Erasmus : The dream of nanotechnology involves the manipulation and control of matter at nanoscale size ranges. Essentially, we can build or engineer anything directly from the building blocks of matter.
We can also change or influence any material object via tools of our intelligence. By being able to manipulate individual atoms and molecules of matter, unique molecules and structures can be developed.
The mechanisms for manipulating this matter are incredibly small. Nano factories that undertake these tasks may be available to everyone and can be the servant of every individual.
Kinkajou : Incredible claims are made for nano manufacturing. Improved efficiency, reduced pollution, lower costs and synthetic functions / factory production completely at the whim of the individual, not the group.
Erasmus : This sounds good. However, many of these claims are made on the basis of “dreams” of the future, not based on the realities of our current knowledge. For instance, the existing competition for mechanical nano engineering is incredible.
To be a viable alternative to the technology we have, nanotechnology needs to be able to do things cheaper or better or faster or to be able to do things that currently cannot be done. To improve on some of our current abilities may well be something that is hundreds of years in the making, if ever to be possible at all.
Currently, we are able to derive power from individual molecules, control the conversion of atoms into many much more complicated molecules, all at incredible power efficiencies and with perfect molecular recycling.
Enzymatic Power Supplies
Kinkajou : And you’re probably going to tell me that these things occur in factories that can self-replicate as well.
Erasmus : Yes. The existing technology that can manipulate matter in this way is called biotechnology. Biotechnology is packaged within cells. Cells are established factories of nanotechnology incorporating their own control systems and self replication mechanisms.
Much cell chemistry is focused on nanoscale technologies and nanoscale chemical reactions, where quantum effects impact on the very nature of the molecules that cells are working with.
The nano biotech existing within the average bacterial cell is capable of burning/processing individual molecules to obtain energy, creating essentially no waste, and capable of creating biological molecules with incredibly low power consumption.
Kinkajou : So there is an existing competition for nanotechnology. To say that nano manufacturing is better, more efficient, less polluting and cheaper is to totally ignore the realities of current biotechnology and bio-tech manufacturing.
What we can do better remains to be seen and will probably be dependent on what we plan to do, and the scale to which we wish to do it.
Erasmus : If we worry about nano tech escaping into the environment and taking over the world, just look out a window and see how the bacteria in the environment are going about it, as they have been for millennia.
As I see it, human designed Systems will always be power dependent.
Also, most systems will depend on human control to supply intelligence, and dependent on humans to provide fuel stocks for molecular reactions and for assistance with replication.
These constraints are substantial and have already been conquered by nanoscale tech factories called bacteria.
We are ready dealing with biotech (bacterial) systems with capacities extensively independent of human input. They power and fuel themselves. They supply their own facsimile of intelligence and are capable of changing the activities of their own internal bio-machinery at a molecular level of activities based on inputs from their environment.
They gather or synthesize their own feedstocks for bio-synthetic processes. They can respond to threats. They can even, at times, turn themselves on or off (via spore formation or entering special “Zen” states of hibernation (such as seen with bacterial "L" forms in steady state).
Kinkajou : Humanity’s next problem in nanotech focuses on powering nano manufacturing and controlling the nano manufacturing process. Everyone dreams of what we can do. However, there is little thought given to how we control these new technologies or how we power them.
I don’t doubt that in the future we can create nano manufacturing technology capable of some incredible processes. However we will still need to power and control them.
Erasmus : So let’s look at some useful definitions of nano tech. Nanotechnology is the manipulation of matter with at least one of its size dimensions existing between 1 nm and 200 nm. This is essentially a practical definition.
One nanometre (nm) is one billionth, or 10−9, of a meter. A double stranded DNA helix has a diameter around 2 nm. The smallest cells such as Mycoplasma bacteria or mycobacteria measure around 200 nm in diameter.
Nano Sized Structures
The lower limit the definition of nanotechnology is set by the size of atoms since nanotechnology must build its devices from atoms and molecules. (Hydrogen has the smallest atoms, which are approximately a quarter of a nm diameter).
