Recent advances of colloidal quantum dots in a physical perspective

In quantum-confined semiconductor nanostructures, electrons show unique behaviors compared to bulk solids, which endows materials with tunable physicochemical and photoelectric properties. Zero-dimensional semiconductor quantum dots (QDs) provide intense light absorption and strong narrowband emission at visible and infrared bands, and have been employed to exhibit optical gain and lasing. These properties are beneficial for different application. Here, we provide a review in the synthesis and understanding of unique properties of colloidal QDs, and discuss their applications in display, lasers, sensing and solar energy conversion.


Introduction
Low-dimensional materials exhibit strong quantum confinement effect (QCE).QCE is the restriction of movement of electrons in space.Two-dimensional QCE is where electrons are restricted to move on a plane, like a single layer of atoms.One-dimensional QCE is where electrons are restricted to move along a line.Zero-dimensional QCE is where electrons are restricted to move in a small space usually in range of a few nano-meters.Engineering how the electrons are confined in the materials can alter the effectivebandgap, thus the optical properties of the materials can be changed.Two-dimensional materials have high surface-to-mass ratio, so they are excellent choices for thermal conductors and one example is graphene.One-dimensional materials have high surface-to-volume ratio, so using one-dimensional materials as catalysts can greatly accelerate chemical reactions.Zero-dimensional materials usually have the shape of sphere, and are relatively easy to fabricate and control their sizes, so their QCE can be controlled, for this reason zero-dimensional materials attract most material scientist's attention.Admittedly, there are already reviews regarding quantum dots (QDs), however, many of them are chemical-focused or biomedical-focused.This paper is an attempt to review QDs from a physical viewpoint, firstly, this paper will describe both physical and chemical methods of fabricating QDs, secondly, this paper will discuss some physical concepts related to the properties of QDs, finally, this paper will present applications of QDs in laser, display panel and solar cell.

Definition of quantum dots
QDs are quasi-spherical crystals with the diameter ranging from 5 nm to 50 nm.They exhibit properties very different from their macroscopic counterparts because their small sizes, such as narrow photoluminescence spectrum, high quantum yield, tunable bandgap and fluorescence lifetime.As shown in Figure 1 (a) and (b) [1], QDs are quasi-spherical crystals.

Top-down
The synthesis of QDs can be divided into to two categories, one is bottom-up, the other is top-down.The top-down method is to build the chemical structure of QDs by removing materials from bulky object, like blocks of silicon crystals, through reactive-ion etching (RIE) and electron beam lithography (EBL) and pulsed-laser ablation (PLA).

Reactive-ion Etching.
Reactive-ion etching (RIE) is a combination of physical and chemical process.Reactive-ions are created by irradiating chemicals that contain reactive components such as carbon tetrachloride (CCl4), carbon tetrafluoride (CF4) and trifluoromethane (CHF3) with microwave.Sample is placed in a vacuum chamber, gas is pump into the chamber, under a powerful radio-frequency source, the molecules in the gas absorbed enough energy become ionized and turned into plasma.The chemical inert parts of the plasma get pump out of the system, left with the reactive ions, they interact with the surface of the sample forming stable byproducts.These byproducts are subsequently vacuumed out of the chamber, leaving the masked portion intact, thus forming a designed pattern.As shown in Figure 2, results of RIE with different etching agents [2].
Tsutsui, Hu et al. investigated the amount of undercut presented in QDs affected by different patterns, with CH4 and H2 as etching gases [3].Rahman, Murad et al. fabricated gallium arsenide QDs (GaAs QDs) using low-power RIE, and they found that compared to other methods low-power RIE had very low levels of residual lattice damage [4].Ribayrol, Tang et al. studied the physical and optical properties of CdTe-CdMnTe QDs with CH4/H2/O2 as etching gases, what they found was dry etched CdTe-CdMnTe QDs had quasi-parabolic potential confinement [5].

