Anisotropic photoresponse properties of single micrometer-sized GeSe nanosheet

Linear Dichroism of the Optical Properties of SnS and SnSe Van der Waals Crystals

Tin monochalcogenides SnS and SnSe, belonging to a family of Van der Waals crystals isoelectronic to black phosphorus, are known as environmentally friendly materials promising for thermoelectric conversion applications. However, they exhibit other desired functionalities, such as intrinsic linear dichroism of the optical and electronic properties originating from strongly anisotropic orthorhombic crystal structures. This property makes them perfect candidates for polarization-sensitive photodetectors working in near-infrared spectral range. A comprehensive study of the SnS and SnSe crystals is presented, performed by means of optical spectroscopy and photoemission spectroscopy, supported by ab initio calculations. The studies reveal the high sensitivity of the optical response of both materials to the incident light polarization, which is interpreted in terms of the electronic band dispersion and orbital composition of the electronic bands, dictating the selection rules. From the photoemission investigation the ionization potential, electron affinity and work function are determined, which are parameters crucial for the design of devices based on semiconductor heterostructures.

Development and challenges of polarization-sensitive photodetectors based on 2D materials

Polarization-sensitive photodetectors based on two-dimensional (2D) materials have garnered significant research attention owing to their distinctive architectures and exceptional photophysical properties. Specifically, anisotropic 2D materials, including semiconductors such as black phosphorus (BP), tellurium (Te), transition metal dichalcogenides (TMDs), and tantalum nickel pentaselenide (Ta2NiSe5), as well as semimetals like 1T'-MoTe2 and PdSe2, and their diverse van der Waals (vdW) heterojunctions, exhibit broad detection spectral ranges and possess inherent functional advantages. To date, numerous polarization-sensitive photodetectors based on 2D materials have been documented. This review initially provides a concise overview of the detection mechanisms and performance metrics of 2D polarization-sensitive photodetectors, which are pivotal for assessing their photodetection capabilities. It then examines the latest advancements in polarization-sensitive photodetectors based on individual 2D materials, 2D vdW heterojunctions, nanoantenna/electrode engineering, and structural strain integrated with 2D structures, encompassing a range of devices from the ultraviolet to infrared bands. However, several challenges persist in developing more comprehensive and functional 2D polarization-sensitive photodetectors. Further research in this area is essential. Ultimately, this review offers insights into the current limitations and challenges in the field and presents general recommendations to propel advancements and guide the progress of 2D polarization-sensitive photodetectors.

Van der Waals GeSe with Strain- and Gate-Tunable Linear Dichroism for Wearable Electronics

The direct detection of light polarization poses a crucial challenge in the field of optoelectronics and photonics. Herein, the tunable linear dichroism (LD) in GeSe-based polarized photodetectors is presented through electronic and structural asymmetry modulation, and demonstrate their application prospects in wearable electronics. An improvement in the dichroic ratio up to 34% is achieved under a gate voltage of 20 V, and the improvement reaches 44% by applying a tensile strain along the zigzag direction. Theoretical calculations reveal that the gate regulation of barrier height between GeSe and Au electrodes is responsible for the electrical-tunable LD, while the anisotropic optical absorption in response to strains leads to the strain-tunable LD. Moreover, flexible GeSe transistors are developed for wearable applications including motion sensors and glucose monitors. This study offers viable approaches for modulating the optical anisotropy of low-dimensional materials and emphasizes the versatility of van der Waals materials for practical applications in wearable electronic devices.

GeSe-embedded metal-oxide double heterojunctions for facilitating self-biased and efficient NIR photodetection

Infrared radiation detection is significantly important in communication, imaging, and sensing fields. Here, we present the integration of germanium selenide (GeSe) with a metal-oxide heterojunction to achieve efficient near-infrared (850 nm) photodetection under zero bias conditions. Nickel oxide (NiO) and silicon (Si) formed a favorable energy band alignment for the efficient separation of photogenerated charge carriers, resulting in a high figure of merits. The additional incorporation of a germanium selenide (GeSe) interlayer between the nickel oxide (NiO) and silicon (Si) heterojunction improved the external responsivity (from 0.22 to 3300 mA W-1), detectivity (from 1.24 × 107 to 20 × 109 Jones), normalized photocurrent to dark current ratio (from 4 × 103 to 3 × 105 W-1), noise equivalent power (from nW to pW), and rise/fall time (from 34/34.5 ms to 14/13 ms). The interlayer introduction of a semiconductor in various heterojunctions can facilitate self-biased and broadband photodetection for widely used optoelectronic applications.

Controlling the Intrinsic Charge Carrier Properties of Two-Dimensional Monochalcogenides (GeSe)

Strong anisotropy exhibited by materials, particularly in their low-dimensional forms, is a highly intriguing characteristic. In this study, we investigate the effects of geometrical potential and thermodynamics on the electronic properties of monolayer monochalcogenide charge carriers. First, the geometrical potential is introduced in a monolayer structure. We discuss the Fermi surface topology of materials and the effects of the geometrical potential on the low energy bands of 2D group-IV monochalcogenides around the Γ-point. Then, the temperature dependence of the carrier mobility of the monolayer is discussed along with predictions for its potential applications as nanomaterials.

Germanium and Tin Precursors for Chalcogenide Materials Containing N-Alkoxy Thioamide Ligands

This study describes the synthesis of germanium and tin complexes Ge(mdpaS)2 (1), Ge(edpaS)2 (2), Ge(bdpaS)2 (3), Ge(empaS)2 (4), Sn(mdpaS)2 (5), Sn(edpaS)2 (6), Sn(bdpaS)2 (7), and Sn(empaS)2 (8) (mdpaSH = (Z)-N-methoxy-2,2-dimethylpropanimidothioic acid; edpaSH = (Z)-N-ethoxy-2,2-dimethylpropanimidothioic acid; bdpaSH = (Z)-N-(tert-butoxy)-2,2-dimethylpropanimidothioic acid; empaSH = (Z)-N-ethoxy-2-methylpropanimidothioic acid), using newly designed N-alkoxy thioamide ligands as precursors for metal chalcogenide materials. All complexes were characterized using various analytical techniques, and the single-crystal structures of complexes 5 and 7 revealed a distorted seesaw geometry in the monomeric SnL2 form. Thermogravimetric (TG) curves showed differences between Ge compounds, which exhibited single-step weight losses, and Sn compounds, which exhibited multistep weight losses. As a result, we suggest that the synthesized complexes 1-8 are potential precursors for group IV metal chalcogenide materials.

