Tiny Structures, Big Ideas and Grey Goo
Professor Chris Binns
Head of the condensed matter physics and professor of Nanoscale Physics (Nanoclusters Group) at the University of Leicester
The Nanoworld contains all structures with a size up to 100nm. In the above picture there is an image of a 13 atom cluster but this is completely artificial.
Nanotechnology breaks down the barriers between biology, chemistry, physics and engineering.
There are three levels of nanotechnology:
Incremental nanotechnology – the special properties of nanoparticles are used to produce new materials. This is not a new technology as our ancient ancestors used it to colour ceramics.
Evolutionary nanotechnology – this involves changing nanoparticles so that they perform individual tasks. Gold nanoparticles are used to form single transistors and research is being carried out on combining them with biological molecules to form a “magic bullet” for the treatment of cancer.
Radical nanotechnology – This involves using nanoscale components to build machines like self-replicating “nanobots”.
The atomic theory
The first people to come up with some form of atomic theory were the ancient Greeks. They believed that there must be a difference between matter and space. They produced a thought experiment that cut matter up into smaller and smaller pieces until it was cut out of existence. They knew this was not possible so they called the smallest particle of matter the atom. The two most famous ancient Greek atomists were Leuccipus and Democritus.
Picture of Leuccipus of Miletus 480 BC – 420 BC (?) on the left and Democritus (Democritus) of Abdera 460 BC – 370 BC on the right.
1. Matter is composed of atoms separated by empty space through which the atoms move. 2. Atoms are solid, homogeneous, indivisible, and unchangeable. 3. All apparent changes in matter result from changes in the groupings of atoms. 4. There are different kinds of atoms that differ in size and shape. 5. The properties of matter reflect the properties of the atoms the matter contains.
Unfortunately their model attracted few supporters among later generations of Greek philosophers. Aristotle, in particular, refused to accept the idea that the natural world could be reduced to a random assortment of atoms moving through a vacuum. As Aristotle was so revered (even by the Catholic Church) it took until the seventeenth century to start moving forward again.
John Dalton FRS (6 September 1766 – 27 July 1844) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory.
Five main points of his atomic theory: 1. Elements are made of extremely small particles called atoms. 2. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. 3. Atoms cannot be subdivided, created, or destroyed. 4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged.
Magnetic materials are magnetic because they contain domains but not all of them are actual magnets because these domains are arranged randomly. A magnetic material can be made magnetic simply by bringing a magnet close to it and the domains line up to make it magnetic too. It is widely known that iron is a magnetic material, but in fact a piece of pure (or “soft”) iron is not magnetized. This is easy to prove by taking a piece of soft Fe and seeing that it does not attract a ball bearing. In contrast, a permanent magnet, which is an alloy, such as neodymium–iron–boron that is permanently magnetized, strongly attracts the ball bearing. A simple and illustrative experiment is to sandwich the ball bearing between the permanent magnet and piece of soft iron and then pull the magnet and the pure iron apart. Oddly, while the ball bearing shows no attraction to the soft iron on its own, in the presence of the magnet it stays glued firmly to the piece of soft iron as it is pulled away, showing that it is magnetized to a greater degree than the actual magnet. Beyond a certain distance from the magnet, the soft iron reverts to its demagnetized state and the ball bearing comes loose.
Lining up the domains also has the effect of making the magnetic field stronger with a greater internal energy.
This energy is reduced by making the material smaller and smaller until you have a single-domain particle. Below a critical size (approx. 100 nm), the energy balance favours just a single domain and the piece of iron stays permanently and fully magnetized.
The Onset of Novel Behaviour (Single-Domain Particles)
Single domain, in magnetism, refers to the state of a ferromagnet in which the magnetization does not vary across the magnet. A magnetic particle that stays in a single domain state for all magnetic fields is called a single domain particle (but other definitions are possible). Such particles are very small (generally below a micrometre in diameter). They are also very important in a lot of applications because they have a high coercivity (the intensity of the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero after it has reached saturation). They are the main source of hardness in hard magnets, the carriers of magnetic memory in tape drives, and the best recorders of the ancient Earth’s magnetic field.
Magnetotactic bacteria (or MTB) are a polyphyletic group of bacteria discovered by Richard P. Blakemore in 1975 that orient along the magnetic field lines of Earth’s magnetic field. To perform this task, these bacteria have organelles called magnetosomes that contain magnetic crystals. They are small enough to form a permanent magnet.
Magnetic Bacterium (magnetospirillum gryphiswaldense) found in river sediments in Northern Germany. The lines of (permanently magnetized) single-domain magnetic nanoparticles, appearing as dark dots, align the body of the bacterium along the local direction of the Earth’s magnetic field, which in Germany is inclined at 55 degrees from horizontal. This means that the bacterium will always swim downwards towards the sediments where it feeds.
Similar features are found in Martian meteorites. These can only be formed by biological processes so it is may be evidence for life on Mars.
Size Matters! (Magnetic clusters)
The magnetic moment of the magnetic material increases as the number of atoms in it decreases (below 100 – 50nm). The magnetism per atom increases and this increase is rapid as the size is reduced below 3nm. It is an example of a size dependent property. A 2nm cluster has half of its atoms on its surface.
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the force (torque) that a magnetic field will exert on it.
Magnetic moments give the periodic table and extra dimension.
The magnetic moment in free nanoparticles can be measured by passing a beam of them through a non-uniform magnetic field and measuring the deflection in their path.
The above graphs show how the magnetic moments of various materials changes with the size of the clusters of nanoparticles.
Lumps (Leicester University Mesoscopic Particle Source)
The apparatus in the above left picture was designed and built at Leicester University as a gas aggregation cluster source which produces high fluxes of magnetic nanoclusters (metal nano particles) such as Fe or Co under UHV conditions. Size selection is achieved via an axially mounted quadrupole filter, also built at Leicester. The above middle picture shows Manganese nanoparticles on bucky balls and the above picture on the right shows Iron nanoparticles on silicon.
Enhanced Magnetism in Free Clusters
Can enhanced magnetism be built into materials?
In the picture below MBE stands for molecular beam epitaxy.
The Slater-Pauling curve describes that the average magnetic moment of an Fex−x alloy increases with increasing Fe concentration starting from x = 0.
Cluster assembled materials is a bit like producing a nano “onion”. The granules and cluster sizes need to be controlled to make them miscible and the nanoparticles give greater magnetism. A granular material with nanoscale grains can be produced by co-depositing Pre-formed nanoparticles with an atomic vapour from a conventional evaporator. The nanoparticles are embedded in a matrix produced by the atomic vapour to form a granular material in which there is independent control over the volume fraction and grain size.
High-Moment Films – definitely a non-trivial problem
Unfortunately rare earth magnets have to be cooled to very low temperatures to be ferromagnetic and hard. This may be improved by embedding transmission nanoparticles into them.
Photovoltaic Materials based on Surface Plasmon Resonance
Surface plasmon resonance (SPR) is the collective oscillation of electrons in a solid or liquid stimulated by incident light. The resonance condition is established when the frequency of light photons matches the natural frequency of surface electrons oscillating against the restoring force of positive nuclei. SPR in nanometer-sized structures is called localized surface plasmon resonance.
The photon has an electric field.
The solar cell produces a photocurrent. The ENSOL system is better than the silicon cell and could be 100% efficient. Another major advantage is that it can actually be painted on surfaces and as it has a transparent insulation layer it could even coat glass to produce a current.
The graph above right show that the smaller the grain size the harder the material. This means that making a material into a nano-structure makes it stronger.
Metal-Organic Frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable during elimination of the guest molecules (often solvents) and can be used for the storage of gases such as hydrogen. Hydrogen is much safer inside a porous material.
Ultra-High Density Magnetic Storage
The bottom left picture shows the process of putting data onto the track. The bottom right picture shows what is required by 2016. Storage on individual magnetic nanoclusters would produce a storage density 1000x existing devices (the length of section 1 is 100nm).
The top left picture shows magnetic data storage on a hard disc. Increasing the density and store information on one nanoparticle will be the equivalent of 1000 existing devices. There is, however, a big problem. Magnetisation in such small clusters isn’t stable at room temperature. The top right picture shows the solution is to coat the surface with a shell of antiferromagnetic material.
Ultimate Storage Density
The above right picture shows a chemically produced array of 6nm diameter FePt nanoparticles. This alloy may bring us closer to the ultimate storage device. Taking us from 1Tb to 1000 Tb discs.
Scanning Probe Read/Write
Molecular scale electronics, also called single molecule electronics, is a branch of nanotechnology that uses single molecules, or nanoscale collections of single molecules, as electronic components. Each component has a conductive or insulator unit. Because single molecules constitute the smallest stable structures imaginable this miniaturization is the ultimate goal for shrinking electrical circuits.
Magnetic Nanoparticle Hyperthermia
Magnetic hyperthermia is the name given to an experimental cancer treatment, although it has also been investigated for the treatment of other ailments, such as bacterial infections. It is based on the fact that magnetic nanoparticles, when subjected to an alternating magnetic field, produce heat. As a consequence, if magnetic nanoparticles are put inside a tumour and the whole patient is placed in an alternating magnetic field of well-chosen amplitude and frequency, the tumour temperature would rise. This treatment method has entered phase II trials (in humans) only in Europe, but research is done in several laboratories around the world to test and develop this technique further.
This will be a gentle, symptom-free treatment based on ‘magic bullet’ approach. It will be generic and able to treat complex fragmented cancers (e.g. liver). It will be a hunter/seeker for the cancer cell. The localised heat should cause a temperature increase of about 5 degrees which is enough to shut down the cancer cell.