Electron-beam lithography (EBL).
EBL is a mask-less method to produce patterns on a surface coated with electron-beam resist that is sensitive to electrons.The resist is spin coated to the sample.The patterning equipment consists of an electron-source that generates electrons, an electromagnetic lens system to focus the electrons to a tiny spot, beam deflectors that control the position of the electron beam.The overall result is that electrons come out of the source and are focused on the resist coated sample, the beam deflectors read the information stored on a computer and scan the resist line by line, where the electrons come in contact with the resist, the resist changed chemically and can be removed subsequently, thus forming a pattern.
EBL as a nanofabrication technique is usually combined with RIE.Rodrigues, Alves et al. used EBL and wet chemical etching to fabricate QDs arrays on InGaAs/GaAs single quantum wells and studies their properties, as they changed the diameter of the QDs, a blue-shift in the emission spectrum was observed for the 120 nm diameter QDs [6].

Pulsed-laser ablation (PLA).
Pulsed-laser beam has high concentration of photon compared to ordinary light.The interaction of atoms with ordinary light is usually one to one, that is one atom absorbs or emits one photon.The interaction of pulsed-laser with atoms is usually one to many, that is one atom absorbs or emits multiple photons, due to the high photon density of pulsed-laser.The sample is usually submerged in liquid, after absorbing the energy from the pulsed-laser beam, atoms changed from solid to plasma, the liquid surrounded the plasma turned into vapor, some nano-sized particles will form inside the plasma.After cooling, the vapor turns back into liquid, and releases nano-sized particles.Figure 3 was an experimental setup for the synthesis of CdS nanoparticles by laser ablation in liquid [7].QDs (SiC QDs) with Ti-sapphire laser, the submersion layer was water.They found there was a correlation between the PL intensities and the laser power settings, the higher the power the higher the PL intensity, and there was no shift in PL spectral maxima [9].Abderrafi, Chirvony et al. fabricated GaAs QDs with Nd-YAG pulsed-laser, they found there was a correlation between the sizes of GaAs QDs and the laser power settings, the higher the laser power the smaller the size of the GaAs QDs [10].Ajimsha, Anoop et al. synthesized zinc oxide QDs (ZnO QDs) with Nd-YAG pulsed-laser and water, methanol, ethanol as confining liquids.They found with the decrease of laser power there was a blueshift in ZnO QDs' PL maximum, specifically ZnO QDs grown in methanol shifted from 2.41 eV to 2.6 eV when the laser power decreased from 45 mJ/pulse to 25 mJ/pulse, and, in ethanol shifted from 2.27 eV to 2.35 eV when the laser power decreased from 45 mJ/pulse to 25 mJ/pulse [11].

Microwave irradiation.
The bottom-up method builds the QDs from chemical precursors.Common methods are microwave irradiation and hot-injection.Microwave is electromagnetic wave, consisting of alternating electric field and magnetic field.The electrons and nuclei are electrically charged particles, can, under the influence of an electric field, be accelerated.Irradiating microwave on chemicals can induce internal vibrations of the molecules, this is heat on the macroscopic scale.The essence of microwave irradiation is heating.Compared to conventional heating methods, such as direct heating, microwave heating is much more efficient as microwave can penetrate deep into the object, heating the object uniformly and reducing the waiting time.
Kim D. and J. Kim synthesized cadmium zinc telluride (CdZnTe) QDs with microwave irradiation and studied their PL spectrum properties by varying the pH, Zn/Cd ratio, reaction time [12].Shahid, Toprak et al. achieved the synthesis of wurtzite zinc sulfide (ZnS) QDs (ZnS QDs) at a low temperature for the first time [13].Xu, Jiang et al. fabricated tin dioxide (SnO 2) QDs with microwave irradiation, they found the time required for the synthesis was much shorter than using the traditional refluxing methods, from hours to minutes [14].

