Large anisotropy of electrical properties in layer-structured In2Se3 nanowires

Simultaneous Characterization of In-Plane and Cross-Plane Resistivities in Highly Anisotropic 2D Layered Heterostructures

Understanding and characterizing the intrinsic properties of charge carrier transport across the interfaces in van der Waals heterostructures is critical to their applications in modern electronics, thermoelectrics, and optoelectronics. However, there are very few published cross-plane resistivity measurements of thin samples because these inherently 2-probe measurements must be corrected for contact and lead resistances. Here, we present a method to extract contact resistances and metal lead resistances by fitting the width dependence of the contact end voltages of top and bottom electrodes of different contact widths to a model based on current crowding. These contributions are then subtracted from the total 2-probe cross-plane resistance to obtain the cross-plane resistance of the material itself without needing multiple devices and/or etching steps. This approach was used to measure cross-plane resistivities of a (PbSe)1(VSe2)1 heterostructure containing alternating layers of PbSe and VSe2 with random in-plane rotational disorder. Several samples measured exhibited a 4 order of magnitude difference between cross-plane and in-plane resistivities over the 6-300 K temperature range. We also reported the observation of charge density wave transition in the cross-plane transport of the (PbSe)1(VSe2)1 heterostructure. The device fabrication process is fully liftoff compatible, and the method developed enables the straightforward measurement of the resistivity anisotropy of most thin film materials with nm thicknesses.

Characteristic Plasmon Energies for 2D In2Se3 Phase Identification at Nanoscale

Two-dimensional (2D) materials with competing polymorphs offer remarkable potential to switch the associated 2D functionalities for novel device applications. Probing their phase transition and competition mechanisms requires nanoscale characterization techniques that can sensitively detect the nucleation of secondary phases down to single-layer thickness. Here we demonstrate nanoscale phase identification on 2D In2Se3 polymorphs, utilizing their distinct plasmon energies that can be distinguished by electron energy-loss spectroscopy (EELS). The characteristic plasmon energies of In2Se3 polymorphs have been validated by first-principles calculations, and also been successfully applied to reveal phase transitions using in situ EELS. Correlating with in situ X-ray diffraction, we further derive a subtle difference in the valence electron density of In2Se3 polymorphs, consistent with their disparate electronic properties. The nanometer resolution and independence of orientation make plasmon-energy mapping a versatile technique for nanoscale phase identification on 2D materials.

Structural Anisotropy-Driven Atomic Mechanisms of Phase Transformations in the Pt-Sn System

Using in situ atomic-resolution scanning transmission electron microscopy, atomic movements and rearrangements associated with diffusive solid to solid phase transformations in the Pt-Sn system are captured to reveal details of the underlying atomistic mechanisms that drive these transformations. In the PtSn4 to PtSn2 phase transformation, a periodic superlattice substructure and a unique intermediate structure precede the nucleation and growth of the PtSn2 phase. At the atomic level, all stages of the transformation are templated by the anisotropic crystal structure of the parent PtSn4 phase. In the case of the PtSn2 to Pt2Sn3 transformation, the anisotropy in the structure of product Pt2Sn3 dictates the path of transformation. Analysis of atomic configurations at the transformation front elucidates the diffusion pathways and lattice distortions required for these phase transformations. Comparison of multiple Pt-Sn phase transformations reveals the structural parameters governing solid to solid phase transformations in this technologically interesting intermetallic system.

The Processing and Electrical Properties of Isotactic Polypropylene/Copper Nanowire Composites

Polypropylene (PP), a promising engineering thermoplastic, possesses the advantages of light weight, chemical resistance, and flexible processability, yet preserving insulative properties. For the rising demand for cost-effective electronic devices and system hardware protections, these applications require the proper conductive properties of PP, which can be easily modified. This study investigates the thermal and electrical properties of isotactic polypropylene/copper nanowires (i-PP/CuNWs). The CuNWs were harvested by chemical reduction of CuCl2 using a reducing agent of glucose, capping agent of hexadecylamine (HDA), and surfactant of PEG-7 glyceryl cocoate. Their morphology, light absorbance, and solution homogeneity were investigated by SEM, UV-visible spectrophotometry, and optical microscopy. The averaged diameters and the length of the CuNWs were 66.4 ± 16.1 nm and 32.4 ± 11.8 µm, respectively. The estimated aspect ratio (L/D, length-to-diameter) was 488 ± 215 which can be recognized as 1-D nanomaterials. Conductive i-PP/CuNWs composites were prepared by solution blending using p-xylene, then melt blending. The thermal analysis and morphology of CuNWs were characterized by DSC, polarized optical microscopy (POM), and SEM, respectively. The melting temperature decreased, but the crystallization temperature increasing of i-PP/CuNWs composites were observed when increasing the content of CuNWs by the melt blending process. The WAXD data reveal the coexistence of Cu2O and Cu in melt-blended i-PP/CuNWs composites. The fit of the electrical volume resistivity (ρ) with the modified power law equation: ρ = ρo (V − Vc)−t based on the percolation theory was used to find the percolation concentration. A low percolation threshold value of 0.237 vol% and high critical exponent t of 2.96 for i-PP/CuNWs composites were obtained. The volume resistivity for i-PP/CuNWs composite was 1.57 × 107 Ω-cm at 1 vol% of CuNWs as a potential candidate for future conductive materials.

Low-Dimensional In2Se3 Compounds: From Material Preparations to Device Applications

Nanostructured In₂Se₃ compounds have been widely used in electronics, optoelectronics, and thermoelectrics. Recently, the revelation of ferroelectricity in low-dimensional (low-D) In₂Se₃ has caused a new upsurge of scientific interest in nanostructured In₂Se₃ and advanced functional devices. The ferroelectric, thermoelectric, and optoelectronic properties of In₂Se₃ are highly correlated with the crystal structure. In this review, we summarize the crystal structures and electronic band structures of the widely interested members of the In₂Se₃ compound family. Recent achievements in the preparation of low-D In₂Se₃ with controlled phases are discussed in detail. General principles for obtaining pure-phased In₂Se₃ nanostructures are described. The excellent ferroelectric, optoelectronic, and thermoelectric properties having been demonstrated using nanostructured and heterostructured In₂Se₃ with different phases are also summarized. Progress and challenges on the applications of In₂Se₃ nanostructures in nonvolatile memories, photodetectors, gas sensors, strain sensors, and photovoltaics are discussed in detail. In the last part of this review, perspectives on the challenges and opportunities in the preparation and applications of In₂Se₃ materials are presented.

