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Quantum dots

Any successful implementation of quantum structures, motivated by their electronic and optical properties – which differ from those of larger particles – relies on controlled fabrication of quantum dots as well as a thorough understanding of their physical properties. In this section, we show different research examples of microwave-assisted colloidal synthesis of various quantum dots. We show how technologies from Anton Paar are used for quantum dots research: microwave irradiation for synthesis of uniform nanoparticles with narrow size distribution; atomic force microscopy (AFM) for characterization of self-assembled indium arsenide quantum dots.

AFM study of self-assembled quantum dots

Introduction

Quantum dots are one of the main topics in nanotechnology, mainly due to their electronic and optical properties, which differ from those of larger particles. There are several methods to fabricate quantum dots such as colloidal synthesis, self-assembly and electrical gating. Self-assembled indium arsenide (InAs) quantum dots (QD) in gallium arsenide (GaAs) are an example of nano-structures obtained by self-assembly. They are of high research interest for several reasons: they provide a confinement for electrons and holes; full quantization makes them also termed artificial atoms; embedded in appropriate heterostructures, they become excellent single-photon emitters.

Experimental

On the single crystal GaAs wafer, quantum dots were grown. The wafer surface has a 100 orientation with a specified crystal-plane misfit of <0.1°. The misfit of the crystalline plane can result in the presence of atomic steps, which, based on a maximum misfit of 0.1°, should occur approximately every 200 nm of lateral distance with a step height of slightly below 300 pm for an atomic step. The goal of this investigation was to image the quantum dots as well as the atomic steps, with Tosca 400 AFM.

Results and discussion

Various locations on the samples were imaged in order to ensure representative results. The results can be seen in Figure 1. In order to shed more light on the vertical and lateral dimensions of the identified surface features, 500 nm x500 nm scans were performed. The linear features and the quantum dots can be clearly observed. We were able to identify 4 different terraces (A, B, C, D). For data processing by Tosca Analysis, the image was levelled parallel to the terraces using a 3-point plane on level C as the reference. Figure 2 shows the analysis of QD dimensions in Fig. 1 using 2D-profile lines. The quantum dots exhibit a radius of about 10 nm, and a height of about 24 nm.

Fig. 1. Topographic area extract of 400 nm x400 nm

Fig. 2. 2D line profile with QDs dimensions

Fig. 3. 3D image of a 500 nm x500 nm scan showing the quantum-dots, and also the atomic-step-structure, of GaAs

Additional information

Instruments:

Source: A. Ludwig, et al: Ultra-low charge and spin noise in self-assembled quantum dots, Journal of Crystal Growth 477, 193 (2017)

Application report: 

Synthesis of carbon dots for crop protection

Introduction

The initiation of RNA interference is an interesting application for plant functional genomics and crop protection. Carbon dots are promising tools for transporting RNAi effectors through the cell wall. Readily accessible amines with high carbon content should be evaluated for the rapid preparation of the desired functionalized carbon dots.

Experimental

A G10 vial was charged with polyethylene imine and a 4:1 mixture of chloroform and methanol. The vial was sealed and subjected to heating. The target temperature was achieved within a 5 min ramp.

After cooling, the solvent was evaporated under a stream of nitrogen and the residue was redispersed in water. Remaining chloroform was removed by centrifugation and the pH of the aqueous phase was adjusted to 8 with HCl (4N). Then the final volume was adjusted to 5 mL with water and the dispersion was stored at room temperature until further use.

Results and discussion

Branched polyethylenimines of different molecular weight were used to generate the fluorescent nitrogen-doped carbon dots. The polyethylenimine precursor served as a carbon source for the nanoparticles and as a nitrogen source for the in-situ surface passivation of the carbon dots.

The size of the obtained carbon dots correlated with the molecular weight of the substrate. The resulting nanoparticles were used in formulation to successfully deliver small interfering RNA into tobacco and tomato plants. The amine-functionalized carbon dots proved a promising tool for gene silencing in plants.

Additional information

Instruments:

Source:

S. H. Schwartz et al., Plant Physiol. 2020, 184, 647-657

Application Database Entry:

Synthesis of fluorescent N-doped carbon dots for optoelectronic devices

Introduction

Carbon dots have attracted considerable interest during the last decade due to their high water-solubility, low toxicity and their remarkable photoluminescent properties. They can be easily accessed by conversion of readily available biomass under various reaction conditions. Microwave irradiation provides a simple and cost-efficient hydrothermal carbonization method to generate valuable nanomaterials from renewable resources. The influence of the starch concentration and reaction temperature on the fluorescent properties was investigated.

