28 Quantum Dots

S.S. islam

Introduction

A quantum dot (QD) is an extremely small particle whose properties can be drastically changed merely by removing or adding an electron. In this parlance, an isolated atom can also be termed as a quantum dot. However, only a cluster of atoms or molecules demonstrates property variations. In biochemistry, QDs are often termed as redox groups. QDs are referred as quantum bits or qubits in nanotechnology. The dimensions of QDs are of the order of few nanometers.

Nanotechnology is a multi-disciplinary field wherein the problem often includes studying biology, chemistry, computer science, electronics, etc. For example, consider a hypothetical biochip, which is grown analogous to a tree from a seed, and which might also include a computer inside it. In this analogy, both terms – a qubit or redox group – both are equally applicable. Besides, the classification of this chip as animate or inanimate, is difficult to accomplish. Each QD present in this chip may account for one or more data bits.

In some cases, an electron within the QD may occupy several states, thus, a QD may also represent a byte of data, instead of a bit (in case of qubit). Alternately, one QD might be simultaneously involved in more than one computations. In addition to these applications, other sophisticated uses of a QD may include nano-mechanics, neural networks, high density data storage, etc.

Applications of Quantum Dots (QDs)

a. Solar Cells

One of the most important features of silicon of use in electronic devices (e.g. transistors) is doping. Wherein small quantities of various elements can be added to silicon in order to generate either an excess (n-type) or deficiency (p-type) of electrons in it, thereby enhancing the material’s conductivity. Additionally, p-n junction can be created by joining n-type and p-type silicon, which are the building blocks for almost all the devices in electronic industry.

Figure 1 TEM images of InAs n-type QDs doped with silver, 3.3nm in diameter.

Doping the QDs is a relatively new area of research, and both n-type and p-type QDs have been produced. Figure 1 shows the indium arsenide QDs which have been doped with silver to create n-type QD. Low cost solar cells can be realised by using QDs, as QDs can be easily prepared via simple and economic chemical processes. Additionally, thin-film photovoltaics can be prepared from QDs which have efficiencies comparable to those of traditional silicon cells. The enhancements in efficiency of QDs can be attributed to the different materials that can be used in their synthesis. Since some semiconductors can emit multiple electrons on absorbing one photon. In addition to this, the manipulation in their size and shape can result in absorption of light having different colours.

However, the synthesis of efficient QD based solar cells has not yet been possible. For preparing solar cell, an n-type, and a p-type nanocrystals are required. When light is incident on a solar cell, electrons and holes are generated as light photons gets absorbed in the material. These electrons and holes must be separated so as to avoid their spontaneous recombination. The separation of these charge carriers results in the flow of electrons out of the semiconductor to the external electrical circuit. However, some of the electrons and holes recombine, this does not cause and electron (and thereby current) flow in the external circuit. This recombination is much pronounced in QDs than the large silicon crystals. Doping of the semiconductor nanocrystals for making p-n junction can separate the electrons and holes more efficiently.

Conventionally, silicon is doped with P or B atoms, however, this cannot be done with the QDs because of their nanometer size. For comparison, a 4 nm QD comprises ~1000 atoms. Addition of few dopant atoms can result in expelling these atoms from the nanocrystal.

Solar Cell Basics

In a typical solar cell, the absorption of light results in the generation of electron-hole (e-h) pairs. When the e-h pair is bound, it is termed as exciton. An internal electric filed (present in p-n junctions or Schottky diodes) can separate this pair, leading to the flow of electrons and holes, which in turn cause an electric current. This internal field can be generated by differential doping of one part of the semiconductor with electron donating atoms (n-type doping), and doping the other part with electron accepting atoms (p-type doping). This results in the formation of a p-n junction within the semiconductor. The e-h pair is generated only when the energy of the absorbing photon in greater than the bandgap of the material. Thus, (a) the photons of lower energy than the bandgap are not absorbed, and (b) photons with higher energy than the bandgap are quickly (within ~10− 13 seconds) thermalised to the band edges, decreasing output. Part (a) causes a reduction in current, and thermalisation decreases the voltage. Consequently, a trade-off exists between voltage and current in a semiconductor cell. This can be partially avoided by employing multiple junctions. Complete balance evaluations envisage a maximum efficiency of 31% for a monolithic solar cell.

A little bit of mathematics can show that the maximum efficiency (i.e., 31%) corresponds to a bandgap of 1.3-1.4 eV, or the near infrared (NIR) spectrum. Silicon’s bandap (1.1 eV) is closest to this value, and this is one of the key factor in the predominance of silicon in solar cell applications. Notwithstanding this advantage, the maximum efficiency achievable with silicon is ~29%. This value can be enhanced via ‘tandem’ or, multi-junction approach, wherein multiple single-junction cells of different bandgaps are vertically stacked. As an example, a 2-layer cell should have one layer of 1.64 eV and second with 0.94 eV, resulting in maximum efficiency of 44%. Similarly, a 3-layered (1.83, 1.16, 0.71 eV) gives the value of 48%. Theoretical calculations have predicted the maximum efficiency of ~86% for an infinite layered cell, where the thermodynamic losses limit the efficiency to this value.