Hot-injection.
Hot-injection (HI) is a method to grow nano-crystals, it was pioneered by C. B. Murray et al. in 1993 [15].The procedure is injecting one chemical precursor X into a hot liquid media containing another chemical precursor Y, or, injecting multiple chemical precursors into a hot liquid media.An instant supersaturation of monomers will result from the sudden increase of precursors, precursors start to nucleate as a result of high concentrations of the precursors.As temperature and concentrations of the monomers decreased, the rate of nucleation will decrease also, and the crystallization will start to take over.Figure 4 shows steps for the synthesis of QDs by hot-injection methods [16].Oberg, Zhang et al. synthesized silver sulfide colloidal (Ag2S) QDs (Ag2S QDs) using HI method, with silver nitrate (AgNO3) dissolved in the mixture of oleic acid and oleylamine, bis(trimethylsilyl) sulfide dissolved in octadecene, to study Ag2S QDs' applications on solar cell, and found their absorption spectrum was broad in the UV-visible region, and Ag2S QDs sizes ranging from 3 nm to 10 nm in diameter [17].Liao, Huang et al. fabricated silver indium sulfide (AgInS2) QDs (AIS QDs) and AgInS2/ZnS QDs (AIS/ZnS QDs) with HI method, and found they can tune the PL spectra by varying the ratio of Ag/In/S/Zn and the liquid media temperature [18].Xiang, Xie et al. synthesized AIS QDs, and studied their PL spectra by varying Ag/In ratio and reaction temperature.They found AIS QDs synthesized at 110 °C had the highest quantum yield, and the absorption spectrum red-shifted as the reaction temperature increased [19].

Properties of QDs
QDs have many properties like narrow photoluminescence (PL) spectrum, high quantum yield (QY), tunable bandgap and fluorescence lifetime.Adjusting the synthesis conditions and chemical composition can change these properties.These properties can be further engineered to meet the needs of display industry, laser industry and solar energy industry.

Optical property: photoluminescence (PL)
Any material, at the fundamental level, is a collection of atoms, and according to quantum theory, there are discrete energy levels inside atomic system.Quantum dot is nano-crystal, having approximately 100 to 1000 atoms, its energy spectrum falls between bulky material and atoms.Photoluminescence is the phenomenon of emission of light after some materials have absorbed light, the absorbed light and emitted light usually have different frequencies.Bound electrons in QDs absorb photons and leap to a high energy level becoming unstable, after a certain amount of time they fall back to a low energy level and release the extra energy in the form of light.The spectrum of QDs is size-dependent, the bigger the size the broader the spectral width.

Tunable Bandgap
According to quantum mechanics, there are well-defined energy levels inside atomic system, electrons can only have these discrete energies.When atoms come together to form solid, the energy levels will change and the energy difference between adjacent levels (ΔE) can either be so small that it can regarded as zero or a finite value.Physicists artificially group levels with zero energy difference into a band, and call it energy band.The finite values between adjacent levels are called bandgaps.Depending on the chemical composition of the materials, the bandgap can be zero, small and big.The bandgaps of conductor, semiconductor and insulator are zero, small and big respectively.Tuning the bandgap can shift PL spectrum and change the width of the spectral lines.There are two ways to tune the bandgap of QDs, namely, size controlling and doping.

Size controlling.
Energy levels can be changed by altering the number of atoms in QDs.One can achieve this by changing the synthesis conditions, like reaction time, temperature, chemical concentration etc.
Lin, Cheng et al. synthesized ZnO QDs with different sizes by controlling the concentration of the chemical precursor.They found that the peak of PL spectrum blue-shifted as the diameters of ZnO QDs decreased, the photons' energies increased from 3.30 eV to 3.43 eV, when the diameters of the ZnO QDs decreased from 12 nm to 3.5 nm [20].Liu, Fu et al. fabricated SnO 2 QDs, and changed their sizes by varying the heating time.They discovered that there was a negative correlation between the bandgaps of SnO2 QDs and the time of hydrothermal treatments, bandgap decreased from 4.20 eV to 4.00 eV when the heating time increased from 0.5 h to 20 h [21].Nair and Prabhash synthesized copper (Cu) QDs (Cu QDs) using citric acid and cetrimonium bromide (CTAB) as the surfactants, the resulting sizes ranging from 3 nm to 10 nm in diameter.They found that Cu QDs synthesized in citric acid had the bandgap ranging from 3.67 eV to 4.27 eV, whereas in CTAB from 4.17 eV to 4.52 eV [22].

