How Nanowires Function

In 1965, engineer Gordon Moore predicted that the number of transistors on an integrated circuit – a forerunner to the microprocessor – would double roughly every two years. Today, this prediction is known as Moore's Law, though it isn’t technically a scientific law. Rather, Moore's Law is more of a self-fulfilling prophecy for the tech industry. Microprocessor manufacturers aim to meet this expectation, because failing to do so would leave them vulnerable to competition [source: Intel].

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To fit more transistors onto a chip, engineers must create smaller transistors. The first chip contained around 2,200 transistors. Today, a single microprocessor chip can house hundreds of millions of transistors. Despite this, companies continue to push for even smaller transistors to fit more on increasingly compact chips. There are already computer chips with nanoscale transistors (ranging from 1 to 100 nanometers – with a nanometer being one billionth of a meter). Future transistors will need to shrink further.

Introducing the nanowire, a structure with an extraordinary length-to-width ratio. Nanowires can be incredibly slender – in fact, a nanowire can have a diameter as small as one nanometer. However, engineers and scientists typically work with nanowires that range from 30 to 60 nanometers in width. There is optimism that nanowires could soon be used to create the tiniest transistors ever, though significant challenges remain.

This article will explore the characteristics of nanowires. We'll dive into the methods engineers use to construct nanowires and the strides they've made in creating electronic chips with nanowire-based transistors. In the concluding section, we'll investigate the potential applications of nanowires, including their possible use in the medical field.

Next, we’ll delve into the unique properties of nanowires.

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Characteristics of Nanowires

Depending on its composition, a nanowire may behave like an insulator, a semiconductor, or a metal. Insulators do not conduct electricity, while metals are highly conductive. Semiconductors, falling in between, conduct electricity under the right conditions. By positioning semiconductor nanowires correctly, engineers can build transistors that function as either a switch or an amplifier.

Nanowires possess some fascinating and unexpected properties due to their incredibly small scale. At the nanoscale and below, the behavior of materials is governed by quantum mechanics, a field that can be perplexing even for experts. Quantum mechanics often contradicts classical physics (also known as Newtonian physics), making it a challenge to fully grasp.

For instance, an electron typically cannot pass through an insulator. However, if the insulator is thin enough, the electron can traverse from one side to the other. This is known as electron tunneling, although the name doesn't quite convey how strange the process is. The electron moves from one side of the insulator to the other without actually passing through the insulator or occupying its internal space. In a way, you could say it teleports. To prevent electron tunneling, thicker layers of insulation can be used, as electrons can only move across very short distances.

An intriguing property of some nanowires is that they function as ballistic conductors. Unlike regular conductors, where electrons collide with atoms in the material, slowing them down and generating heat, ballistic conductors allow electrons to travel unimpeded. This means nanowires could potentially conduct electricity efficiently without producing the excessive heat typically associated with traditional conductors.

At the nanoscale, materials often display properties that are vastly different from what we observe at larger scales. For instance, gold has a melting point exceeding 1,000 degrees Celsius in its bulk form. However, when gold is reduced to nanoparticles, its melting point decreases significantly due to the increased surface-to-volume ratio. Moreover, at the nanoscale, gold acts like a semiconductor, whereas it behaves as a conductor in its bulk state.

Other elements also exhibit unusual behavior at the nanoscale. For example, aluminum is not magnetic in its bulk form, but when reduced to tiny clusters, aluminum atoms become magnetic. The familiar properties of elements that we encounter in our everyday lives may not hold true when these elements are scaled down to the nanometer range.

Our understanding of how different elements behave at the nanoscale is still developing. Some elements, such as silicon, show little change when reduced to the nanoscale, making them perfect for applications like transistors. Other elements remain a mystery and may display unpredictable properties that we cannot yet foresee.

In the next section, we'll explore how engineers create nanowires.

Constructing Nanowires from the Top-Down Approach

Experts in nanoscience refer to two distinct methods for creating structures at the nanoscale: the top-down approach and the bottom-up approach. The top-down approach involves starting with a large quantity of material for the nanowires and carefully carving it down to the desired dimensions. Meanwhile, the bottom-up approach is a process of assembly, where smaller particles come together to form a larger structure.

Although both approaches are viable for constructing nanowires, a way to mass-produce them efficiently has yet to be discovered. Currently, scientists and engineers would need to invest considerable time to create even a fraction of the nanowires required for a microprocessor chip. Additionally, arranging the nanowires correctly once they are fabricated presents a major challenge. The nanoscale makes it extremely difficult to automatically build transistors — engineers typically manipulate the wires into position manually while observing the process through a high-powered microscope.

