9 Along with chemotherapy and surgery, radiation is a common primary treatment for a variety of cancers, including those of the head and neck, breast, lung, prostate, and rectum. Cumulative doses for treatments often range from 20 to 70 Gy, but despite its critical therapeutic value, tissues near the targets of treatment often sustain significant radiation-induced toxicity. Consequently, appropriately dosing cancerous tissues while sparing healthy ones is an important goal. However, existing devices for measuring radiation doses in vivo have a variety of drawbacks, including being invasive, being difficult to handle due to fragility or sensitivity to heat and light, requiring separate read-out devices, or measuring only surface doses. Seeking a better in vivo radiation dosimeter, Pushpavanam et al. (DOI: 10.1021/
acsnano.5b05113) developed a method in which a colorless gold salt solution transforms into colored dispersions of gold nanoparticles upon exposure to radiation, becoming a colorimetric plasmonic radiation nanosensor. Tests show that, upon exposure to radiation with doses as low as 0.5 Gy or as high as 37 Gy, depending on the concentration of the lipid surfactant employed, the solution forms maroon-colored dispersions of plasmonic gold nanoparticles. Differences in color provided a reliable readout of radiation dose. This method was able to detect radiation levels in anthropomorphic prostate phantoms when administered together with endorectal balloons, showing its potential utility as a dosimeter for therapeutic radiation for prostate cancer. The authors suggest that this visible nanosensor might have potential as a reliable
dosimeter for fractionated radiotherapies in a variety of clinical settings.
However, achieving pores below a nanometer with this technique has proven difficult partially due to the self-healing properties of graphene due to highly mobile surface carbon atoms. In a new study, Robertson et al. (DOI: 10.1021/acsnano.5b05700) created sub-nanometer-scale pores in graphene by irradiating sheets at 80 kV with a high beam current density. They were able to prevent these pores from filling with amorphous carbon by heating the graphene to temperatures between 500 and 800 °C. Aberration-corrected transmission electron microscopy (AC-TEM) provided images of pore structures with single atom resolution. Using this method, the researchers found open pore geometries consisting of 4 13 atoms. Bond length measurements showed
that five-membered rings around the pore edge undergo weak reconstruction, with 4 and 6 atom pore structures remaining viable. The authors suggest that gaining better understanding of these structures will be fundamental for achieving the applications envisioned for porous graphene at this length scale.
IN NANO
Getting Radiation Doses Right
Peering Inside Graphene's Pores 9 In addition to applications that capitalize on graphene's extraordinary mechanical, electrical, thermal and optical qualities, researchers have recently suggested that this unusual material could also serve as a filtration membrane for water desalination, translocating DNA and RNA, ionic selection, or gas filtration through the introduction of nanoscale and sub-nanometer scale pores. Though sub-nanometer pores have been introduced directly into graphene by atomic force microscopy indentation and in graphene-like structures through a bottom-up chemical synthesis technique, imaging these structures at the single-atom level has been a challenge. Electron irradiation can create nanometer-size pores while also offering high resolution imaging when combined with transmission electron microscopy.
Carbon Nanomaterials Get Tough 9 Graphene's extreme toughness, stiffness, and electrical conductivity hold promise for applications in a variety of fields. However, assembling microscale nanosheets of this material into the macro-size nanocomposites necessary for many envisioned applications remains a challenge. Nacre, the inner shell layer of some mollusks and the outer layer of pearls, provides an excellent example of how nature assembles macroand nanoscale components into hierarchical structures, leading to synergistic toughening effects. Its two-dimensional aragonite platelets, which are held together with onedimensional nanofigrillar chitin, are a natural inspiration for biomimicry. In a new study, Gong et al. (DOI: 10.1021/ acsnano.5b05252) use the graphene derivative reduced graphene oxide (rGO) and double-
walled carbon nanotubes (DWNTs) to create nanocomposites reminiscent of nacre. These bioinspired materials were created by covalently cross-linking these two materials, with X-ray diffraction and transmission electron microscopy showing the successful introduction of DWNTs into the interlayers of rGO nanosheets. Combining these two materials leads to synergistic toughening. Tests show that the tensile strength and toughness of this nanocomposite reaches 2.6 and 3.3 times that of pure rGO. Further investigation showed that these materials had high conductivities and excellent fatigue-resistant VOL. 9
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properties, suggesting potential uses in aerospace, flexible energy devices, and artificial muscle. The authors suggest that this method for combining building blocks into bioinspired nanocomposites could serve as a strategy for creating other integrated, high-performance nanocomposites that synergistically improve on the qualities of their components.
