Would you like to know what core-shell nanocrystals are and understand how they get their unique electronic as well as optical properties that make them useful in many applications today? Did you know that quantum dots used in everything from the biomedical industry to the manufacture of solar cells and LED displays would not be possible without semiconductor core-shell nanocrystals exhibiting such behaviors as they do?
Let’s dive into a bit of materials science to understand what makes semiconductor nanocrystals different from bulk semiconductor materials. We’ll also find out what the purpose of the shell structure surrounding the core is, and how it’s crucial to enhancing the fundamental properties of nanocrystal quantum dots. And finally, we’ll look at the chemical synthesis of core-shell nanoparticles as well as their potential in optoelectronic applications that were impossible until now.
What Are Core-Shell Semiconductor Nanocrystals And Quantum Dots?
Core-shell semiconducting nanocrystals (CSSNCs) are a type of semiconductor nanoparticles that display fundamental properties that lie between those of individual molecules and bulk crystalline semiconductor materials. The core nanocrystals are what is typically known as the ‘quantum dot’ and are usually made from a distinct semiconducting material than what is used for the shell. Type IV-VI, II-VI, and III–V semiconductors are generally used with a standard core/shell notation used to identify which is which. For example, in InAs/CdSe, has an Indium Arsenide core and a Cadmium Selenide shell.
The electronic structure or distribution of electrons in quantum dots is different from regular bulk semiconductors which have continuous energy levels within bands. They are also different from singular molecules which have a HOMO-LUMO gap. Instead, quantum dots display quantized energy levels with a size-dependent increasing bandgap.
The research into the core-shell structure and its basic optical properties was first conducted in the 1980s with chemical synthesis methods finally developed in the 90s. These nanocrystal quantum dots are small, about 01 to 10 nanometers in diameter. Precise adjustments to the composition of the core and the shell materials, as well as core size, and shell thickness, allow engineers to manipulate the optical properties such as the emission wavelength of these nanocrystal quantum dots.
How The Basic Optical Properties Of Quantum Dots Arise
The electronic properties of semiconductors arise when electrons in the valence band acquire enough energy to jump the band gap to the conduction band. When an electron jumps, it leaves a hole in the valence band. Absorbing light energy can also do this. The electron and hole may exist in a bound state where the pair is called ‘exciton’. When the electron resumes its ground state and recombines with the hole, the result is exciton decay where its energy is released as light. This is what is called ‘fluorescence’.
The energy released is based on the band gap energy, the confinement energies of the excited electron and its hole, and the exciton’s bound energy. The confinement energy is dependent on the size of quantum dots. Synthesis methods have been developed which allow engineers to control the particle size and as a result, the emission wavelength of the fluorescence. Smaller quantum dots absorb and emit higher frequency or higher energy blue light while larger ones can absorb and emit lower frequency or lower energy red light.
To learn more about band theory and how semiconductors conduct electricity, read Understanding the Chemistry of Semiconductors.
The Surface Chemistry Of Semiconducting Nanocrystals
While semiconducting nanocrystals do have the same crystal structure as their bulk crystalline versions, they do exhibit different surface chemistry. The atoms on the surface have fewer atoms bonded to them than what the interior atoms usually have. Their atomic orbitals carry a small positive or negative charge, are localized at the surface, and have their own surface effects. These are called ‘dangling orbitals’ or ‘un-passivated orbitals’.
If the dangling orbital’s energy band lies within the semiconductor’s bandgap, it can act as a trap for charge carriers. In CdSe quantum dots, Cadmium provides the dangling orbitals to trap electrons, while Selenide provides the dangling orbitals for hole traps. In addition to surface trap states created by dangling orbitals, other surface defects could also act as charge carrier traps.
The fluorescence quantum yield is the ratio of the number of photons emitted to the number absorbed. Surface trap states can reduce the quantum yield of core-shell nanocrystals by pushing them towards non-radiative recombination instead of the desired fluorescence caused by exciton decay. The result is Auger recombination or phonon emission which as mentioned is non-radiative and doesn’t emit any light.
