Gas Phase Growth Techniques for Quantum Dots
Weiqiang Wang1,*, Muzhou Jiang2
1Department of Mechanical Engineering, University of Rochester; 2Department of Electrical and Computer Engineering, University of Rochester; *Corresponding author(Email: )
Abstract: An overview of methods for preparing quantum dots (nanoparticles) in the gas phase is given, and recent developments and advances for gas phase synthesis techniques are discussed.Developments in instrumentation for monitoring gas-phase synthesis of nanoparticles, in modeling these processes, and in producing multi-component nanoparticles are alsoincluded. The most important developments relate to improved control and understanding of nanoparticle aggregation andcoalescence during synthesis.
1. Introduction
From the end of last century, researchers in many differentdisciplines trend to pay attention to nano-scale materials and related applications. The term “nanoparticle” came into frequent use in the early 1990s together with the related concepts, “nanoscaled” or “nanosized” particle. Until then, the more general terms submicron and ultrafine particles were used. From a scientific point of view, nanoparticles (Fig. 1) are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials.
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Nanoparticles have been suggested recently for various potential applications in electronics (Fig. 2) where quantum confinement effects may be of advantage. When electrons are confined to a small domain such as a nanoparticle the system is called a “quantum dot” or zero-dimensional structure. Then the electrons are behaving like “particles-in-a-box” and their resulting new energy levels are determined by quantum “confinement” effects.As a result, discrete energy levels are needed to describe the electron excitation and transport in quantum dots.The corresponding wave functions are spatially localized within the quantum dot, but extend over many periods of the crystal lattice.
Being zero-dimensional, quantum dots have a sharper density of states than higher-dimensional structures. As a result, they have superior transport and optical properties, and are being researched for use in diode lasers, amplifiers, and biological sensors. Scientists make efforts to give this invisible matter to boarder applications, for instance,fabrication of optical memories and organic dyesin modern biological analysis.
Methods for the synthesis of nanoparticles are taking place in other than gas-phase growthtechnology. However, gas-phase processing systems may have some advantages over other methods in some cases because of their following inherent advantages:
(a) Gas-phase processes are generally purer than liquid-based processes since even themost ultra-pure water contains traces of minerals, which seem to be avoidable today only in vacuum and gas-phase systems.
(b) Aerosol processes have the potential to create complex chemical structures which are useful in producing multicomponent materials, such as high-temperature superconductors [1].
(c) The process and product control is usually very good in aerosol processes.
(d) Being a nonvacuum technique, aerosol synthesis provides a cheap alternative to expensive vacuum synthesis techniques in thin or thick film synthesis [2]. Furthermore, the much higher deposition rate as compared to vacuum techniques may enable mass production.
(e)An aerosol droplet resembles a very small reactor in which chemical segregation is minimized, as any phases formed cannot leave the particle [3].
(f)Gas-phase processes for particle synthesis are usually continuous processes, while liquid-based synthesis processes or milling processes are often performed in a batch form. Batch processes can result in product characteristics which vary from one batch to another.
2. Synthesis method of quantum dots using gas phase growth technology
Most synthesis methods of nanoparticles in the gas phase are based on homogeneousnucleation in the gas phase and subsequent condensation and coagulation. Once nucleation occurs,remaining supersaturation can be relieved by condensationor reaction of the gas-phase molecules on theresulting particles, and particle growth will occur ratherthan further nucleation. Therefore, to prepare smallparticles, one wants to create a high degree of supersaturation,thereby inducing a high nucleation density, andthen immediately quench the system, either by removingthe source of supersaturation or slowing the kinetics, sothat the particles do not grow. In most cases, this happensrapidly (milliseconds to seconds) in a relatively uncontrolledfashion, and lends itself to continuous or quasi-continuousoperation. This contrasts with many colloidalsyntheses of nanoparticles that are carried out in discretebatches under well-controlled conditions with batchtimes of hours to days.
Finally, initiating homogeneous nucleation synthesis of nanoparticles in the gas phase inside aerosol droplets can result in many nanosized nuclei in the droplet, which upon drying will yield nanoparticles. These methods will be described in detail in the following sections.
