For bulk semiconductors, the carriers (electrons and holes) are free in all three dimensions and have a continuous valence band. When the size of a material becomes smaller than its exciton (electron–hole pair) radius, carriers are confi ned in space. Quantum dots are a unique group of semiconductor particles that are small enough to exhibit quantum confi nement eff ects.
As a result, the density of states that carriers can occupy becomes quantized.
Due to quantum confi nement, electronic and optical properties of materials dramatically change [ 8 ].
Some applications require a core-shell structure (e.g., for sharper lumines- cence) in which case the core compound semiconductor is surrounded by another compound semiconductor (e.g., CdSe/ZnS). Colloidal quantum dots are bound
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to stabilization agents (also called passivation or capping agents) that give the dot its stability in solution. Quantum dots are being investigated for a wide variety of applications, including:
• solar cells,
• displays,
• bioimaging,
• targeted drug delivery,
• polymer nanocomposites,
• single-electron transistors,
• lasers,
• nanosensors,
• infrared/near infrared photodetectors,
• light emitting diodes (LEDs),
Figure 37.1 Comparison of literature sources of quantum dots, cadmium selenide (CdSe) quantum dots, and sources referring to environmental aspects of CdSe quantum dots.
Note: For quantum dot related sources query keywords were quantum dot* or semiconductor nanoparticle* or semiconductor nanocrystal* at ISI Web of Science;
for CdSe, quantum dot keywords and CdSe were combined; for sources referring to environmental aspects, after a query at ISI Web, Google Scholar was also queried to search in full texts rather than limiting the search to abstracts only. Keywords used were environmentally, green, clean, nontoxic; individual records were then scanned to exclude irrelevant records.
5,000 10.0 4,500 4,000 3,500 3,000 2,500 2,000
2001 2002 2003
Quantum dots CdSe
2004 2005 2006 2007
# of sources # of sources referring to environmental aspects (for CdSe quantum dots)1,500
1,000 500 0
9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0
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• optical fi bers,
• optical amplifi ers,
• optical memory, and
• quantum computing.
Some of these applications have already been commercialized or are in the process of being introduced in the market [ 9 ]. Enhanced luminescence and band gap tunability of quantum dots enable them to be applied as nanosensors for environmental analysis and monitoring (screening, diagnostic applications, and monitoring). Several pollutants and pathogens in a sample can be detected simultaneously with high sensitivity [ 10 ]. Recent applications include detection of single cells of E. coli using CdSe/ZnS quantum dots [ 11 ], detection of Cryptosporidium parvum and Giardia lamblia quantum dot–antibody conjugates [ 12 ], and detection of E. coli O157:H7 and S. typhimurium [ 13 ].
Goldman et al. have performed multiplexed sandwich immunoassays by conjugating CdSe/ZnS quantum dots to antibodies to simultaneously detect four toxins that eliminate the need for numerous excitation sources/emission windows and complex processing [ 14 ].
Chemical sensors based on quantum dots are used to detect ions in aqueous solutions. Chen and Rosenzweig [ 15 ] reported the analysis of Cu(II) and Zn(II) ions by CdSe quantum dots capped with polyphosphate, L-cysteine, and thioglycerol in water samples. Jin et al. [ 16 ] reported detection of cyanide ions in water samples by a CdSe quantum dot nanosensor. Konishi and Hiratani [ 17 ] used an oligo (ethylene glycol) capped cadmium sulfi de quantum dot nano- sensor to detect copper ions (Cu[II] and Cu[I]). Gattas-Asfura and Leblanc [ 18 ] reported the detection of Cu(II) and Ag(I) with a peptide-coated cadmium sulfi de quantum dot nanosensor. Sirinakis et al. [ 19 ] used a CdSe/ZnS quantum dot nanosensor to detect aromatic hydrocarbons.
Indirect application of quantum dots toward improving environmental quality includes the use of quantum dot fi lms as active layers for solar cells. As part of the novel so-called third generation photovoltaics that aim to eliminate the shortcomings of conventional solar cells, quantum dot solar cells off er a dual solution for advancing the solar technology by increasing the effi ciency and allowing roll-to-roll production and thus increasing energy production and throughput and lowering manufacturing costs [ 20 , 21 ]. Quantum dot solar cells can absorb nearly all of the incident solar radiation for wavelengths above their absorption onset with a fi lm of only 200 nm thickness [ 22 ]. Candidate materials as quantum dots for solar cells are mostly compound semiconductors of group II–VI, IV–VI, or III–V of the periodic table, including CdSe, CdTe, CdS, InP, InAs, InSb, ZnS, ZnO, ZnSe, ZnTe, PbSe, PbTe, PbS, HgTe, GaN, GaP, GaAs, GaSb, Si, Ge, AlAs, and AlSb.