Improving The Fluorescence Quantum Yield With The Shell
If fewer charge carriers get caught in the traps, more exciton decay is favored instead of Auger recombination. Therefore, the surface traps need to be passivated. In core-shell nanocrystal structures, the shell material is what is used for efficient passivation of the surface traps. This can improve the fluorescence quantum yield, giving the core-shell nanocrystals better emission properties.
Apart from efficient passivation of the surface traps, the shell has another function. It acts as a barrier, physically separating the optically active core from the surrounding medium. This makes the entire structure less sensitive to photo-oxidation and even environmental changes that may occur.
Classifying Core-Shell Semiconductor Nanocrystals
Many of the properties seen in core-shell nanocrystals are due to the relative valence and conduction band edge alignment of the core nanocrystals and the shell materials. Depending on these energy bands, all charge carriers can be localized in the same place such as the core, or have one type of charge carrier in the core and the other in the shell. There are three types of CSSNCs depending on their energy bands and the resulting properties.
Type I CSSNC
When the bandgap of the core is smaller than that of the shell, it is called a Type I CSSNC. As shown in the diagram, the valence and conduction band edges of the core are both inside the band gap of the shell. This means that both electrons and holes will be localized inside the core. CdSe/CdS and CdSe/ZnS are examples of this type.
Reverse Type I CSSNC
When the core has a wider bandgap, the valence and conduction band edges of the shell will lie between those of the core. The charge carriers will be localized in the shell and changing the shell thickness will affect the emission wavelength. CdS/CdSe and ZnSe/CdSe are examples of this type.
Type II CSSNC
In Type II CSSNCs, the core’s valence and conduction band edges will be higher or lower than those of the shell. In ZnTe/CdSe, the holes will be localized in the ZnTe core’s valence band while the electrons will be localized in the CdSe shell’s conduction band. The energy difference between occupied states determines the emission wavelength.
The different types of CSSNCs are identified and characterized using a variety of imaging techniques such as X-ray photoelectron spectroscopy and transmission electron microscopy. To learn more about semiconductor microscopy techniques, read What Does A Semiconductor Microscope Do?
Synthesizing Core-Shell Nanocrystals
Colloidal synthesis is the chemical process used to manufacture semiconductor nanocrystals. Temperature and monomer concentration are two factors that have to be controlled during synthesis to produce nanocrystals that have the desired core size and shell thickness.
When making Cadmium Selenide nanocrystals, the Hydrogen Sulfide (H2S) gas is what is used to control the size of the CdSe core. If the H2S volume is increased, the core size is reduced. Furthermore, rapid cooling after the chemical solution reaches the appropriate reaction temperature can also produce a smaller CdSe core. The thickness of the shell is controlled by manipulating the amount of shell material that is introduced.
After the synthesis is complete, the nanocrystals have to go through a purification process. This is to remove impurities and unreacted materials introduced during crystal growth which will affect the surface chemistry in an undesirable manner if left around. In addition to quantum dots, other semiconductor nanostructures like wires and tubes can be synthesized that also have optoelectronic properties.
Since large batches can be produced, colloidal synthesis is the preferred method used in the manufacture of core-shell nanocrystals for commercial applications. However, there are other forms of synthesis such as using plasma for making Silicon and Germanium dots. There are also electrochemical
Cadmium Selenide, Cadmium Telluride, Indium Arsenide, Lead Selenide, and Indium Phosphide are some examples of compounds from which nanocrystals are made. Apart from quantum dots, semiconducting materials like Indium Phosphide have other applications like laser technology used for LiDAR and fiber optic cables.
To learn more, read The RF Semiconductor and Frequency Range.
Biomedical Applications Of Core-Shell Nanocrystals
The core is usually a quantum dot while the shell can be an organic molecule depending on the biological application. The higher quantum yield of core-shell nanoparticles means less energy can be used to initiate fluorescence. Compared to conventional organic dyes, CSSNCs have a fluorescence efficiency that is 100 to 1000 times higher, making them far superior.
