Opto-electronic Materials
Delft University of Technology
Faculty of Applied Sciences
Chemical Engineering
Van der Maasweg 9
2629 HZ Delft
The Netherlands
Mrs. Wil C.M. Stolwijk
T +31 (0) 15 2783137
Office: D1.240

Dr. A. J. Houtepen

Dr. A.J. Houtepen
Assistant Professor - Tenured



Delft University of Technology

Chemical Engineering

Van der Maasweg 9
2629 HZ Delft

The Netherlands

T: +31 (0) 15 27 82157

Office: D1.140 (Building 58) 


My CV can be found here. An up-to-date list of my publications can be found at, a pdf is available here.
A recent powerpoint presentation about our work can be found here.

Research Interest

Our research focuses on colloidal semiconductor nanocrystals, also known as Quantum Dots (QDs).  Such nanocrystals hold promise for a range of opto-electronic applications, such as solar cells, photodetectors and light emitting devices. These materials are investigated in solution, but since 3 years particular attention is paid to the preparation and investigation of conductive films of these nanostructures.

The research we perform is of a multi-disciplinary nature. It involves synthesis, preparation of highly conductive films, characterization and fundamental (spectroscopic) studies. The disciplines involved range from synthetic chemistry to physical chemistry, optics and solid state physics.
In my view the combination of in-house sample fabrication and advanced spectroscopic investigation is very important for the development of functional materials. There is a strong feedback loop between the spectroscopy and the sample preparation that allows for a rapid advancement.  This feedback works both ways: knowledge on exact sample composition is invaluable to understand the spectroscopic results and these results guide the improvement of synthetic procedures to prepare high quality samples with the desired functionality.

Keywords: quantum dots, nanocrystals, charge injection, charge transport, photoconductivity, disorder, self-assembly, quantum confinement, multiple exciton generation, carrier dynamics, time-resolved spectroscopy.


Funding for our work comes from the following sources:
NWO Veni grant
FOM Joint-Solar-Programme
FOM Functional Nanoparticle Solids programme
Toyota Motor Europe  


For our work on colloidal quantum dots the following facilities have been set up:

  • A lab with facilities for air-free synthesis of colloidal nanocrystals and highly conductive films of those nanocrystals.
  • A lab for combined spectro-electrochemical and dc conductivity measurements
  • A lab for advanced fs spectroscopy with white-light detection capabilities (funded by a Veni grant, together with Ferdinand Grozema)

In addition we make very frequent use of the Time-Resolved Microwave Conductivity setup, the THz conductivity setup and the “Vici” transient absorption setup within the section opto-electronic materials.

Colloidal quantum dots and why they are interesting

Colloidal semiconductor nanocrystals, also called quantum dots, are pieces of semiconductor material that are so small that the electronic properties depend on the size. Charge carriers are literally confined to the space of the nanocrystals. The best-known effect of this so called quantum confinement is that the bandgap (and therefore their colour) of quantum dots depends on their diameter. This is illustrated in Figure 1, which shows CdSe quantum dots of different sizes.
In addition to a size-tunable bandgap, the small size of the QDs also results in enhance Coulomb interaction between charges that reside in the QDs. This enhances electron-electron scattering effects that are governed by this Coulomb interaction and results in enhance Auger recombination and Multiple Exciton Generation.  

1. Synthesis of colloidal nanocrystals

It is a challenge to synthesize quantum dots that have one well-defined size, since this means that all quantum dots have the same electronic properties. Furthermore, such monodisperse nanocrystals can self-organize into 2D or 3D superstructures.
In addition to spherical nanocrystals it is also possible to synthesize more exotic shapes, such a cubes, stars and wires.
The “Nanolab” has all necessary facilities for the synthesis of high quality colloidal nanocrystals. We currently synthesize CdSe, CdTe, CdS, PbSe, PbS, ZnO and FeS2 nanocrystals. Recently much effort is devoted to the synthesis of high quality and non-toxic quantum dots that should ultimately replace Cd and Pb based QDs. Materials include FeS2, Ge and InP nanocrystals.