Chemically tuned intermediate band states in atomically thin CuxGeSe/SnS quantum material for photovoltaic applications

A new generation of quantum material derived from intercalating zerovalent atoms such as Cu into the intrinsic van der Waals gap at the interface of atomically thin two-dimensional GeSe/SnS heterostructure is designed, and their optoelectronic features are explored for next-generation photovoltaic applications. Advanced ab initio modeling reveals that many-body effects induce intermediate band (IB) states, with subband gaps (~0.78 and 1.26 electron volts) ideal for next-generation solar devices, which promise efficiency greater than the Shockley-Queisser limit of ~32%. The charge carriers across the heterojunction are both energetically and spontaneously spatially confined, reducing nonradiative recombination and boosting quantum efficiency. Using this IB material in a solar cell prototype enhances absorption and carrier generation in the near-infrared to visible light range. Tuning the active layer's thickness increases optical activity at wavelengths greater than 600 nm, achieving ~190% external quantum efficiency over a broad solar wavelength range, underscoring its potential in advanced photovoltaic technology.

Robust bond linkage between boron-based coating layer and lithium polyacrylic acid binder enables ultra-stable micro-sized germanium anodes

Micro-sized alloy type germanium (Ge) anodes possess appealing properties for next-generation lithium ions batteries, such as desirable capacity, easy accessibility and greater tapdensity. Nevertheless, volume expansion accompanied by severe pulverization and continuous growth of solid electrolyte interlayer (SEI) still represent fundamental obstacles to their practical applications. Herein, we propose a fresh strategy of constructing robust bond linkage between boron-based coating layer and lithiated polyacrylic acid (PAALi) binder to circumvent the pulverization problems of Ge anodes. Facile pyrolysis of boric acid can introduce an amorphous boron oxide interphase on Ge microparticles (noted as Ge@B2O3). Then in situ crosslinking reaction between B2O3 and PAALi via BOC bond linkage constructs a robust Ge anode (Ge@B-PAALi), which is proved by FTIR and Raman characterizations. Post morphological and compositional investigations reveal the minimized pulverization and a thinner SEI composition. The robust bond linkage strategy endows Ge anode with ultra-stable cycling properties of 1053.8 mAh/g after 500 cycles at 1 A/g vs. 500.7 mAh/g for Ge@PAALi and 372.7 mAh/g for Ge@B2O3, respectively. The proposed bond linkage strategy via artificial coating layer and functional binders unlocks huge potential of alloys and other anodes for next-generation battery applications.

Designing novel monolayer and multilayer h-CSe crystals with tunable photoelectric properties

Using the first-principles method, a new structure of monolayer h-CSe was predicted, exhibiting good dynamical and thermal stability. The geometrical, electronic and optical properties of monolayer h-CSe are examined at the HSE level. Furthermore, the influences of the in-plane strain and layer number on the electric properties of the two dimensional h-CSe material are studied. The results indicate that it possesses an indirect band gap, which exhibits a rich variety of behaviors depending on the small in-plane biaxial strain. The band gap of monolayer h-CSe could be easily tuned in the energy range from 0.82 eV to 2.61 eV under small in-plane biaxial strain (from -3% to 3%). Also, a band gap transition between direct and indirect types is not found. The band gap of the h-CSe materials decreases with the increase of their layer number. In addition, it was found that these h-CSe materials show excellent optical properties, including strong light harvesting ability for the ultra-violet light range of the solar spectrum. The results obtained here indicate that monolayer h-CSe may have significant potential applications in future nanoelectronic fields.

Preparation of Mixed Few-Layer GeSe Nanosheets with High Efficiency by the Thermal Sublimation Method

Two-dimensional (2D) GeSe has been proven promising in fast and broadband optoelectronic applications for its complicated band structure, inert surface property, and excellent stability. The major challenge is the deficiency of the effective technique for controllably prepared large-scale few-to-monolayer GeSe films. For this purpose, a layer-by-layer thinning method by thermal sublimation for manufacturing large-scale mixed few-layer GeSe with direct bandgaps is proposed, and an optimized sublimation temperature of 300 °C in vacuum is evaluated by atomic force microscopy. Scanning electron microscopy, transmission electron microscopy, energy-dispersive spectra, and fluorescence mapping measurements are performed on the thinned GeSe layers, and results are well-indexed to the orthorhombic lattice structure with direct bandgaps with an atomic ratio of Ge/Se ≈ 5:4. Raman and fluorescence spectra show an α-type crystalline structure of the thinned GeSe films, indicating the pure physical process of the sublimation thinning. Both the bulk and few-layer GeSe films demonstrate broadband absorption. Conductivity of the few-layer GeSe device indicates the overall crystalline integrity of the film after thermal thinning. Given the convenience and efficiency, we provide an effective approach for fabrication of large-scale 2D materials that are difficult to be prepared by traditional methods.