Ultrathin Twisted Germanium Sulfide van der Waals Nanowires by Bismuth Catalyzed Vapor-Liquid-Solid Growth

1D nanowires of 2D layered crystals are emerging nanostructures synthesized by combining van der Waals (vdW) epitaxy and vapor-liquid-solid (VLS) growth. Nanowires of the group IV monochalcogenide germanium sulfide (GeS) are of particular interest for twistronics due to axial screw dislocations giving rise to Eshelby twist and precision interlayer twist at helical vdW interfaces. Ultrathin vdW nanowires have not been realized, and it is not clear if confining layered crystals into extremely thin wires is even possible. If axial screw dislocations are still stable, ultrathin vdW nanowires can reach large twists and should display significant quantum confinement. Here it is shown that VLS growth over Bi catalysts yields vdW nanowires down to ≈15 nm diameter while maintaining tens of µm length. Combined electron microscopy and diffraction demonstrate that ultrathin GeS nanowires crystallize in the orthorhombic bulk structure but can realize nonequilibrium stacking that may lead to 1D ferroelectricity. Ultrathin nanowires carry screw dislocations, remain chiral, and achieve very high twist rates. Whenever the dislocation extends to the nanowire tip, it continues into the Bi catalyst. Eshelby twist analysis demonstrates that the ultrathin nanowires follow continuum predictions. Cathodoluminescence on individual nanowires, finally, shows pronounced emission blue shifts consistent with quantum confinement.

Distinct ultrafast carrier dynamics of α-In2Se3 and β-In2Se3: contributions from band filling and bandgap renormalization

As an intrigued layered 2D semiconductor material, indium selenide (In₂Se₃) has attracted widespread attention due to its excellent properties. So far, the carrier dynamics of α-In₂Se₃ and β-In₂Se₃ are still lacking a comprehensive understanding, which is essential to enhancing the performance of In₂Se₃-based optoelectronic devices. In this study, we explored the ultrafast carrier dynamics in thin α-In₂Se₃ and β-In₂Se₃via transient absorption microscopy. For α-In₂Se₃ with a narrower bandgap, band filling and bandgap renormalization jointly governed the time evolution of the differential reflectivity signal, whose magnitude and sign at different delays were determined by the weights between the band filling and bandgap renormalization, depending on the carrier density. For β-In₂Se₃, whose bandgap is close to the probe photon energy, only positive differential reflectivity was detected, which was attributed to strong band filling effect. In both materials, the lifetime decreased and the relative amplitude of the Auger process increased, when the pump fluence was increased. These findings could provide further insights into the optical and optoelectronic properties of In₂Se₃-based devices.

Uncovering Thermal and Electrical Properties of Sb2Te3/GeTe Superlattice Films

Superlattice-like phase change memory (SL-PCM) promises lower switching current than conventional PCM based on Ge₂Sb₂Te₅ (GST); however, a fundamental understanding of SL-PCM requires detailed characterization of the interfaces within such an SL. Here we explore the electrical and thermal transport of SLs with deposited Sb₂Te₃ and GeTe alternating layers of various thicknesses. We find up to an approximately four-fold reduction of the effective cross-plane thermal conductivity of the SL stack (as-deposited polycrystalline) compared with polycrystalline GST (as-deposited amorphous and later annealed) due to the thermal interface resistances within the SL. Thermal measurements with varying periods of our SLs show a signature of phonon coherence with a transition from wave-like to particle-like phonon transport, further described by our modeling. Electrical resistivity measurements of such SLs reveal strong anisotropy (∼2000×) between the in-plane and cross-plane directions due to the weakly interacting van der Waals-like gaps. This work uncovers electrothermal transport in SLs based on Sb₂Te₃ and GeTe for the improved design of low-power PCM.

Correlated and in-situ electrical transmission electron microscopy studies and related membrane-chip fabrication

Understanding the interplay between the structure, composition and opto-electronic properties of semiconductor nano-objects requires combining transmission electron microscopy (TEM) based techniques with electrical and optical measurements on the very same specimen. Recent developments in TEM technologies allow not only the identification and in-situ electrical characterization of a particular object, but also the direct visualization of its modification in-situ by techniques such as Joule heating. Over the past years, we have carried out a number of studies in these fields that are reviewed in this contribution. In particular, we discuss here i) correlated studies where the same unique object is characterized electro-optically and by TEM, ii) in-situ Joule heating studies where a solid-state metal-semiconductor reaction is monitored in the TEM, and iii) in-situ biasing studies to better understand the electrical properties of contacted single nanowires. In addition, we provide detailed fabrication steps for the silicon nitride membrane-chips crucial to these correlated and in-situ measurements.

In2Se3 nanosheets for harmonic mode-locked fiber laser

Two-dimensional materials with a sheet structure have excellent optical, electrical and mechanical properties, and have attracted much attention in recent years, especially In₂Se₃ (the N-type semiconductor compound), which has a rapid development in the fields of materials science and optical communication. In this paper, the nonlinear saturation absorption characteristics of In₂Se₃ are studied. The In₂Se₃ nanosheet dispersion can be used in ultrafast photonics applications. The nonlinear absorption is measured by power dependent method, and the modulation depth and saturation intensity are 3.8% and 246.6 MW cm^-2, respectively. More importantly, In₂Se₃ is used as a saturable absorber (SA) in a passively mode-locked erbium-doped fiber laser. The proposed mode-locked fiber laser is demenstrated with a center wavelength of 1529.4 nm, a fundamental frequency of 5.9 MHz, a spectral width of 3.96 nm, a pulse width of 1.38 ps, and a signal-to-noise ratio of 55 dB. For the first time, harmonic mode-locking with a high-repetition rate of 431 MHz is achieved when the pump power is 360 mW corresponding to 73rd-order harmonic mode locking. It can be seen that In₂Se₃ is indeed a new excellent photonic material, which can be used in fiber optic communication, SAs photonics, laser material processing and light modulators.