2. Preparation of quantum-dot films  

In many instances we study films of nanocrystals, which are prepared by dropcasting, spincoating or layer-by-layer deposition (see figure 3). We can successfully prepare very conductive films by exchanging in a controlled way the original organic ligands on the surface of the quantum dots by much smaller ones. Layer-by-layer assembly provides spatial control over the quantum dots: composite films with a controlled interface between the component or a well-defined size gradient can be made. Figure 4 shows an example of a composite PbSe QD / CdSe QD film on Quartz.  

3. Optical properties of colloidal nanocrystals   

Using white-light transient absorption spectroscopy within the spectral range 450 nm to 1600 nm we investigate the (ultrafast) optical properties of QDs in dispersion and of conductive QD films. An example of such a measurement on very small FeS2 nanocrystals is shown in Figure 5. Many effects can be investigated with this approach, including Multiple Exciton Generation, Auger recombination, Intraband absorption, hot carrier cooling, multiexciton shifts, charge transfer between nanocrystals or to ligands, charge trapping in surface states, etc.  

4. Charge transport in films of colloidal nanocrystals     

The main focus of our work is the study of charge transport in photoconductive films of colloidal nanocrystals. We combine of steady state and fs time-resolved optical spectroscopy with the time-resolved study of the ac photoconductivity of the films. The latter property is studies using the Time-Resolved Microwave Conductivity technique (ns to ms time resolution) or Terahertz spectroscopy (ps to ns time resolution). Often the combination of these techniques on a single sample and under identical conditions provides a lot of insight into the processes that occur in the films. An example is shown in Figure 6, which shows that in highly conductive PbSe QD films the (both real and imaginary) THz conductivity transients are identical to transient absorption transients under identical experimental conductions. This indicates that all charge carriers are mobile and that, hence, all photogenerated excitons dissociate on a ~1 ps timescale. This work was published in Nature Nanotechnology.3


Charge transport between quantum dots usually occurs via tunneling. However at high charge density, or for sufficient strong electronic coupling between neighboring QDs, band-like transport, similar to transport in crystalline solids, may also take place.3

In the regime of electron tunneling the carrier mobility is determined by the distance between the quantum dots and the height of the tunnel barrier. We use layer-by-layer assembly to form films of quantum dots with controlled tunnel barriers between them. The mobility of the charges in these films is many orders of magnitude larger than in films of quantum dots where the original surfactants are still present, and depends exponentially on the length of the ligands used to replace the original ligands (see figure 7).1


We have studied many more electronic effects in these conductive QD films. A brief selection of our recent publications on this topic is given below:

  1. Photoconductivity of Pbse Quantum-Dot Solids: Dependence on Ligand Anchor Group and Length. Gao, Y., Aerts, M., Sandeep, C. S. S., Talgorn, E., Savenije, T. J., Kinge, S., Siebbeles, L. D. A. & Houtepen, A. J. ACS Nano, (2012). IF: 11.421 
  2. Enhanced Hot-Carrier Cooling and Ultrafast Spectral Diffusion in Strongly Coupled Pbse Quantum-Dot Solids. Gao, Y. N., Talgorn, E., Aerts, M., Trinh, M. T., Schins, J. M., Houtepen, A. J. & Siebbeles, L. D. A. Nano Lett. 11, 5471-5476, (2011). IF: 13.198 
  3. Unity Quantum Yield of Photogenerated Charges and Band-Like Transport in Quantum-Dot Solids. Talgorn, E., Gao, Y. N., Aerts, M., Kunneman, L. T., Schins, J. M., Savenije, T. J., van Huis, M. A., van der Zant, H. S. J., Houtepen, A. J. & Siebbeles, L. D. A. Nat. Nanotechnol. 6, 733-739, (2011). IF: 27.270 
  4. Free Charges Produced by Carrier Multiplication in Strongly Coupled Pbse Quantum Dot Films. Aerts, M., Sandeep, C. S. S., Gao, Y. A., Savenije, T. J., Schins, J. M., Houtepen, A. J., Kinge, S. & Siebbeles, L. D. A. Nano Lett. 11, 4485-4489, (2011). IF: 13.198 
  5. Photoconductivity Enhancement in Multilayers of Cdse and Cdte Quantum Dots. Talgorn, E., de Vries, M. A., Siebbeles, L. D. A. & Houtepen, A. J. ACS Nano 5, 3552-3558, (2011). IF: 11.421 

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