stable CdS nanoparticles, with an average size of 3.4 nm or 4.3 nm, respectively
[12] ZnS and CdS nanocrystallites have also been synthesised from aqueous and
methanolic solutions without an organic capping agent [13].
Henglein and Weller made significant progress [14–16] using CdS colloids prepared by controlled precipitation methods [17–22]. However, to obtain highly
monodispersed nanoparticles, post-preparative separation techniques such as size
exclusion chromatography [20] and gel electrophoresis have been employed [21].
Gel electrophoresis was found to be superior to other separation techniques.
Weller et al. synthesised nanocrystallites of Zn 3 P 2 and Cd 3 P 2 by the injection of
phosphine (PH 3 ) into solutions containing metal salts [18, 23]; control of particle
size was achieved by varying the phosphine concentration and the temperature of
the reaction. Samples of both Zn 3 P 2 and Cd 3 P 2 showed remarkable quantum size
effects, as observed by changes in the color of the products. Bulk Cd 3 P 2 is black
whereas a solution containing small nanocrystallites (1.5 nm diameter) is colorless
with a maximum in the electronic spectrum at around 310 nm. The color of such
Cd 3 P 2 changes from the black of macrocrystalline (band gap of 0.53 eV) through
the visible to white for the 1.5 nm sample (with a band gap of ca. 0.8 eV). Hexametaphosphate was used as a stabiliser to prevent particle aggregation. The sample
fluoresced when excited at 300 nm [23].
The relative solubility of inorganic salts can be used to prepare more complex
structures by such methods and examples include: CdS/ZnS [24], CdSe/AgS [25]
HgS/CdS [26], PbS/CdS [27, 28], CdS/HgS [29], ZnS/CdSe [30] and ZnSe/CdSe
[31] particles. The main constraints on the production of such structures involve
the relative solubility of the solids and lattice mismatches between the phases. The
preparation of ‘‘quantum dot quantum well systems’’ such as CdS/HgS/CdS [32,
33], has also been reported, in which a HgS quantum well of 1–3 monolayers
is capped by 1–5 monolayers of CdS. The synthesis grows less soluble HgS on
CdS (5.2 nm) by ion-replacement. The solubility products of CdS and HgS are
5 Â 10
À28 and 1:6 Â 10
À52 respectively. The authors reported fluorescence measurements in which the band edge emission for CdS/HgS/CdS is shifted to lower
energy values with increasing thickness of the HgS well [33].
Other examples of the preparation of semiconductor nanocrystallites by solution
methods can be found in the literature [1–3]. Solution methods provide a cheap
route to many nanoparticle materials. However, a lack of reaction control can be
problematic when larger scale preparations are necessary. Also several important
semiconductors are not easily obtained by this preparative method, with some being air and/or moisture sensitive, e.g. GaAs and InSb.
2.4.2
Reactive Methods in High Boiling Point Solvents
The routine synthesis of well-defined semiconductor nanocrystallites was really
opened up in a landmark paper by Murray, Norris and Bawendi [34]. They reacted
solutions of dimethylcadmium (CH 3 ) 2 Cd (in tri-n-occtylphosphine TOP) and trin-octylphosphine selenide (TOPSe) in hot tri-n-octylphosphine oxide (TOPO) in
2 Strategies for the Scalable Synthesis of Quantum Dots and Related Nanodimensional Materials
20
[12] ZnS and CdS nanocrystallites have also been synthesised from aqueous and
methanolic solutions without an organic capping agent [13].
Henglein and Weller made significant progress [14–16] using CdS colloids prepared by controlled precipitation methods [17–22]. However, to obtain highly
monodispersed nanoparticles, post-preparative separation techniques such as size
exclusion chromatography [20] and gel electrophoresis have been employed [21].
Gel electrophoresis was found to be superior to other separation techniques.
Weller et al. synthesised nanocrystallites of Zn 3 P 2 and Cd 3 P 2 by the injection of
phosphine (PH 3 ) into solutions containing metal salts [18, 23]; control of particle
size was achieved by varying the phosphine concentration and the temperature of
the reaction. Samples of both Zn 3 P 2 and Cd 3 P 2 showed remarkable quantum size
effects, as observed by changes in the color of the products. Bulk Cd 3 P 2 is black
whereas a solution containing small nanocrystallites (1.5 nm diameter) is colorless
with a maximum in the electronic spectrum at around 310 nm. The color of such
Cd 3 P 2 changes from the black of macrocrystalline (band gap of 0.53 eV) through
the visible to white for the 1.5 nm sample (with a band gap of ca. 0.8 eV). Hexametaphosphate was used as a stabiliser to prevent particle aggregation. The sample
fluoresced when excited at 300 nm [23].
The relative solubility of inorganic salts can be used to prepare more complex
structures by such methods and examples include: CdS/ZnS [24], CdSe/AgS [25]
HgS/CdS [26], PbS/CdS [27, 28], CdS/HgS [29], ZnS/CdSe [30] and ZnSe/CdSe
[31] particles. The main constraints on the production of such structures involve
the relative solubility of the solids and lattice mismatches between the phases. The
preparation of ‘‘quantum dot quantum well systems’’ such as CdS/HgS/CdS [32,
33], has also been reported, in which a HgS quantum well of 1–3 monolayers
is capped by 1–5 monolayers of CdS. The synthesis grows less soluble HgS on
CdS (5.2 nm) by ion-replacement. The solubility products of CdS and HgS are
5 Â 10
À28 and 1:6 Â 10
À52 respectively. The authors reported fluorescence measurements in which the band edge emission for CdS/HgS/CdS is shifted to lower
energy values with increasing thickness of the HgS well [33].
Other examples of the preparation of semiconductor nanocrystallites by solution
methods can be found in the literature [1–3]. Solution methods provide a cheap
route to many nanoparticle materials. However, a lack of reaction control can be
problematic when larger scale preparations are necessary. Also several important
semiconductors are not easily obtained by this preparative method, with some being air and/or moisture sensitive, e.g. GaAs and InSb.
2.4.2
Reactive Methods in High Boiling Point Solvents
The routine synthesis of well-defined semiconductor nanocrystallites was really
opened up in a landmark paper by Murray, Norris and Bawendi [34]. They reacted
solutions of dimethylcadmium (CH 3 ) 2 Cd (in tri-n-occtylphosphine TOP) and trin-octylphosphine selenide (TOPSe) in hot tri-n-octylphosphine oxide (TOPO) in
2 Strategies for the Scalable Synthesis of Quantum Dots and Related Nanodimensional Materials
20