2.1 Introduction
2.2 Instrumentation
2.3 Sample Requirements
2.4 Imaging
2.5 Case Studies
2.6 SEM at Binghamton
2.7 References

4.1
4.2
4.3
4.4
4.5
4.6
4.7

6.1 Intoduction
6.2 STM
6.3 AFM
6.4 Case Study
6.5 SPM at BU
6.6 References

7.1 Thermal Analysis of Materials
7.2 Preparation of Nanofibers
7.3 Nanoindentation Analysis of Materials
7.4
7.5
7.6
7.7

3.1
3.2
3.3
3.4
3.5
3.6
3.7

5.1
5.2
5.3
5.4
5.5
5.6
5.7

1.5 Nanophotonics

The next revolution in information technology will dispense with the transistor and use light, not electricity, to carry information. This change will rely on the development of photonic materials, which produce, guide, detect, and process light.
Philip Ball, Made To Measure (1997)

Nanophotonics is an emerging and quickly growing field that incorporates engineering, physics, and chemistry. Nanophotonics first emerged as a field in the 1980s when sub-wavelength optical visualization of sample surfaces was shown to be a practical possibility. The field advanced in the early 1980s with the manipulation of luminescent semiconductor nanoparticles. In the 1990s, sub-wavelength optical visualization became a reality with the use of near-field scanning optical microscopy. This allowed for the generation of images using sub-wavelength light. Recently, the principles of nanophotonics have also been extended into information technology. By utilizing nanophotonics, processes can be made faster and more energy efficient.

1.5.1 Nanophotonic materials

The materials used in nanophotonics are quite diverse; however only the materials involved in the discussed applications will be addressed. In general, most of the nano-scale materials utilized here are nanoparticles. These particles range in size from 2-50 nanometers in diameter. They are made up of many different substances such as metals, semiconductors, and metallic salts.

Most of the nanoparticles discussed here are somewhat spherical in shape; however the nanophotonic switch utilizes nano-cubes. They are aggregates of atoms in a relatively crystalline form. The outer surfaces of these particles, especially those in solution, are usually passivated by some material such as an additional surfactant. A surfactant is a material that decreases the surface tension of the solution in which it is dissolved. This property allows the nanoparticles to remain in solution when they would otherwise not dissolve. Sometimes, the outer surface layer is another semi-conductor material, forming a core/shell particle, Figure 30. Another possibility is that these particles are deposited on a surface, in which case no additional surfactant would be needed.

Figure 30

Many different substances are used in the synthesis of nanophotonic materials, their analysis, and their use. However, a brief introduction to a few major materials such as copper (I) chloride, metals, and semiconductors should outline the important information for the techniques and applications discussed. Copper (I) chloride makes up the nano-cubes used in nanophotonic switches1. Several different metals are used in nanophotonics. Gold and silver are both used to make nanoparticles that act as near-field light condensers. Near-field light is light without wavelength properties and therefore not hindered by the limitations normally imposed on light. These metal particles condense near-field light on their surfaces. Semiconductor nanoparticles are used for their luminescent properties. Specifically, II-VI and III-V semiconductors are the most common ones under investigation. II-VI and III-V refer to the groups of the elements in the substance. A II-VI semiconductor has elements from groups 12 and 16, while III-V refers to groups 13 and 15. An example of a II-VI would be cadmium sulphide whereas a II-V would be indium phosphide. These substances exhibit an interesting property when in the size range under 10 nanometers in diameter. Once the particles are smaller than this threshold size, the color of fluorescence changes along with the size. As these nanoparticles decrease in size, the color shifts towards the blue end of the visible spectrum. Different substances have different spectrums, so indium phosphide which fluoresces red in bulk form, will shift with the particle size down to the yellow/green region of the spectrum. If a substance fluoresces yellow in bulk form, it is possible that the smaller particles may even reach the blue/violet end of the spectrum.2

The diverse materials used in nanophotonics fabrication has lead to several different methods for synthesizing them. One of these methods is solution-phase chemistry. This method is becoming more common in the synthesis of nanoparticles. Basically, it involves mixing the reagents together in a solution and adding heat if necessary. Then the particles will form in the solution (usually passivated by a surfactant). Another common method is solid-state synthesis where the materials are created on a solid substrate. Arrays of metal nanoparticles have been created by treating a surface and then exposing it to a reactant.