Correlated Electro-Optical and Structural Study of Electrically Tunable Nanowire Quantum Dot Emitters

Quantum dots inserted in semiconducting nanowires are an interesting platform for the fabrication of single photon devices. To fully understand the physical properties of these objects, we need to correlate the optical, transport, and structural properties on the same nanostructure. In this work, we study the spectral tunability of the emission of a single quantum dot in a GaN nanowire by applying external bias. The nanowires are dispersed and contacted on electron beam transparent Si₃N₄ membranes, so that transmission electron microscopy observations, photocurrent, and micro-photoluminescence measurements under bias can be performed on the same specimen. The emission from a single dot blue or red shifts when the external electric field compensates or enhances the internal electric field generated by the spontaneous and piezoelectric polarization. A detailed study of two nanowire specimens emitting at 327.5 and 307.5 nm shows spectral shifts at rates of 20 and 12 meV/V, respectively. Theoretical calculations facilitated by the modeling of the exact heterostructure provide a good description of the experimental observations. When the bias-induced band bending is strong enough to favor tunneling of the electron in the dot toward the stem or the cap, the spectral shift saturates and additional transitions associated with charged excitons can be observed.

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.

Atomic-Scale Observation of Reversible Thermally Driven Phase Transformation in 2D In2Se3

Phase transformation in emerging two-dimensional (2D) materials is crucial for understanding and controlling the interplay between structure and electronic properties. In this work, we investigate 2D In₂Se₃ synthesized via chemical vapor deposition, a recently discovered 2D ferroelectric material. We observed that In₂Se₃ layers with thickness ranging from a single layer to ∼20 layers stabilized at the β phase with a superstructure at room temperature. At around 180 K, the β phase converted to a more stable β' phase that was distinct from previously reported phases in 2D In₂Se₃. The kinetics of the reversible thermally driven β-to-β' phase transformation was investigated by temperature-dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, density functional theory calculations reveal in-plane ferroelectricity in the β' phase. Scanning tunneling spectroscopy measurements show that the indirect bandgap of monolayer β' In₂Se₃ is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In₂Se₃ with thickness down to the monolayer limit and the associated electronic properties will provide insights to tune the functionalities of 2D In₂Se₃ and other emerging 2D ferroelectric materials and shed light on their numerous potential applications (e.g., nonvolatile memory devices).

Growth of vertical heterostructures based on orthorhombic SnSe/hexagonal In2Se3 for high-performance photodetectors

Vertical heterostructures based on two-dimensional (2D) layered materials are ideal platforms for electronic structure engineering and novel device applications. However, most of the current heterostructures focus on layered crystals with a similar lattice. In addition, the heterostructures made by 2D materials with different structures are rarely investigated. In this study, we successfully fabricated vertical heterostructures by combining orthorhombic SnSe/hexagonal In₂Se₃ vertical heterostructures using a two-step physical vapor deposition (PVD) method. Structural characterization reveals that the heterostructures are formed of vertically stacked SnSe on the top of the In₂Se₃ film, and vertical heterostructures possess high quality, where In₂Se₃ exposed surface is the (0001) plane and SnSe prefers growing along the [100] direction. Raman maps confirm the precise spatial modulation of the as-grown SnSe/In₂Se₃ heterostructures. In addition, high-performance photodetectors based on the vertical heterostructures were fabricated directly on the substrate, which showed a broadband response, reversibility and stability. Compared with the dark current, the device demonstrated one order magnification of photocurrent, about 186 nA, under 405 nm laser illumination and power of 1.5 mW. Moreover, the device shows an obvious increase in the photocurrent intensity with the changing incident laser power, where I _ph ∝ P ^0.7. Also, the device demonstrated a high responsivity of up to 350 mA W^-1 and a fast response time of about 139 ms. This study broadens the horizon for the synthesis and application of vertical heterostructures based on 2D layered materials with different structures and further develops exciting technologies beyond the reach of the existing materials.

Helical van der Waals crystals with discretized Eshelby twist

The ability to manipulate the twisting topology of van der Waals structures offers a new degree of freedom through which to tailor their electrical and optical properties. The twist angle strongly affects the electronic states, excitons and phonons of the twisted structures through interlayer coupling, giving rise to exotic optical, electric and spintronic behaviours^1-5. In twisted bilayer graphene, at certain twist angles, long-range periodicity associated with moiré patterns introduces flat electronic bands and highly localized electronic states, resulting in Mott insulating behaviour and superconductivity^3,4. Theoretical studies suggest that these twist-induced phenomena are common to layered materials such as transition-metal dichalcogenides and black phosphorus^6,7. Twisted van der Waals structures are usually created using a transfer-stacking method, but this method cannot be used for materials with relatively strong interlayer binding. Facile bottom-up growth methods could provide an alternative means to create twisted van der Waals structures. Here we demonstrate that the Eshelby twist, which is associated with a screw dislocation (a chiral topological defect), can drive the formation of such structures on scales ranging from the nanoscale to the mesoscale. In the synthesis, axial screw dislocations are first introduced into nanowires growing along the stacking direction, yielding van der Waals nanostructures with continuous twisting in which the total twist rates are defined by the radii of the nanowires. Further radial growth of those twisted nanowires that are attached to the substrate leads to an increase in elastic energy, as the total twist rate is fixed by the substrate. The stored elastic energy can be reduced by accommodating the fixed twist rate in a series of discrete jumps. This yields mesoscale twisting structures consisting of a helical assembly of nanoplates demarcated by atomically sharp interfaces with a range of twist angles. We further show that the twisting topology can be tailored by controlling the radial size of the structure.

