An example of why it is so important to correlate the structural and optical properties of semiconductor nanorods directly

Gatan instrument used

The Vulcan™ system, from Gatan, is the first commercial cathodoluminescence system for the (scanning) transmission electron microscope (STEM) that enables direct correlation of optical and structural properties of materials at the nanoscale.

Background

The size, shape, and structure of nanoparticles (NPs) is known to modify their functional properties such as their electrical or optical characteristics. The development of commercial devices built from NPs depends critically on the ability of scientists and engineers to understand the relationship between structure and function. To this end, electron microscopy and optical emission spectroscopies, e.g., photoluminescence (PL) are commonly employed; electron microscopy being used to determine the size, shape, composition, chemistry, and presence of crystal defects of individual NPs at or near atomic resolution and PL used to determine band gap, electronic structure, and electric field. This approach has demonstrated its usefulness. However, the PL technique typically measures the average properties of many hundreds of NPs simultaneously, and there is a growing realization that the very different sample sizes used in the two techniques may lead to unsound conclusions.

Cathodoluminesce (CL) – luminescence initiated by electron excitation – is a particularly valuable characterization technique for NPs bringing the power of optical emission spectroscopy to the electron microscope enabling direct correlation of structure and function. CL also benefits from the ability to focus the electron beam to sub-nanometer dimensions enabling excitation of individual NPs and furthermore, the excitation of specific sites within an NP such as a quantum dot, disk or well embedded within a semiconductor NP.

Nitride semiconductor nanorods (NRs) are highly attractive for applications including high-efficiency light emission and hydrogen harvesting from the water. However, for commercial devices manufacturing costs need reducing, but when doing so, this compromises in the NR structural quality. The literature includes many reports of PL and electron microscopy studies of similar samples though critically, no direct correlation. Here we demonstrate that direct correlation using cathodoluminescence measured in the transmission electron microscope (TEM) enables researchers to draw unambiguous, and avoid incorrect, conclusions.

Materials and Methods

Previous studies revealed anomalous blue light emission from GaN NRs containing nominally 8, 3 nm thick In₁₄Ga₈₆N quantum disk (Q_disk) inserts grown by molecular beam epitaxy (500 – 600 °C). At these low growth temperatures ~60% of the NRs exhibit ideal structures containing “perfect” InGaN Q_disks (Figure 1c), however, the remainder are of low-quality with highly defective structures (Figure 1d). Analysis by PL revealed strong green and blue emission bands (550 and 460 nm, FWHMs 65 nm respectively). The anomalous blue emission was concluded to be due to composition fluctuation within the InGaN Q_disks of the high-quality NRs; the low-quality NRs were thought to be too structurally poor to emit light efficiently.

The integral spectrum recorded by CL from hundreds of NRs replicated the spectrum observed by PL in previous work. However, an investigation by high spatial resolution CL demonstrated that the blue emission band originated from the low-quality NRs only. Moreover, quantum dot (Q_dot)-like emission centers emitting with a range of wavelengths (380 - 480 nm, FWHM 4 – 10 nm) were observed (Figure 1e) and could be correlated directly with small (<10 nm diameter) In₁₄Ga₈₆N nanoclusters formed unintentionally within the Q_disk region. This discovery could be an important step in producing solid-state quantum emitters for encryption and demonstrates the importance of correlating structural and functional properties in nano-objects. 

Summary

Through the use of the Vulcan CL system, the mechanism of anomalous blue emission from nitride semiconductor NRs was revealed to be due to the unintentional fabrication of solid-state quantum dots in highly defective NRs. This is in contradiction to previous studies which had drawn erroneous conclusions and demonstrated the need to correlate structural and functional properties directly.

Credit(s)

A special thank you to A. Prabaswara and Prof. Boon Ooi, King Abdullah University of Science and Technology (KAUST) and the Experimental Technique Centre, Brunel University. For further information, see Prabaswara et al. J. of Nanophotonics, 11(2), 026015 (2017).

Gatan, Inc. is the world’s leading manufacturer of instrumentation and software used to enhance and extend electron microscopes—from specimen preparation and manipulation to imaging and analysis.

The Photonics Laboratory, KAUST aims to deliver compact, and energy saving integrated laser-diode based devices and solutions for applications requiring light spanning the ultraviolet to the visible and near-infrared regime. A plethora of practical laser-based solutions are conceived, and proof-of-concept models built for data collection and testing of a new class of innovative multi-function laser devices.