NFO-CVD (Near-Field Optical Chemical Vapor Deposition) combines near-field optics and chemical vapor deposition to deposit dots or lines of metals on a surface. Chemical vapor deposition is a process that deposits a layer of a material from a gaseous phase onto a surface. Some of these reactions are catalyzed by light. Near-field optics use light that has no wavelength, thus giving it a much smaller resolution. Using a near-field scanning optical microscope, it is possible to illuminate an area of around 20-50 nanometers in diameter, and induce the photochemistry of the deposition process. This technique has yielded dots of zinc and aluminum of around 20-50 nanometers in diameter and 15 nanometers in height.3

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1.5.2 Characterization of Nanophotonics

Electron Microscopy

1. SEM – Scanning electron microscopy (SEM) is a visualization technique whereby a focused electron beam is scanned over a small surface area. The beam knocks off secondary electrons which are picked up by a detector. This technique must be used on a surface which is at least semi-conductive to conduct away the charge that could build up with the beam. SEM is a very common technique to look at surfaces, and potentially has a resolution on the order of a few nanometers.
2. TEM – Transmission electron microscopy (TEM) is another visualization technique utilizing an electron beam. In TEM, the beam passes right through the sample (meaning it must be very thin), and it is possible to get an idea of some of the internal structure of the sample. One of the advantages of the TEM is that its resolution is even better than SEM, down to the order of angstroms.

Scanning Probe Microscopy (SPM)

1. AFM – Atomic Force Microscopy (AFM) uses an atomic scale probe attached to a cantilever. The probe is applied to the surface of the sample to be imaged. The atomic forces between the probe tip and the surface of the sample cause a small deflection in the cantilever. This deflection is tracked by a laser reflecting off of the cantilever. Thus, an image of the surface can be generated on an atomic scale
2. STM – Scanning Tunneling Microscopy (STM) utilizes tunneling current to produce images on the atomic scale. In STM, a fine metallic tip is used to scan the surface of a conductive material. If the tip is close enough to the surface of the material, quantum mechanics allows for a tunneling current to be produced. The magnitude of this current is measured and used to generate an image of the surface
3. NSOM -- Near-field scanning optical microscopy (NSOM) utilizes near-field light to look at a sample surface. Near-field light no longer has the limitations of wavelength that prohibits normal optical visualization of surface structures smaller than the wavelength(s) being used. NSOM illuminates the sample with this near-field light and collects either the reflection or transmission of the light to generate a picture of the sample. (Nanonics Imaging Ltd, http://www.nanonics.co.il/main/twolevels_item1.php?ln=en&item_id=34&main_id=14 - link opens in new browser window)

Semiconductors have an interesting property called a “band gap”. This is an electron energy gap between the valence level and the conduction level of the material (valence band and conduction band). This band gap provides many of semiconductors’ properties, including the ability to convert electronic energy to light energy and vice versa. Light energy of a certain wavelength is absorbed by an electron, bringing it up to the conduction band. The electron can then either lose some energy in vibrations, or it can drop back down to the valence band. When dropping down, it can radiate the energy it loses in the form of electromagnetic radiation, sometimes in the visible spectrum. However, sometimes it will drop down in the form of non-radiative conversion whereby it transfers its energy in another form (such as passing it on to another electron). Figure 31 shows a simplified diagram of how this works.

Figure 31

Semiconductors’ nano-properties can be partially explained through this band gap explanation. All the different energy sub-levels within the bands are due to energy level splitting that occurs when complex materials (such as these semiconductors) form. When two atoms bond, there is a splitting in energy levels. These resulting levels continue to split and form sub-levels until the bulk form of the material is reached, where there are many sub-levels for each band. When creating these nanoparticles below a threshold level, these sub-levels start disappearing. This leads to a larger band gap and higher efficiency (since there is less energy lost in transfers between sub-levels) as the particle gets smaller. Figure 32 shows a simplified version of what this might look like:

Figure 32

In today’s world, a large part of daily life is helped by electronics. From the simple appliances such as a telephone or light source to the more complicated systems that can be found in a computer chip or the workings of a car. Among other factors, these appliances are all limited by the speed by which the electrons in them can travel. A hard-wired telephone signal only travels so fast, and a computer chip can only operate at given maximum speed. A photon can travel much faster than an electron, and because of that property it is possible that in years to come there will be optical fibers carrying optical signals from place to place and a computer may operate by the speeds of a photon rather than an electron.

