1-6 Introduction to nanotechnology and low dimensional nanostructures
Nobel laureate Richard Feynman predicted that by the year 2000 products would be built one molecule or one atom at a time. The basic idea of nanotechnology was stated in 1959 by the famous physical scientist in the lecture “There's Plenty of Room at the Bottom.” This was a truly bold vision because it would represent a new paradigm for manufacturing and constitute a fundamental economic shift analogous to a second industrial revolution. This shift is referred today as the "nanotechnology revolution," and many people consider Dr. Feynman’s quote the birth of nanotechnology..
The computer industry is just one example of the advantages related to miniaturization. Getting small is a means of increasing the power and value of diverse products and services in most industries. For instance, many advances in biotechnology and the development of new drugs are the direct result of miniaturization and utilization of novel materials. The computing power, diagnostic and research power increase as tools decrease in size. Getting small allows biotechnology companies and researchers to do more complex experiments in shorter periods of time, for less money, using less material. This greatly accelerates discovery and ultimately shortens the time from concept to market for new advanced drugs and
other products. Further, nanotechnology enables companies and researchers to design revolutionary new products using new materials and substances not accessible with other technologies.
Nanotechnology, loosely defined as the study of functional structures with dimensions in the 1-100 nanometer range, is emerging as a distinct and promising field of research. Certainly, many organic chemists have designed and fabricated such structures for decades via chemical synthesis. During the last decade, however, developments in the areas of surface microscopy, silicon fabrication, biochemistry, physical chemistry, and computational engineering have converged to provide remarkable capabilities for understanding, fabricating and manipulating structures at the atomic level.
Research in nanoscience is literally exploding, both because of the intellectual allure of constructing matter and molecules one atom at a time, and because the new technical capabilities permit creation of materials and devices with significant societal impact. The rapid evolution of this new science and the opportunities for its application promise that nanotechnology will become one of the dominant technologies of the 21st century.
The structural goals of nanotechnology are frequently more ambitious than
making a single molecule. Often nanotechnologists wish to make arrays of identical or complex molecules, sometimes on a scale that will transcend the boundaries of the microscopic and approach the macroscopic. There are two different approaches to this end, 'top-down' and 'bottom-up.' The top-down approach is exemplified by scientists who build objects and molecular arrays using the techniques of scanning probe microscopy. The bottom-up approach is exemplified by investigators who design two- and three-dimensional chemical systems that cohere according to the rules of chemical interactions. The advantage of the top-down approach is its exquisite precision, but its disadvantage is its lack of extensive parallelism--it requires manipulating atoms and molecules practically one by one. In contrast, the bottom-up approach is massively parallel.
Two-dimensional (2D) nanosturctures (or quanum wells) [1] have been extensively studied by the semiconductor community because they can be conveniently prepared using techniques such as molecular beam epitaxy (MBE) [2].
Thanks to the efforts from many research groups, significant progress has also been made with respect to zero-dimensional (0D) nanostructures (or quantum dots) [3] in the past two decades. With quantum dots as a model system, a lot of intriguing chemistry and physics has been learned by studying the evolution of their fundamental properties with size [4]. Using quantum dots as active components,
various types of nanoscale devices have also been fabricated as prototypes in many research laboratories. Remarkable examples include quantum dot lasers [5], single-electron transistors [6], memory units [7], snsors[8], optical detectors [9], and light emitting diodes [10]. In recent, one-dimensional (1D) nanostructures such as nanowires, nanorods, nanobelts, and nanotubes have also become the focus of intensive research due to their unique applications in mesoscopic physics and fabrication of nanoscale devices. 1D nanostructures provide a good system to investigate the dependence of electrical and thermal transport or optical transition properties on dimensionality and size reduction also quantum confinement effect.
They are also expected to play an important role as both interconnects and functional units in fabricating electronic, optoelectronic, electrochemical, and electromechanical devices with nanoscale dimensions.
In contrast to quantum dots and wells, the advancement of 1D nanostructures have been slow until very recently, since hampered by the difficulties correlated with synthesis and fabrication of these nanostructures with well-controlled dimensions, morphology, phase purity, and chemical composition. Although 1D nanostructure can now be fabricated by using a variety of nanolithographic techniques [11], such electron-beam and focused-ion-beam writing, and x-ray or extreme-UV lithography, further development of these techniques into practical routes to a large quantities of
1D nanostructures from a diversified range of materials, swiftly, and at acceptable low costs, still requires great ingenious efforts. On the contrary, unconventional methods based on the chemical synthesis (will be discussed in the following passage) might provide an alternative and intriguing course for generating 1D nanostructures in terms of material diversity, cost, throughput, and the potential for high-volume production [12].
