Such structures should exhibit the idealized variations in density of electronic states predicted by simple particle in a box type model of elementary quantum mechanics, with the continu
Trang 2Chapter 5 Introduction to Nanoparticles
5.1 General Introduction
About half a century ago, Richard Feyman gave his prophetic lecture “Plenty
of Room at Bottom.” He outlined in this talk the foundations of nanoscience, and the promise that totally synthetic constructions could eventually be built with molecular scale precision Nanoscience research has been rapidly increasing across the globe during the past decade It is now widely accepted by the scientific, industrial, government and business communities, that nanoscience will be integral importance
in the development of future technologies Nanoscience is being touted as the engine that will drive the next industrial revolution
NPs, are referred to particles with size ranges from 1 to 100 nm in diameter Meanwhile, NC is referred to single crystal with size of 1 to 100 nm In this thesis, we are using the term “nanosynthesis” to describe the synthesis that product NP Usually the properties of crystalline solids are ordinarily catalogued without reference to their size It is only in the regime below 10 nm, called Quantum Dots (QDs) where this variable comes into play Independent of large number of surface atoms, NCs with the same interior bonding geometry as a known bulk phase often exhibit strong variations
in their optical and electrical properties with size.1, 2 These changes arise through systematic transformations in the density of electronic energy levels as a function of size of the interior, known as quantum size effect, which pointed out that NCs are lie
in between the atomic and molecular limit of discrete density of electronic state and the extended crystalline limit of continuous bands Clearly shown in Figure 5.1, NPs contain discrete and large “molecular-like” electronic states and their HOMO –
Trang 3LUMO gap widen with decreasing the particle size Therefore band gap of NPs can be tuned by changing the particle size.3
Figure 5.1 Schematic illustration of the density of states, along with the changes in the band gap, in semiconductor clusters.3
In the past decade, enormous range of physical properties afforded by tuning of semiconductor NCs, a class of materials with so many established applications in electronics, optics, and sensors, has drawn the attention of scientists from diverse disciplines, from synthetic and physical chemists to materials scientists, condensed matter physicists, and electrical engineers Recent years, the ability to control the surfaces of semiconductors with near atomic precision has led to a further idealization of semiconductor structures: quantum wells, wire, and dots Such structures should exhibit the idealized variations in density of electronic states predicted by simple particle in a box type model of elementary quantum mechanics, with the continuous levels of the 3d case evolving into the discrete states of the 0-dimensional case as shown in Figure 5.2.4
Trang 4size-Figure 5.2 Idealized density of states for one band of a semiconductor structure of 3 – 0 dimensions.4
5.2 Applications of NPs
Colloidal NCs are sometimes referred to as ‘artificial atoms’ because the density of their electronic states – which controls many physical properties – can be widely and easily tuned by adjusting the crystal’s size and shape The combination of size- and shape-dependent physical properties and ease of fabrication and processing makes NCs promising building blocks for materials with designed functions.5, 6 In the following section, we will present several applications associated with semiconductor NCs and nanostructure engineering
5.2.1 Luminescence
Narrow band (15–20 nm), size-tunable luminescence, with efficiencies at least
of order 10%, is observed at room temperature form semiconductor NCs The origin
of this luminescence remains the topic of some controversy 7-9 As the size is reduced, the shift between the absorbing and emitting state is observed to increase Therefore different emission wavelengths could be obtained in principle by changing the particle size, a well known example will be the CdSe NPs as shown in Figure 5.3.10
Trang 5Independent of the exact origin of the luminescence, it does appear to be one property which can be manipulated in useful ways For example, Korgel and co-workers found that the absorption edge of Si nanowires was significantly blue-shifted as compared with the indirect band gap (~ 1.1 eV) of bulk silicon.11-13 They also observed sharp, discrete features in the absorption spectra and relatively strong “band-edge” photoluminescence They suggested these optical features most likely originated from quantum confinement effects, although surface state might also make additional contributions.14
Figure 5.3 Room-temperature emission (left) and absorption (right) spectra taken from difference sizes CdSe NPs.10
5.2.2 Biological Labeling