Theoretical study on the intrinsic properties of In2Se3/MoS2 as a photocatalyst driven by near-infrared, visible and ultraviolet light

Two-dimensional photocatalysts with full optical absorption have attracted widespread attention for water splitting and pollutant degradation, but only few single materials can meet this criterion.

Ultrasensitive 2D/3D Heterojunction Multicolor Photodetectors: A Synergy of Laterally and Vertically Aligned 2D Layered Materials

In this work, a p-type 2D SnS nanofilm containing both laterally and vertically aligned components was successfully deposited on an n-type Si substrate through pulsed-laser deposition. Energy band analysis demonstrates a typical type-II band alignment between SnS and Si, which is beneficial to the separation of photogenerated carriers. The as-fabricated p-SnS/n-Si heterojunction photodetector exhibits multicolor photoresponse from ultraviolet to near-infrared (370-1064 nm). Importantly, the device manifests a high responsivity of 273 A/W, a large external quantum efficiency of 4.2 × 10⁴%, and an outstanding detectivity of 7× 10¹³ Jones (1 Jones = 1 cm Hz^1/2 W^-1), which far outperforms state-of-the-art 2D/3D heterojunction photodetectors incorporating either laterally or vertically aligned 2D layered materials (2DLMs). The splendid performance is ascribed to lateral SnS's dangling-bond-free interface induced efficient carrier separation, vertical SnS's high-speed carrier transport, and collision ionization induced carrier multiplication. In sum, the current work depicts a unique landscape for revolutionary design and advancement of 2DLM-based heterojunction photodetectors.

Flexible and High-Performance All-2D Photodetector for Wearable Devices

Emerging novel applications at the forefront of innovation horizon raise new requirements including good flexibility and unprecedented properties for the photoelectronic industry. On account of diversity in transport and photoelectric properties, 2D layered materials have proven as competent building blocks toward next-generation photodetectors. Herein, an all-2D Bi₂ Te₃ -SnS-Bi₂ Te₃ photodetector is fabricated with pulsed-laser deposition. It is sensitive to broadband wavelength from ultraviolet (370 nm) to near-infrared (808 nm). In addition, it exhibits great durability to bend, with intact photoresponse after 100 bend cycles. Upon 370 nm illumination, it achieves a high responsivity of 115 A W^-1 , a large external quantum efficiency of 3.9 × 10⁴ %, and a superior detectivity of 4.1 × 10¹¹ Jones. They are among the best figures-of-merit of state-of-the-art 2D photodetectors. The synergistic effect of SnS's strong light-matter interaction, efficient carrier separation of Bi₂ Te₃ -SnS interface, expedite carrier injection across Bi₂ Te₃ -SnS interface, and excellent carrier collection of Bi₂ Te₃ topological insulator electrodes accounts for the superior photodetection properties. In summary, this work depicts a facile all-in-one fabrication strategy toward a Bi₂ Te₃ -SnS-Bi₂ Te₃ photodetector. More importantly, it reveals a novel all-2D concept for construction of flexible, broadband, and high-performance photoelectronic devices by integrating 2D layered metallic electrodes and 2D layered semiconducting channels.

A flexible, transparent and high-performance gas sensor based on layer-materials for wearable technology

Gas sensors play a vital role among a wide range of practical applications. Recently, propelled by the development of layered materials, gas sensors have gained much progress. However, the high operation temperature has restricted their further application. Herein, via a facile pulsed laser deposition (PLD) method, we demonstrate a flexible, transparent and high-performance gas sensor made of highly-crystalline indium selenide (In₂Se₃) film. Under UV-vis-NIR light or even solar energy activation, the constructed gas sensors exhibit superior properties for detecting acetylene (C₂H₂) gas at room temperature. We attribute these properties to the photo-induced charger transfer mechanism upon C₂H₂ molecule adsorption. Moreover, no apparent degradation in the device properties is observed even after 100 bending cycles. In addition, we can also fabricate this device on rigid substrates, which is also capable to detect gas molecules at room temperature. These results unambiguously distinguish In₂Se₃ as a new candidate for future application in monitoring C₂H₂ gas at room temperature and open up new opportunities for developing next generation full-spectrum activated gas sensors.

Controlling Growth High Uniformity Indium Selenide (In2Se3) Nanowires via the Rapid Thermal Annealing Process at Low Temperature

High uniformity Au-catalyzed indium selenide (In₂Se₃₎ nanowires are grown with the rapid thermal annealing (RTA) treatment via the vapor-liquid-solid (VLS) mechanism. The diameters of Au-catalyzed In₂Se₃ nanowires could be controlled with varied thicknesses of Au films, and the uniformity of nanowires is improved via a fast pre-annealing rate, 100 °C/s. Comparing with the slower heating rate, 0.1 °C/s, the average diameters and distributions (standard deviation, SD) of In₂Se₃ nanowires with and without the RTA process are 97.14 ± 22.95 nm (23.63%) and 119.06 ± 48.75 nm (40.95%), respectively. The in situ annealing TEM is used to study the effect of heating rate on the formation of Au nanoparticles from the as-deposited Au film. The results demonstrate that the average diameters and distributions of Au nanoparticles with and without the RTA process are 19.84 ± 5.96 nm (30.00%) and about 22.06 ± 9.00 nm (40.80%), respectively. It proves that the diameter size, distribution, and uniformity of Au-catalyzed In₂Se₃ nanowires are reduced and improved via the RTA pre-treated. The systemic study could help to control the size distribution of other nanomaterials through tuning the annealing rate, temperatures of precursor, and growth substrate to control the size distribution of other nanomaterials. Graphical Abstract Rapid thermal annealing (RTA) process proved that it can uniform the size distribution of Au nanoparticles, and then it can be used to grow the high uniformity Au-catalyzed In₂Se₃ nanowires via the vapor-liquid-solid (VLS) mechanism. Comparing with the general growth condition, the heating rate is slow, 0.1 °C/s, and the growth temperature is a relatively high growth temperature, > 650 °C. RTA pre-treated growth substrate can form smaller and uniform Au nanoparticles to react with the In₂Se₃ vapor and produce the high uniformity In₂Se₃ nanowires. The in situ annealing TEM is used to realize the effect of heating rate on Au nanoparticle formation from the as-deposited Au film. The byproduct of self-catalyzed In₂Se₃ nanoplates can be inhibited by lowering the precursors and growth temperatures.