1-7 Methods to form low dimensional Indium Oxide nanostructures
The essence of 1D nanostructure formation is about crystallization, a process hat has been delved into for hundreds of years. The evolution of a solid from a vapor, liquid, or solid phase comprises two fundamental steps: nucleation and growth. As the concentration of the building blocks of a solid becomes sufficiently high, they aggregate into small clusters or namely, the nuclei, through homogeneous nucleation.
With a continuous supply of the building, these nuclei can serve as seeds for further growth to form larger structures. Though crystallization has been studied for such a long period, very little is known quantitatively about the process. Also, it’s generally accepted that the formation of a perfect crystal requires a reversible pathway between the building blocks on the solid surface and those in a fluid phases (i.e. vapor, solution, or melt). Theses conditions allow the building blocks to easily adopt correct position
in developing the long-range-ordered, crystalline lattice. Moreover, the building blocks also need to be supplied at a well-controlled rate to obtain crystals with a homogeneous composition and uniform morphology.
While developing a synthetic approach for generating nanostructures, the most significant issue to be addressed is the simultaneous control over dimensions, shapes, and the uniformity. In the past several years, multitudes of chemical methods have been reexamined or demonstrated as the “bottom-up” approach for generating 1D nanostructures with different level of control over these parameters. The following figure illustrates some of these synthetic strategies that contain 1) use of intrinsically anisotropic crystallographic structure of a solid to accomplish 1D growth (Fig. 1-2A);
2) introduction of a liquid-solid interface to reduce the symmetry of a seed (Fig. 1-2B);
3) use of various templates with 1D morphologies to direct the formation of 1D nanostructures (Fig. 1-2C); 4) use of supersaturation control to modify the growth habit of a seed; 5) use of appropriate reagents to kinetically control the growth rates of various facets of a seed (Fig. 1-2D); 6) self assembly of 0D nanostructures (Fig. 1-2E) and 7) size reduction of 1D microstructures (Fig. 1-2F).
Fig. 1-2-1. Schematic illustration of six different strategies that have been demonstrated for achieving 1D growth.
The following tips are demonstrating several feasible fabrication methods for Indium Oxide 1D nanostructures. The commonly used methods of fabrication are the Chemical Vapor Deposition (CVD) and the Vapor-Liquid-Solid (VLS) methods, with other less commonly used methods as well (consisting of electrodepositing Indium onto an Alumina template, then adding oxygen, or the method of rapidly heating Indium grains in a mixture of Argon and Oxygen). How these methods work, and their usefulness are explained below.
A. Electrodeposition
This method consists of what is basically described above: An Alumina (Al O
2 3) template is fabricated by anodizing Aluminum (i.e. the Aluminum is electrically charged to about 40 V, making it the anode to a typically Platinum Cathode, in a
acidic electrolytic solution, to create the porous Alumina template of about 30 micrometers thick). After this, Indium Particles are put in the equation, dropped into the recently created Alumina template. The particles are small enough to fit into the template’s pores, and are pulled in and bonded together. After this, the now formed Indium nanowires are oxidized (that is, Oxygen is added into the system), creating Indium Oxide nanowires [13, 14].
Fig. 1-2-2 (a) Cross-section of Alumina template, unfilled. (b) Alumina template, filled with newly created nanowires. (c) Top view of Alumina template, filled with newly created nanowires. Black holes refer to unfilled pores in the template white dots refer to filled pores, and the gray area in between dots is the Alumina template.
The strong points this technique makes lies in its possible high aspect ratio, along with pore size and packing density of the wires being controllable (as a function of acid strength and the amount of voltage used in the anodizing step). However, this technique can only create polycrystalline Indium Oxide nanowires, so other
techniques must be used in the fabrication of single-crystalline nanowires.
B. Rapid Heating
Through this process to create single-crystalline Indium Oxide nanowires, Indium grains (particles) are placed in an atmosphere consisting of a mixture of Argon and Oxygen (one such atmosphere consists of 90 % Argon and 10% Oxygen [15]). This atmosphere is maintained at a very high temperature (about 1030ºC [15]), and from there, using a vapor to solid approach (no catalyst is involved in this process, unlike the CVD approach [15]), the Indium grains combine with the Oxygen in the atmosphere and subsequently combine with other compounds as well to form the nanowires.
