Materials:
All the materials in this world are made of using one or more elements of the periodic table. They can be classified based on either of its mechanical properties (Elastic, Non-elastic, Ductile and Brittle), electrical properties (metals, semiconductors, and insulators), Magnetic properties (Ferro magnets, anti-ferro magnets, Ferri magnets, Para magents and Dia magnets), Dielectric properties (Polarization), thermal properties etc. The property of a certain material changes when its dimensions are changed. Based on the size of the materials, they can be classified into 4 types :
Bulk materials : As such
1D materials (two of its dimensions is less than 100 nm): Nanotubes, Nanowires, Nanosheets, Quantum wires
0D materials (less than 100 nm in all directions): Quantum Dots, nanoparticles
Most of the materials can be fabricated into all the above said dimensions. Depending on the size of the materials, their properties will be changed, and so depending on the applications one can choose the dimension of the materials. The cause and effect of various properties with decrease in a size of material is discussed in detail in the other chapters. In this chapter, I will discuss the available fabrication techniques to manufacture various nanomaterials. With decrease in size of the materials, characterizing the same gets difficult. Here, I will also mention basic principles of few of the various available characterization techniques.
These nanomaterials can be fabricated either by using physical methods or chemical (wet) methods. In the physical methods, one can follow either top-down approach or bottom-up approach. Top-down approach is nothing but breaking/etching the bulk materials until its dimensions are reduced to nano-size. Bottom-up approach is called when the structures are built by assembling atom by atom to the required size.
Fabrication Techniques:
Top-down approach:
The top-down approach is a well matured technology commonly used in the field of micro-electronics. The most important steps involved in this process are Lithography, Etching (either Dry or Chemical) and metal deposition. Depending on the resolution aiming for, one can chose either Photo Lithography or Electron beam Lithography technique. However, there exists a technique so-called soft-Lithography is commonly used in the biological fields to make microfluidics etc., is described in the chapter Bio-Nanotechnology. There are several techniques available to etch the substrates depending on the purpose, the constraints with morphology and the cost, which I will mention briefly in this chapter.
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| http://www.almaden.ibm.com/st/chemistry/lithography/process/ |
http://www.memsnet.org/news/
1. Cleaning the wafer,
2. Apply (e-beam /photo) Resist,
3. Expose pattern,
4. Develop film,
5. Etching / Metal deposition.
Cleaning the wafer:
Most of the wafers are cleaned by rinsing it with acetone followed by ethanol and blown dry with N2. Please note that ethanol should be dispensed on the substrate well before the acetone dries. In other words, one should not let acetone dry on the sample by itself, which leaves the residual marks on the substrate and is difficult to clean.
One can also choose ultrasonic baths to clean the wafer by using acetone followed by ethanol and blow dry with N2/ spin dry.
Resist:
There are two types of resist negative or positive resist.
Bottom-up approach:
Bottom-up approach has become a choice for wide range of materials due to the limitations of the capabilities of the techniques used in top-down approach such as lithography. The most commonly used bottom-up approach methods to fabricate 2D materials such as thin films are CVD, PLD, MOVPE, and MBE etc., where Epitaxy is a process in which the deposited film adapts the crystal phase and orientation identical to those of the substrate. The process of depositing different materials with different band gaps on each other epitaxially is called hetero-epitaxy, and such structures are called heterostructures.
The most commonly used bottom-up approach techniques to fabricate 2-D materials such as NWs, and nanotubes are using growth mechanisms such as selective area epitaxy (SAE), vapor-liquid-solid (VLS), vapor-solid-solid (VSS), and oxide assisted growth (OAG). The SAE technique uses a 30 nm thick SiO2 layer deposited on the (111)B oriented substrates followed by opening holes in the SiO2 layer. The III-V materials deposited on such substrates lead to the growth of NWs from the holes in the SiO2 layer. This technique has been extensively studied by Fukui’s group in Japan using MOVPE deposition technique []. The other commonly used growth technique for NWs is so-called VLS growth technique which uses metal particle as a catalyzing agent aiding the growth of the NWs. Au has been traditionally used as a catalyst particle for the growth of NWs, and hence also called as Au-assisted VLS growth technique. VLS process is so called because the material is supplied in vapor (V) form to the catalyst particle which is liquid (L) and results in the NW which is of solid (S) form. In some cases, the catalyst particles remains solid(S) and is so called VSS process. Recently, Fontcuberta’s group in Germany/Switzerland has demonstrated the growth of GaAs NWs by Ga-assisted VLS growth technique using MBE deposition method [], and has attracted attention of few other groups who demonstrated to fabricate In-assisted InAs NWs and also the growth of these NWs on cheap substrates such as silicon.
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| Schematic drawing of the current MBE growth chamber at NTNU. Only four of the effusion cells are included in the drawing. Ti-ball, ion pump, mass spectrometer, and viewports are not included in the drawing. |
MBE is one of the most versatile and widely used non-equilibrium growth techniques for growing thin, epitaxial films of a wide variety of materials []. The deposition of material on the substrate is performed by evaporating the material from the effusion cells in a chamber maintained under ultrahigh vacuum (UHV). Due to the presence of UHV in the chamber, the mean free path of elements evaporating from the furnaces will be very high which leads to no collisions until they reach the sample surface. So, deposition rates of as low as 0.1 ML/s can be achieved by changing the temperatures of the effusion cells. In addition, a flux of molecules or atoms towards the substrate can be abruptly released (closed) by controlling the shutter in front of the cell. This allows growing heterostructures with abrupt interfaces. Performing the deposition in UHV chamber not only makes it possible to grow highly pure materials but also to install in-situ characterization techniques such as reflection high energy electron diffraction (RHEED).