Lateral Integration of SnS and GeSe van der Waals Semiconductors: Interface Formation, Electronic Structure, and Nanoscale Optoelectronics

The emergence of atomically thin crystals has allowed extending materials integration to lateral heterostructures where different 2D materials are covalently connected in the plane. The concept of lateral heterostructures can be generalized to thicker layered crystals, provided that a suitably faceted seed crystal presents edges to which a compatible second van der Waals material can be attached layer by layer. Here, we examine the possibility of integrating multilayer crystals of the group IV monochalcogenides SnS and GeSe, which have the same crystal structure, small lattice mismatch, and similar bandgaps. In a two-step growth process, lateral epitaxy of GeSe on the sidewalls of multilayer SnS flakes (obtained by vapor transport of a SnS₂ precursor on graphite) yields heterostructures of laterally stitched crystalline GeSe and SnS without any detectable vertical overgrowth of the SnS seeds and with sharp lateral interfaces. Combined cathodoluminescence spectroscopy and ab initio calculations show the effects of small band offsets on carrier transport and radiative recombination near the interface. The results demonstrate the possibility of forming atomically connected lateral interfaces across many van der Waals layers, which is promising for manipulating optoelectronics, photonics, and for managing charge- and thermal transport.

Anisotropic Heavy-Metal-Free Semiconductor Nanocrystals: Synthesis, Properties, and Applications

Heavy-metal (Cd, Hg, and Pb)-containing semiconductor nanocrystals (NCs) have been explored widely due to their unique optical and electrical properties. However, the toxicity risks of heavy metals can be a drawback of heavy-metal-containing NCs in some applications. Anisotropic heavy-metal-free semiconductor NCs are desirable replacements and can be realized following the establishment of anisotropic growth mechanisms. These anisotropic heavy-metal-free semiconductor NCs can possess lower toxicity risks, while still exhibiting unique optical and electrical properties originating from both the morphological and compositional anisotropy. As a result, they are promising light-emitting materials in use various applications. In this review, we provide an overview on the syntheses, properties, and applications of anisotropic heavy-metal-free semiconductor NCs. In the first section, we discuss hazards of heavy metals and introduce the typical heavy-metal-containing and heavy-metal-free NCs. In the next section, we discuss anisotropic growth mechanisms, including solution-liquid-solid (SLS), oriented attachment, ripening, templated-assisted growth, and others. We discuss mechanisms leading both to morphological anisotropy and to compositional anisotropy. Examples of morphological anisotropy include growth of nanorods (NRs)/nanowires (NWs), nanotubes, nanoplatelets (NPLs)/nanosheets, nanocubes, and branched structures. Examples of compositional anisotropy, including heterostructures and core/shell structures, are summarized. Third, we provide insights into the properties of anisotropic heavy-metal-free NCs including optical polarization, fast electron transfer, localized surface plasmon resonances (LSPR), and so on, which originate from the NCs' anisotropic morphologies and compositions. Finally, we summarize some applications of anisotropic heavy-metal-free NCs including catalysis, solar cells, photodetectors, lighting-emitting diodes (LEDs), and biological applications. Despite the huge progress on the syntheses and applications of anisotropic heavy-metal-free NCs, some issues still exist in the novel anisotropic heavy-metal-free NCs and the corresponding energy conversion applications. Therefore, we also discuss the challenges of this field and provide possible solutions to tackle these challenges in the future.

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.

Structure-dependent high-TC ferromagnetism in Mn-doped GeSe

Layered IV-VI diluted magnetic semiconductors (DMSs) have exhibited fascinating ferromagnetism down to atomic layers, but their relatively low Curie temperature (T_C, ≤200 K) significantly hinders their practical application. In this work, Mn-doped GeSe (GeMnSe) DMSs with high-T_C ferromagnetism (FM) are synthesized by chemical vapor deposition. As the Mn concentration varies, the obtained samples exhibit various structures including single-crystalline nanocombs (SC-NCs), polycrystalline nanoparticles (PC-NPs) and amorphous nanoaggregates (a-NAs). All the samples exhibit FM, and their T_C and saturation magnetization (M_S) are correlated to their structures. Notably, GeMnSe SC-NCs show a record high T_C of 309 K and a record strong magnetic moment of 4.37μ_B/Mn compared to all the previously-reported IV-VI DMSs. Further analysis shows that the FM originates from the synergetic effect of the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction, the crystalline order and the shape anisotropy in the samples. Our results provide a panorama of nanostructure-dependent FM in GeSe-based DMSs and suggest a peculiar playground for investigating fundamental physics and spintronic applications in IV-VI compounds.

Ultra-mild synthesis of nanometric metal chalcogenides using organyl chalcogenide precursors

Bis(trialkylsilyl) monochalcogenides and diorganyl dichalcogenides, (R₃Si)₂E and R₂E₂ (E = S, Se or Te and R = alkyl, aryl or allyl group), have emerged in the past decade as excellent reagents for the synthesis of metal chalcogenide nanoparticles (NPs) and clusters owing to their ability to transfer the chalcogenide anion (E^2-) under ultra-mild conditions and versatility in reacting even with non-conventional metal reagents or being employed in a variety of synthetic methods. In comparison, the related non-silylated diorganyl monochalcogenides R₂E have received attention only recently for the solution phase synthesis of metal chalcogenide NPs. In spite of sharing many similarities, these three families of organyl chalcogenides are different in their coordination ability and decomposition behavior, and therefore in reactivities towards metal reagents. This feature article provides a concise overview on the use of these three families as synthons for the ultralow-temperature synthesis of metal chalcogenide nanomaterials, deliberating their different decomposition mechanisms and critically assessing their advantages for certain applications. More specifically, it discusses their usefulness in (i) affording molecular precursors with different kinetic and thermal stabilities, (ii) isolating reactive intermediates for comprehending the mechanism of molecule-to-nanoparticle transformation and, therefore, achieving fine control over the synthesis, (iii) stabilizing isolable metastable or difficult-to-achieve phases, and (iv) yielding complex ternary nanoparticles with controlled stoichiometry or composites with sensitive materials without modifying the characteristics of the latter. Besides providing a perspective on the low-temperature synthesis of nanomaterials, this overview is expected to assist further progress, particularly in the field of R₂E, leading to interesting materials including metastable ones for new applications.