What About Quantum Dots

QDs are semiconductor particles whose dimensions are less than the exciton Bohr radius, resulting in quantum confinement effect being dominant in deciding their properties. Due to the confinement effects, the electron energy levels become finite. The situation is similar to an atom, and that is why, QDs are often termed as ‘artificial atoms’. By adjusting the size and shape of the QDs, these energy levels can be tuned, which then defines the bandgap. QDs can be made with different sizes, thereby changing their bandgap without any modification in the underlying material or the synthesis processes. Ideally, the size of the dot can be changed by simply controlling the synthesis time and temperature.

Owing to the tunable bandgap, QDs are interesting for solar cell applications. Single junction cells of PbS (lead sulfide) colloidal QDs demonstrate bandgaps in the range of far infrared, which is difficult to obtain in traditional cells. Since the maximum portion of the solar energy reaching the Earth is in infrared and near infrared regions, a QD, owing to its bandgap in the right spectrum can make use of this enormous energy.

Furthermore, colloidal QDs can be easily synthesised. Being suspended in a colloidal liquid, they are easily handled throughout production process. Colloidal QDs can be produced in large amounts and the dots can be spread on desired substrate via spin coating.

Dye Sensitized Solar Cell (DSSC)

These are the most recent cell design, wherein a sponge like layer of TiO2 is used as semiconductor value and also provides mechanical support. During construction, an organic dye (e.g., ruthenium-polypyridine) is filled in the sponge which injects electrons into TiO2 upon photoexcitation. Since ruthenium is a rare metal, the cost of the dye is high.

Since the discovery of DSSC, the use of QDs as a substitute to the organic dyes has been considered. Owing to the possibility of tuning the bandgap, other design parameters (such as material sued for other parts of the cell) can also be changed. For instance, the researchers have developed a cell wherein the rear electrode is in contact with a film of QDs, thereby eliminating electrolyte, and resulting in a depleted heterojunction. The cell had an efficiency of 7.0%, which is superior to solid-state DSSC devices, but less than devices using liquid electrolytes.

Multi-junction

In cells absorbing multiple frequencies, cadmium telluride (CdTe) is usually used. A colloidal suspension of CdTe crystals in spin coated over a substrate (e.g., glass slide). Such cells do not employ QDs, since low scale production of QDs is expensive.

Hot-Carrier Capture

The efficiency of the cell can also be improved by capturing the surplus energy of the electron when emitted from a single bandgap material. In materials like silicon, the emission site is at a fair distance from the electrodes (where electrons are collected); electrons interact with he crystal lattice, dissipating their surplus energy as heat. So the electrons do not have extra energy. For increasing these interactions, amorphous silicon was studied, however, the large amount of defects inherent to amorphous materials suppressed their advantages. Modern thin-film cells have less efficiency than conventional silicon cells.

To alleviate these problems due to defects, thin uniform films can be prepared from nanostructured donating species. However, these structures suffer from other issues inherent to QDs, e.g., high resistivity, heat retention.

Multiple Excitons

Researchers in 2004 reported that in QDs, multiple excitons can be created by absorption of one photon. Thus, more solar energy can be captured by collecting them. This approach is termed ‘carrier multiplication’ (CM) or ‘multiple exciton generation’ (MEG), wherein the QD is modified to generate more than one e-h pairs at relatively lower energy than one pair at high energy. This results in enhanced photocurrent, thereby increasing the efficiency.

Non Oxidizing

Recently, researchers from MIT developed solar cells from ZnO/PbS which are stable in air and have an efficiency of 9.2% (in laboratory conditions). The high efficiency was the result of efficient light absorption and charge transport to the electrodes (current collector). The cells are highly stable in air and retained their performance for more than 150 days of storage in air.

b. Display Devices

Quantum dot display employs semiconducting nanocrystals or QDs as display element, where QDs either emit or convert (in case of LED backlit LCDs) light. Presently available display devices (e.g., TV), termed as QLED, use QDs to convert light for LCD backlights, rather than using them for actual display.

The use of QDs as light source was first proposed in 1990s, and typical applications were imaging with QD infrared photodetectors, LEDs, etc. Presently, QDs are intensely investigated for their use in light sources and displays. By controlling the composition and size of a quantum dot, its properties can be controlled. Furthermore, quantum dots are photoactive (photoluminescent) as well as electroactive (electroluminescent), thus, they can be easily incorporated in new emissive display devices.

True QD displays (QLEDs) are at experimental stage and are different from the present commercial devices. QLEDs use QDs or semiconductor nanocrystals as electro-optical display technology, similar to organic LEDs or OLED displays where light is emitted when required. Thus these displays are efficient and the next technology after OLEDs. There are concerns over the use of cadmium to produce QLEDs, thus such devices are still experimental.

Big and flexible displays can be prepared by using QDs. Besides, these displays do not easily degrade like the OLEDs, thus, quantum dots are potential candidates for flat-panel TV screens, digital camera, mobile phones, etc.

