Quantum Dots - Single Photon Emitter

bawendi_vialsII-VI semiconductor colloidal Quantum Dots (QDs) are highly fluorescent nanocrystals which are prepared through organometallic synthesis in solution phase. They have been widely used in practice as fluorescent labels in many bio-imaging applications. Quasi-zero dimensional Wannier exciton states arise in QDs as a result of the strong quantum confinement which makes QDs be very interesting and unique: Just change the size of QDs (no need to change material), emission colors will change. Bigger QDs will emit redder light and smaller QDs will emit bluer. Extensive research has been carried out to exploit their associated electronic/optical properties.

Strong quantum confinement effect also enhance non-radiative Auger recombination processes with respect to their bulk counterpart. Auger processes suppress biexciton and multiexciton emissions in QDs. This effect makes QDs appealing material candidate as room-temperature-operating single photon source owing to its large exciton binding energy commonly seen in II-VI compounds.

1.     Silica Cladding Quantum Dot Synthesis

Our core (CdSe) of Quantum Dot (QD) was synthesized by precipitation reaction first and then the desired multishell CdS/Zn0.5Cd0.5S/ZnS growth was achieved by successive ion layer adhesion and reaction (SILAR) technique. The as-synthesized QDs were capped with octadecylamine (ODA). They had an averaged diameter of 8nm and measured fluorescent quantum yield of about 50%.  A thick shell of silica was subsequently grown onto the QD utilizing water-in-oil (W/O) microemulsion growth technique, which has been widely adopted to synthesize silica colloidal particles. 


 

 Silica-cladding approaches have recently been initiated on QDs as well as on magnetic nanoparticles, but issues related to particle size control and encapsulation uniformity are reported to be considerable challenges.  However, by our technique, we have achieved significant improvement in the final particle size distribution, as seen from the transmission electron microscope (TEM) images where up to 95% of the particles have single QD core precisely positioned at the center. One key to this advance is the choice of the surfactant NP-12 with relatively large unit length polyoxyethylene hydrophilic group. As NP-12 helps to reinforce the stability of the micelle against ethanol, a by-product of the reaction, we could tune the final particle size up to 220nm in a one-pot synthesis without generating secondary silica nuclei.

 
Silica-encapsulated II-VI semiconductor colloidal QDs. a-d, Transmission electron microscope images of the synthesized silica-clad QDs with various total particle diameters of 28nm (a), 75nm (b), 95nm (c), and 180nm (d), obtained via microemulsion synthesis with NP-5 (a) and NP-12 (b-d) as the surfactants, respectively. Single QDs of about 8nm in diameter are visible at the core of the composite particles, appearing as small dark dots. The scale bars in the images correspond to 20nm (a) and 100nm (b-d). e,f, the synthesized silica-clad QDs (right, 180nm diameter) in cyclohexance under ambient (e) and UV (f) exposures. Bare QD controls (left) with similar concentration was used as a reference. g, photoluminescence spectra of the silica-clad QDs in cyclohexane (blue) and in ethanol (green), in comparison to the bare QD control (red), under the excitation at 380nm.

Important for applications, the optical quality of the QDs is largely preserved after thick silica encapsulation. The photoluminescence (PL) spectrum of the QDs after silica encapsulation shows clean QD ground state exciton emission signature centered around 613nm (slightly red-shifted by about 3nm compared to the “bare” QD control), without introducing any noticeable impurity emission background. The PL efficiency of the silica-clad QDs is nearly unchanged if they are dispersed in non-polar solvent; however, it drops by about 60% upon transferring into ethanol.


2.     Electrostatic Force Self-Assembly Method (EFSA):

Our CdSe-core CdS/Zn0.5Cd0.5S/ZnS-multishell QDs were encapsulated with thick SiO2 shell, making them as big as 200nm in diameter. A method of electrostatic force self-assembly (EFSA) was developed to pattern single silica coated QDs.

The EFSA Model for "Well Trap":

EFSA Model

The EFSA Model for "Island Trap":

EFSA Model 2

3.     Single Silica coated QDs Array:

Using EFSA method, we are able to make an array of single silica coated quantum dots. Scanning Electron Microscope Images show excellent results.

Island Trap:

Single Silica coated QD Array

Well Trap: 

EFSA Well trap

 

3.     Antibunched characteristic:

Each element in array shows single photon emission characteristic at room temperature.

Single photon Emitter HBT

 

4.     Device engineering approaches:

 Integrate with micro-LED excitation source arrays:

 

Device Integration

 

 

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