Characterization techniques:
Characterization of nanomaterials involve determining the structural, optical, electrical, magnetic, and mechanical properties, depending on their applications. Structural characterization involves determining the morphology (Shape and size) of nanomaterials, arrangement of atoms (crystal structure), and material composition of nanomaterials. Optical characterization involves determining the light emission and light absorption characteristics of the structures. More details about the light emitting materials and their properties has been discussed in the Nano-photonics page.
XRD:
One can use XRD to determine lattice constant in c-direction and hence the composition of the epilayer, in-plane lattice constants, thickness of epilayers, c-axis and in-plane strain measurements, and crystal orientation.
There are several kind of measurements that can be done using XRD to obtain different kinds of information:
Reflectivity measurements,
theta-2theta measurements,
Reciprocal space mapping (RSM),
phi-scan,
pole-figure measurements.
Reflectivity measurements to measure the thickness of amorphous or polycrystalline layers on a smooth crystalline substrate.
Crystal Structure:
Crystal structure of any material indicates the pattern in which atoms are arranged. The most common crystal structures are SCC, BCC, and FCC. RECIPROCAL SPACE..
Determining the crystal structure, chiral number of CNTs
Electron Microscopy:
The most common technique used to determine the structural properties of the nano materials is by using electron microscopes. An electron microscope is a technique that uses an electron beam to illuminate a specimen and create a highly-magnified image. The advantage of using electrons instead of light is that the better resolution can be achieved, in the ~nm-Å range, compared to μm for light microscopy, due to the lower wavelengths of electron beam (resolution ~ half of the wavelength). The wavelength (λ) of electron beam depends on the energy of the electron beam; λ = 1.22/E^1/2 giving a λ of only 4 pm for 100 keV electrons. However, the spatial resolution of TEM, which uses the electron beam energy of 300 keV is worse than this, ~2 – 3 Å for conventional TEMs because of the aberrations in the lens system used in TEM. More details about the lens aberrations which includes chromatic aberration, spherical aberration can be found else where [http://www.matter.org.uk/tem/].
When an electron beam interacts with a specimen, electrons scatter in different directions as shown in Fig. 3.6. SEM generates the image by detecting the electrons scattered above the surface of a specimen. TEM generates the image by detecting the electrons transmitted through the specimen, and so the TEM sample should be thin enough allowing the electron beam to pass through the sample. The schematic diagrams of SEM and TEM is shown in the Fig. highlighting the basic difference between them. SEM is commonly used for determining the morphology and distribution of nanomaterials, while TEM is used to investigate the crystal structure, crystal defects and crystal composition, and to check the abruptness of the interface of heterostructure materials. Of course, the EDS equipped with SEM enables to determine the composition also by SEM although with less resolution compared to that of TEM.
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| http://barrett-group.mcgill.ca/teaching/nanotechnology/nano02.htm |
Some of the important terms the electron microscopy users usually know is resolution, depth of focus.
More details about the TEM can be found in http://www.matter.org.uk/tem/
and the latest developments in the TEM field can be found in http://superstem.com/
Scanning Probe Microscopy:
Scanning probe microscopy (SPM) involves running a nanometre-sized probe along a surface, and measuring its deflection. This allows the topography of the surface to be mapped out. Variations of the technique allow different aspects of the surface to be mapped out (such as density, viscoelastic response, magnetic behavior, density of electronic states, etc.). The simplest SPM technique is Atomic Force Microscopy (AFM).
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| http://barrett-group.mcgill.ca/teaching/nanotechnology/nano02.htm |
Fluorescence Microscopy:
Fluorescence microscopy is a rapid expanding technique, both in the medical and biological sciences. A fluorescence microscope (colloquially synonymous with epifluorescence microscope) is an optical microscope used to study properties of organic or inorganic substances using the phenomena of fluorescence and phosphorescence instead of, or in addition to, reflection and absorption [Wiki]. The technique has made it possible to identify cells and cellular components with a high degree of specificity. For example, certain antibodies and disease conditions or impurities in inorganic material can be studied with the fluorescence microscopy.
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation.
The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum and thus invisible, and the emitted light is in the visible region.
The samples to be inspected are labelled with fluorescent molecules such as fluorophore (such as green fluorescent protein (GFP), (fluorescein or DyLight 448) enhances the contrast.
Suppliers of Fluorescence microscope:
http://www.mccronemicroscopes.com
http://www.fluorescence-microscopes.com/
Scanning Near Optical Microscopy:
Suppliers of Scanning Near Optical Microscopy:
http://www.attocube.com/
Herbert Kroemer Quantum wells 2000
Charles K. Kao Optical Comm. 2009
Willard S. Boyle, George E. Smith CCD camera 2009
A. Geim and Novoselov Graphene 2010
