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Nanoparticles.

In order to aid in the drive of industry to achieve ever smaller and more efficient devices it has become important for scientists to develop the materials and methods to allow these technologies to advance. One area of pursuit that has risen to meet these challenges is that of nanomaterials science.

How does “Nano” help?

It may seem contrary to everyday experience that by making a piece of material smaller one could effect changes in the materials properties such as colour, melting point etc. However the reasons for these changes are bound up with the Quantum Mechanical rules that are obeyed as the world of molecular dimensions is approached. When the wave motion of a charge carrier, e.g. an electron, is held in a smaller volume of space than it would occupy if it were free to do so the electron is said to be confined or “quantum confined” and it is this confinement that results in the optical properties being altered (see Semiconductor Nanoparticles below). These dimensions are of the nanometre (1-100 * 10-9 m) size range and as such materials which have one of their dimensions in this range are referred to as nanomaterials.
A second aspect of materials on the nanoscale is that they possess large surface area to volume ratios and small radii of curvature. These two facts lead to interesting implications in the area of catalysis, the former yielding larger surface areas upon which the reactions of interest may occur and the latter allows the tuning of the energetics.

Semiconductor Nanoparticles.

Semiconductors, i.e. materials that show an increase in their conductivity as the temperature is increased or under conditions of illumination, are characterised by two primary bands of energies, a valence band and a conduction band which are separated by an energy range for which no levels are present, known as the band gap. The energetic width of this band gap dictates many of the optical and electrical properties of the semiconductor as it is this amount of energy that will be absorbed in order to promote the electron from the valence band to the conduction band and emitted when the electron relaxes directly from the conduction band back to the valence band. When sufficient energy has been supplied to a semiconducting material, the electrons which have been promoted to the conduction band may subsequently be made to move under the influence of an electric field. By judiciously choosing the size of the semiconductor nanoparticles one can alter the energy width of the band gap and consequently tune the optical and electrical response of the material. (see figure below). It is this ability to tune the physical properties of the material by altering the size that promises to make engineering at the nanometre scale potentially very powerful.

One fact that should not be forgotten in all of this is that the chemical compound (material) remains in essence the same and therefore has effectively the same composition and chemistry. This is a major advantage that therefore arises from the use of materials at the nanometre scale as any chemical modification that has been successfully applied to one size (colour) can be further applied to any other. This is unlike the case for dyestuffs, where the optically active centres are molecular and where to change the optical properties one must engineer the molecule. This is by its very nature a more challenging prospect and once achieved still only allows one characteristic absorption and emission profile. On the other hand by stopping the growth of nanoparticles at a particular size one can effectively pre-select the part of the spectrum that one wishes the material to be active in. By removing portions of the growth mixture at different times one can obtain the material at a number of different sizes and therefore, where the different fractions are optically active, obtain material which has been tuned to be active in a number of different regions of the spectrum.
One of the most important criteria that one must consider when dealing with the “quality” of a sample of nanoparticles is the polydispersity i.e. the range of sizes present in the sample. It makes sense of course that if the properties of the material are determined by the size then having a wide range of sizes present will lead to a situation where the required property is smeared out or hidden due to the range of size determined properties present in the sample. This makes it difficult to determine accurately the extent of the property of interest or at the very least the material becomes more difficult to characterise. An added bonus to being able to achieve monodisperse samples is the tendency of such systems to self-assemble giving access to secondary (monolayer) and even tertiary (multilayer) levels of organisation.

What are Semiconductor Nanoparticles good for?