The upper limit is more or less arbitrary but is around the size that Phenomena not observed in larger structures start to become apparent and can be made use of in nano devices.
The significance of quantum effects, interatomic electrostatic effects and atom – atom interactions differentiate nanotechnology from phenomena in larger (microscopic) structures with which we are much more familiar.
Below 200 nm, quantum effects become highly significant. Other effects such as changes in surface area to volume ratios of nano materials alter their mechanical and thermal properties as well as imposing constraints on chemical reactions.
Kinkajou : So nanotechnology is really defined as the science of working with the very very small i.e. nano scale. It really covers many aspects of many sciences.
Erasmus : Nanotechnology is not just about making normal technology smaller. In the nano world many of the processes with which we are familiar do not work. For example, we use lubricant in engines to reduce friction in the big world.
In the nano world, a lubricant is likely to interact on a molecular or atomic level with the engine, stopping its function. A lubricant is no longer a lubricant in the nano world.
Similarly, we cannot just make electric motors really small and expect them to work. We cannot make little robots with little arms and legs. The key issue is that in the nano world, the structures cannot function as they do in the macro world.
Due to the unique aspects arising from nano size, many processes need to be redesigned and to operate in a far different fashion to the way they do in the macro world.
Kinkajou : But aren’t many of the forces we deal with in the big world, also represented in the small (nano) world?
Nano Sized Structures 2
Erasmus : Nano materials impose design problems in manufacturing. The power of a drill is proportional to its volume which is the cube of its radius.
However, surface area predicts friction which is only proportional to the square of the radius of the constructed device.
So reducing the size of a drill by factor of 100 reduces its power by 1 million times while only reducing friction affects by a factor of 10,000. Friction affects have effectively become 100 times greater as impediments to power and performance output. Yes the forces may be the same, but the importance of friction and power substantially alter.
Kinkajou : I see. For this reason new technologies must be used to create nano components.
Erasmus : Electronic bonding and surface tension may make nano fabricated materials sticky, predicating that nano devices cannot simply be a scaled-down version of engines with which we are familiar.
A driveshaft for a nano-machine cannot simply be a rotating propeller driven by a small electric engine. It is more likely to be akin to structures such as cilia or flagella on bacteria which evolution has designed to operate at nanoscale dimensions .
Kinkajou : How about providing power to our devices. I suppose, For instance, powering devices is not a matter of plugging them into an electrical “Power Point”.
Erasmus : At nano scales, field radiation energy such as pioneered by Tesla becomes practical and efficient, capable of powering a plethora of devices. (Tesla had suggested by the early 1900s that power could be distributed by radiating it from a central source.
Power transmission by cable however proved to be the more practical and measurable/tariffable form of energy distribution in the big world). In the nano world, field radiation becomes much more practicable than Power transmission by cable.
There are some other interesting aspects to field radiation of energy.
The energy can be directed to specific devices. By controlling frequency, amplitude and shape of electrical fields, a situation can be created whereby only one of the group devices may be activated on command.
Kinkajou : I think I can see an edge here for nanotechnology. Machine devices function only limited by their power input, unlike bacteria. So if the device does not need to supply its own power (for example being able to collect radiated power), it can function at a very high and constant level, with much greater output.
In short, power usage and power output being the difference between machines and biology. Biology is efficient while machines are powerful.
Erasmus : Nano materials can possess substantially different properties to their larger counterparts, potentially allowing their use in new applications. Opaque substances such as copper can become transparent at nanoscale, stable metals such as aluminium can become combustible.
Nanoscale, insoluble materials such as gold can become soluble at nanoscale and inert materials such as gold can become effective chemical catalysts at nano scales as well. The optical properties of nanoparticles can become dependent on the particle diameter rather than the chemical nature of the particles involved.
For example, opals show colour because of the diffraction from the silicon sub particle boundaries, not due to the nature of the chemicals contained therein. Fluorescence features can alter with changes in particle diameter.
