowires (NWs) has been hindered by numerous problems. The most common method to produce these materials, in which they are grown via a one-step vapor-phase epitaxy mechanism in predetermined areas on substrates guided by SiO2 masks, has numerous challenges. These include a lack of control from seeding and growth being combined into a single step, catalysis using gold (which prevents integration into existing complementary metal-oxide semiconductor (CMOS) processes), and an inability
to use conventional UV lithography because the designated substrate windows are smaller than the UV diffraction limit. In a new study, Zhang et al. (DOI: 10.1021/ acsnano.5b03682) present an approach that bridges the gap between the industrystandard top-down fabrication approaches and nanometer-scale bottom-up approaches. The researchers start by decomposing trimethylindium into indium and its other constituents. The indium adatoms diffuse into windows on InP substrates that are not covered by SiO2 films defined by UV lithography. The NWs are then grown using a vapor liquid solid method. Through a process known as step bunching, the researchers modified the substrate to align indium nanoparticles on the same side of VOL. 9
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the micrometer-scale window, leading to coordinated, orderly growth. They further showed that this method is versatile enough to grow uniform InP/InAs axial heterostructure nanowire arrays. The authors suggest that this technique could greatly improve the potential for mass production of III V semiconductor NWs.
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