2.1. Homogeneous nucleation synthesis
The generation of nanoparticles from the gas phase requires the establishment of supersaturation. A means of achieving the supersaturationrequired to induce homogeneous nucleation of particlesis chemical reaction. Chemical precursors are heatedand/or mixed to induce gas-phase reactions that producea state of supersaturation in the gas phase.
2.1.1 Homogeneous nucleation reactors
Furnace flow reactors Oven sources are the simplest systems to produce a saturated vapor for substances having a large vapor pressure at intermediate temperatures up to about 1700˚C. A crucible containing the source material is placed in a heated flow of inert carrier gas. This has the disadvantage that the operating temperature is limited by the choice of crucible material and that impurities from the crucible might be incorporated in the nanoparticles. Nanoparticles are formed by subsequent cooling, such as natural cooling or dilution cooling. For very small particles a rapid temperature decrease is needed which can be achieved by the free jetexpansion method described later. Materials with too low vapor pressure for obtaining appreciable particle density have to be fed in the form of suitable precursors, such as organometallics or metal carbonyls, in the furnace. A recent developed method of “aerotaxy” utilizing self-limited reaction between III V semiconductor particles in furnace reactor is shown in Fig. 3.
Fig. 3 Schematic diagram of the aerosol generation, sizing and reaction process: aerotaxy. [6]
Plasma reactorsAnother means of providing the energy needed toinduce reactions that lead to supersaturation and particlenucleation is to inject the precursors into thermalplasma. This generally decomposes them fully intoatoms, which can then react or condense to formparticles when cooled by mixing with cool gas orexpansion through a nozzle.(Fig. 4)
Fig. 4 Schematic diagram of a plasma reactor [9]
Laser reactors An alternate means of heating the particles to induce homogeneous nucleation is absorption of laser energy. Compared to heating the gases in a furnace, this allows highly localized heating and rapid cooling, since only the gas (or a portion of the gas) is heated, and its heat capacity is small. Heating is generally done using an infrared (CO2) laser, whose energy is either absorbed by one of the precursors or by an inertphotosensitizer such as sulfur hexafluoride. The ironparticles shown in Fig.6 were prepared by laser pyrolysis.The main advantageof laser-heating in gas-flow systems is the absence of heated walls which reduces thedanger of product contamination.
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Flame reactors Rather than supplying energy externally to inducereaction and particle nucleation, one can carry out theparticle synthesis within a flame, so that the heat neededis produced in situ by the combustion reactions (Fig.7). This isby far the most commercially successful approach tonanoparticle synthesis-producing millions of metrictons per year of carbon black and metal oxides. However,the coupling of the particle production to the flamechemistry makes this a complex process that is ratherdifficult to control. It is primarily useful for makingoxides, since the flame environment is quite oxidizing.Recent advances are expanding flame synthesis to awider variety of materials and providing greater controlover particle morphology. (Fig. 8)
2.1.2. Sputtering
Sputtering is a method of vaporizing materials from a solid surface bybombardment with high-velocity ions of an inert gas, such as Ar or Kr, causing an ejection ofatoms and clusters. So this method is also called inert gas evaporation (IGE) method. This process must be carried out in vacuum systems, below 0.1 Pa, as a higher pressure hindersthe transportation of the sputtered material. Instead of ions, electrons from an electron guncan be also used. As early as in 1982, Iwama et al. [12] operated an electron gun at 10-3Pa separated bya differential pumping system from a 100Pa evaporation chamber in order to evaporate Tiand Al targets in a N2 or NH3 atmosphere, producing TiN and AlN nanoparticles smallerthan 10 nm. Gunther and Kumpmann [13] applied an electron beam to bulk oxides in an inert gasatmosphere with pressures up to 500 Pa in order to produce 5 nm amorphous Al2O3 and SiO2particles and crystalline Y2O3 oxide powders. They also found the primary particle size is rather insensitive to variations in gas pressure and evaporation rate. Magnetron sputtering can be used in a higher pressure level. A schematic picture of magnetron sputtering system is shown below. Hahn and Averback [14] showed that a magnetron sputter source canbe operated in the 100 Pa range, and can be used for metals with high heat of vaporization. They successfully synthesized Al, Mo, Cu91Mn9, Al52Ti48 and ZrO2 nanoparticles with diameters of 7-50nm.Urban et al. [15] recentlydemonstrated formation of nanoparticles of a dozendifferent metals using magnetron sputtering of metaltargets. They formed collimated beams of the nanoparticlesand deposited themas nanostructured films onsilicon substrates. Sputtering has the advantage that it is mainly the target material which is heated and thatthe composition of the sputtered material is the same as that of the target. The low pressure system provided a very clean environment for powder synthesis, but it also makesfurther processing of the nanoparticles in aerosol formdifficult.
