Booming Development of Group IV-VI Semiconductors: Fresh Blood of 2D Family

Two-Dimensional GeS/SnSe2 Tunneling Photodiode with Bidirectional Photoresponse and High Polarization Sensitivity

A two-dimensional (2D) broken-gap (type-III) p-n heterojunction has a unique charge transport mechanism because of nonoverlapping energy bands. In light of this, type-III band alignment can be used in tunneling field-effect transistors (TFETs) and Esaki diodes with tunable operation and low consumption by highlighting the advantages of tunneling mechanisms. In recent years, 2D tunneling photodiodes have gradually attracted attention for novel optoelectronic performance with a combination of strong light-matter interaction and tunable band alignment. However, an in-depth understanding of the tunneling mechanisms should be further investigated, especially for developing electronic and optoelectronic applications. Here, we report a type-III tunneling photodiode based on a 2D multilayered p-GeS/n+-SnSe2 heterostructure, which is first fabricated by the mechanical exfoliation and dry transfer method. Through the Simmons approximation, its various tunneling transport mechanisms dependent on bias and light are demonstrated as the origin of excellent bidirectional photoresponse performance. Moreover, compared to the traditional p-n photodiode, the device enables bidirectional photoresponse capability, including maximum responsivity values of 43 and 8.7 A/W at Vds = 1 and -1 V, respectively, with distinctive photoactive regions from the scanning photocurrent mapping. Noticeably, benefiting from the in-plane anisotropic structure of GeS, the device exhibits an enhanced photocurrent anisotropic ratio of 9, driven by the broader depletion region at Vds = -3 V under 635 nm irradiation. Above all, the results suggest that our designed architecture can be potentially applied to CMOS imaging sensors and polarization-sensitive photodetectors.

2D Free-Standing GeS1-xSex with Composition-Tunable Bandgap for Tailored Polarimetric Optoelectronics

Germanium-based monochalcogenides (i.e., GeS and GeSe) with desirable properties are promising candidates for the development of next-generation optoelectronic devices. However, they are still stuck with challenges, such as relatively fixed electronic band structure, unconfigurable optoelectronic characteristics, and difficulty in achieving free-standing growth. Herein, it is demonstrated that two-dimensional (2D) free-standing GeS1-xSex (0 ≤ x ≤ 1) nanoplates can be grown by low-pressure rapid physical vapor deposition (LPRPVD), fulfilling a continuously composition-tunable optical bandgap and electronic band structure. By leveraging the synergistic effect of composition-dependent modulation and free-standing growth, GeS1-xSex-based optoelectronic devices exhibit significantly configurable hole mobility from 6.22 × 10-4 to 1.24 cm2V-1s⁻1 and tunable responsivity from 8.6 to 311 A W-1 (635 nm), as x varies from 0 to 1. Furthermore, the polarimetric sensitivity can be tailored from 4.3 (GeS0.29Se0.71) to 1.8 (GeSe) benefiting from alloy engineering. Finally, the tailored imaging capability is also demonstrated to show the application potential of GeS1-xSex alloy nanoplates. This work broadens the functionality of conventional binary materials and motivates the development of tailored polarimetric optoelectronic devices.

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.

First-principles study of the electronic structure and optical properties of C-doped SnS2

CONTEXT

Density functional theory (DFT) was used to investigate the effects of varying carbon doping concentrations on the electronic and optical properties of SnS2-doped systems. The findings show that a doping concentration of 3.7% in SnS2 results in the highest structural stability and the lowest formation energy. A pure SnS2 monolayer is an indirect bandgap semiconductor, and the result reveals that increasing carbon doping correlates with a gradual reduction in the system's bandgap. The density of states analysis reveals that the valence band comprises C-2p, S-3p, and Sn-5p orbitals, whereas the conduction band consists of S-3p, Sn-5 s, and C-2p orbitals. Furthermore, doping concentration appears to cause a redshift in both the absorption coefficient and reflection peaks, which both decrease as doping concentration increases.

METHODS

The calculations for this study were performed using DFT within the CASTEP module of Materials Studio Segall et al. J Phys: Condens Matter 14(11):2717, 2002. The system parameters and structures were optimized to determine the electronic structure and optical properties. Geometric optimization and calculations were carried out with the generalized gradient approximation plane-wave pseudopotential method and the Perdew-Burke-Ernzerhof functional Perdew et al. Phys Rev Lett 80(4):891-891, 1998. The parameters for structural optimization included a plane-wave expansion cutoff energy set at 500 eV and a k-point mesh of 6 × 6 × 1 for Brillouin zone integration. The electronic convergence criteria were established at 1.0 × 10-5 eV/atom for the unit cell energy and 1.0 × 10-6 eV/atom for self-consistency. The internal stress deviation was maintained below 0.05 GPa, the atomic force interactions were kept under 0.03 eV/Å, and atomic displacements during geometric optimization were confined to less than 0.001 Å. To calculate the properties of the SnS2 monolayer, a vacuum spacing of 15 Å along the z-axis was introduced to prevent interactions between adjacent layers.

Polarization Reversal of Group IV-VI Semiconductors with Pucker-Like Structure: Mechanism Dissecting and Function Demonstration

Polarization imaging presents advantages in capturing spatial, spectral, and polarization information across various spectral bands. It can improve the perceptual ability of image sensors and has garnered more applications. Despite its potential, challenges persist in identifying band information and implementing image enhancement using polarization imaging. These challenges often necessitate integrating spectrometers or other components, resulting in increased complexities within image processing systems and hindering device miniaturization trends. Here, the characteristics of anisotropic absorption reversal are systematically elucidated in pucker-like group IV-VI semiconductors MX (M = Ge, Sn; X = S, Se) through theoretical predictions and experimental validations. Additionally, the fundamental mechanisms behind anisotropy reversal in different bands are also explored. The photodetector is constructed by utilizing MX as a light-absorbing layer, harnessing polarization-sensitive photoresponse for virtual imaging. The results indicate that the utilization of polarization reversal photodetectors holds advantages in achieving further multifunctional integration within the device structure while simplifying its configuration, including band information identification and image enhancement. This study provides a comprehensive analysis of polarization reversal mechanisms and presents a promising and reliable approach for achieving dual-band image band identification and image enhancement without additional auxiliary components.