The advantage this method has is in no need for a catalyst, so further materials do not need to be brought into the equation. However, this method is rarely used anymore simply because of the fact that there is no way to control the diameter of the nanowires created. Diameters can vary greatly (from 40-120 nanometers [15]), so the need of a more controlled method is apparent, as this method is too varied for any scientific analysis.
C. Chemical Vapor Deposition
This method is very similar to the previously explained method, but holds a few key differences. In this method, the Indium matter (which is introduced from a laser ablation process, which projects a laser onto a Indium Arsenic target, releasing Indium particles along the way due to the incident light [16]) is vaporized in a vacuum tube (similarly to the method above, as the temperature was kept at a very high temperature), then flowed into a specific containment area afterwards. After this, an Argon/Oxygen gas mixture is also flowed into this containment area. The Oxygen reacts with the Indium, creating Indium Oxide particles. Different to the method above, these molecules are then placed onto a Silicon substrate, laced with a Gold catalyst. From each Gold molecule, the nanowires grow radially outward, until the process cannot allow for more molecules on one nanowire. After all of this, the area is cooled down to allow for the solidification of the nanowires [17]..
Fig. 1-2-3 Generalization of the Chemical Vapor Deposition process.
In this process, laser ablation is used to release Indium particles from the target.
These particles, along with an Argon/Oxygen mixture, are flowed into a high
temperature furnace. At the opposite end of the furnace resides a Silicon substrate, with Gold atoms as catalysts. The Indium Oxide molecules, formed earlier in the furnace, is attracted to the catalysts, and forms Indium Oxide wires from it [16].
The key differences between this process and the process described above are what make CVD perform better than its rapid heating counterpart, in the sense that diameter variance is much less (typically 30-50 nanometers [16]). However, even though the variance is down, there is still very little direct control over what diameter will be produced, which makes the following process much more widely used in the fabrication of Indium Oxide nanowires.
D. Vapor-Liquid-Solid
Vapor-Liquid-Solid (VLS) growing is a very sophisticated method, borrowing much from the Chemical Vapor Deposition process, in the fact that it uses the combination of things in vapor form to create the nanowires. Once again, Indium vapor is created using the laser ablation process on the Indium Arsenic target, which is introduced into the high temperature furnace (although there is no Oxygen present yet). However, this is where things get a bit different. Now, a Gold cluster is placed in the system, and is continually bombarded with the Indium vapor. From this continual feeding of the vapor, the Gold cluster forms into a Gold/Indium liquid drop (as the
temperature is high enough to vaporize Indium but not high enough to vaporize the combination of the two, just liquefying them), then the drop reaches supersaturation from all of the Indium, making the Indium particles grow out into a nanowire. After the Indium nanowire is created, Oxygen is then introduced, creating the single-crystalline Indium Oxide nanowire [13].
Fig. 1-2-4 Generalization of the Vapor-Liquid-Solid process.
As Indium vapor is continually supplied to the Gold cluster, the cluster forms a combination with the Indium, followed by the overabundance of Indium forming in a nanowire growth. Oxygen is then supplied to create the single-crystalline Indium Oxide nanowire.
From this process, nanowire diameter is directly controllable, as it is proportional to the size of the Gold cluster put in the system, and is basically equal to the diameter of the Gold/Indium combination. Therefore, creating consistent nanowires is now attainable, and as such, this is the most widely used method, especially when doing analysis on the properties an Indium Oxide nanowire contains [13].
1-8 Properties of Indium Oxide and Indium Oxide compound semiconductors
As far as the elements themselves go, Indium (element number 49 on the periodic table) is a fairly abundant metal, white and silvery in color, and produces a high-pitched sound when bent. A major application of Indium is to make low melting point alloys. Also notable is that Indium is a byproduct of the formation of Lead and Zinc, and many isolation processes (such as the electrolysis of Indium salts in water) are required to make Indium samples pure enough for electronic purposes [18].
