Scanning Tunneling Spectroscopy Method for the Prediction of Semiconductor Heterojunction Performance as a Prequel for Device Development

The prediction of semiconductor device performance is a persistent challenge in materials science, and the ability to anticipate useful specifications prior to construction is crucial for enhancing the overall efficiency. In this study, we investigate the constituents of a solar cell by employing scanning tunneling microscopy (STM) and spectroscopy (STS). Through our observations, we identify a spatial distribution of the dopant type in thin films of materials that were designed to present major p-doping for germanium sulfide (GeS) and dominant n-doping for tin disulfide (SnS2). By generating separate STS maps for each semiconductor film and conducting a statistical analysis of the gap and doping distribution, we determine intrinsic limitations for the solar cell efficiency that must be understood prior to processing. Subsequently, we fabricate a solar cell utilizing these materials (GeS and SnS2) via vapor phase deposition and carry out a characterization using standard J-V curves under both dark/illuminated irradiance conditions. Our devices corroborate the expected reduced efficiency due to doping fluctuation but exhibit stable photocurrent responses. As originally planned, quantum efficiency measurements reveal that the peak efficiency of our solar cell coincides with the range where the standard silicon solar cells sharply decline. Our STS method is suggested as a prequel to device development in novel material junctions or deposition processes where fluctuations of doping levels are retrieved due to intrinsic material characteristics such as the occurrence of defects, roughness, local chemical segregation, and faceting or step bunching.

SnS2 Nanoparticles and Thin Film for Application as an Adsorbent and Photovoltaic Buffer

Energy consumption and environmental pollution are major issues faced by the world. The present study introduces a single solution using SnS₂ for these two major global problems. SnS₂ nanoparticles and thin films were explored as an adsorbent to remove organic toxic materials (Rhodamine B (RhB)) from water and an alternative to the toxic cadmium sulfide (CdS) buffer for thin-film solar cells, respectively. Primary characterization tools such as X-ray photoelectron spectroscopy (XPS), Raman, X-ray diffraction (XRD), and UV-Vis-NIR spectroscopy were used to analyze the SnS₂ nanoparticles and thin films. At a reaction time of 180 min, 0.4 g/L of SnS₂ nanoparticles showed the highest adsorption capacity of 85% for RhB (10 ppm), indicating that SnS₂ is an appropriate adsorbent. The fabricated Cu(In,Ga)Se₂ (CIGS) device with SnS₂ as a buffer showed a conversion efficiency (~5.1%) close to that (~7.5%) of a device fabricated with the conventional CdS buffer, suggesting that SnS₂ has potential as an alternative buffer.

Defect-rich, negatively-charged SnS2 nanosheets for efficient photocatalytic Cr(VI) reduction and organic dye adsorption in water

Nanostructures of layered materials have gained increasing attention in photocatalytic and water-treatment processes. Herein, we report on sub-30 nm SnS₂ nanosheets (NSs) which can perform photocatalytic reduction of Cr(VI) to Cr(III) quite efficiently on one hand, while removes large quantities of toxic organic dye molecules by choosing an adsorption mode of operation over photo-degradation on the other hand, unlike most other SnS₂ nanostructures. The NSs have a highly extended crystallinity growing perpendicular to the (001) lattice direction but exhibit poor X-ray diffraction for the 10 l (1 = 1,2,3…) lattice planes. With such defects, the NSs have a narrow bandgap of 2.21 eV and exhibit a significant photocurrent density at near band-edge illumination. Cr(VI) photo-reduction using the SnS₂ NSs follows a first-order reaction kinetics (rate constant of 0.10 min-1), five-fold higher than commercial TiO₂ (P-25). Furthermore, the NSs adsorb Rhodamine B dye molecules from an aqueous solution by forming a monolayer of dye molecules following a pseudo-second-order kinetic model and exhibit an adsorption capacity of ∼ 53.28 mg/g. We show that the NSs have a Zeta potential of ∼ -22 eV and preferably adsorb cationic dyes only. Thus the SnS₂ NSs can be effective for Cr(VI) contaminated waste-water treatment in a photocatalytic manner and can also act as a potential adsorbent for polluting dye molecules either in the presence or absence of sunlight. While both these activities are known for SnS₂ as well as other materials, the competitive nature of the two mechanisms while each of them is a possibility has never been investigated. Therefore, besides the high activities, the study highlights the presence of different active sites on the material surface that can respond preferentially to either inorganic or organic impurities.