Now days, the QDs are widely employed as targeted fluorescent labels in biomedical research applications due to the quantum size effect of the NPs as discussed in earlier.15-17 A chart as shown in Scheme 5.1 illustrated the applications of QDs as multimodal contrast agents in bioimaging Compared with the organic
Trang 6fluorophores that were previously used as biological labels, QDs do not photobleach Studies have shown that only very small amount of QDs is required to produce a strong signal Indeed, several studies have reported flickering of some specimens, a phenomenon due to the blinking of a small number of QDs.18, 19 This demonstrates that single QDs can still be observed in immunocytological conditions, with an ultimate sensitivity limit of one QD per target molecule In addition, NPs also provide
a readily accessible range of colors Recently, different-sized QDs have been embedded into polymeric microbeads at precisely controlled ratio to achieve multicolor optical coding for biological assays.20
Scheme 5.1. Applications of QDs as multimodal contrast agents in bioimaging.17
Trang 75.2.3 Light Emitting Diodes
Alivisatos et al and Bawendi et al are the first research groups demonstrated
that light-emitting diodes can be made with polymers and CdSe NCs.21, 22 As previously discussed in section 5.2.1, the NCs emission shift with size Thus, the output color of NC LEDs can be tuned by varying the particle size as shown in Figure 5.4 In certain electronic devices, the semiconducting polymers can replace the inorganic semiconductors due to the low processing cost, for instance, organic light-emitting diodes (OLED) The extension of OLEDs into the technologically important near-infrared (NIR) spectral range used in telecommunications is more difficult because organic molecules usually display optical activity only at wavelengths shorter than 1 µm However this shortfall is overcomed by Banin’s group where they created
a near-infrared plastic light-emitting diodes using a conjugated polymers and ZnSe core shell type of NP.23
InAs-Figure 5.4 True color image of CdSe NPs illuminated with UV light (Image was taken from http://ehf.uni-oldenburg.de/pv/nano/index.html)
5.2.4 Laser
Unlike the spherical dots, quantum rods have linearly polarized emission as demonstrated recently by fluorescence measurements on single quantum rods and by
Trang 8theoretical calculations.24 This property of linearly polarized emission, along with the prospect of broad spectral coverage and the chemical accessibility to quantum rods, renders them highly attractive as potential laser materials Recently, an amplified spontaneous emission was observed for spherical colloidal CdSe QDs in close-packed films where pumping with an amplified femtosecond laser source was used to compete with fast non-radiative Auger decay processes.25 Banin’s group have also observed lasing from CdSe/ZnS quantum rods.26 In their study they observed a linearly polarized lasing signal from the quantum rods and a non polarized lasing from QDs (as shown in Figure 5.5) which prove the advantageous for the utility of the rods as laser chromophores Further, the Auger rates in rods may be smaller because
of their larger size while still allowing color tunability through control of the rod diameter
Figure 5.5 Polarized emission measurements for lasing in (a) NCs and (b) NRs.26
5.2.5 Solar Cell
Charge transfer rate is reported to improve the efficiency of polymer photovoltaic devices.27 A faster charge transfer rate can be achieved by chemically bind the organic molecules to the nanocrystalline and bulk inorganic semiconductors, which have a high density of electronic states.28 Alivisatos and his co-workers have
Trang 9demonstrated this concept by showing that CdSe NRs can be used to fabricate readily processed and efficient hybrid solar cells together with conjugated polymer poly(3-hexylthiophene).29 The intrinsic features and thus performance of such a device could
be tuned by controlling the aspect ratios of the NRs They also found that NRs were superior to QDs in photovoltaic applications, because they can provide a direct path for electrical transport at much lower loadings The fabricated photovoltaic device eventually achieved an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers
Trang 10Figure 5.6 (A) Schematic diagram of a platinum mesowire array-based hydrogen sensor or switch (B) SEM image [400 mm(h) by 600 mm (w)] of the active area of a platinum mesowire array-based hydrogen sensor.31, 32
5.3 Synthesis of Nanomaterials
Vast applications of nanomaterials as discussed in the previous section have prompted intensive study of the synthesis of these materials to optimize colloidal semiconductor NCs fabrication As a result, many new concepts for controlling the size, shape, aspect ratio (Length Diameter) and connectivity or coupling of colloidal NCs have been developed first for metal chalcogenide materials, but a unified set of synthesis control concept is now also being applied to other classes of materials, such