Decompression-Driven Superconductivity Enhancement in In2 Se3

An unexpected superconductivity enhancement is reported in decompressed In₂ Se₃ . The onset of superconductivity in In₂ Se₃ occurs at 41.3 GPa with a critical temperature (T_c ) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest T_c that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of T_c is driven by the pressure-induced R-3m to I-43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression-induced superconductivity enhancement implies that it is possible to maintain pressure-induced superconductivity at lower or even ambient pressures with better superconducting performance.

Phase-Defined van der Waals Schottky Junctions with Significantly Enhanced Thermoelectric Properties

We demonstrate a van der Waals Schottky junction defined by crystalline phases of multilayer In₂Se₃. Besides ideal diode behaviors and the gate-tunable current rectification, the thermoelectric power is significantly enhanced in these junctions by more than three orders of magnitude compared with single-phase multilayer In₂Se₃, with the thermoelectric figure-of-merit approaching ∼1 at room temperature. Our results suggest that these significantly improved thermoelectric properties are not due to the 2D quantum confinement effects but instead are a consequence of the Schottky barrier at the junction interface, which leads to hot carrier transport and shifts the balance between thermally and field-driven currents. This "bulk" effect extends the advantages of van der Waals materials beyond the few-layer limit. Adopting such an approach of using energy barriers between van der Waals materials, where the interface states are minimal, is expected to enhance the thermoelectric performance in other 2D materials as well.

Facile Synthesis of γ-In2 Se3 Nanoflowers toward High Performance Self-Powered Broadband γ-In2 Se3 /Si Heterojunction Photodiode

An effective colloidal process involving the hot-injection method is developed to synthesize uniform nanoflowers consisting of 2D γ-In₂ Se₃ nanosheets. By exploiting the narrow direct bandgap and high absorption coefficient in the visible light range of In₂ Se₃ , a high-quality γ-In₂ Se₃ /Si heterojunction photodiode is fabricated. This photodiode shows a high photoresponse under light illumination, short response/recovery times, and long-term durability. In addition, the γ-In₂ Se₃ /Si heterojunction photodiode is self-powered and displays a broadband spectral response ranging from UV to IR with a high responsivity and detectivity. These excellent performances make the γ-In₂ Se₃ /Si heterojunction very interesting as highly efficient photodetectors.

Generation and the role of dislocations in single-crystalline phase-change In2Se3 nanowires under electrical pulses

We report the observation of the generation of dislocations in single-crystal phase-change In2Se3 nanowires under electrical pulses and the impact of these dislocations on electrical properties. Particularly, we correlated the atomic-scale structural characteristics with local electrical resistance variations, by performing transmission electron microscopy and scanning Kelvin probe microscopy on the same nanowires. By coupling the experimental results with first-principles density functional theory calculations, we show that the immobile dislocations are generated via vacancy condensations. Importantly, these dislocations lead to several orders of magnitude increase in the electrical resistance, while maintaining the single crystallinity of the lattice. These results significantly advance the fundamental understanding of the structure-property relation in this phase-change material under transient electrical excitations. From a practical perspective, the significant increase in the electrical resistance, driven by the formation of dislocations, can be exploited as a new electronic state in the single-crystalline phase in this phase-change material.

Synergistic Effect of Hybrid Multilayer In2Se3 and Nanodiamonds for Highly Sensitive Photodetectors

Layered materials have rapidly established themselves as intriguing building blocks for next-generation photodetection platforms in view of their exotic electronic and optical attributes. However, both relatively low mobility and heavier electron effective mass limit layered materials for high-performance applications. Herein, we employed nanodiamonds (NDs) to promote the performance of multilayer In2Se3 photodetectors for the first time. This hybrid NDs-In2Se3 photodetector showed a tremendous promotion of photodetection performance in comparison to pristine In2Se3 ones. This hybrid devices exhibited remarkable detectivity (5.12 × 10(12) jones), fast response speed (less than 16.6 ms), and decent current on/off ratio (∼2285) simultaneously. These parameters are superior to most reported layered materials based photodetectors and even comparable to the state-of-the-art commercial photodetectors. Meanwhile, we attributed this excellent performance to the synergistic effect between NDs and the In2Se3. They can greatly enhance the broad spectrum absorption and promote the injection of photoexcited carrier in NDs to In2Se3. These results actually open up a new scenario for designing and fabricating innovative optoelectronic systems.

Quantification of layered patterns with structural anisotropy: a comparison of biological and geological systems

Large-scale patterns evident from satellite images of aeolian landforms on Earth and other planets; those of intermediate scale in marine and terrestrial sand ripples and sediment profiles; and small-scale patterns such as lamellae in the bones of vertebrates and annuli in fish scales are each represented by layers of different thicknesses and lengths. Layered patterns are important because they form a record of the state of internal and external factors that regulate pattern formation in these geological and biological systems. It is therefore potentially possible to recognize trends, periodicities, and events in the history of the formation of these systems among the incremental sequences. Though the structures and sizes of these 2-D patterns are typically scale-free, they are also characteristically anisotropic; that is, the number of layers and their absolute thicknesses vary significantly during formation. The aim of the present work is to quantify the structure of layered patterns and to reveal similarities and differences in the processing and interpretation of layered landforms and biological systems. To reach this goal we used N-partite graph and Boolean functions to quantify the structure of layers and plot charts for "layer thickness vs. layer number" and "layer area vs. layer number". These charts serve as a source of information about events in the history of formation of layered systems. The concept of synchronization of layer formation across a 2-D plane is introduced to develop the procedure for plotting "layer thickness vs. layer number" and "layer area vs. layer number", which takes into account the structural anisotropy of layered patterns and increase signal-to-noise ratio in charts. Examples include landforms on Mars and Earth and incremental layers in human and iguana bones.