Fig.9. Schematic drawing of the deposition system. [15]
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2.1.3. Inert gas condensation.
One of the earliest methods used to synthesize nanoparticles, which is also perhaps the most straightforward method of achievingsupersaturation, is the evaporation of a material in a cool inert gas, usually He or Ar, at low-pressuresconditions, of the order of 100Pa. It is usually called ‘‘inert gas evaporation’’. This method is well suited forproduction of metal nanoparticles, since many metalsevaporate at reasonable rates at attainable temperatures.By including a reactive gas, such as oxygen, in the coldgas stream, oxides or other compounds of the evaporatedmaterial can be prepared.
Commonvaporization methods are resistive evaporation, [16] laser evaporation and sputtering. A convective flow of inert gaspasses over the evaporation source and transports the nanoparticles formed above theevaporative source via thermophoresis towards a substrate with a liquid N2 cooled surface [17]. A basic experimental system is shown in Fig.10. Later, people developed several modification for this method. One modification from Birringer and Gleiter [18] which consists of a scraper and a collectionfunnel allows the production of relatively large quantities of nanoparticles, which areagglomerated but do not form hard agglomerates and which can be compacted in theapparatus itself without exposing them to air. Increased pressure or increased molecularweight of the inert gas leads to an increase in the mean particle size. Another method replaces the evaporationboat by a hot-wall tubular reactor into which an organometallic precursor in a carriergas is introduced. This process is known as chemical vapor condensation referring to thechemical reactions taking place as opposed to the inert gas condensation method.[19]The gas deposition method is also used in industry. In this method,nanoparticles are formed by evaporation in an inert gas at atmospheric pressure andtransported by a special designed transfer pipe to the spray chamber at a pressure of about 30 Pa. By moving the nozzle at the end of the transfer pipe, the particles which havea mean velocity of 300 m/s can be deposited in required places on the substrate in the spraychamber. Using this technique writing micron-sized patterns was demonstrated [20].
Fig. 10. Cross-section sketch of the inert gas condensation system. [21] Inconel pipe (1), the crucible containing the bismuth melt (2), the furnace (3), the evaporation zone (4), and the cap-free diluter (5). An inset schematic shows the diluter conCguration with gas return cap (6) used to introduce the quenching gas perpendicular to the Bi-laden carrier gas jet.
A systematic modeling study ofthis method is presented by Wegner et al. [21]They applied this method to preparation of bismuth nanoparticles (Fig.11.), and both visualization and computationalfluid dynamics simulation of the flow fields in theirreactor were achieved. They clearly showed that they could control theparticle size distribution by controlling the flow fieldand the mixing of the cold gas with the hot gas carryingthe evaporated metal.
Most recent advances in this methodhave been in preparing composite nanoparticles and incontrolling the morphology of single-component nanoparticlesby controlled sintering after particle formation.Nanda et al. [22] studied the in-flight sintering of PbS nanoparticles.They were able to tune the band-gap of these semiconductornanoparticles by changing the particle sizeand morphology.
Fig. 11. TEM picture ofspherical bismuth particles collected by thermophoretic
sampling from the gas phase.[21]