As previously mentioned the optical properties of semiconductor materials are a function of their size and therefore the optics, absorption and emission can be “tuned”. Hence any device or process that depends on an optical input (absorption) or an optical output (emission) can have nanoparticles incorporated into it. Devices based on nanoparticles and their effects have the potential for higher performance at lower volume, weight, and power consumption. Below are listed some of the devices and processes for which it is envisaged that nanoparticles may play an active role:

Biolabelling:

Biolabelling is the addition of a marking substance, a label, which can subsequently be detected, to a biological sample in order to learn something about the local biological and/or biochemical environment. One example is immunofluorescence which is the labeling of antibodies or antigens with fluorescent dyes. This technique is sometimes used to make viral plaques more readily visible to the human eye. Immunofluorescently labeled tissue sections may then be studied using a fluorescence microscope. The role of the dye is to emit (fluoresce) when the sample is illuminated and the emission can then be detected using a fluorescence detector. These dyes may be substituted by nanoparticles with the following advantages:

• Nanoparticles can be made exceptionally chemically and photo-chemically stable.
• They can deliver the same intensely bright signals in diverse harsh environments and following prolonged periods of irradiation that would completely bleach the signal from conventional dyes.
• Nanoparticles can achieve quantum yields comparable to the brightest traditional dyes available and can absorb 10 - 1000 times more light than competitive dyes. These two properties combined can result in the single brightest class of fluorescence materials available anywhere. This unsurpassed stability and brightness may allow access to observe rare molecules that by using conventional sources would be unobservable.
• The emission from nanoparticles is narrow and symmetric which means spectral overlap with other colors is minimized. That translates into minimal bleed through into adjacent detection channels and attenuated crosstalk, in spite of the fact that many more colors can be used simultaneously.
• Since each emitted color is based upon the same underlying material the chemistry and methods used for one color are easily extrapolated to all of the colors.
• All the nanoparticles can be excited using a single light source and so both narrow laser and broad lamp excitation may be used. Hence three or four-color detection may be achieved without the need for two or more lasers and laborious alignments and compensations.

Electronic Devices:

Electronic devices and applications of semiconducting nanosized materials may be divided into two general categories:
a) Those that make use of the emission properties of the material and
b) Those that make use of the absorption properties of the material.

a) Devices based on emission:

LED’s (light-emitting diode):

LED’s are semiconductor devices that emit narrow-spectrum, incoherent light when a potential is applied in the forward direction and are based on an effect known as electroluminescence (the generation of light by the application of a voltage). As has previously been mentioned when a photon of sufficient energy comes in contact with a semiconductor nanoparticle the photon can be absorbed and an electron promoted from the top of the valance band into the conduction band. The reverse process i.e. direct relaxation of the electron from the conduction band to the valance band results in an emission of a photon. It is a logical extension therefore to imagine that if only the latter process can be engineered to occur then a net gain of photons may be said to result as a consequence (as opposed to one in (absorbtion) + one out (emission) =  zero gained)! Many semiconductor materials may be employed in LED fabrication depending on the colour (frequency) of light one wishes to generate. Amongst the materials presently in common use are: aluminum gallium phosphide (AlGaP) – green, gallium nitride (GaN) – green and blue, indium gallium nitride (InGaN) - near ultraviolet and blue, zinc selenide (ZnSe) – blue and many more materials besides.
One major disadvantage of the conventional LED materials is that the emission wavelengths are material specific and set by nature, the result of which is that not all wavelengths are accessible. Hence the main advantage of using nanoparticles as opposed to the aforementioned materials is that the emission characteristics of nanoparticles are tunable and one does not need to seek a new a material when a different part of the spectrum needs to be addressed. Also conventional materials are produced via high cost processes (from large expensive inflexible wafers which must be designed and cut to shape and which consequently imposes restrictions on the device fabrication and application). Other advantages of LED’s based on nanoparticles are:

• Their extremely small size and versatility allows them to be inserted into many media – plastics, water, organic media etc.
• Using semiconductor nanoparticles it is possible to produce composite semiconductor LED’s that possess the correct spectral characteristics by intermixing red, green and blue emitting particles, something which is impossible using traditional semiconductor materials and processes.
• Nanoparticles can be easily synthesised in large batches using colloidal chemistry and subsequently cast into a variety of final forms giving a huge advantage in cost-effectiveness.