Free-standing large, ultrathin germanium selenide van der Waals ribbons by combined vapor-liquid-solid growth and edge attachment

Among group IV monochalcogenides, layered GeSe is of interest for its anisotropic properties, 1.3 eV direct band gap, ferroelectricity, high mobility, and excellent environmental stability. Electronic, optoelectronic and photovoltaic applications depend on the development of synthesis approaches that yield large quantities of crystalline flakes with controllable size and thickness. Here, we demonstrate the growth of single-crystalline GeSe nanoribbons by a vapor-liquid-solid process over Au catalyst on different substrates at low thermal budget. The nanoribbons crystallize in a layered structure, with ribbon axis along the armchair direction of the van der Waals layers. The ribbon morphology is determined by catalyst driven fast longitudinal growth accompanied by lateral expansion via edge-specific incorporation into the basal planes. This combined growth mechanism enables temperature controlled realization of ribbons with typical widths of up to 30 μm and lengths exceeding 100 μm, while maintaining sub-50 nm thickness. Nanoscale cathodoluminescence spectroscopy on individual GeSe nanoribbons demonstrates intense temperature-dependent band-edge emission up to room temperature, with fundamental bandgap and temperature coefficient of E_g(0) = 1.29 eV and α = 3.0 × 10^-4 eV K^-1, respectively, confirming high quality GeSe with low concentration of non-radiative recombination centers promising for optoelectronic applications including light emitters, photodetectors, and solar cells.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

Strain-engineering on GeSe: Raman spectroscopy study

Among the IV-VI compounds, GeSe has wide applications in nanoelectronics due to its unique photoelectric properties and adjustable band gap. Even though modulation of its physical characteristics, including the band gap, by an external field will be useful for designing novel devices, experimental work is still rare. Here, we report a detailed anisotropic Raman response of GeSe flakes under uniaxial tension strain. Based on theoretical analysis, the anisotropy of the phonon response is attributed to a change in anisotropic bond length and bond angle under in-plane uniaxial strain. An enhancement in anisotropy and band gap is found due to strain along the ZZ or AC directions. This study shows that strain-engineering is an effective method for controlling the GeSe lattice, and paves the way for modulating the anisotropic electric and optical properties of GeSe.

High Anisotropic Optoelectronics in Two Dimensional Layered PbSnX2 (X = S/Se)

We systematically study the giant anisotropic optoelectronics in layered PbSnX₂ (X = S/Se). The highly anisotropic optoelectronics mainly originates from the asymmetric sublattices SnX, resulting in the anisotropy of photoelectronic properties with fascinating visible light absorption range in single-layer and bilayer PbSnX₂. We employ uniaxial strain in both the x and y directions and find an indirect-to-direct band gap transition, while the quasiparticle indirect band gap presents excellent linear scaling with biaxial strain in monolayer PbSnX₂. We also demonstrate ultrahigh anisotropic mobilities of electrons (μ_y > μ_x) and holes (μ_x > μ_y) in both single-layer and bilayer PbSnX₂ (X = S/Se), and spin-orbit coupling effects and the increase of layer number significantly reduce exciton binding energies and band gaps. Finally, the strong layer dependence of the band structure is clearly seen when the film thickness is less than 4 layers. Our results provide a fundamental understanding of highly anisotropic PbSnX₂ (X = S/Se) and show two potential candidates in photoelectric applications.

Anisotropic to Isotropic Transition in Monolayer Group-IV Tellurides

Monolayer group-IV tellurides with phosphorene-derived structures are attracting increasing research interest because of their unique properties. Here, we systematically studied the quasiparticle electronic and optical properties of two-dimensional group-IV tellurides (SiTe, GeTe, SnTe, PbTe) using the GW and Bethe-Salpeter equation method. The calculations revealed that all group-IV tellurides are indirect bandgap semiconductors except for monolayer PbTe with a direct gap of 1.742 eV, while all of them are predicted to have prominent carrier transport ability. We further found that the excitonic effect has a significant impact on the optical properties for monolayer group-IV tellurides, and the predicted exciton binding energy is up to 0.598 eV for SiTe. Interestingly, the physical properties of monolayer group-IV tellurides were subject to an increasingly isotropic trend: from SiTe to PbTe, the differences of the calculated quasiparticle band gap, optical gap, and further exciton binding energy along different directions tended to decrease. We demonstrated that these anisotropic electronic and optical properties originate from the structural anisotropy, which in turn is the result of Coulomb repulsion between non-bonding electron pairs. Our theoretical results provide a deeper understanding of the anisotropic properties of group-IV telluride monolayers.

Electric field controlled type-I and type-II conversion of BP/SnS van der Waals heterostructure

Type-I heterostructure, in which electrons and holes are confined in same region, is widely used in light emitting diodes and semiconductor lasers. Type-II heterostructure is widely used in photovoltaic devices because of its excellent spatial separation property of electrons and holes. Can we integrate photovoltaic, photoelectric properties with luminescent property in one device? Here we report a van der Waals heterostructure formed by black phosphorus (BP) and SnS monolayers. It is expected to realize these functions in one device. By first-principles methods, the structural stability, electronic properties and optical properties are investigated. It was found that the BP/SnS bilayer is type-II heterostructure with an indirect bandgap of 0.56 eV. Thep-like character of the band edge in BP/SnS vdW heterostructure makes it to be an excellent optoelectronic material. The type-II stability of the system can be improved by applying a negative electric field. However, when the positive electric field is bigger than 0.1 V Å^-1, the system begins to transform from type-II to type I. Therefore, by adding a gate voltage the bandgap and band alignment of this system can be controlled. The photovoltaic and photoelectric properties can be integrated in one device based on this heterostructure.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Extrinsic Photoconduction Induced Short-Wavelength Infrared Photodetectors Based on Ge-Based Chalcogenides

2D layered photodetectors have been widely researched for intriguing optoelectronic properties but their application fields are limited by the bandgap. Extending the detection waveband can significantly enrich functionalities and applications of photodetectors. For example, after breaking through bandgap limitation, extrinsic Si photodetectors are used for short-wavelength infrared or even long-wavelength infrared detection. Utilizing extrinsic photoconduction to extend the detection waveband of 2D layered photodetectors is attractive and desirable. However, extrinsic photoconduction has yet not been observed in 2D layered materials. Here, extrinsic photoconduction-induced short-wavelength infrared photodetectors based on Ge-based chalcogenides are reported for the first time and the effectiveness of intrinsic point defects are demonstrated. The detection waveband of room-temperature extrinsic GeSe photodetectors with the assistance of Ge vacancies is broadened to 1.6 µm. Extrinsic GeSe photodetectors have an excellent external quantum efficiency (0.5%) at the communication band of 1.31 µm and polarization-resolved capability to subwaveband radiation. Moreover, room-temperature extrinsic GeS photodetectors with a detection waveband to the communication band of 1.55 µm further verify the versatility of intrinsic point defects. This approach provides design strategies to enrich the functionalities of 2D layered photodetectors.