A good example of the top-down approach is how scientists create fiber-optic nanowires. Fiber-optic cables transmit data using light. To produce a fiber-optic nanowire, engineers begin with a standard fiber-optic cable. There are various techniques for scaling down the cable to the nanoscale. One method involves heating a sapphire rod, wrapping the fiber-optic cable around it, and then pulling the cable to stretch it into a thin nanowire. Another approach uses a small furnace made from sapphire, through which the fiber-optic cable is drawn and stretched into a nanowire. A third technique, known as flame brushing, involves applying a flame beneath the fiber-optic cable while stretching it [source: Gilberto Brambilla and Fei Xu].

In the upcoming section, we will explore the methods scientists use to grow nanowires from the bottom up.

Growing Nanowires

Chemical vapor deposition (CVD) is an example of a bottom-up technique. CVD is a group of processes where solid materials are formed from a gaseous phase. In this process, scientists place catalysts (like gold nanoparticles) on a surface known as a substrate, which attracts the nanowires as they form. The substrate is placed in a chamber with a gas containing the desired element, such as silicon, and the atoms in the gas do the work. First, the gas atoms adhere to the atoms in the catalysts, then more gas atoms bond with those, and the process continues, building a chain or nanowire. Essentially, the nanowires grow autonomously.

A novel approach to constructing nanowires involves printing them directly onto a suitable substrate. This technique was pioneered by a research team in Zurich. They began by carving a silicon wafer so that the raised sections aligned with their intended nanowire arrangement. Using this wafer as a stamp, they pressed it onto a synthetic rubber called PDMS. Then, they spread a liquid containing gold nanoparticles, referred to as a colloidal suspension, across the PDMS. The gold particles settled into the channels formed by the silicon wafer, transforming the PDMS into a mold capable of transferring gold nanowires onto other surfaces. These PDMS molds are reusable and may play a role in future mass production of nanowire circuits. [source: Nature Nanotechnology].

Although several labs have successfully created transistors using nanowires, the process is time-consuming and requires considerable manpower. Nanowire transistors are as efficient as, or even outperform, current transistors. If scientists discover a way to efficiently produce and connect nanowire transistors, it will enable the development of smaller and faster microprocessors, helping the computer industry stay aligned with Moore's Law. This advancement will lead to increasingly compact and powerful computer chips.

Global research into nanowire production continues to progress. Many scientists believe that it is only a matter of time before a feasible method for mass-producing nanowires and nanowire transistors is discovered. When that day arrives, it is hoped that we will also find a way to arrange the nanowires precisely, unlocking their full potential for use in a variety of applications.

In the upcoming section, we will explore the potential applications of nanowire technology.

Applications of Nanowires

One of the most prominent applications for nanowires lies in electronics. Certain nanowires function as excellent conductors or semiconductors, and due to their incredibly small size, manufacturers could potentially fit millions more transistors onto a single microprocessor. This would result in a significant boost in computer processing speed.

Nanowires could be crucial in the advancement of quantum computers. Researchers in the Netherlands developed nanowires made from indium arsenide and connected them to aluminum electrodes. At temperatures close to absolute zero, aluminum turns into a superconductor, enabling it to conduct electricity without any resistance. The nanowires also became superconductors due to the proximity effect. By applying different voltages through the substrate beneath the wires, the team was able to control the superconductivity of the nanowires. [source: New Scientist].

Nanowires might also play a significant role in the development of nanoscale devices such as nanorobots. These tiny robots could be used by doctors to treat diseases like cancer. Some nanorobot designs include internal power systems, and these systems would require components like nanowires to generate and transfer the necessary power.

By utilizing piezoelectric materials, nanoscientists could potentially develop nanowires that generate electricity from kinetic energy. The piezoelectric effect occurs when certain materials produce an electric charge when subjected to physical force. Conversely, if an electric charge is applied to these materials, they begin to vibrate. In the future, piezoelectric nanowires may power nanoscale systems, although no practical uses have been developed yet.

There are countless other possible applications for nanowires in electronics. For instance, researchers in Japan are working on atomic switches that could someday replace semiconductor switches in electronic devices. Scientists at the National Renewable Energy Laboratory are hopeful that coaxial nanowires will enhance the energy efficiency of solar cells. Since we are still exploring the properties of nanowires and other nanoscale structures, there may be thousands of untapped applications waiting to be discovered.