Apart from this, the other desired emission properties of nanocrystals have fueled significant progress in biomedical applications. These are a narrow fluorescence emission that allows for multiple colors to be imaged simultaneously, a 20-second fluorescent lifetime that enables time-resolved imaging, high brightness, a broad absorption profile, and stability against photobleaching.
The CSSNCs’ ability to image multiple colors makes them perfect for in-vitro cell labeling. Getting through the cell membrane is difficult, but once inside, the nanocrystals can stay inside for some time, even after cell division. The nanocrystals can also be laid in a 2D matrix, allowing the tracking of cells that move through this area. To make nanocrystals less toxic and environmentally friendly, research is being conducted on using Silicon instead of Cadmium.
The ability of CSSNCs to emit in the 700–900 nm or near-infrared spectrum can be used in the tracking and diagnosing of cancer cells. Tumors can be targeted by using a quantum dot with a polymer shell that is conjugated with a cancer-specific antibody.
While there is a lack of data about the core’s toxicity, they have been known to cause DNA damage and are toxic to liver cells. To minimize these effects on the human body during in-vivo imaging, rare-earth elements and Silicon are being tested as potential materials for the core. The shell on the other hand has been known to reduce the toxicity caused by core materials.
Applications That Use Optical Properties Of Core-Shell Nanocrystals
The optically active core and the enhancement provided by the shell make CSSNCs great for a variety of optoelectronic applications including light-emitting devices, photo-voltaic systems, and photodetectors.
Light-Emitting Diodes And LED Displays
Perhaps the most famous of light-emitting devices is the light-emitting diode (LED) that has been in use for years. QD-LEDs or QLEDs as they are sometimes called can be easily fabricated on a Silicon substrate, which is what makes them useful in the manufacture of LEDs used for the backlight in televisions and computer monitors.
QLED displays have better color accuracy since the quantum dots can be precisely tuned as mentioned earlier. Only the colors necessary for each image will be displayed on the screen, making for a clearer picture that is also brighter. Since quantum dot technology is very good at producing monochromatic light, they are much more efficient than light emitters that need additional color filtering to produce all the colors necessary.
LEDs made from CSSNCs have shown narrower emission wavelengths of 32 nm which is much less than the standard organic LED (OLED) range between 50 to 100 nm. There is much potential in QLEDs that can be tapped in the future.
Optical gain media which are used to make semiconductor lasers have much to benefit from using Type II CSSNCs. Because this type of nanocrystal structure separates the electrons and holes of the excitons, an electric field is created. Auger recombination can be reduced, which in turn reduces the loss of energy from absorption. CdS/ZnSe for example has been used for optical amplification by stimulated emission of single-exciton states. The lasing threshold can be lowered which is a huge advantage for lasers developed using this material.
For more on optical amplification, read The Semiconductor Optical Amplifier and What It Does.
Dye-Sensitized Solar Cells
Quantum dots have shown promise in increasing the efficiency of photovoltaic systems or solar cells made using Silicon. ZnO/TiO2 nanoparticles have fast electron transport and a high surface area which makes them an ideal candidate for this application. ZnO/MgO can be used for nanowires of these solar cells. The MgO shell provides the insulation which prevents recombination.
The Future Of Core-Shall Semiconductor Nanostructures
CSSNCs still have a long way to go with new materials, structures, and synthesis methods being tested all the time. Another area where significant progress needs to be made is to replace heavy metals like Cadmium from quantum dot technology used in household devices. InP/ZnS as well as Silicon, Germanium, and Carbon has shown similar optical properties in the near-infrared and visible spectrum.
If you wish to learn more about semiconductors, their manufacturing, and other related topics, check out the Inquivix Technologies Blog today!
Core-shell semiconducting nanocrystals (CSSNCs) are nanoparticles made from a core of semiconducting material called a ‘quantum dot’ which has optical properties, and a shell made from a different material. Their physical chemistry is between the properties of bulk crystalline semiconductors and individual molecules.
The shell enhances the optical properties of the core while protecting it from environmental changes, and degradation from photo-oxidation, and adding modularity to the structure.