Advances in low dimensional and 2D materials Free

Graphene analogs of 2D materials has progressed rapidly across the scientific and engineering fields after the isolation of single-layer graphene.¹In this special issue, we compile a series of articles on emerging low-dimensional and 2D materials synthesis, properties, and their applications for sensors, field effect transistors, photo detectors, solar cells, photodiodes, supercapacitors, spin-wave devices, p–n junctions, electrocatalyst for hydrogen evolution reaction, etc. These research articles present an update of the ongoing research in these fields and comprise the strategy for the development of novel low-dimensional and 2D materials and technological devices. The issue is also intended to focus on current challenges for industrial scale synthesis of 2D materials, how materials, physics, and measurements can help; and what new technologies and applications are in prospect.² 

Zhu et al.³ have studied the electronic and transport properties of V-doped zigzag phosphorene nanoribbons (V-ZPNRs) by different edge passivations using a first-principles study based on density functional theory combined with nonequilibrium Green’s function. The devices show a variety of important properties of spintronics, such as negative differential resistance (NDR) behaviors and dual spin filter effects, which is of great significance for the development of spintronic devices based on phosphorene nanoribbons. Li and Matsumoto⁴ reported a computational method to report the electron transport properties of hydrogenated amorphous silicon (a-Si:H). The main outcome of the research was to investigate how the defects, the hydrogen passivation, and the a-Si:H/electrode interface affect the electron transport properties of the material.

Lin et al.⁵ successfully fabricated the yttrium barium copper oxide (YBCO)/Slater-type orbitals SrTiO₃, STO nano-superconducting quantum interference devices (SQUIDs) on a bicrystal STO substrate with a 24° misorientation angle. Based on the electronic measurements with a lot of time spent on this particular work, the authors confirm that the long-time stability and high-performance YBCO nanoSQUIDs can be realized in this YBCO/STO periodic structure, which provides insights into various other nanoelectronic devices. Furthermore, Matsunaga et al.⁶ developed HATCN as a valuable p-dopant for CNT–thin-film transistors (TFTs) that exhibit excellent temperature tolerance. The devices have been readily doped by straight forward spin-coating with hexaazatriphenylenehexacarbonitrile (HATCN) solution. The HATCN-doped carbon nanotubes (CNT)-TFTs retained their p-type character even after 72 h of high-temperature treatment at 200 °C, demonstrating remarkable temperature tolerance when compared to conventional p-dopants, such as oxygen and F4-tetracyanoquinodimethane (TCNQ). Li et al.⁷ reported work on the hydrothermal synthesis of V₂O₅ nanosheets, hoping to improve the electrochemical properties of V₂O₅ powders by adjusting the structure. The V₂O₅ nanosheet morphology was controlled by adjusting the type of the solvent. The size of the V₂O₅ nanosheet was found to be uniform. Due to its good cycling performance, V₂O₅ nanosheet has great application prospects in supercapacitors.

Novák et al.⁸ have developed GaP nanocone (NCs) that were grown at 650 °C, had a large surface area, and were confined to complex crystallographic facets and edges. The quality of the interface between GaP and MoS₂ formed by sulfuration led to the formation of an active PN heterojunction. Importantly, a p-GaP/n-MoS₂ heterostructure was prepared on planar and nanocone-structured GaP substrates for the first time. The heterostructure was characterized by electrical and optical measurements. The GaP/MoS₂ heterostructure on planar and nanocone-structured substrates exhibited turn-on voltages of 1.3 and 0.7 V at a forward current of 1 μA, respectively, and rectifying ratios measured at ±1 V of about 11.4 and 33, respectively. The GaP/MoS₂ heterostructures on the planar and nanocone-structured substrates exhibited short circuit currents of 0.14 and 0.36 μA at an open circuit voltage of ∼0.25 V, respectively. It was reported that edges played a similarly active role in the growth of MoS₂ on graphene, WSe₂, and silicon.

