Harnessing Environmental Sensitivity in SnSe-Based Metal-Semiconductor-Metal Devices: Unveiling Negative Photoconductivity for Enhanced Photodetector Performance and Humidity Sensing

The extreme sensitivity of 2D-layered materials to environmental adsorbates, which is typically seen as a challenge, is harnessed in this study to fine-tune the material properties. This work investigates the impact of environmental adsorbates on electrical properties by studying metal-semiconductor-metal (MSM) devices fabricated on CVD-synthesized SnSe flakes. The freshly prepared devices exhibit positive photoconductivity (PPC), whereas they gradually develop negative photoconductivity (NPC) after being exposed to an ambient environment for ∼1 day. While the photodetectors based on positive photoconductivity exhibit a responsivity and detectivity of 6.1 A/W and 5.06 × 108 Jones, the same for the negative photoconductivity-based photodetector reaches up to 36.3 A/W and 1.49 × 109 Jones, respectively. In addition, the noise-equivalent power of the NPC photodetector decreases by 300 times as compared to the PPC device, which implies a prominent detection capability of the NPC device against weak photo signals. To substantiate the hypothesis that negative photoconductivity stems from the photodesorption of water and oxygen molecules on the dangling bonds of SnSe flakes, the flakes are etched along the most active planes (010) with a focused laser beam in an inert environment, which enhances responsivity by 43%, supporting negative photoconductivity linked to photodesorption. Furthermore, the humidity-dependent dark current variation of the NPC photodetectors is used to design a humidity sensor for human respiration monitoring with faster response and recovery times of 0.72 and 0.68 s, respectively. These findings open up the possibility of tuning the photoelectrical response of layered materials in a facile manner to develop future sensors and optoelectronic multifunctional devices.

Phase-Centric MOCVD Enabled Synthetic Approaches for Wafer-Scale 2D Tin Selenides

Following an initial nucleation stage at the flake level, atomically thin film growth of a van der Waals material is promoted by ultrafast lateral growth and prohibited vertical growth. To produce these highly anisotropic films, synthetic or post-synthetic modifications are required, or even a combination of both, to ensure large-area, pure-phase, and low-temperature deposition. A set of synthetic strategies is hereby presented to selectively produce wafer-scale tin selenides, SnSex (both x = 1 and 2), in the 2D forms. The 2D-SnSe2 films with tuneable thicknesses are directly grown via metal-organic chemical vapor deposition (MOCVD) at 200 °C, and they exhibit outstanding crystallinities and phase homogeneities and consistent film thickness across the entire wafer. This is enabled by excellent control of the volatile metal-organic precursors and decoupled dual-temperature regimes for high-temperature ligand cracking and low-temperature growth. In contrast, SnSe, which intrinsically inhibited from 2D growth, is indirectly prepared by a thermally driven phase transition of an as-grown 2D-SnSe2 film with all the benefits of the MOCVD technique. It is accompanied by the electronic n-type to p-type crossover at the wafer scale. These tailor-made synthetic routes will accelerate the low-thermal-budget production of multiphase 2D materials in a reliable and scalable fashion.

Self-limiting stoichiometry in SnSe thin films

Unique functionalities can arise when 2D materials are scaled down near the monolayer limit. However, in 2D materials with strong van der Waals bonds between layers, such as SnSe, maintaining stoichiometry while limiting vertical growth is difficult. Here, we describe how self-limiting stoichiometry can promote the growth of SnSe thin films deposited by molecular beam epitaxy. The Pnma phase of SnSe was stabilized over a broad range of Sn : Se flux ratios from 1 : 1 to 1 : 5. Changing the flux ratio does not affect the film stoichiometry, but influences the predominant crystallographic orientation. ReaxFF molecular dynamics (MD) simulation demonstrates that, while a mixture of Sn/Se stoichiometries forms initially, SnSe stabilizes as the cluster size evolves. The MD results further show that the excess selenium coalesces into Se clusters that weakly interact with the surface of the SnSe particles, leading to the limited stoichiometric change. Raman spectroscopy corroborates this model showing the initial formation of SnSe2 transitioning into SnSe as experimental film growth progresses. Transmission electron microscopy measurements taken on films deposited with growth rates above 0.25 Å s-1 show a thin layer of SnSe2 that disrupts the crystallographic orientation of the SnSe films. Therefore, using the conditions for self-limiting SnSe growth while avoiding the formation of SnSe2 was found to increase the lateral scale of the SnSe layers. Overall, self-limiting stoichiometry provides a promising avenue for maintaining growth of large lateral-scale SnSe for device fabrication.