Modulation of the electronic properties and photocatalytic performance of black phase monolayer GeSe by noble metal doping

Possible doping positions of noble metal atoms on the surface of monolayer GeSe.

Recent Advances in 2D Metal Monochalcogenides

The family of emerging low-symmetry and structural in-plane anisotropic two-dimensional (2D) materials has been expanding rapidly in recent years. As an important emerging anisotropic 2D material, the black phosphorene analog group IVA-VI metal monochalcogenides (MMCs) have been surged recently due to their distinctive crystalline symmetries, exotic in-plane anisotropic electronic and optical response, earth abundance, and environmentally friendly characteristics. In this article, the recent research advancements in the field of anisotropic 2D MMCs are reviewed. At first, the unique wavy crystal structures together with the optical and electronic properties of such materials are discussed. The Review continues with the various methods adopted for the synthesis of layered MMCs including micromechanical and liquid phase exfoliation as well as physical vapor deposition. The last part of the article focuses on the application of the structural anisotropic response of 2D MMCs in field effect transistors, photovoltaic cells nonlinear optics, and valleytronic devices. Besides presenting the significant research in the field of this emerging class of 2D materials, this Review also delineates the existing limitations and discusses emerging possibilities and future prospects.

Liquid-Phase Exfoliated GeSe Nanoflakes for Photoelectrochemical-Type Photodetectors and Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW-1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV-Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.

Tunable anisotropic plasmon response of monolayer GeSe nanoribbon arrays

Recently, emerging two-dimensional (2D) germanium selenide (GeSe) has drawn lots of attention due to its in-plane anisotropic properties and great potential for optoelectronic applications such as in solar cells. However, methods are still sought to enhance its interaction with light to enable practical applications. Herein, we numerically investigate the localized plasmon response of monolayer GeSe nanoribbon arrays systematically, and the results show that localized surface plasmon polaritons in the far-infrared range with anisotropic behavior can be efficiently excited to enhance the light-matter interaction. We further show that the plasmon response of monolayer GeSe nanoribbons could be tuned effectively through the nanoribbon width, local refractive index, substrate thickness and carrier concentration, pointing out the ways for controlling the localized plasmon response. In the case of monolayer GeSe nanoribbons on a substrate of finite thickness, a Fabry-Pérot-like (FP-like) quantitative model has been proposed to explain the overall spectral response originating from overlapped FP and plasmon modes, and it matches well with the simulation results. All in all, we investigate the plasmon response of the novel 2D GeSe nanoribbons thoroughly for the first time, bringing opportunities for potential applications of novel polarization-dependent optoelectronic devices.

Few-layer metal monochalcogenide saturable absorbers for high-energy Q-switched pulse generation

Two-dimensional layered materials have been widely utilized as nonlinear absorption materials to transfer continue-wave into pulse trains in fiber laser systems. Here, we prepare robust GaSe/GeSe composites with high power bearing capacity as saturable absorbers (SAs) and then investigate their nonlinear optical properties via broadband Z-scan measurement at 800 nm and 1550 nm, respectively. The modulation depths of GaSe/GeSe based SAs are measured to be 11.97% and 7.69% at 1550 nm. After incorporating the GaSe/GeSe SAs into an Erbium-doped fiber laser cavity, passively Q-switched pulse trains could be obtained with repetition rates changing from 83.58 to 136.78 kHz (70.41 to 161.65 kHz). The maximum output power and pulse energy are 52.1 mW/370.67 nJ (GaSe) and 21.6 mW/133.74 nJ (GeSe) under the maximum pump power of 600 mW. The results indicate that GaSe and GeSe possess outstanding thermal stability and could be employed as remarkable saturable absorption materials for high-energy pulses generation.

Fast and Stable Batteries with High Capacity Enabled by Germanium-Phosphorus Binary Nanoparticles Embedded in a Porous Carbon Matrix via Metallothermic Reduction

Lithium-alloyable materials such as Ge and P have attracted considerable attention as promising anode materials for lithium-ion batteries (LIBs) owing to their high theoretical capacity. However, these materials inevitably undergo capacity attenuation caused by large volume expansion in repeated electrochemical processes. Herein, we propose a facile strategy to synthesize germanium-phosphorus binary nanoparticles embedded in porous carbon (GPBN/C) via metallothermic reduction. As an LIB anode, the GPBN/C electrode exhibits outstanding rate performance (368 mAh g^-1 at 40 A g^-1) and remarkable long-term cycling ability (541 mAh g^-1 at 1.0 A g^-1 after 1000 cycles). Besides, the GPBN/C composite electrode presents an outstanding cycling performance at wide temperature ranges, showing reversible capacities of 1030 and 696 mAh g^-1 at 60 and 0 °C, respectively. Attributed to the formation of highly dispersed Ge-P nanoparticles in a porous carbon matrix, the GPBN/C electrode shows exceptional electrochemical performance. Importantly, our strategy provides an effective way to explore alloy-type electrodes to develop fast and stable high-capacity batteries.

