Quantum confinement of 2D perovskite nanoplatelets for light emission

The term "perovskite" refers to a family of compounds with a common crystal structure and the general formula ABX3. Typical metal halide perovskites (MHP) have a monovalent cation at the A site, divalent metal cation at the B site, and halide anion at the X site. The crystal structure of metal halide perovskite is characterised by the BX64- octahedra, where the B element sits at the centre and halide anions occupy the octahedral sites. The A-site cation is caged in between the octahedra, which is important for enabling the perovskite crystal structure to form.
MHPs have attracted significant research interest for optoelectronic applications due to their outstanding optoelectronic properties. One of the main MHP materials is lead halide perovskite, i.e., lead (Pb) as B cation. They have great potential in photovoltaic and light emission applications due to a tuneable band gap, facile production, and high defect tolerance. MHP-based optoelectronic devices have promising performances, for example, external quantum efficiency (EQE) of 20.3% in light-emitting diodes (LEDs) and record power conversion efficiency (PCE) of 25.2% in a single junction solar cell.

While bulk perovskite films have been showing great potential, nanoscale perovskite materials often exhibit other interesting properties. In order to explore more potential applications of perovskite material, nanoscale perovskite morphologies have been investigated extensively. Figure 4 shows perovskite nanomaterials that have been synthesized as nanocubes (NCs), nanoplatelets (NPLs), nanorods (NRs), as well as quantum dots (QDs) classified according to their size and shape.
In nanoscale semiconductor materials, quantum confinement plays a decisive role in the optoelectronic properties by finely manipulating the band gap. The semiconductor forms the "well" while the outer environment serves as the "wall" in the classic quantum well model. Therefore, the energy levels are upshifted to a higher level. As a result, within the same composition, perovskite light emitters can provide different emission wavelengths by nanoscale size tuning, providing alternative choices for targeted emission wavelengths.
Perovskite NCs have typical edge length of 8-15 nm in cubic shape, and is the mostly studied form of perovskite material due to well-established protocols. CsPbX3 NCs offers both size and compositional tuning, covering emission wavelengths from 400-710 nm. It is noted that the exciton Bohr radius for CsPbBr3 materials is calculated as 3.5 nm.
Perovskite nanoplatelets (NPLs) consist of few-layer perovskite lattices and these NPLs been shown to exhibit strong quantum confinement effect, with a significant blue-shift in emission wavelength up to 0.6 eV (~100 nm) relative to bulk perovskite. 2D structures can be represented by their own molecular compositions, e.g., single layer Cs2PbBr4 and double layer CsPb2Br5. The 2D confined structure brings increased exciton binding energy, and it allows emission tuning by thickness control. Charge transfer between stacked NPLs is enhanced due to large contact area. NPLs are proposed to be more stable than NCs against environmental moisture due to large ligand coverage[47]. However, NPLs tends to be less stable in solution as they agglomerate easily, having its unique properties compromised.
Blue emission is a challenging topic for LED lighting and display industry as it suffers from many problems, such as shorter lifetime, lower luminescence quantum yield, and lower efficiency compared with pure green and red LEDs. Blue LED is an essential composition of lighting and display applications as the three primary colour compose white light in the most versatile way.

Our vision is to take graphene from a state of raw potential to a point where it can revolutionise flexible, wearable and transparent (opto)electronics, with a manifold return in innovation and exploitation. Such change in the paradigm of device manufacturing may revolutionise the global industry. The importance of graphene was recognised by the 2011 statement of the Chancellor of the Exchequer launching the initiative that lead to the funding of the Cambridge Graphene Centre, where the proposed Graphene Technology CDT will be based. The aim is take graphene and related materials from "the British laboratory" to the "British factory floor". Not only does our vision align with this mandate, but it also exploits and strengthens several key areas of national importance where the UK has recognised excellence, such as printed electronics, energy and RF & Microwave Communications. Thus, we will strive for both economic impact, by stimulating new UK-manufactured high-value products, and societal benefits, by utilising graphene in potentially many areas including security, energy efficiency and quality of life.
The beneficiaries of our proposal will be of course the cohorts of students that will be trained every year, but will extend more widely. Considering the private sector, we have already indentified tens of companies that will benefit from our work. To achieve the final goal of graphene-technology, and to ease the transition to commercialisation, we have strong alignment with industry needs and engage them as project partners of the CDT: Dyson, Novalia, Plastic Logic, Nokia, Toshiba, BAE Systems, Aixtron, PEL, Nanocyl, IdTechEx, Philips, Dupont, CambridgeIP, Polyfect, Agilent, Nippon Kayaku, Victrex, IMEC. Many more are also partnering with the Cambridge Graphene Centre, and even more are expected to join and benefit directly or indirectly from our work. We consider the civilian sectors of healthcare, telecommunications, energy and homeland security to be those in which applications based on graphene can make significant impact on society at large. There are also applications in defence, especially in secure communications and radars. This will foster competitiveness and enhance quality of life. In particular, the proposed CDT will be of prime interest to industries dealing with the following devices and applications: 1. Mobile communications, wireless sensor networks, including wearable devices. 2. Nano-structured materials for light and microwave energy harvesting. 3. Active and reconfigurable microwave, terahertz and optical materials, including advanced antenna applications for radar and communications.
Policy-makers, within international, national, local government will also benefit. If the vision of graphene as the material of the 21st century is fulfilled, there will be a need for its properties, benefits, applications and advantageousness compared to current technology to be known by the relevant public bodies. For example, any new policy on energy saving, or mobile communications may need to include a reference to the benefits, or limitations, of graphene-based devices.
Economic resilience and innovation require post-doctoral researchers and students trained in new areas. We will contribute to increasing the talent pool for the future graphene industry. The proposed doctoral training centre will provide unique training to students in various aspects of graphene technology: from graphene nanotechnology to energy, RF/microwave and (opto)electronics. This will develop many skilled researchers over the project lifetime, who will stimulate the sustainability of UK graphene engineering research and future commercialisation opportunities across a variety of sectors.