Nanostructures. Lecture 13 OUTLINE

Nanostructures MTX9100 Nanomaterials Lecture 13 OUTLINE -What is quantum confinement? - How can zero-dimensional materials be used? -What are one dimensional structures? -Why does graphene attract so much attention? 1

Zero-dimensional materials Nanocrystals absorb light then reemit the light in a different color the size of the nanocrystal (at the Angstrom scale) determines the color. Six different quantum dot solutions are shown excited with a long wave UV lamp. 3 A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules. The term "Quantum Dot" was coined by Mark Reed at Texas Instruments.

Quantum confinement in semiconductors In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length called the Bohr exciton radius. If the electron and hole are constrained further, then the semiconductor's properties change. This effect is a form of quantum confinement, and it is a key feature in many emerging electronic structures. Specifically, the effect describes the phenomenon results from electrons and electron holes being squeezed into a dimension that approaches a critical quantum measurement. 4 An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor, and such is a Coulomb-correlated electron-hole pair.

Quantum confinement For 0-D nanomaterials, where all the dimensions are at the nanoscale, an electron is confined in 3-D space. Quantum confinement is responsible for the increase of energy difference between energy states and band gap. A phenomenon tightly related with the optical and electronic properties of the materials. 5 No electron delocalization (freedom to move) occurs.

Fundamentals of light Light is a form of electromagnetic (interacting electric and magnetic fields) radiant energy that propagates through space in a way that can be characterized as waves. Electromagnetic radiation can be defined in terms of its wavelength or frequency (reciprocals of one another). The electromagnetic spectrum is very large ranging, from wavelengths at the kilometer scale to those near the atomic scale. 6

Solid-state lighting The direct conversion of electricity to light using semiconductor materials (normally in the form of light emitted diodes - LEDs) LEDs fundamentally produce light via a special form of electroluminescence. One of the fundamental problems with LEDs that of narrow sets of emission frequencies that limit their uses is solvable via quantum dot technologies. 7 A new generation of solid-state lighting quantum light-emitting diodes (QLEDs) made of quantum dot networks are coming into use that would work similarly to traditional LEDs but have greatly improved functionalities and new uses.

Quantum dots Quantum dots offer great potential in the form of QLEDs which are made out of networks of quantum dots and can also build on, yet dramatically improve, existing LED technologies. Quantum dots are essentially nanometer-size crystals of semiconductor materials (e.g., silicon or germanium) for which the electronic properties are strongly dependent on their size. Wavelengths in quantum dots can be controlled in nanocrystalline materials. The energy separation between valence and conduction bands can be altered in nanocrystalline quantum dots by changing the size of the nanoparticles. Resulting energy levels can thus be varied. 8

Making quantum dots quantum dot of gallium arsenide There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. The total electron charge density (shown in green) of a quantum dot of gallium arsenide, containing just 465 atoms. (Image: Lin-Wang Wang) In general, quantum wires, wells and dots are grown by advanced epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by stateof-the-art lithographic techniques 9 www.lbl.gov

Applications Quantum dots are particularly significant for optical applications due to their theoretically high quantum yield. a measure of the efficiency with which absorbed light produces some effect. the probability that a given quantum state is formed from the system initially prepared in some other quantum state In electronic applications they have been proven to operate like a single-electron transistor and show the Coulomb blockade effect. Quantum dots have also been suggested as implementations of qubits for quantum information processing. the increased resistance at small bias voltages of an electronic device comprising at least one lowcapacitance tunnel junction. concerns information science that depends on quantum effects in physics 10

Quantum Dot infrared photodetectors AFM micrographs of: a 1 μm x 1 μm surface imaging of InAs quantum dots on GaAs/InP, (inset) a single InAs quantum dot. Researchers have demonstrated the self-assembled growth of In(Ga)As quantum dots in both InP-based and GaAs based systems using metalorganic chemical vapor deposition. 11 Such QDIPs have many advantages compared to the conventional quantum well infrared photodetector (QWIP), including: higher responsively, higher temperature operation, higher light coupling to normal incidence light, and capability of narrow band te nability.