Thickness-Dependent Dielectric Constant of Few-Layer In₂Se₃ Nanoflakes

The dielectric constant or relative permittivity (ε(r)) of a dielectric material, which describes how the net electric field in the medium is reduced with respect to the external field, is a parameter of critical importance for charging and screening in electronic devices. Such a fundamental material property is intimately related to not only the polarizability of individual atoms but also the specific atomic arrangement in the crystal lattice. In this Letter, we present both experimental and theoretical investigations on the dielectric constant of few-layer In2Se3 nanoflakes grown on mica substrates by van der Waals epitaxy. A nondestructive microwave impedance microscope is employed to simultaneously quantify the number of layers and local electrical properties. The measured ε(r) increases monotonically as a function of the thickness and saturates to the bulk value at around 6-8 quintuple layers. The same trend of layer-dependent dielectric constant is also revealed by first-principles calculations. Our results of the dielectric response, being ubiquitously applicable to layered 2D semiconductors, are expected to be significant for this vibrant research field.

Polarization dependent photocurrent in the Bi2Te3 topological insulator film for multifunctional photodetection

Three dimensional Z₂ Topological insulator (TI) is an unconventional phase of quantum matter possessing insulating bulk state as well as time-reversal symmetry-protected Dirac-like surface state, which is demonstrated by extensive experiments based on surface sensitive detection techniques. This intriguing gapless surface state is theoretically predicted to exhibit many exotic phenomena when interacting with light, and some of them have been observed. Herein, we report the first experimental observation of novel polarization dependent photocurrent of photodetectors based on the TI Bi₂Te₃ film under irradiation of linearly polarized light. This photocurrent is linearly dependent on both the light intensity and the applied bias voltage. To pursue the physical origin of the polarization dependent photocurrent, we establish the basic TI surface state model to treat the light irradiation as a perturbation, and we adopt the Boltzmann equation to calculate the photocurrent. It turns out that the theoretical results are in nice qualitative agreement with the experiment. These findings show that the polycrystalline TI Bi₂Te₃ film working as a multifunctional photodetector can not only detect the light intensity, but also measure the polarization state of the incident light, which is remarkably different from conventional photodetectors that usually only detect the light intensity.

A Roadmap for Controlled Production of Topological Insulator Nanostructures and Thin Films

The group V-VI chalcogenide semiconductors (Bi2 Se3 , Bi2 Te3 , and Sb2 Te3 ) have long been known as thermoelectric materials. Recently, they have been once more generating interest because Bi2 Se3 , Bi2 Te3 and Sb2 Te3 have been crowned as 3D topological insulators (TIs), which have insulating bulk gaps and metallic Dirac surface states. One big challenge in the study of TIs is the lack of high-quality materials with few defects and insulating bulk states. To manifest the topological surface states, it is critical to suppress the contribution from the bulk carriers. Controlled production of TI nanostructures that have a large surface-to-volume ratio is an efficient way to reduce the bulk conductance and to significantly enhance the topological surface conduction. In this review article, the recent progress on the preparation of TI nanostructures is highlighted. Basic production methods for TI nanostructures are introduced in detail. Furthermore, several specific production approaches to reduce the residual bulk carriers from defects are summarized. Finally, the progress and the prospects of the production of TI-based heterostructures, which hold promise in both fundamental study and novel applications are discussed.

In Situ Temperature-Dependent Transmission Electron Microscopy Studies of Pseudobinary mGeTe·Bi₂Te₃ (m = 3-8) Nanowires and First-Principles Calculations

Phase-change nanowires (NWs) have emerged as critical materials for fast-switching nonvolatile memory devices. In this study, we synthesized a series of mGeTe·Bi2Te3 (GBT) pseudobinary alloy NWs-Ge3Bi2Te6 (m = 3), Ge4Bi2Te7 (m = 4), Ge5Bi2Te8 (m = 5), Ge6Bi2Te9 (m = 6), and Ge8Bi2Te11 (m = 8)-and investigated their composition-dependent thermal stabilities and electrical properties. As m decreases, the phase of the NWs evolves from the cubic (C) to the hexagonal (H) phase, which produces unique superlattice structures that consist of periodic 2.2-3.8 nm slabs for m = 3-8. In situ temperature-dependent transmission electron microscopy reveals the higher thermal stability of the compositions with lower m values, and a phase transition from the H phase into the single-crystalline C phase at high temperatures (400 °C). First-principles calculations, performed for the superlattice structures (m = 1-8) of GBT and mGeTe·Sb2Te3 (GST), show an increasing stability of the H phase (versus the C phase) with decreasing m; the difference in stability being more marked for GBT than for GST. The calculations explain remarkably the phase evolution of the GBT and GST NWs as well as the composition-dependent thermal stabilities. Measurement of the current-voltage curves for individual GBT NWs shows that the resistivity is in the range 3-25 mΩ·cm, and the resistivity of the H phase is lower than that of the C phase, which has been supported by the calculations.