The Acoustophotoelectric Effect: Efficient Phonon-Photon-Electron Coupling in Zero-Voltage-Biased 2D SnS2 for Broad-Band Photodetection

Two-dimensional (2D) layered metal dichalcogenides constitute a promising class of materials for photodetector applications due to their excellent optoelectronic properties. The most common photodetectors, which work on the principle of photoconductive or photovoltaic effects, however, require either the application of external voltage biases or built-in electric fields, which makes it challenging to simultaneously achieve high responsivities across broad-band wavelength excitation─especially beyond the material's nominal band gap─while producing low dark currents. In this work, we report the discovery of an intricate phonon-photon-electron coupling─which we term the acoustophotoelectric effect─in SnS2 that facilitates efficient photodetection through the application of 100 MHz order propagating surface acoustic waves (SAWs). This effect not only reduces the band gap of SnS2 but also provides the requisite momentum for indirect band gap transition of the photoexcited charge carriers, to enable broad-band photodetection beyond the visible light range, while maintaining pA-order dark currents─ without the need for any external voltage bias. More specifically, we show in the infrared excitation range that it is possible to achieve up to 8 orders of magnitude improvement in the material's photoresponsivity compared to that previously reported for SnS2-based photodetectors, in addition to exhibiting superior performance compared to most other 2D materials reported to date for photodetection.

Phase-Controllable Growth of Air-Stable SnS Nanostructures for High-Performance Photodetectors with Ultralow Dark Current

The epitaxial growth of low-dimensional tin chalcogenides SnX (X = S, Se) with a controlled crystal phase is of particular interest since it can be utilized to tune optoelectronic properties and exploit potential applications. However, it still remains a great challenge to synthesize SnX nanostructures with the same composition but different crystal phases and morphologies. Herein, we report a phase-controlled growth of SnS nanostructures via physical vapor deposition on mica substrates. The phase transition from α-SnS (Pbnm) nanosheets to β-SnS (Cmcm) nanowires can be tailored by the reduction of growth temperature and precursor concentration, which originates from a delicate competition between SnS-mica interfacial coupling and phase cohesive energy. The phase transition from the α to β phase not only greatly improves the ambient stability of SnS nanostructures but also leads to the band gap reduction from 1.03 to 0.93 eV, which is responsible for fabricated β-SnS devices with an ultralow dark current of 21 pA at 1 V, an ultrafast response speed of ≤14 μs, and broadband spectra response from the visible to near-infrared range under ambient condition. A maximum detectivity of the β-SnS photodetector arrives at 2.01 × 108 Jones, which is about 1 or 2 orders of magnitude larger than that of α-SnS devices. This work provides a new strategy for the phase-controlled growth of SnX nanomaterials for the development of highly stable and high-performance optoelectronic devices.

A universal growth method for high-quality phase-engineered germanium chalcogenide nanosheets

Low-dimensional group IV-VI metal chalcogenide-based semiconductors hold great promise for opto-electronic device applications owing to their diverse crystalline phases and intriguing properties related to thermoelectric and ferroelectric effects. Herein, we demonstrate a universal chemical vapor deposition (CVD) growth method to synthesize stable germanium chalcogenide-based (GeS, GeS₂, GeSe, GeSe₂) nanosheets, which increases the library of the p-type semiconductor. The phase transition between different crystalline polytypes can be deterministically controlled by hydrogen concentration in the reaction chamber. Structural characterization and synthesis experiments identify the behavior, where the higher hydrogen concentration promotes the transiton from germanium dichalcogenides to germanium monochalcogenides. The angle-polarized and temperature-dependent Raman spectra demonstrate the strong interlayer coupling and lattice orientation. Based on the optimized growth scheme and systematic comparison of electrical properties, GeSe nanosheet photodetectors were demonstrated, which exhibit superior device performance on SiO₂/Si and HfO₂/Si substrate with a high photoresponsivity up to 10⁴ A W^-1, fast response time less than 15 ms, and high mobility of 3.2 cm² V^-1 s^-1, which is comparable to the mechanically exfoliated crystals. Our results manifest the hydrogen-mediated deposition strategy as a facile control knob to engineer crystalline phases of germanium chalcogenides for high performance optoelectronic devices.

Two Dimensional Heterostructures for Optoelectronics: Current Status and Future Perspective

Researchers have found various families of two-dimensional (2D) materials and associated heterostructures through detailed theoretical work and experimental efforts. Such primitive studies provide a framework to investigate novel physical/chemical characteristics and technological aspects from micro to nano and pico scale. Two-dimensional van der Waals (vdW) materials and their heterostructures can be obtained to enable high-frequency broadband through a sophisticated combination of stacking order, orientation, and interlayer interactions. These heterostructures have been the focus of much recent research due to their potential applications in optoelectronics. Growing the layers of one kind of 2D material over the other, controlling absorption spectra via external bias, and external doping proposes an additional degree of freedom to modulate the properties of such materials. This mini review focuses on current state-of-the-art material design, manufacturing techniques, and strategies to design novel heterostructures. In addition to a discussion of fabrication techniques, it includes a comprehensive analysis of the electrical and optical properties of vdW heterostructures (vdWHs), particularly emphasizing the energy-band alignment. In the following sections, we discuss specific optoelectronic devices, such as light-emitting diodes (LEDs), photovoltaics, acoustic cavities, and biomedical photodetectors. Furthermore, this also includes a discussion of four different 2D-based photodetector configurations according to their stacking order. Moreover, we discuss the challenges that remain to be addressed in order to realize the full potential of these materials for optoelectronics applications. Finally, as future perspectives, we present some key directions and express our subjective assessment of upcoming trends in the field.

Large Memory Window of van der Waals Heterostructure Devices Based on MOCVD-Grown 2D Layered Ge4 Se9

Van der Waals (vdW) heterostructures have drawn much interest over the last decade owing to their absence of dangling bonds and their intriguing low-dimensional properties. The emergence of 2D materials has enabled the achievement of significant progress in both the discovery of physical phenomena and the realization of superior devices. In this work, the group IV metal chalcogenide 2D-layered Ge₄ Se₉ is introduced as a new selection of insulating vdW material. 2D-layered Ge₄ Se₉ is synthesized with a rectangular shape using the metalcorganic chemical vapor deposition system using a liquid germanium precursor at 240 °C. By stacking the Ge₄ Se₉ and MoS₂ , vdW heterostructure devices are fabricated with a giant memory window of 129 V by sweeping back gate range of ±80 V. The gate-independent decay time reveals that the large hysteresis is induced by the interfacial charge transfer, which originates from the low band offset. Moreover, repeatable conductance changes are observed over the 2250 pulses with low non-linearity values of 0.26 and 0.95 for potentiation and depression curves, respectively. The energy consumption of the MoS₂ /Ge₄ Se₉ device is about 15 fJ for operating energy and the learning accuracy of image classification reaches 88.3%, which further proves the great potential of artificial synapses.

