Inas Quantum Dots Exhibit Stable N-Doping and Intraband Absorption in the 3-5 Micron Range

Colloidal quantum dots based on materials like indium arsenide hold considerable promise for infrared detection and emission, but realising their full potential in the mid-infrared spectrum requires stable electrical doping. Shraman Kumar Saha, Philippe Guyot-Sionnest, and colleagues at the University of Chicago now demonstrate stable n-doping in indium arsenide quantum dots, opening up new avenues for mid-infrared technologies. The team achieves this by carefully controlling the electronic properties of the dots, revealing state-resolved electron behaviour and observing intraband absorption in the 3 to 8 micron range. Notably, films incorporating indium phosphide require less electrical stimulus for doping, resulting in stable performance and sustained light emission at 5 microns, making these materials increasingly attractive for practical mid-infrared applications.

The mid-infrared region presents significant challenges for semiconductor applications unless stable n-doping can be achieved to access intraband transitions. This research investigates mid-infrared intraband transitions in indium arsenide (InAs), InAs/indium phosphide (InAs/InP), and InAs/zinc selenide (InAs/ZnSe) colloidal quantum dots, materials possessing an energy gap at 1. 4μm. Electrochemical methods reveal state-resolved mobility and electron filling within the quantum dot films, alongside observation of intraband absorption in the 3-8μm range.

Colloidal Dot Synthesis and Characterisation for N-Doping

Scientists engineered a novel approach to achieving stable n-doping in III-V colloidal dots, specifically focusing on InAs, InAs/InP, and InAs/ZnSe materials with an energy gap at 1. 4 micron. The study pioneered a synthesis procedure, modified from previously reported methods, to create high-quality core and core/shell quantum dots, carefully controlling reaction conditions and employing oxygen-free techniques to ensure consistent dot quality. The team synthesized InAs cores, followed by the growth of InP and ZnSe shells, meticulously documenting each step to optimize shell thickness and uniformity.

To characterize the resulting structures, researchers employed a combination of spectroscopic and crystallographic techniques. Absorption spectra revealed minimal impact from the thin shells on the electronic states of the InAs cores, confirming successful shell growth without significant alloying. Powdered-XRD analysis confirmed the zinc blend crystal structure of both cores and core/shell dots, and Debye-Scherrer analysis indicated shell thicknesses of approximately 1. 8 monolayers of InP and 2 monolayers of ZnSe. Transmission electron microscopy (TEM) provided direct visualization of the dot shapes and sizes, revealing core diameters of 7.

5 ±0. 8nm for InAs, 8. 4 ±1nm for InAs/InP, and 9. 5 ±1nm for InAs/ZnSe, consistent with the XRD findings. The team further investigated the electronic properties of the dots using electrochemical gating, enabling precise control and measurement of charge injection into the dots and resolving state-filling features. This electrochemical approach allowed the researchers to directly correlate electronic transport with spectroscopic measurements, overcoming limitations of previous solid-state field-effect transistor methods. The technique demonstrated stable n-doping of the 1Se state in InAs/InP dots, resulting in steady-state intraband absorption in the 3-5 micron range and luminescence at 5 micron.

Stable N-Doping of InAs Quantum Dots

This work details a breakthrough in the development of mid-infrared technologies using colloidal quantum dots, specifically indium arsenide (InAs) cores with either indium phosphide (InP) or zinc selenide (ZnSe) shells. Researchers successfully demonstrated stable n-doping of these quantum dots, a crucial step towards utilizing their intraband transitions for mid-infrared applications. The team synthesized InAs cores and core/shell structures, carefully controlling shell growth to maintain crystalline quality and prevent unwanted alloying. Powdered-XRD analysis confirmed the zinc blend structure of both cores and core/shells, with shell growth resulting in a measurable size increase of approximately 1.

8 monolayers of InP and 2 monolayers of ZnSe. Optical absorption spectra revealed minimal impact from the thin shells on the electronic states of the InAs cores, indicating effective confinement of electrons. Notably, the InAs/InP core/shell dots exhibited two distinct absorption shoulders, indicative of intraband transitions, with peak fitting analysis confirming an energy difference corresponding to a desired mid-infrared wavelength of 5 microns. Transmission electron microscopy (TEM) imaging showed that the InAs cores have an average size of 7. 5 ±0.

8nm, increasing to 8. 4 ±1nm for the InAs/InP core/shell and 9. 5 ±1nm for the InAs/ZnSe structures. A key achievement of this study was the demonstration of electrochemical state filling in the quantum dots, allowing researchers to inject electrons into the dots and observe stable n-doping.

Stable N-Doping of InAs Quantum Dots

Researchers have successfully demonstrated electrochemical n-doping of InAs core-based colloidal quantum dots, opening new possibilities for mid-infrared applications. Through careful electrochemical study of thin films, the team observed state-resolved electron mobility and filling, alongside intraband absorption in the 3-8 micron range. Importantly, they found that modifying the quantum dot surface with thin shells of either InP or ZnSe significantly alters the doping characteristics, confirming the crucial role of surface chemistry in controlling electrochemical behaviour. Films incorporating an InP shell exhibited particularly promising results, allowing for stable n-doping even in ambient air, with steady-state absorption and photoluminescence observed around 5 microns.

This stable doping, combined with the low toxicity and high thermal stability of InAs, positions these materials as strong candidates for mid-infrared detection and emission technologies. While the observed intraband luminescence signal is currently lower than that of other mid-infrared materials, the researchers suggest that larger quantum dots with lower reduction potentials could further enhance performance. The team acknowledges that the current signal strength is limited, potentially due to only a fraction of the dots being effectively doped. Future work will likely focus on optimizing dot size and surface passivation to maximize doping efficiency and enhance the observed optical signals. This research expands the potential of InAs quantum dots, enabling their exploration for applications beyond their traditional near-infrared capabilities and offering a potentially advantageous alternative to currently used chalcogenide materials.