Efficacious CO2 Photoconversion to C2 and C3 Hydrocarbons on Upright SnS-SnS2 Heterojunction Nanosheet Frameworks

In this work, SnS-SnS₂ heterostructured upright nanosheet frameworks are constructed on FTO substrates, which demonstrate promising photocatalytic performances for the conversion of CO₂ and water to C2 (acetaldehyde) and C3 (acetone) hydrocarbons without H₂ formation. With post annealing in designated atmospheres, the photocatalytic activity of the SnS-SnS₂ heterostructured nanosheet framework is critically enhanced by increasing the fraction of crystalline SnS in nanosheets through partial transformation of the SnS₂ matrix to SnS but not obviously influenced by improving the crystallinity of the SnS₂ matrix. DFT calculations indicate that transformed SnS possesses the CO₂ adsorption sites with significantly lower activation energy for the rate-determining step to drive efficient CO₂ conversion catalysis. The experimental results and DFT calculations suggest that the SnS-SnS₂ heterojunction nanosheet framework photocatalyst experiences Z-scheme charge transfer dynamic to allow the water oxidation and CO₂ reduction reactions occurring on the surfaces of SnS₂ and SnS, respectively. The Z-scheme SnS-SnS₂ heterostructured nanosheet framework photocatalyst exhibits not only efficient charge separation but also highly catalytic active sites to boost the photocatalytic activity for CO₂ conversion to C2 and C3 hydrocarbons.

Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production

Tin-based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H₂ production in acidic media without any use of sacrificial reagent. P-type SnS nanoplatelet films are coated with the n-CdS buffer layer and the TiO₂ passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm^-2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible-light illumination (λ > 500 nm, P_in = 80 mW cm^-2). The incident photon-to-current efficiency at 0 V versus RHE for H₂ production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H₂ production.

Tin guanidinato complexes: oxidative control of Sn, SnS, SnSe and SnTe thin film deposition

SnS, SnSe and SnTe are potentially important semiconductor materials. We report for the first time the oxidative controlled Aerosol assisted chemical vapor deposition (AA-CVD) of phase pure Sn(ii) chalcogenide thin films, using chalcogenide Sn(iv) guanidinate precursors, containing SnCh bonds (Ch = S, Se and Te).

Structural investigations of SnS1−xSexsolid solution synthesized from chalcogeno-carboxylate complexes of organo-tin by colloidal and solvent-less routes

Tin chalcogenides are important semiconducting materials due to their non-toxic nature, cost effectiveness and layered structure.

Band Gap Engineering of Hexagonal SnSe2 Nanostructured Thin Films for Infra-Red Photodetection

We, for the first time, provide the experimental demonstration on the band gap engineering of layered hexagonal SnSe₂ nanostructured thin films by varying the thickness. For 50 nm thick film, the band gap is ~2.04 eV similar to that of monolayer, whereas the band gap is approximately ~1.2 eV similar to that of bulk for the 1200 nm thick film. The variation of the band gap is consistent with the the theoretically predicted layer-dependent band gap of SnSe₂. Interestingly, the 400–1200 nm thick films were sensitiveto 1064 nm laser iradiation and the sensitivity increases almost exponentiallly with thickness, while films with 50–140 nm thick are insensitive which is due to the fact that the band gap of thinner films is greater than the energy corresponding to 1064 nm. Over all, our results establish the possibility of engineering the band gap of SnSe₂ layered structures by simply controlling the thickness of the film to absorb a wide range of electromagnetic radiation from infra-red to visible range.

Cation Exchange Combined with Kirkendall Effect in the Preparation of SnTe/CdTe and CdTe/SnTe Core/Shell Nanocrystals

Controlling the synthesis of narrow band gap semiconductor nanocrystals (NCs) with a high-quality surface is of prime importance for scientific and technological interests. This Letter presents facile solution-phase syntheses of SnTe NCs and their corresponding core/shell heterostructures. Here, we synthesized monodisperse and highly crystalline SnTe NCs by employing an inexpensive, nontoxic precursor, SnCl2, the reactivity of which was enhanced by adding a reducing agent, 1,2-hexadecanediol. Moreover, we developed a synthesis procedure for the formation of SnTe-based core/shell NCs by combining the cation exchange and the Kirkendall effect. The cation exchange of Sn(2+) by Cd(2+) at the surface allowed primarily the formation of SnTe/CdTe core/shell NCs. Further continuation of the reaction promoted an intensive diffusion of the Cd(2+) ions, which via the Kirkendall effect led to the formation of the inverted CdTe/SnTe core/shell NCs.

Porous nanostructures of SnSe: role of ionic liquid, tuning of nanomorphology and mechanistic studies

Porous SnSe nanoparticles have been synthesized in imidazolium based RTILviaelectron beam irradiation. RTIL provides a stabilizing environment as well as anin situsource of reducing radicals for the reduction of precursors.

Aqueous phase one-pot green synthesis of SnSe nanosheets in a protein matrix: negligible cytotoxicity and room temperature emission in the visible region

A rapid, facile, reproducible and green method for synthesizing SnSe nanosheets in aqueous media is reported. Cyclic voltammetry studies indicate better thermodynamic feasibility for reducing SnSe, while the nanomaterial is nontoxic up to a 100 μM concentration in CHO cells.

Ternary alloy nanocrystals of tin and germanium chalcogenides

Sn_xGe_1−xS, Sn_xGe_1−xSe, GeS_xSe_1−x, and SnS_xSe_1−x alloy nanocrystals were synthesized by novel gas-phase laser photolysis. Their composition-dependent lattice parameters and band gap were thoroughly characterized. The Sn_xGe_1−xS and SnS_xSe_1−x nanocrystals exhibit higher photoconversion efficiency as compared with the end members.