Sulfur precursor and citric acid effect on SnS2 nanoparticles and their influence on the photodegradation activity of selected organic compounds

Semiconductor nanoparticle-mediated photocatalysis is an attractive option for water decontamination, being the semiconductors as SnS₂ with a bandgap in the visible region, the most promising materials. In the present work, we evaluated the influence of important parameters in the photocatalytic application of SnS₂ nanoparticles. Our results show that the presence of citric acid (used as a capping agent) restricts the formation of hexagonal nanoparticles. We also demonstrated that using thioacetamide as a sulfur source results in smaller nanoparticles than thiourea, 24.0 nm and 616 nm respectively. Moreover, small hexagonal nanoparticles play a key role in the photocatalytic activity of SnS₂ nanoparticles. Compared with TiO₂ performance, SnS₂ nanoparticles exhibited faster kinetics for methyl orange (MO) degradation, Kapp = 0.0102 min^-1, and 0.029 min^-1, respectively. We proved that SnS₂ is capable of breaking the azo bond of methyl orange by direct reduction. Furthermore, our analyses indicate that SnS₂ nanoparticles do not degrade atrazine and imazapic, but the photocatalytic route of metribuzin competed with photolysis, resulting in a particular transformation product that was not obtained with light irradiation only. We demonstrated that SnS₂ nanoparticles have high bond selectivity for azo breaking. Furthermore, they represent an advance for the development of designed materials (such as heterostructures), where the properties of SnS₂ can be tuned.

Exploring edge functionalised blue phosphorene nanoribbons as novel photocatalysts for water splitting

1D phosphorene nanoribbon edges activating water molecules under sunlight.

The enhanced photocatalytic performance of SnS2@MoS2 QDs with highly-efficient charge transfer and visible light utilization for selective reduction of mythlen-blue

Molybdenum disulfide (MoS₂) has recently been considered as an effective material for potential photocatalytic applications; however, its photocatalytic activity was limited due to the low density of active sites. In this work, MoS₂ Quantum dots (QDs) were synthesized via the ultrasonication technique to construct heterostructure with SnS₂ nanosheets (SnS₂@MoS₂ QDs) and the prepared materials were tested for photocatalytic applications for Methylene blue (MB). Pristine SnS₂ and SnS₂@MoS₂ QDs nanocomposite were analyzed by XRD, TEM, PL, and Uv-Vis. Both SnS₂ and SnS₂@MoS₂ QDs exhibited a single trigonal phase with the P-3m1 space group. The TEM analysis confirmed the coupling between the pristine SnS₂ and SnS₂@MoS₂ QDs. The results of photocatalytic activity toward MB indicated that SnS₂@MoS₂ QDs material exhibits much superior photocatalytic performance compared to pristine SnS₂. The excellent photodegradation performance of SnS₂@MoS₂ QDs is due in the main to the formation of heterojunction between SnS₂ and MoS₂ QDs with narrow bandgap formation, which results in a facile carriers transfer and thus high photocatalytic efficiency. A representative mechanism of the photodegradation for SnS₂@MoS₂ QDs photocatalyst was proposed. Such an ultrasonic technique is capable of producing small metallic particle size that can be used to construct new heterostructures for water remediation applications.

TiO2 Nanosheet Arrays with Layered SnS2 and CoOx Nanoparticles for Efficient Photoelectrochemical Water Splitting

Converting solar energy into sustainable hydrogen fuel by photoelectrochemical (PEC) water splitting is a promising technology to solve increasingly serious global energy supply and environmental issues. However, the PEC performance based on TiO2 nanomaterials is hindered by the limited sunlight-harvesting ability and its high recombination rate of photogenerated charge carriers. In this work, layered SnS2 absorbers and CoOx nanoparticles decorated two-dimensional (2D) TiO2 nanosheet array photoelectrode have been rationally designed and successfully synthesized, which remarkably enhanced the PEC performance for water splitting. As the result, photoconversion efficiency of TiO2/SnS2/CoOx and TiO2/SnS2 hybrid photoanodes increases by 3.6 and 2.0 times under simulated sunlight illumination, compared with the bare TiO2 nanosheet arrays photoanode. Furthermore, the TiO2/SnS2/CoOx photoanode also presented higher PEC stability owing to CoOx catalyst served as efficient water oxidation catalyst as well as an effective protectant for preventing absorber photocorrosion.

Importance of Clean Surfaces on the Catalyst: SnS2 Nanorings for Environmental Remediation

The focus of the work is the synthesis of SnS₂ nanomaterials with (peg-SnS₂NF) and without (sf-SnS₂NR) the involvement of the organic template and the comparative study of their catalytic activities. The synthesis of these materials was achieved in a single-step procedure aided by hexamethyldisilazane (HMDS). These nanoparticles were subjected to X-ray diffraction, transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, and UV-vis spectroscopy analyses to investigate their structural, topographical, surface, and optical properties. The present work suggests that the surfactant-free SnS₂ nanoring (sf-SnS₂NR) catalyst has lower surface area compared to the poly(ethylene glycol)-stabilized SnS₂ nanoflower (peg-SnS₂NF) catalyst but shows high activity under visible light for the photoreduction of Cr(VI) and the photocatalytic degradation of organic dyes. The work exposed the importance of the clean surfaces on the catalyst and is expected to have a high impact on the photocatalytic activity of the SnS₂ nanomaterial. The study also endorses the utility of the HMDS-assisted synthetic method for the production of multifunctional semiconductor tin disulfide nanomaterials with multiple potential applications.