Synthesis and photoluminescence of high density GeSe triangular nanoplate arrays on Si substrates

We have grown germanium selenide (GeSe) triangular nanoplate arrays (TNAs) with a high density (3.82 × 10⁶ mm^-2) on the Si (111) substrate using a simple thermal evaporation method. The thickness and trilateral lengths of a single triangular nanoplate were statistically estimated by atomic force microscopy as 44 nm, 365 nm, 458 nm and 605 nm, respectively. Transmission electron microscopy (TEM) images and x-ray diffraction patterns show that the TNAs were composed of single crystalline GeSe phase. The Se-related defects in the lattice were also revealed by TEM images and Raman vibration modes. Unlike previously reported GeSe compounds, the GeSe TNAs exhibited temperature-dependent photoluminescence (PL). The PL peak (1.25 eV) of the TNAs at 5 K was in the gaps between those of GeSe monolayers and a few hundred thick films, revealing a close relationship between the PL peak and the thickness of GeSe. The high-density structure and temperature-dependent PL of the TNAs on the Si substrate may be useful for temperature controllable semiconductor nanodevices.

Visible-NIR Photodetectors Based on Low-Dimensional GeSe Micro-Crystals: Designed Morphology and Improved Photoresponsivity

GeSe micro-sheets and micro-belts have been synthesized by a facile one-pot wet chemical method in 1-octadecene solvent and oleic acid solvent, respectively. The adsorption of more oleic acid molecules on the (002) plane promoted growth along [010] direction of the GeSe micro-belts and limited carrier transport in this direction, resulting in higher carrier concentration and mobility of the GeSe micro-belts. The performance of the photodetectors based on the single GeSe micro-sheet and the single GeSe micro-belt was investigated under illumination at 532 nm, 980 nm and 1319 nm. Both, photodetectors based on a single GeSe micro-sheet and a single GeSe micro-belt, exhibit a high photoresponse, short response/recovery times, and long-term durability. Moreover, the photodetector based on a single GeSe micro-belt displays a broadband response with a high responsivity (5562 A/W at 532 nm, 1546 A/W at 980 nm) and detectivity (3.01×10¹² Jones at 532 nm, 8.38×10¹¹ Jones at 980 nm). These excellent characteristics render single GeSe micro-belts very interesting for use as highly efficient photodetectors, especially in the NIR region.

One-dimensional flat bands in twisted bilayer germanium selenide

Experimental advances in the fabrication and characterization of few-layer materials stacked at a relative twist of small angle have recently shown the emergence of flat energy bands. As a consequence electron interactions become relevant, providing inroads into the physics of strongly correlated two-dimensional systems. Here, we demonstrate by combining large scale ab initio simulations with numerically exact strong correlation approaches that an effective one-dimensional system emerges upon stacking two twisted sheets of GeSe, in marked contrast to all moiré systems studied so far. This not only allows to study the necessarily collective nature of excitations in one dimension, but can also serve as a promising platform to scrutinize the crossover from two to one dimension in a controlled setup by varying the twist angle, which provides an intriguing benchmark with respect to theory. We thus establish twisted bilayer GeSe as an intriguing inroad into the strongly correlated physics of lowdimensional systems.

Nanowire Grid Polarization and Polarized Excitonic Emission Observed in Multilayer GaTe

In this study, near-infrared (NIR) x-polarized (P-state) light is created from the transmission ray of monoclinic multilayer GaTe near the band edge. The P-state transmittance photons are produced via the transmission light of a ribbonlike multilayer GaTe with a plurality of nanowire grids being parallel and constructed along the y direction (b axis) from 1.56 to 1.62 eV. At 300 K, a P-state excitonic emission at 1.652 eV can be clearly detected in polarized microphotoluminescence (μPL) measurement. The free-exciton extinction energy and recombination lifetime of the band-edge exciton are evaluated and determined to be ΔE = 32 ± 4 meV and τ ≈ 0.032 ns, respectively, for the multilayer GaTe. Polarized microthermoreflectance (μTR) measurement also verifies that the x-polarized transition is allowed while y-polarized (S-state) transition is forbidden in the multilayer GaTe. An asymmetric p-to-p transition along the x polarization is thus inferred to comprise the band edge of multilayer GaTe to form in-plane optical anisotropy.

Synthesis of low-symmetry 2D Ge(1-x)SnxSe2 alloy flakes with anisotropic optical response and birefringence

Low-symmetry two-dimensional (2D) materials with unique in-plane anisotropy can promote both fundamental science and practical applications in optics, optoelectronics, electronics, and polarization detection. As a member of 2D materials, doping/alloying material systems have gained great attention owing to the tunable bandgap and special properties. However, the in-plane anisotropic optical and electrical properties of these 2D alloy materials have rarely been reported. In this work, low-symmetry 2D Ge_(1-x)Sn_xSe₂ (x = 0-1.0) alloy flakes have been synthesized by chemical vapor deposition (CVD) with a bandgap varying from 1.55 eV (GeSe₂, x = 0) to 1.90 eV (SnSe₂, x = 1.0). Angle-resolved polarized Raman spectroscopy (ARPRS) is used to confirm the in-plane vibrational anisotropy, and azimuth-dependent reflectance difference microscopy (ADRDM) is applied to visualize the in-plane optical anisotropy. Polarization-dependent transmission spectroscopy (PDTS) is carried out to reflect the in-plane absorptional anisotropy and linear dichroism, and birefringence characteristics are also found in the subsequent studies. All of the results indicate the unique in-plane optical anisotropy and birefringence characteristics of the 2D Ge_(1-x)Sn_xSe₂ alloy flakes, providing new opportunities for polarization-controlled devices, optical wave plates, and polarizers.