MAX-phase Derived Tin Diselenide for 2D/2D Heterostructures with Ultralow Surface/Interface Transport Barriers toward Li-/Na-ions Storage

2D tin diselenide and its derived 2D heterostructures have delivered promising potentials in various applications ranging from electronics to energy storage devices. The major challenges associated with large-scale fabrication of SnSe₂ crystals, however, have hindered its engineering applications. Herein, a tin-extraction synthetic method is proposed for producing large-size SnSe₂ bulk crystals. In a typical synthesis, a Sn-containing MAX phase (V₂ SnC) and a Se source are heat-treated under a reducing atmosphere, by which Sn is extracted from the V₂ SnC phase as a rectified Sn source to form SnSe₂ crystals in the cold zone. After the following liquid exfoliation, the obtained 2D SnSe₂ nanosheets have a lateral size of a few centimeters and an atomic thickness. Furthermore, by coupling with 2D graphene to form 2D/2D SnSe₂ /graphene heterostructured electrodes, as validated by theoretical calculation and experimental studies, the superior Li-/Na-ion storage performance with ultralow surface/interface ion transport barriers are achieved for rechargeable Li-/Na-ion batteries. This innovative synthetic strategy opens a new avenue for the large-scale synthesis of selenides and offers more options into the practical application of emerging 2D/2D heterostructure for electrochemical energy storage.

A Decade of Germananes: Four Approaches to Their Functionalization

Since the first synthesis of germanane (GeH) reported in 2013, two-dimensional germanium-based materials have been intensively studied. Over the past decade, several methodologies for the functionalization of germanane have been introduced. The first approach utilized exfoliation of Zintl phase CaGe₂ with alkyl halides. Liu's solvothermal method was used for the synthesis of methyl germanane. Another methodology utilized Ge-H activation with sodium naphthalenide and its subsequent alkylation. All of these methods provide functionalized germananes; thus, a comparison of these methods is needed. In this paper, such a comparison of current synthetic approaches towards alkyl germananes is reported, and additionally, a new method for Ge-H activation utilizing a NaK equimolar alloy is presented as a fourth approach. For this purpose, eight alkyl reagents were chosen representing reactive benzyl bromides as well as linear esters and nitriles because they contain easily trackable functional groups. The materials were characterized using Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and thermogravimetric analysis, and the data were compared. The comparison of all methods revealed not only some drawbacks for each method but also their advantages. The method utilizing sodium naphthalenide provided the lowest degree of surface coverage, whereas the solvothermal method seemed to provide materials with the highest degree of functionalization; unfortunately, the functionalization was also accompanied by a high degree of surface oxidation, i.e., (Ge-OH/Ge═O) formation. The highest degree of surface coverage accompanied by the lowest degree of surface oxidation was achieved employing Goldberger's phase transfer direct exfoliation of CaGe₂ as well as Ge-H activation using the NaK alloy with subsequent alkylation.

Controllable Synthesis of Narrow-Gap van der Waals Semiconductor Nb2GeTe4 with Asymmetric Architecture for Ultrafast Photonics

Ultrafast photonics has become an interdisciplinary topic of great consequence due to the spectacular progress of compact and efficient ultrafast pulse generation. Wide spectrum bandwidth is the key element for ultrafast pulse generation due to the Fourier transform limitation. Herein, monoclinic Nb₂GeTe₄, an emerging class of ternary narrow-gap semiconductors, was used as a real saturable absorber (SA), which manifests superior wide-range optical absorption. The crystallization form and growth mechanism of Nb₂GeTe₄ were revealed by a thermodynamic phase diagram. Furthermore, the Nb₂GeTe₄-SA showed reliable saturation intensity and larger modulation depth, ascribed to a built-in electric field driven by the asymmetric crystal architecture confirmed via X-ray diffraction, polarized Raman spectra, and scanning transmission electron microscopy. Based on the Nb₂GeTe₄-SA, femtosecond mode-locked operation with good overall performance was achieved by a properly designed ring cavity. These results suggest that Nb₂GeTe₄ shows great promise for ultrafast photonic applications and arouse interests in exploring the intriguing properties of the ternary van der Waals material family.

Solution phase growth and analysis of super-thin zigzag tin selenide nanoribbons

Tin selenide (SnSe), a highly promising layered material, has been garnering particular interest in recent times due to its significant promise for future energy devices. Herein we report a simple solution-phase approach for growing highly crystalline layered SnSe nanoribbons. Polyvinylpyrrolidone (PVP) was used as a templating agent to selectively passivates the (100) and (001) facets of the SnSe nanoribbons resulting in the unique growth of nanoribbons along theirb-axis with a defined zigzag edge state along the sidewalls. The SnSe nanoribbons are few layers thick (∼20 layers), with mean widths of ∼40 nm, and achievable length of >1μm. Nanoribbons could be produced in relatively high quantities (>150 mg) in a single batch experiment. The PVP coating also offers some resistance to oxidation, with the removal of the PVP seen to lead to the formation of a SnSe/SnO_xcore-shell structure. The use of non-toxic PVP to replace toxic amines that are typically employed for other 1D forms of SnSe is a significant advantage for sustainable and environmentally friendly applications. Heat transport properties of the SnSe nanoribbons, derived from power-dependent Raman spectroscopy, demonstrate the potential of SnSe nanoribbons as thermoelectric material.

Solution-processed Ge (II)-based chalcogenide thin films with tunable bandgaps for photovoltaics

Solution processes have been widely used to construct chalcogenide-based thin-film optoelectronic and electronic devices that combine high performance with low-cost manufacturing. However, Ge(ii)-based chalcogenide thin films possessing great potential for optoelectronic devices have not been reported using solution-based processes; this is mainly attributed to the easy oxidation of intermediate Ge(ii) to Ge(iv) in the precursor solution. Here we report solution-processed deposition of Ge(ii)-based chalcogenide thin films in the case of GeSe and GeS films by introducing hypophosphorous acid as a suitable reducing agent and strong acid. This enables the generation of Ge(ii) from low-cost and stable GeO₂ powders while suppressing the oxidation of Ge(ii) to Ge(iv) in the precursor solution. We further show that such solution processes can also be used to deposit GeSe_1−xS_x alloy films with continuously tunable bandgaps ranging from 1.71 eV (GeS) to 1.14 eV (GeSe) by adjusting the atomic ratio of S- to Se-precursors in solution, thus allowing the realization of optimal-bandgap single-junction photovoltaic devices and multi-junction devices.