Two-dimensional layered chalcogenides: from rational synthesis to property control via orbital occupation and electron filling

Electron occupation of orbitals in two-dimensional (2D) layered materials controls the magnitude and anisotropy of the interatomic electron transfer and exerts a key influence on the chemical bonding modes of 2D layered lattices. Therefore, their orbital occupations are believed to be responsible for massive variations of the physical and chemical properties from electrocatalysis and energy storage, to charge density waves, superconductivity, spin-orbit coupling, and valleytronics. Especially in nanoscale structures such as nanoribbons, nanoplates, and nanoflakes, 2D layered materials provide opportunities to exploit new quantum phenomena. In this Account, we report our recent progress in the rational design and chemical, electrochemical, and electrical modulations of the physical and chemical properties of layered nanomaterials via modification of the electron occupation in their electronic structures. Here, we start with the growth and fabrication of a group of layered chalcogenides with varied orbital occupation (from 4d/5d electron configuration to 5p/6p electron configuration). The growth techniques include bottom-up methods, such as vapor-liquid-solid growth and vapor-solid growth, and top-down methods, such as mechanical exfoliation with tape and AFM tip scanning. Next, we demonstrate the experimental strategies for the tuning of the chemical potential (orbital occupation tuned with electron filling) and the resulting modulation of the electronic states of layered materials, such as electric-double-layer gating, electrochemical intercalation, and chemical intercalation with molecule and zerovalence metal species. Since the properties of layered chalcogenides are normally dominated by the specific band structure around which the chemical potential is sitting, their desired electronic states and properties can be modulated in a large range, showing unique phenomena including quantum electronic transport and extraordinary optical transmittance. As the most important part of this Account, we further demonstrate some representative examples for the tuning of catalytic, optical, electronic, and spintronic properties of 2D layered chalcogenides, where one can see not only edge-state induced enhancement of catalysis, quantum Aharonov-Bohm interference of the topological surface states, intercalation modulated extraordinary transmittance, and surface plasmonics but also external gating induced superconductivity and spin-coupled valley photocurrent. Since our findings reflect the critical influences of the electron filling of orbital occupation to the properties in 2D layered chalcogenides, we thus last highlight the importance and the prospective of orbital occupation in 2D layered materials for further exploring potential functionalized applications.

Synthesis and characterization of self-assembled three-dimensional flower-like iron(iii) oxide–indium(iii) oxide binary nanocomposites

Three-dimensional flower-like iron(iii) oxide–indium(iii) oxide binary metal oxide nanocomposites were successfully fabricated by a simple and economical route; and it can be used as fillers to significantly enhance the thermal resistance of silicone rubber under nitrogen.

Phototransistor based on single In₂Se₃ nanosheets

Micrometer-sized single-crystalline In₂Se₃ nanosheets are synthesized by epitaxial growth from In₂Se₃nanowires. The In₂Se₃ nanosheets possess anisotropic structural configuration with intralayer covalent bonding and interlayer van der Waals bonding. Phototransistors based on the In₂Se₃ nanosheets are realized, and the devices show high photoresponsivity and high photo On/Off ratio up to two orders. The photo-gating effect can be modulated by the gate bias, indicating potential utility of the In₂Se₃ nanosheets in a variety of optoelectronic applications.

Thermally phase-transformed In2Se3 nanowires for highly sensitive photodetectors

The photoresponse characteristics of In2Se3 nanowire photodetectors with the κ-phase and α-phase structures are investigated. The as-grown κ-phase In2Se3 nanowires by the vapor-liquid-solid technique are phase-transformed to the α-phase nanowires by thermal annealing. The photoresponse performances of the κ-phase and α-phase In2Se3 nanowire photodetectors are characterized over a wide range of wavelengths (300-900 nm). The phase of the nanowires is analyzed using a high-resolution transmission microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The electrical conductivity and photoresponse characteristics are significantly enhanced in the α-phase due to smaller bandgap structure compared to the κ-phase nanowires. The spectral responsivities of the α-phase devices are 200 times larger than those of the κ-phase devices. The superior performance of the thermally phase-transformed In2Se3 nanowire devices offers an avenue to develop highly sensitive photodetector applications.

Indium selenides: structural characteristics, synthesis and their thermoelectric performances

Indium selenides have attracted extensive attention in high-efficiency thermoelectrics for waste heat energy conversion due to their extraordinary and tunable electrical and thermal properties. This Review aims to provide a thorough summary of the structural characteristics (e.g. crystal structures, phase transformations, and structural vacancies) and synthetic methods (e.g. bulk materials, thin films, and nanostructures) of various indium selenides, and then summarize the recent progress on exploring indium selenides as high-efficiency thermoelectric materials. By highlighting challenges and opportunities in the end, this Review intends to shine some light on the possible approaches for thermoelectric performance enhancement of indium selenides, which should open up an opportunity for applying indium selenides in the next-generation thermoelectric devices.

Wide range photodetector based on catalyst free grown indium selenide microwires

We first report the catalyst free growth of indium selenide microwires through a facile approach in a horizontal tube furnace using indium and selenium elemental powders as precursors. The synthesized microwires are γ-phase, high quality, single crystalline and grown along the [112̅0] direction. The wires have a uniform diameter of ∼1 μm and lengths of several micrometers. Photodetectors fabricated from synthesized microwires show reliable and stable photoresponse exhibiting a photoresponsivity of 0.54 A/W, external quantum efficiency of 1.23 at 633 nm with 4 V bias. The photodetector has a reasonable response time of 0.11 s and specific detectivity of 3.94 × 10(10) Jones at 633 nm with a light detection range from 350 to 1050 nm, covering the UV-vis-NIR region. The photoresponse shown by single wire is attributed to direct band gap (Eg = 1.3 eV) and superior single crystalline quality. The photoresponsive studies of single microwires clearly suggest the use of this new and facile growth technique without using catalysts for fabrication of indium selenide microwires in next-generation sensors and detectors for commercial and military applications.

Extraordinary photoresponse in two-dimensional In(2)Se(3) nanosheets

We demonstrate extraordinary photoconductive behavior in two-dimensional (2D) crystalline indium selenide (In2Se3) nanosheets. Photocurrent measurements reveal that semiconducting In2Se3 nanosheets have an extremely high response to visible light, exhibiting a photoresponsivity of 3.95 × 10(2) A·W(-1) at 300 nm with an external quantum efficiency greater than 1.63 × 10(5) % at 5 V bias. The key figures-of-merit exceed that of graphene and other 2D material-based photodetectors reported to date. In addition, the photodetector has a fast response time of 1.8 × 10(-2) s and a specific detectivity of 2.26 × 10(12) Jones. The photoconductive response of α-In2Se3 nanosheets extends into ultraviolet, visible, and near-infrared spectral regions. The high photocurrent response is attributed to the direct band gap (EG = 1.3 eV) of In2Se3 combined with a large surface-area-to-volume ratio and a self-terminated/native-oxide-free surface, which help to reduce carrier recombination while keeping fast response, allowing for real-time detection under very low-light conditions.