Structural changes during water-mediated amorphization of semiconducting two-dimensional thio-stannates

Owing to their combined open-framework structures and semiconducting properties, two-dimensional thio-stannates show great potential for catalytic and sensing applications. One such class of crystalline materials consists of porous polymeric [Sn3S7 2-] n sheets with molecular cations embedded in-between. The compounds are denoted R-SnS-1, where R is the cation. Dependent on the cation, some R-SnS-1 thio-stannates transition into amorphous phases upon dispersion in water. Knowledge about the fundamental chemical properties of the thio-stannates, including their water stability and the nature of the amorphous products, has not yet been established. This paper presents a time-resolved study of the transition from the crystalline to the amorphous phase of two violet-light absorbing thio-stannates, i.e. AEPz-SnS-1 [AEPz = 1-(2-amino-ethyl)-piperazine] and trenH-SnS-1 [tren = tris-(2-amino-ethyl)-amine]. X-ray total scattering data and pair distribution function analysis reveal no change in the local intralayer coordination during the amorphization. However, a rapid decrease in the crystalline domain sizes upon suspension in water is demonstrated. Although scanning electron microscopy shows no significant decrease of the micrometre-sized particles, transmission electron microscopy reveals the formation of small particles (∼200-400 nm) in addition to the larger particles. The amorphization is associated with disorder of the thio-stannate nanosheet stacking. For example, an average decrease in the interlayer distance (from 19.0 to 15.6 Å) is connected to the substantial loss of the organic components as shown by elemental analysis and X-ray photoelectron spectroscopy. Despite the structural changes, the light absorption properties of the amorphisized R-SnS-1 compounds remain intact, which is encouraging for future water-based applications of such materials.

Enhanced visible light absorption performance of SnS2 and SnSe2 via surface charge transfer doping

The layered two-dimensional (2D) SnS₂ and SnSe₂ have received intensive attention due to their sizable band gaps and potential properties. However, it has been shown that the visible light absorption of SnS₂ and SnSe₂ are restricted as photocatalysts and light-harvesting material absorbers for water splitting and high-performance optoelectronic devices. Herein, to enhance the visible light absorption performance of SnS₂ and SnSe₂, we performed a systematic investigation on tuning the electronic and optical properties of monolayers SnS₂ and SnSe₂ via surface charge transfer doping (SCTD) with the adsorption of molybdenum trioxide (MoO₃) and potassium (K) as surface dopants based on density functional theory. Our calculations reveal that MoO₃ molecules and K atoms can draw/donate electrons from/to SnS₂ and SnSe₂ as acceptors and donors, respectively. The adsorption of MoO₃ molecules introduces a new flat impurity state in the gap of the monolayers SnS₂/SnSe₂, and the Fermi level moves correspondingly to the top of valence band, resulting in a p-type doping of the monolayer SnS₂/SnSe₂. With the adsorption of K atoms, the electrons can transfer from K atoms to the monolayer of SnS₂ and SnSe₂, making K an effective electron-donating dopant. Meanwhile, the bandgaps of monolayers SnS₂ and SnSe₂ decrease after the MoO₃ and K doping, which leads to the appearance of appreciable new absorption peaks at around ∼650/480 and ∼600/680 nm, respectively, and yielding an enhanced visible light absorption of SnS₂ and SnSe₂. Our results unveil that SCTD is an effective way to improve the photocatalytic and light-harvesting performance of SnS₂ and SnSe₂, broadening their applications in splitting water and degrading environmental pollutants under sunlight irradiation.

Aqueous Synthesis of High-Quality Cu2ZnSnS4 Nanocrystals and Their Thermal Annealing Characteristics

Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by Sn₂S₆^4- and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes.

Recent advances in 2D materials for photocatalysis

Two-dimensional (2D) materials have attracted increasing attention for photocatalytic applications because of their unique thickness dependent physical and chemical properties. This review gives a brief overview of the recent developments concerning the chemical synthesis and structural design of 2D materials at the nanoscale and their applications in photocatalytic areas. In particular, recent progress on the emerging strategies for tailoring 2D material-based photocatalysts to improve their photo-activity including elemental doping, heterostructure design and functional architecture assembly is discussed.

Fabrication and characterization of Ag2CO3/SnS2 composites with enhanced visible-light photocatalytic activity for the degradation of organic pollutants

The efficient charge transfer at the interfaces of the Ag₂CO₃/SnS₂ composite due to the inner established electric field (E), which effectively facilitated interfacial charge transfer and improved photogenerated electron–hole pairs separation.