Quantum dots can be synthesized through two major routes: vapor-phase and liquid-phase deposition, or colloidal synthesis. In vapor-phase synthesis, quantum dots are grown through epitaxial self-assembly by deposition on the surface of a semiconductor layer that has a lattice structure compatible with
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the quantum dot-compound semiconductor. In colloidal synthesis, precursors of groups II and VI are separately dissolved in organophosphorus solvents such as trioctyl phosphine (TOP), tributyl phosphine (TBP), or triisopropyl phosphine (i-TPP) and injected into a solution of heated solvent (usually a coordinating solvent such as trioctyl phosphine oxide [TOPO]) or a solvent mixture. Quantum dots nucleate and grow instantaneously upon injection of both precursors—or only group V or VI precursors—into a fl ask containing the solvent—or a mixture of solvent plus group II or III precursors. Further growth occurs through Ostwald ripening (i.e., small particles are absorbed by bigger ones).
Colloidal synthesis of CdSe is the most well-known method for the synthesis of quantum dots, and has been extensively researched [ 23– 27 ]. It has become
“a model system” to study colloidal synthesis of quantum dots in general [ 28 ].
Figure 37.2 shows the fl ow for synthesizing CdSe quantum dots using the conventional and most widespread method pioneered by Murray et al. [ 23 ].
Colloidal synthesis is a batch process with low energy and process requirements. The vast majority of environmental impacts are likely to occur in connection with raw material acquisition; thus the choice of precursors and solvents is quite important in reducing the cumulative impact. For the dual precursor route, the source of selenium is selenium powder—there are no alternatives; however, it can be dissolved in diff erent solvents before injection.
For cadmium, alternative cadmium compounds can be employed. Dimethyl cadmium, an organometallic compound, is used as the cadmium precursor in conventional synthesis; however, dimethyl cadmium is an extremely toxic, expensive, and unstable solvent that limits its suitability for large-scale synthesis [ 29 ]. This has led researchers to search for alternative cadmium com- pounds including other organometallics such as alkyl/alkoxy cadmium com pounds (e.g., dineopentylcadmium), cadmium salts of fatty acids (cadmium acetate, cadmium oleate, cadmium laurate), and inorganic forms (cadmium oxide, cadmium carbonate). Instead of using dual precursor sources, single source precursors may also be used although they are less commonly known and applied [ 30– 32 ].
During CdSe quantum dot synthesis the cadmium and selenium precursors are dissolved in TOP, TBP, or i-TPP and form complexes with cadmium and selenium. The solvent precursors are injected in TOPO (termed “the hot matrix”) at high temperatures (250–300°C) [ 33 ]. Multicomponent solvents where phosphonic acids act as cosurfactants may also be used. Addition of tetradecylphosphonic acid (TDPA) to a hexadecyl amine (HDA)-TOPO-TOP stabilizing mixture slows nanocrystal growth resulting in good crystallinity and improved size distribution [ 34 ]. Addition of phosphonic acids also enables morphological control of quantum dots and leads to synthesis of rod- or tetrapod-shapes that show better performance for specifi c applications (e.g., solar cells) [ 35 ]. Trioctyl phosphine oxide surfactant also acts as a capping agent and ensures the solubility of quantum dots in nonpolar solvents, that is, quantum dots do not exist as stand-alone particles, TOPO molecules are
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attached to CdSe in solution [ 36 ]. Methods are also available to render CdSe soluble in aqueous solvents [ 37 , 38 ].
The need to search for alternative solvents stems from the cost of organophosphorus solvents. The price of quantum dots per gram is currently about $2,000 with solvents accounting for up to 90 percent of the cost [ 39 , 40 ].
Recent trends are to use saturated and unsaturated fatty acids (e.g., oleic acid, stearic acid, lauric acid) [ 41 ] and heat transfer fl uids based on phenyls [ 33 ].
The variety of capping agents is far more extensive and application-dependent, including alkylthiols, alkylamines, peptides, and carboxylic acids in addition
Figure 37.2 Liquid-phase synthesis of cadmium selenide (CdSe) quantum dots.
Drying, degassing, and heating TOPO under
Ar flow
Cd (CH3)2- TOP mixture Se-TOP mixture
Synthesis
Isolation
&
Purification
Size selective precipitation Injection of reagents
Flocculation using anhydrous
methanol (x3)
Centrifugation(x2)
Vacuum drying
Dispersion in anhydrous 1-butanol
Dropwise addition of anhydrous methanol
(xN)
Centrifugation (xN)
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to organophosphorus ones. Figure 37.3 presents alternative precursors and solvent pathways for the synthesis of CdSe quantum dots. Table 37.1 provides a listing of sources with varying precursors and solvents/ligands.