Unfortunately it doesn’t quite work at the moment. The body needs to be able to get rid of the particles after a few minutes and the immune system needs protection as metallic iron is toxic. A biocompatible shell needs to be developed.
Scanning Tunnelling Microscope
A scanning tunnelling microscope (STM) is an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer (at IBM Zürich), the Nobel Prize in Physics in 1986. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius.
Atomic Force Microscope
Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometre, more than 1000 times better than the optical diffraction limit. It is an improvement on the scanning tunnelling microscope.
Moving Atoms by STM
The microscope works by moving a sharp metal needle. The tip of the needle is both the eyes and our hands. It senses the atoms to make images of where the atoms are located, and then it moves closer to the atoms to tug them along the surface of a copper sheet to new positions.
Natural building blocks
Buckminsterfullerene (or buckyball) is a spherical fullerene molecule with the formula C60. It has a cage-like fused-ring structure (Truncated icosahedron) which resembles a soccer ball, made of twenty hexagons and twelve pentagons, with a carbon atom at each vertex of each polygon and a bond along each polygon edge.
A molecular machine, or nanomachine, is any discrete number of molecular components that produce quasi-mechanical movements (output) in response to specific stimuli (input). The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level. The term is also common in nanotechnology where a number of highly complex molecular machines have been proposed that are aimed at the goal of constructing a molecular assembler. Molecular machines can be divided into two broad categories; synthetic and biological.
Charles Babbage and his computing engines.
Charles Babbage, FRS (26 December 1791 – 18 October 1871) was an English polymath. He was a mathematician, philosopher, inventor and mechanical engineer, who is best remembered now for originating the concept of a programmable computer.
Mechanical devices which provide functions similar to electronic digital systems are increasingly being sought for a variety of applications specifically where the operating environment is hazardous or under intense radiation.
Synthetic molecular motors are molecular machines capable of rotation under energy input. Although the term “molecular motor” has traditionally referred to a naturally occurring protein that induces motion (via protein dynamics), some groups also use the term when referring to non-biological, non-peptide synthetic motors. Many chemists are pursuing the synthesis of such molecular motors. The prospect of synthetic molecular motors was first raised by the nanotechnology pioneer Richard Feynman in 1959 in his talk “There’s Plenty of Room at the Bottom”.
A virus can be considered a natural nanobot.
Heat is a problem with these nanobots as proteins become de-natured if the temperature gets too high.
The Big Question
Hendrik Brugt Gerhard Casimir FRS (July 15, 1909 – May 4, 2000) was a Dutch physicist best known for his research on the two-fluid model of superconductors (together with C. J. Gorter) in 1934 and the Casimir effect (together with D. Polder) in 1948.
Zero-point energy, also called quantum vacuum zero-point energy, is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature. The uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well. This results in motion even at absolute zero. For example, liquid helium does not freeze under atmospheric pressure at any temperature because of its zero-point energy.
Energy of quantised E-M field
Zero-point energy of E-M field
The Casimir Effect
In quantum field theory, the Casimir effect and the Casimir–Polder force are physical forces arising from a quantized field. They are named after the Dutch physicist Hendrik Casimir.
The magnitude of the force
Dutch physicists Hendrik B. G. Casimir and Dirk Polder at Philips Research Labs proposed the existence of a force between two polarizable atoms and between such an atom and a conducting plate in 1947, and, after a conversation with Niels Bohr who suggested it had something to do with zero-point energy, Casimir alone formulated the theory predicting a force between neutral conducting plates in 1948; the former is called the Casimir–Polder force while the latter is the Casimir effect in the narrow sense.
Contactless Transmission in Nano-Machines
F. Chen, U. Mohideen G. L. Klimchitskaya and V. M. Mostepanenko, Phys. Rev. Lett 88 (2002) 101801
20µm Au sphere on AFM cantilever
NANOCASE was an EU funded project to study one of the most fundamental forces in the universe: The Casimir force.
A physicist in France claims that the Casimir force between two neutral surfaces could be exploited to create tiny ratchets that could someday drive machines built at the micrometre scale.
Repulsive Casimir Force
There are few instances wherein the Casimir effect can give rise to repulsive forces between uncharged objects. Evgeny Lifshitz showed (theoretically) that in certain circumstances (most commonly involving liquids), repulsive forces can arise. This has sparked interest in applications of the Casimir effect toward the development of levitating devices. An experimental demonstration of the Casimir-based repulsion predicted by Lifshitz was recently carried out by Munday et al. Other scientists have also suggested the use of gain media to achieve a similar levitation effect, though this is controversial because these materials seem to violate fundamental causality constraints and the requirement of thermodynamic equilibrium (Kramers-Kronig relations). Casimir and Casimir-Polder repulsion can in fact occur for sufficiently anisotropic electrical bodies; for a review of the issues involved with repulsion see Milton et al.