F A L L 2 0 1 1 WWW.NIST.GOV/CNST The CNST News ExtractingExtracting PhotonsPhotons fromfrom SINGLE QUANTUM DOTS Catalyzing Carbon Nanotube Growth with Gold Carving Diamond into Tougher Micromachines High Contrast Nanoplasmonic Optical Switch Better Nanoparticles for Diagnosing Disease Flower-Shaped Graphene Defects Summer Students Gain Research Experience P A G E 2 F A L L 2 0 1 1 WWW.NIST.GOV/CNST IN THIS ISSUE Charge Transport 2 From the Director As you may be aware, the CNST user facility advances the development of nanotechnology in two distinct in Solar Cells ways. While the NanoFab provides economical access to state-of-the-art commercial nanotechnology Nanoplasmonic 3 tools, our collaborative research staff are dedicated to advancing the state of the art in nanoscale measure- Optical Switch ment and fabrication. This month, I would like to highlight the wide impact of one of our collaborative re- search projects. Catalyzing of 3 There is a world-wide search underway for devices to replace the CMOS switch that is the foundation of Nanostructure current nanoelectronics. Graphene electronic devices are widely considered to be among the front runners, Growth with Gold but progress requires new ways to make and measure circuit elements based on graphene sheets that are Carving Holes in 4 only one atom-thick. Diamond Following the development at Georgia Tech of methods to grow high-quality graphene on silicon carbide, Phase Segrega- 4 Professors de Heer and First sought new ways to characterize the electronic structure of the graphene tion in Organic layers produced and, in particular, correlate it with atomic scale defects. They asked for the help of CNST’s Photovoltaics Joseph Stroscio, who has built two of the world's most advanced ultra-high vacuum scanning tunneling mi- croscope (STM) systems. A collaboration began with a Georgia Tech graduate student bringing the gra- Better Nanoparti- 5 phene samples and growth technology to the CNST, and continued through multiple graduate students and cles for Flow Im- postdocs who performed a highly productive series of collaborative measurements on our STM instru- munoassays ments. Flower-Shaped 5 The results of this five-year collaboration to advance graphene fabrication and measurement technologies Defects in Gra- have been manifold: three Georgia Tech students performed their doctoral research at the CNST; four phene postdoctoral researchers were trained; 14 papers were published, including influential publications in Sci- ence, Nature, and Nature Physics; new commercial instruments were spun off; and, most recently, a postdoc Extracting Pho- 6 left the CNST for Intel. (You can see a highlight from his work on page 5 of this issue.) tons from Quan- tum Dots This example is but one of dozens of collaborative projects underway with CNST researchers. These pro- jects start when someone has a problem not addressed by current nanoscale measurement and fabrication Srinivasan wins 6 technology. The first step in solving such a problem is to contact the Sigma Xi Award most relevant CNST Project Leader (found via our website) to dis- New NanoFab 7 cuss what might be possible. If it is not clear where your solution Tools may lie, contact me and together we will work to find the answer. Summer Stu- 7 –Robert Celotta dents at the CNST Nanoscale Charge Transport in Bulk Heterojunction Solar Cells Researchers in the acceptors. When illuminated by produce electricity. The efficien- CNST have used sunlight, the photoexcited elec- cy is strongly dependent on the photoconductive tron-hole pairs separate at the material morphology, making atomic force mi- interface between the donors measurements that correlate croscopy (PCAFM) and acceptors. The separated nanoscale structure with perfor- to characterize the charges migrate to different con- mance crucial to understanding nanoscale structure tacts, generating an electrical and improving OPVs. Because of organic photo- current. The most efficient OPV PCAFM is now widely used to voltaic (OPV) mate- materials have a homogeneous characterize OPV materials, the rials and have per- mixture of donor and acceptor CNST researchers expect their formed a careful molecules throughout the entire assessment of this measurement A PCAFM map assessment of the strengths and structure, with charge separation technique to be important to shows local hot weaknesses of this technique. By occurring throughout the entire other researchers in the field, spots of photore- varying the device geometry and volume. Unfortunately, the pho- who must consider the tech- sponse correspond- the AFM tip material, the re- toexcited charge must pass nique’s strengths and pitfalls. ing to photogener- searchers clarified how local through a highly disordered envi- Imaging of nanoscale charge transport in nanoscale experimental and ma- ronment, which inhibits their bulk heterojunction solar cells, B. H. ated electron cur- terial factors affect the overall mobility, increases recombina- Hamadani, N. Gergel-Hackett, P. M. rent. Field