Dang et al.⁹ studied the first-principles calculations of electronic and optical properties of small edge-functionalized penta-graphene quantum dots. Their studies reveal that larger penta-graphene quantum dots (PGQDs) are thermodynamically more stable than smaller ones. Raza et al.¹⁰ studied the quantized, stretched, twisted, and twigged electron (subquanta due to quantization of events at 0.1 ≤ nf ≤ 0.9) in the momentum space and gained energy due to harmonic and time-dependent adiabatic perturbations. As a result, Dirac bosons and Dirac fermions are produced. A new configuration of a quantum state for 0.1 ≤ nf ≤ 0.9 from a single electron, in the momentum space, is theoretically suggested, which follows the Kitaev model¹¹ and Majorana fermions.^11,12

Gudi et al.¹³ have optimized the pulsed laser deposition growth conditions for high crystal and electrical quality MoS₂ thin films. The optimal growth temperature for MoS₂ thin films is 800 °C with a 30 min post-growth annealing at 2.2 J/cm² fluence, 5 cm substrate target distance, and 0.5 mTorr Ar partial pressure. For 3 nm (four monolayer) thick MoS₂ films, an rms roughness of 0.17 nm was obtained. In addition, thin film conductivity ∼4000 S/m was achieved, which is dependent on the growth conditions. In addition, the optical transmission and absorption studies demonstrated the characteristic excitonic peaks for MoS₂. The high quality monolayer growth control for MoS₂ is independent of the type of substrate. This paves the path for high-quality 2D semiconductor devices over a large area.

Waghadkar et al.¹⁴ have reported the synthesis of ZnO microspheres grown from heat treatment of ZnS microspheres at different oxidation temperatures. The crystalline and electronic structures were confirmed by physicochemical characterization techniques as well as density functional theory (DFT) studies. Synthesized ZnO microspheres were used as photoanodes to fabricate the dye-sensitized solar cells (DSSCs), and their photovoltaic parameters [J–V, Incident Photon to Current Conversion Efficiency (IPCE)] were measured. The enhanced photovoltaic parameters were obtained for GHZ62 with a photocurrent value of 9.63 mA/cm² and efficiency (η) of 3.38%. This enhanced performance is the result of high surface roughness and porosity, which assisted in improving the efficiency, dye loading, and better current conduction pathway.

Figueroa et al.¹⁵ have reported interesting work on charge transport in graphene in the temperature range 300 < T < 350 K far from the Dirac point (DP) where the effect of electron–hole “puddling” was negligible, and diffusive transport mechanisms predominated. At and near the DP (ΔV ∼ 0 V), σ(T) was non-monotonic. The authors have performed the analysis of the temperature-dependent conductivity at moderate and strong gating. It was observed that at moderate gate voltages (ΔV ∼ −10 V), the conductivity increased monotonically as temperature increased. However, at strong gating (ΔV_g ∼ −40 V), it showed a non-monotonic temperature dependency. In view of the non-monotonicity in σ(T) far from the DP observed in this work, further authors have estimated and compared contributions to the charge transport from different diffusive transport mechanisms. The non-monotonic temperature dependencies of σ observed far away from the DP are manifestations of the scattering of the acoustic phonons, whose part in relation to the scattering of charged impurities becomes enhanced at strong gating. The reported device structure with a top gate polarizable ferroelectric (FE) provided a novel approach to investigate charge transport in graphene via controlled compensation of impurity charges, and in the process revealed non-monotonic behavior in σ(T) not previously seen in SiO₂ back-gated devices.

Ray et al.¹⁶ published their findings on a magnetic anatase-based defective reduced-graphene oxide (r-GO)/TiO₂ composite thin film. The various oxygen functional groups at the interface of r-GO/TiO₂ play a key role in tailoring the magnetic behavior of the r-GO/TiO₂ composite thin film. The composite materials are also suggested for other technological applications, such as photocatalyst activity under sunlight irradiation.

Aleithan et al.¹⁷ used salt to grow the mono-layered or multilayered MoS₂. The salt-assisted CVD method has recently emerged as an effective method for advancing the progression of 2D materials.^18–20 The use of alkali salts, such as NaCl and KCl, influenced the growth of 2D materials that are hard to grow due to their high melting points and low vapor pressure. The reported salt enhances the growth size of films on different substrates via an immediate and direct approach. The authors predicted that the method could be further tested and improved for additional growth applications and purposes related to size, substrate, quality, or doping.