Doping.
The mechanism of doping is to add transitional ions or rare-earth ions to QDs during or after fabrication.The physical effect of adding these ions is that energy levels will be shifted, correspondingly changing the spectrum of QDs.
Firdous, Singh et al. fabricated ZnS QDs and barium-doped ZnS QDs (ZnS/Ba QDs) with chemical precipitation to study their optical and electrical properties.They found that the bandgap of ZnS/Ba QDs decreased from 4.88 eV to 4.43 eV with the introduction of Ba into ZnS QDs [23].Jabeen, Pathak et al. synthesized ZnS QDs, silver-doped ZnS QDs (ZnS/Ag QDs) and gold-doped ZnS QDs (ZnS/Au QDs) with chemical method.They found that pure ZnS QDs exhibited the largest bandgap of 2.5 eV, and ZnS/Au QDs exhibited the smallest bandgap of 2.2 eV, the bandgap of ZnS/Ag QDs was 2.4 eV [24].Wang, Liu et al. synthesized strontium-doped lead selenide QDs (PbSe/Sr QDs) with melt-quenching method and studied their optical properties.They found that the absorption spectrum and PL peak blueshifted as the concentration of strontium (Sr) increased with other synthesize conditions keeping the same, thus the bandgap was enlarged by the increase of Sr [25].

High quantum yield
Quantum yield (QY) is the measure of the light emission efficiency of QDs, roughly speaking QY is the ratio of the emitted photons to the absorbed photons.By changing the chemical structures of QDs, QY can be improved.
Chung, Kim et al. synthesized cadmium selenide (CdSe) QDs (CdSe QDs) with various sizes, and studied their optical and electrical properties.They found the QY ranging from 17.3% to 23.1%, there was a correlation between the QY and the size of the CdSe QDs, the smaller the size, the higher the QY [26].Xie, Pang et al. fabricated CdSe QDs and cadmium selenide zinc sulfide (ZnS) core-shell QDs (CdSe/ZnS QDs).They found the QY of pure CdSe QDs was approximately 20-30%, whereas the QY of CdSe/ZnS QDs was 60-80%, which is much higher than the pure CdSe QDs [27].Samokhvalov, Nabiev et al. synthesized CdSe QDs and coated them with ZnS and cadmium sulfide (CdS).They found that the QY of CdSe QDs with multiple shells reached nearly 100% [28].As shown in Figure 5, the evolution of the QD PL QY during the shell growth [29].

Fluorescence lifetime
The fluorescence lifetime is defined to be the time it takes for 1/e percent of the electrons in the excited state to fall back to the ground state.The curves are very similar to the radioactive material decay curves, with the mathematical expression () =  0 *  − , where N0 is the number of electrons in the excited state at the beginning, λ is material-dependent constant, 1/λ is usually called lifetime (τ).By changing the synthesis conditions and chemical composition, τ can be altered.Kim, Zhao et al. synthesized cadmium selenium sulfur alloyed QDs (CdSeS QDs), with varying Se/S ratio, and studied their fluorescence lifetime.They found that there was a positive correlation between the sulfur percentage and the average lifetime, the lower the sulfur percentage, the shorter the average lifetime [30].Bekasova, Revina et al. synthesized CdS QDs, and studied their optical properties by interacting it with gamma-ray.They found there was a decrease in lifetimes from 1.071 ns to 0.997 ns with increased dosage of gamma-ray [31].

Display panel
Display panels consist of pixels, each pixel is made of three subpixels, namely red, green and blue subpixels.Higher color purity requires optical materials with narrow full width at half maximum (FWHM), FWHM is defined as FWHM=X2-X1 where X2 and X1 are the coordinates where the intensity is half the maximum value.QDs usually have narrow FWHM, moreover, FWHM can be altered by changing the chemical composition and synthesis conditions, hence QDs has a great potential in the display industry.Chen, Jiang et al. synthesized CdS QDs with different Cd/S ratio and chemical reaction time, to study their optical properties.they found different Cd/S ratio corresponded to different FWHM, the smallest FWHM they can reach was 16 nm and the Cd/S ratio was 1 to 3 [32].WANG, Zheng et al. synthesized cadmium telluride QDs (CdTe QDs) to study their optical properties.They found that the synthesis conditions had a strong influence on CdTe QDs' FWHM, CdTe QDs prepared at 2 h had better optical properties [33].