as metals and metal oxides After more than two decades, impressive progress has been made towards the tailored synthesis of colloidal NCs that have well-defined structures A wide variety can now be successfully produced using a number of methods, such as coprecipitation, microemulsion, hydrothermal/solvothermal synthesis and surfactant-controlled growth in a hot organic solvent using either single
or multi-source precursors.34, 35
Trang 115.3.1 Coprecipitation Synthesis
Many of the earliest syntheses of NPs were achieved by the coprecipitation method which involves the simultaneous occurrence of nucleation, growth, coarsening and agglomeration processes Due to the difficulties in isolating each process for independent study, the fundamental mechanisms of this technique is still not thoroughly understood The theory of coprecipiration has been discussed in numerous of books and reviews.36-41 Reaction conditions that influence the mixing process, such as rate of reactant addition and stirring rate, must be considered relevant
to product size, morphology, and particle size distribution The reduction of gold cation to gold metal is the most thoroughly studied metal precipitation reaction Gold cations, usually in the form of AuCl4- are easily reduced by gaseous H2 Although AuCl4- is strong oxidizing (E˚ = +1.022 V) that weaker reducing agents such as carboxylates or alcohols are usually sufficient.34 Tan et al have recently reported the
synthesis of Au, Pt, Pd and Ag NPs by reduction with potassium bitartrate as shown
in Figure 5.7 All the products formed stable colloids with the addition of a suitable stabilizing agent.42 Normally a stronger reducing agent, such as borohydride, is needed to reduce the AuCl4- in a thiol containing solution This is due to the complexes formed between the thiols and AuCl4- are too strong to be reduced by weak
reducing agent e g., citrate However, Yonezawa et al have demonstrated that
reduction of AuCl4- with citrate in the presence of a thiol is possible, if the thiol and citrate are added to the the gold solution simultaneously.43 Gold colloids with 2 – 10
nm dimensions and narrow size distributions are achievable with this method
The stabilization of Au NPs against agglometation in aqueous solutions by
capping ligands such as citrate is a well-documented process Brust et al., however
reported the synthesis of alkanethiol stabilized colloidal Au NPs that are stable in
Trang 12noNPolar solvents Moreover, the particles exhibited rather unusual ability to be fully redispersed into colloids after being isolated as dry powders.44, 45 The two-phase
coprecipitation synthesis introduced by Brust et al have triggered a flurry of research
into the thiol-based stabilization of colloidal NPs.34
Figure 5.7 TEM images of poly(N-vinyl-2-pyrrolidone) capped (a) Au and (b) Pt
NPs.42
5.3.2 Microemulsion Synthesis
Hoar and Schulman noted in 1943 that certain combinations of water, oil, surfactant, and an alcohol- or amine-based cosurfactant produced clear, seemingly homogeneous solutions that Schulman termed “mircoemulsion”.46 Based on their finding, two scenarios can be generated, oil-swollen micelles dispersed in water (oil-in-water microemulsion) or water-swollen micelles dispersed in oil (inversed microemulsion) The size of these micelles can be modified by varying the hydrophile-lipophile balance value of surfactans or the amount of the surfactant These spherical micelle droplets were later being treated as soft template for the growth of NPs
When a reverse micelle solution (inversed microemulsion) contains a dissolved metal salt and a second reverse micelle solution containing a suitable reducing agent is added, the metal cations can be reduced to the metallic state Although the number of metals (e g., Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt and Bi) that can
Trang 13be prepared this way is somewhat limited by the aqueous nature of the reactions,34 this approach has garnered much attention due to the potential application of the products
as catalysts.47 Reverse micelle solutions containing both Fe2+ and Pt2+, have been reported as being simultaneously reduced with BH4- Variation of the Fe/Pt ratio resulted in FePt, Fe2Pt, disordered FePt3, or ordered FePt3.48, 49
Metal chalcogenides can likewise be precipitated using a reverse micelle solution containing dissolved Na2S, Na2Se, or similar salts as the precipitating agent
Hirai et al., for example, recently prepared 2 – 4 nm diameter PbS particles from
micellar solutions of Pb(NO3)2 and Na2S.50 Due to the high solubility of PbS relative
to those of most metal chalcogenides, the authors found that using a large excess of one of the reactants tended to reduce the particle size of the products Similarly, Gan
et al have prepared ~5 nm particles of Mn-doped ZnS in a microemulsion system
consisting of poly(oxyethylene)5 nonyl phenol ether and poly(oxyethylene)9 nonyl phenol ether surfactants and petroleum ether as the oil phase.51 Precipitation was induced by adding a micellar solution of Na2S to a micellar solution of MnCl2 and ZnCl2 Subsequent hydrothermal treatment of the products increased the average diameter to ~12 nm but substantially broadened the size distribution
5.3.3 Hydrothermal/Solvothermal Synthesis
