Utilizing a NaOH Promoter to Achieve Large Single-Domain Monolayer WS₂ Films via Modified Chemical Vapor Deposition

Because of their fascinating properties, two-dimensional (2D) nanomaterials have attracted a lot of attention for developing next-generation electronics and optoelectronics. However, there is still a lack of cost-effective, highly reproducible, and controllable synthesis methods for developing high-quality semiconducting 2D monolayers with a sufficiently large single-domain size. Here, utilizing a NaOH promoter and W foils as the W source, we have successfully achieved the fabrication of ultralarge single-domain monolayer WS2 films via a modified chemical vapor deposition method. With the proper introduction of a NaOH promoter, the single-domain size of monolayer WS2 can be increased to 550 μm, while the WS2 flakes can be well controlled by simply varying the growth duration and oxygen concentration in the carrier gas. Importantly, when they are fabricated into global backgated transistors, WS2 devices exhibit respectable peak electron mobility up to 1.21 cm2 V–1 s–1, which is comparable to those of many state-of-the-art WS2 transistors. Photodetectors based on these single-domain WS2 monolayers give an impressive photodetection performance with a maximum responsivity of 3.2 mA W–1. All these findings do not only provide a cost-effective platform for the synthesis of high-quality large single-domain 2D nanomaterials, but also facilitate their excellent intrinsic material properties for the next-generation electronic and optoelectronic devices

Mechanistic Characteristics of Metal-Assisted Chemical Etching in GaAs

Because of the unique physical properties, various GaAs micro- and nanostructures have attracted increasing research attention for many technical applications such as solar cells, light-emitting diodes, and field-effect transistors. In this regard, numerous fabrication techniques have been explored, and among all, metal-assisted chemical etching is successfully applied to GaAs in order to achieve cost-effective, large-scale, and complex structures. However, the detailed explanations as well as the corresponding etching mechanism have not been reported until now or simply relied on the hole injection model of Si in order to explain the phenomenon. In this work, we perform a more systematic study to further explore and assess the etching phenomenon of GaAs employing the Au catalyst and the [KMnO₄/H₂SO₄] etch system. It is revealed that the anisotropic etching behavior of GaAs is predominantly due to the Au-induced surface defects at the Au/GaAs interface, which makes the particular area more prone to oxidation and thus results in the simple directional wet etching; for that reason, more anisotropic etch is obtained for the Au pattern with higher edge-to-surface-area ratio. All these findings not only offer additional insight into the MacEtch process of GaAs but also provide essential information on different etching parameters in manipulating this anisotropic wet etching to achieve the fabrication of complex GaAs structures for technological applications

Synthesis and Characterizations of Ternary InGaAs Nanowires by a Two-Step Growth Method for High-Performance Electronic Devices

InAs nanowires have been extensively studied for high-speed and high-frequency electronics due to the low effective electron mass and corresponding high carrier mobility. However, further applications still suffer from the significant leakage current in InAs nanowire devices arising from the small electronic band gap. Here, we demonstrate the successful synthesis of ternary InGaAs nanowires in order to tackle this leakage issue utilizing the larger band gap material but at the same time not sacrificing the high electron mobility. In this work, we adapt a two-step growth method on amorphous SiO₂/Si substrates which significantly reduces the kinked morphology and surface coating along the nanowires. The grown nanowires exhibit excellent crystallinity and uniform stoichiometric composition along the entire length of the nanowires. More importantly, the electrical properties of those nanowires are found to be remarkably impressive with <i>I</i>_ON/<i>I</i>_OFF ratio >10⁵, field-effect mobility of ∼2700 cm²/(V·s), and ON current density of ∼0.9 mA/μm. These nanowires are then employed in the contact printing and achieve large-scale assembly of nanowire parallel arrays which further illustrate the potential for utilizing these high-performance nanowires on substrates for the fabrication of future integrated circuits

High-Performance Transparent Ultraviolet Photodetectors Based on InGaZnO Superlattice Nanowire Arrays

