Colloidal nanostructures

When a semiconducting system reduces its size from a bulk material to be on the same order as the exciton Bohr radius, quantum confinement effect occurs. The electronic and optical properties become size dependent, resulting in significant development for advanced nanotechnology [133]. For example, quantum confinement changes the continuous energy bands of a bulk material into discrete, atomic-like energy levels in nanomaterials, leading to a discrete absorption/emission spectrum.

In a nanostructure, the electron/hole is confined in one or more dimensions. Thanks to many successful developments in fabrication, it is possible to control the size of the nanos-tructures in different dimensions. It should be noted that the dimensionality of the material is referred to as its number of degrees of freedom, therefore, it is opposite to the number of confined dimension.

Based on their geometry, nanomaterials can be classified as:

• (i) zero dimensional structures (0D) so called as nanoparticles, nanocrystals, and the most popular name - quantum dots: They are composed of several tens to a few thousand atoms, with all the electrons are localized (3D confinement);

• (ii) one dimensional materials (1D) which are cylinder-like such as nanowires, nanorods, and nanotubes with diameters/cross section in the nanoscale and lengths typically in the micrometer range: The excitons are only free to move along the structure (2D confine-ment);

• (iii) two dimensional (2D) object, for example, nanosheets, nanoribbons and nanoplatelets with the thickness of the order of a few nanometers: The electrons are restricted in the direction perpendicular to the structure (1D confinement).

These three categories are schematized in Figure 5.2.

5.1.2.1 Quantum dots

In 0D nanostructures like quantum dots the carriers are confined in all the three direc-tions. Since strong confinement is provided, these nanocrystals possess properties that are strongly dependent on their size and morphology. Their absorption and photoluminescence

1D confinement 2D confinement

3D confinement

FIGURE 5.2: Schematic representation of three types of quantum confined colloidal nanostruc-tures: quantum dots, nanorods, and nanoplatelets [32].

can be controlled precisely by varying their diameter. Therefore, they have emerged as po-tential light sources in a variety of practical applications, such as photodetectors [134] and biosensing/bioimaging [135].

Quantum dots are normally made from the II-VI or III-V compounds, for example, CdSe, CdS, ZnO, InP, InAs. They could be synthesized by many methods, such as chemical vapor deposition (CVD) [136], physical vapor deposition (PVD) [137], and solution-based approaches.

The wet chemical synthesis has become the most popular because it is simple, low cost, and mass produced. The size and dispersivity of colloidal quantum dots can be well controlled by managing the synthesis conditions, such as reaction time, temperature, and surfactants [138].

These organically passivated quantum dots have suffered from nonradiative surface-related trap states, resulting in low fluorescence quantum yield [139,140]. This could be overcome by growing layers of an other material to form the core/shell structure, which will efficiently passivate the surface trap states [141,142], therefore, give rise to quantum yield [143,144]. Moreover, the shell also acts as a layer protecting the core against the surrounding medium such as environmental changes or photo-oxidation [140]. Synthesis of colloidal core/shell quantum dots is now a very developed field of study with many real life applications [145].

5.1.2.2 Nanorods

One dimensional nanostructures such as rods, wires, and tubes are interesting objects in the fields of nanomaterials. They share many same properties to quantum dots or nanoplatelets, such as quantum confinement effects. However, thanks to their geometry, they possess some characteristics that are difficult to achieve by the other two categories, for example, 1D confined transport of electrons/photons and excellent mechanical properties [146]. Therefore, these cylindrical nanomaterials has been studied and introduced in many fields, such as, batteries [147], solar cells [148], and photoelectrochemical cells [149].

For about twenty years, many researchers have focused on developing one dimensional semiconductor nanostructures [150,151]. They could be fabricated by both physical and chemi-cal approaches, for example, lithography, oxide-assisted growth or solution–liquid–solid growth in organic solvents. In addition, the changeable chemistry of fabricating colloidal core/shell nanorods enables the formation of some new structural semiconductor heterojunctions like CdSe/CdS dot-in-rods. Depending on synthetic methods, the morphology-associated proper-ties and application explorations vary in the vast range. Many oxide II-VI nanomaterials such as ZnO have already been extensively researched and applied in life [152].

5.1.2.3 Nanoplatelets

Nanoplatelets are a new category of two dimensional materials consisting of a thin crys-talline slab of inorganic material [28,153]. They are of great interest for fabricating functional devices due to their high suface to volume ratio, well controlled nanoscale thickness, good opti-cal and photonic properties. Since the thickness of the nanoplatelet is smaller than the exciton Bohr radius, while the lateral dimensions are much larger, they can be considered as an atomic system with high spatial confinement in a single dimension, as shown in Figure 5.2. They have been grown from a wide variety of materials, but those made from semiconductors give very promising optical properties [32,154,155].

These nanoplatelets have electronic properties similar to two-dimensional quantum wells formed by molecular beam epitaxy, and their emission spectra could be tuned precisely by their thickness of growth. One of the most common colloidal nanoplatelets are Cadmium based ones. They are direct semiconductors in the visible region, and synthesized in both crystallite structures. In 2006, Hyeon and coworkers reported the synthesis of wurtzite CdSe nanoribbons.

Zincblende CdSe nanoplatelets were synthesized in 2008 by Ithurria and Dubertret [116]. The synthesis is based on a variation of the protocol developed by Cao and co-workers for the

syn-thesis of zincblende spherical nanocrystals. In contrast with the wurtzite CdSe 2D structures, the zincblende nanoplatelets have their thickness in the [001] direction.