Coupled Nanocrystal Molecules: Quantum coupling effects via chemical coupling of colloidal nanocrystals

Project Information

CoupledNC

Grant agreement ID: 741767

DOI

10.3030/741767

Project closed

EC signature date 18 May 2017

Start date 1 November 2017

End date 30 April 2023

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 499 750,00

EU contribution

€ 2 499 750,00

Coordinated by

THE HEBREW UNIVERSITY OF JERUSALEM
Israel

Periodic Reporting for period 4 - CoupledNC (Coupled Nanocrystal Molecules: Quantum coupling effects via chemical coupling of colloidal nanocrystals)

Reporting period: 2022-05-01 to 2023-04-30

Summary of the context and overall objectives of the project

At the basis of chemistry lies the concept of the chemical bond. By coupling atoms into molecules, we can achieve new structures and rich functionalities in diverse ways, yielding the entire selection of molecules and materials composing our world. In analogy to chemistry with atoms, the main goal of CoupledNC is to introduce a new concept for “Nanocrystals Chemistry”. Using colloidal quantum dots (QDs), as artificial atom building blocks, this project aims to go to the next step and create artificial molecules with such building blocks and to study their unique properties and emergent applications.
QDs are nano-sized chunks of a crystal of a semiconductor material, each particle containing hundreds to thousands of atoms. When viewed through an electron microscope they look like dots. Unlike their Periodic Table counterparts, QD artificial atoms are mercurial in nature, changing their physical, electronic, and optical properties when their size changes. For example, a larger QD will emit red light, while a smaller one, of the same material, will emit green light. In the past twenty years, the level of control over these tiny particles and the understanding of the physical properties of QDs have increased tremendously. This has led to a widespread application of QDs in our daily lives—from bio-imaging and bio-tracking (relying on the fact that QDs emit different colors based on their size) to solar energy harvesting and state of the art displays with exceptional color quality. Forming new artificial molecular structures from such building blocks would thus enrich the functionalities of QDs. We developed a methodology to join and fuse QDs together into artificial molecular structures. Using this method, infinite possibilities could arise from creating artificial molecules from artificial atom building blocks. As with real atoms, when you combine artificial atoms together, they create a new (artificial) molecule with unique properties and characteristics. These molecules are referred to as “artificial” because they’re not one of the 150 million original molecules that have been formed by combining atoms from the 118 known elements in our Periodic Table.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We were able to develop a general method that affords the fusion of any two semiconductor QDs, serving as artificial atoms, together, through control over the distance between both artificial atoms in the newly synthesized artificial molecule. This parameter, along with the identity of the QD monomers, is crucial for determining the unique properties of the newly formed artificial molecule.
Each of the artificial atoms composing the artificial molecule demonstrates specific attributes on its own, yet, while fused together, new optoelectronic features appear, due to that coupling. Therefore, studying the artificial molecules required the study of each of the composing atoms, to exclude the properties associated with the individual nature of the atoms, and to decipher and isolate the newly exhibited properties of the fused molecule.
QD artificial atoms/molecules demonstrate slight variations in their characteristics, which translate to variations in their physical properties, and the resultant functionality. Hence, augmenting ensemble measurements, our study focused on studying the artificial molecules on the single-particle level, to avoid blurring of effects due to averaged measurements. This required using state-of-the-art electron microscopy characterization, to spatially resolve the structure and composition of the artificial molecules on the atomic level. The structural characterization data was correlated with physical and optical characteristics, in particular with the fluorescence spectrum of the QD molecules, their radiative lifetimes, their behavior with time (blinking), their photon correlation statistics, their polarized emission, and their response toward electric field manipulations.
Additionally, a unique two-color emitting QD molecule was revealed, that can change its emission color by the application of an electric field. This has created the tiniest color switch – where one artificial molecule can change the color emitted from it instantaneously on demand.
We were also able to correlate our observations with theoretical models, predicting the extent of coupling, and the structural factors governing this coupling, such as the thickness of the adjoining area between the QDs, its length, the size of both QDs and so on. This was later doctored into synthesis concepts, yielding new structures, with newly found properties, such as a bow-tie structured molecule with an active center, or an electric-filed modulated molecule that can be electrically switched to emit different colors.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Colloidal QDs molecular-like structures were synthesized in the past in a limited way, which involved the use of complicated and expensive machinery, and provided very little control over the varying parameters of the formed structure. Namely, molecular linkers and DNA were used to achieve geometric control of such structures. But in those cases, the coupling is weak, due to the large distance and the presence of a high potential barrier between the two QDs. As a result, their applicability was very limited to specific applications.
We have developed a new and robust methodology, which affords control over the parameters joining the two QDs together. Importantly, this includes also fusing the two QDs to form a coupled molecular-like system with a stable continuous crystal lattice as the connection region between the two QDs. Such a connection is robust and is also providing strong controlled coupling between the two QDs. Considering the wide selection of size and composition of controlled QDs, this opens up vast opportunities to generate different types of artificial molecules using this methodology. At the end of such a process, the surface chemistry of the free-standing artificial molecules can be controlled to enable chemical compatibility with different solvents including water as needed in bio- and medical- applications. They can be embedded in plastic sheets for display applications or printed and deposited on diverse substrates to serve as building blocks for optoelectronic devices.
As was also mentioned above, we developed a method to specifically correlate the structural characterizations of a specific artificial molecule, with its respective photo-physical characterization, on a single-particle level. As the characterization of those properties requires the use of different instruments, associating several characterization methods is challenging due to the nanometric sizes of the artificial molecules. This method can be implemented on a wide range of materials and can assist in deciphering the role of structural defects for example, and their effect on the physical properties of the material, this can aid in the smart engineering of nano-structures for various applications.
We, therefore, envision a new world of opportunities for coupled dimers in various applications. They can be incorporated into optoelectronic applications such as displays, bio-medical applications, optoelectronic devices, and quantum technologies. Special tuning and choice of the nanocrystal building blocks can also lead to tailoring their properties towards photocatalytic applications, and even antibacterial and antimicrobial applications that have become highly relevant in light of the COVID-19 pandemic.

a) forming fused coupled QD molecules b) structural-chemical characterization of the CQDM