Manufacturing QDs arrays.
QDs technology will no doubt revolutionize the display industry in the near future, as its power efficiency and color purity are much superior than the already existing liquid crystal display (LCD) and organic light emitting diode (OLED).There are two ways to manufacture QDs array, one is inkjet-printing, the other is photo-patterning.

Inkjet-printing method.
Inkjet-printing method is to replace the ink in a printer with solution that contains QDs and print the QDs on the surface of desired material.The advantage is that it can place QDs on curved-surfaces, thus inkjet-printing method has a potential in manufacturing wearables like smart glasses and smart watches.The disadvantage of inkjet-printing method is time-consuming, the printer needs to eject the solution drop by drop.Kraus, Yang et al. achieved inkjet-printing of CdSe QDs array with high-resolution, 500 pixel per inch (ppi), and Figure 6 shows their inkjet-printed results [34].Huang, Guo et al. demonstrated the feasibility of inkjet-printed QDs as color converting film.They found the volume ratio of red-QDs to green-QDs was 1:12.9 for the emission intensities to be the same [35].Weng, Zhang et al. synthesized InP/ZnS core/shell QDs (InP/ZnS QDs) and patterned them on glass substrate with pulsed-laser.They discovered that the PL intensities of red and blue reached similar level when the mass ratio of InP/ZnS QDs to the photo-resist was 20% [36].Kim, Ryu et al. patterned QDs on glass substrate with photolithography, the achievable smallest pixel size was 10 μm.They found that the PL spectra of red QDs and green QDs both shifted to the blue end, and FWHMs were broadened when the solvent was evaporated, Figure 7 shows the steps for photo-patterning in their experiment [37].

Laser
Light can either behave like particle or wave, the equation E=hν relates the particle aspect to the wave aspect.The equation describing the wave aspect of wave is  * exp {�2 *  −  �⃗ *  ⃗ + �}, where ν is the frequency,  �⃗ is the wave vector, δ is the phase, A is the amplitude.According to quantum theory, photons will be released when electrons in atomic systems fall back to low energy state from high energy state.Photons as bosons can be in the same quantum state (QS), and the amplitude of light wave can be interpretated as the square root of the number of photons in the same QS.According to quantum theory, excited atoms have high probability to release photons with the same phase information as the already existing ones, and this is called stimulated emission.Compared with normal light, like light emitted from the Sun, laser light has high brightness, high phase correlation.Laser has many industrial applications, because of these features.In medical surgery, it can be used to perform eye surgeries or remove scars.In telecommunication it can be used in optical-fiber as light source to replace copper cables.In manufacturing industries, it can be used to etch marks on the surfaces of products.

5.2.1.
Laser structure and lasing process.The main components of a laser are gain medium, it can be gas or solid; a resonant cavity consisting of a cavity with two parallel mirrors at two ends, one of the mirrors being partially transparent; a power source or pump to provide energy.There are four steps to lasing.
Step one is usually call population inversion, most atoms in the gain medium absorb energy from the power source and transition from the ground state to a high energy state.Step two, a small amount of the high energy atoms spontaneously decays into the ground state and release photons, some of these photons are absorbed by the mirrors.Step three, the majority of atoms remaining in the excited state are forced to release photons and return to the ground state under the influence of the spontaneously decayed photons.
Step four, some of the photons pass through the partially transparent mirror forming a laser beam, when the photons of stimulated emission outnumber the photons being absorbed by the mirrors.