One major problem with creating these optical signal carriers and circuits is that electronics is the operating force and the input and output variable for many systems. To deal with that, a highly efficient electronic/optical converter would be needed. Semiconductor nanoparticles have shown great promise in the area of optoelectronics because of their high-efficiency conversion of electronic energy to light energy. Efficiencies of over 50% have been achieved using core/shell nanoparticles of II-VI and III-V semiconductors. This high-efficiency conversion of energy can also be harnessed in terms of converting light to electronic energy. This ability means that these particles could provide the converter component in input/output systems for optoelectronic devices.4

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1.5.3 Applications for Nanophotonic Materials

Switches. Computers store data through the use of switches. These switches represent either a true or false in memory. When a switch is turned on, this represents true. Conversely, when a switch is turned off, this represents false. These on and off states are controlled through the voltage across the switch. High voltage represents on, and low voltage represents off. If these switches can be made smaller, then computers would be able to store more data. Additionally, if a switch could be controlled by something faster than electric current then computers could run more quickly and more efficiently.

Developments in the field of optical fiber transmission systems require faster transmission rates. Since photons travel faster than electrons, light can be utilized to increase the speed of transmission. Conventional photonic devices, such as diode lasers and optical waveguides, require the usage of an entire wavelength of light. This confines the devices to be at least as large as the wavelength of visible light (400-700 nanometers). Optical devices that utilize sub-wavelength light must be developed in order to make anything in the nano-scale. The advantage of these devices is the ability to perform functions based on local electromagnetic interactions in a much smaller area than is currently possible, leading to smaller nanophotonic circuitry. The development of a nanophotonic switch would be an essential component of a nanophotonic integrated circuit. A nanophotonic switch is a switch that utilizes photons instead of electrons on the nano-scale.

An example of a nanophotonic switch was demonstrated by Kawazoe et al.1 by placing three CuCl quantum cubes in a NaCl matrix. When closely spaced CuCl quantum cubes resonate with each other, near-field light energy transfers between them. Three quantum cubes are used for the input, output, and control terminals of the switch. Figure 33 demonstrates the basic set-up:

Figure 33

In the off position, the excitation energy in the input terminal is transferred to the output terminal and then to the control terminal. Thus, no output signal is generated. In the on state, the excitation is not transferred to the control terminal and the output signal is generated. Figure 34 demonstrates how this would look using an NSOM for visualization:

Figure 34

The area occupied by these particular switches is less than 400 nm². Experimentally, these switches have operated at a few hundred megahertz, 10-100 times faster than conventional optical switches. Therefore, these nanophotonic switches are expected to be a key device in improving optical integrated circuits. The optimal speed of an electronic switch is on the order of gigahertz (109), but the theoretical optimal speed of a photonic switch is estimated to be on the order of petahertz (10¹⁵).

Nanoinscription. Nanoinscription is the process of writing or recording data using nanophotonics. Three nanoinscription methods are nanophotoisomerization, photobleaching and near field optical chemical vapor deposition. These processes can potentially be used for optical data storage. Nanophotoisomerization is the use of light to change the arrangement of cis and trans side chains to create a nano-scaled light-polarizing pattern on a polymer surface. It is being developed as a technique for data storage below the diffraction limit of light. Likodimos et al.5 have used NSOM for nanowriting on azobenzene side-chain polymer thin films based on the isomerization of the side chains. Polarized light activates the isomerization process, realigning the side groups. This creates a pattern, enabling the storage of information. NSOM can be used to create this configuration using a pure polarization interference pattern. Optical patterning on a photosensitive polymer thin film has been demonstrated with features on the order of a micron. Recently, features on the sub-micron scale have been achieved. In fact, using polarization-modulation NSOM, 100 nm patterns have been generated and read back optically on thin-film polymers.

Photobleaching involves exposing nano-sized areas to a one photon or two photon excitation source. This process can be used to create optical data storage. Shen et al.6 used a self-mode-locked titanium/sapphire laser as the excitation source and a dye-doped polymer film as the recording medium. When nano-scale regions are exposed to this excitation source for longer than ten seconds, they become bleached. These nano-areas are clearly distinguishable because they appear black compared to the surrounding fluorescent regions. The difference in intensity can either be used to simply write on the nano-level or to store optical bits, representing a one or zero in recording. Bits with a diameter of 120nm and a center-to-center spacing of 250nm were achieved using a one-photon excitation source. An example of this is shown in Figure 35.