2.1.4. Expansion-cooling.
Expansion of a condensable gas through a nozzle leads tocooling of the gas and a subsequent homogeneous nucleation and condensation. In order to producenanoparticles smaller than 5 nm, supersonic free jets expanding in a vacuum chamber withpressures smaller than 10-2Pa have been used.[23] In the work ofBowles et al. (1981)[24], an inert gas containing a metal vapor was subjected to multipleexpansions. After a first sonic expansion, a mixture of molecular clusters was preparedturbulently with a quench gas and undergoes a second sonic expansion resultingin homogeneous nucleation. Then a cluster growth region in a subsonic, low-pressure,fast-flow reactor produced nanoparticles with mean sizes below 2.5 nm. Also a controlled mean size ranging from the dimer up to several thousand of the monomer species is possible. Converging nozzles which create an adiabatic expansion in a low-pressure flowhave also been used to produce nanoparticles (Bayazitoglu et al., 1996). [25] Although theparticles sizes are larger than in a vacuum expansion, particles of the order of 100 nm wereobtained with a relatively high production rate. They also studied the effects of nozzle initial pressures and
nozzle half angles on the nucleation and, therefore, on size distribution of the exiting particles. And the width of particle size distribution increased with the increase of nozzle pressure. A modified method of producing 4–10 nmsized nanoparticles by expanding a thermal plasma carrying vapor-phase precursorsthrough a ceramic-lined subsonic nozzle has also been developed to obtain a narrow size distribution. [26]
2.1.5 Laser vaporization
This technique uses a laser which evaporates a sampletarget in an inert gas flow reactor. (Fig.12) The source material is locally heatedto a high temperature enabling thus vaporization. The vapor is cooled by collisions with theinert gas molecules and the resulting supersaturation induces nanoparticle formation.Laser vaporization techniques provide several advantages over other heatingmethods such as the production of a high-density vapor of any metal;the generation of a directional high-speed metal vapor from the solidtarget, which can be useful for directional deposition of the particles; thecontrol of the evaporation from specific areas of the target; and thesimultaneous or sequential evaporation of several different targets. [27] Nanocomposites can also be produced, Kato [29] used a continuous-wave CO2 laser with a power of 100 W to prepare nanoparticles between 6 and 100 nm of many complex refractory oxides such as Fe3O4, CaTiO3 and Mg2SiO4 from powders, single crystals orsintered blocks.A modified method which combines laser vaporization of metal targets with controlled condensation in a diffusion cloud chamber is used to synthesize nanoscale metal oxide and metal carbide particles (10-20 nm), and very porous aggregates were obtained.[30]
Fig.12. The schematic diagram of the laser vaporization flowtube reactor. [28]
2.1.6 Spark source
A high-current spark between two solid electrodescan be used to evaporate the electrode material for creating nanoparticles. At theelectrodes a plasma is formed. (Fig.13) This technique is used for materials with a high melting pointsuch as Si or C, which cannot be evaporated in a furnace.Reactive evaporation is also possible by adding a suitablereactant gas.
Fig.13. A schematic diagram of the spark source used to generate luminescent nanometer-scale clusters.[31]
2.2. Laser ablation
Laser ablation is a technique in which a pulsed laser rapidly heats a very thin (<100 nm)layer of substrate material (Fig.14), resulting in the formation of an energetic plasma above thesubstrate. This technique should be distinguished from laser vaporization, as apart fromatoms and ions also fragments of solid or liquid material are ablated from the substratesurface which vary in size from sub-nanometric to micrometric. Therefore, it cannot beconsidered as a pure homogeneous nucleation process. The pulse duration and energydetermines the relative amounts of ablated atoms and particles. The nonequilibrium natureof the short-pulse (10–50 ns) laser heating enables the synthesis of nanoparticles of materialswhich normally would decompose when vaporized directly. The material removal rate by laserablation decreases with longer target exposure times, therefore the target is usually rotated.When used for producing films, this technique is called pulsed laser deposition (PLD).
Fig. 14. Schematic drawing of the laser ablation chamber. [35]
Examples of nanoparticle preparation using this method include magnetic oxidenanoparticles by Shinde S.R. et al. [32], titania nanoparticlesby Harano et al. [33], and hydrogenated-silicon nanoparticlesby Makimura et al. [34].
Theoperating conditions can be altered to select particle formation or film formation. Yamamoto and Mazumder [35] showed that laser ablation of NbAl3at He pressures of 0.1 Torr did not produce any nanoparticles while an operatingpressure of 1 Torr resulted in the formation of 6 nm nanoparticles with the samestoichiometry as the substrate (Fig.15). Typical production rates are in the order of micrograms perpulse with pulse frequencies of about 50 Hz, yielding 10–100 mg powder per hour.