Quantum effects cause alterations in properties in nanoscale structures, unpredictable behavior such as charge tunneling through insulators and hysteresis in usual operating modes for electrically powered devices.
Mech Nano Robot : maybe not
Kinkajou : So the term nano technology encompasses a range of technologies which must deal with the unique aspects of the physical world occurring at very small sizes. So nanotechnology can include diverse fields of science such as: organic chemistry, molecular biology, semiconductor physics, microfabrication, nano lithography, surface tension and surface effects physics.
Each of these specific branches of technology has its own special characteristics and its own esoteric name such as nano-ionics or nano- mechanics or molecular manufacturing.
Nano-ionics studies the transport of ions rather than electrons in nanoscale systems.
In summary, Nanotechnology deals with the control and structuring of atoms or molecules with dimensions between 1 nm and 200 nm. (Some say 100 nm). It is the engineering of functional systems at atomic or molecular dimensions. Unique solutions to problems posed by very small sizes also generate new and unique problems.
Materials can have different effect and toxicity profiles in the macro and the nano world, due to their differing abilities to interact with other structures, especially DNA and cell structures.
Nano Technology Generations
Kinkajou : what is this about four generations of nanotechnology that I’ve heard about?
Erasmus : Current thinking suggests that nanotechnology will developed through four distinctive generations.
Today most applications are limited to the first generation of passive nano materials. This includes materials disbursed in liposomes or colloids (such as water or oils in beauty products), titanium dioxide particles in sunscreen, paint surface coatings, silver nanoparticles used in clothing such as socks or in when dressings, zinc oxide nanoparticles in sunscreens and cosmetics, nanoparticles as catalysts (e.g. cerium oxide) and ferrous nanoparticles in magnetic memory.
First generation products are described as being designed to perform “one” single task.
Second-generation nanostructures include active nanostructures which may perform more than one task. For example biosensors, targeted drug delivery systems (e.g. cancer drugs in liposomes), nano transistors and nano-electronic elements.
Third generation nanostructures will feature nano-systems with a number of interacting components. For example a combination of a biosensor, a nano generator and some type of actuator such as a defibrillator in human patient.
Fourth generation nanostructures include integrated nano-systems, functioning much like eukaryotic cells. Within such structures, multiple nano systems interact. A quantum computer could well be an example of this type of structure, being composed of thousands of integrated nano electronic elements, such as mechanical logic gates, registers, logic arrays, capacity for long range data transmission and millions of transistor elements capable of integrated actions.
Design of an Atomic Force Microscope
Kinkajou : When did the era of nanotechnology begin?
Erasmus : The unofficial start to the modern nanotechnology era coincided with the invention in the 1980s firstly the “scanning tunnelling microscope” and then the “atomic force microscope” (AFM).
These allowed the visualisation of individual atoms and molecules at a resolution of over 1000 times the optical diffraction limit.
Changes in physical properties due to changes in molecular arrangements and active biological systems were able to be measured for the first time at the atomic level.
Atomic Force Microscope Structure
Fig.3: Typical configurations of AFM. Here, (1):Cantilever ,(2):Support(Configured to support cantilever.), (3):Piezoelectric element(Configured to oscillate cantilever at its Eigen frequency.),(4):Tip (Fixed to open end of a cantilever, work as a probe of AFM,(5):Detector (Configured to detect the deflection and motion of the cantilever.),(6): Sample 7):xyz-drive, (8):Stage.
Previously, the electron microscope was the most advanced technique for studying sub microscopic systems. To be studied in an electron microscope, samples needed to be mounted (or specially coated), frozen and imaged within a vacuum to allow for electron transmission. It is obvious that such processing could irreversibly change the structure of the material being studied.
Kinkajou : So if the idea is just to see really small things, what’s the difference between the two methods?
Erasmus : Atomic force microscopes (AFM) by contrast with the electron microscope
- do not have a lens or output radiation,
- do not require the use of vacuum to study samples allowing the imaging of living tissues and biological molecular systems,
- have an improved resolution threshold and
- Are capable of manipulating atoms and molecules directly.