Lasers (light amplification by the stimulated emission of radiation):
Lasers operate on the principle that the light emitted from the source is generated by stimulated emission, as is suggested by the acronym. This process is essentially the result of an incoming photon bringing about the in-phase, equi-energetic and directional emission of another photon. Hence the result of stimulated emission is a very monochromatic (one energy or frequency), coherent (in-phase) and directional beam of light which, depending on the amplification factor achieved, can be made to be very intense when focused. The materials employed for lasing can be a solid state matrix (Ne-YAG, ruby), gases (He-Ne, N2), dyestuffs (Rhodamine 6G, Fluorescin) etc. However all have one property in common – there is a direct transition from a higher energetic level to that of a lower one. This is exactly the situation as was previously described for nanoparticles where the electron can relax from the conduction band to the valance band and emit a photon whose energy is equal to that of the band gap and hence nanoparticles may in principle be used as a lasing source. Amongst the advantages of using nanoparticles as stimulated emission sources in laser devices are:

• Semiconductor quantum dots can be manufactured in high quality (monodispersity)
• They can be made to self-organise
• They offer a new degree of freedom in selecting the working wavelength of photonic elements.
• They allow manufacturers to cover almost completely the entire spectral region from the ultraviolet to the far infrared, with a small number of substrate materials.
• Further advantages of quantum dot lasers are that they have a small energy consumption through low threshold current densities, a high modulation range for high-speed applications as well as an improved temperature stability.

Lighting
The fact that the light emitting centres in nanoparticle devices are, by definition, on the nanometre scale any application that requires at least one dimension which is thin would benefit from the incorporation of emissive nanoparticles. Such applications extend to flatscreen technologies, background lighting displays for mobile phones etc. One added advantage in the area of screen technology is not only the ability to make one dimension small but to increase the definition. By employing masking or lithography techniques in combination with nanoparticle deposition it is in principle possible to envisage pixel sizes that are comparable, or even smaller, than the wavelength of the light emitted!

a) Devices based on absorption:

Detectors:

Detectors are based on the fact that some stimulus (say light) interacts in some way with the detector material(s) and generates a response (usually a current, voltage or change of resistance) that can be measured, often after the initial response has been amplified, by a meter. Ideally, this response should then allow one to work back and quantify the intensity of the stimulus. In fact many optical detectors are already based on semiconducting materials. Once light of sufficient energy has promoted an electron from the valence band to the conduction band this electron can be made to move under the application of an electric field directed around a circuit where it can be detected. More light gives rise to more electrons which in turn gives rise to greater currents or larger potentials. Amongst the advantages of incorporating nanoparticles into detectors and arrays are:

• Lower dark current due to the 3D-confinement of carriers
• Possibilities for higher operating temperatures, which consequently will lead to a cheaper camera system. 
• Absorption of radiation at all angles of incidence, since the electrons are confined in all directions, which will lead to simpler fabrication processes.
• Longer lifetime of excited carriers than in quantum wells, which will increase the probability for an excited electron to contribute to the photocurrent and will therefore increase responsivity.

Sensors:

There are many proposals (far too many for this webpage to hope to be able to provide a comprehensive list) as to how sensor technology can benefit from the use of nanoparticles. The major advantage that nanoparticles bring to this exciting technological field is their large surface to volume ratio. This large surface area may be chemically treated such that receptor molecules for a particular target species are present and when the target interacts with the receptor a signal is generated and detected. The large surface area increases the probability that an interaction will occur and where there are a number of such events possible the intensity of response should correspondingly increase. It is of course also possible that the nanoparticle itself is the receptor.