Experimental

In a G30 vial, the freshly isolated potato starch was suspended in water. Ethylene diamine was added and the vial was subjected to microwave irradiation.

After cooling, the mixture was filtered through a microporous membrane and the desired powdered carbon dots were obtained after freeze-drying for 2 days.

Structural characterization was conducted by transmission electron microscopy, X-ray photoelectron spectroscopy, and FTIR spectroscopy.

Results and discussion

Various reaction conditions with temperatures from 180 °C to 220 °C for 10 min to 50 min were evaluated with starch concentrations of 0.3 to 5 mg/mL, to optimize the process and to obtain the best fluorescence properties. The resulting photoluminescent nitrogen-doped carbon dots with bright blue emission at 360 nm revealed remarkable suitability as phosphors for optoelectronic devices, and were used to prepare white LEDs.

Additional information

Instruments:

Source:

J.-X. Zheng et al., New Carbon Mater. 2018, 33, 276-288

Application Database Entry:

Ferromagnetism in transition metal-doped quantum dots

Introduction

Transition metal-doped diluted magnetic oxide semiconductors (DMOS) are considered promising materials for spintronic devices. The effect of manganese dopant on the magnetic properties of tin oxide quantum dots was investigated to better understand the origin and control of ferromagnetism in these nanomaterials. Microwave irradiation was chosen as a heating technology as it is beneficial for the formation of uniform nanoparticles with narrow size distribution.

Experimental

Manganese chloride and tin(IV) chloride were dissolved in H2O/EtOH (1:1) and mixed accordingly to achieve the required metal ratios. Sodium dodecyl sulfate was added and the precursor solution was pre-heated in an oil bath at 60 °C for 30 min. Then aqueous ammonia solution was added under stirring to adjust the pH to 12, and the mixture was sonicated for another 30 min. A 100 mL PTFE liner was charged with the pre-treated mixture and immersed in the pressure jacket. The reaction vessel was sealed and placed in the rotor, which was closed and subjected to microwave heating.

After cooling, the formed precipitate was filtered and washed with EtOH and water. Then the particles were centrifuged and dried at 80 °C.

Results and discussion

Microwave heating allows for controlled synthesis of uniform nanoparticles. The rapid and controlled synthesis technique facilitated the control over composition and microstructure of the nanoparticles. The content of manganese dopant (varied from 2 to 10 mol%) influenced the ferromagnetic properties of the resulting nanoparticles. A higher manganese content resulted in stronger ferromagnetism.

The prepared quantum dots were characterized by powder X-ray diffraction, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy.

Additional information

Instruments:

Source:

D. Manikandan, et al., Phys. Chem. Chem. Phys. 2018, 20, 6500-6514

D. Manikandan, et al., J. Phys. Chem. C 2019, 123, 3067–3075

Application Database Entry:

Synthesis of luminescent nanoparticles for optoelectronic devices

Introduction

Quantum dots for optoelectronic devices are highly attractive species in nanomaterial research. Since conventional synthesis methods are rather time-consuming, rapid alternatives utilizing microwave irradiation are widely under investigation. Microwave irradiation allows for immediate tuning of reaction conditions and subsequent investigation of the obtained nanoparticles for changes in their physical properties.

Experimental

The selenium precursor was prepared by dissolving selenium powder and trioctylphosphine in octadecene at 250 °C under inert atmosphere. Similarly, the cadmium precursor was prepared by dissolving cadmium oxide and in octadecene at 300 °C under inert atmosphere.

In a glove box, a G30 vial was charged with 10.6 mL Cd precursor and 1 mL Se precursor. The vial was sealed, transferred to the microwave reactor and subjected to microwave irradiation.

After cooling, the mixture was diluted with acetone and centrifuged. The particles were separated, resuspended in toluene and centrifuged again. The particles were stored in toluene suspension under inert atmosphere until further use.

Results and discussion

The desired quantum dots were generated at different temperatures (150 °C to 280 °C) and reaction times (0.5 to 5 min), to investigate particle growth and resulting properties.

With increasing reaction temperature, a significant red shift of the emitted luminescence was observed. Additionally, the particle dimeter increased from approx. 4.5 nm to 7 nm when going from 150 °C to 280 °C.

Since the quantum dot size is related to the band gap, this enables easy tuning of the optical properties to tailor nanoparticles according to their intended application.

Additional information

Instruments:

Source:

D. Thomas et al., J. Nanomater. 2020, 5056875

Application Database Entry: