# An Overview of Emerging Nuclear Sensor Technologies: Challenges, Advancements and Applications
## 2 Emerging Nuclear Sensor Technology
### Quantum Dot-Based Nuclear Sensors  
Quantum dots (QDs) are semiconductor nanocrystals with typical diameters between 2 and 10 nm that confine charge carriers in all three spatial dimensions. This quantum confinement produces discrete energy levels, in contrast to the continuous bands observed in bulk materials, and enables precise tuning of the optical and electronic properties [28]. For example, a decrease in the QD diameter increases the band gap, resulting in a shift of the photoluminescence emission towards the blue end of the spectrum, while larger dots emit at longer (red) wavelengths [29; 30]. This tunability is critical for radiation detection, as it allows QDs to be engineered to emit at wavelengths that correspond to specific energy signatures of ionizing radiation [31].  
When QDs are exposed to high-energy particles (e.g., gamma rays, X-rays, or alpha particles), excitons are generated as electrons are promoted to higher energy states [29]. The radiative recombination of these electron-hole pairs yields photons with energies characteristic of the QD's band gap [30]. This can be seen in the photoluminescence spectra of various QDs in Figure 2. The resulting narrow emission spectra, with full widths at half maximum typically in the 20-30 nm range, enable fine discrimination between different radiation energies [32]. This high energy resolution is very important for identifying and quantifying specific radioactive isotopes in applications such as reactor monitoring and waste management [31; 33].  
The synthesis of quantum dots is achieved primarily by colloidal methods [35] and, to a lesser extent, by molecular beam epitaxy (MBE) [36]. Colloidal synthesis involves the thermal decomposition of organometallic precursors in the presence of surfactants, which control nucleation and growth to yield QDs with relatively narrow size distributions [35]. Although this method is scalable and cost-effective, variations in particle size cause non-uniform optical responses [37]. MBE, while offering improved control over QD size and composition, is more complex and expensive, making it less amenable to large-scale production [37; 38]. Advances in synthesis protocols aim to improve yield and uniformity while reducing production costs.  
In sensor applications, QDs are typically integrated into hybrid device architectures. For instance, QDs may be embedded within a polymer matrix [39] or coupled with conventional semiconductor detectors [40]. In these devices, the QD ensemble absorbs ionizing radiation, and the subsequent emission is collected by a photodetector system. The intensity and spectral characteristics of the emitted light are then correlated with the radiation dose and energy spectrum [32]. This approach allows for the development of compact, high-density sensor arrays capable of spatially resolved radiation mapping--a feature that is particularly beneficial for both fixed installations and portable, wearable radiation monitors [28; 41; 42].  
While QDs generally maintain their structural and optical properties under moderate radiation doses, extended exposure can lead to degradation of the surface passivation layer, resulting in photobleaching and reduced sensitivity [43; 44]. To address these difficulties, advancements are being made towards enhancing the radiation hardness of QDs via improved surface chemistry, such as robust encapsulation techniques and the use of protective coatings that mitigate oxidation and other degradation pathways [45; 46; 47].  
Figure 2: UV illumination of In(Zn)As/ZnSe/ZnS QDs. From left to right, the photoluminescence (PL) and PL quantum efficiency of the QDs are as follows: 60 nm and 58% for 538 nm QD, 63 nm and 66% for 570 nm QD, 62 nm and 50% for 589 nm QD, 71 nm and 37% for 615 nm QD, and 83 nm and 15% for 635 nm QD. Reprinted (adapted) with permission from [34]. Copyright 2021 American Chemical Society.