Progress and Challenges of Bandgap Engineering in One-Dimensional Semiconductor Materials

Since their emergence, low-dimensional nanostructures, particularly semiconductor nanowires and nanobelts, have attracted significant attention due to their unique properties and potential applications in future integrated optoelectronic devices and circuits. With the continuous development of semiconductor technology, the need for miniaturization, high integration, and low energy consumption in devices has led to higher requirements for nanomaterials [1]. Semiconductor material-based integration and the diversification of devices pose major challenges. Thus, the ability to modulate the bandgap along a single nanostructure greatly enhances functionality in optoelectronics. Therefore, studies in this area are essential in order to pave the way for highly integrated future devices.

On the other hand, low-dimensional nanostructures have been reported for use in various optoelectronic devices, including nanoscale lasers, LED, solar cells, waveguides, sensors, photodetectors, spectrometers, and solid-state light sources [2]. However, there are many challenges in their practical application that restrict further development and large-scale application. This Special Issue highlights key areas where significant challenges remain, for example, the structural evaluation and stability of organic semiconductors [3], the doping and stability of inorganic perovskites [4], the external quantum efficiency of light-emitting diodes [5], morphological engineering and semiconductor coupling in photocatalysis [6], and the effects of ions doping in semiconductors [7]. Currently, considering the shortage of available materials and the performance improvement bottleneck, contradictions are particularly prominent [8]. For example, the exponential growth of data in the information age demands enhanced security and privacy protection for image transmission. Photodetectors that convert light signals into electrical signals play a pivotal role in multi-color imaging, optical communications, and logical operations [9]. Consequently, developing high-performance multifunctional photodetectors is essential in order to meet the future demands of information technology. Although relevant research has been reported, achieving multifunctionality and high safety in devices remains a major challenge, highlighting the importance of manufacturing new types of equipment [10]. Interestingly, the gradient structure, with its funnel-shaped band structure, enhances the transport and transmission of carriers, providing a foundation for the development of multifunctional platform optoelectronic devices [11].

So far, significant research efforts have been devoted by scientists around the world to achieving bandgap engineering along low-dimensional semiconductor structures because of their ideal properties and potential applications in photoelectronic circuits [12]. For example, Ga_xIn_1−xAs-InAs heterostructure arrays were fabricated via the molecular beam epitaxy (MBE) approach through hetero-interface diffusion [13]. Si-Ge nanowires with abrupt hetero-interfaces [14] and Si-SiGe nanowire superlattices [15] were developed via the chemical vapor deposition (CVD) approach. GaP-GaAs superlattice nanostructures were prepared using the laser-assisted CVD method, indicating atomic abrupt interfaces along the junctions [16]. InGaN-GaN quantum-well nanowires were synthesized via the metal–organic CVD (MOCVD) approach [17], demonstrating a room-temperature nanoscale laser application. CdS-SnS₂ microwire superlattices were synthesized via the CVD approach and showed weak optical nonlinearities and slow light engineering [18]. CsPbX₃ (X = Cl, Br, I) nanowires were synthesized via an anion exchange process, which showed a white (R-G-B) emission along a single nanowire [19]. In addition, CsPbCl₃-CsPbI₃ heterostructure nanowires were fabricated through a CVD approach and could act as dual-wavelength lasers [20]. Nevertheless, bandgap engineering in a single nanowire has always posed a significant challenge for the scientific community.

Scientists are exploring preparation strategies for nanomaterials and are expected to continuously improve on characteristic methods such as MBE, CVD, MOCVD, MOVPE, laser-assisted CVD, and the ion exchange method. CVD is a useful and reliable technique for synthesizing various one-dimensional nanomaterials with high crystal quality due to its advantages of high controllability, flexible scalability, high yields, and low cost [21]. However, achieving multifunctionality and strong security within a single device is a significant challenge. Thus, achieving compositional modulation along the nanostructure represents a useful strategy in the development of multifunctional devices and circuits.

Based on this consideration, in 2011, the magnetic-pulling, source-moving CVD approach was developed to synthesize numerous compositional gradient nanostructures, for example, CdS_1−xSe_x nanowires [22], CdS/CdS_1−xSe_x heterostructure nanoribbons [9], and ZnCdSSe bandgap gradient nanoribbons [23]. They offer several advantages compared with composition homogenous structures. Firstly, the magnetic-pulling, source-moving CVD method represents an improvement on the traditional one. This method can fabricate bandgap gradient nanowires (e.g., CdS_1−xSe_x nanowires and ZnCdSSe nanowires), which exhibit continuously tunable band gaps ranging from blue to green and then to red [24]. Secondly, photodetectors based on these bandgap gradient nanowires exhibit superior photoelectric performance, such as high responsivity, high detectivity, and fast response speed in the millisecond magnitude, which are superior to traditional devices [25,26]. Thirdly, the synthesis of high-quality on-wire compositional gradient nanostructures with high-performance photoconductance provides an ideal material platform for investigating gradient structural characteristics more clearly and promoting the development of all-optical logic gates, solid-state white light sources, and encrypted communications.

We therefore believe that on-nanowire bandgap modulation has many potential applications in the field of nanoscience and nanotechnology and will make a valuable contribution to the nanoworld in the future.