Some examples of QDs laser.
The working medium of conventional laser is either gas or solid, these two types of materials have high lasing thresholds.They require active cooling when in operation, and consume a large proportion of energy to generate heat.To solve this problem, people proposed to use QDs as gain medium, because QDs have large surface to volume ratio and tunable bandgap.
The power consumed by the laser system, when the gain is greater than the loss, is called lasing threshold.Lowering the lasing threshold can save energy, and the way to achieve this is to shrink down the size of the resonant cavity.
Nomura, Iwamoto et al. synthesized photonic crystal nanocavity with indium arsenic QDs (InAs QDs) in it.They found the irradiated threshold excitation power was very low approximately 2.5 μW, the loss was 375 nW, Figure 8 shows the nanocavity [38].Yan, Shi et al. fabricated cesium lead bromide QDs (CsPbBr3 QDs) and passivated them with 2-hexyldecanoic acid (DA), oleic acid (OA), and deployed silicon dioxide (SiO2) shell as the resonant cavity.Them found that DA-passivated CsPbBr3 QDs had the lowest lasing threshold being 5.47 μJ/cm 2 compared with pristine CsPbBr3 QDs and OA-passivated CsPbBr3 QDs [39].Maximov, Moiseev et al. fabricated InAs/InGaAs/GaAs QDs based laser, with InAs/InGaAs/GaAs QDs as the gain medium, with concentric rings made out of GaAs as the resonant cavity.They found that threshold power depended on the diameter of the inner ring, the lowest threshold power was 1.8 μW, when the inner ring diameter was 0.8 μm [40].Lasing requires a large amount of energy to pump the atoms of the gain medium to the excited state, long term operation of QDs-laser may reduce the stability of QDs through photo-chemical reaction.There are ways to improve photo-stability of QDs in lasers.
Cheng and Mao synthesized CdSe/ZnS QDs to study their photo-stability.They found that passivation of CdSe/ZnS QDs with thin aluminum oxide (Al2O3) layer improved the QDs' photostability both in air and vacuum compared with the pure CdSe/ZnS QDs [41].Chien, Cheng et al. fabricated CdSe/ZnS QDs and studied the photo-stability of the QDs when the resonant cavity is sealed by SiO2 layers and silicon nitride (Si3N4) layers.They found that both the intensity and the peak wavelength were stable in the cases of the sealed CdSe/ZnS QDs, whereas, the lasing intensity of the uncapped QDs decayed with time, and SiO2 was much suited for high power application compared with Si3N4 [42].

Quantum dots-sensitized solar cells (QDSSCs)
5.3.1.Photovoltaic effect.Solar cells are devices that convert light energy, instead of chemical energy, into electrical energy through the photovoltaic effect.The photovoltaic effect is a process where two types of materials directly contact with each other and produce an electric voltage when struck by light, and it is very similar to the photoelectric effect where electrons are ejected from metal faces when struck by light.The key components in a solar cell are a light absorption layer made out of a P-N junction (PNJ), a cathode, an anode.
There is a region inside a PNJ where there is an intrinsic electric field, when light with high enough energy is absorbed by atoms inside this region, pairs of electron-hole will be generated.Electrons and holes move towards the two ends of the PNJ under the influence of the intrinsic electric filed until reached an equilibrant, where the field generated by electrons and holes at the two ends of the PNJ is strong enough to cancel the intrinsic one.The end cumulating with electrons is the cathode, the end cumulating with holes is the anode.5.3.2.Advantages of using QDs as sensitizer.The bandgap of QDs can be altered by doping and size controlling, thus QDs can be engineered to absorb a wide range of light.The area to volume ratio of QDs is very larger, resulting a huge active light absorption area for only a small amount of QDs.Semiconductor QDs are inorganic materials so that they have better photo-stability, and they are easy to manufacture.Figure 9 shows the structure of QDSSCs [43].

Improving QDSSCs.
The key component in QDSSCs is high quality QDs, they will determine the performance of the QDSSCs.Improvement can be achieved by tuning QDs' properties, such as absorption spectrum and photo-stability.
Cai, Zou et al. synthesized AIS QDs with varying Ag/In ratios and deployed them in QDSSCs.They found the absorption spectrum red-shifted as the Ag/In molar ratios increased, the power conversion efficiency (PCE) reached the maximum (2.91%) when the Ag/In ratio was 1 [44].Selopal, Zhao et al. fabricated CdSe/CdS core-shell QDs (CdSe/CdS QDs) and studied their photovoltaic properties by varying the thickness of the CdS shell.They found the CdSe/CdS QDs with 1.96 nm shell thickness had the highest PCE (3.01%).Compared with the pure CdSe QDs sensitized SCs, CdSe/CdS QDs sensitized SCs had better photo-stability [45].Ganguly and Nath synthesized CdS QDs and doped them with manganese (Mn), and studied their photovoltaic properties by varying the concentrations of Mn.They discovered the CdS QDs doped with 2% of Mn had the highest PCE (2.09%) [43].