Direction and strain controlled anisotropic transport behaviors of 2D GeSe-phosphorene vdW heterojunctions

Vertical van der Waals (vdW) heterostructures made up of two or more 2D monolayer materials provide new opportunities for 2D devices. Herein, we study the electronic transport properties of vertical integration of 2D GeSe-phosphorene(GeSe-BP) heterostructure, using the nonequilibrium Green's function formalism combined with the density-functional theory. The results reveal that the directional dependency and strain tunable transport anisotropic behavior appears in GeSe/BP-stacking vdW heterostructures. The current-voltage (I-V) characteristics indicate that the electric current propagates more easily through the perpendicular buckled direction (Y) than the linear atomic chain direction (X) in the low bias regime regardless of the GeSe-BP stacking, which is supported by the underlying electronic structures along Γ-Y and Γ-X lines. The anisotropic transmission spectra indicate an over 10⁵ on/off ratio between the I _Y and I _X in GeSe-BP systems. This anisotropic transmission behavior of 2D GeSe-BP heterojunction is regardless of the considered layer distances. The similar situation can also be found in the I-V characteristics of GeSe-BP nano-device after applying a strain, and a charming behavior that the transport gap can be minished obviously when applied a compressed strain on the perpendicular y-direction or the stretched strain on the x-direction. Moreover, an intriguing semiconductor-metal transition induces by applying the in-plain strain along the y-direction. These results imply that the GeSe-BP nanojunctions may be a promising application in futuristic nano-switching materials.

Fast and Broadband Photoresponse of a Few-Layer GeSe Field-Effect Transistor with Direct Band Gaps

Few-to-monolayer germanium selenide, a new IV-VI group layered material recently fabricated by mechanical exfoliation and subsequent laser thinning, is promising in very fast and broadband optoelectronic applications for its excellent stability, complicated band structures, inert surface properties, and being a natural p-type semiconductor. However, large-scale production of such few-layer GeSe devices with superior performance is still in early stages. In this study, field-effect transistors made of few-layer GeSe with direct band gaps are fabricated. Transistor performance with Schottky contact characteristics is measured at room temperature. A field-effect mobility of 4 cm²/(V s) and drain currents modulated both by holes and electrons are measured. Photoresponses as a function of illumination wavelength, power, and frequency are characterized. The few-layer GeSe transistor shows photoresponse to the illumination wavelengths from visible up to 1400 nm and a photoresponse rise (fall) time of 13 μs (19 μs), demonstrating very broadband and fast detection. The ambipolar behavior and the photoresponse characteristics demonstrate great potential of few-layer GeSe for applications in highly stable, very fast, and very broadband optoelectronic devices.

Polarimetric Three-Dimensional Topological Insulators/Organics Thin Film Heterojunction Photodetectors

As a state of quantum matter with insulating bulk and gapless surface states, topological insulators (TIs) have huge potential in optoelectronic devices. On the other hand, polarization resolution photoelectric devices based on anisotropic materials have overwhelming advantages in practical applications. In this work, the 3D TIs Bi₂Te₃/organics thin film heterojunction polarimetric photodetectors with high anisotropic mobility ratio, fast response time, high responsivity, and EQE in broadband spectra are presented. At first, the maximum anisotropic mobility ratio of the Bi₂Te₃/organics thin film can reach 2.56, which proves that Bi₂Te₃ can serve as a sensitive material for manufacturing polarization photoelectric devices. Moreover, it is found that the device can exhibit a broad bandwidth and ultrahigh response photocurrent from visible to middle wave infrared spectra (405-3500 nm). The highest responsivity (R_i) of optimized devices can reach up to 23.54 AW^-1; surprisingly, the R_i of the device can still reach 1.93 AW^-1 at 3500 nm. In addition, the ultrahigh external quantum efficiency is 4534% with a fast response time (1.42 ms). Excellent properties mentioned above indicate that TIs/organics heterojunction devices are suitable for manufacturing high-performance photoelectric devices in infrared region.

Linear Dichroism Conversion in Quasi-1D Perovskite Chalcogenide

Anisotropic photonic materials with linear dichroism are crucial components in many sensing, imaging, and communication applications. Such materials play an important role as polarizers, filters, and waveplates in photonic devices and circuits. Conventional crystalline materials with optical anisotropy typically show unidirectional linear dichroism over a broad wavelength range. The linear dichroism conversion phenomenon has not been observed in crystalline materials. The investigation of the unique linear dichroism conversion phenomenon in quasi-1D hexagonal perovskite chalcogenide BaTiS₃ is reported. This material shows a record level of optical anisotropy within the visible wavelength range. In contrast to conventional anisotropic optical materials, the linear dichroism polarity in BaTiS₃ makes an orthogonal change at an optical wavelength corresponding to the photon energy of 1.78 eV. First-principles calculations reveal that this anomalous linear dichroism conversion behavior originates from the different selection rules of the parallel energy bands in the BaTiS₃ material. Wavelength-dependent polarized Raman spectroscopy further confirms this phenomenon. Such a material, with linear dichroism conversion properties, could facilitate the sensing and control of the energy and polarization of light, and lead to novel photonic devices such as polarization-wavelength selective detectors and lasers for multispectral imaging, sensing, and optical communication applications.

Kerr Nonlinearity in germanium selenide nanoflakes measured by Z-scan and spatial self-phase modulation techniques and its applications in all-optical information conversion

Germanium selenide (GeSe) has attracted considerable research interest due to its unique photoelectric characteristics: high abundance occurrence, low toxicity, high stability, and environmentally sustainable. To the best of our knowledge, less literature is available on the nonlinear optical (NLO) properties of GeSe and on its significance of the electronic structure. In this work, the GeSe nanoflake ethanol suspensions have been studied by using liquid phase exfoliation method and then characterized by Raman, transmission electron microscopy (TEM), transmittance and atomic force microscopy (AFM). The NLO properties of GeSe suspensions with different concentration are investigated by Z-scan and spatial self-phase modulation (SSPM) methods with continuous wave laser, which are coherent with the parameter nonlinear refractive index n2 and the third order nonlinear polarizabilities χ⁽³⁾. The nonlinear refractive index n2 of GeSe dispersions basically occur in the order of 10^-9 cm²/W for Z-scan methods and 10^-6 cm²/W for SSPM technique, whereas the third-order nonlinear polarizabilities χ⁽³⁾ _total are within the range of 10^-6 esu for SSPM method. On the basis of these substantial characteristics of the NLO response and high stability of the 2D GeSe, we have experimentally studied the applications of the GeSe suspensions on all-optical information conversion technique.