The smallest nanostructures Nanotubes (1 - D): Long, cylindrical tubes of carbon formed by a catalytic growth process. Nanometer-scale drop of molten iron is typical catalyst. Can behave like a conductive metal wire or like a semiconductor Quantum dots (0 D): Crystals containing only a few hundred atoms Electrons are confined to widely separated energy levels -> dot emits one wavelength of light when excited Size of the dot determines electronic, magnetic, and optical properties Used as biological markers (illuminating sample with ultraviolet light crystals will fluoresce at a specific wavelength) 12

13 Carbon structures - publishing

One-dimensional nanomaterials TEM images of ZnO nanorods synthesized at different temperatures: 180C (a, b); room temperature (c); insets indicate crystal orientations. A photoluminescence spectrum of the room-temperature synthesized ZnO nanorods is displayed in (d). 14

Nanostructures Solution synthesis of copper nanowires: (a) an as-prepared Cu nanowire suspension, (b) FESEM image of Cu nanowires, (c) TEM image of a Cu nanowire, and (d) SAED pattern of the nanowire in (c). 15

Nanowire nanosensors for biology Central to detection is the signal transduction associated with selective recognition of a biological or chemical species of interest. The diameters of the nanostructures are comparable to the sizes of biological and chemical species being sensed, and thus intuitively represent excellent primary transducers for producing signals that ultimately interface with macroscopic instruments. The size-tunable colors of semiconductor nanocrystals, together with their highly robust emission properties, are opening up opportunities for labelling and optical-based detection of biological species that offer advantages compared with conventional organic molecular dyes widely used today. 16

Nanowire field-effect sensors The underlying mechanism for nanowire sensors is a field effect that is transduced using fieldeffect transistors (FETs), the ubiquitous switches of the microelectronics industry. The electronic characteristics of nanowires are well controlled during growth in contrast to carbon nanotubes Si nanowires can be prepared as single-crystal structures with diameters as small as 2-3 nm a single Si nanowire sensor device 17

Graphene The atomic structure of isolated, single-layer graphene: hexagonal lattice; high crystal quality 20

What is graphene? Band structure of graphene. The conductance band touches the valence band at the K and K points. charged massless particles (Dirac fermions) Charge carriers can travel thousands of interatomic distances without scattering Intrinsic graphene is a semi-metal or zero-gap semiconductor. 21

22 Graphite - Graphene Graphene is one-atom-thick planar sheet of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. graphite itself consists of many graphene sheets stacked together The carbon-carbon bond length in graphene is about 0.142 nm. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes.

23 SCIENCE, June 2010 If there's a rock star in the world of materials, it's graphene: single-atom thick sheets of carbon prized for its off-the-charts ability to conduct electrons and for being all but transparent. Those qualities make graphene a tantalizing alternative for use as a transparent conductor, the sort now found in everything from computer displays and flat panel TVs to ATM touch screens and solar cells. But the material has been tough to manufacture in anything larger than flakes a few centimeters across. Now researchers have managed to create rectangular sheets of graphene 76 centimeters in the diagonal direction and even use them to create a working touchscreen display

Unique nature of charge carriers In condensed matter physics, the Schrodinger equation rules the world, usually being quite sufficient to describe electronic properties of materials. Graphene is an exception its charge carriers mimic relativistic particles and are more easily and naturally described starting with the Dirac equation rather than the Schrodinger equation. 24

Dirac fermions Although there is nothing particularly relativistic about electrons moving around carbon atoms, their interaction with the periodic potential of graphene s honeycomb lattice gives rise to new quasiparticles that at low energies E are accurately described by the Dirac equation with an effective speed of light. These quasi-particles, called massless Dirac fermions, can be seen as electrons that have lost their rest mass m or as neutrinos that acquired the electron charge e. The relativistic like description of electron waves on honeycomb lattices has been known theoretically for many years, never failing to attract attention, and the experimental discovery of graphene now provides a way to probe quantum electrodynamics (QED) phenomena by measuring graphene s electronic properties. 25

What kind of uses does graphene have? Graphene can be used to make excellent transistors. It is so thin we can easily control whether or not it conducts by applying an electric field. We would like to be able to do this with metals, but we cannot make metal films thin enough to affect their conducting state in this way. Electrons in graphene also travel ballistically over submicron distances. As a result, graphene-based transistors can run at higher frequencies and more efficiently than the silicon transistors we use now. At the present moment we have no way to produce entire integrated circuits from these transistors since we are limited by the size of graphenes we can produce. 26

Gas Sensors Gas molecules that land on graphene affect its electronic properties in a measurable way - in fact, we have measured the effect of a single molecule associating with a graphene. This means that we can create gas sensors which are sensitive to a single atom or molecule! 27

Example: how to make sensors bottom up 28

Thermal conductivity of thin CN films Thin films with oriented nanotube directions can have remarkably high thermal conductivities 29 M. Ashby, P. Ferreira, D. Schodek; Nanomaterials, Nanotechnologies and Design; Copyright 2009 Elsevier Ltd.

30 3-D nanomaterials

Opportunities for using nanomaterials and nanotechnologies in automobiles 31