Zhang et al.²¹ have demonstrated a type of phase shifter based on voltage-controlled magnetic domain walls (DWs) for spin-wave (SW) propagation and phase manipulations. A double-layer magnetic film structure is proposed to effectively maintain the state of the waveguide without an additional magnetic field. This study could be a useful step toward all-voltage-controlled SW devices with ultralow power consumption. Bhattacharya et al.²² conducted important research on defect passivation using achiral cysteamine molecules, which induce n-doping but cannot remove in-gap states caused by the Si/SiO₂ substrate. The reported studies can be further utilized for room temperature valley information-based LEDs and photonic logical devices. Guo et al.²³ demonstrated a new strategy for the simple and effective in situ growth of a graphene lateral p–n junction on a SiC substrate. The authors fabricated a high-performance, UV-enhanced, self-powered photodetector based on a graphene p–n junction. Significantly, the scheme demonstrated in the work reported in this paper is completely compatible with modern semiconductor device synthesis procedures and will allow the fabrication of other graphene-based p–n junction devices with novel characteristics.

Zhang et al.²⁴ reported work on a field effect transistor with a special four source/drain contact geometry method to determine the sheet resistance of the 2D conducting channel independent of the contact resistance. When combined with the device geometry, the resistivity of the WS₂ channel is determined without possible errors due to contact resistance. This sheet resistivity can be used with the geometry of the full channel to determine the total channel resistance, which is then subtracted from the total two-point resistance (measured when the current is forced between the large source and drain contacts and the voltage is measured with the same contacts) to determine the contact resistance of the two contacts. Shinde et al.²⁵ reported work on an efficient hydrogen evaluation reaction (HER) electrocatalyst, VS₂/ZnS/CdS hybrid structure, synthesized via a facile hydrothermal method. The VS₂/ZnS/CdS hybrid exhibits superior HER activity, including high stability, low overpotential of 86 mV, and a smaller Tafel slope of 74.4 mV/dec under acidic conditions. The work demonstrates that transition metal-based sulfides and their hybrids emerge as promising electrocatalysts for future high-performance energy conversion devices. Bakaul et al.²⁶ have described the possible artifacts and precautions that need to be carefully considered to evaluate the ferroelectric (FE) properties of free-standing complex oxides. The free-standing nature of these materials uniquely affects local electromechanical properties due to structural ripples, ultra flexibility, and weak bonding between the substrate and the film.

Choudhari and Jagtap²⁷ reported the growth of pristine and metal-doped SnO₂ nanocomposites using a hydrothermal method and applied them as sensing materials for the determination of lung cancer biomarkers. The findings of this work suggest that doped SnO₂ could be employed as a promising sensing material for the identification of different volatile organic compounds in an effort to diagnose lung cancer quickly and accurately. Wang et al.²⁸ reported square honeycombs, including an ordinary square honeycomb, a gradient square honeycomb, and a buffer square honeycomb, which were designed to enhance the energy absorption performance of the traditional square honeycomb. In-plane dynamic crushing experiments were numerically simulated to examine the differences between the novel gradient square honeycombs and the traditional square honeycomb by using the nonlinear finite element software ABAQUS/Explicit.

Overall, this special issue discusses recent advances in low-dimensional and 2D materials, including their synthesis, properties, and performance for use in sensors, field effect transistors, photodetectors, solar cells, photodiodes, supercapacitors, spin-wave devices, p–n junctions, electrocatalysts for hydrogen evolution reactions, and so on. The authors have demonstrated a lot of new strategies for the development of these materials for emerging devices and technologies. We hope the research papers in this particular issue will provide several new directions for synthesis, properties, and technological application investigations for various new low-dimensional and 2D materials research.

We would like to thank the editorial staff at AIP Advances who supported us all behind the scenes, especially Dr. Diana Schlamadinger and Dr. Jessica Trudeau from the AIP office, for all their help and support. This collection of papers could not have been put together without their hard work and dedication.