Atomic force microscopes (AFM) by contrast with the scanning electron microscope (SEM),
- Can produce a three-dimensional surface profile.
- Contrasting disadvantages include the ability to scan only up to 20 µm in height (cf up to 5 mm) and the ability to scan only 150 µm x 150 µm (cf up to 10 mm²) in area.
Showing an AFM artefact arising from a tip with a high radius of curvature with respect to the feature that is to be visualized. As with any other imaging technique, there is the possibility of image artefacts.
Atomic Force Microscope Scanning Operation
AFM artefact, steep sample topography
Due to the nature of AFM probes, they cannot normally measure steep walls or overhangs.
Atomic Force Microscope Scanning Tip
Atomic Force Microscope Scanning Tip
Kinkajou : Just describe how an atomic force microscope works.
Erasmus : The sharp probe tip is placed at the end of a cantilever arm. This complex is, capable of a curved radius of movement generally to a maximum of about 200 nm, but typically much smaller, around 10 nm.
When the tip comes close to the sample surface, interatomic forces deflect the cantilever. This deflection can be measured by a number of different methods, a common method being the measurement of the degree of deflection of a laser beam from the cantilever.
Many types of beam deflection measurement methods exist, (Optical Interferometry, Piezoelectric detection, Laser Doppler methods, Capacitance detection).
When the sharp probe tip is within 2/10 of a nanometre from the sample, atomic electron clouds overlap resulting in repulsive forces predominating, (these forces being known as repulsive “Van Der Waals” forces). Attractive van der Waals forces occur when charged atoms interact with their electron shells.
These forces can be measured. Other forces that can be measured include electrostatic forces in chemical bonding, surface tension and capillary type forces, magnetic forces, force interactions between molecules and solvents and unusual forces such as Casimir forces operating at quantum states of matter.
All these forces are important in AFM measurements. Specialised probes allow the measurement of even more types of forces as well, including thermal energy, photo-thermal energy, and thermal expansion forces.
Atomic Force Microscope Image Glass Surface
Atomic-force microscope topographical scan of a glass surface. The micro and nano-scale features of the glass can be observed, portraying the roughness of the material. The image space is (x,y,z) = (20 µm × 20 µm × 420 nm).
Beam deflection measurement
AFM beam deflection detection
Electron micrograph of a used AFM cantilever. Image width ~100 micrometers
Atomic Force Microscope Cantilever
Erasmus : AFM operation is usually described as one of three modes, according to the nature of the tip motion: contact mode, tapping mode, and noncontact mode.
- Contact mode, also called static mode (as opposed to the other two modes, which are called dynamic modes). In contact mode, the probe tip is “dragged” across the contours of the sample surface.
At very small distances less than 1 nm, the probe tip can snap in to the surface in certain “charge” situations. Due to this behaviour, contact mode AFM is done at a depth where the overall force on the probe tip is repulsive. This occurs when the probe tip is in firm contact with a solid
Sample surface beneath absorbed solvent layers. In Contact mode imaging it becomes obvious that there are significant frictional and adhesive forces which can damage samples and cause distorted data to be obtained by measurement. This may only be apparent if the sample is rescanned at high distances and greater resolutions after the initial sampling run.
Atomic Force Microscope Imaging Modes
- Subtypes of Contact mode are:
- Constant height mode: In this mode the probe tip height is fixed as it scans, and the degree of cantilever deflection is used directly to generate topographic contour data of the sample under investigation.
It is used predominantly for taking atomic scale images of atomically flat surfaces. It is also used for high-speed scans of changing surfaces, to obtain real-time images.
- Constant force mode (or Topographic image formation mode), (also called Z feedback loop mode): in this mode the cantilever deflection is maintained at a constant level, so the relative distance between the probe tip and sample changes.
The feedback creates high frequency oscillations of the probe tip. Measurements can therefore be made via frequency, vibration or phase amplitude.