In a sealed vessel (bomb, autoclave, etc.), solvents can be brought to temperatures well above their boiling points by the increase in autogenous pressures resulting from heating Performing a chemical reaction under such conditions is
referred to as solvothermal processing or, in the case of water as solvent, hydrothermal processing Review articles devoted specifically to these methods
appear frequently in the literature.52-57 The critical point for water lies at 374 °C and
Trang 14218 atm Above this temperature and pressure, water is said to be supercritical
Supercritical fluids exhibit high viscosities and easily dissolve chemical compounds that have very low solubility under ambient conditions In any event, solvothermal processing allows many inorganic materials to be prepared at temperatures substantially below those required by traditional solid-state reactions Unlike the cases
of coprecipitation and sol-gel methods, which also allow for substantially reduced reaction temperatures, the products of solvothermal reactions are usually crystalline and do not require post-annealing treatments
In 1988, Oguri et al reported the preparation of anatase hydrothermally
processing hydrous titania prepared by the controlled hydrolysis of Ti(OEt)4 in ethanol.58 The reaction conditions leading to monodispersed anatase NPs by this approach were elucidated by others.59 Later, Cheng et al developed a method for
preparing nanoparticulate, phase-pure rutile and anatase (as shown in Figure 5.8) from aqueous TiCl4 by a hydrothermal process.60 They found that acidic conditions favored rutile while basic conditions favored anatase
Figure 5.8 TEM image of rutile (rodlike) and anatase (granulous) TiO2.60
Trang 15Metal chalcogenide NPs have also been successfully prepared using
solvothermal process Qian et al have prepared a series of MSe2 (M = Ni, Co, Fe) NPs by a solvothermal reduction process.61 The starting materials they were using are the appropriate metal chloride, elemental Se, hydrazine, and strongly coordinating solvent such as pyridine, DMF or ethylenediamine By varying the solvent and reaction temperatures, the authors were able to vary the size and morphology of the products from filament-like to octahedral to spherical Although the presence of water can sometimes cause problems with the synthesis of non-oxidic species in hydrothermal reactions, the literature nonetheless contains numerous examples of successful syntheses of NP metal chalcogenides.62 Peng et al developed a method for
the hydrothermal synthesis of ZnSe and CdSe.63 The starting materials in this case were powdered Zn or Cd metal and Se powder that were heated to 180 °C in an autoclave filled to 70% capacity with water The products, while nanoparticulate, were agglomerated due to the absence of a stabilizer or capping agent
While most hydrothermal syntheses of chalcogenides involve the preparation
of binary systems, more complicated ternary compounds have also been reported A series of MIMIIIS2 (MI = Ag+, Cu+; MIII = Ga3+, In3+) semiconductors have been
prepared by Liu et al from the combination of MCl salts with elemental Ga or In
(M’) and S.64, 65
5.3.4 Surfactant-controlled Growth in a Hot Organic Solvent(s)
A typical synthesis system for colloidal NCs consists of three components: precursors, organic surfactants and solvents.66 In some cases, surfactants also serve as solvents Upon heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into active atomic or molecular species (monomers);
Trang 16these then form NCs whose subsequent growth is greatly affected by the presence of surfactant molecules The formation of the NCs involves two steps: nucleation of an initial ‘seed’ and growth In the nucleation step, precursors decompose or react at a relatively high temperature to form a supersaturation of monomers followed by a burst of nucleation of NCs These nuclei then grow by incorporating additional
monomers still present in the reaction medium The classical paper by Bawendi et al
demonstrated this concept for the growth of CdS, CdSe and CdTe NCs in TOPO.67 In their synthesis, the organometallic precursors were rapidly injected into the hot coordinating solvent, which lead to a temporally nucleation stage and permits controlled growth of macroscopic quantities of NCs Size tunable and highly monodispersed CdSe NPs (as shown in Figure 5.9) were prepared in single reaction Their work eventually opens up a new territory in nanosynthesis Other groups have followed the same synthesis and extend the work to core-shell type binary metal chalcogenide NPs, e g., CdSe/ZnS (Figure 5.10) 68, 69 and CdSe/CdS.70
Figure 5.9 A near monolayer of 5 nm CdSe NPs showing short-range hexagonal close packing.67
Trang 17Figure 5.10 HRTEM image of CdSe/ZnS-core/shell NPs.68
The advantage of surfactant-controlled growth is the flexibility on the choice
of surfactant For example, any molecules that contain metal coordinating groups as well as solvophilic groups can be used as surfactants in the nanosynthesis, e g., alkyl phosphines, alkyl phosphonic acids, fatty acids, long chain amine and some nitrogen-