Conclusion
In conclusion, this paper first described two physical methods and two chemical methods of fabricating QDs namely, reactive-ion etching (RIE), pulsed-laser ablation (PLA), microwave irradiation and hotinjection (HI).Secondly, this paper discussed quantities related to QDs like narrow photoluminescence (PL) spectrum, high quantum yield (QY), tunable bandgap and fluorescence lifetime.Finally, this paper discussed laser, display panel and solar cell and the application of QDs to these technologies to enhance their performance.QDs have many applications, some of them were not mentioned in this paper, like medical imaging and drug delivery system, because these applications are primarily biomedical-focused.There is an area where QDs may have a great potential that is quantum computing, due to multiple advantages of QDs it is no doubt that QDs will play an important role in quantum computing industry.The author's personal speculation is that QDs will be in the heart of quantum-processor and quantumphotonic-computing-processor.

Figure 3 .
Figure 3. Setup for the synthesis of CdS nanoparticles by laser ablation in liquid[7].Zhang, Fang et al. synthesized phosphorene QDs (PQDs) with Nd-YAG pulsed-laser, to study the photoluminescence (PL) spectrum of PQDs, and they found the PL peak was independent of the excitation wavelength and the size of the PQDs[8].Zakharko, Lysenko et al. fabricated silicon carbide QDs (SiC QDs) with Ti-sapphire laser, the submersion layer was water.They found there was a correlation between the PL intensities and the laser power settings, the higher the power the higher the PL intensity, and there was no shift in PL spectral maxima[9].Abderrafi, Chirvony et al. fabricated GaAs QDs with Nd-YAG pulsed-laser, they found there was a correlation between the sizes of GaAs QDs and the laser power settings, the higher the laser power the smaller the size of the GaAs QDs[10].Ajimsha, Anoop et al. synthesized zinc oxide QDs (ZnO QDs) with Nd-YAG pulsed-laser and water, methanol, ethanol as confining liquids.They found with the decrease of laser power there was a blueshift in ZnO QDs' PL maximum, specifically ZnO QDs grown in methanol shifted from 2.41 eV to 2.6 eV when the laser power decreased from 45 mJ/pulse to 25 mJ/pulse, and, in ethanol shifted from 2.27 eV to 2.35 eV when the laser power decreased from 45 mJ/pulse to 25 mJ/pulse[11].

Figure 5 .
Figure 5. Evolution of PL QY of QDs in a function of the shell growth stages [29].

Figure 6 .
Figure 6.Inkjet-printed samples, scale bars from e1 to e3 are 5 mm, e4 shows micrograph of the emitting pixels, scale bar is 50 μm [34].5.1.3.Photo-patterning.Photo-patterning can be achieved by dispersing QDs in photosensitive solutions, then spin-coating the solution on the surface of the samples and forming desired patterns with high energy light.The advantage is time-saving as the pattern can be formed with one step.The disadvantage is high energy light may induce photo-chemical reaction or degradation in QDs.Weng, Zhang et al. synthesized InP/ZnS core/shell QDs (InP/ZnS QDs) and patterned them on glass substrate with pulsed-laser.They discovered that the PL intensities of red and blue reached similar level when the mass ratio of InP/ZnS QDs to the photo-resist was 20%[36].Kim, Ryu et al. patterned QDs on glass substrate with photolithography, the achievable smallest pixel size was 10 μm.They found that the PL spectra of red QDs and green QDs both shifted to the blue end, and FWHMs were broadened when the solvent was evaporated, Figure7shows the steps for photo-patterning in their experiment[37].