Finally, plating applications exist for Indium due to its high resistance to corrosion, as well as it being used for doping semiconductors in transistor fabrication [19]. Oxygen (element number 8 on the periodic table) is a very abundant gas (which is colorless, but pale blue in a liquid state) that most life on the planet needs to live. About one fifth of the earth’s atmosphere is comprised of Oxygen, and Oxygen is well known to be a highly reactive element, and forms many compounds with other elements Finally, the combination of the two, Indium Oxide (In2O3), the compound of interest in the fabrication of Indium Oxide Nanowires, is a crystalline yellow solid, with a very high melting point (approximately 1913°C) and also known to be fairly dense as well (approximately 7180 kg/m³) [18]. Because of these properties, it is well suited to be used for electrical and optical applications (as it is an n-type semiconductor, but also can pass over 90% of visible light through it [20]). In addition, it’s a wide bandgap
material with the bandgap of around 3.5~3.6 Ev (340 nm). When it comes to such a wide bandgap material, it may intrigue people for its application on UV LEDs’ and white illumination.
1-9 Review of thus far achievement in applications of Indium Oxide
nanostructures
A. Transistor Applications
Because of the Indium Oxide nanowire’s semiconductor nature, it is well suited for use in field effect transistor applications. At the University of Southern California, testing was conducted using 10-nanometer diameter Indium oxide nanowire as a bridge between two metal electrodes (indicating the source and drain) over a Silicon substrate (creating a FET with a 3 micrometer channel) [13]. Results of such testing can be seen below:
Fig. 1-4-1 (a) Current/Drain Voltage comparison graph at various gate voltages and room temperature. (b) Current/Drain Voltage comparison at a constant gate voltage (0
V) and varying temperatures.
From the figures above, one can see that the Indium Oxide nanowire functions well as a gate for a field effect transistor application, as the output graphs are very similar to that of a regular metal oxide semiconductor field effect transistor (Mainly because the Indium Oxide nanowire is a metal oxide semiconductor) [13].
B. Chemical Sensor Applications
The ability for Indium Oxide nanowires to detect certain things at room temperature, such as chemical compounds, has turned up in recent days as well, especially for detecting the gas with high electron-affinity. Because when exposing the Indium Oxide nanowires to such the atmosphere, the electrons flowing near the surface of nanowires could be easily captures by the gas and effect would also be observed from the I-V measurement due to the slump of conductivity. From the same research team at the University of Southern California, the nanowire transistors created for the above application turned up again in this application. The transistors were placed in a vacuum area (to avoid impurities coming in), and the nanowire’s conductance was monitored as a chemical compound (in this case, NO
2 or NH
3) was diluted in Argon (an inert gas so it would not react with anything) or dry air and flowed in to the vacuumed chamber. Such a test produced these results for NO :
2
Fig. 1-4-2 (a) Current vs. Drain Voltage comparison with no gate voltage applied. The arrows indicate different scales - the before trend uses the left scale, and the after trend uses the right scale. (b) Current vs. Gate Voltage comparison with -.3 V applied to the drain. (c) Time response of the Indium Oxide nanowire under various concentrations of NO . 2
But what do these graphs say about the nanowire? In the first graph, before exposure to the compound, the trend exhibits that of a regular FET, with the well-defined linear and saturation regions. However, after the nanowire is exposed to the chemical compound, the conductance decreases dramatically, somewhere in the range of six orders of magnitude. If using such a difference in conductance as a basis, one could easily determine whether the compound was present or not. In the second graph, the chemical sensing properties of the Indium Oxide nanowire are reaffirmed, as well as other findings. As can be seen above, after exposure to the compound,
nanowire conductance is dramatically decreased. However, other findings pop up as well. For one, it is confirmed that the nanowire is an n-type semiconductor, as when the gate voltage negatively increased, the current drops, indicating a reduced conductance as well. Also, very important to note is the point where each of the curves flatten out, which more or less indicates the threshold point of the FET. As the compound is exposed, the threshold point increases from approximately –45 V to near +20 V, a very noticeable increase. In the final graph, one can note that the nanowire FET can detect even small concentrations of the compound; however the penalty the smaller compound gives is in the form of a much greater amount of time for the conductance to drop. From these experiments, the lowest concentration this FET was able to detect was 5 ppb. But why does the Indium Oxide nanowire act like this? This comes from NO
2 being an oxidizing gas (from it containing Oxygen in its compound).
From this, the Indium Oxide readily adsorbs the molecules also captures the electrons near the nanowire’s surface, which will then leave less room for free electrons logically, since there is less room for free electrons to maneuver, the conductivity
From this, the Indium Oxide readily adsorbs the molecules also captures the electrons near the nanowire’s surface, which will then leave less room for free electrons logically, since there is less room for free electrons to maneuver, the conductivity