The scanning speed in this system is limited by the response time of the feedback circuit. It is the most commonly were generally used AFM scanning system.
Constant force mode is capable of measuring topography as well as measuring other factors such as friction forces or spreading resistance, all simultaneously.
Tapping mode, also called intermittent contact, AC mode, or vibrating mode, or, after the detection mechanism, amplitude modulation AFM
Kinkajou : So you’ve mentioned contact mode and its subtypes of constant height mode and constant force mode. So tell us about the next imaging mode for AFMs (Atomic force microscopes). I think that’s noncontact mode.
Erasmus :Non-contact mode is generally used for lower resolution scans. Scratching damage or surface distortion is avoided for the sample under investigation. Scanning tips last longer in usage.
It is the preferred method for studying biological samples or biological thin films. The presence of surface liquid films substantially affects measurements in this system. Noncontact mode does not penetrate the surface film to reveal topography of sample underneath.
Contact mode will penetrate surface fluids to image subsurface contours. Contaminant water layers can interfere with oscillation, interfering with measurements.
A Probe tip in close proximity to the sample interacts via van der Waals forces. Measurements of Changes in frequency or amplitude of cantilever oscillation requiring correction give data measurements that can be used to map topography.
Single polymer chains (0.4 nm thick) recorded in a tapping mode under aqueous media with different pH
Atomic Force Microscope image molecule
Kinkajou : So you have now mentioned contact and noncontact modes of AFM operation. I think the last mode you need to mention is the “tapping” mode.
Erasmus : Tapping mode is also known as “dynamic contact mode”, intermittent contact mode, or AC mode. The probe tip oscillates (typically 100 to 200 nm) with sufficient energy and amplitude to prevent it from being captured by surface adhesive forces.
Because of the intermittent nature of the contact, frictional forces are generally bypassed, and drag effects are avoided on the surfaces investigated. Image topography is built up from data collected by force measurements at the times of intermittent contact with the sample surface.
Tapping mode is sensitive and gentle enough to allow imaging of cell membrane lipid bilayers as well as imaging of polymer type molecules under liquid media. In optimised media, the conformation of single molecules can remain stable for a number of hours.
Kinkajou : With all these different modes of operation for the AFM (Atomic force microscope), it seems to me that you need to pick the mode of operation that suits the type of sample you are studying and the environment in which it is being studied.
Erasmus : Exactly. Horses for courses!
Kinkajou : When you started the section on nano imaging, you also mentioned the “Scanning Microscope” as well is the Atomic Force Microscope (AFM).
Erasmus : There are several types of Scanning Microscopy used to give high-resolution information on target samples.
These include photo activated localisation microscopy (PA LM), stochastic optical reconstruction microscopy (STORM) and stimulate emission depletion microscopy (STED).
These are “wide field of study” methods using fluorescent proteins or other organic fluorescent molecules allowing images to be obtained with resolution beyond the diffraction limit of light. Of particular importance is that some of these techniques may be compatible with live cell imaging.
The fluorescent molecules used can be complex in terms of using multiple colours or combinations of fluorescent molecules.
Atomic Force Microscope Operation
Kinkajou : My impression is that the technology seems fairly mature. Where next?
Erasmus : The future
- Using multiple parallel probes in AFM can increase scan area size.
- Beam deflection technologies can allow faster measurements. Improved cantilever and probe tip design can allow for sharper and/or faster imaging.
- Faster scanning is required to reduce thermal drift for samples and to reduce damage and decay biological samples.
- Software filtering of noise/real-time correction software can improve image quality, and allow adjustment to sample topography during the scan.
- And something really weird. Perhaps the development of the quantum force microscope. The interesting concept here is that the sample does not need to be investigated by the transmission of particles such as for example light photons. One of a pair of photons can be used to test the sample.
The reading can be taken from the other photon, at a time or place distant from the point at which the first photon was processed / used.
Atomic Force Z Microscope Modes