containing aromatics Peng et al have demonstrated this concept by replaced TOPO
with fatty acid in their synthesis of InAs NPs.71 In some reports, it was found that the choice of surfactant have great impact on the growth of NCs For example, phosphonic acids were only recognized as essential ingredient for shape control of CdSe and other II-IV NCs after their presence in TOPO was finally controlled as demonstrated in Figure 5.11.72
Trang 18Figure 5.11 TEMs of CdSe NPs from the single-injection experiments The surfactant ratio was increased from (a) 8 to (b) 20 to (c) 60% HPA in TOPO For the injection volume experiments (d-f), 20% HPA in TOPO was used, as it was found to provide optimal rod growth conditions.72
A major draw back of the organometallic method is the used of toxic
compound such as dimethylcadmium To overcome this, Peng et al have used the
environmental friendlier precursors; e g., cadmium oxide, cadmium acetate and cadmium carbonate to synthesize high quality CdSe NPs (Figure 5.12).73, 74 Very
recently reports by Knoll et al demonstrated that alloyed semiconductor NCs with
high quantum yield and enhanced stability can be prepared using the growth controlled method.75, 76 The emission of the obtained alloy can be tuned by varying the composition of Cd and Zn Surprisingly the size of the NCs have little effect on the overall emission wavelength
Trang 19surfactant-Figure 5.12 TEM images of various sizes CdSe NPs.73
5.3.4.1 Single Molecular Precursor Approach
Instead of using hazardous ((CH3)2Cd) or inert (e g., CdO) precursors, one can choose to use metal complexes as the single molecular precursors The concept of thermally decomposed single molecular precursor in surfactant to produce NP was first introduced by O’Brien The advantages of using single molecular precursor are, i) relatively stable and easy to handle compared to organometallic precursors; ii) all the essential elements are present in the precursor, for example, [M(E2CN{CH3}C6H13)2] (M = Zn, Cd; E = S, Se) alkyldiseleno carbamates; iii) relatively low decomposition temperature is required In a typical reaction, the molecular precursor is first dissolved in a high boiling solvent and then it was rapidly injected into the hot surfactant at a typical temperature range 200 – 250 ˚C To date, mainly alkaldiseleno- or alkyldithio-carbamate complexes have been used to synthesized CdS,77, 78 CdSe,79 ZnS,79 PtS,80, PbS81 and CuSe82 NPs in TOPO When a
Trang 20nanosynthesis reaction can be carried out O’Brien’s group have demonstrated that CdS NPs can be obtained by thermal decomposed cadmium dithiocarbamate, [Cd{S2CNMe(C18H37)}2] under high temperature in N2 atmosphere and the these NPs are self-capped by HNMe(C18H37).83
Our laboratory and few other research groups have reported using used metal thiocarboxylate precursors to obtain ZnS,84 aqueous and non-aqueous soluble CdS,84,
85
PbS86 and Ag2S87 NPs Besides, Cheon et al also reported the growth of ZnSe and
ZnTe QDs from [Zn(EPh)2(TMEDA)] (E = Se, Te) precursors in TOPO.88, 89 Same group have also reported the syntheses of various anisotropic shaped semiconductor,
e g., CdS (Figure 5.13a & b),90 MnS (Figure 5.13c & d),91 and PbS (Figure 5.13e & f)92 NPs, which were synthesized from single molecular precursors in hexadecylamine surfactant
Figure 5.13 TEM images of various shapes CdS (a – b), MnS (c – d) and PbS (e – f) NPs.90-92
Trang 215.4 Aim and Scope of Part II of This Thesis
As from the introduction, it is seen that nanomaterials offer plenty of interesting properties and what has been covered here is just a small portion of the properties that has been discovered so far Many unique properties are yet to be discovered from these nanomaterials in the years ahead Therefore, the ability to control the uniformity of the size, shape, composition, crystal structure and surface properties of the NCs is vital in the process of uncovering the intrinsic properties of the NPs Many of the reported syntheses required stringent experimental conditions, for example, high temperature (~300 ˚C),67 hazardous precursor67 and high pressure.52-57
Single molecular precursor approach to nanosynthesis is one of the popular synthetic routes employed in the bottom up approach The presence of the metal to chalcogenide bond in the precursor ensures the formation of metal chalcogenide, which illustrates the advantage of this method O’Brien and his co-workers have shown many high quality NCs can be easily synthesized from single precursor route with suitable capping agents Furthermore, by taking the advantage of chemical
reaction, Chin et al have shown that PbS NCs can be obtained at room temperature
from [Pb(SC{O}Ph)2].86 However, until today, relatively few have been developed for the syntheses of metal selenide NPs through molecular precursors As shown in previous chapter, most of the metal selenocarboxylates that have been synthesized are suitable precursors for metal selenide powder based on the TGA and XRPD analysis Hence in the second part of this thesis, we devoted our efforts to employ them as molecular precursor to synthesize the corresponding metal selenide NPs