Salt-Assisted Growth of Ultrathin GeSe Rectangular Flakes for Phototransistors with Ultrahigh Responsivity

Two-dimensional (2D) GeSe is an important IVA-VIA semiconductor for future applications in electronics and optoelectronics because of its high absorption coefficient, mobility, and photoresponsivity. However, the controllable synthesis of 2D GeSe flakes is still a huge problem. Here, high-quality single-crystalline ultrathin 2D GeSe flakes are synthesized by a salt-assisted chemical vapor deposition method. The flakes tend to grow along the [010] crystal orientation presenting a rectangular shape with a thickness down to 5 nm. Then, the electrical and optoelectronic properties have been systematically investigated. A thickness-dependent Schottky barrier is shown in GeSe field-effect transistors. The p-type conductivity of GeSe is mainly caused by the Ge deficiency, which is proven by a variable-temperature experiment and theoretical calculations. In addition, the phototransistors based on as-grown GeSe flakes present an ultrahigh responsivity of 1.8 × 10⁴ A/W and an excellent external quantum efficiency of 4.2 × 10⁶%.

Highly oriented GeSe thin film: self-assembly growth via the sandwiching post-annealing treatment and its solar cell performance

GeSe is considered as a potential absorber material for thin film solar cells owing to its ideal band gap, strong light absorption, remarkable air durability, Earth-abundance and non-toxic constituents. However, the high vapor pressure of GeSe at a temperature below its melting point makes it difficult to synthesize a high-quality GeSe film. To alleviate this limitation, in this work, a thermal evaporation combining a novel sandwiching post-annealing method was introduced to deposit high quality GeSe thin films with (100)-orientation. The self-assembling mechanism of the highly oriented GeSe film was carefully investigated by the systematic experiments and confirmed by the lowest total energy of the (100) crystal plane. Finally, the fully-inorganic, low-cost and non-toxic planar device with the superstrate configuration of FTO/TiO2/GeSe/carbon/Ag was also successfully fabricated. Notably, as a result, an impressive open circuit voltage (VOC) of 340 mV (maximum: 456 mV) was achieved, which is the highest VOC of GeSe solar cells reported so far. Furthermore, through current-voltage, capacitance-voltage profiling and drive level capacitance profiling measurements, it was demonstrated that the limiting factors of the GeSe solar cell performance were the narrow depletion width (138 nm) and the drastic recombination at the TiO2/GeSe interface.

Elastic Anisotropy and Optic Isotropy in Black Phosphorene/Transition-Metal Trisulfide van der Waals Heterostructures

Anisotropic two-dimensional materials with direction-dependent mechanical and optical properties have attracted significant attention in recent years. In this work, based on density functional theory calculations, unexpected elastic anisotropy and optical isotropy in van der Waals (vdW) heterostructures have been theoretically proposed by assembling the well-known anisotropic black phosphorene (BP) and transition-metal trisulfides MS₃ (M = Ti, Hf) together. It is interesting to see that the BP/MS₃ vdW heterostructures show anisotropic flexibility in different directions according to the elastic constants, Young's modulus, and Poisson's ratio. We have further unraveled their physical origin of the type-II band structure nature with their conduction band minimum and valence band maximum separated in different layers. In particular, our results on the optical response functions including the excitonic effects of the BP/MS₃ vdW heterostructures suggest their unexpected optical isotropies together with the enhancements of the solar energy conversion efficiency.

Anisotropic Photoresponse of the Ultrathin GeSe Nanoplates Grown by Rapid Physical Vapor Deposition

Anisotropic materials, especially two-dimensional (2D) layered materials formed by van der Waals force (vdW) with low-symmetry, have become a scientific hot-spot because their electrical, optical, and thermoelectric properties are highly polarization dependent. The 2D GeSe, a typical anisotropic-layered orthorhombic structure and narrow bandgap (1.1-1.2 eV) semiconductor, potentially meets these demands. In this report, the ultrathin elongated hexagonal GeSe nanoplates were successfully synthesized by the rapid physical vapor deposition method developed here. The ultrathin elongated hexagonal GeSe nanoplates have a zigzag edge in the long edge and an armchair edge in the short edge. In addition, the typical Raman mode exhibited 90° periodic vibration, having its maximum intensity between the zigzag direction or the zigzag and armchair direction, indicating an anisotropic electron-phonon interaction. Furthermore, the field effect transistor devices based on the elongated hexagonal GeSe nanoplates were constructed and exhibited the p-type semiconducting behavior with a high photoresponse characteriscs. Finally, the polarized sensitive photocurrent was identified, further revealing the intrinsically anisotropy of the GeSe nanoplate. The results illustrated here may give a useful guidance to synthesize the 2D-layered anisotropic nanomaterials and further advance the development of the polarized photodetector.

Enhancement of photoluminescence efficiency in GeSe ultrathin slab by thermal treatment and annealing: experiment and first-principles molecular dynamics simulations

The effect of thermal treatment and annealing under different temperatures from 100 °C to 250 °C on the photoluminescence spectroscopy of the GeSe ultrathin slab is reported. After the thermal treatment and annealing under 200 °C, we found that the photoluminescence intensity of A exciton and B exciton in GeSe ultrathin slab is increased to twice as much as that in untreated case, while is increased by ~84% in the photoluminescence intensity of C exciton. Combined by our experimental work and theoretical simulations, our study confirms the significant role of thermal treatments and annealing in reducing surface roughness and removing the Se vacancy to form more compact and smoother regions in GeSe ultrathin slab. Our findings imply that the improved quality of GeSe surface after thermal treatments is an important factor for the photoluminescence enhancement.

Contact and Noncontact Measurement of Electronic Transport in Individual 2D SnS Colloidal Semiconductor Nanocrystals

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made toward the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products' inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multipoint contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.