Solution-processed Ge(ii)-based chalcogenide films are achieved by introducing hypophosphorous acid as a suitable reducing agent and strong acid and demonstrated for photovoltaic application.

Atomic layer deposition of SnSex thin films using Sn(N(CH3)2)4 and Se(Si(CH3)3)2 with NH3 co-injection

This study introduces the atomic layer deposition (ALD) of tin selenide thin films using Sn(N(CH₃)₂)₄ and Se(Si(CH₃)₃)₂ with NH₃ co-injection. The co-injection of NH₃ with Se(Si(CH₃)₃)₂ is essential for film growth to convert the precursor into a more reactive form. The most critical feature of this specific ALD process is that the chemical composition (Sn/Se ratio) could be varied by changing the growth temperature, even for the given precursor injection conditions. The composition and morphology of the deposited films varied depending on the process temperature. Below 150 °C, a uniform SnSe₂ thin film was deposited in an amorphous phase, maintaining the oxidation states of its precursors. Above 170 °C, the composition of the film changed to 1 : 1 stoichiometry due to the crystallization of SnSe and desorption of Se. A two-step growth sequence involving a low-temperature seed layer was devised for the high-temperature ALD of SnSe to improve surface roughness.

Investigation of the sublimation mechanism of GeSe and GeS

GeSe and GeS have emerged as promising light-harvesting materials for photovoltaics due to their attractive optoelectronic properties, non-toxic and earth-abundant constituents, and excellent stability. Here we unveil the diatomic molecule sublimation mechanism of GeSe and GeS that directly guides the optimization of GeSe and GeS solar-cell fabricated via the close-space sublimation method.

Surface-Defect States in Photovoltaic Absorber GeSe

GeSe is an emerging promising light-harvesting material for photovoltaics due to its excellent optoelectronic properties, nontoxic and earth-abundant constituents, and high stability. In particular, perovskite-like antibonding states at the valence band maximum arising from Ge-4s and Se-4p coupling enable the bulk-defect-tolerant properties in GeSe. However, a fundamental understanding of surface-defect states in GeSe, another important factor for high-performance photovoltaics, is still lacking. Here, we investigate the surface-defect properties of GeSe through first-principle calculations. We find that different from common semiconductors possessing numerous surface dangling bonds, some GeSe surfaces are prone to reconstruction, thus eliminating the dangling bonds. The rearranged armchair edges exhibit unexpected benign defect properties, similar to those of bulk GeSe, arising from the formation of bulk-like [GeSe₃] tetrahedrons. We further show that the stable exposed (111) surfaces are hard to reconstruct due to the stiff structure but are effectively passivated by the addition of H.

Di-tert-butyltin(IV) 2-pyridyl and 4,6-dimethyl-2-pyrimidyl thiolates: versatile single source precursors for the preparation of SnS nanoplatelets as anode material for lithium ion batteries

New air and moisture stable di-tert-butyltin complexes derived from 2-mercaptopyridine (HSpy), [^tBu₂Sn(Spy)₂], [^tBu₂Sn(Cl)(Spy)] and 4,6-dimethyl-2-mercaptopyrimidine (HSpymMe₂) [^tBu₂Sn(Cl)(SpymMe₂)], have been prepared and utilized as single-source molecular precursors for the preparation of orthorhombic SnS nanoplatelets by a hot injection method and thin films by aerosol assisted chemical vapour deposition (AACVD). The complexes were characterized by NMR (¹H, ¹³C, ¹¹⁹Sn) and elemental analysis and their structures were unambiguously established by the single crystal X-ray diffraction technique. Thermolysis of these complexes in oleylamine (OAm) produced SnS nanoplatelets. The morphologies, elemental compositions, phase purity and crystal structures of the resulting Oam-capped nanoplatelets were determined by electron microscopy (SEM, TEM), energy dispersive X-ray spectroscopy (EDS) and pXRD, while the band gaps of the nanoplatelets were evaluated by diffuse reflectance spectroscopy (DRS) and were blue shifted relative to the bulk material. The morphology and preferential growth of the nanoplatelets were found to be significantly altered by the nature of the molecular precursor employed. The synthesized SnS nanoplatelets were evaluated for their performance as anode material for lithium ion batteries (LIBs). A cell comprised of an SnS electrode could be cycled for 50 cycles. The rate capability of SnS was investigated at different current densities in the range 0.1 to 0.7 A g^-1 which revealed that the initial capacity could be regained.

Phase stability of the tin monochalcogenides SnS and SnSe: a quasi-harmonic lattice-dynamics study

The tin monochalcogenides SnS and SnSe adopt four different crystal structures, viz. orthorhombic Pnma and Cmcm and cubic rocksalt and π-cubic (P2₁3) phases, each of which has optimal properties for a range of potential applications. This rich phase space makes it challenging to identify the conditions under which the different phases are obtained. We have performed first-principles quasi-harmonic lattice-dynamics calculations to assess the relative stabilities of the four phases of SnS and SnSe. We investigate dynamical stability through the presence or absence of imaginary modes in the phonon dispersion curves, and we compute Helmholtz and Gibbs free energies to evaluate the thermodynamic stability. We also consider applied pressures up to 15 GPa to obtain simulated temperature-pressure phase diagrams. Finally, the relationships between the orthorhombic crystal phases are investigated by explicitly mapping the potential-energy surfaces along the imaginary harmonic phonon modes in the Cmcm phase, and the relationships between the cubic phases are found by transition-state modelling using the climbing-image nudged elastic-band method.

Interfacial Strain Engineering in Wide-Bandgap GeS Thin Films for Photovoltaics

Wide-bandgap semiconductors exhibiting a bandgap of ∼1.7-1.9 eV have generated great interest recently due to their important applications in tandem solar cells as top cells and emerging indoor photovoltaics. However, concerns about the stability and toxicity especially in indoor application limit the choice of these materials. Here we report a new member of this family, germanium monosulfide (GeS); this material displays a wide bandgap of 1.7 eV, nontoxic and earth-abundant constituents, and high stability. We find that the little success of GeS solar cells to date is primarily attributed to the challenge in fabricating high-quality polycrystalline GeS films, wherein the high thermal expansion coefficient (α = 3.1 × 10-5 K-1) combined with high crystallization temperature (375 °C) of GeS induces large tensile strain in the GeS film that peels off GeS from the substrate. By introducing a high-α buffer layer between GeS and substrate, we achieve a high-quality polycrystalline GeS thin film that compactly adheres to substrate with no voids. Solar cells fabricated by these GeS films show a power conversion efficiency of 1.36% under AM 1.5G illumination (100 mW cm-2). The unencapsulated devices are stable when stored in ambient atmosphere for 1500 h. Their efficiencies further increase to 3.6% under indoor illumination of 1000 lux.