Published online December 22, 2015 10.1021/acsnano.5b07283 C 2015 American Chemical Society
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9 Being able to image local heat generation and dissipation at the nanoscale is important for understanding the basic thermal properties of micro- and nanostructured materials, including biological systems. It is also critical for developing new nanotechnological applications, such as photothermal therapy with nanoparticles, local chemical reactions of catalytic nanoparticles, and heat dissipation in nanoelectronic circuits. Several technologies have been developed in recent years to probe local heat distribution in nanomaterials. However, no method reported thus far provides sensitive thermal imaging with high temporal and spatial resolution. In a new study, Chen et al. (DOI: 10.1021/ acsnano.5b05306) describe the use of plasmonic thermal microscopy (PTM) to fill this gap. This method is based on the temperature dependence of the refractive index, which is detected with surface plasmon resonance
(SPR) with high sensitivity. The researchers created their PTM setup using an inverted optical microscope with a high numerical aperture oil objective. On this objective, they placed a gold-coated glass slide with a polymer cell to hold the solution containing the nanoscale object to be imaged. By shining p-polarized light from a superluminescent diode onto a gold-coated cover glass, they excited surface plasmons on the gold surface. Reflected light collected through the objective formed a SPR image, which the
researchers recorded with a fast CCD camera. Using a laser to generate heat locally, the researchers successfully imaged photothermal generation from single nanoparticles and graphene pieces, studied spatiotemporal distribution of temperature surrounding a heated nanoparticle, and observed heating at defect sites in graphene. The authors suggest that this method could help elucidate local heat generation and dissipation in a variety of nanoscale systems.
IN NANO
Seeing Heat with Plasmonic Thermal Microscopy
Shining a New Light on Plasmonic Nanolasers 9 Plasmon nanolasers, or spasers (surface plasmon amplification by stimulated emission of radiation), use plasmonic cavities and gain media to compensate loss and achieve amplification of nanolocalized electromagnetic fields. These devices can overcome the diffraction limit and act as coherent nanoscale light sources for miniaturized photonic devices. Though several nanocavity architectures have been reported for spasers over the past few years, most have undesirable characteristics, such as bidirectional lasing emission or emission with limited far-field directionality and large radiative losses. In a new study, Yang et al. (DOI: 10.1021/ acsnano.5b05419) overcome these flaws,
reporting the development of a spaser with unidirectional, tunable lasing from templatestripped 2D plasmonic crystals. The researchers formed this spaser by creating Si templates from aluminum hole arrays on Si wafers. They then deposited thick Au films on this template and stripped the film, which was nanopatterned with an array of cylindrical posts, using polyurethane. The researchers coated these Au plasmonic crystals with IR-140 dye molecules to act as the gain media. After pumping with a pulsed laser, the investigators found that these spasers exhibited lasing in a single emission direction with wavelengths tunable by modulating the dielectric environment. By varying the nanocavity unit-cell structure, metal material, and
gain media, the researchers were able to modulate and customize the lasing response. The authors suggest that this templatestripping technique to create 2D plasmonic crystals offers an ideal way to screen material combinations for new classes of plasmon nanolasers.
In a new study, Lee et al. (DOI: 10.1021/ acsnano.5b06081) developed a switchable surface that acts as a reversibly switchable host for guest molecules. The researchers started with a low-density supramolecular network of 1,3,5-tris(4-carboxyphenyl)benzene (BTB) on highly oriented pyrolytic graphene in a solution containing either coronene (COR) or nanographene (NG). When the polarity of the voltage applied to the substrate with a scanning tunneling microscope tip was switched from positive to negative, the BTB underwent a structural transition from a densely packed linear network to hexagonal symmetry, allowing it to accept molecules of COR or NG. Reversing polarity switched the conformation back again, squeezing out these guest molecules. Temperature could
also induce reversible switching, but this effect was global, unlike the local switching observed with voltage. The authors suggest that physisorbed surfaces with host guest chemistry like this one could be used in numerous potential applications.
Switchable Surface Makes Gracious Host 9 Over the past decade, research has increasingly focused on creating supramolecular surfaces that are able to undergo reversible structural and functional changes in response to external stimuli. Thus far, researchers have developed a variety of dynamic, reversibly switchable surfaces. However, many of these smart surfaces are composed of chemisorbed molecules, which often do not have sufficient space between them to allow for efficient conformational transitions. Though physisorbed monolayers of organic materials offer advantages to get around this bottleneck, the majority of those studied thus far involve a single component adsorbed onto a solid surface rather than reversible switching in multicomponent supramolecular networks.
