The group has a focus on the use of cutting-edge and novel experimental optical techniques to study micro- and nanomaterials with an optoelectronic application. This interest spans:
- 2D materials and devices
- semiconductor nanowires
- exotic photovoltaic materials
If you are interested in collaboration or pursing a PhD in this field, please get in touch.
At present, our most active areas are semiconductor nanowires, 2D photodetectors and photovoltaic materials; more detail is given below.
Semiconductor nanowire optoelectronics
Semiconductors play a key role throughout the field of optoelectronics, providing the active material for photodetectors, light-emitting diodes, and diode lasers. Whilst materials such as silicon are commonly used, a push towards higher speed, lower cost and more tightly integrated devices have led researchers to consider novel material systems with more tunable material properties. Of particular interest are semiconductor nanowires based on III-V materials such as GaAs, InP or InAs.
Nanowires inherently feature nanoscale dimensions, bottom-up fabrication routes and tuneable material parameters through surface or heterostructure engineering, and have been identified as key components for future nanotechnology-enabled optoelectronic devices. However, nanowire-based optoelectronics are a new field, and significant challenges remain for both material characterisation and optoelectronic-relevant testing.
This project builds on recent research at the University of Oxford (Johnston group and Herz group) and a collaboration with the nanowire growth group at the Australian National University, and has four main goals:
- To investigate key material parameters using an integrated range of ultrafast optical techniques, including photocurrent microscopy, photoluminescence microscopy, terahertz photoconductivity, and non-linear optical approaches and correlate these measurements for deeper insight
- To understand inter-wire inhomogeneity
- To implement the best performing nanowire devices based upon the material parameters determined earlier in the project.
- Nanowire statistics: Alanis et al, Nano Letters 17, 4860
- Nanowire terahertz detectors : Peng et al, Nano Letters 16, 4925
- Nanowire lasers : Saxena et al, Nature Photonics, 7, 963
- Nanowire quantum well systems : Davies et al, RSC Nanoscale, 7, 20531
- Nanowire doping : Boland et al, Nano Letters, 15 1336
High speed detectors are essential across a number of fields. In particular, detectors based on InGaAs are essential for telecommunications, where speed directly leads to higher communication rates. At present, mature technology has built up on either silicon in InGaAs on InP architectures, both with advantages in production and implementation.
However, to go beyond the spectral ranges where these materials work – for instance, into the infrared – new materials must be used. Such wavelength ranges are important for either thermal or chemical sensing application.
Working with Tim Echtermeyer in the School of Electrical and Electronic Engineering at the University of Manchester, our group has been using our experience in ultrafast laser microscopy to develop new photodetectors based on 2D materials.
- Substrate modification for photodetectors: Selvi et al. Nanoscale, 10, 3399
- Graphene Shottky-Diode Detectors: Selvi et al. arXiv:1807.00225
Novel photovoltaic materials
Third-generation photovoltaics promise high efficiency, low cost and easily produced solar cells based upon low-temperature or roll-to-roll preparation methods. Key examples include dye-sensitised solar-cells, nanowire-based photovoltaics, colloidal quantum dot sensitised photovoltaics and the rapidly emerging field of perovskite-based devices. These materials are promising due to their use of novel materials or nano/meso-structuring to control the light absorption, charge generation and charge collection processes.
A key aspect of nano or meso-structured devices is the inhomogeneity inherent to such structuring, with critical energy processes occurring at spatially separated positions in three dimensions. Investigation can be hindered by the material in a full device performing differently from the active material alone; we therefore require a non-contact, in-situ probe of photon absorption, charge generation and charge migration processes that is sensitive on the shortest length scales and fastest time scales.
This project will address this challenge by use of recently developed and proof-of-concept techniques in time-resolved microscopy and nanoscopy, utilising visible and terahertz radiation to probe the ultrafast energy dynamics in next generation solar cells.