Recent Advances in 2D Group VB Transition Metal Chalcogenides

2D materials have received considerable research interest owing to their abundant material systems and remarkable properties. Among them, 2D group VB transition metal chalcogenides (GVTMCs) stand out as emerging 2D metallic materials and significantly broaden the research scope of 2D materials. 2D GVTMCs have great advantages in electrical transport, 2D magnetism, charge density wave, sensing, catalysis, and charge storage, making them attractive in the fields of functional devices and energy chemistry. In this review, the recent progress of 2D GVTMCs is summarized systematically from fundamental properties, growth methodologies to potential applications. The challenges and prospects are also discussed for future research in this field.

Fast colloidal synthesis of SnSe2 nanosheets for flexible broad-band photodetection

A new rapid bottom-up colloidal synthetic route has been developed to synthesize SnSe₂ nanosheets within 5 min. A SnSe₂ nanosheet-based flexible photodetector is fabricated for the first time and the resulting device displays a wide photodetection range and high flexibility.

In-plane anisotropic 2D Ge-based binary materials for optoelectronic applications

In-plane anisotropic two-dimensional (2D) materials possess unique in-plane anisotropic physical properties arising from their low crystal lattice symmetry.

Controllable synthesis of SnS2 flakes and MoS2/SnS2 heterostructures by confined-space chemical vapor deposition

A halogen salt-assisted confined-space CVD method is used for the controllable synthesis of SnS₂ flakes, which are parallel to the substrate and have the characteristics of better crystallinity and fewer S vacancies.

Aluminum doping effects on photoresponse characteristics of hydrothermal tin disulfide nanosheets

Photoresponse characteristics of Al-doped SnS₂ nanosheets have been improved significantly by aluminum doping, compared to pristine SnS₂. The response time was reduced by two orders of magnitude and the responsivity was increased one hundredfold.

Defect Engineering in Metastable Phases of Transition-Metal Dichalcogenides for Electrochemical Applications

Metastable metallic phases of transition-metal dichalcogenide (TMD) nanomaterials have displayed excellent performance and emerged as promising candidates for sustainable energy sources low-cost storage and conversion because of their two-dimensional (2D) layered structures and extraordinary physicochemical properties. In order to broaden the range of potential applications, defect engineering is applied to the metastable phases of TMDs for further improvement of their catalytic and electronic properties. According to some recent studies, effective introduction of defects without perturbing the interior conductivity contributes to the development of metastable TMDs. This review provides deep insights into recent progress in electrochemistry using defect engineering in the metastable phases of TMDs. After introducing the structures of metastable phases and methods for defect construction, significant developments in catalysis and energy storage applications are discussed to elucidate structure-function relationships. Key challenges and future directions for defect engineering in the metastable phases of TMDs are also highlighted in the conclusions.

Controlled synthesis of few-layer SnSe2 by chemical vapor deposition

Few-layer SnSe₂ has intrinsic low thermal conductivity and unique phase transition from amorphous to crystalline state under laser irradiation. It has been extensively used in the fields of thermoelectric conversion and information storage. However, the traditional precursors like tin oxide and organic compounds have either high melting points or complex compositions, and the improper deposition temperature of the substrate may lead to mixed products, which impedes controllable synthesis of high-quality few-layer SnSe₂. Here, we propose a chemical vapor deposition (CVD) method, in which the precursor evaporation and deposition have been controlled via the adjustment of precursors/substrate positions, which effectively avoided mixed product growth, thus achieving the growth of high-quality few-layer SnSe₂. The calculated first-order temperature coefficient of the A_1g module is −0.01549 cm⁻¹ K⁻¹, which is superior to other two-dimensional (2D) materials. Meanwhile, two exciton emissions from few-layer SnSe₂ have been found, for which the higher energy one (1.74 eV) has been assigned to near-band-gap emission, while the lower one (1.61 eV) may have roots in the surface state of SnSe₂. The few-layer SnSe₂ also exhibits large exciton binding energies (0.195 and 0.177 eV), which are greater than those of common semiconductors and may contribute to stability of excitons, showing broad application prospects in the field of optoelectronics.

Few-layer SnSe₂ has intrinsic low thermal conductivity and unique phase transition from amorphous to crystalline state under laser irradiation.

Void-free 3D Bioprinting for In-situ Endothelialization and Microfluidic Perfusion

Two major challenges of 3D bioprinting are the retention of structural fidelity and efficient endothelialization for tissue vascularization. We address both of these issues by introducing a versatile 3D bioprinting strategy, in which a templating bioink is deposited layer-by-layer alongside a matrix bioink to establish void-free multimaterial structures. After crosslinking the matrix phase, the templating phase is sacrificed to create a well-defined 3D network of interconnected tubular channels. This void-free 3D printing (VF-3DP) approach circumvents the traditional concerns of structural collapse, deformation and oxygen inhibition, moreover, it can be readily used to print materials that are widely considered "unprintable". By pre-loading endothelial cells into the templating bioink, the inner surface of the channels can be efficiently cellularized with a confluent endothelial layer. This in-situ endothelialization method can be used to produce endothelium with a far greater uniformity than can be achieved using the conventional post-seeding approach. This VF-3DP approach can also be extended beyond tissue fabrication and towards customized hydrogel-based microfluidics and self-supported perfusable hydrogel constructs.

Vertically stacked SnSe homojunctions and negative capacitance for fast low-power tunneling transistors

The two-dimensional (2D) vertical van der Waals (vdW) stacked homojunction is an advantageous configuration for fast low-power tunneling field effect transistors (TFETs). We simulate the device performance of the sub-10 nm vertical SnSe homojunction TFETs with ab initio quantum transport calculations. The vertically stacked device configuration has an effect of decreasing leakage current when compared with its planar counterpart due to the interrupted carrier transport path by the broken connection. A subthreshold swing over four decades (SS_{ave_4 dec}) of 44.2–45.8 mV dec⁻¹ and a drain current at SS = 60 mV dec⁻¹ (I₆₀) of 5–7 μA μm⁻¹ are obtained for the optimal vertical SnSe homojunction TFET with L_g = 10 nm at a supply voltage of 0.5–0.74 V. In terms of the device's main figures of merit (i.e., on-state current, intrinsic delay time, and power delay product), the vertical SnSe TFETs and NCTFETs outperform the 2022 and 2028 targets of the International Technology Roadmap for Semiconductors requirements for low-power application (2013 version), respectively.