Due to the efficient photocarrier separation and collection coming from their distinctive band structures, superlattice nanowires (NWs) have great potential as active materials for high-performance optoelectronic devices. In this work, InGaZnO NWs with superlattice structure and controllable stoichiometry are obtained by ambient-pressure chemical vapor deposition. Along the NW axial direction, perfect alternately stacking of InGaO­(ZnO)4+ blocks and InO2– layers is observed to form a periodic layered structure. Strikingly, when configured into individual NW photodetectors, the Ga concentration is found to significantly influence the amount of oxygen vacancies and oxygen molecules adsorbed on the NW surface, which dictate the photoconducting properties of the NW channels. Based on the optimized Ga concentration (i.e., In1.8Ga1.8Zn2.4O7), the individual NW device exhibits an excellent responsivity of 1.95 × 105 A/W and external quantum efficiency of as high as 9.28 × 107% together with a rise time of 0.93 s and a decay time of 0.2 s for the ultraviolet (UV) photodetection. Besides, the obtained NWs can be fabricated into large-scale parallel arrays on glass substrates as well to achieve fully transparent UV photodetectors, where the performance is on the same level or even better than many transparent photodetectors with high performance. All the results discussed above demonstrate the great potential of InGaZnO superlattice NWs for next-generation advanced optoelectronic devices

Controllable p–n Switching Behaviors of GaAs Nanowires <i>via</i> an Interface Effect

Due to the extraordinary large surface-to-volume ratio, surface effects on semiconductor nanowires have been extensively investigated in recent years for various technological applications. Here, we present a facile interface trapping approach to alter electronic transport properties of GaAs nanowires as a function of diameter utilizing the acceptor-like defect states located between the intrinsic nanowire and its amorphous native oxide shell. Using a nanowire field-effect transistor (FET) device structure, p- to n-channel switching behaviors have been achieved with increasing NW diameters. Interestingly, this oxide interface is shown to induce a space-charge layer penetrating deep into the thin nanowire to deplete all electrons, leading to inversion and thus p-type conduction as compared to the thick and intrinsically n-type GaAs NWs. More generally, all of these might also be applicable to other nanowire material systems with similar interface trapping effects; therefore, careful device design considerations are required for achieving the optimal nanowire device performances

High-Performance GaAs Nanowire Solar Cells for Flexible and Transparent Photovoltaics

Among many available photovoltaic technologies at present, gallium arsenide (GaAs) is one of the recognized leaders for performance and reliability; however, it is still a great challenge to achieve cost-effective GaAs solar cells for smart systems such as transparent and flexible photovoltaics. In this study, highly crystalline long GaAs nanowires (NWs) with minimal crystal defects are synthesized economically by chemical vapor deposition and configured into novel Schottky photovoltaic structures by simply using asymmetric Au–Al contacts. Without any doping profiles such as p–n junction and complicated coaxial junction structures, the single NW Schottky device shows a record high apparent energy conversion efficiency of 16% under air mass 1.5 global illumination by normalizing to the projection area of the NW. The corresponding photovoltaic output can be further enhanced by connecting individual cells in series and in parallel as well as by fabricating NW array solar cells via contact printing showing an overall efficiency of 1.6%. Importantly, these Schottky cells can be easily integrated on the glass and plastic substrates for transparent and flexible photovoltaics, which explicitly demonstrate the outstanding versatility and promising perspective of these GaAs NW Schottky photovoltaics for next-generation smart solar energy harvesting devices

Manipulated Growth of GaAs Nanowires: Controllable Crystal Quality and Growth Orientations via a Supersaturation-Controlled Engineering Process

