Quantum Dots and Their Synthesis Processes

Table of Contents:

Prashont Ambekar1[1] [2] and Josmirkour Randhawa2

Introduction

The possibility of zero-dimensional quantum confinement was realized in the year 1981, when Ekimov and co-workers at Ioffe Physical-Technical Institute, St. Petersburg observed imusual optical spectra for a sample of glass containing CdS and CdSe semiconductors [1]. The first explanation for the unusual optical behaviour has also been given by Ekimov, suggesting that nanocrystallites of the semiconductor got precipitated in glass due to heating and the quantum confinement of electrons in these nauocrystals, which were named quantum dots. A large amount of experimental and theoretical work has been done in the first half of the 1980s decade, studying the size-dependent development of bulk electronic properties in semiconductor crystallites of size ~ 15 to several hundred angstroms [2]. In some initial works, these crystallites have been termed as “clusters” because they were very small to have electronic wave function similar to bulk material despite exhibiting the same unit cell and bond length as the bulk semiconductors. For clusters, it has been concluded that complete delocalization of electrons has not yet occurred [3]. They are fluorescent semiconducting nanocrystals (NCs) with a radius that is comparable to that of the Bohr exciton radius of the material [4]. QDs possess various unique physical properties, such as quantum confinement effects, tunable electronic and optical properties, surface effects, high quantum yields, quantum tunneling effects, etc., due to their- modified energy structure and increased surface to volume ratio. А/The visible change in color from lemon green to blood red, with a gradual decrease in the size of QDs is depicted in Figure 7.1. Atypical example of emission wavelengths of different types and sizes of semiconductor QDs are listed hr Table 7.1 by Reshma et al. [5]. QDs are among the most researched materials at

The CdSe QD samples

Figure 7.1. The CdSe QD samples (A, С, E, G, and I) as seen under normal light and samples (B, D, F, H, and J) as seen under UY light (For mterpretation of the references to color m this figure legend, the reader is referred to the web version of this article). [Repmited from Synthesis, characterization, and application of CdSe quantum dots by Karan Surana, Pramod K. Singh, Нее-Woo Rliee and B. Bhattacharya. 2014. Journal of Industrial and Engineering Chemistry 20: 4188-4193. Copyright (2014) with the permission

from Elsevier],

Table 7.1. Emission wavelength and size of the different QDs (Reprinted with the permission from

ref. [5]).

Sr. No.

Quantum dots

Size/range/diameter (mn)

Emission range (nin)

1.

CdS

2.8-5.4

410-460

2.

CdTe

3.1-9.1

520-750

3.

CdSe

2-8

480-6S0

4.

CdTe/CdSe

4-9.2

650-840

5.

IllP

2.5—4.5

610-710

6,

InAs

3.2-6

860-1270

7.

PbSe

3.2-4.1

1110-1310

8,

DT-Ag,S [u]

5.4-10

1000-1300

present catering to the recent applications, such as light emitting diodes [6, 7], biolables [8, 9], medicine [10], lasers [11]. and sensors [12, 13].

Types of QDs

On the basis of the nature of materials, configuration/structure, and applicability, the QDs developed so far could be listed in different types as—

a. Semiconductor Quantum Dots (SQDs)

b. Carbon-based Quantum Dots (CQDs)

c. Infrared Quantum Dots (IR QDs)

d. Dilute Magnetic Semiconductor Quantum Dots (DMS QDs)

e. Core-Shell Quantum Dots (CSQDs)

Semiconductor Quantum Dots (SQDs): The fust in the genre are nanocrystals of group II-VI, Ш-V, and IV-VI bmaiy, forming a large number of luminescent materials with unique optical, electrical, and physical properties [1]. They demonstrate size, shape, composition, and nauoscale interface-controlled fluorescence properties over a wide range of emission spectra, ranging from 450 to 1500 mil, making them a potential candidate for multiplex optical sensing, in long term in vitro and in vivo imaging [15].

Carbon-based Quantum Dots (CQDs): Xu et al. in 2004 discovered a new class of carbon nanomaterials, i.e., CQDs, while working on the purification of single- walled carbon nanotubes [16]. They show good conductivity, high chemical stability, environmental friendliness, broadband optical absoiption, low toxicity, strong photolumiuescence (PL) emission, optical properties, and can be synthesized easily at a large scale. Their physiochemical properties are seen to be controlled by surface passivation/ functionalization with several polymeric, inorganic, organic, or biological materials [17]. The CQDs so far developed are of three kinds, viz., Polymer Dots (PDs), Carbon Nauodots (CNDs), and Graphene Quantum Dots (GQDs), of which GQDs and CQDs are fluorescent.

Infrared Quantum Dots (IR QDs): Due to the limited penetration depth of visible range photons, IR and near IR quantum dot-based devices are demanded by bio-imaging techniques looking at their performance in deep tissue imaging, wherein the absoiption window of a spectral range of hemoglobin and water are blocked. Moreover, IRQDs are also in demand due to their importance in harvesting the Sun’s infrared energy, which is available most of the time when direct sun rays are not available (~ 480 W/nr). PbS, PbSe, InAs are a few examples of IRQDs developed so far [18].

Dilute Magnetic Semiconductor Quantum Dots (DMSQDs): The ferromagnetism in nanomaterials enhances its usefulness in opto-spintronics [19]. Ferromagnetic DMSs with an energy gap in the visible range are obtained by doping paramagnetic transition metal ions into a wide bandgap of semiconductors, such as ZuO doped with transition metals, viz., V2+, Cr2', Mir', Fe2+, Co2+, Ni2+, of which V2+, Fe2+, and Co2" doping exhibits ferromagnetism at room temperature [20]. This class of materials appears to be extremely sensitive to the conditions of sample preparation and postsynthetic treatment, and the ultimate source of the observed ferromagnetism remains controversial [21].

Core-Shell Quantum Dots (CSQDs): Core-shell QDs (CSQDs) are developed to improve the photoluminescence efficiency of single QDs, as well as to enhance their sensing applications. Achievement and sustenance of quantum confinement is generally obtained by encapsulation with organic surfactant. Organic encapsulation acts as a surface trap state, aiding non-radiative de-excitation of the charge generated by the photon, and hence reduces the fluorescent quantum yield. In CSQDs, epitaxial layers of inorganic material are grown over the core material, which improves the quantum efficiency due to increased confinement of electron-hole pah in the core and dangling bonds on the surface. The core and the shell are typically composed of groups II-YI, IY-YI, and III-V semiconductors, with configurations, such as (CdS) ZnS, (CdSe) ZnS, (CdSe) CdS, and (InAs) CdSe [22]. Different categories of CSQDs are formed depending on the position of the valence and conduction band, and the essential energy gap between them in the semiconductors, as depicted in Figure 7.2.

Types of core-shell quantum dots. [Reprinted from Core-shell quantum dots

Figure 7.2. Types of core-shell quantum dots. [Reprinted from Core-shell quantum dots: Properties and applications by D. Yasudevan, Rolnt Raugauathau Gaddarn, Adrian Trmcln, Ivan Cole. 2015. Journal of Alloys and Compounds, 636: 395-404. Copyright (2015) with the permission from Elsevier],

  • [1] 1 Dharampeth M.P. Deo Memorial Science College, Nagpur, India-440033.- Government College of Eugmeermg, Nagpur, India-441108. Email: This email address is being protected from spam bots, you need Javascript enabled to view it
  • [2] Corresponding author: This email address is being protected from spam bots, you need Javascript enabled to view it
 
Source
< Prev   CONTENTS   Source   Next >