The vertical SnSe homojunction TFETs and NCTFETs are potential candidates for fast low-power application.

Two-dimensional few-layered PC3 as a promising photocatalyst for overall water splitting

Recently, 2D carbon phosphides (PCs) have attracted much attention due to their superior electronic and photovoltaic properties suitable for potential applications in field effect transistors and photodetectors. In this work, we systematically investigate the stability, electronic properties, optical absorption and photocatalytic water splitting performance of few-layered PC3 by using the first principles calculation method. Numerical results indicate that both monolayered and bilayered PC3 can serve as efficient photocatalysts for overall water splitting due to their high stability, moderate band gaps, suitable band edge positions, anisotropic high carrier mobilities and strong capacity of solar absorption. Compared with monolayered PC3, bilayered PC3 displays higher carrier mobilities (2500-23 000 cm2 V-1 s-1) and a wider optical absorption spectrum. Moreover, by applying an in-plane biaxial strain, the utilization of solar energy and the pH range suitable for overall water splitting can be improved effectively for both monolayered and bilayered PC3. Our work reliably expands the potential application of 2D few-layered PC3 in the field of nano-electronics and nano-optoelectronics.

Sub-Angstrom Characterization of the Structural Origin for High In-Plane Anisotropy in 2D GeS2

Materials with layered crystal structures and high in-plane anisotropy, such as black phosphorus, present unique properties and thus promise for applications in electronic and photonic devices. Recently, the layered structures of GeS₂ and GeSe₂ were utilized for high-performance polarization-sensitive photodetection in the short wavelength region due to their high in-plane optical anisotropy and wide band gap. The highly complex, low-symmetric (monoclinic) crystal structures are at the origin of the high in-plane optical anisotropy, but the structural nature of the corresponding nanostructures remains to be fully understood. Here, we present an atomic-scale characterization of monoclinic GeS₂ nanostructures and quantify the in-plane structural anisotropy at the sub-angstrom level in real space by Cs-corrected scanning transmission electron microscopy. We elucidate the origin of this high in-plane anisotropy in terms of ordered and disordered arrangement of [GeS₄] tetrahedra in GeS₂ monolayers, through density functional theory (DFT) calculations and orbital-based bonding analyses. We also demonstrate high in-plane mechanical, electronic, and optical anisotropies in monolayer GeS₂ and envision phase transitions under uniaxial strain that could potentially be exploited for nonvolatile memory applications.

Wafer-size growth of 2D layered SnSe films for UV-Visible-NIR photodetector arrays with high responsitivity

Due to its excellent electrical and optical properties, tin selenide (SnSe), a typical candidate of two-dimensional (2D) semiconductors, has attracted great attention in the field of novel optoelectronics. However, the large-area growth of high-quality SnSe films still remains a great challenge, which limits their practical applications. Here, wafer-size SnSe ultrathin films with high uniformity and crystallization were deposited via a scalable magnetron sputtering method. The results showed that the SnSe photodetector was highly sensitive to a broad range of wavelengths in the UV-visible-NIR range, especially showing an extremely high responsivity of 277.3 A W-1 with the corresponding external quantum efficiency of 8.5 × 104% and detectivity of 7.6 × 1011 Jones. These figures of merits are among the best performances for the sputter-fabricated 2D photodetector devices. The photodetecting mechanisms based on a photogating effect induced by the trapping effect of localized defects are discussed in detail. The results indicate that the few-layered SnSe films obtained from sputtering growth have great potential in the design of high-performance photodetector arrays.

Rapid synthesis of thermoelectric SnSe thin films by MPCVD

Microwave plasma chemical vapor deposition (MPCVD) has been traditionally used to synthesize carbon-based materials such as diamonds, carbon nanotubes and graphene. Here we report that a rapid and catalyst-free growth of SnSe thin films can be achieved by using single-mode MPCVD with appropriate source materials. The analysis combining microscope images, X-ray diffraction patterns and lattice vibration modes shows that the grown thin films were composed of orthorhombic structured SnSe polycrystals. Further thermoelectric (TE) characterization of the SnSe films reveals the high-performance power factor of 3.98 μW cm⁻¹ K⁻² at 600 K. Our results may open an avenue for rapid synthesis of new types of materials such as IV–VI compounds and be useful for TE application of these materials.

Here we report that a rapid and catalyst-free growth of high quality SnSe thin films can be achieved by using single-mode MPCVD with appropriate source materials, the SnSe films exhibit high TE performance.

Thickness of elemental and binary single atomic monolayers

The thickness of monolayers is a fundamental property of two-dimensional (2D) materials that has not found the necessary attention. It plays a crucial role in their mechanical behavior, the determination of related physical properties such as heat transfer, and especially the properties of multilayer systems. Measurements of the thickness of free-standing monolayers are widely lacking and notoriously too large. Consistent thicknesses have been reported for single layers of graphene, boronitrene, and SiC derived from interlayer spacing measured by X-ray diffraction in multilayer systems, first-principles calculations of the interlayer spacing, and tabulated van der Waals (vdW) diameters. Furthermore, the electron density-based volume model agrees with the geometric slab model for graphene and boronitrene. For other single-atom monolayers DFT calculations and molecular dynamics (MD) simulations deliver interlayer distances that are often much smaller than the vdW diameter, owing to further electrostatic and (weak) covalent interlayer interaction. Monolayers strongly bonded to a surface also show this effect. If only weak vdW forces exist, the vdW diameter delivers a reasonable thickness not only for free-standing monolayers but also for few-layer systems and adsorbed monolayers. Adding the usually known corrugation effect of buckled or puckered monolayers to the vdW diameter delivers an upper limit of the monolayer thickness. The study presents a reference database of thickness values for elemental and binary group-IV and group-V monolayers, as well as binary III-V and IV-VI compounds.