Solar cells:
Given the concern about the availability of fossil fuel energy sources and the environmental concerns about emissions during and after their retrieval from the earth much publicity and research funds are being expended on viable, non-polluting long term power generation. Of all the alternatives the hope of eventual cost efficient power generation from the sun appears to fulfil all the required criteria. However this potential source has to date proven not to be cost effective which, to a great extent, is due to the materials employed in their manufacture. First generation photovoltaic (PV) solar cells (those based on silicon) although stable and quite efficient are expensive and due to their inherent inflexibility place limits on the possible device geometries. The problem with second generation materials (based on thin film technologies) is that they do not absorb enough of the available sunlight and consequently are inefficient. Hence a third generation of solar cells based on nanoparticles could be engineered to overcome many of the problems inherent with the present set of materials.

• The main advantage derived from the use of quantum dots stems from their tunable band gap, which allows control of the wavelength at which they will absorb light. The greater the band gap of a semiconductor, the greater the output voltage generated. However a smaller band gap results in a higher output of current but at the expense of a lower output voltage. Both high currents and high voltages are desired for efficient solar-electric conversion.
• Traditional semiconductor devices have band gaps that are not altered cheaply or easily and cannot absorb preferentially in one region of the spectrum. Nanoparticles, on the other hand, can be designed to preferentially absorb in a specific region of the spectrum. Therefore nanoparticles provide a much more precise means of tuning the band gap of the solar cell material to the optimum band gap for energy conversion, resulting in greater efficiencies.
• Nanoparticles can be made to self assemble into ordered 3-D arrays with inter- nanoparticle spacing sufficiently small such that strong electronic coupling occurs and mini-bands are formed to allow for long-range electron transport.
• Nanoparticles can be prepared with a protective shell which increases stability and yields longer lasting solar cells without degradation in performance. This represents an advantage over traditional semiconductor devices, which do not have protective shells to guard against the harmful effects of the sun, and therefore must be replaced more often which drives up material costs.
• Nanoparticles are also unique in their ability to inexpensively capture a large percentage of the sun's energy while retaining great versatility of form.
• Nanoparticles can be made into flexible sheets, put into liquid form, or made to be transparent.
• The relative low cost and high performance nanoparticles compared with that of solar cells made of bulk silicon or thin films allows nanoparticle films to be incorporated into solar cells and make such devices economically competitive.

Another type of solar cell is based on the wide band semiconductor titanium dioxide (TiO2) where dye stuffs are employed to harvest the light and transfer the energy to the TiO2. And so the story starts to get repetitive as by now it should occur to the reader who has being reading these pages from the beginning that if the dye in the system were to be replaced by nanoparticles the world would be a more beautiful place to live…..

Catalysts:

Again one of the main advantages that nanoparticles bring to the area of catalysis is that of a large surface area upon which the reactions of interest can occur. However, one other very important consequence of the particle size for catalysis stems from the ability to tune the band gap.

Photocatalysis:
In order to promote the electron from the valance band to the conduction band a quantum of energy equal to or greater than the band gap energy must be supplied. Equally it can be said that the electron possesses a greater energy after being promoted and that this energy is at least that of the band gap. If the band gap is larger a greater amount of energy must be supplied to the electron and so now this higher energy electron can in principle be made to do work for us. All chemical reactions have an activation energy and if this activation energy is low the reaction is feasible it will occur readily (please note that for the sake of simplicity it is assumed that the reaction is kinetically and thermodynamically possible). However if the energy barrier is high the reaction will not take place (or at least not appreciably so) under the normal conditions of temperature and pressure. If however sufficient energy can be supplied the reaction may be made to proceed and this is where our high energy electron can come to our aid. A corollary of this is that we may also use wide band gap semiconductors to stop the energetic rays of the sun from reaching places where they may do damage. One example of this is the use of white semiconducting materials (the white colour associated with wide band gap semiconductors is an indication of the size of the band gap) such as zinc oxide creams to protect our skin (see pictures of Australian cricketers).
In the case of noble metal catalysis the principle property is that of large a large surface area but the energetic changes due to the radius of curvature and confinement of the plasmon (sea of electrons) cannot be neglected.