Optical Properties of QDs

In contrast to atoms, the energy levels in a QD exhibit strong dependence upon its size. For instance, CdSe QDs emit light of different colours for different sized dots, i.e., a 5nm dot produced red, and 1.5nm dot produced violet light. These changes in colour depending on the size can be attributed to the quantum confinement effects and directly depends on the energy levels of the dot. The energy (and color, in turn) is determined by the bandgap of the QD which varies inversely with the square of the QD size. Thus, larger dots have more energy levels and these are closely spaced, these QDs emit or absorb lower energy photons (redder color). In brief, the energy of the emitted photons increase with decrease in the size of the QD, since more energy is needed to confine the excitation to a smaller volume.

Quantum Dot Light Emitting Diodes (QLEDs)

QLEDs emit characteristically pure and saturated colors having narrow bandwidths. The wavelength of the emitted light can be easily controlled by adjusting the QD’s size. The other advantages of QLEDs include high efficiency, flexibility, relatively lower processing costs than OLEDs. QLEDs can be tuned to operate in the complete visible range, i.e., from 460nm (blue) to 650nm (red). Additionally, by altering the chemical composition of the quantum dots, the emission wavelength can further be extended to UV as well as NIR wavelengths.

QLED technology involves using electroluminescent nanoparticles instead of photoluminescent QDs used in present quantum dots based TV. Thus, in QLED, direct emission of light is used as display, rather than conversion via LED backlights. Instead of using a separate backlight for illumination, QLED TV locally controls the light emitted from individual pixels.

c. Nanoelectronics

Nanoelectronics involves using nanotechnology in electronic components. The devices produced are so small that interatomic interactions and quantum confinement effects are applied to them. The devices include hybrid molecular/semiconductor electronics, nanotubes/nanowires (e.g. SiNWs, CNTs), etc. Recent silicon CMOS devices are also within this length scale. Nanoelectronics is often termed as disruptive technology as present candidates are significantly different from traditional transistors.

Basic Concepts

Gordon Moore, in 1965, proposed a continuous down scaling of silicon transistors, this was later known as Moore’s law. Since then the transistor feature size has been reduced from 10 microns to around 25 nm till 2011. Nanoelectronics is bout continuously realising this law via novel materials and techniques to fabricate electronic devices of nanoscaled dimensions.

An important mathematical relation highly important in nanoelectronics states that, ‘an object’s volume decreases as third power of its linear dimensions, while its surface decreases as the second power’. Thus, the downsizing of objects in nanotechnology must be assessed properly, taking into consideration the above mentioned rule.

Nanoelectronic Devices

Present manufacturing processes use conventional top-down approach for device fabrication. Using this technique, critical length scales in ICs has already reached the nanoscale, for instance, the gate length in transistors widely used in modern electronic devices is of the order of 50nm.

Memory Storage

Traditional memory devices use transistors for storing information. With the help of cross bar switches based electronics, ultra high density storage devices can be produced which have reconfigurable interconnections between vertical and horizontal wiring arrays. This has been achieved by Nantero (developed CNTs based crossbar memory ‘Nano-RAM’), and Hewlett-Packard (proposed memristor to replace flash memory).

Novel Optoelectronic Devices

Optical and optoelectronic devices offer extremely large bandwidths and high capacities; and are replacing the conventional analog devices in modern communications systems. These include photonics crystals and QDs.

In photonic crystals, the refractive index varies periodically with a lattice constant. The lattice constant is the half of the wavelength of light used. They behave like a semiconductor, with the exception that they work with light or photons, rather than the electrons. Thus, photonic crystals have a tunable bandgap for propagating a specific wavelength of light.

QDs are nanosized objects which can also be used to construct lasers. A QD based laser offers the advantage of tunable emission wavelength over the conventional semiconductor lasers. The emission wavelength can be manipulated by changing the diameter of the QD. Additionally, QD lasers are inexpensive and provide high beam quality.

d. Quantum Computers

Quantum mechanical principles can be exploited in quantum computers, thereby enabling the use of fast quantum algorithms. Quantum computers use quantum bit (qubit) as memory space for simultaneous multiple computations. Thus much faster computers can be build.

e. Energy Production

Nanowires and other nanostructured materials can be used to produce inexpensive as well as highly efficient solar cells. More efficient use of solar energy is imperative towards meeting the global energy requirements.

Another application of nanostructured materials in energy production is the use of bio-nano generators which can operate in vivo. These are nanoscaled electrochemical devices analogous to fuel cells. They draw power from the glucose present in the blood of living body, in a similar manner as the body uses food to generate energy. This is achieved by using an enzyme which removes the electrons from the glucose, these free electrons are then used in the device. A bio-nano generator can be used to generate ~100 Watts power (~2000 food calories per day). This is true if the complete food is converted into electricity. Since human body also need some energy for its essential functions, actual electricity produced via bio-nano generator could be less. However, this electricity can be used to power devices embedded within the body (e.g., pacemaker).

f.  Medical Diagnostics

Nanoelectronic devices for the detection of the concentration of biomolecules in realtime presents an attractive application as medical diagnostics. Similarly, devices can be produced which can interact with individual cells. Such devices are known as biosensors and represent a recent area of research for nanoelectronic devices. These applications of nanoelectronic devices are important for health monitoring, defense technology, etc.

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