2D material broadband photodetectors

Our review provides a comprehensive overview of the latest evolution of broadband photodetectors (BBPDs) based on 2D materials (2DMs). We begin with BBPDs built on various 2DM channels, including narrow-bandgap 2DMs, 2D topological semimetals, 2D charge density wave compounds, and 2D heterojunctions. Then, we introduce defect-engineered 2DM BBPDs, including vacancy engineering, heteroatom incorporation, and interfacial engineering. Subsequently, we summarize 2DM based mixed-dimensional (0D-2D, 1D-2D, 2D-3D, and 0D-2D-3D) BBPDs. Finally, we provide several viewpoints for the future development of this burgeoning field.

Polarization-Dependent Optical Properties and Optoelectronic Devices of 2D Materials

The development of optoelectronic devices requires breakthroughs in new material systems and novel device mechanisms, and the demand recently changes from the detection of signal intensity and responsivity to the exploration of sensitivity of polarized state information. Two-dimensional (2D) materials are a rich family exhibiting diverse physical and electronic properties for polarization device applications, including anisotropic materials, valleytronic materials, and other hybrid heterostructures. In this review, we first review the polarized-light-dependent physical mechanism in 2D materials, then present detailed descriptions in optical and optoelectronic properties, involving Raman shift, optical absorption, and light emission and functional optoelectronic devices. Finally, a comment is made on future developments and challenges. The plethora of 2D materials and their heterostructures offers the promise of polarization-dependent scientific discovery and optoelectronic device application.

Layer-Dependent Properties of Ultrathin GeS Nanosheets and Application in UV-Vis Photodetectors

Two-dimensional germanium sulfide (GeS), an analogue of phosphorene, has attracted broad attention owing to its excellent environmental stabilities, fascinating electronic and optical properties, and applications in various nanodevices. In spite of the current achievements on 2D GeS, the report of ultrathin few-layer GeS nanosheets within 5 nm is still lacking. Here in this contribution, we have achieved preparation of ultrathin few-layer GeS nanosheets with thicknesses of 1.3 ± 0.1 nm [approximately three layers (∼3L)], 3.2 ± 0.2 nm (∼6L), and 4.2 ± 0.3 nm (∼8L) via a typical liquid-phase exfoliation (LPE) method. Based on various experimental characterizations and first-principles calculations, the layer-dependent electronic, transport, and optical properties are investigated. For the few-layer GeS nanosheets, enhanced light absorption in the UV-vis region and superior photoresponse behavior with increasing layer number is observed, while for the thin films above 10 nm, the properties degenerate to the bulk feature. In addition, the as-prepared ultrathin nanosheets manifest great potential in the applications of photoelectrochemical (PEC)-type photodetectors, exhibiting excellent and stable periodic photoresponse behavior under the radiation of white light. The ∼8L GeS-based photodetector exhibits superior performance than the thinner GeS nanosheets (<4 nm), even better as compared to the bulk or film (above 10 nm) counterparts in terms of higher photoresponsivity along with remarkable photodetection performance in the UV-vis region. This work not only provides direct and solid evidence of the layer-number evolutionary band structure, mobility, and optical properties of ultrathin 2D GeS nanosheets but also promotes the foreseeable applications of 2D GeS as energy-related photoelectric devices.

High-performance ultra-violet phototransistors based on CVT-grown high quality SnS2 flakes

van der Waals layered two-dimensional (2D) metal dichalcogenides, such as SnS₂, have garnered great interest owing to their new physics in the ultrathin limit, and become potential candidates for the next-generation electronics and/or optoelectronics fields. Herein, we report high-performance UV photodetectors established on high quality SnS₂ flakes and address the relatively lower photodetection capability of the thinner flakes via a compatible gate-controlling strategy. SnS₂ flakes with different thicknesses were mechanically exfoliated from CVT-grown high-quality 2H-SnS₂ single crystals. The photodetectors fabricated using SnS₂ flakes reveal a desired response performance (R _λ ≈ 112 A W^-1, EQE ≈ 3.7 × 10⁴%, and D* ≈ 1.18 × 10¹¹ Jones) under UV light with a very low power density (0.2 mW cm^-2 @ 365 nm). Specifically, SnS₂ flakes present a positive thickness-dependent photodetection behavior caused by the enhanced light absorption capacity of thicker samples. Fortunately, the responsivity of thin SnS₂ flakes (e.g. ∼15 nm) could be indeed enhanced to ∼140 A W^-1 under a gate bias of +20 V, reaching the performance level of thicker samples without gate bias (e.g. ∼144 A W^-1 for a ∼60 nm flake). Our results offer an efficient way to choose 2D crystals with controllable thicknesses as optimal candidates for desirable optoelectronic devices.

Nb2SiTe4: A Stable Narrow-Gap Two-Dimensional Material with Ambipolar Transport and Mid-Infrared Response

Two-dimensional (2D) materials with narrow band gaps (∼0.3 eV) are of great importance for realizing ambipolar transistors and mid-infrared (MIR) detections. However, most of the 2D materials studied to date have band gaps that are too large. A few of the materials with suitable band gaps are not stable under ambient conditions. In this study, the layered Nb₂SiTe₄ is shown to be a stable 2D material with a band gap of 0.39 eV. Field-effect transistors based on few-layer Nb₂SiTe₄ show ambipolar transport with a similar magnitude of electron and hole current and a high charge-carrier mobility of ∼100 cm² V^-1 s^-1 at room temperature. Optoelectronic measurements of the devices show clear response to an MIR wavelength of 3.1 μm with a high responsivity of ∼0.66 AW^-1. These results establish Nb₂SiTe₄ as a good candidate for ambipolar devices and MIR detection.

Three-Dimensional Optical Anisotropy of Low-Symmetry Layered GeS

In-plane anisotropic two-dimensional (2D) materials, especially black phosphorus and ReS₂, have attracted significant interest recently as they can provide one more dimension to manipulate their physical properties when compared with isotropic 2D materials. As a representative anisotropic 2D material, germanium monosulfide (GeS) has emerged as a new research hot topic in this field because of its unique in-plane anisotropic physical properties. Despite the rapid growing progress in the study of GeS, many of their fundamental optical anisotropies are still absent. Here, we report the three-dimensional (3D) optical anisotropy of GeS from theory to experiment. The 3D optical anisotropic properties including extinction, refraction, absorption, and reflection were systematically investigated through density functional calculations. The anisotropic refraction and reflection of GeS were experimentally verified by polarization-resolved optical microscopy and azimuth-dependent reflectance difference microscopy, respectively. Finally, a GeS-based linear dichroic photodetector was demonstrated with a dichroic ratio of 1.45 because of its polarization sensitive absorption. Our results provide deep insights into the optical anisotropy of GeS, which is important for the further development of GeS-based optoelectronic and optical devices.

Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide van der Waals Nanosheets

Because of their earth-abundant, low-cost, and environmentally benign characteristics, two-dimensional (2D) group IV metal chalcogenides (e.g., SnSe₂) with layered structures have shown great potential in optoelectronic, photovoltaic, and thermoelectric applications. However, the intrinsic motion of excited carriers and their coupling with lattice photons, which fundamentally dictates device operation and optimization, remain yet to be unraveled. Herein, we directly follow the ultrafast carrier and photon dynamics of colloidal SnSe₂ nanosheets in real time using ultrafast transient absorption spectroscopy. We show ∼0.3 ps intervalley relaxation process of photoexcited energetic carriers and ∼3 ps carrier defect trapping process with a long-lived trapped carrier (∼1 ns), highlighting the importance of trapped carriers in optoelectronic devices. In addition, ultrashort laser pulse impulsively drives coherent out-of-plane lattice vibration in SnSe₂, indicating strong electron-phonon coupling in SnSe₂. This strong electron-phonon coupling could impose a fundamental limit on SnSe₂ photovoltaic devices but benefit its thermoelectric applications.

2D Metal Chalcogenides for IR Photodetection

Infrared (IR) photodetectors are finding diverse applications in imaging, information communication, military, etc. 2D metal chalcogenides (2DMCs) have attracted increasing interest in view of their unique structures and extraordinary physical properties. They have demonstrated outstanding IR detection performance including high responsivity and detectivity, high on/off ratio, fast response rate, stable room temperature operability, and good mechanical flexibility, which has opened up a new prospect in next-generation IR photodetectors. This Review presents a comprehensive summary of recent progress in advanced IR photodetectors based on 2DMCs. The rationale of the photodetectors containing photocurrent generation mechanisms and performance parameters are briefly introduced. The device performances of 2DMCs-based IR photodetectors are also systematically summarized, and some representative achievements are highlighted as well. Finally, conclusions and outlooks are delivered as a guideline for this thriving field.

Charge Mobility and Recombination Mechanisms in Tellurium van der Waals Solid

Trigonal tellurium is a small band gap elemental semiconductor consisting of van der Waals bound one-dimensional helical chains of tellurium atoms. We study the temperature dependence of the charge carrier mobility and recombination pathways in bulk tellurium. Electrons and holes are generated by irradiation of the sample with 3 MeV electrons and detected by time-resolved microwave conductivity measurements. A theoretical model is used to explain the experimental observations for different charge densities and temperatures. Our analysis reveals a high room temperature mobility of 190 ± 20 cm² V^-1 s^-1. The mobility is thermally deactivated, suggesting a band-like transport mechanism. According to our analysis, the charges predominantly recombine via radiative recombination with a radiative yield close to 98%, even at room temperature. The remaining charges recombine by either trap-assisted (Shockley-Read-Hall) recombination or undergo trapping to deep traps. The high mobility, near-unity radiative yield, and possibility of large-scale production of atomic wires by liquid exfoliation make Te of high potential for next-generation nanoelectronic and optoelectronic applications, including far-infrared detectors and lasers.

Self-passivated ultra-thin SnS layers via mechanical exfoliation and post-oxidation

Remarkable optical/electrical features are expected in two-dimensional group-IV monochalcogenides (MXs; M = Sn/Ge and X = S/Se) with a uniquely distorted layered structure. The lone pair electrons in the group-IV atoms are the origin of this structural distortion, while they also cause a strong interlayer force and high chemical reactivity. The fabrication of chemically stable few-to-monolayer MX has been a significant challenge. We have observed that, once the SnS surface is oxidized, the SnO_x top layer works as a passivation layer for the SnS layer underneath. In this work, the SnO_x/SnS hetero-structure is studied structurally, optically, and electrically. When tape-exfoliated bulk SnS is oxygen-annealed under a reduced pressure at 10 Pa, surface oxidation and SnS sublimation proceed simultaneously, resulting in a monolayer-thick SnS layer with the SnO_x passivation layer. The field-effect transistor of nine-layer SnS prepared via mechanical exfoliation exhibits a p-type characteristic because of intrinsic Sn vacancies, whereas ambipolar behavior is observed for the monolayer-thick SnS obtained via oxygen annealing probably owing to the additional n-type doping by S vacancies. This work on monolayer-thick SnS fabrication can be applied to other unstable lone pair analogues and can facilitate future research on MXs.

Low-Dimensional Materials and State-of-the-Art Architectures for Infrared Photodetection

Infrared photodetectors are gaining remarkable interest due to their widespread civil and military applications. Low-dimensional materials such as quantum dots, nanowires, and two-dimensional nanolayers are extensively employed for detecting ultraviolet to infrared lights. Moreover, in conjunction with plasmonic nanostructures and plasmonic waveguides, they exhibit appealing performance for practical applications, including sub-wavelength photon confinement, high response time, and functionalities. In this review, we have discussed recent advances and challenges in the prospective infrared photodetectors fabricated by low-dimensional nanostructured materials. In general, this review systematically summarizes the state-of-the-art device architectures, major developments, and future trends in infrared photodetection.

π-Phase Tin and Germanium Monochalcogenide Semiconductors: An Emerging Materials System

Cubic π-phase monochalcogenides (MX, M = Sn, Ge; X = S, Se) are an emerging new class of materials that has recently been discovered. Here, their thermodynamic stability, progress in synthetic routes, properties, and prospective applications are reviewed. The thermodynamic stability is demonstrated through density functional theory total energy and phonon spectra calculations, which show that the π-phase polytype is stable across the monochalcogenide family. To date, only π-phase tin monochalcogenides have been observed experimentally while π-phase Ge-monochalcogenides are predicted to be stable but are yet to be experimentally realized. Various synthetic preparation protocols of π-SnS and π-SnSe are described, focusing on surfactant-assisted nanoparticle synthesis and chemical deposition of thin films from aqueous-bath compositions. These techniques provide materials with different surface energies, which are likely to play a major role in stabilizing the π-phase in nanoscale materials. The properties of this newly discovered family of semiconducting materials are discussed in comparison with their conventional orthorhombic polymorphs. These could benefit a number of photovoltaic and optoelectronic applications since, apart from being cubic, they also possess characteristic advantages, such as moderately low toxicity and natural abundance.