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9 Hybrid organic inorganic halide perovskites have attracted increasing attention recently due to the incredible boost in efficiency these materials have given to solar cells, increasing the solar-to-electricity conversion from 3.8% to 20.1% in only five years. Before their role as light harvesters was discovered, these materials were popular research subjects in their two-dimensional form for applications in optoelectronics and microelectronics. Though they had been studied in this capacity for decades, only recently have researchers discovered the large spin orbit coupling of the most frequently used metals predicted from density functional theory (DFT). These findings
have led to further prediction of Rashba or Dresselhaus spin splitting, or both, in these hybrid systems. These effects originally corresponded to spin splitting in zincblende (Rashba) and wurtzite (Dresselhaus) structures, and suggest a promising avenue for spin-based applications. Exploring this possibility, Kepenekian et al. (DOI: 10.1021/acsnano.5b04409) conducted a survey of these effects on two- and three-dimensional hybrid halide perovskites using symmetry analysis and DFT. The researchers find that low-dimensional nanostructures made of Ch3NH3PbX3 (X = I, Br) lead to spin splitting that can be controlled by an applied electrical field. This splitting
can be monitored with temperature as a result of the continuous polarization of the crystal structure from high to low temperatures. The authors suggest that these findings pave the way for perovskite-based spintronics.
IN NANO
Rashba and Dresselhaus Effects from All Angles
Room To Grow Beneath h-BN 9 Research on two-dimensional (2D) materials has flourished over the past decade due to the rise of graphene and other 2D atomic crystals, including hexagonal boron nitride (h-BN) and MoS2. Besides single-component atomic crystals, many studies have focused on 2D heterostructures formed by stacking different kinds of 2D atomic crystals. These layered materials, stabilized by van der Waals forces, often have interesting properties that deviate significantly from those of their components. Two major routes have been used thus far to create these heterostructures: physically placing one layer on top of another, or chemically growing one layer on top of another. In a new study, Yang et al. (DOI: 10.1021/ acsnano.5b05509) discuss a third route to creating 2D layered heterostructures: growing a new layer underneath an existing one. The researchers followed growth of h-BN
islands on Ni(111) surfaces using in situ lowenergy electron microscopy (LEEM). Each of the resulting islands displayed one of two LEEM contrasts, which they found corresponded to growth that was either epitaxial or nonepitaxial to Ni(111). Density functional theory calculations, X-ray photoelectron spectroscopy, and I V curve results suggest that the nonepitaxial islands interacted with the Ni(111) surface more weakly than the epitaxial islands. Further investigation showed that many of the nonepitaxial
islands formed heterostructures during growth due to this weak interaction leaving a 2D space beneath an initial layer that allowed another to grow underneath. The researchers capitalized on this architecture by purposely growing graphene beneath nonepitaxial h-BN islands. The authors suggest that this confined growth method could lead to a new route for synthesizing stacks of 2D atomic crystals.
Bright Future for Carbon Nanotube Optoelectronics 9 Carbon nanotubes (CNTs) have been an intense focus of research efforts for more than two decades, but despite their popularity as study subjects, relatively little work has explored the potential of using CNTs as the basis for optoelectronic devices. Several groups have taken photocurrent spectra of suspended semiconducting nanotubes, reporting excitonic transitions with phonon sidebands, quantum efficiency measurements, polarization dependence, and photothermal effects. Other efforts reported photocurrent spectra of suspended quasimetallic CNT p n junctions, using nominally metallic nanotubes that have finite band gaps. Despite these previous efforts, no studies have compared the field dependence of exciton photocurrent spectra of quasimetallic and semiconducting CNT p n junction devices.
In a new study, Chang et al. (DOI: 10.1021/ acsnano.5b03873) perform this comparison in suspended CNT p n junctions introduced by electrostatic gating. The researchers found that, although the built-in fields of the quasi-metallic CNTs are 1 2 orders of magnitude smaller than those of the semiconducting CNTs, their photocurrent is 2 orders of magnitude higher than corresponding semiconducting devices under the same experimental conditions. The underlying mechanism behind these findings appears to be a large exciton binding energy in semiconducting nanotubes, which makes it difficult for excitons to dissociate into free carriers that can contribute to photocurrents without the aid of a phonon. The significantly lower exciton binding energies of quasi-metallic CNTs and a continuum of electronic states into which the can decay VOL. 9
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make it easier for their excitons to produce a photocurrent. The authors suggest that these results move CNTs closer to optoelectronic applications.
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