of view Haney, and N. B. Zhitenev, Journal of OPV efficiency. OPVs consist of tion, decreases efficiency, and Applied Physics 109, 124501 (2011). is 3.7 µm; height two types of organic molecules, hampers the material’s ability to range is 37.3 nm. electron donors and electron F A L L 2 0 1 1 P A G E 3 Nanoplasmonic Optical Switch Has High Contrast and Low Voltage In a recent article in Nano Letters, tals of the electrochromic dye Prus- material in the CNST researchers describe a new sian Blue inside a gold nanoslit slit (≈ 25 %), high-contrast, low operating- waveguide, where light propagates the switch voltage, electrochemical optical as a surface plasmon polariton operates effi- switch that uses a volume of active (SPP). SPPs are collective charge ciently. Be- dye orders of magnitude smaller oscillations coupled to an external cause the light than that of conventional electro- electromagnetic field that propagate propagates in a chromic devices. Electrochromism along an interface between a metal direction per- refers to a reversible change in the and a dielectric. The dye nanocrys- pendicular to optical absorption of a material tals, deposited on the sidewalls of the direction under an applied voltage. Inorganic the slit by cyclic voltammetry, can of the charge and organic electrochromic materi- be electrochemically switched to transport, the als are used in displays, smart win- provide a transmission change of new switch High resolution scanning electron micrograph of dows, and car rearview mirrors. A ≈ 96 % (in the red) using control design offers two adjacent slits. Low fill fraction (≈ 25 %) and change in light absorption in such a voltages less than one volt. The significant preferential deposition of Prussian Blue nanocrystals material is caused by a change in high switching contrast is enabled promise for on the slit sidewalls are clearly evident. the oxidation state, and requires by the strong spatial overlap be- creating elec- that both ions and electrons diffuse tween the SPPs and the nanocrys- trochromic devices with record- through the material. Decreasing tals confined within the slit. The setting switching speeds. the material’s thickness reduces the contrast is also enhanced by the An integrated electrochromic nanoplasmonic diffusion time, making the electro- unexpectedly high absorption coef- optical switch, A. Agrawal, C. Susut, G. Staf- chromic switch faster, but unfortu- ficient of Prussian Blue nanocrystals ford, U. Bertocci, B. McMorran, H. J. Lezec, nately also reduces the contrast. grown on a gold surface compared and A. A. Talin, Nano Letters 11, 2774-2778 (2011). The NIST and University of Mary- with bulk material. Even with a land researchers have grown crys- relatively low fill fraction of active Catalyzing Carbon Nanotube Growth? Try Some Gold Researchers from the CNST and limitation affects the ultimate densi- sion of car- Arizona State University have ty and placement of the nanostruc- bon in doped demonstrated that the overall cata- tures, an important factor for Ni to 0.07 eV lytic activity of nickel particles for nanofabrication applications. compared to the formation of carbon nanostruc- Using high-resolution images and 1.62 eV for tures is improved by the addition of spectroscopy data collected during pure Ni. a small amount of gold (below 0.2 and after the synthesis, the re- The research- mol fraction). In a recent Nano searchers showed that most of the ers are ex- Letters article, the researchers eval- Au segregates to form an inactive tending this uate Au/SiO2, Ni/SiO2, and Au-Ni/ Au-rich cap, with only a small technique to SiO2 nanoparticles as catalysts for amount of Au present in the active evaluate the carbon nanotube (CNT) and car- region of the particles. They also role of metal bon nanofiber (CNF) formation by showed that the structure of Ni carbide for- measuring the number of particles catalyst particles transforms from mation in the active during tube formation using fcc metal to orthorhombic nickel activity of in situ dynamic imaging in an envi- other metal carbide (Ni3C). They believe that A color overlay of dark-field STEM images from 0.2 ronmental scanning transmission catalysts used carbides form due to the dynamic mol fraction of Au in a Ni sample emphasizes the electron microscope (STEM). Car- equilibrium conditions present for carbon spatial extent of each component (red = Ni-rich, bon nanostructures are generally under these reaction conditions. nanotube syn- green = Au-rich, blue = C-rich). synthesized by catalytic chemical Density functional theory calcula- thesis, such as vapor deposition from carbon tions support the hypothesis that Fe and Co. sources such as acetylene (C2H2) low levels of Au doping (0.06 mol Evaluation of the role of Au in improving and nucleate from catalyst particles, fraction) increases the number of catalytic activity of Ni nanoparticles for the including Ni. However, only some formation of one-dimensional carbon particles active for carbon nanostructures, R.