Colloidal nanomaterials and bottom-up optoelectronic integration

This section presents selected highlights from our work on inorganic colloidal nanomaterials and their integration into functional optoelectronic devices, following a deliberately bottom-up approach. In this strategy, the optical and electronic properties of devices are directly inherited from the physical and chemical characteristics of the constituent nanomaterials and interfaces.

I. Emerging and reconfigurable optoelectronic functions enabled by optical and electrical control

Our recent work has explored the functional exploitation of charge states and interfaces in colloidal nanomaterials to realize emerging and reconfigurable optoelectronic functions. A representative example is the development of optoelectronic memories with optical writing and electrical readout, based on the integration of nitrogen-doped carbon quantum dots onto graphene field-effect transistors.

In these devices, the colloidal nanoparticles play an active role via "optical gating", modulating and storing the electronic state of the transistor, enabling photocurrent control through illumination and electrical biasing. This work demonstrates how simple, solution-processed nanomaterials can be repurposed beyond their conventional uses to perform information storage and processing functions, by leveraging their intrinsic charge trapping and charge dynamics.

Figure caption: Exploitation of the charge trapping properties of nitrogen-doped fluorescent carbon nanoparticles integrated into a graphene-based field-effect transistor to realize an optoelectronic memory with optical write and electrical read operations. (a) FET device architecture; (b) Atomic force microscopy (AFM) characterization of the active channel area; (c) Optical image of the hybrid field-effect transistor under test; (d) Evolution of the source–drain current during successive optical writing cycles under UV illumination and electrical erasing under continuous bias.

Publication :

Chaudhary, M. ; Xin, C. ; Hu, Z. ; Zhang, D. ; Radtke, G. ; Xu, X. ; Billot, L. ; Tripon‐Canseliet, C. ; Chen, Z. Nitrogen‐Doped Carbon Quantum Dots on Graphene for Field‐Effect Transistor Optoelectronic Memories. Adv Electron Mater 2023, 9 (8), 2300159. https://doi.org/10.1002/aelm.202300159.

II. Colloidal nanocrystals as active building blocks for optoelectronics

Our earlier work significantly contributed to the use of semiconductor colloidal nanocrystals as active materials for optoelectronics. In particular, we demonstrated light-emitting diodes based on quasi-two-dimensional colloidal nanoplatelets, exhibiting spectrally narrow emission directly controlled by quantum confinement. These results highlight the potential of fully solution-processed colloidal architectures for fine engineering of electroluminescence properties.

Figure caption: (Left) TEM image of CdSe/CdZnS core/shell colloidal nanoplatelets and photograph of a nanoplatelet-based light-emitting diode operating under continuous bias; (Right) Corresponding photoluminescence (PL) and electroluminescence (EL) spectra.

In parallel, we investigated the fundamental mechanisms of charge transport and recombination in colloidal quantum dot solar cells by combining nanocrystal ligand-tuning with ultrafast electro-optical spectroscopies. These studies revealed the key role of surface chemistry and trap states in photovoltaic performance losses and provided mechanistic insight into charge dynamics in colloidal devices.

Figure caption: (a) Optical absorption and photoluminescence spectra of the investigated PbS quantum dots (QDs), together with a schematic of the corresponding QD solar cell architecture; (b) Pump–push photocurrent spectroscopy results revealing markedly different charge trapping dynamics depending on the organic surface ligands of PbS QDs (MPA and EDT). The inset shows the experimental principle.

Building on this bottom-up approach, we further demonstrated that the charge recombination behavoirs in PbS quantum dot solar cells can be controlled through active layer engineering, for example, by introducing a second population of nanocrystals (Zn-doped CuInS₂). This binary architecture enables partial spatial separation of charge carriers, leading to a measurable reduction in recombination and a significant increase in photocurrent and power conversion efficiency, while remaining compatible with simple solution-processing techniques.

Figure caption: (Left) Schematic of a bilayer TiO₂ / PbS–CuInS₂ quantum dot heterojunction photovoltaic device developed in our group; (Right) Cross-sectional SEM image of the device.

Publications :

⦁ Chen, Z. ; Nadal, B. ; Mahler, B. ; Aubin, H. ; Dubertret, B. Quasi-2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence. Adv Funct Mater 2014, 24 (3), 295–302. https://doi.org/10.1002/adfm.201301711.
⦁ Bakulin, A. A. ; Neutzner, S. ; Bakker, H. J. ; Ottaviani, L. ; Barakel, D. ; Chen, Z. Charge Trapping Dynamics in Pbs Colloidal Quantum Dot Photovoltaic Devices. ACS Nano 2013, 7 (10), 8771–8779. https://doi.org/10.1021/nn403190s.
⦁ Sun, Z. ; Sitbon, G. ; Pons, T. ; Bakulin, A. A. ; Chen, Z. Reduced Carrier Recombination in PbS - CuInS2 Quantum Dot Solar Cells. Sci Rep 2015, 5 (1), 10626. https://doi.org/10.1038/srep10626.

III. Scientific positioning and approach

Overall, these works illustrate a coherent strategy aimed at exploiting defects, charge states, and interfaces in colloidal nanomaterials not necessairly as limitations, but as functional degrees of freedom. They have contributed to establishing a broad expertise ranging from nanomaterial synthesis and surface modification to their integration into complete optoelectronic devices, supported by a detailed understanding of the physical mechanisms governing their operation.