sulfide. The dynamic equilibrium between solvated ions and solid CdS in
acetonitrile as a solvent in the presence of a styrene or maleic anhydride
copolymer allows the preparation of stable CdS nanoparticles (average
size 3.4–4.3 nm). To obtain highly monodispersed nanoparticles,
postpreparative separation techniques like size exclusion chromatography or gel electrophoresis are used.
Weller et al. (1985) injected phosphine (PH 3 ) into solutions containing
metal salts to make particles of Zn 3 P 2 and Cd 3 P 2 . The particle size can be
controlled by varying temperature and phosphine concentration. Even
though this method is cheap, it has several disadvantages. It lacks reaction control, which is problematic in large-scale syntheses. Furthermore,
important semiconductors like GaAs or InSb are not available for use in
this method as they are air and/or moisture sensitive.
9.4.2.2 Reactive methods in high-boiling-point solvents
Another approach for making quantum dots involves reaction of a precursor compound and a high-boiling, hydrophobic solvent. Solutions of
dimethylcadmium, (CH 3 ) 2 Cd (in tri-n-octylphosphine, TOP), and tri-noctylphosphine selenide (TOPSe) in hot tri-n-octylphosphine oxide
(TOPO) at 120°C−300°C produce TOPO-capped nanocrystals of CdSe. The
solvent molecules (e.g., TOPO) both serve as a reaction medium and also
coordinate with unsaturated metal atoms on the QD surface to prevent
the formation of bulk semiconductors.
The TOPO method is one of the most widely used methods of quantum
dot synthesis as it has several advantages over other methods. The size of
the dots can be controlled by temperature (i.e., they are larger at higher
temperatures) and monodispersity can easily be achieved. Additionally,
the yields of the reactions are usually high, producing hundreds of
milligrams in a single experiment. Moreover, this method is readily
adapted to the production of core-shell structures with materials having
high quantum efficiencies.
9.4.2.3 Gas-Phase synthesis of semiconductor nanoparticles
Quantum dots can also be synthesized in the gas phase. This method
involves the atmospheric or low-pressure evaporation of either powders
of a preformed semiconductor, or coevaporation of two elemental components (like Zn metal and sulfur). It is not a very favorable method due
to the large size distribution of the particles (10–200 nm) and the tendency
of particles to aggregate due to the absence of a surface-capping agent.
QUANTUM DOTS 361
acetonitrile as a solvent in the presence of a styrene or maleic anhydride
copolymer allows the preparation of stable CdS nanoparticles (average
size 3.4–4.3 nm). To obtain highly monodispersed nanoparticles,
postpreparative separation techniques like size exclusion chromatography or gel electrophoresis are used.
Weller et al. (1985) injected phosphine (PH 3 ) into solutions containing
metal salts to make particles of Zn 3 P 2 and Cd 3 P 2 . The particle size can be
controlled by varying temperature and phosphine concentration. Even
though this method is cheap, it has several disadvantages. It lacks reaction control, which is problematic in large-scale syntheses. Furthermore,
important semiconductors like GaAs or InSb are not available for use in
this method as they are air and/or moisture sensitive.
9.4.2.2 Reactive methods in high-boiling-point solvents
Another approach for making quantum dots involves reaction of a precursor compound and a high-boiling, hydrophobic solvent. Solutions of
dimethylcadmium, (CH 3 ) 2 Cd (in tri-n-octylphosphine, TOP), and tri-noctylphosphine selenide (TOPSe) in hot tri-n-octylphosphine oxide
(TOPO) at 120°C−300°C produce TOPO-capped nanocrystals of CdSe. The
solvent molecules (e.g., TOPO) both serve as a reaction medium and also
coordinate with unsaturated metal atoms on the QD surface to prevent
the formation of bulk semiconductors.
The TOPO method is one of the most widely used methods of quantum
dot synthesis as it has several advantages over other methods. The size of
the dots can be controlled by temperature (i.e., they are larger at higher
temperatures) and monodispersity can easily be achieved. Additionally,
the yields of the reactions are usually high, producing hundreds of
milligrams in a single experiment. Moreover, this method is readily
adapted to the production of core-shell structures with materials having
high quantum efficiencies.
9.4.2.3 Gas-Phase synthesis of semiconductor nanoparticles
Quantum dots can also be synthesized in the gas phase. This method
involves the atmospheric or low-pressure evaporation of either powders
of a preformed semiconductor, or coevaporation of two elemental components (like Zn metal and sulfur). It is not a very favorable method due
to the large size distribution of the particles (10–200 nm) and the tendency
of particles to aggregate due to the absence of a surface-capping agent.
QUANTUM DOTS 361