CuInSe2 nanowires from facile chemical transformation of In2Se3 and their integration in single-nanowire devices

Nanowire solar cells are receiving a significant amount of attention for their potential to improve light absorption and charge collection in photovoltaics. Single-nanowire solar cells offer the ability to investigate performance limits for macroscale devices, as well as the opportunity for in-depth structural characterization and property measurement in small working devices. Copper indium selenide (CIS) is a material uniquely suited to these investigations. Not only could nanowire solar cells of CIS perhaps allow efficient macroscale photovoltaics to be fabricated while reducing the amount of CIS required, important for a system with possible resource limitations, but it is also a photovoltaic material for which fundamental understanding has been elusive. We here present a recipe for a scaled up vapor liquid solid based synthesis of CIS nanowires, in-depth material and property correlation of single crystalline CIS nanowires, and the first report of a single CIS nanowire solar cell. The synthesis was accomplished by annealing copper-coated In2Se3 nanowires at a moderate temperature of 350 °C, leading to solid-state reaction forming CIS nanowires. These nanowires are p-type with a resitivity of 6.5 Ωcm. Evidence is observed for a strong diameter dependence on the nanowire transport properties. The single-nanowire solar cells have an open-circuit voltage of 500 mV and a short-circuit current of 2 pA under AM 1.5 illumination.

Surface oxide effect on optical sensing and photoelectric conversion of α-In2Se3 hexagonal microplates

The surface formation oxide assists of visible to ultraviolet photoelectric conversion in α-In2Se3 hexagonal microplates has been explored. Hexagonal α-In2Se3 microplates with the sizes of 10s to 100s of micrometers were synthesized and prepared by the chemical vapor transport method using ICl3 as a transport agent. Many vacancies and surface imperfection states have been found in the bulk and on the surface of the microplate because of the intrinsic defect nature of α-In2Se3. To discover physical and chemical properties and finding technological uses of α-In2Se3, several experiments including transmission electron miscopy (TEM), X-ray photoelectron spectroscopy (XPS), surface photovoltage (SPV), photoluminescence (PL), surface photoresponse (SPR), photoconductivity (PC), and thermoreflectance (TR) measurements have been carried out. Experimental results of TEM, XPS, SPV, PL, and SPR measurements show that a surface oxidation layer α-In2Se3-3xO3x (0 ≤ x ≤ 1) has formed on the crystal face of α-In2Se3 in environmental air with the inner layer content close to In2Se3 but the outermost layer content approaching In2O3. The near band edge transitions of α-In2Se3 microplates have been probed experimentally by TR and PC measurements. The direct band gap of α-In2Se3 has been determined to be 1.453 eV. The SPV result shows a maximum quantum efficiency of the surface oxide α-In2Se3-3xO3x (0 ≤ x ≤ 1) that presents a peak photoresponse near 2.18 eV. The analyses of SPV, SPR, PL, TR, and PC measurements revealed that the surface oxide layer facilitates the conversion of the ultraviolet to the visible range while the native defects (Se and In vacancies) sustain photoconductivity in the near-infrared region. On the basis of the experimental results a wide-energy-range photodetector that combines PC- and SPR-mode operations for α-In2Se3 microplate has been made. The testing results show a well-behaved function of photoelectric conversion in the near-infrared to ultraviolet region via the auxiliary forming of surface oxide on the crystalline face of the α-In2Se3 microplates.

Correlation of polarity and crystal structure with optoelectronic and transport properties of GaN/AlN/GaN nanowire sensors

GaN nanowires (NWs) with an AlN insertion were studied by correlated optoelectronic and aberration-corrected scanning transmission electron microscopy (STEM) characterization on the same single NW. Using aberration-corrected annular bright field and high angle annular dark field STEM, we identify the NW growth axis to be the N-polar [000-1] direction. The electrical transport characteristics of the NWs are explained by the polarization-induced asymmetric potential profile and by the presence of an AlN/GaN shell around the GaN base of the wire. The AlN insertion blocks the electron flow through the GaN core, confining the current to the radial GaN outer shell, close to the NW sidewalls, which increases the sensitivity of the photocurrent to the environment and in particular to the presence of oxygen. The desorption of oxygen adatoms in vacuum leads to a reduction of the nonradiative surface trap density, increasing both dark current and photocurrent.

Anisotropic photoresponse properties of single micrometer-sized GeSe nanosheet

Micrometer-sized single-crystal GeSe nanosheets have been synthesized by a solution method. The single GeSe nanosheet exhibits novel anisotropic photoresponse properties in two photodetectors based on individual nanosheet. The on/off switching ratio of the photodetector perpendicular to the nanosheet is 3.5 times higher than that parallel to the nanosheet.

In situ transmission electron microscopy observation of nanostructural changes in phase-change memory

Phase-change memory (PCM) has been researched extensively as a promising alternative to flash memory. Important studies have focused on its scalability, switching speed, endurance, and new materials. Still, reliability issues and inconsistent switching in PCM devices motivate the need to further study its fundamental properties. However, many investigations treat PCM cells as black boxes; nanostructural changes inside the devices remain hidden. Here, using in situ transmission electron microscopy, we observe real-time nanostructural changes in lateral Ge(2)Sb(2)Te(5) (GST) PCM bridges during switching. We find that PCM devices with similar resistances can exhibit distinct threshold switching behaviors due to the different initial distribution of nanocrystalline and amorphous domains, explaining variability of switching behaviors of PCM cells in the literature. Our findings show a direct correlation between nanostructure and switching behavior, providing important guidelines in the design and operation of future PCM devices with improved endurance and lower variability.