Figure 35

Figure 36 shows the letters AF produced by one-photon bleached data bits. Additionally, Figure 37 shows an image of the stripes produced with one-photon photobleaching.

Figure 36

Figure 37

The width of each line is 130 nm, implying that nano-scale fine gratings can be produced using near-field optical techniques. However, using a two photon excitation source limits the effective excitation area. Consequently, a bit diameter of 70nm can be obtained using a two-photon excitation source, as shown in Figure 38.

Figure 38

The last method is the use of near-field optical chemical vapor deposition (NFO-CVD). This method, shown schematically in Figure 39, deposits nanoparticles on a surface using a fiber probe.

Figure 39

This process is enabled by using optical near-fields on the probe tip. Conventional chemical vapor deposition dissociates vapors of organo-metallic compounds by means of far-field light. Using normal methods, it is nearly impossible to deposit particles smaller than the wavelength of light. Because of the absence of the wavelength restriction in near-field optics, the deposition of particles on the nano-scale is possible. NFO-CVD can be used to deposit nanoparticles in patterns capable of storing data in memory.3

Optical data storage has been a field of intense research recently due to the promise of superior operating speed, gain in inscription of information, and noninvasive character, compared to magnetic data storage. The highest possible commercially available bit density with magnetic data storage is 20 megabits per square centimeter. This equates to about one bit for every five square microns. Additionally, traditional optical methods are limited by the wavelength and diffraction of far field light. Therefore, the concepts behind nanophotonics and near-field light must be employed to achieve the goals of optical data storage on the nano-scale. The techniques above are several methods being developed to reach these goals.

Light Sources. Current common light sources operate relatively inefficiently in terms of electrical input and light output. The actual efficiency rating for a standard incandescent light bulb is around 10%. In recent years, semiconductor nanoparticles have shown great promise in their highly efficient fluorescent properties. These fluorescent properties could possibly change the way light sources are made and used in the world.

Semiconductor nanoparticles on the order of 2-8 nanometers, especially the core/shell varieties, have been shown to be very efficient at converting electrical energy to light energy. Some core/shell particles, particles having a thin cap of another material surrounding the inner particle, have been found to have efficiency ratings of over 50%. Though this is still a ways from the “perfect” efficiency of 100%, it is a great increase from the current bulbs in use today.

Each particle of a given material and size gives off a different wavelength of light when electrical energy is supplied to it. For example, a 6-nanometer diameter particle of indium phosphide may fluoresce red, while a much smaller (say 2 or 3-nanometer diameter) particle will fluoresce a yellow-green color. Cadmium sulphide will give a different spectrum ranging from a yellow-orange to the greenish-blue area, and each of the other II-VI and III-V semiconductors will give its own range, Figures 40 and 41.

Figure 40

Figure 41

The different colors come from the different sized band-gaps of the materials. This gap is different for each semiconductor in its bulk form. At the nano-level, a property called quantum confinement appears whereby the band-gap gets larger as the particle gets smaller. This means that the fluorescent color of smaller particles will shift towards the blue end of the spectrum (higher energy).

These particles could be deposited on a surface or suspended in a matrix to create a lighting implement. With arrays of these particles of different materials and sizes, it would be possible to create virtually the entire visible spectrum of light. Also, these particles could be used as highly efficient LED’s or possibly monitors or other displays. A schematic for this projected application is shown in Figure 42. The fact that they are small and efficient gives them the possibility of being quite versatile in their applications.4

Figure 42

Nanophotonics is a relatively new field that opens up numerous possibilities for advancement. Emerging in the last twenty years, nanophotonics has already affected developing technology. Through the manipulation of near-field and far-field light, highly efficient systems and devices are being developed. Light can be used to transmit information and converters can transfer it back into electrical impulses. Near-field light can be used in a nanophotonic switch operating in computers. Additionally, nanophotonic processes such as photobleaching, nanophotoisomerization, and NFO-CVD can be used to store data. Lastly, semiconductor nanoparticles can be used to convert electrical energy into light of variable wavelength for the purpose of light sources. Nanophotonics is just beginning to grow and influence the way information technology operates.

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