Controlling the crystal quality and growth orientation of high performance III–V compound semiconductor nanowires (NWs) in a large-scale synthesis is still challenging, which could restrict the implementation of nanowires for practical applications. Here we present a facile approach to control the crystal structure, defects, orientation, growth rate and density of GaAs NWs via a supersaturation-controlled engineering process by tailoring the chemical composition and dimension of starting Au_<i>x</i>Ga_<i>y</i> catalysts. For the high Ga supersaturation (catalyst diameter < 40 nm), NWs can be manipulated to grow unidirectionally along ⟨111⟩ with the pure zinc blende phase with a high growth rate, density and minimal amount of defect concentration utilizing the low-melting-point catalytic alloys (AuGa, Au₂Ga, and Au₇Ga₃ with Ga atomic concentration > 30%), whereas for the low Ga supersaturation (catalyst diameter > 40 nm), NWs are grown inevitably with a mixed crystal orientation and high concentration of defects from high-melting-point alloys (Au₇Ga₂ with Ga atomic concentration < 30%). In addition to the complicated control of processing parameters, the ability to tune the composition of catalytic alloys by tailoring the starting Au film thickness demonstrates a versatile approach to control the crystal quality and orientation for the uniform NW growth

Rational Design of Inverted Nanopencil Arrays for Cost-Effective, Broadband, and Omnidirectional Light Harvesting

Due to the unique optical properties, three-dimensional arrays of silicon nanostructures have attracted increasing attention as the efficient photon harvesters for various technological applications. In this work, instead of dry etching, we have utilized our newly developed wet anisotropic etching to fabricate silicon nanostructured arrays with different well-controlled geometrical morphologies, ranging from nanopillars, nanorods, and inverted nanopencils to nanocones, followed by systematic investigations of their photon-capturing properties combining experiments and simulations. It is revealed that optical properties of these nanoarrays are predominantly dictated by their geometrical factors including the structural pitch, material filling ratio, and aspect ratio. Surprisingly, along with the proper geometrical design, the inverted nanopencil arrays can couple incident photons into optical modes in the pencil base efficiently in order to achieve excellent broadband and omnidirectional light-harvesting performances even with the substrate thickness down to 10 μm, which are comparable to the costly and technically difficult to achieve nanocone counterparts. Notably, the fabricated nanopencils with both 800 and 380 nm base diameters can suppress the optical reflection well below 5% over a broad wavelength of 400–1000 nm and a wide angle of incidence between 0 and 60°. All these findings not only offer additional insight into the light-trapping mechanism in these complex 3D nanophotonic structures but also provide efficient broadband and omnidirectional photon harvesters for next-generation cost-effective ultrathin nanostructured photovoltaics

GaAs Nanowires: From Manipulation of Defect Formation to Controllable Electronic Transport Properties

Reliable control in the crystal quality of synthesized III–V nanowires (NWs) is particularly important to manipulate their corresponding electronic transport properties for technological applications. In this report, a “two-step” growth process is adopted to achieve single-crystalline GaAs NWs, where an initial high-temperature nucleation process is employed to ensure the formation of high Ga supersaturated Au7Ga3 and Au2Ga alloy seeds, instead of the low Ga supersaturated Au7Ga2 seeds observed in the conventional “single-step” growth. These two-step NWs are long (>60 μm) and thick (>80 nm) with the minimal defect concentrations and uniform growth orientations. Importantly, these NWs exhibit p-type conductivity as compared to the single-step grown n-type NWs for the same diameter range. This NW conductivity difference (p- versus n-channel) is shown to originate from the donor-like crystal defects, such as As precipitates, induced by the low Ga supersaturated multicrystalline Au7Ga2 alloy seeds. Then the well-controlled crystal quality for desired electronic properties is further explored in the application of large-scale p-type GaAs NW parallel array FETs as well as the integration of both p- and n-type GaAs NWs into CMOS inverters. All these illustrate the successful control of NW crystal defects and corresponding electronic transport properties via the manipulation of Ga supersaturation in the catalytic alloy tips with different preparation methods. The understanding of this relationship between NW crystal quality and electronic transport properties is critical and preferential to the future development of nanoelectronic materials, circuit design, and fabrication